team with images generously made available by the bibliothèque nationale de france (bnf/gallica) at http://gallica.bnf.fr the future of astronomy by professor edward c. pickering reprinted from the popular science monthly, august, . the future of astronomy[ ] by professor edward c. pickering harvard college observatory it is claimed by astronomers that their science is not only the oldest, but that it is the most highly developed of the sciences. indeed it should be so, since no other science has ever received such support from royalty, from the state and from the private individual. however this may be, there is no doubt that in recent years astronomers have had granted to them greater opportunities for carrying on large pieces of work than have been entrusted to men in any other department of pure science. one might expect that the practical results of a science like physics would appeal to the man who has made a vast fortune through some of its applications. the telephone, the electric transmission of power, wireless telegraphy and the submarine cable are instances of immense financial returns derived from the most abstruse principles of physics. yet there are scarcely any physical laboratories devoted to research, or endowed with independent funds for this object, except those supported by the government. the endowment of astronomical observatories devoted to research, and not including that given for teaching, is estimated to amount to half a million dollars annually. several of the larger observatories have an annual income of fifty thousand dollars. [ ] commencement address at case school of applied science, cleveland, may , . i once asked the wisest man i know, what was the reason for this difference. he said that it was probably because astronomy appealed to the imagination. a practical man, who has spent all his life in his counting room or mill, is sometimes deeply impressed with the vast distances and grandeur of the problems of astronomy, and the very remoteness and difficulty of studying the stars attract him. my object in calling your attention to this matter is the hope that what i have to say of the organization of astronomy may prove of use to those interested in other branches of science, and that it may lead to placing them on the footing they should hold. my arguments apply with almost equal force to physics, to chemistry, and in fact to almost every branch of physical or natural science, in which knowledge may be advanced by observation or experiment. the practical value of astronomy in the past is easily established. without it, international commerce on a large scale would have been impossible. without the aid of astronomy, accurate boundaries of large tracts of land could not have been defined and standard time would have been impossible. the work of the early astronomers was eminently practical, and appealed at once to every one. this work has now been finished. we can compute the positions of the stars for years, almost for centuries, with all the accuracy needed for navigation, for determining time or for approximate boundaries of countries. the investigations now in progress at the greatest observatories have little, if any, value in dollars and cents. they appeal, however, to the far higher sense, the desire of the intellectual human being to determine the laws of nature, the construction of the material universe, and the properties of the heavenly bodies of which those known to exist far outnumber those that can be seen. three great advances have been made in astronomy. first, the invention of the telescope, with which we commonly associate the name of galileo, from the wonderful results he obtained with it. at that time there was practically no science in america, and for more than two centuries we failed to add materially to this invention. half a century ago the genius of the members of one family, alvan clark and his two sons, placed america in the front rank not only in the construction, but in the possession, of the largest and most perfect telescopes ever made. it is not easy to secure the world's record in any subject. the clarks constructed successively, the -inch lens for chicago, the -inch for washington, the -inch for pulkowa, the -inch for lick and the -inch for yerkes. each in turn was the largest yet made, and each time the clarks were called upon to surpass the world's record, which they themselves had already established. have we at length reached the limit in size? if we include reflectors, no, since we have mirrors of inches aperture at mt. wilson and cambridge, and a still larger one of inches has been undertaken. it is more than doubtful, however, whether a further increase in size is a great advantage. much more depends on other conditions, especially those of climate, the kind of work to be done and, more than all, the man behind the gun. the case is not unlike that of a battleship. would a ship a thousand feet long always sink one of five hundred feet? it seems as if we had nearly reached the limit of size of telescopes, and as if we must hope for the next improvement in some other direction. the second great advance in astronomy originated in america, and was in an entirely different direction, the application of photography to the study of the stars. the first photographic image of a star was obtained in , by george p. bond, with the assistance of mr. j.a. whipple, at the harvard college observatory. a daguerreotype plate was placed at the focus of the -inch equatorial, at that time one of the two largest refracting telescopes in the world. an image of [greek: alpha] lyræ was thus obtained, and for this mr. bond received a gold medal at the first international exhibition, that at the crystal palace, in london, in . in , mr. bond, then professor bond, director of the harvard observatory, again took up the matter with collodion wet plates, and in three masterly papers showed the advantages of photography in many ways. the lack of sensitiveness of the wet plate was perhaps the only reason why its use progressed but slowly. quarter of a century later, with the introduction of the dry plate and the gelatine film, a new start was made. these photographic plates were very sensitive, were easily handled, and indefinitely long exposures could be made with them. as a result, photography has superseded visual observations, in many departments of astronomy, and is now carrying them far beyond the limits that would have been deemed possible a few years ago. the third great advance in astronomy is in photographing the spectra of the stars. the first photograph showing the lines in a stellar spectrum was obtained by dr. henry draper, of new york, in . sir william huggins in had obtained an image of the spectrum of sirius, on a photographic plate, but no lines were visible in it. in he again took up the subject, and, by an early publication, preceded dr. draper. when we consider the attention the photography of stellar spectra is receiving at the present time, in nearly all the great observatories in the world, it may well be regarded as the third great advance in astronomy. what will be the fourth advance, and how will it be brought about? to answer this question we must consider the various ways in which astronomy, and for that matter any other science, may be advanced. first, by educating astronomers. there are many observatories where excellent instruction in astronomy is given, either to the general student or to one who wishes to make it his profession. at almost any active observatory a student would be received as a volunteer assistant. unfortunately, few young men can afford to accept an unpaid position, and the establishment of a number of fellowships each offering a small salary sufficient to support the student would enable him to acquire the necessary knowledge to fill a permanent position. the number of these scholarships should not be large, lest more students should undertake the work than would be required to fill the permanent paying positions in astronomy, as they become vacant. in europe, a favorite method of aiding science is to offer a prize for the best memoir on a specified subject. on theoretical grounds this is extremely objectionable. since the papers presented are anonymous and confidential, no one but the judges know how great is the effort wasted in duplication. the larger the prize, the greater the injury to science, since the greater will be the energy diverted from untried fields. it would be much wiser to invite applications, select the man most likely to produce a useful memoir, and award the prize to him if he achieved success. the award of a medal, if of great intrinsic value, would be an unwise expenditure. the victoria cross is an example of a successful foundation, highly prized, but of small intrinsic value. if made of gold, it would carry no greater honor, and would be more liable to be stolen, melted down or pawned. honorary membership in a famous society, or honorary degrees, have great value if wisely awarded. both are highly prized, form an excellent stimulus to continued work, and as they are both priceless, and without price, they in no way diminish the capacity for work. i recently had occasion to compare the progress in various sciences of different countries, and found that the number of persons elected as foreign associates of the seven great national societies of the world was an excellent test. eighty-seven persons were members of two or more of these societies. only six are residents of the united states, while an equal number come from saxony, which has only a twentieth of the population. of the six residents here, only three were born in the united states. not a single mathematician, or doctor, from this country appears on the list. only in astronomy are we well represented. out of a total of ten astronomers, four come from england, and three from the united states. comparing the results for the last one hundred and fifty years, we find an extraordinary growth for the german races, an equally surprising diminution for the french and other latin races, while the proportion of englishmen has remained unchanged. a popular method of expending money, both by countries and by individuals, is in sending expeditions to observe solar eclipses. these appeal both to donors and recipients. the former believe that they are making a great contribution to science, while the latter enjoy a long voyage to a distant country, and in case of clouds they are not expected to make any scientific return. if the sky is clear at the time of the eclipse, the newspapers of the next day report that great results have been secured, and after that nothing further is ever heard. exceptions should be made of the english eclipse committee and the lick observatory, which, by long continued study and observation, are gradually solving the difficult problems which can be reached in this way only. the gift of a large telescope to a university is of very doubtful value, unless it is accompanied, first, by a sum much greater than its cost, necessary to keep it employed in useful work, and secondly, to require that it shall be erected, not on the university grounds, but in some region, probably mountainous or desert, where results of real value can be obtained. having thus considered, among others, some of the ways in which astronomy is not likely to be much advanced, we proceed to those which will secure the greatest scientific return for the outlay. one of the best of these is to create a fund to be used in advancing research, subject only to the condition that results of the greatest possible value to science shall be secured. one advantage of this method is that excellent results may be obtained at once from a sum, either large or small. whatever is at first given may later be increased indefinitely, if the results justify it. one of the wisest as well as the greatest of donors has said: "find the particular man," but unfortunately, this plan has been actually tried only with some of the smaller funds. any one who will read the list of researches aided by the rumford fund, the elizabeth thompson fund or the bruce fund of will see that the returns are out of all proportion to the money expended. the trustees of such a fund as is here proposed should not regard themselves as patrons conferring a favor on those to whom grants are made, but as men seeking for the means of securing large scientific returns for the money entrusted to them. an astronomer who would aid them in this work, by properly expending a grant, would confer rather than receive a favor. they should search for astronomical bargains, and should try to purchase results where the money could be expended to the best advantage. they should make it their business to learn of the work of every astronomer engaged in original research. a young man who presented a paper of unusual importance at a scientific meeting, or published it in an astronomical journal, would receive a letter inviting him to submit plans to the trustees, if he desired aid in extending his work. in many cases, it would be found that, after working for years under most unfavorable conditions, he had developed a method of great value and had applied it to a few stars, but must now stop for want of means. a small appropriation would enable him to employ an assistant who, in a short time, could do equally good work. the application of this method to a hundred or a thousand stars would then be only a matter of time and money. the american astronomical society met last august at a summer resort on lake erie. about thirty astronomers read papers, and in a large portion of the cases the appropriation of a few hundred dollars would have permitted a great extension in these researches. a sad case is that of a brilliant student who may graduate at a college, take a doctor's degree in astronomy, and perhaps pass a year or two in study at a foreign observatory. he then returns to this country, enthusiastic and full of ideas, and considers himself fortunate in securing a position as astronomer in a little country college. he now finds himself overwhelmed with work as a teacher, without time or appliances for original work. what is worse, no one sympathizes with him in his aspirations, and after a few years he abandons hope and settles down to the dull routine of lectures, recitations and examinations. a little encouragement at the right time, aid by offering to pay for an assistant, for a suitable instrument, or for publishing results, and perhaps a word to the president of his college if the man showed real genius, might make a great astronomer, instead of a poor teacher. for several years, a small fund, yielding a few hundred dollars annually, has been disbursed at harvard in this way, with very encouraging results. a second method of aiding astronomy is through the large observatories. these institutions, if properly managed, have after years of careful study and trial developed elaborate systems of solving the great problems of the celestial universe. they are like great factories, which by taking elaborate precautions to save waste at every point, and by improving in every detail both processes and products, are at length obtaining results on a large scale with a perfection and economy far greater than is possible by individuals, or smaller institutions. the expenses of such an observatory are very large, and it has no pecuniary return, since astronomical products are not salable. a great portion of the original endowment has been spent on the plant, expensive buildings and instruments. current expenditures, like library expenses, heating, lighting, etc., are independent of the output. it is like a man swimming up stream. he may struggle desperately, and yet make no progress. any gain in power effects a real advance. this is the condition of nearly all the larger observatories. their income is mainly used for current expenses, which would be nearly the same whatever their output. a relatively small increase in income can thus be spent to great advantage. the principal instruments are rarely used to their full capacities, and the methods employed could be greatly extended without any addition to the executive or other similar expenses. a man superintending the work of several assistants can often have their number doubled, and his output increased in nearly the same proportion, with no additional expense except the moderate one of their salaries. a single observatory could thus easily do double the work that could be accomplished if its resources were divided between two of half the size. a third, and perhaps the best, method of making a real advance in astronomy is by securing the united work of the leading astronomers of the world. the best example of this is the work undertaken in by the astronomische gesellschaft, the great astronomical society of the world. the sky was divided into zones, and astronomers were invited to measure the positions of all the stars in these zones. the observation of two of the northern and two of the southern zones were undertaken by american observatories. the zone from + ° to + ° was undertaken by the chicago observatory, but was abandoned owing to the great fire of , and the work was assumed and carried to completion by the dudley observatory at albany. the zone from + ° to + ° was undertaken by harvard. an observer and corps of assistants worked on this problem for a quarter of a century. the completed results now fill seven quarto volumes of our annals. of the southern zones, that from - ° to - ° was undertaken by the naval observatory at washington, and is now finished. the zone from - ° to - ° was undertaken at harvard, and a second observer and corps of assistants have been working on it for twenty years. it is now nearly completed, and we hope to begin its publication this year. the other zones were taken by european astronomers. as a result of the whole, we have the precise positions of nearly a hundred and fifty thousand stars, which serve as a basis for the places of all the objects in the sky. another example of cooperative work is a plan proposed by the writer in , at the celebration of the two-hundredth anniversary of the birth of franklin. it was proposed, first to find the best place in the world for an astronomical observatory, which would probably be in south africa, to erect there a telescope of the largest size, a reflector of seven feet aperture. this instrument should be kept at work throughout every clear night, taking photographs according to a plan recommended by an international committee of astronomers. the resulting plates should not be regarded as belonging to a single institution, but should be at the service of whoever could make the best use of them. copies of any, or all, would be furnished at cost to any one who wished for them. as an example of their use, suppose that an astronomer at a little german university should discover a law regulating the stars in clusters. perhaps he has only a small telescope, near the smoke and haze of a large city, and has no means of securing the photographs he needs. he would apply to the committee, and they would vote that ten photographs of twenty clusters, each with an exposure of an hour, should be taken with the large telescope. this would occupy about a tenth part of the time of the telescope for a year. after making copies, the photographs would be sent to the astronomer who would perhaps spend ten years in studying and measuring them. the committee would have funds at their disposal to furnish him, if necessary, with suitable measuring instruments, assistants for reducing the results, and means for publication. they would thus obtain the services of the most skilful living astronomers, each in his own special line of work, and the latter would obtain in their own homes material for study, the best that the world could supply. undoubtedly, by such a combination if properly organized, results could be obtained far better than is now possible by the best individual work, and at a relatively small expense. many years of preparation will evidently be needed to carry out such a plan, and to save time we have taken the first step and have sent a skilful and experienced observer to south africa to study its climate and compare it with the experience he has gained during the last twenty years from a similar study of the climate of south america and the western portion of the united states. the next question to be considered is in what direction we may expect the greatest advance in astronomy will be made. fortunate indeed would be the astronomer who could answer this question correctly. when ptolemy made the first catalogue of the stars, he little expected that his observations would have any value nearly two thousand years later. the alchemists had no reason to doubt that their results were as important as those of the chemists. the astrologers were respected as much as the astronomers. although there is a certain amount of fashion in astronomy, yet perhaps the best test is the judgment of those who have devoted their lives to that science. thirty years ago the field was narrow. it was the era of big telescopes. every astronomer wanted a larger telescope than his neighbors, with which to measure double stars. if he could not get such an instrument, he measured the positions of the stars with a transit circle. then came astrophysics, including photography, spectroscopy and photometry. the study of the motion of the stars along the line of sight, by means of photographs of their spectra, is now the favorite investigation at nearly all the great observatories of the world. the study of the surfaces of the planets, while the favorite subject with the public, next to the destruction of the earth by a comet, does not seem to appeal to astronomers. undoubtedly, the only way to advance our knowledge in this direction is by the most powerful instruments, mounted in the best possible locations. great astronomers are very conservative, and any sensational story in the newspapers is likely to have but little support from them. instead of aiding, it greatly injures real progress in science. there is no doubt that, during the next half century, much time and energy will be devoted to the study of the fixed stars. the study of their motions as indicated by their change in position was pursued with great care by the older astronomers. the apparent motions were so small that a long series of years was required and, in general, for want of early observations of the precise positions of the faint stars, this work was confined mainly to the bright stars. photography is yearly adding a vast amount of material available for this study, but the minuteness of the quantities to be measured renders an accurate determination of their laws very difficult. moreover, we can thus only determine the motions at right angles to the line of sight, the motion towards us or from us being entirely insensible in this way. then came the discovery of the change in the spectrum when a body was in motion, but still this change was so small that visual observations of it proved of but little value. attaching a carefully constructed spectroscope to one of the great telescopes of the world, photographing the spectrum of a star, and measuring it with the greatest care, provided a tool of wonderful efficiency. the motion, which sometimes amounts to several hundreds of miles a second could thus be measured to within a fraction of a mile. the discovery that the motion was variable, owing to the star's revolving around a great dark planet sometimes larger than the star, added greatly not only to the interest of these researches, but also to the labor involved. instead of a single measure for each star, in the case of the so-called spectroscopic binaries, we must make enough measures to determine the dimensions of the orbit, its form and the period of revolution. what has been said of the motions of the stars applies also, in general, to the determination of their distances. a vast amount of labor has been expended on this problem. when at length the distance of a single star was finally determined, the quantity to be measured was so small as to be nearly concealed by the unavoidable errors of measurement. the parallax, or one half of the change in the apparent position of the stars as the earth moves around the sun, has its largest value for the nearest stars. no case has yet been found in which this quantity is as large as a foot rule seen at a distance of fifty miles, and for comparatively few stars is it certainly appreciable. an extraordinary degree of precision has been attained in recent measures of this quantity, but for a really satisfactory solution of this problem, we must probably devise some new method, like the use of the spectroscope for determining motions. two or three illustrations of the kind of methods which might be used to solve this problem may be of interest. there are certain indications of the presence of a selective absorbing medium in space. that is, a medium like red glass, for instance, which would cut off the blue light more than the red light. such a medium would render the blue end of the spectrum of a distant star much fainter, as compared with the red end, than in the case of a near star. a measure of the relative intensity of the two rays would servo to measure the distance, or thickness of the absorbing medium. the effect would be the same for all stars of the same class of spectrum. it could be tested by the stars forming a cluster, like the pleiades, which are doubtless all at nearly the same distance from us. the spectra of stars of the tenth magnitude, or fainter, can be photographed well enough to be measured in this way, so that the relative distances of nearly a million stars could be thus determined. another method which would have a more limited application, would depend on the velocity of light. it has been maintained that the velocity of light in space is not the same for different colors. certain stars, called algol stars, vary in light at regular intervals when partially eclipsed by the interposition of a large dark satellite. recent observations of these eclipses, through glass of different colors, show variations in the time of obscuration. apparently, some of the rays reach the earth sooner than others, although all leave the star at the same time. as the entire time may amount to several centuries, an excessively small difference in velocity would be recognizable. a more delicate test would be to measure the intensity of different portions of the spectrum at a time when the light is changing most rapidly. the effect should be opposite according as the light is increasing or diminishing. it should also show itself in the measures of all spectroscopic binaries. a third method of great promise depends on a remarkable investigation carried on in the physical laboratory of the case school of applied science. according to the undulatory theory of light, all space is filled with a medium called ether, like air, but as much more tenuous than air as air is more tenuous than the densest metals. as the earth is moving through space at the rate of several miles a second, we should expect to feel a breeze as we rush through the ether, like that of the air when in an automobile we are moving with but one thousandth part of this velocity. the problem is one of the greatest delicacy, but a former officer of the case school, one of the most eminent of living physicists, devised a method of solving it. the extraordinary result was reached that no breeze was perceptible. this result appeared to be so improbable that it has been tested again and again, but every time, the more delicate the instrument employed, the more certainly is the law established. if we could determine our motion with reference to the ether, we should have a fixed line of reference to which all other motions could be referred. this would give us a line of ever-increasing length from which to measure stellar distances. still another method depends on the motion of the sun in space. there is some evidence that this motion is not straight, but along a curved line. we see the stars, not as they are now, but as they were when the light left them. in the case of the distant stars this may have occurred centuries ago. accordingly, if we measure the motion of the sun from them, and from near stars, a comparison with its actual motion will give us a clue to their distances. unfortunately, all the stars appear to have large motions whose law we do not know, and therefore we have no definite starting point unless we can refer all to the ether which may be assumed to be at rest. if the views expressed to you this morning are correct, we may expect that the future of astronomy will take the following form: there will be at least one very large observatory employing one or two hundred assistants, and maintaining three stations. two of these will be observing stations, one in the western part of the united states, not far from latitude + °, the other similarly situated in the southern hemisphere, probably in south africa, in latitude - °. the locations will be selected wholly from their climatic conditions. they will be moderately high, from five to ten thousand feet, and in desert regions. the altitude will prevent extreme heat, and clouds or rain will be rare. the range of temperature and unsteadiness of the air will be diminished by placing them on hills a few hundred feet above the surrounding country. the equipment and work of the two stations will be substantially the same. each will have telescopes and other instruments of the largest size, which will be kept at work throughout the whole of every clear night. the observers will do but little work in the daytime, except perhaps on the sun, and will not undertake much of the computation or reductions. this last work will be carried on at a third station, which will be near a large city where the cost of living and of intellectual labor is low. the photographs will be measured and stored at this station, and all the results will be prepared for publication, and printed there. the work of all three stations will be carefully organized so as to obtain the greatest result for a given expenditure. every inducement will be offered to visiting astronomers who wish to do serious work at either of the stations and also to students who intend to make astronomy their profession. in the case of photographic investigations it will be best to send the photographs so that astronomers desiring them can work at home. the work of the young astronomers throughout the world will be watched carefully and large appropriations made to them if it appears that they can spend them to advantage. similar aid will be rendered to astronomers engaged in teaching, and to any one, professional or amateur, capable of doing work of the highest grade. as a fundamental condition for success, no restrictions will be made that will interfere with the greatest scientific efficiency, and no personal or local prejudices that will restrict the work. these plans may seem to you visionary, and too utopian for the twentieth century. but they may be nearer fulfilment than we anticipate. the true astronomer of to-day is eminently a practical man. he does not accept plans of a sensational character. the same qualities are needed in directing a great observatory successfully, as in managing a railroad, or factory. any one can propose a gigantic expenditure, but to prove to a shrewd man of affairs that it is feasible and advisable is a very different matter. it is much more difficult to give away money wisely than to earn it. many men have made great fortunes, but few have learned how to expend money wisely in advancing science, or to give it away judiciously. many persons have given large sums to astronomy, and some day we shall find the man with broad views who will decide to have the advice and aid of the astronomers of the world, in his plans for promoting science, and who will thus expend his money, as he made it, taking the greatest care that not one dollar is wasted. again, let us consider the next great advance, which perhaps will be a method of determining the distances of the stars. many of us are working on this problem, the solution of which may come to some one any day. the present field is a wide one, the prospects are now very bright, and we may look forward to as great an advance in the twentieth century, as in the nineteenth. may a portion of this come to the case school and, with your support, may its enviable record, in the past, be surpassed by its future achievements. [page ii] [illustration: the constellations of orion and taurus. notes.--star a in taurus is red, has eight metals; moves east (page ). at o above tip of right horn is the crab nebula (page ). in orion, a is variable, has five metals; recedes miles per second. b, d, e, x, r, etc., are double stars, the component parts of various colors and magnitudes (page , note). l and i are triple; s, octuple; th, multiple, surrounded by a fine nebula (page ).] [page iii] recreations in astronomy with _directions for practical experiments and telescopic work_ by henry white warren, d.d. author of "sights and insights; or, knowledge by travel," etc. with eighty-three illustrations and maps of stars [page v] [greek: taei psuchaei taei agapaetaei astrapousaei kai isaggedoi] [page vii] preface. all sciences are making an advance, but astronomy is moving at the double-quick. since the principles of this science were settled by copernicus, four hundred years ago, it has never had to beat a retreat. it is rewritten not to correct material errors, but to incorporate new discoveries. once astronomy treated mostly of tides, seasons, and telescopic aspects of the planets; now these are only primary matters. once it considered stars as mere fixed points of light; now it studies them as suns, determines their age, size, color, movements, chemical constitution, and the revolution of their planets. once it considered space as empty; now it knows that every cubic inch of it quivers with greater intensity of force than that which is visible in niagara. every inch of surface that can be conceived of between suns is more wave-tossed than the ocean in a storm. the invention of the telescope constituted one era in astronomy; its perfection in our day, another; and the discoveries of the spectroscope a third--no less important than either of the others. while nearly all men are prevented from practical experimentation in these high realms of knowledge, few [page viii] have so little leisure as to be debarred from intelligently enjoying the results of the investigations of others. this book has been written not only to reveal some of the highest achievements of the human mind, but also to let the heavens declare the glory of the divine mind. in the author's judgment, there is no gulf that separates science and religion, nor any conflict where they stand together. and it is fervently hoped that anyone who comes to a better knowledge of god's works through reading this book, may thereby come to a more intimate knowledge of the worker. i take great pleasure in acknowledging my indebtedness to j. m. van vleck, ll.d., of the u.s. nautical almanac staff, and professor of astronomy at the wesleyan university, for inspecting some of the more important chapters; to dr. s. s. white, of philadelphia, for telescopic advantages; to professor henry draper, for furnishing, in advance of publication, a photograph of the sun's corona in ; and to the excellent work on "popular astronomy," by professor simon newcomb, ll.d., professor u. s. naval observatory, for some of the most recent information, and for the use of the unequalled engravings of jupiter, saturn, and the great nebula of orion. [page ix] contents. chap. i. creative processes ii. creative progress constitution of light chemistry of suns revealed by light creative force of light iii. astronomical instruments the telescope the reflecting telescope the spectroscope iv. celestial measurements celestial movements how to measure v. the sun what the sun does for us vi. the planets, as seen from space the outlook from the earth vii. shooting-stars, meteors, and comets aerolites comets famous comets of what do comets consist? will comets strike the earth? viii. the planets as individuals vulcan mercury venus the earth the aurora borealis [page x] the delicate balance of forces tides the moon telescopic appearance eclipses mars satellites of mars asteroids jupiter satellites of jupiter saturn rings of saturn satellites of saturn uranus neptune ix. the nebular hypothesis. x. the stellar system the open page of the heavens equatorial constellations characteristics of the stars number double and multiple stars colored stars clusters of stars nebulæ variable stars temporary, new, and lost stars movements of stars xi. the worlds and the word xii. the ultimate force summary of latest discoveries and conclusions some elements of the solar system explanation of astronomical symbols signs of the zodiac other abbreviations used in the almanac greek alphabet used indicating the stars chautauqua outline for students glossary of astronomical terms and index [page xi] illustrations fig. the constellations of orion and taurus . an orbit resulting from attraction and projection . the moon's orbit about the earth . changes of orbit by mutual attraction . velocity of light measured by jupiter's satellites . velocity of light measured by fizeau's toothed wheel . white light resolved into colors . showing amount of light received by different planets . measuring intensities of lights . reflection and diffusion of light . manifold reflections . refraction by water . atmospherical reflection . refracting telescope . reflecting telescope . the cambridge equatorial refractor . the new reflecting telescope at paris . spectroscope, with battery of prisms . spectra of glowing hydrogen and of the sun . illustrating arcs and angles . measuring objects by observing angles . mural circle . scale to measure hundredths of an inch . spider-lines to determine star transits . illustrating triangulation [page xii] . measuring distance to an inaccessible object . measuring elevation of an inaccessible object . illustrating parallax . illustrating stellar parallax . mode of ascertaining longitude . relative size of sun, as seen from different planets . zodiacal light . corona of the sun in --brazil . corona of the sun in --colorado . solar prominences of flaming hydrogen . changes in solar cavities during rotation . solar spot . holding telescope to see the sun-spots . orbits and comparative sizes of the planets . orbit of earth, illustrating seasons . inclination of planes of planetary orbits . inclination of orbits of earth and venus . showing the sun's movement among the stars . passage of the sun by star regulus . apparent path of jupiter among the stars . illustrating position of planets . apparent movements of an inferior planet . apparent movements of a superior planet _a_. a swarm of meteors meeting the earth . explosion of a bolide . flight of bolides . the santa rosa aerolite . orbit of november meteors and the comet of . aspects of remarkable comets . phases and apparent dimensions of venus . the earth and moon in space . aurora as waving curtains . tide resulting from centrifugal motion . lunar landscape [page xiii] . telescopic view of the moon . illumination of lunar craters and peaks . lunar crater "copernicus" . eclipses: shadows of earth and moon . apparent sizes of mars, seen from the earth . jupiter . various positions of jupiter's satellites . view of saturn and his rings . perturbations of uranus . map: circumpolar constellations . map of constellations on the meridian in december . map of constellations on the meridian in january . map of constellations on the meridian in april . map of constellations on the meridian in june . map of constellations on the meridian in september . map of constellations on the meridian in november . southern circumpolar constellations . aspects of double stars . sprayed star cluster below ae in hercules . globular star cluster in the centaur . great nebula about th orionis . the crab nebula above z tauri . the ring nebula in lyra . showing place of ring nebula . the horizontal pendulum colored plate representing various specta maps to find the stars [page ] i. creative processes. "in the beginning god created the heaven and the earth. and the earth was without form, and void; and darkness was upon the face of the deep."--_genesis_ i. , . [page ] "not to the domes, where crumbling arch and column attest the feebleness of mortal hand, but to that fane, most catholic and solemn, which god hath planned,-- to that cathedral, boundless as our wonder, whose quenchless lamps the sun and stars supply; its choir the winds and waves, its organ thunder, its dome the sky." h. w. longfellow. "the heavens are a point from the pen of his perfection; the world is a rose-bud from the bower of his beauty; the sun is a spark from the light of his wisdom; and the sky a bubble on the sea of his power." sir w. jones. [page ] recreations in astronomy. * * * * * i. _creative processes._ during all the ages there has been one bright and glittering page of loftiest wisdom unrolled before the eye of man. that this page may be read in every part, man's whole world turns him before it. this motion apparently changes the eternally stable stars into a moving panorama, but it is only so in appearance. the sky is a vast, immovable dial-plate of "that clock whose pendulum ticks ages instead of seconds," and whose time is eternity. the moon moves among the illuminated figures, traversing the dial quickly, like a second-hand, once a month. the sun, like a minute-hand, goes over the dial once a year. various planets stand for hour-hands, moving over the dial in various periods reaching up to one hundred and sixty-four years; while the earth, like a ship of exploration, sails the infinite azure, bearing the observers to different points where they may investigate the infinite problems of this mighty machinery. this dial not only shows present movements, but it keeps the history of uncounted ages past ready to be [page ] read backward in proper order; and it has glorious volumes of prophecy, revealing the far-off future to any man who is able to look thereon, break the seals, and read the record. glowing stars are the alphabet of this lofty page. they combine to form words. meteors, rainbows, auroras, shifting groups of stars, make pictures vast and significant as the armies, angels, and falling stars in the revelation of st. john--changing and progressive pictures of infinite wisdom and power. men have not yet advanced as far as those who saw the pictures john describes, and hence the panorama is not understood. that continuous speech that day after day uttereth is not heard; the knowledge that night after night showeth is not seen; and the invisible things of god from the creation of the world, even his eternal power and godhead, clearly discoverable from things that are made, are not apprehended. the greatest triumphs of men's minds have been in astronomy--and ever must be. we have not learned its alphabet yet. we read only easy lessons, with as many mistakes as happy guesses. but in time we shall know all the letters, become familiar with the combinations, be apt at their interpretation, and will read with facility the lessons of wisdom and power that are written on the earth, blazoned in the skies, and pictured by the flowers below and the rainbows above. in order to know how worlds move and develop, we must create them; we must go back to their beginning, give their endowment of forces, and study the laws of their unfolding. this we can easily do by that faculty wherein man is likest his father, a creative imagination. god creates and embodies; we create, but [page ] it remains in thought only. but the creation is as bright, strong, clear, enduring, and real, as if it were embodied. every one of us would make worlds enough to crush us, if we could embody as well as create. our ambition would outrun our wisdom. let us come into the high and ecstatic frame of mind which shakspeare calls frenzy, in the exigencies of his verse, when "the poet's eye, in a fine frenzy rolling, doth glance from heaven to earth, from earth to heaven; and, as imagination bodies forth the forms of things unknown, the poet's pen turns them to shapes, and gives to airy nothing a local habitation and a name." in the supremacy of our creative imagination let us make empty space, in order that we may therein build up a new universe. let us wave the wand of our power, so that all created things disappear. there is no world under our feet, no radiant clouds, no blazing sun, no silver moon, nor twinkling stars. we look up, there is no light; down, through immeasurable abysses, there is no form; all about, and there is no sound or sign of being--nothing save utter silence, utter darkness. it cannot be endured. creation is a necessity of mind--even of the divine mind. we will now, by imagination, create a monster world, every atom of which shall be dowered with the single power of attraction. every particle shall reach out its friendly hand, and there shall be a drawing together of every particle in existence. the laws governing this attraction shall be two. when these particles are associated together, the attraction shall be in proportion to the mass. a given mass will pull twice [page ] as much as one of half the size, because there is twice as much to pull. and a given mass will be pulled twice as much as one half as large, because there is twice as much to be pulled. a man who weighed one hundred and fifty pounds on the earth might weigh a ton and a half on a body as large as the sun. that shall be one law of attraction; and the other shall be that masses attract inversely as the square of distances between them. absence shall affect friendships that have a material basis. if a body like the earth pulls a man one hundred and fifty pounds at the surface, or four thousand miles from the centre, it will pull the same man one-fourth as much at twice the distance, one-sixteenth as much at four times the distance. that is, he will weigh by a spring balance thirty-seven and a half pounds at eight thousand miles from the centre, and nine pounds six ounces at sixteen thousand miles from the centre, and he will weigh or be pulled by the earth / of a pound at the distance of the moon. but the moon would be large enough and near enough to pull twenty-four pounds on the same man, so the earth could not draw him away. thus the two laws of attraction of gravitation are-- , _gravity is proportioned to the quantity of matter_; and , _the force of gravity varies inversely as the square of the distance from the centre of the attracting body_. the original form of matter is gas. almost as i write comes the announcement that mr. lockyer has proved that all the so-called primary elements of matter are only so many different sized molecules of one original substance--hydrogen. whether that is true or not, let us now create all the hydrogen we can [page ] imagine, either in differently sized masses or in combination with other substances. there it is! we cannot measure its bulk; we cannot fly around it in any recordable eons of time. it has boundaries, to be sure, for we are finite, but we cannot measure them. let it alone, now; leave it to itself. what follows? it is dowered simply with attraction. the vast mass begins to shrink, the outer portions are drawn inward. they rush and swirl in vast cyclones, thousands of miles in extent. the centre grows compact, heat is evolved by impact, as will be explained in chapter ii. dull red light begins to look like coming dawn. centuries go by; contraction goes on; light blazes in insufferable brightness; tornadoes, whirlpools, and tempests scarcely signify anything as applied to such tumultuous tossing. there hangs the only world in existence; it hangs in empty space. it has no tendency to rise; none to fall; none to move at all in any direction. it seethes and, flames, and holds itself together by attractive power, and that is all the force with which we have endowed it. leave it there alone, and withdraw millions of miles into space: it looks smaller and smaller. we lose sight of those distinctive spires of flame, those terrible movements. it only gives an even effulgence, a steady unflickering light. turn one quarter round. still we see our world, but it is at one side. now in front, in the utter darkness, suddenly create another world of the same size, and at the same distance from you. there they stand--two huge, lone bodies, in empty space. but we created them dowered with attraction. each instantly feels the drawing influence of the other. they are mutually attractive, and begin to [page ] move toward each other. they hasten along an undeviating straight line. their speed quickens at every mile. the attraction increases every moment. they fly swift as thought. they dash their flaming, seething foreheads together. and now we have one world again. it is twice as large as before, that is all the difference. there is no variety, neither any motion; just simple flame, and nothing to be warmed thereby. are our creative powers exhausted by this effort? [illustration: fig. .--orbit a d, resulting from attraction, a c, and projectile force, a b.] no, we will create another world, and add another power to it that shall keep them apart. that power shall be what is called the force of inertia, which is literally no power at all; it is an inability to originate or change motion. if a body is at rest, inertia is that quality by which it will forever remain so, unless acted upon by some force from without; and if a body is in motion, it will continue on at the same speed, in a straight line, forever, unless it is quickened, retarded, or turned from its path by some other force. suppose our newly created sun is , miles in diameter. go away , , miles and create an earth eight thousand miles in diameter. it instantly feels the attractive power of the sun drawing it to itself sixty-eight [page ] miles a second. now, just as it starts, give this earth a push in a line at right angles with line of fall to the sun, that shall send it one hundred and eighty-nine miles a second. it obeys both forces. the result is that the world moves constantly forward at the same speed by its inertia from that first push, and attraction momentarily draws it from its straight line, so that the new world circles round the other to the starting-point. continuing under the operation of both forces, the worlds can never come together or fly apart. they circle about each other as long as these forces endure; for the first world does not stand still and the second do all the going; both revolve around the centre of gravity common to both. in case the worlds are equal in mass, they will both take the same orbit around a central stationary point, midway between the two. in case their mass be as one to eighty-one, as in the case of the earth and the moon, the centre of gravity around which both turn will be / of the distance from the earth's centre to the moon's centre. this brings the central point around which both worlds swing just inside the surface of the earth. it is like an apple attached by a string, and swung around the hand; the hand moves a little, the apple very much. thus the problem of two revolving bodies is readily comprehended. the two bodies lie in easy beds, and swing obedient to constant forces. when another body, however, is introduced, with its varying attraction, first on one and then on the other, complications are introduced that only the most masterly minds can follow. introduce a dozen or a million bodies, and complications arise that only omniscience can unravel. [page ] [illustration: fig. .] let the hand swing an apple by an elastic cord. when the apple falls toward the earth it feels another force besides that derived from the hand, which greatly lengthens the elastic cord. to tear it away from the earth's attraction, and make it rise, requires additional force, and hence the string is lengthened; but when it passes over the hand the earth attracts it downward, and the string is very much shortened: so the moon, held by an elastic cord, swings around the earth. from its extreme distance from the earth, at a, fig. , it rushes with increasing speed nearly a quarter of a million of miles toward the sun, feeling its attraction increase with every mile until it reaches b; then it is retarded in its speed, by the same attraction, as it climbs back its quarter of a million of miles away from the sun, in defiance of its power, to c. all the while the invisible elastic force of the earth is unweariedly maintained; and though the moon's distances vary over a range of , miles, the moon is always in a determinable place. a simple revolution of one world about another in a circular orbit would be a problem of easy solution. it would always be at the same distance from its centre, and going with the same velocity. but there are over sixty causes that interfere with such a simple orbit in the case of the moon, all of which causes and their disturbances must be considered in calculating such a simple matter as an eclipse, or predicting the moon's place as the sailors guide. one of the most puzzling of the irregularities [page ] of our night-wandering orb has just been explained by professor hansen, of gotha, as a curious result of the attraction of venus. [illustration: fig. .--changes of orbit by mutual attraction.] take a single instance of the perturbations of jupiter and saturn which can be rendered evident. the times of orbital revolution of saturn and jupiter are nearly as five to two. suppose the orbits of the planets to be, as in fig. , both ellipses, but not necessarily equally distant in all parts. the planets are as near as possible at , . drawn toward each other by mutual attraction, jupiter's orbit bends outward, and saturn's becomes more nearly straight, as shown by the dotted lines. a partial correction of this difficulty immediately follows. as jupiter moves on ahead of saturn it is held back--retarded in its orbit by that body; and saturn is hastened in its orbit by the attraction of jupiter. now greater speed means a straighter orbit. a rifle-ball flies nearer in a straight line than a thrown stone. a greater velocity given to a whirled ball pulls the elastic cord far enough to give the ball a larger orbit. hence, being hastened, saturn stretches out nearer its proper orbit, and, retarded, jupiter approaches the smaller curve that is its true orbit. but if they were always to meet at this point, as they would if jupiter made two revolutions to saturn's one, it would be disastrous. in reality, when saturn has gone around two-thirds of its orbit to , jupiter will have gone once and two-thirds around and overtaken [page ] saturn; and they will be near again, be drawn together, hastened, and retarded, as before; their next conjunction would be at , , etc. now, if they always made their conjunction at points equally distant, or at thirds of their orbits, it would cause a series of increasing deviations; for jupiter would be constantly swelling his orbit at three points, and saturn increasingly contracting his orbit at the same points. disaster would be easily foretold. but as their times of orbital revolutions are not exactly in the ratio of five and two, their points of conjunction slowly travel around the orbit, till, in a period of nine hundred years, the starting-point is again reached, and the perturbations have mutually corrected one another. for example, the total attractive effect of one planet on the other for years is to quicken its speed. the effect for the next years is to retard. the place of saturn, when all the retardations have accumulated for years, is one degree behind what it is computed if they are not considered; and years later it will be one degree before its computed place--a perturbation of two degrees. when a bullet is a little heavier or ragged on one side, it will constantly swerve in that direction. the spiral groove in the rifle, of one turn in forty-five feet, turns the disturbing weight or raggedness from side to side--makes one error correct another, and so the ball flies straight to the bull's-eye. so the place of jupiter and saturn, though further complicated by four moons in the case of jupiter, and eight in the case of saturn, and also by perturbations caused by other planets, can be calculated with exceeding nicety. the difficulties would be greatly increased if the orbits [page ] of saturn and jupiter, instead of being , , miles apart, were interlaced. yet there are the orbits of one hundred and ninety-two asteroids so interlaced that, if they were made of wire, no one could be lifted without raising the whole net-work of them. nevertheless, all these swift chariots of the sky race along the course of their intermingling tracks as securely as if they were each guided by an intelligent mind. _they are guided by an intelligent mind and an almighty arm._ still more complicated is the question of the mutual attractions of all the planets. lagrange has been able to show, by a mathematical genius that seems little short of omniscience in his single department of knowledge, that there is a discovered system of oscillations, affecting the entire planetary system, the periods of which are immensely long. the number of these oscillations is equal to that of all the planets, and their periods range from , to , , years, looking into the open page of the starry heavens we see double stars, the constituent parts of which must revolve around a centre common to them both, or rush to a common ruin. eagerly we look to see if they revolve, and beholding them in the very act, we conclude, not groundlessly, that the same great law of gravitation holds good in distant stellar spaces, and that there the same sufficient mind plans, and the same sufficient power directs and controls all movements in harmony and security. when we come to the perturbations caused by the mutual attractions of the sun, nine planets, twenty moons, one hundred and ninety-two asteroids, millions [page ] of comets, and innumerable meteoric bodies swarming in space, and when we add to all these, that belong to one solar system, the attractions of all the systems of the other suns that sparkle on a brilliant winter night, we are compelled to say, "as high as the heavens are above the earth, so high above our thoughts and ways must be the thoughts and ways of him who comprehends and directs them all." [page ] ii. creative progress. "and god said, let there be light, and there was light."--_genesis_ i., . "god is light."-- _john_, i. . [page ] "hail! holy light, offspring of heaven first born, or of the eternal, co-eternal beam, may i express thee unblamed? since god is light, and never but in unapproached light dwelt from eternity, dwelt then in thee, bright effluence of bright essence increate." milton. "a million torches lighted by thy hand wander unwearied through the blue abyss: they own thy power, accomplish thy command, all gay with life, all eloquent with bliss. what shall we call them? piles of crystal light-- a glorious company of golden streams-- lamps of celestial ether burning bright-- suns lighting systems with their joyous beams? but 'thou to these art as the noon to night." derzhavin, trans. by bowring. [page ] ii. _creative progress._ worlds would be very imperfect and useless when simply endowed with attraction and inertia, if no time were allowed for these forces to work out their legitimate results. we want something more than swirling seas of attracted gases, something more than compacted rocks. we look for soil, verdure, a paradise of beauty, animal life, and immortal minds. let us go on with the process. light is the child of force, and the child, like its father, is full of power. we dowered our created world with but a single quality--a force of attraction. it not only had attraction for its own material substance, but sent out an all-pervasive attraction into space. by the force of condensation it flamed like a sun, and not only lighted its own substance, but it filled all space with the luminous outgoings of its power. a world may be limited, but its influence cannot; its body may have bounds, but its soul is infinite. everywhere is its manifestation as real, power as effective, presence as actual, as at the central point. he that studies ponderable bodies alone is not studying the universe, only its skeleton. skeletons are somewhat interesting in themselves, but far more so when covered with flesh, flushed with beauty, and inspired with soul. the universe [page ] has bones, flesh, beauty, soul, and all is one. it can be understood only by a study of all its parts, and by tracing effect to cause. but how can condensation cause light? power cannot be quiet. the mighty locomotive trembles with its own energy. a smitten piece of iron has all its infinitesimal atoms set in vehement commotion; they surge back and forth among themselves, like the waves of a storm-blown lake. heat is a mode of motion. a heated body commences a vigorous vibration among its particles, and communicates these vibrations to the surrounding air and ether. when these vibrations reach , , , , per second, the human eye, fitted to be affected by that number, discerns the emitted undulations, and the object seems to glow with a dull red light; becoming hotter, the vibrations increase in rapidity. when they reach , , , , per second the color becomes violet, and the eye can observe them no farther. between these numbers are those of different rapidities, which affect the eye--as orange, yellow, green, blue, indigo, in an almost infinite number of shades--according to the sensitiveness of the eye. we now see how our dark immensity of attractive atoms can become luminous. a force of compression results in vibrations within, communicated to the ether, discerned by the eye. illustrations are numerous. if we suddenly push a piston into a cylinder of brass, the force produces heat enough to set fire to an inflammable substance within. strike a half-inch cube of iron a moderate blow and it becomes warm; a sufficient blow, and its vibrations become quick enough to be seen--it is red-hot. attach a thermometer to an extended [page ] arm of a whirling wheel; drive it against the air five hundred feet per second, the mercury rises °. the earth goes , feet per second, or one thousand miles a minute. if it come to an aerolite or mass of metallic rock, or even a cloudlet of gas, standing still in space, its contact with our air evolves , ° of heat. and when the meteor comes toward the world twenty-six miles a second, the heat would become proportionally greater if the meteor could abide it, and not be consumed in fervent heat. it vanishes almost as soon as seen. if there were meteoric masses enough lying in our path, our sky would blaze with myriads of flashes of light. enough have been seen to enable a person to read by them at night. if a sufficient number were present, we should miss their individual flashes as they blend their separate fires in one sea of insufferable glory. the sun is , , times as large as our planet; its attraction proportionally greater; the aerolites more numerous; and hence an infinite hail of stones, small masses and little worlds, makes ceaseless trails of light, whose individuality is lost in one dazzling sea of glory. on the st day of september, , two astronomers, independently of each other, saw a sudden brightening on the surface of the sun. probably two large meteoric masses were travelling side by side at two or three hundred miles per second, and striking the sun's atmosphere, suddenly blazed into light bright enough to be seen on the intolerable light of the photosphere as a background. the earth responded to this new cause of brilliance and heat in the sun. vivid auroras appeared, not only at the north and south poles, but even where such spectacles are seldom seen. the electro-magnetic [page ] disturbances were more distinctly marked. "in many places the telegraphic wires struck work. in washington and philadelphia the electric signalmen received severe electric shocks; at a station in norway the telegraphic apparatus was set fire to; and at boston a flame of fire followed the pen of bain's electric telegraph." there is the best of reason for believing that a continuous succession of such bodies might have gone far toward rendering the earth uncomfortable as a place of residence. of course, the same result of heat and light would follow from compression, if a body had the power of contraction in itself. we endowed every particle of our gas, myriads of miles in extent, with an attraction for every other particle. it immediately compressed itself into a light-giving body, which flamed out through the interstellar spaces, flushing all the celestial regions with exuberant light. but heat exerts a repellent force among particles, and soon an equilibrium is reached, for there comes a time when the contracting body can contract no farther. but heat and light radiate away into cold space, then contraction goes on evolving more light, and so the suns flame on through the millions of years unquenched. it is estimated that the contraction of our sun, from filling immensity of space to its present size, could not afford heat enough to last more than , , years, and that its contraction from its present density (that of a swamp) to such rock as that of which our earth is composed, could supply heat enough for , , years longer. but the far-seeing mind of man knows a time must come when the present force of attraction [page ] shall have produced all the heat it can, and a new force of attraction must be added, or the sun itself will become cold as a cinder, dead as a burned-out char. since light and heat are the product of such enormous cosmic forces, they must partake of their nature, and be force. so they are. the sun has long arms, and they are full of unconquerable strength ninety-two millions, or any other number of millions, of miles away. all this light and heat comes through space that is ° below zero, through utter darkness, and appears only on the earth. so the gas is darkness in the underground pipes, but light at the burner. so the electric power is unfelt by the cable in the bosom of the deep, but is expressive of thought and feeling at the end. having found the cause of light, we will commence a study of its qualities and powers. light is the astronomer's necessity. when the sublime word was uttered, "let there be light!" the study of astronomy was made possible. man can gather but little of it with his eye; so he takes a lens twenty-six inches in diameter, and bends all the light that passes through it to a focus, then magnifies the image and takes it into his eye. or he takes a mirror, six feet in diameter, so hollowed in the middle as to reflect all the rays falling upon it to one point, and makes this larger eye fill his own with light. by this larger light-gathering he discerns things for which the light falling on his pupil one-fifth of an inch in diameter would not be sufficient. we never have seen any sun or stars; we have only seen the light that left them fifty minutes or years ago, more or less. light is the aërial sprite that carries our measuring-rods across the infinite [page ] spaces; light spreads out the history of that far-off beginning; brings us the measure of stars a thousand times brighter than our sun; takes up into itself evidences of the very constitutional elements of the far-off suns, and spreads them at our feet. it is of such capacity that the divine nature, looking for an expression of its own omnipotence, omniscience, and power of revelation, was content to say, "god is light." we shall need all our delicacy of analysis and measurement when we seek to determine the activities of matter so fine and near to spirit as light. [illustration: fig. .--velocity of light measured by eclipses of jupiter's moons.] we first seek the velocity of light. in fig. the earth is , , miles from the sun at e; jupiter is , , miles from the sun at j. it has four moons: the inner one goes around the central body in forty-two hours, and is eclipsed at every revolution. the light that went out from the sun to m ceases to be reflected back to the earth by the intervention of the planet jupiter. we know to a second when these eclipses take place, and they can be seen with a small telescope. but when the earth is on the opposite side of the sun [page ] from jupiter, at e', these eclipses at j' take place sixteen and a half minutes too late. what is the reason? is the celestial chronometry getting deranged? no, indeed; these great worlds swing never an inch out of place, nor a second out of time. by going to the other side of the sun the earth is , , miles farther from jupiter, and the light that brings the intelligence of that eclipse consumes the extra time in going over the extra distance. divide one by the other and we get the velocity, , miles per second. that is probably correct to within a thousand miles. methods of measurement by the toothed wheel of fizeau confirm this result. suppose the wheel, fig. , to have one thousand teeth, making five revolutions to the second. five thousand flashes of light each second will dart out. let each flash travel nine miles to a mirror and return. if it goes that distance in / of a second, or at the rate of , miles a second, the next tooth will have arrived before the eye, and each returning ray be cut off. hasten the revolutions a little, and the next notch will then admit the ray, on its return, that went out of each previous notch: the eighteen miles having been traversed meanwhile. the method of measuring by means of a revolving mirror, used by faucault, is held to be even more accurate. [illustration: fig. .--measuring the velocity of light.] when we take instantaneous photographs by the exposure [page ] of the sensitive plate / part of a second, a stream of light nine miles long dashes in upon the plate in that very brief period of time. the highest velocity we can give a rifle-ball is feet a second, the next second it is only feet, and soon it comes to rest. we cannot compact force enough behind a bit of lead to keep it flying. but light flies unweariedly and without diminution of speed. when it has come from the sun in eight minutes, alpha centauri in three years, polaris in forty-five years, other stars in one thousand, its wings are in nowise fatigued, nor is the rapidity of its flight slackened in the least. it is not the transactions of to-day that we read in the heavens, but it is history, some of it older than the time of adam. those stars may have been smitten out of existence decades of centuries ago, but their poured-out light is yet flooding the heavens. it goes both ways at once in the same place, without interference. we see the light reflected from the new moon to the earth; reflected back from the house-tops, fields, and waters of earth, to the moon again, and from the moon to us once more--three times in opposite directions, in the same place, without interference, and thus we see "the old moon in the arms of the new." _constitution of light._ [illustration: fig. .--white light resolved into colors.] light was once supposed to be corpuscular, or consisting of transmitted particles. it is now known to be the result of undulations in ether. reference has been made to the minuteness of these undulations. their velocity is equally wonderful. put a prism of glass into a ray of light coming into a dark room, and it is [page ] instantly turned out of its course, some parts more and some less, according to the number of vibrations, and appears as the seven colors on different parts of the screen. fig. shows the arrangement of colors, and the number of millions of millions of vibrations per second of each. but the different divisions we call colors are not colors in themselves at all, but simply a different number of vibrations. color is all in the eye. violet has in different places from to , , , , of vibrations per second; red has, in different places, from to , , , , vibrations per second. none of these in any sense are color, but affect the eye differently, and we call these different effects color. they are simply various velocities of vibration. an object, like one kind of stripe in our flag, which absorbs all kinds of vibrations except those between and , , , , , and reflects those, appears red to us. the field for the stars absorbs and destroys all but those vibrations numbering about , , , , of [page ] vibrations per second. a color is a constant creation. light makes momentary color in the flag. drake might have written, in the continuous present as well as in the past, "freedom mingles with its gorgeous dyes the milky baldrick of the skies, and stripes its pore celestial white with streakings of the morning light." every little pansy, tender as fancy, pearled with evanescent dew, fresh as a new creation of sunbeams, has power to suppress in one part of its petals all vibrations we call red, in another those we call yellow, and purple, and reflect each of these in other parts of the same tender petal. "pansies are for thoughts," even more thoughts than poor ophelia knew. an evening cloud that is dense enough to absorb all the faster and weaker vibrations, leaving only the stronger to come through, will be said to be red; because the vibrations that produce the impression we have so named are the only ones that have vigor enough to get through. it is like an army charging upon a fortress. under the deadly fire and fearful obstructions six-sevenths go down, but one-seventh comes through with the glory of victory upon its face. light comes in undulations to the eye, as tones of sound to the ear. must not light also sing? the lowest tone we can hear is made by . vibrations of air per second; the highest, so shrill and "fine that nothing lives 'twixt it and silence," is made by , vibrations per second. between these extremes lie eleven octaves; c of the g clef having - / vibrations to the second, and its octave above - / . not that sound vibrations cease [page ] at , , but our organs are not fitted to hear beyond those limitations. if our ears were delicate enough, we could hear even up to the almost infinite vibrations of light. in one of those semi-inspirations we find in shakspeare's works, he says-- "there's not the smallest orb which thou beholdest, but in his motion like an angel sings, still quiring to the young-eyed cherubim. such harmony is in immortal souls; but, whilst this muddy vesture of decay doth grossly close it in, we cannot hear it." and that older poetry which is always highest truth says, "the morning stars sing together." we misconstrued another passage which we could not understand, and did not dare translate as it was written, till science crept up to a perception of the truth that had been standing there for ages, waiting a mind that could take it in. now we read as it is written--"thou makest the out-goings of the morning and evening to sing." were our senses fine enough, we could hear the separate keynote of every individual star. stars differ in glory and in power, and so in the volume and pitch of their song. were our hearing sensitive enough, we could hear not only the separate key-notes but the infinite swelling harmony of these myriad stars of the sky, as they pour their mighty tide of united anthems in the ear of god: "in reason's ear they all rejoice, and utter forth a glorious voice. forever singing, as they shine, the hand that made us is divine." this music is not monotonous. stars draw near each other, and make a light that is unapproachable by mortals; [page ] then the music swells beyond our ability to endure. they recede far away, making a light so dim that the music dies away, so near to silence that only spirits can perceive it. no wonder god rejoices in his works. they pour into his ear one ceaseless tide of rapturous song. our senses are limited--we have only five, but there is room for many more. some time we shall be taken out of "this muddy vesture of decay," no longer see the universe through crevices of our prison-house, but shall range through wider fields, explore deeper mysteries, and discover new worlds, hints of which have never yet been blown across the wide atlantic that rolls between them and men abiding in the flesh. _chemistry of suns revealed by light._ when we examine the assemblage of colors spread from the white ray of sunlight, we do not find red simple red, yellow yellow, etc., but there is a vast number of fine microscopic lines of various lengths, parallel--here near together, there far apart, always the same number and the same relative distance, when the same light and prism are used. what new alphabets to new realms of knowledge are these! remember, that what we call colors are only various numbers of vibrations of ether. remember, that every little group in the infinite variety of these vibrations may be affected differently from every other group. one number of these is bent by the prism to where we see what we call the violet, another number to the place we call red. all of the vibrations are destroyed when they strike a surface we call black. a part of them are destroyed when [page ] they strike a substance we call colored. the rest are reflected, and give the impression of color. in one place on the flag of our nation all vibrations are destroyed except the red; in another, all but the blue. perhaps on that other gorgeous flag, not of our country but of our sun, the flag we call the solar spectrum, all vibrations are destroyed where these dark lines appear. perhaps this effect is not produced by the surface upon which the rays fall, but by some specific substance in the sun. this is just the truth. light passing through vapor of sodium has the vibrations that would fall on two narrow lines in the yellow utterly destroyed, leaving two black spaces. light passing through vapor of burning iron has some four hundred numbers or kinds of vibrations destroyed, leaving that number of black lines; but if the salt or iron be glowing gas, in the source of the light itself the same lines are bright instead of dark. thus we have brought to our doors a readable record of the very substances composing every world hot enough to shine by its own light. thus, while our flag means all we have of liberty, free as the winds that kiss it, and bright as the stars that shine in it, the flag of the sun means all that it is in constituent elements, all that it is in condition. we find in our sun many substances known to exist in the earth, and some that we had not discovered when the sun wrote their names, or rather made their mark, in the spectrum. thus, also, we find that betelguese and algol are without any perceivable indications of hydrogen, and sirius has it in abundance. what a sense of acquaintanceship it gives us to look up and recognize [page ] the stars whose very substance we know! if we were transported thither, or beyond, we should not be altogether strangers in an unknown realm. but the stars differ in their constituent elements; every ray that flashes from them bears in its very being proofs of what they are. hence the eye of omniscience, seeing a ray of light anywhere in the universe, though gone from its source a thousand years, would be able to tell from what orb it originally came. _creative force of light._ just above the color vibrations of the unbraided sunbeam, above the violet, which is the highest number our eyes can detect, is a chemical force; it works the changes on the glass plate in photography; it transfigures the dark, cold soil into woody fibre, green leaf, downy rose petals, luscious fruit, and far pervasive odor; it flushes the wide acres of the prairie with grass and flowers, fills the valleys with trees, and covers the hills with corn, a single blade of which all the power of man could not make. this power is also fit and able to survive. the engineer stephenson once asked dr. buckland, "what is the power that drives that train?" pointing to one thundering by. "well, i suppose it is one of your big engines." "but what drives the engine?" "oh, very likely a canny newcastle driver." "no, sir," said the engineer, "it is sunshine." the doctor was too dull to take it in. let us see if we can trace such an evident effect to that distant cause. ages ago the warm sunshine, falling on the scarcely lifted hills of pennsylvania, caused the reedy vegetation to grow along the banks of [page ] shallow seas, accumulated vast amounts of this vegetation, sunk it beneath the sea, roofed it over with sand, compacted the sand into rock, and changed this vegetable matter--the products of the sunshine--into coal; and when it was ready, lifted it once more, all garnered for the use of men, roofed over with mighty mountains. we mine the coal, bring out the heat, raise the steam, drive the train, so that in the ultimate analyses it is sunshine that drives the train. these great beds of coal are nothing but condensed sunshine--the sun's great force, through ages gone, preserved for our use to-day. and it is so full of force that a piece of coal that will weigh three pounds (as big as a large pair of fists) has as much power in it as the average man puts into a day's work. three tons of coal will pump as much water or shovel as much sand as the average man will pump or shovel in a lifetime; so that if a man proposes to do nothing but work with his muscles, he had better dig three tons of coal and set that to do his work and then die, because his work will be better done, and without any cost for the maintenance of the doer. come down below the color vibrations, and we shall find that those which are too infrequent to be visible, manifest as heat. naturally there will be as many different kinds of heat as tints of color, because there is as great a range of numbers of vibration. it is our privilege to sift them apart and sort them over, and find what kinds are best adapted to our various uses. take an electric lamp, giving a strong beam of light and heat, and with a plano-convex lens gather it into a single beam and direct it upon a thermometer, twenty feet away, that is made of glass and filled with air. the [page ] expansion or contraction of this air will indicate the varying amounts of heat. watch your air-thermometer, on which the beam of heat is pouring, for the result. there is none. and yet there is a strong current of heat there. put another kind of test of heat beyond it and it appears; coat the air-thermometer with a bit of black cloth, and that will absorb heat and reveal it. but why not at first? because the glass lens stops all the heat that can affect glass. the twenty feet of air absorbs all the heat that affects air, and no kind of heat is left to affect an instrument made of glass and air; but there are kinds of heat enough to affect instruments made of other things. a very strong current of heat may be sent right through the heart of a block of ice without melting the ice at all or cooling off the heat in the least. it is done in this way: send the beam of heat through water in a glass trough, and this absorbs all the heat that can affect water or ice, getting itself hot, and leaving all other kinds of heat to go through the ice beyond; and appropriate tests show that as much heat comes out on the other side as goes in on this side, and it does not melt the ice at all. gunpowder may be exploded by heat sent through ice. dr. kane, years ago, made this experiment. he was coming down from the north, and fell in with some esquimaux, whom he was anxious to conciliate. he said to the old wizard of the tribe, "i am a wizard; i can bring the sun down out of the heavens with a piece of ice." that was a good, deal to say in a country where there was so little sun. "so," he writes, "i took my hatchet, chipped a small piece of ice into the form of a double-convex lens, [page ] smoothed it with my warm hands, held it up to the sun, and, as the old man was blind, i kindly burned a blister on the back of his hand to show him i could do it." these are simple illustrations of the various kinds of heat. the best furnace or stove ever invented consumes fifteen times as much fuel to produce a given amount of heat as the furnace in our bodies consumes to produce a similar amount. we lay in our supplies of carbon at the breakfast, dinner, and supper table, and keep ourselves warm by economically burning it with the oxygen we breathe. heat associated with light has very different qualities from that which is not. sunlight melts ice in the middle, bottom, and top at once. ice in the spring-time is honey-combed throughout. a piece of ice set in the summer sunshine crumbles into separate crystals. dark heat only melts the surface. nearly all the heat of the sun passes through glass without hinderance; but take heat from white-hot platinum and only seventy-six per cent. of it goes through glass, twenty-four per cent. being so constituted that it cannot pass with facility. of heat from copper at ° only six per cent. can go through glass, the other ninety-four per cent. being absorbed by it. the heat of the sun beam goes through glass without [page ] any hinderance whatever. it streams into the room as freely as if there were no glass there. but what if the furnace or stove heat went through glass with equal facility? we might as well try to heat our rooms with the window-panes all out, and the blast of winter sweeping through them. the heat of the sun, by its intense vibrations, comes to the earth dowered with a power which pierces the miles of our atmosphere, but if our air were as pervious to the heat of the earth, this heat would flyaway every night, and our temperature would go down to ° below zero. this heat comes with the light, and then, dissociated from it, the number of its vibrations lessened, it is robbed of its power to get away, and remains to work its beneficent ends for our good. worlds that are so distant as to receive only / of the heat we enjoy, may have atmospheres that retain it all. indeed it is probable that mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours. the poet drew on his imagination when he wrote: "who there inhabit must have other powers, juices, and veins, and sense and life than ours; one moment's cold like theirs would pierce the bone, freeze the heart's-blood, and turn us all to stone." the power that journeys along the celestial spaces in the flashing sunshine is beyond our comprehension. it accomplishes with ease what man strives in vain to do with all his strength. at west point there are some links of a chain that was stretched across the river to prevent british ships from ascending; these links were made of two-and-a-quarter-inch iron. a powerful locomotive might tug in vain at one of them and not stretch it the thousandth part of an inch. but the heat of a single gas-burner, that glows with the preserved sunlight of other ages, when suitably applied to the link, stretches it with ease; such enormous power has a little heat. there is a certain iron bridge across the thames at london, resting on arches. the warm sunshine, acting [page ] upon the iron, stations its particles farther and farther apart. since the bottom cannot give way the arches must rise in the middle. as they become longer they lift the whole bridge, and all the thundering locomotives and miles of goods-trains cannot bring that bridge down again until the power of the sunshine has been withdrawn. there is bunker hill monument, thirty-two feet square at the base, with an elevation of two hundred and twenty feet. the sunshine of every summer's day takes hold of that mighty pile of granite with its aërial fingers, lengthens the side affected, and bends the whole great mass as easily as one would bend a whipstock. a few years ago we hung a plummet from the top of this monument to the bottom. at a.m. it began to move toward the west; at noon it swung round toward the north; in the afternoon it went east of where it first was, and in the night it settled back to its original place. the sunshine says to the sea, held in the grasp of gravitation, "rise from your bed! let millions of tons of water fly on the wings of the viewless air, hundreds of miles to the distant mountains, and pour there those millions of tons that shall refresh a whole continent, and shall gather in rivers fitted to bear the commerce and the navies of nations." gravitation says, "i will hold every particle of this ocean as near the centre of the earth as i can." sunshine speaks with its word of power, and says, "up and away!" and in the wreathing mists of morning these myriads of tons rise in the air, flyaway hundreds of miles, and supply all the niagaras, mississippis and amazons of earth. the sun says to the earth, wrapped in the mantle of winter, [page ] "bloom again;" and the snows melt, the ice retires, and vegetation breaks forth, birds sing, and spring is about us. thus it is evident that every force is constitutionally arranged to be overcome by a higher, and all by the highest. gravitation of earth naturally and legitimately yields to the power of the sun's heat, and then the waters fly into the clouds. it as naturally and legitimately yields to the power of mind, and the waters of the red sea are divided and stand "upright as an heap." water naturally bursts into flame when a bit of potassium is thrown into it, and as naturally when elijah calls the right kind of fire from above. what seems a miracle, and in contravention of law, is only the constitutional exercise of higher force over forces organized to be swayed. if law were perfectly rigid, there could be but one force; but many grades exist from cohesion to mind and spirit. the highest forces are meant to have victory, and thus give the highest order and perfectness. across the astronomic spaces reach all these powers, making creation a perpetual process rather than a single act. it almost seems as if light, in its varied capacities, were the embodiment of god's creative power; as if, having said, "let there be light," he need do nothing else, but allow it to carry forward the creative processes to the end of time. it was newton, one of the earliest and most acute investigators in this study of light, who said, "i seem to have wandered on the shore of truth's great ocean, and to have gathered a few pebbles more beautiful than common; but the vast ocean itself rolls before me undiscovered and unexplored." [page ] experiments with light. a light set in a room is seen from every place; hence light streams in every possible direction. if put in the centre of a hollow sphere, every point of the surface will be equally illumined. if put in a sphere of twice the diameter, the same light will fall on all the larger surface. the surfaces of spheres are as the squares of their diameters; hence, in the larger sphere the surface is illumined only one-quarter as much as the smaller. the same is true of large and small rooms. in fig. it is apparent that the light that falls on the first square is spread, at twice the distance, over the second square, which is four times as large, and at three times the distance over nine times the surface. the varying amount of light received by each planet is also shown in fractions above each world, the amount received by the earth being . [illustration: fig. .] [illustration: fig. .--measuring intensities of light.] the intensity of light is easily measured. let two lights of different brightness, as in fig. , cast shadows on the same screen. arrange them as to distance so that both shadows shall be equally dark. let them fall side by side, and study them carefully. measure the respective distances. suppose one is twenty inches, the other forty. light varies as the square [page ] of the distance: the square of is , of is . divide by , and the result is that one light is four times as bright as the other. [illustration: fig. .--reflection and diffusion of light.] light can be handled, directed, and bent, as well as iron bars. darken a room and admit a beam of sunlight through a shutter, or a ray of lamp-light through the key-hole. if there is dust in the room it will be observed that light goes in straight lines. because of this men are able to arrange houses and trees in rows, the hunter aims his rifle correctly, and the astronomer projects straight lines to infinity. take a hand-mirror, or better, a piece of glass coated on one side with black varnish, and you can send your ray anywhere. by using two mirrors, or having an assistant and using several, you can cause a ray of light to turn as many corners as you please. i once saw mr. tyndall send a ray into a glass jar filled with smoke (fig. ). admitting a slender ray through a small hole in a card over the mouth, one ray appeared; removing the cover, the whole jar was luminous; as the smoke disappeared in spots cavities of darkness appeared. turn the same ray into a tumbler of water, [page ] it becomes faintly visible; stir into it a teaspoonful of milk, then turn in the ray of sunlight, and it glows like a lamp, illuminating the whole room. these experiments show how the straight rays of the sun are diffused in every direction over the earth. set a small light near one edge of a mirror; then, by putting the eye near the opposite edge, you see almost as many flames as you please from the multiplied reflections. how can this be accounted for? into your beam of sunlight, admitted through a half-inch hole, put the mirror at an oblique angle; you can arrange it so as to throw half a dozen bright spots on the opposite wall. [illustration: fig. .--manifold reflections.] in fig. the sunbeam enters at a, and, striking the mirror _m_ at _a_, is partly reflected to on the wall, and partly enters the glass, passes through to the silvered back at b, and is totally reflected to _b_, where it again divides, some of it going to the wall at , and the rest, continuing to make the same reflections and divisions, causes spots , , , etc. the brightest spot is at no. , because the silvered glass at b is the best reflector and has the most light. when the discovery of the moons of mars was announced in , it was also widely published that they could be seen by a mirror. of course this is impossible. the point of light mistaken for the moon in this secondary reflection was caused by holding the mirror in an oblique position. take a small piece of mirror, say an inch in surface, and putting under it three little pellets of wax, putty, or clay, set it on the wrist, with one of the pellets on the pulse. hold the mirror steadily in the beam of light, and the frequency and prominence of each pulse-beat will be indicated by the tossing spot of light on the wall. if the operator becomes excited the fact will be evident to all observers. [illustration: fig. .] place a coin in a basin (fig. ), and set it so that the rim will conceal the coin from the eye. pour in water, and the coin will [page ] appear to rise into sight. when light passes from a medium of one density to a medium of another, its direction is changed. thus a stick in water seems bent. ships below the horizon are sometimes seen above, because of the different density of the layers of air. thus light coming from the interstellar spaces, and entering our atmosphere, is bent down more and more by its increasing density. the effect is greatest when the sun or star is near the horizon, none at all in the zenith. this brings the object into view before it is risen. allowance for this displacement is made in all delicate astronomical observations. [illustration: fig. .--atmospherical refraction.] notice on the floor the shadow of the window-frames. the glass of almost every window is so bent as to turn the sunlight aside enough to obliterate some of the shadows or increase their thickness. decomposition of light. admit the sunbeam through a slit one inch long and one-twentieth of an inch wide. pass it through a prism. either purchase one or make it of three plain pieces of glass one and a half inch wide by six inches long, fastened together in triangular shape--fasten the edges with hot wax and fill it with water; then on a screen or wall you will have the colors of the rainbow, not merely seven but seventy, if your eyes are sharp enough. take a bit of red paper that matches the red color of the spectrum. move it along the line of colors toward the violet. in the orange it is dark, in the yellow darker, in the green and all beyond, black. that is because there are no more red rays to be reflected by it. so a green object is true to its color only in the green rays, and black elsewhere. all these colors may be recombined by a second prism into white light. [page ] iii. astronomical instruments. "the eyes of the lord are in every place."--_proverbs_ xv. . [page ] "man, having one kind of an eye given him by his maker, proceeds to construct two other kinds. he makes one that magnifies invisible objects thousands of times, so that a dull razor-edge appears as thick as three fingers, until the amazing beauty of color and form in infinitesimal objects is entrancingly apparent, and he knows that god's care of least things is infinite. then he makes the other kind four or six feet in diameter, and penetrates the immensities of space thousands of times beyond where his natural eye can pierce, until he sees that god's immensities of worlds are infinite also."--bishop foster. [page ] iii. _the telescope._ frequent allusion has been made in the previous chapter to discovered results. it is necessary to understand more clearly the process by which such results have been obtained. some astronomical instruments are of the simplest character, some most delicate and complex. when a man smokes a piece of glass, in order to see an eclipse of the sun, he makes a simple instrument. ferguson, lying on his back and slipping beads on a string at a certain distance above his eye, measured the relative distances of the stars. the use of more complex instruments commenced when galileo applied the telescope to the heavens. he cannot be said to have invented the telescope, but he certainly constructed his own without a pattern, and used it to good purpose. it consists of a lens, o b (fig. ), which acts as a multiple prism to bend all the rays to one point at r. place the eye there, and it receives as much light as if it were as large as the lens o b. the rays, however, are convergent, and the point difficult to [page ] find. hence there is placed at r a concave lens, passing through which the rays emerge in parallel lines, and are received by the eye. opera-glasses are made upon precisely this principle to-day, because they can be made conveniently short. [illustration: fig. .--refracting telescope.] if, instead of a concave lens at r, converting the converging rays into parallel ones, we place a convex or magnifying lens, the minute image is enlarged as much as an object seems diminished when the telescope is reversed. this is the grand principle of the refracting telescope. difficulties innumerable arise as we attempt to enlarge the instruments. these have been overcome, one after another, until it is now felt that the best modern telescope, with an object lens of twenty-six inches, has fully reached the limit of optical power. _the reflecting telescope_. this is the only kind of instrument differing radically from the refracting one already described. it receives the light in a concave mirror, m (fig. ), which reflects it to the focus f, producing the same result as the lens of the refracting telescope. here a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the newtonian telescope; or a mirror at r may be placed perpendicularly, and send the rays through [page ] an opening in the mirror at m. this form is called the gregorian telescope. or the mirror m may be slightly inclined to the coming rays, so as to bring the point f entirely outside the tube, in which case it is called the herschelian telescope. in either case the image may be magnified, as in the refracting telescope. [illustration: fig. .--reflecting telescope.] reflecting telescopes are made of all sizes, up to the cyclopean eye of the one constructed by lord rosse, which is six feet in diameter. the form of instrument to be preferred depends on the use to which it is to be put. the loss of light in passing through glass lenses is about two-tenths. the loss by reflection is often one-half. in view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector. the mounting of large telescopes demands the highest engineering ability. the whole instrument, with its vast weight of a twenty-six-inch glass lens, with its accompanying tube and appurtenances, must be pointed as nicely as a rifle, and held as steadily as the axis of the globe. to give it the required steadiness, the foundation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observatory is allowed to touch this support. when a star is once found, the earth swiftly rotates the telescope away from it, and it passes out of the field. to avoid this, clock-work is so arranged that the great telescope follows the star by the hour, if required. it will take a star at its eastern rising, and hold it constantly in view while it climbs to the meridian and sinks in the west (fig. ). the reflector demands still more difficult engineering. that of lord rosse has a metallic mirror [page ] weighing six tons, a tube forty feet long, which, with its appurtenances, weighs seven tons more. it moves between two walls only ° east and west. the new paris reflector (fig. ) has a much wider range of movement. [illustration: fig. .--cambridge equatorial.] [illustration: fig. .--new paris reflector.] _the spectroscope._ a spectrum is a collection of the colors which are dispersed by a prism from any given light. if it is sunlight, it is a solar spectrum; if the source of light is a [page ] star, candle, glowing metal, or gas, it is the spectrum of a star, candle, glowing metal, or gas. an instrument to see these spectra is called a spectroscope. considering the infinite variety of light, and its easy modification and absorption, we should expect an immense number of spectra. a mere prism disperses the light so imperfectly that different orders of vibrations, perceived as colors, are mingled. no eye can tell where one commences or ends. such a spectrum is said to be impure. what we want is that each point in the spectrum should be made of rays of the same number of vibrations. as we can let only a small beam of light pass through the prism, in studying celestial objects with a telescope and spectroscope we must, in every instance, contract the aperture of the instrument until we get only a small beam of light. in order to have the colors thoroughly dispersed, the best instruments pass the beam of light through a series of prisms called a battery, each one spreading farther the colors which the previous ones had spread. in fig. the ray is seen entering through the telescope a, which renders the rays parallel, and passing [page ] through the prisms out to telescope b, where the spectrum can be examined on the retina of the eye for a screen. in order to still farther disperse the rays, some batteries receive the ray from the last prism at o upon an oblique mirror, send it up a little to another, which delivers it again to the prism to make its journey back again through them all, and come out to be examined just above where it entered the first prism. [illustration: fig. .--spectroscope, with battery of prisms.] attached to the examining telescope is a diamond-ruled scale of glass, enabling us to fix the position of any line with great exactness. [illustration: fig. .--spectra of glowing hydrogen and the sun.] in fig. is seen, in the lower part, a spectrum of the sun, with about a score of its thousands of lines made evident. in the upper part is seen the spectrum of bright lines given by glowing hydrogen gas. these lines are given by no other known gas; they are its autograph. it is readily observed that they precisely correspond with certain dark lines in the solar spectrum. hence we easily know that a glowing gas gives the same bright lines that it absorbs from the light of another source passing through it--that is, glowing gas gives out the same rays of light that it absorbs when it is not glowing. the subject becomes clearer by a study of the chromolithic plate. no. represents the solar spectrum, with a few of its lines on an accurately graduated scale. [page ] no. shows the bright line of glowing sodium, and, corresponding to a dark line in the solar spectrum, shows the presence of salt in that body. no. shows that potassium has some violet rays, but not all; and there being no dark line to correspond in the solar spectrum, we infer its absence from the sun. no. shows the numerous lines and bands of barium--several red, orange, yellow, and four are very bright green ones. the lines given by any volatilized substances are always in the same place on the scale. a patient study of these signs of substances reveals, richer results than a study of the cuniform characters engraved on assyrian slabs; for one is the handwriting of men, the other the handwriting of god. one of the most difficult and delicate problems solved by the spectroscope is the approach or departure of a light-giving body in the line of sight. stand before a locomotive a mile away, you cannot tell whether it approaches or recedes, yet it will dash by in a minute. how can the movements of the stars be comprehended when they are at such an immeasurable distance? it can best be illustrated by music. the note c of the g clef is made by two hundred and fifty-seven vibrations of air per second. twice as many vibrations per second would give us the note c an octave above. sound travels at the rate of three hundred and sixty-four yards per second. if the source of these two hundred and fifty-seven vibrations could approach us at three hundred and sixty-four yards per second, it is obvious that twice as many waves would be put into a given space, and we should hear the upper c when only waves enough were made for the lower c. the same [page ] result would appear if we carried our ear toward the sound fast enough to take up twice as many valves as though we stood still. this is apparent to every observer in a railway train. the whistle of an approaching locomotive gives one tone; it passes, and we instantly detect another. let two trains, running at a speed of thirty-six yards a second, approach each other. let the whistle of one sound the note e, three hundred and twenty-three vibrations per second. it will be heard on the other as the note g, three hundred and eighty-eight vibrations per second; for the speed of each train crowds the vibrations into one-tenth less room, adding + vibrations per second, making three hundred and eighty-eight in all. the trains pass. the vibrations are put into one-tenth more space by the whistle making them, and the other train allows only nine-tenths of what there are to overtake the ear. each subtracts + vibrations from three hundred and twenty-three, leaving only two hundred and fifty-eight, which is the note c. yet the note e was constantly uttered. [illustration: . solar spectrum. . spectrum of potassium. . spectrum of sodium. . spectrum of strontium. . spectrum of calcium. . spectrum of barium.] if a source of light approach or depart, it will have a similar effect on the light waves. how shall we detect it? if a star approach us, it puts a greater number of waves into an inch, and shortens their length. if it recedes, it increases the length of the wave--puts a less number into an inch. if a body giving only the number of vibrations we call green were to approach sufficiently fast, it would crowd in vibrations enough to appear what we call blue, indigo, or even violet, according to its speed. if it receded sufficiently fast, it would leave behind it only vibrations enough to fill up [page ] the space with what we call yellow, orange, or red, according to its speed; yet it would be green, and green only, all the time. but how detect the change? if red waves are shortened they become orange in color; and from below the red other rays, too far apart to be seen by the eye, being shortened, become visible as red, and we cannot know that anything has taken place. so, if a star recedes fast enough, violet vibrations being lengthened become indigo; and from above the violet other rays, too short to be seen, become lengthened into visible violet, and we can detect no movement of the colors. the dark lines of the spectrum are the cutting out of rays of definite wave-lengths. if the color spectrum moves away, they move with it, and away from their proper place in the ordinary spectrum. if, then, we find them toward the red end, the star is receding; if toward the violet end, it is approaching. turn the instrument on the centre of the sun. the dark lines take their appropriate place, and are recognized on the ruled scale. turn it on one edge, that is approaching us one and a quarter miles a second by the revolution of the sun on its axis, the spectral lines move toward the violet end; turn the spectroscope toward the other edge of the sun, it is receding from us one and a quarter miles a second by reason of the axial revolution, and the spectral lines move toward the red end. turn it near the spots, and it reveals the mighty up-rush in one place and the down-rush in another of one hundred miles a second. we speak of it as an easy matter, but it is a problem of the greatest delicacy, almost defying the mind of man to read the movements of matter. it should be recognized that professor young, of [page ] princeton, is the most successful operator in this recent realm of science. he already proposes to correct the former estimate of the sun's axial revolutions, derived from observing its spots, by the surer process of observing accelerated and retarded light. within a very few years this wonderful instrument, the spectroscope, has made amazing discoveries. in chemistry it reveals substances never known before; in analysis it is delicate to the detection of the millionth of a grain. it is the most deft handmaid of chemistry, the arts, of medical science, and astronomy. it tells the chemical constitution of the sun, the movements taking place, the nature of comets, and nebulæ. by the spectroscope we know that the atmospheres of venus and mars are like our own; that those of jupiter and saturn are very unlike; it tells us which stars approach and which recede, and just how one star differeth from another in glory and substance. in the near future we shall have the brilliant and diversely colored flowers of the sky as well classified into orders and species as are the flowers of the earth. [page ] iv. celestial measurements. "who hath measured the waters in the hollow of his hand, and meted out heaven with the span? mine hand also hath laid the foundation of the earth, and my right hand hath spanned the heavens."--_isa._ xl. ; xlviii. . [page ] "go to yon tower, where busy science plies her vast antennæ, feeling thro' the skies; that little vernier, on whose slender lines the midnight taper trembles as it shines, a silent index, tracks the planets' march in all their wanderings thro' the ethereal arch, tells through the mist where dazzled mercury burns, and marks the spot where uranus returns. "so, till by wrong or negligence effaced, the living index which thy maker traced repeats the line each starry virtue draws through the wide circuit of creation's laws; still tracks unchanged the everlasting ray where the dark shadows of temptation stray; but, once defaced, forgets the orbs of light, and leaves thee wandering o'er the expanse of night." oliver wendell holmes. [page ] iv. _celestial measurements._ we know that astronomy has what are called practical uses. if a ship had been driven by euroclydon ten times fourteen days and nights without sun or star appearing, a moment's glance into the heavens from the heaving deck, by a very slightly educated sailor, would tell within one hundred yards where he was, and determine the distance and way to the nearest port. we know that, in all final and exact surveying, positions must be fixed by the stars. earth's landmarks are uncertain and easily removed; those which we get from the heavens are stable and exact. in the united states steam-ship _enterprise_ was sent to survey the amazon. every night a "star party" went ashore to fix the exact latitude and longitude by observations of the stars. our real landmarks are not the pillars we rear, but the stars millions of miles away. all our standards of time are taken from the stars; every railway train runs by their time to avoid collision; by them all factories start and stop. indeed, we are ruled by the stars even more than the old astrologers imagined. man's finest mechanism, highest thought, and broadest exercise of the creative faculty have been inspired by astronomy. no other instruments approximate in delicacy those which explore the heavens; no other [page ] system of thought can draw such vast and certain conclusions from its premises. "too low they build who build beneath the stars;" we should lay our foundations in the skies, and then build upward. we have been placed on the outside of this earth, instead of the inside, in order that we may look abroad. we are carried about, through unappreciable distance, at the inconceivable velocity of one thousand miles a minute, to give us different points of vision. the earth, on its softly-spinning axle, never jars enough to unnest a bird or wake a child; hence the foundations of our observatories are firm, and our measurements exact. whoever studies astronomy, under proper guidance and in the right spirit, grows in thought and feeling, and becomes more appreciative of the creator. _celestial movements._ let it not be supposed that a mastery of mathematics and a finished education are necessary to understand the results of astronomical research. it took at first the highest power of mind to make the discoveries that are now laid at the feet of the lowliest. it took sublime faith, courage, and the results of ages of experience in navigation, to enable columbus to discover that path to the new world which now any little boat can follow. ages of experience and genius are stored up in a locomotive, but quite an unlettered man can drive it. it is the work of genius to render difficult matters plain, abstruse thoughts clear. [illustration: fig. .] a brief explanation of a few terms will make the principles of world inspection easily understood. imagine a perfect circle thirty feet in diameter--that is, create [page ] one (fig. ). draw through it a diameter horizontally, another perpendicularly. the angles made by the intersecting lines are each said to be ninety degrees, marked thus °. the arc of a circle included between any two of the lines is also °. every circle, great or small, is divided into these °. if the sun rose in the east and came to the zenith at noon, it would have passed °. when it set in the west it would have traversed half the circle, or °. in fig. the angle of the lines measured on the graduated arc is °. the mountain is ° high, the world ° in diameter, the comet moves ° a day, the stars are ° apart. the height of the mountain, the diameter of the world, the velocity of the comet, and the distance between the stars, depend on the distance of each from the point of sight. every degree is divided into minutes (marked '), and every minute into seconds (marked "). [illustration: fig. .--illustration of angles.] imagine yourself inside a perfect sphere one hundred feet in diameter, with the interior surface above, around, and below studded with fixed bright points like stars. the familiar constellations of night might be blazoned there in due proportion. if this star-sprent sphere were made to revolve once in twenty-four hours, all the stars would successively [page ] pass in review. how easily we could measure distances between stars, from a certain fixed meridian, or the equator! how easily we could tell when any particular star would culminate! it is as easy to take all these measurements when our earthly observatory is steadily revolved within the sphere of circumambient stars. stars can be mapped as readily as the streets of a great city. looking down on it in the night, one could trace the lines of lighted streets, and judge something of its extent and regularity. but the few lamps of evening would suggest little of the greatness of the public buildings, the magnificent enterprise and commerce of its citizens, or the intelligence of its scholars. looking up to the lamps of the celestial city, one can judge something of its extent and regularity; but they suggest little of the magnificence of the many mansions. stars are reckoned as so many degrees, minutes, and seconds from each other, from the zenith, or from a given meridian, or from the equator. thus the stars called the pointers, in the great bear, are ° apart; the nearest one is ° from the pole star, which is ° ' " above the horizon at philadelphia. in going to england you creep up toward the north end of the earth, till the pole star is ° high. it stays near its place among the stars continually, "of whose true-fixed and resting quality there is no fellow in the firmament." _how to measure._ suppose a telescope, fixed to a mural circle, to revolve on an axis, as in fig. ; point it horizontally at a star; [page ] turn it up perpendicular to another star. of course the two stars are ° apart, and the graduated scale, which is attached to the outer edge of the circle, shows a revolution of a quarter circle, or °, but a perfect accuracy of measurement must be sought; for to mistake the breadth of a hair, seen at the distance of one hundred and twenty-five feet, would cause an error of , , miles at the distance of the sun, and immensely more at the distance of the stars. the correction of an inaccuracy of no greater magnitude than that has reduced our estimate of the distance of our sun , , miles. [illustration: fig. .--mural circle.] consider the nicety of the work. suppose the graduated scale to be thirty feet in circumference. divided into °, each would be one inch long. divide each degree into ', each one is / of an inch long. it takes good eyesight to discern it. but each minute must be [page ] divided into ", and these must not only be noted, but even tenths and hundredths of seconds must be discerned. of course they are not seen by the naked eye; some mechanical contrivance must be called in to assist. a watch loses two minutes a week, and hence is unreliable. it is taken to a watch-maker that every single second may be quickened / part of itself. now / part of a second would be a small interval of time to measure, but it must be under control. if the temperature of a summer morning rises ten or twenty degrees we scarcely notice it; but the magnetic tastimeter measures / of a degree. come to earthly matters. in , after nearly twenty-eight years' work, the state of massachusetts opened a tunnel nearly five miles long through the hoosac mountains. in the early part of the work the engineers sunk a shaft near the middle feet deep. then the question to be settled was where to go so as to meet the approaching excavations from the east and west. a compass could not be relied on under a mountain. the line must be mechanically fixed. a little divergence at the starting-point would become so great, miles away, that the excavations might pass each other without meeting; the grade must also rise toward the central shaft, and fall in working away from it; but the lines were fixed with such infinitesimal accuracy that, when the one going west from the eastern portal and the one going east from the shaft met in the heart of the mountain, the western line was only one-eighth of an inch too high, and three-sixteenths of an inch too far north. to reach this perfect result they had to triangulate from the eastern portal to distant [page ] mountain peaks, and thence down the valley to the central shaft, and thus fix the direction of the proposed line across the mouth of the shaft. plumb-lines were then dropped one thousand and twenty-eight feet, and thus the line at the bottom was fixed. three attempts were made--in , , and --to fix the exact time-distance between greenwich and washington. these three separate efforts do not differ one-tenth of a second. such demonstrable results on earth greatly increase our confidence in similar measurements in the skies. [illustration: fig. .] a scale is frequently affixed to a pocket-rule, by which we can easily measure one-hundredth of an inch (fig. ). the upper and lower line is divided into tenths of an inch. observe the slanting line at the right hand. it leans from the perpendicular one-tenth of an inch, as shown by noticing where it reaches the top line. when it reaches the second horizontal line it has left the perpendicular one-tenth of that tenth--that is, one-hundredth. the intersection marks / of an inch from one end, and one-hundredth from the other. when division-lines, on measures of great nicety, get too fine to be read by the eye, we use the microscope. by its means we are able to count , lines ruled on a glass plate within an inch. the smallest object that can be seen by a keen eye makes an angle of ", but by putting six microscopes on the scale of the telescope on the mural circle, we are able to reach an exactness of ". , or / of an inch. this instrument is used to measure the declination of stars, or angular [page ] distance north or south of the equator. thus a star's place in two directions is exactly fixed. when the telescope is mounted on two pillars instead of the face of a wall, it is called a transit instrument. this is used to determine the time of transit of a star over the meridian, and if the transit instrument is provided with a graduated circle it can also be used for the same purposes as the mural circle. man's capacity to measure exactly is indicated in his ascertainment of the length of waves of light. it is easy to measure the three hundred feet distance between the crests of storm-waves in the wide atlantic; easy to measure the different wave-lengths of the different tones of musical sounds. so men measure the lengths of the undulations of light. the shortest is of the violet light, . ten-millionths of an inch. by the horizontal pendulum professor root has made / of an inch apparent. the next elements of accuracy must be perfect time and perfect notation of time. as has been said, we get our time from the stars. thus the infinite and heavenly dominates the finite and earthly. clocks are set to the invariable sidereal time. sidereal noon is when we have turned ourselves under the point where the sun crosses the equator in march, called the vernal equinox. sidereal clocks are figured to indicate twenty-four hours in a day: they tick exact seconds. to map stars we wish to know the exact second when they cross the meridian, or the north and south line in the celestial dome above us. the telescope (fig. , p. ) swings exactly north and south. in its focus a set of fine threads of spider-lines is placed (fig. ). the telescope is set just high enough, so that by the rolling over of the earth [page ] the star will come into the field just above the horizontal thread. the observer notes the exact second and tenth of a second when the star reaches each vertical thread in the instrument, adds together the times and divides by five to get the average, and the exact time is reached. [illustration: fig. .--transit of a star noted.] but man is not reliable enough to observe and record with sufficient accuracy. some, in their excitement, anticipate its positive passage, and some cannot get their slow mental machinery in motion till after it has made the transit. moreover, men fall into a habit of estimating some numbers of tenths of a second oftener than others. it will be found that a given observer will say three tenths or seven tenths oftener than four or eight. he is falling into ruts, and not trustworthy. general o. m. mitchel, who had been director of the cincinnati observatory, once told one of his staff-officers that he was late at an appointment. "only a few minutes," said the officer, apologetically. "sir," said the general, "where i have been accustomed to work, hundredths of a second are too important to be neglected." and it is to the rare genius of this astronomer, and to others, that we owe the mechanical accuracy that we now attain. the clock is made to mark its seconds on paper wrapped around a revolving cylinder. under the observer's fingers is an electric key. this he can touch at the instant of the transit of the star [page ] over each wire, and thus put his observation on the same line between the seconds dotted by the clock. of course these distances can be measured to minute fractional parts of a second. but it has been found that it takes an appreciable time for every observer to get a thing into his head and out of his finger-ends, and it takes some observers longer than others. a dozen men, seeing an electric spark, are liable to bring down their recording marks in a dozen different places on the revolving paper. hence the time that it takes for each man to get a thing into his head and out of his fingers is ascertained. this time is called his personal equation, and is subtracted from all of his observations in order to get at the true time; so willing are men to be exact about material matters. can it be thought that moral and spiritual matters have no precision? thus distances east or west from any given star or meridian are secured; those north and south from the equator or the zenith are as easily fixed, and thus we make such accurate maps of the heavens that any movements in the far-off stars--so far that it may take centuries to render the swiftest movements appreciable--may at length be recognized and accounted for. [illustration: fig. .] we now come to a little study of the modes of measuring distances. create a perfect square (fig. ); draw a diagonal line. the square angles are °, the divided angles give two of ° each. now the base a b is equal to the perpendicular a c. now any point--c, where a perpendicular, a c, and a diagonal, b c, meet--will be [page ] as far from a as b is. it makes no difference if a river flows between a and c, and we cannot go over it; we can measure its distance as easily as if we could. set a table four feet by eight out-doors (fig. ); so arrange it that, looking along one end, the line of sight just strikes a tree the other side of the river. go to the other end, and, looking toward the tree, you find the line of sight to the tree falls an inch from the end of the table on the farther side. the lines, therefore, approach each other one inch in every four feet, and will come together at a tree three hundred and eighty-four feet away. [illustration: fig. .--measuring distances.] [illustration: fig. .--measuring elevations.] the next process is to measure the height or magnitude of objects at an ascertained distance. put two pins in a stick half an inch apart (fig. ). hold it up two feet from the eye, and let the upper pin fall in line with your eye and the top of a distant church steeple, and the lower pin in line with the bottom of the church and your eye. if the church is three-fourths of a mile away, it must be eighty-two feet high; if a mile away, it must be one hundred and ten feet high. for if two lines spread [page ] one-half an inch going two feet, in going four feet they will spread an inch, and in going a mile, or five thousand two hundred and eighty feet, they will spread out one-fourth as many inches, viz., thirteen hundred and twenty--that is, one hundred and ten feet. of course these are not exact methods of measurement, and would not be correct to a hair at one hundred and twenty-five feet, but they perfectly illustrate the true methods of measurement. imagine a base line ten inches long. at each end erect a perpendicular line. if they are carried to infinity they will never meet: will be forever ten inches apart. but at the distance of a foot from the base line incline one line toward the other / of an inch, and the lines will come together at a distance of three hundred miles. that new angle differs from the former right angle almost infinitesimally, but it may be measured. its value is about three-tenths of a second. if we lengthen the base line from ten inches to all the miles we can command, of course the point of meeting will be proportionally more distant. the angle made by the lines where they come together will be obviously the same as the angle of divergence from a right angle at this end. that angle is called the parallax of any body, and is the angle that would be made by two lines coming from that body to the two ends of any conventional base, as the semi-diameter of the earth. that that angle would vary according to the various distances is easily seen by fig. . [illustration: fig. .] let o p be the base. this would subtend a greater angle seen from star a than from star b. let b be far enough away, and o p would become invisible, and b [page ] would have no parallax for that base. thus the moon has a parallax of " with the semi-equatorial diameter of the earth for a base. and the sun has a parallax ". on the same base. it is not necessary to confine ourselves to right angles in these measurements, for the same principles hold true in any angles. now, suppose two observers on the equator should look at the moon at the same instant. one is on the top of cotopaxi, on the west coast of south america, and one on the west coast of africa. they are ° apart--half the earth's diameter between them. the one on cotopaxi sees it exactly overhead, at an angle of ° with the earth's diameter. the one on the coast of africa sees its angle with the same line to be ° ' "--that is, its parallax is ". try the same experiment on the sun farther away, as is seen in fig. , and its smaller parallax is found to be only ". . it is not necessary for two observers to actually station themselves at two distant parts of the earth in order to determine a parallax. if an observer could go from one end of the base-line to the other, he could determine both angles. every observer is actually carried along through space by two motions: one is that of the earth's revolution of one thousand miles an hour around the axis; and the other is the movement of the earth around the sun of one thousand miles in a minute. hence we can have the diameter not only of [page ] the earth (eight thousand miles) for a base-line, but the diameter of the earth's orbit ( , , miles), or any part of it, for such a base. two observers at the ends of the earth's diameter, looking at a star at the same instant, would find that it made the same angle at both ends; it has no parallax on so short a base. we must seek a longer one. observe a certain star on the st of march; then let us traverse the realms of space for six months, at one thousand miles a minute. we come round in our orbit to a point opposite where we were six months ago, with , , of miles between the points. now, with this for a base-line, measure the angles of the same stars: it is the same angle. sitting in my study here, i glance out of the window and discern separate bricks, in houses five hundred feet away, with my unaided eye; they subtend a discernible angle. but one thousand feet away i cannot distinguish individual bricks; their width, being only two inches, does not subtend an angle apprehensible to my vision. so at these distant stars the earth's enormous orbit, if lying like a blazing ring in space, with the world set on its edge like a pearl, and the sun blazing like a diamond in the centre, would all shrink to a mere point. not quite to a point from the nearest stars, or we should never be able to measure the distance of any of them. professor airy says that our orbit, seen from the nearest star, would be the same as a circle six-tenths of an inch in diameter seen at the distance of a mile: it would all be hidden by a thread one-twenty-fifth of an inch in diameter, held six hundred and fifty feet from the eye. if a straight line could be drawn from a star, sirius in the east to the star vega in the west, touching our [page ] earth's orbit on one side, as t r a (fig. ), and a line were to be drawn six months later from the same stars, touching our earth's orbit on the other side, as r b t, such a line would not diverge sufficiently from a straight line for us to detect its divergence. numerous vain attempts had been made, up to the year , to detect and measure the angle of parallax by which we could rescue some one or more of the stars from the inconceivable depths of space, and ascertain their distance from us. we are ever impelled to triumph over what is declared to be unconquerable. there are peaks in the alps no man has ever climbed. they are assaulted every year by men zealous of more worlds to conquer. so these greater heights of the heavens have been assaulted, till some ambitious spirits have outsoared even imagination by the certainties of mathematics. [illustration: fig. .] it is obvious that if one star were three times as far from us as another, the nearer one would seem to be displaced by our movement in our orbit three times as much as the other; so, by comparing one star with another, we reach a ground of judgment. the ascertainment of longitude at sea by means of the moon affords a good illustration. along the track where the moon sails, nine bright stars, four planets, and the sun have been selected. the nautical almanacs give the distance of the moon from these successive stars every hour in the night for three years in advance. the sailor can measure the distance at any time by his sextant. looking from the world at d (fig. ), the distance of the moon and [page ] star is a e, which is given in the almanac. looking from c, the distance is only b e, which enables even the uneducated sailor to find the distance, c d, on the earth, or his distance from greenwich. [illustration: fig. .--mode of ascertaining longitude.] so, by comparisons of the near and far stars, the approximate distance of a few of them has been determined. the nearest one is the brightest star in the centaur, never visible in our northern latitudes, which has a parallax of about one second. the next nearest is no. in the swan, or cygni, having a parallax of ". . approximate measurements have been made on sirius, capella, the pole star, etc., about eighteen in all. the distances are immense: only the swiftest agents can traverse them. if our earth were suddenly to dissolve its allegiance to the king of day, and attempt a flight to the north star, and should maintain its flight of one thousand miles a minute, it would flyaway toward polaris for thousands upon thousands of years, till a million years had passed away, before it reached that northern dome of the distant sky, and gave its new allegiance to another sun. the sun it had left behind it would gradually diminish till it was small as arcturus, then small as could be discerned by the naked eye, until at last it would finally fade out in utter darkness long before the new sun was reached. light can traverse the distance around our earth eight times in one second. it comes in eight minutes from the sun, but it takes three and a quarter years to come from alpha [page ] centauri, seven and a quarter years from cygni, and forty-five years from the polar star. sometimes it happens that men steer along a lee shore, dependent for direction on polaris, that light-house in the sky. sometimes it has happened that men have traversed great swamps by night when that star was the light-housse of freedom. in either case the exigency of life and liberty was provided for forty-five years before by a providence that is divine. we do not attempt to name in miles these enormous distances; we must seek another yard-stick. our astronomical unit and standard of measurement is the distance of the earth from the sun-- , , miles. this is the golden reed with which we measure the celestial city. thus, by laying down our astronomical unit , times, we measure to alpha centauri, more than twenty millions of millions of miles. doubtless other suns are as far from alpha centauri and each other as that is from ours. stars are not near or far according to their brightness. cygni is a telescopic star, while sirius, the brightest star in the heavens, is twice as far away from us. one star differs from another star in intrinsic glory. the highest testimonies to the accuracy of these celestial observations are found in the perfect predictions of eclipses, transits of planets over the sun, occultation of stars by the moon, and those statements of the nautical almanac that enable the sailor to know exactly where he is on the pathless ocean by the telling of the stars: "on the trackless ocean this book is the mariner's trusted friend and counsellor; daily and nightly its revelations bring safety to ships in all parts of the [page ] world. it is something more than a mere book; it is an ever-present manifestation of the order and harmony of the universe." another example of this wonderful accuracy is found in tracing the asteroids. within , , or , , miles from the sun, the one hundred and ninety-two minute bodies that have been already discovered move in paths very nearly the same--indeed two of them traverse the same orbit, being one hundred and eighty degrees apart;--they look alike, yet the eye of man in a few observations so determines the curve of each orbit, that one is never mistaken for another. but astronomy has higher uses than fixing time, establishing landmarks, and guiding the sailor. it greatly quickens and enlarges thought, excites a desire to know, leads to the utmost exactness, and ministers to adoration and love of the maker of the innumerable suns. [page ] v. the sun. "and god made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."--_gen._ i. . [page ] "it is perceived that the sun of the world, with all its essence, which is heat and light, flows into every tree, and into every shrub and flower, and into every stone, mean as well as precious; and that every object takes its portion from this common influx, and that the sun does not divide its light and heat, and dispense a part to this and a part to that. it is similar with the sun of heaven, from which the divine love proceeds as heat, and the divine wisdom as light; these two flow into human minds, as the heat and light of the sun of the world into bodies, and vivify them according to the quality of the minds, each of which takes from the common influx as much as is necessary."--swedenborg. [page ] v. _the sun._ suppose we had stood on the dome of boston statehouse november th, , on the night of the great conflagration, and seen the fire break out; seen the engines dash through the streets, tracking their path by their sparks; seen the fire encompass a whole block, leap the streets on every side, surge like the billows of a storm-swept sea; seen great masses of inflammable gas rise like dark clouds from an explosion, then take fire in the air, and, cut off from the fire below, float like argosies of flame in space. suppose we had felt the wind that came surging from all points of the compass to fan that conflagration till it was light enough a mile away to see to read the finest print, hot enough to decompose the torrents of water that were dashed on it, making new fuel to feed the flame. suppose we had seen this spreading fire seize on the whole city, extend to its environs, and, feeding itself on the very soil, lick up worcester with its tongues of flame--albany, new york, chicago, st. louis, cincinnati--and crossing the plains swifter than a prairie fire, making each peak of the rocky mountains hold up aloft a separate torch of flame, and the sierras whiter with heat than they ever were with snow, the waters of the pacific resolve into their constituent elements of oxygen and hydrogen, and [page ] burn with unquenchable fire! we withdraw into the air, and see below a world on fire. all the prisoned powers have burst into intensest activity. quiet breezes have become furious tempests. look around this flaming globe--on fire above, below, around--there is nothing but fire. let it roll beneath us till boston comes round again. no ember has yet cooled, no spire of flame has shortened, no surging cloud has been quieted. not only are the mountains still in flame, but other ranges burst up out of the seething sea. there is no place of rest, no place not tossing with raging flame! yet all this is only a feeble figure of the great burning sun. it is but the merest hint, a million times too insignificant. the sun appears small and quiet to us because we are so far away. seen from the various planets, the relative size of the sun appears as in fig. . looked for from some of the stars about us, the sun could not be seen at all. indeed, seen from the earth, it is not always the same size, because the distance is not always the same. if we represent the size of the sun by one thousand on the d of september or st of march, it would be represented by nine hundred and sixty-seven on the st of july, and by one thousand and thirty-four on the st of january. [illustration: fig. .--relative size of sun as seen from different planets.] we sometimes speak of the sun as having a diameter of , miles. we mean that that is the extent of the body as soon by the eye. but that is a small part of its real diameter. so we say the earth has an equatorial diameter of - / miles, and a polar one of . but the air is as much a part of the earth as the rocks are. the electric currents are as much a part of the [page ] earth as the ores and mountains they traverse. what the diameter of the earth is, including these, no man can tell. we used to say the air extended forty-five miles, but we now know that it reaches vastly farther. so of the sun, we might almost say that its diameter is infinite, for its light and heat reach beyond our measurement. its living, throbbing heart sends out pulsations, keeping all space full of its tides of living light. [page ] [illustration: fig. .--zodiacal light.] we might say with evident truth that the far-off planets are a part of the sun, since the space they traverse is filled with the power of that controlling king; not only with light, but also with gravitating power. but come to more ponderable matters. if we look [page ] into our western sky soon after sunset, on a clear, moonless night in march or april, we shall see a dim, soft light, somewhat like the milky-way, often reaching, well defined, to the pleiades. it is wedge-shaped, inclined to the south, and the smallest star can easily be seen through it. mairan and cassini affirm that they have seen sudden sparkles and movements of light in it. all our best tests show the spectrum of this light to be continuous, and therefore reflected; which indicates that it is a ring of small masses of meteoric matter surrounding the sun, revolving with it and reflecting its light. one bit of stone as large as the end of one's thumb, in a cubic mile, would be enough to reflect what light we see looking through millions of miles of it. perhaps an eye sufficiently keen and far away would see the sun surrounded by a luminous disk, as saturn is with his rings. as it extends beyond the earth's orbit, if this be measured as a part of the sun, its diameter would be about , , miles. come closer. when the sun is covered by the disk of the moon at the instant of total eclipse, observers are startled by strange swaying luminous banners, ghostly and weird, shooting in changeful play about the central darkness (fig. ). these form the corona. men have usually been too much moved to describe them, and have always been incapable of drawing them in the short minute or two of their continuance. but in men travelled eight thousand miles, coming and returning, in order that they might note the three minutes of total eclipse in colorado. each man had his work assigned to him, and he was drilled to attend to that and nothing else. improved instruments were put into his [page ] hands, so that the sun was made to do his own drawing and give his own picture at consecutive instants. fig. is a copy of a photograph of the corona of , by mr. henry draper. it showed much less changeability that year than common, it being very near the time of least sun-spot. the previous picture was taken near the time of maximum sun-spot. [illustration: fig. .--the corona in , brazil.] it was then settled that the corona consists of reflected light, sent to us from dust particles or meteoroids swirling in the vast seas, giving new densities and [page ] rarities, and hence this changeful light. whether they are there by constant projection, and fall again to the sun, or are held by electric influence, or by force of orbital revolution, we do not know. that the corona cannot be in any sense an atmosphere of any continuous gas, is seen from the fact that the comet of , passing within , miles of the body of the sun, was not burned out of existence as a comet, nor in any perceptible degree retarded in its motion. if the sun's diameter is to include the corona, it will be from , , to , , miles. [illustration: fig. .--the corolla in , colorado.] [page ] come closer still. at the instant of the totality of the eclipse red flames of most fantastic shape play along the edge of the moon's disk. they can be seen at any time by the use of a proper telescope with a spectroscope attached. i have seen them with great distinctness and brilliancy with the excellent eleven-inch telescope of the wesleyan university. a description of their appearance is best given in the language of professor young, of princeton college, who has made these flames the object of most successful study. on september th, , he was observing a large hydrogen cloud by the sun's edge. this cloud was about , miles long, and its upper side was some , miles above the sun's surface, the lower side some , miles. the whole had the appearance of being supported on pillars of fire, these seeming pillars being in reality hydrogen jets brighter and more active than the substance of the cloud. at half-past twelve, when professor young chanced to be called away from his observatory, there were no indications of any approaching change, except that one of the connecting stems of the southern extremity of the cloud had grown considerably brighter and more curiously bent to one side; and near the base of another, at the northern end, a little brilliant lump had developed itself, shaped much like a summer thunderhead. [illustration: fig. .--solar prominences of flaming hydrogen.] but when professor young returned, about half an hour later, he found that a very wonderful change had taken place, and that a very remarkable process was actually in progress. "the whole thing had been literally blown to shreds," he says, "by some inconceivable uprush from beneath. in place of the quiet cloud i had [page ] left, the air--if i may use the expression--was filled with the flying _débris_, a mass of detached vertical fusi-form fragments, each from ten to thirty seconds (_i. e._, from four thousand five hundred to thirteen thousand five hundred miles) long, by two or three seconds (nine hundred to thirteen hundred and fifty miles) wide--brighter, and closer together where the pillars had formerly stood, and rapidly ascending. when i looked, some of them had already reached a height of nearly four minutes ( , miles); and while i watched them they arose with a motion almost perceptible to the eye, until, in ten minutes, the uppermost were more than , miles above the solar surface. this was ascertained by careful measurements, the mean of three closely accordant determinations giving , miles as the extreme altitude attained. i am particular in the statement, because, so far as i know, chromatospheric matter (red hydrogen in this case) has never before been observed at any altitude exceeding five minutes, or , miles. the velocity of ascent, also--one hundred and sixty-seven miles per second--is considerably greater than anything hitherto recorded. * * * as the filaments arose, they gradually faded away like a dissolving cloud, and at a quarter past one only a few filmy wisps, with some brighter streamers low down near the chromatosphere, remained to mark the place. but in the mean while the little 'thunder-head' before alluded to had grown and developed wonderfully into a mass of rolling and ever-changing flame, to speak according to appearances. first, it was crowded down, as it were, along the solar surface; later, it arose almost pyramidally , miles in height; then [page ] its summit was drawn down into long filaments and threads, which were most curiously rolled backward and forward, like the volutes of an ionic capital, and finally faded away, and by half-past two had vanished like the other. the whole phenomenon suggested most forcibly the idea of an explosion under the great prominence, acting mainly upward, but also in all directions outward; and then, after an interval, followed by a corresponding in-rush." no language can convey nor mind conceive an idea of the fierce commotion we here contemplate. if we call these movements hurricanes, we must remember that what we use as a figure moves but one hundred miles an hour, while these move one hundred miles a second. such storms of fire on earth, "coming down upon us from the north, would, in thirty seconds after they had crossed the st. lawrence, be in the gulf of mexico, carrying with them the whole surface of the continent in a mass not simply of ruins but of glowing vapor, in which the vapors arising from the dissolution of the materials composing the cities of boston, new york, and chicago would be mixed in a single indistinguishable cloud." in the presence of these evident visions of an actual body in furious flame, we need hesitate no longer in accepting as true the words of st. peter of the time "in which the [atmospheric] heavens shall pass away with a great noise, and the elements shall melt with fervent heat; the earth also, and the works that are therein, shall be burned up." this region of discontinuous flame below the corona is called the chromosphere. hydrogen is the principal material of its upper part; iron, magnesium, and other [page ] metals, some of them as yet unknown on earth, but having a record in the spectrum, in the denser parts below. if these fierce fires are a part of the sun, as they assuredly are, its diameter would be from , , to , , miles. let us approach even nearer. we see a clearly recognized even disk, of equal dimensions in every direction. this is the photosphere. we here reach some definitely measurable data for estimating its visible size. we already know its distance. its disk subtends an angle of ' ". , or a little more than half a degree. three hundred and sixty such suns, laid side by side, would span the celestial arch from east to west with a half circle of light. two lines drawn from our earth at the angle mentioned would be , miles apart at the distance of , , miles. this, then, is the diameter of the visible and measurable part of the sun. it would require one hundred and eight globes like the earth in a line to measure the sun's diameter, and three hundred and thirty-nine, to be strung like the beads of a necklace, to encircle his waist. the sun has a volume equal to , , earths, but being only one-quarter as dense, it has a mass of only , earths. it has seven hundred times the mass of all the planets, asteroids, and satellites put together. thus it is able to control them all by its greater power of attraction. concerning the condition of the surface of the sun many opinions are held. that it is hot beyond all estimate is indubitable. whether solid or gaseous we are not sure. opinions differ: some incline to the first theory, others to the second; some deem the sun composed of solid particles, floating in gas so condensed [page ] by pressure and attraction as to shine like a solid. it has no sensible changes of general level, but has prodigious activity in spots. these spots have been the objects of earnest and almost hourly study on the part of such men as secchi, lockyer, faye, young, and others, for years. but it is a long way off to study an object. no telescope brings it nearer than , miles. theory after theory has been advanced, each one satisfactory in some points, none in all. the facts about the spots are these: they are most abundant on the two sides of the equator. they are gregarious, depressed below the surface, of vast extent, black in the centre, usually surrounded by a region of partial darkness, beyond which is excessive light. they have motion of their own over the surface--motion rotating about an axis, upward and downward about the edges. they change their apparent shape as the sun carries them across its disk by axial revolution, being narrow as they present their edges to us, and rounder as we look perpendicularly into them (fig. ). [illustration: fig. .--change in spots as rotated across the disk, showing cavities.] these spots are also very variable in number, sometimes there being none for nearly two hundred days, and again whole years during which the sun is never without them. the period from minimum to maximum [page ] of spots is about eleven years. we might look for them again and again in vain this year ( ). they will be most numerous in and . the cause of this periodicity was inferred to be the near approach of the enormous planet jupiter, causing disturbance by its attraction. but the periods do not correspond, and the cause is the result of some law of solar action to us as yet unknown. these spots may be seen with almost any telescope, the eye being protected by deeply colored glasses. until within one hundred years they were supposed to be islands of scoriæ floating in the sea of molten matter. but they were depressed below the surface, and showed a notch when on the edge. wilson originated and herschel developed the theory that the sun's real body was dark, cool, and habitable, and that the photosphere was a luminous stratum at a distance from the real body, with openings showing the dark spots below. such a sun would have cooled off in a week, but would previously have annihilated all life below. the solar spots being most abundant on the two sides of the equator, indicates their cyclonic character; the centre of a cyclone is rarefied, and therefore colder, and cold on the sun is darkness. m. faye says: "like our cyclones, they are descending, as i have proved by a special study of these terrestrial phenomena. they carry down into the depths of the solar mass the cooler materials of the upper layers, formed principally of hydrogen, and thus produce in their centre a decided extinction of light and heat as long as the gyratory movement continues. finally, the hydrogen set free at the base of the whirlpool becomes reheated at this [page ] great depth, and rises up tumultuously around the whirlpool, forming irregular jets, which appear above the chromosphere. these jets constitute the protuberances. the whirlpools of the sun, like those on the earth, are of all dimensions, from the scarcely visible pores to the enormous spots which we see from time to time. they have, like those of the earth, a marked tendency, first to increase and then to break up, and thus form a row of spots extending along the same parallel." [illustration: fig. .--solar spot, by langley.] a spot of , miles diameter is quite small; there was one , miles across, visible to the naked eye for a week in . this particular sun-spot somewhat [page ] helped the millerites. on the day of the eclipse, in , a spot over , miles in extent was clearly seen. in such vast tempests, if there were ships built as large as the whole earth, they would be tossed like autumn leaves in an ocean storm. the revolution of the sun carries a spot across its face in about fourteen days. after a lapse of as much more time, they often reappear on the other side, changed but recognizable. they often break ont or disappear under the eye of the observer. they divide like a piece of ice dropped on a frozen pond, the pieces sliding off in every direction, or combine like separate floes driven together into a pack. sometimes a spot will last for more than two hundred days, recognizable through six or eight revolutions. sometimes a spot will last only half an hour. the velocities indicated by these movements are incredible. an up-rush and down-rush at the sides has been measured of twenty miles a second; a side-rush or whirl, of one hundred and twenty miles a second. these tempests rage from a few days to half a year, traversing regions so wide that our indian ocean, the realm of storms, is too small to be used for comparison; then, as they cease, the advancing sides of the spots approach each other at the rate of , miles an hour; they strike together, and the rising spray of fire leaps thousands of miles into space. it falls again into the incandescent surge, rolls over mountains as the sea over pebbles, and all this for eon after eon without sign of exhaustion or diminution. all these swift succeeding himalayas of fire, where one hundred worlds could be buried, do not usually prevent the sun's appearing to our far-off eyes as a perfect sphere. [page ] _what the sun does for us._ to what end does this enormous power, this central source of power, exist? that it could keep all these gigantic forces within itself could not be expected. it is in a system where every atom is made to affect every other atom, and every world to influence every other. the author of all lives only to do good, to send rain on the just and unjust, to cause his sun to rise on the evil and the good, and to give his spirit, like a perpetually widening river, to every man to profit withal. the sun reaches his unrelaxing hand of gravitation to every other world at every instant. the tendency of every world is to fly off in a straight line. this tendency must be momentarily curbed, and the planet held in its true curve about the sun. these giant worlds must be perfectly handled. their speed, amounting to seventy times as fast as that of a rifle-ball, must be managed. each and every world may be said to be lifted momentarily and swung perpetually at arm's-length by the power of the sun. the sun warms us. it would convey but a small idea of the truth to state how many hundreds of millions of cubic miles of ice could be hailed at the sun every second without affecting its heat; but, if any one has any curiosity to know, it is , , cubic miles of ice per second. we journey through space which has a temperature of ° below zero; but we live, as it were, in a conservatory, in the midst of perpetual winter. we are roofed over by the air that treasures the heat, floored under by strata both absorptive and retentive of heat, [page ] and between the earth and air violets grow and grains ripen. the sun has a strange chemical power. it kisses the cold earth, and it blushes with flowers and matures the fruit and grain. we are feeble creatures, and the sun gives us force. by it the light winds move one-eighth of a mile an hour, the storm fifty miles, the hurricane one hundred. the force is as the square of the velocity. it is by means of the sun that the merchant's white-sailed ships are blown safely home. so the sun carries off the miasma of the marsh, the pollution of cities, and then sends the winds to wash and cleanse themselves in the sea-spray. the water-falls of the earth turn machinery, and make lowells and manchesters possible, because the sun lifted all that water to the hills. intermingled with these currents of air are the currents of electric power, all derived from the sun. these have shown their swiftness and willingness to serve man. the sun's constant force displayed on the earth is equal to , , , engines of -horse power each, working day and night; and yet the earth receives only / part of the whole force of the sun. besides all this, the sun, with provident care, has made and given to us coal. this omnipotent worker has stored away in past ages an inexhaustible reservoir of his power which man may easily mine and direct, thus releasing himself from absorbing toil. experiments. any one may see the spots on the sun who has a spy-glass. darken the room and put the glass through an opening toward the sun, as shown in fig. . the eye-piece should be drawn out about half an inch beyond [page ] its usual focusing for distant objects. the farther it is drawn, the nearer must we hold the screen for a perfect image. by holding a paper near the eye-piece, the proper direction of the instrument may be discovered without injury to the eyes. by this means the sun can be studied from day to day, and its spots or the transits of mercury and venus shown to any number of spectators. [illustration: fig. .--holding telescope to see the sun's spots.] first covering the eyes with very dark or smoked glasses, erect a disk of pasteboard four inches in diameter between you and the sun; close one eye; stand near it, and the whole sun is obscured. withdraw from it till the sun's rays just shoot over the edge of the disk on every side. measure the distance from the eye to the disk. you will be able to determine the distance of the sun by the rule of three: thus, as four inches is to , miles, so is distance from eye to disk to distance from disk to the sun. take such measurements at sunrise, noon, and sunset, and see the apparently differing sizes due to refraction. [page ] vi. the planets, as seen from space. "he hangeth the earth upon nothing."--_job_ xxvi. . [page ] "let a power be delegated to a finite spirit equal to the projection of the most ponderous planet in its orbit, and, from an exhaustless magazine, let this spirit select his grand central orb. let him with puissant arm locate it in space, and, obedient to his mandate, there let it remain forever fixed. he proceeds to select his planetary globes, which he is now required to marshal in their appropriate order of distance from the sun. heed well this distribution; for should a single globe be misplaced, the divine harmony is destroyed forever. let us admit that finite intelligence may at length determine the order of combination; the mighty host is arrayed in order. these worlds, like fiery coursers, stand waiting the command to fly. but, mighty spirit, heed well the grand step, ponder well the direction in which thou wilt launch each wailing world; weigh well the mighty impulse soon to be given, for out of the myriads of directions, and the myriads of impulsive forces, there comes but a single combination that will secure the perpetuity of your complex scheme. in vain does the bewildered finite spirit attempt to fathom this mighty depth. in vain does it seek to resolve the stupendous problem. it turns away, and while endued with omnipotent power, exclaims, 'give to me infinite wisdom, or relieve me from the impossible task!'"- . m. mitchel, ll. d. [page ] vi. _the planets, as seen from space_ if we were to go out into space a few millions of miles from either pole of the sun, and were endowed with wonderful keenness of vision, we should perceive certain facts, viz: that space is frightfully dark except when we look directly at some luminous body. there is no air to bend the light out of its course, no clouds or other objects to reflect it in a thousand directions. every star is a brilliant point, even in perpetual sunshine. the cold is frightful beyond the endurance of our bodies. there is no sound of voice in the absence of air, and conversation by means of vocal organs being impossible, it must be carried on by means of mind communication. we see below an unrevolving point on the sun that marks its pole. ranged round in order are the various planets, each with its axis pointing in very nearly the same direction. all planets, except possibly venus, and all moons except those of uranus and neptune, present their equators to the sun. the direction of orbital and axial revolution seen from above the north pole would be opposite to that of the hands of a watch. [illustration: fig. .--orbits and comparative sizes of the planets.] the speed of this orbital revolution must be proportioned to the distance from the sun. the attraction of the sun varies inversely as the square of the distance. [page ] it holds a planet with a certain power; one twice as far off, with one-fourth that power. this attraction must be counterbalanced by centrifugal force; great force from great speed when attraction is great, and small from less [page ] speed when attractive power is diminished by distance. hence mercury must go . miles per second--seventy times as fast as a rifle-ball that goes two-fifths of a mile in a second--or be drawn into the sun; while neptune, seventy-five times as far off, and hence attracted only / as much, must be slowed down to . miles a second to prevent its flying away from the feebler attraction of the sun. the orbital velocity of the various planets in miles per second is as follows: mercury . | jupiter . venus . | saturn . earth . | uranus . mars . | neptune . hence, while the earth makes one revolution in its year, mercury has made over four revolutions, or passed through four years; the slower neptune has made only / of one revolution. the time of axial revolution which determines the length of the day varies with different planets. the periods of the four planets nearest the sun vary only half an hour from that of the earth, while the enormous bodies of jupiter and saturn revolve in ten and ten and a quarter hours respectively. this high rate of speed, and its resultant, centrifugal force, has aided in preventing these bodies from becoming as dense as they would otherwise be--jupiter being only . as dense as the earth, and saturn only . . this extremely rapid revolution produces a great flattening at the poles. if jupiter should rotate four times more rapidly than it does, it could not be held together compactly. as it is, the polar diameter is five thousand miles less than the equatorial: the difference in diameters produced by the [page ] same cause on the earth, owing to the slower motion and smaller mass, being only twenty-six miles. the effect of this will be more specifically treated hereafter. the difference in the size of the planets is very noticeable. if we represent the sun by a gilded globe two feet in diameter, we must represent vulcan and mercury by mustard-seeds; venus, by a pea; earth, by another; mars, by one-half the size; asteroids, by the motes in a sunbeam; jupiter, by a small-sized orange; saturn, by a smaller one; uranus, by a cherry; and neptune, by one a little larger. apply the principle that attraction is in proportion to the mass, and a man who weighs one hundred and fifty pounds on the earth weighs three hundred and ninety-six on jupiter, and only fifty-eight on mars; while on the asteroids he could play with bowlders for marbles, hurl hills like milton's angels, leap into the fifth-story windows with ease, tumble over precipices without harm, and go around the little worlds in seven jumps. [illustration: fig. .--orbit of earth, showing parallelism of axis and seasons.] the seasons of a planet are caused by the inclination of its axis to the plane of its orbit. in fig. the rotating earth is seen at a, with its northern pole turning in constant sunlight, and its southern pole in constant darkness; everywhere south of the equator is more darkness than day, and hence winter. passing on to b, the world is seen illuminated equally on each side of the equator. every place has its twelve hours' darkness and light at each revolution. but at c--the axis of the earth always preserving the same direction--the northern pole is shrouded in continual gloom. every place [page ] north of the equator gets more darkness than light, and hence winter. the varying inclination of the axes of the different planets gives a wonderful variety to their seasons. the sun is always nearly over the equator of jupiter, and every place has nearly its five hours day and five hours night. the seasons of earth, mars, and saturn are so much alike, except in length, that no comment is necessary. the ice-fields at either pole of mars are observed to enlarge and contract, according as it is winter or summer there. saturn's seasons are each seven and a half years long. the alternate darkness and light at the poles is fifteen years long. but the seasons of venus present the greatest anomaly, if its assigned inclination of axis ( °) can be relied on as correct, which is doubtful. its tropic zone extends nearly to the pole, and at the same time the winter at the other pole reaches the equator. the short period of this planet causes it to present the south pole to the sun only one hundred and twelve days after it has been scorching the one at the north. this gives two winters, springs, summers, and autumns to the equator in two hundred and twenty-five days. if each whirling world should leave behind it a trail of light to mark its orbit, and our perceptions of form were sufficiently acute, we should see that these curves of light are not exact circles, but a little flattened into an ellipse, with the sun always in one of the foci. hence each planet is nearer to the sun at one part of its orbit than another; that point is called the perihelion, and the farthest point aphelion. this eccentricity of orbit, or distance of the sun from the centre, is very small. [page ] in the case of venus it is only . of the whole, and in no instance is it more than . , viz., that of mercury. this makes the sun appear twice as large, bright, and hot as seen and felt on mercury at its perihelion than at its aphelion. the earth is , , miles nearer to the sun in our winter than summer. hence the summer in the southern hemisphere is more intolerable than in the northern. but this eccentricity is steadily diminishing at a uniform rate, by reason of the perturbing influence of the other planets. in the case of some other planets it is steadily increasing, and, if it were to go on a sufficient time, might cause frightful extremes of temperature; but lalande has shown that there are limits at which it is said, "thus far shalt thou go, and no farther." then a compensative diminution will follow. conceive a large globe, to represent the sun, floating in a round pond. the axis will be inclined - / ° to the surface of the water, one side of the equator be - / ° below the surface, and the other side the same distance above. let the half-submerged earth sail around the sun in an appropriate orbit. the surface of the water will be the plane of the orbit, and the water that reaches out to the shore, where the stars would be set, will be the plane of the ecliptic. it is the plane of the earth's orbit extended to the stars. the orbits of all the planets do not lie in the same plane, but are differently inclined to the plane of the ecliptic, or the plane of the earth's orbit. going out from the sun's equator, so as to see all the orbits of the planets on the edge, we should see them inclined to that of the earth, as in fig. . [illustration: fig. .--inclination of the planes of orbits.] if the earth, and saturn, and pallas were lying in [page ] the same direction from the sun, and the outer bodies were to start in a direct line for the sun, they would not collide with the earth on their way; but saturn would pass , , and pallas , , miles over our heads. from this same cause we do not see venus and mercury make a transit across the disk of the sun at every revolution. [illustration: fig. .--inclination of orbits of venus and earth. nodal line, d b.] fig. shows a view of the orbits of the earth and venus seen not from the edge but from a position somewhat above. the point e, where venus crosses the plane of the earth's orbit, is called the ascending node. if the earth were at b when venus is at e, venus would be seen on the disk of the sun, making a transit. the same would be true if the earth were at d, and venus at the descending node f. this general view of the flying spheres is full of interest. [page ] while quivering themselves with thunderous noises, all is silent about them; earthquakes may be struggling on their surfaces, but there is no hint of contention in the quiet of space. they are too distant from one another to exchange signals, except, perhaps, the fleet of asteroids that sail the azure between mars and jupiter. some of these come near together, continuing to fill each other's sky for days with brightness, then one gradually draws ahead. they have all phases for each other--crescent, half, full, and gibbous. these hundreds of bodies fill the realm where they are with inexhaustible variety. beyond are vast spaces--cold, dark, void of matter, but full of power. occasionally a little spark of light looms up rapidly into a world so huge that a thousand of our earths could not occupy its vast bulk. it swings its four or eight moons with perfect skill and infinite strength; but they go by and leave the silence unbroken, the darkness unlighted for years. nevertheless, every part of space is full of power. nowhere in its wide orbit can a world find a place; at no time in its eons of flight can it find an instant when the sun does not hold it in safety and life. _the outlook from the earth._ if we come in from our wanderings in space and take an outlook from the earth, we shall observe certain movements, easily interpreted now that we know the system, but nearly inexplicable to men who naturally supposed that the earth was the largest, most stable, and central body in the universe. we see, first of all, sun, moon, and stars rise in the east, mount the heavens, and set in the west. as i [page ] revolve in my pivoted study-chair, and see all sides of the room--library, maps, photographs, telescope, and windows--i have no suspicion that it is the room that whirls; but looking out of a car-window in a depot at another car, one cannot tell which is moving, whether it be his car or the other. in regard to the world, we have come to feel its whirl. we have noticed the pyramids of egypt lifted to hide the sun; the mountains of hymettus hurled down, so as to disclose the moon that was behind them to the watchers on the acropolis; and the mighty mountains of moab removed to reveal the stars of the east. train the telescope on any star; it must be moved frequently, or the world will roll the instrument away from the object. suspend a cannon-ball by a fine wire at the equator; set it vibrating north and south, and it swings all day in precisely the same direction. but suspend it directly over the north pole, and set it swinging toward washington; in six hours after it is swinging toward rome, in italy; in twelve hours, toward siam, in asia; in nineteen hours, toward the sandwich islands; and in twenty-four, toward washington again, not because it has changed the plane of its vibration, but because the earth has whirled beneath it, and the torsion of the wire has not been sufficient to compel the plane of the original direction to change with the turning of the earth. the law of inertia keeps it moving in the same direction. the same experimental proof of revolution is shown in a proportional degree at any point between the pole and the equator. but the watchers on the acropolis do not get turned over so as to see the moon at the same time every night. [page ] we turn down our eastern horizon, but we do not find fair luna at the same moment we did the night before. we are obliged to roll on for some thirty to fifty minutes longer before we find the moon. it must be going in the same direction, and it takes us longer to get round to it than if if it were always in the same spot; so we notice a star near the moon one night--it is ° west of the moon the next night. the moon is going around the earth from west to east, and if it goes ° in one day, it will take a little more than twenty-seven days to go the entire circle of °. [illustration: fig. .--showing the sun's movement among the stars.] [page ] in our outlook we soon observe that we do not by our revolution come to see the same stars rise at the same hour every night. orion and the pleiades, our familiar friends in the winter heavens, are gone from the summer sky. have they fled, or are we turned from them? this is easily understood from fig. . when the observer on the earth at a looks into the midnight sky he sees the stars at e; but as the earth passes on to b, he sees those stars at e three minutes sooner every night; and at midnight the stars at f are over his head. thus in a year, by going around the sun, we have every star of the celestial dome in our midnight sky. we see also how the sun appears among the successive constellations. when we are at a, we see the sun among the stars at g; but as we move toward b, the sun appears to move toward h. if we had observed the sun rise on the th of august, , we should have seen it rise a little before regulus, and a little south of it, in such a relation as circle is to the star in fig. . by sunset the earth had moved enough to make the sun appear to be at circle , and by the next morning at circle , at which time regulus would rise before the sun. thus the earth's motion seems to make the sun traverse a regular circle among the stars once a year: but it is not the sun that moves. [illustration: fig. .] there are certain stars that have such irregular, uncertain, vagarious ways that they were called vagabonds, or planets, by the early astronomers. here is the path of jupiter in the year (fig. ). these bodies go forward for awhile, then stop, start aside, then retrograde, [page ] and go on again. some are never seen far from the sun, and others in all parts of the ecliptic. [illustration: fig. .] first see them as they stand to-day, as in fig. . the observer stands on the earth at a. it has rolled over so far that he cannot see the sun; it has set. but venus is still in sight; jupiter is ° behind venus, and saturn is seen ° farther east. when a has rolled a little farther, if he is awake, he will see mars before he sees the sun; or, in common language, venus will set after, and mars rise before the sun. all these bodies at near and far distances seem set in the starry dome, as the different stars seem in fig. , p. . [illustration: fig. . showing position of planets.] the mysterious movements of advance and retreat are rendered intelligible by fig. . the planet mercury is at a, and, seen from the earth, b is located at _a_, [page ] on the background of the stars it seems to be among. it remains apparently stationary at _a_ for some time, because approaching the earth in nearly a straight line. passing d to c, it appears to retrograde among the stars to _c_; remains apparently stationary for some time, then, in passing from c to e and a, appears to pass back among the stars to _a_. the progress of the earth, meanwhile, although it greatly retards the apparent motion from a to c, greatly hastens it from c to a. [illustration: fig. .--apparent movements of an inferior planet.] it is also apparent that mercury and venus, seen from the earth, can never appear far from the sun. they must be just behind the sun as evening stars, or just before it as heralds of the morning. venus is never more than ° from the sun, and mercury never more than °; indeed, it keeps so near the sun that very few people have ever seen the brilliant sparkler. observe how much larger the planet appears near the earth in conjunction at d than in opposition at e. observe also what phases it must present, and how transits sometimes take place. [page ] the movement of a superior planet, one whose orbit is exterior to the earth, is clear from fig. . when the earth is at a and mars at b, it will appear among the stars at c. when the earth is at d, mars having moved more slowly to e, will have retrograded to f. it remains there while the earth passes on, in a line nearly straight, from mars to g; then, as the earth begins to curve around the sun, mars will appear to retraverse the distance from f to c, and beyond. the farther the superior planet is from the earth the less will be the retrograde movement. [illustration: fig. .--illustrating movements of a superior planet.] the reader should draw the orbits in proportion, and, remembering the relative speed of each planet, note the movement of each in different parts of their orbits. to account for these most simple movements, the earlier astronomers invented the most complex and impossible machinery. they thought the earth the centre, and that the sun, moon, and stars were carried about it, as stoves around a person to warm him. they thought these strange movements of the planets were accomplished by mounting them on subsidiary eccentric wheels in the revolving crystal sphere. all that was [page ] needed to give them a right conception was a sinking of their world and themselves to an appropriate proportion, and an enlargement of their vision, to take in from an exalted stand-point a view of the simplicity of the perfect plan. experiments. fix a rod, or tube, or telescope pointing at a star in the cast or west, and the earth's revolution will be apparent in a moment, turning the tube away from the star. point it at stars about the north pole, and those on one side will be found going in an opposite direction from those on the other, and very much slower than those about the equator. anyone can try the pendulum experiment who has access to some lofty place from which to suspend the ball. it was tried in bunker hill monument a few years ago, and is to be tried in paris, in the summer of , with a seven-hundred-pound pendulum and a suspending wire seventy yards long. the advance and retrograde movements of planets can be illustrated by two persons walking around a centre and noticing the place where the person appears projected on the wall beyond. * * * * * procession of stars and souls. "i stood upon the open casement, and looked upon the night, and saw the westward-going stars pass slowly out of sight. "slowly the bright procession went down the gleaming arch, and my soul discerned the music of the long triumphal march; "till the great celestial army, stretching far beyond the poles, became the eternal symbol of the mighty march of souls. [page ] "onward, forever onward, red mars led on his clan; and the moon, like a mailèd maiden, was riding in the van. "and some were bright in beauty, and some were faint and small, but these might be, in their great heights, the noblest of them all. "downward, forever downward, behind earth's dusky shore, they passed into the unknown night-- they passed, and were no more. "no more! oh, say not so! and downward is not just; for the sight is weak and the sense is dim that looks through heated dust. "the stars and the mailèd moon, though they seem to fall and die, still sweep in their embattled lines an endless reach of sky. "and though the hills of death may hide the bright array, the marshalled brotherhood of souls still keeps its onward way. "upward, forever upward, i see their march sublime, and hear the glorious music of the conquerors of time. "and long let me remember that the palest fainting one may to diviner vision be a bright and blazing sun." thomas buchanan read. [page ] vii. shooting-stars, meteors, and comets. "the lord cast down great stones from heaven upon them unto azekah, and they died."--_joshua_ x. ii. [page ] [illustration: a swarm of meteors meeting the earth. their orbits are all parallel. those coming in direct line to the eye appear as stars, having no motion. those on one side of this line are seen in foreshortened perspective. those furthest from the centre, other things being equal, appear longest. the centre, called the radiant point, of these november meteors is situated in leo; that of the august meteors in perseus. over fifty such radiant points have been discovered. over , meteors have been visible in an hour.] [page ] vii. _shooting-stars, meteors, and comets._ before particularly considering the larger aggregations of matter called planets or worlds as individuals, it is best to investigate a part of the solar system consisting of smaller collections of matter scattered everywhere through space. they are of various densities, from a cloudlet of rarest gas to solid rock; of various sizes, from a grain's weight to little worlds; of various relations to each other, from independent individuality to related streams millions of miles long. when they become visible they are called shooting-stars, which are evanescent star-points darting through the upper air, leaving for an instant a brilliant train; meteors, sudden lights, having a discernible diameter, passing over a large extent of country, often exploding with violence (fig. ), and throwing down upon the earth aerolites; and comets, vast extents of ghostly light, that come we know not whence and go we know not whither. all these forms of matter are governed by the same laws as the worlds, and are an integral part of the solar system--a part of the unity of the universe. [illustration: fig. .--explosion of a bolide.] everyone has seen the so-called shooting-stars. they break out with a sudden brilliancy, shoot a few degrees with quiet speed, and are gone before we can say, "see there!" the cause of their appearance, the [page ] conversion of force into heat by their contact with our atmosphere, has been already explained. other facts remain to be studied. they are found to appear about seventy-three miles above the earth, and to disappear about twenty miles nearer the surface. their average velocity, thirty-five, sometimes rises to one hundred miles a second. they exhibit different colors, according to their different chemical substances, which are consumed. the number of them to be seen on different nights is exceedingly variable; sometimes not more [page ] than five or six an hour, and sometimes so many that a man cannot count those appearing in a small section of sky. this variability is found to be periodic. there are everywhere in space little meteoric masses of matter, from the weight of a grain to a ton, and from the density of gas to rock. the earth meets , , little bodies every day--there is collision--the little meteoroid gives out its lightning sign of extinction, and, consumed in fervent heat, drops to the earth as gas or dust. if we add the number light enough to be seen by a telescope, they cannot be less than , , a day. everywhere we go, in a space as large as that occupied by the earth and its atmosphere, there must be at least , bodies--one in , , cubic miles--large enough to make a light visible to the naked eye, and forty times that number capable of revealing themselves to telescopic vision. professor peirce is about to publish, as the startling result of his investigations, "that the heat which the earth receives directly from meteors is the same in amount which it receives from the sun by radiation, and that the sun receives five-sixths of its heat from the meteors that fall upon it." [illustration: fig. .--bolides.] [page ] in dr. schmidt was fortunate enough to have a telescopic view of a system of bodies which had turned into meteors. these were two larger bodies followed by several smaller ones, going in parallel lines till they were extinguished. they probably had been revolving about each other as worlds and satellites before entering our atmosphere. it is more than probable that the earth has many such bodies, too small to be visible, revolving around it as moons. [illustration: fig. .--santa rosa aerolite.] _aerolites._ sometimes the bodies are large enough to bear the heat, and the unconsumed centre comes to the earth. [page ] their velocity has been lessened by the resisting air, and the excessive heat diminished. still, if found soon after their descent, they are too hot to be handled. these are called aerolites or air-stones. there was a fall in iowa, in february, , from which fragments amounting to five hundred pounds weight were secured. on the evening of december st, , a meteor of unusual size and brilliancy passed over the states of kansas, missouri, illinois, indiana, and ohio. it was first seen in the western part of kansas, at an altitude of about sixty miles. in crossing the state of missouri it began to explode, and this breaking up continued while passing illinois, indiana, and ohio, till it consisted of a large flock of brilliant balls chasing each other across the sky, the number being variously estimated at from twenty to one hundred. it was accompanied by terrific explosions, and was seen along a path of not less than a thousand miles. when first seen in kansas, it is said to have appeared as large as the full moon, and with a train from twenty-five to one hundred feet long. another, very similar in appearance and behavior, passed over a part of the same course in february, . at laigle, france, on april th, , about one o'clock in the day, from two to three thousand fell. the largest did not exceed seventeen pounds weight. one fell in weston, connecticut, in , weighing two hundred pounds. a very destructive shower is mentioned in the book of joshua, chap. x. ver. . these bodies are not evenly distributed through space. in some places they are gathered into systems which circle round the sun in orbits as certain as those of the [page ] planets. the chain of asteroids is an illustration of meteoric bodies on a large scale. they are hundreds in number--meteors are millions. they have their region of travel, and the sun holds them and the giant jupiter by the same power. the power that cares for a world cares for a sparrow. if their orbit so lies that a planet passes through it, and the planet and the meteors are at the point of intersection at the same time, there must be collisions, and the lightning signs of extinction proportioned to the number of little bodies in a given space. it is demonstrated that the earth encounters more than one hundred such systems of meteoric bodies in a single year. it passes through one on the th of august, another on the th of november. in a certain part of the first there is an agglomeration of bodies sufficient to become visible as it approaches the sun, and this is known as the comet of ; in the second is a similar agglomeration, known as temple's comet. it is repeating the same thing to say that meteoroids follow in the train of the comets. the probable orbit of the november meteors and the comet of is an exceedingly elongated ellipse, embracing the orbit of the earth at one end and a portion of the orbit of uranus at the other (fig. ). that of the august meteors and the comet of embraces the orbit of the earth at one end, and thirty per cent. of the other end is beyond the orbit of neptune. [illustration: fig. .--orbit of the november meteors and the comet or .] in january, , biela's comet was observed to be divided. at its next return, in , the parts were , , miles apart. they could not be found on their periodic returns in , , and ; but it [page ] should have crossed the earth's orbit early in september, . the earth itself would arrive at the point of crossing two or three months later. if the law of revolution held, we might still expect to find some of the trailing meteoroids of the comet not gone by on our arrival. it was shown that the point of the earth that would strike them would be toward a certain place in the constellation of andromeda, if the remains of the diluted comet were still there. the prediction was verified in every respect. at the appointed time, place, [page ] and direction, the streaming lights were in our sky. that these little bodies belonged to the original comet none can doubt. by the perturbations of planetary attraction, or by different original velocities, a comet may be lengthened into an invisible stream, or an invisible stream agglomerated till it is visible as a comet. _comets._ comets will be most easily understood by the foregoing considerations. they are often treated as if they were no part of the solar system; but they are under the control of the same laws, and owe their existence, motion, and continuance to the same causes as jupiter and the rest of the planets. they are really planets of wider wandering, greater ellipticity, and less density. they have periodic times less than the earth, and fifty times as great as neptune. they are little clouds of gas or meteoric matter, or both, darting into the solar system from every side, at every angle with the plane of the ecliptic, becoming luminous with reflected light, passing the sun, and returning again to outer darkness. sometimes they have no tail, having a nucleus surrounded by nebulosity like a dim sun with zodiacal light; sometimes one tail, sometimes half a dozen. these follow the comet to perihelion, and precede it afterward (fig. ). the orbits of some comets are enormously elongated; one end may lie inside the earth's orbit, and the other end be as far beyond neptune as that is from the sun. of course only a small part of such a curve can be studied by us: the comet is visible only when near the sun. the same curve around the sun may be an orbit that will bring it back again, [page ] or one that will carry it off into infinite space, never to return. one rate of speed on the curve indicates an elliptical orbit that returns; a greater rate of speed indicates that it will take a parabolic orbit, which never returns. the exact rate of speed is exceedingly difficult to determine; hence it cannot be confidently asserted that any comet ever visible will not return. they may all belong to the solar system; but some will certainly be gone thousands of years before their fiery forms will greet the watchful eyes of dwellers on the earth. a comet that has an elliptic orbit may have it changed to [page ] parabolic by the accelerations of its speed, by attracting planets; or a parabolic comet may become elliptic, and so permanently attracted to the system by the retardations of attracting bodies. a comet of long period may be changed to one of short period by such attraction, or _vice versa_. [illustration: fig. .--aspects of remarkable comets.] the number of comets, like that of meteor streams, is exceedingly large. five hundred have been visible to the naked eye since the christian era. two hundred have been seen by telescopes invented since their invention. some authorities estimate the number belonging to our solar system by millions; professor peirce says more than five thousand millions. _famous comets._ the comet of is perhaps the one that appeared in a.d. , soon after the death of julius cæsar, also in the reign of justinian, a.d. , and in . this is not determined by any recognizable resemblance. it had a tail ° long; it was not all arisen when its head reached the meridian. it is possible, from the shape of its orbit, that it has a periodic time of nine thousand years, or that it may have a parabolic orbit, and never return. observations taken two hundred years ago have not the exactness necessary to determine so delicate a point. on august th, , halley discovered a comet which he soon declared to be one seen by kepler in . looking back still farther, he found that a comet was seen in having the same orbit. still farther, by the same exact period of seventy-five years, he found that it was the same comet that had disturbed [page ] the equanimity of pope calixtus in . calculations were undertaken as to the result of all the accelerations and retardations by the attractions of all the planets for the next seventy-five years. there was not time to finish all the work; but a retardation of six hundred and eighteen days was determined, with a possible error of thirty days. the comet actually came to time within thirty-three days, on march th, . again its return was calculated with more laborious care. it came to time and passed the sun within three days of the predicted time, on the th of november, . it passed from sight of the most powerful telescopes the following may, and has never since been seen by human eye. but the eye of science sees it as having passed its aphelion beyond the orbit of neptune in , and is already hastening back to the warmth and light of the sun. it will be looked for in ; and there is good hope of predicting, long before it is seen, the time of its perihelion within a day. _biela's lost comet._--this was a comet with a periodic time of six years and eight months. it was observed in january, , to have separated into two parts of unequal brightness. the lesser part grew for a month until it equalled the other, then became smaller and disappeared, while the other was visible a month longer. at disappearance the parts were , miles asunder. on its next return, in , the parts were , , miles apart; sometimes one was brighter and sometimes the other; which was the fragment and which was the main body could not be recognized. they vanished in september, , and have never been seen since. three revolutions have been made since that time, but no [page ] trace of it could be discovered. probably the same influence that separated it into parts, separated the particles till too thin and tenuous to be seen. there is ground for believing that the earth passed through a part of it, as before stated under the head of meteors. _the great comet of_ passed nearer the sun than any known body. it almost grazed the sun. if it ever returns, it will be in a.d. . _donati's comet of_ .--this was one of the most magnificent of modern times. during the first three months it showed no tail, but from august to october it had developed one forty degrees in length. its period is about two thousand years. every reader remembers the comet of the summer of . _encke's comet._--this comet has become famous for its supposed confirmation of the theory that space was filled with a substance infinitely tenuous, which resisted the passage of this gaseous body in an appreciable degree, and in long ages would so retard the motion of all the planets that gravitation would draw them all one by one into the sun. we must not be misled by the term retardation to suppose it means behind time, for a retarded body is before time. if its velocity is diminished, the attraction of the sun causes it to take a smaller orbit, and smaller orbits mean increased speed--hence the supposed retardation would shorten its periodic time. this comet was thought to be retarded two and a half hours at each revolution. if it was, it would not prove the existence of the resisting medium. other causes, unknown to us, might account for it. subsequent and more exact calculations fail to find any retardations in at least two revolutions between and [page ] . indications point to a retardation of one and a half hours both before and since. but such discrepancy of result proves nothing concerning a resisting medium, but rather is an argument against its existence. besides, faye's comet, in four revolutions of seven years each, shows no sign of retardation. the truth may be this, that a kind of atmosphere exists around the sun, perhaps revealed by the zodiacal light, that reaches beyond where encke's comet dips inside the orbit of mercury, and thus retards this body, but does not reach beyond the orbit of mars, where faye's comet wheels and withdraws. _of what do comets consist?_ the unsolved problems pertaining to comets are very numerous and exceedingly delicate. whence come they? why did they not contract to centres of nebulæ? are there regions where attractions are balanced, and matter is left to contract on itself, till the movements of suns and planets adds or diminishes attractive force on one side, and so allows them to be drawn slowly toward one planet, and its sun, or another? there is ground for thinking that the comet of and its train of meteors, visible to us in november, was thus drawn into our system by the planet uranus. indeed, leverrier has conjecturally fixed upon the date of a.d. as the time when it occurred; but another and closer observation of its next return, in , will be needed to give confirmation to the opinion. our sun's authority extends at least half-way to the nearest fixed star, one hundred thousand times farther than the orbit of the earth. meteoric and cometary matter lying [page ] there, in a spherical shell about the solar system, balanced between the attraction of different suns, finally feels the power that determines its destiny toward our sun. it would take , , years to come thence to our system. the conditions of matter with which we are acquainted do not cover all the ground presented by these mysterious visitors. we know a gas sixteen times as light as air, but hydrogen is vastly too heavy and dense; for we see the faintest star through thousands of miles of cometary matter; we know that water may become cloudy vapor, but a little of it obscures the vision. into what more ethereal, and we might almost say spiritual, forms matter may be changed we cannot tell. but if we conceive comets to be only gas, it would expand indefinitely in the realms of space, where there is no force of compression but its own. we might say that comets are composed of small separate masses of matter, hundreds of miles apart; and, looking through thousands of miles of them, we see light enough reflected from them all to seem continuous. doubtless that is sometimes the case. but the spectroscope shows another state of things: it reveals in some of these comets an incandescent gas--usually some of the combinations of carbon. the conclusion, then, naturally is that there are both gas and small masses of matter, each with an orbit of its own nearly parallel to those of all the others, and that they afford some attraction to hold the mass of intermingled and confluent gas together. our best judgment, then, is that the nucleus is composed of separate bodies, or matter in a liquid condition, capable of being vaporized by the heat of the sun, and driven off, [page ] as steam from a locomotive, into a tail. indications of this are found in the fact that tails grow smaller at successive returns, as the matter capable of such vaporization becomes condensed. in some instances, as in that of the comet of , the head was diminished by the manufacture of a tail. on the other hand, professor peirce showed that the nucleus of the comets of , , and must have had a tenacity equal to steel, to prevent being pulled apart by the tidal forces caused by its terrible perihelion sweep around the sun. it is likely that there are great varieties of condition in different comets, and in the same comet at times. we see them but a few days out of the possible millions of their periodic time; we see them only close to the sun, under the spur of its tremendous attraction and terrible heat. this gives us ample knowledge of the path of their orbit and time of their revolution, but little ground for judgment of their condition, when they slowly round the uttermost cape of their far-voyaging, in the terrible cold and darkness, to commence their homeward flight. the unsolved problems are not all in the distant sun and more distant stars, but one of them is carried by us, sometimes near, sometimes far off; but our acquaintance with the possible forms and conditions of matter is too limited to enable us to master the difficulties. _will comets strike the earth?_ very likely, since one or two have done so within a recent period. what will be the effect? that depends on circumstances. there is good reason to suppose we passed through the tail of a comet in , and the only [page ] observable effect was a peculiar phosphorescent mist. if the comet were composed of small meteoric masses a brilliant shower would be the result. but if we fairly encountered a nucleus of any considerable mass and solidity, the result would be far more serious. the mass of donati's comet has been estimated by m. faye to be / of that of the earth. if this amount of matter were dense as water, it would make a globe five hundred miles in diameter; and if as dense as professor peirce proved the nucleus of this comet to be, its impact with the earth would develop heat enough to melt and vaporize the hardest rocks. happily there is little fear of this: as professor newcomb says, "so small is the earth in comparison with celestial space, that if one were to shut his eyes and fire at random in the air, the chance of bringing down a bird would be better than that of a comet of any kind striking the earth." besides, we are not living under a government of chance, but under that of an almighty father, who upholdeth all things by the word of his power; and no world can come to ruin till he sees that it is best. [page ] viii. the planets as individuals. "through faith we understand that the worlds [plural] were framed by the word of god, so that things which were seen were not made of things which do appear."--_heb._ xi. . [page ] "o rich and various man! thou palace of sight and sound, carrying in thy senses the morning, and the night, and the unfathomable galaxy; in thy brain the geometry of the city of god; in thy heart the power of love, and the realms of right and wrong. an individual man is a fruit which it costs all the foregoing ages to form and ripen. he is strong, not to do but to live; not in his arms, but in his heart; not as an agent, but as a fact."--emerson. [page ] vii. _the planets as individuals._ how many bodies there may be revolving about the sun we have no means to determine or arithmetic to express. when the new star of the american republic appeared, there were but six planets discovered. since then three regions of the solar system have been explored with wonderful success. the outlying realms beyond saturn yielded the planet uranus in , and neptune in . the middle region between jupiter and mars yielded the little planetoid ceres in , pallas in , and one hundred and ninety others since. the inner region between mercury and the sun is of necessity full of small meteoric bodies; the question is, are there any bodies large enough to be seen? the same great genius of leverrier that gave us neptune from the observed perturbations of uranus, pointed out perturbations in mercury that necessitated either a planet or a group of planetoids between mercury and the sun. theoretical astronomers, aided by the fact that no planet had certainly been seen, and that all asserted discoveries of one had been by inexperienced observers, inclined to the belief in a group, or that the disturbance was caused by the matter reflecting the zodiacal light. when the total eclipse of the sun occurred in , [page ] astronomers were determined that the question of the existence of an intra-mercurial planet should be settled. maps of all the stars in the region of the sun were carefully studied, sections of the sky about the sun were assigned to different observers, who should attend to nothing but to look for a possible planet. it is now conceded that professor watson, of ann arbor, actually saw the sought-for body. vulcan. the god of fire; its sign [symbol], his hammer. distance from the sun, , , miles. orbital revolution, about days. mercury. the swift messenger of the gods; sign [symbol], his caduceus. distance from the sun, , , miles. diameter, miles. orbital revolution, . days. orbital velocity, miles per minute. axial revolution, h. m. mercury shines with a white light nearly as bright as sirius; is always near the horizon. when nearly between us and the sun, as at d (fig. , p. ), its illuminated side nearly opposite to us, we, looking from e, see only a thin crescent of its light. when it is at its greatest angular distance from the sun, as a or c, we see it illuminated like the half-moon. when it is beyond the sun, as at e, we see its whole illuminated face like the full-moon. the variation of its apparent size from the varying distance is very striking. at its extreme distance from the earth it subtends an angle of only five seconds; nearest to us, an angle of twelve seconds. its distance from the earth varies nearly as one to three, and its apparent size in the inverse ratio. [page ] when mercury comes between the earth and the sun, near the line where the planes of their orbits cut each other by reason of their inclination, the dark body of mercury will be seen on the bright surface of the sun. this is called a transit. if it goes across the centre of the sun it may consume eight hours. it goes , miles an hour, and has , miles of disk to cross. the transit of occupied seven and a half hours. the transits for the remainder of the century will occur: november th | november th may th | november th venus. goddess of beauty; its sign [symbol], a mirror. distance from the sun, , , miles. diameter, miles. orbital velocity, miles per minute. axial revolution, h. m. orbital revolution, . days. this brilliant planet is often visible in the daytime. i was once delighted by seeing venus looking down, a little after mid-day through the open space in the dome of the pantheon at rome. it has never since seemed to me as if the home of all the gods was deserted. phoebus, diana, venus and the rest, thronged through that open upper door at noon of night or day. arago relates that bonaparte, upon repairing to luxemburg when the directory was about to give him a _fête_, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. upon inquiry, he learned that these curious persons were observing with astonishment a star which they supposed to be that of the conqueror of italy. the emperor himself was not indifferent when [page ] his piercing eye caught the clear lustre of venus smiling upon him at mid-day. this unusual brightness occurs when venus is about five weeks before or after her inferior conjunction, and also nearest overhead by being north of the sun. this last circumstance occurs once in eight years, and came on february th, . venus may be as near the earth as , , miles, and as far away as , , . this variation of its distances from the earth is obviously much greater than that of mercury, and its consequent apparent size much more changeable. its greatest and least apparent sizes are as ten and sixty-five (fig. ). [illustration: fig. .--phases of venus, and varions apparent dimensions.] when copernicus announced the true theory of the solar system, he said that if the inferior planets could be clearly seen they would show phases like the moon. when galileo turned the little telescope he had made on venus, he confirmed the prophecy of copernicus. desiring to take time for more extended observation, and still be able to assert the priority of his discovery, he published the following anagram, in which his discovery was contained: [page ] "hæc immatura a me jam frustra leguntur o. y." (these unripe things are now vainly gathered by me.) he first saw venus as gibbous; a few months revealed it as crescent, and then he transposed his anagram into: "cynthiæ figuras æmulatur mater amorum." (the mother of loves imitates the phases of cynthia.) many things that were once supposed to be known concerning venus are not confirmed by later and better observations. venus is surrounded by an atmosphere so dense with clouds that it is conceded that her time of rotation and the inclination of her axis cannot be determined. she revealed one of the grandest secrets of the universe to the first seeker; showed her highest beauty to her first ardent lover, and has veiled herself from the prying eyes of later comers. florence has built a kind of shrine for the telescope of galileo. by it he discovered the phases of venus, the spots on the sun, the mountains of the moon, the satellites of jupiter, and some irregularities of shape in saturn, caused by its rings. galileo subsequently became blind, but he had used his eyes to the best purpose of any man in his generation. the earth. its sign [symbol]. distance from the sun, , , miles. diameter, polar, miles; equatorial, - / miles. axial revolution, h. m. . s.; orbital, . . orbital velocity per minute, . miles. let us lift ourselves up a thousand miles from the earth. we see it as a ball hung upon nothing in empty space. as the drop of falling water gathers itself [page ] into a sphere by its own inherent attraction, so the earth gathers itself into a ball. noticing closely, we see forms of continents outlined in bright relief, and oceanic forms in darker surfaces. we see that its axis of revolution is nearly perpendicular to the line of light from the sun. one-half is always dark. the sunrise greets a new thousand miles every hour; the glories of [page ] the sunset follow over an equal space, ° behind. we are glad that the darkness never overtakes the morning. [illustration: fig. .--earth and moon in space.] _the aurora borealis._ while east and west are gorgeous with sunrise and sunset, the north is often more glorious with its aurora borealis. we remember that all worlds have weird and inexplicable appendages. they are not limited to their solid surfaces or their circumambient air. the sun has its fiery flames, corona, zodiacal light, and perhaps a finer kind of atmosphere than we know. the earth is [page ] not without its inexplicable surroundings. it has not only its gorgeous eastern sunrise, its glorious western sunset, high above its surface in the clouds, but it also has its more glorious northern dawn far above its clouds and air. the realm of this royal splendor is as yet an unconquered world waiting for its alexander. there are certain observable facts, viz., it prevails mostly near the arctic circle rather than the pole; it takes on various forms--cloud-like, arched, straight; it streams like banners, waves like curtains in the wind, is inconstant; is either the cause or result of electric disturbance; it is often from four hundred to six hundred miles above the earth, while our air cannot be over one hundred miles. it almost seems like a revelation to human eyes of those vast, changeable, panoramic pictures by which the inhabitants of heaven are taught. [illustration: fig. .--the aurora as waving curtains.] investigation has discovered far more mysteries than it has explained. it is possible that the same cause that produces sun-spots produces aurora in all space, visible in all worlds. if so, we shall see more abundant auroras at the next maximum of sun-spot, between - . _the delicate balance of forces._ a soap-bubble in the wind could hardly be more flexible in form and sensitive to influence than is the earth. on the morning of may th, , the earth's crust at peru gave a few great throbs upward, by the action of expansive gases within. the sea fled, and returned in great waves as the land rose and fell. then these waves fled away over the great mobile surface, and in less than five hours they had covered a space equal to half of europe. the waves ran out to the sandwich islands, six [page ] thousand miles, at the rate of five hundred miles an hour, and arrived there thirty feet high. they not only sped on in straight radial lines, but, having run up the coast to california, were deflected away into the former series of waves, making the most complex undulations. similar beats of the great heart of the earth have sent its pulses as widely and rapidly on previous occasions. the figure of the earth, even on the ocean, is irregular, in consequence of the greater preponderance of land--and hence greater density--in the northern hemisphere. these irregularities are often very perplexing in making exact geodetic measurements. the tendency of matter to fly from the centre by reason of revolution causes the equatorial diameter to be twenty-six, miles longer than the polar one. by this force the mississippi river is enabled to run up a hill nearly three miles high at a very rapid rate. its mouth is that distance farther from the centre of the earth than its source, when but for this rotation both points would be equally distant. if the water became more dense, or if the world were to revolve faster, the oceans would rush to the equator, burying the tallest mountains and leaving polar regions bare. if the water should become lighter in an infinitesimal degree, or the world rotate more slowly, the poles would be submerged and the equator become an arid waste. no balance, turning to / of a grain, is more delicate than the poise of forces on the world. laplace has given us proof that the period of the earth's axial rotation has not changed / of a second of time in two thousand years. [page ] _tides._ but there is an outside influence that is constantly acting upon the earth, and to which it constantly responds. two hundred and forty thousand miles from the earth is the moon, having / the mass of the world. its attractive influence on the earth causes the movable and nearer portions to hurry away from the more stable and distant, and heap themselves up on that part of the earth nearest the moon. gravitation is inversely as the square of the distance; hence the water on the surface of the earth is attracted more than the body of the earth, some parts of which are eight thousand miles farther off; hence the water rises on the side next the moon. but the earth, as a whole, is nearer the moon than the water on the opposite side, and being drawn more strongly, is taken away from the water, leaving it heaped up also on the side opposite to the moon. a subsidiary cause of tides is found in the revolution of the earth and moon about their common centre of gravity. revolution about an axis through the centre of a sphere enlarges the equator by centrifugal force. revolution about an axis touching the surface of a flexible globe converts it into an egg-shaped body, with the longer axis perpendicular to the axis of revolution. in fig. the point of revolution is seen at the centre of gravity at g; hence, in the revolution of earth and moon as one, a strong centrifugal force is caused at d, and a less one at c. this gives greater height to the tides than the attraction of the moon alone could produce. [page ] [illustration: fig. .] if the earth had no axial revolution, the attractive point where the tide rises would be carried around the earth once in twenty-seven days by the moon's revolution about the earth. but since the earth revolves on its axis, it presents a new section to the moon's attraction every hour. if the moon were stationary, that would bring two high tides in exactly twenty-four hours; but as the moon goes forward, we need nearly twenty-five hours for two tides. the attractive influence of the sun also gives us a tide four-tenths as great as that of the moon. when these two influences of the sun and moon combine, as they do, in conjunction--when both bodies are on one side of the earth; or in opposition, sun and moon being on opposite sides of the earth--we have spring or increased tides. when the moon is in its first or third quarter, _i. e._, when a line from the moon to the earth makes a right angle with one from the sun to the earth, these influences antagonize one another, and we have the neap or low tides. it is easy to see that if, when the moon was drawing its usual tide, the sun drew four-tenths of the water in a tide at right angles with it, the moon's tide must be by so much lower. because of the inertia of the water [page ] it does not yield instantly to the moon's influence, and the crest of the tide is some hours behind the advancing moon. the amount of tide in various places is affected by almost innumerable influences, as distance of moon at its apogee or perigee; its position north, south, or at the equator; distance of earth from sun at perihelion and aphelion; the position of islands; the trend of continents, etc. all eastern shores have far greater tides than western. as the earth rolls to the east it leaves the tide-crest under the moon to impinge on eastern shores, hence the tides of from seventy-five to one hundred feet in the bay of fundy. lakes and most seas are too small to have perceptible tides. the spring-tides in the mediterranean sea are only about three inches. this constant ebb and flow of the great sea is a grand provision for its purification. even the wind is sent to the sea to be cleansed. the sea washes every shore, purifies every cove, bay, and river twice every twenty-four hours. all putrescible matter liable to breed a pestilence is carried far from shore and sunk under fathoms of the never-stagnant sea. the distant moon lends its mighty power to carry the burdens of commerce. she takes all the loads that can be floated on her flowing tides, and cheerfully carries them in opposite directions in successive journeys. it must be conceded that the profoundest study has not mastered the whole philosophy of tides. there are certain facts which are apparent, but for an explanation of their true theory such men as laplace, newton, and airy have labored in vain. there are plenty of other worlds still to conquer. [page ] [illustration: fig. .--lunar day.] [page ] the moon. new moon, [symbol]; first quarter, [symbol]; full moon, [symbol]; last quarter, [symbol]. extreme distance from the earth, , miles; least, , miles; mean, , miles. diameter, . miles [ , lockyer]. revolution about the earth, - / days. axial revolution, same time. when the astronomer herschel was observing the southern sky from the cape of good hope, the most clever hoax was perpetrated that ever was palmed upon a credulous public. some new and wonderful instruments were carefully described as having been used by that astronomer, whereby he was enabled to bring the moon so close that he could see thereon trees, houses, animals, and men-like human beings. he could even discern their movements, and gestures that indicated a peaceful race. the extent of the hoax will be perceived when it is stated that no telescope that we are now able to make reveals the moon more clearly than it would appear to the naked eye if it was one hundred or one hundred and fifty miles away. the distance at which a man can be seen by the unaided eye varies according to circumstances of position, background, light, and eye, but it is much inside of five miles. since, however, the moon is our nearest neighbor, a member of our own family in fact, it is a most interesting object of study. a glance at its familiar face reveals its unequal illumination. all ages and races have seen a man in the moon. all lovers have sworn by its constancy, and only part of them have kept their oaths. every twenty-nine or thirty days we see a silver crescent in the west, and are glad if it comes over the right shoulder--so [page ] much tribute does habit pay to superstition. the next night it is thirteen degrees farther east from the sun. we note the stars it occults, or passes by, and leaves behind as it broadens its disk, till it rises full-orbed in the east when the sun sinks in the west. it is easy to see that the moon goes around the earth from west to east. afterward it rises later and smaller each night, till at length, lost from sight, it rises about the same time as the sun, and soon becomes the welcome crescent new moon again. the same peculiarities are always evident in the visible face of the moon; hence we know that it always presents the same side to the earth. obviously it must make just one axial to one orbital revolution. hold any body before you at arm's-length, revolve it one-quarter around you until exactly overhead. if it has not revolved on an axis between the hands, another quarter of the surface is visible; but if in going up it is turned a quarter over, by the hands holding it steady, the same side is visible. three causes enable us to see a little more than half the moon's surface: . the speed with which it traverses the ellipse of its orbit is variable. it sometimes gets ahead of us, sometimes behind, and we see farther around the front or back part. . the axis is a little inclined to the plane of its orbit, and its orbit a little inclined to ours; hence we see a little over its north pole, and then again over the south pole. . the earth being larger, its inhabitants see a little more than half-way around a smaller body. these causes combined enable us to see / of the moon's surface. our eyes will never see the other side of the moon. if, now, being solid, her axial revolution could [page ] be increased enough to make one more revolution in two or three years, that difference between her axial and orbital revolution would give the future inhabitants of the earth a view of the entire circumference of the moon. yet if the moon were once in a fluid state, or had oceans on the surface, the enormous tide caused by the earth would produce friction enough, as they moved over the surface, to gradually retard the axial revolution till the two tidal elevations remained fixed toward and opposite the earth, and then the axial and orbital revolutions would correspond, as at present. in fact, we can prove that the form of the moon is protuberant toward the earth. its centre of gravity is thirty-three miles beyond its centre of magnitude, which is the same in effect as if a mountain of that enormous height rose on the earth side. hence any fluid, as water or air, would flow round to the other side. the moon's day, caused by the sun's light, is - / times as long as ours. the sun shines unintermittingly for fifteen days, raising a temperature as fervid as boiling water. then darkness and frightful cold for the same time succeed, except on that half where the earth acts as a moon. the earth presents the same phases--crescent, full, and gibbous--to the moon as the moon does to us, and for the same causes. lord rosse has been enabled, by his six-foot reflector, to measure the difference of heat on the moon under the full blaze of its noonday and midnight. he finds it to be no less than five hundred degrees. people not enjoying extremes of temperature should shun a lunar residence. the moon gives us only / as much light as the sun. a sky full of moons would scarcely make daylight. [page ] [illustration: fig. .--view of the moon near the third quarter. from a photograph by professor henry draper.] there are no indications of air or water on the moon. when it occults a star it instantly shuts off the light and as instantly reveals it again. an atmosphere would gradually diminish and reveal the light, and by refraction [page ] cause the star to be hidden in much less time than the solid body of the moon would need to pass over it. if the moon ever had air and water, as it probably did, they are now absorbed in the porous lava of its substance. _telescopic appearance._ [illustration: fig. .--illumination of craters and peaks.] probably no one ever saw the moon by means of a good telescope without a feeling of admiration and awe. except at full-moon, we can see where the daylight struggles with the dark along the line of the moon's sunrise or sunset. this line is called the terminator. it is broken in the extreme, because the surface is as rough as possible. in consequence of the small gravitation of the moon, utter absence of the expansive power of ice shivering the cliffs, or the levelling power of rains, precipices can stand in perpendicularity, mountains shoot up like needles, and cavities three miles deep remain unfilled. the light of the sun falling on the rough body of the moon, shown in section (fig. ), illuminates the whole cavity at _a_, part of the one at _b_, casts a long shadow from the mountain at _c_, and touches the tip of the one at _d_, which appears to a distant observer as a point of light beyond the terminator, as the moon revolves the conical cavity, _a_ is illuminated on the forward side only; the light creeps down the backward side of cavity _b_ to the bottom; mountain _c_. comes directly under the sun and casts no shadow, and mountain _d_ casts its long shadow over the plain. knowing the time of revolution, and observing the change of [page ] illumination, we can easily measure the height of mountain and depth of crater. an apple, with excavations and added prominences, revolved on its axis toward the light of a candle, admirably illustrates the crescent light that fills either side of the cavities and the shadows of the mountains on the plain. notice in fig. the crescent forms to the right, showing cavities in abundance. [illustration: fig. .--lunar crater "copernicus," after secchi.] the selenography of one side of the moon is much better known to us than the geography of the earth. our maps of the moon are far more perfect than those of the earth; and the photographs of lunar objects by messrs. draper and de la rue are wonderfully perfect, [page ] and the drawings of padre secchi equally so (fig. ). the least change recognizable from the earth must be speedily detected. there are frequently reports of discoveries of volcanoes on the moon, but they prove to be illusions. the moon will probably look the same to observers a thousand years hence as it does to-day. this little orb, that is only / of the mass of the earth, has twenty-eight mountains that are higher than mont blanc, that "monarch of mountains," in europe. _eclipses._ [illustration: fig. .--eclipses; shadows of earth and moon.] it is evident that if the plane of the moon's orbit were to correspond with that of the earth, as they all lie in the plane of the page (fig. ), the moon must pass between the centres of the earth and sun, and exactly behind the earth at every revolution. such successive and total darkenings would greatly derange all affairs dependent on light. it is easily avoided. venus does [page ] not cross the disk of the sun at every revolution, because of the inclination of the plane of its orbit to that of the earth (see fig. , p. ). so the plane of the orbit of the moon is inclined to the orbit of the earth ° ' "; hence the full-moon is often above or below the earth's shadow, and the earth is below or above the moon's shadow at new moon. it is as if the moon's orbit were pulled up one-quarter of an inch from the page behind the earth, and depressed as much below it between the earth and the sun. the point where the orbit of the moon penetrates the plane of the ecliptic is called a node. if a new moon occur when the line of intersection of the planes of orbits points to the sun, the sun must be eclipsed; if the full-moon occur, the moon must be eclipsed. in any other position the sun or moon will only be partially hidden, or no eclipse will occur. if the new moon be near the earth it will completely obscure the sun. a dime covers it if held close to the eye. it may be so far from the earth as to only partially hide the sun; and, if it cover the centre, leave a ring of sunlight on every side. this is called an annular eclipse. two such eclipses will occur this year ( ). if the full-moon passes near the earth, or is at perigee, it finds the cone of shadow cast by the earth larger, and hence the eclipse is greater; if it is far from the earth, or near apogee, the earth's shadow is smaller, and the eclipse less, or is escaped altogether. there is a certain periodicity in eclipses. whenever the sun, moon, and earth are in a line, as in the total eclipse of july th, , they will be in the same position after the earth has made about eighteen revolutions, [page ] and the moon two hundred and sixteen--that is, eighteen years after. this period, however, is disregarded by astronomers, and each eclipse calculated by itself to the accuracy of a second. how terrible is the fear of ignorance and superstition when the sun or moon appear to be in the process of destruction! how delightful are the joys of knowledge when its prophesies in regard to the heavenly bodies are being fulfilled! mars. the god or war; its sign [symbol], spear and shield. mean distance from the sun, , , miles. diameter, miles. revolution, axial, h. m. . s.; orbital, . days. velocity per minute, miles. satellites, two. [illustration: fig. .--apparent size of mars at mean and extreme distances.] at intervals, on an average of two years one month and nineteen days, we find rising, as the sun goes down, the reddest star in the heavens. its brightness is exceedingly variable; sometimes it scintillates, and sometimes it shines with a steady light. its marked peculiarities demand a close study. we find it to be mars, the fiery god of war. its orbit is far from circular. at perihelion it is , , miles from the sun, and at aphelion , , ; hence its mean distance is about , , . so great a change in its distance from the sun easily accounts for the change in its brilliancy. now, if mars and the earth revolved in circular orbits, the one , , miles from the sun, and the other , , , they would approach at conjunction within , , miles of each other, and at opposition be , , miles apart. but mars at perihelion may be only , , miles from the sun, and earth at [page ] aphelion may be , , miles from the sun. they are, then, but , , miles apart. this favorable opportunity occurs about once in seventy-nine years. at its last occurrence, in , mars introduced to us his two satellites, that had never before been seen by man. in consequence of this greatly varying distance, the apparent size of mars differs very much (fig. ). take a favorable time when the planet is near, also as near overhead as it ever comes, so as to have as little atmosphere as possible to penetrate, and study the planet. the first thing that strikes the observer is a dazzling spot of white near the pole which happens to be toward him, or at both poles when the planet is so situated that they can be seen. when the north pole is turned toward the sun the size of the spot sensibly diminishes, and the spot at the south pole enlarges, and _vice versa_. clearly they are ice-fields. hence mars has water, and air to carry it, and heat to melt ice. it is winter at the south pole when mars is farthest from the sun; therefore the ice-fields are larger than at the north pole. it is summer at the south pole when mars is nearest the sun. hence its ice-fields grow smaller [page ] than those of the north pole in its summer. this carrying of water from pole to pole, and melting of ice over such large areas, might give rise to uncomfortable currents in ocean and air; but very likely an inhabitant of earth might be transported to the surface of mars, and be no more surprised at what he observed there than if he went to some point of the earth to him unknown. day and night would be nearly of the same length; winter would linger longer in the lap of spring; summer would be one hundred and eighty-one days long; but as the seas are more intermingled with the land, and the divisions of land have less of continental magnitude, it may be conjectured that mars might be a comfortable place of residence to beings like men. perhaps the greatest surprise to the earthly visitor would be to find himself weighing only four-tenths as much as usual, able to leap twice as high, and lift considerable bowlders. _satellites of mars._ the night of august th, , is famous in modern astronomy. mars has been a special object of study in all ages; but on that evening professor hall, of washington, discovered a satellite of mars. on the th it was seen again, and its orbital motion followed. on the following night it was hidden behind the body of the planet when the observation began, but at the calculated time--at four o'clock in the morning--it emerged, and established its character as a true moon, and not a fixed star or asteroid. blessings, however, never come singly, for another object soon emerged which proved to be an inner satellite. this is extraordinarily near [page ] the planet--only four thousand miles from the surface--and its revolution is exceedingly rapid. the shortest period hitherto known is that of the inner satellite of saturn, h. m. the inner satellite of mars makes its revolution in h. m.--a rapidity so much surpassing the axial revolution of the planet itself, that it rises in the west and sets in the east, showing all phases of our moon in one night. the outer satellite is , miles from mars, and makes its revolution in h. m. its diameter is six and a quarter miles; that of the inner one is seven and a half miles. this can be estimated only by the amount of light given. asteroids. already discovered ( ), . distances from the sun, from , , to , , miles. diameters, from to miles. mass of all, less than one-quarter of the earth. the sense of infinite variety among the countless number of celestial orbs has been growing rapidly upon us for half a century, and doubtless will grow much more in half a century to come. just as we paused in the consideration of planets to consider meteors and comets, at first thought so different, so must we now pause to consider a ring of bodies, some of which are as small in comparison to jupiter, the next planet, as aerolites are compared to the earth. in an association of astronomers, suspecting that a planet might be found in the great distance between mars and jupiter, divided the zodiac into twenty-four parts, and assigned one part to each astronomer for a thorough search; but, before their organization could commence work, piazzi, an italian astronomer of palermo, [page ] found in taurus a star behaving like a planet. in six weeks it was lost in the rays of the sun. it was rediscovered on its emergence, and named ceres. in march, , a second planet was discovered by olbers in the same gap between mars and jupiter, and named pallas. here was an embarrassment of richness. olbers suggested that an original planet had exploded, and that more pieces could be found. more were found, but the theory is exploded into more pieces than a planet could possibly be. up to one hundred and ninety-two have been discovered, with a prospect of more. between - forty-five were discovered, showing that they are sought for with great skill. in the discovery of these bodies, our american astronomers, professors watson and peters, are without peers. between mars and jupiter is a distance of some , , miles. subtract , , miles next to mars and , , miles next to jupiter, and there is left a zone , , miles wide outside of which the asteroids never wander. if any ever did, the attraction of mars or jupiter may have prevented their return. since the orbits of mars and jupiter show no sign of being affected by these bodies for a century past, it is probable that their number is limited, or at least that their combined mass does not approximate the size of a planet. professor newcomb estimates that if all that are now discovered were put into one planet, it would not be over four hundred miles in diameter; and if a thousand more should exist, of the average size of those discovered since , their addition would not increase the diameter to more than five hundred miles. [page ] that all these bodies, which differ from each other in no respect except in brilliancy, can be noted and fixed so as not to be mistaken one for another, and instantly recognized though not seen for a dozen years, is one of the highest exemplifications of the accuracy of astronomical observation. jupiter. the king of the gods; sign [symbol], the bird of jove. distance from the sun, perihelion, , , miles; aphelion, , , miles. diameter, equatorial, , miles; polar, , miles. volume, earths. mass, earths. axial revolution, h. m s. orbital revolution, years days. velocity, . miles per minute. [illustration: fig. .--jupiter as seen by the great washington telescope. drawn by mr. holden.] jupiter rightly wears the name of the "giant planet." his orbit is more nearly circular than most smaller planets. he could not turn short corners with facility. we know little of his surface. his spots and belts are [page ] changeable as clouds, which they probably are. some spots may be slightly self-luminous, but not the part of the planet we see. it is covered with an enormous depth of atmosphere. since the markings in the belts move about one hundred miles a day, the jovian tempests are probably not violent. it is, however, a singular and unaccountable fact, as remarked by arago, that its trade-winds move in an opposite direction from ours. jupiter receives only one twenty-seventh as much light and heat from the sun as the earth receives. its lighter density, being about that of water, indicates that it still has internal heat of its own. indeed, it is likely that this planet has not yet cooled so as to have any solid crust, and if its dense vapors could be deposited on the surface, its appearance might be more suggestive of the sun than of the earth. _satellites of jupiter._ in one respect jupiter seems like a minor sun--he is royally attended by a group of planets: we call them moons. this system is a favorite object of study to everyone possessing a telescope. indeed, i have known a man who could see these moons with the naked eye, and give their various positions without mistake. galileo first revealed them to ordinary men. we see their orbits so nearly on the edge that the moons seem to be sliding back and forth across and behind the disk, and to varying distances on either side. fig. is the representation of their appearance at successive observations in november, . their motion is so swift, and the means of comparison by one another and the planet so excellent, that they can be seen to change their places, [page ] be occulted, emerge from shadow, and eclipse the planet, in an hour's watching. [illustration: fig. .--_a._ various positions of jupiter's moons; _b._ greatest elongation of each satellite.] elements of jupiter's satellites. +-------------------------------------------------------------+ | | mean distance | | | | | from jupiter. | sidereal period. | diameter.| | |---------------+------------------+----------| | | miles. | days hrs. min. | miles. | | i. io | , | | , | | ii. europa | , | | , | | iii. ganymede | , | | , | | iv. callisto | , , | | , | +-------------------------------------------------------------+ it is seen by the above table that all these moons are larger than ours, one larger than mercury, and the asteroids are hardly large enough to make respectable moons for them. they differ in color: i. and ii. have a bluish tinge; iii. a yellow; and iv. is red. the amount of light given by these satellites varies in the most sudden and inexplicable manner. perhaps it may be owing to the different distributions of land and water on them. the mass of all of them is . of jupiter. [page ] if the jovian system were the only one in existence, it would be a surprising object of wonder and study. a monster planet, , miles in diameter, hung on nothing, revolving its equatorial surface forty-five miles a minute, holding four other worlds in steady orbits, some of them at a speed of seven hundred miles a minute, and the whole system carried through space at five hundred miles a minute. yet the discovery of all this display of power, skill, and stability is only reading the easiest syllables of the vast literature of wisdom and power. saturn. the god or time; sign [symbol], his scythe. mean distance from the sun, , , miles. diameter, polar, , miles; equatorial, , miles. axial revolution, h. m. periodic time, t years. moons, eight. the human mind has used saturn and the two known planets beyond for the last years as a gymnasium. it has exercised itself in comprehending their enormous distances in order to clear those greater spaces, to where the stars are set; it has exercised its ingenuity at interpreting appearances which signify something other than they seem, in order that it may no longer be deluded by any sunrises into a belief that the heavenly dome goes round the earth. that a wandering point of light should develop into such amazing grandeurs under the telescope, is as unexpected as that every tiny seed should show peculiar markings and colors under the microscope. [illustration: fig. .--view of saturn and his rings.] there are certain things that are easy to determine, such as size, density, periodic time, velocity, etc.; but other things are exceedingly difficult to determine. it requires long sight to read when the book is held [page ] , , miles away. only very few, if more than two, opportunities have been found to determine the time of saturn's rotation. on the evening of december th, , professor hall observed a brilliant white spot suddenly show itself on the body of this planet. it was as if an eruption of white hot matter burst up from the interior. it spread eastward, and remained bright till january, when it faded. no such opportunity for getting a basis on which to found a calculation of the time of the rotation of saturn has occurred since sir william herschel's observations; and, very singularly, the two times deduced wonderfully coincide--that of herschel being h. m., that of mr. hall being h. m. [page ] the density of saturn is less than that of water, and its velocity of rotation so great that centrifugal force antagonizes gravitation to such an extent that bodies weigh on it about the same as on the earth. all the fine fancies of the habitability of this vaporous world, all the calculations of the number of people that could live on the square miles of the planet and its enormous rings, are only fancy. nothing could live there with more brains than a fish, at most. it is a world in formative processes. we cannot hear the voice of the creator there, but we can see matter responsive to the voice, and moulded by his word. _rings of saturn._ the eye and mind of man have worked out a problem of marvellous difficulty in finding a true solution of the strange appearance of the rings. galileo has the immortal honor of first having seen something peculiar about this planet. he wrote to the duke of tuscany, "when i view saturn it seems _tricorps_. the central body seems the largest. the two others, situated, the one on the east, and the other on the west, seem to touch it. they are like two supporters, who help old saturn on his way, and always remain at his side." looking a few years later, the rings having turned from view, he said, "it is possible that some demon mocked me;" and he refused to look any more. huyghens, in march, , solved the problem of the triform appearance of saturn. he saw them as handles on the two sides. in a year they had disappeared, and the planet was as round as it seemed to galileo in . he did not, however, despair; and in october, [page ] , he was rewarded by seeing them appear again. he wrote of saturn, "it is girdled by a thin plain ring, nowhere touching, inclined to the ecliptic." since that time discoveries have succeeded one another rapidly. "we have seen by degrees a ring evolved out of a triform planet, and the great division of the ring and the irregularities on it brought to light. enceladus, and coy mimas, faintest of twinklers, are caught by herschel's giant mirrors. and he, too, first of men, realizes the wonderful tenuity of the ring, along which he saw those satellites travelling like pearls strung on a silver thread. then bond comes on the field, and furnishes evidence to show that we must multiply the number of separate rings we know not how many fold. and here we reach the golden age of saturnian discovery, when bond, with the giant refractor of cambridge, and dawes, with his - / -inch munich glass, first beheld that wonderful dark semi-transparent ring, which still remains one of the wonders of our system. but the end is not yet: on the southern surface of the ring, ere summer fades into autumn, otto struve in turn comes upon the field, detects, as dawes had previously done, a division even in the dark ring, and measures it, while it is invisible to lassell's mirror--a proof, if one were needed, of the enormous superiority possessed by refractors in such inquiries. then we approach , when the ring plane again passes through the earth, and struve and wray observe curious nebulous appearances."[*] [footnote *: lockyer.] our opportunities for seeing saturn vary greatly. as the earth at one part of its orbit presents its south pole [page ] to the sun, then its equator, then the north pole, so saturn; and we, in the direction of the sun, see the south side of the rings inclined at an angle of °; next the edge of the rings, like a fine thread of light; then the north side at a similar inclination. on february th, , saturn was between aquarius and pisces, with the edge of the ring to the sun. in , the planet being in taurus, the south side of the rings will be seen at the greatest advantage. from till all circumstances will combine to give most favorable studies of saturn. meanwhile study the picture of it. the outer ring is narrow, dark, showing hints of another division, sometimes more evident than at others, as if it were in a state of flux. the inner, or second, ring is much brighter, especially on the outer edge, and shading off to the dusky edge next to the planet. there is no sign of division into a third dusky innermost ring, as was plainly seen by bond. this, too, may be in a state of flux. the markings of the planet are delicate, difficult of detection, and are not like those stark zebra stripes that are so often represented. the distance between the planet and the second ring seems to be diminished one-half since , and this ring has doubled its breadth in the same time. some of this difference may be owing to our greater telescopic power, enabling us to see the ring closer to the planet; but in all probability the ring is closing in upon the central body, and will touch it by a.d. . thus the whole ring must ultimately fall upon the planet, instead of making a satellite. we are anxious to learn the nature of such a ring. [page ] laplace mathematically demonstrated that it cannot be uniform and solid, and survive. professor peirce showed it could not be fluid, and continue. then professor maxwell showed that it must be formed of clouds of satellites too small to be seen individually, and too near together for the spaces to be discerned, unless, perhaps, we may except the inner dark ring, where they are not near enough to make it positively luminous. indeed, there is some evidence that the meteoroids are far enough apart to make the ring partially transparent. we look forward to the opportunities for observation in with the brightest hope that these difficult questions will be solved. _satellites of saturn._ the first discovered satellite of saturn seen by huyghens was in , and the last by the bonds, father and son, of cambridge, in . these are eight in number, and are named: distant from saturn's centre. i. mimas , miles. ii. enceladus , " iii. tethys , " iv. dione , " v. rhea , " vi. titan , " vii. hyperion , " viii. japetus , , " titan can be seen by almost any telescope; i., ii., and iii., only by the most powerful instrument. all except japetus revolve nearly in the plane of the ring. like the moons of jupiter, they present remarkable and unaccountable variations of brilliancy. an inspection [page ] of the table reveals either an expectation that another moon will be discovered between v. and vi., and about three more between vii. and viii., or that these gaps may be filled with groups of invisible asteroids, as the gap between mars and jupiter. this will become more evident by drawing saturn, the rings, and orbits of the moons all as circles, on a scale of , miles to the inch. saturn will be in the centre, , miles in diameter; then a gap, decreasing twenty-nine miles a year to the first ring, of, say, , miles; a dark ring miles wide; next the brightest ring , miles wide; then a gap of miles; then the outer ring , miles wide; then the orbits of the satellites in order. if the scenery of jupiter is magnificent, that of saturn must be sublime. if one could exist there, he might wander from the illuminated side of the rings, under their magnificent arches, to the darkened side, see the swift whirling moons; one of them presenting ten times the disk of the earth's moon, and so very near as to enable him to watch the advancing line of light that marks the lunar morning journeying round that orb. uranus. sign [symbol]; the initial of herschel, and sign of the world. distance from the sun, , , , miles. diameter, , miles. axial revolution unknown. orbital, years. velocity per minute, miles. moons, four. uranus was presented to the knowledge of man as an unexpected reward for honest work. it was first mistaken by its discoverer for a comet, a mere cloud of vapor; but it proved to be a world, and extended the [page ] boundaries of our solar system, in the moment of its discovery, as much as all investigation had done in all previous ages. sir william herschel was engaged in mapping stars in , when he first observed its sea-green disk. he proposed to call it _georgium sidus_, in honor of his king; but there were too many names of the gods in the sky to allow a mortal name to be placed among them. it was therefore called uranus, since, being the most distant body of our system, as was supposed, it might appropriately bear the name of the oldest god. finding anything in god's realms of infinite riches ought not to lead men to regard that as final, but as a promise of more to follow. this planet had been seen five times by flamsteed before its character was determined--once nearly a century before--and eight times by le monnier. these names, which might easily have been associated with a grand discovery, are associated with careless observation. eyes were made not only to be kept open, but to have minds behind them to interpret their visions. herschel thought he discovered six moons belonging to uranus, but subsequent investigation has limited the number to four. two of these are seen with great difficulty by the most powerful telescopes. if the plane of our moon's orbit were tipped up to a greater inclination, revolving it on the line of nodes as an axis until it was turned °, the moon, still continuing on its orbit in that plane, would go over the poles instead of about the equator, and would go back to its old path when the plane was revolved °; but its revolution would now be from east to west, or [page ] retrograde. the plane of the moons of uranus has been thus inclined till it has passed ° beyond the pole, and the moons' motions are retrograde as regards other known celestial movements. how uranus itself revolves is not known. there are more worlds to conquer. neptune. god of the sea; sign [symbol], his trident. distance from the sun, , , , miles. diameter, , miles. velocity per minute, . miles. axial revolution unknown. orbital, . years. one moon. men sought for neptune as the heroes sought the golden fleece. the place of uranus had been mapped for nearly one hundred years by these accidental observations. on applying the law of universal gravitation, a slight discrepancy was found between its computed place and its observed place. this discrepancy was exceedingly slight. in it was only "; in , "; in , '. two stars that were ' apart would appear as one to the keenest unaided eye, but such an error must not exist in astronomy. years of work were given to its correction. mr. john c. adams, of cambridge, england, finding that the attraction of a planet exterior to uranus would account for its irregularities, computed the place of such a hypothetical body with singular exactness in october, ; but neither he nor the royal astronomer airy looked for it. another opportunity for immortality was heedlessly neglected. meanwhile, m. leverrier, of paris, was working at the same problem. in the summer of leverrier announced the place of the exterior planet. the conclusion was in striking coincidence with that of mr. [page ] clark. mr. challis commenced to search for the planet near the indicated place, and actually saw and mapped the star august th, , but did not recognize its planetary character. dr. galle, of berlin, on the d of september, , found an object with a planetary disk not plotted on the map of stars. it was the sought-for world. it would seem easy to find a world seventy-six times as large as the earth, and easy to recognize it when seen. the fact that it could be discovered only by such care conveys an overwhelming idea of the distance where it moves. [illustration: fig. .--perturbation of uranus.] the effect of these perturbations by an exterior planet is understood from fig. . uranus and neptune were in conjunction, as shown, in . but in it had been found that uranus was too far from the sun, and too much accelerated. since , neptune, in his orbit from f to e, had been hastening uranus in his orbit d from c to b, and also drawing it farther from the sun. after , neptune, in passing from e to d, had been retarding uranus in his orbit from b to a. we have seen it is easy to miss immortality. there is still another instance. lalande saw neptune on may th and th, , noted that it had moved a little, and that the observations did not agree; but, supposing the first was wrong, carelessly missed the glory of once more doubling the bounds of the empire of the sun. [page ] it is time to pause and review our knowledge of this system. the first view reveals a moon and earth endowed with a force of inertia going on in space in straight lines; but an invisible elastic cord of attraction holds them together, just counterbalancing this tendency to fly apart, and hence they circle round their centre of gravity. the revolving earth turns every part of its surface to the moon in each twenty-four hours. by an axial revolution in the same time that the moon goes round the earth, the moon holds the same point of its surface constantly toward the earth. if we were to add one, two, four, eight moons at appropriate distances, the result would be the same. there is, however, another attractive influence--that of the sun. the sun attracts both earth and moon, but their nearer affection for each other keeps them from going apart. they both, revolving on their axes and around their centre of gravity, sweep in a vastly wider curve around the sun. add as many moons as has jupiter or saturn, the result is the same--an orderly carrying of worlds through space. there lies the unsupported sun in the centre, nearer to infinity in all its capacities and intensities of force than our minds can measure, filling the whole dome to where the stars are set with light, heat, and power. it holds five small worlds--vulcan, mercury, venus, earth, and mars--within a space whose radius it would require a locomotive half a thousand years to traverse. it next holds some indeterminate number of asteroids, and the great jupiter, equal in volume to , earths. it holds saturn, uranus, and neptune, and all their variously related satellites and rings. the two thoughts that overwhelm us are distance and power. the period of [page ] man's whole history is not sufficient for an express train to traverse half the distance to neptune. thought wearies and fails in seeking to grasp such distances; it can scarcely comprehend one million miles, and here are thousands of them. even the wings of imagination grow weary and droop. when we stand on that outermost of planets, the very last sentinel of the outposts of the king, the very sun grown dim and small in the distance, we have taken only one step of the infinite distance to the stars. they have not changed their relative position--they have not grown brighter by our approach. neptune carries us round a vast circle about the centre of the dome of stars, but we seem no nearer its sides. in visiting planets, we have been only visiting next-door neighbors in the streets of a seaport town. we know that there are similar neighbors about sirius and arcturus, but a vast sea rolls between. as we said, we stand with the outermost sentinel; but into the great void beyond the king of day sends his comets as scouts, and they fly thousands of years without for one instant missing the steady grasp of the power of the sun. it is nearer almightiness than we are able to think. if we cannot solve the problems of the present existence of worlds, how little can we expect to fathom the unsoundable depths of their creation and development through ages measureless to man! yet the very difficulty provokes the most ambitious thought. we toil at the problem because it has been hitherto unsolvable. every error we make, and discover to be such, helps toward the final solution. every earnest thinker who climbs the shining worlds as steps to a higher thought is trying to solve the problem god has given us to do. [page ] ix. the nebular hypothesis. "and the earth was without form, and void; and darkness was upon the face of the deep."--_genesis_ i. . [page ] "a dark illimitable ocean, without bound, without dimension, where length, breadth, and height, and time, and place are lost."--milton. "it is certain that matter is somehow directed, controlled, and arranged; while no material forces or properties are known to be capable of discharging such functions."--lionel beale. "the laws of nature do not account for their own origin."--john stuart mill. [page ] ix. _the nebular hypothesis._ the method by which the solar system came into its present form was sketched in vast outline by moses. he gave us the fundamental idea of what is called the nebular hypothesis. swedenborg, that prodigal dreamer of vagaries, in threw out some conjectures of the way in which the outlines were to be filled up; buffon followed him closely in ; kant sought to give it an ideal philosophical completeness; as he said, "not as the result of observation and computation," but as evolved out of his own consciousness; and laplace sought to settle it on a mathematical basis. it has been modified greatly by later writers, and must receive still greater modifications before it can be accepted by the best scientists of to-day. it has been called "the grandest generalization of the human mind;" and if it shall finally be so modified as to pass from a tentative hypothesis to an accepted philosophy, declaring the modes of a divine worker rather than the necessities of blind force, it will still be worthy of that high distinction. let it be clearly noted that it never proposes to do more than to trace a portion of the mode of working which brought the universe from one stage to another. it only goes back to a definite point, never to absolute beginning, nor to nothingness. it takes matter from [page ] the hand of the unseen power behind, and merely notes the progress of its development. it finds the clay in the hands of an intelligent potter, and sees it whirl in the process of formation into a vessel. it is not in any sense necessarily atheistic, any more than it is to affirm that a tree grows by vital processes in the sun and dew, instead of being arbitrarily and instantly created. the conclusion reached depends on the spirit of the observer. newton could say, "this most beautiful system of the sun, planets, and comets could only proceed from the counsel and dominion of an intelligent and powerful being!" still it is well to recognize that some of its most ardent defenders have advocated it as materialistic. and laplace said of it to napoleon, "i have no need of the hypothesis of a god." the materialistic statement of the theory is this: that matter is at first assumed to exist as an infinite cloud of fire-mist, dowered with power latent therein to grow of itself into every possibility of world, flower, animal, man, mind, and affection, without any interference or help from without. but it requires far more of the divine worker than any other theory. he must fill matter with capabilities to take care of itself, and this would tax the abilities of the infinite one far more than a constant supervision and occasional interference. instead of making the vase in perfect form, and coloring it with exquisite beauty by an ever-present skill, he must endow the clay with power to make itself in perfect form, adorn itself with delicate beauty, and create other vases. the nebular hypothesis is briefly this: all the matter composing all the bodies of the sun, planets, and satellites once existed in an exceedingly diffused state; [page ] rarer than any gas with which we are acquainted, filling a space larger than the orbit of neptune. gravitation gradually contracted this matter into a condensing globe of immense extent. some parts would naturally be denser than others, and in the course of contraction a rotary motion, it is affirmed, would be engendered. rotation would flatten the globe somewhat in the line of its axis. contracting still more, the rarer gases, aided by centrifugal force, would be left behind as a ring that would ultimately be separated, like saturn's ring, from the retreating body. there would naturally be some places in this ring denser than others; these would gradually absorb all the ring into a planet, and still revolve about the central mass, and still rotate on its own axis, throwing off rings from itself. thus the planet neptune would be left behind in the first sun-ring, to make its one moon; the planet uranus left in the next sun-ring, to make its four moons from four successive planet-rings; saturn, with its eight moons and three rings not made into moons, is left in the third sun-ring; and so on down to vulcan. the outer planets would cool off first, become inhabitable, and, as the sun contracted and they radiated their own heat, become refrigerated and left behind by the retreating sun. of course the outer planets would move slowly; but as that portion of the sun which gave them their motion drew in toward the centre, keeping its absolute speed, and revolving in the lessening circles of a contracting body, it would give the faster motion necessary to be imparted to earth, mercury, and vulcan. the four great classes of facts confirmatory of this hypothesis are as follows: st. all the planets move [page ] in the same direction, and nearly in the same plane, as if thrown off from one equator; d. the motions of the satellites about their primaries are mostly in the same direction as that of their primaries about the sun; d. the rotation of most of these bodies on their axes, and also of the sun, is in the same direction as the motion of the planets about the sun; th. the orbits of the planets, excluding asteroids, and their satellites, have but a comparatively small eccentricity; th. certain nebulæ are observable in the heavens which are not yet condensed into solids, but are still bright gas. the materialistic evolutionist takes up the idea of a universe of material world-stuff without form, and void, but so endowed as to develop itself into orderly worlds, and adds to it this exceeding advance, that when soil, sun, and chemical laws found themselves properly related, a force in matter, latent for a million eons in the original cloud, comes forward, and dead matter becomes alive in the lowest order of vegetable life; there takes place, as herbert spencer says, "a change from an indefinite, incoherent homogeneity, into a definite, coherent heterogeneity, through continuous differentiation and integration." the dead becomes alive; matter passes from unconsciousness to consciousness; passes up from plant to animal, from animal to man; takes on power to think, reason, love, and adore. the theistic evolutionist may think that the same process is gone through, but that an ever-present and working god superintends, guides, and occasionally bestows a new endowment of power that successively gives life, consciousness, mental, affectional, and spiritual capacity. is this world-theory true? and if so, is either of the [page ] evolution theories true also? if the first evolution theory is true, the evolved man will hardly know which to adore most, the being that could so endow matter, or the matter capable of such endowment. there are some difficulties in the way of the acceptance of the nebular hypothesis that compel many of the most thorough scientists of the day to withhold their assent to its entirety. the latest, and one of the most competent writers on the subject, professor newcomb, who is a mathematical astronomer, and not an easy theorist, evolving the system of the universe from the depth of his own consciousness, says: "should any one be sceptical as to the sufficiency of these laws to account for the present state of things, science can furnish no evidence strong enough to overthrow his doubts until the sun shall be found to be growing smaller by actual measurement, or the nebulæ be actually seen to condense into stars and systems." in one of the most elaborate defences of the theory, it is argued that the hypothesis explains why only one of the four planets nearest the sun can have a moon, and why there can be no planet inside of mercury. the discovery of the two satellites to mars and of the planet vulcan makes it all the worse for these facts. some of the objections to the theory should be known by every thinker. laplace must have the cloud "diffused in consequence of excessive heat," etc. helmholtz, in order to account for the heat of the contracting sun, must have the cloud relatively cold. how he and his followers diffused the cloud without heat is not stated. the next difficulty is that of rotation. the laws [page ] of science compel a contraction into one non-rotating body--a central sun, indeed, but no planets about it. laplace cleverly evades the difficulty by not taking from the hand of the creator diffused gas, but a sun with an atmosphere filling space to the orbit of neptune, and _already in revolution_. he says: "it is four millions to one that all motions of the planets, rotations and revolutions, were at once imparted by an original common cause, of which we know neither the nature nor the epoch." helmholtz says of rotation, "the existence of which must be assumed." professor newcomb says that the planets would not be arranged as now, each one twice as far from the sun as the next interior one, and the outer ones made first, but that all would be made into planets at once, and the small inner ones quite likely to cool off more rapidly. it is a very serious difficulty that at least one satellite does not revolve in the right direction. how neptune or uranus could throw their moons backward from its equator is not easily accounted for. it is at least one parthian arrow at the system, not necessarily fatal, but certainly dangerous. a greater difficulty is presented by the recently discovered satellites of mars. the inner one goes round the planet in one-third part of the time of the latter's revolution. how mars could impart three times the speed to a body flying off its surface that it has itself, has caused several defenders of the hypothesis to rush forward with explanations, but none with anything more than mere imaginary collisions with some comet. it is to be noticed that accounting for three times the speed is not enough; for as mars shrunk away from the [page ] ring that formed that satellite, it ought itself to attain more speed, as the sun revolves faster than its planets, and the earth faster than its moon. in defending the hypothesis, mitchel said: "suppose we had discovered that it required more time for saturn or jupiter to rotate on their axes than for their nearest moon to revolve round them in its orbit; this would have falsified the theory." it is also asserted that the newly discovered planet vulcan makes an orbital in less time than the sun makes an axial revolution. in regard to one martial moon, professor kirkwood, on whom proctor conferred the highest title that could be conferred, "the modern kepler," says: "unless some explanation can be given, the short period of the inner satellite will be doubtless regarded as a conclusive argument against the nebular hypothesis." if gravitation be sufficient to account for the various motions of the heavenly bodies, we have a perplexing problem in the star known as groombridge, now in the hunting dogs of bootes. it is thought to have a speed of two hundred miles per second--a velocity that all the known matter in the universe could not give to the star by all its combined attraction. neither could all that attraction stop the motion of the star, or bend it into an orbit. its motion must be accounted for on some hypothesis other than the nebular. the nebulæ which we are able to observe are not altogether confirmatory of the hypothesis under consideration. they have the most fantastic shapes, as if they had no relation to rotating suns in the formative stages. there are vast gaps in the middle, where they ought to be densest. mr. plumer, in the _natural science review_, [page ] says, in regard to the results of the spectroscopic revelations: "we are furnished with distinct proof that the gases so examined are not only of nearly equal density, but that they exist in a low state of _tension. this fact is fatal to the nebular theory._" in the autumn of a star blazed out in cygnus, which promised to throw a flood of light on the question of world-making. its spectrum was like some of the fixed stars. it probably blazed ont by condensation from some previously invisible nebula. but its brilliancy diminished swiftly, when it ought to have taken millions of years to cool. if the theory was true, it ought to have behaved very differently. it should have regularly condensed from gas to a solid sun by slow process. but, worst of all, after being a star awhile, it showed unmistakable proofs of turning into a cloud-mist--a star into a nebula, instead of _vice versa_. a possible explanation will be considered under variable stars. such are a few of the many difficulties in the way of accepting the nebular hypothesis, as at present explained, as being the true mode of development of the solar system. doubtless it has come from a hot and diffused condition into its present state; but when such men as proctor, newcomb, and kirkwood see difficulties that cannot be explained, contradictions that cannot be reconciled by the principles of this theory, surely lesser men are obliged to suspend judgment, and render the scotch verdict of "not proven." whatever truth there may be in the theory will survive, and be incorporated into the final solution of the problem; which solution will be a much grander generalization of the human mind than the nebular hypothesis. [page ] of some things we feel very sure: that matter was once without form and void, and darkness rested on the face of the mighty deeps; that, instead of chaos, we have now cosmos and beauty; and that there is some process by which matter has been brought from one state to the other. whether, however, the nebular hypothesis lays down the road travelled to this transfiguration, we are not sure. some of it seems like solid rock, and some like shifting quicksand. doubtless there is a road from that chaos to this fair cosmos. the nebular hypothesis has surveyed, worked, and perfected many long reaches of this road, but the rivers are not bridged, the chasms not filled, nor the mountains tunnelled. when men attempt to roll the hypothesis of evolution along the road of the nebular hypothesis of worlds, and even beyond to the production of vegetable and animal life, mind and affection, the gaps in the road become evident, and disastrous. a soul that has reached an adoration for the supreme father cares not how he has made him. doubtless the way god chose was the best. it is as agreeable to have been thought of and provided for in the beginning, to have had a myriad ages of care, and to have come from the highest existent life at last, as to have been made at once, by a single act, out of dust. the one who is made is not to say to the maker, "why hast thou formed me in this or that manner?" we only wish the question answered in what manner we were really made. evolution, without constant superintendence and occasional new inspiration of power, finds some tremendous chasms in the road it travels. these must be spanned by the power of a present god or the airy imagination [page ] of man. dr. mccosh has happily enumerated some of these tremendous gaps over which mere force cannot go. given, then, matter with mechanical power only, what are the gaps between it and spirituality? " . chemical action cannot be produced by mechanical power. " . life, even in the lowest forms, cannot be produced from unorganized matter. " . protoplasm can be produced only by living matter. " . organized matter is made up of cells, and can be produced only by cells. whence the first cell? " . a living being can be produced only from a seed or germ. whence the first vegetable seed? " . an animal cannot be produced from a plant. whence the first animal? " . sensation cannot be produced in insentient matter. " . the genesis of a new species of plant or animal has never come under the cognizance of man, either in pre-human or post-human ages, either in pre-scientific or scientific times. darwin acknowledges this, and says that, should a new species suddenly arise, we have no means of knowing that it is such. " . consciousness--that is, a knowledge of self and its operations--cannot be produced out of mere matter or sensation. " . we have no knowledge of man being generated out of the lower animals. " . all human beings, even savages, are capable of forming certain high ideas, such as those of god and duty. the brute creatures cannot be made to entertain these by any training. [page ] "with such tremendous gaps in the process, the theory which would derive all things out of matter by development is seen to be a very precarious one. the truth, according to the best judgment to be formed in the present state of knowledge, would seem to be about this: the nebular hypothesis is correct in all the main facts on which it is based; but that neither the present forces of matter, nor any other forces conceivable to the mind of man, with which it can possibly be endowed, can account for all the facts already observed. there is a demand for a personal volition, for an exercise of intelligence, for the following of a divine plan that embraces a final perfection through various and changeful processes. the five great classes of facts that sustain the nebular hypothesis seem set before us to show the regular order of working. the several facts that will not, so far as at present known, accord with that plan, seem to be set before us to declare the presence of a divine will and power working his good pleasure according to the exigencies of time and place. [page ] x. the stellar system. "the heavens number out the glory of the strong god."--david. [page ] richter says that "an angel once took a man and stripped him of his flesh, and lifted him up into space to show him the glory of the universe. when the flesh was taken away the man ceased to be cowardly, and was ready to fly with the angel past galaxy after galaxy, and infinity after infinity, and so man and angel passed on, viewing the universe, until the sun was out of sight--until our solar system appeared as a speck of light against the black empyrean, and there was only darkness. and they looked onward, and in the infinities of light before, a speck of light appeared, and suddenly they were in the midst of rushing worlds. but they passed beyond that system, and beyond system after system, and infinity after infinity, until the human heart sank, and the man cried out: 'end is there none of the universe of god?' the angel strengthened the man by words of counsel and courage, and they flew on again until worlds left behind them were out of sight, and specks of light in advance were transformed, as they approached them, into rushing systems; they moved over architraves of eternities, over pillars of immensities, over architecture of galaxies, unspeakable in dimensions and duration, and the human heart sank again and called ont: 'end is there none of the universe of god?' and all the stars echoed the question with amazement: 'end is there none of the universe of god?' and this echo found no answer. they moved on again past immensities of immensities, and eternities of eternities, until in the dizziness of uncounted galaxies the human heart sank for the last time, and called out: 'end is there none of the universe of god?' and again all the stars repeated the question, and the angel answered: 'end is there none of the universe of god. lo, also, there is no beginning.'" [page ] x. _the open page of the heavens._ the greeks set their mythological deities in the skies, and read the revolving pictures as a starry poem. not that they were the first to set the blazonry of the stars as monuments of their thought; we read certain allusions to stars and asterisms as far back as the time of job. and the pleiades, arcturus, and orion are some of the names used by him who "calleth all the stars by their names, in the greatness of his power." homer and hesiod, b.c., allude to a few stars and groups. the arabians very early speak of the great bear; but the greeks completely nationalized the heavens. they colonized the earth widely, but the heavens completely; and nightly over them marched the grand procession of their apotheosized divinities. there hercules perpetually wrought his mighty labors for the good of man; there flashed and faded the changeful star algol, as an eye in the head of the snaky-haired medusa; over them flew pegasus, the winged horse of the poet, careering among the stars; there the ship argo, which had explored all strange seas of earth, nightly sailed in the infinite realms of heaven; there perseus perpetually killed the sea-monster by celestial aid, and perpetually won the chained andromeda for his bride. very evident was their recognition of divine help: equally evident was [page ] their assertion of human ability and dominion. they gathered the illimitable stars, and put uncountable suns into the shape of the great bear--the most colossal form of animal ferocity and strength--across whose broad forehead imagination grows weary in flying; but they did not fail to put behind him a representative of themselves, who forever drives him around a sky that never sets--a perpetual type that man's ambition and expectation correspond to that which has always been revealed as the divine. the heavens signify much higher power and wisdom to us; we retain the old pictures and groupings for the convenience of finding individual stars. it is enough for the astronomer that we speak of a star as situated right ascension ' ", declination ° '. but for most people, if not all, it is better to call it polaris. so we might speak of a lake in latitude ° ', longitude ° ', but it would be clearer to most persons to say chatauqua. for exact location of a star, right ascension and declination must be given; but for general indication its name or place in a constellation is sufficiently exact. the heaven is rather indeterminably laid out in irregular tracts, and the mythological names are preserved. the brightest stars are then indicated in order by the letters of the greek alphabet--alpha (a), beta (b), gamma (g), etc. after these are exhausted, the roman alphabet is used in the same manner, and then numbers are resorted to; so that the famous star cygni is the th star in brightness in that one constellation. an acquaintance with the names, peculiarities, and movements of the stars visible at different seasons of the year is an unceasing source of pleasure. it [page ] is not vision alone that is gratified, for one fine enough may hear the morning stars sing together, and understand the speech that day uttereth unto day, and the knowledge that night showeth unto night. one never can be alone if he is familiarly acquainted with the stars. he rises early in the summer morning, that he may see his winter friends; in winter, that he may gladden himself with a sight of the summer stars. he hails their successive rising as he does the coming of his personal friends from beyond the sea. on the wide ocean he is commercing with the skies, his rapt soul sitting in his eyes. under the clear skies of the east he hears god's voice speaking to him, as to abraham, and saying, "look now toward the heavens, and tell the number of the stars, if thou be able to number them." a general acquaintance with the stars will be first attempted; a more particular knowledge afterward. fig. (page ) is a map of the circumpolar region, which is in full view every clear night. it revolves daily round polaris, its central point. toward this star, the two end stars of the great dipper ever point, and are in consequence called "the pointers." the map may be held toward the northern sky in such a position as the stars may happen to be. the great bear, or dipper, will be seen at nine o'clock in the evening above the pole in april and may; west of the pole, the pointers downward, in july and august; close to the north horizon in october and november; and east of the pole the pointers highest, in january and february. the names of such constantly visible stars should be familiar. in order, from the end of the tail of the great bear, we have benetnasch ae, mizar z, little alcor close to it, [page ] alioth, e megrez, d at the junction, has been growing dimmer for a century, phad, g dubhe and merak. it is best to get some facility at estimating distances in degrees. dubhe and merak, "the pointers," are five degrees apart. eighteen degrees forward of dubhe is the bear's nose; and three pairs of stars, fifteen degrees apart, show the position of the bear's three feet. follow "the pointers" twenty-nine degrees from dubhe, and we come to the pole-star. this star is double, made of two suns, both appearing as one to the naked eye. it is a test of an excellent three-inch telescope to resolve it into two. three stars beside it make the curved-up handle of the little dipper of ursa minor. between the two bears, thirteen degrees from megrez, and eleven degrees from mizar, are two stars in the tail of the dragon, which curves about to appropriate all the stars not otherwise assigned. follow a curve of fifteen stars, doubling back to a quadrangle from five to three degrees on a side, and thirty-five degrees from the pole, for his head. his tongue runs out to a star four degrees in front. we shall find, hereafter, that the foot of hercules stands on this head. this is the dragon slain by cadmus, and whose teeth produced such a crop of sanguinary men. the star thuban was once the pole-star. in the year b.c. it was ten times nearer the pole than polaris is now. in the year a.d. the pole will be within ' of polaris; in a.d. , it will be at a of cepheus; in a.d. , , within ° of vega; in a.d. , , at the star in the tongue of draco; in a.d. , , at thuban; in a.d. , , back to polaris. this indicates no change in the position of the dome [page ] of stars, but a change in the direction of the axis of the earth pointing to these various places as the cycles pass. as the earth goes round its orbit, the axis, maintaining nearly the same direction, really points to every part of a circle near the north star as large as the earth's orbit, that is, , , miles in diameter. but, as already shown, that circle is too small to be discernible at our distance. the wide circle of the pole through the ages is really made up of the interlaced curves of the annual curves continued through , years. the stem of the spinning top wavers, describes a circle, and finally falls; the axis of the spinning earth wavers, describes a circle of nearly , years, and never falls. the star g draconis, also called etanin, is famous in modern astronomy, because observations on this star led to the discovery of the _aberration of light_. if we held a glass tube perpendicularly out of the window of a car at rest, when the rain was falling straight down, we could see the drops pass directly through. put the car in motion, and the drops would seem to start toward us, and the top of the tube must be bent forward, or the drops entering would strike on the backside of the tube carried toward them. so our telescopes are bent forward on the moving earth, to enable the entered light to reach the eye-piece. hence the star does not appear just where it is. as the earth moves faster in some parts of its orbit than others, this aberration is sometimes greater than at others. it is fortunate that light moves with a uniform velocity, or this difficult, problem would be still further complicated. the displacement of a star from this course is about ". . [page ] on the side of polaris, opposite to ursa major, is king cepheus, made of a few dim stars in the form of the letter k. near by is his brilliant wife cassiopeia, sitting on her throne of state. they were the graceless parents who chained their daughter to a rock for the sea-monster to devour; but perseus, swift with the winged sandals of mercury, terrible with his avenging sword, and invincible with the severed head of medusa, whose horrid aspect of snaky hair and scaly body turned to stone every beholder, rescues the maiden from chains, and leads her away by the bands of love. nothing could be more poetical than the life of perseus. when he went to destroy the dreadful gorgon, medusa, pluto lent him his helmet, which would make him invisible at will; minerva loaned her buckler, impenetrable, and polished like a mirror; mercury gave him a dagger of diamonds, and his winged sandals, which would carry him through the air. coming to the loathsome thing, he would not look upon her, lest he, too, be turned to stone; but, guided by the reflection in the buckler, smote off her head, carried it high over libya, the dropping blood turning to serpents, which have infested those deserts ever since. [illustration: fig. .--circumpolar constellations. always visible. in this position.--january th, at o'clock; february th, at o'clock; and february th, at o'clock.] the human mind has always been ready to deify and throne in the skies the heroes that labor for others. both perseus and hercules are divine by one parent, and human by the other. they go up and down the earth, giving deliverance to captives, and breaking every yoke. they also seek to purge away all evil; they slay dragons, gorgons, devouring monsters, cleanse the foul places of earth, and one of them so wrestles with death as to win a victim from his grasp. finally, by [page ] an ascension in light, they go up to be in light forever. they are not ideally perfect. they right wrong by slaying wrong-doers, rather than by being crucified themselves; they are just murderers; but that only plucks the fruit from the tree of evil. they never attempted to infuse a holy life. they punished rather than regenerated. it must be confessed, also, that they were not sinless. but they were the best saviors the race could imagine, and are examples of that perpetual effort of the human mind to incarnate a divine helper who shall labor and die for the good of men. [page ] [illustration: fig. .--algol is on the meridian, ° south of pole.--at o'clock, december th; o'clock, december d; o'clock, january th.] _equatorial constellations._ if we turn our backs on polaris on the th of november, at o'clock in the evening, and look directly overhead, we shall see the beautiful constellation of andromeda. together with the square of pegasus, it makes another enormous dipper. the star a alpheratz is in her face, the three at the left cross her breast. b and the two above mark the girdle of her loins, and g is in the foot. perseus is near enough for help; and cetus, the sea-monster, is far enough away to do no harm. below, and east of andromeda, is the ram of the golden fleece, recognizable by the three stars in an acute triangle. the brightest is called arietis, or hamel. east of this are the pleiades, and the v-shaped hyades in taurus, or the bull. the pleiades rise about o'clock on the evening of the th of september, and at o'clock a.m. on june th. [page ] [illustration: fig. .--capella ( ° from the pole) and rigel ( °) are on the meridian at o'clock february th, o'clock january d, and at o'clock january th.] fig. extends east and south of our last map. it is the most gorgeous section of our heavens. (see the notes to the frontispiece.) note the triangle, ° on a side, made by betelguese, sirius, and procyon. a line from procyon to pollux leads quite near to polaris. orion is the mighty hunter. under his feet is a hare, behind him are two dogs, and before him is the rushing bull. the curve of stars to the right of bellatrix, g, represents his shield of the nemean lion's hide. the three stars of his belt make a measure ° long; the upper one, mintaker, is less than ' south of the equinoctial. the ecliptic passes between aldebaran and the pleiades. sirius rises about o'clock p.m. on the st of december, and about o'clock a.m. on the th of august. procyon rises about half an hour earlier. [page ] [illustration: fig. --regulus comes on the meridian, ° south from the pole, at o'clock march d, o'clock april th, and at o'clock april d.] fig. continues eastward. note the sickle in the head and neck of the lion. the star b is denebola, in his tail. arcturus appears by the word bootes, at the edge of the map. these two stars make a triangle with spica, about ° on a side. the geometric head of hydra is easily discernible east of procyon: the star g in the virgin is double, with a period of years. z is just above the equinoctial. there is a fine nebula two-thirds of the way from d to ae, and a little above the line connecting the two. coma berenices is a beautiful cluster of faint stars. spica rises at o'clock on the th of february, at o'clock a.m. on the th of november. [page ] [illustration: fig. l.--arcturus comes to the meridian, ° from the pole, at o'clock may th, o'clock june th, and at o'clock june th.] fig. represents the sky to the eastward and northward of the last. a line drawn from polaris and benetnasch comes east of arcturus to the little triangle called his sons. bootes drives the great bear round the pole. arcturus and denebola make a triangle with a, also called cor coroli, in the hunting dogs. this triangle, and the one having the same base, with spica for its apex, is called the "diamond of the virgin." hercules appears head down--a in the face, b, g, d; in his shoulders, p; and ae; in the loins, t in the knee, the foot being bent to the stars at the right. the serpent's head, making an x, is just at the right of the g of hercules, and the partial circle of the northern crown above. the head of draco is seen at b on the left of the map. arcturus rises at o'clock about the th of february, and at a.m. on the d of october; regulus h. m. earlier. [page ] [illustration: fig. .--altair comes to the meridian, ° from the pole, at o'clock p.m. august th, at o'clock september d, and at o'clock september th.] fig. portrays the stars eastward and southward. scorpio is one of the most brilliant and easily traced constellations. antares, a, in the heart, is double. in sagittarius is the little milk-dipper, and west of it the bended bow. vega is at the top of the map. near it observe z, a double, and e, a quadruple star. the point to which the solar system is tending is marked by the sign of the earth below p; herculis. the serpent, west of hercules, and coiled round nearly to aquila, is very traceable. in the right-hand lower corner is the centaur. below, and always out of our sight, is the famous a centauri. the diamond form of the dolphin is sometimes called "job's coffin." the ecliptic passes close [page ] to b of scorpio, which star is in the head. antares, in scorpio, rises at o'clock p.m. on may th, and at o'clock a.m. on january th. [illustration: fig. .--fomalhaut comes to the meridian, only ° from the horizon, at o'clock november th.] in fig. we recognize the familiar stars of pegasus, which tell us we have gone quite round the heavens. note the beautiful cross in the swan. b in the bill is named albireo, and is a beautiful double to almost any glass. its yellow and blue colors are very distinct. the place of the famous double star cygni is seen. the first magnitude star in the lower left-hand corner is fomalhaut, in the southern fish. a pegasi is in the diagonal corner from alpharetz, in andromeda. the star below altair is b aquilæ, and is called alschain; the one above is g aquilæ, named tarazed. this is not a brilliant section of the sky. altair rises at o'clock on the th of may, and at o'clock a.m. on the th of january. [page ] [illustration: fig. .--southern circumpolar constellations invisible north of the equator.] fig. gives the stars that are never seen by persons north of the earth's equator. in the ship is brilliant canopus, and the remarkable variable ae. below it is the beautiful southern cross, near the pole of the southern heavens. just below are the two first magnitude stars bungala, a, and achernar, b, of the centaur. such a number of unusually brilliant stars give the southern sky an unequalled splendor. in the midst of them, as if for contrast, is the dark hole, called by the sailors the "coal-sack," where even the telescope reveals no sign of light. here, also, are the two magellanic clouds, both easily discernible by the naked eye; the larger two hundred times the apparent size of the moon, lying between the pole and canopus, and the other between achernar and the pole. the smaller cloud is only one-fourth the size of the other. both are mostly resolvable into groups of stars from the fifth to the fifteenth magnitude. [page ] for easy out-door finding of the stars above the horizon at any time, see star-maps at end of the book. _characteristics of the stars._ such a superficial examination of stars as we have made scarcely touches the subject. it is as the study of the baptismal register, where the names were anciently recorded, without any knowledge of individuals. the heavens signify much more to us than to the greeks. we revolve under a dome that investigation has infinitely enlarged from their estimate. their little lights were turned by clumsy machinery, held together by material connections. our vast worlds are connected by a force so fine that it seems to pass out of the realm of the material into that of the spiritual. animal ferocity or a human hercules could image their idea of power. ours finds no symbol, but rises to the almighty. their heavens were full of fighting orions, wild bulls, chained andromedas, and devouring monsters. our heavens are significant of harmony and unity; all worlds carried by one force, and all harmonized into perfect music. all their voices blend their various significations into a personal speaking, which says, "hast thou not heard that the everlasting god, the lord, the creator of the ends of the earth, fainteth not, neither is weary?" there is no searching of his understanding. lift up your eyes on high, and behold who hath created all these things, that brought out their host by number, that calleth them all by their names in the greatness of his power; for that he is strong in power not one faileth. [page ] _number._ we find about five thousand stars visible to the naked eye in the whole heavens, both north and south. of these twenty are of the first magnitude, sixty-five of the second, two hundred of the third, four hundred of the fourth, eleven hundred of the fifth, and three thousand two hundred of the sixth. we think we can easily number the stars; but train a six-inch telescope on a little section of the twins, where six faint stars are visible, and over three thousand luminous points appear. the seventh magnitude has , stars; the eighth, , ; the ninth, , . there are , , stars in the zone called the milky way. when our eyes are not sensitive enough to be affected by the light of far-off stars the tastimetre feels their heat, and tells us the word of their maker is true--"they are innumerable."[*] [footnote *: _telescopic work._--look at the hyades and pleiades in taurus. notice the different colors of stars in them both. find the cluster præsepe in fig. , just a trifle above a point midway between procyon and regulus. it is equally distant from procyon and a point a little below pollux. sweep along the milky way almost anywhere, and observe the distribution of stars; in some places perfect crowds, in others more sparsely scattered. find with the naked eye the rich cluster in perseus. draw a line from algol to a of perseus (fig. ); turn at right angles to the right, at a distance of once and four-tenths the first line a brightness will be seen. the telescope reveals a gorgeous cluster.] _double and multiple stars._ if we look up during the summer months nearly overhead at the star e lyra, east of vega (fig. ), we shall see with the naked eye that the star appears a little [page ] elongated. turn your opera-glass upon it, and two stars appear. turn a larger telescope on this double star, and each of the components separate into two. it is a double double star. we know that if two stars are near in reality, and not simply apparently so by being in the same line of sight, they must revolve around a common centre of gravity, or rush to a common ruin. eagerly we watch to see if they revolve. a few years suffice to show them in actual revolution. nay, the movement of revolution has been decided before the companion star was discovered. sirius has long been known to have a proper motion, such as it would have if another sun were revolving about it. even the direction of the unseen body could always be indicated. in february, , alvan clark, artist, poet, and maker of telescopes (which requires even greater genius than to be both poet and artist), discovered the companion of sirius just in its predicted place. as a matter of fact, one of mr. clark's sons saw it first; but their fame is one. the time of revolution of this pair is fifty years. but one companion does not meet the conditions of the movements. here must also be one or more planets too small or dark to be seen. the double star x in the great bear (see fig. ) makes a revolution in fifty-eight years. procyon moves in an orbit which requires the presence of a companion star, but it has as yet eluded our search. castor is a double star; but a third star or planet, as yet undiscovered, is required to account for its perturbations. men who discovered neptune by the perturbations of uranus are capable of judging the cause of the perturbations of suns. we have spoken of [page ] the whole orbit of the earth being invisible from the stars. the nearest star in our northern hemisphere, cygni, is a telescopic double star; the constituent parts of it are forty-five times as far from each other as the earth is from the sun, yet it takes a large telescope to show any distance between the stars.[*] [footnote *: _telescopic work._--only such work will be laid out here as can be done by small telescopes of from two to four inch object-glasses. the numbers in fig. correspond to those of the table. -------------------------------------------------------------------- | | | |dist. of|magni-| | |no.| name. | fig. | parts. |tudes.| remarks. | |---|------------|-------------|--------|------|---------------------| | .| e lyræ | | ' " | |quadruple. | | .| z lyræ | | | & |topaz and green. | | .| b cygni | | - / | & |yellow and blue. | | .| cygni | | | & |nearest star but one.| | .| mizar | | | & |both white. | | .| polaris | | - / | & |test object of eye | | | | | | | and glass. | | .| r orionis |frontispiece.| | & |yellow and blue. | | .| b orionis | " | | & | rigel. | | .| d " | " | | & | red and white. | | .| th " | " | | |septuple. | | .| l " | " | | |white and violet. | | .| s " | " a, b.| | & |octuple. | | .| castor | | - / | & |white. | | .| pollux | | |triple|orange, gray, lilac. | | .| g virginis | | | & |both yellow. | -------------------------------------------------------------------- ] when g virginis was observed in by bradley, the component parts were " asunder. he incidentally remarked in his note-book that the line of their connection was parallel to the line of the two stars spica, or a and d virginis. by they were not more than " apart, and the line of their connection greatly changed. the appearance of the star is given in fig. ( ), commencing at the left, for the years ' ' ' ' ' ' and ' . also a conjectural [page ] orbit, placed obliquely, and the position of the stars at the times mentioned, commencing at the top. the time of its complete revolution is one hundred and fifty years. [illustration: fig. .--aspects and revolution of double stars.] the meaning of these double stars is that two or more suns revolve about their centre of gravity, as the moon and earth about their centre. if they have planets, as doubtless they have, the movement is no more complicated than the planets we call satellites of saturn revolving about their central body, and also about the sun. kindle saturn and jupiter to a blaze, or let out their possible light, and our system would appear a triple star in the distance. doubtless, in the far past, before these giant planets were cooled, it so appeared. we find some stars double, others triple, quadruple, octuple, and multiple. it is an extension of the same principles that govern our system. some of these suns are so far asunder that they can swing their neptunes between them, with less perturbation than uranus and neptune have in ours. light all our planets, and there would be a multiple star with more or less suns seen, [page ] according to the power of the instrument. perhaps the octuple star s in orion differs in no respect from our system, except in the size and distance of its separate bodies, and less cooling, either from being younger, or from the larger bodies cooling more slowly. suns are of all ages. infinite variety fills the sky. it is as preposterous to expect that every system or world should have analogous circumstances to ours at the present time, as to insist that every member of a family should be of the same age, and in the same state of development. there are worlds that have not yet reached the conditions of habitability by men, and worlds that have passed these conditions long since. let them go. there are enough left, and an infinite number in the course of preparation. some are fine and lasting enough to be eternal mansions. _colored stars._ in the cloudy morning we get only red light, but the sun is white. so aldebaran and betelguese may be girt by vapors, that only the strong red rays can pass. again, an iron moderately heated gives out dull red light; becoming hotter, it emits white light. sirius, regulus, vega, and spica may be white from greater intensity of vibration. procyon, capella, and polaris are yellow from less intensity of vibration. again, burn salt in a white flame, and it turns to yellow; mix alcohol and boracic acid, ignite them, and a beautiful green flame results; alcohol and nitrate of strontia give red flame; alcohol and nitrate of barytes give yellow flame. so the composition of a sun, or the special development of anyone substance thereof at any time, may determine the color of a star. [page ] the special glory of color in the stars is seen in the marked contrasts presented in the double and multiple stars. the larger star is usually white, still in the intensity of heat and vibration; the others, smaller, are somewhat cooled off, and hence present colors lower down the scale of vibration, as green, yellow, orange, and even red. that stars should change color is most natural. many causes would produce this effect. the ancients said sirius was red. it is now white. the change that would most naturally follow mere age and cooling would be from white, through various colors, to red. we are charmed with the variegated flowers of our gardens of earth, but he who makes the fields blush with flowers under the warm kisses of the sun has planted his wider gardens of space with colored stars. "the rainbow flowers of the footstool, and the starry flowers of the throne," proclaim one being as the author of them all. _clusters of stars._ from double and multiple we naturally come to groups and clusters. allusion has been made to the hyades, pleiades, etc. everyone has noticed the milky way. it seems like two irregular streams of compacted stars. it is not supposed that they are necessarily nearer together than the stars in the sparse regions about the pole. but the , , suns belonging to our system are arranged within a space represented by a flattened disk. if one hundred lights, three inches apart, are arranged on a hoop ten feet in diameter, they would be in a circle. add a thousand or two more the same distance apart, filling up the centre, and [page ] extending a few inches on each side of the inner plane of the hoop: an eye in the centre, looking out toward the edge, would see a milky way of lights; looking out toward the sides or poles, would see comparatively few. it would seem as if this oblate spheroidal arrangement was the result of a revolution of all the suns composing the system. jupiter and earth are flattened at the poles for the same reason. [illustration: fig. .--sprayed cluster below ae in hercules.] [illustration: fig. .--globular cluster.] in various parts of the heavens there are small globular well-defined clusters, and clusters very irregular in form, marked with sprays of stars. there is a cluster of this latter class in hercules, just under the s, in fig. . "probably no one ever saw it with a good telescope without a shout of wonder." here is a cluster of the former class represented in fig. . "the noble globular cluster, o centauri is beyond all comparison the richest and largest object of the kind in the heavens. its stars are literally innumerable; and as their total light, when received by the naked eye, affects it hardly more than a star of the fifth to fourth [page ] magnitude, the minuteness of each star may be imagined." there are two possibilities of thought concerning these clusters. either that they belong to our stellar system, and hence the stars must be small and young, or they are another universe of millions of suns, so far way that the inconceivable distances between the stars are shrunken to a hand's-breadth, and their unbearable splendor of innumerable suns can only make a gray haze at the distance at which we behold them. the latter is the older and grander thought; the former the newer and better substantiated. _nebulæ._ the gorgeous clusters we have been considering appear to the eye or the small telescope as little cloudlets of hazy light. one after another were resolved into stars; and the natural conclusion was, that all would yield and reveal themselves to be clustered suns, when we had telescopes of sufficient power. but the spectroscope, seeing not merely form but substance also, shows that some of them are not stars in any sense, but masses of glowing gas. two of these nebulæ are visible to the naked eye: one in andromeda (see fig. ), and one around the middle star of the sword of orion, shown in fig. . a three-inch telescope resolves th orionis into the famous trapezium, and a nine-inch instrument sees two stars more. the shape of the nebula is changeable, and is hardly suggestive of the moulding influence of gravitation. it is probably composed of glowing nitrogen and hydrogen gases. nebulæ are of all conceivable shapes--circular, annular, oval, lenticular, [page ] conical, spiral, snake-like, looped, and nameless. compare the sprays of the crab nebulæ above z tauri, seen in fig. , and the ring nebula, fig. . this last possibly consists of stars, and is situated, as shown in fig. , midway between b and g lyræ. [illustration: fig. .--the great nebula about the multiple star th orionis. (see frontispiece.)] when herschel was sweeping the heavens with his telescope, and saw but few stars, he often said to his assistant, "prepare to write; the nebulæ are coming." they are most abundant where the stars are least so. a zone about the heavens ° wide, with the milky way in the centre, would include one-fourth of the celestial sphere; but instead of one-fourth, we find nine-tenths [page ] of the stars in this zone, and but one-tenth of the nebulæ. these immense masses of unorganized matter are noticed to change their forms, vary their light greatly, but not quickly; they change through the ages. "god works slowly." he takes a thousand years to lift his hand off. [illustration: fig. .--crab nebula, near z tauri. (see frontispiece.)] there are many unsolved problems connected with these strange bodies. whether they belong to our system, or are beyond it, is not settled; the weight of evidence leans to the first view. [page ] _variable stars._ [illustration: fig. .--the ring nebula.] our sun gives a variable amount of light, changing through a period of eleven years. probably every star, if examined by methods sufficiently delicate and exact, would be found to be variable. the variations of some [page ] stars are so marked as to challenge investigation. b lyræ (fig. ) has two maxima and minima of light. in three days it rises from magnitude - / to - / ; in a week falls to , and rises to - / ; and in three days more drops to - / : it makes all these changes in thirteen days; but this period is constantly increasing. the variations of one hundred and forty-three stars have been well ascertained. [illustration: fig. .--constellation lyra, showing place of the ring nebula.] mira, or the wonderful, in the whale (fig. ), is easily found when visible. align from capella to the pleiades, and as much farther, and four stars will be seen, situated thus: * * * * the right-hand one is mira. for half a month it shines as a star of the second magnitude. then for three months it fades away, and lost to sight; going down even to the eleventh magnitude. but after five months its resurrection morning mes; and in three months more--eleven months in all--our wonderful is in its full glory in the heavens. it its period and brilliancy are also variable. the star megrez, d in the great bear, has been growing dim [page ] for a century. in betelguese was exceedingly variable, and continued so till , when the changes became much less conspicuous. algol (fig. ) has been already referred to. this slowly winking eye is of the second magnitude during d. h. then it dozes off toward sleep for h. m., when it is nearly invisible. it wakes up during the same time; so that its period from maximum brilliancy to the same state again is d. h. m. its recognizable changes are within five or six hours. as i write, march th, , algol gives its minimum light at h. m. p.m. it passes fifteen minima in d. m. there will therefore be another minimum may th, at h. m. its future periods are easy to estimate. perhaps it has some dark body revolving about it at frightful speed, in a period of less than three days. the period of its variability is growing shorter at an increasing rate. if its variability is caused by a dark body revolving about it, the orbit of that body is contracting, and the huge satellite will soon, as celestial periods are reckoned, commence to graze the surface of the sun itself, rebound again and again, and at length plunge itself into the central fire. such an event would evolve heat enough to make algol flame up into a star of the first magnitude, and perhaps out-blaze sirius or capella in our winter sky. none of the causes for these changes we have been able to conjecture seem very satisfactory. the stars may have opaque planets revolving about them, shutting off their light; they may rotate, and have unequally illuminated sides; they may revolve in very elliptical orbits, so as to greatly alter their distance from us; they may be so situated in regard to zones of meteorites as [page ] to call down periodically vast showers; but none or all of these suppositions apply to all cases, if they do to any. _temporary, new, and lost stars._ besides regular movements to right and left, up and down, to and from us--changes in the intensity of illumination by changes of distance--besides variations occurring at regular and ascertainable intervals, there are stars called _temporary_, shining awhile and then disappearing; _new_, coming to a definite brightness, and so remaining; and _lost_, those whose first appearance was not observed, but which have utterly disappeared. in november, , a new star blazed out in cassiopeia. its place is shown in fig. , ch g being the stars d * g ch in the seat of the chair, and d being the first one in the back. this star was visible at noonday, and was brighter than any other star in the heavens. in january, , it was less bright than jupiter; in april it was below the second magnitude, and the last of may it utterly disappeared. it was as variable in color as in brilliancy. during its first two months, the period of greatest brightness, it was dazzling white, then became yellow, and finally as red as mars or aldebaran, and so expired. a bright star was seen very near to the place of the _pilgrim_, as the star of was called, in a.d. and . a star of the tenth magnitude is now seen brightening slowly almost exactly in the same place. it is possible that this is a variable star of a period of about three hundred and ten years, and will blaze out again about . but we have had, within a few years, fine opportunities [page ] to study, with improved instruments, two new stars; on the evening of may th, , a star of the second magnitude was observed in the northern crown, where no star above the fifth magnitude had been twenty-four hours before. in argelander's chart a star of the tenth magnitude occupies the place. may th it had declined to the third magnitude, may th to the fourth, may th to the fifth, may th to the seventh, may st to the ninth, and has since diminished to the tenth. the spectroscope showed it to be a star in the usual condition; but through the usual colored spectrum, crossed with bright lines, shone four bright lines, two of which indicated glowing hydrogen. here was plenty of proof that an unusual amount of this gas had given this sun its sudden flame. as the hydrogen burned out the star grew dim. two theories immediately presented themselves: first, that vast volumes had been liberated from within the orb by some sudden breaking up of the doors of its great deeps; or, second, this star had precipitated upon itself, by attraction, some other sun or planet, the force of whose impact had been changed into heat. though we see the liberated hydrogen of our sun burst up with sudden flame, it can hardly be supposed that enough could be liberated at once to increase the light and heat one hundred-fold. in regard to the second theory, it is capable of proof that two suns half as large as ours, moving at a velocity of four hundred and seventy-six miles per second, would evolve heat enough to supply the radiation of our sun for fifty million years. how could it be possible for a sun like this newly blazing orb to cool off to such a [page ] degree in a month? besides, there would not be one chance in a thousand for two orbs to come directly together. they would revolve about each other till a kind of grazing contact of grinding worlds would slowly kindle the ultimate heat. it is far more likely that this star encountered an enormous stream of meteoric bodies, or perhaps absorbed a whole comet, that laid its million leagues of tail as fuel on the central fire. only let it be remembered that the fuel is far more force than substance. allusion has already been made to the sudden brightening of our sun on the first day of september, . that was caused, no doubt, by the fall of large meteors, following in the train of the comet of , or some other comet. what the effect would have been, had the whole mass of the comet been absorbed, cannot be imagined. another new star lately appeared in cygnus, near the famous star --the first star in the northern hemisphere whose distance was determined. it was first seen november th, , as a third magnitude star of a yellow color. by december d it had sunk to the fourth magnitude, and changed to a greenish color. it had then three bright hydrogen lines, the strong double sodium line, and others, which made, it strongly resemble the spectrum of the chromosphere of our sun. an entirely different result appeared in the fading of these two stars. in the case of the star in the crown, the extraordinary light was the first to fade, leaving the usual stellar spectrum. in the case of the star in cygnus, the part of the spectrum belonging to stellar light was the first to fade, leaving the bright lines; that is, the gas of one gave way to regular starlight, and the starlight [page ] of the other having faded, the regular light of the glowing gas continued. by some strange oversight, no one studied the star again for six months. in september and november, , the light of this star was found to be blue, and not to be starlight at all. it had no rainbow spectrum, only one kind of rays, and hence only one color. its sole spectroscopic line is believed to be that of glowing nitrogen gas. we have then, probably, in the star of , a body shining by a feeble and undiscernible light, surrounded by a discernible immensity of light of nitrogen gas. this is its usual condition; but if a flight of meteors should raise the heat of the central body so as to outshine the nebulous envelope, we should have the conditions we discovered in november, . but a rapid cooling dissipates the observable light of all colors, and leaves only the glowing gas of one color. _movements of stars._ we call the stars _fixed_, but motion and life are necessary to all things. besides the motion in the line of sight described already, there is motion in every other conceivable direction. we knew sirius moved before we had found the cause. we know that our sun moves back and forth in his easy bed one-half his vast diameter, as the larger planets combine their influence on one side or the other. the sun has another movement. we find the stars in hercules gradually spreading from each other. hercules's brawny limbs grow brawnier every century. there can be but one cause: we are approaching that quarter of the heavens. (see [symbol], fig. .) we are even [page ] able to compute the velocity of our approach; it is four miles a second. the stars in the opposite quarter of the heavens in argo are drawing nearer together. this movement would have no effect on the apparent place of the stars at either pole, if they were all equally distant; but it must greatly extend or contract the apparent space between them, since they are situated at various distances. independent of this, the stars themselves are all in motion, but so vast is the distance from which we observe them that it has taken an accumulation of centuries before they could be made measurable. a train going forty miles an hour, seen from a distance of two miles, almost seems to stand still. arcturus moves through space three times as fast as the earth, but it takes a century to appear to move the eighth part of the diameter of the moon. there is a star in the hunting dogs, known as groombridge, which has a velocity beyond what all the attraction of the matter of the known universe could give it. by the year it may be in berenice's hair. some stars have a common movement, being evidently related together. a large proportion of the brighter stars between aldebaran and the pleiades have a common motion eastward of about ten seconds a century. all the angles marked by a, b, g, ch orionis will be altered in different directions; l is moving toward g. l and e will appear as a double star. in a.d. , procyon will be nearer ch orionis than rigel now is, and sirius will be in line with a and ch orionis. all the stars of the great dipper, except benetnasch and dubhe, have a common motion somewhat in the direction [page ] of thuban (fig. ), while the two named have a motion nearly opposite. in , years the end of the dipper will have fallen out so that it will hold no water, and the handle will be broken square off at mizar. "the southern cross," says humboldt, "will not always keep its characteristic form, for its four stars travel in different directions with unequal velocities. at the present time it is not known how many myriads of years must elapse before its entire dislocation." these movements are not in fortuitous or chaotic ways, but are doubtless in accordance with some perfect plan. we have climbed up from revolving earth and moon to revolving planets and sun, in order to understand how two or ten suns can revolve about a common centre. let us now leap to the grander idea that all the innumerable stars of a winter night not only loan, but must revolve about some centre of gravity. men have been looking for a central sun of suns, and have not found it. none is needed. two suns can balance about a point; all suns can swing about a common centre. that one unmoving centre may be that city more gorgeous than eastern imagination ever conceived, whose pavement is transparent gold, whose walls are precious stones, whose light is life, and where no dark planetary bodies ever cast shadows. there reigns the king and lord of all, and ranged about are the far-off provinces of his material systems. they all move in his sight, and receive power from a mind that never wearies. [page ] xi. the worlds and the word. "the worlds were framed by the word of god."--_heb._ xi., . [page ] "mysterious night! when our first parent knew thee from report divine, and heard thy name, did he not tremble for this lovely frame, this glorious canopy of light and blue? yet, 'neath a curtain of translucent dew, bathed in the rays of the great setting flame, hesperus, with all the host of heaven, came, and lo! creation widened in man's view. who could have thought such darkness lay concealed within thy beams, o sun! oh who could find, whilst fruit and leaf and insect stood revealed, that to such countless worlds thou mad'st us blind! why do we then shun death with anxious strife? if light conceal so much, wherefore not life?" blanco white. [page ] xi. _the worlds and the word._ men have found the various worlds to be far richer than they originally thought. they have opened door after door in their vast treasuries, have ascended throne after throne of power, and ruled realms of increasing extent. we have no doubt that unfoldings in the future will amaze even those whose expectations have been quickened by the revealings of the past. what if it be found that the word is equally inexhaustible? after ages of thought and discovery we have come out of the darkness and misconceptions of men. we believe in no serpent, turtle, or elephant supporting the world; no atlas holding up the heavens; no crystal domes, "with cycles and epicycles scribbled o'er." what if it be found that one book, written by ignorant men, never fell into these mistakes of the wisest! nay, more, what if some of the greatest triumphs of modern science are to be found plainly stated in a book older than the writings of homer? if suns, planets, and satellites, with all their possibilities of life, changes of flora and fauna, could be all provided for, as some scientists tell us, in the fiery star-dust of a cloud, why may not the same author provide a perpetually widening river of life in his word? as we believe he is perpetually present in his worlds, we know he has [page ] promised to be perpetually present in his word, making it alive with spirit and life. the wise men of the past could not avoid alluding to ideas the falsity of which subsequent discovery has revealed; but the writers of the bible did avoid such erroneous allusion. of course they referred to some things, as sunrise and sunset, according to appearance; but our most scientific books do the same to-day. that the bible could avoid teaching the opposite of scientific truth proclaims that a higher than human wisdom was in its teaching. that negative argument is strong, but the affirmative argument is much stronger. the bible declares scientific truth far in advance of its discovery, far in advance of man's ability to understand its plain declarations. take a few conspicuous illustrations: the bible asserted from the first that the present order of things had a beginning. after ages of investigation, after researches in the realms of physics, arguments in metaphysics, and conclusions by the necessities of resistless logic, science has reached the same result. the bible asserted from the first that creation of matter preceded arrangement. it was chaos--void--without form--darkness; arrangement was a subsequent work. the world was not created in the form it was to have; it was to be moulded, shaped, stratified, coaled, mountained, valleyed, subsequently. all of which science utters ages afterward. the bible did not hesitate to affirm that light existed before the sun, though men did not believe it, and used it as a weapon against inspiration. now we praise men for having demonstrated the oldest record. [page ] it is a recently discovered truth of science that the trata of the earth were formed by the action of water, and the mountains were once under the ocean. it is an idea long familiar to bible readers: "thou coverest the earth with the deep as with a garment. the waters stood above the mountains. at thy rebuke they fled; at the voice of thy thunder they hasted away. the mountains ascend; the valleys descend into the place thou hast founded for them." here is a whole volume of geology in a paragraph. the thunder of continental convulsions is god's voice; the mountains rise by god's power; the waters haste away unto the place god prepared for them. our slowness of geological discovery is perfectly accounted for by peter. "for of this they are _willingly ignorant_, that by the word of god there were heavens of old, and land framed out of water, and by means of water, whereby the world that then was, being overflowed by water, perished." we recognize these geological subsidences, but we read them from the testimony of the rocks more willingly than from the testimony of the word. science exults in having discovered what it is pleased to call an order of development on earth--tender grass, herb, tree; moving creatures that have life in the waters; bird, reptile, beast, cattle, man. the bible gives the same order ages before, and calls it god's successive creations. during ages on ages man's wisdom held the earth to be flat. meanwhile, god was saying, century after century, of himself, "he sitteth upon the sphere of the earth" (gesenius). men racked their feeble wits for expedients to uphold [page ] the earth, and the best they could devise were serpents, elephants, and turtles; beyond that no one had ever gone to see what supported them. meanwhile, god was perpetually telling men that he had hung the earth upon nothing. men were ever trying to number the stars. hipparchus counted one thousand and twenty-two; ptolemy one thousand and twenty-six; and it is easy to number those visible to the naked eye. but the bible said, when there were no telescopes to make it known, that they were as the sands of the sea, "innumerable." science has appliances of enumeration unknown to other ages, but the space-penetrating telescopes and tastimeters reveal more worlds--eighteen millions in a single system, and systems beyond count--till men acknowledge that the stars are innumerable to man. it is god's prerogative "to number all the stars; he also calleth them all by their names." torricelli's discovery that the air had weight was received with incredulity. for ages the air had propelled ships, thrust itself against the bodies of men, and overturned their works. but no man ever dreamed that weight was necessary to give momentum. during all the centuries it had stood in the bible, waiting for man's comprehension: "he gave to the air its weight" (job xxviii. ). the pet science of to-day is meteorology. the fluctuations and variations of the weather have hitherto baffled all attempts at unravelling them. it has seemed that there was no law in their fickle changes. but at length perseverance and skill have triumphed, and a single man in one place predicts the weather and winds [page ] for a continent. but the bible has always insisted that the whole department was under law; nay, it laid down that law so clearly, that if men had been willing to learn from it they might have reached this wisdom ages ago. the whole moral law is not more clearly crystallized in "thou shalt love the lord thy god with all thy heart, and thy neighbor as thyself," than all the fundamentals of the science of meteorology are crystallized in these words: "the wind goeth toward the south (equator), and turneth about (up) unto the north; it whirleth about continually, and the wind returneth again according to his circuits (established routes). all the rivers run into the sea; yet the sea is not full: unto the place from whence the rivers come, thither they return again" (eccles. i. , ). those scientific queries which god propounded to job were unanswerable then; most of them are so now. "whereon are the sockets of the earth made to sink?" job never knew the earth turned in sockets; much less could he tell where they were fixed. god answered this question elsewhere. "he stretcheth the north (one socket) over the empty place, and hangeth the earth upon nothing." speaking of the day-spring, god says the earth is _turned_ to it, as clay to the seal. the earth's axial revolution is clearly recognized. copernicus declared it early; god earlier. no man yet understands the balancing of the clouds, nor the suspension of the frozen masses of hail, any more than job did. had god asked if he had perceived the _length_ of the earth, many a man to-day could have answered yes. but the eternal ice keeps us from perceiving the _breadth_ [page ] of the earth, and shows the discriminating wisdom of the question. the statement that the sun's going is from the end of the heaven, and his circuit to the ends of it, has given edge to many a sneer at its supposed assertion that the sun went round the earth. it teaches a higher truth--that the sun itself obeys the law it enforces on the planets, and flies in an orbit of its own, from one end of heaven in argo to the other in hercules. so eminent an astronomer and so true a christian as general mitchell, who understood the voices in which the heavens declare the glory of god, who read with delight the word of god em bodied in worlds, and who fed upon the written word of god as his daily bread, declared, "we find an aptness and propriety in all these astronomical illustrations, which are not weakened, but amazingly strengthened, when viewed in the clear light of our present knowledge." herschel says, "all human discoveries seem to be made only for the purpose of confirming more strongly the truths that come from on high, and are contained in the sacred writings." the common authorship of the worlds and the word becomes apparent; their common unexplorable wealth is a necessary conclusion. since the opening revelations of the past show an unsearchable wisdom in the word, has that word any prophecy concerning mysteries not yet understood, and events yet in the future? there are certain problems as yet insolvable. we have grasped many clews, and followed them far into labyrinths of darkness, but not yet through into light. we ask in vain, "what is matter?" no man can [page ] answer. we trace it up through the worlds, till its increasing fineness, its growing power, and possible identity of substance, seem as if the next step would reveal its spirit origin. what we but hesitatingly stammer, the word boldly asserts. we ask, "what is force?" no man can answer. we recognize its various grades, each subordinate to the higher--cohesion dissolvable by heat; the affinity of oxygen and hydrogen in water overcome by the piercing intensity of electric fire; rivers seeking the sea by gravitation carried back by the sun; rock turned to soil, soil to flowers; and all the forces in nature measurably subservient to mind. hence we partly understand what the word has always taught us, that all lower forces must be subject to that which is highest. how easily can seas be divided, iron made to swim, water to burn, and a dead body to live again, if the highest force exert itself over forces made to be mastered. when we have followed force to its highest place, we always find ourselves considering the forces of mind and spirit, and say, in the words of the scriptures, "god is spirit." we ask in vain what is the end of the present condition of things. we have read the history of our globe with great difficulty--its prophecy is still more difficult. we have asked whether the stars form a system, and if so, whether that system is permanent. we are not able to answer yet. we have said that the sun would in time become as icy cold and dead as the moon, and then the earth would wander darkling in the voids of space. but the end of the earth, as prophesied in the word, is different: "the heavens will pass away with [page ] a rushing noise, and the elements will be dissolved with burning heat, and the earth and the works therein will be burned up." the latest conclusions of science point the same way. the great zones of uncondensed matter about the sun seem to constitute a resisting medium as far as they reach. encke's comet, whose orbit comes near the sun, is delayed. this gives gravitation an overwhelming power, and hence the orbit is lessened and a revolution accomplished more quickly. faye's comet, which wheels beyond the track of mars, is not retarded. if the earth moves through a resisting substance, its ultimate fall into the sun is certain. whether in that far future the sun shall have cooled off, or will be still as hot as to-day, peter's description would admirably portray the result of the impact. peters description, however, seems rather to indicate an interference of divine power at an appropriate time before a running down of the system at present in existence, and a re-endowment of matter with new capabilities. after thousands of years, science discovered the true way to knowledge. it is the baconian way of experiment, of trial, of examining the actual, instead of imagining the ideal. it is the acceptance of the scriptural plan. "if a man wills to do god's will, he shall know." oh taste and see! in science men try hypotheses, think the best they can, plan broadly as possible, and then see if facts sustain the theory. they have adopted the scriptural idea of accepting a plan, and then working in faith, in order to acquire knowledge. fortunately, in the work of salvation the plan is always perfect. but, in order to make the trial under the most favorable circumstances, there must be faith. the faith of [page ] science is amazing; its assertions of the supersensual are astounding. it affirms a thousand things that cannot be physically demonstrated: that the flight of a rifle-ball is parabolic; that the earth has poles; that gages are made of particles; that there are atoms; that an electric light gives ten times as many rays as are visible; that there are sounds to which we are deaf, sights to which we are blind; that a thousand objects and activities are about us, for the perception of which we need a hundred senses instead of five. these faiths have nearly all led to sight; they have been rewarded, and the world's wealth of knowledge is the result. the word has ever asserted the supersensuous, solicited man's faith, and ever uplifted every true faith into sight. lowell is partly right when he sings: "science was faith once; faith were science now, would she but lay her bow and arrows by, and aim her with the weapons of the time." faith laid her bow and arrows by before men in pursuit of worldly knowledge discovered theirs. what becomes of the force of the sun that is being spent to-day? it is one of the firmest rocks of science that there can be no absolute destruction of force. it is all conserved somehow. but how? the sun contracts, light results, and leaps swiftly into all encircling space. it can never be returned. heat from stars invisible by the largest telescope enters the tastimeter, and declares that that force has journeyed from its source through incalculable years. there is no encircling dome to reflect all this force back upon its sources. is it lost? science, in defence of its own dogma, should [page ] assign light a work as it flies in the space which we have learned cannot be empty. there ought to be a realm where light's inconceivable energy is utilized in building a grander universe, where there is no night. christ said, as he went out of the seen into the unseen, "i go to prepare a place for you;" and when john saw it in vision the sun had disappeared, the moon was gone, but the light still continued. science finds matter to be capable of unknown refinement; water becomes steam full of amazing capabilities: we add more heat, superheat the steam, and it takes on new aptitudes and uncontrollable energy. zinc burned in acid becomes electricity, which enters iron as a kind of soul, to fill all that body with life. all matter is capable of transformation, if not transfiguration, till it shines by the light of an indwelling spirit. scripture readers know that bodies and even garments can be transfigured, be made astrapton (luke xxiv. ), shining with an inner light. they also look for new heavens and a new earth endowed with higher powers, fit for perfect beings. when god made matter, so far as our thought permits us to know, he simply made force stationary and unconscious. thereafter he moves through it with his own will. he can at any time change these forces, making air solid, water and rock gaseous, a world a cloud, or a fire-mist a stone. he may at some time restore all force to consciousness again, and make every part of the universe thrill with responsive joy. "then shall the mountains and the hills break forth before you into singing, and all the trees of the field clap their hands." one of these changes is to come to the earth. [page ] amidst great noise the heaven shall flee, the earth be burned up, and all their forces be changed to new forms. perhaps it will not then be visible to mortal eyes. perhaps force will then be made conscious, and the flowers thereafter return our love as much as lower creatures do now. a river and tree of life may be consciously alive, as well as give life. poets that are nearest to god are constantly hearing the sweet voices of responsive feeling in nature. "for his gayer hours she has a voice of gladness and a smile, and eloquence of beauty; and she glides into his darker musings with a mild and gentle sympathy, that steals away their sharpness ere he is aware." prophets who utter god's voice of truth say, "the wilderness and the solitary place shall be glad for holy men, and the desert shall rejoice and blossom as the rose. it shall blossom abundantly and rejoice, even with joy and singing." distinguish clearly between certainty and surmise. the certainty is that the world will pass through catastrophic changes to a perfect world. the grave of uniformitarianism is already covered with grass. he that creates promises to complete. the invisible, imponderable, inaudible ether is beyond our apprehension; it transmits impressions , miles a second; it is millions of times more capable and energetic than air. what may be the bounds of its possibility none can imagine, for law is not abrogated nor designs disregarded as we ascend into higher realms. law works out more beautiful designs with more absolute certainty. why [page ] should there not be a finer universe than this, and disconnected from this world altogether--a fit home for immortal souls? it is a necessity. god filleth all in all, is everywhere omnipotent and wise. why should there be great vacuities, barren of power and its creative outgoings? god has fixed the stars as proofs of his agency at some points in space. but is it in points only? science is proud of its discovery that what men once thought to be empty space is more intensely active than the coarser forms of matter can be. but in the long times which are past job glanced at earth, seas, clouds, pillars of heaven, stars, day, night, all visible things, and then added: "lo! these are only the outlying borders of his works. what a whisper of a word we hear of _him!_ the thunder of his power who can comprehend?" science discovers that man is adapted for mastery in this world. he is of the highest order of visible creatures. neither is it possible to imagine an order of beings generically higher to be connected with the conditions of the material world. this whole secret was known to the author of the oldest writing. "and god blessed them, and god said unto them: be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth." the idea is never lost sight of in the sacred writings. and while every man knows he must fail in one great contest, and yield himself to death, the later portions of the divine word offer him victory even here. the typical man is commissioned to destroy even death, and make man a sharer in the victory. [page ] science babbles at this great truth of man's position like a little child; scripture treats it with a breadth of perfect wisdom we are only beginning to grasp. science tells us that each type is prophetic of a higher one. the whale has bones prophetic of a human hand. has man reached perfection? is there no prophecy in him? not in his body, perhaps; but how his whole soul yearns for greater beauty. as soon as he has found food, the savage begins to carve his paddle, and make himself gorgeous with feathers. how man yearns for strength, subduing animal and cosmic forces to his will! how he fights against darkness and death, and strives for perfection and holiness! these prophecies compel us to believe there is a world where powers like those of electricity and luminiferous ether are ever at hand; where its waters are rivers of life, and its trees full of perfect healing, and from which all unholiness is forever kept. what we infer, scripture affirms. science tells us there has been a survival of the fittest. doubtless this is so. so in the future there will be a survival of the fittest. what is it? wisdom, gentleness, meekness, brotherly kindness, and charity. over those who have these traits death hath no permanent power. the caterpillar has no fear as he weaves his own shroud; for there is life within fit to survive, and ere long it spreads its gorgeous wings, and flies in the air above where once it crawled. man has had two states of being already. one confined, dark, peculiarly nourished, slightly conscious; then he was born into another--wide, differently nourished, and intensely [page ] conscious. he knows he may be born again into a life wider yet, differently nourished, and even yet more intensely conscious. science has no hint how a long ascending series of developments crowned by man may advance another step, and make man isaggelos--equal to angels. but the simplest teaching of scripture points out a way so clear that a child need not miss the glorious consummation. when uranus hastened in one part of its orbit, and then retarded, and swung too wide, men said there must be another attracting world beyond; and, looking there, neptune was found. so, when individual men are so strong that nations or armies cannot break down their wills; so brave, that lions have no terrors; so holy, that temptation cannot lure nor sin defile them; so grand in thought, that men cannot follow; so pure in walk, that god walks with them--let us infer an attracting world, high and pure and strong as heaven. the eleventh chapter of hebrews is a roll-call of heroes of whom this world was not worthy. they were tortured, not accepting deliverance, that they might obtain a better resurrection. the world to come influenced, as it were, the orbits of their souls, and when their bodies fell off, earth having no hold on them, they sped on to their celestial home. the tendency of such souls necessitates such a world. the worlds and the word speak but one language, teach but one set of truths. how was it possible that the writers of the earlier scriptures described physical phenomena with wonderful sublimity, and with such penetrative truth? they gazed upon the same heaven that those men saw who ages afterward led the world in knowledge. these latter were near-sighted, and absorbed [page ] in the pictures on the first veil of matter; the former were far-sighted, and penetrated a hundred strata of thickest material, and saw the immaterial power behind. the one class studied the present, and made the gravest mistakes; the other pierced the uncounted ages of the past, and uttered the profoundest wisdom. there is but one explanation. he that planned and made the worlds inspired the word. science and religion are not two separate departments, they are not even two phases of the same truth. science has a broader realm in the unseen than in the seen, in the source of power than in the outcomes of power, in the sublime laws of spirit than in the laws of matter; and religion sheds its beautiful light over all stages of life, till, whether we eat or whether we drink, or whatsoever we do, we may do all for the glory of god. science and religion make common confession that the great object of life is to learn and to grow. both will come to see the best possible means, for the attainment of this end is a personal relation to a teacher who is the way, the truth, and the life. [page ] xii. the ultimate force. "in the beginning was the word, and the word was with god, and the word was god. the same was in the beginning with god. all things became by him, and without him was not anything made that was made * * * and by him all things stand together." [page ] "o thou eternal one; whose presence blight all space doth occupy--all motion guide-- thou from primeval nothingness didst call first chaos, then existence. lord, on thee eternity had its foundation: all sprung forth from thee--of light, joy, harmony, sole origin: all life, all beauty thine. thy word created all, and doth create; thy splendor fills all space with rays divine; thou art and wert, and shalt be glorious, great; life-giving, life-sustaining potentate, thy chains the unmeasured universe surround-- upheld by thee, by thee inspired with breath." derzhavin. [page ] xii. _the ultimate force._ the universe is god's name writ large. thought goes up the shining suns as golden stairs, and reads the consecutive syllables--all might, and wisdom, and beauty; and if the heart be fine enough and pure enough, it also reads everywhere the mystic name of love. let us learn to read the hieroglyphics, and then turn to the blazonry of the infinite page. that is the key-note; the heavens and the earth declaring the glory of god, and men with souls attuned listening. to what voices shall we listen first? stand on the shore of a lake set like an azure gem among the bosses of green hills. the patter of rain means an annual fall of four cubic feet of water on every square foot of it. it weighs two hundred and forty pounds to the cubic foot, one hundred million tons on the surface of a little sheet of water twenty miles long by three wide. now, all that weight of falling rain had to be lifted, a work compared to which taking up mountains and casting them into the sea is pastime. all that water had to be taken up before it could be cast down, and carried hundreds of miles before it could be there. you have heard niagara's thunder; have stood beneath the falling immensity; seen it ceaselessly poured from an infinite hand; felt that you would be ground to atoms if you fell into that resistless flood. well, all that infinity of [page ] water had to be lifted by main force, had to be taken up out of the far pacific, brought over the rocky mountains; and the mississippi keeps bearing its wide miles of water to the gulf, and niagara keeps thundering age after age, because there is power somewhere to carry the immeasurable floods all the time the other way in the upper air. but this is only the alpha of power. professor clark, of amherst, massachusetts, found that such a soft and pulpy thing as a squash had so great a power of growth that it lifted three thousand pounds, and held it day and night for months. it toiled and grew under the growing weight, compacting its substance like oak to do the work. all over the earth this tremendous power and push of life goes on--in the little star-eyed flowers that look up to god only on the alpine heights, in every tuft of grass, in every acre of wheat, in every mile of prairie, and in every lofty tree that wrestles with the tempests of one hundred winters. but this is only the b in the alphabet of power. rise above the earth, and you find the worlds tossed like playthings, and hurled seventy times as fast as a rifle-ball, never an inch out of place or a second out of time. but this is only the c in the alphabet of power. rise to the sun. it is a quenchless reservoir of high-class energy. our tornadoes move sixty miles an hour, those of the sun twenty thousand miles an hour. a forest on fire sends its spires of flame one hundred feet in air, the sun sends its spires of flame two hundred thousand miles. all our fires exhaust the fuel and burn out. if the sun were pure coal, it would burn out in five thousand years; and yet this sea of unquenchable [page ] flame seethes and burns, and rolls and vivifies a dozen worlds, and flashes life along the starry spaces for a million years without any apparent diminution. it sends out its power to every planet, in the vast circle in which it lies. it fills with light not merely a whole circle, but a dome; not merely a dome above, but one below, and on every side. at our distance of ninety-two and a half millions of miles, the great earth feels that power in gravitation, tides, rains, winds, and all possible life--every part is full of power. fill the earth's orbit with a circle of such receptive worlds--seventy thousand instead of one--everyone would be as fully supplied with power from this central source. more. fill the whole dome, the entire extent of the surrounding sphere, bottom, sides, top, a sphere one hundred and eighty-five million miles in diameter, and everyone of these uncountable worlds would be touched with the same power as one; each would thrill with life. this is only the d of the alphabet of power. and glancing up to the other suns, one hundred, five hundred, twelve hundred times as large, double, triple, septuple, multiple suns, we shall find power enough to go through the whole alphabet in geometrical ratio; and then in the clustered suns, galaxies, and nebulæ, power enough still unrepresented by single letters to require all combinations of the alphabet of power. what is the significance of this single element of power? the answer of science to-day is "correlation," the constant evolution of one force from another. heat is a mode of motion, motion a result of heat. so far so good. but are we mere reasoners in a circle? then we would be lost men, treading our round of death in a limitless forest. what is the ultimate? reason [page ] out in a straight line. no definition of matter allows it to originate force; only mind can do that. hence the ultimate force is always mind. carry your correlation as far as you please--through planets, suns, nebulæ, concretionary vortices, and revolving fire-mist--there must always be mind and will beyond. some of that willpower that works without exhaustion must take its own force and render it static, apparent. it may do this in such correlated relation that that force shall go on year after year to a thousand changing forms; but that force must originate in mind. go out in the falling rain, stand under the thunderous niagara, feel the immeasurable rush of life, see the hanging worlds, and trace all this--the carried rain, the terrific thunder with god's bow of peace upon it, and the unfailing planets hung upon nothing--trace all this to the orb of day blazing in perpetual strength, but stop not there. who _made_ the sun? contrivance fills all thought. _who_ made the sun? nature says there is a mind, and that mind is almighty. then you have read the first syllables, viz., being and power. what is the continuous relation of the universe to the mind from which it derived its power? some say that it is the relation of a wound-up watch to the winder. it was dowered with sufficient power to revolve its ceaseless changes, and its maker is henceforth an absentee god. is it? let us have courage to see. for twenty years one devotes ten seconds every night to putting a little force into a watch. it is so arranged that it distributes that force over twenty-four hours. in that twenty years more power has been put into that watch than a horse could exert at once. but suppose [page ] one had tried to put all that force into the watch at once: it would have pulverized it to atoms. but supposing the universe had been dowered with power at first to run its enormous rounds for twenty millions of years. it is inconceivable; steel would be as friable as sand, and strengthless as smoke, in such strain. we have discovered some of the laws of the force we call gravitation. but what do we know of its essence? how it appears to act we know a little, what it is we are profoundly ignorant. few men ever discuss this question. all theories are sublimely ridiculous, and fail to pass the most primary tests. how matter can act where it is not, and on that with which it has no connection, is inconceivable. newton said that anyone who has in philosophical matters a competent faculty of thinking, could not admit for a moment the possibility of a sun reaching through millions of miles, and exercising there an attractive power. a watch may run if wound up, but how the watch-spring in one pocket can run the watch in another is hard to see. a watch is a contrivance for distributing a force outside of itself, and if the universe runs at all on that principle, it distributes some force outside of itself. le sage's theory of gravitation by the infinitive hail of atoms cannot stand a minute, hence we come back as a necessity of thought to herschel's statement. "it is but reasonable to regard gravity as a result of a consciousness and a will existent somewhere." where? i read an old book speaking of these matters, and it says of god, he hangeth the earth upon nothing; he upholdeth constantly all things by the word of his power. [page ] by him all things consist or hold together. it teaches an imminent mind; an almighty, constantly exerted power. proof of this starts up on every side. there is a recognized tendency in all high-class energy to deteriorate to a lower class. there is steam in the boiler, but it wastes without fuel. there is electricity in the jar, but every particle of air steals away a little, unless our conscious force is exerted to regather it. there is light in the sun, but infinite space waits to receive it, and takes it swift as light can leap. we said that if the sun were pure coal, it would burn out in five thousand years, but it blazes undimmed by the million. how can it? there have been various theories: chemical combustion, it has failed; meteoric impact, it is insufficient; condensation, it is not proved; and if it were, it is an intermediate step back to the original cause of condensation. the far-seeing eyes see in the sun the present active power of him who first said, "let there be light," and who at any moment can meet a saul in the way to damascus with a light above the brightness of the sun--another noon arisen on mid-day; and of whom it shall be said in the eternal state of unclouded brightness, where sun and moon are no more, "the glory of the lord shall lighten it, and the lamb is the light thereof." but suppose matter could be dowered, that worlds could have a gravitation, one of two things must follow: it must have conscious knowledge of the position, exact weight, and distance of every atom, mass, and world, in order to proportion the exact amount of gravity, or it must fill infinity with an omnipresent attractive power, pulling in myriads of places at nothing; in [page ] a few places at worlds. every world must exert an infinitely extended power, but myriads of infinities cannot be in the same space. the solution is, one infinite power and conscious will. to see the impossibility of every other solution, join in the long and microscopic hunt for the ultimate particle, the atom; and if found, or if not found, to a consideration of its remarkable powers. bring telescopes and microscopes, use all strategy, for that atom is difficult to catch. make the first search with the microscope: we can count , lines ruled on a glass plate inside of an inch. but we are here looking at mountain ridges and valleys, not atoms. gold can be beaten to the / of an inch. it can be drawn as the coating of a wire a thousand times thinner, to the / of an inch. but the atoms are still heaped one upon another. take some of the infusorial animals. alonzo gray says millions of them would not equal in bulk a grain of sand. yet each of them performs the functions of respiration, circulation, digestion, and locomotion. some of our blood-vessels are not a millionth of our size. what must be the size of the ultimate particles that freely move about to nourish an animal whose totality is too small to estimate? a grain of musk gives off atoms enough to scent every part of the air of a room. you detect it above, below, on every side. then let the zephyrs of summer and the blasts of winter sweep through that room for forty years, bearing out into the wide world miles on miles of air, all perfumed from the atoms of that grain of musk, and at the end of the forty years the weight of musk has not appreciably diminished. [page ] yet uncountable myriads on myriads of atoms have gone. our atom is not found yet. many are the ways of searching for it which we cannot stop to consider. we will pass in review the properties with which materialists preposterously endow it. it is impenetrable and indivisible, though some atoms are a hundred times larger than others. each has definite shape; some one shape, and some another. they differ in weight, in quantity of combining power, in quality of combining power. they combine with different substances, in certain exact assignable quantities. thus one atom of hydrogen combines with eighty of bromine, one hundred and sixty of mercury, two hundred and forty of boron, three hundred and twenty of silicon, etc. hence our atom of hydrogen must have power to count, or at least to measure, or be cognizant of bulk. again, atoms are of different sorts, as positive or negative to electric currents. they have power to take different shapes with different atoms in crystallization; that is, there is a power in them, conscious or otherwise, that the same bricks shall make themselves into stables or palaces, sewers or pavements, according as the mortar varies. "no, no," you cry out; "it is only according as the builder varies his plan." there is no need to rehearse these powers much further; though not one-tenth of the supposed innate properties of this infinitesimal infinite have been recited--properties which are expressed by the words atomicity, quantivilence, monad, dryad, univalent, perissad, quadrivalent, and twenty other terms, each expressing some endowment of power in this in visible atom. refer to one more presumed ability, an ability [page ] to keep themselves in exact relation of distance and power to each other, without touching. it is well known that water does not fill the space it occupies. we can put eight or ten similar bulks of different substances into a glass of water without greatly increasing its bulk, some actually diminishing it. a philosopher has said that the atoms of oxygen and hydrogen are probably not nearer to each other in water than one hundred and fifty men would be if scattered over the surface of england, one man to four hundred square miles. the atoms of the luminiferous ether are infinitely more diffused, and yet its interactive atoms can give four hundred millions of light-waves a second. and now, more preposterous than all, each atom has an attractive power for every other atom of the universe. the little mote, visible only in a sunbeam streaming through a dark room, and the atom, infinitely smaller, has a grasp upon the whole world, the far-off sun, and the stars that people infinite space. the sage of concord advises you to hitch your wagon to a star. but this is hitching all stars to an infinitesimal part of a wagon. such an atom, so dowered, so infinite, so conscious, is an impossible conception. but if matter could be so dowered as to produce such results by mechanism, could it be dowered to produce the results of intelligence? could it be dowered with power of choice without becoming mind? if oxygen and hydrogen could be made able to combine into water, could the same unformed matter produce in one case a plant, in another a bird, in a third a man; and in each of these put bone, brain, blood, and nerve in [page ] proper relations? matter must be mind, or subject to a present working mind, to do this. there must be a present intelligence directing the process, laying the dead bricks, marble, and wood in an intelligent order for a living temple. if we do put god behind a single veil in dead matter, in all living things he must be apparent and at work. if, then, such a thing as an infinite atom is impossible, shall we not best understand matter by saying it is a visible representation of god's personal will and power, of his personal force, and perhaps knowledge, set aside a little from himself, still possessed somewhat of his personal attributes, still responsive to his will. what we call matter may be best understood as god's force, will, knowledge, rendered apparent, static, and unweariably operative. unless matter is eternal, which is unthinkable, there was nothing out of which the world could be made, but god himself; and, reverently be it said, matter seems to retain fit capabilities for such source. is not this the teaching of the bible? i come to the old book. i come to that man who was taken up into the arcana of the third heaven, the holy of holies, and heard things impossible to word. i find he makes a clear, unequivocal statement of this truth as god's revelation to him. "by faith," says the author of hebrews, "we understand the worlds were framed by the word of god, so that things which are seen were not made of things which do appear." in corinthians, paul says--but to us there is but one god, the father, of whom [as a source] are all things; and one lord jesus christ, by whom [as a creative worker] are all things. so in romans he says--"for out of him, and through him, and to him are all things, to whom be glory forever. amen." [page ] god's intimate relation to matter is explained. no wonder the forces respond to his will; no wonder pantheism--the idea that matter is god--has had such a hold upon the minds of men. matter, derived from him, bears marks of its parentage, is sustained by him, and when the divine will shall draw it nearer to himself the new power and capabilities of a new creation shall appear. let us pay a higher respect to the attractions and affinities; to the plan and power of growth; to the wisdom of the ant; the geometry of the bee; the migrating instinct that rises and stretches its wings toward a provided south--for it is all god's present wisdom and power. let us come to that true insight of the old prophets, who are fittingly called seers; whose eyes pierced the veil of matter, and saw god clothing the grass of the field, feeding the sparrows, giving snow like wool and scattering hoar-frost like ashes, and ever standing on the bow of our wide-sailing world, and ever saying to all tumultuous forces, "peace, be still." let us, with more reverent step, walk the leafy solitudes, and say: "father, thy hand hath reared these venerable columns: thou did'st weave this verdant roof. thou did'st look down upon the naked earth, and forthwise rose all these fair ranks of trees. they in thy sun budded, and shook their green leaves in thy breeze. "that delicate forest flower, with scented breath and looks so like a smile, seems, as it issues from the shapeless mould, an emanation of the indwelling life, a visible token of the unfolding love that are the soul of this wide universe."--bryant. [page ] philosophy has seen the vast machine of the universe, wheel within wheel, in countless numbers and hopeless intricacy. but it has not had the spiritual insight of ezekiel to see that they were everyone of them full of eyes--god's own emblem of the omniscient supervision. what if there are some sounds that do not seem to be musically rhythmic. i have seen where an avalanche broke from the mountain side and buried a hapless city; have seen the face of a cliff shattered to fragments by the weight of its superincumbent mass, or pierced by the fingers of the frost and torn away. all these thunder down the valley and are pulverized to sand. is this music? no, but it is a tuning of instruments. the rootlets seize the sand and turn it to soil, to woody fibre, leafy verdure, blooming flowers, and delicious fruit. this asks life to come, partake, and be made strong. the grass gives itself to all flesh, the insect grows to feed the bird, the bird to nourish the animal, the animal to develop the man. notwithstanding the tendency of all high-class energy to deteriorate, to find equilibrium, and so be strengthless and dead, there is, somehow, in nature a tremendous push upward. ask any philosopher, and he will tell you that the tendency of all endowed forces is to find their equilibrium and be at rest--that is, dead. he draws a dismal picture of the time when the sun shall be burned out, and the world float like a charnel ship through the dark, cold voids of space--the sun a burned-out char, a dead cinder, and the world one dismal silence, cold beyond measure, and dead beyond consciousness. the philosopher has wailed a dirge without [page ] hope, a requiem without grandeur, over the world's future. but nature herself, to all ears attuned, sings pæans, and shouts to men that the highest energy, that of life, does not deteriorate. mere nature may deteriorate. the endowments of force must spend themselves. wound-up watches and worlds must run down. but nature sustained by unexpendable forces must abide. nature filled with unexpendable forces continues in form. nature impelled by a magnificent push of life must ever rise. study her history in the past. sulphurous realms of deadly gases become solid worlds; surplus sunlight becomes coal, which is reserved power; surplus carbon becomes diamonds; sediments settle until the heavens are azure, the air pure, the water translucent. if that is the progress of the past, why should it deteriorate in the future? there is a system of laws in the universe in which the higher have mastery over the lower. lower powers are constitutionally arranged to be overcome; higher powers are constitutionally arranged for mastery. at one time the water lies in even layers near the ocean's bed, in obedience to the law or power of gravitation. at another time it is heaved into mountain billows by the shoulders of the wind. again it flies aloft in the rising mists of the morning, transfigured by a thousand rain bows by the higher powers of the sun. again it develops the enormous force of steam by the power of heat. again it divides into two light flying airs by electricity. again it stands upright as a heap by the power of some law in the spirit realm, whose mode of working we are not yet large enough [page ] to comprehend. the water is solid, liquid, gaseous on earth, and in air according to the grade of power operating upon it. the constant invention of man finds higher and higher powers. once he throttled his game, and often perished in the desperate struggle; then he trapped it; then pierced it with the javelin; then shot it with an arrow, or set the springy gases to hurl a rifle-ball at it. sometime he may point at it an electric spark, and it shall be his. once he wearily trudged his twenty miles a day, then he took the horse into service and made sixty; invoked the winds, and rode on their steady wings two hundred and forty; tamed the steam, and made almost one thousand; and if he cannot yet send his body, he can his mind, one thousand miles a second. it all depends upon the grade of power he uses. now, hear the grand truth of nature: as the years progress the higher grades of power increase. either by discovery or creation, there are still higher class forces to be made available. once there was no air, no usable electricity. there is no lack of those higher powers now. the higher we go the more of them we find. mr. lockyer says that the past ten years have been years of revelation concerning the sun. a man could not read in ten years the library of books created in that time concerning the sun. but though we have solved certain problems and mysteries, the mysteries have increased tenfold. we do not know that any new and higher forces have been added to matter since man's acquaintance with it. but it would be easy to add any number of them, or change any lower into higher. that is the [page ] meaning of the falling granite that becomes soil, of the pulverized lava that decks the volcano's trembling sides with flowers; that is the meaning of the grass becoming flesh, and of all high forces constitutionally arranged for mastery over lower. take the ore from the mountain. it is loose, friable, worthless in itself. raise it in capacity to cast-iron, wrought-iron, steel, it becomes a highway for the commerce of nations, over the mountains and under them. it becomes bones, muscles, body for the inspiring soul of steam. it holds up the airy bridge over the deep chasm. it is obedient in your hand as blade, hammer, bar, or spring. it is inspirable by electricity, and bears human hopes, fears, and loves in its own bosom. it has been raised from valueless ore. change it again to something as far above steel as that is above ore. change all earthly ores to highest possibility; string them to finest tissues, and the new result may fit god's hand as tools, and thrill with his wisdom and creative processes, a body fitted for god's spirit as well as the steel is fitted to your hand. from this world take opacity, gravity, darkness, bring in more mind, love, and god, and then we will have heaven. an immanent god makes a plastic world. when man shall have mastered the forces that now exist, the original creator and sustainer will say, "behold, i create all things new." nature shall be called nearer to god, be more full of his power. to the long-wandering Æneas, his divine mother sometimes came to cheer his heart and to direct his steps. but the goddess only showed herself divine by her departure; only when he stood in desolation did the hero know he had [page ] stood face to face with divine power, beauty, and love. not so the christian scholars, the wanderers in nature's bowers to-day. in the first dawn of discovery, we see her full of beauty and strength; in closer communion, we find her full of wisdom; to our perfect knowledge, she reveals an indwelling god in her; to our ardent love, she reveals an indwelling god in us. but the evidence of the progressive refinements of habitation is no more clear than that of progressive refinement of the inhabitant: there must be some one to use these finer things. an empty house is not god's ideal nor man's. the child may handle a toy, but a man must mount a locomotive; and before there can be new jerusalems with golden streets, there must be men more avaricious of knowledge than of gold, or they would dig them up; more zealous for love than jewels, or they would unhang the pearly gates. the uplifting refinement of the material world has been kept back until there should appear masterful spirits able to handle the higher forces. doors have opened on every side to new realms of power, when men have been able to wield them. if men lose that ability they close again, and shut out the knowledge and light. then ages, dark and feeble, follow. some explore prophecy for the date of the grand transformation of matter by the coming of the son of man, for a new creation. a little study of nature would show that the date cannot be fixed. a little study of peter would show the same thing. he says, "what manner of persons ought ye to be, in all holy conversation and godliness, looking for and hastening the coming [page ] of the day of god, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat? nevertheless we, according to his promise, look for a new heaven and a new earth." the idea is, that the grand transformation of matter waits the readiness of man. the kingdom waits the king. the scattered cantons of italy were only prostrate provinces till victor emanuel came, then they were developed into united italy. the prostrate provinces of matter are not developed until the man is victor, able to rule there a realm equal to ten cities here. every good man hastens the coming of the day of god and nature's renovation. not only does inference teach that there must be finer men, but fact affirms that transformation has already taken place. life is meant to have power over chemical forces. it separates carbon from its compounds and builds a tree, separates the elements and builds the body, holds them separate until life withdraws. more life means higher being. certainly men can be refined and recapacitated as well as ore. in ovid's "metamorphoses" he represents the lion in process of formation from earth, hind quarters still clay, but fore quarters, head, erect mane, and blazing eye--live lion--and pawing to get free. we have seen winged spirits yet linked to forms of clay, but beating the celestial air, endeavoring to be free; and we have seen them, dowered with new sight, filled with new love, break loose and rise to higher being. in this grand apotheosis of man which nature teaches, progress lias already been made. man has already outgrown his harmony with the environment of mere matter. he has given his hand to science, and been lifted up above the earth into the voids of infinite space. he [page ] has gone on and on, till thought, wearied amidst the infinities of velocity and distance, has ceased to note them. but he is not content; all his faculties are not filled. he feels that his future self is in danger of not being satisfied with space, and worlds, and all mental delights, even as his manhood fails to be satisfied with the materiel toys of his babyhood. he asks for an author and maker of things, infinitely above them. he has seen wisdom unsearchable, power illimitable; but he asks for personal sympathy and love. paul expresses his feeling: every creature--not the whole creation--groaneth and travaileth in pain together until now, waiting for the adoption--the uplifting from orphanage to parentage--a translation out of darkness into the kingdom of god's dear son. he hears that a man in christ is a new creation: old things pass away, all things become new. there is then a possibility of finding the author of nature, and the father of man. he begins his studies anew. now he sees that all lines of knowledge converge as they go out toward the infinite mystery; sees that these converging lines are the reins of government in this world; sees the converging lines grasped by an almighty hand; sees a loving face and form behind; sees that these lines of knowledge and power are his personal nerves, along which flashes his will, and every force in the universe answers like a perfect muscle. then he asks if this personality is as full of love as of power. he is told of a tenderness too deep for tears, a love that has the cross for its symbol, and a dying cry for its expression: seeking it, he is a new creation. he sees more wondrous things in the word than in the [page ] world. he comes to know god with his heart, better than he knows god's works by his mind. every song closes with the key-note with which it began, and the brief cadence at the close hints the realms of sound through which it has tried its wings. the brief cadence at the close is this: all force runs back into mind for its source, constant support, and uplifts into higher grades. mr. grove says, "causation is the will, creation is the act, of god." creation is planned and inspired for the attainment of constantly rising results. the order is chaos, light, worlds, vegetable forms, animal life, then man. there is no reason to pause here. this is not perfection, not even perpetuity. original plans are not accomplished, nor original force exhausted. in another world, free from sickness, sorrow, pain, and death, perfection of abode is offered. perfection of inhabitant is necessary; and as the creative power is everywhere present for the various uplifts and refinements of matter, it is everywhere present with appropriate power for the uplifting and refinement of mind and spirit. [page ] summary of latest discoveries and conclusions. _movements on the sun._--the discovery and measurement of the up-rush, down-rush, and whirl of currents about the sunspots, also of the determination of the velocity of rotation by means of the spectroscope, as described (page ), is one of the most delicate and difficult achievements of modern science. _movement of stars in line of sight_ (page ).--the following table shows this movement of stars, so far as at present known: --------------------------------------------------------------- | aproaching. || receding. | |------------------------------||-------------------------------| | map. | name. | rate || map. | name. | rate | | | | per sec. || | | per sec. | |-------|-----------|----------||--------|-----------|----------| |fig. |arcturus | miles ||fig. |sirius | miles | | " |vega | " ||fr'piece|betelguese | " | | " |a cygni | " || " |rigel | " | | " |pollux | " ||fig. |castor | " | | " |dubhe | " || " |regulus | " | --------------------------------------------------------------- _sun's appearance._--this was formerly supposed to be an even, regular, dazzling brightness, except where the spots appeared. but the sun's surface is now known to be mottled with what are called rice grains or willow leaves. but the rice grains are as large as the continent of america. the spaces between are called pores. they constitute an innumerable number of small spots. this appearance of the general surface is well portrayed in the cut on page . _close relation between sun and earth._-men always knew that the earth received light from the sun. they subsequently discovered that the earth was momentarily held by the power [page ] of gravitation. but it is a recent discovery that the light is one of the principal agents in chemical changes, in molecular grouping and world-building, thus making all kinds of life possible (p. - ). the close connection of the sun and the earth will be still farther shown in the relation of sun-spots and auroras. one of the most significant instances is related on page , when the earth felt the fall of bolides upon the sun. members of the body no more answer to the heart than the planets do to the sun. _hydrogen flames._--it has been demonstrated that the sun flames , miles high are hydrogen in a state of flaming incandescence (page ). _sun's distance._--the former estimate, , , miles, has been reduced by nearly one-thirtieth. lockyer has stated it as low as , , miles, and proctor, in "encyclopædia britannica," at , , miles, but discovered errors show that these estimates are too small. newcomb gives , , as within , miles of the correct distance. the data for a new determination of this distance, obtained from the transit of venus, december th, , have not yet been deciphered; a fact that shows the difficulty and laboriousness of the work. meanwhile it begins to be evident that observations of the transit of venus do not afford the best basis for the most perfect determination of the sun's distance. since the earth's distance is our astronomical unit of measure, it follows that all other distances will be changed, when expressed in miles, by this ascertained change of the value of the standard. _oxygen in the sun._--in professor draper announced the discovery of oxygen lines in the spectrum of the sun. the discovery was doubted, and the methods used were criticised by lockyer and others, but later and more delicate experiments substantiate professor draper's claim to the discovery. the elements known to exist in the sun are salt, iron, hydrogen, [page ] magnesium, barium, copper, zinc, cromium, and nickel. some elements in the sun are scarcely, if at all, discoverable on the earth, and some on the earth not yet discernible in the sun. _substance of stars._--aldebaran (_frontispiece_) shows salt, magnesium, hydrogen, calcium, iron, bismuth, tellurium, antimony, and mercury. some of the sun's metals do not appear. stars differ in their very substance, and will, no doubt, introduce new elements to us unknown before. the theory that all nebulæ are very distant clusters of stars is utterly disproved by the clearest proof that some of them are only incandescent gases of one or two kinds. _discoveries of new bodies._--vulcan, the planet nearest the sun (page ). the two satellites of mars were discovered by mr. hall, u. s. naval observatory, august th, (page ). "the outer one is called diemas; the inner, phobus. sir william herschel thought he discovered six satellites of uranus. the existence of four of them has been disproved by the researches of men with larger telescopes. two new ones, however, were discovered by mr. lassell in . _saturn's rings_ are proved to be in a state of fluidity and contraction (page ). _meteors and comets._--the orbits of over one hundred swarms of meteoric bodies are fixed: their relation to, and in some cases indentity with, comets determined. some comets are proved to be masses of great weight and solidity (page ). _aerolites._-some have a texture like our lowest strata of rocks. there is a geology of stars and meteors as well as of the earth. m. meunier has just received the lalande medal from the paris academy for his treatise showing that, so far as our present knowledge can determine, some of these meteors once belonged to a globe developed in true geological epochs, and which has been separated into fragments by agencies with which we are not acquainted. [illustration: fig. .--horizontal pendulum.] _the horizontal pendulum._--this delicate instrument is [page ] represented in fig. . it consists of an upright standard, strongly braced; a weight, _m_, suspended by the hair-spring of a watch, b d, and held in a horizontal position by another watch-spring, a c. the weight is deflected from side to side by the slightest influence. the least change in the level of a base thirty-nine inches long that could be detected by a spirit-level is ". of an arc--equal to raising one end / of an inch. but the pendulum detects a raising of one end / of an inch. to observe the movements of the pendulum, it is kept in a dark room, and a ray of light is directed to the mirror, _m_, and thence reflected upon a screen. thus the least movement may be enormously magnified, and read and measured by the moving spot on the screen. it has been discovered that when the sun rises it has sufficient attraction to incline this instrument to the east; when it sets, to incline it to the west. the same is true of the moon. when either is exactly overhead or underfoot, of course there is no deflection. the mean deflection caused by the moon at rising or setting is ". ; by the sun, ". . great results are expected from this instrument hardly known as yet: among others, whether gravitation acts instantly or consumes time in coming from the sun. this will be shown by the time of the change of the pendulum from east to west when the sun reaches the zenith, and _vice versa_ when it crosses the nadir. the sun will be best studied without light, in the quiet and darkness of some deep mine. [page ] _light of unseen stars._--from careful examination, it appears that three-fourths of the light on a fine starlight night comes from stars that cannot be discerned by the naked eye. the whole amount of star light is about one-eightieth of that of the full moon. _lateral movements of stars_, page - . _future discoveries_--_a trans-neptunian planet._--professor asaph hall says: "it is known to me that at least two american astronomers, armed with powerful telescopes, have been searching quite recently for a trans-neptunian planet. these searches have been caused by the fact that professor newcomb's tables of uranus and neptune already begin to differ from observation. but are we to infer from these errors of the planetary tables the existence of a trans-neptunian planet? it is possible that such a planet may exist, but the probability is, i think, that the differences are caused by errors in the theories of these planets. * * * a few years ago the remark was frequently made that the labors of astronomers on the solar system were finished, and that henceforth they could turn their whole attention to sidereal astronomy. but to-day we have the lunar theory in a very discouraging condition, and the theories of mercury, jupiter, saturn, uranus, and neptune all in need of revision; unless, indeed, leverrier's theories of the last two planets shall stand the test of observation. but, after all, such a condition of things is only the natural result of long and accurate series of observations, which make evident the small inequalities in the motions, and bring to light the errors of theory." future discoveries will mostly reveal the laws and conditions of the higher and finer forces. already professor loomis telegraphs twenty miles without wire, by the electric currents between mountains. we begin to use electricity for light, and feel after it for a motor. comets and auroras show its presence between worlds, and in the interstellar spaces. let another newton arise. [page ] some elements of the solar system ------------------------------------------------------------------------ | | | | mean dist. | | | | | | | from sun. | | | | | | |-------------------| mean |density.| | | | | earth's| |diameter |[earth] | | name. | sign. | masses. | dist. | millions |in miles.| = . | | | | | as . | of miles.| | | |-----------|--------|------------|--------|----------|---------|--------| | sun |[symbol]| unity | | | , | . | | mercury |[symbol]| / (?)| . | - / | , | . | | venus |[symbol]| / | . | - / | , | . | | earth |[symbol]| / | . | - / | , | . | | mars |[symbol]| / | . | | , | . | | asteroids | (no.) | | | | | | | jupiter |[symbol]| / | . | | , | . | | saturn |[symbol]| / | . | | , | . | | uranus |[symbol]| / | . | | , | . | | neptune |[symbol]| / | . | | , | . | ------------------------------------------------------------------------ ------------------------------------------------------------- | | | gravity | | | | | axial | at | | orbital | | | revolu- | surface. | periodic | velocity | | name. | tion | [earth] | time. | in miles | | | | = | | per sec. | |-----------|---------------|----------|-----------|----------| | sun | to d | . | | | | mercury | h m(?) | . | . d | . | | venus | h m(?) | . | . d | . | | earth | h m s | . | . d | . | | mars | h m . s | . | . d | . | | asteroids | | | | | | jupiter | h m s | . | . yrs | . | | saturn | h m | . | . yrs | . | | uranus | unknown. | . | . yrs | . | | neptune | unknown. | . | . yrs | . | ------------------------------------------------------------- [page ] explanation of astronomical symbols. signs of the zodiac . [symbol] aries ° | vi. [symbol] libra ° i. [symbol] taurus | vii. [symbol] scorpio ii. [symbol] gemini | viii. [symbol] sagittarius iii. [symbol] cancer | ix. [symbol] capricornus iv. [symbol] leo | x. [symbol] aquarius v. [symbol] virgo | xi. [symbol] pisces * * * * * [symbol] conjunction. | s. seconds of time. [symbol] quadrature. | ° degrees. [symbol] opposition. | ' minutes of arc. [symbol] ascending node. | " seconds of arc. [symbol] descending node. | r. a. right ascension. h. hours. | decl. or d. declination. m. minutes of time. | n. p. d. dist. from north pole. other abbreviations used in the almanac. s., south, _i.e._, crosses the meridian; m., morning; a, afternoon; gr. h. l. n., greatest heliocentric latitude north, _i.e._, greatest distance north of the ecliptic, as seen from the sun. [symbols] inf., inferior conjunction; sup., superior conjunction. greek alphabet used indicating the stars. a, alpha. | ae, eta. | n, nu. | t, tau. b, beta. | th, theta. | x, xi. | u, upsilon. g, gamma. | i, iota. | o, omicron. | ph, phi. d, delta. | k, kappa. | p, pi. | ch, chi. e, epsilon. | l, lambda. | r, rho. | ps, psi. z, zeta. | m, mu. | s, sigma. | o, omega. [page ] chautauqua outline for students. as an aid to comprehension, every student should draw illustrative figures of the various circles, planes, and situations described. (for example, see fig. , page .) as an aid to memory, the portion of this outline referring to each chapter should be examined at the close of the reading, and this mere sketch filled up to a perfect picture from recollection. i. _creative processes._--the dial-plate of the sky. cause or different weights--on sun, moon. two laws of gravity. inertia. fall of earth to sun per second. forward motion. elastic attraction. perturbation of moon; of jupiter and saturn. oscillations of planets. ii. _light._--from condensation. number of vibrations of red; violet. thermometer against air. aerolite against earth. two bolides against the sun. large eye. velocity of light. prism. color means different vibrations. music of light. light reports substance of stars. force of; bridge, rain, dispersion, intensities, reflection, refraction, decomposition. iii. _astronomical instruments._--refracting telescope. reflecting; largest. spectroscope. spectra of sun, hydrogen, sodium, etc. e made g by approach; c by departure. stars approach and recede. iv. _celestial measurements._-place and time by stars. degrees, minutes, seconds. mapping stars. mural circle. slow watch. hoosac tunnel. fine measurements. sidereal time. spider-lines. personal equation. measure distance--height. ten-inch base line. parallax of sun, stars. longitude at sea. distance of polaris, a centauri, cygni. orbits of asteroids. v. _the sun._--world on fire. apparent size from planets. zodiacal light. corona. hydrogen--how high? size. how many earths? spots: . motion; . edges; . variable; . periodic; . cyclonic; . size; . velocities. what the sun does. experiments. vi. _the planets from space._--north pole. speed. sizes. axial revolution. man's weight on. seasons. parallelism of axis. earth near [page ] sun in winter. plane of ecliptic. orbits inclined to. earth rotates. proof. sun's path among stars. position of planets. motion--direct, retrograde. experiments. vii. _meteors._--size; number; cause of; above earth; velocity; colors; number in space; telescopic view of. aerolites: systems of; how many known. comets: orbits; number of comets; halley's; biela's lost; encke's. resisting medium. whence come comets? composed of what? amount of matter in. [symbol]. viii. _the planets._--how many? uranus discovered? neptune? asteroids? vulcan? distance from sun. periodic time. mercury: elements; shapes, as seen from earth; transits. venus: elements; seen by day; how near earth? how far from? phases; galileo. earth: elements; in space; aurora; balance of forces. tides: main and subsidiary causes; eastern shores; mediterranean sea. moon: elements; hoax; moves east; see one side; three causes help to see more than half. revolution: why twenty-nine and a half days: heat--cold; how much light? craters and peaks lighted; measured. eclipses--why not every new and full moon? periodicity. mars: elements; how near earth? how far from? apparent size; ice-fields; which end most? satellites--asteroids: how found? when? by whom? how many? jupiter: elements; trade-winds; how much light received? own heat. satellites: how many? colors. saturn: elements; habitability; rings; flux; satellites. uranus: elements; discoverer; seen by; moon's motion. neptune: elements; discovered by; how? review system. ix. _the nebular hypothesis._--state it; facts confirmatory. objections-- . heat; . rotation; . retrograde; . martial moons; . star of . evolution: gaps in; conclusion. x. _the stellar system._-motto. man among stars; open page; starry poem; stars located; named. thuban. etanin. constellations: know them; number of stars; double; e lyræ, sirius, procyon, castor, cygni, g virginis. colored stars; change color. clusters: two theories. nebulæ: two visible; composed of; shapes; where? variable stars. sun. b lyræ, mira, betelguese, algol; cause. temporary; . new star of : two theories. star of . movements of stars; sirius; sun; groombridge. stars near pleiades: orion, great dipper, southern cross. centre of gravity. xi. _the worlds and the word._--rich. number. erroneous allusions. truth before discovery: . a beginning; . creation before arrangement; . light before sun; . mountains under water; . order of development; [page ] . sphere of earth; . how upheld; . number of stars; . weight of air; . meteorology; . queries to job; . sun to end of heaven; . view of mitchell; . herschel. what is matter? force? end of earth. way to knowledge. work of light. transfiguration of matter. uniformitarianism. a whisper of him. man for mastery. each a type of higher. survival of fittest. uranus. worlds and word one language. xii. _the ultimate force._--universe shows power: . rain; niagara; . vegetable growth; . worlds carried; . sun; fill dome with worlds; . double suns; . galaxies. correlation. what ultimate? mind and will. what continuous relation? watch. theories of gravitation: newton's, le sage's, bible's. high-class energy deteriorates. search for atoms: . microscope; . gold; . infusoria; . musk. properties of atoms: . impenetrable; . indivisible; . shape; . quality; . crystallization; . not touch each other; . active; . attractive; . intelligent. whose? relation of matter to god; rock to soil. push upward. highest has mastery. man advances by highest. matter recapacitated. refined habitations. inhabitants. all force leads back to mind. personal and infinite. [page ] glossary of astronomical terms and index. abbreviations used in astronomies, . aberration of light (_a wandering away_), an apparent displacement of a star, owing to the progressive motion of light combined with that of the earth and its orbit, . aerolite (_air-stone_), . air, refraction of the, . algol, the variable star, . almanac, nautical, ; explanation of signs used, . alphabet, greek, . altitude, angular elevation of a body above the horizon. angle, difference in directions of two straight lines that meet. annular (_ring-shaped_) eclipses, ; nebulæ, , . aphelion, the point in an orbit farthest from the sun. apogee, the point of an orbit which is farthest from the earth. apsis, plural _apsides_, the line joining the aphelion and perihelion points; or the major axis of elliptical orbits. arc, a part of a circle. ascension, right, the angular distance of a heavenly body from the first point of aries, measured on the equator. asteroids (_star-like_), ; orbits of interlaced, . astronomical instruments, . astronomy, use of, . atom, size of, ; power of, . aurora borealis, . axis, the line about which a body rotates. azimuth, the angular distance of any point or body in the horizon from the north or south points. bailey's beads, dots of light on the edge of the moon seen in a solar eclipse, caused by the moon's inequalities of surface. base line, . biela's comet, . binary system, a double star, the component parts of which revolve around their centre of gravity. bode's law of planetary distances is no law at all, but a study of coincidences. bolides, small masses of matter in space. they are usually called meteors when luminous by contact with air, . [page ] celestial sphere, the apparent dome in which the heavenly bodies seem to be set; appears to revolve, . centre of gravity, the point on which a body, or two or more related bodies, balances. centrifugal force (_centre fleeing_). chromolithic plate of spectra of metals, to face . circumpolar stars, map of north, . colors of stars, . colures, the four principal meridians of the celestial sphere passing from the pole, one through each equinox, and one through each solstice. comets, ; halley's, ; biela's lost, ; encke's, ; constitution of, ; will they strike the earth? . conjunction. two or more bodies are in conjunction when they are in a straight line (disregarding inclination of orbit) with the sun. planets nearer the sun than the earth are in inferior conjunction when they are between the earth and the sun; superior conjunction when they are beyond the sun. constellation, a group of stars supposed to represent some figure: circumpolar, ; equatorial, for december, ; for january, ; april, ; june, ; september, ; november, ; southern circumpolar, . culmination, the passage of a heavenly body across the meridian or south point of a place; it is the highest point reached in its path. cusp, the extremities of the crescent form of the moon or an interior planet. declination, the angular distance of a celestial body north or south from the celestial equator. degree, the / part of a circle. direct motion, a motion from west to east among stars. disk, the visible surface of sun, moon, or planets. distance of stars, . double stars, . earth, revolution of, ; in space, ; irregular figure, . eccentricity of an ellipse, the distance of either focus from centre divided by half the major axis. eclipse (_a disappearance_), . ecliptic, the apparent annual path of the sun among the stars; plane of, . egress, the passing of one body off the disk of another. elements, the quantities which determine the motion of a planet: data for predicting astronomical phenomena; table of solar, . elements, chemical, present in the sun, . elongation, the angular distance of a planet from the sun. emersion, the reappearance of a body after it has been eclipsed or occulted by another. [page ] equator, terrestrial, the great circle half-way between the poles of the earth. when the plane of this is extended to the heavens, the line of contact is called the celestial equator. equinox, either of the points in which the sun, in its apparent annual course among the stars, crosses the equator, making days and nights of equal length. evolution, materialistic, ; insufficient, . fizeau determines the velocity of light, . forces, delicate balance of, . galileo, construction of his telescope, . geocentric, a position of a heavenly body as seen or measured from the earth's centre. geodesy, the art of measuring the earth without reference to the heavenly bodies. god, relation of, to the universe, . gravitation, laws of, ; extends to the stars, ; theories of, . gravity on different bodies, , . helical, rising or setting of a star, as near to sunrise or sunset as it can be seen. heliocentric, as seen from the centre of the sun. hoosac tunnel, example of accuracy, . horizontal pendulum, . immersion, the disappearance of one body behind another, or in its shadow. inclination of an orbit, the angle between its plane and the plane of the ecliptic. inferior conjunction, when an interior planet is between the earth and the sun. jupiter, apparent path of, in , ; elements of, ; satellites of, ; positions of satellites, ; elements of satellites, ; the jovian system, . kepler's laws-- st, that the orbits of planets are ellipses, having the sun or central body in one of the foci; d, the radius-vector passes over equal spaces in equal times; d, the squares of the periodic times of the planets are in proportion to the cubes of their mean distances from the sun. latitude, the angular distance of a heavenly body from the ecliptic. light, the child of force, ; number of vibrations of, , ; velocity of, ; undulatory and musical, ; chemical force of, ; experiments with, ; approach and departure of a light-giving body measured, ; aberration of, . limb, the edge of the disk of the moon, sun, or a planet. longitude. if a perpendicular be dropped from a body to the ecliptic, its celestial longitude is the distance of the foot of the perpendicular from the vertical equinox, counted toward the east; mode of ascertaining terrestrial, . magellanic clouds, . [page ] mars, ; snow spots of, ; satellites of, . mass, the quantity of matter a body contains. mean distance of a planet, half the sum of the aphelion and perihelion distances. measurements, celestial, . mercury, . meridian, terrestrial, of a place, a great circle of the heavens passing through the poles, the zenith, and the north and south points of the horizon; celestial, any great circle passing from one pole to the other. meteors, ; swarm of, meeting the earth, ; explosion of, ; systems of, ; relation of, to comets, . micrometer, any instrument for the accurate measurement of very small distances or angles. mind, origin of force, ; continuous relation of, to the universe, . milky way, , . mira, the wonderful, . moon, the, ; greatest and least distance from the earth, ; telescopic appearance of, . mural circle, . nadir, the point in the celestial sphere directly beneath our feet, opposite to zenith. nebulÆ, . nebular hypothesis, not atheistic, ; stated, ; confirmatory facts, ; objections to, . neptune, elements of, . node, the point in which an orbit intersects the ecliptic, or other plane of reference; ascending, descending, line of, . occultation, the hiding of a star, planet, or satellite by the interposition of a nearer body of greater angular magnitude. opposition. a superior planet is in opposition when the sun, earth, and the planet are in a line, the earth being in the middle. orbit, the path of a planet, comet, or meteor around the sun, or of a satellite around a primary; inclination of, ; earth's, seen from the stars, . outline for students, . parallax, the difference of direction of a heavenly body as seen from two points, as the centre of the earth and some point of its surface, . parallels, imaginary circles on the earth or in the heavens parallel to the equator, having the poles for their centre. perigee, nearest the earth; said of a point in an orbit. perihelion, the point of an orbit nearest the sun. periodic time, time of a planet's, comet's, or satellite's revolution. personal equation, . perturbation, the effect of the attractions of the planets or other [page ] bodies upon each other, disturbing their regular motion; of saturn and jupiter, ; of asteroids, ; of uranus and neptune, . phases, the portions of the illuminated half of the moon or interior planet, as seen from the earth, called crescent, full, and gibbous. photosphere of the sun, . planet (_a wanderer_), as seen from space, ; speed of, ; size of, ; movements retrograde and direct, . pointers, the, . pole, north, movement of, . poles, the extremities of an imaginary line on which a celestial body rotates. quadrant, the fourth part of the circumference of a circle, or °. quadrature, a position of the moon or other body when ° from the sun. radiant point, that point of the heavens from which meteors seem to diverge, . radius-vector, an imaginary line joining the sun and a planet or comet in any part of its orbit. rain, weight of, . reflecting telescope, . refracting telescope, . refraction, a bending of light by passing through any medium, as air, water, prism. retrograde motion, the apparent movement of a planet from east to west among the stars. revolution, the movement of bodies about their centre of gravity. rotation, the motion of a body around its axis. satellites, smaller bodies revolving around planets and stars. saturn, elements of, ; revolution of, ; rings of, ; decreasing, ; nature of, ; satellites of, . seasons, of the earth, ; of other planets, . selenography (_lunography_), a description of the moon's surface. signs of the zodiac, the twelve equal parts, of ° each, into which the zodiac is divided. solar system, view of, , . solstices, those points of the ecliptic which are most distant from the equator. the sun passes one about june st, and the other about december st, giving the longest days and nights. spectroscope, . spectrum of sun and metals, . stars, chemistry of, ; distance of, - ; mode of naming, ; number of, ; double and multiple, ; colored, ; clusters of, ; variable, ; temporary, new, and lost, ; movements of lateral, ; in line of sight, . stationary points, places in a planet's orbit at which it has no motion among the stars. [page ] stellar system, the, . summary of recent discoveries, . sun, fall of two meteoric bodies into, ; light from contraction of, ; as seen from planets, ; corona, ; hydrogen flames of, ; condition of, ; spots, ; experiments, ; apparent path among the stars, ; power of, . symbols used in astronomy, . telescope, refracting, ; reflecting, ; cambridge equatorial, . telescopic work, clusters, ; double stars, . temporary stars, . terminator, the boundary-line between light and darkness on the moon or a planet. tides, . transit, the passage of an object across some fixed line, as the meridian, or between the eye of an observer and an apparently larger object, as that of mercury or venus over the disk of the sun, and the satellites of jupiter over its disk; of a star, . ultimate force, the, . uranus, elements of, ; moons of, retrograde, ; perturbed by neptune, . variable stars, . venus, . vernier, a scale to measure very minute distances. vertical circle, one that passes through the zenith and nadir of the celestial sphere. the prime vertical circle passes through the east and west points of the horizon. vulcan, discovery of, . worlds, the, and the word, teach the same truth, - . year, the, length of, on any planet, is determined by the periodic time. zenith, the point in the celestial sphere directly overhead. zodiac, a belt ° wide encircling the heavens, the ecliptic being the middle. in this belt the larger planets always appear. in the older astronomy it was divided into twelve parts of ° each, called signs of the zodiac. zodiacal light, . to find the stars in the sky. detach any of the following maps, appropriate to the time of year, hold it between you and a lantern out-of-doors, and you have an exact miniature of the sky. or, better, cut squares of suitable sizes from the four sides of a box; put a map over each aperture; provide for ventilation, and turn the box over a lamp or candle out-of-doors. use an opera glass to find the smaller stars, if one is accessible. [illustration: circumpolar constellations. always visible. in this position.--january th, at o'clock; february th, at o'clock; and february th, at o'clock.] [illustration: algol is on the meridian, ° south of pole.--at o'clock, december th; o'clock, december d; o'clock, january th.] [illustration: capella ( ° from pole) and rigel ( °) are on the meridian at o'clock february th, o'clock january d, and at o'clock january th.] [illustration: regulus comes on the meridian, ° south from the pole, at o'clock march d, o'clock april th, and at o'clock april d.] [illustration: arcturus comes to the meridian, ° from the pole, at o'clock may th, o'clock june th, and at o'clock june th.] [illustration: altair comes to the meridian, ° from the pole, at o'clock p.m. august th, at o'clock september d, and at o'clock september th.] [illustration: fomalhaut comes to the meridian, only ° from the horizon, at o'clock november th.] [illustration: fig. . the constellation of orion (hubble). photographed with a small camera lens of inch aperture and inches focal length. the three bright stars in the centre of the picture form the belt of orion. just below, in the sword handle, is an irregular white patch about one-eighth of an inch in diameter. this is a small-scale image of the great nebula in orion, shown on a larger scale in fig. .] the new heavens by george ellery hale director of the mount wilson observatory of the carnegie institution of washington with numerous illustrations new york charles scribner's sons to my wife preface fourteen years ago, in a book entitled "the study of stellar evolution" (university of chicago press, ), i attempted to give in untechnical language an account of some modern methods of astrophysical research. this book is now out of print, and the rapid progress of science has left it completely out of date. as i have found no opportunity to prepare a new edition, or to write another book of similar purpose, i have adopted the simpler expedient of contributing occasional articles on recent developments to _scribner's magazine_, three of which are included in the present volume. i am chiefly indebted, for the illustrations, to the mount wilson observatory and the present and former members of its staff whose names appear in the captions. special thanks are due to mr. ferdinand ellerman, who made all of the photographs of the observatory buildings and instruments, and prepared all material for reproduction. the cut of the original cavendish apparatus is copied from the _philosophical transactions for _ with the kind permission of the royal society, and i am also indebted to the royal society and to professor fowler and father cortie for the privilege of reproducing from the _proceedings_ two illustrations of their spectroscopic results. g. e. h. january, . contents chapter i. the new heavens ii. giant stars iii. cosmic crucibles illustrations fig. . the constellation of orion (hubble) . the great nebula in orion (pease) . model by ellerman of summit of mount wilson, showing the observatory buildings among the trees and bushes . the -inch hooker telescope . erecting the polar axis of the -inch telescope . lowest section of tube of -inch telescope, ready to leave pasadena for mount wilson . section of a steel girder for dome covering the -inch telescope, on its way up mount wilson . erecting the steel building and revolving dome that cover the hooker telescope . building and revolving dome, feet in diameter, covering the -inch hooker telescope . one-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (seen above) . the driving-clock and worm-gear that cause the -inch hooker telescope to follow the stars . large irregular nebula and star cluster in sagittarius (duncan) . faint spiral nebula in the constellation of the hunting dogs (pease) . spiral nebula in andromeda, seen edge on (ritchey) . photograph of the moon made on september , , with the -inch hooker telescope (pease) . photograph of the moon made on september , , with the -inch hooker telescope (pease) . hubble's variable nebula. one of the few nebulæ known to vary in brightness and form . ring nebula in lyra, photographed with the -inch (ritchey) and -inch (duncan) telescopes . gaseous prominence at the sun's limb, , miles high (ellerman) . the sun, , miles in diameter, from a direct photograph showing many sun-spots (whitney) . great sun-spot group, august , (whitney) . photograph of the hydrogen atmosphere of the sun (ellerman) . diagram showing outline of the -inch hooker telescope, and path of the two pencils of light from a star when under observation with the -foot michelson interferometer . twenty-foot michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the -inch hooker telescope . the giant betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of orion (hubble) . arcturus (within the white circle), known to the arabs as the "lance bearer," and to the chinese as the "great horn" or the "palace of the emperors" (hubble) . the giant star antares (within the white circle), notable for its red color in the constellation scorpio, and named by the greeks "a rival of mars" (hubble) . diameters of the sun, arcturus, betelgeuse, and antares compared with the orbit of mars . aldebaran, the "leader" (of the pleiades), was also known to the arabs as "the eye of the bull," "the heart of the bull," and "the great camel" (hubble) . solar prominences, photographed with the spectroheliograph without an eclipse (ellerman) . the -foot tower telescope of the mount wilson observatory . pasadena laboratory of the mount wilson observatory . sun-spot vortex in the upper hydrogen atmosphere (benioff) . splitting of spectrum lines by a magnetic field (bacock) . electric furnace in the pasadena laboratory of the mount wilson observatory . titanium oxide in red stars . titanium oxide in sun-spots . the cavendish experiment . the trifid nebula in sagittarius (ritchey) . spiral nebula in ursa major (ritchey) . mount san antonio as seen from mount wilson chapter i the new heavens go out under the open sky, on a clear and moon-less night, and try to count the stars. if your station lies well beyond the glare of cities, which is often strong enough to conceal all but the brighter objects, you will find the task a difficult one. ranging through the six magnitudes of the greek astronomers, from the brilliant sirius to the faintest perceptible points of light, the stars are scattered in great profusion over the celestial vault. their number seems limitless, yet actual count will show that the eye has been deceived. in a survey of the entire heavens, from pole to pole, it would not be possible to detect more than from six to seven thousand stars with the naked eye. from a single viewpoint, even with the keenest vision, only two or three thousand can be seen. so many of these are at the limit of visibility that ptolemy's "almagest," a catalogue of all the stars whose places were measured with the simple instruments of the greek astronomers, contains only , stars. back of ptolemy, through the speculations of the greek philosophers, the mysteries of the egyptian sun-god, and the observations of the ancient chaldeans, the rich and varied traditions of astronomy stretch far away into a shadowy past. all peoples, in the first stirrings of their intellectual youth, drawn by the nightly splendor of the skies and the ceaseless motions of the planets, have set up some system of the heavens, in which the sense of wonder and the desire for knowledge were no less concerned than the practical necessities of life. the measurement of time and the needs of navigation have always stimulated astronomical research, but the intellectual demand has been keen from the first. hipparchus and the greek astronomers of the alexandrian school, shaking off the vagaries of magic and divination, placed astronomy on a scientific basis, though the reaction of the middle ages caused even such a great astronomer as tycho brahe himself to revert for a time to the practice of astrology. early instruments the transparent sky of egypt, rarely obscured by clouds, greatly favored ptolemy's observations. here was prepared his great star catalogue, based upon the earlier observations of hipparchus, and destined to remain alone in its field for more than twelve centuries, until ulugh bey, prince of samarcand, repeated the work of his greek predecessor. throughout this period the stars were looked upon mainly as points of reference for the observation of planetary motions, and the instruments of observation underwent little change. the astrolabe, which consists of a circle divided into degrees, with a rotating diametral arm for sighting purposes, embodies their essential principle. in its simple form, the astrolabe was suspended in a vertical plane, and the stars were observed by bringing the sights on the movable diameter to bear upon them. their altitude was then read off on the circle. ultimately, the circle of the astrolabe, mounted with one of its diameters parallel to the earth's axis, became the armillary sphere, the precursor of our modern equatorial telescope. great stone quadrants fixed in the meridian were also employed from very early times. out of such furnishings, little modified by the lapse of centuries, was provided the elaborate instrumental equipment of uranibourg, the great observatory built by tycho brahe on the danish island of huen in . in this "city of the heavens," still dependent solely upon the unaided eye as a collector of starlight, tycho made those invaluable observations that enabled kepler to deduce the true laws of planetary motion. but after all these centuries the sidereal world embraced no objects, barring an occasional comet or temporary star, that lay beyond the vision of the earliest astronomers. the conceptions of the stellar universe, except those that ignored the solid ground of observation, were limited by the small aperture of the human eye. but the dawn of another age was at hand. [illustration: fig. . the great nebula in orion (pease). photographed with the -inch telescope. this short-exposure photograph shows only the bright central part of the nebula. a longer exposure reveals a vast outlying region.] the dominance of the sun as the central body of the solar system, recognized by aristarchus of samos nearly three centuries before the christian era, but subsequently denied under the authority of ptolemy and the teachings of the church, was reaffirmed by the polish monk copernicus in . kepler's laws of the motions of the planets, showing them to revolve in ellipses instead of circles, removed the last defect of the copernican system, and left no room for its rejection. but both the world and the church clung to tradition, and some visible demonstration was urgently needed. this was supplied by galileo through his invention of the telescope. [illustration: fig. . model by ellerman of summit of mount wilson, showing the observatory buildings among the trees and bushes. the -foot tower on the extreme left, which is at the edge of a precipitous cañon , feet deep, is the vertical telescope of the smithsonian astrophysical observatory. above it are the "monastery" and other buildings used as quarters by the astronomers of the mount wilson observatory while at work on the mountain. (the offices, computing-rooms, laboratories, and shops are in pasadena.) following the ridge, we come successively to the dome of the -inch photographic telescope, the power-house, laboratory, snow horizontal telescope, -foot-tower telescope, and -foot-tower telescope, these last three used for the study of the sun. the dome of the -inch reflecting telescope is just below the -foot tower, while that of the -inch telescope is farther to the right. the altitude of mount wilson is about , feet.] the crystalline lens of the human eye, limited by the iris to a maximum opening about one-quarter of an inch in diameter, was the only collector of starlight available to the greek and arabian astronomers. galileo's telescope, which in suddenly pushed out the boundaries of the known stellar universe and brought many thousands of stars into range, had a lens about - / inches in diameter. the area of this lens, proportional to the square of its diameter, was about eighty-one times that of the pupil of the eye. this great increase in the amount of light collected should bring to view stars down to magnitude . , of which nearly half a million are known to exist. it is not too much to say that galileo's telescope revolutionized human thought. turned to the moon, it revealed mountains, plains, and valleys, while the sun, previously supposed immaculate in its perfection, was seen to be blemished with dark spots changing from day to day. jupiter, shown to be accompanied by four encircling satellites, afforded a picture in miniature of the solar system, and strongly supported the copernican view of its organization, which was conclusively demonstrated by galileo's discovery of the changing phases of venus and the variation of its apparent diameter during its revolution about the sun. galileo's proof of the copernican theory marked the downfall of mediævalism and established astronomy on a firm foundation. but while his telescope multiplied a hundredfold the number of visible stars, more than a century elapsed before the true possibilities of sidereal astronomy were perceived. [illustration: fig. . the -inch hooker telescope.] structure of the universe sir william herschel was the first astronomer to make a serious attack upon the problem of the structure of the stellar universe. in his first memoir on the "construction of the heavens," read before the royal society in , he wrote as follows: "hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere in the centre of which the eye of an observer might be supposed to be placed.... in future we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed as well as consisting of very different materials." on turning his -inch reflecting telescope to a part of the milky way in orion, he found its whitish appearance to be completely resolved into small stars, not separately seen with his former telescopes. "the glorious multitude of stars of all possible sizes that presented themselves here to my view are truly astonishing; but as the dazzling brightness of glittering stars may easily mislead us so far as to estimate their number greater than it really is, i endeavored to ascertain this point by counting many fields, and computing from a mean of them, what a certain given portion of the milky way might contain." by this means, applied not only to the milky way but to all parts of the heavens, herschel determined the approximate number and distribution of all the stars within reach of his instrument. by comparing many hundred gauges or counts of stars visible in a field of about one-quarter of the area of the moon, herschel found that the average number of stars increased toward the great circle which most nearly conforms with the course of the milky way. ninety degrees from this plane, at the pole of the milky way, only four stars, on the average, were seen in the field of the telescope. in approaching the milky way this number increased slowly at first, and then more and more rapidly, until it rose to an average of stars per field. [illustration: fig. . erecting the polar axis of the -inch telescope.] these observations were made in the northern hemisphere, and subsequently sir john herschel, using his father's telescope at the cape of good hope, found an almost exactly similar increase of apparent star density for the southern hemisphere. according to his estimates, the total number of stars in both hemispheres that could be seen distinctly enough to be counted in this telescope would probably be about five and one-half millions. the herschels concluded that "the stars of our firmament, instead of being scattered in all directions indifferently through space, form a stratum of which the thickness is small, in comparison with its length and breadth; and in which the earth occupies a place somewhere about the middle of its thickness, between the point where it subdivides into two principal laminæ inclined at a small angle to each other." this view does not differ essentially from our modern conception of the form of the galaxy; but as the herschels were unable to see stars fainter than the fifteenth magnitude, it is evident that their conclusions apply only to a restricted region surrounding the solar system, in the midst of the enormously extended sidereal universe which modern instruments have brought within our range. modern methods the remarkable progress of modern astronomy is mainly due to two great instrumental advances: the rise and development of the photographic telescope, and the application of the spectroscope to the study of celestial objects. these new and powerful instruments, supplemented by many accessories which have completely revolutionized observatory equipment, have not only revealed a vastly greater number of stars and nebulæ: they have also rendered feasible observations of a type formerly regarded as impossible. the chemical analysis of a faint star is now so easy that it can be accomplished in a very short time--as quickly, in fact, as an equally complex substance can be analyzed in the laboratory. the spectroscope also measures a star's velocity, the pressure at different levels in its atmosphere, its approximate temperature, and now, by a new and ingenious method, its distance from the earth. it determines the velocity of rotation of the sun and of nebulæ, the existence and periods of orbital revolution of binary stars too close to be separated by any telescope, the presence of magnetic fields in sunspots, and the fact that the entire sun, like the earth, is a magnet. [illustration: fig. . lowest section of tube of -inch telescope, ready to leave pasadena for mount wilson.] such new possibilities, with many others resulting from the application of physical methods of the most diverse character, have greatly enlarged the astronomer's outlook. he may now attack two great problems: ( ) the structure of the universe and the motions of its constituent bodies, and ( ) the evolution of the stars: their nature, origin, growth, and decline. these two problems are intimately related and must be studied as one.[*] [footnote *: a third great problem open to the astronomer, the study of the constitution of matter, is described in chapter iii.] if space permitted, it would be interesting to survey the progress already accomplished by modern methods of astronomical research. hundreds of millions of stars have been photographed, and the boundaries of the stellar universe have been pushed far into space, but have not been attained. globular star clusters, containing tens of thousands of stars, are on so great a scale (according to shapley) that light, travelling at the rate of , miles per second, may take years to cross one of them, while the most distant of these objects may be more than , light-years from the earth. the spiral nebulæ, more than a million in number, are vast whirling masses in process of development, but we are not yet certain whether they should be regarded as "island universes" or as subordinate to the stellar system which includes our minute group of sun and planets, the great star clouds of the milky way, and the distant globular star clusters. [illustration: fig. . section of a steel girder for dome covering the -inch telescope, on its way up mount wilson.] these few particulars may give a slight conception of the scale of the known universe, but a word must be added regarding some of its most striking phenomena. the great majority of the stars whose motions have been determined belong to one or the other of two great star streams, but the part played by these streams in the sidereal system as a whole is still obscure. the stars have been grouped in classes, presumably in the order of their evolutional development, as they pass from the early state of gaseous masses, of low density, through the successive stages resulting from loss of heat by radiation and increased density due to shrinkage. strangely enough, their velocities in space show a corresponding change, increasing as they grow older or perhaps depending upon their mass. it is impossible within these limits to do more than to give some indication of the scope of the new astronomy. enough has been said, however, to assist in appreciating the increased opportunity for investigation, and the nature of the heavy demands made upon the modern observatory. but before passing on to describe one of the latest additions to the astronomer's instrumental equipment, a word should be added regarding the chief classes of telescopes. refractors and reflectors astronomical telescopes are of two types: refractors and reflectors. a refracting telescope consists of an object-glass composed of two or more lenses, mounted at the upper end of a tube, which is pointed at the celestial object. the light, after passing through the lenses, is brought to a focus at the lower end of the tube, where the image is examined visually with an eyepiece, or photographed upon a sensitive plate. the largest instruments of this type are the -inch lick telescope and the -inch refractor of the yerkes observatory. [illustration: fig. . erecting the steel building and revolving dome that cover the hooker telescope.] reflecting telescopes, which are particularly adapted for photographic work, though also excellent for visual observations, are very differently constructed. no lens is used. the telescope tube is usually built in skeleton form, open at its upper end, and with a large concave mirror supported at its base. this mirror serves in place of a lens. its upper surface is paraboloidal in shape, as a spherical surface will not unite in a sharp focus the rays coming from a distant object. the light passes through no glass--a great advantage, especially for photography, as the absorption in lenses cuts out much of the blue and violet light, to which photographic plates are most sensitive. the reflection occurs on the _upper_ surface of the mirror, which is covered with a coat of pure silver, renewed several times a year and always kept highly burnished. silvered glass is better than metals or other substances for telescope mirrors, chiefly because of the perfection with which glass can be ground and polished, and the ease of renewing its silvered surface when tarnished. the great reflectors of herschel and lord rosse, which were provided with mirrors of speculum metal, were far inferior to much smaller telescopes of the present day. with these instruments the star images were watched as they were carried through the field of view by the earth's rotation, or kept roughly in place by moving the telescope with ropes or chains. photographic plates, which reveal invisible stars and nebulæ when exposed for hours in modern instruments, were not then available. in any case they could not have been used, in the absence of the perfect mechanism required to keep the star images accurately fixed in place upon the sensitive film. [illustration: fig. . building and revolving dome, feet in diameter, covering the -inch hooker telescope. photographed from the summit of the -foot-tower telescope.] it would be interesting to trace the long contest for supremacy between refracting and reflecting telescopes, each of which, at certain stages in its development, appeared to be unrivalled. in modern observatories both types are used, each for the purpose for which it is best adapted. for the photography of nebulæ and the study of the fainter stars, the reflector has special advantages, illustrated by the work of such instruments as the crossley and mills reflectors of the lick observatory; the great -inch reflector, recently brought into effective service at the dominion observatory in canada; and the -inch and -inch reflectors of the mount wilson observatory. the unaided eye, with an available area of one-twentieth of a square inch, permits us to see stars of the sixth magnitude. herschel's -inch reflector, with an area , times as great, rendered visible stars of the fifteenth magnitude. the -inch reflector, with an area , times that of the eye, reveals stars of the eighteenth magnitude, while to reach stars of about the twentieth magnitude, photographic exposures of four or five hours suffice with this instrument. every gain of a magnitude means a great gain in the number of stars rendered visible. stars of the second magnitude are . times as numerous as those of the first, those of the eighth magnitude are three times as numerous as those of the seventh, while the sixteenth magnitude stars are only . as numerous as those of the fifteenth magnitude. this steadily decreasing ratio is probably due to an actual thinning out of the stars toward the boundaries of the stellar universe, as the most exhaustive tests have failed to give any evidence of absorption of light in its passage through space. but in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the -inch reflector. here is one of the chief sources of interest in the possibilities of a -inch reflecting telescope. -inch telescope [illustration: fig. . one-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (seen above.)] in the late john d. hooker, of los angeles, gave the carnegie institution of washington a sum sufficient to construct a telescope mirror inches in diameter, and thus large enough to collect , times the light received by the eye. (fig. .) the casting and annealing of a suitable glass disk, inches in diameter and inches thick, weighing four and one-half tons, was a most difficult operation, finally accomplished by a great french glass company at their factory in the forest of st. gobain. a special optical laboratory was erected at the pasadena headquarters of the mount wilson observatory, and here the long task of grinding, figuring, and testing the mirror was successfully carried out by the observatory opticians. this operation, which is one of great delicacy, required years for its completion. meanwhile the building, dome, and mounting for the telescope were designed by members of the observatory staff, and the working drawings were prepared. an opportune addition by mr. carnegie to the endowment of the carnegie institution of washington, of which the observatory is a branch, permitted the necessary appropriations to be made for the completion and erection of the telescope. though delayed by the war, during which the mechanical and optical facilities of the observatory shops were utilized for military and naval purposes, the telescope is now in regular use on mount wilson. the instrument is mounted on a massive pier of reinforced concrete, feet high and feet in diameter at the top. a solid wall extends south from this pier a distance of feet, on the west side of which a very powerful spectrograph, for photographing the spectra of the brightest stars, will be mounted. within the pier are a photographic dark room, a room for silvering the large mirror (which can be lowered into the pier), and the clock-room, where stands the powerful driving-clock, with which the telescope is caused to follow the apparent motion of the stars. (fig. .) [illustration: fig. . the driving-clock and worm-gear that cause the -inch hooker telescope to follow the stars.] the telescope mounting is of the english type, in which the telescope tube is supported by the declination trunnions between the arms of the polar axis, built in the form of a rectangular yoke carried by bearings on massive pedestals to the north and south. these bearings must be aligned exactly parallel to the axis of the earth, and must support the polar axis so freely that it can be rotated with perfect precision by the driving-clock, which turns a worm-wheel feet in diameter, clamped to the lower end of the axis. as this motion must be sufficiently uniform to counteract exactly the rotation of the earth on its axis, and thus to maintain the star images accurately in position in the field of view, the greatest care had to be taken in the construction of the driving-clock and in the spacing and cutting of the teeth in the large worm-wheel. here, as in the case of all of the more refined parts of the instrument, the work was done by skilled machinists in the observatory shops in pasadena or on mount wilson after the assembling of the telescope. the massive sections of the instrument, some of which weigh as much as ten tons each, were constructed at quincy, mass., where machinery sufficiently large to build battleships was available. they were then shipped to california, and transported to the summit of mount wilson over a road built for this purpose by the construction division of the observatory, which also built the pier on which the telescope stands, and erected the steel building and dome that cover it. [illustration: fig. . large irregular nebula and star cluster in sagittarius (duncan). photographed with the -inch telescope.] [illustration: fig. . faint spiral nebula in the constellation of the hunting dogs (pease). photographed with the -inch telescope.] the parts of the telescope which are moved by the driving-clock weigh about tons, and it was necessary to provide means of reducing the great friction on the bearings of the polar axis. to accomplish this, large hollow steel cylinders, floating in mercury held in cast-iron tanks, were provided at the upper and lower ends of the polar axis. almost the entire weight of the instrument is thus floated in mercury, and in this way the friction is so greatly reduced that the driving-clock moves the instrument with perfect ease and smoothness. the -inch mirror rests at the bottom of the telescope tube on a special support system, so designed as to prevent any bending of the glass under its own weight. electric motors, forty in number, are provided to move the telescope rapidly or slowly in right ascension (east or west) and in declination (north or south), for focussing the mirrors, and for many other purposes. they are also used for rotating the dome, feet in diameter, under which the telescope is mounted, and for opening the shutter, feet wide, through which the observations are made. a telescope of this kind can be used in several different ways. the -inch mirror has a focal length of about feet, and in one of the arrangements of the instrument, the photographic plate is mounted at the centre of the telescope tube near its upper end, where it receives directly the image formed by the large mirror. in another arrangement, a silvered glass mirror, with plane surface, is supported near the upper end of the tube at an angle of °, so as to form the image at the side of the tube, where the photographic plate can be placed. in this case, the observer stands on a platform, which is moved up and down by electric motors in front of the opening in the dome through which the observations are made. [illustration: fig. . spiral nebula in andromeda, seen edge on (ritchey). photographed with the -inch telescope.] other arrangements of the telescope, for which auxiliary convex mirrors carried near the upper end of the tube are required, permit the image to be photographed at the side of the tube near its lower end, either with or without a spectrograph; or with a very powerful spectrograph mounted within a constant-temperature chamber south of the telescope pier. in this last case, the light of a star is so reflected by auxiliary mirrors that it passes down through a hole in the south end of the polar axis and brings the star to a focus on the slit of the fixed spectrograph. atmospheric limitations the huge dimensions of such a powerful engine of research as the hooker telescope are not in themselves a source of satisfaction to the astronomer, for they involve a decided increase in the labor of observation and entail very heavy expense, justifiable only in case important results, beyond the reach of other instruments, can be secured. the construction of a telescope of these dimensions was necessarily an experiment, for it was by no means certain, after the optical and mechanical difficulties had been overcome, that even the favorable atmosphere of california would be sufficiently tranquil to permit sharply defined celestial images to be obtained with so large an aperture. it is therefore important to learn what the telescope will actually accomplish under customary observing conditions. fortunately we are able to measure the performance of the instrument with certainty. close beside it on mount wilson stands the -inch reflector, of similar type, erected in . the two telescopes can thus be rigorously compared under identical atmospheric conditions. the large mirror of the -inch telescope has an area about . times that of the -inch, and therefore receives nearly three times as much light from a star. under atmospheric conditions perfect enough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with a given exposure, stars about one magnitude fainter than the faintest stars within reach of the -inch. the increased focal length, permitting such objects as the moon to be photographed on a larger scale, should also reveal smaller details of structure and render possible higher accuracy of measurement. finally, the greater theoretical resolving power of the larger aperture, providing it can be utilized, should permit the separation of the members of close double stars beyond the range of the smaller instrument. critical tests the many tests already made indicate that the advantages expected of the new telescope will be realized in practice. the increased light-gathering power will mean the addition of many millions of stars to those already known. spectroscopic observations now in regular progress have carried the range of these investigations far beyond the possibilities of the -inch telescope. a great class of red stars, for example, almost all the members of which were inaccessible to the -inch, are now being made the subject of special study. and in other fields of research equal advantages have been gained. the increase in the scale of the images over those given by the -inch telescope is illustrated by two photographs of the ring nebula in lyra, reproduced in fig. . the great nebula in orion, photographed with the -inch telescope with a comparatively short exposure, sufficient to bring out the brighter regions, is reproduced in fig. . it is interesting to compare this picture with the small-scale image of the same nebula shown in fig. . [illustration: fig. . photograph of the moon made on september , , with the -inch hooker telescope (pease). the ring-like formations are the so-called craters, most of them far larger than anything similar on the earth. that in the lower left corner with an isolated mountain in the centre is albategnius, sixty-four miles in diameter. peaks in the ring rise to a height of fifteen thousand feet above the central plain. note the long sunset shadows cast by the mountains on the left. the level region below on the right is an extensive plain, the mare nubium.] [illustration: fig. . photograph of the moon made on september , , with the -inch hooker telescope (pease). the mountains above and to the left are the lunar apennines; those on the left just below the centre are the alps. both ranges include peaks from fifteen thousand to twenty thousand feet in height. in the upper right corner is copernicus, about fifty miles in diameter. the largest of the conspicuous group of three just below the apennines is archimedes and at the lower end of the alps is plato. note the long sunset shadows cast by the isolated peaks on the left. the central portion of the picture is a vast plain, the mare imbrium.] the sharpness of the images given by the new telescope may be illustrated by some recent photographs of the moon, obtained with an equivalent focal length of feet. in fig. is shown a rugged region of the moon, containing many ring-like mountains or craters. fig. shows the great arc of the lunar apennines (above) and the alps (below), to the left of the broad plain of the mare imbrium. the starlike points along the moon's terminator, which separates the dark area from the region upon which the sun (on the right) shines, are the mountain peaks, about to disappear at sunset. the long shadows cast by the mountains just within the illuminated area are plainly seen. some of the peaks of the lunar apennines attain a height of , feet. in less powerful telescopes the stars at the centre of the great globular clusters are so closely crowded together that they cannot be studied separately with the spectrograph. moreover, most of them are much too faint for examination with this instrument. at the -foot focus the -inch telescope gives a large-scale image of such clusters, and permits the spectra of stars as faint as the fifteenth magnitude to be separately photographed. [illustration: fig. . hubble's variable nebula. one of the few nebulæ known to vary in brightness and form. photographed with the -inch telescope (hubble).] close double stars a remarkable use of the -inch telescope, which permits its full theoretical resolving power to be not merely attained but to be doubled, has been made possible by the first application of michelson's interference method to the measurement of very close double stars. when employing this, the -inch mirror is completely covered, except for two slits. beams of light from a star, entering by the slits, unite at the focus of the telescope, where the image is examined by an eyepiece magnifying about five thousand diameters. across the enlarged star image a series of fine, sharp fringes is seen, even when the atmospheric conditions are poor. if the star is single the fringes remain visible, whatever the distance between the slits. but in the case of a star like capella, previously inferred to be double from the periodic displacement of the lines in its spectrum, but with components too close together to be distinguished separately, the fringes behave differently. as the slits are moved apart a point is reached where the fringes completely disappear, only to reappear as the separation is continued. this effect is obtained when the slits are at right angles to the line joining the two stars of the pair, found by this method to be . of a second of arc apart (on december , ). subsequent measures, of far greater precision than those obtainable by other methods in the case of easily separated double stars, show the rapid orbital motion of the components of the system. this device will be applied to other close binaries, hitherto beyond the reach of measurement. [illustration: fig. . ring nebula in lyra, photographed with the -inch (ritchey) and -inch (duncan) telescopes. showing the increased scale of the images given by the larger instrument.] without entering into further details of the tests, it is evident that the new telescope will afford boundless possibilities for the study of the stellar universe.[*] the structure and extent of the galactic system, and the motions of the stars comprising it; the distribution, distances, and dimensions of the spiral nebulæ, their motions, rotation, and mode of development; the origin of the stars and the successive stages in their life history: these are some of the great questions which the new telescope must help to answer. in such an embarrassment of riches the chief difficulty is to withstand the temptation toward scattering of effort, and to form an observing programme directed toward the solution of crucial problems rather than the accumulation of vast stores of miscellaneous data. this programme will be supplemented by an extensive study of the sun, the only star near enough the earth to be examined in detail, and by a series of laboratory investigations involving the experimental imitation of solar and stellar conditions, thus aiding in the interpretation of celestial phenomena. [footnote *: it is not adapted for work on the sun, as the mirrors would be distorted by its heat. three other telescopes, especially designed for solar observations, are in use on mount wilson.] chapter ii giant stars our ancestral sun, as pictured by laplace, originally extended in a state of luminous vapor beyond the boundaries of the solar system. rotating upon its axis, it slowly contracted through loss of heat by radiation, leaving behind it portions of its mass, which condensed to form the planets. still gaseous, though now denser than water, it continues to pour out the heat on which our existence depends, as it shrinks imperceptibly toward its ultimate condition of a cold and darkened globe. laplace's hypothesis has been subjected in recent years to much criticism, and there is good reason to doubt whether his description of the mode of evolution of our solar system is correct in every particular. all critics agree, however, that the sun was once enormously larger than it now is, and that the planets originally formed part of its distended mass. even in its present diminished state, the sun is huge beyond easy conception. our own earth, though so minute a fragment of the primeval sun, is nevertheless so large that some parts of its surface have not yet been explored. seen beside the sun, by an observer on one of the planets, the earth would appear as an insignificant speck, which could be swallowed with ease by the whirling vortex of a sun-spot. if the sun were hollow, with the earth at its centre, the moon, though , miles from us, would have room and to spare in which to describe its orbit, for the sun is , miles in diameter, so that its volume is more than a million times that of the earth. [illustration: fig. . gaseous prominence at the sun's limb, , miles high (ellerman). photographed with the spectroheliograph, using the light emitted by glowing calcium vapor. the comparative size of the earth is indicated by the white circle.] but what of the stars, proved by the spectroscope to be self-luminous, intensely hot, and formed of the same chemical elements that constitute the sun and the earth? are they comparable in size with the sun? do they occur in all stages of development, from infancy to old age? and if such stages can be detected, do they afford indications of the gradual diminution in volume which laplace imagined the sun to experience? [illustration: fig. . the sun, , miles in diameter, from a direct photograph showing many sun-spots (whitney) the small black disk in the centre represents the comparative size of the earth, while the circle surrounding it corresponds in diameter to the orbit of the moon.] star images prior to the application of the powerful new engine of research described in this article we have had no means of measuring the diameters of the stars. we have measured their distances and their motions, determined their chemical composition, and obtained undeniable evidence of progressive development, but even in the most powerful telescopes their images are so minute that they appear as points rather than as disks. in fact, the larger the telescope and the more perfect the atmospheric conditions at the observer's command, the smaller do these images appear. on the photographic plate, it is true, the stars are recorded as measurable disks, but these are due to the spreading of the light from their bright point-like images, and their diameters increase as the exposure time is prolonged. from the images of the brighter stars rays of light project in straight lines, but these also are instrumental phenomena, due to diffraction of light by the steel bars that support the small mirror in the tube of reflecting telescopes. in a word, the stars are so remote that the largest and most perfect telescopes show them only as extremely minute needle-points of light, without any trace of their true disks. [illustration: fig. . great sun-spot group, august , (whitney). the disk in the corner represents the comparative size of the earth.] how, then, may we hope to measure their diameters? by using, as the man of science must so often do, indirect means when the direct attack fails. most of the remarkable progress of astronomy during the last quarter-century has resulted from the application of new and ingenious devices borrowed from the physicist. these have multiplied to such a degree that some of our observatories are literally physical laboratories, in which the sun and stars are examined by powerful spectroscopes and other optical instruments that have recently advanced our knowledge of physics by leaps and bounds. in the present case we are indebted for our star-measuring device to the distinguished physicist professor albert a. michelson, who has contributed a long array of novel apparatus and methods to physics and astronomy. the interferometer the instrument in question, known as the interferometer, had previously yielded a remarkable series of results when applied in its various forms to the solution of fundamental problems. to mention only a few of those that have helped to establish michelson's fame, we may recall that our exact knowledge of the length of the international metre at sevres, the world's standard of measurement, was obtained by him with an interferometer in terms of the invariable length of light-waves. a different form of interferometer has more recently enabled him to measure the minute tides within the solid body of the earth--not the great tides of the ocean, but the slight deformations of the earth's body, which is as rigid as steel, that are caused by the varying attractions of the sun and moon. finally, to mention only one more case, it was the michelson-morley experiment, made years ago with still another form of interferometer, that yielded the basic idea from which the theory of relativity was developed by lorentz and einstein. [illustration: fig. . photograph of the hydrogen atmosphere of the sun (ellerman). made with the spectroheliograph, showing the immense vortices, or whirling storms like tornadoes, that centre in sun-spots. the comparative size of the earth is shown by the white circle traced on the largest sun-spot.] the history of the method of measuring star diameters is a very curious one, showing how the most promising opportunities for scientific progress may lie unused for decades. the fundamental principle of the device was first suggested by the great french physicist fizeau in . in the theory was developed by the french astronomer stéphan, who observed interference fringes given by a large number of stars, and rightly concluded that their angular diameters must be much smaller than . of a second of arc, the smallest measurable with his instrument. in michelson, unaware of the earlier work, published in the _philosophical magazine_ a complete description of an interferometer capable of determining with surprising accuracy the distance between the components of double stars so close together that no telescope can separate them. he also showed how the same principle could be applied to the measurement of star diameters if a sufficiently large interferometer could be built for this purpose, and developed the theory much more completely than stéphan had done. a year later he measured the diameters of jupiter's satellites by this means at the lick observatory. but nearly thirty years elapsed before the next step was taken. two causes have doubtless contributed to this delay. both theory and experiment have demonstrated the extreme sensitiveness of the "interference fringes," on the observation of which the method depends, and it was generally supposed by astronomers that disturbances in the earth's atmosphere would prevent them from being clearly seen with large telescopes. furthermore, a very large interferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double stars could have been easily studied by this device with several of the large telescopes of the early nineties. but whatever the reasons, a powerful method of research lay unused. the approaching completion of the -inch telescope of the mount wilson observatory led me to suggest to professor michelson, before the united states entered the war, that the method be thoroughly tested under the favorable atmospheric conditions of southern california. he was at that time at work on a special form of interferometer, designed to determine whether atmospheric disturbances could be disregarded in planning large-scale experiments. but the war intervened, and all of our efforts were concentrated for two years on the solution of war problems.[*] in , as soon as the -inch telescope had been completed and tested, the work was resumed on mount wilson. [footnote *: professor michelson's most important contribution during the war period was a new and very efficient form of range-finder, adopted for use by the u. s. navy.] a laboratory experiment the principle of the method can be most readily seen by the aid of an experiment which any one can easily perform for himself with simple apparatus. make a narrow slit, a few thousandths of an inch in width, in a sheet of black paper, and support it vertically before a brilliant source of light. observe this from a distance of or feet with a small telescope magnifying about diameters. the object-glass of the telescope should be covered with an opaque cap, pierced by two circular holes about one-eighth of an inch in diameter and half an inch apart. the holes should be on opposite sides of the centre of the object-glass and equidistant from it, and the line joining the holes should be horizontal. when this cap is removed the slit appears as a narrow vertical band with much fainter bands on both sides of it. with the cap in place, the central bright band appears to be ruled with narrow vertical lines or fringes produced by the "interference"[*] of the two pencils of light coming through different parts of the object-glass from the distant slit. cover one of the holes, and the fringes instantly disappear. their production requires the joint effect of the two light-pencils. [footnote *: for an explanation of the phenomena of interference, see any encyclopæedia or book on physics.] now suppose the two holes over the object-glass to be in movable plates, so that their distance apart can be varied. as they are gradually separated the narrow vertical fringes become less and less distinct, and finally vanish completely. measure the distance between the holes and divide this by the wavelength of light, which we may call / of an inch. the result is the angular width of the distant slit. knowing the distance of the slit, we can at once calculate its linear width. if for the slit we substitute a minute circular hole, the method of measurement remains the same, but the angular diameter as calculated above must be multiplied by . .[*] [footnote *: more complete details may be found in michelson's lowell lectures on "light-waves and their uses," university of chicago press, .] to measure the diameter of a star we proceed in a similar way, but, as the angle it subtends is so small, we must use a very large telescope, for the smaller the angle the farther apart must be the two holes over the object-glass (or the mirror, in case a reflecting telescope is employed). in fact, when the holes are moved apart to the full aperture of the -inch hooker telescope, the interference fringes are still visible even with the star betelgeuse, though its angular diameter is perhaps as great as that of any other star. thus, we must build an attachment for the telescope, so arranged as to permit us to move the openings still farther apart. [illustration: fig. . diagram showing outline of the -inch hooker telescope, and path of the two pencils of light from a star when under observation with the -foot michelson interferometer. a photograph of the interferometer is shown in fig. .] the -foot instrument the -foot interferometer designed by messrs. michelson and pease, and constructed in the mount wilson observatory instrument-shop, is shown in the diagram (fig. ) and in a photograph of the upper end of the skeleton tube of the telescope (fig. ). the light from the star is received by two flat mirrors (ml, m ) which project beyond the tube and can be moved apart along the supporting arm. these take the place of the two holes over the object-glass in our experiment. from these mirrors the light is reflected to a second pair of flat mirrors (m , m ), which send it toward the -inch concave mirror (m ) at the bottom of the telescope tube. after this the course of the light is exactly as it would be if the mirrors m , m were replaced by two holes over the -inch mirror. it is reflected to the convex mirror (m ), then back in a less rapidly convergent beam toward the large mirror. before reaching it the light is caught by the plane mirror (m ) and reflected through an opening at the side of the telescope tube to the eye-piece e. here the fringes are observed with a magnification ranging from , to , diameters. [illustration: fig. . twenty-foot michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the -inch hooker telescope. the path of the two pencils of light from the star is shown in fig. . for a photograph of the entire telescope, see fig. .] in the practical application of this method to the measurement of star diameters, the chief problem was whether the atmosphere would be quiet enough to permit sharp interference fringes to be produced with light-pencils more than inches apart. after successful preliminary tests with the -inch refracting telescope of the yerkes observatory, professor michelson made the first attempt to see the fringes with the -inch and -inch reflectors on mount wilson in september, . he was surprised and delighted to find that the fringes were perfectly sharp and distinct with the full aperture of both these instruments. doctor anderson, of the observatory staff, then devised a special form of interferometer for the measurement of close double stars, and applied it with the -inch telescope to the measurement of the orbital motion of the close components of capella, with results of extraordinary accuracy, far beyond anything attainable by previous methods. the success of this work strongly encouraged the more ambitious project of measuring the diameter of a star, and the -foot interferometer was built for this purpose. the difficult and delicate problem of adjusting the mirrors of this instrument with the necessary extreme accuracy was solved by professor michelson during his visit to mount wilson in the summer of , and with the assistance of mr. pease, of the observatory staff, interference fringes were observed in the case of certain stars when the mirrors were as much as feet apart. all was thus in readiness for a decisive test as soon as a suitable star presented itself. the giant betelgeuse russell, shapley, and eddington had pointed out betelgeuse (arabic for "the giant's shoulder"), the bright red star in the constellation of orion (fig. ), as the most favorable of all stars for measurement, and the last-named had given its angular diameter as . of a second of arc. this deduction from theory appeared in his recent presidential address before the british association for the advancement of science, in which professor eddington remarked: "probably the greatest need of stellar astronomy at the present day, in order to make sure that our theoretical deductions are starting on the right lines, is some means of measuring the apparent angular diameter of stars." he then referred to the work already in progress on mount wilson, but anticipated "that atmospheric disturbance will ultimately set the limit to what can be accomplished." [illustration: fig. . the giant betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of orion (hubble). measures with the interferometer show its angular diameter to be . of a second of arc, corresponding to a linear diameter of , , miles, if the best available determination of its distance can be relied upon. this determination shows betelgeuse to be light-years from the earth. light travels at the rate of , miles per second, and yet spends years on its journey to us from this star.] on december , , mr. pease successfully measured the diameter of betelgeuse with the -foot interferometer. as the outer mirrors were separated the interference fringes gradually became less distinct, as theory requires, and as doctor merrill had previously seen when observing betelgeuse with the interferometer used for capella. at a separation of feet the fringes disappeared completely, giving the data required for calculating the diameter of the star. to test the perfection of the adjustment, the telescope was turned to other stars, of smaller angular diameter, which showed the fringes with perfect clearness. turning back to betelgeuse, they were seen beyond doubt to be absent. assuming the mean wave-length of the light of this star to be / of a millimetre, its angular diameter comes out . of a second of arc, thus falling between the values-- . and . of a second--predicted by eddington and russell from slightly different assumptions. subsequent corrections and repeated measurement will change mr. pease's result somewhat, but it is almost certainly within or per cent of the truth. we may therefore conclude that the angular diameter of betelgeuse is very nearly the same as that of a ball one inch in diameter, seen at a distance of seventy miles. [illustration: fig. . arcturus (within the white circle), known to the arabs as the "lance bearer," and to the chinese as the "great horn" or the "palace of the emperors" (hubble). its angular diameter, measured at mount wilson by pease with the -foot michelson interferometer on april , , is . of a second, in close agreement with russell's predicted value of . of a second. the mean parallax of arcturus, based upon several determinations, is . of a second, corresponding to a distance of light-years. the linear diameter, computed from pease's measure and this value of the distance is about million miles.] but this represents only the angle subtended by the star's disk. to learn its linear diameter, we must know its distance. four determinations of the parallax, which determines the distance, have been made. elkin, with the yale heliometer, obtained . of a second of arc. schlesinger, from photographs taken with the -inch allegheny refractor, derived . . adams, by his spectroscopic method applied with the -inch mount wilson reflector, obtained . . lee's recent value, secured photographically with the -inch yerkes refractor, is . . the heliometer parallax is doubtless less reliable than the photographic ones, and doctor adams states that the spectral type and luminosity of betelgeuse make his value less certain than in the case of most other stars. if we take a (weighted) mean value of . of a second, we shall probably not be far from the truth. this parallax represents the angle subtended by the radius of the earth's orbit ( , , miles) at the distance of betelgeuse. by comparing it with . , the angular diameter of the star, we see that the linear diameter is about two and one-third times as great as the distance from the earth to the sun, or approximately , , miles. thus, if this measure of its distance is not considerably in error, betelgeuse would nearly fill the orbit of mars. all methods of determining the distances of the stars are subject to uncertainty, however, and subsequent measures may reduce this figure very appreciably. but there can be no doubt that the diameter of betelgeuse exceeds , , miles, and it is probably much greater. the extremely small angle subtended by this enormous disk is explained by the great distance of the star, which is about light-years. that is to say, light travelling at the rate of , miles per second spends years in crossing the space that lies between us and betelgeuse, whose tremendous proportions therefore seem so minute even in the most powerful telescopes. stellar evolution this actual measure of the diameter of betelgeuse supplies a new and striking test of russell's and hertzsprung's theory of dwarf and giant stars. just before the war russell showed that our old methods of classifying the stars according to their spectra must be radically changed. stars in an early stage of their life history may be regarded as diffuse gaseous masses, enormously larger than our sun, and at a much lower temperature. their density must be very low, and their state that of a perfect gas. these are the "giants." in the slow process of time they contract through constant loss of heat by radiation. but, despite this loss, the heat produced by contraction and from other sources (see p. ) causes their temperature to rise, while their color changes from red to bluish white. the process of shrinkage and rise of temperature goes on so long as they remain in the state of a perfect gas. but as soon as contraction has increased the density of the gas beyond a certain point the cycle reverses and the temperature begins to fall. the bluish-white light of the star turns yellowish, and we enter the dwarf stage, of which our own sun is a representative. the density increases, surpassing that of water in the case of the sun, and going far beyond this point in later stages. in the lapse of millions of years a reddish hue appears, finally turning to deep red. the falling temperature permits the chemical elements, existing in a gaseous state in the outer atmosphere of the star, to unite into compounds, which are rendered conspicuous by their characteristic bands in the spectrum. finally comes extinction of light, as the star approaches its ultimate state of a cold and solid globe. [illustration: fig. . the giant star antares (within the white circle), notable for its red color in the constellation scorpio, and named by the greeks "a rival of mars" (hubble). the distance of antares, though not very accurately known, is probably not far from light-years. its angular diameter of . of a second would thus correspond to a linear diameter of about million miles.] we may thus form a new picture of the two branches of the temperature curve, long since suggested by lockyer, on very different grounds, as the outline of stellar life. on the ascending side are the giants, of vast dimensions and more diffuse than the air we breathe. there are good reasons for believing that the mass of betelgeuse cannot be more than ten times that of the sun, while its volume is at least a million times as great and may exceed eight million times the sun's volume. therefore, its average density must be like that of an attenuated gas in an electric vacuum tube. three-quarters of the naked-eye stars are in the giant stage, which comprises such familiar objects as betelgeuse, antares, and aldebaran, but most of them are much denser than these greatly inflated bodies. the pinnacle is reached in the intensely hot white stars of the helium class, in whose spectra the lines of this gas are very conspicuous. the density of these stars is perhaps one-tenth that of the sun. sirius, also very hot, is nearly twice as dense. then comes the cooling stage, characterized, as already remarked, by increasing density, and also by increasing chemical complexity resulting from falling temperature. this life cycle is probably not followed by all stars, but it may hold true for millions of them. the existence of giant and dwarf stars has been fully proved by the remarkable work of adams and his associates on mount wilson, where his method of determining a star's distance and intrinsic luminosity by spectroscopic observations has already been applied to , stars. discussion of the results leads at once to the recognition of the two great classes of giants and dwarfs. now comes the work of michelson and pease to cap the climax, giving us the actual diameter of a typical giant star, in close agreement with predictions based upon theory. from this diameter we may conclude that the density of betelgeuse is extremely low, in harmony with russell's theory, which is further supported by spectroscopic analysis of the star's light, revealing evidence of the comparatively low temperature called for by the theory at this early stage of stellar existence. two other giants the diameter of arcturus was successfully measured by mr. pease at mount wilson on april . as the mirrors of the interferometer were moved apart, the fringes gradually decreased in visibility until they finally disappeared at a mirror separation of . feet. adopting a mean wave-length of / of a millimetre for the light of arcturus, this gives a value of . of a second of arc for the angular diameter of the star. if we use a mean value of . of a second for the parallax, the corresponding linear diameter comes out , , miles. the angular diameter, as in the case of betelgeuse, is in remarkably close agreement with the diameter predicted from theory. antares, the third star measured by mr. pease, is the largest of all. if it is actually a member of the scorpius-centaurus group, as we have strong reason to believe, it is fully light-years from the earth, and its diameter is about , , miles. [illustration: fig. . diameters of the sun, arcturus, betelgeuse, and antares compared with the orbit of mars. sun, diameter, , miles. arcturus, diameter, , , miles. betelgeuse, diameter, , , miles. antares, diameter, , , miles.] it now remains to make further measures of betelgeuse, especially because its marked changes in brightness suggest possible variations in diameter. we must also apply the interferometer method to stars of the various spectral types, in order to afford a sure basis for future studies of stellar evolution. unfortunately, only a few giant stars are certain to fall within the range of our present instrument. an interferometer of -feet aperture would be needed to measure sirius accurately, and one of twice this size to deal with less brilliant white stars. a -foot instrument, if feasible to build, would permit objects representing most of the chief stages of stellar development to be measured, thus contributing in the highest degree to the progress of our knowledge of the life history of the stars. fortunately, though the mechanical difficulties are great, the optical problem is insignificant, and the cost of the entire apparatus, though necessarily high, would be only a small fraction of that of a telescope of corresponding aperture, if such could be built. a -foot interferometer might be designed in many different forms, and one of these may ultimately be found to be within the range of possibility. meanwhile the -foot interferometer has been improved so materially that it now promises to yield approximate measures of stars at first supposed to be beyond its capacity. [illustration: fig. . aldebaran, the "leader" (of the pleiades), was also known to the arabs as "the eye of the bull," "the heart of the bull," and "the great camel" (hubble). like betelgeuse and antares, it is notable for its red color, which accounts for the fact that its image on this photograph is hardly more conspicuous than the images of stars which are actually much fainter but contain a larger proportion of blue light, to which the photographic plates here employed are more sensitive than to red or yellow. aldebaran is about light-years from the earth. interferometer measures, now in progress on mount wilson, indicate that its angular diameter is about . of a second.] while the theory of dwarf and giant stars and the measurements just described afford no direct evidence bearing on laplace's explanation of the formation of planets, they show that stars exist which are comparable in diameter with our solar system, and suggest that the sun must have shrunk from vast dimensions. the mode of formation of systems like our own, and of other systems numerously illustrated in the heavens, is one of the most fascinating problems of astronomy. much light has been thrown on it by recent investigations, rendered possible by the development of new and powerful instruments and by advances in physics of the most fundamental character. all the evidence confirms the existence of dwarf and giant stars, but much work must be done before the entire course of stellar evolution can be explained. chapter iii cosmic crucibles "shelter during raids," marking the entrance to underground passages, was a sign of common occurrence and sinister suggestion throughout london during the war. with characteristic ingenuity and craftiness, ostensibly for purposes of peace but with bomb-carrying capacity as a prime specification, the zeppelin had been developed by the germans to a point where it seriously threatened both london and paris. searchlights, range-finders, and anti-aircraft guns, surpassed by the daring ventures of british and french airmen, would have served but little against the night invader except for its one fatal defect--the inflammable nature of the hydrogen gas that kept it aloft. a single explosive bullet served to transform a zeppelin into a heap of scorched and twisted metal. this characteristic of hydrogen caused the failure of the zeppelin raids. had the war lasted a few months longer, however, the work of american scientists would have made our counter-attack in the air a formidable one. at the signing of the armistice hundreds of cylinders of compressed helium lay at the docks ready for shipment abroad. extracted from the natural gas of texas wells by new and ingenious processes, this substitute for hydrogen, almost as light and absolutely uninflammable, produced in quantities of millions of cubic feet, would have made the dirigibles of the allies masters of the air. the special properties of this remarkable gas, previously obtainable only in minute quantities, would have sufficed to reverse the situation. solar helium helium, as its name implies, is of solar origin. in , when lockyer first directed his spectroscope to the great flames or prominences that rise thousands of miles, sometimes hundreds of thousands, above the surface of the sun, he instantly identified the characteristic red and blue radiations of hydrogen. in the yellow, close to the position of the well-known double line of sodium, but not quite coincident with it, he detected a new line, of great brilliancy, extending to the highest levels. its similarity in this respect with the lines of hydrogen led him to recognize the existence of a new and very light gas, unknown to terrestrial chemistry. many years passed before any chemical laboratory on earth was able to match this product of the great laboratory of the sun. in ramsay at last succeeded in separating helium, recognized by the same yellow line in its spectrum, in minute quantities from the mineral uraninite. once available for study under electrical excitation in vacuum tubes, helium was found to have many other lines in its spectrum, which have been identified in the spectra of solar prominences, gaseous nebulæ, and hot stars. indeed, there is a stellar class known as helium stars, because of the dominance of this gas in their atmospheres. [illustration: fig. . solar prominences, photographed with the spectroheliograph without an eclipse (ellerman). in these luminous gaseous clouds, which sometimes rise to elevations exceeding half the sun's diameter, the new gas helium was discovered by lockyer in . helium was not found on the earth until . since then it has been shown to be a prominent constituent of nebulæ and hot stars.] the chief importance of helium lies in the clue it has afforded to the constitution of matter and the transmutation of the elements. radium and other radioactive substances, such as uranium, spontaneously emit negatively charged particles of extremely small mass (electrons), and also positively charged particles of much greater mass, known as alpha particles. rutherford and geiger actually succeeded in counting the number of alpha particles emitted per second by a known mass of radium, and showed that these were charged helium atoms. to discuss more at length the extraordinary characteristics of helium, which plays so large a part in celestial affairs, would take us too far afield. let us therefore pass to another case in which a fundamental discovery, this time in physics, was first foreshadowed by astronomical observation. sun-spots as magnets no archæologist, whether young or champollion deciphering the rosetta stone, or rawlinson copying the cuneiform inscription on the cliff of behistun, was ever faced by a more fascinating problem than that which confronts the solar physicist engaged in the interpretation of the hieroglyphic lines of sun-spot spectra. the colossal whirling storms that constitute sun-spots, so vast that the earth would make but a moment's scant mouthful for them, differ materially from the general light of the sun when examined with the spectroscope. observing them visually many years ago, the late professor young, of princeton, found among their complex features a number of double lines which he naturally attributed, in harmony with the physical knowledge of the time, to the effect of "reversal" by superposed layers of vapors of different density and temperature. what he actually saw, however, as was proved at the mount wilson observatory in , was the effect of a powerful magnetic field on radiation, now known as the zeeman effect. [illustration: fig. . the -foot tower telescope of the mount wilson observatory. an image of the sun about inches in diameter is formed in the laboratory at the base of the tower. below this, in a well extending feet into the earth, is the powerful spectroscope with which the magnetic fields in sun-spots and the general magnetic field of the sun are studied.] faraday was the first to detect the influence of magnetism on light. between the poles of a large electromagnet, powerful for those days ( ), he placed a block of very dense glass. the plane of polarization of a beam of light, which passed unaffected through the glass before the switch was closed, was seen to rotate when the magnetic field was produced by the flow of the current. a similar rotation is now familiar in the well-known tests of sugars--lævulose and dextrose--which rotate plane-polarized light to left and right, respectively. but in this first discovery of a relationship between light and magnetism faraday had not taken the more important step that he coveted--to determine whether the vibration period of a light-emitting particle is subject to change in a magnetic field. he attempted this in --the last experiment of his life. a sodium flame was placed between the poles of a magnet, and the yellow lines were watched in a spectroscope when the magnet was excited. no change could be detected, and none was found by subsequent investigators until zeeman, of leiden, with more powerful instruments made his famous discovery, the twenty-fifth anniversary of which has recently been celebrated. [illustration: fig. . pasadena laboratory of the mount wilson observatory. showing the large magnet (on the left) and the spectroscopes used for the study of the effect of magnetism on radiation. a single line in the spectrum is split by the magnetic field into from three to twenty-one components, as illustrated in fig. . the corresponding lines in the spectra of sun-spots are split up in precisely the same way, thus indicating the presence of powerful magnetic fields in the sun.] his method of procedure was similar to faraday's, but his magnet and spectroscope were much more powerful, and a theory due to lorentz, predicting the nature of the change to be expected, was available as a check on his results. when the current was applied the lines were seen to widen. in a still more powerful magnetic field each of them split into two components (when the observation was made along the lines of force), and the light of the components of each line was found to be circularly polarized in opposite directions. strictly in harmony with lorentz's theory, this splitting and polarization proved the presence in the luminous vapor of exactly such negatively charged electrons as had been indicated there previously by very different experimental methods. in great cyclonic storms, or vortices, were discovered at the mount wilson observatory centring in sun-spots. such whirling masses of hot vapors, inferred from sir joseph thomson's results to contain electrically charged particles, should give rise to a magnetic field. this hypothesis at once suggested that the double lines observed by young might really represent the zeeman effect. the test was made, and all the characteristic phenomena of radiation in a magnetic field were found. thus a great physical experiment is constantly being performed for us in the sun. every large sunspot contains a magnetic field covering many thousands of square miles, within which the spectrum lines of iron, manganese, chromium, titanium, vanadium, calcium, and other metallic vapors are so powerfully affected that their widening and splitting can be seen with telescopes and spectroscopes of moderate size. the tower telescope both of these illustrations show how the physicist and chemist, when adequately armed for astronomical attack, can take advantage in their studies of the stupendous processes visible in cosmic crucibles, heated to high temperatures and influenced, as in the case of sun-spots, by intense magnetic fields. certain modern instruments, like the -foot and -foot tower telescopes on mount wilson, are especially designed for observing the course of these experiments. the second of these telescopes produces at a fixed point in a laboratory an image of the sun about inches in diameter, thus enlarging the sun-spots to such a scale that the magnetic phenomena of their various parts can be separately studied. this analysis is accomplished with a spectroscope feet in length, mounted in a subterranean chamber beneath the tower. the varied results of such investigations cannot be described here. only one of them may be mentioned--the discovery that the entire sun, rotating on its axis, is a great magnet. hence we may reasonably infer that every star, and probably every planet, is also a magnet, as the earth has been known to be since the days of gilbert's "de magnete." here lies one of the best clues for the physicist who seeks the cause of magnetism, and attempts to produce it, as barnett has recently succeeded in doing, by rapidly whirling masses of metal in the laboratory. [illustration: fig. . sun-spot vortex in the upper hydrogen atmosphere. (benioff). photographed with the spectroheliograph. the electric vortex that causes the magnetic field of the spot lies at a lower level, and is not shown by such photographs.] perhaps a word of caution should be interpolated at this point. solar magnetism in no wise accounts for the sun's gravitational power. indeed, its attraction cannot be felt by the most delicate instruments at the distance of the earth, and would still be unknown were it not for the influence of magnetism on light. auroras, magnetic storms, and such electric currents as those that recently deranged several atlantic cables are due, not to the magnetism of the sun or its spots, but probably to streams of electrons, shot out from highly disturbed areas of the solar surface surrounding great sun-spots, traversing ninety-three million miles of the ether of space, and penetrating deep into the earth's atmosphere. these striking phenomena lead us into another chapter of physics, which limitations of space forbid us to pursue. stellar chemistry let us turn again to chemistry, and see where experiments performed in cosmic laboratories can serve as a guide to the investigator. a spinning solar tornado, incomparably greater in scale than the devastating whirlwinds that so often cut narrow paths of destruction through town and country in the middle west, gradually gives rise to a sun-spot. the expansion produced by the centrifugal force at the centre of the storm cools the intensely hot gases of the solar atmosphere to a point where chemical union can occur. titanium and oxygen, too hot to combine in most regions of the sun, join to form the vapor of titanium oxide, characterized in the sunspot spectrum by fluted bands, made up of hundreds of regularly spaced lines. similarly magnesium and hydrogen combine as magnesium hydride and calcium and hydrogen form calcium hydride. none of these compounds, stable at the high temperatures of sun-spots, has been much studied in the laboratory. the regions in which they exist, though cooler than the general atmosphere of the sun, are at temperatures of several thousand degrees, attained in our laboratories only with the aid of such devices as powerful electric furnaces. [illustration: fig. . splitting of spectrum lines by a magnetic field (babcock). the upper and lower strips show lines in the spectrum of chromium, observed without a magnetic field. when subjected to the influence of magnetism, these single lines are split into several components. thus the first line on the right is resolved by the field into three components, one of which (plane polarized) appears in the second strip, while the other two, which are polarized in a plane at right angles to that of the middle component, are shown on the third strip. the next line is split by the magnetic field into twelve components, four of which appear in the second strip and eight in the third. the magnetic fields in sun-spots affect these lines in precisely the same way.] it is interesting to follow our line of reasoning to the stars, which differ widely in temperature at various stages in their life-cycle.[*] a sun-spot is a solar tornado, wherein the intensely hot solar vapors are cooled by expansion, giving rise to the compounds already named. a red star, in russell's scheme of stellar evolution, is a cooler sun, vast in volume and far more tenuous than atmospheric air when in the initial period of the "giant" stage, but compressed and denser than water in the "dwarf" stage, into which our sun has already entered as it gradually approaches the last phases of its existence. therefore we should find, throughout the entire atmosphere of such stars, some of the same compounds that are produced within the comparatively small limits of a sun-spot. this, of course, on the correct assumption that sun and stars are made of the same substances. fowler has already identified the bands of titanium oxide in such red stars as the giant betelgeuse, and in others of its class. it is safe to predict that an interesting chapter in the chemistry of the future will be based upon the study of such compounds, both in the laboratory and under the progressive temperature conditions afforded by the countless stellar "giants" and "dwarfs" that precede and follow the solar state. [footnote *: see chapter ii.] [illustration: fig. . electric furnace in the pasadena laboratory of the mount wilson observatory. with which the chemical phenomena observed in sun-spots and red stars are experimentally imitated.] astrophysical laboratories it is precisely in this long sequence of physical and chemical changes that the astrophysicist and the astrochemist can find the means of pushing home their attack. it is true, of course, that the laboratory investigator has a great advantage in his ability to control his experiments, and to vary their progress at will. but by judicious use of the transcendental temperatures, far out ranging those of his furnaces, and extreme conditions, which he can only partially imitate, afforded by the sun, stars, and nebulæ, he may greatly widen the range of his inquiries. the sequence of phenomena seen during the growth of a sun-spot, or the observation of spots of different sizes, and the long series of successive steps that mark the rise and decay of stellar life, resemble the changes that the experimenter brings about as he increases and diminishes the current in the coils of his magnet or raises and lowers the temperature of his electric furnace, examining from time to time the spectrum of the glowing vapors, and noting the changes shown by the varying appearance of their lines. [illustration: fig. . titanium oxide in red stars. the upper spectrum is that of titanium in the flame of the electric arc, where its combination with oxygen gives rise to the bands of titanium oxide (fowler). the lower strip shows the spectrum of the red star mira (omicron ceti), as drawn by cortie at stonyhurst. the bands of titanium oxide are clearly present in the star.] [illustration: fig. . titanium oxide in sun-spots. the upper strip shows a portion of the spectrum of a sun-spot (ellerman); the lower one the corresponding region of the spectrum of titanium oxide (king). the fluted bands of the oxide spectrum are easily identified in the spot, where they indicate that titanium and oxygen, too hot to combine in the solar atmosphere, unite in the spot because of the cooling produced by expansion in the vortex.] astronomical observations of this character, it should be noted, are most effective when constantly tested and interpreted by laboratory experiment. indeed, a modern astrophysical observatory should be equipped like a great physical laboratory, provided on the one hand with telescopes and accessory apparatus of the greatest attainable power, and on the other with every device known to the investigator of radiation and the related physical and chemical phenomena. its telescopes, especially designed with the aims of the physicist and chemist in view, bring images of sun, stars, nebulæ, and other heavenly bodies within the reach of powerful spectroscopes, sensitive bolometers and thermopiles, and the long array of other appliances available for the measurement and analysis of radiation. its electric furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing and decreasing pressure, varying chemical conditions, and subjecting luminous gases and vapors to the influence of electric and magnetic fields, provide the means of imitating celestial phenomena, and of repeating and interpreting the experiments observed at the telescope. and the advantage thus derived, as we have seen, is not confined to the astronomer, who has often been able, by making fundamental physical and chemical discoveries, to repay his debt to the physicist and chemist for the apparatus and methods which he owes to them. newton and einstein take, for another example, the greatest law of physics--newton's law of gravitation. huge balls of lead, as used by cavendish, produce by their gravitational effect a minute rotation of a delicately suspended bar, carrying smaller balls at its extremities. but no such feeble means sufficed for newton's purpose. to prove the law of gravitation he had recourse to the tremendous pull on the moon of the entire mass of the earth, and then extended his researches to the mutual attractions of all the bodies of the solar system. later herschel applied this law to the suns which constitute double stars, and to-day adams observes from mount wilson stars falling with great velocity toward the centre of the galactic system under the combined pull of the millions of objects that compose it. thus full advantage has been taken of the possibility of utilizing the great masses of the heavenly bodies for the discovery and application of a law of physics and its reciprocal use in explaining celestial motions. [illustration: fig. . the cavendish experiment. two lead balls, each two inches in diameter, are attached to the ends of a torsion rod six feet long, which is suspended by a fine wire. the experiment consists in measuring the rotation of the suspended system, caused by the gravitational attraction of two lead spheres, each twelve inches in diameter, acting on the two small lead balls.] or consider the einstein theory of relativity, the truth or falsity of which is no less fundamental to physics. its inception sprang from the michelson-morley experiment, made in a laboratory in cleveland, which showed that motion of the earth through the ether of space could not be detected. all of the three chief tests of einstein's general theory are astronomical--because of the great masses required to produce the minute effects predicted: the motion of the perihelion of mercury, the deflection of the light of a star by the attraction of the sun, and the shift of the lines of the solar spectrum toward the red--questions not yet completely answered. but it is in the study of the constitution of matter and the evolution of the elements, the deepest and most critical problem of physics and chemistry, that the extremes of pressure and temperature in the heavenly bodies, and the prevalence of other physical conditions not yet successfully imitated on earth, promise the greatest progress. it fortunately happens that astrophysical research is now at the very apex of its development, founded as it is upon many centuries of astronomical investigation, rejuvenated by the introduction into the observatory of all the modern devices of the physicist, and strengthened with instruments of truly extraordinary range and power. these instruments bring within reach experiments that are in progress on some minute region of the sun's disk, or in some star too distant even to be glimpsed with ordinary telescopes. indeed, the huge astronomical lenses and mirrors now available serve for these remote light-sources exactly the purpose of the lens or mirror employed by the physicist to project upon the slit of his spectroscope the image of a spark or arc or vacuum tube within which atoms and molecules are exposed to the influence of the electric discharge. the physicist has the advantage of complete control over the experimental conditions, while the astrophysicist must observe and interpret the experiments performed for him in remote laboratories. in actual practice, the two classes of work must be done in the closest conjunction, if adequate utilization is to be made of either. and this is only natural, for the trend of recent research has made clear the fact that one of the three greatest problems of modern astronomy and astrophysics, ranking with the structure of the universe and the evolution of celestial bodies, is the constitution of matter. let us see why this is so. transmutation of the elements the dream of the alchemist was to transmute one element into another, with the prime object of producing gold. such transmutation has been actually accomplished within the last few years, but the process is invariably one of disintegration--the more complex elements being broken up into simpler constituents. much remains to be done in this same direction; and here the stars and nebulæ, which show the spectra of the elements under a great variety of conditions, should help to point the way. the progressive changes in spectra, from the exclusive indications of the simple elements hydrogen, helium, nitrogen, possibly carbon, and the terrestrially unknown gas nebulium in the gaseous nebulæ, to the long list of familiar substances, including several chemical compounds, in the red stars, may prove to be fundamentally significant when adequately studied from the standpoint of the investigator of atomic structure. the existing evidence seems to favor the view, recently expressed by saha, that many of these differences are due to varying degrees of ionization, the outer electrons of the atoms being split off by high temperature or electrical excitation. it is even possible that cosmic crucibles, unrivalled by terrestrial ones, may help materially to reveal the secret of the formation of complex elements from simpler ones. physicists now believe that all of the elements are compounded of hydrogen atoms, bound together by negative electrons. thus helium is made up of four hydrogen atoms, yet the atomic weight of helium ( ) is less than four times that of hydrogen ( . ). the difference may represent the mass of the electrical energy released when the transmutation occurred. [illustration: fig. . the trifid nebula in sagittarius (ritchey). the gas "nebulium," not yet found on the earth, is the most characteristic constituent of irregular nebulæ. nebulium is recognized by two green lines in its spectrum, which cause the green color of nebulæ of the gaseous type.] eddington has speculated in a most interesting way on this possible source of stellar heat in his recent presidential address before the british association for the advancement of science (see _nature_, september , ). he points out that the old contraction hypothesis, according to which the source of solar and stellar heat was supposed to reside in the slow condensation of a radiating mass of gas under the action of gravity, is wholly inadequate to explain the observed phenomena. if the old view were correct, the earlier history of a star, from the giant stage of a cool and diaphanous gas to the period of highest temperature, would be run through within eighty thousand years, whereas we have the best of evidence that many thousands of centuries would not suffice. some other source of energy is imperatively needed. if per cent of a star's mass consists originally of hydrogen atoms, which gradually combine in the slow process of time to form more complex elements, the total heat thus liberated would more than suffice to account for all demands, and it would be unnecessary to assume the existence of any other source of heat. [illustration: fig. . spiral nebula in ursa major (ritchey). luminous matter, in every variety of physical and chemical state, is available for study in the most diverse celestial objects, from the spiral and irregular nebulæ through all the types of stars. doctor van maanen's measures of the mount wilson photographs indicate outward motion along the arms of spiral nebulæ, while the spectroscope shows them to be whirling at enormous velocities.] cosmic pressures this, it may fairly be said, is very speculative, but the fact remains that celestial bodies appear to be the only places in which the complex elements may be in actual process of formation from their known source--hydrogen. at least we may see what a vast variety of physical conditions these cosmic crucibles afford. at one end of the scale we have the excessively tenuous nebulæ, the luminosity of which, mysterious in its origin, resembles the electric glow in our vacuum tubes. here we can detect only the lightest and simplest of the elements. in the giant stars, also extremely tenuous (the density of betelgeuse can hardly exceed one-thousandth of an atmosphere) we observe the spectra of iron, manganese, titanium, calcium, chromium, magnesium, vanadium, and sodium, in addition to titanium oxide. the outer part of these bodies, from which light reaches us, must therefore be at a temperature of only a few thousand degrees, but vastly higher temperatures must prevail at their centres. in passing up the temperature curve more and more elements appear, the surface temperature rises, and the internal temperature may reach millions of degrees. at the same time the pressure within must also rise, reaching enormous figures in the last stages of stellar life. cook has calculated that the pressure at the centre of the earth is between , and , tons per square inch, and this must be only a very small fraction of that attained within larger celestial bodies. jeans has computed the pressure at the centre of two colliding stars as they strike and flatten, and finds it may be of the order of , , , tons per square inch--sufficient, if their diameter be equal to that of the sun--to vaporize them , times over. compare these pressures with the highest that can be produced on earth. if the german gun that bombarded paris were loaded with a solid steel projectile of suitable dimensions, a muzzle velocity of , feet per second could be reached. suppose this to be fired into a tapered hole in a great block of steel. the instantaneous pressure, according to cook, would be about , tons per square inch, only / of that possible through the collision of the largest stars. [illustration: fig. . mount san antonio as seen from mount wilson. michelson is measuring the velocity of light between stations on mount wilson and mount san antonio. astronomical observations afford the best means, however, of detecting any possible difference between the velocities of light of different colors. from studies of variable stars in the cluster messier shapley concludes that if there is any difference between the velocities of blue and yellow light in free space it cannot exceed two inches in one second, the time in which light travels , miles.] finally, we may compare the effects of light pressure on the earth and stars. twenty years ago nichols and hull succeeded, with the aid of the most sensitive apparatus, in measuring the minute displacements produced by the pressure of light. the effect is so slight, even with the brightest light-sources available, that great experimental skill is required to measure it. yet in the case of some of the larger stars eddington calculates that one-half of their mass is supported by radiation pressure, and this against their enormous gravitational attraction. in fact, if their mass were as great as ten times that of the sun, the radiation pressure would so nearly overcome the pull of gravitation that they would be likely to break up. but enough has been said to illustrate the wide variety of experimental devices that stand at our service in the laboratories of the heavens. here the physicist and chemist of the future will more and more frequently supplement their terrestrial apparatus, and find new clues to the complex problems which the amazing progress of recent years has already done so much to solve. practical value of researches on the constitution of matter the layman has no difficulty in recognizing the practical value of researches directed toward the improvement of the incandescent lamp or the increased efficiency of the telephone. he can see the results in the greatly decreased cost of electric illumination and the rapid extension of the range of the human voice. but the very men who have made these advances, those who have succeeded beyond all expectation in accomplishing the economic purposes in view, are most emphatic in their insistence upon the importance of research of a more fundamental character. thus vice-president j. j. carty, of the american telephone and telegraph company, who directs its great department of development and research, and doctor w. j. whitney, director of the research laboratory of the general electric company, have repeatedly expressed their indebtedness to the investigations of the physicist, made with no thought of immediate practical return. faraday, studying the laws of electricity, discovered the principle which rendered the dynamo possible. maxwell, henry, and hertz, equally unconcerned with material advantage, made wireless telegraphy practicable. in fact, all truly great advances are thus derived from fundamental science, and the future progress of the world will be largely dependent upon the provision made for scientific research, especially in the fields of physics and chemistry, which underlie all branches of engineering. the constitution of matter, therefore, instead of appealing as a subject to research only to the natural philosopher or to the general student of science, is a question of the greatest practical concern. already the by-products of investigations directed toward its elucidation have been numerous and useful in the highest degree. helium has been already cited; x-rays hardly require mention; radium, which has so materially aided sufferers from cancer, is still better known. wireless telephony and transcontinental telephony with wires were both rendered possible by studies of the nature of the electric discharge in vacuum tubes. thus the "practical man," with his distrust of "pure" science, need not resent investments made for the purpose of advancing our knowledge of such fundamental subjects as physics and chemistry. on the contrary, if true to his name, he should help to multiply them many fold in the interest of economic and commercial development. generously made available by the internet archive/american libraries.) transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. the story of the heavens [illustration: plate i. the planet saturn, in .] the story of the heavens sir robert stawell ball, ll.d. d.sc. _author of_ "_star-land_" fellow of the royal society of london, honorary fellow of the royal society of edinburgh, fellow of the royal astronomical society, scientific adviser to the commissioners of irish lights, lowndean professor of astronomy and geometry in the university of cambridge, and formerly royal astronomer of ireland _with twenty-four coloured plates and numerous illustrations_ new and revised edition cassell and company, limited _london, paris, new york & melbourne_ all rights reserved [illustration: la·belle sauvage] preface to original edition. i have to acknowledge the kind aid which i have received in the preparation of this book. mr. nasmyth has permitted me to use some of the beautiful drawings of the moon, which have appeared in the well-known work published by him in conjunction with mr. carpenter. to this source i am indebted for plates vii., viii., ix., x., and figs. , , . professor pickering has allowed me to copy some of the drawings made at harvard college observatory by mr. trouvelot, and i have availed myself of his kindness for plates i., iv., xii., xv. i am indebted to professor langley for plate ii., to mr. de la rue for plates iii. and xiv., to mr. t.e. key for plate xvii., to professor schiaparelli for plate xviii., to the late professor c. piazzi smyth for fig. , to mr. chambers for fig. , which has been borrowed from his "handbook of descriptive astronomy," to dr. stoney for fig. , and to dr. copeland and dr. dreyer for fig. . i have to acknowledge the valuable assistance derived from professor newcomb's "popular astronomy," and professor young's "sun." in revising the volume i have had the kind aid of the rev. maxwell close. i have also to thank dr. copeland and mr. steele for their kindness in reading through the entire proofs; while i have also occasionally availed myself of the help of mr. cathcart. robert s. ball. observatory, dunsink, co. dublin. _ th may, ._ note to this edition. i have taken the opportunity in the present edition to revise the work in accordance with the recent progress of astronomy. i am indebted to the royal astronomical society for the permission to reproduce some photographs from their published series, and to mr. henry f. griffiths, for beautiful drawings of jupiter, from which plate xi. was prepared. robert s. ball. cambridge, _ st may, _. contents. page introduction chapter i. the astronomical observatory ii. the sun iii. the moon iv. the solar system v. the law of gravitation vi. the planet of romance vii. mercury viii. venus ix. the earth x. mars xi. the minor planets xii. jupiter xiii. saturn xiv. uranus xv. neptune xvi. comets xvii. shooting stars xviii. the starry heavens xix. the distant suns xx. double stars xxi. the distances of the stars xxii. star clusters and nebulÆ xxiii. the physical nature of the stars xxiv. the precession and nutation of the earth's axis xxv. the aberration of light xxvi. the astronomical significance of heat xxvii. the tides appendix list of plates. plate i. the planet saturn _frontispiece_ ii. a typical sun-spot _to face page_ a. the sun " " iii. spots and faculæ on the sun " " iv. solar prominences or flames " " v. the solar corona " " vi. chart of the moon's surface " " b. portion of the moon " " vii. the lunar crater triesnecker " " viii. a normal lunar crater " " ix. the lunar crater plato " " x. the lunar crater tycho " " xi. the planet jupiter " " xii. coggia's comet " " c. comet a., , swift " " xiii. spectra of the sun and of three stars " " d. the milky way, near messier ii. " " xiv. the great nebula in orion " " xv. the great nebula in andromeda " " e. nebulæ in the pleiades " " f. ô centauri " " xvi. nebulæ observed with lord rosse's telescope " " xvii. the comet of " " xviii. schiaparelli's map of mars " " list of illustrations. fig. page . principle of the refracting telescope . dome of the south equatorial at dunsink observatory, co. dublin . section of the dome of dunsink observatory . the telescope at yerkes observatory, chicago . principle of herschel's reflecting telescope . south front of the yerkes observatory, chicago . lord rosse's telescope . meridian circle . the great bear . comparative sizes of the earth and the sun . the sun, photographed september , . photograph of the solar surface . an ordinary sun-spot . scheiner's observations on sun-spots . zones on the sun's surface in which spots appear . texture of the sun and a small spot . the prism . dispersion of light by the prism . prominences seen in total eclipses . view of the corona in a total eclipse . view of corona during eclipse of january , . the zodiacal light in . comparative sizes of the earth and the moon . the moon's path around the sun . the phases of the moon . the earth's shadow and penumbra . key to chart of the moon (plate vi.) . lunar volcano in activity: nasmyth's theory . lunar volcano: subsequent feeble activity . " " formation of the level floor by lava . orbits of the four interior planets . the earth's movement . orbits of the four giant planets . apparent size of the sun from various planets . comparative sizes of the planets . illustration of the moon's motion . drawing an ellipse . varying velocity of elliptic motion . equal areas in equal times . transit of the planet of romance . variations in phase and apparent size of mercury . mercury as a crescent . venus, may , . different aspects of venus in the telescope . venus on the sun at the transit of . paths of venus across the sun in the transits of and . a transit of venus, as seen from two localities . orbits of the earth and of mars . apparent movements of mars in . relative sizes of mars and the earth , . drawings of mars . elevations and depressions on the terminator of mars . the southern polar cap on mars . the zone of minor planets between mars and jupiter . relative dimensions of jupiter and the earth - . the occultation of jupiter . jupiter and his four satellites . disappearances of jupiter's satellites . mode of measuring the velocity of light . saturn . relative sizes of saturn and the earth . method of measuring the rotation of saturn's rings . method of measuring the rotation of saturn's rings . transit of titan and its shadow . parabolic path of a comet . orbit of encke's comet . tail of a comet directed from the sun . bredichin's theory of comets' tails . tails of the comet of . the comet of . the path of the fireball of november , . the orbit of a shoal of meteors . radiant point of shooting stars . the history of the leonids . section of the chaco meteorite . the great bear and pole star . the great bear and cassiopeia . the great square of pegasus . perseus and its neighbouring stars . the pleiades . orion, sirius, and neighbouring stars . castor and pollux . the great bear and the lion . boötes and the crown . virgo and neighbouring constellations . the constellation of lyra . vega, the swan, and the eagle . the orbit of sirius . the parallactic ellipse . cygni and the comparison stars . parallax in declination of cygni . globular cluster in hercules . position of the great nebula in orion . the multiple star th orionis . the nebula n.g.c. . star-map, showing precessional movement . illustration of the motion of precession the story of the heavens. "the story of the heavens" is the title of our book. we have indeed a wondrous story to narrate; and could we tell it adequately it would prove of boundless interest and of exquisite beauty. it leads to the contemplation of grand phenomena in nature and great achievements of human genius. let us enumerate a few of the questions which will be naturally asked by one who seeks to learn something of those glorious bodies which adorn our skies: what is the sun--how hot, how big, and how distant? whence comes its heat? what is the moon? what are its landscapes like? how does our satellite move? how is it related to the earth? are the planets globes like that on which we live? how large are they, and how far off? what do we know of the satellites of jupiter and of the rings of saturn? how was uranus discovered? what was the intellectual triumph which brought the planet neptune to light? then, as to the other bodies of our system, what are we to say of those mysterious objects, the comets? can we discover the laws of their seemingly capricious movements? do we know anything of their nature and of the marvellous tails with which they are often decorated? what can be told about the shooting-stars which so often dash into our atmosphere and perish in a streak of splendour? what is the nature of those constellations of bright stars which have been recognised from all antiquity, and of the host of smaller stars which our telescopes disclose? can it be true that these countless orbs are really majestic suns, sunk to an appalling depth in the abyss of unfathomable space? what have we to tell of the different varieties of stars--of coloured stars, of variable stars, of double stars, of multiple stars, of stars that seem to move, and of stars that seem at rest? what of those glorious objects, the great star clusters? what of the milky way? and, lastly, what can we learn of the marvellous nebulæ which our telescopes disclose, poised at an immeasurable distance? such are a few of the questions which occur when we ponder on the mysteries of the heavens. the history of astronomy is, in one respect, only too like many other histories. the earliest part of it is completely and hopelessly lost. the stars had been studied, and some great astronomical discoveries had been made, untold ages before those to which our earliest historical records extend. for example, the observation of the apparent movement of the sun, and the discrimination between the planets and the fixed stars, are both to be classed among the discoveries of prehistoric ages. nor is it to be said that these achievements related to matters of an obvious character. ancient astronomy may seem very elementary to those of the present day who have been familiar from childhood with the great truths of nature, but, in the infancy of science, the men who made such discoveries as we have mentioned must have been sagacious philosophers. of all the phenomena of astronomy the first and the most obvious is that of the rising and the setting of the sun. we may assume that in the dawn of human intelligence these daily occurrences would form one of the first problems to engage the attention of those whose thoughts rose above the animal anxieties of everyday existence. a sun sets and disappears in the west. the following morning a sun rises in the east, moves across the heavens, and it too disappears in the west; the same appearances recur every day. to us it is obvious that the sun, which appears each day, is the same sun; but this would not seem reasonable to one who thought his senses showed him that the earth was a flat plain of indefinite extent, and that around the inhabited regions on all sides extended, to vast distances, either desert wastes or trackless oceans. how could that same sun, which plunged into the ocean at a fabulous distance in the west, reappear the next morning at an equally great distance in the east? the old mythology asserted that after the sun had dipped in the western ocean at sunset (the iberians, and other ancient nations, actually imagined that they could hear the hissing of the waters when the glowing globe was plunged therein), it was seized by vulcan and placed in a golden goblet. this strange craft with its astonishing cargo navigated the ocean by a northerly course, so as to reach the east again in time for sunrise the following morning. among the earlier physicists of old it was believed that in some manner the sun was conveyed by night across the northern regions, and that darkness was due to lofty mountains, which screened off the sunbeams during the voyage. in the course of time it was thought more rational to suppose that the sun actually pursued his course below the solid earth during the course of the night. the early astronomers had, moreover, learned to recognise the fixed stars. it was noticed that, like the sun, many of these stars rose and set in consequence of the diurnal movement, while the moon obviously followed a similar law. philosophers thus taught that the various heavenly bodies were in the habit of actually passing beneath the solid earth. by the acknowledgment that the whole contents of the heavens performed these movements, an important step in comprehending the constitution of the universe had been decidedly taken. it was clear that the earth could not be a plane extending to an indefinitely great distance. it was also obvious that there must be a finite depth to the earth below our feet. nay, more, it became certain that whatever the shape of the earth might be, it was at all events something detached from all other bodies, and poised without visible support in space. when this discovery was first announced it must have appeared a very startling truth. it was so difficult to realise that the solid earth on which we stand reposed on nothing! what was to keep it from falling? how could it be sustained without tangible support, like the legendary coffin of mahomet? but difficult as it may have been to receive this doctrine, yet its necessary truth in due time commanded assent, and the science of astronomy began to exist. the changes of the seasons and the recurrence of seed-time and harvest must, from the earliest times, have been associated with certain changes in the position of the sun. in the summer at mid-day the sun rises high in the heavens, in the winter it is always low. our luminary, therefore, performs an annual movement up and down in the heavens, as well as a diurnal movement of rising and setting. but there is a third species of change in the sun's position, which is not quite so obvious, though it is still capable of being detected by a few careful observations, if combined with a philosophical habit of reflection. the very earliest observers of the stars can hardly have failed to notice that the constellations visible at night varied with the season of the year. for instance, the brilliant figure of orion, though so well seen on winter nights, is absent from the summer skies, and the place it occupied is then taken by quite different groups of stars. the same may be said of other constellations. each season of the year can thus be characterised by the sidereal objects that are conspicuous by night. indeed, in ancient days, the time for commencing the cycle of agricultural occupations was sometimes indicated by the position of the constellations in the evening. by reflecting on these facts the early astronomers were enabled to demonstrate the apparent annual movement of the sun. there could be no rational explanation of the changes in the constellations with the seasons, except by supposing that the place of the sun was altering, so as to make a complete circuit of the heavens in the course of the year. this movement of the sun is otherwise confirmed by looking at the west after sunset, and watching the stars. as the season progresses, it may be noticed each evening that the constellations seem to sink lower and lower towards the west, until at length they become invisible from the brightness of the sky. the disappearance is explained by the supposition that the sun appears to be continually ascending from the west to meet the stars. this motion is, of course, not to be confounded with the ordinary diurnal rising and setting, in which all the heavenly bodies participate. it is to be understood that besides being affected by the common motion our luminary has a slow independent movement in the opposite direction; so that though the sun and a star may set at the same time to-day, yet since by to-morrow the sun will have moved a little towards the east, it follows that the star must then set a few minutes before the sun.[ ] the patient observations of the early astronomers enabled the sun's track through the heavens to be ascertained, and it was found that in its circuit amid the stars and constellations our luminary invariably followed the same path. this is called the _ecliptic_, and the constellations through which it passes form a belt around the heavens known as the _zodiac_. it was anciently divided into twelve equal portions or "signs," so that the stages on the sun's great journey could be conveniently indicated. the duration of the year, or the period required by the sun to run its course around the heavens, seems to have been first ascertained by astronomers whose names are unknown. the skill of the early oriental geometers was further evidenced by their determination of the position of the ecliptic with regard to the celestial equator, and by their success in the measurement of the angle between these two important circles on the heavens. the principal features of the motion of the moon have also been noticed with intelligence at an antiquity more remote than history. the attentive observer perceives the important truth that the moon does not occupy a fixed position in the heavens. during the course of a single night the fact that the moon has moved from west to east across the heavens can be perceived by noting its position relatively to adjacent stars. it is indeed probable that the motion of the moon was a discovery prior to that of the annual motion of the sun, inasmuch as it is the immediate consequence of a simple observation, and involves but little exercise of any intellectual power. in prehistoric times also, the time of revolution of the moon had been ascertained, and the phases of our satellite had been correctly attributed to the varying aspect under which the sun-illuminated side is turned towards the earth. but we are far from having exhausted the list of great discoveries which have come down from unknown antiquity. correct explanations had been given of the striking phenomenon of a lunar eclipse, in which the brilliant surface is plunged temporarily into darkness, and also of the still more imposing spectacle of a solar eclipse, in which the sun itself undergoes a partial or even a total obscuration. then, too, the acuteness of the early astronomers had detected the five wandering stars or planets: they had traced the movements of mercury and venus, mars, jupiter, and saturn. they had observed with awe the various configurations of these planets: and just as the sun, and in a lesser degree the moon, were intimately associated with the affairs of daily life, so in the imagination of these early investigators the movements of the planets were thought to be pregnant with human weal or human woe. at length a certain order was perceived to govern the apparently capricious movements of the planets. it was found that they obeyed certain laws. the cultivation of the science of geometry went hand in hand with the study of astronomy: and as we emerge from the dim prehistoric ages into the historical period, we find that the theory of the phenomena of the heavens possessed already some degree of coherence. ptolemy, following pythagoras, plato, and aristotle, acknowledged that the earth's figure was globular, and he demonstrated it by the same arguments that we employ at the present day. he also discerned how this mighty globe was isolated in space. he admitted that the diurnal movement of the heavens could be accounted for by the revolution of the earth upon its axis, but unfortunately he assigned reasons for the deliberate rejection of this view. the earth, according to him, was a fixed body; it possessed neither rotation round an axis nor translation through space, but remained constantly at rest in what he supposed to be the centre of the universe. according to ptolemy's theory the sun and the moon moved in circular orbits around the earth in the centre. the explanation of the movements of the planets he found to be more complicated, because it was necessary to account for the fact that a planet sometimes advanced and that it sometimes retrograded. the ancient geometers refused to believe that any movement, except revolution in a circle, was possible for a celestial body: accordingly a contrivance was devised by which each planet was supposed to revolve in a circle, of which the centre described another circle around the earth. although the ptolemaic doctrine is now known to be framed on quite an extravagant estimate of the importance of the earth in the scheme of the heavens, yet it must be admitted that the apparent movements of the celestial bodies can be thus accounted for with considerable accuracy. this theory is described in the great work known as the "almagest," which was written in the second century of our era, and was regarded for fourteen centuries as the final authority on all questions of astronomy. such was the system of astronomy which prevailed during the middle ages, and was only discredited at an epoch nearly simultaneous with that of the discovery of the new world by columbus. the true arrangement of the solar system was then expounded by copernicus in the great work to which he devoted his life. the first principle established by these labours showed the diurnal movement of the heavens to be due to the rotation of the earth on its axis. copernicus pointed out the fundamental difference between real motions and apparent motions; he proved that the appearances presented in the daily rising and setting of the sun and the stars could be accounted for by the supposition that the earth rotated, just as satisfactorily as by the more cumbrous supposition of ptolemy. he showed, moreover, that the latter supposition must attribute an almost infinite velocity to the stars, so that the rotation of the entire universe around the earth was clearly a preposterous supposition. the second great principle, which has conferred immortal glory on copernicus, assigned to the earth its true position in the universe. copernicus transferred the centre, about which all the planets revolve, from the earth to the sun; and he established the somewhat humiliating truth, that our earth is merely a planet pursuing a track between the paths of venus and of mars, and subordinated like all the other planets to the supreme sway of the sun. this great revolution swept from astronomy those distorted views of the earth's importance which arose, perhaps not unnaturally, from the fact that we happen to be domiciled on that particular planet. the achievements of copernicus were soon to be followed by the invention of the telescope, that wonderful instrument by which the modern science of astronomy has been created. to the consideration of this important subject we shall devote the first chapter of our book. [illustration: plate ii. a typical sun-spot. (after langley.)] chapter i. the astronomical observatory. early astronomical observations--the observatory of tycho brahe--the pupil of the eye--vision of faint objects--the telescope--the object-glass--advantages of large telescopes--the equatorial--the observatory--the power of a telescope--reflecting telescopes--lord rosse's great reflector at parsonstown--how the mighty telescope is used--instruments of precision--the meridian circle--the spider lines--delicacy of pointing a telescope--precautions necessary in making observations--the ideal instrument and the practical one--the elimination of error--greenwich observatory--the ordinary opera-glass as an astronomical instrument--the great bear--counting the stars in the constellation--how to become an observer. the earliest rudiments of the astronomical observatory are as little known as the earliest discoveries in astronomy itself. probably the first application of instrumental observation to the heavenly bodies consisted in the simple operation of measuring the shadow of a post cast by the sun at noonday. the variations in the length of this shadow enabled the primitive astronomers to investigate the apparent movements of the sun. but even in very early times special astronomical instruments were employed which possessed sufficient accuracy to add to the amount of astronomical knowledge, and displayed considerable ingenuity on the part of the designers. professor newcomb[ ] thus writes: "the leader was tycho brahe, who was born in , three years after the death of copernicus. his attention was first directed to the study of astronomy by an eclipse of the sun on august st, , which was total in some parts of europe. astonished that such a phenomenon could be predicted, he devoted himself to a study of the methods of observation and calculation by which the prediction was made. in the king of denmark founded the celebrated observatory of uraniborg, at which tycho spent twenty years assiduously engaged in observations of the positions of the heavenly bodies with the best instruments that could then be made. this was just before the invention of the telescope, so that the astronomer could not avail himself of that powerful instrument. consequently, his observations were superseded by the improved ones of the centuries following, and their celebrity and importance are principally due to their having afforded kepler the means of discovering his celebrated laws of planetary motion." the direction of the telescope to the skies by galileo gave a wonderful impulse to the study of the heavenly bodies. this extraordinary man is prominent in the history of astronomy, not alone for his connection with this supreme invention, but also for his achievements in the more abstract parts of astronomy. he was born at pisa in , and in the first telescope used for astronomical observation was constructed. galileo died in , the year in which newton was born. it was galileo who laid with solidity the foundations of that science of dynamics, of which astronomy is the most splendid illustration; and it was he who, by promulgating the doctrines taught by copernicus, incurred the wrath of the inquisition. the structure of the human eye in so far as the exquisite adaptation of the pupil is concerned presents us with an apt illustration of the principle of the telescope. to see an object, it is necessary that the light from it should enter the eye. the portal through which the light is admitted is the pupil. in daytime, when the light is brilliant, the iris decreases the size of the pupil, and thus prevents too much light from entering. at night, or whenever the light is scarce, the eye often requires to grasp all it can. the pupil then expands; more and more light is admitted according as the pupil grows larger. the illumination of the image on the retina is thus effectively controlled in accordance with the requirements of vision. a star transmits to us its feeble rays of light, and from those rays the image is formed. even with the most widely-opened pupil, it may, however, happen that the image is not bright enough to excite the sensation of vision. here the telescope comes to our aid: it catches all the rays in a beam whose original dimensions were far too great to allow of its admission through the pupil. the action of the lenses concentrates those rays into a stream slender enough to pass through the small opening. we thus have the brightness of the image on the retina intensified. it is illuminated with nearly as much light as would be collected from the same object through a pupil as large as the great lenses of the telescope. [illustration: fig. .--principle of the refracting telescope.] in astronomical observatories we employ telescopes of two entirely different classes. the more familiar forms are those known as _refractors_, in which the operation of condensing the rays of light is conducted by refraction. the character of the refractor is shown in fig. . the rays from the star fall upon the object-glass at the end of the telescope, and on passing through they become refracted into a converging beam, so that all intersect at the focus. diverging from thence, the rays encounter the eye-piece, which has the effect of restoring them to parallelism. the large cylindrical beam which poured down on the object-glass has been thus condensed into a small one, which can enter the pupil. it should, however, be added that the composite nature of light requires a more complex form of object-glass than the simple lens here shown. in a refracting telescope we have to employ what is known as the achromatic combination, consisting of one lens of flint glass and one of crown glass, adjusted to suit each other with extreme care. [illustration: fig. .--the dome of the south equatorial at dunsink observatory co dublin.] [illustration: fig. .--section of the dome of dunsink observatory.] the appearance of an astronomical observatory, designed to accommodate an instrument of moderate dimensions, is shown in the adjoining figures. the first (fig. ) represents the dome erected at dunsink observatory for the equatorial telescope, the object-glass of which was presented to the board of trinity college, dublin, by the late sir james south. the main part of the building is a cylindrical wall, on the top of which reposes a hemispherical roof. in this roof is a shutter, which can be opened so as to allow the telescope in the interior to obtain a view of the heavens. the dome is capable of revolving so that the opening may be turned towards that part of the sky where the object happens to be situated. the next view (fig. ) exhibits a section through the dome, showing the machinery by which the attendant causes it to revolve, as well as the telescope itself. the eye of the observer is placed at the eye-piece, and he is represented in the act of turning a handle, which has the power of slowly moving the telescope, in order to adjust the instrument accurately on the celestial body which it is desired to observe. the two lenses which together form the object-glass of this instrument are twelve inches in diameter, and the quality of the telescope mainly depends on the accuracy with which these lenses have been wrought. the eye-piece is a comparatively simple matter. it consists merely of one or two small lenses; and various eye-pieces can be employed, according to the magnifying power which may be desired. it is to be observed that for many purposes of astronomy high magnifying powers are not desirable. there is a limit, too, beyond which the magnification cannot be carried with advantage. the object-glass can only collect a certain quantity of light from the star; and if the magnifying power be too great, this limited amount of light will be thinly dispersed over too large a surface, and the result will be found unsatisfactory. the unsteadiness of the atmosphere still further limits the extent to which the image may be advantageously magnified, for every increase of power increases in the same degree the atmospheric disturbance. a telescope mounted in the manner here shown is called an _equatorial_. the convenience of this peculiar style of supporting the instrument consists in the ease with which the telescope can be moved so as to follow a star in its apparent journey across the sky. the necessary movements of the tube are given by clockwork driven by a weight, so that, once the instrument has been correctly pointed, the star will remain in the observer's field of view, and the effect of the apparent diurnal movement will be neutralised. the last refinement in this direction is the application of an electrical arrangement by which the driving of the instrument is controlled from the standard clock of the observatory. [illustration: fig. .--the telescope at yerkes observatory, chicago. (_from the astrophysical journal, vol. vi., no. ._)] the power of a refracting telescope--so far as the expression has any definite meaning--is to be measured by the diameter of its object-glass. there has, indeed, been some honourable rivalry between the various civilised nations as to which should possess the greatest refracting telescope. among the notable instruments that have been successfully completed is that erected in by sir howard grubb, of dublin, at the splendid observatory at vienna. its dimensions may be estimated from the fact that the object-glass is two feet and three inches in diameter. many ingenious contrivances help to lessen the inconvenience incident to the use of an instrument possessing such vast proportions. among them we may here notice the method by which the graduated circles attached to the telescope are brought within view of the observer. these circles are necessarily situated at parts of the instrument which lie remote from the eye-piece where the observer is stationed. the delicate marks and figures are, however, easily read from a distance by a small auxiliary telescope, which, by suitable reflectors, conducts the rays of light from the circles to the eye of the observer. [illustration: fig. .--principle of herschel's refracting telescope.] numerous refracting telescopes of exquisite perfection have been produced by messrs. alvan clark, of cambridgeport, boston, mass. one of their most famous telescopes is the great lick refractor now in use on mount hamilton in california. the diameter of this object-glass is thirty-six inches, and its focal length is fifty-six feet two inches. a still greater effort has recently been made by the same firm in the refractor of forty inches aperture for the yerkes observatory of the university of chicago. the telescope, which is seventy-five feet in length, is mounted under a revolving dome ninety feet in diameter, and in order to enable the observer to reach the eye-piece without using very large step-ladders, the floor of the room can be raised and lowered through a range of twenty-two feet by electric motors. this is shown in fig. , while the south front of the yerkes observatory is represented in fig. . [illustration: fig. .--south front of the yerkes observatory, chicago. (_from the astrophysical journal, vol. vi., no. ._)] [illustration: fig. .--lord rosse's telescope.] within the last few years two fine telescopes have been added to the instrumental equipment of the royal observatory, greenwich, both by sir h. grubb. one of these, containing a -inch object-glass, has been erected on a mounting originally constructed for a smaller instrument by sir g. airy. the other, presented by sir henry thompson, is of inches aperture, and is adapted for photographic work. there is a limit to the size of the refractor depending upon the material of the object-glass. glass manufacturers seem to experience unusual difficulties in their attempts to form large discs of optical glass pure enough and uniform enough to be suitable for telescopes. these difficulties are enhanced with every increase in the size of the discs, so that the cost has a tendency to increase at a very much greater rate. it may be mentioned in illustration that the price paid for the object-glass of the lick telescope exceeded ten thousand pounds. there is, however, an alternative method of constructing a telescope, in which the difficulty we have just mentioned does not arise. the principle of the simplest form of _reflector_ is shown in fig. , which represents what is called the herschelian instrument. the rays of light from the star under observation fall on a mirror which is both carefully shaped and highly polished. after reflection, the rays proceed to a focus, and diverging from thence, fall on the eye-piece, by which they are restored to parallelism, and thus become adapted for reception in the eye. it was essentially on this principle (though with a secondary flat mirror at the upper end of the tube reflecting the rays at a right angle to the side of the tube, where the eye-piece is placed) that sir isaac newton constructed the little reflecting telescope which is now treasured by the royal society. a famous instrument of the newtonian type was built, half a century ago, by the late earl of rosse, at parsonstown. it is represented in fig. . the colossal aperture of this instrument has never been surpassed; it has, indeed, never been rivalled. the mirror or speculum, as it is often called, is a thick metallic disc, composed of a mixture of two parts of copper with one of tin. this alloy is so hard and brittle as to make the necessary mechanical operations difficult to manage. the material admits, however, of a brilliant polish, and of receiving and retaining an accurate figure. the rosse speculum--six feet in diameter and three tons in weight--reposes at the lower end of a telescope fifty-five feet long. the tube is suspended between two massive castellated walls, which form an imposing feature on the lawn at birr castle. this instrument cannot be turned about towards every part of the sky, like the equatorials we have recently been considering. the great tube is only capable of elevation in altitude along the meridian, and of a small lateral movement east and west of the meridian. every star or nebula visible in the latitude of parsonstown (except those very near the pole) can, however, be observed in the great telescope, if looked for at the right time. [illustration: fig. .--meridian circle.] before the object reaches the meridian, the telescope must be adjusted at the right elevation. the necessary power is transmitted by a chain from a winch at the northern end of the walls to a point near the upper end of the tube. by this contrivance the telescope can be raised or lowered, and an ingenious system of counterpoises renders the movement equally easy at all altitudes. the observer then takes his station in one of the galleries which give access to the eye-piece; and when the right moment has arrived, the star enters the field of view. powerful mechanism drives the great instrument, so as to counteract the diurnal movement, and thus the observer can retain the object in view until he has made his measurements or finished his drawing. of late years reflecting telescopes have been generally made with mirrors of glass covered with a thin film of silver, which is capable of reflecting much more light than the surface of a metallic mirror. among great reflectors of this kind we may mention two, of three and five feet aperture respectively, with which dr. common has done valuable work. we must not, however, assume that for the general work in an observatory a colossal instrument is the most suitable. the mighty reflector, or refractor, is chiefly of use where unusually faint objects are being examined. for work in which accurate measurements are made of objects not particularly difficult to see, telescopes of smaller dimensions are more suitable. the fundamental facts about the heavenly bodies have been chiefly learned from observations obtained with instruments of moderate optical power, specially furnished so as to enable precise measures of position to be secured. indeed, in the early stages of astronomy, important determinations of position were effected by contrivances which showed the direction of the object without any telescopic aid. perhaps the most valuable measurements obtained in our modern observatories are yielded by that instrument of precision known as the _meridian circle_. it is impossible, in any adequate account of the story of the heavens, to avoid some reference to this indispensable aid to astronomical research, and therefore we shall give a brief account of one of its simpler forms, choosing for this purpose a great instrument in the paris observatory, which is represented in fig. . the telescope is attached at its centre to an axis at right angles to its length. pivots at each extremity of this axis rotate upon fixed bearings, so that the movements of the telescope are completely restricted to the plane of the meridian. inside the eye-piece of the telescope extremely fine vertical fibres are stretched. the observer watches the moon, or star, or planet enter the field of view; and he notes by the clock the exact time, to the fraction of a second, at which the object passes over each of the lines. a silver band on the circle attached to the axis is divided into degrees and subdivisions of a degree, and as this circle moves with the telescope, the elevation at which the instrument is pointed will be indicated. for reading the delicately engraved marks and figures on the silver, microscopes are necessary. these are shown in the sketch, each one being fixed into an aperture in the wall which supports one end of the instrument. at the opposite side is a lamp, the light from which passes through the perforated axis of the pivot, and is thence ingeniously deflected by mirrors so as to provide the requisite illumination for the lines at the focus. the fibres which the observer sees stretched over the field of view of the telescope demand a few words of explanation. we require for this purpose a material which shall be very fine and fairly durable, as well as somewhat elastic, and of no appreciable weight. these conditions cannot be completely fulfilled by any metallic wire, but they are exquisitely realised in the beautiful thread which is spun by the spider. the delicate fibres are stretched with nice skill across the field of view of the telescope, and cemented in their proper places. with instruments so beautifully appointed we can understand the precision attained in modern observations. the telescope is directed towards a star, and the image of the star is a minute point of light. when that point coincides with the intersection of the two central spider lines the telescope is properly sighted. we use the word sighted designedly, because we wish to suggest a comparison between the sighting of a rifle at the target and the sighting of a telescope at a star. instead of the ordinary large bull's-eye, suppose that the target only consisted of a watch-dial, which, of course, the rifleman could not see at the distance of any ordinary range. but with the telescope of the meridian circle the watch-dial would be visible even at the distance of a mile. the meridian circle is indeed capable of such precision as a sighting instrument that it could be pointed separately to each of two stars which subtend at the eye an angle no greater than that subtended by an adjoining pair of the sixty minute dots around the circumference of a watch-dial a mile distant from the observer. this power of directing the instrument so accurately would be of but little avail unless it were combined with arrangements by which, when once the telescope has been pointed correctly, the position of the star can be ascertained and recorded. one element in the determination of the position is secured by the astronomical clock, which gives the moment when the object crosses the central vertical wire; the other element is given by the graduated circle which reads the angular distance of the star from the zenith or point directly overhead. superb meridian instruments adorn our great observatories, and are nightly devoted to those measurements upon which the great truths of astronomy are mainly based. these instruments have been constructed with refined skill; but it is the duty of the painstaking astronomer to distrust the accuracy of his instrument in every conceivable way. the great tube may be as rigid a structure as mechanical engineers can produce; the graduations on the circle may have been engraved by the most perfect of dividing machines; but the conscientious astronomer will not be content with mere mechanical precision. that meridian circle which, to the uninitiated, seems a marvellous piece of workmanship, possessing almost illimitable accuracy, is viewed in a very different light by the astronomer who makes use of it. no one can appreciate more fully than he the skill of the artist who has made that meridian circle, and the beautiful contrivances for illumination and reading off which give to the instrument its perfection; but while the astronomer recognises the beauty of the actual machine he is using, he has always before his mind's eye an ideal instrument of absolute perfection, to which the actual meridian circle only makes an approximation. contrasted with the ideal instrument, the finest meridian circle is little more than a mass of imperfections. the ideal tube is perfectly rigid, the actual tube is flexible; the ideal divisions of the circle are perfectly uniform, the actual divisions are not uniform. the ideal instrument is a geometrical embodiment of perfect circles, perfect straight lines, and perfect right angles; the actual instrument can only show approximate circles, approximate straight lines, and approximate right angles. perhaps the spider's part of the work is on the whole the best; the stretched web gives us the nearest mechanical approach to a perfectly straight line; but we mar the spider's work by not being able to insert those beautiful threads with perfect uniformity, while our attempts to adjust two of them across the field of view at right angles do not succeed in producing an angle of exactly ninety degrees. nor are the difficulties encountered by the meridian observer due solely to his instrument. he has to contend against his own imperfections; he has often to allow for personal peculiarities of an unexpected nature; the troubles that the atmosphere can give are notorious; while the levelling of his instrument warns him that he cannot even rely on the solid earth itself. we learn that the earthquakes, by which the solid ground is sometimes disturbed, are merely the more conspicuous instances of incessant small movements in the earth which every night in the year derange the delicate adjustment of the instrument. when the existence of these errors has been recognised, the first great step has been taken. by an alliance between the astronomer and the mathematician it is possible to measure the discrepancies between the actual meridian circle and the instrument that is ideally perfect. once this has been done, we can estimate the effect which the irregularities produce on the observations, and finally, we succeed in purging the observations from the grosser errors by which they are contaminated. we thus obtain results which are not indeed mathematically accurate, but are nevertheless close approximations to those which would be obtained by a perfect observer using an ideal instrument of geometrical accuracy, standing on an earth of absolute rigidity, and viewing the heavens without the intervention of the atmosphere. in addition to instruments like those already indicated, astronomers have other means of following the motions of the heavenly bodies. within the last fifteen years photography has commenced to play an important part in practical astronomy. this beautiful art can be utilised for representing many objects in the heavens by more faithful pictures than the pencil of even the most skilful draughtsman can produce. photography is also applicable for making charts of any region in the sky which it is desired to examine. when repeated pictures of the same region are made from time to time, their comparison gives the means of ascertaining whether any star has moved during the interval. the amount and direction of this motion may be ascertained by a delicate measuring apparatus under which the photographic plate is placed. if a refracting telescope is to be used for taking celestial photographs, the lenses of the object-glass must be specially designed for this purpose. the rays of light which imprint an image on the prepared plate are not exactly the same as those which are chiefly concerned in the production of the image on the retina of the human eye. a reflecting mirror, however, brings all the rays, both those which are chemically active and those which are solely visual, to one and the same focus. the same reflecting instrument may therefore be used either for looking at the heavens or for taking pictures on a photographic plate which has been substituted for the observer's eye. a simple portrait camera has been advantageously employed for obtaining striking photographs of larger areas of the sky than can be grasped in a long telescope; but for purposes of accurate measurement those taken with the latter are incomparably better. it is needless to say that the photographic apparatus, whatever it may be, must be driven by delicately-adjusted clockwork to counteract the apparent daily motion of the stars caused by the rotation of the earth. the picture would otherwise be spoiled, just as a portrait is ruined if the sitter does not remain quiet during the exposure. among the observatories in the united kingdom the royal observatory at greenwich is of course the most famous. it is specially remarkable among all the similar institutions in the world for the continuity of its labours for several generations. greenwich observatory was founded in for the promotion of astronomy and navigation, and the observations have from the first been specially arranged with the object of determining with the greatest accuracy the positions of the principal fixed stars, the sun, the moon, and the planets. in recent years, however, great developments of the work of the observatory have been witnessed, and the most modern branches of the science are now assiduously pursued there. the largest equatorial at greenwich is a refractor of twenty-eight inches aperture and twenty-eight feet long, constructed by sir howard grubb. a remarkable composite instrument from the same celebrated workshop has also been recently added to our national institution. it consists of a great refractor specially constructed for photography, of twenty-six inches aperture (presented by sir henry thompson) and a reflector of thirty inches diameter, which is the product of dr. common's skill. the huge volume published annually bears witness to the assiduity with which the astronomer royal and his numerous staff of assistant astronomers make use of the splendid means at their disposal. the southern part of the heavens, most of which cannot be seen in this country, is watched from various observatories in the southern hemisphere. foremost among them is the royal observatory at the cape of good hope, which is furnished with first-class instruments. we may mention a great photographic telescope, the gift of mr. m'clean. astronomy has been greatly enriched by the many researches made by dr. gill, the director of the cape observatory. [illustration: fig. .--the great bear.] it is not, however, necessary to use such great instruments to obtain some idea of the aid the telescope will afford. the most suitable instrument for commencing astronomical studies is within ordinary reach. it is the well-known binocular that a captain uses on board ship; or if that cannot be had, then the common opera-glass will answer nearly as well. this is, no doubt, not so powerful as a telescope, but it has some compensating advantages. the opera-glass will enable us to survey a large region of the sky at one glance, while a telescope, generally speaking, presents a much smaller field of view. let us suppose that the observer is provided with an opera-glass and is about to commence his astronomical studies. the first step is to become acquainted with the conspicuous group of seven stars represented in fig. . this group is often called the plough, or charles's wain, but astronomers prefer to regard it as a portion of the constellation of the great bear (ursa major). there are many features of interest in this constellation, and the beginner should learn as soon as possible to identify the seven stars which compose it. of these the two marked a and b, at the head of the bear, are generally called the "pointers." they are of special use, because they serve to guide the eye to that most important star in the whole sky, known as the "pole star." fix the attention on that region in the great bear, which forms a sort of rectangle, of which the stars a b g d are the corners. the next fine night try to count how many stars are visible within that rectangle. on a very fine night, without a moon, perhaps a dozen might be perceived, or even more, according to the keenness of the eyesight. but when the opera-glass is directed to the same part of the constellation an astonishing sight is witnessed. a hundred stars can now be seen with the greatest ease. but the opera-glass will not show nearly all the stars in this region. any good telescope will reveal many hundreds too faint for the feebler instrument. the greater the telescope the more numerous the stars: so that seen through one of the colossal instruments the number would have to be reckoned in thousands. we have chosen the great bear because it is more generally known than any other constellation. but the great bear is not exceptionally rich in stars. to tell the number of the stars is a task which no man has accomplished; but various estimates have been made. our great telescopes can probably show at least , , stars. the student who uses a good refracting telescope, having an object-glass not less than three inches in diameter, will find occupation for many a fine evening. it will greatly increase the interest of his work if he have the charming handbook of the heavens known as webb's "celestial objects for common telescopes." chapter ii. the sun. the vast size of the sun--hotter than melting platinum--is the sun the source of heat for the earth?--the sun is , , miles distant--how to realise the magnitude of this distance--day and night--luminous and non-luminous bodies--contrast between the sun and the stars--the sun a star--granulated appearance of the sun--the spots on the sun--changes in the form of a spot--the faculæ--the rotation of the sun on its axis--view of a typical sun-spot--periodicity of the sun-spots--connection between the sun-spots and terrestrial magnetism--principles of spectrum analysis--substances present in the sun--spectrum of a spot--the prominences surrounding the sun--total eclipse of the sun--size and movement of the prominences--their connection with the spots--spectroscopic measurement of motion on the sun--the corona surrounding the sun--constitution of the sun. in commencing our examination of the orbs which surround us, we naturally begin with our peerless sun. his splendid brilliance gives him the pre-eminence over all other celestial bodies. the dimensions of our luminary are commensurate with his importance. astronomers have succeeded in the difficult task of ascertaining the exact figures, but they are so gigantic that the results are hard to realise. the diameter of the orb of day, or the length of the axis, passing through the centre from one side to the other, is , miles. yet this bare statement of the dimensions of the great globe fails to convey an adequate idea of its vastness. if a railway were laid round the sun, and if we were to start in an express train moving sixty miles an hour, we should have to travel for five years without intermission night or day before we had accomplished the journey. when the sun is compared with the earth the bulk of our luminary becomes still more striking. suppose his globe were cut up into one million parts, each of these parts would appreciably exceed the bulk of our earth. fig. exhibits a large circle and a very small one, marked s and e respectively. these circles show the comparative sizes of the two bodies. the mass of the sun does not, however, exceed that of the earth in the same proportion. were the sun placed in one pan of a mighty weighing balance, and were , bodies as heavy as our earth placed in the other, the luminary would turn the scale. [illustration: fig. .--comparative size of the earth and the sun.] the sun has a temperature far surpassing any that we artificially produce, either in our chemical laboratories or our metallurgical establishments. we can send a galvanic current through a piece of platinum wire. the wire first becomes red hot, then white hot; then it glows with a brilliance almost dazzling until it fuses and breaks. the temperature of the melting platinum wire could hardly be surpassed in the most elaborate furnaces, but it does not attain the temperature of the sun. it must, however, be admitted that there is an apparent discrepancy between a fact of common experience and the statement that the sun possesses the extremely high temperature that we have just tried to illustrate. "if the sun were hot," it has been said, "then the nearer we approach to him the hotter we should feel; yet this does not seem to be the case. on the top of a high mountain we are nearer to the sun, and yet everybody knows that it is much colder up there than in the valley beneath. if the mountain be as high as mont blanc, then we are certainly two or three miles nearer the glowing globe than we were at the sea-level; yet, instead of additional warmth, we find eternal snow." a simple illustration may help to lessen this difficulty. in a greenhouse on a sunshiny day the temperature is much hotter than it is outside. the glass will permit the hot sunbeams to enter, but it refuses to allow them out again with equal freedom, and consequently the temperature rises. the earth may, from this point of view, be likened to a greenhouse, only, instead of the panes of glass, our globe is enveloped by an enormous coating of air. on the earth's surface, we stand, as it were, inside the greenhouse, and we benefit by the interposition of the atmosphere; but when we climb very high mountains, we gradually pass through some of the protecting medium, and then we suffer from the cold. if the earth were deprived of its coat of air, it seems certain that eternal frost would reign over whole continents as well as on the tops of the mountains. the actual distance of the sun from the earth is about , , miles; but by merely reciting the figures we do not receive a vivid impression of the real magnitude. it would be necessary to count as quickly as possible for three days and three nights before one million was completed; yet this would have to be repeated nearly ninety-three times before we had counted all the miles between the earth and the sun. every clear night we see a vast host of stars scattered over the sky. some are bright, some are faint, some are grouped into remarkable forms. with regard to this multitude of brilliant points we have now to ask an important question. are they bodies which shine by their own light like the sun, or do they only shine with borrowed light like the moon? the answer is easily stated. most of those bodies shine by their own light, and they are properly called _stars_. suppose that the sun and the multitude of stars, properly so called, are each and all self-luminous brilliant bodies, what is the great distinction between the sun and the stars? there is, of course, a vast and obvious difference between the unrivalled splendour of the sun and the feeble twinkle of the stars. yet this distinction does not necessarily indicate that our luminary has an intrinsic splendour superior to that of the stars. the fact is that we are nestled up comparatively close to the sun for the benefit of his warmth and light, while we are separated from even the nearest of the stars by a mighty abyss. if the sun were gradually to retreat from the earth, his light would decrease, so that when he had penetrated the depths of space to a distance comparable with that by which we are separated from the stars, his glory would have utterly departed. no longer would the sun seem to be the majestic orb with which we are familiar. no longer would he be a source of genial heat, or a luminary to dispel the darkness of night. our great sun would have shrunk to the insignificance of a star, not so bright as many of those which we see every night. momentous indeed is the conclusion to which we are now led. that myriad host of stars which studs our sky every night has been elevated into vast importance. each one of those stars is itself a mighty sun, actually rivalling, and in many cases surpassing, the splendour of our own luminary. we thus open up a majestic conception of the vast dimensions of space, and of the dignity and splendour of the myriad globes by which that space is tenanted. there is another aspect of the picture not without its utility. we must from henceforth remember that our sun is only a star, and not a particularly important star. if the sun and the earth, and all which it contains, were to vanish, the effect in the universe would merely be that a tiny star had ceased its twinkling. viewed simply as a star, the sun must retire to a position of insignificance in the mighty fabric of the universe. but it is not as a star that we have to deal with the sun. to us his comparative proximity gives him an importance incalculably transcending that of all the other stars. we imagined ourselves to be withdrawn from the sun to obtain his true perspective in the universe; let us now draw near, and give him that attention which his supreme importance to us merits. [illustration: fig. .--the sun, photographed on september , .] to the unaided eye the sun appears to be a flat circle. if, however, it be examined with the telescope, taking care of course to interpose a piece of dark-coloured glass, or to employ some similar precaution to screen the eye from injury, it will then be perceived that the sun is not a flat surface, but a veritable glowing globe. the first question which we must attempt to answer enquires whether the glowing matter which forms the globe is a solid mass, or, if not solid, which is it, liquid or gaseous? at the first glance we might think that the sun cannot be fluid, and we might naturally imagine that it was a solid ball of some white-hot substance. but this view is not correct; for we can show that the sun is certainly not a solid body in so far at least as its superficial parts are concerned. a general view of the sun as shown by a telescope of moderate dimensions may be seen in fig. , which is taken from a photograph obtained by mr. rutherford at new york on the nd of september, . it is at once seen that the surface of the luminary is by no means of uniform texture or brightness. it may rather be described as granulated or mottled. this appearance is due to the luminous clouds which float suspended in a somewhat less luminous layer of gas. it is needless to say that these solar clouds are very different from the clouds which we know so well in our own atmosphere. terrestrial clouds are, of course, formed from minute drops of water, while the clouds at the surface of the sun are composed of drops of one or more chemical elements at an exceedingly high temperature. the granulated appearance of the solar surface is beautifully shown in the remarkable photographs on a large scale which m. janssen, of meudon, has succeeded in obtaining during the last twenty years. we are enabled to reproduce one of them in fig. . it will be observed that the interstices between the luminous dots are of a greyish tint, the general effect (as remarked by professor young) being much like that of rough drawing paper seen from a little distance. we often notice places over the surface of such a plate where the definition seems to be unsatisfactory. these are not, however, the blemishes that might at first be supposed. they arise neither from casual imperfections of the photographic plate nor from accidents during the development; they plainly owe their origin to some veritable cause in the sun itself, nor shall we find it hard to explain what that cause must be. as we shall have occasion to mention further on, the velocities with which the glowing gases on the sun are animated must be exceedingly great. even in the hundredth part of a second (which is about the duration of the exposure of this plate) the movements of the solar clouds are sufficiently great to produce the observed indistinctness. [illustration: fig. .--photograph of the solar surface. (_by janssen._)] irregularly dispersed over the solar surface small dark objects called sun-spots are generally visible. these spots vary greatly both as to size and as to number. sun-spots were first noticed in the beginning of the seventeenth century, shortly after the invention of the telescope. their general appearance is shown in fig. , in which the dark central nucleus appears in sharp contrast with the lighter margin or penumbra. fig. shows a small spot developing out of one of the pores or interstices between the granules. [illustration: fig. .--an ordinary sun-spot.] the earliest observers of these spots had remarked that they seem to have a common motion across the sun. in fig. we give a copy of a remarkable drawing by father scheiner, showing the motion of two spots observed by him in march, . the figure indicates the successive positions assumed by the spots on the several days from the nd to the th march. those marks which are merely given in outline represent the assumed positions on the th and the th, on which days it happened that the weather was cloudy, so that no observations could be made. it is invariably found that these objects move in the same direction--namely, from the eastern to the western limb[ ] of the sun. they complete the journey across the face of the sun in twelve or thirteen days, after which they remain invisible for about the same length of time until they reappear at the eastern limb. these early observers were quick to discern the true import of their discovery. they deduced from these simple observations the remarkable fact that the sun, like the earth, performs a rotation on its axis, and in the same direction. but there is the important difference between these rotations that whereas the earth takes only twenty-four hours to turn once round, the solar globe takes about twenty-six days to complete one of its much more deliberate rotations. [illustration: plate iii. spots and faculÆ on the sun. (from a photograph by mr. warren de la rue, th sept., .)] if we examine sun-spots under favourable atmospheric conditions and with a telescope of fairly large aperture, we perceive a great amount of interesting detail which is full of information with regard to the structure of the sun. the penumbra of a spot is often found to be made up of filaments directed towards the middle of the spot, and generally brighter at their inner ends, where they adjoin the nucleus. in a regularly formed spot the outline of the penumbra is of the same general form as that of the nucleus, but astronomers are frequently deeply interested by witnessing vast spots of very irregular figure. in such cases the bright surface-covering of the sun (the photosphere, as it is called) often encroaches on the nucleus and forms a peninsula stretching out into, or even bridging across, the gloomy interior. this is well shown in professor langley's fine drawing (plate ii.) of a very irregular spot which he observed on december - , . the details of a spot vary continually; changes may often be noticed even from day to day, sometimes from hour to hour. a similar remark may be made with respect to the bright streaks or patches which are frequently to be observed especially in the neighbourhood of spots. these bright marks are known by the name of _faculæ_ (little torches). they are most distinctly seen near the margin of the sun, where the light from its surface is not so bright as it is nearer to the centre of the disc. the reduction of light at the margin is due to the greater thickness of absorbing atmosphere round the sun, through which the light emitted from the regions near the margin has to pass in starting on its way towards us. none of the markings on the solar disc constitute permanent features on the sun. some of these objects may no doubt last for weeks. it has, indeed, occasionally happened that the same spot has marked the solar globe for many months; but after an existence of greater or less duration those on one part of the sun may disappear, while as frequently fresh marks of the same kind become visible in other places. the inference from these various facts is irresistible. they tell us that the visible surface of the sun is not a solid mass, is not even a liquid mass, but that the globe, so far as we can see it, consists of matter in the gaseous, or vaporous, condition. [illustration: fig. .--scheiner's observations on sun-spots.] it often happens that a large spot divides into two or more separate portions, and these have been sometimes seen to fly apart with a velocity in some cases not less than a thousand miles an hour. "at times, though very rarely" (i quote here professor young,[ ] to whom i am frequently indebted), "a different phenomenon of the most surprising and startling character appears in connection with these objects: patches of intense brightness suddenly break out, remaining visible for a few minutes, moving, while they last, with velocities as great as one hundred miles _a second_." [illustration: fig. .--zones on the sun's surface in which spots appear.] "one of these events has become classical. it occurred on the forenoon (greenwich time) of september st, , and was independently witnessed by two well-known and reliable observers--mr. carrington and mr. hodgson--whose accounts of the matter may be found in the monthly notices of the royal astronomical society for november, . mr. carrington at the time was making his usual daily observations upon the position, configuration, and size of the spots by means of an image of the solar disc upon a screen--being then engaged upon that eight years' series of observations which lie at the foundation of so much of our present solar science. mr. hodgson, at a distance of many miles, was at the same time sketching details of sun-spot structure by means of a solar eye-piece and shade-glass. they simultaneously saw two luminous objects, shaped something like two new moons, each about eight thousand miles in length and two thousand wide, at a distance of some twelve thousand miles from each other. these burst suddenly into sight at the edge of a great sun-spot with a dazzling brightness at least five or six times that of the neighbouring portions of the photosphere, and moved eastward over the spot in parallel lines, growing smaller and fainter, until in about five minutes they disappeared, after traversing a course of nearly thirty-six thousand miles." the sun-spots do not occur at all parts of the sun's surface indifferently. they are mainly found in two zones (fig. ) on each side of the solar equator between the latitudes of ° and °. on the equator the spots are rare except, curiously enough, near the time when there are few spots elsewhere. in high latitudes they are never seen. closely connected with these peculiar principles of their distribution is the remarkable fact that spots in different latitudes do not indicate the same values for the period of rotation of the sun. by watching a spot near the sun's equator carrington found that it completed a revolution in twenty-five days and two hours. at a latitude of ° the period is about twenty-five days and eighteen hours, at ° it is no less than twenty-six days and twelve hours, while the comparatively few spots observed in the latitude of ° require twenty-seven and a half days to complete their circuit. as the sun, so far at least as its outer regions are concerned, is a mass of gas and not a solid body, there would be nothing incredible in the supposition that spots are occasionally endowed with movements of their own like ships on the ocean. it seems, however, from the facts before us that the different zones on the sun, corresponding to what we call the torrid and temperate zones on the earth, persist in rotating with velocities which gradually decrease from the equator towards the poles. it seems probable that the interior parts of the sun do not rotate as if the whole were a rigidly connected mass. the mass of the sun, or at all events its greater part, is quite unlike a rigid body, and the several portions are thus to some extent free for independent motion. though we cannot actually see how the interior parts of the sun rotate, yet here the laws of dynamics enable us to infer that the interior layers of the sun rotate more rapidly than the outer layers, and thus some of the features of the spot movements can be accounted for. but at present it must be confessed that there are great difficulties in the way of accounting for the distribution of spots and the law of rotation of the sun. in the year schwabe, a german astronomer, commenced to keep a regular register of the number of spots visible on the sun. after watching them for seventeen years he was able to announce that the number of spots seemed to fluctuate from year to year, and that there was a period of about ten years in their changes. subsequent observations have confirmed this discovery, and old books and manuscripts have been thoroughly searched for information of early date. thus a more or less complete record of the state of the sun as regards spots since the beginning of the seventeenth century has been put together. this has enabled astronomers to fix the period of the recurring maximum with greater accuracy. the course of one of the sun-spot cycles may be described as follows: for two or three years the spots are both larger and more numerous than on the average; then they begin to diminish, until in about six or seven years from the maximum they decline to a minimum; the number of the spots then begins to increase, and in about four and a half years the maximum is once more attained. the length of the cycle is, on an average, about eleven years and five weeks, but both its length and the intensity of the maxima vary somewhat. for instance, a great maximum occurred in the summer of , after which a very low minimum occurred in , followed by a feeble maximum at the end of ; next came an average minimum about august, , followed by the last observed maximum in january, . it is not unlikely that a second period of about sixty or eighty years affects the regularity of the eleven-year period. systematic observations carried on through a great many years to come will be required to settle this question, as the observations of sun-spots previous to are far too incomplete to decide the issues which arise. a curious connection seems to exist between the periodicity of the spots and their distribution over the surface of the sun. when a minimum is about to pass away the spots generally begin to show themselves in latitudes about ° north and south of the sun's equator; they then gradually break out somewhat nearer to the equator, so that at the time of maximum frequency most of them appear at latitudes not greater than °. this distance from the sun's equator goes on decreasing till the time of minimum. indeed, the spots linger on very close to the equator for a couple of years more, until the outbreak signalising the commencement of another period has commenced in higher latitudes. we have still to note an extraordinary feature which points to an intimate connection between the phenomena of sun-spots and the purely terrestrial phenomena of magnetism. it is of course well known that the needle of a compass does not point exactly to the north, but diverges from the meridian by an angle which is different in different places and is not even constant at the same place. for instance, at greenwich the needle at present points in a direction ° west of north, but this amount is subject to very slow and gradual changes, as well as to very small daily oscillations. it was found about fifty years ago by lamont (a bavarian astronomer, but a native of scotland) that the extent of this daily oscillation increases and decreases regularly in a period which he gave as - / years, but which was subsequently found to be - / years, exactly the same as the period of the spots on the sun. from a diligent study of the records of magnetic observations it has been found that the time of sun-spot maximum always coincides almost exactly with that of maximum daily oscillation of the compass needle, while the minima agree similarly. this close relationship between the periodicity of sun-spots and the daily movements of the magnetic needle is not the sole proof we possess that there is a connection of some sort between solar phenomena and terrestrial magnetism. a time of maximum sun-spots is a time of great magnetic activity, and there have even been special cases in which a peculiar outbreak on the sun has been associated with remarkable magnetic phenomena on the earth. a very interesting instance of this kind is recorded by professor young, who, when observing at sherman on the rd august, , perceived a very violent disturbance of the sun's surface. he was told the same day by a member of his party, who was engaged in magnetic observations and who was quite in ignorance of what professor young had seen, that he had been obliged to desist from his magnetic work in consequence of the violent motion of his magnet. it was afterwards found from the photographic records at greenwich and stonyhurst that the magnetic "storm" observed in america had simultaneously been felt in england. a similar connection between sun-spots and the aurora borealis has also been noticed, this fact being a natural consequence of the well-known connection between the aurora and magnetic disturbances. on the other hand, it must be confessed that many striking magnetic storms have occurred without any corresponding solar disturbance,[ ] but even those who are inclined to be sceptical as to the connection between these two classes of phenomena in particular cases can hardly doubt the remarkable parallelism between the general rise and fall in the number of sun-spots and the extent of the daily movements of the compass needle. [illustration: fig. .--the texture of the sun and a small spot.] we have now described the principal solar phenomena with which the telescope has made us acquainted. but there are many questions connected with the nature of the sun which not even the most powerful telescope would enable us to solve, but which the spectroscope has given us the means of investigating. what we receive from the sun is warmth and light. the intensely heated mass of the sun radiates forth its beams in all directions with boundless prodigality. each beam we feel to be warm, and we see to be brilliantly white, but a more subtle analysis than mere feeling or mere vision is required. each sunbeam bears marks of its origin. these marks are not visible until a special process has been applied, but then the sunbeam can be made to tell its story, and it will disclose to us much of the nature of the constitution of the great luminary. we regard the sun's light as colourless, just as we speak of water as tasteless, but both of those expressions relate rather to our own feelings than to anything really characteristic of water or of sunlight. we regard the sunlight as colourless because it forms, as it were, the background on which all other colours are depicted. the fact is, that white is so far from being colourless that it contains every known hue blended together in certain proportions. the sun's light is really extremely composite; nature herself tells us this if we will but give her the slightest attention. whence come the beautiful hues with which we are all familiar? look at the lovely tints of a garden; the red of the rose is not in the rose itself. all the rose does is to grasp the sunbeams which fall upon it, extract from these beams the red which they contain, and radiate that red light to our eyes. were there not red rays conveyed with the other rays in the sunbeam, there could be no red rose to be seen by sunlight. the principle here involved has many other applications; a lady will often say that a dress which looks very well in the daylight does not answer in the evening. the reason is that the dress is intended to show certain colours which exist in the sunlight; but these colours are not contained to the same degree in gaslight, and consequently the dress has a different hue. the fault is not in the dress, the fault lies in the gas; and when the electric light is used it sends forth beams more nearly resembling those from the sun, and the colours of the dress appear with all their intended beauty. the most glorious natural indication of the nature of the sunlight is seen in the rainbow. here the sunbeams are refracted and reflected from tiny globes of water in the clouds; these convey to us the sunlight, and in doing so decompose the white beams into the seven primary hues--red, orange, yellow, green, blue, indigo, and violet. [illustration: plate a. the sun. _royal observatory, greenwich, july , ._] [illustration: fig. .--the prism.] the bow set in the cloud is typical of that great department of modern science of which we shall now set forth the principles. the globes of water decompose the solar beams; and we follow the course suggested by the rainbow, and analyse the sunlight into its constituents. we are enabled to do this with scientific accuracy when we employ that remarkable key to nature's secrets known as the spectroscope. the beams of white sunlight consist of innumerable beams of every hue in intimate association. every shade of red, of yellow, of blue, and of green, can be found in a sunbeam. the magician's wand, with which we strike the sunbeam and sort the tangled skein into perfect order, is the simple instrument known as the glass prism. we have represented this instrument in its simplest form in the adjoining figure (fig. ). it is a piece of pure and homogeneous glass in the shape of a wedge. when a ray of light from the sun or from any source falls upon the prism, it passes through the transparent glass and emerges on the other side; a remarkable change is, however, impressed on the ray by the influence of the glass. it is bent by refraction from the path it originally pursued, and is compelled to follow a different path. if, however, the prism bent all rays of light equally, then it would be of no service in the analysis of light; but it fortunately happens that the prism acts with varying efficiency on the rays of different hues. a red ray is not refracted so much as a yellow ray; a yellow ray is not refracted so much as a blue one. it consequently happens that when the composite beam of sunlight, in which all the different rays are blended, passes through the prism, they emerge in the manner shown in the annexed figure (fig. ). here then we have the source of the analysing power of the prism; it bends the different hues unequally and consequently the beam of composite sunlight, after passing through the prism, no longer shows mere white light, but is expanded into a coloured band of light, with hues like the rainbow, passing from deep red at one end through every intermediate grade to the violet. [illustration: fig. .--dispersion of light by the prism.] we have in the prism the means of decomposing the light from the sun, or the light from any other source, into its component parts. the examination of the quality of the light when analysed enables us to learn something of the constitution of the body from which this light has emanated. indeed, in some simple cases the mere colour of a light will be sufficient to indicate the source from which it has come. there is, for instance, a splendid red light sometimes seen in displays of fireworks, due to the metal strontium. the eye can identify the element by the mere colour of the flame. there is also a characteristic yellow light produced by the flame of common salt burned with spirits of wine. sodium is the important constituent of salt, so here we recognise another substance merely by the colour it emits when burning. we may also mention a third substance, magnesium, which burns with a brilliant white light, eminently characteristic of the metal. [illustration: plate xiii. spectra of the sun and stars. i. sun. ii. sirius. iii. aldebaran. iv. betelgeuze.] the three metals, strontium, sodium, and magnesium, may thus be identified by the colours they produce when incandescent. in this simple observation lies the germ of the modern method of research known as spectrum analysis. we may now examine with the prism the colours of the sun and the colours of the stars, and from this examination we can learn something of the materials which enter into their composition. we are not restricted to the use of merely a single prism, but we may arrange that the light which it is desired to analyse shall pass through several prisms in succession in order to increase the _dispersion_ or the spreading out of the different colours. to enter the spectroscope the light first passes through a narrow slit, and the rays are then rendered parallel by passing through a lens; these parallel rays next pass through one or more prisms, and are finally viewed through a small telescope, or they may be intercepted by a photographic plate on which a picture will then be made. if the beam of light passing through the slit has radiated from an incandescent solid or liquid body, or from a gas under high pressure, the coloured band or _spectrum_ is found to contain all the colours indicated on plate xiii., without any interruption between the colours. this is known as a continuous spectrum. but if we examine light from a gas under low pressure, as can be done by placing a small quantity of the gas in a glass tube and making it glow by an electric current, we find that it does not emit rays of all colours, but only rays of certain distinct colours which are different for different gases. the spectrum of a gas, therefore, consists of a number of detached luminous lines. when we study the sunlight through the prism, it is found that the spectrum does not extend quite continuously from one end to the other, but is shaded over by a multitude of dark lines, only a few of which are shown in the adjoining plate. (plate xiii.) these lines are a permanent feature in the solar spectrum. they are as characteristic of the sunlight as the prismatic colours themselves, and are full of interest and information with regard to the sun. these lines are the characters in which the history and the nature of the sun are written. viewed through an instrument of adequate power, dark lines are to be found crossing the solar spectrum in hundreds and in thousands. they are of every variety of strength and faintness; their distribution seems guided by no simple law. at some parts of the spectrum there are but few lines; in other regions they are crowded so closely together that it is difficult to separate them. they are in some places exquisitely fine and delicate, and they never fail to excite the admiration of every one who looks at this interesting spectacle in a good instrument. there can be no better method of expounding the rather difficult subject of spectrum analysis than by actually following the steps of the original discovery which first gave a clear demonstration of the significance of the dark "fraunhofer" lines. let us concentrate our attention specially upon that line of the solar spectrum marked d. this, when seen in the spectroscope, is found to consist of two lines, very delicately separated by a minute interval, one of these lines being slightly thicker than the other. suppose that while the attention is concentrated on these lines the flame of an ordinary spirit-lamp coloured by common salt be held in front of the instrument, so that the ray of direct solar light passes through the flame before entering the spectroscope. the observer sees at once the two lines known as d flash out with a greatly increased blackness and vividness, while there is no other perceptible effect on the spectrum. a few trials show that this intensification of the d lines is due to the vapour of sodium arising from the salt burning in the lamp through which the sunlight has passed. it is quite impossible that this marvellous connection between sodium and the d lines of the spectrum can be merely casual. even if there were only a single line concerned, it would be in the highest degree unlikely that the coincidence should arise by accident; but when we find the sodium affecting both of the two close lines which form d, our conviction that there must be some profound connection between these lines and sodium rises to absolute certainty. suppose that the sunlight be cut off, and that all other light is excluded save that emanating from the glowing vapour of sodium in the spirit flame. we shall then find, on looking through the spectroscope, that we no longer obtain all the colours of the rainbow; the light from the sodium is concentrated into two bright yellow lines, filling precisely the position which the dark d lines occupied in the solar spectrum, and the darkness of which the sodium flame seemed to intensify. we must here endeavour to remove what may at first sight appear to be a paradox. how is it, that though the sodium flame produces two _bright_ lines when viewed in the absence of other light, yet it actually appears to intensify the two _dark_ lines in the sun's spectrum? the explanation of this leads us at once to the cardinal doctrine of spectrum analysis. the so-called dark lines in the solar spectrum are only dark _by contrast_ with the brilliant illumination of the rest of the spectrum. a good deal of solar light really lies in the dark lines, though not enough to be seen when the eye is dazzled by the brilliancy around. when the flame of the spirit-lamp charged with sodium intervenes, it sends out a certain amount of light, which is entirely localised in these two lines. so far it would seem that the influence of the sodium flame ought to be manifested in diminishing the darkness of the lines and rendering them less conspicuous. as a matter of fact, they are far more conspicuous with the sodium flame than without it. this arises from the fact that the sodium flame possesses the remarkable property of cutting off the sunlight which was on its way to those particular lines; so that, though the sodium contributes some light to the lines, yet it intercepts a far greater quantity of the light that would otherwise have illuminated those lines, and hence they became darker with the sodium flame than without it. we are thus conducted to a remarkable principle, which has led to the interpretation of the dark lines in the spectrum of the sun. we find that when the sodium vapour is heated, it gives out light of a very particular type, which, viewed through the prism, is concentrated in two lines. but the sodium vapour possesses also this property, that light from the sun can pass through it without any perceptible absorption, except of those particular rays which are of the same characters as the two lines in question. in other words, we say that if the heated vapour of a substance gives a spectrum of bright lines, corresponding to lights of various kinds, this same vapour will act as an opaque screen to lights of those special kinds, while remaining transparent to light of every other description. this principle is of such importance in the theory of spectrum analysis that we add a further example. let us take the element iron, which in a very striking degree illustrates the law in question. in the solar spectrum some hundreds of the dark lines are known to correspond with the spectrum of iron. this correspondence is exhibited in a vivid manner when, by a suitable contrivance, the light of an electric spark from poles of iron is examined in the spectroscope side by side with the solar spectrum. the iron lines in the sun are identical in position with the lines in the spectrum of glowing iron vapour. but the spectrum of iron, as here described, consists of bright lines; while those with which it is compared in the sun are dark on a bright background. they can be completely understood if we suppose the vapour arising from intensely heated iron to be present in the atmosphere which surrounds the luminous strata on the sun. this vapour would absorb or stop precisely the same rays as it emits when incandescent, and hence we learn the important fact that iron, no less than sodium, must, in one form or another, be a constituent of the sun. such is, in brief outline, the celebrated discovery of modern times which has given an interpretation to the dark lines of the solar spectrum. the spectra of a large number of terrestrial substances have been examined in comparison with the solar spectrum, and thus it has been established that many of the elements known on the earth are present in the sun. we may mention calcium, iron, hydrogen, sodium, carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium, manganese, copper, zinc, cadmium, silver, tin, lead, potassium. some of the elements which are of the greatest importance on the earth would appear to be missing from the sun. sulphur, phosphorus, mercury, gold, nitrogen may be mentioned among the elements which have hitherto given no indication of their being solar constituents. it is also possible that the lines of a substance in the sun's atmosphere may be so very bright that the light of the continuous spectrum, on which they are superposed, is not able to "reverse" them--_i.e._ turn them into dark lines. we know, for instance, that the bright lines of sodium vapour may be made so intensely bright that the spectrum of an incandescent lime-cylinder placed behind the sodium vapour does not reverse these lines. if, then, we make the sodium lines fainter, they may be reduced to exactly the intensity prevailing in that part of the spectrum of the lime-light, in which case the lines, of course, could not be distinguished. the question as to what elements are really missing from the sun must therefore, like many other questions concerning our great luminary, at present be considered an open one. we shall shortly see that an element previously unknown has actually been discovered by means of a line representing it in the solar spectrum. let us now return to the sun-spots and see what the spectroscope can teach us as to their nature. we attach a powerful spectroscope to the eye-end of a telescope in order to get as much light as possible concentrated on the slit; the latter has therefore to be placed exactly at the focus of the object-glass. the instrument is then pointed to a spot, so that its image falls on the slit, and the presence of the dark central part called the _umbra_ reveals itself by a darkish stripe which traverses the ordinary sun-spectrum from end to end. it is bordered on both sides by the spectrum of the _penumbra_, which is much brighter than that of the umbra, but fainter than that of the adjoining regions of the sun. from the fact that the spectrum is darkened we learn that there is considerable general absorption of light in the umbra. this absorption is not, however, such as would be caused by the presence of volumes of minute solid or liquid particles like those which constitute smoke or cloud. this is indicated by the fact, first discovered by young in , that the spectrum is not uniformly darkened as it would be if the absorption were caused by floating particles. in the course of examination of many large and quiescent spots, he perceived that the middle green part of the spectrum was crossed by countless fine, dark lines, generally touching each other, but here and there separated by bright intervals. each line is thicker in the middle (corresponding to the centre of the spot) and tapers to a fine thread at each end; indeed, most of these lines can be traced across the spectrum of the penumbra and out on to that of the solar surface. the absorption would therefore seem to be caused by gases at a much lower temperature than that of the gases present outside the spot. in the red and yellow parts of the spot-spectrum, which have been specially studied for many years by sir norman lockyer at the south kensington observatory, interesting details are found which confirm this conclusion. many of the dark lines are not thicker and darker in the spot than they are in the ordinary sun-spectrum, while others are very much thickened in the spot-spectrum, such as the lines of iron, calcium, and sodium. the sodium lines are sometimes both widened and doubly reversed--that is, on the thick dark line a bright line is superposed. the same peculiarity is not seldom seen in the notable calcium lines h and k at the violet end of the spectrum. these facts indicate the presence of great masses of the vapours of sodium and calcium over the nucleus. the observations at south kensington have also brought to light another interesting peculiarity of the spot-spectra. at the time of minimum frequency of spots the lines of iron and other terrestrial elements are prominent among the most widened lines; at the maxima these almost vanish, and the widening is found only amongst lines of unknown origin. the spectroscope has given us the means of studying other interesting features on the sun, which are so faint that in the full blaze of sunlight they cannot be readily observed with a mere telescope. we can, however, see them easily enough when the brilliant body of the sun is obscured during the rare occurrence of a total eclipse. the conditions necessary for the occurrence of an eclipse will be more fully considered in the next chapter. for the present it will be sufficient to observe that by the movement of the moon it may so happen that the moon completely hides the sun, and thus for certain parts of the earth produces what we call a total eclipse. the few minutes during which a total eclipse lasts are of much interest to the astronomer. darkness reigns over the landscape, and in that darkness rare and beautiful sights are witnessed. [illustration: fig. .--prominences seen in total eclipse.] we have in fig. a diagram of a total eclipse, showing some of the remarkable objects known as prominences (_a_, _b_, _c_, _d_, _e_) which project from behind the dark body of the moon. that they do not belong to the moon, but are solar appendages of some sort, is easily demonstrated. they first appear on the eastern limb at the commencement of totality. those first seen are gradually more or less covered by the advancing moon, while others peep out behind the western limb of the moon, until totality is over and the sunlight bursts out again, when they all instantly vanish. the first total eclipse which occurred after the spectroscope had been placed in the hands of astronomers was in . on the th august in that year a total eclipse was visible in india. several observers, armed with spectroscopes, were on the look-out for the prominences, and were able to announce that their spectrum consisted of detached bright lines, thus demonstrating that these objects were masses of glowing gas. on the following day the illustrious astronomer, janssen, one of the observers of the eclipse, succeeded in seeing the lines in full sunlight, as he now knew exactly where to look for them. many months before the eclipse sir norman lockyer had been preparing to search for the prominences, as he expected them to yield a line spectrum which would be readily visible, if only the sun's ordinary light could be sufficiently winnowed away. he proposed to effect this by using a spectroscope of great dispersion, which would spread out the continuous spectrum considerably and make it fainter. the effect of the great dispersion on the isolated bright lines he expected to see would be only to widen the intervals between them without interfering with their brightness. the new spectroscope, which he ordered to be constructed for this purpose, was not completed until some weeks after the eclipse was over, though before the news of janssen's achievement reached europe from india. when that news did arrive sir n. lockyer had already found the spectrum of unseen prominences at the sun's limb. the honour of the practical application of a method of observing solar prominences without the help of an eclipse must therefore be shared between the two astronomers. when a spectroscope is pointed to the margin of the sun so that the slit is radial, certain short luminous lines become visible which lie exactly in the prolongation of the corresponding dark lines in the solar spectrum. from due consideration of the circumstances it can be shown that the gases which form the prominences are also present as a comparatively shallow atmospheric layer all round the great luminary. this layer is about five or six thousand miles deep, and is situated immediately above the dense layer of luminous clouds which forms the visible surface of the sun and which we call the photosphere. the gaseous envelope from which the prominences spring has been called the chromosphere on account of the coloured lines displayed in its spectrum. such lines are very numerous, but those pertaining to the single substance, hydrogen, predominate so greatly that we may say the chromosphere consists chiefly of this element. it is, however, to be noted that calcium and one other element are also invariably present, while iron, manganese and magnesium are often apparent. the remarkable element, of which we have not yet mentioned the name, has had an astonishing history. during the eclipse of a fine yellow line was noticed among the lines of the prominence spectrum, and it was not unnaturally at first assumed that it must be the yellow sodium line. but when careful observations were afterwards made without hurry in full sunshine, and accurate measures were obtained, it was at once remarked that this line was not identical with either of the components of the double sodium line. the new line was, no doubt, quite close to the sodium lines, but slightly towards the green part of the spectrum. it was also noticed there was not generally any corresponding line to be seen among the dark lines in the ordinary solar spectrum, though a fine dark one has now and then been detected, especially near a sun-spot. sir norman lockyer and sir edward frankland showed that this was not produced by any known terrestrial element. it was, therefore, supposed to be caused by some hitherto unknown body to which the name of _helium_, or the sun element, was given. about a dozen less conspicuous lines were gradually identified in the spectrum of the prominences and the chromosphere, which appeared also to be caused by this same mysterious helium. these same remarkable lines have in more recent years also been detected in the spectra of various stars. this gas so long known in the heavens was at last detected on earth. in april, , professor ramsay, who with lord rayleigh had discovered the new element argon, detected the presence of the famous helium line in the spectrum of the gas liberated by heating the rare mineral known as cleveite, found in norway. thus this element, the existence of which had first been detected on the sun, ninety-three million miles away, has at last been proved to be a terrestrial element also. when it was announced by runge that the principal line in the spectrum of the terrestrial helium had a faint and very close companion line on the red-ward side, some doubt seemed at first to be cast on the identity of the new terrestrial gas discovered by ramsay with the helium of the chromosphere. the helium line of the latter had never been noticed to be double. subsequently, however, several observers provided with very powerful instruments found that the famous line in the chromosphere really had a very faint companion line. thus the identity between the celestial helium and the gas found on our globe was established in the most remarkable manner. certain circumstances have seemed to indicate that the new gas might possibly be a mixture of two gases of different densities, but up to the present this has not been proved to be the case. after it had been found possible to see the spectra of prominences without waiting for an eclipse, sir w. huggins, in an observation on the th of february, , successfully applied a method for viewing the remarkable solar objects themselves instead of their mere spectra in full sunshine. it is only necessary to adjust the spectroscope so that one of the brightest lines--_e.g._ the red hydrogen line--is in the middle of the field of the viewing telescope, and then to open wide the slit of the spectroscope. a red image of the prominence will then be displayed instead of the mere line. in fact, when the slit is opened wide, the prisms produce a series of detached images of the prominence under observation, one for each kind of light which the object emits. we have spoken of the spectroscope as depending upon the action of glass prisms. it remains to be added that in the highest class of spectroscopes the prisms are replaced by ruled gratings from which the light is reflected. the effect of the ruling is to produce by what is known as diffraction the required breaking up of the beam of light into its constituent parts. [illustration: plate iv. solar prominences. (drawn by trouvelot at harvard college, cambridge, u.s., in .)] majestic indeed are the proportions of some of those mighty prominences which leap from the luminous surface; yet they flicker, as do our terrestrial flames, when we allow them time comparable to their gigantic dimensions. drawings of the same prominence made at intervals of a few hours, or even less, often show great changes. the magnitude of the displacements that have been noticed sometimes attains many thousands of miles, and the actual velocity with which such masses move frequently exceeds miles a second. still more violent are the convulsions when, from the surface of the chromosphere, as from a mighty furnace, vast incandescent masses of gas are projected upwards. plate iv. gives a view of a number of prominences as seen by trouvelot at harvard college observatory, cambridge, u.s.a. trouvelot has succeeded in exhibiting in the different pictures the wondrous variety of aspect which these objects assume. the dimensions of the prominences may be inferred from the scale appended to the plate. the largest of those here shown is fully , miles high; and trustworthy observers have recorded prominences of an altitude even much greater. the rapid changes which these objects sometimes undergo are well illustrated in the two sketches on the left of the lowest line, which were drawn on april th, . these are both drawings of the same prominence taken at an interval no greater than twenty minutes. this mighty flame is so vast that its length is ten times as great as the diameter of the earth, yet in this brief period it has completely changed its aspect; the upper part of the flame has, indeed, broken away, and is now shown in that part of the drawing between the two figures on the line above. the same plate also shows various instances of the remarkable spike-like objects, taken, however, at different times and at various parts of the sun. these spikes attain altitudes not generally greater than , miles, though sometimes they soar aloft to stupendous distances. we may refer to one special object of this kind, the remarkable history of which has been chronicled by professor young. on october th, , a prominence was seen, at about . a.m., on the south-east limb of the sun. it was then about , miles high, and attracted no special attention. half an hour later a marvellous transformation had taken place. during that brief interval the prominence became very brilliant and doubled its length. for another hour the mighty flame still soared upwards, until it attained the unprecedented elevation of , miles--a distance more than one-third the diameter of the great luminary itself. at this climax the energy of the mighty outbreak seems to have at last become exhausted: the flame broke up into fragments, and by . --an interval of only two hours from the time when it was first noticed--the phenomenon had completely faded away. no doubt this particular eruption was exceptional in its vehemence, and in the vastness of the changes of which it was an indication. the velocity of upheaval must have been at least , miles an hour, or, to put it in another form, more than fifty miles a second. this mighty flame leaped from the sun with a velocity more than times as great as that of the swiftest bullet ever fired from a rifle. the prominences may be generally divided into two classes. we have first those which are comparatively quiescent, and in form somewhat resemble the clouds which float in our earth's atmosphere. the second class of prominences are best described as eruptive. they are, in fact, thrown up from the chromosphere like gigantic jets of incandescent material. these two classes of objects differ not only in appearance but also in the gases of which they are composed. the cloud-like prominences consist mainly of hydrogen, with helium and calcium, while many metals are present in the eruptive discharges. the latter are never seen in the neighbourhood of the sun's poles, but generally appear close to a sun-spot, thus confirming the conclusion that the spots are associated with violent disturbances on the surface of the sun. when a spot has reached the limb of the sun it is frequently found to be surrounded by prominences. it has even been possible in a few instances to detect powerful gaseous eruptions in the neighbourhood of a spot, the spectroscope rendering them visible against the background of the solar surface just as the prominences are observed at the limb against the background of the sky. in order to photograph a prominence we have, of course, to substitute a photographic plate for the observer's eye. owing, however, to the difficulty of preventing the feeble light from the prominence from being overpowered by extraneous light, the photography of these bodies was not very successful until professor hale, of chicago, designed his spectro-heliograph. in this instrument there is (in addition to the usual slit through which the light falls on the prisms, or grating,) a second slit immediately in front of the photographic plate through which the light of a given wave-length can be permitted to pass to the exclusion of all the rest. the light chosen for producing an image of the prominences is that radiated in the remarkable "k line," due to calcium. this lies at the extreme end of the violet. the light from that part of the spectrum, though it is invisible to the eye, is much more active photographically than the light from the red, yellow, or green parts of the spectrum. the front slit is adjusted so that the k line falls upon the second slit, and as the front slit is slowly swept by clockwork over the whole of a prominence, the second slit keeps pace with it by a mechanical contrivance. if the image of the solar disc is hidden by a screen of exactly the proper size, the slits may be made to sweep over the whole sun, thus giving us at one exposure a picture of the chromospheric ring round the sun's limb with its prominences. the screen may now be withdrawn, and the slits may be made to sweep rapidly over the disc itself. they reveal the existence of glowing calcium vapours in many parts of the surface of the sun. thus we get a striking picture of the sun as drawn by this particular light. in this manner professor hale confirmed the observation made long before by professor young, that the spectra of faculæ always show the two great calcium bands. the velocity with which a prominence shoots upward from the sun's limb can, of course, be measured directly by observations of the ordinary kind with a micrometer. the spectroscope, however, enables us to estimate the speed with which disturbances at the surface of the sun travel in the direction towards the earth or from the earth. we can measure this speed by watching the peculiar behaviour of the spectral lines representing the rapidly moving masses. this opens up a remarkable line of investigation with important applications in many branches of astronomy. it is, of course, now generally understood that the sensation of light is caused by waves or undulations which impinge on the retina of the eye after having been transmitted through that medium which we call the ether. to the different colours correspond different wave-lengths--that is to say, different distances between two successive waves. a beam of white light is formed by the union of innumerable different waves whose lengths have almost every possible value lying between certain limits. the wave-length of red light is such that there are , waves in an inch, while that of violet light is but little more than half that of red light. the position of a line in the spectrum depends solely on the wave-length of the light to which it is due. suppose that the source of light is approaching directly towards the observer; obviously the waves follow each other more closely than if the source were at rest, and the number of undulations which his eye receives in a second must be proportionately increased. thus the distance between two successive ether waves will be very slightly diminished. a well-known phenomenon of a similar character is the change of pitch of the whistle of a locomotive engine as it rushes past. this is particularly noticeable if the observer happens to be in a train which is moving rapidly in the opposite direction. in the case of sound, of course, the vibrations or waves take place in the air and not in the ether. but the effect of motion to or from the observer is strictly analogous in the two cases. as, however, light travels , miles a second, the source of light will also have to travel with a very high velocity in order to produce even the smallest perceptible change in the position of a spectral line. we have already seen that enormously high velocities are by no means uncommon in some of these mighty disturbances on the sun; accordingly, when we examine the spectrum of a sun-spot, we often see that some of the lines are shifted a little towards one end of the spectrum and sometimes towards the other, while in other cases the lines are seen to be distorted or twisted in the most fantastic manner, indicating very violent local commotions. if the spot happens to be near the centre of the sun's disc, the gases must be shooting upwards or downwards to produce these changes in the lines. the velocities indicated in observations of this class sometimes amount to as much as two or even three hundred miles per second. we find it difficult to conceive the enormous internal pressures which are required to impel such mighty masses of gases aloft from the photosphere with speeds so terrific, or the conditions which bring about the downrush of such gigantic masses of vapour from above. in the spectra of the prominences on the sun's limb also we often see the bright lines bent or shifted to one side. in such cases what we witness is evidently caused by movements along the surface of the chromosphere, conveying materials towards us or away from us. an interesting application of this beautiful method of measuring the speed of moving bodies has been made in various attempts to determine the period of rotation of the sun spectroscopically. as the sun turns round on its axis, a point on the eastern limb is moving towards the observer and a point on the western limb is moving away from him. in each case the velocity is a little over a mile per second. at the eastern limb the lines in the solar spectrum are very slightly shifted towards the violet end of the spectrum, while the lines in the spectrum of the western limb are equally shifted towards the red end. by an ingenious optical contrivance it is possible to place the spectra from the two limbs side by side, which doubles the apparent displacement, and thus makes it much more easy to measure. even with this contrivance the visual quantities to be measured remain exceedingly minute. all the parts of the instrument have to be most accurately adjusted, and the observations are correspondingly delicate. they have been attempted by various observers. among the most successful investigations of this kind we may mention that of the swedish astronomer, dunér, who, by pointing his instrument to a number of places on the limb, found values in good agreement with the peculiar law of rotation which has been deduced from the motion of sun-spots. this result is specially interesting, as it shows that the atmospheric layers, in which that absorption takes place which produces the dark lines in the spectrum, shares in the motion of the photosphere at the same latitude. [illustration: fig. .--view of the corona (and a comet) in a total eclipse.] [illustration: plate v. total solar eclipse, july th, . the corona from the photographs. (harkness.)] we have yet to mention one other striking phenomenon which is among the chief attractions to observers of total eclipses, and which it has hitherto not been found possible to see in full daylight. this is the corona or aureole of light which is suddenly seen to surround the sun in an eclipse when the moon has completely covered the last remaining crescent of the sun. a general idea of the appearance of the corona is given in fig. , and we further present in plate v. the drawing of the corona made by professor harkness from a comparison of a large number of photographs obtained at different places in the united states during the total eclipse of july th, . in fig. we are permitted by the kindness of mr. and mrs. maunder to reproduce the remarkable photograph of the corona which they obtained in india during the eclipse of january nd, . [illustration: fig. .--view of corona during the eclipse of jan. nd, (_reproduced by kind permission of mr. and mrs. maunder and of the proprietors of "knowledge._")] the part of the corona nearest the sun is very bright, though not so brilliant as the prominences, which (as professor young says) blaze through it like carbuncles. this inner portion is generally of fairly regular outline, forming a white ring about a tenth part of the solar diameter in width. the outer parts of the corona are usually very irregular and very extensive. they are often interrupted by narrow "rifts," or narrow dark bands, which reach from the limb of the sun through the entire corona. on the other hand, there are also sometimes narrow bright streamers, inclined at various angles to the limb of the sun and not seldom curved. in the eclipses of , , and , all of which occurred at periods of sun-spot minimum, the corona showed long and faint streamers nearly in the direction of the sun's equator, and short but distinct brushes of light near the poles. in the eclipses of , , and , near sun-spot maxima, the corona was more regularly circular, and chiefly developed over the spot zones. we have here another proof (if one were necessary) of the intimate connection between the periodicity of the spots and the development of all other solar phenomena. in the spectrum of the corona there is a mysterious line in the green, as to the origin of which nothing is at present certainly known. it is best seen during eclipses occurring near the time of sun-spot maximum. it is presented in the ordinary solar spectrum as a very thin, dark line, which generally remains undisturbed even when lines of hydrogen and other substances are twisted and distorted by the violent rush of disturbed elements. the line is always present among the bright lines of the chromosphere spectrum. in addition to it the corona shows a few other bright lines, belonging, no doubt, to the same unknown element ("coronium"), and also a faint continuous spectrum, in which even a few of the more prominent dark lines of the solar spectrum have been sometimes detected. this shows that in addition to glowing gas (represented by the bright lines) the corona also contains a great deal of matter like dust, or fog, the minute particles of which are capable of reflecting the sunlight and thereby producing a feeble continuous spectrum. this matter seems to form the principal constituent of the long coronal rays and streamers, as the latter are not visible in the detached images of the corona which appear instead of the bright lines when the corona is viewed, or photographed, during an eclipse, in a spectroscope without a slit. if the long rays were composed of the gas or gases which constitute the inner corona, it is evident that they ought to appear in these detached images. as to the nature of the forces which are continually engaged in shooting out these enormously long streamers, we have at present but little information. it is, however, certain that the extensive atmospheric envelope round the sun, which shows itself as the inner corona, must be extremely attenuated. comets have on several occasions been known to rush through this coronal atmosphere without evincing the slightest appreciable diminution in their speed from the resistance to which they were exposed. we have accumulated by observation a great number of facts concerning the sun, but when we try to draw from these facts conclusions as to the physical constitution of that great body, it cannot be denied that the difficulties seem to be very great indeed. we find that the best authorities differ considerably in the opinions they entertain as to its nature. we shall here set forth the principal conclusions as to which there is little or no controversy. we shall see in a following chapter that astronomers have been able to determine the relative densities of the bodies in the solar system; in other words, they have found the relation between the quantities of matter contained in an equally large volume of each. it has thus been ascertained that the average density of the sun is about a quarter that of the earth. if we compare the weight of the sun with that of an equally great globe of water, we find that the luminary would be barely one and a half times as heavy as the water. of course, the actual mass of the sun is very enormous; it is no less than , times as great as that of the earth. the solar material itself is, however, relatively light, so that the sun is four times as big as it would have to be if, while its weight remained the same, its density equalled that of the earth. bearing in mind this lightness of the sun, and also the exceedingly high temperature which we know to prevail there, no other conclusion seems possible than that the body of the sun must be in a gaseous state. the conditions under which such gases exist in the sun are, no doubt, altogether different from those with which we are acquainted on the earth. at the surface of the sun the force of gravity is more than twenty-seven times as great as it is on the earth. a person who on the earth could just lift twenty-seven equal pieces of metal would, if he were transferred to the sun, only be able to lift one of the pieces at a time. the pressure of the gases below the surface must therefore be very great, and it might be supposed that they would become liquefied in consequence. it was, however, discovered by andrews that so long as a gas is kept at a temperature higher than a certain point, known as the "critical temperature" (which is different for different gases), the gas will not be turned into a liquid however great be the pressure to which it is submitted. the temperature on the sun cannot be lower than the critical temperatures of the gases there existing; so it would seem that even the enormous pressure can hardly reduce the gases in the great luminary to the liquid form. of the interior of the sun we can, of course, expect to learn little or nothing. what we observe is the surface-layer, the so-called photosphere, in which the cold of space produces the condensation of the gases into those luminous clouds which we see in our drawings and photographs as "rice grains" or "willow leaves." it has been suggested by dr. johnstone stoney (and afterwards by professor hastings, of baltimore) that these luminous clouds are mainly composed of carbon with those of the related elements silicon and boron, the boiling points of which are much higher than those of other elements which might be considered likely to form the photospheric clouds. the low atomic weight of carbon must also have the effect of giving the molecules of this element a very high velocity, and thereby enabling them to work their way into the upper regions, where the temperature has so fallen that the vapour becomes chilled into cloud. a necessary consequence of the rapid cooling of these clouds, and the consequent radiation of heat on a large scale, would be the formation of what we may perhaps describe as smoke, which settles by degrees through the intervals between the clouds (making these intervals appear darker) until it is again volatilised on reaching a level of greater heat below the clouds. this same smoke is probably the cause of the well-known fact that the solar limb is considerably fainter than the middle of the disc. this seems to arise from the greater absorption caused by the longer distance which a ray of light from a point near the limb has to travel through this layer of smoke before reaching the earth. it is shown that this absorption cannot be attributed to a gaseous atmosphere, since this would have the effect of producing more dark absorption lines in the spectrum. there would thus be a marked difference between the solar spectrum from a part near the middle of the disc and the spectrum from a part near the limb. this, however, we do not find to be the case. with regard to the nature of sun-spots, the idea first suggested by secchi and lockyer, that they represent down rushes of cooler vapours into the photosphere (or to its surface), seems on the whole to accord best with the observed phenomena. we have already mentioned that the spots are generally accompanied by faculæ and eruptive prominences in their immediate neighbourhood, but whether these eruptions are caused by the downfall of the vapour which makes the photospheric matter "splash up" in the vicinity, or whether the eruptions come first, and by diminishing the upward pressure from below form a "sink," into which overlying cooler vapour descends, are problems as to which opinions are still much divided. a remarkable appendage to the sun, which extends to a distance very much greater than that of the corona, produces the phenomenon of the zodiacal light. a pearly glow is sometimes seen in the spring to spread over a part of the sky in the vicinity of the point where the sun has disappeared after sunset. the same spectacle may also be witnessed before sunrise in the autumn, and it would seem as if the material producing the zodiacal light, whatever it may be, had a lens-shaped form with the sun in the centre. the nature of this object is still a matter of uncertainty, but it is probably composed of a kind of dust, as the faint spectrum it affords is of a continuous type. a view of the zodiacal light is shown in fig. . in all directions the sun pours forth, with the most prodigal liberality, its torrents of light and of heat. the earth can only grasp the merest fraction, less than the , , , th part of the whole. our fellow planets and the moon also intercept a trifle; but how small is the portion of the mighty flood which they can utilise! the sip that a flying swallow takes from a river is as far from exhausting the water in the river as are the planets from using all the heat which streams from the sun. the sun's gracious beams supply the magic power that enables the corn to grow and ripen. it is the heat of the sun which raises water from the ocean in the form of vapour, and then sends down that vapour as rain to refresh the earth and to fill the rivers which bear our ships down to the ocean. it is the heat of the sun beating on the large continents which gives rise to the breezes and winds that waft our vessels across the deep; and when on a winter's evening we draw around the fire and feel its invigorating rays, we are only enjoying sunbeams which shone on the earth countless ages ago. the heat in those ancient sunbeams developed the mighty vegetation of the coal period, and in the form of coal that heat has slumbered for millions of years, till we now call it again into activity. it is the power of the sun stored up in coal that urges on our steam-engines. it is the light of the sun stored up in coal that beams from every gaslight in our cities. for the power to live and move, for the plenty with which we are surrounded, for the beauty with which nature is adorned, we are immediately indebted to one body in the countless hosts of space, and that body is the sun. [illustration: fig. .--the zodiacal light in .] chapter iii. the moon. the moon and the tides--the use of the moon in navigation--the changes of the moon--the moon and the poets--whence the light of the moon?--sizes of the earth and the moon--weight of the moon--changes in apparent size--variations in its distance--influence of the earth on the moon--the path of the moon--explanation of the moon's phases--lunar eclipses--eclipses of the sun, how produced--visibility of the moon in a total eclipse--how eclipses are predicted--uses of the moon in finding longitude--the moon not connected with the weather--topography of the moon--nasmyth's drawing of triesnecker--volcanoes on the moon--normal lunar crater--plato--the shadows of lunar mountains--the micrometer--lunar heights--former activity on the moon--nasmyth's view of the formation of craters--gravitation on the moon--varied sizes of the lunar craters--other features of the moon--is there life on the moon?--absence of water and of air--dr. stoney's theory--explanation of the rugged character of lunar scenery--possibility of life on distant bodies in space. if the moon were suddenly struck out of existence, we should be immediately apprised of the fact by a wail from every seaport in the kingdom. from london and from liverpool we should hear the same story--the rise and fall of the tide had almost ceased. the ships in dock could not get out; the ships outside could not get in; and the maritime commerce of the world would be thrown into dire confusion. the moon is the principal agent in causing the daily ebb and flow of the tide, and this is the most important work which our satellite has to do. the fleets of fishing boats around the coasts time their daily movements by the tide, and are largely indebted to the moon for bringing them in and out of harbour. experienced sailors assure us that the tides are of the utmost service to navigation. the question as to how the moon causes the tides is postponed to a future chapter, in which we shall also sketch the marvellous part which the tides seem to have played in the early history of our earth. who is there that has not watched, with admiration, the beautiful series of changes through which the moon passes every month? we first see her as an exquisite crescent of pale light in the western sky after sunset. if the night is fine, the rest of the moon is visible inside the crescent, being faintly illumined by light reflected from our own earth. night after night she moves further and further to the east, until she becomes full, and rises about the same time that the sun sets. from the time of the full the disc of light begins to diminish until the last quarter is reached. then it is that the moon is seen high in the heavens in the morning. as the days pass by, the crescent shape is again assumed. the crescent wanes thinner and thinner as the satellite draws closer to the sun. finally she becomes lost in the overpowering light of the sun, again to emerge as the new moon, and again to go through the same cycle of changes. the brilliance of the moon arises solely from the light of the sun, which falls on the not self-luminous substance of the moon. out of the vast flood of light which the sun pours forth with such prodigality into space the dark body of the moon intercepts a little, and of that little it reflects a small fraction to illuminate the earth. the moon sheds so much light, and seems so bright, that it is often difficult at night to remember that the moon has no light except what falls on it from the sun. nevertheless, the actual surface of the brightest full moon is perhaps not much brighter than the streets of london on a clear sunshiny day. a very simple observation will suffice to show that the moon's light is only sunlight. look some morning at the moon in daylight, and compare the moon with the clouds. the brightness of the moon and of the clouds are directly comparable, and then it can be readily comprehended how the sun which illuminates the clouds has also illumined the moon. an attempt has been made to form a comparative estimate of the brightness of the sun and the full moon. if , full moons were shining at once, their collective brilliancy would equal that of the sun. the beautiful crescent moon has furnished a theme for many a poet. indeed, if we may venture to say so, it would seem that some poets have forgotten that the moon is not to be seen every night. a poetical description of evening is almost certain to be associated with the appearance of the moon in some phase or other. we may cite one notable instance in which a poet, describing an historical event, has enshrined in exquisite verse a statement which cannot be correct. every child who speaks our language has been taught that the burial of sir john moore took place "by the struggling moonbeams' misty light." there is an appearance of detail in this statement which wears the garb of truth. we are not inclined to doubt that the night was misty, nor as to whether the moonbeams had to struggle into visibility; the question at issue is a much more fundamental one. we do not know who was the first to raise the point as to whether any moon shone on that memorable event at all or not; but the question having been raised, the nautical almanac immediately supplies an answer. from it we learn in language, whose truthfulness constitutes its only claim to be poetry, that the moon was new at one o'clock in the morning of the day of the battle of corunna ( th january, ). the ballad evidently implies that the funeral took place on the night following the battle. we are therefore assured that the moon can hardly have been a day old when the hero was consigned to his grave. but the moon in such a case is practically invisible, and yields no appreciable moonbeams at all, misty or otherwise. indeed, if the funeral took place at the "dead of night," as the poet asserts, then the moon must have been far below the horizon at the time.[ ] in alluding to this and similar instances, mr. nasmyth gives a word of advice to authors or to artists who desire to bring the moon on a scene without knowing as a matter of fact that our satellite was actually present. he recommends them to follow the example of bottom in _a midsummer's night's dream_, and consult "a calendar, a calendar! look in the almanac; find out moonshine, find out moonshine!" [illustration: fig. .--comparative sizes of the earth and the moon.] among the countless host of celestial bodies--the sun, the moon, the planets, and the stars--our satellite enjoys one special claim on our attention. the moon is our nearest permanent neighbour. it is just possible that a comet may occasionally approach the earth more closely than the moon but with this exception the other celestial bodies are all many hundreds or thousands, or even many millions, of times further from us than the moon. it is also to be observed that the moon is one of the smallest visible objects which the heavens contain. every one of the thousands of stars that can be seen with the unaided eye is enormously larger than our satellite. the brilliance and apparent vast proportions of the moon arise from the fact that it is only , miles away, which is a distance almost immeasurably small when compared with the distances between the earth and the stars. fig. exhibits the relative sizes of the earth and its attendant. the small globe shows the moon, while the larger globe represents the earth. when we measure the actual diameters of the two globes, we find that of the earth to be , miles and of the moon , miles, so that the diameter of the earth is nearly four times greater than the diameter of the moon. if the earth were cut into fifty pieces, all equally large, then one of these pieces rolled into a globe would equal the size of the moon. the superficial extent of the moon is equal to about one thirteenth part of the surface of the earth. the hemisphere our neighbour turns towards us exhibits an area equal to about one twenty-seventh part of the area of the earth. this, to speak approximately, is about double the actual extent of the continent of europe. the average materials of the earth are, however, much heavier than those contained in the moon. it would take more than eighty globes, each as ponderous as the moon, to weigh down the earth. amid the changes which the moon presents to us, one obvious fact stands prominently forth. whether our satellite be new or full, at first quarter or at last, whether it be high in the heavens or low near the horizon, whether it be in process of eclipse by the sun, or whether the sun himself is being eclipsed by the moon, the apparent size of the latter is nearly constant. we can express the matter numerically. a globe one foot in diameter, at a distance of feet from the observer, would under ordinary circumstances be just sufficient to hide the disc of the moon; occasionally, however, the globe would have to be brought in to a distance of only feet, or occasionally it might have to be moved out to so much as feet, if the moon is to be exactly hidden. it is unusual for the moon to approach either of its extreme limits of position, so that the distance from the eye at which the globe must be situated so as to exactly cover the moon is usually more than feet, and less than feet. these fluctuations in the apparent size of our satellite are contained within such narrow limits that in the first glance at the subject they may be overlooked. it will be easily seen that the apparent size of the moon must be connected with its real distance from the earth. suppose, for the sake of illustration, that the moon were to recede into space, its size would seem to dwindle, and long ere it had reached the distance of even the very nearest of the other celestial bodies it would have shrunk into insignificance. on the other hand, if the moon were to come nearer to the earth, its apparent size would gradually increase until, when close to our globe, it would seem like a mighty continent stretching over the sky. we find that the apparent size of the moon is nearly constant, and hence we infer that the average distance of the same body is also nearly constant. the average value of that distance is , miles. in rare circumstances it may approach to a distance but little more than , miles, or recede to a distance hardly less than , miles, but the ordinary fluctuations do not exceed more than about , miles on either side of its mean value. from the moon's incessant changes we perceive that she is in constant motion, and we now further see that whatever these movements may be, the earth and the moon must at present remain at _nearly_ the same distance apart. if we further add that the path pursued by the moon around the heavens lies nearly in a plane, then we are forced to the conclusion that our satellite must be revolving in a nearly circular path around the earth at the centre. it can, indeed, be shown that the constant distance of the two bodies involves as a necessary condition the revolution of the moon around the earth. the attraction between the moon and the earth tends to bring the two bodies together. the only way by which such a catastrophe can be permanently avoided is by making the satellite move as we actually find it to do. the attraction between the earth and the moon still exists, but its effect is not then shown in bringing the moon in towards the earth. the attraction has now to exert its whole power in restraining the moon in its circular path; were the attraction to cease, the moon would start off in a straight line, and recede never to return. [illustration: fig. .--the moon's path around the sun.] the fact of the moon's revolution around the earth is easily demonstrated by observations of the stars. the rising and setting of our satellite is, of course, due to the rotation of the earth, and this apparent diurnal movement the moon possesses in common with the sun and with the stars. it will, however, be noticed that the moon is continually changing its place among the stars. even in the course of a single night the displacement will be conspicuous to a careful observer without the aid of a telescope. the moon completes each revolution around the earth in a period of · days. [illustration: fig. .--the phases of the moon.] in fig. we have a view of the relative positions of the earth, the sun, and the moon, but it is to be observed that, for the convenience of illustration, we have been obliged to represent the orbit of the moon on a much larger scale than it ought to be in comparison with the distance of the sun. that half of the moon which is turned towards the sun is brilliantly illuminated, and, according as we see more or less of that brilliant half, we say that the moon is more or less full, the several "phases" being visible in the succession shown by the numbers in fig. . a beginner sometimes finds considerable difficulty in understanding how the light on the full moon at night can have been derived from the sun. "is not," he will say, "the earth in the way? and must it not intercept the sunlight from every object on the other side of the earth to the sun?" a study of fig. will explain the difficulty. the plane in which the moon revolves does not coincide with the plane in which the earth revolves around the sun. the line in which the plane of the earth's motion is intersected by that of the moon divides the moon's path into two semicircles. we must imagine the moon's path to be tilted a little, so that the upper semicircle is somewhat above the plane of the paper, and the other semicircle below. it thus follows that when the moon is in the position marked full, under the circumstances shown in the figure, the moon will be just above the line joining the earth and the sun; the sunlight will thus pass over the earth to the moon, and the moon will be illuminated. at new moon, the moon will be under the line joining the earth and the sun. as the relative positions of the earth and the sun are changing, it happens twice in each revolution that the sun comes into the position of the line of intersection of the two planes. if this occurs at the time of full moon, the earth lies directly between the moon and the sun; the moon is thus plunged into the shadow of the earth, the light from the sun is intercepted, and we say that the moon is eclipsed. the moon sometimes only partially enters the earth's shadow, in which case the eclipse is a partial one. when, on the other hand, the sun is situated on the line of intersection at the time of new moon, the moon lies directly between the earth and the sun, and the dark body of the moon will then cut off the sunlight from the earth, producing a solar eclipse. usually only a part of the sun is thus obscured, forming the well-known partial eclipse; if, however, the moon pass centrally over the sun, then we must have one or other of two very remarkable kinds of eclipse. sometimes the moon entirely blots out the sun, and thus is produced the sublime spectacle of a total eclipse, which tells us so much as to the nature of the sun, and to which we have already referred in the last chapter. even when the moon is placed centrally over the sun, a thin rim of sunlight is occasionally seen round the margin of the moon. we then have what is known as an annular eclipse. it is remarkable that the moon is sometimes able to hide the sun completely, while on other occasions it fails to do so. it happens that the average apparent size of the moon is nearly equal to the average apparent size of the sun, but, owing to the fluctuations in their distances, the actual apparent sizes of both bodies undergo certain changes. on certain occasions the apparent size of the moon is greater than that of the sun. in this case a central passage produces a total eclipse; but it may also happen that the apparent size of the sun exceeds that of the moon, in which case a central passage can only produce an annular eclipse. [illustration: fig. .--form of the earth's shadow, showing the penumbra, or partially shaded region. within the penumbra, the moon is visible; in the shadow it is nearly invisible.] there are hardly any more interesting celestial phenomena than the different descriptions of eclipses. the almanac will always give timely notice of the occurrence, and the more striking features can be observed without a telescope. in an eclipse of the moon (fig. ) it is interesting to note the moment when the black shadow is first detected, to watch its gradual encroachment over the bright surface of the moon, to follow it, in case the eclipse is total, until there is only a thin crescent of moonlight left, and to watch the final extinction of that crescent when the whole moon is plunged into the shadow. but now a spectacle of great interest and beauty is often manifested; for though the moon is so hidden behind the earth that not a single direct ray of the sunlight could reach its surface, yet we often find that the moon remains visible, and, indeed, actually glows with a copper-coloured hue bright enough to permit several of the markings on the surface to be discerned. this illumination of the moon even in the depth of a total eclipse is due to the sunbeams which have just grazed the edge of the earth. in doing so they have become bent by the refraction of the atmosphere, and have thus been turned inwards into the shadow. such beams have passed through a prodigious thickness of the earth's atmosphere, and in this long journey through hundreds of miles of air they have become tinged with a ruddy or copper-like hue. nor is this property of our atmosphere an unfamiliar one. the sun both at sunrise and at sunset glows with a light which is much more ruddy than the beams it dispenses at noonday. but at sunset or at sunrise the rays which reach our eyes have much more of our atmosphere to penetrate than they have at noon, and accordingly the atmosphere imparts to them that ruddy colour so characteristic and often so lovely. if the spectrum of the sun when close to the horizon is examined it is seen to be filled with numerous dark lines and bands situated chiefly towards the blue and violet end. these are caused by the increased absorption which the light suffers in the atmosphere, and give rise to the preponderating red light on the sun under such conditions. in the case of the eclipsed moon, the sunbeams have to take an atmospheric journey more than double as long as that at sunrise or sunset, and hence the ruddy glow of the eclipsed moon may be accounted for. the almanacs give the full particulars of each eclipse that happens in the corresponding year. these predictions are reliable, because astronomers have been carefully observing the moon for ages, and have learned from these observations not only how the moon moves at present, but also how it will move for ages to come. the actual calculations are so complicated that we cannot here discuss them. there is, however, one leading principle about eclipses which is so simple that we must refer to it. the eclipses occurring this year have no very obvious relation to the eclipses that occurred last year, or to those that will occur next year. yet, when we take a more extended view of the sequence of these phenomena, a very definite principle becomes manifest. if we observe all the eclipses in a period of eighteen years, or nineteen years, then we can predict, with at least an approximation to the truth, all the future eclipses for many years. it is only necessary to recollect that in , - / days after one eclipse a nearly similar eclipse follows. for instance, a beautiful eclipse of the moon occurred on the th of december, . if we count back , days from that date, or, that is, eighteen years and eleven days, we come to november th, , and a similar eclipse of the moon took place then. again, there were four eclipses in the year . if we add , - / days to the date of each eclipse, it will give the dates of all the four eclipses in the year . it was this rule which enabled the ancient astronomers to predict the recurrence of eclipses, at a time when the motions of the moon were not understood nearly so well as they now are. during a long voyage, and perhaps in critical circumstances, the moon will often render invaluable information to the sailor. to navigate a ship, suppose from liverpool to china, the captain must frequently determine the precise position which his ship then occupies. if he could not do this, he would never find his way across the trackless ocean. observations of the sun give him his latitude and tell him his local time, but the captain further requires to know the greenwich time before he can place his finger at a point of the chart and say, "my ship is here." to ascertain the greenwich time the ship carries a chronometer which has been carefully rated before starting, and, as a precaution, two or three chronometers are usually provided to guard against the risk of error. an unknown error of a minute in the chronometer might perhaps lead the vessel fifteen miles from its proper course. [illustration: plate vi. chart of the moon's surface.] [illustration: fig. .--key to chart of the moon (plate vi.).] it is important to have the means of testing the chronometers during the progress of the voyage; and it would be a great convenience if every captain, when he wished, could actually consult some infallible standard of greenwich time. we want, in fact, a greenwich clock which may be visible over the whole globe. there is such a clock; and, like any other clock, it has a face on which certain marks are made, and a hand which travels round that face. the great clock at westminster shrinks into insignificance when compared with the mighty clock which the captain uses for setting his chronometer. the face of this stupendous dial is the face of the heavens. the numbers engraved on the face of a clock are replaced by the twinkling stars; while the hand which moves over the dial is the beautiful moon herself. when the captain desires to test his chronometer, he measures the distance of the moon from a neighbouring star. for example, he may see that the moon is three degrees from the star regulus. in the nautical almanac he finds the greenwich time at which the moon was three degrees from regulus. comparing this with the indications of the chronometer, he finds the required correction. there is one widely-credited myth about the moon which must be regarded as devoid of foundation. the idea that our satellite and the weather bear some relation has no doubt been entertained by high authority, and appears to be an article in the belief of many an excellent mariner. careful comparison between the state of the weather and the phases of the moon has, however, quite discredited the notion that any connection of the kind does really exist. we often notice large blank spaces on maps of africa and of australia which indicate our ignorance of parts of the interior of those great continents. we can find no such blank spaces in the map of the moon. astronomers know the surface of the moon better than geographers know the interior of africa. every spot on the face of the moon which is as large as an english parish has been mapped, and all the more important objects have been named. a general map of the moon is shown in plate vi. it has been based upon drawings made with small telescopes, and it gives an entire view of that side of our satellite which is presented towards us. the moon is shown as it appears in an astronomical telescope, which inverts everything, so that the south is at the top and the north at the bottom (to show objects upright a telescope requires an additional pair of lenses in the eye-piece, and as this diminishes the amount of light reaching the eye they are dispensed with in astronomical telescopes). we can see on the map some of the characteristic features of lunar scenery. those dark regions so conspicuous in the ordinary full moon are easily recognised on the map. they were thought to be seas by astronomers before the days of telescopes, and indeed the name "mare" is still retained, though it is obvious that they contain no water at present. the map also shows certain ridges or elevated portions, and when we apply measurement to these objects we learn that they must be mighty mountain ranges. but the most striking features on the moon are those ring-like objects which are scattered over the surface in profusion. these are known as the lunar craters. to facilitate reference to the chief points of interest we have arranged an index map (fig. ) which will give a clue to the names of the several objects depicted upon the plate. the so-called seas are represented by capital letters; so that a is the mare crisium, and h the oceanus procellarum. the ranges of mountains are indicated by small letters; thus a on the index is the site of the so-called caucasus mountains, and similarly the apennines are denoted by _c_. the numerous craters are distinguished by numbers; for example, the feature on the map corresponding to on the index is the crater designated ptolemy. a. mare crisium. b. mare foecunditatis. c. mare tranquillitatis. d. mare serenitatis. e. mare imbrium. f. sinus iridum. g. mare vaporum. h. oceanus procellarum. i. mare humorum. j. mare nubium. k. mare nectaris. _a._ caucasus. _b._ alps. _c._ apennines. _d._ carpathians. _f._ cordilleras & d'alembert mountains. _g._ rook mountains. _h._ doerfel mountains. _i._ leibnitz mountains. . posidonius. . linné. . aristotle. . great valley of the alps. . aristillus. . autolycus. . archimedes. . plato. . eratosthenes. . copernicus. . kepler. . aristarchus. . grimaldi. . gassendi. . schickard. . wargentin. . clavius. . tycho. . alphonsus. . ptolemy. . catharina. . cyrillus. . theophilus. . petavius. . hyginus. . triesnecker. in every geographical atlas there is a map showing the two hemispheres of the earth, the eastern and the western. in the case of the moon we can only give a map of one hemisphere, for the simple reason that the moon always turns the same side towards us, and accordingly we never get a view of the other side. this is caused by the interesting circumstance that the moon takes exactly the same time to turn once round its own axis as it takes to go once round the earth. the rotation is, however, performed with uniform speed, while the moon does not move in its orbit with a perfectly uniform velocity (_see_ chapter iv.). the consequence is that we now get a slight glimpse round the east limb, and now a similar glimpse round the west limb, as if the moon were shaking its head very gently at us. but it is only an insignificant margin of the far side of the moon which this _libration_ permits us to examine. lunar objects are well suited for observation when the sunlight falls upon them in such a manner as to exhibit strongly contrasted lights and shadows. it is impossible to observe the moon satisfactorily when it is full, for then no conspicuous shadows are cast. the most opportune moment for seeing any particular lunar object is when it lies just at the illuminated side of the boundary between light and shade, for then the features are brought out with exquisite distinctness. plate vii.[ ] gives an illustration of lunar scenery, the object represented being known to astronomers by the name of triesnecker. the district included is only a very small fraction of the entire surface of the moon, yet the actual area is very considerable, embracing as it does many hundreds of square miles. we see in it various ranges of lunar mountains, while the central object in the picture is one of those remarkable lunar craters which we meet with so frequently in every lunar landscape. this crater is about twenty miles in diameter, and it has a lofty mountain in the centre, the peak of which is just illuminated by the rising sun in that phase of our satellite which is represented in the picture. a typical view of a lunar crater is shown in plate viii. this is, no doubt, a somewhat imaginary sketch. the point of view from which the artist is supposed to have taken the picture is one quite unattainable by terrestrial astronomers, yet there can be little doubt that it is a fair representation of objects on the moon. we should, however, recollect the scale on which it is drawn. the vast crater must be many miles across, and the mountain at its centre must be thousands of feet high. the telescope will, even at its best, only show the moon as well as we could see it with the unaided eye if it were miles away instead of being , . we must not, therefore, expect to see any details on the moon even with the finest telescopes, unless they were coarse enough to be visible at a distance of miles. england from such a point of view would only show london as a coloured spot, in contrast with the general surface of the country. we return, however, from a somewhat fancy sketch to a more prosaic examination of what the telescope does actually reveal. plate ix. represents the large crater plato, so well known to everyone who uses a telescope. the floor of this remarkable object is nearly flat, and the central mountain, so often seen in other craters, is entirely wanting. we describe it more fully in the general list of lunar objects. the mountain peaks on the moon throw long, well-defined shadows, characterised by a sharpness which we do not find in the shadows of terrestrial objects. the difference between the two cases arises from the absence of air from the moon. our atmosphere diffuses a certain amount of light, which mitigates the blackness of terrestrial shadows and tends to soften their outline. no such influences are at work on the moon, and the sharpness of the shadows is taken advantage of in our attempts to measure the heights of the lunar mountains. it is often easy to compute the altitude of a church steeple, a lofty chimney, or any similar object, from the length of its shadow. the simplest and the most accurate process is to measure at noon the number of feet from the base of the object to the end of the shadow. the elevation of the sun at noon on the day in question can be obtained from the almanac, and then the height of the object follows by a simple calculation. indeed, if the observations can be made either on the th of april or the th of september, at or near the latitude of london, then calculations would be unnecessary. the noonday length of the shadow on either of the dates named is equal to the altitude of the object. in summer the length of the noontide shadow is less than the altitude; in winter the length of the shadow exceeds the altitude. at sunrise or sunset the shadows are, of course, much longer than at noon, and it is shadows of this kind that we observe on the moon. the necessary measurements are made by that indispensable adjunct to the equatorial telescope known as the _micrometer_. this word denotes an instrument for measuring _small_ distances. in one sense the term is not a happy one. the objects to which the astronomer applies the micrometer are usually anything but small. they are generally of the most transcendent dimensions, far exceeding the moon or the sun, or even our whole system. still, the name is not altogether inappropriate, for, vast though the objects may be, they generally seem minute, even in the telescope, on account of their great distance. we require for such measurements an instrument capable of the greatest nicety. here, again, we invoke the aid of the spider, to whose assistance in another department we have already referred. in the filar micrometer two spider lines are parallel, and one intersects them at right angles. one or both of the parallel lines can be moved by means of screws, the threads of which have been shaped by consummate workmanship. the distance through which the line has been moved is accurately indicated by noting the number of revolutions and parts of a revolution of the screw. suppose the two lines be first brought into coincidence, and then separated until the apparent length of the shadow of the mountain on the moon is equal to the distance between the lines: we then know the number of revolutions of the micrometer screw which is equivalent to the length of the shadow. the number of miles on the moon which correspond to one revolution of the screw has been previously ascertained by other observations, and hence the length of the shadow can be determined. the elevation of the sun, as it would have appeared to an observer at this point of the moon, at the moment when the measures were being made, is also obtainable, and hence the actual elevation of the mountain can be calculated. by measurements of this kind the altitudes of other lunar objects, such, for example, as the height of the rampart surrounding a circular-walled plane, can be determined. the beauty and interest of the moon as a telescopic object induces us to give to the student a somewhat detailed account of the more remarkable features which it presents. most of the objects we are to describe can be effectively exhibited with very moderate telescopic power. it is, however, to be remembered that all of them cannot be well seen at one time. the region most distinctly shown is the boundary between light and darkness. the student will, therefore, select for observation such objects as may happen to lie near that boundary at the time when he is observing. . _posidonius._--the diameter of this large crater is nearly miles. although its surrounding wall is comparatively slender, it is so distinctly marked as to make the object very conspicuous. as so frequently happens in lunar volcanoes, the bottom of the crater is below the level of the surrounding plain, in the present instance to the extent of nearly , feet. . _linné._--this small crater lies in the mare serenitatis. about sixty years ago it was described as being about - / miles in diameter, and seems to have been sufficiently conspicuous. in schmidt, of athens, announced that the crater had disappeared. since then an exceedingly small shallow depression has been visible, but the whole object is now very inconsiderable. this seems to be the most clearly attested case of change in a lunar object. apparently the walls of the crater have tumbled into the interior and partly filled it up, but many astronomers doubt that a change has really taken place, as schröter, a hanoverian observer at the end of the eighteenth century, appears not to have seen any conspicuous crater in the place, though it must be admitted that his observations are rather incomplete. to give some idea of schmidt's amazing industry in lunar researches, it may be mentioned that in six years he made nearly , individual settings of his micrometer in the measurement of lunar altitudes. his great chart of the mountains in the moon is based on no less than , drawings and sketches, if those are counted twice that may have been used for two divisions of the map. . _aristotle._--this great philosopher's name has been attached to a grand crater miles in diameter, the interior of which, although very hilly, shows no decidedly marked central cone. but the lofty wall of the crater, exceeding , feet in height, overshadows the floor so continuously that its features are never seen to advantage. . _the great valley of the alps._--a wonderfully straight valley, with a width ranging from - / to miles, runs right through the lunar alps. it is, according to mädler, at least , feet deep, and over miles in length. a few low ridges which are parallel to the sides of the valley may possibly be the result of landslips. . _aristillus._--under favourable conditions lord rosse's great telescope has shown the exterior of this magnificent crater to be scored with deep gullies radiating from its centre. aristillus is about miles wide and , feet in depth. . _autolycus_ is somewhat smaller than the foregoing, to which it forms a companion in accordance with what mädler thought a well-defined relation amongst lunar craters, by which they frequently occurred in pairs, with the smaller one more usually to the south. towards the edge this arrangement is generally rather apparent than real, and is merely a result of foreshortening. . _archimedes._--this large plain, about miles in diameter, has its vast smooth interior divided by unequally bright streaks into seven distinct zones, running east and west. there is no central mountain or other obvious internal sign of former activity, but its irregular wall rises into abrupt towers, and is marked outside by decided terraces. [illustration: plate b. portion of the moon. (alps, archimedes, apennines.) _messrs. loewy & puiseux_.] . _plato._--we have already referred to this extensive circular plain, which is noticeable with the smallest telescope. the average height of the rampart is about , feet on the eastern side; the western side is somewhat lower, but there is one peak rising to the height of nearly , feet. the plain girdled by this vast rampart is of ample proportions. it is a somewhat irregular circle, about miles in diameter, and containing an area of , square miles. on its floor the shadows of the western wall are shown in plate ix., as are also three of the small craters, of which a large number have been detected by persevering observers. the narrow sharp line leading from the crater to the left is one of those remarkable "clefts" which traverse the moon in so many directions. another may be seen further to the left. above plato are several detached mountains, the loftiest of which is pico, about , feet in height. its long and pointed shadow would at first sight lead one to suppose that it must be very steep; but schmidt, who specially studied the inclinations of the lunar slopes, is of opinion that it cannot be nearly so steep as many of the swiss mountains that are frequently ascended. as many as thirty minute craters have been carefully observed on the floor of plato, and variations have been thought by mr. w.h. pickering to be perceptible. . _eratosthenes._--this profound crater, upwards of miles in diameter, lies at the end of the gigantic range of the apennines. not improbably, eratosthenes once formed the volcanic vent for the stupendous forces that elevated the comparatively craterless peaks of these great mountains. . _copernicus._--of all the lunar craters this is one of the grandest and best known. the region to the west is dotted over with innumerable minute craterlets. it has a central many-peaked mountain about , feet in height. there is good reason to believe that the terracing shown in its interior is mainly due to the repeated alternate rise, partial congelation, and subsequent retreat of a vast sea of lava. at full moon the crater of copernicus is seen to be surrounded by radiating streaks. . _kepler._--although the internal depth of this crater is scarcely less than , feet, it has but a very low surrounding wall, which is remarkable for being covered with the same glistening substance that also forms a system of bright rays not unlike those surrounding the last object. . _aristarchus_ is the most brilliant of the lunar craters, being specially vivid with a low power in a large telescope. so bright is it, indeed, that it has often been seen on the dark side just after new moon, and has thus given rise to marvellous stories of active lunar volcanoes. to the south-east lies another smaller crater, herodotus, north of which is a narrow, deep valley, nowhere more than - / miles broad, which makes a remarkable zigzag. it is one of the largest of the lunar "clefts." . _grimaldi_ calls for notice as the darkest object of its size in the moon. under very exceptional circumstances it has been seen with the naked eye, and as its area has been estimated at nearly , square miles, it gives an idea of how little unaided vision can discern in the moon; it must, however, be added that we always see grimaldi considerably foreshortened. . the great crater _gassendi_ has been very frequently mapped on account of its elaborate system of "clefts." at its northern end it communicates with a smaller but much deeper crater, that is often filled with black shadow after the whole floor of gassendi has been illuminated. . _schickard_ is one of the largest walled plains on the moon, about miles in breadth. within its vast expanse mädler detected minor craters. with regard to this object chacornac pointed out that, owing to the curvature of the surface of the moon, a spectator at the centre of the floor "would think himself in a boundless desert," because the surrounding wall, although in one place nearly , feet high, would lie entirely beneath his horizon. . close to the foregoing is _wargentin_. there can be little doubt that this is really a huge crater almost filled with congealed lava, as there is scarcely any fall towards the interior. . _clavius._--near the th parallel of lunar south latitude lies this enormous enclosure, the area of which is not less than , square miles. both in its interior and on its walls are many peaks and secondary craters. the telescopic view of a sunrise upon the surface of clavius is truly said by mädler to be indescribably magnificent. one of the peaks rises to a height of , feet above the bottom of one of the included craters. mädler even expressed the opinion that in this wild neighbourhood there are craters so profound that no ray of sunlight ever penetrated their lowest depths, while, as if in compensation, there are peaks whose summits enjoy a mean day almost twice as long as their night. . if the full moon be viewed through an opera-glass or any small hand-telescope, one crater is immediately seen to be conspicuous beyond all others, by reason of the brilliant rays or streaks that radiate from it. this is the majestic _tycho_, , feet in depth and miles in diameter (plate x.). a peak , feet in height rises in the centre of its floor, while a series of terraces diversity its interior slopes; but it is the mysterious bright rays that chiefly surprise us. when the sun rises on tycho, these streaks are utterly invisible; indeed, the whole object is then so obscure that it requires a practised eye to recognise tycho amidst its mountainous surroundings. but as soon as the sun has attained a height of about ° above its horizon, the rays emerge from their obscurity and gradually increase in brightness until the moon becomes full, when they are the most conspicuous objects on her surface. they vary in length, from a few hundred miles to two or, in one instance, nearly three thousand miles. they extend indifferently across vast plains, into the deepest craters, or over the loftiest elevations. we know of nothing on our earth to which they can be compared. as these rays are only seen about the time of full moon, their visibility obviously depends on the light falling more or less closely in the line of sight, quite regardless of the inclination of the surfaces, mountains or valleys, on which they appear. each small portion of the surface of the streak must therefore be of a form which is symmetrical to the spectator from whatever point it is seen. the sphere alone appears to fulfil this condition, and professor copeland therefore suggests that the material constituting the surface of the streak must be made up of a large number of more or less completely spherical globules. the streaks must represent parts of the lunar surface either pitted with minute cavities of spherical figure, or strewn over with minute transparent spheres.[ ] near the centre of the moon's disc is a fine range of ring plains fully open to our view under all illuminations. of these, two may be mentioned--_alphonsus_ ( ), the floor of which is strangely characterised by two bright and several dark markings which cannot be explained by irregularities in the surface.--_ptolemy_ ( ). besides several small enclosed craters, its floor is crossed by numerous low ridges, visible when the sun is rising or setting. , , .--when the moon is five or six days old this beautiful group of three craters will be favourably placed for observation. they are named _catharina_, _cyrillus_, and _theophilus_. catharina, the most southerly of the group, is more than , feet deep, and connected with cyrillus by a wide valley; but between cyrillus and theophilus there is no such connection. indeed, cyrillus looks as if its huge surrounding ramparts, as high as mont blanc, had been completely finished before the volcanic forces commenced the formation of theophilus, the rampart of which encroaches considerably on its older neighbour. theophilus stands as a well-defined circular crater about miles in diameter, with an internal depth of , to , feet, and a beautiful central group of mountains, one-third of that height, on its floor. although theophilus is the deepest crater we can see in the moon, it has suffered little or no deformation from secondary eruptions, while the floor and wall of catharina show complete sequences of lesser craters of various sizes that have broken in upon and partly destroyed each other. in the spring of the year, when the moon is somewhat before the first quarter, this instructive group of extinct volcanoes can be seen to great advantage at a convenient hour in the evening. [illustration: plate vii. triesnecker. (after nasmyth.)] . _petavius_ is remarkable not only for its great size, but also for the rare feature of having a double rampart. it is a beautiful object soon after new moon, or just after full moon, but disappears absolutely when the sun is more than ° above its horizon. the crater floor is remarkably convex, culminating in a central group of hills intersected by a deep cleft. . _hyginus_ is a small crater near the centre of the moon's disc. one of the largest of the lunar chasms passes right through it, making an abrupt turn as it does so. . _triesnecker._--this fine crater has been already described, but is again alluded to in order to draw attention to the elaborate system of chasms so conspicuously shown in plate vii. that these chasms are depressions is abundantly evident by the shadows inside. very often their margins are appreciably raised. they seem to be fractures in the moon's surface. of the various mountains that are occasionally seen as projections on the actual edge of the moon, those called after leibnitz (_i_) seem to be the highest. schmidt found the highest peak to be upwards of , feet above a neighbouring valley. in comparing these altitudes with those of mountains on our earth, we must for the latter add the depth of the sea to the height of the land. reckoned in this way, our highest mountains are still higher than any we know of in the moon. we must now discuss the important question as to the origin of these remarkable features on the surface of the moon. we shall admit at the outset that our evidence on this subject is only indirect. to establish by unimpeachable evidence the volcanic origin of the remarkable lunar craters, it would seem almost necessary that volcanic outbursts should have been witnessed on the moon, and that such outbursts should have been seen to result in the formation of the well-known ring, with or without the mountain rising from the centre. to say that nothing of the kind has ever been witnessed would be rather too emphatic a statement. on certain occasions careful observers have reported the occurrence of minute local changes on the moon. as we have already remarked, a crater named linné, of dimensions respectable, no doubt, to a lunar inhabitant, but forming a very inconsiderable telescopic object, was thought to have undergone some change. on another occasion a minute crater was thought to have arisen near the well-known object named hyginus. the mere enumeration of such instances gives real emphasis to the statement that there is at the present time no appreciable source of disturbance of the moon's surface. even were these trifling cases of suspected change really established--and this is perhaps rather farther than many astronomers would be willing to go--they are still insignificant when compared with the mighty phenomena that gave rise to the host of great craters which cover so large a portion of the moon's surface. we are led inevitably to the conclusion that our satellite must have once possessed much greater activity than it now displays. we can also give a reasonable, or, at all events, a plausible, explanation of the cessation of that activity in recent times. let us glance at two other bodies of our system, the earth and the sun, and compare them with the moon. of the three bodies, the sun is enormously the largest, while the moon is much less than the earth. we have also seen that though the sun must have a very high temperature, there can be no doubt that it is gradually parting with its heat. the surface of the earth, formed as it is of solid rocks and clay, or covered in great part by the vast expanse of ocean, bears but few obvious traces of a high temperature. nevertheless, it is highly probable from ordinary volcanic phenomena that the interior of the earth still possesses a temperature of incandescence. a large body when heated takes a longer time to cool than does a small body raised to the same temperature. a large iron casting will take days to cool; a small casting will become cold in a few hours. whatever may have been the original source of heat in our system--a question which we are not now discussing--it seems demonstrable that the different bodies were all originally heated, and have now for ages been gradually cooling. the sun is so vast that he has not yet had time to cool; the earth, of intermediate bulk, has become cold on the outside, while still retaining vast stores of internal heat; while the moon, the smallest body of all, has lost its heat to such an extent that changes of importance on its surface can no longer be originated by internal fires. we are thus led to refer the origin of the lunar craters to some ancient epoch in the moon's history. we have no moans of knowing the remoteness of that epoch, but it is reasonable to surmise that the antiquity of the lunar volcanoes must be extremely great. at the time when the moon was sufficiently heated to originate those convulsions, of which the mighty craters are the survivals, the earth must also have been much hotter than it is at present. when the moon possessed sufficient heat for its volcanoes to be active, the earth was probably so hot that life was impossible on its surface. this supposition would point to an antiquity for the lunar craters far too great to be estimated by the centuries and the thousands of years which are adequate for such periods as those with which the history of human events is concerned. it seems not unlikely that millions of years may have elapsed since the mighty craters of plato or of copernicus consolidated into their present form. we shall now attempt to account for the formation of the lunar craters. the most probable views on the subject seem to be those which have been set forth by mr. nasmyth, though it must be admitted that his doctrines are by no means free from difficulty. according to his theory we can explain how the rampart around the lunar crater has been formed, and how the great mountain arose which so often adorns the centre of the plain. the view in fig. contains an imaginary sketch of a volcanic vent on the moon in the days when the craters were active. the eruption is here shown in the fulness of its energy, when the internal forces are hurling forth ashes or stones which fall at a considerable distance from the vent. the materials thus accumulated constitute the rampart surrounding the crater. the second picture (fig. ) depicts the crater in a later stage of its history. the prodigious explosive power has now been exhausted, and has perhaps been intermitted for some time. again, the volcano bursts into activity, but this time with only a small part of its original energy. a comparatively feeble eruption now issues from the same vent, deposits materials close around the orifice, and raises a mountain in the centre. finally, when the activity has subsided, and the volcano is silent and still, we find the evidence of the early energy testified to by the rampart which surrounds the ancient crater, and by the mountain which adorns the interior. the flat floor which is found in some of the craters may not improbably have arisen from an outflow of lava which has afterwards consolidated. subsequent outbreaks have also occurred in many cases. one of the principal difficulties attending this method of accounting for the structure of a crater arises from the great size which some of these objects attain. there are ancient volcanoes on the moon forty or fifty miles in diameter; indeed, there is one well-formed ring, with a mountain rising in the centre, the diameter of which is no less than seventy-eight miles (petavius). it seems difficult to conceive how a blowing cone at the centre could convey the materials to such a distance as the thirty-nine miles between the centre of petavius and the rampart. the explanation is, however, facilitated when it is borne in mind that the force of gravitation is much less on the moon than on the earth. [illustration: plate viii. a normal lunar crater.] [illustration: fig. .--volcano in activity.] [illustration: fig. .--subsequent feeble activity.] have we not already seen that our satellite is so much smaller than the earth that eighty moons rolled into one would not weigh as much as the earth? on the earth an ounce weighs an ounce and a pound weighs a pound; but a weight of six ounces here would only weigh one ounce on the moon, and a weight of six pounds here would only weigh one pound on the moon. a labourer who can carry one sack of corn on the earth could, with the same exertion, carry six sacks of corn on the moon. a cricketer who can throw a ball yards on the earth could with precisely the same exertion throw the same ball yards on the moon. hiawatha could shoot ten arrows into the air one after the other before the first reached the ground; on the moon he might have emptied his whole quiver. the volcano, which on the moon drove projectiles to the distance of thirty-nine miles, need only possess the same explosive power as would have been sufficient to drive the missiles six or seven miles on the earth. a modern cannon properly elevated would easily achieve this feat. [illustration: fig. .--formation of the level floor by lava.] it must also be borne in mind that there are innumerable craters on the moon of the same general type but of the most varied dimensions; from a tiny telescopic object two or three miles in diameter, we can point out gradually ascending stages until we reach the mighty petavius just considered. with regard to the smaller craters, there is obviously little or no difficulty in attributing to them a volcanic origin, and as the continuity from the smallest to the largest craters is unbroken, it seems quite reasonable to suppose that even the greatest has arisen in the same way. it should, however, be remarked that some lunar features might be explained by actions from without rather than from within. mr. g.k. gilbert has marshalled the evidence in support of the belief that lunar sculptures arise from the impact of bodies falling on the moon. the mare imbrium, according to this view, has been the seat of a collision to which the surrounding lunar scenery is due. mr. gilbert explains the furrows as hewn out by mighty projectiles moving with such velocities as meteors possess. the lunar landscapes are excessively weird and rugged. they always remind us of sterile deserts, and we cannot fail to notice the absence of grassy plains or green forests such as we are familiar with on our globe. in some respects the moon is not very differently circumstanced from the earth. like it, the moon has the pleasing alternations of day and night, though the day in the moon is as long as twenty-nine of our days, and the night of the moon is as long as twenty-nine of our nights. we are warmed by the rays of the sun; so, too, is the moon; but, whatever may be the temperature during the long day on the moon, it seems certain that the cold of the lunar night would transcend that known in the bleakest regions of our earth. the amount of heat radiated to us by the moon has been investigated by lord rosse, and more recently by professor langley. though every point on the moon's surface is exposed to the sunlight for a fortnight without any interruption, the actual temperature to which the soil is raised cannot be a high one. the moon does not, like the earth, possess a warm blanket, in the shape of an atmosphere, which can keep in and accumulate the heat received. even our largest telescopes can tell nothing directly as to whether life can exist on the moon. the mammoth trees of california might be growing on the lunar mountains, and elephants might be walking about on the plains, but our telescopes could not show them. the smallest object that we can see on the moon must be about as large as a good-sized cathedral, so that organised beings resembling in size any that we are familiar with, if they existed, could not make themselves visible as telescopic objects. we are therefore compelled to resort to indirect evidence as to whether life would be possible on the moon. we may say at once that astronomers believe that life, as we know it, could not exist. among the necessary conditions of life, water is one of the first. take every form of vegetable life, from the lichen which grows on the rock to the giant tree of the forest, and we find the substance of every plant contains water, and could not exist without it. nor is water less necessary to the existence of animal life. deprived of this element, all organic life, the life of man himself, would be inconceivable. unless, therefore, water be present in the moon, we shall be bound to conclude that life, as we know it, is impossible. if anyone stationed on the moon were to look at the earth through a telescope, would he be able to see any water here? most undoubtedly he would. he would see the clouds and he would notice their incessant changes, and the clouds alone would be almost conclusive evidence of the existence of water. an astronomer on the moon would also see our oceans as coloured surfaces, remarkably contrasted with the land, and he would perhaps frequently see an image of the sun, like a brilliant star, reflected from some smooth portion of the sea. in fact, considering that much more than half of our globe is covered with oceans, and that most of the remainder is liable to be obscured by clouds, the lunar astronomer in looking at our earth would often see hardly anything but water in one form or other. very likely he would come to the conclusion that our globe was only fitted to be a residence for amphibious animals. but when we look at the moon with our telescopes we see no direct evidence of water. close inspection shows that the so-called lunar seas are deserts, often marked with small craters and rocks. the telescope reveals no seas and no oceans, no lakes and no rivers. nor is the grandeur of the moon's scenery ever impaired by clouds over her surface. whenever the moon is above our horizon, and terrestrial clouds are out of the way, we can see the features of our satellite's surface with distinctness. there are no clouds in the moon; there are not even the mists or the vapours which invariably arise wherever water is present, and therefore astronomers have been led to the conclusion that the surface of the globe which attends the earth is a sterile and a waterless desert. another essential element of organic life is also absent from the moon. our globe is surrounded with a deep clothing of air resting on the surface, and extending above our heads to the height of about or miles. we need hardly say how necessary air is to life, and therefore we turn with interest to the question as to whether the moon can be surrounded with an atmosphere. let us clearly understand the problem we are about to consider. imagine that a traveller started from the earth on a journey to the moon; as he proceeded, the air would gradually become more and more rarefied, until at length, when he was a few hundred miles above the earth's surface, he would have left the last perceptible traces of the earth's envelope behind him. by the time he had passed completely through the atmosphere he would have advanced only a very small fraction of the whole journey of , miles, and there would still remain a vast void to be traversed before the moon would be reached. if the moon were enveloped in the same way as the earth, then, as the traveller approached the end of his journey, and came within a few hundred miles of the moon's surface, he would meet again with traces of an atmosphere, which would gradually increase in density until he arrived at the moon's surface. the traveller would thus have passed through one stratum of air at the beginning of his journey, and through another at the end, while the main portion of the voyage would have been through space more void than that to be found in the exhausted receiver of an air-pump. such would be the case if the moon were coated with an atmosphere like that surrounding our earth. but what are the facts? the traveller as he drew near the moon would seek in vain for air to breathe at all resembling ours. it is possible that close to the surface there are faint traces of some gaseous material surrounding the moon, but it can only be equal to a very small fractional part of the ample clothing which the earth now enjoys. for all purposes of respiration, as we understand the term, we may say that there is no air on the moon, and an inhabitant of our earth transferred thereto would be as certainly suffocated as he would be in the middle of space. it may, however, be asked how we learn this. is not air transparent, and how, therefore, could our telescopes be expected to show whether the moon really possessed such an envelope? it is by indirect, but thoroughly reliable, methods of observation that we learn the destitute condition of our satellite. there are various arguments to be adduced; but the most conclusive is that obtained on the occurrence of what is called an "occultation." it sometimes happens that the moon comes directly between the earth and a star, and the temporary extinction of the latter is an "occultation." we can observe the moment when the phenomenon takes place, and the suddenness of the disappearance of the star is generally remarked. if the moon were enveloped in a copious atmosphere, the interposition of this gaseous mass by the movement of the moon would produce a gradual evanescence of the star wholly wanting the abruptness which marks the obscuration.[ ] let us consider how we can account for the absence of an atmosphere from the moon. what we call a gas has been found by modern research to be a collection of an immense number of molecules, each of which is in exceedingly rapid motion. this motion is only pursued for a short distance in one direction before a molecule comes into collision with some other molecule, whereby the directions and velocities of the individual molecules are continually changed. there is a certain average speed for each gas which is peculiar to the molecules of that gas at a certain temperature. when several gases are mixed, as oxygen and nitrogen are in our atmosphere, the molecules of each gas continue to move with their own characteristic velocities. so far as we can estimate the temperature at the boundary of the earth's atmosphere, we may assume that the average of the velocities of the oxygen molecules there found is about a quarter of a mile per second. the velocities for nitrogen are much the same, while the average speed of a molecule of hydrogen is about one mile per second, being, in fact, by far the greatest molecular velocity possessed by any gas. [illustration: plate ix. plato. (after nasmyth.)] a stone thrown into the air soon regains the earth. a rifle bullet fired vertically upwards will ascend higher and higher, until at length its motion ceases, it begins to return, and falls to the ground. let us for the moment suppose that we had a rifle of infinite strength and gunpowder of unlimited power. as we increase the charge we find that the bullet will ascend higher and higher, and each time it will take a longer period before it returns to the ground. the descent of the bullet is due to the attraction of the earth. gravitation must necessarily act on the projectile throughout its career, and it gradually lessens the velocity, overcomes the upward motion, and brings the bullet back. it must be remembered that the efficiency of the attraction decreases when the height is increased. consequently when the body has a prodigiously great initial velocity, in consequence of which it ascends to an enormous height, its return is retarded by a twofold cause. in the first place, the distance through which it has to be recalled is greatly increased, and in the second place the efficiency of gravitation in effecting its recall has decreased. the greater the velocity, the feebler must be the capacity of gravitation for bringing back the body. we can conceive the speed to be increased to that point at which the gravitation, constantly declining as the body ascends, is never quite able to neutralise the velocity, and hence we have the remarkable case of a body projected away never to return. it is possible to exhibit this reasoning in a numerical form, and to show that a velocity of six or seven miles a second directed upwards would suffice to convey a body entirely away from the gravitation of the earth. this speed is far beyond the utmost limits of our artillery. it is, indeed, at least a dozen times as swift as a cannon shot; and even if we could produce it, the resistance of the air would present an insuperable difficulty. such reflections, however, do not affect the conclusion that there is for each planet a certain specific velocity appropriate to that body, and depending solely upon its size and mass, with which we should have to discharge a projectile, in order to prevent the attraction of that body from pulling the projectile back again. it is a simple matter of calculation to determine this "critical velocity" for any celestial body. the greater the body the greater in general must be the initial speed which will enable the projectile to forsake for ever the globe from which it has been discharged. as we have already indicated, this speed is about seven miles per second on the earth. it would be three on the planet mercury, three and a half on mars, twenty-two on saturn, and thirty-seven on jupiter; while for a missile to depart from the sun without prospect of return, it must leave the brilliant surface at a speed not less than miles per second. supposing that a quantity of free hydrogen was present in our atmosphere, its molecules would move with an average velocity of about one mile per second. it would occasionally happen by a combination of circumstances that a molecule would attain a speed which exceeded seven miles a second. if this happened on the confines of the atmosphere where it escaped collision with other molecules, the latter object would fly off into space, and would not be recaptured by the earth. by incessant repetitions of this process, in the course of countless ages, all the molecules of hydrogen gas would escape from the earth, and in this manner we may explain the fact that there is no free hydrogen present in the earth's atmosphere.[ ] the velocities which can be attained by the molecules of gases other than hydrogen are far too small to permit of their escape from the attraction of the earth. we therefore find oxygen, nitrogen, water vapour, and carbon dioxide remaining as permanent components of our air. on the other hand, the enormous mass of the sun makes the "critical velocity" at the surface of that body to be so great ( miles per second) that not even the molecules of hydrogen can possibly emulate it. consequently, as we have seen, hydrogen is a most important component of the sun's atmospheric envelope. if we now apply this reasoning to the moon, the critical velocity is found by calculation to be only a mile and a half per second. this seems to be well within the maximum velocities attainable by the molecules of oxygen, nitrogen, and other gases. it therefore follows that none of these gases could remain permanently to form an atmosphere at the surface of so small a body as the moon. this seems to be the reason why there are no present traces of any distinct gaseous surroundings to our satellite. the absence of air and of water from the moon explains the sublime ruggedness of the lunar scenery. we know that on the earth the action of wind and of rain, of frost and of snow, is constantly tending to wear down our mountains and reduce their asperities. no such agents are at work on the moon. volcanoes sculptured the surface into its present condition, and, though they have ceased to operate for ages, the traces of their handiwork seem nearly as fresh to-day as they were when the mighty fires were extinguished. "the cloud-capped towers, the gorgeous palaces, the solemn temples" have but a brief career on earth. it is chiefly the incessant action of water and of air that makes them vanish like the "baseless fabric of a vision." on the moon these causes of disintegration and of decay are all absent, though perhaps the changes of temperature in the transition from lunar day to lunar night would be attended with expansions and contractions that might compensate in some slight degree for the absence of more potent agents of dissolution. it seems probable that a building on the moon would remain for century after century just as it was left by the builders. there need be no glass in the windows, for there is no wind and no rain to keep out. there need not be fireplaces in the rooms, for fuel cannot burn without air. dwellers in a lunar city would find that no dust could rise, no odours be perceived, no sounds be heard. man is a creature adapted for life under circumstances which are very narrowly limited. a few degrees of temperature more or less, a slight variation in the composition of air, the precise suitability of food, make all the difference between health and sickness, between life and death. looking beyond the moon, into the length and breadth of the universe, we find countless celestial globes with every conceivable variety of temperature and of constitution. amid this vast number of worlds with which space is tenanted, are there any inhabited by living beings? to this great question science can make no response: we cannot tell. yet it is impossible to resist a conjecture. we find our earth teeming with life in every part. we find life under the most varied conditions that can be conceived. it is met with under the burning heat of the tropics and in the everlasting frost at the poles. we find life in caves where not a ray of light ever penetrates. nor is it wanting in the depths of the ocean, at the pressure of tons on the square inch. whatever may be the external circumstances, nature generally provides some form of life to which those circumstances are congenial. it is not at all probable that among the million spheres of the universe there is a single one exactly like our earth--like it in the possession of air and of water, like it in size and in composition. it does not seem probable that a man could live for one hour on any body in the universe except the earth, or that an oak-tree could live in any other sphere for a single season. men can dwell on the earth, and oak-trees can thrive therein, because the constitutions of the man and of the oak are specially adapted to the particular circumstances of the earth. could we obtain a closer view of some of the celestial bodies, we should probably find that they, too, teem with life, but with life specially adapted to the environment--life in forms strange and weird; life far stranger to us than columbus found it to be in the new world when he first landed there. life, it may be, stranger than ever dante described or doré sketched. intelligence may also have a home among those spheres no less than on the earth. there are globes greater and globes less--atmospheres greater and atmospheres less. the truest philosophy on this subject is crystallised in the language of tennyson:-- "this truth within thy mind rehearse, that in a boundless universe is boundless better, boundless worse. "think you this mould of hopes and fears could find no statelier than his peers in yonder hundred million spheres?" [illustration: plate x. tycho and its surroundings. (after nasmyth.)] chapter iv. the solar system. exceptional importance of the sun and moon--the course to be pursued--the order of distance--the neighbouring orbs--how are they to be discriminated?--the planets venus and jupiter attract notice by their brilliancy--sirius not a neighbour--the planets saturn and mercury--telescopic planets--the criterion as to whether a body is to be ranked as a neighbour--meaning of the word _planet_--uranus and neptune--comets--the planets are illuminated by the sun--the stars are not--the earth is really a planet--the four inner planets, mercury, venus, the earth, and mars--velocity of the earth--the outer planets, jupiter, saturn, uranus, neptune--light and heat received by the planets from the sun--comparative sizes of the planets--the minor planets--the planets all revolve in the same direction--the solar system--an island group in space. in the two preceding chapters of this work we have endeavoured to describe the heavenly bodies in the order of their relative importance to mankind. could we doubt for a moment as to which of the many orbs in the universe should be the first to receive our attention? we do not now allude to the intrinsic significance of the sun when compared with other bodies or groups of bodies scattered through space. it may be that numerous globes rival the sun in real splendour, in bulk, and in mass. we shall, in fact, show later on in this volume that this is the case; and we shall then be in a position to indicate the true rank of the sun amid the countless hosts of heaven. but whatever may be the importance of the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation in asserting how immeasurably his influence on the earth surpasses that of all other bodies in the universe together. it was therefore natural--indeed inevitable--that our first examination of the orbs of heaven should be directed to that mighty body which is the source of our life itself. nor could there be much hesitation as to the second step which ought to be taken. the intrinsic importance of the moon, when compared with other celestial bodies, may be small; it is, indeed, as we shall afterwards see, almost infinitesimal. but in the economy of our earth the moon has played, and still plays, a part second only in importance to that of the sun himself. the moon is so close to us that her brilliant rays pale to invisibility countless orbs of a size and an intrinsic splendour incomparably greater than her own. the moon also occupies an exceptional position in the history of astronomy; for the law of gravitation, the greatest discovery that science has yet witnessed, was chiefly accomplished by observations of the moon. it was therefore natural that an early chapter in our story of the heavens should be devoted to a body the interest of which approximated so closely to that of the sun himself. but the sun and the moon having been partly described (we shall afterwards have to refer to them again), some hesitation is natural in the choice of the next step. the two great luminaries being abstracted from our view, there remains no other celestial body of such exceptional interest and significance as to make it quite clear what course to pursue; we desire to unfold the story of the heavens in the most natural manner. if we made the attempt to describe the celestial bodies in the order of their actual magnitude, our ignorance must at once pronounce the task to be impossible. we cannot even make a conjecture as to which body in the heavens is to stand first on the list. even if that mightiest body be within reach of our telescopes (in itself a highly improbable supposition), we have not the least idea in what part of the heavens it is to be sought. and even if this were possible--if we were able to arrange all the visible bodies rank by rank in the order of their magnitude and their splendour--still the scheme would be impracticable, for of most of them we know little or nothing. we are therefore compelled to adopt a different method of procedure, and the simplest, as well as the most natural, will be to follow as far as possible the order of distance of the different bodies. we have already spoken of the moon as the nearest neighbour to the earth; we shall next consider some of the other celestial bodies which are comparatively near to us; then, as the subject unfolds, we shall discuss the objects further and further away, until towards the close of the volume we shall be engaged in considering the most distant bodies in the universe which the telescope has yet revealed to us. even when we have decided on this principle, our course is still not free from ambiguity. many of the bodies in the heavens are in motion, so that their relative distances from the earth are in continual change; this is, however, a difficulty which need not detain us. we shall make no attempt to adhere closely to the principle in all details. it will be sufficient if we first describe those great bodies--not a very numerous class--which are, comparatively speaking, in our vicinity, though still at varied distances; and then we shall pass on to the uncounted bodies which are separated from us by distances so vast that the imagination is baffled in the attempt to realise them. let us, then, scan the heavens to discover those orbs which lie in our neighbourhood. the sun has set, the moon has not risen; a cloudless sky discloses a heaven glittering with countless gems of light. some are grouped together into well-marked constellations; others seem scattered promiscuously, with every degree of lustre, from the very brightest down to the faintest point that the eye can just glimpse. amid all this host of objects, how are we to identify those which lie nearest to the earth? look to the west: and there, over the spot where the departing sunbeams still linger, we often see the lovely evening star shining forth. this is the planet venus--a beauteous orb, twin-sister to the earth. the brilliancy of this planet, its rapid changes both in position and in lustre, would suggest at once that it was much nearer to the earth than other star-like objects. this presumption has been amply confirmed by careful measurements, and therefore venus is to be included in the list of the orbs which constitute our neighbours. another conspicuous planet--almost rivalling venus in lustre, and vastly surpassing venus in the magnificence of its proportions and its retinue--has borne from antiquity the majestic name of jupiter. no doubt jupiter is much more distant from us than venus. indeed, he is always at least twice as far, and sometimes as much as ten times. but still we must include jupiter among our neighbours. compared with the host of stars which glitter on the heavens, jupiter must be regarded as quite contiguous. the distance of the great planet requires, it is true, hundreds of millions of miles for its expression; yet, vast as is that distance, it would have to be multiplied by tens of thousands, or hundreds of thousands, before it would be long enough to span the abyss which intervenes between the earth and the nearest of the stars. venus and jupiter have invited our attention by their exceptional brilliancy. we should, however, fall into error if we assumed generally that the brightest objects were those nearest to the earth. an observer unacquainted with astronomy might not improbably point to the dog star--or sirius, as astronomers more generally know it--as an object whose exceptional lustre showed it to be one of our neighbours. this, however, would be a mistake. we shall afterwards have occasion to refer more particularly to this gem of our southern skies, and then it will appear that sirius is a mighty globe far transcending our own sun in size as well as in splendour, but plunged into the depths of space to such an appalling distance that his enfeebled rays, when they reach the earth, give us the impression, not of a mighty sun, but only of a brilliant star. the principle of selection, by which the earth's neighbours can be discriminated, will be explained presently; in the meantime, it will be sufficient to observe that our list is to be augmented first by the addition of the unique object known as saturn, though its brightness is far surpassed by that of sirius, as well as by a few other stars. then we add mars, an object which occasionally approaches so close to the earth that it shines with a fiery radiance which would hardly prepare us for the truth that this planet is intrinsically one of the smallest of the celestial bodies. besides the objects we have mentioned, the ancient astronomers had detected a fifth, known as mercury--a planet which is usually invisible amid the light surrounding the sun. mercury, however, occasionally wanders far enough from our luminary to be seen before sunrise or after sunset. these five--mercury, venus, mars, jupiter, and saturn--comprised the planets known from remote antiquity. we can, however, now extend the list somewhat further by adding to it the telescopic objects which have in modern times been found to be among our neighbours. here we must no longer postpone the introduction of the criterion by which we can detect whether a body is near the earth or not. the brighter planets can be recognised by the steady radiance of their light as contrasted with the incessant twinkling of the stars. a little attention devoted to any of the bodies we have named will, however, point out a more definite contrast between the planets and the stars. observe, for instance, jupiter, on any clear night when the heavens can be well seen, and note his position with regard to the constellations in his neighbourhood--how he is to the right of this star, or to the left of that; directly between this pair, or directly pointed to by that. we then mark down the place of jupiter on a celestial map, or we make a sketch of the stars in the neighbourhood showing the position of the planet. after a month or two, when the observations are repeated, the place of jupiter is to be compared again with those stars by which it was defined. it will be found that, while the stars have preserved their relative positions, the place of jupiter has changed. hence this body is with propriety called a _planet_, or a wanderer, because it is incessantly moving from one part of the starry heavens to another. by similar comparisons it can be shown that the other bodies we have mentioned--venus and mercury, saturn and mars--are also wanderers, and belong to that group of heavenly bodies known as planets. here, then, we have the simple criterion by which the earth's neighbours are readily to be discriminated from the stars. each of the bodies near the earth is a planet, or a wanderer, and the mere fact that a body is a wanderer is alone sufficient to prove it to be one of the class which we are now studying. provided with this test, we can at once make an addition to our list of neighbours. amid the myriad orbs which the telescope reveals, we occasionally find one which is a wanderer. two other mighty planets, known as uranus and neptune, must thus be added to the five already mentioned, making in all a group of seven great planets. a vastly greater number may also be reckoned when we admit to our view bodies which not only seem to be minute telescopic objects, but really are small globes when compared with the mighty bulk of our earth. these lesser planets, to the number of more than four hundred, are also among the earth's neighbours. we should remark that another class of heavenly bodies widely differing from the planets must also be included in our system. these are the comets, and, indeed, it may happen that one of these erratic bodies will sometimes draw nearer to the earth than even the closest approach ever made by a planet. these mysterious visitors will necessarily engage a good deal of our attention later on. for the present we confine our attention to those more substantial globes, whether large or small, which are always termed planets. imagine for a moment that some opaque covering could be clasped around our sun so that all his beams were extinguished. that our earth would be plunged into the darkness of midnight is of course an obvious consequence. a moment's consideration will show that the moon, shining as it does by the reflected rays of the sun, would become totally invisible. but would this extinction of the sunlight have any other effect? would it influence the countless brilliant points that stud the heavens at midnight? such an obscuration of the sun would indeed produce a remarkable effect on the sky at night, which a little attention would disclose. the stars, no doubt, would not exhibit the slightest change in brilliancy. each star shines by its own light and is not indebted to the sun. the constellations would thus twinkle on as before, but a wonderful change would come over the planets. were the sun to be obscured, the planets would also disappear from view. the midnight sky would thus experience the effacement of the planets one by one, while the stars would remain unaltered. it may seem difficult to realise how the brilliancy of venus or the lustre of jupiter have their origin solely in the beams which fall upon these bodies from the distant sun. the evidence is, however, conclusive on the question; and it will be placed before the reader more fully when we come to discuss the several planets in detail. suppose that we are looking at jupiter high in mid-heavens on a winter's night, it might be contended that, as the earth lies between jupiter and the sun, it must be impossible for the rays of the sun to fall upon the planet. this is, perhaps, not an unnatural view for an inhabitant of this earth to adopt until he has become acquainted with the relative sizes of the various bodies concerned, and with the distances by which those bodies are separated. but the question would appear in a widely different form to an inhabitant of the planet jupiter. if such a being were asked whether he suffered much inconvenience by the intrusion of the earth between himself and the sun, his answer would be something of this kind:--"no doubt such an event as the passage of the earth between me and the sun is possible, and has occurred on rare occasions separated by long intervals; but so far from the transit being the cause of any inconvenience, the whole earth, of which you think so much, is really so minute, that when it did come in front of the sun it was merely seen as a small telescopic point, and the amount of sunlight which it intercepted was quite inappreciable." the fact that the planets shine by the sun's light points at once to the similarity between them and our earth. we are thus led to regard our sun as a central fervid globe associated with a number of much smaller bodies, each of which, being dark itself, is indebted to the sun both for light and for heat. that was, indeed, a grand step in astronomy which demonstrated the nature of the solar system. the discovery that our earth must be a globe isolated in space was in itself a mighty exertion of human intellect; but when it came to be recognised that this globe was but one of a whole group of similar objects, some smaller, no doubt, but others very much larger, and when it was further ascertained that these bodies were subordinated to the supreme control of the sun, we have a chain of discoveries that wrought a fundamental transformation in human knowledge. we thus see that the sun presides over a numerous family. the members of that family are dependent upon the sun, and their dimensions are suitably proportioned to their subordinate position. even jupiter, the largest member of that family, does not contain one-thousandth part of the material which forms the vast bulk of the sun. yet the bulk of jupiter alone would exceed that of the rest of the planets were they all rolled together. around the central luminary in fig. we have drawn four circles in dotted lines which sufficiently illustrate the orbits in which the different bodies move. the innermost of these four paths represents the orbit of the planet mercury. the planet moves around the sun in this path, and regains the place from which it started in eighty-eight days. the next orbit, proceeding outwards from the sun, is that of the planet venus, which we have already referred to as the well-known evening star. venus completes the circuit of its path in days. one step further from the sun and we come to the orbit of another planet. this body is almost the same size as venus, and is therefore much larger than mercury. the planet now under consideration accomplishes each revolution in days. this period sounds familiar to our ears. it is the length of the year; and the planet is the earth on which we stand. there is an impressive way in which to realise the length of the road along which the earth has to travel in each annual journey. the circumference of a circle is about three and one-seventh times the diameter of the same figure; so that taking the distance from the earth to the centre of the sun as , , miles, the diameter of the circle which the earth describes around the sun will be , , miles, and consequently the circumference of the mighty circle in which the earth moves round the sun is fully , , miles. the earth has to travel this distance every year. it is merely a sum in division to find how far we have to move each second in order to accomplish this long journey in a twelvemonth. it will appear that the earth must actually complete eighteen miles every second, as otherwise it would not finish its journey within the allotted time. [illustration: fig. .--the orbits of the four interior planets.] pause for a moment to think what a velocity of eighteen miles a second really implies. can we realise a speed so tremendous? let us compare it with our ordinary types of rapid movement. look at that express train how it crashes under the bridge, how, in another moment, it is lost to view! can any velocity be greater than that? let us try it by figures. the train moves a mile a minute; multiply that velocity by eighteen and it becomes eighteen miles a _minute_, but we must further multiply it by sixty to make it eighteen miles a _second_. the velocity of the express train is not even the thousandth part of the velocity of the earth. let us take another illustration. we stand at the rifle ranges to see a rifle fired at a target , feet away, and we find that a second or two is sufficient to carry the bullet over that distance. the earth moves nearly one hundred times as fast as the rifle bullet. [illustration: fig. .--the earth's movement.] viewed in another way, the stupendous speed of the earth does not seem immoderate. the earth is a mighty globe, so great indeed that even when moving at this speed it takes almost eight minutes to pass over its own diameter. if a steamer required eight minutes to traverse a distance equal to its own length, its pace would be less than a mile an hour. to illustrate this method of considering the subject, we show here a view of the progress made by the earth (fig. ). the distance between the centres of these circles is about six times the diameter; and, accordingly, if they be taken to represent the earth, the time required to pass from one position to the other is about forty-eight minutes. outside the path of the earth, we come to the orbit of the fourth planet, mars, which requires days, or nearly two years, to complete its circuit round the sun. with our arrival at mars we have gained the limit to the inner portion of the solar system. the four planets we have mentioned form a group in themselves, distinguished by their comparative nearness to the sun. they are all bodies of moderate dimensions. venus and the earth are globes of about the same size. mercury and mars are both smaller objects which lie, so far as bulk is concerned, between the earth and the moon. the four planets which come nearest to the sun are vastly surpassed in bulk and weight by the giant bodies of our system--the stately group of jupiter and saturn, uranus and neptune. [illustration: fig. .--the orbits of the four giant planets.] these giant planets enjoy the sun's guidance equally with their weaker brethren. in the diagram on this page (fig. ) parts of the orbits of the great outer planets are represented. the sun, as before, presides at the centre, but the inner planets would on this scale be so close to the sun that it is only possible to represent the orbit of mars. after the orbit of mars comes a considerable interval, not, however, devoid of planetary activity, and then follow the orbits of jupiter and saturn; further still, we have uranus, a great globe on the verge of unassisted vision; and, lastly, the whole system is bounded by the grand orbit of neptune--a planet of which we shall have a marvellous story to narrate. the various circles in fig. show the apparent sizes of the sun as seen from the different planets. taking the circle corresponding to the earth to represent the amount of heat and light which the earth derives from the sun then the other circles indicate the heat and the light enjoyed by the corresponding planets. the next outer planet to the earth is mars, whose share of solar blessings is not so very inferior to that of the earth; but we fail to see how bodies so remote as jupiter or saturn can enjoy climates at all comparable with those of the planets which are more favourably situated. [illustration: fig. .--comparative apparent size of the sun as seen from the various planets.] fig. shows a picture of the whole family of planets surrounding the sun--represented on the same scale, so as to exhibit their comparative sizes. measured by bulk, jupiter is more than , times as great as the earth, so that it would take at least , earths rolled into one to form a globe equal to the globe of jupiter. measured by weight, the disparity between the earth and jupiter, though still enormous, is not quite so great; but this is a matter to be discussed more fully in a later chapter. [illustration: fig. .--comparative sizes of the planets.] even in this preliminary survey of the solar system we must not omit to refer to the planets which attract our attention, not by their bulk, but by their multitude. in the ample zone bounded on the inside by the orbit of mars and on the outside by the orbit of jupiter it was thought at one time that no planet revolved. modern research has shown that this region is tenanted, not by one planet, but by hundreds. the discovery of these planets is a charge which has been undertaken by various diligent astronomers of the present day, while the discussion of their movements affords labour to other men of science. we shall find something to learn from the study of these tiny bodies, and especially from another small planet called eros, which lies nearer to the earth than the limit above indicated. a chapter will be devoted to these objects. but we do not propose to enter deeply into the mere statistics of the planetary system at present. were such our intention, the tables at the end of the volume would show that ample materials are available. astronomers have taken an inventory of each of the planets. they have measured their distances, the shapes of their orbits and the positions of those orbits, their times of revolution, and, in the case of all the larger planets, their sizes and their weights. such results are of interest for many purposes. it is, however, the more general features of the science which at present claim our attention. let us, in conclusion, note one or two important truths with reference to our planetary system. we have seen that all the planets revolve in nearly circular paths around the sun. we have now to add another fact possessing much significance. each of the planets pursues its path in the same direction. it thus happens that one such body may overtake another, but it can never happen that two planets pass by each other as do the trains on adjacent lines of railway. we shall subsequently find that the whole welfare of our system, nay, its continuous existence, is dependent upon this remarkable uniformity taken in conjunction with other features of the system. such is our solar system; a mighty organised group of planets circulating under the control of the sun, and completely isolated from all external interference. no star, no constellation, has any appreciable influence on our solar system. we constitute a little island group, separated from the nearest stars by the most amazing distances. it may be that as the other stars are suns, so they too may have systems of planets circulating around them; but of this we know nothing. of the stars we can only say that they appear to us as points of light, and any planets they may possess must for ever remain invisible to us, even if they were many times larger than jupiter. we need not repine at this limitation to our possible knowledge, for just as we find in the solar system all that is necessary for our daily bodily wants, so shall we find ample occupation for whatever faculties we may possess in endeavouring to understand those mysteries of the heavens which lie within our reach. chapter v. the law of gravitation. gravitation--the falling of a stone to the ground--all bodies fall equally, sixteen feet in a second--is this true at great heights?--fall of a body at a height of a quarter of a million miles--how newton obtained an answer from the moon--his great discovery--statement of the law of gravitation--illustrations of the law--how is it that all the bodies in the universe do not rush together?--the effect of motion--how a circular path can be produced by attraction--general account of the moon's motion--is gravitation a force of great intensity?--two weights of lbs.--two iron globes, yards in diameter, and a mile apart, attract with a force of lb.--characteristics of gravitation--orbits of the planets not strictly circles--the discoveries of kepler--construction of an ellipse--kepler's first law--does a planet move uniformly?--law of the changes of velocity--kepler's second law--the relation between the distances and the periodic times--kepler's third law--kepler's laws and the law of gravitation--movement in a straight line--a body unacted on by disturbing forces would move in a straight line with constant velocity--application to the earth and the planets--the law of gravitation deduced from kepler's laws--universal gravitation. our description of the heavenly bodies must undergo a slight interruption, while we illustrate with appropriate detail an important principle, known as the law of gravitation, which underlies the whole of astronomy. by this law we can explain the movements of the moon around the earth, and of the planets around the sun. it is accordingly incumbent upon us to discuss this subject before we proceed to the more particular account of the separate planets. we shall find, too, that the law of gravitation sheds some much-needed light on the nature of the stars situated at the remotest distances in space. it also enables us to cast a glance through the vistas of time past, and to trace with plausibility, if not with certainty, certain early phases in the history of our system. the sun and the moon, the planets and the comets, the stars and the nebulæ, all alike are subject to this universal law, which is now to engage our attention. what is more familiar than the fact that when a stone is dropped it will fall to the ground? no one at first thinks the matter even worthy of remark. people are often surprised at seeing a piece of iron drawn to a magnet. yet the fall of a stone to the ground is the manifestation of a force quite as interesting as the force of magnetism. it is the earth which draws the stone, just as the magnet draws the iron. in each case the force is one of attraction; but while the magnetic attraction is confined to a few substances, and is of comparatively limited importance, the attraction of gravitation is significant throughout the universe. let us commence with a few very simple experiments upon the force of gravitation. hold in the hand a small piece of lead, and then allow it to drop upon a cushion. the lead requires a certain time to move from the fingers to the cushion, but that time is always the same when the height is the same. take now a larger piece of lead, and hold one piece in each hand at the same height. if both are released at the same moment, they will both reach the cushion simultaneously. it might have been thought that the heavy body would fall more quickly than the light body; but when the experiment is tried, it is seen that this is not the case. repeat the experiment with various other substances. an ordinary marble will be found to fall in the same time as the piece of lead. with a piece of cork we again try the experiment, and again obtain the same result. at first it seems to fail when we compare a feather with the piece of lead; but that is solely on account of the air, which resists the feather more than it resists the lead. if, however, the feather be placed upon the top of a penny, and the penny be horizontal when dropped, it will clear the air out of the way of the feather in its descent, and then the feather will fall as quickly as the penny, as quickly as the marble, or as quickly as the lead. if the observer were in a gallery when trying these experiments, and if the cushion were sixteen feet below his hands, then the time the marble would take to fall through the sixteen feet would be one second. the time occupied by the cork or by the lead would be the same; and even the feather itself would fall through sixteen feet in one second, if it could be screened from the interference of the air. try this experiment where we like, in london, or in any other city, in any island or continent, on board a ship at sea, at the north pole, or the south pole, or the equator, it will always be found that any body, of any size or any material, will fall about sixteen feet in one second of time. lest any erroneous impression should arise, we may just mention that the distance traversed in one second does vary slightly at different parts of the earth, but from causes which need not at this moment detain us. we shall for the present regard sixteen feet as the distance through which any body, free from interference, would fall in one second at any part of the earth's surface. but now let us extend our view above the earth's surface, and enquire how far this law of sixteen feet in a second may find obedience elsewhere. let us, for instance, ascend to the top of a mountain and try the experiment there. it would be found that at the top of the mountain a marble would take a little longer to fall through sixteen feet than the same marble would if let fall at its base. the difference would be very small; but yet it would be measurable, and would suffice to show that the power of the earth to pull the marble to the ground becomes somewhat weakened at a point high above the earth's surface. whatever be the elevation to which we ascend, be it either the top of a high mountain, or the still greater altitudes that have been reached in balloon ascents, we shall never find that the tendency of bodies to fall to the ground ceases, though no doubt the higher we go the more is that tendency weakened. it would be of great interest to find how far this power of the earth to draw bodies towards it can really extend. we cannot attain more than about five or six miles above the earth's surface in a balloon; yet we want to know what would happen if we could ascend miles, or , miles, or still further, into the regions of space. conceive that a traveller were endowed with some means of soaring aloft for miles and thousands of miles, still up and up, until at length he had attained the awful height of nearly a quarter of a million of miles above the ground. glancing down at the surface of that earth, which is at such a stupendous depth beneath, he would be able to see a wonderful bird's-eye view. he would lose, no doubt, the details of towns and villages; the features in such a landscape would be whole continents and whole oceans, in so far as the openings between the clouds would permit the earth's surface to be exposed. at this stupendous elevation he could try one of the most interesting experiments that was ever in the power of a philosopher. he could test whether the earth's attraction was felt at such a height, and he could measure the amount of that attraction. take for the experiment a cork, a marble, or any other object, large or small; hold it between the fingers, and let it go. everyone knows what would happen in such a case down here; but it required sir isaac newton to tell what would happen in such a case up there. newton asserts that the power of the earth to attract bodies extends even to this great height, and that the marble would fall. this is the doctrine that we can now test. we are ready for the experiment. the marble is released, and, lo! our first exclamation is one of wonder. instead of dropping instantly, the little object appears to remain suspended. we are on the point of exclaiming that we must have gone beyond the earth's attraction, and that newton is wrong, when our attention is arrested; the marble is beginning to move, so slowly that at first we have to watch it carefully. but the pace gradually improves, so that the attraction is beyond all doubt, until, gradually acquiring more and more velocity, the marble speeds on its long journey of a quarter of a million of miles to the earth. but surely, it will be said, such an experiment must be entirely impossible; and no doubt it cannot be performed in the way described. the bold idea occurred to newton of making use of the moon itself, which is almost a quarter of a million of miles above the earth, for the purpose of answering the question. never was our satellite put to such noble use before. it is actually at each moment falling in towards the earth. we can calculate how much it is deflected towards the earth in each second, and thus obtain a measure of the earth's attractive power. from such enquiries newton was able to learn that a body released at the distance of , miles above the surface of the earth would still be attracted by the earth, that in virtue of the attraction the body would commence to move off towards the earth--not, indeed, with the velocity with which a body falls in experiments on the surface, but with a very much lesser speed. a body dropped down from the distance of the moon would commence its long journey so slowly that a _minute_, instead of a _second_, would have elapsed before the distance of sixteen feet had been accomplished.[ ] it was by pondering on information thus won from the moon that newton made his immortal discovery. the gravitation of the earth is a force which extends far and wide through space. the more distant the body, the weaker the gravitation becomes; here newton found the means of determining the great problem as to the law according to which the intensity of the gravitation decreased. the information derived from the moon, that a body , miles away requires a minute to fall through a space equal to that through which it would fall in a second down here, was of paramount importance. in the first place, it shows that the attractive power of the earth, by which it draws all bodies earthwards, becomes weaker at a distance. this might, indeed, have been anticipated. it is as reasonable to suppose that as we retreated further and further into the depths of space the power of attraction should diminish, as that the lustre of light should diminish as we recede from it; and it is remarkable that the law according to which the attraction of gravitation decreases with the increase of distance is precisely the same as the law according to which the brilliancy of a light decreases as its distance increases. the law of nature, stated in its simplest form, asserts that the intensity of gravitation varies inversely as the square of the distance. let me endeavour to elucidate this somewhat abstract statement by one or two simple illustrations. suppose a body were raised above the surface of the earth to a height of nearly , miles, so as to be at an altitude equal to the radius of the earth. in other words, a body so situated would be twice as far from the centre of the earth as a body which lay on the surface. the law of gravitation says that the intensity of the attraction is then to be decreased to one-fourth part, so that the pull of the earth on a body , miles high is only one quarter of the pull of the earth on that body so long as it lies on the ground. we may imagine the effect of this pull to be shown in different ways. allow the body to fall, and in the interval of one second it will only drop through four feet, a mere quarter of the distance that gravity would cause near the earth's surface. we may consider the matter in another way by supposing that the attraction of the earth is measured by one of those little weighing machines known as a spring balance. if a weight of four pounds be hung on such a contrivance, at the earth's surface, the index of course shows a weight of four pounds; but conceive this balance, still bearing the weight appended thereto, were to be carried up and up, the _indicated_ strain would become less and less, until by the time the balance reached , miles high, where it was _twice_ as far away from the earth's centre as at first, the indicated strain would be reduced to the _fourth_ part, and the balance would only show one pound. if we could imagine the instrument to be carried still further into the depths of space, the indication of the scale would steadily continue to decline. by the time the apparatus had reached a distance of , miles high, being then _three_ times as far from the earth's centre as at first, the law of gravitation tells us that the attraction must have decreased to one-ninth part. the strain thus shown on the balance would be only the ninth part of four pounds, or less than half a pound. but let the voyage be once again resumed, and let not a halt be made this time until the balance and its four-pound weight have retreated to that orbit which the moon traverses in its monthly course around the earth. the distance thus attained is about sixty times the radius of the earth, and consequently the attraction of gravitation is diminished in the proportion of one to the square of sixty; the spring will then only be strained by the inappreciable fraction of - , part of four pounds. it therefore appears that a weight which on the earth weighed a ton and a half would, if raised , miles, weigh less than a pound. but even at this vast distance we are not to halt; imagine that we retreat still further and further; the strain shown by the balance will ever decrease, but it will still exist, no matter how far we go. astronomy appears to teach us that the attraction of gravitation can extend, with suitably enfeebled intensity, across the most profound gulfs of space. the principle of gravitation is of far wider scope than we have yet indicated. we have spoken merely of the attraction of the earth, and we have stated that this force extends throughout space. but the law of gravitation is not so limited. not only does the earth attract every other body, and every other body attract the earth, but each of these bodies attracts the other; so that in its more complete shape the law of gravitation announces that "every body in the universe attracts every other body with a force which varies inversely as the square of the distance." it is impossible for us to over-estimate the importance of this law. it supplies the clue by which we can unravel the complicated movements of the planets. it has led to marvellous discoveries, in which the law of gravitation has enabled us to anticipate the telescope, and to feel the existence of bodies before those bodies have even been seen. an objection which may be raised at this point must first be dealt with. it seems to be, indeed, a plausible one. if the earth attracts the moon, why does not the moon tumble down on the earth? if the earth is attracted by the sun, why does it not tumble into the sun? if the sun is attracted by other stars, why do they not rush together with a frightful collision? it may not unreasonably be urged that if all these bodies in the heavens are attracting each other, it would seem that they must all rush together in consequence of that attraction, and thus weld the whole material universe into a single mighty mass. we know, as a matter of fact, that these collisions do not often happen, and that there is extremely little likelihood of their taking place. we see that although our earth is said to have been attracted by the sun for countless ages, yet the earth is just as far from the sun as ever it was. is not this in conflict with the doctrine of universal gravitation? in the early days of astronomy such objections would be regarded, and doubtless were regarded, as well-nigh insuperable; even still we occasionally hear them raised, and it is therefore the more incumbent on us to explain how it happens that the solar system has been able to escape from the catastrophe by which it seems to be threatened. there can be no doubt that if the moon and the earth had been initially placed _at rest_, they would have been drawn together by their mutual attraction. so, too, if the system of planets surrounding the sun had been left initially _at rest_ they would have dashed into the sun, and the system would have been annihilated. it is the fact that the planets are _moving_, and that the moon is _moving_, which has enabled these bodies successfully to resist the attraction in so far, at least, as that they are not drawn thereby to total destruction. it is so desirable that the student should understand clearly how a central attraction is compatible with revolution in a nearly circular path, that we give an illustration to show how the moon pursues its monthly orbit under the guidance and the control of the attracting earth. [illustration: fig. .--illustration of the moon's motion.] the imaginary sketch in fig. denotes a section of the earth with a high mountain thereon.[ ] if a cannon were stationed on the top of the mountain at c, and if the cannonball were fired off in the direction c e with a moderate charge of powder, the ball would move down along the first curved path. if it be fired a second time with a heavier charge, the path will be along the second curved line, and the ball would again fall to the ground. but let us try next time with a charge still further increased, and, indeed, with a far stronger cannon than any piece of ordnance ever yet made. the velocity of the projectile must now be assumed to be some miles per second, but we can conceive that the speed shall be so adjusted that the ball shall move along the path c d, always at the same height above the earth, though still curving, as every projectile must curve, from the horizontal line in which it moved at the first moment. arrived at d, the ball will still be at the same height above the surface, and its velocity must be unabated. it will therefore continue in its path and move round another quadrant of the circle without getting nearer to the surface. in this manner the projectile will travel completely round the whole globe, coming back again to c and then taking another start in the same path. if we could abolish the mountain and the cannon at the top, we should have a body revolving for ever around the earth in consequence of the attraction of gravitation. make now a bold stretch of the imagination. conceive a terrific cannon capable of receiving a round bullet not less than , miles in diameter. discharge this enormous bullet with a velocity of about , feet per second, which is two or three times as great as the velocity actually attainable in modern artillery. let this notable bullet be fired horizontally from some station nearly a quarter of a million miles above the surface of the earth. that fearful missile would sweep right round the earth in a nearly circular orbit, and return to where it started in about four weeks. it would then commence another revolution, four weeks more would find it again at the starting point, and this motion would go on for ages. do not suppose that we are entirely romancing. we cannot indeed show the cannon, but we can point to a great projectile. we see it every month; it is the beautiful moon herself. no one asserts that the moon was ever shot from such a cannon; but it must be admitted that she moves as if she had been. in a later chapter we shall enquire into the history of the moon, and show how she came to revolve in this wonderful manner. as with the moon around the earth, so with the earth around the sun. the illustration shows that a circular or nearly circular motion harmonises with the conception of the law of universal gravitation. we are accustomed to regard gravitation as a force of stupendous magnitude. does not gravitation control the moon in its revolution around the earth? is not even the mighty earth itself retained in its path around the sun by the surpassing power of the sun's attraction? no doubt the actual force which keeps the earth in its path, as well as that which retains the moon in our neighbourhood, is of vast intensity, but that is because gravitation is in such cases associated with bodies of enormous mass. no one can deny that all bodies accessible to our observation appear to attract each other in accordance with the law of gravitation; but it must be confessed that, unless one or both of the attracting bodies is of gigantic dimensions, the intensity is almost immeasurably small. let us attempt to illustrate how feeble is the gravitation between masses of easily manageable dimensions. take, for instance, two iron weights, each weighing about lb., and separated by a distance of one foot from centre to centre. there is a certain attraction of gravitation between these weights. the two weights are drawn together, yet they do not move. the attraction between them, though it certainly exists, is an extremely minute force, not at all comparable as to intensity with magnetic attraction. everyone knows that a magnet will draw a piece of iron with considerable vigour, but the intensity of gravitation is very much less on masses of equal amount. the attraction between these two lb. weights is less than the ten-millionth part of a single pound. such a force is utterly infinitesimal in comparison with the friction between the weights and the table on which they stand, and hence there is no response to the attraction by even the slightest movement. yet, if we can conceive each of these weights mounted on wheels absolutely devoid of friction, and running on absolutely perfect horizontal rails, then there is no doubt that the bodies would slowly commence to draw together, and in the course of time would arrive in actual contact. if we desire to conceive gravitation as a force of measurable intensity, we must employ masses immensely more ponderous than those lb. weights. imagine a pair of globes, each composed of , tons of cast iron, and each, if solid, being about yards in diameter. imagine these globes placed at a distance of one mile apart. each globe attracts the other by the force of gravitation. it does not matter that buildings and obstacles of every description intervene; gravitation will pass through such impediments as easily as light passes through glass. no screen can be devised dense enough to intercept the passage of this force. each of these iron globes will therefore under all circumstances attract the other; but, notwithstanding their ample proportions, the intensity of that attraction is still very small, though appreciable. the attraction between these two globes is a force no greater than the pressure exerted by a single pound weight. a child could hold back one of these massive globes from its attraction by the other. suppose that all was clear, and that friction could be so neutralised as to permit the globes to follow the impulse of their mutual attractions. the two globes will then commence to approach, but the masses are so large, while the attraction is so small, that the speed will be accelerated very slowly. a microscope would be necessary to show when the motion has actually commenced. an hour and a half must elapse before the distance is diminished by a single foot; and although the pace improves subsequently, yet three or four days must elapse before the two globes will come together. the most remarkable characteristic of the force of gravitation must be here specially alluded to. the intensity appears to depend only on the quantity of matter in the bodies, and not at all on the nature of the substances of which these bodies are composed. we have described the two globes as made of cast iron, but if either or both were composed of lead or copper, of wood or stone, of air or water, the attractive power would still be the same, provided only that the masses remain unaltered. in this we observe a profound difference between the attraction of gravitation and magnetic attraction. in the latter case the attraction is not perceptible at all in the great majority of substances, and is only considerable in the case of iron. in our account of the solar system we have represented the moon as revolving around the earth in a _nearly_ circular path, and the planets as revolving around the sun in orbits which are also approximately circular. it is now our duty to give a more minute description of these remarkable paths; and, instead of dismissing them as being _nearly_ circles, we must ascertain precisely in what respects they differ therefrom. if a planet revolved around the sun in a truly circular path, of which the sun was always at the centre, it is then obvious that the distance from the sun to the planet, being always equal to the radius of the circle, must be of constant magnitude. now, there can be no doubt that the distance from the sun to each planet is approximately constant; but when accurate observations are made, it becomes clear that the distance is not absolutely so. the variations in distance may amount to many millions of miles, but, even in extreme cases, the variation in the distance of the planet is only a small fraction--usually a very small fraction--of the total amount of that distance. the circumstances vary in the case of each of the planets. the orbit of the earth itself is such that the distance from the earth to the sun departs but little from its mean value. venus makes even a closer approach to perfectly circular movement; while, on the other hand, the path of mars, and much more the path of mercury, show considerable relative fluctuations in the distance from the planet to the sun. it has often been noticed that many of the great discoveries in science have their origin in the nice observation and explanation of minute departures from some law approximately true. we have in this department of astronomy an excellent illustration of this principle. the orbits of the planets are nearly circles, but they are not exactly circles. now, why is this? there must be some natural reason. that reason has been ascertained, and it has led to several of the grandest discoveries that the mind of man has ever achieved in the realms of nature. in the first place, let us see the inferences to be drawn from the fact that the distance of a planet from the sun is not constant. the motion in a circle is one of such beauty and simplicity that we are reluctant to abandon it, unless the necessity for doing so be made clearly apparent. can we not devise any way by which the circular motion might be preserved, and yet be compatible with the fluctuations in the distance from the planet to the sun? this is clearly impossible with the sun at the centre of the circle. but suppose the sun did not occupy the centre, while the planet, as before, revolved around the sun. the distance between the two bodies would then necessarily fluctuate. the more eccentric the position of the sun, the larger would be the proportionate variation in the distance of the planet when at the different parts of its orbit. it might further be supposed that by placing a series of circles around the sun the various planetary orbits could be accounted for. the centre of the circle belonging to venus is to coincide very nearly with the centre of the sun, and the centres of the orbits of all the other planets are to be placed at such suitable distances from the sun as will render a satisfactory explanation of the gradual increase and decrease of the distance between the two bodies. there can be no doubt that the movements of the moon and of the planets would be, to a large extent, explained by such a system of circular orbits; but the spirit of astronomical enquiry is not satisfied with approximate results. again and again the planets are observed, and again and again the observations are compared with the places which the planets would occupy if they moved in accordance with the system here indicated. the centres of the circles are moved hither and thither, their radii are adjusted with greater care; but it is all of no avail. the observations of the planets are minutely examined to see if they can be in error; but of errors there are none at all sufficient to account for the discrepancies. the conclusion is thus inevitable--astronomers are forced to abandon the circular motion, which was thought to possess such unrivalled symmetry and beauty, and are compelled to admit that the orbits of the planets are not circular. then if these orbits be not circles, what are they? such was the great problem which kepler proposed to solve, and which, to his immortal glory, he succeeded in solving and in proving to demonstration. the great discovery of the true shape of the planetary orbits stands out as one of the most conspicuous events in the history of astronomy. it may, in fact, be doubted whether any other discovery in the whole range of science has led to results of such far-reaching interest. we must here adventure for a while into the field of science known as geometry, and study therein the nature of that curve which the discovery of kepler has raised to such unparalleled importance. the subject, no doubt, is a difficult one, and to pursue it with any detail would involve us in many abstruse calculations which would be out of place in this volume; but a general sketch of the subject is indispensable, and we must attempt to render it such justice as may be compatible with our limits. the curve which represents with perfect fidelity the movements of a planet in its revolution around the sun belongs to that well-known group of curves which mathematicians describe as the conic sections. the particular form of conic section which denotes the orbit of a planet is known by the name of the _ellipse_: it is spoken of somewhat less accurately as an oval. the ellipse is a curve which can be readily constructed. there is no simpler method of doing so than that which is familiar to draughtsmen, and which we shall here briefly describe. we represent on the next page (fig. ) two pins passing through a sheet of paper. a loop of twine passes over the two pins in the manner here indicated, and is stretched by the point of a pencil. with a little care the pencil can be guided so as to keep the string stretched, and its point will then describe a curve completely round the pins, returning to the point from which it started. we thus produce that celebrated geometrical figure which is called an ellipse. it will be instructive to draw a number of ellipses, varying in each case the circumstances under which they are formed. if, for instance, the pins remain placed as before, while the length of the loop is increased, so that the pencil is farther away from the pins, then it will be observed that the ellipse has lost some of its elongation, and approaches more closely to a circle. on the other hand, if the length of the cord in the loop be lessened, while the pins remain as before, the ellipse will be found more oval, or, as a mathematician would say, its _eccentricity_ is increased. it is also useful to study the changes which the form of the ellipse undergoes when one of the pins is altered, while the length of the loop remains unchanged. if the two pins be brought nearer together the eccentricity will decrease, and the ellipse will approximate more closely to the shape of a circle. if the pins be separated more widely the eccentricity of the ellipse will be increased. that the circle is an extreme form of ellipse will be evident, if we suppose the two pins to draw in so close together that they become coincident; the point will then simply trace out a circle as the pencil moves round the figure. [illustration: fig. .--drawing an ellipse.] the points marked by the pins obviously possess very remarkable relations with respect to the curve. each one is called a _focus_, and an ellipse can only have one pair of foci. in other words, there is but a single pair of positions possible for the two pins, when an ellipse of specified size, shape, and position is to be constructed. the ellipse differs principally from a circle in the circumstance that it possesses variety of form. we can have large and small ellipses just as we can have large and small circles, but we can also have ellipses of greater or less eccentricity. if the ellipse has not the perfect simplicity of the circle it has, at least, the charm of variety which the circle has not. the oval curve has also the beauty derived from an outline of perfect grace and an association with ennobling conceptions. the ancient geometricians had studied the ellipse: they had noticed its foci; they were acquainted with its geometrical relations; and thus kepler was familiar with the ellipse at the time when he undertook his celebrated researches on the movements of the planets. he had found, as we have already indicated, that the movements of the planets could not be reconciled with circular orbits. what shape of orbit should next be tried? the ellipse was ready to hand, its properties were known, and the comparison could be made; memorable, indeed, was the consequence of this comparison. kepler found that the movement of the planets could be explained, by supposing that the path in which each one revolved was an ellipse. this in itself was a discovery of the most commanding importance. on the one hand it reduced to order the movements of the great globes which circulate round the sun; while on the other, it took that beautiful class of curves which had exercised the geometrical talents of the ancients, and assigned to them the dignity of defining the highways of the universe. but we have as yet only partly enunciated the first discovery of kepler. we have seen that a planet revolves in an ellipse around the sun, and that the sun is, therefore, at some point in the interior of the ellipse--but at what point? interesting, indeed, is the answer to this question. we have pointed out how the foci possess a geometrical significance which no other points enjoy. kepler showed that the sun must be situated in one of the foci of the ellipse in which each planet revolves. we thus enunciate the first law of planetary motion in the following words:-- _each planet revolves around the sun in an elliptic path, having the sun at one of the foci._ we are now enabled to form a clear picture of the orbits of the planets, be they ever so numerous, as they revolve around the sun. in the first place, we observe that the ellipse is a plane curve; that is to say, each planet must, in the course of its long journey, confine its movements to one plane. each planet has thus a certain plane appropriated to it. it is true that all these planes are very nearly coincident, at least in so far as the great planets are concerned; but still they are distinct, and the only feature in which they all agree is that each one of them passes through the sun. all the elliptic orbits of the planets have one focus in common, and that focus lies at the centre of the sun. it is well to illustrate this remarkable law by considering the circumstances of two or three different planets. take first the case of the earth, the path of which, though really an ellipse, is very nearly circular. in fact, if it were drawn accurately to scale on a sheet of paper, the difference between the elliptic orbit and the circle would hardly be detected without careful measurement. in the case of venus the ellipse is still more nearly a circle, and the two foci of the ellipse are very nearly coincident with the centre of the circle. on the other hand, in the case of mercury, we have an ellipse which departs from the circle to a very marked extent, while in the orbits of some of the minor planets the eccentricity is still greater. it is extremely remarkable that every planet, no matter how far from the sun, should be found to move in an ellipse of some shape or other. we shall presently show that necessity compels each planet to pursue an elliptic path, and that no other form of path is possible. started on its elliptic path, the planet pursues its stately course, and after a certain duration, known as the _periodic time_, regains the position from which its departure was taken. again the planet traces out anew the same elliptic path, and thus, revolution after revolution, an identical track is traversed around the sun. let us now attempt to follow the body in its course, and observe the history of its motion during the time requisite for the completion of one of its circuits. the dimensions of a planetary orbit are so stupendous that the planet must run its course very rapidly in order to finish the journey within the allotted time. the earth, as we have already seen, has to move eighteen miles a second to accomplish one of its voyages round the sun in the lapse of - / days. the question then arises as to whether the rate at which a planet moves is uniform or not. does the earth, for instance, actually move at all times with the velocity of eighteen miles a second, or does our planet sometimes move more rapidly and sometimes more slowly, so that the average of eighteen miles a second is still maintained? this is a question of very great importance, and we are able to answer it in the clearest and most emphatic manner. the velocity of a planet is _not_ uniform, and the variations of that velocity can be explained by the adjoining figure (fig. ). [illustration: fig. .--varying velocity of elliptic motion.] let us first of all imagine the planet to be situated at that part of its path most distant from the sun towards the right of the figure. in this position the body's velocity is at its lowest; as the planet begins to approach the sun the speed gradually improves until it attains its mean value. after this point has been passed, and the planet is now rapidly hurrying on towards the sun, the velocity with which it moves becomes gradually greater and greater, until at length, as it dashes round the sun, its speed attains a maximum. after passing the sun, the distance of the planet from the luminary increases, and the velocity of the motion begins to abate; gradually it declines until the mean value is again reached, and then it falls still lower, until the body recedes to its greatest distance from the sun, by which time the velocity has abated to the value from which we supposed it to commence. we thus observe that the nearer the planet is to the sun the quicker it moves. we can, however, give numerical definiteness to the principle according to which the velocity of the planet varies. the adjoining figure (fig. ) shows a planetary orbit, with, of course, the sun at the focus s. we have taken two portions, a b and c d, round the ellipse, and joined their extremities to the focus. kepler's second law may be stated in these words:-- "_every planet moves round the sun with such a velocity at every point, that a straight line drawn from it to the sun passes over equal areas in equal times._" [illustration: fig. .--equal areas in equal times.] for example, if the two shaded portions, a b s and d c s, are equal in area, then the times occupied by the planet in travelling over the portions of the ellipse, a b and c d, are equal. if the one area be greater than the other, then the times required are in the proportion of the areas. this law being admitted, the reason of the increase in the planet's velocity when it approaches the sun is at once apparent. to accomplish a definite area when near the sun, a larger arc is obviously necessary than at other parts of the path. at the opposite extremity, a small arc suffices for a large area, and the velocity is accordingly less. these two laws completely prescribe the motion of a planet round the sun. the first defines the path which the planet pursues; the second describes how the velocity of the body varies at different points along its path. but kepler added to these a third law, which enables us to compare the movements of two different planets revolving round the same sun. before stating this law, it is necessary to explain exactly what is meant by the _mean_ distance of a planet. in its elliptic path the distance from the sun to the planet is constantly changing; but it is nevertheless easy to attach a distinct meaning to that distance which is an average of all the distances. this average is called the mean distance. the simplest way of finding the mean distance is to add the greatest of these quantities to the least, and take half the sum. we have already defined the periodic time of the planet; it is the number of days which the planet requires for the completion of a journey round its path. kepler's third law establishes a relation between the mean distances and the periodic times of the various planets. that relation is stated in the following words:-- "_the squares of the periodic times are proportional to the cubes of the mean distances._" kepler knew that the different planets had different periodic times; he also saw that the greater the mean distance of the planet the greater was its periodic time, and he was determined to find out the connection between the two. it was easily found that it would not be true to say that the periodic time is merely proportional to the mean distance. were this the case, then if one planet had a distance twice as great as another, the periodic time of the former would have been double that of the latter; observation showed, however, that the periodic time of the more distant planet exceeded twice, and was indeed nearly three times, that of the other. by repeated trials, which would have exhausted the patience of one less confident in his own sagacity, and less assured of the accuracy of the observations which he sought to interpret, kepler at length discovered the true law, and expressed it in the form we have stated. to illustrate the nature of this law, we shall take for comparison the earth and the planet venus. if we denote the mean distance of the earth from the sun by unity then the mean distance of venus from the sun is · . omitting decimals beyond the first place, we can represent the periodic time of the earth as · days, and the periodic time of venus as · days. now the law which kepler asserts is that the square of · is to the square of · in the same proportion as unity is to the cube of · . the reader can easily verify the truth of this identity by actual multiplication. it is, however, to be remembered that, as only four figures have been retained in the expressions of the periodic times, so only four figures are to be considered significant in making the calculations. the most striking manner of making the verification will be to regard the time of the revolution of venus as an unknown quantity, and deduce it from the known revolution of the earth and the mean distance of venus. in this way, by assuming kepler's law, we deduce the cube of the periodic time by a simple proportion, and the resulting value of · days can then be obtained. as a matter of fact, in the calculations of astronomy, the distances of the planets are usually ascertained from kepler's law. the periodic time of the planet is an element which can be measured with great accuracy; and once it is known, then the square of the mean distance, and consequently the mean distance itself, is determined. such are the three celebrated laws of planetary motion, which have always been associated with the name of their discoverer. the profound skill by which these laws were elicited from the mass of observations, the intrinsic beauty of the laws themselves, their widespread generality, and the bond of union which they have established between the various members of the solar system, have given them quite an exceptional position in astronomy. as established by kepler, these planetary laws were merely the results of observation. it was found, as a matter of fact, that the planets did move in ellipses, but kepler assigned no reason why they should adopt this curve rather than any other. still less was he able to offer a reason why these bodies should sweep over equal areas in equal times, or why that third law was invariably obeyed. the laws as they came from kepler's hands stood out as three independent truths; thoroughly established, no doubt, but unsupported by any arguments as to why these movements rather than any others should be appropriate for the revolutions of the planets. it was the crowning triumph of the great law of universal gravitation to remove this empirical character from kepler's laws. newton's grand discovery bound together the three isolated laws of kepler into one beautiful doctrine. he showed not only that those laws are true, but he showed why they must be true, and why no other laws could have been true. he proved to demonstration in his immortal work, the "principia," that the explanation of the famous planetary laws was to be sought in the attraction of gravitation. newton set forth that a power of attraction resided in the sun, and as a necessary consequence of that attraction every planet must revolve in an elliptic orbit round the sun, having the sun as one focus; the radius of the planet's orbit must sweep over equal areas in equal times; and in comparing the movements of two planets, it was necessary to have the squares of the periodic times proportional to the cubes of the mean distances. as this is not a mathematical treatise, it will be impossible for us to discuss the proofs which newton has given, and which have commanded the immediate and universal acquiescence of all who have taken the trouble to understand them. we must here confine ourselves only to a very brief and general survey of the subject, which will indicate the character of the reasoning employed, without introducing details of a technical character. let us, in the first place, endeavour to think of a globe freely poised in space, and completely isolated from the influence of every other body in the universe. let us imagine that this globe is set in motion by some impulse which starts it forward on a rapid voyage through the realms of space. when the impulse ceases the globe is in motion, and continues to move onwards. but what will be the path which it pursues? we are so accustomed to see a stone thrown into the air moving in a curved path, that we might naturally think a body projected into free space will also move in a curve. a little consideration will, however, show that the cases are very different. in the realms of free space we find no conception of upwards or downwards; all paths are alike; there is no reason why the body should swerve to the right or to the left; and hence we are led to surmise that in these circumstances a body, once started and freed from all interference, would move in a straight line. it is true that this statement is one which can never be submitted to the test of direct experiment. circumstanced as we are on the surface of the earth, we have no means of isolating a body from external forces. the resistance of the air, as well as friction in various other forms, no less than the gravitation towards the earth itself, interfere with our experiments. a stone thrown along a sheet of ice will be exposed to but little resistance, and in this case we see that the stone will take a straight course along the frozen surface. a stone similarly cast into empty space would pursue a course absolutely rectilinear. this we demonstrate, not by any attempts at an experiment which would necessarily be futile, but by indirect reasoning. the truth of this principle can never for a moment be doubted by one who has duly weighed the arguments which have been produced in its behalf. admitting, then, the rectilinear path of the body, the next question which arises relates to the velocity with which that movement is performed. the stone gliding over the smooth ice on a frozen lake will, as everyone has observed, travel a long distance before it comes to rest. there is but little friction between the ice and the stone, but still even on ice friction is not altogether absent; and as that friction always tends to stop the motion, the stone will at length be brought to rest. in a voyage through the solitudes of space, a body experiences no friction; there is no tendency for the velocity to be reduced, and consequently we believe that the body could journey on for ever with unabated speed. no doubt such a statement seems at variance with our ordinary experience. a sailing ship makes no progress on the sea when the wind dies away. a train will gradually lose its velocity when the steam has been turned off. a humming-top will slowly expend its rotation and come to rest. from such instances it might be plausibly argued that when the force has ceased to act, the motion that the force generated gradually wanes, and ultimately vanishes. but in all these cases it will be found, on reflection, that the decline of the motion is to be attributed to the action of resisting forces. the sailing ship is retarded by the rubbing of the water on its sides; the train is checked by the friction of the wheels, and by the fact that it has to force its way through the air; and the atmospheric resistance is mainly the cause of the stopping of the humming-top, for if the air be withdrawn, by making the experiment in a vacuum, the top will continue to spin for a greatly lengthened period. we are thus led to admit that a body, once projected freely in space and acted upon by no external resistance, will continue to move on for ever in a straight line, and will preserve unabated to the end of time the velocity with which it originally started. this principle is known as the _first law of motion_. let us apply this principle to the important question of the movement of the planets. take, for instance, the case of our earth, and let us discuss the consequences of the first law of motion. we know that the earth is moving each moment with a velocity of about eighteen miles a second, and the first law of motion assures us that if this globe were submitted to no external force, it would for ever pursue a straight track through the universe, nor would it depart from the precise velocity which it possesses at the present moment. but is the earth moving in this manner? obviously not. we have already found that our globe is moving round the sun, and the comprehensive laws of kepler have given to that motion the most perfect distinctness and precision. the consequence is irresistible. the earth cannot be free from external force. some potent influence on our globe must be in ceaseless action. that influence, whatever it may be, constantly deflects the earth from the rectilinear path which it tends to pursue, and constrains it to trace out an ellipse instead of a straight line. the great problem to be solved is now easily stated. there must be some external agent constantly influencing the earth. what is that agent, whence does it proceed, and to what laws is it submitted? nor is the question confined to the earth. mercury and venus, mars, jupiter, and saturn, unmistakably show that, as they are not moving in rectilinear paths, they must be exposed to some force. what is this force which guides the planets in their paths? before the time of newton this question might have been asked in vain. it was the splendid genius of newton which supplied the answer, and thus revolutionised the whole of modern science. the data from which the question is to be answered must be obtained from observation. we have here no problem which can be solved by mere mathematical meditation. mathematics is no doubt a useful, indeed, an indispensable, instrument in the enquiry; but we must not attribute to mathematics a potency which it does not possess. in a case of this kind, all that mathematics can do is to interpret the results obtained by observation. the data from which newton proceeded were the observed phenomena in the movement of the earth and the other planets. those facts had found a succinct expression by the aid of kepler's laws. it was, accordingly, the laws of kepler which newton took as the basis of his labours, and it was for the interpretation of kepler's laws that newton invoked the aid of that celebrated mathematical reasoning which he created. the question is then to be approached in this way: a planet being subject to _some_ external influence, we have to determine what that influence is, from our knowledge that the path of each planet is an ellipse, and that each planet sweeps round the sun over equal areas in equal times. the influence on each planet is what a mathematician would call a force, and a force must have a line of direction. the most simple conception of a force is that of a pull communicated along a rope, and the direction of the rope is in this case the direction of the force. let us imagine that the force exerted on each planet is imparted by an invisible rope. kepler's laws will inform us with regard to the direction of this rope and the intensity of the strain transmitted through it. the mathematical analysis of kepler's laws would be beyond the scope of this volume. we must, therefore, confine ourselves to the results to which they lead, and omit the details of the reasoning. newton first took the law which asserted that the planet moved over equal areas in equal times, and he showed by unimpeachable logic that this at once gave the direction in which the force acted on the planet. he showed that the imaginary rope by which the planet is controlled must be invariably directed towards the sun. in other words, the force exerted on each planet was at all times pointed from the planet towards the sun. it still remained to explain the intensity of the force, and to show how the intensity of that force varied when the planet was at different points of its path. kepler's first law enables this question to be answered. if the planet's path be elliptic, and if the force be always directed towards the sun at one focus of that ellipse, then mathematical analysis obliges us to say that the intensity of the force must vary inversely as the square of the distance from the planet to the sun. the movements of the planets, in conformity with kepler's laws, would thus be accounted for even in their minutest details, if we admit that an attractive power draws the planet towards the sun, and that the intensity of this attraction varies inversely as the square of the distance. can we hesitate to say that such an attraction does exist? we have seen how the earth attracts a falling body; we have seen how the earth's attraction extends to the moon, and explains the revolution of the moon around the earth. we have now learned that the movement of the planets round the sun can also be explained as a consequence of this law of attraction. but the evidence in support of the law of universal gravitation is, in truth, much stronger than any we have yet presented. we shall have occasion to dwell on this matter further on. we shall show not only how the sun attracts the planets, but how the planets attract each other; and we shall find how this mutual attraction of the planets has led to remarkable discoveries, which have elevated the law of gravitation beyond the possibility of doubt. admitting the existence of this law, we can show that the planets must revolve around the sun in elliptic paths with the sun in the common focus. we can show that they must sweep over equal areas in equal times. we can prove that the squares of the periodic times must be proportional to the cubes of their mean distances. still further, we can show how the mysterious movements of comets can be accounted for. by the same great law we can explain the revolutions of the satellites. we can account for the tides, and for other phenomena throughout the solar system. finally, we shall show that when we extend our view beyond the limits of our solar system to the beautiful starry systems scattered through space, we find even there evidence of the great law of universal gravitation. chapter vi. the planet of romance. outline of the subject--is mercury the planet nearest the sun?--transit of an interior planet across the sun--has a transit of vulcan ever been seen?--visibility of planets during a total eclipse of the sun--professor watson's researches in . provided with a general survey of the solar system, and with such an outline of the law of universal gravitation as the last chapter has afforded us, we commence the more detailed examination of the planets and their satellites. we shall begin with the planets nearest to the sun, and then we shall gradually proceed outwards to one planet after another, until we reach the confines of the system. we shall find much to occupy our attention. each planet is itself a globe, and it will be for us to describe as much as is known of it. the satellites by which so many of the planets are accompanied possess many points of interest. the circumstances of their discovery, their sizes, their movements, and their distances must be duly considered. it will also be found that the movements of the planets present much matter for reflection and examination. we shall have occasion to show how the planets mutually disturb each other, and what remarkable consequences have arisen from these influences. we must also occasionally refer to the important problems of celestial measuring and celestial weighing. we must show how the sizes, the weights, and the distances of the various members of our system are to be discovered. the greater part of our task will fortunately lead us over ground which is thoroughly certain, and where the results have been confirmed by frequent observation. it happens, however, that at the very outset of our course we are obliged to deal with observations which are far from certain. the existence of a planet much closer to the sun than those hitherto known has been asserted by competent authority. the question is still unsettled, but the planet cannot at present be found. hence it is that we have called the subject of this chapter, the planet of romance. it had often been thought that mercury, long supposed to be the nearest planet to the sun, was perhaps not really the body entitled to that distinction. mercury revolves round the sun at an average distance of about , , miles. in the interval between it and the sun there might have been one or many other planets. there might have been one revolving at ten million miles, another at fifteen, and so on. but did such planets exist? did even one planet revolve inside the orbit of mercury? there were certain reasons for believing in such a planet. in the movements of mercury indications were perceptible of an influence that it was at one time thought might have been accounted for by the supposition of an interior planet.[ ] but there was necessarily a great difficulty about seeing this object. it must always be close to the sun, and even in the best telescope it is generally impossible to see a star-like point in that position. nor could such a planet be seen after sunset, for under the most favourable conditions it would set almost immediately after the sun, and a like difficulty would make it invisible at sunrise. our ordinary means of observing a planet have therefore completely failed. we are compelled to resort to extraordinary methods if we would seek to settle the great question as to the existence of the intra-mercurial planets. there are at least two lines of observation which might be expected to answer our purpose. an opportunity for the first would arise when it happened that the unknown planet came directly between the earth and the sun. in the diagram (fig. ) we show the sun at the centre; the internal dotted circle denotes the orbit of the unknown planet, which has received the name of vulcan before even its very existence has been at all satisfactorily established. the outer circle denotes the orbit of the earth. as vulcan moves more rapidly than the earth, it will frequently happen that the planet will overtake the earth, so that the three bodies will have the positions represented in the diagram. it would not, however, necessarily follow that vulcan was exactly between the earth and the luminary. the path of the planet may be tilted, so that, as seen from the earth, vulcan would be over or under the sun, according to circumstances. if, however, vulcan really does exist, we might expect that sometimes the three bodies will be directly in line, and this would then give the desired opportunity of making the telescopic discovery of the planet. we should expect on such an occasion to observe the planet as a dark spot, moving slowly across the face of the sun. the two other planets interior to the earth, namely, mercury and venus, are occasionally seen in the act of transit; and there cannot be a doubt that if vulcan exists, its transits across the sun must be more numerous than those of mercury, and far more numerous than those of venus. on the other hand, it may reasonably be anticipated that vulcan is a small globe, and as it will be much more distant from us than mercury at the time of its transit, we could not expect that the transit of the planet of romance would be at all comparable as a spectacle with those of either of the two other bodies we have named. the question arises as to whether telescopic research has ever disclosed anything which can be regarded as a transit of vulcan. on this point it is not possible to speak with any certainty. it has, on more than one occasion, been asserted by observers that a spot has been seen traversing the sun, and from its shape and general appearance they have presumed it to have been an intra-mercurial planet. but a close examination of the circumstances in which such observations have been made has not tended to increase confidence in this presumption. such discoveries have usually been made by persons little familiar with telescopic observations. it is certainly a significant fact that, notwithstanding the diligent scrutiny to which the sun has been subjected during the past century by astronomers who have specially devoted themselves to this branch of research, no telescopic discovery of vulcan on the sun has been announced by any really experienced astronomer. the last announcement of a planet having crossed the sun dates from , and was made by a german amateur, but what he thought to have been a planet was promptly shown to have been a small sun-spot, which had been photographed at greenwich in the course of the daily routine work, and had also been observed at madrid. from an examination of the whole subject, we are inclined to believe that there is not at this moment any reliable telescopic evidence of the transit of an intra-mercurial planet over the face of the central luminary. [illustration: fig. .--the transit of the planet of romance.] but there is still another method by which we might reasonably hope to detect new planets in the vicinity of the sun. this method is, however, but seldom available. it is only possible when the sun is totally eclipsed. when the moon is interposed directly between the earth and the sun, the brightness of day is temporarily exchanged for the gloom of night. if the sky be free from clouds the stars spring forth, and can be seen around the obscured sun. even if a planet were quite close to the luminary it would be visible on such an occasion if its magnitude were comparable with that of mercury. careful preparation is necessary when it is proposed to make a trial of this kind. the danger to be specially avoided is that of confounding the planet with the ordinary stars, which it will probably resemble. the late distinguished american astronomer, professor watson, specially prepared to devote himself to this research during the notable total eclipse in . when the eclipse occurred the light of the sun vanished and the stars burst forth. among them professor watson saw an object which to him seemed to be the long-sought intra-mercurial planet. we should add that this zealous observer saw another object which he at first took to be the star known as zeta in the constellation cancer. when he afterwards found that the recorded place of this object did not agree so well as he expected with the known position of this star, he came to the conclusion that it could not be zeta but must be some other unknown planet. the relative positions of the two objects which he took to be planets agree, however, sufficiently well, considering the difficulties of the observation, with the relative positions of the stars theta and zeta cancri, and it can now hardly be doubted that watson merely saw these two stars. he maintained, however, that he had noticed theta cancri as well as the two planets, but without recording its position. there is, however, a third star, known as cancri, near the same place, and this watson probably mistook for theta. it is necessary to record that vulcan has not been observed, though specially looked for, during the eclipses which have occurred since , and it is accordingly the general belief among astronomers that no planet has yet been detected within the orbit of mercury. chapter vii. mercury. the ancient astronomical discoveries--how mercury was first found--not easily seen--mercury was known from the earliest ages--skill necessary in the discovery--the distinction of mercury from a star--mercury in the east and in the west--the prediction--how to observe mercury--its telescopic appearance--difficulty of observing its appearance--orbit of mercury--velocity of the planet--can there be life on the planet?--changes in its temperature--transit of mercury over the sun--gassendi's observations--rotation of mercury--the weight of mercury. long and glorious is the record of astronomical discovery. the discoveries of modern days have succeeded each other with such rapidity, they have so often dazzled our imaginations with their brilliancy, that we are sometimes apt to think that astronomical discovery is a purely modern product. but no idea could be more fundamentally wrong. while we appreciate to the utmost the achievements of modern times, let us endeavour to do justice to the labours of the astronomers of antiquity. and when we speak of the astronomers of antiquity, let us understand clearly what is meant. the science is now growing so rapidly that each century witnesses a surprising advance; each generation, each decade, each year, has its own rewards for those diligent astronomers by whom the heavens are so carefully scanned. we must, however, project our glance to a remote epoch in time past, if we would view the memorable discovery of mercury. compared with it, the discoveries of newton are to be regarded as very modern achievements; even the announcement of the copernican system of the heavens is itself a recent event in comparison with the detection of this planet now to be discussed. by whom was this great discovery made? let us see if the question can be answered by the examination of astronomical records. at the close of his memorable life copernicus was heard to express his sincere regret that he never enjoyed an opportunity of beholding the planet mercury. he had specially longed to see this body, the movements of which were to such a marked extent illustrative of the theory of the celestial motions which it was his immortal glory to have established, but he had never been successful. mercury is not generally to be seen so easily as are some of the other planets, and it may well have been that the vapours from the immense lagoon at the mouth of the vistula obscured the horizon at frauenburg, where copernicus dwelt, and thus his opportunities of viewing mercury were probably even rarer than they are at other places. the existence of mercury was certainly quite a familiar fact in the time of copernicus, and therefore we must look to some earlier epoch for its discovery. in the scanty astronomical literature of the middle ages we find occasional references to the existence of this object. we can trace observations of mercury through remote centuries to the commencement of our era. records from dates still earlier are not wanting, until at length we come on an observation which has descended to us for more than , years, having been made in the year before the christian era. it is not pretended, however, that this observation records the _discovery_ of the planet. earlier still we find the chief of the astronomers at nineveh alluding to mercury in a report which he made to assurbanipal, the king of assyria. it does not appear in the least degree likely that the discovery was even then a recent one. it may have been that the planet was independently discovered in two or more localities, but all records of such discoveries are totally wanting; and we are ignorant alike of the names of the discoverers, of the nations to which they belonged, and of the epochs at which they lived. although this discovery is of such vast antiquity, although it was made before correct notions were entertained as to the true system of the universe, and, it is needless to add, long before the invention of the telescope, yet it must not be assumed that the detection of mercury was by any means a simple or obvious matter. this will be manifest when we try to conceive the manner in which the discovery must probably have been made. some primæval astronomer, long familiar with the heavens, had learned to recognise the various stars and constellations. experience had impressed upon him the permanence of these objects; he had seen that sirius invariably appeared at the same seasons of the year, and he had noticed how it was placed with regard to orion and the other neighbouring constellations. in the same manner each of the other bright stars was to him a familiar object always to be found in a particular region of the heavens. he saw how the stars rose and set in such a way, that though each star appeared to move, yet the relative positions of the stars were incapable of alteration. no doubt this ancient astronomer was acquainted with venus; he knew the evening star; he knew the morning star; and he may have concluded that venus was a body which oscillated from one side of the sun to the other. we can easily imagine how the discovery of mercury was made in the clear skies over an eastern desert. the sun has set, the brief twilight has almost ceased, when lo, near that part of the horizon where the glow of the setting sun still illuminates the sky, a bright star is seen. the primæval astronomer knows that there is no bright star at this place in the heavens. if the object of his attention be not a star, what, then, can it be? eager to examine this question, the heavens are watched next night, and there again, higher above the horizon, and more brilliant still, is the object seen the night before. each successive night the body gains more and more lustre, until at length it becomes a conspicuous gem. perhaps it will rise still higher and higher; perhaps it will increase till it attains the brilliancy of venus itself. such were the surmises not improbably made by those who first watched this object; but they were not realised. after a few nights of exceptional splendour the lustre of this mysterious orb declines. the planet again draws near the horizon at sunset, until at length it sets so soon after the sun that it has become invisible. is it lost for ever? years may elapse before another opportunity of observing the object after sunset may be available; but then again it will be seen to run through the same series of changes, though, perhaps, under very different circumstances. the greatest height above the horizon and the greatest brightness both vary considerably. long and careful observations must have been made before the primæval astronomer could assure himself that the various appearances might all be attributed to a single body. in the eastern deserts the phenomena of sunrise must have been nearly as familiar as those of sunset, and in the clear skies, at the point where the sunbeams were commencing to dawn above the horizon, a bright star-like point might sometimes be perceived. each successive day this object rose higher and higher above the horizon before the moment of sunrise, and its lustre increased with the distance; then again it would draw in towards the sun, and return for many months to invisibility. such were the data which were presented to the mind of the primitive astronomer. one body was seen after sunset, another body was seen before sunrise. to us it may seem an obvious inference from the observed facts that the two bodies were identical. the inference is a correct one, but it is in no sense an obvious one. long and patient observation established the remarkable law that one of these bodies was never seen until the other had disappeared. hence it was inferred that the phenomena, both at sunrise and at sunset, were due to the same body, which oscillated to and fro about the sun. we can easily imagine that the announcement of the identity of these two objects was one which would have to be carefully tested before it could be accepted. how are the tests to be applied in a case of this kind? there can hardly be a doubt that the most complete and convincing demonstration of scientific truth is found in the fulfilment of prediction. when mercury had been observed for years, a certain regularity in the recurrence of its visibility was noticed. once a periodicity had been fully established, prediction became possible. the time when mercury would be seen after sunset, the time when it would be seen before sunrise, could be foretold with accuracy! when it was found that these predictions were obeyed to the letter--that the planet was always seen when looked for in accordance with the predictions--it was impossible to refuse assent to the hypothesis on which these predictions were based. underlying that hypothesis was the assumption that all the various appearances arose from the oscillations of a single body, and hence the discovery of mercury was established on a basis as firm as the discovery of jupiter or of venus. in the latitudes of the british islands it is generally possible to see mercury some time during the course of the year. it is not practicable to lay down, within reasonable limits, any general rule for finding the dates at which the search should be made; but the student who is determined to see the planet will generally succeed with a little patience. he must first consult an almanac which gives the positions of the body, and select an occasion when mercury is stated to be an evening or a morning star. such an occasion during the spring months is especially suitable, as the elevation of mercury above the horizon is usually greater then than at other seasons; and in the evening twilight, about three-quarters of an hour after sunset, a view of this shy but beautiful object will reward the observer's attention. to those astronomers who are provided with equatorial telescopes such instructions are unnecessary. to enjoy a telescopic view of mercury, we first turn to the nautical almanac, and find the position in which the planet lies. if it happen to be above the horizon, we can at once direct the telescope to the place, and even in broad daylight the planet will very often be seen. the telescopic appearance of mercury is, however, disappointing. though it is much larger than the moon, yet it is sufficiently far off to seem insignificant. there is, however, one feature in a view of this planet which would immediately attract attention. mercury is not usually observed to be a circular object, but more or less crescent-shaped, like a miniature moon. the phases of the planet are also to be accounted for on exactly the same principles as the phases of the moon. mercury is a globe composed, like our earth, of materials possessing in themselves no source of illumination. one hemisphere of the planet must necessarily be turned towards the sun, and this side is accordingly lighted up brilliantly by the solar rays. when we look at mercury we see nothing of the non-illuminated side, and the crescent is due to the foreshortened view which we obtain of the illuminated part. the planet is such a small object that, in the glitter of the naked-eye view, the _shape_ of the luminous body cannot be defined. indeed, even in the much larger crescent of venus, the aid of the telescope has to be invoked before the characteristic form can be observed. beyond, however, the fact that mercury is a crescent, and that it undergoes varying phases in correspondence with the changes in its relative position to the earth and the sun, we cannot see much of the planet. it is too small and too bright to admit of easy delineation of details on its surface. no doubt attempts have been made, and observations have been recorded, as to certain very faint and indistinct markings on the planet, but such statements must be received with great hesitation. [illustration: fig. .--the movement of mercury, showing the variations in phase and in apparent size.] [illustration: fig. .--mercury as a crescent.] the facts which have been thoroughly established with regard to mercury are mainly numerical statements as to the path it describes around the sun. the time taken by the planet to complete one of its revolutions is eighty-eight days nearly. the average distance from the sun is about , , miles, and the mean velocity with which the body moves is over twenty-nine miles a second. we have already alluded to the most characteristic and remarkable feature of the orbit of mercury. that orbit differs from the paths of all the other large planets by its much greater departure from the circular form. in the majority of cases the planetary orbits are so little elliptic that a diagram of the orbit drawn accurately to scale would not be perceived to differ from a circle unless careful measurements were made. in the case of mercury the circumstances are different. the elliptic form of the path would be quite unmistakable by the most casual observer. the distance from the sun to the planet fluctuates between very considerable limits. the lowest value it can attain is about , , miles; the highest value is about , , miles. in accordance with kepler's second law, the velocity of the planet must exhibit corresponding changes. it must sweep rapidly around that part of his path near the sun, and more slowly round the remote parts of his path. the greatest velocity is about thirty-five miles a second, and the least is twenty-three miles a second. for an adequate conception of the movements of mercury we ought not to dissociate the velocity from the true dimensions of the body by which it is performed. no doubt a speed of twenty-nine miles a second is enormous when compared with the velocities with which daily life makes us familiar. the speed of the planet is not less than a hundred times as great as the velocity of the rifle bullet. but when we compare the sizes of the bodies with their velocities, the velocity of mercury seems relatively much less than that of the bullet. a rifle bullet traverses a distance equal to its own diameter many thousands of times in a second. but even though mercury is moving so much faster, yet the dimensions of the planet are so considerable that a period of two minutes will be required for it to move through a distance equal to its diameter. viewing the globe of the planet as a whole, the velocity of its movement is but a stately and dignified progress appropriate to its dimensions. as we can learn little or nothing of the true surface of mercury, it is utterly impossible for us to say whether life can exist on the surface of that planet. we may, however, reasonably conclude that there cannot be life on mercury in any respect analogous to the life which we know on the earth. the heat of the sun and the light of the sun beat down on mercury with an intensity many times greater than that which we experience. when this planet is at its utmost distance from the sun the intensity of solar radiation is even then more than four times greater than the greatest heat which ever reaches the earth. but when mercury, in the course of its remarkable changes of distance, draws in to the warmest part of its orbit, it is exposed to a terrific scorching. the intensity of the sun's heat must then be not less than nine times as great as the greatest radiation to which we are exposed. these tremendous climatic changes succeed each other much more rapidly than do the variations of our seasons. on mercury the interval between midsummer and midwinter is only forty-four days, while the whole year is only eighty-eight days. such rapid variations in solar heat must in themselves exercise a profound effect on the habitability of mercury. mr. ledger well remarks, in his interesting work,[ ] that if there be inhabitants on mercury the notions of "perihelion" and "aphelion," which are here often regarded as expressing ideas of an intricate or recondite character, must on the surface of that planet be familiar to everybody. the words imply "near the sun," and "away from the sun;" but we do not associate these expressions with any obvious phenomena, because the changes in the distance of the earth from the sun are inconsiderable. but on mercury, where in six weeks the sun rises to more than double his apparent size, and gives more than double the quantity of light and of heat, such changes as are signified by perihelion and aphelion embody ideas obviously and intimately connected with the whole economy of the planet. it is nevertheless rash to found any inferences as to climate merely upon the proximity or the remoteness of the sun. climate depends upon other matters besides the sun's distance. the atmosphere surrounding the earth has a profound influence on heat and cold, and if mercury have an atmosphere--as has often been supposed--its climate may be thereby modified to any necessary extent. it seems, however, hardly possible to suppose that any atmosphere could form an adequate protection for the inhabitants from the violent and rapid fluctuations of solar radiation. all we can say is, that the problem of life in mercury belongs to the class of unsolved, and perhaps unsolvable, mysteries. it was in the year that kepler made an important announcement as to impending astronomical events. he had been studying profoundly the movements of the planets; and from his study of the past he had ventured to predict the future. kepler announced that in the year the planets venus and mercury would both make a transit across the sun, and he assigned the dates to be november th for mercury, and december th for venus. this was at the time a very remarkable prediction. we are so accustomed to turn to our almanacs and learn from them all the astronomical phenomena which are anticipated during the year, that we are apt to forget how in early times this was impossible. it has only been by slow degrees that astronomy has been rendered so perfect as to enable us to foretell, with accuracy, the occurrence of the more delicate phenomena. the prediction of those transits by kepler, some years before they occurred, was justly regarded at the time as a most remarkable achievement. the illustrious gassendi prepared to apply the test of actual observation to the announcements of kepler. we can now assign the time of the transit accurately to within a few minutes, but in those early attempts equal precision was not practicable. gassendi considered it necessary to commence watching for the transit of mercury two whole days before the time indicated by kepler, and he had arranged an ingenious plan for making his observations. the light of the sun was admitted into a darkened room through a hole in the shutter, and an image of the sun was formed on a white screen by a lens. this is, indeed, an admirable and a very pleasing way of studying the surface of the sun, and even at the present day, with our best telescopes, one of the methods of viewing our luminary is founded on the same principle. gassendi commenced his watch on the th of november, and carefully studied the sun's image at every available opportunity. it was not, however, until five hours after the time assigned by kepler that the transit of mercury actually commenced. gassendi's preparations had been made with all the resources which he could command, but these resources seem very imperfect when compared with the appliances of our modern observatories. he was anxious to note the time when the planet appeared, and for this purpose he had stationed an assistant in the room beneath, who was to observe the altitude of the sun at the moment indicated by gassendi. the signal to the assistant was to be conveyed by a very primitive apparatus. gassendi was to stamp on the floor when the critical moment had arrived. in spite of the long delay, which exhausted the patience of the assistant, some valuable observations were obtained, and thus the first passage of a planet across the sun was observed. the transits of mercury are not rare phenomena (there have been thirteen of them during the nineteenth century), and they are chiefly of importance on account of the accuracy which their observation infuses into our calculations of the movements of the planet. it has often been hoped that the opportunities afforded by a transit would be available for procuring information as to the physical character of the globe of mercury, but these hopes have not been realised. spectroscopic observations of mercury are but scanty. they seem to indicate that water vapour is a probable constituent in the atmosphere of mercury, as it is in our own. a distinguished italian astronomer, professor schiaparelli, some years ago announced a remarkable discovery with respect to the rotation of the planet mercury. he found that the planet rotates on its axis in the same period as it revolves around the sun. the practical consequence of the identity between these two periods is that mercury always turns the same face to the sun. if our earth were to rotate in a similar fashion, then the hemisphere directed to the sun would enjoy eternal day, while the opposite hemisphere would be relegated to perpetual night. according to this discovery, mercury revolves around the sun in the same way as the moon revolves around the earth. as the velocity with which mercury travels round the sun is very variable, owing to the highly elliptic shape of its orbit, while the rotation about its axis is performed with uniform speed, it follows that rather more than a hemisphere (about five-eighths of the surface) enjoys more or less the light of the sun in the course of a mercurial year. this important discovery of schiaparelli has lately been confirmed by an american astronomer, mr. lowell, of arizona, u.s.a., who observed the planet under very favourable conditions with a refractor of twenty-four inches aperture. he has detected on the globe of mercury certain narrow, dark lines, the very slow shifting of which points to a period of rotation about its axis exactly coincident with the period of revolution round the sun. the same observer shows that the axis of rotation of mercury is perpendicular to the plane of the orbit. mr. lowell has perceived no sign of clouds or obscurations, and indeed no indication of any atmospheric envelope; the surface of mercury is colourless, "a geography in black and white." we may assert that, there is a strong _à priori_ probability in favour of the reality of schiaparelli's discovery. mercury, being one of the planets devoid of a moon, will be solely influenced by the sun in so far as tidal phenomena are concerned. owing, moreover, to the proximity of mercury to the sun, the solar tides on that planet possess an especial vehemence. as the tendency of tides is to make mercury present a constant face to the sun, there need be little hesitation in accepting testimony that tides have wrought exactly the result that we know they were competent to perform. here we take leave of the planet mercury--an interesting and beautiful object, which stimulates our intellectual curiosity, while at the same time it eludes our attempts to make a closer acquaintance. there is, however, one point of attainable knowledge which we must mention in conclusion. it is a difficult, but not by any means an impossible, task to weigh mercury in the celestial balance, and determine his mass in comparison with the other globes of our system. this is a delicate operation, but it leads us through some of the most interesting paths of astronomical discovery. the weight of the planet, as recently determined by von asten, is about one twenty-fourth part of the weight of the earth, but the result is more uncertain than the determinations of the mass of any of the other larger planets. chapter viii. venus. interest attaching to this planet--the unexpectedness of its appearance--the evening star--visibility in daylight--lighted only by the sun--the phases of venus--why the crescent is not visible to the unaided eye--variations in the apparent size of the planet--the rotation of venus--resemblance of venus to the earth--the transit of venus--why of such especial interest--the scale of the solar system--orbits of the earth and venus not in the same plane--recurrence of the transits in pairs--appearance of venus in transit--transits of and --the early transits of and --the observations of horrocks and crabtree--the announcement of halley--how the track of the planet differs from different places--illustrations of parallax--voyage to otaheite--the result of encke--probable value of the sun's distance--observations at dunsink of the last transit of venus--the question of an atmosphere to venus--other determinations of the sun's distance--statistics about venus. it might, for one reason, have been not inappropriate to have commenced our review of the planetary system by the description of the planet venus. this body is not especially remarkable for its size, for there are other planets hundreds of times larger. the orbit of venus is no doubt larger than that of mercury, but it is much smaller than that of the outer planets. venus has not even the splendid retinue of minor attendants which gives such dignity and such interest to the mighty planets of our system. yet the fact still remains that venus is peerless among the planetary host. we speak not now of celestial bodies only seen in the telescope; we refer to the ordinary observation which detected venus ages before telescopes were invented. who has not been delighted with the view of this glorious object? it is not to be seen at all times. for months together the star of evening is hidden from mortal gaze. its beauties are even enhanced by the caprice and the uncertainty which attend its appearance. we do not say that there is any caprice in the movements of venus, as known to those who diligently consult their almanacs. the movements of the lovely planet are there prescribed with a prosaic detail hardly in harmony with the character usually ascribed to the goddess of love. but to those who do not devote particular attention to the stars, the very unexpectedness of its appearance is one of its greatest charms. venus has not been noticed, not been thought of, for many months. it is a beautifully clear evening; the sun has just set. the lover of nature turns to admire the sunset, as every lover of nature will. in the golden glory of the west a beauteous gem is seen to glitter; it is the evening star--the planet venus. a few weeks later another beautiful sunset is seen, and now the planet is no longer a point low down in the western glow; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. again, a little later, and venus has gained its full brilliancy and splendour. all the heavenly host--even sirius and even jupiter--must pale before the splendid lustre of venus, the unrivalled queen of the firmament. after weeks of splendour, the height of venus at sunset diminishes, and its lustre begins gradually to decline. it sinks to invisibility, and is forgotten by the great majority of mankind; but the capricious goddess has only moved from one side of the sky to the other. ere the sun rises, the morning star will be seen in the east. its splendour gradually augments until it rivals the beauty of the evening star. then again the planet draws near to the sun, and remains lost to view for many months, until the same cycle of changes recommences, after an interval of a year and seven months. when venus is at its brightest it can be easily seen in broad daylight with the unaided eye. this striking spectacle proclaims in an unmistakable manner the unrivalled supremacy of this planet as compared with its fellow-planets and with the fixed stars. indeed, at this time venus is from forty to sixty times more brilliant than any stellar object in the northern heavens. the beautiful evening star is often such a very conspicuous object that it may seem difficult at first to realise that the body is not self-luminous. yet it is impossible to doubt that the planet is really only a dark globe, and to that extent resembles our own earth. the brilliance of the planet is not so very much greater than that of the earth on a sunshiny day. the splendour of venus entirely arises from the reflected light of the sun, in the manner already explained with respect to the moon. we cannot distinguish the characteristic crescent shape of the planet with the unaided eye, which merely shows a brilliant point too small to possess sensible form. this is to be explained on physiological grounds. the optical contrivances in the eye form an image of the planet on the retina which is necessarily very small. even when venus is nearest to the earth the diameter of the planet subtends an angle not much more than one minute of arc. on the delicate membrane a picture of venus is thus drawn about one six-thousandth part of an inch in diameter. great as may be the delicacy of the retina, it is not adequate to the perception of form in a picture so minute. the nervous structure, which has been described as the source of vision, forms too coarse a canvas for the reception of the details of this tiny picture. hence it is that to the unaided eye the brilliant venus appears merely as a bright spot. ordinary vision cannot tell what shape it has; still less can it reveal the true beauty of the crescent. if the diameter of venus were several times as great as it actually is; were this body, for instance, as large as jupiter or some of the other great planets, then its crescent could be readily discerned by the unaided eye. it is curious to speculate on what might have been the history of astronomy had venus only been as large as jupiter. were everyone able to see the crescent form without a telescope, it would then have been an elementary and almost obvious truth that venus must be a dark body revolving round the sun. the analogy between venus and our earth would have been at once perceived; and the doctrine which was left to be discovered by copernicus in comparatively modern times might not improbably have been handed down to us with the other discoveries which have come from the ancient nations of the east. [illustration: fig. . venus, may th, .] perhaps the most perfect drawing of venus that has been hitherto obtained is that made (fig. ) by professor e.e. barnard, on th may, , with a -inch equatorial, at the lick observatory, which for this purpose and on this occasion professor barnard found to be superior to the -inch. the markings shown seem undoubtedly to exist on the planet, and in professor barnard writes: "the circumstances under which this drawing was made are memorable with me, for i never afterwards had such perfect conditions to observe venus." in fig. we show three views of venus under different aspects. the planet is so much closer to the earth when the crescent is seen, that it appears to be part of a much larger circle than that made by venus when more nearly full. this drawing shows the different aspects of the globe in their true relative proportions. it is very difficult to perceive distinctly any markings on the brilliantly lighted surface. sometimes observers have seen spots or other features, and occasionally the pointed extremities of the horns have been irregular, as if to show that the surface of venus is not smooth. some observers report having seen white spots at the poles of venus, in some degree resembling the more conspicuous features of the same character to be seen on mars. [illustration: fig. .--different aspects of venus in the telescope.] as it is so very difficult to see any markings on venus, we are hardly yet able to give a definite answer to the important question as to the period of rotation of this planet round its axis. various observers during the last two hundred years have from very insufficient data concluded that venus rotated in about twenty-three hours. schiaparelli, of milan, turned his attention to this planet in and noticed a dark shade and two bright spots, all situated not far from the southern end of the crescent. this most painstaking astronomer watched these markings for three months, and found that there was no change perceptible in the position which they occupied. this was particularly the case when he continued his watch for some consecutive hours. this fact seemed to show conclusively that venus could not rotate in twenty-three hours nor in any other short period. week after week the spots remained unaltered, until schiaparelli felt convinced that his observations could only be reconciled with a period of rotation between six and nine months. he naturally concluded that the period was days--that is to say, the period which venus takes to complete one revolution round the sun; in other words, venus always turns the same face to the sun. this remarkable result was confirmed by observations made at nice; but it has been vigorously assailed by several observers, who maintain that their own drawings can only agree with a period about equal to that of the rotation of our own earth. schiaparelli's result is, however, well supported by the letters of mr. lowell. he has published a number of drawings of venus made with his -inch refractor, and he finds that the rotation is performed in the same time as the planet's orbital revolution, the axis of rotation being perpendicular to the plane of the orbit. the markings seen by mr. lowell were long and streaky, and they were always visible whenever his own atmospheric conditions were fairly good. we have seen that the moon revolves so as to keep the same face always turned towards the earth. we have now seen that the planets venus and mercury each appear to revolve in such a way that they keep the same face towards the sun. all these phenomena are of profound interest in the higher departments of astronomical research. they are not mere coincidences. they arise from the operation of the tides, in a manner that will be explained in a later chapter. it happens that our earth and venus are very nearly equal in bulk. the difference is hardly perceptible, but the earth has a diameter a few miles greater than that of venus. there are indications of the existence of an atmosphere around venus, and the evidence of the spectroscope shows that water vapour is there present. if there be oxygen in the atmosphere of venus, then it would seem possible that there might be life on that globe not essentially different in character from some forms of life on the earth. no doubt the sun's heat on venus is greatly in excess of the sun's heat with which we are acquainted, but this is not an insuperable difficulty. we see at present on the earth, life in very hot regions and life in very cold regions. indeed, with each approach to the equator we find life more and more exuberant; so that, if water be present on the surface of venus and if oxygen be a constituent of its atmosphere, we might expect to find in that planet a luxuriant tropical life, of a kind perhaps analogous in some respects to life on the earth. in our account of the planet mercury, as well as in the brief description of the hypothetical planet vulcan, it has been necessary to allude to the phenomena presented by the transit of a planet over the face of the sun. such an event is always of interest to astronomers, and especially so in the case of venus. we have in recent years had the opportunity of witnessing two of these rare occurrences. it is perhaps not too much to assert that the transits of and have received a degree of attention never before accorded to any astronomical phenomenon. the transit of venus cannot be described as a very striking or beautiful spectacle. it is not nearly so fine a sight as a great comet or a shower of shooting stars. why is it, then, that it is regarded as of so much scientific importance? it is because the phenomenon helps us to solve one of the greatest problems which has ever engaged the mind of man. by the transit of venus we may determine the scale on which our solar system is constructed. truly this is a noble problem. let us dwell upon it for a moment. in the centre of our system we have the sun--a majestic globe more than a million times as large as the earth. circling round the sun we have the planets, of which our earth is but one. there are hundreds of small planets. there are a few comparable with our earth; there are others vastly surpassing the earth. besides the planets there are other bodies in our system. many of the planets are accompanied by systems of revolving moons. there are hundreds, perhaps thousands, of comets. each member of this stupendous host moves in a prescribed orbit around the sun, and collectively they form the solar system. it is comparatively easy to learn the proportions of this system, to measure the relative distances of the planets from the sun, and even the relative sizes of the planets themselves. peculiar difficulties are, however, experienced when we seek to ascertain the actual _size_ of the system as well as its shape. it is this latter question which the transit of venus offers us a method of solving. look, for instance, at an ordinary map of europe. we see the various countries laid down with precision; we can tell the courses of the rivers; we can say that france is larger than england, and russia larger than france; but no matter how perfectly the map be constructed, something else is necessary before we can have a complete conception of the dimensions of the country. we must know _the scale on which the map is drawn_. the map contains a reference line with certain marks upon it. this line is to give the scale of the map. its duty is to tell us that an inch on the map corresponds with so many miles on the actual surface. unless it be supplemented by the scale, the map would be quite useless for many purposes. suppose that we consulted it in order to choose a route from london to vienna, we can see at once the direction to be taken and the various towns and countries to be traversed; but unless we refer to the little scale in the corner, the map will not tell how many miles long the journey is to be. a map of the solar system can be readily constructed. we can draw on it the orbits of some of the planets and of their satellites, and we can include many of the comets. we can assign to the planets and to the orbits their proper proportions. but to render the map quite efficient something more is necessary. we must have the scale which is to tell us how many millions of miles on the heavens correspond to one inch of the map. it is at this point we encounter a difficulty. there are, however, several ways of solving the problem, though they are all difficult and laborious. the most celebrated method (though far from the best) is that presented on an occasion of the transit of venus. herein, then, lies the importance of this rare event. it is one of the best-known means of finding the actual scale on which our system is constructed. observe the full importance of the problem. once the scale has been determined, then all is known. we know the size of the sun; we know his distance; we know the bulk of jupiter, and the distances at which his satellites revolve; we know the dimensions of the comets, and the number of miles to which they recede in their wanderings; we know the velocity of the shooting stars; and we learn the important lesson that our earth is but one of the minor members of the sun's family. as the path of venus lies inside that of the earth, and as venus moves more quickly than the earth, it follows that the earth is frequently passed by the planet, and just at the critical moment it will sometimes happen that the earth, the planet, and the sun lie in the same straight line. we can then see venus on the face of the sun, and this is the phenomenon which we call the _transit of venus_. it is, indeed, quite plain that if the three bodies were exactly in a line, an observer on the earth, looking at the planet, would see it brought out vividly against the brilliant background of the sun. considering that the earth is overtaken by venus once every nineteen months, it might seem that the transits of the planet should occur with corresponding frequency. this is not the case; the transit of venus is an exceedingly rare occurrence, and a hundred years or more will often elapse without a single one taking place. the rarity of these phenomena arises from the fact that the path of the planet is inclined to the plane of the earth's orbit; so that for half of its path venus is above the plane of the earth's orbit, and in the other half it is below. when venus overtakes the earth, the line from the earth to venus will therefore usually pass over or under the sun. if, however, it should happen that venus overtakes the earth at or near either of the points in which the plane of the orbit of venus passes through that of the earth, then the three bodies will be in line, and a transit of venus will be the consequence. the rarity of the occurrence of a transit need no longer be a mystery. the earth passes through one of the critical parts every december, and through the other every june. if it happens that the conjunction of venus occurs on, or close to, june th or december th, then a transit of venus will occur at that conjunction, but in no other circumstances. the most remarkable law with reference to the repetition of the phenomenon is the well-known eight-year interval. the transits may be all grouped together into pairs, the two transits of any single pair being separated by an interval of eight years. for instance, a transit of venus took place in , and again in . no further transits occurred until those witnessed in and in . then, again, comes a long interval, for another transit will not occur until , but it will be followed by another in . this arrangement of the transits in pairs admits of a very simple explanation. it happens that the periodic time of venus bears a remarkable relation to the periodic time of the earth. the planet accomplishes thirteen revolutions around the sun in very nearly the same time that the earth requires for eight revolutions. if, therefore, venus and the earth were in line with the sun in , then in eight years more the earth will again be found in the same place; and so will venus, for it has just been able to accomplish thirteen revolutions. a transit of venus having occurred on the first occasion, a transit must also occur on the second. it is not, however, to be supposed that every eight years the planets will again resume the same position with sufficient precision for a regular eight-year transit interval. it is only approximately true that thirteen revolutions of venus are coincident with eight revolutions of the earth. each recurrence of conjunction takes place at a slightly different position of the planets, so that when the two planets came together again in the year the point of conjunction was so far removed from the critical point that the line from the earth to venus did not intersect the sun, and thus, although venus passed very near the sun, yet no transit took place. [illustration: fig. .--venus on the sun at the transit of .] fig. represents the transit of venus in . it is taken from a photograph obtained, during the occurrence, by m. janssen. his telescope was directed towards the sun during the eventful minutes while it lasted, and thus an image of the sun was depicted on the photographic plate placed in the telescope. the lighter circle represents the disc of the sun. on that disc we see the round, sharp image of venus, showing the characteristic appearance of the planet during the progress of the transit. the only other features to be noticed are a few of the solar spots, rather dimly shown, and a network of lines which were marked on a glass plate across the field of view of the telescope to facilitate measurements. the adjoining sketch (fig. ) exhibits the course which the planet pursued in its passage across the sun on the two occasions in and . our generation has had the good fortune to witness the two occurrences indicated on this picture. the white circle denotes the disc of the sun; the planet encroaches on the white surface, and at first is like a bite out of the sun's margin. gradually the black spot steals in front of the sun, until, after nearly half an hour, the black disc is entirely visible. slowly the planet wends its way across, followed by hundreds of telescopes from every accessible part of the globe whence the phenomenon is visible, until at length, in the course of a few hours, it emerges at the other side. it will be useful to take a brief retrospect of the different transits of venus of which there is any historical record. they are not numerous. hundreds of such phenomena have occurred since man first came on the earth. it was not until the approach of the year that attention began to be directed to the matter, though the transit which undoubtedly occurred in that year was not noticed by anyone. the success of gassendi in observing the transit of mercury, to which we have referred in the last chapter, led him to hope that he would be equally fortunate in observing the transit of venus, which kepler had also foretold. gassendi looked at the sun on the th, th, and th december. he looked at it again on the th, but he saw no sign of the planet. we now know the reason. the transit of venus took place during the night, between the th and the th, and must therefore have been invisible to european observers. kepler had not noticed that another transit would occur in . this discovery was made by another astronomer, and it is the one with which the history of the subject may be said to commence. it was the first occasion on which the phenomenon was ever actually witnessed; nor was it then seen by many. so far as is known, it was witnessed by only two persons. [illustration: fig. .--the path of venus across the sun in the transits of and .] a young and ardent english astronomer, named horrocks, had undertaken some computations about the motions of venus. he made the discovery that the transit of venus would be repeated in , and he prepared to verify the fact. the sun rose bright on the morning of the day--which happened to be a sunday. the clerical profession, which horrocks followed, here came into collision with his desires as an astronomer. he tells us that at nine he was called away by business of the highest importance--referring, no doubt, to his official duties; but the service was quickly performed, and a little before ten he was again on the watch, only to find the brilliant face of the sun without any unusual feature. it was marked with a spot, but nothing that could be mistaken for a planet. again, at noon, came an interruption; he went to church, but he was back by one. nor were these the only impediments to his observations. the sun was also more or less clouded over during part of the day. however, at a quarter past three in the afternoon his clerical work was over; the clouds had dispersed, and he once more resumed his observations. to his intense delight he then saw on the sun the round, dark spot, which was at once identified as the planet venus. the observations could not last long; it was the depth of winter, and the sun was rapidly setting. only half an hour was available, but he had made such careful preparations beforehand that it sufficed to enable him to secure some valuable measurements. horrocks had previously acquainted his friend, william crabtree, with the impending occurrence. crabtree was therefore on the watch, and succeeded in seeing the transit; a striking picture of crabtree's famous observation is shown in one of the beautiful frescoes in the town hall at manchester. but to no one else had horrocks communicated the intelligence; as he says, "i hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, rather preferring their hawks and hounds, to say no worse; and although england is not without votaries of astronomy, with some of whom i am acquainted, i was unable to convey to them the agreeable tidings, having myself had so little notice." it was not till long afterwards that the full importance of the transit of venus was appreciated. nearly a century had rolled away when the great astronomer, halley ( - ), drew attention to the subject. the next transit was to occur in , and forty-five years before that event halley explained his celebrated method of finding the distance of the sun by means of the transit of venus.[ ] he was then a man sixty years of age; he could have no expectation that he would live to witness the event; but in noble language he commends the problem to the notice of the learned, and thus addresses the royal society of london:--"and this is what i am now desirous to lay before this illustrious society, which i foretell will continue for ages, that i may explain beforehand to young astronomers, who may, perhaps, live to observe these things, a method by which the immense distance of the sun may be truly obtained.... i recommend it, therefore, again and again to those curious astronomers who, when i am dead, will have an opportunity of observing these things, that they would remember this my admonition, and diligently apply themselves with all their might in making the observations, and i earnestly wish them all imaginable success--in the first place, that they may not by the unseasonable obscurity of a cloudy sky be deprived of this most desirable sight, and then that, having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory." halley lived to a good old age, but he died nineteen years before the transit occurred. the student of astronomy who desires to learn how the transit of venus will tell the distance from the sun must prepare to encounter a geometrical problem of no little complexity. we cannot give to the subject the detail that would be requisite for a full explanation. all we can attempt is to render a general account of the method, sufficient to enable the reader to see that the transit of venus really does contain all the elements necessary for the solution of the problem. we must first explain clearly the conception which is known to astronomers by the name of _parallax_; for it is by parallax that the distance of the sun, or, indeed, the distance of any other celestial body, must be determined. let us take a simple illustration. stand near a window whence you can look at buildings, or the trees, the clouds, or any distant objects. place on the glass a thin strip of paper vertically in the middle of one of the panes. close the right eye, and note with the left eye the position of the strip of paper relatively to the objects in the background. then, while still remaining in the same position, close the left eye and again observe the position of the strip of paper with the right eye. you will find that the position of the paper on the background has changed. as i sit in my study and look out of the window i see a strip of paper, with my right eye, in front of a certain bough on a tree a couple of hundred yards away; with my left eye the paper is no longer in front of that bough, it has moved to a position near the outline of the tree. this apparent displacement of the strip of paper, relatively to the distant background, is what is called parallax. move closer to the window, and repeat the observation, and you find that _the apparent displacement of the strip increases_. move away from the window, and the displacement decreases. move to the other side of the room, the displacement is much less, though probably still visible. we thus see that the change in the apparent place of the strip of paper, as viewed with the right eye or the left eye, varies in amount as the distance changes; but it varies in the opposite way to the distance, for as either becomes greater the other becomes less. we can thus associate with each particular distance a corresponding particular displacement. from this it will be easy to infer that if we have the means of measuring the amount of displacement, then we have the means of calculating the distance from the observer to the window. it is this principle, applied on a gigantic scale, which enables us to measure the distances of the heavenly bodies. look, for instance, at the planet venus; let this correspond to the strip of paper, and let the sun, on which venus is seen in the act of transit, be the background. instead of the two eyes of the observer, we now place two observatories in distant regions of the earth; we look at venus from one observatory, we look at it from the other; we measure the amount of the displacement, and from that we calculate the distance of the planet. all depends, then, on the means which we have of measuring the displacement of venus as viewed from the two different stations. there are various ways of accomplishing this, but the most simple is that originally proposed by halley. from the observatory at a venus seems to pursue the upper of the two tracks shown in the adjoining figure (fig. ). from the observatory at b it follows the lower track, and it is for us to measure the distance between the two tracks. this can be accomplished in several ways. suppose the observer at a notes the time that venus has occupied in crossing the disc, and that similar observations be made at b. as the track seen from b is the larger, it must follow that the time observed at b will be greater than that at a. when the observations from the different hemispheres are compared, the _times_ observed will enable the lengths of the tracks to be calculated. the lengths being known, their places on the circular disc of the sun are determined, and hence the amount of displacement of venus in transit is ascertained. thus it is that the distance of venus is measured, and the scale of the solar system is known. [illustration: fig. .--to illustrate the observation of the transit of venus from two localities, a and b, on the earth.] the two transits to which halley's memorable researches referred occurred in the years and . the results of the first were not very successful, in spite of the arduous labours of those who undertook the observations. the transit of is of particular interest, not only for the determination of the sun's distance, but also because it gave rise to the first of the celebrated voyages of captain cook. it was to see the transit of venus that captain cook was commissioned to sail to otaheite, and there, on the rd of june, on a splendid day in that exquisite climate, the phenomenon was carefully observed and measured by different observers. simultaneously with these observations others were obtained in europe and elsewhere, and from the combination of all the observations an approximate knowledge of the sun's distance was gained. the most complete discussion of these observations did not, however, take place for some time. it was not until the year that the illustrious encke computed the distance of the sun, and gave as the definite result , , miles. for many years this number was invariably adopted, and many of the present generation will remember how they were taught in their school-days that the sun was , , miles away. at length doubts began to be whispered as to the accuracy of this result. the doubts arose in different quarters, and were presented with different degrees of importance; but they all pointed in one direction, they all indicated that the distance of the sun was not really so great as the result which encke had obtained. it must be remembered that there are several ways of finding the distance of the sun, and it will be our duty to allude to some other methods later on. it has been ascertained that the result obtained by encke from the observations made in and , with instruments inferior to our modern ones, was too great, and that the distance of the sun may probably be now stated at , , miles. i venture to record our personal experience of the last transit of venus, which we had the good fortune to view from dunsink observatory on the afternoon of the th of december, . the morning of the eventful day appeared to be about as unfavourable for a grand astronomical spectacle as could well be imagined. snow, a couple of inches thick, covered the ground, and more was falling, with but little intermission, all the forenoon. it seemed almost hopeless that a view of the phenomenon could be obtained from that observatory; but it is well in such cases to bear in mind the injunction given to the observers on a celebrated eclipse expedition. they were instructed, no matter what the day should be like, that they were to make all their preparations precisely as they would have done were the sun shining with undimmed splendour. by this advice no doubt many observers have profited; and we acted upon it with very considerable success. there were at that time at the observatory two equatorials, one of them an old, but tolerably good, instrument, of about six inches aperture; the other the great south equatorial, of twelve inches aperture, already referred to. at eleven o'clock the day looked worse than ever; but we at once proceeded to make all ready. i stationed mr. rambaut at the small equatorial, while i myself took charge of the south instrument. the snow was still falling when the domes were opened; but, according to our prearranged scheme, the telescopes were directed, not indeed upon the sun, but to the place where we knew the sun was, and the clockwork was set in motion which carried round the telescopes, still constantly pointing towards the invisible sun. the predicted time of the transit had not yet arrived. the eye-piece employed on the south equatorial must also receive a brief notice. it will, of course, be obvious that the full glare of the sun has to be greatly mitigated before the eye can view it with impunity. the light from the sun falls upon a piece of transparent glass inclined at a certain angle, and the chief portion of the sun's heat, as well as a certain amount of its light, pass through the glass and are lost. a certain fraction of the light is, however, reflected from the glass, and enters the eye-piece. this light is already much reduced in intensity, but it undergoes as much further reduction as we please by an ingenious contrivance. the glass which reflects the light does so at what is called the polarising angle, and between the eye-piece and the eye is a plate of tourmaline. this plate of tourmaline can be turned round by the observer. in one position it hardly interferes with the polarised light at all, while in the position at right angles thereto it cuts off nearly the whole of it. by simply adjusting the position of the tourmaline, the observer has it in his power to render the image of any brightness that may be convenient, and thus the observations of the sun can be conducted with the appropriate degree of illumination. but such appliances seemed on this occasion to be a mere mockery. the tourmaline was all ready, but up to one o'clock not a trace of the sun could be seen. shortly after one o'clock, however, we noticed that the day was getting lighter; and, on looking to the north, whence the wind and the snow were coming, we saw, to our inexpressible delight, that the clouds were clearing. at length, the sky towards the south began to improve, and at last, as the critical moment approached, we could detect the spot where the sun was becoming visible. but the predicted moment arrived and passed, and still the sun had not broken through the clouds, though every moment the certainty that it would do so became more apparent. the external contact was therefore missed. we tried to console ourselves by the reflection that this was not, after all, a very important phase, and hoped that the internal contact would be more successful. at length the struggling beams pierced the obstruction, and i saw the round, sharp disc of the sun in the finder, and eagerly glanced at the point on which attention was concentrated. some minutes had now elapsed since the predicted moment of first contact, and, to my delight, i saw the small notch in the margin of the sun showing that the transit had commenced, and that the planet was then one-third on the sun. but the critical moment had not yet arrived. by the expression "first internal contact" we are to understand the moment when the planet has completely entered _on_ the sun. this first contact was timed to occur twenty-one minutes later than the external contact already referred to. but the clouds again disappointed our hope of seeing the internal contact. while steadily looking at the exquisitely beautiful sight of the gradual advance of the planet, i became aware that there were other objects besides venus between me and the sun. they were the snowflakes, which again began to fall rapidly. i must admit the phenomenon was singularly beautiful. the telescopic effect of a snowstorm with the sun as a background i had never before seen. it reminded me of the golden rain which is sometimes seen falling from a flight of sky-rockets during pyrotechnic displays; i would gladly have dispensed with the spectacle, for it necessarily followed that the sun and venus again disappeared from view. the clouds gathered, the snowstorm descended as heavily as ever, and we hardly dared to hope that we should see anything more; hr. min. came and passed, the first internal contact was over, and venus had fully entered on the sun. we had only obtained a brief view, and we had not yet been able to make any measurements or other observations that could be of service. still, to have seen even a part of a transit of venus is an event to remember for a lifetime, and we felt more delight than can be easily expressed at even this slight gleam of success. but better things were in store. my assistant came over with the report that he had also been successful in seeing venus in the same phase as i had. we both resumed our posts, and at half-past two the clouds began to disperse, and the prospect of seeing the sun began to improve. it was now no question of the observations of contact. venus by this time was well on the sun, and we therefore prepared to make observations with the micrometer attached to the eye-piece. the clouds at length dispersed, and at this time venus had so completely entered on the sun that the distance from the edge of the planet to the edge of the sun was about twice the diameter of the planet. we measured the distance of the inner edge of venus from the nearest limb of the sun. these observations were repeated as frequently as possible, but it should be added that they were only made with very considerable difficulty. the sun was now very low, and the edges of the sun and of venus were by no means of that steady character which is suitable for micrometrical measurement. the margin of the luminary was quivering, and venus, though no doubt it was sometimes circular, was very often distorted to such a degree as to make the measures very uncertain. we succeeded in obtaining sixteen measures altogether; but the sun was now getting low, the clouds began again to interfere, and we saw that the pursuit of the transit must be left to the thousands of astronomers in happier climes who had been eagerly awaiting it. but before the phenomena had ceased i spared a few minutes from the somewhat mechanical work at the micrometer to take a view of the transit in the more picturesque form which the large field of the finder presented. the sun was already beginning to put on the ruddy hues of sunset, and there, far in on its face, was the sharp, round, black disc of venus. it was then easy to sympathise with the supreme joy of horrocks, when, in , he for the first time witnessed this spectacle. the intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of venus helps us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the june of a.d. . the occasion of a transit of venus also affords an opportunity of studying the physical nature of the planet, and we may here briefly indicate the results that have been obtained. in the first place, a transit will throw some light on the question as to whether venus is accompanied by a satellite. if venus were attended by a small body in close proximity, it would be conceivable that in ordinary circumstances the brilliancy of the planet would obliterate the feeble beam of rays from the minute companion, and thus the satellite would remain undiscovered. it was therefore a matter of great interest to scrutinise the vicinity of the planet while in the act of transit. if a satellite existed--and the existence of one or more of such bodies has often been suspected--then it would be capable of detection against the brilliant background of the sun. special attention was directed to this point during the recent transits, but no satellite of venus was to be found. it seems, therefore, to be very unlikely that venus can be attended by any companion globe of appreciable dimensions. the observations directed to the investigation of the atmosphere surrounding venus have been more successful. if the planet were devoid of an atmosphere, then it would be totally invisible just before commencing to enter on the sun, and would relapse into total invisibility as soon as it had left the sun. the observations made during the transits are not in conformity with such suppositions. special attention has been directed to this point during the recent transits. the result has been very remarkable, and has proved in the most conclusive manner the existence of an atmosphere around venus. as the planet gradually moved off the sun, the circular edge of the planet extending out into the darkness was seen to be bounded by a circular arc of light, and dr. copeland, who observed this transit in very favourable circumstances, was actually able to follow the planet until it had passed entirely away from the sun, at which time the globe, though itself invisible, was distinctly marked by the girdle of light by which it was surrounded. this luminous circle is inexplicable save by the supposition that the globe of venus is surrounded by an atmospheric shell in the same way as the earth. it may be asked, what is the advantage of devoting so much time and labour to a celestial phenomenon like the transit of venus which has so little bearing on practical affairs? what does it matter whether the sun be , , miles off, or whether it be only , , , or any other distance? we must admit at once that the enquiry has but a slender bearing on matters of practical utility. no doubt a fanciful person might contend that to compute our nautical almanacs with perfect accuracy we require a precise knowledge of the distance of the sun. our vast commerce depends on skilful navigation, and one factor necessary for success is the reliability of the "nautical almanac." the increased perfection of the almanac must therefore bear some relation to increased perfection in navigation. now, as good authorities tell us that in running for a harbour on a tempestuous night, or in other critical emergencies, even a yard of sea-room is often of great consequence, so it may conceivably happen that to the infinitesimal influence of the transit of venus on the "nautical almanac" is due the safety of a gallant vessel. but the time, the labour, and the money expended in observing the transit of venus are really to be defended on quite different grounds. we see in it a fruitful source of information. it tells us the distance of the sun, which is the foundation of all the great measurements of the universe. it gratifies the intellectual curiosity of man by a view of the true dimensions of the majestic solar system, in which the earth is seen to play a dignified, though still subordinate, part; and it leads us to a conception of the stupendous scale on which the universe is constructed. it is not possible for us, with a due regard to the limits of this volume, to protract any longer our discussion of the transit of venus. when we begin to study the details of the observations, we are immediately confronted with a multitude of technical and intricate matters. unfortunately, there are very great difficulties in making the observations with the necessary precision. the moments when venus enters on and leaves the solar disc cannot be very accurately observed, partly owing to a peculiar optical illusion known as "the black drop," whereby venus seems to cling to the sun's limb for many seconds, partly owing to the influence of the planet's atmosphere, which helps to make the observed time of contact uncertain. these circumstances make it difficult to determine the distance of the sun from observations of transits of venus with the accuracy which modern science requires. it seems therefore likely that the final determination of the sun's distance will be obtained in quite a different manner. this will be explained in chapter xi., and hence we feel the less reluctance in passing any from the consideration of the transit of venus as a method of celestial surveying. we must now close our description of this lovely planet; but before doing so, let us add--or in some cases repeat--a few statistical facts as to the size and the dimensions of the planet and its orbit. the diameter of venus is about , miles, and the planet shows no measurable departure from the globular form, though we can hardly doubt that its polar diameter must really be somewhat shorter than the equatorial diameter. this diameter is only about miles less than that of the earth. the mass of venus is about three-quarters of the mass of the earth; or if, as is more usual, we compare the mass of venus with the sun, it is to be represented by the fraction divided by , . it is to be observed that the mass of venus is not quite so great in comparison with its bulk as might have been expected. the density of this planet is about · of that of the earth. venus would weigh · times as much as a globe of water of equal size. the gravitation at its surface will, to a slight extent, be less than the gravitation at the surface of the earth. a body here falls sixteen feet in a second; a body let fall at the surface of venus would fall about three feet less. it seems not unlikely that the time of rotation of venus may be equal to the period of its revolution around the sun. the orbit of venus is remarkable for the close approach which it makes to a circle. the greatest distance of this planet from the sun does not exceed the least distance by one per cent. its mean distance from the sun is about , , miles, and the movement in the orbit amounts to a mean velocity of nearly miles per second, the entire journey being accomplished in · days. chapter ix. the earth. the earth is a great globe--how the size of the earth is measured--the base line--the latitude found by the elevation of the pole--a degree of the meridian--the earth not a sphere--the pendulum experiment--is the motion of the earth slow or fast?--coincidence of the axis of rotation and the axis of figure--the existence of heat in the earth--the earth once in a soft condition--effects of centrifugal force--comparison with the sun and jupiter--the protuberance of the equator--the weighing of the earth--comparison between the weight of the earth and an equal globe of water--comparison of the earth with a leaden globe--the pendulum--use of the pendulum in measuring the intensity of gravitation--the principle of isochronism--shape of the earth measured by the pendulum. that the earth must be a round body is a truth immediately suggested by simple astronomical considerations. the sun is round, the moon is round, and telescopes show that the planets are round. no doubt comets are not round, but then a comet seems to be in no sense a solid body. we can see right through one of these frail objects, and its weight is too small for our methods of measurement to appreciate. if, then, all the solid bodies we can see are round globes, is it not likely that the earth is a globe also? but we have far more direct information than mere surmise. there is no better way of actually seeing that the surface of the ocean is curved than by watching a distant ship on the open sea. when the ship is a long way off and is still receding, its hull will gradually disappear, while the masts will remain visible. on a fine summer's day we can often see the top of the funnel of a steamer appearing above the sea, while the body of the steamer is below. to see this best the eye should be brought as close as possible to the surface of the sea. if the sea were perfectly flat, there would be nothing to obscure the body of the vessel, and it would therefore be visible so long as the funnel remains visible. if the sea be really curved, the protuberant part intercepts the view of the hull, while the funnel is still to be seen. we thus learn how the sea is curved at every part, and therefore it is natural to suppose that the earth is a sphere. when we make more careful measurements we find that the globe is not perfectly round. it is flattened to some extent at each of the poles. this may be easily illustrated by an indiarubber ball, which can be compressed on two opposite sides so as to bulge out at the centre. the earth is similarly flattened at the poles, and bulged out at the equator. the divergence of the earth from the truly globular form is, however, not very great, and would not be noticed without very careful measurements. the determination of the size of the earth involves operations of no little delicacy. very much skill and very much labour have been devoted to the work, and the dimensions of the earth are known with a high degree of accuracy, though perhaps not with all the precision that we may ultimately hope to attain. the scientific importance of an accurate measurement of the earth can hardly be over-estimated. the radius of the earth is itself the unit in which many other astronomical magnitudes are expressed. for example, when observations are made with the view of finding the distance of the moon, the observations, when discussed and reduced, tell us that the distance of the moon is equal to fifty-nine times the equatorial radius of the earth. if we want to find the distance of the moon in miles, we require to know the number of miles in the earth's radius. a level part of the earth's surface having been chosen, a line a few miles long is measured. this is called the base, and as all the subsequent measures depend ultimately on the base, it is necessary that this measurement shall be made with scrupulous accuracy. to measure a line four or five miles long with such precision as to exclude any errors greater than a few inches demands the most minute precautions. we do not now enter upon a description of the operations that are necessary. it is a most laborious piece of work, and many ponderous volumes have been devoted to the discussion of the results. but when a few base lines have been obtained in different places on the earth's surface, the measuring rods are to be laid aside, and the subsequent task of the survey of the earth is to be conducted by the measurement of angles from one station to another and trigonometrical calculations based thereon. starting from a base line a few miles long, distances of greater length are calculated, until at length stretches miles long, or even more, can be accomplished. it is thus possible to find the length of a long line running due north and south. so far the work has been merely that of the terrestrial surveyor. the distance thus ascertained is handed over to the astronomer to deduce from it the dimensions of the earth. the astronomer fixes his observatory at the northern end of the long line, and proceeds to determine his latitude by observation. there are various ways by which this can be accomplished. they will be found fully described in works on practical astronomy. we shall here only indicate in a very brief manner the principle on which such observations are to be made. everyone ought to be familiar with the pole star, which, though by no means the most brilliant, is probably the most important star in the whole heavens. in these latitudes we are accustomed to find the pole star at a considerable elevation, and there we can invariably find it, always in the same place in the northern sky. but suppose we start on a voyage to the southern hemisphere: as we approach the equator we find, night after night, the pole star coming closer to the horizon. at the equator it is on the horizon; while if we cross the line, we find on entering the southern hemisphere that this useful celestial body has become invisible. this is in itself sufficient to show us that the earth cannot be the flat surface that untutored experience seems to indicate. on the other hand, a traveller leaving england for norway observes that the pole star is every night higher in the heavens than he has been accustomed to see it. if he extend his journey farther north, the same object will gradually rise higher and higher, until at length, when approaching the pole of the earth, the pole star is high up over his head. we are thus led to perceive that the higher our latitude, the higher, in general, is the elevation of the pole star. but we cannot use precise language until we replace the twinkling point by the pole of the heavens itself. the pole of the heavens is near the pole star, which itself revolves around the pole of the heavens, as all the other stars do, once every day. the circle described by the pole star is, however, so small that, unless we give it special attention, the motion will not be perceived. the true pole is not a visible point, but it is capable of being accurately defined, and it enables us to state with the utmost precision the relation between the pole and the latitude. the statement is, that the elevation of the pole above the horizon is equal to the latitude of the place. the astronomer stationed at one end of the long line measures the elevation of the pole above the horizon. this is an operation of some delicacy. in the first place, as the pole is invisible, he has to obtain its position indirectly. he measures the altitude of the pole star when that altitude is greatest, and repeats the operation twelve hours later, when the altitude of the pole star is least; the mean between the two, when corrected in various ways which it is not necessary for us now to discuss, gives the true altitude of the pole. suffice it to say that by such operations the latitude of one end of the line is determined. the astronomer then, with all his equipment of instruments, moves to the other end of the line. he there repeats the process, and he finds that the pole has now a different elevation, corresponding to the different latitude. the difference of the two elevations thus gives him an accurate measure of the number of degrees and fractional parts of a degree between the latitudes of the two stations. this can be compared with the actual distance in miles between the two stations, which has been ascertained by the trigonometrical survey. a simple calculation will then show the number of miles and fractional parts of a mile corresponding to one degree of latitude--or, as it is more usually expressed, the length of a degree of the meridian. this operation has to be repeated in different parts of the earth--in the northern hemisphere and in the southern, in high latitudes and in low. if the sea-level over the entire earth were a perfect sphere, an important consequence would follow--the length of a degree of the meridian would be everywhere the same. it would be the same in peru as in sweden, the same in india as in england. but the lengths of the degrees are not all the same, and hence we learn that our earth is not really a sphere. the measured lengths of the degrees enable us to see to what extent the shape of the earth departs from a perfect sphere. near the pole the length of a degree is longer than near the equator. this shows that the earth is flattened at the poles and protuberant at the equator, and it provides the means by which we may calculate the actual lengths of the polar and the equatorial axes. in this way the equatorial diameter has been found equal to , miles, while the polar diameter is miles shorter. the polar axis of the earth may be defined as the diameter about which the earth rotates. this axis intersects the surface at the north and south poles. the time which the earth occupies in making a complete rotation around this axis is called a sidereal day. the sidereal day is a little shorter than the ordinary day, being only hours, minutes, and seconds. the rotation is performed just as if a rigid axis passed through the centre of the earth; or, to use the old and homely illustration, the earth rotates just as a ball of worsted may be made to rotate around a knitting-needle thrust through its centre. it is a noteworthy circumstance that the axis about which the earth rotates occupies a position identical with that of the shortest diameter of the earth as found by actual surveying. this is a coincidence which would be utterly inconceivable if the shape of the earth was not in some way physically connected with the fact that the earth is rotating. what connection can then be traced? let us enquire into the subject, and we shall find that the shape of the earth is a consequence of its rotation. the earth at the present time is subject, at various localities, to occasional volcanic outbreaks. the phenomena of such eruptions, the allied occurrence of earthquakes, the well-known fact that the heat increases the deeper we descend into the earth, the existence of hot springs, the geysers found in iceland and elsewhere, all testify to the fact that heat exists in the interior of the earth. whether that heat be, as some suppose, universal in the interior of the earth, or whether it be merely local at the several places where its manifestations are felt, is a matter which need not now concern us. all that is necessary for our present purpose is the admission that heat is present to some extent. this internal heat, be it much or little, has obviously a different origin from the heat which we know on the surface. the heat we enjoy is derived from the sun. the internal heat cannot have been derived from the sun; its intensity is far too great, and there are other insuperable difficulties attending the supposition that it has come from the sun. where, then, has this heat come from? this is a question which at present we can hardly answer--nor, indeed, does it much concern our argument that we should answer it. the fact being admitted that the heat is there, all that we require is to apply one or two of the well-known thermal laws to the interpretation of the facts. we have first to consider the general principle by which heat tends to diffuse itself and spread away from its original source. the heat, deep-seated in the interior of the earth, is transmitted through the superincumbent rocks, and slowly reaches the surface. it is true that the rocks and materials with which our earth is covered are not good conductors of heat; most of them are, indeed, extremely inefficient in this way; but, good or bad, they are in some shape conductors, and through them the heat must creep to the surface. it cannot be urged against this conclusion that we do not feel this heat. a few feet of brickwork will so confine the heat of a mighty blast furnace that but little will escape through the bricks; but _some_ heat does escape, and the bricks have never been made, and never could be made, which would absolutely intercept all the heat. if a few feet of brickwork can thus nearly mask the heat of a furnace, cannot some scores of miles of rock nearly mask the heat in the depths of the earth, even though that heat were seven times hotter than the mightiest furnace that ever existed? the heat would escape slowly, and perhaps imperceptibly, but, unless all our knowledge of nature is a delusion, no rocks, however thick, can prevent, in the course of time, the leakage of the heat to the surface. when this heat arrives at the surface of the earth it must, in virtue of another thermal law, gradually radiate away and be lost to the earth. it would lead us too far to discuss fully the objections which may perhaps be raised against what we have here stated. it is often said that the heat in the interior of the earth is being produced by chemical combination or by mechanical process, and thus that the heat may be constantly renewed as fast or even faster than it escapes. this, however, is more a difference in form than in substance. if heat be produced in the way just supposed (and there can be no doubt that there may be such an origin for some of the heat in the interior of the globe) there must be a certain expenditure of chemical or mechanical energies that produces a certain exhaustion. for every unit of heat which escapes there will either be a loss of an unit of heat from the globe, or, what comes nearly to the same thing, a loss of an unit of heat-making power from the chemical or the mechanical energies. the substantial result is the same; the heat, actual or potential, of the earth must be decreasing. it should, of course, be observed that a great part of the thermal losses experienced by the earth is of an obvious character, and not dependent upon the slow processes of conduction. each outburst of a volcano discharges a stupendous quantity of heat, which disappears very speedily from the earth; while in the hot springs found in so many places there is a perennial discharge of the same kind, which in the course of years attains enormous proportions. the earth is thus losing heat, while it never acquires any fresh supplies of the same kind to replace the losses. the consequence is obvious; the interior of the earth must be growing colder. no doubt this is an extremely slow process; the life of an individual, the life of a nation, perhaps the life of the human race itself, has not been long enough to witness any pronounced change in the store of terrestrial heat. but the law is inevitable, and though the decline in heat may be slow, yet it is continuous, and in the lapse of ages must necessarily produce great and important results. it is not our present purpose to offer any forecast as to the changes which must necessarily arise from this process. we wish at present rather to look back into past time and see what consequences we may legitimately infer. such intervals of time as we are familiar with in ordinary life, or even in ordinary history, are for our present purpose quite inappreciable. as our earth is daily losing internal heat, or the equivalent of heat, it must have contained more heat yesterday than it does to-day, more last year than this year, more twenty years ago than ten years ago. the effect has not been appreciable in historic time; but when we rise from hundreds of years to thousands of years, from thousands of years to hundreds of thousands of years, and from hundreds of thousands of years to millions of years, the effect is not only appreciable, but even of startling magnitude. there must have been a time when the earth contained much more heat than at present. there must have been a time when the surface of the earth was sensibly hot from this source. we cannot pretend to say how many thousands or millions of years ago this epoch must have been; but we may be sure that earlier still the earth was even hotter, until at length we seem to see the temperature increase to a red heat, from a red heat we look back to a still earlier age when the earth was white hot, back further till we find the surface of our now solid globe was actually molten. we need not push the retrospect any further at present, still less is it necessary for us to attempt to assign the probable origin of that heat. this, it will be observed, is not required in our argument. we find heat now, and we know that heat is being lost every day. from this the conclusion that we have already drawn seems inevitable, and thus we are conducted back to some remote epoch in the abyss of time past when our solid earth was a globe molten and soft throughout. a dewdrop on the petal of a flower is nearly globular; but it is not quite a globe, because the gravitation presses it against the flower and somewhat distorts the shape. a falling drop of rain is a globe; a drop of oil suspended in a liquid with which it does not mix forms a globe. passing from small things to great things, let us endeavour to conceive a stupendous globe of molten matter. let that globe be as large as the earth, and let its materials be so soft as to obey the forces of attraction exerted by each part of the globe on all the other parts. there can be no doubt as to the effect of these attractions; they would tend to smooth down any irregularities on the surface just in the same way as the surface of the ocean is smooth when freed from the disturbing influences of the wind. we might, therefore, expect that our molten globe, isolated from all external interference, would assume the form of a sphere. but now suppose that this great sphere, which we have hitherto assumed to be at rest, is made to rotate round an axis passing through its centre. we need not suppose that this axis is a material object, nor are we concerned with any supposition as to how the velocity of rotation was caused. we can, however, easily see what the consequence of the rotation would be. the sphere would become deformed, the centrifugal force would make the molten body bulge out at the equator and flatten down at the poles. the greater the velocity of rotation the greater would be the bulging. to each velocity of rotation a certain degree of bulging would be appropriate. the molten earth thus bulged out to an extent which was dependent upon the fact that it turned round once a day. now suppose that the earth, while still rotating, commences to pass from the liquid to the solid state. the form which the earth would assume on consolidation would, no doubt, be very irregular on the surface; it would be irregular in consequence of the upheavals and the outbursts incident to the transformation of so mighty a mass of matter; but irregular though it be, we can be sure that, on the whole, the form of the earth's surface would coincide with the shape which it had assumed by the movement of rotation. hence we can explain the protuberant form of the equator of the earth, and we can appeal to that form in corroboration of the view that this globe was once in a soft or molten condition. the argument may be supported and illustrated by comparing the shape of our earth with the shapes of some of the other celestial bodies. the sun, for instance, seems to be almost a perfect globe. no measures that we can make show that the polar diameter of the sun is shorter than the equatorial diameter. but this is what we might have expected. no doubt the sun is rotating on its axis, and, as it is the rotation that causes the protuberance, why should not the rotation have deformed the sun like the earth? the probability is that a difference really does exist between the two diameters of the sun, but that the difference is too small for us to measure. it is impossible not to connect this with the _slowness_ of the sun's rotation. the sun takes twenty-five days to complete a rotation, and the protuberance appropriate to so low a velocity is not appreciable. on the other hand, when we look at one of the quickly-rotating planets, we obtain a very different result. let us take the very striking instance which is presented in the great planet jupiter. viewed in the telescope, jupiter is at once seen not to be a globe. the difference is so conspicuous that accurate measures are not necessary to show that the polar diameter of jupiter is shorter than the equatorial diameter. the departure of jupiter from the truly spherical shape is indeed much greater than the departure of the earth. it is impossible not to connect this with the much more rapid rotation of jupiter. we shall presently have to devote a chapter to the consideration of this splendid orb. we may, however, so far anticipate what we shall then say as to state that the time of jupiter's rotation is under ten hours, and this notwithstanding the fact that jupiter is more than one thousand times greater than the earth. his enormously rapid rotation has caused him to bulge out at the equator to a remarkable extent. the survey of our earth and the measurement of its dimensions having been accomplished, the next operation for the astronomer is the determination of its weight. here, indeed, is a problem which taxes the resources of science to the very uttermost. of the interior of the earth we know little--i might almost say we know nothing. no doubt we sink deep mines into the earth. these mines enable us to penetrate half a mile, or even a whole mile, into the depths of the interior. but this is, after all, only a most insignificant attempt to explore the interior of the earth. what is an advance of one mile in comparison with the distance to the centre of the earth? it is only about one four-thousandth part of the whole. our knowledge of the earth merely reaches to an utterly insignificant depth below the surface, and we have not a conception of what may be the nature of our globe only a few miles below where we are standing. seeing, then, our almost complete ignorance of the solid contents of the earth, does it not seem a hopeless task to attempt to weigh the entire globe? yet that problem has been solved, and the result is known--not, indeed, with the accuracy attained in other astronomical researches, but still with tolerable approximation. it is needless to enunciate the weight of the earth in our ordinary units. the enumeration of billions of tons does not convey any distinct impression. it is a far more natural course to compare the mass of the earth with that of an equal globe of water. we should be prepared to find that our earth was heavier than a like volume of water. the rocks which form its surface are heavier, bulk for bulk, than the oceans which repose on those rocks. the abundance of metals in the earth, the gradual increase in the density of the earth, which must arise from the enormous pressure at great depths--all these considerations will prepare us to learn that the earth is very much heavier than a globe of water of equal size. newton supposed that the earth was between five and six times as heavy as an equal bulk of water. nor is it hard to see that such a suggestion is plausible. the rocks and materials on the surface are usually about two or three times as heavy as water, but the density of the interior must be much greater. there is good reason to believe that down in the remote depths of the earth there is a very large proportion of iron. an iron earth would weigh about seven times as much as an equal globe of water. we are thus led to see that the earth's weight must be probably more than three, and probably less than seven, times an equal globe of water; and hence, in fixing the density between five and six, newton adopted a result plausible at the moment, and since shown to be probably correct. several methods have been proposed by which this important question can be solved with accuracy. of all these methods we shall here only describe one, because it illustrates, in a very remarkable manner, the law of universal gravitation. in the chapter on gravitation it was pointed out that the intensity of this force between two masses of moderate dimensions was extremely minute, and the difficulty in weighing the earth arises from this cause. the practical application of the process is encumbered by multitudinous details, which it will be unnecessary for us to consider at present. the principle of the process is simple enough. to give definiteness to our description, let us conceive a large globe about two feet in diameter; and as it is desirable for this globe to be as heavy as possible, let us suppose it to be made of lead. a small globe brought near the large one is attracted by the force of gravitation. the amount of this attraction is extremely small, but, nevertheless, it can be measured by a refined process which renders extremely small forces sensible. the intensity of the attraction depends both on the masses of the globes and on their distance apart, as well as on the force of gravitation. we can also readily measure the attraction of the earth upon the small globe. this is, in fact, nothing more nor less than the weight of the small globe in the ordinary acceptation of the word. we can thus compare the attraction exerted by the leaden globe with the attraction exerted by the earth. if the centre of the earth and the centre of the leaden globe were at the same distance from the attracted body, then the intensity of their attractions would give at once the ratio of their masses by simple proportion. in this case, however, matters are not so simple: the leaden ball is only distant by a few inches from the attracted ball, while the centre of the earth's attraction is nearly , miles away at the centre of the earth. allowance has to be made for this difference, and the attraction of the leaden sphere has to be reduced to what it would be were it removed to a distance of , miles. this can fortunately be effected by a simple calculation depending upon the general law that the intensity of gravitation varies inversely as the square of the distance. we can thus, partly by calculation and partly by experiment, compare the intensity of the attraction of the leaden sphere with the attraction of the earth. it is known that the attractions are proportional to the masses, so that the comparative masses of the earth and of the leaden sphere have been measured; and it has been ascertained that the earth is about half as heavy as a globe of lead of equal size would be. we may thus state finally that the mass of the earth is about five and a half times as great as the mass of a globe of water equal to it in bulk. in the chapter on gravitation we have mentioned the fact that a body let fall near the surface of the earth drops through sixteen feet in the first second. this distance varies slightly at different parts of the earth. if the earth were a perfect sphere, then the attraction would be the same at every part, and the body would fall through the same distance everywhere. the earth is not round, so the distance which the body falls in one second differs slightly at different places. at the pole the radius of the earth is shorter than at the equator, and accordingly the attraction of the earth at the pole is greater than at the equator. had we accurate measurements showing the distance a body would fall in one second both at the pole and at the equator, we should have the means of ascertaining the shape of the earth. it is, however, difficult to measure correctly the distance a body will fall in one second. we have, therefore, been obliged to resort to other means for determining the force of attraction of the earth at the equator and other accessible parts of its surface. the methods adopted are founded on the pendulum, which is, perhaps, the simplest and certainly one of the most useful of philosophical instruments. the ideal pendulum is a small and heavy weight suspended from a fixed point by a fine and flexible wire. if we draw the pendulum aside from its vertical position and then release it, the weight will swing to and fro. for its journey to and fro the pendulum requires a small period of time. it is very remarkable that this period does not depend appreciably on the length of the circular arc through which the pendulum swings. to verify this law we suspend another pendulum beside the first, both being of the same length. if we draw both pendulums aside and then release them, they swing together and return together. this might have been expected. but if we draw one pendulum a great deal to one side, and the other only a little, the two pendulums still swing sympathetically. this, perhaps, would not have been expected. try it again, with even a still greater difference in the arc of vibration, and still we see the two weights occupy the same time for the swing. we can vary the experiment in another way. let us change the weights on the pendulums, so that they are of unequal size, though both of iron. shall we find any difference in the periods of vibration? we try again: the period is the same as before; swing them through different arcs, large or small, the period is still the same. but it may be said that this is due to the fact that both weights are of the same material. try it again, using a leaden weight instead of one of the iron weights; the result is identical. even with a ball of wood the period of oscillation is the same as that of the ball of iron, and this is true no matter what be the arc through which the vibration takes place. if, however, we change the _length_ of the wire by which the weight is supported, then the period will not remain unchanged. this can be very easily illustrated. take a short pendulum with a wire only one-fourth of the length of that of the long one; suspend the two close together, and compare the periods of vibration of the short pendulum with that of the long one, and we find that the former has a period only half that of the latter. we may state the result generally, and say that the time of vibration of a pendulum is proportional to the square root of its length. if we quadruple the length of the suspending cord we double the time of its vibration; if we increase the length of the pendulum ninefold, we increase its period of vibration threefold. it is the gravitation of the earth which makes the pendulum swing. the greater the attraction, the more rapidly will the pendulum oscillate. this may be easily accounted for. if the earth pulls the weight down very vigorously, the time will be short; if the power of the earth's attraction be lessened, then it cannot pull the weight down so quickly, and the period will be lengthened. the time of vibration of the pendulum can be determined with great accuracy. let it swing for , oscillations, and measure the time that these oscillations have consumed. the arc through which the pendulum swings may not have remained quite constant, but this does not appreciably affect the _time_ of its oscillation. suppose that an error of a second is made in the determination of the time of , oscillations; this will only entail an error of the ten-thousandth part of the second in the time of a single oscillation, and will afford a correspondingly accurate determination of the force of gravity at the place where the experiment was made. take a pendulum to the equator. let it perform , oscillations, and determine carefully the _time_ that these oscillations have required. bring the same pendulum to another part of the earth, and repeat the experiment. we have thus a means of comparing the gravitation at the two places. there are, no doubt, a multitude of precautions to be observed which need not here concern us. it is not necessary to enter into details as to the manner in which the motion of the pendulum is to be sustained, nor as to the effect of changes of temperature in the alteration of its length. it will suffice for us to see how the time of the pendulum's swing can be measured accurately, and how from that measurement the intensity of gravitation can be calculated. the pendulum thus enables us to make a gravitational survey of the surface of the earth with the highest degree of accuracy. we cannot, however, infer that gravity alone affects the oscillations of the pendulum. we have seen how the earth rotates on its axis, and we have attributed the bulging of the earth at the equator to this influence. but the centrifugal force arising from the rotation has the effect of decreasing the apparent weight of bodies, and the change is greatest at the equator, and lessens gradually as we approach the poles. from this cause alone the attraction of the pendulum at the equator is less than elsewhere, and therefore the oscillations of the pendulum will take a longer time there than at other localities. a part of the apparent change in gravitation is accordingly due to the centrifugal force; but there is, in addition, a real alteration. in a work on astronomy it does not come within our scope to enter into further detail on the subject of our planet. the surface of the earth, its contour and its oceans, its mountain chains and its rivers, are for the physical geographer; while its rocks and their contents, its volcanoes and its earthquakes, are to be studied by the geologists and the physicists. chapter x. mars. our nearer neighbours in the heavens--surface of mars can be examined in the telescope--remarkable orbit of mars--resemblance of mars to a star--meaning of opposition--the eccentricity of the orbit of mars--different oppositions of mars--apparent movements of the planet--effect of the earth's movement--measurement of the distance of mars--theoretical investigation of the sun's distance--drawings of the planet--is there snow on mars?--the rotation of the planet--gravitation on mars--has mars any satellites?--prof. asaph hall's great discovery--the revolutions of the satellites--deimos and phobos--"gulliver's travels." the special relation in which we stand to one planet of our system has necessitated a somewhat different treatment of that globe from the treatment appropriate to the others. we discussed mercury and venus as distant objects known chiefly by telescopic research, and by calculations of which astronomical observations were the foundation. our knowledge of the earth is of a different character, and attained in a different way. yet it was necessary for symmetry that we should discuss the earth after the planet venus, in order to give to the earth its true position in the solar system. but now that the earth has been passed in our outward progress from the sun, we come to the planet mars; and here again we resume, though in a somewhat modified form, the methods that were appropriate to venus and to mercury. venus and mars have, from one point of view, quite peculiar claims on our attention. they are our nearest planetary neighbours, on either side. we may naturally expect to learn more of them than of the other planets farther off. in the case of venus, however, this anticipation can hardly be realised, for, as we have already pointed out, its dense atmosphere prevents us from making a satisfactory telescopic examination. when we turn to our other planetary neighbour, mars, we are enabled to learn a good deal with regard to his appearance. indeed, with the exception of the moon, we are better acquainted with the details of the surface of mars than with those of any other celestial body. this beautiful planet offers many features for consideration besides those presented by its physical structure. the orbit of mars is one of remarkable proportions, and it was by the observations of this orbit that the celebrated laws of kepler were discovered. during the occasional approaches of mars to the earth it has been possible to measure its distance with accuracy, and thus another method of finding the sun's distance has arisen which, to say the least, may compete in precision with that afforded by the transit of venus. it must also be observed that the greatest achievement in pure telescopic research which this century has witnessed was that of the discovery of the satellites of mars. to the unaided eye this planet generally appears like a star of the first magnitude. it is usually to be distinguished by its ruddy colour, but the beginner in astronomy cannot rely on its colour only for the identification of mars. there are several stars nearly, if not quite, as ruddy as this globe. the bright star aldebaran, the brightest star in the constellation of the bull, has often been mistaken for the planet. it often resembles betelgeuze, a brilliant point in the constellation of orion. mistakes of this kind will be impossible if the learner has first studied the principal constellations and the more brilliant stars. he will then find great interest in tracing out the positions of the planets, and in watching their ceaseless movements. [illustration: fig. .--the orbits of the earth and of mars, showing the favourable opposition of .] the position of each orb can always be ascertained from the almanac. sometimes the planet will be too near the sun to be visible. it will rise with the sun and set with the sun, and consequently will not be above the horizon during the night. the best time for seeing one of the planets situated like mars will be during what is called its opposition. this state of things occurs when the earth intervenes directly between the planet and the sun. in this case, the distance from mars to the earth is less than at any other time. there is also another advantage in viewing mars during opposition. the planet is then at one side of the earth and the sun at the opposite side, so that when mars is high in the heavens the sun is directly beneath the earth; in other words, the planet is then at its greatest elevation above the horizon at midnight. some oppositions of mars are, however, much more favourable than others. this is distinctly shown in fig. , which represents the orbit of mars and the orbit of the earth accurately drawn to scale. it will be seen that while the orbit of the earth is very nearly circular, the orbit of mars has a very decided degree of eccentricity; indeed, with the exception of the orbit of mercury, that of mars has the greatest eccentricity of any orbit of the larger planets in our system. the value of an opposition of mars for telescopic purposes will vary greatly according to circumstances. the favourable oppositions will be those which occur as near as possible to the th of august. the other extreme will be found in an opposition which occurs near the nd of february. in the latter case the distance between the planet and the earth is nearly twice as great as the former. the last opposition which was suitable for the highest class of work took place in the year . mars was then a magnificent object, and received much, and deserved, attention. the favourable oppositions follow each other at somewhat irregular intervals; the last occurred in the year , and another will take place in the year . the apparent movements of mars are by no means simple. we can imagine the embarrassment of the early astronomer who first undertook the task of attempting to decipher these movements. the planet is seen to be a brilliant and conspicuous object. it attracts the astronomer's attention; he looks carefully, and he sees how it lies among the constellations with which he is familiar. a few nights later he observes the same body again; but is it exactly in the same place? he thinks not. he notes more carefully than before the place of the planet. he sees how it is situated with regard to the stars. again, in a few days, his observations are repeated. there is no longer a trace of doubt about the matter--mars has decidedly changed his position. it is veritably a wanderer. night after night the primitive astronomer is at his post. he notes the changes of mars. he sees that it is now moving even more rapidly than it was at first. is it going to complete the circuit of the heavens? the astronomer determines to watch the orb and see whether this surmise is justified. he pursues his task night after night, and at length he begins to think that the body is not moving quite so rapidly as at first. a few nights more, and he is sure of the fact: the planet is moving more slowly. again a few nights more, and he begins to surmise that the motion may cease; after a short time the motion does cease, and the object seems to rest; but is it going to remain at rest for ever? has its long journey been finished? for many nights this seems to be the case, but at length the astronomer suspects that the planet must be commencing to move backwards. a few nights more, and the fact is confirmed beyond possibility of doubt, and the extraordinary discovery of the direct and the retrograde movement of mars has been accomplished. [illustration: fig. .--the apparent movements of mars in .] in the greater part of its journey around the heavens mars seems to move steadily from the west to the east. it moves backwards, in fact, as the moon moves and as the sun moves. it is only during a comparatively small part of its path that those elaborate movements are accomplished which presented such an enigma to the primitive observer. we show in the adjoining picture (fig. ) the track of the actual journey which mars accomplished in the opposition of . the figure only shows that part of its path which presents the anomalous features; the rest of the orbit is pursued, not indeed with uniform velocity, but with unaltered direction. this complexity of the apparent movements of mars seems at first sight fatal to the acceptance of any simple and elementary explanation of the planetary motion. if the motion of mars were purely elliptic, how, it may well be said, could it perform this extraordinary evolution? the elucidation is to be found in the fact that the earth on which we stand is itself in motion. even if mars were at rest, the fact that the earth moves would make the planet appear to move. the apparent movements of mars are thus combined with the real movements. this circumstance will not embarrass the geometer. he is able to disentangle the true movement of the planet from its association with the apparent movement, and to account completely for the complicated evolutions exhibited by mars. could we transfer our point of view from the ever-shifting earth to an immovable standpoint, we should then see that the shape of the orbit of mars was an ellipse, described around the sun in conformity with the laws which kepler discovered by observations of this planet. mars takes days to travel round the sun, its average distance from that body being , , miles. under the most favourable circumstances the planet, at the time of opposition, may approach the earth to a distance not greater than about , , miles. no doubt this seems an enormous distance, when estimated by any standard adapted for terrestrial measurements; it is, however, hardly greater than the distance of venus when nearest, and it is much less than the distance from the earth to the sun. we have explained how the _form_ of the solar system is known from kepler's laws, and how the absolute size of the system and of its various parts can be known when the direct measurement of any one part has been accomplished. a close approach of mars affords a favourable opportunity for measuring his distance, and thus, in a different way, solving the same problem as that investigated by the transit of venus. we are thus led a second time to a knowledge of the distance of the sun and the distances of the planets generally, and to many other numerical facts about the solar system. on the occasion of the opposition of mars in a successful attempt was made to apply this refined process to the solution of the problem of celestial measurement. it cannot be said to have been the first occasion on which this method was suggested, or even practically attempted. the observations of were, however, conducted with such skill and with such minute attention to the necessary precautions as to render them an important contribution to astronomy. dr. david gill, now her majesty's astronomer at the cape of good hope, undertook a journey to the island of ascension for the purpose of observing the parallax of mars in . on this occasion mars approached to the earth so closely as to afford an admirable opportunity for the application of the method. dr. gill succeeded in obtaining a valuable series of measurements, and from them he concluded the distance of the sun with an accuracy somewhat superior to that attainable by the transit of venus. there is yet another method by which mars can be made to give us information as to the distance of the sun. this method is one of some delicacy, and is interesting from its connection with the loftiest enquiries in mathematical astronomy. it was foreshadowed in the dynamical theory of newton, and was wrought to perfection by le verrier. it is based upon the great law of gravitation, and is intimately associated with the splendid discoveries in planetary perturbation which form so striking a chapter in modern astronomical discovery. there is a certain relation between two quantities which at first sight seems quite independent. these quantities are the mass of the earth and the distance of the sun. the distance of the sun bears to a certain distance (which can be calculated when we know the intensity of gravitation at the earth's surface, the size of the earth and the length of the year) the same proportion that the cube root of the sun's mass bears to the cube root of that of the earth. there is no uncertainty about this result, and the consequence is obvious. if we have the means of weighing the earth in comparison with the sun, then the distance of the sun can be immediately deduced. how are we to place our great earth in the weighing scales? this is the problem which le verrier has shown us how to solve, and he does so by invoking the aid of the planet mars. if mars in his revolution around the sun were solely swayed by the attraction of the sun, he would, in accordance with the well-known laws of planetary motion, follow for ever the same elliptic path. at the end of one century, or even of many centuries, the shape, the size, and the position of that ellipse would remain unaltered. fortunately for our present purpose, a disturbance in the orbit of mars is produced by the earth. although the mass of our globe is so much less than that of the sun, yet the earth is still large enough to exercise an appreciable attraction on mars. the ellipse described by the planet is consequently not invariable. the shape of that ellipse and its position gradually change, so that the position of the planet depends to some extent upon the mass of the earth. the place in which the planet is found can be determined by observation; the place which the planet would have had if the earth were absent can be found by calculation. the difference between the two is due to the attraction of the earth, and, when it has been measured, the mass of the earth can be ascertained. the amount of displacement increases from one century to another, but as the rate of growth is small, ancient observations are necessary to enable the measures to be made with accuracy. a remarkable occurrence which took place more than two centuries ago fortunately enables the place of mars to be determined with great precision at that date. on the st of october, , three independent observers witnessed the occultation of a star in aquarius by the ruddy planet. the place of the star is known with accuracy, and hence we are provided with the means of indicating the exact point in the heavens occupied by mars on the day in question. from this result, combined with the modern meridian observations, we learn that the displacement of mars by the attraction of the earth has, in the lapse of two centuries, grown to about five minutes of arc ( seconds). it has been maintained that this cannot be erroneous to the extent of more than a second, and hence it would follow that the earth's mass is determined to about one three-hundredth part of its amount. if no other error were present, this would give the sun's distance to about one nine-hundredth part. [illustration: fig. .--relative sizes of mars and the earth.] notwithstanding the intrinsic beauty of this method, and the very high auspices under which it has been introduced, it is, we think, at present hardly worthy of reliance in comparison with some of the other methods. as the displacement of mars, due to the perturbing influence of the earth, goes on increasing continually, it will ultimately attain sufficient magnitude to give a very exact value of the earth's mass, and then this method will give us the distance of the sun with great precision. but interesting and beautiful though this method may be, we must as yet rather regard it as a striking confirmation of the law of gravitation than as affording an accurate means of measuring the sun's distance. [illustration: fig. .--drawing of mars (july th, ).] [illustration: fig. .--drawing of mars (august th, ).] [illustration: fig. .--elevations and depressions on the "terminator" of mars (august th, ).] [illustration: fig. .--the southern polar cap on mars (july , ).] the close approaches of mars to the earth afford us opportunities for making a careful telescopic scrutiny of his surface. it must not be expected that the details on mars could be inspected with the same minuteness as those on the moon. even under the most favourable circumstances, mars is still more than a hundred times as far as the moon, and, therefore, the features of the planet have to be at least one hundred times as large if they are to be seen as distinctly as the features on the moon. mars is much smaller than the earth. the diameter of the planet is , miles, but little more than half that of the earth. fig. shows the comparative sizes of the two bodies. we here reproduce two of the remarkable drawings[ ] of mars made by professor william h. pickering at the lowell observatory, flagstaff a.t. fig. was taken on the th of july, , and fig. on the th of august, . the southern polar cap on mars, as seen by professor william h. pickering at lowell observatory on the st of july, , is represented in fig. .[ ] the remarkable black mark intruding into the polar area will be noticed. in fig. are shown a series of unusually marked elevations and depressions upon the "terminator" of the planet, drawn as accurately as possible to scale by the same skilful hand on the th of august, . in making an examination of the planet it is to be observed that it does not, like the moon, always present the same face towards the observer. mars rotates upon an axis in exactly the same manner as the earth. it is not a little remarkable that the period required by mars for the completion of one rotation should be only about half an hour greater than the period of rotation of the earth. the exact period is hours, minutes, - / seconds. it therefore follows that the aspect of the planet changes from hour to hour. the western side gradually sinks from view, the eastern side gradually assumes prominence. in twelve hours the aspect of the planet is completely changed. these changes, together with the inevitable effects of foreshortening, render it often difficult to correlate the objects on the planet with those on the maps. the latter, it must be confessed, fall short of the maps of the moon in definiteness and in certainty; yet there is no doubt that the main features of the planet are to be regarded as thoroughly established, and some astronomers have given names to all the prominent objects. the markings on the surface of mars are of two classes. some of them are of an iron-grey hue verging on green, while the others are generally dark yellow or orange, occasionally verging on white. the former have usually been supposed to represent the tracts of ocean, the latter the continental masses on the ruddy planet. we possess a great number of drawings of mars, the earliest being taken in the middle of the seventeenth century. though these early sketches are very rough, and are not of much value for the solution of questions of topography, they have been found very useful in aiding us to fix the period of rotation of the planet on its axis by comparison with our modern drawings. early observers had already noticed that each of the poles of mars is distinguished by a white spot. it is, however, to william herschel that we owe the first systematic study of these remarkable polar caps. this illustrious astronomer was rewarded by a very interesting discovery. he found that these arctic tracts on mars vary both in extent and distinctness with the seasons of the hemisphere on which they are situated. they attain a maximum development from three to six months after the winter solstice on that planet, and then diminish until they are smallest about three to six months after the summer solstice. the analogy with the behaviour of the masses of snow and ice which surround our own poles is complete, and there has until lately been hardly any doubt that the white polar spots of mars are somewhat similarly constituted. as the period of revolution of mars around the sun is so much longer than our year, days instead of , the seasons of the planet are, of course, also much longer than the terrestrial seasons. in the northern hemisphere of mars the summer lasts for no fewer than days, and the winter must be days. in both hemispheres the white polar cap in the course of the long winter season increases until it reaches a diameter of ° to °, while the long summer reduces it to a small area only ° or ° in diameter. it is remarkable that one of these white regions--that at the south pole--seems not to be concentric with the pole, but is placed so much to one side that the south pole of mars appears to be quite free from ice or snow once a year. although many valuable observations of mars were made in the course of the nineteenth century, it is only since the very favourable opposition of that the study of the surface of mars has made that immense progress which is one of the most remarkable features of modern astronomy. among the observers who produced valuable drawings of the planet in we may mention mr. green, whose exquisite pictures were published by the royal astronomical society, and professor schiaparelli, of milan, who almost revolutionised our knowledge of this planet. schiaparelli had a refractor of only eight inches aperture at his disposal, but he was doubtless much favoured by the purity of the italian sky, which enabled him to detect in the bright portions of the surface of mars a considerable number of long, narrow lines. to these he gave the name of canals, inasmuch as they issued from the so-called oceans, and could be traced across the reputed continents for considerable distances, which sometimes reached thousands of miles. the canals seemed to form a kind of network, which connected the various seas with each other. a few of the more conspicuous of these so-called canals appeared indeed on some of the drawings made by dawes and others before schiaparelli's time. it was, however, the illustrious italian astronomer who detected that these narrow lines are present in such great numbers as to form a notable feature of the planet. some of these remarkable features are shown in figs. and , which are copied from drawings made by professor william h. pickering at the lowell observatory in . great as had been the surprise of astronomers when schiaparelli first proclaimed the discovery of these numerous canals, it was, perhaps, surpassed by the astonishment with which his announcement was received in that most of the canals had become double. between december, , and february, , thirty of these duplications appear to have taken place. nineteen of these were cases of a well-traced parallel line being formed near a previously existing canal. the remaining canals were less certainly established, or were cases where the two lines did not seem to be quite parallel. a copy of the map of mars which schiaparelli formed from his observations of - is given in plate xviii. it brings out clearly these strange double canals, so unlike any features that we know on any other globe. [illustration: plate xviii. schiaparelli's map of mars in - .] subsequent observations by schiaparelli and several other observers seem to indicate that this phenomenon of the duplication of the canals is of a periodic character. it is produced about the times when mars passes through its equinoxes. one of the two parallel lines is often superposed as exactly as possible upon the track of the old canal. it does, however, sometimes happen that both the lines occupy opposite sides of the former canal and are situated on entirely new ground. the distance between the two lines varies from about miles as a maximum down to the smallest limit distinguishable in our large telescopes, which is something less than thirty miles. the breadth of each of these remarkable channels may range from the limits of visibility, say, up to more than sixty miles. the duplication of the canals is perhaps the most difficult problem which mars offers to us for solution. even if we admit that the canals themselves represent inlets or channels through which the melted polar snow makes its way across the equatorial continents, it is not easy to see how the duplicate canals can arise. this is especially true in those cases where the original channel seems to vanish and to be replaced by two quite new canals, each about the breadth of the english channel, and lying one on each side of the course of the old one. the very obvious explanation that the whole duplication is an optical illusion has been brought forward more than once, but never in a conclusive manner. we must, perhaps, be content to let the solution of this matter rest for the present, in the hope that the extraordinary attention which this planet is now receiving will in due time explain the present enigma. the markings on the surface of this planet are, generally speaking, of a permanent character, so that when we compare drawings made one or two hundred years ago with drawings made more recently we can recognise in each the same features. this permanence is, however, not nearly so absolute as it is in the case of the moon. in addition to the canals which we have already considered, many other parts of the surface of mars alter their outlines from time to time. this is particularly the case with those dark spots which we call oceans, the contours of which sometimes undergo modifications in matters of detail which are quite unmistakable. changes of colour are often observed on parts of the planet, and though some of these observations may perhaps be attributed to the influence of our own atmosphere on the planet's appearance, they cannot be all thus accounted for. some of the phenomena must certainly be due to actual changes which have taken place on the surface of mars. as an example of such changes, we may refer to the north-western part of the notable feature, to which schiaparelli has given the name of _syrtis major_.[ ] this has at various times been recorded as grey, green, blue, brown, and even violet. when this region (about the time of the autumnal equinox of the northern hemisphere) is situated in the middle of the visible disc, the eastern part is distinctly greener than the western. as the season progresses this characteristic colour gets feebler, until the green tint is to be perceived only on the shores of the syrtis. the atmosphere of mars is usually very transparent, and fortunately allows us to scrutinise the surface of the planet without putting obstacles in the way m the shape of martian clouds. such clouds, however, are not invariably absent. our view of the surface is occasionally obstructed in such a manner as to make it certain that clouds or mist in the atmosphere of mars must be the cause of the trouble. would we form an idea of the physical constitution of the surface of mars, then the question as to the character of the atmosphere of the planet is among the first to be considered. spectroscopic observations do not in this case render us much assistance. of course, we know that the planet has no intrinsic light. it merely shines by reflected sunlight. the hemisphere which is turned towards the sun is bright, and the hemisphere which is turned away from the sun is dark. the spectrum ought, therefore, like that of the moon, to be an exact though faint copy of the solar spectrum, unless the sun's rays, by passing twice through the atmosphere of mars, suffered some absorption which could give rise to additional dark lines. some of the earlier observers thought that they could distinctly make out some such lines due, as was supposed, to water vapour. the presence of such lines is, however, denied by mr. campbell, of the lick observatory, and professor keeler, at the allegheny observatory,[ ] who, with their unrivalled opportunities, both instrumental and climatic, could find no difference between the spectra of mars and the moon. if mars had an atmosphere of appreciable extent, its absorptive effect should be noticeable, especially at the limb of the planet; but mr. campbell's observations do not show any increased absorption at the limb. it would therefore seem that mars cannot have an extensive atmosphere, and this conclusion is confirmed in several other ways. the distinctness with which we see the surface of this planet tends to show that the atmosphere must be very thin as compared with our own. there can hardly be any doubt that an observer on mars with a good telescope would be unable to distinguish much of the features of the earth's surface. this would be the case not only by reason of the strong absorption of the light during the double passage through our atmosphere, but also on account of the great diffusion of the light caused by this same atmosphere. also, it is needless to say, the great amount of cloud generally floating over the earth would totally obscure many parts of our planet from a martian observer. but though, as already mentioned, we occasionally find parts of mars rendered indistinct, it must be acknowledged that the clouds on mars are very slight. we should expect that the polar caps, if composed of snow, would, when melting, produce clouds which would more or less hide the polar regions from our inspection; yet nothing of the kind has ever been seen. we have seen that there are very grave doubts as to the existence of water on mars. no doubt we have frequently spoken of the dark markings as "oceans" and of the bright parts as "continents." that this language was just has been the opinion of astronomers for a very long time. a few years ago mr. schaeberle, of the lick observatory, came to the very opposite conclusion. he contended that the dark parts were the continents and the bright ones were the oceans of water, or some other fluid. he pointed to the irregular shading of the dark parts, which does not suggest the idea of light reflected from a spherical surface of water, especially as the contrasts between light and shade are strongest about the middle of the disc. it is also to be noticed that the dark regions are not infrequently traversed by still darker streaks, which can be traced for hundreds of miles almost in straight lines, while the so-called canals in the bright parts often seem to be continuations of these same lines. mr. schaeberle therefore suggests that the canals may be chains of mountains stretching over sea and land! the late professor phillips and mr. h.d. taylor have pointed out that if there were lakes or seas in the tropical regions of mars we should frequently see the sun directly reflected from them, thus producing a bright, star-like point which could not escape observation. even moderately disturbed water would make its presence known in this manner, and yet nothing of the kind has ever been recorded. on the question as to the possibility of life on mars a few words may be added. if we could be certain of the existence of water on mars, then one of the fundamental conditions would be fulfilled; and even though the atmosphere on mars had but few points of resemblance either in composition or in density to the atmosphere of the earth, life might still be possible. even if we could suppose that a man would find suitable nutriment for his body and suitable air for his respiration, it seems very doubtful whether he would be able to live. owing to the small size of mars and the smallness of its mass in comparison with the earth, the intensity of the gravitation on the neighbouring planet would be different from the attraction on the surface of the earth. we have already alluded to the small gravitation on the moon, and in a lesser degree the same remarks will apply to mars. a body which weighs on the earth two pounds would on the surface of mars weigh rather less than one pound. nearly the same exertion which will raise a -lb. weight on the earth would lift two similar weights on mars. the earth is attended by one moon. jupiter is attended by four conspicuous moons. mars is a planet revolving between the orbits of the earth and of jupiter. it is a body of the same general type as the earth and jupiter. it is ruled by the same sun, and all three planets form part of the same system; but as the earth has one moon and jupiter four moons, why should not mars also have a moon? no doubt mars is a small body, less even than the earth, and much less than jupiter. we could not expect mars to have large moons, but why should it be unlike its two neighbours, and not have any moon at all? so reasoned astronomers, but until modern times no satellite of mars could be found. for centuries the planet has been diligently examined with this special object, and as failure after failure came to be recorded, the conclusion seemed almost to be justified that the chain of analogical reasoning had broken down. the moonless mars was thought to be an exception to the rule that all the great planets outside venus were dignified by an attendant retinue of satellites. it seemed almost hopeless to begin again a research which had often been tried, and had invariably led to disappointment; yet, fortunately, the present generation has witnessed still one more attack, conducted with perfect equipment and with consummate skill this attempt has obtained the success it so well merited, and the result has been the memorable detection of two satellites of mars. this discovery was made by professor asaph hall, the distinguished astronomer at the observatory of washington. mr. hall was provided with an instrument of colossal proportions and of exquisite workmanship, known as the great washington refractor. it is the product of the celebrated workshop of messrs. alvan clark and sons, from which so many large telescopes have proceeded, and in its noble proportions far surpassed any other telescope ever devoted to the same research. the object-glass measures twenty-six inches in diameter, and is hardly less remarkable for the perfection of its definition than for its size. but even the skill of mr. hall, and the space-penetrating power of his telescope, would not have been able on ordinary occasions to discover the satellites of mars. advantage was accordingly taken of that memorable opposition of mars in , when, as we have already described, the planet came unusually near the earth. had mars been attended by a moon one-hundredth part of the bulk of our moon it must long ago have been discovered. mr. hall, therefore, knew that if there were any satellites they must be extremely small bodies, and he braced himself for a severe and diligent search. the circumstances were all favourable. not only was mars as near as it well could be to the earth; not only was the great telescope at washington the most powerful refractor then in existence; but the situation of washington is such that mars was seen from the observatory at a high elevation. it was while the british association were meeting at plymouth, in , that a telegram flashed across the atlantic. brilliant success had rewarded mr. hall's efforts. he had hoped to discover one satellite. the discovery of even one would have made the whole scientific world ring; but fortune smiled on mr. hall. he discovered first one satellite, and then he discovered a second; and, in connection with these satellites, he further discovered a unique fact in the solar system. deimos, the outer of the satellites, revolves around the planet in the period of hours, mins., secs.; it is the inner satellite, phobos, which has commanded the more special attention of every astronomer in the world. mars turns round on his axis in a martial day, which is very nearly the same length as our day of twenty-four hours. the inner satellite of mars moves round in hours, mins., secs. phobos, in fact, revolves three times round mars in the same time that mars can turn round once. this circumstance is unparalleled in the solar system; indeed, as far as we know, it is unparalleled in the universe. in the case of our own planet, the earth rotates twenty-seven times for one revolution of the moon. to some extent the same may be said of jupiter and of saturn; while in the great system of the sun himself and the planets, the sun rotates on his axis several times for each revolution of even the most rapidly moving of the planets. there is no other known case where the satellite revolves around the primary more quickly than the primary rotates on its axis. the anomalous movement of the satellite of mars has, however, been accounted for. in a subsequent chapter we shall again allude to this, as it is connected with an important department of modern astronomy. the satellites are so small that we are unable to measure their diameters directly, but from observations of their brightness it is evident that their diameters cannot exceed twenty or thirty miles, and may be even smaller. owing to their rapid motion the two satellites must present some remarkable peculiarities to an observer on mars. phobos rises in the west, passes across the heavens, and sets in the east after about five and a half hours, while deimos rises in the east and remains more than two days above the horizon. as the satellites revolve in paths vertically above the equator of their primary, the one less than , miles and the other only some , miles above the surface, it follows that they can never be visible from the poles of mars; indeed, to see phobos, the observer's planetary latitude must not be above - / °. if it were so, the satellite would be hidden by the body of mars, just as we, in the british islands, would be unable to see an object revolving round the earth a few hundred miles above the equator. before passing from the attractive subject of the satellites, we may just mention two points of a literary character. mr. hall consulted his classical friends as to the designation to be conferred on the two satellites. homer was referred to, and a passage in the "iliad" suggested the names of deimos and phobos. these personages were the attendants of mars, and the lines in which they occur have been thus construed by my friend professor tyrrell:-- "mars spake, and called dismay and rout to yoke his steeds, and he did on his harness sheen." a curious circumstance with respect to the satellites of mars will be familiar to those who are acquainted with "gulliver's travels." the astronomers on board the flying island of laputa had, according to gulliver, keen vision and good telescopes. the traveller says that they had found two satellites to mars, one of which revolved around him in ten hours, and the other in twenty-one and a half. the author has thus not only made a correct guess about the number of the satellites, but he actually stated the periodic time with considerable accuracy! we do not know what can have suggested the latter guess. a few years ago any astronomer reading the voyage to laputa would have said this was absurd. there might be two satellites to mars, no doubt; but to say that one of them revolves in ten hours would be to assert what no one could believe. yet the truth has been even stranger than the fiction. and now we must bring to a close our account of this beautiful and interesting planet. there are many additional features over which we are tempted to linger, but so many other bodies claim our attention in the solar system, so many other bodies which exceed mars in size and intrinsic importance, that we are obliged to desist. our next step will not, however, at once conduct us to the giant planets. we find outside mars a host of objects, small indeed, but of much interest; and with these we shall find abundant occupation for the following chapter. chapter xi. the minor planets. the lesser members of our system--bode's law--the vacant region in the planetary system--the research--the discovery of piazzi--was the small body a planet?--the planet becomes invisible--gauss undertakes the search by mathematics--the planet recovered--further discoveries--number of minor planets now known--the region to be searched--the construction of the chart for the search for small planets--how a minor planet is discovered--physical nature of the minor planets--small gravitation on the minor planets--the berlin computations--how the minor planets tell us the distance of the sun--accuracy of the observations--how they may be multiplied--victoria and sappho--the most perfect method. in our chapters on the sun and moon, on the earth and venus, and on mercury and mars, we have been discussing the features and the movements of globes of vast dimensions. the least of all these bodies is the moon, but even that globe is , miles from one side to the other. in approaching the subject of the minor planets we must be prepared to find objects of dimensions quite inconsiderable in comparison with the great spheres of our system. no doubt these minor planets are all of them some few miles, and some of them a great many miles, in diameter. were they close to the earth they would be conspicuous, and even splendid, objects; but as they are so distant they do not, even in our greatest telescopes, become very remarkable, while to the unaided eye they are almost all invisible. in the diagram (p. ) of the orbits of the various planets, it is shown that a wide space exists between the orbit of mars and that of jupiter. it was often surmised that this ample region must be tenanted by some other planet. the presumption became much stronger when a remarkable law was discovered which exhibited, with considerable accuracy, the relative distances of the great planets of our system. take the series of numbers, , , , , , , , whereof each number (except the second) is double of the number which precedes it. if we now add four to each, we have the series , , , , , , . with the exception of the fifth of these numbers ( ), they are all sensibly proportional to the distances of the various planets from the sun. in fact, the distances are as follows:--mercury, · ; venus, · ; earth, ; mars, · ; jupiter, · ; saturn, · . although we have no physical reason to offer why this law--generally known as bode's--should be true, yet the fact that it is so nearly true in the case of all the known planets tempts us to ask whether there may not also be a planet revolving around the sun at the distance represented by . so strongly was this felt at the end of the eighteenth century that some energetic astronomers decided to make a united effort to search for the unknown planet. it seemed certain that the planet could not be a large one, as otherwise it must have been found long ago. if it should exist, then means were required for discriminating between the planet and the hosts of stars strewn along its path. the search for the small planet was soon rewarded by a success which has rendered the evening of the first day in the nineteenth century memorable in astronomy. it was in the pure skies of palermo that the observatory was situated where the memorable discovery of the first known minor planet was made by piazzi. this laborious and accomplished astronomer had organised an ingenious system of exploring the heavens which was eminently calculated to discriminate a planet among the starry host. on a certain night he would select a series of stars to the number of fifty, more or less, according to circumstances. with his meridian circle he determined the places of the chosen objects. the following night, or, at all events, as soon as convenient, he re-observed the whole fifty stars with the same instrument and in the same manner, and the whole operation was afterwards repeated on two, or perhaps more, nights. when the observations were compared together he was in possession of some four or more places of each one of the stars on different nights, and the whole series was complete. he was persevering enough to carry on these observations for very many groups, and at length he was rewarded by a success which amply compensated him for all his toil. it was on the st of january, , that piazzi commenced for the one hundred and fifty-ninth time to observe a new series. fifty stars this night were viewed in his telescope, and their places were carefully recorded. of these objects the first twelve were undoubtedly stellar, and so to all appearance was the thirteenth, a star of the eighth magnitude in the constellation of taurus. there was nothing to distinguish the telescopic appearance of this object from all the others which preceded or followed it. the following night piazzi, according to his custom, re-observed the whole fifty stars, and he did the same again on the rd of january, and once again on the th. he then, as usual, brought together the four places he had found for each of the several bodies. when this was done it was at once seen that the thirteenth object on the list was quite a different body from the remainder and from all the other stars which he had ever observed before. the four places of this mysterious object were all different; in other words, it was in movement, and was therefore a planet. a few days' observation sufficed to show how this little body, afterwards called ceres, revolved around the sun, and how it circulated in that vacant path intermediate between the path of mars and the path of jupiter. great, indeed, was the interest aroused by this discovery and the influence which it has exercised on the progress of astronomy. the majestic planets of our system had now to admit a much more humble object to a share of the benefits dispensed by the sun. after piazzi had obtained a few further observations, the season for observing this part of the heavens passed away, and the new planet of course ceased to be visible. in a few months, no doubt, the same part of the sky would again be above the horizon after dark, and the stars would of course be seen as before. the planet, however, was moving, and would continue to move, and by the time the next season had arrived it would have passed off into some distant region, and would be again confounded with the stars which it so closely resembled. how, then, was the planet to be pursued through its period of invisibility and identified when it again came within reach of observation? this difficulty attracted the attention of astronomers, and they sought for some method by which the place of the planet could be recovered so as to prevent piazzi's discovery from falling into oblivion. a young german mathematician, whose name was gauss, opened his distinguished career by a successful attempt to solve this problem. a planet, as we have shown, describes an ellipse around the sun, and the sun lies at a focus of that curve. it can be demonstrated that when three positions of a planet are known, then the ellipse in which the planet moves is completely determined. piazzi had on each occasion measured the place which it then occupied. this information was available to gauss, and the problem which he had to solve may be thus stated. knowing the place of the planet on three nights, it is required, without any further observations, to tell what the place of the planet will be on a special occasion some months in the future. mathematical calculations, based on the laws of kepler, will enable this problem to be solved, and gauss succeeded in solving it. gauss demonstrated that though the telescope of the astronomer was unable to detect the wanderer during its season of invisibility, yet the pen of the mathematician could follow it with unfailing certainty. when, therefore, the progress of the seasons permitted the observations to be renewed, the search was recommenced. the telescope was directed to the point which gauss's calculations indicated, and there was the little ceres. ever since its re-discovery, the planet has been so completely bound in the toils of mathematical reasoning that its place every night of the year can be indicated with a fidelity approaching to that attainable in observing the moon or the great planets of our system. the discovery of one minor planet was quickly followed by similar successes, so that within seven years pallas, juno, and vesta were added to the solar system. the orbits of all these bodies lie in the region between the orbit of mars and of jupiter, and for many years it seems to have been thought that our planetary system was now complete. forty years later systematic research was again commenced. planet after planet was added to the list; gradually the discoveries became a stream of increasing volume, until in the total number reached about . their distribution in the solar system is somewhat as represented in fig. . by the improvement of astronomical telescopes, and by the devotion with which certain astronomers have applied themselves to this interesting research, a special method of observing has been created for the distinct purpose of searching out these little objects. it is known that the paths in which all the great planets move through the heavens coincide very nearly with the path which the sun appears to follow among the stars, and which is known as the ecliptic. it is natural to assume that the small planets also move in the same great highway, which leads them through all the signs of the zodiac in succession. some of the small planets, no doubt, deviate rather widely from the track of the sun, but the great majority are approximately near it. this consideration at once simplifies the search for new planets. a certain zone extending around the heavens is to be examined, but there is in general little advantage in pushing the research into other parts of the sky. the next step is to construct a map containing all the stars in this region. this is a task of very great labour; the stars visible in the large telescopes are so numerous that many tens of thousands, perhaps we should say hundreds of thousands, are included in the region so narrowly limited. the fact is that many of the minor planets now known are objects of extreme minuteness; they can only be seen with very powerful telescopes, and for their detection it is necessary to use charts on which even the faintest stars have been depicted. many astronomers have concurred in the labour of producing these charts; among them may be mentioned palisa, of vienna, who by means of his charts has found eighty-three minor planets, and the late professor peters, of clinton, new york, who in a similar way found forty-nine of these bodies. [illustration: fig. .--the zone of minor planets between mars and jupiter.] the astronomer about to seek for a new planet directs his telescope towards that part of the sun's path which is on the meridian at midnight; there, if anywhere, lies the chance of success, because that is the region in which such a body is nearer to the earth than at any other part of its course. he steadfastly compares his chart with the heavens, and usually finds the stars in the heavens and the stars in the chart to correspond; but sometimes it will happen that a point in the heavens is missing from the chart. his attention is at once arrested; he follows the object with care, and if it moves it is a planet. still he cannot be sure that he has really made a discovery; he has found a planet, no doubt, but it may be one of the large number already known. to clear up this point he must undertake a further, and sometimes a very laborious, enquiry; he must search the berlin year-book and other ephemerides of such planets and see whether it is possible for one of them to have been in the position on the night in question. if he can ascertain that no previously discovered body could have been there, he is then entitled to announce to his brother astronomers the discovery of a new member of the solar system. it seems certain that all the more important of the minor planets have been long since discovered. the recent additions to the list are generally extremely minute objects, beyond the powers of small telescopes. since the method of searching for minor planets which we have just described has been almost abandoned in favour of a process greatly superior. it has been found feasible to employ photography for making charts of the heavens. a photographic plate is exposed in the telescope to a certain region of the sky sufficiently long to enable very faint telescopic stars to imprint their images. care has to be taken that the clock which moves the camera shall keep pace most accurately with the rotation of the earth, so that fixed stars appear on the plate as sharp points. if, on developing the plate, a star is found to have left a trail, it is evident that this star must during the time of exposure (generally some hours) have had an independent motion of its own; in other words, it must be a planet. for greater security a second picture is generally taken of the same region after a short interval. if the place occupied by the trail on the first plate is now vacant, while on the second plate a new trail appears in a line with the first one, there remains no possible doubt that we have genuine indications of a planet, and that we have not been led astray by some impurity on the plate or by a few minute stars which happened to lie very closely together. wolf, of heidelberg, and following in his footsteps charlois, of nice, have in this manner discovered a great number of new minor planets, while they have also recovered a good many of those which had been lost sight of owing to an insufficiency of observations. on the th of august, , herr g. witt, of the observatory of urania in berlin, discovered a new asteroid by the photographic method. this object was at first regarded merely as forming an addition of no special importance to the asteroids whose discovery had preceded it. it received, as usual, a provisional designation in accordance with a simple alphabetical device. this temporary label affixed to witt's asteroid was "d q." but the formal naming of the asteroid has now superseded this label. herr witt has given to his asteroid the name of "eros." this has been duly accepted by astronomers, and thus for all time the planet is to be known. the feature which makes the discovery of eros one of the most remarkable incidents in recent astronomy is that on those rare occasions when this asteroid comes nearest to the earth it is closer to the earth than the planet mars can ever be. closer than the planet venus can ever be. closer than any other known asteroid can ever be. thus we assign to eros the exceptional position of being our nearest planetary neighbour in the whole host of heaven. under certain circumstances it will have a distance from the earth not exceeding one-seventh of the mean distance of the sun. of the physical composition of the asteroids and of the character of their surfaces we are entirely ignorant. it may be, for anything we can tell, that these planets are globes like our earth in miniature, diversified by continents and by oceans. if there be life on such bodies, which are often only a few miles in diameter, that life must be something totally different from anything with which we are familiar. setting aside every other difficulty arising from the possible absence of water and from the great improbability of finding there an atmosphere of a density and a composition suitable for respiration, gravitation itself would prohibit organic beings adapted for this earth from residing on a minor planet. let us attempt to illustrate this point, and suppose that we take the case of a minor planet eight miles in diameter, or, in round numbers, one-thousandth part of the diameter of the earth. if we further suppose that the materials of the planet are of the same nature as the substances in the earth, it is easy to prove that the gravity on the surface of the planet will be only one-thousandth part of the gravity of the earth. it follows that the weight of an object on the earth would be reduced to the thousandth part if that object were transferred to the planet. this would not be disclosed by an ordinary weighing scales, where the weights are to be placed in one pan and the body to be weighed in the other. tested in this way, a body would, of course, weigh precisely the same anywhere; for if the gravitation of the body is altered, so is also in equal proportion the gravitation of the counterpoising weights. but, weighed with a spring balance, the change would be at once evident, and the effort with which a weight could be raised would be reduced to one-thousandth part. a load of one thousand pounds could be lifted from the surface of the planet by the same effort which would lift one pound on the earth; the effects which this would produce are very remarkable. in our description of the moon it was mentioned (p. ) that we can calculate the velocity with which it would be necessary to discharge a projectile so that it would never again fall back on the globe from which it was expelled. we applied this reasoning to explain why the moon has apparently altogether lost any atmosphere it might have once possessed. if we assume for the sake of illustration that the densities of all planets are identical, then the law which expresses the critical velocity for each planet can be readily stated. it is, in fact, simply proportional to the diameter of the globe in question. thus, for a minor planet whose diameter was one-thousandth part of that of the earth, or about eight miles, the critical velocity would be the thousandth part of six miles a second--that is, about thirty feet per second. this is a low velocity compared with ordinary standards. a child easily tosses a ball up fifteen or sixteen feet high, yet to carry it up this height it must be projected with a velocity of thirty feet per second. a child, standing upon a planet eight miles in diameter, throws his ball vertically upwards; up and up the ball will soar to an amazing elevation. if the original velocity were less than thirty feet per second, the ball would at length cease to move, would begin to turn, and fall with a gradually accelerating pace, until at length it regained the surface with a speed equal to that with which it had been projected. if the original velocity had been as much as, or more than, thirty feet per second, then the ball would soar up and up never to return. in a future chapter it will be necessary to refer again to this subject. a few of the minor planets appear in powerful telescopes as discs with appreciable dimensions, and they have even been measured with the micrometer. in this way professor barnard, late of the lick observatory, determined the following values for the diameters of the four first discovered minor planets:-- ceres miles. pallas miles. juno miles. vesta miles. the value for juno is, however, very uncertain, and by far the greater number of the minor planets are very much smaller than the figures here given would indicate. it is possible by a certain calculation to form an estimate of the aggregate mass of all the minor planets, inasmuch as observations disclose to us the extent of their united disturbing influences on the motion of mars. in this manner le verrier concluded that the collected mass of the small planets must be about equal to one-fourth of the mass of the earth. harzer, repeating the enquiry in an improved manner, deduced a collected mass one-sixth of that of the earth. there can be no doubt that the total mass of all the minor planets at present known is not more than a very small fraction of the amount to which these calculations point. we therefore conclude that there must be a vast number of minor planets which have not yet been recognised in the observatory. these unknown planets must be extremely minute. the orbits of this group of bodies differ in remarkable characteristics from those of the larger planets. some of them are inclined at angles of ° to the plane of the earth's orbit, the inclinations of the great planets being not more than a few degrees. some of the orbits of the minor planets are also greatly elongated ellipses, while, of course, the orbits of the large planets do not much depart from the circular form. the periods of revolution of these small objects round the sun range from three years to nearly nine years. a great increase in the number of minor planets has rewarded the zeal of those astronomers who have devoted their labours to this subject. their success has entailed a vast amount of labour on the computers of the "berlin year-book." that useful work occupies in this respect a position which has not been taken by our own "nautical almanac," nor by the similar publications of other countries. a skilful band of computers make it their duty to provide for the "berlin year-book" detailed information as to the movements of the minor planets. as soon as a few complete observations have been obtained, the little object passes into the secure grasp of the mathematician; he is able to predict its career for years to come, and the announcements with respect to all the known minor planets are to be found in the annual volumes of the work referred to. the growth of discovery has been so rapid that the necessary labour for the preparation of such predictions is now enormous. it must be confessed that many of the minor planets are very faint and otherwise devoid of interest, so that astronomers are sometimes tempted to concur with the suggestion that a portion of the astronomical labour now devoted to the computation of the paths of these bodies might be more profitably applied. for this it would be only necessary to cast adrift all the less interesting members of the host, and allow them to pursue their paths unwatched by the telescope, or by the still more ceaseless tables of the mathematical computer. the sun, which controls the mighty orbs of our system, does not disdain to guide, with equal care, the tiny globes which form the minor planets. at certain times some of them approach near enough to the earth to merit the attention of those astronomers who are specially interested in determining the dimensions of the solar system. the observations are of such a nature that they can be made with considerable precision; they can also be multiplied to any extent that may be desired. some of these little bodies have consequently a great astronomical future, inasmuch as they seem destined to indicate the true distance from the earth to the sun more accurately than venus or than mars. the smallest of these planets will not answer for this purpose; they can only be seen in powerful telescopes, and they do not admit of being measured with the necessary accuracy. it is also obvious that the planets to be chosen for observation must come as near the earth as possible. in favourable circumstances, some of the minor planets will approach the earth to a distance which is about three-quarters of the distance of the sun. these various conditions limit the number of bodies available for this purpose to about a dozen, of which one or two will usually be suitably placed each year. for the determination of the sun's distance this method by the minor planets offers unquestionable advantages. the orb itself is a minute star-like point in the telescope, and the measures are made from it to the stars which are seen near it. a few words will, perhaps, be necessary at this place as to the nature of the observations referred to. when we speak of the measures from the planet to the star, we do not refer to what would be perhaps the most ordinary acceptation of the expression. we do _not_ mean the actual measurement of the number of miles in a straight line between the planet and the star. this element, even if attainable, could only be the result of a protracted series of observations of a nature which will be explained later on when we come to speak of the distances of the stars. the measures now referred to are of a more simple character; they are merely to ascertain the apparent distance of the objects expressed in angular measure. this angular measurement is of a wholly different character from the linear measurement, and the two methods may, indeed, lead to results that would at first seem paradoxical. we may take, as an illustration, the case of the group of stars forming the pleiades, and those which form the great bear. the latter is a large group, the former is a small one. but why do we think the words large and small rightly applied here? each pair of stars of the great bear makes a large angle with the eye. each pair of stars in the pleiades makes a small angle, and it is these angles which are the direct object of astronomical measurement. we speak of the distance of two stars, meaning thereby the angle which is bounded by the two lines from the eye to the two stars. this is what our instruments are able to measure, and it is to be observed that no reference to linear magnitude is implied. indeed, if we are to mention actual dimensions, it is quite possible, for anything we can tell, that the pleiades may form a much larger group than the great bear, and that the apparent superiority of the latter is merely due to its being closer to us. the most accurate of these angular measures are obtained when two stars, or two star-like points, are so close together as to enable them to be included in one field of view of the telescope. there are special forms of apparatus which enable the astronomer in this case to give to his observations a precision unattainable in the measurement of objects less definitely marked, or at a greater apparent distance. the determination of the distance of the small star-like planet from a star is therefore characterised by great accuracy. but there is another and, perhaps, a weightier argument in favour of the determination of the scale of the solar system by this process. the real strength of the minor planet method rests hardly so much on the individual accuracy of the observations, as on the fact that from the nature of the method a considerable number of repetitions can be concentrated on the result. it will, of course, be understood that when we speak of the accuracy of an observation, it is not to be presumed that it can ever be entirely free from error. errors always exist, and though they may be small, yet if the quantity to be measured is minute, an error of intrinsic insignificance may amount to an appreciable fraction of the whole. the one way by which their effect can be subdued is by taking the mean of a large number of observations. this is the real source of the value of the minor planet method. we have not to wait for the occurrence of rare events like the transit of venus. each year will witness the approach of some one or more minor planets sufficiently close to the earth to render the method applicable. the varied circumstances attending each planet, and the great variety of the observations which may be made upon it, will further conduce to eliminate error. as the planet pursues its course through the sky, which is everywhere studded over with countless myriads of minute stars, it is evident that this body, itself so like a star, will always have some stars in its immediate neighbourhood. as the movements of the planet are well known, we can foretell where it will be on each night that it is to be observed. it is thus possible to prearrange with observers in widely-different parts of the earth as to the observations to be made on each particular night. an attempt has been made, on the suggestion of dr. gill, to carry out this method on a scale commensurate with its importance. the planets iris, victoria, and sappho happened, in the years and , to approach so close to the earth that arrangements were made for simultaneous measurements in both the northern and the southern hemispheres. a scheme was completely drawn up many months before the observations were to commence. each observer who participated in the work was thus advised beforehand of the stars which were to be employed each night. viewed from any part of the earth, from the cape of good hope or from great britain, the positions of the stars remain absolutely unchanged. their distance is so stupendous that a change of place on the earth displaces them to no appreciable extent. but the case is different with a minor planet. it is hardly one-millionth part of the distance of the stars, and the displacement of the planet when viewed from the cape and when viewed from europe is a measurable quantity. the magnitude we are seeking is to be elicited by comparison between the measurements made in the northern hemisphere with those made in the southern. the observations in the two localities must be as nearly simultaneous as possible, due allowance being made for the motion of the planet in whatever interval may have elapsed. although every precaution is taken to eliminate the errors of each observation, yet the fact remains that we compare the measures made by observers in the northern hemisphere with those made by different observers, using of course different instruments, thousands of miles away. but in this respect we are at no greater disadvantage than in observing the transit of venus. it is, however, possible to obviate even this objection, and thus to give the minor planet method a supremacy over its rival which cannot be disputed. the difficulty would be overcome if we could arrange that an astronomer, after making a set of observations on a fine night in the northern hemisphere, should be instantly transferred, instruments and all, to the southern station, and there repeat the observations. an equivalent transformation can be effected without any miraculous agency, and in it we have undoubtedly the most perfect mode of measuring the sun's distance with which we are acquainted. this method has already been applied with success by dr. gill in the case of juno, and there are other members of the host of minor planets still more favourably circumstanced. consider, for instance, a minor planet, which sometimes approaches to within , , miles of the earth. when the opposition is drawing near, a skilled observer is to be placed at some suitable station near the equator. the instrument he is to use should be that marvellous piece of mechanical and optical skill known as the heliometer.[ ] it can be used to measure the angular distance between objects too far apart for the filar micrometer. the measurements are to be made in the evening as soon as the planet has risen high enough to enable it to be seen distinctly. the observer and the observatory are then to be transferred to the other side of the earth. how is this to be done? say, rather, how we could prevent it from being done. is not the earth rotating on its axis, so that in the course of a few hours the observatory on the equator is carried bodily round for thousands of miles? as the morning approaches the observations are to be repeated. the planet is found to have changed its place very considerably with regard to the stars. this is partly due to its own motion, but it is also largely due to the parallactic displacement arising from the rotation of the earth, which may amount to so much as twenty seconds. the measures on a single night with the heliometer should not have a mean error greater than one-fifth of a second, and we might reasonably expect that observations could be secured on about twenty-five nights during the opposition. four such groups might be expected to give the sun's distance without any uncertainty greater than the thousandth part of the total amount. the chief difficulty of the process arises from the movement of the planet during the interval which divides the evening from the morning observations. this drawback can be avoided by diligent and repeated measurements of the place of the planet with respect to the stars among which it passes. in the monumental piece of work which issued in from the cape observatory, under the direction of dr. gill, the final results from the observations of iris, victoria, and sappho have been obtained. from this it appears that the angle which the earth's equatorial radius subtends at the centre of the sun when at its mean distance has the value "· . if we employ the best value of the earth's equatorial radius we obtain , , miles as the mean distance of the centre of the sun from the centre of the earth. this is probably the most accurate determination of the scale of the solar system which has yet been made. chapter xii. jupiter. the great size of jupiter--comparison of his diameter with that of the earth--dimensions of the planet and his orbit--his rotation--comparison of his weight and bulk with that of the earth--relative lightness of jupiter--how explained--jupiter still probably in a heated condition--the belts on jupiter--spots on his surface--time of rotation of different spots various--storms on jupiter--jupiter not incandescent--the satellites--their discovery--telescopic appearance--their orbits--the eclipses and occultations--a satellite in transit--the velocity of light discovered--how is this velocity to be measured experimentally?--determination of the sun's distance by the eclipses of jupiter's satellites--jupiter's satellites demonstrating the copernican system. in our exploration of the beautiful series of bodies which form the solar system, we have proceeded step by step outwards from the sun. in the pursuit of this method we have now come to the splendid planet jupiter, which wends its majestic way in a path immediately outside those orbits of the minor planets which we have just been considering. great, indeed, is the contrast between these tiny globes and the stupendous globe of jupiter. had we adopted a somewhat different method of treatment--had we, for instance, discussed the various bodies of our planetary system in the order of their magnitude--then the minor planets would have been the last to be considered, while the leader of the host would be jupiter. to this position jupiter is entitled without an approach to rivalry. the next greatest on the list, the beautiful and interesting saturn, comes a long distance behind. another great descent in the scale of magnitude has to be made before we reach uranus and neptune, while still another step downwards must be made before we reach that lesser group of planets which includes our earth. so conspicuously does jupiter tower over the rest, that even if saturn were to be augmented by all the other globes of our system rolled into one, the united mass would still not equal the great globe of jupiter. [illustration: fig. .--the relative dimensions of jupiter and the earth.] the adjoining picture (fig. ) shows the relative dimensions of jupiter and the earth, and it conveys to the eye a more vivid impression of the enormous bulk of jupiter than we can readily obtain by merely considering the numerical statements by which his bulk is to be accurately estimated. as, however, it will be necessary to place the numerical facts before our readers, we do so at the outset of this chapter. jupiter revolves in an elliptic orbit around the sun in the focus, at a mean distance of , , miles. the path of jupiter is thus about · times as great in diameter as the path pursued by the earth. the shape of jupiter's orbit departs very appreciably from a circle, the greatest distance from the sun being · , while the least distance is about · , the earth's distance from the sun being taken as unity. in the most favourable circumstances for seeing jupiter at opposition, it must still be about four times as far from the earth as the earth is from the sun. this great globe will also illustrate the law that the more distant a planet is, the slower is the velocity with which its orbital motion is accomplished. while the earth passes over eighteen miles each second, jupiter only accomplishes eight miles. thus for a twofold reason the time occupied by an exterior planet in completing a revolution is greater than the period of the earth. not only has the outer planet to complete a longer course than the earth, but the speed is less; it thus happens that jupiter requires , · days, or about fifty days less than twelve years, to make a circuit of the heavens. the mean diameter of the great planet is about , miles. we say the _mean_ diameter, because there is a conspicuous difference in the case of jupiter between his equatorial and his polar diameters. we have already seen that there is a similar difference in the case of the earth, where we find the polar diameter to be shorter than the equatorial; but the inequality of these two dimensions is very much larger in jupiter than in the earth. the equatorial diameter of jupiter is , miles, while the polar is not more than , miles. the ellipticity of jupiter indicated by these figures is sufficiently marked to be obvious without any refined measures. around the shortest diameter the planet spins with what must be considered an enormous velocity when we reflect on the size of the globe. each rotation is completed in about hrs. mins. we may naturally contrast the period of rotation of jupiter with the much slower rotation of our earth in twenty-four hours. the difference becomes much more striking if we consider the relative speeds at which an object on the equator of the earth and on that of jupiter actually moves. as the diameter of jupiter is nearly eleven times that of the earth, it will follow that the speed of the equator on jupiter must be about twenty-seven times as great as that on the earth. it is no doubt to this high velocity of rotation that we must ascribe the extraordinary ellipticity of jupiter; the rapid rotation causes a great centrifugal force, and this bulges out the pliant materials of which he seems to be formed. jupiter is not, so far as we can see, a solid body. this is an important circumstance; and therefore it will be necessary to discuss the matter at some little length, as we here perceive a wide contrast between this great planet and the other planets which have previously occupied our attention. from the measurements already given it is easy to calculate the bulk or the volume of jupiter. it will be found that this planet is about , times as large as the earth; in other words, it would take , globes, each as large as our earth, all rolled into one, to form a single globe as large as jupiter. if the materials of which jupiter is composed were of a nature analogous to the materials of the earth, we might expect that the weight of the planet would exceed the weight of the earth in something like the proportion of their volumes. this is the matter now proposed to be brought to trial. here we may at once be met with the query, as to how we are to find the weight of jupiter. it is not even an easy matter to weigh the earth on which we stand. how, then, can we weigh a mighty planet vastly larger than the earth, and distant from us by some hundreds of millions of miles? truly, this is a bold problem. yet the intellectual resources of man have proved sufficient to achieve this feat of celestial engineering. they are not, it is true, actually able to make the ponderous weighing scales in which the great planet is to be cast, but they are able to divert to this purpose certain natural phenomena which yield the information that is required. such investigations are based on the principle of universal gravitation. the mass of jupiter attracts other masses in the solar system. the efficiency of that attraction is more particularly shown on the bodies which are near the planet. in virtue of this attraction certain movements are performed by those bodies. we can observe their character with our telescopes, we can ascertain their amount, and from our measurements we can calculate the mass of the body by which the movements have been produced. this is the sole method which we possess for the investigation of the masses of the planets; and though it may be difficult in its application--not only from the observations which are required, but also from the intricacy and the profundity of the calculations to which those observations must be submitted--yet, in the case of jupiter at least, there is no uncertainty about the result. the task is peculiarly simplified in the case of the greatest planet of our system by the beautiful system of moons with which he is attended. these little moons revolve under the guidance of jupiter, and their movements are not otherwise interfered with so as to prevent their use for our present purpose. it is from the observations of the satellites of jupiter that we are enabled to measure his attractive power, and thence to calculate the mass of the mighty planet. to those not specially conversant with the principles of mechanics, it may seem difficult to realise the degree of accuracy of which such a method is capable. yet there can be no doubt that his moons inform us of the mass of jupiter, and do not leave a margin of inaccuracy so great as one hundredth part of the total amount. if other confirmation be needed, then it is forthcoming in abundance. a minor planet occasionally draws near the orbit of jupiter and experiences his attraction; the planet is forced to swerve from its path, and the amount of the deviation can be measured. from that measurement the mass of jupiter can be computed by a calculation, of which it would be impossible to give an account in this place. the mass of jupiter, as determined by this method, agrees with the mass obtained in a totally different manner from the satellites. nor have we yet exhausted the resources of astronomy in its bearing on this question. we can discard the planetary system, and invite the assistance of a comet which, flashing through the orbits of the planets, occasionally experiences large and sometimes enormous disturbances. for the present it suffices to remark, that on one or two occasions it has happened that venturous comets have been near enough to jupiter to be much disturbed by his attraction, and then to proclaim in their altered movements the magnitude of the mass which has affected them. the satellites of jupiter, the minor planets, and the comets, all tell the weight of the giant orb; and, as they all concur in the result (at least within extremely narrow limits), we cannot hesitate to conclude that the mass of the greatest planet of our system has been determined with accuracy. the results of these measures must now be stated. they show, of course, that jupiter is vastly inferior to the sun--that, in fact, it would take about , jupiters, all rolled into one, to form a globe equal in _weight_ to the sun. they also show us that it would take globes as heavy as our earth to counterbalance the weight of jupiter. no doubt this proves jupiter to be a body of magnificent proportions; but the remarkable circumstance is not that jupiter should be times as heavy as the earth, but that he is not a great deal more. have we not stated that jupiter is , times as _large_ as the earth? how then comes it that he is only times as _heavy_? this points at once to some fundamental contrast between the constitution of jupiter and of the earth. how are we to account for this difference? we can conceive of two explanations. in the first place, it might be supposed that jupiter is constituted of materials partly or wholly unknown on the earth. there is, however, an alternative supposition at once more philosophical and more consistent with the evidence. it is true that we know little or nothing of what the elementary substances on jupiter may be, but one of the great discoveries of modern astronomy has taught us something of the elementary bodies present in other bodies of the universe, and has demonstrated that to a large extent they are identical with the elementary bodies on the earth. if jupiter be composed of bodies resembling those on the earth, there is one way, and only one, in which we can account for the disparity between his size and his mass. perhaps the best way of stating the argument will be found in a glance at the remote history of the earth itself, for it seems not impossible that the present condition of jupiter was itself foreshadowed by the condition of our earth countless ages ago. in a previous chapter we had occasion to point out how the earth seemed to be cooling from an earlier and highly heated condition. the further we look back, the hotter our globe seems to have been; and if we project our glance back to an epoch sufficiently remote, we see that it must once have been so hot that life on its surface would have been impossible. back still earlier, we find the heat to have been such that water could not rest on the earth; and hence it seems likely that at some incredibly remote epoch all the oceans now reposing in the deeps on the surface, and perhaps a considerable portion of its now solid crust, must have been in a state of vapour. such a transformation of the globe would not alter its _mass_, for the materials weigh the same whatever be their condition as to temperature, but it would alter the _size_ of our globe to a very considerable extent. if these oceans were transformed into vapour, then the atmosphere, charged with mighty clouds, would have a bulk some hundreds of times greater than that which it has at present. viewed from a distant planet, the cloud-laden atmosphere would indicate the visible size of our globe, and its average density would accordingly appear to be very much less than it is at present. from these considerations it will be manifest that the discrepancy between the size and the weight of jupiter, as contrasted with our earth, would be completely removed if we supposed that jupiter was at the present day a highly heated body in the condition of our earth countless ages ago. every circumstance of the case tends to justify this argument. we have assigned the smallness of the moon as a reason why the moon has cooled sufficiently to make its volcanoes silent and still. in the same way the smallness of the earth, as compared with jupiter, accounts for the fact that jupiter still retains a large part of its original heat, while the smaller earth has dissipated most of its store. this argument is illustrated and strengthened when we introduce other planets into the comparison. as a general rule we find that the smaller bodies, like the earth and mars, have a high density, indicative of a low temperature, while the giant planets, like jupiter and saturn, have a low density, suggesting that they still retain a large part of their original heat. we say "original heat" for the want, perhaps, of a more correct expression; it will, however, indicate that we do not in the least refer to the solar heat, of which, indeed, the great outer planets receive much less than those nearer the sun. where the original heat may have come from is a matter still confined to the province of speculation. a complete justification of these views with regard to jupiter is to be found when we make a minute telescopic scrutiny of its surface; and it fortunately happens that the size of the planet is so great that, even at a distance of more millions of miles than there are days in the year, we can still trace on its surface some significant features. plate xi. gives a series of four different views of jupiter. they have been taken from a series of admirable drawings of the great planet made by mr. griffiths in . the first picture shows the appearance of the globe at h. m. greenwich time on february th, , through a powerful refracting telescope. we at once notice in this drawing that the outline of jupiter is distinctly elliptical. the surface of the planet usually shows the remarkable series of belts here represented. they are nearly parallel to each other and to the planet's equator. when jupiter is observed for some hours, the appearance of the belts undergoes certain changes. these are partly due to the regular rotation of the planet on its axis, which, in a period of less than five hours, will completely carry away the hemisphere we first saw, and replace it by the hemisphere originally at the other side. but besides the changes thus arising, the belts and other features on the planet are also very variable. sometimes new stripes or marks appear, and old ones disappear; in fact, a thorough examination of jupiter will demonstrate the remarkable fact that there are no permanent features whatever to be discerned. we are here immediately struck by the contrast between jupiter and mars; on the smaller planet the main topographical outlines are almost invariable, and it has been feasible to construct maps of the surface with tolerably accurate detail; a map of jupiter is, however, an impossibility--the drawing of the planet which we make to-night will be different from the drawing of the same hemisphere made a few weeks hence. it should, however, be noticed that objects occasionally appear on the planet which seem of a rather more persistent character than the belts. we may especially mention the object known as the great oblong red spot, which has been a very remarkable feature upon the southern hemisphere of jupiter since . this object, which has attracted a great deal of attention from observers, is about , miles long by about , in breadth. professor barnard remarks that the older the spots on jupiter are, the more ruddy do they tend to become. the conclusion is irresistibly forced upon us that when we view the surface of jupiter we are not looking at any solid body. the want of permanence in the features of the planet would be intelligible if what we see be merely an atmosphere laden with clouds of impenetrable density. the belts especially support this view; we are at once reminded of the equatorial zones on our own earth, and it is not at all unlikely that an observer sufficiently remote from the earth to obtain a just view of its appearance would detect upon its surface more or less perfect cloud-belts suggestive of those on jupiter. a view of our earth would be, as it were, intermediate between a view of jupiter and of mars. in the latter case the appearance of the permanent features of the planet is only to a trifling extent obscured by clouds floating over the surface. our earth would always be partly, and often perhaps very largely, covered with cloud, while jupiter seems at all times completely enveloped. from another class of observations we are also taught the important truth that jupiter is not, superficially at least, a solid body. the period of rotation of the planet around its axis is derived from the observation of certain marks, which present sufficient definiteness and sufficient permanence to be suitable for the purpose. suppose one of these objects to lie at the centre of the planet's disc; its position is carefully measured, and the time is noted. as the hours pass on, the mark moves to the edge of the disc, then round the other side of the planet, and back again to the visible disc. when it has returned to the position originally occupied the time is again taken, and the interval which has elapsed is called the period of rotation of the spot. if jupiter were a solid, and if these features were engraved upon its surface, then it is perfectly clear that the time of rotation as found by any one spot must coincide precisely with the time yielded by any other spot; but this is not observed to be the case. in fact, it would be nearer the truth to say that each spot gives a special period of its own. nor are the differences very minute. it has been found that the time in which the red spot (the latitude of which is about ° south) is carried round is five minutes longer than that required by some peculiar white marks near the equator. the red spot has now been watched for about twenty years, and during most of that time has had a tendency to rotate more and more slowly, as may be seen from the following values of its rotation period:-- in , h. m. · s. in , h. m. · s. in , h. m. · s. since this tendency seems to have ceased, while the spot has been gradually fading away. generally speaking, we may say that the equatorial regions rotate in about h. m. s., and the temperate zones in about h. m. s. remarkable exceptions are occasionally met with. some small black spots in north latitude °, which broke out in and again in , rotated in h. m. to h. - / m. it may, therefore, be regarded as certain that the globe of jupiter, so far as we can see it, is not a solid body. it consists, on the exterior at all events, of clouds and vaporous masses, which seem to be agitated by storms of the utmost intensity, if we are to judge from the ceaseless changes of the planet's surface. [illustration: plate xi. feb. nd. feb. th. feb. th. feb. th. the planet jupiter. .] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] various photographs of jupiter have been obtained; those which have been taken at the lick observatory being specially interesting and instructive. pictures of the planet obtained with the camera in remarkable circumstances are represented in figs. - , which were taken by professor wm. h. pickering at arequipa, peru, on the th of august, .[ ] the small object with the belts is the planet jupiter. the large advancing disc (of which only a small part can be shown) is the moon. the phenomenon illustrated is called the "occultation" of the planet. the planet is half-way behind the moon in fig. , while in fig. half of the planet is still hidden by the dark limb of the moon. it is well known that the tempests by which the atmosphere surrounding the earth is convulsed are to be ultimately attributed to the heat of the sun. it is the rays from the great luminary which, striking on the vast continents, warm the air in contact therewith. this heated air becomes lighter, and rises, while air to supply its place must flow in along the surface. the currents so produced form a breeze or a wind; while, under exceptional circumstances, we have the phenomena of cyclones and hurricanes, all originated by the sun's heat. need we add that the rains, which so often accompany the storms, have also arisen from the solar beams, which have distilled from the wide expanse of ocean the moisture by which the earth is refreshed? the storms on jupiter seem to be vastly greater than those on the earth. yet the intensity of the sun's heat on jupiter is only a mere fraction--less, indeed, than the twenty-fifth part--of that received by the earth. it is incredible that the motive power of the appalling tempests on the great planet can be entirely, or even largely, due to the feeble influence of solar heat. we are, therefore, led to seek for some other source of such disturbances. what that source is to be will appear obvious when we admit that jupiter still retains a large proportion of primitive internal heat. just as the sun itself is distracted by violent tempests as an accompaniment of its intense internal fervour, so, in a lesser degree, do we observe the same phenomena in jupiter. it may also be noticed that the spots on the sun usually lie in more or less regular zones, parallel to its equator, the arrangement being in this respect not dissimilar to that of the belts on jupiter. it being admitted that the mighty planet still retains some of its internal heat, the question remains as to how much. it is, of course, obvious that the heat of the planet is inconsiderable when compared with the heat of the sun. the brilliance of jupiter, which makes it an object of such splendour in our midnight sky, is derived from the same great source which illuminates the earth, the moon, or the other planets. jupiter, in fact, shines by reflected sunlight, and not in virtue of any intrinsic light in his globe. a beautiful proof of this truth is familiar to every user of a telescope. the little satellites of the planet sometimes intrude between him and the sun, and cast a shadow on jupiter. the shadow is black, or, at all events, it seems black, relatively to the brilliant surrounding surface of the planet. it must, therefore, be obvious that jupiter is indebted to the sun for its brilliancy. the satellites supply another interesting proof of this truth. one of these bodies sometimes enters the shadow of jupiter and lo! the little body vanishes. it does so because jupiter has cut off the supply of sunlight which previously rendered the satellite visible. but the planet is not himself able to offer the satellite any light in compensation for the sunlight which he has intercepted.[ ] enough, however, has been demonstrated to enable us to pronounce on the question as to whether the globe of jupiter can be inhabited by living creatures resembling those on this earth. obviously this cannot be so. the internal heat and the fearful tempests seem to preclude the possibility of organic life on the great planet, even were there not other arguments tending to the same conclusion. it may, however, be contended, with perhaps some plausibility, that jupiter has in the distant future the prospect of a glorious career as the residence of organic life. the time will assuredly come when the internal heat must decline, when the clouds will gradually condense into oceans. on the surface dry land may then appear, and jupiter be rendered habitable. from this sketch of the planet itself we now turn to the interesting and beautiful system of five satellites by which jupiter is attended. we have, indeed, already found it necessary to allude more than once to these little bodies, but not to such an extent as to interfere with the more formal treatment which they are now to receive. the discovery of the four chief satellites may be regarded as an important epoch in the history of astronomy. they are objects situated in a remarkable manner on the border-line which divides the objects visible to the unaided eye from those which require telescopic aid. it has been frequently asserted that these objects have been seen with the unaided eye; but without entering into any controversy on the matter, it is sufficient to recite the well-known fact that, although jupiter had been a familiar object for countless centuries, yet the sharpest eyes under the clearest skies never discovered the satellites until galileo turned the newly invented telescope upon them. this tube was no doubt a very feeble instrument, but very little power suffices to show objects so dose to the limit of visibility. [illustration: fig. .--jupiter and his four satellites as seen in a telescope of low power.] the view of the planet and its elaborate system of satellites as shown in a telescope of moderate power, is represented in fig. . we here see the great globe, and nearly in a line parsing through its centre lie four small objects, three on one side and one on the other. these little bodies resemble stars, but they can be distinguished therefrom by their ceaseless movements around the planet, which they never fail to accompany during his entire circuit of the heavens. there is no more pleasing spectacle for the student than to follow with his telescope the movements of this beautiful system. [illustration: fig. .--disappearances of jupiter's satellites.] in fig. we have represented some of the various phenomena which the satellites present. the long black shadow is that produced by the interposition of jupiter in the path of the sun's rays. in consequence of the great distance of the sun this shadow will extend, in the form of a very elongated cone, to a distance far beyond the orbit of the outer satellite. the second satellite is immersed in this shadow, and consequently eclipsed. the eclipse of a satellite must not be attributed to the intervention of the body of jupiter between the satellite and the earth. such an occurrence is called an occultation, and the third satellite is shown in this condition. the second and the third satellites are thus alike invisible, but the cause of the invisibility is quite different in the two cases. the eclipse is much the more striking phenomenon of the two, because the satellite, at the moment it plunges into the darkness, may be still at some apparent distance from the edge of the planet, and is thus seen up to the moment of the eclipse. in an occultation the satellite in passing behind the planet is, at the time of disappearance, close to the planet's bright edge, and the extinction of the light from the small body cannot be observed with the same impressiveness as the occurrence of an eclipse. a satellite also assumes another remarkable situation when in the course of transit over the face of the planet. the satellite itself is not always very easy to see in such circumstances, but the beautiful shadow which it casts forms a sharp black spot on the brilliant orb: the satellite will, indeed, frequently cast a visible shadow when it passes between the planet and the sun, even though it be not actually at the moment in front of the planet, as it is seen from the earth. the periods in which the four principal moons of jupiter revolve around their primary are respectively, day hrs. min. secs. for the first; days hrs. min. secs., for the second; days hrs. min. secs, for the third; and days hrs. min. secs. for the fourth. we thus observe that the periods of jupiter's satellites are decidedly briefer than that of our moon. even the satellite most distant from the great planet requires for each revolution less than two-thirds of an ordinary lunar month. the innermost of these bodies, revolving as it does in less than two days, presents a striking series of ceaseless and rapid changes, and it becomes eclipsed during every revolution. the distance from the centre of jupiter to the orbit of the innermost of these four attendants is a quarter of a million miles, while the radius of the outermost is a little more than a million miles. the second of the satellites proceeding outwards from the planet is almost the same size as our moon; the other three bodies are larger; the third being the greatest of all (about , miles in diameter). owing to the minuteness of the satellites as seen from the earth, it is extremely difficult to perceive any markings on their surfaces, but the few observations made seem to indicate that the satellites (like our moon) always turn the same face towards their primary. professor barnard has, with the great lick refractor, seen a white equatorial belt on the first satellite, while its poles were very dark. mr. douglass, observing with mr. lowell's great refractor, has also reported certain streaky markings on the third satellite. a very interesting astronomical discovery was that made by professor e.e. barnard in . he detected with the -inch lick refractor an extremely minute fifth satellite to jupiter at a distance of , miles, and revolving in a period of hrs. min. · secs. it can only be seen with the most powerful telescopes. the eclipses of jupiter's satellites had been observed for many years, and the times of their occurrence had been recorded. at length it was perceived that a certain order reigned among the eclipses of these bodies, as among all other astronomical phenomena. when once the laws according to which the eclipses recurred had been perceived, the usual consequence followed. it became possible to foretell the time at which the eclipses would occur in future. predictions were accordingly made, and it was found that they were approximately verified. further improvements in the calculations were then perfected, and it was sought to predict the times with still greater accuracy. but when it came to naming the actual minute at which the eclipse should occur, expectations were not always realised. sometimes the eclipse was five or ten minutes too soon. sometimes it was five or ten minutes too late. discrepancies of this kind always demand attention. it is, indeed, by the right use of them that discoveries are often made, and one of the most interesting examples is that now before us. the irregularity in the occurrence of the eclipses was at length perceived to observe certain rules. it was noticed that when the earth was near to jupiter the eclipse generally occurred before the predicted time; while when the earth happened to be at the side of its orbit away from jupiter, the eclipse occurred after the predicted time. once this was proved, the great discovery was quickly made by roemer, a danish astronomer, in . when the satellite enters the shadow, its light gradually decreases until it disappears. it is the last ray of light from the eclipsed satellite that gives the time of the eclipse; but that ray of light has to travel from the satellite to the earth, and enter our telescope, before we can note the occurrence. it used to be thought that light travelled instantaneously, so that the moment the eclipse occurred was assumed to be the moment when the eclipse was seen in the telescope. this was now perceived to be incorrect. it was found that light took time to travel. when the earth was comparatively near jupiter the light had only a short journey, the intelligence of the eclipse arrived quickly, and the eclipse was seen sooner than the calculations indicated. when the earth occupied a position far from jupiter, the light had a longer journey, and took more than the average time, so that the eclipse was later than the prediction. this simple explanation removed the difficulty attending the predictions of the eclipses of the satellites. but the discovery had a significance far more momentous. we learned from it that light had a measurable velocity, which, according to recent researches, amounts to , miles per second. one of the most celebrated attempts to ascertain the distance of the sun is derived from a combination of experiments on the velocity of light with astronomical measurements. this is a method of considerable refinement and interest, and although it does not so fulfil all the necessary conditions as to make it perfectly satisfactory, yet it is impossible to avoid some reference to it here. notwithstanding that the velocity of light is so stupendous, it has been found possible to measure that velocity by actual trial. this is one of the most delicate experimental researches that have ever been undertaken. if it be difficult to measure the speed of a rifle bullet, what shall we say of the speed of a ray of light, which is nearly a million times as great? how shall we devise an apparatus subtle enough to determine the velocity which would girdle the earth at the equator no less than seven times in a single second of time? ordinary contrivances for measurement are here futile; we have to devise an instrument of a wholly different character. in the attempt to discover the speed of a moving body we first mark out a certain distance, and then measure the time which the body requires to traverse that distance. we determine the velocity of a railway train by the time it takes to pass from one mile-post to the next. we learn the speed of a rifle bullet by an ingenious contrivance really founded on the same principle. the greater the velocity, the more desirable is it that the distance traversed during the experiment shall be as large as possible. in dealing with the measurement of the velocity of light, we therefore choose for our measured distance the greatest length that may be convenient. it is, however, necessary that the two ends of the line shall be visible from each other. a hill a mile or two away will form a suitable site for the distant station, and the distance of the selected point on the hill from the observer must be carefully measured. the problem is now easily stated. a ray of light is to be sent from the observer to the distant station, and the time occupied by that ray in the journey is to be measured. we may suppose that the observer, by a suitable contrivance, has arranged a lantern from which a thin ray of light issues. let us assume that this travels all the way to the distant station, and there falls upon the surface of a reflecting mirror. instantly it will be diverted by reflection into a new direction depending upon the inclination of the mirror. by suitable adjustment the latter can be so placed that the light shall fall perpendicularly upon it, in which case the ray will of course return along the direction in which it came. let the mirror be fixed in this position throughout the course of the experiments. it follows that a ray of light starting from the lantern will be returned to the lantern after it has made the journey to the distant station and back again. imagine, then, a little shutter placed in front of the lantern. we open the shutter, the ray streams forth to the remote reflector, and back again through the opening. but now, after having allowed the ray to pass through the shutter, suppose we try and close it before the ray has had time to get back again. what fingers could be nimble enough to do this? even if the distant station were ten miles away, so that the light had a journey of ten miles in going to the mirror and ten miles in coming back, yet the whole course would be accomplished in about the nine-thousandth part of a second--a period so short that even were it a thousand times as long it would hardly enable manual dexterity to close the aperture. yet a shutter can be constructed which shall be sufficiently delicate for the purpose. [illustration: fig. .--mode of measuring the velocity of light.] the principle of this beautiful method will be sufficiently obvious from the diagram on this page (fig. ), which has been taken from newcomb's "popular astronomy." the figure exhibits the lantern and the observer, and a large wheel with projecting teeth. each tooth as it passes round eclipses the beam of light emerging from the lantern, and also the eye, which is of course directed to the mirror at the distant station. in the position of the wheel here shown the ray from the lantern will pass to the mirror and back so as to be visible to the eye; but if the wheel be rotating, it may so happen that the beam after leaving the lantern will not have time to return before the next tooth of the wheel comes in front of the eye and screens it. if the wheel be urged still faster, the next tooth may have passed the eye, so that the ray again becomes visible. the speed at which the wheel is rotating can be measured. we can thus determine the time taken by one of the teeth to pass in front of the eye; we have accordingly a measure of the time occupied by the ray of light in the double journey, and hence we have a measurement of the velocity of light. it thus appears that we can tell the velocity of light either by the observations of jupiter's satellites or by experimental enquiry. if we take the latter method, then we are entitled to deduce remarkable astronomical consequences. we can, in fact, employ this method for solving that great problem so often referred to--the distance from the earth to the sun--though it cannot compete in accuracy with some of the other methods. the dimensions of the solar system are so considerable that a sunbeam requires an appreciable interval of time to span the abyss which separates the earth from the sun. eight minutes is approximately the duration of the journey, so that at any moment we see the sun as it appeared eight minutes earlier to an observer in its immediate neighbourhood. in fact, if the sun were to be suddenly blotted out it would still be seen shining brilliantly for eight minutes after it had really disappeared. we can determine this period from the eclipses of jupiter's satellites. so long as the satellite is shining it radiates a stream of light across the vast space between jupiter and the earth. when the eclipse has commenced, the little orb is no longer luminous, but there is, nevertheless, a long stream of light on its way, and until all this has poured into our telescopes we still see the satellite shining as before. if we could calculate the moment when the eclipse really took place, and if we could observe the moment at which the eclipse is seen, the difference between the two gives the time which the light occupies on the journey. this can be found with some accuracy; and, as we already know the velocity of light, we can ascertain the distance of jupiter from the earth; and hence deduce the scale of the solar system. it must, however, be remarked that at both extremities of the process there are characteristic sources of uncertainty. the occurrence of the eclipse is not an instantaneous phenomenon. the satellite is large enough to require an appreciable time in crossing the boundary which defines the shadow, so that the observation of an eclipse cannot be sufficiently precise to form the basis of an important and accurate measurement.[ ] still greater difficulties accompany the attempt to define the true moment of the occurrence of the eclipse as it would be seen by an observer in the vicinity of the satellite. for this we should require a far more perfect theory of the movements of jupiter's satellites than is at present attainable. this method of finding the sun's distance holds out no prospect of a result accurate to the one-thousandth part of its amount, and we may discard it, inasmuch as the other methods available seem to admit of much higher accuracy. the four chief satellites of jupiter have special interest for the mathematician, who finds in them a most striking instance of the universality of the law of gravitation. these bodies are, of course, mainly controlled in their movements by the attraction of the great planet; but they also attract each other, and certain curious consequences are the result. the mean motion of the first satellite in each day about the centre of jupiter is °· . that of the second is °· , and that of the third is °· . these quantities are so related that the following law will be found to be observed: the mean motion of the first satellite added to twice the mean motion of the third is exactly equal to three times the mean motion of the second. there is another law of an analogous character, which is thus expressed (the mean longitude being the angle between a fixed line and the radius to the mean place of the satellite): if to the mean longitude of the first satellite we add twice the mean longitude of the third, and subtract three times the mean longitude of the second, the difference is always °. it was from observation that these principles were first discovered. laplace, however, showed that if the satellites revolved nearly in this way, then their mutual perturbations, in accordance with the law of gravitation, would preserve them in this relative position for ever. we shall conclude with the remark, that the discovery of jupiter's satellites afforded the great confirmation of the copernican theory. copernicus had asked the world to believe that our sun was a great globe, and that the earth and all the other planets were small bodies revolving round the great one. this doctrine, so repugnant to the theories previously held, and to the immediate evidence of our senses, could only be established by a refined course of reasoning. the discovery of jupiter's satellites was very opportune. here we had an exquisite ocular demonstration of a system, though, of course, on a much smaller scale, precisely identical with that which copernicus had proposed. the astronomer who had watched jupiter's moons circling around their primary, who had noticed their eclipses and all the interesting phenomena attendant on them, saw before his eyes, in a manner wholly unmistakable, that the great planet controlled these small bodies, and forced them to revolve around him, and thus exhibited a miniature of the great solar system itself. "as in the case of the spots on the sun, galileo's announcement of this discovery was received with incredulity by those philosophers of the day who believed that everything in nature was described in the writings of aristotle. one eminent astronomer, clavius, said that to see the satellites one must have a telescope which would produce them; but he changed his mind as soon as he saw them himself. another philosopher, more prudent, refused to put his eye to the telescope lest he should see them and be convinced. he died shortly afterwards. 'i hope,' said the caustic galileo, 'that he saw them while on his way to heaven'"[ ] chapter xiii. saturn. the position of saturn in the system--saturn one of the three most interesting objects in the heavens--compared with jupiter--saturn to the unaided eye--statistics relating to the planet--density of saturn--lighter than water--the researches of galileo--what he found in saturn--a mysterious object--the discoveries made by huyghens half a century later--how the existence of the ring was demonstrated--invisibility of the rings every fifteen years--the rotation of the planet--the celebrated cypher--the explanation--drawing of saturn--the dark line--w. herschel's researches--is the division in the ring really a separation?--possibility of deciding the question--the ring in a critical position--are there other divisions in the ring?--the dusky ring--physical nature of saturn's rings--can they be solid?--can they even be slender rings?--a fluid?--true nature of the rings--a multitude of small satellites--analogy of the rings of saturn to the group of minor planets--problems suggested by saturn--the group of satellites to saturn--the discoveries of additional satellites--the orbit of saturn not the frontier of our system. at a profound distance in space, which, on an average, is , , miles, the planet saturn performs its mighty revolution around the sun in a period of twenty-nine and a half years. this gigantic orbit formed the boundary to the planetary system, so far as it was known to the ancients. although saturn is not so great a body as jupiter, yet it vastly exceeds the earth in bulk and in mass, and is, indeed, much greater than any one of the planets, jupiter alone excepted. but while saturn must yield the palm to jupiter so far as mere dimensions are concerned, yet it will be generally admitted that even jupiter, with all the retinue by which he is attended, cannot compete in beauty with the marvellous system of saturn. to the present writer it has always seemed that saturn is one of the three most interesting celestial objects visible to observers in northern latitudes. the other two will occupy our attention in future chapters. they are the great nebula in orion, and the star cluster in hercules. so far as the globe of saturn is concerned, we do not meet with any features which give to the planet any exceptional interest. the globe is less than that of jupiter, and as the latter is also much nearer to us, the apparent size of saturn is in a twofold way much smaller than that of jupiter. it should also be noticed that, owing to the greater distance of saturn from the sun, its intrinsic brilliancy is less than that of jupiter. there are, no doubt, certain marks and bands often to be seen on saturn, but they are not nearly so striking nor so characteristic as the ever-variable belts upon jupiter. the telescopic appearance of the globe of saturn must also be ranked as greatly inferior in interest to that of mars. the delicacy of detail which we can see on mars when favourably placed has no parallel whatever in the dim and distant saturn. nor has saturn, regarded again merely as a globe, anything like the interest of venus. the great splendour of venus is altogether out of comparison with that of saturn, while the brilliant crescent of the evening star is infinitely more pleasing than any telescopic view of the globe of saturn. yet even while we admit all this to the fullest extent, it does not invalidate the claim of saturn to be one of the most supremely beautiful and interesting objects in the heavens. this interest is not due to his globe; it is due to that marvellous system of rings by which saturn is surrounded--a system wonderful from every point of view, and, so far as our knowledge goes, without a parallel in the wide extent of the universe. [illustration: fig. . saturn. (july nd, . -in. equatorial.) (prof. e.e. barnard.)] to the unaided eye saturn usually appears like a star of the first magnitude. its light alone would hardly be sufficient to discriminate it from many of the brighter fixed stars. yet the ancients were acquainted with saturn, and they knew it as a planet. it was included with the other four great planets--mercury, venus, mars, and jupiter--in the group of wanderers, which were bound to no fixed points of the sky like the stars. on account of the great distance of saturn, its movements are much slower than those of the other planets known to the ancients. twenty-nine years and a half are required for this distant object to complete its circuit of the heavens; and, though this movement is slow compared with the incessant changes of venus, yet it is rapid enough to attract the attention of any careful observer. in a single year saturn moves through a distance of about twelve degrees, a quantity sufficiently large to be conspicuous to casual observation. even in a month, or sometimes in a week, the planet traverses an arc of the sky which can be detected by anyone who will take the trouble to mark the place of the planet with regard to the stars in its vicinity. those who are privileged to use accurate astronomical instruments can readily detect the motion of saturn in a few hours. the average distance from the sun to saturn is about millions of miles. the path of saturn, as of every other planet, is really an ellipse with the sun in one focus. in the case of saturn the shape of this ellipse is very appreciably different from a purely circular path. around this path saturn moves with an average velocity of · miles per second. the mean diameter of the globe of saturn is about , miles. its equatorial diameter is about , miles, and its polar diameter , miles--the ratio of these numbers being approximately that of to . it is thus obvious that saturn departs from the truly spherical shape to a very marked extent. the protuberance at its equator must, no doubt, be attributed to the high velocity with which the planet is rotating. the velocity of rotation of saturn is more than double as fast as that of the earth, though it is not quite so fast as that of jupiter. saturn makes one complete rotation in about hrs. min. mr. stanley williams has, however, observed with great care a number of spots which he has discovered, and he finds that some of these spots in about ° north latitude indicate rotation in a period of hrs. mins. to min., while equatorial spots require no more than hrs. min. to min. there is, however, the peculiarity that spots in the same latitude, but at different parts of the planet, rotate at rates which differ by a minute or more, while the period found by various groups of spots seems to change from year to year. these facts prove that saturn and the spots do not form a rigid system. the lightness of this planet is such as to be wholly incompatible with the supposition that its globe is constituted of solid materials at all comparable with those of which the crust of our earth is composed. the satellites, which surround saturn and form a system only less interesting than the renowned rings themselves, enable us to weigh the planet in comparison with the sun, and hence to deduce its actual mass relatively to the earth. the result is not a little remarkable. it appears that the density of the earth is eight times as great as that of saturn. in fact, the density of the latter is less than that of water itself, so that a mighty globe of water, equal in bulk to saturn, would actually weigh more. if we could conceive a vast ocean into which a globe equal to saturn in size and weight were cast, the great globe would not sink like our earth or like any of the other planets; it would float buoyantly at the surface with one-fourth of its bulk out of the water. we thus learn with high probability that what our telescopes show upon saturn is not a solid surface, but merely a vast envelope of clouds surrounding a heated interior. it is impossible to resist the suggestion that this planet, like jupiter, has still retained its heat because its mass is so large. we must, however, allude to a circumstance which perhaps may seem somewhat inconsistent with the view here taken. we have found that jupiter and saturn are, both of them, much less dense than the earth. when we compare the two planets together, it appears that saturn is much less dense than jupiter. in fact, every cubic mile of jupiter weighs nearly twice as much as each cubic mile of saturn. this would seem to point to the conclusion that saturn is the more heated of the two bodies. yet, as jupiter is the larger, it might more reasonably have been expected to be hotter than the other planet. we do not attempt to reconcile this discrepancy; in fact, in our ignorance as to the material constitution of these bodies, it would be idle to discuss the question. even if we allow for the lightness of saturn, as compared bulk for bulk with the earth, yet the volume of saturn is so enormous that the planet weighs more than ninety-five times as much as the earth. the adjoining view represents the relative sizes of saturn and the earth (fig. ). [illustration: fig. .--relative sizes of saturn and the earth.] as the unaided eye discloses none of those marvels by which saturn is surrounded, the interest which attaches to this planet may be said to commence from the time when it began to be observed with the telescope. the history must be briefly alluded to, for it was only by degrees that the real nature of this complicated object was understood. when galileo completed his little refracting telescope, which, though it only magnified thirty times, was yet an enormous addition to the powers of unaided vision, he made with it his memorable review of the heavens. he saw the spots on the sun and the mountains on the moon; he noticed the crescent of venus and the satellites of jupiter. stimulated and encouraged by such brilliant discoveries, he naturally sought to examine the other planets, and accordingly directed his telescope to saturn. here, again, galileo at once made a discovery. he saw that saturn presented a visible form like the other planets, but that it differed from any other telescopic object, inasmuch as it appeared to him to be composed of three bodies which always touched each other and always maintained the same relative positions. these three bodies were in a line--the central one was the largest, and the two others were east and west of it. there was nothing he had hitherto seen in the heavens which filled his mind with such astonishment, and which seemed so wholly inexplicable. in his endeavours to understand this mysterious object, galileo continued his observations during the year , and, to his amazement, he saw the two lesser bodies gradually become smaller and smaller, until, in the course of the two following years, they had entirely vanished, and the planet simply appeared with a round disc like jupiter. here, again, was a new source of anxiety to galileo. he had at that day to contend against the advocates of the ancient system of astronomy, who derided his discoveries and refused to accept his theories. he had announced his observation of the composite nature of saturn; he had now to tell of the gradual decline and the ultimate extinction of these two auxiliary globes, and he naturally feared that his opponents would seize the opportunity of pronouncing that the whole of his observations were illusory.[ ] "what," he remarks, "is to be said concerning so strange a metamorphosis? are the two lesser stars consumed after the manner of the solar spots? have they vanished and suddenly fled? has saturn perhaps, devoured his own children? or were the appearances indeed illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom i have shown them? now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of their existence. i do not know what to say in a case so surprising, so unlooked for, and so novel. the shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me." but galileo was not mistaken. the objects were really there when he first began to observe, they really did decline, and they really disappeared; but this disappearance was only for a time--they again came into view. they were then subjected to ceaseless examination, until gradually their nature became unfolded. with increased telescopic power it was found that the two bodies which galileo had described as globes on either side of saturn were not really spherical--they were rather two luminous crescents with the concavity of each turned towards the central globe. it was also perceived that these objects underwent a remarkable series of periodic changes. at the beginning of such a series the planet was found with a truly circular disc. the appendages first appeared as two arms extending directly outwards on each side of the planet; then these arms gradually opened into two crescents, resembling handles to the globe, and attained their maximum width after about seven or eight years; then they began to contract, until after the lapse of about the same time they vanished again. the true nature of these objects was at length discovered by huyghens in , nearly half a century after galileo had first detected their appearance. he perceived the shadow thrown by the ring upon the globe, and his explanation of the phenomena was obtained in a very philosophical manner. he noticed that the earth, the sun, and the moon rotated upon their axes, and he therefore regarded it as a general law that each one of the bodies in the system rotates about an axis. it is true, observations had not yet been made which actually showed that saturn was also rotating; but it would be highly, nay, indeed, infinitely, improbable that any planet should be devoid of such movement. all the analogies of the system pointed to the conclusion that the velocity of rotation would be considerable. one satellite of saturn was already known to revolve in a period of sixteen days, being little more than half our month. huyghens assumed--and it was a most reasonable assumption--that saturn in all probability rotated rapidly on its axis. it was also to be observed that if these remarkable appendages were attached by an actual bodily connection to the planet they must rotate with saturn. if, however, the appendages were not actually attached it would still be necessary that they should rotate if the analogy of saturn to other objects in the system were to be in any degree preserved. we see satellites near jupiter which revolve around him. we see, nearer home, how the moon revolves around the earth. we see how all the planetary system revolves around the sun. all these considerations were present to huyghens when he came to the conclusion that, whether the curious appendages were actually attached to the planet or were physically free from it, they must still be in rotation. provided with such reasonings, it soon became easy to conjecture the true nature of the saturnian system. we have seen how the appendages declined to invisibility once every fifteen years, and then gradually reappeared in the form, at first, of rectilinear arms projecting outwards from the planet. the progressive development is a slow one, and for weeks and months, night after night, the same appearance is presented with but little change. but all this time both saturn and the mysterious objects around him are rotating. whatever these may be, they present the same appearance to the eye, notwithstanding their ceaseless motion of rotation. what must be the shape of an object which satisfies the conditions here implied? it will obviously not suffice to regard the projections as two spokes diverging from the planet. they would change from visibility to invisibility in every rotation, and thus there would be ceaseless alterations of the appearance instead of that slow and gradual change which requires fifteen years for a complete period. there are, indeed, other considerations which preclude the possibility of the objects being anything of this character, for they are always of the same length as compared with the diameter of the planet. a little reflection will show that one supposition--and indeed only one--will meet all the facts of the case. if there were a thin symmetrical ring rotating in its own plane around the equator of saturn, then the persistence of the object from night to night would be accounted for. this at once removes the greater part of the difficulty. for the rest, it was only necessary to suppose that the ring was so thin that when turned actually edgewise to the earth it became invisible, and then as the illuminated side of the plane became turned more and more towards the earth the appendages to the planet gradually increased. the handle-shaped appearance which the object periodically assumed demonstrated that the ring could not be attached to the globe. at length huyghens found that he had the clue to the great enigma which had perplexed astronomers for the last fifty years. he saw that the ring was an object of astonishing interest, unique at that time, as it is, indeed, unique still. he felt, however, that he had hardly demonstrated the matter with all the certainty which it merited, and which he thought that by further attention he could secure. yet he was loath to hazard the loss of his discovery by an undue postponement of its announcement, lest some other astronomer might intervene. how, then, was he to secure his priority if the discovery should turn out correct, and at the same time be enabled to perfect it at his leisure? he adopted the course, usual at the time, of making his first announcement in cipher, and accordingly, on march th, , he published a tract, which contained the following proposition:-- aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu perhaps some of those curious persons whose successors now devote so much labour to double acrostics may have pondered on this renowned cryptograph, and even attempted to decipher it. but even if such attempts were made, we do not learn that they were successful. a few years of further study were thus secured to huyghens. he tested his theory in every way that he could devise, and he found it verified in every detail. he therefore thought that it was needless for him any longer to conceal from the world his great discovery, and accordingly in the year --about three years after the appearance of his cryptograph--he announced the interpretation of it. by restoring the letters to their original arrangement the discovery was enunciated in the following words:--"_annulo cingitur_, _tenui_, _plano_, _nusquam cohærente_, _ad eclipticam inclinato_," which may be translated into the statement:--"the planet is surrounded by a slender flat ring everywhere distinct from its surface, and inclined to the elliptic." huyghens was not content with merely demonstrating how fully this assumption explained all the observed phenomena. he submitted it to the further and most delicate test which can be applied to any astronomical theory. he attempted by its aid to make a prediction the fulfilment of which would necessarily give his theory the seal of certainty. from his calculations he saw that the planet would appear circular about july or august in . this anticipation was practically verified, for the ring was seen to vanish in may of that year. no doubt, with our modern calculations founded on long-continued and accurate observation, we are now enabled to make forecasts as to the appearance or the disappearance of saturn's ring with far greater accuracy; but, remembering the early stage in the history of the planet at which the prediction of huyghens was made, we must regard its fulfilment as quite sufficient, and as confirming in a satisfactory manner the theory of saturn and his ring. the ring of saturn having thus been thoroughly established as a fact in celestial architecture, each generation of astronomers has laboured to find out more and more of its marvellous features. in the frontispiece (plate i.) we have a view of the planet as seen at the harvard college observatory, u.s.a., between july th and october th, . it has been drawn by the skilful astronomer and artist--mr. l. trouvelot--and gives a faithful and beautiful representation of this unique object. fig. is a drawing of the same object taken on july nd, , by prof. e.e. barnard, at the lick observatory. the next great discovery in the saturnian system after those of huyghens showed that the ring surrounding the planet was marked by a dark concentric line, which divided it into two parts--the outer being narrower than the inner. this line was first seen by j.d. cassini, when saturn emerged from the rays of the sun in . that this black line is not merely a black mark on the ring, but that it is actually a separation, was rendered very probable by the researches of maraldi in , followed many years later by those of sir william herschel, who, with that thoroughness which was a marked characteristic of the man, made a minute and scrupulous examination of saturn. night after night he followed it for hours with his exquisite instruments, and considerably added to our knowledge of the planet and his system. herschel devoted very particular attention to the examination of the line dividing the ring. he saw that the colour of this line was not to be distinguished from the colour of the space intermediate between the globe and the ring. he observed it for ten years on the northern face of the ring, and during that time it continued to present the same breadth and colour and sharpness of outline. he was then fortunate enough to observe the southern side of the ring. there again could the black line be seen, corresponding both in appearance and in position with the dark line as seen on the northern side. no doubt could remain as to the fact that saturn was girdled by two concentric rings equally thin, the outer edge of one closely approaching to the inner edge of the other. at the same time it is right to add that the only absolutely indisputable proof of the division between the rings has not yet been yielded by the telescope. the appearances noted by herschel would be consistent with the view that the black line was merely a part of the ring extending through its thickness, and composed of materials very much less capable of reflecting light than the rest of the ring. it is still a matter of doubt how far it is ever possible actually to see through the dark line. there is apparently only one satisfactory method of accomplishing this. it would only occur in rare circumstances, and it does not seem that the opportunity has as yet arisen. suppose that in the course of its motion through the heavens the path of saturn happened to cross directly between the earth and a fixed star. the telescopic appearance of a star is merely a point of light much smaller than the globes and rings of saturn. if the ring passed in front of the star and the black line on the ring came over the star, we should, if the black line were really an opening, see the star shining through the narrow aperture. up to the present, we believe, there has been no opportunity of submitting the question of the duplex character of the ring to this crucial test. let us hope that as there are now so many telescopes in use adequate to deal with the subject, there may, ere long, be observations made which will decide the question. it can hardly be expected that a very small star would be suitable. no doubt the smallness of the star would render the observations more delicate and precise if the star were visible; but we must remember that it will be thrown into contrast with the bright rings of saturn on each margin so that unless the star were of considerable magnitude it would hardly answer. it has, however, been recently observed that the globe of the planet can be, in some degree, discerned through the dark line; this is practically a demonstration of the fact that the line is at all events partly transparent. the outer ring is also divided into two by a line much fainter than that just described. it requires a good telescope and a fine night, combined with a favourable position of the planet, to render this line a well-marked object. it is most easily seen at the extremities of the ring most remote from the planet. to the present writer, who has examined the planet with the twelve-inch refractor of the south equatorial at dunsink observatory, this outer line appears as broad as the well-known line; but it is unquestionably fainter, and has a more shaded appearance. it certainly does not suggest the appearance of being actually an opening in the ring, and it is often invisible for a long time. it seems rather as if the ring were at this place thinner and less substantial without being actually void of substance. on these points it may be expected that much additional information will be acquired when next the ring places itself in such a position that its plane, if produced, would pass between the earth and the sun. such occasions are but rare, and even when they do occur it may happen that the planet will not be well placed for observation. the next really good opportunity will not be till . in this case the sunlight illuminates one side of the ring, while it is the other side of the ring that is presented towards the earth. powerful telescopes are necessary to deal with the planet under such circumstances; but it may be reasonably hoped that the questions relating to the division of the ring, as well as to many other matters, will then receive some further elucidation. occasionally, other divisions of the ring, both inner and outer, have been recorded. it may, at all events, be stated that no such divisions can be regarded as permanent features. yet their existence has been so frequently enunciated by skilful observers that it is impossible to doubt that they have been sometimes seen. it was about years after huyghens had first explained the true theory of saturn that another very important discovery was effected. it had, up to the year , been always supposed that the two rings, divided by the well-known black line, comprised the entire ring system surrounding the planet. in the year just mentioned, professor bond, the distinguished astronomer of cambridge, mass., startled the astronomical world by the announcement of his discovery of a third ring surrounding saturn. as so often happens in such cases, the same object was discovered independently by another--an english astronomer named dawes. this third ring lies just inside the inner of the two well-known rings, and extends to about half the distance towards the body of the planet. it seems to be of a totally different character from the two other rings in so far as they present a comparatively substantial appearance. we shall, indeed, presently show that they are not solid--not even liquid bodies--but still, when compared with the third ring, the others were of a substantial character. they can receive and exhibit the deeply-marked shadow of saturn, and they can throw a deep and black shadow upon saturn themselves; but the third ring is of a much less compact texture. it has not the brilliancy of the others, it is rather of a dusky, semi-transparent appearance, and the expression "crape ring," by which it is often designated, is by no means inappropriate. it is the faintness of this crape ring which led to its having been so frequently overlooked by the earlier observers of saturn. it has often been noticed that when an astronomical discovery has been made with a good telescope, it afterwards becomes possible for the same object to be observed with instruments of much inferior power. no doubt, when the observer knows what to look for, he will often be able to see what would not otherwise have attracted his attention. it may be regarded as an illustration of this principle, that the crape ring of saturn has become an object familiar to those who are accustomed to work with good telescopes; but it may, nevertheless, be doubted whether the ease and distinctness with which the crape ring is now seen can be entirely accounted for by this supposition. indeed, it seems possible that the crape ring has, from some cause or other, gradually become more and more visible. the supposed increased brightness of the crape ring is one of those arguments now made use of to prove that in all probability the rings of saturn are at this moment undergoing gradual transformation; but observations of hadley show that the crape ring was seen by him in , and it was previously seen by campani and picard, as a faint belt crossing the planet. the partial transparency of the crape ring was beautifully illustrated in an observation by professor barnard of the eclipse of iapetus on november st, . the satellite was faintly visible in the shadow of the crape ring, while wholly invisible in the shadow of the better known rings. the various features of the rings are well shown in the drawing of trouvelot already referred to. we here see the inner and the outer ring, and the line of division between them. we see in the outer ring the faint traces of the line by which it is divided, and inside the inner ring we have a view of the curious and semi-transparent crape ring. the black shadow of the planet is cast upon the ring, thus proving that the ring, no less than the body of the planet, shines only in virtue of the sunlight which falls upon it. this shadow presents some anomalous features, but its curious irregularity may be, to some extent, an optical illusion. there can be no doubt that any attempt to depict the rings of saturn only represents the salient features of that marvellous system. we are situated at such a great distance that all objects not of colossal dimensions are invisible. we have, indeed, only an outline, which makes us wish to be able to fill in the details. we long, for instance, to see the actual texture of the rings, and to learn of what materials they are made; we wish to comprehend the strange and filmy crape ring, so unlike any other object known to us in the heavens. there is no doubt that much may even yet be learned under all the disadvantageous conditions of our position; there is still room for the labour of whole generations of astronomers provided with splendid instruments. we want accurate drawings of saturn under every conceivable aspect in which it may be presented. we want incessantly repeated measurements, of the most fastidious accuracy. these measures are to tell us the sizes and the shapes of the rings; they are to measure with fidelity the position of the dark lines and the boundaries of the rings. these measures are to be protracted for generations and for centuries; then and then only can terrestrial astronomers learn whether this elaborate system has really the attributes of permanence, or whether it may be undergoing changes. we have been accustomed to find that the law of universal gravitation pervades every part of our system, and to look to gravitation for the explanation of many phenomena otherwise inexplicable. we have good reasons for knowing that in this marvellous saturnian system the law of gravitation is paramount. there are satellites revolving around saturn as well as a ring; these satellites move, as other satellites do, in conformity with the laws of kepler; and, therefore, any theory as to the nature of saturn's ring must be formed subject to the condition that it shall be attracted by the gigantic planet situated in its interior. to a hasty glance nothing might seem easier than to reconcile the phenomena of the ring with the attraction of the planet. we might suppose that the ring stands at rest symmetrically around the planet. at its centre the planet pulls in the ring equally on all sides, so that there is no tendency in it to move in one way rather than another; and, therefore, it will stay at rest. this will not do. a ring composed of materials almost infinitely rigid might possibly, under such circumstances, be for a moment at rest; but it could not remain permanently at rest any more than can a needle balanced vertically on its point. in each case the equilibrium is unstable. if the slightest cause of disturbance arise, the equilibrium is destroyed, and the ring would inevitably fall in upon the planet. such causes of derangement are incessantly present, so that unstable equilibrium cannot be an appropriate explanation of the phenomena. even if this difficulty could be removed, there is still another, which would be quite insuperable if the ring be composed of any materials with which we are acquainted. let us ponder for a moment on the matter, as it will lead up naturally to that explanation of the rings of saturn which is now most generally accepted. imagine that you stood on the planet saturn, near his equator; over your head stretches the ring, which sinks down to the horizon in the east and in the west. the half-ring above your horizon would then resemble a mighty arch, with a span of about a hundred thousand miles. every particle of this arch is drawn towards saturn by gravitation, and if the arch continue to exist, it must do so in obedience to the ordinary mechanical laws which regulate the railway arches with which we are familiar. the continuance of these arches depends upon the resistance of the stones forming them to a crushing force. each stone of an arch is subjected to a vast pressure, but stone is a material capable of resisting such pressure, and the arch remains. the wider the span of the arch the greater is the pressure to which each stone is exposed. at length a span is reached which corresponds to a pressure as great as the stones can safely bear, and accordingly we thus find the limiting span over which a single arch of masonry can be constructed. apply these principles to the stupendous arch formed by the ring of saturn. it can be shown that the pressure on the materials of the arch capable of spanning an abyss of such awful magnitude would be something so enormous that no materials we know of would be capable of bearing it. were the ring formed of the toughest steel that was ever made, the pressure would be so great that the metal would be squeezed like a liquid, and the mighty structure would collapse and fall down on the surface of the planet. it is not credible that any materials could exist capable of sustaining a stress so stupendous. the law of gravitation accordingly bids us search for a method by which the intensity of this stress can be mitigated. one method is at hand, and is obviously suggested by analogous phenomena everywhere in our system. we have spoken of the ring as if it were at rest; let us now suppose it to be animated by a motion of rotation in its plane around saturn as a centre. instantly we have a force developed antagonistic to the gravitation of saturn. this force is the so-called centrifugal force. if we imagine the ring to rotate, the centrifugal force at all points acts in an opposite direction to the attractive force, and hence the enormous stress on the ring can be abated and one difficulty can be overcome. we can thus attribute to each ring a rotation which will partly relieve it from the stress the arch would otherwise have to sustain. but we cannot admit that the difficulty has been fully removed. suppose that the outer ring revolve at such a rate as shall be appropriate to neutralise the gravitation on its outer edge, the centrifugal force will be less at the interior of the ring, while the gravitation will be greater; and hence vast stresses will be set up in the interior parts of the outer ring. suppose the ring to rotate at such a rate as would be adequate to neutralise the gravitation at its inner margin; then the centrifugal force at the outer parts will largely exceed the gravitation, and there will be a tendency to disruption of the ring outwards. to obviate this tendency we may assume the outer parts of each ring to rotate more slowly than the inner parts. this naturally requires that the parts of the ring shall be mobile relatively to one another, and thus we are conducted to the suggestion that perhaps the rings are really composed of matter in a fluid state. the suggestion is, at first sight, a plausible one; each part of each ring would then move with an appropriate velocity, and the rings would thus exhibit a number of concentric circular currents with different velocities. the mathematician can push this inquiry a little farther, and he can study how this fluid would behave under such circumstances. his symbols can pursue the subject into the intricacies which cannot be described in general language. the mathematician finds that waves would originate in the supposed fluid, and that as these waves would lead to disruption of the rings, the fluid theory must be abandoned. but we can still make one or two more suppositions. what if it be really true that the ring consist of an incredibly large number of concentric rings, each animated precisely with the velocity which would be suitable to the production of a centrifugal force just adequate to neutralise the attraction? no doubt this meets many of the difficulties: it is also suggested by those observations which have shown the presence of several dark lines on the ring. here again dynamical considerations must be invoked for the reply. such a system of solid rings is not compatible with the laws of dynamics. we are, therefore, compelled to make one last attempt, and still further to subdivide the ring. it may seem rather startling to abandon entirely the supposition that the ring is in any sense a continuous body, but there remains no alternative. look at it how we will, we seem to be conducted to the conclusion that the ring is really an enormous shoal of extremely minute bodies; each of these little bodies pursues an orbit of its own around the planet, and is, in fact, merely a satellite. these bodies are so numerous and so close together that they seem to us to be continuous, and they may be very minute--perhaps not larger than the globules of water found in an ordinary cloud over the surface of the earth, which, even at a short distance, seems like a continuous body. until a few years ago this theory of the constitution of saturn's rings, though unassailable from a mathematical point of view, had never been confirmed by observation. the only astronomer who maintained that he had actually seen the rings rotate was w. herschel, who watched the motion of some luminous points on the ring in , at which time the plane of the ring happened to pass through the earth. from these observations herschel concluded that the ring rotated in ten hours and thirty-two minutes. but none of the subsequent observers, even though they may have watched saturn with instruments very superior to that used by herschel, were ever able to succeed in verifying his rotation of these appendages of saturn. if the ring were composed of a vast number of small bodies, then the third law of kepler will enable us to calculate the time which these tiny satellites would require to travel completely round the planet. it appears that any satellite situated at the outer edge of the ring would require as long a period as hrs. min., those about the middle would not need more than hrs. min., while those at the inner edge of the ring would accomplish their rotation in hrs. min. even our mightiest telescopes, erected in the purest skies and employed by the most skilful astronomers, refuse to display this extremely delicate phenomenon. it would, indeed, have been a repetition on a grand scale of the curious behaviour of the inner satellite of mars, which revolves round its primary in a shorter time than the planet itself takes to turn round on its own axis. [illustration: fig. .--prof. keeler's method of measuring the rotation of saturn's ring.] but what the telescope could not show, the spectroscope has lately demonstrated in a most effective and interesting manner. we have explained in the chapter on the sun how the motion of a source of light along the line of vision, towards or away from the observer, produces a slight shift in the position of the lines of the spectrum. by the measurement of the displacement of the lines the direction and amount of the motion of the source of light may be determined. we illustrated the method by showing how it had actually been used to measure the speed of rotation of the solar surface. in professor keeler,[ ] director of the allegheny observatory, succeeded in measuring the rotation of saturn's ring in this manner. he placed the slit of his spectroscope across the ball, in the direction of the major axis of the elliptic figure which the effect of perspective gives the ring as shown by the parallel lines in fig. stretching from e to w. his photographic plate should then show three spectra close together, that of the ball of saturn in the middle, separated by dark intervals from the narrower spectra above and below it of the two handles (or ansæ, as they are generally called) of the ring. in fig. we have represented the behaviour of any one line of the spectrum under various suppositions as to rotation or non-rotation of saturn and the ring. at the top ( ) we see how each line would look if there was no rotatory motion; the three lines produced by ring, planet, and ring are in a straight line. of course the spectrum, which is practically a very faint copy of the solar spectrum, shows the principal dark fraunhofer lines, so that the reader must imagine these for himself, parallel to the one we show in the figure. but saturn and the ring are not standing still, they are rotating, the eastern part (at e) moving towards us, and the western part (w) moving away from us.[ ] at e the line will therefore be shifted towards the violet end of the spectrum and at w towards the red, and as the actual linear velocity is greater the further we get away from the centre of saturn (assuming ring and planet to rotate together), the lines would be turned as in fig. ( ), but the three would remain in a straight line. if the ring consisted of two independent rings separated by cassini's division and rotating with different velocities, the lines would be situated as in fig. ( ), the lines due to the inner ring being more deflected than those due to the outer ring, owing to the greater velocity of the inner ring. [illustration: fig. .--prof. keeler's method of measuring the rotation of saturn's ring.] finally, let us consider the case of the rings, consisting of innumerable particles moving round the planet in accordance with kepler's third law. the actual velocities of these particles would be per second:-- at outer edge of ring · miles. at middle of ring · miles. at inner edge of ring · miles. rotation speed at surface of planet · miles. the shifting of the lines of the spectrum should be in accordance with these velocities, and it is easy to see that the lines ought to lie as in the fourth figure. when professor keeler came to examine the photographed spectra, he found the lines of the three spectra tilted precisely in this manner, showing that the outer edge of the ring was travelling round the planet with a smaller linear velocity than the inner one, as it ought to do if the sources of light (or, rather, the reflectors of sunlight) were independent particles free to move according to kepler's third law, and as it ought not to do if the ring, or rings, were rigid, in which case the outer edge would have the greatest linear speed, as it had to travel through the greatest distance. here, at last, was the proof of the meteoritic composition of saturn's ring. professor keeler's beautiful discovery has since been verified by repeated observations at the allegheny, lick, paris, and pulkova observatories; the actual velocities resulting from the observed displacements of the lines have been measured and found to agree well (within the limits of the errors of observation) with the calculated velocities, so that this brilliant confirmation of the mathematical deductions of clerk maxwell is raised beyond the possibility of doubt. the spectrum of saturn is so faint that only the strongest lines of the solar spectrum can be seen in it, but the atmosphere of the planet seems to exert a considerable amount of general absorption in the blue and violet parts of the spectrum, which is especially strong near the equatorial belt, while a strong band in the red testifies to the density of the atmosphere. this band is not seen in the spectrum of the rings, around which there can therefore be no atmosphere. as saturn's ring is itself unique, we cannot find elsewhere any very pertinent illustration of a structure so remarkable as that now claimed for the ring. yet the solar system does show some analogous phenomena. there is, for instance, one on a very grand scale surrounding the sun himself. we allude to the multitude of minor planets, all confined within a certain region of the system. imagine these planets to be vastly increased in number, and those orbits which are much inclined to the rest flattened down and otherwise adjusted, and we should have a ring surrounding the sun, thus producing an arrangement not dissimilar from that now attributed to saturn. it is tempting to linger still longer over this beautiful system, to speculate on the appearance which the ring would present to an inhabitant of saturn, to conjecture whether it is to be regarded as a permanent feature of our system in the same way as we attribute permanence to our moon or to the satellites of jupiter. looked at from every point of view, the question is full of interest, and it provides occupation abundant for the labours of every type of astronomer. if he be furnished with a good telescope, then has he ample duties to fulfil in the task of surveying, of sketching, and of measuring. if he be one of those useful astronomers who devote their energies not to actual telescopic work, but to forming calculations based on the observations of others, then the beautiful system of saturn provides copious material. he has to foretell the different phases of the ring, to announce to astronomers when each feature can be best seen, and at what hour each element can be best determined. he has also to predict the times of the movements of saturn's satellites, and the other phenomena of a system more elaborate than that of jupiter. lastly, if the astronomer be one of that class--perhaps, from some points of view, the highest class of all--who employ the most profound researches of the human intellect to unravel the dynamical problems of astronomy, he, too, finds in saturn problems which test to the utmost, even if they do not utterly transcend, the loftiest flights of analysis. he discovers in saturn's ring an object so utterly unlike anything else, that new mathematical weapons have to be forged for the encounter. he finds in the system so many extraordinary features, and such delicacy of adjustment, that he is constrained to admit that if he did not actually see saturn's rings before him, he would not have thought that such a system was possible. the mathematician's labours on this wondrous system are at present only in their infancy. not alone are the researches of so abstruse a character as to demand the highest genius for this branch of science, but even yet the materials for the inquiry have not been accumulated. in a discussion of this character, observation must precede calculation. the scanty observations hitherto obtained, however they may illustrate the beauty of the system, are still utterly insufficient to form the basis of that great mathematical theory of saturn which must eventually be written. but saturn possesses an interest for a far more numerous class of persons than those who are specially devoted to astronomy. it is of interest, it must be of interest, to every cultivated person who has the slightest love for nature. a lover of the picturesque cannot behold saturn in a telescope without feelings of the liveliest emotion; while, if his reading and reflection have previously rendered him aware of the colossal magnitude of the object at which he is looking, he will be constrained to admit that no more remarkable spectacle is presented in the whole of nature. we have pondered so long over the fascinations of saturn's ring that we can only give a very brief account of that system of satellites by which the planet is attended. we have already had occasion to allude more than once to these bodies; it only remains now to enumerate a few further particulars. it was on the th of march, , that the first satellite of saturn was detected by huyghens, to whose penetration we owe the discovery of the true form of the ring. on the evening of the day referred to, huyghens was examining saturn with a telescope constructed with his own hands, when he observed a small star-like object near the planet. the next night he repeated his observations, and it was found that the star was accompanying the planet in its progress through the heavens. this showed that the little object was really a satellite to saturn, and further observations revealed the fact that it was revolving around him in a period of days, hours, minutes. such was the commencement of that numerous series of discoveries of satellites which accompany saturn. one by one they were detected, so that at the present time no fewer than nine are known to attend the great planet through his wanderings. the subsequent discoveries were, however, in no case made by huyghens, for he abandoned the search for any further satellites on grounds which sound strange to modern ears, but which were quite in keeping with the ideas of his time. it appears that from some principle of symmetry, huyghens thought that it would accord with the fitness of things that the number of satellites, or secondary planets, should be equal in number to the primary planets themselves. the primary planets, including the earth, numbered six; and huyghens' discovery now brought the total number of satellites to be also six. the earth had one, jupiter had four, saturn had one, and the system was complete. nature, however, knows no such arithmetical doctrines as those which huyghens attributed to her. had he been less influenced by such prejudices, he might, perhaps, have anticipated the labours of cassini, who, by discovering other satellites of saturn, demonstrated the absurdity of the doctrine of numerical equality between planets and satellites. as further discoveries were made, the number of satellites was at first raised above the number of planets; but in recent times, when the swarm of minor planets came to be discovered, the number of planets speedily reached and speedily passed the number of their attendant satellites. it was in , about sixteen years after the discovery of the first satellite of saturn, that a second was discovered by cassini. this is the outermost of the older satellites; it takes days to travel round saturn. in the following year he discovered another; and twelve years later, in , still two more; thus making a total of five satellites to this planet. [illustration: fig. .--transit of titan and its shadow, by f. terby louvain, th april, .] the complexity of the saturnian system had now no rival in the heavens. saturn had five satellites, and jupiter had but four, while at least one of the satellites of saturn, named titan, was larger than any satellite of jupiter.[ ] some of the discoveries of cassini had been made with telescopes of quite monstrous dimensions. the length of the instrument, or rather the distance at which the object-glass was placed, was one hundred feet or more from the eye of the observer. it seemed hardly possible to push telescopic research farther with instruments of this cumbrous type. at length, however, the great reformation in the construction of astronomical instruments began to dawn. in the hands of herschel, it was found possible to construct reflecting telescopes of manageable dimensions, which were both more powerful and more accurate than the long-focussed lenses of cassini. a great instrument of this kind, forty feet long, just completed by herschel, was directed to saturn on the th of august, . never before had the wondrous planet been submitted to a scrutiny so minute. herschel was familiar with the labours of his predecessors. he had often looked at saturn and his five moons in inferior telescopes; now again he saw the five moons and a star-like object so near the plane of the ring that he conjectured this to be a sixth satellite. a speedy method of testing this conjecture was at hand. saturn was then moving rapidly over the heavens. if this new object were in truth a satellite, then it must be carried on by saturn. herschel watched with anxiety to see whether this would be the case. a short time sufficed to answer the question; in two hours and a half the planet had moved to a distance quite appreciable, and had carried with him not only the five satellites already known, but also this sixth object. had this been a star it would have been left behind; it was not left behind, and hence it, too, was a satellite. thus, after the long lapse of a century, the telescopic discovery of satellites to saturn recommenced. herschel, as was his wont, observed this object with unremitting ardour, and discovered that it was much nearer to saturn than any of the previously known satellites. in accordance with the general law, that the nearer the satellite the shorter the period of revolution, herschel found that this little moon completed a revolution in about day, hours, minutes. the same great telescope, used with the same unrivalled skill, soon led herschel to a still more interesting discovery. an object so small as only to appear like a very minute point in the great forty-foot reflector was also detected by herschel, and was by him proved to be a satellite, so close to the planet that it completed a revolution in the very brief period of hours and minutes. this is an extremely delicate object, only to be seen by the best telescopes in the brief intervals when it is not entirely screened from view by the ring. again another long interval elapsed, and for almost fifty years the saturnian system was regarded as consisting of the series of rings and of the seven satellites. the next discovery has a singular historical interest. it was made simultaneously by two observers--professor bond, of cambridge, mass., and mr. lassell, of liverpool--for on the th september, , both of these astronomers verified that a small point which they had each seen on previous nights was really a satellite. this object is, however, at a considerable distance from the planet, and requires days, hours, minutes for each revolution; it is the seventh in order from the planet. yet one more extremely faint outer satellite was discerned by photography on the th, th, and th august, , by professor w.h. pickering. this object is much more distant from the planet than the larger and older satellites. its motion has not yet been fully determined, but probably it requires not less than days to perform a single revolution. from observations of the satellites it has been found that , globes as heavy as saturn would weigh as much as the sun. a law has been observed by professor kirkwood, which connects together the movements of the four interior satellites of saturn. this law is fulfilled in such a manner as leads to the supposition that it arises from the mutual attraction of the satellites. we have already described a similar law relative to three of the satellites of jupiter. the problem relating to saturn, involving as it does no fewer than four satellites, is one of no ordinary complexity. it involves the theory of perturbations to a greater degree than that to which mathematicians are accustomed in their investigation of the more ordinary features of our system. to express this law it is necessary to have recourse to the daily movements of the satellites; these are respectively-- satellite. daily movement. i. °· . ii. °· . iii. °· . iv. °· . the law states that if to five times the movement of the first satellite we add that of the third and four times that of the fourth, the whole will equal ten times the movement of the second satellite. the calculation stands thus:-- times i. equals °· iii. equals °· ii. °· times iv. equals °· -------- -------- °· equal °· nearly. nothing can be simpler than the verification of this law; but the task of showing the physical reason why it should be fulfilled has not yet been accomplished. saturn was the most distant planet known to the ancients. it revolves in an orbit far outside the other ancient planets, and, until the discovery of uranus in the year , the orbit of saturn might well be regarded as the frontier of the solar system. the ringed planet was indeed a worthy object to occupy a position so distinguished. but we now know that the mighty orbit of saturn does not extend to the frontiers of the solar system; a splendid discovery, leading to one still more splendid, has vastly extended the boundary, by revealing two mighty planets, revolving in dim telescopic distance, far outside the path of saturn. these objects have not the beauty of saturn; they are, indeed, in no sense effective telescopic pictures. yet these outer planets awaken an interest of a most special kind. the discovery of each is a classical event in the history of astronomy, and the opinion has been maintained, and perhaps with reason, that the discovery of neptune, the more remote of the two, is the greatest achievement in astronomy made since the time of newton. chapter xiv uranus. contrast between uranus and the other great planets--william herschel--his birth and parentage--herschel's arrival in england--his love of learning--commencement of his astronomical studies--the construction of telescopes--construction of mirrors--the professor of music becomes an astronomer--the methodical research--the th march, --the discovery of uranus--delicacy of observation--was the object a comet?--the significance of this discovery--the fame of herschel--george iii. and the bath musician--the king's astronomer at windsor--the planet uranus--numerical data with reference thereto--the four satellites of uranus--their circular orbits--early observations of uranus--flamsteed's observations--lemonnier saw uranus--utility of their measurements--the elliptic path--the great problem thus suggested. to the present writer it has always seemed that the history of uranus, and of the circumstances attending its discovery, forms one of the most pleasing and interesting episodes in the whole history of science. we here occupy an entirely new position in the study of the solar system. all the other great planets were familiarly known from antiquity, however erroneous might be the ideas entertained in connection with them. they were conspicuous objects, and by their movements could hardly fail to attract the attention of those whose pursuits led them to observe the stars. but now we come to a great planet, the very existence of which was utterly unknown to the ancients; and hence, in approaching the subject, we have first to describe the actual discovery of this object, and then to consider what we can learn as to its physical nature. we have, in preceding pages, had occasion to mention the revered name of william herschel in connection with various branches of astronomy; but we have hitherto designedly postponed any more explicit reference to this extraordinary man until we had arrived at the present stage of our work. the story of uranus, in its earlier stages at all events, is the story of the early career of william herschel. it would be alike impossible and undesirable to attempt to separate them. william herschel, the illustrious astronomer, was born at hanover in . his father was an accomplished man, pursuing, in a somewhat humble manner, the calling of a professor of music. he had a family of ten children, of whom william was the fourth; and it may be noted that all the members of the family of whom any record has been preserved inherited their father's musical talents, and became accomplished performers. pleasing sketches have been given of this interesting family, of the unusual aptitude of william, of the long discussions on music and on philosophy, and of the little sister caroline, destined in later years for an illustrious career. william soon learned all that his master could teach him in the ordinary branches of knowledge, and by the age of fourteen he was already a competent performer on the oboe and the viol. he was engaged in the court orchestra at hanover, and was also a member of the band of the hanoverian guards. troublous times were soon to break up herschel's family. the french invaded hanover, the hanoverian guards were overthrown in the battle of hastenbeck, and young william herschel had some unpleasant experience of actual warfare. his health was not very strong, and he decided that he would make a change in his profession. his method of doing so is one which his biographers can scarcely be expected to defend; for, to speak plainly, he deserted, and succeeded in making his escape to england. it is stated on unquestionable authority that on herschel's first visit to king george iii., more than twenty years afterwards, his pardon was handed to him by the king himself, written out in due form. at the age of nineteen the young musician began to seek his fortunes in england. he met at first with very considerable hardship, but industry and skill conquered all difficulties, and by the time he was twenty-six years of age he was thoroughly settled in england, and doing well in his profession. in the year we find herschel occupying a position of some distinction in the musical world; he had become the organist of the octagon chapel at bath, and his time was fully employed in giving lessons to his numerous pupils, and with his preparation for concerts and oratorios. notwithstanding his busy professional life, herschel still retained that insatiable thirst for knowledge which he had when a boy. every moment he could snatch from his musical engagements was eagerly devoted to study. in his desire to perfect his knowledge of the more abstruse parts of the theory of music he had occasion to learn mathematics; from mathematics the transition to optics was a natural one; and once he had commenced to study optics, he was of course brought to a knowledge of the telescope, and thence to astronomy itself. his beginnings were made on a very modest scale. it was through a small and imperfect telescope that the great astronomer obtained his first view of the celestial glories. no doubt he had often before looked at the heavens on a clear night, and admired the thousands of stars with which they were adorned; but now, when he was able to increase his powers of vision even to a slight extent, he obtained a view which fascinated him. the stars he had seen before he now saw far more distinctly; but, more than this, he found that myriads of others previously invisible were now revealed to him. glorious, indeed, is this spectacle to anyone who possesses a spark of enthusiasm for natural beauty. to herschel this view immediately changed the whole current of his life. his success as a professor of music, his oratorios, and his pupils were speedily to be forgotten, and the rest of his life was to be devoted to the absorbing pursuit of one of the noblest of the sciences. herschel could not remain contented with the small and imperfect instrument which first interested him. throughout his career he determined to see everything for himself in the best manner which his utmost powers could command. he at once decided to have a better instrument, and he wrote to a celebrated optician in london with the view of making a purchase. but the price which the optician demanded seemed more than herschel thought he could or ought to give. instantly his resolution was taken. a good telescope he must have, and as he could not buy one he resolved to make one. it was alike fortunate, both for herschel and for science, that circumstances impelled him to this determination. yet, at first sight, how unpromising was the enterprise! that a music teacher, busily employed day and night, should, without previous training, expect to succeed in a task where the highest mechanical and optical skill was required, seemed indeed unlikely. but enthusiasm and genius know no insuperable difficulties. from conducting a brilliant concert in bath, when that city was at the height of its fame, herschel would rush home, and without even delaying to take off his lace ruffles, he would plunge into his manual labours of grinding specula and polishing lenses. no alchemist of old was ever more deeply absorbed in a project for turning lead into gold than was herschel in his determination to have a telescope. he transformed his home into a laboratory; of his drawing-room he made a carpenter's shop. turning lathes were the furniture of his best bedroom. a telescope he must have, and as he progressed he determined, not only that he should have a good telescope, but a very good one; and as success cheered his efforts he ultimately succeeded in constructing the greatest telescope that the world had up to that time ever seen. though it is as an astronomer that we are concerned with herschel, yet we must observe even as a telescope maker also great fame and no small degree of commercial success flowed in upon him. when the world began to ring with his glorious discoveries, and when it was known that he used no other telescopes than those which were the work of his own hands, a demand sprang up for instruments of his construction. it is stated that he made upwards of eighty large telescopes, as well as many others of smaller size. several of these instruments were purchased by foreign princes and potentates.[ ] we have never heard that any of these illustrious personages became celebrated astronomers, but, at all events, they seem to have paid herschel handsomely for his skill, so that by the sale of large telescopes he was enabled to realise what may be regarded as a fortune in the moderate horizon of the man of science. up to the middle of his life herschel was unknown to the public except as a laborious musician, with considerable renown in his profession, not only in bath, but throughout the west of england. his telescope-making was merely the occupation of his spare moments, and was unheard of by most of those who knew and respected his musical attainments. it was in that herschel first enjoyed a view of the heavens through an instrument built with his own hands. it was but a small one in comparison with those which he afterwards fashioned, but at once he experienced the advantage of being his own instrument maker. night after night he was able to add the improvements which experience suggested; at one time he was enlarging the mirrors; at another he was reconstructing the mounting, or trying to remedy defects in the eye-pieces. with unwearying perseverance he aimed at the highest excellence, and with each successive advance he found that he was able to pierce further into the sky. his enthusiasm attracted a few friends who were, like himself, ardently attached to science. the mode in which he first made the acquaintance of sir william watson, who afterwards became his warmest friend, was characteristic of both. herschel was observing the mountains in the moon, and as the hours passed on, he had occasion to bring his telescope into the street in front of his house to enable him to continue his work. sir william watson happened to pass by, and was arrested by the unusual spectacle of an astronomer in the public street, at the dead of night, using a large and quaint-looking instrument. having a taste for astronomy, sir william stopped, and when herschel took his eye from the telescope, asked if he might be allowed to have a look at the moon. the request was readily granted. probably herschel found but few in the gay city who cared for such matters; he was quickly drawn to sir w. watson, who at once reciprocated the feeling, and thus began a friendship which bore important fruit in herschel's subsequent career. at length the year approached, which was to witness his great achievement. herschel had made good use of seven years' practical experience in astronomy, and he had completed a telescope of exquisite optical perfection, though greatly inferior in size to some of those which he afterwards erected. with this reflector herschel commenced a methodical piece of observation. he formed the scheme of systematically examining all the stars which were above a certain degree of brightness. it does not quite appear what object herschel proposed to himself when he undertook this labour, but, in any case, he could hardly have anticipated the extraordinary success with which the work was to be crowned. in the course of this review the telescope was directed to a star; that star was examined; then another was brought into the field of view, and it too was examined. every star under such circumstances merely shows itself as a point of light; the point may be brilliant or not, according as the star is bright or not; the point will also, of course, show the colour of the star, but it cannot exhibit recognisable size or shape. the greater, in fact, the perfection of the telescope, the smaller is the telescopic image of a star. how many stars herschel inspected in this review we are not told; but at all events, on the ever-memorable night of the th of march, , he was pursuing his self-allotted task among the hosts in the constellation gemini. doubtless, one star after another was admitted to view, and was allowed to pass away. at length, however, an object was placed in the field which differed from every other star. it was not a mere point of light; it had a minute, but still a perfectly recognisable, disc. we say the disc was perfectly recognisable, but we should be careful to add that it was so in the excellent telescope of herschel alone. other astronomers had seen this object before. its position had actually been measured no fewer than nineteen times before the bath musician, with his home-made telescope, looked at it, but the previous observers had only seen it in small meridian instruments with low magnifying powers. even after the discovery was made, and when well-trained observers with good instruments looked again under the direction of herschel, one after another bore testimony to the extraordinary delicacy of the great astronomer's perception, which enabled him almost at the first glance to discriminate between it and a star. if not a star, what, then, could it be? the first step to enable this question to be answered was to observe the body for some time. this herschel did. he looked at it one night after another, and soon he discovered another fundamental difference between this object and an ordinary star. the stars are, of course, characterised by their fixity, but this object was not fixed; night after night the place it occupied changed with respect to the stars. no longer could there be any doubt that this body was a member of the solar system, and that an interesting discovery had been made; many months, however, elapsed before herschel knew the real merit of his achievement. he did not realise that he had made the superb discovery of another mighty planet revolving outside saturn; he thought that it could only be a comet. no doubt this object looked very different from a great comet, decorated with a tail. it was not, however, so entirely different from some forms of telescopic comets as to make the suggestion of its being a body of this kind unlikely; and the discovery was at first announced in accordance with this view. time was necessary before the true character of the object could be ascertained. it must be followed for a considerable distance along its path, and measures of its position at different epochs must be effected, before it is practicable for the mathematician to calculate the path which the body pursues; once, however, attention was devoted to the subject, many astronomers aided in making the necessary observations. these were placed in the hands of mathematicians, and the result was proclaimed that this body was not a comet, but that, like all the planets, it revolved in nearly a circular path around the sun, and that the path lay millions of miles outside the path of saturn, which had so long been regarded as the boundary of the solar system. it is hardly possible to over-estimate the significance of this splendid discovery. the five planets had been known from all antiquity; they were all, at suitable seasons, brilliantly conspicuous to the unaided eye. but it was now found that, far outside the outermost of these planets revolved another splendid planet, larger than mercury or mars, larger--far larger--than venus and the earth, and only surpassed in bulk by jupiter and by saturn. this superb new planet was plunged into space to such a depth that, notwithstanding its noble proportions, it seemed merely a tiny star, being only on rare occasions within reach of the unaided eye. this great globe required a period of eighty-four years to complete its majestic path, and the diameter of that path was , , , miles. although the history of astronomy is the record of brilliant discoveries--of the labours of copernicus, and of kepler--of the telescopic achievements of galileo, and the splendid theory of newton--of the refined discovery of the aberration of light--of many other imperishable triumphs of intellect--yet this achievement of the organist at the octagon chapel occupies a totally different position from any other. there never before had been any historic record of the discovery of one of the bodies of the particular system to which the earth belongs. the older planets were no doubt discovered by someone, but we can say little more about these discoveries than we can about the discovery of the sun or of the moon; all are alike prehistoric. here was the first recorded instance of the discovery of a planet which, like the earth, revolves around the sun, and, like our earth, may conceivably be an inhabited globe. so unique an achievement instantly arrested the attention of the whole scientific world. the music-master at bath, hitherto unheard of as an astronomer, was speedily placed in the very foremost rank of those entitled to the name. on all sides the greatest interest was manifested about the unknown philosopher. the name of herschel, then unfamiliar to english ears, appeared in every journal, and a curious list has been preserved of the number of blunders which were made in spelling the name. the different scientific societies hastened to convey their congratulations on an occasion so memorable. tidings of the discovery made by the hanoverian musician reached the ears of george iii., and he sent for herschel to come to the court, that the king might learn what his achievement actually was from the discoverer's own lips. herschel brought with him one of his telescopes, and he provided himself with a chart of the solar system, with which to explain precisely wherein the significance of the discovery lay. the king was greatly interested in herschel's narrative, and not less in herschel himself. the telescope was erected at windsor, and, under the astronomer's guidance, the king was shown saturn and other celebrated objects. it is also told how the ladies of the court the next day asked herschel to show them the wonders which had so pleased the king. the telescope was duly erected in a window of one of the queen's apartments, but when evening arrived the sky was found to be overcast with clouds, and no stars could be seen. this was an experience with which herschel, like every other astronomer, was unhappily only too familiar. but it is not every astronomer who would have shown the readiness of herschel in escaping gracefully from the position. he showed to his lady pupils the construction of the telescope; he explained the mirror, and how he had fashioned it and given the polish; and then, seeing the clouds were inexorable, he proposed that, as he could not show them the real saturn, he should exhibit an artificial one as the best substitute. the permission granted, herschel turned the telescope away from the sky, and pointed it towards the wall of a distant garden. on looking into the telescope there was saturn, his globe and his system of rings, so faithfully shown that, says herschel, even a skilful astronomer might have been deceived. the fact was that during the course of the day herschel saw that the sky would probably be overcast in the evening, and he had provided for the emergency by cutting a hole in a piece of cardboard, the shape of saturn, which was then placed against the distant garden wall, and illuminated by a lamp at the back. this visit to windsor was productive of consequences momentous to herschel, momentous to science. he had made so favourable an impression, that the king proposed to create for him the special appointment of king's astronomer at windsor. the king was to provide the means for erecting the great telescopes, and he allocated to herschel a salary of £ a year, the figures being based, it must be admitted, on a somewhat moderate estimate of the requirements of an astronomer's household. herschel mentioned these particulars to no one save to his constant and generous friend, sir w. watson, who exclaimed, "never bought monarch honour so cheap." to other enquirers, herschel merely said that the king had provided for him. in accepting this post, the great astronomer took no doubt a serious step. he at once sacrificed entirely his musical career, now, from many sources, a lucrative one; but his determination was speedily taken. the splendid earnest that he had already given of his devotion to astronomy was, he knew, only the commencement of a series of memorable labours. he had indeed long been feeling that it was his bounden duty to follow that path in life which his genius indicated. he was no longer a young man. he had attained middle age, and the years had become especially precious to one who knew that he had still a life-work to accomplish. he at one stroke freed himself from all distractions; his pupils and concerts, his whole connection at bath, were immediately renounced; he accepted the king's offer with alacrity, and after one or two changes settled permanently at slough, near windsor. it has, indeed, been well remarked that the most important event in connection with the discovery of uranus was the discovery of herschel's unrivalled powers of observation. uranus must, sooner or later, have been found. had herschel not lived, we would still, no doubt, have known uranus long ere this. the really important point for science was that herschel's genius should be given full scope, by setting him free from the engrossing details of an ordinary professional calling. the discovery of uranus secured all this, and accordingly obtained for astronomy all herschel's future labours.[ ] uranus is so remote that even the best of our modern telescopes cannot make of it a striking picture. we can see, as herschel did, that it has a measurable disc, and from measurements of that disc we conclude that the diameter of the planet is about , miles. this is about four times as great as the diameter of the earth, and we accordingly see that the volume of uranus must be about sixty-four times as great as that of the earth. we also find that, like the other giant planets, uranus seems to be composed of materials much lighter, on the whole, than those we find here; so that, though sixty-four times as large as the earth, uranus is only fifteen times as heavy. if we may trust to the analogies of what we see everywhere else in our system, we can feel but little doubt that uranus must rotate about an axis. the ordinary means of demonstrating this rotation can be hardly available in a body whose surface appears so small and so faint. the period of rotation is accordingly unknown. the spectroscope tells us that a remarkable atmosphere, containing apparently some gases foreign to our own, deeply envelops uranus. there is, however, one feature about uranus which presents many points of interest to those astronomers who are possessed of telescopes of unusual size and perfection. uranus is accompanied by a system of satellites, some of which are so faint as to require the closest scrutiny for their detection. the discovery of these satellites was one of the subsequent achievements of herschel. it is, however, remarkable that even his penetration and care did not preserve him from errors with regard to these very delicate objects. some of the points which he thought to be satellites must, it would now seem, have been merely stars enormously more distant, which happened to lie in the field of view. it has been since ascertained that the known satellites of uranus are four in number, and their movements have been made the subject of prolonged and interesting telescopic research. the four satellites bear the names of ariel, umbriel, titania, and oberon. arranged in order of their distance from the central body, ariel, the nearest, accomplishes its journey in days and hours. oberon, the most distant, completes its journey in days and hours. the law of kepler declares that the path of a satellite around its primary, no less than of the primary around the sun, must be an ellipse. it leaves, however, boundless latitude in the actual eccentricity of the curve. the ellipse may be nearly a circle, it may be absolutely a circle, or it may be something quite different from a circle. the paths pursued by the planets are, generally speaking, nearly circles; but we meet with no exact circle among planetary orbits. so far as we at present know, the closest approach made to a perfectly circular movement is that by which the satellites of uranus revolve around their primary. we are not prepared to say that these paths are absolutely circular. all that can be said is that our telescopes fail to show any measurable departure therefrom. it is also to be noted as an interesting circumstance that the orbits of the satellites of uranus all lie in the same plane. this is not true of the orbits of the planets around the sun, nor is it true of the orbits of any other system of satellites around their primary. the most singular circumstance attending the uranian system is, however, found in the position which this plane occupies. this is indeed almost as great an anomaly in our system as are the rings of saturn themselves. we have already had occasion to notice that the plane in which the earth revolves around the sun is very nearly coincident with the planes in which all the other great planets revolve. the same is true, to a large extent, of the orbits of the minor planets; though here, no doubt, we meet with a few cases in which the plane of the orbit is inclined at no inconsiderable angle to the plane in which the earth moves. the plane in which the moon revolves also approximates to this system of planetary planes. so, too, do the orbits of the satellites of saturn and of jupiter, while even the more recently discovered satellites of mars form no exception to the rule. the whole solar system--at least so far as the great planets are concerned--would require comparatively little alteration if the orbits were to be entirely flattened down into one plane. there are, however, some notable exceptions to this rule. the satellites of uranus revolve in a plane which is far from coinciding with the plane to which all other orbits approximate. in fact, the paths of the satellites of uranus lie in a plane nearly at right angles to the orbit of uranus. we are not in a position to give any satisfactory explanation of this circumstance. it is, however, evident that in the genesis of the uranian system there must have been some influence of a quite exceptional and local character. soon after the discovery of the planet uranus, in , sufficient observations were accumulated to enable the orbit it follows to be determined. when the path was known, it was then a mere matter of mathematical calculation to ascertain where the planet was situated at any past time, and where it would be situated at any future time. an interesting enquiry was thus originated as to how far it might be possible to find any observations of the planet made previously to its discovery by herschel. uranus looks like a star of the sixth magnitude. not many astronomers were provided with telescopes of the perfection attained by herschel, and the personal delicacy of perception characteristic of herschel was a still more rare possession. it was, therefore, to be expected that, if such previous observations existed, they would merely record uranus as a star visible, and indeed bright, in a moderate telescope, but still not claiming any exceptional attention over thousands of apparently similar stars. many of the early astronomers had devoted themselves to the useful and laborious work of forming catalogues of stars. in the preparation of a star catalogue, the telescope was directed to the heavens, the stars were observed, their places were carefully measured, the brightness of the star was also estimated, and thus the catalogue was gradually compiled in which each star had its place faithfully recorded, so that at any future time it could be identified. the stars were thus registered, by hundreds and by thousands, at various dates from the birth of accurate astronomy till the present time. the suggestion was then made that, as uranus looked so like a star, and as it was quite bright enough to have engaged the attention of astronomers possessed of even very moderate instrumental powers, there was a possibility that it had already been observed, and thus actually lay recorded as a star in some of the older catalogues. this was indeed an idea worthy of every attention, and pregnant with the most important consequences in connection with the immortal discovery to be discussed in our next chapter. but how was such an examination of the catalogues to be conducted? uranus is constantly moving about; does it not seem that there is every element of uncertainty in such an investigation? let us consider a notable example. the great national observatory at greenwich was founded in , and the first astronomer-royal was the illustrious flamsteed, who in commenced that series of observations of the heavenly bodies which has been continued to the present day with such incalculable benefits to science. at first the instruments were of a rather primitive description, but in the course of some years flamsteed succeeded in procuring instruments adequate to the production of a catalogue of stars, and he devoted himself with extraordinary zeal to the undertaking. it is in this memorable work, the "historia coelestis" of flamsteed, that the earliest observation of uranus is recorded. in the first place it was known that the orbit of this body, like the orbit of every other great planet, was inclined at a very small angle to the ecliptic. it hence follows that uranus is at all times only to be met with along the ecliptic, and it is possible to calculate where the planet has been in each year. it was thus seen that in the planet was situated in that part of the ecliptic where flamsteed was at the same date making his observations. it was natural to search the observations of flamsteed, and see whether any of the so-called stars could have been uranus. an object was found in the "historia coelestis" which occupied a position identical with that which uranus must have filled on the same date. could this be uranus? a decisive test was at once available. the telescope was directed to the spot in the heavens where flamsteed saw a sixth-magnitude star. if that were really a star, then would it still be visible. the trial was made: no such star could be found, and hence the presumption that this was really uranus could hardly be for a moment doubted. speedily other confirmation flowed in. it was shown that uranus had been observed by bradley and by tobias mayer, and it also became apparent that flamsteed had observed uranus not only once, but that he had actually measured its place four times in the years and . yet flamsteed was never conscious of the discovery that lay so nearly in his grasp. he was, of course, under the impression that all these observations related to different stars. a still more remarkable case is that of lemonnier, who had actually observed uranus twelve times, and even recorded it on four consecutive days in january, . if lemonnier had only carefully looked over his own work; if he had perceived, as he might have done, how the star he observed yesterday was gone to-day, while the star visible to-day had moved away by to-morrow, there is no doubt that uranus would have been discovered, and william herschel would have been anticipated. would lemonnier have made as good use of his fame as herschel did? this seems a question which can never be decided, but those who estimate herschel as the present writer thinks he ought to be estimated, will probably agree in thinking that it was most fortunate for science that lemonnier did _not_ compare his observations.[ ] these early accidental observations of uranus are not merely to be regarded as matters of historical interest or curiosity. that they are of the deepest importance with regard to the science itself a few words will enable us to show. it is to be remembered that uranus requires no less than eighty-four years to accomplish his mighty revolution around the sun. the planet has completed one entire revolution since its discovery, and up to the present time ( ) has accomplished more than one-third of another. for the careful study of the nature of the orbit, it was desirable to have as many measurements as possible, and extending over the widest possible interval. this was in a great measure secured by the identification of the early observations of uranus. an approximate knowledge of the orbit was quite capable of giving the places of the planet with sufficient accuracy to identify it when met with in the catalogues. but when by their aid the actual observations have been discovered, they tell us precisely the place of uranus; and hence, instead of our knowledge of the planet being limited to but little more than one revolution, we have at the present time information with regard to it extending over considerably more than two revolutions. from the observations of the planet the ellipse in which it moves can be ascertained. we can compute this ellipse from the observations made during the time since the discovery. we can also compute the ellipse from the early observations made before the discovery. if kepler's laws were rigorously verified, then, of course, the ellipse performed in the present revolution must differ in no respect from the ellipse performed in the preceding, or indeed in any other revolution. we can test this point in an interesting manner by comparing the ellipse derived from the ancient observations with that deduced from the modern ones. these ellipses closely resemble each other; they are nearly the same; but it is most important to observe that they are not _exactly_ the same, even when allowance has been made for every known source of disturbance in accordance with the principles explained in the next chapter. the law of kepler seems thus not absolutely true in the case of uranus. here is, indeed, a matter demanding our most earnest and careful attention. have we not repeatedly laid down the universality of the laws of kepler in controlling the planetary motions? how then can we reconcile this law with the irregularities proved beyond a doubt to exist in the motions of uranus? let us look a little more closely into the matter. we know that the laws of kepler are a consequence of the laws of gravitation. we know that the planet moves in an elliptic path around the sun, in virtue of the sun's attraction, and we know that the ellipse will be preserved without the minutest alteration if the sun and the planet be left to their mutual attractions, and if no other force intervene. we can also calculate the influence of each of the known planets on the form and position of the orbit. but when allowance is made for all such perturbing influences it is found that the observed and computed orbits do not agree. the conclusion is irresistible. uranus does not move solely in consequence of the sun's attraction and that of the planets of our system interior to uranus; there must therefore be some further influence acting upon uranus besides those already known. to the development of this subject the next chapter will be devoted. chapter xv. neptune. discovery of neptune--a mathematical achievement--the sun's attraction--all bodies attract--jupiter and saturn--the planetary perturbations--three bodies--nature has simplified the problem--approximate solution--the sources of success--the problem stated for the earth--the discoveries of lagrange--the eccentricity--necessity that all the planets revolve in the same direction--lagrange's discoveries have not the dramatic interest of the more recent achievements--the irregularities of uranus--the unknown planet must revolve outside the path of uranus--the data for the problem--le verrier and adams both investigate the question--adams indicates the place of the planet--how the search was to be conducted--le verrier also solves the problem--the telescopic discovery of the planet--the rival claims--early observation of neptune--difficulty of the telescopic study of neptune--numerical details of the orbit--is there any outer planet?--contrast between mercury and neptune. we describe in this chapter a discovery so extraordinary that the whole annals of science may be searched in vain for a parallel. we are not here concerned with technicalities of practical astronomy. neptune was first revealed by profound mathematical research rather than by minute telescopic investigation. we must develop the account of this striking epoch in the history of science with the fulness of detail which is commensurate with its importance; and it will accordingly be necessary, at the outset of our narrative, to make an excursion into a difficult but attractive department of astronomy, to which we have as yet made little reference. the supreme controlling power in the solar system is the attraction of the sun. each planet of the system experiences that attraction, and, in virtue thereof, is constrained to revolve around the sun in an elliptic path. the efficiency of a body as an attractive agent is directly proportional to its mass, and as the mass of the sun is more than a thousand times as great as that of jupiter, which, itself, exceeds that of all the other planets collectively, the attraction of the sun is necessarily the chief determining force of the movements in our system. the law of gravitation, however, does not merely say that the sun attracts each planet. gravitation is a doctrine much more general, for it asserts that every body in the universe attracts every other body. in obedience to this law, each planet must be attracted, not only by the sun, but by innumerable bodies, and the movement of the planet must be the joint effect of all such attractions. as for the influence of the stars on our solar system, it may be at once set aside as inappreciable. the stars are no doubt enormous bodies, in many cases possibly transcending the sun in magnitude, but the law of gravitation tells us that the intensity of the attraction decreases as the square of the distance increases. most of the stars are a million times as remote as the sun, and consequently their attraction is so slight as to be absolutely inappreciable in the discussion of this question. the only attractions we need consider are those which arise from the action of one body of the system upon another. let us take, for instance, the two largest planets of our system, jupiter and saturn. each of these globes revolves mainly in consequence of the sun's attraction, but every planet also attracts every other, and the consequence is that each one is slightly drawn away from the position it would have otherwise occupied. in the language of astronomy, we would say that the path of jupiter is perturbed by the attraction of saturn; and, conversely, that the path of saturn is perturbed by the attraction of jupiter. for many years these irregularities of the planetary motions presented problems with which astronomers were not able to cope. gradually, however, one difficulty after another has been vanquished, and though there are no doubt some small irregularities still outstanding which have not been completely explained, yet all the larger and more important phenomena of the kind are well understood. the subject is one of the most difficult which the astronomer has to encounter in the whole range of his science. he has here to calculate what effect one planet is capable of producing on another planet. such calculations bristle with formidable difficulties, and can only be overcome by consummate skill in the loftiest branches of mathematics. let us state what the problem really is. when two bodies move in virtue of their mutual attraction, both of them will revolve in a curve which admits of being exactly ascertained. each path is, in fact, an ellipse, and they must have a common focus at the centre of gravity of the two bodies, considered as a single system. in the case of a sun and a planet, in which the mass of the sun preponderates enormously over the mass of the planet, the centre of gravity of the two lies very near the centre of the sun; the path of the great body is in such a case very small in comparison with the path of the planet. all these matters admit of perfectly accurate calculation of a somewhat elementary character. but now let us add a third body to the system which attracts each of the others and is attracted by them. in consequence of this attraction, the third body is displaced, and accordingly its influence on the others is modified; they in turn act upon it, and these actions and reactions introduce endless complexity into the system. such is the famous "problem of three bodies," which has engaged the attention of almost every great mathematician since the time of newton. stated in its mathematical aspect, and without having its intricacy abated by any modifying circumstances, the problem is one that defies solution. mathematicians have not yet been able to deal with the mutual attractions of three bodies moving freely in space. if the number of bodies be greater than three, as is actually the case in the solar system, the problem becomes still more hopeless. nature, however, has in this matter dealt kindly with us. she has, it is true, proposed a problem which cannot be accurately solved; but she has introduced into the problem, as proposed in the solar system, certain special features which materially reduce the difficulty. we are still unable to make what a mathematician would describe as a rigorous solution of the question; we cannot solve it with the completeness of a sum in arithmetic; but we can do what is nearly if not quite as useful. we can solve the problem approximately; we can find out what the effect of one planet on the other is _very nearly_, and by additional labour we can reduce the limits of uncertainty to as low a point as may be desired. we thus obtain a practical solution of the problem adequate for all the purposes of science. it avails us little to know the place of a planet with absolute mathematical accuracy. if we can determine what we want with so close an approximation to the true position that no telescope could possibly disclose the difference, then every practical end will have been attained. the reason why in this case we are enabled to get round the difficulties which we cannot surmount lies in the exceptional character of the problem of three bodies as exhibited in the solar system. in the first place, the sun is of such pre-eminent mass that many matters may be overlooked which would be of moment were he rivalled in mass by any of the planets. another source of our success arises from the small inclinations of the planetary orbits to each other; while the fact that the orbits are nearly circular also greatly facilitates the work. the mathematicians who may reside in some of the other parts of the universe are not equally favoured. among the sidereal systems we find not a few cases where the problem of three bodies, or even of more than three, would have to be faced without any of the alleviating circumstances which our system presents. in such groups as the marvellous star th orionis, we have three or four bodies comparable in size, which must produce movements of the utmost complexity. even if terrestrial mathematicians shall ever have the hardihood to face such problems, there is no likelihood of their being able to do so for ages to come; such researches must repose on accurate observations as their foundation; and the observations of these distant systems are at present utterly inadequate for the purpose. the undisturbed revolution of a planet around the sun, in conformity with kepler's law, would assure for that planet permanent conditions of climate. the earth, for instance, if guided solely by kepler's laws, would return each day of the year exactly to the same position which it had on the same day of last year. from age to age the quantity of heat received by the earth would remain constant if the sun continued unaltered, and the present climate might thus be preserved indefinitely. but since the existence of planetary perturbation has become recognised, questions arise of the gravest importance with reference to the possible effects which such perturbations may have. we now see that the path of the earth is not absolutely fixed. that path is deranged by venus and by mars; it is deranged, it must be deranged, by every planet in our system. it is true that in a year, or even in a century, the amount of alteration produced is not very great; the ellipse which represents the path of our earth this year does not differ considerably from the ellipse which represented the movement of the earth one hundred years ago. but the important question arises as to whether the slight difference which does exist may not be constantly increasing, and may not ultimately assume such proportions as to modify our climates, or even to render life utterly impossible. indeed, if we look at the subject without attentive calculation, nothing would seem more probable than that such should be the fate of our system. this globe revolves in a path inside that of the mighty jupiter. it is, therefore, constantly attracted by jupiter, and when it overtakes the vast planet, and comes between him and the sun, then the two bodies are comparatively close together, and the earth is pulled outwards by jupiter. it might be supposed that the tendency of such disturbances would be to draw the earth gradually away from the sun, and thus to cause our globe to describe a path ever growing wider and wider. it is not, however, possible to decide a dynamical question by merely superficial reasoning of this character. the question has to be brought before the tribunal of mathematical analysis, where every element in the case is duly taken into account. such an enquiry is by no means a simple one. it worthily occupied the splendid talents of lagrange and laplace, whose discoveries in the theory of planetary perturbation are some of the most remarkable achievements in astronomy. we cannot here attempt to describe the reasoning which these great mathematicians employed. it can only be expressed by the formulæ of the mathematician, and would then be hardly intelligible without previous years of mathematical study. it fortunately happens, however, that the results to which lagrange and laplace were conducted, and which have been abundantly confirmed by the labours of other mathematicians, admit of being described in simple language. let us suppose the case of the sun, and of two planets circulating around him. these two planets are mutually disturbing each other, but the amount of the disturbance is small in comparison with the effect of the sun on each of them. lagrange demonstrated that, though the ellipse in which each planet moved was gradually altered in some respects by the attraction of the other planet, yet there is one feature of the curve which the perturbation is powerless to alter permanently: the longest axis of the ellipse, and, therefore, the mean distance of the planet from the sun, which is equal to one-half of it, must remain unchanged. this is really a discovery as important as it was unexpected. it at once removes all fear as to the effect which perturbations can produce on the stability of the system. it shows that, notwithstanding the attractions of mars and of venus, of jupiter and of saturn, our earth will for ever continue to revolve at the same mean distance from the sun, and thus the succession of the seasons and the length of the year, so far as this element at least is concerned, will remain for ever unchanged. but lagrange went further into the enquiry. he saw that the mean distance did not alter, but it remained to be seen whether the eccentricity of the ellipse described by the earth might not be affected by the perturbations. this is a matter of hardly less consequence than that just referred to. even though the earth preserved the same average distance from the sun, yet the greatest and least distance might be widely unequal: the earth might pass very close to the sun at one part of its orbit, and then recede to a very great distance at the opposite part. so far as the welfare of our globe and its inhabitants is concerned, this is quite as important as the question of the mean distance; too much heat in one half of the year would afford but indifferent compensation for too little during the other half. lagrange submitted this question also to his analysis. again he vanquished the mathematical difficulties, and again he was able to give assurance of the permanence of our system. it is true that he was not this time able to say that the eccentricity of each path will remain constant; this is not the case. what he does assert, and what he has abundantly proved, is that the eccentricity of each orbit will always remain small. we learn that the shape of the earth's orbit gradually swells and gradually contracts; the greatest length of the ellipse is invariable, but sometimes it approaches more to a circle, and sometimes becomes more elliptical. these changes are comprised within narrow limits; so that, though they may probably correspond with measurable climatic changes, yet the safety of the system is not imperilled, as it would be if the eccentricity could increase indefinitely. once again lagrange applied the resources of his calculus to study the effect which perturbations can have on the inclination of the path in which the planet moves. the result in this case was similar to that obtained with respect to the eccentricities. if we commence with the assumption that the mutual inclinations of the planets are small, then mathematics assure us that they must always remain small. we are thus led to the conclusion that the planetary perturbations are unable to affect the stability of the solar system. we shall perhaps more fully appreciate the importance of these memorable researches if we consider how easily matters might have been otherwise. let us suppose a system resembling ours in every respect save one. let that system have a sun, as ours has; a system of planets and of satellites like ours. let the masses of all the bodies in this hypothetical system be identical with the masses in our system, and let the distances and the periodic times be the same in the two cases. let all the planes of the orbits be similarly placed; and yet this hypothetical system might contain seeds of decay from which ours is free. there is one point in the imaginary scheme which we have not yet specified. in our system all the planets revolve in the _same direction_ around the sun. let us suppose this law violated in the hypothetical system by reversing one planet on its path. that slight change alone would expose the system to the risk of destruction by the planetary perturbations. here, then, we find the necessity of that remarkable uniformity of the directions in which the planets revolve around the sun. had these directions not been uniform, our system must, in all probability, have perished ages ago, and we should not be here to discuss perturbations or any other subject. great as was the success of the eminent french mathematician who made these beautiful discoveries, it was left for this century to witness the crowning triumph of mathematical analysis applied to the law of gravitation. the work of lagrange lacks the dramatic interest of the discovery made by le verrier and adams, which gave still wider extent to the solar system by the discovery of the planet neptune revolving far outside uranus. we have already alluded to the difficulties which were experienced when it was sought to reconcile the early observations of uranus with those made since its discovery. we have shown that the path in which this planet revolved experienced change, and that consequently uranus must be exposed to the action of some other force besides the sun's attraction. the question arises as to the nature of these disturbing forces. from what we have already learned of the mutual deranging influence between any two planets, it seems natural to inquire whether the irregularities of uranus could not be accounted for by the attraction of the other planets. uranus revolves just outside saturn. the mass of saturn is much larger than the mass of uranus. could it not be that saturn draws uranus aside, and thus causes the changes? this is a question to be decided by the mathematician. he can compute what saturn is able to do, and he finds, no doubt, that saturn is capable of producing some displacement of uranus. in a similar manner jupiter, with his mighty mass, acts on uranus, and produces a disturbance which the mathematician calculates. when the figures had been worked out for all the known planets they were applied to uranus, and we might expect to find that they would fully account for the observed irregularities of his path. this was, however, not the case. after every known source of disturbance had been carefully allowed for, uranus was still shown to be influenced by some further agent; and hence the conclusion was established that uranus must be affected by some unknown body. what could this unknown body be, and where must it be situated? analogy was here the guide of those who speculated on this matter. we know no cause of disturbance of a planet's motion except it be the attraction of another planet. could it be that uranus was really attracted by some other planet at that time utterly unknown? this suggestion was made by many astronomers, and it was possible to determine some conditions which the unknown body should fulfil. in the first place its orbit must lie outside the orbit of uranus. this was necessary, because the unknown planet must be a large and massive one to produce the observed irregularities. if, therefore, it were nearer than uranus, it would be a conspicuous object, and must have been discovered long ago. other reasonings were also available to show that if the disturbances of uranus were caused by the attraction of a planet, that body must revolve outside the globe discovered by herschel. the general analogies of the planetary system might also be invoked in support of the hypothesis that the path of the unknown planet, though necessarily elliptic, did not differ widely from a circle, and that the plane in which it moved must also be nearly coincident with the plane of the earth's orbit. the measured deviations of uranus at the different points of its orbit were the sole data available for the discovery of the new planet. we have to fit the orbit of the unknown globe, as well as the mass of the planet itself, in such a way as to account for the various perturbations. let us, for instance, assume a certain distance for the hypothetical body, and try if we can assign both an orbit and a mass for the planet, at that distance, which shall account for the perturbations. our first assumption is perhaps too great. we try again with a lesser distance. we can now represent the observations with greater accuracy. a third attempt will give the result still more closely, until at length the distance of the unknown planet is determined. in a similar way the mass of the body can be also determined. we assume a certain value, and calculate the perturbations. if the results seem greater than those obtained by observations, then the assumed mass is too great. we amend the assumption, and recompute with a lesser amount, and so on until at length we determine a mass for the planet which harmonises with the results of actual measurement. the other elements of the unknown orbit--its eccentricity and the position of its axis--are all to be ascertained in a similar manner. at length it appeared that the perturbations of uranus could be completely explained if the unknown planet had a certain mass, and moved in an orbit which had a certain position, while it was also manifest that no very different orbit or greatly altered mass would explain the observed facts. these remarkable computations were undertaken quite independently by two astronomers--one in england and one in france. each of them attacked, and each of them succeeded in solving, the great problem. the scientific men of england and the scientific men of france joined issue on the question as to the claims of their respective champions to the great discovery; but in the forty years which have elapsed since these memorable researches the question has gradually become settled. it is the impartial verdict of the scientific world outside england and france, that the merits of this splendid triumph of science must be divided equally between the late distinguished professor j.c. adams, of cambridge, and the late u.j.j. le verrier, the director of the paris observatory. shortly after mr. adams had taken his degree at cambridge, in , when he obtained the distinction of senior wrangler, he turned his attention to the perturbations of uranus, and, guided by these perturbations alone, commenced his search for the unknown planet. long and arduous was the enquiry--demanding an enormous amount of numerical calculation, as well as consummate mathematical resource; but gradually mr. adams overcame the difficulties. as the subject unfolded itself, he saw how the perturbations of uranus could be fully explained by the existence of an exterior planet, and at length he had ascertained, not alone the orbit of this outer body, but he was even able to indicate the part of the heavens in which the unknown globe must be sought. with his researches in this advanced condition, mr. adams called on the astronomer-royal, sir george airy, at greenwich, in october, , and placed in his hands the computations which indicated with marvellous accuracy the place of the yet unobserved planet. it thus appears that seven months before anyone else had solved this problem mr. adams had conquered its difficulties, and had actually located the planet in a position but little more than a degree distant from the spot which it is now known to have occupied. all that was wanted to complete the discovery, and to gain for professor adams and for english science the undivided glory of this achievement, was a strict telescopic search through the heavens in the neighbourhood indicated. why, it may be said, was not such an enquiry instituted at once? no doubt this would have been done, if the observatories had been generally furnished forty years ago with those elaborate star-charts which they now possess. in the absence of a chart (and none had yet been published of the part of the sky where the unknown planet was) the search for the planet was a most tedious undertaking. it had been suggested that the new globe could be detected by its visible disc; but it must be remembered that even uranus, so much closer to us, had a disc so small that it was observed nearly a score of times without particular notice, though it did not escape the eagle glance of herschel. there remained then only one available method of finding neptune. it was to construct a chart of the heavens in the neighbourhood indicated, and then to compare this chart night after night with the stars in the heavens. before recommending the commencement of a labour so onerous, the astronomer-royal thought it right to submit mr. adams's researches to a crucial preliminary test. mr. adams had shown how his theory rendered an exact account of the perturbations of uranus in longitude. the astronomer-royal asked mr. adams whether he was able to give an equally clear explanation of the notable variations in the distance of uranus. there can be no doubt that his theory would have rendered a satisfactory account of these variations also; but, unfortunately, mr. adams seems not to have thought the matter of sufficient importance to give the astronomer-royal any speedy reply, and hence it happened that no less than nine months elapsed between the time when mr. adams first communicated his results to the astronomer-royal and the time when the telescopic search for the planet was systematically commenced. up to this time no account of mr. adams's researches had been published. his labours were known to but few besides the astronomer-royal and professor challis of cambridge, to whom the duty of making the search was afterwards entrusted. in the meantime the attention of le verrier, the great french mathematician and astronomer, had been specially directed by arago to the problem of the perturbations of uranus. with exhaustive analysis le verrier investigated every possible known source of disturbance. the influences of the older planets were estimated once more with every precision, but only to confirm the conclusion already arrived at as to their inadequacy to account for the perturbations. le verrier then commenced the search for the unknown planet by the aid of mathematical investigation, in complete ignorance of the labours of adams. in november, , and again on the st of june, , portions of the french astronomer's results were announced. the astronomer-royal then perceived that his calculations coincided practically with those of adams, insomuch that the places assigned to the unknown planet by the two astronomers were not more than a degree apart! this was, indeed, a remarkable result. here was a planet unknown to human sight, yet felt, as it were, by mathematical analysis with a certainty so great that two astronomers, each in total ignorance of the other's labours, concurred in locating the planet in almost the same spot of the heavens. the existence of the new globe was thus raised nearly to a certainty, and it became incumbent on practical astronomers to commence the search forthwith. in june, , the astronomer-royal announced to the visitors of the greenwich observatory the close coincidence between the calculations of le verrier and of adams, and urged that a strict scrutiny of the region indicated should be at once instituted. professor challis, having the command of the great northumberland equatorial telescope at cambridge, was induced to undertake the work, and on the th july, , he began his labours. the plan of search adopted by professor challis was an onerous one. he first took the theoretical place of the planet, as given by mr. adams, and after allowing a very large margin for the uncertainties of a calculation so recondite, he marked out a certain region of the heavens, near the ecliptic, in which it might be anticipated that the unknown planet must be found. he then determined to observe all the stars in this region and measure their relative positions. when this work was once done it was to be repeated a second time. his scheme even contemplated a third complete set of observations of the stars contained within this selected region. there could be no doubt that this process would determine the planet if it were bright enough to come within the limits of stellar magnitude which professor challis adopted. the globe would be detected by its motion relatively to the stars, when the three series of measures came to be compared. the scheme was organised so thoroughly that it must have led to the expected discovery--in fact, it afterwards appeared that professor challis did actually observe the planet more than once, and a subsequent comparison of its positions must infallibly have led to the detection of the new globe. le verrier was steadily maturing his no less elaborate investigations in the same direction. he felt confident of the existence of the planet, and he went so far as to predict not only the situation of the globe but even its actual appearance. he thought the planet would be large enough (though still of course only a telescopic object) to be distinguished from the stars by the possession of a disc. these definite predictions strengthened the belief that we were on the verge of another great discovery in the solar system, so much so that when sir john herschel addressed the british association on the th of september, , he uttered the following words:--"the past year has given to us the new planet astræa--it has done more, it has given us the probable prospect of another. we see it as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis, with a certainty hardly inferior to ocular demonstration." the time of the discovery was now rapidly approaching. on the th of september, , le verrier wrote to dr. galle of the berlin observatory, describing the place of the planet indicated by his calculations, and asking him to make its telescopic discovery. the request thus preferred was similar to that made on behalf of adams to professor challis. both at berlin and at cambridge the telescopic research was to be made in the same region of the heavens. the berlin astronomers were, however, fortunate in possessing an invaluable aid to the research which was not at the time in the hands of professor challis. we have mentioned how the search for a telescopic planet can be facilitated by the use of a carefully-executed chart of the stars. in fact, a mere comparison of the chart with the sky is all that is necessary. it happened that the preparation of a series of star charts had been undertaken by the berlin academy of sciences some years previously. on these charts the place of every star, down even to the tenth magnitude, had been faithfully engraved. this work was one of much utility, but its originators could hardly have anticipated the brilliant discovery which would arise from their years of tedious labour. it was found convenient to publish such an extensive piece of surveying work by instalments, and accordingly, as the chart was completed, it issued from the press sheet by sheet. it happened that just before the news of le verrier's labours reached berlin the chart of that part of the heavens had been engraved and printed. it was on the rd of september that le verrier's letter reached dr. galle at berlin. the sky that night was clear, and we can imagine with what anxiety dr. galle directed his telescope to the heavens. the instrument was pointed in accordance with le verrier's instructions. the field of view showed a multitude of stars, as does every part of the heavens. one of these was really the planet. the new chart was unrolled, and, star by star, the heavens were compared with it. as the identification of the stars went on, one object after another was found to lie in the heavens as it was engraved on the chart, and was of course rejected. at length a star of the eighth magnitude--a brilliant object--was brought into review. the chart was examined, but there was no star there. this object could not have been in its present place when the chart was formed. the object was therefore a wanderer--a planet. yet it was necessary to be cautious in such a matter. many possibilities had to be guarded against. it was, for instance, at least conceivable that the object was really a star which, by some mischance, eluded the careful eye of the astronomer who had constructed the map. it was even possible that the star might be one of the large class of variables which alternate in brightness, and it might have been too faint to have been visible when the chart was made. or it might be one of the minor planets moving between mars and jupiter. even if none of these explanations would answer, it was still necessary to show that the object was moving with that particular velocity and in that particular direction which the theory of le verrier indicated. the lapse of a single day was sufficient to dissipate all doubts. the next night the object was again observed. it had moved, and when its motion was measured it was found to accord precisely with what le verrier had foretold. indeed, as if no circumstance in the confirmation should be wanting, the diameter of the planet, as measured by the micrometers at berlin, proved to be practically coincident with that anticipated by le verrier. the world speedily rang with the news of this splendid achievement. instantly the name of le verrier rose to a pinnacle hardly surpassed by that of any astronomer of any age or country. the circumstances of the discovery were highly dramatic. we picture the great astronomer buried in profound meditation for many months; his eyes are bent, not on the stars, but on his calculations. no telescope is in his hand; the human intellect is the instrument he alone uses. with patient labour, guided by consummate mathematical artifice, he manipulates his columns of figures. he attempts one solution after another. in each he learns something to avoid; by each he obtains some light to guide him in his future labours. at length he begins to see harmony in those results where before there was but discord. gradually the clouds disperse, and he discerns with a certainty little short of actual vision the planet glittering in the far depths of space. he rises from his desk and invokes the aid of a practical astronomer; and lo! there is the planet in the indicated spot. the annals of science present no such spectacle as this. it was the most triumphant proof of the law of universal gravitation. the newtonian theory had indeed long ere this attained an impregnable position; but, as if to place its truth in the most conspicuous light, this discovery of neptune was accomplished. for a moment it seemed as if the french were to enjoy the undivided honour of this splendid triumph; nor would it, indeed, have been unfitting that the nation which gave birth to lagrange and to laplace, and which developed the great newtonian theory by their immortal labours, should have obtained this distinction. up to the time of the telescopic discovery of the planet by dr. galle at berlin, no public announcement had been made of the labours of challis in searching for the planet, nor even of the theoretical researches of adams on which those observations were based. but in the midst of the pæans of triumph with which the enthusiastic french nation hailed the discovery of le verrier, there appeared a letter from sir john herschel in the _athenæum_ for rd october, , in which he announced the researches made by adams, and claimed for him a participation in the glory of the discovery. subsequent enquiry has shown that this claim was a just one, and it is now universally admitted by all independent authorities. yet it will easily be imagined that the french _savants_, jealous of the fame of their countryman, could not at first be brought to recognise a claim so put forward. they were asked to divide the unparalleled honour between their own illustrious countryman and a young foreigner of whom but few had ever heard, and who had not even published a line of his work, nor had any claim been made on his part until after the work had been completely finished by le verrier. the demand made on behalf of adams was accordingly refused any acknowledgment in france; and an embittered controversy was the consequence. point by point the english astronomers succeeded in establishing the claim of their countryman. it was true that adams had not published his researches to the world, but he had communicated them to the astronomer-royal, the official head of the science in this country. they were also well known to professor challis, the professor of astronomy at cambridge. then, too, the work of adams was published, and it was found to be quite as thorough and quite as successful as that of le verrier. it was also found that the method of search adopted by professor challis not only must have been eventually successful, but that it actually was in a sense already successful. when the telescopic discovery of the planet had been achieved, challis turned naturally to see whether he had observed the new globe also. it was on the st october that he heard of the success of dr. galle, and by that time challis had accumulated observations in connection with this research of no fewer than , stars. among them he speedily found that an object observed on the th of august was not in the same place on the th of july. this was really the planet; and its discovery would thus have been assured had challis had time to compare his measurements. in fact, if he had only discussed his observations at once, there cannot be much doubt that the entire glory of the discovery would have been awarded to adams. he would then have been first, no less in the theoretical calculations than in the optical verification of the planet's existence. it may also be remarked that challis narrowly missed making the discovery of neptune in another way. le verrier had pointed out in his paper the possibility of detecting the sought-for globe by its disc. challis made the attempt, and before the intelligence of the actual discovery at berlin had reached him he had made an examination of the region indicated by le verrier. about stars passed through the field of view, and among them he selected one on account of its disc; it afterwards appeared that this was indeed the planet. if the researches of le verrier and of adams had never been undertaken it is certain that the distant neptune must have been some time discovered; yet that might have been made in a manner which every true lover of science would now deplore. we hear constantly that new minor planets are observed, yet no one attaches to such achievements a fraction of the consequence belonging to the discovery of neptune. the danger was, that neptune should have been merely dropped upon by simple survey work, just as uranus was discovered, or just as the hosts of minor planets are now found. in this case theoretical astronomy, the great science founded by newton, would have been deprived of its most brilliant illustration. neptune had, in fact, a very narrow escape on at least one previous occasion of being discovered in a very simple way. this was shown when sufficient observations had been collected to enable the path of the planet to be calculated. it was then possible to trace back the movements of the planet among the stars and thus to institute a search in the catalogues of earlier astronomers to see whether they contained any record of neptune, erroneously noted as a star. several such instances have been discovered. i shall, however, only refer to one, which possesses a singular interest. it was found that the place of the planet on may th, , must have coincided with that of a so-called star recorded on that day in the "histoire céleste" of lalande. by actual examination of the heavens it further appeared that there was no star in the place indicated by lalande, so the fact that here was really an observation of neptune was placed quite beyond doubt. when reference was made to the original manuscripts of lalande, a matter of great interest was brought to light. it was there found that he had observed the same star (for so he regarded it) both on may th and on may th; on each day he had determined its position, and both observations are duly recorded. but when he came to prepare his catalogue and found that the places on the two occasions were different, he discarded the earlier result, and merely printed the latter. had lalande possessed a proper confidence in his own observations, an immortal discovery lay in his grasp; had he manfully said, "i was right on the th of may and i was right on the th of may; i made no mistake on either occasion, and the object i saw on the th must have moved between that and the th," then he must without fail have found neptune. but had he done so, how lamentable would have been the loss to science! the discovery of neptune would then merely have been an accidental reward to a laborious worker, instead of being one of the most glorious achievements in the loftiest department of human reason. besides this brief sketch of the discovery of neptune, we have but little to tell with regard to this distant planet. if we fail to see in uranus any of those features which make mars or venus, jupiter or saturn, such attractive telescopic objects, what can we expect to find in neptune, which is half as far again as uranus? with a good telescope and a suitable magnifying power we can indeed see that neptune has a disc, but no features on that disc can be identified. we are consequently not in a position to ascertain the period in which neptune rotates around its axis, though from the general analogy of the system we must feel assured that it really does rotate. more successful have been the attempts to measure the diameter of neptune, which is found to be about , miles, or more than four times the diameter of the earth. it would also seem that, like jupiter and like saturn, the planet must be enveloped with a vast cloud-laden atmosphere, for the mean density of the globe is only about one-fifth that of the earth. this great globe revolves around the sun at a mean distance of no less than , millions of miles, which is about thirty times as great as the mean distance from the earth to the sun. the journey, though accomplished at the rate of more than three miles a second, is yet so long that neptune requires almost years to complete one revolution. since its discovery, some fifty years ago, neptune has moved through about one-third of its path, and even since the date when it was first casually seen by lalande, in , it has only had time to traverse three-fifths of its mighty circuit. neptune, like our earth, is attended by a single satellite; this delicate object was discovered by mr. lassell with his two-foot reflecting telescope shortly after the planet itself became known. the motion of the satellite of neptune is nearly circular. its orbit is inclined at an angle of about ° to the ecliptic, and it is specially noteworthy that, like the satellites of uranus, the direction of the motion runs counter to the planetary movements generally. the satellite performs its journey around neptune in a period of a little less than six days. by observing the motions of this moon we are enabled to determine the mass of the planet, and thus it appears that the weight of neptune is about one nineteen-thousandth part of that of the sun. no planets beyond neptune have been seen, nor is there at present any good ground for believing in their existence as visual objects. in the chapter on the minor planets i have entered into a discussion of the way in which these objects are discovered. it is by minute and diligent comparison of the heavens with elaborate star charts that these bodies are brought to light. such enquiries would be equally efficacious in searching for an ultra-neptunian planet; in fact, we could design no better method to seek for such a body, if it existed, than that which is at this moment in constant practice at many observatories. the labours of those who search for small planets have been abundantly rewarded with discoveries now counted by hundreds. yet it is a noteworthy fact that all these planets are limited to one region of the solar system. it has sometimes been conjectured that time may disclose perturbations in the orbit of neptune, and that these perturbations may lead to the discovery of a planet still more remote, even though that planet be so distant and so faint that it eludes all telescopic research. at present, however, such an enquiry can hardly come within the range of practical astronomy. its movements have no doubt been studied minutely, but it must describe a larger part of its orbit before it would be feasible to conclude, from the perturbations of its path, the existence of an unknown and still more remote planet. we have thus seen that the planetary system is bounded on one side by mercury and on the other by neptune. the discovery of mercury was an achievement of prehistoric times. the early astronomer who accomplished that feat, when devoid of instrumental assistance and unsupported by accurate theoretical knowledge, merits our hearty admiration for his untutored acuteness and penetration. on the other hand, the discovery of the exterior boundary of the planetary system is worthy of special attention from the fact that it was founded solely on profound theoretical learning. though we here close our account of the planets and their satellites, we have still two chapters to add before we shall have completed what is to be said with regard to the solar system. a further and notable class of bodies, neither planets nor satellites, own allegiance to the sun, and revolve round him in conformity with the laws of universal gravitation. these bodies are the comets, and their somewhat more humble associates, the shooting stars. we find in the study of these objects many matters of interest, which we shall discuss in the ensuing chapters. chapter xvi. comets. comets contrasted with planets in nature as well as in their movements--coggia's comet--periodic returns--the law of gravitation--parabolic and elliptic orbits--theory in advance of observations--most cometary orbits are sensibly parabolic--the labours of halley--the comet of --halley's memorable prediction--the retardation produced by disturbance--successive returns of halley's comet--encke's comet--effect of perturbations--orbit of encke's comet--attraction of mercury and of jupiter--how the identity of the comet is secured--how to weigh mercury--distance from the earth to the sun found by encke's comet--the disturbing medium--remarkable comets--spectrum of a comet--passage of a comet between the earth and the stars--can the comet be weighed?--evidence of the small mass of the comet derived from the theory of perturbation--the tail of the comet--its changes--views as to its nature--carbon present in comets--origin of periodic comets. in our previous chapters, which treated of the sun and the moon, the planets and their satellites, we found in all cases that the celestial bodies with which we were concerned were nearly globular in form, and many are undoubtedly of solid substance. all these objects possess a density which, even if in some cases it be much less than that of the earth, is still hundreds of times greater than the density of merely gaseous materials. we now, however, approach the consideration of a class of objects of a widely different character. we have no longer to deal with globular objects possessing considerable mass. comets are of altogether irregular shape; they are in large part, at all events, formed of materials in the utmost state of tenuity, and their masses are so small that no means we possess have enabled them to be measured. not only are comets different in constitution from planets or from the other more solid bodies of our system, but the movements of such bodies are quite distinct from the orderly return of the planets at their appointed seasons. the comets appear sometimes with almost startling unexpectedness; they rapidly swell in size to an extent that in superstitious ages called forth the utmost terror; presently they disappear, in many cases never again to return. modern science has, no doubt, removed a great deal of the mystery which once invested the whole subject of comets. their movements are now to a large extent explained, and some additions have been made to our knowledge of their nature, though we must still confess that what we do know bears but a very small proportion to what remains unknown. let me first describe in general terms the nature of a comet, in so far as its structure is disclosed by the aid of a powerful refracting telescope. we represent in plate xii. two interesting sketches made at harvard college observatory of the great comet of , distinguished by the name of its discoverer coggia. we see here the head of the comet, containing as its brightest spot what is called the nucleus, and in which the material of the comet seems to be much denser than elsewhere. surrounding the nucleus we find certain definite layers of luminous material, the coma, or head, from , to , , miles in diameter, from which the tail seems to stream away. this view may be regarded as that of a typical object of this class, but the varieties of structure presented by different comets are almost innumerable. in some cases we find the nucleus absent; in other cases we find the tail to be wanting. the tail is, no doubt, a conspicuous feature in those great comets which receive universal attention; but in the small telescopic objects, of which a few are generally found every year, this feature is usually absent. not only do comets present great varieties in appearance, but even the aspect of a single object undergoes great change. the comet will sometimes increase enormously in bulk; sometimes it will diminish; sometimes it will have a large tail, or sometimes no tail at all. measurements of a comet's size are almost futile; they may cease to be true even during the few hours in which a comet is observed in the course of a night. it is, in fact, impossible to identify a comet by any description of its personal appearance. yet the question as to identity of a comet is often of very great consequence. we must provide means by which it can be established, entirely apart from what the comet may look like. it is now well known that several of these bodies make periodic returns. after having been invisible for a certain number of years, a comet comes into view, and again retreats into space to perform another revolution. the question then arises as to how we are to recognise the body when it does come back? the personal features of its size or brightness, the presence or absence of a tail, large or small, are fleeting characters of no value for such a purpose. fortunately, however, the law of elliptic motion established by kepler has suggested the means of defining the identity of a comet with absolute precision. after newton had made his discovery of the law of gravitation, and succeeded in demonstrating that the elliptic paths of the planets around the sun were necessary consequences of that law, he was naturally tempted to apply the same reasoning to explain the movements of comets. here, again, he met with marvellous success, and illustrated his theory by completely explaining the movements of the remarkable body which was visible from december, , to march, . [illustration: fig. .--the parabolic path of a comet.] there is a certain beautiful curve known to geometricians by the name of the parabola. its form is shown in the adjoining figure; it is a curved line which bends in towards and around a certain point known as the focus. this would not be the occasion for any allusion to the geometrical properties of this curve; they should be sought in works on mathematics. it will here be only necessary to point to the connection which exists between the parabola and the ellipse. in a former chapter we have explained the construction of the latter curve, and we have shown how it possesses two foci. let us suppose that a series of ellipses are drawn, each of which has a greater distance between its foci than the preceding one. imagine the process carried on until at length the distance between the foci became enormously great in comparison with the distance from each focus to the curve, then each end of this long ellipse will practically have the same form as a parabola. we may thus look on the latter curve represented in fig. as being one end of an ellipse of which the other end is at an indefinitely great distance. in doerfel, a clergyman of saxony, proved that the great comet then recently observed moved in a parabola, in the focus of which the sun was situated. newton showed that the law of gravitation would permit a body to move in an ellipse of this very extreme type no less than in one of the more ordinary proportions. an object revolving in a parabolic orbit about the sun at the focus moves in gradually towards the sun, sweeps around the great luminary, and then begins to retreat. there is a necessary distinction between parabolic and elliptic motion. in the latter case the body, after its retreat to a certain distance, will turn round and again draw in towards the sun; in fact, it must make periodic circuits of its orbit, as the planets are found to do. but in the case of the true parabola the body can never return; to do so it would have to double the distant focus, and as that is infinitely remote, it could not be reached except in the lapse of infinite time. the characteristic feature of the movement in a parabola may be thus described. the body draws in gradually towards the focus from an indefinitely remote distance on one side, and after passing round the focus gradually recedes to an indefinitely remote distance on the other side, never again to return. when newton had perceived that parabolic motion of this type could arise from the law of gravitation, it at once occurred to him (independently of doerfel's discovery, of which he was not aware) that by its means the movements of a comet might be explained. he knew that comets must be attracted by the sun; he saw that the usual course of a comet was to appear suddenly, to sweep around the sun and then retreat, never again to return. was this really a case of parabolic motion? fortunately, the materials for the trial of this important suggestion were ready to his hand. he was able to avail himself of the known movements of the comet of , and of observations of several other bodies of the same nature which had been collected by the diligence of astronomers. with his usual sagacity, newton devised a method by which, from the known facts, the path which the comet pursues could be determined. he found that it was a parabola, and that the velocity of the comet was governed by the law that the straight line from the sun to the comet swept over equal areas in equal times. here was another confirmation of the law of universal gravitation. in this case, indeed, the theory may be said to have been actually in advance of calculation. kepler had determined from observation that the paths of the planets were ellipses, and newton had shown how this fact was a consequence of the law of gravitation. but in the case of the comets their highly erratic orbits had never been reduced to geometrical form until the theory of newton showed him that they were parabolic, and then he invoked observation to verify the anticipations of his theory. [illustration: plate xii. coggia's comet. (as seen on june th and july th, .)] the great majority of comets move in orbits which cannot be sensibly discriminated from parabolæ, and any body whose orbit is of this character can only be seen at a single apparition. the theory of gravitation, though it admits the parabola as a possible orbit for a comet, does not assert that the path must necessarily be of this type. we have pointed out that this curve is only a very extreme type of ellipse, and it would still be in perfect accordance with the law of gravitation for a comet to pursue a path of any elliptical form, provided that the sun was placed at the focus, and that the comet obeyed the rule of describing equal areas in equal times. if a body move in an elliptic path, then it will return to the sun again, and consequently we shall have periodical visits from the same object. an interesting field of enquiry was here presented to the astronomer. nor was it long before the discovery of a periodic comet was made which illustrated, in a striking manner, the soundness of the anticipation just expressed. the name of the celebrated astronomer halley is, perhaps, best known from its association with the great comet whose periodicity was discovered by his calculations. when halley learned from the newtonian theory the possibility that a comet might move in an elliptic orbit, he undertook a most laborious investigation; he collected from various records of observed comets all the reliable particulars that could be obtained, and thus he was enabled to ascertain, with tolerable accuracy, the nature of the paths pursued by about twenty-four large comets. one of these was the great body of , which halley himself observed, and whose path he computed in accordance with the principles of newton. halley then proceeded to investigate whether this comet of could have visited our system at any previous epoch. to answer this question he turned to the list of recorded comets which he had so carefully compiled, and he found that his comet very closely resembled, both in appearance and in orbit, a comet observed in , and also another observed in . could these three bodies be identical? it was only necessary to suppose that a comet, instead of revolving in a parabolic orbit, really revolved in an extremely elongated ellipse, and that it completed each revolution in a period of about seventy-five or seventy-six years. he submitted this hypothesis to every test that he could devise; he found that the orbits, determined on each of the three occasions, were so nearly identical that it would be contrary to all probability that the coincidence should be accidental. accordingly, he decided to submit his theory to the most supreme test known to astronomy. he ventured to make a prediction which posterity would have the opportunity of verifying. if the period of the comet were seventy-five or seventy-six years, as the former observations seemed to show, then halley estimated that, if unmolested, it ought to return in or . there were, however, certain sources of disturbance which he pointed out, and which would be quite powerful enough to affect materially the time of return. the comet in its journey passes near the path of jupiter, and experiences great perturbations from that mighty planet. halley concluded that the expected return might be accordingly delayed till the end of or the beginning of . this prediction was a memorable event in the history of astronomy, inasmuch as it was the first attempt to foretell the apparition of one of those mysterious bodies whose visits seemed guided by no fixed law, and which were usually regarded as omens of awful import. halley felt the importance of his announcement. he knew that his earthly course would have run long before the comet had completed its revolution; and, in language almost touching, the great astronomer writes: "wherefore if it should return according to our prediction about the year , impartial posterity will not refuse to acknowledge that this was first discovered by an englishman." as the time drew near when this great event was expected, it awakened the liveliest interest among astronomers. the distinguished mathematician clairaut undertook to compute anew, by the aid of improved methods, the effect which would be wrought on the comet by the attraction of the planets. his analysis of the perturbations was sufficient to show that the object would be kept back for days by saturn, and for days by jupiter. he therefore gave some additional exactness to the prediction of halley, and finally concluded that this comet would reach the perihelion, or the point of its path nearest to the sun, about the middle of april, . the sagacious astronomer (who, we must remember, lived long before the discovery of uranus and of neptune) further adds that as this body retreats so far, it may possibly be subject to influences of which we do not know, or to the disturbance even of some planet too remote to be ever perceived. he, accordingly, qualified his prediction with the statement that, owing to these unknown possibilities, his calculations might be a month wrong one way or the other. clairaut made this memorable communication to the academy of sciences on the th of november, . the attention of astronomers was immediately quickened to see whether the visitor, who last appeared seventy-six years previously, was about to return. night after night the heavens were scanned. on christmas day in the comet was first detected, and it passed closest to the sun about midnight on the th of march, just a month earlier than the time announced by clairaut, but still within the limits of error which he had assigned as being possible. the verification of this prediction was a further confirmation of the theory of gravitation. since then, halley's comet has returned once again, in , in circumstances somewhat similar to those just narrated. further historical research has also succeeded in identifying halley's comet with numerous memorable apparitions of comets in former times. it has even been shown that a splendid object, which appeared eleven years before the commencement of the christian era, was merely halley's comet in one of its former returns. among the most celebrated visits of this body was that of , when the apparition attracted universal attention. a picture of the comet on this occasion forms a quaint feature in the bayeux tapestry. the next return of halley's comet is expected about the year . there are now several comets known which revolve in elliptic paths, and are, accordingly, entitled to be termed periodic. these objects are chiefly telescopic, and are thus in strong contrast to the splendid comet of halley. most of the other periodic comets have periods much shorter than that of halley. of these objects, by far the most celebrated is that known as encke's comet, which merits our careful attention. the object to which we refer has had a striking career during which it has provided many illustrations of the law of gravitation. we are not here concerned with the prosaic routine of a mere planetary orbit. a planet is mainly subordinated to the compelling sway of the sun's gravitation. it is also to some slight extent affected by the attractions which it experiences from the other planets. mathematicians have long been accustomed to anticipate the movements of these globes by actual calculation. they know how the place of the planet is approximately decided by the sun's attraction, and they can discriminate the different adjustments which that place is to receive in consequence of the disturbances produced by the other planets. the capabilities of the planets for producing disturbance are greatly increased when the disturbed body follows the eccentric path of a comet. it is frequently found that the path of such a body comes very near the track of a planet, so that the comet may actually sweep by the planet itself, even if the two bodies do not actually run into collision. on such an occasion the disturbing effect is enormously augmented, and we therefore turn to the comets when we desire to illustrate the theory of planetary perturbations by some striking example. having decided to choose a comet, the next question is, _what_ comet? there cannot here be much room for hesitation. those splendid comets which appear so capriciously may be at once excluded. they are visitors apparently coming for the first time, and retreating without any distinct promise that mankind shall ever see them again. a comet of this kind moves in a parabolic path, sweeps once around the sun, and thence retreats into the space whence it came. we cannot study the effect of perturbations on a comet completely until it has been watched during successive returns to the sun. our choice is thus limited to the comparatively small class of objects known as periodic comets; and, from a survey of the entire group, we select the most suitable to our purpose. it is the object generally known as encke's comet, for, though encke was not the discoverer, yet it is to his calculations that the comet owes its fame. this body is rendered more suitable for our purpose by the researches to which it has recently given rise. in the year a comet was discovered by the painstaking astronomer pons at marseilles. we are not to imagine that this body produced a splendid spectacle. it was a small telescopic object, not unlike one of those dim nebulæ which are scattered in thousands over the heavens. the comet is, however, readily distinguished from a nebula by its movement relatively to the stars, while the nebula remains at rest for centuries. the position of this comet was ascertained by its discoverer, as well as by other astronomers. encke found from the observations that the comet returned to the sun once in every three years and a few months. this was a startling announcement. at that time no other comet of short period had been detected, so that this new addition to the solar system awakened the liveliest interest. the question was immediately raised as to whether this comet, which revolved so frequently, might not have been observed during previous returns. the historical records of the apparitions of comets are counted by hundreds, and how among this host are we to select those objects which were identical with the comet discovered by pons? [illustration: fig. .--the orbit of encke's comet.] we may at once relinquish any hope of identification from drawings of the object, but, fortunately, there is one feature of a comet on which we can seize, and which no fluctuations of the actual structure can modify or disguise. the path in which the body travels through space is independent of the bodily changes in its structure. the shape of that path and its position depend entirely upon those other bodies of the solar system which are specially involved in the theory of encke's comet. in fig. we show the orbits of three of the planets. they have been chosen with such proportions as shall make the innermost represent the orbit of mercury; the next is the orbit of the earth, while the outermost is the orbit of jupiter. besides these three we perceive in the figure a much more elliptical path, representing the orbit of encke's comet, projected down on the plane of the earth's motion. the sun is situated at the focus of the ellipse. the comet is constrained to revolve in this curve by the attraction of the sun, and it requires a little more than three years to accomplish a complete revolution. it passes close to the sun at perihelion, at a point inside the path of mercury, while at its greatest distance it approaches the path of jupiter. this elliptic orbit is mainly determined by the attraction of the sun. whether the comet weighed an ounce, a ton, a thousand tons, or a million tons, whether it was a few miles, or many thousands of miles in diameter, the orbit would still be the same. it is by the shape of this ellipse, by its actual size, and by the position in which it lies, that we identify the comet. it had been observed in , , and , but on these occasions it had not been noticed that the comet's path deviated from the parabola. encke's comet is usually so faint that even the most powerful telescope in the world would not show a trace of it. after one of its periodical visits, the body withdraws until it recedes to the outermost part of its path, then it will turn, and again approach the sun. it would seem that it becomes invigorated by the sun's rays, and commences to dilate under their genial influence. while moving in this part of its path the comet lessens its distance from the earth. it daily increases in splendour, until at length, partly by the intrinsic increase in brightness and partly by the decrease in distance from the earth, it comes within the range of our telescopes. we can generally anticipate when this will occur, and we can tell to what point of the heavens the telescope is to be pointed so as to discern the comet at its next return to perihelion. the comet cannot elude the grasp of the mathematician. he can tell when and where the comet is to be found, but no one can say what it will be like. were all the other bodies of the system removed, then the path of encke's comet must be for ever performed in the same ellipse and with absolute regularity. the chief interest for our present purpose lies not in the regularity of its path, but in the _irregularities_ introduced into that path by the presence of the other bodies of the solar system. let us, for instance, follow the progress of the comet through its perihelion passage, in which the track lies near that of the planet mercury. it will usually happen that mercury is situated in a distant part of its path at the moment the comet is passing, and the influence of the planet will then be comparatively small. it may, however, sometimes happen that the planet and the comet come close together. one of the most interesting instances of a close approach to mercury took place on the nd november, . on that day the comet and the planet were only separated by an interval of about one-thirtieth of the earth's distance from the sun, _i.e._ about , , miles. on several other occasions the distance between encke's comet and mercury has been less than , , miles--an amount of trifling import in comparison with the dimensions of our system. approaches so close as this are fraught with serious consequences to the movements of the comet. mercury, though a small body, is still sufficiently massive. it always attracts the comet, but the efficacy of that attraction is enormously enhanced when the comet in its wanderings comes near the planet. the effect of this attraction is to force the comet to swerve from its path, and to impress certain changes upon its velocity. as the comet recedes, the disturbing influence of mercury rapidly abates, and ere long becomes insensible. but time cannot efface from the orbit of the comet the effect which the disturbance of mercury has actually accomplished. the disturbed orbit is different from the undisturbed ellipse which the comet would have occupied had the influence of the sun alone determined its shape. we are able to calculate the movements of the comet as determined by the sun. we can also calculate the effects arising from the disturbance produced by mercury, provided we know the mass of the latter. though mercury is one of the smallest of the planets, it is perhaps the most troublesome to the astronomer. it lies so close to the sun that it is seen but seldom in comparison with the other great planets. its orbit is very eccentric, and it experiences disturbances by the attraction of other bodies in a way not yet fully understood. a special difficulty has also been found in the attempt to place mercury in the weighing scales. we can weigh the whole earth, we can weigh the sun, the moon, and even jupiter and other planets, but mercury presents difficulties of a peculiar character. le verrier, however, succeeded in devising a method of weighing it. he demonstrated that our earth is attracted by this planet, and he showed how the amount of attraction may be disclosed by observations of the sun, so that, from an examination of the observations, he made an approximate determination of the mass of mercury. le verrier's result indicated that the weight of the planet was about the fourteenth part of the weight of the earth. in other words, if our earth was placed in a balance, and fourteen globes, each equal to mercury, were laid in the other, the scales would hang evenly. it was necessary that this result should be received with great caution. it depended upon a delicate interpretation of somewhat precarious measurements. it could only be regarded as of provisional value, to be discarded when a better one should be obtained. the approach of encke's comet to mercury, and the elaborate investigations of von asten and backlund, in which the observations of the body were discussed, have thrown much light on the subject; but, owing to a peculiarity in the motion of this comet, which we shall presently mention, the difficulties of this investigation are enormous. backlund's latest result is, that the sun is , , times as heavy as mercury, and he considers that this is worthy of great confidence. there is a considerable difference between this result (which makes the earth about thirty times as heavy as mercury) and that of le verrier; and, on the other hand, haerdtl has, from the motion of winnecke's periodic comet, found a value of the mass of mercury which is not very different from le verrier's. mercury is, however, the only planet about the mass of which there is any serious uncertainty, and this must not make us doubt the accuracy of this delicate weighing-machine. look at the orbit of jupiter, to which encke's comet approaches so nearly when it retreats from the sun. it will sometimes happen that jupiter and the comet are in close proximity, and then the mighty planet seriously disturbs the pliable orbit of the comet. the path of the latter bears unmistakable traces of the jupiter perturbations, as well as of the mercury perturbations. it might seem a hopeless task to discriminate between the influences of the two planets, overshadowed as they both are by the supreme control of the sun, but contrivances of mathematical analysis are adequate to deal with the problem. they point out how much is due to mercury, how much is due to jupiter; and the wanderings of encke's comet can thus be made to disclose the mass of jupiter as well as that of mercury. here we have a means of testing the precision of our weighing appliances. the mass of jupiter can be measured by his moons, in the way mentioned in a previous chapter. as the satellites revolve round and round the planet, they furnish a method of measuring his weight by the rapidity of their motion. they tell us that if the sun were placed in one scale of the celestial balance, it would take , bodies equal to jupiter in the other to weigh him down. hardly a trace of uncertainty clings to this determination, and it is therefore of great interest to test the theory of encke's comet by seeing whether it gives an accordant result. the comparison has been made by von asten. encke's comet tells us that the sun is , times as heavy as jupiter; so the results are practically identical, and the accuracy of the indications of the comet are confirmed. but the calculation of the perturbations of encke's comet is so extremely intricate that asten's result is not of great value. from the motion of winnecke's periodic comet, haerdtl has found that the sun is , · times as heavy as jupiter, in perfect accordance with the best results derived from the attraction of jupiter on his satellites and the other planets. we have hitherto discussed the adventures of encke's comet in cases where they throw light on questions otherwise more or less known to us. we now approach a celebrated problem, on which encke's comet is our only authority. every , days that comet revolves completely around its orbit, and returns again to the neighbourhood of the sun. the movements of the comet are, however, somewhat irregular. we have already explained how perturbations arise from mercury and from jupiter. further disturbances arise from the attraction of the earth and of the other remaining planets; but all these can be allowed for, and then we are entitled to expect, if the law of gravitation be universally true, that the comet shall obey the calculations of mathematics. encke's comet has not justified this anticipation; at each revolution the period is getting steadily shorter! each time the comet comes back to perihelion in two and a half hours less than on the former occasion. two and a half hours is, no doubt, a small period in comparison with that of an entire revolution; but in the region of its path visible to us the comet is moving so quickly that its motion in two and a half hours is considerable. this irregularity cannot be overlooked, inasmuch as it has been confirmed by the returns during about twenty revolutions. it has sometimes been thought that the discrepancies might be attributed to some planetary perturbations omitted or not fully accounted for. the masterly analysis of von asten and backlund has, however, disposed of this explanation. they have minutely studied the observations down to , but only to confirm the reality of this diminution in the periodic time of encke's comet. an explanation of these irregularities was suggested by encke long ago. let us briefly attempt to describe this memorable hypothesis. when we say that a body will move in an elliptic path around the sun in virtue of gravitation, it is always assumed that the body has a free course through space. it is assumed that there is no friction, no air, or other source of disturbance. but suppose that this assumption should be incorrect; suppose that there really is some medium pervading space which offers resistance to the comet in the same way as the air impedes the flight of a rifle bullet, what effect ought such a medium to produce? this is the idea which encke put forward. even if the greater part of space be utterly void, so that the path of the filmy and almost spiritual comet is incapable of feeling resistance, yet in the neighbourhood of the sun it was supposed that there might be some medium of excessive tenuity capable of affecting so light a body. it can be demonstrated that a resisting medium such as we have supposed would lessen the size of the comet's path, and diminish the periodic time. this hypothesis has, however, now been abandoned. it has always appeared strange that no other comet showed the least sign of being retarded by the assumed resisting medium. but the labours of backlund have now proved beyond a doubt that the acceleration of the motion of encke's comet is not a constant one, and cannot be accounted for by assuming a resisting medium distributed round the sun, no matter how we imagine this medium to be constituted with regard to density at different distances from the sun. backlund found that the acceleration was fairly constant from to ; it commenced to decrease between and , and continued to diminish till some time between and , since which time it has remained fairly constant. he considers that the acceleration can only be produced by the comet encountering periodically a swarm of meteors, and if we could only observe the comet during its motion through the greater part of its orbit we should be able to point out the locality where this encounter takes place. we have selected the comets of halley and of encke as illustrations of the class of periodic comets, of which, indeed, they are the most remarkable members. another very remarkable periodic comet is that of biela, of which we shall have more to say in the next chapter. of the much more numerous class of non-periodic comets, examples in abundance may be cited. we shall mention a few which have appeared during the present century. there is first the splendid comet of , which appeared suddenly in february of that year, and was so brilliant that it could be seen during full daylight. this comet followed a path which could not be certainly distinguished from a parabola, though there is no doubt that it might have been a very elongated ellipse. it is frequently impossible to decide a question of this kind, during the brief opportunities available for finding the place of the comet. we can only see the object during a very small arc of its orbit, and even then it is not a very well-defined point which admits of being measured with the precision attainable in observations of a star or a planet. this comet of is, however, especially remarkable for the rapidity with which it moved, and for the close approach which it made to the sun. the heat to which it was exposed during its passage around the sun must have been enormously greater than the heat which can be raised in our mightiest furnaces. if the materials had been agate or cornelian, or the most infusible substances known on the earth, they would have been fused and driven into vapour by the intensity of the sun's rays. the great comet of was one of the celestial spectacles of modern times. it was first observed on june nd of that year by donati, whose name the comet has subsequently borne; it was then merely a faint nebulous spot, and for about three months it pursued its way across the heavens without giving any indications of the splendour which it was so soon to attain. the comet had hardly become visible to the unaided eye at the end of august, and was then furnished with only a very small tail, but as it gradually drew nearer and nearer to the sun in september, it soon became invested with splendour. a tail of majestic proportions was quickly developed, and by the middle of october, when the maximum brightness was attained, its length extended over an arc of forty degrees. the beauty and interest of this comet were greatly enhanced by its favourable position in the sky at a season when the nights were sufficiently dark. on the nd may, , mr. tebbutt, of windsor, in new south wales, discovered a comet which speedily developed into one of the most interesting celestial objects seen by this generation. about the nd of june it became visible from these latitudes in the northern sky at midnight. gradually it ascended higher and higher until it passed around the pole. the nucleus of the comet was as bright as a star of the first magnitude, and its tail was about ° long. on the nd of september it ceased to be visible to the unaided eye, but remained visible in telescopes until the following february. this was the first comet which was successfully photographed, and it may be remarked that comets possess very little actinic power. it has been estimated that moonlight possesses an intensity , times greater than that of a comet where the purposes of photography are concerned. another of the bodies of this class which have received great and deserved attention was that discovered in the southern hemisphere early in september, . it increased so much in brilliancy that it was seen in daylight by mr. common on the th of that month, while on the same day the astronomers at the cape of good hope were fortunate enough to have observed the body actually approach the sun's limb, where it ceased to be visible. we know that the comet must have passed between the earth and the sun, and it is very interesting to learn from the cape observers that it was totally invisible when it was actually projected on the sun's disc. the following day it was again visible to the naked eye in full daylight, not far from the sun, and valuable spectroscopic observations were secured at dunecht and palermo. at that time the comet was rushing through the part of its orbit closest to the sun, and about a week later it began to be visible in the morning before sunrise, near the eastern horizon, exhibiting a fine long tail. (_see_ plate xvii.) the nucleus gradually lengthened until it broke into four separate pieces, lying in a straight line, while the comet's head became enveloped in a sort of faint, nebulous tube, pointing towards the sun. several small detached nebulous masses became also visible, which travelled along with the comet, though not with the same velocity. the comet became invisible to the naked eye in february, and was last observed telescopically in south america on the st june, . there is a remarkable resemblance between the orbit of this comet and the orbits in which the comet of , the great comet of , and a great comet seen in in the southern hemisphere, travelled round the sun. in fact, these four comets moved along very nearly the same track and rushed round the sun within a couple of hundred thousand miles of the surface of the photosphere. it is also possible that the comet which, according to aristotle, appeared in the year b.c. followed the same orbit. and yet we cannot suppose that all these were apparitions of one and the same comet, as the observations of the comet of give the period of revolution of that body equal to about years. it is not impossible that the comets of and are one and the same, but in both years the observations extend over too short a time to enable us to decide whether the orbit was a parabola or an ellipse. but as the comet of was in any case a distinct body, it seems more likely that we have here a family of comets approaching the sun from the same region of space and pursuing almost the same course. we know a few other instances of such resemblances between the orbits of distinct comets. of other interesting comets seen within the last few years we may mention one discovered by mr. holmes in london on the th november, . it was then situated not far from the bright nebula in the constellation andromeda, and like it was just visible to the naked eye. the comet became gradually fainter and more diffused, but on the th january following it appeared suddenly with a central condensation, like a star of the eighth magnitude, surrounded by a small coma. gradually it expanded again, and grew fainter, until it was last observed on the th april.[ ] the orbit was found to be an ellipse more nearly circular than the orbit of any other known comet, the period being nearly seven years. another comet of is remarkable as having been discovered by professor barnard, of the lick observatory, on a photograph of a region in aquila; he was at once able to distinguish the comet from a nebula by its motion. since the light of every comet which has made its appearance has been analysed by the spectroscope. the slight surface-brightness of these bodies renders it necessary to open the slit of the spectroscope rather wide, and the dispersion employed cannot be very great, which again makes accurate measurements difficult. the spectrum of a comet is chiefly characterised by three bright bands shading gradually off towards the violet, and sharply defined on the side towards the red. this appearance is caused by a large number of fine and close lines, whose intensity and distance apart decrease towards the violet. these three bands reveal the existence of hydrocarbon in comets. the important _rôle_ which we thus find carbon playing in the constitution of comets is especially striking when we reflect on the significance of the same element on the earth. we see it as the chief constituent of all vegetable life, we find it to be invariably present in animal life. it is an interesting fact that this element, of such transcendent importance on the earth, should now have been proved to be present in these wandering bodies. the hydrocarbon bands are, however, not always the only features visible in cometary spectra. in a comet seen in the spring months of , professor copeland discovered that a new bright yellow line, coinciding in position with the d-line of sodium, had suddenly appeared, and it was subsequently, both by him and by other observers, seen beautifully double. in fact, sodium was so strongly represented in this comet, that both the head and the tail could be perfectly well seen in sodium light by merely opening the slit of the spectroscope very wide, just as a solar prominence may be seen in hydrogen light. the sodium line attained its greatest brilliance at the time when the comet was nearest to the sun, while the hydrocarbon bands were either invisible or very faint. the same connection between the intensity of the sodium line and the distance from the sun was noticed in the great september comet of . the spectrum of the great comet of was observed by copeland and lohse on the th september in daylight, and, in addition to the sodium line, they saw a number of other bright lines, which seemed to be due to iron vapour, while the only line of manganese visible at the temperature of a bunsen burner was also seen. this very remarkable observation was made less than a day after the perihelion passage, and illustrates the wonderful activity in the interior of a comet when very close to the sun. [illustration: plate xvii. the comet of , as seen from streatham, nov. th, a.m. from a drawing by t.e. key.] in addition to the bright lines comets generally show a faint continuous spectrum, in which dark fraunhofer lines can occasionally be distinguished. of course, this shows that the continuous spectrum is to a great extent due to reflected sunlight, but there is no doubt that part of it is often due to light actually developed in the comets. this was certainly the case in the first comet of , as a sudden outburst of light in this body was accompanied by a considerable increase of brightness of the continuous spectrum. a change in the relative brightness of the three hydrocarbon bands indicated a considerable rise of temperature, during the continuance of which the comet emitted white light. as comets are much nearer to the earth than the stars, it will occasionally happen that the comet must arrive at a position directly between the earth and a star. there is quite a similar phenomenon in the movement of the moon. a star is frequently occulted in this way, and the observations of such phenomena are familiar to astronomers; but when a comet passes in front of a star the circumstances are widely different. the star is indeed seen nearly as well through the comet as it would be if the comet were entirely out of the way. this has often been noticed. one of the most celebrated observations of this kind was made by the late sir john herschel on biela's comet, which is one of the periodic class, and will be alluded to in the next chapter. the illustrious astronomer saw on one occasion this object pass over a star cluster. it consisted of excessively minute stars, which could only be seen by a powerful telescope, such as the one sir john was using. the faintest haze or the merest trace of a cloud would have sufficed to hide all the stars. it was therefore with no little interest that the astronomer watched the progress of biela's comet. gradually the wanderer encroached on the group of stars, so that if it had any appreciable solidity the numerous twinkling points would have been completely screened. but what were the facts? down to the most minute star in that cluster, down to the smallest point of light which the great telescope could show, every object in the group was distinctly seen to twinkle right through the mass of biela's comet. this was an important observation. we must recollect that the veil drawn between the cluster and the telescope was not a thin curtain; it was a volume of cometary substance many thousands of miles in thickness. contrast, then, the almost inconceivable tenuity of a comet with the clouds to which we are accustomed. a cloud a few hundred feet thick will hide not only the stars, but even the great sun himself. the lightest haze that ever floated in a summer sky would do more to screen the stars from our view than would one hundred thousand miles of such cometary material as was here interposed. the great comet of donati passed over many stars which were visible distinctly through its tail. among these stars was a very bright one--the well-known arcturus. the comet, fortunately, happened to pass over arcturus, and though nearly the densest part of the comet was interposed between the earth and the star, yet arcturus twinkled on with undiminished lustre through the thickness of this stupendous curtain. recent observations have, however, shown that stars in some cases experience change in lustre when the denser part of the comet passes over them. it is, indeed, difficult to imagine that a star would remain visible if the nucleus of a really large comet passed over it; but it does not seem that an opportunity of testing this supposition has yet arisen. as a comet contains transparent gaseous material we might expect that the place of a star would be deranged when the comet approached it. the refractive power of air is very considerable. when we look at the sunset, we see the sun appearing to pass below the horizon; yet the sun has actually sunk beneath the horizon before any part of its disk appears to have commenced its descent. the refractive power of the air bends the luminous rays round and shows the sun, though it is directly screened by the intervening obstacles. the refractive power of the material of comets has been carefully tested. a comet has been observed to approach two stars; one of which was seen through the comet, while the other could be observed directly. if the body had any appreciable quantity of gas in its composition the relative places of the two stars would be altered. this question has been more than once submitted to the test of actual measurement. it has sometimes been found that no appreciable change of position could be detected, and that accordingly in such cases the comet has no perceptible density. careful measurements of the great comet in showed, however, that in the neighbourhood of the nucleus there was some refractive power, though quite insignificant in comparison with the refraction of our atmosphere. [illustration: plate c. comet a , . swift. _photographed by e.e. barnard, th april, ._] from these considerations it will probably be at once admitted that the _mass_ of a comet must be indeed a very small quantity in comparison with its bulk. when we attempt actually to weigh the comet, our efforts have proved abortive. we have been able to weigh the mighty planets jupiter and saturn; we have been even able to weigh the vast sun himself; the law of gravitation has provided us with a stupendous weighing apparatus, which has been applied in all these cases with success, but the same methods applied to comets are speedily seen to be illusory. no weighing machinery known to the astronomer is delicate enough to determine the weight of a comet. all that we can accomplish in any circumstances is to weigh one heavenly body in comparison with another. comets seem to be almost imponderable when estimated by such robust masses as those of the earth, or any of the other great planets. of course, it will be understood that when we say the weight of a comet is inappreciable, we mean with regard to the other bodies of our system. perhaps no one now doubts that a great comet must really weigh tons; though whether those tons are to be reckoned in tens, in hundreds, in thousands, or in millions, the total seems quite insignificant when compared with the weight of a body like the earth. the small mass of comets is also brought before us in a very striking way when we recall what has been said in the last chapter on the important subject of the planetary perturbations. we have there treated of the permanence of our system, and we have shown that this permanence depends upon certain laws which the planetary motions must invariably fulfil. the planets move nearly in circles, their orbits are all nearly in the same plane, and they all move in the same direction. the permanence of the system would be imperilled if any one of these conditions was not fulfilled. in that discussion we made no allusion to the comets. yet they are members of our system, and they far outnumber the planets. the comets repudiate these rules of the road which the planets so rigorously obey. their orbits are never like circles; they are, indeed, more usually parabolic, and thus differ as widely as possible from the circular path. nor do the planes of the orbits of comets affect any particular aspect; they are inclined at all sorts of angles, and the directions in which they move seem to be mere matters of caprice. all these articles of the planetary convention are violated by comets, but yet our system lasts; it has lasted for countless ages, and seems destined to last for ages to come. the comets are attracted by the planets, and conversely, the comets must attract the planets, and must perturb their orbits to some extent; but to what extent? if comets moved in orbits subject to the same general laws which characterise planetary motion, then our argument would break down. the planets might experience considerable derangements from cometary attraction, and yet in the lapse of time those disturbances would neutralise each other, and the permanence of the system would be unaffected. but the case is very different when we deal with the actual cometary orbits. if comets could appreciably disturb planets, those disturbances would not neutralise each other, and in the lapse of time the system would be wrecked by a continuous accumulation of irregularities. the facts, however, show that the system has lived, and is living, notwithstanding comets; and hence we are forced to the conclusion that their masses must be insignificant in comparison with those of the great planetary bodies. these considerations exhibit the laws of universal gravitation and their relations to the permanence of our system in a very striking light. if we include the comets, we may say that the solar system includes many thousands of bodies, in orbits of all sizes, shapes, and positions, only agreeing in the fact that the sun occupies a focus common to all. the majority of these bodies are imponderable in comparison with planets, and their orbits are placed anyhow, so that, although they may suffer much from the perturbations of the other bodies, they can in no case inflict any appreciable disturbance. there are, however, a few great planets capable of producing vast disturbances; and if their orbits were not properly adjusted, chaos would sooner or later be the result. by the mutual adaptations of their orbits to a nearly circular form, to a nearly coincident plane, and to a uniformity of direction, a permanent truce has been effected among the great planets. they cannot now permanently disorganise each other, while the slight mass of the comets renders them incompetent to do so. the stability of the great planets is thus assured; but it is to be observed that there is no guarantee of stability for comets. their eccentric and irregular paths may undergo the most enormous derangements; indeed, the history of astronomy contains many instances of the vicissitudes to which a cometary career is exposed. great comets appear in the heavens in the most diverse circumstances. there is no part of the sky, no constellation or region, which is not liable to occasional visits from these mysterious bodies. there is no season of the year, no hour of the day or of the night when comets may not be seen above the horizon. in like manner, the size and aspect of the comets are of every character, from the dim spot just visible to an eye fortified by a mighty telescope, up to a gigantic and brilliant object, with a tail stretching across the heavens for a distance which is as far as from the horizon to the zenith. so also the direction of the tail of the comet seems at first to admit of every possible position: it may stand straight up in the heavens, as if the comet were about to plunge below the horizon; it may stream down from the head of the comet, as if the body had been shot up from below; it may slope to the right or to the left. amid all this variety and seeming caprice, can we discover any feature common to the different phenomena? we shall find that there is a very remarkable law which the tails of comets obey--a law so true and satisfactory, that if we are given the place of a comet in the heavens, it is possible at once to point out in what direction the tail will lie. a beautiful comet appears in summer in the northern sky. it is near midnight; we are gazing on the faintly luminous tail, which stands up straight and points towards the zenith; perhaps it may be curved a little or possibly curved a good deal, but still, on the whole, it is directed from the horizon to the zenith. we are not here referring to any particular comet. every comet, large or small, that appears in the north must at midnight have its tail pointed up in a nearly vertical direction. this fact, which has been verified on numerous occasions, is a striking illustration of the law of direction of comets' tails. think for one moment of the facts of the case. it is summer; the twilight at the north shows the position of the sun, and the tail of the comet points directly away from the twilight and away from the sun. take another case. it is evening; the sun has set, the stars have begun to shine, and a long-tailed comet is seen. let that comet be high or low, north or south, east or west, its tail invariably points _away_ from that point in the west where the departing sunlight still lingers. again, a comet is watched in the early morning, and if the eye be moved from the place where the first streak of dawn is appearing to the head of the comet, then along that direction, streaming away from the sun, is found the tail of the comet. this law is of still more general application. at any season, at any hour of the night, the tail of a comet is directed away from the sun. more than three hundred years ago this fact in the movement of comets arrested the attention of those who pondered on the movements of the heavenly bodies. it is a fact patent to ordinary observation, it gives some degree of consistency to the multitudinous phenomena of comets, and it must be made the basis of our enquiries into the structure of the tails. in the adjoining figure, fig. , we show a portion of the parabolic orbit of a comet, and we also represent the position of the tail of the comet at various points of its path. it would be, perhaps, going too far to assert that throughout the whole vast journey of the comet, its tail must always be directed from the sun. in the first place, it must be recollected that we can only see the comet during that small part of its journey when it is approaching to or receding from the sun. it is also to be remembered that, while actually passing round the sun, the brilliancy of the comet is so overpowered by the sun that the comet often becomes invisible, just as the stars are invisible in daylight. indeed, in certain cases, jets of cometary material are actually projected towards the sun. [illustration: fig. .--the tail of a comet directed from the sun.] in a hasty consideration of the subject, it might be thought that as the comet was dashing along with enormous velocity the tail was merely streaming out behind, just as the shower of sparks from a rocket are strewn along the path which it follows. this would be an entirely erroneous analogy; the comet is moving not through an atmosphere, but through open space, where there is no medium sufficient to sweep the tail into the line of motion. another very remarkable feature is the gradual growth of the tail as the comet approaches the sun. while the body is still at a great distance it has usually no perceptible tail, but as it draws in the tail gradually develops, and in some cases reaches stupendous dimensions. it is not to be supposed that this increase is a mere optical consequence of the diminution of distance. it can be shown that the growth of the tail takes place much more rapidly than it would be possible to explain in this way. we are thus led to connect the formation of the tail with the approach to the sun, and we are accordingly in the presence of an enigma without any analogy among the other bodies of our system. that the comet as a whole is attracted by the sun there can be no doubt whatever. the fact that the comet moves in an ellipse or in a parabola proves that the two bodies act and react on each other in obedience to the law of universal gravitation. but while this is true of the comet as a whole, it is no less certain that the tail of the comet is _repelled_ by the sun. it is impossible to speak with certainty as to how this comes about, but the facts of the case seem to point to an explanation of the following kind. we have seen that the spectroscope has proved with certainty the presence of hydrocarbon and other gases in comets. but we are not to conclude from this that comets are merely masses of gas moving through space. though the total quantity of matter in a comet, as we have seen, is exceedingly small, it is quite possible that the comet may consist of a number of widely scattered particles of appreciable density; indeed, we shall see in the next chapter, when describing the remarkable relationship between comets and meteors, that we have reason to believe this to be the case. we may therefore look on a comet as a swarm of tiny solid particles, each surrounded by gas. when we watch a great comet approaching the sun the nucleus is first seen to become brighter and more clearly defined; at a later stage luminous matter appears to be projected from it towards the sun, often in the shape of a fan or a jet, which sometimes oscillates to and fro like a pendulum. in the head of halley's comet, for instance, bessel observed in october, , that the jet in the course of eight hours swung through an angle of °. on other occasions concentric arcs of light are formed round the nucleus, one after another, getting fainter as they travel further from the nucleus. evidently the material of the fan or the arcs is repelled by the nucleus of the comet; but it is also repelled by the sun, and this latter repulsive force compels the luminous matter to overcome the attraction of gravitation, and to turn back all round the nucleus in the direction away from the sun. in this manner the tail is formed. (_see_ plate xii.) the mathematical theory of the formation of comets' tails has been developed on the assumption that the matter which forms the tail is repelled both by the nucleus and by the sun. this investigation was first undertaken by the great astronomer bessel, in his memoir on the appearance of halley's comet in , and it has since been considerably developed by roche and the russian astronomer bredichin. though we are, perhaps, hardly in a position to accept this theory as absolutely true, we can assert that it accounts well for the principal phenomena observed in the formation of comets' tails. professor bredichin has conducted his labours in the philosophical manner which has led to many other great discoveries in science. he has carefully collated the measurements and drawings of the tails of various comets. one result has been obtained from this preliminary part of his enquiry, which possesses a value that cannot be affected even if the ulterior portion of his labours should be found to require qualification. in the examination of the various tails, he observed that the curvilinear shapes of the outlines fall into one or other of three special types. in the first we have the straightest tails, which point almost directly away from the sun. in the second are classed tails which, after starting away from the sun, are curved backwards from the direction towards which the comet is moving. in the third we find the appendage still more curved in towards the comet's path. it can be shown that the tails of comets can almost invariably be identified with one or other of these three types; and in cases where the comet exhibits two tails, as has sometimes happened, then they will be found to belong to two of the types. the adjoining diagram (fig. ) gives a sketch of an imaginary comet furnished with tails of the three different types. the direction in which the comet is moving is shown by the arrow-head on the line passing through the nucleus. bredichin concludes that the straightest of the three tails, marked as type i., is most probably due to the element hydrogen; the tails of the second form are due to the presence of some of the hydrocarbons in the body of the comet; while the small tails of the third type may be due to iron or to some other element with a high atomic weight. it will, of course, be understood that this diagram does not represent any actual comet. [illustration: fig. .--bredichin's theory of comets' tails.] [illustration: fig. .--tails of the comet of .] an interesting illustration of this theory is afforded in the case of the celebrated comet of already referred to, of which a drawing is shown in fig. . we find here, besides the great tail, which is the characteristic feature of the body, two other faint streaks of light. these are the edges of the hollow cone which forms a tail of type i. when we look through the central regions it will be easily understood that the light is not sufficiently intense to be visible; at the edges, however, a sufficient thickness of the cometary matter is presented, and thus we have the appearance shown in this figure. it would seem that donati's comet possessed one tail due to hydrogen, and another due to some of the compounds of carbon. the carbon compounds involved appear to be of considerable variety, and there is, in consequence, a disposition in the tails of the second type to a more indefinite outline than in the hydrogen tails. cases have been recorded in which several tails have been seen simultaneously on the same comet. the most celebrated of these is that which appeared in the year . professor bredichin has devoted special attention to the theory of this marvellous object, and he has shown with a high degree of probability how the multiform tail could be accounted for. the adjoining figure (fig. ) is from a sketch of this object made on the morning of the th march by mademoiselle kirch at the berlin observatory. the figure shows eleven streaks, of which the first ten (counting from the left) represent the bright edges of five of the tails, while the sixth and shortest tail is at the extreme right. sketches of this rare phenomenon were also made by chéseaux at lausanne and de l'isle at st. petersburg. before the perihelion passage the comet had only had one tail, but a very splendid one. [illustration: fig. .--the comet of .] it is possible to submit some of the questions involved to the test of calculation, and it can be shown that the repulsive force adequate to produce the straight tail of type i. need only be about twelve times as large as the attraction of gravitation. tails of the second type could be produced by a repulsive force which was about equal to gravitation, while tails of the third type would only require a repulsive force about one-quarter the power of gravitation.[ ] the chief repulsive force known in nature is derived from electricity, and it has naturally been surmised that the phenomena of comets' tails are due to the electric condition of the sun and of the comet. it would be premature to assert that the electric character of the comet's tail has been absolutely demonstrated; all that can be said is that, as it seems to account for the observed facts, it would be undesirable to introduce some mere hypothetical repulsive force. it must be remembered that on quite other grounds it is known that the sun is the seat of electric phenomena. as the comet gradually recedes from the sun the repulsive force becomes weaker, and accordingly we find that the tail of the comet declines. if the comet be a periodic one, the same series of changes may take place at its next return to perihelion. a new tail is formed, which also gradually disappears as the comet regains the depths of space. if we may employ the analogy of terrestrial vapours to guide us in our reasoning, then it would seem that, as the comet retreats, its tail would condense into myriads of small particles. over these small particles the law of gravitation would resume its undivided sway, no longer obscured by the superior efficiency of the repulsion. the mass of the comet is, however, so extremely small that it would not be able to recall these particles by the mere force of attraction. it follows that, as the comet at each perihelion passage makes a tail, it must on each occasion expend a corresponding quantity of tail-making material. let us suppose that the comet was endowed in the beginning with a certain capital of those particular materials which are adapted for the production of tails. each perihelion passage witnesses the formation of a tail, and the expenditure of a corresponding amount of the capital. it is obvious that this operation cannot go on indefinitely. in the case of the great majority of comets the visits to perihelion are so extremely rare that the consequences of the extravagance are not very apparent; but to those periodic comets which have short periods and make frequent returns, the consequences are precisely what might have been anticipated: the tail-making capital has been gradually squandered, and thus at length we have the spectacle of a comet without any tail at all. we can even conceive that a comet may in this manner be completely dissipated, and we shall see in the next chapter how this fate seems to have overtaken biela's periodic comet. but as it sweeps through the solar system the comet may chance to pass very near one of the larger planets, and, in passing, its motion may be seriously disturbed by the attraction of the planet. if the velocity of the comet is accelerated by this disturbing influence, the orbit will be changed from a parabola into another curve known as a hyperbola, and the comet will swing round the sun and pass away never to return. but if the planet is so situated as to retard the velocity of the comet, the parabolic orbit will be changed into an ellipse, and the comet will become a periodic one. we can hardly doubt that some periodic comets have been "captured" in this manner and thereby made permanent members of our solar system, if we remark that the comets of short periods (from three to eight years) come very near the orbit of jupiter at some point or other of their paths. each of them must, therefore, have been near the giant planet at some moment during their past history. similarly the other periodic comets of longer period approach near to the orbits of either saturn, uranus, or neptune, the last-mentioned planet being probably responsible for the periodicity of halley's comet. we have, indeed, on more than one occasion, actually witnessed the violent disturbance of a cometary orbit. the most interesting case is that of lexell's comet. in the french astronomer messier (who devoted himself with great success to the discovery of comets) detected a comet for which lexell computed the orbit, and found an ellipse with a period of five years and some months. yet the comet had never been seen before, nor did it ever come back again. long afterwards it was found, from most laborious investigations by burckhardt and le verrier, that the comet had moved in a totally different orbit previous to . but at the beginning of the year it happened to come so close to jupiter that the powerful attraction of this planet forced it into a new orbit, with a period of five and a half years. it passed the perihelion on the th august, , and again in , but in the latter year it was not conveniently situated for being seen from the earth. in the summer of the comet was again in the neighbourhood of jupiter, and was thrown out of its elliptic orbit, so that we have never seen it since, or, perhaps, it would be safer to say that we have not with certainty identified lexell's comet with any comet observed since then. we are also, in the case of several other periodic comets, able to fix in a similar way the date when they started on their journeys in their present elliptic orbits. such is a brief outline of the principal facts known with regard to these interesting but perplexing bodies. we must be content with the recital of what we know, rather than hazard guesses about matters beyond our reach. we see that they are obedient to the great laws of gravitation, and afford a striking illustration of their truth. we have seen how modern science has dissipated the superstition with which, in earlier ages, the advent of a comet was regarded. we no longer regard such a body as a sign of impending calamity; we may rather look upon it as an interesting and a beautiful visitor, which comes to please us and to instruct us, but never to threaten or to destroy. chapter xvii. shooting stars. small bodies of our system--their numbers--how they are observed--the shooting star--the theory of heat--a great shooting star--the november meteors--their ancient history--the route followed by the shoal--diagram of the shoal of meteors--how the shoal becomes spread out along its path--absorption of meteors by the earth--the discovery of the relation between meteors and comets--the remarkable investigations concerning the november meteors--two showers in successive years--no particles have ever been identified from the great shooting star showers--meteoric stones--chladni's researches--early cases of stone-falls--the meteorite at ensisheim--collections of meteorites--the rowton siderite--relative frequency of iron and stony meteorites--fragmentary character of meteorites--tschermak's hypothesis--effects of gravitation on a missile ejected from a volcano--can they have come from the moon?--the claims of the minor planets to the parentage of meteorites--possible terrestrial origin--the ovifak iron. in the preceding chapters we have dealt with the gigantic bodies which form the chief objects in what we know as the solar system. we have studied mighty planets measuring thousands of miles in diameter, and we have followed the movements of comets whose dimensions are often to be told by millions of miles. once, indeed, in a previous chapter we have made a descent to objects much lower in the scale of magnitude, and we have examined that numerous class of small bodies which we call the minor planets. it is now, however, our duty to make a still further, and this time a very long step, downwards in the scale of magnitude. even the minor planets must be regarded as colossal objects when compared with those little bodies whose presence is revealed to us in an interesting and sometimes in a striking manner. these small bodies compensate in some degree for their minute size by the profusion in which they exist. no attempt, indeed, could be made to tell in figures the myriads in which they swarm throughout space. they are probably of very varied dimensions, some of them being many pounds or perhaps tons in weight, while others seem to be not larger than pebbles, or even than grains of sand. yet, insignificant as these bodies may seem, the sun does not disdain to undertake their control. each particle, whether it be as small as the mote in a sunbeam or as mighty as the planet jupiter, must perforce trace out its path around the sun in conformity with the laws of kepler. who does not know that beautiful occurrence which we call a shooting star, or which, in its more splendid forms, is sometimes called a meteor or fireball? it is to objects of this class that we are now to direct our attention. a small body is moving round the sun. just as a mighty planet revolves in an ellipse, so even a small object will be guided round and round in an ellipse with the sun in the focus. there are, at the present moment, inconceivable myriads of such meteors moving in this manner. they are too small and too distant for our telescopes, and we never see them except under extraordinary circumstances. when the meteor flashes into view it is moving with such enormous velocity that it often traverses more than twenty miles in a second of time. such a velocity is almost impossible near the earth's surface: the resistance of the air would prevent it. aloft, in the emptiness of space, there is no air to impede its flight. it may have been moving round and round the sun for thousands, perhaps for millions of years, without suffering any interference; but the supreme moment arrives, and the meteor perishes in a streak of splendour. in the course of its wanderings the body comes near the earth, and within a few hundred miles of its surface begins to encounter the upper surface of the atmosphere with which the earth is enclosed. to a body moving with the appalling velocity of a meteor, a plunge into the atmosphere is usually fatal. even though the upper layers of air are excessively attenuated, yet they suddenly check the velocity almost as a rifle bullet would be checked when fired into water. as the meteor rushes through the atmosphere the friction of the air warms its surface; gradually it becomes red-hot, then white-hot, and is finally driven off into vapour with a brilliant light, while we on the earth, one or two hundred miles below, exclaim: "oh, look, there is a shooting star!" we have here an experiment illustrating the mechanical theory of heat. it may seem incredible that mere friction should be sufficient to generate heat enough to produce so brilliant a display, but we must recollect two facts: first, that the velocity of the meteor is, perhaps, one hundred times that of a rifle bullet; and, second, that the efficiency of friction in developing heat is proportional to the square of the velocity. the meteor in passing through the air may therefore develop by the friction of the air about ten thousand times as much heat as the rifle bullet. we do not make an exaggerated estimate in supposing that the latter missile becomes heated ten degrees by friction; yet if this be admitted, we must grant that there is such an enormous development of heat attending the flight of the meteor that even a fraction of it would be sufficient to drive the object into vapour. let us first consider the circumstances in which these external bodies are manifested to us, and, for the sake of illustration, we may take a remarkable fireball which occurred on november th, . this body was seen from many different places in england; and by combining and comparing these observations, we obtain accurate information as to the height of the object and the velocity with which it travelled. it appears that this meteor commenced to be visible at a point ninety miles above frome, in somersetshire, and that it vanished twenty-seven miles over the sea, near st. ives, in cornwall. the path of the body, and the principal localities from which it was observed, are shown in the map (fig. ). the whole length of its visible course was about miles, which was performed in a period of five seconds, thus giving an average velocity of thirty-four miles per second. a remarkable feature in the appearance which this fireball presented was the long persistent streak of luminous cloud, about fifty miles long and four miles wide, which remained in sight for fully fifty minutes. we have in this example an illustration of the chief features of the phenomena of a shooting star presented on a very grand scale. it is, however, to be observed that the persistent luminous streak is not a universal, nor, indeed, a very common characteristic of a shooting star. [illustration: fig. .--the path of the fireball of november th, .] the small objects which occasionally flash across the field of the telescope show us that there are innumerable telescopic shooting stars, too small and too faint to be visible to the unaided eye. these objects are all dissipated in the way we have described; it is, in fact, only at the moment, and during the process of their dissolution, that we become aware of their existence. small as these missiles probably are, their velocity is so prodigious that they would render the earth uninhabitable were they permitted to rain down unimpeded on its surface. we must, therefore, among the other good qualities of our atmosphere, not forget that it constitutes a kindly screen, which shields us from a tempest of missiles, the velocity of which no artillery could equal. it is, in fact, the very fury of these missiles which is the cause of their utter destruction. their anxiety to strike us is so great, that friction dissolves them into harmless vapour. next to a grand meteor such as that we have just described, the most striking display in connection with shooting stars is what is known as a shower. these phenomena have attracted a great deal of attention within the last century, and they have abundantly rewarded the labour devoted to them by affording some of the most interesting astronomical discoveries of modern times. the showers of shooting stars do not occur very frequently. no doubt the quickened perception of those who especially attend to meteors will detect a shower when others see only a few straggling shooting stars; but, speaking generally, we may say that the present generation can hardly have witnessed more than two or three such occurrences. i have myself seen two great showers, one of which, in november, , has impressed itself on my memory as a glorious spectacle. to commence the history of the november meteors it is necessary to look back for nearly a thousand years. on the th of october, in the year , occurred the death of a moorish king, and in connection with this event an old chronicler relates how "that night there were seen, as it were lances, an infinite number of stars, which scattered themselves like rain to right and left, and that year was called the year of the stars." no one now believes that the heavens intended to commemorate the death of the king by that display. the record is, however, of considerable importance, for it indicates the year as one in which a great shower of shooting stars occurred. it was with the greatest interest astronomers perceived that this was the first recorded instance of that periodical shower, the last of whose regular returns were seen in , , and . further diligent literary research has revealed here and there records of startling appearances in the heavens, which fit in with successive returns of the november meteors. from the first instance, in , to the present day there have been twenty-nine visits of the shower; and it is not unlikely that these may have all been seen in some parts of the earth. sometimes they may have been witnessed by savages, who had neither the inclination nor the means to place on record an apparition which to them was a source of terror. sometimes, however, these showers were observed by civilised communities. their nature was not understood, but the records were made; and in some cases, at all events, these records have withstood the corrosion of time, and have now been brought together to illustrate this curious subject. we have altogether historical notices of twelve of these showers, collected mainly by the industry of professor h.a. newton whose labours have contributed so much to the advancement of our knowledge of shooting stars. let us imagine a swarm of small objects roaming through space. think of a shoal of herrings in the ocean, extending over many square miles, and containing countless myriads of individuals; or think of those enormous flocks of wild pigeons in the united states of which audubon has told us. the shoal of shooting stars is perhaps much more numerous than the herrings or the pigeons. the shooting stars are, however, not very close together; they are, on an average, probably some few miles apart. the actual bulk of the shoal is therefore prodigious; and its dimensions are to be measured by hundreds of thousands of miles. [illustration: fig. .--the orbit of a shoal of meteors.] the meteors cannot choose their own track, like the shoal of herrings, for they are compelled to follow the route which is prescribed to them by the sun. each one pursues its own ellipse in complete independence of its neighbours, and accomplishes its journey, thousands of millions of miles in length, every thirty-three years. we cannot observe the meteors during the greater part of their flight. there are countless myriads of these bodies at this very moment coursing round their path. we never see them till the earth catches them. every thirty-three years the earth makes a haul of these meteors just as successfully as the fisherman among the herrings, and in much the same way, for while the fisherman spreads his net in which the fishes meet their doom, so the earth has an atmosphere wherein the meteors perish. we are told that there is no fear of the herrings becoming exhausted, for those the fishermen catch are as nothing compared to the profusion in which they abound in ocean. we may say the same with regard to the meteors. they exist in such myriads, that though the earth swallows up millions every thirty-three years, plenty are left for future showers. the diagram (fig. ) will explain the way in which the earth makes her captures. we there see the orbit in which our globe moves around the sun, as well as the elliptic path of the meteors, though it should be remarked that it is not convenient to draw the figure exactly to scale, so that the path of the meteors is relatively much larger than here represented. once each year the earth completes its revolution, and between the th and the th of november crosses the track in which the meteors move. it will usually happen that the great shoal is not at this point when the earth is passing. there are, however, some stragglers all along the path, and the earth generally catches a few of these at this date. they dart into our atmosphere as shooting stars, and form what we usually speak of as the november meteors. it will occasionally happen that when the earth is in the act of crossing the track it encounters the bulk of the meteors. through the shoal our globe then plunges, enveloped, of course, with the surrounding coat of air. into this net the meteors dash in countless myriads, never again to emerge. in a few hours' time, the earth, moving at the rate of eighteen miles a second, has crossed the track and emerges on the other side, bearing with it the spoils of the encounter. some few meteors, which have only narrowly escaped capture, will henceforth bear evidence of the fray by moving in slightly different orbits, but the remaining meteors of the shoal continue their journey without interruption; perhaps millions have been taken, but probably hundreds of millions have been left. such was the occurrence which astonished the world on the night between november th and th, . we then plunged into the middle of the shoal. the night was fine; the moon was absent. the meteors were distinguished not only by their enormous multitude, but by their intrinsic magnificence. i shall never forget that night. on the memorable evening i was engaged in my usual duty at that time of observing nebulæ with lord rosse's great reflecting telescope. i was of course aware that a shower of meteors had been predicted, but nothing that i had heard prepared me for the splendid spectacle so soon to be unfolded. it was about ten o'clock at night when an exclamation from an attendant by my side made me look up from the telescope, just in time to see a fine meteor dash across the sky. it was presently followed by another, and then again by more in twos and in threes, which showed that the prediction of a great shower was likely to be verified. at this time the earl of rosse (then lord oxmantown) joined me at the telescope, and, after a brief interval, we decided to cease our observations of the nebulæ and ascend to the top of the wall of the great telescope (fig. , p. ), whence a clear view of the whole hemisphere of the heavens could be obtained. there, for the next two or three hours, we witnessed a spectacle which can never fade from my memory. the shooting stars gradually increased in number until sometimes several were seen at once. sometimes they swept over our heads, sometimes to the right, sometimes to the left, but they all diverged from the east. as the night wore on, the constellation leo ascended above the horizon, and then the remarkable character of the shower was disclosed. all the tracks of the meteors radiated from leo. (_see_ fig. , p. .) sometimes a meteor appeared to come almost directly towards us, and then its path was so foreshortened that it had hardly any appreciable length, and looked like an ordinary fixed star swelling into brilliancy and then as rapidly vanishing. occasionally luminous trains would linger on for many minutes after the meteor had flashed across, but the great majority of the trains in this shower were evanescent. it would be impossible to say how many thousands of meteors were seen, each one of which was bright enough to have elicited a note of admiration on any ordinary night. the adjoining figure (fig. ) shows the remarkable manner in which the shooting stars of this shower diverged from a point. it is not to be supposed that all these objects were in view at the same moment. the observer of a shower is provided with a map of that part of the heavens in which the shooting stars appear. he then fixes his attention on one particular shooting star, and observes carefully its track with respect to the fixed stars in its vicinity. he then draws a line upon his map in the direction in which the shooting star moved. repeating the same observation for several other shooting stars belonging to the shower, his map will hardly fail to show that their different tracks almost all tend from one point or region of the figure. there are, it is true, a few erratic ones, but the majority observe this law. it certainly looks, at first sight, as if all the shooting stars did actually dart from this point; but a little reflection will show that this is a case in which the real motion is different from the apparent. if there actually were a point from which these meteors diverged, then from different parts of the earth the point would be seen in different positions with respect to the fixed stars; but this is not the case. the radiant, as this point is called, is seen in the same part of the heavens from whatever station the shower is visible. [illustration: fig. .--the radiant point of shooting stars.] we are, therefore, led to accept the simple explanation afforded by the theory of perspective. those who are acquainted with the principles of this science know that when a number of parallel lines in an object have to be represented in a drawing, they must all be made to pass through the same point in the plane of the picture. when we are looking at the shooting stars, we see the projections of their paths upon the surface of the heavens. from the fact that those paths pass through the same point, we are to infer that the shooting stars belonging to the same shower are moving in parallel lines. we are now able to ascertain the actual direction in which the shooting stars are moving, because a line drawn from the eye of the observer to the radiant point must be parallel to that direction. of course, it is not intended to convey the idea that throughout all space the shooting stars of one shower are moving in parallel lines; all we mean is that during the short time in which we see them the motion of each of the shooting stars is sensibly a straight line, and that all these straight lines are parallel. in the year the great meteor shoal of the leonids (for so this shower is called) attained its greatest distance from the sun, and then commenced to return. each year the earth crossed the orbit of the meteors; but the shoal was not met with, and no noteworthy shower of stars was perceived. every succeeding year found the meteors approaching the critical point, and the year brought the shoal to the earth's track. in that year a brilliant meteoric shower was expected, but the result fell far short of expectation. the shoal of meteors is of such enormous length that it takes more than a year for the mighty procession to pass through the critical portion of its orbit which lies across the track of the earth. we thus see that the meteors cannot escape the earth. it may be that when the shoal begins to reach this neighbourhood the earth will have just left this part of its path, and a year will have elapsed before the earth gets round again. those meteors that have the good fortune to be in the front of the shoal will thus escape the net, but some of those behind will not be so fortunate, and the earth will again devour an incredible host. it has sometimes happened that casts into the shoal have been obtained in two consecutive years. if the earth happened to pass through the front part in one year, then the shoal is so long that the earth will have moved right round its orbit of , , miles, and will again dash through the critical spot before the entire number have passed. history contains records of cases when, in two consecutive novembers, brilliant showers of leonids have been seen. as the earth consumes such myriads of leonids each thirty-three years, it follows that the total number must be decreasing. the splendour of the showers in future ages will, no doubt, be affected by this circumstance. they cannot be always so bright as they have been. it is also of interest to notice that the shape of the shoal is gradually changing. each meteor of the shoal moves in its own ellipse round the sun, and is quite independent of the rest of these bodies. each one has thus a special period of revolution which depends upon the length of the ellipse in which it happens to revolve. two meteors will move around the sun in the same time if the lengths of their ellipses are exactly equal, but not otherwise. the lengths of these ellipses are many hundreds of millions of miles, and it is impossible that they can be all absolutely equal. in this may be detected the origin of a gradual change in the character of the shower. suppose two meteors a and b be such that a travels completely round in thirty-three years, while b takes thirty-four years. if the two start together, then when a has finished the first round b will be a year behind; the next time b will be two years behind, and so on. the case is exactly parallel to that of a number of boys who start for a long race, in which they have to run several times round the course before the distance has been accomplished. at first they all start in a cluster, and perhaps for the first round or two they may remain in comparative proximity; gradually, however, the faster runners get ahead and the slower ones lag behind, so the cluster becomes elongated. as the race continues, the cluster becomes dispersed around the entire course, and perhaps the first boy will even overtake the last. such seems the destiny of the november meteors in future ages. the cluster will in time come to be spread out around the whole of this mighty track, and no longer will a superb display have to be recorded every thirty-three years. it was in connection with the shower of november meteors in that a very interesting and beautiful discovery in mathematical astronomy was made by professor adams. we have seen that the leonids must move in an elliptic path, and that they return every thirty-three years, but the telescope cannot follow them during their wanderings. all that we know by observation is the date of their occurrence, the point of the heavens from which they radiate, and the great return every thirty-three years. putting these various facts together, it is possible to determine the ellipse in which the meteors move--not exactly: the facts do not go so far--they only tell us that the ellipse must be one of five possible orbits. these five possible orbits are--firstly, the immense ellipse in which we now know the meteorites do revolve, and for which they require the whole thirty-three years to complete a revolution; secondly, a nearly circular orbit, very little larger than the earth's path, which the meteors would traverse in a few days more than a year; another similar orbit, in which the time would be a few days short of the year; and two other small elliptical orbits lying inside the earth's orbit. it was clearly demonstrated by professor newton, of new haven, u.s.a., that the observed facts would be explained if the meteors moved in any one of these paths, but that they could not be explained by any other hypothesis. it remained to see which of these orbits was the true one. professor newton himself made the suggestion of a possible method of solving the problem. the test he proposed was one of some difficulty, for it involved certain intricate calculations in the theory of perturbations. fortunately, however, professor adams undertook the inquiry, and by his successful labours the path of the leonids has been completely ascertained. [illustration: fig. .--the history of the leonids.] when the ancient records of the appearance of great leonid showers were examined, it was found that the date of their occurrence undergoes a gradual and continuous change, which professor newton fixed at one day in seventy years. it follows as a necessary consequence that the point where the path of the meteors crosses the earth's track is not fixed, but that at each successive return they cross at a point about half a degree further on in the direction in which the earth is travelling. it follows that the orbit in which the meteors are revolving is undergoing change; the path they follow in one revolution varies slightly from that pursued in the next. as, however, these changes proceed in the same direction, they may gradually attain considerable dimensions; and the amount of change which is produced in the path of the meteors in the lapse of centuries may be estimated by the two ellipses shown in fig. . the continuous line represents the orbit in a.d. ; the dotted line represents it at present. this unmistakable change in the orbit is one that astronomers attribute to what we have already spoken of as perturbation. it is certain that the elliptic motion of these bodies is due to the sun, and that if they were only acted on by the sun the ellipse would remain absolutely unaltered. we see, then, in this gradual change of the ellipse the influence of the attractions of the planets. it was shown that if the meteors moved in the large orbit, this shifting of the path must be due to the attraction of the planets jupiter, saturn, uranus, and the earth; while if the meteors followed one of the smaller orbits, the planets that would be near enough and massive enough to act sensibly on them would be the earth, venus, and jupiter. here, then, we see how the question may be answered by calculation. it is difficult, but it is possible, to calculate what the attraction of the planets would be capable of producing for each of the five different suppositions as to the orbit. this is what adams did. he found that if the meteors moved in the great orbit, then the attraction of jupiter would account for two-thirds of the observed change, while the remaining third was due to the influence of saturn, supplemented by a small addition on account of uranus. in this way the calculation showed that the large orbit was a possible one. professor adams also computed the amount of displacement in the path that could be produced if the meteors revolved in any of the four smaller ellipses. this investigation was one of an arduous character, but the results amply repaid the labour. it was shown that with the smaller ellipses it would be impossible to obtain a displacement even one-half of that which was observed. these four orbits must, therefore, be rejected. thus the demonstration was complete that it is in the large path that the meteors revolve. the movements in each revolution are guided by kepler's laws. when at the part of its path most distant from the sun the velocity of a meteor is at its lowest, being then but little more than a mile a second; as it draws in, the speed gradually increases, until, when the meteor crosses the earth's track, its velocity is no less than twenty-six miles a second. the earth is moving very nearly in the opposite direction at the rate of eighteen miles a second, so that, if the meteor happen to strike the earth's atmosphere, it does so with the enormous velocity of nearly forty-four miles a second. if a collision is escaped, then the meteor resumes its onward journey with gradually declining velocity, and by the time it has completed its circuit a period of thirty-three years and a quarter will have elapsed. the innumerable meteors which form the leonids are arranged in an enormous stream, of a breadth very small in comparison with its length. if we represent the orbit by an ellipse whose length is seven feet, then the meteor stream will be represented by a thread of the finest sewing-silk, about a foot and a half or two feet long, creeping along the orbit.[ ] the size of this stream may be estimated from the consideration that even its width cannot be less than , miles. its length may be estimated from the circumstance that, although its velocity is about twenty-six miles a second, yet the stream takes about two years to pass the point where its orbit crosses the earth's track. on the memorable night between the th and th of november, , the earth plunged into this stream near its head, and did not emerge on the other side until five hours later. during that time it happened that the hemisphere of the earth which was in front contained the continents of europe, asia, and africa, and consequently it was in the old world that the great shower was seen. on that day twelvemonth, when the earth had regained the same spot, the shoal had not entirely passed, and the earth made another plunge. this time the american continent was in the van, and consequently it was there that the shower of was seen. even in the following year the great shoal had not entirely passed, and since then a few stragglers along the route have been encountered at each annual transit of the earth across this meteoric highway. the diagram is also designed to indicate a remarkable speculation which was put forward on the high authority of le verrier, with the view of explaining how the shoal came to be introduced into the solar system. the orbit in which the meteors revolve does not intersect the paths of jupiter, saturn, or mars, but it does intersect the orbit of uranus. it must sometimes happen that uranus is passing through this point of its path just as the shoal arrives there. le verrier has demonstrated that such an event took place in the year a.d. , but that it has not happened since. we thus seem to have a clue to a very wonderful history by which the meteors are shown to have come into our system in the year named. the expectations or a repetition of the great shower in which had been widely entertained, and on good grounds, were not realised. hardly more than a few meteors of the ordinary type were observed. assuming that the orbit of the august meteors was a parabola, schiaparelli computed the dimensions and position in space of this orbit, and when he had worked this out, he noticed that the orbit corresponded in every particular with the orbit of a fine comet which had appeared in the summer of . this could not be a mere matter of accident. the plane in which the comet moved coincided exactly with that in which the meteors moved; so did the directions of the axes of their orbits, while the direction of the motion is the same, and the shortest distance from the sun to the orbit is also in the two cases identical. this proved to demonstration that there must be some profound physical connection between comets and swarms of meteors. and a further proof of this was shortly afterwards furnished, when le verrier had computed the orbit of the november meteors, for this was at once noticed to be precisely the same as the orbit of a comet which had passed its perihelion in january, , and for which the period of revolution had been found to be thirty-three years and two months. among the leonids we see occasionally fireballs brighter than venus, and even half the apparent size of the moon, bursting out with lightning-like flashes, and leaving streaks which last from a minute to an hour or more. but the great majority are only as bright as stars of the second, third, or fourth magnitude. as the amount of light given by a meteor depends on its mass and velocity, we can form some idea as to the actual weight of one of these meteors, and it appears that most of them do not weigh nearly as much as a quarter of an ounce; indeed, it is probable that many do not weigh a single grain. but we have seen that a comet in all probability is nothing but a very loose swarm of small particles surrounded by gas of very slight density, and we have also seen that the material of a comet must by degrees be more or less dissipated through space. we have still to tell a wonderful story of the breaking up of a comet and what appears to have become of the particles thereof. a copious meteoric shower took place on the night of the th november, . on this occasion the shooting stars diverged from a radiant point in the constellation of andromeda. as a spectacle, it was unquestionably inferior to the magnificent display of , but it is difficult to say which of the two showers has been of greater scientific importance. it surely is a remarkable coincidence that the earth should encounter the andromedes (for so this shower is called) at the very moment when it is crossing the track of biela's comet. we have observed the direction from which the andromedes come when they plunge into the atmosphere; we can ascertain also the direction in which biela's comet is moving when it passes the earth's track, and we find that the direction in which the comet moves and the direction in which the meteors move are identical. this is, in itself, a strong and almost overwhelming presumption that the comet and the shooting stars are connected; but it is not all. we have observations of this swarm dating back to the eighteenth century, and we find that the date of its appearance has changed from the th or th of december to the end of november in perfect accordance with the retrograde motion of the crossing-point of the earth's orbit and the orbit of biela's comet. this comet was observed in , and again in - , before its periodic return every seven years was discovered. it was discovered by biela in , and was observed again in . in the astronomical world was startled to find that there were now two comets in place of one, and the two fragments were again perceived at the return in . in biela's comet could not be seen, owing to its unfavourable situation with regard to the earth. no trace of biela's comet was seen in - , when its return was also due, nor has it ever been seen since. it therefore appears that in the autumn of the time had arrived for the return of biela's comet, and thus the occurrence of the great shower of the andromedes took place about the time when biela's comet was actually due. the inference is irresistible that the shooting stars, if not actually a part of the comet itself, are at all events most intimately connected therewith. this shower is also memorable for the telegram sent from professor klinkerfues to mr. pogson at madras. the telegram ran as follows:--"biela touched earth on th. search near theta centauri." pogson did search and did find a comet, but, unfortunately, owing to bad weather he only secured observations of it on two nights. as we require three observations to determine the orbit of a planet or comet, it is not possible to compute the orbit of pogson's, but it seems almost certain that the latter cannot be identical with either of the two components of biela's comet. it is, however, likely that it really was a comet moving along the same track as biela and the meteors. another display of the biela meteors took place in , just giving time for two complete revolutions of the swarm since . the display on the th november, , was magnificent; professor newton estimated that at the time of maximum the meteors came on at the rate of , per hour. in the comet ought again to have returned to perihelion, but in that year no meteors were seen on the th november, while many were seen on the rd from the same radiant. the change in the point of intersection between the orbit of the meteors and the orbit of the earth indicated by this difference of four days was found by bredichin to be due to the perturbing action of jupiter on the motion of the swarm. it is a noticeable circumstance that the great meteoric showers seem never yet to have projected a missile which has reached the earth's surface. out of the myriads of leonids, of perseids, or of andromedes, not one particle has ever been seized and identified.[ ] those bodies which fall from the sky to the earth, and which we call meteorites, do not seem to come from the great showers, so far as we know. they may, indeed, have quite a different origin from that of the periodic meteors. it is somewhat curious that the belief in the celestial origin of meteorites is of modern growth. in ancient times there were, no doubt, rumours of wonderful stones which had fallen down from the heavens to the earth, but these reports seem to have obtained but little credit. they were a century ago regarded as perfectly fabulous, though there was abundant testimony on the subject. eye-witnesses averred that they had seen the stones fall. the bodies themselves were unlike other objects in the neighbourhood, and cases were even authenticated where men had been killed by these celestial visitors. no doubt the observations were generally made by ignorant and illiterate persons. the true parts of the record were so mixed up with imaginary additions, that cautious men refused to credit the statements that such objects really fell from the sky. even at the present day it is often extremely difficult to obtain accurate testimony on such matters. for instance, the fall of a meteorite was observed by a hindoo in the jungle. the stone was there, its meteoric character was undoubted, and the witness was duly examined as to the details of the occurrence; but he was so frightened by the noise and by the danger he believed himself to have narrowly escaped, that he could tell little or nothing. he felt certain, however, that the meteorite had hunted him for two hours through the jungle before it fell to the earth! in the year chladni published an account of the remarkable mass of iron which the traveller pallas had discovered in siberia. it was then for the first time recognised that this object and others similar to it must have had a celestial origin. but even chladni's reputation and the arguments he brought forward failed to procure universal assent. shortly afterwards a stone of fifty-six pounds was exhibited in london, which several witnesses declared they had seen fall at wold cottage, in yorkshire, in . this body was subsequently deposited in our national collection, and is now to be seen in the natural history museum at south kensington. the evidence then began to pour in from other quarters; portions of stone from italy and from benares were found to be of identical composition with the yorkshire stone. the incredulity of those who had doubted the celestial origin of these objects began to give way. a careful memoir on the benares meteorite, by howard, was published in the "philosophical transactions" for , while, as if to complete the demonstration, a great shower of stones took place in the following year at l'aigle, in normandy. the french academy deputed the physicist biot to visit the locality and make a detailed examination of the circumstances attending this memorable shower. his enquiry removed every trace of doubt, and the meteoric stones have accordingly been transferred from the dominions of geology to those of astronomy. it may be noted that the recognition of the celestial origin of meteorites happens to be simultaneous with the discovery of the first of the minor planets. in each case our knowledge of the solar system has been extended by the addition of numerous minute bodies, which, notwithstanding their insignificant dimensions, are pregnant with information. when the possibility of stone-falls has been admitted, we can turn to the ancient records, and assign to them the credit they merit, which was withheld for so many centuries. perhaps the earliest of all these stone-falls which can be said to have much pretension to historical accuracy is that of the shower which livy describes as having fallen, about the year b.c., on the alban mount, near rome. among the more modern instances, we may mention one which was authenticated in a very emphatic manner. it occurred in the year at ensisheim, in alsace. the emperor maximilian ordered a minute narrative of the circumstances to be drawn up and deposited with the stone in the church. the stone was suspended in the church for three centuries, until in the french revolution it was carried off to colmar, and pieces were broken from it, one of which is now in our national collection. fortunately, this interesting object has been restored to its ancient position in the church at ensisheim, where it remains an attraction to sight-seers at this day. the account is as follows:--"in the year of the lord , on the wednesday before st. martin's day, november th, a singular miracle occurred, for between eleven o'clock and noon there was a loud clap of thunder and a prolonged confused noise, which was heard at a great distance, and a stone fell from the air in the jurisdiction of ensisheim which weighed pounds, and the confused noise was at other places much louder than here. then a boy saw it strike on ploughed ground in the upper field towards the rhine and the ill, near the district of gisgang, which was sown with wheat, and it did no harm, except that it made a hole there; and then they conveyed it from the spot, and many pieces were broken from it, which the land vogt forbade. they therefore caused it to be placed in the church, with the intention of suspending it as a miracle, and there came here many people to see this stone, so there were many remarkable conversations about this stone; the learned said they knew not what it was, for it was beyond the ordinary course of nature that such a large stone should smite from the height of the air, but that it was really a miracle from god, for before that time never was anything heard like it, nor seen, nor written. when they found that stone, it had entered into the earth to half the depth of a man's stature, which everybody explained to be the will of god that it should be found, and the noise of it was heard at lucerne, at villingen, and at many other places, so loud that the people thought that the houses had been overturned; and as the king maximilian was here, the monday after st. catherine's day of the same year, his royal excellency ordered the stone which had fallen to be brought to the castle, and after having conversed a long time about it with the noblemen, he said that the people of ensisheim should take it and order it to be hung up in the church, and not to allow anybody to take anything from it. his excellency, however, took two pieces of it, of which he kept one, and sent the other to duke sigismund of austria, and there was a great deal of talk about the stone, which was suspended in the choir, where it still is, and a great many people came to see it." admitting the celestial origin of the meteorites, they surely claim our closest attention. they afford the only direct method we possess of obtaining a knowledge of the materials of bodies exterior to our planet. we can take a meteorite in our hands, we can analyse it, and find the elements of which it is composed. we shall not attempt to enter into any very detailed account of the structure of meteorites; it is rather a matter for the consideration of chemists and mineralogists than for astronomers. a few of the more obvious features will be all that we require. they will serve as a preliminary to the discussion of the probable origin of these bodies. in the natural history museum at south kensington we may examine a superb collection of meteorites. they have been brought together from all parts of the earth, and vary in size from bodies not much larger than a pin's head up to vast masses weighing many hundredweights. there are also models of celebrated meteorites, of which the originals are dispersed through various other museums. many meteorites have nothing very remarkable in their external appearance. if they were met with on the sea beach, they would be passed by without more notice than would be given to any other stone. yet, what a history a meteorite might tell us if we could only manage to obtain it! it fell; it was seen to fall from the sky; but what was its course anterior to that movement? where was it years ago, , years ago? through what regions of space has it wandered? why did it never fall before? why has it actually now fallen? such are some of the questions which crowd upon us as we ponder over these most interesting bodies. some of these objects are composed of very characteristic materials; take, for example, one of the more recent arrivals, known as the rowton siderite. this body differs very much from the more ordinary kind of stony meteorite. it is an object which even a casual passer-by would hardly pass without notice. its great weight would also attract attention, while if it be scratched or rubbed with a file, it would appear to be a mass of nearly pure iron. we know the circumstances in which that piece of iron fell to the earth. it was on the th of april, , about . p.m., that a strange rumbling noise, followed by a startling explosion, was heard over an area of several miles in extent among the villages in shropshire, eight or ten miles north of the wrekin. about an hour after this occurrence a farmer noticed that the ground in one of his grass-fields had been disturbed, and he probed the hole which the meteorite had made, and found it, still warm, about eighteen inches below the surface. some men working at no great distance had heard the noise made in its descent. this remarkable object, weighs - / lbs. it is an irregular angular mass of iron, though all its edges seem to have been rounded by fusion in its transit through the air. it is covered with a thick black pellicle of the magnetic oxide of iron, except at the point where it first struck the ground. the duke of cleveland, on whose property it fell, afterwards presented it to our national institution already referred to, where, as the rowton siderite, it attracts the attention of everyone who is interested in these wonderful bodies. this siderite is specially interesting on account of its distinctly metallic character. falls of objects of this particular type are not so frequent as are those of the stony meteorites; in fact, there are only a few known instances of meteoric irons having been actually seen to fall, while the observed falls of stony meteorites are to be counted in scores or in hundreds. the inference is that the iron meteorites are much less frequent than the stony ones. this is, however, not the impression that the visitor to the museum would be likely to receive. in that extensive collection the meteoric irons are by far the most striking objects. the explanation is not difficult. those gigantic masses of iron are unquestionably meteoric: no one doubts that this is the case. yet the vast majority of them have never been seen to fall; they have simply been found, in circumstances which point unmistakably to their meteoric nature. suppose, for instance, that a traveller on one of the plains of siberia or of central america finds a mass of metallic iron lying on the surface of the ground, what explanation can be rendered of such an occurrence? no one has brought the iron there, and there is no iron within hundreds of miles. man never fashioned that object, and the iron is found to be alloyed with nickel in a manner that is always observed in known meteorites, and is generally regarded as a sure indication of a meteoric origin. observe also, that as iron perishes by corrosion in our atmosphere, that great mass of iron cannot have lain where it is for indefinite ages; it must have been placed there at some finite time. only one source for such an object is conceivable; it must have fallen from the sky. on the same plains the stony meteorites have also fallen in hundreds and in thousands, but they crumble away in the course of time, and in any case would not arrest the attention of the traveller as the irons are likely to do. hence it follows, that although the stony meteorites seem to fall much more frequently, yet, unless they are actually observed at the moment of descent, they are much more liable to be overlooked than the meteoric irons. hence it is that the more prominent objects of the british collection are the meteoric irons. we have said that a noise accompanied the descent of the rowton siderite, and it is on record that a loud explosion took place when the meteorite fell at ensisheim. in this we have a characteristic feature of the phenomenon. nearly all the descents of meteorites that have been observed seem to have been ushered in by a detonation. we do not, however, assert that this is quite an invariable feature; and it is also the case that meteors often detonate without throwing down any solid fragments that have been collected. the violence associated with the phenomenon is forcibly illustrated by the butsura meteorite. this object fell in india in . a loud explosion was heard, several fragments of stone were collected from distances three or four miles apart; and when brought together, they were found to fit, so as to enable the primitive form of the meteorite to be reconstructed. a few of the pieces are wanting (they were, no doubt, lost by falling unobserved into localities from which they could not be recovered), but we have obtained pieces quite numerous enough to permit us to form a good idea of the irregular shape of the object before the explosion occurred which shattered it into fragments. this is one of the ordinary stony meteorites, and is thus contrasted with the rowton siderite which we have just been considering. there are also other types of meteorites. the breitenbach iron, as it is called, is a good representative of a class of these bodies which lie intermediate between the meteoric irons and the stones. it consists of a coarsely cellular mass of iron, the cavities being filled with mineral substances. in the museum, sections of intermediate forms are shown in which this structure is exhibited. look first at the most obvious characteristic of these meteorites. we do not now allude to their chemical composition, but to their external appearance. what is the most remarkable feature in the shape of these objects?--surely it is that they are fragments. they are evidently pieces that are _broken_ from some larger object. this is apparent by merely looking at their form; it is still more manifest when we examine their mechanical structure. it is often found that meteorites are themselves composed of smaller fragments. such a structure may be illustrated by a section of an aërolite found on the sierra of chaco, weighing about lbs. (fig. ). the section here represented shows the composite structure of this object, which belongs to the class of stony meteorites. its shape shows that it was really a fragment with angular edges and corners. no doubt it may have been much more considerable when it first dashed into the atmosphere. the angular edges now seen on the exterior may be due to an explosion which then occurred; but this will not account for the structure of the interior. we there see irregular pieces of varied form and material agglomerated into a single mass. if we would seek for analogous objects on the earth, we must look to some of the volcanic rocks, where we have multitudes of irregular angular fragments cemented together by a matrix in which they are imbedded. the evidence presented by this meteorite is conclusive as to one circumstance with regard to the origin of these objects. they must have come as fragments, from some body of considerable, if not of vast, dimensions. in this meteorite there are numerous small grains of iron mingled with mineral substances. the iron in many meteorites has, indeed, characters resembling those produced by the actual blasting of iron by dynamite. thus, a large meteoric iron from brazil has been found to have been actually shivered into fragments at some time anterior to its fall on the earth. these fragments have been cemented together again by irregular veins of mineral substances. [illustration: fig. .--section of the chaco meteorite.] for an aërolite of a very different type we may refer to the carbonaceous meteorite of orgueil, which fell in france on the th may, . on the occasion of its descent a splendid meteor was seen, rivalling the full moon in size. the actual diameter of this globe of fire must have been some hundreds of yards. nearly a hundred fragments of the body were found scattered over a tract of country fifteen miles long. this object is of particular interest, inasmuch as it belongs to a rare group of aërolites, from which metallic iron is absent. it contains many of the same minerals which are met with in other meteorites, but in these fragments they are _associated with carbon_, and with substances of a white or yellowish crystallisable material, soluble in ether, and resembling some of the hydrocarbons. such a substance, if it had not been seen falling to the earth, would probably be deemed a product resulting from animal or vegetable life! we have pointed out how a body moving with great velocity and impinging upon the air may become red-hot and white-hot, or even be driven off into vapour. how, then, does it happen that meteorites escape this fiery ordeal, and fall down to the earth, with a great velocity, no doubt, but still, with very much less than that which would have sufficed to drive them off into vapour? had the rowton siderite, for instance, struck our atmosphere with a velocity of twenty miles a second, it seems unquestionable that it would have been dissipated by heat, though, no doubt, the particles would ultimately coalesce so as to descend slowly to the earth in microscopic beads of iron. how has the meteorite escaped this fate? it must be remembered that our earth is also moving with a velocity of about eighteen miles per second, and that the _relative_ velocity with which the meteorite plunges into the air is that which will determine the degree to which friction is operating. if the meteorite come into direct collision with the earth, the velocity of the collision will be extremely great; but it may happen that though the actual velocities of the two bodies are both enormous, yet the relative velocity may be comparatively small. this is, at all events, one conceivable explanation of the arrival of a meteorite on the surface of the earth. we have shown in the earlier parts of the chapter that the well-known star showers are intimately connected with comets. in fact, each star shower revolves in the path pursued by a comet, and the shooting star particles have, in all probability, been themselves derived from the comet. showers of shooting stars have, therefore, an intimate connection with comets, but it is doubtful whether meteorites have any connection with comets. it has already been remarked that meteorites have never been known to fall in the great star showers. no particle of a meteorite is known to have dropped from the countless host of the leonids or of the perseids; as far as we know, the lyrids never dropped a meteorite, nor did the quadrantids, the geminids, or the many other showers with which every astronomer is familiar. there is no reason to connect meteorites with these showers, and it is, therefore, doubtful whether we should connect meteorites with comets. with reference to the origin of meteorites it is difficult to speak with any great degree of confidence. every theory of meteorites presents difficulties, so it seems that the only course open to us is to choose that view of their origin which seems least improbable. it appears to me that this condition is fulfilled in the theory entertained by the austrian mineralogist, tschermak. he has made a study of the meteorites in the rich collection at vienna, and he has come to the conclusion that the "meteorites have had a volcanic source on some celestial body." let us attempt to pursue this reasoning and discuss the problem, which may be thus stated:--assuming that at least some of the meteorites have been ejected from volcanoes, on what body or bodies in the universe must these volcanoes be situated? this is really a question for astronomers and mathematicians. once the mineralogists assure us that these bodies are volcanic, the question becomes one of calculation and of the balance of probabilities. the first step in the enquiry is to realise distinctly the dynamical conditions of the problem. conceive a volcano to be located on a planet. the volcano is supposed to be in a state of eruption, and in one of its mighty throes projects a missile aloft: this missile will ascend, it will stop, and fall down again. such is the case at present in the eruptions of terrestrial volcanoes. cotopaxi has been known to hurl prodigious stones to a vast height, but these stones assuredly return to earth. the gravitation of the earth has gradually overcome the velocity produced by the explosion, and down the body falls. but let us suppose that the eruption is still more violent, and that the stones are projected from the planet to a still greater height above its surface. suppose, for instance, that the stone should be shot up to a height equal to the planet's radius, the attraction of gravitation will then be reduced to one-fourth of what it was at the surface, and hence the planet will find greater difficulty in pulling back the stone. not only is the distance through which the stone has to be pulled back increased as the height increases, but the efficiency of gravitation is weakened, so that in a twofold way the difficulty of recalling the stone is increased. we have already more than once alluded to this subject, and we have shown that there is a certain critical velocity appropriate to each planet, and depending on its mass and its radius. if the missile be projected upwards with a velocity equal to or greater than this, then it will ascend never to return. we all recollect jules verne's voyage to the moon, in which he described the columbiad, an imaginary cannon, capable of shooting out a projectile with a velocity of six or seven miles a second. this is the critical velocity for the earth. if we could imagine the air removed, then a cannon of seven-mile power would project a body upwards which would never fall down. the great difficulty about tschermak's view of the volcanic origin of the meteorites lies in the tremendous initial velocity which is required. the columbiad is a myth, and we know no agent, natural or artificial, at the present time on the earth, adequate to the production of a velocity so appalling. the thunders of krakatoa were heard thousands of miles away, but in its mightiest throes it discharged no missiles with a velocity of six miles a second. we are therefore led to enquire whether any of the other celestial bodies are entitled to the parentage of the meteorites. we cannot see volcanoes on any other body except the moon; all the other bodies are too remote for an inspection so minute. does it seem likely that volcanoes on the moon can ever launch forth missiles which fall upon the earth? this belief was once sustained by eminent authority. the mass of the moon is about one-eightieth of the mass of the earth. it would not be true to assert that the critical velocity of projection varies directly as the mass of the planet. the correct law is, that it varies directly as the square root of the mass, and inversely as the square root of the radius. it is hence shown that the velocity required to project a missile away from the moon is only about one-sixth of that which would be required to project a missile away from the earth. if the moon had on its surface volcanoes of one-mile power, it is quite conceivable that these might be the source of meteorites. we have seen how the whole surface of the moon shows traces of intense volcanic activity. a missile thus projected from the moon could undoubtedly fall on the earth, and it is not impossible that some of the meteorites may really have come from this source. there is, however, one great difficulty about the volcanoes on the moon. suppose an object were so projected, it would, under the attraction of the earth, in accordance with kepler's laws, move around the earth as a focus. if we set aside the disturbances produced by all other bodies, as well as the disturbance produced by the moon itself, we see that the meteorite if it once misses the earth can never fall thereon. it would be necessary that the shortest distance of the earth's centre from the orbit of the projectile should be less than the radius of the earth, so that if a lunar meteorite is to fall on the earth, it must do so the first time it goes round. the journey of a meteorite from the moon to the earth is only a matter of days, and therefore, as meteorites are still falling, it would follow that they must still be constantly ejected from the moon. the volcanoes on the moon are, however, not now active; observers have long studied its surface, and they find no reliable traces of volcanic activity at the present day. it is utterly out of the question, whatever the moon may once have been able to do, that at the present date she could still continue to launch forth meteorites. it is just possible that a meteorite expelled from the moon in remote antiquity, when its volcanoes were active, may, under the influence of the disturbances of the other bodies of the system, have its orbit so altered, that at length it comes within reach of the atmosphere and falls to the earth, but in no circumstances could the moon send us a meteorite at present. it is therefore reasonable to look elsewhere in our search for volcanoes fulfilling the conditions of the problem. let us now direct our attention to the planets, and examine the circumstances in which volcanoes located thereon could eject a meteorite which should ultimately tumble on the earth. we cannot see the planets well enough to tell whether they have or ever had any volcanoes; but the almost universal presence of heat in the large celestial masses seems to leave us in little doubt that some form of volcanic action might be found in the planets. we may at once dismiss the giant planets, such as jupiter or saturn: their appearance is very unlike a volcanic surface; while their great mass would render it necessary to suppose that the meteorites were expelled with terrific velocity if they should succeed in escaping from the gravitation of the planet. applying the rule already given, a volcano on jupiter would have to be five or six times as powerful as the volcano on the earth. to avoid this difficulty, we naturally turn to the smaller planets of the system; take, for instance, one of that innumerable host of minor planets, and let us enquire how far this body is likely to have ejected a missile which should fall upon the earth. some of these globes are only a few miles in diameter. there are bodies in the solar system so small that a very moderate velocity would be sufficient to project a missile away from them altogether. we have, indeed, already illustrated this point in discussing the minor planets. it has been suggested that a volcano placed on one of the minor planets might be quite powerful enough to start the meteorites on a long ramble through space until the chapter of accidents brought them into collision with the earth. there is but little difficulty in granting that there might be such volcanoes, and that they might be sufficiently powerful to drive bodies from the surface of the planet; but we must remember that the missiles are to fall on the earth, and dynamical considerations are involved which merit our close attention. to concentrate our ideas, we shall consider one of the minor planets, and for this purpose let us take ceres. if a meteorite is to fall upon the earth, it must pass through the narrow ring, some , miles wide, which marks the earth's path; it will not suffice for the missile to pass through the ecliptic on the inside or on the outside of the ring, it must be actually through this narrow strip, and then if the earth happens to be there at the same moment the meteorite will fall. the first condition to be secured is, therefore, that the path of the meteorite shall traverse this narrow ring. this is to be effected by projection from some point in the orbit of ceres. but it can be shown on purely dynamical grounds that although the volcanic energy sufficient to remove the projectile from ceres may be of no great account, yet if that projectile is to cross the earth's track, the dynamical requirements of the case demand a volcano on ceres at the very least of three-mile power. we have thus gained but little by the suggestion of a minor planet, for we have not found that a moderate volcanic power would be adequate. but there is another difficulty in the case of ceres, inasmuch as the ring on the ecliptic is very narrow in comparison with the other dimensions of the problem. ceres is a long way off, and it would require very great accuracy in volcanic practice on ceres to project a missile so that it should just traverse this ring and fall neither inside nor outside, neither above nor below. there must be a great many misses for every hit. we have attempted to make the calculation by the aid of the theory of probabilities, and we find that the chances against this occurrence are about , to , so that out of every , projectiles hurled from a point in the orbit of ceres only a single one can be expected to satisfy even the first of the conditions necessary if it is ever to tumble on our globe. it is thus evident that there are two objections to ceres (and the same may be said of the other minor planets) as a possible source of the meteorites. firstly, that notwithstanding the small mass of the planet a very powerful volcano would still be required; and secondly, that we are obliged to assume that for every one which ever reached the earth at least , must have been ejected. it is thus plain that if the meteorites have really been driven from some planet of the solar system, large or small, the volcano must, from one cause or another, have been a very powerful one. we are thus led to enquire which planet possesses on other grounds the greatest probability in its favour. we admit of course that at the present time the volcanoes on the earth are utterly devoid of the necessary power; but were the terrestrial volcanoes always so feeble as they are in these later days? grounds are not wanting for the belief that in the very early days of geological time the volcanic energy on the earth was much greater than at present. we admit fully the difficulties of the view that the meteorites have really come from the earth; but they must have some origin, and it is reasonable to indicate the source which seems to have most probability in its favour. grant for a moment that in the primæval days of volcanic activity there were some mighty throes which hurled forth missiles with the adequate velocity: these missiles would ascend, they would pass from the gravitation of the earth, they would be seized by the gravitation of the sun, and they would be compelled to revolve around the sun for ever after. no doubt the resistance of the air would be a very great difficulty, but this resistance would be greatly lessened were the crater at a very high elevation above the sea level, while, if a vast volume of ejected gases or vapours accompanied the more solid material, the effect of the resistance of the air would be still further reduced. some of these objects might perhaps revolve in hyperbolic orbits, and retreat never to return; while others would be driven into elliptic paths. round the sun these objects would revolve for ages, but at each revolution--and here is the important point--they would traverse the point from which they were originally launched. in other words, every object so projected from the earth would at each revolution cross the track of the earth. we have in this fact an enormous probability in favour of the earth as contrasted with ceres. only one ceres-ejected meteorite out of every , would probably cross the earth's track, while every earth-projected meteorite would necessarily do so. if this view be true, then there must be hosts of meteorites traversing space in elliptic orbits around the sun. these orbits have one feature in common: they all intersect the track of the earth. it will sometimes happen that the earth is found at this point at the moment the meteorite is crossing; when this is the case the long travels of the little body are at an end, and it tumbles back on the earth from which it parted so many ages ago. it is well to emphasise the contrast between the lunar theory of meteorites (which we think improbable) and the terrestrial theory (which appears to be probable). for the lunar theory it would, as we have seen, be necessary that some of the lunar volcanoes should be still active. in the terrestrial theory it is only necessary to suppose that the volcanoes on the earth once possessed sufficient explosive power. no one supposes that the volcanoes at present on the earth eject now the fragments which are to form future meteorites; but it seems possible that the earth may be now slowly gathering back, in these quiet times, the fragments she ejected in an early stage of her history. assuming, therefore, with tschermak, that many meteorites have had a volcanic origin on some considerable celestial body, we are led to agree with those who think that most probably that body is the earth. it is interesting to notice a few circumstances which seem to corroborate the view that many meteorites are of ancient terrestrial origin. the most characteristic constituent of these bodies is the alloy of iron and nickel, which is almost universally present. sometimes, as in the rowton siderite, the whole object consists of little else; sometimes this alloy is in grains distributed through the mass. when nordenskjöld discovered in greenland a mass of native iron containing nickel, this was at once regarded as a celestial visitor. it was called the ovifak meteorite, and large pieces of the iron were conveyed to our museums. there is, for instance, in the national collection a most interesting exhibit of the ovifak substance. close examination shows that this so-called meteorite lies in a bed of basalt which has been vomited from the interior of the earth. those who believe in the meteoric origin of the ovifak iron are constrained to admit that shortly after the eruption of the basalt, and while it was still soft, this stupendous iron meteorite of gigantic mass and bulk happened to fall into this particular soft bed. the view is, however, steadily gaining ground that this great iron mass was no celestial visitor at all, but that it simply came forth from the interior of the earth with the basalt itself. the beautiful specimens in the british museum show how the iron graduates into the basalt in such a way as to make it highly probable that the source of the iron is really to be sought in the earth and not external thereto. should further research establish this, as now seems probable, a most important step will have been taken in proving the terrestrial origin of meteorites. if the ovifak iron be really associated with the basalt, we have a proof that the iron-nickel alloy is indeed a terrestrial substance, found deep in the interior of the earth, and associated with volcanic phenomena. this being so, it will be no longer difficult to account for the iron in undoubted meteorites. when the vast volcanoes were in activity they ejected masses of this iron-alloy, which, having circulated round the sun for ages, have at last come back again. as if to confirm this view, professor andrews discovered particles of native iron in the basalt of the giant's causeway, while the probability that large masses of iron are there associated with the basaltic formation was proved by the researches on magnetism of the late provost lloyd. besides the more solid meteorites there can be no doubt that the _débris_ of the ordinary shooting stars must rain down upon the earth in gentle showers of celestial dust. the snow in the arctic regions has often been found stained with traces of dust which contains particles of iron. similar particles have been found on the towers of cathedrals and in many other situations where it could only have been deposited from the air. there can be hardly a doubt that some of the motes in the sunbeam, and many of the particles which good housekeepers abhor as dust, have indeed a cosmical origin. in the famous cruise of the _challenger_ the dredges brought up from the depths of the atlantic no "wedges of gold, great anchors, heaps of pearl," but among the mud which they raised are to be found numerous magnetic particles which there is every reason to believe fell from the sky, and thence subsided to the depths of the ocean. sand from the deserts of africa, when examined under the microscope, yield traces of minute iron particles which bear the marks of having experienced a high temperature. the earth draws in this cosmic dust continuously, but the earth now never parts with a particle of its mass. the consequence is inevitable; the mass of the earth must be growing, and though the change may be a small one, yet to those who have studied darwin's treatise on "earth-worms," or to those who are acquainted with the modern theory of evolution, it will be manifest that stupendous results can be achieved by slight causes which tend in one direction. it is quite probable that an appreciable part of the solid substance of our globe may have been derived from meteoric matter which descends in perennial showers upon its surface. chapter xviii. the starry heavens. the constellations--the great bear and the pointers--the pole star--cassiopeia--andromeda, pegasus, and perseus--the pleiades: auriga, capella, aldebaran--taurus, orion, sirius; castor and pollux--the lion--boötes, corona, and hercules--virgo and spica--vega and lyra--the swan. the student of astronomy should make himself acquainted with the principal constellations in the heavens. this is a pleasing acquirement, and might well form a part of the education of every child in the kingdom. we shall commence our discussion of the sidereal system with a brief account of the principal constellations visible in the northern hemisphere, and we accompany our description with such outline maps of the stars as will enable the beginner to identify the chief features of the starry heavens. in an earlier chapter we directed the attention of the student to the remarkable constellation of stars which is known to astronomers as ursa major, or the great bear. it forms the most conspicuous group in the northern skies, and in northern latitudes it never sets. at eleven p.m. in the month of april the great bear is directly overhead (for an observer in the united kingdom); at the same hour in september it is low down in the north; at the same hour july it is in the west; by christmas it is at the east. from the remotest antiquity this group of stars has attracted attention. the stars in the great bear were comprised in a great catalogue of stars, made two thousand years ago, which has been handed down to us. from the positions of the stars given in this catalogue it is possible to reconstruct the great bear as it appeared in those early days. this has been done, and it appears that the seven principal stars have not changed in this lapse of time to any large extent, so that the configuration of the great bear remains practically the same now as it was then. the beginner must first obtain an acquaintance with this group of seven stars, and then his further progress in this branch of astronomy will be greatly facilitated. the great bear is, indeed, a splendid constellation, and its only rival is to be found in orion, which contains more brilliant stars, though it does not occupy so large a region in the heavens. [illustration: fig. .--the great bear and pole star.] [illustration: fig. .--the great bear and cassiopeia.] in the first place, we observe how the great bear enables the pole star, which is the most important object in the northern heavens, to be readily found. the pole star is very conveniently indicated by the direction of the two stars, b and a, of the great bear, which are, accordingly, generally known as the "pointers." this use of the great bear is shown on the diagram in fig. , in which the line b a, produced onwards and slightly curved, will conduct to the pole star. there is no likelihood of making any mistake in this star, as it is the only bright one in the neighbourhood. once it has been seen it will be readily identified on future occasions, and the observer will not fail to notice how constant is the position which it preserves in the heavens. the other stars either rise or set, or, like the great bear, they dip down low in the north without actually setting, but the pole star exhibits no considerable changes. in summer or winter, by night or by day, the pole star is ever found in the same place--at least, so far as ordinary observation is concerned. no doubt, when we use the accurate instruments of the observatory the notion of the fixity of the pole star is abandoned; we then see that it has a slow motion, and that it describes a small circle every twenty-four hours around the true pole of the heavens, which is not coincident with the pole star, though closely adjacent thereto. the distance is at present a little more than a degree, and it is gradually lessening, until, in the year a.d. , the distance will be under half a degree. the pole star itself belongs to another inconsiderable group of stars known as the little bear. the two principal members of this group, next in brightness to the pole star, are sometimes called the "guards." the great bear and the little bear, with the pole star, form a group in the northern sky not paralleled by any similarly situated constellation in the southern heavens. at the south pole there is no conspicuous star to indicate its position approximately--a circumstance disadvantageous to astronomers and navigators in the southern hemisphere. it will now be easy to add a third constellation to the two already acquired. on the opposite side of the pole star to the great bear, and at about the same distance, lies a very pleasing group of five bright stars, forming a w. these are the more conspicuous members of the constellation cassiopeia, which contains altogether about sixty stars visible to the naked eye. when the great bear is low down in the north, then cassiopeia is high overhead. when the great bear is high overhead, then cassiopeia is to be looked for low down in the north. the configuration of the leading stars is so striking that once the eye has recognised them future identification will be very easy--the more so when it is borne in mind that the pole star lies midway between cassiopeia and the great bear (fig. ). these important constellations will serve as guides to the rest. we shall accordingly show how the learner may distinguish the various other groups visible from the british islands or similar northern latitudes. the next constellation to be recognised is the imposing group which contains the great square of pegasus. this is not, like ursa major, or like cassiopeia, said to be "circumpolar." the great square of pegasus sets and rises daily. it cannot be seen conveniently during the spring and the summer, but in autumn and in winter the four stars which mark the corners of the square can be easily recognised. there are certain small stars within the region so limited; perhaps about thirty can be counted by an unaided eye of ordinary power in these latitudes. in the south of europe, with its pure and bright skies, the number of visible stars appears to be greatly increased. an acute observer at athens has counted in the same region. [illustration: fig. .--the great square of pegasus.] the great square of pegasus can be reached by a line from the pole star over the end of cassiopeia. if it be produced about as far again it will conduct the eye to the centre of the great square of pegasus (fig. ). the line through b and a in pegasus continued ° to the south points out the important star fomalhaut in the mouth of the southern fish. to the right of this line, nearly half-way down, is the rather vague constellation of aquarius, where a small equilateral triangle with a star in the centre may be noticed. the square of pegasus is not a felicitous illustration of the way in which the boundaries of the constellations should be defined. there can be no more naturally associated group than the four stars of this square, and they ought surely to be included in the same constellation. three of the stars--marked a, b, g--do belong to pegasus; but that at the fourth corner--also marked a--is placed in a different figure, known as andromeda, whereof it is, indeed, the brightest member. the remaining bright stars of andromeda are marked b and g, and they are readily identified by producing one side of the square of pegasus in a curved direction. we have thus a remarkable array of seven stars, which it is both easy to identify and easy to remember, notwithstanding that they are contributed to by three different constellations. they are respectively a, b, and g of pegasus; a, b, and g of andromeda; and a of perseus. the three form a sort of handle, as it were, extending from one side of the square, and are a group both striking in appearance, and useful in the further identification of celestial objects. b andromedæ, with two smaller stars, form the girdle of the unfortunate heroine. a persei lies between two other stars (g and d) of the same constellation. if we draw a curve through these three and prolong it in a bold sweep, we are conducted to one of the gems of the northern heavens--the beautiful star capella, in auriga (fig. ). close to capella are three small stars forming an isosceles triangle--these are the hoedi or kids. capella and vega are, with the exception of arcturus, the two most brilliant stars in the northern heavens; and though vega is probably the more lustrous of the two, yet the opposite opinion has been entertained. different eyes will frequently form various estimates of the relative brilliancy of stars which approach each other in brightness. the difficulty of making a satisfactory comparison between vega and capella is greatly increased by the wide distance in the heavens at which they are separated, as well as by a slight difference in colour, for vega is distinctly whiter than capella. this contrast between the colour of stars is often a source of uncertainty in the attempt to compare their relative brilliancy; so that when actual measurements have to be effected by instrumental means, it is necessary to compare the two stars alternately with some object of intermediate hue. [illustration: fig. .--perseus and its neighbouring stars.] on the opposite side of the pole to capella, but not quite so far away, will be found four small stars in a quadrilateral. they form the head of the dragon, the rest of whose form coils right round the pole. if we continue the curve formed by the three stars g, a, and d in perseus, and if we bend round this curve gracefully into one of an opposite flexion, in the manner shown in fig. , we are first conducted to two other principal stars in perseus, marked e and z. the region of perseus is one of the richest in the heavens. we have here a most splendid portion of the milky way, and the field of the telescope is crowded with stars beyond number. even a small telescope or an opera-glass directed to this teeming constellation cannot fail to delight the observer, and convey to him a profound impression of the extent of the sidereal heavens. we shall give in a subsequent paragraph a brief enumeration of some of the remarkable telescopic objects in perseus. pursuing in the same figure the line e and z, we are conducted to the remarkable little group known as the pleiades. [illustration: fig. .--the pleiades.] the pleiades form a group so universally known and so easily identified that it hardly seems necessary to give any further specific instructions for their discovery. it may, however, be observed that in these latitudes they cannot be seen before midnight during the summer. let us suppose that the search is made at about p.m. at night: on the st of january the pleiades will be found high up in the sky in the south-west; on the st of march, at the same hour, they will be seen to be setting in the west. on the st of may they are not visible; on the st of july they are not visible; on the st of september they will be seen low down in the east. on the st of november they will be high in the heavens in the south-east. on the ensuing st of january the pleiades will be in the same position as they were on the same date in the previous year, and so on from year to year. it need, perhaps, hardly be explained here that these changes are not really due to movements of the constellations; they are due, of course, to the apparent annual motion of the sun among the stars. [illustration: fig. .--orion, sirius, and the neighbouring stars.] the pleiades are shown in the figure (fig. ), where a group of ten stars is represented, this being about the number visible with the unaided eye to those who are gifted with very acute vision. the lowest telescopic power will increase the number of stars to thirty or forty (galileo saw more than forty with his first telescope), while with telescopes of greater power the number is largely increased; indeed, no fewer than have been counted with the aid of a powerful telescope. the group is, however, rather too widely scattered to make an effective telescopic object, except with a large field and low power. viewed through an opera-glass it forms a very pleasing spectacle. [illustration: fig. .--castor and pollux.] if we draw a ray from the pole star to capella, and produce it sufficiently far, as shown in fig. , we come to the great constellation of our winter sky, the splendid group of orion. the brilliancy of the stars in orion, the conspicuous belt, and the telescopic objects which it contains, alike render this group remarkable, and place it perhaps at the head of the constellations. the leading star in orion is known either as a orionis, or as betelgeuze, by which name it is here designated. it lies above the three stars, d, e, z, which form the belt. betelgeuze is a star of the first magnitude, and so also is rigel, on the opposite side of the belt. orion thus enjoys the distinction of containing two stars of the first magnitude in its group, while the five other stars shown in fig. are of the second magnitude. the neighbourhood of orion contains some important stars. if we carry on the line of the belt upwards to the right, we are conducted to another star of the first magnitude, aldebaran, which strongly resembles betelgeuze in its ruddy colour. aldebaran is the brightest star in the constellation of taurus. it is this constellation which contains the pleiades already referred to, and another more scattered group known as the hyades, which can be discovered near aldebaran. [illustration: fig. .--the great bear and the lion.] the line of the belt of orion continued downwards to the left conducts the eye to the gem of the sky, the splendid sirius, which is the most brilliant star in the heavens. it has, indeed, been necessary to create a special order of magnitude for the reception of sirius alone; all the other first magnitude stars, such as vega and capella, betelgeuze and aldebaran, coming a long way behind. sirius, with a few other stars of much less lustre, form the constellation of canis major. it is useful for the learner to note the large configuration, of an irregular lozenge shape, of which the four corners are the first magnitude stars, aldebaran, betelgeuze, sirius, and rigel (fig. ). the belt of orion is placed symmetrically in the centre of the group, and the whole figure is so striking that once perceived it is not likely to be forgotten. about half way from the square of pegasus to aldebaran is the chief star in the ram--a bright orb of the second magnitude; with two others it forms a curve, at the other end of which will be found g of the same constellation, which was the first double star ever noticed. we can again invoke the aid of the great bear to point out the stars in the constellation of gemini (fig. ). if the diagonal joining the stars d and b of the body of the bear be produced in the direction opposite to the tail, it will lead to castor and pollux, two remarkable stars of the second magnitude. this same line carried a little further on passes near the star procyon, of the first magnitude, which is the only conspicuous object in the constellation of the little dog. [illustration: fig. .--boötes and the crown.] [illustration: fig. .--virgo and the neighbouring constellations.] the pointers in the great bear marked a b will also serve to indicate the constellation of the lion. if we produce the line joining them in the direction opposite from that used in finding the pole, we are brought into the body of the lion. this group will be recognised by the star of the first magnitude called regulus. it is one of a series of stars forming an object somewhat resembling a sickle: three of the group are of the second magnitude. the sickle has a special claim on our notice because it contains the radiant point from which the periodic shooting star shower known as the leonids diverges. regulus lies alongside the sun's highway through the stars, at a point which he passes on the st of august every year. between gemini and leo the inconspicuous constellation of the crab may be found; the most striking object it contains is the misty patch called præsepe or the bee-hive, which the smallest opera-glass will resolve into its component stars. [illustration: fig. .--the constellation of lyra.] the tail of the great bear, when prolonged with a continuation of the curve which it possesses, leads to a brilliant star of the first magnitude known as arcturus, the principal star in the constellation of boötes (fig. ). a few other stars, marked b, g, d, and e in the same constellation, are also shown in the figure. among the stars visible in these latitudes arcturus is to be placed next to sirius in point of brightness. two stars in the southern hemisphere, invisible in these latitudes, termed a centauri and canopus, are nearly as bright as vega and capella, but not quite as bright as arcturus. in the immediate neighbourhood of boötes is a striking semicircular group known as the crown or corona borealis. it will be readily found from its position as indicated in the figure, or it may be identified by following the curved line indicated by b, d, e, and z in the great bear. [illustration: fig. .--vega, the swan, and the eagle.] the constellation of virgo is principally characterised by the first magnitude star called spica, or a virginis. this may be found from the great bear; for if the line joining the two stars a and g in that constellation be prolonged with a slight curve, it will conduct the eye to spica. we may here notice another of those large configurations which are of great assistance in the study of the stars. there is a fine equilateral triangle, whereof arcturus and spica form two of the corners, while the third is indicated by denebola, the bright star near the tail of the lion (fig. ). in the summer evenings when the crown is overhead, a line from the pole star through its fainter edge, continued nearly to the southern horizon, encounters the brilliant red star cor scorpionis, or the scorpion's heart (antares), which was the first star mentioned as having been seen with the telescope in the daytime. the first magnitude star, vega, in the constellation of the lyre, can be readily found at the corner of a bold triangle, of which the pole star and arcturus form the base (fig. ). the brilliant whiteness of vega will arrest the attention, while the small group of neighbouring stars which form the lyre produces one of the best defined constellations. near vega is another important constellation, known as the swan or cygnus. the brightest star will be identified as the vertex of a right-angled triangle, of which the line from vega to the pole star is the base, as shown in fig. . there are in cygnus five principal stars, which form a constellation of rather remarkable form. the last constellation which we shall here describe is that of aquila or the eagle, which contains a star of the first magnitude, known as altair; this group can be readily found by a line from vega over b cygni, which passes near the line of three stars, forming the characteristic part of the eagle. we have taken the opportunity to indicate in these sketches of the constellations the positions of some other remarkable telescopic objects, the description of which we must postpone to the following chapters. chapter xix. the distant suns. sirius contrasted with the sun--stars can be weighed, but not in general measured--the companion of sirius--determination of the weights of sirius and his companion--dark stars--variable and temporary stars--enormous number of stars. the splendid pre-eminence of sirius has caused it to be observed with minute care from the earliest times in the history of astronomy. each generation of astronomers devoted time and labour to determine the exact places of the brightest stars in the heavens. a vast mass of observations as to the place of sirius among the stars had thus been accumulated, and it was found that, like many other stars, sirius had what astronomers call _proper motion_. comparing the place of sirius with regard to the other stars now with the place which it occupied one hundred years ago, there is a difference of two minutes ( ") in its situation. this is a small quantity: it is so small that the unaided eye could not see it. could we now see the sky as it appeared one century ago, we should still see this star in its well-known place to the left of orion. careful alignment by the eye would hardly detect that sirius was moving in two, or even in three or in four centuries. but the accuracy of the meridian circle renders these minute quantities evident, and gives to them their true significance. to the eye of the astronomer, sirius, instead of creeping along with a movement which centuries will not show, is pursuing its majestic course with a velocity appropriate to its dimensions. though the velocity of sirius is _about_ , miles a minute, yet it is sometimes a little more and sometimes a little less than its mean value. to the astronomer this fact is pregnant with information. were sirius an isolated star, attended only by planets of comparative insignificance, there could be no irregularity in its motion. if it were once started with a velocity of , miles a minute, then it must preserve that velocity. neither the lapse of centuries nor the mighty length of the journey could alter it. the path of sirius would be inflexible in its direction; and it would be traversed with unalterable velocity. [illustration: fig. .--the orbit of sirius (professor burnham).] the fact that sirius had not been moving uniformly was of such interest that it arrested the attention of bessel when he discovered the irregularities in . believing, as bessel did, that there must be some adequate cause for these disturbances, it was hardly possible to doubt what the cause must be. when motion is disturbed there must be force in action, and the only force that we recognise in such cases is that known as gravitation. but gravity can only act from one body to another body; so that when we seek for the derangement of sirius by gravitation, we are obliged to suppose that there must be some mighty and massive body near sirius. the question was taken up again by peters and by auwers, who were able to discover, from the irregularities of sirius, the nature of the path of the disturbing body. they were able to show that it must revolve around sirius in a period of about fifty years, and although they could not tell its distance from sirius, yet they were able to point out the direction in which it must lie. fig. shows the orbit of sirius as given by mr. burnham, of yerkes observatory. the detection of the attendant of sirius, and the measures which have been made thereon, enable us to determine the weight of this famous star. let us attempt to illustrate this subject. it must, no doubt, be admitted that the numerical estimates we employ have to be received with a certain degree of caution. the companion of sirius is a difficult object to observe, and previous to it had only been followed through an arc of °. we are, therefore, hardly as yet in a position to speak with absolute accuracy as to the periodic time in which the companion completes its revolution. we may, however, take this time to be fifty-two years. we also know the distance from sirius to his companion, and we may take it to be about twenty-one times the distance from the earth to the sun. it is useful, in the first place, to compare the revolution of the companion around sirius with the revolution of the planet uranus around the sun. taking the earth's distance as unity, the radius of the orbit of uranus is about nineteen, and uranus takes eighty-four years to accomplish a complete revolution. we have no planet in the solar system at a distance of twenty-one; but from kepler's third law it may be shown that, if there were such a planet, its periodic time would be about ninety-nine years. we have now the necessary materials for making the comparison between the mass of sirius and the mass of the sun. a body revolving around sirius at a certain distance completes its journey in fifty-two years. to revolve around the sun at the same distance a body should complete its journey in ninety-nine years. the quicker the body is moving the greater must be the centrifugal force, and the greater must be the attractive power of the central body. it can be shown from the principles of dynamics that the attractive power is inversely proportional to the square of the periodic time. hence, then, the attractive power of sirius must bear to the attractive power of the sun the proportion which the square of ninety-nine has to the square of fifty-two. as the distances are in each case supposed to be equal, the attractive powers will be proportional to the masses, and hence we conclude that the mass of sirius, together with that of his companion, is to the mass of the sun, together with that of his planet, in the ratio of three and a half to one. we had already learned that sirius was much brighter than the sun; now we have learned that it is also much more massive. before we leave the consideration of sirius, there is one additional point of very great interest which it is necessary to consider. there is a remarkable contrast between the brilliancy of sirius and his companion. sirius is a star far transcending all other stars of the first magnitude, while his companion is extremely faint. even if it were completely withdrawn from the dazzling proximity of sirius, the companion would be only a small star of the eighth or ninth magnitude, far below the limits of visibility to the unaided eye. to put the matter in numerical language, sirius is , times as bright as its companion, but only about twice as heavy! here is a very great contrast; and this point will appear even more forcible if we contrast the companion of sirius with our sun. the companion is slightly heavier than our sun; but in spite of its slightly inferior bulk, our sun is much more powerful as a light-giver. one hundred of the companions of sirius would not give as much light as our sun! this is a result of very considerable significance. it teaches us that besides the great bodies in the universe which attract attention by their brilliancy, there are also other bodies of stupendous mass which have but little brilliancy--probably some of them possess none at all. this suggests a greatly enhanced conception of the majestic scale of the universe. it also invites us to the belief that the universe which we behold bears but a small ratio to the far larger part which is invisible in the sombre shades of night. in the wide extent of the material universe we have here or there a star or a mass of gaseous matter sufficiently heated to be luminous, and thus to become visible from the earth; but our observation of these luminous points can tell us little of the remaining contents of the universe. the most celebrated of all the variable stars is that known as algol, whose position in the constellation of perseus is shown in fig. . this star is conveniently placed for observation, being visible every night in our latitude, and its interesting changes can be observed without any telescopic aid. everyone who desires to become acquainted with the great truths of astronomy should be able to recognise this star, and should have also followed it during one of its periods of change. algol is usually a star of the second magnitude; but in a period between two and three days, or, more accurately, in an interval of days hours minutes and seconds, its brilliancy goes through a most remarkable cycle of variations. the series commences with a gradual decline of the star's brightness, which in the course of four and a half hours falls from the second magnitude down to the fourth. at this lowest stage of brightness algol remains for about twenty minutes, and then begins to increase, until in three and a half hours it regains the second magnitude, at which it continues for about days hours, when the same series commences anew. it seems that the period required by algol to go through its changes is itself subject to a slow but certain variation. we shall see in a following chapter how it has been proved that the variability of algol is due to the occasional interposition of a dark companion which cuts off a part of the lustre of the star. all the circumstances can thus be accounted for, and even the weight and the size of algol and its dark companion be determined. there are, however, other classes of variable stars, the fluctuation of whose light can hardly be due to occasional obscuration by dark bodies. this is particularly the case with those variables which are generally faint, but now and then flare up for a short time, after which temporary exaltation they again sink down to their original condition. the periods of such changes are usually from six months to two years. the best known example of a star of this class was discovered more than three hundred years ago. it is situated in the constellation cetus, a little south of the equator. this object was the earliest known case of a variable star, except the so-called temporary stars, to which we shall presently refer. the variable in cetus received the name of mira, or the wonderful. the period of the fluctuations of mira ceti is about eleven months, during the greater part of which time the star is of the ninth magnitude, and consequently invisible to the naked eye. when the proper time has arrived, its brightness begins to increase rather suddenly. it soon becomes a conspicuous object of the second or third magnitude. in this condition it remains for eight or ten days, and then declines more slowly than it rose until it is reduced to its original faintness, about three hundred days after the rise commenced. more striking to the general observer than the ordinary variable stars are the _temporary stars_ which on rare occasions suddenly make their appearance in the heavens. the most famous object of this kind was that which blazed out in the beginning of november, , and which when first seen was as bright as venus at its maximum brightness. it could, indeed, be seen in full daylight by sharp-sighted people. as far as history can tell us, no other temporary star has ever been as bright as this one. it is specially associated with the name of tycho brahe, for although he was not the discoverer, he made the best observations of the object, and he proved that it was at a distance comparable with that of the ordinary fixed stars. tycho described carefully the gradual decline of the wonderful star until it disappeared from his view about the end of march, , for the telescope, by which it could doubtless have been followed further, had not yet been invented. during the decline the colour of the object gradually changed; at first it was white, and by degrees became yellow, and in the spring of reddish, like aldebaran. about may, , we are told somewhat enigmatically that it "became like lead, or somewhat like saturn," and so it remained as long as it was visible. what a fund of information our modern spectroscopes and other instruments would supply us with if so magnificent a star were to burst out in these modern days! but though we have not in our own times been favoured with a view of a temporary star as splendid as the one seen by tycho brahe and his contemporaries, it has been our privilege to witness several minor outbursts of this kind. it seems likely that we should possess more records of temporary stars from former times if a better watch had been kept for them. that is at any rate the impression we get when we see how several of the modern stars of this kind have nearly escaped us altogether, notwithstanding the great number of telescopes which are now pointed to the sky on every clear night. in a star of the second magnitude suddenly appeared in the constellation of the crown (corona borealis). it was first seen on the th may, and a few days afterwards it began to fade away. argelander's maps of the northern heavens had been published some years previously, and when the position of the new star had been accurately determined, it was found that it was identical with an insignificant looking star marked on one of the maps as of the - / magnitude. the star exists in the same spot to this day, and it is of the same magnitude as it was prior to its spasmodic outburst in . this was the first new star which was spectroscopically examined. we shall give in chapter xxiii. a short account of the features of its spectrum. the next of these temporary bright stars, nova cygni, was first seen by julius schmidt at athens on the th november, , when it was between the third and fourth magnitudes, and he maintains that it cannot have been conspicuous four days earlier, when he was looking at the same constellation. by some inadvertence the news of the discovery was not properly circulated, and the star was not observed elsewhere for about ten days, when it had already become considerably fainter. the decrease of brightness went on very slowly; in october, , the star was only of the tenth magnitude, and it continued getting fainter until it reached the fifteenth magnitude; in other words, it became a minute telescopic star, and it is so still in the very same spot. as this star did not reach the first or second magnitude it would probably have escaped notice altogether if schmidt had not happened to look at the swan on that particular evening. we are not so likely to miss seeing a new star since astronomers have pressed the photographic camera into their service. this became evident in , when the last conspicuous temporary star appeared in auriga. on the th january, dr. anderson, an astronomer in edinburgh, noticed a yellowish star of the fifth magnitude in the constellation auriga, and a week later, when he had compared a star-map with the heavens and made sure that the object was really a new star, he made his discovery public. in the case of this star we are able to fix fairly closely the moment when it first blazed out. in the course of the regular photographic survey of the heavens undertaken at the harvard college observatory (cambridge, massachusetts) the region of the sky where the new star appeared had been photographed on thirteen nights from october st to december st, , and on twelve nights from december th to january th, . on the first series of plates there was no trace of the nova, while it was visible on the very first plate of the second series as a star of the fifth magnitude. fortunately it turned out that professor max wolf of heidelberg, a most successful celestial photographer, had photographed the same region on the th december, and this photograph does not show the star, so that it cannot on that night have been as bright as the ninth magnitude. nova auriga must therefore have flared up suddenly between the th and the th of december. according to the harvard photographs, the first maximum of brightness occurred about the th of december, when the magnitude was - / . the decrease of the brightness was very irregular; the star fluctuated for the five weeks following the first of february between the fourth and the sixth magnitude, but after the beginning of march, , the brightness declined very rapidly, and at the end of april the star was seen as an exceedingly faint one (sixteenth magnitude) with the great lick refractor. when this mighty instrument was again pointed to the nova in the following august, it had risen nearly to the tenth magnitude, after which it gradually became extremely faint again, and is so still. the temporary and the variable stars form but a very small section of the vast number of stars with which the vault of the heavens is studded. that the sun is no more than a star, and the stars are no less than suns, is a cardinal doctrine of astronomy. the imposing magnificence of this truth is only realised when we attempt to estimate the countless myriads of stars. this is a problem on which our calculations are necessarily vain. let us, therefore, invoke the aid of the poet to attempt to express the innumerable, and conclude this chapter with the following lines of mr. allingham:-- "but number every grain of sand, wherever salt wave touches land; number in single drops the sea; number the leaves on every tree, number earth's living creatures, all that run, that fly, that swim, that crawl; of sands, drops, leaves, and lives, the count add up into one vast amount, and then for every separate one of all those, let a flaming sun whirl in the boundless skies, with each its massy planets, to outreach all sight, all thought: for all we see encircled with infinity, is but an island." chapter xx. double stars. interesting stellar objects--stars optically double--the great discovery of the binary stars made by herschel--the binary stars describe elliptic paths--why is this so important?--the law of gravitation--special double stars--castor--mizar--the coloured double stars--b cygni. the sidereal heavens contain few more interesting objects for the telescope than can be found in the numerous class of double stars. they are to be counted in thousands; indeed, _many_ thousands can be found in the catalogues devoted to this special branch of astronomy. many of these objects are, no doubt, small and comparatively uninteresting, but some of them are among the most conspicuous stars in the heavens, such as sirius, whose system we have already described. we shall in this brief account select for special discussion and illustration a few of the more remarkable double stars. we shall particularly notice some of those that can be readily observed with a small telescope, and we have indicated on the sketches of the constellations in a previous chapter how the positions of these objects in the heavens can be ascertained. it had been shown by cassini in that certain stars, which appeared to the unaided eye as single points of light, really consisted of two or more stars, so close together that the telescope was required for their separation.[ ] the number of these objects was gradually increased by fresh discoveries, until in (the same year in which herschel discovered uranus) a list containing eighty double stars was published by the astronomer bode. these interesting objects claimed the attention of herschel during his memorable researches. the list of known doubles rapidly swelled. herschel's discoveries are to be enumerated by hundreds, while he also commenced systematic measurements of the distance by which the stars were separated, and the direction in which the line joining them pointed. it was these measurements which ultimately led to one of the most important and instructive of all herschel's discoveries. when, in the course of years, his observations were repeated, herschel found that in some cases the relative position of the stars had changed. he was thus led to the discovery that in many of the double stars the components are so related that they revolve around each other. mark the importance of this result. we must remember that the stars are suns, comparable, it may be, with our sun in magnitude; so that here we have the astonishing spectacle of pairs of suns in mutual revolution. there is nothing very surprising in the fact that movements should be observed, for in all probability every body in the universe is in motion. it is the particular character of the movement which is specially interesting and instructive. it had been imagined that the proximity of the two stars forming a double must be only accidental. it was thought that amid the vast host of stars in the heavens it not unfrequently happened that one star was so nearly behind another (as seen from the earth) that when the two were viewed in the telescope they produced the effect of a double star. no doubt many of the so-called double stars are produced in this way. herschel's discovery shows that this explanation will not always answer, but that in many cases we really have two stars close together, and in motion round their common centre of gravity. when the measurements of the distances and the positions of double stars had been accumulated during many years, they were taken over by the mathematicians to be treated by their methods. there is one peculiarity about double star observations: they have not--they cannot have--the accuracy which the computer of an orbit demands. if the distance between the pair of stars forming a binary be four seconds, the orbit we have to scrutinise is only as large as the apparent size of a penny-piece at the distance of one mile. it would require very careful measurement to make out the form of a penny a mile off, even with good telescopes. if the penny were tilted a little, it would appear, not circular, but oval; and it would be possible, by measuring this oval, to determine how much the penny was tilted. all this requires skilful work: the errors, viewed intrinsically, may not be great, but viewed with reference to the whole size of the quantities under consideration, they are very appreciable. we therefore find the errors of observation far more prominent in observations of this class than is generally the case when the mathematician assumes the task of discussing the labours of the observer. the interpretation of herschel's discovery was not accomplished by himself; the light of mathematics was turned on his observations of the binary stars by savary, and afterwards by other mathematicians. under their searching enquiries the errors of the measurements were disclosed, and the observations were purified from the grosser part of their inaccuracy. mathematicians could then apply to their corrected materials the methods of enquiry with which they were familiar; they could deduce with fair precision the actual shape of the orbit of the binary stars, and the position of the plane in which that orbit is contained. the result is not a little remarkable. it has been proved that the motion of each of the stars is performed in an ellipse which contains the centre of gravity of the two stars in its focus. this has been actually shown to be true in many binary stars; it is believed to be true in all. but why is this so important? is not motion in an ellipse common enough? does not the earth revolve in an ellipse round the sun? and do not the planets also revolve in ellipses? it is this very fact that elliptic motion is so common in the planets of the solar system which renders its discovery in binary stars of such importance. from what does the elliptic motion in the solar system arise? is it not due to the law of attraction, discovered by newton, which states that every mass attracts every other mass with a force which varies inversely as the square of the distance? that law of attraction had been found to pervade the whole solar system, and it explained the movements of the bodies of our system with marvellous fidelity. but the solar system, consisting of the sun, and the planets, with their satellites, the comets, and a host of smaller bodies, formed merely a little island group in the universe. in the economy of this tiny cosmical island the law of gravitation reigns supreme; before herschel's discovery we never could have known whether that law was not merely a piece of local legislation, specially contrived for the exigencies of our particular system. this discovery gave us the knowledge which we could have gained from no other source. from the binary stars came a whisper across the vast abyss of space. that whisper told us that the law of gravitation was not peculiar to the solar system. it told us the law extended to the distant shores of the abyss in which our island is situated. it gives us grounds for believing that the law of gravitation is obeyed throughout the length, breadth, and depth of the entire visible universe. one of the finest binary stars is that known as castor, the brighter of the two principal stars in the constellation of gemini. the position of castor on the heavens is indicated in fig. , page . viewed by the unaided eye, castor resembles a single star; but with a moderately good telescope it is found that what seems to be one star is really two separate stars, one of which is of the third magnitude, while the other is somewhat less. the angular distance of these two stars in the heavens is not so great as the angle subtended by a line an inch long viewed at a distance of half a mile. castor is one of the double stars in which the components have been observed to possess a motion of revolution. the movement is, however, extremely slow, and the lapse of centuries will be required before a revolution is completely effected. a beautiful double star can be readily identified in the constellation of ursa major (_see_ fig. , page ). it is known as mizar, and is the middle star (z) of the three which form the tail. in the close neighbourhood of mizar is the small star alcor, which can be readily seen with the unaided eye; but when we speak of mizar as a double star, it is not to be understood that alcor is one of the components of the double. under the magnifying power of the telescope alcor is seen to be transferred a long way from mizar, while mizar itself is split up into two suns close together. these components are of the second and the fourth magnitudes respectively, and as the apparent distance is nearly three times as great as in castor, they are observed with facility even in a small telescope. this is, indeed, the best double star in the heavens for the beginner to commence his observations upon. we cannot, however, assert that mizar is a binary, inasmuch as observations have not yet established the existence of a motion of revolution. still less are we able to say whether alcor is also a member of the same group, or whether it may not merely be a star which happens to fall nearly in the line of vision. recent spectroscopic observations have shown that the larger component of mizar is itself a double, consisting of a pair of suns so close together that there is not the slightest possibility of their ever being seen separately by the most powerful telescope in the world. a pleasing class of double stars is that in which we have the remarkable phenomenon of colours, differing in a striking degree from the colours of ordinary stars. among the latter we find, in the great majority of cases, no very characteristic hue; some are, however, more or less tinged with red, some are decidedly ruddy, and some are intensely red. stars of a bluish or greenish colour are much more rare,[ ] and when a star of this character does occur, it is almost invariably as one of a pair which form a double. the other star of the double is sometimes of the same hue, but more usually it is yellow or ruddy. one of the loveliest of these objects, which lies within reach of telescopes of very moderate pretensions, is that found in the constellation of the swan, and known as b cygni (fig. ). this exquisite object is composed of two stars. the larger, about the third magnitude, is of a golden-yellow, or topaz, colour; the smaller, of the sixth magnitude, is of a light blue. these colours are nearly complementary, but still there can be no doubt that the effect is not merely one of contrast. that these two stars are both tinged with the hues we have stated can be shown by hiding each in succession behind a bar placed in the field of view. it has also been confirmed in a very striking manner by spectroscopic investigation; for we see that the blue star has experienced a special absorption of the red rays, while the more ruddy light of the other star has arisen from the absorption of the blue rays. the contrast of the colours in this object can often be very effectively seen by putting the eye-piece out of focus. the discs thus produced show the contrast of colours better than when the telescope exhibits merely two stellar points. such are a few of these double and multiple stars. their numbers are being annually augmented; indeed, one observer--mr. burnham, formerly on the staff of the lick observatory, and now an observer in the yerkes observatory--has added by his own researches more than , new doubles to the list of those previously known. the interest in this class of objects must necessarily be increased when we reflect that, small as the stars appear to be in our telescopes, they are in reality suns of great size and splendour, in many cases rivalling our own sun, or, perhaps, even surpassing him. whether these suns have planets attending upon them we cannot tell; the light reflected from the planet would be utterly inadequate to the penetration of the vast extent of space which separates us from the stars. if there be planets surrounding these objects, then, instead of a single sun, such planets will be illuminated by two, or, perhaps, even more suns. what wondrous effects of light and shade must be the result! sometimes both suns will be above the horizon together, sometimes only one sun, and sometimes both will be absent. especially remarkable would be the condition of a planet whose suns were of the coloured type. to-day we have a red sun illuminating the heavens, to-morrow it would be a blue sun, and, perhaps, the day after both the red sun and the blue sun will be in the firmament together. what endless variety of scenery such a thought suggests! there are, however, grave dynamical reasons for doubting whether the conditions under which such a planet would exist could be made compatible with life in any degree resembling the life with which we are familiar. the problem of the movement of a planet under the influence of two suns is one of the most difficult that has ever been proposed to mathematicians, and it is, indeed, impossible in the present state of analysis to solve with accuracy all the questions which it implies. it seems not at all unlikely that the disturbances of the planet's orbit would be so great that it would be exposed to vicissitudes of light and of temperature far transcending those experienced by a planet moving, like the earth, under the supreme control of a single sun. chapter xxi. the distances of the stars. sounding-line for space--the labours of bessel--meaning of annual parallax--minuteness of the parallactic ellipse illustrated--the case of cygni--different comparison stars used--the proper motion of the star--struve's investigations--can they be reconciled?--researches at dunsink--conclusion obtained--accuracy which such observations admit examined--the proper motion of cygni--the permanence of the sidereal heavens--the new star in cygnus--its history--no appreciable parallax--a mighty outburst of light--the movement of the solar system through space--herschel's discovery--journey towards lyra--probabilities. we have long known the dimensions of the solar system with more or less accuracy. our knowledge includes the distances of the planets and the comets from the sun, as well as their movements. we have also considerable knowledge of the diameters and the masses of many of the different bodies which belong to the solar system. we have long known, in fact, many details of the isolated group nestled together under the protection of the sun. the problem for consideration in the present chapter involves a still grander survey than is required for measures of our solar system. we propose to carry the sounding-line across the vast abyss which separates the group of bodies closely associated about our sun from the other stars which are scattered through the realms of space. for centuries the great problem of star distance has engaged the attention of those who have studied the heavens. it would be impossible to attempt here even an outline of the various researches which have been made on the subject. in the limited survey which we can make, we must glance first at the remarkable speculative efforts which have been directed to the problem, and then we shall refer to those labours which have introduced the problem into the region of accurate astronomy. no attempt to solve the problem of the absolute distances of the stars was successful until many years after herschel's labours were closed. fresh generations of astronomers, armed with fresh appliances, have for many years pursued the subject with unremitting diligence, but for a long time the effort seemed hopeless. the distances of the stars were so great that they could not be ascertained until the utmost refinements of mechanical skill and the most elaborate methods of mathematical calculation were brought to converge on the difficulty. at last it was found that the problem was beginning to yield. a few stars have been induced to disclose the secret of their distance. we are able to give some answer to the question--how far are the stars? though it must be confessed that our reply up to the present moment is both hesitating and imperfect. even the little knowledge which has been gained possesses interest and importance. as often happens in similar cases, the discovery of the distance of a star was made independently about the same time by two or three astronomers. the name of bessel stands out conspicuously in this memorable chapter of astronomy. bessel proved ( ) that the distance of the star known as cygni was a measurable quantity. his demonstration possessed such unanswerable logic that universal assent could not be withheld. almost simultaneously with the classical labours of bessel we have struve's measurement of the distance of vega, and henderson's determination of the distance of the southern star a centauri. great interest was excited in the astronomical world by these discoveries, and the royal astronomical society awarded its gold medal to bessel. it appropriately devolved on sir john herschel to deliver the address on the occasion of the presentation of the medal: that address is a most eloquent tribute to the labours of the three astronomers. we cannot resist quoting the few lines in which sir john said:-- "gentlemen of the royal astronomical society,--i congratulate you and myself that we have lived to see the great and hitherto impassable barrier to our excursion into the sidereal universe, that barrier against which we have chafed so long and so vainly--_æstuantes angusto limite mundi_--almost simultaneously overleaped at three different points. it is the greatest and most glorious triumph which practical astronomy has ever witnessed. perhaps i ought not to speak so strongly; perhaps i should hold some reserve in favour of the bare possibility that it may be all an illusion, and that future researches, as they have repeatedly before, so may now fail to substantiate this noble result. but i confess myself unequal to such prudence under such excitement. let us rather accept the joyful omens of the time, and trust that, as the barrier has begun to yield, it will speedily be effectually prostrated." before proceeding further, it will be convenient to explain briefly how the distance of a star can be measured. the problem is one of a wholly different character from that of the sun's distance, which we have already discussed in these pages. the observations for the determination of stellar parallax are founded on the familiar truth that the earth revolves around the sun. we may for our present purpose assume that the earth revolves in a circular path. the centre of that path is at the centre of the sun, and the radius of the path is , , miles. owing to our position on the earth, we observe the stars from a point of view which is constantly changing. in summer the earth is , , miles distant from the position which it occupied in winter. it follows that the apparent positions of the stars, as projected on the background of the sky, must present corresponding changes. we do not now mean that the actual positions of the stars are really displaced. the changes are only apparent, and while oblivious of our own motion, which produces the displacements, we attribute the changes to the stars. on the diagram in fig. is an ellipse with certain months--viz., january, april, july, october--marked upon its circumference. this ellipse may be regarded as a miniature picture of the earth's orbit around the sun. in january the earth is at the spot so marked; in april it has moved a quarter of the whole journey; and so on round the whole circle, returning to its original position in the course of one year. when we look from the position of the earth in january, we see the star a projected against the point of the sky marked . three months later the observer with his telescope is carried round to april; but he now sees the star projected to the position marked . thus, as the observer moves around the whole orbit in the annual revolution of the earth, so the star appears to move round in an ellipse on the background of the sky. in the technical language of astronomers, we speak of this as the parallactic ellipse, and it is by measuring the major axis of this ellipse that we determine the distance of the star from the sun. half of this major axis, or, what comes to the same thing, the angle which the radius of the earth's orbit subtends as seen from the star, is called the star's "annual parallax." [illustration: fig. .--the parallactic ellipse.] the figure shows another star, b, more distant from the earth and the solar system generally than the star previously considered. this star also describes an elliptic path. we cannot, however, fail to notice that the parallactic ellipse belonging to b is much smaller than that of a. the difference in the sizes of the ellipses arises from the different distances of the stars from the earth. the nearer the star is to the earth the greater is the ellipse, so that the nearest star in the heavens will describe the largest ellipse, while the most distant star will describe the smallest ellipse. we thus see that the distance of the star is inversely proportional to the size of the ellipse, and if we measure the angular value of the major axis of the ellipse, then, by an exceedingly simple mathematical manipulation, the distance of the star can be expressed as a multiple of a radius of the earth's orbit. assuming that radius to be , , miles, the distance of the star is obtained by simple arithmetic. the difficulty in the process arises from the fact that these ellipses are so small that our micrometers often fail to detect them. how shall we adequately describe the extreme minuteness of the parallactic ellipses in the case of even the nearest stars? in the technical language of astronomers, we may state that the longest diameter of the ellipse never subtends an angle of more than one and a half seconds. in a somewhat more popular manner, we would say that one thousand times the major axis of the very largest parallactic ellipse would not be as great as the diameter of the full moon. for a still more simple illustration, let us endeavour to think of a penny-piece placed at a distance of two miles. if looked at edgeways it will be linear, if tilted a little it would be elliptic; but the ellipse would, even at that distance, be greater than the greatest parallactic ellipse of any star in the sky. suppose a sphere described around an observer, with a radius of two miles. if a penny-piece were placed on this sphere, in front of each of the stars, every parallactic ellipse would be totally concealed. the star in the swan known as cygni is not remarkable either for its size or for its brightness. it is barely visible to the unaided eye, and there are some thousands of stars which are apparently larger and brighter. it is, however, a very interesting example of that remarkable class of objects known as double stars. it consists of two nearly equal stars close together, and evidently connected by a bond of mutual attraction. the attention of astronomers is also specially directed towards the star by its large proper motion. in virtue of that proper motion, the two components are carried together over the sky at the rate of five seconds annually. a proper motion of this magnitude is extremely rare, yet we do not say it is unparalleled, for there are some few stars which have a proper motion even more rapid; but the remarkable duplex character of cygni, combined with the large proper motion, render it an unique object, at all events, in the northern hemisphere. when bessel proposed to undertake the great research with which his name will be for ever connected, he determined to devote one, or two, or three years to the continuous observations of one star, with the view of measuring carefully its parallactic ellipse. how was he to select the object on which so much labour was to be expended? it was all-important to choose a star which should prove sufficiently near to reward his efforts by exhibiting a measurable parallax. yet he could have but little more than surmise and analogy as a guide. it occurred to him that the exceptional features of cygni afforded the necessary presumption, and he determined to apply the process of observation to this star. he devoted the greater part of three years to the work, and succeeded in discovering its distance from the earth. since the date of sir john herschel's address, cygni has received the devoted and scarcely remitted attention of astronomers. in fact, we might say that each succeeding generation undertakes a new discussion of the distance of this star, with the view of confirming or of criticising the original discovery of bessel. the diagram here given (fig. ) is intended to illustrate the recent history of cygni. when bessel engaged in his labours, the pair of stars forming the double were at the point indicated on the diagram by the date . the next epoch occurred fifteen years later, when otto struve undertook his researches, and the pair of stars had by that time moved to the position marked . finally, when the same object was more recently observed at dunsink observatory, the pair had made still another advance, to the position indicated by the date . thus, in forty years this double star had moved over an arc of the heavens upwards of three minutes in length. the actual path is, indeed, more complicated than a simple rectilinear movement. the two stars which form the double have a certain relative velocity, in consequence of their mutual attraction. it will not, however, be necessary to take this into account, as the displacement thus arising in the lapse of a single year is far too minute to produce any inconvenient effect on the parallactic ellipse. [illustration: fig. .-- cygni and the comparison stars.] the case of cygni is, however, exceptional. it is one of our nearest neighbours in the heavens. we can never find its distance accurately to one or two billions of miles; but still we have a consciousness that an uncertainty amounting to twenty billions is too large a percentage of the whole. we shall presently show that we believe struve was right, yet it does not necessarily follow that bessel was wrong. the apparent paradox can be easily explained. it would not be easily explained if struve had used the _same comparison star_ as bessel had done; but struve's comparison star was different from either of bessel's, and this is probably the cause of the discrepancy. it will be recollected that the essence of the process consists of the comparison of the small ellipse made by the distant star with the larger ellipse made by the nearer star. if the two stars were at the same distance, the process would be wholly inapplicable. in such a case, no matter how near the stars were to the earth, no parallax could be detected. for the method to be completely successful, the comparison star should be at least eight times as far as the principal star. bearing this in mind, it is quite possible to reconcile the measures of bessel with those of struve. we need only assume that bessel's comparison stars are about three times as far as cygni, while struve's comparison star is at least eight or ten times as far. we may add that, as the comparison stars used by bessel are brighter than that of struve, there really is a presumption that the latter is the most distant of the three. we have here a characteristic feature of this method of determining parallax. even if all the observations and the reductions of a parallax series were mathematically correct, we could not with strict propriety describe the final result as the parallax of one star. it is only the _difference_ between the parallax of the star and that of the comparison star. we can therefore only assert that the parallax sought cannot be less than the quantity determined. viewed in this manner, the discrepancy between struve and bessel vanishes. bessel asserted that the distance of cygni could not be _more_ than sixty billions of miles. struve did not contradict this--nay, he certainly confirmed it--when he showed that the distance could not be more than forty billions. nearly half a century has elapsed since struve made his observations. those observations have certainly been challenged; but they are, on the whole, confirmed by other investigations. in a critical review of the subject auwers showed that struve's determination is worthy of considerable confidence. yet, notwithstanding this authoritative announcement, the study of cygni has been repeatedly resumed. dr. brünnow, when astronomer royal of ireland, commenced a series of observations on the parallax of cygni, which were continued and completed by the present writer, his successor. brünnow chose a fourth comparison star (marked on the diagram), different from any of those which had been used by the earlier observers. the method of observing which brünnow employed was quite different from that of struve, though the filar micrometer was used in both cases. brünnow sought to determine the parallactic ellipse by measuring the difference in declination between cygni and the comparison star.[ ] in the course of a year it is found that the difference in declination undergoes a periodic change, and from that change the parallactic ellipse can be computed. in the first series of observations i measured the difference of declination between the preceding star of cygni and the comparison star; in the second series i took the other component of cygni and the same comparison star. we had thus two completely independent determinations of the parallax resulting from two years' work. the first of these makes the distance forty billions of miles, and the second makes it almost exactly the same. there can be no doubt that this work supports struve's determination in correction of bessel's, and therefore we may perhaps sum up the present state of our knowledge of this question by saying that the distance of cygni is much nearer to the forty billions of miles which struve found than to the sixty billions which bessel found.[ ] it is desirable to give the reader the means of forming his own opinion as to the quality of the evidence which is available in such researches. the diagram in fig. here shown has been constructed with this object. it is intended to illustrate the second series of observations of difference of declination which i made at dunsink. each of the dots represents one night's observations. the height of the dot is the observed difference of declination between (b) cygni and the comparison star. the distance along the horizontal line--or the abscissa, as a mathematician would call it--represents the date. these observations are grouped more or less regularly in the vicinity of a certain curve. that curve expresses where the observations should have been, had they been absolutely perfect. the distances between the dots and the curve may be regarded as the errors which have been committed in making the observations. [illustration: fig. .--parallax in declination of cygni.] perhaps it will be thought that in many cases these errors appear to have attained very undesirable dimensions. let us, therefore, hasten to say that it was precisely for the purpose of setting forth these errors that this diagram has been shown; we have to exhibit the weakness of the case no less than its strength. the errors of the observations are not, however, intrinsically so great as might at first sight be imagined. to perceive this, it is only necessary to interpret the scale on which this diagram has been drawn by comparison with familiar standards. the distance from the very top of the curve to the horizontal line denotes an angle of only four-tenths of a second. this is about the apparent diameter of a penny-piece at a distance of _ten miles_! we can now appraise the true magnitude of the errors which have been made. it will be noticed that no one of the dots is distant from the curve by much more than half of the height of the curve. it thus appears that the greatest error in the whole series of observations amounts to but two or three tenths of a second. this is equivalent to our having pointed the telescope to the upper edge of a penny-piece fifteen or twenty miles off, instead of to the lower edge. this is not a great blunder. a rifle team whose errors in pointing were more than a hundred times as great might still easily win every prize at bisley. we have entered into the history of cygni with some detail, because it is the star whose distance has been most studied. we do not say that cygni is the nearest of all the stars; it would, indeed, be very rash to assert that any particular star was the nearest of all the countless millions in the heavenly host. we certainly know one star which seems nearer than cygni; it lies in one of the southern constellations, and its name is a centauri. this star is, indeed, of memorable interest in the history of the subject. its parallax was first determined at the cape of good hope by henderson; subsequent researches have confirmed his observations, and the elaborate investigations of dr. gill have proved that the parallax of this star is about three-quarters of a second, so that it is only two-thirds of the distance of cygni. cygni arrested our attention, in the first instance, by the circumstance that it had the large proper motion of five seconds annually. we have also ascertained that the annual parallax is about half a second. the combination of these two statements leads to a result of considerable interest. it teaches us that cygni must each year traverse a distance of not less than ten times the radius of the earth's orbit. translating this into ordinary figures, we learn that this star must travel nine hundred and twenty million miles per annum. it must move between two and three million miles each day, but this can only be accomplished by maintaining the prodigious velocity of thirty miles per second. there seems to be no escape from this conclusion. the facts which we have described, and which are now sufficiently well established, are inconsistent with the supposition that the velocity of cygni is less than thirty miles per second; the velocity may be greater, but less it cannot be. for the last hundred and fifty years we know that cygni has been moving in the same direction and with the same velocity. prior to the existence of the telescope we have no observation to guide us; we cannot, therefore, be absolutely certain as to the earlier history of this star, yet it is only reasonable to suppose that cygni has been moving from remote antiquity with a velocity comparable with that it has at present. if disturbing influences were entirely absent, there could be no trace of doubt about the matter. _some_ disturbing influence, however, there must be; the only question is whether that disturbing influence is sufficient to modify seriously the assumption we have made. a powerful disturbing influence might greatly alter the velocity of the star; it might deflect the star from its rectilinear course; it might even force the star to move around a closed orbit. we do not, however, believe that any disturbing influence of this magnitude need be contemplated, and there can be no reasonable doubt that cygni moves at present in a path very nearly straight, and with a velocity very nearly uniform. as the distance of cygni from the sun is forty billions of miles, and its velocity is thirty miles a second, it is easy to find how long the star would take to accomplish a journey equal to its distance from the sun. the time required will be about , years. in the last , years cygni will have moved over a distance ten times as great as its present distance from the sun, whatever be the direction of motion. this star must therefore have been about ten times as far from the earth , years ago as it is at present. though this epoch is incredibly more remote than any historical record, it is perhaps not incomparable with the duration of the human race; while compared with the vast lapse of geological time, such periods seem trivial and insignificant. geologists have long ago repudiated mere thousands of years; they now claim millions, and many millions of years, for the performance of geological phenomena. if the earth has existed for the millions of years which geologists assert, it becomes reasonable for astronomers to speculate on the phenomena which have transpired in the heavens in the lapse of similar ages. by the aid of our knowledge of star distances, combined with an assumed velocity of thirty miles per second, we can make the attempt to peer back into the remote past, and show how great are the changes which our universe seems to have undergone. in a million years cygni will apparently have moved through a distance which is twenty-five times as great as its present distance from the sun. whatever be the direction in which cygni is moving--whether it be towards the earth or from the earth, to the right or to the left, it must have been about twenty-five times as far off a million years ago as it is at present; but even at its present distance cygni is a small star; were it ten times as far it could only be seen with a good telescope; were it twenty-five times as far it would barely be a visible point in our greatest telescopes. the conclusions arrived at with regard to cygni may be applied with varying degrees of emphasis to other stars. we are thus led to the conclusion that many of the stars with which the heavens are strewn are apparently in slow motion. but this motion though apparently slow may really be very rapid. when standing on the sea-shore, and looking at a steamer on the distant horizon, we can hardly notice that the steamer is moving. it is true that by looking again in a few minutes we can detect a change in its place; but the motion of the steamer seems slow. yet if we were near the steamer we would find that it was rushing along at the rate of many miles an hour. it is the distance which causes the illusion. so it is with the stars: they seem to move slowly because they are very distant, but were we near them, we could see that in the majority of cases their motions are a thousand times as fast as the quickest steamer that ever ploughed the ocean. it thus appears that the permanence of the sidereal heavens, and the fixity of the constellations in their relative positions, are only ephemeral. when we rise to the contemplation of such vast periods of time as the researches of geology disclose, the durability of the constellations vanishes! in the lapse of those stupendous ages stars and constellations gradually dissolve from view, to be replaced by others of no greater permanence. it not unfrequently happens that a parallax research proves abortive. the labour has been finished, the observations are reduced and discussed, and yet no value of the parallax can be obtained. the distance of the star is so vast that our base-line, although it is nearly two hundred millions of miles long, is too short to bear any appreciable ratio to the distance of the star. even from such failures, however, information may often be drawn. let me illustrate this by an account derived from my own experience at dunsink. we have already mentioned that on the th november, , a well-known astronomer--dr. schmidt, of athens--noticed a new bright star of the third magnitude in the constellation cygnus. on the th of november nova cygni was invisible. whether it first burst forth on the st, nd, or rd no one can tell; but on the th it was discovered. its brilliancy even then seemed to be waning; so, presumably, it was brightest at some moment between the th and th of november. the outbreak must thus have been comparatively sudden, and we know of no cause which would account for such a phenomenon more simply than a gigantic collision. the decline in the brilliancy was much more tardy than its growth, and more than a fortnight passed before the star relapsed into insignificance--two or three days (or less) for the rise, two or three weeks for the fall. yet even two or three weeks was a short time in which to extinguish so mighty a conflagration. it is comparatively easy to suggest an explanation of the sudden outbreak; it is not equally easy to understand how it can have been subdued in a few weeks. a good-sized iron casting in one of our foundries takes nearly as much time to cool as sufficed to abate the celestial fires in nova cygni! on this ground it seemed not unreasonable to suppose that perhaps nova cygni was not really a very extensive conflagration. but, if such were the case, the star must have been comparatively _near_ to the earth, since it presented so brilliant a spectacle and attracted so much attention. it therefore appeared a plausible object for a parallax research; and consequently a series of observations were made some years ago at dunsink. i was at the time too much engaged with other work to devote very much labour to a research which might, after all, only prove illusory. i simply made a sufficient number of micrometric measurements to test whether a _large_ parallax existed. it has been already pointed out how each star appears to describe a minute parallactic ellipse, in consequence of the annual motion of the earth, and by measurement of this ellipse the parallax--and therefore the distance--of the star can be determined. in ordinary circumstances, when the parallax of a star is being investigated, it is necessary to measure the position of the star in its ellipse on many different occasions, distributed over a period of at least an entire year. the method we adopted was much less laborious. it was sufficiently accurate to test whether or not nova cygni had a _large_ parallax, though it might not have been delicate enough to disclose a small parallax. at a certain date, which can be readily computed, the star is at one end of the parallactic ellipse, and six months later the star is at the other end. by choosing suitable times in the year for our observations, we can measure the star in those two positions when it is most deranged by parallax. it was by observations of this kind that i sought to detect the parallax of nova cygni. its distance from a neighbouring star was carefully measured by the micrometer at the two seasons when, if parallax existed, those distances should show their greatest discrepancy; but no certain difference between these distances could be detected. the observations, therefore, failed to reveal the existence of a parallactic ellipse--or, in other words, the distance of nova cygni was too great to be measured by observations of this kind. it is certain that if nova cygni had been one of the nearest stars these observations would not have been abortive. we are therefore entitled to believe that nova cygni must be at least , , , , miles from the solar system; and the suggestion that the brilliant outburst was of small dimensions must, it seems, be abandoned. the intrinsic brightness of nova cygni, when at its best, cannot have been greatly if at all inferior to the brilliancy of our sun himself. if the sun were withdrawn from us to the distance of nova cygni, it would seemingly have dwindled down to an object not more brilliant than the variable star. how the lustre of such a stupendous object declined so rapidly remains, therefore, a mystery not easy to explain. have we not said that the outbreak of brilliancy in this star occurred between the th and the th of november, ? it would be more correct to say that the tidings of that outbreak reached our system at the time referred to. the real outbreak must have taken place at least three years previously. indeed, at the time that the star excited such commotion in the astronomical world here, it had already relapsed again into insignificance. in connection with the subject of the present chapter we have to consider a great problem which was proposed by sir william herschel. he saw that the stars were animated by proper motion; he saw also that the sun is a star, one of the countless host of heaven, and he was therefore led to propound the stupendous question as to whether the sun, like the other stars which are its peers, was also in motion. consider all that this great question involves. the sun has around it a retinue of planets and their attendant satellites, the comets, and a host of smaller bodies. the question is, whether all this superb system is revolving around the sun _at rest_ in the middle, or whether the whole system--sun, planets, and all--is not moving on bodily through space. herschel was the first to solve this noble problem; he discovered that our sun and the splendid retinue by which it is attended are moving in space. he not only discovered this, but he ascertained the direction in which the system was moving, as well as the approximate velocity with which that movement was probably performed. it has been shown that the sun and his system is now hastening towards a point of the heavens near the constellation lyra. the velocity with which the motion is performed corresponds to the magnitude of the system; quicker than the swiftest rifle-bullet that was ever fired, the sun, bearing with it the earth and all the other planets, is now sweeping onwards. we on the earth participate in that motion. every half hour we are something like ten thousand miles nearer to the constellation of lyra than we should have been if the solar system were not animated by this motion. as we are proceeding at this stupendous rate towards lyra, it might at first be supposed that we ought soon to get there; but the distances of the stars in that neighbourhood seem not less than those of the stars elsewhere, and we may be certain that the sun and his system must travel at the present rate for far more than a million years before we have crossed the abyss between our present position and the frontiers of lyra. it must, however, be acknowledged that our estimate of the actual _speed_ with which our solar system is travelling is exceedingly uncertain, but this does not in the least affect the fact that we are moving in the direction first approximately indicated by herschel (_see_ chapter xxiii.). it remains to explain the method of reasoning which herschel adopted, by which he was able to make this great discovery. it may sound strange to hear that the detection of the motion of the sun was not made by looking at the sun; all the observations of the luminary itself with all the telescopes in the world would never tell us of that motion, for the simple reason that the earth, whence our observations must be made, participates in it. a passenger in the cabin of a ship usually becomes aware that the ship is moving by the roughness of the sea; but if the sea be perfectly calm, then, though the tables and chairs in the cabin are moving as rapidly as the ship, yet we do not see them moving, because we are also travelling with the ship. if we could not go out of the cabin, nor look through the windows, we would never know whether the ship was moving or at rest; nor could we have any idea as to the direction in which the ship was going, or as to the velocity with which that motion was performed. the sun, with his attendant host of planets and satellites, may be likened to the ship. the planets may revolve around the sun just as the passengers may move about in the cabin, but as the passengers, by looking at objects on board, can never tell whither the ship is going, so we, by merely looking at the sun, or at the other planets or members of the solar system, can never tell if our system as a whole is in motion. the conditions of a perfectly uniform movement along a perfectly calm sea are not often fulfilled on the waters with which we are acquainted, but the course of the sun and his system is untroubled by any disturbance, so that the majestic progress is conducted with absolute uniformity. we do not feel the motion; and as all the planets are travelling with us, we can get no information from them as to the common motion by which the whole system is animated. the passengers are, however, at once apprised of the ship's motion when they go on deck, and when they look at the sea surrounding them. let us suppose that their voyage is nearly accomplished, that the distant land appears in sight, and, as evening approaches, the harbour is discerned into which the ship is to enter. let us suppose that the harbour has, as is often the case, a narrow entrance, and that its mouth is indicated by a lighthouse on each side. when the harbour is still a long way off, near the horizon, the two lights are seen close together, and now that the evening has closed in, and the night has become quite dark, these two lights are all that remain visible. while the ship is still some miles from its destination the two lights seem close together, but as the distance decreases the two lights seem to open out; gradually the ship gets nearer, while the lights are still opening, till finally, when the ship enters the harbour, instead of the two lights being directly in front, as at the commencement, one of the lights is passed by on the right hand, while the other is similarly found on the left. if, then, we are to discover the motion of the solar system, we must, like the passenger, look at objects unconnected with our system, and learn our own motion by their apparent movements. but are there any objects in the heavens unconnected with our system? if all the stars were like the earth, merely the appendages of our sun, then we never could discover whether we were at rest or whether we were in motion: our system might be in a condition of absolute rest, or it might be hurrying on with an inconceivably great velocity, for anything we could tell to the contrary. but the stars do not belong to the system of our sun; they are, rather, suns themselves, and do not recognise the sway of our sun, as this earth is obliged to do. the stars will, therefore, act as the external objects by which we can test whether our system is voyaging through space. with the stars as our beacons, what ought we to expect if our system be really in motion? remember that when the ship was approaching the harbour the lights gradually opened out to the right and left. but the astronomer has also lights by which he can observe the navigation of that vast craft, our solar system, and these lights will indicate the path along which he is borne. if our solar system be in motion, we should expect to find that the stars were gradually spreading away from that point in the heavens towards which our motion tends. this is precisely what we do find. the stars in the constellations are gradually spreading away from a central point near the constellation of lyra, and hence we infer that it is towards lyra that the motion of the solar system is directed. there is one great difficulty in the discussion of this question. have we not had occasion to observe that the stars themselves are in actual motion? it seems certain that every star, including the sun himself as a star, has each an individual motion of its own. the motions of the stars as we see them are partly apparent as well as partly real; they partly arise from the actual motion of each star and partly from the motion of the sun, in which we partake, and which produces an apparent motion of the star. how are these to be discriminated? our telescopes and our observations can never effect this decomposition directly. to accomplish the analysis, herschel resorted to certain geometrical methods. his materials at that time were but scanty, but in his hands they proved adequate, and he boldly announced his discovery of the movement of the solar system. so astounding an announcement demanded the severest test which the most refined astronomical resources could suggest. there is a certain powerful and subtle method which astronomers use in the effort to interpret nature. bishop butler has said that probability is the guide of life. the proper motion of a star has to be decomposed into two parts, one real and the other apparent. when several stars are taken, we may conceive an infinite number of ways into which the movements of each star can be so decomposed. each one of these conceivable divisions will have a certain element of probability in its favour. it is the business of the mathematician to determine the amount of that probability. the case, then, is as follows:--among all the various systems one must be true. we cannot lay our finger for certain on the true one, but we can take that which has the highest degree of probability in its favour, and thus follow the precept of butler to which we have already referred. a mathematician would describe his process by calling it the method of least squares. since herschel's discovery, one hundred years ago, many an astronomer using observations of hundreds of stars has attacked the same problem. mathematicians have exhausted every refinement which the theory of probabilities can afford, but only to confirm the truth of that splendid theory which seems to have been one of the flashes of herschel's genius. chapter xxii. star clusters and nebulÆ. interesting sidereal objects--stars not scattered uniformly--star clusters--their varieties--the cluster in perseus--the globular cluster in hercules--the milky way--a cluster of minute stars--the magellanic clouds--nebulæ distinct from clouds--number of known nebulæ--the constellation of orion--the position of the great nebula--the wonderful star th orionis--the drawing of the great nebula in lord rosse's telescope--photographs of this wonderful object--the great nebula in andromeda--the annular nebula in lyra--resemblance to vortex rings--planetary nebulæ--drawings of several remarkable nebulæ--nature of nebulæ--spectra of nebulæ--their distribution; the milky way. we have already mentioned saturn as one of the most glorious telescopic spectacles in the heavens. setting aside the obvious claims of the sun and of the moon, there are, perhaps, two other objects visible from these latitudes which rival saturn in the splendour and the interest of their telescopic picture. one of these objects is the star cluster in hercules; the other is the great nebula in orion. we take these objects as typical of the two great classes of bodies to be discussed in this chapter, under the head of star clusters and nebulæ. the stars, which to the number of several millions bespangle the sky, are not scattered uniformly. we can see that while some regions are comparatively barren, others contain stars in profusion. sometimes we have a small group, like the pleiades; sometimes we have a stupendous region of the heavens strewn over with stars, as in the milky way. such objects are called star clusters. we find every variety in the clusters; sometimes the stars are remarkable for their brilliancy, sometimes for their enormous numbers, and sometimes for the remarkable form in which they are grouped. sometimes a star cluster is adorned with brilliantly-coloured stars; sometimes the luminous points are so close together that their separate rays cannot he disentangled; sometimes the stars are so minute or so distant that the cluster is barely distinguishable from a nebula. of the clusters remarkable at once both for richness and brilliancy of the individual stars, we may mention the cluster in the sword-handle of perseus. the position of this object is marked on fig. , page . to the unaided eye a hazy spot is visible, which in the telescope expands into two clusters separated by a short distance. in each of them we have innumerable stars, crowded together so as to fill the field of view of the telescope. the splendour of this object may be appreciated when we reflect that each one of these stars is itself a brilliant sun, perhaps rivalling our own sun in lustre. there are, however, regions in the heavens near the southern cross, of course invisible from northern latitudes, in which parts of the milky way present a richer appearance even than the cluster in perseus. the most striking type of star cluster is well exhibited in the constellation of hercules. in this case we have a group of minute stars apparently in a roughly globular form. fig. represents this object as seen in lord rosse's great telescope, and it shows three radiating streaks, in which the stars seem less numerous than elsewhere. it is estimated that this cluster must contain from , to , stars, all concentrated into an extremely small part of the heavens. viewed in a very small telescope, this object resembles a nebula. the position of the cluster in hercules is shown in a diagram previously given (fig. , page ). we have already referred to this glorious aggregation of stars as one of the three especially interesting objects in the heavens. [illustration: plate d. milky way near messier ii. _photographed by e.e. barnard, th june, ._] the milky way forms a girdle which, with more or less regularity, sweeps completely around the heavens; and when viewed with the telescope, is seen to consist of myriads of minute stars. in some places the stars are much more numerous than elsewhere. all these stars are incomparably more distant than the sun, which they surround, so it is evident that our sun and, of course, the system which attends him lie actually inside the milky way. it seems tempting to pursue the thought here suggested, and to reflect that the whole milky way may, after all, be merely a star cluster, comparable in size with some of the other star clusters which we see, and that, viewed from a remote point in space, the milky way would seem to be but one of the many clusters of stars containing our sun as an indistinguishable unit. [illustration: fig. .--the globular cluster in hercules.] in the southern hemisphere there are two immense masses which are conspicuously visible to the naked eye, and resemble detached portions of the milky way. they cannot be seen by observers in our latitude, and are known as the magellanic clouds or the two nubeculæ. their structure, as revealed to an observer using a powerful telescope, is of great complexity. sir john herschel, who made a special study of these remarkable objects, gives the following description of them: "the general ground of both consists of large tracts and patches of nebulosity in every stage of resolution, from light irresolvable, in a reflector of eighteen inches aperture, up to perfectly separated stars like the milky way, and clustering groups sufficiently insulated and condensed to come under the designation of irregular and in some cases pretty rich clusters. but besides these there are also nebulæ in abundance and globular clusters in every state of condensation." it can hardly be doubted that the two nubeculæ, which are, roughly speaking, round, or, rather, oval, are not formed accidentally by a vast number of very different objects being ranged at various distances along the same line of sight, but that they really represent two great systems of objects, widely different in constitution, which here are congregated in each other's neighbourhood, whereas they generally do not co-exist close to each other in the milky way, with which the mere naked-eye view would otherwise lead us to associate the magellanic clouds. when we direct a good telescope to the heavens, we shall occasionally meet with one of the remarkable celestial objects which are known as nebulæ. they are faint cloudy spots, or stains of light on the black background of the sky. they are nearly all invisible to the naked eye. these celestial objects must not for a moment be confounded with clouds, in the ordinary meaning of the word. the latter exist only suspended in the atmosphere, while nebulæ are immersed in the depths of space. clouds shine by the light of the sun, which they reflect to us; nebulæ shine with no borrowed light; they are self-luminous. clouds change from hour to hour; nebulæ do not change even from year to year. clouds are far smaller than the earth; while the smallest nebula known to us is incomparably greater than the sun. clouds are within a few miles of the earth; the nebulæ are almost inconceivably remote. immediately after herschel and his sister had settled at slough he commenced his review of the northern heavens in a systematic manner. for observations of this kind it is essential that the sky be free from cloud, while even the light of the moon is sufficient to obliterate the fainter and more interesting objects. it was in the long and fine winter nights, when the stars were shining brilliantly and the pale path of the milky way extended across the heavens, that the labour was to be done. the telescope being directed to the heavens, the ordinary diurnal motion by which the sun and stars appear to rise and set carries the stars across the field of view in a majestic panorama. the stars enter slowly into the field of view, slowly move across it, and slowly leave it, to be again replaced by others. thus the observer, by merely remaining passive at the eye-piece, sees one field after another pass before him, and is enabled to examine their contents. it follows, that even without moving the telescope a long narrow strip of the heavens is brought under review, and by moving the telescope slightly up and down the width of this strip can be suitably increased. on another night the telescope is brought into a different position, and another strip of the sky is examined; so that in the course of time the whole heavens can be carefully scrutinised. herschel stands at the eye-piece to watch the glorious procession of celestial objects. close by, his sister caroline sits at her desk, pen in hand, to take down the observations as they fall from her brother's lips. in front of her is a chronometer from which she can note the time, and a contrivance which indicates the altitude of the telescope, so that she can record the exact position of the object in connection with the description which her brother dictated. such was the splendid scheme which this brother and sister had arranged to carry out as the object of their life-long devotion. the discoveries which herschel was destined to make were to be reckoned not by tens or by hundreds, but by thousands. the records of these discoveries are to be found in the "philosophical transactions of the royal society," and they are among the richest treasures of those volumes. it was left to sir john herschel, the only son of sir william, to complete his father's labour by repeating the survey of the northern heavens and extending it to the southern hemisphere. he undertook with this object a journey to the cape of good hope, and sojourned there for the years necessary to complete the great work. [illustration: fig. .--the constellation of orion, showing the position of the great nebula.] as the result of the gigantic labours thus inaugurated and continued by other observers, there are now about eight thousand nebulæ known to us, and with every improvement of the telescope fresh additions are being made to the list. they differ from one another as eight thousand pebbles selected at random on a sea-beach might differ--namely, in form, size, colour, and material--but yet, like the pebbles, bear a certain generic resemblance to each other. to describe this class of bodies in any detail would altogether exceed the limits of this chapter; we shall merely select a few of the nebulæ, choosing naturally those of the most remarkable character, and also those which are representatives of the different groups into which nebulæ may be divided. [illustration: plate xiv. the great nebula in orion.] we have already stated that the great nebula in the constellation of orion is one of the most interesting objects in the heavens. it is alike remarkable whether we consider its size or its brilliancy, the care with which it has been studied, or the success which has attended the efforts to learn something of its character. to find this object, we refer to fig. for the sketch of the chief stars in this constellation, where the letter a indicates the middle one of the three stars which form the sword-handle of orion. above the handle will be seen the three stars which form the well-known belt so conspicuous in the wintry sky. the star a, when viewed attentively with the unaided eye, presents a somewhat misty appearance. in the year cysat directed a telescope to this star, and saw surrounding it a curious luminous haze, which proved to be the great nebula. ever since his time this object has been diligently studied by many astronomers, so that very many observations have been made of the great nebula, and even whole volumes have been written which treat of nothing else. any ordinary telescope will show the object to some extent, but the more powerful the telescope the more are the curious details revealed. [illustration: fig. .--the multiple star (th orionis) in the great nebula of orion.] in the first place, the object which we have denoted by a (th orionis, also called the trapezium of orion) is in itself the most striking multiple star in the whole heavens. it consists really of six stars, represented in the next diagram (fig. ). these points are so close together that their commingled rays cannot be distinguished without a telescope. four of them are, however, easily seen in quite small instruments, but the two smaller stars require telescopes of considerable power. and yet these stars are suns, comparable, it may be, with our sun in magnitude. it is not a little remarkable that this unrivalled group of six suns should be surrounded by the renowned nebula; the nebula or the multiple star would, either of them alone, be of exceptional interest, and here we have a combination of the two. it seems impossible to resist drawing the conclusion that the multiple star really lies in the nebula, and not merely along the same line of vision. it would, indeed, seem to be at variance with all probability to suppose that the presentation of these two exceptional objects in the same field of view was merely accidental. if the multiple star be really in the nebula, then this object affords evidence that in one case at all events the distance of a nebula is a quantity of the same magnitude as the distance of a star. this is unhappily almost the entire extent of our knowledge of the distances of the nebulæ from the earth. the great nebula of orion surrounds the multiple star, and extends out to a vast distance into the neighbouring space. the dotted circle drawn around the star marked a in fig. represents approximately the extent of the nebula, as seen in a moderately good telescope. the nebula is of a faint bluish colour, impossible to represent in a drawing. its brightness is much greater in some places than in others; the central parts are, generally speaking, the most brilliant, and the luminosity gradually fades away as the edge of the nebula is approached. in fact, we can hardly say that the nebula has any definite boundary, for with each increase of telescopic power faint new branches can be seen. there seems to be an empty space in the nebula immediately surrounding the multiple star, but this is merely an illusion, produced by the contrast of the brilliant light of the stars, as the spectroscopic examination of the nebula shows that the nebulous matter is continuous between the stars. the plate of the great nebula in orion which is here shown (plate xiv.) represents, in a reduced form, the elaborate drawing of this object, which has been made with the earl of rosse's great reflecting telescope at parsonstown.[ ] a telescopic view of the nebula shows two hundred stars or more, scattered over its surface. it is not necessary to suppose that these stars are immersed in the substance of the nebula as the multiple star appears to be; they may be either in front of it, or, less probably, behind it, so as to be projected on the same part of the sky. [illustration: plate xv. photograph of the nebula m andromedÆ exposure hours, enlarged times. taken by mr. isaac roberts, december, .] a considerable number of drawings of this unique object have been made by other astronomers. among these we must mention that executed by professor bond, in cambridge, mass., which possesses a faithfulness in detail that every student of this object is bound to acknowledge. of late years also successful attempts have been made to photograph the great nebula. the late professor draper was fortunate enough to obtain some admirable photographs. in england mr. common was the first to take most excellent photographs of the nebula, and superb photographs of the same object have also been obtained by dr. roberts and mr. w.e. wilson, which show a vast extension of the nebula into regions which it was not previously known to occupy. the great nebula in andromeda, which is faintly visible to the unaided eye, is shown in plate xv., which has been copied with permission from one of the astonishing photographs that dr. isaac roberts has obtained. two dark channels in the nebula cannot fail to be noticed, and the number of faint stars scattered over its surface is also a point to which attention may be drawn. to find this object we must look out for cassiopeia and the great square of pegasus, and then the nebula will be easily perceived in the position shown on p. . in the year a new star of the seventh magnitude suddenly appeared close to the brightest part of the nebula, and declined again to invisibility after the lapse of a few months. the nebula in lyra is the most conspicuous ring nebula in the heavens, but it is not to be supposed that it is the only member of this class. altogether, there are about a dozen of these objects. it seems difficult to form any adequate conception of the nature of such a body. it is, however, impossible to view the annular nebulæ without being, at all events, reminded of those elegant objects known as vortex rings. who has not noticed a graceful ring of steam which occasionally escapes from the funnel of a locomotive, and ascends high into the air, only dissolving some time after the steam not so specialised has disappeared? such vortex rings can be produced artificially by a cubical box, one open side of which is covered with canvas, while on the opposite side of the box is a circular hole. a tap on the canvas will cause a vortex ring to start from the hole; and if the box be filled with smoke, this ring will be visible for many feet of its path. it would certainly be far too much to assert that the annular nebulæ have any real analogy to vortex rings; but there is, at all events, no other object known to us with which they can be compared. the heavens contain a number of minute but brilliant objects known as the planetary nebulæ. they can only be described as globes of glowing bluish-coloured gas, often small enough to be mistaken for a star when viewed through a telescope. one of the most remarkable of these objects lies in the constellation draco, and can be found half-way between the pole star and the star g draconis. some of the more recently discovered planetary nebulæ are extremely small, and they have indeed only been distinguished from small stars by the spectroscope. it is also to be noticed that such objects are a little out of the stellar focus in the refracting telescope in consequence of their blue colour. this remark does not apply to a reflecting telescope, as this instrument conducts all the rays to a common focus. there are many other forms of nebulæ: there are long nebulous rays; there are the wondrous spirals which have been disclosed in lord rosse's great reflector; there are the double nebulæ. but all these various objects we must merely dismiss with this passing reference. there is a great difficulty in making pictorial representations of such nebulæ. most of them are very faint--so faint, indeed, that they can only be seen with close attention even in powerful instruments. in making drawings of these objects, therefore, it is impossible to avoid intensifying the fainter features if an intelligible picture is to be made. with this caution, however, we present plate xvi., which exhibits several of the more remarkable nebulæ as seen through lord rosse's great telescope. [illustration: fig. .--the nebula n.g.c., , . (_by e.e. barnard, lick observatory, september , ._)] the actual nature of the nebulæ offers a problem of the greatest interest, which naturally occupied the mind of the first assiduous observer of nebulæ, william herschel, for many years. at first he assumed all nebulæ to be nothing but dense aggregations of stars--a very natural conclusion for one who had so greatly advanced the optical power of telescopes, and was accustomed to see many objects which in a small telescope looked nebulous become "resolved" into stars when scrutinised with a telescope of large aperture. but in , when sir william huggins first directed a telescope armed with a spectroscope to one of the planetary nebulæ, it became evident that at least some nebulæ were really clouds of fiery mist and not star clusters. we shall in our next chapter deal with the spectra of the fixed stars, but we may here in anticipation remark that these spectra are continuous, generally showing the whole length of spectrum, from red to violet, as in the sun's spectrum, though with many and important differences as to the presence of dark and bright lines. a star cluster must, of course, give a similar spectrum, resulting from the superposition of the spectra of the single stars in the cluster. many nebulæ give a spectrum of this kind; for instance, the great nebula in andromeda. but it does not by any means follow from this that these objects are only clusters of ordinary stars, as a continuous spectrum may be produced not only by matter in the liquid or solid state, or by gases at high pressure, but also by gases at lower pressure but high temperature under certain conditions. a continuous spectrum in the case of a nebula, therefore, need not indicate that the nebula is a cluster of bodies comparable in size and general constitution with our sun. but if a spectrum of bright lines is given by a nebula, we can be certain that gases at low pressure are present in the object under examination. and this was precisely what sir william huggins discovered to be the case in many nebulæ. when he first decided to study the spectra of nebulæ, he selected for observation those objects known as planetary nebulæ--small, round, or slightly oval discs, generally without central condensation, and looking like ill-defined planets. the colour of their light, which often is blue tinted with green, is remarkable, since this is a colour very rare among single stars. the spectrum was found to be totally different to that of any star, consisting merely of three or four bright lines. the brightest one is situated in the bluish-green part of the spectrum, and was at first thought to be identical with a line of the spectrum of nitrogen, but subsequent more accurate measures have shown that neither this nor the second nebular line correspond to any dark line in the solar spectrum, nor can they be produced experimentally in the laboratory, and we are therefore unable to ascribe them to any known element. the third and fourth lines were at once seen to be identical with the two hydrogen lines which in the solar spectrum are named f and g. [illustration: plate e. nebulÆ in the pleiades. _from a photograph by dr. isaac roberts._] spectrum analysis has here, as on so many other occasions, rendered services which no telescope could ever have done. the spectra of nebulæ have, after huggins, been studied, both visually and photographically, by vogel, copeland, campbell, keeler, and others, and a great many very faint lines have been detected in addition to those four which an instrument of moderate dimensions shows. it is remarkable that the red c-line of hydrogen, ordinarily so bright, is either absent or excessively faint in the spectra of nebulæ, but experiments by frankland and lockyer have shown that under certain conditions of temperature and pressure the complicated spectrum of hydrogen is reduced to one green line, the f-line. it is, therefore, not surprising that the spectra of gaseous nebulæ are comparatively simple, as the probably low density of the gases in them and the faintness of these bodies would tend to reduce the spectra to a small number of lines. some gaseous nebulæ also show faint continuous spectra, the place of maximum brightness of which is not in the yellow (as in the solar spectrum), but about the green. it is probable that these continuous spectra are really an aggregate of very faint luminous lines. a list of all the nebulæ known to have a gaseous spectrum would now contain about eighty members. in addition to the planetary nebulæ, many large and more diffused nebulæ belong to this class, and this is also the case with the annular nebula in lyra and the great nebula of orion. it is needless to say that it is of special interest to find this grand object enrolled among the nebulæ of a gaseous nature. in this nebula copeland detected the wonderful d line of helium at a time when "helium" was a mere name, a hypothetical something, but which we now know to be an element very widely distributed through the universe. it has since been found in several other nebulæ. the ease with which the characteristic gaseous spectrum is recognised has suggested the idea of sweeping the sky with a spectroscope in order to pick up new planetary nebulæ, and a number of objects have actually been discovered by pickering and copeland in this manner, as also more recently by pickering by examining spectrum photographs of various regions of the sky. most of these new objects when seen through a telescope look like ordinary stars, and their real nature could never have been detected without the spectroscope. when we look up at the starry sky on a clear night, the stars seem at first sight to be very irregularly distributed over the heavens. here and there a few bright stars form characteristic groups, like orion or the great bear, while other equally large tracts are almost devoid of bright stars and only contain a few insignificant ones. if we take a binocular, or other small telescope, and sweep the sky with it, the result seems to be the same--now we come across spaces rich in stars; now we meet with comparatively empty places. but when we approach the zone of the milky way, we are struck with the rapid increase of the number of stars which fill the field of the telescope; and when we reach the milky way itself, the eye is almost unable to separate the single points of light, which are packed so closely together that they produce the appearance to the naked eye of a broad, but very irregular, band of dim light, which even a powerful telescope in some places can hardly resolve into stars. how are we to account for this remarkable arrangement of the stars? what is the reason of our seeing so few at the parts of the heavens farthest from the milky way, and so very many in or near that wonderful belt? the first attempt to give an answer to these questions was made by thomas wright, an instrument maker in london, in a book published in . he supposed the stars of our sidereal system to be distributed in a vast stratum of inconsiderable thickness compared with its length and breadth. if we had a big grindstone made of glass, in which had become uniformly imbedded a vast quantity of grains of sand or similar minute particles, and if we were able to place our eye somewhere near the centre of this grindstone, it is easy to see that we should see very few particles near the direction of the axle of the grindstone, but a great many if we looked towards any point of the circumference. this was wright's idea of the structure of the milky way, and he supposed the sun to be situated not very far from the centre of this stellar stratum. [illustration: plate f. ô centauri. _from a drawing in the publications of harvard college observatory._] if the milky way itself did not exist--and we had simply the fact to build on that the stars appeared to increase rapidly in number towards a certain circle (almost a great circle) spanning the heavens--then the disc theory might have a good deal in its favour. but the telescopic study of the milky way, and even more the marvellous photographs of its complicated structure produced by professor barnard, have given the death blow to the old theory, and have made it most reasonable to conclude that the milky way is really, and not only apparently, a mighty stream of stars encircling the heavens. we shall shortly mention a few facts which point in this direction. a mere glance is sufficient to show that the milky way is not a single belt of light; near the constellation aquila it separates into two branches with a fairly broad interval between them, and these branches do not meet again until they have proceeded far into the southern hemisphere. the disc theory had, in order to explain this, to assume that the stellar stratum was cleft in two nearly to the centre. but even if we grant this, how can we account for the numerous more or less dark holes in the milky way, the largest and most remarkable of which is the so-called "coal sack" in the southern hemisphere? obviously we should have to assume the existence of a number of tunnels, drilled through the disc-like stratum, and by some strange sympathy all directed towards the spot where our solar system is situated. and the many small arms which stretch out from the milky way would have to be either planes seen edgeways or the convexities of curved surfaces viewed tangentially. the improbability of these various assumptions is very great. but evidence is not wanting that the relatively bright stars are crowded together along the same zone where the excessively faint ones are so closely packed. the late mr. proctor plotted all the stars which occur in argelander's great atlas of the northern hemisphere, , in number, on a single chart, and though these stars are all above the tenth magnitude, and thus superior in brightness to that innumerable host of stars of which the individual members are more or less lost in the galactic zone, and on the hypothesis of uniform distribution ought to be relatively near to us, the chart shows distinctly the whole course of the milky way by the clustering of these stars. this disposes sufficiently of the idea that the milky way is nothing but a disc-like stratum seen projected on the heavenly sphere; after this it is hardly necessary to examine professor barnard's photographs and see how fairly bright and very faint regions alternate without any attempt at regularity, in order to become convinced that the milky way is more probably a stream of stars clustered together, a stream or ring of incredibly enormous dimensions, inside which our solar system happens to be situated. but it must be admitted that it is premature to attempt to find the actual figure of this stream or to determine the relative distance of the various portions of it. [illustration: plate xvi. nebulÆ observed with lord rosse's great telescope.] chapter xxiii. the physical nature of the stars. star spectroscopes--classification of stellar spectra--type i., with very few absorption lines--type ii., like the sun--type iii., with strongly marked dark bands--distribution of these classes over the heavens--motion in the line of sight--orbital motion discovered with the spectroscope: new class of binaries--spectra of temporary stars--nature of these bodies. we have frequently in the previous chapters had occasion to refer to the revelations of the spectroscope, which form an important chapter in the history of modern science. by its aid a mighty stride has been taken in our attempt to comprehend the physical constitution of the sun. in the present chapter we propose to give an account of what the spectroscope tells us about the physical constitution of the fixed stars. quite a new phase of astronomy is here opened up. every improvement in telescopes revealed fainter and fainter objects, but all the telescopes in the world could not answer the question as to whether iron and other elements are to be found in the stars. the ordinary star is a mighty glowing globe, hotter than a bessemer converter or a siemens furnace; if iron is in the star, it must be not only white-hot and molten, but actually converted into vapour. but the vapour of iron is not visible in the telescope. how would you recognise it? how would you know if it commingled with the vapour of many other metals or other substances? it is, in truth, a delicate piece of analysis to discriminate iron in the glowing atmosphere of a star. but the spectroscope is adequate to the task, and it renders its analysis with an amount of evidence that is absolutely convincing. that the spectra of the moon and planets are practically nothing but faint reproductions of the spectrum of the sun was discovered by the great german optician fraunhofer about the year . by placing a prism in front of the object glass of a small theodolite (an instrument used for geodetic measurements) he was able to ascertain that venus and mars showed the same spectrum as the sun, while sirius gave a very different one. this important observation encouraged him to procure better instrumental means with which to continue the work, and he succeeded in distinguishing the chief characteristics of the various types of stellar spectra. the form of instrument which fraunhofer adopted for this work, in which the prism was placed outside the object glass of the telescope, has not been much used until within the last few years, owing to the difficulty of obtaining prisms of large dimensions (for it is obvious that the prism ought to be as large as the object glass if the full power of the latter is to be made use of), but this is the simplest form of spectroscope for observing spectra of objects of no sensible angular diameter, like the fixed stars. the parallel rays from the stars are dispersed by the prism into a spectrum, and this is viewed by means of the telescope. but as the image of the star in the telescope is nothing but a luminous point, its spectrum will be merely a line in which it would not be possible to distinguish any lines crossing it laterally such as those we see in the spectrum of the sun. a cylindrical lens is, therefore, placed before the eye-piece of the telescope, and as this has the effect of turning a point into a line and a line into a band, the narrow spectrum of the star is thereby broadened out into a luminous band in which we can distinguish any details that exist. in other forms of stellar spectroscope we require a slit which must be placed in the focus of the object glass, and the general arrangement is similar to that which we have described in the chapter on the sun, except that a cylindrical lens is required. the study of the spectra of the fixed stars made hardly any progress until the principles of spectrum analysis had been established by kirchhoff in . when the dark lines in the solar spectrum had been properly interpreted, it was at once evident that science had opened wide the gates of a new territory for human exploration, of the very existence of which hardly anyone had been aware up to that time. we have seen to what splendid triumphs the study of the sun has led the investigators in this field, and we have seen how very valuable results have been obtained by the new method when applied to observations of comets and nebulæ. we shall now give some account of what has been learned with regard to the constitution of the fixed stars by the researches which were inaugurated by sir william huggins and continued and developed by him, as well as by secchi, vogel, pickering, lockyer, dunér, scheiner and others. here, as in the other modern branches of astronomy, photography has played a most important part, not only because photographed spectra of stars extend much farther at the violet end than the observer can follow them with his eye, but also because the positions of the lines can be very accurately measured on the photographs. the first observer who reduced the apparently chaotic diversity of stellar spectra to order was secchi, who showed that they might all be grouped according to four types. within the last thirty years, however, so many modifications of the various types have been found that it has become necessary to subdivide secchi's types, and most observers now make use of vogel's classification, which we shall also for convenience adopt in this chapter. _type i._--in the spectra of stars of this class the metallic lines, which are so very numerous and conspicuous in the sun's violet spectrum, are very faint and thin, or quite invisible, and the blue and white parts are very intensely bright. vogel subdivides the class into three groups. in the first (i.a) the hydrogen lines are present, and are remarkably broad and intense; sirius, vega, and regulus are examples of this group. the great breadth of the lines probably indicates that these stars are surrounded by hydrogen atmospheres of great dimensions. it is generally acknowledged that stars of this group must be the hottest of all, and support is lent to this view by the appearance in their spectra of a certain magnesium line, which, as sir norman lockyer showed many years ago, by laboratory experiments, does not appear in the ordinary spectrum of magnesium, but is indicative of the presence of the substance at a very high temperature. in the spectra of stars of group i.b the hydrogen lines and the few metallic lines are of equal breadth, and the magnesium line just mentioned is the strongest of all. rigel and several other bright stars in orion belong to this group, and it is remarkable that helium is present at least in some of these stars, so that (as professor keeler remarks) the spectrum of rigel may almost be regarded as the nebular spectrum reversed (lines dark instead of bright), except that the two chief nebular lines are not reversed in the star. this fact will doubtless eventually be of great importance to our understanding the successive development of a star from a nebula; and a star like rigel is no doubt also of very high temperature. this is probably not the case with stars of the third subdivision of type i. (i.c), the spectra of which are distinguished by the presence of bright hydrogen lines and the bright helium line d . among the stars having this very remarkable kind of spectrum is a very interesting variable star in the constellation lyra (b) and the star known as g cassiopeiæ, both of which have been assiduously observed, their spectra possessing numerous peculiarities which render an explanation of the physical constitution of the stars of this subdivision a very difficult matter. passing to _type ii._, we find spectra in which the metallic lines are strong. the more refrangible end of the spectrum is fainter than in the previous class, and absorption bands are sometimes found towards the red end. in its first subdivision (ii.a) are contained spectra with a large number of strong and well-defined lines due to metals, the hydrogen lines being also well seen, though they are not specially conspicuous. among the very numerous stars of this group are capella, aldebaran, arcturus, pollux, etc. the spectra of these stars are in fact practically identical with the spectrum of our own sun, as shown, for instance, by dr. scheiner, of the potsdam astrophysical observatory, who has measured several hundred lines on photographs of the spectrum of capella, and found a very close agreement between these lines and corresponding ones in the solar spectrum. we can hardly doubt that the physical constitution of these stars is very similar to that of our sun. this cannot be the case with the stars of the second subdivision (ii.b), the spectra of which are very complex, each consisting of a continuous spectrum crossed by numerous dark lines, on which is superposed a second spectrum of bright lines. upwards of seventy stars are known to possess this extraordinary spectrum, the only bright one among them being a star of the third magnitude in the southern constellation argus. here again we have hydrogen and helium represented by bright lines, while the origin of the remaining bright lines is doubtful. with regard to the physical constitution of the stars of this group it is very difficult to come to a definite conclusion, but it would seem not unlikely that we have here to do with stars which are not only surrounded by an atmosphere of lower temperature, causing the dark lines, but which, outside of that, have an enormous envelope of hydrogen and other gases. in one star at least of this group professor campbell, of the lick observatory, has seen the f line as a long line extending a very appreciable distance on each side of the continuous spectrum, and with an open slit it was seen as a large circular disc about six seconds in diameter; two other principal hydrogen lines showed the same appearance. as far as this observation goes, the existence of an extensive gaseous envelope surrounding the star seems to be indicated. _type iii._ contains comparatively few stars, and the spectra are characterised by numerous dark bands in addition to dark lines, while the more refrangible parts are very faint, for which reason the stars are more or less red in colour. this class has two strongly marked subdivisions. in the first (iii.a) the principal absorption lines coincide with similar ones in the solar spectrum, but with great differences as to intensity, many lines being much stronger in these stars than in the sun, while many new lines also appear. these dissimilarities are, however, of less importance than the peculiar absorption bands in the red, yellow, and green parts of the spectrum, overlying the metallic lines, and being sharply defined on the side towards the violet and shading off gradually towards the red end of the spectrum. bands of this kind belong to chemical combinations, and this appears to show that somewhere in the atmospheres of these distant suns the temperature is low enough to allow stable chemical combinations to be formed. the most important star of this kind is betelgeuze or a orionis, the red star of the first magnitude in the shoulder of orion; but it is of special importance to note that many variable stars of long period have spectra of type iii.a. sir norman lockyer predicted in that bright lines, probably of hydrogen, would eventually be found to appear at the maximum of brightness, when the smaller swarm is supposed to pass through the larger one, and this was soon afterwards confirmed by the announcement that professor pickering had found a number of hydrogen lines bright on photographs, obtained at harvard college observatory, of the spectrum of the remarkable variable, mira ceti, at the time of maximum. professor pickering has since then reported that bright lines have been found on the plates of forty-one previously known variables of this class, and that more than twenty other stars have been detected as variables by this peculiarity of their spectrum; that is, bright lines being seen in them suggested that the stars were variable, and further photometric investigations corroborated the fact. the second subdivision (iii.b) contains only comparatively faint stars, of which none exceed the fifth magnitude, and is limited to a small number of red stars. the strongly marked bands in their spectra are sharply defined and dark on the red side, while they fade away gradually towards the violet, exactly the reverse of what we see in the spectra of iii.a. these bands appear to arise from the absorption due to hydrocarbon vapours present in the atmospheres of these stars; but there are also some lines visible which indicate the presence of metallic vapours, sodium being certainly among these. there can be little doubt that these stars represent the last stage in the life of a sun, when it has cooled down considerably and is not very far from actual extinction, owing to the increasing absorption of its remaining light in the atmosphere surrounding it. the method employed for the spectroscopic determination of the motion of a star in the line of sight is the same as the method we have described in the chapter on the sun. the position of a certain line in the spectrum of a star is compared with the position of the corresponding bright line of an element in an artificially produced spectrum, and in this manner a displacement of the stellar line either towards the violet (indicating that the star is approaching us) or towards the red (indicating that it is receding) may be detected. the earliest attempt of this sort was made in by sir william huggins, who compared the f line in the spectrum of sirius with the same line of the spectrum of hydrogen contained in a vacuum tube reflected into the field of his astronomical spectroscope, so that the two spectra appeared side by side. the work thus commenced and continued by him was afterwards taken up at the greenwich observatory; but the results obtained by these direct observations were never satisfactory, as remarkable discrepancies appeared between the values obtained by different observers, and even by the same observer on different nights. this is not to be wondered at when we bear in mind that the velocity of light is so enormous compared with any velocity with which a heavenly body may travel, that the change of wave length resulting from the latter motion can only be a very minute one, difficult to perceive, and still more difficult to measure. but since photography was first made use of for these investigations by dr. vogel, of potsdam, much more accordant and reliable results have been obtained, though even now extreme care is required to avoid systematic errors. to give some idea of the results obtainable, we present in the following table the values of the velocity per second of a number of stars observed in and by mr. h.f. newall with the bruce spectrograph attached to the great -inch newall refractor of the cambridge observatory, and we have added the values found at potsdam by vogel and scheiner. the results are expressed in kilometres ( km. = · english mile). the sign + means that the star is receding from us,-that it is approaching. newall. vogel. scheiner. aldebaran + · + · + · betelgeuze + · + · + · procyon - · - · - · pollux - · + · + · g leonis - · - · - · arcturus - · - · - · these results have been corrected for the earth's orbital motion round the sun, but not for the sun's motion through space, as the amount of the latter is practically unknown, or at least very uncertain; so that the above figures really represent the velocity per second of the various stars relative to the sun. we may add that the direction and velocity of the sun's motion may eventually be ascertained from spectroscopic measures of a great number of stars, and it seems likely that the sun's velocity will be much more accurately found in this way than by the older method of combining proper motions of stars with speculations as to the average distances of the various classes of stars. this has already been attempted by dr. kempf, who from the potsdam spectrographic observations found the sun's velocity to be · kilometres, or · miles per second, a result which is probably not far from the truth. but the spectra of the fixed stars can also tell us something about orbital motion in these extremely distant systems. if one star revolved round another in a plane passing through the sun, it must on one side of the orbit move straight towards us and on the other side move straight away from us, while it will not alter its distance from us while it is passing in front of, or behind, the central body. if we therefore find from the spectroscopic observations that a star is alternately moving towards and away from the earth in a certain period, there can be no doubt that this star is travelling round some unseen body (or, rather, round the centre of gravity of both) in the period indicated by the shifting of the spectral lines. in chapter xix. we mentioned the variable star algol in the constellation perseus, which is one of a class of variable stars distinguished by the fact that for the greater part of the period they remain of unaltered brightness, while for a very short time they become considerably fainter. that this was caused by some sort of an eclipse--or, in other words, by the periodic passage of a dark body in front of the star, hiding more or less of the latter from us--was the simplest possible hypothesis, and it had already years ago been generally accepted. but it was not possible to prove that this was the true explanation of the periodicity of stars like algol until professor vogel, from the spectroscopic observations made at potsdam, found that before every minimum algol is receding from the sun, while it is approaching us after the minimum. assuming the orbit to be circular, the velocity of algol was found to be twenty-six miles per second. from this and the length of the period ( d. h. m. s.) and the time of obscuration it was easy to compute the size of the orbit and the actual dimensions of the two bodies. it was even possible to go a step further and to calculate from the orbital velocities the masses of the two bodies,[ ] assuming them to be of equal density--an assumption which is no doubt very uncertain. the following are the approximate elements of the algol system found by vogel:-- diameter of algol , , miles. diameter of companion , miles. distance between their centres , , miles. orbital velocity of algol miles per sec. orbital velocity of companion miles per sec. mass of algol / of sun's mass. mass of companion / of sun's mass. the period of algol has been gradually decreasing during the last century (by six or seven seconds), but whether this is caused by the motion of the pair round a third and very much more distant body, as suggested by mr. chandler, has still to be found out. we have already mentioned that in order to produce eclipses, and thereby variations of light, it is necessary that the line of sight should lie nearly in the plane of the orbit. it is also essential that there should be a considerable difference of brightness between the two bodies. these conditions must be fulfilled in the fifteen variable stars of the algol class now known; but according to the theory of probability, there must be many more binary systems like that of algol where these conditions are not fulfilled, and in those cases no variations will occur in the stars' brightness. of course, we know many cases of a luminous star travelling round another, but there must also be cases of a large companion travelling round another at so small a distance that our telescopes are unable to "divide" the double star. this has actually been discovered by means of the spectroscope. if we suppose an extremely close double star to be examined with the spectroscope, the spectra of the two components will be superposed, and we shall not be aware that we really see two different spectra. but during the revolution of the two bodies round their common centre of gravity there must periodically come a time when one body is moving towards us and the other moving from us, and consequently the lines in the spectrum of the former will be subject to a minute, relative shift towards the violet end of the spectrum, and those of the other to a minute shift towards the red. those lines which are common to the two spectra will therefore periodically become double. a discovery of this sort was first made in by professor pickering from photographs of the spectrum of mizar, or z ursa majoris, the larger component of the well-known double star in the tail of the great bear. certain of the lines were found to be double at intervals of fifty-two days. the maximum separation of the two components of each line corresponds to a relative velocity of one star as compared with the other of about a hundred miles per second, but subsequent observations have shown the case to be very complicated, either with a very eccentric elliptic orbit or possibly owing to the presence of a third body. the harvard college photographs also showed periodic duplicity of lines in the star b aurigæ, the period being remarkably short, only three days and twenty-three hours and thirty-seven minutes. in vogel found, from photographs of the spectrum of spica, the first magnitude star in virgo, that this star alternately recedes from and approaches to the solar system, the period being four days. certain other "spectroscopic binaries" have since then been found, notably one component of castor, with a period of three days, found by m. belopolsky, and a star in the constellation scorpio, with a period of only thirty-four hours, detected on the harvard spectrograms. quite recently mr. h.f. newall, at cambridge, and mr. campbell, of the lick observatory, have shown that a aurigæ, or capella, consists of a sun-like star and a procyon-like star, revolving in days. at first sight there is something very startling in the idea of two suns circling round each other, separated by an interval which, in comparison with their diameters, is only a very small one. in the algol system, for instance, we have two bodies, one the size of our own sun and the other slightly larger, moving round their common centre of gravity in less than three days, and at a distance between their surfaces equal to only twice the diameter of the larger one. again, in the system of spica we have two great suns swinging round each other in only four days, at a distance equal to that between saturn and his sixth satellite. but although we have at present nothing analogous to this in our solar system, it can be proved mathematically that it is perfectly possible for a system of this kind to preserve its stability, if not for ever, at any rate for ages, and we shall see in our last chapter that there was in all probability a time when the earth and the moon formed a peculiar system of two bodies revolving rapidly at a very small distance compared to the diameters of the bodies. it is possible that we have a more complicated system in the star known as b lyræ. this is a variable star of great interest, having a period of twelve days and twenty-two hours, in which time it rises from magnitude - / to a little above - / , sinks nearly to the fourth magnitude, rises again to fully - / , and finally falls to magnitude - / . in professor pickering discovered that the bright lines in the spectrum of this star changed their position from time to time, appearing now on one side, now on the other side of corresponding dark lines. obviously these bright lines change their wave length, the light-giving source alternately receding from and approaching to the earth, and the former appeared to be the case during one-half of the period of variation of the star's light, the latter during the other half. the spectrum of this star has been further examined by belopolsky and others, who have found that the lines are apparently double, but that one of the components either disappears or becomes very narrow from time to time. on the assumption that these lines were really single (the apparent duplicity resulting from the superposition of a dark line), belopolsky determined the amount of their displacement by measuring the distances from the two edges of a line of hydrogen (f) to the artificial hydrogen line produced by gas glowing in a tube and photographed along with the star-spectrum. assuming the alternate approach and recession to be caused by orbital revolution, belopolsky found that the body emitting the light of the bright lines moved with an orbital velocity of forty-one miles. he succeeded in in observing the displacement of a dark line due to magnesium, and found that the body emitting it was also moving in an orbit, but while the velocities given by the bright f line are positive after the principal minimum of the star's light, those given by the dark line are negative. therefore, during the principal minimum it is a star giving the dark line which is eclipsed, and during the secondary minimum another star giving the bright line is eclipsed. this wonderful variable will, however, require more observatioêns before the problem of its constitution is finally solved, and the same may be said of several variable stars, _e.g._ ê aquilæ and d cephei, in which a want of harmony has been found between the changes of velocity and the fluctuations of the light. there are some striking analogies between the complicated spectrum of b lyræ and the spectra of temporary stars. the first "new star" which could be spectroscopically examined was that which appeared in corona borealis in , and which was studied by sir w. huggins. it showed a continuous spectrum with dark absorption lines, and also the bright lines of hydrogen; practically the same spectrum as the stars of type ii.b. this was also the case with schmidt's star of , which showed the helium line (d ) and the principal nebula line in addition to the lines of hydrogen; but in the autumn of , when the star had fallen to the tenth magnitude, dr. copeland was surprised to find that only one line was visible, the principal nebula line, in which almost the whole light of the star was concentrated, the continuous spectrum being hardly traceable. it seemed, in fact, that the star had been transformed into a planetary nebula, but later the spectrum seems to have lost this peculiar monochromatic character, the nebula line having disappeared and a faint continuous spectrum alone being visible, which is also the case with the star of since it sank down to the tenth magnitude. a continuous spectrum was all that could be seen of the new star which broke out in the nebula of andromeda in , much the same as the spectrum of the nebula itself. when the new star in auriga was announced, in february, , astronomers were better prepared to observe it spectroscopically, as it was now possible by means of photography to study the ultra-violet part of the spectrum which to the eye is invisible. the visible spectrum was very like that of nova cygni of , but when the wave-lengths of all the bright lines seen and photographed at the lick observatory and at potsdam were measured, a strong resemblance to the bright line spectrum of the chromosphere of the sun became very evident. the hydrogen lines were very conspicuous, while the iron lines were very numerous, and calcium and magnesium were also represented. the most remarkable revelation made by the photographs was, however, that the bright lines were in many cases accompanied, on the side next the violet, by broad dark bands, while both bright and dark lines were of a composite character. many of the dark lines had a thin bright line superposed in the middle, while on the other hand many of the bright lines had two or three points maxima of brightness. the results of the measures of motion in the line of sight were of special importance. they showed that the source of light, whence came the thin bright lines within the dark ones, was travelling towards the sun at the enormous rate of miles per second, and if the bright lines were actual "reversals" of the dark ones, then the source of the absorption spectrum must have been endowed with much the same velocity. on the other hand, if the two or three maxima of brightness in the bright lines really represent two or three separate bodies giving bright lines, the measures indicate that the principal one was almost at rest as regards the sun, while the others were receding from us at the extraordinary rates of and miles per second. and as if this were not sufficiently puzzling, the star on its revival in august, , as a tenth magnitude star had a totally different spectrum, showing nothing but a number of the bright lines belonging to planetary nebulæ! it is possible that the principal ones of these were really present in the spectrum from the first, but that their wave lengths had been different owing to change of the motion in the line of sight, so that the nebula lines seen in the autumn were identical with others seen in the spring at slightly different places. subsequent observations of these nebula lines seemed to point to a motion of the nova towards the solar system (of about miles per second) which gradually diminished. but although we are obliged to confess our inability to say for certain why a temporary star blazes up so suddenly, we have every cause to think that these strange bodies will by degrees tell us a great deal about the constitution of the fixed stars. the great variety of spectra which we see in the starry universe, nebula spectra with bright lines, stellar spectra of the same general character, others with broad absorption bands, or numerous dark lines like our sun, or a few absorption lines only--all this shows us the universe as teeming with bodies in various stages of evolution. we shall have a few more words to say on this matter when we come to consider the astronomical significance of heat; but we have reached a point where man's intellect can hardly keep pace with the development of our instrumental resources, and where our imagination stands bewildered when we endeavour to systematise the knowledge we have gained. that great caution will have to be exercised in the interpretation of the observed phenomena is evident from the recent experience of professor rowland, of baltimore, from which we learn that spectral lines are not only widened by increased pressure of the light-giving vapour, but that they may be bodily shifted thereby. dr. zeeman's discovery, that a line from a source placed in a strong magnetic field may be both widened, broadened, and doubled, will also increase our difficulties in the interpretation of these obscure phenomena. chapter xxiv. the precession and nutation of the earth's axis. the pole is not a fixed point--its effect on the apparent places of the stars--the illustration of the peg-top--the disturbing force which acts on the earth--attraction of the sun on a globe--the protuberance at the equator--the attraction of the protuberance by the sun and by the moon produces precession--the efficiency of the precessional agent varies inversely as the cube of the distance--the relative efficiency of the sun and the moon--how the pole of the earth's axis revolves round the pole of the ecliptic--variation of latitude. the position of the pole of the heavens is most conveniently indicated by the bright star known as the pole star, which lies in its immediate vicinity. around this pole the whole heavens appear to rotate once in a sidereal day; and we have hitherto always referred to the pole as though it were a fixed point in the heavens. this language is sufficiently correct when we embrace only a moderate period of time in our review. it is no doubt true that the pole lies near the pole star at the present time. it did so during the lives of the last generation, and it will do so during the lives of the next generation. all this time, however, the pole is steadily moving in the heavens, so that the time will at length come when the pole will have departed a long way from the present pole star. this movement is incessant. it can be easily detected and measured by the instruments in our observatories, and astronomers are familiar with the fact that in all their calculations it is necessary to hold special account of this movement of the pole. it produces an apparent change in the position of a star, which is known by the term "precession." [illustration: fig. .] the movement of the pole is very clearly shown in the accompanying figure (fig. ), for which i am indebted to the kindness of the late professor c. piazzi smyth. the circle shows the track along which the pole moves among the stars. the centre of the circle in the constellation of draco is the pole of the ecliptic. a complete journey of the pole occupies the considerable period of about , years. the drawing shows the position of the pole at the several dates from b.c. to a.d. a glance at this map brings prominently before us how casual is the proximity of the pole to the pole star. at present, indeed, the distance of the two is actually lessening, but afterwards the distance will increase until, when half of the revolution has been accomplished, the pole will be at a distance of twice the radius of the circle from the pole star. it will then happen that the pole will be near the bright star vega or a lyræ, so that our successors some , years hence may make use of vega for many of the purposes for which the pole star is at present employed! looking back into past ages, we see that some , or , years b.c. the star a draconis was suitably placed to serve as the pole star, when b and d of the great bear served as pointers. it need hardly be added, that since the birth of accurate astronomy the course of the pole has only been observed over a very small part of the mighty circle. we are not, however, entitled to doubt that the motion of the pole will continue to pursue the same path. this will be made abundantly clear when we proceed to render an explanation of this very interesting phenomenon. the north pole of the heavens is the point of the celestial sphere towards which the northern end of the axis about which the earth rotates is directed. it therefore follows that this axis must be constantly changing its position. the character of the movement of the earth, so far as its rotation is concerned, may be illustrated by a very common toy with which every boy is familiar. when a peg-top is set spinning, it has, of course, a very rapid rotation around its axis; but besides this rotation there is usually another motion, whereby the axis of the peg-top does not remain in a constant direction, but moves in a conical path around the vertical line. the adjoining figure (fig. ) gives a view of the peg-top. it is, of course, rotating with great rapidity around its axis, while the axis itself revolves around the vertical line with a very deliberate motion. if we could imagine a vast peg-top which rotated on its axis once a day, and if that axis were inclined at an angle of twenty-three and a half degrees to the vertical, and if the slow conical motion of the axis were such that the revolution of the axis were completed in about , years, then the movements would resemble those actually made by the earth. the illustration of the peg-top comes, indeed, very close to the actual phenomenon of precession. in each case the rotation about the axis is far more rapid than that of the revolution of the axis itself; in each case also the slow movement is due to an external interference. looking at the figure of the peg-top (fig. ) we may ask the question, why does it not fall down? the obvious effect of gravity would seem to say that it is impossible for the peg-top to be in the position shown in the figure. yet everybody knows that this is possible so long as the top is spinning. if the top were not spinning, it would, of course, fall. it therefore follows that the effect of the rapid rotation of the top so modifies the effect of gravitation that the latter, instead of producing its apparently obvious consequence, causes the slow conical motion of the axis of rotation. this is, no doubt, a dynamical question of some difficulty, but it is easy to verify experimentally that it is the case. if a top be constructed so that the point about which it is spinning shall coincide with the centre of gravity, then there is no effect of gravitation on the top, and there is no conical motion perceived. [illustration: fig. .--illustration of the motion of precession.] if the earth were subject to no external interference, then the direction of the axis about which it rotates must remain for ever constant; but as the direction of the axis does not remain constant, it is necessary to seek for a disturbing force adequate to the production of the phenomena which are observed. we have invariably found that the dynamical phenomena of astronomy can be accounted for by the law of universal gravitation. it is therefore natural to enquire how far gravitation will render an account of the phenomenon of precession; and to put the matter in its simplest form, let us consider the effect which a distant attracting body can have upon the rotation of the earth. to answer this question, it becomes necessary to define precisely what we mean by the earth; and as for most purposes of astronomy we regard the earth as a spherical globe, we shall commence with this assumption. it seems also certain that the interior of the earth is, on the whole, heavier than the outer portions. it is therefore reasonable to assume that the density increases as we descend; nor is there any sufficient ground for thinking that the earth is much heavier in one part than at any other part equally remote from the centre. it is therefore usual in such calculations to assume that the earth is formed of concentric spherical shells, each one of which is of uniform density; while the density decreases from each shell to the one exterior thereto. a globe of this constitution being submitted to the attraction of some external body, let us examine the effects which that external body can produce. suppose, for instance, the sun attracts a globe of this character, what movements will be the result? the first and most obvious result is that which we have already so frequently discussed, and which is expressed by kepler's laws: the attraction will compel the earth to revolve around the sun in an elliptic path, of which the sun is in the focus. with this movement we are, however, not at this moment concerned. we must enquire how far the sun's attraction can modify the earth's rotation around its axis. it can be demonstrated that the attraction of the sun would be powerless to derange the rotation of the earth so constituted. this is a result which can be formally proved by mathematical calculation. it is, however, sufficiently obvious that the force of attraction of any distant point on a symmetrical globe must pass through the centre of that globe: and as the sun is only an enormous aggregate of attracting points, it can only produce a corresponding multitude of attractive forces; each of these forces passes through the centre of the earth, and consequently the resultant force which expresses the joint result of all the individual forces must also be directed through the centre of the earth. a force of this character, whatever other potent influence it may have, will be powerless to affect the rotation of the earth. if the earth be rotating on an axis, the direction of that axis would be invariably preserved; so that as the earth revolves around the sun, it would still continue to rotate around an axis which always remained parallel to itself. nor would the attraction of the earth by any other body prove more efficacious than that of the sun. if the earth really were the symmetrical globe we have supposed, then the attraction of the sun and moon, and even the influence of all the planets as well, would never be competent to make the earth's axis of rotation swerve for a single second from its original direction. we have thus narrowed very closely the search for the cause of the "precession." if the earth were a perfect sphere, precession would be inexplicable. we are therefore forced to seek for an explanation of precession in the fact that the earth is not a perfect sphere. this we have already demonstrated to be the case. we have shown that the equatorial axis of the earth is longer than the polar axis, so that there is a protuberant zone girdling the equator. the attraction of external bodies is able to grasp this protuberance, and thereby force the earth's axis of rotation to change its direction. there are only two bodies in the universe which sensibly contribute to the precessional movement of the earth's axis: these bodies are the sun and the moon. the shares in which the labour is borne by the sun and the moon are not what might have been expected from a hasty view of the subject. this is a point on which it will be desirable to dwell, as it illustrates a point in the theory of gravitation which is of very considerable importance. the law of gravitation asserts that the intensity of the attraction which a body can exercise is directly proportional to the mass of that body, and inversely proportional to the square of its distance from the attracted point. we can thus compare the attraction exerted upon the earth by the sun and by the moon. the mass of the sun exceeds the mass of the moon in the proportion of about , , to . on the other hand, the moon is at a distance which, on an average, is about one- th part of that of the sun. it is thus an easy calculation to show that the efficiency of the sun's attraction on the earth is about times as great as the attraction of the moon. hence it is, of course, that the earth obeys the supremely important attraction of the sun, and pursues an elliptic path around the sun, bearing the moon as an appendage. but when we come to that particular effect of attraction which is competent to produce precession, we find that the law by which the efficiency of the attracting body is computed assumes a different form. the measure of efficiency is, in this case, to be found by taking the mass of the body and dividing it by the _cube_ of the distance. the complete demonstration of this statement must be sought in the formulæ of mathematics, and cannot be introduced into these pages; we may, however, adduce one consideration which will enable the reader in some degree to understand the principle, though without pretending to be a demonstration of its accuracy. it will be obvious that the nearer the disturbing body approaches to the earth the greater is the _leverage_ (if we may use the expression) which is afforded by the protuberance at the equator. the efficiency of a given force will, therefore, on this account alone, increase in the inverse proportion of the distance. the actual intensity of the force itself augments in the inverse square of the distance, and hence the capacity of the attracting body for producing precession will, for a double reason, increase when the distance decreases. suppose, for example, that the disturbing body is brought to half its original distance from the disturbed body, the leverage is by this means doubled, while the actual intensity of the force is at the same time quadrupled according to the law of gravitation. it will follow that the effect produced in the latter case must be eight times as great as in the former case. and this is merely equivalent to the statement that the precession-producing capacity of a body varies inversely as the cube of the distance. it is this consideration which gives to the moon an importance as a precession-producing agent to which its mere attractive capacity would not have entitled it. even though the mass of the sun be , , times as great as the mass of the moon, yet when this number is divided by the cube of the relative value of the distances of the bodies ( ), it is seen that the efficiency of the moon is more than twice as great as that of the sun. in other words, we may say that one-third of the movement of precession is due to the sun, and two-thirds to the moon. for the study of the joint precessional effect due to the sun and the moon acting simultaneously, it will be advantageous to consider the effect produced by the two bodies separately; and as the case of the sun is the simpler of the two, we shall take it first. as the earth travels in its annual path around the sun, the axis of the earth is directed to a point in the heavens which is - / ° from the pole of the ecliptic. the precessional effect of the sun is to cause this point--the pole of the earth--to revolve, always preserving the same angular distance from the pole of the ecliptic; and thus we have a motion of the type represented in the diagram. as the ecliptic occupies a position which for our present purpose we may regard as fixed in space, it follows that the pole of the ecliptic is a fixed point on the surface of the heavens; so that the path of the pole of the earth must be a small circle in the heavens, fixed in its position relatively to the surrounding stars. in this we find a motion strictly analogous to that of the peg-top. it is the gravitation of the earth acting upon the peg-top which forces it into the conical motion. the immediate effect of the gravitation is so modified by the rapid rotation of the top, that, in obedience to a profound dynamical principle, the axis of the top revolves in a cone rather than fall down, as it would do were the top not spinning. in a similar manner the immediate effect of the sun's attraction on the protuberance at the equator would be to bring the pole of the earth's axis towards the pole of the ecliptic, but the rapid rotation of the earth modifies this into the conical movement of precession. the circumstances with regard to the moon are much more complicated. the moon describes a certain orbit around the earth; that orbit lies in a certain plane, and that plane has, of course, a certain pole on the celestial sphere. the precessional effect of the moon would accordingly tend to make the pole of the earth's axis describe a circle around that point in the heavens which is the pole of the moon's orbit. this point is about ° from the pole of the ecliptic. the pole of the earth is therefore solicited by two different movements--one a revolution around the pole of the ecliptic, the other a revolution about another point ° distant, which is the pole of the moon's orbit. it would thus seem that the earth's pole should make a certain composite movement due to the two separate movements. this is really the case, but there is a point to be very carefully attended to, which at first seems almost paradoxical. we have shown how the potency of the moon as a precessional agent exceeds that of the sun, and therefore it might be thought that the composite movement of the earth's pole would conform more nearly to a rotation around the pole of the plane of the moon's orbit than to a rotation around the pole of the ecliptic; but this is not the case. the precessional movement is represented by a revolution around the pole of the ecliptic, as is shown in the figure. here lies the germ of one of those exquisite astronomical discoveries which delight us by illustrating some of the most subtle phenomena of nature. the plane in which the moon revolves does not occupy a constant position. we are not here specially concerned with the causes of this change in the plane of the moon's orbit, but the character of the movement must be enunciated. the inclination of this plane to the ecliptic is about °, and this inclination does not vary (except within very narrow limits); but the line of intersection of the two planes does vary, and, in fact, varies so quickly that it completes a revolution in about - / years. this movement of the plane of the moon's orbit necessitates a corresponding change in the position of its pole. we thus see that the pole of the moon's orbit must be actually revolving around the pole of the ecliptic, always remaining at the same distance of °, and completing its revolution in - / years. it will, therefore, be obvious that there is a profound difference between the precessional effect of the sun and of the moon in their action on the earth. the sun invites the earth's pole to describe a circle around a fixed centre; the moon invites the earth's pole to describe a circle around a centre which is itself in constant motion. it fortunately happens that the circumstances of the case are such as to reduce considerably the complexity of the problem. the movement of the moon's plane, only occupying about - / years, is a very rapid motion compared with the whole precessional movement, which occupies about , years. it follows that by the time the earth's axis has completed one circuit of its majestic cone, the pole of the moon's plane will have gone round about , times. now, as this pole really only describes a comparatively small cone of ° in radius, we may for a first approximation take the average position which it occupies; but this average position is, of course, the centre of the circle which it describes--that is, the pole of the ecliptic. we thus see that the average precessional effect of the moon simply conspires with that of the sun to produce a revolution around the pole of the ecliptic. the grosser phenomena of the movements of the earth's axis are to be explained by the uniform revolution of the pole in a circular path; but if we make a minute examination of the track of the earth's axis, we shall find that though it, on the whole, conforms with the circle, yet that it really traces out a sinuous line, sometimes on the inside and sometimes on the outside of the circle. this delicate movement arises from the continuous change in the place of the pole of the moon's orbit. the period of these undulations is - / years, agreeing exactly with the period of the revolution of the moon's nodes. the amount by which the pole departs from the circle on either side is only about · seconds--a quantity rather less than the twenty-thousandth part of the radius of the sphere. this phenomenon, known as "nutation," was discovered by the beautiful telescopic researches of bradley, in . whether we look at the theoretical interest of the subject or at the refinement of the observations involved, this achievement of the "vir incomparabilis," as bradley has been called by bessel, is one of the masterpieces of astronomical genius. the phenomena of precession and nutation depend on movements of the earth itself, and not on movements of the axis of rotation within the earth. therefore the distance of any particular spot on the earth from the north or south pole is not disturbed by either of these phenomena. the latitude of a place is the distance of the place from the earth's equator, and this quantity remains unaltered in the course of the long precession cycle of , years. but it has been discovered within the last few years that latitudes are subject to a small periodic change of a few tenths of a second of arc. this was first pointed out about by dr. küstner, of berlin, and by a masterly analysis of all available observations, made in the course of many years past at various observatories, dr. chandler, of boston, has shown that the latitude of every point on the earth is subject to a double oscillation, the period of one being days and the other about a year, the mean amplitude of each being o"· . in other words, the spot in the arctic regions, directly in the prolongation of the earth's axis of rotation, is not absolutely fixed; the end of the imaginary axis moves about in a complicated manner, but always keeping within a few yards of its average position. this remarkable discovery is not only of value as introducing a new refinement in many astronomical researches depending on an accurate knowledge of the latitude, but theoretical investigations show that the periods of this variation are incompatible with the assumption that the earth is an absolutely rigid body. though this assumption has in other ways been found to be untenable, the confirmation of this view by the discovery of dr. chandler is of great importance. chapter xxv. the aberration of light. the real and apparent movements of the stars--how they can be discriminated--aberration produces effects dependent on the position of the stars--the pole of the ecliptic--aberration makes stars seem to move in a circle, an ellipse, or a straight line according to position--all the ellipses have equal major axes--how is this movement to be explained?--how to be distinguished from annual parallax--the apex of the earth's way--how this is to be explained by the velocity of light--how the scale of the solar system can be measured by the aberration of light. we have in this chapter to narrate a discovery of a recondite character, which illustrates in a forcible manner some of the fundamental truths of astronomy. our discussion of it will naturally be divided into two parts. in the first part we must describe the nature of the phenomenon, and then we must give the extremely elegant explanation afforded by the properties of light. the telescopic discovery of aberration, as well as its explanation, are both due to the illustrious bradley. the expression _fixed_ star, so often used in astronomy, is to be received in a very qualified sense. the stars are, no doubt, well fixed in their places, so far as coarse observation is concerned. the lineaments of the constellations remain unchanged for centuries, and, in contrast with the ceaseless movements of the planets, the stars are not inappropriately called fixed. we have, however, had more than one occasion to show throughout the course of this work that the expression "fixed star" is not an accurate one when minute quantities are held in estimation. with the exact measures of modern instruments, many of these quantities are so perceptible that they have to be always reckoned with in astronomical enquiry. we can divide the movements of the stars into two great classes: the real movements and the apparent movements. the proper motion of the stars and the movements of revolution of the binary stars constitute the real movements of these bodies. these movements are special to each star, so that two stars, although close together in the heavens, may differ in the widest degree as to the real movements which they possess. it may, indeed, sometimes happen that stars in a certain region are animated with a common movement. in this phenomenon we have traces of a real movement shared by a number of stars in a certain group. with this exception, however, the real movements of the stars seem to be governed by no systematic law, and the rapidly moving stars are scattered here and there indiscriminately over the heavens. the apparent movements of the stars have a different character, inasmuch as we find the movement of each star determined by the place which it occupies in the heavens. it is by this means that we discriminate the real movements of the star from its apparent movements, and examine the character of both. in the present chapter we are concerned with the apparent movements only, and of these there are three, due respectively to precession, to nutation, and to aberration. each of these apparent movements obeys laws peculiar to itself, and thus it becomes possible to analyse the total apparent motion, and to discriminate the proportions in which the precession, the nutation, and the aberration have severally contributed. we are thus enabled to isolate the effect of aberration as completely as if it were the sole agent of apparent displacement, so that, by an alliance between mathematical calculation and astronomical observation, we can study the effects of aberration as clearly as if the stars were affected by no other motions. concentrating our attention solely on the phenomena of aberration we shall describe its particular effect upon stars in different regions of the sky, and thus ascertain the laws according to which the effects of aberration are exhibited. when this step has been taken, we shall be in a position to give the beautiful explanation of those laws dependent upon the velocity of light. at one particular region of the heavens the effect of aberration has a degree of simplicity which is not manifested anywhere else. this region lies in the constellation draco, at the pole of the ecliptic. at this pole, or in its immediate neighbourhood, each star, in virtue of aberration, describes a circle in the heavens. this circle is very minute; it would take something like , of these circles together to form an area equal to the area of the moon. expressed in the usual astronomical language, we should say that the diameter of this small circle is about · seconds of arc. this is a quantity which, though small to the unaided eye, is really of great relative magnitude in the present state of telescopic research. it is not only large enough to be perceived, but it can be measured, with an accuracy which actually does not admit of a doubt, to the hundredth part of the whole. it is also observed that each star describes its little circle in precisely the same period of time; and that period is one year, or, in other words, the time of the revolution of the earth around the sun. it is found that for all stars in this region, be they large stars or small, single or double, white or coloured, the circles appropriate to each have all the same size, and are all described in the same time. even from this alone it would be manifest that the cause of the phenomenon cannot lie in the star itself. this unanimity in stars of every magnitude and distance requires some simpler explanation. further examination of stars in different regions sheds new light on the subject. as we proceed from the pole of the ecliptic, we still find that each star exhibits an annual movement of the same character as the stars just considered. in one respect, however, there is a difference. the apparent path of the star is no longer a circle; it has become an ellipse. it is, however, soon perceived that the shape and the position of this ellipse are governed by the simple law that the further the star is from the pole of the ecliptic the greater is the eccentricity of the ellipse. the apparent path of the stars at the same distance from the pole have equal eccentricity, and of the axes of the ellipse the shorter is always directed to the pole, the longer being, of course, perpendicular to it. it is, however, found that no matter how great the eccentricity may become, the major axis always retains its original length. it is always equal to about · seconds--that is, to the diameter of the circle of aberration at the pole itself. as we proceed further and further from the pole of the ecliptic, we find that each star describes a path more and more eccentric, until at length, when we examine a star on the ecliptic, the ellipse has become so attenuated that it has flattened into a line. each star which happens to lie on the ecliptic oscillates to and fro along the ecliptic through an amplitude of · seconds. half a year accomplishes the journey one way, and the other half of the year restores the star to its original position. when we pass to stars on the southern side of the ecliptic, we see the same series of changes proceed in an inverse order. the ellipse, from being actually linear, gradually grows in width, though still preserving the same length of major axis, until at length the stars near the southern pole of the ecliptic are each found to describe a circle equal to the paths pursued by the stars at the north pole of the ecliptic. the circumstance that the major axes of all those ellipses are of equal length suggests a still further simplification. let us suppose that every star, either at the pole of the ecliptic or elsewhere, pursues an absolutely circular path, and that all these circles agree not only in magnitude, but also in being all parallel to the plane of the ecliptic: it is easy to see that this simple supposition will account for the observed facts. the stars at the pole of the ecliptic will, of course, show their circles turned fairly towards us, and we shall see that they pursue circular paths. the circular paths of the stars remote from the pole of the ecliptic will, however, be only seen somewhat edgewise, and thus the apparent paths will be elliptical, as we actually find them. we can even calculate the degree of ellipticity which this surmise would require, and we find that it coincides with the observed ellipticity. finally, when we observe stars actually moving in the ecliptic, the circles they follow would be seen edgewise, and thus the stars would have merely the linear movement which they are seen to possess. all the observed phenomena are thus found to be completely consistent with the supposition that every star of all the millions in the heavens describes once each year a circular path; and that, whether the star be far or near, this circle has always the same apparent diameter, and lies in a plane always parallel to the plane of the ecliptic. we have now wrought the facts of observation into a form which enables us to examine into the cause of a movement so systematic. why is it that each star should seem to describe a small circular path? why should that path be parallel to the ecliptic? why should it be completed exactly in a twelvemonth? we are at once referred to the motion of the earth around the sun. that movement takes place in the ecliptic. it is completed in a year. the coincidences are so obvious that we feel almost necessarily compelled to connect in some way this apparent movement of the stars with the annual movement of the earth around the sun. if there were no such connection, it would be in the highest degree improbable that the planes of the circles should be all parallel to the ecliptic, or that the time of revolution of each star in its circle should equal that of the revolution of the earth around the sun. as both these conditions are fulfilled, the probability of the connection rises to a value almost infinite. the important question has then arisen as to why the movement of the earth around the sun should be associated in so remarkable a manner with this universal star movement. there is here one obvious point to be noticed and to be dismissed. we have in a previous chapter discussed the important question of the annual parallax of stars, and we have shown how, in virtue of annual parallax, each star describes an ellipse. it can further be demonstrated that these ellipses are really circles parallel to the ecliptic; so that we might hastily assume that annual parallax was the cause of the phenomenon discovered by bradley. a single circumstance will, however, dispose of this suggestion. the circle described by a star in virtue of annual parallax has a magnitude dependent on the distance of the star, so that the circles described by various stars are of various dimensions, corresponding to the varied distances of different stars. the phenomena of aberration, however, distinctly assert that the circular path of each star is of the same size, quite independently of what its distance may be, and hence annual parallax will not afford an adequate explanation. it should also be noticed that the movements of a star produced by annual parallax are much smaller than those due to aberration. there is not any known star whose circular path due to annual parallax has a diameter one-twentieth part of that of the circle due to aberration; indeed, in the great majority of cases the parallax of the star is an absolutely insensible quantity. there is, however, a still graver and quite insuperable distinction between the parallactic path and the aberrational path. let us, for simplicity, think of a star situated near the pole of the ecliptic, and thus appearing to revolve annually in a circle, whether we regard either the phenomenon of parallax or of aberration. as the earth revolves, so does the star appear to revolve; and thus to each place of the earth in its orbit corresponds a certain place of the star in its circle. if the movement arise from annual parallax, it is easy to see where the place of the star will be for any position of the earth. it is, however, found that in the movement discovered by bradley the star never has the position which parallax assigns to it, but is, in fact, a quarter of the circumference of its little circle distant therefrom. a simple rule will find the position of the star due to aberration. draw from the centre of the ellipse a radius parallel to the direction in which the earth is moving at the moment in question, then the extremity of this radius gives the point on its ellipse where the star is to be found. tested at all seasons, and with all stars, this law is found to be always verified, and by its means we are conducted to the true explanation of the phenomenon. we can enunciate the effects of aberration in a somewhat different manner, which will show even more forcibly how the phenomenon is connected with the motion of the earth in its orbit. as the earth pursues its annual course around the sun, its movement at any moment may be regarded as directed towards a certain point of the ecliptic. from day to day, and even from hour to hour, the point gradually moves along the ecliptic, so as to complete the circuit in a year. at each moment, however, there is always a certain point in the heavens towards which the earth's motion is directed. it is, in fact, the point on the celestial sphere towards which the earth would travel continuously if, at the moment, the attraction of the sun could be annihilated. it is found that this point is intimately connected with the phenomenon of aberration. in fact, the aberration is really equivalent to drawing each star from its mean place towards the apex of the earth's way, as the point is sometimes termed. it can also be shown by observation that the amount of aberration depends upon the distance from the apex. a star which happened to lie on the ecliptic will not be at all deranged by aberration from its mean place when it happens that the apex coincides with the star. all the stars ° from the apex will be displaced each by the same amount, and all directly in towards the apex. a star ° from the apex will undergo a larger degree of displacement, though still in the same direction, exactly towards the apex; and all stars at the same distance will be displaced by the same amount. proceeding thus from the apex, we come to stars at a distance of ° therefrom. here the amount of displacement will be a maximum. each one will be about twenty seconds from its average place; but in every case the imperative law will be obeyed, that the displacement of the star from its mean place lies towards the apex of the earth's way. we have thus given two distinct descriptions of the phenomenon of aberration. in the first we find it convenient to speak of a star as describing a minute circular path; in the other we have regarded aberration as merely amounting to a derangement of the star from its mean place in accordance with specified laws. these descriptions are not inconsistent: they are, in fact, geometrically equivalent; but the latter is rather the more perfect, inasmuch as it assigns completely the direction and extent of the derangement caused by aberration in any particular star at any particular moment. the question has now been narrowed to a very definite form. what is it which makes each star seem to close in towards the point towards which the earth is travelling? the answer will be found when we make a minute enquiry into the circumstances in which we view a star in the telescope. the beam of rays from a star falls on the object-glass of a telescope; those rays are parallel, and after they pass through the object-glass they converge to a focus near the eye end of the instrument. let us first suppose that the telescope is at rest; then if the telescope be pointed directly towards the star, the rays will converge to a point at the centre of the field of view where a pair of cross wires are placed, whose intersection defines the axis of the telescope. the case will, however, be altered if the telescope be moved after the light has passed through the objective; the rays of light in the interior of the tube will pursue a direct path, as before, and will proceed to a focus at the same precise point as before. as, however, the telescope has moved, it will, of course, have carried with it the pair of cross wires; they will no longer be at the same point as at first, and consequently the image of the star will not now coincide with their intersection. the movement of the telescope arises from its connection with the earth: for as the earth hurries along at a speed of eighteen miles a second, the telescope is necessarily displaced with this velocity. it might at first be thought, that in the incredibly small fraction of time necessary for light to pass from the object-glass to the eye-piece, the change in the position of the telescope must be too minute to be appreciable. let us suppose, for instance, that the star is situated near the pole of the ecliptic, then the telescope will be conveyed by the earth's motion in a direction perpendicular to its length. if the tube of the instrument be about twenty feet long, it can be readily demonstrated that during the time the light travels down the tube the movement of the earth will convey the telescope through a distance of about one-fortieth of an inch.[ ] this is a quantity very distinctly measurable with the magnifying power of the eye-piece, and hence this derangement of the star's place is very appreciable. it therefore follows that if we wish the star to be shown at the centre of the instrument, the telescope is not to be pointed directly at the star, as it would have to be were the earth at rest, but the telescope must be pointed a little in advance of the star's true position; and as we determine the apparent place of the star by the direction in which the telescope is pointed, it follows that the apparent place of the star is altered by the motion of the earth. every circumstance of the change in the star's place admits of complete explanation in this manner. take, for instance, the small circular path which each star appears to describe. we shall, for simplicity, refer only to a star at the pole of the ecliptic. suppose that the telescope is pointed truly to the place of the star, then, as we have shown, the image of the star will be at a distance of one-fortieth of an inch from the cross wires. this distance will remain constant, but each night the direction of the star from the cross wires will change, so that in the course of the year it completes a circle, and returns to its original position. we shall not pursue the calculations relative to other stars; suffice it here to say that the movement of the earth has been found adequate to account for the phenomena, and thus the doctrine of the aberration of light is demonstrated. it remains to allude to one point of the utmost interest and importance. we have seen that the magnitude of the aberration can be measured by astronomical observation. the amount of this aberration depends upon the velocity of light, and on the velocity with which the earth's motion is performed. we can measure the velocity of light by independent measurements, in the manner already explained in chapter xii. we are thus enabled to calculate what the velocity of the earth must be, for there is only one particular velocity for the earth which, when combined with the measured velocity of light, will give the measured value of aberration. the velocity of the earth being thus ascertained, and the length of the year being known, it is easy to find the circumference of the earth's path, and therefore its radius; that is, the distance from the earth to the sun. here is indeed a singular result, and one which shows how profoundly the various phenomena of science are interwoven. we make experiments in our laboratory, and find the velocity of light. we observe the fixed stars, and measure the aberration. we combine these results, and deduce therefrom the distance from the earth to the sun! although this method of finding the sun's distance is one of very great elegance, and admits of a certain amount of precision, yet it cannot be relied upon as a perfectly unimpeachable method of deducing the great constant. a perfect method must be based on the operations of mere surveying, and ought not to involve recondite physical considerations. we cannot, however, fail to regard the discovery of aberration by bradley as a most pleasing and beautiful achievement, for it not only greatly improves the calculations of practical astronomy, but links together several physical phenomena of the greatest interest. chapter xxvi. the astronomical significance of heat. heat and astronomy--distribution of heat--the presence of heat in the earth--heat in other celestial bodies--varieties of temperature--the law of cooling--the heat of the sun--can its temperature be measured?--radiation connected with the sun's bulk--can the sun be exhausting his resources?--no marked change has occurred--geological evidence as to the changes of the sun's heat doubtful--the cooling of the sun--the sun cannot be merely an incandescent solid cooling--combustion will not explain the matter--some heat is obtained from meteoric matter, but this is not adequate to the maintenance of the sun's heat--the contraction of a heated globe of gas--an apparent paradox--the doctrine of energy--the nebular theory--evidence in support of this theory--sidereal evidence of the nebular theory--herschel's view of sidereal aggregation--the nebulæ do not exhibit changes within the limits of our observation. that a portion of a work on astronomy should bear the title placed at the head of this chapter will perhaps strike some of our readers as unusual, if not actually inappropriate. is not heat, it may be said, a question merely of experimental physics? and how can it be legitimately introduced into a treatise upon the heavenly bodies and their movements? whatever weight such objections might have once had need not now be considered. the recent researches on heat have shown not only that heat has important bearings on astronomy, but that it has really been one of the chief agents by which the universe has been moulded into its actual form. at the present time no work on astronomy could be complete without some account of the remarkable connection between the laws of heat and the astronomical consequences which follow from those laws. in discussing the planetary motions and the laws of kepler, or in discussing the movements of the moon, the proper motions of the stars, or the revolutions of the binary stars, we proceed on the supposition that the bodies we are dealing with are rigid particles, and the question as to whether these particles are hot or cold does not seem to have any especial bearing. no doubt the ordinary periodic phenomena of our system, such as the revolution of the planets in conformity with kepler's laws, will be observed for countless ages, whether the planets be hot or cold, or whatever may be the heat of the sun. it must, however, be admitted that the laws of heat introduce certain modifications into the statement of these laws. the effects of heat may not be immediately perceptible, but they exist--they are constantly acting; and in the progress of time they are adequate to effecting the mightiest changes throughout the universe. let us briefly recapitulate the circumstances of our system which give to heat its potency. look first at our earth, which at present seems--on its surface, at all events--to be a body devoid of internal heat; a closer examination will dispel this idea. have we not the phenomena of volcanoes, of geysers, and of hot springs, which show that in the interior of the earth heat must exist in far greater intensity than we find on the surface? these phenomena are found in widely different regions of the earth. their origin is, no doubt, involved in a good deal of obscurity, but yet no one can deny that they indicate vast reservoirs of heat. it would indeed seem that heat is to be found everywhere in the deep inner regions of the earth. if we take a thermometer down a deep mine, we find it records a temperature higher than at the surface. the deeper we descend the higher is the temperature; and if the same rate of progress should be maintained through those depths of the earth which we are not able to penetrate, it can be demonstrated that at twenty or thirty miles below the surface the temperature must be as great as that of red-hot iron. we find in the other celestial bodies abundant evidence of the present or the past existence of heat. our moon, as we have already mentioned, affords a very striking instance of a body which must once have been very highly heated. the extraordinary volcanoes on its surface place this beyond any doubt. it is equally true that those volcanoes have been silent for ages, so that, whatever may be the interior condition of the moon, the surface has now cooled down. extending our view further, we see in the great planets jupiter and saturn evidence that they are still endowed with a temperature far in excess of that which the earth has retained; while, when we look at our sun, we see a body in a state of brilliant incandescence, and glowing with a fervour to which we cannot approximate in our mightiest furnaces. the various fixed stars are bodies which glow with heat, like our sun; while we have in the nebulæ objects the existence of which is hardly intelligible to us, unless we admit that they are possessed of heat. from this rapid survey of the different bodies in our universe one conclusion is obvious. we may have great doubts as to the actual temperature of any individual body of the system; but it cannot be doubted that there is a wide range of temperature among the different bodies. some are hotter than others. the stars and suns are perhaps the hottest of all; but it is not improbable that they may be immeasurably outnumbered by the cold and dark bodies of the universe, which are to us invisible, and only manifest their existence in an indirect and casual manner. the law of cooling tells us that every body radiates heat, and that the quantity of heat which it radiates increases when the temperature of the body increases relatively to the surrounding medium. this law appears to be universal. it is obeyed on the earth, and it would seem that it must be equally obeyed by every other body in space. we thus see that each of the planets and each of the stars is continuously pouring forth in all directions a never-ceasing stream of heat. this radiation of heat is productive of very momentous consequences. let us study them, for instance, in the case of the sun. our great luminary emits an incessant flood of radiant heat in all directions. a minute fraction of that heat is intercepted by our earth, and is, directly or indirectly, the source of all life, and of nearly all movement, on our earth. to pour forth heat as the sun does, it is necessary that his temperature be enormously high. and there are some facts which permit us to form an estimate of what that temperature must actually be. it is difficult to form any numerical statement of the actual temperature of the sun. the intensity of that temperature vastly transcends the greatest artificial heat, and any attempt to clothe such estimates in figures is necessarily very precarious. but assuming the greatest artificial temperature to be about , ° fahr., we shall probably be well within the truth if we state the effective temperature of the sun to be about , ° fahr. this is the result of a recent investigation by messrs. wilson and gray, which seems to be entitled to considerable weight. the copious outflow of heat from the sun corresponds with its enormous temperature. we can express the amount of heat in various ways, but it must be remembered that considerable uncertainty still attaches to such measurements. the old method of measuring heat by the quantity of ice melted may be used as an illustration. it is computed that a shell of ice - / feet thick surrounding the whole sun would in one minute be melted by the sun's heat underneath. a somewhat more elegant illustration was also given by sir john herschel, who showed that if a cylindrical glacier miles in diameter were to be continually flowing into the sun with the velocity of light, the end of that glacier would be melted as quickly as it advanced. from each square foot in the surface of the sun emerges a quantity of heat as great as could be produced by the daily combustion of sixteen tons of coal. this is, indeed, an amount of heat which, properly transformed into work, would keep an engine of many hundreds of horse-power running from one year's end to the other. the heat radiated from a few acres on the sun would be adequate to drive all the steam engines in the world. when we reflect on the vast intensity of the radiation from each square foot of the sun's surface, and when we combine with this the stupendous dimensions of the sun, imagination fails to realise how vast must be the actual expenditure of heat. in presence of the prodigal expenditure of the sun's heat, we are tempted to ask a question which has the most vital interest for the earth and its inhabitants. we live from hour to hour by the sun's splendid generosity; and, therefore, it is important for us to know what security we possess for the continuance of his favours. when we witness the terrific disbursement of the sun's heat each hour, we are compelled to ask whether our great luminary may not be exhausting its resources; and if so, what are the prospects of the future? this question we can partly answer. the whole subject is indeed of surpassing interest, and redolent with the spirit of modern scientific thought. our first attempt to examine this question must lie in an appeal to the facts which are attainable. we want to know whether the sun is showing any symptoms of decay. are the days as warm and as bright now as they were last year, ten years ago, one hundred years ago? we can find no evidence of any change since the beginning of authentic records. if the sun's heat had perceptibly changed within the last two thousand years, we should expect to find corresponding changes in the distribution of plants and of animals; but no such changes have been detected. there is no reason to think that the climate of ancient greece or of ancient rome was appreciably different from the climates of the greece and the rome that we know at this day. the vine and the olive grow now where they grew two thousand years ago. we must not, however, lay too much stress on this argument; for the effects of slight changes in the sun's heat may have been neutralised by corresponding adaptations in the pliable organisms of cultivated plants. all we can certainly conclude is that no marked change has taken place in the heat of the sun during historical time. but when we come to look back into much earlier ages, we find copious evidence that the earth has undergone great changes in climate. geological records can on this question hardly be misinterpreted. yet it is curious to note that these changes are hardly such as could arise from the gradual exhaustion of the sun's radiation. no doubt, in very early times we have evidence that the earth's climate must have been much warmer than at present. we had the great carboniferous period, when the temperature must almost have been tropical in arctic latitudes. yet it is hardly possible to cite this as evidence that the sun was then much more powerful; for we are immediately reminded of the glacial period, when our temperate zones were overlaid by sheets of solid ice, as northern greenland is at present. if we suppose the sun to have been hotter than it is at present to account for the vegetation which produced coal, then we ought to assume the sun to be colder than it is now to account for the glacial period. it is not reasonable to attribute such phenomena to fluctuations in the radiation from the sun. the glacial periods prove that we cannot appeal to geology in aid of the doctrine that a secular cooling of the sun is now in progress. the geological variations of climate may have been caused by changes in the earth itself, or by changes in its actual orbit; but however they have been caused, they hardly tell us much with regard to the past history of our sun. the heat of the sun has lasted countless ages; yet we cannot credit the sun with the power of actually creating heat. we must apply to the tremendous mass of the sun the same laws which we have found by our experiments on the earth. we must ask, whence comes the heat sufficient to supply this lavish outgoing? let us briefly recount the various suppositions that have been made. place two red-hot spheres of iron side by side, a large one and a small one. they have been taken from the same fire; they were both equally hot; they are both cooling, but the small sphere cools more rapidly. it speedily becomes dark, while the large sphere is still glowing, and would continue to do so for some minutes. the larger the sphere, the longer it will take to cool; and hence it has been supposed that a mighty sphere of the prodigious dimensions of our sun would, if once heated, cool gradually, but the duration of the cooling would be so long that for thousands and for millions of years it could continue to be a source of light and heat to the revolving system of planets. this suggestion will not bear the test of arithmetic. if the sun had no source of heat beyond that indicated by its high temperature, we can show that radiation would cool the sun a few degrees every year. two thousand years would then witness a very great decrease in the sun's heat. we are certain that no such decrease can have taken place. the source of the sun's radiation cannot be found in the mere cooling of an incandescent mass. can the fires in the sun be maintained by combustion, analogous to that which goes on in our furnaces? here we would seem to have a source of gigantic heat; but arithmetic also disposes of this supposition. we know that if the sun were made of even solid coal itself, and if that coal were burning in pure oxygen, the heat that could be produced would only suffice for , years. if the sun which shone upon the builders of the great pyramid had been solid coal from surface to centre, it must by this time have been in great part burned away in the attempt to maintain its present rate of expenditure. we are thus forced to look to other sources for the supply of the sun's heat, since neither the heat of incandescence nor the heat of combustion will suffice. there is probably--indeed, we may say certainly--one external source from which the heat of the sun is recruited. it will be necessary for us to consider this source with some care, though i think we shall find it to be merely an auxiliary of comparatively trifling moment. according to this view, the solar heat receives occasional accessions from the fall upon the sun's surface of masses of meteoric matter. there can be hardly a doubt that such masses do fall upon the sun; there is certainly no doubt that if they do, the sun must gain some heat thereby. we have experience on the earth of a very interesting kind, which illustrates the development of heat by meteoric matter. there lies a world of philosophy in a shooting star. some of these myriad objects rush into our atmosphere and are lost; others, no doubt, rush into the sun with the same result. we also admit that the descent of a shooting star into the atmosphere of the sun must be attended with a flash of light and of heat. the heat acquired by the earth from the flashing of the shooting stars through our air is quite insensible. it has been supposed, however, that the heat accruing to the sun from the same cause may be quite sensible--nay, it has been even supposed that the sun may be re-invigorated from this source. here, again, we must apply the cold principles of weights and measures to estimate the plausibility of this suggestion. we first calculate the actual weight of meteoric indraught to the sun which would be adequate to sustain the fires of the sun at their present vigour. the mass of matter that would be required is so enormous that we cannot usefully express it by imperial weights; we must deal with masses of imposing magnitude. it fortunately happens that the weight of our moon is a convenient unit. conceive that our moon--a huge globe, , miles in diameter--were crushed into a myriad of fragments, and that these fragments were allowed to rain in on the sun; there can be no doubt that this tremendous meteoric shower would contribute to the sun rather more heat than would be required to supply his radiation for a whole year. if we take our earth itself, conceive it comminuted into dust, and allow that dust to fall on the sun as a mighty shower, each fragment would instantly give out a quantity of heat, and the whole would add to the sun a supply of heat adequate to sustain the present rate of radiation for nearly one hundred years. the mighty mass of jupiter treated in the same way would generate a meteoric display greater in the ratio in which the mass of jupiter exceeds the mass of earth. were jupiter to fall into the sun, enough heat would be thereby produced to scorch the whole solar system; while all the planets together would be capable of producing heat which, if properly economised, would supply the radiation of the sun for , years. it must be remembered that though the moon could supply one year's heat, and jupiter , years' heat, yet the practical question is not whether the solar system could supply the sun's heat, but whether it does. is it likely that meteors equal in mass to the moon fall into the sun every year? this is the real question, and i think we are bound to reply to it in the negative. it can be shown that the quantity of meteors which could be caught by the sun in any one year can be only an excessively minute fraction of the total amount. if, therefore, a moon-weight of meteors were caught every year, there must be an incredible mass of meteoric matter roaming at large through the system. there must be so many meteors that the earth would be incessantly pelted with them, and heated to such a degree as to be rendered uninhabitable. there are also other reasons which preclude the supposition that a stupendous quantity of meteoric matter exists in the vicinity of the sun. such matter would produce an appreciable effect on the movement of the planet mercury. there are, no doubt, some irregularities in the movements of mercury not yet fully explained, but these irregularities are very much less than would be the case if meteoric matter existed in quantity adequate to the sustentation of the sun. astronomers, then, believe that though meteors may provide a rate in aid of the sun's current expenditure, yet that the greater portion of that expenditure must be defrayed from other resources. it is one of the achievements of modern science to have effected the solution of the problem--to have shown how it is that, notwithstanding the stupendous radiation, the sun still maintains its temperature. the question is not free from difficulty in its exposition, but the matter is one of such very great importance that we are compelled to make the attempt. let us imagine a vast globe of heated gas in space. this is not an entirely gratuitous supposition, inasmuch as there are globes apparently of this character; they have been already alluded to as planetary nebulæ. this globe will radiate heat, and we shall suppose that it emits more heat than it receives from the radiation of other bodies. the globe will accordingly lose heat, or what is equivalent thereto, but it will be incorrect to assume that the globe will necessarily fall in temperature. that the contrary is, indeed, the case is a result almost paradoxical at the first glance; but yet it can be shown to be a necessary consequence of the laws of heat and of gases. let us fix our attention on a portion of the gas lying on the surface of the globe. this is, of course, attracted by all the rest of the globe, and thus tends in towards the centre of the globe. if equilibrium subsists, this tendency must be neutralised by the pressure of the gas beneath; so that the greater the gravitation, the greater is the pressure. when the globe of gas loses heat by radiation, let us suppose that it grows colder--that its temperature accordingly falls; then, since the pressure of a gas decreases when the temperature falls, the pressure beneath the superficial layer of the gas will decrease, while the gravitation is unaltered. the consequence will inevitably be that the gravitation will now conquer the pressure, and the globe of gas will accordingly contract. there is, however, another way in which we can look at the matter. we know that heat is equivalent to energy, so that when the globe radiates forth heat, it must expend energy. a part of the energy of the globe will be due to its temperature; but another, and in some respects a more important, part is that due to the separation of its particles. if we allow the particles to come closer together we shall diminish the energy due to separation, and the energy thus set free can take the form of heat. but this drawing in of the particles necessarily involves a shrinking of the globe. and now for the remarkable consequence, which seems to have a very important application in astronomy. as the globe contracts, a part of its energy of separation is changed into heat; that heat is partly radiated away, but not so rapidly as it is produced by the contraction. the consequence is, that although the globe is really losing heat and really contracting, yet that its temperature is actually rising.[ ] a simple case will suffice to demonstrate this result, paradoxical as it may at first seem. let us suppose that by contraction of the sphere it had diminished to one-half its diameter; and let us fix our attention on a cubic inch of the gaseous matter in any point of the mass. after the contraction has taken place each edge of the cube would be reduced to half an inch, and the volume would therefore be reduced to one-eighth part of its original amount. the law of gases tells us that if the temperature be unaltered the pressure varies inversely as the volume, and consequently the internal pressure in the cube would in that case be increased eightfold. as, however, in the case before us, the distance between every two particles is reduced to one-half, it will follow that the gravitation between every two particles is increased fourfold, and as the area is also reduced to one-fourth, it will follow that the pressure inside the reduced cube is increased sixteenfold; but we have already seen that with a constant temperature it only increases eightfold, and hence the temperature cannot be constant, but must rise with the contraction. we thus have the somewhat astonishing result that a gaseous globe in space radiating heat, and thereby growing smaller, is all the time actually increasing in temperature. but, it may be said, surely this cannot go on for ever. are we to suppose that the gaseous mass will go on contracting and contracting with a temperature ever fiercer and fiercer, and actually radiating out more and more heat the more it loses? where lies the limit to such a prospect? as the body contracts, its density must increase, until it either becomes a liquid, or a solid, or, at any rate, until it ceases to obey the laws of a purely gaseous body which we have supposed. once these laws cease to be observed the argument disappears; the loss of heat may then really be attended with a loss of temperature, until in the course of time the body has sunk to the temperature of space itself. it is not assumed that this reasoning can be applied in all its completeness to the present state of the sun. the sun's density is now so great that the laws of gases cannot be there strictly followed. there is, however, good reason to believe that the sun was once more gaseous than at present; possibly at one time he may have been quite gaseous enough to admit of this reasoning in all its fulness. at present the sun appears to be in some intermediate stage of its progress from the gaseous condition to the solid condition. we cannot, therefore, say that the temperature of the sun is now increasing in correspondence with the process of contraction. this may be true or it may not be true; we have no means of deciding the point. we may, however, feel certain that the sun is still sufficiently gaseous to experience in some degree the rise of temperature associated with the contraction. that rise in temperature may be partly or wholly obscured by the fall in temperature which would be the more obvious consequence of the radiation of heat from the partially solid body. it will, however, be manifest that the cooling of the sun may be enormously protracted if the fall of temperature from the one cause be nearly compensated by the rise of temperature from the other. it can hardly be doubted that in this we find the real explanation of the fact that we have no historical evidence of any appreciable alteration in the radiation of heat from the sun. this question is one of such interest that it may be worth while to look at it from a slightly different point of view. the sun contains a certain store of energy, part of which is continually disappearing in the form of radiant heat. the energy remaining in the sun is partly transformed in character; some of it is transformed into heat, which goes wholly or partly to supply the loss by radiation. the total energy of the sun must, however, be decreasing; and hence it would seem the sun must at some time or other have its energy exhausted, and cease to be a source of light and of heat. it is true that the rate at which the sun contracts is very slow. we are, indeed, not able to measure with certainty the decrease in the sun's bulk. it is a quantity so minute, that the contraction since the birth of accurate astronomy is not large enough to be perceptible in our telescopes. it is, however, possible to compute what the contraction of the sun's bulk must be, on the supposition that the energy lost by that contraction just suffices to supply the daily radiation of heat. the change is very small when we consider the present size of the sun. at the present time the sun's diameter is about , miles. if each year this diameter decreases by about feet, sufficient energy will be yielded to account for the entire radiation. this gradual decrease is always in progress. these considerations are of considerable interest when we apply them retrospectively. if it be true that the sun is at this moment shrinking, then in past times his globe must have been greater than it is at present. assuming the figures already given, it follows that one hundred years ago the diameter of the sun must have been nearly six miles greater than it is now; one thousand years ago the diameter was fifty-seven miles greater; ten thousand years ago the diameter of the sun was five hundred and seventy miles greater than it is to-day. when man first trod this earth it would seem that the sun must have been many hundreds, perhaps many thousands, of miles greater than it is at this time. we must not, however, over-estimate the significance of this statement. the diameter of the sun is so great, that a diminution of , miles would be but little more than the hundredth part of its diameter. if it were suddenly to shrink to the extent of , miles, the change would not be appreciable to ordinary observation, though a much smaller change would not elude delicate astronomical measurement. it does not necessarily follow that the climates on our earth in these early times must have been very different from those which we find at this day, for the question of climate depends upon other matters besides sunbeams. yet we need not abruptly stop our retrospect at any epoch, however remote. we may go back earlier and earlier, through the long ages which geologists claim for the deposition of the stratified rocks; and back again still further, to those very earliest epochs when life began to dawn on the earth. still we can find no reason to suppose that the law of the sun's decreasing heat is not maintained; and thus we would seem bound by our present knowledge to suppose that the sun grows larger and larger the further our retrospect extends. we cannot assume that the rate of that growth is always the same. no such assumption is required; it is sufficient for our purpose that we find the sun growing larger and larger the further we peer back into the remote abyss of time past. if the present order of things in our universe has lasted long enough, then it would seem that there was a time when the sun must have been twice as large as it is at present; it must once have been ten times as large. how long ago that was no one can venture to say. but we cannot stop at the stage when the sun was even ten times as large as it is at present; the arguments will still apply in earlier ages. we see the sun swelling and swelling, with a corresponding decrease in its density, until at length we find, instead of our sun as we know it, a mighty nebula filling a gigantic region of space. such is, in fact, the doctrine of the origin of our system which has been advanced in that celebrated speculation known as the nebular theory of laplace. nor can it be ever more than a speculation; it cannot be established by observation, nor can it be proved by calculation. it is merely a conjecture, more or less plausible, but perhaps in some degree necessarily true, if our present laws of heat, as we understand them, admit of the extreme application here required, and if also the present order of things has reigned for sufficient time without the intervention of any influence at present unknown to us. this nebular theory is not confined to the history of our sun. precisely similar reasoning may be extended to the individual planets: the farther we look back, the hotter and the hotter does the whole system become. it has been thought that if we could look far enough back, we should see the earth too hot for life; back further still, we should find the earth and all the planets red-hot; and back further still, to an exceedingly remote epoch, when the planets would be heated just as much as our sun is now. in a still earlier stage the whole solar system is thought to have been one vast mass of glowing gas, from which the present forms of the sun, with the planets and their satellites, have been gradually evolved. we cannot be sure that the course of events has been what is here indicated; but there are sufficient grounds for thinking that this doctrine substantially represents what has actually occurred. many of the features in the solar system harmonise with the supposition that the origin of the system has been that suggested by the nebular theory. we have already had occasion in an earlier chapter to allude to the fact that all the planets perform their revolutions around the sun in the same direction. it is also to be observed that the rotation of the planets on their axes, as well as the movements of the satellites around their primaries, all follow the same law, with two slight exceptions in the case of the uranian and neptunian systems. a coincidence so remarkable naturally suggests the necessity for some physical explanation. such an explanation is offered by the nebular theory. suppose that countless ages ago a mighty nebula was slowly rotating and slowly contracting. in the process of contraction, portions of the condensed matter of the nebula would be left behind. these portions would still revolve around the central mass, and each portion would rotate on its axis in the same direction. as the process of contraction proceeded, it would follow from dynamical principles that the velocity of rotation would increase; and thus at length these portions would consolidate into planets, while the central mass would gradually contract to form the sun. by a similar process on a smaller scale the systems of satellites were evolved from the contracting primary. these satellites would also revolve in the same direction, and thus the characteristic features of the solar system could be accounted for. the nebular origin of the solar system receives considerable countenance from the study of the sidereal heavens. we have already dwelt upon the resemblance between the sun and the stars. if, then, our sun has passed through such changes as the nebular theory requires, may we not anticipate that similar phenomena should be met with in other stars? if this be so, it is reasonable to suppose that the evolution of some of the stars may not have progressed so far as has that of the sun, and thus we may be able actually to witness stars in the earlier phases of their development. let us see how far the telescope responds to these anticipations. the field of view of a large telescope usually discloses a number of stars scattered over a black background of sky; but the blackness of the background is not uniform: the practised eye of the skilled observer will detect in some parts of the heavens a faint luminosity. this will sometimes be visible over the whole extent of the field, or it may even occupy several fields. years may pass on, and still there is no perceptible change. there can be no illusion, and the conclusion is irresistible that the object is a stupendous mass of faintly luminous glowing gas or vapour. this is the simplest type of nebula; it is characterised by extreme faintness, and seems composed of matter of the utmost tenuity. on the other hand we are occasionally presented with the beautiful and striking phenomenon of a definite and brilliant star surrounded by a luminous atmosphere. between these two extreme types of a faint diffused mass on the one hand, and a bright star with a nebula surrounding it on the other, a graduated series of various other nebulæ can be arranged. we thus have a series of links passing by imperceptible gradations from the most faintly diffused nebulæ on the one side, into stars on the other. the nebulæ seemed to herschel to be vast masses of phosphorescent vapour. this vapour gradually cools down, and ultimately condenses into a star, or a cluster of stars. when the varied forms of nebulæ were classified, it almost seemed as if the different links in the process could be actually witnessed. in the vast faint nebulæ the process of condensation had just begun; in the smaller and brighter nebulæ the condensation had advanced farther; while in others, the star, or stars, arising from the condensation had already become visible. but, it may be asked, how did herschel know this? what is his evidence? let us answer this question by an illustration. go into a forest, and look at a noble old oak which has weathered the storm for centuries; have we any doubt that the oak-tree was once a young small plant, and that it grew stage by stage until it reached maturity? yet no one has ever followed an oak-tree through its various stages; the brief span of human life has not been long enough to do so. the reason why we believe the oak-tree to have passed through all these stages is, because we are familiar with oak-trees of every gradation in size, from the seedling up to the noble veteran. having seen this gradation in a vast multitude of trees, we are convinced that each individual passes through all these stages. it was by a similar train of reasoning that herschel was led to adopt the view of the origin of the stars which we have endeavoured to describe. the astronomer's life is not long enough, the life of the human race might not be long enough, to watch the process by which a nebula condenses down so as to form a solid body. but by looking at one nebula after another, the astronomer thinks he is able to detect the various stages which connect the nebula in its original form with the final form. he is thus led to believe that each of the nebulæ passes, in the course of ages, through these stages. and thus herschel adopted the opinion that stars--some, many, or all--have each originated from what was once a glowing nebula. such a speculation may captivate the imagination, but it must be carefully distinguished from the truths of astronomy, properly so called. remote posterity may perhaps obtain evidence on the subject which to us is inaccessible: our knowledge of nebulæ is too recent. there has not yet been time enough to detect any appreciable changes: for the study of nebulæ can only be said to date from messier's catalogue in . since herschel's time, no doubt, many careful drawings and observations of the nebulæ have been obtained; but still the interval has been much too short, and the earlier observations are too imperfect, to enable any changes in the nebulæ to be investigated with sufficient accuracy. if the human race lasts for very many centuries, and if our present observations are preserved during that time for comparison, then herschel's theory may perhaps be satisfactorily tested. a hundred years have passed since laplace, with some diffidence, set forth his hypothesis as to the mode of formation of the solar system. on the whole it must be said that this "nebular hypothesis" has stood the test of advancing science well, though some slight modifications have become necessary in the light of more recent discoveries. laplace (and herschel also) seems to have considered a primitive nebula to consist of a "fiery mist" or glowing gas at a very high temperature. but this is by no means necessary, as we have seen that the gradual contraction of the vast mass supplies energy which may be converted into heat, and the spectroscopic evidence seems also to point to the existence of a moderate temperature in the gaseous nebulæ, which must be considered to be representatives of the hypothetical primitive chaos out of which our sun and planets have been evolved. another point which has been reconsidered is the formation of the various planets. it was formerly thought that the rotation of the original mass had by degrees caused a number of rings of different dimensions to be separated from the central part, the material of which rings in time collected into single planets. the ring of saturn was held to be a proof of this process, since we here have a ring, the condensation of which into one or more satellites has somehow been arrested. but while it is not impossible that matter in the shape of rings may have been left behind during the contraction of the nebulous mass (indeed, the minor planets between mars and jupiter have perhaps originated in this way), it seems likely that the larger planets were formed from the agglomeration of matter at a point on the equator of the rotating nebula. the actual steps of the process by which the primeval nebula became transformed into the solar system seem to lie beyond reach of discovery. chapter xxvii. the tides.[ ] mathematical astronomy--lagrange's theories: how far they are really true--the solar system not made of rigid bodies--kepler's laws true to observation, but not absolutely true when the bodies are not rigid--the errors of observation--the tides--how the tides were observed--discovery of the connection between the tides and the moon--solar and lunar tides--work done by the tides--whence do the tides obtain the power to do the work?--tides are increasing the length of the day--limit to the shortness of the day--early history of the earth-moon system--unstable equilibrium--ratio of the month to the day--the future course of the system--equality of the month and the day--the future critical epoch--the constant face of the moon accounted for--the other side of the moon--the satellites of mars--their remarkable motions--have the tides possessed influence in moulding the solar system generally?--moment of momentum--tides have had little or no appreciable effect on the orbit of jupiter--conclusion. that the great discoveries of lagrange on the stability of the planetary system are correct is in one sense strictly true. no one has ever ventured to impugn the mathematics of lagrange. given the planetary system in the form which lagrange assumed and the stability of that system is assured for all time. there is, however, one assumption which lagrange makes, and on which his whole theory was founded: his assumption is that the planets are _rigid_ bodies. no doubt our earth seems a rigid body. what can be more solid and unyielding than the mass of rocks and metals which form the earth, so far as it is accessible to us? in the wide realms of space the earth is but as a particle; it surely was a natural and a legitimate assumption to suppose that that particle was a rigid body. if the earth were absolutely rigid--if every particle of the earth were absolutely at a fixed distance from every other particle--if under no stress of forces, and in no conceivable circumstance, the earth experienced even the minutest change of form--if the same could be said of the sun and of all the other planets--then lagrange's prediction of the eternal duration of our system must be fulfilled. but what are the facts of the case? is the earth really rigid? we know from experiment that a rigid body in the mathematical sense of the word does not exist. rocks are not rigid; steel is not rigid; even a diamond is not perfectly rigid. the whole earth is far from being rigid even on the surface, while part of the interior is still, perhaps, more or less fluid. the earth cannot be called a perfectly rigid body; still less can the larger bodies of our system be called rigid. jupiter and saturn are perhaps hardly even what could be called solid bodies. the solar system of lagrange consisted of a rigid sun and a number of minute rigid planets; the actual solar system consists of a sun which is in no sense rigid, and planets which are only partially so. the question then arises as to whether the discoveries of the great mathematicians of the last century will apply, not only to the ideal solar system which they conceived, but to the actual solar system in which our lot has been cast. there can be no doubt that these discoveries are approximately true: they are, indeed, so near the absolute truth, that observation has not yet satisfactorily shown any departure from them. but in the present state of science we can no longer overlook the important questions which arise when we deal with bodies not rigid in the mathematical sense of the word. let us, for instance, take the simplest of the laws to which we have referred, the great law of kepler, which asserts that a planet will revolve for ever in an elliptic path of which the sun is one focus. this is seen to be verified by actual observation; indeed, it was established by observation before any theoretical explanation of that movement was propounded. if, however, we state the matter with a little more precision, we shall find that what newton really demonstrated was, that if two _rigid_ particles attract each other by a law of force which varies with the inverse square of the distance between the particles, then each of the particles will describe an ellipse with the common centre of gravity in the focus. the earth is, to some extent, rigid, and hence it was natural to suppose that the relative behaviour of the earth and the sun would, to a corresponding extent, observe the simple elliptic law of kepler; as a matter of fact, they do observe it with such fidelity that, if we make allowance for other causes of disturbance, we cannot, even by most careful observation, detect the slightest variation in the motion of the earth arising from its want of rigidity. there is, however, a subtlety in the investigations of mathematics which, in this instance at all events, transcends the most delicate observations which our instruments enable us to make. the principles of mathematics tell us that though kepler's laws may be true for bodies which are absolutely and mathematically rigid, yet that if the sun or the planets be either wholly, or even in their minutest part, devoid of perfect rigidity, then kepler's laws can be no longer true. do we not seem here to be in the presence of a contradiction? observation tells us that kepler's laws are true in the planetary system; theory tells us that these laws cannot be true in the planetary system, because the bodies in that system are not perfectly rigid. how is this discrepancy to be removed? or is there really a discrepancy at all? there is not. when we say that kepler's laws have been proved to be true by observation, we must reflect on the nature of the proofs which are attainable. we observe the places of the planets with the instruments in our observatories; these places are measured by the help of our clocks and of the graduated circles on the instruments. these observations are no doubt wonderfully accurate; but they do not, they cannot, possess absolute accuracy in the mathematical sense of the word. we can, for instance, determine the place of a planet with such precision that it is certainly not one second of arc wrong; and one second is an extremely small quantity. a foot-rule placed at a distance of about forty miles subtends an angle of a second, and it is surely a delicate achievement to measure the place of a planet, and feel confident that no error greater than this can have intruded into our result. when we compare the results of observation with the calculations conducted on the assumption of the truth of kepler's laws, and when we pronounce on the agreement of the observations with the calculations, there is always a reference, more or less explicit, to the inevitable errors of the observations. if the calculations and observations agree so closely that the differences between the two are minute enough to have arisen in the errors inseparable from the observations, then we are satisfied with the accordance; for, in fact, no closer agreement is attainable, or even conceivable. the influence which the want of rigidity exercises on the fulfilment of the laws of kepler can be estimated by calculation; it is found, as might be expected, to be extremely small--so small, in fact, as to be contained within that slender margin of error by which observations are liable to be affected. we are thus not able to discriminate by actual measurement the effects due to the absence of rigidity; they are inextricably hid among the small errors of observation. the argument on which we are to base our researches is really founded on a very familiar phenomenon. there is no one who has ever visited the sea-side who is not familiar with that rise and fall of the sea which we call the tide. twice every twenty-four hours the sea advances on the beach to produce high tide; twice every day the sea again retreats to produce low tide. these tides are not merely confined to the coasts; they penetrate for miles up the courses of rivers; they periodically inundate great estuaries. in a maritime country the tides are of the most profound practical importance; they also possess a significance of a far less obvious character, which it is our object now to investigate. these daily pulses of the ocean have long ceased to be a mystery. it was in the earliest times perceived that there was a connection between the tides and the moon. ancient writers, such as pliny and aristotle, have referred to the alliance between the times of high water and the age of the moon. i think we sometimes do not give the ancient astronomers as much credit as their shrewdness really entitles them to. we have all read--we have all been taught--that the moon and the tides are connected together; but how many of us are in a position to say that we have actually noticed that connection by direct personal observation? the first man who studied this matter with sufficient attention to convince himself and to convince others of its reality must have been a great philosopher. we know not his name, we know not his nation, we know not the age in which he lived; but our admiration of his discovery must be increased by the reflection that he had not the theory of gravitation to guide him. a philosopher of the present day who had never seen the sea could still predict the necessity of tides as a consequence of the law of universal gravitation; but the primitive astronomer, who knew not of the invisible bond by which all bodies in the universe are drawn together, made a splendid--indeed, a typical--inductive discovery, when he ascertained the relation between the moon and the tides. we can surmise that this discovery, in all probability, first arose from the observations of experienced navigators. in all matters of entering port or of leaving port, the state of the tide is of the utmost concern to the sailor. even in the open sea he has sometimes to shape his course in accordance with the currents produced by the tides; or, in guiding his course by taking soundings, he has always to bear in mind that the depth varies with the tide. all matters relating to the tide would thus come under his daily observation. his daily work, the success of his occupation, the security of his life, depend often on the tides; and hence he would be solicitous to learn from his observation all that would be useful to him in the future. to the coasting sailor the question of the day is the time of high water. that time varies from day to day; it is an hour or more later to-morrow than to-day, and there is no very simple rule which can be enunciated. the sailor would therefore welcome gladly any rule which would guide him in a matter of such importance. we can make a conjecture as to the manner in which such a rule was first discovered. let us suppose that a sailor at calais, for example, is making for harbour. he has a beautiful night--the moon is full; it guides him on his way; he gets safely into harbour; and the next morning he finds the tide high between and .[ ] he often repeats the same voyage, but he finds sometimes a low and inconvenient tide in the morning. at length, however, it occurs to him that _when he has a moonlight night_ he has a high tide at . this occurs once or twice: he thinks it but a chance coincidence. it occurs again and again. at length he finds it always occurs. he tells the rule to other sailors; they try it too. it is invariably found that when the moon is full, the high tide always recurs at the same hour at the same place. the connection between the moon and the tide is thus established, and the intelligent sailor will naturally compare other phases of the moon with the times of high water. he finds, for example, that the moon at the first quarter always gives high water at the same hour of the day; and finally, he obtains a practical rule, by which, from the state of the moon, he can at once tell the time when the tide will be high at the port where his occupation lies. a diligent observer will trace a still further connection between the moon and the tides; he will observe that some high tides rise higher than others, that some low tides fall lower than others. this is a matter of much practical importance. when a dangerous bar has to be crossed, the sailor will feel much additional security in knowing that he is carried over it on the top of a spring tide; or if he has to contend against tidal currents, which in some places have enormous force, he will naturally prefer for his voyage the neap tides, in which the strength of these currents is less than usual. the spring tides and the neap tides will become familiar to him, and he will perceive that the spring tides occur when the moon is full or new--or, at all events, that the spring tides are within a certain constant number of days of the full or new moon. it was, no doubt, by reasoning such as this, that in primitive times the connection between the moon and the tides came to be perceived. it was not, however, until the great discovery of newton had disclosed the law of universal gravitation that it became possible to give a physical explanation of the tides. it was then seen how the moon attracts the whole earth and every particle of the earth. it was seen how the fluid particles which form the oceans on the earth were enabled to obey the attraction in a way that the solid parts could not. when the moon is overhead it tends to draw the water up, as it were, into a heap underneath, and thus to give rise to the high tide. the water on the opposite side of the earth is also affected in a way that might not be at first anticipated. the moon attracts the solid body of the earth with greater intensity than it attracts the water at the other side which lies more distant from it. the earth is thus drawn away from the water, and there is therefore a tendency to a high tide as well on the side of the earth away from the moon as on that towards the moon. the low tides occupy the intermediate positions. the sun also excites tides on the earth; but owing to the great distance of the sun, the difference between its attraction on the sea and on the solid interior of the earth is not so appreciable. the solar tides are thus smaller than the lunar tides. when the two conspire, they cause a spring tide; when the solar and lunar tides are opposed, we have the neap tide. there are, however, a multitude of circumstances to be taken into account when we attempt to apply this general reasoning to the conditions of a particular case. owing to local peculiarities the tides vary enormously at the different parts of the coast. in a confined area like the mediterranean sea, the tides have only a comparatively small range, varying at different places from one foot to a few feet. in mid-ocean also the tidal rise and fall is not large, amounting, for instance, to a range of three feet at st. helena. near the great continental masses the tides become very much modified by the coasts. we find at london a tide of eighteen or nineteen feet; but the most remarkable tides in the british islands are those in the bristol channel, where, at chepstow or cardiff, there is a rise and fall during spring tides to the height of thirty-seven or thirty-eight feet, and at neap tides to a height of twenty-eight or twenty-nine. these tides are surpassed in magnitude at other parts of the world. the greatest of all tides are those in the bay of fundy, at some parts of which the rise and fall at spring tides is not less than fifty feet. the rising and falling of the tide is necessarily attended with the formation of currents. such currents are, indeed, well known, and in some of our great rivers they are of the utmost consequence. these currents of water can, like water-streams of any other kind, be made to do useful work. we can, for instance, impound the rising water in a reservoir, and as the tide falls we can compel the enclosed water to work a water-wheel before it returns to the sea. we have, indeed, here a source of actual power; but it is only in very unusual circumstances that it is found to be economical to use the tides for this purpose. the question can be submitted to calculation, and the area of the reservoir can be computed which would retain sufficient water to work a water-wheel of given horse-power. it can be shown that the area of the reservoir necessary to impound water enough to produce horse-power would be acres. the whole question is then reduced to the simple one of expense: would the construction and the maintenance of this reservoir be more or less costly than the erection and the maintenance of a steam-engine of equivalent power? in most cases it would seem that the latter would be by far the cheaper; at all events, we do not practically find tidal engines in use, so that the power of the tides is now running to waste. the economical aspects of the case may, however, be very profoundly altered at some remote epoch, when our stores of fuel, now so lavishly expended, give appreciable signs of approaching exhaustion. the tides are, however, _doing work_ of one kind or another. a tide in a river estuary will sometimes scour away a bank and carry its materials elsewhere. we have here work done and energy consumed, just as much as if the same task had been accomplished by engineers directing the powerful arms of navvies. we know that work cannot be done without the consumption of energy in some of its forms; whence, then, comes the energy which supplies the power of the tides? at a first glance the answer to this question seems a very obvious one. have we not said that the tides are caused by the moon? and must not the energy, therefore, be derived from the moon? this seems plain enough, but, unfortunately, it is not true. it is one of those cases by no means infrequent in dynamics, where the truth is widely different from that which seems to be the case. an illustration will perhaps make the matter clearer. when a rifle is fired, it is the finger of the rifleman that pulls the trigger; but are we, then, to say that the energy by which the bullet has been driven off has been supplied by the rifleman? certainly not; the energy is, of course, due to the gunpowder, and all the rifleman did was to provide the means by which the energy stored up in the powder could be liberated. to a certain extent we may compare this with the tidal problem; the tides raised by the moon are the originating cause whereby a certain store of energy is drawn upon and applied to do such work as the tides are competent to perform. this store of energy, strange to say, does not lie in the moon; it is in the earth itself. indeed, it is extremely remarkable that the moon actually gains energy from the tides by itself absorbing some of the store which exists in the earth. this is not put forward as an obvious result; it depends upon a refined dynamical theorem. we must clearly understand the nature of this mighty store of energy from which the tides draw their power, and on which the moon is permitted to make large and incessant drafts. let us see in what sense the earth is said to possess a store of energy. we know that the earth rotates on its axis once every day. it is this rotation which is the source of the energy. let us compare the rotation of the earth with the rotation of the fly-wheel belonging to a steam-engine. the rotation of the fly-wheel is really a reservoir, into which the engine pours energy at each stroke of the piston. the various machines in the mill worked by the engine merely draw upon the store of energy accumulated in the fly-wheel. the earth may be likened to a gigantic fly-wheel detached from the engine, though still connected with the machines in the mill. from its stupendous dimensions and from its rapid velocity, that great fly-wheel possesses an enormous store of energy, which must be expended before the fly-wheel comes to rest. hence it is that, though the tides are caused by the moon, yet the energy they require is obtained by simply appropriating some of the vast supply available from the rotation of the earth. there is, however, a distinction of a very fundamental character between the earth and the fly-wheel of an engine. as the energy is withdrawn from the fly-wheel and consumed by the various machines in the mill, it is continually replaced by fresh energy, which flows in from the exertions of the steam-engine, and thus the velocity of the fly-wheel is maintained. but the earth is a fly-wheel without the engine. when the tides draw upon the store of energy and expend it in doing work, that energy is not replaced. the consequence is irresistible: the energy in the rotation of the earth must be decreasing. this leads to a consequence of the utmost significance. if the engine be cut off from the fly-wheel, then, as everyone knows, the massive fly-wheel may still give a few rotations, but it will speedily come to rest. a similar inference must be made with regard to the earth; but its store of energy is so enormous, in comparison with the demands which are made upon it, that the earth is able to hold out. ages of countless duration must elapse before the energy of the earth's rotation can be completely exhausted by such drafts as the tides are capable of making. nevertheless, it is necessarily true that the energy is decreasing; and if it be decreasing, then the speed of the earth's rotation must be surely, if slowly, abating. now we have arrived at a consequence of the tides which admits of being stated in the simplest language. if the speed of rotation be abating, then the length of the day must be increasing; and hence we are conducted to the following most important statement: that the _tides are increasing the length of the day_. to-day is longer than yesterday--to-morrow will be longer than to-day. the difference is so small that even in the course of ages it can hardly be said to have been distinctly established by observation. we do not pretend to say how many centuries have elapsed since the day was even one second shorter than it is at present; but centuries are not the units which we employ in tidal evolution. a million years ago it is quite probable that the divergence of the length of the day from its present value may have been very considerable. let us take a glance back into the profound depths of times past, and see what the tides have to tell us. if the present order of things has lasted, the day must have been shorter and shorter the farther we look back into the dim past. the day is now twenty-four hours; it was once twenty hours, once ten hours; it was once six hours. how much farther can we go? once the six hours is past, we begin to approach a limit which must at some point bound our retrospect. the shorter the day the more is the earth bulged at the equator; the more the earth is bulged at the equator the greater is the strain put upon the materials of the earth by the centrifugal force of its rotation. if the earth were to go too fast it would be unable to cohere together; it would separate into pieces, just as a grindstone driven too rapidly is rent asunder with violence. here, therefore, we discern in the remote past a barrier which stops the present argument. there is a certain critical velocity which is the greatest that the earth could bear without risk of rupture, but the exact amount of that velocity is a question not very easy to answer. it depends upon the nature of the materials of the earth; it depends upon the temperature; it depends upon the effect of pressure, and on other details not accurately known to us. an estimate of the critical velocity has, however, been made, and it has been shown mathematically that the shortest period of rotation which the earth could have, without flying into pieces, is about three or four hours. the doctrine of tidal evolution has thus conducted us to the conclusion that, at some inconceivably remote epoch, the earth was spinning round its axis in a period approximating to three or four hours. we thus learn that we are indebted to the moon for the gradual elongation of the day from its primitive value up to twenty-four hours. in obedience to one of the most profound laws of nature, the earth has reacted on the moon, and the reaction of the earth has taken a tangible form. it has simply consisted in gradually driving the moon away from the earth. you may observe that this driving away of the moon resembles a piece of retaliation on the part of the earth. the consequence of the retreat of the moon is sufficiently remarkable. the path in which the moon is revolving has at the present time a radius of , miles. this radius must be constantly growing larger, in consequence of the tides. provided with this fact, let us now glance back into the past history of the moon. as the moon's distance is increasing when we look forwards, so we find it decreasing when we look backwards. the moon must have been nearer the earth yesterday than it is to-day; the difference is no doubt inappreciable in years, in centuries, or in thousands of years; but when we come to millions of years, the moon must have been significantly closer than it is at present, until at length we find that its distance, instead of , miles, has dwindled down to , , to , , to , miles. nor need we stop--nor can we stop--until we find the moon actually close to the earth's surface. if the present laws of nature have operated long enough, and if there has been no external interference, then it cannot be doubted that the moon and the earth were once in immediate proximity. we can, indeed, calculate the period in which the moon must have been revolving round the earth. the nearer the moon is to the earth the quicker it must revolve; and at the critical epoch when the satellite was in immediate proximity to our earth it must have completed each revolution in about three or four hours. this has led to one of the most daring speculations which has ever been made in astronomy. we cannot refrain from enunciating it; but it must be remembered that it is only a speculation, and to be received with corresponding reserve. the speculation is intended to answer the question, what brought the moon into that position, close to the surface of the earth? we will only say that there is the gravest reason to believe that the moon was, at some very early period, fractured off from the earth when the earth was in a soft or plastic condition. at the beginning of the history we found the earth and the moon close together. we found that the rate of rotation of the earth was only a few hours, instead of twenty-four hours. we found that the moon completed its journey round the primitive earth in exactly the same time as the primitive earth rotated on its axis, so that the two bodies were then constantly face to face. such a state of things formed what a mathematician would describe as a case of unstable dynamical equilibrium. it could not last. it may be compared to the case of a needle balanced on its point; the needle must fall to one side or the other. in the same way, the moon could not continue to preserve this position. there were two courses open: the moon must either have fallen back on the earth, and been reabsorbed into the mass of the earth, or it must have commenced its outward journey. which of these courses was the moon to adopt? we have no means, perhaps, of knowing exactly what it was which determined the moon to one course rather than to another, but as to the course which was actually taken there can be no doubt. the fact that the moon exists shows that it did not return to the earth, but commenced its outward journey. as the moon recedes from the earth it must, in conformity with kepler's laws, require a longer time to complete its revolution. it has thus happened that, from the original period of only a few hours, the duration has increased until it has reached the present number of hours. the rotation of the earth has, of course, also been modified, in accordance with the retreat of the moon. once the moon had commenced to recede, the earth was released from the obligation which required it constantly to direct the same face to the moon. when the moon had receded to a certain distance, the earth would complete the rotation in less time than that required by the moon for one revolution. still the moon gets further and further away, and the duration of the revolution increases to a corresponding extent, until three, four, or more days (or rotations of the earth) are identical with the month (or revolution of the moon). although the number of days in the month increases, yet we are not to suppose that the rate of the earth's rotation is increasing; indeed, the contrary is the fact. the earth's rotation is getting slower, and so is the revolution of the moon, but the retardation of the moon is greater than that of the earth. even though the period of rotation of the earth has greatly increased from its primitive value, yet the period of the moon has increased still more, so that it is several times as large as that of the rotation of the earth. as ages roll on the moon recedes further and further, its orbit increases, the duration of the revolution augments, until at length a very noticeable epoch is attained, which is, in one sense, a culminating point in the career of the moon. at this epoch the revolution periods of the moon, when measured in rotation periods of the earth, attain their greatest value. it would seem that the month was then twenty-nine days. it is not, of course, meant that the month and the day at that epoch were the month and the day as our clocks now measure time. both were shorter then than now. but what we mean is, that at this epoch the earth rotated twenty-nine times on its axis while the moon completed one circuit. this epoch has now been passed. no attempt can be made at present to evaluate the date of that epoch in our ordinary units of measurement. at the same time, however, no doubt can be entertained as to the immeasurable antiquity of the event, in comparison with all historic records; but whether it is to be reckoned in hundreds of thousands of years, in millions of years, or in tens of millions of years, must be left in great degree to conjecture. this remarkable epoch once passed, we find that the course of events in the earth-moon system begins to shape itself towards that remarkable final stage which has points of resemblance to the initial stage. the moon still continues to revolve in an orbit with a diameter steadily, though very slowly, growing. the length of the month is accordingly increasing, and the rotation of the earth being still constantly retarded, the length of the day is also continually growing. but the ratio of the length of the month to the length of the day now exhibits a change. that ratio had gradually increased, from unity at the commencement, up to the maximum value of somewhere about twenty-nine at the epoch just referred to. the ratio now begins again to decline, until we find the earth makes only twenty-eight rotations, instead of twenty-nine, in one revolution of the moon. the decrease in the ratio continues until the number twenty-seven expresses the days in the month. here, again, we have an epoch which it is impossible for us to pass without special comment. in all that has hitherto been said we have been dealing with events in the distant past; and we have at length arrived at the present state of the earth-moon system. the days at this epoch are our well-known days, the month is the well-known period of the revolution of our moon. at the present time the month is about twenty-seven of our days, and this relation has remained sensibly true for thousands of years past. it will continue to remain sensibly true for thousands of years to come, but it will not remain true indefinitely. it is merely a stage in this grand transformation; it may possess the attributes of permanence to our ephemeral view, just as the wings of a gnat seem at rest when illuminated by the electric spark; but when we contemplate the history with time conceptions sufficiently ample for astronomy we realise how the present condition of the earth-moon system can have no greater permanence than any other stage in the history. our narrative must, however, now assume a different form. we have been speaking of the past; we have been conducted to the present; can we say anything of the future? here, again, the tides come to our assistance. if we have rightly comprehended the truth of dynamics (and who is there now that can doubt them?), we shall be enabled to make a forecast of the further changes of the earth-moon system. if there be no interruption from any external source at present unknown to us, we can predict--in outline, at all events--the subsequent career of the moon. we can see how the moon will still follow its outward course. the path in which it revolves will grow with extreme slowness, but yet it will always grow; the progress will not be reversed, at all events, before the final stage of our history has been attained. we shall not now delay to dwell on the intervening stages; we will rather attempt to sketch the ultimate type to which our system tends. in the dim future--countless millions of years to come--this final stage will be approached. the ratio of the month to the day, whose decline we have already referred to, will continue to decline. the period of revolution of the moon will grow longer and longer, but the length of the day will increase much more rapidly than the increase in the duration of the moon's period. from the month of twenty-seven days we shall pass to a month of twenty-six days, and so on, until we shall reach a month of ten days, and, finally, a month of one day. let us clearly understand what we mean by a month of one day. we mean that the time in which the moon revolves around the earth will be equal to the time in which the earth rotates around its axis. the length of this day will, of course, be vastly greater than our day. the only element of uncertainty in these enquiries arises when we attempt to give numerical accuracy to the statements. it seems to be as true as the laws of dynamics that a state of the earth-moon system in which the day and the month are equal must be ultimately attained; but when we attempt to state the length of that day we introduce a hazardous element into the enquiry. in giving any estimate of its length, it must be understood that the magnitude is stated with great reserve. it may be erroneous to some extent, though, perhaps, not to any considerable amount. the length of this great day would seem to be about equal to fifty-seven of our days. in other words, at some critical time in the excessively distant future, the earth will take something like , hours to perform a rotation, while the moon will complete its journey precisely in the same time. we thus see how, in some respects, the first stage of the earth-moon system and the last stage resemble each other. in each case we have the day equal to the month. in the first case the day and the month were only a small fraction of our day; in the last stage the day and the month are each a large multiple of our day. there is, however, a profound contrast between the first critical epoch and the last. we have already mentioned that the first epoch was one of unstability--it could not last; but this second state is one of dynamical stability. once that state has been acquired, it would be permanent, and would endure for ever if the earth and the moon could be isolated from all external interference. there is one special feature which characterises the movement when the month is equal to the day. a little reflection will show that when this is the case the earth must constantly direct the same face towards the moon. if the day be equal to the month, then the earth and moon must revolve together, as if bound by invisible bands; and whatever hemisphere of the earth be directed to the moon when this state of things commences will remain there so long as the day remains equal to the month. at this point it is hardly possible to escape being reminded of that characteristic feature of the moon's motion which has been observed from all antiquity. we refer, of course, to the fact that the moon at the present time constantly turns the same face to the earth. it is incumbent upon astronomers to provide a physical explanation of this remarkable fact. the moon revolves around our earth once in a definite number of seconds. if the moon always turns the same face to the earth, then it is demonstrated that the moon rotates on its axis once in the same number of seconds also. now, this would be a coincidence wildly improbable unless there were some physical cause to account for it. we have not far to seek for a cause: the tides on the moon have produced the phenomenon. we now find the moon has a rugged surface, which testifies to the existence of intense volcanic activity in former times. those volcanoes are now silent--the internal fires in the moon seem to have become exhausted; but there was a time when the moon must have been a heated and semi-molten mass. there was a time when the materials of the moon were so hot as to be soft and yielding, and in that soft and yielding mass the attraction of our earth excited great tides. we have no historical record of these tides (they were long anterior to the existence of telescopes, they were probably long anterior to the existence of the human race), but we know that these tides once existed by the work they have accomplished, and that work is seen to-day in the constant face which the moon turns towards the earth. the gentle rise and fall of the oceans which form our tides present a picture widely different from the tides by which the moon was once agitated. the tides on the moon were vastly greater than those of the earth. they were greater because the weight of the earth is greater than that of the moon, so that the earth was able to produce much more powerful tides in the moon than the moon has ever been able to raise on the earth. that the moon should bend the same face to the earth depends immediately upon the condition that the moon shall rotate on its axis in precisely the same period as that which it requires to revolve around the earth. the tides are a regulating power of unremitting efficiency to ensure that this condition shall be observed. if the moon rotated more slowly than it ought, then the great lava tides would drag the moon round faster and faster until it attained the desired velocity; and then, but not till then, they would give the moon peace. or if the moon were to rotate faster on its axis than in its orbit, again the tides would come furiously into play; but this time they would be engaged in retarding the moon's rotation, until they had reduced the speed of the moon to one rotation for each revolution. can the moon ever escape from the thraldom of the tides? this is not very easy to answer, but it seems perhaps not impossible that the moon may, at some future time, be freed from tidal control. it is, indeed, obvious that the tides, even at present, have not the extremely stringent control over the moon which they once exercised. we now see no ocean on the moon, nor do the volcanoes show any trace of molten lava. there can hardly be tides _on_ the moon, but there may be tides _in_ the moon. it may be that the interior of the moon is still hot enough to retain an appreciable degree of fluidity, and if so, the tidal control would still retain the moon in its grip; but the time will probably come, if it have not come already, when the moon will be cold to the centre--cold as the temperature of space. if the materials of the moon were what a mathematician would call absolutely rigid, there can be no doubt that the tides could no longer exist, and the moon would be emancipated from tidal control. it seems impossible to predicate how far the moon can ever conform to the circumstances of an actual rigid body, but it may be conceivable that at some future time the tidal control shall have practically ceased. there would then be no longer any necessary identity between the period of rotation and that of revolution. a gleam of hope is thus projected over the astronomy of the distant future. we know that the time of revolution of the moon is increasing, and so long as the tidal governor could act, the time of rotation must increase sympathetically. we have now surmised a state of things in which the control is absent. there will then be nothing to prevent the rotation remaining as at present, while the period of revolution is increasing. the privilege of seeing the other side of the moon, which has been withheld from all previous astronomers, may thus in the distant future be granted to their successors. the tides which the moon raises in the earth act as a brake on the rotation of the earth. they now constantly tend to bring the period of rotation of the earth to coincide with the period of revolution of the moon. as the moon revolves once in twenty-seven days, the earth is at present going too fast, and consequently the tidal control at the present moment endeavours to retard the rotation of the earth. the rotation of the moon long since succumbed to tidal control, but that was because the moon was comparatively small and the tidal power of the earth was enormous. but this is the opposite case. the earth is large and more massive than the moon, the tides raised by the moon are but small and weak, and the earth has not yet completely succumbed to the tidal action. but the tides are constant, they never for an instant relax the effort to control, and they are gradually tending to render the day and the month coincident, though the progress is a very slow one. the theory of the tides leads us to look forward to a remote state of things, in which the moon revolves around the earth in a period equal to the day, so that the two bodies shall constantly bend the same face to each other, provided the tidal control be still able to guide the moon's rotation. so far as the mutual action of the earth and the moon is concerned, such an arrangement possesses all the attributes of permanence. if, however, we venture to project our view to a still more remote future, we can discern an external cause which must prevent this mutual accommodation between the earth and the moon from being eternal. the tides raised by the moon on the earth are so much greater than those raised by the sun, that we have, in the course of our previous reasoning, held little account of the sun-raised tides. this is obviously only an approximate method of dealing with the question. the influence of the solar tide is appreciable, and its importance relatively to the lunar tide will gradually increase as the earth and moon approach the final critical stage. the solar tides will have the effect of constantly applying a further brake to the rotation of the earth. it will therefore follow that, after the day and the month have become equal, a still further retardation awaits the length of the day. we thus see that in the remote future we shall find the moon revolving around the earth in a shorter time than that in which the earth rotates on its axis. a most instructive corroboration of these views is afforded by the discovery of the satellites of mars. the planet mars is one of the smaller members of our system. it has a mass which is only the eighth part of the mass of the earth. a small planet like mars has much less energy of rotation to be destroyed than a larger one like the earth. it may therefore be expected that the small planet will proceed much more rapidly in its evolution than the large one; we might, therefore, anticipate that mars and his satellites have attained a more advanced stage of their history than is the case with the earth and her satellite. when the discovery of the satellites of mars startled the world, in , there was no feature which created so much amazement as the periodic time of the interior satellite. we have already pointed out in chapter x. how phobos revolves around mars in a period of hours minutes. the period of rotation of mars himself is hours minutes, and hence we have the fact, unparalleled in the solar system, that the satellite is actually revolving three times as rapidly as the planet is rotating. there can hardly be a doubt that the solar tides on mars have abated its velocity of rotation in the manner just suggested. it has always seemed to me that the matter just referred to is one of the most interesting and instructive in the whole history of astronomy. we have, first, a very beautiful telescopic discovery of the minute satellites of mars, and we have a determination of the anomalous movement of one of them. we have then found a satisfactory physical explanation of the cause of this phenomenon, and we have shown it to be a striking instance of tidal evolution. finally, we have seen that the system of mars and his satellite is really a forecast of the destiny which, after the lapse of ages, awaits the earth-moon system. it seems natural to enquire how far the influence of tides can have contributed towards moulding the planetary orbits. the circumstances are here very different from those we have encountered in the earth-moon system. let us first enunciate the problem in a definite shape. the solar system consists of the sun in the centre, and of the planets revolving around the sun. these planets rotate on their axes; and circulating round some of the planets we have their systems of satellites. for simplicity, we may suppose all the planets and their satellites to revolve in the same plane, and the planets to rotate about axes which are perpendicular to that plane. in the study of the theory of tidal evolution we must be mainly guided by a profound dynamical principle known as the conservation of the "moment of momentum." the proof of this great principle is not here attempted; suffice it to say that it can be strictly deduced from the laws of motion, and is thus only second in certainty to the fundamental truths of ordinary geometry or of algebra. take, for instance, the giant planet, jupiter. in one second he moves around the sun through a certain angle. if we multiply the mass of jupiter by that angle, and if we then multiply the product by the square of the distance from jupiter to the sun, we obtain a certain definite amount. a mathematician calls this quantity the "orbital" moment of momentum of jupiter.[ ] in the same way, if we multiply the mass of saturn by the angle through which the planet moves in one second, and this product by the square of the distance between the planet and the sun, then we have the orbital moment of momentum of saturn. in a similar manner we ascertain the moment of momentum for each of the other planets due to revolution around the sun. we have also to define the moment of momentum of the planets around their axes. in one second jupiter rotates through a certain angle; we multiply that angle by the mass of jupiter, and by the square of a certain line which depends on his internal constitution: the product forms the "rotational" moment of momentum. in a similar manner we find the rotational moment of momentum for each of the other planets. each satellite revolves through a certain angle around its primary in one second; we obtain the moment of momentum of each satellite by multiplying its mass into the angle described in one second, and then multiplying the product into the square of the distance of the satellite from its primary. finally, we compute the moment of momentum of the sun due to its rotation. this we obtain by multiplying the angle through which the sun turns in one second by the whole mass of the sun, and then multiplying the product by the square of a certain line of prodigious length, which depends upon the details of the sun's internal structure. if we have succeeded in explaining what is meant by the moment of momentum, then the statement of the great law is comparatively simple. we are, in the first place, to observe that the moment of momentum of any planet may alter. it would alter if the distance of the planet from the sun changed, or if the velocity with which the planet rotates upon its axis changed; so, too, the moment of momentum of the sun may change, and so may those of the satellites. in the beginning a certain total quantity of moment of momentum was communicated to our system, and not one particle of that total can the solar system, as a whole, squander or alienate. no matter what be the mutual actions of the various bodies of the system, no matter what perturbations they may undergo--what tides may be produced, or even what mutual collisions may occur--the great law of the conservation of moment of momentum must be obeyed. if some bodies in the solar system be losing moment of momentum, then other bodies in the system must be gaining, so that the total quantity shall remain unaltered. this consideration is one of supreme importance in connection with the tides. the distribution of moment of momentum in the system is being continually altered by the tides; but, however the tides may ebb or flow, the total moment of momentum can never alter so long as influences external to the system are absent. we must here point out the contrast between the endowment of our system with energy and with moment of momentum. the mutual actions of our system, in so far as they produce heat, tend to squander the energy, a considerable part of which can be thus dissipated and lost; but the mutual actions have no power of dissipating the moment of momentum. the total moment of momentum of the solar system being taken to be , this is at present distributed as follows:-- orbital moment of momentum of jupiter orbital moment of momentum of saturn orbital moment of momentum of uranus orbital moment of momentum of neptune rotational moment of momentum of sun -- the contributions of the other items are excessively minute. the orbital moments of momentum of the few interior planets contain but little more than one thousandth part of the total amount. the rotational contributions of all the planets and of their satellites is very much less, being not more than one sixty-thousandth part of the whole. when, therefore, we are studying the general effects of tides on the planetary orbits these trifling matters may be overlooked. we shall, however, find it desirable to narrow the question still more, and concentrate our attention on one splendid illustration. let us take the sun and the planet jupiter, and, supposing all other bodies of our system to be absent, let us discuss the influence of tides produced in jupiter by the sun, and of tides in the sun by jupiter. it might be hastily thought that, just as the moon was born of the earth, so the planets were born of the sun, and have gradually receded by tides into their present condition. we have the means of enquiry into this question by the figures just given, and we shall show that it is impossible that jupiter, or any of the other planets, can ever have been very much closer to the sun than they are at present. in the case of jupiter and the sun we have the moment of momentum made up of three items. by far the largest of these items is due to the orbital revolution of jupiter, the next is due to the sun, the third is due to the rotation of jupiter on its axis. we may put them in round numbers as follows:-- orbital moment of momentum of jupiter , rotational moment of momentum of sun , rotational moment of momentum of jupiter the sun produces tides in jupiter, those tides retard the rotation of jupiter. they make jupiter rotate more and more slowly, therefore the moment of momentum of jupiter is decreasing, therefore its present value of must be decreasing. even the mighty sun himself may be distracted by tides. jupiter raises tides in the sun, those tides retard the motion of the sun, and therefore the moment of momentum of the sun is decreasing, and it follows from both causes that the item of , must be increasing; in other words, the orbital motion of jupiter must be increasing, or jupiter must be receding from the sun. to this extent, therefore, the sun-jupiter system is analogous to the earth-moon system. as the tides on the earth are driving away the moon, so the tides in jupiter and the sun are gradually driving the two bodies apart. but there is a profound difference between the two cases. it can be proved that the tides produced in jupiter by the sun are more effective than those produced in the sun by jupiter. the contribution of the sun may, therefore, be at present omitted; so that, practically, the augmentations of the orbital moment of momentum of jupiter are now achieved at the expense of that stored up by jupiter's rotation. but what is compared with , . even when the whole of jupiter's rotational moment of momentum and that of his satellites has become absorbed into the orbital motion, there will hardly be an appreciable difference in the latter. in ancient days we may indeed suppose that jupiter being hotter was larger than at present, and that he had considerably more rotational moment of momentum. but it is hardly credible that jupiter can ever have had one hundred times the moment of momentum that he has at present. yet even if , units of rotational momentum had been transferred to the orbital motion it would only correspond with the most trivial difference in the distance of jupiter from the sun. we are hence assured that the tides have not appreciably altered the dimensions of the orbit of jupiter, or of the other great planets. the time will, however, come when the rotation of jupiter on his axis will be gradually abated by the influence of the tides. it will then be found that the moment of momentum of the sun's rotation will be gradually expended in increasing the orbits of the planets, but as this reserve only holds about two per cent. of the whole amount in our system it cannot produce any considerable effect. the theory of tidal evolution, which in the hands of professor darwin has taught us so much with regard to the past history of the systems of satellites in the solar system, will doubtless also, as pointed out by dr. see, be found to account for the highly eccentric orbits of double star systems. in the earth-moon system we have two bodies exceedingly different in bulk, the mass of the earth being about eighty times as great as that of the moon. but in the case of most double stars we have to do with two bodies not very different as regards mass. it can be demonstrated that the orbit must have been originally of slight eccentricity, but that tidal friction is capable not only of extending, but also of elongating it. the accelerating force is vastly greater at periastron (when the two bodies are nearest each other) than at apastron (when their distance is greatest). at periastron the disturbing force will, therefore, increase the apastron distance by an enormous amount, while at apastron it increases the periastron distance by a very small amount. thus, while the ellipse is being gradually expanded, the orbit grows more and more eccentric, until the axial rotations have been sufficiently reduced by the transfer of axial to orbital moment of momentum. and now we must draw this chapter to a close, though there are many other subjects that might be included. the theory of tidal evolution is, indeed, one of quite exceptional interest. the earlier mathematicians expended their labour on the determination of the dynamics of a system which consisted of rigid bodies. we are indebted to contemporary mathematicians for opening up celestial mechanics upon the more real supposition that the bodies are not rigid; in other words, that they are subject to tides. the mathematical difficulties are enormously enhanced, but the problem is more true to nature, and has already led to some of the most remarkable astronomical discoveries made in modern times. * * * * * our story of the heavens has now been told. we commenced this work with some account of the mechanical and optical aids to astronomy; we have ended it with a brief description of an intellectual method of research which reveals some of the celestial phenomena that occurred ages before the human race existed. we have spoken of those objects which are comparatively near to us, and then, step by step, we have advanced to the distant nebulæ and clusters which seem to lie on the confines of the visible universe. yet how little can we see with even our greatest telescopes, when compared with the whole extent of infinite space! no matter how vast may be the depth which our instruments have sounded, there is yet a beyond of infinite extent. imagine a mighty globe described in space, a globe of such stupendous dimensions that it shall include the sun and his system, all the stars and nebulæ, and even all the objects which our finite capacities can imagine. yet, what ratio must the volume of this great globe bear to the whole extent of infinite space? the ratio is infinitely less than that which the water in a single drop of dew bears to the water in the whole atlantic ocean. appendix. astronomical quantities. the sun. the sun's mean distance from the earth is , , miles; his diameter is , miles; his mean density, as compared with water, is · ; his ellipticity is insensible; he rotates on his axis in a period between and days. the moon. the moon's mean distance from the earth is , miles. the diameter of the moon is , miles; and her mean density, as compared with water, is · . the time of a revolution around the earth is · days. the planets. ___________________________________________________________________________ | |distance from the sun in | | mean | |density | | | millions of miles. | periodic |diameter| axial |compared| | |-------------------------| time | in | rotation.| with | | | mean. | least.|greatest.| in days. | miles. | | water. | |-------+-------+-------+---------+----------+--------+----------+--------| |mercury| · | · | · | · | , | (?) | · (?)| |venus | · | · | · | · | , | (?) | · | |earth | · | · | · | · | , | · | · | |mars | | | | · | , | · | · | |jupiter| | | | , · | , | -- | · | |saturn | | | | , | , | -- | · | |uranus | , | , | , | , | , | unknown | · | |neptune| , | , | , | , | , | unknown | · | --------------------------------------------------------------------------- the satellites of mars. mean distance from periodic time. name. centre of mars. hrs. mins. secs. phobos , miles deimos , miles the satellites of jupiter. mean distance from periodic time. name. centre of jupiter. days. hrs. mins. secs. new inner satellite barnard , miles i. , miles ii. , miles iii. , miles iv. , , miles the satellites of saturn. mean distance from periodic time. name. centre of saturn. days. hrs. mins. secs. mimas , miles enceladus , miles tethys , miles dione , miles rhea , miles titan , miles hyperion , miles iapetus , , miles the satellites of uranus. mean distance from periodic time. name. centre of uranus. days. hrs. mins. secs. ariel , miles umbriel , miles titania , miles oberon , miles the satellite of neptune. mean distance from periodic time. name. centre of neptune. days. hrs. mins. secs. satellite , miles index. a aberration of light, - ; and the apparent movements of stars, , ; bradley's discoveries, ; causes, - ; circles of stars, - ; dependent upon the velocity of light, ; effect on draco, ; telescopic investigation, achromatic combination of glasses, adams, professor j.c., and the discovery of neptune, - , - ; and the ellipse of the leonids, aërolite, the chaco, ; the orgueil, airy, sir george, alban mount meteorites, the, alcor, aldebaran, , , ; spectrum of, ; value of velocity of, algol, , almagest, the, alphonsus, alps, the great valley of the (lunar), altair, aluminium in the sun, ancients, astronomy of the, - andrews, professor, and basaltic formation at giant's causeway, andromeda, ; nebula in, , andromedes, the, shooting star shower, and biela's comet, antares, apennines (lunar), aphelion, aquarius, , aquila, or the eagle, arago, archimedes, arcturus, , ; value of velocity of, argelander's catalogue of stars, , argus, ariel, , aristarchus, aristillus, aristotle, lunar crater named after him, ; credulity respecting his writings, ; the moon and the tides and, asteroids, - astrea, astronomers of nineveh, astronomical quantities, astronomy, ancient, - ; galileo's achievements in, ; the first phenomenon of, _athenæum_, the, and sir john herschel's letter on adams's share in the discovery of neptune, atmosphere, height of the earth's, attraction, between the moon and the earth, ; between the planets, ; between the sun and the planets, , ; of jupiter, , ; producing precession, auriga, , aurora borealis, autolycus, auwers and star distances, ; and the irregularity in movement of sirius, axis, polar, , ; precession and nutation of the earth's, - b backlund, and encke's comet, , barnard, professor e.e., and saturn, , , ; and titan, ; and the comet of , ; and the milky way, beehive, the, belopolsky, m., and binaries, , benares meteorite, the, bessel, and bradley, ; and the distance of cygni, , , ; and the distances of stars, ; and the irregular movements of sirius, ; receives gold medal of royal astronomical society, betelgeuze, , , , ; value of velocity of, biela's comet, and sir john herschel, ; and the andromedes, binaries, spectroscopic, binocular glass, biot and the l'aigle meteorites, bode's law, ; list of double stars, bond, professor, and saturn's satellites, ; and the nebula in orion, ; and the third ring of saturn, boötes, bradley, and nutation, ; and the aberration of light, ; his observations of uranus, bredichin, professor, and the tails of comets, , , breitenbach iron, the, bristol channel, tides in the, brünnow, dr., observations on the parallax of cygni, _burial of sir john moore_, burnham, mr., and the orbit of sirius, ; his additions to the known number of double stars, butler, bishop, and probability, butsura meteorite, c cadmium in the sun, calais, tides at, calcium in the sun, campbell, mr., and argus, ; and mars, canals on mars, cancri , cancri, z, cancri, th, canis major, canopus, cape observatory, capella, , , carboniferous period, cardiff, tides at, cassini, j.d., and double stars, ; and saturn's satellites, ; and the rings of saturn, cassiopeia, castor, , ; a binary star, ; revolution of, catalogues of stars, , ; messier's, catharina, centauri, a, ; dr. gill's observations of, ; henderson's measurement of distance of, , ceres, , , ; and meteorites, , chaco meteorite, the, chacornac, and the lunar crater schickard, _challenger_, the cruise of the, and magnetic particles in the atlantic, challis, professor, ; his search for neptune, , , , chandler, mr., and algol, charles's wain, chepstow, tides at, chéseaux, discoverer of comet of , chicago, telescope at yerkes observatory, chladni and the meteorite of siberia, chromium in the sun, chromosphere, the, chronometers tested by the moon, clairaut and the attraction of planets on comets, , clavius, ; and jupiter's satellites, clock, astronomical, clusters, star, - cobalt in the sun, coggia's comet, , colour of light and indication of its source, colours, the seven primary, columbiad, the, columbus, comets, , , , ; and the spectroscope, ; attraction from planets, , ; biela's, ; biela's and the andromedes, ; clairaut's investigations, , ; coggia's, ; common's ( ), ; connection of, with shooting star showers, ; constitution of, ; containing sodium and iron, ; donati's ( ), , , ; eccentricity of, ; encke's, - ; existence of carbon in, , ; gravitation and, , ; halley's investigations about, - ; head or nucleus of, ; lexell's, ; mass of, ; movements of, ; newton's explanations of, ; non-periodic, - ; of , ; of , ; of , , ; of , ; of (chéseaux's), ; of , ; of , ; of , ; of , ; of , ; origin of, ; parabolic orbits of, - , ; periodic return of, - ; shape of, ; size of, ; tailless, ; tails of, , ; bredichin's researches, ; chéseaux's, ; composition of, , ; condensation of, ; electricity and, ; gradual growth of, ; law of direction of, ; repelled by the sun, ; repulsive force of, , ; various types of, ; tebbutt's ( ), ; tenuity of, common, dr., constructor of reflectors, ; and the comet of , ; and the nebula in orion, cook, captain, and the transit of venus, copeland, dr., and schmidt's star, ; and the lunar crater, tycho, ; and the spectra of nebula, ; and the transit of venus, copernicus and mercury, ; confirmation of his theory by the discovery of jupiter's satellites, ; his theory of astronomy, ; lunar crater called after him, copper in the sun, cor scorpionis, corona borealis, , corona of sun, during an eclipse, - , coronium, cotopaxi and meteorites, crab, the, crabtree, and the transit of venus, crape ring of saturn, craters in the moon, - , - critical velocity, , , crown, the, cryptograph of huyghens, the, cygni, b, cygni , annual parallax of, ; bessel's measurement of distance of, , , ; brünnow's observations of, ; distance from the sun of, ; disturbing influence of, ; double, ; professor a. hall's measurement of, ; professor pritchard's photographic researches concerning, ; proper motion of, ; struve's observations of, , ; velocity of, cygnus, cyrillus, cysat, and the belt of orion, d d line in solar spectrum, darwin, professor g.h., and tidal evolution, dawes, professor, and saturn's third ring, day, length of, and the moon, ; and the tides, deimos, , denebola, diffraction, dione, dispersion of colours, distances, astronomical, , doerfel, and comets, dog star (_see_ sirius) dog, the little, donati's comet, , ; tails, double stars, - d q, draco, nebula in, dragon, the, draper, professor, and the nebula in orion, dunsink observatory, , , , dynamical stability, ; theory of newton, dynamics and the earth-moon system, dynamics, galileo the founder of, e eagle, the, earth, the, ancient ideas respecting, ; annual movement of, and the apparent movement of the stars, , ; attraction of jupiter, ; attraction of on encke's comet, ; attraction of, on the leonids, ; attraction of saturn, ; attraction of the moon, , ; attraction of the sun, ; axial rotation of, ; carboniferous period on, ; change of climate on, ; composition of, ; contact of atmosphere of, with meteors, - ; density of, ; diameter of, ; distance of, from mars, ; distance of, from the moon, , ; distance of, from the sun, , , , , , , , ; energy from rotation of, ; formerly a molten globe, , ; geological records and, ; glacial period on, ; gravitation and, , , , ; heat in the interior of, , , , , ; how it is measured, - ; its mass increasing owing to the fall of meteoric matter, ; its oceans once vapour, ; once in immediate proximity to the moon, ; orbit of, ; orbit of, its elliptic form, ; path of deranged by venus and mars, ; periodic time of, ; plane of orbit of, ; polar axis of, , - ; position of, relatively to the sun and the moon, , ; precession and nutation of axis of, - ; radius of, , ; rotation of, , , , , ; shape of, , , , , ; size of, compared with jupiter, , and with other planets, ; size and weight of, compared with those of the sun, , and moon, , ; velocity of, , , , , and periodic time, ; volcanic outbreaks on, , and the origin of meteorites, ; weight of, , , as compared with saturn, , earthquakes, astronomical instruments disturbed by, eccentricity of planetary ellipses, , eclipse of jupiter's satellites, , , - ellipse of the moon, - ; of the sun, eclipses, ancient explanations of, ; calculations of the recurrence of, , ecliptic, the, , ; pole of the, , , electric light, the, ellipse, the, ; eccentricity of, ; focus of, ; kepler's discoveries respecting, , , - , ; the form which the orbit of a planet takes, ; the parallactic, ; variety of form of, enceladus, encke, and the distance of the sun from the earth, , ; his comet, - encke's comet, - ; approach to jupiter of, ; and mercury, ; and the sun, ; diminution in periodic time of, ; distance from mercury of, ; disturbed by the earth, , and by mercury, ; irregularities of, , ; orbit of, ; periodical return of, ; von asten's calculations concerning, - energy supplying the tides, ensisheim meteorite, the, equatorial diameter, , ; telescope, eratosthenes, eros, eruptions, evening star, , eye, structure of the, f faculæ of the sun, fire ball of , fire balls, "fixed" stars, flamsteed, first astronomer-royal, ; his _historia coelestis_, focus of planetary ellipse, - fomalhaut, fraunhofer, fraunhofer lines, fundy, bay of, tides in, g galileo, achievements of, ; and jupiter's satellites, ; and saturn's rings, , ; and the pleiades, galle, dr., and neptune, - gassendi, and the transit of mercury, ; and the transit of venus, ; lunar crater named after him, gauss, and the minor planet ceres, gemini, constellation of, , geminids, the, geologists and the lapse of time, geometers, oriental, geometry, cultivation by the ancients of, george iii. and sir w. herschel, , giant's causeway, gill, dr. d., ; and juno, ; and the minor planets, ; and the parallax of a centauri, ; and the parallax of mars, glacial period, gravitation, law of, - ; and binary stars, ; and precession, ; and the earth's axis, , , ; and the parabolic path of comets, ; and the periodical return of comets, ; and the weight of the earth, , ; illustrated by experiments, , , , - ; its discovery aided by lunar observations, , ; its influence on the satellites, ; its influence on stars, ; its influence on tides, ; le verrier's triumphant proof of, ; newton's discoveries, , , ; on the moon, ; universality amongst the heavenly bodies, , great bear, , , ; configuration, ; double star in the, ; positions of, , green, mr., and mars, greenwich observatory, , griffiths, mr., and jupiter, grimaldi, grubb, sir howard, "guards," the, _gulliver's travels_ and the satellites of mars, h hadley's observations of saturn, hall, professor asaph, and the satellites of mars, halley, and the periodicity of comets, - ; and the transit of venus, heat, bearings on astronomy, ; in the interior of the earth, - , ; of the sun, - heliometer, the, helium, henderson, and the distance of a centauri, , hercules, star cluster in, , herodotus (lunar crater), herschel, caroline, , herschel, sir john, address to british association, ; address on the presentation of gold medal to bessel, ; and biela's comet, ; and nebulæ, ; letter to _athenoeum_ on adams's share in the discovery of neptune, herschel, sir w., and double stars, , ; and saturn, ; and saturn's satellites, ; and the empress catherine, ; and the movement of solar system towards lyra, ; discovery of satellite of uranus by, , ; discovery of uranus by, , ; early life of, ; friendship with sir w. watson of ; he makes his own telescopes, ; "king's astronomer," ; method of making his telescopes, ; musical talent of, ; organist of octagon chapel, bath, ; pardon for desertion from george iii., ; passion for astronomy of, , ; relinquishes musical profession, ; sidereal aggregation theory of, ; study of the nebulæ by, - , herschelian telescope, _historia coelestis_, hoedi, the, holmes's, mr., comet ( ), horrocks, and the transit of venus, howard, mr., and the benares meteorite, huggins, sir w., , ; and nebulæ, huyghens, and saturn's rings, - ; discovers first satellite of saturn, hyades, the, hydrogen in sirius and vega, ; in the sun, hyginus, hyperion, i iapetus, iberians, the, inquisition, the, and galileo, iris, iron, dust in the arctic regions, ; in the sun, ; of meteorites, the, ; spectrum of, j janssen, m., , ; and the transit of venus, juno, , jupiter, ancient study of, ; and the leonids, ; attraction of, ; axial rotation of, ; belts of, ; brilliancy of, ; composition of, ; covered with an atmosphere of clouds, , ; density of, ; diameter of, , ; distance from the earth of, , ; distance from the sun of, , ; habitability of, ; heat received from the sun by, ; internal heat of, , , ; lack of permanent features of, ; lack of solidity of, , , ; moment of momentum of, , ; occultation of, ; orbit of, , , ; path of, perturbed by the attraction of saturn, ; periodic time of, ; a planet, or "wanderer," ; red spot in , ; revolution of, ; rotation of, , ; satellites of, , , - , , ; satellites of, and gravitation, ; satellites of, and the copernican theory, ; shadow from satellites of, ; shape of, , , , ; size of, compared with the earth, , , , and other planets, ; and the sun, ; storms on, ; tides on, ; weight of, , , and encke's comet, k keeler, professor, and saturn's ring, kempf, dr., and the sun's velocity, kepler, and comets, ; and laws of planetary motion, ; and meteors, ; and the orbit of mars, ; explanation of his laws, , , ; his discovery of the shape of the planetary orbits, , ; his first planetary law, ; lunar crater called after him, ; prediction of the transit of venus and mercury, , ; second law, ; third law, kids, the, kirchhoff, and spectrum analysis, kirkwood, professor, and the movements of saturn's satellites, klinkerfues, professor, l lagrange, and the theory of planetary perturbation, - ; his assumption of planetary rigidity, l'aigle meteorites, the, lalande, and neptune, , landscapes, lunar, lane, mr. j. homer, laplace, and the nebular theory, ; and the satellites of jupiter, ; and the theory of planetary perturbation, lassell, mr., and saturn's eighth satellite, ; discovers neptune's satellite, law of gravitation (_see_ gravitation) laws of planetary motion (_see_ planetary motion) lead in the sun, ledger, mr., and mercury, leibnitz, lunar mountains named after him, lemonnier, and uranus, leo, and shooting stars, , leonids, attractions of planets on, ; breadth of stream of, ; change of shape of, ; decrease of, ; enormous number of, ; historical records, ; length of stream of, ; le verrier, and the cause of their introduction into the solar system, ; meteor shoal of, ; periodic return of, ; their connection with comets and professor, schiaparelli, leonis g, value of velocity of, leverage by equatorial protuberance, le verrier, and mars, ; and the discovery of neptune, - ; and the introduction of the leonids into the solar system, ; and the weight of mercury, lexell's comet, libration, lick observatory, light, aberration of, - ; velocity of, , , , , linné, , lion, the, , little bear, the, little dog, the, livy, and meteorites, lloyd, provost, lockyer, sir norman, and betelgeuze, ; and solar light, london, tides at, louvain, f. terby, and titan, lowell, mr., and mercury, lunar tides, , lyra, motion of solar system towards, lyre, the, ; nebula in, lyrids, the, m mädler, and the lunar craters, , , magellanic clouds, magnesium, colour of flame from, ; in the sun, magnetism, connection with sun spots, manganese in the sun, maraldi, and the rings of saturn, mare crisium, ; foecunditatis, ; humorum, ; imbrium, , ; nectaris ; nubium, ; serenitatis, ; tranquillitatis, ; vaporum, mars, ancient study of, ; appearance of, through the telescope, ; atmosphere of, ; axial rotation of, ; canals on, ; density of, ; diameter of, ; distance, from the earth of, ; distance from the sun of, , ; gravitation on, ; le verrier's discovery of, ; life improbable on, ; marking on, ; movements of, - ; opposition of, - ; orbit of, , , , ; orbit of, and the laws of kepler, ; parallax ( ), and dr. d. gill, ; periodic time of, ; a planet or "wanderer," ; "polar caps" on, , ; proximity to the earth of, ; rising and setting of, ; rotation of, ; satellites of, - , ; size of compared with other planets, , ; tides on, ; water and ice on, , maximilian, emperor, mayer, tobias, and uranus, measurement of the earth, - mediterranean, tides in the, mercury, ancient study of, ; antiquity of its discovery, - ; atmosphere of, ; attraction on comets of, ; climate of, ; comparative proximity to the earth of, ; composition of, ; crescent-shaped, ; density of, ; diameter of, ; distance from the sun of, , ; habitability of, ; movement of, , ; its elliptic form, , ; orbit of, ; period of revolution of, ; periodic appearances of, ; periodic time of, ; perturbations of, ; a planet or "wanderer," ; revolution of, ; rotation of, and professor schiaparelli, ; size of, compared with other planets, ; surface of, ; transit of, ; transit of, and gassendi's observations, ; transit of, predicted by kepler, ; velocity of, ; weight of, , meridian circle, , messier's catalogue of stars, meteors (_see_ stars, shooting) meteorites, ; alban mount, ; ancient accounts, , ; benares, ; butsura, ; chaco, ; characteristics of, ; chladni's account of discovery in siberia, ; composition of, - ; ensisheim ( ), ; hindoo account of, ; l'aigle, ; not connected with comets, ; not connected with star showers, ; orgueil, ; origin, - ; ovifak, ; rowton, - ; wold cottage, micrometer, milky way, - , - mimas, minor planets, - mira ceti, , mizar, , moment of momentum, the, - month of one day, moon, the, absence of air on, , ; absence of heat on, ; agent in causing the tides, , - ; ancient discoveries respecting, ; apparent size of, ; attraction to the earth of, ; brightness of, as compared with that of the sun, ; changes during the month of, , ; chart of surface of, ; craters on, , , - , ; density of, ; diameter of, ; distance from the earth of, , , ; eclipses of, , - ; illustration of the law of gravitation, , , ; landscapes on, ; life impossible on, ; measuring heights of mountains, etc., of, , ; micrometer, ; motion of, ; mountains on, , , , , , ; phases of, , ; plane of orbit of, , , ; poets and artists and, ; pole, ; possibility of ejecting meteorites, ; possibly fractured off from the earth, ; prehistoric tides on, , ; produces precession, - ; proximity to the earth of, , ; receding from the earth, ; relative position of with regard to the earth and the sun, , ; revolution of, round the earth, , , ; "seas" on, , ; shadows of, ; size of, compared with that of the earth, ; test for chronometers, a, ; thraldom of terrestrial tides, ; waterless, ; weather not a affected by the phases of, ; weight of, motion, laws of planetary, , , , , mountains of the moon, , , n nasmyth, mr., and the formation of lunar craters, natural history museum, meteorites, _nautical almanack_, neap tides, nebula, in andromeda, ; annular, in lyra, ; in orion, , , - ; colour of, ; magnitude of, ; nature of, ; planetary, in draco, ; simplest type of a, ; various grades of, nebulæ, - ; condensation, ; distances of, ; double, ; herschel's labours respecting, - , , ; number of, ; planetary, ; self-luminous, ; smallest greater than the sun, ; spiral, nebular theory, the, neptune, ; adams's researches, - , ; challis's observations of, - ; density of, ; diameter of, , ; disc of, ; discovery ( ) of, ; distance from the sun of, , ; lalande's observations of, , ; le verrier's calculations, - ; moment of momentum of, ; orbit of, ; periodic time of, ; revolution of, ; rotation of uncertain, ; satellite of, discovered by mr. lassell, ; size of, compared with other planets, ; vaporous atmosphere of, ; weight of, newall, mr. h.f., and capella, ; and the values of velocity of stars, newcomb, professor, , , , newton, professor, and meteoric showers, , newton, sir isaac, discovery of gravitation verified kepler's laws, ; dynamical theory, ; illustrations of his teaching, - ; law of gravitation and, , , ; parabolic path of comets and, - ; reflecting telescope, ; weight of the earth and, nickel in the sun, nineveh, astronomers of, nordenskjöld, and the ovifak meteorite, nova cygni, ; brilliancy of, ; decline of, ; distance of, ; parallax of, november meteors, , , nutation, and bradley, o oberon, , object-glasses, , , , , observatories, - observatory, cape of good hope, ; dunsink, , ; greenwich, , ; lick, ; paris, ; uraniborg, ; vienna, ; washington, ; yerkes, occultation, , oceanus procellarum, opera-glass, , opposition of mars, orbital moment of momentum, orbits of planets, , , ; dimensions, - ; elliptical form, - ; minor planets, , , ; not exactly circles, ; of satellites of uranus, ; sun the common focus, orgueil meteorite, the, orion, , orion, belt of, , ; brilliancy of, ; nebula in, , , - orionis, a, , orionis, th, a multiple star, , ovifak meteorite, the, p palisa and the minor planets, pallas, , parabolic path of comets, - parallactic ellipse, parallax, , , , ; of stars, paris telescope, , pegasus, great square of, , peg-top, the, and the rotation of the earth, pendulum for determining the force of the earth's attraction, penumbra of sun-spot, perihelion, periodic times of planets, - , periodicity of sun-spots, perseids, perseus, , , ; sword-handle, perturbation, planetary, - , perturbations, theory of, petavius, peters, professor, and charts of minor planets, ; and the derangement of sirius, phases of the moon, , phobos, , , photography, and practical astronomy, ; and the distance of cygni, ; dr. roberts and the nebula in andromeda, ; mr. common and the nebula in orion, ; sir w. huggins and the spectra of nebulæ, photosphere, the, , physical nature of the stars, piazzi, discoverer of the first known minor planet, pickering, professor, , , , ; and betelgeuze, ; and planetary nebulæ, ; and saturn's satellites, ; and spectroscopic binaries, , pico, planetary motion, kepler's laws of, , , , , planetary nebulæ, planetary perturbation, - planets, ancient ideas respecting, , ; approximate number of, ; attract each other, , ; attracted by comets, ; bode's law, ; comparative sizes of, , ; distance of, from the earth, - ; distance of, from the sun, ; how distinguished from stars, ; irregularity of motions of, - ; lagrange's theory of rigidity of, ; light of, derived from the sun, ; minor, - ; orbits of the four giant, ; orbits of the four interior, ; orbits have their focus in the centre of the sun, ; orbits not exactly circles, ; orbits take the form of an ellipse, - ; origin of, as suggested by the nebular theory, ; periodic times of, - , ; relative distances of, ; uniformity of direction in their revolution, , ; velocity of, - , , , plato (lunar crater), pleiades, , ; invisible in the summer, pliny, the tides and the moon, plough, the, pogson, mr., pointers in the great bear, , polar axis, polar caps on mars, , pole, the, distance of from pole star lessening, ; elevation of, ; movement of, ; near a draconis, ; near vega or a lyra, pole star, ; belongs to the little bear, ; distance of, from the pole of the heavens, , , ; position of, ; slow motion of, pollux, , ; value of velocity of, pons, and the comet of , posidonius, potassium in the sun, præsepe, precession and nutation of the earth's axis, - proctor, and the stars in argelander's atlas, prism, the, ; its analysing power, pritchard, professor, stellar photographic researches of, procyon, ; value of velocity of, prominences on the sun, - ptolemy, his theory of astronomy, ; lunar crater named after him, q quarantids, the, r radius of the earth, , rainbow, the, ram, the, reflectors, , , refraction by the prism, refractors, , , regulus, , reservoir formed from tidal water, retina, the, and the telescope, , rhea, rigel, , , rigidity of the planets, , roberts, dr. isaac, and the nebula in andromeda, ; and the nebula in orion, roemer, and the velocity of light, romance, planet of, - rosse telescope, the, , , , rotational moment of momentum, rowland, professor, and spectral lines, rowton siderite, royal astronomical society and bessel, s sappho, satellites of jupiter, , , - , , ; confirmation of the copernican theory, satellites of mars, , - , , satellites of neptune, , satellites of saturn, ; bond's discoveries, ; cassini's discoveries, ; distances, ; herschel's discoveries, ; huyghens' discovery, ; kirkwood's deduction, ; lassell's deduction, ; movements, ; origin as suggested by the nebular theory, satellites of uranus, , , , saturn, ancient study of, ; attraction on uranus, ; axial rotation of, ; beauty of, ; comparative proximity to the earth of, ; density of, ; diameter of, , ; distance of, from the sun, , , ; elliptic path of, ; gravitation paramount, ; internal heat of, , ; leonids and, ; low density of, ; moment of momentum of, ; motion of, ; orbit of, , ; path of, perturbed by the attraction of jupiter, ; periodic time of, ; period of revolution of, ; picturesqueness of, ; position of, in the solar system, ; rings of, ; rings, bonds discovery, ; rings, cassini's discovery, ; rings, consistency, ; rings, dawes's discovery, ; rings, galileo's discovery, , ; rings, hadley's observations, ; rings, herschel's researches, ; rings, huyghens' discovery, - ; rings, keeler's measurement of the rotation, ; rings, maraldi's researches, ; rings, rotation of, , ; rings, spectrum of, ; rings, trouvelot's drawing, ; satellites of, , , , , ; size of, compared with other planets, , , ; spectrum of, ; unequal in appearance to mars and venus, ; velocity of, ; weight of, compared with the earth, savary and binary stars, schaeberle, mr., and mars, scheiner, and the values of velocity of stars, ; observations on sun-spots, schiaparelli, professor, and mars, ; and the connection between shooting-star showers and comets, ; and the rotation of mercury, schickard, schmidt, and nova cygni, , ; and the crater linné, ; and the leibnitz mountains, schröter, and the crater posidonius, schwabe, and sun-spots, seas in the moon, secchi, and stellar spectra, shoal of shooting stars, ; dimensions, shooting stars (_see_ stars, shooting) sickle, the, sidereal aggregation theory of sir w. herschel, siderite, rowton, sinus iridum, sirius, change in position of, ; companion of, , ; exceptional lustre of, ; irregularities of movement of, ; larger than the sun, ; most brilliant star, ; periodical appearances of, ; proper motion of, ; spectrum of, ; velocity of, ; weight of, smyth, professor c.p., sodium, colour of flame from, ; in the sun, solar corona, prominences etc. (_see under_ sun) solar system, - ; copernican exposition of the, ; influence of gravitation on, ; information respecting, obtained by observing the transit of venus, ; island in the universe, ; minor planets, - ; moment of momentum, ; movement of, towards lyra, ; origin of, as suggested by the nebular theory, ; position of saturn and uranus in, , south, sir james, spectra of stars, spectro-heliograph, spectroscope, - ; detection of iron in the sun by the, spectroscopic binaries, spectrum analysis, ; dark lines, , ; gaseous nebulæ, ; line d, , speculum, the rosse, spica, , spider-threads for adjusting the micrometer, ; for sighting telescopes, spots on the sun, - ; connection with magnetism, ; cycles, ; duration, ; epochs of maximum, ; motion, ; period of revolution, ; scheiner's observations, ; zones in which they occur, star clusters, - ; in hercules, ; in perseus, stars, apparent movements due to precession, nutation, and aberration, ; approximate number of, ; attraction inappreciable, ; catalogues of, , , , ; charts of, , ; circular movement of, - stars, distances of, ; bessel's labours, - ; henderson's labours, ; method of measuring, - ; struve's work, , , ; parallactic ellipse, - stars, double, ; bode's list, ; burnham's additions, ; cassini, ; herschel, , ; measurement, , ; revolution, ; savary, ; shape of orbit, ; variation in colour, stars, elliptic movement of, ; gravitation and, ; how distinguished from planets, ; physical nature of, ; probability of their possessing a planetary system, ; real and apparent movements of, ; really suns, , stars, shooting, attractions of the planets, ; connection with comets, - ; countless in number, ; dimensions of shoal, ; features of, ; length of orbit, ; orbit, ; orbit, gradual change, ; period of revolution, ; periodic return, , ; shower of november, , , - ; shower of november, , and professor adams, , ; shower of november, , radiation of tracks from leo, ; shower of november, , ; showers, ; showers and professor newton, ; track, ; transformed into vapour by friction with the earth's atmosphere, , ; velocity, , stars, spectra of, ; teaching of ancients respecting, ; temperature of, ; temporary, , ; values of velocity of, ; variable, stoney, dr. g.j., strontium, flame from, ; in the sun, struve, otto, and the distance of vega, , ; and the distance of cygni, , sun, the, and the velocity of light, ; apparent size of, as seen from the planets, , ; as a star, ; axial rotation of, ; compared with the earth, ; connection of, with the seasons, ; corona of, during eclipse, - ; density of, , ; diameter of, ; distance of, from mars, ; distance of, from saturn, ; distance of, from the earth, , , , , ; eclipse of, , ; ellipticity of, ; faculæ on surface of, ; focus of planets' orbits, ; gradually parting with its heat, ; granules on surface of, ; heat of, and its sources, - ; heat of, thrown on jupiter, ; minor planets and, ; movement of, towards lyra, ; nebular theory of its heat, ; photographed, ; precession of the earth's axis, ; prominences of, - ; relation of, to the moon, ; rising and setting of, ; rotation of, , ; size of, ; spectrum of, ; spots on, - ; spots, connection with magnetism, ; storms and convulsions on, , ; surface of, gaseous matter, ; surface of, mottled, ; teaching of early astronomers concerning, - ; temperature of, , , ; texture of, ; tides on, ; velocity of, ; weight of, compared with jupiter, , ; zodiacal light and, ; zones on the surface of, sunbeam, revelations of a, swan, the, , , sword-handle of perseus, syrtis major, t taurus, constellation of, , tebbutt's comet, telescope, construction of the first, ; equatorial (dunsink), - , ; greenwich, ; herschelian, ; lick, , ; paris, , ; reflecting, , ; refracting, , ; rosse, , , , ; sighting of a, ; structure of the eye illustrates the principle of the, ; vienna, - ; washington, ; yerkes, temporary stars, , tethys, theophilus, tides, the, actual energy derived from the earth, ; affected by the law of gravitation, , ; affected by the moon, , - ; at bay of fundy, ; at cardiff, ; at chepstow, ; at london, ; at st. helena, ; excited by the sun, ; formation of currents, ; in bristol channel, ; in mediterranean, ; in mid-ocean, ; jupiter and, ; length of the day and, ; lunar, , ; moment of momentum and, ; neap, ; rotation of the earth, and revolution of the moon, ; satellites of mars, ; solar, ; spring, ; variations in, ; waste of water power, ; work effected, tin in the sun, titan, , , titania, , transit of mercury, , , transit of venus, ; captain cook, ; copeland's observations of, ; crabtree's observations of, ; gassendi's observations of, ; halley's method, , ; horrocks' observations of, , ; importance of, ; kepler's prediction of, ; observations of, at dunsink, - transit of vulcan, - triesnecker, , trouvelot, mr. l., and saturn's rings, tschermak, and the origin of meteorites, , tycho (lunar crater), tycho brahe, and the observatory of uraniborg, , , u umbra of sun-spot, umbriel, , unstable dynamical equilibrium, uraniborg, observatory of, uranus, ; attraction of saturn, ; bradley's observations of, ; composition of, ; density of, ; diameter of, , ; diameter of orbit of, ; disc of, ; discovery of, by herschel, , ; distance from sun of, ; ellipse of, ; first taken for a comet, ; flamsteed's observations of, , ; formerly regarded as a star, , ; investigations to discover a planet outside the orbit, - ; irregular motion of, , ; lemonnier's observations of, ; leonids and, ; mayer's observations of, ; moment of momentum of, ; orbit of, , ; periodic time of, ; period of revolution of, ; rotation of, ; satellites of, ; satellites, discovery by herschel, ; satellites, movement nearly circular, ; satellites, periodical movements, ; satellites, plane of orbits, , ; size of compared with the earth, ; and with other planets, ; subject to another attraction besides the sun, ursa major (_see_ great bear) v variable stars, vega, , , , ; struve's measurement of, velocity, of light, , , ; of light, laws dependent upon, ; of planets, - , , ; of stars, values of, - venus, ancient study of, ; aspects of, ; atmosphere of, ; brilliancy of, ; density of, ; diameter of, , ; distance of, from the sun, , ; habitability of, ; movement of, ; neighbour to the earth, ; orbit of, , ; orbit form of, , ; periodic time of, ; a planet or "wanderer," ; rotation of, ; shape of, ; size of, compared with other planets, , ; surface of, ; transit of, , - ; transit, importance of, ; transit predicted by kepler, ; velocity and periodic time of, , , ; view of the ancients about, vesta, , victoria, vienna telescope, - virgo, vogel and algol, ; and spica, , ; and the spectra of the stars, , volcanic origin of meteorites, ; outbreaks on the earth, von asten and encke's comet, , ; and the distance of the sun, ; and the weight of mercury, vortex rings, vulcan, , ; and the sun, w wargentin, watson, professor, and mercury, watson, sir william, friendship with herschel, wave-lengths, weather, not affected by the moon, wilson, mr. w.e., and the nebula in orion, witt, herr g., and eros, wold cottage meteorite, the, wright, thomas, and the milky way, y "year of stars," the, yerkes observatory, chicago, young, professor, account of a marvellous sun-prominence, ; and sun-spots, ; observations on magnetic storms, z zeeman, dr., and spectral lines, zinc in the sun, zodiac, the, zodiacal light, zone of minor planets, printed by cassell & company, limited, la belle sauvage, london, e.c. footnotes: [ ] it may, however, be remarked that a star is never _seen_ to set, as, owing to our atmosphere, it ceases to be visible before it reaches the horizon. [ ] "popular astronomy," p. . [ ] _limb_ is the word used by astronomers to denote the _edge_ or circumference of the apparent disc of a heavenly body. [ ] "the sun," p. . [ ] it has been frequently stated that the outburst in , witnessed by carrington and hodgson, was immediately followed by an unusually intense magnetic storm, but the records at kew and greenwich show that the magnetic disturbances on that day were of a very trivial character. [ ] some ungainly critic has observed that the poet himself seems to have felt a doubt on the matter, because he has supplemented the dubious moonbeams by the "lantern dimly burning." the more generous, if somewhat a sanguine remark has been also made, that "the time will come when the evidence of this poem will prevail over any astronomical calculations." [ ] this sketch has been copied by permission from the very beautiful view in messrs. nasmyth and carpenter's book, of which it forms plate xi. so have also the other illustrations of lunar scenery in plates viii., ix. the photographs were obtained by mr. nasmyth from models carefully constructed from his drawings to illustrate the features on the moon. during the last twenty years photography has completely superseded drawing by eye in the delineation of lunar objects. long series of magnificent photographs of lunar scenery have been published by the paris and lick observatories. [ ] at the british association's meeting at cardiff in , prof. copeland exhibited a model of the moon, on which the appearance of the streaks near full moon was perfectly shown by means of small spheres of transparent glass attached to the surface. [ ] the duration of an occultation, or, in other words, the length of time during which the moon hides the star, would be slightly shorter than the computed time, if the moon had an atmosphere capable of sensibly refracting the light from the star. but, so far, our observations do not indicate this with certainty. [ ] i owe my knowledge of this subject to dr. g. johnstone stoney, f.r.s. there has been some controversy as to who originated the ingenious and instructive doctrine here sketched. [ ] the space described by a falling body is proportional to the product of the force and the square of the time. the force varies inversely as the square of the distance from the earth, so that the space will vary as the square of the time, and inversely as the square of the distance. if, therefore, the distance be increased sixty-fold, the time must also be increased sixty-fold, if the space fallen through is to remain the same. [ ] see newcomb's "popular astronomy," p. . [ ] recent investigation by newcomb on the motion of mercury have led to the result that the hypothesis of a planet or a ring of very small planets between the orbit of mercury and the sun cannot account for the difference between theory and observation in the movements of mercury. harzer has come to the same result, and has shown that the disturbing element may possibly be the material of the solar corona. [ ] "the sun: its planets, and their satellites." london: (page ). [ ] james gregory, in a book on optics written in , had already suggested the use of the transit of venus for this purpose. [ ] _see_ "astronomy and astrophysics," no. . [ ] _see_ "astronomy and astrophysics," no. . [ ] this is the curved marking which on plate xviii. appears in longitude ° and north of (that is, below) the equator. here, as in all astronomical drawings, north is at the foot and south at the top. _see_ above, p. (chapter iii.). [ ] now director of the lick observatory. [ ] the heliometer is a telescope with its object-glass cut in half along a diameter. one or both of these halves is movable transversely by a screw. each half gives a complete image of the object. the measures are effected by observing how many turns of the screw convey the image of the star formed by one half of the object-glass to coincide with the image of the planet formed by the other. [ ] see "astronomy and astrophysics," no. . [ ] it is only right to add that some observers believe that, in exceptional circumstances, points of jupiter have shown some slight degree of intrinsic light. [ ] professor pickering, of cambridge, mass., has, however, effected the important improvement of measuring the decline of light of the satellite undergoing eclipse by the photometer. much additional precision may be anticipated in the results of such observations. [ ] "newcomb's popular astronomy," p. . [ ] _see_ grant, "history of physical astronomy," page . [ ] now director of the lick observatory. [ ] we are here neglecting the orbital motion of saturn, by which the whole system is moved towards or from the earth, but as this motion is common to the ball and the ring, it will not disturb the relative positions of the three spectra. [ ] according to prof. barnard's recent measures, the diameter of titan is , miles. this is the satellite discovered by huyghens; it is the sixth in order from the planet. [ ] extract from "three cities of russia," by c. piazzi smyth, vol. ii., p. : "in the year . it then chanced that george iii., of great britain, was pleased to send as a present to the empress catharine of russia a ten-foot reflecting telescope constructed by sir william herschel. her majesty immediately desired to try its powers, and roumovsky was sent for from the academy to repair to tsarskoe-selo, where the court was at the time residing. the telescope was accordingly unpacked, and for eight long consecutive evenings the empress employed herself ardently in observing the moon, planets, and stars; and more than this, in inquiring into the state of astronomy in her dominions. then it was that roumovsky set before the imperial view the academy's idea of removing their observatory, detailing the necessity for, and the advantages of, such a proceeding. graciously did the 'semiramis of the north,' the 'polar star,' enter into all these particulars, and warmly approve of the project; but death closed her career within a few weeks after, and prevented her execution of the design." [ ] _see_ professor holden's "sir william herschel, his life and works." [ ] arago says that "lemonnier's records were the image of chaos." bouvard showed to arago one of the observations of uranus which was written on a paper bag that in its time had contained hair-powder. [ ] the first comet of also suddenly increased in brightness, while a distinct disc, which hitherto had formed the nucleus, became transformed into a fine point of light. [ ] the three numbers , , and / are nearly inversely proportional to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour, and it is for this reason that bredichin suggested the above-mentioned composition of the various types of tail. spectroscopic evidence of the presence of hydrogen is yet wanting. [ ] this illustration, as well as the figure of the path of the meteors, has been derived from dr. g.j. stoney's interesting lecture on "the story of the november meteors," at the royal institution, in . [ ] on the th november, , a piece of meteoric iron fell at mazapil, in mexico, during the shower of andromedes, but whether it formed part of the swarm is not known. it is, however, to be noticed that meteorites are said to have fallen on several other occasions at the end of november. [ ] hooke had noticed, in , that the star gamma arietis was double. [ ] perhaps if we could view the stars without the intervention of the atmosphere, blue stars would be more common. the absorption of the atmosphere specially affects the greenish and bluish colours. professor langley gives us good reason for believing that the sun itself would be blue if it were not for the effect of the air. [ ] the declination of a star is the arc drawn from the star to the equator at right angles to the latter. [ ] the distance of cygni has, however, again been investigated by professor asaph hall, of washington, who has obtained a result considerably less than had been previously supposed; on the other hand, professor pritchard's photographic researches are in confirmation of struve's and those obtained at dunsink. [ ] i am indebted for this drawing to the kindness of messrs. de la rue. [ ] _see_ chapter xix., on the mass of sirius and his satellite. [ ] as the earth carries on the telescope at the rate of miles a second, and as light moves with the velocity of , miles a second very nearly, it follows that the velocity of the telescope is about one ten-thousandth part of that of light. while the light moves down the tube feet long, the telescope will therefore have moved the ten-thousandth part of feet--_i.e._, the fortieth of an inch. [ ] _see_ newcomb's "popular astronomy," p. , where the discovery of this law is attributed to mr. j. homer lane, of washington. the contraction theory is due to helmholtz. [ ] the theory of tidal evolution sketched in this chapter is mainly due to the researches of professor g.h. darwin, f.r.s. [ ] the hour varies with the locality: it would be . at calais; at liverpool, . ; at swansea bay, . , etc. [ ] having decided upon the units of mass, of angle, and of distance which we intend to use for measuring these quantities, then any mass, or angle, or distance is expressed by a certain definite number. thus if we take the mass of the earth as the unit of mass, the angle through which it moves in a second as the unit of angle, and its distance from the sun as the unit of distance, we shall find that the similar quantities for jupiter are expressed by the numbers , · , and · respectively. hence its orbital moment of momentum is × · × ( · )². generously made available by the internet archive/american libraries.) transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. the advertisement from the beginning of the book has been joined with the other advertisements near the end of the book. greek words are spelled out and represented as [word]. greek letters are represented as [a] "for alpha". astronomy of to-day [illustration: the total eclipse of the sun of august th, . the corona; from a water-colour sketch, made at burgos, in spain, during the total phase, by the french artist, mdlle. andrÉe moch.] astronomy of to-day _a popular introduction in non-technical language_ by cecil g. dolmage, m.a., ll.d., d.c.l. fellow of the royal astronomical society; member of the british astronomical association; member of the astronomical society of the pacific; membre de la société astronomique de france; membre de la société belge d'astronomie with a frontispiece in colour and illustrations & diagrams _third edition_ london seeley and co. limited great russell street preface the object of this book is to give an account of the science of astronomy, as it is known at the present day, in a manner acceptable to the _general reader_. it is too often supposed that it is impossible to acquire any useful knowledge of astronomy without much laborious study, and without adventuring into quite a new world of thought. the reasoning applied to the study of the celestial orbs is, however, of no different order from that which is employed in the affairs of everyday life. the science of mathematics is perhaps responsible for the idea that some kind of difference does exist; but mathematical processes are, in effect, no more than ordinary logic in concentrated form, the _shorthand of reasoning_, so to speak. i have attempted in the following pages to take the main facts and theories of astronomy out of those mathematical forms which repel the general reader, and to present them in the _ordinary language of our workaday world_. the few diagrams introduced are altogether supplementary, and are not connected with the text by any wearying cross-references. each diagram is complete in itself, being intended to serve as a pictorial aid, in case the wording of the text should not have perfectly conveyed the desired meaning. the full page illustrations are also described as adequately as possible at the foot of each. as to the coloured frontispiece, this must be placed in a category by itself. it is the work of the _artist_ as distinct from the scientist. the book itself contains incidentally a good deal of matter concerned with the astronomy of the past, the introduction of which has been found necessary in order to make clearer the astronomy of our time. it would be quite impossible for me to enumerate here the many sources from which information has been drawn. but i acknowledge my especial indebtedness to professor f.r. moulton's _introduction to astronomy_ (macmillan, ), to the works on eclipses of the late rev. s.j. johnson and of mr. w.t. lynn, and to the excellent _journals of the british astronomical association_. further, for those grand questions concerned with the stellar universe at large, i owe a very deep debt to the writings of the famous american astronomer, professor simon newcomb, and of our own countryman, mr. john ellard gore; to the latter of whom i am under an additional obligation for much valuable information privately rendered. in my search for suitable illustrations, i have been greatly aided by the kindly advice of mr. w. h. wesley, the assistant secretary of the royal astronomical society. to those who have been so good as to permit me to reproduce pictures and photographs, i desire to record my best thanks as follows:--to the french artist, mdlle. andrée moch; to the astronomer royal; to sir david gill, k.c.b., ll.d., f.r.s.; to the council of the royal astronomical society; to professor e.b. frost, director of the yerkes observatory; to m.p. puiseux, of the paris observatory; to dr. max wolf, of heidelberg; to professor percival lowell; to the rev. theodore e.r. phillips, m.a., f.r.a.s.; to mr. w.h. wesley; to the warner and swasey co., of cleveland, ohio, u.s.a.; to the publishers of _knowledge_, and to messrs. sampson, low & co. for permission to reproduce the beautiful photograph of the spiral nebula in canes venatici (plate xxii.), i am indebted to the distinguished astronomer, the late dr. w.e. wilson, d.sc., f.r.s., whose untimely death, i regret to state, occurred in the early part of this year. finally, my best thanks are due to mr. john ellard gore, f.r.a.s., m.r.i.a., to mr. w.h. wesley, and to mr. john butler burke, m.a., of cambridge, for their kindness in reading the proof-sheets. cecil g. dolmage. london, s.w., _august , ._ prefatory note to the second edition the author of this book lived only long enough to hear of the favour with which it had been received, and to make a few corrections in view of the second edition which it has so soon reached. _december ._ contents chapter i page the ancient view chapter ii the modern view chapter iii the solar system chapter iv celestial mechanism chapter v celestial distances chapter vi celestial measurement chapter vii eclipses and kindred phenomena chapter viii famous eclipses of the sun chapter ix famous eclipses of the moon chapter x the growth of observation chapter xi spectrum analysis chapter xii the sun chapter xiii the sun--_continued_ chapter xiv the inferior planets chapter xv the earth chapter xvi the moon chapter xvii the superior planets chapter xviii the superior planets--_continued_ chapter xix comets chapter xx remarkable comets chapter xxi meteors or shooting stars chapter xxii the stars chapter xxiii the stars--_continued_ chapter xxiv systems of stars chapter xxv the stellar universe chapter xxvi the stellar universe--_continued_ chapter xxvii the beginning of things chapter xxviii the end of things index list of illustrations list of plates plate the total eclipse of the sun of august , _frontispiece_ i. the total eclipse of the sun of may , _to face page_ ii. great telescope of hevelius " " iii. a tubeless, or "aerial" telescope " " iv. the great yerkes telescope " " v. the sun, showing several groups of spots " " vi. photograph of a sunspot " " vii. forms of the solar corona at the epochs of sunspot maximum and sunspot minimum respectively. (a) the total eclipse of the sun of december , . (b) the total eclipse of the sun of may , " " viii. the moon _to face page_ ix. map of the moon, showing the principal "craters," mountain ranges and "seas" " " x. one of the most interesting regions on the moon " " xi. the moon (showing systems of "rays") " " xii. a map of the planet mars " " xiii. minor planet trails " " xiv. the planet jupiter " " xv. the planet saturn " " xvi. early representations of saturn " " xvii. donati's comet " " xviii. daniel's comet of " " xix. the sky around the north pole " " xx. orion and his neighbours " " xxi. the great globular cluster in the southern constellation of centaurus " " xxii. spiral nebula in the constellation of canes venatici " " xxiii. the great nebula in the constellation of andromeda _to face page_ xxiv. the great nebula in the constellation of orion " " list of diagrams fig. page . the ptolemaic idea of the universe . the copernican theory of the solar system . total and partial eclipses of the moon . total and partial eclipses of the sun . "baily's beads" . map of the world on mercator's projection, showing a portion of the progress of the total solar eclipse of august , , across the surface of the earth . the "ring with wings" . the various types of telescope . the solar spectrum . a section through the sun, showing how the prominences rise from the chromosphere . orbit and phases of an inferior planet . the "black drop" . summer and winter . orbit and phases of the moon . the rotation of the moon on her axis . laplace's "perennial full moon" . illustrating the author's explanation of the apparent enlargement of celestial objects . showing how the tail of a comet is directed away from the sun . the comet of , as represented in the bayeux tapestry . passage of the earth through the thickest portion of a meteor swarm astronomy of to-day chapter i the ancient view it is never safe, as we know, to judge by appearances, and this is perhaps more true of astronomy than of anything else. for instance, the idea which one would most naturally form of the earth and heaven is that the solid earth on which we live and move extends to a great distance in every direction, and that the heaven is an immense dome upon the inner surface of which the stars are fixed. such must needs have been the idea of the universe held by men in the earliest times. in their view the earth was of paramount importance. the sun and moon were mere lamps for the day and for the night; and these, if not gods themselves, were at any rate under the charge of special deities, whose task it was to guide their motions across the vaulted sky. little by little, however, this simple estimate of nature began to be overturned. difficult problems agitated the human mind. on what, for instance, did the solid earth rest, and what prevented the vaulted heaven from falling in upon men and crushing them out of existence? fantastic myths sprang from the vain attempts to solve these riddles. the hindoos, for example, imagined the earth as supported by four elephants which stood upon the back of a gigantic tortoise, which, in its turn, floated on the surface of an elemental ocean. the early western civilisations conceived the fable of the titan atlas, who, as a punishment for revolt against the olympian gods, was condemned to hold up the expanse of sky for ever and ever. later on glimmerings of the true light began to break in upon men. the greek philosophers, who busied themselves much with such matters, gradually became convinced that the earth was spherical in shape, that is to say, round like a ball. in this opinion we now know that they were right; but in their other important belief, viz. that the earth was placed at the centre of all things, they were indeed very far from the truth. by the second century of the christian era, the ideas of the early philosophers had become hardened into a definite theory, which, though it appears very incorrect to us to-day, nevertheless demands exceptional notice from the fact that it was everywhere accepted as the true explanation until so late as some four centuries ago. this theory of the universe is known by the name of the ptolemaic system, because it was first set forth in definite terms by one of the most famous of the astronomers of antiquity, claudius ptolemæus pelusinensis ( - a.d.), better known as ptolemy of alexandria. in his system the earth occupied the centre; while around it circled in order outwards the moon, the planets mercury and venus, the sun, and then the planets mars, jupiter, and saturn. beyond these again revolved the background of the heaven, upon which it was believed that the stars were fixed-- "stellis ardentibus aptum," as virgil puts it (see fig. ). [illustration: fig. .--the ptolemaic idea of the universe.] the ptolemaic system persisted unshaken for about fourteen hundred years after the death of its author. clearly men were flattered by the notion that their earth was the most important body in nature, that it stood still at the centre of the universe, and was the pivot upon which all things revolved. chapter ii the modern view it is still well under four hundred years since the modern, or copernican, theory of the universe supplanted the ptolemaic, which had held sway during so many centuries. in this new theory, propounded towards the middle of the sixteenth century by nicholas copernicus ( - ), a prussian astronomer, the earth was dethroned from its central position and considered merely as one of a number of planetary bodies which revolve around the sun. as it is not a part of our purpose to follow in detail the history of the science, it seems advisable to begin by stating in a broad fashion the conception of the universe as accepted and believed in to-day. the sun, the most important of the celestial bodies so far as we are concerned, occupies the central position; not, however, in the whole universe, but only in that limited portion which is known as the solar system. around it, in the following order outwards, circle the planets mercury, venus, the earth, mars, jupiter, saturn, uranus, and neptune (see fig. , p. ). at an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. the two first-mentioned members of the solar system, mercury and venus, are known as the inferior planets; and in their courses about the sun, they always keep well inside the path along which our earth moves. the remaining members (exclusive of the earth) are called superior planets, and their paths lie all outside that of the earth. [illustration: fig. .--the copernican theory of the solar system.] the five planets, mercury, venus, mars, jupiter, and saturn, have been known from all antiquity. nothing then can bring home to us more strongly the immense advance which has taken place in astronomy during modern times than the fact that it is only years since observation of the skies first added a planet to that time-honoured number. it was indeed on the th of march , while engaged in observing the constellation of the twins, that the justly famous sir william herschel caught sight of an object which he did not recognise as having met with before. he at first took it for a comet, but observations of its movements during a few days showed it to be a planet. this body, which the power of the telescope alone had thus shown to belong to the solar family, has since become known to science under the name of uranus. by its discovery the hitherto accepted limits of the solar system were at once pushed out to twice their former extent, and the hope naturally arose that other planets would quickly reveal themselves in the immensities beyond. for a number of years prior to herschel's great discovery, it had been noticed that the distances at which the then known planets circulated appeared to be arranged in a somewhat orderly progression outwards from the sun. this seeming plan, known to astronomers by the name of bode's law, was closely confirmed by the distance of the new planet uranus. there still lay, however, a broad gap between the planets mars and jupiter. had another planet indeed circuited there, the solar system would have presented an appearance of almost perfect order. but the void between mars and jupiter was unfilled; the space in which one would reasonably expect to find another planet circling was unaccountably empty. on the first day of the nineteenth century the mystery was however explained, a body being discovered[ ] which revolved in the space that had hitherto been considered planetless. but it was a tiny globe hardly worthy of the name of planet. in the following year a second body was discovered revolving in the same space; but it was even smaller in size than the first. during the ensuing five years two more of these little planets were discovered. then came a pause, no more such bodies being added to the system until half-way through the century, when suddenly the discovery of these so-called "minor planets" began anew. since then additions to this portion of our system have rained thick and fast. the small bodies have received the name of asteroids or planetoids; and up to the present time some six hundred of them are known to exist, all revolving in the previously unfilled space between mars and jupiter. in the year the outer boundary of the solar system was again extended by the discovery that a great planet circulated beyond uranus. the new body, which received the name of neptune, was brought to light as the result of calculations made at the same time, though quite independently, by the cambridge mathematician adams, and the french astronomer le verrier. the discovery of neptune differed, however, from that of uranus in the following respect. uranus was found merely in the course of ordinary telescopic survey of the heavens. the position of neptune, on the other hand, was predicted as the result of rigorous mathematical investigations undertaken with the object of fixing the position of an unseen and still more distant body, the attraction of which, in passing by, was disturbing the position of uranus in its revolution around the sun. adams actually completed his investigation first; but a delay at cambridge in examining that portion of the sky, where he announced that the body ought just then to be, allowed france to snatch the honour of discovery, and the new planet was found by the observer galle at berlin, very near the place in the heavens which le verrier had mathematically predicted for it. nearly fifty years later, that is to say, in our own time, another important planetary discovery was made. one of the recent additions to the numerous and constantly increasing family of the asteroids, a tiny body brought to light in , turned out after all not to be circulating in the customary space between mars and jupiter, but actually in that between our earth and mars. this body is very small, not more than about twenty miles across. it has received the name of eros (the greek equivalent for cupid), in allusion to its insignificant size as compared with the other leading members of the system. this completes the list of the planets which, so far, have revealed themselves to us. whether others exist time alone will show. two or three have been suspected to revolve beyond the path of neptune; and it has even been asserted, on more than one occasion, that a planet circulates nearer to the sun than mercury. this supposed body, to which the name of "vulcan" was provisionally given, is said to have been "discovered" in by a french doctor named lescarbault, of orgères near orleans; but up to the present there has been no sufficient evidence of its existence. the reason why such uncertainty should exist upon this point is easy enough to understand, when we consider the overpowering glare which fills our atmosphere all around the sun's place in the sky. mercury, the nearest known planet to the sun, is for this reason always very difficult to see; and even when, in its course, it gets sufficiently far from the sun to be left for a short time above the horizon after sunset, it is by no means an easy object to observe on account of the mists which usually hang about low down near the earth. one opportunity, however, offers itself from time to time to solve the riddle of an "intra-mercurial" planet, that is to say, of a planet which circulates within the path followed by mercury. the opportunity in question is furnished by a total eclipse of the sun; for when, during an eclipse of that kind, the body of the moon for a few minutes entirely hides the sun's face, and the dazzling glare is thus completely cut off, astronomers are enabled to give an unimpeded, though all too hurried, search to the region close around. a goodly number of total eclipses of the sun have, however, come and gone since the days of lescarbault, and no planet, so far, has revealed itself in the intra-mercurial space. it seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet mercury. next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of satellites. the name _satellite_ is derived from a latin word signifying _an attendant_; for the bodies so-called move along always in close proximity to their respective "primaries," as the planets which they accompany are technically termed. the satellites cannot be considered as allotted with any particular regularity among the various members of the system; several of the planets, for instance, having a goodly number of these bodies accompanying them, while others have but one or two, and some again have none at all. taking the planets in their order of distance outward from the sun, we find that neither mercury nor venus are provided with satellites; the earth has only one, viz. our neighbour the moon; while mars has but two tiny ones, so small indeed that one might imagine them to be merely asteroids, which had wandered out of their proper region and attached themselves to that planet. for the rest, so far as we at present know, jupiter possesses seven,[ ] saturn ten, uranus four, and neptune one. it is indeed possible, nay more, it is extremely probable, that the two last-named planets have a greater number of these secondary bodies revolving around them; but, unfortunately, the uranian and neptunian systems are at such immense distances from us, that even the magnificent telescopes of to-day can extract very little information concerning them. from the distribution of the satellites, the reader will notice that the planets relatively near to the sun are provided with few or none, while the more distant planets are richly endowed. the conclusion, therefore, seems to be that nearness to the sun is in some way unfavourable either to the production, or to the continued existence, of satellites. a planet and its satellites form a repetition of the solar system on a tiny scale. just as the planets revolve around the sun, so do these secondary bodies revolve around their primaries. when galileo, in , turned his newly invented telescope upon jupiter, he quickly recognised in the four circling moons which met his gaze, a miniature edition of the solar system. besides the planets and their satellites, there are two other classes of bodies which claim membership of the solar system. these are comets and meteors. comets differ from the bodies which we have just been describing in that they appear filmy and transparent, whereas the others are solid and opaque. again, the paths of the planets around the sun and of the satellites around their primaries are not actually circles; they are ovals, but their ovalness is not of a marked degree. the paths of comets on the other hand are usually _very_ oval; so that in their courses many of them pass out as far as the known limits of the solar system, and even far beyond. it should be mentioned that nowadays the tendency is to consider comets as permanent members of the system, though this was formerly not by any means an article of faith with astronomers. meteors are very small bodies, as a rule perhaps no larger than pebbles, which move about unseen in space, and of which we do not become aware until they arrive very close to the earth. they are then made visible to us for a moment or two in consequence of being heated to a white heat by the friction of rushing through the atmosphere, and are thus usually turned into ashes and vapour long before they reach the surface of our globe. though occasionally a meteoric body survives the fiery ordeal, and reaches the earth more or less in a solid state to bury itself deep in the soil, the majority of these celestial visitants constitute no source of danger whatever for us. any one who will take the trouble to gaze at the sky for a short time on a clear night, is fairly certain to be rewarded with the view of a meteor. the impression received is as if one of the stars had suddenly left its accustomed place, and dashed across the heavens, leaving in its course a trail of light. it is for this reason that meteors are popularly known under the name of "shooting stars." [ ] by the italian astronomer, piazzi, at palermo. [ ] probably eight. (see note, page .) chapter iii the solar system we have seen, in the course of the last chapter, that the solar system is composed as follows:--there is a central body, the sun, around which revolve along stated paths a number of important bodies known as planets. certain of these planets, in their courses, carry along in company still smaller bodies called satellites, which revolve around them. with regard, however, to the remaining members of the system, viz. the comets and the meteors, it is not advisable at this stage to add more to what has been said in the preceding chapter. for the time being, therefore, we will devote our attention merely to the sun, the planets, and the satellites. of what shape then are these bodies? of one shape, and that one alone which appears to characterise all solid objects in the celestial spaces: they are spherical, which means _round like a ball_. each of these spherical bodies rotates; that is to say, turns round and round, as a top does when it is spinning. this rotation is said to take place "upon an axis," a statement which may be explained as follows:--imagine a ball with a knitting-needle run right through its centre. then imagine this needle held pointing in one fixed direction while the ball is turned round and round. well, it is the same thing with the earth. as it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it from north pole to south pole. in reality, however, there is no such material axis to regulate the constant direction of the rotation, just as there are no actual supports to uphold the earth itself in space. the causes which keep the celestial spheres poised, and which control their motions, are far more wonderful than any mechanical device. at this juncture it will be well to emphasise the sharp distinction between the terms _rotation_ and _revolution_. the term "rotation" is invariably used by astronomers to signify the motion which a celestial body has upon an axis; the term "revolution," on the other hand, is used for the movement of one celestial body around another. speaking of the earth, for instance, we say, that it _rotates_ on its axis, and that it _revolves_ around the sun. so far, nothing has been said about the sizes of the members of our system. is there any stock size, any pattern according to which they may be judged? none whatever! they vary enormously. very much the largest of all is the sun, which is several hundred times larger than all the planets and satellites of the system rolled together. next comes jupiter and afterwards the other planets in the following order of size:--saturn, uranus, neptune, the earth, venus, mars, and mercury. very much smaller than any of these are the asteroids, of which ceres, the largest, is less than miles in diameter. it is, by the way, a strange fact that the zone of asteroids should mark the separation of the small planets from the giant ones. the following table, giving roughly the various diameters of the sun and the principal planets in miles, will clearly illustrate the great discrepancy in size which prevails in the system. sun , miles mercury , " venus , " earth , " mars , " zone of asteroids jupiter , " saturn , " uranus[ ] , " neptune[ ] , " it does not seem possible to arrive at any generalisation from the above data, except it be to state that there is a continuous increase in size from mercury to the earth, and a similar decrease in size from jupiter outwards. were mars greater than the earth, the planets could then with truth be said to increase in size up to jupiter, and then to decrease. but the zone of asteroids, and the relative smallness of mars, negative any attempt to regard the dimensions of the planets as an orderly sequence. next with respect to relative distance from the sun, venus circulates nearly twice as far from it as mercury, the earth nearly three times as far, and mars nearly four times. after this, just as we found a sudden increase in size, so do we meet with a sudden increase in distance. jupiter, for instance, is about thirteen times as far as mercury, saturn about twenty-five times, uranus about forty-nine times, and neptune about seventy-seven. (see fig. , p. .) it will thus be seen how enormously the solar system was enlarged in extent by the discovery of the outermost planets. the finding of uranus plainly doubled its breadth; the finding of neptune made it more than half as broad again. nothing indeed can better show the import of these great discoveries than to take a pair of compasses and roughly set out the above relative paths in a series of concentric circles upon a large sheet of paper, and then to consider that the path of saturn was the supposed boundary of our solar system prior to the year . we have seen that the usual shape of celestial bodies themselves is spherical. of what form then are their paths, or _orbits_, as these are called? one might be inclined at a venture to answer "circular," but this is not the case. the orbits of the planets cannot be regarded as true circles. they are ovals, or, to speak in technical language, "ellipses." their ovalness or "ellipticity" is, however, in each case not by any means of the same degree. some orbits--for instance, that of the earth--differ only slightly from circles; while others--those of mars or mercury, for example--are markedly elliptic. the orbit of the tiny planet eros is, however, far and away the most elliptic of all, as we shall see when we come to deal with that little planet in detail. it has been stated that the sun and planets are always rotating. the various rates at which they do so will, however, be best appreciated by a comparison with the rate at which the earth itself rotates. but first of all, let us see what ground we have, if any, for asserting that the earth rotates at all? if we carefully watch the heavens we notice that the background of the sky, with all the brilliant objects which sparkle in it, appears to turn once round us in about twenty-four hours; and that the pivot upon which this movement takes place is situated somewhere near what is known to us as the _pole star_. this was one of the earliest facts noted with regard to the sky; and to the men of old it therefore seems as if the heavens and all therein were always revolving around the earth. it was natural enough for them to take this view, for they had not the slightest idea of the immense distance of the celestial bodies, and in the absence of any knowledge of the kind they were inclined to imagine them comparatively near. it was indeed only after the lapse of many centuries, when men had at last realised the enormous gulf which separated them from even the nearest object in the sky, that a more reasonable opinion began to prevail. it was then seen that this revolution of the heavens about the earth could be more easily and more satisfactorily explained by supposing a mere rotation of the solid earth about a fixed axis, pointed in the direction of the polar star. the probability of such a rotation on the part of the earth itself was further strengthened by the observations made with the telescope. when the surfaces of the sun and planets were carefully studied these bodies were seen to be rotating. this being the case, there could not surely be much hesitation in granting that the earth rotated also; particularly when it so simply explained the daily movement of the sky, and saved men from the almost inconceivable notion that the whole stupendous vaulted heaven was turning about them once in every twenty-four hours. if the sun be regularly observed through a telescope, it will gradually be gathered from the slow displacement of sunspots across its face, their disappearance at one edge and their reappearance again at the other edge, that it is rotating on an axis in a period of about twenty-six days. the movement, too, of various well-known markings on the surfaces of the planets mars, jupiter, and saturn proves to us that these bodies are rotating in periods, which are about twenty-four hours for the first, and about ten hours for each of the other two. with regard, however, to uranus and neptune there is much more uncertainty, as these planets are at such great distances that even our best telescopes give but a confused view of the markings which they display; still a period of rotation of from ten to twelve hours appears to be accepted for them. on the other hand the constant blaze of sunlight in the neighbourhood of mercury and venus equally hampers astronomers in this quest. the older telescopic observers considered that the rotation periods of these two planets were about the same as that of the earth; but of recent years the opinion has been gaining ground that they turn round on their axes in exactly the same time as they revolve about the sun. this question is, however, a very doubtful one, and will be again referred to later on; but, putting it on one side, it will be seen from what we have said above, that the rotation periods of the other planets of our system are usually about twenty-four hours, or under. the fact that the rotation period of the sun should run into _days_ need not seem extraordinary when one considers its enormous size. the periods taken by the various planets to revolve around the sun is the next point which has to be considered. here, too, it is well to start with the earth's period of revolution as the standard, and to see how the periods taken by the other planets compare with it. the earth takes about - / days to revolve around the sun. this period of time is known to us as a "year." the following table shows in days and years the periods taken by each of the other planets to make a complete revolution round the sun:-- mercury about days. venus " " mars " year and days. jupiter " years and days. saturn " years and days. uranus " years and days. neptune " years and days. from these periods we gather an important fact, namely, that the nearer a planet is to the sun the faster it revolves. compared with one of our years what a long time does an uranian, or neptunian, "year" seem? for instance, if a "year" had commenced in neptune about the middle of the reign of george ii., that "year" would be only just coming to a close; for the planet is but now arriving back to the position, with regard to the sun, which it then occupied. uranus, too, has only completed a little more than - / of its "years" since herschel discovered it. having accepted the fact that the planets are revolving around the sun, the next point to be inquired into is:--what are the positions of their orbits, or paths, relatively to each other? suppose, for instance, the various planetary orbits to be represented by a set of hoops of different sizes, placed one within the other, and the sun by a small ball in the middle of the whole; in what positions will these hoops have to be arranged so as to imitate exactly the true condition of things? first of all let us suppose the entire arrangement, ball and hoops, to be on one level, so to speak. this may be easily compassed by imagining the hoops as floating, one surrounding the other, with the ball in the middle of all, upon the surface of still water. such a set of objects would be described in astronomical parlance as being _in the same plane_. suppose, on the other hand, that some of these floating hoops are tilted with regard to the others, so that one half of a hoop rises out of the water and the other half consequently sinks beneath the surface. this indeed is the actual case with regard to the planetary orbits. they do not by any means lie all exactly in the same plane. each one of them is tilted, or _inclined_, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the _level_ of the solar system. this tilting, or "inclination," is, in the larger planets, greatest for the orbit of mercury, least for that of uranus. mercury's orbit is inclined to that of the earth at an angle of about °, that of venus at a little over °, that of saturn - / °; while in those of mars, neptune, and jupiter the inclination is less than °. but greater than any of these is the inclination of the orbit of the tiny planet eros, viz. nearly °. the systems of satellites revolving around their respective planets being, as we have already pointed out, mere miniature editions of the solar system, the considerations so far detailed, which regulate the behaviour of the planets in their relations to the sun, will of necessity apply to the satellites very closely. in one respect, however, a system of satellites differs materially from a system of planets. the central body around which planets are in motion is self-luminous, whereas the planetary body around which a satellite revolves is not. true, planets shine, and shine very brightly too; as, for instance, venus and jupiter. but they do not give forth any light of their own, as the sun does; they merely reflect the sunlight which they receive from him. putting this one fact aside, the analogy between the planetary system and a satellite system is remarkable. the satellites are spherical in form, and differ markedly in size; they rotate, so far as we know, upon their axes in varying times; they revolve around their governing planets in orbits, not circular, but elliptic; and these orbits, furthermore, do not of necessity lie in the same plane. last of all the satellites revolve around their primaries at rates which are directly comparable with those at which the planets revolve around the sun, the rule in fact holding good that the nearer a satellite is to its primary the faster it revolves. [ ] as there seems to be much difference of opinion concerning the diameters of uranus and neptune, it should here be mentioned that the above figures are taken from professor f.r. moulton's _introduction to astronomy_ ( ). they are there stated to be given on the authority of "barnard's many measures at the lick observatory." chapter iv celestial mechanism as soon as we begin to inquire closely into the actual condition of the various members of the solar system we are struck with a certain distinction. we find that there are two quite different points of view from which these bodies can be regarded. for instance, we may make our estimates of them either as regards _volume_--that is to say, the mere room which they take up; or as regards _mass_--that is to say, the amount of matter which they contain. let us imagine two globes of equal volume; in other words, which take up an equal amount of space. one of these globes, however, may be composed of material much more tightly put together than in the other; or of greater _density_, as the term goes. that globe is said to be the greater of the two in mass. were such a pair of globes to be weighed in scales, one globe in each pan, we should see at once, by its weighing down the other, which of the two was composed of the more tightly packed materials; and we should, in astronomical parlance, say of this one that it had the greater mass. volume being merely another word for size, the order of the members of the solar system, with regard to their volumes, will be as follows, beginning with the greatest:--the sun, jupiter, saturn, uranus, neptune, the earth, venus, mars, and mercury. with regard to mass the same order strangely enough holds good. the actual densities of the bodies in question are, however, very different. the densest or closest packed body of all is the earth, which is about five and a half times as dense as if it were composed entirely of water. venus follows next, then mars, and then mercury. the remaining bodies, on the other hand, are relatively loose in structure. saturn is the least dense of all, less so than water. the density of the sun is a little greater than that of water. this method of estimating is, however, subject to a qualification. it must be remembered that in speaking of the sun, for instance, as being only a little denser than water, we are merely treating the question from the point of view of an average. certain parts of it in fact will be ever so much denser than water: those are the parts in the centre. other portions, for instance, the outside portions, will be very much less dense. it will easily be understood that in all such bodies the densest or most compressed portions are to be found towards the centre; while the portions towards the exterior being less pressed upon, will be less dense. we now reach a very important point, the question of gravitation. _gravitation_, or _gravity_, as it is often called, is the attractive force which, for instance, causes objects to fall to the earth. now it seems rather strange that one should say that it is owing to a certain force that things fall towards the earth. all things seem to us to fall so of their own accord, as if it were quite natural, or rather most unnatural if they did not. why then require a "force" to make them fall? the story goes that the great sir isaac newton was set a-thinking on this subject by seeing an apple fall from a tree to the earth. he then carried the train of thought further; and, by studying the movements of the moon, he reached the conclusion that a body even so far off as our satellite would be drawn towards the earth in the same manner. this being the case, one will naturally ask why the moon herself does not fall in upon the earth. the answer is indeed found to be that the moon is travelling round and round the earth at a certain rapid pace, and it is this very same rapid pace which keeps her from falling in upon us. any one can test this simple fact for himself. if we tie a stone to the end of a string, and keep whirling it round and round fast enough, there will be a strong pull from the stone in an outward direction, and the string will remain tight all the time that the stone is being whirled. if, however, we gradually slacken the speed at which we are making the stone whirl, a moment will come at length when the string will become limp, and the stone will fall back towards our hand. it seems, therefore, that there are two causes which maintain the stone at a regular distance all the time it is being steadily whirled. one of these is the continual pull inward towards our hand by means of the string. the other is the continual pull away from us caused by the rate at which the stone is travelling. when the rate of whirling is so regulated that these pulls exactly balance each other, the stone travels comfortably round and round, and shows no tendency either to fall back upon our hand or to break the string and fly away into the air. it is indeed precisely similar with regard to the moon. the continual pull of the earth's gravitation takes the place of the string. if the moon were to go round and round slower than it does, it would tend to fall in towards the earth; if, on the other hand, it were to go faster, it would tend to rush away into space. the same kind of pull which the earth exerts upon the objects at its surface, or upon its satellite, the moon, exists through space so far as we know. every particle of matter in the universe is found in fact to attract every other particle. the moon, for instance, attracts the earth also, but the controlling force is on the side of the much greater mass of the earth. this force of gravity or attraction of gravitation, as it is also called, is perfectly regular in its action. its power depends first of all exactly upon the mass of the body which exerts it. the gravitational pull of the sun, for instance, reaches out to an enormous distance, controlling perhaps, in their courses, unseen planets circling far beyond the orbit of neptune. again, the strength with which the force of gravity acts depends upon distance in a regularly diminishing proportion. thus, the nearer an object is to the earth, for instance, the stronger is the gravitational pull which it gets from it; the farther off it is, the weaker is this pull. if then the moon were to be brought nearer to the earth, the gravitational pull of the latter would become so much stronger that the moon's rate of motion would have also to increase in due proportion to prevent her from being drawn into the earth. last of all, the point in a body from which the attraction of gravitation acts, is not necessarily the centre of the body, but rather what is known as its _centre of gravity_, that is to say, the balancing point of all the matter which the body contains. it should here be noted that the moon does not actually revolve around the centre of gravity of the earth. what really happens is that both orbs revolve around their _common_ centre of gravity, which is a point within the body of the earth, and situated about three thousand miles from its centre. in the same manner the planets and the sun revolve around the centre of gravity of the solar system, which is a point within the body of the sun. the neatly poised movements of the planets around the sun, and of the satellites around their respective planets, will therefore be readily understood to result from a nice balance between gravitation and speed of motion. the mass of the earth is ascertained to be about eighty times that of the moon. our knowledge of the mass of a planet is learned from comparing the revolutions of its satellite or satellites around it, with those of the moon around the earth. we are thus enabled to deduce what the mass of such a planet would be compared to the earth's mass; that is to say, a study, for instance, of jupiter's satellite system shows that jupiter must have a mass nearly three hundred and eighteen times that of our earth. in the same manner we can argue out the mass of the sun from the movements of the planets and other bodies of the system around it. with regard, however, to venus and mercury, the problem is by no means such an easy one, as these bodies have no satellites. for information in this latter case we have to rely upon such uncertain evidence as, for instance, the slight disturbances caused in the motion of the earth by the attraction of these planets when they pass closest to us, or their observed effect upon the motions of such comets as may happen to pass near to them. mass and weight, though often spoken of as one and the same thing, are by no means so. mass, as we have seen, merely means the amount of matter which a body contains. the weight of a body, on the other hand, depends entirely upon the gravitational pull which it receives. the force of gravity at the surface of the earth is, for instance, about six times as great as that at the surface of the moon. all bodies, therefore, weigh about six times as much on the earth as they would upon the moon; or, rather, a body transferred to the moon's surface would weigh only about one-sixth of what it did on the terrestrial surface. it will therefore be seen that if a body of given _mass_ were to be placed upon planet after planet in turn, its _weight_ would regularly alter according to the force of gravity at each planet's surface. gravitation is indeed one of the greatest mysteries of nature. what it is, the means by which it acts, or why such a force should exist at all, are questions to which so far we have not had even the merest hint of an answer. its action across space appears to be instantaneous. the intensity of gravitation is said in mathematical parlance "to vary inversely with the square of the distance." this means that at _twice_ the distance the pull will become only _one-quarter_ as strong, and not one-half as otherwise might be expected. at _four_ times the distance, therefore, it will be _one-sixteenth_ as strong. at the earth's surface a body is pulled by the earth's gravitation, or "falls," as we ordinarily term it, through feet in one _second_ of time; whereas at the distance of the moon the attraction of the earth is so very much weakened that a body would take as long as one _minute_ to fall through the same space. newton's investigations showed that if a body were to be placed _at rest_ in space entirely away from the attraction of any other body it would remain always in a motionless condition, because there would plainly be no reason why it should move in any one direction rather than in another. and, similarly, if a body were to be projected in a certain direction and at a certain speed, it would move always in the same direction and at the same speed so long as it did not come within the gravitational attraction of any other body. the possibility of an interaction between the celestial orbs had occurred to astronomers before the time of newton; for instance, in the ninth century to the arabian musa-ben-shakir, to camillus agrippa in , and to kepler, who suspected its existence from observation of the tides. horrox also, writing in , spoke of the moon as moved by an _emanation_ from the earth. but no one prior to newton attempted to examine the question from a mathematical standpoint. notwithstanding the acknowledged truth and far-reaching scope of the law of gravitation--for we find its effects exemplified in every portion of the universe--there are yet some minor movements which it does not account for. for instance, there are small irregularities in the movement of mercury which cannot be explained by the influence of possible intra-mercurial planets, and similarly there are slight unaccountable deviations in the motions of our neighbour the moon. chapter v celestial distances up to this we have merely taken a general view of the solar system--a bird's-eye view, so to speak, from space. in the course of our inquiry we noted in a rough way the _relative_ distances at which the various planets move around the sun. but we have not yet stated what these distances _actually_ are, and it were therefore well now to turn our attention to this important matter. each of us has a fair idea of what a mile is. it is a quarter of an hour's sharp walk, for instance; or yonder village or building, we know, lies such and such a number of miles away. the measurements which have already been given of the diameters of the various bodies of the solar system appear very great to us, who find that a walk of a few miles at a time taxes our strength; but they are a mere nothing when we consider the distances from the sun at which the various planets revolve in their orbits. the following table gives these distances in round numbers. as here stated they are what are called "mean" distances; for, as the orbits are oval, the planets vary in their distances from the sun, and we are therefore obliged to strike a kind of average for each case:-- mercury about , , miles. venus " , , " earth " , , " mars " , , " jupiter " , , " saturn " , , " uranus " , , , " neptune " , , , " from the above it will be seen at a glance that we have entered upon a still greater scale of distance than in dealing with the diameters of the various bodies of the system. in that case the distances were limited to thousands of miles; in this, however, we have to deal with millions. a million being ten hundred thousand, it will be noticed that even the diameter of the huge sun is well under a million miles. how indeed are we to get a grasp of such distances, when those to which we are ordinarily accustomed--the few miles' walk, the little stretch of sea or land which we gaze upon around us--are so utterly minute in comparison? the fact is, that though men may think that they can picture in their minds such immense distances, they actually can not. in matters like these we unconsciously employ a kind of convention, and we estimate a thing as being two or three or more times the size of another. more than this we are unable to do. for instance, our ordinary experience of a mile enables us to judge, in a way, of a stretch of several miles, such as one can take in with a glance; but in our estimation of a thousand miles, or even of one hundred, we are driven back upon a mental trick, so to speak. in our attempts to realise such immense distances as those in the solar system we are obliged to have recourse to analogies; to comparisons with other and simpler facts, though this is at the best a mere self-cheating device. the analogy which seems most suited to our purpose here, and one which has often been employed by writers, is borrowed from the rate at which an express train travels. let us imagine, for instance, that we possess an express train which is capable of running anywhere, never stops, never requires fuel, and always goes along at sixty miles an hour. suppose we commence by employing it to gauge the size of our own planet, the earth. let us send it on a trip around the equator, the span of which is about , miles. at its sixty-miles-an-hour rate of going, this journey will take nearly days. next let us send it from the earth to the moon. this distance, , miles, being ten times as great as the last, will of course take ten times as long to cover, namely, days; that is to say, nearly half a year. again, let us send it still further afield, to the sun, for example. here, however, it enters upon a journey which is not to be measured in thousands of miles, as the others were, but in millions. the distance from the earth to the sun, as we have seen in the foregoing table, is about millions of miles. our express train would take about _years_ to traverse this. having arrived at the sun, let us suppose that our train makes a tour right round it. this will take more than five years. supposing, finally, that our train were started from the sun, and made to run straight out to the known boundaries of the solar system, that is to say, as far as the orbit of neptune, it would take over years to traverse the distance. that sixty miles an hour is a very great speed any one, i think, will admit who has stood upon the platform of a country station while one of the great mail trains has dashed past. but are not the immensities of space appalling to contemplate, when one realises that a body moving incessantly at such a rate would take so long as , years to traverse merely the breadth of our solar system? ten thousand years! just try to conceive it. why, it is only a little more than half that time since the pyramids were built, and they mark for us the dawn of history. and since then half-a-dozen mighty empires have come and gone! having thus concluded our general survey of the appearance and dimensions of the solar system, let us next inquire into its position and size in relation to what we call the universe. a mere glance at the night sky, when it is free from clouds, shows us that in every direction there are stars; and this holds good, no matter what portion of the globe we visit. the same is really true of the sky by day, though in that case we cannot actually see the stars, for their light is quite overpowered by the dazzling light of the sun. we thus reach the conclusion that our earth, that our solar system in fact, lies plunged within the midst of a great tangle of stars. what position, by the way, do we occupy in this mighty maze? are we at the centre, or anywhere near the centre, or where? it has been indeed amply proved by astronomical research that the stars are bodies giving off a light of their own, just as our sun does; that they are in fact suns, and that our sun is merely one, perhaps indeed a very unimportant member, of this great universe of stars. each of these stars, or suns, besides, may be the centre of a system similar to what we call our solar system, comprising planets and satellites, comets and meteors;--or perchance indeed some further variety of attendant bodies of which we have no example in our tiny corner of space. but as to whether one is right in a conjecture of this kind, there is up to the present no proof whatever. no telescope has yet shown a planet in attendance upon one of these distant suns; for such bodies, even if they do exist, are entirely out of the range of our mightiest instruments. on what then can we ground such an assumption? merely upon analogy; upon the common-sense deduction that as the stars have characteristics similar to our particular star, the sun, it would seem unlikely that ours should be the only such body in the whole of space which is attended by a planetary system. "the stars," using that expression in its most general sense, do not lie at one fixed distance from us, set here and there upon a background of sky. there is in fact no background at all. the brilliant orbs are all around us in space, at different distances from us and from each other; and we can gaze between them out into the blackness of the void which, perhaps, continues to extend unceasingly long after the very outposts of the stellar universe has been left behind. shall we then start our imaginary express train once more, and send it out towards the nearest of the stars? this would, however, be a useless experiment. our express-train method of gauging space would fail miserably in the attempt to bring home to us the mighty gulf by which we are now faced. let us therefore halt for a moment and look back upon the orders of distance with which we have been dealing. first of all we dealt with thousands of miles. next we saw how they shrank into insignificance when we embarked upon millions. we found, indeed, that our sixty-mile-an-hour train, rushing along without ceasing, would consume nearly the whole of historical time in a journey from the sun to neptune. in the spaces beyond the solar system we are faced, however, by a new order of distance. from sun to planets is measured in millions of miles, but from sun to sun is measured in billions. but does the mere stating of this fact convey anything? i fear not. for the word "billion" runs as glibly off the tongue as "million," and both are so wholly unrealisable by us that the actual difference between them might easily pass unnoticed. let us, however, make a careful comparison. what is a million? it is a thousand thousands. but what is a billion? it is a million millions. consider for a moment! a million of millions. that means a million, each unit of which is again a million. in fact every separate " " in this million is itself a million. here is a way of trying to realise this gigantic number. a million seconds make only eleven and a half days and nights. but a billion seconds will make actually more than thirty thousand years! having accepted this, let us try and probe with our express train even a little of the new gulf which now lies before us. at our old rate of going it took almost two years to cover a million miles. to cover a billion miles--that is to say, a million times this distance--would thus take of course nearly two million years. alpha centauri, the nearest star to our earth, is some twenty-five billions of miles away. our express train would thus take about fifty millions of years to reach it! this shows how useless our illustration, appropriate though it seemed for interplanetary space, becomes when applied to the interstellar spaces. it merely gives us millions in return for billions; and so the mind, driven in upon itself, whirls round and round like a squirrel in its revolving cage. there is, however, a useful illustration still left us, and it is the one which astronomers usually employ in dealing with the distances of the stars. the illustration in question is taken from the velocity of light. light travels at the tremendous speed of about , miles a second. it therefore takes only about a second and a quarter to come to us from the moon. it traverses the , , of miles which separate us from the sun in about eight minutes. it travels from the sun out to neptune in about four hours, which means that it would cross the solar system from end to end in eight. to pass, however, across the distance which separates us from alpha centauri it would take so long as about four and a quarter years! astronomers, therefore, agree in estimating the distances of the stars from the point of view of the time which light would take to pass from them to our earth. they speak of that distance which light takes a year to traverse as a "light year." according to this notation, alpha centauri is spoken of as being about four and a quarter light years distant from us. now as the rays of light coming from alpha centauri to us are chasing one another incessantly across the gulf of space, and as each ray left that star some four years before it reaches us, our view of the star itself must therefore be always some four years old. were then this star to be suddenly removed from the universe at any moment, we should continue to see it still in its place in the sky for some four years more, after which it would suddenly disappear. the rays which had already started upon their journey towards our earth must indeed continue travelling, and reaching us in their turn until the last one had arrived; after which no more would come. we have drawn attention to alpha centauri as the nearest of the stars. the majority of the others indeed are ever so much farther. we can only hazard a guess at the time it takes for the rays from many of them to reach our globe. suppose, for instance, we see a sudden change in the light of any of these remote stars, we are inclined to ask ourselves when that change did actually occur. was it in the days of queen elizabeth, or at the time of the norman conquest; or was it when rome was at the height of her glory, or perhaps ages before that when the pyramids of egypt were being built? even the last of these suppositions cannot be treated lightly. we have indeed no real knowledge of the distance from us of those stars which our giant telescopes have brought into view out of the depths of the celestial spaces. chapter vi celestial measurement had the telescope never been invented our knowledge of astronomy would be trifling indeed. prior to the year , when galileo first turned the new instrument upon the sky, all that men knew of the starry realms was gathered from observation with their own eyes unaided by any artificial means. in such researches they had been very much at a disadvantage. the sun and moon, in their opinion, were no doubt the largest bodies in the heavens, for the mere reason that they looked so! the mighty solar disturbances, which are now such common-places to us, were then quite undreamed of. the moon displayed a patchy surface, and that was all; her craters and ring-mountains were surprises as yet in store for men. nothing of course was known about the surfaces of the planets. these objects had indeed no particular characteristics to distinguish them from the great host of the stars, except that they continually changed their positions in the sky while the rest did not. the stars themselves were considered as fixed inalterably upon the vault of heaven. the sun, moon, and planets apparently moved about in the intermediate space, supported in their courses by strange and fanciful devices. the idea of satellites was as yet unknown. comets were regarded as celestial portents, and meteors as small conflagrations taking place in the upper air. in the entire absence of any knowledge with regard to the actual sizes and distances of the various celestial bodies, men naturally considered them as small; and, concluding that they were comparatively near, assigned to them in consequence a permanent connection with terrestrial affairs. thus arose the quaint and erroneous beliefs of astrology, according to which the events which took place upon our earth were considered to depend upon the various positions in which the planets, for instance, found themselves from time to time. it must, however, be acknowledged that the study of astrology, fallacious though its conclusions were, indirectly performed a great service to astronomy by reason of the accurate observations and diligent study of the stars which it entailed. we will now inquire into the means by which the distances and sizes of the celestial orbs have been ascertained, and see how it was that the ancients were so entirely in the dark in this matter. there are two distinct methods of finding out the distance at which any object happens to be situated from us. one method is by actual measurement. the other is by moving oneself a little to the right or left, and observing whether the distant object appears in any degree altered in position by our own change of place. one of the best illustrations of this relative change of position which objects undergo as a result of our own change of place, is to observe the landscape from the window of a moving railway carriage. as we are borne rapidly along we notice that the telegraph posts which are set close to the line appear to fly past us in the contrary direction; the trees, houses, and other things beyond go by too, but not so fast; objects a good way off displace slowly; while some spire, or tall landmark, in the far distance appears to remain unmoved during a comparatively long time. actual change of position on our own part is found indeed to be invariably accompanied by an apparent displacement of the objects about us, such apparent displacement as a result of our own change of position being known as "parallax." the dependence between the two is so mathematically exact, that if we know the amount of our own change of place, and if we observe the amount of the consequent displacement of any object, we are enabled to calculate its precise distance from us. thus it comes to pass that distances can be measured without the necessity of moving over them; and the breadth of a river, for instance, or the distance from us of a ship at sea, can be found merely by such means. it is by the application of this principle to the wider field of the sky that we are able to ascertain the distance of celestial bodies. we have noted that it requires a goodly change of place on our own part to shift the position in which some object in the far distance is seen by us. to two persons separated by, say, a few hundred yards, a ship upon the horizon will appear pretty much in the same direction. they would require, in fact, to be much farther apart in order to displace it sufficiently for the purpose of estimating their distance from it. it is the same with regard to the moon. two observers, standing upon our earth, will require to be some thousands of miles apart in order to see the position of our satellite sufficiently altered with regard to the starry background, to give the necessary data upon which to ground their calculations. the change of position thus offered by one side of the earth's surface at a time is, however, not sufficient to displace any but the nearest celestial bodies. when we have occasion to go farther afield we have to seek a greater change of place. this we can get as a consequence of the earth's movement around the sun. observations, taken several days apart, will show the effect of the earth's change of place during the interval upon the positions of the other bodies of our system. but when we desire to sound the depths of space beyond, and to reach out to measure the distance of the nearest star, we find ourselves at once thrown upon the greatest change of place which we can possibly hope for; and this we get during the long journey of many millions of miles which our earth performs around the sun during the course of each year. but even this last change of place, great as it seems in comparison with terrestrial measurements, is insufficient to show anything more than the tiniest displacements in a paltry forty-three out of the entire host of the stars. we can thus realise at what a disadvantage the ancients were. the measuring instruments at their command were utterly inadequate to detect such small displacements. it was reserved for the telescope to reveal them; and even then it required the great telescopes of recent times to show the slight changes in the position of the nearer stars, which were caused by the earth's being at one time at one end of its orbit, and some six months later at the other end--stations separated from each other by a gulf of about one hundred and eighty-six millions of miles. the actual distances of certain celestial bodies being thus ascertainable, it becomes a matter of no great difficulty to determine the actual sizes of the measurable ones. it is a matter of everyday experience that the size which any object appears to have, depends exactly upon the distance it is from us. the farther off it is the smaller it looks; the nearer it is the bigger. if, then, an object which lies at a known distance from us looks such and such a size, we can of course ascertain its real dimensions. take the moon, for instance. as we have already shown, we are able to ascertain its distance. we observe also that it looks a certain size. it is therefore only a matter of calculation to find what its actual dimensions should be, in order that it may look that size at that distance away. similarly we can ascertain the real dimensions of the sun. the planets, appearing to us as points of light, seem at first to offer a difficulty; but, by means of the telescope, we can bring them, as it were, so much nearer to us, that their broad expanses may be seen. we fail, however, signally with regard to the stars; for they are so very distant, and therefore such tiny points of light, that our mightiest telescopes cannot magnify them sufficiently to show any breadth of surface. instead of saying that an object looks a certain breadth across, such as a yard or a foot, a statement which would really mean nothing, astronomers speak of it as measuring a certain angle. such angles are estimated in what are called "degrees of arc"; each degree being divided into sixty minutes, and each minute again into sixty seconds. popularly considered the moon and sun _look_ about the same size, or, as an astronomer would put it, they measure about the same angle. this is an angle, roughly, of thirty-two minutes of arc; that is to say, slightly more than half a degree. the broad expanse of surface which a celestial body shows to us, whether to the naked eye, as in the case of the sun and moon, or in the telescope, as in the case of other members of our system, is technically known as its "disc." chapter vii eclipses and kindred phenomena since some members of the solar system are nearer to us than others, and all are again much nearer than any of the stars, it must often happen that one celestial body will pass between us and another, and thus intercept its light for a while. the moon, being the nearest object in the universe, will, of course, during its motion across the sky, temporarily blot out every one of the others which happen to lie in its path. when it passes in this manner across the face of the sun, it is said to _eclipse_ it. when it thus hides a planet or star, it is said to _occult_ it. the reason why a separate term is used for what is merely a case of obscuring light in exactly the same way, will be plain when one considers that the disc of the sun is almost of the same apparent size as that of the moon, and so the complete hiding of the sun can last but a few minutes at the most; whereas a planet or a star looks so very small in comparison, that it is always _entirely swallowed up for some time_ when it passes behind the body of our satellite. the sun, of course, occults planets and stars in exactly the same manner as the moon does, but we cannot see these occultations on account of the blaze of sunlight. by reason of the small size which the planets look when viewed with the naked eye, we are not able to note them in the act of passing over stars and so blotting them out; but such occurrences may be seen in the telescope, for the planetary bodies then display broad discs. there is yet another occurrence of the same class which is known as a _transit_. this takes place when an apparently small body passes across the face of an apparently large one, the phenomenon being in fact the exact reverse of an occultation. as there is no appreciable body nearer to us than the moon, we can never see anything in transit across her disc. but since the planets venus and mercury are both nearer to us than the sun, they will occasionally be seen to pass across his face, and thus we get the well-known phenomena called transits of venus and transits of mercury. as the satellites of jupiter are continually revolving around him, they will often pass behind or across his disc. such occultations and transits of satellites can be well observed in the telescope. there is, however, a way in which the light of a celestial body may be obscured without the necessity of its being hidden from us by one nearer. it will no doubt be granted that any opaque object casts a shadow when a strong light falls directly upon it. thus the earth, under the powerful light which is directed upon it from the sun, casts an extensive shadow, though we are not aware of the existence of this shadow until it falls upon something. the shadow which the earth casts is indeed not noticeable to us until some celestial body passes into it. as the sun is very large, and the earth in comparison very small, the shadow thrown by the earth is comparatively short, and reaches out in space for only about a million miles. there is no visible object except the moon, which circulates within that distance from our globe, and therefore she is the only body which can pass into this shadow. whenever such a thing happens, her surface at once becomes dark, for the reason that she never emits any light of her own, but merely reflects that of the sun. as the moon is continually revolving around the earth, one would be inclined to imagine that once in every month, namely at what is called _full moon_, when she is on the other side of the earth with respect to the sun, she ought to pass through the shadow in question. but this does not occur every time, because the moon's orbit is not quite _upon the same plane_ with the earth's. it thus happens that time after time the moon passes clear of the earth's shadow, sometimes above it, and sometimes below it. it is indeed only at intervals of about six months that the moon can be thus obscured. this darkening of her light is known as an _eclipse of the moon_. it seems a great pity that custom should oblige us to employ the one term "eclipse" for this and also for the quite different occurrence, an eclipse of the sun; in which the sun's face is hidden as a consequence of the moon's body coming directly _between_ it and our eyes. the popular mind seems always to have found it more difficult to grasp the causes of an eclipse of the moon than an eclipse of the sun. as mr. j.e. gore[ ] puts it: "the darkening of the sun's light by the interposition of the moon's body seems more obvious than the passing of the moon through the earth's shadow." eclipses of the moon furnish striking spectacles, but really add little to our knowledge. they exhibit, however, one of the most remarkable evidences of the globular shape of our earth; for the outline of its shadow when seen creeping over the moon's surface is always circular. [illustration: fig. .--total and partial eclipses of the moon. the moon is here shown in two positions; i.e. _entirely_ plunged in the earth's shadow and therefore totally eclipsed, and only _partly_ plunged in it or partially eclipsed.] _eclipses of the moon_, or lunar eclipses, as they are also called, are of two kinds--_total_, and _partial_. in a total lunar eclipse the moon passes entirely into the earth's shadow, and the whole of her surface is consequently darkened. this darkening lasts for about two hours. in a partial lunar eclipse, a portion only of the moon passes through the shadow, and so only _part_ of her surface is darkened (see fig. ). a very striking phenomenon during a total eclipse of the moon, is that the darkening of the lunar surface is usually by no means so intense as one would expect, when one considers that the sunlight at that time should be _wholly_ cut off from it. the occasions indeed upon which the moon has completely disappeared from view during the progress of a total lunar eclipse are very rare. on the majority of these occasions she has appeared of a coppery-red colour, while sometimes she has assumed an ashen hue. the explanations of these variations of colour is to be found in the then state of the atmosphere which surrounds our earth. when those portions of our earth's atmosphere through which the sun's rays have to filter on their way towards the moon are free from watery vapour, the lunar surface will be tinged with a reddish light, such as we ordinarily experience at sunset when our air is dry. the ashen colour is the result of our atmosphere being laden with watery vapour, and is similar to what we see at sunset when rain is about. lastly, when the air around the earth is thickly charged with cloud, no light at all can pass; and on such occasions the moon disappears altogether for the time being from the night sky. _eclipses of the sun_, otherwise known as solar eclipses, are divided into _total_, _partial_, and _annular_. a total eclipse of the sun takes place when the moon comes between the sun and the earth, in such a manner that it cuts off the sunlight _entirely_ for the time being from a _portion_ of the earth's surface. a person situated in the region in question will, therefore, at that moment find the sun temporarily blotted out from his view by the body of the moon. since the moon is a very much smaller body than the sun, and also very much the nearer to us of the two, it will readily be understood that the portion of the earth from which the sun is seen thus totally eclipsed will be of small extent. in places not very distant from this region, the moon will appear so much shifted in the sky that the sun will be seen only partially eclipsed. the moon being in constant movement round the earth, the portion of the earth's surface from which an eclipse is seen as total will be always a comparatively narrow band lying roughly from west to east. this band, known as the _track of totality_, can, at the utmost, never be more than about miles in width, and as a rule is very much less. for about miles on either side of it the sun is seen partially eclipsed. outside these limits no eclipse of any kind is visible, as from such regions the moon is not seen to come in the way of the sun (see fig. (i.), p. ). it may occur to the reader that eclipses can also take place in the course of which the positions, where the eclipse would ordinarily be seen as total, will lie outside the surface of the earth. such an eclipse is thus not dignified with the name of total eclipse, but is called a partial eclipse, because from the earth's surface the sun is only seen _partly eclipsed at the utmost_ (see fig. (ii.), p. ). [illustration: (i.) total eclipse of the sun.] [illustration: (ii.) partial eclipse of the sun. fig. .--total and partial eclipses of the sun. from the position a the sun cannot be seen, as it is entirely blotted out by the moon. from b it is seen partially blotted out, because the moon is to a certain degree in the way. from c no eclipse is seen, because the moon does not come in the way. it is to be noted that in a partial eclipse of the sun, the position a lies _outside_ the surface of the earth.] an _annular eclipse_ is an eclipse which just fails to become total for yet another reason. we have pointed out that the orbits of the various members of the solar system are not circular, but oval. such oval figures, it will be remembered, are technically known as ellipses. in an elliptic orbit the controlling body is situated not in the middle of the figure, but rather towards one of the ends; the actual point which it occupies being known as the _focus_. the sun being at the focus of the earth's orbit, it follows that the earth is, at times, a little nearer to him than at others. the sun will therefore appear to us to vary a little in size, looking sometimes slightly larger than at other times. it is so, too, with the moon, at the focus of whose orbit the earth is situated. she therefore also appears to us at times to vary slightly in size. the result is that when the sun is eclipsed by the moon, and the moon at the time appears the larger of the two, she is able to blot out the sun completely, and so we can get a total eclipse. but when, on the other hand, the sun appears the larger, the eclipse will not be quite total, for a portion of the sun's disc will be seen protruding all around the moon like a ring of light. this is what is known as an annular eclipse, from the latin word _annulus_, which means a ring. the term is consecrated by long usage, but it seems an unfortunate one on account of its similarity to the word "annual." the germans speak of this kind of eclipse as "ring-formed," which is certainly much more to the point. there can never be a year without an eclipse of the sun. indeed there must be always two such eclipses _at least_ during that period, though there need be no eclipse of the moon at all. on the other hand, the greatest number of eclipses which can ever take place during a year are seven; that is to say, either five solar eclipses and two lunar, or four solar and three lunar. this general statement refers merely to eclipses in their broadest significance, and informs us in no way whether they will be total or partial. of all the phenomena which arise from the hiding of any celestial body by one nearer coming in the way, a total eclipse of the sun is far the most important. it is, indeed, interesting to consider how much poorer modern astronomy would be but for the extraordinary coincidence which makes a total solar eclipse just possible. the sun is about times farther off from us than the moon, and enormously greater than her in bulk. yet the two are relatively so distanced from us as to look about the same size. the result of this is that the moon, as has been seen, can often blot out the sun entirely from our view for a short time. when this takes place the great blaze of sunlight which ordinarily dazzles our eyes is completely cut off, and we are thus enabled, unimpeded, to note what is going on in the immediate vicinity of the sun itself. in a total solar eclipse, the time which elapses from the moment when the moon's disc first begins to impinge upon that of the sun at his western edge until the eclipse becomes total, lasts about an hour. during all this time the black lunar disc may be watched making its way steadily across the solar face. notwithstanding the gradual obscuration of the sun, one does not notice much diminution of light until about three-quarters of his disc are covered. then a wan, unearthly appearance begins to pervade all things, the temperature falls noticeably, and nature seems to halt in expectation of the coming of something unusual. the decreasing portion of sun becomes more and more narrow, until at length it is reduced to a crescent-shaped strip of exceeding fineness. strange, ill-defined, flickering shadows (known as "shadow bands") may at this moment be seen chasing each other across any white expanse such as a wall, a building, or a sheet stretched upon the ground. the western side of the sky has now assumed an appearance dark and lowering, as if a rainstorm of great violence were approaching. this is caused by the mighty mass of the lunar shadow sweeping rapidly along. it flies onward at the terrific velocity of about half a mile a second. if the gradually diminishing crescent of sun be now watched through a telescope, the observer will notice that it does not eventually vanish all at once, as he might have expected. rather, it breaks up first of all along its length into a series of brilliant dots, known as "baily's beads." the reason of this phenomenon is perhaps not entirely agreed upon, but the majority of astronomers incline to the opinion that the so-called "beads" are merely the last remnants of sunlight peeping between those lunar mountain peaks which happen at the moment to fringe the advancing edge of the moon. the beads are no sooner formed than they rapidly disappear one after the other, after which no portion of the solar surface is left to view, and the eclipse is now total (see fig. ). [illustration: _in a total eclipse_ _in an annular eclipse_ fig. .--"baily's beads."] but with the disappearance of the sun there springs into view a new and strange appearance, ordinarily unseen because of the blaze of sunlight. it is a kind of aureole, or halo, pearly white in colour, which is seen to surround the black disc of the moon. this white radiance is none other than the celebrated phenomenon widely known as the _solar corona_. it was once upon a time thought to belong to the moon, and to be perhaps a lunar atmosphere illuminated by the sunlight shining through it from behind. but the suddenness with which the moon always blots out stars when occulting them, has amply proved that she possesses no atmosphere worth speaking about. it is now, however, satisfactorily determined that the corona belongs to the sun, for during the short time that it remains in view the black body of the moon can be seen creeping across it. all the time that the _total phase_ (as it is called) lasts, the corona glows with its pale unearthly light, shedding upon the earth's surface an illumination somewhat akin to full moonlight. usually the planet venus and a few stars shine out the while in the darkened heaven. meantime around the observer animal and plant life behave as at nightfall. birds go to roost, bats fly out, worms come to the surface of the ground, flowers close up. in the norwegian eclipse of fish were seen rising to the surface of the water. when the total phase at length is over, and the moon in her progress across the sky has allowed the brilliant disc of the sun to spring into view once more at the other side, the corona disappears. there is another famous accompaniment of the sun which partly reveals itself during total solar eclipses. this is a layer of red flame which closely envelops the body of the sun and lies between it and the corona. this layer is known by the name of the _chromosphere_. just as at ordinary times we cannot see the corona on account of the blaze of sunlight, so are we likewise unable to see the chromosphere because of the dazzling white light which shines through from the body of the sun underneath and completely overpowers it. when, however, during a solar eclipse, the lunar disc has entirely hidden the brilliant face of the sun, we are still able for a few moments to see an edgewise portion of the chromosphere in the form of a narrow red strip, fringing the advancing border of the moon. later on, just before the moon begins to uncover the face of the sun from the other side, we may again get a view of a strip of chromosphere. the outer surface of the chromosphere is not by any means even. it is rough and billowy, like the surface of a storm-tossed sea. portions of it, indeed, rise at times to such heights that they may be seen standing out like blood-red points around the black disc of the moon, and remain thus during a good part of the total phase. these projections are known as the _solar prominences_. in the same way as the corona, the chromosphere and prominences were for a time supposed to belong to the moon. this, however, was soon found not to be the case, for the lunar disc was noticed to creep slowly across them also. the total phase, or "totality," as it is also called, lasts for different lengths of time in different eclipses. it is usually of about two or three minutes' duration, and at the utmost it can never last longer than about eight minutes. when totality is over and the corona has faded away, the moon's disc creeps little by little from the face of the sun, light and heat returns once more to the earth, and nature recovers gradually from the gloom in which she has been plunged. about an hour after totality, the last remnant of moon draws away from the solar disc, and the eclipse is entirely at an end. the corona, the chromosphere, and the prominences are the most important of these accompaniments of the sun which a total eclipse reveals to us. our further consideration of them must, however, be reserved for a subsequent chapter, in which the sun will be treated of at length. every one who has had the good fortune to see a total eclipse of the sun will, the writer feels sure, agree with the verdict of sir norman lockyer that it is at once one of the "grandest and most awe-inspiring sights" which man can witness. needless to say, such an occurrence used to cause great consternation in less civilised ages; and that it has not in modern times quite parted with its terrors for some persons, is shown by the fact that in iowa, in the united states, a woman died from fright during the eclipse of . to the serious observer of a total solar eclipse every instant is extremely precious. many distinct observations have to be crowded into a time all too limited, and this in an eclipse-party necessitates constant rehearsals in order that not a moment may be wasted when the longed-for totality arrives. such preparation is very necessary; for the rarity and uncommon nature of a total eclipse of the sun, coupled with its exceeding short duration, tends to flurry the mind, and to render it slow to seize upon salient points of detail. and, even after every precaution has been taken, weather possibilities remain to be reckoned with, so that success is rather a lottery. above all things, therefore, a total solar eclipse is an occurrence for the proper utilisation of which personal experience is of absolute necessity. it was manifestly out of the question that such experience could be gained by any individual in early times, as the imperfection of astronomical theory and geographical knowledge rendered the predicting of the exact position of the track of totality well-nigh impossible. thus chance alone would have enabled one in those days to witness a total phase, and the probabilities, of course, were much against a second such experience in the span of a life-time. and even in more modern times, when the celestial motions had come to be better understood, the difficulties of foreign travel still were in the way; for it is, indeed, a notable fact that during many years following the invention of the telescope the tracks were placed for the most part in far-off regions of the earth, and europe was visited by singularly few total solar eclipses. thus it came to pass that the building up of a body of organised knowledge upon this subject was greatly delayed. nothing perhaps better shows the soundness of modern astronomical theory than the almost exact agreement of the time predicted for an eclipse with its actual occurrence. similarly, by calculating backwards, astronomers have discovered the times and seasons at which many ancient eclipses took place, and valuable opportunities have thus arisen for checking certain disputed dates in history. it should not be omitted here that the ancients were actually able, _in a rough way_, to predict eclipses. the chaldean astronomers had indeed noticed very early a curious circumstance, _i.e._ that eclipses tend to repeat themselves after a lapse of slightly more than eighteen years. in this connection it must, however, be pointed out, in the first instance, that the eclipses which occur in any particular year are in no way associated with those which occurred in the previous year. in other words, the mere fact that an eclipse takes place upon a certain day this year will not bring about a repetition of it at the same time next year. however, the nicely balanced behaviour of the solar system, an equilibrium resulting from æons of orbital ebb and flow, naturally tends to make the members which compose that family repeat their ancient combinations again and again; so that after definite lapses of time the same order of things will _almost exactly_ recur. thus, as a consequence of their beautifully poised motions, the sun, the moon, and the earth tend, after a period of years and - / days,[ ] to occupy very nearly the same positions with regard to each other. the result of this is that, during each recurring period, the eclipses comprised within it will be repeated in their order. to give examples:-- the total solar eclipse of august , , was a repetition of that of august , . the partial solar eclipse of february , , corresponded to that which took place on february , . the annular eclipse of july , , was a recurrence of that of june , . in this way we can go on until the eighteen year cycle has run out, and we come upon a total solar eclipse predicted for september , , which will repeat the above-mentioned ones of and ; and so on too with the others. from mere observation alone, extending no doubt over many ages, those time-honoured watchers of the sky, the early chaldeans, had arrived at this remarkable generalisation; and they used it for the rough prediction of eclipses. to the period of recurrence they give the name of "saros." and here we find ourselves led into one of the most interesting and fascinating by-paths in astronomy, to which writers, as a rule, pay all too little heed. in order not to complicate matters unduly, the recurrence of solar eclipses alone will first be dealt with. this limitation will, however, not affect the arguments in the slightest, and it will be all the more easy in consequence to show their application to the case of eclipses of the moon. the reader will perhaps have noticed that, with regard to the repetition of an eclipse, it has been stated that the conditions which bring it on at each recurrence are reproduced _almost exactly_. here, then, lies the _crux_ of the situation. for it is quite evident that were the conditions _exactly_ reproduced, the recurrences of each eclipse would go on for an indefinite period. for instance, if the lapse of a saros period found the sun, moon, and earth again in the precise relative situations which they had previously occupied, the recurrences of a solar eclipse would tend to duplicate its forerunner with regard to the position of the shadow upon the terrestrial surface. but the conditions _not_ being exactly reproduced, the shadow-track does not pass across the earth in quite the same regions. it is shifted a little, so to speak; and each time the eclipse comes round it is found to be shifted a little farther. every solar eclipse has therefore a definite "life" of its own upon the earth, lasting about years, or saros returns, and working its way little by little across our globe from north to south, or from south to north, as the case may be. let us take an imaginary example. a _partial_ eclipse occurs, say, somewhere near the north pole, the edge of the "partial" shadow just grazing the earth, and the "track of totality" being as yet cast into space. here we have the beginning of a series. at each saros recurrence the partial shadow encroaches upon a greater extent of earth-surface. at length, in its turn, the track of totality begins to impinge upon the earth. this track streaks across our globe at each return of the eclipse, repeating itself every time in a slightly more southerly latitude. south and south it moves, passing in turn the tropic of cancer, the equator, the tropic of capricorn, until it reaches the south pole; after which it touches the earth no longer, but is cast into space. the rear portion of the partial shadow, in its turn, grows less and less in extent; and it too in time finally passes off. our imaginary eclipse series is now no more--its "life" has ended. we have taken, as an example, an eclipse series moving from north to south. we might have taken one moving from south to north, for they progress in either direction. from the description just given the reader might suppose that, if the tracks of totality of an eclipse series were plotted upon a chart of the world, they would lie one beneath another like a set of steps. this is, however, _not_ the case, and the reason is easily found. it depends upon the fact that the saros does not comprise an exact number of days, but includes, as we have seen, one-third of a day in addition. it will be granted, of course, that if the number of days was exact, the _same_ parts of the earth would always be brought round by the axial rotation _to front the sun_ at the moment of the recurrence of the eclipse. but as there is still one-third of a day to complete the saros period, the earth has yet to make one-third of a rotation upon its axis before the eclipse takes place. thus at every recurrence the track of totality finds itself placed one-third of the earth's circumference to the _westward_. three of the recurrences will, of course, complete the circuit of the globe; and so the fourth recurrence will duplicate the one which preceded it, three saros returns, or years and month before. this duplication, as we have already seen, will, however, be situated in a latitude to the south or north of its predecessor, according as the eclipse series is progressing in a southerly or northerly direction. lastly, every eclipse series, after working its way across the earth, will return again to go through the same process after some , years; so that, at the end of that great lapse of time, the entire "life" of every eclipse should repeat itself, provided that the conditions of the solar system have not altered appreciably during the interval. we are now in a position to consider this gradual southerly or northerly progress of eclipse recurrences in its application to the case of eclipses of the moon. it should be evident that, just as in solar eclipses the lunar shadow is lowered or raised (as the case may be) each time it strikes the terrestrial surface, so in lunar eclipses will the body of the moon shift its place at each recurrence relatively to the position of the earth's shadow. every lunar eclipse, therefore, will commence on our satellite's disc as a partial eclipse at the northern or southern extremity, as the case may be. let us take, as an example, an imaginary series of eclipses of the moon progressing from north to south. at each recurrence the partial phase will grow greater, its boundary encroaching more and more to the southward, until eventually the whole disc is enveloped by the shadow, and the eclipse becomes total. it will then repeat itself as total during a number of recurrences, until the entire breadth of the shadow has been passed through, and the northern edge of the moon at length springs out into sunlight. this illuminated portion will grow more and more extensive at each succeeding return, the edge of the shadow appearing to recede from it until it finally passes off at the south. similarly, when a lunar eclipse commences as partial at the south of the moon, the edge of the shadow at each subsequent recurrence finds itself more and more to the northward. in due course the total phase will supervene, and will persist during a number of recurrences until the southerly trend of the moon results in the uncovering of the lunar surface at the south. thus, as the boundary of the shadow is left more and more to the northward, the illuminated portion on the southern side of the moon becomes at each recurrence greater and the darkened portion on the northern side less, until the shadow eventually passes off at the north. the "life" of an eclipse of the moon happens, for certain reasons, to be much shorter than that of an eclipse of the sun. it lasts during only about years, or saros returns. fig. , p. , is a map of the world on mercator's projection, showing a portion of the march of the total solar eclipse of august , , across the surface of the earth. the projection in question has been employed because it is the one with which people are most familiar. this eclipse began by striking the neighbourhood of the north pole in the guise of a partial eclipse during the latter part of the reign of queen elizabeth, and became total on the earth for the first time on the th of june . its next appearance was on the th of july . it has not been possible to show the tracks of totality of these two early visitations on account of the distortion of the polar regions consequent on the _fiction_ of mercator's projection. it is therefore made to commence with the track of its third appearance, viz. on july , . in consequence of those variations in the apparent sizes of the sun and moon, which result, as we have seen, from the variations in their distances from the earth, this eclipse will change from a total into an annular eclipse towards the end of the twenty-first century. by that time the track will have passed to the southern side of the equator. the track will eventually leave the earth near the south pole about the beginning of the twenty-sixth century, and the rear portion of the partial shadow will in its turn be clear of the terrestrial surface by about a.d., when the series comes to an end. [illustration: fig. .--map of the world on mercator's projection, showing a portion of the progress of the total solar eclipse of august , , across the surface of the earth.] [ ] astronomical essays (p. ), london, . [ ] in some cases the periods between the dates of the corresponding eclipses _appear_ to include a greater number of days than ten; but this is easily explained when allowance is made for intervening _leap_ years (in each of which an _extra_ day has of course been added), and also for variations in local time. chapter viii famous eclipses of the sun what is thought to be the earliest reference to an eclipse comes down to us from the ancient chinese records, and is over four thousand years old. the eclipse in question was a solar one, and occurred, so far as can be ascertained, during the twenty-second century b.c. the story runs that the two state astronomers, ho and hi by name, being exceedingly intoxicated, were unable to perform their required duties, which consisted in superintending the customary rites of beating drums, shooting arrows, and the like, in order to frighten away the mighty dragon which it was believed was about to swallow up the lord of day. this eclipse seems to have been only partial; nevertheless a great turmoil ensued, and the two astronomers were put to death, no doubt with the usual _celestial_ cruelty. the next eclipse mentioned in the chinese annals is also a solar eclipse, and appears to have taken place more than a thousand years later, namely in b.c. records of similar eclipses follow from the same source; but as they are mere notes of the events, and do not enter into any detail, they are of little interest. curiously enough the chinese have taken practically no notice of eclipses of the moon, but have left us a comparatively careful record of comets, which has been of value to modern astronomy. the earliest mention of a _total_ eclipse of the sun (for it should be noted that the ancient chinese eclipse above-mentioned was merely partial) was deciphered in , on a very ancient babylonian tablet, by mr. l.w. king of the british museum. this eclipse took place in the year b.c. assyrian tablets record three solar eclipses which occurred between three and four hundred years later than this. the first of these was in b.c.; the total phase being visible near nineveh. the next record of an eclipse of the sun comes to us from a grecian source. this eclipse took place in b.c., and has been the subject of much investigation. herodotus, to whom we are indebted for the account, tells us that it occurred during a battle in a war which had been waging for some years between the lydians and medes. the sudden coming on of darkness led to a termination of the contest, and peace was afterwards made between the combatants. the historian goes on to state that the eclipse had been foretold by thales, who is looked upon as the founder of grecian astronomy. this eclipse is in consequence known as the "eclipse of thales." it would seem as if that philosopher were acquainted with the chaldean saros. the next solar eclipse worthy of note was an annular one, and occurred in b.c., the first year of the peloponnesian war. plutarch relates that the pilot of the ship, which was about to convey pericles to the peloponnesus, was very much frightened by it; but pericles calmed him by holding up a cloak before his eyes, and saying that the only difference between this and the eclipse was that something larger than the cloak prevented his seeing the sun for the time being. an eclipse of great historical interest is that known as the "eclipse of agathocles," which occurred on the morning of the th of august, b.c. agathocles, tyrant of syracuse, had been blockaded in the harbour of that town by the carthaginian fleet, but effected the escape of his squadron under cover of night, and sailed for africa in order to invade the enemy's territory. during the following day he and his vessels experienced a total eclipse, in which "day wholly put on the appearance of night, and the stars were seen in all parts of the sky." a few solar eclipses are supposed to be referred to in early roman history, but their identity is very doubtful in comparison with those which the greeks have recorded. additional doubt is cast upon them by the fact that they are usually associated with famous events. the birth and death of romulus, and the passage of the rubicon by julius cæsar, are stated indeed to have been accompanied by these marks of the approval or disapproval of the gods! reference to our subject in the bible is scanty. amos viii. is thought to refer to the nineveh eclipse of b.c., to which allusion has already been made; while the famous episode of hezekiah and the shadow on the dial of ahaz has been connected with an eclipse which was partial at jerusalem in b.c. the first solar eclipse, recorded during the christian era, is known as the "eclipse of phlegon," from the fact that we are indebted for the account to a pagan writer of that name. this eclipse took place in a.d. , and the total phase was visible a little to the north of palestine. it has sometimes been confounded with the "darkness of the crucifixion," which event took place near the date in question; but it is sufficient here to say that the crucifixion is well known to have occurred during the passover of the jews, which is always celebrated at the _full_ moon, whereas an eclipse of the sun can only take place at _new_ moon. dion cassius, commenting on the emperor claudius about the year a.d. , writes as follows:-- "as there was going to be an eclipse on his birthday, through fear of a disturbance, as there had been other prodigies, he put forth a public notice, not only that the obscuration would take place, and about the time and magnitude of it, but also about the causes that produce such an event." this is a remarkable piece of information; for the romans, an essentially military nation, appear hitherto to have troubled themselves very little about astronomical matters, and were content, as we have seen, to look upon phenomena, like eclipses, as mere celestial prodigies. what is thought to be the first definite mention of the solar corona occurs in a passage of plutarch. the eclipse to which he refers is probably one which took place in a.d. . he says that the obscuration caused by the moon "has no time to last and no extensiveness, but some light shows itself round the sun's circumference, which does not allow the darkness to become deep and complete." no further reference to this phenomenon occurs until near the end of the sixteenth century. it should, however, be here mentioned that mr. e.w. maunder has pointed out the probability[ ] that we have a very ancient symbolic representation of the corona in the "winged circle," "winged disc," or "ring with wings," as it is variously called, which appears so often upon assyrian and egyptian monuments, as the symbol of the deity (fig. ). [illustration: fig. .--the "ring with wings." the upper is the assyrian form of the symbol, the lower the egyptian. (from _knowledge_.) compare the form of the corona on plate vii. (b), p. .] the first solar eclipse recorded to have been seen in england is that of a.d. , mention of which is found in the _anglo-saxon chronicle_. the track of totality did not, however, come near our islands, for only two-thirds of the sun's disc were eclipsed at london. in a great eclipse took place in europe, which was total for more than five minutes across what is now bavaria. terror at this eclipse is said to have hastened the death of louis le debonnaire, emperor of the west, who lay ill at worms. in --_temp._ king alfred--an eclipse of the sun took place which was total at london. from this until no other eclipse was total at london itself; though this does not apply to other portions of england. an eclipse, generally known as the "eclipse of stiklastad," is said to have taken place in , during the sea-fight in which olaf of norway is supposed to have been slain. longfellow, in his _saga of king olaf_, has it that "the sun hung red as a drop of blood," but, as in the case of most poets, the dramatic value of an eclipse seems to have escaped his notice. in the year there occurred a total eclipse of the sun, the last to be visible in england for more than five centuries. indeed there have been only two such since--namely, those of and , to which we shall allude in due course. the eclipse of took place on the th march, and is thus referred to in the _anglo-saxon chronicle_:-- "in the lent, the sun and the day darkened, about the noon-tide of the day, when men were eating, and they lighted candles to eat by. that was the th day before the calends of april. men were very much struck with wonder." several of the older historians speak of a "fearful eclipse" as having taken place on the morning of the battle of crecy, . lingard, for instance, in his _history of england_, has as follows:-- "never, perhaps, were preparations for battle made under circumstances so truly awful. on that very day the sun suffered a partial eclipse: birds, in clouds, the precursors of a storm, flew screaming over the two armies, and the rain fell in torrents, accompanied by incessant thunder and lightning. about five in the afternoon the weather cleared up; the sun in full splendour darted his rays in the eyes of the enemy." calculations, however, show that no eclipse of the sun took place in europe during that year. this error is found to have arisen from the mistranslation of an obsolete french word _esclistre_ (lightning), which is employed by froissart in his description of the battle. in an eclipse was total over scotland and part of north germany. it was observed at torgau by jessenius, an hungarian physician, who noticed a bright light around the moon during the time of totality. this is said to be the first reference to the corona since that of plutarch, to which we have already drawn attention. mention of scotland recalls the fact that an unusual number of eclipses happen to have been visible in that country, and the occult bent natural to the scottish character has traditionalised a few of them in such terms as the "black hour" (an eclipse of ), "black saturday" (the eclipse of which has been alluded to above), and "mirk monday" ( ). the track of the last-named also passed over carrickfergus in ireland, where it was observed by a certain dr. wybord, in whose account the term "corona" is first employed. this eclipse is the last which has been total in scotland, and it is calculated that there will not be another eclipse seen as total there until the twenty-second century. an eclipse of the sun which took place on may , , is recorded as having been seen "through a tube." this probably refers to the then recent invention--the telescope. the eclipses which we have been describing are chiefly interesting from an historical point of view. the old mystery and confusion to the beholders seem to have lingered even into comparatively enlightened times, for we see how late it is before the corona attracts definite attention for the sake of itself alone. it is not a far cry from notice of the corona to that of other accompaniments of a solar eclipse. thus the eclipse of , the total phase of which was visible in switzerland, is of great interest; for it was on this occasion that the famous red prominences seem first to have been noted. a certain captain stannyan observed this eclipse from berne in switzerland, and described it in a letter to flamsteed, the then astronomer royal. he says the sun's "getting out of his eclipse was preceded by a blood-red streak of light from its left limb, which continued not longer than six or seven seconds of time; then part of the sun's disc appeared all of a sudden, as bright as venus was ever seen in the night, nay brighter; and in that very instant gave a light and shadow to things as strong as moonlight uses to do." how little was then expected of the sun is, however, shown by flamsteed's words, when communicating this information to the royal society:-- "the captain is the first man i ever heard of that took notice of a red streak of light preceding the emersion of the sun's body from a total eclipse. and i take notice of it to you because it infers that _the moon has an atmosphere_; and its short continuance of only six or seven seconds of time, tells us that _its height is not more than the five or six hundredth part of her diameter_." what a change has since come over the ideas of men! the sun has proved a veritable mine of discovery, while the moon has yielded up nothing new. the eclipse of , the first total at london since that of , was observed by the famous astronomer, edmund halley, from the rooms of the royal society, then in crane court, fleet street. on this occasion both the corona and a red projection were noted. halley further makes allusion to that curious phenomenon, which later on became celebrated under the name of "baily's beads." it was also on the occasion of this eclipse that the _earliest recorded drawings of the corona_ were made. cambridge happened to be within the track of totality; and a certain professor cotes of that university, who is responsible for one of the drawings in question, forwarded them to sir isaac newton together with a letter describing his observations. in there occurred an eclipse, the total phase of which was visible from the south-west of england, but not from london. the weather was unfavourable, and the eclipse consequently appears to have been seen by only one person, a certain dr. stukeley, who observed it from haraden hill near salisbury plain. this is the last eclipse of which the total phase was seen in any part of england. the next will not be until june , , and will be visible along a line across north wales and lancashire. the discs of the sun and moon will just then be almost of the same apparent size, and so totality will be of extremely short duration; in fact only a few seconds. london itself will not see a totality until the year --a circumstance which need hardly distress any of us personally! it is only from the early part of the nineteenth century that serious scientific attention to eclipses of the sun can be dated. an _annular_ eclipse, visible in in the south of scotland, drew the careful notice of francis baily of jedburgh in roxburghshire to that curious phenomenon which we have already described, and which has ever since been known by the name of "baily's beads." spurred by his observation, the leading astronomers of the day determined to pay particular attention to a total eclipse, which in the year was to be visible in the south of france and the north of italy. the public interest aroused on this occasion was also very great, for the region across which the track of totality was to pass was very populous, and inhabited by races of a high degree of culture. this eclipse occurred on the morning of the th july, and from it may be dated that great enthusiasm with which total eclipses of the sun have ever since been received. airy, our then astronomer royal, observed it from turin; arago, the celebrated director of the paris observatory, from perpignan in the south of france; francis baily from pavia; and sir john herschel from milan. the corona and three large red prominences were not only well observed by the astronomers, but drew tremendous applause from the watching multitudes. the success of the observations made during this eclipse prompted astronomers to pay similar attention to that of july , , the total phase of which was to be visible in the south of norway and sweden, and across the east of prussia. this eclipse was also a success, and it was now ascertained that the red prominences belonged to the sun and not to the moon; for the lunar disc, as it moved onward, was seen to cover and to uncover them in turn. it was also noted that these prominences were merely uprushes from a layer of glowing gaseous matter, which was seen closely to envelop the sun. the total eclipse of july , , was observed in spain, and photography was for the first time _systematically_ employed in its observation.[ ] in the photographs taken the stationary appearance of both the corona and prominences with respect to the moving moon, definitely confirmed the view already put forward that they were actual appendages of the sun. the eclipse of august , , the total phase of which lasted nearly six minutes, was visible in india, and drew thither a large concourse of astronomers. in this eclipse the spectroscope came to the front, and showed that both the prominences, and the chromospheric layer from which they rise, are composed of glowing vapours--chief among which is the vapour of hydrogen. the direct result of the observations made on this occasion was the spectroscopic method of examining prominences at any time in full daylight, and without a total eclipse. this method, which has given such an immense impetus to the study of the sun, was the outcome of independent and simultaneous investigation on the part of the french astronomer, the late m. janssen, and the english astronomer, professor (now sir norman) lockyer, a circumstance strangely reminiscent of the discovery of neptune. the principles on which the method was founded seem, however, to have occurred to dr. (now sir william) huggins some time previously. the eclipse of december , , was total for a little more than two minutes, and its track passed across the mediterranean. m. janssen, of whom mention has just been made, escaped in a balloon from then besieged paris, taking his instruments with him, and made his way to oran, in algeria, in order to observe it; but his expectations were disappointed by cloudy weather. the expedition sent out from england had the misfortune to be shipwrecked off the coast of sicily. but the occasion was redeemed by a memorable observation made by the american astronomer, the late professor young, which revealed the existence of what is now known as the "reversing layer." this is a shallow layer of gases which lies immediately beneath the chromosphere. an illustration of the corona, as it was seen during the above eclipse, will be found on plate vii. (a), p. . in the eclipse of december , , total across southern india, the photographs of the corona obtained by mr. davis, assistant to lord lindsay (now the earl of crawford), displayed a wealth of detail hitherto unapproached. the eclipse of july , , total across the western states of north america, was a remarkable success, and a magnificent view of the corona was obtained by the well-known american astronomer and physicist, the late professor langley, from the summit of pike's peak, colorado, over , feet above the level of the sea. the coronal streamers were seen to extend to a much greater distance at this altitude than at points less elevated, and the corona itself remained visible during more than four minutes after the end of totality. it was, however, not entirely a question of altitude; the coronal streamers were actually very much longer on this occasion than in most of the eclipses which had previously been observed. the eclipse of may , , observed in upper egypt, is notable from the fact that, in one of the photographs taken by dr. schuster at sohag, a bright comet appeared near the outer limit of the corona (see plate i., p. ). the comet in question had not been seen before the eclipse, and was never seen afterwards. this is the third occasion on which attention has been drawn to a comet _merely_ by a total eclipse. the first is mentioned by seneca; and the second by philostorgius, in an account of an eclipse observed at constantinople in a.d. . a fourth case of the kind occurred in , when faint evidences of one of these filmy objects were found on photographs of the corona taken by the american astronomer, professor schaeberle, during the total eclipse of april of that year. the eclipse of may , , had a totality of over five minutes, but the central track unfortunately passed across the pacific ocean, and the sole point of land available for observing it from was one of the marquesas group, caroline island, a coral atoll seven and a half miles long by one and a half broad. nevertheless astronomers did not hesitate to take up their posts upon that little spot, and were rewarded with good weather. the next eclipse of importance was that of april , . it stretched from chili across south america and the atlantic ocean to the west coast of africa, and, as the weather was fine, many good results were obtained. photographs were taken at both ends of the track, and these showed that the appearance of the corona remained unchanged during the interval of time occupied by the passage of the shadow across the earth. it was on the occasion of this eclipse that professor schaeberle found upon his photographs those traces of the presence of a comet, to which allusion has already been made. extensive preparations were made to observe the eclipse of august , . totality lasted from two to three minutes, and the track stretched from norway to japan. bad weather disappointed the observers, with the exception of those taken to nova zembla by sir george baden powell in his yacht _otaria_. the eclipse of january , , across india _viâ_ bombay and benares, was favoured with good weather, and is notable for a photograph obtained by mrs. e.w. maunder, which showed a ray of the corona extending to a most unusual distance. [illustration: plate i. the total eclipse of the sun of may th, a comet is here shown in the immediate neighbourhood of the corona. drawn by mr. w.h. wesley from the photographs. (page )] of very great influence in the growth of our knowledge with regard to the sun, is the remarkable piece of good fortune by which the countries around the mediterranean, so easy of access, have been favoured with a comparatively large number of total eclipses during the past sixty years. tracks of totality have, for instance, traversed the spanish peninsula on no less than five occasions during that period. two of these are among the most notable eclipses of recent years, namely, those of may , , and of august , . in the former the track of totality stretched from the western seaboard of mexico, through the southern states of america, and across the atlantic ocean, after which it passed over portugal and spain into north africa. the total phase lasted for about a minute and a half, and the eclipse was well observed from a great many points along the line. a representation of the corona, as it appeared on this occasion, will be found on plate vii. (b), p. . the track of the other eclipse to which we have alluded, _i.e._ that of august , , crossed spain about miles to the northward of that of . it stretched from winnipeg in canada, through labrador, and over the atlantic; then traversing spain, it passed across the balearic islands, north africa, and egypt, and ended in arabia (see fig. , p. ). much was to be expected from a comparison between the photographs taken in labrador and egypt on the question as to whether the corona would show any alteration in shape during the time that the shadow was traversing the intervening space--some miles. the duration of the total phase in this eclipse was nearly four minutes. bad weather, however, interfered a good deal with the observations. it was not possible, for instance, to do anything at all in labrador. in spain the weather conditions were by no means favourable; though at burgos, where an immense number of people had assembled, the total phase was, fortunately, well seen. on the whole, the best results were obtained at guelma in algeria. the corona on the occasion of this eclipse was a very fine one, and some magnificent groups of prominences were plainly visible to the naked eye (see the frontispiece). the next total eclipse after that of was one which occurred on january , . it passed across central asia and siberia, and had a totality lasting two and a half minutes at most; but it was not observed as the weather was extremely bad, a circumstance not surprising with regard to those regions at that time of year. the eclipse of january , , passed across the pacific ocean. only two small coral islands--hull island in the phoenix group, and flint island about miles north of tahiti--lay in the track. two expeditions set out to observe it, _i.e._ a combined american party from the lick observatory and the smithsonian institution of washington, and a private one from england under mr. f.k. mcclean. as hull island afforded few facilities, both parties installed their instruments on flint island, although it was very little better. the duration of the total phase was fairly long--about four minutes, and the sun very favourably placed, being nearly overhead. heavy rain and clouds, however, marred observation during the first minute of totality, but the remaining three minutes were successfully utilised, good photographs of the corona being obtained. the next few years to come are unfortunately by no means favourable from the point of view of the eclipse observer. an eclipse will take place on june , , the track stretching from greenland across the north polar regions into siberia. the geographical situation is, however, a very awkward one, and totality will be extremely short--only six seconds in greenland and twenty-three seconds in siberia. the eclipse of may , , will be visible in tasmania. totality will last so long as four minutes, but the sun will be at the time much too low in the sky for good observation. the eclipse of the following year, april , , will also be confined, roughly speaking, to the same quarter of the earth, the track passing across the old convict settlement of norfolk island, and then out into the pacific. the eclipse of april , , will stretch from portugal, through france and belgium into north germany. it will, however, be of practically no service to astronomy. totality, for instance, will last for only three seconds in portugal; and, though paris lies in the central track, the eclipse, which begins as barely total, will have changed into an _annular_ one by the time it passes over that city. the first really favourable eclipse in the near future will be that of august , . its track will stretch from greenland across norway, sweden, and russia. this eclipse is a return, after one saros, of the eclipse of august , . the last solar eclipse which we will touch upon is that predicted for june , . it has been already alluded to as the first of those in the future to be _total_ in england. the central line will stretch from wales in a north-easterly direction. stonyhurst observatory, in lancashire, will lie in the track; but totality there will be very short, only about twenty seconds in duration. [ ] _knowledge_, vol. xx. p. , january . [ ] the _first photographic representation of the corona_ had, however, been made during the eclipse of . this was a daguerreotype taken by dr. busch at königsberg in prussia. chapter ix famous eclipses of the moon the earliest lunar eclipse, of which we have any trustworthy information, was a total one which took place on the th march, b.c., and was observed from babylon. for our knowledge of this eclipse we are indebted to ptolemy, the astronomer, who copied it, along with two others, from the records of the reign of the chaldean king, merodach-baladan. the next eclipse of the moon worth noting was a total one, which took place some three hundred years later, namely, in b.c. this eclipse was observed at athens, and is mentioned by aristophanes in his play, _the clouds_. plutarch relates that a total eclipse of the moon, which occurred in b.c., so greatly frightened nicias, the general of the athenians, then warring in sicily, as to cause a delay in his retreat from syracuse which led to the destruction of his whole army. seven years later--namely, in b.c., the twenty-sixth year of the peloponnesian war--there took place another total lunar eclipse of which mention is made by xenophon. omitting a number of other eclipses alluded to by ancient writers, we come to one recorded by josephus as having occurred a little before the death of herod the great. it is probable that the eclipse in question was the total lunar one, which calculation shows to have taken place on the th september b.c., and to have been visible in western asia. this is very important, for we are thus enabled to fix that year as the date of the birth of christ, for herod is known to have died in the early part of the year following the nativity. in those accounts of total lunar eclipses, which have come down to us from the dark and middle ages, the colour of the moon is nearly always likened to "blood." on the other hand, in an account of the eclipse of january , a.d. , our satellite is described as "covered with a horrid black shield." we thus have examples of the two distinct appearances alluded to in chapter vii., _i.e._ when the moon appears of a coppery-red colour, and when it is entirely darkened. it appears, indeed, that, in the majority of lunar eclipses on record, the moon has appeared of a ruddy, or rather of a coppery hue, and the details on its surface have been thus rendered visible. one of the best examples of a _bright_ eclipse of this kind is that of the th march , when the illumination of our satellite was so great that many persons could not believe that an eclipse was actually taking place. a certain mr. foster, who observed this eclipse from bruges, states that the markings on the lunar disc were almost as visible as on an "ordinary dull moonlight night." he goes on to say that the british consul at ghent, not knowing that there had been any eclipse, wrote to him for an explanation of the red colour of the moon on that evening. out of the _dark_ eclipses recorded, perhaps the best example is that of may , , observed by wargentin at stockholm. on this occasion the lunar disc is said to have disappeared so completely, that it could not be discovered even with the telescope. another such instance is the eclipse of june , , observed from london. the summer of that year was particularly wet--a point worthy of notice in connection with the theory that these different appearances are due to the varying state of our earth's atmosphere. sometimes, indeed, it has happened that an eclipse of the moon has partaken of both appearances, part of the disc being visible and part invisible. an instance of this occurred in the eclipse of july , , when the late rev. s.j. johnson, one of the leading authorities on eclipses, who observed it, states that he found one-half the moon's surface quite invisible, both with the naked eye and with the telescope. in addition to the examples given above, there are three total lunar eclipses which deserve especial mention. . a.d. , november . during the progress of this eclipse the moon occulted the star aldebaran in the constellation of taurus. . a.d. , april . this is the celebrated eclipse which is said to have so well served the purposes of christopher columbus. certain natives having refused to supply him with provisions when in sore straits, he announced to them that the moon would be darkened as a sign of the anger of heaven. when the event duly came to pass, the savages were so terrified that they brought him provisions as much as he needed. . a.d. , july . the eclipse in question is notable as having been seen through the telescope, then a recent invention. it was without doubt the first so observed, but unfortunately the name of the observer has not come down to us. chapter x the growth of observation the earliest astronomical observations must have been made in the dawn of historic time by the men who tended their flocks upon the great plains. as they watched the clear night sky they no doubt soon noticed that, with the exception of the moon and those brilliant wandering objects known to us as the planets, the individual stars in the heaven remained apparently fixed with reference to each other. these seemingly changeless points of light came in time to be regarded as sign-posts to guide the wanderer across the trackless desert, or the voyager upon the wide sea. just as when looking into the red coals of a fire, or when watching the clouds, our imagination conjures up strange and grotesque forms, so did the men of old see in the grouping of the stars the outlines of weird and curious shapes. fed with mythological lore, they imagined these to be rough representations of ancient heroes and fabled beasts, whom they supposed to have been elevated to the heavens as a reward for great deeds done upon the earth. we know these groupings of stars to-day under the name of the constellations. looking up at them we find it extremely difficult to fit in the majority with the figures which the ancients believed them to represent. nevertheless, astronomy has accepted the arrangement, for want of a better method of fixing the leading stars in the memory. our early ancestors lived the greater part of their lives in the open air, and so came to pay more attention in general to the heavenly orbs than we do. their clock and their calendar was, so to speak, in the celestial vault. they regulated their hours, their days, and their nights by the changing positions of the sun, the moon, and the stars; and recognised the periods of seed-time and harvest, of calm and stormy weather, by the rising or setting of certain well-known constellations. students of the classics will recall many allusions to this, especially in the odes of horace. as time went on and civilisation progressed, men soon devised measuring instruments, by means of which they could note the positions of the celestial bodies in the sky with respect to each other; and, from observations thus made, they constructed charts of the stars. the earliest complete survey of this kind, of which we have a record, is the great catalogue of stars which was made, in the second century b.c., by the celebrated greek astronomer, hipparchus, and in which he is said to have noted down about stars. it is unnecessary to follow in detail the tedious progress of astronomical discovery prior to the advent of the telescope. certain it is that, as time went on, the measuring instruments to which we have alluded had become greatly improved; but, had they even been perfect, they would have been utterly inadequate to reveal those minute displacements, from which we have learned the actual distance of the nearest of the celestial orbs. from the early times, therefore, until the mediæval period of our own era, astronomy grew up upon a faulty basis, for the earth ever seemed so much the largest body in the universe, that it continued from century to century to be regarded as the very centre of things. to the arabians is due the credit of having kept alive the study of the stars during the dark ages of european history. they erected some fine observatories, notably in spain and in the neighbourhood of bagdad. following them, some of the oriental peoples embraced the science in earnest; ulugh beigh, grandson of the famous tamerlane, founding, for instance, a great observatory at samarcand in central asia. the mongol emperors of india also established large astronomical instruments in the chief cities of their empire. when the revival of learning took place in the west, the europeans came to the front once more in science, and rapidly forged ahead of those who had so assiduously kept alight the lamp of knowledge through the long centuries. the dethronement of the older theories by the copernican system, in which the earth was relegated to its true place, was fortunately soon followed by an invention of immense import, the invention of the telescope. it is to this instrument, indeed, that we are indebted for our knowledge of the actual scale of the celestial distances. it penetrated the depths of space; it brought the distant orbs so near, that men could note the detail on the planets, or measure the small changes in their positions in the sky which resulted from the movement of our own globe. it was in the year that the telescope was first constructed. a year or so previous to this a spectacle-maker of middleburgh in holland, one hans lippershey, had, it appears, hit upon the fact that distant objects, when viewed through certain glass lenses suitably arranged, looked nearer.[ ] news of this discovery reached the ears of galileo galilei, of florence, the foremost philosopher of the day, and he at once applied his great scientific attainments to the construction of an instrument based upon this principle. the result was what was called an "optick tube," which magnified distant objects some few times. it was not much larger than what we nowadays contemptuously refer to as a "spy-glass," yet its employment upon the leading celestial objects instantly sent astronomical science onward with a bound. in rapid succession galileo announced world-moving discoveries; large spots upon the face of the sun; crater-like mountains upon the moon; four subordinate bodies, or satellites, circling around the planet jupiter; and a strange appearance in connection with saturn, which later telescopic observers found to be a broad flat ring encircling that planet. and more important still, the magnified image of venus showed itself in the telescope at certain periods in crescent and other forms; a result which copernicus is said to have announced should of necessity follow if his system were the true one. the discoveries made with the telescope produced, as time went on, a great alteration in the notions of men with regard to the universe at large. it must have been, indeed, a revelation to find that those points of light which they called the planets, were, after all, globes of a size comparable with the earth, and peopled perchance with sentient beings. even to us, who have been accustomed since our early youth to such an idea, it still requires a certain stretch of imagination to enlarge, say, the bright star of eve, into a body similar in size to our earth. the reader will perhaps recollect tennyson's allusion to this in _locksley hall, sixty years after_:-- "hesper--venus--were we native to that splendour or in mars, we should see the globe we groan in, fairest of their evening stars. "could we dream of wars and carnage, craft and madness, lust and spite, roaring london, raving paris, in that point of peaceful light?" the form of instrument as devised by galileo is called the refracting telescope, or "refractor." as we know it to-day it is the same in principle as his "optick tube," but it is not quite the same in construction. the early _object-glass_, or large glass at the end, was a single convex lens (see fig. , p. , "galilean"); the modern one is, on the other hand, composed of two lenses fitted together. the attempts to construct large telescopes of the galilean type met in course of time with a great difficulty. the magnified image of the object observed was not quite pure; its edges, indeed, were fringed with rainbow-like colours. this defect was found to be aggravated with increase in the size of object-glasses. a method was, however, discovered of diminishing this colouration, or _chromatic aberration_ as it is called from the greek word [chrôma] (_chroma_), which means colour, viz. by making telescopes of great length and only a few inches in width. but the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. attempts were made to evade this unwieldiness by constructing them with skeleton tubes (see plate ii., p. ), or, indeed, even without tubes at all; the object-glass in the tubeless or "aerial" telescope being fixed at the top of a high post, and the _eye-piece_, that small lens or combination of lenses, which the eye looks directly into, being kept in line with it by means of a string and manoeuvred about near the ground (plate iii., p. ). the idea of a telescope without a tube may appear a contradiction in terms; but it is not really so, for the tube adds nothing to the magnifying power of the instrument, and is, in fact, no more than a mere device for keeping the object-glass and eye-piece in a straight line, and for preventing the observer from being hindered by stray lights in his neighbourhood. it goes without saying, of course, that the image of a celestial object will be more clear and defined when examined in the darkness of a tube. the ancients, though they knew nothing of telescopes, had, however, found out the merit of a tube in this respect; for they employed simple tubes, blackened on the inside, in order to obtain a clearer view of distant objects. it is said that julius cæsar, before crossing the channel, surveyed the opposite coast of britain through a tube of this kind. [illustration: plate ii. great telescope of hevelius this instrument, feet in length, with a _skeleton_ tube, was constructed by the celebrated seventeenth century astronomer, hevelius of danzig. from an illustration in the _machina celestis_. (page )] a few of the most famous of the immensely long telescopes above alluded to are worthy of mention. one of these, feet in length, was presented to the royal society of london by the dutch astronomer huyghens. hevelius of danzig constructed a skeleton one of feet in length (see plate ii., p. ). bradley used a tubeless one feet long to measure the diameter of venus in ; while one of feet is said to have been constructed, but to have proved quite unworkable! such difficulties, however, produced their natural result. they set men at work to devise another kind of telescope. in the new form, called the reflecting telescope, or "reflector," the light coming from the object under observation was _reflected_ into the eye-piece from the surface of a highly polished concave metallic mirror, or _speculum_, as it was called. it is to sir isaac newton that the world is indebted for the reflecting telescope in its best form. that philosopher had set himself to investigate the causes of the rainbow-like, or prismatic colours which for a long time had been such a source of annoyance to telescopic observers; and he pointed out that, as the colours were produced in the passage of the rays of light _through_ the glass, they would be entirely absent if the light were reflected from the _surface_ of a mirror instead. the reflecting telescope, however, had in turn certain drawbacks of its own. a mirror, for instance, can plainly never be polished to such a high degree as to reflect as much light as a piece of transparent glass will let through. further, the position of the eye-piece is by no means so convenient. it cannot, of course, be pointed directly towards the mirror, for the observer would then have to place his head right in the way of the light coming from the celestial object, and would thus, of course, cut it off. in order to obviate this difficulty, the following device was employed by newton in his telescope, of which he constructed his first example in . a small, flat mirror was fixed by thin wires in the centre of the tube of the telescope, and near to its open end. it was set slant-wise, so that it reflected the rays of light directly into the eye-piece, which was screwed into a hole at the side of the tube (see fig. , p. , "newtonian"). although the newtonian form of telescope had the immense advantage of doing away with the prismatic colours, yet it wasted a great deal of light; for the objection in this respect with regard to loss of light by reflection from the large mirror applied, of course, to the small mirror also. in addition, the position of the "flat," as the small mirror is called, had the further effect of excluding from the great mirror a certain proportion of light. but the reflector had the advantage, on the other hand, of costing less to make than the refractor, as it was not necessary to procure flawless glass for the purpose. a disc of a certain metallic composition, an alloy of copper and tin, known in consequence as _speculum metal_, had merely to be cast; and this had to be ground and polished _upon one side only_, whereas a lens has to be thus treated _upon both its sides_. it was, therefore, possible to make a much larger instrument at a great deal less labour and expense. [illustration: plate iii. a tubeless, or "aerial" telescope from an illustration in the _opera varia_ of christian huyghens. (page )] [illustration: fig. .--the various types of telescope. all the above telescopes are _pointed_ in the same direction; that is to say, the rays of light from the object are coming from the left-hand side.] we have given the newtonian form as an example of the principle of the reflecting telescope. a somewhat similar instrument had, however, been projected, though not actually constructed, by james gregory a few years earlier than newton's, _i.e._ in . in this form of reflector, known as the "gregorian" telescope, a hole was made in the big concave mirror; and a small mirror, also concave, which faced it at a certain distance, received the reflected rays, and reflected them back again through the hole in question into the eye-piece, which was fixed just behind (see fig. , p. , "gregorian"). the gregorian had thus the sentimental advantage of being _pointed directly at the object_. the hole in the big mirror did not cause any loss of light, for the central portion in which it was made was anyway unable to receive light through the small mirror being directly in front of it. an adaptation of the gregorian was the "cassegrainian" telescope, devised by cassegrain in , which differed from it chiefly in the small mirror being convex instead of concave (see fig. , p. , "cassegrainian"). these _direct-view_ forms of the reflecting telescope were much in vogue about the middle of the eighteenth century, when many beautiful examples of gregorians were made by the famous optician, james short, of edinburgh. an adaptation of the newtonian type of telescope is known as the "herschelian," from being the kind favoured by sir william herschel. it is, however, only suitable in immense instruments, such as herschel was in the habit of employing. in this form the object-glass is set at a slight slant, so that the light coming from the object is reflected straight into the eye-piece, which is fixed facing it in the side of the tube (see fig. , p. , "herschelian"). this telescope has an advantage over the other forms of reflector through the saving of light consequent on doing away with the _second_ reflection. there is, however, the objection that the slant of the object-glass is productive of some distortion in the appearance of the object observed; but this slant is of necessity slight when the length of the telescope is very great. the principle of this type of telescope had been described to the french academy of sciences as early as by le maire, but no one availed himself of the idea until , when herschel tried it. at first, however, he rejected it; but in he seems to have found that it suited the huge instruments which he was then making. herschel's largest telescope, constructed in , was about four feet in diameter and forty feet in length. it is generally spoken of as the "forty-foot telescope," though all other instruments have been known by their _diameters_, rather than by their lengths. to return to the refracting telescope. a solution of the colour difficulty was arrived at in (two years after newton's death) by an essex gentleman named chester moor hall. he discovered that by making a double object-glass, composed of an outer convex lens and an inner concave lens, made respectively of different kinds of glass, _i.e._ _crown_ glass and _flint_ glass, the troublesome colour effects could be, _to a very great extent_, removed. hall's investigations appear to have been rather of an academic nature; and, although he is believed to have constructed a small telescope upon these lines, yet he seems to have kept the matter so much to himself that it was not until the year that the first example of the new instrument was given to the world. this was done by john dollond, founder of the well-known optical firm of dollond, of ludgate hill, london, who had, quite independently, re-discovered the principle. this "achromatic" telescope, or telescope "free from colour effects," is the kind ordinarily in use at present, whether for astronomical or for terrestrial purposes (see fig. , p. , "achromatic"). the expense of making large instruments of this type is very great, for, in the object-glass alone, no less than _four_ surfaces have to be ground and polished to the required curves; and, usually, the two lenses of which it is composed have to fit quite close together. with the object of evading the expense referred to, and of securing _complete_ freedom from colour effects, telescopes have even been made, the object-glasses of which were composed of various transparent liquids placed between thin lenses; but leakages, and currents set up within them by changes of temperature, have defeated the ingenuity of those who devised these substitutes. the solution of the colour difficulty by means of dollond's achromatic refractor has not, however, ousted the reflecting telescope in its best, or newtonian form, for which great concave mirrors made of glass, covered with a thin coating of silver and highly polished, have been used since about instead of metal mirrors. they are very much lighter in weight and cheaper to make than the old specula; and though the silvering, needless to say, deteriorates with time, it can be renewed at a comparatively trifling cost. also these mirrors reflect much more light, and give a clearer view, than did the old metallic ones. when an object is viewed through the type of astronomical telescope ordinarily in use, it is seen _upside down_. this is, however, a matter of very small moment in dealing with celestial objects; for, as they are usually round, it is really not of much consequence which part we regard as top and which as bottom. such an inversion would, of course, be most inconvenient when viewing terrestrial objects. in order to observe the latter we therefore employ what is called a terrestrial telescope, which is merely a refractor with some extra lenses added in the eye portion for the purpose of turning the inverted image the right way up again. these extra lenses, needless to say, absorb a certain amount of light; wherefore it is better in astronomical observation to save light by doing away with them, and putting up with the slight inconvenience of seeing the object inverted. this inversion of images by the astronomical telescope must be specially borne in mind with regard to the photographs of the moon in chapter xvi. in the year the largest achromatic refractor in existence was one of nine and a half inches in diameter constructed by fraunhofer for the observatory of dorpat in russia. the largest refractors in the world to-day are in the united states, _i.e._ the forty-inch of the yerkes observatory (see plate iv., p. ), and the thirty-six inch of the lick. the object-glasses of these and of the thirty-inch telescope of the observatory of pulkowa, in russia, were made by the great optical house of alvan clark & sons, of cambridge, massachusetts, u.s.a. the tubes and other portions of the yerkes and lick telescopes were, however, constructed by the warner and swasey co., of cleveland, ohio. the largest reflector, and so the largest telescope in the world, is still the six-foot erected by the late lord rosse at parsonstown in ireland, and completed in the year . it is about fifty-six feet in length. next come two of five feet, with mirrors of silver on glass; one of them made by the late dr. common, of ealing, and the other by the american astronomer, professor g.w. ritchey. the latter of these is installed in the solar observatory belonging to carnegie institution of washington, which is situated on mount wilson in california. the former is now at the harvard college observatory, and is considered by professor moulton to be probably the most efficient reflector in use at present. another large reflector is the three-foot made by dr. common. it came into the possession of mr. crossley of halifax, who presented it to the lick observatory, where it is now known as the "crossley reflector." although to the house of clark belongs, as we have seen, the credit of constructing the object-glasses of the largest refracting telescopes of our time, it has nevertheless keen competitors in sir howard grubb, of dublin, and such well-known firms as cooke of york and steinheil of munich. in the four-foot reflector, made in for the observatory of melbourne by the firm of grubb, the cassegrainian principle was employed. with regard to the various merits of refractors and reflectors much might be said. each kind of instrument has, indeed, its special advantages; though perhaps, on the whole, the most perfect type of telescope is the achromatic refractor. [illustration: plate iv. the great yerkes telescope great telescope at the yerkes observatory of the university of chicago, williams bay, wisconsin, u.s.a. it was erected in - , and is the largest refracting telescope in the world. diameter of object-glass, inches; length of telescope, about feet. the object-glass was made by the firm of alvan clark and sons, of cambridge, massachusetts; the other portions of the instrument by the warner and swasey co., of cleveland, ohio. (page )] in connection with telescopes certain devices have from time to time been introduced, but these merely aim at the _convenience_ of the observer and do not supplant the broad principles upon which are based the various types of instrument above described. such, for instance, are the "siderostat," and another form of it called the "coelostat," in which a plane mirror is made to revolve in a certain manner, so as to reflect those portions of the sky which are to be observed, into the tube of a telescope kept fixed. such too are the "equatorial coudé" of the late m. loewy, director of the paris observatory, and the "sheepshanks telescope" of the observatory of cambridge, in which a telescope is separated into two portions, the eye-piece portion being fixed upon a downward slant, and the object-glass portion jointed to it at an angle and pointed up at the sky. in these two instruments (which, by the way, differ materially) an arrangement of slanting mirrors in the tubes directs the journey of the rays of light from the object-glass to the eye-piece. the observer can thus sit at the eye-end of his telescope in the warmth and comfort of his room, and observe the stars in the same unconstrained manner as if he were merely looking down into a microscope. needless to say, devices such as these are subject to the drawback that the mirrors employed sap a certain proportion of the rays of light. it will be remembered that we made allusion to loss of light in this way, when pointing out the advantage in light grasp of the herschelian form of telescope, where only _one_ reflection takes place, over the newtonian in which there are _two_. it is an interesting question as to whether telescopes can be made much larger. the american astronomer, professor g.e. hale, concludes that the limit of refractors is about five feet in diameter, but he thinks that reflectors as large as nine feet in diameter might now be made. as regards refractors there are several strong reasons against augmenting their proportions. first of all comes the great cost. secondly, since the lenses are held in position merely round their rims, they will bend by their weight in the centres if they are made much larger. on the other hand, attempts to obviate this, by making the lenses thicker, would cause a decrease in the amount of light let through. but perhaps the greatest stumbling-block to the construction of larger telescopes is the fact that the unsteadiness of the air will be increasingly magnified. and further, the larger the tubes become, the more difficult will it be to keep the air within them at one constant temperature throughout their lengths. it would, indeed, seem as if telescopes are not destined greatly to increase in size, but that the means of observation will break out in some new direction, as it has already done in the case of photography and the spectroscope. the direct use of the eye is gradually giving place to indirect methods. we are, in fact, now _feeling_ rather than seeing our way about the universe. up to the present, for instance, we have not the slightest proof that life exists elsewhere than upon our earth. but who shall say that the twentieth century has not that in store for us, by which the presence of life in other orbs may be perceived through some form of vibration transmitted across illimitable space? there is no use speaking of the impossible or the inconceivable. after the extraordinary revelations of the spectroscope--nay, after the astounding discovery of röntgen--the word impossible should be cast aside, and inconceivability cease to be regarded as any criterion. [ ] the principle upon which the telescope is based appears to have been known _theoretically_ for a long time previous to this. the monk roger bacon, who lived in the thirteenth century, describes it very clearly; and several writers of the sixteenth century have also dealt with the idea. even lippershey's claims to a practical solution of the question were hotly contested at the time by two of his own countrymen, _i.e._ a certain jacob metius, and another spectacle-maker of middleburgh, named jansen. chapter xi spectrum analysis if white light (that of the sun, for instance) be passed through a glass prism, namely, a piece of glass of triangular shape, it will issue from it in rainbow-tinted colours. it is a common experience with any of us to notice this when the sunlight shines through cut-glass, as in the pendant of a chandelier, or in the stopper of a wine-decanter. the same effect may be produced when light passes through water. the rainbow, which we all know so well, is merely the result of the sunlight passing through drops of falling rain. white light is composed of rays of various colours. red, orange, yellow, green, blue, indigo, and violet, taken all together, go, in fact, to make up that effect which we call white. it is in the course of the _refraction_, or bending of a beam of light, when it passes in certain conditions through a transparent and denser medium, such as glass or water, that the constituent rays are sorted out and spread in a row according to their various colours. this production of colour takes place usually near the edges of a lens; and, as will be recollected, proved very obnoxious to the users of the old form of refracting telescope. it is, indeed, a strange irony of fate that this very same production of colour, which so hindered astronomy in the past, should have aided it in recent years to a remarkable degree. if sunlight, for instance, be admitted through a narrow slit before it falls upon a glass prism, it will issue from the latter in the form of a band of variegated colour, each colour blending insensibly with the next. the colours arrange themselves always in the order which we have mentioned. this seeming band is, in reality, an array of countless coloured images of the original slit ranged side by side; the colour of each image being the slightest possible shade different from that next to it. this strip of colour when produced by sunlight is called the "solar spectrum" (see fig. , p. ). a similar strip, or _spectrum_, will be produced by any other light; but the appearance of the strip, with regard to preponderance of particular colours, will depend upon the character of that light. electric light and gas light yield spectra not unlike that of sunlight; but that of gas is less rich in blue and violet than that of the sun. the spectroscope, an instrument devised for the examination of spectra, is, in its simplest form, composed of a small tube with a narrow slit and prism at one end, and an eye-piece at the other. if we drop ordinary table salt into the flame of a gas light, the flame becomes strongly yellow. if, then, we observe this yellow flame with the spectroscope, we find that its spectrum consists almost entirely of two bright yellow transverse lines. chemically considered ordinary table salt is sodium chloride; that is to say, a compound of the metal sodium and the gas chlorine. now if other compounds of sodium be experimented with in the same manner, it will soon be found that these two yellow lines are characteristic of sodium when turned into vapour by great heat. in the same manner it can be ascertained that every element, when heated to a condition of vapour, gives as its spectrum a set of lines peculiar to itself. thus the spectroscope enables us to find out the composition of substances when they are reduced to vapour in the laboratory. [illustration: fig. .--the solar spectrum.] in order to increase the power of a spectroscope, it is necessary to add to the number of prisms. each extra prism has the effect of lengthening the coloured strip still more, so that lines, which at first appeared to be single merely through being crowded together, are eventually drawn apart and become separately distinguishable. on this principle it has gradually been determined that the sun is composed of elements similar to those which go to make up our earth. further, the composition of the stars can be ascertained in the same manner; and we find them formed on a like pattern, though with certain elements in greater or less proportion as the case may be. it is in consequence of our thus definitely ascertaining that the stars are self-luminous, and of a sun-like character, that we are enabled to speak of them as _suns_, or to call the sun a _star_. in endeavouring to discover the elements of which the planets and satellites of our system are composed, we, however, find ourselves baffled, for the simple reason that these bodies emit no real light of their own. the light which reaches us from them, being merely reflected sunlight, gives only the ordinary solar spectrum when examined with the spectroscope. but in certain cases we find that the solar spectrum thus viewed shows traces of being weakened, or rather of suffering absorption; and it is concluded that this may be due to the sunlight having had to pass through an atmosphere on its way to and from the surface of the planet from which it is reflected to us. since the sun is found to be composed of elements similar to those which go to make up our earth, we need not be disheartened at this failure of the spectroscope to inform us of the composition of the planets and satellites. we are justified, indeed, in assuming that more or less the same constituents run through our solar system; and that the elements of which these bodies are composed are similar to those which are found upon our earth and in the sun. the spectroscope supplies us with even more information. it tells us, indeed, whether the sun-like body which we are observing is moving away from us or towards us. a certain slight shifting of the lines towards the red or violet end of the spectrum respectively, is found to follow such movement. this method of observation is known by the name of _doppler's method_,[ ] and by it we are enabled to confirm the evidence which the sunspots give us of the rotation of the sun; for we find thus that one edge of that body is continually approaching us, and the other edge is continually receding from us. also, we can ascertain in the same manner that certain of the stars are moving towards us, and certain of them away from us. [ ] the idea, initiated by christian doppler at prague in , was originally applied to sound. the approach or recession of a source from which sound is coming is invariably accompanied by alterations of pitch, as the reader has no doubt noticed when a whistling railway-engine has approached him or receded from him. it is to sir william huggins, however, that we are indebted for the application of the principle to spectroscopy. this he gave experimental proof of in the year . chapter xii the sun the sun is the chief member of our system. it controls the motions of the planets by its immense gravitative power. besides this it is the most important body in the entire universe, so far as we are concerned; for it pours out continually that flood of light and heat, without which life, as we know it, would quickly become extinct upon our globe. light and heat, though not precisely the same thing, may be regarded, however, as next-door neighbours. the light rays are those which directly affect the eye and are comprised in the visible spectrum. we _feel_ the heat rays, the chief of which are beyond the red portion of the spectrum. they may be investigated with the _bolometer_, an instrument invented by the late professor langley. chemical rays--for instance, those radiations which affect the photographic plate--are for the most part also outside the visible spectrum. they are, however, at the other end of it, namely, beyond the violet. such a scale of radiations may be compared to the keyboard of an imaginary piano, the sound from only one of whose octaves is audible to us. the brightest light we know on the earth is dull compared with the light of the sun. it would, indeed, look quite dark if held up against it. it is extremely difficult to arrive at a precise notion of the temperature of the body of the sun. however, it is far in excess of any temperature which we can obtain here, even in the most powerful electric furnace. a rough idea of the solar heat may be gathered from the calculation that if the sun's surface were coated all over with a layer of ice feet thick, it would melt through this completely in one hour. the sun cannot be a hot body merely cooling; for the rate at which it is at present giving off heat could not in such circumstances be kept up, according to professor moulton, for more than years. further, it is not a mere burning mass, like a coal fire, for instance; as in that case about a thousand years would show a certain drop in temperature. no perceptible diminution of solar heat having taken place within historic experience, so far as can be ascertained, we are driven to seek some more abstruse explanation. the theory which seems to have received most acceptance is that put forward by helmholtz in . his idea was that gravitation produces continual contraction, or falling in of the outer parts of the sun; and that this falling in, in its turn, generates enough heat to compensate for what is being given off. the calculations of helmholtz showed that a contraction of about feet a year from the surface towards the centre would suffice for the purpose. in recent years, however, this estimate has been extended to about feet. nevertheless, even with this increased figure, the shrinkage required is so slight in comparison with the immense girth of the sun, that it would take a continual contraction at this rate for about years, to show even in our finest telescopes that any change in the size of that body was taking place at all. upon this assumption of continuous contraction, a time should, however, eventually be reached when the sun will have shrunk to such a degree of solidity, that it will not be able to shrink any further. then, the loss of heat not being made up for any longer, the body of the sun should begin to grow cold. but we need not be distressed on this account; for it will take some , , years, according to the above theory, before the solar orb becomes too cold to support life upon our earth. since the discovery of radium it has, on the other hand, been suggested, and not unreasonably, that radio-active matter may possibly play an important part in keeping up the heat of the sun. but the body of scientific opinion appears to consider the theory of contraction as a result of gravitation, which has been outlined above, to be of itself quite a sound explanation. indeed, the late lord kelvin is said to have held to the last that it was amply sufficient to account for the underground heat of the earth, the heat of the sun, and that of all the stars in the universe. one great difficulty in forming theories with regard to the sun, is the fact that the temperature and gravitation there are enormously in excess of anything we meet with upon our earth. the force of gravity at the sun's surface is, indeed, about twenty-seven times that at the surface of our globe. the earth's atmosphere appears to absorb about one-half of the radiations which come to us from the sun. this absorptive effect is very noticeable when the solar orb is low down in our sky, for its light and heat are then clearly much reduced. of the light rays, the blue ones are the most easily absorbed in this way; which explains why the sun looks red when near the horizon. it has then, of course, to shine through a much greater thickness of atmosphere than when high up in the heavens. what astonishes one most about the solar radiation, is the immense amount of it that is apparently wasted into space in comparison with what falls directly upon the bodies of the solar system. only about the one-hundred-millionth is caught by all the planets together. what becomes of the rest we cannot tell. that brilliant white body of the sun, which we see, is enveloped by several layers of gases and vaporous matter, in the same manner as our globe is enveloped by its atmosphere (see fig. , p. ). these are transparent, just as our atmosphere is transparent; and so we see the white bright body of the sun right through them. this white bright portion is called the _photosphere_. from it comes most of that light and heat which we see and feel. we do not know what lies under the photosphere, but, no doubt, the more solid portions of the sun are there situated. just above the photosphere, and lying close upon it, is a veil of smoke-like haze. next upon this is what is known as the _reversing layer_, which is between and miles in thickness. it is cooler than the underlying photosphere, and is composed of glowing gases. many of the elements which go to make up our earth are present in the reversing layer in the form of vapour. the _chromosphere_, of which especial mention has already been made in dealing with eclipses of the sun, is another layer lying immediately upon the last one. it is between and , miles in thickness. like the reversing layer, it is composed of glowing gases, chief among which is the vapour of hydrogen. the colour of the chromosphere is, in reality, a brilliant scarlet; but, as we have already said, the intensely white light of the photosphere shines through it from behind, and entirely overpowers its redness. the upper portion of the chromosphere is in violent agitation, like the waves of a stormy sea, and from it rise those red prominences which, it will be recollected, are such a notable feature in total solar eclipses. [illustration: fig. .--a section through the sun, showing how the prominences rise from the chromosphere.] the _corona_ lies next in order outside the chromosphere, and is, so far as we know, the outermost of the accompaniments of the sun. this halo of pearly-white light is irregular in outline, and fades away into the surrounding sky. it extends outwards from the sun to several millions of miles. as has been stated, we can never see the corona unless, when during a total solar eclipse, the moon has, for the time being, hidden the brilliant photosphere completely from our view. the solar spectrum is really composed of three separate spectra commingled, _i.e._ those of the photosphere, of the reversing layer, and of the chromosphere respectively. if, therefore, the photosphere could be entirely removed, or covered up, we should see only the spectra of those layers which lie upon it. such a state of things actually occurs in a total eclipse of the sun. when the moon's body has crept across the solar disc, and hidden the last piece of photosphere, the solar spectrum suddenly becomes what is technically called "reversed,"--the dark lines crossing it changing into bright lines. this occurs because a strip of those layers which lie immediately upon the photosphere remains still uncovered. the lower of these layers has therefore been called the "reversing layer," for want of a better name. after a second or two this reversed spectrum mostly vanishes, and an altered spectrum is left to view. taking into consideration the rate at which the moon is moving across the face of the sun, and the very short time during which the spectrum of the reversing layer lasts, the thickness of that layer is estimated to be not more than a few hundred miles. in the same way the last of the three spectra--namely, that of the chromosphere--remains visible for such a time as allows us to estimate its depth at about ten times that of the reversing layer, or several thousand miles. when the chromosphere, in its turn during a total eclipse, has been covered by the moon, the corona alone is left. this has a distinct spectrum of its own also; wherein is seen a strange line in the green portion, which does not tally with that of any element we are acquainted with upon the earth. this unknown element has received for the time being the name of "coronium." chapter xiii the sun--_continued_ the various parts of the sun will now be treated of in detail. i. photosphere. the photosphere, or "light-sphere," from the greek [phôs] (_phos_), which means _light_, is, as we have already said, the innermost portion of the sun which can be seen. examined through a good telescope it shows a finely mottled structure, as of brilliant granules, somewhat like rice grains, with small dark spaces lying in between them. it has been supposed that we have here the process of some system of circulation by which the sun keeps sending forth its radiations. in the bright granules we perhaps see masses of intensely heated matter, rising from the interior of the sun. the dark interspaces may represent matter which has become cooled and darkened through having parted with its heat and light, and is falling back again into the solar furnace. the _sun spots_, so familiar to every one nowadays, are dark patches which are often seen to break out in the photosphere (see plate v., p. ). they last during various periods of time; sometimes only for a few days, sometimes so long as a month or more. a spot is usually composed of a dark central portion called the _umbra_, and a less dark fringe around this called the _penumbra_ (see plate vi., p. ). the umbra ordinarily has the appearance of a deep hole in the photosphere; but, that it is a hole at all, has by no means been definitely proved. [illustration: plate v. the sun, showing several groups of spots from a photograph taken at the royal observatory, greenwich. the cross-lines seen on the disc are in no way connected with the sun, but belong to the telescope through which the photograph was taken. (page )] sun spots are, as a rule, some thousands of miles across. the umbra of a good-sized spot could indeed engulf at once many bodies the size of our earth. sun spots do not usually appear singly, but in groups. the total area of a group of this kind may be of immense extent; even so great as to cover the one-hundredth part of the whole surface of the sun. very large spots, when such are present, may be seen without any telescope; either through a piece of smoked glass, or merely with the naked eye when the air is misty, or the sun low on the horizon. the umbra of a spot is not actually dark. it only appears so in contrast with the brilliant photosphere around. spots form, grow to a large size in comparatively short periods of time, and then quickly disappear. they seem to shrink away as a consequence of the photosphere closing in upon them. that the sun is rotating upon an axis, is shown by the continual change of position of all spots in one constant direction across his disc. the time in which a spot is carried completely round depends, however, upon the position which it occupies upon the sun's surface. a spot situated near the equator of the sun goes round once in about twenty-five days. the further a spot is situated from this equator, the longer it takes. about twenty-seven days is the time taken by a spot situated midway between the equator and the solar poles. spots occur to the north of the sun's equator, as well as to the south; though, since regular observations have been made--that is to say, during the past fifty years or so--they appear to have broken out a little more frequently in the southern parts. from these considerations it will be seen that the sun does not rotate as the earth does, but that different portions appear to move at different speeds. whether in the neighbourhood of the solar poles the time of rotation exceeds twenty-seven days we are unable to ascertain, for spots are not seen in those regions. no explanation has yet been given of this peculiar rotation; and the most we can say on the subject is that the sun is not by any means a solid body. _faculæ_ (latin, little torches) are brilliant patches which appear here and there upon the sun's surface, and are in some way associated with spots. their displacement, too, across the solar face confirms the evidence which the spots give us of the sun's rotation. our proofs of this rotation are still further strengthened by the doppler spectroscopic method of observation alluded to in chapter xi. as was then stated, one edge of the sun is thus found to be continually approaching us, and the other side continually receding from us. the varying rates of rotation, which the spots and faculæ give us, are duly confirmed by this method. [illustration: plate vi. photograph of a sunspot this fine picture was taken by the late m. janssen. the granular structure of the sun's surface is here well represented. (from _knowledge_.) (page )] the first attempt to bring some regularity into the question of sunspots was the discovery by schwabe, in , that they were subject to a regular variation. as a matter of fact they wax and wane in their number, and the total area which they cover, in the course of a period, or cycle, of on an average about - / years; being at one part of this period large and abundant, and at another few and small. this period of - / years is known as the sun spot cycle. no explanation has yet been given of the curious round of change, but the period in question seems to govern most of the phenomena connected with the sun. ii. reversing layer. this is a layer of relatively cool gases lying immediately upon the photosphere. we never see it directly; and the only proof we have of its presence is that remarkable reversal of the spectrum already described, when during an instant or two in a total eclipse, the advancing edge of the moon, having just hidden the brilliant photosphere, is moving across the fine strip which the layer then presents edgewise towards us. the fleeting moments during which this reversed spectrum lasts, informs us that the layer is comparatively shallow; little more indeed than about miles in depth. the spectrum of the reversing layer, or "flash spectrum," as it is sometimes called on account of the instantaneous character with which the change takes place, was, as we have seen, first noticed by young in ; and has been successfully photographed since then during several eclipses. the layer itself appears to be in a fairly quiescent state; a marked contrast to the seething photosphere beneath, and the agitated chromosphere above. iii. the chromosphere. the chromosphere--so called from the greek [chrôma] (_chroma_), which signifies _colour_--is a layer of gases lying immediately upon the preceding one. its thickness is, however, plainly much the greater of the two; for whereas the reversing layer is only revealed to us _indirectly_ by the spectroscope, a portion of the chromosphere may clearly be _seen_ in a total eclipse in the form of a strip of scarlet light. the time which the moon's edge takes to traverse it tells us that it must be about ten times as deep as the reversing layer, namely, from to , miles in depth. its spectrum shows that it is composed chiefly of hydrogen, calcium and helium, in the state of vapour. its red colour is mainly due to glowing hydrogen. the element helium, which it also contains, has received its appellation from [hêlios] (_helios_), the greek name for the sun; because, at the time when it first attracted attention, there appeared to be no element corresponding to it upon our earth, and it was consequently imagined to be confined to the sun alone. sir william ramsay, however, discovered it to be also a terrestrial element in , and since then it has come into much prominence as one of the products given off by radium. taking into consideration the excessive force of gravity on the sun, one would expect to find the chromosphere and reversing layer growing gradually thicker in the direction of the photosphere. this, however, is not the case. both these layers are strangely enough of the same densities all through; which makes it suspected that, in these regions, the force of gravity may be counteracted by some other force or forces, exerting a powerful pressure outwards from the sun. iv. the prominences. we have already seen, in dealing with total eclipses, that the exterior surface of the chromosphere is agitated like a stormy sea, and from it billows of flame are tossed up to gigantic heights. these flaming jets are known under the name of prominences, because they were first noticed in the form of brilliant points projecting from behind the rim of the moon when the sun was totally eclipsed. prominences are of two kinds, _eruptive_ and _quiescent_. the eruptive prominences spurt up directly from the chromosphere with immense speeds, and change their shape with great rapidity. quiescent prominences, on the other hand, have a form somewhat like trees, and alter their shape but slowly. in the eruptive prominences glowing masses of gas are shot up to altitudes sometimes as high as , miles,[ ] with velocities even so great as from to miles a second. it has been noticed that the eruptive prominences are mostly found in those portions of the sun where spots usually appear, namely, in the regions near the solar equator. the quiescent prominences, on the other hand, are confined, as a rule, to the neighbourhood of the sun's poles. prominences were at first never visible except during total eclipses of the sun. but in the year , as we have already seen, a method of employing the spectroscope was devised, by means of which they could be observed and studied at any time, without the necessity of waiting for an eclipse. a still further development of the spectroscope, the _spectroheliograph_, an instrument invented almost simultaneously by professor hale and the french astronomer, m. deslandres, permits of photographs being taken of the sun, with the light emanating from _only one_ of its glowing gases at a time. for instance, we can thus obtain a record of what the glowing hydrogen alone is doing on the solar body at any particular moment. with this instrument it is also possible to obtain a series of photographs, showing what is taking place upon the sun at various levels. this is very useful in connection with the study of the spots; for we are, in consequence, enabled to gather more evidence on the subject of their actual form than is given us by their highly foreshortened appearances when observed directly in the telescope. v. corona. (latin, _a crown_.) this marvellous halo of pearly-white light, which displays itself to our view only during the total phase of an eclipse of the sun, is by no means a layer like those other envelopments of the sun of which we have just been treating. it appears, on the other hand, to be composed of filmy matter, radiating outwards in every direction, and fading away gradually into space. its structure is noted to bear a strong resemblance to the tails of comets, or the streamers of the aurora borealis. our knowledge concerning the corona has, however, advanced very slowly. we have not, so far, been as fortunate with regard to it as with regard to the prominences; and, for all we can gather concerning it, we are still entirely dependent upon the changes and chances of total solar eclipses. all attempts, in fact, to apply the spectroscopic method, so as to observe the corona at leisure in full sunlight in the way in which the prominences can be observed, have up to the present met with failure. the general form under which the corona appears to our eyes varies markedly at different eclipses. sometimes its streamers are many, and radiate all round; at other times they are confined only to the middle portions of the sun, and are very elongated, with short feathery-looking wisps adorning the solar poles. it is noticed that this change of shape varies in close accordance with that - / year period during which the sun spots wax and wane; the many-streamered regular type corresponding to the time of great sunspot activity, while the irregular type with the long streamers is present only when the spots are few (see plate vii., p. ). streamers have often been noted to issue from those regions of the sun where active prominences are at the moment in existence; but it cannot be laid down that this is always the case. no hypothesis has yet been formulated which will account for the structure of the corona, or for its variation in shape. the great difficulty with regard to theorising upon this subject, is the fact that we see so much of the corona under conditions of marked foreshortening. assuming, what indeed seems natural, that the rays of which it is composed issue in every direction from the solar body, in a manner which may be roughly imitated by sticking pins all over a ball; it is plainly impossible to form any definite idea concerning streamers, which actually may owe most of the shape they present to us, to the mixing up of multitudes of rays at all kinds of angles to the line of sight. in a word, we have to try and form an opinion concerning an arrangement which, broadly speaking, is _spherical_, but which, on account of its distance, must needs appear to us as absolutely _flat_. the most known about the composition of the corona is that it is made up of particles of matter, mingled with a glowing gas. it is an element in the composition of this gas which, as has been stated, is not found to tally with any known terrestrial element, and has, therefore, received the name of coronium for want of a better designation. one definite conclusion appears to be reached with regard to the corona, _i.e._ that the matter of which it is composed, must be exceedingly rarefied; as it is not found, for instance, to retard appreciably the speed of comets, on occasions when these bodies pass very close to the sun. a calculation has indeed been made which would tend to show that the particles composing the coronal matter, are separated from each other by a distance of perhaps between two and three yards! the density of the corona is found not to increase inwards towards the sun. this is what has already been noted with regard to the layers lying beneath it. powerful forces, acting in opposition to gravity, must hold sway here also. [illustration: (a.) the total eclipse of the sun of december nd, drawn by mr. w.h. wesley from a photograph taken at syracuse by mr. brothers. this is the type of corona seen at the time of _greatest_ sunspot activity. the coronas of (plate i., p. ) and of (frontispiece) are of the same type. (b.) the total eclipse of the sun of may th, drawn by mr. w.h. wesley from photographs taken by mr. e.w. maunder. this is the type of corona seen when the sunspots are _least_ active. compare the "ring with wings," fig. , p. . plate vii. forms of the solar corona at the epochs of sunspot maximum and sunspot minimum, respectively (page )] the - / year period, during which the sun spots vary in number and size, appears to govern the activities of the sun much in the same way that our year does the changing seasonal conditions of our earth. not only, as we have seen, does the corona vary its shape in accordance with the said period, but the activity of the prominences, and of the faculæ, follow suit. further, this constant round of ebb and flow is not confined to the sun itself, but, strangely enough, affects the earth also. the displays of the aurora borealis, which we experience here, coincide closely with it, as does also the varying state of the earth's magnetism. the connection may be still better appreciated when a great spot, or group of spots, has made its appearance upon the sun. it has, for example, often been noted that when the solar rotation carries a spot, or group of spots, across the middle of the visible surface of the sun, our magnetic and electrical arrangements are disturbed for the time being. the magnetic needles in our observatories are, for instance, seen to oscillate violently, telegraphic communication is for a while upset, and magnificent displays of the aurora borealis illumine our night skies. mr. e.w. maunder, of greenwich observatory, who has made a very careful investigation of this subject, suspects that, when elongated coronal streamers are whirled round in our direction by the solar rotation, powerful magnetic impulses may be projected upon us at the moments when such streamers are pointing towards the earth. some interesting investigations with regard to sunspots have recently been published by mrs. e.w. maunder. in an able paper, communicated to the royal astronomical society on may , , she reviews the greenwich observatory statistics dealing with the number and extent of the spots which have appeared during the period from to --a whole sunspot cycle. from a detailed study of the dates in question, she finds that the number of those spots which are formed on the side of the sun turned away from us, and die out upon the side turned towards us, is much greater than the number of those which are formed on the side turned towards us and die out upon the side turned away. it used, for instance, to be considered that the influence of a planet might _produce_ sunspots; but these investigations make it look rather as if some influence on the part of the earth tends, on the contrary, to _extinguish_ them. mrs. maunder, so far, prefers to call the influence thus traced an _apparent_ influence only, for, as she very fairly points out, it seems difficult to attribute a real influence in this matter to the earth, which is so small a thing in comparison not only with the sun, but even with many individual spots. the above investigation was to a certain degree anticipated by mr. henry corder in ; but mrs. maunder's researches cover a much longer period, and the conclusions deduced are of a wider and more defined nature. with regard to its chemical composition, the spectroscope shows us that thirty-nine of the elements which are found upon our earth are also to be found in the sun. of these the best known are hydrogen, oxygen, helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and lead. some elements of the metallic order have, however, not been found there, as, for instance, gold and mercury; while a few of the other class of element, such as nitrogen, chlorine, and sulphur, are also absent. it must not, indeed, be concluded that the elements apparently missing do not exist at all in the solar body. gold and mercury have, in consequence of their great atomic weight, perhaps sunk away into the centre. again, the fact that we cannot find traces of certain other elements, is no real proof of their entire absence. some of them may, for instance, be resolved into even simpler forms, under the unusual conditions which exist in the sun; and so we are unable to trace them with the spectroscope, the experience of which rests on laboratory experiments conducted, at best, in conditions which obtain upon the earth. [ ] on november , , dr. a. rambaut, radcliffe observer at oxford university, noted a prominence which rose to a height of , miles. chapter xiv the inferior planets starting from the centre of the solar system, the first body we meet with is the planet mercury. it circulates at an average distance from the sun of about thirty-six millions of miles. the next body to it is the planet venus, at about sixty-seven millions of miles, namely, about double the distance of mercury from the sun. since our earth comes next again, astronomers call those planets which circulate within its orbit, _i.e._ mercury and venus, the inferior planets, while those which circulate outside it they call the superior planets.[ ] in studying the inferior planets, the circumstances in which we make our observations are so very similar with regard to each, that it is best to take them together. let us begin by considering the various positions of an inferior planet, as seen from the earth, during the course of its journeys round the sun. when furthest from us it is at the other side of the sun, and cannot then be seen owing to the blaze of light. as it continues its journey it passes to the left of the sun, and is then sufficiently away from the glare to be plainly seen. it next draws in again towards the sun, and is once more lost to view in the blaze at the time of its passing nearest to us. then it gradually comes out to view on the right hand, separates from the sun up to a certain distance as before, and again recedes beyond the sun, and is for the time being once more lost to view. to these various positions technical names are given. when the inferior planet is on the far side of the sun from us, it is said to be in _superior conjunction_. when it has drawn as far as it can to the left hand, and is then as east as possible of the sun, it is said to be at its _greatest eastern elongation_. again, when it is passing nearest to us, it is said to be in _inferior conjunction_; and, finally, when it has drawn as far as it can to the right hand, it is spoken of as being at its _greatest western elongation_ (see fig. , p. ). the continual variation in the distance of an interior planet from us, during its revolution around the sun, will of course be productive of great alterations in its apparent size. at superior conjunction it ought, being then farthest away, to show the smallest disc; while at inferior conjunction, being the nearest, it should look much larger. when at greatest elongation, whether eastern or western, it should naturally present an appearance midway in size between the two. [illustration: various positions, and illumination by the sun, of an inferior planet in the course of its orbit. corresponding views of the same situations of an inferior planet as seen from the earth, showing consequent phases and alterations in apparent size. fig. .--orbit and phases of an inferior planet.] from the above considerations one would be inclined to assume that the best time for studying the surface of an interior planet with the telescope is when it is at inferior conjunction, or, nearest to us. but that this is not the case will at once appear if we consider that the sunlight is then falling upon the side away from us, leaving the side which is towards us unillumined. in superior conjunction, on the other hand, the light falls full upon the side of the planet facing us; but the disc is then so small-looking, and our view besides is so dazzled by the proximity of the sun, that observations are of little avail. in the elongations, however, the sunlight comes from the side, and so we see one half of the planet lit up; the right half at eastern elongation, and the left half at western elongation. piecing together the results given us at these more favourable views, we are enabled, bit by bit, to gather some small knowledge concerning the surface of an inferior planet. from these considerations it will be seen at once that the inferior planets show various phases comparable to the waxing and waning of our moon in its monthly round. superior conjunction is, in fact, similar to full moon, and inferior conjunction to new moon; while the eastern and western elongations may be compared respectively to the moon's first and last quarters. it will be recollected how, when these phases were first seen by the early telescopic observers, the copernican theory was felt to be immensely strengthened; for it had been pointed out that if this system were the correct one, the planets venus and mercury, were it possible to see them more distinctly, would of necessity present phases like these when viewed from the earth. it should here be noted that the telescope was not invented until nearly seventy years after the death of copernicus. the apparent swing of an inferior planet from side to side of the sun, at one time on the east side, then passing into and lost in the sun's rays to appear once more on the west side, is the explanation of what is meant when we speak of an _evening_ or a _morning star_. an inferior planet is called an evening star when it is at its eastern elongation, that is to say, on the left-hand of the sun; for, being then on the eastern side, it will set after the sun sets, as both sink in their turn below the western horizon at the close of day. similarly, when such a planet is at its western elongation, that is to say, to the right-hand of the sun, it will go in advance of him, and so will rise above the eastern horizon before the sun rises, receiving therefore the designation of morning star. in very early times, however, before any definite ideas had been come to with regard to the celestial motions, it was generally believed that the morning and evening stars were quite distinct bodies. thus venus, when a morning star, was known to the ancients under the name of phosphorus, or lucifer; whereas they called it hesperus when it was an evening star. since an inferior planet circulates between us and the sun, one would be inclined to expect that such a body, each time it passed on the side nearest to the earth, should be seen as a black spot against the bright solar disc. now this would most certainly be the case were the orbit of an inferior planet in the same plane with the orbit of the earth. but we have already seen how the orbits in the solar system, whether those of planets or of satellites, are by no means in the one plane; and that it is for this very reason that the moon is able to pass time after time in the direction of the sun, at the epoch known as new moon, and yet not to eclipse him save after the lapse of several such passages. transits, then, as the passages of an inferior planet across the sun's disc are called, take place, for the same reason, only after certain regular lapses of time; and, as regards the circumstances of their occurrence, are on a par with eclipses of the sun. the latter, however, happen much more frequently, because the moon passes in the neighbourhood of the sun, roughly speaking, once a month, whereas venus comes to each inferior conjunction at intervals so long apart as a year and a half, and mercury only about every four months. from this it will be further gathered that transits of mercury take place much oftener than transits of venus. until recent years _transits of venus_ were phenomena of great importance to astronomers, for they furnished the best means then available of calculating the distance of the sun from the earth. this was arrived at through comparing the amount of apparent displacement in the planet's path across the solar disc, when the transit was observed from widely separated stations on the earth's surface. the last transit of venus took place in , and there will not be another until the year . _transits of mercury_, on the other hand, are not of much scientific importance. they are of no interest as a popular spectacle; for the dimensions of the planet are so small, that it can be seen only with the aid of a telescope when it is in the act of crossing the sun's disc. the last transit of mercury took place on november , , and there will be another on november , . the first person known to have observed a transit of an inferior planet was the celebrated french philosopher, gassendi. this was the transit of mercury which took place on the th of december . the first time a transit of venus was ever seen, so far as is known, was on the th of november . the observer was a certain jeremiah horrox, curate of hoole, near preston, in lancashire. the transit in question commenced shortly before sunset, and his observations in consequence were limited to only about half-an-hour. horrox happened to have a great friend, one william crabtree, of manchester, whom he had advised by letter to be on the look out for the phenomenon. the weather in crabtree's neighbourhood was cloudy, with the result that he only got a view of the transit for about ten minutes before the sun set. that this transit was observed at all is due entirely to the remarkable ability of horrox. according to the calculations of the great kepler, no transit could take place that year ( ), as the planet would just pass clear of the lower edge of the sun. horrox, however, not being satisfied with this, worked the question out for himself, and came to the conclusion that the planet would _actually_ traverse the lower portion of the sun's disc. the event, as we have seen, proved him to be quite in the right. horrox is said to have been a veritable prodigy of astronomical skill; and had he lived longer would, no doubt, have become very famous. unfortunately he died about two years after his celebrated transit, in his _twenty-second_ year only, according to the accounts. his friend crabtree, who was then also a young man, is said to have been killed at the battle of naseby in . there is an interesting phenomenon in connection with transits which is known as the "black drop." when an inferior planet has just made its way on to the face of the sun, it is usually seen to remain for a short time as if attached to the sun's edge by what looks like a dark ligament (see fig. , p. ). this gives to the planet for the time being an elongated appearance, something like that of a pear; but when the ligament, which all the while keeps getting thinner and thinner, has at last broken, the black body of the planet is seen to stand out round against the solar disc. [illustration: fig. .--the "black drop."] this appearance may be roughly compared to the manner in which a drop of liquid (or, preferably, of some glutinous substance) tends for a while to adhere to an object from which it is falling. when the planet is in turn making its way off the face of the sun, the ligament is again seen to form and to attach it to the sun's edge before its due time. the phenomenon of the black drop, or ligament, is entirely an illusion, and, broadly speaking, of an optical origin. something very similar will be noticed if one brings one's thumb and forefinger _slowly_ together against a very bright background. this peculiar phenomenon has proved one of the greatest drawbacks to the proper observation of transits, for it is quite impossible to note the exact instant of the planet's entrance upon and departure from the solar disc in conditions such as these. the black drop seems to bear a family resemblance, so to speak, to the phenomenon of baily's beads. in the latter instance the lunar peaks, as they approach the sun's edge, appear to lengthen out in a similar manner and bridge the intervening space before their time, thus giving prominence to an effect which otherwise should scarcely be noticeable. the last transit of mercury, which, as has been already stated, took place on november , , was not successfully observed by astronomers in england, on account of the cloudiness of the weather. in france, however, professor moye, of montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully a minute. the transit was also observed in the united states, the reports from which speak of the black drop as very "troublesome." before leaving the subject of transits it should be mentioned that it was in the capacity of commander of an expedition to otaheite, in the pacific, to observe the transit of venus of june , , that captain cook embarked upon the first of his celebrated voyages. in studying the surfaces of venus and mercury with the telescope, observers are, needless to say, very much hindered by the proximity of the sun. venus, when at the greatest elongations, certainly draws some distance out of the glare; but her surface is, even then, so dazzlingly bright, that the markings upon it are difficult to see. mercury, on the other hand, is much duller in contrast, but the disc it shows in the telescope is exceedingly small; and, in addition, when that planet is left above the horizon for a short time after sunset, as necessarily happens after certain intervals, the mists near the earth's surface render observation of it very difficult. until about twenty-five years ago, it was generally believed that both these planets rotated on their axes in about twenty-four hours, a notion, no doubt, originally founded upon an unconscious desire to bring them into some conformity with our earth. but schiaparelli, observing in italy, and percival lowell, in the clear skies of arizona and mexico, have lately come to the conclusion that both planets rotate upon their axes in the same time as they revolve in their orbits,[ ] the result being that they turn one face ever towards the sun in the same manner that the moon turns one face ever towards the earth--a curious state of things, which will be dealt with more fully when we come to treat of our satellite. the marked difference in the brightness between the two planets has already been alluded to. the surface of venus is, indeed, about five times as bright as that of mercury. the actual brightness of mercury is about equivalent to that of our moon, and astronomers are, therefore, inclined to think that it may resemble her in having a very rugged surface and practically no atmosphere. this probable lack of atmosphere is further corroborated by two circumstances. one of these is that when mercury is just about to transit the face of the sun, no ring of diffused light is seen to encircle its disc as would be the case if it possessed an atmosphere. such a lack of atmosphere is, indeed, only to be expected from what is known as the _kinetic theory of gases_. according to this theory, which is based upon the behaviour of various kinds of gas, it is found that these elements tend to escape into space from the surface of bodies whose force of gravitation is weak. hydrogen gas, for example, tends to fly away from our earth, as any one may see for himself when a balloon rises into the air. the gravitation of the earth seems, however, powerful enough to hold down other gases, as, for instance, those of which the air is chiefly composed, namely, oxygen and nitrogen. in due accordance with the kinetic theory, we find the moon and mercury, which are much about the same size, destitute of atmospheres. mars, too, whose diameter is only about double that of the moon, has very little atmosphere. we find, on the other hand, that venus, which is about the same size as our earth, clearly possesses an atmosphere, as just before the planet is in transit across the sun, the outline of its dark body is seen to be surrounded by a bright ring of light. the results of telescopic observation show that more markings are visible on mercury than on venus. the intense brilliancy of venus is, indeed, about the same as that of our white clouds when the sun is shining directly upon them. it has, therefore, been supposed that the planet is thickly enveloped in cloud, and that we do not ever see any part of its surface, except perchance the summit of some lofty mountain projecting through the fleecy mass. with regard to the great brilliancy of venus, it may be mentioned that she has frequently been seen in england, with the naked eye in full sunshine, when at the time of her greatest brightness. the writer has seen her thus at noonday. needless to say, the sky at the moment was intensely blue and clear. the orbit of mercury is very oval, and much more so than that of any other planet. the consequence is that, when mercury is nearest to the sun, the heat which it receives is twice as great as when it is farthest away. the orbit of venus, on the other hand, is in marked contrast with that of mercury, and is, besides, more nearly of a circular shape than that of any of the other planets. venus, therefore, always keeps about the same distance from the sun, and so the heat which she receives during the course of her year can only be subject to very slight variations. [ ] in employing the terms inferior and superior the writer bows to astronomical custom, though he cannot help feeling that, in the circumstances, interior and exterior would be much more appropriate. [ ] this question is, however, uncertain, for some very recent spectroscopic observations of venus seem to show a rotation period of about twenty-four hours. chapter xv the earth we have already seen (in chapter i.) how, in very early times, men naturally enough considered the earth to be a flat plane extending to a very great distance in every direction; but that, as years went on, certain of the greek philosophers suspected it to be a sphere. one or two of the latter are, indeed, said to have further believed in its rotation about an axis, and even in its revolution around the sun; but, as the ideas in question were founded upon fancy, rather than upon any direct evidence, they did not generally attract attention. the small effect, therefore, which these theories had upon astronomy, may well be gathered from the fact that in the ptolemaic system the earth was considered as fixed and at the centre of things; and this belief, as we have seen, continued unaltered down to the days of copernicus. it was, indeed, quite impossible to be certain of the real shape of the earth or the reality of its motions until knowledge became more extended and scientific instruments much greater in precision. we will now consider in detail a few of the more obvious arguments which can be put forward to show that our earth is a sphere. if, for instance, the earth were a plane surface, a ship sailing away from us over the sea would appear to grow smaller and smaller as it receded into the distance, becoming eventually a tiny speck, and fading gradually from our view. this, however, is not at all what actually takes place. as we watch a vessel receding, its hull appears bit by bit to slip gently down over the horizon, leaving the masts alone visible. then, in their turn, the masts are seen to slip down in the same manner, until eventually every trace of the vessel is gone. on the other hand, when a ship comes into view, the masts are the first portions to appear. they gradually rise up from below the horizon, and the hull follows in its turn, until the whole vessel is visible. again, when one is upon a ship at sea, a set of masts will often be seen sticking up alone above the horizon, and these may shorten and gradually disappear from view without the body of the ship to which they belong becoming visible at all. since one knows from experience that there is no _edge_ at the horizon over which a vessel can drop down, the appearance which we have been describing can only be explained by supposing that the surface of the earth is always curving gradually in every direction. the distance at which what is known as the _horizon_ lies away from us depends entirely upon the height above the earth's surface where we happen at the moment to be. a ship which has appeared to sink below the horizon for a person standing on the beach, will be found to come back again into view if he at once ascends a high hill. experiment shows that the horizon line lies at about three miles away for a person standing at the water's edge. the curving of the earth's surface is found, indeed, to be at the rate of eight inches in every mile. now it can be ascertained, by calculation, that a body curving at this rate in every direction must be a globe about miles in diameter. again, the fact that, if not stopped by such insuperable obstacles as the polar ice and snow, those who travel continually in any one direction upon the earth's surface always find themselves back again at the regions from which they originally set out, is additional ground for concluding that the earth is a globe. we can find still further evidence. for instance, in an eclipse of the moon the earth's shadow, when seen creeping across the moon's face, is noted to be _always_ circular in shape. one cannot imagine how such a thing could take place unless the earth were a sphere. also, it is found from observation that the sun, the planets, and the satellites are, all of them, round. this roundness cannot be the roundness of a flat plate, for instance, for then the objects in question would sometimes present their thin sides to our view. it happens, also, that upon the discs which these bodies show, we see certain markings shifting along continually in one direction, to disappear at one side and to reappear again at the other. such bodies must, indeed, be spheres in rotation. the crescent and other phases, shown by the moon and the inferior planets, should further impress the truth of the matter upon us, as such appearances can only be caused by the sunlight falling from various directions upon the surfaces of spherical bodies. another proof, perhaps indeed the weightiest of all, is the continuous manner in which the stars overhead give place to others as one travels about the surface of the earth. when in northern regions the pole star and its neighbours--the stars composing the plough, for instance--are over our heads. as one journeys south these gradually sink towards the northern horizon, while other stars take their place, and yet others are uncovered to view from the south. the regularity with which these changes occur shows that every point on the earth's surface faces a different direction of the sky, and such an arrangement would only be possible if the earth were a sphere. the celebrated greek philosopher, aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that it was so. the idea of the sphericity of the earth does not appear, however, to have been generally accepted until the voyages of the great navigators showed that it could be sailed round. the next point we have to consider is the rotation of the earth about its axis. from the earliest times men noticed that the sky and everything in it appeared to revolve around the earth in one fixed direction, namely, towards what is called the west, and that it made one complete revolution in the period of time which we know as twenty-four hours. the stars were seen to come up, one after another, from below the eastern horizon, to mount the sky, and then to sink in turn below the western horizon. the sun was seen to perform exactly the same journey, and the moon, too, whenever she was visible. one or two of the ancient greek philosophers perceived that this might be explained, either by a movement of the entire heavens around the earth, or by a turning motion on the part of the earth itself. of these diverse explanations, that which supposed an actual movement of the heavens appealed to them the most, for they could hardly conceive that the earth should continually rotate and men not be aware of its movement. the question may be compared to what we experience when borne along in a railway train. we see the telegraph posts and the trees and buildings near the line fly past us one after another in the contrary direction. either these must be moving, or we must be moving; and as we happen to _know_ that it is, indeed, we who are moving, there can be no question therefore about the matter. but it would not be at all so easy to be sure of this movement were one unable to see the objects close at hand displacing themselves. for instance, if one is shut up in a railway carriage at night with the blinds down, there is really nothing to show that one is moving, except the jolting of the train. and even then it is hard to be sure in which direction one is actually travelling. the way we are situated upon the earth is therefore as follows. there are no other bodies sufficiently near to be seen flying past us in turn; our earth spins without a jolt; we and all things around us, including the atmosphere itself, are borne along together with precisely the same impetus, just as all the objects scattered about a railway carriage share in the forward movement of the train. such being the case, what wonder that we are unconscious of the earth's rotation, of which we should know nothing at all, were it not for that slow displacement of the distant objects in the heavens, as we are borne past them in turn. if the night sky be watched, it will be soon found that its apparent turning movement seems to take place around a certain point, which appears as if fixed. this point is known as the north pole of the heavens; and a rather bright star, which is situated very close to this hub of movement, is in consequence called the pole star. for the dwellers in southern latitudes there is also a point in their sky which appears to remain similarly fixed, and this is known as the south pole of the heavens. since, however, the heavens do not turn round at all, but the earth does, it will easily be seen that these apparently stationary regions in the sky are really the points towards which the axis of the earth is directed. the positions on the earth's surface itself, known as the north and south poles, are merely the places where the earth's axis, if there were actually such a thing, would be expected to jut out. the north pole of the earth will thus be situated exactly beneath the north pole of the heavens, and the south pole of the earth exactly beneath the south pole of the heavens. we have seen that the earth rotates upon its imaginary axis once in about every twenty-four hours. this means that everything upon the surface of the earth is carried round once during that time. the measurement around the earth's equator is about , miles; and, therefore, an object situated at the equator must be carried round through a distance of about , miles in each twenty-four hours. everything at the equator is thus moving along at the rapid rate of about miles an hour, or between sixteen and seventeen times as fast as an express train. if, however, one were to take measurements around the earth parallel to the equator, one would find these measurements becoming less and less, according as the poles were approached. it is plain, therefore, that the speed with which any point moves, in consequence of the earth's rotation, will be greatest at the equator, and less and less in the direction of the poles; while at the poles themselves there will be practically no movement, and objects there situated will merely turn round. the considerations above set forth, with regard to the different speeds at which different portions of a rotating globe will necessarily be moving, is the foundation of an interesting experiment, which gives us further evidence of the rotation of our earth. the measurement around the earth at any distance below the surface, say, for instance, at the depth of a mile, will clearly be less than a similar measurement at the surface itself. the speed of a point at the bottom of a mine, which results from the actual rotation of the earth, must therefore be less than the speed of a point at the surface overhead. this can be definitely proved by dropping a heavy object down a mine shaft. the object, which starts with the greater speed of the surface, will, when it reaches the bottom of the mine, be found, as might be indeed expected, to be a little ahead (_i.e._ to the east) of the point which originally lay exactly underneath it. the distance by which the object gains upon this point is, however, very small. in our latitudes it amounts to about an inch in a fall of feet. the great speed at which, as we have seen, the equatorial regions of the earth are moving, should result in giving to the matter there situated a certain tendency to fly outwards. sir isaac newton was the first to appreciate this point, and he concluded from it that the earth must be _bulged_ a little all round the equator. this is, indeed, found to be the case, the diameter at the equator being nearly twenty-seven miles greater than it is from pole to pole. the reader will, no doubt, be here reminded of the familiar comparison in geographies between the shape of the earth and that of an orange. in this connection it is interesting to consider that, were the earth to rotate seventeen times as fast as it does (_i.e._ in one hour twenty-five minutes, instead of twenty-four hours), bodies at the equator would have such a strong tendency to fly outwards that the force of terrestrial gravity acting upon them would just be counterpoised, and they would virtually have _no weight_. and, further, were the earth to rotate a little faster still, objects lying loose upon its surface would be shot off into space. the earth is, therefore, what is technically known as an _oblate spheroid_; that is, a body of spherical shape flattened at the poles. it follows of course from this, that objects at the polar regions are slightly nearer to the earth's centre than objects at the equatorial regions. we have already seen that gravitation acts from the central parts of a body, and that its force is greater the nearer are those central parts. the result of this upon our earth will plainly be that objects in the polar regions will be pulled with a slightly stronger pull, and will therefore _weigh_ a trifle more than objects in the equatorial regions. this is, indeed, found by actual experiment to be the case. as an example of the difference in question, professor young, in his _manual of astronomy_, points out that a man who weighs pounds at the equator would weigh at the pole. in such an experiment the weighing would, however, have to be made with a _spring balance_, and _not with scales_; for, in the latter case, the "weights" used would alter in their weight in exactly the same degree as the objects to be weighed. it used to be thought that the earth was composed of a relatively thin crust, with a molten interior. scientific men now believe, on the other hand, that such a condition cannot after all prevail, and that the earth must be more or less solid all through, except perhaps in certain isolated places where collections of molten matter may exist. the _atmosphere_, or air which we breathe, is in the form of a layer of limited depth which closely envelops the earth. actually, it is a mixture of several gases, the most important being nitrogen and oxygen, which between them practically make up the air, for the proportion of the other gases, the chief of which is carbonic acid gas, is exceedingly small. it is hard to picture our earth, as we know it, without this atmosphere. deprived of it, men at once would die; but even if they could be made to go on living without it by any miraculous means, they would be like unto deaf beings, for they would never hear any sound. what we call _sounds_ are merely vibrations set up in the air, which travel along and strike upon the drum of the ear. the atmosphere is densest near the surface of the earth, and becomes less and less dense away from it, as a result of diminishing pressure of air from above. the greater portion of it is accumulated within four or five miles of the earth's surface. it is impossible to determine exactly at what distance from the earth's surface the air ceases altogether, for it grows continually more and more rarefied. there are, however, two distinct methods of ascertaining the distance beyond which it can be said practically not to exist. one of these methods we get from twilight. twilight is, in fact, merely light reflected to us from those upper regions of the air, which still continue to be illuminated by the sun after it has disappeared from our view below the horizon. the time during which twilight lasts, shows us that the atmosphere must be at least fifty miles high. but the most satisfactory method of ascertaining the height to which the atmosphere extends is from the observation of meteors. it is found that these bodies become ignited, by the friction of passing into the atmosphere, at a height of about miles above the surface of the earth. we thus gather that the atmosphere has a certain degree of density even at this height. it may, indeed, extend as far as about miles. the layer of atmosphere surrounding our earth acts somewhat in the manner of the glass covering of a greenhouse, bottling in the sun's rays, and thus storing up their warmth for our benefit. were this not so, the heat which we get from the sun would, after falling upon the earth, be quickly radiated again into space. it is owing to the unsteadiness of the air that stars are seen to twinkle. a night when this takes place, though it may please the average person, is worse than useless to the astronomer, for the unsteadiness is greatly magnified in the telescope. this twinkling is, no doubt, in a great measure responsible for the conventional "points" with which art has elected to embellish stars, and which, of course, have no existence in fact. the phenomena of _refraction_,[ ] namely, that bending which rays of light undergo, when passing _slant-wise_ from a rare into a dense transparent medium, are very marked with regard to the atmosphere. the denser the medium into which such rays pass, the greater is this bending found to be. since the layer of air around us becomes denser and denser towards the surface of the earth, it will readily be granted that the rays of light reaching our eyes from a celestial object, will suffer the greater bending the lower the object happens to be in the sky. celestial objects, unless situated directly overhead, are thus not seen in their true places, and when nearest to the horizon are most out of place. the bending alluded to is upwards. thus the sun and the moon, for instance, when we see them resting upon the horizon, are actually _entirely_ beneath it. when the sun, too, is sinking towards the horizon, the lower edge of its disc will, for the above reason, look somewhat more raised than the upper. the result is a certain appearance of flattening; which may plainly be seen by any one who watches the orb at setting. in observations to determine the exact positions of celestial objects correction has to be made for the effects of refraction, according to the apparent elevation of these objects in the sky. such effects are least when the objects in question are directly overhead, for then the rays of light, coming from them to the eye, enter the atmosphere perpendicularly, and not at any slant. a very curious effect, due to refraction, has occasionally been observed during a total eclipse of the moon. to produce an eclipse of this kind, _the earth must, of course, lie directly between the sun and the moon_. therefore, when we see the shadow creeping over the moon's surface, the sun should actually be well below the horizon. but when a lunar eclipse happens to come on just about sunset, the sun, although really sunk below the horizon, appears still above it through refraction, and the eclipsed moon, situated, of course, exactly opposite to it in the sky, is also lifted up above the horizon by the same cause. pliny, writing in the first century of the christian era, describes an eclipse of this kind, and refers to it as a "prodigy." the phenomenon is known as a "horizontal eclipse." it was, no doubt, partly owing to it that the ancients took so long to decide that an eclipse of the moon was really caused by the shadow cast by the earth. plutarch, indeed, remarks that it was easy enough to understand that a solar eclipse was caused by the interposition of the moon, but that one could not imagine by the interposition _of what body_ the moon itself could be eclipsed. in that apparent movement of the heavens about the earth, which men now know to be caused by the mere rotation of the earth itself, a slight change is observed to be continually taking place. the stars, indeed, are always found to be gradually drawing westward, _i.e._ towards the sun, and losing themselves one after the other in the blaze of his light, only to reappear, however, on the other side of him after a certain lapse of time. this is equivalent to saying that the sun itself seems always creeping slowly _eastward_ in the heaven. the rate at which this appears to take place is such that the sun finds itself back again to its original position, with regard to the starry background, at the end of a year's time. in other words, the sun seems to make a complete tour of the heavens in the course of a year. here, however, we have another illusion, just as the daily movement of the sky around the earth was an illusion. the truth indeed is, that this apparent movement of the sun eastward among the stars during a year, arises merely from a _continuous displacement of his position_ caused by an actual motion of the earth itself around him in that very time. in a word, it is the earth which really moves around the sun, and not the sun around the earth. the stress laid upon this fundamental point by copernicus, marks the separation of the modern from the ancient view. not that copernicus, indeed, had obtained any real proof that the earth is merely a planet revolving around the sun; but it seemed to his profound intellect that a movement of this kind on the part of our globe was the more likely explanation of the celestial riddle. the idea was not new; for, as we have already seen, certain of the ancient greeks (aristarchus of samos, for example) had held such a view; but their notions on the subject were very fanciful, and unsupported by any good argument. what copernicus, however, really seems to have done was to _insist_ upon the idea that the sun occupied the _centre_, as being more consonant with common sense. no doubt, he was led to take up this position by the fact that the sun appeared entirely of a different character from the other members of the system. the one body in the scheme, which performed the important function of dispenser of light and heat, would indeed be more likely to occupy a position apart from the rest; and what position more appropriate for its purposes than the centre! but here copernicus only partially solved the difficult question. he unfortunately still clung to an ancient belief, which as yet remained unquestioned; _i.e._ the great virtue, one might almost say, the _divineness_, of circular motion. the ancients had been hag-ridden, so to speak, by the circle; and it appeared to them that such a perfectly formed curve was alone fitted for the celestial motions. ptolemy employed it throughout his system. according to him the "planets" (which included, under the ancient view, both the sun and the moon), moved around the earth in circles; but, as their changing positions in the sky could not be altogether accounted for in this way, it was further supposed that they performed additional circular movements, around peculiarly placed centres, during the course of their orbital revolutions. thus the ptolemaic system grew to be extremely complicated; for astronomers did not hesitate to add new circular movements whenever the celestial positions calculated for the planets were found not to tally with the positions observed. in this manner, indeed, they succeeded in doctoring the theory, so that it fairly satisfied the observations made with the rough instruments of pre-telescopic times. although copernicus performed the immense service to astronomy of boldly directing general attention to the central position of the sun, he unfortunately took over for the new scheme the circular machinery of the ptolemaic system. it therefore remained for the famous kepler, who lived about a century after him, to find the complete solution. just as copernicus, for instance, had broken free from tradition with regard to the place of the sun; so did kepler, in turn, break free from the spell of circular motion, and thus set the coping-stone to the new astronomical edifice. this astronomer showed, in fact, that if the paths of the planets around the sun, and of the moon around the earth, were not circles, but _ellipses_, the movements of these bodies about the sky could be correctly accounted for. the extreme simplicity of such an arrangement was far more acceptable than the bewildering intricacy of movement required by the ptolemaic theory. the copernican system, as amended by kepler, therefore carried the day; and was further strengthened, as we have already seen, by the telescopic observations of galileo and the researches of newton into the effects of gravitation. and here a word on the circle, now fallen from its high estate. the ancients were in error in supposing that it stood entirely apart--the curve of curves. as a matter of fact it is merely _a special kind of ellipse_. to put it paradoxically, it is an ellipse which has no ellipticity, an oval without any ovalness! notwithstanding all this, astronomy had to wait yet a long time for a definite proof of the revolution of the earth around the sun. the leading argument advanced by aristotle, against the reality of any movement of the earth, still held good up to about seventy years ago. that philosopher had pointed out that the earth could not move about in space to any great extent, or the stars would be found to alter their apparent places in the sky, a thing which had never been observed to happen. centuries ran on, and instruments became more and more perfect, yet no displacements of stars were noted. in accepting the copernican theory men were therefore obliged to suppose these objects as immeasurably distant. at length, however, between the years and , it was discovered by the prussian astronomer, bessel, that a star known as cygni--that is to say, the star marked in celestial atlases as no. in the constellation of the swan--appeared, during the course of a year, to perform a tiny circle in the heavens, such as would result from a movement on our own part around the sun. since then about forty-three stars have been found to show minute displacements of a similar kind, which cannot be accounted for upon any other supposition than that of a continuous revolution of the earth around the sun. the triumph of the copernican system is now at last supreme. if the axis of the earth stood "straight up," so to speak, while the earth revolved in its orbit, the sun would plainly keep always on a level with the equator. this is equivalent to stating that, in such circumstances, a person at the equator would see it rise each morning exactly in the east, pass through the _zenith_, that is, the point directly overhead of him, at midday, and set in the evening due in the west. as this would go on unchangingly at the equator every day throughout the year, it should be clear that, at any particular place upon the earth, the sun would in these conditions always be seen to move in an unvarying manner across the sky at a certain altitude depending upon the latitude of the place. thus the more north one went upon the earth's surface, the more southerly in the sky would the sun's path lie; while at the north pole itself, the sun would always run round and round the horizon. similarly, the more south one went from the equator the more northerly would the path of the sun lie, while at the south pole it would be seen to skirt the horizon in the same manner as at the north pole. the result of such an arrangement would be, that each place upon the earth would always have one unvarying climate; in which case there would not exist any of those beneficial changes of season to which we owe so much. the changes of season, which we fortunately experience, are due, however, to the fact that the sun does not appear to move across the sky each day at one unvarying altitude, but is continually altering the position of its path; so that at one period of the year it passes across the sky low down, and remains above the horizon for a short time only, while at another it moves high up across the heavens, and is above the horizon for a much longer time. actually, the sun seems little by little to creep up the sky during one half of the year, namely, from mid-winter to mid-summer, and then, just as gradually, to slip down it again during the other half, namely, from mid-summer to mid-winter. it will therefore be clear that every region of the earth is much more thoroughly warmed during one portion of the year than during another, _i.e._ when the sun's path is high in the heavens than when it is low down. once more we find appearances exactly the contrary from the truth. the earth is in this case the real cause of the deception, just as it was in the other cases. the sun does not actually creep slowly up the sky, and then slowly dip down it again, but, owing to the earth's axis being set aslant, different regions of the earth's surface are presented to the sun at different times. thus, in one portion of its orbit, the northerly regions of the earth are presented to the sun, and in the other portion the southerly. it follows of course from this, that when it is summer in the northern hemisphere it is winter in the southern, and _vice versâ_ (see fig. , p. ). [illustration: fig. .--summer and winter.] the fact that, in consequence of this slant of the earth's axis, the sun is for part of the year on the north side of the equator and part of the year on the south side, leads to a very peculiar result. the path of the moon around the earth is nearly on the same plane with the earth's path around the sun. the moon, therefore, always keeps to the same regions of the sky as the sun. the slant of the earth's axis thus regularly displaces the position of both the sun and the moon to the north and south sides of the equator respectively in the manner we have been describing. were the earth, however, a perfect sphere, such change of position would not produce any effect. we have shown, however, that the earth is not a perfect sphere, but that it is bulged out all round the equator. the result is that this bulged-out portion swings slowly under the pulls of solar and lunar gravitation, in response to the displacements of the sun and moon to the north and to the south of it. this slow swing of the equatorial regions results, of course, in a certain slow change of the direction of the earth's axis, so that the north pole does not go on pointing continually to the same region of the sky. the change in the direction of the axis is, however, so extremely slight, that it shows up only after the lapse of ages. the north pole of the heavens, that is, the region of the sky towards which the north pole of the earth's axis points, displaces therefore extremely slowly, tracing out a wide circle, and arriving back again to the same position in the sky only after a period of about , years. at present the north pole of the heavens is quite close to a bright star in the tail of the constellation of the little bear, which is consequently known as the pole star; but in early greek times it was at least ten times as far away from this star as it is now. after some , years the pole will point to the constellation of lyra, and vega, the most brilliant star in that constellation, will then be considered as the pole star. this slow twisting of the earth's axis is technically known as _precession_, or the _precession of the equinoxes_ (see plate xix., p. ). the slow displacement of the celestial pole appears to have attracted the attention of men in very early times, but it was not until the second century b.c. that precession was established as a fact by the celebrated greek astronomer, hipparchus. for the ancients this strange cyclical movement had a mystic significance; and they looked towards the end of the period as the end, so to speak, of a "dispensation," after which the life of the universe would begin anew:-- "magnus ab integro sæclorum nascitur ordo. jam redit et virgo, redeunt saturnia regna; . . . . . . alter erit tum tiphys, et altera quæ vehat argo delectos heroas; erunt etiam altera bella, atque iterum ad trojam magnus mittetur achilles." we have seen that the orbit of the earth is an ellipse, and that the sun is situated at what is called the _focus_, a point not in the middle of the ellipse, but rather towards one of its ends. therefore, during the course of the year the distance of the earth from the sun varies. the sun, in consequence of this, is about , , miles _nearer_ to us in our northern _winter_ than it is in our northern summer, a statement which sounds somewhat paradoxical. this variation in distance, large as it appears in figures, can, however, not be productive of much alteration in the amount of solar heat which we receive, for during the first week in january, when the distance is least, the sun only looks about _one-eighteenth_ broader than at the commencement of july, when the distance is greatest. the great disparity in temperature between winter and summer depends, as we have seen, upon causes of quite another kind, and varies between such wide limits that the effects of this slight alteration in the distance of the sun from the earth may be neglected for practical purposes. the tides are caused by the gravitational pull of the sun and moon upon the water of the earth's surface. of the two, the moon, being so much the nearer, exerts the stronger pull, and therefore may be regarded as the chief cause of the tides. this pull always draws that portion of the water, which happens to be right underneath the moon at the time, into a heap; and there is also a _second_ heaping of water at the same moment _at the contrary side of the earth_, the reasons for which can be shown mathematically, but cannot be conveniently dealt with here. as the earth rotates on its axis each portion of its surface passes beneath the moon, and is swelled up by this pull; the watery portions being, however, the only ones to yield visibly. a similar swelling up, as we have seen, takes place at the point exactly away from the moon. thus each portion of our globe is borne by the rotation through two "tide-areas" every day, and this is the reason why there are two tides during every twenty-four hours. the crest of the watery swelling is known as high tide. the journey of the moon around the earth takes about a month, and this brings her past each place in turn by about fifty minutes later each day, which is the reason why high tide is usually about twenty-five minutes later each time. the moon is, however, not the sole cause of the tides, but the sun, as we have said, has a part in the matter also. when it is new moon the gravitational attractions of both sun and moon are clearly acting together from precisely the same direction, and, therefore, the tide will be pulled up higher than at other times. at full moon, too, the same thing happens; for, although the bodies are now acting from opposite directions, they do not neutralise each other's pulls as one might imagine, since the sun, in the same manner as the moon, produces a tide both under it and also at the opposite side of the earth. thus both these tides are actually increased in height. the exceptionally high tides which we experience at new and full moons are known as _spring tides_, in contradistinction to the minimum high tides, which are known as _neap tides_. the ancients appear to have had some idea of the cause of the tides. it is said that as early as b.c. the chinese noticed that the moon exerted an influence upon the waters of the sea. the greeks and romans, too, had noticed the same thing; and cæsar tells us that when he was embarking his troops for britain the tide was high _because_ the moon was full. pliny went even further than this, in recognising a similar connection between the waters and the sun. from casual observation one is inclined to suppose that the high tide always rises many feet. but that this is not the case is evidenced by the fact that the tides in the midst of the great oceans are only from three to four feet high. however, in the seas and straits around our isles, for instance, the tides rise very many feet indeed, but this is merely owing to the extra heaping up which the large volumes of water undergo in forcing their passage through narrow channels. as the earth, in rotating, is continually passing through these tide-areas, one might expect that the friction thus set up would tend to slow down the rotation itself. such a slowing down, or "tidal drag," as it is called, is indeed continually going on; but the effects produced are so exceedingly minute that it will take many millions of years to make the rotation appreciably slower, and so to lengthen the day. recently it has been proved that the axis of the earth is subject to a very small displacement, or rather, "wobbling," in the course of a period of somewhat over a year. as a consequence of this, the pole shifts its place through a circle of, roughly, a few yards in width during the time in question. this movement is, perhaps, the combined result of two causes. one of these is the change of place during the year of large masses of material upon our earth; such as occurs, for instance, when ice and snow melt, or when atmospheric and ocean currents transport from place to place great bodies of air and water. the other cause is supposed to be the fact that the earth is not absolutely rigid, and so yields to certain strains upon it. in the course of investigation of this latter point the interesting conclusion has been reached by the famous american astronomer, professor simon newcomb, that our globe as a whole is _a little more rigid than steel_. we will bring this chapter to a close by alluding briefly to two strange appearances which are sometimes seen in our night skies. these are known respectively as the zodiacal light and the gegenschein. the _zodiacal light_ is a faint cone-shaped illumination which is seen to extend upwards from the western horizon after evening twilight has ended, and from the eastern horizon before morning twilight has begun. it appears to rise into the sky from about the position where the sun would be at that time. the proper season of the year for observing it during the evening is in the spring, while in autumn it is best seen in the early morning. in our latitudes its light is not strong enough to render it visible when the moon is full, but in the tropics it is reported to be very bright, and easily seen in full moonlight. one theory regards it as the reflection of light from swarms of meteors revolving round the sun; another supposes it to be a very rarefied extension of the corona. the _gegenschein_ (german for "counter-glow") is a faint oval patch of light, seen in the sky exactly opposite to the place of the sun. it is usually treated of in connection with the zodiacal light, and one theory regards it similarly as of meteoric origin. another theory, however--that of mr. evershed--considers it a sort of _tail_ to the earth (like a comet's tail) composed of hydrogen and helium--the two _lightest_ gases we know--driven off from our planet in the direction contrary to the sun. [ ] every one knows the simple experiment in which a coin lying at the bottom of an empty basin, and hidden from the eye by its side, becomes visible when a certain quantity of water has been poured in. this is an example of refraction. the rays of light coming from the coin ought _not_ to reach the eye, on account of the basin's side being in the way; yet by the action of the water they are _refracted_, or bent over its edge, in such a manner that they do. chapter xvi the moon what we call the moon's "phases" are merely the various ways in which we see the sun shining upon her surface during the course of her monthly revolutions around the earth (see fig. , p. ). when she passes in the neighbourhood of the sun all his light falls upon that side which is turned away from us, and so the side which is turned towards us is unillumined, and therefore invisible. when in this position the moon is spoken of as _new_. as she continues her motion around the earth, she draws gradually to the east of the sun's place in the sky. the sunlight then comes somewhat from the side; and so we see a small portion of the right side of the lunar disc illuminated. this is the phase known as the _crescent_ moon. as she moves on in her orbit more and more of her illuminated surface is brought into view; and so the crescent of light becomes broader and broader, until we get what is called half-moon, or _first quarter_, when we see exactly one-half of her surface lit up by the sun's rays. as she draws still further round yet more of her illuminated surface is brought into view, until three-quarters of the disc appear lighted up. she is then said to be _gibbous_. eventually she moves round so that she faces the sun completely, and the whole of her disc appears illuminated. she is then spoken of as _full_. when in this position it is clear that she is on the contrary side of the earth to the sun, and therefore rises about the same time that he is setting. she is now, in fact, at her furthest from the sun. [illustration: direction from which the sun's rays are coming. various positions and illumination of the mooon by the sun during her revolution around the earth. the corresponding positions as viewed from the earth, showing the consequent phases. fig. .--orbit and phases of the moon.] after this, the motion of the moon in her orbit carries her on back again in the direction of the sun. she thus goes through her phases as before, only these of course are _in the reverse order_. the full phase is seen to give place to the gibbous, and this in turn to the half-moon and to the crescent; after which her motion carries her into the neighbourhood of the sun, and she is once more new, and lost to our sight in the solar glare. following this she draws away to the east of the sun again, and the old order of phases repeat themselves as before. the early babylonians imagined that the moon had a bright and a dark side, and that her phases were caused by the bright side coming more and more into view during her movement around the sky. the greeks, notably aristotle, set to work to examine the question from a mathematical standpoint, and came to the conclusion that the crescent and other appearances were such as would necessarily result if the moon were a dark body of spherical shape illumined merely by the light of the sun. although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures--advertisement posters, for instance--in which stars appear _within_ the horns of a crescent moon! can it be that there are to-day educated persons who believe that the moon is a thing which _grows_ to a certain size and then wastes away again; who, in fact, do not know that the entire body of the moon is there all the while? when the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. this appearance is popularly known as the "old moon in the new moon's arms." the dark part of her surface must, indeed, be to some degree illumined, or we should not be able to see it at all. whence then comes the light which illumines it, since it clearly cannot come from the sun? the riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. he would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sunlight. the lunar landscape all around would, therefore, be bathed in what to _him_ would be "earthlight," which of course takes the place there of what _we_ call moonlight. if, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. it is considered, indeed, to be some twenty times brighter. it is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth. the ancients were greatly exercised in their minds to account for this "earthlight," or "earthshine," as it is also called. posidonius ( - b.c.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. it was not, however, until the fifteenth century that the correct solution was arrived at. [illustration: one side of the moon only is ever presented to the earth. this side is here indicated by the letters s.f.e. (side facing earth). by placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth. fig. .--the rotation of the moon on her axis.] perhaps the most remarkable thing which one notices about the moon is that she always turns the same side towards us, and so we never see her other side. one might be led from this to jump to the conclusion that she does not rotate upon an axis, as do the other bodies which we see; but, paradoxical as it may appear, the fact that she turns one face always towards the earth, is actually a proof that she _does_ rotate upon an axis. the rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolves around the earth (see fig. ). in order to understand the matter clearly, let the reader place an object in the centre of a room and walk around it once, _keeping his face turned towards it the whole time_, while he is doing this, it is evident that he will face every one of the four walls of the room in succession. now in order to face each of the four walls of a room in succession one would be obliged _to turn oneself entirely round_. therefore, during the act of walking round an object with his face turned directly towards it, a person at the same time turns his body once entirely round. in the long, long past the moon must have turned round much faster than this. her rate of rotation has no doubt been slowed down by the action of some force. it will be recollected how, in the course of the previous chapter, we found that the tides were tending, though exceedingly gradually, to slow down the rotation of the earth upon its axis. but, on account of the earth's much greater mass, the force of gravitation exercised by it upon the surface of the moon is, of course, much more powerful than that which the moon exercises upon the surface of the earth. the tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. it is, in consequence, probable that such a tidal drag, extending over a very long period of time, has resulted in slowing down the moon's rotation to its present rate. the fact that we never see but one side of the moon has given rise from time to time to fantastic speculations with regard to the other side. some, indeed, have wished to imagine that our satellite is shaped like an egg, the more pointed end being directed away from us. we are here, of course, faced with a riddle, which is all the more tantalising from its appearing for ever insoluble to men, chained as they are to the earth. however, it seems going too far to suppose that any abnormal conditions necessarily exist at the other side of the moon. as a matter of fact, indeed, small portions of that side are brought into our view from time to time in consequence of slight irregularities in the moon's movement; and these portions differ in no way from those which we ordinarily see. on the whole, we obtain a view of about per cent. of the entire lunar surface; that is to say, a good deal more than one-half. the actual diameter of the moon is about miles, which is somewhat more than one-quarter the diameter of the earth. for a satellite, therefore, she seems very large compared with her primary, the earth; when we consider that jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of jupiter. furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year. seen from space, therefore, she would not give the impression of a circling body, as other satellites do. her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its orbit. in view of all this, some astronomers are inclined to regard the earth and moon rather as a "double planet" than as a system of planet and satellite. when the moon is full she attracts more attention perhaps than in any of her other phases. the moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. the sun _appears_ to go once entirely round the sky in the course of a year, and the moon performs the same journey in the space of about a month. the moon, when full, having got half-way round this journey, occupies, therefore, that region of the sky which the sun itself will occupy half a year later. thus in winter the full moon will be found roughly to occupy the sun's summer position in the sky, and in summer the sun's winter position. it therefore follows that the full moon in winter time is high up in the heavens, while in summer time it is low down. we thus get the greatest amount of full moonlight when it is the most needed. the great french astronomer, laplace, being struck by the fact that the "lesser light" did not rule the night to anything like the same extent that the "greater light" ruled the day, set to work to examine the conditions under which it might have been made to do so. the result of his speculations showed that if the moon were removed to such a distance that she took a year instead of a month to revolve around the earth; and if she were started off in her orbit at full moon, she would always continue to remain full--a great advantage for us. whewell, however, pointed out that in order to get the moon to move with the requisite degree of slowness, she would have to revolve so far from the earth that she would only look one-sixteenth as large as she does at present, which rather militates against the advantage laplace had in mind! finally, however, it was shown by m. liouville, in , that the position of a _perennial full moon_, such as laplace dreamed of, would be unstable--that is to say, the body in question could not for long remain undisturbed in the situation suggested (see fig. , p. ). [illustration: various positions of laplace's "moon" with regard to the earth and sun during the course of a year. the same positions of laplace's "moon," arranged around the earth, show that it would make only one revolution in a year. fig. .--laplace's "perennial full moon."] there is a well-known phenomenon called _harvest moon_, concerning the nature of which there seems to be much popular confusion. an idea in fact appears to prevail among a good many people that the moon is a harvest moon when, at rising, it looks bigger and redder than usual. such an appearance has, however, nothing at all to say to the matter; for the moon always _looks_ larger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, when there is more mist and fog about than there is in summer. what astronomers actually term the harvest moon is, indeed, something entirely different from this. about the month of september the slant at which the full moon comes up from below the horizon happens to be such that, during several evenings together, she _rises almost at the same hour_, instead of some fifty minutes later, as is usually the case. as the harvest is being gathered in about that time, it has come to be popularly considered that this is a provision of nature, according to which the sunlight is, during several evenings, replaced without delay by more or less full-moonlight, in order that harvesters may continue their work straight on into the night, and not be obliged to break off after sunset to wait until the moon rises. the same phenomenon is almost exactly repeated a month later, but by reason of the pursuits then carried on it is known as the "hunter's moon." in this connection should be mentioned that curious phenomenon above alluded to, and which seems to attract universal notice, namely, that the moon _looks much larger when near the horizon_--at its rising, for instance, than when higher up in the sky. this seeming enlargement is, however, by no means confined to the moon. that the sun also looks much larger when low down in the sky than when high up, seems to strike even the most casual watcher of a sunset. the same kind of effect will, indeed, be noted if close attention be paid to the stars themselves. a constellation, for instance, appears more spread out when low down in the sky than when high up. this enlargement of celestial objects when in the neighbourhood of the horizon is, however, only _apparent_ and not real. it must be entirely an _illusion_; for the most careful measurements of the discs of the sun and of the moon fail to show that the bodies are any larger when near the horizon than when high up in the sky. in fact, if there be any difference in measurements with regard to the moon, it will be found to be the other way round; for her disc, when carefully measured, is actually the slightest degree _greater_ when _high_ in the sky, than when low down. the reason for this is that, on account of the rotundity of the earth's surface, she is a trifle nearer the observer when overhead of him. this apparent enlargement of celestial objects, when low down in the sky, is granted on all sides to be an illusion; but although the question has been discussed with animation time out of mind, none of the explanations proposed can be said to have received unreserved acceptance. the one which usually figures in text-books is that we unconsciously compare the sun and moon, when low down in the sky, with the terrestrial objects in the same field of view, and are therefore inclined to exaggerate the size of these orbs. some persons, on the other hand, imagine the illusion to have its source in the structure of the human eye; while others, again, put it down to the atmosphere, maintaining that the celestial objects in question _loom_ large in the thickened air near the horizon, in the same way that they do when viewed through fog or mist. the writer[ ] ventures, however, to think that the illusion has its origin in our notion of the shape of the celestial vault. one would be inclined, indeed, to suppose that this vault ought to appear to us as the half of a hollow sphere; but he maintains that it does not so appear, as a consequence of the manner in which the eyes of men are set quite close together in their heads. if one looks, for instance, high up in the sky, the horizon cannot come within the field of view, and so there is nothing to make one think that the expanse then gazed upon is other than quite _flat_--let us say like the ceiling of a room. but, as the eyes are lowered, a portion of the _encircling_ horizon comes gradually into the field of view, and the region of the sky then gazed upon tends in consequence to assume a _hollowed-out_ form. from this it would seem that our idea of the shape of the celestial vault is, that it is _flattened down over our heads and hollowed out all around in the neighbourhood of the horizon_ (see fig. , p. ). now, as a consequence of their very great distance, all the objects in the heavens necessarily appear to us to move as if they were placed on the background of the vault; the result being that the mind is obliged to conceive them as expanded or contracted, in its unconscious attempts to make them always fill their due proportion of space in the various parts of this abnormally shaped sky. from such considerations the writer concludes that the apparent enlargement in question is merely the natural consequence of the idea we have of the shape of the celestial vault--an idea gradually built up in childhood, to become later on what is called "second nature." and in support of this contention, he would point to the fact that the enlargement is not by any means confined to the sun and moon, but is every whit as marked in the case of the constellations. to one who has not noticed this before, it is really quite a revelation to compare the appearance of one of the large constellations (orion, for instance) when high up in the sky and when low down. the widening apart of the various stars composing the group, when in the latter position, is very noticeable indeed. [illustration: fig. .--illustrating the author's explanation of the apparent enlargement of celestial objects.] further, if a person were to stand in the centre of a large dome, he would be exactly situated as if he were beneath the vaulted heaven, and one would consequently expect him to suffer the same illusion as to the shape of the dome. objects fixed upon its background would therefore appear to him under the same conditions as objects in the sky, and the illusions as to their apparent enlargement should hold good here also. some years ago a belgian astronomer, m. stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very conditions just detailed. to various portions of the inner surface of a large dome he attached pairs of electric lights; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down! he does not, however, seem to have sought for the cause in the vaulted expanse. on the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us estimate objects as larger when in front than when overhead. in connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. even skilled observers find themselves in error in attempting to do so. this seems to bear out the writer's contention that the form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments. before leaving this question, it should also be mentioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. the full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc. [illustration: plate viii. the moon from a photograph taken at the paris observatory by m.p. puiseux. (page )] the moon is just too far off to allow us to see the actual detail on her surface with the naked eye. when thus viewed she merely displays a patchy appearance,[ ] and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term "man in the moon." an examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye. even with an ordinary opera-glass, an observer will be able to note a good deal of detail upon the lunar disc. if it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "cain and his dog," the "man in the moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations (see plate viii., p. ). the dark areas retain even to the present day their ancient name of "seas," for galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago; as, for instance, "sea of showers," "bay of rainbows," "lake of dreams."[ ] the improved telescopes of later times showed, however, that they were not really seas (there is no water on the moon), but merely areas of darker material. the crater-like formations above alluded to are the "lunar mountains." a person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. but once he has realised this novel point of view, he will no doubt marvel at the formations which lie scattered as it were at his feet. the type of lunar mountain is indeed in striking contrast to the terrestrial type. on our earth the range-formation is supreme; on the moon the crater-formation is the rule, and is so-called from analogy to our volcanoes. a typical lunar crater may be described as a circular wall, enclosing a central plain, or "floor," which is often much depressed below the level of the surface outside. these so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic proportions. for instance, the central plain of one of them, known as ptolemæus,[ ] is about miles across, while that of plato is about . the walls of craters often rise to great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. nevertheless, a person posted at the centre of one of the larger craters might be surprised to find that he could not see the encompassing crater-walls, which would in every direction be below his horizon. this would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth. [illustration: plate ix. map of the moon, showing the principal "craters," mountain ranges, and "seas" in this, as in the other plates of the moon, the _south_ will be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seen _inverted_. (page )] we have mentioned ptolemæus as among the very large craters, or ring-mountains, on the moon. its encompassing walls rise to nearly , feet, and it has the further distinction of being almost in the centre of the lunar disc. there are, however, several others much wider, but they are by no means in such a conspicuous position. for instance, schickard, close to the south-eastern border, is nearly miles in diameter, and its wall rises in one point to over , feet. grimaldi, almost exactly at the east point, is nearly as large as schickard. another crater, clavius, situated near the south point, is about miles across; while its neighbour bailly--named after a famous french astronomer of the eighteenth century--is , and the largest of those which we can see (see plate ix., p. ). many of the lunar craters encroach upon one another; in fact there is not really room for them all upon the visible hemisphere of the moon. about , have been mapped; but this is only a small portion, for according to the american astronomer, professor w.h. pickering, there are more than , in all. notwithstanding the fact that the crater is the type of mountain associated in the mind with the moon, it must not be imagined that upon our satellite there are no mountains at all of the terrestrial type. there are indeed many isolated peaks, but strangely enough they are nearly always to be found in the centres of craters. some of these peaks are of great altitude, that in the centre of the crater copernicus being over , feet high. a few mountain ranges also exist; the best known of which are styled, the lunar alps and lunar apennines (see plate x., p. ). since the _mass_ of the moon is only about one-eightieth that of the earth, it will be understood that the force of gravity which she exercises is much less. it is calculated that, at her surface, this is only about one-sixth of what we experience. a man transported to the moon would thus be able to jump _six times as high_ as he can here. a building could therefore be six times as tall as upon our earth, without causing any more strain upon its foundations. it should not, then, be any subject for wonder, that the highest peaks in the lunar apennines attain to such heights as , feet. such a height, upon a comparatively small body like the moon, for her _volume_ is only one-fiftieth that of the earth, is relatively very much in excess of the , feet of himalayan structure, mount everest, the boast of our planet, miles across! high as are the lunar apennines, the highest peaks on the moon are yet not found among them. there is, for instance, on the extreme southern edge of the lunar disc, a range known as the leibnitz mountains; several peaks of which rise to a height of nearly , feet, one peak in particular being said to attain to , feet (see plate ix., p. ). [illustration: plate x. one of the most interesting regions on the moon we have here (see "map," plate ix., p. ) the mountain ranges of the apennines, the caucasus and the alps; also the craters plato, aristotle, eudoxus, cassini, aristillus, autolycus, archimedes and linné. the crater linné is the very bright spot in the dark area at the upper left hand side of the picture. from a photograph taken at the paris observatory by m.m. loewy and puiseux. (page )] but the reader will surely ask the question: "how is it possible to determine the actual height of a lunar mountain, if one cannot go upon the moon to measure it?" the answer is, that we can calculate its height from noting the length of the shadow which it casts. any one will allow that the length of a shadow cast by the sun depends upon two things: firstly, upon the height of the object which causes the shadow, and secondly, upon the elevation of the sun at the moment in the sky. the most casual observer of nature upon our earth can scarcely have failed to notice that shadows are shortest at noonday, when the sun is at its highest in the sky; and that they lengthen out as the sun declines towards its setting. here, then, we have the clue. to ascertain, therefore, the height of a lunar mountain, we have first to consider at what elevation the sun is at that moment above the horizon of the place where the mountain in question is situated. then, having measured the actual length in miles of the shadow extended before us, all that is left is to ask ourselves the question: "what height must an object be whose shadow cast by the sun, when at that elevation in the sky, will extend to this length?" there is no trace whatever of water upon the moon. the opinion, indeed, which seems generally held, is that water has never existed upon its surface. erosions, sedimentary deposits, and all those marks which point to a former occupation by water are notably absent. similarly there appears to be no atmosphere on the moon; or, at any rate, such an excessively rare one, as to be quite inappreciable. of this there are several proofs. for instance, in a solar eclipse the moon's disc always stands out quite clear-cut against that of the sun. again during occultations, stars disappear behind the moon with a suddenness, which could not be the case were there any appreciable atmosphere. lastly, we see no traces of twilight upon the lunar surface, nor any softening at the edges of shadows; both which effects would be apparent if there were an atmosphere. the moon's surface is rough and rocky, and displays no marks of the "weathering" that would necessarily follow, had it possessed anything of an atmosphere in the past. this makes us rather inclined to doubt that it ever had one at all. supposing, however, that it did possess an atmosphere in the past, it is interesting to inquire what may have become of it. in the first place it might have gradually disappeared, in consequence of the gases which composed it uniting chemically with the materials of which the lunar body is constructed; or, again, its constituent gases may have escaped into space, in accordance with the principles of that kinetic theory of which we have already spoken. the latter solution seems, indeed, the most reasonable of the two, for the force of gravity at the lunar surface appears too weak to hold down any known gases. this argument seems also to dispose of the question of absence of water; for dr. george johnstone stoney, in a careful investigation of the subject, has shown that the liquid in question, when in the form of vapour, will escape from a planet if its mass is less than _one-fourth_ that of our earth. and the mass of the moon is very much less than this; indeed only the _one-eightieth_, as we have already stated. in consequence of this lack of atmosphere, the condition of things upon the moon will be in marked contrast to what we experience upon the earth. the atmosphere here performs a double service in shielding us from the direct rays of the sun, and in bottling the heat as a glass-house does. on the moon, however, the sun beats down in the day-time with a merciless force; but its rays are reflected away from the surface as quickly as they are received, and so the cold of the lunar night is excessive. it has been calculated that the day temperature on the moon may, indeed, be as high as our boiling-point, while the night temperature may be more than twice as low as the greatest cold known in our arctic regions. that a certain amount of solar heat is reflected to us from the moon is shown by the sharp drop in temperature which certain heat-measuring instruments record when the moon becomes obscured in a lunar eclipse. the solar heat which is thus reflected to us by the moon is, however, on the whole extremely small; more light and heat, indeed, reach us _direct_ from the sun in half a minute than we get by _reflection_ from the moon during the entire course of the year. with regard to the origin of the lunar craters there has been much discussion. some have considered them to be evidence of violent volcanic action in the dim past; others, again, as the result of the impact of meteorites upon the lunar surface, when the moon was still in a plastic condition; while a third theory holds that they were formed by the bursting of huge bubbles during the escape into space of gases from the interior. the question is, indeed, a very difficult one. though volcanic action, such as would result in craters of the size of ptolemæus, is hard for us to picture, and though the lone peaks which adorn the centres of many craters have nothing reminiscent of them in our terrestrial volcanoes, nevertheless the volcanic theory seems to receive more favour than the others. in addition to the craters there are two more features which demand notice, namely, what are known as _rays_ and _rills_. the rays are long, light-coloured streaks which radiate from several of the large craters, and extend to a distance of some hundreds of miles. that they are mere markings on the surface is proved by the fact that they cast no shadows of any kind. one theory is, that they were originally great cracks which have been filled with lighter coloured material, welling up from beneath. the rills, on the other hand, are actually fissures, about a mile or so in width and about a quarter of a mile in depth. the rays are seen to the best advantage in connection with the craters tycho and copernicus (see plate xi., p. ). in consequence of its fairly forward position on the lunar disc, and of the remarkable system of rays which issue from it like spokes from the axle of a wheel, tycho commands especial attention. the late rev. t.w. webb, a famous observer, christened it, very happily, the "metropolitan crater of the moon." [illustration: plate xi. the moon the systems of rays from the craters tycho, copernicus and kepler are well shown here. from a photograph taken at the paris observatory by m.p. puiseux. (page )] a great deal of attention is, and has been, paid by certain astronomers to the moon, in the hope of finding out if any changes are actually in progress at present upon her surface. sir william herschel, indeed, once thought that he saw a lunar volcano in eruption, but this proved to be merely the effect of the sunlight striking the top of the crater aristarchus, while the region around it was still in shadow--sunrise upon aristarchus, in fact! no change of any real importance has, however, been noted, although it is suspected that some minor alterations have from time to time taken place. for instance, slight variations of tint have been noticed in certain areas of the lunar surface. professor w.h. pickering puts forward the conjecture that these may be caused by the growth and decay of some low form of vegetation, brought into existence by vapours of water, or carbonic acid gas, making their way out from the interior through cracks near at hand. again, during the last hundred years one small crater known as linné (linnæus), situated in the mare serenitatis (sea of serenity), has appeared to undergo slight changes, and is even said to have been invisible for a while (see plate x., p. ). it is, however, believed that the changes in question may be due to the varying angles at which the sunlight falls upon the crater; for it is an understood fact that the irregularities of the moon's motion give us views of her surface which always differ slightly. the suggestion has more than once been put forward that the surface of the moon is covered with a thick layer of ice. this is generally considered improbable, and consequently the idea has received very little support. it first originated with the late mr. s.e. peal, an english observer of the moon, and has recently been resuscitated by the german observer, herr fauth. the most unfavourable time for telescopic study of the moon is when she is full. the sunlight is then falling directly upon her visible hemisphere, and so the mountains cast no shadows. we thus do not get that impression of hill and hollow which is so very noticeable in the other phases. the first map of the moon was constructed by galileo. tobias mayer published another in ; while during the nineteenth century greatly improved ones were made by beer and mädler, schmidt, neison and others. in , professor w.h. pickering brought out a complete photographic lunar atlas; and a similar publication has recently appeared, the work of mm. loewy and puiseux of the observatory of paris. the so-called "seas" of the moon are, as we have seen, merely dark areas, and there appears to be no proof that they were ever occupied by any liquid. they are for the most part found in the _northern_ portion of the moon; a striking contrast to our seas and oceans, which take up so much of the _southern_ hemisphere of the earth. there are many erroneous ideas popularly held with regard to certain influences which the moon is supposed to exercise upon the earth. for instance, a change in the weather is widely believed to depend upon a change in the moon. but the word "change" as here used is meaningless, for the moon is continually changing her phase during the whole of her monthly round. besides, the moon is visible over a great portion of the earth _at the same moment_, and certainly all the places from which it can then be seen do not get the same weather! further, careful observations, and records extending over the past one hundred years and more, fail to show any reliable connection between the phases of the moon and the condition of the weather. it has been stated, on very good authority, that no telescope ever shows the surface of the moon as clearly as we could see it with the naked eye were it only miles distant from us. supposing, then, that we were able to approach our satellite, and view it without optical aid at such comparatively close quarters, it is interesting to consider what would be the smallest detail which our eye could take in. the question of the limit of what can be appreciated with the naked eye is somewhat uncertain, but it appears safe to say that at a distance of miles the _minutest speck_ visible would have to be _at least_ some yards across. atmosphere and liquid both wanting, the lunar surface must be the seat of an eternal calm; where no sound breaks the stillness and where change, as we know it, does not exist. the sun beats down upon the arid rocks, and inky shadows lie athwart the valleys. there is no mellowing of the harsh contrasts. we cannot indeed absolutely affirm that life has no place at all upon this airless and waterless globe, since we know not under what strange conditions it may manifest its presence; and our most powerful telescopes, besides, do not bring the lunar surface sufficiently near to us to disprove the existence there of even such large creatures as disport themselves upon our planet. still, we find it hard to rid ourselves of the feeling that we are in the presence of a dead world. on she swings around the earth month after month, with one face ever turned towards us, leaving a certain mystery to hang around that hidden side, the greater part of which men can never hope to see. the rotation of the moon upon her axis--the lunar day--has become, as we have seen, equal to her revolution around the earth. an epoch may likewise eventually be reached in the history of our own planet, when the length of the terrestrial day has been so slowed down by tidal friction that it will be equal to the year. then will the earth revolve around the central orb, with one side plunged in eternal night and the other in eternal sunshine. but such a vista need not immediately distress us. it is millions of years forward in time. [ ] _journal of the british astronomical association_, vol. x. ( - ), nos. and . [ ] certain of the ancient greeks thought the markings on the moon to be merely the reflection of the seas and lands of our earth, as in a badly polished mirror. [ ] mare imbrium, sinus iridum, lacus somniorum. [ ] the lunar craters have, as a rule, received their names from celebrated persons, usually men of science. this system of nomenclature was originated by riccioli, in . chapter xvii the superior planets having, in a previous chapter, noted the various aspects which an inferior planet presents to our view, in consequence of its orbit being nearer to the sun than the orbit of the earth, it will be well here to consider in the same way the case of a superior planet, and to mark carefully the difference. to begin with, it should be quite evident that we cannot ever have a transit of a superior planet. the orbit of such a body being entirely _outside_ that of the earth, the body itself can, of course, never pass between us and the sun. a superior planet will be at its greatest distance from us when on the far side of the sun. it is said then to be in _conjunction_. as it comes round in its orbit it eventually passes, so to speak, at the _back_ of us. it is then at its nearest, or in _opposition_, as this is technically termed, and therefore in the most favourable position for telescopic observation of its surface. being, besides, seen by us at that time in the direction of the heavens exactly opposite to where the sun is, it will thus at midnight be high up in the south side of the sky, a further advantage to the observer. last of all, a superior planet cannot show crescent shapes like an interior; for whether it be on the far side of the sun, or behind us, or again to our right or left, the sunlight must needs appear to fall more or less full upon its face. the planetoid eros the nearest to us of the superior planets is the tiny body, eros, which, as has been already stated, was discovered so late as the year . in point of view, however, of its small size, it can hardly be considered as a true planet, and the name "planetoid" seems much more appropriate to it. eros was not discovered, like uranus, in the course of telescopic examination of the heavens, nor yet, like neptune, as the direct result of difficult calculations, but was revealed by the impress of its light upon a photographic plate, which had been exposed for some length of time to the starry sky. since many of the more recent additions to the asteroids have been discovered in the same manner, we shall have somewhat more to say about this special employment of photography when we come to deal with those bodies later on. the path of eros around the sun is so very elliptical, or, to use the exact technical term, so very "eccentric," that the planetoid does not keep all the time entirely in the space between our orbit and that of mars, which latter happens to be the next body in the order of planetary succession outwards. in portions of its journey eros, indeed, actually goes outside the martian orbit. the paths of the planetoid and of mars are, however, _not upon the same plane_, so the bodies always pass clear of each other, and there is thus as little chance of their dashing together as there would be of trains which run across a bridge at an upper level, colliding with those which pass beneath it at a lower level. when eros is in opposition, it comes within about - / million miles of our earth, and, after the moon, is therefore by a long way our nearest neighbour in space. it is, however, extremely small, not more, perhaps, than twenty miles in diameter, and is subject to marked variations in brightness, which do not appear up to the present to meet with a satisfactory explanation. but, insignificant as is this little body, it is of great importance to astronomy; for it happens to furnish the best method known of calculating the sun's distance from our earth--a method which galle, in , and sir david gill, in , suggested that asteroids might be employed for, and which has in consequence supplanted the old one founded upon transits of venus. the sun's distance is now an ascertained fact to within , miles, or less than half the distance of the moon. the planet mars we next come to the planet mars. this body rotates in a period of slightly more than twenty-four hours. the inclination, or slant, of its axis is about the same as that of the earth, so that, putting aside its greater distance from the sun, the variations of season which it experiences ought to be very much like ours. the first marking detected upon mars was the notable one called the syrtis major, also known, on account of its shape, as the hour-glass sea. this observation was made by the famous huyghens in ; and, from the movement of the marking in question across the disc, he inferred that the planet rotated on its axis in a period of about twenty-four hours. there appears to be very little atmosphere upon mars, the result being that we almost always obtain a clear view of the detail on its surface. indeed, it is only to be expected from the kinetic theory that mars could not retain much of an atmosphere, as the force of gravity at its surface is less than one-half of what we experience upon the earth. it should here be mentioned that recent researches with the spectroscope seem to show that, whatever atmosphere there may be upon mars, its density at the surface of the planet cannot be more than the one-fourth part of the density of the air at the surface of the earth. professor lowell, indeed, thinks it may be more rarefied than that upon our highest mountain-tops. seen with the naked eye, mars appears of a red colour. viewed in the telescope, its surface is found to be in general of a ruddy hue, varied here and there with darker patches of a bluish-green colour. these markings are permanent, and were supposed by the early telescopic observers to imply a distribution of the planet's surface into land and water, the ruddy portions being considered as continental areas (perhaps sandy deserts), and the bluish-green as seas. the similarity to our earth thus suggested was further heightened by the fact that broad white caps, situated at the poles, were seen to vary with the planet's seasons, diminishing greatly in extent during the martian summer (the southern cap in even disappearing altogether), and developing again in the martian winter.[ ] readers of oliver wendell holmes will no doubt recollect that poet's striking lines:-- "the snows that glittered on the disc of mars have melted, and the planet's fiery orb rolls in the crimson summer of its year." a state of things so strongly analogous to what we experience here, naturally fired the imaginations of men, and caused them to look on mars as a world like ours, only upon a much smaller scale. being smaller, it was concluded to have cooled quicker, and to be now long past its prime; and its "inhabitants" were, therefore, pictured as at a later stage of development than the inhabitants of our earth. notwithstanding the strong temptation to assume that the whiteness of the martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. the deposition of water in the form of snow, or even of hoar frost, would at least imply that the atmosphere of mars should now and then display traces of aqueous vapour, which it does not appear to do.[ ] it has, indeed, been suggested that the whiteness may not after all be due to this cause, but to carbonic acid gas (carbon dioxide), which is known to freeze at a _very low_ temperature. the suggestion is plainly based upon the assumption that, as mars is so much further from the sun than we are, it would receive much less heat, and that the little thus received would be quickly radiated away into space through lack of atmosphere to bottle it in. we now come to those well-known markings, popularly known as the "canals" of mars, which have been the subject of so much discussion since their discovery thirty years ago. it was, in fact, in the year , when mars was in opposition, and thus at its nearest to us, that the famous italian astronomer, schiaparelli, announced to the world that he had found that the ruddy areas, thought to be continents, were intersected by a network of straight dark lines. these lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty to sixty miles in width. he christened the lines _channels_, the italian word for which, "canali," was unfortunately translated into english as "canals." the analogy, thus accidentally suggested, gave rise to the idea that they might be actual waterways.[ ] in the winter of - , when mars was again in opposition, schiaparelli further announced that he had found some of these lines doubled; that is to say, certain of them were accompanied by similar lines running exactly parallel at no great distance away. there was at first a good deal of scepticism on the subject of schiaparelli's discoveries, but gradually other observers found themselves seeing both the lines and their doublings. we have in this a good example of a curious circumstance in astronomical observation, namely, the fact that when fine detail has once been noted by a competent observer, it is not long before other observers see the same detail with ease. an immense amount of close attention has been paid to the planet mars during recent years by the american observer, professor percival lowell, at his famous observatory, feet above the sea, near the town of flagstaff, arizona, u.s.a. his observations have not, like those of most astronomers, been confined merely to "oppositions," but he has systematically kept the planet in view, so far as possible, since the year . the instrumental equipment of his observatory is of the very best, and the "seeing" at flagstaff is described as excellent. in support of the latter statement, mr. lampland, of the lowell observatory, maintains that the faintest stars shown on charts made at the lick observatory with the -inch telescope there, are _perfectly visible_ with the -inch telescope at flagstaff. professor lowell is, indeed, generally at issue with the other observers of mars. he finds the canals extremely narrow and sharply defined, and he attributes the blurred and hazy appearance, which they have presented to other astronomers, to the unsteady and imperfect atmospheric conditions in which their observations have been made. he assigns to the thinnest a width of two or three miles, and from fifteen to twenty to the larger. relatively to their width, however, he finds their length enormous. many of them are miles long, while one is even as much as . such lengths as these are very great in comparison with the smallness of the planet. he considers that the canals stand in some peculiar relation to the polar cap, for they crowd together in its neighbourhood. in place, too, of ill-defined condensations, he sees sharp black spots where the canals meet and intersect, and to these he gives the name of "oases." he further lays particular stress upon a dark band of a blue tint, which is always seen closely to surround the edges of the polar caps all the time that they are disappearing; and this he takes to be a proof that the white material is something which actually _melts_. of all substances which we know, water alone, he affirms, would act in such a manner. the question of melting at all may seem strange in a planet which is situated so far from the sun, and possesses such a rarefied atmosphere. but professor lowell considers that this very thinness of the atmosphere allows the direct solar rays to fall with great intensity upon the planet's surface, and that this heating effect is accentuated by the great length of the martian summer. in consequence he concludes that, although the general climate of mars is decidedly cold, it is above the freezing point of water. the observations at flagstaff appear to do away with the old idea that the darkish areas are seas, for numerous lines belonging to the so-called "canal system" are seen to traverse them. again, there is no star-like image of the sun reflected from them, as there would be, of course, from the surface of a great sheet of water. lastly, they are observed to vary in tone and colour with the changing martian seasons, the blue-green changing into ochre, and later on back again into blue-green. professor lowell regards these areas as great tracts of vegetation, which are brought into activity as the liquid reaches them from the melting snows. [illustration: plate xii. a map of the planet mars we see here the syrtis major (or "hour-glass sea"), the polar caps, several "oases," and a large number of "canals," some of which are double. the south is at the top of the picture, in accordance with the _inverted_ view given by an astronomical telescope. from a drawing by professor percival lowell. (page )] with respect to the canals, the lowell observations further inform us that these are invisible during the martian winter, but begin to appear in the spring when the polar cap is disappearing. professor lowell, therefore, inclines to the view that in the middle of the so-called canals there exist actual waterways which serve the purposes of irrigation, and that what we see is not the waterways themselves, for they are too narrow, but the fringe of vegetation which springs up along the banks as the liquid is borne through them from the melting of the polar snows. he supports this by his observation that the canals begin to appear in the neighbourhood of the polar caps, and gradually grow, as it were, in the direction of the planet's equator. it is the idea of life on mars which has given this planet such a fascination in the eyes of men. a great deal of nonsense has, however, been written in newspapers upon the subject, and many persons have thus been led to think that we have obtained some actual evidence of the existence of living beings upon mars. it must be clearly understood, however, that professor lowell's advocacy of the existence of life upon that planet is by no means of this wild order. at the best he merely indulges in such theories as his remarkable observations naturally call forth. his views are as follows:--he considers that the planet has reached a time when "water" has become so scarce that the "inhabitants" are obliged to employ their utmost skill to make their scanty supply suffice for purposes of irrigation. the changes of tone and colour upon the martian surface, as the irrigation produces its effects, are similar to what a telescopic observer--say, upon venus--would notice on our earth when the harvest ripens over huge tracts of country; that is, of course, if the earth's atmosphere allowed a clear view of the terrestrial surface--a very doubtful point indeed. professor lowell thinks that the perfect straightness of the lines, and the geometrical manner in which they are arranged, are clear evidences of artificiality. on a globe, too, there is plainly no reason why the liquid which results from the melting of the polar caps should trend at all in the direction of the equator. upon our earth, for instance, the transference of water, as in rivers, merely follows the slope of the ground, and nothing else. the lowell observations show, however, that the martian liquid is apparently carried from one pole towards the equator, and then past it to the other pole, where it once more freezes, only to melt again in due season, and to reverse the process towards and across the equator as before. professor lowell therefore holds, and it seems a strong point in favour of his theory, that the liquid must, in some artificial manner, as by pumping, for instance, be _helped_ in its passage across the surface of the planet. a number of attempts have been made to explain the _doubling_ of the canals merely as effects of refraction or reflection; and it has even been suggested that it may arise from the telescope not being accurately focussed. the actual doubling of the canals once having been doubted, it was an easy step to the casting of doubt on the reality of the canals themselves. the idea, indeed, was put forward that the human eye, in dealing with detail so very close to the limit of visibility, may unconsciously treat as an actual line several point-like markings which merely happen to lie in a line. in order to test this theory, experiments were carried out in by mr. e.w. maunder of greenwich observatory, and mr. j.e. evans of the royal hospital school at greenwich, in which certain schoolboys were set to make drawings of a white disc with some faint markings upon it. the boys were placed at various distances from the disc in question; and it was found that the drawings made by those who were just too far off to see distinctly, bore out the above theory in a remarkable manner. recently, however, the plausibility of the _illusion_ view has been shaken by photographs of mars taken during the opposition of by mr. lampland at the lowell observatory, in which a number of the more prominent canals come out as straight dark lines. further still, in some photographs made there quite lately, several canals are said to appear visibly double. following up the idea alluded to in chapter xvi., that the moon may be covered with a layer of ice, mr. w.t. lynn has recently suggested that this may be the case on mars; and that, at certain seasons, the water may break through along definite lines, and even along lines parallel to these. this, he maintains, would account for the canals becoming gradually visible across the disc, without the necessity of professor lowell's "pumping" theory. and now for the views of professor lowell himself with regard to the doubling of the canals. from his observations, he considers that no pairs of railway lines could apparently be laid down with greater parallelism. he draws attention to the fact that the doubling does not take place by any means in every canal; indeed, out of canals seen at flagstaff, only fifty-one--or, roughly, one-eighth--have at any time been seen double. he lays great stress upon this, which he considers points strongly against the duplication being an optical phenomenon. he finds that the distance separating pairs of canals is much less in some doubles than in others, and varies on the whole from to miles. according to him, the double canals appear to be confined to within degrees of the equator: or, to quote his own words, they are "an equatorial feature of the planet, confined to the tropic and temperate belts." finally, he points out that they seem to _avoid_ the blue-green areas. but, strangely enough, professor lowell does not so far attempt to fit in the doubling with his body of theory. he makes the obvious remark that they may be "channels and return channels," and with that he leaves us. the conclusions of professor lowell have recently been subjected to strenuous criticism by professor w.h. pickering and dr. alfred russel wallace. it was professor pickering who discovered the "oases," and who originated the idea that we did not see the so-called "canals" themselves, but only the growth of vegetation along their borders. he holds that the oases are craterlets, and that the canals are cracks which radiate from them, as do the rifts and streaks from craters upon the moon. he goes on to suggest that vapours of water, or of carbonic acid gas, escaping from the interior, find their way out through these cracks, and promote the growth of a low form of vegetation on either side of them. in support of this view he draws attention to the existence of long "steam-cracks," bordered by vegetation, in the deserts of the highly volcanic island of hawaii. we have already seen, in an earlier chapter, how he has applied this idea to the explanation of certain changes which are suspected to be taking place upon the moon. in dealing with the lowell canal system, professor pickering points out that under such a slight atmospheric pressure as exists on mars, the evaporation of the polar caps--supposing them to be formed of snow--would take place with such extraordinary rapidity that the resulting water could never be made to travel along open channels, but that a system of gigantic tubes or water-mains would have to be employed! as will be gathered from his theories regarding vegetation, professor pickering does not deny the existence of a form of life upon mars. but he will not hear of civilisation, or of anything even approaching it. he thinks, however, that as mars is intermediate physically between the moon and earth, the form of life which it supports may be higher than that on the moon and lower than that on the earth. in a small book published in the latter part of , and entitled _is mars habitable?_ dr. alfred russel wallace sets himself, among other things, to combat the idea of a comparatively high temperature, such as professor lowell has allotted to mars. he shows the immense service which the water-vapour in our atmosphere exercises, through keeping the solar heat from escaping from the earth's surface. he then draws attention to the fact that there is no spectroscopic evidence of water-vapour on mars[ ]; and points out that its absence is only to be expected, as dr. george johnstone stoney has shown that it will escape from a body whose mass is less than one-quarter the mass of the earth. the mass of mars is, in fact, much less than this, _i.e._ only one-ninth. dr. wallace considers, therefore, that the temperature of mars ought to be extremely low, unless the constitution of its atmosphere is very different from ours. with regard to the latter statement, it should be mentioned that the swedish physicist, arrhenius, has recently shown that the carbonic acid gas in our atmosphere has an important influence upon climate. the amount of it in our air is, as we have seen, extremely small; but arrhenius shows that, if it were doubled, the temperature would be more uniform and much higher. we thus see how futile it is, with our present knowledge, to dogmatise on the existence or non-existence of life in other celestial orbs. as to the canals dr. wallace puts forward a theory of his own. he contends that after mars had cooled to a state of solidity, a great swarm of meteorites and small asteroids fell in upon it, with the result that a thin molten layer was formed all over the planet. as this layer cooled, the imprisoned gases escaped, producing vents or craterlets; and as it attempted to contract further upon the solid interior, it split in fissures radiating from points of weakness, such, for instance, as the craterlets. and he goes on to suggest that the two tiny martian satellites, with which we shall deal next, are the last survivors of his hypothetical swarm. finally, with regard to the habitability of mars, dr. wallace not only denies it, but asserts that the planet is "absolutely uninhabitable." for a long time it was supposed that mars did not possess any satellites. in , however, during that famous opposition in which schiaparelli first saw the canals, two tiny satellites were discovered at the washington observatory by an american astronomer, professor asaph hall. these satellites are so minute, and so near to the planet, that they can only be seen with very large telescopes; and even then the bright disc of the planet must be shielded off. they have been christened phobos and deimos (fear and dread); these being the names of the two subordinate deities who, according to homer, attended upon mars, the god of war. it is impossible to measure the exact sizes of these satellites, as they are too small to show any discs, but an estimate has been formed from their brightness. the diameter of phobos was at first thought to be six miles, and that of deimos, seven. as later estimates, however, considerably exceed this, it will, perhaps, be not far from the truth to state that they are each roughly about the size of the planetoid eros. phobos revolves around mars in about - / hours, at a distance of about only miles from the planet's surface, and deimos in about hours, at a distance of about , miles. as mars rotates on its axis in about hours, it will be seen that phobos makes more than three revolutions while the planet is rotating once--a very interesting condition of things. a strange foreshadowing of the discovery of the satellites of mars will be familiar to readers of _gulliver's travels_. according to dean swift's hero, the astronomers on the flying island of laputa had found two tiny satellites to mars, one of which revolved around the planet in ten hours. the correctness of this guess is extraordinarily close, though at best it is, of course, nothing more than a pure coincidence. it need not be at all surprising that much uncertainty should exist with regard to the actual condition of the surface of mars. the circumstances in which we are able to see that planet at the best are, indeed, hardly sufficient to warrant us in propounding any hard and fast theories. one of the most experienced of living observers, the american astronomer, professor e.e. barnard, considers that the view we get of mars with the best telescope may be fairly compared with our naked eye view of the moon. since we have seen that a view with quite a small telescope entirely alters our original idea of the lunar surface, a slight magnification revealing features of whose existence we had not previously the slightest conception, it does not seem too much to say that a further improvement in optical power might entirely subvert the present notions with regard to the martian canals. therefore, until we get a still nearer view of these strange markings, it seems somewhat futile to theorise. the lines which we see are perhaps, indeed, a foreshortened and all too dim view of some type of formation entirely novel to us, and possibly peculiar to mars. differences of gravity and other conditions, such as obtain upon different planets, may perhaps produce very diverse results. the earth, the moon, and mars differ greatly from one another in size, gravitation, and other such characteristics. mountain-ranges so far appear typical of our globe, and ring-mountains typical of the moon. may not the so-called "canals" be merely some special formation peculiar to mars, though quite a natural result of its particular conditions and of its past history? the asteroids (or minor planets) we now come to that belt of small planets which are known by the name of asteroids. in the general survey of the solar system given in chapter ii., we saw how it was long ago noticed that the distances of the planetary orbits from the sun would have presented a marked appearance of orderly sequence, were it not for a gap between the orbits of mars and jupiter where no large planet was known to circulate. the suspicion thus aroused that some planet might, after all, be moving in this seemingly empty space, gave rise to the gradual discovery of a great number of small bodies; the largest of which, ceres, is less than miles in diameter. up to the present day some of these bodies have been discovered; the four leading ones, in order of size, being named ceres, pallas, juno, and vesta. all the asteroids are invisible to the naked eye, with the exception of vesta, which, though by no means the largest, happens to be the brightest. it is, however, only just visible to the eye under favourable conditions. no trace of an atmosphere has been noted upon any of the asteroids, but such a state of things is only to be expected from the kinetic theory. for a good many years the discoveries of asteroids were made by means of the telescope. when, in the course of searching the heavens, an object was noticed which did not appear upon any of the recognised star charts, it was kept under observation for several nights to see whether it changed its place in the sky. since asteroids move around the sun in orbits, just as planets do, they, of course, quickly reveal themselves by their change of position against the starry background. the year started a new era in the discovery of asteroids. it occurred to the heidelberg observer, dr. max wolf, one of the most famous of the hunters of these tiny planets, that photography might be employed in the quest with success. this photographic method, to which allusion has already been made in dealing with eros, is an extremely simple one. if a photograph of a portion of the heavens be taken through an "equatorial"--that is, a telescope, moving by machinery, so as to keep the stars, at which it is pointed, always exactly in the field of view during their apparent movement across the sky--the images of these stars will naturally come out in the photograph as sharply defined points. if, however, there happens to be an asteroid, or other planetary body, in the same field of view, its image will come out as a short white streak; because the body has a comparatively rapid motion of its own, and will, during the period of exposure, have moved sufficiently against the background of the stars to leave a short trail, instead of a dot, upon the photographic plate. by this method wolf himself has succeeded in discovering more than a hundred asteroids (see plate xiii., p. ). it was, indeed, a little streak of this kind, appearing upon a photograph taken by the astronomer witt, at berlin, in , which first informed the world of the existence of eros. [illustration: plate xiii. minor planet trails two trails of minor planets (asteroids) imprinted _at the same time_ upon one photographic plate. in the white streak on the left-hand side of the picture we witness the _discovery_ of a new minor planet. the streak on the right was made by a body already known--the minor planet "fiducia." this photograph was taken by dr. max wolf, at heidelberg, on the th of november, , with the aid of a -inch telescope. the time of exposure was two hours. (page )] it has been calculated that the total mass of the asteroids must be much less than one-quarter that of the earth. they circulate as a rule within a space of some , , miles in breadth, lying about midway between the paths of mars and jupiter. two or three, however, of the most recently discovered of these small bodies have been found to pass quite close to jupiter. the orbits of the asteroids are by no means in the one plane, that of pallas being the most inclined to the plane of the earth's orbit. it is actually three times as much inclined as that of eros. two notable theories have been put forward to account for the origin of the asteroids. the first is that of the celebrated german astronomer, olbers, who was the discoverer of pallas and vesta. he suggested that they were the fragments of an exploded planet. this theory was for a time generally accepted, but has now been abandoned in consequence of certain definite objections. the most important of these objections is that, in accordance with the theory of gravitation, the orbits of such fragments would all have to pass through the place where the explosion originally occurred. but the wide area over which the asteroids are spread points rather against the notion that they all set out originally from one particular spot. another objection is that it does not appear possible that, within a planet already formed, forces could originate sufficiently powerful to tear the body asunder. the second theory is that for some reason a planet here failed in the making. possibly the powerful gravitational action of the huge body of jupiter hard by, disturbed this region so much that the matter distributed through it was never able to collect itself into a single mass. [ ] sir william herschel was the first to note these polar changes. [ ] quite recently, however, professor lowell has announced that his observer, mr. e.c. slipher, finds with the spectroscope faint traces of water vapour in the martian atmosphere. [ ] in a somewhat similar manner the term "crater," as applied to the ring-mountain formation on the moon, has evidently given a bias in favour of the volcanic theory as an explanation of that peculiar structure. [ ] mr. slipher's results (see note , page ) were not then known. chapter xviii the superior planets--_continued_ the planets, so far, have been divided into inferior and superior. such a division, however, refers merely to the situation of their orbits with regard to that of our earth. there is, indeed, another manner in which they are often classed, namely, according to size. on this principle they are divided into two groups; one group called the _terrestrial planets_, or those which have characteristics like our earth, and the other called the _major planets_, because they are all of very great size. the terrestrial planets are mercury, venus, the earth, and mars. the major planets are the remainder, namely, jupiter, saturn, uranus, and neptune. as the earth's orbit is the boundary which separates the inferior from the superior planets, so does the asteroidal belt divide the terrestrial from the major planets. we found the division into inferior and superior useful for emphasising the marked difference in aspect which those two classes present as seen from our earth; the inferior planets showing phases like the moon when viewed in the telescope, whereas the superior planets do not. but the division into terrestrial and major planets is the more far-reaching classification of the two, for it includes the whole number of planets, whereas the other arrangement necessarily excludes the earth. the members of each of these classes have many definite characteristics in common. the terrestrial planets are all of them relatively small in size, comparatively near together, and have few or no satellites. they are, moreover, rather dense in structure. the major planets, on the other hand, are huge bodies, circulating at great distances from each other, and are, as a rule, provided with a number of satellites. with respect to structure, they may be fairly described as being loosely put together. further, the markings on the surfaces of the terrestrial planets are permanent, whereas those on the major planets are continually shifting. the planet jupiter jupiter is the greatest of the major planets. it has been justly called the "giant" planet, for both in volume and in mass it exceeds all the other planets put together. when seen through the telescope it exhibits a surface plentifully covered with markings, the most remarkable being a series of broad parallel belts. the chief belt lies in the central parts of the planet, and is at present about , miles wide. it is bounded on either side by a reddish brown belt of about the same width. bright spots also appear upon the surface of the planet, last for a while, and then disappear. the most notable of the latter is one known as the "great red spot." this is situated a little beneath the southern red belt, and appeared for the first time about thirty years ago. it has undergone a good many changes in colour and brightness, and is still faintly visible. this spot is the most permanent marking which has yet been seen upon jupiter. in general, the markings change so often that the surface which we see is evidently not solid, but of a fleeting nature akin to cloud (see plate xiv., p. ). [illustration: plate xiv. the planet jupiter the giant planet as seen at . p.m., on the th of january, , with a - / -inch reflecting telescope. the extensive oval marking in the upper portion of the disc is the "great red spot." the south is at the top of the picture, the view being the _inverted_ one given by an astronomical telescope. from a drawing by the rev. theodore e.r. phillips, m.a., f.r.a.s., director of the jupiter section of the british astronomical association. (page )] observations of jupiter's markings show that on an average the planet rotates on its axis in a period of about hours minutes. the mention here of _an average_ with reference to the rotation will, no doubt, recall to the reader's mind the similar case of the sun, the different portions of which rotate with different velocities. the parts of jupiter which move quickest take hours minutes to go round, while those which move slowest take hours minutes. the middle portions rotate the fastest, a phenomenon which the reader will recollect was also the case with regard to the sun. jupiter is a very loosely packed body. its density is on an average only about - / times that of water, or about one-fourth the density of the earth; but its bulk is so great that the gravitation at that surface which we see is about - / times what it is on the surface of the earth. in accordance, therefore, with the kinetic theory, we may expect the planet to retain an extensive layer of gases around it; and this is confirmed by the spectroscope, which gives evidence of the presence of a dense atmosphere. all things considered, it may be safely inferred that the interior of jupiter is very hot, and that what we call its surface is not the actual body of the planet, but a voluminous layer of clouds and vapours driven upwards from the heated mass underneath. the planet was indeed formerly thought to be self-luminous; but this can hardly be the case, for those portions of the surface which happen to lie at any moment in the shadows cast by the satellites appear to be quite black. again, when a satellite passes into the great shadow cast by the planet it becomes entirely invisible, which would not be the case did the planet emit any perceptible light of its own. in its revolutions around the sun, jupiter is attended, so far as we know, by seven[ ] satellites. four of these were among the first celestial objects which galileo discovered with his "optick tube," and he named them the "medicean stars" in honour of his patron, cosmo de medici. being comparatively large bodies they might indeed just be seen with the naked eye, were it not for the overpowering glare of the planet. it was only in quite recent times, namely, in , that a fifth satellite was added to the system of jupiter. this body, discovered by professor e.e. barnard, is very small. it circulates nearer to the planet than the innermost of galileo's moons; and, on account of the glare, is a most difficult object to obtain a glimpse of, even in the best of telescopes. in december and january respectively, two more moons were added to the system, these being found by _photography_, by the american astronomer, professor c.d. perrine. both the bodies in question revolve at a greater distance from the planet than the outermost of the older known satellites. galileo's moons, though the largest bodies of jupiter's satellite system, are, as we have already pointed out, very small indeed when compared with the planet itself. the diameters of two of them, europa and io, are, however, about the same as that of our moon, while those of the other two, callisto and ganymede, are more than half as large again. the recently discovered satellites are, on the other hand, insignificant; that found by barnard, for example, being only about miles in diameter. of the four original satellites io is the nearest to jupiter, and, seen from the planet, it would show a disc somewhat larger than that of our moon. the others would appear somewhat smaller. however, on account of the great distance of the sun, the entire light reflected to jupiter by all the satellites should be very much less than what we get from our moon. barnard's satellite circles around jupiter at a distance less than our moon is from us, and in a period of about hours. galileo's four satellites revolve in periods of about , - / , , and - / days respectively, at distances lying roughly between a quarter of a million and one million miles. perrine's two satellites are at a distance of about seven million miles, and take about nine months to complete their revolutions. the larger satellites, when viewed in the telescope, exhibit certain defined markings; but the bodies are so far away from us, that only those details which are of great extent can be seen. the satellite io, according to professor barnard, shows a darkish disc, with a broad white belt across its middle regions. mr. douglass, one of the observers at the lowell observatory, has noted upon ganymede a number of markings somewhat resembling those seen on mars, and he concludes, from their movement, that this satellite rotates on its axis in about seven days. professor barnard, on the other hand, does not corroborate this, though he claims to have discovered bright polar caps on both ganymede and callisto. in an earlier chapter we dealt at length with eclipses, occultations, and transits, and endeavoured to make clear the distinction between them. the system of jupiter's satellites furnishes excellent examples of all these phenomena. the planet casts a very extensive shadow, and the satellites are constantly undergoing obscuration by passing through it. such occurrences are plainly comparable to our lunar eclipses. again, the satellites may, at one time, be occulted by the huge disc of the planet, and at another time seen in transit over its face. a fourth phenomenon is what is known as an _eclipse of the planet by a satellite_, which is the exact equivalent of what we style on the earth an eclipse of the sun. in this last case the shadow, cast by the satellite, appears as a round black spot in movement across the planet's surface. in the passages of these attendant bodies behind the planet, into its shadow, or across its face, respectively, it occasionally happens that galileo's four satellites all disappear from view, and the planet is then seen for a while in the unusual condition of being apparently without its customary attendants. an instance of this phenomenon took place on the rd of october . on that occasion, the satellites known as i. and iii. (_i.e._ io and ganymede) were eclipsed, that is to say, obscured by passing into the planet's shadow; satellite iv. (callisto) was occulted by the planet's disc; while satellite ii. (europa), being at the same moment in transit across the planet's face, was invisible against that brilliant background. a number of instances of this kind of occurrence are on record. galileo, for example, noted one on the th of march , while herschel observed another on the rd of may . it was indirectly to jupiter's satellites that the world was first indebted for its knowledge of the velocity of light. when the periods of revolution of the satellites were originally determined, jupiter happened, at the time, to be at his nearest to us. from the periods thus found tables were made for the prediction of the moments at which the eclipses and other phenomena of the satellites should take place. as jupiter, in the course of his orbit, drew further away from the earth, it was noticed that the disappearances of the satellites into the shadow of the planet occurred regularly later than the time predicted. in the year , roemer, a danish astronomer, inferred from this, not that the predictions were faulty, but that light did not travel instantaneously. it appeared, in fact, to take longer to reach us, the greater the distance it had to traverse. thus, when the planet was far from the earth, the last ray given out by the satellite, before its passage into the shadow, took a longer time to cross the intervening space, than when the planet was near. modern experiments in physics have quite confirmed this, and have proved for us that light does not travel across space in the twinkling of an eye, as might hastily be supposed, but actually moves, as has been already stated, at the rate of about , miles per second. the planet saturn seen in the telescope the planet saturn is a wonderful and very beautiful object. it is distinguished from all the other planets, in fact from all known celestial bodies, through being girt around its equator by what looks like a broad, flat ring of exceeding thinness. this, however, upon closer examination, is found to be actually composed of three concentric rings. the outermost of these is nearly of the same brightness as the body of the planet itself. the ring which comes immediately within it is also bright, and is separated from the outer one all the way round by a relatively narrow space, known as "cassini's division," because it was discovered by the celebrated french astronomer, j.d. cassini, in the year . inside the second ring, and merging insensibly into it, is a third one, known as the "crape ring," because it is darker in hue than the others and partly transparent, the body of saturn being visible through it. the inner boundary of this third and last ring does not adjoin the planet, but is everywhere separated from it by a definite space. this ring was discovered _independently_[ ] in by bond in america and dawes in england. [illustration: plate xv. the planet saturn from a drawing made by professor barnard with the great lick telescope. the black band fringing the outer ring, where it crosses the disc, is portion of the _shadow which the rings cast upon the planet_. the black wedge-shaped mark, where the rings disappear behind the disc at the left-hand side, is portion of the _shadow which the planet casts upon the rings_. (page )] as distinguished from the crape ring, the bright rings must have a considerable closeness of texture; for the shadow of the planet may be seen projected upon them, and their shadows in turn projected upon the surface of the planet (see plate xv., p. ). according to professor barnard, the entire breadth of the ring system, that is to say, from one side to the other of the outer ring, is , miles, or somewhat more than double the planet's diameter. in the varying views which we get of saturn, the system of the rings is presented to us at very different angles. sometimes we are enabled to gaze upon its broad expanse; at other times, however, its thin edge is turned exactly towards us, an occurrence which takes place after intervals of about fifteen years. when this happened in the rings are said to have disappeared entirely from view in the great lick telescope. we thus get an idea of their small degree of thickness, which would appear to be only about miles. the last time the system of rings was exactly edgewise to the earth was on the rd of october . the question of the composition of these rings has given rise to a good deal of speculation. it was formerly supposed that they were either solid or liquid, but in it was proved by clerk maxwell that a structure of this kind would not be able to stand. he showed, however, that they could be fully explained by supposing them to consist of an immense number of separate solid particles, or, as one might otherwise put it, extremely small satellites, circling in dense swarms around the middle portions of the planet. it is therefore believed that we have here the materials ready for the formation of a satellite or satellites; but that the powerful gravitative action, arising through the planet's being so near at hand, is too great ever to allow these materials to aggregate themselves into a solid mass. there is, as a matter of fact, a minimum distance from the body of any planet within which it can be shown that a satellite will be unable to form on account of gravitational stress. this is known as "roche's limit," from the name of a french astronomer who specially investigated the question. there thus appears to be a certain degree of analogy between saturn's rings and the asteroids. empty spaces, too, exist in the asteroidal zone, the relative position of one of which bears a striking resemblance to that of "cassini's division." it is suggested, indeed, that this division had its origin in gravitational disturbances produced by the attraction of the larger satellites, just as the empty spaces in the asteroidal zone are supposed to be the result of perturbations caused by the giant planet hard by. it has long been understood that the system of the rings must be rotating around saturn, for if they were not in motion his intense gravitational attraction would quickly tear them in pieces. this was at length proved to be the fact by the late professor keeler, director of the lick observatory, who from spectroscopic observations found that those portions of the rings situated near to the planet rotated faster than those farther from it. this directly supports the view that the rings are composed of satellites; for, as we have already seen, the nearer a satellite is to its primary the faster it will revolve. on the other hand, were the rings solid, their outer portions would move the fastest; as we have seen takes place in the body of the earth, for example. the mass of the ring system, however, must be exceedingly small, for it does not appear to produce any disturbances in the movements of saturn's satellites. from the kinetic theory, therefore, one would not expect to find any atmosphere on the rings, and the absence of it is duly shown by spectroscopic observations. the diameter of saturn, roughly speaking, is about one-fifth less than that of jupiter. the planet is very flattened at the poles, this flattening being quite noticeable in a good telescope. for instance, the diameter across the equator is about , miles, while from pole to pole it is much less, namely, , . the surface of saturn bears a strong resemblance to that of jupiter. its markings, though not so well defined, are of the same belt-like description; and from observation of them it appears that the planet rotates _on an average_ in a little over ten hours. the rotation is in fact of the same peculiar kind as that of the sun and jupiter; but the difference of speed at which the various portions of saturn go round are even more marked than in the case of the giant planet. the density of saturn is less than that of jupiter; so that it must be largely in a condition of vapour, and in all probability at a still earlier stage of planetary evolution. up to the present we know of as many as ten satellites circling around saturn, which is more than any other planet of the solar system can lay claim to. two of these, however, are very recent discoveries; one, phoebe, having been found by photography in august , and the other, themis, in , also by the same means. for both of these we are indebted to professor w.h. pickering. themis is said to be _the faintest object in the solar system_. it cannot be _seen_, even with the largest telescope in existence; a fact which should hardly fail to impress upon one the great advantage the photographic plate possesses in these researches over the human eye. the most important of the whole saturnian family of satellites are the two known as titan and japetus. these were discovered respectively by huyghens in and by cassini in . japetus is about the same size as our moon; while the diameter of titan, the largest of the satellites, is about half as much again. titan takes about sixteen days to revolve around saturn, while japetus takes more than two months and a half. the former is about three-quarters of a million miles distant from the planet, and the latter about two and a quarter millions. to sir william herschel we are indebted for the discovery of two more satellites, one of which he found on the evening that he used his celebrated -foot telescope for the first time. the ninth satellite, phoebe, one of the two discovered by professor pickering, is perhaps the most remarkable body in the solar system, for all the other known members of that system perform their revolutions in one fixed direction, whereas this satellite revolves in the _contrary_ direction. in consequence of the great distance of saturn, the sun, as seen from the planet, would appear so small that it would scarcely show any disc. the planet, indeed, only receives from the sun about one-ninetieth of the heat and light which the earth receives. owing to this diminished intensity of illumination, the combined light reflected to saturn by the whole of its satellites must be very small. with the sole exception of jupiter, not one of the planets circulating nearer to the sun could be seen from saturn, as they would be entirely lost in the solar glare. for an observer upon saturn, jupiter would, therefore, fill much the same position as venus does for us, regularly displaying phases and being alternately a morning and an evening star. it is rather interesting to consider the appearances which would be produced in our skies were the earth embellished with a system of rings similar to those of saturn. in consequence of the curving of the terrestrial surface, they would not be seen at all from within the arctic or antarctic circles, as they would be always below the horizon. from the equator they would be continually seen edgewise, and so would appear merely as line of light stretching right across the heaven and passing through the zenith. but the dwellers in the remaining regions would find them very objectionable, for they would cut off the light of the sun during lengthy periods of time. saturn was a sore puzzle to the early telescopic observers. they did not for a long time grasp the fact that it was surrounded by a ring--so slow is the human mind to seek for explanations out of the ordinary course of things. the protrusions of the ring on either side of the planet, at first looked to galileo like two minor globes placed on opposite sides of it, and slightly overlapping the disc. he therefore informed kepler that "saturn consists of three stars in contact with one another." yet he was genuinely puzzled by the fact that the two attendant bodies (as he thought them) always retained the same position with regard to the planet's disc, and did not appear to revolve around it, nor to be in any wise shifted as a consequence of the movements of our earth. about a year and a half elapsed before he again examined saturn; and, if he was previously puzzled, he was now thoroughly amazed. it happened just then to be one of those periods when the ring is edgewise towards the earth, and of course he only saw a round disc like that of jupiter. what, indeed, had become of the attendant orbs? was some demon mocking him? had saturn devoured his own children? he was, however, fated to be still more puzzled, for soon the minor orbs reappeared, and, becoming larger and larger as time went on, they ended by losing their globular appearance and became like two pairs of arms clasping the planet from each side! (see plate xvi., p. ). galileo went to his grave with the riddle still unsolved, and it remained for the famous dutch astronomer, huyghens, to clear up the matter. it was, however, some little time before he hit upon the real explanation. having noticed that there were dark spaces between the strange appendages and the body of the planet, he imagined saturn to be a globe fitted with handles at each side; "ansæ" these came to be called, from the latin _ansa_, which means a handle. at length, in the year , he solved the problem, and this he did by means of that -foot tubeless telescope, of which mention has already been made. the ring happened then to be at its edgewise period, and a careful study of the behaviour of the ansæ when disappearing and reappearing soon revealed to huyghens the true explanation. [illustration: plate xvi. early representations of saturn from an illustration in the _systema saturnium_ of christian huyghens. (page )] the planets uranus and neptune we have already explained (in chapter ii.) the circumstances in which both uranus and neptune were discovered. it should, however, be added that after the discovery of uranus, that planet was found to have been already noted upon several occasions by different observers, but always without the least suspicion that it was other than a mere faint star. again, with reference to the discovery of neptune, it may here be mentioned that the apparent amount by which that planet had pulled uranus out of its place upon the starry background was exceedingly small--so small, indeed, that no eye could have detected it without the aid of a telescope! of the two predictions of the place of neptune in the sky, that of le verrier was the nearer. indeed, the position calculated by adams was more than twice as far out. but adams was by a long way the first in the field with his results, and only for unfortunate delays the prize would certainly have fallen to him. for instance, there was no star-map at cambridge, and professor challis, the director of the observatory there, was in consequence obliged to make a laborious examination of the stars in the suspected region. on the other hand, all that galle had to do was to compare that part of the sky where le verrier told him to look with the berlin star-chart which he had by him. this he did on september , , with the result that he quickly noted an eighth magnitude star which did not figure in that chart. by the next night this star had altered its position in the sky, thus disclosing the fact that it was really a planet. six days later professor challis succeeded in finding the planet, but of course he was now too late. on reviewing his labours he ascertained that he had actually noted down its place early in august, and had he only been able to sift his observations as he made them, the discovery would have been made then. later on it was found that neptune had only just missed being discovered about fifty years earlier. in certain observations made during , the famous french astronomer, lalande, found that a star, which he had mapped in a certain position on the th of may of that year, was in a different position two days later. the idea of a planet does not appear to have entered his mind, and he merely treated the first observation as an error! the reader will, no doubt, recollect how the discovery of the asteroids was due in effect to an apparent break in the seemingly regular sequence of the planetary orbits outwards from the sun. this curious sequence of relative distances is usually known as "bode's law," because it was first brought into general notice by an astronomer of that name. it had, however, previously been investigated mathematically by titius in . long before this, indeed, the unnecessarily wide space between the orbits of mars and jupiter had attracted the attention of the great kepler to such a degree, that he predicted that a planet would some day be found to fill the void. notwithstanding the service which the so-called law of bode has indirectly rendered to astronomy, it has strangely enough been found after all not to rest upon any scientific foundation. it will not account for the distance from the sun of the orbit of neptune, and the very sequence seems on the whole to be in the nature of a mere coincidence. neptune is invisible to the naked eye; uranus is just at the limit of visibility. both planets are, however, so far from us that we can get but the poorest knowledge of their condition and surroundings. uranus, up to the present, is known to be attended by four satellites, and neptune by one. the planets themselves are about equal in size; their diameters, roughly speaking, being about one-half that of saturn. some markings have, indeed, been seen upon the disc of uranus, but they are very indistinct and fleeting. from observation of them, it is assumed that the planet rotates on its axis in a period of some ten to twelve hours. no definite markings have as yet been seen upon neptune, which body is described by several observers as resembling a faint planetary nebula. with regard to their physical condition, the most that can be said about these two planets is that they are probably in much the same vaporous state as jupiter and saturn. on account of their great distance from the sun they can receive but little solar heat and light. seen from neptune, in fact, the sun would appear only about the size of venus at her best, though of a brightness sufficiently intense to illumine the neptunian landscape with about seven hundred times our full moonlight. [ ] mr. p. melotte, of greenwich observatory, while examining a photograph taken there on february , , discovered upon it a very faint object which it is firmly believed will prove to be an _eighth_ satellite of jupiter. this object was afterwards found on plates exposed as far back as january . it has since been photographed several times at greenwich, and also at heidelberg (by dr. max wolf) and at the lick observatory. its movement is probably _retrograde_, like that of phoebe (p. ). [ ] in the history of astronomy two salient points stand out. the first of these is the number of "independent" discoveries which have taken place; such, for instance, as in the cases of le verrier and adams with regard to neptune, and of lockyer and janssen in the matter of the spectroscopic method of observing solar prominences. the other is the great amount of "anticipation." copernicus, as we have seen, was anticipated by the greeks; kepler was not actually the first who thought of elliptic orbits; others before newton had imagined an attractive force. both these points furnish much food for thought! chapter xix comets the reader has, no doubt, been struck by the marked uniformity which exists among those members of the solar system with which we have dealt up to the present. the sun, the planets, and their satellites are all what we call solid bodies. the planets move around the sun, and the satellites around the planets, in orbits which, though strictly speaking, ellipses, are yet not in any instance of a very oval form. two results naturally follow from these considerations. firstly, the bodies in question hide the light coming to us from those further off, when they pass in front of them. secondly, the planets never get so far from the sun that we lose sight of them altogether. with the objects known as comets it is, however, quite the contrary. these objects do not conform to our notions of solidity. they are so transparent that they can pass across the smallest star without dimming its light in the slightest degree. again, they are only visible to us during a portion of their orbits. a comet may be briefly described as an illuminated filmy-looking object, made up usually of three portions--a head, a nucleus, or brighter central portion within this head, and a tail. the heads of comets vary greatly in size; some, indeed, appear quite small, like stars, while others look even as large as the moon. occasionally the nucleus is wanting, and sometimes the tail also. [illustration: fig. .--showing how the tail of a comet is directed away from the sun.] these mysterious visitors to our skies come up into view out of the immensities beyond, move towards the sun at a rapidly increasing speed, and, having gone around it, dash away again into the depths of space. as a comet approaches the sun, its body appears to grow smaller and smaller, while, at the same time, it gradually throws out behind it an appendage like a tail. as the comet moves round the central orb this tail is always directed _away_ from the sun; and when it departs again into space the tail goes in advance. as the comet's distance from the sun increases, the tail gradually shrinks away and the head once more grows in size (see fig. ). in consequence of these changes, and of the fact that we lose sight of comets comparatively quickly, one is much inclined to wonder what further changes may take place after the bodies have passed beyond our ken. the orbits of comets are, as we have seen, very elliptic. in some instances this ellipticity is so great as to take the bodies out into space to nearly six times the distance of neptune from the sun. for a long time, indeed, it was considered that comets were of two kinds, namely, those which actually _belonged_ to the solar system, and those which were merely _visitors_ to it for the first and only time--rushing in from the depths of space, rapidly circuiting the sun, and finally dashing away into space again, never to return. on the contrary, nowadays, astronomers are generally inclined to regard comets as permanent members of the solar system. the difficulty, however, of deciding absolutely whether the orbits of comets are really always _closed_ curves, that is to say, curves which must sooner or later bring the bodies back again towards the sun, is, indeed, very great. comets, in the first place, are always so diffuse, that it is impossible to determine their exact position, or, rather, the exact position of that important point within them, known as the centre of gravity. secondly, that stretch of its orbit along which we can follow a comet, is such a very small portion of the whole path, that the slightest errors of observation which we make will result in considerably altering our estimate of the actual shape of the orbit. comets have been described as so transparent that they can pass across the sky without dimming the lustre of the smallest stars, which the thinnest fog or mist would do. this is, indeed, true of every portion of a comet except the nucleus, which is, as its name implies, the densest part. and yet, in contrast to this ghostlike character, is the strange fact that when comets are of a certain brightness they may actually be seen in full daylight. as might be gathered from their extreme tenuity, comets are so exceedingly small in mass that they do not appear to exert any gravitational attraction upon the other bodies of our system. it is, indeed, a known fact that in the year a comet passed right amidst the satellites of jupiter without disturbing them in the slightest degree. the attraction of the planet, on the other hand, so altered the comet's orbit, as to cause it to revolve around the sun in a period of seven years, instead of twenty-seven, as had previously been the case. also, in , the comet known as lexell's passed quite close to jupiter, and its orbit was so changed by that planet's attraction that it has never been seen since. the density of comets must, as a rule, be very much less than the one-thousandth part of that of the air at the surface of our globe; for, if the density of the comet were even so small as this, its mass would _not_ be inappreciable. if comets are really undoubted members of the solar system, the circumstances in which they were evolved must have been different from those which produced the planets and satellites. the axial rotations of both the latter, and also their revolutions, take place in one certain direction;[ ] their orbits, too, are ellipses which do not differ much from circles, and which, furthermore, are situated fairly in the one plane. comets, on the other hand, do not necessarily travel round the sun in the same fixed direction as the planets. their orbits, besides, are exceedingly elliptic; and, far from keeping to one plane, or even near it, they approach the sun from all directions. broadly speaking, comets may be divided into two distinct classes, or "families." in the first class, the same orbit appears to be shared in common by a series of comets which travel along it, one following the other. the comets which appeared in the years , , , , and are instances of a number of different bodies pursuing the same path around the sun. the members of a comet family of this kind are observed to have similar characteristics. the idea is that such comets are merely portions of one much larger cometary body, which became broken up by the gravitational action of other bodies in the system, or through violent encounter with the sun's surroundings. the second class is composed of comets which are supposed to have been seized by the gravitative action of certain planets, and thus forced to revolve in short ellipses around the sun, well within the limits of the solar system. these comets are, in consequence, spoken of as "captures." they move around the sun in the same direction as the planets do. jupiter has a fairly large comet family of this kind attached to him. as a result of his overpowering gravitation, it is imagined that during the ages he must have attracted a large number of these bodies on his own account, and, perhaps, have robbed other planets of their captures. his family at present numbers about thirty. of the other planets, so far as we know, saturn possesses a comet family of two, uranus three, and neptune six. there are, indeed, a few comets which appear as if under the influence of some force situated outside the known bounds of the solar system, a circumstance which goes to strengthen the idea that other planets may revolve beyond the orbit of neptune. the terrestrial planets, on the other hand, cannot have comet families; because the enormous gravitative action of the sun in their vicinity entirely overpowers the attractive force which they exert upon those comets which pass close to them. besides this, a comet, when in the inner regions of the solar system, moves with such rapidity, that the gravitational pull of the planets there situated is not powerful enough to deflect it to any extent. it must not be presumed, however, that a comet once captured should always remain a prisoner. further disturbing causes might unsettle its newly acquired orbit, and send it out again into the celestial spaces. with regard to the matter of which comets are composed, the spectroscope shows the presence in them of hydrocarbon compounds (a notable characteristic of these bodies), and at times, also, of sodium and iron. some of the light which we get from comets is, however, merely reflected sunlight. the fact that the tails of comets are always directed away from the sun, has given rise to the idea that this is caused by some repelling action emanating from the sun itself, which is continually driving off the smallest particles. two leading theories have been formulated to account for the tails themselves upon the above assumption. one of these, first suggested by olbers in , and now associated with the name of the russian astronomer, the late professor brédikhine, who carefully worked it out, presumes an electrical action emanating from the sun; the other, that of arrhenius, supposes a pressure exerted by the solar light in its radiation outwards into space. it is possible, indeed, that repelling forces of both these kinds may be at work together. minute particles are probably being continually produced by friction and collisions among the more solid parts in the heads of comets. supposing that such particles are driven off altogether, one may therefore assume that the so-called captured comets are disintegrating at a comparatively rapid rate. kepler long ago maintained that "comets die," and this actually appears to be the case. the ordinary periodic ones, such, for instance, as encke's comet, are very faint, and becoming fainter at each return. certain of these comets have, indeed, failed altogether to reappear. it is notable that the members of jupiter's comet family are not very conspicuous objects. they have small tails, and even in some cases have none at all. the family, too, does not contain many members, and yet one cannot but suppose that jupiter, on account of his great mass, has had many opportunities for making captures adown the ages. of the two theories to which allusion has above been made, that of brédikhine has been worked out so carefully, and with such a show of plausibility, that it here calls for a detailed description. it appears besides to explain the phenomena of comets' tails so much more satisfactorily than that of arrhenius, that astronomers are inclined to accept it the more readily of the two. according to brédikhine's theory the electrical repulsive force, which he assumes for the purposes of his argument, will drive the minutest particles of the comet in a direction away from the sun much more readily than the gravitative action of that body will pull them towards it. this may be compared to the ease with which fine dust may be blown upwards, although the earth's gravitation is acting upon it all the time. the researches of brédikhine, which began seriously with his investigation of coggia's comet of , led him to classify the tails of comets in _three types_. presuming that the repulsive force emanating from the sun did not vary, he came to the conclusion that the different forms assumed by cometary tails must be ascribed to the special action of this force upon the various elements which happen to be present in the comet. the tails which he classes as of the first type, are those which are long and straight and point directly away from the sun. examples of such tails are found in the comets of , , and . tails of this kind, he thinks, are in all probability formed of _hydrogen_. his second type comprises those which are pointed away from the sun, but at the same time are considerably curved, as was seen in the comets of donati and coggia. these tails are formed of _hydrocarbon gas_. the third type of tail is short, brush-like, and strongly bent, and is formed of the _vapour of iron_, mixed with that of sodium and other elements. it should, however, be noted that comets have occasionally been seen which possess several tails of these various types. we will now touch upon a few of the best known comets of modern times. the comet of was the first whose orbit was calculated according to the laws of gravitation. this was accomplished by newton, and he found that the comet in question completed its journey round the sun in a period of about years. in there appeared a great comet, which has become famous under the name of halley's comet, in consequence of the profound investigations made into its motion by the great astronomer, edmund halley. he fixed its period of revolution around the sun at about seventy-five years, and predicted that it would reappear in the early part of . he did not, however, live to see this fulfilled, but the comet duly returned--_the first body of the kind to verify such a prediction_--and was detected on christmas day, , by george palitzch, an amateur observer living near dresden. halley also investigated the past history of the comet, and traced it back to the year . the orbit of halley's comet passes out slightly beyond the orbit of neptune. at its last visit in , this comet passed comparatively close to us, namely, within five million miles of the earth. according to the calculations of messrs p.h. cowell and a.c.d. crommelin of greenwich observatory, its next return will be in the spring of ; the nearest approach to the earth taking place about may . on the th of march, , a great comet appeared, which remained visible for nearly a year and a half. it was a magnificent object; the tail being about millions of miles in length, and the head about , miles in diameter. a detailed study which he gave to this comet prompted olbers to put forward that theory of electrical repulsion which, as we have seen, has since been so carefully worked out by brédikhine. olbers had noticed that the particles expelled from the head appeared to travel to the end of the tail in about eleven minutes, thus showing a velocity per second very similar to that of light. the discovery in of the comet known as encke's, because its orbit was determined by an astronomer of that name, drew attention for the first time to jupiter's comet family, and, indeed, to short-period comets in general. this comet revolves around the sun in the shortest known period of any of these bodies, namely, - / years. encke predicted that it would return in . this duly occurred, the comet passing at its nearest to the sun within three hours of the time indicated; being thus the second instance of the fulfilment of a prediction of the kind. a certain degree of irregularity which encke's comet displays in the dates of its returns to the sun, has been supposed to indicate that it passes in the course of its orbit through some retarding medium, but no definite conclusions have so far been arrived at in this matter. a comet, which appeared in , goes by the name of biela's comet, because of its discovery by an austrian military officer, wilhelm von biela. this comet was found to have a period of between six and seven years. certain calculations made by olbers showed that, at its return in , it would pass _through the earth's orbit_. the announcement of this gave rise to a panic; for people did not wait to inquire whether the earth would be anywhere near that part of its orbit when the comet passed. the panic, however, subsided when the french astronomer, arago, showed that at the moment in question the earth would be some millions of miles away from the point indicated! [illustration: plate xvii. donati's comet from a drawing made on october th, , by g.p. bond, of harvard college observatory, u.s.a. a good illustration of brédikhine's theory: note the straight tails of his _first_ type, and the curved tail of his _second_. (page )] in , shortly after one of its returns, biela's comet divided into two portions. at its next appearance ( ) these portions had separated to a distance of about - / millions of miles from each other. this comet, or rather its constituents, have never since been seen. perhaps the most remarkable comet of recent times was that of , known as donati's, it having been discovered at florence by the italian astronomer, g.b. donati. this comet, a magnificent object, was visible for more than three months with the naked eye. its tail was then millions of miles in length. it was found to revolve around the sun in a period of over years, and to go out in its journey to about - / times the distance of neptune. its motion is retrograde, that is to say, in the contrary direction to the usual movement in the solar system. a number of beautiful drawings of donati's comet were made by the american astronomer, g.p. bond. one of the best of these is reproduced on plate xvii., p. . in there appeared a great comet. on the th of june of that year the earth and moon actually passed through its tail; but no effects were noticed, other than a peculiar luminosity in the sky. in the year there appeared another large comet, known as tebbutt's comet, from the name of its discoverer. this was the _first comet of which a satisfactory photograph was obtained_. the photograph in question was taken by the late m. janssen. the comet of was of vast size and brilliance. it approached so close to the sun that it passed through some , miles of the solar corona. though its orbit was not found to have been altered by this experience, its nucleus displayed signs of breaking up. some very fine photographs of this comet were obtained at the cape of good hope by mr. (now sir david) gill. the comet of was followed with the telescope nearly up to the orbit of saturn, which seems to be the greatest distance at which a comet has ever been seen. the _first discovery of a comet by photographic means_[ ] was made by professor barnard in ; and, since then, photography has been employed with marked success in the detection of small periodic comets. the best comet seen in the northern hemisphere since that of , appears to have been daniel's comet of (see plate xviii., p. ). this comet was discovered on june , , by mr. z. daniel, at princeton observatory, new jersey, u.s.a. it became visible to the naked eye about mid-july of that year, and reached its greatest brilliancy about the end of august. it did not, however, attract much popular attention, as its position in the sky allowed it to be seen only just before dawn. [ ] with the exception, of course, of such an anomaly as the retrograde motion of the ninth satellite of saturn. [ ] if we except the case of the comet which was photographed near the solar corona in the eclipse of . [illustration: plate xviii. daniel's comet of from a photograph taken, on august th, , by dr. max wolf, at the astrophysical observatory, heidelberg. the instrument used was a -inch reflecting telescope, and the time of exposure was fifteen minutes. as the telescope was guided to follow the moving comet, the stars have imprinted themselves upon the photographic plate as short trails. this is clearly the opposite to what is depicted on plate xiii. (page )] chapter xx remarkable comets if eclipses were a cause of terror in past ages, comets appear to have been doubly so. their much longer continuance in the sight of men had no doubt something to say to this, and also the fact that they arrived without warning; it not being then possible to give even a rough prediction of their return, as in the case of eclipses. as both these phenomena were occasional, and out of the ordinary course of things, they drew exceptional attention as unusual events always do; for it must be allowed that quite as wonderful things exist, but they pass unnoticed merely because men have grown accustomed to them. for some reason the ancients elected to class comets along with meteors, the aurora borealis, and other phenomena of the atmosphere, rather than with the planets and the bodies of the spaces beyond. the sudden appearance of these objects led them to be regarded as signs sent by the gods to announce remarkable events, chief among these being the deaths of monarchs. shakespeare has reminded us of this in those celebrated lines in _julius cæsar_:-- "when beggars die there are no comets seen, the heavens themselves blaze forth the death of princes." numbed by fear, the men of old blindly accepted these presages of fate; and did not too closely question whether the threatened danger was to their own nation or to some other, to their ruler or to his enemy. now and then, as in the case of the roman emperor vespasian, there was a cynical attempt to apply some reasoning to the portent. that emperor, in alluding to the comet of a.d. , is reported to have said: "this hairy star does not concern me; it menaces rather the king of the parthians, for he is hairy and i am bald." vespasian, all the same, died shortly afterwards! pliny, in his natural history, gives several instances of the terrible significance which the ancients attached to comets. "a comet," he says, "is ordinarily a very fearful star; it announces no small effusion of blood. we have seen an example of this during the civil commotion of octavius." a very brilliant comet appeared in b.c., and about the same time an earthquake caused helicè and bura, two towns in achaia, to be swallowed up by the sea. the following remark made by seneca concerning it shows that the ancients did not consider comets merely as precursors, but even as actual _causes_ of fatal events: "this comet, so anxiously observed by every one, _because of the great catastrophe which it produced as soon as it appeared_, the submersion of bura and helicè." comets are by no means rare visitors to our skies, and very few years have elapsed in historical times without such objects making their appearance. in the dark and middle ages, when europe was split up into many small kingdoms and principalities, it was, of course, hardly possible for a comet to appear without the death of some ruler occurring near the time. critical situations, too, were continually arising in those disturbed days. the end of louis le debonnaire was hastened, as the reader will, no doubt, recollect, by the great eclipse of ; but it was firmly believed that a comet which had appeared a year or two previously presaged his death. the comet of is reported to have _influenced_ the abdication of the emperor charles v.; but curiously enough, this event had already taken place before the comet made its appearance! such beliefs, no doubt, had a very real effect upon rulers of a superstitious nature, or in a weak state of health. for instance, gian galeazzo visconti, duke of milan, was sick when the comet of appeared. after seeing it, he is said to have exclaimed: "i render thanks to god for having decreed that my death should be announced to men by this celestial sign." his malady then became worse, and he died shortly afterwards. it is indeed not improbable that such superstitious fears in monarchs were fanned by those who would profit by their deaths, and yet did not wish to stain their own hands with blood. evil though its effects may have been, this morbid interest which past ages took in comets has proved of the greatest service to our science. had it not been believed that the appearance of these objects was attended with far-reaching effects, it is very doubtful whether the old chroniclers would have given themselves the trouble of alluding to them at all; and thus the modern investigators of cometary orbits would have lacked a great deal of important material. we will now mention a few of the most notable comets which historians have recorded. a comet which appeared in b.c. was thought to betoken the success of the expedition undertaken in that year by timoleon of corinth against sicily. "the gods by an extraordinary prodigy announced his success and future greatness: a burning torch appeared in the heavens throughout the night and preceded the fleet of timoleon until it arrived off the coast of sicily." the comet of b.c. was generally believed to be the soul of cæsar on its way to heaven. josephus tells us that in a.d. several prodigies, and amongst them a comet in the shape of a sword, announced the destruction of jerusalem. this comet is said to have remained over the city for the space of a year! a comet which appeared in a.d. was considered to have announced the death of the emperor constantine. but perhaps the most celebrated comet of early times was the one which appeared in a.d. . that year was, in more than one way, big with portent, for there had long been a firm belief that the christian era could not possibly run into four figures. men, indeed, steadfastly believed that when the thousand years had ended, the millennium would immediately begin. therefore they did not reap neither did they sow, they toiled not, neither did they spin, and the appearance of the comet strengthened their convictions. the fateful year, however, passed by without anything remarkable taking place; but the neglect of husbandry brought great famine and pestilence over europe in the years which followed. in april , that year fraught with such immense consequences for england, a comet appeared. no one doubted but that it was a presage of the success of the conquest, and perhaps, indeed, it had its due weight in determining the minds and actions of the men who took part in the expedition. _nova stella, novus rex_ ("a new star, a new sovereign") was a favourite proverb of the time. the chroniclers, with one accord, have delighted to relate that the normans, "guided by a comet," invaded england. a representation of this object appears in the bayeux tapestry (see fig. , p. ).[ ] [illustration: fig. .--the comet of , as represented in the bayeux tapestry. (from the _world of comets_.)] we have mentioned halley's comet of , and how it revisits the neighbourhood of the earth at intervals of seventy-six years. the comet of has for many years been supposed to be halley's comet on one of its visits. the identity of these two, however, was only quite recently placed beyond all doubt by the investigations of messrs cowell and crommelin. this comet appeared also in , when john huniades was defending belgrade against the turks led by mahomet ii., the conqueror of constantinople, and is said to have paralysed both armies with fear. the middle ages have left us descriptions of comets, which show only too well how the imagination will run riot under the stimulus of terror. for instance, the historian, nicetas, thus describes the comet of the year : "after the romans were driven from constantinople a prognostic was seen of the excesses and crimes to which andronicus was to abandon himself. a comet appeared in the heavens similar to a writhing serpent; sometimes it extended itself, sometimes it drew itself in; sometimes, to the great terror of the spectators, it opened a huge mouth; it seemed that, as if thirsting for human blood, it was upon the point of satiating itself." and, again, the celebrated ambrose paré, the father of surgery, has left us the following account of the comet of , which appeared in his own time: "this comet," said he, "was so horrible, so frightful, and it produced such great terror in the vulgar, that some died of fear, and others fell sick. it appeared to be of excessive length, and was of the colour of blood. at the summit of it was seen the figure of a bent arm, holding in its hand a great sword, as if about to strike. at the end of the point there were three stars. on both sides of the rays of this comet were seen a great number of axes, knives, blood-coloured swords, among which were a great number of hideous human faces, with beards and bristling hair." paré, it is true, was no astronomer; yet this shows the effect of the phenomenon, even upon a man of great learning, as undoubtedly he was. it should here be mentioned that nothing very remarkable happened at or near the year . concerning the comet of , the extraordinary story got about that, at rome, a hen had laid an egg on which appeared a representation of the comet! but the superstitions with regard to comets were now nearing their end. the last blow was given by halley, who definitely proved that they obeyed the laws of gravitation, and circulated around the sun as planets do; and further announced that the comet of had a period of seventy-six years, which would cause it to reappear in the year . we have seen how this prediction was duly verified. we have seen, too, how this comet appeared again in , and how it is due to return in the early part of . [ ] with regard to the words "isti mirant stella" in the figure, mr. w.t. lynn suggests that they may not, after all, be the grammatically bad latin which they appear, but that the legend is really "isti mirantur stellam," the missing letters being supposed to be hidden by the building and the comet. chapter xxi meteors or shooting stars any one who happens to gaze at the sky for a short time on a clear night is pretty certain to be rewarded with a view of what is popularly known as a "shooting star." such an object, however, is not a star at all, but has received its appellation from an analogy; for the phenomenon gives to the inexperienced in these matters an impression as if one of the many points of light, which glitter in the vaulted heaven, had suddenly become loosened from its place, and was falling towards the earth. in its passage across the sky the moving object leaves behind a trail of light which usually lasts for a few moments. shooting stars, or meteors, as they are technically termed, are for the most part very small bodies, perhaps no larger than peas or pebbles, which, dashing towards our earth from space beyond, are heated to a white heat, and reduced to powder by the friction resulting from their rapid passage into our atmosphere. this they enter at various degrees of speed, in some cases so great as miles a second. the speed, of course, will depend greatly upon whether the earth and the meteors are rushing towards each other, or whether the latter are merely overtaking the earth. in the first of these cases the meteors will naturally collide with the atmosphere with great force; in the other case they will plainly come into it with much less rapidity. as has been already stated, it is from observations of such bodies that we are enabled to estimate, though very imperfectly, the height at which the air around our globe practically ceases, and this height is imagined to be somewhere about miles. fortunate, indeed, is it for us that there is a goodly layer of atmosphere over our heads, for, were this not so, these visitors from space would strike upon the surface of our earth night and day, and render existence still more unendurable than many persons choose to consider it. to what a bombardment must the moon be continually subject, destitute as she is of such an atmospheric shield! it is only in the moment of their dissolution that we really learn anything about meteors, for these bodies are much too small to be seen before they enter our atmosphere. the débris arising from their destruction is wafted over the earth, and, settling down eventually upon its surface, goes to augment the accumulation of that humble domestic commodity which men call dust. this continual addition of material tends, of course, to increase the mass of the earth, though the effect thus produced will be on an exceedingly small scale. the total number of meteors moving about in space must be practically countless. the number which actually dash into the earth's atmosphere during each year is, indeed, very great. professor simon newcomb, the well-known american astronomer, has estimated that, of the latter, those large enough to be seen with the naked eye cannot be in all less than , , , per annum. ten times more numerous still are thought to be those insignificant ones which are seen to pass like mere sparks of light across the field of an observer's telescope. until comparatively recent times, perhaps up to about a hundred years ago, it was thought that meteors were purely terrestrial phenomena which had their origin in the upper regions of the air. it, however, began to be noticed that at certain periods of the year these moving objects appeared to come from definite areas of the sky. considerations, therefore, respecting their observed velocities, directions, and altitudes, gave rise to the theory that they are swarms of small bodies travelling around the sun in elongated elliptical orbits, all along the length of which they are scattered, and that the earth, in its annual revolution, rushing through the midst of such swarms at the same epoch each year, naturally entangles many of them in its atmospheric net. the dates at which the earth is expected to pass through the principal meteor-swarms are now pretty well known. these swarms are distinguished from one another by the direction of the sky from which the meteors seem to arrive. many of the swarms are so wide that the earth takes days, and even weeks, to pass through them. in some of these swarms, or streams, as they are also called, the meteors are distributed with fair evenness along the entire length of their orbits, so that the earth is greeted with a somewhat similar shower at each yearly encounter. in others, the chief portions are bunched together, so that, in certain years, the display is exceptional (see fig. , p. ). that part of the heavens from which a shower of meteors is seen to emanate is called the "radiant," or radiant point, because the foreshortened view we get of the streaks of light makes it appear as if they radiated outwards from this point. in observations of these bodies the attention of astronomers is directed to registering the path and speed of each meteor, and to ascertaining the position of the radiant. it is from data such as these that computations concerning the swarms and their orbits are made. [illustration: fig. .--passage of the earth through the thickest portion of a meteor swarm. the earth and the meteors are here represented as approaching each other from opposite directions.] for the present state of knowledge concerning meteors, astronomy is largely indebted to the researches of mr. w.f. denning, of bristol, and of the late professor a.s. herschel. during the course of each year the earth encounters a goodly number of meteor-swarms. three of these, giving rise to fine displays, are very well known--the "perseids," or august meteors, and the "leonids" and "bielids," which appear in november. of the above three the _leonid_ display is by far the most important, and the high degree of attention paid to it has laid the foundation of meteoric astronomy in much the same way that the study of the fascinating corona has given such an impetus to our knowledge of the sun. the history of this shower of meteors may be traced back as far as a.d. , which was known as the "year of the stars." it is related that in that year, on the night of october th--the shower now comes about a month later--whilst the moorish king, ibrahim ben ahmed, lay dying before cosenza, in calabria, "a multitude of falling stars scattered themselves across the sky like rain," and the beholders shuddered at what they considered a dread celestial portent. we have, however, little knowledge of the subsequent history of the leonids until , since which time the maximum shower has appeared with considerable regularity at intervals of about thirty-three years. but it was not until that they sprang into especial notice. on the th november in that year a splendid display was witnessed at cumana, in south america, by the celebrated travellers, humboldt and bonpland. finer still, and surpassing all displays of the kind ever seen, was that of november , , when the meteors fell thick as snowflakes, , being estimated to have appeared during seven hours. some of them were even so bright as to be seen in full daylight. the radiant from which the meteors seem to diverge was ascertained to be situated in the head of the constellation of the lion, or "sickle of leo," as it is popularly termed, whence their name--leonids. it was from a discussion of the observations then made that the american astronomer, olmsted, concluded that these meteors sprang upon us from interplanetary space, and were not, as had been hitherto thought, born of our atmosphere. later on, in , olbers formulated the theory that the bodies in question travelled around the sun in an elliptical orbit, and at the same time he established the periodicity of the maximum shower. the periodic time of recurrence of this maximum, namely, about thirty-three years, led to eager expectancy as drew near. hopes were then fulfilled, and another splendid display took place, of which sir robert ball, who observed it, has given a graphic description in his _story of the heavens_. the display was repeated upon a smaller scale in the two following years. the leonids were henceforth deemed to hold an anomalous position among meteor swarms. according to theory the earth cut through their orbit at about the same date each year, and so a certain number were then seen to issue from the radiant. but, in addition, after intervals of thirty-three years, as has been seen, an exceptional display always took place; and this state of things was not limited to one year alone, but was repeated at each meeting for about three years running. the further assumption was, therefore, made that the swarm was much denser in one portion of the orbit than elsewhere,[ ] and that this congested part was drawn out to such an extent that the earth could pass through the crossing place during several annual meetings, and still find it going by like a long procession (see fig. , p. ). in accordance with this ascertained period of thirty-three years, the recurrence of the great leonid shower was timed to take place on the th of november . but there was disappointment then, and the displays which occurred during the few years following were not of much importance. a good deal of comment was made at the time, and theories were accordingly put forward to account for the failure of the great shower. the most probable explanation seems to be, that the attraction of one of the larger planets--jupiter perhaps--has diverted the orbit somewhat from its old position, and the earth does not in consequence cut through the swarm in the same manner as it used to do. the other november display alluded to takes place between the rd and th of that month. it is called the _andromedid_ shower, because the meteors appear to issue from the direction of the constellation of andromeda, which at that period of the year is well overhead during the early hours of the night. these meteors are also known by the name of _bielids_, from a connection which the orbit assigned to them appears to have with that of the well-known comet of biela. m. egenitis, director of the observatory of athens, accords to the bielids a high antiquity. he traces the shower back to the days of the emperor justinian. theophanes, the chronicler of that epoch, writing of the famous revolt of nika in the year a.d. , says:--"during the same year a great fall of stars came from the evening till the dawn." m. egenitis notes another early reference to these meteors in a.d. , during the reign of the eastern emperor, constantine copronymous. writing of that year, nicephorus, a patriarch of constantinople, has as follows:--"all the stars appeared to be detached from the sky, and to fall upon the earth." the bielids, however, do not seem to have attracted particular notice until the nineteenth century. attention first began to be riveted upon them on account of their suspected connection with biela's comet. it appeared that the same orbit was shared both by that comet and the bielid swarm. it will be remembered that the comet in question was not seen after its appearance in . since that date, however, the bielid shower has shown an increased activity; which was further noticed to be especially great in those years in which the comet, had it still existed, would be due to pass near the earth. the third of these great showers to which allusion has above been made, namely, the _perseids_, strikes the earth about the th of august; for which reason it is known on the continent under the name of the "tears of st. lawrence," the day in question being sacred to that saint. this shower is traceable back many centuries, even as far as the year a.d. . the name given to these meteors, "perseids," arises from the fact that their radiant point is situated in the constellation of perseus. this shower is, however, not by any means limited to the particular night of august th, for meteors belonging to the swarm may be observed to fall in more or less varying quantities from about july th to august nd. the perseid meteors sometimes fall at the rate of about sixty per hour. they are noted for their great rapidity of motion, and their trails besides often persist for a minute or two before being disseminated. unlike the other well-known showers, the radiants of which are stationary, that of the perseids shifts each night a little in an easterly direction. the orbit of the perseids cuts that of the earth almost perpendicularly. the bodies are generally supposed to be the result of the disintegration of an ancient comet which travelled in the same orbit. tuttle's comet, which passed close to the earth in , also belongs to this orbit; and its period of revolution is calculated to be years. the perseids appear to be disseminated all along this great orbit, for we meet them in considerable quantities each year. the bodies in question are in general particularly small. the swarm has, however, like most others, a somewhat denser portion, and through this the earth passed in . the _aphelion_, or point where the far end of the orbit turns back again towards the sun, is situated right away beyond the path of neptune, at a distance of forty-eight times that of the earth from the sun. the comet of also belongs to the perseid orbit. it revisited the neighbourhood of the earth in , and should have returned in . but we have no record of it in that year; for which omission the then politically disturbed state of europe may account. if not already disintegrated, this comet is due to return in . this supposed connection between comets and meteor-swarms must be also extended to the case of the leonids. these meteors appear to travel along the same track as tempel's comet of . it is considered that the attractions of the various bodies of the solar system upon a meteor swarm must eventually result in breaking up the "bunched" portion, so that in time the individual meteors should become distributed along the whole length of the orbit. upon this assumption the perseid swarm, in which the meteors are fairly well scattered along its path, should be of greater age than the leonid. as to the leonid swarm itself, le verrier held that it was first brought into the solar system in a.d. , having been captured from outer space by the gravitative action of the planet uranus. the acknowledged theory of meteor swarms has naturally given rise to an idea, that the sunlight shining upon such a large collection of particles ought to render a swarm visible before its collision with the earth's atmosphere. several attempts have therefore been made to search for approaching swarms by photography, but, so far, it appears without success. it has also been proposed, by mr. w.h.s. monck, that the stars in those regions from which swarms are due, should be carefully watched, to see if their light exhibits such temporary diminutions as would be likely to arise from the momentary interposition of a cloud of moving particles. between ten and fifteen years ago it happened that several well-known observers, employed in telescopic examination of the sun and moon, reported that from time to time they had seen small dark bodies, sometimes singly, sometimes in numbers, in passage across the discs of the luminaries. it was concluded that these were meteors moving in space beyond the atmosphere of the earth. the bodies were called "dark meteors," to emphasise the fact that they were seen in their natural condition, and not in that momentary one in which they had hitherto been always seen; _i.e._ when heated to white heat, and rapidly vaporised, in the course of their passage through the upper regions of our air. this "discovery" gave promise of such assistance to meteor theories, that calculations were made from the directions in which they had been seen to travel, and the speeds at which they had moved, in the hope that some information concerning their orbits might be revealed. but after a while some doubt began to be thrown upon their being really meteors, and eventually an australian observer solved the mystery. he found that they were merely tiny particles of dust, or of the black coating on the inner part of the tube of the telescope, becoming detached from the sides of the eye-piece and falling across the field of view. he was led to this conclusion by having noted that a gentle tapping of his instrument produced the "dark" bodies in great numbers! thus the opportunity of observing meteors beyond our atmosphere had once more failed. _meteorites_, also known as ærolites and fireballs, are usually placed in quite a separate category from meteors. they greatly exceed the latter in size, are comparatively rare, and do not appear in any way connected with the various showers of meteors. the friction of their passage through the atmosphere causes them to shine with a great light; and if not shattered to pieces by internal explosions, they reach the ground to bury themselves deep in it with a great rushing and noise. when found by uncivilised peoples, or savages, they are, on account of their celestial origin, usually regarded as objects of wonder and of worship, and thus have arisen many mythological legends and deifications of blackened stones. on the other hand, when they get into the possession of the civilised, they are subjected to careful examinations and tests in chemical laboratories. the bodies are, as a rule, composed of stone, in conjunction with iron, nickel, and such elements as exist in abundance upon our earth; though occasionally specimens are found which are practically pure metal. in the museums of the great capitals of both continents are to be seen some fine collections of meteorites. several countries--greenland and mexico, for instance--contain in the soil much meteoric iron, often in masses so large as to baffle all attempts at removal. blocks of this kind have been known to furnish the natives in their vicinity for many years with sources of workable iron. the largest meteorite in the world is one known as the anighito meteorite. it was brought to the united states by the explorer peary, who found it at cape york in greenland. he estimates its weight at from to tons. one found in mexico, called the bacubirito, comes next, with an estimated weight of - / tons. the third in size is the willamette meteorite, found at willamette in oregon in . it measures × - / × - / feet, and weighs about - / tons. [ ] the "gem" of the meteor ring, as it has been termed. chapter xxii the stars in the foregoing chapters we have dealt at length with those celestial bodies whose nearness to us brings them into our especial notice. the entire room, however, taken up by these bodies, is as a mere point in the immensities of star-filled space. the sun, too, is but an ordinary star; perhaps quite an insignificant one[ ] in comparison with the majority of those which stud that background of sky against which the planets are seen to perform their wandering courses. dropping our earth and the solar system behind, let us go afield and explore the depths of space. we have seen how, in very early times, men portioned out the great mass of the so-called "fixed stars" into divisions known as constellations. the various arrangements, into which the brilliant points of light fell as a result of perspective, were noticed and roughly compared with such forms as were familiar to men upon the earth. imagination quickly saw in them the semblances of heroes and of mighty fabled beasts; and, around these monstrous shapes, legends were woven, which told how the great deeds done in the misty dawn of historical time had been enshrined by the gods in the sky as an example and a memorial for men. though the centuries have long outlived such fantasies, yet the constellation figures and their ancient names have been retained to this day, pretty well unaltered for want of any better arrangement. the great and little bears, cassiopeia, perseus, and andromeda, orion and the rest, glitter in our night skies just as they did centuries and centuries ago. many persons seem to despair of gaining any real knowledge of astronomy, merely because they are not versed in recognising the constellations. for instance, they will say:--"what is the use of my reading anything about the subject? why, i believe i couldn't even point out the great bear, were i asked to do so!" but if such persons will only consider for a moment that what we call the great bear has no existence in fact, they need not be at all disheartened. could we but view this familiar constellation from a different position in space, we should perhaps be quite unable to recognise it. mountain masses, for instance, when seen from new directions, are often unrecognisable. it took, as we have seen, a very long time for men to acknowledge the immense distances of the stars from our earth. their seeming unchangeableness of position was, as we have seen, largely responsible for the idea that the earth was immovable in space. it is a wonder that the copernican system ever gained the day in the face of this apparent fixity of the stars. as time went on, it became indeed necessary to accord to these objects an almost inconceivable distance, in order to account for the fact that they remained apparently quite undisplaced, notwithstanding the journey of millions of miles which the earth was now acknowledged to make each year around the sun. in the face of the gradual and immense improvement in telescopes, this apparent immobility of the stars was, however, not destined to last. the first ascertained displacement of a star, namely that of cygni, noted by bessel in the year , definitely proved to men the truth of the copernican system. since then some forty more stars have been found to show similar tiny displacements. we are, therefore, in possession of the fact, that the actual distances of a few out of the great host can be calculated. to mention some of these. the nearest star to the earth, so far as we yet know, is alpha centauri, which is distant from us about billions of miles. the light from this star, travelling at the stupendous rate of about , miles per second, takes about - / years to reach our earth, or, to speak astronomically, alpha centauri is about - / "light years" distant from us. sirius--the brightest star in the whole sky--is at twice this distance, _i.e._ about - / light years. vega is about light years distant from us, capella about , and arcturus about . the displacements, consequent on the earth's movement, have, however, plainly nothing to say to any real movements on the part of the stars themselves. the old idea was that the stars were absolutely fixed; hence arose the term "fixed stars"--a term which, though inaccurate, has not yet been entirely banished from the astronomical vocabulary. but careful observations extending over a number of years have shown slight changes of position among these bodies; and such alterations cannot be ascribed to the revolution of the earth in its orbit, for they appear to take place in every direction. these evidences of movement are known as "proper motions," that is to say, actual motions in space proper to the stars themselves. stars which are comparatively near to us show, as a rule, greater proper motions than those which are farther off. it must not, however, be concluded that these proper motions are of any very noticeable amounts. they are, as a matter of fact, merely upon the same apparently minute scale as other changes in the heavens; and would largely remain unnoticed were it not for the great precision of modern astronomical instruments. one of the swiftest moving of the stars is a star of the sixth magnitude in the constellation of the great bear; which is known as " groombridge," because this was the number assigned to it in a catalogue of stars made by an astronomer of that name. it is popularly known as the "runaway star," a name given to it by professor newcomb. its speed is estimated to be at least miles per second. it may be actually moving at a much greater rate, for it is possible that we see its path somewhat foreshortened. a still greater proper motion--the greatest, in fact, known--is that of an eighth magnitude star in the southern hemisphere, in the constellation of pictor. nothing, indeed, better shows the enormous distance of the stars from us, and the consequent inability of even such rapid movements to alter the appearance of the sky during the course of ages, than the fact that it would take more than two centuries for the star in question to change its position in the sky by a space equal to the apparent diameter of the moon; a statement which is equivalent to saying that, were it possible to see this star with the naked eye, which it is not, at least twenty-five years would have to elapse before one would notice that it had changed its place at all! both the stars just mentioned are very faint. that in pictor is, as has been said, not visible to the naked eye. it appears besides to be a very small body, for sir david gill finds a parallax which makes it only as far off from us as sirius. the groombridge star, too, is just about the limit of ordinary visibility. it is, indeed, a curious fact that the fainter stars seem, on the average, to be moving more rapidly than the brighter. investigations into proper motions lead us to think that every one of the stars must be moving in space in some particular direction. to take a few of the best known. sirius and vega are both approaching our system at a rate of about miles per second, arcturus at about miles per second, while capella is receding from us at about miles per second. of the twin brethren, castor and pollux, castor is moving away from us at about - / miles per second, while pollux is coming towards us at about miles per second. much of our knowledge of proper motions has been obtained indirectly by means of the spectroscope, on the doppler principle already treated of, by which we are enabled to ascertain whether a source from which light is coming is approaching or receding. the sun being, after all, a mere star, it will appear only natural for it also to have a proper motion of its own. this is indeed the case; and it is rushing along in space at a rate of between ten and twelve miles per second, carrying with it its whole family of planets and satellites, of comets and meteors. the direction in which it is advancing is towards a point in the constellation of lyra, not far from its chief star vega. this is shown by the fact that the stars about the region in question appear to be opening out slightly, while those in the contrary portion of the sky appear similarly to be closing together. sir william herschel was the first to discover this motion of the sun through space; though in the idea that such a movement might take place he seems to have been anticipated by mayer in , by michell in , and by lalande in . a suggestion has been made that our solar system, in its motion through the celestial spaces, may occasionally pass through regions where abnormal magnetic conditions prevail, in consequence of which disturbances may manifest themselves throughout the system at the same instant. thus the sun may be getting the credit of _producing_ what it merely reacts to in common with the rest of its family. but this suggestion, plausible though it may seem, will not explain why the magnetic disturbances experienced upon our earth show a certain dependence upon such purely local facts, as the period of the sun's rotation, for instance. one would very much like to know whether the movement of the sun is along a straight line, or in an enormous orbit around some centre. the idea has been put forward that it may be moving around the centre of gravity of the whole visible stellar universe. mädler, indeed, propounded the notion that alcyone--the chief star in the group known as the pleiades--occupied this centre, and that everything revolved around it. he went even further to proclaim that here was the place of the almighty, the mansion of the eternal! but mädler's ideas upon this point have long been shelved. to return to the general question of the proper motion of stars. in several instances these motions appear to take place in groups, as if certain stars were in some way associated together. for example, a large number of the stars composing the pleiades appear to be moving through space in the same direction. also, of the seven stars composing the plough, all but two--the star at the end of its "handle," and that one of the "pointers," as they are called, which is the nearer to the pole star--have a common proper motion, _i.e._ are moving in the same direction and nearly at the same rate. further still, the well-known dutch astronomer, professor kapteyn, of groningen, has lately reached the astonishing conclusion that a great part of the visible universe is occupied by two vast streams of stars travelling in opposite directions. in both these great streams, the individual bodies are found, besides, to be alike in design, alike in chemical constitution, and alike in the stage of their development. a fable related by the persian astronomer, al sufi (tenth century, a.d.) shows well the changes in the face of the sky which proper motions are bound to produce after great lapses of time. according to this fable the stars sirius and procyon were the sisters of the star canopus. canopus married rigel (another star,) but, having murdered her, he fled towards the south pole, fearing the anger of his sisters. the fable goes on to relate, among other things, that sirius followed him across the milky way. mr. j. e. gore, in commenting on the story, thinks that it may be based upon a tradition of sirius having been seen by the men of the stone age on the opposite side of the milky way to that on which it now is. sirius is in that portion of the heavens _from_ which the sun is advancing. its proper motion is such that it is gaining upon the earth at the rate of about ten miles per second, and so it must overtake the sun after the lapse of great ages. vega, on the other hand, is coming towards us from that part of the sky _towards_ which the sun is travelling. it should be about half a million years before the sun and vega pass by one another. those who have specially investigated this question say that, as regards the probability of a near approach, it is much more likely that vega will be then so far to one side of the sun, that her brightness will not be much greater than it is at this moment. considerations like these call up the chances of stellar collisions. such possibilities need not, however, give rise to alarm; for the stars, as a rule, are at such great distances from each other, that the probability of relatively near approaches is slight. we thus see that the constellations do not in effect exist, and that there is in truth no real background to the sky. we find further that the stars are strewn through space at immense distances from each other, and are moving in various directions hither and thither. the sun, which is merely one of them, is moving also in a certain direction, carrying the solar system along with it. it seems, therefore, but natural to suppose that many a star may be surrounded by some planetary system in a way similar to ours, which accompanies it through space in the course of its celestial journeyings. [ ] vega, for instance, shines one hundred times more brightly than the sun would do, were it to be removed to the distance at which that star is from us. chapter xxiii the stars--_continued_ the stars appear to us to be scattered about the sky without any orderly arrangement. further, they are of varying degrees of brightness; some being extremely brilliant, whilst others can but barely be seen. the brightness of a star may arise from either of two causes. on the one hand, the body may be really very bright in itself; on the other hand, it may be situated comparatively near to us. sometimes, indeed, both these circumstances may come into play together. since variation in brightness is the most noticeable characteristic of the stars, men have agreed to class them in divisions called "magnitudes." this term, it must be distinctly understood, is employed in such classification without any reference whatever to actual size, being merely taken to designate roughly the amount of light which we receive from a star. the twenty brightest stars in the sky are usually classed in the first magnitude. in descending the scale, each magnitude will be noticed to contain, broadly speaking, three times as many stars as the one immediately above it. thus the second magnitude contains , the third , the fourth , the fifth , and the sixth . the last of these magnitudes is about the limit of the stars which we are able to see with the naked eye. adding, therefore, the above numbers together, we find that, without the aid of the telescope, we cannot see more than about stars in the entire sky--northern and southern hemispheres included. quite a small telescope will, however, allow us to see down to the ninth magnitude, so that the total number of stars visible to us with such very moderate instrumental means will be well over , . it must not, however, be supposed that the stars included within each magnitude are all of exactly the same brightness. in fact, it would be difficult to say if there exist in the whole sky two stars which send us precisely the same amount of light. in arranging the magnitudes, all that was done was to make certain broad divisions, and to class within them such stars as were much on a par with regard to brightness. it may here be noted that a standard star of the first magnitude gives us about one hundred times as much light as a star of the sixth magnitude, and about one million times as much as one of the sixteenth magnitude--which is near the limit of what we can see with the very best telescope. though the first twenty stars in the sky are popularly considered as being of the first magnitude, yet several of them are much brighter than an average first magnitude star would be. for instance, sirius--the brightest star in the whole sky--is equal to about eleven first magnitude stars, like, say, aldebaran. in consequence of such differences, astronomers are agreed in classifying the brightest of them as _brighter_ than the standard first magnitude star. on this principle sirius would be about two and a half magnitudes _above_ the first. this notation is usefully employed in making comparisons between the amount of light which we receive from the sun, and that which we get from an individual star. thus the sun will be about twenty-seven and a half magnitudes _above_ the first magnitude. the range, therefore, between the light which we receive from the sun (considered merely as a very bright star) and the first magnitude stars is very much greater than that between the latter and the faintest star which can be seen with the telescope, or even registered upon the photographic plate. to classify stars merely by their magnitudes, without some definite note of their relative position in the sky, would be indeed of little avail. we must have some simple method of locating them in the memory, and the constellations of the ancients here happily come to our aid. a system combining magnitudes with constellations was introduced by bayer in , and is still adhered to. according to this the stars in each constellation, beginning with the brightest star, are designated by the letters of the greek alphabet taken in their usual order. for example, in the constellation of canis major, or the greater dog, the brightest star is the well-known sirius, called by the ancients the "dog star"; and this star, in accordance with bayer's method, has received the greek letter [a] (alpha), and is consequently known as alpha canis majoris.[ ] as soon as the greek letters are used up in this way the roman alphabet is brought into requisition, after which recourse is had to ordinary numbers. notwithstanding this convenient arrangement, some of the brightest stars are nearly always referred to by certain proper names given to them in old times. for instance, it is more usual to speak of sirius, arcturus, vega, capella, procyon, aldebaran, regulus, and so on, than of [a] canis majoris, [a] boötis, [a] lyræ, [a] aurigæ, [a] canis minoris, [a] tauri, [a] leonis, &c. &c. in order that future generations might be able to ascertain what changes were taking place in the face of the sky, astronomers have from time to time drawn up catalogues of stars. these lists have included stars of a certain degree of brightness, their positions in the sky being noted with the utmost accuracy possible at the period. the earliest known catalogue of this kind was made, as we have seen, by the celebrated greek astronomer, hipparchus, about the year b.c. it contained stars. it was revised and brought up to date by ptolemy in a.d. . another celebrated list was that drawn up by the persian astronomer, al sufi, about the year a.d. . in it stars were noted down. a catalogue of stars was made in by the famous danish astronomer, tycho brahe. among modern catalogues that of argelander ( - ) contained as many as , stars. it was extended by schönfeld so as to include a portion of the southern hemisphere, in which way , more stars were added. in recent years a project was placed on foot of making a photographic survey of the sky, the work to be portioned out among various nations. a great part of this work has already been brought to a conclusion. about , , stars will appear upon the plates; but, so far, it has been proposed to catalogue only about a million and a quarter of the brightest of them. this idea of surveying the face of the sky by photography sprang indirectly from the fine photographs which sir david gill took, when at the cape of good hope, of the comet of . the immense number of star-images which had appeared upon his plates suggested the idea that photography could be very usefully employed to register the relative positions of the stars. the arrangement of seven stars known as the "plough" is perhaps the most familiar configuration in the sky (see plate xix., p. ). in the united states it is called the "dipper," on account of its likeness to the outline of a saucepan, or ladle. "charles' wain" was the old english name for it, and readers of cæsar will recollect it under _septentriones_, or the "seven stars," a term which that writer uses as a synonym for the north. though identified in most persons' minds with _ursa major_, or the great bear, the plough is actually only a small portion of that famous constellation. six out of the seven stars which go to make up the well-known figure are of the second magnitude, while the remaining one, which is the middle star of the group, is of the third. the greek letters, as borne by the individual stars of the plough, are a plain transgression of bayer's method as above described, for they have certainly not been allotted here in accordance with the proper order of brightness. for instance, the third magnitude star, just alluded to as being in the middle of the group, has been marked with the greek letter [d] (delta); and so is made to take rank _before_ the stars composing what is called the "handle" of the plough, which are all of the second magnitude. sir william herschel long ago drew attention to the irregular manner in which bayer's system had been applied. it is, indeed, a great pity that this notation was not originally worked out with greater care and correctness; for, were it only reliable, it would afford great assistance to astronomers in judging of what changes in relative brightness have taken place among the stars. though we may speak of using the constellations as a method of finding our way about the sky, it is, however, to certain marked groupings in them of the brighter stars that we look for our sign-posts. most of the constellations contain a group or so of noticeable stars, whose accidental arrangement dimly recalls the outline of some familiar geometrical figure and thus arrests the attention.[ ] for instance, in an almost exact line with the two front stars of the plough, or "pointers" as they are called,[ ] and at a distance about five times as far away as the interval between them, there will be found a third star of the second magnitude. this is known as polaris, or the pole star, for it very nearly occupies that point of the heaven towards which the north pole of the earth's axis is _at present_ directed (see plate xix., p. ). thus during the apparently daily rotation of the heavens, this star looks always practically stationary. it will, no doubt, be remembered how shakespeare has put into the mouth of julius cæsar these memorable words:-- "but i am constant as the northern star, of whose true-fix'd and resting quality there is no fellow in the firmament." [illustration: plate xix. the sky around the north pole we see here the plough, the pole star, ursa minor, auriga, cassiopeia's chair, and lyra. also the circle of precession, along which the pole makes a complete revolution in a period of , years, and the temporary star discovered by tycho brahe in the year . (page )] on account of the curvature of the earth's surface, the height at which the pole star is seen above the horizon at any place depends regularly upon the latitude; that is to say, the distance of the place in question from the equator. for instance, at the north pole of the earth, where the latitude is greatest, namely, °, the pole star will appear directly overhead; whereas in england, where the latitude is about °, it will be seen a little more than half way up the northern sky. at the equator, where the latitude is _nil_, the pole star will be on the horizon due north. in consequence of its unique position, the pole star is of very great service in the study of the constellations. it is a kind of centre around which to hang our celestial ideas--a starting point, so to speak, in our voyages about the sky. according to the constellation figures, the pole star is in _ursa minor_, or the little bear, and is situated at the end of the tail of that imaginary figure (see plate xix., p. ). the chief stars of this constellation form a group not unlike the plough, except that the "handle" is turned in the contrary direction. the americans, in consequence, speak of it as the "little dipper." before leaving this region of the sky, it will be well to draw attention to the second magnitude star [z] in the great bear (zeta ursæ majoris), which is the middle star in the "handle" of the plough. this star is usually known as mizar, a name given to it by the arabians. a person with good eyesight can see quite near to it a fifth magnitude star, known under the name of alcor. we have here a very good example of that deception in the estimation of objects in the sky, which has been alluded to in an earlier chapter. alcor is indeed distant from mizar by about one-third the apparent diameter of the moon, yet no one would think so! on the other side of polaris from the plough, and at about an equal apparent distance, will be found a figure in the form of an irregular "w", made up of second and third magnitude stars. this is the well-known "cassiopeia's chair"--portion of the constellation of _cassiopeia_ (see plate xix., p. ). on either side of the pole star, about midway between the plough and cassiopeia's chair, but a little further off from it than these, are the constellations of _auriga_ and _lyra_ (see plate xix., p. ). the former constellation will be easily recognised, because its chief features are a brilliant yellowish first magnitude star, with one of the second magnitude not far from it. the first magnitude star is capella, the other is [b] aurigæ. lyra contains only one first magnitude star--vega, pale blue in colour. this star has a certain interest for us from the fact that, as a consequence of that slow shift of direction of the earth's axis known as precession, it will be very near the north pole of the heavens in some , years, and so will then be considered the pole star (see plate xix., p. ). the constellation of lyra itself, it must also be borne in mind, occupies that region of the heavens towards which the solar system is travelling. the handle of the plough points roughly towards the constellation of _boötes_, in which is the brilliant first magnitude star arcturus. this star is of an orange tint. between boötes and lyra lie the constellations of _corona borealis_ (or the northern crown) and _hercules_. the chief feature of corona borealis, which is a small constellation, is a semicircle of six small stars, the brightest of which is of the second magnitude. the constellation of hercules is very extensive, but contains no star brighter than the third magnitude. near to lyra, on the side away from hercules, are the constellations of _cygnus_ and _aquila_. of the two, the former is the nearer to the pole star, and will be recognised by an arrangement of stars widely set in the form of a cross, or perhaps indeed more like the framework of a boy's kite. the position of aquila will be found through the fact that three of its brightest stars are almost in a line and close together. the middle of these is altair, a yellowish star of the first magnitude. at a little distance from ursa major, on the side away from the pole star, is the constellation of _leo_, or the lion. its chief feature is a series of seven stars, supposed to form the head of that animal. the arrangement of these stars is, however, much more like a sickle, wherefore this portion of the constellation is usually known as the "sickle of leo." at the end of the handle of the sickle is a white first magnitude star--regulus. the reader will, no doubt, recollect that it is from a point in the sickle of leo that the leonid meteors appear to radiate. the star second in brightness in the constellation of leo is known as denebola. this star, now below the second magnitude, seems to have been very much brighter in the past. it is noted, indeed, as a brilliant first magnitude star by al sufi, that famous persian astronomer who lived, as we have seen, in the tenth century. ptolemy also notes it as of the first magnitude. in the neighbourhood of auriga, and further than it from the pole star, are several remarkable constellations--taurus, orion, gemini, canis minor, and canis major (see plate xx., p. ). the first of these, _taurus_ (or the bull), contains two conspicuous star groups--the pleiades and the hyades. the pleiades are six or seven small stars quite close together, the majority of which are of the fourth magnitude. this group is sometimes occulted by the moon. the way in which the stars composing it are arranged is somewhat similar to that in the plough, though of course on a scale ever so much smaller. the impression which the group itself gives to the casual glance is thus admirably pictured in tennyson's _locksley hall_:-- "many a night i saw the pleiads, rising through the mellow shade, glitter like a swarm of fire-flies tangled in a silver braid." [illustration: plate xx. orion and his neighbours we see here that magnificent region of the sky which contains the brightest star of all--sirius. note also especially the milky way, the pleiades, the hyades, and the "belt" and "sword" of orion. (page )] the group of the hyades occupies the "head" of the bull, and is much more spread out than that of the pleiades. it is composed besides of brighter stars, the brightest being one of the first magnitude, aldebaran. this star is of a red colour, and is sometimes known as the "eye of the bull." the constellation of _orion_ is easily recognised as an irregular quadrilateral formed of four bright stars, two of which, betelgeux (reddish) and rigel (brilliant white), are of the first magnitude. in the middle of the quadrilateral is a row of three second magnitude stars, known as the "belt" of orion. jutting off from this is another row of stars called the "sword" of orion. the constellation of _gemini_, or the twins, contains two bright stars--castor and pollux--close to each other. pollux, though marked with the greek letter [b], is the brighter of the two, and nearly of the standard first magnitude. just further from the pole than gemini, is the constellation of _canis minor_, or the lesser dog. its chief star is a white first magnitude one--procyon. still further again from the pole than canis minor is the constellation of _canis major_, or the greater dog. it contains the brightest star in the whole sky, the first magnitude star sirius, bluish-white in colour, also known as the "dog star." this star is almost in line with the stars forming the belt of orion, and is not far from that constellation. taken in the following order, the stars capella, [b] aurigæ, castor, pollux, procyon, and sirius, when they are all above the horizon at the same time, form a beautiful curve stretching across the heaven. the groups of stars visible in the southern skies have by no means the same fascination for us as those in the northern. the ancients were in general unacquainted with the regions beyond the equator, and so their scheme of constellations did not include the sky around the south pole of the heavens. in modern times, however, this part of the celestial expanse was also portioned out into constellations for the purpose of easy reference; but these groupings plainly lack that simplicity of conception and legendary interest which are so characteristic of the older ones. the brightest star in the southern skies is found in the constellation of _argo_, and is known as canopus. in brightness it comes next to sirius, and so is second in that respect in the entire heaven. it does not, however, rise above the english horizon. of the other southern constellations, two call for especial notice, and these adjoin each other. one is _centaurus_ (or the centaur), which contains the two first magnitude stars, [a] and [b] centauri. the first of these, alpha centauri, comes next in brightness to canopus, and is notable as being the nearest of all the stars to our earth. the other constellation is called _crux_, and contains five stars set in the form of a rough cross, known as the "southern cross." the brightest of these, [a] crucis, is of the first magnitude. owing to the precession of the equinoxes, which, as we have seen, gradually shifts the position of the pole among the stars, certain constellations used to be visible in ancient times in more northerly latitudes than at present. for instance, some five thousand years ago the southern cross rose above the english horizon, and was just visible in the latitude of london. it has, however, long ago even ceased to be seen in the south of europe. the constellation of crux happens to be situated in that remarkable region of the southern skies, in which are found the stars canopus and alpha centauri, and also the most brilliant portion of the milky way. it is believed to be to this grand celestial region that allusion is made in the book of job (ix. ), under the title of the "chambers of the south." the "cross" must have been still a notable feature in the sky of palestine in the days when that ancient poem was written. there is no star near enough to the southern pole of the heavens to earn the distinction of south polar star. the galaxy, or _milky way_ (see plate xx., p. ), is a broad band of diffused light which is seen to stretch right around the sky. the telescope, however, shows it to be actually composed of a great host of very faint stars--too faint, indeed, to be separately distinguished with the naked eye. along a goodly stretch of its length it is cleft in two; while near the south pole of the heavens it is entirely cut across by a dark streak. in this rapid survey of the face of the sky, we have not been able to do more than touch in the broadest manner upon some of the most noticeable star groups and a few of the most remarkable stars. to go any further is not a part of our purpose; our object being to deal with celestial bodies as they actually are, and not in those groupings under which they display themselves to us as a mere result of perspective. [ ] attention must here be drawn to the fact that the name of the constellation is always put in the genitive case. [ ] the early peoples, as we have seen, appear to have been attracted by those groupings of the stars which reminded them in a way of the figures of men and animals. we moderns, on the other hand, seek almost instinctively for geometrical arrangements. this is, perhaps, symptomatic of the evolution of the race. in the growth of the individual we find, for example, something analogous. a child, who has been given pencil and paper, is almost certain to produce grotesque drawings of men and animals; whereas the idle and half-conscious scribblings which a man may make upon his blotting-paper are usually of a geometrical character. [ ] because the line joining them _points_ in the direction of the pole star. chapter xxiv systems of stars many stars are seen comparatively close together. this may plainly arise from two reasons. firstly, the stars may happen to be almost in the same line of sight; that is to say, seen in nearly the same direction; and though one star may be ever so much nearer to us than the other, the result will give all the appearance of a related pair. a seeming arrangement of two stars in this way is known as a "double," or double star; or, indeed, to be very precise, an "optical double." secondly, in a pair of stars, both bodies may be about the same distance from us, and actually connected as a system like, for instance, the moon and the earth. a pairing of stars in this way, though often casually alluded to as a double star, is properly termed a "binary," or binary system. but collocations of stars are by no means limited to two. we find, indeed, all over the sky such arrangements in which there are three or more stars; and these are technically known as "triple" or "multiple" stars respectively. further, groups are found in which a great number of stars are closely massed together, such a massing together of stars being known as a "cluster." the pole star (polaris) is a double star, one of the components being of a little below the second magnitude, and the other a little below the ninth. they are so close together that they appear as one star to the naked eye, but they may be seen separate with a moderately sized telescope. the brighter star is yellowish, and the faint one white. this brighter star is found _by means of the spectroscope_ to be actually composed of three stars so very close together that they cannot be seen separately even with a telescope. it is thus a triple star, and the three bodies of which it is composed are in circulation about each other. two of them are darker than the third. the method of detecting binary stars by means of the spectroscope is an application of doppler's principle. it will, no doubt, be remembered that, according to the principle in question, we are enabled, from certain shiftings of the lines in the spectrum of a luminous body, to ascertain whether that body is approaching us or receding from us. now there are certain stars which always appear single even in the largest telescopes, but when the spectroscope is directed to them a spectrum _with two sets of lines_ is seen. such stars must, therefore, be double. further, if the shiftings of the lines, in a spectrum like this, tell us that the component stars are making small movements to and from us which go on continuously, we are therefore justified in concluding that these are the orbital revolutions of a binary system greatly compressed by distance. such connected pairs of stars, since they cannot be seen separately by means of any telescope, no matter how large, are known as "spectroscopic binaries." in observations of spectroscopic binaries we do not always get a double spectrum. indeed, if one of the components be below a certain magnitude, its spectrum will not appear at all; and so we are left in the strange uncertainty as to whether this component is merely faint or actually dark. it is, however, from the shiftings of the lines in the spectrum of the other component that we see that an orbital movement is going on, and are thus enabled to conclude that two bodies are here connected into a system, although one of these bodies resolutely refuses directly to reveal itself even to the all-conquering spectroscope. mizar, that star in the handle of the plough to which we have already drawn attention, will be found with a small telescope to be a fine double, one of the components being white and the other greenish. actually, however, as the american astronomer, professor f.r. moulton, points out, these stars are so far from each other that if we could be transferred to one of them we should see the other merely as an ordinary bright star. the spectroscope shows that the brighter of these stars is again a binary system of two huge suns, the components revolving around each other in a period of about twenty days. this discovery made by professor e.c. pickering, the _first_ of the kind by means of the spectroscope, was announced in from the harvard observatory in the united states. a star close to vega, known as [e] (epsilon) lyræ (see plate xix., p. ), is a double, the components of which may be seen separately with the naked eye by persons with very keen eyesight. if this star, however, be viewed with the telescope, the two companions will be seen far apart; and it will be noticed that each of them is again a double. by means of the spectroscope capella is shown to be really composed of two stars (one about twice as bright as the other) situated very close together and forming a binary system. sirius is also a binary system; but it is what is called a "visual" one, for its component stars may be _seen_ separately in very large telescopes. its double, or rather binary, nature, was discovered in by the celebrated optician alvan g. clark, while in the act of testing the -inch refracting telescope, then just constructed by his firm, and now at the dearborn observatory, illinois, u.s.a. the companion is only of the tenth magnitude, and revolves around sirius in a period of about fifty years, at a mean distance equal to about that of uranus from the sun. seen from sirius, it would shine only something like our full moon. it must be self-luminous and not a mere planet; for mr. gore has shown that if it shone only by the light reflected from sirius, it would be quite invisible even in the great yerkes telescope. procyon is also a binary, its companion having been discovered by professor j.m. schaeberle at the lick observatory in . the period of revolution in this system is about forty years. observations by mr. t. lewis of greenwich seem, however, to point to the companion being a small nebula rather than a star. the star [ê] (eta) cassiopeiæ (see plate xix., p. ), is easily seen as a fine double in telescopes of moderate size. it is a binary system, the component bodies revolving around their common centre of gravity in a period of about two hundred years. this system is comparatively near to us, _i.e._ about nine light years, or a little further off than sirius. in a small telescope the star castor will be found double, the components, one of which is brighter than the other, forming a binary system. the fainter of these was found by belopolsky, with the spectroscope, to be composed of a system of two stars, one bright and the other either dark or not so bright, revolving around each other in a period of about three days. the brighter component of castor is also a spectroscopic binary, with a period of about nine days; so that the whole of what we see with the naked eye as castor, is in reality a remarkable system of four stars in mutual orbital movement. alpha centauri--the nearest star to the earth--is a visual binary, the component bodies revolving around each other in a period of about eighty-one years. the extent of this system is about the same as that of sirius. viewed from each other, the bodies would shine only like our sun as seen from neptune. among the numerous binary stars the orbits of some fifty have been satisfactorily determined. many double stars, for which this has not yet been done, are, however, believed to be, without doubt, binary. in some cases a parallax has been found; so that we are enabled to estimate in miles the actual extent of such systems, and the masses of the bodies in terms of the sun's mass. most of the spectroscopic binaries appear to be upon a smaller scale than the telescopic ones. some are, indeed, comparatively speaking, quite small. for instance, the component stars forming [b] aurigæ are about eight million miles apart, while in [z] geminorum, the distance between the bodies is only a little more than a million miles. spectroscopic binaries are probably very numerous. professor w.w. campbell, director of the lick observatory, estimates, for instance, that, out of about every half-a-dozen stars, one is a spectroscopic binary. it is only in the case of binary systems that we can discover the masses of stars at all. these are ascertained from their movements with regard to each other under the influence of their mutual gravitative attractions. in the case of simple stars we have clearly nothing of the kind to judge by; though, if we can obtain a parallax, we may hazard a guess from their brightness. binary stars were incidentally discovered by sir william herschel. in his researches to get a stellar parallax he had selected a number of double stars for test purposes, on the assumption that, if one of such a pair were much nearer than the other, it might show a displacement with regard to its neighbour as a direct consequence of the earth's orbital movement around the sun. he, however, failed entirely to obtain any parallaxes, the triumph in this being, as we have seen, reserved for bessel. but in some of the double stars which he had selected, he found certain alterations in the relative positions of the bodies, which plainly were not a consequence of the earth's motion, but showed rather that there was an actual circling movement of the bodies themselves under their mutual attractions. it is to be noted that the existence of such connected pairs had been foretold as probable by the rev. john michell, who lived a short time before herschel. the researches into binary systems--both those which can be seen with the eye and those which can be observed by means of the spectroscope, ought to impress upon us very forcibly the wide sway of the law of gravitation. of star clusters about are known, and such systems often contain several thousand stars. they usually cover an area of sky somewhat smaller than the moon appears to fill. in most clusters the stars are very faint, and, as a rule, are between the twelfth and sixteenth magnitudes. it is difficult to say whether these are actually small bodies, or whether their faintness is due merely to their great distance from us, since they are much too far off to show any appreciable parallactic displacement. mr. gore, however, thinks there is good evidence to show that the stars in clusters are really close, and that the clusters themselves fill a comparatively small space. one of the finest examples of a cluster is the great globular one, in the constellation of hercules, discovered by halley in . it contains over stars, and upon a clear, dark night is visible to the naked eye as a patch of light. in the telescope, however, it is a wonderful object. there are also fine clusters in the constellations of auriga, pegasus, and canes venatici. in the southern heavens there are some magnificent examples of globular clusters. this hemisphere seems, indeed, to be richer in such objects than the northern. for instance, there is a great one in the constellation of the centaur, containing some stars (see plate xxi., p. ). [illustration: plate xxi. the great globular cluster in the southern constellation of centaurus from a photograph taken at the cape observatory, on may th, . time of exposure, hour. (page )] certain remarkable groups of stars, of a nature similar to clusters, though not containing such faint or densely packed stars as those we have just alluded to, call for a mention in this connection. the best example of such star groups are the pleiades and the hyades (see plate xx., p. ), coma berenices, and præsepe (or the beehive), the last-named being in the constellation of cancer. stars which alter in their brightness are called _variable stars_, or "variables." the first star whose variability attracted attention is that known as omicron ceti, namely, the star marked with the greek letter [o] (omicron) in the constellation of cetus, or the whale, a constellation situated not far from taurus. this star, the variability of which was discovered by fabricius in , is also known as mira, or the "wonderful," on account of the extraordinary manner in which its light varies from time to time. the star known by the name of algol,[ ] popularly called the "demon star"--whose astronomical designation is [b] (beta) persei, or the star second in brightness in the constellation of perseus--was discovered by goodricke, in the year , to be a variable star. in the following year [b] lyræ, the star in lyra next in order of brightness after vega, was also found by the same observer to be a variable. it may be of interest to the reader to know that goodricke was deaf and dumb, and that he died in at the early age of twenty-one years! it was not, however, until the close of the nineteenth century that much attention was paid to variable stars. now several hundreds of these are known, thanks chiefly to the observations of, amongst others, professor s.c. chandler of boston, u.s.a., mr. john ellard gore of dublin, and dr. a.w. roberts of south africa. this branch of astronomy has not, indeed, attracted as much popular attention as it deserves, no doubt because the nature of the work required does not call for the glamour of an observatory or a large telescope. the chief discoveries with regard to variable stars have been made by the naked eye, or with a small binocular. the amount of variation is estimated by a comparison with other stars. as in many other branches of astronomy, photography is now employed in this quest with marked success; and lately many variable stars have been found to exist in clusters and nebulæ. it was at one time considered that a variable star was in all probability a body, a portion of whose surface had been relatively darkened in some manner akin to that in which sun spots mar the face of the sun; and that when its axial rotation brought the less illuminated portions in turn towards us, we witnessed a consequent diminution in the star's general brightness. herschel, indeed, inclined to this explanation, for his belief was that all the stars bore spots like those of the sun. it appears preferably thought nowadays that disturbances take place periodically in the atmosphere or surroundings of certain stars, perhaps through the escape of imprisoned gases, and that this may be a fruitful cause of changes of brilliancy. the theory in question will, however, apparently account for only one class of variable star, namely, that of which mira ceti is the best-known example. the scale on which it varies in brightness is very great, for it changes from the second to the ninth magnitude. for the other leading type of variable star, algol, of which mention has already been made, is the best instance. the shortness of the period in which the changes of brightness in such stars go their round, is the chief characteristic of this latter class. the period of algol is a little under three days. this star when at its brightest is of about the second magnitude, and when least bright is reduced to below the third magnitude; from which it follows that its light, when at the minimum, is only about one-third of what it is when at the maximum. it seems definitely proved by means of the spectroscope that variables of this kind are merely binary stars, too close to be separated by the telescope, which, as a consequence of their orbits chancing to be edgewise towards us, eclipse each other in turn time after time. if, for instance, both components of such a pair are bright, then when one of them is right behind the other, we will not, of course, get the same amount of light as when they are side by side. if, on the other hand, one of the components happens to be dark or less luminous and the other bright, the manner in which the light of the bright star will be diminished when the darker star crosses its face should easily be understood. it is to the second of these types that algol is supposed to belong. the algol system appears to be composed of a body about as broad as our sun, which regularly eclipses a brighter body which has a diameter about half as great again. since the companion of algol is often spoken of as a _dark_ body, it were well here to point out that we have no evidence at all that it is entirely devoid of light. we have already found, in dealing with spectroscopic binaries, that when one of the component stars is below a certain magnitude[ ] its spectrum will not be seen; so one is left in the glorious uncertainty as to whether the body in question is absolutely dark, or darkish, or faint, or indeed only just out of range of the spectroscope. it is thought probable by good authorities that the companion of algol is not quite dark, but has some inherent light of its own. it is, of course, much too near algol to be seen with the largest telescope. there is in fact a distance of only from two to three millions of miles between the bodies, from which mr. gore infers that they would probably remain unseparated even in the largest telescope which could ever be constructed by man. the number of known variables of the algol type is, so far, small; not much indeed over thirty. in some of them the components are believed to revolve touching each other, or nearly so. an extreme example of this is found in the remarkable star v. puppis, an algol variable of the southern hemisphere. both its components are bright, and the period of light variation is about one and a half days. dr. a. w. roberts finds that the bodies are revolving around each other in actual contact. _temporary stars_ are stars which have suddenly blazed out in regions of the sky where no star was previously seen, and have faded away more or less gradually. it was the appearance of such a star, in the year b.c., which prompted hipparchus to make his celebrated catalogue, with the object of leaving a record by which future observers could note celestial changes. in another star of this kind flashed out in the constellation of cassiopeia (see plate xix., p. ), and was detected by tycho brahe. it became as bright as the planet venus, and eventually was visible in the day-time. two years later, however, it disappeared, and has never since been seen. in kepler recorded a similar star in the constellation of ophiuchus which grew to be as bright as jupiter. it also lasted for about two years, and then faded away, leaving no trace behind. it is rarely, however, that temporary stars attain to such a brilliance; and so possibly in former times a number of them may have appeared, but not have risen to a sufficient magnitude to attract attention. even now, unless such a star becomes clearly visible to the naked eye, it runs a good chance of not being detected. a curious point, worth noting, with regard to temporary stars is that the majority of them have appeared in the milky way. these sudden visitations have in our day received the name of _novæ_; that is to say, "new" stars. two, in recent years, attracted a good deal of attention. the first of these, known as nova aurigæ, or the new star in the constellation of auriga, was discovered by dr. t.d. anderson at edinburgh in january . at its greatest brightness it attained to about the fourth magnitude. by april it had sunk to the twelfth, but during august it recovered to the ninth magnitude. after this last flare-up it gradually faded away. the startling suddenness with which temporary stars usually spring into being is the groundwork upon which theories to account for their origin have been erected. that numbers of dark stars, extinguished suns, so to speak, may exist in space, there is a strong suspicion; and it is just possible that we have an instance of one dark stellar body in the companion of algol. that such dark stars might be in rapid motion is reasonable to assume from the already known movements of bright stars. two dark bodies might, indeed, collide together, or a collision might take place between a dark star and a star too faint to be seen even with the most powerful telescope. the conflagration produced by the impact would thus appear where nothing had been seen previously. again, a similar effect might be produced by a dark body, or a star too faint to be seen, being heated to incandescence by plunging in its course through a nebulous mass of matter, of which there are many examples lying about in space. the last explanation, which is strongly reminiscent of what takes place in shooting stars, appears more probable than the collision theory. the flare-up of new stars continues, indeed, only for a comparatively short time; whereas a collision between two bodies would, on the other hand, produce an enormous nebula which might take even millions of years to cool down. we have, indeed, no record of any such sudden appearance of a lasting nebula. the other temporary star, known as nova persei, or the new star in the constellation of perseus, was discovered early in the morning of february , , also by dr. anderson. a day later it had grown to be brighter than capella. photographs which had been taken, some three days previous to its discovery, of the very region of the sky in which it had burst forth, were carefully examined, and it was not found in these. at the end of two days after its discovery nova persei had lost one-third of its light. during the ensuing six months it passed through a series of remarkable fluctuations, varying in brightness between the third and fifth magnitudes. in the month of august it was seen to be surrounded by luminous matter in the form of a nebula, which appeared to be gradually spreading to some distance around. taking into consideration the great way off at which all this was taking place, it looked as if the new star had ejected matter which was travelling outward with a velocity equivalent to that of light. the remarkable theory was, however, put forward by professor kapteyn and the late dr. w.e. wilson that there might be after all no actual transmission of matter; but that perhaps the real explanation was the gradual _illumination_ of hitherto invisible nebulous matter, as a consequence of the flare-up which had taken place about six months before. it was, therefore, imagined that some dark body moving through space at a very rapid rate had plunged through a mass of invisible nebulous matter, and had consequently become heated to incandescence in its passage, very much like what happens to a meteor when moving through our atmosphere. the illumination thus set up temporarily in one point, being transmitted through the nebulous wastes around with the ordinary velocity of light, had gradually rendered this surrounding matter visible. on the assumptions required to fit in with such a theory, it was shown that nova persei must be at a distance from which light would take about three hundred years in coming to us. the actual outburst of illumination, which gave rise to this temporary star, would therefore have taken place about the beginning of the reign of james i. some recent investigations with regard to nova persei have, however, greatly narrowed down the above estimate of its distance from us. for instance, bergstrand proposes a distance of about ninety-nine light years; while the conclusions of mr. f.w. very would bring it still nearer, _i.e._ about sixty-five light years. the last celestial objects with which we have here to deal are the _nebulæ_. these are masses of diffused shining matter scattered here and there through the depths of space. nebulæ are of several kinds, and have been classified under the various headings of spiral, planetary, ring, and irregular. a typical _spiral_ nebula is composed of a disc-shaped central portion, with long curved arms projecting from opposite sides of it, which give an impression of rapid rotatory movement. the discovery of spiral nebulæ was made by lord rosse with his great -foot reflector. two good examples of these objects will be found in ursa major, while there is another fine one in canes venatici (see plate xxii., p. ), a constellation which lies between ursa major and boötes. but the finest spiral of all, perhaps the most remarkable nebula known to us, is the great nebula in the constellation of andromeda, (see plate xxiii., p. )--a constellation just further from the pole than cassiopeia. when the moon is absent and the night clear this nebula can be easily seen with the naked eye as a small patch of hazy light. it is referred to by al sufi. [illustration: plate xxii. spiral nebula in the constellation of canes venatici from a photograph by the late dr. w.e. wilson, d.sc., f.r.s. (page )] spiral nebulæ are white in colour, whereas the other kinds of nebula have a greenish tinge. they are also by far the most numerous; and the late professor keeler, who considered this the normal type of nebula, estimated that there were at least , of such spirals within the reach of the crossley reflector of the lick observatory. professor perrine has indeed lately raised this estimate to half a million, and thinks that with more sensitive photographic plates and longer exposures the number of spirals would exceed a million. the majority of these objects are very small, and appear to be distributed over the sky in a fairly uniform manner. _planetary_ nebulæ are small faint roundish objects which, when seen in the telescope, recall the appearance of a planet, hence their name. one of these nebulæ, known astronomically as g.c. , has recently been found to be rushing through space towards the earth at a rate of between thirty and forty miles per second. it seems strange, indeed, that any gaseous mass should move at such a speed! what are known as _ring_ nebulæ were until recently believed to form a special class. these objects have the appearance of mere rings of nebulous matter. much doubt has, however, been thrown upon their being rings at all; and the best authorities regard them merely as spiral nebulæ, of which we happen to get a foreshortened view. very few examples are known, the most famous being one in the constellation of lyra, usually known as the annular nebula in lyra. this object is so remote from us as to be entirely invisible to the naked eye. it contains a star of the fifteenth magnitude near to its centre. from photographs taken with the crossley reflector, professor schaeberle finds in this nebula evidences of spiral structure. it may here be mentioned that the great nebula in andromeda, which has now turned out to be a spiral, had in earlier photographs the appearance of a ring. there also exist nebulæ of _irregular_ form, the most notable being the great nebula in the constellation of orion (see plate xxiv., p. ). it is situated in the centre of the "sword" of orion (see plate xx., p. ). in large telescopes it appears as a magnificent object, and in actual dimensions it must be much on the same scale as the andromeda nebula. the spectroscope tells us that it is a mass of glowing gas. the trifid nebula, situated in the constellation of sagittarius, is an object of very strange shape. three dark clefts radiate from its centre, giving it an appearance as if it had been torn into shreds. the dumb-bell nebula, a celebrated object, so called from its likeness to a dumb-bell, turns out, from recent photographs taken by professor schaeberle, which bring additional detail into view, to be after all a great spiral. there is a nest, or rather a cluster of nebulæ in the constellation of coma berenices; over a hundred of these objects being here gathered into a space of sky about the size of our full moon. [illustration: plate xxiii. the great nebula in the constellation of andromeda from a photograph taken at the yerkes observatory. (page )] the spectroscope informs us that spiral nebulæ are composed of partially-cooled matter. their colour, as we have seen, is white. nebulæ of a greenish tint are, on the other hand, found to be entirely in a gaseous condition. just as the solar corona contains an unknown element, which for the time being has been called "coronium," so do the gaseous nebulæ give evidence of the presence of another unknown element. to this sir william huggins has given the provisional name of "nebulium." the _magellanic clouds_ are two patches of nebulous-looking light, more or less circular in form, which are situated in the southern hemisphere of the sky. they bear a certain resemblance to portions of the milky way, but are, however, not connected with it. they have received their name from the celebrated navigator, magellan, who seems to have been one of the first persons to draw attention to them. "nubeculæ" is another name by which they are known, the larger cloud being styled _nubecula major_ and the smaller one _nubecula minor_. they contain within them stars, clusters, and gaseous nebulæ. no parallax has yet been found for any object which forms part of the nubeculæ, so it is very difficult to estimate at what distance from us they may lie. they are, however, considered to be well within our stellar universe. having thus brought to a conclusion our all too brief review of the stars and the nebulæ--of the leading objects in fine which the celestial spaces have revealed to man--we will close this chapter with a recent summation by sir david gill of the relations which appear to obtain between these various bodies. "huggins's spectroscope," he says, "has shown that many nebulæ are not stars at all; that many well-condensed nebulæ, as well as vast patches of nebulous light in the sky, are but inchoate masses of luminous gas. evidence upon evidence has accumulated to show that such nebulæ consist of the matter out of which stars (_i.e._ suns) have been and are being evolved. the different types of star spectra form such a complete and gradual sequence (from simple spectra resembling those of nebulæ onwards through types of gradually increasing complexity) as to suggest that we have before us, written in the cryptograms of these spectra, the complete story of the evolution of suns from the inchoate nebula onwards to the most active sun (like our own), and then downward to the almost heatless and invisible ball. the period during which human life has existed upon our globe is probably too short--even if our first parents had begun the work--to afford observational proof of such a cycle of change in any particular star; but the fact of such evolution, with the evidence before us, can hardly be doubted."[ ] [ ] the name al gûl, meaning the demon, was what the old arabian astronomers called it, which looks very much as if they had already noticed its rapid fluctuations in brightness. [ ] mr. gore thinks that the companion of algol may be a star of the sixth magnitude. [ ] presidential address to the british association for the advancement of science (leicester, ), by sir david gill, k.c.b., ll.d., f.r.s., &c. &c. [illustration: plate xxiv. the great nebula in the constellation of orion from a photograph taken at the yerkes observatory. (page )] chapter xxv the stellar universe the stars appear fairly evenly distributed all around us, except in one portion of the sky where they seem very crowded, and so give one an impression of being very distant. this portion, known as the milky way, stretches, as we have already said, in the form of a broad band right round the entire heavens. in those regions of the sky most distant from the milky way the stars appear to be thinly sown, but become more and more closely massed together as the milky way is approached. this apparent distribution of the stars in space has given rise to a theory which was much favoured by sir william herschel, and which is usually credited to him, although it was really suggested by one thomas wright of durham in ; that is to say, some thirty years or more before herschel propounded it. according to this, which is known as the "disc" or "grindstone" theory, the stars are considered as arranged in space somewhat in the form of a thick disc, or grindstone, close to the _central_ parts of which our solar system is situated.[ ] thus we should see a greater number of stars when we looked out through the _length_ of such a disc in any direction, than when we looked out through its _breadth_. this theory was, for a time, supposed to account quite reasonably for the milky way, and for the gradual increase in the number of stars in its vicinity. it is quite impossible to verify directly such a theory, for we know the actual distance of only about forty-three stars. we are unable, therefore, definitely to assure ourselves whether, as the grindstone theory presupposes, the stellar universe actually reaches out very much further from us in the direction of the milky way than in the other parts of the sky. the theory is clearly founded upon the supposition that the stars are more or less equal in size, and are scattered through space at fairly regular distances from each other. brightness, therefore, had been taken as implying nearness to us, and faintness great distance. but we know to-day that this is not the case, and that the stars around us are, on the other hand, of various degrees of brightness and of all orders of size. some of the faint stars--for instance, the galloping star in pictor--are indeed nearer to us than many of the brighter ones. sirius, on the other hand, is twice as far off from us as [a] centauri, and yet it is very much brighter; while canopus, which in brightness is second only to sirius out of the whole sky, is too far off for its distance to be ascertained! it must be remembered that no parallax had yet been found for any star in the days of herschel, and so his estimations of stellar distances were necessarily of a very circumstantial kind. he did not, however, continue always to build upon such uncertain ground; but, after some further examination of the milky way, he gave up his idea that the stars were equally disposed in space, and eventually abandoned the grindstone theory. since we have no means of satisfactorily testing the matter, through finding out the various distances from us at which the stars are really placed, one might just as well go to the other extreme, and assume that the thickening of stars in the region of the milky way is not an effect of perspective at all, but that the stars in that part of the sky are actually more crowded together than elsewhere--a thing which astronomers now believe to be the case. looked at in this way, the shape of the stellar universe might be that of a globe-shaped aggregation of stars, in which the individuals are set at fairly regular distances from each other; the whole being closely encircled by a belt of densely packed stars. it must, however, be allowed that the gradual increase in the number of stars towards the milky way appears a strong argument in favour of the grindstone theory; yet the belt theory, as above detailed, seems to meet with more acceptance. there is, in fact, one marked circumstance which is remarkably difficult of explanation by means of the grindstone theory. this is the existence of vacant spaces--holes, so to speak, in the groundwork of the milky way. for instance, there is a cleft running for a good distance along its length, and there is also a starless gap in its southern portion. it seems rather improbable that such a great number of stars could have arranged themselves so conveniently, as to give us a clear view right out into empty space through such a system in its greatest thickness; as if, in fact, holes had been bored, and clefts made, from the boundary of the disc clean up to where our solar system lies. sir john herschel long ago drew attention to this point very forcibly. it is plain that such vacant spaces can, on the other hand, be more simply explained as mere holes in a belt; and the best authorities maintain that the appearance of the milky way confirms a view of this kind. whichever theory be indeed the correct one, it appears at any rate that the stars do not stretch out in every direction to an infinite distance; but that _the stellar system is of limited extent_, and has in fact a boundary. in the first place, science has no grounds for supposing that light is in any way absorbed or destroyed merely by its passage through the "ether," that imponderable medium which is believed to transmit the luminous radiations through space. this of course is tantamount to saying that all the direct light from all the stars should reach us, excepting that little which is absorbed in its passage through our own atmosphere. if stars, and stars, and stars existed in every direction outwards without end, it can be proved mathematically that in such circumstances there could not remain the tiniest space in the sky without a star to fill it, and that therefore the heavens would always blaze with light, and the night would be as bright as the noonday.[ ] how very far indeed this is from being the case, may be gathered from an estimate which has been made of the general amount of light which we receive from the stars. according to this estimate the sky is considered as more or less dark, the combined illumination sent to us by all the stars being only about the one-hundreth part of what we get from the full moon.[ ] secondly, it has been suggested that although light may not suffer any extinction or diminution from the ether itself, still a great deal of illumination may be prevented from reaching us through myriads of extinguished suns, or dark meteoric matter lying about in space. the idea of such extinguished suns, dark stars in fact, seems however to be merely founded upon the sole instance of the invisible companion of algol; but, as we have seen, there is no proof whatever that it is a dark body. again, some astronomers have thought that the dark holes in the milky way, "coal sacks," as they are called, are due to masses of cool, or partially cooled matter, which cuts off the light of the stars beyond. the most remarkable of these holes is one in the neighbourhood of the southern cross, known as the "coal sack in crux." but mr. gore thinks that the cause of the holes is to be sought for rather in what sir william herschel termed "clustering power," _i.e._ a tendency on the part of stars to accumulate in certain places, thus leaving others vacant; and the fact that globular and other clusters are to be found very near to such holes certainly seems corroborative of this theory. in summing up the whole question, professor newcomb maintains that there does not appear any evidence of the light from the milky way stars, which are apparently the furthest bodies we see, being intercepted by dark bodies or dark matter. as far as our telescopes can penetrate, he holds that we see the stars _just as they are_. also, if there did exist an infinite number of stars, one would expect to find evidence in some direction of an overpoweringly great force,--the centre of gravity of all these bodies. it is noticed, too, that although the stars increase in number with decrease in magnitude, so that as we descend in the scale we find three times as many stars in each magnitude as in the one immediately above it, yet this progression does not go on after a while. there is, in fact, a rapid falling off in numbers below the twelfth magnitude; which looks as if, at a certain distance from us, the stellar universe were beginning to _thin out_. again, it is estimated, by mr. gore and others, that only about millions of stars are to be seen in the whole of the sky with the best optical aids. this shows well the limited extent of the stellar system, for the number is not really great. for instance, there are from fifteen to sixteen times as many persons alive upon the earth at this moment! last of all, there appears to be strong photographic evidence that our sidereal system is limited in extent. two photographs taken by the late dr. isaac roberts of a region rich in stellar objects in the constellation of cygnus, clearly show what has been so eloquently called the "darkness behind the stars." one of these photographs was taken in , and the other in . on both occasions the state of the atmosphere was practically the same, and the sensitiveness of the films was of the same degree. the exposure in the first case was only one hour; in the second it was about two hours and a half. and yet both photographs show _exactly the same stars, even down to the faintest_. from this one would gather that the region in question, which is one of the most thickly star-strewn in the milky way, is _penetrable right through_ with the means at our command. dr. roberts himself in commenting upon the matter drew attention to the fact, that many astronomers seemed to have tacitly adopted the assumption that the stars extend indefinitely through space. from considerations such as these the foremost astronomical authorities of our time consider themselves justified in believing that the collection of stars around us is _finite_; and that although our best telescopes may not yet be powerful enough to penetrate to the final stars, still the rapid decrease in numbers as space is sounded with increasing telescopic power, points strongly to the conclusion that the boundaries of the stellar system may not lie very far beyond the uttermost to which we can at present see. is it possible then to make an estimate of the extent of this stellar system? whatever estimates we may attempt to form cannot however be regarded as at all exact, for we know the actual distances of such a very few only of the nearest of the stars. but our knowledge of the distances even of these few, permits us to assume that the stars close around us may be situated, on an average, at about eight light-years from each other; and that this holds good of the stellar spaces, with the exception of the encircling girdle of the milky way, where the stars seem actually to be more closely packed together. this girdle further appears to contain the greater number of the stars. arguing along these lines, professor newcomb reaches the conclusion that the farthest stellar bodies which we see are situated at about between and light-years from us. starting our inquiry from another direction, we can try to form an estimate by considering the question of proper motions. it will be noticed that such motions do not depend entirely upon the actual speed of the stars themselves, but that some of the apparent movement arises indirectly from the speed of our own sun. the part in a proper motion which can be ascribed to the movement of our solar system through space is clearly a displacement in the nature of a parallax--sir william herschel called it "_systematic_ parallax"; so that knowing the distance which we move over in a certain lapse of time, we are able to hazard a guess at the distances of a good many of the stars. an inquiry upon such lines must needs be very rough, and is plainly based upon the assumption that the stars whose distances we attempt to estimate are moving at an average speed much like that of our own sun, and that they are not "runaway stars" of the groombridge order. be that as it may, the results arrived at by professor newcomb from this method of reasoning are curiously enough very much on a par with those founded on the few parallaxes which we are really certain about; with the exception that they point to somewhat closer intervals between the individual stars, and so tend to narrow down our previous estimate of the extent of the stellar system. thus far we get, and no farther. our solar system appears to lie somewhere near the centre of a great collection of stars, separated each one from the other, on an average, by some billions of miles; the whole being arranged in the form of a mighty globular cluster. light from the nearest of these stars takes some four years to come to us. it takes about times as long to reach us from the confines of the system. this globe of stars is wrapt around closely by a stellar girdle, the individual stars in which are set together more densely than those in the globe itself. the entire arrangement appears to be constructed upon a very regular plan. here and there, as professor newcomb points out, the aspect of the heavens differs in small detail; but generally it may be laid down that the opposite portions of the sky, whether in the milky way itself, or in those regions distant from it, show a marked degree of symmetry. the proper motions of stars in corresponding portions of the sky reveal the same kind of harmony, a harmony which may even be extended to the various colours of the stars. the stellar system, which we see disposed all around us, appears in fine to bear all the marks of an _organised whole_. the older astronomers, to take sir william herschel as an example, supposed some of the nebulæ to be distant "universes." sir william was led to this conclusion by the idea he had formed that, when his telescopes failed to show the separate stars of which he imagined these objects to be composed, he must put down the failure to their stupendous distance from us. for instance, he thought the orion nebula, which is now known to be made up of glowing gas, to be an external stellar system. later on, however, he changed his mind upon this point, and came to the conclusion that "shining fluid" would better account both for this nebula, and for others which his telescopes had failed to separate into component stars. the old ideas with regard to external systems and distant universes have been shelved as a consequence of recent research. all known clusters and nebulæ are now firmly believed to lie _within_ our stellar system. this view of the universe of stars as a sort of island in the immensities, does not, however, give us the least idea about the actual extent of space itself. whether what is called space is really infinite, that is to say, stretches out unendingly in every direction, or whether it has eventually a boundary somewhere, are alike questions which the human mind seems utterly unable to picture to itself. [ ] the ptolemaic idea dies hard! [ ] even the milky way itself is far from being a blaze of light, which shows that the stars composing it do not extend outwards indefinitely. [ ] mr. gore has recently made some remarkable deductions, with regard to the amount of light which we get from the stars. he considers that most of this light comes from stars below the sixth magnitude; and consequently, if all the stars visible to the naked eye were to be blotted out, the glow of the night sky would remain practically the same as it is at present. going to the other end of the scale, he thinks also that the combined light which we get from all the stars below the seventeenth magnitude is so very small, that it may be neglected in such an estimation. he finds, indeed, that if there are stars so low as the twentieth magnitude, one hundred millions of them would only be equal in brightness to a single first-magnitude star like vega. on the other hand, it is possible that the light of the sky at night is not entirely due to starlight, but that some of it may be caused by phosphorescent glow. chapter xxvi the stellar universe--_continued_ it is very interesting to consider the proper motions of stars with reference to such an isolated stellar system as has been pictured in the previous chapter. these proper motions are so minute as a rule, that we are quite unable to determine whether the stars which show them are moving along in straight lines, or in orbits of immense extent. it would, in fact, take thousands of years of careful observation to determine whether the paths in question showed any degree of curving. in the case of the more distant stars, the accurate observations which have been conducted during the last hundred years have not so far revealed any proper motions with regard to them; but one cannot escape the conclusion that these stars move as the others do. if space outside our stellar system is infinite in extent, and if all the stars within that system are moving unchecked in every conceivable direction, the result must happen that after immense ages these stars will have drawn apart to such a distance from each other, that the system will have entirely disintegrated, and will cease to exist as a connected whole. eventually, indeed, as professor newcomb points out, the stars will have separated so far from each other that each will be left by itself in the midst of a black and starless sky. if, however, a certain proportion of stars have a speed sufficiently slow, they will tend under mutual attraction to be brought to rest by collisions, or forced to move in orbits around each other. but those stars which move at excessive speeds, such, for instance, as groombridge, or the star in the southern constellation of pictor, seem utterly incapable of being held back in their courses by even the entire gravitative force of our stellar system acting as a whole. these stars must, therefore, move eventually right through the system and pass out again into the empty spaces beyond. add to this; certain investigations, made into the speed of groombridge, furnish a remarkable result. it is calculated, indeed, that had this star been _falling through infinite space for ever_, pulled towards us by the combined gravitative force of our entire system of stars, it could not have gathered up anything like the speed with which it is at present moving. no force, therefore, which we can conjure out of our visible universe, seems powerful enough either to have impressed upon this runaway star the motion which it now has, or to stay it in its wild course. what an astounding condition of things! speculations like this call up a suspicion that there may yet exist other universes, other centres of force, notwithstanding the apparent solitude of our stellar system in space. it will be recollected that the idea of this isolation is founded upon such facts as, that the heavens do not blaze with light, and that the stars gradually appear to thin out as we penetrate the system with increasing telescopic power. but perchance there is something which hinders us from seeing out into space beyond our cluster of stars; which prevents light, in fact, from reaching us from other possible systems scattered through the depths beyond. it has, indeed, been suggested by mr. gore[ ] that the light-transmitting ether may be after all merely a kind of "atmosphere" of the stars; and that it may, therefore, thin off and cease a little beyond the confines of our stellar system, just as the air thins off and practically ceases at a comparatively short distance from the earth. a clashing together of solid bodies outside our atmosphere could plainly send us no sound, for there is no air extending the whole way to bear to our ears the vibrations thus set up; so light emitted from any body lying beyond our system of stars, would not be able to come to us if the ether, whose function it is to convey the rays of light, ceased at or near the confines of that system. perchance we have in this suggestion the key to the mystery of how our sun and the other stellar bodies maintain their functions of temperature and illumination. the radiations of heat and light arriving at the limits of this ether, and unable to pass any further, may be thrown back again into the system in some altered form of energy. but these, at best, are mere airy and fascinating speculations. we have, indeed, no evidence whatever that the luminiferous ether ceases at the boundary of the stellar system. if, therefore, it extends outwards infinitely in every direction, and if it has no absorbing or weakening effect on the vibrations which it transmits, we cannot escape from the conclusion that practically all the rays of light ever emitted by all the stars must chase one another eternally through the never-ending abysses of space. [ ] _planetary and stellar studies_, by john ellard gore, f.r.a.s., m.r.i.a., london, . chapter xxvii the beginning of things laplace's nebular hypothesis dwelling upon the fact that all the motions of revolution and rotation in the solar system, as known in his day, took place in the same direction and nearly in the same plane, the great french astronomer, laplace, about the year , put forward a theory to account for the origin and evolution of that system. he conceived that it had come into being as a result of the gradual contraction, through cooling, of an intensely heated gaseous lens-shaped mass, which had originally occupied its place, and had extended outwards beyond the orbit of the furthest planet. he did not, however, attempt to explain how such a mass might have originated! he went on to suppose that this mass, _in some manner_, perhaps by mutual gravitation among its parts, had acquired a motion of rotation in the same direction as the planets now revolve. as this nebulous mass parted with its heat by radiation, it contracted towards the centre. becoming smaller and smaller, it was obliged to rotate faster and faster in order to preserve its equilibrium. meanwhile, in the course of contraction, rings of matter became separated from the nucleus of the mass, and were left behind at various intervals. these rings were swept up into subordinate masses similar to the original nebula. these subordinate masses also contracted in the same manner, leaving rings behind them which, in turn, were swept up to form satellites. saturn's ring was considered, by laplace, as the only portion of the system left which still showed traces of this evolutionary process. it is even probable that it may have suggested the whole of the idea to him. laplace was, however, not the first philosopher who had speculated along these lines concerning the origin of the world. nearly fifty years before, in to be exact, thomas wright, of durham, had put forward a theory to account for the origin of the whole sidereal universe. in his theory, however, the birth of our solar system was treated merely as an incident. shortly afterwards the subject was taken up by the famous german philosopher, kant, who dealt with the question in a still more ambitious manner, and endeavoured to account in detail for the origin of the solar system as well as of the sidereal universe. something of the trend of such theories may be gathered from the remarkable lines in tennyson's _princess_:-- "this world was once a fluid haze of light, till toward the centre set the starry tides, and eddied into suns, that wheeling cast the planets." the theory, as worked out by kant, was, however, at the best merely a _tour de force_ of philosophy. laplace's conception was much less ambitious, for it did not attempt to explain the origin of the entire universe, but only of the solar system. being thus reasonably limited in its scope, it more easily obtained credence. the arguments of laplace were further founded upon a mathematical basis. the great place which he occupied among the astronomers of that time caused his theory to exert a preponderating influence on scientific thought during the century which followed. a modification of laplace's theory is the meteoritic hypothesis of sir norman lockyer. according to the views of that astronomer, the material of which the original nebula was composed is presumed to have been in the meteoric, rather than in the gaseous, state. sir norman lockyer holds, indeed, that nebulæ are, in reality, vast swarms of meteors, and the light they emit results from continual collisions between the constituent particles. the french astronomer, faye, also proposed to modify laplace's theory by assuming that the nebula broke up into rings all at once, and not in detail, as laplace had wished to suppose. the hypothesis of laplace fits in remarkably well with the theory put forward in later times by helmholtz, that the heat of the sun is kept up by the continual contraction of its mass. it could thus have only contracted to its present size from one very much larger. plausible, however, as laplace's great hypothesis appears on the surface, closer examination shows several vital objections, a few of those set forth by professor moulton being here enumerated-- although laplace held that the orbits of the planets were sufficiently near to being in the one plane to support his views, yet later investigators consider that their very deviations from this plane are a strong argument against the hypothesis. again, it is thought that if the theory were the correct explanation, the various orbits of the planets would be much more nearly circular than they are. it is also thought that such interlaced paths, as those in which the asteroids and the little planet eros move, are most unlikely to have been produced as a result of laplace's nebula. further, while each of the rings was sweeping up its matter into a body of respectable dimensions, its gravitative power would have been for the time being so weak, through being thus spread out, that any lighter elements, as, for instance, those of the gaseous order, would have escaped into space in accordance with the principles of the kinetic theory. _the idea that rings would at all be left behind at certain intervals during the contraction of the nebula is, perhaps, one of the weakest points in laplace's hypothesis._ mathematical investigation does not go to show that the rings, presuming they could be left behind during the contraction of the mass, would have aggregated into planetary bodies. indeed, it rather points to the reverse. lastly, such a discovery as that the ninth satellite of saturn revolves in a _retrograde_ direction--that is to say, in a direction contrary to the other revolutions and rotations in our solar system--appears directly to contradict the hypothesis. although laplace's hypothesis seems to break down under the keen criticism to which it has been subjected, yet astronomers have not relinquished the idea that our solar system has probably had its origin from a nebulous mass. but the apparent failure of the laplacian theory is emphasised by the fact, that _not a single example of a nebula, in the course of breaking up into concentric rings, is known to exist in the entire heaven_. indeed, as we saw in chapter xxiv., there seems to be no reliable example of even a "ring" nebula at all. mr. gore has pointed this out very succinctly in his recently published work, _astronomical essays_, where he says:--"to any one who still persists in maintaining the hypothesis of ring formation in nebulæ, it may be said that the whole heavens are against him." the conclusions of keeler already alluded to, that the spiral is the normal type of nebula, has led during the past few years to a new theory by the american astronomers, professors chamberlin and moulton. in the detailed account of it which they have set forth, they show that those anomalies which were stumbling-blocks to laplace's theory do not contradict theirs. to deal at length with this theory, to which the name of "planetesimal hypothesis" has been given, would not be possible in a book of this kind. but it may be of interest to mention that the authors of the theory in question remount the stream of time still further than did laplace, and seek to explain the _origin_ of the spiral nebulæ themselves in the following manner:-- having begun by assuming that the stars are moving apparently in every direction with great velocities, they proceed to point out that sooner or later, although the lapse of time may be extraordinarily long, collisions or near approaches between stars are bound to occur. in the case of collisions the chances are against the bodies striking together centrally, it being very much more likely that they will hit each other rather towards the side. the nebulous mass formed as a result of the disintegration of the bodies through their furious impact would thus come into being with a spinning movement, and a spiral would ensue. again, the stars may not actually collide, but merely approach near to each other. if very close, the interaction of gravitation will give rise to intense strains, or tides, which will entirely disintegrate the bodies, and a spiral nebula will similarly result. as happens upon our earth, two such tides would rise opposite to each other; and, consequently, it is a noticeable fact that spiral nebulæ have almost invariably two opposite branches (see plate xxii., p ). even if not so close, the gravitational strains set up would produce tremendous eruptions of matter; and in this case, a spiral movement would also be generated. on such an assumption the various bodies of the solar system may be regarded as having been ejected from parent masses. the acceptance of the planetesimal hypothesis in the place of the hypothesis of laplace will not, as we have seen, by any means do away with the probability that our solar system, and similar systems, have originated from a nebulous mass. on the contrary it puts that idea on a firmer footing than before. the spiral nebulæ which we see in the heavens are on a vast scale, and may represent the formation of stellar systems and globular clusters. our solar system may have arisen from a small spiral. we will close these speculations concerning the origin of things with a short sketch of certain investigations made in recent years by sir george h. darwin, of cambridge university, into the question of the probable birth of our moon. he comes to the conclusion that at least fifty-four millions of years ago the earth and moon formed one body, which had a diameter of a little over miles. this body rotated on an axis in about five hours, namely, about five times as fast as it does at present. the rapidity of the rotation caused such a tremendous strain that the mass was in a condition of, what is called, unstable equilibrium; very little more, in fact, being required to rend it asunder. the gravitational pull of the sun, which, as we have already seen, is in part the cause of our ordinary tides, supplied this extra strain, and a portion of the mass consequently broke off, which receded gradually from the rest and became what we now know as the moon. sir george darwin holds that the gravitational action of the sun will in time succeed in also disturbing the present apparent harmony of the earth-moon system, and will eventually bring the moon back towards the earth, so that after the lapse of great ages they will re-unite once again. in support of this theory of the terrestrial origin of the moon, professor w.h. pickering has put forward a bold hypothesis that our satellite had its origin in the great basin of the pacific. this ocean is roughly circular, and contains no large land masses, except the australian continent. he supposes that, prior to the moon's birth, our globe was already covered with a slight crust. in the tearing away of that portion which was afterwards destined to become the moon the remaining area of the crust was rent in twain by the shock; and thus were formed the two great continental masses of the old and new worlds. these masses floated apart across the fiery ocean, and at last settled in the positions which they now occupy. in this way professor pickering explains the remarkable parallelism which exists between the opposite shores of the atlantic. the fact of this parallelism had, however, been noticed before; as, for example, by the late rev. s.j. johnson, in his book _eclipses, past and future_, where we find the following passage:-- "if we look at our maps we shall see the parts of one continent that jut out agree with the indented portions of another. the prominent coast of africa would fit in the opposite opening between north and south america, and so in numerous other instances. a general rending asunder of the world would seem to have taken place when the foundations of the great deep were broken up." although professor pickering's theory is to a certain degree anticipated in the above words, still he has worked out the idea much more fully, and given it an additional fascination by connecting it with the birth of the moon. he points out, in fact, that there is a remarkable similarity between the lunar volcanoes and those in the immediate neighbourhood of the pacific ocean. he goes even further to suggest that australia is another portion of the primal crust which was detached out of the region now occupied by the indian ocean, where it was originally connected with the south of india or the east of africa. certain objections to the theory have been put forward, one of which is that the parallelism noticed between the opposite shores of the atlantic is almost too perfect to have remained through some sixty millions of years down to our own day, in the face of all those geological movements of upheaval and submergence, which are perpetually at work upon our globe. professor pickering, however, replies to this objection by stating that many geologists believe that the main divisions of land and water on the earth are permanent, and that the geological alterations which have taken place since these were formed have been merely of a temporary and superficial nature. chapter xxviii the end of things we have been trying to picture the beginning of things. we will now try to picture the end. in attempting this, we find that our theories must of necessity be limited to the earth, or at most to the solar system. the time-honoured expression "end of the world" really applies to very little beyond the end of our own earth. to the people of past ages it, of course, meant very much more. for them, as we have seen, the earth was the centre of everything; and the heavens and all around were merely a kind of minor accompaniment, created, as they no doubt thought, for their especial benefit. in the ancient view, therefore, the beginning of the earth meant the beginning of the universe, and the end of the earth the extinction of all things. the belief, too, was general that this end would be accomplished through fire. in the modern view, however, the birth and death of the earth, or indeed of the solar system, might pass as incidents almost unnoticed in space. they would be but mere links in the chain of cosmic happenings. a number of theories have been forward from time to time prognosticating the end of the earth, and consequently of human life. we will conclude with a recital of a few of them, though which, if any, is the true one, the last men alone can know. just as a living creature may at any moment die in the fulness of strength through sudden malady or accident, or, on the other hand, may meet with death as a mere consequence of old age, so may our globe be destroyed by some sudden cataclysm, or end in slow processes of decay. barring accidents, therefore, it would seem probable that the growing cold of the earth, or the gradual extinction of the sun, should after many millions of years close the chapter of life, as we know it. on the former of these suppositions, the decrease of temperature on our globe might perhaps be accelerated by the thinning of the atmosphere, through the slow escape into space of its constituent gases, or their gradual chemical combination with the materials of the earth. the subterranean heat entirely radiated away, there would no longer remain any of those volcanic elevating forces which so far have counteracted the slow wearing down of the land surface of our planet, and thus what water remained would in time wash over all. if this preceded the growing cold of the sun, certain strange evolutions of marine forms of life would be the last to endure, but these, too, would have to go in the end. should, however, the actual process be the reverse of this, and the sun cool down the quicker, then man would, as a consequence of his scientific knowledge, tend in all probability to outlive the other forms of terrestrial life. in such a vista we can picture the regions of the earth towards the north and south becoming gradually more and more uninhabitable through cold, and human beings withdrawing before the slow march of the icy boundary, until the only regions capable of habitation would lie within the tropics. in such a struggle between man and destiny science would be pressed to the uttermost, in the devising of means to counteract the slow diminution of the solar heat and the gradual disappearance of air and water. by that time the axial rotation of our globe might possibly have been slowed down to such an extent that one side alone of its surface would be turned ever towards the fast dying sun. and the mind's eye can picture the last survivors of the human race, huddled together for warmth in a glass-house somewhere on the equator, waiting for the end to come. the mere idea of the decay and death of the solar system almost brings to one a cold shudder. all that sun's light and heat, which means so much to us, entirely a thing of the past. a dark, cold ball rushing along in space, accompanied by several dark, cold balls circling ceaselessly around it. one of these a mere cemetery, in which there would be no longer any recollection of the mighty empires, the loves and hates, and all that teeming play of life which we call history. tombstones of men and of deeds, whirling along forgotten in the darkness and silence. _sic transit gloria mundi._ in that brilliant flight of scientific fancy, the _time machine_, mr. h.g. wells has pictured the closing years of the earth in some such long-drawn agony as this. he has given us a vision of a desolate beach by a salt and almost motionless sea. foul monsters of crab-like form crawl slowly about, beneath a huge hull of sun, red and fixed in the sky. the rocks around are partly coated with an intensely green vegetation, like the lichen in caves, or the plants which grow in a perpetual twilight. and the air is now of an exceeding thinness. he dips still further into the future, and thus predicts the final form of life:-- "i saw again the moving thing upon the shoal--there was no mistake now that it was a moving thing--against the red water of the sea. it was a round thing, the size of a football perhaps, or it may be bigger, and tentacles trailed down from it; it seemed black against the weltering blood-red water, and it was hopping fitfully about." what a description of the "heir of all the ages!" to picture the end of our world as the result of a cataclysm of some kind, is, on the other hand, a form of speculation as intensely dramatic as that with which we have just been dealing is unutterably sad. it is not so many years ago, for instance, that men feared a sudden catastrophe from the possible collision of a comet with our earth. the unreasoning terror with which the ancients were wont to regard these mysterious visitants to our skies had, indeed, been replaced by an apprehension of quite another kind. for instance, as we have seen, the announcement in that biela's comet, then visible, would cut through the orbit of the earth on a certain date threw many persons into a veritable panic. they did not stop to find out the real facts of the case, namely, that, at the time mentioned, the earth would be nearly a month's journey from the point indicated! it is, indeed, very difficult to say what form of damage the earth would suffer from such a collision. in it passed, as we have seen, through the tail of the comet without any noticeable result. but the head of a comet, on the other hand, may, for aught we know, contain within it elements of peril for us. a collision with this part might, for instance, result in a violent bombardment of meteors. but these meteors could not be bodies of any great size, for the masses of comets are so very minute that one can hardly suppose them to contain any large or dense constituent portions. the danger, however, from a comet's head might after all be a danger to our atmosphere. it might precipitate, into the air, gases which would asphyxiate us or cause a general conflagration. it is scarcely necessary to point out that dire results would follow upon any interference with the balance of our atmosphere. for instance, the well-known french astronomer, m. camille flammarion,[ ] has imagined the absorption of the nitrogen of the air in this way; and has gone on to picture men and animals reduced to breathing only oxygen, first becoming excited, then mad, and finally ending in a perfect saturnalia of delirium. lastly, though we have no proof that stars eventually become dark and cold, for human time has so far been all too short to give us even the smallest evidence as to whether heat and light are diminishing in our own sun, yet it seems natural to suppose that such bodies must at last cease their functions, like everything else which we know of. we may, therefore, reasonably presume that there are dark bodies scattered in the depths of space. we have, indeed, a suspicion of at least one, though perhaps it partakes rather of a planetary nature, namely, that "dark" body which continually eclipses algol, and so causes the temporary diminution of its light. as the sun rushes towards the constellation of lyra such an extinguished sun may chance to find itself in his path; just as a derelict hulk may loom up out of the darkness right beneath the bows of a vessel sailing the great ocean. unfortunately a collision between the sun and a body of this kind could not occur with such merciful suddenness. a tedious warning of its approach would be given from that region of the heavens whither our system is known to be tending. as the dark object would become visible only when sufficiently near our sun to be in some degree illuminated by his rays, it might run the chance at first of being mistaken for a new planet. if such a body were as large, for instance, as our own sun, it should, according to mr. gore's calculations, reveal itself to the telescope some fifteen years before the great catastrophe. steadily its disc would appear to enlarge, so that, about nine years after its discovery, it would become visible to the naked eye. at length the doomed inhabitants of the earth, paralysed with terror, would see their relentless enemy shining like a second moon in the northern skies. rapidly increasing in apparent size, as the gravitational attractions of the solar orb and of itself interacted more powerfully with diminishing distance, it would at last draw quickly in towards the sun and disappear in the glare. it is impossible for us to conceive anything more terrible than these closing days, for no menace of catastrophe which we can picture could bear within it such a certainty of fulfilment. it appears, therefore, useless to speculate on the probable actions of men in their now terrestrial prison. hope, which so far had buoyed them up in the direst calamities, would here have no place. humanity, in the fulness of its strength, would await a wholesale execution from which there could be no chance at all of a reprieve. observations of the approaching body would have enabled astronomers to calculate its path with great exactness, and to predict the instant and character of the impact. eight minutes after the moment allotted for the collision the resulting tide of flame would surge across the earth's orbit, and our globe would quickly pass away in vapour. and what then? a nebula, no doubt; and after untold ages the formation possibly from it of a new system, rising phoenix-like from the vast crematorium and filling the place of the old one. a new central sun, perhaps, with its attendant retinue of planets and satellites. and teeming life, perchance, appearing once more in the fulness of time, when temperature in one or other of these bodies had fallen within certain limits, and other predisposing conditions had supervened. "the world's great age begins anew, the golden years return, the earth doth like a snake renew her winter weeds outworn: heaven smiles, and faiths and empires gleam like wrecks of a dissolving dream. a brighter hellas rears its mountains from waves serener far; a new peneus rolls his fountains against the morning star; where fairer tempes bloom, there sleep young cyclads on a sunnier deep. a loftier argo cleaves the main, fraught with a later prize; another orpheus sings again, and loves, and weeps, and dies; a new ulysses leaves once more calypso for his native shore. * * * * * oh cease! must hate and death return? cease! must men kill and die? cease! drain not to its dregs the urn of bitter prophecy! the world is weary of the past,-- oh might it die or rest at last!" [ ] see his work, _la fin du monde_, wherein the various ways by which our world may come to an end are dealt with at length, and in a profoundly interesting manner. index achromatic telescope, , adams, , , aerial telescopes, , agathocles, eclipse of, agrippa, camillus, ahaz, dial of, air, airy, sir g.b., al gûl, al sufi, , , , alcor, alcyone, aldebaran, , , , algol, , - , , , alpha, centauri, - , , - , , alpha crucis, alps, lunar, altair, altitude of objects in sky, aluminium, amos viii. , anderson, t.d., - andromeda (constellation), , ; great nebula in, , andromedid meteors, anglo-saxon chronicle, - anighito meteorite, annular eclipse, - , , , annular nebula in lyra, - annulus, ansæ, - anticipation in discovery, - apennines, lunar, aphelion, apparent enlargement of celestial objects, - apparent size of celestial objects deceptive, , apparent sizes of sun and moon, variations in, , , aquila (constellation), arabian astronomers, , arago, , arc, degrees minutes and seconds of, arcturus, , , , argelander, argo (constellation), aristarchus of samos, aristarchus (lunar crater), aristophanes, aristotle, , , arrhenius , - assyrian tablet, asteroidal zone, analogy of, to saturn's rings, asteroids (or minor planets), - , - , ; discovery of the, , ; wolf's method of discovering, - astrology, _astronomical essays_, , astronomical society, royal, _astronomy, manual of_, atlantic ocean, parallelism of opposite shores, - atlas, the titan, atmosphere, absorption by earth's, - ; ascertainment of, by spectroscope, - , ; height of earth's, , ; of asteroids, ; of earth, , , - , , , , ; of mars, , , ; of mercury, ; of moon, - , , - ; of jupiter, ; of planets, ; of saturn's rings, "atmosphere" of the stars, atmospheric layer and "glass-house" compared, , august meteors (perseids), auriga (constellation), - , , ; new star in, aurigæ, [b] (beta), , , aurora borealis, , , australia, suggested origin of, axis, - ; of earth, , ; small movement of earth's, - babylonian tablet, babylonian idea of the moon, bacon, roger, bacubirito meteorite, bagdad, baily, francis, "baily's beads," , , - , bailly (lunar crater), ball, sir robert, barnard, e.e., , , - , , "bay of rainbows," bayer's classification of stars, , - bayeux tapestry, bear, great (constellation). _see_ ursa major; little, _see_ ursa minor beehive (præsepe), beer, belopolsky, "belt" of orion, belt theory of milky way, belts of jupiter, bergstrand, berlin star chart, bessel, , , beta ([b]) lyræ, beta ([b]) persei. _see_ algol betelgeux, bible, eclipses in, biela's comet, - , - , bielids, , - billion, - binary stars, spectroscopic, - , ; visual, , - "black drop," - "black hour," "black saturday," blood, moon in eclipse like, blue (rays of light), , bode's law, - , - bolometer, bond, g.p., , bonpland, boötes (constellation), , bradley, brahe, tycho, , brédikhine's theory of comets' tails, - , bright eclipses of moon, , british association for the advancement of science, _british astronomical association, journal of_, british museum, bull (constellation). _see_ taurus; "eye" of the, ; "head" of the, burgos, busch, cæsar, julius, , , , , , , calcium, , callisto, - cambridge, , , , campbell, canali, "canals" of mars, - , - cancer (constellation), canes venatici (constellation), , canis major (constellation), , - ; minor, - canopus, , - , capella, , , , , , , carbon, carbon dioxide. _see_ carbonic acid gas carbonic acid gas, , , - carnegie institution, solar observatory of, cassegrainian telescope, , cassini, j.d., , "cassini's division" in saturn's ring, , cassiopeia (constellation), , , , cassiopeiæ, [ê] (eta), cassiopeia's chair, cassius, dion, castor, , , catalogues of stars, , - , centaur. _see_ centaurus centaurus (constellation), , centre of gravity, , - , ceres, diameter of, , ceti, omicron (or mira), - cetus, or the whale (constellation), chaldean astronomers, , challis, - chamberlin, "chambers of the south," chandler, charles v., "charles' wain," chemical rays, chinese and eclipses, chloride of sodium, chlorine, , christ, birth of, christian era, first recorded solar eclipse in, chromatic aberration, chromosphere, - , - , - , - circle, - clark, alvan, & sons, - , claudius, emperor, clavius (lunar crater), clerk maxwell, "clouds" (of aristophanes), clustering power, clusters of stars, , , , coal sacks. _see_ holes in milky way coelostat, coggia's comet, colour, production of, in telescopes, - , , collision of comet with earth, - ; of dark star with sun, - ; of stars, , columbus, coma berenices (constellation), , comet, first discovery of by photography, ; first orbit calculated, ; first photograph of, - ; furthest distance seen, ; passage of among satellites of jupiter, ; passage of earth and moon through tail of, , comet of a.d., ; , - ; , , ; , - ; , , , ; , - ; , , , ; , ; , comets, - , , chaps. xix. and xx., - ; ancient view of, - ; captured, - ; chinese records of, - ; composition of, ; contrasted with planets, ; families of, - , ; meteor swarms and, ; revealed by solar eclipses, - ; tails of, , , , - common, telescopes of dr. a.a., conjunction, constellations, , - , , contraction theory of sun's heat, - , cook, captain, cooke, copernican system, , , , - , , copernicus, , , , , - , copernicus (lunar crater), , copper, corder, h., corona, - , , - , , - , ; earliest drawing of, ; earliest employment of term, ; earliest mention of, ; earliest photograph of, ; illumination given by, ; possible change in shape of during eclipse, - ; structure of, - ; variations in shape of, corona borealis (constellation), coronal matter, ; streamers, - , - coronium, , , cotes, coudé, equatorial, cowell, p.h., , crabtree, crape ring of saturn, - craterlets on mars, craters (ring-mountains) on moon, - , , ; suggested origin of, - , crawford, earl of, crecy, supposed eclipse at battle of, - crescent moon, , crommelin, a.c.d., , crossley reflector, , - crown glass, crucifixion, darkness of, crucis, [a] (alpha), crux, or "southern cross" (constellation), - , cycle, sunspot, - , , - cygni, , , cygnus, or the swan (constellation), , daniel's comet of , danzig, dark ages, , , dark eclipses of moon, , - dark matter in space, dark meteors, - dark stars, - , , , - "darkness behind the stars," darwin, sir g.h., davis, dawes, dearborn observatory, death from fright at eclipse, debonnaire, louis le, , deimos, deity, symbol of the, "demon star." _see_ algol denebola, denning, w.f., densities of sun and planets, density, deslandres, diameters of sun and planets, disappearance of moon in lunar eclipse, , - disc, "disc" theory. _see_ "grindstone" theory discoveries, independent, discovery, anticipation in, - ; indirect methods of, "dipper," the, ; the "little," distance of a celestial body, how ascertained, - ; of sun from earth, how determined, , distances of planets from sun, distances of sun and moon, relative, dog, the greater. _see_ canis major; the lesser, _see_ canis minor "dog star," , dollond, john, - donati's comet, , doppler's method, , , , - dorpat, double canals of mars, - , - double planet, earth and moon a, double stars, douglass, "dreams, lake of," dumb-bell nebula, earth, , , , , , , chap. xv., ; cooling of, ; diameter of, ; interior of, ; mean distance of from sun, ; rigidity of, ; rotation of, , , - , ; shape of, ; "tail" to, "earthlight," or "earthshine," earth's axis, precessional movement of, - , , - earth's shadow, circular shape of, , eclipse, eclipse knowledge, delay of, eclipse party, work of, eclipse of sun, advance of shadow in total, ; animal and plant life during, ; earliest record of total, ; description of total, - ; duration of total, , ; importance of total, eclipses, ascertainment of dates of past, ; experience a necessity in solar, - ; of moon, - , chap. ix., ; photography in, ; prediction of future, ; recurrence of, - eclipses of sun, , - , chap. viii., - , ; a.d., ; , , ; , , ; , ; , - ; , , ; , ; , ; , ; , ; , ; , - ; , - ; , , ; , , ; , ; , - , - , - ; , ; , ; , ; , , - _eclipses, past and future_, egenitis, electric furnace, electric light, spectrum of, elements composing sun, - ellipses, , , - , - elliptic orbit, , ellipticity, elongation, eastern, , ; western, , encke's comet, , "end of the world," england, solar eclipses visible in, - , - epsilon, ([e]) lyræ, equator, equatorial telescope, equinoxes. _see_ precession of eros, - , , - ; discovery of, , , ; importance of, ; orbit of, , , , eruptive prominences, _esclistre_, ether, - , - europa, , evans, j.e., evening star, - , everest, mount, evershed, eye-piece, fabricius, faculæ, , fauth, faye, _fin du monde_, first quarter, "fixed stars," flagstaff, - , flammarion, camille, flamsteed, "flash spectrum," "flat," flint glass, focus, , "forty-foot telescope," foster, fraunhofer, french academy of sciences, froissart, "full moon" of laplace, galaxy. _see_ milky way. galilean telescope, galileo, , , , , , - , galle, , , ganymede, - gas light, spectrum of, gegenschein, - "gem" of meteor ring, gemini, or the twins (constellation), , - geminorum, [z] (zeta), geometrical groupings of stars, "giant" planet, , - gibbous, , gill, sir david, , , , - gold, goodricke, gore, j.e., , , , - , , - , , , granulated structure of photosphere, gravitation (or gravity), , - , , greek ideas, , , - , , , green (rays of light), greenwich observatory, - , , , gregorian telescope, - grimaldi (lunar crater), "grindstone" theory, - "groombridge, ," - , , groups of stars, - grubb, sir howard, _gulliver's travels_, hale, g.e., , half moon, , hall, asaph, hall, chester moor, halley, edmund, , , - , halley's comet, , - haraden hill, harvard, , harvest moon, - hawaii, heat rays, heidelberg, , height of lunar mountains, how determined, height of objects in sky, estimation of, helium, , , helmholtz, , hercules (constellation), herod the great, - herodotus, herschel, a.s., herschel, sir john, , herschel, sir william, , , - , , , , , , , - , - herschelian telescope, , hesper, hesperus, hevelius, hezekiah, hi, hindoos, hipparchus, , , , ho, holes in milky way, - holmes, oliver wendell, homer, horace, odes of, horizon, horizontal eclipse, horrox, , - hour glass sea, huggins, sir william, , , humboldt, "hunter's moon," huyghens, - , , - hyades, - , hydrocarbon gas, hydrogen, , , , , , , , ibrahim ben ahmed, ice-layer theory: mars, ; moon, , illusion theory of martian canals, imbrium, mare, inclination of orbits, - indigo (rays of light), inferior conjunction, , inferior planets, , , chap. xiv., instruments, pre-telescopic, - , international photographic survey of sky, - intra-mercurial planet, - _introduction to astronomy_, inverted view in astronomical telescope, - io, - iridum, sinus, iron, , _is mars habitable?_ jansen, janssen, , , japetus, jessenius, job, book of, johnson, s.j., , josephus, , juno, jupiter, , - , , , , , - , - , , , ; comet family of, - , ; discovery of eighth satellite, , ; eclipse of, by satellite, ; without satellites, - jupiter, satellites of, , , , , - ; their eclipses, - ; their occultations, , ; their transits, , kant, kapteyn, , keeler, , kelvin, lord, kepler, , , , , , , , kinetic theory, , , , , , , king, l.w., _knowledge_, labrador, lacus somniorum, "lake of dreams," lalande, , lampland, , langley, , laplace, , laputa, le maire, le verrier, , , - , lead, leibnitz mountains (lunar), leo (constellation), , - leonids, - , - lescarbault, lewis, t., lexell's comet, lick observatory, , , - , , , , , ; great telescope of, , , "life" of an eclipse of the moon, ; of the sun, - life on mars, lowell's views, - ; pickering's, ; wallace's, - light, no extinction of, - ; rays of, ; velocity of, , - ; white, "light year," , lindsay, lord, linné (lunar crater), liouville, lippershey, liquid-filled lenses, _locksley hall_, ; _sixty years after_, lockyer, sir norman, , , , loewy, , london, eclipses visible at, - , - longfellow, lowell observatory, , , - lowell, percival, , - , - lucifer, lynn, w.t., , lyra (constellation), , , - , , , mädler, , magellanic clouds, magnetism, disturbances of terrestrial, , magnitudes of stars, - major planets, - "man in the moon," _manual of astronomy_, maps of the moon, mare imbrium, mare serenitatis, mars, , - , - , , , , , - , ; compared with earth and moon, , ; polar caps of, - , ; satellites of, , - ; temperature of, , , - mass, ; of a star, how determined, masses of celestial bodies, how ascertained, ; of earth and moon compared, ; of sun and planets compared, maunder, e.w., , , maunder, mrs., e.w., , maxwell, clerk, mayer, tobias, , mcclean, f.k., mean distance, "medicean stars," mediterranean, eclipse tracks across, , melbourne telescope, melotte, p., mercator's projection, - mercury (the metal), mercury (the planet), , , - , - , , , chap. xiv.; markings on, ; possible planets within orbit of, - ; transit of, , , metals in sun, meteor swarms, - , , - meteors, , , , , chap. xxi. meteors beyond earth's atmosphere, - meteorites, - meteoritic hypothesis, metius, jacob, michell, , middle ages, , , middleburgh, milky way (or galaxy), , , , , - ; penetration of, by photography, million, , - minor planets. _see_ asteroids. mira ceti, - "mirk monday," mirror (speculum), , mizar, , monck, w.h.s., mongol emperors of india, moon, , chap. xvi.; appearance of, in lunar eclipse, , - ; diameter of, ; distance of, how ascertained, ; distance of, from earth, ; full, , , , , , , ; mass of, , ; mountains on, - ; how their height is determined, ; movement of, - ; new, , , , ; origin of, - ; plane of orbit of, ; possible changes on, - , ; "seas" of, , ; smallest detail visible on, ; volume of, morning star, - , moulton, f.r., , , , , , moye, multiple stars, musa-ben-shakir, mythology, neap-tides, nebulæ, - , , , ; evolution of stars from, - nebular hypothesis of laplace, - nebular hypotheses, chap. xxvii. nebulium, neison, neptune, , , , , , - , , , , ; discovery of, - , , , , - ; lalande and, ; possible planets beyond, , ; satellite of, , ; "year" in, - "new" (or temporary), stars, - newcomb, simon, , , , , - , newton, sir isaac, , , , - , , , , , newtonian telescope, , , , nineveh eclipse, - nitrogen, , , , northern crown, nova aurigæ, nova persei, - novæ. _see_ new (or temporary) stars nubeculæ, "oases" of mars, , object-glass, oblate spheroid, occultation, - , , _olaf, saga of king_, olbers, , , , "old moon in new moon's arms," olmsted, omicron (or "mira") ceti, - opposition, "optick tube," - , orange (rays of light), orbit of moon, plane of, orbits, , - , , , oriental astronomy, orion (constellation), , , - , ; great nebula in, , oxford, oxygen, , , , pacific ocean, origin of moon in, palitzch, pallas, , parallax, , , , , , paré, ambrose, - peal, s.e., peary, pegasus (constellation), penumbra of sunspot, perennial full moon of laplace, pericles, perrine, c.d., - , perseids, , - perseus (constellation), , , , phases of an inferior planet, , ; of the moon, , , - phlegon, eclipse of, - phobos, phoebe, retrograde motion of, , , phosphorescent glow in sky, phosphorus (venus), photographic survey of sky, international, - photosphere, - , piazzi, pickering, e.c., pickering, w.h., , - , - , , - pictor, "runaway star" in constellation of, - , , plane of orbit, , planetary nebulæ, , _planetary and stellar studies_, planetesimal hypothesis, - planetoids. _see_ asteroids planets, classification of, ; contrasted with comets, ; in ptolemaic scheme, ; relative distances of, from sun, - plato (lunar crater), pleiades, , - , pliny, , plough, , - , plutarch, , , , "pointers," polaris. _see_ pole star pole of earth, precessional movement of, - , , - pole star, , , , - , - poles, , - ; of earth, speed of point at, pollux, , posidonius, powell, sir george baden, præsepe (the beehive), precession of the equinoxes, , , - pre-telescopic notions, primaries, _princess, the_ (tennyson), princeton observatory, prism, prismatic colours, , procyon, , , , prominences, solar, , , , - , ; first observation of, with spectroscope, , , proper motions of stars, , - , , - ptolemæus (lunar crater), - , ptolemaic idea, ; system, , , , - ptolemy, , , , , puiseux, p., pulkowa telescope, puppis, v., quiescent prominences, radcliffe observer, "radiant," or radiant point, radiation from sun, , radium, , rainbow, "rainbows, bay of," rambaut, a., ramsay, sir william, rays (on moon), recurrence of eclipses, - red (rays of light), , , , red spot, the great, reflecting telescope, - ; future of, reflector. _see_ reflecting telescope refracting and reflecting telescopes contrasted, refracting telescope, - , - ; limits to size of, - refraction, , - refractor, _see_ refracting telescope regulus, , retrograde motion of phoebe, , , "reversing layer," , , , - revival of learning, revolution, ; of earth around sun, - ; periods of sun and planets, riccioli, rice-grain structure of photosphere, rigel, , rills (on moon), ring-mountains of moon. _see_ craters "ring" nebulæ, , "ring with wings," rings of saturn, , - , - , ritchey, g.w., roberts, a.w., , roberts, isaac, "roche's limit," roemer, roman history, eclipses in, - romulus, röntgen, rosse, great telescope of lord, , rotation, ; of earth, , - , ; of sun, , , - , ; periods of sun and planets, royal society of london, - , rubicon, passage of the, "runaway" stars, , , sagittarius (constellation), salt, spectrum of table, samarcand, "saros," chaldean, - , satellites, - , saturn, , , , , , - , ; comet family of, ; a puzzle to the early telescope observers, - ; retrograde motion of satellite phoebe, , , ; ring system of, ; satellites of, , - ; shadows of planet on rings and of rings on planet, schaeberle, - , , schiaparelli, , , schickhard (lunar crater), schmidt, schönfeld, schuster, schwabe, scotland, solar eclipses visible in, - , sea of serenity, "sea of showers," "seas" of moon, , seasons on earth, - ; on mars, secondary bodies, seneca, , _septentriones_, serenitatis, mare, "seven stars," "shadow bands," shadow of earth, circular shape of, - shadows on moon, inky blackness of, shakespeare, , sheepshanks telescope, "shining fluid" of sir w. herschel, "shooting stars." _see_ meteors short (of edinburgh), "showers, sea of," sickle of leo, - , siderostat, silver, silvered mirrors for reflecting telescopes, sinus iridum, sirius, , , - , - , , - , ; companion of, ; stellar magnitude of, size of celestial bodies, how ascertained, skeleton telescopes, sky, international photographic survey of, - ; light of the, slipher, e.c., , smithsonian institution of washington, snow on mars, sodium, , , sohag, solar system, - , - ; centre of gravity of, ; decay and death of, somniorum, lacus, sound, , , south pole of heavens, , , - southern constellations, - southern cross. _see_ crux space, spain, early astronomy in, ; eclipse tracks across , - spectroheliograph, spectroscope, , , - , - , , ; prominences first observed with, , , spectrum of chromosphere, - ; of corona, ; of photosphere, ; of reversing layer, , ; solar, - , , speculum, , ; metal, spherical bodies, spherical shape of earth, proofs of, - spherical shapes of sun, planets, and satellites, spiral nebulæ, - , - spring balance, spring tides, spy-glass, "square of the distance," - stannyan, captain, star, mass of, how determined, ; parallax of, first ascertained, , stars, the, , , , _et seq._; brightness of, , ; distances between, - ; distances of some, , , ; diminution of, below twelfth magnitude, ; evolution of, from nebulæ, - ; faintest magnitude of, ; number of those visible altogether, ; number of those visible to naked eye, "steam cracks," steinheil, stellar system, estimated extent of, - ; an organised whole, ; limited extent of, - , ; possible disintegration of, stiklastad, eclipse of, stone age, stoney, g.j., , stonyhurst observatory, _story of the heavens_, streams of stars, kapteyn's two, stroobant, stukeley, sulphur, summer, , sun, chaps xii. and xiii.; as a star, , , ; as seen from neptune, , ; chemical composition of, - ; distance of, how ascertained, , ; equator of, - , ; gravitation at surface of, , - ; growing cold of, - ; mean distance of, from earth, , ; motion of, through space, - , ; not a solid body, ; poles of, ; radiations from, ; revolution of earth around, - ; stellar magnitude of, - ; variation in distance of, , sunspots, , , - , - , - , ; influence of earth on, suns and possible systems, , superior conjunction, - superior planets, , , - , swan (constellation). _see_ cygnus swift, dean, "sword" of orion, , syrtis major. _see_ hour glass sea "_systematic_ parallax," systems, other possible, , tails of comets, tamerlane, taurus (constellation), , - , "tears of st. lawrence," tebbutt's comet, - telescope, , , - , ; first eclipse of moon seen through, ; of sun, telescopes, direct view reflecting, ; gigantic, ; great constructors of, - ; great modern, - tempel's comet, temperature on moon, ; of sun, temporary (or new) stars, - tennyson, lord, , , terrestrial planets, - terrestrial telescope, thales, eclipse of, themis, "tidal drag," , , , tide areas, - tides, - , - _time machine_, tin, titan, titius, total phase, - totality, ; track of, trail of a minor planet, - transit, , - ; of mercury, , , ; of venus, , - , , trifid nebula, triple stars, tubeless telescopes, - , tubes used by ancients, tuttle's comet, twilight, , twinkling of stars, twins (constellation). _see_ gemini tycho brahe, , tycho (lunar crater), ulugh beigh, umbra of sunspot, - universe, early ideas concerning, - , , , universes, possibility of other, - uranus, - , , , , , ; comet family of, ; discovery of, , , ; rotation period of , ; satellites of, , ; "year" in, - ursa major (constellation), , , , , ; minor, , , - ursæ majoris, ([z]) zeta. _see_ mizar variable stars, - variations in apparent sizes of sun and moon, , , vault, shape of the celestial, - vega, , , , - , , , , , , vegetation on mars, , - ; on moon, venus, , , , , , - , , chap. xiv., , ; rotation period of, , very, f.w., vesta, , violet (rays of light), - , virgil, volcanic theory of lunar craters, - , volume, volumes of sun and planets compared, - "vulcan," wallace, a.r., on mars, - water, lack of, on moon, - water vapour, , , wargentin, warner and swasey co., weather, moon and, - weathering, webb, rev. t.w., weight, , - wells, h.g., whale (constellation). _see_ cetus whewell, willamette meteorite, wilson, mount, wilson, w.e., "winged circle" (or "disc"), winter, , witt, wolf, max, - , wright, thomas, , wybord, xenophon, year, "year" in uranus and neptune, - year, number of eclipses in a, "year of the stars," yellow (rays of light), - , yerkes telescope great, , young, , , zenith, zinc, zodiacal light, zone of asteroids, - , the end printed by ballantyne, hanson & co. edinburgh & london the science of to-day series _with many illustrations. extra crown vo. s. net._ botany of to-day. a popular account of the evolution of modern botany. by prof. g.f. scott elliot, m.a., b.sc., author of "the romance of plant life," _&c. &c._ "one of the books that turn botany from a dryasdust into a fascinating study."--_evening standard._ aerial navigation of to-day. a popular account of the evolution of aeronautics. by charles c. turner. 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"mr. gibson has given us one of the best examples of popular scientific exposition that we remember seeing. his aim has been to produce an account of the chief modern applications of electricity without using technical language or making any statements which are beyond the comprehension of any reader of ordinary intelligence. in this he has succeeded to admiration, and his book may be strongly commended to all who wish to realise what electricity means and does in our daily life."--_the tribune._ seeley & co., ltd., great russell street. the romance of modern electricity describing in non-technical language what is known about electricity & many of its interesting applications by charles r. gibson, a.i.e.e. author or "electricity of to-day," etc. _extra crown vo._ _with illustrations and diagrams._ _s._ "everywhere mr. charles r. gibson makes admirable use of simple analogies which bespeak the practised lecturer, and bring the matter home without technical detail. the attention is further sustained by a series of surprises. the description of electric units, the volt, the ohm, and especially the ampere, is better than we have found in more pretentious works."--_academy._ "mr. gibson's style is very unlike the ordinary text-book. it is fresh, and is non-technical. its facts are strictly scientific, however, and thoroughly up to date. if we wish to gain a thorough knowledge of electricity pleasantly and without too much trouble on our own part, we will read mr. gibson's 'romance.'"--_expository times._ "a book which the merest tyro totally unacquainted with elementary electrical principles can understand, and should therefore especially appeal to the lay reader. especial interest attaches to the chapter on wireless telegraphy, a subject which is apt to 'floor' the uninitiated. the author reduces the subject to its simplest aspect, and describes the fundamental principles underlying the action of the coherer in language so simple that anyone can grasp them."--_electricity._ "contains a clear and concise account of the various forms in which electricity is used at the present day, and the working of the telephone, wireless telegraphy, tramcars, and dynamos is explained with the greatest possible lucidity, while the marvels of the x-rays and of radium receive their due notice. now that electricity plays such an all-important part in our daily life, such a book as this should be in the hands of every boy. indeed, older people would learn much from its pages. for instance, how few people could explain the principles of wireless telegraphy in a few words if suddenly questioned on the subject. the book is well and appropriately illustrated."--_graphic._ "mr. gibson sets out to describe in non-technical language the marvellous discoveries and adaptation of this pervasive and powerful essence, and being a most thorough master of the subject, he leads the reader through its mazes with a sure hand. throughout he preserves a clear and authoritative style of exposition which will be understood by any intelligent reader."--_yorkshire observer._ "a popular and eminently readable manual for those interested in electrical appliances. it describes in simple and non-technical language what is known about electricity and many of its interesting applications. there are a number of capital illustrations and diagrams which will help the reader greatly in the study of the book."--_record._ seeley & co., ltd., great russell street. the romance of savage life describing the habits, customs, everyday life, &c., of primitive man by prof. g.f. scott elliot, m.a., b.sc., &c. _with thirty illustrations._ _extra crown vo._ _s._ "mr. scott elliot has hit upon a good idea in this attempt to set forth the life of the primitive savage. on the whole, too, he has carried it out well and faithfully.... we can recommend the book as filling a gap."--_athenæum._ "a readable contribution to the excellent series of which it forms a part. mr. scott elliot writes pleasantly ... he possesses a sufficiently vivid imagination to grasp the relation of a savage to his environment."--_nature._ "there are things of remarkable interest in this volume, and it makes excellent reading and represents much research."--_spectator._ the romance of plant life describing the curious and interesting in the plant world by prof. g.f. scott elliot, m.a., b.sc., &c. _with thirty-four illustrations._ _extra crown vo._ _s._ "the author has worked skilfully into his book details of the facts and inferences which form the groundwork of modern botany. the illustrations are striking, and cover a wide field of interest, and the style is lively."--_athenæum._ "in twenty-nine fascinating, well-printed, and well-illustrated chapters, prof. scott elliot describes a few of the wonders of plant life. a very charming and interesting volume."--_daily telegraph._ "mr. scott elliot is of course a well-known authority on all that concerns plants, and the number of facts he has brought together will not only surprise but fascinate all his readers."--_westminster gazette._ seeley & co., ltd., great russell street. the romance of insect life describing the curious & interesting in the insect world by edmund selous author of "the romance of the animal world," etc. _with sixteen illustrations._ _extra crown vo._ _s._ "an entertaining volume, one more of a series which seeks with much success to describe the wonders of nature and science in simple, attractive form."--_graphic._ "offers most interesting descriptions of the strange and curious inhabitants of the insect world, sure to excite inquiry and to foster observation. there are ants white and yellow, locusts and cicadas, bees and butterflies, spiders and beetles, scorpions and cockroaches--and especially ants--with a really scientific investigation of their wonderful habits not in dry detail, but in free and charming exposition and narrative. an admirable book to put in the hands of a boy or girl with a turn for natural science--and whether or not."--_educational times._ "both interesting and instructive. such a work as this is genuinely educative. there are numerous illustrations."--_liverpool courier._ "with beautiful original drawings by carton moore park and lancelot speed, and effectively bound in dark blue cloth, blazoned with scarlet and gold."--_lady._ "admirably written and handsomely produced. mr. selous's volume shows careful research, and the illustrations of insects and the results of their powers are well done."--_world._ the romance of modern mechanism interesting descriptions in non-technical language of wonderful machinery, mechanical. devices, & marvellously delicate scientific instruments by archibald williams, b.a., f.r.g.s. author of "the romance of modern exploration," etc. _with twenty-six illustrations._ _extra crown vo._ _s._ "no boy will be able to resist the delights of this book, full to the brim of instructive and wonderful matter."--_british weekly._ "this book has kept your reviewer awake when he reasonably expected to be otherwise engaged. we do not remember coming across a more fascinating volume, even to a somewhat blasé reader whose business it is to read all that comes in his way. the marvels miracles they should be called, of the modern workshop are here exploited by mr. williams for the benefit of readers who have not the opportunity of seeing these wonders or the necessary mathematical knowledge to understand a scientific treatise on their working. only the simplest language is used and every effort is made, by illustration or by analogy, to make sufficiently clear to the non-scientific reader how the particular bit of machinery works and what its work really is. delicate instruments, calculating machines, workshop machinery, portable tools, the pedrail, motors ashore and afloat, fire engines, automatic machines, sculpturing machines--these are a few of the chapters which crowd this splendid volume."--_educational news._ "it is difficult to make descriptions of machinery and mechanism interesting, but mr. williams has the enviable knack of doing so, and it is hardly possible to open this book at any page without turning up something which you feel you must read; and then you cannot stop till you come to the end of the chapter."--_electricity._ "this book is full of interest and instruction, and is a welcome addition to messrs. seeley and company's romance series."--_leeds mercury._ "a book of absorbing interest for the boy with a mechanical turn, and indeed for the general reader."--_educational times._ "an instructive and well-written volume."--_hobbies._ seeley & co., ltd., great russell street. a catalogue of books on art, history, and general literature published by seeley, service & co ltd. great russell st. london _some of the contents_ crown library, the elzevir library, the events of our own times series illuminated series, the miniature library of devotion, the miniature portfolio monographs, the missions, the library of new art library, the portfolio monographs science of to-day series, the seeley's illustrated pocket library seeley's standard library story series, the "things seen" series, the _the publishers will be pleased to post their complete catalogue or their illustrated miniature catalogue on receipt of a post-card_ catalogue of books _arranged alphabetically under the names of authors and series_ abbott, rev. e.a., d.d. how to parse. an english grammar. fcap. vo, s. d. how to tell the parts of speech. an introduction to english grammar. fcap. vo, s. how to write clearly. rules and exercises on english composition. s. d. latin gate, the. a first latin translation book. crown vo, s. d. via latina. a first latin grammar. crown vo, s. d. abbott, rev. e.a., and sir j.r. seeley. english lessons for english people. crown vo, s. d. ady, mrs. _see_ cartwright, julia. À kempis, thomas. of the imitation of christ. with illuminated frontispiece and title page, and illuminated sub-titles to each book. in white or blue cloth, with inset miniatures. gilt top; crown vo, s. nett; also bound in same manner in real classic vellum. each copy in a box, s. d. nett; antique leather with clasps, s. d. nett. "it may well be questioned whether the great work of thomas à kempis has ever been presented to better advantage."--_the guardian._ anderson, prof. w. japanese wood engravings. coloured illustrations. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also small to, cloth, s. nett; lambskin, s. nett. armstrong, sir walter. the art of velazquez. illustrated. super-royal vo, s. d. nett. the life of velazquez. illustrated. super-royal vo, s. d. nett. velazquez. a study of his life and art. with eight copper plates and many minor illustrations. super-royal vo, cloth, s. nett. thomas gainsborough. illustrated. super-royal vo, half-linen, s. d. nett. also new edition small to, cloth, s. nett; leather, s. nett and s. nett. the peel collection and the dutch school of painting. with illustrations in photogravure and half-tone. super-royal vo, sewed, s. nett; cloth, s. nett. w.q. orchardson. super-royal vo, sewed, s. d.; half-linen, s. d. nett. augustine, s. confessions of s. augustine. with illuminated pages. in white or blue cloth, gilt top, crown vo, s. nett; also in vellum, s. d. nett. baker, captain b. granville the passing of the turkish empire in europe. with thirty-two illustrations. demy vo, s. nett. baring-gould, rev. s. family names and their story. demy vo, s. d. nett. s. nett. bedford, rev. w.k.r. malta and the knights hospitallers. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett. benham, rev. canon d.d., f.s.a. the tower of london. with four plates in colours and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. mediæval london. with a frontispiece in photogravure, four plates in colour, and many other illustrations. super-royal vo, sewed, s. nett; cloth, gilt top, s. nett. also extra crown vo, s. d. nett. old st. paul's cathedral. with a frontispiece in photogravure, four plates printed in colour, and many other illustrations. super-royal vo, sewed, s. nett, or cloth, gilt top, s. nett. bennett, edward. the post office and its story. an interesting account of the activities of a great government department. with twenty-five illustrations. ex. crn. vo, s. nett. bickersteth, rev. e. family prayers for six weeks. crown vo, s. d. a companion to the holy communion. mo, cloth, s. binyon, laurence. dutch etchers of the seventeenth century. illustrated. super-royal vo, sewed, s. d.; half-linen, s. d. nett. john crome and john sell cotman. illustrated. super-royal vo sewed, s. d. nett. birch, g.h. london on thames in bygone days. with four plates printed in colour and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. bridges, rev. c. an exposition of psalm cxix. crown vo, s. butcher, e.l. things seen in egypt. with fifty illustrations. small to, cloth, s. nett; lambskin, s. nett; velvet leather, in box, s. nett. poems, s. d. nett. cachemaille, rev. e.p., m.a. xxvi present-day papers on prophecy. an explanation of the visions of daniel and of the revelation, on the continuous historic system. with maps and diagrams. pp. s. nett. cartwright, julia. jules bastien-lepage. super-royal vo, sewed, s. d.; cloth, s. d. nett. sacharissa. some account of dorothy sidney, countess of sunderland, her family and friends. with five portraits. demy vo, s. d. raphael in rome. illustrated. super-royal vo, sewed, s. d.; half-linen, s. d. nett; also in small to, cloth, s. nett; leather, s. nett and s. nett. the early work of raphael. illustrated. super-royal vo, sewed s. d.; half-linen, s. d. also new edition, revised, in small to, in cloth, s. nett; leather, s. nett. raphael: a study of his life and work. with eight copper plates and many other illustrations. super-royal vo, s. d. nett. cesaresco, the countess martinengo the liberation of italy. with portraits on copper. crown vo, s. chatterton, e. keble. fore and aft. the story of the fore and aft rig from the earliest times to the present day. sq. ex. royal vo. with illustrations and coloured frontispiece by c. dixon, r.i. s. nett. through holland in the "vivette." the cruise of a -tonner from the solent to the zuyder zee, through the dutch waterways. with sixty illustrations and charts, s. nett. chitty, j.r. things seen in china. with fifty illustrations. small to; cloth, s.; leather, s.; velvet leather in a box, s. nett. choral service-book for parish churches, the. compiled and edited by j.w. elliott, organist and choirmaster of st. mark's, hamilton terrace, london. with some practical counsels taken by permission from "notes on the church service," by bishop walsham how. a. royal vo, sewed, s.; cloth, s. d. b. mo, sewed, d.; cloth, d. _the following portions may be had separately:_-- the ferial and festal responses and the litany. arranged by j.w. elliott. sewed, d. the communion service, kyrie, credo, sanctus, and gloria in excelsis. set to music by dr. j. naylor, organist of york minster. sewed, d. church, sir arthur h., f.r.s. josiah wedgwood, master potter. with many illustrations. super-royal vo, sewed, s. nett; cloth, s. nett; also small to, cloth, s. nett; leather, s. and s. nett. the chemistry of paints and painting. third edition. crown vo, s. church, rev. a.j. nicias, and the sicilian expedition. crown vo, s. d. for other books by professor church see complete catalogue. clark, j.w., m.a. cambridge. with a coloured frontispiece and many other illustrations by a. brunet-debaines and h. toussaint &c. extra crown vo, s.; also crown vo, cloth, s. nett; leather, s.; special leather, in box, s. nett. cody, rev. h.a. an apostle of the north. the biography of the late bishop bompas, first bishop of athabasca, and with an introduction by the archbishop of ruperts-land. with illustrations. demy vo, s. d. nett. s. nett. corbin, t.w. engineering of to-day. with seventy-three illustrations and diagrams. extra crown vo, s. nett. mechanical inventions of to-day. ex. crown vo; with ninety-four illustrations, s. nett. cornish, c.j. animals of to-day: their life and conversation. with illustrations from photographs by c. reid of wishaw. crown vo, s. the isle of wight. illustrated. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also a new edition, small to, cloth, s.; leather, s. and s. life at the zoo. notes and traditions of the regent's park gardens. illustrated from photographs by gambier bolton. fifth edition. crown vo, s. the naturalist on the thames. many illustrations. demy vo, s. d. the new forest. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also new edition, small to, cloth, s.; leather, s. nett; and special velvet leather, each copy in a box, s. the new forest and the isle of wight. with eight plates and many other illustrations. super-royal vo, s. d. nett. nights with an old gunner, and other studies of wild life. with sixteen illustrations by lancelot speed, charles whymper, and from photographs. crown vo, s. * * * * * the crown library a series of notable copyright books issued in uniform binding. extra crown vo. with many illustrations, s. nett. _just issued. second and cheaper edition._ swann, a.j. fighting the slave hunters in central africa. a record of twenty-six years of travel and adventure round the great lakes, and of the overthrow of tip-pu-tib, rumaliza, and other great slave traders. with illustrations and a map, s. nett. _recently issued._ grubb, w. barbrooke. an unknown people in an unknown land. an account of the life and customs of the lengua indians of the paraguayan chaco, with adventures and experiences met with during twenty years' pioneering and exploration amongst them. with twenty-four illustrations and a map. extra crown vo, s. nett. fraser, sir a.h.l., k.c.s.i., m.a., ll.d., litt.d., ex-lieutenant-governor of bengal. among indian rajahs and ryots. a civil servants' recollections and impressions of thirty-seven years of work and sport in the central provinces and bengal. third edition, s. nett. cody, rev. h.a. an apostle of the north. the story of bishop bompas's life amongst the red indians & eskimo. third edition, s. nett. pennell, t.l., m.d., b.sc. among the wild tribes of the afghan frontier. a record of sixteen years' close intercourse with the natives of afghanistan and the north-west frontier. introduction by earl roberts. extra crown vo. twenty-six illustrations and map. fifth edition, s. net. * * * * * cust, lionel. the engravings of albert dürer. illustrated. super-royal vo, half-linen, s. d. nett. paintings and drawings of albert dürer. illustrated. super-royal vo, sewed, s. d. nett. albrecht dürer. a study of his life and work. with eight copper plates and many other illustrations. super-royal vo, s. d. davenport, cyril. cameos. with examples in colour and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. royal english bookbindings. with coloured plates and many other illustrations. super-royal vo, sewed, s. d.; cloth, s. d. davies, randall, f.s.a. english society of the eighteenth century in contemporary art. with four coloured and many other illustrations. super royal vo, sewed, s. nett; cloth, s. nett. dawson, rev. e.c. the life of bishop hannington. crown vo, paper boards, s. d.; or with map and illustrations, cloth, s. d. destrÉe, o.g. the renaissance of sculpture in belgium. illustrated. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett. dolmage, cecil g., m.a., d.c.l., ll.d., f.r.a.s. astronomy of to-day. a popular account in non-technical language. with forty-six illustrations and diagrams. extra crown vo, s. nett. domville-fife, charles w. submarine engineering of to-day. extra crown vo, s. nett. elzevir library, the. selections from the choicest english writers. exquisitely illustrated, with frontispiece and title-page in colours by h.m. brock, and many other illustrations. half bound in cloth, coloured top, s. nett; full leather, s. d. nett; velvet leather, gilt edges, in a box, s. d. nett. volume i. fancy & humour of lamb. " ii. wit & imagination of disraeli. " iii. vignettes from oliver goldsmith. " iv. wit & sagacity of dr. johnson. " v. insight & imagination of john ruskin. " vi. vignettes of london life from dickens. " vii. xviiith century vignettes from thackeray. " viii. vignettes of country life from dickens. " ix. wisdom & humour of carlyle. "decidedly natty and original in get-up."--_the saturday review._ evans, willmott, m.d. medical science of to-day. ex. crn. vo; illustrations, s. nett. wilmot, eardley, rear-admiral s. our fleet to-day and its development during the last half century. with many illustrations. crown vo, s. events of our own times crown vo. with illustrations, s. each. the war in the crimea. by general sir e. hamley, k.c.b. the indian mutiny. by colonel malleson, c.s.i. the afghan wars, - , and - . by archibald forbes. our fleet to-day and its development during the last half-century. by rear-admiral s. eardley wilmot. the refounding of the german empire. by colonel malleson, c.s.i. the liberation of italy. by the countess martinengo cesaresco. great britain in modern africa. by edgar sanderson, m.a. the war in the peninsula. by a. innes shand. fletcher, w.y. bookbinding in france. coloured plates. super-royal, sewed, s. d. nett; half-linen, s. d. nett. forbes, archibald. the afghan wars of - and - . with four portraits on copper, and maps and plans. crown vo, s. fraser, sir andrew h.l. among indian rajahs and ryots. with illustrations and a map. demy vo, s. nett. third and cheaper edition, s. nett. fraser, donald. winning a primitive people. illustrated. extra crown vo, s. nett. fripp, sir alfred d., k.c.v.o., & r. thompson, f.r.c.s. human anatomy for art students. profusely illustrated with photographs and drawings by innes fripp, a.r.c.a. square extra crown vo, s. d. nett. frobenius, leo. the childhood of man. a popular account of the lives and thoughts of primitive races. translated by prof. a.h. keane, ll.d. with illustrations. demy vo, s. nett. fry, roger. discourses delivered to the students of the royal academy by sir joshua reynolds. with an introduction and notes by roger fry. with thirty-three illustrations. square crown vo, s. d. nett. gardner, j. starkie. armour in england. with eight coloured plates and many other illustrations. super-royal vo, sewed, s. d. nett. foreign armour in england. with eight coloured plates and many other illustrations. super-royal vo, sewed, s. d. nett. armour in england. with sixteen coloured plates and many other illustrations. the two parts in one volume. super-royal vo, cloth, gilt top, s. nett. garnett, r., ll.d. richmond on thames. illustrated. super-royal vo, sewed, s. d. nett. giberne, agnes. beside the waters of comfort. crown vo, s. d. gibson, charles r., f.r.s.e. electricity of to-day. its works and mysteries described in non-technical language. with illustrations. extra crown vo, s. nett. scientific ideas of to-day. a popular account in non-technical language of the nature of matter, electricity, light, heat, &c., &c. with illustrations. extra crown vo, s. nett. how telegraphs and telephones work. with many illustrations. crown vo, s. d. nett. the autobiography of an electron. with illustrations. long vo, s. d. nett. wireless telegraphy. with many illustrations. ex. crn. vo, s. nett. godley, a.d. socrates and athenian society in his day. crown vo, s. d. aspects of modern oxford. with many illustrations. crown vo, cloth, s. nett; lambskin, s. nett; velvet leather, in box, s. nett. golden reciter (_see_ james, prof. cairns.) gomes, edwin h., m.a. seventeen years among the sea dyaks of borneo. with illustrations and a map. demy vo, s. nett. grahame, george. claude lorrain. illustrated. super-royal vo, s. d. nett; half-linen, s. d. nett. griffith, m.e. hume. behind the veil in persia and turkish arabia. an account of an englishwoman's eight years' residence amongst the women of the east. with illustrations and a map. demy vo, s. nett. grindon, leo. lancashire. brief historical and descriptive notes. with many illustrations. crown vo, s. grubb, w. barbrooke (pioneer and explorer of the chaco). an unknown people in an unknown land. with sixty illustrations and a map. demy vo, s. nett. third and cheaper edition, s. a church in the wilds. illustrated. extra crown vo, s. nett. hadow, w.h. a croatian composer. notes toward the study of joseph haydn. crown vo, s. d. nett. studies in modern music. first series. berlioz, schumann, wagner. with an essay on music and musical criticism. with five portraits. crown vo, s. d. studies in modern music. second series. chopin, dvoràk, brahms. with an essay on musical form. with four portraits. crown vo, s. d. hamerton, p.g. the etchings of rembrandt, and dutch etchers of the seventeenth century. by p.g. hamerton and laurence binton. with eight copper plates and many other illustrations. super-royal vo, s. d. nett. the mount. narrative of a visit to the site of a gaulish city on mount beuvray. with a description of the neighbouring city of autun. crown vo, s. d. round my house. notes on rural life in peace and war. crown vo, with illustrations, s. d. nett. cheaper edition, s. nett. paris. illustrated. new edition. cloth, s. nett; leather, s. nett in special leather, full gilt, in box, s. nett. hamley, gen. sir e. the war in the crimea. with copper plates and other illustrations. crown vo, s. hanoum zeyneb (heroine of pierre loti's novel "les désenchantées.") a turkish woman's european impressions. edited by grace ellison. with a portrait by auguste rodin and other illustrations from photographs. crown vo, s. nett. hartley, c. gasquoine. things seen in spain. with fifty illustrations. small to, cloth, s.; leather, s.; velvet leather in a box, s. nett. haywood, capt. a.h.w. through timbuctu & across the great sahara. demy vo, with illustrations and a map. s. nett. henderson, major percy e. a british officer in the balkans. through dalmatia, montenegro, turkey in austria, magyarland, bosnia and herzegovina. with illustrations and a map. gilt top. demy vo, s. nett. herbert, george. the temple. sacred poems and ejaculations. the text reprinted from the first edition. with seventy-six illustrations after albert dÜrer, holbrin, and other masters. crown vo, cloth, s. nett; 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[illustration: lilly's hieroglyphs (published in )] myths and marvels of astronomy by richard a. proctor author of "rough ways made smooth," "the expanse of heaven," "our place among infinities," "pleasant ways in science," etc., etc. _new edition_ longmans, green, and co. london, new york, and bombay _printed by_ ballantyne, hanson & co _at the ballantyne press_ preface. the chief charm of astronomy, with many, does not reside in the wonders revealed to us by the science, but in the lore and legends connected with its history, the strange fancies with which in old times it has been associated, the half-forgotten myths to which it has given birth. in our own times also, astronomy has had its myths and fancies, its wild inventions, and startling paradoxes. my object in the present series of papers has been to collect together the most interesting of these old and new astronomical myths, associating with them, in due proportion, some of the chief marvels which recent astronomy has revealed to us. to the former class belong the subjects of the first four and the last five essays of the present series, while the remaining essays belong to the latter category. throughout i have endeavoured to avoid technical expressions on the one hand, and ambiguous phraseology (sometimes resulting from the attempt to avoid technicality) on the other. i have, in fact, sought to present my subjects as i should wish to have matters outside the range of my special branch of study presented for my own reading. richard a. proctor. contents. page i. astrology ii. the religion of the great pyramid iii. the mystery of the pyramids iv. swedenborg's visions of other worlds v. other worlds and other universes vi. suns in flames vii. the rings of saturn viii. comets as portents ix. the lunar hoax x. on some astronomical paradoxes xi. on some astronomical myths xii. the origin of the constellation-figures myths and marvels of astronomy i. _astrology._ signs and planets, in aspects sextile, quartile, trine, conjoined, or opposite; houses of heaven, with their cusps, hours, and minutes; almuten, almochoden, anahibazon, catahibazon; a thousand terms of equal sound and significance.--_guy mannering._ ... come and see! trust thine own eyes. a fearful sign stands in the house of life, an enemy: a fiend lurks close behind the radiance of thy planet--oh! be warned!--coleridge. astrology possesses a real interest even in these days. it is true that no importance attaches now even to the discussion of the considerations which led to the rejection of judicial astrology. none but the most ignorant, and therefore superstitious, believe at present in divination of any sort or kind whatsoever. divination by the stars holds no higher position than palmistry, fortune-telling by cards, or the indications of the future which foolish persons find in dreams, tea-dregs, salt-spilling, and other absurdities. but there are two reasons which render the history of astrology interesting. in the first place, faith in stellar influences was once so widespread that astrological terminology came to form a part of ordinary language, insomuch that it is impossible rightly to understand many passages of ancient and mediæval literature, or rightly to apprehend the force of many allusions and expressions, unless the significance of astrological teachings to the men of those times be recognised. in the second place, it is interesting to examine how the erroneous teachings of astrology were gradually abandoned, to note the way in which various orders of mind rejected these false doctrines or struggled to retain them, and to perceive how, with a large proportion of even the most civilised races, the superstitions of judicial astrology were long retained, or are retained even to this very day. the world has still to see some superstitions destroyed which are as widely received as astrology ever was, and which will probably retain their influence over many minds long after the reasoning portion of the community have rejected them. even so far back as the time of eudoxus the pretensions of astrologers were rejected, as cicero informs us ('de div.' ii. ). and though the romans were strangely superstitious in such matters, cicero reasons with excellent judgment against the belief in astrology. gassendi quotes the argument drawn by cicero against astrology, from the predictions of the chaldæans that cæsar, crassus, and pompey would die 'in a full old age, in their own houses, in peace and honour,' whose deaths, nevertheless, were 'violent, immature, and tragical.' cicero also used an argument whose full force has only been recognised in modern times. 'what contagion,' he asked, 'can reach us from the planets, whose distance is almost infinite?' it is singular that seneca, who was well acquainted with the uniform character of the planetary motions, seems to have entertained no doubt respecting their influence. tacitus expresses some doubts, but was on the whole inclined to believe in astrology. 'certainly,' he says, 'the majority of mankind cannot be weaned from the opinion that at the birth of each man his future destiny is fixed; though some things may fall out differently from the predictions, by the ignorance of those who profess the art; and thus the art is unjustly blamed, confirmed as it is by noted examples in all ages.'[ ] probably, the doubt suggested by the different fortunes and characters of men born at the same time must have occurred to many before cicero dwelt upon it. pliny, who followed cicero in this, does not employ the argument quite correctly, for he says that, 'in every hour, in every part of the world, are born lords and slaves, kings and beggars.' but of course, according to astrological principles, it would be necessary that two persons, whose fortunes were to be alike, should be born, not only in the same hour, but in the same place. the fortunes and character of jacob and esau, however, should manifestly have been similar, which was certainly not the case, if their history has been correctly handed down to us. an astrologer of the time of julius cæsar, named publius nigidius figulus, used a singular argument against such reasoning. when an opponent urged the different fortunes of men born nearly at the same instant, nigidius asked him to make two contiguous marks on a potter's wheel which was revolving rapidly. when the wheel was stopped, the two marks were found to be far apart. nigidius is said to have received the name of figulus (the potter), in remembrance of the story; but more probably he was a potter by trade, and an astrologer only during those leisure hours which he could devote to charlatanry. st. augustine, who relates the story (which i borrow from whewell's 'history of the inductive sciences'), says, justly, that the argument of nigidius was as fragile as the ware made on the potter's wheel. the belief must have been all but universal in those days that at the birth of any person who was to hold an important place in the world's history the stars would either be ominously conjoined, or else some blazing comet or new star would make its appearance. for we know that some such object having appeared, or some unusual conjunction of planets having occurred, near enough to the time of christ's birth to be associated in men's minds with that event, it came eventually to be regarded as belonging to his horoscope, and as actually indicating to the wise men of the east (chaldæan astrologers, doubtless) the future greatness of the child then born. it is certain that that is what the story of the star in the east means as it stands. theologians differ as to its interpretation in points of detail. some think the phenomenon was meteoric, others that a comet then made its appearance, others that a new star shone out, and others that the account referred to a conjunction of jupiter, saturn, and mars, which occurred at about that time. as a matter of detail it may be mentioned, that none of these explanations in the slightest degree corresponds with the account, for neither meteor, nor comet, nor new star, nor conjoined planets, would go before travellers from the east, to show them their way to any place. yet the ancients sometimes regarded comets as guides. whichever view we accept, it is abundantly clear that an astrological significance was attached by the narrator to the event. and not so very long ago, when astrologers first began to see that their occupation was passing from them, the wise men of the east were appealed to against the enemies of astrology,[ ]--very much as moses was appealed to against copernicus and galileo, and more recently to protect us against certain relationships which darwin, wallace, and huxley unkindly indicate for the human race divine. although astronomers now reject altogether the doctrines of judicial astrology, it is impossible for the true lover of that science to regard astrology altogether with contempt. astronomy, indeed, owes much more to the notions of believers in astrology than is commonly supposed. astrology bears the same relation to modern astronomy that alchemy bears to modern chemistry. as it is probable that nothing but the hope of gain, literally in this case _auri sacra fames_, would have led to those laborious researches of the alchemists which first taught men how to analyse matter into its elementary constituents, and afterwards to combine these constituents afresh into new forms, so the belief that, by carefully studying the stars, men might acquire the power of predicting future events, first directed attention to the movements of the celestial bodies. kepler's saying, that astrology, though a fool, was the daughter of a wise mother,[ ] does not by any means present truly the relationship between astrology and astronomy. rather we may say that astrology and alchemy, though foolish mothers, gave birth to those wise daughters, astronomy and chemistry. even this way of speaking scarcely does justice to the astrologers and alchemists of old times. their views appear foolish in the light of modern scientific knowledge, but they were not foolish in relation to what was known when they were entertained. modern analysis goes far to demonstrate the immutability, and, consequently, the non-transmutability of the metals, though it is by no means so certain as many suppose that the present position of the metals in the list of _elements_ is really correct. certainly a chemist of our day would be thought very unwise who should undertake a series of researches with the object of discovering a mineral having such qualities as the alchemists attributed to the philosopher's stone. but when as yet the facts on which the science of chemistry is based were unknown, there was nothing unreasonable in supposing that such a mineral might exist, or the means of compounding it be discovered. nay, many arguments from analogy might be urged to show that the supposition was altogether probable. in like manner, though the known facts of astronomy oppose themselves irresistibly to any belief in planetary influences upon the fates of men and nations, yet before those facts were discovered it was not only not unreasonable, but was in fact, highly reasonable to believe in such influences, or at least that the sun, and moon, and stars moved in the heavens in such sort as to indicate what would happen. if the wise men of old times rejected the belief that 'the stars in their courses fought' for or against men, they yet could not very readily abandon the belief that the stars were for signs in the heavens of what was to befall mankind. if we consider the reasoning now commonly thought valid in favour of the doctrine that other orbs besides our earth are inhabited, and compare it with the reasoning on which judicial astrology was based, we shall not find much to choose between the two, so far as logical weight is concerned. because the only member of the solar system which we can examine closely is inhabited, astronomers infer a certain degree of probability for the belief that the other planets of the system are also inhabited. and because the only sun we know much about is the centre of a system of planets, astronomers infer that probably the stars, those other suns which people space, are also the centres of systems; although no telescope which man can make would show the members of a system like ours, attending on even the nearest of all the stars. the astrologer had a similar argument for his belief. the moon, as she circles around the earth, exerts a manifest influence upon terrestrial matter--the tidal wave rising and sinking synchronously with the movements of the moon, and other consequences depending directly or indirectly upon her revolution around the earth. the sun's influence is still more manifest; and, though it may have required the genius of a herschel or of a stephenson to perceive that almost every form of terrestrial energy is derived from the sun, yet it must have been manifest from the very earliest times that the greater light which rules the day rules the seasons also, and, in ruling them, provides the annual supplies of vegetable food, on which the very existence of men and animals depends. if these two bodies, the sun and moon, are thus potent, must it not be supposed, reasoned the astronomers of old, that the other celestial bodies exert corresponding influences? _we_ know, but they did not know, that the moon rules the tides effectually because she is near to us, and that the sun is second only to the moon in tidal influence because of his enormous mass and attractive energy. we know also that his position as fire, light, and life of the earth and its inhabitants, is due directly to the tremendous heat with which the whole of his mighty frame is instinct. not knowing this, the astronomers of old times had no sufficient reason for distinguishing the sun and moon from the other celestial bodies, so far at least as the general question of celestial influences was concerned. so far as particulars were concerned, it was not altogether so clear to them as it is to us, that the influence of the sun must be paramount in all respects save tidal action, and that of the moon second only to the sun's in other respects, and superior to his in tidal sway alone. many writers on the subject of life in other worlds are prepared to show (as brewster attempts to do, for example) that jupiter and saturn are far nobler worlds than the earth, because superior in this or that circumstance. so the ancient astronomers, in their ignorance of the actual conditions on which celestial influences depend, found abundant reasons for regarding the feeble influences exerted by saturn, jupiter, and mars, as really more potent than those exerted by the sun himself upon the earth. they reasoned, as milton afterwards made raphaël reason, that 'great or bright infers not excellence,' that saturn or jupiter, though 'in comparison so small, nor glist'ring' to like degree, may yet 'of solid good contain more plenty than the sun.' supposing the influence of a celestial body to depend on the magnitude of its sphere, in the sense of the old astronomy (according to which each planet had its proper sphere, around the earth as centre), then the influence of the sun would be judged to be inferior to that of either saturn, jupiter, or mars; while the influences of venus and mercury, though inferior to the influence of the sun, would still be held superior to that of the moon. for the ancients measured the spheres of the seven planets of their system by the periods of the apparent revolution of those bodies around the celestial dome, and so set the sphere of the moon innermost, enclosed by the sphere of mercury, around which in turn was the sphere of venus, next the sun's, then, in order, those of mars, jupiter, and saturn. we can readily understand how they might come to regard the slow motions of the sphere of saturn and jupiter, taking respectively some thirty and twelve years to complete a revolution, as indicating power superior to the sun's, whose sphere seemed to revolve once in a single year. many other considerations might have been urged, before the copernican theory was established, to show that, possibly, some of the planets exert influences more effective than those of the sun and moon. it is, indeed, clear that the first real shock sustained by astrology came from the arguments of copernicus. so long as the earth was regarded as the centre round which all the celestial bodies move, it was hopeless to attempt to shake men's faith in the influences of the stars. so far as i know, there is not a single instance of a believer in the old ptolemaic system who rejected astrology absolutely. the views of bacon--the last of any note who opposed the system of copernicus[ ]--indicate the extreme limits to which a ptolemaist could go in opposition to astrology. it may be worth while to quote bacon's opinion in this place, because it indicates at once very accurately the position held by believers in astrology in his day, and the influence which the belief in a central fixed earth could not fail to exert on the minds of even the most philosophical reasoners. 'astrology,' he begins, 'is so full of superstition that scarce anything sound can be discovered in it; though we judge it should rather be purged than absolutely rejected. yet if any one shall pretend that this science is founded not in reason and physical contemplations, but in the direct experience and observation of past ages, and therefore not to be examined by physical reasons, as the chaldæans boasted, he may at the same time bring back divination, auguries, soothsaying, and give in to all kinds of fables; for these also were said to descend from long experience. but we receive astrology as a part of physics, without attributing more to it than reason and the evidence of things allow, and strip it of its superstition and conceits. thus we banish that empty notion about the horary reign of the planets, as if each resumed the throne thrice in twenty-four hours, so as to leave three hours supernumerary; and yet this fiction produced the division of the week,[ ] a thing so ancient and so universally received. thus likewise we reject as an idle figment the doctrine of horoscopes, and the distribution of the houses, though these are the darling inventions of astrology, which have kept revel, as it were, in the heavens. and lastly, for the calculation of nativities, fortunes, good or bad hours of business, and the like fatalities, they are mere levities, that have little in them of certainty and solidity, and may be plainly confuted by physical reasons. but here we judge it proper to lay down some rules for the examination of astrological matters, in order to retain what is useful therein, and reject what is insignificant. thus, . let the greater revolutions be retained, but the lesser, of horoscopes and houses, be rejected--the former being like ordnance which shoot to a great distance, whilst the other are but like small bows, that do no execution. . the celestial operations affect not all kinds of bodies, but only the more sensible, as humours, air, and spirits. . all the celestial operations rather extend to masses of things than to individuals, though they may obliquely reach some individuals also which are more sensible than the rest, as a pestilent constitution of the air affects those bodies which are least able to resist it. . all the celestial operations produce not their effects instantaneously, and in a narrow compass, but exert them in large portions of time and space. thus predictions as to the temperature of a year may hold good, but not with regard to single days. . there is no fatal necessity in the stars; and this the more prudent astrologers have constantly allowed. . we will add one thing more, which, if amended and improved, might make for astrology--viz. that we are certain the celestial bodies have other influences besides heat and light, but these influences act not otherwise than by the foregoing rules, though they lie so deep in physics as to require a fuller explanation. so that, upon the whole, we must register as needed,[ ] an astrology written in conformity with these principles, under the name of _astrologia sana_.' he then proceeds to show what this just astrology should comprehend--as, , the doctrine of the commixture of rays; , the effect of nearest approaches and farthest removes of planets to and from the point overhead (the planets, like the sun, having their summer and winter); , the effects of distance, 'with a proper enquiry into what the vigour of the planets may perform of itself, and what through their nearness to us; for,' he adds, but unfortunately without assigning any reason for the statement, 'a planet is more brisk when most remote, but more communicative when nearest;' , the other accidents of the planet's motions as they pursue their wand'ring course, now high, now low, then hid, progressive, retrograde, or standing still; , all that can be discovered of the general nature of the planets and fixed stars, considered in their own essence and activity; , lastly, let this just astrology, he says, 'contain, from tradition, the particular natures and alterations of the planets and fixed stars; for' (here is a reason indeed) 'as these are delivered with general consent, they are not lightly to be rejected, unless they directly contradict physical considerations. of such observations let a just astrology be formed; and according to these alone should schemes of the heavens be made and interpreted.' the astrology thus regarded by bacon as sane and just did not differ, as to its primary object, from the false systems which now seem to us so absurd. 'let this astrology be used with greater confidence in prediction,' says bacon, 'but more cautiously in election, and in both cases with due moderation. thus predictions may be made of comets, and all kinds of meteors, inundations, droughts, heats, frosts, earthquakes, fiery eruptions, winds, great rains, the seasons of the year, plagues, epidemic diseases, plenty, famine, wars, seditions, sects, transmigrations of people, and all commotions, or great innovations of things, natural and civil. predictions may possibly be made more particular, though with less certainty, if, when the general tendencies of the times are found, a good philosophical or political judgment applies them to such things as are most liable to accidents of this kind. for example, from a foreknowledge of the seasons of any year, they might be apprehended more destructive to olives than grapes, more hurtful in distempers of the lungs than the liver, more pernicious to the inhabitants of hills than valleys, and, for want of provisions, to monks than courtiers, etc. or if any one, from a knowledge of the influence which the celestial bodies have upon the spirits of mankind, should find it would affect the people more than their rulers, learned and inquisitive men more than the military, etc. for there are innumerable things of this kind that require not only a general knowledge gained from the stars which are the agents, but also a particular one of the passive subjects. nor are elections to be wholly rejected, though not so much to be trusted as predictions; for we find in planting, sowing, and grafting, observations of the moon are not absolutely trifling, and there are many particulars of this kind. but elections are more to be curbed by our rules than predictions; and this must always be remembered, that election only holds in such cases where the virtue of the heavenly bodies, and the action of the inferior bodies also, is not transient, as in the examples just mentioned; for the increases of the moon and planets are not sudden things. but punctuality of time should here be absolutely rejected. and perhaps there are more of these instances to be found in civil matters than some would imagine.' the method of inquiry suggested by bacon as proper for determining the just rules of the astrology he advocated, was, as might be expected, chiefly inductive. there are, said he, 'but four ways of arriving at this science, viz.-- , by future experiments; , past experiments; , traditions; , physical reasons.' but he was not very hopeful as to the progress of the suggested researches. it is vain, he said, to think at present of future experiments, because many ages are required to procure a competent stock of them. as for the past, it is true that past experiments are within our reach, 'but it is a work of labour and much time to procure them. thus astrologers may, if they please, draw from real history all greater accidents, as inundations, plagues, wars, seditions, deaths of kings, etc., as also the positions of the celestial bodies, not according to fictitious horoscopes, but the above-mentioned rules of their revolutions, or such as they really were at the time, and, when the event conspires, erect a probable rule of prediction.' traditions would require to be carefully sifted, and those thrown out which manifestly clashed with physical considerations, leaving those in full force which complied with such considerations. lastly, the physical reasons worthiest of being enquired into are those, said bacon, 'which search into the universal appetites and passions of matter, and the simple genuine motions of the heavenly bodies.' it is evident there was much which, in our time at least, would be regarded as wild and fanciful in the 'sound and just astrology' advocated by bacon. yet, in passing, it may be noticed that even in our own time we have seen similar ideas promulgated, not by common astrologers and fortune-tellers (who, indeed, know nothing about such matters), but by persons supposed to be well-informed in matters scientific. in a roundabout way, a new astrology has been suggested, which is not at all unlike bacon's 'astrologia sana,' though not based, as he proposed that astrology should be, on experiment, or tradition, or physical reasons. it has been suggested, first, that the seasons of our earth are affected by the condition of the sun in the matter of spots, and very striking evidence has been collected to show that this must be the case. for instance, it has been found that years when the sun has been free from spots have been warmer than the average; and it has also been found that such years have been cooler than the average: a double-shotted argument wholly irresistible, especially when it is also found that when the sun has many spots the weather has sometimes been exceptionally warm and sometimes exceptionally cold. if this be not considered sufficient, then note that in one country or continent or hemisphere the weather, when the sun is most spotted (or least, as the case may be), may be singularly hot, while in another country, continent, or hemisphere, the weather may be as singularly cold. so with wind and calm, rain and drought, and so forth. always, whether the sun is very much spotted or quite free from spots, something unusual in the way of weather must be going on somewhere, demonstrating in the most significant way the influence of sun-spots or the want of sun-spots on the weather. it is true that captious minds might say that this method of reasoning proved too much in many ways, as, for example, thus--always, whether the sun is very much spotted or quite free from spots, some remarkable event, as a battle, massacre, domestic tragedy on a large scale, or the like, may be going on, demonstrating in the most significant way the influence of sun-spots or the want of sun-spots on the passions of men--which sounds absurd. but the answer is twofold. first, such reasoning is captious, and secondly, it is not certain that sun-spots, or the want of them, may not influence human passions; it may be worth while to enquire into this possible solar influence as well as the other, which can be done by crossing the hands of the new fortune-tellers with a sufficient amount of that precious metal which astrologers have in all ages dedicated to the sun. that the new system of divination is not solely solar, but partly planetary also, is seen when we remember that the sun-spots wax and wane in periods of time which are manifestly referable to the planetary motions. thus, the great solar spot-period lasts about eleven years, the successive spotless epochs being separated on the average by about that time; and so nearly does this period agree with the period of the planet jupiter's revolution around the sun, that during eight consecutive spot-periods the spots were most numerous when jupiter was farthest from the sun, and it is only by going back to the periods preceding these eight that we find a time when the reverse happened, the spots being most numerous when jupiter was nearest to the sun. so with various other periods which the ingenuity of messrs. de la rue and balfour stewart has detected, and which, under the closest scrutiny, exhibit almost exact agreement for many successive periods, preceded and followed by almost exact disagreement. here, again, the captious may argue that such alternate agreements and disagreements may be noted in every case where two periods are not very unequal, whether there be any connection between them or not; but much more frequently when there is no connection: and that the only evidence really proving a connection between planetary motions and the solar spots would be constant agreement between solar spot periods and particular planetary periods. but the progress of science, and especially the possible erection of a new observatory for finding out ('for a consideration') how sun-spots affect the weather, etc., ought not to be interfered with by captious reasoners in this objectionable manner. nor need any other answer be given them. seeing, then, that sun-spots manifestly affect the weather and the seasons, while the planets rule the sun-spots, it is clear that the planets really rule the seasons. and again, seeing that the planets rule the seasons, while the seasons largely affect the well-being of men and nations (to say nothing of animals), it follows that the planets influence the fates of men and nations (and animals). _quod erat demonstrandum._ let us return, however, to the more reasonable astrology of the ancients, and enquire into some of the traditions which bacon considered worthy of attention in framing the precepts of a sound and just astrology. it was natural that the astrologers of old should regard the planetary influences as depending in the main on the position of the celestial bodies on the sky above the person or place whose fortunes were in question. thus two men at the same moment in rome and in persia would by no means have the same horoscope cast for their nativities, so that their fortunes, according to the principles of judicial astrology, would be quite different. in fact it might happen that two men, born at the same instant of time, would have all the principal circumstances of their lives contrasted--planets riding high in the heavens of one being below the horizon of the other, and _vice versâ_. the celestial sphere placed as at the moment of the native's birth was divided into twelve parts by great circles supposed to pass through the point overhead, and its opposite, the point vertically beneath the feet. these twelve divisions were called 'houses.' their position is illustrated in the following figure, taken from raphaël's astrology. [illustration: particular significations of the _twelve celestial houses_, according to various astrological authors. sun-rise. cusp of the _ascendant_. life and health cusp of the _second house_. riches cusp of the _third house_. kindred and short journeys cusp of the _fourth house_. inheritances mid-night. cusp of the _fifth house_. children cusp of the _sixth house_. sickness cusp of the _seventh house_. marriage sun-set. cusp of the _eighth house_. death cusp of the _ninth house_. long journeys cusp of the _mid-heaven_. honor noon-day. cusp of the _eleventh house_. friends cusp of the _twelfth house_. enemies ] the first, called the ascendant house, was the portion rising above the horizon at the east. it was regarded as the house of life, the planets located therein at the moment of birth having most potent influence on the life and destiny of the native. such planets were said to rule the ascendant, being in the ascending house; and it is from this usage that our familiar expression that such and such an influence is 'in the ascendant' is derived. the next house was the house of riches, and was one-third of the way from the east below the horizon towards the place of the sun at midnight. the third was the house of kindred, short journeys, letters, messages, etc. it was two-thirds of the way towards the place of the midnight sun. the fourth was the house of parents, and was the house which the sun reached at midnight. the fifth was the house of children and women, also of all sorts of amusements, theatres, banquets, and merry-making. the sixth was the house of sickness. the seventh was the house of love and marriage. these three houses (the fifth, sixth, and seventh) followed in order from the fourth, so as to correspond to the part of the sun's path below the horizon, between his place at midnight and his place when descending in the west. the seventh, opposite to the first, was the descendant. the eighth house was the first house above the horizon, lying to the west, and was the house of death. the ninth house, next to the mid-heaven on the west, was the house of religion, science, learning, books, and long voyages. the tenth, which was in the mid-heaven, or region occupied by the sun at midday, was the house of honour, denoting credit, renown, profession or calling, trade, preferment, etc. the eleventh house, next to the mid-heaven on the east, was the house of friends. lastly, the twelfth house was the house of enemies. the houses were not all of equal potency. the _angular_ houses, which are the first, the fourth, the seventh, and the tenth--lying east, north, west, and south--were first in power, whether for good or evil. the second, fifth, eighth, and eleventh houses were called _succedents_, as following the angular houses, and next to them in power. the remaining four houses--viz. the third, sixth, ninth, and twelfth houses--were called _cadents_, and were regarded as weakest in influence. the houses were regarded as alternately masculine and feminine: the first, third, fifth, etc., being masculine; while the second, fourth, sixth, etc., were feminine. the more particular significations of the various houses are shown in the accompanying figure from the same book. [illustration: a celestial diagram representing at one view the various symbolical significations of the _twelve heavenly houses_; according to ancient manuscript writers of the twelfth century; _and not to be found in authors_. brethren of friends, fathers of kings, sickness of public enemies, wives of enemies, death of servants, long journeys of children, friends of brethren, thoughts of the asker. the end of youth, brethren of private enemies, fathers and grandsires of friends, king's sons, enemies of wives, magistery of children, private enemies of brethren. sects, dreams, churches, fathers of private enemies, sons of friends, sickness of kings, enemies of the religious, trade of servants, private enemies of fathers. dead men's goods, castles, treasure hid, the fate of the corpse in the grave, money of brethren, children of private enemies, sickness of friends, king's enemies, friends of servants. cards, dice, brethren's brethren, father's money, sickness of private enemies, enemies of friends, death of kings, friends of enemies, enemies of servants. vassals, children's money, brethren's fathers, father's brethren, enemies' enemies, death of friends, journeys and religion of kings, lay dignities, enemies of wives. fines, pleas, laws, nuptials, death of enemies, friends of brethren, sons of friends, sisters of brethren, death of enemies and of great beasts, religion of friends. labour, sorrow, inheritance of the dead, money of enemies, brethren of servants, sickness of brethren, dignity of friends, king's friends, enemies of religious persons. prophets, prayers, visions, omens, divine worship, wife's brethren, fathers of servants, children's children, sickness of fathers, enemies of brethren, friends of friends, enemies of kings. judges, brethren of enemies, servants, fathers of enemies, children of servants, sickness of sons, death of brethren, friends of enemies, enemies of friends. knights, esquires, children of enemies, sickness of servants, enemies and wives of offspring, death of fathers, journeys of brethren, enemies of enemies. envy, sorrow, guile, long hidden wrath, money of friends, brethren of kings, sickness of wives, servants' enemies, death of children, trade of brethren, a prison. ] it will be easily understood how these houses were dealt with in erecting a scheme of nativity. the position of the planets at the moment of the native's birth, in the several houses, determined his fortunes with regard to the various matters associated with these houses. thus planets of good influence in the native's ascendant, or first house, signified generally a prosperous life; but if at the same epoch a planet of malefic influence was in the seventh house, then the native, though on the whole prosperous, would be unfortunate in marriage. a good planet in the tenth house signified good fortune and honour in office or business, and generally a prosperous career as distinguished from a happy life; but evil planets in the ninth house would suggest to the native caution in undertaking long voyages, or entering upon religious or scientific controversies. similar considerations applied to questions relating to horary astronomy, in which the position of the planets in the various houses at some epoch guided the astrologer's opinion as to the fortune of that hour, either in the life of a man or the career of a state. in such inquiries, however, not only the position of the planets, etc., at the time had to be considered, but also the original horoscope of the person, or the special planets and signs associated with particular states. thus if jupiter, the most fortunate of all the planets, was in the ascendant, or in the house of honour, at the time of the native's birth, and at some epoch this planet was ill-aspected or afflicted by other planets potent for evil in the native's horoscope, then that epoch would be a threatening one in the native's career. the sign gemini was regarded by astrologers as especially associated with the fortunes of london, and accordingly they tell us that the great fire of london, the plague, the building of london bridge, and other events interesting to london, all occurred when this sign was in the ascendant, or when special planets were in this sign.[ ] the signs of the zodiac in the various houses were in the first place to be noted, because not only had these signs special powers in special houses, but the effects of the planets in particular houses varied according to the signs in which the planets were situated. if we were to follow the description given by the astrologers themselves, not much insight would be thrown upon the meaning of the zodiacal signs. for instance, astrologers say that aries is a vernal, dry, fiery, masculine, cardinal, equinoctial, diurnal, movable, commanding, eastern, choleric, violent, and quadrupedalian sign. we may, however, infer generally from their accounts the influences which they assigned to the zodiacal signs. aries is the house and joy of mars, signifies a dry constitution, long face and neck, thick shoulders, swarthy complexion, and a hasty, passionate temper. it governs the head and face, and all diseases relating thereto. it reigns over england, france, switzerland, germany, denmark, lesser poland, syria, naples, capua, verona, etc. it is a masculine sign, and is regarded as fortunate. taurus gives to the native born under his auspices a stout athletic frame, broad bull-like forehead, dark curly hair, short neck, and so forth, and a dull apathetic temper, exceedingly cruel and malicious if once aroused. it governs the neck and throat, and reigns over ireland, great poland, part of russia, holland, persia, asia minor, the archipelago, mantua, leipsic, etc. it is a feminine sign, and unfortunate. gemini is the house of mercury. the native of gemini will have a sanguine complexion and tall, straight figure, dark eyes quick and piercing, brown hair, active ways, and will be of exceedingly ingenious intellect. it governs the arms and shoulders, and rules over the south-west parts of england, america, flanders, lombardy, sardinia, armenia, lower egypt, london, versailles, brabant, etc. it is a masculine sign, and fortunate. cancer is the house of the moon and exaltation of jupiter, and its native will be of fair but pale complexion, round face, grey or mild blue eyes, weak voice, the upper part of the body large, slender arms, small feet, and an effeminate constitution. it governs the breast and the stomach, and reigns over scotland, holland, zealand, burgundy, africa, algiers, tunis, tripoli, constantinople, new york, etc. it is a feminine sign, and unfortunate. the native born under leo will be of large body, broad shoulders, austere countenance, with dark eyes and tawny hair, strong voice, and leonine character, resolute and ambitious, but generous, free, and courteous. leo governs the heart and back, and reigns over italy, bohemia, france, sicily, rome, bristol, bath, taunton, philadelphia, etc. it is a masculine sign, and fortunate. virgo is the joy of mercury. its natives are of moderate stature, seldom handsome, slender but compact, thrifty and ingenious. it governs the abdomen, and reigns over turkey both in europe and asia, greece, and mesopotamia, crete, jerusalem, paris, lyons, etc. it is a feminine sign, and generally unfortunate. libra is the house of venus. the natives of libra are tall and well made, elegant in person, round-faced and ruddy, but plain-featured and 'inclined to eruptions that disfigure the face when old; they' (the natives) 'are of sweet disposition, just and upright in dealing.' it governs the lumbar regions, and reigns over austria, alsace, savoy, portugal, livonia, india, ethiopia, lisbon, vienna, frankfort, antwerp, charleston, etc. it is a masculine sign, and fortunate. scorpio is, like aries, the house of mars, 'and also his joy.' its natives are strong, corpulent, and robust, with large bones, 'dark curly hair and eyes' (presumably the eyes dark only, not curly), middle stature, dusky complexion, active bodies; they are usually reserved in speech. it governs the region of the groin, and reigns over judæa, mauritania, catalonia, norway, west silesia, upper batavia, barbary, morocco, valentia, messina, etc. it is feminine, and unfortunate. (it would appear likely, by the way, that astrology was a purely masculine science.) sagittarius is the house and joy of jupiter. its natives are well formed and tall, ruddy, handsome, and jovial, with fine clear eyes, chestnut hair, and oval fleshy face. they are 'generally jolly fellows at either bin or board,' active, intrepid, generous, and obliging. it governs the legs and thighs,[ ] and reigns over arabia felix, spain, hungary, moravia, liguria, narbonne, cologne, avignon, etc. it is masculine, and of course fortunate. capricorn is the house of saturn and exaltation of mars. this sign gives to its natives a dry constitution and slender make, with a long thin visage, thin beard (a generally goaty aspect, in fact), dark hair, long neck, narrow chin, and weak knees. it governs, nevertheless, the knees and hams, and reigns over india, macedonia, thrace and greece, mexico, saxony, wilna, mecklenburgh, brandenburg, and oxford. it is feminine, and unfortunate. aquarius also is the house of saturn. its natives are robust, steady, strong, healthy, and of middle stature; delicate complexion, clear but not pale, sandy hair, hazel eyes, and generally an honest disposition. it governs the legs and ankles, and reigns over arabia, petræa, tartary, russia, denmark, lower sweden, westphalia, hamburg, and bremen. it is masculine, and fortunate. pisces is the house of jupiter and exaltation of venus. its natives are short, pale, thick-set, and round-shouldered (like fish), its character phlegmatic and effeminate. it governs the feet and toes, and reigns over portugal, spain, egypt, normandy, galicia, ratisbon, calabria, etc. it is feminine, and therefore, naturally, unfortunate. let us next consider the influences assigned to the various planets and constellations. though we can understand that in old times the planets and stars were regarded as exercising very potent influences upon the fates of men and nations,[ ] it is by no means easy to understand how astrologers came to assign to each planet its special influence. that is, it is not easy to understand how they could have been led to such a result by actual reasoning, still less by any process of observation.[ ] there was a certain scientific basis for the belief in the possibility of determining the special influences of the stars; and we should have expected to find some scientific process adopted for the purpose. yet, so far as can be judged, the influences assigned to the planets depended on entirely fanciful considerations. in some cases we seem almost to see the line along which the fancies of the old astrologers led them, just as in some cases we can perceive how mythological superstitions (which are closely related to astrological ideas) had their origin; though it is not quite clear whether the planets were first regarded as deities with special qualities, and these qualities afterwards assigned to the planetary influences, or whether the planetary influences were first assigned, and came eventually to be regarded as the qualities of the deities associated with the several planets. it is easy, for instance, to understand why astrologers should have regarded the sun as the emblem of kingly power and dignity, and equally easy to understand why, to the sun regarded as a deity, corresponding qualities should have been ascribed; but it is not easy to determine whether the astrological or the sabaistic superstitions were the earlier. and in like manner of the moon and planets. there seems to me no sufficient evidence in favour of whewell's opinion, that 'in whatever manner the sun, moon, and planets came to be identified with gods and goddesses, the characters ascribed to these gods and goddesses, regulated the virtues and powers of the stars which bear their names.' as he himself very justly remarks, 'we do not possess any of the speculations of the earlier astrologers; and we cannot, therefore, be certain that the notions which operated in men's minds when the art had its birth, agreed with the views on which it was afterwards defended.' he does not say why he infers that, though at later periods supported by physical analogies, it was originally suggested by mythological beliefs. quite as probably mythological beliefs were suggested by astrological notions. some of these beliefs, indeed, seem manifestly to have been so suggested; as the character of the deity mercury, from the rapid motions of the planet mercury, and the difficulty of detecting it; the character of mars from the blood-red hue of the planet when close to the horizon, and so forth. let us examine, however, the characteristics ascribed by astrologers to various planets. it is unfortunate for astrology that, despite the asserted careful comparison of events with the planetary positions preceding and indicating them, nothing was ever observed which seemed to suggest the possibility that there may be an unknown planet ruling very strongly the affairs of men. astrologers tell us now that uranus is a very potent planet; yet the old astrologers seem to have got on very well without him. by the way, one of the moderns, the grave raphaël, gives a very singular account of the discovery of uranus, in a book published sixteen years before neptune was discovered by just such a process as raphaël imagined in the case of uranus. he says that drs. halley, bradley, and others, having frequently observed that saturn was disturbed in his motion by some force exerted from beyond his orbit, and being unable to account for the disturbance on the known principles of gravitation, pursued their enquiry into the matter, 'till at length the discovery of this hitherto unknown planet covered their labours with success, and has enabled us to enlarge our present solar system to nearly double its bounds.' of course there is not a word of truth in this; uranus having been discovered by accident long after halley and bradley were in the grave. but the account suggests what might have been, and curiously anticipates the actual manner in which neptune was discovered. astrologers agree in attributing evil effects to uranus. but the evil he does is always peculiarly strange, unaccountable, and totally unexpected. he causes the native born under his influence to be of a very eccentric and original disposition, romantic, unsettled, addicted to change, a seeker after novelty; though, if the moon or mercury have a good aspect towards uranus, the native will be profound in the secret sciences, magnanimous, and lofty of mind. but let all beware of marriage when uranus is in the seventh house, or afflicting the moon. and in general, let the fair sex remember that uranus is peculiarly hostile to them, and very evil in love. saturn is the greater infortune of the old system of astrology, and is by universal experience acknowledged to be the most potent, evil, and malignant of all the planets. those born under him are of dark and pale complexion, with small, black, leering eyes, thick lips and nostrils, large ears, thin face, lowering looks, cloudy aspect, and seemingly melancholy and unhappy; and though they have broad shoulders, they have but short lips and a thin beard, they are in character austere and reserved, covetous, laborious, and revengeful; constant in friendship, and good haters. the most remarkable and certain characteristic of the saturnine man is that, as an old author observes 'he will never look thee in the face.' 'if they have to love any one, these saturnines,' says another old author, 'they love most constantly; and if they hate, they hate to the death.' the persons signified symbolically by saturn are grandparents, and other old persons, day labourers, paupers, beggars, clowns, husbandmen of the meaner sort, and especially undertakers, sextons, and gravediggers. chaucer thus presents the chief effects which saturn produces in the fortunes of men and nations--saturn himself being the speaker:-- ... quod saturne my cours, that hath so wide for to turne, hath more power than wot any man. min is the drenching in the sea so wan, min is the prison in the derke cote, min is the strangel and hanging by the throte, the murmure and the cherles rebelling, the groyning, and the prive empoysoning, i do vengaunce and pleine correction, while i dwell in the signe of the leon; min is the ruine of the high halles, the falling of the toures and of the walles upon the minour or the carpenter: i slew sampson in shaking the piler. min ben also the maladies colde, the derke tresons, and the castes olde: my loking is the fader of pestilence. jupiter, on the contrary, though saturn's next neighbour in the solar system, produces effects of an entirely contrary kind. he is, in fact, the most propitious of all the planets, and the native born under his influence has every reason to be jovial in fact as he is by nature. such a native will be tall and fair, handsome and erect, robust, ruddy, and altogether a good-looking person, whether male or female. the native will also be religious, or at least a good moral honest man, unless jupiter be afflicted by the aspects of saturn, mars, or uranus; in which case he may still be a jolly fellow, no man's enemy but his own--only he will probably be his own enemy to a very considerable extent, squandering his means and ruining his health by gluttony and intoxication. the persons represented by jupiter (when he is not afflicted) are judges, counsellors, church dignitaries, from cardinals to curates, scholars, chancellors, barristers, and the highest orders of lawyers, woollendrapers (possibly there may be some astral significance in the woolsack), and clothiers. when jupiter is afflicted, however, he denotes quacks and mountebanks, knaves, cheats, and drunkards. the influence of the planet on the fortunes is nearly always good. astrologers, who to a man reverence dignities, consider great britain fortunate in that the lady whom, with customary effusion, they term 'our most gracious queen,' was born when jupiter was riding high in the heavens near his culmination, this position promising a most fortunate and happy career. the time has passed when the fortunes of this country were likely to be affected by such things; but we may hope, for the lady's own sake, that this prediction has been fulfilled. astrologers assert the same about the duke of wellington, assigning midnight, may , , as the hour of his birth. there is some doubt both as to the date and place of the great soldier's birth; but the astrologer finds in the facts of his life the means of removing all such doubts.[ ] next in order comes mars, inferior only in malefic influence to saturn, and called by the old astrologers the lesser infortune. the native born under the influence of mars is usually of fierce countenance, his eyes sparkling, or sharp and darting, his complexion fiery or yellowish, and his countenance scarred or furrowed. his hair is reddish or sandy, unless mars chances to be in a watery sign, in which case the hair will be flaxen; or in an earthly sign, in which case the hair will be chestnut. the martialist is broad-shouldered, steady, and strong, but short,[ ] and often bony and lean. in character the martialist is fiery and choleric, naturally delighting in war and contention, but generous and magnanimous. this when mars is well aspected; should the planet be evil aspected, then will the native be treacherous, thievish, treasonable, cruel, and wicked. the persons signified by mars are generals, soldiers, sailors (if he is in a watery sign), surgeons, chemists, doctors, armourers, barbers, curriers, smiths, carpenters, bricklayers, sculptors, cooks, and tailors. when afflicted with mercury or the moon, he denotes thieves, hangmen, and 'all cut throat people.' in fact, except the ploughboy, who belongs to saturn, all the members of the old septet, 'tinker, tailor, soldier, sailor, apothecary, ploughboy, thief,' are favourites with mars. the planet's influence is not quite so evil as saturn's, nor are the effects produced by it so long-lasting. 'the influence of saturn,' says an astrologer, 'may be compared to a lingering but fatal consumption; that of mars to a burning fever.' he is the cause of anger, quarrels, violence, war, and slaughter. the sun comes next; for it must be remembered that, according to the old system of astronomy, the sun was a planet. persons born under the sun as the planet ruling their ascendant, would be more apt to be aware of the fact than saturnine, jovial, martial, or any other folk, because the hour of birth, if remembered, at once determines whether the native is a solar subject or not. the solar native has generally a round face (like pictures of the sun in old books of astronomy), with a short chin; his complexion somewhat sanguine; curling sandy hair, and a white tender skin. as to character, he is bold and resolute, desirous of praise, of slow speech and composed judgment; outwardly decorous, but privately not altogether virtuous. the sun, in fact, according to astrologers, is the natural significator of respectability; for which i can discover no reason, unless it be that the sun travelling always in the ecliptic has no latitude, and so solar folk are allowed none. when the sun is ill aspected, the native is both proud and mean, tyrannical and sycophantic, exceedingly unamiable, and generally disliked because of his arrogance and ignorant pomposity. the persons signified by the sun are emperors, kings, and titled folk generally, goldsmiths, jewellers, and coiners. when 'afflicted,' the sun signifies pretenders either to power or knowledge. the sun's influence is not in itself either good or evil, but is most powerful for good when he is favourably aspected, and for evil when he is afflicted by other planets. venus, the next in order, bore the same relation to the greater fortune jupiter which mars bore to saturn the greater ill-fortune. she was the lesser fortune, and her influence was in nearly all respects benevolent. the persons born under the influence of this planet are handsome, with beautiful sparkling hazel or black eyes (but another authority assigns the subject of venus, 'a full eye, usually we say goggle-eyed,' by which we do not usually imply beauty), ruddy lips, the upper lip short, soft smooth hair, dimples in the cheek and chin, an amorous look and a sweet voice. one old astrologer puts the matter thus pleasantly:--'the native of venus hath,' quoth he, 'a love-dimple in the chin, a lovely mouth, cherry lips, and a right merry countenance.' in character the native of venus is merry 'to a fault,' but of temper engaging, sweet and cheerful, unless she be ill aspected, when her native is apt to be too fond of pleasure and amusement. that her influence is good is shown (in the opinion of raphaël, writing in ) by the character of george iv., 'our present beloved monarch and most gracious majesty, who was born just as this benevolent star' was in the ascendant; 'for it is well known to all europe what a refined and polished genius, and what exquisite taste, the king of england possesses, which therefore may be cited as a most illustrious proof of the celestial science; a proof likewise which is palpably demonstrable, even to the most casual observer, since the time of his nativity is taken from the public journals of the period, and cannot be gainsaid.' 'this illustrious and regal horoscope is replete with wonderful verifications of planetary influence, and england cannot but prosper while she is blessed with the mild and beneficent sway of this potent monarch.' strengthened in faith by this convincing proof of the celestial science, we proceed to notice that venus is the protectrice of musicians, embroiderers, perfumers, classic modellers, and all who work in elegant attire or administer to the luxuries of the great; but when she is afflicted, she represents 'the lower orders of the votaries of voluptuousness.' mercury is considered by astrologers 'a cold, dry, melancholy star.' the mercurial is neither dark nor fair, but between both, long-faced, with high forehead and thin sharp nose, 'thin beard (many times none at all), slender of body, and with small weak eyes;' long slender hands and fingers are 'especial marks of mercury,' says raphaël. in character the mercurial is busy and prattling. but when well affected, mercury gives his subjects a strong, vigorous, active mind, searching and exhaustive, a retentive memory, a natural thirst for knowledge.[ ] the persons signified by mercury are astrologers, philosophers, mathematicians, politicians, merchants, travellers, teachers, poets, artificers, men of science, and all ingenious, clever men. when he is ill affected, however, he represents pettifoggers, cunning vile persons, thieves, messengers, footmen, and servants, etc. the moon comes last in planetary sequence, as nearest to the earth. she is regarded by astrologers as a cold, moist, watery, phlegmatic planet, variable to an extreme, and, like the sun, partaking of good or evil according as she is aspected favourably or the reverse. her natives are of good stature, fair, and pale, moon-faced, with grey eyes, short arms, thick hands and feet, smooth, corpulent and phlegmatic body. when she is in watery signs, the native has freckles on the face, or, says lilly, 'he or she is blub-cheeked, not a handsome body, but a muddling creature.' unless the moon is very well aspected, she ever signifies an ordinary vulgar person. she signifies sailors (not as mars does, the fighting-men of war-ships, but nautical folk generally) and all persons connected with water or any kind of fluid; also all who are engaged in inferior and common offices. we may note, in passing, that to each planet a special metal is assigned, as also particular colours. chaucer, in the chanones yemannes' tale, succinctly describes the distribution of the metals among the planets:-- sol gold is, and luna silver we threpe; mars iren, mercurie silver we clepe: saturnus led, and jupiter is tin, and venus coper, by my [the chanones yemannes'] faderkin. the colours are thus assigned:--to saturn, black; to jupiter, mixed red and green; to mars, red; to the sun, yellow or yellow-purple; to venus, white or purple; to mercury, azure blue; to the moon, a colour spotted with white and other mixed colours. again, the planets were supposed to have special influence on the seven ages of human life. the infant, 'mewling and puking in the nurse's arms,' was very appropriately dedicated to the moist moon; the whining schoolboy (did schoolboys whine in the days of good queen bess?) was less appropriately assigned to mercury, the patron of those who eagerly seek after knowledge: then very naturally, the lover sighing like furnace was regarded as the special favourite of venus. thus far the order has been that of the seven planets of the ancient astrology, in supposed distance. now, however, we have to pass over the sun, finding mars the patron of mid life, appropriately (in this respect) presiding over the soldier full of strange oaths, and so forth; the 'justice in fair round belly with good capon lined' is watched over by the respectable sun; maturer age by jupiter; and, lastly, old age by saturn. colours were also assigned to the twelve zodiacal signs--to aries, white and red; to taurus, white and lemon; to gemini, white and red (the same as aries); to cancer, green or russet; to leo, red or green; to virgo, black speckled with blue; to libra, black, or dark crimson, or tawny colour; to scorpio, brown; to sagittarius, yellow, or a green sanguine (this is as strange a colour as the _gris rouge_ of molière's _l'avare_); capricorn, black or russet, or a swarthy brown; to aquarius, a sky-coloured blue; to pisces, white glistening colour (like a fish just taken out of the water). the chief fixed stars had various influences assigned to them by astrologers. these influences were mostly associated with the imaginary figures of the constellations. thus the bright star in the head of aries, called by some the ram's horn, was regarded as dangerous and evil, denoting bodily hurts. the star menkar in the whale's jaw denoted sickness, disgrace, and ill-fortune, with danger from great beasts. betelgeux, the bright star on orion's right shoulder, denoted martial honours or wealth; bellatrix, the star on orion's left shoulder, denoted military or civic honours; rigel, on orion's left foot, denoted honours; sirius and procyon, the greater and lesser dog stars, both implied wealth and renown. star clusters seem to have portended loss of sight; at least we learn that the pleiades were 'eminent stars,' but denoting accidents to the sight or blindness, while the cluster præsepe or the beehive in like manner threatened blindness. the cluster in perseus does not seem to have been noticed by astrologers. the variable star algol or caput medusæ, which marks the head of gorgon, was accounted 'the most unfortunate, violent, and dangerous star in the heavens.' it is tolerably clear that the variable character of this star had been detected long before montanari (to whom the discovery is commonly attributed) noticed the phenomenon. the name algol is only a variation of al-ghúl, the monster or demon, and it cannot be doubted that the demoniac, gorgonian character assigned to this star was suggested by its ominous change, as though it were the eye of some fierce monster slowly winking amid the gloom of space. the two stars called the aselli, which lie on either side of the cluster præsepe, 'are said' (by astrologers) 'to be of a burning nature, and to give great indications of a violent death, or of violent and severe accidents by fire.' the star called cor hydræ, or the serpent's heart, denotes trouble through women (said i not rightly that astrology was a masculine science?); the lion's heart, regulus, implied glory and riches; deneb, the lion's tail, misfortune and disgrace. the southern scale of libra meant bad fortune, while the northern was eminently fortunate. astrology was divided into three distinct branches--the doctrine of nativities, horary astrology, and state astrology. the first assigned the rules for determining the general fortunes of the native, by drawing up his scheme of nativity or casting his horoscope. it took into account the positions of the various planets, signs, stars, etc., at the time of the native's birth; and as the astrologer could calculate the movements of the planets thereafter, he could find when those planets which were observed by the horoscope to be most closely associated with the native's fortunes would be well aspected or the reverse. thus the auspicious and unlucky epochs of the native's life could be predetermined. the astrologer also claimed some degree of power to rule the planets, not by modifying their movements in any way, but by indicating in what way the ill effects portended by their positions could be prevented. the arabian and persian astrologers, having less skill than the followers of ptolemy, made use of a different method of determining the fortunes of men, not calculating the positions of the planets for many years following the birth of the native, but assigning to every day after his birth a whole year of his life and for every two hours' motion of the moon one month. thus the positions of the stars and planets, twenty-one days after the birth of the native, would indicate the events corresponding to the time when he would have completed his twenty-first year. there was another system called the placidian, in which the effects of the positions of the planets were judged with sole reference to the motion of the earth upon her axis. it is satisfactory to find astrologers in harmony amongst each other as to these various methods, which one would have supposed likely to give entirely different results. 'each of them,' says a modern astrologer, 'is not only correct and approved by long-tried practice, but may be said to defy the least contradiction from those who will but take the pains to examine them (and no one else should deliver an opinion upon the subject). although each of the above methods are different, yet they by no means contradict each other, but each leads to _true results_, and in many instances they each lead to the foreknowledge of the same event; in which respect they may be compared to the ascent of a mountain by different paths, where, although some paths are longer and more difficult than others, they notwithstanding all lead to the same object.' all which, though plausible in tone labours under the disadvantage of being untrue. ptolemy is careful to point out, in his celebrated work the 'tetrabiblos,' that, of all events whatsoever which take place after birth, the most essential is the continuance of life. 'it is useless,' he says, 'to consider what events might happen to the native in later years if his life does not extend, for instance, beyond one year. so that the enquiry into the duration of life takes precedence of all others.' in order to deal properly with this question, it is necessary to determine what planet shall be regarded as the hyleg, apheta, or lord of life, for the native. next the anareta, or destroyer of life, must be ascertained. the anaretic planets are, by nature, saturn, mars, and uranus, though the sun, moon, and mercury may be endowed with the same fatal influence, if suitably afflicted. the various ways in which the hyleg, or giver of life, may be afflicted by the anareta, correspond to the various modes of death. but astrologers have always been singularly careful, in casting horoscopes, to avoid definite reference to the native's death. there are but few cases where the actual day of death is said to have been assigned. one is related in clarendon's 'history of the rebellion.' he tells us that william earl of pembroke died at the age of fifty, on the day upon which his tutor sandford had predicted his decease. burton, the author of the 'anatomy of melancholy,' having cast his own horoscope, and ascertained that he was to die on january , , is said to have committed suicide in order that the accuracy of his calculations might not be called in question. a similar story is related of cardan by dr. young (sidrophel vapulans), on the authority of gassendi, who, however, says only that either cardan starved himself, or, being confident in his art, took the predicted day for a fatal one, and by his fears made it so. gassendi adds that while cardan pretended to describe the fates of his children in his voluminous commentaries, he all the while never suspected, from the rules of his great art, that his dearest son would be condemned in the flower of his youth to be beheaded on a scaffold, by an executioner of justice, for destroying his own wife by poison. horary astrology relates to particular questions, and is a comparatively easy branch of the science. the art of casting nativities requires many years of study; but horary astrology 'may be well understood,' says lilly, 'in less than a quarter of a year.' 'if a proposition of any nature,' he adds, 'be made to any individual, about the result of which he is anxious, and therefore uncertain whether to accede to it or not, let him but note the hour and minute when it was _first_ made, and erect a figure of the heavens, and his doubts will be instantly resolved. he may thus in five minutes learn whether the affair will succeed or not: and consequently whether it is prudent to accept the offer made or not. if he examine the sign on the first house of the figure, the planet therein, or the planet ruling the sign, _will exactly describe the party making the offer_, both in person and character, and this may at once convince the enquirer for truth of the reality of the principles of the science. moreover, the descending sign, etc., _will describe his own person and character_--a farther proof of the truth of the science.' there is one feature of horary astrology which is probably almost as ancient as any portion of the science, yet which remains even to the present day, and will probably remain for many years to come. i refer to the influence which the planets were supposed to exert on the successive hours of every day--a belief from which the division of time into weeks of seven days unquestionably had its origin--though we may concede that the subdivision of the lunar month into four equal parts was also considered in selecting this convenient measure of time. every hour had its planet. now dividing twenty-four by seven, we get three and three over; whence, each day containing twenty-four hours, it follows that in each day the complete series of seven planets was run through three times, and three planets of the next series were used. the order of the planets was that of their distances, as indicated above. saturn came first, then jupiter, mars, the sun, venus, mercury, and the moon. beginning with saturn, as ruling the first hour of saturn's day (saturday), we get through the above series three times, and have for the last three hours of the day, saturn, jupiter, and mars. thus the next hour, the first hour of the next day, belongs to the sun--sunday follows saturday. we again run three times through the series, and the three remaining hours are governed by the sun, venus, and mercury,--giving the moon as the first planet for the next day. monday thus follows sunday. the last three hours of monday are ruled by the moon, saturn, and jupiter; leaving mars to govern the next day--martis dies, mardi, tuesday or tuisco's day. proceeding in the same way, we get mercury for the next day, mercurii dies, mercredi, wednesday or woden's day; jupiter for the next day, jovis dies, jeudi, thursday or thor's day; venus for the next day, veneris dies, vendredi, friday or freya's day; and so we come to saturday again.[ ] the period of seven days, which had its origin in, and derived its nomenclature from astrological ideas, shows by its wide prevalence how widely astrological superstitions were once spread among the nations. as whewell remarks (though, for reasons which will readily be understood he was by no means anxious to dwell upon the true origin of the sabbatical week), 'the usage is found over all the east; it existed among the arabians, assyrians, and egyptians. the same week is found in india, among the brahmins; it has there also its days marked by the names of the heavenly bodies; and it has been ascertained that the same day has, in that country, the name corresponding with its designation in other nations.... the period has gone on without interruption or irregularity from the earliest recorded times to our own days, traversing the extent of ages and the revolutions of empires; the names of ancient deities, which were associated with the stars, were replaced by those of the objects of the worship of our teutonic ancestors, according to their views of the correspondence of the two mythologies; and the quakers, in rejecting these names of days, have cast aside the most ancient existing relic of astrological as well as idolatrous superstition. not only do the names remain, but some of the observances connected with the old astrological systems remain even to this day. as ceremonies derived from pagan worship are still continued, though modified in form, and with a different interpretation, in christian and especially roman catholic observances, so among the jews and among christians the rites and ceremonies of the old egyptian and chaldæan astrology are still continued, though no longer interpreted as of yore. the great jewish lawgiver and those who follow him seem, for example, to have recognised the value of regular periods of rest (whether really required by man or become a necessity through long habit), but to have been somewhat in doubt how best to continue the practice without sanctioning the superstitions with which it had been connected. at any rate two different and inconsistent interpretations were given in the earlier and later codes of law. but whether the jews accepted the sabbath because they believed that an all-powerful being, having created the world in six days, required and took rest ('and was refreshed') on the seventh, as stated in exodus (xx. and xxxi. ), or whether they did so in remembrance of their departure from egypt, as stated in deuteronomy (v. ), there can be no question that among the egyptians the sabbath or saturn's day was a day of rest because of the malignant nature of the powerful planet-deity who presided over that day. nor can it be seriously doubted that the jews descended from the old chaldæans, among whom (as appears from stone inscriptions recently discovered) the very word sabbath was in use for a seventh day of rest connected with astrological observances, were familiar with the practice even before their sojourn in egypt. they had then probably regarded it as a superstitious practice to be eschewed like those idolatrous observances which had caused terah to remove with abraham and lot from ur of the chaldees. at any rate, we find no mention of the seventh day of rest as a religious observance until after the exodus.[ ] it was not their only religious observance having in reality an astrological origin. indeed, if we examine the jewish sacrificial system as described in numbers xxviii. and elsewhere, we shall find throughout a tacit reference to the motions or influences of the celestial bodies. there was the morning and evening sacrifice guided by the movements of the sun; the sabbath offering, determined by the predominance of saturn; the offering of the new moon, depending on the motions of the moon; and lastly, the paschal sacrifice, depending on the combined movements of the sun and moon--made, in fact, during the lunation following the sun's ascending passage of the equator at the sign of aries. let us return, however, after this somewhat long digression, to astrological matters. horary astrology is manifestly much better fitted than the casting of nativities for filling the pocket of the astrologer himself; because only one nativity can be cast, but any number of horary questions can be asked. it is on account of their skill in horary astrology that the zadkiels of our own time have occasionally found their way into the twelfth house, or house of enemies. even lilly himself, not devoting, it would seem, five minutes to inquire into the probable success of the affair, was indicted in by a half-witted young woman, because he had given judgment respecting stolen goods, receiving two shillings and sixpence, contrary to an act made under and provided by the wise and virtuous king james, first of england and sixth of scotland. state astrology relates to the destinies of kingdoms, thrones, empires, and may be regarded as a branch of horary science relating to subjects (and rulers) of more than ordinary importance. in former ages all persons likely to occupy an important position in the history of the world had their horoscopes erected; but in these degenerate days neither the casting of nativities nor the art of ruling the planets flourishes as it should do. our zadkiels and raphaëls publish, indeed, the horoscopes of kings and emperors, princes and princesses, and so forth; but their fate is as that of benedict (according to beatrice)--men 'wonder they will still be talking, for nobody marks them.' even those whose horoscopes have been erected show no proper respect for the predictions made in their behalf. thus the prince of wales being born when sagittarius was in the ascendant should have been, according to zadkiel, a tall man, with oval face, ruddy complexion, somewhat dusky, and so forth; but i understand he has by no means followed these directions as to his appearance. the sun, being well aspected, prognosticated honours--a most remarkable and unlooked-for circumstance, strangely fulfilled by the event; but then being in cancer, in sextile with mars, the prince of wales was to be partial to maritime affairs and attain naval glory, whereas as a field-marshal he can only win military glory. (i would not be understood to say that he is not quite as competent to lead our fleets as our battalions into action.) the house of wealth was occupied by jupiter, aspected by saturn, which betokened great wealth through inheritance--a prognostication, says professor miller, which is not unlikely to come true. the house of marriage was unsettled by the conflicting influences of venus, mars, and saturn; but the first predominating, the prince, after some trouble in his matrimonial speculations, was to marry a princess of high birth, and one not undeserving of his kindest and most affectionate attention, probably in . as to the date, an almanack informs me that the prince married a danish princess in march , which looks like a most culpable neglect of the predictions of our national astrologer. again, in may , when saturn was stationary in the ascending degree, the prince ought to have been injured by a horse, and also to have received a blow on the left side of the head, near the ear; but reprehensibly omitted both these ceremonies. a predisposition to fever and epileptic attacks was indicated by the condition of the house of sickness. the newspapers described, a few years since, a serious attack of fever; but as most persons have some experience of the kind, the fulfilment of the prediction can hardly be regarded as very wonderful. epileptic attacks, which, as less common, might have saved the credit of the astrologers, have not visited 'this royal native.' the position of saturn in capricorn betokened loss or disaster in one or other of the places ruled over by capricorn--which, as we have seen, are india, macedonia, thrace, greece, mexico, saxony, wilna, mecklenburgh, brandenburgh, and oxford. professor miller expresses the hope that oxford was the place indicated, and the disaster nothing more serious than some slight scrape with the authorities of christchurch. but princes never get into scrapes with college dons. probably some one or other of the 'hair-breadth 'scapes' chronicled by the reporters of his travels in india was the event indicated by the ominous position of saturn in capricorn. a remarkable list of characteristics were derived by zadkiel from the positions of the various planets and signs in the twelve houses of the 'royal native.' some, of course, were indicated in more ways than one, which will explain the parenthetical notes in the following alphabetical table which professor miller has been at the pains to draw up from zadkiel's predictions. the prince was to be 'acute, affectionate, amiable, amorous, austere, avaricious, beneficent, benevolent, brave, brilliant, calculated for government' (a quality which may be understood two ways), 'candid, careful of his person, careless, compassionate, courteous (twice over), delighting in eloquence, discreet, envious, fond of glory, fond of learning, fond of music, fond of poetry, fond of sports, fond of the arts and sciences, frank, full of expedients, generous (three times), gracious, honourable, hostile to crime, impervious, ingenious, inoffensive, joyous, just (twice), laborious, liberal, lofty, magnanimous, modest, noble, not easy to be understood (!), parsimonious, pious (twice), profound in opinion, prone to regret his acts, prudent, rash, religious, reverent, self-confident, sincere, singular in mode of thinking, strong, temperate, unreserved, unsteady, valuable in friendship, variable, versatile, violent, volatile, wily, and worthy.' zadkiel concludes thus:--'the square of saturn to the moon will add to the gloomy side of the picture, and give a tinge of melancholy at times to the native's character, and also a disposition to look at the dark side of things, and lead him to despondency; nor will he be at all of a sanguine character, but cool and calculating, though occasionally rash. yet, all things considered, though firm and sometimes positive in opinion, this royal native, if he live to mount the throne, will sway the sceptre of these realms in moderation and justice, and be a pious and benevolent man, and a merciful sovereign.' fortunately, the time has long since passed when swaying the sceptre of these realms had any but a figurative meaning, or when englishmen who obeyed their country's laws depended on the mercy of any man, or when even bad citizens were judged by princes. but we still prefer that princes should be well-mannered gentlemen, and therefore it is sincerely to be hoped that zadkiel's prediction, so far as it relates to piety and benevolence, may be fulfilled, should this 'royal native' live to mount the throne. as for mercy, it is a goodly quality even in these days and in this country; for if the law no longer tolerates cruelty to men, even on the part of princes, who once had prescribed rights in that direction, there are still some cruel, nay brutal sports in which 'royal natives' might sometimes be tempted to take part. wherefore let us hope that, even in regard to mercy, the predictions of astrologers respecting this 'royal native' may be fulfilled. passing however, from trivialities, let us consider the lessons which the history of astrology teaches us respecting the human mind, its powers and weaknesses. it has been well remarked by whewell that for many ages 'mysticism in its various forms was a leading character both of the common mind and the speculations of the most intelligent and profound reasoners.' thus mysticism was the opposite of that habit of thought which science requires, 'namely, clear ideas, distinctly employed to connect well-ascertained facts; inasmuch as the ideas in which it dealt were vague and unstable, and the temper in which they were contemplated was an urgent and aspiring enthusiasm, which could not submit to a calm conference with experience upon even terms.' we have seen what has been the history of one particular form of the mysticism of ancient and mediæval ages. if we had followed the history of alchemy, magic, and other forms of mysticism, we should have seen similar results. true science has gradually dispossessed science falsely so called, until now none but the weaker minds hold by the tenets formerly almost universally adopted. in mere numbers, believers in the ancient superstitions may be by no means insignificant; but they no longer have any influence. it has become a matter of shame to pay any attention to what those few say or do who not merely hold but proclaim the ancient faith in these matters. we can also see why this has been. in old times enthusiasm usurped the place of reason in these cases; but opinions so formed and so retained could not maintain their ground in the presence of reasoning and experience. so soon as intelligent and thoughtful men perceived that facts were against the supposed mysterious influences of the stars, the asserted powers of magicians, the pretended knowledge of alchemists, the false teachings of magic, alchemy, and astrology, were rejected. the lesson thus learned respecting erroneous doctrines which were once widely prevalent has its application in our time, when, though the influence of those teachings has passed away, other doctrines formerly associated with them still hold their ground. men in old times, influenced by erroneous teachings, wasted their time and energies in idle questionings of the stars, vain efforts to find arcana of mysterious power, and to acquire magical authority over the elements. is it altogether clear that in these our times men are not hampered, prevented to some degree from doing all the good they might do in the short life-time allotted to them, by doctrines of another kind? is there in our day no undue sacrifice of present good in idle questionings? is there no tendency to trust in a vain fetishism to prevent or remove evils which energy could avert or remedy? the time will come, in my belief, when the waste of those energies which in these days are devoted (not merely with the sanction, but the high approval, of some of the best among us) to idle aims, will be deplored as regretfully--but, alas, as idly--as the wasted speculations and labours of those whom whewell has justly called the most intelligent and profound reasoners of the 'stationary age' of science. the words with which whewell closes his chapter on the 'mysticism of the middle ages' have their application to the mysticism of the nineteenth century:--'experience collects her stores in vain, or ceases to collect them, when she can only pour them into the flimsy folds of the lap of mysticism, who is, in truth, so much absorbed in looking for the treasures which are to fall from the skies, that she heeds little how scantily she obtains, or how loosely she holds, such riches as she might find beside her.' ii. _the religion of the great pyramid._ during the last few years a new sect has appeared which, though as yet small in numbers, is full of zeal and fervour. the faith professed by this sect may be called the religion of the great pyramid, the chief article of their creed being the doctrine that that remarkable edifice was built for the purpose of revealing--in the fulness of time, now nearly accomplished--certain noteworthy truths to the human race. the founder of the pyramid religion is described by one of the present leaders of the sect as 'the late worthy john taylor, of gower street, london;' but hitherto the chief prophets of the new faith have been in this country professor smyth, astronomer royal for scotland, and in france the abbé moigno. i propose to examine here some of the facts most confidently urged by pyramidalists in support of their views. but it will be well first to indicate briefly the doctrines of the new faith. they may be thus presented: the great pyramid was erected, it would seem, under the instructions of a certain semitic king, probably no other than melchizedek. by supernatural means, the architects were instructed to place the pyramid in latitude ° north; to select for its figure that of a square pyramid, carefully oriented; to employ for their unit of length the sacred cubit corresponding to the , , th part of the earth's polar axis; and to make the side of the square base equal to just so many of these sacred cubits as there are days and parts of a day in a year. they were further, by supernatural help, enabled to square the circle, and symbolised their victory over this problem by making the pyramid's height bear to the perimeter of the base the ratio which the radius of a circle bears to the circumference. moreover, the great precessional period, in which the earth's axis gyrates like that of some mighty top around the perpendicular to the ecliptic, was communicated to the builders with a degree of accuracy far exceeding that of the best modern determinations, and they were instructed to symbolise that relation in the dimensions of the pyramid's base. a value of the sun's distance more accurate by far than modern astronomers have obtained (even since the recent transit) was imparted to them, and they embodied that dimension in the height of the pyramid. other results which modern science has achieved, but which by merely human means the architects of the pyramid could not have obtained, were also supernaturally communicated to them; so that the true mean density of the earth, her true shape, the configuration of land and water, the mean temperature of the earth's surface, and so forth, were either symbolised in the great pyramid's position, or in the shape and dimensions of its exterior and interior. in the pyramid also were preserved the true, because supernaturally communicated, standards of length, area, capacity, weight, density, heat, time, and money. the pyramid also indicated, by certain features of its interior structure, that when it was built the holy influences of the pleiades were exerted from a most effective position--the meridian, through the points where the ecliptic and equator intersect. and as the pyramid thus significantly refers to the past, so also it indicates the future history of the earth, especially in showing when and where the millennium is to begin. lastly, the apex or crowning stone of the pyramid was no other than the antitype of that stone of stumbling and rock of offence, rejected by builders who knew not its true use, until it was finally placed as the chief stone of the corner. whence naturally, 'whosoever shall fall upon it'--that is, upon the pyramid religion--'shall be broken; but on whomsoever it shall fall it will grind him to powder.' if we examine the relations actually presented by the great pyramid--its geographical position, dimensions, shape, and internal structure--without hampering ourselves with the tenets of the new faith on the one hand, or on the other with any serious anxiety to disprove them, we shall find much to suggest that the builders of the pyramid were ingenious mathematicians, who had made some progress in astronomy, though not so much as they had made in the mastery of mechanical and scientific difficulties. the first point to be noticed is the geographical position of the great pyramid, so far, at least, as this position affects the aspect of the heavens, viewed from the pyramid as from an observatory. little importance, i conceive, can be attached to purely geographical relations in considering the pyramid's position. professor smyth notes that the pyramid is peculiarly placed with respect to the mouth of the nile, standing 'at the southern apex of the delta-land of egypt.' this region being shaped like a fan, the pyramid, set at the part corresponding to the handle, was, he considers, 'that monument pure and undefiled in its religion through an idolatrous land, alluded to by isaiah; the monument which was both "an altar to the lord in the midst of the land of egypt, and a pillar at the border thereof," and destined withal to become a witness in the latter days, and before the consummation of all things, to the same lord, and to what he hath purposed upon man kind.' still more fanciful are some other notes upon the pyramid's geographical position: as (i.) that there is more land along the meridian of the pyramid than on any other all the world round; (ii.) that there is more land in the latitude of the pyramid than in any other; and (iii.) that the pyramid territory of lower egypt is at the centre of the dry land habitable by man all the world over. it does not seem to be noticed by those who call our attention to these points that such coincidences prove too much. it might be regarded as not a mere accident that the great pyramid stands at the centre of the arc of shore-line along which lie the outlets of the nile; or it might be regarded as not a mere coincidence that the great pyramid stands at the central point of all the habitable land-surface of the globe; or, again, any one of the other relations above mentioned might be regarded as something more than a mere coincidence. but if, instead of taking only one or other of these four relations, we take all four of them, or even any two of them, together, we must regard peculiarities of the earth's configuration as the result of special design which certainly have not hitherto been so regarded by geographers. for instance, if it was by a special design that the pyramid was placed at the centre of the nile delta, and also by special design that the pyramid was placed at the centre of the land-surface of the earth, if these two relations are each so exactly fulfilled as to render the idea of mere accidental coincidence inadmissible, then it follows, of necessity, that it is through no merely accidental coincidence that the centre of the nile delta lies at the centre of the land-surface of the earth; in other words, the shore-line along which lie the mouths of the nile has been designedly curved so as to have its centre so placed. and so of the other relations. the very fact that the four conditions _can_ be fulfilled simultaneously is evidence that a coincidence of the sort may result from mere accident.[ ] indeed, the peculiarity of geographical position which really seems to have been in the thoughts of the pyramid architects, introduces yet a fifth condition which by accident could be fulfilled along with the four others. it would seem that the builders of the pyramid were anxious to place it in latitude °, as closely as their means of observation permitted. let us consider what result they achieved, and the evidence thus afforded respecting their skill and scientific attainments. in our own time, of course, the astronomer has no difficulty in determining with great exactness the position of any given latitude-parallel. but at the time when the great pyramid was built it must have been a matter of very serious difficulty to determine the position of any required latitude-parallel with a great degree of exactitude. the most obvious way of dealing with the difficulty would have been by observing the length of shadows thrown by upright posts at noon in spring and autumn. in latitude ° north, the sun at noon in spring (or, to speak precisely, on the day of the vernal equinox) is just twice as far from the horizon as he is from the point vertically overhead; and if a pointed post were set exactly upright at true noon (supposed to occur at the moment of the vernal or autumnal equinox), the shadow of the post would be exactly half as long as a line drawn from the top of the pole to the end of the shadow. but observations based on this principle would have presented many difficulties to the architects of the pyramid. the sun not being a point of light, but a globe, the shadow of a pointed rod does not end in a well-defined point. the moment of true noon, which is not the same as ordinary or civil noon, never does agree exactly with the time of the vernal or autumnal equinox, and may be removed from it by any interval of time not exceeding twelve hours. and there are many other circumstances which would lead astronomers, like those who doubtless presided over the scientific preparations for building the great pyramid, to prefer a means of determining the latitude depending on another principle. the stellar heavens would afford practically unchanging indications for their purpose. the stars being all carried round the pole of the heavens, as if they were fixed points in the interior of a hollow revolving sphere, it becomes possible to determine the position of the pole of the star sphere, even though no bright conspicuous star actually occupies that point. any bright star close by the pole is seen to revolve in a very small circle, whose centre is the pole itself. such a star is our present so-called pole-star; and, though in the days when the great pyramid was built, that star was not near the pole, another, and probably a brighter star lay near enough to the pole[ ] to serve as a pole-star, and to indicate by its circling motion the position of the actual pole of the heavens. this was at that time, and for many subsequent centuries, the leading star of the great constellation called the dragon. the pole of the heavens, we know, varies in position according to the latitude of the observer. at the north pole it is exactly overhead; at the equator the poles of the heavens are both on the horizon; and, as the observer travels from the equator towards the north or south pole of the earth, the corresponding pole of the heavens rises higher and higher above the horizon. in latitude ° north, or one-third of the way from the equator to the pole, the pole of the heavens is raised one-third of the way from the horizon to the point vertically overhead; and when this is the case the observer knows that he is in latitude °. the builders of the great pyramid, with the almost constantly clear skies of egypt, may reasonably be supposed to have adopted this means of determining the true position of that thirtieth parallel on which they appear to have designed to place the great building they were about to erect. it so happens that we have the means of forming an opinion on the question whether they used one method or the other; whether they employed the sun or the stars to guide them to the geographical position they required. in fact, were it not for this circumstance, i should not have thought it worth while to discuss the qualities of either method. it will presently be seen that the discussion bears importantly on the opinion we are to form of the skill and attainments of the pyramid architects. every celestial object is apparently raised somewhat above its true position by the refractive power of our atmosphere, being most raised when nearest the horizon and least when nearest the point vertically overhead. this effect is, indeed, so marked on bodies close to the horizon that if the astronomers of the pyramid times had observed the sun, moon, and stars attentively when so placed, they could not have failed to discover the peculiarity. probably, however, though they noted the time of rising and setting of the celestial bodies, they only made instrumental observations upon them when these bodies were high in the heavens. thus they remained ignorant of the refractive powers of the air.[ ] now, if they had determined the position of the thirtieth parallel of latitude by observations of the noonday sun (in spring or autumn), then since, owing to refraction, they would have judged the sun to be higher than he really was, it follows that they would have supposed the latitude of any station from which they observed to be lower than it really was. for the lower the latitude the higher is the noonday sun at any given season. thus, when really in latitude ° they would have supposed themselves in a latitude lower than °, and would have travelled a little further north to find the proper place, as they would have supposed, for erecting the great pyramid. on the other hand, if they determined the place from observations of the movements of stars near the pole of the heavens, they would make an error of a precisely opposite nature. for the higher the latitude the higher is the pole of the heavens; and refraction, therefore, which apparently raises the pole of the heavens, gives to a station the appearance of being in a higher latitude than it really is, so that the observer would consider he was in latitude north when in reality somewhat south of that latitude. we have only then to inquire whether the great pyramid was set north or south of latitude °, to ascertain whether the pyramid architects observed the noonday sun or circumpolar stars to determine their latitude; always assuming (as we reasonably may) that those architects did propose to set the pyramid in that particular latitude, and that they were able to make very accurate observations of the apparent positions of the celestial bodies, but that they were not acquainted with the refractive effects of the atmosphere. the answer comes in no doubtful terms. the centre of the great pyramid's base lies about one mile and a third _south_ of the thirtieth parallel of latitude; and from this position the pole of the heavens, as raised by refraction, would appear to be very near indeed to the required position. in fact, if the pyramid had been set about half a mile still farther south the pole would have _seemed_ just right. of course, such an explanation as i have here suggested appears altogether heretical to the pyramidalists. according to them the pyramid architects knew perfectly well where the true thirtieth parallel lay, and knew also all that modern science has discovered about refraction; but set the pyramid south of the true parallel and north of the position where refraction would just have made the apparent elevation of the pole correct, simply in order that the pyramid might correspond as nearly as possible to each of two conditions, whereof both could not be fulfilled at once. the pyramid would indeed, they say, have been set even more closely midway between the true and the apparent parallels of ° north, but that the jeezeh hill on which it is set does not afford a rock foundation any farther north. 'so very close,' says professor smyth, 'was the great pyramid placed to the northern brink of its hill, that the edges of the cliff might have broken off under the terrible pressure had not the builders banked up there most firmly the immense mounds of rubbish which came from their work, and which strabo looked so particularly for years ago, but could not find. here they were, however, and still are, utilised in enabling the great pyramid to stand on the very utmost verge of its commanding hill, within the limits of the _two_ required latitudes, as well as over the centre of the land's physical and radial formation, and at the same time on the sure and proverbially wise foundation of rock.' the next circumstance to be noted in the position of the great pyramid (as of all the pyramids) is that the sides are carefully oriented. this, like the approximation to a particular latitude, must be regarded as an astronomical rather than a geographical relation. the accuracy with which the orientation has been effected will serve to show how far the builders had mastered the methods of astronomical observation by which orientation was to be secured. the problem was not so simple as might be supposed by those who are not acquainted with the way in which the cardinal points are correctly determined. by solar observations, or rather by the observations of shadows cast by vertical shafts before and after noon, the direction of the meridian, or north and south line, can theoretically be ascertained. but probably in this case, as in determining the latitude, the builders took the stars for their guide. the pole of the heavens would mark the true north; and equally the pole-star, when below or above the pole, would give the true north, but, of course, most conveniently when below the pole. nor is it difficult to see how the builders would make use of the pole-star for this purpose. from the middle of the northern side of the intended base they would bore a slant passage tending always from the position of the pole-star at its lower meridional passage, that star at each successive return to that position serving to direct their progress; while its small range, east and west of the pole, would enable them most accurately to determine the star's true mid-point below the pole; that is, the true north. when they had thus obtained a slant tunnel pointing truly to the meridian, and had carried it down to a point nearly below the middle of the proposed square base, they could, from the middle of the base, bore vertically downwards, until by rough calculation they were near the lower end of the slant tunnel; or both tunnels could be made at the same time. then a subterranean chamber would be opened out from the slant tunnel. the vertical boring, which need not be wider than necessary to allow a plumb-line to be suspended down it, would enable the architects to determine the point vertically below the point of suspension. the slant tunnel would give the direction of the true north, either from that point or from a point at some known small distance east or west of that point.[ ] thus, a line from some ascertained point near the mouth of the vertical boring to the mouth of the slant tunnel would lie due north and south, and serve as the required guide for the orientation of the pyramid's base. if this base extended beyond the opening of the slant tunnel, then, by continuing this tunnelling through the base tiers of the pyramid, the means would be obtained of correcting the orientation. this, i say, would be the course naturally suggested to astronomical architects who had determined the latitude in the manner described above. it may even be described as the only very accurate method available before the telescope had been invented. so that if the accuracy of the orientation appears to be greater than could be obtained by the shadow method, the natural inference, even in the absence of corroborative evidence, would be that the stellar method, and no other, had been employed. now, in , nouet, by refined observations, found the error of orientation measured by less than minutes of arc, corresponding roughly to a displacement of the corners by about - / inches from their true position, as supposed to be determined from the centre; or to a displacement of a southern corner by inches on an east and west line from a point due south of the corresponding northern corner. this error, for a base length of inches, would not be serious, being only one inch in about five yards (when estimated in the second way). yet the result is not quite worthy of the praise given to it by professor smyth. he himself, however, by much more exact observations, with an excellent altazimuth, reduced the alleged error from minutes to only - / , or to - ths of its formerly supposed value. this made the total displacement of a southern corner from the true meridian through the corresponding northern corner, almost exactly one foot, or one inch in about twenty-one yards--a degree of accuracy rendering it practically certain that some stellar method was used in orienting the base. now there _is_ a slanting tunnel occupying precisely the position of the tunnel which should, according to this view, have been formed in order accurately to orient the pyramid's base, assuming that the time of the building of the pyramid corresponded with one of the epochs when the star alpha draconis was distant ° ' from the pole of the heavens. in other words, there is a slant tunnel directed northwards and upwards from a point deep down below the middle of the pyramid's base, and inclined ° ' to the horizon, the elevation of alpha draconis at its lower culmination when ° ' from the pole. the last epoch when the star was thus placed was _circiter_ b.c.; the epoch next before that was b.c. between these two we should have to choose, on the hypothesis that the slant tunnel was really directed to that star when the foundations of the pyramid were laid. for the next epoch before the earlier of the two named was about , b.c., and the pyramid's date cannot have been more remote than b.c. the slant tunnel, while admirably fulfilling the requirements suggested, seems altogether unsuited for any other. its transverse height (that is, its width in a direction perpendicular to its upper and lower faces) did not amount to quite four feet; its breadth was not quite three feet and a half. it was, therefore, not well fitted for an entrance passage to the subterranean chamber immediately under the apex of the pyramid (with which chamber it communicates in the manner suggested by the above theory). it could not have been intended to be used for observing meridian transits of the stars in order to determine sidereal time; for close circumpolar stars, by reason of their slow motion, are the least suited of all for such a purpose. as professor smyth says, in arguing against this suggested use of the star, 'no observer in his senses, in any existing observatory, when seeking to obtain the time, would observe the transit of a circumpolar star for anything else than _to get the direction of the meridian to adjust his instrument by_.' (the italics are his.) it is precisely such a purpose (the adjustment, however, not of an instrument, but of the entire structure of the pyramid itself), that i have suggested for this remarkable passage--this 'cream-white, stone-lined, long tube,' where it traverses the masonry of the pyramid, and below that dug through the solid rock to a distance of more than feet. let us next consider the dimensions of the square base thus carefully placed in latitude ° north to the best of the builders' power, with sides carefully oriented. it seems highly probable that, whatever special purpose the pyramid was intended to fulfil, a subordinate idea of the builders would have been to represent symbolically in the proportions of the building such mathematical and astronomical relations as they were acquainted with. from what we know by tradition of the men of the remote time when the pyramid was built, and what we can infer from the ideas of those who inherited, however remotely, the modes of thought of the earliest astronomers and mathematicians, we can well believe that they would look with superstitious reverence on special figures, proportions, numbers, and so forth. apart from this, they may have had a quasi-scientific desire to make a lasting record of their discoveries, and of the collected knowledge of their time. it seems altogether probable, then, that the smaller unit of measurement used by the builders of the great pyramid was intended, as professor smyth thinks, to be equal to the , , th part of the earth's diameter, determined from their geodetical observations. it was perfectly within the power of mechanicians and mathematicians so experienced as they undoubtedly were--the pyramid attests so much--to measure with considerable accuracy the length of a degree of latitude. they could not possibly (always setting aside the theory of divine inspiration) have known anything about the compression of the earth's globe, and therefore could not have intended, as professor smyth supposes, to have had the , , th part of the earth's polar axis, as distinguished from any other, for their unit of length. but if they made observations in or near latitude ° north on the supposition that the earth is a globe, their probable error would exceed the difference even between the earth's polar and equatorial diameters. both differences are largely exceeded by the range of difference among the estimates of the actual length of the sacred cubit, supposed to have contained twenty-five of these smaller units. and, again, the length of the pyramid base-side, on which smyth bases his own estimate of the sacred cubit, has been variously estimated, the largest measure being inches, and the lowest inches. the fundamental theory of the pyramidalists, that the sacred cubit was exactly one , , th part of the earth's polar diameter, and that the side of the base contained as many cubits and parts of a cubit as there are days and parts of a day in the tropical year (or year of seasons), requires that the length of the side should be inches, lying between the limits indicated, but still so widely removed from either that it would appear very unsafe to base a theory on the supposition that the exact length is or was inches. if the measures inches and inches were inferior, and several excellent measures made by practised observers ranged around the length inches, the case would be different. but the best recent measures gave respectively and inches; and smyth exclaims against the unfairness of sir h. james in taking as 'therefore the [probable] true length of the side of the great pyramid when perfect,' calling this 'a dishonourable shelving of the honourable older observers with their larger results.' the only other measures, besides these two, are two by colonel howard vyse and by the french _savants_, giving respectively and · inches. the pyramidalists consider inches a fair mean value from these four. the natural inference, however, is, that the pyramid base is not now in a condition to be satisfactorily measured; and assuredly no such reliance can be placed on the mean value inches that, on the strength of it, we should believe what otherwise would be utterly incredible, viz. that the builders of the great pyramid knew 'both the size and shape of the earth exactly.' 'humanly, or by human science, finding it out in that age was, of course, utterly impossible,' says professor smyth. but he is so confident of the average value derived from widely conflicting base measures as to assume that this value, not being humanly discoverable, was of necessity 'attributable to god and to his divine inspiration.' we may agree, in fine, with smyth, that the builders of the pyramid knew the earth to be a globe; that they took for their measure of length the sacred cubit, which, by their earth measures, they made very fairly approximate to the , , th part of the earth's mean diameter; but there seems no reason whatever for supposing (even if the supposition were not antecedently of its very nature inadmissible) that they knew anything about the compression of the earth, or that they had measured a degree of latitude in their own place with very wonderful accuracy.[ ] but here a very singular coincidence may be noticed, or, rather, is forced upon our notice by the pyramidalists, who strangely enough recognise in it fresh evidence of design, while the unbeliever finds in it proof that coincidences are no sure evidence of design. the side of the pyramid containing - / times the sacred cubit of pyramid inches, it follows that the diagonal of the base contains , such inches, and the two diagonals together contain , pyramid inches, or almost exactly as many inches as there are years in the great precessional period. 'no one whatever amongst men,' says professor smyth after recording various estimates of the precessional period, 'from his own or school knowledge, knew anything about such a phenomenon, until hipparchus, some years after the great pyramid's foundation, had a glimpse of the fact; and yet it had been ruling the heavens for ages, and was recorded in jeezeh's ancient structure.' to minds not moved to most energetic forgetfulness by the spirit of faith, it would appear that when a square base had been decided upon, and its dimensions fixed, with reference to the earth's diameter and the year, the diagonals of the square base were determined also; and, if it so chanced that they corresponded with some other perfectly independent relation, the fact was not to be credited to the architects. moreover it is manifest that the closeness of such a coincidence suggests grave doubts how far other coincidences can be relied upon as evidence of design. it seems, for instance, altogether likely that the architects of the pyramid took the sacred cubit equal to one , , th part of the earth's diameter for their chief unit of length, and intentionally assigned to the side of the pyramid's square base a length of just so many cubits as there are days in the year; and the closeness of the coincidence between the measured length and that indicated by this theory strengthens the idea that this was the builder's purpose. but when we find that an even closer coincidence immediately presents itself, which manifestly is a coincidence _only_, the force of the evidence before derived from mere coincidence is _pro tanto_ shaken. for consider what this new coincidence really means. its nature may be thus indicated: take the number of days in the year, multiply that number by , and increase the result in the same degree that the diagonal of a square exceeds the side--then the resulting number represents very approximately the number of years in the great precessional period. the error, according to the best modern estimates, is about one th part of the true period. this is, of course, a merely accidental coincidence, for there is no connection whatever in nature between the earth's period of rotation, the shape of a square, and the earth's period of gyration. yet this merely accidental coincidence is very much closer than the other supposed to be designed could be proved to be. it is clear, then, that mere coincidence is a very unsafe evidence of design. of course the pyramidalists find a ready reply to such reasoning. they argue that, in the first place, it may have been by express design that the period of the earth's rotation was made to bear this particular relation to the period of gyration in the mighty precessional movement: which is much as though one should say that by express design the height of monte rosa contains as many feet as there are miles in the th part of the sun's distance.[ ] then, they urge, the architects were not bound to have a square base for the pyramid; they might have had an oblong or a triangular base, and so forth--all which accords very ill with the enthusiastic language in which the selection of a square base had on other accounts been applauded. next let us consider the height of the pyramid. according to the best modern measurements, it would seem that the height when (if ever) the pyramid terminated above in a pointed apex, must have been about feet. and from the comparison of the best estimates of the base side with the best estimates of the height, it seems very likely indeed that the intention of the builders was to make the height bear to the perimeter of the base the same ratio which the radius of a circle bears to the circumference. remembering the range of difference in the base measures it might be supposed that the exactness of the approximation to this ratio could not be determined very satisfactorily. but as certain casing stones have been discovered which indicate with considerable exactness the slope of the original plane-surfaces of the pyramid, the ratio of the height to the side of the base may be regarded as much more satisfactorily determined than the actual value of either dimension. of course the pyramidalists claim a degree of precision indicating a most accurate knowledge of the ratio between the diameter and the circumference of a circle; and the angle of the only casing stone measured being diversely estimated at ° ' and ° - / ', they consider ° ' · " the true value, and infer that the builders regarded the ratio as · to . the real fact is, that the modern estimates of the dimensions of the casing stones (which, by the way, ought to agree better if these stones are as well made as stated) indicate the values · and · for the ratio; and all we can say is, that the ratio really used lay _probably_ between these limits, though it may have been outside either. now the approximation of either is not remarkably close. it requires no mathematical knowledge at all to determine the circumference of a circle much more exactly. 'i thought it very strange,' wrote a circle-squarer once to de morgan (_budget of paradoxes_, p. ), 'that so many great scholars in all ages should have failed in finding the true ratio, and have been determined to try myself.' 'i have been informed,' proceeds de morgan, 'that this trial makes the diameter to the circumference as to , giving the ratio equal to · exactly. the result was obtained by the discoverer in three weeks after he first heard of the existence of the difficulty. this quadrator has since published a little slip and entered it at stationers' hall. he says he has done it by actual measurement; and i hear from a private source that he uses a disc of twelve inches diameter which he rolls upon a straight rail.' the 'rolling is a very creditable one; it is as much below the mark as archimedes was above it. its performer is a joiner who evidently knows well what he is about when he measures; he is not wrong by in .' such skilful mechanicians as the builders of the pyramid could have obtained a closer approximation still by mere measurement. besides, as they were manifestly mathematicians, such an approximation as was obtained by archimedes must have been well within their power; and that approximation lies well within the limits above indicated. professor smyth remarks that the ratio was 'a quantity which men in general, and all human science too, did not begin to trouble themselves about until long, long ages, languages, and nations had passed away after the building of the great pyramid; and after the sealing up, too, of that grand primeval and prehistoric monument of the patriarchal age of the earth according to scripture.' i do not know where the scripture records the sealing up of the great pyramid; but it is all but certain that during the very time when the pyramid was being built astronomical observations were in progress which, for their interpretation, involved of necessity a continual reference to the ratio in question. no one who considers the wonderful accuracy with which, nearly two thousand years before the christian era, the chaldæans had determined the famous cycle of the saros, can doubt that they must have observed the heavenly bodies for several centuries before they could have achieved such a success; and the study of the motions of the celestial bodies compels 'men to trouble themselves' about the famous ratio of the circumference to the diameter. we now come upon a new relation (contained in the dimensions of the pyramid as thus determined) which, by a strange coincidence, causes the height of the pyramid to appear to symbolise the distance of the sun. there were pyramid inches, or british inches, in the height of the pyramid according to the relations already indicated. now, in the sun's distance, according to an estimate recently adopted and freely used,[ ] there are , , miles or thousand millions of inches--that is, there are approximately as many thousand millions of inches in the sun's distance as there are inches in the height of the pyramid. if we take the relation as exact we should infer for the sun's distance thousand millions of inches, or , , miles--an immense improvement on the estimate which for so many years occupied a place of honour in our books of astronomy. besides, there is strong reason for believing that, when the results of recent observations are worked out, the estimated sun distance will be much nearer this pyramid value than even to the value , , recently adopted. this result, which one would have thought so damaging to faith in the evidence from coincidence--nay, quite fatal after the other case in which a close coincidence had appeared by merest accident--is regarded by the pyramidalist as a perfect triumph for their faith. they connect it with another coincidence, viz. that, assuming the height determined in the way already indicated, then it so happens that the height bears to half a diagonal of the base the ratio to . seeing that the perimeter of the base symbolises the annual motion of the earth round the sun, while the height represents the radius of a circle with that perimeter, it follows that the height should symbolise the sun's distance. 'that line, further,' says professor smyth (speaking on behalf of mr. w. petrie, the discoverer of this relation), 'must represent' this radius 'in the proportion of to , , , ' (or _ten_ raised to power _nine_), 'because amongst other reasons to is practically the shape of the great pyramid.' for this building 'has such an angle at the corners, that for every ten units its structure advances inwards on the diagonal of the base, it practically rises upwards, or points to sunshine' (_sic_) 'by _nine_. nine, too, out of the ten characteristic parts (viz. five angles and five sides) being the number of those parts which the sun shines on in such a shaped pyramid, in such a latitude near the equator, out of a high sky, or, as the peruvians say, when the sun sets on the pyramid with all its rays.' the coincidence itself on which this perverse reasoning rests is a singular one--singular, that is, as showing how close an accidental coincidence may run. it amounts to this, that if the number of days in the year be multiplied by , and a circle be drawn with a circumference containing times as many inches as there are days in the year, the radius of the circle will be very nearly one , , , th part of the sun's distance. remembering that the pyramid inch is assumed to be one , , th part of the earth's diameter, we shall not be far from the truth in saying that, as a matter of fact, the earth by her orbital motion traverses each day a distance equal to two hundred times her own diameter. but, of course, this relation is altogether accidental. it has no real cause in nature.[ ] such relations show that mere numerical coincidences, however close, have little weight as evidence, except where they occur in series. even then they require to be very cautiously regarded, seeing that the history of science records many instances where the apparent law of a series has been found to be falsified when the theory has been extended. of course this reason is not quoted in order to throw doubt on the supposition that the height of the pyramid was intended to symbolise the sun's distance. that supposition is simply inadmissible if the hypothesis, according to which the height was already independently determined in another way, is admitted. either hypothesis might be admitted were we not certain that the sun's distance could not possibly have been known to the builders of the pyramid; or both hypotheses may be rejected: but to admit both is out of the question. considering the multitude of dimensions of length, surface, capacity, and position, the great number of shapes, and the variety of material existing within the pyramid, and considering, further, the enormous number of relations (presented by modern science) from among which to choose, can it be wondered at if fresh coincidences are being continually recognised? if a dimension will not serve in one way, use can be found for it in another; for instance, if some measure of length does not correspond closely with any known dimension of the earth or of the solar system (an unlikely supposition), then it can be understood to typify an interval of time. if, even after trying all possible changes of that kind, no coincidence shows itself (which is all but impossible), then all that is needed to secure a coincidence is that the dimensions should be manipulated a little. let a single instance suffice to show how the pyramidalists (with perfect honesty of purpose) hunt down a coincidence. the slant tunnel already described has a transverse height, once no doubt uniform, now giving various measures from · pyramid inches to · inches, so that the vertical height from the known inclination of the tunnel would be estimated at somewhere between · inches and · . neither dimension corresponds very obviously with any measured distance in the earth or solar system. nor when we try periods, areas, etc., does any very satisfactory coincidence present itself. but the difficulty is easily turned into a new proof of design. putting all the observations together (says professor smyth), 'i deduced · pyramid inches to be the transverse height of the entrance passage; and computing from thence with the observed angle of inclination the vertical height, that came out · of the same inches. but the sum of those two heights, or the height taken up and down, equals inches, which length, as elsewhere shown, is the general pyramid linear representation of a day of twenty-four hours. and the mean of the two heights, or the height taken one way only, and impartially to the middle point between them, equals fifty inches; which quantity is, therefore, the general pyramid linear representation of only half a day. in which case, let us ask what the entrance passage has to do with half rather than a whole day?' on relations such as these, which, if really intended by the architect, would imply an utterly fatuous habit of concealing elaborately what he desired to symbolise, the pyramidalists base their belief that 'a mighty intelligence did both think out the plans for it, and compel unwilling and ignorant idolators, in a primal age of the world, to work mightily both for the future glory of the one true god of revelation, and to establish lasting prophetic testimony touching a further development, still to take place, of the absolutely divine christian dispensation.' iii. _the mystery of the pyramids._ few subjects of inquiry have proved more perplexing than the question of the purpose for which the pyramids of egypt were built. even in the remotest ages of which we have historical record, nothing seems to have been known certainly on this point. for some reason or other, the builders of the pyramids concealed the object of these structures, and this so successfully that not even a tradition has reached us which purports to have been handed down from the epoch of the pyramids' construction. we find, indeed, some explanations given by the earliest historians; but they were professedly only hypothetical, like those advanced in more recent times. including ancient and modern theories, we find a wide range of choice. some have thought that these buildings were associated with the religion of the early egyptians; others have suggested that they were tombs; others, that they combined the purposes of tombs and temples, that they were astronomical observatories, defences against the sands of the great desert, granaries like those made under joseph's direction, places of resort during excessive overflows of the nile; and many other uses have been suggested for them. but none of these ideas are found on close examination to be tenable as representing the sole purpose of the pyramids, and few of them have strong claims to be regarded as presenting even a chief object of these remarkable structures. the significant and perplexing history of the three oldest pyramids--the great pyramid of cheops, shofo, or suphis, the pyramid of chephren, and the pyramid of mycerinus; and the most remarkable of all the facts known respecting the pyramids generally, viz., the circumstance that one pyramid after another was built as though each had become useless soon after it was finished, are left entirely unexplained by all the theories above mentioned, save one only, the tomb theory, and that does not afford by any means a satisfactory explanation of the circumstances. i propose to give here a brief account of some of the most suggestive facts known respecting the pyramids, and, after considering the difficulties which beset the theories heretofore advanced, to indicate a theory (new so far as i know) which seems to me to correspond better with the facts than any heretofore advanced; i suggest it, however, rather for consideration than because i regard it as very convincingly supported by the evidence. in fact, to advance any theory at present with confident assurance of its correctness, would be simply to indicate a very limited acquaintance with the difficulties surrounding the subject. let us first consider a few of the more striking facts recorded by history or tradition, noting, as we proceed, whatever ideas they may suggest as to the intended character of these structures. it is hardly necessary to say, perhaps, that the history of the great pyramid is of paramount importance in this inquiry. whatever purpose pyramids were originally intended to subserve, must have been conceived by the builders of _that_ pyramid. new ideas may have been superadded by the builders of later pyramids, but it is unlikely that the original purpose can have been entirely abandoned. some great purpose there was, which the rulers of ancient egypt proposed to fulfil by building very massive pyramidal structures on a particular plan. it is by inquiring into the history of the first and most massive of these structures, and by examining its construction, that we shall have the best chance of finding out what that great purpose was. according to herodotus, the kings who built the pyramids reigned not more than twenty-eight centuries ago; but there can be little doubt that herodotus misunderstood the egyptian priests from whom he derived his information, and that the real antiquity of the pyramid-kings was far greater. he tells us that, according to the egyptian priests, cheops 'on ascending the throne plunged into all manner of wickedness. he closed the temples, and forbade the egyptians to offer sacrifice, compelling them instead to labour one and all in his service, viz., in building the great pyramid.' still following his interpretation of the egyptian account, we learn that one hundred thousand men were employed for twenty years in building the great pyramid, and that ten years were occupied in constructing a causeway by which to convey the stones to the place and in conveying them there. 'cheops reigned fifty years; and was succeeded by his brother chephren, who imitated the conduct of his predecessor, built a pyramid--but smaller than his brother's--and reigned fifty-six years. thus during one hundred and six years, the temples were shut and never opened.' moreover, herodotus tells us that 'the egyptians so detested the memory of these kings, that they do not much like even to mention their names. hence they commonly call the pyramids after philition, a shepherd who at that time fed his flocks about the place.' 'after chephren, mycerinus, son of cheops, ascended the throne, he reopened the temples, and allowed the people to resume the practice of sacrifice. he, too, left a pyramid, but much inferior in size to his father's. it is built, for half of its height, of the stone of ethiopia,' or, as professor smyth (whose extracts from rawlinson's translation i have here followed) adds 'expensive red granite.' 'after mycerinus, asychis ascended the throne. he built the eastern gateway of the temple of vulcan (phtha); and, being desirous of eclipsing all his predecessors on the throne, left as a monument of his reign a pyramid of brick.' this account is so suggestive, as will presently be shown, that it may be well to inquire whether it can be relied on. now, although there can be no doubt that herodotus misunderstood the egyptians in some matters, and in particular as to the chronological order of the dynasties, placing the pyramid kings far too late, yet in other respects he seems not only to have understood them correctly, but also to have received a correct account from them. the order of the kings above named corresponds with the sequence given by manetho, and also found in monumental and hieroglyphic records. manetho gives the names suphis i., suphis ii., and mencheres, instead of cheops, chephren, and mycerinus; while, according to the modern egyptologists, herodotus's cheops was shofo, shufu, or koufou; chephren was shafre, while he was also called nou-shofo or noum-shufu as the brother of shofo; and mycerinus was menhere or menkerre. but the identity of these kings is not questioned. as to the true dates there is much doubt, and it is probable that the question will long continue open; but the determination of the exact epochs when the several pyramids were built is not very important in connection with our present inquiry. we may, on the whole, fairly take the points quoted above from herodotus, and proceed to consider the significance of the narrative, with sufficient confidence that in all essential respects it is trustworthy. there are several very strange features in the account. in the first place, it is manifest that cheops (to call the first king by the name most familiar to the general reader) attached great importance to the building of his pyramid. it has been said, and perhaps justly, that it would be more interesting to know the plan of the architect who devised the pyramid than the purpose of the king who built it. but the two things are closely connected. the architect must have satisfied the king that some highly important purpose in which the king himself was interested, would be subserved by the structure. whether the king was persuaded to undertake the work as a matter of duty, or only to advance his own interests, may not be so clear. but that the king was most thoroughly in earnest about the work is certain. a monarch in those times would assuredly not have devoted an enormous amount of labour and material to such a scheme unless he was thoroughly convinced of its great importance. that the welfare of his people was not considered by cheops in building the great pyramid is almost equally certain. he might, indeed, have had a scheme for their good which either he did not care to explain to them or which they could not understand. but the most natural inference from the narrative is that his purpose had no reference whatever to their welfare. for though one could understand his own subjects hating him while he was all the time working for their good, it is obvious that his memory would not have been hated if some important good had eventually been gained from his scheme. many a far-seeing ruler has been hated while living on account of the very work for which his memory has been revered. but the memory of cheops and his successors was held in detestation. may we, however, suppose that, though cheops had not the welfare of his own people in his thoughts, his purpose was nevertheless not selfish, but intended in some way to promote the welfare of the human race? i say his purpose, because, whoever originated the scheme, cheops carried it out; it was by means of his wealth and through his power that the pyramid was built. this is the view adopted by professor piazzi smyth and others, in our own time, and first suggested by john taylor. 'whereas other writers,' says smyth, 'have generally esteemed that the mysterious persons who directed the building of the great pyramid (and to whom the egyptians, in their traditions, and for ages afterwards, gave an immoral and even abominable character) must therefore have been very bad indeed, so that the world at large has always been fond of standing on, kicking, and insulting that dead lion, whom they really knew not; he, mr. john taylor, seeing how religiously bad the egyptians themselves were, was led to conclude, on the contrary, that those _they_ hated (and could never sufficiently abuse) might, perhaps, have been pre-eminently good; or were, at all events, of _different religious faith_ from themselves.' 'combining this with certain unmistakable historical facts,' mr. taylor deduced reasons for believing that the directors of the building designed to record in its proportions, and in its interior features, certain important religious and scientific truths, not for the people then living, but for men who were to come years or so after. i have already considered at length (see the preceding essay) the evidence on which this strange theory rests. but there are certain matters connecting it with the above narrative which must here be noticed. the mention of the shepherd philition, who fed his flocks about the place where the great pyramid was built, is a singular feature of herodotus's narrative. it reads like some strange misinterpretation of the story related to him by the egyptian priests. it is obvious that if the word philition did not represent a people, but a person, this person must have been very eminent and distinguished--a shepherd-king, not a mere shepherd. rawlinson, in a note on this portion of the narrative of herodotus, suggests that philitis was probably a shepherd-prince from palestine, perhaps of philistine descent, 'but so powerful and domineering, that it may be traditions of his oppressions in that earlier age which, mixed up afterwards in the minds of later egyptians with the evils inflicted on their country by the subsequent shepherds of better known dynasties, lent so much fear to their religious hate of shepherd times and that name.' smyth, somewhat modifying this view, and considering certain remarks of manetho respecting an alleged invasion of egypt by shepherd-kings, 'men of an ignoble race (from the egyptian point of view) who had the confidence to invade our country, and easily subdued it to their power without a battle,' comes to the conclusion that some shemite prince, 'a contemporary of, but rather older than, the patriarch abraham,' visited egypt at this time, and obtained such influence over the mind of cheops as to persuade him to erect the pyramid. according to smyth, the prince was no other than melchizedek, king of salem, and the influence he exerted was supernatural. with such developments of the theory we need not trouble ourselves. it seems tolerably clear that certain shepherd-chiefs who came to egypt during cheops' reign were connected in some way with the designing of the great pyramid. it is clear also that they were men of a different religion from the egyptians, and persuaded cheops to abandon the religion of his people. taylor, smyth, and the pyramidalists generally, consider this sufficient to prove that the pyramid was erected for some purpose connected with religion. 'the pyramid,' in fine, says smyth, 'was charged by god's inspired shepherd-prince, in the beginning of human time, to keep a certain message secret and inviolable for years, and it has done so; and in the next thousand years it was to enunciate that message to all men, with more than traditional force, more than all the authenticity of copied manuscripts or reputed history; and that part of the pyramid's usefulness is now beginning.' there are many very obvious difficulties surrounding this theory; as, for example (i.) the absurd waste of power in setting supernatural machinery at work years ago with cumbrous devices to record its object, when the same machinery, much more simply employed now, would effect the alleged purpose far more thoroughly; (ii.) the enormous amount of human misery and its attendant hatreds brought about by this alleged divine scheme; and (iii.) the futility of an arrangement by which the pyramid was only to subserve its purpose when it had lost that perfection of shape on which its entire significance depended, according to the theory itself. but, apart from these, there is a difficulty, nowhere noticed by smyth or his followers, which is fatal, i conceive, to this theory of the pyramid's purpose. the second pyramid, though slightly inferior to the first in size, and probably far inferior in quality of masonry, is still a structure of enormous dimensions, which must have required many years of labour from tens of thousands of workmen. now, it seems impossible to explain why chephren built this second pyramid, if we adopt smyth's theory respecting the first pyramid. for either chephren knew the purpose for which the great pyramid was built, or he did not know it. if he knew that purpose, and it was that indicated by smyth, then he also knew that no second pyramid was wanted. on that hypothesis, all the labour bestowed on the second pyramid was wittingly and wilfully wasted. this, of course is incredible. but, on the other hand, if chephren did not know what was the purpose for which the great pyramid was built, what reason could chephren have had for building a pyramid at all? the only answer to this question seems to be that chephren built the second pyramid in hopes of finding out why his brother had built the first, and this answer is simply absurd. it is clear enough that whatever purpose cheops had in building the first pyramid, chephren must have had a similar purpose in building the second; and we require a theory which shall at least explain why the first pyramid did not subserve for chephren the purpose which it subserved or was meant to subserve for cheops. the same reasoning may be extended to the third pyramid, to the fourth, and in fine to all the pyramids, forty or so in number, included under the general designation of the pyramids of ghizeh or jeezeh. the extension of the principle to pyramids later than the second is especially important as showing that the difference of religion insisted on by smyth has no direct bearing on the question of the purpose for which the great pyramid itself was constructed. for mycerinus either never left or else returned to the religion of the egyptians. yet he also built a pyramid, which, though far inferior in size to the pyramids built by his father and uncle, was still a massive structure, and relatively more costly even than theirs, because built of expensive granite. the pyramid built by asychis, though smaller still, was remarkable as built of brick; in fact, we are expressly told that asychis desired to eclipse all his predecessors in such labours, and accordingly left this brick pyramid as a monument of his reign. we are forced, in fact, to believe that there was some special relation between the pyramid and its builder, seeing that each one of these kings wanted a pyramid of his own. this applies to the great pyramid quite as much as to the others, despite the superior excellence of that structure. or rather, the argument derives its chief force from the superiority of the great pyramid. if chephren, no longer perhaps having the assistance of the shepherd-architects in planning and superintending the work, was unable to construct a pyramid so perfect and so stately as his brother's, the very fact that he nevertheless built a pyramid shows that the great pyramid did not fulfil for chephren the purpose which it fulfilled for cheops. but, if smyth's theory were true, the great pyramid would have fulfilled finally and for all men the purpose for which it was built. since this was manifestly not the case, that theory is, i submit, demonstrably erroneous. it was probably the consideration of this point, viz. that each king had a pyramid constructed for himself, which led to the theory that the pyramids were intended to serve as tombs. this theory was once very generally entertained. thus we find humboldt, in his remarks on american pyramids, referring to the tomb theory of the egyptian pyramids as though it were open to no question. 'when we consider,' he says, 'the pyramidical monuments of egypt, of asia, and of the new continent, from the same point of view, we see that, though their form is alike, their destination was altogether different. the group of pyramids of ghizeh and at sakhara in egypt; the triangular pyramid of the queen of the scythians, zarina, which was a stadium high and three in circumference, and which was decorated with a colossal figure; the fourteen etruscan pyramids, which are said to have been enclosed in the labyrinth of the king porsenna, at clusium--were reared to serve as the sepulchres of the illustrious dead. nothing is more natural to men than to commemorate the spot where rest the ashes of those whose memory they cherish whether it be, as in the infancy of the race, by simple mounds of earth, or, in later periods, by the towering height of the tumulus. those of the chinese and of thibet have only a few metres of elevation. farther to the west the dimensions increase; the tumulus of the king alyattes, father of croesus, in lydia, was six stadia, and that of ninus was more than ten stadia in diameter. in the north of europe the sepulchre of the scandinavian king gormus and the queen daneboda, covered with mounds of earth, are three hundred metres broad, and more than thirty high.' but while we have abundant reason for believing that in egypt, even in the days of cheops and chephren, extreme importance was attached to the character of the place of burial for distinguished persons, there is nothing in what is known respecting earlier egyptian ideas to suggest the probability that any monarch would have devoted many years of his subjects' labour, and vast stores of material, to erect a mass of masonry like the great pyramid, solely to receive his own body after death. far less have we any reason for supposing that many monarchs in succession would do this, each having a separate tomb built for him. it might have been conceivable, had only the great pyramid been erected, that the structure had been raised as a mausoleum for all the kings and princes of the dynasty. but it seems utterly incredible that such a building as the great pyramid should have been erected for one king's body only--and that, not in the way described by humboldt, when he speaks of men commemorating the spot where rest the remains of those whose memory they cherish, but at the expense of the king himself whose body was to be there deposited. besides, the first pyramid, the one whose history must be regarded as most significant of the true purpose of these buildings, was not built by an egyptian holding in great favour the special religious ideas of his people, but by one who had adopted other views and those not belonging, so far as can be seen, to a people among whom sepulchral rites were held in exceptional regard. a still stronger objection against the exclusively tombic theory resides in the fact that this theory gives no account whatever of the characteristic features of the pyramids themselves. these buildings are all, without exception, built on special astronomical principles. their square bases are so placed as to have two sides lying east and west, and two lying north and south, or, in other words, so that their four faces front the four cardinal points. one can imagine no reason why a tomb should have such a position. it is not, indeed, easy to understand why any building at all, except an astronomical observatory, should have such a position. a temple perhaps devoted to sun-worship, and generally to the worship of the heavenly bodies, might be built in that way. for it is to be noticed that the peculiar figure and position of the pyramids would bring about the following relations:--when the sun rose and set south of the east and west points, or (speaking generally) between the autumn and the spring equinoxes, the rays of the rising and setting sun illuminated the southern face of the pyramid; whereas during the rest of the year, that is, during the six months between the spring and autumn equinoxes, the rays of the rising and setting sun illuminated the northern face. again, all the year round the sun's rays passed from the eastern to the western face at solar noon. and lastly, during seven months and a half of each year, namely, for three months and three quarters before and after midsummer, the noon rays of the sun fell on all four faces of the pyramid, or, according to a peruvian expression (so smyth avers), the sun shone on the pyramid 'with all his rays.' such conditions as these might have been regarded as very suitable for a temple devoted to sun-worship. yet the temple theory is as untenable as the tomb theory. for, in the first place, the pyramid form--as the pyramids were originally built, with perfectly smooth slant-faces, not terraced into steps as now through the loss of the casing-stones--was entirely unsuited for all the ordinary requirements of a temple of worship. and further, this theory gives no explanation of the fact that each king built a pyramid, and each king only one. similar difficulties oppose the theory that the pyramids were intended to serve as astronomical observatories. for while their original figure, however manifestly astronomical in its relations, was quite unsuited for observatory work, it is manifest that if such had been the purpose of pyramid-building, so soon as the great pyramid had once been built, no other would be needed. certainly none of the pyramids built afterwards could have subserved any astronomical purpose which the first did not subserve, or have subserved nearly so well as the great pyramid those purposes (and they are but few) which that building may be supposed to have fulfilled as an astronomical observatory. of the other theories mentioned at the beginning of this paper none seem to merit special notice, except perhaps the theory that the pyramids were made to receive the royal treasures, and this theory rather because of the attention it received from arabian literati, during the ninth and tenth centuries, than because of any strong reasons which can be suggested in its favour. 'emulating,' says professor smyth, 'the enchanted tales of bagdad,' the court poets of al mamoun (son of the far-famed haroun al raschid) 'drew gorgeous pictures of the contents of the pyramid's interior.... all the treasures of sheddad ben ad the great antediluvian king of the earth, with all his medicines and all his sciences, they declared were there, told over and over again. others, though, were positive that the founder-king was no other than saurid ibn salhouk, a far greater one than the other; and these last gave many more minute particulars, some of which are at least interesting to us in the present day, as proving that, amongst the egypto-arabians of more than a thousand years ago, the jeezeh pyramids, headed by the grand one, enjoyed a pre-eminence of fame vastly before all the other pyramids of egypt put together; and that if any other is alluded to after the great pyramid (which has always been the notable and favourite one, and chiefly was known then as the east pyramid), it is either the second one at jeezeh, under the name of the west pyramid; or the third one, distinguished as the coloured pyramid, in allusion to its red granite, compared with the white limestone casings of the other two (which, moreover, from their more near, but by no means exact, equality of size, went frequently under the affectionate designation of "the pair").' the report of ibn abd alkohm, as to what was to be found in each of these three pyramids, or rather of what, according to him, was put into them originally by king saurid, runs as follows: 'in the western pyramid, thirty treasuries filled with store of riches and utensils, and with signatures made of precious stones, and with instruments of iron and vessels of earth, and with arms which rust not, and with glass which might be bended and yet not broken, and with strange spells, and with several kinds of _alakakirs_ (magical precious stones) single and double, and with deadly poisons, and with other things besides. he made also in the east' (the great pyramid) 'divers celestial spheres and stars, and what they severally operate in their aspects, and the perfumes which are to be used to them, and the books which treat of these matters. he put also into the coloured pyramid the commentaries of the priests in chests of black marble, and with every priest a book, in which the wonders of his profession and of his actions and of his nature were written, and what was done in his time, and what is and what shall be from the beginning of time to the end of it.' the rest of this worthy's report relates to certain treasurers placed within these three pyramids to guard their contents, and (like all or most of what i have already quoted) was a work of imagination. ibn abd alkohm, in fact, was a romancist of the first water. perhaps the strongest argument against the theory that the pyramids were intended as strongholds for the concealment of treasure, resides in the fact that, search being made, no treasure has been discovered. when the workmen employed by caliph al mamoun, after encountering manifold difficulties, at length broke their way into the great ascending passage leading to the so-called king's chamber, they found 'a right noble apartment, thirty-four feet long, seventeen broad, and nineteen high, of polished red granite throughout, walls, floor, and ceiling, in blocks squared and true, and put together with such exquisite skill that the joints are barely discernible to the closest inspection. but where is the treasure--the silver and the gold, the jewels, medicines, and arms?--these fanatics look wildly around them, but can see nothing, not a single _dirhem_ anywhere. they trim their torches, and carry them again and again to every part of that red-walled, flinty hall, but without any better success. nought but pure polished red granite, in mighty slabs, looks upon them from every side. the room is clean, garnished too, as it were, and, according to the ideas of its founders, complete and perfectly ready for its visitors so long expected, so long delayed. but the gross minds who occupy it now, find it all barren, and declare that there is nothing whatever for them in the whole extent of the apartment from one end to another; nothing except an empty stone chest without a lid.' it is, however, to be noted that we have no means of learning what had happened between the time when the pyramid was built and when caliph al mamoun's workmen broke their way into the king's chamber. the place may, after all, have contained treasures of some kind; nor, indeed, is it incompatible with other theories of the pyramid to suppose that it was used as a safe receptacle for treasures. it is certain, however, that this cannot have been the special purpose for which the pyramids were designed. we should find in such a purpose no explanation whatever of any of the most stringent difficulties encountered in dealing with other theories. there could be no reason why strangers from the east should be at special pains to instruct an egyptian monarch how to hide and guard his treasures. nor, if the great pyramid had been intended to receive the treasures of cheops, would chephren have built another for his own treasures, which must have included those gathered by cheops. but, apart from this, how inconceivably vast must a treasure-hoard be supposed to be, the safe guarding of which would have repaid the enormous cost of the great pyramid in labour and material! and then, why should a mere treasure-house have the characteristics of an astronomical observatory? manifestly, if the pyramids were used at all to receive treasures, it can only have been as an entirely subordinate though perhaps convenient means of utilising these gigantic structures. having thus gone through all the suggested purposes of the pyramids save two or three which clearly do not possess any claim to serious consideration, and having found none which appear to give any sufficient account of the history and principal features of these buildings, we must either abandon the inquiry or seek for some explanation quite different from any yet suggested. let us consider what are the principal points of which the true theory of the pyramids should give an account. in the first place, the history of the pyramids shows that the erection of the first great pyramid was in all probability either suggested to cheops by wise men who visited egypt from the east, or else some important information conveyed to him by such visitors caused him to conceive the idea of building the pyramid. in either case we may suppose, as the history indeed suggests, that these learned men, whoever they may have been, remained in egypt to superintend the erection of the structure. it may be that the architectural work was not under their supervision; in fact, it seems altogether unlikely that shepherd-rulers would have much to teach the egyptians in the matter of architecture. but the astronomical peculiarities which form so significant a feature of the great pyramid were probably provided for entirely under the instructions of the shepherd chiefs who had exerted so strange an influence upon the mind of king cheops. next, it seems clear that self-interest must have been the predominant reason in the mind of the egyptian king for undertaking this stupendous work. it is true that his change of religion implies that some higher cause influenced him. but a ruler who could inflict such grievous burdens on his people in carrying out his purpose that for ages afterwards his name was held in utter detestation, cannot have been solely or even chiefly influenced by religious motives. it affords an ample explanation of the behaviour of cheops, in closing the temples and forsaking the religion of his country, to suppose that the advantages which he hoped to secure by building the pyramid depended in some way on his adopting this course. the visitors from the east may have refused to give their assistance on any other terms, or may have assured him that the expected benefit could not be obtained if the pyramid were erected by idolaters. it is certain, in any case, that they were opposed to idolatry; and we have thus some means of inferring who they were and whence they came. we know that one particular branch of one particular race in the east was characterised by a most marked hatred of idolatry in all its forms. terah and his family, or, probably, a sect or division of the chaldæan people, went forth from ur of the chaldees, to go into the land of canaan--and the reason why they went forth we learn from a book of considerable historical interest (the book of judith) to have been because 'they would not worship the gods of their fathers who were in the land of the chaldæans.' the bible record shows that members of this branch of the chaldæan people visited egypt from time to time. they were shepherds, too, which accords well with the account of herodotus above quoted. we can well understand that persons of this family would have resisted all endeavours to secure their acquiescence in any scheme associated with idolatrous rites. neither promises nor threats would have had much influence on them. it was a distinguished member of the family, the patriarch abraham, who said: 'i have lift up mine hand unto the lord, the most high god, the possessor of heaven and earth, that i will not take from a thread even to a shoe-latchet, and that i will not take anything that is thine, lest thou shouldest say, i have made abram rich.' vain would all the promises and all the threats of cheops have been to men of this spirit. such men might help him in his plans, suggested, as the history shows, by teachings of their own, but it must be on their own conditions, and those conditions would most certainly include the utter rejection of idolatrous worship by the king in whose behalf they worked, as well as by all who shared in their labours. it seems probable that they convinced both cheops and chephren, that unless these kings gave up idolatry, the purpose, whatever it was, which the pyramid was erected to promote, would not be fulfilled. the mere fact that the great pyramid was built either directly at the suggestion of these visitors, or because they had persuaded cheops of the truth of some important doctrine, shows that they must have gained great influence over his mind. rather we may say that he must have been so convinced of their knowledge and power as to have accepted with unquestioning confidence all that they told him respecting the particular subject over which they seemed to possess so perfect a mastery. but having formed the opinion, on grounds sufficiently assured, that the strangers who visited egypt and superintended the building of the great pyramid were kinsmen of the patriarch abraham, it is not very difficult to decide what was the subject respecting which they had such exact information. they or their parents had come from the land of the chaldæans, and they were doubtless learned in all the wisdom of their chaldæan kinsmen. they were masters, in fact, of the astronomy of their day, a science for which the chaldæans had shown from the earliest ages the most remarkable aptitude. what the actual extent of their astronomical knowledge may have been it would be difficult to say. but it is certain, from the exact knowledge which later chaldæans possessed respecting long astronomical cycles, that astronomical observations must have been carried on continuously by that people for many hundreds of years. it is highly probable that the astronomical knowledge of the chaldæans in the days of terah and abraham was much more accurate than that possessed by the greeks even after the time of hipparchus.[ ] we see indeed, in the accurate astronomical adjustment of the great pyramid, that the architects must have been skilful astronomers and mathematicians; and i may note here, in passing, how strongly this circumstance confirms the opinion that the visitors were kinsmen of terah and abraham. all we know from herodotus and manetho, all the evidence from the circumstances connected with the religion of the pyramid-kings, and the astronomical evidence given by the pyramids themselves, tends to assure us that members of that particular branch of the chaldæan family which went out from ur of the chaldees because they would not worship the gods of the chaldæans, extended their wanderings to egypt, and eventually superintended the erection of the great pyramid so far as astronomical and mathematical relations were concerned. but not only have we already decided that the pyramids were not intended solely or chiefly to sub serve the purpose of astronomical observatories, but it is certain that cheops would not have been personally much interested in any astronomical information which these visitors might be able to communicate. unless he saw clearly that something was to be gained from the lore of his visitors, he would not have undertaken to erect any astronomical buildings at their suggestion, even if he had cared enough for their knowledge to pay any attention to them whatever. most probably the reply cheops would have made to any communications respecting mere astronomy, would have run much in the style of the reply made by the turkish cadi, imaum ali zadè to a friend of layard's who had apparently bored him about double stars and comets: 'oh my soul! oh my lamb!' said ali zadè, 'seek not after the things which concern thee not. thou camest unto us, and we welcomed thee: go in peace. of a truth thou hast spoken many words; and there is no harm done, for the speaker is one and the listener is another. after the fashion of thy people thou hast wandered from one place to another until thou art happy and content in none. listen, oh my son! there is no wisdom equal unto the belief in god! he created the world, and shall we liken ourselves unto him in seeking to penetrate into the mysteries of his creation? shall we say, behold this star spinneth round that star, and this other star with a tail goeth and cometh in so many years! let it go! he from whose hand it came will guide and direct it. but thou wilt say unto me, stand aside, oh man, for i am more learned than thou art, and have seen more things. if thou thinkest that thou art in this respect better than i am, thou art welcome. i praise god that i seek not that which i require not. thou art learned in the things i care not for; and as for that which thou hast seen, i defile it. will much knowledge create thee a double belly, or wilt thou seek paradise with thine eyes?' such, omitting the references to the creator, would probably have been the reply of cheops to his visitors, had they only had astronomical facts to present him with. or, in the plenitude of his kingly power, he might have more decisively rejected their teaching by removing their heads. but the shepherd-astronomers had knowledge more attractive to offer than a mere series of astronomical discoveries. their ancestors had watched from the centres of their sleeping flocks those radiant mercuries, that seemed to move carrying through æther in perpetual round decrees and resolutions of the gods; and though the visitors of king cheops had themselves rejected the sabaistic polytheism of their kinsmen, they had not rejected the doctrine that the stars in their courses affect the fortunes of men. we know that among the jews, probably the direct descendants of the shepherd-chiefs who visited cheops, and certainly close kinsmen of theirs, and akin to them also in their monotheism, the belief in astrology was never regarded as a superstition. in fact, we can trace very clearly in the books relating to this people that they believed confidently in the influences of the heavenly bodies. doubtless the visitors of king cheops shared the belief of their chaldæan kinsmen that astrology is a true science, 'founded' indeed (as bacon expresses their views) 'not in reason and physical contemplations, but in the direct experience and observation of past ages.' josephus records the jewish tradition (though not as a tradition but as a fact) that 'our first father, adam, was instructed in astrology by divine inspiration,' and that seth so excelled in the science, that, 'foreseeing the flood and the destruction of the world thereby, he engraved the fundamental principles of his art (astrology) in hieroglyphical emblems, for the benefit of after ages, on two pillars of brick and stone.' he says farther on that the patriarch abraham, 'having learned the art in chaldæa, when he journeyed into egypt taught the egyptians the sciences of arithmetic and astrology.' indeed, the stranger called philitis by herodotus may, for aught that appears, have been abraham himself; for it is generally agreed that the word philitis indicated the race and country of the visitors, regarded by the egyptians as of philistine descent and arriving from palestine. however, i am in no way concerned to show that the shepherd-astronomers who induced cheops to build the great pyramid were even contemporaries of abraham and melchizedek. what seems sufficiently obvious is all that i care to maintain, namely, that these shepherd-astronomers were of chaldæan birth and training, and therefore astrologers, though, unlike their chaldæan kinsmen, they rejected sabaism or star-worship, and taught the belief in one only deity. now, if these visitors were astrologers, who persuaded cheops, and were honestly convinced themselves, that they could predict the events of any man's life by the chaldæan method of casting nativities, we can readily understand many circumstances connected with the pyramids which have hitherto seemed inexplicable. the pyramid built by a king would no longer be regarded as having reference to his death and burial, but to his birth and life, though after his death it might receive his body. each king would require to have his own nativity-pyramid, built with due symbolical reference to the special celestial influences affecting his fortunes. every portion of the work would have to be carried out under special conditions, determined according to the mysterious influences ascribed to the different planets and their varying positions-- now high, now low, then hid. progressive, retrograde, or standing still. if the work had been intended only to afford the means of predicting the king's future, the labour would have been regarded by the monarch as well bestowed. but astrology involved much more than the mere prediction of future events. astrologers claimed the power of ruling the planets--that is, of course, not of ruling the motions of those bodies, but of providing against evil influences or strengthening good influences which they supposed the celestial orbs to exert in particular aspects. thus we can understand that while the mere basement layers of the pyramid would have served for the process of casting the royal nativity, with due mystic observances, the further progress of building the pyramid would supply the necessary means and indications for ruling the planets most potent in their influence upon the royal career. remembering the mysterious influence which astrologers ascribed to special numbers, figures, positions, and so forth, the care with which the great pyramid was so proportioned as to indicate particular astronomical and mathematical relations is at once explained. the four sides of the square base were carefully placed with reference to the cardinal points, precisely like the four sides of the ordinary square scheme of nativity.[ ] the eastern side faced the ascendant, the southern faced the mid-heaven, the western faced the descendant, and the northern faced the imum coeli. again, we can understand that the architects would have made a circuit of the base correspond in length with the number of days in the year--a relation which, according to prof. p. smyth, is fulfilled in this manner, that the four sides contain one hundred times as many pyramid inches as there are days in the year. the pyramid inch, again, is itself mystically connected with astronomical relations, for its length is equal to the five hundred millionth part of the earth's diameter, to a degree of exactness corresponding well with what we might expect chaldæan astronomers to attain. prof. smyth, indeed, believes that it was exactly equal to that proportion of the earth's polar diameter--a view which would correspond with his theory that the architects of the great pyramid were assisted by divine inspiration; but what is certainly known about the sacred cubit, which contained twenty-five of these inches, corresponds better with the diameter which the chaldæan astronomers, if they worked very carefully, would have deduced from observations made in their own country, on the supposition which they would naturally have made that the earth is a perfect globe, not compressed at the poles. it is not indeed at all certain that the sacred cubit bore any reference to the earth's dimensions; but this seems tolerably well made out--that the sacred cubit was about inches in length, and that the circuit of the pyramid's base contained a hundred inches for every day of the year. relations such as these are precisely what we might expect to find in buildings having an astrological significance. similarly, it would correspond well with the mysticism of astrology that the pyramid should be so proportioned as to make the height be the radius of a circle whose circumference would equal the circuit of the pyramid's base. again, that long slant tunnel, leading downwards from the pyramid's northern face, would at once find a meaning in this astrological theory. the slant tunnel pointed to the pole-star of cheops' time, when due north below the true pole of the heavens. this circumstance had no observational utility. it could afford no indication of time, because a pole-star moves very slowly, and the pole-star of cheops' day must have been in view through that tunnel for more than an hour at a time. but, apart from the mystical significance which an astrologer would attribute to such a relation, it may be shown that this slant tunnel is precisely what the astrologer would require in order to get the horoscope correctly. another consideration remains to be mentioned which, while strengthening the astrological theory of the pyramids, may bring us even nearer to the true aim of those who planned and built these structures. it is known also that the chaldæans from the earliest times pursued the study of alchemy in connection with astrology, not hoping to discover the philosopher's stone by chemical investigations alone, but by carrying out such investigations under special celestial influence. the hope of achieving this discovery, by which he would at once have had the means of acquiring illimitable wealth, would of itself account for the fact that cheops expended so much labour and material in the erection of the great pyramid, seeing that, of necessity, success in the search for the philosopher's stone would be a main feature of his fortunes, and would therefore be astrologically indicated in his nativity-pyramid, or perhaps even be secured by following mystical observances proper for ruling his planets. the elixir of life may also have been among the objects which the builders of the pyramids hoped to discover. it may be noticed, as a somewhat significant circumstance, that, in the account given by ibn abd alkohm of the contents of the various pyramids, those assigned to the great pyramid relate entirely to astrology and associated mysteries. it is, of course, clear that abd alkohm drew largely on his imagination. yet it seems probable that there was also some basis of tradition for his ideas. and certainly one would suppose that, as he assigned a treasurer to the east pyramid ('a statue of black agate, his eyes open and shining, sitting on a throne with a lance'), he would have credited the building with treasure also, had not some tradition taught otherwise. but he says that king saurid placed in the east pyramid, not treasures, but 'divers celestial spheres and stars, and what they severally operate in their aspects, and the perfumes which are to be used to them, and the books which treat of these matters.'[ ] but, after all, it must be admitted that the strongest evidence in favour of the astrological (and alchemical) theory of the pyramids is to be found in the circumstance that all other theories seem untenable. the pyramids were undoubtedly erected for some purpose which was regarded by their builders as most important. this purpose certainly related to the personal fortunes of the kingly builders. it was worth an enormous outlay of money, labour, and material. this purpose was such, furthermore, that each king required to have his own pyramid. it was in some way associated with astronomy, for the pyramids are built with most accurate reference to celestial aspects. it also had its mathematical and mystical bearings, seeing that the pyramids exhibit mathematical and symbolical peculiarities not belonging to their essentially structural requirements. and lastly, the erection of the pyramids was in some way connected with the arrival of certain learned persons from palestine, and presumably of chaldæan origin. all these circumstances accord well with the theory i have advanced; while only some of them, and these not the most characteristic, accord with any of the other theories. moreover, no fact known respecting the pyramids or their builders is inconsistent with the astrological (and alchemical) theory. on the whole, then, if it cannot be regarded as demonstrated (in its general bearing, of course, for we cannot expect any theory about the pyramids to be established in minute details), the astrological theory may fairly be described as having a greater degree of probability in its favour than any hitherto advanced. iv. _swedenborg's visions of other worlds._ if it were permitted to men to select a sign whereby they should know that a message came from the supreme being, probably the man of science would select for the sign the communication of some scientific fact beyond the knowledge of the day, but admitting of being readily put to the test. the evidence thus obtained in favour of a revelation would correspond in some sense to that depending on prophecies; but it would be more satisfactory to men having that particular mental bent which is called the scientific. whether this turn of mind is inherent or the result of training, it certainly leads men of science to be more exacting in considering the value of evidence than any men, except perhaps lawyers. in the case of the student of science, st. paul's statement that 'prophecies' 'shall fail' has been fulfilled, whereas it may be doubted whether evidence from 'knowledge' would in like manner 'vanish away.' on the contrary, it would grow stronger and stronger, as knowledge from observation, from experiment, and from calculation continually increased. it can scarcely be said that this has happened with such quasi-scientific statements as have actually been associated with revelation. if we regard st. paul's reference to knowledge as relating to such statements as these, then nothing could be more complete than the fulfilment of his own prediction, 'whether there be prophecies, they shall fail; whether there be tongues, they shall cease; whether there be knowledge, it shall vanish away.' the evidence from prophecies fails for the exact inquirer, who perceives the doubts which exist (among the most earnest believers) as to the exact meaning of the prophetic words, and even in some cases as to whether prophecies have been long since fulfilled or relate to events still to come. the evidence from 'tongues' has ceased, and those are dust who are said to have spoken in strange tongues. the knowledge which was once thought supernatural has utterly vanished away. but if, in the ages of faith, some of the results of modern scientific research had been revealed, as the laws of the solar system, the great principle of the conservation of energy, or the wave theory of light, or if some of the questions which still remain for men of science to solve had been answered in those times, the evidence for the student of science would have been irresistible. of course he will be told that even then he would have hardened his heart; that the inquiry after truth tending naturally to depravity of mind, he would reject even evidence based on his beloved laws of probability; that his 'wicked and adulterous generation seeketh "in vain" after a sign,' and that if he will not accept moses and the prophets, neither would he believe though one rose from the dead. still the desire of the student of science to base his faith on convincing evidence (in a matter as important to him as to those who abuse him) does seem to have something reasonable in it after all. the mental qualities which cause him to be less easily satisfied than others, came to him in the same way as his bodily qualities; and even if the result to which his mental training leads him is as unfortunate as some suppose, that training is not strictly speaking so heinously sinful that nothing short of the eternal reprobation meted out to him by earthly judges can satisfy divine justice. so that it may be thought not a wholly unpardonable sin to speak of a sign which, had it been accorded, would have satisfied even the most exacting student of science. apart, too, from all question of faith, the mere scientific interest of divinely inspired communications respecting natural laws and processes would justify a student of science in regarding them as most desirable messages from a being of superior wisdom and benevolence. if prophecies and tongues, why not knowledge, as evidence of a divine mission? such thoughts are suggested by the claim of some religious teachers to the possession of knowledge other than that which they could have gained by natural means. the claim has usually been quite honest. the teacher of religion tests the reality of his mission in simple _à priori_ confidence that he has such a mission, and that therefore some one or other of the tests he applies will afford the required evidence. to one, says st. paul, is given the word of wisdom; to another, the word of knowledge; to another, faith; to another, the gift of healing; to another, the working of miracles; to another, prophecy; to another, the discerning of spirits; to another, divers kinds of tongues: and so forth. if a man like mahomet, who believes in his mission to teach, finds that he cannot satisfactorily work miracles--that mountains will not be removed at his bidding--then some other evidence satisfies him of the reality of his mission. swedenborg, than whom, perhaps, no more honest man ever lived, said and believed that to him had been granted the discerning of spirits. 'it is to be observed,' he said, 'that a man may be instructed by spirits and angels if his interiors be so open as to enable him to speak and be in company with them, for man in his essence is a spirit, and is with spirits as to his interiors; so that he whose interiors are opened by the lord may converse with them, as man with man. _this privilege i have enjoyed daily now for twelve years._' it indicates the fulness of swedenborg's belief in this privilege that he did not hesitate to describe what the spirits taught him respecting matters which belong rather to science than to faith; though it must be admitted that probably he supposed there was small reason for believing that his statements could ever be tested by the results of scientific research. the objects to which his spiritual communications related were conveniently remote. i do not say this as desiring for one moment to suggest that he purposely selected those objects, and not others which might be more readily examined. he certainly believed in the reality of the communications he described. but possibly there is some law in things visionary, corresponding to the law of mental operation with regard to scientific theories; and as the mind theorises freely about a subject little understood, but cautiously where many facts have been ascertained, so probably exact knowledge of a subject prevents the operation of those illusions which are regarded as supernatural communications. it is in a dim light only that the active imagination pictures objects which do not really exist; in the clear light of day they can no longer be imagined. so it is with mental processes. probably there is no subject more suitable in this sense for the visionary than that of life in other worlds. it has always had an attraction for imaginative minds, simply because it is enwrapped in so profound a mystery; and there has been little to restrain the fancy, because so little is certainly known of the physical condition of other worlds. recently, indeed, a somewhat sudden and severe check has been placed on the liveliness of imagination which had enabled men formerly to picture to themselves the inhabitants of other orbs in space. spectroscopic analysis and exact telescopic scrutiny will not permit some speculations to be entertained which formerly met with favour. yet even now there has been but a slight change of scene and time. if men can no longer imagine inhabitants of one planet because it is too hot, or of another because it is too cold, of one body because it is too deeply immersed in vaporous masses, or of another because it has neither atmosphere nor water, we have only to speculate about the unseen worlds which circle round those other suns, the stars; or, instead of changing the region of space where we imagine worlds, we can look backward to the time when planets now cold and dead were warm with life, or forward to the distant future when planets now glowing with fiery heat shall have cooled down to a habitable condition. swedenborg's imaginative mind seems to have fully felt the charm of this interesting subject. it was, indeed, because of the charm which he found in it, that he was readily persuaded into the belief that knowledge had been supernaturally communicated to him respecting it. 'because i had a desire,' he says, 'to know if there are other earths, and to learn their nature and the character of their inhabitants, it was granted me by the lord to converse and have intercourse with spirits and angels who had come from other earths, with some for a day, with some for a week, and with some for months. from them i have received information respecting the earths from and near which they are, the modes of life, customs and worship of their inhabitants, besides various other particulars of interest, all which, having come to my knowledge in this way, i can describe as things which i have seen and heard.' it is interesting (psychologically) to notice how the reasoning which had convinced swedenborg of the existence of other inhabited worlds is attributed by him to the spirits. 'it is well known in the other life,' he says, 'that there are many earths with men upon them; for there (that is, in the spiritual life) every one who, from a love of truth and consequent use, desires it, is allowed to converse with the spirits of other earths, so as to be assured that there is a plurality of worlds, and be informed that the human race is not confined to one earth only, but extends to numberless earths.... i have occasionally conversed on this subject with the spirits of our earth, and the result of our conversation was that a man of enlarged understanding may conclude from various considerations that there are many earths with human inhabitants upon them. for it is an inference of reason that masses so great as the planets are, some of which exceed this earth in magnitude, are not empty bodies, created only to be carried in their motion round the sun, and to shine with their scanty light for the benefit of one earth only; but that they must have a nobler use. he who believes, as every one ought to believe, that the deity created the universe for no other end than the existence of the human race, and of heaven from it (for the human race is the seminary of heaven), must also believe that wherever there is an earth there are human inhabitants. that the planets which are visible to us, being within the boundary of our solar system, are earths, may appear from various considerations. they are bodies of earthy matter, because they reflect the sun's light, and when seen through the telescope appear, not as stars shining with a flaming lustre, but as earths, variegated with obscure spots. like our earth, they are carried round the sun by a progressive motion, through the path of the zodiac, whence they have years and seasons of the year, which are spring, summer, autumn, and winter; and they rotate upon their axes, which makes days, and times of the day, as morning, midday, evening, and night. some of them also have satellites, which perform their revolutions about their globes, as the moon does about ours. the planet saturn, as being farthest from the sun, has besides an immense luminous ring, which supplies that earth with much, though reflected, light. how is it possible for anyone acquainted with these facts, and who thinks from reason, to assert that such bodies are uninhabited?' remembering that this reasoning was urged by the spirits, and that during twelve years swedenborg's interiors had been opened in such sort that he could converse with spirits from other worlds, it is surprising that he should have heard nothing about uranus or neptune, to say nothing of the zone of asteroids, or again, of planets as yet unknown which may exist outside the path of neptune. he definitely commits himself, it will be observed, to the statement that saturn is the planet farthest from the sun. and elsewhere, in stating where in these spiritual communications the 'idea' of each planet was conceived to be situated, he leaves no room whatever for uranus and neptune, and makes no mention of other bodies in the solar system than those known in his day. this cannot have been because the spirits from then unknown planets did not feel themselves called upon to communicate with the spirit of one who knew nothing of their home, for he received visitors from worlds in the starry heavens far beyond human ken. it would almost seem, though to the faithful swedenborgian the thought will doubtless appear very wicked, that the system of swedenborg gave no place to uranus and neptune, simply because he knew nothing about those planets. otherwise, what a noble opportunity there would have been for establishing the truth of swedenborgian doctrines by revealing to the world the existence of planets hitherto unknown. before the reader pronounces this a task beneath the dignity of the spirits and angels who taught swedenborg it will be well for him to examine the news which they actually imparted. i may as well premise, however, that it does not seem to me worth while to enter here at any length into swedenborg's descriptions of the inhabitants of other worlds, because what he has to say on this subject is entirely imaginative. there is a real interest for us in his ideas respecting the condition of the planets, because those ideas were based (though unconsciously) upon the science of his day, in which he was no mean proficient. and even where his mysticism went beyond what his scientific attainments suggested, a psychological interest attaches to the workings of his imagination. it is as curious a problem to trace his ideas to their origin as it sometimes is to account for the various phases of a fantastic dream, such a dream, for instance, as that which armadale, the doctor, and midwinter, in 'armadale,' endeavour to connect with preceding events. but swedenborg's visions of the behaviour and appearance of the inhabitants of other earths have little interest, because it is hopeless to attempt to account for even their leading features. for instance, what can we make of such a passage as the following, relating to the spirits who came from mercury?--'some of them are desirous to appear, not like the spirits of other earths as men, but as crystalline globes. their desire to appear so, although they do not, arises from the circumstance that the knowledges of things immaterial are in the other life represented by crystals.' yet some even of these more fanciful visions significantly indicate the nature of swedenborg's philosophy. one can recognise his disciples and his opponents among the inhabitants of various favoured and unhappy worlds, and one perceives how the wiser and more dignified of his spiritual visitors are made to advocate his own views, and to deride those of his adversaries. some of the teachings thus circuitously advanced are excellent. for instance, swedenborg's description of the inhabitants of mercury and their love of abstract knowledge contains an instructive lesson. 'the spirits of mercury imagine,' he says, 'that they know so much, that it is almost impossible to know more. but it has been told them by the spirits of our earth, that they do not know many things, but few, and that the things which they know not are comparatively infinite, and in relation to those they do know are as the waters of the largest ocean to those of the smallest fountain; and further, that the first advance to wisdom is to know, acknowledge, and perceive that what we do know, compared with what we do not know, is so little as hardly to amount to anything.'[ ] so far we may suppose that swedenborg presents his own ideas, seeing that he is describing what has been told the mercurial spirits by the spirits of our earth, of whom (during these spiritual conversations) he was one. but he proceeds to describe how angels were allowed to converse with the mercurial spirits in order to convince them of their error. 'i saw another angel,' says he, after describing one such conversation, 'conversing with them; he appeared at some altitude to the right; he was from our earth, and he enumerated very many things of which they were ignorant.... as they had been proud on account of their knowledges, on hearing this they began to humble themselves. their humiliation was represented by the sinking of the company which they formed, for that company then appeared as a volume or roll, ... as if hollowed in the middle and raised at the sides.... they were told what that signified, that is, what they thought in their humiliation, and that those who appeared elevated at the sides were not as yet in any humiliation. then i saw that the volume was separated, and that those who were not in humiliation were remanded back towards their earth, the rest remaining.' little being known to swedenborg, as indeed little is known to the astronomers of our own time, about mercury, we find little in the visions relating to that planet which possesses any scientific interest. he asked the inhabitants who were brought to him in visions about the sun of the system, and they replied that it looks larger from mercury than as seen from other worlds. this of course was no news to swedenborg. they explained further, that the inhabitants enjoy a moderate temperature, without extremes of heat or cold. 'it was given to me,' proceeds swedenborg, 'to tell them that it was so provided by the lord, that they might not be exposed to excessive heat from their greater proximity to the sun, since heat does not arise from the sun's nearness, but from the height and density of the atmosphere, as appears from the cold on high mountains even in hot climates; also that heat is varied according to the direct or oblique incidence of the sun's rays, as is plain from the seasons of winter and summer in every region.' it is curious to find thus advanced, in a sort of lecture addressed to visionary mercurials, a theory which crops up repeatedly in the present day, because the difficulty which suggests it is dealt with so unsatisfactorily for the most part in our text-books of science. continually we hear of some new paradoxist who propounds as a novel doctrine the teaching that the atmosphere, and not the sun, is the cause of heat. the mistake was excusable in swedenborg's time. in fact it so chanced that, apart from the obvious fact on which the mistake is usually based--the continued presence, namely, of snow on the summits of high mountains even in the torrid zone--it had been shown shortly before by newton, that the light fleecy clouds seen sometimes even in the hottest weather above the wool-pack or cumulus clouds are composed of minute crystals of ice. seeing that these tiny crystals can exist under the direct rays of the sun in hot summer weather, many find it difficult to understand how those rays can of themselves have any heating power. yet in reality the reasoning addressed by swedenborg to his mercurial friends was entirely erroneous. if he could have adventured as far forth into time as he did into space, and could have attended in the spirit the lectures of one john tyndall, a spirit of our earth, he would have had this matter rightly explained to him. in reality the sun's heat is as effective directly at the summit of the highest mountain as at the sea-level. a thermometer exposed to the sun in the former position indicates indeed a slightly higher temperature than one similarly exposed to the sun (when at the same altitude) at the sea-level. but the air does not get warmed to the same degree, simply because, owing to its rarity and relative dryness, it fails to retain any portion of the heat which passes through it. it is interesting to notice how swedenborg's scientific conceptions of the result of the (relatively) airless condition of our moon suggested peculiar fancies respecting the lunar inhabitants. interesting, i mean, psychologically: for it is curious to see scientific and fanciful conceptions thus unconsciously intermingled. of the conscious intermingling of such conceptions instances are common enough. the effects of the moon's airless condition have been often made the subject of fanciful speculations. the reader will remember how scheherazade, in 'the poet at the breakfast table,' runs on about the moon. 'her delight was unbounded, and her curiosity insatiable. if there were any living creatures there, what odd things they must be. they couldn't have any lungs nor any hearts. what a pity! did they ever die? how could they expire if they didn't breathe? burn up? no air to burn in. tumble into some of those horrid pits, perhaps, and break all to bits. she wondered how the young people there liked it, or whether there were any young people there. perhaps nobody was young and nobody was old, but they were like mummies all of them--what an idea!--two mummies making love to each other! so she went on in a rattling, giddy kind of way, for she was excited by the strange scene in which she found herself, and quite astonished the young astronomer with her vivacity.' but swedenborg's firm belief that the fancies engendered in his mind were scientific realities is very different from the conscious play of fancy in the passage just quoted. it must be remembered that swedenborg regarded his visions with as much confidence as though they were revelations made by means of scientific instruments; nay, with even more confidence, for he knew that scientific observations may be misunderstood, whereas he was fully persuaded that his visions were miraculously provided for his enlightenment, and that therefore he would not be allowed to misunderstand aught that was thus revealed to him. 'it is well known to spirits and angels,' he says, 'that there are inhabitants in the moon, and in the moons or satellites which revolve about jupiter and saturn. even those who have not seen and conversed with spirits who are from them entertain no doubt of their being inhabited, for they, too, are earths, and where there is an earth there is man; man being the end for which every earth exists, and without an end nothing was made by the great creator. every one who thinks from reason in any degree enlightened, must see that the human race is the final cause of creation.' the moon being inhabited then by human beings, but being very insufficiently supplied with air, it necessarily follows that these human beings must be provided in some way with the means of existing in that rare and tenuous atmosphere. tremendous powers of inspiration and expiration would be required to make that air support the life of the human body. although swedenborg could have had no knowledge of the exact way in which breathing supports life (for priestley was his junior by nearly half a century), yet he must clearly have perceived that the quantity of air inspired has much to do with the vitalising power of the indraught. no ordinary human lungs could draw in an adequate supply of air from such an atmosphere as the moon's; but by some great increase of breathing power it might be possible to live there: at least, in swedenborg's time there was no reason for supposing otherwise. reason, then, having convinced him that the lunar inhabitants must possess extraordinary breathing apparatus, and presumably most powerful voices, imagination presented them to him accordingly. 'some spirits appeared overhead,' he says, 'and thence were heard voices like thunder; for their voices sounded precisely like thunder from the clouds after lightning. i supposed it was a great multitude of spirits who had the art of giving voices with such a sound. the more simple spirits who were with me derided them, which greatly surprised me. but the cause of their derision was soon discovered, which was, that the spirits who thundered were not many, but few, and were as little as children, and that on former occasions they (the thunderers) had terrified them by such sounds, and yet were unable to do them the least harm. that i might know their character, some of them descended from on high, where they thundered; and, what surprised me, one carried another on his back, and the two thus approached me. their faces appeared not unhandsome, but longer than those of other spirits. in stature they were like children of seven years old, but the frame was more robust, so that they were like men. it was told me by the angels that they were from the moon. he who was carried by the other came to me, applying himself to my left side under the elbow, and thence spoke. he said, that when they utter their voices they thunder in this way,'--and it seems likely enough that if there are any living speaking beings in the moon, their voice, could they visit the earth, would be found to differ very markedly from the ordinary human voice. 'in the spiritual world their thunderous voices have their use. for by their thundering the spirits from the moon terrify spirits who are inclined to injure them, so that the lunar spirits go in safety where they will. to convince me the sound they make was of this kind, he (the spirit who was carried by the other) retired, but not out of sight, and thundered in like manner. they showed, moreover, that the voice was thundered by being uttered from the abdomen like an eructation. it was perceived that this arose from the circumstance that the inhabitants of the moon do not, like the inhabitants of other earths, speak from the lungs, but from the abdomen, and thus from air collected there, the reason of which is that the atmosphere with which the moon is surrounded is not like that of other earths.' in his intercourse with spirits from jupiter, swedenborg heard of animals larger than those that live on the earth. it has been a favourite idea of many believers in other worlds than ours, that though in each world the same races of animals exist, they would be differently proportioned; and there has been much speculation as to the probable size of men and other animals in worlds much larger or much smaller than the earth. when as yet ideas about other worlds were crude, the idea prevailed that giants exist in the larger orbs, and pygmies in the smaller. whether this idea had its origin in conceptions as to the eternal fitness of things or not, does not clearly appear. it seems certainly at first view natural enough to suppose that the larger beings would want more room and so inhabit the larger dwelling-places. it was a pleasing thought that, if we could visit jupiter or saturn, we should find the human inhabitants there in bigness to surpass earth's giant sons; but that if we could visit our moon or mercury, or whatever smaller worlds there are, we should find men now less than smallest dwarfs, in narrow room throng numberless, like that pygmæan race beyond the indian mount; or fairy elves, whose midnight revels, by a forest side or fountain, some belated peasant sees, or dreams he sees. later the theory was started that the size of beings in various worlds depends on the amount of light received from the central sun. thus wolfius asserted that the inhabitants of jupiter are nearly fourteen feet high, which he proved by comparing the quantity of sunlight which reaches the jovians with that which we terrenes receive. recently, however, it has been noted that the larger the planet, the smaller in all probability must be the inhabitants, if any. for if there are two planets of the same density but unequal size, gravity must be greater at the surface of the larger planet, and where gravity is great large animals are cumbered by their weight. it is easy to see this by comparing the muscular strength of two men similarly proportioned, but unequal in height. suppose one man five feet in height, the other six; then the cross section of any given muscle will be less for the former than for the latter in the proportion of twenty-five (five times five) to thirty-six (six times six). roughly, the muscular strength of the bigger man will be half as great again as that of the smaller. but the weights of the men will be proportioned as (five times five times five) to (six times six times six), so that the weight of the bigger man exceeds that of the smaller nearly as seven exceeds four, or by three-fourths. the taller man exceeds the smaller, then, much more in weight than he does in strength; he is accordingly less active in proportion to his size. within certain limits, of course, size increases a man's effective as well as his real strength. for instance, our tall man in the preceding illustration cannot lift his own weight as readily as the small man can lift his; but he can lift a weight of three hundred pounds as easily as the small man can lift a weight of two hundred pounds. when we get beyond certain limits of height, however, we get absolute weakness as the result of the increase of weight. swift's brobdingnags, for instance, would have been unable to stand upright; for they were six times as tall as men, and therefore each brobdingnag would have weighed times as much as a man, but would have possessed only thirty-six times the muscular power. their weight would have been greater, then, in a sixfold greater degree than their strength, and, so far as their mere weight was concerned, their condition would have resembled that of an ordinary man under a load five times exceeding his own weight. as no man could walk or stand upright under such a load, so the brobdingnags would have been powerless to move, despite, or rather because of, their enormous stature. applying the general considerations here enunciated to the question of the probable size of creatures like ourselves in other planets, we see that men in jupiter should be much smaller, men in mercury much larger, than men on the earth. so also with other animals. but swedenborg's spirit visitors from these planets taught differently. 'the horses of our earth,' he says, 'when seen by the spirits of jupiter, appeared to me smaller than usual, though rather robust; which arose from the idea those spirits had respecting them. they informed me that among them there are animals similar, though much larger; but that they are wild, and in the woods, and that when they come in sight they cause terror though they are harmless; they added that their terror of them is natural or innate.'[ ] on the other hand the inhabitants of mercury, who might be thirteen feet high yet as active as our men, appeared slenderer than terrene men. 'i was desirous to know,' says swedenborg, 'what kind of face and person the people in mercury have, compared with those of the people on our earth. there therefore stood before me a female exactly resembling the women on that earth. her face was beautiful, but it was smaller than that of a woman of our earth; she was more slender, but of equal height; she wore a linen head-dress, not artfully yet gracefully disposed. a man also was presented. he, too, was more slender than the men of our earth; he wore a garment of deep blue, closely fitted to his body without folds or flowing skirts. such, i learn, were the personal form and costume of the humans of that earth. afterwards there was shown me a species of the oxen and cows, which did not indeed differ much from those on our earth, except that they were smaller, and made some approach to the stag and hind species.' we have seen, too, that the lunar spirits were no larger than children seven years old. one passage of swedenborg's description of jupiter is curious. 'although on that earth,' he says, 'spirits speak with men' (_i.e._ with jovian men) 'man in his turn does not speak with spirits, except to say, when instructed, _that he will do so no more_,'--which we should regard as a bull if it were not news from the jovian spirit world. 'nor is man allowed to tell anyone that a spirit has spoken to him; if he does so, he is punished. those spirits of jupiter when they were with me, at first supposed they were with a man of their own earth; but when in my turn i spoke with them, and thought of publishing what passed between us and so relating it to others, then, because they were not allowed to chastise me, they discovered they were with a stranger.' it has been a favourite idea with those who delight in the argument from design, that the moons of the remoter planets have been provided for the express purpose of making up for the small amount of sunlight which reaches those planets. jupiter receives only about one twenty-seventh part of the light which we receive from the sun; but then, has he not four moons to make his nights glorious? saturn is yet farther away from the sun, and receives only the ninetieth part of the light we get from the sun; but then he has eight moons and his rings, and the nocturnal glory of his skies must go far to compensate the saturnians for the small quantity of sunlight they receive. the saturnian spirits who visited swedenborg were manifestly indoctrinated with these ideas. for they informed him that the nocturnal light of saturn is so great that some saturnians worship it, calling it the lord. these wicked spirits are separated from the rest, and are not tolerated by them. 'the nocturnal light,' say the spirits, 'comes from the immense ring which at a distance encircles that earth, and from the moons which are called the satellites of saturn.' and again, being questioned further 'concerning the great ring which appears from our earth to rise above the horizon of that planet, and to vary its situations, they said that it does not appear to them as a ring, but only as a snow-white substance in heaven in various directions.' unfortunately for our faith in the veracity of these spirits, it is certain that the moons of saturn cannot give nearly so much light as ours, while the rings are much more effective as darkeners than as illuminators. one can readily calculate the apparent size of each of the moons as seen from saturn, and thence show that the eight discs of the moons together are larger than our moon's disc in about the proportion of forty-five to eight. so that if they were all shining as brightly as our full moon and all full at the same time, their combined light would exceed hers in that degree. but they are not illuminated as our moon is. they are illuminated by the same remote sun which illuminates saturn, while our moon is illuminated by a sun giving her as much light as we ourselves receive. our moon then is illuminated ninety times more brightly than the moons of saturn, and as her disc is less than all theirs together, not as one to ninety, but as sixteen to ninety, it follows that all the saturnian moons, if full at the same time, would reflect to saturn one-sixteenth part of the light which we receive from the full moon.[ ] as regards the rings of saturn, nothing can be more certain than that they tend much more to deprive saturn of light then to make up by reflection for the small amount of light which saturn receives directly from the sun. the part of the ring which lies between the planet and the sun casts a black shadow upon saturn, this shadow sometimes covering an extent of surface many times exceeding the entire surface of our earth. the shadow thus thrown upon the planet creeps slowly, first one way, then another, northwards and southwards over the illuminated hemisphere of the planet (as pictured in the th plate of my treatise on saturn), requiring for its passage from the arctic to the antarctic regions and back again to the arctic regions of the planet, a period nearly equal to that of a generation of terrestrial men. nearly thirty of our years the process lasts, during half of which time the northern hemisphere suffers, and during the other half the southern. the shadow band, which be it remembered stretches right athwart the planet from the extreme eastern to the extreme western side of the illuminated hemisphere, is so broad during the greater part of the time that in some regions (those corresponding to our temperate zones) the shadow takes two years in passing, during which time the sun cannot be seen at all, unless for a few moments through some chinks in the rings, which are known to be not solid bodies, but made up of closely crowded small moons. and the slow passage of this fearful shadow, which advances at the average rate of some twenty miles a day, but yet hangs for years over the regions athwart which it sweeps, occurs in the very season when the sun's small direct supply of heat would require to be most freely compensated by nocturnal light--in the winter season, namely, of the planet. moreover, not only during the time of the shadow's passage, but during the entire winter half of the saturnian year, the ring reflects no light during the night time, the sun being on the other or summer side of the ring's plane.[ ] the only nocturnal effect which would be observable would be the obliteration of the stars covered by the ring system. it is strange that, this being so, the spirits from saturn should have made no mention of the circumstance; and even more strange that these spirits and others should have asserted that the moons and rings of saturn compensate for the small amount of light directly received from the sun. most certainly a swedenborg of our own time would find the spirits from saturn more veracious and more communicative about these matters, though even what _he_ would hear from the spirits would doubtless appear to sceptics of the twenty-first century to be no more than he could have inferred from the known facts of the science of his day. but swedenborg was not content merely to receive visits from the inhabitants of other planets in the solar system. he was visited also by the spirits of earths in the starry heaven; nay, he was enabled to visit those earths himself. for man, even while living in the world, 'is a spirit as to his interiors, the body which he carries about in the world only serving him for performing functions in this natural or terrestrial sphere, which is the lowest.' and to certain men it is granted not only to converse as a spirit with angels and spirits, but to traverse in a spiritual way the vast distances which separate world from world and system from system, all the while remaining in the body. swedenborg was one of these. 'the interiors of my spirit,' he says, 'are opened by the lord, so that while i am in the body i can at the same time be with angels in heaven, and not only converse with them, but behold the wonderful things which are there and describe them, that henceforth it may no more be said, "who ever came from heaven to assure us it exists and tell us what is there?" he who is unacquainted with the arcana of heaven cannot believe that man can see earths so remote, and give any account of them from sensible experience. but let him know that spaces and distances, and consequently progressions, existing in the natural world, in their origin and first causes are changes of the state of the interiors; that with angels and spirits progressions appear according to changes of state; and that by changes of state they may be apparently translated from one place to another, and from one earth to another, even to earths at the boundaries of the universe; so likewise may man as to his spirit, his body still remaining in its place. this has been the case with me.' before describing his visits to earths in the starry heavens, swedenborg is careful to indicate the probability that such earths exist. 'it is well known to the learned world,' he says, 'that every star is a sun in its place, remaining fixed like the sun of our earth.' the proper motions of the stars had, alas! not been discovered in swedenborg's day, nor does he seem to have been aware what a wild chase he was really entering upon in his spiritual progressions. conceive the pursuit of sirius or vega as either sun rushed through space with a velocity of thirty or forty miles in every second of time! to resume, however, the account which swedenborg gives of the ideas of the learned world of his day. 'it is the distance which makes a star appear in a small form; consequently' (the logical necessity is not manifest, however) 'each star, like the sun of our system, has around it planets which are earths; and the reason these are not visible to us is because of their immense distance and their having no light but from their own star, which light cannot be reflected so far as to reach us.' 'to what other end,' proceeds this most convincing reasoning, 'can be so immense a heaven with such a multitude of stars? for man is the end for which the universe was created. it has been ascertained by calculation that supposing there were in the universe a million earths, and on every earth three hundred millions of men and two hundred generations within six thousand years, and that to every man or spirit was allotted a space of three cubic ells, the collective number of men or spirits could not occupy a space equal to a thousandth part of this earth, thus not more than that occupied by one of the satellites of jupiter or saturn; a space on the universe almost undiscernible, for a satellite is hardly visible to the naked eye. what would this be for the creator of the universe, to whom the whole universe filled with earths could not be enough' (for what?), 'seeing that he is infinite.' however, it is not on this reasoning alone that swedenborg relies. he tells us, honestly beyond all doubt, that he knows the truth of what he relates. 'the information i am about to give,' he says, 'respecting the earths in the starry heaven is from experimental testimony; from which it will likewise appear how i was translated thither as to my spirit, the body remaining in its place.' his progress in his first star-hunt was to the right, and continued for about two hours. he found the boundary of our solar system marked first by a white but thick cloud, next by a fiery smoke ascending from a great chasm. here some guards appeared, who stopped some of the company, because these had not, like swedenborg and the rest, received permission to pass. they not only stopped those unfortunates, but tortured them, conduct for which terrestrial analogues might possibly be discovered. having reached another system, he asked the spirits of one of the earths there how large their sun was and how it appeared. they said it was less than the sun of our earth, and has a flaming appearance. our sun, in fact, is larger than other suns in space, for from that earth starry heavens are seen, and a star larger than the rest appears, which, say those spirits, 'was declared from heaven' to be the sun of swedenborg's earthly home. what swedenborg saw upon that earth has no special interest. the men there, though haughty, are loved by their respective wives because they, the men, are good. but their goodness does not appear very manifest from anything in the narrative. the only man seen by swedenborg took from his wife 'the garment which she wore, and threw it over his own shoulders; loosening the lower part, which flowed down to his feet like a robe (much as a man of our earth might be expected to loosen the tie-back of the period, if he borrowed it in like manner) he thus walked about clad.' he next visited an earth circling round a star, which he learned was one of the smaller sort, not far from the equator. its greater distance was plain from the circumstance that swedenborg was two days in reaching it. in this earth he very nearly fell into a quarrel with the spirits. for hearing that they possess remarkable keenness of vision, he 'compared them with eagles which fly aloft, and enjoy a clear and extensive view of objects beneath.' at this they were indignant, supposing, poor spirits, 'that he compared them to eagles as to their rapacity, and consequently thought them wicked.' he hastened to explain, however, that he 'did not liken them to eagles as to their rapacity, but as to sharpsightedness.' swedenborg's account of a third earth in the star-depths contains a very pretty idea for temples and churches. the temples in that earth 'are constructed,' he says, of trees, not cut down, but growing in the place where they were first planted. on that earth, it seems, there are trees of an extraordinary size and height; these they set in rows when young, and arrange in such an order that they may serve when they grow up to form porticoes and colonnades. in the meanwhile, by cutting and pruning, they fit and prepare the tender shoots to entwine one with another, and join together so as to form the groundwork and floor of the temple to be constructed, and to rise at the sides as walls, and above to bend into arches to form the roof. in this manner they construct the temple with admirable art, elevating it high above the ground. they prepare also an ascent into it, by continuous branches of the trees, extended from the trunk and firmly connected together. moreover, they adorn the temple without and within in various ways, by disposing the foliage into particular forms; thus they build entire groves. but it was not permitted me to see the nature of these temples, only i was informed that the light of their sun is let in by apertures amongst the branches, and is everywhere transmitted through crystals; whereby the light falling on the walls is refracted in colours like those of the rainbow, particularly blue and orange, of which they are fondest. such is their architecture, which they prefer to the most magnificent palaces of our earth.' other earths in the starry heavens were visited by swedenborg, but the above will serve sufficiently to illustrate the nature of his observations. one statement, by the way, was made to him which must have seemed unlikely ever to be contravened, but which has been shown in our time to be altogether erroneous. in the fourth star-world he visited, he was told that that earth, which travels round its sun in days of fifteen hours each, is one of the least in the universe, being scarcely german miles, say english miles, in circumference. this would make its diameter about english miles. but there is not one of the whole family of planetoids which has a diameter so great as this, and many of these earths must be less than fifty miles in diameter. now swedenborg remarks that he had his information from the angels, 'who made a comparison in all these particulars with things of a like nature on our earth, according to what they saw in me or in my memory. their conclusions were formed by angelic ideas, whereby are instantly known the measure of space and time in a just proportion with respect to space and time elsewhere. angelic ideas, which are spiritual, in such calculations infinitely excel human ideas, which are natural.' he must therefore have met, unfortunately, with untruthful angels. the real source of swedenborg's inspirations will be tolerably obvious--to all, at least, who are not swedenborgians. but our account of his visions would not be complete in a psychological sense without a brief reference to the personal allusions which the spirits and angels made during their visits or his wanderings. his distinguished rival, christian wolf, was encountered as a spirit by spirits from mercury, who 'perceived that what he said did not rise above the sensual things of the natural man, because in speaking he thought of honour, and was desirous, as in the world (for in the other world every one is like his former self), to connect various things into series, and from these again continually to deduce others, and so form several chains of such, which they did not see or acknowledge to be true, and which, therefore, they declared to be chains which neither cohered in themselves nor with the conclusions, calling them the obscurity of authority;' so they ceased to question him further, and presently left him. similarly, a spirit who in this world had been a 'prelate and a preacher,' and 'very pathetic, so that he could deeply move his hearers,' got no hearing among the spirits of a certain earth in the starry heavens; for they said they could tell 'from the tone of the voice whether a discourse came from the heart or not;' and as his discourse came not from the heart, 'he was unable to teach them, whereupon he was silent.' convenient thus to have spirits and angels to confirm our impressions of other men, living or dead. apart from the psychological interest attaching to swedenborg's strange vision, one cannot but be strongly impressed by the idea pervading them, that to beings suitably constituted all that takes place in other worlds might be known. modern science recognises a truth here; for in that mysterious ether which occupies all space, messages are at all times travelling by which the history of every orb is constantly recorded. no world, however remote or insignificant; no period, however distant--but has its history thus continually proclaimed in ever widening waves. nay, by these waves also (to beings who could read their teachings aright) the future is constantly indicated. for, as the waves which permeate the ether could only be situated as they actually are, at any moment, through past processes, each one of which is consequently indicated by those ethereal waves, so also there can be but one series of events in the future, as the sequel of the relations actually indicated by the ethereal undulations. these, therefore, speak as definitely and distinctly of the future as of the past. could we but rid us of the gross habiliments of flesh, and by some new senses be enabled to feel each order of ethereal undulations, even of those only which reach our earth, all knowledge of the past and future would be within our power. the consciousness of this underlies the fancies of swedenborg, just as it underlies the thought of him who sang-- there's not an orb which thou behold'st but in his motion like an angel sings, still quiring to the young-eyed cherubim. but while this muddy vesture of decay doth grossly close us in, we cannot hear it. v. _other worlds and other universes._ if any one shall gravely tell me that i have spent my time idly in a vain and fruitless inquiry after what i can never become sure of, the answer is that at this rate he would put down all natural philosophy, as far as it concerns itself in searching into the nature of such things. in such noble and sublime studies as these, 'tis a glory to arrive at probability, and the search itself rewards the pains. but there are many degrees of probable, some nearer to the truth than others, in the determining of which lies the chief exercise of our judgment. and besides the nobleness and pleasure of the studies, may we not be so bold as to say that they are no small help to the advancement of wisdom and morality?--huyghens, _conjectures concerning the planetary worlds_. the interest with which astronomy is studied by many who care little or nothing for other sciences is due chiefly to the thoughts which the celestial bodies suggest respecting life in other worlds than ours. there is no feeling more deeply seated in the human heart--not the belief in higher than human powers, not the hope of immortality, not even the fear of death--than the faith in realms of life where other conditions are experienced than those we are acquainted with here. it is not vulgar curiosity or idle fancy that suggests the possibilities of life in other worlds. it has been the conviction of the profoundest thinkers, of men of highest imagination. the mystery of the star-depths has had its charm for the mathematician as well as for the poet; for the exact observer as for the most fruitful theoriser; nay, for the man of business as for him whose life is passed in communing with nature. if we analyse the interest with which the generality of men inquire into astronomical matters apparently not connected with the question of life in other worlds, we find in every case that it has been out of this question alone or chiefly that that interest has sprung. the great discoveries made during the last few years respecting the sun for example, might seem remote from the subject of life in other worlds. it is true that sir william herschel thought the sun might be the abode of living creatures; and sir john herschel even suggested the possibility that the vast streaks of light called the solar willow-leaves, objects varying from two hundred to a thousand miles in length, might be living creatures whose intense lustre was the measure of their intense vitality. but modern discoveries had rendered all such theories untenable. the sun is presented to us as a mighty furnace, in whose fires the most stubborn elements are not merely melted but vaporised. the material of the sun has been analysed, the motions and changes taking place on his surface examined, the laws of his being determined. how, it might be asked, is the question of life in other worlds involved in these researches? the faith of sir david brewster in the sun as the abode of life being dispelled, how could discoveries respecting the sun interest those who care about the subject of the plurality of worlds? the answer to these questions is easily found. the real interest which solar researches have possessed for those who are not astronomers has resided in the evidence afforded respecting the sun's position as the fire, light, and life of the system of worlds whereof our world is one. the mere facts discovered respecting the sun would be regarded as so much dry detail were they not brought directly into relation with our earth and its wants, and therefore with the wants of the other earths which circle round the sun; but when thus dealt with they immediately excite attention and interest. i do not speak at random in asserting this, but describe the result of widely ranging observation. i have addressed hundreds of audiences in great britain and america on the subject of recent solar discoveries, and i have conversed with many hundreds of persons of various capacity and education, from men almost uncultured to men of the highest intellectual power; and my invariable experience has been that solar research derives its chief interest when viewed in relation to the sun's position as the mighty ruler, the steadfast sustainer, the beneficent almoner of the system of worlds to which our earth belongs. it is the same with other astronomical subjects. few care for the record of lunar observations, save in relation to the question whether the moon is or has been the abode of living creatures. the movements of comets and meteors, and the discoveries recently made respecting their condition, have no interest except in relation to the position of these bodies in the economy of solar systems, or to the possible part which they may at one time have performed in building up worlds and suns. none save astronomers, and few only of these, care for researches into the star-depths, except in connection with the thought that every star is a sun and therefore probably the light and fire of a system of worlds like those which circle around our own sun. it is singular how variously this question of life in other worlds has been viewed at various stages of astronomical progress. from the time of pythagoras, who first, so far as is known, propounded the general theory of the plurality of worlds, down to our own time, when brewster and chalmers on the one hand, and whewell on the other, have advocated rival theories probably to be both set aside for a theory at once intermediate to and more widely ranging in time and space than either, the aspect of the subject has constantly varied, as new lights have been thrown upon it from different directions. it may be interesting briefly to consider what has been thought in the past on this strangely attractive question, and then to indicate the view towards which modern discoveries seem manifestly to point--a view not likely to undergo other change than that resulting from clearer vision and closer approach. in other words, i shall endeavour to show that the theory to which we are now led by all the known facts is correct in general, though, as fresh knowledge is obtained, it may undergo modification in details. we now see the subject from the right point of view, though as science progresses we may come to see it more clearly and definedly. when men believed the earth to be a flat surface above which the heavens were arched as a tent or canopy, they were not likely to entertain the belief in other worlds than ours. during the earlier ages of mankind ideas such as these prevailed. the earth had been fashioned into its present form and condition, the heavens had been spread over it, the sun, and moon, and stars had been set in the heavens for its use and adornment, and there was no thought of any other world. but while this was the general belief, there was already a school of philosophy where another doctrine had been taught. pythagoras had adopted the belief of apollonius pergæus that the sun is the centre of the planetary paths, the earth one among the planets--a belief inseparable from the doctrine of the plurality of worlds. much argument has been advanced to show that this belief never was adopted before the time of copernicus, and unquestionably it must be admitted that the theory was not presented in the clear and simple form to which we have become accustomed. but it is not necessary to weigh the conflicting arguments for and against the opinion that pythagoras and others regarded the earth as not the fixed centre of the universe. the certain fact that the doctrine of the plurality of worlds was entertained (i do not say adopted) by them, proves sufficiently that they cannot have believed the earth to be fixed and central. the idea of other worlds like our earth is manifestly inconsistent with the belief that the earth is the central body around which the whole universe revolves. that this is so is well illustrated by the fate of the unfortunate giordano bruno. he was one of the first disciples of copernicus, and, having accepted the doctrine that the earth travels round the sun as one among his family of planets, was led very naturally to the belief that the other planets are inhabited. he went farther, and maintained that as the earth is not the only inhabited world in the solar system, so the sun is not the only centre of a system of inhabited worlds, but each star a sun like him, about which many planets revolve. this was one of the many heresies for which bruno was burned at the stake. it is easy, also, to recognise in the doctrine of many worlds as the natural sequel of the copernican theory, rather than in the features of this theory itself, the cause of the hostility with which theologians regarded it, until, finding it proved, they discovered that it is directly taught in the books which they interpret for us so variously. the copernican theory was not rejected--nay, it was even countenanced--until this particular consequence of the theory was recognised. but within a few years from the persecution of bruno, galileo was imprisoned, and the last years of his life made miserable, because it had become clear that in setting the earth adrift from its position as centre of the universe, he and his brother copernicans were sanctioning the belief in other worlds than ours. again and again, in the attacks made by clericals and theologians upon the copernican theory, this lamentable consequence was insisted upon. unconscious that they were advancing the most damaging argument which could be conceived for the cause they had at heart, they maintained, honestly but unfortunately, that with the new theory came the manifest inference that our earth is not the only and by no means the most important world in the universe--a doctrine manifestly inconsistent (so they said) with the teachings of the scriptures. it was naturally only by a slow progression that men were able to advance into the domain spread before them by the copernican theory, and to recognise the real minuteness of the earth both in space and time. they more quickly recognised the earth's insignificance in space, because the new theory absolutely forced this fact upon them. if the earth, whose globe they knew to be minute compared with her distance from the sun, is really circling around the sun in a mighty orbit many millions of miles in diameter, it follows of necessity that the fixed stars must lie so far away that even the span of the earth's orbit is reduced to nothing by comparison with the vast depths beyond which lie even the nearest of those suns. this was tycho brahe's famous and perfectly sound argument against the copernican theory. 'the stars remain fixed in apparent position all the time, yet the copernicans tell us that the earth from which we view the stars is circling once a year in an orbit many millions of miles in diameter; how is it that from so widely ranging a point of view we do not see widely different celestial scenery? who can believe that the stars are so remote that by comparison the span of the earth's path is a mere point?' tycho's argument was of course valid.[ ] of two things one. either the earth does not travel round the sun, or the stars are much farther away than men had conceived possible in tycho's time. his mistake lay in rejecting the correct conclusion because simply it made the visible universe seem many millions of times vaster than he had supposed. yet the universe, even as thus enlarged, was but a point to the universe visible in our day, which in turn will dwindle to a point compared with the universe as men will see it a few centuries hence; while that or the utmost range of space over which men can ever extend their survey is doubtless as nothing to the real universe of occupied space. such has been the progression of our ideas as to the position of the earth in space. forced by the discoveries of copernicus to regard our earth as a mere point compared with the distances of the nearest fixed stars, men gradually learned to recognise those distances which at first had seemed infinite as in their turn evanescent even by comparison with that mere point of space over which man is able by instrumental means to extend his survey. though there has been a similar progression in men's ideas as to the earth's position in time, that progression has not been carried to a corresponding extent. men have not been so bold in widening their conceptions of time as in widening their conceptions of space. it is here and thus that, in my judgment, the subject of life in other worlds has been hitherto incorrectly dealt with. men have given up as utterly idle the idea that the existence of worlds is to be limited to the special domain of space to which our earth belongs; but they are content to retain the conception that the domain of time to which our earth's history belongs, 'this bank and shoal of time' on which the life of the earth is cast, is the period to which the existence of other worlds than ours should be referred. this, which is to be noticed in nearly all our ordinary treatises on astronomy, appears as a characteristic peculiarity of works advocating the theory of the plurality of worlds. brewster and dick and chalmers, all in fact who have taken that doctrine under their special protection, reason respecting other worlds as though, if they failed to prove that other orbs are inhabited _now_, or are at least _now_ supporting life in some way or other, they failed of their purpose altogether. the idea does not seem to have occurred to them that there is room and verge enough in eternity of time not only for activity but for rest. they must have all the orbs of space busy at once in the one work which they seem able to conceive as the possible purpose of those bodies--the support of life. the argument from analogy, which they had found effective in establishing the general theory of the plurality of worlds, is forgotten when its application to details would suggest that not _all_ orbs are _at all times_ either the abode of life or in some way subserving the purposes of life. we find, in all the forms of life with which we are acquainted, three characteristic periods--first the time of preparation for the purposes of life; next, the time of fitness for those purposes; and thirdly, the time of decadence tending gradually to death. we see among all objects which exist in numbers, examples of all these stages existing at the same time. in every race of living creatures there are the young as yet unfit for work, the workers, and those past work; in every forest there are saplings, seed-bearing trees, and trees long past the seed-bearing period. we know that planets, or rather, speaking more generally, the orbs which people space, pass through various stages of development, during some only of which they can reasonably be regarded as the abode of life or supporting life; yet the eager champion of the theory of many worlds will have them all in these life-bearing or life-supporting stages, none in any of the stages of preparation, none in any of the stages of decrepitude or death. this has probably had its origin in no small degree from the disfavour with which in former years the theory of the growth and development of planets and systems of planets was regarded. until the evidence became too strong to be resisted, the doctrine that our earth was once a baby world, with many millions of years to pass through before it could be the abode of life, was one which only the professed atheist (so said too many divines) could for a moment entertain; while the doctrine that not the earth alone, but the whole of the solar system, had developed from a condition utterly unlike that through which it is now passing, could have had its origin only in the suggestions of the evil one. both doctrines were pronounced to be so manifestly opposed to the teachings of moses, and not only so, but so manifestly inconsistent with the belief in a supreme being, that--that further argument was unnecessary, and denunciation only was required. so confident were divines on these points, that it would not have been very wonderful if some few students of science had mistaken assertion for proof, and so concluded that the doctrines towards which science was unmistakably leading them really were inconsistent with what they had been taught to regard as the word of god. whether multiplied experiences taught men of science to wait before thus deciding, or however matters fell out, it certainly befell before very long that the terrible doctrine of cosmical development was supported by such powerful evidence, astronomical and terrestrial, as to appear wholly irresistible. then, not only was the doctrine accepted by divines, but shown to be manifestly implied in the sacred narrative of the formation of the earth and heavens, sun, and moon, and stars; while upon those unfortunate students of science who had not changed front in good time, and were found still arguing on the mistaken assumption that the development of our system was not accordant with that ancient narrative, freshly forged bolts were flung from the olympus of orthodoxy. so far as the other argument--from the inconsistency of the development theory with belief in a supreme being--was concerned, the student of science was independent of the interpretations which divines claim the sole right of assigning to the ancient books. science has done so much more than divinity (which in fact has done nothing) to widen our conceptions of space and time, that she may justly claim full right to deal with any difficulties arising from such enlargement of our ideas. with the theological difficulty science would not care to deal at all, were she not urged to do so by the denunciations of divines; and when, so urged, she touches that difficulty, she is quickly told that the difficulty is insuperable, and not long after that it has no existence, and (on both accounts) that it should have been left alone. but with the difficulty arising from the widening of our ideas respecting space and time, science may claim good, almost sole, right to deal. the path to a solution of the problem is not difficult to find. at a first view, it does seem to those whose vision had been limited to a contracted field, that the wide domain of time and space in which processes of development are found to take place is the universe itself, that to deny the formation of our earth by a special creative act is to deny the existence of a creator, that to regard the beginning of our earth as a process of development is to assert that development has been in operation from the beginning of all things. but when we recognise clearly that vastness and minuteness, prolonged and brief duration, are merely relative, we perceive that in considering our earth's history we have to deal only with small parts of space and brief periods of time, by comparison with all space and all time. our earth is very large compared with a tree or an animal, but very small compared with the solar system, a mere point compared with the system of stars to which the sun belongs, and absolutely as nothing compared with the universe of space; and in like manner, while the periods of her growth and development occupy periods very long-lasting compared with those required for the growth and development of a tree or an animal, they are doubtless but brief compared with the eras of the development of our solar system, a mere instant compared with the eras of the development of star-systems, and absolutely evanescent compared with eternity. we have no more reason for rejecting the belief in a creator because our earth or the solar system is found to have developed to its present condition from an embryonic primordial state, than we have had ever since men first found that animals and trees are developed from the germ. the region of development is larger, the period of development lasts longer, but neither the one nor the other is infinite; and being finite, both one and the other are simply nothing by comparison with infinity. it is a startling thought, doubtless, that periods of time compared with which the life of a man, the existence of a nation, nay, the duration of the human race itself, sink into insignificance, should themselves in turn be dwarfed into nothingness by comparison with periods of a still higher order. but the thought is not more startling than that other thought which we have been compelled to admit--the thought that the earth on which we live, and the solar system to which it belongs, though each so vast that all known material objects are as nothing by comparison, are in turn as nothing compared with the depths of space separating us from even the nearest among the fixed stars. one thought, as i have said, we have been compelled to admit, the other has not as yet been absolutely forced upon us. though men have long since given up the idea that the earth and heavens have endured but a few thousand years, it is still possible to believe that the birth of our solar system, whether by creative act or by the beginning of processes of development, belongs to the beginning of all time. but this view cannot be regarded as even probable. although it has never been proved that any definite relation must subsist between time (occupied by events) and space (occupied by matter), the mind naturally accepts the belief that such a relation exists. as we find the universe enlarging under the survey of science, our conceptions of the duration of the universe enlarge also. when the earth was supposed to be the most important object in creation, men might reasonably assign to time itself (regarded as the interval between the beginning of the earth and the consummation of all things when the earth should perish) a moderate duration; but it is equally reasonable that, as the insignificance of the earth's domain in space is recognised, men should recognise also the presumable insignificance of the earth's existence in time. in this respect, although we have nothing like the direct evidence afforded by the measurement of space, we yet have evidence which can scarcely be called in question. we find in the structure of our earth the signs of its former condition. we see clearly that it was once intensely hot! and we know from experimental researches on the cooling of various earths that many millions of years must have been required by the earth in cooling down from its former igneous condition. we may doubt whether bischoff's researches can be relied upon in details, and so be unwilling to assign with him a period of millions of years to a single stage of the process of cooling. but that the entire process lasted tens of millions and probably hundreds of millions of years cannot be doubted. recognising such enormous periods as these in the development of one of the smallest fruits of the great solar tree of life, we cannot but admit at least the reasonableness of believing that the larger fruits (jupiter, for instance, with times as much matter, and saturn with times) must require periods still vaster, probably many times larger. indeed, science shows not only that this view is reasonable, but that no other view is possible. for the mighty root of the tree of life, the great orb of the sun, containing _thousand_ times as much matter as the earth, yet mightier periods would be needed. the growth and development of these, the parts of the great system, must of necessity require much shorter time-intervals than the growth and development of the system regarded as a whole. the enormous period when the germs only of the sun and planets existed as yet, when the chaotic substance of the system had not yet blossomed into worlds, the mighty period which is to follow the death of the last surviving member of the system, when the whole scheme will remain as the dead trunk of a tree remains after the last leaf has fallen, after the last movement of sap within the trunk--these periods must be infinite compared with those which measure the duration of even the mightiest separate members of the system. but all this has been left unnoticed by those who have argued in support of the brewsterian doctrine of a plurality of worlds. they argue as if it had never been shown that every member of the solar system, as of all other such systems in space, has to pass through an enormously long period of preparation before becoming fit to be the abode of life, and that after being fit for life (for a period very long to our conceptions, but by comparison with the other exceedingly short) it must for countless ages remain as an extinct world. or else they reason as though it had been proved that the relatively short life-bearing periods in the existence of the several planets must of necessity synchronise, instead of all the probabilities lying overwhelmingly the other way. while this has been (in my judgment) a defect in what may be called the brewsterian theory of other worlds, a defect not altogether dissimilar has characterised the opposite or whewellite theory. very useful service was rendered to astronomy by whewell's treatise upon, or rather against, the plurality of worlds, calling attention as it did to the utter feebleness of the arguments on which men had been content to accept the belief that other planets and other systems are inhabited. but some among the most powerfully urged arguments against that belief tacitly relied on the assumption of a similarity of general condition among the members of the solar system. for instance, the small mean density of jupiter and saturn had, on the brewsterian theory, been explained as probably due to vast hollow spaces in those planets' interiors--an explanation which (if it could be admitted) would leave us free to believe that jupiter and saturn may be made of the same materials as our own earth. with this was pleasantly intermixed the conception that the inhabitant of these planets may have his 'home in subterranean cities warmed by central fires, or in crystal caves cooled by ocean tides, or may float with the nereids upon the deep, or mount upon wings as eagles, or rise upon the pinions of the dove, that he may flee away and be at rest,' with much more in the same fanciful vein. we now know that there can be no cavities more than a few miles below the crust of a planet, simply because, under the enormous pressures which would exist, the most solid matter would be perfectly plastic. but while whewell's general objection to the theory that jupiter or saturn is in the same condition as our earth thus acquires new force, the particular explanation which he gave of the planet's small density is open to precisely the same general objection. for he assumes that, because the planet's mean density is little greater than that of water, the planet is probably a world of water and ice with a cindery nucleus, or in fact just such a world as would be formed if a sufficient quantity of water in the same condition as the water of our seas were placed at jupiter's greater distance from the sun, around a nucleus of earthy or cindery matter large enough to make the density of the entire planet thus formed equal to that of jupiter, or about one-third greater than the density of water. in this argument there are in reality two assumptions, of precisely the same nature as those which whewell set himself to combat. it is first assumed that some material existing on a large scale in our earth, and nearly of the same density as jupiter, must constitute the chief bulk of that planet, and secondly that the temperature of jupiter's globe must be that which a globe of such material would have if placed where jupiter is. the possibility that jupiter may be in an entirely different stage of planetary life--or, in other words, that the youth, middle life, and old age of that planet may belong to quite different eras from the corresponding periods of our earth's life--is entirely overlooked. rather, indeed, it may be said that the extreme probability of this, on any hypothesis respecting the origin of the solar system, and its absolute certainty on the hypothesis of the development of that system, are entirely overlooked. a fair illustration of the erroneous nature of the arguments which have been used, not only in advocating rival theories respecting the plurality of worlds, but also in dealing with subordinate points, may be presented as follows: imagine a wide extent of country covered with scattered trees of various size, and with plants and shrubs, flowers and herbs, down to the minutest known. let us suppose a race of tiny creatures to subsist on one of the fruits of a tree of moderate size, their existence as a race depending entirely on the existence of the fruit on which they subsist, while the existence of the individuals of their race lasts but for a few minutes. furthermore, let there be no regular fruit season either on their tree or in their region of vegetable life, but fruits forming, growing, and decaying all the time. let us next conceive these creatures to be possessed of a power of reasoning respecting themselves, their fruit world, the tree on which it hangs, and to some degree even respecting such other trees, plants, flowers, and so forth, as the limited range of their vision might be supposed to include. it would be a natural thought with them, when first they began to exercise this power of reasoning, that their fruit home was the most important object in existence, and themselves the chief and noblest of living beings. it would also be very natural that they should suppose the formation of their world to correspond with the beginning of time, and the formation of their race to have followed the formation of their world by but a few seconds. they would conclude that a supreme being had fashioned their world and themselves by special creative acts, and that what they saw outside their fruit world had been also specially created, doubtless to subserve their wants. let us now imagine that gradually, by becoming more closely observant than they had been, by combining together to make more complete observations, and above all by preserving the records of observations made by successive generations, these creatures began to obtain clearer ideas respecting their world and the surrounding regions of space. they would find evidence that the fruit on which they lived had not been formed precisely as they knew it, but had undergone processes of development. the distressing discovery would be made that this development could not possibly have taken place in a few seconds, but must have required many hours, nay, even several of those enormous periods called by us days. this, however, would only be the beginning of their troubles. gradually the more advanced thinkers and the closest observers would perceive that not only had their world undergone processes of development, but that its entire mass had been formed by such processes--that in fact it had not been created at all, in the sense in which they had understood the word, but had _grown_. this would be very dreadful to these creatures, because they would not readily be able to dispossess their minds of the notion that they were the most important beings in the universe, their domain of space coextensive with the universe, the duration of their world coextensive with time. but passing over the difficulties thus arising, and the persecution and abuse to which those would be subjected who maintained the dangerous doctrine that their fruit home had been developed, not created, let us consider how these creatures would regard the question of other worlds than their own. at first they would naturally be unwilling to admit the possibility that other worlds as important as their own could exist. but if after a time they found reason to believe that their world was only one of several belonging to a certain tree system, the idea would occur to them, and would gradually come to be regarded as something more than probable, that those other fruit worlds, like their own, might be the abode of living creatures. and probably at first, while as yet the development of their own world was little understood, they would conceive the notion that all the fruits, large or small, upon their tree system were in the same condition as their own, and either inhabited by similar races or at least in the same full vigour of life-bearing existence. but so soon as they recognised the law of development of their own world, and the relation between such development and their own requirements, they would form a different opinion, if they found that only during certain stages of their world's existence life could exist upon it. if, for instance, they perceived that their fruit world must once have been so bitter and harsh in texture that no creatures in the least degree like themselves could have lived upon it, and that it was passing slowly but surely through processes by which it would become one day dry and shrivelled and unable to support living creatures, they would be apt, if their reasoning powers were fairly developed, to inquire whether other fruits which they saw around them on their tree system were either in the former or in the latter condition. if they found reason to believe certain fruits were in one or other of these stages, they would regard such fruits as not yet the abode of life or as past the life-supporting era. it seems probable even that another idea would suggest itself to some among their bolder thinkers. recognising in their own world in several instances what to their ideas resembled absolute waste of material or of force, it might appear to them quite possible that some, perhaps even a large proportion, of the fruits upon their tree were not only not supporting life at the particular epoch of observation, but never had supported life and never would--that, through some cause or other, life would never appear upon such fruits even when they were excellently fitted for the support of life. they might even conceive that some among the fruits of their tree had failed or would fail to come to the full perfection of fruit life. looking beyond their own tree--that is, the tree to which their own fruit world belonged--they would perceive other trees, though their visual powers might not enable them to know whether such trees bore fruit, whether they were in other respects like their own, whether those which seemed larger or smaller were really so, or owed their apparent largeness to nearness, or their apparent smallness to great distance. they would be apt perhaps to generalise a little too daringly respecting these remote tree systems, concluding too confidently that a shrub or a flower was a tree system like their own, or that a great tree, every branch of which was far larger than their entire tree system, belonged to the same order and bore similar fruit. they might mistake, also, in forgetting the probable fact that as every fruit in their own tree system had its own period of life, very brief compared with the entire existence of the fruit, so every tree might have its own fruit-bearing season. thus, contemplating a tree which they supposed to be like their own in its nature, they might say, 'yonder is a tree system crowded with fruits, each the abode of many myriads of creatures like ourselves:' whereas in reality the tree might be utterly unlike their own, might not yet have reached or might long since have passed the fruit-bearing stage, might when in that stage bear fruit utterly unlike any they could even imagine, and each such fruit during its brief life-bearing condition might be inhabited by living beings utterly unlike any creatures they could conceive. yet again, we can very well imagine that the inhabitants of our fruit world, though they might daringly overleap the narrow limits of space and time within which their actual life or the life of their race was cast, though they might learn to recognise the development of their own world and of others like it, even from the very blossom, would be utterly unable to conceive the possibility that the tree itself to which their world belonged had developed by slow processes of growth from a time when it was less even than their own relatively minute home. still less would it seem credible to them, or even conceivable, that the whole forest region to which they belonged, containing many orders of trees differing altogether from their own tree system, besides plants and shrubs, and flowers and herbs (forms of vegetation of whose use they could form no just conception whatever), had itself grown; that once the entire forest domain had been under vast masses of water--the substance which occasionally visited their world in the form of small drops; that such changes were but minute local phenomena of a world infinitely higher in order than their own; that that world in turn was but one of the least of the worlds forming a yet higher system; and so on _ad infinitum_. such ideas would seem to them not merely inconceivable, but many degrees beyond the widest conceptions of space and time which they could regard as admissible. our position differs only in degree, not in kind, from that of these imagined creatures, and the reasoning which we perceive (though they could not) to be just for such creatures is just for us also. it was perfectly natural that before men recognised the evidences of development in the structure of our earth they should regard the earth and all things upon the earth and visible from the earth as formed by special creative acts precisely as we see them now. but so soon as they perceived that the earth is undergoing processes of development and has undergone such processes in the past, it was reasonable, though at first painful, to conclude that on this point they had been mistaken. yet as we recognise the absurdity of the supposition that, because fruits and trees grow, and were not made in a single instant as we know them, therefore there is no supreme being, so may we justly reject as absurd the same argument, enlarged in scale, employed to induce the conclusion that because planets and solar systems have been developed to their present condition, and were not created in their present form, therefore there is no creator, no god. i do not know that the argument ever has been used in this form; but it has been used to show that those who believe in the development of worlds and systems must of necessity be atheists, an even more mischievous conclusion than the other; for none who had not examined the subject would be likely to adopt the former conclusion, but many might be willing to believe that a number of their fellow-men hold obnoxious tenets, without inquiring closely or at all into the reasoning on which the assertion had been based. but it is more important to notice how our views respecting other worlds should be affected by those circumstances in the evidence _we_ have, which correspond with the features of the evidence on which the imagined inhabitants of the fruit world would form their opinion. it was natural that when men first began to reason about themselves and their home they should reject the idea of other worlds like ours, and perhaps it was equally natural that when first the idea was entertained that the planets may be worlds like ours, men should conceive that all those worlds are in the same condition as ours. but it would be, or rather it _is_, as unreasonable for men to maintain such an opinion now, when the laws of planetary development are understood, when the various dimensions of the planets are known, and when the shortness of the life-supporting period of a planet's existence compared with the entire duration of the planet has been clearly recognised, as it would be for the imagined inhabitants of a small fruit on a tree to suppose that all the other fruits on the tree, though some manifestly far less advanced in development and others far more advanced than their own, were the abode of the same forms of life, though these forms were seen to require those conditions, and no other, corresponding to the stage of development through which their own world was passing. viewing the universe of suns and worlds in the manner here suggested, we should adopt a theory of other worlds which would hold a position intermediate between the brewsterian and the whewellite theories. (it is not on this account that i advocate it, let me remark in passing, but simply because it accords with the evidence, which is not the case with the others.) rejecting on the one hand the theory of the plurality of worlds in the sense implying that all existing worlds are inhabited, and on the other hand the theory of but one world, we should accept a theory which might be entitled the paucity of worlds, only that relative not absolute paucity must be understood. it is absolutely certain that this theory is the correct one, if we admit two postulates, neither of which can be reasonably questioned--viz., first, that the life-bearing era of any world is short compared with the entire duration of that world; and secondly, that there can have been no cause which set all the worlds in existence, not simultaneously, which would be amazing enough, but (which would be infinitely more surprising) in such a way that after passing each through its time of preparation, longer for the large worlds and shorter for the small worlds, they all reached at the same time the life-bearing era. but quite apart from this antecedent probability, amounting as it does to absolute certainty if these two highly probably postulates are admitted, we have the actual evidence of the planets we can examine--that evidence proving incontestably, as i have shown elsewhere, that such planets as jupiter and saturn are still in the state of preparation, still so intensely hot that no form of life could possibly exist upon them, and that such bodies as our moon have long since passed the life-bearing stage, and are to all intents and purposes defunct. but may we not go farther? recognising in our own world, in many instances, what to our ideas resembles waste--waste seeds, waste lives, waste races, waste regions, waste forces--recognising superfluity and superabundance in all the processes and in all the works of nature, should it not appear at least possible that some, perhaps even a large proportion, of the worlds in the multitudinous systems peopling space, are not only not now supporting life, but never have supported life and never will? does this idea differ in kind, however largely to our feeble conceptions it may seem to differ in degree, from the idea of the imagined creatures on a fruit, that some or even many fruits excellently fitted for the support of life might not subserve that purpose? and as those creatures might conceive (as we _know_) that some fruits, even many, fail to come to the full perfection of fruit life, may not we without irreverence conceive (as higher beings than ourselves may _know_) that a planet or a sun may fail in the making? we cannot say that in such a case there would be a waste or loss of material, though we may be unable to conceive how the lost sun or planet could be utilised. our imagined insect reasoners would be unable to imagine that fruits plucked from their tree system were otherwise than wasted, for they would conceive that their idea of the purpose of fruits was the only true one; yet they would be altogether mistaken, as we may be in supposing the main purpose of planetary existence is the support of life. in like manner, when we pass in imagination beyond the limits of our own system, we may learn a useful lesson from the imagined creatures' reasoning about other tree systems than that to which their world belonged. astronomers have been apt to generalise too daringly respecting remote stars and star systems, as though our solar system were a true picture of all solar systems, the system of stars to which our sun belongs a true picture of all star systems. they have been apt to forget that, as every world in our own system has its period of life, short by comparison with the entire duration of the world, so each solar system, each system of such systems, may have its own life-bearing season, infinitely long according to our conceptions, but very short indeed compared with the entire duration of which the life-bearing season would be only a single era. lastly, though men may daringly overleap the limits of time and space within which their lives are cast, though they may learn to recognise the development of their own world and of others like it even from the blossom of nebulosity, they seem unable to rise to the conception that the mighty tree which during remote æons bore those nebulous blossoms sprang itself from cosmical germs. we are unable to conceive the nature of such germs; the processes of development affecting them belong to other orders than any processes we know of, and required periods compared with which the inconceivable, nay, the inexpressible periods required for the development of the parts of our universe, are as mere instants. yet have we every reason which analogy can afford to believe that even the development of a whole universe such as ours should be regarded as but a minute local phenomenon of a universe infinitely higher in order, that universe in turn but a single member of a system of such universes, and so on, even _ad infinitum_. to reject the belief that this is possible is to share the folly of beings such as we have conceived regarding their tiny world as a fit centre whence to measure the universe, while yet, from such a stand-point, this little earth on which we live would be many degrees beyond the limits where for them the inconceivable would begin. to reject the belief that this is not only possible, but real, is to regard the few short steps by which man has advanced towards the unknown as a measurable approach towards limits of space, towards the beginning and the end of all things. until it can be shown that space is bounded by limits beyond which neither matter nor void exists, that time had a beginning before which it was not and tends to an end after which it will exist no more, we may confidently accept the belief that the history of our earth is as evanescent in time as the earth itself is evanescent in space, and that nothing we can possibly learn about our earth, or about the system it belongs to, or about systems of such systems, can either prove or disprove aught respecting the scheme and mode of government of the universe itself. it is true now as it was in days of yore, and it will remain true as long as the earth and those who dwell on it endure, that what men know is nothing, the unknown infinite. vi. _suns in flames._ in november news arrived of a catastrophe the effects of which must in all probability have been disastrous, not to a district, or a country, or a continent, or even a world, but to a whole system of worlds. the catastrophe happened many years ago--probably at least a hundred--yet the messenger who brought the news has not been idle on his way, but has sped along at a rate which would suffice to circle this earth eight times in the course of a second. that messenger has had, however, to traverse millions of millions of miles, and only reached our earth november . the news he brought was that a sun like our own was in conflagration; and on a closer study of his message something was learned as to the nature of the conflagration, and a few facts tending to throw light on the question (somewhat interesting to ourselves) whether our own sun is likely to undergo a similar mishap at any time. what would happen if he did, we know already. the sun which has just met with this disaster--that is, which so suffered a few generations ago--blazed out for a time with several hundred times its former lustre. if our sun were to increase as greatly in light and heat, the creatures on the side of our earth turned towards him at the time would be destroyed in an instant. those on the dark or night hemisphere would not have to wait for their turn till the earth, by rotating, carried them into view of the destroying sun. in much briefer space the effect of his new fires would be felt all over the earth's surface. the heavens would be dissolved and the elements would melt with fervent heat. in fact no description of such a catastrophe, as affecting the night half of the earth, could possibly be more effective and poetical than st. peter's account of the day of the lord, coming 'as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein being burned up;' though i imagine the apostle would have been scarce prepared to admit that the earth was in danger from a solar conflagration. indeed, according to another account, the sun was to be turned into darkness and the moon into blood, before that great and notable day of the lord came--a description corresponding well with solar and lunar eclipses, the most noteworthy 'signs in the heavens,' but agreeing very ill with the outburst of a great solar conflagration. before proceeding to inquire into the singular and significant circumstances of the recent outburst, it may be found interesting to examine briefly the records which astronomy has preserved of similar catastrophes in former years. these may be compared to the records of accidents on the various railway lines in a country or continent. those other suns which we can stars are engines working the mighty mechanism of planetary systems, as our sun maintains the energies of our own system; and it is a matter of some interest to us to inquire in how many cases, among the many suns within the range of vision, destructive explosions occur. we may take the opportunity, later, to inquire into the number of cases in which the machinery of solar systems appears to have broken down. the first case of a solar conflagration on record is that of the new star observed by hipparchus some years ago. in his time, and indeed until quite recently, an object of this kind was called a new star, or a temporary star. but we now know that when a star makes its appearance where none had before been visible, what has really happened has been that a star too remote to be seen has become visible through some rapid increase of splendour. when the new splendour dies out again, it is not that a star has ceased to exist; but simply that a faint star which had increased greatly in lustre has resumed its original condition. hipparchus's star must have been a remarkable object, for it was visible in full daylight, whence we may infer that it was many times brighter than the blazing dog-star. it is interesting in the history of science, as having led hipparchus to draw up a catalogue of stars, the first on record. some moderns, being sceptical, rejected this story as a fiction; but biot examining chinese chronicles[ ] relating to the times of hipparchus, finds that in b.c. (about nine years before the date of hipparchus's catalogue) a new star was recorded as having appeared in the constellation scorpio. the next new star (that is, stellar conflagration) on record is still more interesting, as there appears some reason for believing that before long we may see another outburst of the same star. in the years , , and , brilliant stars appeared in the region of the heavens between cepheus and cassiopeia. sir j. herschel remarks, that, 'from the imperfect account we have of the places of the two earlier, as compared with that of the last, which was well determined, as well as from the tolerably near coincidence of the intervals of their appearance, we may suspect them, with goodricke, to be one and the same star, with a period of or perhaps of years.' the latter period may very reasonably be rejected, as one can perceive no reason why the intermediate returns of the star to visibility should have been overlooked, the star having appeared in a region which never sets. it is to be noted that, the period from to being years, and that from to only years, the period of this star (if goodricke is correct in supposing the three outbursts to have occurred in the same star) would seem to be diminishing. at any time, then, this star might now blaze out in the region between cassiopeia and cepheus, for more than years have already passed since its last outburst. as the appearance of a new star led hipparchus to undertake the formation of his famous catalogue, so did the appearance of the star in cassiopeia, in , lead the danish astronomer tycho brahe to construct a new and enlarged catalogue. (this, be it remembered, was before the invention of the telescope.) returning one evening (november , , old style) from his laboratory to his dwelling-house, he found, says sir j. herschel, 'a group of country people gazing at a star, which he was sure did not exist an hour before. this was the star in question.' the description of the star and its various changes is more interesting at the present time, when the true nature of these phenomena is understood, than it was even in the time when the star was blazing in the firmament. it will be gathered from that description and from what i shall have to say farther on about the results of recent observations on less splendid new stars, that, if this star should reappear in the next few years, our observers will probably be able to obtain very important information from it. the message from it will be much fuller and more distinct than any we have yet received from such stars, though we have learned quite enough to remain in no sort of doubt as to their general nature. the star remained visible, we learn, about sixteen months, during which time it kept its place in the heavens without the least variation. 'it had all the radiance of the fixed stars, and twinkled like them; and was in all respects like sirius, except that it surpassed sirius in brightness and magnitude.' it appeared larger than jupiter, which was at that time at his brightest, and was scarcely inferior to venus. _it did not acquire this lustre gradually_, but shone forth at once of its full size and brightness, 'as if,' said the chroniclers of the time, 'it had been of instantaneous creation.' for three weeks it shone with full splendour, during which time it could be seen at noonday 'by those who had good eyes, and knew where to look for it.' but before it had been seen a month, it became visibly smaller, and from the middle of december till march , when it entirely disappeared, it continually diminished in magnitude. 'as it decreased in size, it varied in colour: at first its light was white and extremely bright; it then became yellowish; afterwards of a ruddy colour like mars; and finished with a pale livid white resembling the colour of saturn.' all the details of this account should be very carefully noted. it will presently be seen that they are highly characteristic. those who care to look occasionally at the heavens to know whether this star has returned to view may be interested to learn whereabouts it should be looked for. the place may be described as close to the back of the star-gemmed chair in which cassiopeia is supposed to sit--a little to the left of the seat of the chair, supposing the chair to be looked at in its normal position. but as cassiopeia's chair is always inverted when the constellation is most conveniently placed for observation, and indeed as nine-tenths of those who know the constellation suppose the chair's legs to be the back, and _vice versâ_, it may be useful to mention that the star was placed somewhat thus with respect to the straggling w formed by the five chief stars of cassiopeia. there is a star not very far from the place here indicated, but rather nearer to the middle angle of the w. this, however, is not a bright star; and cannot possibly be mistaken for the expected visitant. (the place of tycho's star is indicated in my school star-atlas and also in my larger library atlas. the same remark applies to both the new stars in the serpent-bearer, presently to be described.) [illustration] in august the astronomer fabricius observed a new star in the neck of the whale, which also after a time disappeared. it was not noticed again till the year , when an observer rejoicing in the name of phocyllides holwarda observed it, and, keeping a watch, after it had vanished, upon the place where it had appeared, saw it again come into view nine months after its disappearance. since then it has been known as a variable star with a period of about days hours. when brightest this star is of the second magnitude. it indicates a somewhat singular remissness on the part of the astronomers of former days, that a star shining so conspicuously for a fortnight, once in each period of - / days, should for so many years have remained undetected. it may, perhaps, be thought that, noting this, i should withdraw the objection raised above against sir j. herschel's idea that the star in cassiopeia may return to view once in years, instead of once in years. but there is a great difference between a star which at its brightest shines only as a second-magnitude star, so that it has twenty or thirty companions of equal or greater lustre above the horizon along with it, and a star which surpasses three-fold the splendid sirius. we have seen that even in tycho brahe's day, when probably the stars were not nearly so well known by the community at large, the new star in cassiopeia had not shone an hour before the country people were gazing at it with wonder. besides, cassiopeia and the whale are constellations very different in position. the familiar stars of cassiopeia are visible on every clear night, for they never set. the stars of the whale, at least of the part to which the wonderful variable star belongs, are below the horizon during rather more than half the twenty-four hours; and a new star there would only be noticed, probably (unless of exceeding splendour), if it chanced to appear during that part of the year when the whale is high above the horizon between eventide and midnight, or in the autumn and early winter. it is a noteworthy circumstance about the variable star in the whale, deservedly called mira, or the wonderful, that it does not always return to the same degree of brightness. sometimes it has been a very bright second-magnitude star when at its brightest, at others it has barely exceeded the third magnitude. hevelius relates that during the four years between october and december , mira did not show herself at all! as this star fades out, it changes in colour from white to red. towards the end of september , a new star made its appearance in the constellation ophiuchus, or the serpent-bearer. its place was near the heel of the right foot of 'ophiuchus huge.' kepler tells us that it had no hair or tail, and was certainly not a comet. moreover, like the other fixed stars, it kept its place unchanged, showing unmistakably that it belonged to the star-depths, not to nearer regions. 'it was exactly like one of the stars, except that in the vividness of its lustre, and the quickness of its sparkling, it exceeded anything that he had ever seen before. it was every moment changing into some of the colours of the rainbow, as yellow, orange, purple, and red; though it was generally white when it was at some distance from the vapours of the horizon.' in fact, these changes of colour must not be regarded as indicating aught but the star's superior brightness. every very bright star, when close to the horizon, shows these colours, and so much the more distinctly as the star is the brighter. sirius, which surpasses the brightest stars of the northern hemisphere full four times in lustre, shows these changes of colour so conspicuously that they were regarded as specially characteristic of this star, insomuch that homer speaks of sirius (not by name, but as the 'star of autumn') shining most beautifully 'when laved of ocean's wave'--that is, when close to the horizon. and our own poet, tennyson, following the older poet, sings how the fiery sirius alters hue, and bickers into red and emerald. the new star was brighter than sirius, and was about five degrees lower down, when at its highest above the horizon, than sirius when _he_ culminates. five degrees being equal to nearly ten times the apparent diameter of the moon, it will be seen how much more favourable the conditions were in the case of kepler's star for those coloured scintillations which characterised that orb. sirius never rises very high above the horizon. in fact, at his highest (near midnight in winter, and, of course, near midday in summer) he is about as high above the horizon as the sun at midday in the first week in february. kepler's star's greatest height above the horizon was little more than three-fourths of this, or equal to about the sun's elevation at midday on january or in any year. like tycho brahe's star, kepler's was brighter even than jupiter, and only fell short of venus in splendour. it preserved its lustre for about three weeks, after which time it gradually grew fainter and fainter until some time between october and february , when it disappeared. the exact day is unknown, as during that interval the constellation of the serpent-bearer is above the horizon in the day-time only. but in february , when it again became possible to look for the new star in the night-time, it had vanished. it probably continued to glow with sufficient lustre to have remained visible, but for the veil of light under which the sun concealed it, for about sixteen months altogether. in fact, it seems very closely to have resembled tycho's star, not only in appearance and in the degree of its greatest brightness, but in the duration of its visibility. in the year a new star appeared in the constellation cygnus, attaining the third magnitude. it remained visible, but not with this lustre, for nearly two years. after it had faded almost out of view, it flickered up again for awhile, but soon after it died out, so as to be entirely invisible. whether a powerful telescope would still have shown it is uncertain, but it seems extremely probable. it may be, indeed, that this new star in the swan is the same which has made its appearance within the last few weeks; but on this point the evidence is uncertain. on april , , mr. hind (superintendent of the nautical almanac, and discoverer of ten new members of the solar system) noticed a new star of the fifth magnitude in the serpent-bearer, but in quite another part of that large constellation than had been occupied by kepler's star. a few weeks later, it rose to the fourth magnitude. but afterwards its light diminished until it became invisible to ordinary eyesight. it did not vanish utterly, however. it is still visible with telescopic power, shining as a star of the eleventh magnitude, that is five magnitudes below the faintest star discernible with the unaided eye. this is the first new star which has been kept in view since its apparent creation. but we are now approaching the time when it was found that as so-called new stars continue in existence long after they have disappeared from view, so also they are not in reality new, but were in existence long before they became visible to the naked eye. on may , , shortly before midnight, mr. birmingham, of tuam, noticed a star of the second magnitude in the northern crown, where hitherto no star visible to the naked eye had been known. dr. schmidt, of athens, who had been observing that region of the heavens the same night, was certain that up to p.m., athens local time, there was no star above the fourth magnitude in the place occupied by the new star. so that, if this negative evidence can be implicitly relied on, the new star must have sprung at least from the fourth, and probably from a much lower magnitude, to the second, in less than three hours--eleven o'clock at athens corresponding to about nine o'clock by irish railway time. a mr. barker, of london, canada, put forward a claim to having seen the new star as early as may --a claim not in the least worth investigating, so far as the credit of first seeing the new star is concerned, but exceedingly important in its bearing on the nature of the outburst affecting the star in corona. it is unpleasant to have to throw discredit on any definite assertion of facts; unfortunately, however, mr. barker, when his claim was challenged, laid before mr. stone, of the greenwich observatory, such very definite records of observations made on may , , , and , that we have no choice but either to admit these observations, or to infer that he experienced the delusive effects of a very singular trick of memory. he mentions in his letter to mr. stone that he had sent full particulars of his observations on those early dates to professor watson, of ann arbor university, on may ; but (again unfortunately) instead of leaving that letter to tell its own story in professor watson's hands, he asked professor watson to return it to him: so that when mr. stone very naturally asked professor watson to furnish a copy of this important letter, professor watson had to reply, 'about a month ago, mr. barker applied to me for this letter, and i returned it to him, as requested, without preserving a copy. i can, however,' he proceeded, 'state positively that he did not mention any actual observation earlier than may . he said he thought he had noticed a strange star in the crown about two weeks before the date of his first observation--may --but not particularly, and that he did not recognise it until the th. he did not give any date, and did not even seem positive as to identity.... when i returned the letter of may , i made an endorsement across the first page, in regard to its genuineness, and attached my signature. i regret that i did not preserve a copy of the letter in question; but if the original is produced, it will appear that my recollection of its contents is correct.' i think no one can blame mr. stone, if, on the receipt of this letter, he stated that he had not the 'slightest hesitation' in regarding mr. barker's earlier observations as 'not entitled to the slightest credit.'[ ] it may be fairly taken for granted that the new star leapt very quickly, if not quite suddenly, to its full splendour. birmingham, as we have seen, was the first to notice it, on may . on the evening of may , schmidt of athens discovered it independently, and a few hours later it was noticed by a french engineer named courbebaisse. afterwards, baxendell of manchester, and others independently saw the star. schmidt, examining argelander's charts of , stars (charts which i have had the pleasure of mapping in a single sheet), found that the star was not a new one, but had been set down by argelander as between the ninth and tenth magnitudes. referring to argelander's list, we find that the star had been twice observed--viz., on may , , and on march , . birmingham wrote at once to mr. huggins, who, in conjunction with the late dr. miller, had been for some time engaged in observing stars and other celestial objects with the spectroscope. these two observers at once directed their telescope armed with spectroscopic adjuncts--the telespectroscope is the pleasing name of the compound instrument--to the new-comer. the result was rather startling. it may be well, however, before describing it, to indicate in a few words the meaning of various kinds of spectroscopic evidence. the light of the sun, sifted out by the spectroscope, shows all the colours but not all the tints of the rainbow. it is spread out into a large rainbow-tinted streak, but at various places (a few thousand) along the streak there are missing tints; so that in fact the streak is crossed by a multitude of dark lines. we know that these lines are due to the absorptive action of vapours existing in the atmosphere of the sun, and from the position of the lines we can tell what the vapours are. thus, hydrogen by its absorptive action produces four of the bright lines. the vapour of iron is there, the vapour of sodium, magnesium, and so on. again, we know that these same vapours, which, by their absorptive action, cut off rays of certain tints, emit light of just those tints. in fact, if the glowing mass of the sun could be suddenly extinguished, leaving his atmosphere in its present intensely heated condition, the light of the faint sun which would thus be left us would give (under spectroscopic scrutiny) those very rays which now seem wanting. there would be a spectrum of multitudinous bright lines, instead of a rainbow-tinted spectrum crossed by multitudinous dark lines. it is, indeed, only by contrast that the dark lines appear dark, just as it is only by contrast that the solar spots seem dark. not only the penumbra but the umbra of a sun-spot, not only the umbra but the nucleus, not only the nucleus but the deeper black which seems to lie at the core of the nucleus, shine really with a lustre far exceeding that of the electric light, though by contrast with the rest of the sun's surface the penumbra looks dark, the umbra darker still, the nucleus deep black, and the core of the nucleus jet black. so the dark lines across the solar spectrum mark where certain rays are relatively faint, though in reality intensely lustrous. conceive another change than that just imagined. conceive the sun's globe to remain as at present, but the atmosphere to be excited to many times its present degree of light and splendour: then would all these dark lines become bright, and the rainbow-tinted background would be dull or even quite dark by contrast. this is not a mere fancy. at times, local disturbances take place in the sun which produce just such a change in certain constituents of the sun's atmosphere, causing the hydrogen, for example, to glow with so intense a heat that, instead of its lines appearing dark, they stand out as bright lines. occasionally, too, the magnesium in the solar atmosphere (over certain limited regions only, be it remembered) has been known to behave in this manner. it was so during the intensely hot summer of , insomuch that the italian observer tacchini, who noticed the phenomenon, attributed to such local overheating of the sun's magnesium vapour the remarkable heat from which we then for a time suffered. now, the stars are suns, and the spectrum of a star is simply a miniature of the solar spectrum. of course, there are characteristic differences. one star has more hydrogen, at least more hydrogen at work absorbing its rays, and thus has the hydrogen lines more strongly marked than they are in the solar spectrum. another star shows the lines of various metals more conspicuously, indicating that the glowing vapours of such elements, iron, copper, mercury, tin, and so forth, either hang more densely in the star's atmosphere than in our sun's, or, being cooler, absorb their special tints more effectively. but speaking generally, a stellar spectrum is like the solar spectrum. there is the rainbow-tinted streak, which implies that the source of light is glowing solid, liquid, or highly compressed vaporous matter, and athwart the streak there are the multitudinous dark lines which imply that around the glowing heart of the star there are envelopes of relatively cool vapours. we can understand, then, the meaning of the evidence obtained from the new star in the northern crown. in the first place, the new star showed the rainbow-tinted streak crossed by dark lines, which indicated its sun-like nature. _but, standing out on that rainbow-tinted streak as on a dark background, were four exceedingly bright lines--lines so bright, though fine, that clearly most of the star's light came from the glowing vapours to which these lines belonged._ three of the lines belonged to hydrogen, the fourth was not identified with any known line. let us distinguish between what can certainly be concluded from this remarkable observation, and what can only be inferred with a greater or less degree of probability. it is absolutely certain that when messrs. huggins and miller made their observation (by which time the new star had faded from the second to the third magnitude), enormous masses of hydrogen around the star were glowing with a heat far more intense than that of the star itself within the hydrogen envelope. it is certain that the increase in the star's light, rendering the star visible which before had been far beyond the range of ordinary eyesight, was due to the abnormal heat of the hydrogen surrounding that remote sun. but it is not so clear whether the intense glow of the hydrogen was caused by combustion or by intense heat without combustion. the difference between the two causes of increased light is important; because on the opinion we form on this point must depend our opinion as to the probability that our sun may one day experience a similar catastrophe, and also our opinion as to the state of the sun in the northern crown after the outburst. to illustrate the distinction in question, let us take two familiar cases of the emission of light. a burning coal glows with red light, and so does a piece of iron placed in a coal fire. but the coal and the iron are undergoing very different processes. the coal is burning, and will presently be consumed; the iron is not burning (except in the sense that it is burning hot, which means only that it will make any combustible substance burn which is brought into contact with it), and it will not be consumed though the coal fire be maintained around it for days and weeks and months. so with the hydrogen flames which play at all times over the surface of our own sun. they are not burning like the hydrogen flames which are used for the oxy-hydrogen lantern. were the solar hydrogen so burning, the sun would quickly be extinguished. they are simply aglow with intensity of heat, as a mass of red-hot iron is aglow; and, so long as the sun's energies are maintained, the hydrogen around him will glow in this way without being consumed. as the new fires of the star in the crown died out rapidly, it is possible that in their case there was actual combustion. on the other hand, it is also possible, and perhaps on the whole more probable, that the hydrogen surrounding the star was simply set glowing with increased lustre owing to some cause not as yet ascertained. let us see how these two theories have been actually worded by the students of science themselves who have maintained them. 'the sudden blazing forth of this star,' says mr. huggins, 'and then the rapid fading away of its light, suggest the rather bold speculation that in consequence of some great internal convulsion, a large volume of hydrogen and other gases was evolved from it, the hydrogen, by its combination with some other element,' in other words, by _burning_, 'giving out the light represented by the bright lines, and at the same time heating to the point of vivid incandescence the solid matter of the star's surface.' 'as the liberated hydrogen gas became exhausted' (i now quote not huggins's own words, but words describing his theory in a book which he has edited) 'the flame gradually abated, and, with the consequent cooling, the star's surface became less vivid, and the star returned to its original condition.' on the other hand, the german physicists, meyer and klein, consider the sudden development of hydrogen, in quantities sufficient to explain such an outburst, exceedingly unlikely. they have therefore adopted the opinion, that the sudden blazing out of the star was occasioned by the violent precipitation of some mighty mass, perhaps a planet, upon the globe of that remote sun, 'by which the momentum of the falling mass would be changed into molecular motion, or in other words into heat and light.' it might even be supposed, they urge, that the star in the crown, by its swift motion, may have come in contact with one of the star clouds which exist in large numbers in the realms of space. 'such a collision would necessarily set the star in a blaze and occasion the most vehement ignition of its hydrogen.' fortunately, our sun is safe for many millions of years to come from contact from any one of its planets. the reader must not, however, run away with the idea that the danger consists only in the gradual contraction of planetary orbits sometimes spoken of. that contraction, if it is taking place at all, of which we have not a particle of evidence, would not draw mercury to the sun's surface for at least ten million millions of years. the real danger would be in the effects which the perturbing action of the larger planets might produce on the orbit of mercury. that orbit is even now very eccentric, and must at times become still more so. it might, but for the actual adjustment of the planetary system, become so eccentric that mercury could not keep clear of the sun; and a blow from even small mercury (only weighing, in fact, millions of millions of millions of tons), with a velocity of some miles per second, would warm our sun considerably. but there is no risk of this happening in mercury's case--though the unseen and much more shifty vulcan (in which planet i beg to express here my utter disbelief) might, perchance, work mischief if he really existed. as for star clouds lying in the sun's course, we may feel equally confident. the telescope assures us that there are none immediately on the track, and we know, also, that, swiftly though the sun is carrying us onwards through space,[ ] many millions of years must pass before he is among the star families towards which he is rushing. of the danger from combustion, or from other causes of ignition than those considered by meyer and klein, it still remains to speak. but first, let us consider what new evidence has been thrown upon the subject by the observations made on the star which flamed out last november. the new star was first seen by professor schmidt, who has had the good fortune of announcing to astronomers more than one remarkable phenomenon. it was he who discovered in november that a lunar crater had disappeared, an announcement quite in accordance with the facts of the case. we have seen that he was one of the independent discoverers of the outburst in the northern crown. on november , at the early hour of . in the evening (showing that schmidt takes time by the forelock at his observatory), he noticed a star of the third magnitude in the constellation of the swan, not far from the tail of that southward-flying celestial bird. he is quite sure that on november , the last preceding clear evening, the star was not there. at midnight its light was very yellow, and it was somewhat brighter than the neighbouring star eta pegasi, on the flying horse's southernmost knee (if anatomists will excuse my following the ordinary usage which calls the wrist of the horse's fore-arm the knee). he sent news of the discovery forthwith to leverrier, the chief of the paris observatory; and the observers there set to work to analyse the light of the stranger. unfortunately the star's suddenly acquired brilliancy rapidly faded. m. paul henry estimated the star's brightness on december as equal only to that of a fifth-magnitude star. moreover, the colour, which had been very yellow on november , was by this time 'greenish, almost blue.' on december , m. cornu, observing during a short time when the star was visible through a break between clouds, found that the star's spectrum consisted almost entirely of bright lines. on december , he was able to determine the position of these lines, though still much interrupted by clouds. he found three bright lines of hydrogen, the strong (really double) line of sodium, the (really triple) line of magnesium, and two other lines. one of these last seemed to agree exactly in position with a bright line belonging to the corona seen around the sun during total eclipse.[ ] the star has since faded gradually in lustre until, at present, it is quite invisible to the naked eye. we cannot doubt that the catastrophe which befell this star is of the same general nature as is that which befell the star in the northern crown. it is extremely significant that all the elements which manifested signs of intense heat in the case of the star in the swan, are characteristic of our sun's outer appendages. we know that the coloured flames seen around the sun during total solar eclipse consist of glowing hydrogen, and of glowing matter giving a line so near the sodium line that in the case of a stellar spectrum it would, probably, not be possible to distinguish one from the other. into the prominences there are thrown from time to time masses of glowing sodium, magnesium, and (in less degree) iron and other metallic vapours. lastly, in that glorious appendage, the solar corona, which extends for hundreds of thousands of miles from the sun's surface, there are enormous quantities of some element, whose nature is as yet unknown, showing under spectroscopic analysis the bright line which seems to have appeared in the spectrum of the flaming sun in the swan. this evidence seems to me to suggest that the intense heat which suddenly affected this star had its origin from without. at the same time, i cannot agree with meyer and klein in considering that the cause of the heat was either the downfall of a planetary mass on the star, or the collision of the star with a star-cloudlet, or nebula, traversing space in one direction while the star swept onwards in another. a planet could not very well come into final conflict with its sun at one fell swoop. it would gradually draw nearer and nearer, not by the narrowing of its path, but by the change of the path's shape. the path would, in fact, become more and more eccentric; until, at length, at its point of nearest approach, the planet would graze its primary, exciting an intense heat where it struck, but escaping actual destruction that time. the planet would make another circuit, and again graze its sun, at or near the same part of the planet's path. for several circuits this would continue, the grazes not becoming more effective each time, but rather less. the interval between them, however, would grow continually less and less. at last the time would come when the planet's path would be reduced to the circular form, its globe touching its sun's all the way round, and then the planet would very quickly be reduced to vapour, and partly burned up, its substance being absorbed by its sun. but all the successive grazes would be indicated to us by accessions in the star's lustre, the period between each seeming outburst being only a few months at first, and becoming gradually less and less (during a long course of years, perhaps even of centuries), until the planet was finally destroyed. nothing of this sort has happened in the case of any so-called new star. as for the rush of a star through a nebulous mass, that is a theory which would scarcely be entertained by any one acquainted with the enormous distances separating the gaseous star-clouds properly called nebulæ. there may be small clouds of the same sort scattered much more densely through space; but we have not a particle of evidence that this actually is the case. all we certainly _know_ about star-cloudlets suggest that the distances separating them from each other are comparable with those which separate star from star, in which case the idea of a star coming into collision with a star-cloudlet, and still more the idea of this occurring several times in a century, is wild in the extreme. on the whole, the theory seems more probable than any of these, that enormous flights of large meteoric masses travel around those stars which thus occasionally break forth in conflagration, such flights travelling on exceedingly eccentric paths, and requiring enormously long periods to complete each circuit of their vast orbits. in conceiving this, we are not imagining anything new. such a meteoric flight would differ only in degree not kind from meteoric flights which are known to circle around our own sun. i am not sure, indeed, that it can be definitely asserted that our sun has no meteoric appendages of the same nature as those which, if this theory be true, excite to intense periodic activity the sun round which they circle. we know that comets and meteors are closely connected, every comet being probably (many certainly) attended by flights of meteoric masses. the meteors which produce the celebrated november showers of falling stars follow in the track of a comet invisible to the naked eye. may we not reasonably suppose, then, that those glorious comets which have not only been visible but conspicuous, shining even in the day-time, and brandishing round tails which, like that of the 'wonder in heaven, the great dragon,' seemed to 'draw the third part of the stars of heaven,' are followed by much denser flights of much more massive meteors? now some among these giant comets have paths which carry them very close to our sun. newton's comet, with its tail a hundred millions of miles in length, all but grazed the sun's globe. the comet of , whose tail, says sir j. herschel, 'stretched half-way across the sky,' must actually have grazed the sun, though but lightly, for its nucleus was within , miles of his surface, and its head was more than , miles in diameter. and these are only two among the few comets whose paths are known. at any time we might be visited by a comet mightier than either, travelling on an orbit intersecting the sun's surface, followed by flights of meteoric masses enormous in size and many in number, which, falling on the sun's globe with the enormous velocity corresponding to their vast orbital range and their near approach to the sun--a velocity of some miles per second--would, beyond all doubt, excite his whole frame, and especially his surface regions, to a degree of heat far exceeding what he now emits. we have had evidence of the tremendous heat to which the sun's surface would be excited by the downfall of a shower of large meteoric masses. carrington and hodgson, on september , , observed (independently) the passage of two intensely bright bodies across a small part of the sun's surface--the bodies first increasing in brightness, then diminishing, then fading away. it is generally believed that these were meteoric masses raised to fierce heat by frictional resistance. now so much brighter did they appear, or rather did that part of the sun's surface appear through which they had rushed, that carrington supposed the dark glass screen used to protect the eye had broken, and hodgson described the brightness of this part of the sun as such that the part shone like a brilliant star on the background of the glowing solar surface. mark, also, the consequences of the downfall of those two bodies only. a magnetic disturbance affected the whole frame of the earth at the very time when the sun had been thus disturbed. vivid auroras were seen not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. 'by degrees,' says sir j. herschel, 'accounts began to pour in of great auroras seen not only in these latitudes, but at rome, in the west indies, in the tropics within eighteen degrees of the equator (where they hardly ever appear); nay, what is still more striking, in south america and in australia--where, at melbourne, on the night of september , the greatest aurora ever seen there made its appearance. these auroras were accompanied with unusually great electro-magnetic disturbances in every part of the world. in many places the telegraph wires struck work. they had too many private messages of their own to convey. at washington and philadelphia, in america, the electric signal-men received severe electric shocks. at a station in norway the telegraphic apparatus was set fire to; and at boston, in north america, a flame of fire followed the pen of bain's electric telegraph, which writes down the message upon chemically prepared paper.' seeing that where the two meteors fell the sun's surface glowed thus intensely, and that the effect of this accession of energy upon our earth was thus well marked, can it be doubted that a comet, bearing in its train a flight of many millions of meteoric masses, and falling directly upon the sun, would produce an accession of light and heat whose consequences would be disastrous? when the earth has passed through the richer portions (not the actual nuclei, be it remembered) of meteor systems, the meteors visible from even a single station have been counted by tens of thousands, and it has been computed that millions must have fallen upon the whole earth. these were meteors following in the train of very small comets. if a very large comet followed by no denser a flight of meteors, but each meteoric mass much larger, fell directly upon the sun, it would not be the outskirts but the nucleus of the meteoric train which would impinge upon him. they would number thousands of millions. the velocity of downfall of each mass would be more than miles per second. and they would continue to pour in upon him for several days in succession, millions falling every hour. it seems not improbable that, under this tremendous and long-continued meteoric hail, his whole surface would be caused to glow as intensely as that small part whose brilliancy was so surprising in the observation made by carrington and hodgson. in that case, our sun, seen from some remote star whence ordinarily he is invisible, would shine out as a new sun, for a few days, while all things living on our earth, and whatever other members of the solar system are the abode of life, would inevitably be destroyed. the reader must not suppose that this idea has been suggested merely in the attempt to explain outbursts of stars. the following passage from a paper of considerable scientific interest by professor kirkwood, of bloomington, indiana, a well-known american astronomer, shows that the idea had occurred to him for a very different reason. he speaks here of a probable connection between the comet of and the great sun-spot which appeared in june . i am not sure, however, but that we may regard the very meteors which seem to have fallen on the sun on september , , as bodies travelling in the track of the comet of --just as the november meteors seen in - , , etc., until , were bodies certainly following in the track of the telescopic comet of . 'the opinion has been expressed by more than one astronomer,' he says, speaking of carrington's observation, 'that this phenomenon was produced by the fall of meteoric matter upon the sun's surface. now, the fact may be worthy of note that the comet of actually grazed the sun's atmosphere about three months before the appearance of the great sun-spot of the same year. had it approached but little nearer, the resistance of the atmosphere would probably have brought its entire mass to the solar surface. even at its actual distance it must have produced considerable atmospheric disturbance. but the recent discovery that a number of comets are associated with meteoric matter, travelling in nearly the same orbits, suggests the inquiry whether an enormous meteorite following in the comet's train, and having a somewhat less perihelion distance, may not have been precipitated upon the sun, thus producing the great disturbance observed so shortly after the comet's perihelion passage.' there are those, myself among the number, who consider the periodicity of the solar spots, that tide of spots which flows to its maximum and then ebbs to its minimum in a little more than eleven years, as only explicable on the theory that a small comet having this period, and followed by a meteor train, has a path intersecting the sun's surface. in an article entitled 'the sun a bubble,' which appeared in the 'cornhill magazine' for october , i remarked that from the observed phenomena of sun-spots we might be led to suspect the existence of some as yet undetected comet with a train of exceptionally large meteoric masses, travelling in a period of about eleven years round the sun, and having its place of nearest approach to that orb so close to the solar surface that, when the main flight is passing, the stragglers fall upon the sun's surface. in this case, we could readily understand that, as this small comet unquestionably causes our sun to be variable to some slight degree in brilliancy, in a period of about eleven years, so some much larger comet circling around mira, in a period of about days, may occasion those alternations of brightness which have been described above. it may be noticed in passing, that it is by no means certain that the time when the sun is most spotted is the time when he gives out least light. though at such times his surface is dark where the spots are, yet elsewhere it is probably brighter than usual; at any rate, all the evidence we have tends to show that when the sun is most spotted, his energies are most active. it is then that the coloured flames leap to their greatest height and show their greatest brilliancy, then also that they show the most rapid and remarkable changes of shape. supposing there really is, i will not say danger, but a possibility, that our sun may one day, through the arrival of some very large comet travelling directly towards him, share the fate of the suns whose outbursts i have described above, we might be destroyed unawares, or we might be aware for several weeks of the approach of the destroying comet. suppose, for example, the comet, which might arrive from any part of the heavens, came from out that part of the star-depths which is occupied by the constellation taurus--then, if the arrival were so timed that the comet, which might reach the sun at any time, fell upon him in may or june, we should know nothing of that comet's approach: for it would approach in that part of the heavens which was occupied by the sun, and his splendour would hide as with a veil the destroying enemy. on the other hand, if the comet, arriving from the same region of the heavens, so approached as to fall upon the sun in november or december, we should see it for several weeks. for it would then approach from the part of the heavens high above the southern horizon at midnight. astronomers would be able in a few days after it was discovered to determine its path and predict its downfall upon the sun, precisely as newton calculated the path of _his_ comet and predicted its near approach to the sun. it would be known for weeks then that the event which newton contemplated as likely to cause a tremendous outburst of solar heat, competent to destroy all life upon the surface of our earth, was about to take place; and, doubtless, the minds of many students of science would be exercised during that interval in determining whether newton was right or wrong. for my own part, i have very little doubt that, though the change in the sun's condition in consequence of the direct downfall upon his surface of a very large comet would be but temporary, and in that sense slight--for what are a few weeks in the history of an orb which has already existed during thousands of millions of years?--yet the effect upon the inhabitants of the earth would be by no means slight. i do not think, however, that any students of science would remain, after the catastrophe, to estimate or to record its effects. fortunately, all that we have learned hitherto from the stars favours the belief that, while a catastrophe of this sort may be possible, it is exceedingly unlikely. we may estimate the probabilities precisely in the same way that an insurance company estimates the chance of a railway accident. such a company considers the number of accidents which occur among a given number of railway journeys, and from the smallness of the number of accidents compared with the largeness of the number of journeys estimates the safety of railway travelling. our sun is one among many millions of suns, any one of which (though all but a few thousands are actually invisible) would become visible to the naked eye, if exposed to the same conditions as have affected the suns in flames described in the preceding pages. seeing, then, that during the last two thousand years or thereabouts, only a few instances of the kind, certainly not so many as twenty, have been recorded, while there is reason to believe that some of these relate to the same star which has blazed out more than once, we may fairly consider the chance exceedingly small that during the next two thousand, or even the next twenty thousand years, our sun will be exposed to a catastrophe of the kind. we might arrive at this conclusion independently of any considerations tending to show that our sun belongs to a safe class of system-rulers, and that all, or nearly all, the great solar catastrophes have occurred among suns of a particular class. there are, however, several considerations of the kind which are worth noting. in the first place, we may dismiss as altogether unlikely the visit of a comet from the star-depths to our sun, on a course carrying the comet directly upon the sun's surface. but if, among the comets travelling in regular attendance upon the sun, there be one whose orbit intersects the sun's globe, then that comet must several times ere this have struck the sun, raising him temporarily to a destructive degree of heat. now, such a comet must have a period of enormous length, for the races of animals now existing upon the earth must all have been formed since that comet's last visit--on the assumption, be it remembered, that the fall of a large comet upon the sun, or rather the direct passage of the sun through the meteoric nucleus of a large comet, would excite the sun to destructive heat. if all living creatures on the earth are to be destroyed when some comet belonging to the solar system makes its next return to the sun, that same comet at its last visit must have raised the sun to an equal, or even greater intensity of heat, so that either no such races as at present exist had then come into being, or, if any such existed, they must at that time have been utterly destroyed. we may fairly believe that all comets of the destructive sort have been eliminated. judging from the evidence we have on the subject, the process of the formation of the solar system was one which involved the utilisation of cometic and meteoric matter; and it fortunately so chanced that the comets likely otherwise to have been most mischievous--those, namely, which crossed the track of planets, and still more those whose paths intersected the globe of the sun--were precisely those which would be earliest and most thoroughly used up in this way. secondly, it is noteworthy that all the stars which have blazed out suddenly, except one, have appeared in a particular region of the heavens--the zone of the milky way (all, too, on one half of that zone). the single exception is the star in the northern crown, and that star appeared in a region which i have found to be connected with the milky way by a well-marked stream of stars, not a stream of a few stars scattered here and there, but a stream where thousands of stars are closely aggregated together, though not quite so closely as to form a visible extension of the milky way. in my map of , stars this stream can be quite clearly recognised; but, indeed, the brighter stars scattered along it form a stream recognisable with the naked eye, and have long since been regarded by astronomers as such, forming the stars of the serpent and the crown, or a serpentine streak followed by a loop of stars shaped like a coronet. now the milky way, and the outlying streams of stars connected with it, seem to form a region of the stellar universe where fashioning processes are still at work. as sir w. herschel long since pointed out, we can recognise in various parts of the heavens various stages of development, and chief among the regions where as yet nature's work seems incomplete, is the galactic zone--especially that half of it where the milky way consists of irregular streams and clouds of stellar light. as there is no reason for believing that our sun belongs to this part of the galaxy, but on the contrary good ground for considering that he belongs to the class of insulated stars, few of which have shown signs of irregular variation, while none have ever blazed suddenly out with many hundred times their former lustre, we may fairly infer a very high degree of probability in favour of the belief that, for many ages still to come, the sun will continue steadily to discharge his duties as fire, light, and life of the solar system. vii. _the rings of saturn._ the rings of saturn, always among the most interesting objects of astronomical research, have recently been subjected to close scrutiny under high telescopic powers by mr. trouvelot, of the harvard observatory, cambridge, u.s. the results which he has obtained afford very significant evidence respecting these strange appendages, and even throw some degree of light on the subject of cosmical evolution. the present time, when saturn is the ruling planet of the night, seems favourable for giving a brief account of recent speculations respecting the saturnian ring-system, especially as the observations of mr. trouvelot appear to remove all doubt as to the true nature of the rings, if indeed any doubt could reasonably be entertained after the investigations made by european and american astronomers when the dark inner ring had but recently been recognised. it may be well to give a brief account of the progress of observation from the time when the rings were first discovered. in passing, i may remark that the failure of galileo to ascertain the real shape of these appendages has always seemed to me to afford striking evidence of the importance of careful reasoning upon all observations whose actual significance is not at once apparent. if galileo had been thus careful to analyse his observations of saturn, he could not have failed to ascertain their real meaning. he had seen the planet apparently attended by two large satellites, one on either side, 'as though supporting the aged saturn upon his slow course around the sun.' night after night he had seen these attendants, always similarly placed, one on either side of the planet, and at equal distances from it. then in he had again examined the planet, and lo, the attendants had vanished, 'as though saturn had been at his old tricks, and had devoured his children.' but after a while the attendant orbs had reappeared in their former positions, had seemed slowly to grow larger, until at length they had presented the appearance of two pairs of mighty arms encompassing the planet. if galileo had reasoned upon these changes of appearance, he could not have failed, as it seems to me, to interpret their true meaning. the three forms under which the rings had been seen by him sufficed to indicate the true shape of the appendage. because saturn was seen with two attendants of apparently equal size and always equi-distant from him, it was certain that there must be some appendage surrounding him, and extending to that distance from his globe. because this appendage disappeared, it was certain that it must be thin and flat. because it appeared at another time with a dark space between the arms and the planet, it was certain that the appendage is separated by a wide gap from the body of the planet. so that galileo might have concluded--not doubtfully, but with assured confidence--that the appendage is a thin flat ring nowhere attached to the planet, or, as huyghens said some forty years later, saturn '_annulo cingitur tenui, plano, nusquam cohærente_.' whether such reasoning would have been accepted by the contemporaries of galileo may be doubtful. the generality of men are not content with reasoning which is logically sound, but require evidence which they can easily understand. very likely huyghens' proof from direct observation, though in reality not a whit more complete and far rougher, would have been regarded as the first true proof of the existence of saturn's ring, just as sir w. herschel's observation of one star actually moving round another was regarded as the first true proof of the physical association of certain stars, a fact which michell had proved as completely and far more neatly half a century earlier, by a method, however, which was 'caviare to the general.' however, as matters chanced, the scientific world was not called upon to decide between the merits of a discovery made by direct observation and one effected by means of abstract reasoning. it was not until saturn had been examined with much higher telescopic power than galileo could employ, that the appendage which had so perplexed the florentine astronomer was seen to be a thin flat ring, nowhere touching the planet, and considerably inclined to the plane in which saturn travels. we cannot wonder that the discovery was regarded as a most interesting one. astronomers had heretofore had to deal with solid masses, either known to be spheroidal, like the earth, the sun, the moon, jupiter, and venus, or presumed to be so, like the stars. the comets might be judged to be vaporous masses of various forms; but even these were supposed to surround or to attend upon globe-shaped nuclear masses. here, however, in the case of saturn's ring, was a quoit-shaped body travelling around the sun in continual attendance upon saturn, whose motions, no matter how they varied in velocity or direction, were so closely followed by this strange attendant that the planet remained always centrally poised within the span of its ring-girdle. to appreciate the interest with which this strange phenomenon was regarded, we must remember that as yet the law of gravity had not been recognised. huyghens discovered the ring (or rather perceived its nature) in , but it was not till that newton first entertained the idea that the moon is retained in its orbit about the earth by the attractive energy which causes unsupported bodies to fall earthwards; and he was unable to demonstrate the law of gravity before . now, in a general sense, we can readily understand in these days how a ring around a planet continues to travel along with the planet despite all changes of velocity or direction of motion. for the law of gravity teaches that the same causes which tend to change the direction and velocity of the planet's motion tend in precisely the same degree to change the direction and velocity of the ring's motion. but when huyghens made his discovery it must have appeared a most mysterious circumstance that a ring and planet should be thus constantly associated--that during thousands of years no collision should have occurred whereby the relatively delicate structure of the ring had been destroyed. only six years later a discovery was made by two english observers, william and thomas ball, which enhanced the mystery. observing the northern face of the ring, which was at that time turned earthwards, they perceived a black stripe of considerable breadth dividing the ring into two concentric portions. the discovery did not attract so much attention as it deserved, insomuch that when cassini, ten years later, announced the discovery of a corresponding dark division on the southern surface, none recalled the observation made by the brothers ball. cassini expressed the opinion that the ring is really divided into two, not merely marked by a dark stripe on its southern face. this conclusion would, of course, have been an assured one, had the previous observation of a dark division on the northern face been remembered. with the knowledge which we now possess, indeed, the darkness of the seeming stripe would be sufficient evidence that there must be a real division there between the rings; for we know that no mere darkness of the ring's substance could account for the apparent darkness of the stripe. it has been well remarked by professor tyndall, that if the moon's whole surface could be covered with black velvet, she would yet appear white when seen on the dark background of the sky. and it may be doubted whether a circular strip of black velvet miles wide, placed where we see the dark division between the rings, would appear nearly as dark as that division. since we could only admit the possibility of some substance resembling our darker rocks occupying this position (for we know of nothing to justify the supposition that a substance as dark as lampblack or black velvet could be there), we are manifestly precluded from supposing that the dark space is other than a division between two distinct rings. yet sir w. herschel, in examining the rings of saturn with his powerful telescopes, for a long time favoured the theory that there is no real division. he called it the 'broad black mark,' and argued that it can neither indicate the existence of a zone of hills upon the ring, nor of a vast cavernous groove, for in either case it would present changes of appearance (according to the ring's changes of position) such as he was unable to detect. it was not until the year , eleven years after his observations had commenced, that, perceiving a corresponding broad black mark upon the ring's southern face, herschel expressed a 'suspicion' that the ring is divided into two concentric portions by a circular gap nearly miles in width. he expressed at the same time, very strongly, his belief that this division was the only one in saturn's ring-system. a special interest attached at that time to the question whether the ring is divided or not, for laplace had then recently published the results of his mathematical inquiry into the movements of such a ring as saturn's, and, having _proved_ that a single solid ring of such enormous width could not continue to move around the planet, had expressed the _opinion_ that saturn's ring consists in reality of many concentric rings, each turning, with its own proper rotation rate, around the central planet. it is singular that herschel, who, though not versed in the methods of the higher mathematics, had considerable native power as a mathematician, was unable to perceive the force of laplace's reasoning. indeed, this is one of those cases where clearness of perception rather than profundity of mathematical insight was required. laplace's equations of motion did not express all the relations involved, nor was it possible to judge, from the results he deduced, how far the stability of the saturnian rings depended on the real structure of these appendages. one who was well acquainted with mechanical matters, and sufficiently versed in mathematics to understand how to estimate generally the forces acting upon the ring-system, could have perceived as readily the general conditions of the problem as the most profound mathematician. one may compare the case to the problem of determining whether the action of the moon in causing the tidal wave modifies in any manner the earth's motion of rotation. we know that as a mathematical question this is a very difficult one. the astronomer royal, for example, not long ago dealt with it analytically, and deduced the conclusion that there is no effect on the earth's rotation, presently however, discovering by a lucky chance a term in the result which indicates an effect of that kind. but if we look at the matter in its mechanical aspect, we perceive at once, without any profound mathematical research, that the retardation so hard to detect mathematically must necessarily take place. as sir e. beckett says in his masterly work, _astronomy without mathematics_, 'the conclusion is as evident without mathematics as with them, when once it has been suggested.' so when we consider the case of a wide flat ring surrounding a mighty planet like saturn, we perceive that nothing could possibly save such a ring from destruction if it were really one solid structure. to recognise this the more clearly, let us first notice the dimensions of the planet and rings. we have in saturn a globe about , miles in mean diameter, an equatorial diameter being about , miles, the polar diameter , miles. the attractive force of this mighty mass upon bodies placed on its surface is equal to about one-fifth more than terrestrial gravity if the body is near the pole of saturn, and is almost exactly the same as terrestrial gravity if the body is at the planet's equator. its action on the matter of the ring is, of course, very much less, because of the increased distance, but still a force is exerted on every part of the ring which is comparable with the familiar force of terrestrial gravity. the outer edge of the outer ring lies about , miles from the planet's centre, the inner edge of the inner ring (i speak throughout of the ring-system as known to sir w. herschel and laplace) about , miles from the centre, the breadth of the system of bright rings being about , miles. between the planet's equator and the inner edge of the innermost bright ring there intervenes a space of about , miles. roughly speaking, it may be said that the attraction of the planet on the substance of the ring's inner edge is less than gravity at saturn's equator (or, which is almost exactly the same thing, is less than terrestrial gravity) in about the proportion of to ; or, still more roughly, the inner edge of saturn's inner bright ring is drawn inwards by about half the force of gravity at the earth's surface. the outer edge is drawn towards saturn by a force less than terrestrial gravity in the proportion of about to --say roughly that the force thus exerted by saturn on the matter of the outer edge of the ring-system is equivalent to about one-fifth of the force of gravity at the earth's surface. it is clear, first, that if the ring-system did not rotate, the forces thus acting on the material of the rings would immediately break them into fragments, and, dragging these down to the planet's equator, would leave them scattered in heaps upon that portion of saturn's surface. the ring would in fact be in that case like a mighty arch, each portion of which would be drawn towards saturn's centre by its own weight. this weight would be enormous if bessel's estimate of the mass of the ring-system is correct. he made the mass of the ring rather greater than the mass of the earth--an estimate which i believe to be greatly in excess of the truth. probably the rings do not amount in mass to more than a fourth part of the earth's mass. but even that is enormous, and subjected as is the material of the rings to forces varying from one-half to a fifth of terrestrial gravity, the strains and pressures upon the various parts of the system would exceed thousands of times those which even the strongest material built up into their shape could resist. the system would no more be able to resist such strains and pressures than an arch of iron spanning the atlantic would be able to sustain its own weight against the earth's attraction. it would be necessary then that the ring-system should rotate around the planet. but it is clear that the proper rate of rotation for the outer portion would be very different from the rate suited for the inner portion. in order that the inner portion should travel around saturn entirely relieved of its weight, it should complete a revolution in about seven hours twenty-three minutes. the outer portion, however, should revolve in about thirteen hours fifty-eight minutes, or nearly fourteen hours. thus the inner part should rotate in little more than half the time required by the outer part. the result would necessarily be that the ring-system would be affected by tremendous strains, which it would be quite unable to resist. the existence of the great division would manifestly go far to diminish the strains. it is easily shown that the rate of turning where the division is, would be once in about eleven hours and twenty-five minutes, not differing greatly from the mean between the rotation-periods for the outside and for the inside edges of the system. even then, however, the strains would be hundreds of times greater than the material of the ring could resist. a mass comparable in weight to our earth, compelled to rotate in (say) nine hours when it ought to rotate in eleven or in seven, would be subjected to strains exceeding many times the resistances which the cohesive power of its substance could afford. that would be the condition of the inner ring. and in like manner the outer ring, if it rotated in about twelve hours and three-quarters, would have its outer portions rotating too fast and its inner portions too slowly, because their proper periods would be fourteen hours and eleven hours and a half respectively. nothing but the division of the ring into a number of narrow hoops could possibly save it from destruction through the internal strains and pressures to which its material would be subjected. even this complicated arrangement, however, would not save the ring-system. if we suppose a fine hoop to turn around a central attracting body as the rings of saturn rotate around the planet, it may be shown that unless the hoop is so weighted that its centre of gravity is far from the planet, there will be no stability in the resulting motions; the hoop will before long be made to rotate eccentrically, and eventually be brought into destructive collision with the central planet. it was here that laplace left the problem. nothing could have been more unsatisfactory than his result, though it was accepted for nearly half a century unquestioned. he had shown that a weighted fine hoop may possibly turn around a central attracting mass without destructive changes of position, but he had not proved more than the bare possibility of this, while nothing in the appearance of saturn's rings suggests that any such arrangement exists. again, manifestly a multitude of narrow hoops, so combined as to form a broad flat system of rings, would be constantly in collision _inter se_. besides, each one of them would be subjected to destructive strains. for though a fine uniform hoop set rotating at a proper rate around an attracting mass at its centre would be freed from all strains, the case is very different with a hoop so weighted as to have its centre of gravity greatly displaced. laplace had saved the theoretical stability of the motions of a fine ring at the expense of the ring's power of resisting the strains to which it would be exposed. it seems incredible that such a result (expressed, too, very doubtingly by the distinguished mathematician who had obtained it) should have been accepted so long almost without question. there is nothing in nature in the remotest degree resembling the arrangement imagined by laplace, which indeed appears on _à priori_ grounds impossible. it was not claimed for it that it removed the original difficulties of the problem; and it introduced others fully as serious. so strong, however, is authority in the scientific world that none ventured to express any doubts except sir w. herschel, who simply denied that the two rings were divided into many, as laplace's theory required. as time went on and the signs of many divisions were at times recognised, it was supposed that laplace's reasoning had been justified; and despite the utter impossibility of the arrangement he had suggested, that arrangement was ordinarily described as probably existing. at length, however, a discovery was made which caused the whole question to be reopened. on november , , w. bond, observing the planet with the telescope of the harvard observatory, perceived within the inner bright ring a feeble illumination which he was at a loss to understand. on the next night the faint light was better seen. on the th, tuttle, who was observing with bond, suggested the idea that the light within the inner bright ring was due to a dusky ring inside the system of bright rings. on november , mr. dawes in england perceived this dusky ring, and announced the discovery before the news had reached england that bond had already seen the dark ring. the credit of the discovery is usually shared between bond and dawes, though the usual rule in such matters would assign the discovery to bond alone. it was found that the dark ring had already been seen at rome so far back as , and again by galle at berlin in may . the roman observations were not satisfactory. those by galle, however, were sufficient to have established the fact of the ring's existence; indeed, in galle measured the dark ring. but very little attention was attracted to this interesting discovery, insomuch that when bond and dawes announced their observation of the dark ring in , the news was received by astronomers with all the interest attaching to the detection of before unnoted phenomena. it may be well to notice under what conditions the dark ring was detected in . in september the ring had been turned edgewise towards the sun, and as rather more than seven years are occupied in the apparent gradual opening out of the ring from that edge view to its most open appearance (when the outline of the ring-system is an eclipse whose lesser axis is nearly equal to half the greater), it will be seen that in november the rings were but slightly opened. thus the recognition of the dark ring within the bright system was made under unfavourable conditions. for four preceding years--that is, from the year --the rings had been as little or less opened; and again for several years preceding , though the rings had been more open, the planet had been unfavourably placed for observation in northern latitudes, crossing the meridian at low altitudes. still, in and , when the rings were most open, although the planet was never seen under favourable conditions, the opening of the rings, then nearly at its greatest, made the recognition of the dark ring possible; and we have seen that galle then made the discovery. when bond rediscovered the dark ring, everything promised that before long the appendage would be visible with telescopes far inferior in power to the great harvard refractor. year after year the planet was becoming more favourably placed for observation, while all the time the rings were opening out. accordingly it need not surprise us to learn that in the dark ring was seen with a telescope less than three inches and a half in aperture. even so early as , mr. hartnup, observing the planet with a telescope eight inches and a half in aperture, found that 'the dark ring could not be overlooked for an instant.' but while this increase in the distinctness of the dark ring was to be expected, from the mere fact that the ring was discovered under relatively unfavourable conditions, yet the fact that saturn was thus found to have an appendage of a remarkable character, perfectly obvious even with moderate telescopic power, was manifestly most surprising. the planet had been studied for nearly two centuries with telescopes exceeding in power those with which the dark ring was now perceived. some among these telescopes were not only of great power, but employed by observers of the utmost skill. the elder herschel had for a quarter of a century studied saturn with his great reflectors eighteen inches in aperture, and had at times turned on the planet his monstrous (though not mighty) four-feet mirror. schröter had examined the dark space within the inner bright ring for the special purpose of determining whether the ring-system is really disconnected from the globe. he had used a mirror nineteen inches in aperture, and he had observed that the dark space seen on either side of saturn inside the ring-system not only appeared dark, but actually darker than the surrounding sky. this was presumably (though not quite certainly) an effect of contrast only, the dark space being bounded all round by bright surfaces. if real, the phenomenon signified that whereas the space outside the ring, where the satellites of the planet travel, was occupied by some sort of cosmical dust, the space within the ring-system was, as it were, swept and garnished, as though all the scattered matter which might otherwise have occupied that region had been either attracted to the body of the planet or to the rings.[ ] but manifestly the observation was entirely inconsistent with the supposition that there existed in schröter's time a dark or dusky ring within the bright system. again, the elder struve made the most careful measurement of the whole of the ring-system in , when the system was as well placed for observation as in (or, in other words, as well placed as it can possibly be); but though he used a telescope nine inches and a half in aperture, and though his attention was specially attracted to the inner edge of the inner bright ring (_which seemed to him indistinct_), he did not detect the dark ring. yet we have seen that in , under much less favourable conditions, a less practised observer, using a telescope of less aperture, found that the dark ring could not be overlooked for an instant. it is manifest that all these considerations point to the conclusion that the dark ring is a new formation, or, at the least, that it has changed notably in condition during the present century. i have hitherto only considered the appearance of the dusky ring as seen on either side of the planet's globe within the bright rings. the most remarkable feature of the appendage remains still to be mentioned--the fact, namely, that the bright body of the planet can be seen through this dusky ring. where the dark ring crosses the planet, it appears as a rather dark belt, which might readily be mistaken for a belt upon the planet's surface; for the outline of the planet can be seen through the ring as through a film of smoke or a crape veil. now it is worthy of notice that whereas the dark ring was not detected outside the planet's body until , nor generally recognised by astronomers until , the dark belt across the planet, really caused by the dusky ring, was observed more than a century earlier. in the younger cassini saw it, and perceived that it was not curved enough for a belt really belonging to the planet. hadley again observed that the belt attended the ring as this opened out and closed, or, in other words, that the dark belt belonged to the ring, not to the body of the planet. and in many pictures of saturn's system a dark band is shown along the inner edge of the inner bright ring where it crosses the body of the planet. it seems to me that we have here a most important piece of evidence respecting the rings. it is clear that the inner part of the inner bright ring has for more than a century and a half (how much more we do not know) been partially transparent, and it is probable that within its inner edge there has been all the time a ring of matter; but this ring has only within the last half-century gathered consistency enough to be discernible. it is manifest that the existence of the dark belt shown in the older pictures would have led directly to the detection of the dark ring, had not this appendage been exceedingly faint. thus, while the observation of the dark belt across the planet's face proves the dusky ring to have existed in some form long before it was perceived, the same fact only helps to render us certain that the dark ring has changed notably in condition during the present century. the discovery of this singular appendage, an object unique in the solar system, naturally attracted fresh attention to the question of the stability of the rings. the idea was thrown out by the elder bond that the new ring may be fluid, or even that the whole ring-system may be fluid, and the dark ring simply thinner than the rest. it was thought possible that the ring-system is of the nature of a vast ocean, whose waves are steadily advancing upon the planet's globe. the mathematical investigation of the subject was also resumed by professor benjamin pierce, of harvard, and it was satisfactorily demonstrated that the stability of a system of actual rings of solid matter required so nice an adjustment of so many narrow rings as to render the system far more complex than even laplace had supposed. 'a stable formation can,' he said, 'be nothing other than a very great number of separate narrow rigid rings, each revolving with its proper relative velocity.' as was well remarked by the late professor nichol, 'if this arrangement or anything like it were real, how many new conditions of instability do we introduce. observation tells us that the division between such rings must be extremely narrow, so that the slightest disturbance by external or internal causes would cause one ring to impinge upon another; and we should thus have the seed of perpetual catastrophes.' nor would such a constitution protect the system against dissolution. 'there is no escape from the difficulties, therefore, but through the final rejection of the idea that saturn's rings are rigid or in any sense a solid formation.' the idea that the ring-system may be fluid came naturally next under mathematical scrutiny. strangely enough, the physical objections to the theory of fluidity appear to have been entirely overlooked. before we could accept such a theory, we must admit the existence of elements differing entirely from those with which we are familiar. no fluid known to us could retain the form of the rings of saturn under the conditions to which they are exposed. but the mathematical examination of the subject disposed so thoroughly of the theory that the rings can consist of continuous fluid masses, that we need not now discuss the physical objections to the theory. there remains only the theory that the saturnian ring-system consists of discrete masses analogous to the streams of meteors known to exist in great numbers within the solar system. the masses may be solid or fluid, may be strewn in relatively vacant space, or may be surrounded by vaporous envelopes; but that they are discrete, each free to travel on its own course, seemed as completely demonstrated by pierce's calculations as anything not actually admitting of direct observation could possibly be. the matter was placed beyond dispute by the independent analysis to which clerk maxwell subjected the mathematical problem. it had been selected in as the subject for the adams prize essay at cambridge, and clerk maxwell's essay, which obtained the prize, showed conclusively that only a system of many small bodies, each free to travel upon its course under the varying attractions to which it was subjected by saturn itself, and by the saturnian satellites, could possibly continue to girdle a planet as the rings of saturn girdle him. it is clear that all the peculiarities hitherto observed in the saturnian ring-system are explicable so soon as we regard that system as made up of multitudes of small bodies. varieties of brightness simply indicate various degrees of condensation of these small satellites. thus the outer ring had long been observed to be less bright than the inner. of course it did not seem impossible that the outer ring might be made of different materials; yet there was something bizarre in the supposition that two rings forming the same system were thus different in substance. it would not have been at all noteworthy if different parts of the same ring differed in luminosity--in fact, it was much more remarkable that each zone of the system seemed uniformly bright all round. but that one zone should be of one tint, another of an entirely different tint, was a strange circumstance so long as the only available interpretation seemed to be that one zone was made (throughout) of one substance, the other of another. if this was strange when the difference between the inner and outer bright rings was alone considered, how much stranger did it seem when the multitudinous divisions in the rings were taken into account! why should the ring-system, , miles in width, be thus divided into zones of different material? an arrangement so artificial is quite unlike all that is elsewhere seen among the subjects of the astronomer's researches. but when the rings are regarded as made up of multitudes of small bodies, we can quite readily understand how the nearly circular movements of all of these, at different rates, should result in the formation of rings of aggregation and rings of segregation, appearing at the earth's distance as bright rings and faint rings. the dark ring clearly corresponds in appearance with a ring of thinly scattered satellites. indeed, it seems impossible otherwise to account for the appearance of a dusky belt across the globe of the planet where the dark ring crosses the disc. if the material of the dark ring were some partly transparent solid or fluid substance, the light of the planet received through the dark ring added to the light reflected by the dark ring itself, would be so nearly equivalent to the light received from the rest of the planet's disc, that either no dark belt would be seen, or the darkening would be barely discernible. in some positions a bright belt would be seen, not a dark one. but a ring of scattered satellites would cast as its shadow a multitude of black spots, which would give to the belt in shadow a dark grey aspect. a considerable proportion of these spots would be hidden by the satellites forming the dark ring, and in every case where a spot was wholly or partially hidden by a satellite, the effect (at our distant station where the separate satellites of the dark ring are not discernible) would simply be to reduce _pro tanto_ the darkness of the grey belt of shadow. but certainly more than half the shadows of the satellites would remain in sight; for the darkness of the ring at the time of its discovery showed that the satellites were very sparsely strewn. and these shadows would be sufficient to give to the belt a dusky hue, such as it presented when first discovered.[ ] the observations which have recently been made by mr. trouvelot indicate changes in the ring-system, and especially in the dark ring, which place every other theory save that to which we have thus been led entirely out of the question. it should be noted that mr. trouvelot has employed telescopes of unquestionable excellence and varying in aperture from six inches to twenty-six inches, the latter aperture being that of the great telescope of the washington observatory (the largest refractor in the world). he has noted in the first place that the interior edge of the outer bright ring, which marks the outer limit of the great division, is irregular, but whether the irregularity is permanent or not he does not know. the great division itself is found not to be actually black, but, as was long since noted by captain jacob, of the madras observatory, a very dark brown, as though a few scattered satellites travelled along this relatively vacant zone of the system. mr. trouvelot has further noticed that the shadow of the planet upon the rings, and especially upon the outer ring, changes continually in shape, a circumstance which he attributes to irregularities in the surface of the rings. for my own part, i should be disposed to attribute these changes in the shape of the planet's shadow (noted by other observers also) to rapid changes in the deep cloud-laden atmosphere of the planet. passing on, however, to less doubtful observations, we find that the whole system of rings has presented a clouded and spotted aspect during the last four years. mr. trouvelot specially describes this appearance as observed on the parts of the ring outside the disc, called by astronomers the _ansæ_ (because of their resemblance to handles), and it would seem, therefore, that the spotted and cloudy portions are seen only where the background on which the rings are projected is black. this circumstance clearly suggests that the darkness of these parts is due to the background, or, in other words, that the sky is in reality seen through those parts of the ring-system, just as the darkness of the slate-coloured interior ring is attributed, on the satellite theory, to the background of sky visible through the scattered flight of satellites forming the dark ring. the matter composing the dark ring has been observed by mr. trouvelot to be gathered in places into compact masses, which prevent the light of the planet from being seen through those portions of the dark ring where the matter is thus massed together. it is clear that such peculiarities could not possibly present themselves in the case of a continuous solid or fluid ring-system, whereas they would naturally occur in a ring formed of multitudes of minute bodies travelling freely around the planet. the point next to be mentioned is still more decisive. when the dark ring was carefully examined with powerful telescopes during the ten years following its discovery by bond, at which time it was most favourably placed for observation, it was observed that the outline of the planet could be seen across the entire breadth of the dark ring. all the observations agreed in this respect. it was, indeed, noticed by dawes that outside the planet's disc the dark ring showed varieties of tint, its inner half being darker than its outer portion. lassell, observing the planet under most favourable conditions with his two-feet mirror at malta, could not perceive these varieties of tint, which therefore we may judge to have been either not permanent or very slightly marked. but, as i have said, all observers agreed that the outline of the planet could be seen athwart the entire width of the dark ring. mr. trouvelot, however, has found that during the last four years the planet has not been visible through the whole width of the dark ring, but only through the inner half of the ring's breadth. it appears, then, that either the inner portion is getting continually thinner and thinner--that is, the satellites composing it are becoming continually more sparsely strewn--or that the outer portion is becoming more compact, doubtless by receiving stray satellites from the interior of the inner bright ring. it is clear that in saturn's ring-system, if not in the planet itself, mighty changes are still taking place. it may be that the rings are being so fashioned under the forces to which they are subjected as to be on their way to becoming changed into separate satellites, inner members of that system which at present consists of eight secondary planets. but, whatever may be the end towards which these changes are tending, we see processes of evolution taking place which may be regarded as typifying the more extensive and probably more energetic processes whereby the solar system itself reached its present condition. i ventured more than ten years ago, in the preface to my treatise upon the planet saturn, to suggest the possibility 'that in the variations perceptibly proceeding in the saturnian ring-system a key may one day be found to the law of development under which the solar system has reached its present condition.' this suggestion seems to me strikingly confirmed by the recent discoveries. the planet saturn and its appendages, always interesting to astronomers, are found more than ever worthy of close investigation and scrutiny. we may here, as it were, seize nature in the act, and trace out the actual progress of developments which at present are matters rather of theory than of observation. viii. _comets as portents_ the blazing star, threat'ning the world with famine, plague, and war; to princes death; to kingdoms many curses; to all estates inevitable losses; to herdsmen rot; to ploughmen hapless seasons; to sailors storms; to cities civil treasons. although comets are no longer regarded with superstitious awe as in old times, mystery still clings to them. astronomers can tell what path a comet is travelling upon, and say whence it has come and whither it will go, can even in many cases predict the periodic returns of a comet, can analyse the substance of these strange wanderers, and have recently discovered a singular bond of relationship between comets and those other strange visitants from the celestial depths, the shooting stars. but astronomy has hitherto proved unable to determine the origin of comets, the part they perform in the economy of the universe, their real structure, the causes of the marvellous changes of shape which they undergo as they approach the sun, rush round him, and then retreat. as sir john herschel has remarked: 'no one, hitherto, has been able to assign any single point in which we should be a bit better or worse off, materially speaking, if there were no such thing as a comet. persons, even thinking persons, have busied themselves with conjectures; such as that they may serve for fuel for the sun (into which, however, they never fall), or that they may cause warm summers, which is a mere fancy, or that they may give rise to epidemics, or potato-blights, and so forth.' and though, as he justly says, 'this is all wild talking,' yet it will probably continue until astronomers have been able to master the problems respecting comets which hitherto have foiled their best efforts. the unexplained has ever been and will ever be marvellous to the general mind. just as unexplored regions of the earth have been tenanted in imagination by anthropophagi and men whose heads do grow beneath their shoulders, so do wondrous possibilities exist in the unknown and the ill-understood phenomena of nature. in old times, when the appearance and movements of comets were supposed to be altogether uncontrolled by physical laws, it was natural that comets should be regarded as signs from heaven, tokens of divine wrath towards some, and of the interposition of divine providence in favour of others. as seneca well remarked: 'there is no man so dull, so obtuse, so turned to earthly things, who does not direct all the powers of his mind towards things divine when some novel phenomenon appears in the heavens. while all follows its usual course up yonder, familiarity robs the spectacle of its grandeur. for so is man made. however wonderful may be what he sees day after day, he looks on it with indifference; while matters of very little importance attract and interest him if they depart from the accustomed order. the host of heavenly constellations beneath the vault of heaven, whose beauty they adorn, attract no attention; but if any unusual appearance be noticed among them, at once all eyes are turned heavenwards. the sun is only looked on with interest when he is undergoing eclipse. men observe the moon only under like conditions.... so thoroughly is it a part of our nature to admire the new rather than the great. the same is true of comets. when one of these fiery bodies of unusual form appears, every one is eager to know what it means; men forget other objects to inquire about the new arrival; they know not whether to wonder or to tremble; for many spread fear on all sides, drawing from the phenomenon most grave prognostics.' there is no direct reference to comets in the bible, either in the old testament or the new. it is possible that some of the signs from heaven recorded in the bible pages were either comets or meteors, and that even where in some places an angel or messenger from god is said to have appeared and delivered a message, what really happened was that some remarkable phenomenon in the heavens was interpreted in a particular manner by the priests, and the interpretation afterwards described as the message of an angel. the image of the 'flaming sword which turned every way' may have been derived from a comet; but we can form no safe conclusion about this, any more than we can upon the question whether the 'horror of great darkness' which fell upon abraham (genesis xv. ) when the sun was going down, was caused by an eclipse;[ ] or whether the going back of the shadow upon the dial of ahaz was caused by a mock sun. the star seen by the wise men from the east may have been a comet, since the word translated 'star' signifies any bright object seen in the heavens, and is in fact the same word which homer, in a passage frequently referred to, uses to signify either a comet or a meteor. the way in which it appeared to go before them, when (directed by herod, be it noticed) they went to bethlehem, almost due south of jerusalem, would correspond to a meridian culmination low down--for the star had manifestly not been visible in the earlier evening, since we are told that they rejoiced when they saw the star again. it was probably a comet travelling southwards; and, as the wise men had travelled from the east, it had very likely been first seen in the west as an evening star, wherefore its course was retrograde--that is, supposing it _was_ a comet.[ ] it may possibly have been an apparition of halley's comet, following a course somewhat similar to that which it followed in the year , when the perihelion passage was made on november , and the comet running southwards disappeared from northern astronomers, though in january it was '_received_' by sir j. herschel, to use his own expression, 'in the southern hemisphere.' there was an apparition of halley's comet in the year , or seventy years after the nativity; and the period of the comet varies, according to the perturbing influences affecting the comet's motion, from sixty-nine to eighty years. homer does not, to the best of my recollection, refer anywhere directly to comets. pope, indeed, who made very free with homer's references to the heavenly bodies,[ ] introduces a comet--and a red one, too!--into the simile of the heavenly portent in book iv.:-- as the red comet from saturnius sent to fright the nations with a dire portent (a fatal sign to armies in the plain, or trembling sailors on the wintry main), with sweeping glories glides along in air, and shakes the sparkles from its blazing hair: between two armies thus, in open sight, shot the bright goddess in a trail of light. but homer says nothing of this comet. if homer had introduced a comet, we may be sure it would not have shaken sparkles from its blazing tail. homer said simply that 'pallas rushed from the peaks of heaven, like the bright star sent by the son of crafty-counselled kronus (as a sign either to sailors, or the broad array of the nations), from which many sparks proceed.' strangely enough, pingré and lalande, the former noted for his researches into ancient comets, the latter a skilful astronomer, agree in considering that homer really referred to a comet, and they even regard this comet as an apparition of the comet of . they cite in support of this opinion the portent which followed the prayer of anchises, 'Æneid,' book ii. , etc.: 'scarce had the old man ceased from praying, when a peal of thunder was heard on the left, and a star, gliding from the heavens amid the darkness, rushed through space followed by a long train of light; we saw the star,' says Æneas, 'suspended for a moment above the roof, brighten our home with its fires, then, tracing out a brilliant course, disappear in the forests of ida; then a long train of flame illuminated us, and the place around reeked with the smell of sulphur. overcome by these startling portents, my father arose, invoked the gods, and worshipped the holy star.' it is impossible to recognise here the description of a comet. the noise, the trail of light, the visible motion, the smell of sulphur, all correspond with the fall of a meteorite close by; and doubtless virgil simply introduced into the narrative the circumstances of some such phenomenon which had been witnessed in his own time. to base on such a point the theory that the comet of was visible at the time of the fall of troy, the date of which is unknown, is venturesome in the extreme. true, the period calculated for the comet of , when pingré and lalande agreed in this unhappy guess, was years; and if we multiply this period by five we obtain years, taking from which leaves years b.c., near enough to the supposed date of the capture of troy. unfortunately, encke (the eminent astronomer to whom we owe that determination of the sun's distance which for nearly half a century held its place in our books, but has within the last twenty years been replaced by a distance three millions of miles less) went over afresh the calculations of the motions of that famous comet, and found that, instead of years, the most probable period is about years. the difference amounts only to years; but even this small difference rather impairs the theory of lalande and pingré.[ ] three hundred and seventy-one years before the christian era, a comet appeared which aristotle (who was a boy at the time) has described. diodorus siculus writes thus respecting it: 'in the first year of the d olympiad, alcisthenes being archon of athens, several prodigies announced the approaching humiliation of the lacedæmonians; a blazing torch of extraordinary size, which was compared to a flaming beam, was seen during several nights.' guillemin, from whose interesting work on comets i have translated the above passage, remarks that this same comet was regarded by the ancients as having not merely presaged but produced the earthquakes which caused the towns of helice and bura to be submerged. this was clearly in the thoughts of seneca when he said of this comet that as soon as it appeared it brought about the submergence of bura and helice. in those times, however, comets were not regarded solely as signs of disaster. as the misfortunes of one nation were commonly held to be of advantage to other nations, so the same comet might be regarded very differently by different nations or different rulers. thus the comet of the year b.c. was regarded by timoleon of corinth as presaging the success of his expedition against corinth. 'the gods announced,' said diodorus siculus, 'by a remarkable portent, his success and future greatness; a blazing torch appeared in the heavens at night, and went before the fleet of timoleon until he arrived in sicily.' the comets of the years b.c. and b.c. were not regarded as portents of death, but as signalising, the former the birth, the latter the accession, of mithridates. the comet of b.c. was held by some to be the soul of julius cæsar on its way to the abode of the gods. bodin, a french lawyer of the sixteenth century, regarded this as the usual significance of comets. he was, indeed, sufficiently modest to attribute the opinion to democritus, but the whole credit of the discovery belonged to himself. he maintained that comets only indicate approaching misfortunes because they are the spirits or souls of illustrious men, who for many years have acted the part of guardian angels, and, being at last ready to die, celebrate their last triumph by voyaging to the firmament as flaming stars. 'naturally,' he says, 'the appearance of a comet is followed by plague, pestilence, and civil war; for the nations are deprived of the guidance of their worthy rulers, who, while they were alive, gave all their efforts to prevent intestine disorders.' pingré comments justly on this, saying that 'it must be classed among base and shameful flatteries, not among philosophic opinions.' usually, however, it must be admitted that the ancients, like the men of the middle ages, regarded comets as harbingers of evil. 'a fearful star is the comet,' says pliny, 'and not easily appeased, as appeared in the late civil troubles when octavius was consul; a second time by the intestine war of pompey and cæsar; and, in our own time, when, claudius cæsar having been poisoned, the empire was left to domitian, in whose reign there appeared a blazing comet.' lucan tells us of the second event here referred to, that during the war 'the darkest nights were lit up by unknown stars' (a rather singular way of saying that there were no dark nights); 'the heavens appeared on fire, flaming torches traversed in all directions the depths of space; a comet, that fearful star which overthrows the powers of the earth, showed its horrid hair.' seneca also expressed the opinion that some comets portend mischief: 'some comets,' he said, 'are very cruel and portend the worst misfortunes; they bring with them and leave behind them the seeds of blood and slaughter.' it was held, indeed, by many in those times a subject for reproach that some were too hard of heart to believe when these signs were sent. it was a point of religious faith that 'god worketh' these 'signs and wonders in heaven.' when troubles were about to befall men, 'nation rising against nation, and kingdom against kingdom, with great earthquakes in divers places, and famines, and pestilences, and fearful sights,' then 'great signs shall there be from heaven.' says josephus, commenting on the obstinacy of the jews in such matters, 'when they were at any time premonished from the lips of truth itself, by prodigies and other premonitory signs of their approaching ruin, they had neither eyes nor ears nor understanding to make a right use of them, but passed them over without heeding or so much as thinking of them; as, for example, what shall we say of the comet in the form of a sword that hung over jerusalem for a whole year together?' this was probably the comet described by dion cassius (_hist. roman._ lxv. ) as having been visible between the months of april and december in the year a.d. this or the comet of a.d. might have been halley's comet. the account of josephus as to the time during which it was visible would not apply to halley's, or, indeed, to any known comet whatever; doubtless he exaggerated. he says: 'the comet was of the kind called _xiphias_, because their tail resembles the blade of a sword,' and this would apply fairly well to halley's comet as seen in , , and ; though it is to be remembered that comets vary very much even at successive apparitions, and it would be quite unsafe to judge from the appearance of a comet seen eighteen centuries ago that it either was or was not the same as some comet now known to be periodic. the comet of a.d. is interesting as having given rise to a happy retort from vespasian, whose death the comet was held to portend. seeing some of his courtiers whispering about the comet, 'that hairy star,' he said, 'does not portend evil to me. it menaces rather the king of the parthians. he is a hairy man, but i am bald.' anna comnena goes even beyond josephus. he only rebuked other men for not believing so strongly as he did himself in the significance of comets--a rebuke little needed, indeed, if we can judge from what history tells us of the terrors excited by comets. but the judicious daughter of alexius was good enough to approve of the wisdom which provided these portents. speaking of a remarkable comet which appeared before the irruption of the gauls into the roman empire, she says: 'this happened by the usual administration of providence in such cases; for it is not fit that so great and strange an alteration of things as was brought to pass by that irruption of theirs should be without some previous denunciation and admonishment from heaven.' socrates, the historian (b. , c. ), says that when gainas besieged constantinople, 'so great was the danger which hung over the city, that it was presignified and portended by a huge blazing comet which reached from heaven to the earth, the like whereof no man had ever seen before.' and cedrenus, in his 'compendium of history,' states that a comet appeared before the death of johannes tzimicas, the emperor of the east, which foreshadowed not alone his death, but the great calamities which were to befall the roman empire by reason of their civil wars. in like manner, the comet of announced the death of attila, that of the death of valentinian. the death of merovingius was announced by the comet of , of chilperic by that of , of the emperor maurice by that of , of mahomet by that of , of louis the debonair by that of , and of the emperor louis ii. by that of . nay, so confidently did men believe that comets indicated the approaching death of great men, that they did not believe a very great man _could_ die without a comet. so they inferred that the death of a very great man indicated the arrival of a comet; and if the comet chanced not to be visible, so much the worse--not for the theory, but--for the comet. 'a comet of this kind,' says pingré, 'was that of the year , presaging the death of charlemagne.' so guillemin quotes pingré; but he should rather have said, such was the comet whose arrival was announced by charlemagne's death--and in no other way, for it was not seen by mortal man. the reader who chances to be strong as to his dates may have observed that some of the dates above mentioned for comets do not accord exactly with the dates of the events associated with those comets. thus louis the debonair did not die in , but in . this, however, is a matter of very little importance. if some men, after their comet has called for them, are 'an unconscionable time in dying,' as charles ii. said of himself, it surely must not be considered the fault of the comet. louis himself regarded the comet of as his death-warrant; the astrologers admitted as much: what more could be desired? the account of the matter given in a chronicle of the time, by a writer who called himself 'the astronomer,' is curious enough: 'during the holy season of easter, a phenomenon, ever fatal and of gloomy foreboding, appeared in the heavens. as soon as the emperor, who paid attention to such phenomena, received the first announcement of it, he gave himself no rest until he had called a certain learned man and myself before him. as soon as i arrived, he anxiously asked me what i thought of such a sign. i asked time of him, in order to consider the aspect of the stars, and to discover the truth by their means, promising to acquaint him on the morrow; but the emperor, persuaded that i wished to gain time, which was true, in order not to be obliged to announce anything fatal to him, said to me: "go on the terrace of the palace, and return at once to tell me what you have seen, for i did not see this star last evening, and you did not point it out to me; but i know that it is a comet; tell me what you think it announces to me." then, scarcely allowing me time to say a word, he added: "there is still another thing you keep back: it is that a change of reign and the death of a prince are announced by this sign." and as i advanced the testimony of the prophet, who said: "fear not the signs of the heavens as the nations fear them," the prince, with his grand nature and the wisdom which never forsook him, said: "we must only fear him who has created both us and this star. but, as this phenomenon may refer to us, let us acknowledge it as a warning from heaven."' accordingly, louis himself and all his court fasted and prayed, and he built churches and monasteries. but all was of no avail. in little more than three years he died; showing, as the historian raoul glaber remarked, that 'these phenomena of the universe are never presented to man without surely announcing some wonderful and terrible event.' with a range of three years in advance, and so many kings and princes as there were about in those days, and are still, it would be rather difficult for a comet to appear without announcing some such wonderful and terrible event as a royal death. the year a.d. was by all but common consent regarded as the date assigned for the end of the world. for a thousand years satan had been chained, and now he was to be loosened for a while. so that when a comet made its appearance, and, terrible to relate, continued visible for nine days, the phenomenon was regarded as something more than a nine days' wonder. besides the comet, a very wonderful meteor was seen. 'the heavens opened, and a kind of flaming torch fell upon the earth, leaving behind a long track of light like the path of a flash of lightning. its brightness was so great that it frightened not only those who were in the fields, but even those who were in their houses. as this opening in the sky slowly closed men saw with horror the figure of a dragon, whose feet were blue, and whose head' [like that of dickens's dwarf] 'seemed to grow larger and larger.' a picture of this dreadful meteor accompanies the account given by the old chronicler. for fear the exact likeness of the dragon might not be recognised (and, indeed, to see it one must 'make believe a good deal'), there is placed beside it a picture of a dragon to correspond, which picture is in turn labelled 'serpens cum ceruleis pedibus.' it was considered very wicked in the year to doubt that the end of all things was at hand. but somehow the world escaped that time. in the year halley's comet appeared to announce to the saxons the approaching conquest of england by william the norman. a contemporary poet made a singular remark, which may have some profound poetical meaning, but certainly seems a little indistinct on the surface. he said that 'the comet had been more favourable to william than nature had been to cæsar; the latter had no hair, but william had received some from the comet.' this is the only instance, so far as i know, in which a comet has been regarded as a perruquier. a monk of malmesbury spoke more to the purpose, according to then received ideas, in thus apostrophising the comet: 'here art thou again, cause of tears to many mothers! it is long since i saw thee last, but i see thee now more terrible than ever; thou threatenest my country with complete ruin.' halley's comet, with its inconveniently short period of about seventy-seven years, has repeatedly troubled the nations and been regarded as a sign sent from heaven: ten million cubic miles of head, ten billion leagues of tail, all provided for the sole purpose of warning one petty race of earth-folks against the evils likely to be brought against them by another. this comet has appeared twenty-four times since the date of its first recorded appearance, which some consider to have been b.c., and others refer to a few years later. it may be interesting to quote here babinet's description of the effects ascribed in to this comet, often the terror of nations, but the triumph of mathematicians, as the first whose motions were brought into recognisable obedience to the laws of gravity.[ ] 'the mussulmans, with mahomet ii. at their head, were besieging belgrade, which was defended by huniade, surnamed the exterminator of the turks. halley's comet appeared and the two armies were seized with equal fear. pope calixtus iii., himself seized by the general terror, ordered public prayers and timidly anathematised the comet and the enemies of christianity. he established the prayer called the noon _angelus_, the use of which is continued in all catholic churches. the franciscans (_frères mineurs_) brought , defenders to belgrade, besieged by the conqueror of constantinople, the destroyer of the eastern empire. at last the battle began; it continued two days without ceasing. a contest of two days caused , combatants to bite the dust. the franciscans, unarmed, crucifix in hand, were in the front rank, invoking the papal exorcism against the comet, and turning upon the enemy that heavenly wrath of which none in those times dared doubt.' the great comet of has been regarded as the occasion of the emperor charles v.'s abdication of the imperial throne; a circumstance which seems rendered a little doubtful by the fact that he had already abdicated when the comet appeared--a mere detail, perhaps, but suggesting the possibility that cause and effect may have been interchanged by mistake, and that it was charles's abdication which occasioned the appearance of the comet. according to gemma's account the comet was conspicuous rather from its great light than from the length of its tail or the strangeness of its appearance. 'its head equalled jupiter in brightness, and was equal in diameter to nearly half the apparent diameter of the moon.' it appeared about the end of february, and in march presented a terrible appearance, according to ripamonte. 'terrific indeed,' says sir j. herschel, 'it might well have been to the mind of a prince prepared by the most abject superstition to receive its appearance as a warning of approaching death, and as specially sent, whether in anger or in mercy, to detach his thoughts from earthly things, and fix them on his eternal interests. such was its effect on the emperor charles v., whose abdication is distinctly ascribed by many historians to this cause, and whose words on the occasion of his first beholding it have even been recorded-- "his ergo indiciis me mea fata vocant"-- the language and the metrical form of which exclamation afford no ground for disputing its authenticity, when the habits and education of those times are fairly considered.' it is quite likely that, having already abdicated the throne, charles regarded the comet as signalling his retirement from power--an event which he doubtless considered a great deal too important to be left without some celestial record. but the words attributed to him are in all probability apocryphal. the comet of was remarkable for the strangeness of its aspect, which in some respects resembled that of the comet of , called donati's. it required only the terror with which such portentous objects were witnessed in the middle ages to transform the various streamers, curved and straight, extending from such an object, into swords and spears, and other signs of war and trouble. doubtless, we owe to the fears of the middle ages the strange pictures claiming to present the actual aspect of some of the larger comets. halley's comet did not escape. it was compared to a straight sword at one visit, to a curved scimitar in , and even at its last return in there were some who recognised in the comet a resemblance to a misty head. other comets have been compared to swords of fire, bloody crosses, flaming daggers, spears, serpents, fiery dragons, fish, and so forth. but in this respect no comet would seem to have been comparable with that of , of which andrew paré writes as follows: 'this comet was so horrible and dreadful, and engendered such terror in the minds of men, that they died, some from fear alone, others from illness engendered by fear. it was of immense length and blood-red colour; at its head was seen the figure of a curved arm, holding a large sword in the hand as if preparing to strike. at the point of this sword were three stars; and on either side a number of axes, knives, and swords covered with blood, amongst which were many hideous human faces with bristling beards and hair.' such peculiarities of shape, and also those affecting the position and movements of comets, were held to be full of meaning. as bayle pointed out in his 'thoughts about the comet of ,' these fancies are of great antiquity. pliny tells us that in his time astrologers claimed to interpret the meaning of a comet's position and appearance, and that also of the direction towards which its rays pointed. they could, moreover, explain the effects produced by the fixed stars whose rays were conjoined with the comet's. if a comet resembles a flute, then musicians are aimed at; when comets are in the less dignified parts of the constellations, they presage evil to immodest persons; if the head of a comet forms an equilateral triangle or a square with fixed stars, then it is time for mathematicians and men of science to tremble. when they are in the sign of the ram, they portend great wars and widespread mortality, the abasement of the great and the elevation of the small, besides fearful droughts in regions over which that sign predominates; in the virgin, they imply many grievous ills to the female portion of the population; in the scorpion, they portend a plague of reptiles, especially locusts; in the fishes, they indicate great troubles from religious differences, besides war and pestilence. when, like the one described by milton, they 'fire the length of ophiuchus huge,' they show that there will be much mortality caused by poisoning. the comet of , which led bayle to write the treatise to which reference has just been made, was one well calculated to inspire terror. indeed, if the truth were known, that comet probably brought greater danger to the inhabitants of the earth than any other except the comet of --the danger not, however, being that derived from possible collision between the earth and a comet, but that arising from the possible downfall of a large comet upon the sun, and the consequent enormous increase of the sun's heat. that, according to newton, is the great danger men have to fear from comets; and the comet of was one which in that sense was a very dangerous one. there is no reason why a comet from outer space should not fall straight towards the sun, as at one time the comet of was supposed to be doing. all the comfort that science can give the world on that point is that such a course for a comet is only one out of many millions of possible courses, all fully as likely; and that, therefore, the chance of a comet falling upon the sun is only as one in many millions. still, the comet of made a very fair shot at the sun, and a very slight modification of its course by jupiter or saturn might have brought about the catastrophe which newton feared. whether, if a comet actually fell upon the sun, anything very dreadful would happen, is not so clear. newton's ideas respecting comets were formed in ignorance of many physical facts and laws which in our day render reasoning upon the subject comparatively easy. yet, even in our time, it is not possible to assert confidently that such fears are idle. during the solar outburst witnessed by carrington and hodgson in september , it is supposed that the sun swallowed a large meteoric mass; and, as great cornets are probably followed by many such masses, it seems reasonable to infer that if such a comet fell upon the sun, his surface being pelted with such exceptionally large masses, stoned with these mighty meteoric balls, would glow all over (or nearly so) as brightly as a small spot of that surface glowed upon that occasion. now that portion was so bright that carrington thought 'that by some chance a ray of light had penetrated a hole in the screen attached to the object-glass by which the general image is thrown in shade, for the brilliancy was fully equal to that of direct sunlight.' manifestly, if the whole surface of the sun, or any large portion of the surface, were caused to glow with that exceeding brilliancy, surpassing ordinary sunlight in the same degree that ordinary sunlight surpassed the shaded solar image in carrington's observations, the result would be disastrous in the extreme for the inhabitants of that half of the earth which chanced to be in sunlight at the time; and if (as could scarcely fail to happen) the duration of that abnormal splendour were more than half a day, then the whole earth would probably be depopulated by the intense heat. the danger, as i have said, is slight--partly because there is small chance of a collision between the sun and a comet, partly because we have no certain reasons for assuming that a collision would be followed by the heating of the sun for a while to a very high temperature. looking around at the suns which people space, and considering their history, so far as it has been made known to us, for the last two thousand years, we find small occasion for fear. those suns seem to have been for the most part safe from any sudden or rapid accessions of heat; and if they travel thus safely in their mighty journeys through space, we may well believe that our sun also is safe. nevertheless, there _have_ been catastrophes here and there. now one sun and now another has blazed out with a hundred times its usual lustre, gradually losing its new fires and returning to its customary brightness; but after what destruction among those peopling its system of worlds who shall say? spectroscopic analysis, that powerful help to the modern astronomical inquirer, has shown in one of these cases that just such changes had taken place as we might fairly expect would follow if a mighty comet fell into the sun. if this interpretation be correct, then we are not wholly safe. any day might bring us news of a comet sailing full upon our sun from out the depths of space. then astronomers would perhaps have the opportunity of ascertaining the harmlessness of a collision between the ruler of our system and one of the long-tailed visitors from the celestial spaces. or possibly, astronomers and the earth's inhabitants generally might find out the reverse, though the knowledge would not avail them much, seeing that the messenger who would bring it would be the king of terrors himself. it was well, perhaps, that newton's discovery of the law of gravitation, and the application of this law to the comets of and (the latter our old friend halley's comet, then properly so called as studied by him), came in time to aid in removing to some slight degree the old superstitions respecting comets. for in england many remembered the comets of the great plague and of the great fire of london. these comets came so closely upon the time of the plague and the fire respectively, that it was not wonderful if even the wiser sort were struck by the coincidence and could scarcely regard it as accidental. it is not easy for the student of science in our own times, when the movements of comets are as well understood as those of the most orderly planets, to place himself in the position of men in the times when no one knew on what paths comets came, or whither they retreated after they had visited our sun. taught as men were, on the one hand, that it was wicked to question what seemed to be the teaching of the scriptures, that changes or new appearances in the heavens were sent to warn mankind of approaching troubles, and perplexed as they were, on the other, by the absence of any real knowledge respecting comets and meteors, it was not so easy as we might imagine from our own way of viewing these matters, to shake off a superstition which had ruled over men's minds for thousands of years. no sect had been free from this superstition. popes and priests had taught their followers to pray against the evil influences of comets and other celestial portents; luther and melanchthon had condemned in no measured terms the rashness and impiety of those who had striven to show that the heavenly bodies and the earth move in concordance with law--those 'fools who wish to reverse the entire science of astronomy.' a long interval had elapsed between the time when the copernican theory was struggling for existence--when, but that more serious heresies engaged men's attention and kept religious folk by the ears, that astronomical heresy would probably have been quenched in blood--and the forging by newton of the final link of the chain of reasoning on which modern astronomy is based; but in those times the minds of men moved more slowly than in ours. the masses still held to the old beliefs about the heavenly bodies. defoe, indeed, speaking of the terror of men at the time of the great plague, says that they 'were more addicted to prophecies and astrological conjurations, dreams, and old wives' tales, than ever they were before or since.' but in reality, it was only because of the great misery then prevailing that men seemed more superstitious than usual; for misery brings out the superstitions--the fetishisms, if we may so speak--which are inherent in many minds, but concealed from others in prosperous times, out of shame, or perhaps a worthier feeling. even in our own times great national calamities would show that many superstitions exist which had been thought extinct, and we should see excited among the ill-educated that particular form of persecution which arises, not from zeal for religion and not from intolerance, but from the belief that the troubles have been sent because of unbelief and the fear that unless some expiation be made the evil will not pass away from the midst of the people. it is at such times of general affliction that minds of the meaner sort have proved 'zealous even to slaying.' the influence of strange appearances in the heavens on even thoughtful and reasoning minds, at such times of universal calamity, is well shown by defoe's remarks on the comets of the years and . 'the old women,' he says, 'and the phlegmatic, hypochondriacal part of the other sex, whom i could almost call old women too, remarked that those two comets passed directly over the city' [though that appearance must have depended on the position whence these old women, male and female, observed the comet], 'and that so very near the houses, that it was plain they imported something peculiar to the city alone; and that the comet before the pestilence was of a faint, dull, languid colour, and its motion very heavy, solemn, and slow; but that the comet before the fire was bright and sparkling, or, as others said, flaming, and its motion swift and furious: and that accordingly one foretold a heavy judgment, slow but severe, terrible and frightful, as was the plague; but the other foretold a stroke, sudden, swift, and fiery, as was the conflagration. nay, so particular some people were, that, as they looked upon that comet preceding the fire, they fancied that they not only saw it pass swiftly and fiercely, and could perceive the motion with their eye, but even that they heard it; that it made a mighty rushing noise, fierce and terrible, though at a distance and but just perceivable. i saw both these stars, and must confess had i had so much the common notion of such things in my head, that i was apt to look upon them as the forerunners and warnings of god's judgments, and especially when, the plague having followed the first, i yet saw another of the same kind, i could not but say, god had not yet sufficiently scourged the city' [london]. the comets of and , though they did not bring plagues or conflagrations immediately, yet were not supposed to have been altogether without influence. the convenient fiction, indeed, that some comets operate quickly and others slowly, made it very difficult for a comet to appear to which some evil effects could not be ascribed. if any one can find a single date, since the records of history have been carefully kept, which was so fortunately placed that, during no time following it within five years, no prince, king, emperor, or pope died, no war was begun, or ended disastrously for one side or the other engaged in it, no revolution was effected, neither plague nor pestilence occurred, neither droughts nor floods afflicted any nation, no great hurricanes, earthquakes, volcanic outbursts, or other trouble was recorded, he will then have shown the bare possibility that a comet might have appeared which seemed to presage neither abrupt nor slow-moving calamities. but it is not possible to name such a date, nor even a date which was not followed within two years at the utmost by a calamity such as superstition might assign to a comet. and so closely have such calamities usually followed, that scarce a comet could appear which might not be regarded as the precursor of very quickly approaching calamity. even if a comet had come which seemed to bring no trouble, nay, if many such comets had come, men would still have overlooked the absence of any apparent fulfilment of the predicted troubles. henry iv. well remarked, when he was told that astrologers predicted his death because a certain comet had been observed: 'one of these days they will predict it truly, and people will remember better the single occasion when the prediction will be fulfilled than the many other occasions when it has been falsified by the event.' the troubles connected with the comets of and were removed farther from the dates of the events themselves than usual, at least so far as the english interpretation of the comets was concerned. 'the great comet in ,' says one, 'followed by a lesser comet in , was evidently the forerunner of all those remarkable and disastrous events that ended in the revolution of . it also evidently presaged the revocation of the edict of nantes, and the cruel persecution of the protestants, by the french king louis xiv., afterwards followed by those terrible wars which, with little intermission, continued to ravage the finest parts of europe for nearly twenty-four years.' if in some respects the fears inspired by comets have been reduced by modern scientific discoveries respecting these bodies, yet in other respects the very confidence engendered by the exactness of modern astronomical computations has proved a source of terror. there is nothing more remarkable, for instance, in the whole history of cometary superstition, than the panic which spread over france in the year , in consequence of a rumour that the mathematician lalande had predicted the occurrence of a collision between a comet and the earth, and that disastrous effects would inevitably follow. the foundation of the rumour was slight enough in all conscience. it had simply been announced that lalande would read before the academy of sciences a paper entitled 'reflections on those comets which can approach the earth.' that was absolutely all; yet, from that one fact, not only were vague rumours of approaching cometic troubles spread abroad, but the statement was definitely made that on may or , , 'a comet would encounter the earth.'[ ] so great was the fear thus excited, that, in order to calm it, lalande inserted in the 'gazette de france' of may , , the following advertisement:--'m. lalande had not time to read his memoir upon comets which may approach the earth and cause changes in her motions; but he would observe that it is impossible to assign the epochs of such events. the next comet whose return is expected is the one which should return in eighteen years; but it is not one of those which can hurt the earth.' this note had not the slightest effect in restoring peace to the minds of unscientific frenchmen. m. lalande's study was crowded with anxious persons who came to inquire about his memoir. certain devout folk, 'as ignorant as they were imbecile,' says a contemporary journal, begged the archbishop of paris to appoint forty hours' prayer to avert the danger and prevent the terrible deluge. for this was the particular form most men agreed that the danger would take. that prelate was on the point, indeed, of complying with their request, and would have done so, but that some members of the academy explained to him that by so doing he would excite ridicule. far more effective, and, to say truth, far better judged, was the irony of voltaire, in his deservedly celebrated 'letter on the pretended comet.' it ran as follows:-- 'grenoble, may , . 'certain parisians who are not philosophers, and who, if we are to believe them, will not have time to become such, have informed me that the end of the world approaches, and will occur without fail on the th of this present month of may. they expect, that day, a comet, which is to take our little globe from behind and reduce it to impalpable powder, according to a certain prediction of the academy of sciences which has not yet been made. 'nothing is more likely than this event; for james bernouilli, in his "treatise upon the comet" of , predicted expressly that the famous comet of would return with terrible uproar (_fracas_) on may , ; he assured us that in truth its perruque would signify nothing mischievous, but that its tail would be an infallible sign of the wrath of heaven. if james bernouilli mistook, it is, after all, but a matter of fifty-four years and three days. 'now, so small an error as this being regarded by all geometricians as of little moment in the immensity of ages, it is manifest that nothing can be more reasonable than to hope (_sic, espérer_) for the end of the world on the th of this present month of may , or in some other year. if the thing should not come to pass, "omittance is no quittance" (_ce qui est différé, n'est pas perdu_). 'there is certainly no reason for laughing at m. trissotin, triple idiot though he is (_tout trissotin qu'il est_), when he says to madame philaminte (molière's "femmes savantes," acte iv. scène ), 'nous l'avons en dormant, madame, échappé belle; un monde près de nous a passé tout du long, est chu tout au travers de notre tourbillon; et, s'il eût en chemin rencontré notre terre, elle eût été brisée en morceaux comme verre. 'a comet coursing along its parabolic orbit may come full tilt against our earth. but then, what will happen? either that comet will have a force equal to that of our earth, or greater, or less. if equal, we shall do the comet as much harm as it will do us, action and reaction being equal; if greater, the comet will bear us away with it; if less, we shall bear away the comet. 'this great event may occur in a thousand ways, and no one can affirm that our earth and the other planets have not experienced more than one revolution, through the mischance of encountering a comet on their path. 'the parisians will not desert their city on the th inst.; they will sing songs, and the play of "the comet and the world's end" will be performed at the opéra comique.' the last touch is as fine in its way as sydney smith's remark that, if london were destroyed by an earthquake, the surviving citizens would celebrate the event by a public dinner among the ruins. voltaire's prediction was not fulfilled exactly to the letter, but what actually happened was even funnier than what his lively imagination had suggested. it was stated by a parisian professor in (as a reason why the academy of sciences should refute an assertion then rife to the effect that biela's comet would encounter the earth that year) that during the cometic panic of 'there were not wanting people who knew too well the art of turning to their advantage the alarm inspired by the approaching comet, and _places in paradise were sold at a very high rate_.[ ] the announcement of the comet of may produce similar effects,' he said, 'unless the authority of the academy apply a prompt remedy; and this salutary intervention is at this moment implored by many benevolent persons.' in recent years the effects produced on the minds of men by comets have been less marked than of yore, and appear to have depended a good deal on circumstances. the comet of the year (called donati's), for example, occasioned no special fears, at least until napoleon iii. made his famous new-year's day speech, after which many began to think the comet had meant mischief. but the comet of , though less conspicuous, occasioned more serious fears. it was held by many in italy to presage a very great misfortune indeed, viz. the restoration of francis ii. to the throne of the two sicilies. others thought that the downfall of the temporal power of the papacy and the death of pope pius ix. were signified. i have not heard that any very serious consequences were expected to follow the appearance of coggia's comet in . the great heat which prevailed during parts of the summer of was held by many to be connected in some way with a comet which some very unskilful telescopist constructed in his imagination out of the glare of jupiter in the object-glass of his telescope. another benighted person, seeing the pleiades low down through a fog, turned them into a comet, about the same time. possibly the idea was, that since comets are supposed to cause great heats, great heats may be supposed to indicate a comet somewhere; and with minds thus prepared, it was not wonderful, perhaps, that telescopic glare, or an imperfect view of our old friends the pleiades, should have been mistaken for a vision of the heat-producing comet. it should be a noteworthy circumstance to those who still continue to look on comets as signs of great catastrophes, that a war more remarkable in many respects than any which has ever yet been waged between two great nations--a war swift in its operations and decisive in its effects--a war in which three armies, each larger than all the forces commanded by napoleon i. during the campaign of , were captured bodily--should have been begun and carried on to its termination without the appearance of any great comet. the civil war in america, a still more terrible calamity to that great nation than the success of moltke's operations to the french, may be regarded by believers as presignified by the great comet of . but it so chances that the war between france and germany occurred near the middle of one of the longest intervals recorded in astronomical annals as unmarked by a single conspicuous comet--the interval between the years and . if the progress of just ideas respecting comets has been slow, it must nevertheless be regarded as on the whole satisfactory. when we remember that it was not a mere idle fancy which had to be opposed, not mere terrors which had to be calmed, but that the idea of the significance of changes in the heavens had come to be regarded by mankind as a part of their religion, it cannot but be thought a hopeful sign that all reasoning men in our time have abandoned the idea that comets are sent to warn the inhabitants of this small earth. obeying in their movements the same law of gravitation which guides the planets in their courses, the comets are tracked by the skilful mathematician along those remote parts of their course where even the telescope fails to keep them in view. not only are they no longer regarded as presaging the fortunes of men on this earth, but men on this earth are able to predict the fortunes of comets. not only is it seen that they cannot influence the fates of the earth or other planets, but we perceive that the earth and planets by their attractive energies influence, and in no unimportant degree, the fates of these visitants from outer space. encouraging, truly, is the lesson taught us by the success of earnest study and careful inquiry in determining some at least among the laws which govern bodies once thought the wildest and most erratic creatures in the whole of god's universe. ix. _the lunar hoax._ then he gave them an account of the famous moon hoax, which came out in . it was full of the most barefaced absurdities, yet people swallowed it all; and even arago is said to have treated it seriously as a thing that could not well be true, for mr. herschel would have certainly notified him of these marvellous discoveries. the writer of it had not troubled himself to invent probabilities, but had borrowed his scenery from the 'arabian nights' and his lunar inhabitants from 'peter wilkins.'--oliver wendell holmes (in _the poet at the breakfast-table_). in one of the earliest numbers of 'macmillan's magazine, the late professor de morgan, in an article on scientific hoaxing, gave a brief account of the so-called 'lunar hoax'--an instance of scientific trickery frequently mentioned, though probably few are familiar with the real facts. de morgan himself possessed a copy of the second english edition of the pamphlet, published in london in . but the original pamphlet edition, published in america in september , is not easily to be obtained. the proprietors of the new york 'sun,' in which the fictitious narrative first appeared, published an edition of , copies, and every copy was sold in less than a month. lately a single copy of that edition was sold for three dollars seventy-five cents.[ ] the pamphlet is interesting in many respects, and i propose to give here a brief account of it. but first it may be well to describe briefly the origin of the hoax. it is said that after the french revolution of nicollet, a french astronomer of some repute, especially for certain lunar observations of a very delicate and difficult kind, left france in debt and also in bad odour with the republican party. according to this story, arago the astronomer was especially obnoxious to nicollet, and it was as much with the view of revenging himself on his foe as from a wish to raise a little money that nicollet wrote the moon-fable. it is said further that arago was entrapped, as nicollet desired, and circulated all over paris the wonders related in the pamphlet, until nicollet wrote to his friend bouvard explaining the trick. so runs the story, but the story cannot be altogether true. nicollet may have prepared the narrative and partly written it, but there are passages in the pamphlet as published in america which no astronomer could have written. possibly there is some truth in de morgan's supposition that the original work was french. this may have been nicollet's: and the american edition was probably enlarged by the translator, who, according to this account, was richard alton locke,[ ] to whom in america the whole credit, or discredit, of the hoax is commonly attributed. there can be no doubt that either the french version was much more carefully designed than the american, or there was no truth in the story that arago was deceived by the narrative; for in its present form the story, though clever, could not for an instant have deceived any one acquainted with the most elementary laws of optics. the whole story turns on optical rather than on astronomical considerations; but every astronomer of the least skill is acquainted with the principles on which the construction of optical instruments depends. though the success of the deception recently practised on m. chasles by the forger of the pascal papers has been regarded as showing how easily mathematicians may be entrapped, yet even m. chasles would not have been deceived by bad mathematics; and arago, a master of the science of optics, could not but have detected optical blunders which would be glaring to the average cambridge undergraduate. but let us turn to the story itself. the account opens with a passage unmistakably from an american hand, though purporting, be it remembered, to be quoted from the 'supplement to the edinburgh journal of science.' 'in this unusual addition to our journal, we have the happiness of making known to the british public, and thence to the whole civilised world, recent discoveries in astronomy which will build an imperishable monument to the age in which we live, and confer upon the present generation of the human race a proud distinction through all future time. it has been poetically said' [where and by whom?] 'that the stars of heaven are the hereditary regalia of man, as the intellectual sovereign of the animal creation. he may now fold the zodiac around him with a loftier consciousness of his mental supremacy.' to the american mind enwrapment in the star-jewelled zodiac may appear as natural as their ordinary oratorical references to the star-spangled banner; but the idea is essentially transatlantic, and not even the most poetical european astronomer could have risen to such a height of imagery. passing over several pages of introductory matter, we come to the description of the method by which a telescope of sufficient magnifying power to show living creatures in the moon was constructed by sir john herschel. it had occurred, it would seem, to the elder herschel to construct an improved series of parabolic and spherical reflectors 'uniting all the meritorious points in the gregorian and newtonian instruments, with the highly interesting achromatic discovery of dolland'(_sic_). [this is much as though one should say that a clever engineer had conceived the idea of constructing an improved series of railway engines, combining all the meritorious points in stationary and locomotive engines, with _isaac_ watts' highly ingenious discovery of screw propulsion. for the gregorian and newtonian instruments simply differ in sending the rays received from the great mirror in different directions, and dolland's discovery relates to the ordinary forms of telescopes with large lens, not with large mirror.] however, accumulating infirmities and eventually death prevented sir william herschel from applying his plan, which 'evinced the most profound research in optical science, and the most dexterous ingenuity in mechanical contrivance. but his son, sir john herschel, nursed and cradled in the observatory, and a practical astronomer from his boyhood, determined upon testing it at whatever cost. within two years of his father's death he completed his new apparatus, and adapted it to the old telescope with nearly perfect success.' a short account of the observations made with this instrument, now magnifying six thousand times, follows, in which most of the astronomical statements are very correctly and justly worded, being, in fact, borrowed from a paper by sir w. herschel on observation of the moon with precisely that power. but this great improvement upon all former telescopes still left the observer at a distance of forty miles from the moon; and at that distance no object less than about twenty yards in diameter could be distinguished, and even objects of that size 'would appear only as feeble, shapeless points.' sir john 'had the satisfaction to know that if he could leap astride a cannon-ball, and travel upon its wings of fury for the respectable period of several millions of years, he would not obtain a more enlarged view of the more distant stars than he could now possess in a few minutes of time; and that it would require an ultra-railroad speed of fifty miles an hour for nearly the livelong year, to secure him a more favourable inspection of the gentle luminary of the night;' but 'the exciting question whether this "observed" of all the sons of men, from the days of eden to those of edinburgh, be inhabited by beings, like ourselves, of consciousness and curiosity, was left to the benevolent index of natural analogy, or to the severe tradition that the moon is tenanted only by the hoary _solitaire_, whom the criminal code of the nursery had banished thither for collecting fuel on the sabbath-day.'[ ] but the time had arrived when the great discovery was to be made, by which at length the moon could be brought near enough, by telescopic power, for living creatures on her surface to be seen if any exist. the account of the sudden discovery of the new method, during a conversation between sir john herschel and sir david brewster, is one of the most cleverly conceived (though also one of the absurdest) passages in the pamphlet. 'about three years ago, in the course of a conversational discussion with sir david brewster upon the merits of some ingenious suggestions by the latter, in his article on optics in the "edinburgh encyclopædia," p. , for improvements in newtonian reflectors, sir john herschel adverted to the convenient simplicity of the old astronomical telescopes that were without tubes, and the object-glass of which, placed upon a high pole, threw the focal image to a distance of and even feet. dr. brewster readily admitted that a tube was not necessary, provided the focal image were conveyed into a dark apartment and there properly received by reflectors.... the conversation then became directed to that all-invincible enemy, the paucity of light in powerful magnifiers. after a few moments' silent thought, sir john diffidently enquired whether it would not be possible to effect _a transfusion of artificial light through the focal object of vision_! sir david, somewhat startled at the originality of the idea, paused awhile, and then hesitatingly referred to the refrangibility of rays, and the angle of incidence. sir john, grown more confident, adduced the example of the newtonian reflector, in which the refrangibility was corrected by the second speculum, and the angle of incidence restored by the third.' all this part of the narrative is simply splendid in absurdity. hesitating references to refrangibility and the angle of incidence would have been sheerly idiotic under the supposed circumstances; and in the newtonian reflector (which has only two specula or mirrors) there is no refrangibility to be corrected; apart from which, 'correcting refrangibility' has no more meaning than 'restoring the angle of incidence.' '"and," continued sir john, "why cannot the illuminating microscope, say the hydro-oxygen, be applied to render distinct, and, if necessary, even to magnify, the focal object?" sir david sprung from his chair' [and well he might, though not] 'in an ecstasy of conviction, and, leaping half-way to the ceiling, exclaimed, "thou art the man!" each philosopher anticipated the other in presenting the prompt illustration that if the rays of the hydro-oxygen microscope, passed through a drop of water containing the larvæ of a gnat and other objects invisible to the naked eye, rendered them not only keenly but firmly magnified to dimensions of many feet; so could the same artificial light, passed through the faintest focal object of a telescope, both distinctify (to coin a new word for an extraordinary occasion) and magnify its feeblest component members. the only apparent desideratum was a recipient for the focal image which should transfer it, without refranging it, to the surface on which it was to be viewed under the revivifying light of the microscopic reflectors.' singularly enough, the idea here mentioned does not appear to many so absurd as it is in reality. it is known that the image formed by the large lens of an ordinary telescope or the large mirror of a reflecting telescope is a real image; not a merely virtual image like that which is seen in a looking-glass. it can be received on a sheet of paper or other white surface just as the image of surrounding objects can be thrown upon the white table of the camera obscura. it is this real image, in fact, which we look at in using a telescope of any sort, the portion of such a telescope nearest to the eye being in reality a microscope for viewing the image formed by the great lens or mirror, as the case may be. and it does not seem to some altogether absurd to speak of illuminating this image by transfused light, or of casting by means of an illuminating microscope a vastly enlarged picture of this image upon a screen. but of course the image being simply formed by the passage of rays (which originally came from the object whose image they form) through a certain small space, to send _other_ rays (coming from some other luminous object) through the same small space, is not to improve, but, so far as any effect is produced at all, to impair, the distinctness of the image. in fact, if these illuminating rays reached the eye, they would seriously impair the distinctness of the image. their effect may be compared exactly with the effect of rays of light cast upon the image in a camera obscura; and, to see what the effect of such rays would be, we need only consider why it is that the camera _is_ made 'obscura,' or dark. the effect of the transfusion of light through a telescopic image may be easily tried by any one who cares to make the experiment. he has only to do away with the tube of his telescope (substituting two or three straight rods to hold the glass in its place), and then in the blaze of a strong sun to direct the telescope on some object lying nearly towards the sun. or if he prefer artificial light for the experiment, then at night let him direct the telescope so prepared upon the moon, while a strong electric light is directed upon the place where the focal image is formed (close in front of the eye). the experiment will not suggest very sanguine hopes of good result from the transfusion of artificial light. yet, to my own knowledge, not a few who were perfectly well aware that the lunar hoax was not based on facts, have gravely reasoned that the principle suggested might be sound, and, in fact, that they could see no reason why astronomers should not try it, even though it had been first suggested as a joke. to return, however, to the narrative. 'the co-operative philosophers, having hit upon their method, determined to test it practically. they decided that a medium of the purest plate-glass (which it is said they obtained, by consent, be it observed, from the shop-window of m. desanges, the jeweller to his ex-majesty charles x., in high street) was the most eligible they could discover. it answered perfectly with a telescope which magnified a hundred times, and a microscope of about thrice that power.' thus fortified by experiment, and 'fully sanctioned by the high optical authority of sir david brewster, sir john laid his plan before the royal society, and particularly directed to it the attention of his royal highness the duke of sussex, the ever munificent patron of science and the arts. it was immediately and enthusiastically approved by the committee chosen to investigate it, and the chairman, who was the royal president' (this continual reference to royalty is manifestly intended to give a british tone to the narrative), 'subscribed his name for a contribution of £ , , with a promise that he would zealously submit the proposed instrument as a fit object for the patronage of the privy purse. he did so without delay; and his majesty, on being informed that the estimated expense was £ , , naïvely enquired if the costly instrument would conduce to any improvement in _navigation_. on being informed that it undoubtly would, the sailor king promised a _carte blanche_ for any amount which might be required.' all this is very clever. the 'sailor king' comes in as effectively to give _vraisemblance_ to the narrative as 'crabtree's little bronze shakspeare that stood over the fireplace,' and the 'postman just come to the door with a double letter from northamptonshire.' then comes a description of the construction of the object-glass, twenty-four feet in diameter, 'just six times the size of the elder herschel's;' who, by the way, never made a telescope with an object-glass. the account of sir john herschel's journey from england, and even some details of the construction of the observatory, were based on facts, indeed, so many persons in america as well as in england were acquainted with some of these circumstances, that it was essential to follow the facts as closely as possible. of course, also, some explanation had to be given of the circumstance that nothing had before been heard respecting the gigantic instrument taken out by sir john herschel. 'whether,' says the story, 'the british government were sceptical concerning the promised splendour of the discoveries, or wished them to be scrupulously veiled until they had accumulated a full-orbed glory for the nation and reign in which they originated, is a question which we can only conjecturally solve. but certain it is that the astronomer's royal patrons enjoined a masonic taciturnity upon him and his friends until he should have officially communicated the results of his great experiment.' it was not till the night of january , , that the mighty telescope was at length directed towards our satellite. the part of the moon selected was on the eastern part of her disc. 'the whole immense power of the telescope was applied, and to its focal image about one half of the power of the microscope. on removing the screen of the latter, the field of view was covered throughout its entire area with a beautifully distinct and even vivid representation of _basaltic rock_. its colour was a greenish brown; and the width of the columns, as defined by their interstices on the canvas, was invariably twenty-eight inches. no fracture whatever appeared in the mass first presented; but in a few seconds a shelving pile appeared, of five or six columns' width, which showed their figure to be hexagonal, and their articulations similar to those of the basaltic formation at staffa. this precipitous cliff was profusely covered with a dark red flower, precisely similar, says dr. grant, to the papaver rhoeus, or rose poppy, of our sublunary cornfields; and this was the first organic production of nature in a foreign world ever revealed to the eyes of men.' it would be wearisome to go through the whole series of observations thus fabled, and only a few of the more striking features need be indicated. the discoveries are carefully graduated in interest. thus we have seen how, after recognising basaltic formations, the observers discovered flowers: they next see a lunar forest, whose 'trees were of one unvaried kind, and unlike any on earth except the largest kind of yews in the english churchyards.' (there is an american ring in this sentence, by the way, as there is in one, a few lines farther on, where the narrator having stated that by mistake the observers had the sea of clouds instead of a more easterly spot in the field of view, proceeds to say: 'however, the moon was a free country, and we not as yet attached to any particular province.') next a lunar ocean is described, 'the water nearly as blue as that of the deep sea, and breaking in large white billows upon the strand, while the action of very high tides was quite manifest upon the face of the cliffs for more than a hundred miles.' after a description of several valleys, hills, mountains and forests, we come to the discovery of animal life. an oval valley surrounded by hills, red as the purest vermilion, is selected as the scene. 'small collections of trees, of every imaginable kind, were scattered about the whole of this luxuriant area; and here our magnifiers blessed our panting hopes with specimens of conscious existence. in the shade of the woods we beheld brown quadrupeds having all the external characteristics of the bison, but more diminutive than any species of the bos genus in our natural history.' then herds of agile creatures like antelopes are described, 'abounding on the acclivitous glades of the woods.' in the contemplation of these sprightly animals the narrator becomes quite lively. 'this beautiful creature,' says he, 'afforded us the most exquisite amusement. the mimicry of its movements upon our white painted canvas was as faithful and luminous as that of animals within a few yards of the camera obscura. frequently, when attempting to put our fingers upon its beard, it would suddenly bound away as if conscious of our earthly impertinence; but then others would appear, whom we could not prevent nibbling the herbage, say or do to them what we would.' a strange amphibious creature, of a spherical form, rolling with great velocity along a pebbly beach, is the next object of interest, but is presently lost sight of in a strong current setting off from the angle of an island. after this there are three or four pages descriptive of various lunar scenes and animals, the latter showing a tendency, singular considering the circumstances, though very convenient for the narrator, to become higher and higher in type as the discoveries proceed, until an animal somewhat of the nature of the missing link is discovered. it is found in the endymion (a circular walled plain) in company with a small kind of reindeer, the elk, the moose, and the horned bear, and is described as the biped beaver. it 'resembles the beaver of the earth in every other respect than in its destitution of a tail, and its invariable habit of walking upon only two feet. it carries its young in its arms like a human being, and moves with an easy gliding motion. its huts are constructed better and higher than those of many tribes of human savages, and, from the appearance of smoke in nearly all of them, there is no doubt of its being acquainted with the use of fire. still, its head and body differ only in the points stated from that of the beaver; and it was never seen except on the borders of lakes and rivers, in which it has been observed to immerse for a period of several seconds.' the next step towards the climax brings us to domestic animals, 'good large sheep, which would not have disgraced the farms of leicestershire or the shambles of leadenhall market; we fairly laughed at the recognition of so familiar an acquaintance in so distant a land. presently they appeared in great numbers, and, on reducing the lenses, we found them in flocks over a great part of the valley. i need not say how desirous we were of finding shepherds to these flocks, and even a man with blue apron and rolled-up sleeves would have been a welcome sight to us, if not to the sheep; but they fed in peace, lords of their own pastures, without either protector or destroyer in human shape.' in the meantime, discussion had arisen as to the lunar locality where men, or creatures resembling them, would most likely be found. herschel had a theory on the subject--viz., that just where the balancing or libratory swing of the moon brings into view the greatest extent beyond the eastern or western parts of that hemisphere which is turned earthwards in the moon's mean or average position, lunar inhabitants would probably be found, and nowhere else. this, by the way (speaking seriously), is a rather curious anticipation of a view long subsequently advanced by hansen, and for a time adopted by sir j. herschel, that possibly the remote hemisphere of the moon may be a fit abode for living creatures, the oceans and atmosphere which are wanting on the nearer hemisphere having been (on this hypothesis) drawn over to the remoter because of a displacement of the moon's centre of gravity. i ventured in one of my first books on astronomy to indicate objections to this theory, the force of which sir j. herschel admitted in a letter addressed to me on the subject. taking, then, an opportunity when the moon had just swung to the extreme limit of her balancing, or, to use technical terms, when she had attained her maximum libration in longitude, the observers approached the level opening to lake langrenus, as the narrator calls this fine walled plain, which, by the way, is fully thirty degrees of lunar longitude within the average western limit of the moon's visible hemisphere. 'here the valley narrows to a mile in width, and displays scenery on both sides picturesque and romantic beyond the powers of a prose description. imagination, borne on the wings of poetry, could alone gather similes to portray the wild sublimity of this landscape, where dark behemoth crags stood over the brows of lofty precipices, as if a rampart in the sky; and forests seemed suspended in mid-air. on the eastern side there was one soaring crag, crested with trees, which hung over in a curve like three-fourths of a gothic arch, and being of a rich crimson colour, its effect was most strange upon minds unaccustomed to the association of such grandeur with such beauty. but, whilst gazing upon them in a perspective of about half a mile, we were thrilled with astonishment to perceive four successive flocks of large winged creatures, wholly unlike any kind of birds, descend with a slow even motion from the cliffs on the western side and alight upon the plain. they were first noticed by dr. herschel, who exclaimed: "now, gentlemen, my theories against your proofs, which you have often found a pretty even bet, we have here something worth looking at. i was confident that if ever we found beings in human shape it would be in this longitude, and that they would be provided by their creator with some extraordinary powers of locomotion." ... we counted three parties of these creatures, of twelve, nine, and fifteen in each, walking erect towards a small wood near the base of the eastern precipices. certainly they _were_ like human beings, for their wings had now disappeared, and their attitude in walking was both erect and dignified.... they averaged four feet in height, were covered, except on the face, with short and glossy copper-coloured hair, lying snugly upon their backs, from the top of the shoulders to the calves of the legs. the face, which was of a yellowish flesh colour, was a slight improvement upon that of the large orang outang, being more open and intelligent in its expression, and having a much greater expansion of forehead. the mouth, however, was very prominent, though somewhat relieved by a thick beard upon the lower jaw, and by lips far more human than those of any species of the simia genus. in general symmetry of body and limbs they were infinitely superior to the orang outang; so much so, that, but for their long wings, lieutenant drummond said they would look as well on a parade ground as some of the old cockney militia.... these creatures were evidently engaged in conversation; their gesticulation, more particularly the varied action of their hands and arms, appeared impassioned and emphatic. we hence inferred that they were rational beings, and, although not perhaps of so high an order as others which we discovered the next month on the shores of the bay of rainbows, that they were capable of producing works of art and contrivance.... they possessed wings of great expansion, similar in construction to those of the bat, being a semi-transparent membrane united in curvilinear divisions by means of straight radii, united at the back by the dorsal integuments. but what astonished us very much was the circumstance of this membrane being continued from the shoulders to the legs, united all the way down, though gradually decreasing in width' (very much as fuseli depicted the wings of his satanic majesty, though h.s.m. would seem to have the advantage of the lunar bat-men in not being influenced by gravity[ ]). 'the wings seemed completely under the command of volition, for those of the creatures whom we saw bathing in the water spread them instantly to their full width, waved them as ducks do theirs to shake off the water, and then as instantly closed them again in a compact form. our further observation of the habits of these creatures, who were of both sexes, led to results so very remarkable, that i prefer they should be first laid before the public in dr. herschel's own work, where i have reason to know they are fully and faithfully stated, however incredulously they may be received.... we scientifically denominated them the vespertilio-homo or bat-man; and they are doubtless innocent and happy creatures, notwithstanding that some of their amusements would but ill comport with our terrestrial notions of decorum.' the omitted passages were suppressed in obedience to dr. grant's private injunction. 'these, however, and other prohibited passages,' were to be presently 'published by dr. herschel, with the certificates of the civil and military authorities of the colony, and of several episcopal, wesleyan, and other ministers, who in the month of march last were permitted, under stipulation of temporary secrecy, to visit the observatory, and become eye-witnesses of the wonders which they were requested to attest. we are confident that his forthcoming volumes will be at once the most sublime in science, and the most intense in general interest, that ever issued from the press.' the actual climax of the narrative, however, is not yet reached. the inhabitants of langrenus, though rational, do not belong to the highest orders of intelligent lunarians. herschel, ever ready with theories, had pointed out that probably the most cultivated races would be found residing on the slopes of some active volcano, and, in particular, that the proximity of the flaming mountain bullialdus (about twenty degrees south and ten east of the vast crater tycho, the centre whence extend those great radiations which give to the moon something of the appearance of a peeled orange) 'must be so great a local convenience to dwellers in this valley during the long periodical absence of solar light, as to render it a place of popular resort for the inhabitants of all the adjacent regions, more especially as its bulwark of hills afforded an infallible security against any volcanic eruption that could occur.' our observers therefore applied their full power to explore it. 'rich, indeed, was our reward. the very first object in this valley that appeared upon our canvas was a magnificent work of art. it was a temple--a fane of devotion or of science, which, when consecrated to the creator, is devotion of the loftiest order, for it exhibits his attributes purely, free from the masquerade attire and blasphemous caricature of controversial creeds, and has the seal and signature of his own hand to sanction its aspirations. it was an equi-angular temple, built of polished sapphire, or of some resplendent blue stone, which, like it, displayed a myriad point of golden light twinkling and scintillating in the sunbeams.... the roof was composed of yellow metal, and divided into three compartments, which were not triangular planes inclining to the centre, but subdivided, curved, and separated so as to present a mass of violently agitated flames rising from a common source of conflagration, and terminating in wildly waving points. this design was too manifest and too skilfully executed to be mistaken for a single moment. through a few openings in these metallic flames we perceived a large sphere of a darker kind of metal nearly of a clouded copper colour, which they enclosed and seemingly raged around, as if hieroglyphically consuming it.... what did the ingenious builders mean by the globe surrounded by flames? did they, by this, record any past calamity of _their_ world, or predict any future one of _ours_?' (why, by the way, should the past theory be assigned to the moon and the future one to our earth?) 'i by no means despair of ultimately solving not only these but a thousand other questions which present themselves respecting the objects in this planet; for not the millionth part of her surface has yet been explored, and we have been more desirous of collecting the greatest possible number of new facts than of indulging in speculative theories, however seductive to the imagination.' after this we have an account of the behaviour of the vespertilio-homo at meals. 'they seemed eminently happy, and even polite; for individuals would select large and bright specimens of fruit, and throw them archwise across to some friend who had extracted the nutriment from those scattered around him.' however, the lunar men are not on the whole particularly interesting beings according to this account. 'so far as we could judge, they spent their happy hours in collecting various fruits in the woods, in eating, flying, bathing, and loitering about the summits of precipices.' one may say of them what huxley is reported to have said of the spirits as described by spiritualists, that no student of science would care to waste his time inquiring about such a stupid set of people. such are the more interesting and characteristic portions of a narrative, running in the original to forty or fifty large octavo pages. in its day the story attracted a good deal of notice, and, even when every one had learned the trick, many were still interested in a _brochure_ which was so cleverly conceived and had deceived so many. to this day the lunar hoax is talked of in america, where originally it had its chief--or, one may rather say, its only real--success as a hoax. it reached england too late to deceive any but those who were unacquainted with herschel's real doings, and no editors of public journals, i believe, gave countenance to it at all. in america, on the contrary, many editors gave the narrative a distinguished place in their columns. some indeed expressed doubts, and others followed the safe course of the 'philadelphia inquirer,' which informed its readers that 'after an attentive perusal of the whole story they could decide for themselves;' adding that, 'whether true or false, the narrative is written with consummate ability and possesses intense interest.' but others were more credulous. according to the 'mercantile advertiser' the story carried 'intrinsic evidence of being an authentic document.' the 'albany daily advertiser' had read the article 'with unspeakable emotions of pleasure and astonishment.' the 'new york times' announced that 'the writer (dr. andrew grant) displays the most extensive and accurate knowledge of astronomy; and the description of sir john's recently improved instruments, the principle on which the inestimable improvements were founded, the account of the wonderful discoveries in the moon, etc., all are probable and plausible, and have an air of intense verisimilitude.' the 'new yorker' considered the discoveries 'of astounding interest, creating a new era in astronomy and science generally.'[ ] in our time a trick of the kind could hardly be expected to succeed so well, even if as cleverly devised and as well executed. the facts of popular astronomy and of general popular science have been more widely disseminated. america, too, more than any other great nation, has advanced in the interval. it was about two years after this pamphlet had appeared, that j. quincy adams used the following significant language in advocating the erection of an astronomical observatory at washington: 'it is with no feeling of pride as an american that the remark may be made, that on the comparatively small territorial surface of europe there are existing more than of these lighthouses of the skies; while throughout the whole american hemisphere there is but one.' at present, some of the finest observatories in the world belong to american cities, or are attached to american colleges; and much of the most interesting astronomical work of this country has been achieved by american observers. yet we still hear from time to time of the attempted publication of hoaxes of greater or less ingenuity. it is singular (and i think significant) how often these relate to the moon. there would seem to be some charm about our satellite for the minds of paradoxists and hoaxers generally. nor are these tricks invariably detected at once by the general public, or even by persons of some culture. i remember being gravely asked (in january ) whether an account given in the 'new york world,' purporting to describe how the moon's frame was gradually cracking, threatening eventually to fall into several separate fragments, was in reality based on fact. in the far west, at lincoln, nebraska, a lawyer asked me, not long since, why i had not described the great discoveries recently made by means of a powerful reflector erected near paris. according to the 'chicago times,' this powerful instrument had shown buildings in the moon, and bands of workers could be seen with it who manifestly were undergoing some kind of penal servitude, for they were chained together. it was clear, from the presence of these and the absence of other inhabitants, that the side of the moon turned earthwards is a dreary and unpleasant place of abode, the real 'happy hunting grounds' of the moon lying on her remote and unseen hemisphere. as gauges of general knowledge, scientific hoaxes have their uses, just as paradoxical works have. no one, certainly no student of science, can thoroughly understand how little some persons know about science, until he has observed how much will be believed, if only published with the apparent authority of a few known names, and announced with a sufficient parade of technical verbiage; nor is it so easy as might be thought, even for those who are acquainted with the facts, to disprove either a hoax or a paradox. nothing, indeed, can much more thoroughly perplex and confound a student of science than to be asked to prove, for example, that the earth is not flat, or the moon not inhabited by creatures like ourselves; for the circumstance that such a question is asked implies ignorance so thorough of the very facts on which the proof must be based, as to render argument all but hopeless from the outset. i have had a somewhat wide experience of paradoxists, and have noted the experience of de morgan and others who, like him, have tried to convince them of their folly. the conclusion at which i have arrived is, that to make a rope of sand were an easy task compared with the attempt to instil the simpler facts of science into paradoxical heads. i would make some remarks, in conclusion, upon scientific or quasi-scientific papers not intended to deceive, but yet presenting imaginary scenes, events, and so forth, described more or less in accordance with scientific facts. imaginary journeys to the sun, moon, planets, and stars; travels over regions on the earth as yet unexplored; voyages under the sea, through the bowels of the earth, and other such narratives, may, perhaps, be sometimes usefully written and read, so long as certain conditions are fulfilled by the narrator. in the first place, while adopting, to preserve the unities, the tone of one relating facts which actually occurred, he should not suffer even the simplest among his readers to lie under the least misapprehension as to the true nature of the narrative. again, since of necessity established facts must in such a narrative appear in company with the results of more or less probable surmise, the reader should have some means of distinguishing where fact ends and surmise begins. for example, in a paper i once wrote, entitled 'a journey to saturn,' i was not sufficiently careful to note that while the appearances described in the approach towards the planet were in reality based on the observed appearances as higher and higher telescopic powers are applied to the planet, others supposed to have been seen by the visitors to saturn when actually within his system, were only such as might possibly or probably be seen, but for which we have no real evidence. in consequence of this omission, i received several inquiries about these matters. 'is it true,' some wrote, 'that the small satellite hyperion' (scarce discernible in powerful telescopes, while titan and japetus on either side are large) 'is only one of a ring of small satellites travelling between the orbits of the larger moons?'--as the same planets travel between the paths of mars and jupiter. others asked on what grounds it was said that the voyagers found small moons circling about titan, the giant moon of the saturnian system, as the moons of jupiter and saturn circle around those giant members of the solar system. in each case, i was reduced to the abject necessity of explaining that there was no evidence for the alleged state of things, which, however, might nevertheless exist. scientific fiction which has to be interpreted in that way is as bad as a joke that has to be explained. in my 'journey to the sun' i was more successful (it was the earlier essay, however); insomuch that professor young, of dartmouth college (hanover, n.h.), one of the most skilful solar observers living, assured me that, with scarcely a single exception, the various phenomena described corresponded exactly with the ideas he had formed respecting the probable condition of our luminary.[ ] but i must confess that my own experience has not been, on the whole, favourable to that kind of popular science writing. it appears to me that the more thoroughly the writer of such an essay has studied any particular scientific subject, the less able must he be to write a fictitious narrative respecting it. just as those ignorant of any subject are often the readiest to theorise about it, because least hampered by exact knowledge, so i think that the careful avoidance of any exact study of the details of a scientific subject must greatly facilitate the writing of a fictitious narrative respecting it. but unfortunately a narrative written under such conditions, however interesting to the general reader, can scarcely forward the propagation of scientific knowledge, one of the qualities claimed for fables of the kind. as an instance in point, i may cite jules verne's 'voyage to the moon,' where (apart, of course, from the inherent and intentional absurdity of the scheme itself), the circumstances which are described are calculated to give entirely erroneous ideas about the laws of motion. nothing could be more amusing, but at the same time nothing more scientifically absurd, than the story of the dead dog satellite, which, flung out of the travelling projectile, becomes a veritable satellite, moving always beside the voyagers; for, with whatever velocity the dog had been expelled by them, with that same velocity would he have retreated continually from their projectile abode, whose own attraction on the dog would have had no appreciable effect in checking his departure. again, the scene when the projectile reaches the neutral point between the earth and moon, so that there is no longer any gravity to keep the travellers on the floor of their travelling car, is well conceived (though, in part, somewhat profane); but in reality the state of things described as occurring there would have prevailed throughout the journey. the travellers would no more be drawn earthwards (as compared with the projectile itself) than we travellers on the earth are drawn sunwards with reference to the earth. the earth's attracting force on the projectile and on the travellers would be equal all through the journey, not solely when the projectile reached the neutral point; and being equal on both, would not draw them together. it may be argued that the attractions were equal before the projectile set out on its journey, and therefore, if the reasoning just given were correct, the travellers ought not to have had any weight keeping them on the floor of the projectile before it started, 'which is absurd.' but the pressure upon the floor of the projectile at rest is caused by the floor being kept from moving; let it be free to obey gravity, and there will no longer be any pressure: and throughout the journey to the moon, the projectile, like the travellers it contains, is obeying the action of gravity. unfortunately, those who are able to follow the correct reasoning in such matters are not those to whom jules verne's account would suggest wrong ideas about matters dynamical; the young learner who _is_ misled by such narratives is neither able to reason out the matter for himself, nor to understand the true reasoning respecting it. he is, therefore, apt to be set quite at sea by stories of the kind, and especially by the specious reasoning introduced to explain the events described. in fine, it would seem that such narratives must be valued for their intrinsic interest, just like other novels or romances, not for the quality sometimes claimed for them of combining instruction with amusement. x. _on some astronomical paradoxes._ for many years the late professor de morgan contributed to the columns of the 'athenæum' a series of papers in which he dealt with the strange treatises in which the earth is flattened, the circle squared, the angle divided into three, the cube doubled (the famous problem which the delphic oracle set astronomers), and the whole of modern astronomy shown to be a delusion and a snare. he treated these works in a quaint fashion: not unkindly, for his was a kindly nature; not even earnestly, though he was thoroughly in earnest; yet in such sort as to rouse the indignation of the unfortunate paradoxists. he was abused roundly for what he said, but much more roundly when he declined further controversy. paradoxists of the ignorant sort (for it must be remembered that not all are ignorant) are, indeed, well practised in abuse, and have long learned to call mathematicians and astronomers cheats and charlatans. they freely used their vocabulary for the benefit of de morgan, whom they denounced as a scurrilous scribbler, a defamatory, dishonest, abusive, ungentlemanly, and libellous trickster. he bore this shower of abuse with exceeding patience and good nature. he had not been wholly unprepared for it, in fact; and, as he had a purpose in dealing with the paradoxists, he was satisfied to continue that quiet analysis of their work which so roused their indignation. he found in them a curious subject of study; and he found an equally curious subject of study in their disciples. the simpler--not to say more foolish--paradoxists, whose wonderful discoveries are merely amazing misapprehensions, were even more interesting to de morgan than the craftier sort who make a living, or try to make a living, out of their pretended theories. indeed, these last he treated, as they deserved, with a scathing satire quite different from his humorous and not ungenial comments on the wonderful theories of the honest paradoxists. there is one special use to which the study of paradox-literature may be applied, which--so far as i know--has not hitherto been much attended to. it may be questioned whether half the strange notions into which paradoxists fall must not be ascribed to the vagueness of too many of our scientific treatises. a half-understood explanation, or a carelessly worded account of some natural phenomenon, leads the paradoxist, whose nature is compounded of conceit and simplicity, to originate a theory of his own on the subject. once such a theory has been devised, it takes complete possession of the paradoxist's mind. all the facts of which he thenceforward hears, which bear in the least on his favourite craze, appear to give evidence in its favour, even though in reality they are most obviously opposed to it. he learns to look upon himself as an unappreciated newton, and to see the bitterest malevolence in those who venture to question his preposterous notions. he is fortunate if he do not suffer his theories to withdraw him from his means of earning a livelihood, or if he do not waste his substance in propounding and defending them. one of the favourite subjects for paradox-forming is the accepted theory of the solar system. our books on astronomy too often present this theory in such sort that it seems only a _successor_ of ptolemy's; and the impression is conveyed that, like ptolemy's, it may be one day superseded by some other theory. this is quite enough for the paradoxist. if a new theory is to replace the one now accepted, why should not _he_ be the new copernicus? he starts upon the road without a tithe of the knowledge that old ptolemy possessed, unaware of the difficulties which ptolemy met and dealt with--free, therefore, because of his perfect ignorance, to form theories at which ptolemy would have smiled. he has probably heard of the centrics and eccentrics scribbled o'er cycle and epicycle, orb in orb, which disfigured the theories of the ancients; but he is quite unconscious that every one of those scribblings had a real meaning, each being intended to account for some observed peculiarity of planetary motion, which _must_ be accounted for by any theory which is to claim acceptance. in this happy unconsciousness that there are any peculiarities requiring explanation, knowing nothing of the strange paths which the planets are seen to follow on the heavenly vault, their wand'ring course now high, now low, then hid, progressive, retrograde, or standing still, he placidly puts forward--and presently very vehemently urges--a theory which accounts for none of these things. it has often seemed to me that a large part of the mischief--for let it be remembered that the published errors of the paradoxist are indicative of much unpublished misapprehension--arises from the undeserved contempt with which our books of astronomy too often treat the labours of ptolemy, tycho brahe, and others who advocated erroneous theories. if the simple truth were told, that the theory of ptolemy was a masterpiece of ingenuity and that it was worked out by his followers in a way which merits the highest possible praise, while the theory of tycho brahe was placed in reality on a sounder basis than that of copernicus, and accounted as well and as simply for observed appearances, the student would begin to realise the noble nature of the problem which those great astronomers dealt with. and again, if stress were laid upon the fact that tycho brahe devoted years upon years of his life to secure such observations of the planets as might settle the questions at issue, the student would learn something of the spirit in which the true lover of science proceeds. it seems to me, also, that far too little is said about the kind of work by which kepler and newton finally established the accepted theories. there is a strange charm in the history of those twenty years of kepler's life during which he was analysing the observations made by tycho brahe. surrounded with domestic trials and anxieties, which might well have claimed his whole attention, tried grievously by ill-health and bodily anguish, he laboured all those years upon erroneous theories. the very worst of these had infinitely more evidence in its favour than the best which the paradoxists have brought forth. there was not one of those theories which nine out of ten of his scientific contemporaries would not have accepted ungrudgingly. yet he wrought these theories one after another to their own disproof. _nineteen_ of them he tried and rejected--the twentieth was the true theory of the solar system. perhaps nothing in the whole history of astronomy affords a nobler lesson to the student of science--unless, indeed, it be the calm philosophy with which newton for eighteen years suffered the theory of the universe to remain in abeyance, because faulty measurements of the earth prevented his calculations from agreeing with observed facts. but, as professor tyndall has well remarked--and the paradoxist should lay the lesson well to heart--'newton's action in this matter was the normal action of the scientific mind. if it were otherwise--if scientific men were not accustomed to demand verification, if they were satisfied with the imperfect while the perfect is attainable--their science, instead of being, as it is, a fortress of adamant, would be a house of clay, ill fitted to bear the buffetings of the theologic storms to which it has been from time to time, and is at present, exposed.' the fame of newton has proved to many paradoxists an irresistible attraction; it has been to these unfortunates as the candle to the fluttering moth. circle-squaring, as we shall presently see, has had its attractions, nor have earth-fixing and earth-flattening been neglected; but attacking the law of gravitation has been the favourite work of paradoxists. newton has been praised as surpassing the whole human race in genius; mathematicians and astronomers have agreed to laud him as unequalled; why should not paradoxus displace him and be praised in like manner? it would be unfair, perhaps, to say that the paradoxist consciously argues thus. he doubtless in most instances convinces himself that he has really detected some flaw in the theory of gravitation. yet it is impossible not to recognise, as the real motive of every paradox-monger, the desire to have that said of him which has been said of newton: '_genus humanum ingenio superavit._' i remember a curious instance of this which occurred soon after the appearance of the comet of . it chanced that, while that object was under discussion, reference was made to the action of a repulsive force exerted by the sun upon the matter of the comet's tail. on this, some one addressed a long letter to a glasgow newspaper, announcing that he had long ago proved that the sun's attraction alone is insufficient to account for the planetary motions. his reasoning was amazingly simple. if the sun's attraction is powerful enough to keep the outer planets in their course, it must be too powerful for venus and mercury close by the sun; if it only just suffices to keep these in their course, it cannot possibly be powerful enough to restrain the outer planets. the writer of this letter said that he had been very badly treated by scientific bodies. he had announced his discovery to the royal astronomical society, the royal society, the imperial academy at paris, and other scientific bodies; but they had one and all refused to listen to him. he had forsaken or neglected his trade for several years in order to give attention to the new and (as he thought) the true theory of the universe. he complained in a specially bitter manner of the unfavourable comments which men of science had made upon his views in private letters addressed to him in reply to his communications. there is something melancholy even in what is most ridiculous in cases of this sort. the simplicity which supposes that considerations so obvious as those adduced could escape the scrutiny, not of newton only, but of all who have followed in the same track during two centuries, is certainly stupendous; nor can one fail to smile at seeing a difficulty, such as might naturally suggest itself to a beginner, and such as half-a-dozen words from an expert would clear up, regarded gravely as a discovery calculated to make its author famous for all time. yet, when one considers the probable consequences of the blunder to the unhappy enthusiast, and perchance to his family, it is difficult not to feel a sense of pity, quite apart from that pity allied to contempt which is excited by his mistake. a few words added to the account of newton's theory, which the paradoxist had probably read in some astronomical treatise, would have prevented all this mischief. indeed, this difficulty, which, as we have said, is a natural one, should be dealt with and removed in any account of the planetary system intended for beginners. the simple statement that the outer planets move more slowly than the inner, and so _require_ a smaller force to keep them in their course, would have sufficed, not, perhaps, altogether to remove the difficulty, but to show the beginner where the explanation was to be looked for. it was in connection with this subject of gravitation that one of the most well-meaning of the paradoxists--the late mr. james reddie--came under professor de morgan's criticism. mr. reddie was something more than well-meaning. he was earnestly desirous of advancing the interests of science, as well as of defending religion from what he mistakenly supposed to be the dangerous teachings of the newtonians. he founded for these purposes the victoria institute, of which society he was the secretary from the time of its institution until his decease, some years since; and, probably, many who declined to join that society because of the anti-newtonian proclivities of its secretary, were unaware that to that secretary the institute owed its existence. it so chanced that i had myself a good deal of correspondence with mr. reddie (who was, however, personally unknown to me). this correspondence served to throw quite a new light on the mental habitudes and ways of thinking of the honest paradoxist. i believe that professor de morgan hardly gave mr. reddie credit for the perfect honesty which he really possessed. it may have been that a clear reasoner like de morgan could hardly (despite his wide experience) appreciate the confusion of mind which is the normal characteristic of the paradoxist. but certainly the very candid way in which mr. reddie admitted, in the correspondence above named, that he had not known some facts and had misunderstood others, afforded to my mind the most satisfactory proofs of his straightforwardness. it may be instructive to consider a few of those paradoxes of mr. reddie's which professor de morgan found chief occasion to pulverise. in a letter to the astronomer-royal mr. reddie announced that he was about to write 'a paper intended to be hereafter published, elaborating more minutely and discussing more rigidly than before the glaring fallacies, dating from the time of newton, relating to the motion of the moon.' he proceeded to 'indicate the nature of the issues he intended to raise.' he had discovered that the moon does not, as a matter of fact, go round the earth at the rate of miles an hour, as astronomers say, but follows an undulatory path round the sun at a rate varying between , and , miles an hour; because, while the moon seems to go round the earth, the latter is travelling onwards at the rate of , miles an hour round the sun. of course he was quite right in his facts, and quite wrong in his inferences; as the astronomer-royal pointed out in a brief letter, closing with the remark that, 'as a very closely occupied man,' mr. airy could 'not enter further into the matter.' but further mr. reddie persisted in going, though he received no more letters from greenwich. his reply to sir g. airy contained, in fact, matter enough for a small pamphlet. now here was certainly an amazing fact. a well-known astronomical relation, which astronomers have over and over again described and explained, is treated as though it were something which had throughout all ages escaped attention. it is not here the failure to comprehend the _rationale_ of a simple explanation which is startling, but the notion that an obvious fact had been wholly overlooked. of like nature was the mistake which brought mr. reddie more especially under professor de morgan's notice. it is known that the sun, carrying with him his family of planets, is speeding swiftly through space--his velocity being estimated as probably not falling short of , miles per hour. it follows, of course, that the real paths of the planets in space are not closed curves, but spirals of different orders. how, then, can the theory of copernicus be right, according to which the planets circle in closed orbits round the sun? here was mr. reddie's difficulty; and like the other, it appeared to his mind as a great discovery. he was no whit concerned by the thought that astronomers ought surely to have noticed the difficulty before. it did not seem in the least wonderful that he, lightly reading a book or two of popular astronomy, should discover that which laplace, the herschels, leverrier, airy, adams, and a host of others, who have given their whole lives to astronomy, had failed to notice. accordingly, mr. reddie forwarded to the british association (in session at newcastle) a paper controverting the theory of the sun's motion. the paper was declined with thanks by that bigoted body 'as opposed to newtonian astronomy.' 'that paper i published,' says mr. reddie, 'in september , with an appendix, in both thoroughly exhibiting the illogical reasoning and absurdities involved in the theory; and with what result? the members of section a of the british association, and fellows of the royal society and of the royal astronomical society, to whom i sent copies of my paper, were, without exception, _dumb_.' professor de morgan, however, having occasion to examine mr. reddie's publications some time after, was in no sort dumb, but in very plain and definite terms exhibited their absurdity. after all, however, the real absurdity consisted, not in the statements which mr. reddie made, nor even in the conclusions which he drew from them, but in the astounding simplicity which could suppose that astronomers were unaware of the facts which their own labours had revealed. in my correspondence with mr. reddie i recognised the real source of the amazing self-complacency displayed by the true paradoxist. the very insufficiency of the knowledge which a paradoxist possesses of his subject, affords the measure of his estimate of the care with which other men have studied that subject. because the paradoxist is ready to pronounce an opinion about matters he has not studied, it does not seem strange to him that newton and his followers should be equally ready to discuss subjects they had not inquired into. another very remarkable instance was afforded by mr. reddie's treatment of the subject of comets. and here, by the way, i shall quote a remark made by sir john herschel soon after the appearance of the comet of . 'i have received letters,' he said, 'about the comets of the last few years, enough to make one's hair stand on end at the absurdity of the theories they propose, and at the ignorance of the commonest laws of optics, of motion, of heat, and of general physics, they betray in their writers.' in the present instance, the correspondence showed that the paradoxist supposed the parabolic paths of some comets to be regarded by astronomers as analogous to the parabolic paths traversed by projectiles. he expressed considerable astonishment when i informed him that, in the first place, projectiles do not travel on truly parabolic paths; and secondly, that in all respects their motion differs essentially from that which astronomers ascribe to comets. these last move more and more quickly until they reach what is called the vertex of the parabola (the point of such a path which lies nearest to the sun): projectiles, on the contrary, move more and more slowly as they approach the corresponding point of their path; and further, the comet first approaches and then recedes from the centre of attraction--the projectile first recedes from and then approaches the attracting centre. the earth-flatteners form a considerable section of the paradoxical family. they experienced a practical rebuff, a few years since, which should to some degree have shaken their faith in the present chief of their order. to do this chief justice, he is probably far less confident about the flatness of the earth than any of his disciples. under the assumed name of parallax he visited most of the chief towns of england, propounding what he calls his system of zetetic astronomy. why he should call himself parallax it would be hard to say; unless it be that the verb from which the word is derived signifies primarily to shift about or dodge, and secondarily to alter a little, especially for the worse. his employment of the word zetetic is less doubtful, as he claims for his system that it alone is founded on the true seeking out of nature's secrets. the experimental basis of the theory of parallax is mainly this: having betaken himself to a part of the bedford canal, where there is an uninterrupted water-line of about six miles, he tested the water surface for signs of curvature, and (as he said) found none. it chanced, unfortunately, that a disciple--mr. john hampden, of swindon--accepted the narrative of this observation in an unquestioning spirit; and was so confident that the bedford canal has a truly plane surface, that he wagered five hundred pounds on his opinion, challenging the believers in the earth's rotundity to repeat the experiment. the challenge was accepted by mr. wallace, the eminent naturalist; and the result may be anticipated. three boats were to be moored in a line, three miles or so between each. each carried a mast of given length. if, when the summits of the first and last masts were seen in a line through a telescope, the summit of the middle mast was not found to be above the line, then mr. hampden was to receive five hundred pounds from mr. wallace. if, on the contrary, the top of the middle mast was found, as the accepted theory said it should be, to be several feet above the line joining the tops of the two outer masts, then mr. hampden was to lose the five hundred pounds he had so rashly ventured. everything was conducted in accordance with the arrangements agreed upon. the editor of a well-known sporting paper acted as stakeholder, and unprejudiced umpires were to decide as to what actually was seen through the telescope. it need scarcely be said that the accepted theory held its own, and that mr. hampden lost his money. he scarcely bore the loss with so good a grace as was to have been expected from a philosopher merely desirous of ascertaining the truth. his wrath was not expended on parallax, whom he might have suspected of having led him astray; nor does he seem to have been angry with himself, as would have seemed natural. all his anger was reserved for those who still continued to believe in the earth's rotundity. whether he believed that the bedford water had risen under the middle boat to oblige mr. wallace, or how it came to pass that his own chosen experiment had failed him, does not appear. the subsequent history of this matter has been unpleasant. it illustrates, unfortunately but too well, the mischief which may ensue from the tricks of those who make a trade of paradox--tricks which would be scarce possible, however, if text-books of science were more carefully written, and by those only who are really acquainted with the subject of which they treat. the book which originally led to mr. hampden's misfortunes, and has misled not a few, ought to have deceived none. i have already mentioned the statement on which parallax (whose true name is rowbotham) rested his theory. of course, if that statement had been true--if he had, with his eye a few inches from the surface of the water of the bedford canal, seen an object close to the surface six miles from him--there manifestly would have been something wrong in the accepted theory about the earth's rotundity. so, also, if a writer were to announce a new theory of gravity, stating as the basis of his theory that a heavy missile which he had thrown into the air had gone upwards on a serpentine course to the moon, any one who accepted the statement would be logically bound to admit at least that the fact described was inconsistent with the accepted theory. but no one would accept such a statement; and no one should have accepted mr. rowbotham's statement. his statement was believed, however, and perhaps is still believed by many. twenty years ago de morgan wrote that 'the founder of the zetetic astronomy gained great praise from provincial newspapers for his ingenuity in proving that the earth is a flat, surrounded by ice,' with the north polar ice in the middle. 'some of the journals rather incline to this view; but the "leicester advertiser" thinks that the statement "would seem to invalidate some of the most important conclusions of modern astronomy;" while the "norfolk herald" is clear that "there must be great error on one side or the other." ... the fact is worth noting that from - arguments on the roundness or flatness of the earth did itinerate. i have no doubt they did much good, for very few persons have any distinct idea of the evidence for the rotundity of the earth. the "blackburn standard" and "preston guardian" (december and , ) unite in stating that the lecturer ran away from his second lecture at burnley, having been rather too hard pressed, at the end of his first lecture, to explain why the large hull of a ship disappeared before the masts. the persons present and waiting for the second lecture assuaged their disappointment by concluding that the lecturer had slipped off the ice edge of his flat disc, and that he would not be seen again till he peeped up on the opposite side.' ... 'the zetetic system,' proceeds de morgan, 'still lives in lectures and books; as it ought to do, for there is no way of teaching a truth comparable to opposition. the last i heard of it was in lectures at plymouth, in october . since this time a prospectus has been issued of a work entitled "the earth not a globe;" but whether it has been published i do not know.' the book was published soon after the above was written, and de morgan gives the following quaint account of it: 'august , . the zetetic astronomy has come into my hands. when in i went to see the great exhibition i heard an organ played by a performer who seemed very desirous of exhibiting one particular stop. "what do you think of that stop?" i was asked. "that depends on the name of it," said i "oh! what can the name of it have to do with the sound? 'that which we call a rose,' etc." "the name has everything to do with it: if it be a flute stop i think it very harsh; but if it be a railway-whistle stop, i think it very sweet." so as to this book: if it be childish, it is clever; if it be mannish, it is unusually foolish. the flat earth floating tremulously on the sea; the sun moving always over the flat, giving day when near enough, and night when too far off; the self-luminous moon, with a semi-transparent invisible moon created to give her an eclipse now and then; the new law of perspective, by which the vanishing of the hull before the masts, usually thought to prove the earth globular, really proves it flat;--all these and other things are well fitted to form exercises for a person who is learning the elements of astronomy. the manner in which the sun dips into the sea, especially in tropical climates, upsets the whole. mungo park, i think, gives an african hypothesis which explains phenomena better than this. the sun dips into the western ocean, and the people there cut him in pieces, fry him in a pan, and then join him together again; take him round the under way, and set him up in the east. i hope this book will be read, and that many will be puzzled by it; for there are many whose notions of astronomy deserve no better fate. there is no subject on which there is so little accurate conception as on that of the motions of the heavenly bodies.[ ] the author, though confident in the extreme, neither impeaches the honesty of those whose opinion he assails, nor allots them any future inconvenience: in these points he is worthy to live on a globe and to rotate in twenty-four hours.' i chanced to reside near plymouth when mr. rowbotham lectured there in october . it will readily be understood that, in a town where there are so many naval men, his lectures were not altogether so successful as they have sometimes been in small inland towns. numbers of naval officers, however, who were thoroughly well assured of the fact that the earth is a globe, were not able to demolish the crafty arguments of parallax publicly, during the discussions which he challenged at the close of each lecture. he was too skilled in that sort of evasion which his assumed name (as interpreted by liddell and scott) suggests, to be readily cornered. when an argument was used which he could not easily meet, or seem to meet, he would say simply: 'well, sir, you have now had your fair share of the discussion; let some one else have his turn.' it was stated in the newspapers that one of his audience was so wrathful with the lecturer on account of these evasions, that he endeavoured to strike parallax with a knobbed stick at the close of the second lecture; but probably there was no real foundation for the story. mr. rowbotham did a very bold thing, however, at plymouth. he undertook to prove, by observations made with a telescope upon the eddystone lighthouse from the hoe and from the beach, that the surface of the water is flat. from the beach usually only the lantern can be seen. from the hoe the whole of the lighthouse is visible under favourable conditions. duly on the morning appointed, mr. rowbotham appeared. from the hoe a telescope was directed towards the lighthouse, which was well seen, the morning being calm and still, and tolerably clear. on descending to the beach it was found that, instead of the whole lantern being visible as usual, only half could be seen--a circumstance doubtless due to the fact that the air's refractive power, which usually diminishes the dip due to the earth's curvature by about one-sixth part, was less efficient that morning than usual. the effect of the peculiarity was manifestly unfavourable to mr. rowbotham's theory. the curvature of the earth produced a greater difference than usual between the appearance of a distant object as seen from a certain high station and from a certain low station (though still the difference fell short of that which would be shown if there were no air). but parallax claimed the peculiarity observable that morning as an argument in favour of his flat earth. it is manifest, he said, that there is something wrong about the accepted theory; for it tells us that so much less of the lighthouse should be seen from the beach than from the hoe, whereas less still was seen. and many of the plymouth folk went away from the hoe that morning, and from the second lecture, in which parallax triumphantly quoted the results of the observation, with the feeling which had been expressed seven years before in the 'leicester advertiser,' that 'some of the most important conclusions of modern astronomy had been seriously invalidated.' if our books of astronomy, in referring to the effects of the earth's curvature, had only been careful to point out how surveyors and sailors and those who build lighthouses take into account the modifying effects of atmospheric refraction, and how these effects have long been known to vary with the temperature and pressure of the air, this mischief would have been avoided. it would not be fair to say of the persons misled on that occasion by parallax that they deserved no better; since the fault is not theirs as readers, but that of careless or ill-informed writers. another experiment conducted by parallax the same morning was creditable to his ingenuity. nothing better, perhaps, was ever devised to deceive people, apparently by ocular evidence, into the belief that the earth is flat--nor is there any clearer evidence of the largeness of the earth's globe compared with our ordinary measures. on the hoe, some ninety or a hundred feet above the sea-level, he had a mirror suspended in a vertical position facing the sea, and invited the bystanders to look in that mirror at the sea-horizon. to all appearance the line of the horizon corresponded exactly with the level of the eye-pupils of the observer. now, of course, when we look into a mirror whose surface is exactly vertical, the line of sight to the eye-pupils of our image in the mirror is exactly horizontal; whereas the line of sight from the eyes to the image of the sea-horizon is depressed exactly as much as the line from the eyes to the real sea-horizon. here, then, seemed to be proof positive that there is no depression of the sea-horizon; for the horizontal line to the image of the eye-pupil seemed to coincide exactly with the line to the image of the sea-horizon. it is not necessary to suppose here that the mirror was wrongly adjusted, though the slightest error of adjustment would affect the result either favourably or unfavourably for parallax's flat-earth theory. it is a matter of fact that, if the mirror were perfectly vertical, only very acute vision could detect the depression of the image of the sea-horizon below the image of the eye-pupil. the depression can easily be calculated for any given circumstances. parallax encouraged observers to note very closely the position of the eye-pupil in the image, so that most of them approached the image within about ten inches, or the glass within about five. now, in such a case, for a height of one hundred feet above the sea-level the image of the sea-horizon would be depressed below the image of the eye-pupil by less than three hundredths of an inch--an amount which could not be detected by one eye in a hundred. the average diameter of the pupil itself is one-fifth of an inch, or about seven times as great as the depression of the sea-horizon in the case supposed. it would require very close observation and a good eye to determine whether a horizontal line seen on either side of the head were on the level of the centres of the eye-pupils, or lower by about one-seventh of the breadth of either pupil. the experiment is a pretty one, however, and well worth trying by any one who lives near to the sea-shore and sea-cliffs. but there is a much more effective experiment which can be much more easily tried--only it is open to the disadvantage that it at once demolishes the argument of our friend parallax. it occurred to me while i was writing the above paragraph. let a very small mirror (it need not be larger than a sixpence) be so suspended to a small support and so weighted that when left to itself it hangs with its face perfectly vertical--an arrangement which any competent optician will easily secure--and let a fine horizontal line or several horizontal lines be marked on the mirror; which, by the way, should be a metallic one, as its indications will then be altogether more trustworthy. this mirror can be put into the waistcoat pocket and conveniently carried to much greater height than the mirror used by parallax. now, at some considerable height--say five or six hundred feet above the sea-level, but a hundred or even fifty will suffice--look into this small mirror while _facing_ the sea. the true horizon will then be seen to be visibly below the centre of the eye-pupil--visibly in this case because the horizontal line traced on the mirror can be made to coincide with the sea-horizon exactly, and will then be found _not_ to coincide with the centre of the eye-pupil. such an instrument could be readily made to show the distance of the sea-horizon, which at once determines the height of the observer above the sea-level. for this purpose all that would be necessary would be a means of placing the eye at some definite distance from the small mirror, and a fine vertical scale on the mirror to show the exact depression of the sea-horizon. for balloonists such an instrument would sometimes be useful, as showing the elevation independently of the barometer, whenever any portion of the sea-horizon was in view. the mention of balloon experiences leads me to another delusive argument of the earth-flatteners.[ ] it has been the experience of all aeronauts that, as the balloon rises, the appearance of the earth is by no means what would be expected from the familiar teachings in our books of astronomy. there is a picture in most of these books representing the effect of ascent above the sea-level in depressing the line of sight to the horizon, and bringing more and more into view the convexity of the earth's globe. one would suppose, from the picture, that when an observer is at a great height the earth would appear to rise under him, like some great round and well-curved shield whose convexity was towards him. instead of this, the aeronaut finds the earth presenting the appearance of a great hollow basin, or of the concave side of a well-curved shield. the horizon seems to rise as he rises, while the earth beneath him sinks lower and lower. a somewhat similar phenomenon may be noted when, after ascending the landward side of a high cliff, we come suddenly upon a view of the sea--invariably the sea-horizon is higher than we expected to find it. _only_, in this case, the surface of the sea seems to rise from the beach below towards the distant horizon convexly not concavely; the reason of which i take to be this, that the waves, and especially long rollers or uniform large ripples, teach the eye to form true conceptions of the shape of the sea-surface even when the eye is deceived as to the position of the sea-horizon. indeed, i should much like to know what would be the appearance of the sea from a balloon when no land was in sight (though i do not particularly wish to make the observation myself): the convexity discernible, for the reason just named, would contend strangely with the concavity imagined, for the reason now to be indicated. the deception arises from the circumstance that the scene displayed below and around the balloon is judged by the eye from the experience of more familiar scenes. the horizon is depressed, but so little that the eye cannot detect the depression, especially where the boundary of the horizon is irregular. it is here that the text-book pictures mislead; for they show the depression as far too great to be overlooked, setting the observer sometimes about two thousand miles above the sea-level. the eye, then, judges the horizon to be where it usually is--on the same level as the observer; but looking downwards, the eye perceives, and at once appreciates if it does not even exaggerate, the great depth at which the earth lies below the balloon. the appearance, then, as judged by the eye, is that of a mighty basin whose edge rises up all round to the level of the balloon, while its bottom lies two or three miles or more below the balloon. the zetetic faithful reason about this matter as though the impressions of the senses were trustworthy under all conditions, familiar or otherwise; whereas, in point of fact, we know that the senses often deceive, even under familiar conditions, and almost always deceive under conditions, which are not familiar. a person, for example, accustomed to the mist and haze of our british air, is told by the sense of sight, when he is travelling where a clearer atmosphere prevails, that a mountain forty miles from him is a hill a few miles away. on the other hand, an italian travelling through the highlands is impressed with the belief that all the features of the scenery are much larger (because he supposes them much more remote) than they really are. a hundred such instances of deception might easily be cited. the conditions under which the aeronaut observes the earth are certainly less familiar than those under which the briton views the alps and apennines, or the italian views ben lomond or ben lawers. it would be rash, therefore, even if no other evidence were available, to reject the faith that the earth is a globe because, as seen from a balloon, it looks like a basin. indeed, to be strictly logical, the followers of parallax ought on this account to adopt the faith that the earth is not flat, but basin-shaped, which hitherto they have not been ready to do. we have seen that parallax describes a certain experiment on the bedford level, which, if made as he states, would have shown certainly that something was wrong in the accepted system--for a six-mile straight-edge along water would be as severe a blow to the belief in a round earth, as a straight line on the sea-surface from queenstown to new york. another curious experiment adorns his little book, which, if it could be repeated successfully before a dozen trustworthy witnesses, would rather astonish men of science. having, he says, by certain reasoning--altogether erroneous, but that is a detail--convinced himself that, on the accepted theory, a bullet fired vertically upwards ought to fall far to the west of the place whence it was fired, he carefully fixed an air-gun in a vertical position, and fired forty bullets vertically upwards. all these fell close to the gun--which is not surprising, though it must have made such an experiment rather dangerous; but two fell back into the barrel itself--which certainly was very surprising indeed. one might fairly challenge the most experienced gunner in the world to achieve one such vertical shot in a thousand trials; two in forty bordered on the miraculous. the earth-flatteners i have been speaking of claim, as one of their objects, the defence of scripture. but some of the earth-flatteners of the last generation (or a little farther back) took quite another view of the matter. for instance, sir richard phillips, a more vehement earth-flattener than parallax, was so little interested in defending the scriptures, that in he was sentenced to a year's imprisonment for selling a book regarded as atheistic. in he attempted the conversion of professor de morgan, opening the correspondence with the remark that he had 'an inveterate abhorrence of all the pretended wisdom of philosophy derived from the monks and doctors of the middle ages, and not less those of higher name who merely sought to make the monkish philosophy more plausible, or so to disguise it as to mystify the mob of small thinkers.' he seems himself to have succeeded in mystifying many of those whom he intended to convert. admiral smyth gives the following account of an interview he had with phillips: 'this pseudo-mathematical knight once called upon me at bedford, without any previous acquaintance, to discuss "those errors of newton, which he almost blushed to name," and which were inserted in the "principia" to "puzzle the vulgar." he sneered with sovereign contempt at the "trinity of gravitating force, projectile force, and void space," and proved that all change of place is accounted for by motion.' [startling hypothesis!] 'he then exemplified the conditions by placing some pieces of paper on a table, and slapping his hand down close to them, thus making them fly off, which he termed applying the momentum. all motion, he said, is in the direction of the forces; and atoms seek the centre by "terrestrial centripetation"--a property which causes universal pressure; but in what these attributes of pushing and pulling differ from gravitation and attraction was not expounded. many of his "truths" were as mystified as the conundrums of rabelais; so nothing was made of the motion.' a favourite subject of paradoxical ideas has been the moon's motion of rotation. strangely enough, de morgan, who knew more about past paradoxists than any man of his time, seems not to have heard of the dispute between keill and bentley over this matter in . he says, 'there was a dispute on the subject, in , between james ferguson and an anonymous opponent; and i think there have been others;' but the older and more interesting dispute he does not mention. bentley, who was no mathematician, pointed out in a lecture certain reasons for believing that the moon does not turn on her axis, or has no axis on which she turns. keill, then only nineteen years old, pointed out that the arguments used by bentley proved that the moon does rotate instead of showing that she does not. (twenty years later keill was appointed savilian professor of astronomy at oxford. he was the first holder of that office to teach the newtonian astronomy.) in recent times, as most of my readers know, the paradox that the moon does not rotate has been revived more than once. in it was sustained by mr. jellinger symons, one of whose staunchest supporters, mr. h. perigal, had commenced the attack a few years earlier. of course, the gist of the argument against the moon's rotation lies in the fact that the moon always keeps the same face turned towards the earth, or very nearly so. if she did so exactly, and if her distance from the earth were constantly the same, then her motion would be exactly the same as though she were rigidly connected with the earth, and turned round an axis at the earth. the case may be thus illustrated: through the middle of a large orange thrust one short rod vertically, and another long rod horizontally; thrust the further end of the latter through a small apple, and now turn the whole affair round the short vertical rod as an axis. then the apple will move with respect to the orange as the moon would move with respect to the earth on the suppositions just made. no one in this case would say that the apple was turning round on its axis, since its motion would be one of rotation round the upright axis through the orange. therefore, say the opponents of the moon's rotation, no one should say that the moon turns round on her axis. of course, the answer would be obvious even if the moon's motions were as supposed. the moon is not connected with the earth as the apple is with the orange in the illustrative case. if the apple, without rigid connection with the orange, were carried round the orange so as to move precisely as if it were so connected, it would unquestionably have to rotate on its axis, as any one will find who may try the experiment. thus for the straight rod thrust through the apple substitute a straight horizontal bar carrying a small basin of water in which the apple floats. sway the bar steadily and slowly round, and it will be found (if a mark is placed on the apple) that the apple no longer keeps the same face towards the centre of motion; but that, to cause it to do so, a slow motion of rotation must be communicated to the apple in the same direction and at the same rate (neglecting the effects of the friction of the water against the sides of the basin) as the bar is rotating. in my 'treatise on the moon' i have described and pictured a simple apparatus by which this experiment may easily be made. but, of course, such experiments are not essential to the argument by which the paradox is overthrown. this argument simply is, that the moon as she travels on her orbit round the sun--the real centre of her motion--turns every part of her equator in succession towards him once in a lunar month. at the time of new moon the sun illuminates the face of the moon turned from us; at the time of full moon he illuminates the face which has been gradually brought round to him as the moon has passed through her first two quarters. as she passes onwards to new moon again, the face we see is gradually turned from him until he shines full upon the other face. and so on during successive lunations. this could not happen unless the moon rotated. again, if we lived on the moon we should find the heaven of the fixed stars turning round from east to west once in rather more than twenty-seven days; and unless we supposed, as we should probably do for a long time, that our small world was the centre of the universe, and that the stars turned round it, we should be compelled to admit that it was turning on its own axis from west to east once in the time just named. there would be no escape. the mere fact that all the time the stars thus seemed to be turning round the moon, the earth would not so seem to move, but would lie always in the same direction, would in no sort help to remove the difficulty. lunarian paradoxists would probably argue that she was in some way rigidly connected with the moon; but even they would never think of arguing that their world did not turn on its axis, _unless_ they maintained that it was the centre of the universe. this, i think, they would very probably do; but as yet terrestrial paradoxists have not, i believe, maintained this hypothesis. i once asked mr. perigal whether that was the true theory of the universe--the moon central, the earth, sun, and heavens carried round her. he admitted that his objections to accepted views were by no means limited to the moon's rotation; and, if i remember rightly, he said that the idea i had thrown out in jest was nearer the truth than i thought, or used words to that effect. but as yet the theory has not been definitely enunciated that the moon is the boss of the universe. comets, as already mentioned, have been the subjects of paradoxes innumerable; but as yet comets have been so little understood, even by astronomers, that paradoxes respecting them cannot be so readily dealt with as those relating to well-established facts. among thoroughly paradoxical ideas respecting comets, however, may be mentioned one whose author is a mathematician of well-deserved repute--professor tait's 'sea-bird theory' of comets' tails. according to this theory, the rapid formation of long tails and the rapid changes of their position may be explained on the same principle that we explain the rapid change of appearance of a flight of sea-birds, when, from having been in a position where the eye looks athwart it, the flight assumes a position where the eye looks at it edgewise. in the former position it is scarcely visible (when at a distance), in the latter it is seen as a well-defined streak; and as a very slight change of position of each bird may often suffice to render an extensive flight thus visible throughout its entire length, which but a few moments before had been invisible, so the entire length of a comet's tail may be brought into view, and apparently be formed in a few hours, through some comparatively slight displacement of the individual meteorites composing it. this paradox--for paradox it unquestionably is--affords a curious illustration of the influence which mathematical power has on the minds of men. every one knows that professor tait has potential mathematical energy competent to dispose, in a very short time, of all the difficulties involved in his theory; therefore few seem to inquire whether this potential energy has ever been called into action. it is singular, too, that other mathematicians of great eminence have been content to take the theory on trust. thus sir w. thomson, at the meeting of the british association at edinburgh, described the theory as disposing easily of the difficulties presented by newton's comet in . glashier, in his translation of guillemin's 'les comètes,' speaks of the theory as one not improbably correct, though only to be established by rigid investigation of the mathematical problems involved. in reality, not five minutes' inquiry is needed to show any one acquainted with the history of long-tailed comets that tait's theory is quite untenable. take newton's comet. it had a tail ninety millions of miles long, extending directly from the sun as the comet approached him, and seen, four days later, extending to the same distance, and still directly from the sun, as the comet receded from him in an entirely different direction. according to tait's sea-bird theory, the earth was at both these epochs in the plane of a sheet of meteorites forming the tail; but on each occasion the sun also was in the same plane, for the edge of the sheet of meteorites was seen to be directly in a line with the sun. the comet's head, of course, was in the same plane; but three points, not in a straight line, determine a plane. hence we have, as the definite result of the sea-bird theory, that the layer or stratum of meteorites, forming the tail of newton's comet, lay in the same plane which contained the sun, the earth, and the comet. but the comet crossed the ecliptic (the plane in which the earth travels round the sun) between the epochs named, crossing it at a great angle. when crossing it, then, the great layer of meteorites was in the plane of the ecliptic; before crossing it the layer was greatly inclined to that plane one way, and after crossing it the layer was greatly inclined to that plane another way. so that we have in no way escaped the difficulty which the sea-bird theory was intended to remove. if it was a startling and, indeed, incredible thing that the particles along a comet's tail should have got round in four days from the first to the second position of the tail considered above, it is as startling and incredible that a mighty layer of meteorites should have shifted bodily in the way required by the sea-bird theory. nay, there is an element in our result which is still more startling than any of the difficulties yet mentioned; and that is, the singular care which the great layer of meteorites would seem to have shown to keep its plane always passing through the earth, with which it was in no way connected. why should this preference have been shown by the meteor flock for our earth above all the other members of the solar system?--seeing that the sea-bird theory _requires_ that this comet, and not newton's comet alone but all others having tails, should not only be thus complaisant with respect to our little earth, but should behave in a totally different way with respect to every other member of the sun's family. we can understand that, while several have been found who have applauded the sea-bird paradox for what it _might_ do in explaining comets' tails, its advocates have as yet not done much to reconcile it with cometic observation. the latest astronomical paradox published is perhaps still more startling. it relates to the planet venus, and is intended to explain the appearance presented by this planet when crossing the sun's face, or, technically, when in transit. at this time she is surrounded by a ring of light, which appears somewhat brighter than the disc of the sun itself. before fully entering on the sun's face, also, the part of venus's globe as yet outside the sun's disc is seen to be girt round by a ring of exceedingly bright light--so bright, indeed, that it has left its record in photographs where the exposure was only for the small fraction of a second allowable in the case of so intensely brilliant a body as the sun. astronomers have not found it difficult to explain either peculiarity. it has been proved clearly in other ways that venus has an atmosphere like our own, but probably denser. as the sun is raised into view above the horizon (after he has really passed below the horizon plane) by the bending power of our air upon his rays, so the bending power of venus's air brings the sun into our view round the dark body of the planet. but the new paradox advances a much bolder theory. instead of an atmosphere such as ours, venus has a glass envelope; and instead of a surface of earth and water, in some cases covered with clouds, venus has a surface shining with metallic lustre.[ ] the author of this theory, mr. jos. brett, startled astronomers by announcing, a few years ago, that with an ordinary telescope he could see the light of the sun's corona without the aid of an eclipse, though astronomers had observed that the delicate light of the corona fades out of view with the first returning rays of the sun after total eclipse. the latest paradoxist, misled by the incorrect term 'centrifugal force,' proposes to 'modify, if not banish,' the old-fashioned astronomy. what is called centrifugal force is in truth only inertia. in the familiar instance of a body whirled round by a string, the breaking of the string no more implies that an active force has pulled away the body, than the breaking of a rope by which a weight is pulled implies that the weight has exerted an active resistance. of course, here again the text-books are chiefly in fault. such are a few among the paradoxes of various orders by which astronomers, like the students of other sciences, have been from time to time amused. it is not altogether, as it may seem at first sight, 'a sin against the twenty-four hours' to consider such matters; for much may be learned not only from the study of the right road in science, but from observing where and how men may go astray. i know, indeed, few more useful exercises for the learner than to examine a few paradoxes, when leisure serves, and to consider how, if left to his own guidance, he would confute them. xi. _on some astronomical myths._ the expression 'astronomical myth' has recently been used, on the title-page of a translation from the french, as synonymous with false systems of astronomy. it is not, however, in that sense that i here use it. the history of astronomy presents the records of some rather perplexing observations, not confirmed by later researches, but yet not easily to be explained away or accounted for. such observations humboldt described as belonging to the myths of an uncritical period; and it is in that sense that i employ the term 'astronomical myth' in this essay. i propose briefly to describe and comment on some of the more interesting of these observations, which, in whatever sense they are to be interpreted, will be found to afford a useful lesson. it is hardly necessary, perhaps, to point out that the cases which i include here i regard as really cases in which astronomers have been deceived by illusory observations. other students of astronomy may differ from me as respects some of these instances. i do not wish to dogmatise, but simply to describe the facts as i see them, and the impressions which i draw from them. those who view the facts differently will not, i think, have to complain that i have incorrectly described them. at the outset, let me point out that some observations which were for a long time regarded as mythical have proved to be exact. for instance, when as yet very few telescopes existed, and those very feeble, galileo's discovery of moons travelling round jupiter was rejected as an illusion for which satan received the chief share of credit. there is an amusing and yet in one aspect almost pathetic reference to this in his account of his earlier observations of saturn. he had seen the planet apparently attended on either side by two smaller planets, as if helping old saturn along. but on december , ,[ ] turning his telescope on the planet, he found to his infinite amazement not a trace of the companion planets could be seen; there in the field of view of his telescope was the golden-tinted disc of the planet as smoothly rounded as the disc of mars or jupiter. 'what,' he wrote, 'is to be said concerning so strange a metamorphosis? are the two lesser stars consumed after the manner of the solar spots? have they vanished or suddenly fled? has saturn, perhaps, devoured his children? or were the appearances, indeed, illusion or fraud with which the glasses have so long deceived me as well as many others to whom i have shown them? now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of the existence of those things which the telescope appears to show. i do not know what to say in a case so surprising, so unlooked for, and so novel. the shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me.' we now know that these observations, as well as those made soon after by hevelius, though wrongly interpreted, were correct enough. nay, we know that if either galileo or hevelius had been at the pains to reason out the meaning of the alternate visibility and disappearance of objects looking like attendant planets, they must have anticipated the discovery made in by huyghens, that saturn's globe is girdled about by a thin flat ring so vast that, if a score of globes like our earth were set side by side, the range of that row of worlds would be less than the span of the saturnian ring system. there is a reference in galileo's letter to the solar spots; 'are the two lesser stars,' he says, 'consumed after the manner of the solar spots?' when he thus wrote the spots were among the myths or fables of astronomy, and an explanation was offered, by those who did not reject them utterly, which has taken its place among forsaken doctrines, those broken toys of astronomers. it is said that when scheiner, himself a jesuit, communicated to the provincial of the jesuits his discovery of the spots on the sun, the latter, a staunch aristotelian, cautioned him not to see these things. 'i have read aristotle's writings from beginning to end many times,' he said, 'and i can assure you i have nowhere found in them anything similar to what you mention' [amazing circumstances!] 'go, therefore, my son, tranquillise yourself; be assured that what you take for spots on the sun are the faults of your glasses or your eyes.' as the idea was obviously inadmissible that a celestial body could be marked by spots, the theory was started that the dark objects apparently seen on the sun's body were in reality small planets revolving round the sun, and a contest arose for the possession of these mythical planets. tardé maintained that they should be called _astra borbonia_, in honour of the royal family of france; but c. malapert insisted that they should be called _sidera austriaca_. meantime the outside world laughed at the spots, and their names, and the astronomers who were thought to have invented both. 'fabritius puts only three spots,' wrote burton in his 'anatomy of melancholy,' 'and those in the sun; apelles , and those without the sun, floating like the cyanean isles in the euxine sea. tardé the frenchman hath observed , and those neither spots nor clouds as galileus supposed, but planets concentric with the sun, and not far from him, with regular motions. christopher schemer' [a significant way of spelling scheiner's name], 'a german suisser jesuit, divides them _in maculas et faculas_, and will have them to be fixed _in solis superficie_ and to absolve their periodical and regular motions in or dayes; holding withall the rotation of the sun upon his centre, and are all so confident that they have made schemes and tables of their motions. the hollander censures all; and thus they disagree among themselves, old and new, irreconcilable in their opinions; thus aristarchus, thus hipparchus, thus ptolomæus, thus albategnius, etc., with their followers, vary and determine of these celestial orbs and bodies; and so whilst these men contend about the sun and moon, like the philosophers in lucian, it is to be feared the sun and moon will hide themselves, and be as much offended as she was with those, and send another message to jupiter, by some new-fangled icaromenippus, to make an end of all these curious controversies, and scatter them abroad.' it is well to notice how in this, as in many other instances, the very circumstance which makes scientific research trustworthy caused the unscientific to entertain doubt. if men of science were to arrange beforehand with each other what observations they should publish, how their accounts should be ended, what theories they would endeavour to establish, their results would seem far more trustworthy, their theories far more probable, than according to the method actually adopted. science, which should be exact, seems altogether inexact, because one observer seems to obtain one result, another a different result. scientific theories seem unworthy of reliance because scientific men entertain for a long time rival doctrines. but in another and a worthier sense than as the words are used in the 'critic,' when men of science do agree their agreement is wonderful. it _is_ wonderful, worthy of all admiration, because before it has been attained errors long entertained have had to be honestly admitted; because the taunt of inconsistency is not more pleasant to the student of science than to others, and the man who having a long time held one doctrine adopts and enforces another (one perhaps which he had long resisted), is sure to be accused by the many of inconsistency, the truly scientific nature of his procedure being only recognised by the few. the agreement of men of science ought to be regarded also as most significant in another sense. so long as there is room for refusing to admit an important theory advanced by a student of science, it is natural that other students of science should refuse to do so; for in admitting the new theory they are awarding the palm to a rival. in strict principle, of course, this consideration ought to have no influence whatever; as a matter of fact, however, men of science, being always men and not necessarily strengthened by scientific labours against the faults of humanity, the consideration has and must always have influence. therefore, when the fellow-writers and rivals of newton or of his followers gave in their adhesion to the newtonian theory; when in our own time--but let us leave our own time alone, in this respect--when, speaking generally, a novel doctrine, or some new generalisation, or some great and startling discovery, is admitted by rival students of the branch of astronomy to which it belongs, the probability is great that the weight of evidence has been found altogether overwhelming. let us now, however, turn to cases in which, while many observations seem to point to some result, it has appeared that, after all, those observations must have been illusory. a striking instance in point is found in the perplexing history of the supposed satellite of venus. on january , , the celebrated astronomer, j.d. cassini saw a crescent shaped and posited like venus, but smaller, on the western side of the planet. more than fourteen years later, he saw a crescent east of the planet. the object continued visible in the latter case for half an hour, when the approach of daylight obliterated the planet and this phantom moon from view. the apparent distance of the moon from venus was in both cases small, viz., only one diameter of the planet in the former case, and only three-fifths of that diameter in the latter. next, on october , , old style, the optician short, who had had considerable experience in observation, saw a small star perfectly defined but less luminous than venus, at a distance from the planet equal to about one-third of the apparent diameter of our moon. this is a long distance, and would correspond to a distance from venus certainly not less than the moon's distance from the earth. short was aware of the risk of optical illusion in such matters, and therefore observed venus with a second telescope; he also used four eye-pieces of different magnifying power. he says that venus was very distinct, the air very pure, insomuch that he was able to use a power of . the seeming moon had a diameter less than a third of venus's, and showed the same phase as the planet. its disc was exceedingly well defined. he observed it several times during a period of about one hour. still more convincing, to all appearance, is the account of the observations made by m. montaigne, as presented to the academy of sciences at paris by m. baudouin in . the transit of venus which was to take place on june in that year led to some inquiry as to the satellite supposed to have been seen by cassini and short, for of course a transit would be a favourable occasion for observing the satellite. m. montaigne, who had no faith in the existence of such an attendant, was persuaded to look for it early in . on may he saw a little crescent moon about twenty minutes of arc (nearly two-thirds the apparent diameter of our moon) from the planet. he repeated his observation several times that night, always seeing the small body, but not quite certain, despite its crescent shape, whether it might not be a small star. on the next evening, and again on may and , he saw the small companion apparently somewhat farther from venus and in a different position. he found that it could be seen when venus was not in the field of view. the following remarks were made respecting these observations in a french work, 'dictionnaire de physique,' published in :--'the year will be celebrated in astronomy in consequence of the discovery that was made on may of a satellite circulating round venus. we owe it to m. montaigne, member of the society of limoges. m. baudouin read before the academy of sciences at paris a very interesting memoir, in which he gave a determination of the revolution and distance of the satellite. from the calculations of this expert astronomer we learn that the new star has a diameter about one-fourth that of venus, is distant from venus almost as far as the moon from our earth, has a period of nine days seven hours' [much too short, by the way, to be true, expert though m. baudouin is said to have been], 'and its ascending node'--but we need not trouble ourselves about its ascending node. three years later rödkier, at copenhagen, march and , , saw the satellite of venus with a refracting telescope feet long, which should have been effective if longitude has any virtue. he could not see the satellite with another telescope which he tried. but several of his friends saw it with the long telescope. amongst others, horrebow, professor of astronomy, saw the satellite on march and , after taking several precautions to prevent optical illusion. a few days later montbaron, at auxerre, who had heard nothing of these observations, saw a satellite, and again on march and it appeared, always in a different position. it should be added that scheuten asserted that during the transit of venus was accompanied by a small satellite in her motion across the sun's face. so confidently did many believe in this satellite of venus that frederick the great, who for some reason imagined that he was entitled to dispose as he pleased of the newly discovered body, proposed to assign it away to the mathematician d'alembert, who excused himself from accepting the questionable honour in the following terms:-- 'your majesty does me too much honour in wishing to baptize this new planet with my name. i am neither great enough to become the satellite of venus in the heavens, nor well enough (_assez bien portant_) to be so on the earth, and i am too well content with the small place i occupy in this lower world to be ambitious of a place in the firmament.' it is not at all easy to explain how this phantom satellite came to be seen. father hell, of vienna--the same astronomer whom sir g. airy suspects of falling asleep during the progress of the transit of venus in --made some experiments showing how a false image of the planet might be seen beside the true one, the false image being smaller and fainter, like the moons seen by schort (as hell called short), cassini, and the rest. and more recently sir david brewster stated that wargentin 'had in his possession a good achromatic telescope, which always showed venus with such a satellite.' but hell admitted that the falsehood of the unreal venus was easily detected, and brewster adds to his account of wargentin's phantom moon, that 'the deception was discovered by turning the telescope about its axis.' as admiral smyth well remarks, to endeavour to explain away in this manner the observations made by cassini and short 'must be a mere pleasantry, for it is impossible such accurate observers could have been deceived by so gross a neglect.' smyth, by the way, was a believer in the moon of venus. 'the contested satellite is perhaps extremely minute,' he says, 'while some parts of its body may be less capable of reflecting light than others; and when the splendour of its primary and our inconvenient station for watching it are considered, it must be conceded that, however slight the hope may be, search ought not to be relinquished.' setting aside scheuten's asserted recognition of a dark body near venus during the transit of , venus has always appeared without any attendant when in transit. as no one else claimed to have seen what scheuten saw in , though the transit was observed by hundreds, of whom many used far finer telescopes than he, we must consider that he allowed his imagination to deceive him. during the transit of , and again on december - , , venus certainly had no companion during her transit. what, then, was it that cassini, short, montaigne, and the rest supposed they saw? the idea has been thrown out by mr. webb that mirage caused the illusion. but he appears to have overlooked the fact that though an image of venus formed by mirage would be fainter than the planet, it would not be smaller. it might, according to the circumstances, be above venus or below, or even somewhat towards either side, and it might be either a direct or an inverted image, but it could not possibly be a diminished image. single observations like cassini's or short's might be explained as subjective phenomena, but this explanation will not avail in the case of the copenhagen observations. i reject, as every student of astronomy will reject, the idea of wilful deception. occasionally an observer may pretend to see what he has not seen, though i believe this very seldom happens. but even if cassini and the rest had been notoriously untrustworthy persons instead of being some of them distinguished for the care and accuracy with which their observations were made and recorded, these occasional views of a phantom satellite are by no means such observations as they would have invented. no distinction was to be gained by observations which could not be confirmed by astronomers possessing more powerful telescopes. cassini, for example, knew well that nothing but his well-earned reputation could have saved him from suspicion or ridicule when he announced that he had seen venus attended by a satellite. it seems to me probable that the false satellite was an optical illusion brought about in a different way from those referred to by hell and brewster, though among the various circumstances which in an imperfect instrument might cause such a result i do not undertake to make a selection. it is certain that venus's satellite has vanished with the improvement of telescopes, while it is equally certain that even with the best modern instruments illusions occasionally appear which deceive even the scientific elect. three years have passed since i heard the eminent observer otto struve, of pulkowa, give an elaborate account of a companion to the star procyon, describing the apparent brightness, distance, and motions of this companion body, for the edification of the astronomer-royal and many other observers. i had visited but a few months before the observatory at washington, where, with a much more powerful telescope, that companion to procyon had been systematically but fruitlessly sought for, and i entertained a very strong opinion, notwithstanding the circumstantial nature of struve's account and his confidence (shared in unquestioningly by the observers present), that he had been in some way deceived. but i could not then see, nor has any one yet explained, how this could be. the fact, however, that he had been deceived is now undoubted. subsequent research has shown that the pulkowa telescope, though a very fine instrument, possesses the undesirable quality of making a companion orb for all first-class stars in the position where o. struve and his assistant lindenau saw the supposed companion of procyon. i may as well point out, however, that theories so wild have recently been broached respecting venus, that far more interesting explanations of the enigma than this optical one may be looked for presently. it has been gravely suggested by mr. jos. brett, the artist, that venus has a surface of metallic brilliancy, with a vitreous atmosphere,--which can only be understood to signify a glass case. this stupendous theory has had its origin in an observation of considerable interest which astronomers (it is perhaps hardly necessary to say) explain somewhat differently. when venus has made her entry in part upon the sun's face at the beginning of transit, there is seen all round the portion of her disc which still remains outside the sun an arc of light so brilliant that it records its photographic trace during the instantaneous exposure required in solar photography. it is mathematically demonstrable that this arc of light is precisely what _should_ be seen if venus has an atmosphere like our earth's. but mathematical demonstration is not sufficient (or perhaps we may say it is too much) for some minds. therefore, to simplify matters, venus has been provided with a mirror surface and a glass case. (see preceding essay, on astronomical paradoxes, for further details.) the enigma next to be considered is of a more doubtful character than the myth relating to the satellite of venus. astronomers are pretty well agreed that venus has no moon, but many, including some deservedly eminent, retain full belief in the story of the planet vulcan. more than seventeen years ago the astronomical world was startled by the announcement that a new planet had been discovered, under circumstances unlike any which had heretofore attended the discovery of fresh members of the solar system. at that time astronomers had already become accustomed to the discovery, year after year, of several asteroids, which are in reality planets, though small ones. in fact, no less than fifty-six of these bodies were then known, whereof fifty-one had been discovered during the years - inclusive, not one of these years having passed without the detection of an asteroid. but all these planets belonged to one family, and as there was every reason to believe that thousands more travel in the same region of the solar system, the detection of a few more among the number had no longer any special interest for astronomers. the discovery of the first known member of the family had indeed been full of interest, and had worthily inaugurated the present century, on the first day of which it was made. for it had been effected in pursuance of a set scheme, and astronomers had almost given up all hopes of success in that scheme when piazzi announced his detection of little ceres. again the discovery of the next few members of the family had been interesting as revealing the existence of a new order of bodies in the solar system. no one had suspected the possibility that besides the large bodies which travel round the sun, either singly or attended by subordinate families of moons, there might be a ring of many planets. this was what the discovery of ceres, pallas, juno, and vesta seemed to suggest, unless--still stranger thought--these were but fragments of a mighty planet which had been shattered in long-past ages by some tremendous explosion. since then, however, this startling theory has been (itself) exploded. year after year new members of the ring of multitudinous planets are discovered, and that, not as was recently predicted, in numbers gradually decreasing, but so rapidly that more have been discovered during the last ten years than during the preceding twenty. the discovery of the giant planet uranus, an orb exceeding our earth twelve and a half times in mass and seventy-four times in volume, was a matter of much greater importance, so far as the dignity of the planetary system was concerned, for it is known that the whole ring of asteroids together does not equal one-tenth part of the earth in mass, while uranus exceeds many times in volume the entire family of terrestrial planets--mercury, venus, the earth, and mars. the detection of uranus, unlike that of ceres, was effected by accident. sir w. herschel was looking for double stars of a particular kind in the constellation gemini when by good fortune the stranger was observed. the interest with which astronomers received the announcement of the discovery of uranus, though great, was not to be compared with that with which they deservedly welcomed the discovery of neptune, a larger and more massive planet, revolving at a distance one-half greater even than the mighty space which separates uranus from the sun, a space so great that by comparison with it the range of , , of miles, which forms the diameter of our earth's orbit, seems quite insignificant. it was not, however, the vastness of neptune's mass or volume, or the awful remoteness of the path along which he pursues his gloomy course, which attracted the interest of astronomers, but the strangeness of the circumstances under which the planet had been detected. his influence had been felt for many years before astronomers thought of looking for him, and even when the idea had occurred to one or two, it was considered, and that, too, by an astronomer as deservedly eminent as sir g. airy, too chimerical to be reasonably entertained. all the world now knows how leverrier, the greatest living master of physical astronomy, and adams, then scarce known outside cambridge, both conceived the idea of finding the planet, not by the simple method of looking for it with a telescope, but by the mathematical analysis of the planet's disturbing influence upon known members of the solar system. all know, too, that these mathematicians succeeded in their calculations, and that the planet was found in the very region and close to the very point indicated first by adams, and later, but independently, and (fortunately for him more publicly) by leverrier. none of these instances of the discovery of members of the solar system resembled in method or details the discovery announced early in the year . it was not amid the star-depths and in the darkness of night that the new planet was looked for, but in broad day, and on the face of the sun himself. it was not on the outskirts of the solar system that the planet was supposed to be travelling, but within the orbit of mercury, hitherto regarded as of all planets the nearest to the sun. it was not hoped that any calculation of the perturbations of other planets would show the place of the stranger, though certain changes in the orbit of mercury seemed clearly enough to indicate the stranger's existence. early in leverrier had announced that the position of mercury's path was not precisely in agreement with calculations based on the adopted estimates of the masses of those planets which chiefly disturb the motions of mercury. the part of the path where mercury is nearest to the sun, and where, therefore, he travels fastest, had slightly shifted from its calculated place. this part of the path was expected to move, but it had moved more than was expected; and of course mercury having his region of swiftest motion somewhat differently placed than was anticipated, himself moved somewhat differently. leverrier found that to explain this feature of mercury's motion either the mass of venus must be regarded as one-tenth greater than had been supposed, or some unknown cause must be regarded as affecting the motion of mercury. a planet as large as mercury, about midway between mercury and the sun, would account for the observed disturbance; but leverrier rejected the belief that such a planet exists, simply because he could not 'believe that it would be invisible during total eclipses of the sun.' 'all difficulties disappear,' he added, 'if we admit, in place of a single planet, small bodies circulating between mercury and the sun.' considering their existence as not at all improbable, he advised astronomers to watch for them. it was on january , , that leverrier thus wrote. on december , , a letter had been addressed by a m. lescarbault of orgères to leverrier, through m. vallée, hon. inspector-general of roads and bridges, announcing that on march , , about four in the afternoon, lescarbault had seen a round black spot on the face of the sun, and had watched it as it passed across like a planet in transit--not with the slow motion of an ordinary sun-spot. the actual time during which the round spot was visible was one hour, seventeen minutes, nine seconds, the rate of motion being such that, had the spot crossed the middle of the sun's disc, at the same rate, the transit would have lasted more than four hours. the spot thus merely skirted the sun's disc, being at no time more than about one forty-sixth part of the sun's apparent diameter from the edge of the sun. lescarbault expressed his conviction that on a future day, a black spot, perfectly round and very small, will be seen passing over the sun, and 'this point will very probably be the planet whose path i observed on march , .' 'i am persuaded,' he added, 'that this body is the planet, or one of the planets, whose existence in the vicinity of the sun m. leverrier had made known a few months ago' (referring to the preliminary announcement of results which leverrier published afterwards more definitely). leverrier, when the news of lescarbault's observation first reached him, was surprised that the observation should not have been announced earlier. he did not consider the delay sufficiently justified by lescarbault's statement that he wished to see the spot again. he therefore set out for orgères, accompanied by m. vallée. 'the predominant feeling in leverrier's mind,' says abbé moigno, 'was the wish to unmask an attempt to impose upon him, as the person more likely than any other astronomer to listen to the allegation that his prophecy had been fulfilled.' 'one should have seen m. lescarbault,' says moigno, 'so small, so simple, so modest, and so timid, in order to understand the emotion with which he was seized, when leverrier, from his great height, and with that blunt intonation which he can command, thus addressed him: "it is then you, sir, who pretend to have observed the intra-mercurial planet, and who have committed the grave offence of keeping your observation secret for nine months. i warn you that i have come here with the intention of doing justice to your pretensions, and of demonstrating either that you have been dishonest or deceived. tell me, then, unequivocally, what you have seen."' this singular address did not bring the interview, as one might have expected, to an abrupt end. the lamb, as the abbé calls the doctor, trembling, stammered out an account of what he had seen. he explained how he had timed the passage of the black spot. 'where is your chronometer?' asked leverrier. 'it is this watch, the faithful companion of my professional journeys.' 'what! with that old watch, showing only minutes, dare you talk of estimating seconds. my suspicions are already too well confirmed.' 'pardon me, i have a pendulum which beats seconds.' 'show it me.' the doctor brings down a silk thread to which an ivory ball is attached. fixing the upper end to a nail, he draws the ball a little from the vertical, counts the number of oscillations, and shows that his pendulum beats seconds; he explains also how his profession, requiring him to feel pulses and count pulsations, he has no difficulty in mentally keeping record of successive seconds. having been shown the telescope with which the observation was made, the record of the observation (on a piece of paper covered with grease and laudanum, and doing service as a marker in the 'connaissance des temps,' or french nautical almanac), leverrier presently inquired if lescarbault had attempted to deduce the planet's distance from the sun from the period of its transit. the doctor admitted that he had attempted this, but, being no mathematician, had failed to achieve success with the problem. he showed the rough draughts of his futile attempts at calculation on a board in his workshop, 'for,' said he naïvely, 'i am a joiner as well as an astronomer.' the interview satisfied leverrier that a new planet, travelling within the orbit of mercury, had really been discovered. 'with a grace and dignity full of kindness,' says a contemporary narrative of these events,[ ] 'he congratulated lescarbault on the important discovery which he had made.' anxious to obtain some mark of respect for the discoverer of vulcan, leverrier made inquiry concerning his private character, and learned from the village curé, the juge de paix, and other functionaries, that he was a skilful physician and a worthy man. with such high recommendations, m. leverrier requested from m. rouland, the minister of public instruction, the decoration of the legion of honour for m. lescarbault. the minister, in a brief but interesting statement of his claim, communicated this request to the emperor, who, by a decree dated january , conferred upon the village astronomer the honours so justly due to him. his professional brethren in paris were equally solicitous to testify their regard; and mm. felix roubaud, legrande, and caffe, as delegates of the scientific press, proposed to the medical body, and to the scientific world in paris, to invite lescarbault to a banquet in the hôtel du louvre on january . the announcement of the supposed discovery caused astronomers to re-examine records of former observations of black spots moving across the sun. several such records existed, but they had gradually come to be regarded as of no real importance. wolff of zurich published a list of no fewer than twenty such observations made since . carrington added many other cases. comparing together three of these observations, wolff found that they would be satisfied by a planet having a period of revolution of days, agreeing fairly with the period of rather more than - / days inferred by leverrier for lescarbault's planet. but the entire set of observations of black spots require that there should be at least three new planets travelling between mercury and the sun. many observers also set themselves the task of searching for vulcan, as the supposed new planet was called. they have continued fruitlessly to observe the sun for this purpose until the present time. while the excitement over lescarbault's discovery was at its height, another observer impugned not only the discovery but the honesty of the discoverer. m. liais, a french astronomer of considerable skill, formerly of the paris observatory, but at the time of lescarbault's achievement in the service of the brazilian government, published a paper, 'sur la nouvelle planète annoncée par m. lescarbault,' in which he endeavoured to establish the four following points:-- first, the observation of lescarbault was never made. secondly, leverrier was mistaken in considering that a planet such as vulcan might have escaped detection when off the sun's face. thirdly, that vulcan would certainly have been seen during total solar eclipses, if the planet had a real objective existence. fourthly, m. leverrier's reasons for believing that the planet exists are based on the supposition that astronomical observations are more precise than they really are. probably, liais's objections would have had more weight with leverrier had the fourth point been omitted. it was rash in a former subordinate to impugn the verdict of the chief of the paris observatory on a matter belonging to that special department of astronomy which an observatory chief might be expected to understand thoroughly. it is thought daring in the extreme for one outside the circles of official astronomy (as newton in flamstead's time, sir w. herschel in maskelyne's, and sir j. herschel in the present century), to advance or maintain an opinion adverse to that of some official chief, but for a subordinate (even though no longer so), to be guilty of such rash procedure 'is most tolerable and not to be endured,' as a typical official has said. accordingly, very little attention was paid by leverrier to liais's objections. yet, in some respects, what m. liais had to say was very much to the point. at the very time when lescarbault was watching the black spot on the sun's face, liais was examining the sun with a telescope of much greater magnifying power, and saw no such spot. his attention was specially directed to the edge of the sun (where lescarbault saw the spot) because he was engaged in determining the decrease of the sun's brightness near the edge. moreover, he was examining the very part of the sun's edge where lescarbault saw the planet enter, at a time when it must have been twelve minutes in time upon the face of the sun, and well within the margin of the solar disc. the negative evidence here is strong; though it must always be remembered that negative evidence requires to be overwhelmingly strong before it can be admitted as effective against positive evidence. it seems at a first view utterly impossible that liais, examining with a more powerful telescope the region where lescarbault saw the spot, could have failed to see it had it been there; but experience shows that it is not impossible for an observer engaged in examining phenomena of one class to overlook a phenomenon of another class, even when glaringly obvious. all we can say is that liais was not likely to have overlooked lescarbault's planet had it been there; and we must combine this probability against vulcan's existence with arguments derived from other considerations. there is also the possibility of an error in time. as the writer in the 'north british review' remarks, 'twelve minutes is so short a time that it is just possible that the planet may not have entered upon the sun during the time that liais observed it.' the second and third arguments are stronger. in fact, i do not see how they can be resisted. it is, in the first place, clear from lescarbault's account that vulcan must have a considerable diameter--certainly if vulcan's diameter in miles were only half the diameter of mercury, it would have been all but impossible for lescarbault with his small telescope to see vulcan at all, whereas he saw the black spot very distinctly. say vulcan has half the diameter of mercury, and let us compare the brightness of these two planets when at their greatest apparent distances from the sun, that is, when each looks like a half-moon. the distance of mercury exceeds the estimated distance of vulcan from the sun as exceeds , so that vulcan is more strongly illuminated in the proportion of times to times , or to --say at least to . but having a diameter but half as large the disc of vulcan could be but about a fourth of mercury's at the same distance from us (and they would be at about the same distance from us when seen as half-moons). hence vulcan would be brighter than mercury in the proportion of to . of course being so near the sun he would not be so easily seen; and we could never expect to see him at all, perhaps, with the naked eye--though even this is not certain. but mercury, when at the same apparent distance from the sun, and giving less light than at his greatest seeming distance, is quite easily seen in the telescope. much more easily, then, should vulcan be seen, if a telescope were rightly directed at such a time, or when vulcan was anywhere near his greatest seeming distance from the sun. now it is true astronomers do not know precisely when or where to look for him. but he passes from his greatest distance on one side of the sun to his greatest distance on the other in less than ten days, according to the computed period, and certainly (that is, if the planet exists) in a very short time. the astronomer has then only to examine day after day a region of small extent on either side of the sun, for ten or twelve days in succession (an hour's observation each day would suffice), to be sure of seeing vulcan. yet many astronomers have made such search many times over, without seeing any trace of the planet. during total solar eclipses, again, the planet has been repeatedly looked for unsuccessfully--though it should at such a time be a very conspicuous object, when favourably placed, and could scarcely fail of being very distinctly seen wherever placed. the fourth argument of lescarbault's is not so effective, and in fact he gets beyond his depth in dealing with it. but it is to be noticed that a considerable portion of the discrepancy between mercury's observed and calculated motions has long since been accounted for by the changed estimate of the earth's mass as compared with the sun's, resulting from the new determination of the sun's distance. however, the arguments depending on this consideration would not be suited to these pages. there was one feature in liais's paper which was a little unfortunate. he questioned lescarbault's honesty. he said 'lescarbault contradicts himself in having first asserted that he saw the planet enter upon the sun's disc, and having afterwards admitted to leverrier that it had been on the disc some seconds before he saw it, and that he had merely inferred the time of its entry from the rate of its motion afterwards. if this one assertion be fabricated, the whole may be so.' 'he considers these arguments to be strengthened,' says the 'north british review,' 'by the assertion which, as we have seen, perplexed leverrier himself, that if m. lescarbault had actually seen a planet on the sun, he could not have kept it secret for nine months.' this charge of dishonesty, unfortunate in itself, had the unfortunate effect of preventing lescarbault or the abbé moigno from replying. the latter simply remarked that the accusation was of such a nature as to dispense him from any obligation to refute it. this was an error of judgment, i cannot but think, if an effective reply was really available. the remarks with which the north british reviewer closes his account may be repeated now, so far as they relate to the force of the negative evidence, with tenfold effect. 'since the first notice of the discovery in the beginning of january the sun has been anxiously observed by astronomers; and the limited area around him in which the planet _must be_, if he is not upon the sun, has doubtless been explored with equal care by telescopes of high power, and processes by which the sun's direct light has been excluded from the tube of the telescope as well as the eye of the observer, and yet no planet has been found. this fact would entitle us to conclude that no such planet exists if its existence had been merely conjectured, or if it had been deduced from any of the laws of planetary distance, or even if leverrier or adams had announced it as the probable result of planetary perturbations. if the finest telescopes cannot rediscover a planet which with the small power used by lescarbault has a visible disc, within so limited an area of which the sun is the centre, or rather within a narrow belt of that circle, we should unhesitatingly declare that no such planet exists. but the question assumes a very different aspect when it involves moral considerations. if,' proceeds the reviewer, writing in august , 'after the severe scrutiny which the sun and its vicinity will undergo before and after and during his total eclipse in july, no planet shall be seen; and if no round black spot distinctly separable from the usual solar spots shall be seen on the solar spots' (_sic_, presumably solar disc was intended), 'we will not dare to say that it does not exist. we cannot doubt the honesty of m. lescarbault, and we can hardly believe that he was mistaken. no solar spot, no floating scoria, could maintain in its passage over the sun a circular and uniform shape, and we are confident that no other hypothesis but that of an intra-mercurial planet can explain the phenomena seen and measured by m. lescarbault, a man of high character, possessing excellent instruments, and in every way competent to use them well, and to describe clearly and correctly the results of his observations. time, however, tries facts as well as speculations. the phenomena observed by the french astronomer may never be again seen, and the disturbance of mercury which rendered it probable may be otherwise explained. should this be the case, we must refer the round spot on the sun to some of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.' the evidence which has accumulated against vulcan in the interval since this was written is not negative only, but partly positive, as the following instance, which i take from my own narrative at the time in a weekly journal, serves to show:--after more than sixteen years of fruitless watching, astronomers learned last august ( ) that in the month of april vulcan had been seen on the sun's disc in china. on april , it appeared, herr weber, an observer of considerable skill, stationed at pecheli, had seen a small round spot on the sun, looking very much as a small planet might be expected to look. a few hours later he turned his telescope upon the sun, and lo! the spot had vanished, precisely as though the planet had passed away after the manner of planets in transit. he forwarded the news of his observation to europe. the astronomer wolff, well known for his sun-spot studies, carefully calculated the interval which had passed since lescarbault saw vulcan on march , , and to his intense satisfaction was enabled to announce that this interval contained the calculated period of the planet an exact number of times. leverrier at paris received the announcement still more joyfully; while the abbé moigno, who gave vulcan its name, and has always staunchly believed in the planet's existence, congratulated lescarbault warmly upon this new view of the shamefaced vulcan. not one of those who already believed in the planet had the least doubt as to the reality of weber's observations, and of these only lescarbault himself received the news without pleasure. he, it seems, has never forgiven the germans for destroying his observatory and library during the invasion of france in , and apparently would prefer that his planet should never be seen again rather than that a german astronomer should have seen it. but the joy of the rest and lescarbault's sorrow were alike premature. it was found that the spot seen by weber had not only been observed at the madrid observatory, where careful watch is kept upon the sun, but had been photographed at greenwich; and when the description of its appearance, as seen in a powerful telescope at one station, and its picture as photographed by a fine telescope at the other, came to be examined, it was proved unmistakably that the spot was an ordinary sun-spot (not even quite round), which had after a few hours disappeared, as even larger sun-spots have been known to do in even a shorter time. it is clear that had not weber's spot been fortunately seen at madrid and photographed at greenwich, his observation would have been added to the list of recorded apparitions of vulcan in transit, for it fitted in perfectly with the theory of vulcan's real existence. i think, indeed, for my own part, that the good fortune was weber's. had it so chanced that thick weather in madrid and at greenwich had destroyed the evidence actually obtained to show that what weber described he really saw, although it was not what he thought, some of the more suspicious would have questioned whether, in the euphonious language of the north british reviewer, 'the round spot on the sun' was not due 'to one of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.' of course no one acquainted with m. weber's antecedents would imagine for a moment that he had invented the observation, even though the objective reality of his spot had not been established. but if a person who is entirely unknown, states that he has seen vulcan, there is antecedently some degree of probability in favour of the belief that the observation is as much a myth as the planet itself. some observations of vulcan have certainly been invented. i have received several letters purporting to describe observations of bodies in transit over the sun's face, either the rate of transit, the size of the body, or the path along which it was said to move, being utterly inconsistent with the theory that it was an intra-mercurial planet, while yet (herein is the suspicious circumstance of such narratives) the epoch of transit accorded in the most remarkable manner with the period assigned to vulcan. a paradoxist in america (of louisville, kentucky) who had invented a theory of the weather, in which the planets, by their influence on the sun, were supposed to produce all weather-changes, the nearer planets being the most effective, found his theory wanted vulcan very much. accordingly, he saw vulcan crossing the sun's face in september, which, being half a year from march, is a month wherein, according to lescarbault's observation, vulcan may be seen in transit, and by a strange coincidence the interval between our paradoxist's observation and lescarbault's exactly contained a certain number of times the period calculated by leverrier for vulcan. this was a noble achievement on the part of our paradoxist. at one stroke it established his theory of the weather, and promised to ensure him text-book immortality as one of the observers of vulcan. but, unfortunately, a student of science residing in st. louis, after leaving the louisville paradoxist full time to parade his discovery, heartlessly pointed out that an exact number of revolutions of vulcan after lescarbault's march observation, must of necessity have brought the planet on that side of the sun on which the earth lies in march, so that to see vulcan so placed on the sun's face in september was to see vulcan through the sun, a very remarkable achievement indeed. the paradoxist was abashed, the reader perhaps imagines. not in the least. the planet's period must have been wrongly calculated by leverrier--that was all: the real period was less than half as long as leverrier had supposed; and instead of having gone a certain number of times round since lescarbault had seen it, vulcan had gone twice as many times round and half once round again. the circumstance that if vulcan's period had been thus short, the time of crossing the sun's face would have been much less than, according to lescarbault's account, it actually was, had not occurred to the louisville weather-prophet.[ ] leverrier's faith in vulcan, however, has remained unshaken. he has used all the observations of spots which, like weber's, have been seen only for a short time. at least he has used all which have not, like weber's, been proved to be only transient sun-spots. selecting those which fit in well with lescarbault's observation, he has pointed out how remarkable it is that they show this accord. the possibility that some of them might be explicable as weber's proved to be, and that some even may have been explicable as completely, but less satisfactorily, in another way, seems to have been thought scarcely worth considering. using the imperfect materials available, but with exquisite skill--as a phidias might model an exquisite figure of materials that would presently crumble into dust--leverrier came to the conclusion that vulcan would cross the sun's disc on or about march , . 'he, therefore,' said sir g. airy, addressing the astronomical society, 'circulated a despatch among his friends, asking them carefully to observe the sun on march .' sir g. airy, humouring his honoured friend, sent telegrams to india, australia, and new zealand, requesting that observations might be made every two hours or oftener. leverrier himself wrote to santiago de chili and other places, so that, including american and european observations, the sun could be watched all through the twenty-four hours on march , , and . 'without saying positively that he believed or disbelieved in the existence of the planet,' proceeds the report, 'sir g. airy thought, since m. leverrier was so confident, that the opportunity ought not to be neglected by anybody who professed to take an interest in the progress of planetary astronomy.' it is perhaps unnecessary to add that observations were made as requested. many photographs of the sun also were taken during the hours when vulcan, if he exists at all, might be expected to cross the sun's face. but the 'planet of romance,' as abbé moigno has called vulcan, failed to appear, and the opinion i had expressed last october ('english mechanic and world of science,' for october , , p. ), that vulcan might perhaps better be called the 'planet of fiction' was _pro tanto_ confirmed. nevertheless, i would not be understood to mean by the word 'fiction' aught savouring of fraud so far as lescarbault is concerned--i prefer the north briton's view of lescarbault's spot, that so to speak, it was ... the blot upon his brain, that _would_ show itself without. i have left small space to treat of other fancied discoveries among the orbs of heaven. yet there are some which are not only interesting but instructive, as showing how even the most careful observers may be led astray. in this respect the mistakes into which observers of great and well deserved eminence have fallen are specially worthy of attention. with the description of three such mistakes, made by no less an astronomer than sir w. herschel, i shall bring this paper to a close. when sir w. herschel examined the planet uranus with his most powerful telescope he saw the planet to all appearance girt about by two rings at right angles to one another. the illusion was so complete that herschel for several years remained in the belief that the rings were real. they were, however, mere optical illusions, due to the imperfect defining qualities of the telescope with which he observed the planet. later he wrote that 'the observations which tend to ascertain' (indicate?) 'the existence of rings not being satisfactorily supported, it will be proper that surmises of them should either be given up, as ill-founded, or at least reserved till superior instruments can be provided.' sir w. herschel was more completely misled by the false uranian satellites. he had seen, as he supposed, no less than six of these bodies. as only two of these had been seen again, while two more were discovered by lassell, the inference was that uranus has eight satellites in all. these for a long time flourished in our text-books of astronomy; and many writers, confident in the care and skill of sir w. herschel, were unable for a long time to believe that he had been deceived. thus admiral smyth, in his 'celestial cycle,' wrote of those who doubted the extra satellites:--'they must have but a meagre notion of sir w. herschel's powerful means, his skill in their application, and his method of deliberate procedure. so far from doubting there being six satellites' (this was before lassell had discovered the other two) 'it is highly probable that there are still more.' whewell, also, in his 'bridgewater treatise,' says, 'that though it no longer appears probable that uranus has a ring like saturn, he has at least five satellites which are visible to us, and we believe that the astronomer will hardly deny that he' (uranus, not the astronomer), 'may possibly have thousands of smaller ones circulating about him.' but in this case sir w. herschel, anxiously though he endeavoured to guard against the possibility of error, was certainly mistaken. uranus may, for anything that is known to the contrary, have many small satellites circulating about him, but he certainly has not four satellites (besides those known) which could have been seen by sir w. herschel with the telescope he employed. for the neighbourhood of the planet has been carefully examined with telescopes of much greater power by observers who with those telescopes have seen objects far fainter than the satellites supposed to have been seen by the elder herschel. the third of the herschelian myths was the lunar volcano in eruption, which he supposed he had seen in progress in that part of the moon which was not at the time illuminated by the sun's rays. he saw a bright star-like point of light, which corresponded in position with the crater of the lunar mountain aristarchus. he inferred that a volcano was in active eruption because the brightness of the point of light varied from time to time, and also because he did not remember to have seen it before under the same conditions. there is no doubt something very remarkable in the way in which this part of the moon's surface shines when not illumined by the sun. if it were always bright we should conclude at once that the earth-light shining upon it rendered it visible. for it must be remembered that the part of the moon which looks dark (or seems wanting to the full disc) is illuminated by our earth, shining in the sky of the moon as a disc thirteen times as large as that of the moon we see, and with the same proportion of its disc sunlit as is dark in the moon's disc. thus when the moon is nearly new our earth is shining in the lunar skies as a nearly full moon thirteen times as large as ours. the light of this noble moon must illumine the moon's surface much more brightly than a terrestrial landscape is illumined by the full moon, and if any parts of her surface are very white they will shine out from the surface around, just as the snow-covered peak of a mountain shines out upon a moonlit night from among the darker hills and dales and rocks and forests of the landscape. but herschel considered that the occasional brightness of the crater aristarchus could not be thus explained. the spot had been seen before the time of herschel's observations by cassini and others. it has been seen since by captain kater, francis baily, and many others. dr. maskelyne tells us that in march it was seen by the naked eye by two persons. baily thus describes the appearance presented by this lunar crater on december , : 'directed telescope to the moon, and pointing it to the dark part in the vicinity of aristarchus, soon saw the outline of that mountain very distinctly, formed like an irregular nebula. nearly in the centre was a light resembling that of a star of the ninth or tenth magnitude. it appeared by glimpses, but at times was brilliant, and visible for several seconds together.' there can be little doubt, however, that the apparent brightness of this lunar crater, or rather of its summit, is due to some peculiar quality in the surface, which may perhaps be covered by some crystalline or vitreous matter poured out in the far distant time when the crater was an active one. prof. shaler, who examined the crater when it was illuminated only by earthshine, with the fine -inch telescope of the harvard observatory (cambridge u.s.), says that he has been able to recognise nearly all the craters over miles in diameter in the dark part. 'there are several degrees of brightness,' he says, 'observable in the different objects which shine out by the earth-light. this fact probably explains the greater part of the perplexing statements concerning the illumination of certain craters. it certainly accounts for the volcanic activity which has so often been supposed to be manifested by aristarchus. under the illumination by the earth-light this is by far the brightest object on the dark part of the moon's face, and is visible much longer and with poorer glasses than any other object there.' here my record of astronomical myths must be brought to a close. it will be noticed that in every instance either the illusion has affected the actual observations of eminent and skilful astronomers, or has caused such astronomers to put faith for a while in illusory observations. had i cared to include the mistakes which have been made by or have misled observers of less experience, i could have filled many sheets for each page of the present article. but it has seemed to me more instructive to show how error may affect the observations even of the most careful and deservedly eminent astronomers, how even the most cautious may be for a time misled by the mistakes of inferior observers, especially when the fact supposed to have been observed accords with preconceived opinions. xii. _the origin of the constellation-figures._ although the strange figures which astronomers still allow to straggle over their star maps no longer have any real scientific interest, they still possess a certain charm, not only for the student of astronomy, but for many who care little or nothing about astronomy as a science. when i was giving a course of twelve lectures in boston, america, a person of considerable culture said to me, 'i wish you would lecture about the constellations; i care little about the sun and moon and the planets, and not much more about comets; but i have always felt great interest in the bears and lions, the chained and chaired ladies, king cepheus and the rescuer perseus, orion, ophiuchus, hercules, and the rest of the mythical and fanciful beings with which the old astronomers peopled the heavens. i say with carlyle, "why does not some one teach me the constellations, and make me at home in the starry heavens, which are always overhead, and which i don't half know to this day."' we may notice, too, that the poets by almost unanimous consent have recognised the poetical aspect of the constellations, while they have found little to say about subjects which belong especially to astronomy as a science. milton has indeed made an archangel reason (not unskilfully for milton's day) about the ptolemaic and copernican systems, while tennyson makes frequent reference to astronomical theories. 'there sinks the nebulous star we call the sun, if that hypothesis of theirs be sound,' said ida; but she said no more, save 'let us down and rest,' as though the subject were wearisome to her. again, in the palace of art the soul of the poet having built herself that 'great house so royal, rich, and wide,' thither-- ... when all the deep unsounded skies shuddered with silent stars, she clomb, and as with optic glasses her keen eyes pierced through the mystic dome, regions of lucid matter taking forms, brushes of fire, hazy gleams, clusters and beds of worlds and beelike swarms of suns, and starry streams: she saw the snowy poles of moonless mars, that marvellous round of milky light below orion, and those double stars whereof the one more bright is circled by the other. but the poet's soul so wearied of these astronomical researches that the beautiful lines i have quoted disappeared (more's the pity) from the second and all later editions. such exceptions, indeed, prove the rule. poets have been chary in referring to astronomical researches and results, full though these have been of unspeakable poetry; while from the days of homer to those of tennyson, the constellations which 'garland the heavens' have always been favourite subjects of poetic imagery. it is not my present purpose, however, to discuss the poetic aspect of the constellations. i propose to inquire how these singular figures first found their way to the heavens, and, so far as facts are available for the purpose, to determine the history and antiquity of some of the more celebrated constellations. long before astronomy had any existence as a science men watched the stars with wonder and reverence. those orbs, seemingly countless--which bespangle the dark robe of night--have a charm and beauty of their own apart from the significance with which the science of astronomy has invested them. the least fanciful mind is led to recognise on the celestial concave the emblems of terrestrial objects, pictured with more or less distinctness among the mysterious star-groupings. we can imagine that long before the importance of the study of the stars was recognised, men had begun to associate with certain star-groups the names of familiar objects animate or inanimate. the flocks and herds which the earliest observers of the heavens tended would suggest names for certain sets of stars, and thus the bull, the ram, the kids, would appear in the heavens. other groups would remind those early observers of the animals from whom they had to guard their flocks, or of the animals to whose vigilance they trusted for protection, and thus the bear, the lion, and the dogs would find their place among the stars. the figures of men and horses, and of birds and fishes, would naturally enough be recognised, nor would either the implements of husbandry, or the weapons by which the huntsman secured his prey, remain unrepresented among the star-groupings. and lastly, the altar on which the first-fruits of harvest and vintage were presented, or the flesh of lambs and goats consumed, would be figured among the innumerable combinations which a fanciful eye can recognise among the orbs of heaven. in thus suggesting that the first observers of the heavens were shepherds, huntsmen, and husbandmen, i am not advancing a theory on the difficult questions connected with the origin of exact astronomy. the first observations of the heavens were of necessity made by men who depended for their subsistence on a familiarity with the progress and vicissitudes of the seasons, and doubtless preceded by many ages the study of astronomy as a science. and yet the observations made by those early shepherds and hunters, unscientific though they must have been in themselves, are full of interest to the student of modern exact astronomy. the assertion may seem strange at first sight, but is nevertheless strictly true, that if we could but learn with certainty the names assigned to certain star-groups, before astronomy had any real existence, we could deduce lessons of extreme importance from the rough observations which suggested those old names. in these days, when observations of such marvellous exactness are daily and nightly made, when instruments capable of revealing the actual constitution of the stars are employed, and observers are so numerous, it may seem strange to attach any interest to the question whether half-savage races recognised in such and such a star-group the likeness of a bear, or in another group the semblance of a ship. but though we could learn more, of course, from exacter observations, yet even such rough and imperfect records would have their value. if we could be certain that in long-past ages a star-group really resembled some known object, we should have in the present resemblance of that group to the same object evidence of the general constancy of stellar lustre, or if no resemblance could be recognised we should have reason to doubt whether other suns (and therefore our own sun) may not be liable to great changes. the subject of the constellation-figures as first known is interesting in other ways. for instance, it is full of interest to the antiquarian (and most of us are to some degree antiquarians) as relating to the most ancient of all human sciences. the same mental quality which causes us to look with interest on the buildings raised in long-past ages, or on the implements and weapons of antiquity, renders the thought impressive that the stars which we see were gazed on perhaps not less wonderingly in the very infancy of the human race. it is, again, a subject full of interest to the chronologist to inquire in what era of the world's history exact astronomy began, the moon was assigned her twenty-eight zodiacal mansions, the sun his twelve zodiacal signs. it is well known, indeed, that newton himself did not disdain to study the questions thus suggested; and the speculations of the ingenious dupuis found favour with the great mathematician laplace. unfortunately, the evidence is not sufficiently exact to be very trustworthy. in considering, for instance, the chronological inquiries of newton, one cannot but feel that the reliance placed by him on the statements made by different writers is not justified by the nature of those statements, which were for the most part vague in the extreme. we owe many of them to poets who, knowing little of astronomy, mixed up the phenomena of their own time with those which they found recorded in the writings of astronomers. some of the statements left by ancient writers are indeed ludicrously incongruous; insomuch that grotius not unjustly said of the account of the constellations given by the poet aratus, that it could be assigned to no fixed epoch and to no fixed place. however, this would not be the place to discuss details such as are involved in exact inquiries. i have indicated some of these in an appendix to my treatise on 'saturn,' and others in the preface to my 'gnomonic star atlas'; but for the most part they do not admit very readily of familiar description. let us turn to less technical considerations, which fortunately are in this case fully as much to the point as exact inquiries, seeing that there is no real foundation for such inquiries in any of the available evidence. the first obvious feature of the old constellations is one which somehow has not received the attention it deserves. it is as instructive as any of those which have been made the subject of profound research. there is a great space in the heavens over which none of the old constellations extend, except the river eridanus as now pictured, but we do not know where this winding stream of stars was supposed by the old observers to come to an end. this great space surrounds the southern pole of the heavens, and thus shows that the first observers of the stars were not acquainted with the constellations which can be seen only from places far south of chaldæa, persia, egypt, india, china, and indeed of all the regions to which the invention of astronomy has been assigned. whatever the first astronomers were, however profound their knowledge of astronomy may have been (as some imagine), they had certainly not travelled far enough towards the south to know the constellations around the southern pole. if they had been as well acquainted with geography as some assert, if even any astronomer had travelled as far south as the equator, we should certainly have had pictured in the old star charts some constellations in that region of the heavens wherein modern astronomers have placed the octant, the bird of paradise, the sword-fish, the flying-fish, toucan, the net, and other uncelestial objects. in passing i may note that this fact disposes most completely of a theory lately advanced that the constellations were invented in the southern hemisphere, and that thus is to be explained the ancient tradition that the sun and stars have changed their courses. for though all the northern constellations would have been more or less visible from parts of the southern hemisphere near the equator, it is absurd to suppose that a southern observer would leave untenanted a full fourth of the heavens round the southern or visible pole, while carefully filling up the space around the northern or unseen pole with incomplete constellations whose northern unknown portions would include that pole. supposing it for a moment to be true, as a modern advocate of the southern theory remarks, that 'one of the race migrating from one side to the other of the equator would take his position from the sun, and fancy he was facing the same way when he looked at it at noon, and so would think the motion of the stars to have altered instead of his having turned round,' the theory that astronomy was brought to us from south of the equator cannot possibly be admitted in presence of that enormous vacant region around the southern pole. i think, however, that, apart from this, a race so profoundly ignorant as to suppose any such thing, to imagine they were looking north when in reality they were looking south, can hardly be regarded as the first founders of the science of astronomy. the great gap i have spoken of has long been recognised. but one remarkable feature in its position has not, to the best of my remembrance, been considered--the vacant space is eccentric with regard to the southern pole of the heavens. the old constellations, the altar, the centaur, and the ship argo, extend within twenty degrees of the pole, while the southern fish and the great sea-monster cetus, which are the southernmost constellations on the other side, do not reach within some sixty degrees of the pole. of course, in saying that this peculiarity has not been considered, i am not suggesting that it has not been noticed, or that its cause is in any way doubtful or unknown. we know that the earth, besides whirling once a day on its axis, and rushing on its mighty orbit around the sun (spanning some , , of miles) reels like a gigantic top, with a motion so slow that , years are required for a single circuit of the swaying axis around an imaginary line upright to the plane in which the earth travels. and we know that in consequence of this reeling motion the points of the heavens opposite the earth's poles necessarily change. so that the southern pole, now eccentrically placed amid the region where there were no constellations in old times, was once differently situated. but the circumstance which seems to have been overlooked is this, that by calculating backwards to the time when the southern pole was in the centre of that vacant region, we have a much better chance of finding the date (let us rather say the century) when the older constellations were formed, than by any other process. we may be sure not to be led very far astray; for we are not guided by one constellation but by several, whereas all the other indications which have been followed depend on the supposed ancient position of single constellations. and then most of the other indications are such as might very well have belonged to periods following long after the invention of the constellations themselves. an astronomer might have ascertained, for instance, that the sun in spring was in some particular part of the ram or of the fishes, and later a poet like aratus might describe that relation (erroneously for his own epoch) as characteristic of one or other constellation; but who is to assure us that the astronomer who noted the relation correctly may not have made his observation many hundreds of years after those constellations were invented? whereas, there was one period, and only one period, when the most southernmost of the old constellations could have marked the limits of the region of sky visible from some northern region. thus, too, may we form some idea of the latitude in which the first observers lived. for in high latitudes the southernmost of the old constellations would not have been visible at all, and in latitudes much lower than a certain latitude, presently to be noted, these constellations would have ridden high above the southern horizon, other star-groups showing below them which were not included among the old constellations. i have before me as i write a picture of the southern heavens, drawn by myself, in which this vacant space--eccentric in position but circular in shape--is shown. the centre lies close by the lesser magellanic cloud--between the stars kappa toucani and eta hydri of our modern maps, but much nearer to the last named. near this spot, then, we may be sure, lay the southern pole of the star-sphere when the old constellations, or at least the southern ones, were invented. (if there had been astronomers in the southern hemisphere eta hydri would certainly have been their pole-star.) now it is a matter of no difficulty whatever to determine the epoch when the southern pole of the heavens was thus placed.[ ] between and years before the christian era the southern constellations had the position described, the invisible southern pole lying at the centre of the vacant space of the star-sphere--or rather of the space free from constellations. it is noteworthy that for other reasons this period, or rather a definite epoch within it, is indicated as that to which must be referred the beginning of exact astronomy. amongst others must be mentioned this--that in the year b.c. _quam proximè_, the pleiades rose to their highest above the horizon at noon (or technically made their noon culmination), at the spring equinox. we can readily understand that to minds possessed with full faith in the influence of the stars on the earth, this fact would have great significance. the changes which are brought about at that season of the year, in reality, of course, because of the gradual increase in the effect of the sun's rays as he rises higher and higher above the celestial equator, would be attributed, in part at least, to the remarkable star-cluster coming then close by the sun on the heavens, though unseen. thus we can readily understand the reference in job to the 'sweet influences of the pleiades.' again at that same time, b.c. when the sun and the pleiades opened the year (with commencing spring) together, the star alpha of the dragon, which was the pole-star of the period, had that precise position with respect to the true pole of the heavens which is indicated by the slope of the long passage extending downwards aslant from the northern face of the great pyramid; that is to say, when due north below the pole (or at what is technically called its sub-polar meridional passage) the pole-star of the period shone directly down that long passage, and i doubt not could be seen not only when it came to that position during the night, but also when it came there during the day-time. but some other singular relations are to be noted in connection with the particular epoch i have indicated. it is tolerably clear that in imagining figures of certain objects in the heavens, the early observers would not be apt to picture these objects in unusual positions. a group of stars may form a figure so closely resembling that of a familiar object that even a wrong position would not prevent the resemblance from being noticed, as for instance the 'chair,' the 'plough,' and so forth. but such cases are not numerous; indeed, to say the truth, one must 'make believe a good deal' to see resemblance between the star-groups and _most_ of the constellation-figures, even under the most favourable conditions. when there is no very close resemblance, as is the case with all the large constellations, position must have counted for something in determining the association between a star-group and a known object. now the constellations north of the equator assume so many and such various positions that this special consideration does not apply very forcibly to them. but those south of the equator are only seen above the southern horizon, and change little in position during their progress from east to west of the south point. the lower down they are the less they change in position. and the very lowest--such as those were, for instance, which i have been considering in determining the position of the southern pole--are only fully visible when due south. they must, then, in all probability, have stood upright or in their natural position when so placed, for if they were not rightly placed then they only were so when below the horizon and consequently invisible. let us, then, inquire what was the position of the southernmost constellations when fully seen above the southern horizon at midnight. the centaur stood then as he does now, upright; only--whereas now in egypt, chaldæa, india, persia, and china, only the upper portions of his figure rise above the horizon, he then stood, the noblest save orion of all the constellations, with his feet (marked by the bright alpha and beta still belonging to the constellation, and by the stars of the southern cross which have been taken from it) upon the horizon itself. in latitude twenty degrees or so north he may still be seen thus placed when due south. the centaur was represented in old times as placing an offering upon the altar, which was pictured, says manilius, as bearing a fire of incense represented by stars. this to a student of our modern charts seems altogether perplexing. the centaur carries the wolf on the end of his spear; but instead of placing the wolf (not a very acceptable meat offering, one would suppose) upon the altar, he is directing this animal towards the base of the altar, whose top is downwards, the flames represented there tending (naturally) downwards also. it is quite certain the ancient observers did not imagine anything of this sort. as i have said, aratus tells us the celestial centaur was placing an offering _upon_ the altar, which was therefore upright, and manilius describes the altar as ferens thuris, stellis imitantibus, ignem, so that the fire was where it should be, on the top of an upright altar, where also on the sky itself were stars looking like the smoke from incense fires. now that was precisely the appearance presented by the stars forming the constellation at the time i have indicated, some years b.c. setting the altar upright above the southern horizon (that is, inverting the absurd picture at present given of it) we see it just where it should be placed to receive the centaur's offering. a most remarkable portion of the milky way is then seen to be directly above the altar in such a way as to form a very good imitation of smoke ascending from it. this part of the milky way is described by sir j. herschel, who studied it carefully during his stay at the cape of good hope, as forming a complicated system of interlaced streaks and masses which covers the tail of scorpio (extending from the altar which lies immediately south of the scorpion's tail). the milky way divides, in fact, just above the altar as the constellation was seen years ago above the southern horizon, one branch being that just described, the other (like another stream of smoke) 'passing,' says herschel, 'over the stars iota of the altar, theta and iota of the scorpion, etc., to gamma of the archer, where it suddenly collects into a vivid oval mass, so very rich in stars that a very moderate calculation makes their number exceed , .' nothing could accord better with the descriptions of aratus and manilius. but there is another constellation which shows in a more marked way than either the centaur or the altar that the date when the constellations were invented must have been near that which i have named. both ara and centaurus look now in suitable latitudes (about twenty degrees north) as they looked in higher latitudes (about forty degrees north) years ago. for, the reeling motion of our earth has changed the place of the celestial pole in such a way as only to depress these constellations southwards without much changing their _position_; they are nearly upright when due south now as they were years ago, only lower down. but the great ship argo has suffered a much more serious displacement. one cannot now see this ship _like_ a ship at any time or from any place on the earth's surface. if we travel south till the whole constellation comes into visibility above the southern horizon at the proper season (january and february for the midnight hours) the keel of the ship is aslant, the stern being high above the waist (the fore part is wanting). if we travel still further south, we can indeed reach places where the course of the ship is so widened, and the changes of position so increased, that she appears along part of her journey on an even keel, but then she is high above the horizon. now years ago she stood on the horizon itself at her southern culmination, with level keel and upright mast. in passing i may note that for my own part i imagine that this great ship represented the ark, its fore part being originally the portion of the centaur now forming the horse, so that the centaur was represented as a man (not as a man-horse) offering a gift on the altar. thus in this group of constellations i recognise the ark, and noah going up from the ark towards the altar 'which he builded unto the lord; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.' i consider further that the constellation-figures of the ship, the man with an offering, and the altar, painted or sculptured in some ancient astrological temple, came at a later time to be understood as picturing a certain series of events, interpreted and expanded by a poetical writer into a complete narrative. without venturing to insist on so heterodox a notion, i may remark as an odd coincidence that probably such a picture or sculpture would have shown the smoke ascending from the altar which i have already described, and in this smoke there would be shown the bow of sagittarius; which, interpreted and expanded in the way i have mentioned, might have accounted for the 'bow set in the clouds, for a token of a covenant.' it is noteworthy that all the remaining constellations forming the southern limit of the old star-domes or charts, were watery ones--the southern fish, over which aquarius is pouring a quite unnecessary stream of water, the great sea monster towards which in turn flow the streams of the river eridanus. the equator, too, was then occupied along a great part of its length by the great sea serpent hydra, which reared its head above the equator, very probably indicated then by a water horizon, for nearly all the signs below it were then watery. at any rate, as the length of hydra then lay horizontally above the ship, whose masts reached it, we may well believe that this part of the picture of the heavens showed a sea-horizon and a ship, the great sea serpent lying along the horizon. on the back of hydra is the raven, which again may be supposed by those who accept the theory mentioned above to have suggested the raven which went forth to and fro from the ark. he is close enough to the rigging of argo to make an easy journey of it. the dove, however, must not be confounded with the modern constellation columba, though this is placed (suitably enough) near the ark. we must suppose the idea of the dove was suggested by a bird pictured in the rigging of the celestial ship. the sequence in which the constellations came above the horizon as the year went round corresponded very satisfactorily with the theory, fanciful though this seem to some. first aquarius pouring streams of water, the three fishes (pisces and piscis australis), and the great sea monster cetus, showing how the waters prevailed over the highest hills, then the ark sailing on the waters, a little later the raven (corvus), the man descending from the ark and offering a gift on the altar, and last the bow set amid the clouds. the theory just described may not meet with much favour. but wilder theories of the story of the deluge have been adopted and advocated with considerable confidence. one of the wildest, i fear, is the astronomer-royal's, that the deluge was simply a great rising of the nile; and sir g. airy is so confident respecting this that he says, 'i cannot entertain the smallest doubt that the flood of noah was a flood of the nile;' precisely as he might say, 'i cannot entertain the smallest doubt that the earth moves round the sun.' on one point we can entertain very little doubt indeed. if it ever rained before the flood, which seems probable, and if the sun ever shone on falling rain, which again seems likely, nothing short of a miracle could have prevented the rainbow from making its appearance before the flood. the wildest theory that can be invented to explain the story of the deluge cannot be wilder than the supposition that the rays of sunlight shining on falling raindrops could have ever failed to show the prismatic colours. the theory i have suggested above, without going so far as strongly to advocate it, far less insist upon it, is free at any rate from objection on this particular score, which cannot be said of the ordinary theory. i am not yet able, however, to say that 'i cannot entertain the smallest doubt' about my theory. we may feel tolerably sure that the period when the old southern constellations were formed must have been between and years before the present era, a period, by the way, including the date usually assigned to the deluge,--which, however, must really occupy our attention no further. in fact, let us leave the watery constellations lying below the equator of those remote times and seek at once the highest heavens above them. here, at the northern pole of these days, we find the great dragon, which in any astrological temple of the time must have formed the highest or crowning constellation, surrounding the very key-stone of the dome. he has fallen away from that proud position since. in fact, even years ago he only held to the pole, so to speak, by his tail, and we have to travel back years or so to find the pole situate in a portion of the length of the dragon which can be regarded as central. one might almost, if fancifully disposed, recognise the gradual displacement of the dragon from his old place of honour, in certain traditions of the downfall of the great dragon whose 'tail drew the third part of the stars of heaven.' the central position of the dragon, for even when the pole-star had drawn near to the dragon's tail the constellation was still central, will remind the classical reader of homer's description of the shield of hercules-- the scaly horror of a dragon, coil'd full in the central field, unspeakable, with eyes oblique retorted, that ascant shot gleaming fire. (_elton's translation._) i say homer's description, for i cannot understand how any one who compares together the description of the shield of achilles in the iliad and that of the shield of hercules in the fragmentary form in which we have it, can doubt for a moment that both descriptions came from the same hand. (the theory that hesiod composed the latter poem can scarcely be entertained by any scholar.) as i long since pointed out in my essay 'a new theory of achilles' shield' ('light science,' first series), no poet so inferior as actually to borrow homer's words in part of the description of the shield of hercules could have written the other parts not found in the shield of achilles. 'i cannot for my own part entertain the slightest doubt'--that is to say, i think it altogether probable--that homer composed the lines supposed to describe the shield of hercules long before he introduced the description, pruned and strengthened, into that particular part of the iliad where it served his purpose best. and i have as little doubt that the original description, of which we only get fragments in either poem, related to something far more important than a shield. the constellations are not suitable adornments for the shield of fighting man, even though he was under the special care of a celestial mother and had armour made for him by a celestial smith. yet we learn that achilles' shield displayed-- the starry lights that heav'n's high convex crown'd the pleiads, hyads, and the northern beam, and great orion's more refulgent beam,-- to which, around the cycle of the sky, the bear revolving, points his golden eye,-- still shines exalted. and so forth. the shield of hercules displayed at its centre the polar constellation the dragon. we read also that-- there was the knight of fair-hair'd danae born, perseus. orion is not specially mentioned, but orion, lepus, and the dogs seem referred to:-- men of chase were taking the fleet hares; two keen-toothed dogs bounded beside. homer would find no difficulty in pluralising the mighty hunter and the hare into huntsmen and hares when utilising a description originally referring to the constellation. i conceive that the original description related to one of those zodiac temples whose remains are still found in egypt, though the egyptian temples of this kind were probably only copies of more ancient chaldæan temples. we know from assyrian sculptures that representations of the constellations (and especially the zodiacal constellations) were common among the babylonians; and, as i point out in the essay above referred to, 'it seems probable that in a country where sabæanism or star-worship was the prevailing form of religion, yet more imposing proportions would be given to zodiac temples than in egypt.' my theory, then, respecting the two famous 'shields' is that homer in his eastern travels visited imposing temples devoted to astronomical observation and star-worship, and that nearly every line in both descriptions is borrowed from a poem in which he described a temple of this sort, its domed zodiac, and those illustrations of the labours of different seasons and of military or judicial procedures which the astrological proclivities of star-worshippers led them to associate with the different constellations. for the arguments on which this theory is based i have not here space. they are dealt with in the essay from which i have quoted. one point only i need touch upon here, besides those i have mentioned already. it may be objected that the description of a zodiac temple has nothing to connect it with the subject of the iliad. this is certainly true; but no one who is familiar with homer's manner can doubt that he would work in, if he saw the opportunity, a poem on some subject outside that of the iliad, so modifying the language that the description would correspond with the subject in hand. there are many passages, though none of such length, in both the iliad and the odyssey, which seem thus to have been brought into the poem; and other passages not exactly of this kind yet show that homer was not insensible to the advantage of occasionally using memory instead of invention. any one who considers attentively the aspect of the constellation draco in the heavens, will perceive that the drawing of the head in the maps is not correct; the head is no longer pictured as it must have been conceived by those who first formed the constellation. the two bright stars beta and gamma are now placed on a head in profile. formerly they marked the two eyes. i would not lay stress on the description of the dragon in the shield of hercules, 'with eyes oblique retorted, that askant shot gleaming fire;' for all readers may not be prepared to accept my opinion that that description related to the constellation draco. but the description of the constellation itself by aratus suffices to show that the two bright stars i have named marked the eyes of the imagined monster--in fact, aratus's account singularly resembles that given in the shield of hercules. 'swol'n is his neck,' says aratus of the dragon-- ... eyes charg'd with sparkling fire his crested head illume. as if in ire, to helice he turns his foaming jaw, and darts his tongue, barb'd with a blazing star. and the dragon's head with sparkling eyes can be recognised to this day, so soon as this change is made in its configuration, whereas no one can recognise the remotest resemblance to a dragon's head in profile. the star barbing the dragon's tongue would be xi of the dragon according to aratus's account, for so only would the eyes be turned towards helice the bear. but when aratus wrote, the practice of separating the constellations from each other had been adopted; in fact, he derived his knowledge of them chiefly from eudoxus, the astronomer and mathematician, who certainly would not have allowed the constellations to be intermixed. in the beginning, there are reasons for believing it was different, and if a group of stars resembled any known object it would be called after that object, even though some of the stars necessary to make up the figure belonged already to some other figure. this being remembered, we can have no difficulty in retorting the dragon's head more naturally--not to the star xi of the dragon, but to the star iota of hercules. the four stars are situated thus, [illustration] the larger ones representing the eyes; and so far as the head is concerned it is a matter of indifference whether the lower or the upper small star be taken to represent the tongue. but, as any one will see who looks at these stars when the dragon is best placed for ordinary (non-telescopic) observation, the attitude of the animal is far more natural when the star iota of hercules marks the tongue, for then the creature is situated like a winged serpent hovering above the horizon and looking downwards, whereas when the star xi marks the tongue, the hovering dragon is looking upwards and is in an unnaturally constrained position. (i would not, indeed, claim to understand perfectly all the ways of dragons; still it may be assumed that a dragon hovering above the horizon would rather look downwards in a natural position than upwards in an awkward one.) the star iota of hercules marks the heel of this giant, called the kneeler (engonasin) from time immemorial. he must have been an important figure on the old zodiac temples, and not improbably his presence there as one of the largest and highest of the human figures may have caused a zodiac-dome to be named after hercules. the dome of hercules would come near enough to the title, 'the shield of hercules,' borne by the fragmentary poem dealt with above. the foot of the kneeling man was represented on the head of the dragon, the dragon having hold of the heel. and here, again, some imagine that a sculptured representation of these imagined figures in the heavens may have been interpreted and expanded into the narrative of a contest between the man and the old serpent the dragon, ophiuchus the serpent-bearer being supposed to typify the eventual defeat of the dragon. this fancy might be followed out like that relating to the deluge; but the present place would be unsuitable for further inquiries in that particular direction. some interest attaches to the constellation ophiuchus, to my mind, in the evidence it affords respecting the way in which the constellations were at first intermixed. i have mentioned one instance in which, as i think, the later astronomers separated two constellations which had once been conjoined. many others can be recognised when we compare the actual star-groups with the constellation-figures as at present depicted. no one can recognise the poop of a ship in the group of stars now assigned to the stern of argo, but if we include the stars of the greater dog, and others close by, a well-shaped poop can be clearly seen. the head of the lion of our maps is as the head of a dog, so far as stars are concerned; but if stars from the crab on one side and from virgo on the other be included in the figure, and especially berenice's hair to form the tuft of the lion's tail, a very fine lion with waving mane can be discerned, with a slight effort of the imagination. so with bootes the herdsman. he was of old 'a fine figure of a man,' waving aloft his arms, and, as his name implies, shouting lustily at the retreating bear. now, and from some time certainly preceding that of eudoxus, one arm has been lopped off to fashion the northern crown, and the herdsman holds his club as close to his side as a soldier holds his shouldered musket. the constellation of the great bear, once i conceive the only bear (though the lesser bear is a very old constellation), has suffered wofully. originally it must have been a much larger bear, the stars now forming the tail marking part of the outline of the back; but first some folks who were unacquainted with the nature of bears turned the three stars (the horses of the plough) into a long tail, abstracting from the animal all the corresponding portion of his body, and then modern astronomers finding a great vacant space where formerly the bear's large frame extended, incontinently formed the stars of this space into a new constellation, the hunting dogs. no one can recognise a bear in the constellation as at present shaped, but any one who looks attentively at the part of the skies occupied by the constellation will recognise (always 'making believe a good deal') a monstrous bear, with the proper small head of creatures of the bear family, and with exceedingly well-developed plantigrade feet. of course this figure cannot at all times be recognised with equal facility; but before midnight during the last four or five months in the year, the bear occupies positions favouring his recognition, being either upright on his feet, or as if descending a slope, or squatting on his great haunches. as a long-tailed animal the creature is more like one of those wooden toy-monkeys which used to be made for children, and may be now, in which the sliding motion of a ringed rod carried the monkey over the top of a stick. the little bear has i think been borrowed from the dragon, which was certainly a winged monster originally. now the astronomers who separated from each other, and in so doing spoiled the old constellation-figures, seem to have despaired of freeing ophiuchus from his entanglements. the serpent is twined around his body, the scorpion is clawing at one leg. the constellation makers have _per fas et nefas_ separated scorpio from the serpent holder, spoiling both figures. but the serpent has been too much for them, insomuch that they have been reduced to the abject necessity of leaving one part of the serpent on one side of the region they allow to ophiuchus, and the other part of the serpent to the other. a group of constellations whose origin and meaning are little understood remains to be mentioned. close by the dragon is king cepheus, beside him his wife cassiopeia (the seated lady), near whom is andromeda the chained lady. the sea monster cetus is not far away, though not near enough to threaten her safety, the ram and triangle being between the monster's head and her feet, the fishes intervening between the body of the monster and her fair form. close at hand is perseus, the rescuer, with a sword (looking very much like a reaping-hook in all the old pictures) in his right hand, and bearing in his left the head of medusa. the general way of accounting for the figures thus associated has been by supposing that, having a certain tradition about cepheus and his family, men imagined in the heavens the pictorial representation of the events of the tradition. i have long believed that the actual order in this and other cases was the reverse of this, that men imagined certain figures in the heavens, pictured these figures in their astronomical temples or observatories, and made stories to fit the pictures afterwards, probably many generations afterwards. be this as it may, we can at present give no satisfactory explanation of the group of constellations. wilford gives an account, in his 'asiatic researches,' of a conversation with a pundit or astronomer respecting the names of the indian constellations. 'asking him,' he says, 'to show me in the heavens the constellation antarmada, he immediately pointed to andromeda, though i had not given him any information about it beforehand. he afterwards brought me a very rare and curious work in sanscrit, which contained a chapter devoted to _upanachatras_, or extra-zodiacal constellations, with drawings of _capuja_ (cepheus) and of _casyapi_ (cassiopeia) seated and holding a lotus-flower in her hand, of antarmada charmed with the fish beside her, and last of _paraseia_ (perseus), who, according to the explanation of the book, held the head of a monster which he had slain in combat; blood was dropping from it, and for hair it had snakes.' some have inferred from the circumstance that the indian charts thus showed the cassiopeian set of constellations, that the origin of these figures is to be sought in india. but probably both the indian and the greek constellation-figures were derived from a much older source. the zodiacal twelve are in some respects the most important and interesting of all the ancient constellations. if we could determine the origin of these figures, their exact configuration as at first devised, and the precise influences assigned to them in the old astrological systems, we should have obtained important evidence as to the origin of astronomy itself. not indeed that the twelve signs of the zodiac were formed at the beginning or even in the early infancy of astronomy. it seems abundantly clear that the division of the zodiac (which includes the moon's track as well as the sun's) had reference originally to the moon's motions. she circuits the star-sphere in about twenty-seven days and a third, while the lunation or interval from new moon to new moon is, as we all know, about twenty-nine days and a half in length. it would appear that the earliest astronomers, who were of course astrologers also, of all nations--the indian, egyptian, chinese, persian, and chaldæan astronomers--adopted twenty-eight days (probably as a rough mean between the two periods just named) for their chief lunar period, and divided the moon's track round the ecliptic into twenty-eight portions or mansions. how they managed about the fractions of days outstanding--whether the common lunation was considered or the moon's motion round the star-sphere--is not known. the very circumstance, however, that they were for a long time content with their twenty-eight lunar mansions shows that they did not seek great precision at first. doubtless they employed some rough system of 'leap-months' by which, as occasion required, the progress of the month was reconciled with the progress of the moon, just as by our leap-years the progress of the year is reconciled with the progress of the sun or seasons. the use of the twenty-eight-day period naturally suggested the division of time into weeks of seven days each. the ordinary lunar month is divided in a very obvious manner into four equal parts by the lunar aspects. every one can recognise roughly the time of full moon and the times of half moon before and after full, while the time of new moon is recognised from these two last epochs. thus the four quarters of the month, or roughly the four weeks of the month, would be the first time-measure thought of;--after the day, which is the necessary foundation of all time measures. the nearest approach which can be made to a quarter-month in days is the week of seven days; and although some little awkwardness arose from the fact that four weeks differ appreciably from a lunar month, this would not long prevent the adoption of the week as a measure of time. in fact, just as our years begin on different days of the week without causing any inconvenience, so the ancient months might be made to begin with different week-days. all that would be necessary to make the week measure fairly well the quarters of the month, would be to start each month on the proper or nearest week-day. to inform people about this, some ceremony could be appointed for the day of the new moon, and some signal employed to indicate the time when this ceremony was to take place. this--the natural and obvious course--we find was the means actually adopted, the festival of the new moon and the blowing of trumpets in the new moon being an essential part of the arrangements adopted by nations who used the week as a chief measure of time. the seven days were not affected by the new moons so far as the nomenclature of these days, or special duties connected with any one of them, might be concerned. originally the idea may have been to have festivals and sacrifices at the time of new moon, first quarter, full moon, and third quarter; but this arrangement would naturally (and did, as we know, actually) give way before long to a new moon festival regulating the month and seventh-day festivals, each class of festival having its appropriate sacrifices and duties. this, i say, was the natural course. its adoption _may_ have been aided by the recognition of the fact that the seven planets of the old system of astronomy might conveniently be taken to rule the days and the hours in the way described in the essay on astrology. that that nomenclature and that system of association between the planets and the hours, days, and weeks of time-measurement was eventually adopted, is certain; but whether the convenience and apparent mystical fitness of this arrangement led to the use of weekly festivals in conjunction with monthly ones, or whether those weekly festivals were first adopted in the way described above, or whether (which seems altogether more likely) both sets of considerations led to the arrangement, we cannot certainly tell. the arrangement was in every way a natural one; and one may say, considering all the circumstances, that it was almost an inevitable one. there was, however, another possible arrangement, viz., the division of time into ten-day periods, three to each month, with corresponding new moon festivals. but as the arrival of the moon at the _thirds_ of her progress are not at all so well marked as her arrival at the quarters, and as there is no connection between the number ten and the planets, this arrangement was far less likely to be adopted than the other. accordingly we find that only one or two nations adopted it. six sets of five days would be practically the same arrangement; five sets of six for each month would scarcely be thought of, as with that division the use of simple direct observations of the moon for time measurement, which was the real aim of all such divisions, would not be convenient or indeed even possible for the generality of persons. few could tell easily when the moon is two-fifths or four-fifths full, whereas every one can tell when she is half-full or quite full (the requisite for weekly measurement); and it would be possible to guess pretty nearly when she is one-third or two-thirds full, the requisite for the tridecennial division. my object in the above discussion of the origin of the week (as distinguished from the origin of the sabbath, which i considered in the essay on astrology), has been to show that the use of the twelve zodiacal signs was in every case preceded by the use of the twenty-eight lunar mansions. it has been supposed that those nations in whose astronomy the twenty-eight mansions still appear, adopted one system, while the use of the twelve signs implies that another system had been adopted. thus the following passage occurs in mr. blake's version of flammarion's 'history of the heavens:'--'the chinese have twenty-eight constellations, though the word _sion_ does not mean a group of stars, but simply a mansion or hotel. in the coptic and ancient egyptian the word for constellations has the same meaning. they also have twenty-eight, and the same number is found among the arabians, persians, and indians. among the chaldæans or accadians we find no sign of the number twenty-eight. the ecliptic, or "yoke of the sky," with them, as we see in the newly-discovered tablet, was divided into twelve divisions, as now, and the only connection that can be imagined between this and the twenty-eight is the opinion of m. biot, who thinks that the chinese had originally only twenty-four mansions, four more being added by chenkung, b.c., and that they corresponded with the twenty-four stars, twelve to the north and twelve to the south, that marked the twelve signs of the zodiac amongst the chaldæans. but under this supposition the twenty-eight has no reference to the moon, whereas we have every reason to believe it has.' the last observation is undoubtedly correct--the twenty-eight mansions have been mansions of the moon from the beginning. but in this very circumstance, as also in the very tablets referred to in the preceding passage, we find all the evidence needed to show that originally the chaldæans divided the zodiac into twenty-eight parts. for we find from the tablets that, like the other nations who had twenty-eight zodiacal mansions, the chaldæans used a seven-day period, derived from the moon's motions, every seventh day being called _sabbatu_, and held as a day of rest. we may safely infer that the chaldæan astronomers, advancing beyond those of other nations, recognised the necessity of dividing the zodiac with reference to the sun's motions instead of the moon's. they therefore discarded the twenty-eight lunar mansions, and adopted instead twelve solar signs; this number twelve, like the number twenty-eight itself, being selected merely as the most convenient approximation to the number of parts into which the zodiac was naturally divided by another period. thus the twenty-eighth part of the zodiac corresponds roughly with the moon's daily motion, and the twelfth part of the zodiac corresponds roughly with the moon's monthly motion; and both the numbers twenty-eight and twelve admit of being subdivided, while twenty-nine (a nearer approach than twenty-eight to the number of days in a lunation) and thirteen (almost as near an approach as twelve to the number of months in a year) do not. it seems to me highly probable that the date to which all inquiries into the origin of the constellations and the zodiacal signs seems to point--viz. b.c.--was the date at which the chaldæan astronomers definitely adopted the new system, the lunisolar instead of lunar division of the zodiac and of time. one of the objects which the architects of the great pyramid (not the king who built it) may have had was not improbably this--the erection of a building indicating the epoch when the new system was entered upon, and defining in its proportions, its interior passages, and other features, fundamental elements of the new system. the great difficulty, an overwhelming difficulty it has always seemed to me, in accepting the belief that the year b.c. defined the beginning of exact astronomy, has been this, that several of the circumstances insisted upon as determining that date imply a considerable knowledge of astronomy. thus astronomers must have made great progress in their science before they could select as a day for counting from, the epoch when the slow reeling motion of the earth (the so-called precessional motion) brought the pleiades centrally south, at noon, at the time of the vernal equinox. the construction of the great pyramid, again, in all its astronomical features, implies considerable proficiency in astronomical observation. thus the year b.c. may very well be regarded as defining the introduction of a new system of astronomy, but certainly not the beginning of astronomy itself. of course we may cut the knot of this difficulty, as prof. smyth and abbé moigno do, by saying that astronomy began b.c., the first astronomers being instructed supernaturally, so that the astronomical minerva came into full-grown being. but i apprehend that argument against such a belief is as unnecessary as it would certainly be useless. and now let us consider how this theory accords with the result to which we were led by the position of the great vacant space around the southern pole. so far as the date is concerned, we have already seen that the epoch b.c. accords excellently with the evidence of the vacant space. but this evidence, as i mentioned at the outset, establishes more than the date; it indicates the latitude of the place where the most ancient of ptolemy's forty-eight constellations were first definitely adopted by astronomers. if we assume that at this place the southernmost constellations were just fully seen when due south, we find for the latitude about thirty-eight degrees north. (the student of astronomy who may care to test my results may be reminded here that it is not enough to show that every star of a constellation would when due south be above the horizon of the place--what is wanted is, that the whole constellation when towards the south should be visible at a single view. however, the whole constellation may not have included all the stars now belonging to it.) the station of the astronomers who founded the new system can scarcely have been more than a degree or two north of this latitude. on the other side, we may go a little further, for by so doing we only raise the constellations somewhat higher above the southern horizon, to which there is less objection than to a change thrusting part of the constellations below the horizon. still it may be doubted whether the place where the constellations were first formed was less than or degrees north of the equator. the great pyramid, as we know, is about degrees north of the equator; but we also know that its architects travelled southwards to find a suitable place for it. one of their objects may well have been to obtain a fuller view of the star-sphere south of their constellations. i think from to degrees north would be about the most probable limits, and from to degrees north the certain limits of the station of the first founders of solar zodiacal astronomy. what their actual station may have been is not so easily established. some think the region lay between the sources of the oxus (amoor) and indus, others that the station of these astronomers was not very far from mount ararat--a view to which i was led long ago by other considerations discussed in the first appendix to my treatise on 'saturn and its system.' at the epoch indicated, the first constellation of the zodiac was not, as now, the fishes, nor, as when a fresh departure was made by hipparchus, the ram, but the bull, a trace of which is found in virgil's words-- candidus auratis aperit cum cornibus annum taurus. the bull then was the spring sign, the pleiades and ruddy aldebaran joining their rays with the sun's at the time of the vernal equinox. the midsummer sign was the lion (the bright cor leonis nearly marking the sun's highest place). the autumn sign was the scorpion, the ruddy antares and the stars clustering in the head of the scorpion joining their rays with the sun's at the time of the autumnal equinox. and lastly the winter sign was the water bearer, the bright fomalhaut conjoining his rays with the sun's at midwinter. it is noteworthy that all these four constellations really present some resemblance to the objects after which they are named. the scorpion is in the best drawing, but the bull's head is well marked, and, as already mentioned, a leaping lion can be recognised. the streams of stars from the urn of aquarius and the urn itself are much better defined than the urn bearer. i have not left myself much space to speak of the finest of all the constellations, the glorious orion--the giant in his might, as he was called of old. in this noble asterism the figure of a giant ascending a slope can be readily discerned when the constellation is due south. at the time to which i have referred the constellation orion was considerably below the equator, and instead of standing nearly upright when due south high above the horizon, as now in our northern latitudes, he rose upright above the south-eastern horizon. the resemblance to a giant figure must then have been even more striking than it is at present (except in high northern latitudes, where orion, when due south, is just fully above the horizon). the giant orion has long been identified with nimrod; and those who recognise the antitypes of the ark in argo, of the old dragon in draco, and of the first and second adams in the kneeling hercules defeated by the serpent and the upright ophiuchus triumphant over the serpent, may, if they so please, find in the giant orion, the two dogs, the hare, and the bull (whom orion is more directly dealing with), the representations of nimrod, that mighty hunter before the lord, his hunting dogs, and the animals he hunted. pegasus, formerly called the horse, was regarded in very ancient times as the steed of nimrod. in modern astronomy the constellations no longer have the importance which once attached to them. they afford convenient means for naming the stars, though i think many observers would prefer the less attractive but more business-like methods adopted by piazzi and others, according to which a star rejoices in no more striking title than 'piazzi xiiih. ,' or 'struve, .' they still serve, however, to teach beginners the stars, and probably many years will pass before even exact astronomy dismisses them altogether to the limbo of discarded symbolisms. it is, indeed, somewhat singular that astronomers find it easier to introduce new absurdities among the constellations than to get rid of these old ones. the new and utterly absurd figures introduced by bode still remain in many charts despite such inconvenient names as _honores frederici_, _globum Ærostaticum_ and _machina pneumatica_; and i have very little doubt that a new constellation, if it only had a specially inconvenient title, would be accepted. but when francis baily tried to simplify the heavens by removing many of bode's absurd constellations, he was abused by many as violently as though he had proposed the rejection of the newtonian system. i myself tried a small measure of reform in the three first editions of my 'library atlas,' but have found it desirable to return to the old nomenclature in the fourth. the end. _printed by_ ballantyne, hanson & co. _edinburgh and london_ footnotes: [ ] these reflections were suggested to tacitus by the conduct of thrasyllus (chief astrologer of the emperor tiberius), when his skill was tested by his imperial employer after a manner characteristic of that agreeable monarch. the story runs thus (i follow whewell's version): 'those who were brought to tiberius on any important matter, were admitted to an interview in an apartment situated on a lofty cliff in the island of capreæ. they reached this place by a narrow path, accompanied by a single freedman of great bodily strength; and on their return, if the emperor had conceived any doubts of their trustworthiness, a single blow buried the secret and its victim in the ocean below. after thrasyllus had, in this retreat, stated the results of his art as they concerned the emperor, tiberius asked him whether he had calculated how long he himself had to live. the astrologer examined the aspect of the stars, and while he did this showed hesitation, alarm, increasing terror, and at last declared that "the present hour was for him critical, perhaps fatal." tiberius embraced him, and told him "he was right in supposing he had been in danger, but that he should escape it," and made him henceforward his confidential counsellor.' it is evident, assuming the story to be true (as seems sufficiently probable), that the emperor was no match for the charlatan in craft. it was a natural thought on the former's part to test the skill of his astrologer by laying for him a trap such as the story indicates--a thought so natural, indeed, that it probably occurred to thrasyllus himself long before tiberius put the plan into practice. even if thrasyllus had not been already on the watch for such a trick, he would have been but a poor trickster himself if he had not detected it the moment it was attempted, or failed to see the sole safe course which was left open to him. probably, with a man of the temper of tiberius, such a counter-trick as galeotti's in _quentin durward_ would have been unsafe. [ ] the belief in the influence of the stars and the planets on the fortunes of the new-born child was still rife when shakespeare made glendower boast: at my nativity the front of heaven was full of fiery shapes of burning cressets; know, that at my birth the frame and huge foundation of the earth shook like a coward. and shakespeare showed himself dangerously tainted with freethought in assigning (even to the fiery hotspur) the reply: so it would have done at the same season, if your mother's cat had kittened, though yourself had ne'er been born. in a similar vein butler, in _hudibras_ ridiculed the folly of those who believe in horoscopes and nativities: as if the planet's first aspect the tender infant did infect in soul and body, and instil all future good and future ill; which in their dark fatalities lurking, at destined periods fall a-working, and break out, like the hidden seeds of long diseases, into deeds, in friendships, enmities, and strife. and all th' emergencies of life. [ ] preface to the _rudolphine tables_. [ ] it is commonly stated that bacon opposed the copernican theory because he disliked gilbert, who had advocated it. 'bacon,' says one of his editors, 'was too jealous of gilbert to entertain one moment any doctrine that he advanced.' but, apart from the incredible littleness of mind which this explanation imputes to bacon, it would also have been an incredible piece of folly on bacon's part to advocate an inferior theory while a rival was left to support a better theory. bacon saw clearly enough that men were on their way to the discovery of the true theory, and, so far as in him lay, he indicated how they should proceed in order most readily to reach the truth. it must, then, have been from conviction, not out of mere contradiction, that bacon declared himself in favour of the ptolemaic system. in fact, he speaks of the diurnal motion of the earth as 'an opinion which we can demonstrate to be most false;' doubtless having in his thoughts some such arguments as misled tycho brahe. [ ] to bacon's theological contemporaries this must have seemed a dreadful heresy, and possibly in our own days the assertion would be judged scarcely less harshly, seeing that the observance of the (so-called) sabbath depends directly upon the belief in quite another origin of the week. yet there can be little question that the week really had its origin in astrological formulæ. [ ] in bohn's edition the word 'defective' is here used, entirely changing the meaning of the sentence. bacon registers an _astrologia sana_ amongst the things needed for the advancement of learning, whereas he is made to say that such an astrology must be registered as defective. [ ] the astrologers were exceedingly ingenious in showing that their art had given warning of the great plague and fire of london. thus, the star which marks the bull's northern horn--and which is described by ptolemy as like mars--was, they say, exactly in that part of the sign gemini which is the ascendant of london, in . lilly, however, for whom they claim the credit of predicting the year of this calamity, laid no claim himself to that achievement; nay, specially denied that he knew when the fire was to happen. the story is rather curious. in lilly had published his _monarchy or no monarchy_, which contained a number of curious hieroglyphics. amongst these were two (see frontispiece) which appeared to portend plague and fire respectively. the hieroglyphic of the plague represents three dead bodies wrapped in death-clothes, and for these bodies two coffins lie ready and two graves are being dug; whence it was to be inferred that the number of deaths would exceed the supply of coffins and graves. the hieroglyphic of the fire represents several persons, gentlefolk on one side and commonfolk on the other, emptying water vessels on a furious fire into which two children are falling headlong. the occurrence of the plague in attracted no special notice to lilly's supposed prediction of that event, though probably many talked of the coincidence as remarkable. but when in the great fire occurred, the house of commons summoned lilly to attend the committee appointed to enquire into the cause of the fire. 'at two of the clock on friday, the th of october ,' he attended in the speaker's chamber, 'to answer such questions as should then and there be asked him.' sir robert brooke spoke to this effect: 'mr. lilly, this committee thought fit to summon you to appear before them this day, to know if you can say anything as to the cause of the late fire, or whether there might be any design therein. you are called the rather hither, because in a book of yours long since printed, you hinted some such thing by one of your hieroglyphics.' unto which he replied: 'may it please your honours, after the beheading of the late king, considering that in the three subsequent years the parliament acted nothing which concerned the settlement of the nation's peace, and seeing the generality of the people dissatisfied, the citizens of london discontented, and the soldiery prone to mutiny, i was desirous, according to the best knowledge god had given me, to make enquiry by the art i studied, what might, from that time, happen unto the parliament and nation in general. at last, having satisfied myself as well as i could, and perfected my judgment therein, i thought it most convenient to signify my intentions and conceptions thereof in forms, shapes, types, hieroglyphics, etc., without any commentary, that so my judgment might be concealed from the vulgar, and made manifest only unto the wise; i herein imitating the examples of many wise philosophers who had done the like. having found, sir, that the great city of london should be sadly afflicted with a great plague, and not long after with an exorbitant fire, i framed these two hieroglyphics, as represented in the book, which in effect have proved very true.' 'did you foresee the year?' said one. 'i did not,' said lilly; 'nor was desirous; of that i made no scrutiny. now, sir, whether there was any design of burning the city, or any employed to that purpose, i must deal ingenuously with you, that since the fire i have taken much pains in the search thereof, but cannot or could not give myself the least satisfaction therein. i conclude that it was the finger of god only; but what instruments he used thereunto i am ignorant.' [ ] sir toby belch and sir andrew aguecheek were evidently not well taught in astrology. 'shall we set about some revels?' says the latter. 'what shall we do else?' says toby; 'were we not born under taurus?' 'taurus, that's sides and heart,' says sapient andrew. 'no, sir,' responds toby, 'it's legs and thighs. let me see thee caper.' [ ] 'this is the excellent foppery of the world, that, when we are sick in fortune (often the surfeit of our own behaviour), we make guilty of our disasters the sun, moon, and stars: as if we were villains on necessity; fools by heavenly compulsion; knaves, thieves, and treacherous by spherical predominance; drunkards, liars, and adulterers, by inforced obedience of planetary influence; and all that we are, evil, by a divine thrusting on.'--shakespeare (_king lear_). [ ] there are few things more remarkable, or to reasoning minds more inexplicable, than the readiness with which men undertook in old times, and even now undertake, to interpret omens and assign prophetic significance to casual events. one can understand that foolish persons should believe in omens, and act upon the ideas suggested by their superstitions. the difficulty is to comprehend how these superstitions came into existence. for instance, who first conceived the idea that a particular line in the palm of the hand is the line of life; and what can possibly have suggested so absurd a notion? to whom did the thought first present itself that the pips on playing-cards are significant of future events; and why did he think so? how did the 'grounds' of a teacup come to acquire that deep significance which they now possess for mrs. gamp and betsy prig? if the believers in these absurdities be asked _why_ they believe, they answer readily enough either that they themselves or their friends have known remarkable fulfilments of the ominous indications of cards or tea-dregs, which must of necessity be the case where millions of forecasts are daily made by these instructive methods. but the persons who first invented those means of divination can have had no such reasons. they must have possessed imaginations of singular liveliness and not wanting in ingenuity. it is a pity that we know so little of them. [ ] wellington lived too long for the astrologers, his death within the year having unfortunately been predicted by them many times during the last fifteen years of his life. some astrologers were more cautious, however. i have before me his horoscope, carefully calculated, _secundum artem_, by raphaël in , with results 'sufficiently evincing the surprising verity and singular accuracy of astrological calculations, when founded on the correct time of birth, and mathematically calculated. i have chosen,' he proceeds, 'the nativity of this illustrious native, in preference to others, as the subject is now living, and, consequently, all possibility of making up any fictitious horoscope is at once set aside; thus affording me a most powerful shield against the insidious representations of the envious and ignorant traducer of my sublime science.' by some strange oversight, however, raphaël omits to mention anything respecting the future fortunes of wellington, showing only how wonderfully wellington's past career had corresponded with his horoscope. [ ] 'i have still observed,' says an old author, 'that your right martialist doth seldom exceed in height, or be at the most above a yard or a yard and a half in height' (which is surely stint measure). 'it hath been always thus,' said that right martialist sir geoffrey hudson to julian peveril; 'and in the history of all ages, the clean tight dapper little fellow hath proved an overmatch for his burly antagonist. i need only instance, out of holy writ, the celebrated downfall of goliath and of another lubbard, who had more fingers in his hand, and more inches to his stature, than ought to belong to an honest man, and who was slain by a nephew of good king david; and of many others whom i do not remember; nevertheless, they were all philistines of gigantic stature. in the classics, also, you have tydeus, and other tight compact heroes, whose diminutive bodies were the abode of large minds.' [ ] it is likely that swedenborg in his youth studied astrology, for in his visions the mercurial folk have this desire of knowledge as their distinguishing characteristic. [ ] it is singular that, when there is this perfectly simple explanation of the origin of the nomenclature of the days of the week, an explanation given by ancient historians and generally received, whewell should have stated that 'various accounts are given, all the methods proceeding upon certain arbitrary arithmetical processes connected in some way with astrological views.' speaking of the arrangement of the planets in the order of their supposed distances, and of the order in which the planets appear in the days of the week, he says, 'it would be difficult to determine with certainty why the former order was adopted, and how and why the latter was derived from it.' but, in reality, there is no difficulty about either point. the former arrangement corresponded precisely with the periodic times of the seven planets of the old egyptian system (unquestionably far more ancient than the system adopted by the greeks), while the latter springs directly from the former. assign to the hours of the day, successively, the seven planets in the former order, continuing the sequence without interruption day after day, and in the course of seven days each one of the planets will have ruled the first hour of a day, in the order,--saturn, the sun, the moon, mars, mercury, jupiter, and venus. what arbitrary arithmetical process there is in this it would be difficult to conceive. arithmetic does not rule the method at all. nor has any other method ever been suggested; though this method has been presented in several ways, some arithmetical and some geometrical. we need then have no difficulty in understanding what seems so perplexing to whewell, the universality, namely, of the notions 'which have produced this result,' for the notions were not fantastic, but such as naturally sprang from the ideas on which astrology itself depends. [ ] the following remarks by the astronomer-royal on this subject seem to me just, in the main. they accord with what i had said earlier in my essay on saturn and the sabbath of the jews ('our place among infinities,' th essay). 'the importance which moses attached to it [the hebdomadal rest] is evident; and, with all reverence, i recognise to the utmost degree the justice of his views. no direction was given for religious ceremonial' (he seems to have overlooked numbers xxviii. , and cognate passages), 'but it was probably seen that the health given to the mind by a rest from ordinary cares, and by the opportunity of meditation, could not fail to have a most beneficial religious effect. but, to give sanction to this precept, the authority of at least a myth was requisite. i believe it was simply for this reason that the myth of the six days of creation was preserved. it is expressly cited in the first delivery of the commandments, as the solemn authority (exodus xxxi. ) for the command. it is remarkable that at the second mention of the commandment (deuteronomy v.) no reference is made to the creation; perhaps, after the complete establishment of jehovistic ideas in the minds of the israelites, they had nearly lost the recollection of the elohistic account, and it was not thought desirable to refer to it' (airy, 'on the early hebrew scriptures,' p. ). it must be regarded as a singular instance of the persistency of myths, if this view be correct, that a myth which had become obsolete for the jews between the time of moses and that of the writer (whoever he may have been) who produced the so-called mosaic book of deuteronomy, should thereafter have been revived, and have come to be regarded by the jews themselves and by christians as the word of god. [ ] of course it may be argued that nothing in the world is the result of _mere_ accident, and some may assert that even matters which are commonly regarded as entirely casual have been specially designed. it would not be easy to draw the precise line dividing events which all men would regard as to all intents and purposes accidental from those which some men would regard as results of special providence. but common sense draws a sufficient distinction, at least for our present purpose. [ ] this star, called _thuban_ from the arabian _al-thúban_, the dragon, is now not very bright, being rated at barely above the fourth magnitude, but it was formerly the brightest star of the constellation, as its name indicates. bayer also assigned to it the first letter of the greek alphabet; though this is not absolutely decisive evidence that so late as his day it retained its superiority over the second magnitude stars to which bayer assigned the second and third greek letters. in the year b.c., or thereabouts, the star was at its nearest to the true north pole of the heavens, the diameter of the little circle in which it then moved being considerably less than one-fourth the apparent diameter of the moon. at that time the star must have seemed to all ordinary observation an absolutely fixed centre, round which all the other stars revolved. at the time when the pyramid was built this star was about sixty times farther removed from the true pole, revolving in a circle whose apparent diameter was about seven times as great as the moon's. yet it would still be regarded as a very useful pole-star, especially as there are very few conspicuous stars in the neighbourhood. [ ] even that skilful astronomer hipparchus, who may be justly called the father of observational astronomy, overlooked this peculiarity, which ptolemy would seem to have been the first to recognise. [ ] it would only be by a lucky accident, of course, that the direction of the slant tunnel's axis and that of the vertical from the selected central point would lie in the same vertical plane. the object of the tunnelling would, in fact, be to determine how far apart the vertical planes through these points lay, and the odds would be great against the result proving to be zero. [ ] it may, perhaps, occur to the reader to inquire what diameter of the earth, supposed to be a perfect sphere, would be derived from a degree of latitude measured with absolute accuracy near latitude °. a degree of latitude measured in polar regions would indicate a diameter greater even than the equatorial; one measured in equatorial regions would indicate a diameter less even than the polar. near latitude ° the measurement of a degree of latitude would indicate a diameter very nearly equal to the true polar diameter of the earth. in fact, if it could be proved that the builders of the pyramid used for their unit of length an exact subdivision of the polar diameter, the inference would be that, while the coincidence itself was merely accidental, their measurement of a degree of latitude in their own country had been singularly accurate. by an approximate calculation i find that, taking the earth's compression at - , the diameter of the earth, estimated from the accurate measurement of a degree of latitude in the neighbourhood of the great pyramid, would have made the sacred cubit--taken at one , , th of the diameter--equal to · british inches; a closer approximation than professor smyth's to the estimated mean probable value of the sacred cubit. [ ] it is, however, almost impossible to mark any limits to what may be regarded as evidence of design by a coincidence-hunter. i quote the following from the late professor de morgan's _budget of paradoxes_. having mentioned that occurs less frequently than any other digit in the number expressing the ratio of circumference to diameter of a circle, he proceeds: 'a correspondent of my friend piazzi smyth notices that is the number of most frequency, and that - / is the nearest approximation to it in simple digits. professor smyth, whose work on egypt is paradox of a very high order, backed by a great quantity of useful labour, the results of which will be made available by those who do not receive the paradoxes, is inclined to see confirmation for some of his theory in these phenomena.' in passing, i may mention as the most singular of these accidental digit relations which i have yet noticed, that in the first digits of the square root of , the number occurs more than twice as often as either or , which each occur eight times, and occurring each nine times, and occurring no less than eighteen times. [ ] i have substituted this value in the article 'astronomy,' of the _british encyclopædia_, for the estimate formerly used, viz. , , miles. but there is good reason for believing that the actual distance is nearly , , miles. [ ] it may be matched by other coincidences as remarkable and as little the result of the operation of any natural law. for instance, the following strange relation, introducing the dimensions of the sun himself, nowhere, so far as i have yet seen, introduced among pyramid relations, even by pyramidalists: 'if the plane of the ecliptic were a true surface, and the sun were to commence rolling along that surface towards the part of the earth's orbit where she is at her mean distance, while the earth commenced rolling upon the sun (round one of his great circles), each globe turning round in the same time,--then, by the time the earth had rolled its way once round the sun, the sun would have almost exactly reached the earth's orbit. this is only another way of saying that the sun's diameter exceeds the earth's in almost exactly the same degree that the sun's distance exceeds the sun's diameter.' [ ] it has been remarked that, though hipparchus had the enormous advantage of being able to compare his own observations with those recorded by the chaldæans, he estimated the length of the year less correctly than the chaldæans. it has been thought by some that the chaldæans were acquainted with the true system of the universe, but i do not know that there are sufficient grounds for this supposition. diodorus siculus and apollonius myndius mention, however, that they were able to predict the return of comets, and this implies that their observations had been continued for many centuries with great care and exactness. [ ] the language of the modern zadkiels and raphaëls, though meaningless and absurd in itself, yet, as assuredly derived from the astrology of the oldest times, may here be quoted. (it certainly was not invented to give support to the theory i am at present advocating.) thus runs the jargon of the tribe: 'in order to illustrate plainly to the reader what astrologers mean by the "houses of heaven," it is proper for him to bear in mind the four cardinal points. the eastern, facing the rising sun, has at its centre the first grand angle or first house, termed the horoscope or ascendant. the northern, opposite the region where the sun is at midnight, or the _cusp_ of the lower heaven or nadir, is the imum coeli, and has at its centre the fourth house. the western, facing the setting sun, has at its centre the third grand angle or seventh house or descendant. and lastly, the southern, facing the noonday sun, has at its centre the astrologer's tenth house, or mid-heaven, the most powerful angle or house of honour.' 'and although,' proceeds the modern astrologer, 'we cannot in the ethereal blue discern these lines or terminating divisions, both reason and experience assure us that they certainly exist; therefore the astrologer has certain grounds for the choice of his four angular houses' (out of twelve in all) 'which, resembling the palpable demonstration they afford, are in the astral science esteemed the most powerful of the whole. '--raphaël's _manual of astrology_. [ ] arabian writers give the following account of egyptian progress in astrology and the mystical arts: nacrawasch, the progenitor of misraim, was the first egyptian prince, and the first of the magicians who excelled in astrology and enchantment. retiring into egypt with his family of eighty persons, he built essous, the most ancient city of egypt, and commenced the first dynasty of misraimitish princes, who excelled as cabalists, diviners, and in the mystic arts generally. the most celebrated of the race were naerasch, who first represented by images the twelve signs of the zodiac; gharnak, who openly described the arts before kept secret; hersall, who first worshipped idols; sehlouk, who worshipped the sun; saurid (king saurid of ibn abd alkohm's account), who erected the first pyramids and invented the magic mirror; and pharaoh, the last king of the dynasty, whose name was afterwards taken as a kingly title, as cæsar later became a general imperial title. [ ] it is noteworthy how swedenborg here anticipates a saying of laplace, the greatest mathematician the world has known, save newton alone. newton's remark that he seemed but as a child who had gathered a few shells on the shores of ocean, is well known. laplace's words, '_ce que nous connaissons est peu de chose; ce que nous ignorons est immense_,' were not, as is commonly stated, his last. de morgan gives the following account of laplace's last moments, on the authority of laplace's friend and pupil, the well-known mathematician poisson: 'after the publication (in ) of the fifth volume of the mécanique céleste, laplace became gradually weaker, and with it musing and abstracted. he thought much on the great problems of existence, and often muttered to himself, "_qu'est-ce que c'est que tout cela!_" after many alternations he appeared at last so permanently prostrated that his family applied to his favourite pupil, m. poisson, to try to get a word from him. poisson paid a visit, and after a few words of salutation, said, "j'ai une bonne nouvelle à vous annoncer: on a reçu au bureau des longitudes une lettre d'allemagne annonçant que m. bessel a vérifié par l'observation vos découvertes théoriques sur les satellites de jupiter." laplace opened his eyes and answered with deep gravity. "_l'homme ne poursuit que des chimères._" he never spoke again. his death took place march , .' [ ] the reason assigned by swedenborg is fanciful enough. 'in the spiritual sense,' he says, 'a horse signifies the intellectual principle formed from scientifics, and as they are afraid of cultivating the intellectual faculties by worldly sciences, from this comes an influx of fear. they care nothing for scientifics which are of human erudition.' [ ] similar reasoning applies to the moons of jupiter, and it so chances that the result in their case comes out exactly the same as in the case of saturn; all the jovian moons, if full together, would reflect only the sixteenth part of the light which we receive from the full moon. it is strange that scientific men of considerable mathematical power have used the argument from design apparently supplied by the satellites, without being at the pains to test its validity by the simple mathematical calculations necessary to determine the quantity of light which these bodies can reflect to the planets round which they travel. brewster and whewell, though they took opposite sides in the controversy about other inhabited worlds, agreed in this. brewster, of course, holding the theory that all the planets are inhabited, very naturally accepted the argument from design in this case. whewell, in opposing that theory, did not dwell at all upon the subjects of the satellites. but in his 'bridgewater treatise on astronomy and general physics,' he says, 'taking only the ascertained cases of venus, the earth, jupiter, and saturn, we conceive that a person of common understanding will be strongly impressed with the persuasion that the satellites are placed in the system with a view to compensate for the diminished light of the sun at greater distances. mars is an exception; some persons might conjecture from this case that the arrangement itself, like other useful arrangements, has been brought about by some wider law which we have not yet detected. but whether or not we entertain such a guess (it can be nothing more), we see in other parts of creation so many examples of apparent exceptions to rules, which are afterwards found to be capable of explanation, or to be provided for by particular contrivances, that no one familiar with such contemplations will, by one anomaly, be driven from the persuasion that the end which the arrangements of the satellites seem suited to answer is really one of the ends of their creation.' [ ] the reader who cares enough about such subjects to take the necessary trouble, can easily make a little model of saturn and his ring system, which will very prettily illustrate the effect of the rings both in reflecting light to the planet's darkened hemisphere and in cutting off light from the planet's illuminated hemisphere. take a ball, say an ordinary hand-ball, and pierce it through the centre with a fine knitting-needle. cut out a flat ring of card, proportioned to the ball as the ring system of saturn to his ball. (if the ball is two inches in diameter, strike out on a sheet of cardboard two concentric circles, one of them with a radius of a little more than an inch and a half, the other with a radius of about two inches and three-eights, and cut out the ring between these two circles.) thrust the knitting-needle through this ring in such a way that the ball shall lie in the middle of the ring, as the globe of saturn hangs (without knitting-needle connections) in the middle of his ring system. thrust another knitting-needle centrally through the ball square to the plane of the ring, and use this second needle, which we may call the polar one, as a handle. now take the ball and ring into sunlight, or the light of a lamp or candle, holding them so that the shadow of the ring is as thin as possible. this represents the position of the shadow at the time of saturnian spring or autumn. cause the shadow slowly to shift until it surrounds the part of the ball through which the polar needle passes on one side. this will represent the position of the shadow at the time of midwinter for the hemisphere corresponding to that side of the ball. notice that while the shadow is traversing this half of the ball, the side of the ring which lies towards that half is in shadow, so that a fly or other small insect on that half of the ball would see the darkened side of the ring. a saturnian correspondingly placed would get no reflected sunlight from the ring system. move the ball and ring so that the shadow slowly returns to its first position. you will then have illustrated the changes taking place during one half of a saturnian year. continue the motion so that the shadow passes to the other half of the ball, and finally surrounds the other point through which the polar needle passes. the polar point which the shadow before surrounded will now be seen to be in the light, and this half of the ball will illustrate the hemisphere of saturn where it is midsummer. it will also be seen that the side of the ring towards this half of the ball is now in the light, so that a small insect on this half of the ball would see the bright side of the ring. a saturnian correspondingly placed would get reflected sunlight from the ring system _both by day and by night_. moving the ball and ring so that the shadow returns to its first position, an entire saturnian year will have been illustrated. these changes can be still better shown with a saturnian orrery (see plate viii. of my saturn), which can be very easily constructed. [ ] not 'of course' because tycho used it, for, like other able students of science, he made mistakes from time to time. thus he argued that the earth cannot rotate on her axis, because if she did bodies raised above her surface would be left behind--an argument which even the mechanical knowledge of his own time should have sufficed to invalidate, though it is still used from time to time by paradoxers of our own day. [ ] chinese chronicles contain other references to new stars. the annals of ma-touan-lin, which contain the official records of remarkable appearances in the heavens, include some phenomena which manifestly belong to this class. thus they record that in the year a star appeared between the stars which mark the hind feet of the centaur. this star remained visible from december in that year until july in the next (about the same time as tycho brahe's and kepler's new stars, presently to be described). another star, assigned by these annals to the year , seems to be the same as a star referred to by hepidannus as appearing a.d. . it was of extraordinary brilliancy, and remained visible in the southern part of the heavens during three months. the annals of ma-touan-lin assign to it a position low down in sagittarius. [ ] still a circumstance must be mentioned which tends to show that the star may have been visible a few hours earlier than dr. schmidt supposed. mr. m. walter, surgeon of the th regiment, then stationed in north india, wrote (oddly enough, on may , , the first anniversary of mr. birmingham's discovery) as follows to mr. stone:--'i am certain that this same conflagration was distinctly perceptible here at least six hours earlier. my knowledge of the fact came about in this wise. the night of the th of may last year was exceedingly sultry, and about eight o'clock on that evening i got up from the tea-table and rushed into my garden to seek a cooler atmosphere. as my door opens towards the east, the first object that met my view was the northern crown. my attention was at once arrested by the sight of a strange star outside the crown' (that is, outside the circlet of stars forming the diadem, not outside the constellation itself). the new star 'was then certainly quite as bright--i rather thought more so--as its neighbour alphecca,' the chief gem of the crown. 'i was so much struck with its appearance, that i exclaimed to those indoors, "why, here is a new comet!'" he made a diagram of the constellation, showing the place of the new star correctly. unfortunately, mr. walter does not state why he is so confident, a year after the event, that it was on the th of may, and not on the th, that he noticed the new star. if he fixed the date only by the star's appearance as a second-magnitude star, his letter proves nothing; for we know that on the th it was still shining as brightly as alphecca, though on the th it was perceptibly fainter. [ ] the velocity of three or four miles per second inferred by the elder struve must now be regarded (as i long since pointed out would prove to be the case) as very far short of the real velocity of our system's motion through stellar space. [ ] m. cornu's observations are full of interest, and he deserves considerable credit for his energy in availing himself of the few favourable opportunities he had for making them. but he goes beyond his province in adding to his account of them some remarks, intended apparently as a reflection on mr. huggins's speculations respecting the star in the northern crown. '_i_,' says m. cornu, 'will not try to form any hypothesis about the cause of the outburst. to do so would be unscientific, and such speculations, though interesting, cumber science wofully.' this is sheer nonsense, and comes very ill from an observer whose successes in science have been due entirely to the employment of methods of observation which would have had no existence had others been as unready to think out the meaning of observed facts as he appears to be himself. [ ] the same peculiarity has been noticed since the discovery of the dark ring, the space within that ring being observed by coolidge and g. bond at harvard in to be apparently darker than the surrounding sky. [ ] i cannot understand why mr. webb, in his interesting little work, _celestial objects for common telescopes_, says that the satellite theory of the rings certainly seems insufficient to account for the phenomena of the dark ring. it seems, on the contrary, manifest that the dark ring can scarcely be explained in any other way. the observations recently made are altogether inexplicable on any other theory. [ ] a gentleman, whose acquaintance i made in returning from america last spring, assured me that he had found demonstrative evidence showing that a total eclipse of the moon then occurred; for he could prove that abraham's vision occurred at the time of full moon, so that it could not otherwise have been dark when the sun went down (v. ). but the horror of great darkness occurred when the sun was going down, and total eclipses of the moon do not behave that way--at least, in our time. [ ] it is not easy to understand what else it could have been. the notion that a conjunction of three planets, which took place shortly before the time of christ's birth, gave rise to the tradition of the star in the east, though propounded by a former president of the astronomical society, could hardly be entertained by an astronomer, unless he entirely rejected matthew's account, which the author of this theory, being a clergyman, can scarcely have done. [ ] as, for instance, when he makes homer say of the moon that around her throne the vivid planets roll, and stars unnumbered gild the glowing pole. it is difficult, indeed, to understand how so thorough an astronomer as the late admiral smyth could have called the passage in which these lines occur one of the finest bursts of poetry in our language, except on the principle cleverly cited by waller when charles ii. upbraided him for the warmth of his panegyric on cromwell, that 'poets succeed better with fiction than with truth.' macaulay, though not an astronomer, speaks more justly of the passage in saying that this single passage contains more inaccuracies than can be found in all wordsworth's 'excursion.' [ ] it may be necessary to throw in here a few words of explanation, lest the non-astronomical reader should run away with the idea that the so-called exact science is a very inexact science indeed, so far as comets are concerned. the comet of was one of those which travel on a very eccentric orbit. coming, indeed, from out depths many times more remote than the path even of the remotest planet, neptune, this comet approached nearer to the sun than any which astronomers have ever seen, except only the comet of . when at its nearest its nucleus was only a sixth part of the sun's diameter from his surface. thus the part of the comet's orbit along which astronomers traced its motion was only a small part at one end of an enormously long oval, and very slight errors of observation were sufficient to produce very large errors in the determination of the nature of the comet's orbit. encke admitted that the period might, so far as the comparatively imperfect observations made in were concerned, be any whatever, from years to many millions of years, or even to infinity--that is, the comet might have a path not re-entering into itself, but carrying the comet for ever away from the sun after its one visit to our system. [ ] for a portion of the passages which i have quoted in this essay i am indebted to guillemin's 'treatise on comets,' a useful contribution to the literature of the subject, though somewhat inadequate so far as exposition is concerned. [ ] something very similar happened only a few years ago, so that we cannot afford to laugh too freely at the terrors of france in . it was reported during the winter of - , that plantamour, the swiss astronomer, had predicted the earth's destruction by a comet on august , . yet there was no other foundation for this rumour than the fact that plantamour, in a lecture upon comets and meteors, had stated that the meteors seen on august , , and are bodies following in the track of a comet whose orbit passes very near to the earth's. it was very certainly known to astronomers that there could be no present danger of a collision with this comet, for the comet has a period of at least years, and had last passed close to the earth's orbit (not to the earth herself, be it understood) in . but it was useless to point this out. many people insisted on believing that on august , , the earth would come into collision, possibly disastrous, with a mighty comet, which plantamour was said to have detected and to have shown by a profound calculation to be rushing directly upon our unfortunate earth. [ ] a rather amusing mistake was made by the stenographers of a new york paper in reporting the above sentence, which i happened to quote in a lecture upon comets and meteors. instead of paradise they wrote paris. those acquainted with pitman's system of short-hand, the one most commonly employed by reporters, will easily understand how the mistake was made, the marks made to represent the consonants p, r, d, and s differing little from those made to represent the consonants p, r, and s (the 'd' or 't' sound is represented, or may be represented, by simply shortening the length of the sign for the preceding consonant). the mistake led naturally to my remarking in my next lecture that i had not before known how thoroughly synonymous the words are in america, though i had heard it said that 'good americans, when they die, go to paris.' [ ] on the occasion of my first visit to america, in , i for the first time succeeded in obtaining a copy of this curious pamphlet. it had been mentioned to me (by emerson, i think) as an amusing piece of trickery played off by a scientific man on his brethren; and dr. wendell holmes, who was present, remarked that he had a copy in his possession. this he was good enough to lend me. soon after, a valued friend in new york presented me with a copy. [ ] this locke must not be confounded with richard lock, the circle-squarer and general paradoxist, who flourished a century earlier. [ ] the nurses' tale is, that the man was sent to the moon by moses for gathering sticks on the sabbath, and they refer to the cheerful story in numbers xv. - . according to german nurses the day was not the sabbath, but sunday. their tale runs as follows: 'ages ago there went one sunday an old man into the woods to hew sticks. he cut a faggot and slung it on a stout staff, cast it over his shoulder, and began to trudge home with his burthen. on his way he met a handsome man in sunday suit, walking towards the church. the man stopped, and asked the faggot-bearer; "do you know that this is sunday on earth, when all must rest from their labours?" "sunday on earth or monday in heaven, it's all one to me?" laughed the wood-cutter. "then bear your bundle for ever!" answered the stranger. "and as you value not sunday on earth, yours shall be a perpetual moon-day in heaven; you shall stand for eternity in the moon, a warning to all sabbath-breakers." thereupon the stranger vanished; and the man was caught up with his staff and faggot into the moon, where he stands yet.' according to some narrators the stranger was christ; but whether from german laxity in such matters or for some other reason, no text is quoted in evidence, as by the more orthodox british nurses. luke vi. - might serve. [ ] milton's opinion may be quoted against me here; and as received ideas respecting angels, good and bad, the fall of man, and many other such matters, are due quite as much to milton as to any other authority, his opinion must not be lightly disregarded. but though, when milton's satan 'meets a vast vacuity' where his wings are of no further service to him, 'all unawares flutt'ring his pennons vain, plumb down he drops ten thousand fathoms deep, and to this hour down had been falling, had not by ill chance the strong rebuff of some tumultuous cloud, instinct with fire and nitre, hurried him as many miles aloft,' yet this was written nearly a quarter of a century before newton had established the law of gravity. moreover, there is no evidence to show in what direction satan fell; 'above is below and below above,' says richter, 'to one stripped of gravitating body;' and whether satan was under the influence of gravity or not, he would be practically exempt from its action when in the midst of that 'dark, illimitable ocean' of space, 'without bound, without dimensions, where length, breadth, and height, and time and place are lost.' his lighting 'on niphates' top,' and overleaping the gate of paradise, may be used as arguments either way. on the whole, i must (according to my present lights) claim for satan a freedom from all scientific restraints. this freedom is exemplified by his showing all the kingdoms of the world from an exceeding high mountain, thus affording the first practical demonstration of the flat-earth theory, the maintenance of which led to poor mr. hampden's incarceration. [ ] the _sun_ itself claimed to have established the veracity of the account in a manner strongly recalling a well-known argument used by orthodox believers in the bible account of the cosmogony. either, say these, moses discovered how the world was made, or the facts were revealed to him by some one who had made the discovery: but moses could not have made the discovery, knowing nothing of the higher departments of science; therefore, the account came from the only being who could rationally be supposed to know anything about the beginning of the world. 'either,' said the _new york sun_, speaking of a mathematical problem discussed in the article, 'that problem was predicated by us or some other person, who has thereby made the greatest of all modern discoveries in mathematical astronomy. we did not make it, for we know nothing of mathematics whatever; therefore, it was made by the only person to whom it can rationally be ascribed, namely herschel the astronomer, its only avowed and undeniable author.' in reality, notwithstanding this convincing argument, the problem was stolen by locke from a paper by olbers, shortly before published, and gave the method followed by beer and mädler throughout their selenographical researches in - . [ ] i had at the same time the good fortune to satisfy in equal degree, though quite unexpectedly, an english student of the sun, who at that time bore me no great good-will. something in the article chanced to suggest that it came from another, presumably a rival, hand; while an essay which appeared about the same time (the spring of ) was commonly but erroneously attributed to me. accordingly, a leading article in _nature_ was devoted to the annihilation of the writer supposed to be myself, and to the lavish and quite undeserved laudation of the article i had written, which was selected as typifying all the good qualities which an article of the kind should possess. those acquainted with the facts were not a little amused by the mistake. [ ] the astronomer-royal once told me that he had found that few persons have a clear conception of the fact that the stars rise and set. still fewer know how the stars move, which stars rise and set, which are always above the horizon, which move on large circles, which on small ones; though a few hours' observation on half-a-dozen nights in the year (such observations being continuous, but made only at hourly intervals) would show dearly how the stars move. it is odd to find even some who write about astronomy making mistakes on matters so elementary. for instance, in a primer of astronomy recently published, it is stated that the stars which pass overhead in london rise and set on a slant--the real fact being that _those_ stars never rise or set at all, never coming within some two dozen moon-breadths of the horizon. [ ] in passing let me note that, of course, i am not discussing the arguments of paradoxists with the remotest idea of disproving them. they are not, in reality, worth the trouble. but they show where the general reader of astronomical text-books, and other such works, is likely to go astray, and thus conveniently indicate matters whose explanation may be useful or interesting. [ ] sterne anticipated this paradoxist in (jestingly) attributing glassiness to an inferior planet. he made the inhabitants, however, not the air, glassy. 'the intense heat of the country,' he says, speaking of the planet mercury, 'must, i think, long ago have vitrified the bodies of the inhabitants to suit them for the climate; so that all the tenements of their souls may be nothing else, for aught the soundest philosophy can show to the contrary, but one fine transparent body of clear glass; so that till the inhabitant grows old and tolerably wrinkled, whereby the rays of light become monstrously refracted, or return reflected from the surface, etc., his soul might as well play the fool out o' doors as in her own house.' [ ] it will be seen from table x. of my treatise on saturn that the ring disappeared on december , remaining invisible (because turning its dark side earthwards) till the spring of . but on december , the ring must have been quite invisible in a telescope so feeble as galileo's. the ring then would have been little more than a fine line of light as seen with one of our powerful modern telescopes. [ ] _north british review_ for august . [ ] he had, indeed, at an earlier stage, shown a marvellous ignorance of astronomy by the remark, which doubtless appeared to him a safe one, that when he saw a planet on the sun in september he supposed it was mercury; a september transit of mercury being as impossible as an eclipse of the sun during the moon's third quarter. [ ] it is, by the way, somewhat amusing to find baron humboldt referring a question of this sort to the great mathematician gauss, and describing the problem as though it involved the most profound calculations. ten minutes should suffice to deal with any problem of the kind. transcriber's note the following typographical errors were corrected. page error correction julias julius genuis genius artficers artificers signfies signifies footnote preplexing perplexing chaldean chaldæan chaldeans chaldæans peruquier perruquier peruque perruque northfolk norfolk ascant askant none curiosities of the sky by garrett serviss curiosities of the sky was first published in and the text is in the public domain. the transcription was done by william mcclain (info@sattre-press.com), . a printed version of this book is available from sattre press (http://csky.sattre-press.com). it includes extensive annotations, a new introduction and all the original photographs and diagrams. _________________________________________________________________ preface what froude says of history is true also of astronomy: it is the most impressive where it transcends explanation. it is not the mathematics of astronomy, but the wonder and the mystery that seize upon the imagination. the calculation of an eclipse owes all its prestige to the sublimity of its data; the operation, in itself, requires no more mental effort than the preparation of a railway time-table. the dominion which astronomy has always held over the minds of men is akin to that of poetry; when the former becomes merely instructive and the latter purely didactic, both lose their power over the imagination. astronomy is known as the oldest of the sciences, and it will be the longest-lived because it will always have arcana that have not been penetrated. some of the things described in this book are little known to the average reader, while others are well known; but all possess the fascination of whatever is strange, marvelous, obscure, or mysterious -- magnified, in this case, by the portentous scale of the phenomena. the idea of the author is to tell about these things in plain language, but with as much scientific accuracy as plain language will permit, showing the wonder that is in them without getting away from the facts. most of them have hitherto been discussed only in technical form, and in treatises that the general public seldom sees and never reads. among the topics touched upon are: * the strange unfixedness of the ``fixed stars,'' the vast migrations of the suns and worlds constituting the universe. * the slow passing out of existence of those collocations of stars which for thousands of years have formed famous ``constellations,'' preserving the memory of mythological heroes and heroines, and perhaps of otherwise unrecorded history. * the tendency of stars to assemble in immense clouds, swarms, and clusters. * the existence in some of the richest regions of the universe of absolutely black, starless gaps, deeps, or holes, as if one were looking out of a window into the murkiest night. * the marvelous phenomena of new, or temporary, stars, which appear as suddenly as conflagrations, and often turn into something else as eccentric as themselves. * the amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' and ``lace,'' or ``tress,'' nebulæ. * the strange surroundings of the sun, only seen in particular circumstances, but evidently playing a constant part in the daily phenomena of the solar system. * the mystery of the zodiacal light and the gegenschein. * the extraordinary transformations undergone by comets and their tails. * the prodigies of meteorites and masses of stone and metal fallen from the sky. * the cataclysms that have wrecked the moon. * the problem of life and intelligence on the planet mars. * the problematical origin and fate of the asteroids. * the strange phenomena of the auroral lights. an attempt has been made to develop these topics in an orderly way, showing their connection, so that the reader may obtain a broad general view of the chief mysteries and problems of astronomy, and an idea of the immense field of discovery which still lies, almost unexplored, before it. the windows of absolute night to most minds mystery is more fascinating than science. but when science itself leads straight up to the borders of mystery and there comes to a dead stop, saying, ``at present i can no longer see my way,'' the force of the charm is redoubled. on the other hand, the illimitable is no less potent in mystery than the invisible, whence the dramatic effect of keats' ``stout cortez'' staring at the boundless pacific while all his men look at each other with a wild surmise, ``silent upon a peak in darien.'' it is with similar feelings that the astronomer regards certain places where from the peaks of the universe his vision seems to range out into endless empty space. he sees there the shore of his little isthmus, and, beyond, unexplored immensity. the name, ``coal-sacks,'' given to these strange voids is hardly descriptive. rather they produce upon the mind the effect of blank windows in a lonely house on a pitch-dark night, which, when looked at from the brilliant interior, become appalling in their rayless murk. infinity seems to acquire a new meaning in the presence of these black openings in the sky, for as one continues to gaze it loses its purely metaphysical quality and becomes a kind of entity, like the ocean. the observer is conscious that he can actually see the beginning of its ebon depths, in which the visible universe appears to float like an enchanted island, resplendent within with lights and life and gorgeous spectacles, and encircled with screens of crowded stars, but with its dazzling vistas ending at the fathomless sea of pure darkness which encloses all. the galaxy, or milky way, surrounds the borders of our island in space like a stellar garland, and when openings appear in it they are, by contrast, far more impressive than the general darkness of the interstellar expanse seen in other directions. yet even that expanse is not everywhere equally dark, for it contains gloomy deeps discernable with careful watching. here, too, contrast plays an important part, though less striking than within the galactic region. some of sir william herschel's observations appear to indicate an association between these tenebrious spots and neighboring star clouds and nebulæ. it is an illuminating bit of astronomical history that when he was sweeping the then virgin heavens with his great telescopes he was accustomed to say to his sister who, note-book in hand, waited at his side to take down his words, fresh with the inspiration of discovery: ``prepare to write; the nebulæ are coming; here space is vacant.'' the most famous of the ``coal-sacks,'' and the first to be brought to general attention before astronomers had awakened to the significance of such things, lies adjacent to the ``southern cross,'' and is truly an amazing phenomenon. it is not alone the conspicuousness of this celestial vacancy, opening suddenly in the midst of one of the richest parts of the galaxy, that has given it its fame, but quite as much the superstitious awe with which it was regarded by the early explorers of the south seas. to them, as well as to those who listened in rapt wonder to their tales, the ``coal-sack'' seemed to possess some occult connection with the mystic ``cross.'' in the eyes of the sailors it was not a vacancy so much as a sable reality in the sky, and as, shuddering, they stared at it, they piously crossed themselves. it was another of the magical wonders of the unknown south, and as such it formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. scientific investigation has not diminished its prestige, and today no traveler in the southern hemisphere is indifferent to its fascinating strangeness, while some find it the most impressive spectacle of the antarctic heavens. all around, up to the very edge of the yawning gap, the sheen of the milky way is surpassingly glorious; but there, as if in obedience to an almighty edict, everything vanishes. a single faint star is visible within the opening, producing a curious effect upon the sensitive spectator, like the sight of a tiny islet in the midst of a black, motionless, waveless tarn. the dimensions of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by five, so that it occupies a space in the sky about one hundred and thirty times greater than the area of the full moon. it attracts attention as soon as the eye is directed toward the quarter where it exists, and by virtue of the rarity of such phenomena it appears a far greater wonder than the drifts of stars that are heaped around it. now that observatories are multiplying in the southern hemisphere, the great austral ``coal-sack'' will, no doubt, receive attention proportioned to its importance as one of the most significant features of the sky. already at the sydney observatory photographs have shown that the southern portion of this dead sea of space is not quite ``bottomless,'' although its northern part defies the longest sounding lines of the astronomer. there is a similar, but less perfect, ``coal-sack'' in the northern hemisphere, in the constellation of ``the swan,'' which, strange to say, also contains a well-marked figure of a cross outlined by stars. this gap lies near the top of the cross-shaped figure. it is best seen by averted vision, which brings out the contrast with the milky way, which is quite brilliant around it. it does not, however, exercise the same weird attraction upon the eye as the southern ``coal-sack,'' for instead of looking like an absolute void in the sky, it rather appears as if a canopy of dark gauze had been drawn over the stars. we shall see the possible significance of this appearance later. just above the southern horizon of our northern middle latitudes, in summer, where the milky way breaks up into vast sheets of nebulous luminosity, lying over and between the constellations scorpio and sagittarius, there is a remarkable assemblage of ``coal-sacks,'' though none is of great size. one of them, near a conspicuous star-cluster in scorpio, m , is interesting for having been the first of these strange objects noted by herschel. probably it was its nearness to m which suggested to his mind the apparent connection of such vacancies with star-clusters which we have already mentioned. but the most marvelous of the ``coal-sacks'' are those that have been found by photography in sagittarius. one of barnard's earliest and most excellent photographs includes two of them, both in the star-cluster m . the larger, which is roughly rectangular in outline, contains one little star, and its smaller neighbor is lune-shaped -- surely a most singular form for such an object. both are associated with curious dark lanes running through the clustered stars like trails in the woods. along the borders of these lanes the stars are ranked in parallel rows, and what may be called the bottoms of the lanes are not entirely dark, but pebbled with faint stellar points. one of them which skirts the two dark gaps and traverses the cluster along its greatest diameter is edged with lines of stars, recalling the alignment of the trees bordering a french highway. this road of stars cannot be less than many billions of miles in length! all about the cluster the bed of the galaxy is strangely disturbed, and in places nearly denuded, as if its contents had been raked away to form the immense stack and the smaller accumulations of stars around it. the well-known ``trifid nebula'' is also included in the field of the photograph, which covers a truly marvelous region, so intricate in its mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark vacancies that no description can do it justice. yet, chaotic as it appears, there is an unmistakable suggestion of unity about it, impressing the beholder with the idea that all the different parts are in some way connected, and have not been fortuitously thrown together. miss agnes m. clerke made the striking remark that the dusky lanes in m are exemplified on the largest scale in the great rift dividing the milky way, from cygnus in the northern hemisphere all the way to the ``cross'' in the southern. similar lanes are found in many other clusters, and they are generally associated with flanking rows of stars, resembling in their arrangement the thick-set houses and villas along the roadways that traverse the approaches to a great city. but to return to the black gaps. are they really windows in the star-walls of the universe? some of them look rather as if they had been made by a shell fired through a luminous target, allowing the eye to range through the hole into the void space beyond. if science is discretely silent about these things, what can the more venturesome and less responsible imagination suggest? would a huge ``runaway sun,'' like arcturus, for instance, make such an opening if it should pass like a projectile through the milky way? it is at least a stimulating inquiry. being probably many thousands of times more massive than the galactic stars, such a stellar missile would not be stopped by them, though its direction of flight might be altered. it would drag the small stars lying close to its course out of their spheres, but the ultimate tendency of its attraction would be to sweep them round in its wake, thus producing rather a star-swarm than a vacancy. those that were very close to it might be swept away in its rush and become its satellites, careering away with it in its flight into outer space; but those that were farther off, and they would, of course, greatly outnumber the nearer ones, would tend inward from all sides toward the line of flight, as dust and leaves collect behind a speeding motor (though the forces operating would be different), and would fill up the hole, if hole it were. a swarm thus collected should be rounded in outline and bordered with a relatively barren ring from which the stars had been ``sucked'' away. in a general sense the m cluster answers to this description, but even if we undertook to account for its existence by a supposition like the above, the black gaps would remain unexplained, unless one could make a further draft on the imagination and suggest that the stars had been thrown into a vast eddy, or system of eddies, whose vortices appear as dark holes. only a maelstrom-like motion could keep such a funnel open, for without regard to the impulse derived from the projectile, the proper motions of the stars themselves would tend to fill it. perhaps some other cause of the whirling motion may be found. as we shall see when we come to the spiral nebulæ, gyratory movements are exceedingly prevalent throughout the universe, and the structure of the milky way is everywhere suggestive of them. but this is hazardous sport even for the imagination -- to play with suns as if they were but thistle-down in the wind or corks in a mill-race. another question arises: what is the thickness of the hedge of stars through which the holes penetrate? is the depth of the openings proportionate to their width? in other words, is the milky way round in section like a rope, or flat and thin like a ribbon? the answer is not obvious, for we have little or no information concerning the relative distances of the faint galactic stars. it would be easier, certainly, to conceive of openings in a thin belt than in a massive ring, for in the first case they would resemble mere rifts and breaks, while in the second they would be like wells or bore-holes. then, too, the fact that the milky way is not a continuous body but is made up of stars whose actual distances apart is great, offers another quandary; persistent and sharply bordered apertures in such an assemblage are a priori as improbable, if not impossible, as straight, narrow holes running through a swarm of bees. the difficulty of these questions indicates one of the reasons why it has been suggested that the seeming gaps, or many of them, are not openings at all, but opaque screens cutting off the light from stars behind them. that this is quite possible in some cases is shown by barnard's later photographs, particularly those of the singular region around the star rho ophiuchi. here are to be seen somber lanes and patches, apparently forming a connected system which covers an immense space, and which their discoverer thinks may constitute a ``dark nebula.'' this seems at first a startling suggestion; but, after all, why should their not be dark nebulæ as well as visible ones? in truth, it has troubled some astronomers to explain the luminosity of the bright nebulæ, since it is not to be supposed that matter in so diffuse a state can be incandescent through heat, and phosphorescent light is in itself a mystery. the supposition is also in accord with what we know of the existence of dark solid bodies in space. many bright stars are accompanied by obscure companions, sometimes as massive as themselves; the planets are non-luminous; the same is true of meteors before they plunge into the atmosphere and become heated by friction; and many plausible reasons have been found for believing that space contains as many obscure as shining bodies of great size. it is not so difficult, after all, then, to believe that there are immense collections of shadowy gases and meteoric dust whose presence is only manifested when they intercept the light coming from shining bodies behind them. this would account for the apparent extinguishment of light in open space, which is indicated by the falling off in relative number of telescopic stars below the tenth magnitude. even as things are, the amount of light coming to us from stars too faint to be seen with the naked eye is so great that the statement of it generally surprises persons who are unfamiliar with the inner facts of astronomy. it has been calculated that on a clear night the total starlight from the entire celestial sphere amounts to one-sixtieth of the light of the full moon; but of this less than one-twenty-fifth is due to stars separately distinguished by the eye. if there were no obscuring medium in space, it is probable that the amount of starlight would be noticeably and perhaps enormously increased. but while it seems certain that some of the obscure spots in the milky way are due to the presence of ``dark nebulæ,'' or concealing veils of one kind or another, it is equally certain that there are many which are true apertures, however they may have been formed, and by whatever forces they may be maintained. these, then, are veritable windows of the galaxy, and when looking out of them one is face to face with the great mystery of infinite space. there the known universe visibly ends, but manifestly space itself does not end there. it is not within the power of thought to conceive an end to space, for the instant we think of a terminal point or line the mind leaps forward to the beyond. there must be space outside as well as inside. eternity of time and infinity of space are ideas that the intellect cannot fully grasp, but neither can it grasp the idea of a limitation to either space or time. the metaphysical conceptions of hypergeometry, or fourth-dimensional space, do not aid us. having, then, discovered that the universe is a thing contained in something indefinitely greater than itself; having looked out of its windows and found only the gloom of starless night outside -- what conclusions are we to draw concerning the beyond? it seems as empty as a vacuum, but is it really so? if it be, then our universe is a single atom astray in the infinite; it is the only island in an ocean without shores; it is the one oasis in an illimitable desert. then the milky way, with its wide-flung garland of stars, is afloat like a tiny smoke-wreath amid a horror of immeasurable vacancy, or it is an evanescent and solitary ring of sparkling froth cast up for a moment on the viewless billows of immensity. from such conclusions the mind instinctively shrinks. it prefers to think that there is something beyond, though we cannot see it. even the universe could not bear to be alone -- a crusoe lost in the cosmos! as the inhabitants of the most elegant château, with its gardens, parks, and crowds of attendants, would die of loneliness if they did not know that they have neighbors, though not seen, and that a living world of indefinite extent surrounds them, so we, when we perceive that the universe has limits, wish to feel that it is not solitary; that beyond the hedges and the hills there are other centers of life and activity. could anything be more terrible than the thought of an isolated universe? the greater the being, the greater the aversion to seclusion. only the infinite satisfies; in that alone the mind finds rest. we are driven, then, to believe that the universal night which envelopes us is not tenantless; that as we stare out of the star-framed windows of the galaxy and see nothing but uniform blackness, the fault is with our eyes or is due to an obscuring medium. since our universe is limited in extent, there must be other universes beyond it on all sides. perhaps if we could carry our telescopes to the verge of the great ``coal-sack'' near the ``cross,'' being then on the frontier of our starry system, we could discern, sparkling afar off in the vast night, some of the outer galaxies. they may be grander than ours, just as many of the suns surrounding us are immensely greater than ours. if we could take our stand somewhere in the midst of immensity and, with vision of infinite reach, look about us, we should perhaps see a countless number of stellar systems, amid which ours would be unnoticeable, like a single star among the multitude glittering in the terrestial sky on a clear night. some might be in the form of a wreath, like our own; some might be globular, like the great star-clusters in hercules and centaurus; some might be glittering circles, or disks, or rings within rings. if we could enter them we should probably find a vast variety of composition, including elements unknown to terrestrial chemistry; for while the visible universe appears to contain few if any substances not existing on the earth or in the sun, we have no warrant to assume that others may not exist in infinite space. and how as to gravitation? we do not know that gravitation acts beyond the visible universe, but it is reasonable to suppose that it does. at any rate, if we let go its sustaining hand we are lost, and can only wander hopelessly in our speculations, like children astray. if the empire of gravitation is infinite, then the various outer systems must have some, though measuring by our standards an imperceptible, attractive influence upon each other, for gravitation never lets go its hold, however great the space over which it is required to act. just as the stars about us are all in motion, so the starry systems beyond our sight may be in motion, and our system as a whole may be moving in concert with them. if this be so, then after interminable ages the aspect of the entire system of systems must change, its various members assuming new positions with respect to one another. in the course of time we may even suppose that our universe will approach relatively close to one of the others; and then, if men are yet living on the earth, they may glimpse through the openings which reveal nothing to us now, the lights of another nearing star system, like the signals of a strange squadron, bringing them the assurance (which can be but an inference at present) that the ocean of space has other argosies venturing on its limitless expanse. there remains the question of the luminiferous ether by whose agency the waves of light are borne through space. the ether is as mysterious as gravitation. with regard to ether we only infer its existence from the effects which we ascribe to it. evidently the ether must extend as far as the most distant visible stars. but does it continue on indefinitely in outer space? if it does, then the invisibility of the other systems must be due to their distance diminishing the quantity of light that comes from them below the limit of perceptibility, or to the interposition of absorbing media; if it does not, then the reason why we cannot see them is owing to the absence of a means of conveyance for the light waves, as the lack of an interplanetary atmosphere prevents us from hearing the thunder of sun-spots. (it is interesting to recall that mr edison was once credited with the intention to construct a gigantic microphone which should render the roar of sun-spots audible by transforming the electric vibrations into sound-waves). on this supposition each starry system would be enveloped in its own globule of ether, and no light could cross from one to another. but the probability is that both the ether and gravitation are ubiquitous, and that all the stellar systems are immersed in the former like clouds of phosphorescent organisms in the sea. so astronomy carries the mind from height to greater height. men were long in accepting the proofs of the relative insignificance of the earth; they were more quickly convinced of the comparative littleness of the solar system; and now the evidence assails their reason that what they had regarded as the universe is only one mote gleaming in the sunbeams of infinity. star-clouds, star-clusters, and star-streams in the preceding chapter we have seen something of the strangely complicated structure of the galaxy, or milky way. we now proceed to study more comprehensively that garlanded ``pathway of the gods.'' judged by the eye alone, the milky way is one of the most delicately beautiful phenomena in the entire realm of nature -- a shimmer of silvery gauze stretched across the sky; but studied in the light of its revelations, it is the most stupendous object presented to human ken. let us consider, first, its appearance to ordinary vision. its apparent position in the sky shifts according to the season. on a serene, cloudless summer evening, in the absence of the moon, whose light obscures it, one sees the galaxy spanning the heavens from north to southeast of the zenith like a phosphorescent arch. in early spring it forms a similar but, upon the whole, less brilliant arch west of the zenith. between spring and summer it lies like a long, faint, twilight band along the northern horizon. at the beginning of winter it again forms an arch, this time spanning the sky from east to west, a little north of the zenith. these are its positions as viewed from the mean latitude of the united states. even the beginner in star-gazing does not have to watch it throughout the year in order to be convinced that it is, in reality, a great circle, extending entirely around the celestial sphere. we appear to be situated near its center, but its periphery is evidently far away in the depths of space. although to the casual observer it seems but a delicate scarf of light, brighter in some places than in others, but hazy and indefinite at the best, such is not its appearance to those who study it with care. they perceive that it is an organic whole, though marvelously complex in detail. the telescope shows that it consists of stars too faint and small through excess of distance to be separately visible. of the hundred million suns which some estimates have fixed as the probable population of the starry universe, the vast majority (at least thirty to one) are included in this strange belt of misty light. but they are not uniformly distributed in it; on the contrary, they are arrayed in clusters, knots, bunches, clouds, and streams. the appearance is somewhat as if the galaxy consisted of innumerable swarms of silver-winged bees, more or less intermixed, some massed together, some crossing the paths of others, but all governed by a single purpose which leads them to encircle the region of space in which we are situated. from the beginning of the systematic study of the heavens, the fact has been recognized that the form of the milky way denotes the scheme of the sidereal system. at first it was thought that the shape of the system was that of a vast round disk, flat like a cheese, and filled with stars, our sun and his relatively few neighbors being placed near the center. according to this view, the galactic belt was an effect of perspective; for when looking in the direction of the plane of the disk, the eye ranged through an immense extension of stars which blended into a glimmering blur, surrounding us like a ring; while when looking out from the sides of the disk we saw but few stars, and in those directions the heavens appeared relatively blank. finally it was recognized that this theory did not correspond with the observed appearances, and it became evident that the milky way was not a mere effect of perspective, but an actual band of enormously distant stars, forming a circle about the sphere, the central opening of the ring (containing many scattered stars) being many times broader than the width of the ring itself. our sun is one of the scattered stars in the central opening. as already remarked, the ring of the galaxy is very irregular, and in places it is partly broken. with its sinuous outline, its pendant sprays, its graceful and accordant curves, its bunching of masses, its occasional interstices, and the manifest order of a general plan governing the jumble of its details, it bears a remarkable resemblance to a garland -- a fact which appears the more wonderful when we recall its composition. that an elm-tree should trace the lines of beauty with its leafy and pendulous branches does not surprise us; but we can only gaze with growing amazement when we behold a hundred million suns imitating the form of a chaplet! and then we have to remember that this form furnishes the ground-plan of the universe. as an indication of the extraordinary speculations to which the mystery of the milky way has given rise, a theory recently ( ) proposed by prof. george c. comstock may be mentioned. starting with the data (first) that the number of stars increases as the milky way is approached, and reaches a maximum in its plane, while on the other hand the number of nebulæ is greatest outside the milky way and increases with distance from it, and (second) that the milky way, although a complete ring, is broad and diffuse on one side through one-half its course -- that half alone containing nebulæ -- and relatively narrow and well defined on the opposite side, the author of this singular speculation avers that these facts can best be explained by supposing that the invisible universe consists of two interpenetrating parts, one of which is a chaos of indefinite extent, strewn with stars and nebulous dust, and the other a long, broad but comparatively thin cluster of stars, including the sun as one of its central members. this flat star-cluster is conceived to be moving edgewise through the chaos, and, according to professor comstock, it acts after the manner of a snow-plough sweeping away the cosmic dust and piling it on either hand above and below the plane of the moving cluster. it thus forms a transparent rift, through which we see farther and command a view of more stars than through the intensified dust-clouds on either hand. this rift is the milky way. the dust thrown aside toward the poles of the milky way is the substance of the nebulæ which abound there. ahead, where the front of the star-plough is clearing the way, the chaos is nearer at hand, and consequently there the rift subtends a broader angle, and is filled with primordial dust, which, having been annexed by the vanguard of the star-swarm, forms the nebulæ seen only in that part of the milky way. but behind, the rift appears narrow because there we look farther away between dust-clouds produced ages ago by the front of the plough, and no scattered dust remains in that part of the rift. in quoting an outline of this strikingly original theory the present writer should not be understood as assenting to it. that it appears bizarre is not, in itself, a reason for rejecting it, when we are dealing with so problematical and enigmatical a subject as the milky way; but the serious objection is that the theory does not sufficiently accord with the observed phenomena. there is too much evidence that the milky way is an organic system, however fantastic its form, to permit the belief that it can only be a rift in chaotic clouds. as with every organism, we find that its parts are more or less clearly repeated in its ensemble. among all the strange things that the milky way contains there is nothing so extraordinary as itself. every astronomer must many times have found himself marveling at it in those comparatively rare nights when it shows all its beauty and all its strangeness. in its great broken rifts, divisions, and spirals are found the gigantic prototypes of similar forms in its star-clouds and clusters. as we have said, it determines the general shape of the whole sidereal system. some of the brightest stars in the sky appear to hang like jewels suspended at the ends of tassels dropped from the galaxy. among these pendants are the pleiades and the hyades. orion, too, the ``mighty hunter,'' is caught in ``a loop of light'' thrown out from it. the majority of the great first-magnitude stars seem related to it, as if they formed an inner ring inclined at an angle of some twenty degrees to its plane. many of the long curves that set off from it on both sides are accompanied by corresponding curves of lucid stars. in a word, it offers every appearance of structural connection with the entire starry system. that the universe should have assumed the form of a wreath is certainly a matter for astonishment; but it would have been still more astonishing if it had been a cube, a rhomboid, or a dodecahedron, for then we should have had to suppose that something resembling the forces that shape crystals had acted upon the stars, and the difficulty of explaining the universe by the laws of gravitation would have been increased. from the milky way as a whole we pass to the vast clouds, swarms, and clusters of stars of which it is made up. it may be, as some astronomers hold, that most of the galactic stars are much smaller than the sun, so that their faintness is not due entirely to the effect of distance. still, their intrinsic brilliance attests their solar character, and considering their remoteness, which has been estimated at not less than ten thousand to twenty thousand light-years (a light-year is equal to nearly six thousand thousand million miles) their actual masses cannot be extremely small. the minutest of them are entitled to be regarded as real suns, and they vary enormously in magnitude. the effects of their attractions upon one another can only be inferred from their clustering, because their relative movements are not apparent on account of the brevity of the observations that we can make. but imagine a being for whom a million years would be but as a flitting moment; to him the milky way would appear in a state of ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' the cloud-like aspect of large parts of the galaxy must always have attracted attention, even from naked-eye observers, but the true star-clouds were first satisfactorily represented in barnard's photographs. the resemblance to actual clouds is often startling. some are close-packed and dense, like cumuli; some are wispy or mottled, like cirri. the rifts and modulations, as well as the general outlines, are the same as those of clouds of vapor or dust, and one notices also the characteristic thinning out at the edges. but we must beware of supposing that the component suns are thickly crowded as the particles forming an ordinary cloud. they look, indeed, as if they were matted together, because of the irradiation of light, but in reality millions and billions of miles separate each star from its neighbors. nevertheless they form real assemblages, whose members are far more closely related to one another than is our sun to the stars around him, and if we were in the milky way the aspect of the nocturnal sky would be marvelously different from its present appearance. stellar clouds are characteristic of the galaxy and are not found beyond its borders, except in the ``magellanic clouds'' of the southern hemisphere, which resemble detached portions of the milky way. these singular objects form as striking a peculiarity of the austral heavens as does the great ``coal-sack'' described in chapter . but it is their isolation that makes them so remarkable, for their composition is essentially galactic, and if they were included within its boundaries they would not appear more wonderful than many other parts of the milky way. placed where they are, they look like masses fallen from the great stellar arch. they are full of nebulæ and star-clusters, and show striking evidences of spiral movement. star-swarms, which are also characteristic features of the galaxy, differ from star-clouds very much in the way that their name would imply -- i.e., their component stars are so arranged, even when they are countless in number, that the idea of an exceedingly numerous assemblage rather than that of a cloud is impressed on the observer's mind. in a star-swarm the separate members are distinguishable because they are either larger or nearer than the stars composing a ``cloud.'' a splendid example of a true star-swarm is furnished by chi persei, in that part of the milky way which runs between the constellations perseus and cassiopeia. this swarm is much coarser than many others, and can be seen by the naked eye. in a small telescope it appears double, as if the suns composing it had divided into two parties which keep on their way side by side, with some commingling of their members where the skirts of the two companies come in contact. smaller than either star-clouds or star-swarms, and differing from both in their organization, are star-clusters. these, unlike the others, are found outside as well as inside the milky way, although they are more numerous inside its boundaries than elsewhere. the term star-cluster is sometimes applied, though improperly, to assemblages which are rather groups, such, for instance, as the pleiades. in their most characteristic aspect star-clusters are of a globular shape -- globes of suns! a famous example of a globular star-cluster, but one not included in the milky way, is the ``great cluster in hercules.'' this is barely visible to the naked eye, but a small telescope shows its character, and in a large one it presents a marvelous spectacle. photographs of such clusters are, perhaps, less effective than those of star-clouds, because the central condensation of stars in them is so great that their light becomes blended in an indistinguishable blur. the beautiful effect of the incessant play of infinitesimal rays over the apparently compact surface of the cluster, as if it were a globe of the finest frosted silver shining in an electric beam, is also lost in a photograph. still, even to the eye looking directly at the cluster through a powerful telescope, the central part of the wonderful congregation seems almost a solid mass in which the stars are packed like the ice crystals in a snowball. the same question rises to the lips of every observer: how can they possibly have been brought into such a situation? the marvel does not grow less when we know that, instead of being closely compacted, the stars of the cluster are probably separated by millions of miles; for we know that their distances apart are slight as compared with their remoteness from the earth. sir william herschel estimated their number to be about fourteen thousand, but in fact they are uncountable. if we could view them from a point just within the edge of the assemblage, they would offer the appearance of a hollow hemisphere emblazoned with stars of astonishing brilliancy; the near-by ones unparalleled in splendor by any celestial object known to us, while the more distant ones would resemble ordinary stars. an inhabitant of the cluster would not know, except by a process of ratiocination, that he was dwelling in a globular assemblage of suns; only from a point far outside would their spherical arrangement become evident to the eye. imagine fourteen-thousand fire-balloons with an approach to regularity in a spherical space -- say, ten miles in diameter; there would be an average of less than thirty in every cubic mile, and it would be necessary to go to a considerable distance in order to see them as a globular aggregation; yet from a point sufficiently far away they would blend into a glowing ball. photographs show even better than the best telescopic views that the great cluster is surrounded with a multitude of dispersed stars, suggestively arrayed in more or less curving lines, which radiate from the principle mass, with which their connection is manifest. these stars, situated outside the central sphere, look somewhat like vagrant bees buzzing round a dense swarm where the queen bee is sitting. yet while there is so much to suggest the operation of central forces, bringing and keeping the members of the cluster together, the attentive observer is also impressed with the idea that the whole wonderful phenomenon may be the result of explosion. as soon as this thought seizes the mind, confirmation of it seems to be found in the appearance of the outlying stars, which could be as readily explained by the supposition that they have been blown apart as that they have flocked together toward a center. the probable fact that the stars constituting the cluster are very much smaller than our sun might be regarded as favoring the hypothesis of an explosion. of their real size we know nothing, but, on the basis of an uncertain estimate of their parallax, it has been calculated that they may average forty-five thousand miles in diameter -- something more than half the diameter of the planet jupiter. assuming the same mean density, fourteen thousand such stars might have been formed by the explosion of a body about twice the size of the sun. this recalls the theory of olbers, which has never been altogether abandoned or disproved, that the asteroids were formed by the explosion of a planet circulating between the orbits of mars and jupiter. the asteroids, whatever their manner of origin, form a ring around the sun; but, of course, the explosion of a great independent body, not originally revolving about a superior center of gravitational force, would not result in the formation of a ring of small bodies, but rather of a dispersed mass of them. but back of any speculation of this kind lies the problem, at present insoluble: how could the explosion be produced? (see the question of explosions in chapters and ). then, on the other hand, we have the observation of herschel, since abundantly confirmed, that space is unusually vacant in the immediate neighborhood of condensed star-clusters and nebulæ, which, as far as it goes, might be taken as an indication that the assembled stars had been drawn together by their mutual attractions, and that the tendency to aggregation is still bringing new members toward the cluster. but in that case there must have been an original condensation of stars at that point in space. this could probably have been produced by the coagulation of a great nebula into stellar nuclei, a process which seems now to be taking place in the orion nebula. a yet more remarkable globular star-cluster exists in the southern hemisphere, omega centauri. in this case the central condensation of stars presents an almost uniform blaze of light. like the hercules cluster, that in centaurus is surrounded with stars scattered over a broad field and showing an appearance of radial arrangement. in fact, except for its greater richness, omega centauri is an exact duplicate of its northern rival. each appears to an imaginative spectator as a veritable ``city of suns.'' mathematics shrinks from the task of disentangling the maze of motions in such an assemblage. it would seem that the chance of collisions is not to be neglected, and this idea finds a certain degree of confirmation in the appearance of ``temporary stars'' which have more than once blazed out in, or close by, globular star-clusters. this leads up to the notable fact, first established by professor bailey a few years ago, that such clusters are populous with variable stars. omega centauri and the hercules cluster are especially remarkable in this respect. the variables found in them are all of short period and the changes of light show a noteworthy tendency to uniformity. the first thought is that these phenomena must be due to collisions among the crowded stars, but, if so, the encounters cannot be between the stars themselves, but probably between stars and meteor swarms revolving around them. such periodic collisions might go on for ages without the meteors being exhausted by incorporation with the stars. this explanation appears all the more probable because one would naturally expect that flocks of meteors would abound in a close aggregation of stars. it is also consistent with perrine's discovery -- that the globular star clusters are powdered with minute stars strewn thickly among the brighter ones. in speaking of professor comstock's extraordinary theory of the milky way, the fact was mentioned that, broadly speaking, the nebulæ are less numerous in the galactic belt than in the comparatively open spaces on either side of it, but that they are, nevertheless, abundant in the broader half of the milky way which he designates as the front of the gigantic ``plough'' supposed to be forcing its way through the enveloping chaos. in and around the sagittarius region the intermingling of nebulæ and galactic star clouds and clusters is particularly remarkable. that there is a causal connection no thoughtful person can doubt. we are unable to get away from the evidence that a nebula is like a seed-ground from which stars spring forth; or we may say that nebulæ resemble clouds in whose bosom raindrops are forming. the wonderful aspect of the admixtures of nebulæ and star-clusters in sagittarius has been described in chapter . we now come to a still more extraordinary phenomenon of this kind -- the pleiades nebulæ. the group of the pleiades, although lying outside the main course of the galaxy, is connected with it by a faint loop, and is the scene of the most remarkable association of stars and nebulous matter known in the visible universe. the naked eye is unaware of the existence of nebulæ in the pleiades, or, at the best, merely suspects that there is something of the kind there; and even the most powerful telescopes are far from revealing the full wonder of the spectacle; but in photographs which have been exposed for many hours consecutively, in order to accumulate the impression of the actinic rays, the revelation is stunning. the principle stars are seen surrounded by, and, as it were, drowned in, dense nebulous clouds of an unparalleled kind. the forms assumed by these clouds seem at first sight inexplicable. they look like fleeces, or perhaps more like splashes and daubs of luminous paint dashed carelessly from a brush. but closer inspection shows that they are, to a large extent, woven out of innumerable threads of filmy texture, and there are many indications of spiral tendencies. each of the bright stars of the group -- alcyone, merope, maia, electra, taygeta, atlas -- is the focus of a dense fog (totally invisible, remember, alike to the naked eye and to the telescope), and these particular stars are veiled from sight behind the strange mists. running in all directions across the relatively open spaces are nebulous wisps and streaks of the most curious forms. on some of the nebular lines, which are either straight throughout, or if they change direction do so at an angle, little stars are strung like beads. in one case seven or eight stars are thus aligned, and, as if to emphasize their dependence upon the chain which connects them, when it makes a slight bend the file of stars turns the same way. many other star rows in the group suggest by their arrangement that they, too, were once strung upon similar threads which have now disappeared, leaving the stars spaced along their ancient tracks. we seem forced to the conclusion that there was a time when the pleiades were embedded in a vast nebula resembling that of orion, and that the cloud has now become so rare by gradual condensation into stars that the merest trace of it remains, and this would probably have escaped detection but for the remarkable actinic power of the radiant matter of which it consists. the richness of many of these faint nebulous masses in ultra-violet radiations, which are those that specifically affect the photographic plate, is the cause of the marvelous revelatory power of celestial photography. so the veritable unseen universe, as distinguished from the ``unseen universe'' of metaphysical speculation, is shown to us. a different kind of association between stars and nebulæ is shown in some surprising photographic objects in the constellation cygnus, where long, wispy nebulæ, billions of miles in length, some of them looking like tresses streaming in a breeze, lie amid fields of stars which seem related to them. but the relation is of a most singular kind, for notwithstanding the delicate structure of the long nebulæ they appear to act as barriers, causing the stars to heap themselves on one side. the stars are two, three, or four times as numerous on one side of the nebulæ as on the other. these nebulæ, as far as appearance goes, might be likened to rail fences, or thin hedges, against which the wind is driving drifts of powdery snow, which, while scattered plentifully all around, tends to bank itself on the leeward side of the obstruction. the imagination is at a loss to account for these extraordinary phenomena; yet there they are, faithfully giving us their images whenever the photographic plate is exposed to their radiations. thus the more we see of the universe with improved methods of observation, and the more we invent aids to human senses, each enabling us to penetrate a little deeper into the unseen, the greater becomes the mystery. the telescope carried us far, photography is carrying us still farther; but what as yet unimagined instrument will take us to the bottom, the top, and the end? and then, what hitherto untried power of thought will enable us to comprehend the meaning of it all? stellar migrations to the untrained eye the stars and the planets are not distinguishable. it is customary to call them all alike ``stars.'' but since the planets more or less rapidly change their places in the sky, in consequence of their revolution about the sun, while the stars proper seem to remain always in the same relative positions, the latter are spoken of as ``fixed stars.'' in the beginnings of astronomy it was not known that the ``fixed stars'' had any motion independent of their apparent annual revolution with the whole sky about the earth as a seeming center. now, however, we know that the term ``fixed stars'' is paradoxical, for there is not a single really fixed object in the whole celestial sphere. the apparent fixity in the positions of the stars is due to their immense distance, combined with the shortness of the time during which we are able to observe them. it is like viewing the plume of smoke issuing from a steamer, hull down, at sea: if one does not continue to watch it for a long time it appears to be motionless, although in reality it may be traveling at great speed across the line of sight. even the planets seem fixed in position if one watches them for a single night only, and the more distant ones do not sensibly change their places, except after many nights of observation. neptune, for instance, moves but little more than two degrees in the course of an entire year, and in a month its change of place is only about one-third of the diameter of the full moon. yet, fixed as they seem, the stars are actually moving with a speed in comparison with which, in some cases, the planets might almost be said to stand fast in their tracks. jupiter's speed in his orbit is about eight miles per second, neptune's is less than three and one-half miles, and the earth's is about eighteen and one-half miles; while there are ``fixed stars'' which move two hundred or three hundred miles per second. they do not all, however, move with so great a velocity, for some appear to travel no faster than the planets. but in all cases, notwithstanding their real speed, long-continued and exceedingly careful observations are required to demonstrate that they are moving at all. no more overwhelming impression of the frightful depths of space in which the stars are buried can be obtained than by reflecting upon the fact that a star whose actual motion across the line of sight amounts to two hundred miles per second does not change its apparent place in the sky, in the course of a thousand years, sufficiently to be noticed by the casual observer of the heavens! there is one vast difference between the motions of the stars and those of the planets to which attention should be at once called: the planets, being under the control of a central force emanating from their immediate master, the sun, all move in the same direction and in orbits concentric about the sun; the stars, on the other hand, move in every conceivable direction and have no apparent center of motion, for all efforts to discover such a center have failed. at one time, when theology had finally to accept the facts of science, a grandiose conception arose in some pious minds, according to which the throne of god was situated at the exact center of his creation, and, seated there, he watched the magnificent spectacle of the starry systems obediently revolving around him. astronomical discoveries and speculations seemed for a time to afford some warrant for this view, which was, moreover, an acceptable substitute for the abandoned geocentric theory in minds that could only conceive of god as a superhuman artificer, constantly admiring his own work. no longer ago than the middle of the nineteenth century a german astronomer, maedler, believed that he had actually found the location of the center about which the stellar universe revolved. he placed it in the group of the pleiades, and upon his authority an extraordinary imaginative picture was sometimes drawn of the star alcyone, the brightest of the pleiades, as the very seat of the almighty. this idea even seemed to gain a kind of traditional support from the mystic significance, without known historical origin, which has for many ages, and among widely separated peoples, been attached to the remarkable group of which alcyone is the chief. but since maedler's time it has been demonstrated that the pleiades cannot be the center of revolution of the universe, and, as already remarked, all attempts to find or fix such a center have proved abortive. yet so powerful was the hold that the theory took upon the popular imagination, that even today astronomers are often asked if alcyone is not the probable site of ``jerusalem the golden.'' if there were a discoverable center of predominant gravitative power, to which the motions of all the stars could be referred, those motions would appear less mysterious, and we should then be able to conclude that the universe was, as a whole, a prototype of the subsidiary systems of which it is composed. we should look simply to the law of gravitation for an explanation, and, naturally, the center would be placed within the opening enclosed by the milky way. if it were there the milky way itself should exhibit signs of revolution about it, like a wheel turning upon its hub. no theory of the star motions as a whole could stand which failed to take account of the milky way as the basis of all. but the very form of that divided wreath of stars forbids the assumption of its revolution about a center. even if it could be conceived as a wheel having no material center it would not have the form which it actually presents. as was shown in chapter , there is abundant evidence of motion in the milky way; but it is not motion of the system as a whole, but motion affecting its separate parts. instead of all moving one way, the galactic stars, as far as their movements can be inferred, are governed by local influences and conditions. they appear to travel crosswise and in contrary directions, and perhaps they eddy around foci where great numbers have assembled; but of a universal revolution involving the entire mass we have no evidence. most of our knowledge of star motions, called ``proper motions,'' relates to individual stars and to a few groups which happen to be so near that the effects of their movements are measurable. in some cases the motion is so rapid (not in appearance, but in reality) that the chief difficulty is to imagine how it can have been imparted, and what will eventually become of the ``runaways.'' without a collision, or a series of very close approaches to great gravitational centers, a star traveling through space at the rate of two hundred or three hundred miles per second could not be arrested or turned into an orbit which would keep it forever flying within the limits of the visible universe. a famous example of these speeding stars is `` groombridge,'' a star of only the sixth magnitude, and consequently just visible to the naked eye, whose motion across the line of sight is so rapid that it moves upon the face of the sky a distance equal to the apparent diameter of the moon every years. the distance of this star is at least , , , , miles, and may be two or three times greater, so that its actual speed cannot be less than two hundred, and may be as much as four hundred, miles per second. it could be turned into a new course by a close approach to a great sun, but it could only be stopped by collision, head-on, with a body of enormous mass. barring such accidents it must, as far as we can see, keep on until it has traversed our stellar system, whence in may escape and pass out into space beyond, to join, perhaps, one of those other universes of which we have spoken. arcturus, one of the greatest suns in the universe, is also a runaway, whose speed of flight has been estimated all the way from fifty to two hundred miles per second. arcturus, we have every reason to believe, possesses hundreds of times the mass of our sun -- think, then, of the prodigious momentum that its motion implies! sirius moves more moderately, its motion across the line of sight amounting to only ten miles per second, but it is at the same time approaching the sun at about the same speed, its actual velocity in space being the resultant of the two displacements. what has been said about the motion of sirius brings us to another aspect of this subject. the fact is, that in every case of stellar motion the displacement that we observe represents only a part of the actual movement of the star concerned. there are stars whose motion carries them straight toward or straight away from the earth, and such stars, of course, show no cross motion. but the vast majority are traveling in paths inclined from a perpendicular to our line of sight. taken as a whole, the stars may be said to be flying about like the molecules in a mass of gas. the discovery of the radial component in the movements of the stars is due to the spectroscope. if a star is approaching, its spectral lines are shifted toward the violet end of the spectrum by an amount depending upon the velocity of approach; if it is receding, the lines are correspondingly shifted toward the red end. spectroscopic observation, then, combined with micrometric measurements of the cross motion, enables us to detect the real movement of the star in space. sometimes it happens that a star's radial movement is periodically reversed; first it approaches, and then it recedes. this indicates that it is revolving around a near-by companion, which is often invisible, and superposed upon this motion is that of the two stars concerned, which together may be approaching or receding or traveling across the line of sight. thus the complications involved in the stellar motions are often exceedingly great and puzzling. yet another source of complication exists in the movement of our own star, the sun. there is no more difficult problem in astronomy than that of disentangling the effects of the solar motion from those of the motions of the other stars. but the problem, difficult as it is, has been solved, and upon its solution depends our knowledge of the speed and direction of the movement of the solar system through space, for of course the sun carries its planets with it. one element of the solution is found in the fact that, as a result of perspective, the stars toward which we are going appear to move apart toward all points of the compass, while those behind appear to close up together. then the spectroscopic principle already mentioned is invoked for studying the shift of the lines, which is toward the violet in the stars ahead of us and toward the red in those that we are leaving behind. of course the effects of the independent motions of the stars must be carefully excluded. the result of the studies devoted to this subject is to show that we are traveling at a speed of twelve to fifteen miles per second in a northerly direction, toward the border of the constellations hercules and lyra. a curious fact is that the more recent estimates show that the direction is not very much out of a straight line drawn from the sun to the star vega, one of the most magnificent suns in the heavens. but it should not be inferred from this that vega is drawing us on; it is too distant for its gravitation to have such an effect. many unaccustomed thoughts are suggested by this mighty voyage of the solar system. whence have we come, and whither do we go? every year of our lives we advance at least , , miles. since the traditional time of adam the sun has led his planets through the wastes of space no less than , , , miles, or more than times the distance that separates him from the earth. go back in imagination to the geologic ages, and try to comprehend the distance over which the earth has flown. where was our little planet when it emerged out of the clouds of chaos? where was the sun when his ``thunder march'' began? what strange constellations shone down upon our globe when its masters of life were the monstrous beasts of the ``age of reptiles''? a million years is not much of a span of time in geologic reckoning, yet a million years ago the earth was farther from its present place in space than any of the stars with a measurable parallax are now. it was more than seven times as far as sirius, nearly fourteen times as far as alpha centauri, three times as far as vega, and twice as far as arcturus. but some geologists demand two hundred, three hundred, even one thousand million years to enable them to account for the evolutionary development of the earth and its inhabitants. in a thousand million years the earth would have traveled farther than from the remotest conceivable depths of the milky way! other curious reflections arise when we think of the form of the earth's track as it follows the lead of the sun, in a journey which has neither known beginning nor conceivable end. there are probably many minds which have found a kind of consolation in the thought that every year the globe returns to the same place, on the same side of the sun. this idea may have an occult connection with our traditional regard for anniversaries. when that period of the year returns at which any great event in our lives has occurred we have the feeling that the earth, in its annual round, has, in a manner, brought us back to the scene of that event. we think of the earth's orbit as a well-worn path which we traverse many times in the course of a lifetime. it seems familiar to us, and we grow to have a sort of attachment to it. the sun we are accustomed to regard as a fixed center in space, like the mill or pump around which the harnessed patient mule makes his endless circuits. but the real fact is that the earth never returns to the place in space where it has once quitted. in consequence of the motion of the sun carrying the earth and the other planets along, the track pursued by our globe is a vast spiral in space continually developing and never returning upon its course. it is probable that the tracks of the sun and the others stars are also irregular, and possibly spiral, although, as far as can be at present determined, they appear to be practically straight. every star, wherever it may be situated, is attracted by its fellow-stars from many sides at once, and although the force is minimized by distance, yet in the course of many ages its effects must become manifest. looked at from another side, is there not something immensely stimulating and pleasing to the imagination in the idea of so stupendous a journey, which makes all of us the greatest of travelers? in the course of a long life a man is transported through space thirty thousand million miles; halley's comet does not travel one-quarter as far in making one of its immense circuits. and there are adventures on this voyage of which we are just beginning to learn to take account. space is full of strange things, and the earth must encounter some of them as it advances through the unknown. many singular speculations have been indulged in by astronomers concerning the possible effects upon the earth of the varying state of the space that it traverses. even the alternation of hot and glacial periods has sometimes been ascribed to this source. when tropical life flourished around the poles, as the remains in the rocks assure us, the needed high temperature may, it has been thought, have been derived from the presence of the earth in a warm region of space. then, too, there is a certain interest for us in the thought of what our familiar planet has passed through. we cannot but admire it for its long journeying as we admire the traveler who comes to us from remote and unexplored lands, or as we gaze with a glow of interest upon the first locomotive that has crossed a continent, or a ship that has visited the arctic or antarctic regions. if we may trust the indications of the present course, the earth, piloted by the sun, has come from the milky way in the far south and may eventually rejoin that mighty band of stars in the far north. while the stars in general appear to travel independently of one another, except when they are combined in binary or trinary systems, there are notable exceptions to this rule. in some quarters of the sky we behold veritable migrations of entire groups of stars whose members are too widely separated to show any indications of revolution about a common center of gravity. this leads us back again to the wonderful group of the pleiades. all of the principle stars composing that group are traveling in virtually parallel lines. whatever force set them going evidently acted upon all alike. this might be explained by the assumption that when the original projective force acted upon them they were more closely united than they are at present, and that in drifting apart they have not lost the impulse of the primal motion. or it may be supposed that they are carried along by some current in space, although it would be exceedingly difficult, in the present state of our knowledge, to explain the nature of such a current. yet the theory of a current has been proposed. as to an attractive center around which they might revolve, none has been found. another instance of similar ``star-drift'' is furnished by five of the seven stars constituting the figure of the ``great dipper.'' in this case the stars concerned are separated very widely, the two extreme ones by not less than fifteen degrees, so that the idea of a common motion would never have been suggested by their aspect in the sky; and the case becomes the more remarkable from the fact that among and between them there are other stars, some of the same magnitude, which do not share their motion, but are traveling in other directions. still other examples of the same phenomenon are found in other parts of the sky. of course, in the case of compact star-clusters, it is assumed that all the members share a like motion of translation through space, and the same is probably true of dense star-swarms and star-clouds. the whole question of star-drift has lately assumed a new phase, in consequence of the investigations of kapteyn, dyson, and eddington on the ``systematic motions of the stars.'' this research will, it is hoped, lead to an understanding of the general law governing the movements of the whole body of stars constituting the visible universe. taking about eleven hundred stars whose proper motions have been ascertained with an approach to certainty, and which are distributed in all parts of the sky, it has been shown that there exists an apparent double drift, in two independent streams, moving in different and nearly opposed directions. the apex of the motion of what is called ``stream i'' is situated, according to professor kapteyn, in right ascension °, declination south °, which places it just south of the constellation orion; while the apex of ``stream ii'' is in right ascension °, declination south °, placing it in the constellation ara, south of scorpio. the two apices differ very nearly ° in right ascension and about ° in declination. the discovery of these vast star-streams, if they really exist, is one of the most extraordinary in modern astronomy. it offers the correlation of stellar movements needed as the basis of a theory of those movements, but it seems far from revealing a physical cause for them. as projected against the celestial sphere the stars forming the two opposite streams appear intermingled, some obeying one tendency and some the other. as professor dyson has said, the hypothesis of this double movement is of a revolutionary character, and calls for further investigation. indeed, it seems at first glance not less surprising than would be the observation that in a snow-storm the flakes over our heads were divided into two parties and driving across each other's course in nearly opposite directions, as if urged by interpenetrating winds. but whatever explanation may eventually be found for the motions of the stars, the knowledge of the existence of those motions must always afford a new charm to the contemplative observer of the heavens, for they impart a sense of life to the starry system that would otherwise be lacking. a stagnant universe, with every star fixed immovably in its place, would not content the imagination or satisfy our longing for ceaseless activity. the majestic grandeur of the evolutions of the celestial hosts, the inconceivable vastness of the fields of space in which they are executed, the countless numbers, the immeasurable distances, the involved convolutions, the flocking and the scattering, the interpenetrating marches and countermarches, the strange community of impulsion affecting stars that are wide apart in space and causing them to traverse the general movement about them like aides and despatch-bearers on a battle-field -- all these arouse an intensity of interest which is heightened by the mystery behind them. the passing of the constellations from a historical and picturesque point of view, one of the most striking results of the motions of the stars described in the last chapter is their effect upon the forms of the constellations, which have been watched and admired by mankind from a period so early that the date of their invention is now unknown. the constellations are formed by chance combinations of conspicuous stars, like figures in a kaleidoscope, and if our lives were commensurate with the æons of cosmic existence we should perceive that the kaleidoscope of the heavens was ceaselessly turning and throwing the stars into new symmetries. even if the stars stood fast, the motion of the solar system would gradually alter the configurations, as the elements of a landscape dissolve and recombine in fresh groupings with the traveler's progress amid them. but with the stars themselves all in motion at various speeds and in many directions, the changes occur more rapidly. of course, ``rapid'' is here understood in a relative sense; the wheel of human history to an eye accustomed to the majestic progression of the universe would appear to revolve with the velocity of a whirling dynamo. only the deliberation of geological movements can be contrasted with the evolution and devolution of the constellations. and yet this secular fluctuation of the constellation figures is not without keen interest for the meditative observer. it is another reminder of the swift mutability of terrestial affairs. to the passing glance, which is all that we can bestow upon these figures, they appear so immutable that they have been called into service to form the most lasting records of ancient thought and imagination that we possess. in the forms of the constellations, the most beautiful, and, in imaginative quality, the finest, mythology that the world has ever known has been perpetuated. yet, in a broad sense, this scroll of human thought imprinted on the heavens is as evanescent as the summer clouds. although more enduring than parchment, tombs, pyramids, and temples, it is as far as they from truly eternizing the memory of what man has fancied and done. before studying the effects that the motions of the stars have had and will have upon the constellations, it is worth while to consider a little further the importance of the stellar pictures as archives of history. to emphasize the importance of these effects it is only necessary to recall that the constellations register the oldest traditions of our race. in the history of primeval religions they are the most valuable of documents. leaving out of account for the moment the more familiar mythology of the greeks, based on something older yet, we may refer for illustration to that of the mysterious maya race of america. at izamal, in yucatan, says mr stansbury hagar, is a group of ruins perched, after the mexican and central-american plan, on the summits of pyramidal mounds which mark the site of an ancient theogonic center of the mayas. here the temples all evidently refer to a cult based upon the constellations as symbols. the figures and the names, of course, were not the same as those that we have derived from our aryan ancestors, but the star groups were the same or nearly so. for instance, the loftiest of the temples at izamal was connected with the sign of the constellation known to us as cancer, marking the place of the sun at the summer solstice, at which period the sun was supposed to descend at noon like a great bird of fire and consume the offerings left upon the altar. our scorpio was known to the mayas as a sign of the ``death god.'' our libra, the ``balance,'' with which the idea of a divine weighing out of justice has always been connected, seems to be identical with the mayan constellation teoyaotlatohua, with which was associated a temple where dwelt the priests whose special business it was to administer justice and to foretell the future by means of information obtained from the spirits of the dead. orion, the ``hunter'' of our celestial mythology, was among the mayas a ``warrior,'' while sagittarius and others of our constellations were known to them (under different names, of course), and all were endowed with a religious symbolism. and the same star figures, having the same significance, were familiar to the peruvians, as shown by the temples at cuzco. thus the imagination of ancient america sought in the constellations symbols of the unchanging gods. but, in fact, there is no nation and no people that has not recognized the constellations, and at one period or another in its history employed them in some symbolic or representative capacity. as handled by the greeks from prehistoric times, the constellation myths became the very soul of poetry. the imagination of that wonderful race idealized the principal star groups so effectively that the figures and traditions thus attached to them have, for civilized mankind, displaced all others, just as greek art in its highest forms stands without parallel and eclipses every rival. the romans translated no heroes and heroines of the mythical period of their history to the sky, and the deified cæsars never entered that lofty company, but the heavens are filled with the early myths of the greeks. herakles nightly resumes his mighty labors in the stars; zeus, in the form of the white ``bull,'' taurus, bears the fair europa on his back through the celestial waves; andromeda stretches forth her shackled arms in the star-gemmed ether, beseeching aid; and perseus, in a blaze of diamond armor, revives his heroic deeds amid sparkling clouds of stellar dust. there, too, sits queen cassiopeia in her dazzling chair, while the great king, cepheus, towers gigantic over the pole. professor young has significantly remarked that a great number of the constellations are connected in some way or other with the argonautic expedition -- that strangely fascinating legend of earliest greek story which has never lost its charm for mankind. in view of all this, we may well congratulate ourselves that the constellations will outlast our time and the time of countless generations to follow us; and yet they are very far from being eternal. let us now study some of the effects of the stellar motions upon them. we begin with the familiar figure of the ``great dipper.'' he who has not drunk inspiration from its celestial bowl is not yet admitted to the circle of olympus. this figure is made up of seven conspicuous stars in the constellation ursa major, the ``greater bear.'' the handle of the ``dipper'' corresponds to the tail of the imaginary ``bear,'' and the bowl lies upon his flank. in fact, the figure of a dipper is so evident and that of a bear so unevident, that to most persons the ``great dipper'' is the only part of the constellation that is recognizable. of the seven stars mentioned, six are of nearly equal brightness, ranking as of the second magnitude, while the seventh is of only the third magnitude. the difference is very striking, since every increase of one magnitude involves an increase of two-and-a-half times in brightness. there appears to be little doubt that the faint star, which is situated at the junction of the bowl and the handle, is a variable of long period, since three hundred years ago it was as bright as its companions. but however that may be, its relative faintness at the present time interferes but little with the perfection of the ``dipper's'' figure. in order the more readily to understand the changes which are taking place, it will be well to mention both the names and the greek letters which are attached to the seven stars. beginning at the star in the upper outer edge of the rim of the bowl and running in regular order round the bottom and then out to the end of the handle, the names and letters are as follows: dubhe ({\alpha}), merak ({\beta}), phaed ({\gamma}), megrez ({\delta}), alioth ({\epsilon}), mizar ({\zeta}), and benetnasch ({\eta}). megrez is the faint star already mentioned at the junction of the bowl and handle, and mizar, in the middle of the handle, has a close, naked-eye companion which is named alcor. the arabs called this singular pair of stars ``the horse and rider.'' merak and duhbe are called ``the pointers,'' because an imaginary line drawn northward through them indicates the pole star. now it has been found that five of these stars -- viz., merak, phaed, megrez, alioth, and mizar (with its comrade) -- are moving with practically the same speed in an easterly direction, while the other two, dubhe and benetnasch, are simultaneously moving westward, the motions of benetnasch being apparently more rapid. the consequence of these opposed motions is, of course, that the figure of the ``dipper'' cannot always have existed and will not continue to exist. in the accompanying diagrams it has been thought interesting to show the relative positions of these seven stars, as seen from the point which the earth now occupies, both in the past and in the future. arrows attached to the stars in the figure representing the present appearance of the ``dipper'' indicate the directions of the motions and the distances over which they will carry the stars in a period of about five hundred centuries. the time, no doubt, seems long, but remember the vast stretch of ages through which the earth has passed, and then reflect that no reason is apparent why our globe should not continue to be a scene of animation for ten thousand centuries yet to come. the fact that the little star alcor placed so close to mizar should accompany the latter in its flight is not surprising, but that two of the principal stars of the group should be found moving in a direction directly opposed to that pursued by the other five is surprising in the highest degree; and it recalls the strange theory of a double drift affecting all the stars, to which attention was called in the preceding chapter. it would appear that benetnasch and dubhe belong to one ``current,'' and merak, phaed, megrez, alioth, and mizar to the other. as far as is known, the motion of the seven stars are not shared by the smaller stars scattered about them, but on the theory of currents there should be such a community of motion, and further investigation may reveal it. from the ``great dipper'' we turn to a constellation hardly less conspicuous and situated at an equal distance from the pole on the other side -- cassiopeia. this famous star-group commemorating the romantic queen of ethiopia whose vain boasting of her beauty was punished by the exposure of her daughter andromeda to the ``sea monster,'' is well-marked by five stars which form an irregular letter ``w'' with its open side toward the pole. three of these stars are usually ranked as of the second magnitude, and two of the third; but to ordinary observation they appear of nearly equal brightness, and present a very striking picture. they mark out the chair and a part of the figure of the beautiful queen. beginning at the right-hand, or western, end of the ``w,'' their greek letter designations are: beta ({\beta}), alpha ({\alpha}), gamma ({\gamma}), delta ({\delta}), and epsilon ({\epsilon}). four of them, beta, alpha, delta, and epsilon are traveling eastwardly at various speeds, while the fifth, gamma, moves in a westerly direction. the motion of beta is more rapid than that of any of the others. it should be said, however, that no little uncertainty attaches to the estimates of the rate of motion of stars which are not going very rapidly, and different observers often vary considerably in their results. in the beautiful ``northern crown,'' one of the most perfect and charming of all the figures to be found in the stars, the alternate combining and scattering effects of the stellar motions are shown by comparing the appearance which the constellation must have had five hundred centuries ago with that which it has at present and that which it will have in the future. the seven principle stars of the asterism, forming a surprisingly perfect coronet, have movements in three directions at right angles to one another. that in these circumstances they should ever have arrived at positions giving them so striking an appearance of definite association is certainly surprising; from its aspect one would have expected to find a community of movement governing the brilliants of the ``crown,'' but instead of that we find evidence that they will inevitably drift apart and the beautiful figure will dissolve. a similar fate awaits such asterisms as the ``northern cross'' in cygnus; the ``crow'' (corvus), which stands on the back of the great ``sea serpent,'' hydra, and pecks at his scales; ``job's coffin'' (delphinus); the ``great square of pegasus''; the ``twins'' (gemini); the beautiful ``sickle'' in leo; and the exquisite group of the hyades in taurus. in the case of the hyades, two controlling movements are manifest: one, affecting five of the stars which form the well-known figure of a letter ``v,'' is directed northerly; the other, which controls the direction of two stars, has an easterly trend. the chief star of the group, aldebaran, one of the finest of all stars both for its brilliance and its color, is the most affected by the easterly motion. in time it will drift entirely out of connection with its present neighbors. although the hyades do not form so compact a group as the pleiades in the same constellation, yet their appearance of relationship is sufficient to awaken a feeling of surprise over the fact that, as with the stars of the ``dipper,'' their association is only temporary or apparent. the great figure of orion appears to be more lasting, not because its stars are physically connected, but because of their great distance, which renders their movements too deliberate to be exactly ascertained. two of the greatest of its stars, betelgeuse and rigel, possess, as far as has been ascertained, no perceptible motion across the line of sight, but there is a little movement perceptible in the ``belt.'' at the present time this consists of an almost perfect straight line, a row of second-magnitude stars about equally spaced and of the most striking beauty. in the course of time, however, the two right-hand stars, mintaka and alnilam (how fine are these arabic star names!) will approach each other and form a naked-eye double, but the third, alnita, will drift away eastward, so that the ``belt'' will no longer exist. for one more example, let us go to the southern hemisphere, whose most celebrated constellation, the ``southern cross,'' has found a place in all modern literatures, although it has no claim to consideration on account of association with ancient legends. this most attractive asterism, which has never ceased to fascinate the imagination of christendom since it was first devoutly described by the early explorers of the south, is but a passing collocation of brilliant stars. yet even in its transfigurations it has been for hundreds of centuries, and will continue to be for hundreds of centuries to come, a most striking object in the sky. our figures show its appearance in three successive phases: first, as it was fifty thousand years ago (viewed from the earth's present location); second, as it is in our day; and, third, as it will be an equal time in the future. the nearness of these bright stars to one another -- the length of the longer beam of the ``cross'' is only six degrees -- makes this group very noticeable, whatever the arrangement of its components may be. the largest star, at the base of the ``cross,'' is of the first magnitude, two of the others are of the second magnitude, and the fourth is of the third. other stars, not represented in the figures, increase the effect of a celestial blazonry, although they do not help the resemblance to a cross. but since the motion of the solar system itself will, in the course of so long a period as fifty thousand years, produce a great change in the perspective of the heavens as seen from the earth, by carrying us nearly nineteen trillion miles from our present place, why, it may be asked, seek to represent future appearances of the constellations which we could not hope to see, even if we could survive so long? the answer is: because these things aid the mind to form a picture of the effects of the mobility of the starry universe. only by showing the changes from some definite point of view can we arrive at a due comprehension of them. the constellations are more or less familiar to everybody, so that impending changes of their forms must at once strike the eye and the imagination, and make clearer the significance of the movements of the stars. if the future history of mankind is to resemble its past and if our race is destined to survive yet a million years, then our remote descendents will see a ``new heavens'' if not a ``new earth,'' and will have to invent novel constellations to perpetuate their legends and mythologies. if our knowledge of the relative distances of the stars were more complete, it would be an interesting exercise in celestial geometry to project the constellations probably visible to the inhabitants of worlds revolving around some of the other suns of space. our sun is too insignificant for us to think that he can make a conspicuous appearance among them, except, perhaps, in a few cases. as seen, for instance, from the nearest known star, alpha centauri, the sun would appear of the average first magnitude, and consequently from that standpoint he might be the gem of some little constellation which had no sirius, or arcturus, or vega to eclipse him with its superior splendor. but from the distance of the vast majority of the stars the sun would probably be invisible to the naked eye, and as seen from nearer systems could only rank as a fifth or sixth magnitude star, unnoticed and unknown except by the star-charting astronomer. conflagrations in the heavens suppose it were possible for the world to take fire and burn up -- as some pessimists think that it will do when the divine wrath shall have sufficiently accumulated against it -- nobody out of our own little corner of space would ever be aware of the catastrophe! with all their telescopes, the astronomers living in the golden light of arcturus or the diamond blaze of canopus would be unable to detect the least glimmer of the conflagration that had destroyed the seat of adam and his descendents, just as now they are totally ignorant of its existence. but at least fifteen times in the course of recorded history men looking out from the earth have beheld in the remote depths of space great outbursts of fiery light, some of them more splendidly luminous than anything else in the firmament except the sun! if they were conflagrations, how many million worlds like ours were required to feed their blaze? it is probable that ``temporary'' or ``new'' stars, as these wonderful apparitions are called, really are conflagrations; not in the sense of a bonfire or a burning house or city, but in that of a sudden eruption of inconceivable heat and light, such as would result from the stripping off the shell of an encrusted sun or the crashing together of two mighty orbs flying through space with a hundred times the velocity of the swiftest cannon-shot. temporary stars are the rarest and most erratic of astronomical phenomena. the earliest records relating to them are not very clear, and we cannot in every instance be certain that it was one of these appearances that the ignorant and superstitious old chroniclers are trying to describe. the first temporary star that we are absolutely sure of appeared in , and is known as ``tycho's star,'' because the celebrated danish astronomer (whose remains, with his gold-and-silver artificial nose -- made necessary by a duel -- still intact, were disinterred and reburied in ) was the first to perceive it in the sky, and the most assiduous and successful in his studies of it. as the first fully accredited representative of its class, this new star made its entry upon the scene with becoming éclat. it is characteristic of these phenomena that they burst into view with amazing suddenness, and, of course, entirely unexpectedly. tycho's star appeared in the constellation cassiopeia, near a now well-known and much-watched little star named kappa, on the evening of november , . the story has often been repeated, but it never loses interest, how tycho, going home that evening, saw people in the street pointing and staring at the sky directly over their heads, and following the direction of their hands and eyes he was astonished to see, near the zenith, an unknown star of surpassing brilliance. it outshone the planet jupiter, and was therefore far brighter than the first magnitude. there was not another star in the heavens that could be compared with it in splendor. tycho was not in all respects free from the superstitions of his time -- and who is? -- but he had the true scientific instinct, and immediately he began to study the stranger, and to record with the greatest care every change in its aspect. first he determined as well as he could with the imperfect instruments of his day, many of which he himself had invented, the precise location of the phenomena in the sky. then he followed the changes that it underwent. at first it brightened until its light equaled or exceeded that of the planet venus at her brightest, a statement which will be appreciated at its full value by anyone who has ever watched venus when she plays her dazzling rôle of ``evening star,'' flaring like an arc light in the sunset sky. it even became so brilliant as to be visible in full daylight, since, its position being circumpolar, it never set in the latitude of northern europe. finally it began to fade, turning red as it did so, and in march, , it disappeared from tycho's searching gaze, and has never been seen again from that day to this. none of the astronomers of the time could make anything of it. they had not yet as many bases of speculation as we possess today. tycho's star has achieved a romantic reputation by being fancifully identified with the ``star of bethlehem,'' said to have led the wondering magi from their eastern deserts to the cradle-manger of the savior in palestine. many attempts have been made to connect this traditional ``star'' with some known phenomenon of the heavens, and none seems more idle than this. yet it persistently survives, and no astronomer is free from eager questions about it addressed by people whose imagination has been excited by the legend. it is only necessary to say that the supposition of a connection between the phenomenon of the magi and tycho's star is without any scientific foundation. it was originally based on an unwarranted assumption that the star of tycho was a variable of long period, appearing once every three hundred and fifteen years, or thereabout. if that were true there would have been an apparition somewhere near the traditional date of the birth of christ, a date which is itself uncertain. but even the data on which the assumption was based are inconsistent with the theory. certain monkish records speak of something wonderful appearing in the sky in the years and , and these were taken to have been outbursts of tycho's star. investigation shows that the records more probably refer to comets, but even if the objects seen were temporary stars, their dates do not suit the hypothesis; from to there is a gap of years, and from to one of only years; moreover years have now ( ) elapsed since tycho saw the last glimmer of his star. upon a variability so irregular and uncertain as that, even if we felt sure that it existed, no conclusion could be found concerning an apparition occurring years ago. in the year (the year in which giordano bruno was burned at the stake for teaching that there is more than one physical world), a temporary star of the third magnitude broke out in the constellation cygnus, and curiously enough, considering the rarity of such phenomena, only four years later another surprisingly brilliant one appeared in the constellation ophiuchus. this is often called ``kepler's star,'' because the great german astronomer devoted to it the same attention that tycho had given to the earlier phenomenon. it, too, like tycho's, was at first the brightest object in the stellar heavens, although it seems never to have quite equaled its famous predecessor in splendor. it disappeared after a year, also turning of a red color as it became more faint. we shall see the significance of this as we go on. some of kepler's contemporaries suggested that the outburst of this star was due to a meeting of atoms in space, and idea bearing a striking resemblance to the modern theory of ``astronomical collisions.'' in , , and temporary stars made their appearance, but none of them was of great brilliance. in one of the second magnitude broke forth in the ``northern crown'' and awoke much interest, because by that time the spectroscope had begun to be employed in studying the composition of the stars, and huggins demonstrated that the new star consisted largely of incandescent hydrogen. but this star, apparently unlike the others mentioned, was not absolutely new. before its outburst it had shown as a star of the ninth magnitude (entirely invisible, of course, to the naked eye), and after about six weeks it faded to its original condition in which it has ever since remained. in a temporary star appeared in the constellation cygnus, and attained at one time the brightness of the second magnitude. its spectrum and its behavior resembled those of its immediate predecessor. in , astronomers were surprised to see a sixth-magnitude star glimmering in the midst of the hazy cloud of the great andromeda nebula. it soon absolutely disappeared. its spectrum was remarkable for being ``continuous,'' like that of the nebula itself. a continuous spectrum is supposed to represent a body, or a mass, which is either solid or liquid, or composed of gas under great pressure. in january, , a new star was suddenly seen in the constellation auriga. it never rose much above the fourth magnitude, but it showed a peculiar spectrum containing both bright and dark lines of hydrogen. but a bewildering surprise was now in store; the world was to behold at the opening of the twentieth century such a celestial spectacle as had not been on view since the times of tycho and kepler. before daylight on the morning of february , , the rev. doctor anderson, of edinburgh, an amateur astronomer, who had also been the first to see the new star in auriga, beheld a strange object in the constellation perseus not far from the celebrated variable star algol. he recognized its character at once, and immediately telegraphed the news, which awoke the startled attention of astronomers all over the world. when first seen the new star was no brighter than algol (less than the second magnitude), but within twenty-four hours it was ablaze, outshining even the brilliant capella, and far surpassing the first magnitude. at the spot in the sky where it appeared nothing whatever was visible on the night before its coming. this is known with certainty because a photograph had been made of that very region on february , and this photograph showed everything down to the twelfth magnitude, but not a trace of the stranger which burst into view between the st and the nd like the explosion of a rocket. upon one who knew the stars the apparition of this intruder in a well-known constellation had the effect of a sudden invasion. the new star was not far west of the zenith in the early evening, and in that position showed to the best advantage. to see capella, the hitherto unchallenged ruler of that quarter of the sky, abased by comparison with this stranger of alien aspect, for there was always an unfamiliar look about the ``nova,'' was decidedly disconcerting. it seemed to portend the beginning of a revolution in the heavens. one could understand what the effect of such an apparition must have been in the superstitious times of tycho. the star of tycho had burst forth on the northern border of the milky way; this one was on its southern border, some forty-five degrees farther east. astronomers were well-prepared this time for the scientific study of the new star, both astronomical photography and spectroscopy having been perfected, and the results of their investigations were calculated to increase the wonder with which the phenomenon was regarded. the star remained at its brightest only a few days; then, like a veritable conflagration, it began to languish; and, like the reflection of a dying fire, as it sank it began to glow with the red color of embers. but its changes were spasmodic; once about every three days it flared up only to die away again. during these fluctuations its light varied alternately in the ratio of one to six. finally it took a permanent downward course, and after a few months the naked eye could no longer perceive it; but it remained visible with telescopes, gradually fading until it had sunk to the ninth magnitude. then another astonishing change happened: in august photographs taken at the yerkes observatory and at heidelberg showed that the ``nova'' was surrounded by a spiral nebula! the nebula had not been there before, and no one could doubt that it represented a phase of the same catastrophe that had produced the outburst of the new star. at one time the star seemed virtually to have disappeared, as if all its substance had been expanded into the nebulous cloud, but always there remained a stellar nucleus about which the misty spiral spread wider and ever wider, like a wave expanding around a center of disturbance. the nebula too showed a variability of brightness, and four condensations which formed in it seemed to have a motion of revolution about the star. as time went on the nebula continued to expand at a rate which was computed to be not less than twenty thousand miles per second! and now the star itself, showing indications of having turned into a nebula, behaved in a most erratic manner, giving rise to the suspicion that it was about to burst out again. but this did not occur, and at length it sunk into a state of lethargy from which it has to the present time not recovered. but the nebulous spiral has disappeared, and the entire phenomena as it now ( ) exists consists of a faint nebulous star of less than the ninth magnitude. the wonderful transformations just described had been forecast in advance of the discovery of the nebulous spiral encircling the star by the spectroscopic study of the latter. at first there was no suggestion of a nebular constitution, but within a month or two characteristic nebular lines began to appear, and in less than six months the whole spectrum had been transformed to the nebular type. in the mean time the shifting of the spectral lines indicated a complication of rapid motions in several directions simultaneously. these motions were estimated to amount to from one hundred to five hundred miles per second. the human mind is so constituted that it feels forced to seek an explanation of so marvelous a phenomenon as this, even in the absence of the data needed for a sound conclusion. the most natural hypothesis, perhaps, is that of a collision. such a catastrophe could certainly happen. it has been shown, for instance, that in infinity of time the earth is sure to be hit by a comet; in the same way it may be asserted that, if no time limit is fixed, the sun is certain to run against some obstacle in space, either another star, or a dense meteor swarm, or one of the dark bodies which there is every reason to believe abound around us. the consequences of such a collision are easy to foretell, provided that we know the masses and the velocities of the colliding bodies. in a preceding chapter we have discussed the motions of the sun and stars, and have seen that they are so swift that an encounter between any two of them could not but be disastrous. but this is not all; for as soon as two stars approached within a few million miles their speed would be enormously increased by their reciprocal attractions and, if their motion was directed radially with respect to their centers, they would come together with a crash that would reduce them both to nebulous clouds. it is true that the chances of such a ``head-on'' collision are relatively very small; two stars approaching each other would most probably fall into closed orbits around their common center of gravity. if there were a collision it would most likely be a grazing one instead of a direct front-to-front encounter. but even a close approach, without any actual collision, would probably prove disastrous, owing to the tidal influence of each of the bodies on the other. suns, in consequence of their enormous masses and dimensions and the peculiarities of their constitution, are exceedingly dangerous to one another at close quarters. propinquity awakes in them a mutually destructive tendency. consisting of matter in the gaseous, or perhaps, in some cases, liquid, state, their tidal pull upon each other if brought close together might burst them asunder, and the photospheric envelope being destroyed the internal incandescent mass would gush out, bringing fiery death to any planets that were revolving near. without regard to the resulting disturbance of the earth's orbit, the close approach of a great star to the sun would be in the highest degree perilous to us. but this is a danger which may properly be regarded as indefinitely remote, since, at our present location in space, we are certainly far from every star except the sun, and we may feel confident that no great invisible body is near, for if there were one we should be aware of its presence from the effects of its attraction. as to dark nebulæ which may possibly lie in the track that the solar system is pursuing at the rate of , , miles per year, that is another question -- and they, too, could be dangerous! this brings us directly back to ``nova persei,'' for among the many suggestions offered to explain its outburst, as well as those of other temporary stars, one of the most fruitful is that of a collision between a star and a vast invisible nebula. professor seeliger, of munich, first proposed this theory, but it afterward underwent some modifications from others. stated in a general form, the idea is that a huge dark body, perhaps an extinguished sun, encountered in its progress through space a widespread flock of small meteors forming a dark nebula. as it plunged into the swarm the friction of the innumerable collisions with the meteors heated its surface to incandescence, and being of vast size it then became visible to us as a new star. meanwhile the motion of the body through the nebula, and its rotation upon itself, set up a gyration in the blazing atmosphere formed around it by the vaporized meteors; and as this atmosphere spread wider, under the laws of gyratory motion a rotation in the opposite direction began in the inflamed meteoric cloud outside the central part of the vortex. thus the spectral lines were caused to show motion in opposite directions, a part of the incandescent mass approaching the earth simultaneously with the retreat of another part. so the curious spectroscopic observations before mentioned were explained. this theory might also account for the appearance of the nebulous spiral first seen some six months after the original outburst. the sequent changes in the spectrum of the ``nova'' are accounted for by this theory on the assumption, reasonable enough in itself, that at first the invading body would be enveloped in a vaporized atmosphere of relatively slight depth, producing by its absorption the fine dark lines first observed; but that as time went on and the incessant collisions continued, the blazing atmosphere would become very deep and extensive, whereupon the appearance of the spectral lines would change, and bright lines due to the light of the incandescent meteors surrounding the nucleus at a great distance would take the place of the original dark ones. the vortex of meteors once formed would protect the flying body within from further immediate collisions, the latter now occurring mainly among the meteors themselves, and then the central blaze would die down, and the original splendor of the phenomenon would fade. but the theories about nova persei have been almost as numerous as the astronomers who have speculated about it. one of the most startling of them assumed that the outburst was caused by the running amuck of a dark star which had encountered another star surrounded with planets, the renewed outbreaks of light after the principal one had faded being due to the successive running down of the unfortunate planets! yet another hypothesis is based on what we have already said of the tidal influence that two close approaching suns would have upon each other. supposing two such bodies which had become encrusted, but remained incandescent and fluid within, to approach within almost striking distance; they would whirl each other about their common center of gravity, and at the same time their shells would burst under the tidal strain, and their glowing nuclei being disclosed would produce a great outburst of light. applying this theory to a ``nova,'' like that of in the ``northern crown,'' which had been visible as a small star before the outbreak, and which afterward resumed its former aspect, we should have to assume that a yet shining sun had been approached by a dark body whose attraction temporarily burst open its photosphere. it might be supposed that in this case the dark body was too far advanced in cooling to suffer the same fate from the tidal pull of its victim. but a close approach of that kind would be expected to result in the formation of a binary system, with orbits of great eccentricity, perhaps, and after the lapse of a certain time the outburst should be renewed by another approximation of the two bodies. a temporary star of that kind would rather be ranked as a variable. the celebrated french astronomer, janssen, had a different theory of nova persei, and of temporary stars in general. according to his idea, such phenomena might be the result of chemical changes taking place in a sun without interference by, or collision with, another body. janssen was engaged for many years in trying to discover evidence of the existence of oxygen in the sun, and he constructed his observatory on the summit of mount blanc specially to pursue that research. he believed that oxygen must surely exist in the sun since we find so many other familiar elements included in the constitution of the solar globe, and as he was unable to discover satisfactory evidence of its presence he assumed that it existed in a form unknown on the earth. if it were normally in the sun's chromosphere, or coronal atmosphere, he said, it would combine with the hydrogen which we know is there and form an obscuring envelope of water vapor. it exists, then, in a special state, uncombined with hydrogen; but let the temperature of the sun sink to a critical point and the oxygen will assume its normal properties and combine with the hydrogen, producing a mighty outburst of light and heat. this, janssen thought, might explain the phenomena of the temporary stars. it would also, he suggested, account for their brief career, because the combination of the elements would be quickly accomplished, and then the resulting water vapor would form an atmosphere cutting off the radiation from the star within. this theory may be said to have a livelier human interest than some of the others, since, according to it, the sun may carry in its very constitution a menace to mankind; one does not like to think of it being suddenly transformed into a gigantic laboratory for the explosive combination of oxygen and hydrogen! but while janssen's theory might do for some temporary stars, it is inadequate to explain all the phenomena of nova persei, and particularly the appearance of the great spiral nebula that seemed to exhale from the heart of the star. upon the whole, the theory of an encounter between a star and a dark nebula seems best to fit the observations. by that hypothesis the expanding billow of light surrounding the core of the conflagration is very well accounted for, and the spectroscopic peculiarities are also explained. dr gustov le bon offers a yet more alarming theory, suggesting that temporary stars are the result of atomic explosion; but we shall touch upon this more fully in chapter . twice in the course of this discussion we have called attention to the change of color invariably undergone by temporary stars in the later stages of their career. this was conspicuous with nova persei which glowed more and more redly as it faded, until the nebulous light began to overpower that of the stellar nucleus. nothing could be more suggestive of the dying out of a great fire. moreover, change of color from white to red is characteristic of all variable stars of long period, such as ``mira'' in cetus. it is also characteristic of stars believed to be in the later stages of evolution, and consequently approaching extinction, like antares and betelgeuse, and still more notably certain small stars which ``gleam like rubies in the field of the telescope.'' these last appear to be suns in the closing period of existence as self-luminous bodies. between the white stars, such as sirius and rigel, and the red stars, such as aldebaran and alpha herculis, there is a progressive series of colors from golden yellow through orange to deep red. the change is believed to be due to the increase of absorbing vapors in the stellar atmosphere as the body cools down. in the case of ordinary stars these changes no doubt occupy many millions of years, which represent the average duration of solar life; but the temporary stars run through similar changes in a few months: they resemble ephemeral insects -- born in the morning and doomed to perish with the going down of the sun. explosive and whirling nebulæ one of the most surprising triumphs of celestial photography was professor keeler's discovery, in , that the great majority of the nebulæ have a distinctly spiral form. this form, previously known in lord rosse's great ``whirlpool nebula,'' had been supposed to be exceptional; now the photographs, far excelling telescopic views in the revelation of nebular forms, showed the spiral to be the typical shape. indeed, it is a question whether all nebulæ are not to some extent spiral. the extreme importance of this discovery is shown in the effect that it has had upon hitherto prevailing views of solar and planetary evolution. for more than three-quarters of a century laplace's celebrated hypothesis of the manner of origin of the solar system from a rotating and contracting nebula surrounding the sun had guided speculation on that subject, and had been tentatively extended to cover the evolution of systems in general. the apparent forms of some of the nebulæ which the telescope had revealed were regarded, and by some are still regarded, as giving visual evidence in favor of this theory. there is a ``ring nebula'' in lyra with a central star, and a ``planetary nebula'' in gemini bearing no little resemblance to the planet saturn with its rings, both of which appear to be practical realizations of laplace's idea, and the elliptical rings surrounding the central condensation of the andromeda nebula may be cited for the same kind of proof. but since keeler's discovery there has been a decided turning away of speculation another way. the form of the spiral nebulæ seems to be entirely inconsistent with the theory of an originally globular or disk-shaped nebula condensing around a sun and throwing or leaving off rings, to be subsequently shaped into planets. some astronomers, indeed, now reject laplace's hypothesis in toto, preferring to think that even our solar system originated from a spiral nebula. since the spiral type prevails among the existing nebulæ, we must make any mechanical theory of the development of stars and planetary systems from them accord with the requirements which that form imposes. a glance at the extraordinary variations upon the spiral which professor keeler's photographs reveal is sufficient to convince one of the difficulty of the task of basing a general theory upon them. in truth, it is much easier to criticize laplace's hypothesis than to invent a satisfactory substitute for it. if the spiral nebulæ seem to oppose it there are other nebulæ which appear to support it, and it may be that no one fixed theory can account for all the forms of stellar evolution in the universe. our particular planetary system may have originated very much as the great french mathematician supposed, while others have undergone, or are now undergoing, a different process of development. there is always a too strong tendency to regard an important new discovery and the theories and speculations based upon it as revolutionizing knowledge, and displacing or overthrowing everything that went before. upon the plea that ``laplace only made a guess'' more recent guesses have been driven to extremes and treated by injudicious exponents as ``the solid facts at last.'' before considering more recent theories than laplace's, let us see what the nature of the photographic revelations is. the vast celestial maelstrom discovered by lord rosse in the ``hunting dogs'' may be taken as the leading type of the spiral nebulæ, although there are less conspicuous objects of the kind which, perhaps, better illustrate some of their peculiarities. lord rosse's nebula appears far more wonderful in the photographs than in his drawings made with the aid of his giant reflecting telescope at parsonstown, for the photographic plate records details that no telescope is capable of showing. suppose we look at the photograph of this object as any person of common sense would look at any great and strange natural phenomenon. what is the first thing that strikes the mind? it is certainly the appearance of violent whirling motion. one would say that the whole glowing mass had been spun about with tremendous velocity, or that it had been set rotating so rapidly that it had become the victim of ``centrifugal force,'' one huge fragment having broken loose and started to gyrate off into space. closer inspection shows that in addition to the principal focus there are various smaller condensations scattered through the mass. these are conspicuous in the spirals. some of them are stellar points, and but for the significance of their location we might suppose them to be stars which happen to lie in a line between us and the nebula. but when we observe how many of them follow most faithfully the curves of the spirals we cannot but conclude that they form an essential part of the phenomenon; it is not possible to believe that their presence in such situations is merely fortuitous. one of the outer spirals has at least a dozen of these star-like points strung upon it; some of them sharp, small, and distinct, others more blurred and nebulous, suggesting different stages of condensation. even the part which seems to have been flung loose from the main mass has, in addition to its central condensation, at least one stellar point gleaming in the half-vanished spire attached to it. some of the more distant stars scattered around the ``whirlpool'' look as if they too had been shot out of the mighty vortex, afterward condensing into unmistakable solar bodies. there are at least two curved rows of minute stars a little beyond the periphery of the luminous whirl which clearly follow lines concentric with those of the nebulous spirals. such facts are simply dumbfounding for anyone who will bestow sufficient thought upon them, for these are suns, though they may be small ones; and what a birth is that for a sun! look now again at the glowing spirals. we observe that hardly have they left the central mass before they begin to coagulate. in some places they have a ``ropy'' aspect; or they are like peascods filled with growing seeds, which eventually will become stars. the great focus itself shows a similar tendency, especially around its circumference. the sense that it imparts of a tremendous shattering force at work is overwhelming. there is probably more matter in that whirling and bursting nebula than would suffice to make a hundred solar systems! it must be confessed at once that there is no confirmation of the laplacean hypothesis here; but what hypothesis will fit the facts? there is one which it has been claimed does so, but we shall come to that later. in the meanwhile, as a preparation, fix in the memory the appearance of that second spiral mass spinning beside its master which seems to have spurned it away. for a second example of the spiral nebulæ look at the one in the constellation triangulum. god, how hath the imagination of puny man failed to comprehend thee! here is creation through destruction with a vengeance! the spiral form of the nebula is unmistakable, but it is half obliterated amid the turmoil of flying masses hurled away on all sides with tornadic fury. the focus itself is splitting asunder under the intolerable strain, and in a little while, as time is reckoned in the cosmos, it will be gyrating into stars. and then look at the cyclonic rain of already finished stars whirling round the outskirts of the storm. observe how scores of them are yet involved in the fading streams of the nebulous spirals; see how they have been thrown into vast loops and curves, of a beauty that half redeems the terror of the spectacle enclosed within their lines -- like iridescent cirri hovering about the edges of a hurricane. and so again are suns born! let us turn to the exquisite spiral in ursa major; how different its aspect from that of the other! one would say that if the terrific coil in triangulum has all but destroyed itself in its fury, this one on the contrary has just begun its self-demolition. as one gazes one seems to see in it the smooth, swift, accelerating motion that precedes catastrophe. the central part is still intact, dense, and uniform in texture. how graceful are the spirals that smoothly rise from its oval rim and, gemmed with little stars, wind off into the darkness until they have become as delicate as threads of gossamer! but at bottom the story told here is the same -- creation by gyration! compare with the above the curious mass in cetus. here the plane of the whirling nebula nearly coincides with our line of sight and we see the object at a low angle. it is far advanced and torn to shreds, and if we could look at it perpendicularly to its plane it is evident that it would closely resemble the spectacle in triangulum. then take the famous andromeda nebula (see frontispiece), which is so vast that notwithstanding its immense distance even the naked eye perceives it as an enigmatical wisp in the sky. its image on the sensitive plate is the masterpiece of astronomical photography; for wild, incomprehensible beauty there is nothing that can be compared with it. here, if anywhere, we look upon the spectacle of creation in one of its earliest stages. the andromeda nebula is apparently less advanced toward transformation into stellar bodies than is that in triangulum. the immense crowd of stars sprinkled over it and its neighborhood seem in the main to lie this side of the nebula, and consequently to have no connection with it. but incipient stars (in some places clusters of them) are seen in the nebulous rings, while one or two huge masses seem to give promise of transformation into stellar bodies of unusual magnitude. i say ``rings'' because although the loops encompassing the andromeda nebula have been called spirals by those who wish utterly to demolish laplace's hypothesis, yet they are not manifestly such, as can be seen on comparing them with the undoubted spirals of the lord rosse nebula. they look quite as much like circles or ellipses seen at an angle of, say, fifteen or twenty degrees to their plane. if they are truly elliptical they accord fairly well with laplace's idea, except that the scale of magnitude is stupendous, and if the andromeda nebula is to become a solar system it will surpass ours in grandeur beyond all possibility of comparison. there is one circumstance connected with the spiral nebulæ, and conspicuous in the andromeda nebula on account of its brightness, which makes the question of their origin still more puzzling; they all show continuous spectra, which, as we have before remarked, indicate that the mass from which the light comes is either solid or liquid, or a gas under heavy pressure. thus nebulæ fall into two classes: the ``white'' nebulæ, giving a continuous spectrum; and the ``green'' nebulæ whose spectra are distinctly gaseous. the andromeda nebula is the great representative of the former class and the orion nebula of the latter. the spectrum of the andromeda nebula has been interpreted to mean that it consists not of luminous gas, but of a flock of stars so distant that they are separately indistinguishable even with powerful telescopes, just as the component stars of the milky way are indistinguishable with the naked eye; and upon this has been based the suggestion that what we see in andromeda is an outer universe whose stars form a series of elliptical garlands surrounding a central mass of amazing richness. but this idea is unacceptable if for no other reason than that, as just said, all the spiral nebulæ possess the same kind of spectrum, and probably no one would be disposed to regard them all as outer universes. as we shall see later, the peculiarity of the spectra of the spiral nebulæ is appealed to in support of a modern substitute for laplace's hypothesis. finally, without having by any means exhausted the variety exhibited by the spiral nebulæ, let us turn to the great representative of the other species, the orion nebula. in some ways this is even more marvelous than the others. the early drawings with the telescope failed to convey an adequate conception either of its sublimity or of its complication of structure. it exists in a nebulous region of space, since photographs show that nearly the whole constellation is interwoven with faintly luminous coils. to behold the entry of the great nebula into the field even of a small telescope is a startling experience which never loses its novelty. as shown by the photographs, it is an inscrutable chaos of perfectly amazing extent, where spiral bands, radiating streaks, dense masses, and dark yawning gaps are strangely intermingled without apparent order. in one place four conspicuous little stars, better seen in a telescope than in the photograph on account of the blurring produced by over-exposure, are suggestively situated in the midst of a dark opening, and no observer has ever felt any doubt that these stars have been formed from the substance of the surrounding nebula. there are many other stars scattered over its expanse which manifestly owe their origin to the same source. but compare the general appearance of this nebula with the others that we have studied, and remark the difference. if the unmistakably spiral nebulæ resemble bursting fly-wheels or grindstones from whose perimeters torrents of sparks are flying, the orion nebula rather recalls the aspect of a cloud of smoke and fragments produced by the explosion of a shell. this idea is enforced by the look of the outer portion farthest from the bright half of the nebula, where sharply edged clouds with dark spaces behind seem to be billowing away as if driven by a wind blowing from the center. next let us consider what scientific speculation has done in the effort to explain these mysteries. laplace's hypothesis can certainly find no standing ground either in the orion nebula or in those of a spiral configuration, whatever may be its situation with respect to the grand nebula of andromeda, or the ``ring'' and ``planetary'' nebulæ. some other hypothesis more consonant with the appearances must be found. among the many that have been proposed the most elaborate is the ``planetesimal hypothesis'' of professors chamberlin and moulton. it is to be remarked that it applies to the spiral nebulæ distinctively, and not to an apparently chaotic mass of gas like the vast luminous cloud in orion. the gist of the theory is that these curious objects are probably the result of close approaches to each other of two independent suns, reminding us of what was said on this subject when we were dealing with temporary stars. of the previous history of these appulsing suns the theory gives us no account; they are simply supposed to arrive within what may be called an effective tide-producing distance, and then the drama begins. some of the probable consequences of such an approach have been noticed in chapter ; let us now consider them a little more in detail. tides always go in couples; if there is a tide on one side of a globe there will be a corresponding tide on the other side. the cause is to be found in the law that the force of gravitation varies inversely as the square of the distance; the attraction on the nearest surface of the body exercised by another body is greater than on its center, and greater yet than on its opposite surface. if two great globes attract each other, each tends to draw the other out into an ellipsoidal figure; they must be more rigid than steel to resist this -- and even then they cannot altogether resist. if they are liquid or gaseous they will yield readily to the force of distortion, the amount of which will depend upon their distance apart, for the nearer they are the greater becomes the tidal strain. if they are encrusted without and liquid or gaseous in the interior, the internal mass will strive to assume the figure demanded by the tidal force, and will, if it can, burst the restraining envelope. now this is virtually the predicament of the body we call a sun when in the immediate presence of another body of similarly great mass. such a body is presumably gaseous throughout, the component gases being held in a state of rigidity by the compression produced by the tremendous gravitational force of their own aggregate mass. at the surface such a body is enveloped in a shell of relatively cool matter. now suppose a great attracting body, such as another sun, to approach near enough for the difference in its attraction on the two opposite sides of the body and on its center to become very great; the consequence will be a tidal deformation of the whole body, and it will lengthen out along the line of the gravitational pull and draw in at the sides, and if its shell offers considerable resistance, but not enough to exercise a complete restraint, it will be violently burst apart, or blown to atoms, and the internal mass will leap out on the two opposite sides in great fiery spouts. in the case of a sun further advanced in cooling than ours the interior might be composed of molten matter while the exterior crust had become rigid like the shell of an egg; then the force of the ``tidal explosion'' produced by the appulse of another sun would be more violent in consequence of the greater resistance overcome. such, then, is the mechanism of the first phase in the history of a spiral nebula according to the planetesimal hypothesis. two suns, perhaps extinguished ones, have drawn near together, and an explosive outburst has occured in one or both. the second phase calls for a more agile exercise of the imagination. to simplify the case, let us suppose that only one of the tugging suns is seriously affected by the strain. its vast wings produced by the outburst are twisted into spirals by their rotation and the contending attractions exercised upon them, as the two suns, like battleships in desperate conflict, curve round each other, concentrating their destructive energies. then immense quantities of débris are scattered about in which eddies are created, and finally, as the sun that caused the damage goes on its way, leaving its victim to repair its injuries as it may, the dispersed matter cools, condenses, and turns into streams of solid particles circling in elliptical paths about their parent sun. these particles, or fragments, are the ``planetesimals'' of the theory. in consequence of the inevitable intersection of the orbits of the planetesimals, nodes are formed where the flying particles meet, and at these nodes large masses are gradually accumulated. the larger the mass the greater its attraction, and at last the nodal points become the nuclei of great aggregations from which planets are shaped. this, in very brief form, is the planetesimal hypothesis which we are asked to substitute for that based on laplace's suggestion as an explanation of the mode of origin of the solar system; and the phenomena of the spiral nebulæ are appealed to as offering evident support to the new hypothesis. we are reminded that they are elliptical in outline, which accords with the hypothesis; that their spectra are not gaseous, which shows that they may be composed of solid particles like the planetesimals; and that their central masses present an oval form, which is what would result from the tidal effects, as just described. we also remember that some of them, like the lord rosse and the andromeda nebulæ, are visually double, and in these cases we might suppose that the two masses represent the tide-burst suns that ventured into too close proximity. it may be added that the authors of the theory do not insist upon the appulse of two suns as the only way in which the planetesimals may have originated, but it is the only supposition that has been worked out. but serious questions remain. it needs, for instance, but a glance at the triangulum monster to convince the observer that it cannot be a solar system which is being evolved there, but rather a swarm of stars. many of the detached masses are too vast to admit of the supposition that they are to be transformed into planets, in our sense of planets, and the distances of the stars which appear to have been originally ejected from the focal masses are too great to allow us to liken the assemblage that they form to a solar system. then, too, no nodes such as the hypothesis calls for are visible. moreover, in most of the spiral nebulæ the appearances favor the view that the supposititious encountering suns have not separated and gone each rejoicing on its way, after having inflicted the maximum possible damage on its opponent, but that, on the contrary, they remain in close association like two wrestlers who cannot escape from each other's grasp. and this is exactly what the law of gravitation demands; stars cannot approach one another with impunity, with regard either to their physical make-up or their future independence of movement. the theory undertakes to avoid this difficulty by assuming that in the case of our system the approach of the foreign body to the sun was not a close one -- just close enough to produce the tidal extrusion of the relatively insignificant quantity of matter needed to form the planets. but even then the effect of the appulse would be to change the direction of flight, both of the sun and of its visitor, and there is no known star in the sky which can be selected as the sun's probable partner in their ancient pas deux. that there are unconquered difficulties in laplace's hypothesis no one would deny, but in simplicity of conception it is incomparably more satisfactory, and with proper modifications could probably be made more consonant with existing facts in our solar system than that which is offered to replace it. even as an explanation of the spiral nebulæ, not as solar systems in process of formation, but as the birthplaces of stellar clusters, the planetesimal hypothesis would be open to many objections. granting its assumptions, it has undoubtedly a strong mathematical framework, but the trouble is not with the mathematics but with the assumptions. laplace was one of the ablest mathematicians that ever lived, but he had never seen a spiral nebula; if he had, he might have invented a hypothesis to suit its phenomena. his actual hypothesis was intended only for our solar system, and he left it in the form of a ``note'' for the consideration of his successors, with the hope that they might be able to discover the full truth, which he confessed was hidden from him. it cannot be said that that truth has yet been found, and when it is found the chances are that intuition and not logic will have led to it. the spiral nebulæ, then, remain among the greatest riddles of the universe, while the gaseous nebulæ, like that of orion, are no less mysterious, although it seems impossible to doubt that both forms give birth to stars. it is but natural to look to them for light on the question of the origin of our planetary system; but we should not forget that the scale of the phenomena in the two cases is vastly different, and the forces in operation may be equally different. a hill may have been built up by a glacier, while a mountain may be the product of volcanic forces or of the upheaval of the strata of the planet. the banners of the sun as all the world knows, the sun, a blinding globe pouring forth an inconceivable quantity of light and heat, whose daily passage through the sky is caused by the earth's rotation on its axis, constitutes the most important phenomenon of terrestial existence. viewed with a dark glass to take off the glare, or with a telescope, its rim is seen to be a sharp and smooth circle, and nothing but dark sky is visible around it. except for the interference of the moon, we should probably never have known that there is any more of the sun than our eyes ordinarily see. but when an eclipse of the sun occurs, caused by the interposition of the opaque globe of the moon, we see its immediate surroundings, which in some respects are more wonderful than the glowing central orb. these surroundings, although not in the sense in which we apply the term to the gaseous envelope of the earth, may be called the sun's atmosphere. they consist of two very different parts -- first, the red ``prominences,'' which resemble tongues of flame ascending thousands of miles above the sun's surface; and, second, the ``corona,'' which extends to distances of millions of miles from the sun, and shines with a soft, glowing light. the two combined, when well seen, make a spectacle without parallel among the marvels of the sky. although many attempts have been made to render the corona visible when there is no eclipse, all have failed, and it is to the moon alone that we owe its revelation. to cover the sun's disk with a circular screen will not answer the purpose because of the illumination of the air all about the observer. when the moon hides the sun, on the other hand, the sunlight is withdrawn from a great cylinder of air extending to the top of the atmosphere and spreading many miles around the observer. there is then no glare to interfere with the spectacle, and the corona appears in all its surprising beauty. the prominences, however, although they were discovered during an eclipse, can now, with the aid of the spectroscope, be seen at any time. but the prominences are rarely large enough to be noticed by the naked eye, while the streamers of the corona, stretching far away in space, like ghostly banners blown out from the black circle of the obscuring moon, attract every eye, and to this weird apparition much of the fear inspired by eclipses has been due. but if the corona has been a cause of terror in the past it has become a source of growing knowledge in our time. the story of the first scientific observation of the corona and the prominences is thrillingly interesting, and in fact dramatic. the observation was made during the eclipse of , which fortunately was visible all over central and southern europe so that scores of astronomers saw it. the interest centers in what happened at pavia in northern italy, where the english astronomer francis baily had set up his telescope. the eclipse had begun and bailey was busy at his telescope when, to quote his own words in the account which he wrote for the memoirs of the royal astronomical society: i was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints... pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which ``made the welkin ring,'' and for the moment withdrew my attention from the object with which i was immediately occupied. i had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but i did not expect, from any of the accounts of preceding eclipses that i had read, to witness so magnificent an exhibition as that which took place... splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet i must confess that there was at the same time something in its singular and wonderful appearance that was appalling... but the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. they had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. they somewhat resembled the tops of the snowy alpine mountains when colored by the rising or the setting sun. they resembled the alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona... the whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored. i have quoted nearly all of this remarkable description not alone for its intrinsic interest, but because it is the best depiction that can be found of the general phenomena of a total solar eclipse. still, not every such eclipse offers an equally magnificent spectacle. the eclipses of and , for instance, which were seen by the writer, the first in south carolina and the second in spain, fell far short of that described by bailey in splendor and impressiveness. of course, something must be allowed for the effect of surprise; bailey had not expected to see what was so suddenly disclosed to him. but both in and the amount of scattered light in the sky was sufficient in itself to make the corona appear faint, and there were no very conspicuous prominences visible. yet on both occasions there was manifest among the spectators that mingling of admiration and awe of which bailey speaks. the south carolinians gave a cheer and the ladies waved their handkerchiefs when the corona, ineffably delicate of form and texture, melted into sight and then in two minutes melted away again. the spaniards, crowded on the citadel hill of burgos, with their king and his royal retinue in their midst, broke out with a great clapping of hands as the awaited spectacle unfolded itself in the sky; and on both occasions, before the applause began, after an awed silence a low murmur ran through the crowds. at burgos it is said many made the sign of the cross. it was not long before bailey's idea that the prominences were a part of the corona was abandoned, and it was perceived that the two phenomena were to a great extent independent. at the eclipse of , which the astronomers, aroused by the wonderful scene of , and eager to test the powers of the newly invented spectroscope, flocked to india to witness, janssen conceived the idea of employing the spectroscope to render the prominences visible when there was no eclipse. he succeeded the very next day, and these phenomena have been studied in that way ever since. there are recognized two kinds of prominences -- the ``erruptive'' and the ``quiescent.'' the latter, which are cloud-like in form, may be seen almost anywhere along the edge of the sun; but the former, which often shoot up as if hurled from mighty volcanoes, appear to be associated with sun-spots, and appear only above the zones where spots abound. either of them, when seen in projection against the brilliant solar disk, appears white, not red, as against a background of sky. the quiescent prominences, whose elevation is often from forty thousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. the latter, it will be remembered, is an element which was known to be in the sun many years before the discovery that it also exists in small quantities on the earth. a fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. the eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors -- i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. the velocity of their ascent occasionally amounts to three hundred or four hundred miles per second. it is known from mathematical considerations that the gravitation of the sun would not be able to bring back any body that started from its surface with a velocity exceeding three hundred and eighty-three miles per second; so it is evident that some of the matter hurled forth in eruptive prominences may escape from solar control and go speeding out into space, cooling and condensing into solid masses. there seems to be no reason why some of the projectiles from the sun might not reach the planets. here, then, we have on a relatively small scale, explosions recalling those which it has been imagined may be the originating cause of some of the sudden phenomena of the stellar heavens. of the sun-spots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptive prominences, as well as some relation, not yet fully understood, with the corona. of the real cause of sun-spots we know virtually nothing, but recent studies by professor hale and others have revealed a strange state of things in the clouds of metallic vapors floating above them and their surroundings. evidences of a cyclonic tendency have been found, and professor hale has proved that sun-spots are strong magnetic fields, and consist of columns of ionized vapors rotating in opposite directions in the two hemispheres. a fact which may have the greatest significance is that titanium and vanadium have been found both in sun-spots and in the remarkable variable mira ceti, a star which every eleven months, or thereabout, flames up with great brilliancy and then sinks back to invisibility with the naked eye. it has been suggested that sun-spots are indications of the beginning of a process in the sun which will be intensified until it falls into the state of such a star as mira. stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so that there is much reason for regarding sunspots as emblems of advancing age. the association of the corona with sun-spots is less evident than that of the eruptive prominences; still such an association exists, for the form and extent of the corona vary with the sun-spot period of which we shall presently speak. the constitution of the corona remains to be discovered. it is evidently in part gaseous, but it also probably contains matter in the form of dust and small meteors. it includes one substance altogether mysterious -- ``coronium.'' there are reasons for thinking that this may be the lightest of all the elements, and professor young, its discoverer, said that it was ``absolutely unique in nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical.'' the enormous extent of the corona is one of its riddles. since the development of the curious subject of the ``pressure of light'' it has been proposed to account for the sustentation of the corona by supposing that it is borne upon the billows of light continually poured out from the sun. experiment has proved, what mathematical considerations had previously pointed out as probable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon is sufficiently small. in that case the light pressure will prevail over the attraction of gravitation, and propel the attenuated matter away from the sun in the teeth of its attraction. the earth itself would be driven away if, instead of consisting of a solid globe of immense aggregate mass, it were a cloud of microscopic particles. the reason is that the pressure varies in proportion to the surface of the body acted upon, while the gravitational attraction is proportional to the volume, or the total amount of matter in the body. but the surface of any body depends upon the square of its diameter, while the volume depends upon the cube of the diameter. if, for instance, the diameter is represented by , the surface will be proportional to × , or , and the volume to × × , or ; but if the diameter is taken as , the surface will be × , or , and the volume × × , or . now, the ratio of to is twice as great as that of to . if the diameter is still further decreased, the ratio of the surface to the volume will proportionally grow larger; in other words, the pressure will gain upon the attraction, and whatever their original ratio may have been, a time will come, if the diminution of size continues, when the pressure will become more effective than the attraction, and the body will be driven away. supposing the particles of the corona to be below the critical size for the attraction of a mass like that of the sun to control them, they would be driven off into the surrounding space and appear around the sun like the clouds of dust around a mill. we shall return to this subject in connection with the zodiacal light, the aurora, and comets. on the other hand, there are parts of the corona which suggest by their forms the play of electric or magnetic forces. this is beautifully shown in some of the photographs that have been made of the corona during recent eclipses. take, for instance, that of the eclipse of . the sheaves of light emanating from the poles look precisely like the ``lines of force'' surrounding the poles of a magnet. it will be noticed in this photograph that the corona appears to consist of two portions: one comprising the polar rays just spoken of, and the other consisting of the broader, longer, and less-defined masses of light extending out from the equatorial and middle-latitude zones. yet even in this more diffuse part of the phenomenon one can detect the presence of submerged curves bearing more or less resemblance to those about the poles. just what part electricity or electro-magnetism plays in the mechanism of the solar radiation it is impossible to say, but on the assumption that it is a very important part is based the hypothesis that there exists a direct solar influence not only upon the magnetism, but upon the weather of the earth. this hypothesis has been under discussion for half a century, and still we do not know just how much truth it represents. it is certain that the outbreak of great disturbances on the sun, accompanied by the formation of sun-spots and the upshooting of eruptive prominences (phenomena which we should naturally expect to be attended by action), have been instantly followed by corresponding ``magnetic storms'' on the earth and brilliant displays of the auroral lights. there have been occasions when the influence has manifested itself in the most startling ways, a great solar outburst being followed by a mysterious gripping of the cable and telegraph systems of the world, as if an invisible and irresistible hand had seized them. messages are abruptly cut off, sparks leap from the telegraph instruments, and the entire earth seems to have been thrown into a magnetic flurry. these occurrences affect the mind with a deep impression of the dependence of our planet on the sun, such as we do not derive from the more familiar action of the sunlight on the growth of plants and other phenomena of life depending on solar influences. perhaps the theory of solar magnetic influence upon the weather is best known in connection with the ``sun-spot cycle.'' this, at any rate, is, as already remarked, closely associated with the corona. its existence was discovered in by the german astronomer schwabe. it is a period of variable length, averaging about eleven years, during which the number of spots visible on the sun first increases to a maximum, then diminishes to a minimum, and finally increases again to a maximum. for unknown reasons the period is sometimes two or three years longer than the average and sometimes as much shorter. nevertheless, the phenomena always recur in the same order. starting, for instance, with a time when the observer can find few or no spots, they gradually increase in number and size until a maximum, in both senses, is reached, during which the spots are often of enormous size and exceedingly active. after two or three years they begin to diminish in number, magnitude, and activity until they almost or quite disappear. a strange fact is that when a new period opens, the spots appear first in high northern and southern latitudes, far from the solar equator, and as the period advances they not only increase in number and size, but break out nearer and nearer to the equator, the last spots of a vanishing period sometimes lingering in the equatorial region after the advance-guard of its successor has made its appearance in the high latitudes. spots are never seen on the equator nor near the poles. it was not very long after the discovery of the sun-spot cycle that the curious observation was made that a striking coincidence existed between the period of the sun-spots and another period affecting the general magnetic condition of the earth. when a curved line representing the varying number of sun-spots was compared with another curve showing the variations in the magnetic state of the earth the two were seen to be in almost exact accord, a rise in one curve corresponding to a rise in the other, and a fall to a fall. continued observation has proved that this is a real coincidence and not an accidental one, so that the connection, although as yet unexplained, is accepted as established. but does the influence extend further, and directly affect the weather and the seasons as well as the magnetic elements of the earth? a final answer to this question cannot yet be given, for the evidence is contradictory, and the interpretations put upon it depend largely on the predilections of the judges. but, in a broad sense, the sun-spots and the phenomena connected with them must have a relation to terrestial meteorology, for they prove the sun to be a variable star. reference was made, a few lines above, to the resemblance of the spectra of sun-spots to those of certain stars which seem to be failing through age. this in itself is extremely suggestive; but if this resemblance had never been discovered, we should have been justified in regarding the sun as variable in its output of energy; and not only variable, but probably increasingly so. the very inequalities in the sun-spot cycle are suspicious. when the sun is most spotted its total light may be reduced by one-thousandth part, although it is by no means certain that its outgiving of thermal radiations is then reduced. a loss of one-thousandth of its luminosity would correspond to a decrease of . of a stellar magnitude, considering the sun as a star viewed from distant space. so slight a change would not be perceptible; but it is not alone sun-spots which obscure the solar surface, its entire globe is enveloped with an obscuring veil. when studied with a powerful telescope the sun's surface is seen to be thickly mottled with relatively obscure specks, so numerous that it has been estimated that they cut off from one-tenth to one-twentieth of the light that we should receive from it if the whole surface were as brilliant as its brightest parts. the condition of other stars warrants the conclusion that this obscuring envelope is the product of a process of refrigeration which will gradually make the sun more and more variable until its history ends in extinction. looking backward, we see a time when the sun must have been more brilliant than it is now. at that time it probably shone with the blinding white splendor of such stars as sirius, spica, and vega; now it resembles the relatively dull procyon; in time it will turn ruddy and fall into the closing cycle represented by antares. considering that once it must have been more radiantly powerful than at present, one is tempted to wonder if that could have been the time when tropical life flourished within the earth's polar circles, sustained by a vivific energy in the sun which it has now lost. the corona, as we have said, varies with the sun-spot cycle. when the spots are abundant and active the corona rises strong above the spotted zones, forming immense beams or streamers, which on one occasion, at least, had an observed length of ten million miles. at the time of a spot minimum the corona is less brilliant and has a different outline. it is then that the curved polar rays are most conspicuous. thus the vast banners of the sun, shaken out in the eclipse, are signals to tell of its varying state, but it will probably be long before we can read correctly their messages. the zodiacal light mystery there is a singular phenomenon in the sky -- one of the most puzzling of all -- which has long arrested the attention of astronomers, defying their efforts at explanation, but which probably not one in a hundred, and possibly not one in a thousand, of the readers of this book has ever seen. yet its name is often spoken, and it is a conspicuous object if one knows when and where to look for it, and when well seen it exhibits a mystical beauty which at the same time charms and awes the beholder. it is called ``the zodiacal light,'' because it lies within the broad circle of the zodiac, marking the sun's apparent annual path through the stars. what it is nobody has yet been able to find out with certainty, and books on astronomy usually speak of it with singular reserve. but it has given rise to many remarkable theories, and a true explanation of it would probably throw light on a great many other celestial mysteries. the milky way is a more wonderful object to look upon, but its nature can be comprehended, while there is a sort of uncanniness about the zodiacal light which immediately impresses one upon seeing it, for its part in the great scheme of extra-terrestrial affairs is not evident. if you are out-of-doors soon after sunset -- say, on an evening late in the month of february -- you may perceive, just after the angry flush of the dying winter's day has faded from the sky, a pale ghostly presence rising above the place where the sun went down. the writer remembers from boyhood the first time it was pointed out to him and the unearthly impression that it made, so that he afterward avoided being out alone at night, fearful of seeing the spectral thing again. the phenomenon brightens slowly with the fading of the twilight, and soon distinctly assumes the shape of an elongated pyramid of pearly light, leaning toward the south if the place of observation is in the northern hemisphere. it does not impress the observer at all in the same manner as the milky way; that looks far off and is clearly among the stars, but the zodiacal light seems closer at hand, as if it were something more intimately concerning the earth. to all it immediately suggests a connection, also, with the sunken sun. if the night is clear and the moon absent (and if you are in the country, for city lights ruin the spectacles of the sky), you will be able to watch the apparition for a long time. you will observe that the light is brightest near the horizon, gradually fading as the pyramidal beam mounts higher, but in favorable circumstances it may be traced nearly to the meridian south of the zenith, where its apex at last vanishes in the starlight. it continues visible during the evenings of march and part of april, after which, ordinarily, it is seen no more, or if seen is relatively faint and unimpressive. but when autumn comes it appears again, this time not like a wraith hovering above the westward tomb of the day-god, but rather like a spirit of the morning announcing his reincarnation in the east. the reason why the zodiacal light is best seen in our latitudes at the periods just mentioned is because at those times the zodiac is more nearly perpendicular to the horizon, first in the west and then in the east; and, since the phenomenon is confined within the borders of the zodiac, it cannot be favorably placed for observation when the zodiacal plane is but slightly inclined to the horizon. its faint light requires the contrast of a background of dark sky in order to be readily perceptible. but within the tropics, where the zodiac is always at a favorable angle, the mysterious light is more constantly visible. nearly all observant travelers in the equatorial regions have taken particular note of this phenomenon, for being so much more conspicuous there than in the temperate zones it at once catches the eye and holds the attention as a novelty. humboldt mentions it many times in his works, for his genius was always attracted by things out of the ordinary and difficult of explanation, and he made many careful observations on its shape, its brilliancy, and its variations; for there can be no doubt that it does vary, and sometimes to an astonishing degree. it is said that it once remained practically invisible in europe for several years in succession. during a trip to south africa in an english astronomer, mr e. w. maunder, found a remarkable difference between the appearance of the zodiacal light on his going and coming voyages. in fact, when crossing the equator going south he did not see it at all; but on returning he had, on march th, when one degree south of the equator, a memorable view of it. it was a bright, clear night, and the zodiacal light was extraordinarily brilliant -- brighter than he had ever seen it before. the milky way was not to be compared with it. the brightest part extended ° from the sun. there was a faint and much narrower extension which they could just make out beyond the pleiades along the ecliptic, but the greater part of the zodiacal light showed as a broad truncated column, and it did not appear nearly as conical as he had before seen it. when out of the brief twilight of intertropical lands, where the sun drops vertically to the horizon and night rushes on like a wave of darkness, the zodiacal light shoots to the very zenith, its color is described as a golden tint, entirely different from the silvery sheen of the milky way. if i may venture again to refer to personal experiences and impressions, i will recall a view of the zodiacal light from the summit of the cone of mt etna in the autumn of the year (more briefly described in astronomy with the naked eye). there are few lofty mountains so favorably placed as etna for observations of this kind. it was once resorted to by prof. george e. hale, in an attempt to see the solar corona without an eclipse. rising directly from sea-level to an elevation of nearly eleven thousand feet, the observer on its summit at night finds himself, as it were, lost in the midst of the sky. but for the black flanks of the great cone on which he stands he might fancy himself to be in a balloon. on the occasion to which i refer the world beneath was virtually invisible in the moonless night. the blaze of the constellations overhead was astonishingly brilliant, yet amid all their magnificence my attention was immediately drawn to a great tapering light that sprang from the place on the horizon where the sun would rise later, and that seemed to be blown out over the stars like a long, luminous veil. it was the finest view of the zodiacal light that i had ever enjoyed -- thrilling in its strangeness -- but i was almost disheartened by the indifference of my guide, to whom it was only a light and nothing more. if he had no science, he had less poetry -- rather a remarkable thing, i thought, for a child of his clime. the light appeared to me to be distinctly brighter than the visible part of the milky way which included the brilliant stretches in auriga and perseus, and its color, if one may speak of color in connection with such an object, seemed richer than that of the galactic band; but i did not think of it as yellow, although humboldt has described it as resembling a golden curtain drawn over the stars, and du chaillu in equatorial africa found it of a bright yellow color. it may vary in color as in conspicuousness. the fascination of that extraordinary sight has never faded from my memory. i turned to regard it again and again, although i had never seen the stellar heavens so brilliant, and it was one of the last things i looked for when the morning glow began softly to mount in the east, and sicily and the mediterranean slowly emerged from the profound shadow beneath us. the zodiacal light seems never to have attracted from astronomers in general the amount of careful attention that it deserves; perhaps because so little can really be made of it as far as explanation is concerned. i have referred to the restraint that scientific writers apparently feel in speaking of it. the grounds for speculation that it affords may be too scanty to lead to long discussions, yet it piques curiosity, and as we shall see in a moment has finally led to a most interesting theory. once it was the subject of an elaborate series of studies which carried the observer all round the world. that was in -- , during the united states exploring expedition that visited the then little known japan. the chaplain of the fleet, the rev. mr jones, went out prepared to study the mysterious light in all its phases. he saw it from many latitudes on both sides of the equator, and the imagination cannot but follow him with keen interest in his world-circling tour, keeping his eyes every night fixed upon the phantasm overhead, whose position shifted with that of the hidden sun. he demonstrated that the flow extends at times completely across the celestial dome, although it is relatively faint directly behind the earth. on his return the government published a large volume of his observations, in which he undertook to show that the phenomenon was due to the reflection of sunlight from a ring of meteoric bodies encircling the earth. but, after all, this elaborate investigation settled nothing. prof. e. e. barnard has more recently devoted much attention to the zodiacal light, as well as to a strange attendant phenomenon called the ``gegenschein,'' or counterglow, because it always appears at that point in the sky which is exactly opposite the sun. the gegenschein is an extremely elusive phenomenon, suitable only for eyes that have been specially trained to see it. professor newcomb has cautiously remarked that it is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light... this phenomenon is so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside. it certainly cannot be set aside at all since the observations of barnard. i recall an attempt to see it under his guidance during a visit to mount hamilton, when he was occupied there with the lick telescope. of course, both the gegenschein and the zodiacal light are too diffuse to be studied with telescopes, which, so to speak, magnify them out of existence. they can only be successfully studied with the naked eye, since every faintest glimmer that they afford must be utilized. this is especially true of the gegenschein. at mount hamilton, mr barnard pointed out to me its location with reference to certain stars, but with all my gazing i could not be sure that i saw it. to him, on the contrary, it was obvious; he had studied it for months, and was able to indicate its shape, its boundaries, its diameter, and the declination of its center with regard to the ecliptic. there is not, of course, the shadow of a doubt of the existence of the gegenschein, and yet i question if one person in a million has ever seen or ever will see it. the zodiacal light, on the other hand, is plain enough, provided that the time and the circumstances of the observation are properly chosen. in the attempts to explain the zodiacal light, the favorite hypothesis has been that it is an appendage of the sun -- perhaps simply an extension of the corona in the plane of the ecliptic, which is not very far from coinciding with that of the sun's equator. this idea is quite a natural one, because of the evident relation of the light to the position of the sun. the vast extension of the equatorial wings of the corona in gave apparent support to this hypothesis; if the substance of the corona could extend ten million miles from the sun, why might it not extend even one hundred million, gradually fading out beyond the orbit of the earth? a variation of this hypothesis assumes that the reflection is due to swarms of meteors circling about the sun, in the plane of its equator, all the way from its immediate neighborhood to a distance exceeding that of the earth. but in neither form is the hypothesis satisfactory; there is nothing in the appearance of the corona to indicate that it extends even as far as the planet mercury, while as to meteors, the orbits of the known swarms do not accord with the hypothesis, and we have no reason to believe that clouds of others exist traveling in the part of space where they would have to be in order to answer the requirements of the theory. the extension of the corona in did not resemble in its texture the zodiacal light. now, it has so often happened in the history of science that an important discovery in one branch has thrown unexpected but most welcome light upon some pending problem in some other branch, that a strong argument might be based upon that fact alone against the too exclusive devotion of many investigators to the narrow lines of their own particular specialty; and the zodiacal light affords a case in point, when it is considered in connection with recent discoveries in chemistry and physics. from the fact that atoms are compound bodies made up of corpuscles at least a thousand times smaller than the smallest known atom -- a fact which astounded most men of science when it was announced a few years ago -- a new hypothesis has been developed concerning the nature of the zodiacal light (as well as other astronomical riddles), and this hypothesis comes not from an astronomer, but from a chemist and physicist, the swede, svante arrhenius. in considering an outline of this new hypothesis we need neither accept nor reject it; it is a case rather for suspension of judgment. to begin with, it carries us back to the ``pressure of light'' mentioned in the preceding chapter. the manner in which this pressure is believed generally to act was there sufficiently explained, and it only remains to see how it is theoretically extended to the particles of matter supposed to constitute the zodiacal light. we know that corpuscles, or ``fragments of atoms'' negatively electrified, are discharged from hot bodies. streams of these ``ions'' pour from many flames and from molten metals; and the impact of the cathode and ultra-violet rays causes them to gush even from cold bodies. in the vast laboratory of the sun it is but reasonable to suppose that similar processes are taking place. ``as a very hot metal emits these corpuscles,'' says prof. j. j. thomson, ``it does not seem an improbable hypothesis that they are emitted by that very hot body, the sun.'' let it be assumed, then, that the sun does emit them; what happens next? negatively charged corpuscles, it is known, serve as nuclei to which particles of matter in the ordinary state are attracted, and it is probable that those emitted from the sun immediately pick up loads in this manner and so grow in bulk. if they grow large enough the gravitation of the sun draws them back, and they produce a negative charge in the solar atmosphere. but it is probable that many of the particles do not attain the critical size which, according to the principles before explained, would enable the gravitation of the sun to retain them in opposition to the pressure of the waves of light, and with these particles the light pressure is dominant. clouds of them may be supposed to be continually swept away from the sun into surrounding space, moving mostly in or near the plane of the solar equator, where the greatest activity, as indicated by sunspots and related phenomena, is taking place. as they pass outward into space many of them encounter the earth. if the earth, like the moon, had no atmosphere the particles would impinge directly on its surface, giving it a negative electric charge. but the presence of the atmosphere changes all that, for the first of the flying particles that encounter it impart to it their negative electricity, and then, since like electric charges repel like, the storm of particles following will be sheered off from the earth, and will stream around it in a maze of hyperbolic paths. those that continue on into space beyond the earth may be expected to continue picking up wandering particles of matter until their bulk has become so great that the solar attraction prevails again over the light pressure acting upon them, and they turn again sunward. passing the earth on their return they will increase the amount of dust-clouds careering round it; and these will be further increased by the action of the ultra-violet rays of the sunlight causing particles to shoot radially away from the earth when the negative charge of the upper atmosphere has reached a certain amount, which particles, although starting sunward, will be swept back to the earth with the oncoming streams. as the final result of all this accumulation of flying and gyrating particles in the earth's neighborhood, we are told that the latter must be transformed into the semblance of a gigantic solid-headed comet provided with streaming tails, the longest of them stretching away from the direction of the sun, while another shorter one extends toward the sun. this shorter tail is due to the particles that we have just spoken of as being driven sunward from the earth by the action of ultra-violet light. no doubt this whole subject is too technical for popular statement; but at any rate the general reader can understand the picturesque side of the theory, for its advocates assure us that if we were on the moon we would doubtless be able to see the comet-like tails of the earth, and then we could appreciate the part that they play in producing the phenomenon of the zodiacal light. that the light as we see it could be produced by the reflection of sunlight from swarms of particles careering round the earth in the manner supposed by arrhenius' hypothesis is evident enough; and it will be observed that the new theory, after all, is only another variant of the older one which attributes the zodiacal light to an extension of the solar corona. but it differs from the older theory in offering an explanation of the manner in which the extension is effected, and it differentiates between the corona proper and the streams of negative particles shot away from the sun. in its details the hypothesis of arrhenius also affords an explanation of many peculiarities of the zodiacal light, such as that it is confined to the neighborhood of the ecliptic, and that it is stronger on the side of the earth which is just turning away from a position under the sun than on the other side; but it would carry us beyond our limits to go into these particulars. the gegenschein, according to this theory, is a part of the same phenomenon as the zodiacal light, for by the laws of perspective it is evident that the reflection from the streams of particles situated at a point directly opposite to the sun would be at a maximum, and this is the place which the gegenschein occupies. apart from its geometrical relations to the position of the sun, the variability of the zodiacal light appears to affirm its solar dependence, and this too would be accounted for by arrhenius' hypothesis better than by the old theory of coronal extension. the amount of corpuscular discharge from the sun must naturally be governed by the state of relative activity or inactivity of the latter, and this could not but be reflected in the varying splendor of the zodiacal light. but much more extended study than has yet been given to the subject will be required before we can feel that we know with reasonable certainty what this mysterious phenomenon really is. by the hypothesis of arrhenius every planet that has an atmosphere must have a zodiacal light attending it, but the phenomenon is too faint for us to be able to see it in the case, for instance, of venus, whose atmosphere is very abundant. the moon has no corresponding ``comet's tail'' because, as already explained, of the lack of a lunar atmosphere to repel the streams by becoming itself electrified; but if there were a lunar zodiacal light, no doubt we could see it because of the relative nearness of our satellite. marvels of the aurora one of the most vivid recollections of my early boyhood is that of seeing my father return hastily into the house one evening and call out to the family: ``come outside and look at the sky!'' ours was a country house situated on a commanding site, and as we all emerged from the doorway we were dumbfounded to see the heavens filled with pale flames which ran licking and quivering over the stars. instantly there sprang into my terrified mind the recollection of an awful description of ``the day of judgment'' (the dies iræ), which i had heard with much perturbation of spirit in the dutch reformed church from the lips of a tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned type. my heart literally sank at sight of the spectacle, for it recalled the preacher's very words; it was just as he had said it would be, and it needed the assured bearing of my elders finally to convince me that that day of wrath, o dreadful day, when heaven and earth shall pass away, as david and the sibyl say had not actually come upon us. and even the older members of the household were not untouched with misgivings when menacing spots of crimson appeared, breaking out now here, now there, in the shuddering sky. toward the north the spectacle was appalling. a huge arch spanned an unnaturally dark segment resting on the horizon, and above this arch sprang up beams and streamers in a state of incessant agitation, sometimes shooting up to the zenith with a velocity that took one's breath, and sometimes suddenly falling into long ranks, and marching, marching, marching, like an endless phalanx of fiery specters, and moving, as i remember, always from east to west. the absolute silence with which these mysterious evolutions were performed and the quavering reflections which were thrown upon the ground increased the awfulness of the exhibition. occasionally enormous curtains of lambent flame rolled and unrolled with a majestic motion, or were shaken to and fro as if by a mighty, noiseless wind. at times, too, a sudden billowing rush would be made toward the zenith, and for a minute the sky overhead would glow so brightly that the stars seemed to have been consumed. the spectacle continued with varying intensity for hours. this exhibition occurred in central new york, a latitude in which the aurora borealis is seldom seen with so much splendor. i remember another similar one seen from the city of new york in november, . on this last occasion some observers saw a great upright beam of light which majestically moved across the heavens, stalking like an apparition in the midst of the auroral pageant, of whose general movements it seemed to be independent, maintaining always its upright posture, and following a magnetic parallel from east to west. this mysterious beam was seen by no less than twenty-six observers in different parts of the country, and a comparison of their observations led to a curious calculation indicating that the apparition was about one hundred and thirty-three miles tall and moved at the speed of ten miles per second! but, as everybody knows, it is in the arctic regions that the aurora, or the ``northern lights,'' can best be seen. there, in the long polar night, when for months together the sun does not rise, the strange coruscations in the sky often afford a kind of spectral daylight in unison with the weird scenery of the world of ice. the pages in the narratives of arctic exploration that are devoted to descriptions of the wonderful effects of the northern lights are second to none that man has ever penned in their fascination. the lights, as i have already intimated, display astonishing colors, particularly shades of red and green, as they flit from place to place in the sky. the discovery that the magnetic needle is affected by the aurora, quivering and darting about in a state of extraordinary excitement when the lights are playing in the sky, only added to the mystery of the phenomenon until its electro-magnetic nature had been established. this became evident as soon as it was known that the focus of the displays was the magnetic pole; and when the far south was visited the aurora australis was found, having its center at the south magnetic pole. then, if not before, it was clear that the earth was a great globular magnet, having its poles of opposite magnetism, and that the auroral lights, whatever their precise cause might be, were manifestations of the magnetic activity of our planet. after the invention of magnetic telegraphy it was found that whenever a great aurora occurred the telegraph lines were interrupted in their operation, and the ocean cables ceased to work. such a phenomenon is called a ``magnetic storm.'' the interest excited by the aurora in scientific circles was greatly stimulated when, in the last half of the nineteenth century, it was discovered that it is a phenomenon intimately associated with disturbances on the sun. the ancient ``zurich chronicles,'' extending from the year to the year , in which both sun-spots visible to the naked eye and great displays of the auroral lights were recorded, first set rudolf wolf on the track of this discovery. the first notable proof of the suspected connection was furnished with dramatic emphasis by an occurrence which happened on september , . near noon on that day two intensely brilliant points suddenly broke out in a group of sun-spots which were under observation by mr r. c. carrington at his observatory at redhill, england. the points remained visible for not more than five minutes, during which interval they moved thirty-five thousand miles across the solar disk. mr r. hodgson happened to see the same phenomenon at his observatory at highgate, and thus all possibility of deception was removed. but neither of the startled observers could have anticipated what was to follow, and, indeed, it was an occurrence which has never been precisely duplicated. i quote the eloquent account given by miss clerke in her history of astronomy during the nineteenth century. this unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. from august to september , , a magnetic storm of unparalleled intensity, extent, and duration was in progress over the entire globe. telegraphic communication was everywhere interrupted -- except, indeed, that it was in some cases found practicable to work the lines without batteries by the agency of the earth-currents alone; sparks issued from the wires; gorgeous auroras draped the skies in solemn crimson over both hemispheres, and even in the tropics; the magnetic needle lost all trace of continuity in its movements and darted to and fro as if stricken with inexplicable panic. the coincidence was even closer. at the very instant of the solar outburst witnessed by carrington and hodgson the photographic apparatus at kew registered a marked disturbance of all the three magnetic elements; while shortly after the ensuing midnight the electric agitation culminated, thrilling the whole earth with subtle vibrations, and lighting up the atmosphere from pole to pole with coruscating splendors which perhaps dimly recall the times when our ancient planet itself shone as a star. if this amazing occurrence stood alone, and as i have already said it has never been exactly duplicated, doubt might be felt concerning some of the inferences drawn from it; but in varying forms it has been repeated many times, so that now hardly anyone questions the reality of the assumed connection between solar outbursts and magnetic storms accompanied by auroral displays on the earth. it is true that the late lord kelvin raised difficulties in the way of the hypothesis of a direct magnetic action of the sun upon the earth, because it seemed to him that an inadmissible quantity of energy was demanded to account for such action. but no calculation like that which he made is final, since all calculations depend upon the validity of the data; and no authority is unshakable in science, because no man can possess omniscience. it was lord kelvin who, but a few years before the thing was actually accomplished, declared that aerial navigation was an impracticable dream, and demonstrated its impracticability by calculation. however the connection may be brought about, it is as certain as evidence can make it that solar outbursts are coincident with terrestial magnetic disturbances, and coincident in such a way as to make the inference of a causal connection irresistible. the sun is only a little more than a hundred times its own diameter away from the earth. why, then, with the subtle connection between them afforded by the ether which conveys to us the blinding solar light and the life-sustaining solar heat, should it be so difficult to believe that the sun's enormous electric energies find a way to us also? no doubt the impulse coming from the sun acts upon the earth after the manner of a touch upon a trigger, releasing energies which are already stored up in our planet. but besides the evidence afforded by such occurrences as have been related of an intimate connection between solar outbreaks and terrestial magnetic flurries, attended by magnificent auroral displays, there is another line of proof pointing in the same direction. thus, it is known that the sun-spot period, as remarked in a preceding chapter, coincides in a most remarkable manner with the periodic fluctuations in the magnetic state of the earth. this coincidence runs into the most astonishing details. for instance, when the sun-spot period shortens, the auroral period shortens to precisely the same extent; as the short sun-spot periods usually bring the most intense outbreaks of solar activity, so the corresponding short auroral periods are attended by the most violent magnetic storms; a secular period of about two hundred and twenty-two years affecting sun-spots is said to have its auroral duplicate; a shorter period of fifty-five and a half years, which some observers believe that they have discovered appears also to be common to the two phenomena; and yet another ``superposed'' period of about thirty-five years, which some investigators aver exists, affects sun-spots and aurora alike. in short, the coincidences are so numerous and significant that one would have to throw the doctrine of probability to the winds in order to be able to reject the conclusion to which they so plainly lead. but still the question recurs: how is the influence transmitted? here arrhenius comes once more with his hypothesis of negative corpuscles, or ions, driven away from the sun by light-pressure -- a hypothesis which seems to explain so many things -- and offers it also as an explanation of the way in which the sun creates the aurora. he would give the aurora the same lineage with the zodiacal light. to understand the application of this theory we must first recall the fact that the earth is a great magnet having its two opposite poles of magnetism, one near the arctic and the other near the antarctic circle. like all magnets, the earth is surrounded with ``lines of force,'' which, after the manner of the curved rays we saw in the photograph of a solar eclipse, start from a pole, rising at first nearly vertically, then bend gradually over, passing high above the equator, and finally descending in converging sheaves to the opposite pole. now the axis of the earth is so placed in space that it lies at nearly a right angle to the direction of the sun, and as the streams of negatively charged particles come pouring on from the sun (see the last preceding chapter), they arrive in the greatest numbers over the earth's equatorial regions. there they encounter the lines of magnetic force at the place where the latter have their greatest elevation above the earth, and where their direction is horizontal to the earth's surface. obeying a law which has been demonstrated in the laboratory, the particles then follow the lines of force toward the poles. while they are above the equatorial regions they do not become luminescent, because at the great elevation that they there occupy there is virtually no atmosphere; but as they pass on toward the north and the south they begin to descend with the lines of force, curving down to meet at the poles; and, encountering a part of the atmosphere comparable in density with what remains in an exhausted crookes tube, they produce a glow of cathode rays. this glow is conceived to represent the aurora, which may consequently be likened to a gigantic exhibition of vacuum-tube lights. anybody who recalls his student days in the college laboratory and who has witnessed a display of northern lights will at once recognize the resemblance between them in colors, forms, and behavior. this resemblance had often been noted before arrhenius elaborated his hypothesis. without intending to treat his interesting theory as more than a possibly correct explanation of the phenomena of the aurora, we may call attention to some apparently confirmatory facts. one of the most striking of these relates to a seasonal variation in the average number of auroræ. it has been observed that there are more in march and september than at any other time of the year, and fewer in june and december; moreover (and this is a delicate test as applied to the theory), they are slightly rarer in june than in december. now all these facts seem to find a ready explanation in the hypothesis of arrhenius, thus: ( ) the particles issuing from the sun are supposed to come principally from the regions whose excitement is indicated by the presence of sun-spots (which accords with hale's observation that sun-spots are columns of ionized vapors), and these regions have a definite location on either side of the solar equator, seldom approaching it nearer than within ° or ° north or south, and never extending much beyond ° toward either pole; ( ) the equator of the sun is inclined about ° to the plane of the earth's orbit, from which it results that twice in a year -- viz., in june and december -- the earth is directly over the solar equator, and twice a year -- viz., in march and september -- when it is farthest north or south of the solar equator, it is over the inner edge of the sun-spot belts. since the corpuscles must be supposed to be propelled radially from the sun, few will reach the earth when the latter is over the solar equator in june and december, but when it is over, or nearly over, the spot belts, in march and september, it will be in the line of fire of the more active parts of the solar surface, and relatively rich streams of particles will reach it. this, as will be seen from what has been said above, is in strict accord with the observed variations in the frequency of auroræ. even the fact that somewhat fewer auroræ are seen in june than in december also finds its explanation in the known fact that the earth is about three million miles nearer the sun in the winter than in the summer, and the number of particles reaching it will vary, like the intensity of light, inversely as the square of the distance. these coincidences are certainly very striking, and they have a cumulative force. if we accept the theory, it would appear that we ought to congratulate ourselves that the inclination of the sun's equator is so slight, for as things stand the earth is never directly over the most active regions of the sun-spots, and consequently never suffers from the maximum bombardment of charged particles of which the sun is capable. incessant auroral displays, with their undulating draperies, flitting colors, and marching columns might not be objectionable from the point of view of picturesqueness, but one magnetic storm of extreme intensity following closely upon the heels of another, for months on end, crazing the magnetic needle and continually putting the telegraph and cable lines out of commission, to say nothing of their effect upon ``wireless telegraphy'', would hardly add to the charms of terrestrial existence. one or two other curious points in connection with arrhenius' hypothesis may be mentioned. first, the number of auroræ, according to his explanation, ought to be greatest in the daytime, when the face of the earth on the sunward side is directly exposed to the atomic bombardment. of course visual observation can give us no information about this, since the light of the aurora is never sufficiently intense to be visible in the presence of daylight, but the records of the magnetic observatories can be, and have been, appealed to for information, and they indicate that the facts actually accord with the theory. behind the veil of sunlight in the middle of the afternoon, there is good reason to believe, auroral exhibitions often take place which would eclipse in magnificence those seen at night if we could behold them. observation shows, too, that auroræ are more frequent before than after midnight, which is just what we should expect if they originate in the way that arrhenius supposes. second, the theory offers an explanation of the alleged fact that the formation of clouds in the upper air is more frequent in years when auroræ are most abundant, because clouds are the result of the condensation of moisture upon floating particles in the atmosphere (in an absolutely dustless atmosphere there would be no clouds), and it has been proved that negative ions like those supposed to come from the sun play a master part in the phenomena of cloud formation. yet another singular fact, almost mystical in its suggestions, may be mentioned. it seems that the dance of the auroral lights occurs most frequently during the absence of the moon from the hemisphere in which they appear, and that they flee, in greater part, to the opposite hemisphere when the moon's revolution in an orbit considerably inclined to the earth's equator brings her into that where they have been performing. arrhenius himself discovered this curious relation of auroral frequency to the position of the moon north or south of the equator, and he explains it in this way. the moon, like the earth, is exposed to the influx of the ions from the sun; but having no atmosphere, or almost none, to interfere with them, they descend directly upon her surface and charge her with an electric negative potential to a very high degree. in consequence of this she affects the electric state of the upper parts of the earth's atmosphere where they lie most directly beneath her, and thus prevents, to a large extent, the negative discharges to which the appearance of the aurora is due. and so ``the extravagant and erring spirit'' of the aurora avoids the moon as hamlet's ghost fled at the voice of the cock announcing the awakening of the god of day. there are even other apparent confirmations of the hypothesis, but we need not go into them. we shall, however, find one more application of it in the next chapter, for it appears to be a kind of cure-all for astronomical troubles; at any rate it offers a conceivable solution of the question, how does the sun manage to transmit its electric influence to the earth? and this solution is so grandiose in conception, and so novel in the mental pictures that it offers, that its acceptance would not in the least detract from the impression that the aurora makes upon the imagination. strange adventures of comets the fears and legends of ancient times before science was born, and the superstitions of the dark ages, sedulously cultivated for theological purposes by monks and priests, have so colored our ideas of the influence that comets have had upon the human mind that many readers may be surprised to learn that it was the apparition of a wonderful comet, that of , which led to the foundation of our greatest astronomical institution, the harvard college observatory. no doubt the comet superstition existed half a century ago, as, indeed, it exists yet today, but in this case the marvelous spectacle in the sky proved less effective in inspiring terror than in awakening a desire for knowledge. even in the sixteenth century the views that enlightened minds took of comets tended powerfully to inspire popular confidence in science, and halley's prediction, after seeing and studying the motion of the comet which appeared in , that it would prove to be a regular member of the sun's family and would be seen returning after a period of about seventy-six years, together with the fulfillment of that prediction, produced a revulsion from the superstitious notions which had so long prevailed. then the facts were made plain that comets are subject to the law of gravitation equally with the planets; that there are many which regularly return to the neighborhood of the sun (perihelion); and that these travel in orbits differing from those of the planets only in their greater eccentricity, although they have the peculiarity that they do not, like the planets, all go round the sun in the same direction, and do not keep within the general plane of the planetary system, but traverse it sometimes from above and sometimes from below. other comets, including most of the ``great'' ones, appear to travel in parabolic or, in a few cases, hyperbolic orbits, which, not being closed curves, never bring them back again. but it is not certain that these orbits may not be extremely eccentric ellipses, and that after the lapse of hundreds, or thousands, of years the comets that follow them may not reappear. the question is an interesting one, because if all orbits are really ellipses, then all comets must be permanent members of the solar system, while in the contrary case many of them are simply visitors, seen once and never to be seen again. the hypothesis that comets are originally interlopers might seem to derive some support from the fact that the certainly periodic ones are associated, in groups, with the great outer planets, whose attraction appears to have served as a trap for them by turning them into elliptical orbits and thus making them prisoners in the solar system. jupiter, owing to his great mass and his commanding situation in the system, is the chief ``comet-catcher;'' but he catches them not for himself, but for the sun. yet if comets do come originally from without the borders of the planetary system, it does not, by any means, follow that they were wanderers at large in space before they yielded to the overmastering attraction of the sun. investigation of the known cometary orbits, combined with theoretical considerations, has led some astronomers to the conclusion that as the sun travels onward through space he ``picks up en route'' cometary masses which, without belonging strictly to his empire, are borne along in the same vast ``cosmical current'' that carries the solar system. but while no intelligent person any longer thinks that the appearance of a great comet is a token from the heavenly powers of the approaching death of a mighty ruler, or the outbreak of a devastating war, or the infliction of a terrible plague upon wicked mankind, science itself has discovered mysteries about comets which are not less fascinating because they are more intellectual than the irrational fancies that they have displaced. to bring the subject properly before the mind, let us see what the principal phenomena connected with a comet are. at the present day comets are ordinarily ``picked up'' with the telescope or the photographic plate before any one except their discoverer is aware of their existence, and usually they remain so insignificant in appearance that only astronomers ever see them. yet so great is the prestige of the word ``comet'' that the discovery of one of these inconspicuous wanderers, and its subsequent movements, become items of the day's news which everybody reads with the feeling, perhaps, that at least he knows what is going on in the universe even if he doesn't understand it. but a truly great comet presents quite a different proposition. it, too, is apt to be detected coming out of the depths of space before the world at large can get a glimpse of it, but as it approaches the sun its aspect undergoes a marvelous change. agitated apparently by solar influence, it throws out a long streaming tail of nebulous light, directed away from the sun and looking as if blown out like a pennon by a powerful wind. whatever may be the position of the comet with regard to the sun, as it circles round him it continually keeps its tail on the off side. this, as we shall soon see, is a fact of capital importance in relation to the probable nature of comets' tails. almost at the same time that the formation of the tail is observed a remarkable change takes place in the comet's head, which, by the way, is invariably and not merely occasionally its most important part. on approaching the sun the head usually contracts. coincidently with this contraction a nucleus generally makes its appearance. this is a bright, star-like point in the head, and it probably represents the totality of solid matter that the comet possesses. but it is regarded as extremely unlikely that even the nucleus consists of a uniformly solid mass. if it were such, comets would be far more formidable visitors when they pass near the planets than they have been found to be. the diameter of the nucleus may vary from a few hundred up to several thousand miles; the heads, on the average, are from twenty-five thousand to one hundred thousand miles in diameter, although a few have greatly exceeded these dimensions; that of the comet of , one of the most stupendous ever seen, was a million and a quarter miles in diameter! as to the tails, not withstanding their enormous length -- some have been more than a hundred million miles long -- there is reason to believe that they are of extreme tenuity, ``as rare as vacuum.'' the smallest stars have been seen shining through their most brilliant portions with undiminished luster. after the nucleus has been formed it begins to throw out bright jets directed toward the sun. a stream, and sometimes several streams, of light also project sunward from the nucleus, occasionally appearing like a stunted tail directed oppositely to the real tail. symmetrical envelopes which, seen in section, appear as half circles or parabolas, rise sunward from the nucleus, forming a concentric series. the ends of these stream backward into the tail, to which they seem to supply material. ordinarily the formation of these ejections and envelopes is attended by intense agitation of the nucleus, which twists and turns, swinging and gyrating with an appearance of the greatest violence. sometimes the nucleus is seen to break up into several parts. the entire heads of some comets have been split asunder in passing close around the sun; the comet of retreated into space after its perihelion passage with five heads instead of the one that it had originally, and each of these heads had its own tail! the possession of the spectroscope has enabled astronomers during later years to study the chemical composition of comets by analyzing their light. at first the only substances thus discovered in them were hydro-carbon compounds, due evidently to the gaseous envelopes in which some combination of hydrogen with carbon existed. behind this gaseous spectrum was found a faint continuous spectrum ascribed to the nucleus, which apparently both reflects the sunlight and gives forth the light of a glowing solid or liquid. subsequently sodium and iron lines were found in cometary spectra. the presence of iron would seem to indicate that some of these bodies may be much more massive than observations on their attractive effects have indicated. in some recent comets, such as morehouse's, in , several lines have been found, the origin of which is unknown. without going back of the nineteenth century we may find records of some of the most extraordinary comets that man has ever looked upon. in , still spoken of as ``the year of the comet,'' because of the wonderful vintage ascribed to the skyey visitor, a comet shaped like a gigantic sword amazed the whole world, and, as it remained visible for seventeen months, was regarded by superstitious persons as a symbol of the fearful happenings of napoleon's russian campaign. this comet, the extraordinary size of whose head, greatly exceeding that of the sun itself, has already been mentioned, was also remarkable for exhibiting so great a brilliancy without approaching even to the earth's distance from the sun. but there was once a comet (and only once -- in the year ) which never got nearer to the sun than four times the distance of the earth and yet appeared as a formidable object in the sky. as professor young has remarked, ``it must have been an enormous comet to be visible from such a distance.'' and we are to remember that there were no great telescopes in the year . that comet affects the imagination like a phantom of space peering into the solar system, displaying its enormous train afar off (which, if it had approached as near as other comets, would probably have become the celestial wonder of all human memory), and then turning away and vanishing in the depths of immensity. in a comet appeared which was so brilliant that it could be seen in broad day close beside the sun! this was the first authenticated instance of that kind, but the occurrence was to be repeated, as we shall see in a moment, less than forty years later. the splendid comet of , usually called donati's, is remembered by many persons yet living. it was, perhaps, both as seen by the naked eye and with the telescope, the most beautiful comet of which we have any record. it too marked a rich vintage year, still remembered in the vineyards of france, where there is a popular belief that a great comet ripens the grape and imparts to the wine a flavor not attainable by the mere skill of the cultivator. there are ``comet wines,'' carefully treasured in certain cellars, and brought forth only when their owner wishes to treat his guests to a sip from paradise. the year saw another very remarkable comet, of an aspect strangely vast and diffuse, which is believed to have swept the earth with its immense tail when it passed between us and the sun on the night of june th, an event which produced no other known effect than the appearance of an unwonted amount of scattered light in the sky. the next very notable comet was the ``great southern comet'' of , which was not seen from the northern hemisphere. it mimicked the aspect of the famous comet of , and to the great surprise of astronomers appeared to be traveling in the same path. this proved to be the rising of the curtain for an astronomical sensation unparalleled in its kind; for two years later another brilliant comet appeared, first in the southern hemisphere, and it too followed the same track. the startling suggestion was now made that this comet was identical with those of and , its return having been hastened by the resistance experienced in passing twice through the coronal envelope, and there were some who thought that it would now swing swiftly round and then plunge straight into the sun, with consequences that might be disastrous to us on account of the ``flash of heat'' that would be produced by the impact. nervous people were frightened, but observation soon proved that the danger was imaginary, for although the comet almost grazed the sun, and must have rushed through two or three million miles of the coronal region, no retardation of its immense velocity was perceptible, and it finally passed away in a damaged condition, as before remarked, and has never since appeared. then the probable truth was perceived -- viz., that the three comets ( , , and ) were not one identical body, but three separate ones all traveling in the same orbit. it was found, too, that a comet seen in bore similar insignia of relationship. the natural inference was that these four bodies had once formed a single mass which had been split apart by the disruptive action of the sun. strength was lent to this hypothesis by the fact that the comet of was apparently torn asunder during its perihelion passage, retreating into space in a dissevered state. but prof. george forbes has a theory that the splitting of the original cometary mass was effected by an unknown planet, probably greater than jupiter, situated at a hundred times the earth's distance from the sun, and revolving in a period of a thousand years. he supposes that the original comet was not that of , but one seen in , which has since been ``missing,'' and that its disruption occurred from an encounter with the supposititious planet about the year . truly from every point of view comets are the most extraordinary of adventurers! the comet of was likewise remarkable for being visible, like its predecessor of , in full daylight in close proximity to the sun. the story of its detection when almost in contact with the solar disk is dramatic. it had been discovered in the southern hemisphere only a couple of weeks before its perihelion, which occurred on september th, and on the forenoon of that day it was seen by doctor common in england, and by doctor elkin and mr finlay at the cape of good hope, almost touching the sun. it looked like a dazzling white bird with outspread wings. the southern observers watched it go right into the sun, when it instantly disappeared. what had happened was that the comet in passing its perihelion point had swung exactly between the earth and the sun. on the following morning it was seen from all parts of the world close by the sun on the opposite side, and it remained thus visible for three days, gradually receding from the solar disk. it then became visible for northern observers in the morning sky before sunrise, brandishing a portentous sword-shaped tail which, if it had been in the evening sky, would have excited the wonder of hundreds of millions, but situated where it was, comparatively few ever saw it. the application of photography to the study of comets has revealed many curious details which might otherwise have escaped detection, or at best have remained subject to doubt. it has in particular shown not only the precise form of the tails, but the remarkable vicissitudes that they undergo. professor barnard's photographs of brooks' comet in suggested, by the extraordinary changes in the form of the tail which they revealed, that the comet was encountering a series of obstructions in space which bent and twisted its tail into fantastic shapes. the reader will observe the strange form into which the tail was thrown on the night of october st. a cloud of meteors through which the comet was passing might have produced such deformations of its tail. in the photograph of daniels' comet of , a curious striping of the tail will be noticed. the short bright streaks seen in the photograph, it may be explained, are the images of stars which are drawn out into lines in consequence of the fact that the photographic telescope was adjusted to follow the motion of the comet while the stars remained at rest. but the adventures of comets are not confined to possible encounters with unknown obstacles. we have referred to the fact that the great planets, and especially jupiter, frequently interfere with the motions of comets. this interference is not limited to the original alteration of their orbits from possible parabolas to ellipses, but is sometimes exercised again and again, turning the bewildered comets into elliptical paths of all degrees of eccentricity. a famous example of this kind of planetary horse-play is furnished by the story of lexell's missing comet. this comet was first seen in . investigation showed that it was moving in an orbit which should bring it back to perihelion every five and a half years; yet it had never been seen before and, although often searched for, has never been seen since. laplace and leverrier proved mathematically that in it had approached so close to jupiter as to be involved among the orbits of his satellites. what its track had been before is not known, but on that occasion the giant planet seized the interloper, threw it into a short elliptic orbit and sent it, like an arrested vagrant, to receive sentence at the bar of the sun. on this journey it passed within less than , , miles of the earth. the form of orbit which jupiter had impressed required, as we have said, its return in about five and a half years; but soon after it had the misfortune a second time to encounter jupiter at close range, and he, as if dissatisfied with the leniency of the sun, or indignant at the stranger's familiarity, seized the comet and hurled it out of the system, or at any rate so far away that it has never since been able to rejoin the family circle that basks in the immediate rays of the solar hearth. nor is this the only instance in which jupiter has dealt summarily with small comets that have approached him with too little deference. the function which jupiter so conspicuously fulfills as master of the hounds to the sun is worth considering a little more in detail. to change the figure, imagine the sun in its voyage through space to be like a majestic battleship surrounded by its scouts. small vessels (the comets, as they are overhauled by the squadron, are taken in charge by the scouts, with jupiter for their chief, and are forced to accompany the fleet, but not all are impressed. if a strange comet undertakes to run across jupiter's bows the latter brings it to, and makes prize of it by throwing it into a relatively small ellipse with the sun for its focus. thenceforth, unless, as happened to the unhappy comet of lexell, it encounters jupiter again in such a way as to be diverted by him into a more distant orbit, it can never get away. about thirty comets are now known to have thus been captured by the great planet, and they are called ``jupiter's comet family.'' but, on the other hand, if a wandering comet crosses the wake of the chief planetary scout the latter simply drives it away by accelerating its motion and compels it to steer off into open space. the transformation of comets into meteors will be considered in the next chapter, but here, in passing, mention may be made of the strange fate of one member of jupiter's family, biela's comet, which, having become over bold in its advances to its captor, was, after a few revolutions in is impressed orbit, torn to pieces and turned into a flock of meteors. and now let us return to the mystery of comets' tails. that we are fully justified in speaking of the tails of comets as mysterious is proved by the declaration of sir john herschel, who averred, in so many words, that ``there is some profound secret and mystery of nature concerned in this phenomenon,'' and this profound secret and mystery has not yet been altogether cleared up. nevertheless, the all-explaining hypothesis of arrhenius offers us once more a certain amount of aid. comets' tails, arrhenius assures us, are but another result of the pressure of light. the reader will recall the applications of this theory to the zodiacal light and the aurora. in the form in which we now have to deal with it, the supposition is made that as a comet approaches the sun eruptions of vapor, due to the solar heat, occur in its nucleus. these are naturally most active on the side which is directly exposed to the sun, whence the appearance of the immense glowing envelopes that surround the nucleus on the sunward side. among the particles of hydro-carbon, and perhaps solid carbon in the state of fine dust, which are thus set free there will be many whose size is within the critical limit which enables the light-waves from the sun to drive them away. clouds of such particles, then, will stream off behind the advancing comet, producing the appearance of a tail. this accounts for the fact that the tails of comets are always directed away from the sun, and it also explains the varying forms of the tails and the extraordinary changes that they undergo. the speed of the particles driven before the light-waves must depend upon their size and weight, the lightest of a given size traveling the most swiftly. by accretion certain particles might grow, thus losing velocity and producing the appearance of bunches in the tail, such as have been observed. the hypothesis also falls in with the researches of bredichin, who has divided the tails of comets into three principal classes -- viz.: ( ) those which appear as long, straight rays; ( ) those which have the form of curved plumes or scimitars; ( ) those which are short, brushy, and curved sharply backward along the comet's path. in the first type he calculates the repulsive force at from twelve to fifteen times the force of gravity; in the second at from two to four times; and in the third at about one and a half times. the straight tails he ascribes to hydrogen because the hydrogen atom is the lightest known; the sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. it will be seen that, if the force driving off the tails is that which arrhenius assumes it to be, the forms of those appendages would accord with those that bredichin's theory calls for. at the same time we have an explanation of the multiple tails with which some comets have adorned themselves. the comet of , for instance, had at one time no less than seven tails spread in a wide curved brush behind it. donati's comet of also had at least two tails, the principal one sword-shaped and the other long, narrow, and as straight as a rule. according to bredichin, the straight tail must have been composed of hydrogen, and the other of some form of hydro-carbon whose atoms are heavier than those of hydrogen, and, consequently, when swept away by the storm of light-waves, followed a curvature depending upon the resultant of the forces operating upon them. the seven tails of the comet of presented a kind of diagram graphically exhibiting its complex composition, and, if we knew a little more about the constituents of a comet, we might be able to say from the amount of curvature of the different tails just what were the seven substances of which that comet consisted. if these theories seem to the reader fantastic, at any rate they are no more fantastic than the phenomena that they seek to explain. meteors, fire-balls, and meteorites one of the most terrorizing spectacles with which the heavens have ever caused the hearts of men to quake occurred on the night of november , . on that night north america, which faced the storm, was under a continual rain of fire from about ten o'clock in the evening until daybreak. the fragments of a comet had struck the earth. but the meaning of what had happened was not discovered until long afterward. to the astronomers who, with astonishment not less than that of other people, watched the wonderful scene, it was an unparalleled ``shower of meteors.'' they did not then suspect that those meteors had once formed the head of a comet. light dawned when, a year later, prof. denison olmsted, of yale college, demonstrated that the meteors had all moved in parallel orbits around the sun, and that these orbits intersected that of the earth at the point where our planet happened to be on the memorable night of november th. professor olmsted even went so far as to suggest that the cloud of meteors that had encountered the earth might form a diffuse comet; but full recognition of the fact that they were cometary débris came later, as the result of further investigation. the key to the secret was plainly displayed in the spectacle itself, and was noticed without being understood by thousands of the terror-stricken beholders. it was an umbrella of fire that had opened overhead and covered the heavens; in other words, the meteors all radiated from a particular point in the constellation leo, and, being countless as the snowflakes in a winter tempest, they ribbed the sky with fiery streaks. professor olmsted showed that the radiation of the meteors from a fixed point was an effect of perspective, and in itself a proof that they were moving in parallel paths when they encountered the earth. the fact was noted that there had been a similar, but incomparably less brilliant, display of meteors on the same day of november, , and it was rightly concluded that these had belonged to the same stream, although the true relationship of the phenomena was not immediately apprehended. olmsted ascribed to the meteors a revolution about the sun once in every six months, bringing them to the intersection of their orbit with that of the earth every november th; but later investigators found that the real period was about thirty-three and one-quarter years, so that the great displays were due three times in a century, and their return was confidently predicted for the year . the appearance of the meteors in , a year before the great display, was ascribed to the great length of the stream which they formed in space -- so great that they required more than two years to cross the earth's orbit. in the earth had encountered a relatively rare part of the stream, but in , on returning to the crossing-place, it found there the richest part of the stream pouring across its orbit. this explanation also proved to be correct, and the predicted return in was duly witnessed, although the display was much less brilliant than in . it was followed by another in . in the mean time olmsted's idea of a cometary relationship of the meteors was demonstrated to be correct by the researches of schiaparelli and others, who showed that not only the november meteors, but those of august, which are seen more or less abundantly every year, traveled in the tracks of well-known comets, and had undoubtedly an identical origin with those comets. in other words the comets and the meteor-swarms were both remnants of original masses which had probably been split up by the action of the sun, or of some planet to which they had made close approaches. the annual periodicity of the august meteors was ascribed to the fact that the separation had taken place so long ago that the meteors had become distributed all around the orbit, in consequence of which the earth encountered some of them every year when it arrived at the crossing-point. then leverrier showed that the original comet associated with the november meteors was probably brought into the system by the influence of the planet uranus in the year of the christian era. afterward alexander herschel identified the tracks of no less than seventy-six meteor-swarms (most of them inconspicuous) with those of comets. the still more recent researches of mr w. f. denning make it probable that there are no meteors which do not belong to a flock or system probably formed by the disintegration of a cometary mass; even the apparently sporadic ones which shoot across the sky, ``lost souls in the night,'' being members of flocks which have become so widely scattered that the earth sometimes takes weeks to pass through the region of space where their paths lie. the november meteors should have exhibited another pair of spectacles in and , and their failure to do so caused at first much disappointment, until it was made plain that a good reason existed for their absence. it was found that after their last appearance, in , they had been disturbed in their movements by the planets jupiter and saturn, whose attractions had so shifted the position of their orbit that it no longer intersected that of the earth, as it did before. whether another planetary interference will sometime bring the principal mass of the november meteors back to the former point of intersection with the earth's orbit is a question for the future to decide. it would seem that there may be several parallel streams of the november meteors, and that some of them, like those of august, are distributed entirely around the orbit, so that every mid-november we see a few of them. we come now to a very remarkable example of the disintegration of a comet and the formation of a meteor-stream. in biela, of josephstadt, austria, discovered a comet to which his name was given. calculation showed that it had an orbital period of about six and a half years, belonging to jupiter's ``family.'' on one of its returns, in , it astonished its watchers by suddenly splitting in two. the two comets thus formed out of one separated to a distance of about one hundred and sixty thousand miles, and then raced side by side, sometimes with a curious ligature connecting them, like siamese twins, until they disappeared together in interplanetary space. in they came back, still nearly side by side, but now the distance between them had increased to a million and a quarter of miles. after that, at every recurrence of their period, astronomers looked for them in vain, until , when an amazing thing happened. on the night of november th, when the earth was crossing the plane of the orbit of the missing comet, a brilliant shower of meteors burst from the northern sky, traveling nearly in the track which the comet should have pursued. the astronomers were electrified. klinkerfues, of göttingen, telegraphed to pogson, of madras: ``biela touched earth; search near theta centauri.'' pogson searched in the place indicated and saw a cometary mass retreating into the southern heavens, where it was soon swallowed from sight! since then the biela meteors have been among the recognized periodic spectacles of the sky, and few if any doubt that they represent a portion of the missing comet whose disintegration began with the separation into two parts in . the comet itself has never since been seen. the first display of these meteors, sometimes called the ``andromedes,'' because they radiate from the constellation andromeda, was remarkable for the great brilliancy of many of the fire-balls that shot among the shower of smaller sparks, some of which were described as equaling the full moon in size. none of them is known to have reached the earth, but during the display of the same meteors in a meteoric mass fell at mazapil in northern mexico (it is now in the museum at vienna), which many have thought may actually be a piece of the original comet of biela. this brings us to the second branch of our subject. more rare than meteors or falling stars, and more startling, except that they never appear in showers, are the huge balls of fire which occasionally dart through the sky, lighting up the landscapes beneath with their glare, leaving trains of sparks behind them, often producing peals of thunder when they explode, and in many cases falling upon the earth and burying themselves from a few inches to several feet in the soil, from which, more than once, they have been picked up while yet hot and fuming. these balls are sometimes called bolides. they are not really round in shape, although they often look so while traversing the sky, but their forms are fragmentary, and occasionally fantastic. it has been supposed that their origin is different from that of the true meteors; it has even been conjectured that they may have originated from the giant volcanoes of the moon or have been shot out from the sun during some of the tremendous explosions that accompany the formation of eruptive prominences. by the same reasoning some of them might be supposed to have come from some distant star. others have conjectured that they are wanderers in space, of unknown origin, which the earth encounters as it journeys on, and lord kelvin made a suggestion which has become classic because of its imaginative reach -- viz., that the first germs of life may have been brought to the earth by one of these bodies, ``a fragment of an exploded world.'' it is a singular fact that astronomers and scientific men in general were among the last to admit the possibility of solid masses falling from the sky. the people had believed in the reality of such phenomena from the earliest times, but the savants shook their heads and talked of superstition. this was the less surprising because no scientifically authenticated instance of such an occurrence was known, and the stones popularly believed to have fallen from the sky had become the objects of worship or superstitious reverence, a fact not calculated to recommend them to scientific credence. the celebrated ``black stone'' suspended in the kaaba at mecca is one of these reputed gifts from heaven; the ``palladium'' of ancient troy was another; and a stone which fell near ensisheim, in germany, was placed in a church as an object to be religiously venerated. many legends of falling stones existed in antiquity, some of them curiously transfigured by the imagination, like the ``lion of the peloponnesus,'' which was said to have sprung down from the sky upon the isthmus of corinth. but near the beginning of the nineteenth century, in , a veritable shower of falling stones occurred at l'aigle, in northern france, and this time astronomers took note of the phenomenon and scientifically investigated it. thousands of the strange projectiles came from the sky on this occasion, and were scattered over a wide area of country, and some buildings were hit. four years later another shower of stones occurred at weston, conn., numbering thousands of individuals. the local alarm created in both cases was great, as well it might be, for what could be more intimidating than to find the blue vault of heaven suddenly hurling solid missiles at the homes of men? after these occurrences it was impossible for the most skeptical to doubt any longer, and the regular study of ``aerolites,'' or ``meteorites,'' began. one of the first things recognized was the fact that fire-balls are solid meteorites in flight, and not gaseous exhalations in the air, as some had assumed. they burn in the air during their flight, and sometimes, perhaps, are entirely consumed before reaching the ground. their velocity before entering the earth's atmosphere is equal to that of the planets in their orbits -- viz., from twenty to thirty miles per second -- a fact which proves that the sun is the seat of the central force governing them. their burning in the air is not difficult to explain; it is the heat of friction which so quickly brings them to incandescence. calculation shows that a body moving through the air at a velocity of about a mile per second will be brought, superficially, to the temperature of ``red heat'' by friction with the atmosphere. if its velocity is twenty miles per second the temperature will become thousands of degrees. this is the state of affairs with a meteorite rushing into the earth's atmosphere; its surface is liquefied within a few seconds after the friction begins to act, and the melted and vaporized portion of its mass is swept backward, forming the train of sparks that follows every great fire-ball. however, there is one phenomenon connected with the trains of meteorites which has never been satisfactorily explained: they often persist for long periods of time, drifting and turning with the wind, but not ceasing to glow with a phosphorescent luminosity. the question is, whence comes this light? it must be light without heat, since the fine dust or vapor of which the train can only consist would not retain sufficient heat to render it luminous for so long a time. an extremely remarkable incident of this kind occurred on february , , when an immense fire-ball that passed over southern england left a train that remained visible during two hours, assuming many curious shapes as it was drifted about by currents in the air. but notwithstanding the enormous velocity with which meteorites enter the air they are soon slowed down to comparatively moderate speed, so that when they disappear they are usually traveling not faster than a mile a second. the courses of many have been traced by observers situated along their track at various points, and thus a knowledge has been obtained of their height above the ground during their flight and of the length of their visible courses. they generally appear at an elevation of eighty or a hundred miles, and are seldom visible after having descended to within five miles of the ground, unless the observer happens to be near the striking-point, when he may actually witness the fall. frequently they burst while high in the air and their fragments are scattered like shrapnel over the surface of the ground, sometimes covering an area of several square miles, but of course not thickly; different fragments of the same meteorite may reach the ground at points several miles apart. the observed length of their courses in the atmosphere varies from fifty to five hundred miles. if they continued a long time in flight after entering the air, even the largest of them would probably be consumed to the last scrap, but their fiery career is so short on account of their great speed that the heat does not have time to penetrate very deeply, and some that have been picked up immediately after their fall have been found cold as ice within. their size after reaching the ground is variable within wide limits; some are known which weigh several tons, but the great majority weigh only a few pounds and many only a few ounces. meteorites are of two kinds: stony meteorites and iron meteorites. the former outnumber the latter twenty to one; but many stone meteorites contain grains of iron. nickel is commonly found in iron meteorites, so that it might be said that that redoubtable alloy nickel-steel is of cosmical invention. some twenty-five chemical elements have been found in meteorites, including carbon and the ``sun-metal,'' helium. the presence of the latter is certainly highly suggestive in connection with the question of the origin of meteorites. the iron meteorites, besides metallic iron and nickel, of which they are almost entirely composed, contain hydrogen, helium, and carbonic oxide, and about the only imaginable way in which these gases could have become absorbed in the iron would be through the immersion of the latter while in a molten or vaporized state in a hot and dense atmosphere composed of them, a condition which we know to exist only in the envelopes of the sun and the stars. the existence of carbon in the canyon diablo iron meteorites is attended by a circumstance of the most singular character -- a very ``fairy tale of science.'' in some cases the carbon has become diamond! these meteoric diamonds are very small; nevertheless, they are true diamonds, resembling in many ways the little black gems produced by moissan's method with the aid of the electric furnace. the fact that they are found embedded in these iron meteorites is another argument in favor of the hypothesis of the solar or stellar origin of the latter. to appreciate this it is necessary to recall the way in which moissan made his diamonds. it was by a combination of the effects of great heat, great pressure, and sudden or rapid superficial cooling on a mass of iron containing carbon. when he finally broke open his iron he found it a pudding stuffed with miniature black diamonds. when a fragment of the canyon diablo meteoric iron was polished in philadelphia over fifteen years ago it cut the emery-wheel to pieces, and examination showed that the damage had been effected by microscopic diamonds peppered through the mass. how were those diamonds formed? if the sun or sirius was the laboratory that prepared them, we can get a glimpse at the process of their formation. there is plenty of heat, plenty of pressure, and an abundance of vaporized iron in the sun and the stars. when a great solar eruption takes place, masses of iron which have absorbed carbon may be shot out with a velocity which forbids their return. plunged into the frightful cold of space, their surfaces are quickly cooled, as moissan cooled his prepared iron by throwing it into water, and thus the requisite stress is set up within, and, as the iron solidifies, the included carbon crystallizes into diamonds. whether this explanation has a germ of truth in it or not, at any rate it is evident that iron meteorites were not created in the form in which they come to us; they must once have been parts of immeasurably more massive bodies than themselves. the fall of meteorites offers an appreciable, though numerically insignificant, peril to the inhabitants of the earth. historical records show perhaps three or four instances of people being killed by these bodies. but for the protection afforded by the atmosphere, which acts as a very effective shield, the danger would doubtless be very much greater. in the absence of an atmosphere not only would more meteorites reach the ground, but their striking force would be incomparably greater, since, as we have seen, the larger part of their original velocity is destroyed by the resistance of the air. a meteorite weighing many tons and striking the earth with a velocity of twenty or thirty miles per second, would probably cause frightful havoc. it is a singular fact that recent investigations seem to have proved that an event of this kind actually happened in north america -- perhaps not longer than a thousand or two thousand years ago. the scene of the supposed catastrophe is in northern central arizona, at coon butte, where there is a nearly circular crater in the middle of a circular elevation or small mountain. the crater is somewhat over four thousand feet in diameter, and the surrounding rim, formed of upturned strata and ejected rock fragments, rises at its highest point one hundred and sixty feet above the plain. the crater is about six hundred feet in depth -- that is, from the rim to the visible floor or bottom of the crater. there is no evidence that volcanic action has ever taken place in the immediate neighborhood of coon butte. the rock in which the crater has been made is composed of horizontal sandstone and limestone strata. between three hundred and four hundred million tons of rock fragments have been detached, and a large portion hurled by some cause out of the crater. these fragments lie concentrically distributed around the crater, and in large measure form the elevation known as coon butte. the region has been famous for nearly twenty years on account of the masses of meteoric iron found scattered about and known as the ``canyon diablo'' meteorites. it was one of these masses, which consist of nickel-iron containing a small quantity of platinum, and of which in all some ten tons have been recovered for sale to the various collectors throughout the world, that as before mentioned destroyed the grinding-tool at philadelphia through the cutting power of its embedded diamonds. these meteoric irons are scattered about the crater-hill, in concentric distribution, to a maximum distance of about five miles. when the suggestion was first made in that a monster meteorite might have created by its fall this singular lone crater in stratified rocks, it was greeted with incredulous smiles; but since then the matter has assumed a different aspect. the standard iron company, formed by messrs. d. m. barringer, b. c. tilghman, e. j. bennitt, and s. j. holsinger, having become, in , the owner of this freak of nature, sunk shafts and bored holes to a great depth in the interior of the crater, and also trenched the slopes of the mountain, and the result of their investigations has proved that the meteoric hypothesis of origin is correct. (see the papers published in the proceedings of the academy of natural sciences of philadelphia, december, , wherein it is proved that the united states geological survey was wrong in believing this crater to have been due to a steam explosion. since that date there has been discovered a great amount of additional confirmatory proof). material of unmistakably meteoric origin was found by means of the drills, mixed with crushed rock, to a depth of six hundred to seven hundred feet below the floor of the crater, and a great deal of it has been found admixed with the ejected rock fragments on the outer slopes of the mountain, absolutely proving synchronism between the two events, the formation of this great crater and the falling of the meteoric iron out of the sky. the drill located in the bottom of the crater was sent, in a number of cases, much deeper (over one thousand feet) into unaltered horizontal red sandstone strata, but no meteoric material was found below this depth (seven hundred feet, or between eleven and twelve hundred feet below the level of the surrounding plain), which has been assumed as being about the limit of penetration. it is not possible to sink a shaft at present, owing to the water which has drained into the crater, and which forms, with the finely pulverized sandstone, a very troublesome quicksand encountered at about two hundred feet below the visible floor of the crater. as soon as this water is removed by pumping it will be easy to explore the depths of the crater by means of shafts and drifts. the rock strata (sandstone and limestone) of which the walls consist present every appearance of having been violently upturned by a huge body penetrating the earth like a cannon-ball. the general aspect of the crater strikingly resembles the impression made by a steel projectile shot into an armor-plate. mr tilghman has estimated that a meteorite about five hundred feet in diameter and moving with a velocity of about five miles per second would have made just such a perforation upon striking rocks of the character of those found at this place. there was some fusion of the colliding masses, and the heat produced some steam from the small amount of water in the rocks. as a result there has been found at depth a considerable amount of fused quartz (original sandstone), and with it innumerable particles or sparks of fused nickel-iron (original meteorite). a projectile of that size penetrating eleven to twelve hundred feet into the rocky shell of the globe must have produced a shock which was perceptible several hundred miles away. the great velocity ascribed to the supposed meteorite at the moment of striking could be accounted for by the fact that it probably plunged nearly vertically downward, for it formed a circular crater in the rocky crust of the earth. in that case it would have been less retarded by the resistance of the atmosphere than are meteorites which enter the air at a lower angle and shoot ahead hundreds of miles until friction has nearly destroyed their original motion when they drop upon the earth. some meteoric masses of great size, such as peary's iron meteorite found at cape york, greenland, and the almost equally large mass discovered at bacubirito, mexico, appear to have penetrated but slightly on striking the earth. this may be explained by supposing that they pursued a long, horizontal course through the air before falling. the result would be that, their original velocity having been practically destroyed, they would drop to the ground with a velocity nearly corresponding to that which gravity would impart within the perpendicular distance of their final fall. a six-hundred-and-sixty-pound meteorite, which fell at knyahinya, hungary, striking at an angle of ° from the vertical, penetrated the ground to a depth of eleven feet. it has been remarked that the coon butte meteorite may have fallen not longer ago than a few thousand years. this is based upon the fact that the geological indications favor the supposition that the event did not occur more than five thousand years ago, while on the other hand the rings of growth in the cedar-trees growing on the slopes of the crater show that they have existed there about seven hundred years. prof. william h. pickering has recently correlated this with an ancient chronicle which states that at cairo, egypt, in the year , ``many stars passed with a great noise.'' he remarks that cairo is about °, by great circle, from coon butte, so that if the meteorite that made the crater was a member of a flock of similar bodies which encountered the earth moving in parallel lines, some of them might have traversed the sky tangent to the earth's surface at cairo. that the spectacle spoken of in the chronicle was caused by meteorites he deems exceedingly probable because of what is said about ``a great noise;'' meteorites are the only celestial phenomena attended with perceptible sounds. professor pickering conjectures that this supposed flock of great meteorites may have formed the nucleus of a comet which struck the earth, and he finds confirmation of the idea in the fact that out of the ten largest meteorites known, no less than seven were found within nine hundred miles of coon butte. it would be interesting if we could trace back the history of that comet, and find out what malicious planet caught it up in its innocent wanderings and hurled it with so true an aim at the earth! this remarkable crater is one of the most interesting places in the world, for there is absolutely no record of such a mass, possibly an iron-headed comet, from outer space having come into collision with our earth. the results of the future exploration of the depths of the crater will be awaited with much interest. the wrecking of the moon there are sympathetic moods under whose influence one gazes with a certain poignant tenderness at the worn face of the moon; that little ``fossil world'' (the child of our mother earth, too) bears such terrible scars of its brief convulsive life that a sense of pity is awakened by the sight. the moon is the wonder-land of the telescope. those towering mountains, whose ``proud aspiring peaks'' cast silhouettes of shadow that seem drawn with india-ink; those vast plains, enchained with gentle winding hills and bordered with giant ranges; those oval ``oceans,'' where one looks expectant for the flash of wind-whipped waves; those enchanting ``bays'' and recesses at the seaward feet of the alps; those broad straits passing between guardian heights incomparably mightier than gibraltar; those locket-like valleys as secluded among their mountains as the vale of cashmere; those colossal craters that make us smile at the pretensions of vesuvius, etna, and cotopaxi; those strange white ways which pass with the unconcern of roman roads across mountain, gorge, and valley -- all these give the beholder an irresistible impression that it is truly a world into which he is looking, a world akin to ours, and yet no more like our world than pompeii is like naples. its air, its waters, its clouds, its life are gone, and only a skeleton remains -- a mute but eloquent witness to a cosmical tragedy without parallel in the range of human knowledge. one cannot but regret that the moon, if it ever was the seat of intelligent life, has not remained so until our time. think what the consequences would have been if this other world at our very door had been found to be both habitable and inhabited! we talk rather airily of communicating with mars by signals; but mars never approaches nearer than , , miles, while the moon when nearest is only a little more than , miles away. given an effective magnifying power of five thousand diameters, which will perhaps be possible at the mountain observatories as telescopes improve, and we should be able to bring the moon within an apparent distance of about forty miles, while the corresponding distance for mars would be more than seven thousand miles. but even with existing telescopic powers we can see details on the moon no larger than some artificial constructions on the earth. st peter's at rome, with the vatican palace and the great piazza, if existing on the moon, would unquestionably be recognizable as something else than a freak of nature. large cities, with their radiating lines of communication, would at once betray their real character. cultivated tracts, and the changes produced by the interference of intelligent beings, would be clearly recognizable. the electric illumination of a large town at night would probably be markedly visible. gleams of reflected sunlight would come to us from the surfaces of the lakes and oceans, and a huge ``liner'' traversing a lunar sea could probably be followed by its trail of smoke. as to communications by ``wireless'' signals, which certain enthusiasts have thought of in connection with mars, in the case of the moon they should be a relatively simple matter, and the feat might actually be accomplished. think what a literature would grow up about the moon if it were a living world! its very differences from the earth would only accentuate its interest for us. night and day on the moon are each two weeks in length; how interesting it would be to watch the manner in which the lunarians dealt with such a situation as that. lunar and terrestrial history would keep step with each other, and we should record them both. truly one might well wish to have a neighbor world to study; one would feel so much the less alone in space. it is not impossible that the moon did at one time have inhabitants of some kind. but, if so, they vanished with the disappearance of its atmosphere and seas, or with the advent of its cataclysmic age. at the best, its career as a living world must have been brief. if the water and air were gradually absorbed, as some have conjectured, by its cooling interior rocks, its surface might, nevertheless, have retained them for long ages; but if, as others think, their disappearance was due to the escape of their gaseous molecules in consequence of the inability of the relatively small lunar gravitation to retain them, then the final catastrophe must have been as swift as it was inevitable. accepting darwin's hypothesis, that the moon was separated from the earth by tidal action while both were yet plastic or nebulous, we may reasonably conclude that it began its career with a good supply of both water and air, but did not possess sufficient mass to hold them permanently. yet it may have retained them long enough for life to develop in many forms upon its surface; in fact, there are so many indications that air and water have not always been lacking to the lunar world that we are driven to invent theories to explain both their former presence and their present absence. but whatever the former condition of the moon may have been, its existing appearance gives it a resistless fascination, and it bears so clearly the story of a vast catastrophe sculptured on its rocky face that the thoughtful observer cannot look upon it without a feeling of awe. the gigantic character of the lunar features impresses the beholder not less than the universality of the play of destructive forces which they attest. let us make a few comparisons. take the lunar crater called ``tycho'', which is a typical example of its kind. in the telescope tycho appears as a perfect ring surrounding a circular depression, in the center of which rises a group of mountains. its superficial resemblance to some terrestrial volcanic craters is very striking. vesuvius, seen from a point vertically above, would no doubt look something like that (the resemblance would have been greater when the monte del cavallo formed a more complete circuit about the crater cone). but compare the dimensions. the remains of the outer crater ring of vesuvius are perhaps half a mile in diameter, while the active crater itself is only two or three hundred feet across at the most; tycho has a diameter of fifty-four miles! the group of relatively insignificant peaks in the center of the crater floor of tycho is far more massive than the entire mountain that we call vesuvius. the largest known volcanic crater on the earth, aso san, in japan, has a diameter of seven miles; it would take sixty craters like aso san to equal tycho in area! and tycho, though one of the most perfect, is by no means the largest crater on the moon. another, called ``theophilus,'' has a diameter of sixty-four miles, and is eighteen thousand feet deep. there are hundreds from ten to forty miles in diameter, and thousands from one to ten miles. they are so numerous in many places that they break into one another, like the cells of a crushed honeycomb. the lunar craters differ from those of the earth more fundamentally than in the matter of mere size; they are not situated on the tops of mountains. if they were, and if all the proportions were the same, a crater like tycho might crown a conical peak fifty or one hundred miles high! instead of being cavities in the summits of mountains, the lunar craters are rather gigantic sink-holes whose bottoms in many cases lie two or three miles below the general surface of the lunar world. around their rims the rocks are piled up to a height of from a few hundred to two or three thousand feet, with a comparatively gentle inclination, but on the inner side they fall away in gigantic broken precipices which make the dizzy cliffs of the matterhorn seem but ``lover's leaps.'' down they drop, ridge below ridge, crag under crag, tottering wall beneath wall, until, in a crater named ``newton,'' near the south lunar pole, they attain a depth where the rays of the sun never reach. nothing more frightful than the spectacle which many of these terrible chasms present can be pictured by the imagination. as the lazy lunar day slowly advances, the sunshine, unmitigated by clouds or atmospheric veil of any kind, creeps across their rims and begins to descend the opposite walls. presently it strikes the ragged crest of a ridge which had lain hidden in such darkness as we never know on the earth, and runs along it like a line of kindling fire. rocky pinnacles and needles shoot up into the sunlight out of the black depths. down sinks the line of light, mile after mile, and continually new precipices and cliffs are brought into view, until at last the vast floor is attained and begins to be illuminated. in the meanwhile the sun's rays, darting across the gulf, have touched the summits of the central peaks, twenty or thirty miles from the crater's inmost edge, and they immediately kindle and blaze like huge stars amid the darkness. so profound are some of these awful craters that days pass before the sun has risen high enough above them to chase the last shadows from their depths. although several long ranges of mountains resembling those of the earth exist on the moon, the great majority of its elevations assume the crateriform aspect. sometimes, instead of a crater, we find an immense mountain ring whose form and aspect hardly suggest volcanic action. but everywhere the true craters are in evidence, even on the sea-beds, although they attain their greatest number and size on those parts of the moon -- covering sixty per cent of its visible surface -- which are distinctly mountainous in character and which constitute its most brilliant portions. broadly speaking, the southwestern half of the moon is the most mountainous and broken, and the northeastern half the least so. right down through the center, from pole to pole, runs a wonderful line of craters and crateriform valleys of a magnitude stupendous even for the moon. another similar line follows the western edge. three or four ``seas'' are thrust between these mountainous belts. by the effects of ``libration'' parts of the opposite hemisphere of the moon which is turned away from the earth are from time to time brought into view, and their aspect indicates that that hemisphere resembles in its surface features the one which faces the earth. there are many things about the craters which seem to give some warrant for the hypothesis which has been particularly urged by mr g. k. gilbert, that they were formed by the impact of meteors; but there are also many things which militate against that idea, and, upon the whole, the volcanic theory of their origin is to be preferred. the enormous size of the lunar volcanoes is not so difficult to account for when we remember how slight is the force of lunar gravity as compared with that of the earth. with equal size and density, bodies on the moon weigh only one-sixth as much as on the earth. impelled by the same force, a projectile that would go ten miles on the earth would go sixty miles on the moon. a lunar giant thirty-five feet tall would weigh no more than an ordinary son of adam weighs on his greater planet. to shoot a body from the earth so that it would not drop back again, we should have to start it with a velocity of seven miles per second; a mile and a half per second would serve on the moon. it is by no means difficult to believe, then, that a lunar volcano might form a crater ring eight or ten times broader than the greatest to be found on the earth, especially when we reflect that in addition to the relatively slight force of gravity, the materials of the lunar crust are probably lighter than those of our terrestrial rocks. for similar reasons it seems not impossible that the theory mentioned in a former chapter -- that some of the meteorites that have fallen upon the earth originated from the lunar volcanoes -- is well founded. this would apply especially to the stony meteorites, for it is hardly to be supposed that the moon, at least in its superficial parts, contains much iron. it is surely a scene most strange that is thus presented to the mind's eye -- that little attendant of the earth's (the moon has only one-fiftieth of the volume, and only one-eightieth of the mass of the earth) firing great stones back at its parent planet! and what can have been the cause of this furious outbreak of volcanic forces on the moon? evidently it was but a passing stage in its history; it had enjoyed more quiet times before. as it cooled down from the plastic state in which it parted from the earth, it became incrusted after the normal manner of a planet, and then oceans were formed, its atmosphere being sufficiently dense to prevent the water from evaporating and the would-be oceans from disappearing continually in mist. this, if any, must have been the period of life in the lunar world. as we look upon the vestiges of that ancient world buried in the wreck that now covers so much of its surface, it is difficult to restrain the imagination from picturing the scenes which were once presented there; and, in such a case, should the imagination be fettered? we give it free rein in terrestrial life, and it rewards us with some of our greatest intellectual pleasures. the wonderful landscapes of the moon offer it an ideal field with just enough half-hidden suggestions of facts to stimulate its powers. the great plains of the mare imbrium and the mare serenitatis (the ``sea of showers'' and the ``sea of serenity''), bordered in part by lofty mountain ranges precisely like terrestrial mountains, scalloped along their shores with beautiful bays curving back into the adjoining highlands, and united by a great strait passing between the nearly abutting ends of the ``lunar apennines'' and the ``lunar caucasus,'' offer the elements of a scene of world beauty such as it would be difficult to match upon our planet. look at the finely modulated bottom of the ancient sea in mr ritchey's exquisite photograph of the western part of the mare serenitatis, where one seems to see the play of the watery currents heaping the ocean sands in waving lines, making shallows, bars, and deeps for the mariner to avoid or seek, and affording a playground for the creatures of the main. what geologist would not wish to try his hammer on those rocks with their stony pages of fossilized history? there is in us an instinct which forbids us to think that there was never any life there. if we could visit the moon, there is not among us a person so prosaic and unimaginative that he would not, the very first thing, begin to search for traces of its inhabitants. we would look for them in the deposits on the sea bottoms; we would examine the shores wherever the configuration seemed favorable for harbors and the sites of maritime cities -- forgetting that it may be a little ridiculous to ascribe to the ancient lunarians the same ideas that have governed the development of our race; we would search through the valleys and along the seeming courses of vanished streams; we would explore the mountains, not the terrible craters, but the pinnacled chains that recall our own alps and rockies; seeking everywhere some vestige of the transforming presence of intelligent life. perhaps we should find such traces, and perhaps, with all our searching, we should find nothing to suggest that life had ever existed amid that universal ruin. look again at the border of the ``sea of serenity'' -- what a name for such a scene! -- and observe how it has been rent with almost inconceivable violence, the wall of the colossal crater posidonius dropping vertically upon the ancient shore and obliterating it, while its giant neighbor, le monnier, opens a yawning mouth as if to swallow the sea itself. a scene like this makes one question whether, after all, those may not be right who have imagined that the so-called sea bottoms are really vast plains of frozen lava which gushed up in floods so extensive that even the mighty volcanoes were half drowned in the fiery sea. this suggestion becomes even stronger when we turn to another of the photographs of mr ritchey's wonderful series, showing a part of the mare tranquilitatis (``sea of tranquility''!). notice how near the center of the picture the outline of a huge ring with radiating ridges shows through the sea bottom; a fossil volcano submerged in a petrified ocean! this is by no means the only instance in which a buried world shows itself under the great lunar plains. yet, as the newer craters in the sea itself prove, the volcanic activity survived this other catastrophe, or broke out again subsequently, bringing more ruin to pile upon ruin. yet notwithstanding the evidence which we have just been considering in support of the hypothesis that the ``seas'' are lava floods, messrs. loewy and puiseux, the selenographers of the paris observatory, are convinced that these great plains bear characteristic marks of the former presence of immense bodies of water. in that case we should be forced to conclude that the later oceans of the moon lay upon vast sheets of solidified lava; and thus the catastrophe of the lunar world assumes a double aspect, the earliest oceans being swallowed up in molten floods issuing from the interior, while the lands were reduced to chaos by a universal eruption of tremendous volcanoes; and then a period of comparative quiet followed, during which new seas were formed, and new life perhaps began to flourish in the lunar world, only to end in another cataclysm, which finally put a term to the existence of the moon as a life-supporting world. suppose we examine two more of mr ritchey's illuminating photographs, and, first, the one showing the crater theophilus and its surroundings. we have spoken of theophilus before, citing the facts that it is sixty-four miles in diameter and eighteen thousand feet deep. it will be noticed that it has two brother giants -- cyrillus the nearer, and catharina the more distant; but theophilus is plainly the youngest of the trio. centuries, and perhaps thousands of years, must have elapsed between the periods of their upheaval, for the two older craters are partly filled with débris, while it is manifest at a glance that when the south eastern wall of theophilus was formed, it broke away and destroyed a part of the more ancient ring of cyrillus. there is no more tremendous scene on the moon than this; viewed with a powerful telescope, it is absolutely appalling. the next photograph shows, if possible, a still wilder region. it is the part of the moon lying between tycho and the south pole. tycho is seen in the lower left-hand part of the picture. to the right, at the edge of the illuminated portion of the moon, are the crater-rings, longomontanus and wilhelm i, the former being the larger. between them are to be seen the ruins of two or three more ancient craters which, together with portions of the walls of wilhelm i and longomontanus, have been honeycombed with smaller craters. the vast crateriform depression above the center of the picture is clavius, an unrivaled wonder of lunar scenery, a hundred and forty-two miles in its greatest length, while its whole immense floor has sunk two miles below the general surface of the moon outside the ring. the monstrous shadow-filled cavity above clavius toward the right is blancanus, whose aspect here gives a good idea of the appearance of these chasms when only their rims are in the sunlight. but observe the indescribable savagery of the entire scene. it looks as though the spirit of destruction had gone mad in this spot. the mighty craters have broken forth one after another, each rending its predecessor; and when their work was finished, a minor but yet tremendous outbreak occurred, and the face of the moon was gored and punctured with thousands of smaller craters. these relatively small craters (small, however, only in a lunar sense, for many of them would appear gigantic on the earth) recall once more the theory of meteoric impact. it does not seem impossible that some of them may have been formed by such an agency. one would not wish for our planet such a fate as that which has overtaken the moon, but we cannot be absolutely sure that something of the kind may not be in store for it. we really know nothing of the ultimate causes of volcanic activity, and some have suggested that the internal energies of the earth may be accumulating instead of dying out, and may never yet have exhibited their utmost destructive power. perhaps the best assurance that we can find that the earth will escape the catastrophe that has overtaken its satellite is to be found in the relatively great force of its gravitation. the moon has been the victim of its weakness; given equal forces, and the earth would be the better able to withstand them. it is significant, in connection with these considerations, that the little planet mercury, which seems also to have parted with its air and water, shows to the telescope some indications that it is pitted with craters resembling those that have torn to pieces the face of the moon. upon the whole, after studying the dreadful lunar landscapes, one cannot feel a very enthusiastic sympathy with those who are seeking indications of the continued existence of some kind of life on the moon; such a world is better without inhabitants. it has met its fate; let it go! fortunately, it is not so near that it cannot hide its scars and appear beautiful -- except when curiosity impels us to look with the penetrating eyes of the astronomer. the great mars problem let any thoughtful person who is acquainted with the general facts of astronomy look up at the heavens some night when they appear in their greatest splendor, and ask himself what is the strongest impression that they make upon his mind. he may not find it easy to frame an answer, but when he has succeeded it will probably be to the effect that the stars give him an impression of the universality of intelligence; they make him feel, as the sun and the moon cannot do, that his world is not alone; that all this was not made simply to form a gorgeous canopy over the tents of men. if he is of a devout turn of mind, he thinks, as he gazes into those fathomless deeps and among those bewildering hosts, of the infinite multitude of created beings that the almighty has taken under his care. the narrow ideas of the old geocentric theology, which made the earth god's especial footstool, and man his only rational creature, fall away from him like a veil that had obscured his vision; they are impossible in the presence of what he sees above. thus the natural tendency, in the light of modern progress, is to regard the universe as everywhere filled with life. but science, which is responsible for this broadening of men's thoughts concerning the universality of life, itself proceeds to set limits. of spiritual existences it pretends to know nothing, but as to physical beings, it declares that it can only entertain the supposition of their existence where it finds evidence of an environment suited to their needs, and such environment may not everywhere exist. science, though repelled by the antiquated theological conception of the supreme isolation of man among created beings, regards with complacency the probability that there are regions in the universe where no organic life exists, stars which shine upon no inhabited worlds, and planets which nourish no animate creatures. the astronomical view of the universe is that it consists of matter in every stage of evolution: some nebulous and chaotic; some just condensing into stars (suns) of every magnitude and order; some shaped into finished solar bodies surrounded by dependent planets; some forming stars that perhaps have no planets, and will have none; some constituting suns that are already aging, and will soon lose their radiant energy and disappear; and some aggregated into masses that long ago became inert, cold, and rayless, and that can only be revivified by means about which we can form conjectures, but of which we actually know nothing. as with the stars, so with the planets, which are the satellites of stars. all investigations unite to tell us that the planets are not all in the same state of development. as some are large and some small, so some are, in an evolutionary sense, young, and some old. as they depend upon the suns around which they revolve for their light, heat, and other forms of radiant energy, so their condition varies with their distance from those suns. many may never arrive at a state suitable for the maintenance of life upon their surfaces; some which are not at present in such a state may attain it later; and the forms of life themselves may vary with the peculiar environment that different planets afford. thus we see that we are not scientifically justified in affirming that life is ubiquitous, although we are thus justified in saying that it must be, in a general sense, universal. we might liken the universe to a garden known to contain every variety of plant. if on entering it we see no flowers, we examine the species before us and find that they are not of those which bloom at this particular season, or perhaps they are such as never bear flowers. yet we feel no doubt that we shall find flowers somewhere in the garden, because there are species which bloom at this season, and the garden contains all varieties. while it is tacitly assumed that there are planets revolving around other stars than the sun, it would be impossible for us to see them with any telescope yet invented, and no instrument now in the possession of astronomers could assure us of their existence; so the only planetary system of which we have visual knowledge is our own. excluding the asteroids, which could not from any point of view be considered as habitable, we have in the solar system eight planets of various sizes and situated at various distances from the sun. of these eight we know that one, the earth, is inhabited. the question, then, arises: are there any of the others which are inhabited or habitable? since it is our intention to discuss the habitability of only one of the seven to which the question applies, the rest may be dismissed in a few words. the smallest of them, and the nearest to the sun, is mercury, which is regarded as uninhabitable because it has no perceptible supply of water and air, and because, owing to the extraordinary eccentricity of its orbit, it is subjected to excessive and very rapid alterations in the amount of solar heat and light poured upon its surface, such alterations being inconsistent with the supposition that it can support living beings. even its average temperature is more than six and a half times that prevailing on the earth! another circumstance which militates against its habitability is that, according to the results of the best telescopic studies, it always keeps the same face toward the sun, so that one half of the planet is perpetually exposed to the fierce solar rays, and the other half faces the unmitigated cold of open space. venus, the next in distance from the sun, is almost the exact twin of the earth in size, and many arguments may be urged in favor of its habitability, although it is suspected of possessing the same peculiarity as mercury, in always keeping the same side sunward. unfortunately its atmosphere appears to be so dense that no permanent markings on its surface are certainly visible, and the question of its actual condition must, for the present, be left in abeyance. mars, the first planet more distant from the sun than the earth, is the special subject of this chapter, and will be described and discussed a few lines further on. jupiter, saturn, uranus, and neptune, the four giant planets, all more distant than mars, and each more distant than the other in the order named, are all regarded as uninhabitable because none of them appears to possess any degree of solidity. they may have solid or liquid nuclei, but exteriorly they seem to be mere balls of cloud. of course, one can imagine what he pleases about the existence of creatures suited to the physical constitution of such planets as these, but they must be excluded from the category of habitable worlds in the ordinary sense of the term. we go back, then, to mars. it will be best to begin with a description of the planet. mars is miles in diameter; its surface is not much more than one-quarter as extensive as that of the earth (. ). its mean distance from the sun is , , miles, , , miles greater than that of the earth. since radiant energy varies inversely as the square of distance, mars receives less than half as much solar light and heat as the earth gets. mars' year (period of revolution round the sun) is days. its mean density is per cent of the earth's, and the force of gravity on its surface is per cent of that on the surface of the earth; i.e., a body weighing one hundred pounds on the earth would, if transported to mars, weigh but thirty-eight pounds. the inclination of its equator to the plane of its orbit differs very little from that of the earth's equator, and its axial rotation occupies hours minutes. so that the length of day and night, and the extent of the seasonal changes on mars, are almost precisely the same as on the earth. but owing to the greater length of its year, the seasons of mars, while occurring in the same order, are almost twice as long as ours. the surface of the planet is manifestly solid, like that of our globe, and the telescope reveals many permanent markings on it, recalling the appearance of a globe on which geographical features have been represented in reddish and dusky tints. around the poles are plainly to be seen rounded white areas, which vary in extent with the martian seasons, nearly vanishing in summer and extending widely in winter. the most recent spectroscopic determinations indicate that mars has an atmosphere perhaps as dense as that to be found on our loftiest mountain peaks, and there is a perceptible amount of watery vapor in this atmosphere. the surface of the planet appears to be remarkably level, and it has no mountain ranges. no evidences of volcanic action have been discovered on mars. the dusky and reddish areas were regarded by the early observers as respectively seas and lands, but at present it is not believed that there are any bodies of water on the planet. there has never been much doubt expressed that the white areas about the poles represent snow. it will be seen from this brief description that many remarkable resemblances exist between mars and the earth, and there is nothing wonderful in the fact that the question of the habitability of the former has become one of extreme and wide-spread interest, giving rise to the most diverse views, to many extraordinary speculations, and sometimes to regrettably heated controversy. the first champion of the habitability of mars was sir william herschel, although even before his time the idea had been suggested. he was convinced by the revelations of his telescopes, continually increasing in power, that mars was more like the earth than any other planet. he could not resist the testimony of the polar snows, whose suggestive conduct was in such striking accord with what occurs upon the earth. gradually, as telescopes improved and observers increased in number, the principal features of the planet were disclosed and charted, and ``areography,'' as the geography of mars was called, took its place among the recognized branches of astronomical study. but it was not before that a fundamentally new discovery in areography gave a truly sensational turn to speculation about life on ``the red planet.'' in that year mars made one of its nearest approaches to the earth, and was so situated in its orbit that it could be observed to great advantage from the northern hemisphere of the earth. the celebrated italian astronomer, schiaparelli, took advantage of this opportunity to make a trigonometrical survey of the surface of mars -- as coolly and confidently as if he were not taking his sights across a thirty-five-million-mile gulf of empty space -- and in the course of this survey he was astonished to perceive that the reddish areas, then called continents, were crossed in many directions by narrow, dusky lines, to which he gave the suggestive name of ``canals.'' thus a kind of firebrand was cast into the field of astronomical speculation, which has ever since produced disputes that have sometimes approached the violence of political faction. at first the accuracy of schiaparelli's observations was contested; it required a powerful telescope, and the most excellent ``seeing,'' to render the enigmatical lines visible at all, and many searchers were unable to detect them. but schiaparelli continued his studies in the serene sky of italy, and produced charts of the gridironed face of mars containing so much astonishing detail that one had either to reject them in toto or to confess that schiaparelli was right. as subsequent favorable oppositions of mars occurred, other observers began to see the ``canals'' and to confirm the substantial accuracy of the italian astronomer's work, and finally few were found who would venture to affirm that the ``canals'' did not exist, whatever their meaning might be. when schiaparelli began his observations it was generally believed, as we have said, that the dusky areas on mars were seas, and since schiaparelli thought that the ``canals'' invariably began and ended at the shores of the ``seas,'' the appropriateness of the title given to the lines seemed apparent. their artificial character was immediately assumed by many, because they were too straight and too suggestively geometrical in their arrangement to permit the conclusion that they were natural watercourses. a most surprising circumstance noted by schiaparelli was that the ``canals'' made their appearance after the melting of the polar snow in the corresponding hemisphere had begun, and that they grew darker, longer, and more numerous in proportion as the polar liquidation proceeded; another very puzzling observation was that many of them became double as the season advanced; close beside an already existing ``canal,'' and in perfect parallelism with it, another would gradually make its appearance. that these phenomena actually existed and were not illusions was proved by later observations, and today they are seen whenever mars is favorably situated for observation. in the closing decade of the nineteenth century, mr percival lowell took up the work where schiaparelli had virtually dropped it, and soon added a great number of ``canals'' to those previously known, so that in his charts the surface of the wonderful little planet appears covered as with a spider's web, the dusky lines criss-crossing in every direction, with conspicuous knots wherever a number of them come together. mr lowell has demonstrated that the areas originally called seas, and thus named on the earlier charts, are not bodies of water, whatever else they may be. he has also found that the mysterious lines do not, as schiaparelli supposed, begin and end at the edges of the dusky regions, but often continue on across them, reaching in some cases far up into the polar regions. but schiaparelli was right in his observation that the appearance of the ``canals'' is synchronous with the gradual disappearance of the polar snows, and this fact has become the basis of the most extraordinary theory that the subject of life in other worlds has ever given birth to. now, the effect of such discoveries, as we have related, depends upon the type of mind to whose attention they are called. many are content to accept them as strange and inexplicable at present, and to wait for further light upon them; others insist upon an immediate inquiry concerning their probable nature and meaning. such an inquiry can only be based upon inference proceeding from analogy. mars, say mr lowell and those who are of his opinion, is manifestly a solidly incrusted planet like the earth; it has an atmosphere, though one of great rarity; it has water vapor, as the snows in themselves prove; it has the alternation of day and night, and a succession of seasons closely resembling those of the earth; its surface is suggestively divided into regions of contrasting colors and appearance, and upon that surface we see an immense number of lines geometrically arranged, with a system of symmetrical intersections where the lines expand into circular and oval areas -- and all connected with the annual melting of the polar snows in a way which irresistibly suggests the interference of intelligence directed to a definite end. why, with so many concurrent circumstances to support the hypothesis, should we not regard mars as an inhabited globe? but the differences between mars and the earth are in many ways as striking as their resemblances. mars is relatively small; it gets less than half as much light and heat as we receive; its atmosphere is so rare that it would be distressing to us, even if we could survive in it at all; it has no lakes, rivers, or seas; its surface is an endless prairie. and its ``canals'' are phenomena utterly unlike anything on the earth. yet it is precisely upon these divergences between the earth and mars, this repudiation of terrestrial standards, that the theory of ``life on mars,'' for which mr lowell is mainly responsible, is based. because mars is smaller than the earth, we are told it must necessarily be more advanced in planetary evolution, the underlying cause of which is the gradual cooling and contraction of the planet's mass. mars has parted with its internal heat more rapidly than the earth; consequently its waters and its atmosphere have been mostly withdrawn by chemical combinations, but enough of both yet remain to render life still possible on its surface. as the globe of mars is evolutionally older than that of the earth, so its forms of organic life may be proportionally further advanced, and its inhabitants may have attained a degree of cultivated intelligence much superior to what at present exists upon the earth. understanding the nature and the causes of the desiccation of their planet, and possessing engineering science and capabilities far in advance of ours, they may be conceived to have grappled with the stupendous problem of keeping their world in a habitable condition as long as possible. supposing them to have become accustomed to live in their rarefied atmosphere (a thing not inconceivable, since men can live for a time at least in air hardly less rare), the most pressing problem for them is that of a water-supply, without which plant life cannot exist, while animal life in turn depends for its existence upon vegetation. the only direction in which they can seek water is that of the polar regions, where it is alternately condensed into snow and released in the liquid form by the effect of the seasonal changes. it is, then, to the annual melting of the polar snow-fields that the martian engineers are supposed to have recourse in supplying the needs of their planet, and thus providing the means of prolonging their own existence. it is imagined that they have for this purpose constructed a stupendous system of irrigation extending over the temperate and equatorial regions of the planet. the ``canals'' represent the lines of irrigation, but the narrow streaks that we see are not the canals themselves, but the irrigated bands covered by them. their dark hue, and their gradual appearance after the polar melting has begun, are due to the growth of vegetation stimulated by the water. the rounded areas visible where several ``canals'' meet and cross are called by mr lowell ``oases.'' these are supposed to be the principal centers of population and industry. it must be confessed that some of them, with their complicated systems of radiating lines, appear to answer very well to such a theory. no attempt to explain them by analogy with natural phenomena on the earth has proved successful. but a great difficulty yet remains: how to explain the seemingly miraculous powers of the supposed engineers? here recourse is had once more to the relative smallness of the planet. we have remarked that the force of gravity on mars is only thirty-eight per cent of that on the earth. a steam-shovel driven by a certain horse-power would be nearly three times as effective there as here. a man of our stature on mars would find his effective strength increased in the same proportion. but just because of the slight force of gravity there, a martian might attain to the traditional stature of goliath without finding his own weight an encumbrance to his activity, while at the same time his huge muscles would come into unimpeded play, enabling him single-handed to perform labors that would be impossible to a whole gang of terrestrial workmen. the effective powers of huge machines would be increased in the same way; and to all this must be added the fact that the mean density of the materials of which mars is composed is much less than that of the constituents of the earth. combining all these considerations, it becomes much less difficult to conceive that public works might be successfully undertaken on mars which would be hopelessly beyond the limits of human accomplishment. certain other difficulties have also to be met; as, for instance, the relative coldness of the climate of mars. at its distance it gets considerably less than half as much light and heat as we receive. in addition to this, the rarity of its atmosphere would naturally be expected to decrease the effective temperature at the planet's surface, since an atmosphere acts somewhat like the glass cover of a hot-house in retaining the solar heat which has penetrated it. it has been calculated that, unless there are mitigating circumstances of which we know nothing, the average temperature at the surface of mars must be far below the freezing-point of water. to this it is replied that the possible mitigating circumstances spoken of evidently exist in fact, because we can see that the watery vapor condenses into snow around the poles in winter, but melts again when summer comes. the mitigating agent may be supposed to exist in the atmosphere where the presence of certain gases would completely alter the temperature gradients. it might also be objected that it is inconceivable that the martian engineers, however great may be their physical powers, and however gigantic the mechanical energies under their control, could force water in large quantities from the poles to the equator. this is an achievement that measures up to the cosmical standard. it is admitted by the champions of the theory that the difficulty is a formidable one; but they call attention to the singular fact that on mars there can be found no chains of mountains, and it is even doubtful if ranges of hills exist there. the entire surface of the planet appears to be almost ``as smooth as a billiard ball,'' and even the broad regions which were once supposed to be seas apparently lie at practically the same level as the other parts, since the ``canals'' in many cases run uninterruptedly across them. lowell's idea is that these sombre areas may be expanses of vegetation covering ground of a more or less marshy character, for while the largest of them appear to be permanent, there are some which vary coincidently with the variations of the canals. as to the kind of machinery employed to force the water from the poles, it has been conjectured that it may have taken the form of a gigantic system of pumps and conduits; and since the martians are assumed to be so far in advance of us in their mastery of scientific principles, the hypothesis will at least not be harmed by supposing that they have learned to harness forces of nature whose very existence in a manageable form is yet unrecognized on the earth. if we wish to let the imagination loose, we may conjecture that they have conquered the secret of those intra-atomic forces whose resistless energy is beginning to become evident to us, but the possibility of whose utilization remains a dream, the fulfillment of which nobody dares to predict. such, in very brief form, is the celebrated theory of mars as an inhabited world. it certainly captivates the imagination, and if we believe it to represent the facts, we cannot but watch with the deepest sympathy this gallant struggle of an intellectual race to preserve its planet from the effects of advancing age and death. we may, indeed, wonder whether our own humanity, confronted by such a calamity, could be counted on to meet the emergency with equal stoutness of heart and inexhaustibleness of resource. up to the present time we certainly have shown no capacity to confront nature toe to toe, and to seize her by the shoulders and turn her round when she refuses to go our way. if we could get into wireless telephonic communication with the martians we might learn from their own lips the secret of their more than ``roman recovery.'' the riddle of the asteroids between the orbits of mars and jupiter revolves the most remarkable system of little bodies with which we are acquainted -- the asteroids, or minor planets. some six hundred are now known, and they may actually number thousands. they form virtually a ring about the sun. the most striking general fact about them is that they occupy the place in the sky which should be occupied, according to bode's law, by a single large planet. this fact, as we shall see, has led to the invention of one of the most extraordinary theories in astronomy -- viz., that of the explosion of a world! bode's law, so-called, is only an empiric formula, but until the discovery of neptune it accorded so well with the distances of the planets that astronomers were disposed to look upon it as really representing some underlying principle of planetary distribution. they were puzzled by the absence of a planet in the space between mars and jupiter, where the ``law'' demanded that there should be one, and an association of astronomers was formed to search for it. there was a decided sensation when, in , piazzi, of palermo, announced that he had found a little planet which apparently occupied the place in the system which belonged to the missing body. he named it ceres, and it was the first of the asteroids. the next year olbers, of bremen, while looking for ceres with his telescope, stumbled upon another small planet which he named pallas. immediately he was inspired with the idea that these two planets were fragments of a larger one which had formerly occupied the vacant place in the planetary ranks, and he predicted that others would be found by searching in the neighborhood of the intersection of the orbits of the two already discovered. this bold prediction was brilliantly fulfilled by the finding of two more -- juno in , and vesta in . olbers would seem to have been led to the invention of his hypothesis of a planetary explosion by the faith which astronomers at that time had in bode's law. they appear to have thought that several planets revolving in the gap where the ``law'' called for but one could only be accounted for upon the theory that the original one had been broken up to form the several. gravitation demanded that the remnants of a planet blown to pieces, no matter how their orbits might otherwise differ, should all return at stated periods to the point where the explosion had occurred; hence olbers' prediction that any asteroids that might subsequently be discovered would be found to have a common point of orbital intersection. and curiously enough all of the first asteroids found practically answered to this requirement. olbers' theory seemed to be established. after the first four, no more asteroids were found until , when one was discovered; then, in , three more were added to the list; and after that searchers began to pick them up with such rapidity that by the close of the century hundreds were known, and it had become almost impossible to keep track of them. the first four are by far the largest members of the group, but their actual sizes remained unknown until less than twenty years ago. it was long supposed that vesta was the largest, because it shines more brightly than any of the others; but finally, in , barnard, with the lick telescope, definitely measured their diameters, and proved to everybody's surprise that ceres is really the chief, and vesta only the third in rank. his measures are as follows: ceres, miles; pallas, miles; vesta, miles; and juno, miles. they differ greatly in the reflective power of their surfaces, a fact of much significance in connection with the question of their origin. vesta is, surface for surface, rather more than three times as brilliant as ceres, whence the original mistake about its magnitude. nowadays new asteroids are found frequently by photography, but physically they are most insignificant bodies, their average diameter probably not exceeding twenty miles, and some are believed not to exceed ten. on a planet only ten miles in diameter, assuming the same mean density as the earth's, which is undoubtedly too much, the force of gravity would be so slight that an average man would not weigh more than three ounces, and could jump off into space whenever he liked. although the asteroids all revolve around the sun in the same direction as that pursued by the major planets, their orbits are inclined at a great variety of angles to the general plane of the planetary system, and some of them are very eccentric -- almost as much so as the orbits of many of the periodic comets. it has even been conjectured that the two tiny moons of mars and the four smaller satellites of jupiter may be asteroids gone astray and captured by those planets. two of the asteroids are exceedingly remarkable for the shapes and positions of their orbits; these are eros, discovered in , and t. g., , found eight years later. the latter has a mean distance from the sun slightly greater than that of jupiter, while the mean distance of eros is less than that of mars. the orbit of eros is so eccentric that at times it approaches within , , miles of the earth, nearer than any other regular member of the solar system except the moon, thus affording an unrivaled means of measuring the solar parallax. but for our present purpose the chief interest of eros lies in its extraordinary changes of light. these changes, although irregular, have been observed and photographed many times, and there seems to be no doubt of their reality. their significance consists in their possible connection with the form of the little planet, whose diameter is generally estimated at not more than twenty miles. von oppolzer found, in , that eros lost three-fourths of its brilliancy once in every two hours and thirty-eight minutes. other observers have found slightly different periods of variability, but none as long as three hours. the most interesting interpretation that has been offered of this phenomenon is that it is due to a great irregularity of figure, recalling at once olbers' hypothesis. according to some, eros may be double, the two bodies composing it revolving around each other at very close quarters; but a more striking, and it may be said probable, suggestion is that eros has a form not unlike that of a dumb-bell, or hour-glass, turning rapidly end over end so that the area of illuminated surface presented to our eyes continually changes, reaching at certain times a minimum when the amount of light that it reflects toward the earth is reduced to a quarter of its maximum value. various other bizarre shapes have been ascribed to eros, such, for instance, as that of a flat stone revolving about one of its longer axes, so that sometimes we see its face and sometimes its edge. all of these explanations proceed upon the assumption that eros cannot have a simple globular figure like that of a typical planet, a figure which is prescribed by the law of gravitation, but that its shape is what may be called accidental; in a word, it is a fragment, for it seems impossible to believe that a body formed in interplanetary space, either through nebular condensation or through the aggregation of particles drawn together by their mutual attractions, should not be practically spherical in shape. nor is eros the only asteroid that gives evidence by variations of brilliancy that there is something abnormal in its constitution; several others present the same phenomenon in varying degrees. even vesta was regarded by olbers as sufficiently variable in its light to warrant the conclusion that it was an angular mass instead of a globe. some of the smaller ones show very notable variations, and all in short periods, of three or four hours, suggesting that in turning about one of their axes they present a surface of variable extent toward the sun and the earth. the theory which some have preferred -- that the variability of light is due to the differences of reflective power on different parts of the surface -- would, if accepted, be hardly less suggestive of the origin of these little bodies by the breaking up of a larger one, because the most natural explanation of such differences would seem to be that they arose from variations in the roughness or smoothness of the reflecting surface, which would be characteristic of fragmentary bodies. in the case of a large planet alternating expanses of land and water, or of vegetation and desert, would produce a notable variation in the amount of reflection, but on bodies of the size of the asteroids neither water nor vegetation could exist, and an atmosphere would be equally impossible. one of the strongest objections to olbers' hypothesis is that only a few of the first asteroids discovered travel in orbits which measurably satisfy the requirement that they should all intersect at the point where the explosion occurred. to this it was at first replied that the perturbations of the asteroidal orbits, by the attractions of the major planets, would soon displace them in such a manner that they would cease to intersect. one of the first investigations undertaken by the late prof. simon newcomb was directed to the solution of this question, and he arrived at the conclusion that the planetary perturbations could not explain the actual situation of the asteroidal orbits. but afterward it was pointed out that the difficulty could be avoided by supposing that not one but a series of explosions had produced the asteroids as they now are. after the primary disruption the fragments themselves, according to this suggestion, may have exploded, and then the resulting orbits would be as ``tangled'' as the heart could wish. this has so far rehabilitated the explosion theory that it has never been entirely abandoned, and the evidence which we have just cited of the probably abnormal shapes of eros and other asteroids has lately given it renewed life. it is a subject that needs a thorough rediscussion. we must not fail to mention, however, that there is a rival hypothesis which commends itself to many astronomers -- viz., that the asteroids were formed out of a relatively scant ring of matter, situated between mars and jupiter and resembling in composition the immensely more massive rings from which, according to laplace's hypothesis, the planets were born. it is held by the supporters of this theory that the attraction of the giant jupiter was sufficient to prevent the small, nebulous ring that gave birth to the asteroids from condensing like the others into a single planet. but if we accept the explosion theory, with its corollary that minor explosions followed the principal one, we have still an unanswered question before us: what caused the explosions? the idea of a world blowing up is too titanic to be shocking; it rather amuses the imagination than seriously impresses it; in a word, it seems essentially chimerical. we can by no appeal to experience form a mental picture of such an occurrence. even the moon did not blow up when it was wrecked by volcanoes. the explosive nebulæ and new stars are far away in space, and suggest no connection with such a catastrophe as the bursting of a planet into hundreds of pieces. we cannot conceive of a great globe thousands of miles in diameter resembling a pellet of gunpowder only awaiting the touch of a match to cause its sudden disruption. somehow the thought of human agency obtrudes itself in connection with the word ``explosion,'' and we smile at the idea that giant powder or nitro-glycerine could blow up a planet. yet it would only need enough of them to do it. after all, we may deceive ourselves in thinking, as we are apt to do, that explosive energies lock themselves up only in small masses of matter. there are many causes producing explosions in nature, every volcanic eruption manifests the activity of some of them. think of the giant power of confined steam; if enough steam could be suddenly generated in the center of the earth by a downpour of all the waters of the oceans, what might not the consequences be for our globe? in a smaller globe, and it has never been estimated that the original asteroid was even as large as the moon, such a catastrophe would, perhaps, be more easily conceivable; but since we are compelled in this case to assume that there was a series of successive explosions, steam would hardly answer the purpose; it would be more reasonable to suppose that the cause of the explosion was some kind of chemical reaction, or something affecting the atoms composing the exploding body. here dr gustav le bon comes to our aid with a most startling suggestion, based on his theory of the dissipation of intra-atomic energy. it will be best to quote him at some length from his book on the evolution of forces. ``it does not seem at first sight,'' says doctor le bon, very comprehensible that worlds which appear more and more stable as they cool could become so unstable as to afterward dissociate entirely. to explain this phenomenon, we will inquire whether astronomical observations do not allow us to witness this dissociation. we know that the stability of a body in motion, such as a top or a bicycle, ceases to be possible when its velocity of rotation descends below a certain limit. once this limit is reached it loses its stability and falls to the ground. prof. j. j. thomson even interprets radio-activity in this manner, and points out that when the speed of the elements composing the atoms descends below a certain limit they become unstable and tend to lose their equilibria. there would result from this a commencement of dissociation, with diminution of their potential energy and a corresponding increase of their kinetic energy sufficient to launch into space the products of intra-atomic disintegration. it must not be forgotten that the atom being an enormous reservoir of energy is by this very fact comparable with explosive bodies. these last remain inert so long as their internal equilibria are undisturbed. so soon as some cause or other modifies these, they explode and smash everything around them after being themselves broken to pieces. atoms, therefore, which grow old in consequence of the diminution of a part of their intra-atomic energy gradually lose their stability. a moment, then, arrives when this stability is so weak that the matter disappears by a sort of explosion more or less rapid. the bodies of the radium group offer an image of this phenomenon -- a rather faint image, however, because the atoms of this body have only reached a period of instability when the dissociation is rather slow. it probably precedes another and more rapid period of dissociation capable of producing their final explosion. bodies such as radium, thorium, etc., represent, no doubt, a state of old age at which all bodies must some day arrive, and which they already begin to manifest in our universe, since all matter is slightly radio-active. it would suffice for the dissociation to be fairly general and fairly rapid for an explosion to occur in a world where it was manifested. these theoretical considerations find a solid support in the sudden appearances and disappearances of stars. the explosions of a world which produce them reveal to us, perhaps, how the universes perish when they become old. as astronomical observations show the relative frequency of these rapid destructions, we may ask ourselves whether the end of a universe by a sudden explosion after a long period of old age does not represent its most general ending. here, perhaps, it will be well to stop, since, entrancing as the subject may be, we know very little about it, and doctor le bon's theory affords a limitless field for the reader's imagination. _________________________________________________________________ a printed version of this book is available from sattre press (http://csky.sattre-press.com). it includes extensive annotations, a new introduction and all the original photographs and diagrams. [illustration: photo, mt. wilson solar observatory _an active prominence of the sun, , miles high, photographed july , ._] astronomy the science of the heavenly bodies by david todd director emeritus, amherst college observatory [illustration: harper & brothers logo] new york and london harper & brothers publishers mcmxxii copyright by p. f. collier & son company sir william rowan hamilton, the eminent mathematician of dublin, has, of all writers ancient and modern, most fittingly characterized the ideal science of astronomy as man's golden chain connecting the heavens to the earth, by which we "learn the language and interpret the oracles of the universe." the oldest of the sciences, astronomy is also the broadest in its relations to human knowledge and the interests of mankind. many are the cognate sciences upon which the noble structure of astronomy has been erected: foremost of all, geometry and the higher mathematics, which tell us of motions, magnitudes and distances; physics and chemistry, of the origin, nature, and destinies of planets, sun, and star; meteorology, of the circulation of their atmospheres; geology, of the structure of the moon's surface; mineralogy, of the constitution of meteorites; while, if we attack, even elementally, the fascinating, though perhaps forever unsolvable, problem of life in other worlds, the astronomer must invoke all the resources that his fellow biologists and their many-sided science can afford him. the progress of astronomy from age to age has been far from uniform--rather by leaps and bounds: from the earliest epoch when man's planet earth was the center about which the stupendous cosmos wheeled, for whom it was created, and for whose edification it was maintained--down to the modern age whose discoveries have ascertained that even our stellar universe, the vast region of the solar domain, is but one of the thousands of island universes that tenant the inconceivable immensities of space. such results have been attainable only through the successful construction and operation of monster telescopes that bring to the eye and visualize on photographic plates the faintest of celestial objects which were the despair of astronomers only a few years ago. but the end is not yet; astronomy to-day is but passing from infancy to youth. and with new and greater telescopes, with new photographic processes of higher sensitivity, with the help of modern invention in overcoming the obstacle of the air--that constant foe of the astronomer--who will presume to set down any limit to the leaps and bounds of astronomy in the future? so rapid, indeed, has been the progress of astronomy in very recent years that the present is especially favorable for setting forth its salient features; and this book is an attempt to present the wide range of astronomy in readable fashion, as if a story with a definite plot, from its origin with the shepherds of ancient chaldea down to present-day ascertainment of the actual scale of the universe, and definite measures of the huge volume of supersolar giants among the stars. david todd amherst college observatory november, contents chapter page i. astronomy a living science ii. the first astronomers iii. pyramid, tomb, and temple iv. origin of greek astronomy v. measuring the earth--eratosthenes vi. ptolemy and his great book vii. astronomy of the middle ages viii. copernicus and the new era ix. tycho, the great observer x. kepler, the great calculator xi. galileo, the great experimenter xii. after the great masters xiii. newton and motion xiv. newton and gravitation xv. after newton xvi. halley and his comet xvii. bradley and aberration xviii. the telescope xix. reflectors--mirror telescopes xx. the story of the spectroscope xxi. the story of astronomical photography xxii. mountain observatories xxiii. the program of a great observatory xxiv. our solar system xxv. the sun and observing it xxvi. sun spots and prominences xxvii. the inner planets xxviii. the moon and her surface xxix. eclipses of the moon xxx. total eclipses of the sun xxxi. the solar corona xxxii. the ruddy planet xxxiii. the canals of mars xxxiv. life in other worlds xxxv. the little planets xxxvi. the giant planet xxxvii. the ringed planet xxxviii. the farthest planets xxxix. the trans-neptunian planet xl. comets--the hairy stars xli. where do comets come from? xlii. meteors and shooting stars xliii. meteorites xliv. the universe of stars xlv. star charts and catalogues xlvi. the sun's motion toward lyra xlvii. stars and their spectral type xlviii. star distances xlix. the nearest stars l. actual dimensions of the stars li. the variable stars lii. the novÆ, or new stars liii. the double stars liv. the star clusters lv. moving clusters lvi. the two star streams lvii. the galaxy or milky way lviii. star clouds and nebulÆ lix. the spiral nebulÆ lx. cosmogony lxi. cosmogony in transition list of illustrations active prominence of the sun, , miles high _frontispiece_ facing page nicholas copernicus galileo galilei johann kepler sir isaac newton the hundred-inch reflecting telescope at mount wilson the forty-inch refracting telescope, yerkes observatory -foot tower, mount wilson, a diagram of tower and pit -foot tower--exterior view view looking down into the pit beneath -foot tower mount wilson solar observatory--the -foot dome mount chimborazo, the best site in the world for an observatory lick observatory, mount hamilton, california photographing with the -inch refractor great sunspot group of august , calcium flocculi on the sun eclipse of the moon, with the lunar surface visible moon's surface in the region of copernicus south central portion of the moon, at last quarter corona of the sun during an eclipse venus, in the crescent phase mars, showing bright polar cap jupiter, the giant planet neptune and its satellites saturn, with edge of rings only in view saturn, with rings displayed to fullest extent two views of halley's comet swift's comet, which showed remarkable transformations meteor trail in field with fine nebulÆ ring nebula in lyra dumb-bell nebula star clouds and black holes in sagittarius great nebula in andromeda chapter i astronomy a living science like life itself we do not know when astronomy began; we cannot conceive a time when it was not. man of the early stone age must have begun to observe sun, moon, and stars, because all the bodies of the cosmos were there, then as now. with his intellectual birth astronomy was born. onward through the childhood of the race he began to think on the things he observed, to make crude records of times and seasons; the chaldeans and chinese began each their own system of astronomy, the causes of things and the reasons underlying phenomena began to attract attention, and astronomy was cultivated not for its own sake, but because of its practical utility in supplying the data necessary to accurate astrological prediction. belief in astrology was universal. the earth set in the midst of the wonders of the sky was the reason for it all. clearly the earth was created for humanity; so, too, the heavens were created for the edification of the race. all was subservient to man; naturally all was geocentric, or earth-centered. from the savage who could count only to five, the digits of one hand, civilized man very slowly began to evolve; he noted the progress of the seasons; the old records of eclipses showed thales, an early greek, how to predict their happenings, and true science had its birth when man acquired the power to make forecasts that always came true. few ancient philosophers were greater than pythagoras, and his conceptions of the order of the heavens and the shape and motion of the earth were so near the truth that we sometimes wonder how they could have been rejected for twenty centuries. we must remember, however, that man had not yet learned the art of measuring things, and the world could not be brought into subjection to him until he had. to measure he must have tools--instruments; to have instruments he must learn the art of working in metals, and all this took time; it was a slow and in large part imperceptible process; it is not yet finished. the earliest really sturdy manifestation of astronomical life came with the birth of greek science, culminating with aristarchus, hipparchus and ptolemy. the last of these great philosophers, realizing that only the art of writing prevents man's knowledge from perishing with him, set down all the astronomical knowledge of that day in one of the three greatest books on astronomy ever written, the almagest, a name for it derived through the arabic, and really meaning "the greatest." the system of earth and heaven seemed as if finished, and the authority of ptolemy and his almagest were as holy writ for the unfortunate centuries that followed him. with fatal persistence the fundamental error of his system delayed the evolutionary life of the science through all that period. but man had begun to measure. geometry had been born and eratosthenes had indeed measured the size of the earth. tools in bronze and iron were fashioned closely after the models of tools of stone; astrolabes and armillary spheres were first built on geometric spheres and circles; and science was then laid away for the slumber of the dark ages. nevertheless, through all this dreary period the life of the youthful astronomical giant was maintained. time went on, the heavens revolved; sun, moon, and stars kept their appointed places, and arab and moor and the savage monarchs of the east were there to observe and record, even if the world-mind was lying fallow, and no genius had been born to inspire anew that direction of human intellect on which the later growth of science and civilization depends. with the growth of the collective mind of mankind, from generation to generation, we note that ordered sequence of events which characterizes the development of astronomy from earliest peoples down to the age of newton, herschel, and the present. it is the unfolding of a story as if with a definite plot from the beginning. leaving to philosophical writers the great fundamental reason underlying the intellectual lethargy of the dark ages, we only note that astronomy and its development suffered with every other department of human activity that concerned the intellectual progress of the race. to knowledge of every sort the medieval spirit was hostile. but with the founding and growth of universities, a new era began. the time was ripe for copernicus and a new system of the heavens. the discovery of the new world and the revival of learning through the universities added that stimulus and inspiration which marked the transition from the middle ages to our modern era, and the life of astronomy, long dormant, was quickened to an extraordinary development. it fell to the lot of copernicus to write the second great book on astronomy, "de revolutionibus orbium coelestium." but the new heliocentric or sun-centered system of copernicus, while it was the true system bidding fair to replace the false, could not be firmly established except on the basis of accurate observation. how fortunate was the occurrence of the new star of , that turned the keen intellect of tycho brahe toward the heavens! without the observational labors of tycho's lifetime, what would the mathematical genius of kepler have availed in discovery of his laws of motion of the planets? historians dwell on the destruction and violent conflicts of certain centuries of the middle ages, quite overlooking the constructive work in progress through the entire era. much of this was of a nature absolutely essential to the new life that was to manifest itself in astronomy. the arabs had made important improvements in mathematical processes, european artisans had made great advances in the manufacture of glass and in the tools for working in metals. then came galileo with his telescope revealing anew the universe to mankind. it was the north of italy where the renaissance was most potent, recalling the vigorous life of ancient greece. copernicus had studied here; it was the home of galileo. columbus was a genoese, and the compass which guided him to the western world was a product of deft italian artisans whose skill with that of their successors was now available to construct the instruments necessary for further progress in the accurate science of astronomical observation. even before copernicus, johann müller, better known as regiomontanus, had imbibed the learning of the greeks while studying in italy, and founded an observatory and issued nautical almanacs from nuremberg, the basis of those by which columbus was guided over untraversed seas. about this time, too, the art of printing was invented, and the interrelation of all the movements then in progress led up to a general awakening of the mind of man, and eventually an outburst in science and learning, which has continued to the present day. naturally it put new life into astronomy, and led directly up from galileo and his experimental philosophy to newton and the _principia_, the third in the trinity of great astronomical books of all time. to get to the bottom of things, one must study intimately the history of the intellectual development of europe through the fifteenth and sixteenth centuries. many of the western countries were ruled by sovereigns of extraordinary vigor and force of character, and their activities tended strongly toward that firm basis on which the foundations of modern civilization were securely laid. contemporaneously with this era, and following on through the seventeenth century, came the measurements of the earth by french geodesists, the construction of greater and greater telescopes and the wonderful discoveries with them by huygens, cassini, and many others. most important of all was the application of telescopes to the instruments with which angles are measured. then for the first time man had begun to find out that by accurate measures of the heavenly bodies, their places among the stars, their sizes and distances, he could attain to complete knowledge of them and so conquer the universe. but he soon realized the insufficiency of the mathematical tools with which he worked--how unsuited they were to the solution of the problem of three bodies (sun, earth, and moon) under the newtonian law of gravitation, let alone the problem of n-bodies, mutually attracting each the other; and every one perturbing the motion of every other one. so the invention of new mathematical tools was prosecuted by newton and his rival leibnitz, who, by the way, showed himself as great a man as mathematician: "taking mathematics," wrote leibnitz, "from the beginning of the world to the times when newton lived, what he had done was much the better half." newton was the greatest of astronomers who, since the revival of learning, had observed the motions of the heavenly bodies and sought to find out why they moved. copernicus, tycho brahe, galileo, kepler, newton, all are bound together as in a plot. not one of them can be dissociated from the greatest of all discoveries. but newton, the greatest of them all, revealed his greatness even more by saying: "if i have seen further than other men, it is because i have been standing on the shoulders of giants." elsewhere he says: "all this was in the two plague years of and [he was then but twenty-four], for in those days i was in the prime of my age for invention, and minded mathematics and philosophy more than at any time since." all school children know these as the years of the plague and the fire; but very few, in school or out, connect these years with two other far-reaching events in the world's history, the invention of the infinitesimal calculus and the discovery of the law of gravitation. we have passed over the name of descartes, almost contemporary with galileo, the founder of modern dynamics, but his initiation of one of the greatest improvements of mathematical method cannot be overlooked. this era was the beginning of the golden age of mathematics that embraced the lives of the versatile euler, equally at home in dynamics and optics and the lunar theory; of la grange, author of the elegant "mécanique analytique"; and la place, of the unparalleled "mécanique céleste." with them and a fully elaborated calculus newton's universal law had been extended to all the motions of the cosmos. even the tides and precession of the equinoxes and bradley's nutation were accounted for and explained. mathematical or gravitational astronomy had attained its pinnacle--it seemed to be a finished science: all who were to come after must be but followers. the culmination of one great period, however, proved to be but the inception of another epoch in the development of the living science. the greatest observer of all time, with a telescope built by his own hands, had discovered a great planet far beyond the then confines of the solar system. mathematicians would take care of uranus, and herschel was left free to build bigger telescopes still, and study the construction of the stellar universe. down to his day astronomy had dealt almost wholly with the positions and motions of the celestial bodies--astronomy was a science of _where_. to inquire _what_ the heavenly bodies _are_, seemed to herschel worthy of his keenest attention also. while "a knowledge of the construction of the heavens has always been the ultimate object of my observations," as he said, and his ingenious method of star-gauging was the first practicable attempt to investigate the construction of the sidereal universe, he nevertheless devoted much time to the description of nebulæ and their nature, as well as their distribution in space. he was the founder of double-star astronomy, and his researches on the light of the stars by the simple method of sequences were the inception of the vast fields of stellar photometry and variable stars. the physics of the sun, also, was by no means neglected; and his lifework earned for him the title of father of descriptive astronomy. while progress and discovery in the earlier fields of astronomy were going on, the initial discoveries in the vast group of small planets were made at the beginning of the nineteenth century. the great bessel added new life to the science by revolutionizing the methods and instruments of accurate observation, his work culminating in the measure of the distance of cygni, first of all the stars whose distance from the sun became known. wonderful as was this achievement, however, a greater marvel still was announced just before the middle of the century--a new planet far beyond uranus, whose discovery was made as a direct result of mathematical researches by adams and le verrier, and affording an extraordinary verification of the great newtonian law. these were the days of great discoveries, and about this time the giant of all the astronomical tools of the century was erected by lord rosse, the "leviathan" reflector with a speculum six feet in diameter, which remained for more than half a century the greatest telescope in the world, and whose epochal discovery of spiral nebulæ has greater significance than we yet know or perhaps even surmise. the living science was now at the height of a vigorous development, when a revolutionary discovery was announced by kirchhoff which had been hanging fire nearly half a century--the half century, too, which had witnessed the invention of photography, the steam engine, the railroad, and the telegraph: three simple laws by which the dark absorption lines of a spectrum are interpreted, and the physical and chemical constitution of sun and stars ascertained, no matter what their distance from us. huggins in england and secchi in italy were quick to apply the discovery to the stars, and draper and pickering by masterly organization have photographed and classified the spectra of many hundred thousand stars of both hemispheres, a research of the highest importance which has proved of unique service in studies of stellar movements and the structure of the universe by eddington and shapley, campbell and kapteyn, with many others who are still engaged in pushing our knowledge far beyond the former confines of the universe. few are the branches of astronomy that have not been modified by photography and the spectroscope. it has become a measuring tool of the first order of accuracy; measuring the speed of stars and nebulæ toward and from us; measuring the rotational speed of sun and planets, corona and saturnian ring; measuring the distances of whole classes of stars from the solar system; measuring afresh even the distance of the sun--the yardstick of our immediate universe; measuring the drift of the sun with his entire family of planets twelve miles every second in the direction of alpha lyræ; and discovering and measuring the speed of binary suns too close together for our telescopes, and so making real the astronomy of the invisible. impatient of the handicap of a turbulent atmosphere, the living science has sought out mountain tops and there erected telescopes vastly greater than the "leviathan" of a past century. there the sun in every detail of disk and spectrum is photographed by day, and stars with their spectra and the nebulæ by night. great streams of stars are discovered and the speed and direction of their drift ascertained. the marvels of the spiral nebulæ are unfolded, their multitudinous forms portrayed and deciphered. and their distances? and the distances of the still more wonderful clusters? far, inconceivably far beyond the milky way. and are they "island universes"? and can man, the measurer, measure the distance of the "mainland" beyond? chapter ii the first astronomers who were the first astronomers? and who wrote the first treatise on astronomy, oldest of the sciences? questions not easy to answer in our day. with the progress of archæological research, or inquiry into the civilization and monuments of early peoples, it becomes certain that man has lived on this planet earth for tens of thousands of years in the past as an intelligent, observing, intellectual being; and it is impossible to assign any time so remote that he did not observe and philosophize upon the firmament above. we can hardly imagine a people so primitive that they would fail to regard the sun as "lord of the day," and therefore all important in the scheme of things terrestrial. says anne bradstreet of the sun in her "contemplations": what glory's like to thee? soul of this world, this universe's eye, no wonder some made thee deity. to the babylonians belongs the credit of the oldest known work on astronomy. it was written nearly six thousand years ago, about b. c. , by their monarch sargon the first, king of agade. only the merest fragments of this historic treatise have survived, and they indicate the reverence of the babylonians for the sun. another work by sargon is entitled "omens," which shows the intimate relationship of astronomy to mysticism and superstitious worship at this early date, and which persists even at the present day. as remotely as b. c. , the sun-god shamash and his wife aya are carved upon the historic cylinders of hematite and lapis lazuli, and one of the oldest designs on these cylinders represents the sun-god coming out of the door of sunrise, while a porter is opening the gate of the east. the semitic religion had as its basis a reverence for the bodies of the sky; and samson, hebrew for sun, was probably the sun-god of the hebrews. the phoenician deity, baal, was a sun-god under differing designations; and at the epoch of the shepherd kings, about b. c. , during the hyksos dynasty, the sun-god was represented by a circle or disk with extended rays ending in hands, possibly the precursor of the frequently recurring egyptian design of the winged disk or winged solar globe. hittites, persians, and assyrians, as well as the phoenicians, frequently represented the sun-god in similar fashion in their sacred glyphs or carvings. for a long period in early human history, astronomy and astrology were pretty much the same. we can trace the history of astrology back as far as b. c. in ancient babylonia. the motions of the sun, moon, and the five lucid planets of that time indicated the activity of the various gods who influenced human affairs. so the babylonian priests devised an elaborate system of interpreting the phenomena of the heavens; and attaching the proper significance in human terms to everything that took place in the sky. in babylonia and assyria it was the king and his people for whom the prognostications were made out. it was the same in egypt. later, about the fifth century b. c., astrology spread through greece, where astrologers developed the idea of the influence of planets upon individual concerns. astrology persisted through the dark ages, and the great astronomers copernicus, tycho, kepler, gassendi, and huygens were all astrologers as well. milton makes many references to planetary influence, our language has many words with a direct origin in astrology, and in our great cities to-day are many astrologers who prepare individual horoscopes of more than ordinary interest. it is difficult to assign the antiquity of the chinese astronomy with any approach to definiteness. their earliest records appear to have been total eclipses of the sun, going back nearly , years before the christian era; and nearly a thousand years earlier the hindu astronomy sets down a conjunction of all the planets, concerning which, however, there is doubt whether it was actually observed or merely calculated backward. owing to a colossal misfortune, the burning of all native scientific books by order of the emperor tsin-chi-hwang-ti, in b. c. , excepting only the volumes relating to agriculture, medicine, and astrology, the chinese lost a precious mass of astronomical learning, accumulated through the ages. no less an authority than wells williams credits them with observing solar eclipses between b. c. and a. d. , and there must have been some centuries of eclipses observed and recorded anterior to b. c. , as this is the date assigned to the eclipse which came unheralded by the astronomers royal, hi and ho, who had become intoxicated and forgot to warn the court, in accord with their duty. china was thereby exposed to the anger of the gods, and hi and ho were executed by his majesty's command. it is doubtful if there is an earlier record of any celestial phenomenon. chapter iii pyramid, tomb, and temple inquiry into the beginnings of astronomy in ancient egypt reveals most interesting relations of the origins of the science to the life and work and worship of the people. their astronomers were called the "mystery teachers of heaven"; their monuments indicate a civilization more or less advanced; and their temples were built on astronomical principles and dedicated to purpose of worship. the egyptian records carry us back many thousands of years, and we find that in egypt, as in other early civilizations, observation of the heavenly bodies may be embraced in three pretty distinct stages. awe, fear, wonder and worship were the first. then came utility: a calendar was necessary to tell men when "to plow and sow, to reap and mow," and a calendar necessitated astronomical observations of some sort. following this, the third direction required observations of celestial positions and phenomena also, because astrology, in which the potentates of every ancient realm believed, could only thrive as it was based on astronomy. sun worship was preeminent in early egypt as in india, where the primal antithesis between night and day struck terror in the unformed mind of man. in one of the vedas occurs this significant song to the god of day: "will the sun rise again? will our old friend the dawn come back again? will the power of darkness be conquered by the god of light?" quite different from india, however, is egypt in matters of record: in india, records in papyrus, but no monuments of very great antiquity; in egypt, no papyrus, but monuments of exceeding antiquity in abundance. herodotus and pliny have told us of the great antiquity of these monuments, even in their own day, and research by archæologist and astronomer has made it certain that the pyramids were built by a race possessing great knowledge of astronomy. their temples, too, were constructed in strict relation to stars. not only are the temples, as edfu and denderah, of exceeding interest in themselves, but associated with them are often huge monoliths of syenite, obelisks of many hundred tons in weight, which the astronomer recognizes as having served as observation pillars or gnomons. specimens of these have wandered as far from home as central park and the bank of the thames. but there is an even more remarkable wealth of temple inscriptions, zodiacs especially. next to the sun himself was the worship of the dawn and sunrise, the great revelations of nature. there were numerous hymns to the still more numerous sun-gods and the powers of sunlight. ra was the sun-god in his noontide strength; osiris, the dying sun of sunset. only two gods were associated with the moon, and for the stars a special goddess, sesheta. sacrifices were made at day-break; and the stars that heralded the dawn were the subjects of careful observation by the sacrificial priests, who must therefore have possessed a good knowledge of star places and names, doubtless in belts of stars extending clear around the heavens. these decans, as they were called, are the exact counterparts of the moon stations devised by the arabians, indians, and other peoples for a like purpose. the plane or circle of observation, both in egypt and india, was always the horizon, whether the sun was observed or moon or stars. so the sun was often worshiped by the ancient egyptians as the "lord of the two horizons." it is sometimes difficult to keep in mind the fact, in regard to all temples of the ancients, whether in egypt or elsewhere, that in studying them we must deal with the risings or settings of the heavenly bodies in quite different fashion from that of the astronomer of to-day, who is mainly concerned only with observing them on the meridian. the axis of the temple shows by its direction the place of rising or setting: if the temple faces directly east or west, its amplitude is . now the sun, moon, and planets are, as everyone knows, very erratic as to their amplitudes (i. e., horizon points) of rising and setting; so it must have been the stars that engrossed the attention of the earliest builders of temples. after that, temples were directed to the rising sun, at the equinox or solstices. then came the necessity of finding out about the inclination or obliquity of the ecliptic, and this is where the gnomon was employed. at karnak are many temples of the solstitial order: the wonderful temple of amen-ra is so oriented that its axis stands in amplitude degrees north of west, which is the exact amplitude of the sun at thebes at sunset of the summer solstice. the axis of a lesser temple adjacent points to degrees south of east, which is the exact amplitude of sunrise at the winter solstice. at gizeh we find the temples oriented, not solstitially, but by the equinoxes, that is, they face due east and west. peoples who worshiped the sun at the solstice must have begun their year at the solstice; and sir norman lockyer shows how the rise of the nile, which took place at the summer solstice, dominated not only the industry but the astronomy and religion of egypt. looking into the question of temple orientation in other countries, as china, for example, lockyer finds that the most important temple of that country, the temple of the sun at peking, is oriented to the winter solstice; and stonehenge, as has long been known, is oriented to sunrise at the summer solstice. in like fashion the rising and setting of many stars were utilized by the egyptians, in both temple and pyramid; and no astronomer who has ever seen these ancient structures and studied their orientations can doubt that they were built by astronomers for use by astronomers of that day. the priests were the astronomers, and the temples had a deep religious significance, with a ceremony of exceeding magnificence wherever observations of heavenly bodies were undertaken, whether of sun or stars. hindu and persian astronomy must be passed over very briefly. interesting as their systems are historically, there were few, if any, original contributions of importance, and the indian treatises bear strong evidence of greek origin. chapter iv origin of greek astronomy while the greeks laid the foundations of modern scientific astronomy, they were not as a whole observers: rather philosophers, we should say. the later representatives of the greek school, however, saw the necessity of observation as a basis of true induction; and they discovered that real progress was not possible unless their speculative ideas were sufficiently developed and made definite by the aid of geometry, so that they became capable of detailed comparison with observation. this was the necessary and ultimate test with them, and the same is true to-day. the early greek philosophers were, however, mainly interested, not in observations, but in guessing the causes of phenomena. thales of miletus, founder of the ionian school, introduced the system of egyptian astronomy into greece, about the end of the seventh century b. c. he is universally known as the first astronomer who ever predicted a total eclipse of the sun that happened when he said it would: the eclipse of b. c. . this he did by means of the chaldean eclipse cycle of years known as the saros. aristarchus of samos was the first and most eminent of the alexandrian astronomers, and his treatise "on the magnitudes and distances of the sun and moon" is still extant. this method of ascertaining how many times farther the sun is than the moon is very simple, and geometrically exact. unfortunately it is impossible, even to-day, to observe with accuracy the precise time when the moon "quarters," (an observation essential to his method), because the moon's terminal, or line between day and night, is not a straight line as required by theory, but a jagged one. by his observation, the sun was only twenty times farther away than the moon, a distance which we know to be nearly twenty times too small. his views regarding other astronomical questions were right, although they found little favor among contemporaries. not only was the earth spherical, he said, but it rotated on its axis and also traveled round the sun. aristarchus was, indeed, the true originator of the modern doctrine of motions in the solar system, and not copernicus, seventeen centuries later; but seleucus appears to have been his only follower in these very advanced conceptions. aristarchus made out the apparent diameters of sun and moon as practically equal to one another, and inferred correctly that their real diameters are in proportion to their distances from the earth. also he estimated, from observations during an eclipse of the moon, that the moon's diameter is about one-third that of the earth. aristarchus appears to have been one of the clearest and most accurate thinkers among the ancient astronomers; even his views concerning the distances of the stars were in accord with the fact that they are immeasurably distant as compared with the distances of the sun, moon, and planets. practically contemporary with aristarchus were timocharis and aristillus, who were excellent observers, and left records of position of sun and planets which were exceedingly useful to their successors, hipparchus and ptolemy in particular. indeed their observations of star positions were such that, in a way, they deserve the fame of having made the first catalogue, rather than hipparchus, to whom is universally accorded that honor. spherical astronomy had its origin with the alexandrian school, many famous geometers, and in particular euclid, pointing the way. spherics, or the doctrine of the sphere, was the subject of numerous treatises, and the foundations were securely laid for that department of astronomical research which was absolutely essential to farther advance. the artisans of that day began to build rude mechanical adaptations of the geometric conceptions as concrete constructions in wood and metal, and it became the epoch of the origin of astrolabes and armillary spheres. chapter v measuring the earth--eratosthenes all told, the greek philosophers were probably the keenest minds that ever inhabited the planet, and we cannot suppose them so stupid as to reject the doctrine of a spherical earth. in fact so certain were they that the earth's true figure is a sphere that eratosthenes in the third century b. c. made the first measure of the dimensions of the terrestrial sphere by a method geometrically exact. at syene in upper egypt the sun at the summer solstice was known to pass through the zenith at noon, whereas at alexandria eratosthenes estimated its distance as seven degrees from the zenith at the same time. this difference being about one-fiftieth of the entire circumference of a meridian, eratosthenes correctly inferred that the distance between alexandria and syene must be one-fiftieth of the earth's circumference. so he measured the distance between the two and found it , stadia. this figured out the size of the earth with a percentage of error surprisingly small when we consider the rough means with which eratosthenes measured the sun's zenith distance and the distance between the two stations. greatest of all the greek astronomers and one of the greatest in the history of the science was hipparchus who had an observatory at rhodes in the middle of the second century b. c. his activities covered every department of astronomy; he made extensive series of observations which he diligently compared with those handed down to him by the earlier astronomers, especially aristillus and timocharis. this enabled him to ascertain the motion of the equinoxial points, and his value of the constant of precession of the equinoxes is exceedingly accurate for a first determination. in b. c. a new star blazed out in the constellation scorpio, and this set hipparchus at work on a catalogue of the brighter stars of the firmament, a monumental work of true scientific conception, because it would enable the astronomers of future generations to ascertain what changes, if any, were taking place in the stellar universe. there were , stars in his catalogue, and he referred their positions to the ecliptic and the equinoxes. also he originated the present system of stellar magnitudes or orders of brightness, and his catalogue was in use as a standard for many centuries. hipparchus was a great mathematician as well, and he devoted himself to the improvement of the method of applying numerical calculations to geometrical figures: trigonometry, both plane and spherical, that is; and by some authorities he is regarded as the inventor of original methods in trigonometry. the system of spheres of eudoxus did not satisfy him, so he devised a method of representing the paths of the heavenly bodies by perfectly uniform motion in circles. there is slight evidence that apollonius of perga may have been the originator of the system, but it was reserved for hipparchus to work it out in final form. this enabled him to ascertain the varying length of the seasons, and he fixed the true length of the year as - / days. he had almost equal success in dealing with the irregularities of the moon's motion, although the problem is much more complicated. the distance and size of the moon, by the method of aristarchus, were improved by him, and he worked out, for the distance of the sun, , radii of the earth--a classic for many centuries. hipparchus devoted much attention to eclipses of both sun and moon, and we owe to him the first elucidation of the subject of parallax, or the effect of difference of position of an observer on the earth's surface as affecting the apparent projection of the moon against the sun when a solar eclipse takes place; whereas an eclipse of the moon is unaffected by parallax and can be seen at the same time by observers everywhere, no matter what their location on the earth. indeed, with all that hipparchus achieved, we need not be surprised that astronomy was regarded as a finished science, and made practically no progress whatever for centuries after his time. then came claudius ptolemæus, generally known as ptolemy, the last great name in greek astronomy. he lived in alexandria about the middle of the second century a. d. and wrote many minor astronomical and astrological treatises, also works on geography and optics, in the last of which the atmospheric refraction of rays of light from the heavenly bodies, apparently elevating them toward the zenith, is first dealt with in true form. chapter vi ptolemy and his great book ptolemy was an observer of the heavens, though not of the highest order; but he had all the work of his predecessors, best of all hipparchus, to build upon. ptolemy's greatest work was the "megale syntaxis," generally known as the almagest. it forms a nearly complete compendium of the ancient astronomy, and although it embodies much error, because built on a wrong theory, the almagest nevertheless is competent to follow the motions of all the bodies in the sky with a close approach to accuracy, even at the present day. this marvelous work written at this critical epoch became as authoritative as the philosophy of aristotle, and for many centuries it was the last word in the science. the old astrology held full sway, and the ptolemaic theory of the universe supplied everything necessary: further progress, indeed, was deemed impossible. the almagest comprises in all thirteen books, the first two of which deal with the simpler observations of the celestial sphere, its own motion and the apparent motions of sun, moon, and planets upon it. he discusses, too, the postulates of his system and exhibits great skill as an original geometer and mathematician. in the third book he takes up the length of the year, and in the fourth book similarly the moon and the length of the month. here his mathematical powers are at their best, and he made a discovery of an inequality in the moon's motion known as the evection. book five describes the construction and use of the astrolabe, a combination of graduated circles with which ptolemy made most of his observations. in the sixth book he follows mainly hipparchus in dealing with eclipses of sun and moon. in the seventh and eighth books he discusses the motion of the equinox, and embodies a catalogue of , stars, substantially as in hipparchus. the five remaining books of the almagest deal with the planetary motions, and are the most important of all of ptolemy's original contributions to astronomy. ptolemy's fundamental doctrines were that the heavens are spherical in form, all the heavenly motions being in circles. in his view, the earth too is spherical, and it is located at the center of the universe, being only a point, as it were, in comparison. all was founded on mere appearance combined with the philosophical notion that the circle being the only perfect curve, all motions of heavenly bodies must take place in earth-centered circles. for fourteen or fifteen centuries this false theory persisted, on the authority of ptolemy and the almagest, rendering progress toward the development of the true theory impossible. ptolemy correctly argued that the earth itself is a sphere that is curved from east to west, and from north to south as well, clinching his argument, as we do to-day, by the visibility of objects at sea, the lower portions of which are at first concealed from our view by the curved surface of the water which intervenes. to ptolemy also the earth is at the center of the celestial sphere, and it has no motion of translation from that point; but his argument fails to prove this. truth and error, indeed, are so deftly intermingled that one is led to wonder why the keen intelligence of this great philosopher permitted him to reject the simple doctrine of the earth's rotation on its axis. but if we reflect that there was then no science of natural philosophy or physics proper, and that the age was wholly undeveloped along the lines of practical mechanics, we shall see why the astronomers of ptolemy's time and subsequent centuries were content to accept the doctrines of the heavens as formulated by him. when it came to explaining the movements of the "wandering stars," or planets, as we term them, the ptolemaic theory was very happy in so far as accuracy was concerned, but very unhappy when it had to account for the actual mechanics of the cosmos in space. sun and moon were the only bodies that went steadily onward, easterly: whereas all the others, mercury, venus, mars, jupiter, saturn, although they moved easterly most of the time, nevertheless would at intervals slow down to stationary points, where for a time they did not move at all, and then actually go backward to the west, or retrograde, then become stationary again, finally resuming their regular onward motion to the east. to help out of this difficulty, the worst possible mechanical scheme was invented, that known as the epicycle. each of the five planets was supposed to have a fictitious "double," which traveled eastward with uniformity, attached to the end of a huge but mechanically impossible bar. the earth-centered circle in which this traveled round was called the "deferent." what this bar was made of, what stresses it would be subjected to, or what its size would have to be in order to keep from breaking--none of these questions seems to have agitated the ancient and medieval astronomers, any more than the flat-earth astronomy of the hindu is troubled by the necessity of something to hold up the tortoise that holds up the elephant that holds up the earth. but at the end of this bar is jointed or swiveled another shorter bar, to the revolving end of which is attached the actual planet itself; and the second bar, by swinging once round the end of the primary advancing bar, would account for the backward or retrograde motion of the planet as seen in the sky. for every new irregularity that was found, in the motion of mars, for instance, a new and additional bar was requisitioned, until interplanetary space was hopelessly filled with revolving bars, each producing one of the epicycles, some large, some small, that were needed to take up the vagaries of the several planets. the arabic astronomers who kept the science alive through the middle ages added epicycle to epicycle, until there was every justification for milton's verses descriptive of the sphere: with centric and eccentric scribbled o'er, cycle and epicycle, orb in orb. chapter vii astronomy of the middle ages with the fall of alexandria and the victory of mohammed throughout the west, and a consequent decline in learning, supremacy in science passed to the east and centered round the caliphs of bagdad in the seventh and eighth centuries. they were interested in astronomy only as a practical, and to them useful, science, in adjusting the complicated lunar calendar of the mohammedans, in ascertaining the true direction of mecca which every mohammedan must know, and in the revival of astrology, to which the greeks had not attached any particular significance. harun al-rashid ordered the almagest and many other greek works translated, of which the modern world would otherwise no doubt never have heard, as the greek originals are not extant. splendid observatories were built at damascus and bagdad, and fine instruments patterned after greek models were continuously used in observing. the arab astronomers, although they had no clocks, were nevertheless so fully impressed with the importance of time that they added extreme value to their observations of eclipses, for example, by setting down the altitudes of sun or stars at the same time. on very important occasions the records were certified on oath by a body of barristers and astronomers conjointly--a precedent which fortunately has never been followed. about the middle of the ninth century, the caliph al-mamun directed his astronomers to revise the greek measures of the earth's dimensions, and they had less reverence for the almagest than existed in later centuries: indeed, tabit ben korra invented and applied to the tables of the almagest a theoretical fluctuation in the position of the ecliptic which he called "trepidation," which brought sad confusion into astronomical tables for many succeeding centuries. albategnius was another arab prince whose record in astronomy in the ninth and tenth centuries was perhaps the best: the ptolemaic values of the precession of the equinoxes and of the obliquity of the ecliptic were improved by new observations, and his excellence as mathematician enabled him to make permanent improvements in the astronomical application of trigonometry. abul wefa was the last of the bagdad astronomers in the latter half of the tenth century, and his great treatise on astronomy known as the almagest is sometimes confused with ptolemy's work. following him was ibn yunos of cairo, whose labors culminated in the famous hakemite tables, which became the standard in mathematical and astronomical computations for several centuries. mohammedan astronomy thrived, too, in spain and northern africa. arzachel of toledo published the toledan tables, and his pupils made improvements in instruments and the methods of calculation. the giralda was built by the moors in seville in , the first astronomical observatory on the continent of europe; but within the next half century both seville and cordova became christian again, and arab astronomy was at an end. through many centuries, however, the science had been kept alive, even if no great original advances had been achieved; and arab activities have modified our language very materially, adding many such words as almanac, zenith, and radii, and a wealth of star names, as aldebaran, rigel, betelgeuse, vega, and so on. meanwhile, other schools of astronomy had developed in the east, one at meraga near the modern persia, where nassir eddin, the astronomer of hulagu khan, grandson of the mongol emperor genghis khan, built and used large and carefully constructed instruments, translated all the greek treatises on astronomy, and published a laborious work known as the ilkhanic tables, based on the hakemite tables of ibn yunos. more important still was the tartar school of astronomy under ulugh beg, a grandson of tamerlane, who built an observatory at samarcand in , published new tables of the planets, and made with his excellent instruments the observations for a new catalogue of stars, the first since hipparchus, the star places being recorded with great precision. the european astronomy of the middle ages amounted to very little besides translation from the arabic authors into latin, with commentaries. astronomers under the patronage of alfonso x of leon and castile published in the alfonsine tables, which superseded the toledan tables and were accepted everywhere throughout europe. alfonso published also the "libros del saber," perhaps the first of all astronomical cyclopedias, in which is said to occur the earliest diagram representing a planetary orbit as an ellipse: mercury's supposed path round the earth as a center. purbach of vienna about the middle of the th century began his "epitome of astronomy" based on the "almagest" of ptolemy, which was finished by his collaborator regiomontanus, who was an expert in mathematics and published a treatise on trigonometry with the first table of sines calculated for every minute from ° to °, a most helpful contribution to theoretical astronomy. regiomontanus had a very picturesque career, finally taking up his residence in nuremberg, where a wealthy citizen named walther became his patron, pupil, and collaborator. the artisans of the city were set at work on astronomical instruments of the greatest accuracy, and the comet of was the first to be observed and studied in true scientific fashion. regiomontanus was very progressive and the invention of the new art of printing gave him an opportunity to publish purbach's treatise, which went through several editions and doubtless had much to do in promoting dissatisfaction with the ancient ptolemaic system, and was thus most significant in preparing a background for the coming of the new copernican order. the nuremberg presses popularized astronomy in other important ways, issuing almanacs, the first precursors of our astronomical ephemerides. regiomontanus was practical as well, and invented a new method of getting a ship's position at sea, with tables so accurate that they superseded all others in the great voyages of discovery, and it is probable that they were employed by columbus in his discovery of the american continent. regiomontanus had died several years earlier, in at rome, where he had gone by invitation of the pope to effect a reformation in the calendar. he was only forty, and his patron walther kept on with excellent observations, the first probably to be corrected for the effect of atmospheric refraction, although its influence had been known since ptolemy. the nuremberg school lasted for nearly two centuries. nearly contemporary with regiomontanus were fracastoro and peter apian, whose original observations on comets are worthy of mention because they first noticed that the tails of these bodies always point away from the sun. leonardo da vinci was the first to give the true explanation of earth-shine on the moon, and similarly the moon-illumination of the earth; and this no doubt had great weight in disposing of the popular notion of an essential difference of nature between the earth and celestial bodies--all of which helped to prepare the way for copernicus and the great revolution in astronomical thought. chapter viii copernicus and the new era throughout the middle ages the progress of astronomy was held back by a combination of untoward circumstances. a prolonged reaction from the heights attained by the greek philosophers was to be expected. the uprising of the mohammedan world, and the savage conquerors in the east did not produce conditions favorable to the origin and development of great ideas. at the birth of copernicus, however, in , the time was ripening for fundamental changes from the ancient system, the error of which had helped to hold back the development of the science for centuries. the fifteenth century was most fruitful in a general quickening of intelligence, the invention of printing had much to do with this, as it spread a knowledge of the greek writers, and led to conflict of authorities. even aristotle and ptolemy were not entirely in harmony, yet each was held inviolate. it was the age of the reformation, too, and near the end of the century the discovery of america exerted a powerful stimulus in the advance of thought. copernicus searched the works of the ancient writers and philosophers, and embodied in this new order such of their ideas as commended themselves in the elaboration of his own system. pythagoras alone and his philosophy looked in the true direction. many believe that he taught that the sun, not the earth, is at the center of our solar system; but his views were mingled with the speculative philosophy of the greeks, and none of his writings, barring a few meager fragments, have come down to our modern age. to many philosophers, through all these long centuries, the true theory of the celestial motions must have been obvious, but their views were not formulated, nor have they been preserved in writing. so the fact remains that copernicus alone first proved the truth of the system which is recognized to-day. this he did in his great treatise entitled "de revolutionibus orbium coelestium," the first printed copy of which was dramatically delivered to him on his deathbed, in may, . the seventy years of his life were largely devoted to the preparation of this work, which necessitated many observations as well as intricate calculations based upon them. being a canon in the church, he naturally hesitated about publishing his revolutionary views, his friend rheticus first doing this for him in outline in . so simple are the great principles that they may be embodied in very few words; what appears to us as the daily revolution of the heavens is not a real motion, but only an apparent one; that is, the heavens are at rest, while the earth itself is in motion, turning round an axis which passes through its center. and the second proposition is that the earth is simply one of the six known planets; and they all revolve round the sun as the true center. the solar system, therefore, is "heliocentric," or sun-centered, not "geocentric" or earth-centered, as taught by the ptolemaic theory. copernicus demonstrates clearly how his system explains the retrograde motion of the planets and their stationary points, no matter whether they are within the orbit of the earth, as mercury and venus, or outside of it, as mars, jupiter, and saturn. his system provides also the means of ascertaining with accuracy the proportions of the solar system, or the relative distances of the planets from the sun and from each other. in this respect also his system possessed a vast advantage over that of ptolemy, and the planetary distances which copernicus computed are very close approximations to the measures of the present day. reinhold revised the calculations of copernicus and prepared the "tabulæ prutenicæ," based on the "de revolutionibus," which proved far superior to the alfonsine tables, and were only supplanted by the rudolphine tables of kepler. on the whole we may regard the lifework of copernicus as fundamentally the most significant in the history and progress of astronomy. chapter ix tycho, the great observer clear as copernicus had made the demonstration of the truth of his new system, it nevertheless failed of immediate and universal acceptance. the ptolemaic system was too strongly intrenched, and the motions of all the bodies in the sky were too well represented by it. accurate observations were greatly needed, and the landgrave william iv. of hesse built the cassel observatory, which made a new catalogue of stars, and introduced the use of clocks to carry on the time as measured by the uniform motion of the celestial sphere. three years after the death of copernicus, tycho brahe was born, and when he was the king of denmark built for him the famous observatory of uraniborg, where the great astronomer passed nearly a quarter of a century in critically observing the positions of the stars and planets. tycho was celebrated as a designer and constructor of new types of astronomical instruments, and he printed a large volume of these designs, which form the basis of many in use at the present day. unfortunately for the genius of tycho and the significance of his work, the invention of the telescope had not yet been made, so that his observations had not the modern degree of accuracy. nevertheless, they were destined to play a most important part in the progress of astronomy. tycho was sadly in error in his rejection of the copernican system, although his reasons, in his day, seemed unanswerable. if the outer planets were displaced among the stars by the annual motion of the earth round the sun, he argued, then the fixed stars must be similarly displaced--unless indeed they be at such vast distances that their motions would be too slight to be visible. of course we know now that this is really true, and that no instruments that tycho was able to build could possibly have detected the motions, the effects of which we now recognize in the case of the nearer fixed stars in their annual, or parallactic, orbits. the remarkably accurate instruments devised by tycho brahe and employed by him in improving the observations of the positions of the heavenly bodies were no doubt built after descriptions of astrolabes such as hipparchus used, as described by ptolemy. in his "astronomiæ instauratæ mechanica" we find illustrations and descriptions of many of them. one is a polar astrolabe, mounted somewhat as a modern equatorial telescope is, and the meridian circle is adjustable so that it can be used in any place, no matter what its latitude might be. there is a graduated equatorial ring at right angles to the polar axis, so that the astrolabe could be used for making observations outside the meridian as well as on it. this equatorial circle slides through grooves, and is furnished with movable sights, and a plumb line from the zenith or highest point of the meridian circle makes it possible to give the necessary adjustment in the vertical. screws for adjustment at the bottom are provided, just as in our modern instruments, and two observers were necessary, taking their sights simultaneously; unless, as in one type of the instrument, a clock, or some sort of measure of time, was employed. another early type of instrument is called by tycho the ecliptic astrolabe (_armillæ zodiacales_, or the zodiacal rings). it resembles the equatorial astrolabe somewhat, but has a second ring inclined to the equatorial one at an angle equal to the obliquity of the ecliptic. in observing, the equatorial ring was revolved round till the ecliptic ring came into coincidence with the plane of the ecliptic in the sky. then the observation of a star's longitude and latitude, as referred to the ecliptic plane, could be made, quite as well as that of right ascension and declination on the equatorial plane. but it was necessary to work quickly, as the adjustment on the ecliptic would soon disappear and have to be renewed. tycho is often called the father of the science of astronomical observation, because of the improvements in design and construction of the instruments he used. his largest instrument was a mural quadrant, a quarter-circle of copper, turning parallel to the north-and-south face of a wall, its axis turning on a bearing fixed in the wall. the radius of this quadrant was nine feet, and it was graduated or divided so as to read the very small angle of ten seconds of arc--an extraordinary degree of precision for his day. tycho built also a very large alt-azimuth quadrant, of six feet radius. its operation was very much as if his mural quadrant could be swung round in azimuth. at several of the great observatories of the present day, as greenwich and washington, there are instruments of a similar type, but much more accurate, because the mechanical work in brass and steel is executed by tools that are essentially perfect, and besides this the power of the telescope is superadded to give absolute direction, or pointing on the object under observation. excellent clocks are necessary for precise observation with such an instrument; but neither tycho brahe, nor hevelius was provided with such accessories. hevelius did not avail himself of the telescope as an aid to precision of observation, claiming that pinhole sights gave him more accurate results. it was a dispute concerning this question that halley was sent over from london to danzig to arbitrate. there could be but one way to decide; the telescope with its added power magnifies any displacement of the instrument, and thereby enables the observer to point his instrument more exactly. so he can detect smaller errors and differences of direction than he can without it. and what is of great importance in more modern astronomy, the telescope makes it possible to observe accurately the position of objects so faint that they are wholly invisible to the naked eye. chapter x kepler, the great calculator most fortunate it was for the later development of astronomical theory that tycho brahe not only was a practical or observational astronomer of the highest order, but that he confined himself studiously for years to observations of the places of the planets. of mars he accumulated an especially long and accurate series, and among those who assisted him in his work was a young and brilliant pupil named johann kepler. strongly impressed with the truth of the copernican system, kepler was free to reject the erroneous compromise system devised by tycho brahe, and soon after tycho's death kepler addressed himself seriously to the great problem that no one had ever attempted to solve, viz: to find out what the laws of motion of the planets round the sun really are. of course he took the fullest advantage of all that ptolemy and copernicus had done before him, and he had in addition the splendid observations of tycho brahe as a basis to work upon. copernicus, while he had effected the tremendous advance of substituting the sun for the earth as the center of motion, nevertheless clung to the erroneous notion of ptolemy that all the bodies of the sky must perforce move at uniform speeds, and in circular curves, the circle being the only "perfect curve." kepler was not long in finding out that this could not be so, and he found it out because tycho brahe's observations were much more accurate than any that copernicus had employed. naturally he attempted the nearest planet first, and that was mars--the planet that tycho had assigned to him for research. how fortunate that the orbit of mars was the one, of all the planets, to show practically the greatest divergence from the ancient conditions of uniform motion in a perfectly circular orbit! had the orbit of mars chanced to be as nearly circular as is that of venus, kepler might well have been driven to abandon his search for the true curve of planetary motion. however, the facts of the cosmos were on his side, but the calculations essential in testing his various hypotheses were of the most tedious nature, because logarithms were not yet known in his day. his first discovery was that the orbit of mars is certainly not a circle, but oval or elliptic in figure. and the sun, he soon found, could not be in the center of the ellipse, so he made a series of trial calculations with the sun located in one of the foci of the ellipse instead. then he found he could make his calculated places of mars agree quite perfectly with tycho brahe's observed positions, if only he gave up the other ancient requisite of perfectly uniform motion. on doing this, it soon appeared that mars, when in perihelion, or nearest the sun, always moved swiftest, while at its greatest distance from the sun, or aphelion, its orbital velocity was slowest. kepler did not busy himself to inquire why these revolutionary discoveries of his were as they were; he simply went on making enough trials on mars, and then on the other planets in turn, to satisfy himself that all the planetary orbits are elliptical, not circular in form, and are so located in space that the center of the sun is at one of the two foci of each orbit. this is known as kepler's first law of planetary motion. the second one did not come quite so easy; it concerned the variable speed with which the planet moves at every point of the orbit. we must remember how handicapped he was in solving this problem: only the geometry of euclid to work with, and none of the refinements of the higher mathematics of a later day. but he finally found a very simple relation which represented the velocity of the planet everywhere in its orbit. it was this: if we calculate the area swept, or passed over, by the planet's radius vector (that is, the line joining its center to the sun's center) during a week's time near perihelion, and then calculate the similar area for a week near aphelion, or indeed for a week when mars is in any intermediate part of its orbit, we shall find that these areas are all equal to each other. so kepler formulated his second great law of planetary motion very simply: the radius vector of any planet describes, or sweeps over, equal areas in equal times. and he found this was true for all the planets. but the real genius of the great mathematician was shown in the discovery of his third law, which is more complex and even more significant than the other two--a law connecting the distances of the planets from the sun with their periods of revolution about the sun. this cost kepler many additional years of close calculation, and the resulting law, his third law of planetary motion is this: the cubes of the mean or average distances of the planets from the sun are proportional to the squares of their times of revolution around him. so kepler had not only disposed of the sacred theories of motion of the planets held by the ancients as inviolable, but he had demonstrated the truth of a great law which bound all the bodies of the solar system together. so accurately and completely did these three laws account for all the motions, that the science of astronomy seemed as if finished; and no matter how far in the future a time might be assigned, kepler's laws provided the means of calculating the planet's position for that epoch as accurately as it would be possible to observe it. kepler paused here, and he died in . chapter xi galileo, the great experimenter the fifteenth and sixteenth centuries, containing the lives and work of copernicus, tycho, galileo, kepler, huygens, halley, and newton, were a veritable golden age of astronomy. all these men were truly great and original investigators. none had a career more picturesque and popular than did galileo. born a few years earlier and dying a few years later than kepler, the work of each of these two great astronomers was wholly independent of the other and in entirely different fields. kepler was discovering the laws of planetary motion, while galileo was laying the secure foundations of the new science of dynamics, in particular the laws of falling bodies, that was necessary before kepler's laws could be fully understood. when only eighteen galileo's keen power of observation led to his discovery of the laws of pendulum motion, suggested by the oscillation to and fro of a lamp in the cathedral of pisa. the world-famous leaning tower of this place, where he was born, served as a physical laboratory from the top of which he dropped various objects, and thus was led to formulate the laws of falling bodies. he proved that aristotle was all wrong in saying that a heavy body must fall swifter in proportion to its weight than a lighter one. these and other discoveries rendered him unpopular with his associates, who christened him the "wrangler." the new system of copernicus appealed to him; and when he, first of all men, turned a telescope on the heavenly bodies, there was venus with phases like those of the moon, and jupiter with satellites traveling about it--a copernican system in miniature. nothing could have happened that would have provided a better demonstration of the truth of the new system and the falsity of the old. his marvelous discoveries caused the greatest excitement--consternation even, among the anti-copernicans. galileo published the "sidereus nuncius," with many observations and drawings of the moon, which he showed to be a body not wholly dissimilar to the earth: this, too, was obviously of great moment in corroboration of the copernican order and in contradiction to the ptolemaic, which maintained sharp lines of demarcation between things terrestrial and things celestial. his telescopes, small as they were, revealed to him anomalous appearances on both sides of the planet saturn which he called _ansæ_, or handles. but their subsequent disappearance was unaccountable to him, and later observers, who kept on guessing ineffectively till huygens, nearly a half century after, showed that the true nature of the appendage was a ring. spots on the sun were frequently observed by galileo and led to bitter controversies. he proved, however, that they were objects on the sun itself, not outside it, and by noticing their repeated transits across the sun's disk, he showed that the sun turned round on his axis in a little less than a month--another analogy to the like motion of the earth on the copernican plan. galileo's appointment in as "first philosopher and mathematician" to the grand duke of tuscany gave him abundant time for the pursuit of original investigations and the preparation of books and pamphlets. his first visit to rome the year following was the occasion of a reception with great honor by many cardinals and others of high rank. his lack of sympathy with others whose views differed from his, and his naturally controversial spirit, had begun to lead him headlong into controversies with the jesuits and the church, which culminated in his censure by the authorities of the church and persecution by the inquisition. in three comets appeared, and galileo was again in controversial hot water with the jesuits. but it led to the publication five years later of "il saggiatore" (the assayer), of no great scientific value, but only a brilliant bit of controversial literature dedicated to the newly elevated pope, urban viii. later he wrote through several years a great treatise, more or less controversial in character, entitled a "dialogue on the two chief systems of the world" between three speakers, and extending through four successive days. simplicio argues for the aristotelians, salviati for the copernicans, while sagredo does his best to be neutral. it will always be a very readable book, and we are fortunate to have a recent translation by professor crew of evanston. here we find the first suggestion of the modern method of getting stellar parallaxes, the relative parallax, that is, of two stars in the same field--a method not put into service till bessel's time, two centuries later. but the most important chapters of the "dialogue" deal with galileo's investigations of the laws of motion of bodies in general, which he applied to the problem of the earth's motion. in this he really anticipated newton in the first of his three laws of motion, and in a subsequent work, dealing with the theory of projectiles, he reaches substantially the results of newton's second law of motion, although he gave no general statement of the principle. nevertheless, in the epoch where his life was lived and his work done, his telescopic discoveries, combined with his dynamic researches in untrodden fields, resulted in the complete and final overthrow of the ancient system of error, and the secure establishment of the copernican system beyond further question and discussion. only then could the science of astronomy proceed unhampered to the fullest development by the master minds of succeeding centuries. chapter xii after the great masters following kepler and galileo was a half century of great astronomical progress along many lines laid out by the work of the great masters. the telescope seemed only a toy, but its improvement in size and quality showed almost inconceivable possibilities of celestial discoveries. hevelius of danzig took up the study of the moon, and his "selenographia" was finely illustrated by plates which he not only drew but engraved himself. lunar names of mountains, plains, and craters we owe very largely to him. also he published among other works two on comets, the second of which was published in and called the "cometographia," the first detailed account of all the comets observed and recorded to date. many were the telescopes turned on the planet saturn, and every variety of guess was made as to the actual shape and physical nature of the weird appendages discovered by galileo. the true solution was finally reached by huygens, whose mechanical genius had enabled him to grind and polish larger and better lenses than his contemporaries; in he published the "systema saturnium" interpreting the ring and the cause of its various configurations, and the first discovery of a saturnian satellite is due to him. gascoigne in england about was the first to make the important application of the micrometer to enhance the accuracy of measurement of small angles in the telescopic field; an invention made and applied independently many years later by huygens in holland and auzout and picard in france, where the instrument was first regularly employed as an accessory in the work of an observatory. another englishman, jeremiah horrocks, was the first observer of a transit of venus over the disk of the sun, in . horrocks was possessed of great ability in calculational astronomy also. this was about the time of the invention of the pendulum clock by huygens, which in conjunction with the later invention of the transit instrument by roemer wrought a revolution in the exacting art of practical astronomy. this was because it enabled the time to be carried along continuously, and the revolution of the earth could be utilized in making precise measures of the position of sun, moon, and stars. louis xiv had just founded the new observatory at paris in , and picard was the first to establish regular time-observations there. huygens followed up the motion of the pendulum in theory as well as practice in his "horologium oscillatorium" ( ), showing the way to measure the force of gravity, and his study of circular motion showed the fundamental necessity of some force directed toward the center in planetary motions. the doctrine of the sphericity of the earth being no longer in doubt, the great advance in accuracy of astronomical observation indicated to willebrord snell in holland the best way to measure an arc of meridian by triangulation. picard repeated the measurements near paris with even greater accuracy, and his results were of the utmost significance to newton in establishing his law of gravitation. domenico cassini, an industrious observer, voluminous writer, and a strong personality, devised telescopes of great size, discovered four saturnian satellites and the main division in the ring of saturn, determined the rotation periods of mars and jupiter, and prepared tables of the eclipses of jupiter's satellites. at his suggestion richer undertook an expedition to cayenne in latitude degrees north, where it was found that the intensity of gravity was less than at paris, and his clock therefore lost time, thus indicating that the earth was not a perfect sphere as had been thought, but a spheroid instead. the planet mars passed a near opposition, and richer's observations of it from cayenne, when combined with those of cassini and others in france, gave a new value of the sun's parallax and distance, really the first actual measurement worth the name in the history of astronomy. to close this era of signal advance in astronomy we may cite a discovery by roemer of the first order: no less than that of the velocity of transmission of light through space. at the instigation of picard, roemer in studying the motions of jupiter's satellites found that the intervals between eclipses grew less and less as jupiter and the earth approached each other, and greater and greater than the average as the two planets separated farther and farther. roemer correctly attributed this difference to the progressive motion of light and a rough value of its velocity was calculated, though not accepted by astronomers generally for more than a century. why the laws of kepler should be true, kepler himself was unable to say. nor could anyone else in that day answer these questions: ( ) the planets move in orbits that are elliptical not circular--why should they move in an imperfect curve, rather than the perfect one in which it had always been taught that they moved? ( ) why should our planet vary its velocity at all, and travel now fast, now slow; especially why should the speed so vary that the line of varying length, joining the planet to the sun, always passes over areas proportional to the time of describing them? and ( ) why should there be any definite relation of the distances of planets from the sun to their times of revolution about him? why should it be exactly as the cube of one to the square of the other? we must remember that the copernican system itself was not yet, in the beginning of the seventeenth century, accepted universally; and the great minds of that period were most concerned in overturning the erroneous theory of ptolemy. the next step in logical order was to find a basic explanation of the planetary motions, and descartes and his theory of vortices are worthy of mention, among many unsuccessful attempts in this direction. descartes was a brilliant french philosopher and mathematician, but his hypothesis of a multitude of whirlpools in the ether, while ingenious in theory, was too vague and indefinite to account for the planetary motions with any approach to the precision with which the laws of kepler represented them. another great astronomer whose labors helped immensely in preparing the way for the signal discoveries that were soon to come was huygens, a man of versatility as natural philosopher, mechanician, and astronomical observer. huygens was born thirteen years before the death of galileo, and to the discovery of the laws of motion by the latter huygens added researches on the laws of action of centrifugal forces. neither of them, however, appeared to see the immediate bearing on the great general problem of celestial motions in its true light, and it was reserved for another generation, and an astronomer of another country, to make the one fundamental discovery that should explain the whole by a single simple law. chapter xiii newton and motion "how is it that you are able to make these great discoveries?" was once asked of sir isaac newton, _facile princeps_ of all philosophers, and the discoverer of the great law of universal gravitation. "by perpetually thinking about them," was newton's terse and illuminating reply. he had set for himself the definite problem of kepler's laws: why is it that they are true, and is there not some single, general law that will embody all the circumstances of the planetary motions? newton was born in , the year after the death of galileo. he had a thorough training in the mathematics of his day, and addressed himself first to an investigation and definite formulation of the general laws of motion, which he found to be three in number, and which he was able to put in very simple terms. the first one is: any body, once it is set in motion, will continue to move forward in a straight line with a uniform velocity forever, provided it is acted upon by no force whatever. in other words, a state of motion is as natural as a state of rest (rest in relation to things everywhere adjacent) in which we find all things in general. here on earth where gravity itself pulls all objects downward toward the earth, and where resistance of the air tends to hold a moving body back and bring it to rest, and where friction from contact with whatever material substance may be in its path is perpetually tending to neutralize all motion--with all three of these forces always at work to stop a moving body, the truth of this first and fundamental law of motion was not apparent on the surface. till galileo's time everyone had made the mistake of supposing that some force or other must be acting continually on every moving body to keep it in motion. ptolemy, copernicus, kepler, leonardo da vinci--all failed to see the truth of this law which newton developed in the immortal _principia_. and at the present day it is not always easy to accept at first, although the progress of mechanical science, by reducing friction and resistance, has produced machines in which motion of large masses may be kept up indefinitely with the application of only the merest minimum of force. once a planet is set in motion round the sun, it would go on forever through frictionless, non-resistant space; but there must be a central force, as huygens saw clearly, to hold it in its orbit. otherwise it would at any moment take the direction of a tangent to the orbit. here is where newton's second law of motion comes in, and he formulated it with great definiteness. when any force acts on a moving body, its deviation from a straight line will be in the direction of the force applied and proportional to that force. in accord with this law, newton first began to inquire whether the force of attraction here on earth, which everyone commonly recognizes as gravity, drawing all things down toward the center of the earth, might not extend upward indefinitely. it is found in operation on the summits of mountain peaks, and the clouds above them and the rain falling from them are obviously drawn downward by the same force. may it not extend outward into space, even as far as the moon? this was an audacious question, but newton not only asked, but tried to answer it in the year , when he was only twenty-three. on the surface of the earth this attraction is strong enough to draw a falling body downward through a vertical space of sixteen feet in a second of time. what ought it to be at the distance of the moon. the distance of the moon in newton's time was better known in terms of the earth's size than was the size of the earth itself: the earth's radius was known to be one-sixtieth of the moon's distance, but the earth's diameter was thought to be something under , miles, so that newton's first calculations were most disappointing, and he laid them aside for nearly twenty years. meanwhile the french astronomers led by picard had measured the earth anew, and showed it to be nearly , miles in diameter. as soon as newton learned of this, he revised his calculations, and found that by the law of the inverse square the moon, in one second, should fall away from a tangent to its orbit one thirty-six hundredth of sixteen feet. this accorded exactly with his original supposition that the earth's attraction extended to the moon. so he concluded that the force which makes a stone fall, or an apple, as the story goes, is the same force that holds the moon in its orbit, and that this force diminishes in the exact proportion that the square of the distance from the earth's center increases. the moon, indeed, becomes a falling body; only, as kingdon clifford puts it: "she is going so fast and is so far off that she falls quite around to the other side of the earth, instead of hitting it; and so goes on forever." [illustration: nicholas copernicus] [illustration: galileo galilei] [illustration: johann kepler] [illustration: sir isaac newton] newton goes on in the _principia_ to explain the extension of gravitation to the other bodies of the solar system beyond the earth and moon. clearly the same gravitation that holds the moon in its orbit round the earth, must extend outward from the sun also, and hold all the planets in their orbits centered about him. newton demonstrates by calculation based on kepler's third law that ( ) the forces drawing the planets toward the sun are inversely as the squares of their mean distances from him; and ( ) if the force be constantly directed toward the sun, the radius vector in an elliptic orbit must pass over equal areas in equal times. chapter xiv newton and gravitation so all of kepler's laws could be embodied in a single law of gravitation toward a central body, whose force of attraction decreases outward in exact proportion as the square of the distance increases. only one farther step had to be taken, and this the most complicated of all: he must make all the bodies of the sky conform to his third law of motion. this is: action and reaction are equal, or the mutual actions of any two bodies are always equal and oppositely directed. there must be mutual attractions everywhere: earth for sun as well as sun for earth, moon for sun and sun for moon, earth for venus and venus for earth, jupiter for saturn and saturn for jupiter, and so on. the motions of the planets in the undisturbed ellipses of kepler must be impossible. as observations of the planets became more accurate, it was found that they really did fail to move in exact accord with kepler's laws unmodified. newton was unable, with the imperfect processes of the mathematics of his day to ascertain whether the deviations then known could be accounted for by his law of gravitation; but he nevertheless formulated the law with entire precision, as follows: every particle of matter in the universe attracts every other particle with a force exactly proportioned to the product of their masses, and inversely as the square of the distance between their centers. the centuries of astronomical research since newton's day, however, have verified the great law with the utmost exactness. practically every irregularity of lunar and planetary motion is accounted for; indeed, the intricacies of the problems involved, and the nicety of their solution, have led to the invention of new mathematical processes adequate to the difficulties encountered. and about the middle of the last century, when uranus departed from the path laid out for it by the mathematical astronomers, its orbital deviations were made the basis of an investigation which soon led to the assignment of the position where a great planet could be found that would account for the unexplained irregularities of the motion of uranus. and the immediate discovery of this planet, neptune, became the most striking verification of the newtonian law that the solar system could possibly afford. the astronomers of still later days investigating the statelier motions of stellar systems find the newtonian law regnant everywhere among the stars where our most powerful telescopes have as yet reached. so that newton's law is known as the law of universal gravitation, and its author is everywhere held as the greatest scientist of the ages. newton's _principia_ may be regarded as the culminating research of the inductive method, and further outline of its contents is desirable. it is divided into three books following certain introductory sections. the first book treats of the problems of moving bodies, the solutions being worked out generally and not with special reference to astronomy. the second book deals with the motion of bodies through resistant media, as fluids, and has very little significance in astronomy. the third book is the all important one, and applies his general principles to the case of the actual solar system, providing a full explanation of the motions of all the bodies of the system known in his day. anyone who critically reads the _principia_ of newton will be forced to conclude that its author was a genius in the highest sense of the word. the elegance and thoroughness of the demonstrations, and the completeness of application of the law of gravitation are especially impressive. the universality of his new law was the feature to which he gave particular attention. it was clear to him that the gravitation of a planet, although it acted as if wholly concentrated at the center, was nevertheless resident in every one of the particles of which the planet is composed. indeed, his universal law was so formulated as to make every particle attract every other particle; and an investigation known as the cavendish experiment--a research of great delicacy of manipulation--not only proves this, but leads also to a measurement of the earth's mean density, from which we can calculate approximately how much the earth actually weighs. another way to attack the same problem is by measuring the attraction of mountains, as maskelyne, astronomer royal of scotland did on mount schehallien in scotland, which was selected because of its sheer isolation. the attraction of the mountain deflected the plumb-lines by measurable amounts, the volume of the mountain was carefully ascertained by surveys, and geologists found out what rocks composed it. so the weight of the entire mountain became pretty well known, and combining this with the observed deflection, an independent value of the earth's weight was found. still other methods have been applied to this question, and as an average it is found that the materials composing the earth are about five and a half times as heavy as water, and the total weight of the earth is something like six sextillions of tons. what is the true shape of the earth? and does the earth's turning round on its axis affect this shape? newton saw the answer to these questions in his law of gravitation. a spherical figure followed as a matter of course from the mutual attraction of all materials composing the earth, providing it was at rest, or did not turn round on its axis. but rotation bulges it at the equator and draws it in at the poles, by an amount which calculation shows to be in exact agreement with the amount ascertained by actual measurement of the earth itself. another curious effect, not at first apparent, was that all bodies carried from high latitudes toward the equator would get lighter and lighter, in consequence of the centrifugal force of rotation. this was unexpectedly demonstrated by richer when the french academy sent him south to observe mars in . his clock had been regulated exactly in paris, and he soon found that it lost time when set up at cayenne. the amount of loss was found by observation, and it was exactly equal to the calculated effect that the reduction of gravity by centrifugal action should produce. also newton saw that his law of gravitation would afford an explanation of the rise and fall of the tides. the water on the side of the earth toward the moon, being nearer to the moon, would be more strongly attracted toward it, and therefore raised in a tide. and the water on the farther side of the earth away from the moon, being at a greater distance than the earth itself, the moon would attract the earth more strongly than this mass of water, tending therefore to draw the earth away from the water, and so raising at the same time a high tide on the side of the earth away from the moon. as the earth turns round on its axis, therefore, two tidal waves continually follow each other at intervals of about twelve hours. the sun, too, joins its gravitating force with that of the moon, raising tides nearly half as high as those which the moon produces, because the sun's vaster mass makes up in large part for its much greater distance. at first and third quarters of the moon, the sun acts against the moon, and the difference of their tide-producing forces gives us "neap tides"; while at new moon and full, sun and moon act together, and produce the maximum effect known as "spring tides." newton passed on to explain, by the action of gravitation also, the precession of the equinoxes, a phenomenon of the sky discovered by hipparchus, who pretty well ascertained its amount, although no reason for it had ever been assigned. the plane of the earth's equator extended to the celestial sphere marks out the celestial equator, and the two opposite points where it intersects the plane of the ecliptic, or the earth's path round the sun, are called the equinoctial points, or simply the equinoxes. and precession of the equinoxes is the motion of these points westward or backward, about seconds each year, so that a complete revolution round the ecliptic would take place in about , years. newton saw clearly how to explain this: it is simply due to the attraction of the sun's gravitation upon the protuberant bulge around the earth's equator, acting in conjunction with the earth's rotation on its axis, the effect being very similar to that often seen in a spinning top, or in a gyroscope. the moon moving near the ecliptic produces a precessional effect, as also do the planets to a very slight degree; and the observed value of precession is the same as that calculated from gravitation, to a high degree of precision. newton died in , too early to have witnessed that complete and triumphant verification of his law which ultimately has accounted for practically every inequality in the planetary motions caused by their mutual attractions. the problems involved are far beyond the complexity of those which the mathematical astronomer has to deal with, and the mathematicians of france deserve the highest credit for improving the processes of their science so that obstacles which appeared insuperable were one after another overcome. newton's method of dealing with these problems was mainly geometric, and the insufficiency of this method was apparent. only when the french mathematicians began to apply the higher methods of algebra was progress toward the ultimate goal assured. d'alembert and clairaut for a time were foremost in these researches, but their places were soon taken by lagrange, who wrote the "mécanique analytique," and laplace, whose "mécanique céleste" is the most celebrated work of all. in large part these works are the basis of the researches of subsequent mathematical astronomers who, strictly speaking, cannot as yet be said to have arrived at a complete and rigorous solution of all the problems which the mutual attractions of all the bodies of the solar system have originated. it may well be that even the mathematics of the present day are incompetent to this purpose. when the brilliant genius of sir william hamilton invented quaternion analysis and showed the marvelous facility with which it solved the intricate problems of physics, there was the expectation that its application to the higher problems of mathematical astronomy might effect still greater advances; but nothing in that direction has so far eventuated. some astronomers look for the invention of new functions with numerical tables bearing perhaps somewhat the relation to present tables of logarithms, sines, tangents, and so on, that these tables do to the simple multiplication table of pythagoras. chapter xv after newton we have said that practically all the motions in the solar system have been accounted for by the newtonian law of gravitation. it will be of interest to inquire into the instances that lead to qualification of this absolute statement. one relates to the planet mercury, whose orbit or path round the sun is the most elliptical of all the planetary orbits. this will be explained a little later. the moon has given the mathematical astronomers more trouble than any other of the celestial bodies, for one reason because it is nearest to us and very minute deviations in its motion are therefore detectible. halley it was who ascertained two centuries ago that the moon's motion round the earth was not uniform, but subject to a slight acceleration which greatly puzzled lagrange and laplace, because they had proved exactly this sort of thing to be impossible, unless indeed the body in question should be acted on by some other force than gravitation. but laplace finally traced the cause to the secular or very slow reduction in the eccentricity of the earth's own orbit. the sun's action on the moon was indeed progressively changing from century to century in such manner as to accelerate the moon's own motion in its orbit round the earth. adams, the eminent english astronomer, revised the calculations of laplace, and found the effect in question only half as great as laplace had done; and for years a great mathematical battle was on between the greatest of astronomical experts in this field of research. adams, in conjunction with delaunay, the greatest of the french mathematicians a half century ago, won the battle in so far as the mathematical calculations were concerned; but the moon continues to the present day her slight and perplexing deviation, as if perhaps our standard time-keeper, the earth, by its rotation round its axis, were itself subject to variation. although many investigations have been made of the uniformity of the earth's rotation, no such irregularity has been detected, and this unexplained variation of the moon's motion is one of the unsolved problems of the gravitational astronomer of to-day. but we are passing over the most impressive of all the earlier researches of lagrange and laplace, which concerned the exceedingly slow changes, technically called the secular variations of the elements of the planetary orbits. these elements are geometrical relations which indicate the form of the orbit, the size of the orbit, and its position in space; and it was found that none of these relations or quantities are constant in amount or direction, but that all, with but one exception, are subject to very slow, or secular, change, or oscillation. this question assumed an alarming significance at an early day, particularly as it affected the eccentricity of the earth's orbit round the sun. should it be possible for this element to go on increasing for indefinite ages, clearly the earth's orbit would become more and more elliptical, and the sun would come nearer and nearer at perihelion, and the earth would drift farther and farther from the sun at aphelion, until the extremes of temperature would bring all forms of life on the earth to an end. the refined and powerful analysis of lagrange, however, soon allayed the fears of humanity by accounting for these slow progressive changes as merely part of the regular system of mere oscillations, in entire accord with the operation of the law of gravitation; and extending throughout the entire planetary system. indeed, the periods of these oscillations were so vast that none of them were shorter than , years, while they ranged up to two million years in length--"great clocks of eternity which beat ages as ours beat seconds." about a century ago, an eminent lecturer on astronomy told his audience that the problem of weighing the planets might readily be one that would seem wholly impossible to solve. to measure their sizes and distances might well be done, but actually to ascertain how many tons they weigh--never! yet if a planet is fortunate enough to have one satellite or more, the astronomer's method of weighing the planet is exceedingly simple; and all the major planets have satellites except the two interior ones, mercury and venus. as the satellite travels round its primary, just as the moon does round the earth, two elements of its orbit need to be ascertained, and only two. first, the mean distance of the satellite from its primary, and second the time of revolution round it. now it is simply a case of applying kepler's third law. first take the cube of the satellite's distance and divide it by the square of the time of revolution. similarly take the cube of the planet's distance from the sun and divide by the square of the planet's time of revolution round him. the proportion, then, of the first quotient to the second shows the relation of the mass (that is the weight) of the planet to that of the sun. in the case of jupiter, we should find it to be , , in that of saturn , , and so on. the range of planetary masses, in fact, is very curious, and is doubtless of much significance in the cosmogony, with which we deal later. if we consider the sun and his eight planets, the mass or weight of each of the nine bodies far exceeds the combined mass of all the others which are lighter than itself. to illustrate: suppose we take as our unit of weight the one-billionth part of the sun's weight; then the planets in the order of their masses will be mercury, mars, venus, earth, uranus, neptune, saturn, and jupiter. according to their relative masses, then, mercury being a five-millionth part the weight of the sun will be represented by ; similarly venus, a four hundred and twenty-five thousandth part by , , and so on. then we have mercury mars ------ sum of weights of mercury and mars venus , ------ sum of weights of mercury, mars, and venus , the earth , ------ sum of weights of four inner planets , uranus , ------ sum of weights of five planets , neptune , ------- sum of weights of six planets , saturn , --------- sum of weights of seven planets , jupiter , --------- sum of weights of all the planets , , mass or weight of the sun , , , curious and interesting it is that saturn is nearly three times as heavy as the six lighter planets taken together, jupiter between two and three times heavier than all the other planets combined, while the sun's mass is times that of all the great planets of his system rolled into one. all the foregoing masses, except those of mercury and venus, are pretty accurately known because they were found by the satellite method just indicated. mercury's mass is found by its disturbing effects on encke's comet whenever it approaches very near. the mass of venus is ascertained by the perturbations in the orbital motion of the earth. in such cases the newtonian law of gravitation forms the basis of the intricate and tedious calculations necessary to find out the mass by this indirect method. its inferiority to the satellite method was strikingly shown at the observatory in washington soon after the satellites of mars were discovered in . the inaccurate mass of that planet, as previously known by months of computation based upon years and years of observation, was immediately discarded in favor of the new mass derived from the distance and period of the outer satellite by only a few minutes' calculation. in weighing the planets, astronomers always use the sun as the unit. what then is the sun's own weight? obviously the law of gravitation answers this question, if we compare the sun's attraction with the earth's at equal distances. first we conceive of the sun's mass as if all compressed into a globe the size of the earth, and calculate how far a body at the surface of this globe would fall in one second. the relation of this number to . feet, the distance a body falls in one second on the actual earth, is about , , which is therefore the number of times the sun's weight exceeds that of the earth. a word may be added regarding the force of gravitation and what it really is. as a matter of fact newton did not concern himself in the least with this inquiry, and says so very definitely. what he did was to discover the law according to which gravitation acts everywhere throughout the solar system. and although many physicists have endeavored to find out what gravitation really is, its cause is not yet known. in some manner as yet mysterious it acts instantaneously over distances great and small alike, and no substance has been found which, if we interpose it between two bodies, has in any degree the effect of interrupting their gravitational tendency toward each other. while the newtonian law of gravitation has been accepted as true because it explained and accounted for all the motions of the heavenly bodies, even including such motions of the stars as have been subjected to observation, astronomers have for a long time recognized that quite possibly the law might not be absolutely exact in a mathematical sense, and that deviations from it would surely make their appearance in time. a crude instance of this was suggested about a century ago, when the planet uranus was found to be deviating from the path marked out for it by bouvard's tables based on the newtonian law; and the theory was advocated by many astronomers that this law, while operant at the medium distances from the sun where the planets within jupiter and saturn travel, could not be expected to hold absolutely true at the vast distance of uranus and beyond. the discovery of neptune in , however, put an end to all such speculation, and has universally been regarded as an extraordinary verification of the law, as indeed it is. when, however, le verrier investigated the orbit of mercury he found an excess of motion in the perihelion point of the planet's orbit which neither he nor subsequent investigators have been able to account for by newtonian gravitation, pure and simple. if newton's theory is absolutely true, the excess motion of mercury's perihelion remains a mystery. only one theory has been advanced to account for this discrepancy, and that is the einstein theory of gravitation. this ingenious speculation was first propounded in comprehensive form nearly fifteen years ago, and its author has developed from it mathematical formulæ which appear to yield results even more precise than those based on the newtonian theory. in expressing the difference between the law of gravitation and his own conception, einstein says: "imagine the earth removed, and in its place suspended a box as big as a moon or a whole house and inside a man naturally floating in the center, there being no force whatever pulling him. imagine, further, this box being, by a rope or other contrivance, suddenly jerked to one side, which is scientifically termed 'difform motion,' as opposed to 'uniform motion.' the person would then naturally reach bottom on the opposite side. the result would consequently be the same as if he obeyed newton's law of gravitation, while, in fact, there is no gravitation exerted whatever, which proves that difform motion will in every case produce the same effects as gravitation.... the term relativity refers to time and space. according to galileo and newton, time and space were absolute entities, and the moving systems of the universe were dependent on this absolute time and space. on this conception was built the science of mechanics. the resulting formulas sufficed for all motions of a slow nature; it was found, however, that they would not conform to the rapid motions apparent in electrodynamics.... briefly the theory of special relativity discards absolute time and space, and makes them in every instance relative to moving systems. by this theory all phenomena in electrodynamics, as well as mechanics, hitherto irreducible by the old formulæ, were satisfactorily explained." natural phenomena, then, involving gravitation and inertia, as in the planetary motions, and electro-magnetic phenomena, including the motion of light, are to be regarded as interrelated, and not independent of one another. and the einstein theory would appear to have received a striking verification in both these fields. on this theory the newtonian dynamics fails when the velocities concerned are a near approach to that of light. the newtonian theory, then, is not to be considered as wrong, but in the light of a first approximation. applying the new theory to the case of the motion of mercury's perihelion, it is found to account for the excess quite exactly. on the electro-magnetic side, including also the motion of light, a total eclipse of the sun affords an especially favorable occasion for applying the critical test, whether a huge mass like the sun would or would not deflect toward itself the rays of light from stars passing close to the edge of its disk, or limb. a total eclipse of exceptional duration occurred on may , , and the two eclipse parties sent out by the royal society of london and the royal astronomical society were equipped especially with apparatus for making this test. their stations were one on the east coast of brazil and the other on the west coast of africa. accurate calculation beforehand showed just where the sun would be among the stars at the time of the eclipse; so that star plates of this region were taken in england before the expeditions went out. then, during the total eclipse, the same regions were photographed with the eclipsed sun and the corona projected against them. to make doubly sure, the stars were a third time photographed some weeks after the eclipse, when the sun had moved away from that particular region. measuring up the three sets of plates, it was found that an appreciable deflection of the light of the stars nearest alongside the sun actually exists; and the amount of it is such as to afford a fair though not absolutely exact verification of the theory. the observed deflection may of course be due to other causes, but the english astronomers generally regard the near verification as a triumph for the einstein theory. astronomers are already beginning preparations for a repetition of the eclipse programme with all possible refinement of observation, when the next total eclipse of the sun occurs, september , , visible in australia and the islands of the indian ocean. a third test of the theory is perhaps more critical than either of the others, and this necessitates a displacement of spectral lines in a gravitational field toward the red end of the spectrum; but the experts who have so far made measures for detecting such displacement disagree as to its actual existence. the work of st. john at mt. wilson is unfavorable to the theory, as is that of evershed of kodiakanal, who has made repeated tests on the spectrum of venus, as well as in the cyanogen bands of the sun. the enthusiastic advocates of the einstein theory hold that, as newton proved the three laws of kepler to be special cases of his general law, so the "universal relativity theory" will enable eventually the newtonian law to be deduced from the einstein theory. "this is the way we go on in science, as in everything else," wrote sir george airy, astronomer royal; "we have to make out that something is true; then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained." meanwhile, the prudent person keeps the open mind. chapter xvi halley and his comet halley is one of the most picturesque characters in all astronomical history. next to newton himself he was most intimately concerned in giving the newtonian law to the world. edmund halley was born ( ) in stirring times. charles i. had just been executed, and it was the era of cromwell's lord protectorate and the wars with spain and holland. then followed ( ) the promising but profligate charles ii. (who nevertheless founded at greenwich the greatest of all observatories when halley was nineteen), the frightful ravages of the black plague, the tyrannies of james ii., and the revolution of --all in the early manhood of halley, whose scientific life and works marched with much of the vigor of the contending personalities of state. the telescope had been invented a half century earlier, and galileo's discoveries of jupiter's moons and the phases of venus had firmly established the sun-centered theory of copernicus. the sun's distance, though, was known but crudely; and why the stars seemed to have no yearly orbits of their own corresponding to that of the earth was a puzzle. newton was well advanced toward his supreme discovery of the law of universal gravitation; and the authority of kepler taught that comets travel helter-skelter through space in straight lines past the earth, a perpetual menace to humanity. "ugly monsters," that comets always were to the ancient world, the medieval church perpetuated this misconception so vigorously that even now these harmless, gauzy visitors from interstellar space possess a certain "wizard hold upon our imagination." this entertaining phase of the subject is excellently treated in president andrew d. white's "history of the doctrine of comets," in the papers of the american historical association. halley's brilliant comet at its earlier apparitions had been no exception. halley's father was a wealthy london soap maker, who took great pride in the growing intellectuality of his son. graduating at queen's college, oxford, the latter began his astronomical labors at twenty by publishing a work on planetary orbits; and the next year he voyaged to st. helena to catalogue the stars of the southern firmament, to measure the force of terrestrial gravity, and observe a transit of mercury over the disk of the sun. while clouds seriously interfered with his observations on that lonely isle, what he saw of the transit led to his invention of "halley's method," which, as applied to the transit of venus, though not till long after his death, helped greatly in the accurate determination of the sun's distance from the earth. halley's researches on the proper motions of the stars of both hemispheres soon made him famous, and it was said of him, "if any star gets displaced on the globe, halley will presently find it out." his return to london and election to the royal society (of which he was many years secretary) added much to his fame, and he was commissioned by the society to visit danzig and arbitrate an astronomical controversy between hooke and hevelius, both his seniors by a generation. on the continent he associated with other great astronomers, especially cassini, who had already found three saturnian moons; and it was then he observed the great comet of , which led up to the most famous event of halley's life. the seerlike seneca may almost be said to have predicted the advent of halley, when he wrote ("quaestiones naturales," vii): "some day there will arise a man who will demonstrate in what region of the heavens comets pursue their way; why they travel apart from the planets; and what their sizes and constitution are. then posterity will be amazed that simple things of this sort were not explained before." to newton it appeared probable that cometary voyagers through space might have orbits of their own; and he proved that the comet of never swerved from such a path. as it could nowhere approach within the moon's orbit, clearly threats of its wrecking the earth and punishing its inhabitants ought to frighten no more. halley then became intensely interested in comets, and gathered whatever data concerning the paths of all these bodies he could find. his first great discovery was that the comets seen in by apian, and in by kepler, traveled round the sun in identical paths with one he had himself observed in . a still earlier appearance of halley's comet ( ) seems to have given rise to a popular and long-reiterated myth of a papal bull excommunicating "the devil, the turk, and the comet." no longer room for doubt: so certain was halley that all three were one and the same comet, completing the round of its orbit in about seventy-six years, that he fearlessly predicted that it would be seen again in or . and with equal confidence he might have foretold its return in and ; for all three predictions have come true to the letter. halley's span of existence did not permit his living to see even the first of these now historic verifications. but we in our day may emphatically term the epoch of the third verified return _annus halleianus_. says turner, halley's successor in the savilian chair at oxford to-day: "there can be no more complete or more sensational proof of a scientific law, than to predict events by means of it. halley was deservedly the first to perform this great service for newton's law of gravitation, and he would have rejoiced to think how conspicuous a part england was to play in the subsequent prediction of the existence of neptune." halley rose rapidly among the chief astronomical figures of his day. but he had little veneration for mere authority, and the significant veering of his religious views toward heterodoxy was for years an obstacle to his advance. still halley the astronomer was great enough to question any contemporary dicta that seemed to rest on authority alone. everyone called the stars "fixed" stars; but halley doubting this, made the first discovery of a star's individual motion--proper motion, as astronomers say. to-day, two hundred years after, every star is considered to be in motion, and astronomers are ascertaining their real motions in the celestial spaces to a nicety undreamed of by even the exacting halley. the moon, of priceless service to the early navigator, was regarded by all astronomers as endowed with an average rate of motion round the earth that did not vary from age to age. but halley questioned this too; and on comparing with the ancient value from chaldean eclipses, he made another discovery--the secular acceleration of the moon's mean motion, as it is technically termed. this was a colossal discovery in celestial dynamics; and the reason underlying it lay hidden in newton's law for yet another century, till the keener mathematics of laplace detected its true origin. * * * * * with newton, halley laid down the firm foundations of celestial mechanics, and they pushed the science as far as the mathematics of their day would permit. halley, however, was not content with elucidating the motion of bodies nearest the earth, and pressed to the utmost confines of the solar system known to him. here, too, he made a signal discovery of that mutual disturbance of the planets in their motion round the sun, called the great inequality of jupiter and saturn. halley's versatile genius attacked all the great problems of the day. his observation of the sun's total eclipse in is the earliest reliable account of such a phenomenon by a trained astronomer. he described the corona minutely and was the first to see that other interesting phenomenon which only an alert observer can detect, which a great astronomer of a later day compared to the "ignition of a fine train of gunpowder," and which has ever since borne the name of "baily's beads." besides being a great astronomer, halley was a man of affairs as well, which newton, although the greater mathematician, was not. without halley, newton's superb discovery might easily have been lost to the age and nation, for the latter was bent merely on making discoveries, and on speculative contemplation of them, with never a thought of publishing to the world. halley, more practical and businesslike, insisted on careful writing out and publication. newton was then only forty-two, and halley fully fourteen years his junior. but the philosophers of that day were keenly alive to the mystery of kepler's laws, and halley was fully conscious of the grandeur and far-reaching significance of newton's great generalization which embodied all three of kepler's laws in one. newton at last yielded, though reluctantly, and the "principia" was given to the world, though wholly at halley's private charges. but halley was far from being completely engrossed with the absorbing problems of the sky; things terrestrial held for years his undivided attention. imagine present-day lords commissioners of the admiralty intrusting a ship of the british navy to civilian command. yet such was their confidence in halley that he was commissioned as captain of h. m.'s pink _paramour_ in , with instructions to proceed to southern seas for geographical discoveries, and for improving knowledge of the longitude problem, and of the variations of the compass. trade winds and monsoons, charts of magnetic variation, tides and surveys of the channel coast, and experiments with diving bells were practical activities that occupied his attention. halley in became astronomer royal. he was the second incumbent of this great office, but the first to supply the royal observatory with instruments of its own, some of which adorn its walls even to-day. his long series of lunar observations and his magnetic researches were of immense practical value in navigation. halley lived to a ripe old age and left the world vastly better than he found it. his rise from humblest obscurity was most remarkable, and he lived to gratify all the ambitions of his early manhood. "of attractive appearance, pleasing manners, and ready wit," says one of his biographers, "loyal, generous, and free from self-seeking, he was one of the most personally engaging men who ever held the office of astronomer royal." he died in office at greenwich in . "halley was buried," says chambers, "in the churchyard of st. margaret's, lee, not far from greenwich, and it has lately been announced that the admiralty have decided to repair his tomb at the public expense, no descendants of his being known." there is no suitable monument in england to the memory of one of her greatest scientific men. in any event the collection and republication of his epoch-making papers would be welcomed by astronomers of every nation. chapter xvii bradley and aberration living at kew in london early in the th century was an enthusiastic young astronomer, james bradley. he is famous chiefly for his accurate observations of star places which have been invaluable to astronomers of later epochs in ascertaining the proper motions of stars. the latitude of bradley's house in kew was very nearly the same as the declination of the bright star gamma draconis, so that it passed through his zenith once every day. bradley had a zenith sector, and with this he observed with the greatest care the zenith distance of gamma draconis at every possible opportunity. this he did by pointing the telescope on the star and then recording the small angle of its inclination to a fine plumb line. so accurate were his measures that he was probably certain of the star's position to the nearest second of arc. what he hoped to find was the star's motion round a very slight orbit once each year, and due to the earth's motion in its orbit round the sun. in other words, he sought to find the star's parallax if it turned out to be a measurable quantity. it is just as well now that his method of observation proved insufficiently delicate to reveal the parallax of gamma draconis; but his assiduity in observation led him to an unexpected discovery of greater moment at that time. what he really found was that the star had a regular annual orbit; but wholly different from what he expected, and very much larger in amount. this result was most puzzling to bradley. the law of relative motion would require that the star's motion in its expected orbit should be opposite to that of the earth in its annual orbit; instead of which the star was all the time at right angles to the earth's motion. bradley was a frequent traveler by boat on the thames, and the apparent change in the direction of the wind when the boat was in motion is said to have suggested to him what caused the displacement of gamma draconis. the progressive motion of light had been roughly ascertained by roemer: let that be the velocity of the wind. and the earth's motion in its orbit round the sun, let that be the speed of the boat. then as the wind (to an observer on the moving boat) always seems to come from a point in advance of the point it actually proceeds from (to an observer at rest), so the star should be constantly thrown forward by an angle given by the relation of the velocity of light to the speed of the earth in orbital revolution round the sun. the apparent places of all stars are affected in this manner, and this displacement is called the aberration of light. astronomers since bradley's discovery of aberration in have devoted a great deal of attention to this astronomical constant, as it is called, and the arc value of it is very nearly ". . this means that light travels more than ten thousand times as fast as the earth in its orbit ( , miles per second as against the earth's . ). and we can ascertain the sun's distance by aberration also because the exact values of the velocity of light and of the constant of aberration when properly combined give the exact orbital speed of the earth; and this furnishes directly by geometry the radius of the earth's orbit, that is the distance of the sun. in fact, this is one of the more accurate modern methods of ascertaining the distance of the sun. as early as it enabled the writer to calculate the sun's parallax equal to ". , a value absolutely identical with that adopted by the paris conference of , and now universally accepted as the standard. in whatever part of the sky we observe, every star is affected by aberration. at the poles of the ecliptic, - / degrees from the earth's poles, the annual aberration orbits of the stars are very small circles, " in diameter. toward the ecliptic the aberration orbits become more and more oval, ellipses in fact of greater and greater eccentricity, but with their major axes all of the same length, until we reach the ecliptic itself; and then the ellipse is flattened into a straight line " in length, in which the star travels forth and back once a year. exact correspondence of the aberration ellipses of the stars with the annual motion of the earth round the sun affords indisputable proof of this motion, and as every star partakes of the movement, this proof of our motion round the sun becomes many million-fold. indeed, if we were to push a little farther the refinement of our analysis of the effect of aberration on stellar positions, we could prove also the rotation of the earth on its axis, because that motion is swift enough to bear an appreciable ratio to the velocity of light. diurnal aberration is the term applied to this slight effect, and as every star partakes of it, demonstration of the earth's turning round on its axis becomes many million-fold also. chapter xviii the telescope had anyone told ptolemy that his earth-centered system of sun, moon, and stars would ultimately be overthrown, not by philosophy but by the overwhelming evidence furnished by a little optical instrument which so aided the human eye that it could actually see systems of bodies in revolution round each other in the sky, he would no doubt have vehemently denied that any such thing was possible. to be sure, it took fourteen centuries to bring this about, and the discovery even then was without much doubt due to accident. through all this long period when astronomy may be said to have merely existed, practically without any forward step or development, its devotees were unequipped with the sort of instruments which were requisite to make the advance possible. there were astrolabes and armillary spheres, with crudely divided circles, and the excellent work done with them only shows the genius of many of the early astronomers who had nothing better to work with. regarding star-places made with instruments fixed in the meridian, bessel, often called the father of practical astronomy, used to say that, even if you provided a bad observer with the best of instruments, a genius could surpass him with a gun barrel and a cart wheel. before the days of telescopes, that is, prior to the seventeenth century, it was not known whether any of the planets except the earth had a moon or not; consequently the masses of these planets were but very imperfectly ascertained; the phases of mercury and venus were merely conjectured; what were the actual dimensions of the planets could only be guessed at; the approximate distances of sun, moon, and planets were little better than guesses; the distances of the stars were wildly inaccurate; and the positions of the stars on the celestial sphere, and of sun, moon, and planets among them were far removed from modern standards of precision--all because the telescope had not yet become available as an optical adjunct to increase the power of the human eye and enable it to see as if distances were in considerable measure annihilated. galileo almost universally is said to have been the inventor of the telescope, but intimate research into the question would appear to give the honor of that original invention to another, in another country. what galileo deserves the highest praise for, however, is the reinvention independently of an "optick tube" by which he could bring distant objects apparently much nearer to him; and being an astronomer, he was by universal acknowledgment first of all men to turn a telescope on the heavenly bodies. this was in the year , and his first discovery was the phase of venus, his second the four medicean moons or satellites of jupiter, discoveries which at that epoch were of the highest significance in establishing the truth of the copernican system beyond the shadow of doubt. but the first telescopes of which we have record were made, so far as can now be ascertained, in holland very early in the th century. metius, a professor of mathematics, and jansen and lipperhey, who were opticians in middelburg--all three are entitled to consideration as claimants of the original invention of the telescope. but that such an instrument was pretty well known would appear to be shown by his government's refusal of a patent to lipperhey in ; while the officials recognizing the value of such an instrument for purposes of war, got him to construct several telescopes and ordered him to keep the invention a secret. within a year galileo heard that an instrument was in use in holland by which it was possible to see distant objects as if near at hand. skilled in optics as he was, the reinvention was a task neither long nor difficult for him. one of his first instruments magnified but three times; still it made a great sensation in venice where he exhibited the little tube to the authorities of that city, in which he first invented it. galileo's telescope was of the simplest type, with but two lenses; the one a double convex lens with which an image of the distant object is formed, the other a double concave lens, much smaller which was the eye-lens for examining the image. it is this simple form of galilean telescope that is still used in opera glasses and field glasses, because of the shorter tube necessary. galileo carried on the construction of telescopes, all the time improving their quality and enlarging their power until he built one that magnified thirty times. what the diameter of the object glass was we do not know, perhaps two inches or possibly a little more. glass of a quality good enough to make a telescope of cannot have been abundant or even obtainable except with great difficulty in those early days. other discoveries by this first of celestial observers were the spots on the sun, the larger mountains of the moon, the separate stars of which the milky way is composed, and, greatest wonder of all, the anomalous "handles" (_ansæ_, he called them) of saturn, which we now know as the planet's ring, the most wonderful of all the bodies in the sky. since galileo's time, only three centuries past, the progress in size and improvement in quality of the telescope have been marvelous. and this advance would not have been possible except for, first, the discoveries still kept in large part secret by the makers of optical glass which have enabled them to make disks of the largest size; second, the consummate skill of modern opticians in fashioning these disks into perfect lenses; and third, the progress in the mechanical arts and engineering, by which telescope tubes of many tons' weight are mounted or poised so delicately that the thrust of a finger readily swerves them from one point of the heavens to another. as the telescope is the most important of all astronomical instruments, it is necessary to understand its construction and adjustment and how the astronomer uses it. telescopes are optical instruments, and nothing but optical parts would be requisite in making them, if only the optical conditions of their perfect working could be obtained without other mechanical accessories. in original principle, all telescopes are as simple as galileo's; first, an object glass to form the image of the distant object; second the eyepiece usually made of two lenses, but really a microscope, to magnify that image, and working in the same way that any microscope magnifies an object close at hand; and third, a tube to hold all the necessary lenses in the true relative positions. [illustration: the -inch hooker telescope, largest reflector in the world, on mt. wilson. (_photo, mt. wilson solar observatory._)] [illustration: the largest refractor, the -inch telescope at yerkes observatory. dome ft. in diameter. (_photo, yerkes observatory._)] [illustration: the -ft. tower at the mt. wilson solar observatory. at the left is a diagram of tower, telescope and pit. at the upper right is an exterior view of the tower; below a view looking down into the pit, ft. deep. (_photo, mt. wilson solar observatory._)] the focal lengths of object glass and eyepiece will determine just what distance apart the lenses must be in order to give perfect vision. but it is quite as important that the axes of all the lenses be adjusted into one and the same straight line, and then held there rigidly and permanently. otherwise vision with the telescope will be very imperfect and wholly unsatisfactory. the distance from the objective, or object glass to its focal point is called its focal length; and if we divide this by the focal length of the eyepiece, we shall have the magnifying power of the telescope. the eyepiece will usually be made of two lenses, or more, and we use its focal length considered as a single lens, in getting the magnifying power. a telescope will generally have many eyepieces of different focal lengths, so that it will have a corresponding range of magnifying powers. the lowest magnifying power will be not less than four or five diameters for each inch of aperture of the objective; otherwise the eye will fail to receive all the light which falls upon the glass. a -inch telescope will therefore have no eyepiece with a lower magnifying power than about diameters. the highest magnifying power advantageous for a glass of this size will be about to , the working rule being about diameters to each inch of aperture, although the theoretical limit is regarded as . the reason for a variety of eyepieces with different magnifying powers soon becomes apparent on using the telescope. comets and nebulæ call for very low powers, while double stars and the planetary surfaces require the higher powers, provided the state of the atmosphere at the moment will allow it. if there is much quivering and unsteadiness, nothing is gained by trying the higher powers, because all the waves of unsteadiness are magnified also in the same proportion, and sharpness of vision, or fine definition, or "good seeing," as it is called, becomes impossible. the vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found. quite another power of the telescope is dependent on its objective solely: its light-gathering power. light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. in the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. but the object-glass of a telescope, by focusing the rays from a star, pours into the eye, almost as a funnel acts with water, all the light which falls on its larger surface. and as geometry has settled it for us that areas of surfaces are proportioned to the squares of their diameters, a two-inch object glass focuses upon the retina of the eye times as much light as the unassisted eye would receive. and the great -inch objective of the yerkes telescope would, theoretically, yield , times as much light as the eye alone. but there would be a noticeable percentage of this lost through absorption by the glasses of the telescope and scattering by their surfaces. the first makers of telescopes soon encountered a most discouraging difficulty, because it seemed to them absolutely insuperable. this is known as chromatic aberration, or the scattering of light in a telescope due simply to its color or wave length. when light passes through a prism, red is refracted the least and violet the most. through a lens it is the same, because a lens may be regarded as an indefinite system of prisms. the image of a star or planet, then, formed by a single lens cannot be optically perfect; instead it will be a confused intermingling of images of various colors. with low powers this will not be very troublesome, but great indistinctness results from the use of high magnifying powers. the early makers and users of telescopes in the latter part of the seventeenth century found that the troublesome effects of chromatic aberration could be much reduced by increasing the focal length of the objective. this led to what we term engineering difficulties of a very serious nature, because the tubes of great length were very awkward in pointing toward celestial objects, especially near the zenith, where the air is quietest. and it was next to impossible to hold an object steadily in the field, even after all the troubles of getting it there had been successfully overcome. bianchini and cassini, hevelius and huygens were among the active observers of that epoch who built telescopes of extraordinary length, a hundred feet and upward. one tube is said to have been built feet in length, but quite certainly it could never have been used. so-called aerial telescopes were also constructed, in which the objective was mounted on top of a tower or a pole, and the eyepiece moved along near the ground. but it is difficult to see how anything but fleeting glimpses of the heavenly bodies could have been obtained with such contrivances, even if the lenses had been perfect. newton indeed, who was expert in optics, gave up the problem of improving the refracting telescope, and turned his energies toward the reflector. in , half a century after newton and a century and a quarter after galileo, chester more hall, an englishman, found by experiment that chromatic aberration could be nearly eliminated by making the objective of two lenses instead of one, and the same invention was made independently by dollond, an english optician, who took out letters patent about . so the size of telescopes seemed to be limited only by the skill of the glassmaker and the size of disks that he might find it practicable to produce. what hall and dollond did was to make the outer or crown lens of the objective as before, and place behind it a plano-concave lens of dense flint glass. this had the effect of neutralizing the chromatic effect, or color aberration, while at the same time only part of the refractive effect of the crown lens was destroyed. this ingenious but costly combination prepared the way for the great refracting telescopes of the present day, because it solved, or seemed to solve, the important problem of getting the necessary refraction of light rays without harmful dispersion or decomposition of them. through the th century and the first years of the th many telescopes of a size very great for that day were built, and their success seemed complete. with large increase in the size of the disks, however, a new trouble arose, quite inherent in the glass itself. the two kinds of glass, flint and crown, do not decompose white light with uniformity, so that when the so-called achromatic objective was composed of flint and crown, there was an effect known as irrationality of dispersion, or secondary spectrum, which produced a very troublesome residuum of blue light surrounding the images of bright objects. this is the most serious defect of all the great refractors of the day, and effectively it limits their size to about inches of aperture, with present types of flint and crown. it is expected by present experimenters, however, that further improvements in optical glass will do much to extend this limit; so that a refracting telescope of much greater size than any now in existence will be practicable. improvements in mounting telescopes, too, are still possible. within recent years, hartness, of springfield, vermont, has erected a new and ingenious type of turret telescope which protects the observer from wind and cold while his instrument is outside. it affords exceptional facilities for rapid and convenient observing, as for variable stars, and is adaptable to both refractors and reflectors. the captivating study of the heavens can of course be begun with the naked eye alone, but very moderate optical assistance is a great help and stimulates. an opera-glass affords such assistance; a field-glass does still better, and best of all, for certain purposes, is a modern prism-binocular. chapter xix reflectors--mirror telescopes cherished with the utmost care in the rooms of the royal society of london is a world-famous telescope, a diminutive reflector made by the hands of sir isaac newton. we have already mentioned his connection with the refractor; and how he abandoned that type of telescope in favor of the reflecting mirror, or reflector in which the obstacles to great size appeared to be purely mechanical. by many, indeed, newton is regarded as the inventor of the reflector. by the principles of optics, all the rays from a star that strike a concave mirror will be reflected to the geometric focal point, provided a section of that mirror is a parabola. such a mirror is called a speculum, and is an alloy of tin, copper, and bismuth. its surface takes a very high polish, reflecting when newly polished nearly per cent of the light that falls upon it. but the focus where the eyepiece must be used is in front of the mirror, and if the eye were placed there, the observer's head would intercept all or much of the light that would otherwise reach the mirror. gregory, probably the real inventor of the reflector, was the first to dodge this difficulty by perforating the mirror at the center and applying the eyepiece there, at the back of the speculum; but it was necessary to first send the rays to that point by reflection from a second or smaller mirror, in the optical axis of the speculum. this reflects the rays backward down the tube to the eyepiece, or spectroscope, or camera. another english optician, cassegrain, improved on this design somewhat by placing the secondary mirror inside the focus of the speculum, or nearer to it, so that the tube is shorter. this form is preferable for many kinds of astronomical work, especially photography. herschel sought to do away with the secondary reflector entirely and save the loss of light by tilting the speculum slightly, so as to throw the image at one side of the tube; but this modification introduces bad definition of the image and has never been much used. a better plan is that of newton, who placed a small plane speculum at an angle of degrees in the optical axis where the secondary mirror of the gregory-cassegrainian type is placed. the rays are then received by the eyepiece at the side of the upper end of the tube, the observer looking in at right angles to the axis. and a modern improvement first used by draper is a small rectangular prism in place of the little plane speculum, effecting a saving of five to ten per cent of the light. it is not easy to say which type of telescope, the refractor or the reflector, is the more famous. nor which is the better or more useful, or the more likely to lead in the astronomy of the future. when the successors of dollond had carried the achromatic refractor to the limit enforced by the size of the glass disks they were able to secure, they found these instruments not so great an improvement after all. the single-lens telescopes of great focal length were nearly as good optically, though much more awkward to handle. but the quality of the glass obtainable in that day appeared to set an arbitrary limit to that great amplification of size and power which progress in observational astronomy demanded. then came the elder herschel, best known and perhaps the greatest of all astronomers. at bath, england, music was his profession, especially the organ. but he was dissatisfied with his little gregorian reflector, and being a very clever mechanician he set out to build a reflector for himself. it is said that he cast and polished nearly mirrors, in the course of experiments on the most highly reflective type of alloys, and the sort of mechanism that would enable him to give them the highest polish. in all his work he was ably and enthusiastically aided by his sister, caroline herschel, most famous of all women astronomers. upward in size of his mirrors he advanced, till he had a speculum of two feet diameter with a tube feet long. twelve to fifteen years had elapsed when in , while testing one of these reflectors on stars in the constellation gemini, he made the first discovery of a planet since the invention of the telescope--the great planet now known as uranus. under the patronage of king george, he advanced to telescopes of still greater size, his largest being no less than forty feet in length, with a speculum of four feet in diameter. two new satellites of saturn were discovered with this giant reflector, which was dismantled by sir john herschel with appropriate ceremonies, including the singing of an ode by the herschel family assembled inside of the tube, on new year's eve, - . we have record of but few attempts to improve the size and definition of great reflectors by the continental astronomers during this era. in england and ireland, however, great progress was made. about lassell built a two-foot reflector, with which he discovered two new satellites of uranus, and which he subsequently set up in the island of malta. ten years later thomas grubb and son of dublin constructed a four-foot reflector, now at the observatory in melbourne, australia. calver in conjunction with common of ealing, london, about - built several large reflectors, the largest of five feet diameter, now owned by harvard college observatory; and, rather earlier, martin of paris completed a four-foot reflector. the mirrors of these latter instruments were not made of speculum metal, but of solid glass, which must be very thick (one-seventh their diameter) in order to prevent flexure or bending by their own weight. so sensitive is the optical surface to distortion that unless a complicated series of levers and counterpoises is supplied, to support the under surface of the mirror, the perfection of its optical figure disappears when the telescope is directed to objects at different altitudes in the sky. the upper or outer surface of the glass is the one which receives the optical polish on a heavy coat of silver chemically deposited on the polished glass after its figure has been tested and found satisfactory. but far and away the most famous reflecting telescope of all is the "leviathan" of lord rosse, built at birr castle, parsonstown, ireland, about the middle of the last century. his lordship made many ingenious improvements in grinding the mirror, which was of speculum metal, six feet in diameter and weighed seven tons. it was ground to a focal length of fifty-four feet and mounted between heavy walls of masonry, so that the motion of the great tube was restricted to a few degrees on both sides of the meridian. the huge mechanism was very cumbersome in operation, and photography was not available in those days; nevertheless lord rosse's telescope made the epochal discovery of the spiral nebulæ, which no other telescope of that day could have done. in america the reflector has always kept at least even pace with the refractor. as early as , mason and smith, two students at yale college, enthused by denison olmsted, built a -inch speculum with which they made unsurpassed observations of the nebulæ. dr. henry draper, returning from a visit to lord rosse, began about the construction of two silver-on-glass reflectors, one of inches diameter, the other of inches, with which he did important work for many years in photography and spectroscopy, and his mirrors are now the property of harvard college observatory. alvan clark and sons have in later years built a -inch mirror for the lowell observatory in arizona, and very recently a -foot silver-on-glass mirror has been set up in the dominion of canada astrophysical observatory at victoria, british columbia, where it is doing excellent work in the hands of plaskett, its designer. the huge glass disk for the reflector weighs two tons, and it must be cast so that there are no internal strains; otherwise it is liable to burst in fragments in the process of grinding. it should be free from air-bubbles, too; so the glass is cast in one melting, if possible. this disk was made by the st. gobain plate glass company, whose works have been ruthlessly destroyed by the enemy during the war; but fortunately the great disk had been shipped from antwerp only a week before declaration of hostilities. brashear of allegheny was intrusted with the optical parts, which occupied many months of critical work. the finished mirror is inches in diameter, its focal length is feet, and its thickness inches. a central hole inches in diameter makes possible its use as a gregorian or cassegrainian type, as well as newtonian. the mechanical parts of this great telescope are by warner and swasey of cleveland, after the well-known equatorial mounting of the melbourne reflector by grubb of dublin. friction of the polar and declination axes is reduced by ball bearings. the -foot dome has an opening feet wide and extending six feet beyond the zenith. all motions of the telescope, dome shutters, and observing platform are under complete control by electric motors. spectroscopic binaries form one of the special fields of research with this powerful instrument, and many new binaries have already been detected. the great reflectors designed and constructed by ritchey, formerly of chicago and now of pasadena, deserve especial mention. while connected with the yerkes observatory he constructed a two-foot reflector for that institution, with which he had exceptional success in photography of the stars and nebulæ. later he built a -foot reflector, now at the carnegie observatory on mount wilson, california, with which the spiral nebulæ and many other celestial objects have been especially well photographed. ritchey's later years have been spent on the construction of an even greater mirror, no less than inches in diameter, which was completed in , and has already yielded photographic results dealt with farther on, and far surpassing anything previously obtained. theoretically this huge mirror, if its surface were perfectly reflective so that it would transmit all the rays falling upon it, would gather , times as much light as the unaided eye alone. whether a -inch refractor, should it ever be constructed, would surpass the -inch reflector as an all-round engine for astronomical research, is a question that can only be fully answered by building it and trying the two instruments alongside. probably three-quarters of all the really great astronomical work in the past has been done by refractors. they are always ready and convenient for use, and the optical surfaces rarely require cleaning and readjustment. with increase of size, however, the secondary spectrum becomes very bothersome in the great lenses; and the larger they are, the more light is lost by absorption on account of the increasing thickness of the lenses. with the reflector on the other hand, while there is clearly a greater range of size, the reflective surface retains its high polish only a brief period, so that mere tarnish effectively reduces the aperture; and the great mirror is more or less ineffective in consequence of flexure uncompensated by the lever system that supports the back of the mirror. both types of telescope still have their enthusiastic devotees; and the next great reflector would doubtless be a gratifying success, if mounted in some elevated region of the world, like the andes of northern chile, where the air is exceptionally steady and the sky very clear a large part of the year. the highest magnifying powers suitable for work with such a telescope could then be employed, and new discoveries added as well as important work done in extension of lines already begun on the universe of stars. on the authority of clark, even a six-foot objective would not necessitate a combined thickness of its glasses in excess of six inches. present disks are vastly superior to the early ones in transparency, and there is reason to expect still greater improvement. the engineering troubles incident to execution of the mechanical side of the scheme need not stand in the way; they never have, indeed the astronomer has but just begun to invoke the fertile resources of the modern engineer. not long before his death the younger clark who had just finished the great lenses of the -inch yerkes telescope, ventured this prevision, already in part come true: "the new astronomy, as well as the old, demands more power. problems wait for their solution, and theories to be substantiated or disproved. the horizon of science has been greatly broadened within the last few years, but out upon the borderland i see the glimmer of new lights that await for their interpretation, and the great telescopes of the future must be their interpreters." practically all the great telescopes of the world have in turn signalized the new accession of power by some significant astronomical discovery: to specify, one of herschel's reflectors first revealed the planet uranus; lord rosse's "leviathan" the spiral nebulæ; the -inch cambridge lens the crape, or dusky ring of saturn; the - / -inch chicago refractor the companion of sirius; the washington -inch telescope the satellites of mars; the -inch pulkowa glass the nebulosities of the pleiades; and the -inch lick telescope brought to light a fifth satellite of jupiter. at the time these discoveries were made, each of these great telescopes was the only instrument then in existence with power enough to have made the discovery possible. so we may advance to still farther accessions of power with the expectation that greater discoveries will continue to gratify our confidence. chapter xx the story of the spectroscope sir isaac newton ought really to have been the inventor of the spectroscope, because he began by analyzing light in the rough with prisms, was very expert in optics, and was certainly enough of a philosopher to have laid the foundations of the science. what newton did was to admit sunlight into a darkened room through a small round aperture, then pass the rays through a glass prism and receive the band of color on a screen. he noticed the succession of colors correctly--violet, indigo, blue, green, yellow, orange, red; also that they were not pure colors, but overlapping bands of color. apparently neither he nor any other experimenter for more than a century went any further, when the next essential step was taken by wollaston about in england. he saw that by receiving the light through a narrow slit instead of a round hole, he got a purer spectrum, spectrum being the name given to the succession of colors into which the prism splits up or decomposes the original beam of white sunlight. this seemingly insignificant change, a narrow slit replacing the round hole, made wollaston and not newton the discoverer of the dark lines crossing the spectrum at various irregular intervals, and these singularly neglected lines meant the basis of a new and most important science. even wollaston, however, passed them by, and it was fraunhofer who in - first made a chart of them. consequently they are known as fraunhofer lines, or dark absorption lines. sending the beam of light through a succession of prisms gives greater dispersion and increases the power of the spectroscope. the greater the dispersion the greater the number of absorption lines; and it is the number and intensity of these lines, with their accurate position throughout the range of the spectrum which becomes the basis of spectrum analysis. the half century that saw the invention of the steam engine, photography, the railroad and the telegraph elapsed without any farther developments than mere mapping of the fundamental lines, a, b, c, d, e, f, g, h of the solar spectrum. the moon, too, was examined and its spectrum found the same, as was to be expected from sunlight simply reflected. sir john herschel and other experimenters came near guessing the significance of the dark lines, but the problem of unraveling their mystery was finally solved by bunsen and kirchhoff who ascertained that an incandescent gas emits rays of exactly the same degree of refrangibility which it absorbs when white light is passed through it. this great discovery was at once received as the secure basis of spectrum analysis, and kirchhoff in put in compact and comprehensive form the three following principles underlying the theory of the science: ( ) solid and liquid bodies, also gases under high pressure, give when incandescent a continuous spectrum, that is one with a mere succession of colors, and neither bright nor dark lines; ( ) gases under low pressure give a discontinuous spectrum, crossed by bright lines whose number and position in the spectrum differ according to the substances vaporized; ( ) when white light passes through a gas, this medium absorbs or quenches rays of identical wave-length with those composing its own bright-line spectrum. clearly then it makes no difference where the light originates whether it comes from sun or star. only it must be bright enough so that we can analyze it with the spectroscope. but our analysis of sun and star could not proceed until the chemist had vaporized in the laboratory all the elements, and charted their spectra with accuracy. when this had been done, every substance became at once recognizable by the number and position of its lines, with practical certainty. how then can we be sure of the chemical and physical composition of sun and stars? only by detailed and critical comparison of their spectra with the laboratory spectra of elements which chemical and physical research have supplied. as in the sun, so in the stars, each of which is encircled by a gaseous absorptive layer or atmosphere, the light rays from the self-luminous inner sphere must pass through this reversing layer, which absorbs light of exactly the same wave-length as the lines that make up its own bright line spectrum. whatever substances are here found in gaseous condition, the same will be evident by dark lines in the spectrum of sun or star, and the position of these dark lines will show, by coincidence with the position of the laboratory bright lines, all the substances that are vaporized in the atmospheres of the self-luminous bodies of the sky. here then originated the science of the new astronomy: the old astronomy had concerned itself mainly with positions of the heavenly bodies, _where_ they are; the new astronomy deals with their chemical composition and physical constitution, and _what_ they are. between and the fundamental application of the basic principles was well advanced by the researches of sir william huggins in england, of father angelo secchi in rome, of jules janssen in paris, and of dr. henry draper in new york. in analyzing the spectrum of the sun, many thousands of dark absorption lines are found, and their coincidences with the bright lines of terrestrial elements show that iron, for instance, is most prominently identified, with rather more than , coincidences of bright and dark lines. calcium, too, is indicated by peculiar intensity of its lines, as well as their great number. next in order are hydrogen, nickel and sodium. by prolonged and minute comparison of the solar spectrum with spectra of terrestrial elements, something like forty elemental substances are now known to exist in the sun. rowland's splendid photographs of the solar spectrum have contributed most effectively. about half of these elements, though not in order of certainty, are aluminum, cadmium, calcium, carbon, chromium, cobalt, copper, hydrogen, iron, magnesium, manganese, nickel, scandium, silicon, silver, sodium, titanium, vanadium, yttrium, zinc, and zirconium. oxygen, too, is pretty surely indicated; but certain elements abundant on earth, as nitrogen and chlorine, together with gold, mercury, phosphorus, and sulphur, are not found in the sun. the two brilliant red stars, aldebaran in taurus, and betelgeuse in orion, were the first stars whose chemical constitution was revealed to the eye of man, and sir william huggins of london was the astronomer who achieved this epoch-making result. father secchi of the vatican observatory proceeded at once with the visual examination of the spectra of hundreds of the brighter stars, and he was the first to provide a classification of stellar spectra. there were four types. secchi's type i is characterized chiefly by the breadth and intensity of dark hydrogen lines, together with a faintness or entire absence of metallic lines. these are bluish or white stars and they are very abundant, nearly half of all the stars. vega, altair, and numerous other bright stars belong to this type, and especially sirius, which gives to the type the name "sirians." type ii is characterized by a multitude of fine dark metallic lines, closely resembling the lines of the solar spectrum. these stars are somewhat yellowish in tinge like the sun, and from this similarity of spectra they are called "solars." arcturus and capella are "solars," and on the whole the solars are rather less numerous than the sirians. stars nearest to the solar system are mostly of this type, and, according to kapteyn of groningen, the absolute luminous power of first type stars exceeds that of second type stars seven-fold. secchi's type iii is characterized by many dark bands, well defined on the side toward the blue end of the spectrum, but shading off toward the red--a "colonnaded spectrum", as miss clerke aptly terms it. alpha herculis, antares, and mira, together with orange and reddish stars and most of the variable stars, belong in type iii. type iv is also characterized by dark bands, often called "flutings," similar to those of type iii, but reversed as to shading, that is, well defined on the side toward the red, but fading out toward the blue. their atmospheres contain carbon; but they are not abundant, besides being faint and nearly all blood-red in tint. following up the brilliant researches of draper, who in obtained the first successful photograph of a star's spectrum, that of vega, pickering of harvard supplemented secchi's classification by type v, a spectrum characterized by bright lines. they, too, are not abundant and are all found near the middle of the galaxy. these are usually known as wolf-rayet stars, from the two paris astronomers who first investigated their spectra. type v stars are a class of objects seemingly apart from the rest of the stellar universe, and many of the planetary nebulæ yield the same sort of a spectrum. the late mrs. anna palmer draper, widow of dr. henry draper, established the henry draper memorial at harvard, and investigation of the photographic spectra of all the brighter stars of the entire heavens has been prosecuted on a comprehensive scale, those of the northern hemisphere at cambridge, and of the southern at arequipa, peru. these researches have led to a broad reclassification of the stars into eight distinct groups, a work of exceptional magnitude begun by the late mrs. fleming and recently completed by miss annie cannon, who classified the photographic spectra of more than , stars on the new system, as follows:-- the letters o, b, a, f, g, k, m, n represent a continuous gradation in the supposed order of stellar evolution, and farther subdivision is indicated by tenths, g k meaning a type half way between g and k, and usually written g simply. b would indicate a type between b and a, but nearer to b than a, and so on. on this system, the spectrum of a star in the earliest stages of its evolution is made up of diffuse bright bands on a faint continuous background. as these bands become fewer and narrower, very faint absorption lines begin to appear, first the helium lines, followed by several series of hydrogen lines. on the disappearance of the bright bands, the spectrum becomes wholly absorptive bands and lines. then comes a very great increase in intensity of the true hydrogen spectrum, with wide and much diffused lines, and few if any other lines. then the h and k calcium lines and other lines peculiar to the sun become more and more intense. then the hydrogen lines go through their long decline. the calcium spectrum becomes intense, and later the spectrum becomes quite like that of the sun with a great wealth of lines. following this stage the spectrum shortens from the ultra violet, the hydrogen lines fade out still farther, and bands due to metallic compounds make their appearance, the entire spectrum finally resembling that of sun spots. to designate these types rather more categorically:-- type o--bright bands on a faint continuous background, with five subdivisions, oa, ob, oc, od, oe, according to the varying width and intensity of the bands. type b--the orion type, or helium type, with additional lines of origin unknown as yet, but without any of the bright bands of type o. type a--the sirian type, the regular balmer series of hydrogen lines being very intense, with a few other lines not conspicuously marked. type f--the calcium type, hydrogen lines less strongly marked, but with the narrow calcium lines h and k very intense. type g--the solar type, with multitudes of metallic lines. type k--in some respects similar to g, but with the hydrogen lines fading out, and the metallic lines relatively more prominent. type m--spectrum with peculiar flutings due to titanium oxide, with subdivisions ma and mb, and the variable stars of long period, with a few bright hydrogen lines additional, in a separate class md. type n--similar to m, in that both are pronouncedly reddish, but with characteristic flutings probably indicating carbon compounds. the draper classification being based on photographic spectra, and the original secchi classification being visual, the relation of the two systems is approximately as follows: secchi type i includes draper b & a ii includes draper f, g & k iii includes draper m iv includes draper n pickering's marked success in organization and execution of this great programme was due to his adoption of the "slitless spectroscope," which made it possible to photograph stellar spectra in vast numbers on a single plate. the first observers of stellar spectra placed the spectroscope beyond the focus of the telescope with which it was used, thereby limiting the examination to but one star at a time. in the slitless spectroscope, a large prism is mounted in front of the objective (of short focus), so that the star's rays pass through it first, and then are brought to the same focus on the photographic plate, for all the stars within the field of view, sometimes many thousand in number. this arrangement provides great advantages in the comparison and classification of stellar spectra. when spectroscopic methods were first introduced into astronomy, there was no expectation that the field of the old or so-called exact astronomy would be invaded. physicists were sometimes jocularly greeted among astronomers as "ribbon men," and no one even dreamed that their researches were one day to advance to equal recognition with results derived from micrometer, meridian circle, and heliometer. the first step in this direction was taken in by sir william huggins of london, who noticed small displacements in the lines of spectra of very bright stars. in fact the whole spectrum appeared to be shifted; in the case of sirius it was shifted toward the red, while the whole spectrum of arcturus was shifted by three times this amount toward the violet end of the spectrum. the reason was not difficult to assign. as early as doppler had enunciated the principle that when we are approaching or are approached by a body which is emitting regular vibrations, then the number of waves we receive in a second is increased, and their wave-length correspondingly diminished; and just the reverse of this occurs when the distance of the vibrating body is increasing. it is the same with light as with sound, and everyone has noticed how the pitch of a locomotive whistle suddenly rises as it passes, and falls as suddenly on retreating from us. so huggins drew the immediate inference that the distance between the earth and sirius was increasing at the rate of nearly twenty miles per second, while arcturus was nearing us with a velocity of sixty miles per second. these pioneer observations of motions in the line of sight, or radial velocities as they are now called, led directly to the acceptance of the high value of spectroscopic work as an adjunct of exact astronomy in stellar research. nor has it been found wanting in application to a great variety of exact problems in the solar system which would have been wholly impossible to solve without it. foremost is the sun, of course, because of the overplus of light. young early measured the displacement of lines in the spectra of the prominences, and found velocities sometimes exceeding miles per second. many astronomers, dunér among them, investigated the rotation of the sun by the spectroscopic method. the sun's east limb is coming toward us, while the west is going from us; and by measuring the sum of the displacements, the rate of rotation has been calculated, not only at the sun's equator but at many solar latitudes also, both north and south. as was to be expected, these results agree well with the sun's rotation as found by the transits of sun spots in the lower latitudes where they make their appearance. bélopolsky has applied the same method to the rotation of the planet venus, and keeler, by measuring the displacement of lines in the spectrum of saturn, on opposite sides of the ring, provided a brilliant observational proof of the physical constitution of the rings; because he showed that the inner ring traveled round more swiftly than the outer one, thus demonstrating that the ring could not be solid, but must be composed of multitudes of small particles traveling around the ball of saturn, much as if they were satellites. indeed, keeler ascertained the velocity of their orbital motion and found that in each case it agreed exactly with that required by the keplerian law. even the filmy corona of the sun was investigated in similar fashion by deslandres at the total eclipse of , and he found that it rotates bodily with the sun. but the complete vindication of the spectroscopic method as an adjunct of the old astronomy came with its application to measurement of the distance of the sun. the method is very interesting and was first suggested by campbell in . spectrum-line measurements have become very accurate with the introduction of dry-plate photography, and ecliptic stars were spectrographed, toward and from which the earth is traveling by its orbital motion round the sun. by accurate measurement of these displacements, the orbital velocity of the earth is calculated; and as we know the exact length of the year, or a complete period, the length of the orbit itself in miles becomes known, and thus, by simple mensuration, the length of the radius of the orbit--which is the distance of the sun. if we pass from sun to star, the triumph of the spectroscope has been everywhere complete and significant. as the spectroscopic survey of the stars grew toward completeness, it became evident that the swarming hosts of the stellar universe are in constant motion through space, not only athwart the line of vision as their proper motions had long disclosed, but some stars are swiftly moving toward our solar system and others as swiftly from it. fixed stars, strictly speaking--there are no such. all are in relative motion. exact astronomy by discussion of the proper motions had assigned a region of the sky toward which the sun and planets are moving. spectrography soon verified this direction not only, but gave a determination of the velocity of our motion of twelve miles per second in a direction approximately that of the constellation lyra. from corresponding radial velocities, we draw the ready conclusion that certain groups or clusters of stars are actually connected in space and moving as related systems, as in the pleiades and ursa major. rather more than a quarter century ago, the spectroscope came to the assistance of the telescope in helping to solve the intricate problem of stellar distribution. kapteyn, by combining the proper motions of certain stars with their classification in the draper catalogue of stellar spectra, drew the conclusion that, as stars having very small proper motions show a condensation toward the galaxy, the stars composing this girdle are mostly of the sirian type, and are at vast distances from the solar system. the proper motion of a star near to us will ordinarily be large, and, in the case of solar stars, the larger their proper motion the greater their number. so it would appear that the solar stars are aggregated round the sun himself, and this conclusion is greatly strengthened by the fact that of stars whose distances and spectral type are both ascertained, seven of the eight nearest to us are solar stars. in the spectroscope achieved an unexpected triumph by enabling the late professor pickering to make the first discovery of a spectroscopic double, or binary star, a type of object now quite abundant. unlike the visual binary systems whose periods are years in length, the spectroscopic binaries have short periods, reckoned in some cases in days, or hours even. if the orbit of a very close binary is seen edge on, the light of the two stars will coalesce twice in every revolution. halfway between these points there are two times when the two stars will be moving, one toward the earth and the other from it. at all times the light of the star, in so far as the telescope shows it, proceeds from a single object. now photograph the star's spectrum at each of the four critical points above indicated: in the first pair the lines are sharply defined and single, because at conjunction the stars are simply moving athwart the line of sight, while at the intermediate points the lines are double. doppler's principle completely accounts for this: the light from the receding companion is giving lines displaced toward the red, while the approaching companion yields lines displaced toward the violet. mizar, the double star at the bend of the handle in the great dipper was the first star to yield this peculiar type of spectrum, and the period of its invisible companion is about days. the relative velocity of the components is miles a second, and applying newton's law we find its mass exceeds that of the sun forty-fold. capella has been found to be a spectroscopic binary; also the pole star. spectroscopic binaries have relatively short periods, one of the shortest known being only hours in length. it is in the constellation scorpio. beta aurigæ is another whose lines double on alternate nights, giving a period of four days; and the combined mass of both stars is more than twice that of the sun. the catalogue of spectroscopic binaries is constantly enlarging; but thousands doubtless exist that can never be discovered by this method, as is evident if their orbits are perpendicular to the line of sight or nearly so. the history of the spectroscopic binaries is one of the most interesting chapters in astronomy, and affords a marvelous confirmation of the prediction of bessel who first wrote of "the astronomy of the invisible." find a star's distance by the spectroscope? impossible, everyone would have said, even a very few years ago. now, however, the thing is done, and with increasing accuracy. adams of mount wilson has found, after protracted investigation, that the relative intensity of certain spectral lines varies according to the absolute brightness of a star; indeed, so close is the correspondence that the spectroscopic observations are employed to provide in certain cases a good determination of the absolute magnitude, and therefore of the distance. to test this relation, the spectroscopic parallaxes have been compared with the measured parallaxes in numerous instances, and an excellent agreement is shown. this new method is adding extensively to our knowledge of stellar luminosities and distances, and even the vast distances of globular clusters and spiral nebulæ are becoming known. in fact, but few departments of the old astronomy are left which the new astronomy has not invaded, and this latest triumph of the spectroscope in determining accurately the distances of even the remotest stars is enthusiastically welcomed by advocates of the old and new astronomy alike. chapter xxi the story of astronomical photography the most powerful ally of both telescope and spectroscope is photography. without it the marvelous researches carried on with both these types of instrument would have been essentially impossible. even the great telescopes of herschel and lord rosse, notwithstanding their splendid record as optical instruments, might have achieved vastly more had photography been developed in their time to the point where the astronomer could have employed its wonderful capabilities as he does to-day. and, with the spectroscope, it is hardly too much to say that no investigator ever observes visually with that instrument any more: practically every spectrum is made a matter of photographic record first. the observing, or nowadays the measuring, is all done afterward. all telescopes and cameras are alike, in that each must form or have formed within it an image by means of a lens or mirror. in the telescope the eye sees the fleeting image, in the camera the process of registering the image on a plate or film is known as photography. daguerre first invented the process (silver film on a copper plate) in . the year following it was first employed on the moon, in the first star was photographed, in the first total eclipse of the sun; all by the primitive daguerreotype process, which, notwithstanding its awkwardness and the great length of exposure required, was found to possess many advantages for astronomical work. about the middle of the last century the wet plate process, so called because the sensitized collodion film must be kept moist during exposure, came into general use, and the astronomers of that period were not slow to avail themselves of the advantages of a more sensitive process, which in , in the skillful hands of henry draper, produced the first spectrum of a star. in a nebula was first photographed, and in a comet. before this time, however, the new dry-plate process had been developed to the point where astronomers began to avail of its greater convenience and increased sensitiveness, even in spite of the coarseness of grain of the film. forty years of dry-plate service have brought a wealth of advantages scarcely dreamed of in the beginning, and nearly every department of astronomical research has been enhanced thereby, while many entirely new photographic methods of investigation have been worked out. continued improvement in photographic processes has provided the possibility of pictures of fainter and fainter celestial objects, and all the larger telescopes have photographed stars and nebulæ of such exceeding faintness that the human eye, even if applied to the same instrument, would never be able to see them. this is because the eye, in ten or twelve seconds of keen watching, becomes fatigued and must be rested, whereas the action of very faint light rays is cumulative on the highly sensitive film; so that a continuous exposure of many hours' duration becomes readily visible to the eye on development. so a supersensitive dry plate will often record many thousand stars in a region where the naked eye can see but one. perhaps the greatest amplification of photography has taken place at the harvard observatory under pickering, where a library of many hundred thousand plates has accumulated; and at groningen, holland, where kapteyn has established an astronomical laboratory without instruments except such as are necessary to measure photographic plates, whenever and wherever taken. so it is possible to select the clearest of skies, all over the world, for exposure of the plates, and bring back the photographs for expert discussion. of course the sun was the celestial body first photographed, and its surpassing brilliance necessitates reduction of exposure to a minimum. in moments of exceptional steadiness of the atmosphere, a very high degree of magnification of the solar surface on the photographic plate is permitted, and the details in formation, development, and ending of sun spots are faithfully registered. nevertheless, it cannot be said that photography has yet entirely replaced the eye in this work, and careful drawings of sun spots at critical stages in their life are capable of registering fine detail which the plate has so far been unable to record. janssen of paris took photographs of the solar photosphere so highly magnified that the granulation or willow-leaf structure of the surface was clearly visible, and its variations traceable from hour to hour. the advantages of sun spot photography in ascertaining the sun's rotation, keeping count of the spots, and in a permanent record for measurement of position of the sun's axis and the spot zones, are obvious. in direct portrayal of the sun's corona during total eclipses, photography has offered superior advantages over visual sketching, in the form and exact location of the coronal streamers; but the extraordinary differences of intensity between the inner corona and its outlying extensions are such that halation renders a complete picture on a single plate practically impossible. the filamentous detail of the inner corona, and the faintest outlying extensions or streamers, the eye must still reveal directly. in solar spectrum photography, research has been especially benefited; indeed, exact registry of the multitudinous lines was quite impossible without it. photographic maps of the spectrum by thollon, mcclean and rowland are so complete and accurate that no visual charts can approach them. rowland's great photographic map of the solar spectrum spread out into a band about forty feet in length; and in the infra-red, langley's spectrobolometer extended the invisible heat spectrum photographically to many times that length. at the other end of the spectrum, special photographic processes have extended the ultra-violet spectrum far beyond the ocular limit, to a point where it is abruptly cut off by absorption of the earth's atmosphere. on the same plate with certain regions of the sun's spectrum, the spectra of terrestrial metals are photographed side by side, and exact coincidences of lines show that about forty elemental substances known to terrestrial chemistry are vaporized in the sun. [illustration: a view of the -foot dome in which the largest telescope in the world is housed. (_courtesy, mt. wilson solar observatory._)] [illustration: mount chimborazo, near the equator. an observatory located on this mountain would make it possible to study the phenomena of northern and southern skies from the same point. (_courtesy, pan-american union._)] [illustration: lick observatory, on the summit of mt. hamilton, about twenty-five miles s. w. of san jose, california. it contains the famous lick telescope, a -inch refractor.] [illustration: near view of the eye-end of the yerkes telescope. the eyepiece is removed and its place taken by a photographic plate.] young was the first to photograph a solar prominence in , and twenty years later deslandres of paris and hale of chicago independently invented the spectroheliograph, by which the chromosphere and prominences of the sun, as well as the disk of the sun itself, are all photographed by monochromatic light on a single plate. hale has developed this instrument almost to the limit, first at the yerkes observatory of the university of chicago, and more recently at the mount wilson observatory of the carnegie institution, where spectroheliograms of marvelous perfection are daily taken. it was with this instrument that hale discovered the effect of an electro-magnetic field in sun spots which has revolutionized solar theories, a research impossible to conceive of without the aid of photography. when we apply doppler's principle, photography becomes doubly advantageous, whether we determine, as dunér did and more recently adams, the sun's own rotation and find it to vary in different solar latitudes, the equator going fastest; or apply the method to the sun's corona at the east and west limbs of the sun, which deslandres in proved to be rotating bodily with the sun, because of the measured displacement of spectral lines of the corona in juxtaposition on the photographic plate. in the solar astronomy of measurement, too, photography has been helpfully utilized, as in registering the transits of mercury over the sun's disk, for correcting the tables of the planet's orbital motion; and most prominently in the action taken by the principal governments of the world in sending out expeditions to observe the transits of venus in and , for the purpose of determining the parallax of venus and so the distance of the earth from the sun. in our studies of the moon, photography has almost completely superseded ocular work during the past sixty years. rutherfurd and draper of new york about obtained very excellent lunar photographs with wet plates, which were unexcelled for nearly half a century. the harvard, lick, and paris observatories have published pretty complete photographic atlases of the moon, and the best negatives of these series show nearly everything that the eye can discern, except under unusual circumstances. later lunar photography was taken up at the yerkes observatory, and exceptionally fine photographs on a large scale were obtained with the -inch refractor, using a color screen. more recently the -inch and -inch mirrors of the mount wilson observatory have taken a series of photographs of the moon far surpassing everything previously done, as was to be expected from the unique combination of a tranquil mountain atmosphere with the extraordinary optical power of the instruments, and a special adaptation of photographic methods. during lunar eclipses, pickering has made a photographic search for a possible satellite of the moon, occultations of stars by the moon have been recorded by photography, and russell of princeton has shown how the position of the moon among the stars can be determined by the aid of photography with a high order of precision. the story of planetary photography is on the whole disappointing. much has been done, but there is much that is within reach, or ought to be, that remains undone. from mercury nothing ought perhaps to be expected. on many of the photographs of the transit of venus, especially those taken under the writer's direction at the lick observatory in , we have unmistakable evidence of the planet's atmosphere. here again the wet plate process, although more clumsy, demonstrated its superiority over the dry process used by other expeditions. in spectroscopy, bélopolsky has sought to determine the period of rotation of venus on her axis. at the lowell observatory, douglass succeeded in photographing the faint zodiacal light, and very successful photographs of mars were taken at this institution as early as by slipher. two years later these were much improved upon by the writer's expedition to the andes of chile, when , exposures of mars were made, many of them showing the principal _canali_, and other prominent features of the planet's disk. at subsequent oppositions of the planet, barnard at the yerkes observatory and the mount wilson observers have far surpassed all these photographs. for future oppositions a more sensitive film is highly desired, in connection with instruments possessing greater light-gathering power, so permitting a briefer exposure that will be less influenced by irregularities and defects of the atmosphere. the spectrum of mars is of course that of sunlight, very much reduced, and modified to a slight extent by its passing twice through the atmosphere of mars. what amount of aqueous vapor that atmosphere may contain is a question that can be answered only by critical comparison of the martian spectrum with the spectrum of the moon, and photography affords the only method by which this can be done. many are the ways in which photography has aided research on the asteroid group. since more than of them have been discovered by photography, and it is many times easier to find the new object on the photographic plate than to detect it in the sky as was formerly done by means of star charts. the planet by its motion during the exposure of the plate produces a trail, whereas the surrounding stars are all round dots or images. or by moving the plate slightly during exposure, as in metcalf's ingenious method, we may catch the planet at that point where it will give a nearly circular image, and thus be quite as easy to detect, because all the stars on the same plate will then be trails. photographic photometry of the asteroids has revealed marked variations in their light, due perhaps to irregularities of figure. on account of their faint light, the asteroids are especially suited, as mars is not, to exact photography for ascertaining their parallax, and from this the sun's distance when the asteroid's distance has been found. many asteroids have been utilized in this way, in particular eros ( ). in it approaches the earth within million miles, when the photographic method will doubtless give the sun's distance with the utmost accuracy. photographs of jupiter have been very successfully taken at the yerkes and lowell observatories and elsewhere, but the great depth of the planet's atmosphere is highly absorptive, so that the impression is very weak in the neighborhood of the limb, if the exposure is correctly timed for the center of the disk. the striking detail of the belts, however, is excellently shown. wood of baltimore has obtained excellent results by monochromatic photography of jupiter and saturn with the -inch reflector on mount wilson. jupiter's satellites have not been neglected photographically, and pickering has observed hundreds of the eclipses of the satellites by a sort of cinematographic method of repeated exposures, around the time of disappearance and reappearance by eclipse. the newest outer satellites of jupiter were all discovered by photography, and it is extremely doubtful if they would have been found otherwise. saturn has long been a favorite object with the astronomical photographer, and there are many fine pictures in spite of his yellowish light, relatively weak photographically. the marvelous ring system with the cassini division, the oblateness of the ball, the occasional markings on it--all are well shown in the best photographs; but the call is for more light and a more sensitive photographic process. pickering's ninth satellite (phoebe) was discovered by photography, one of the faintest moons in the solar system. like the faint outer moons of jupiter, few existing telescopes are powerful enough to show it. its orbit has been found from photographic observations, and its position is checked up from time to time by photography. but the crowning achievement of spectrum photography in the saturnian system is keeler's application of doppler's principle in determining the rate of orbital motion of particles in different zones of the rings, thereby establishing the maxwellian theory of the constitution of the rings beyond the possibility of doubt. for uranus and neptune photography has availed but little, except to negative the existence of additional satellites of these planets, which doubtless would have been discovered by the thorough photographic search which has been made for them by w. h. pickering without result. as with the asteroids, so with comets: several of these bodies have been discovered by photography; none more spectacular than the egyptian comet of may th, , which impressed itself on the plates of the corona of that date. withdrawal of the sun's light by total eclipse made the comet visible, and it had never been seen before, nor is it known whether it will ever return. in cometary photography, much the same difficulties are present as in photographing the corona: if the plate is exposed long enough to get the faint extensions of the tail, the fine filaments of the coma or head are obliterated by halation and overexposure. no one has had greater success in this work than barnard, whose photographs of comets, particularly at the lick observatory, are numerous and unexcelled. his photographs of the brooks comet of revealed rapid and violent changes in the tail, as if shattered by encounter with meteors; and the tail of halley's comet in showed the rapid propagation of luminous waves down the tail, similar to phenomena sometimes seen in streamers of the aurora. draper obtained the first photograph of a comet's spectrum in , disclosing an identity with hydrocarbons burning in a bunsen flame, also bands in the violet due to carbon compounds. the photographic spectra of subsequent comets have shown bright lines due to sodium and the vapor of iron and magnesium. even the elusive meteor has been caught by photography, first by wolf in , who was exposing a plate on stars in the milky way. on developing it, he found a fine, dark nearly uniform line crossing it, due to the accidental flight across the field of a meteor of varying brightness. since then meteor trails have been repeatedly photographed, and even the trail spectra of meteors have been registered on the harvard plates. at yale in elkin employed a unique apparatus for securing photographic trails of meteors: six photographic cameras mounted at different angles on a long polar axis driven by clockwork, the whole arranged so as to cover a large area of the sky where meteors were expected. when we pass from the solar system to the stellar universe the advantages of photography and the amplification of research due to its employment as accessory in nearly every line of investigation are enormous. so extensively has photography been introduced that plates, and to a slight extent films, are now almost exclusively used in securing original records. regrettably so in case of the nebulæ, because the numerous photographs of the brighter nebulæ taken since when draper got the first photograph of the nebula of orion, are as a rule not comparable with each other. differences of instruments, of plates, of exposure, and development--all have occasioned differences in portrayal of a nebula which do not exist. when we consider faithful accuracy of portrayal of the nebulæ for purposes of critical comparison from age to age, many of our nebular photographs of the past forty years, fine as they are and marvelous as they are, must fail to serve the purpose of revealing progressive changes in nebular features in the future. roberts and common in england were among the first to obtain nebular photographs with extraordinary detail, also the brothers henry of paris. as early as roberts revealed the true nature of the great nebula in andromeda, which had never been suspected of being spiral; and keeler and perrine at the lick observatory pushed the photographic discovery of spiral nebulæ so far that their estimates fill the sky with many hundred thousands of these objects. in the southern hemisphere the -inch bruce telescope of harvard college observatory has obtained many very remarkable photographs of nebulæ, particularly in the vicinity of eta carinæ. but the great reflectors of the mount wilson observatory, on account of their exceptional location and extraordinary power, have surpassed all others in the photographic portrayal of these objects, especially of the spiral nebulæ which appear to show all stages in transition from nebula to star. no less remarkable are the photographs of such wonderful clusters as omega centauri, a perfect visual representation of which is wholly impossible. intercomparison of the photographs of clusters has afforded bailey of harvard, shapley of mount wilson and others the opportunity of discovery that hundreds of the component stars are variable. what is the longest photographic exposure ever made? at the cape of good hope, under the direction of the late sir david gill, exposures on nebulæ were made, utilizing the best part of several nights, and totaling as high as seventeen, or even twenty-three hours. but the mount wilson observers have far surpassed this duration. to study the rotation and radial velocity of the central part of the nebula of andromeda, an exposure of no less than hours' total duration was made on the exceedingly faint spectrum, and even that record has since been exceeded. the eye cannot be removed from the guiding star for a moment while the exposure is in progress, and this tedious piece of work was rewarded by determining the velocity of the center of the nucleus as a motion of approach at the rate of kilometers per second. but when the stars, their magnitudes and their special peculiarities are to be investigated _en masse_, photography provides the facile means for researches that would scarcely have been dreamed of without it. the international photographic chart of the entire heavens, in progress at twenty observatories since , the photographic charts of the northern heavens at harvard and of the southern sky at cape town, the manifold investigations that have led up to the harvard photometry, and the unparalleled photographic researches of the henry draper memorial, enabling the spectra of many hundred thousand stars to be examined and classified--all this is but a part of the astronomical work in stellar fields that photography has rendered possible. then there are the stellar parallaxes, now observed for many stars at once photographically, when formerly only one star's parallax could be measured at a time and with the eye at the telescope. and photo-electric photometry, measuring smaller differences of light than any other method, and providing more accurate light-curves of the variable stars. and perhaps most remarkable of all, the radial velocity work on both stars and nebulæ, giving us the distance of whole classes of stars, discovering large numbers of spectroscopic binaries and checking up the motion of the solar system toward lyra within a fraction of a mile per second. all told, photography has been the most potent adjunct in astronomical research, and it is impossible to predict the future with more powerful apparatus and photographic processes of higher sensitiveness. the field of research is almost boundless, and the possibilities practically without limit. what would herschel have done with £ , --and photography! chapter xxii mountain observatories the century that has elapsed since the time of sir william herschel, known as the father of the new or descriptive astronomy, has witnessed all the advances of the science that have been made possible by adopting the photographic method of making the record, instead of depending upon the human eye. only one eye can be looking at the eyepiece at a time: the photograph can be studied by a thousand eyes. at mountain elevations telescopes are now extensively employed, and there the camera is of especial and additional value, because the photograph taken on the mountain can be brought down for the expert to study, at ease and in the comfort of a lower elevation. we shall next trace the movement that has led the astronomer to seek the summits of mountains for his observatories, and the photographer to follow him. not only did the genius of newton discover the law of universal gravitation, and make the first experiments in optics essential to the invention of the spectroscope, but he was the real originator also of the modern movement for the occupation of mountain elevations for astronomical observatories. his keen mind followed a ray of light all the way from its celestial source to the eye of the observer, and analyzed the causes of indistinct and imperfect vision. endeavoring to improve on the telescope as galileo and his followers had left it, he found such inherent difficulties in glass itself that he abandoned the refracting type of telescope for the reflector, to the construction of which he devoted many years. but he soon found out, what every astronomer and optician knew to their keen regret, that a telescope, no matter how perfectly the skill of the optician's hand may make it, cannot perform perfectly unless it has an optically perfect atmosphere to look through. so newton conceived the idea of a mountain observatory, on the summit of which, as he thought, the air would be not only cloudless, but so steady and equable that the rays of light from the heavenly bodies might reach the eye undisturbed by atmospheric tremors and quiverings which are almost always present in the lower strata of the great ocean of air that surrounds our planet. this is the way newton puts the question in his treatise on _opticks_--he says: "the air through which we look upon the stars, is in a perpetual tremor; as may be seen by the tremulous motion of shadows cast from high towers, and by the twinkling of the fix'd stars.... the only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds." newton's suggestion is that the _highest_ mountains may afford the best conditions for tranquillity; and it is an interesting coincidence that the summits of the highest mountains, about , feet in elevation, are at about the same level where the turbulence of the atmosphere most likely ceases, according to the indications of recent meteorological research. these heights are far above any elevations permanently occupied as yet, but a good beginning has been made and results of great value have already been reached. curiously, investigation of mountain peaks and their suitability for this purpose was not undertaken till nearly two centuries after newton, when piazzi smyth in organized his expedition to the summit of a mountain of quite moderate elevation, and published his "teneriffe: an astronomer's experiment." teneriffe is an accessible peak of about , feet, on an island of the canaries off the african coast, where smyth fancied that conditions of equability would exist; and on reaching the summit with his apparatus and spending a few days and nights there, he was not disappointed. could he have reached an elevation of , feet, he would have had fully one-third of all the atmosphere in weight below him, and that the most turbulent portion of all. nevertheless, the gain in steadiness of the atmosphere, providing "better seeing," as the astronomer's expression is, even at , feet, was most encouraging, and led to attempts on other peaks by other astronomers, a few of whom we shall mention. davidson, an observer of the united states coast survey, with a broad experience of many years in mountain observing, investigated the summit of the sierra nevada mountains as early as , at an elevation of , feet. his especial object was to make an accurate comparison between elevated stations at different heights. he found the seeing excellent, especially on the sun; but the excessive snowfall at his station, feet annually, was a condition very adverse to permanent occupation. in the summer of , young spent several weeks at sherman, wyoming, at an elevation exceeding , feet. he carried with him the . -inch telescope of dartmouth college, where he was then professor, and this was the first expedition on which a large glass was used by a very skillful observer at great elevation. he found the number of good days and nights small, but the sky was exceedingly favorable when clear. many th magnitude stars could be detected with the naked eye. young's observations at sherman were mainly spectroscopic, however, and they demonstrated the immense advantage of a high-level station, far above the dust and haze of the lower atmosphere. he pronounced the . -inch glass at , feet the full equivalent of a -inch at sea level. mont blanc of , feet elevation was another summit where the veteran janssen of paris maintained a station for many years; but the continental conditions of atmospheric moisture and circulation were not favorable on the whole. janssen was mainly interested in the sun, and the daylight seeing is rarely benefited, owing to the strong upward currents of warm air set in motion by the sun itself. mountains in the beautiful climate of california were among the earliest investigated, and when in the trustees of mr. james lick's estate were charged with equipping an observatory with the most powerful telescope in existence, they wisely located on the summit of mount hamilton. it is , feet above sea level, and burnham and other astronomers made critical tests of the steadiness of vision there by observing double stars, which afford perhaps the best means of comparing the optical quality of the atmosphere of one region with another. the writer was fortunate in having charge of the observations of the transit of venus in on the mountain, when the observatory was in process of construction, and the quality of the photographs obtained on that occasion demonstrated anew the excellence of the site. particularly at night, for about nine months of the year, the seeing is exceptionally good, especially when fog banks rolling in from the pacific, cover the valleys below like a blanket, preventing harmful radiation from the soil below. the great telescope mounted in , a -inch refractor by alvan clark, has fulfilled every expectation of its projectors, and justified the selection of the site in every particular. the elevation, although moderate, is still high enough to secure very marked advantage in clearness and steadiness of the air, and at the same time not so high that the health and activities of the observers are appreciably affected by the thinner air of the summit. this telescope is known the world over for the monumental contributions to science made by the able astronomers who have worked with it: among them barnard who discovered the fifth satellite of jupiter in ; burnham, hussey, and aitken, who have discovered and measured thousands of close double stars; keeler, who spent many faithful years on the summit; and campbell, the present director, whose spectroscopic researches on stellar movements have added greatly to our knowledge of the structure of the universe. among the many lines of research now in progress at the lick observatory and in the d. o. mills observatory at santiago, chile, are the discoveries of stars whose velocities in space are not constant, but variable with the spectral type of the star. mr. lick's bequest for the observatory was about $ , . so ably has this scientific trust been administered that he might well have endowed it with his entire estate, exceeding $ , , . another california mountain that was early investigated is mount whitney. its summit elevation is nearly , feet, and in langley made its ascent for the purpose of measuring the solar constant. he found conditions much more favorable than on mount etna, sicily--elevation about , feet--which he had visited the year before. but the height of mount whitney was such as to occasion him much inconvenience from mountain sickness, an ailment which is most distressing and due partly to lack of oxygen and partly to mere diminution of mechanical pressure. mount whitney was also visited many years after by campbell for investigating the spectrum of mars in comparison with that of the moon. langley found on mount whitney an excellent station lower down, at about , feet elevation; and by equipping the two stations with like apparatus for measuring the solar heat, he obtained very important data on the selective absorption of the atmosphere. returning from the transit of venus in , copeland of edinburgh visited several sites in the andes of peru, ascending on the railway from mollendo. vincocaya was one of the highest, something over , feet elevation. his report was most enthusiastic, not only as to clearness and transparency of the atmosphere, but also as to its steadiness, which for planetary and double star observations is almost as important. copeland's investigation of this region of the andes has led many other astronomers to make critical tests in the same general region. climatic conditions are particularly favorable, and the sites for high-level research are among the best known, the atmosphere being not only clear a large part of the year, but in certain favored spots exceedingly steady. in the writer ascended the summit of fujiyama, japan, , feet elevation. the early september conditions as to steadiness of atmosphere were extraordinarily fine, but the mountain is covered by cloud many months in each year. there is a saddle on the inside of the crater that would form an ideal location for a high-level observatory. this expedition was undertaken at the request of the late professor pickering, director of harvard college observatory, which had recently received a bequest from uriah a. boyden, amounting to nearly a quarter of a million dollars, to "establish and maintain, in conjunction with others, an astronomical observatory on some mountain peak." great elevations were systematically investigated in colorado and california, the chilean desert of atacama was visited, and a temporary station established at chosica, peru, elevation about , feet. atmospheric conditions becoming unfavorable, a permanent station was established in at arequipa, peru, elevation , feet, which has been maintained as an annex to the harvard observatory ever since. the cloud conditions have been on the whole less favorable than was expected, but the steadiness of the air has been very satisfactory. in addition to planetary researches conducted there in the earlier years by w. h. pickering, many large programs of stellar research have been executed, especially relating to the magnitudes and spectra of the stars. in conjunction with the home observatory in the northern hemisphere, this afforded a vast advantage in embracing all the stars of the entire heavens, on a scale not attempted elsewhere. the bruce photographic telescope of -inch aperture has been employed for many years at arequipa, and with it the plates were taken which enabled pickering to discover the ninth satellite of saturn (phoebe), and the splendid photographs of southern globular clusters in which bailey has found numerous variable stars of very short periods--very faint objects, but none the less interesting, and of much significance in modern study of the evolution and structure of the stellar universe. the crowning research of the observatory is the henry draper catalogue of stellar spectra, now in process of publication, which is of the first order of importance in statistical studies of stellar distribution with reference to spectral type, and in studying the relation of parallax and distance, proper motion, radial velocity and its variation to the spectral characteristics of the stars. perrine of cordova is now establishing on sierra chica about twenty-five miles southwest of cordova, a great reflecting telescope comparable in size with the instruments of the northern hemisphere, for investigation of the southern nebulæ and clusters, and motions of the stars. the elevation of this new argentine observatory will be , feet above sea level. another observatory at mountain elevation and in a highly favorable climate is the lowell observatory, located at about , feet elevation at flagstaff, arizona. many localities were visited and the atmosphere tested especially for steadiness, an optical quality very essential for research on the planetary surfaces. mexico was one of these stations, but local air currents and changes of temperature there were such that good seeing was far from prevalent, as had been expected. at flagstaff, on the other hand, conditions have been pretty uniformly good, and an enormous amount of work on the planet mars has been accumulated and published. the first successful photographs of this planet were taken there in , and jupiter, saturn, the zodiacal light and many other test objects have been photographed, which demonstrates the excellence of the site for astronomical research. within recent years spectrum research by slipher, especially on the nebulæ, has been added to the program, and the rotation and radial velocities of many nebulæ have been determined. on mount wilson, near pasadena, california, at an elevation of nearly , feet, is the carnegie solar observatory, founded and equipped under the direction of professor george e. hale, as a department of the carnegie institution of washington, of which dr. john campbell merriam is president. the climatology of the region was carefully investigated and tests of the seeing made by hussey and others. although equipped primarily for study of the sun, the program of the observatory has been widely amplified to include the stars and nebulæ. the instrumental equipment is unique in many respects. to avoid the harmful effect of unsteadiness of air strata close to the ground a tower feet high was erected, with a dome surmounting it and covering a coelostat with mirror for reflecting the sun's rays vertically downward. underneath the tower a dry well was excavated to a depth equal to / the height of the tower above it. in the subterranean chamber is the spectroheliograph of exceptional size and power. the sun's original image is nearly inches in diameter on the plate, and the solar chromosphere and prominences, together with the photosphere and faculæ, are all recorded by monochromatic light. connected with the observatory on mount wilson are the laboratories, offices and instrument shops in pasadena, miles distant, where the remarkable apparatus for use on the mountain is constructed. a reflecting telescope with silver-on-glass mirror inches in diameter was first built by ritchey and thoroughly tested by stellar photographs. also the northern spiral nebulæ were photographed, exhibiting an extraordinary wealth of detail in apparent star formation. the success of this instrument paved the way for one similar in design, but with a mirror inches in diameter, provided by gift of the late john d. hooker of los angeles. the telescope was completed in . notwithstanding its huge size and enormous weight, the mounting is very successful, as well as the mirror. mercurial bearings counterbalance the weight of the polar axis in large part. this great telescope, by far the largest and most powerful ever constructed, is now employed on a program of research in which its vast light-gathering power will be utilized to the full. under the skillful management of hale and his enthusiastic and capable colleagues, the confines of the stellar heavens will be enormously extended, and secrets of evolution of the universe and of its structure no doubt revealed. in all the mountain stations hitherto established, as the lick observatory at , feet, the mount wilson observatory at , feet, the lowell observatory at , feet, the harvard observatory at , feet; and teneriffe and etna at , , fujiyama at , , pike's peak at , , mont blanc and mount whitney at , , the researches that have been carried on have fully demonstrated the vast advantage of increased elevation in localities where climatological conditions as well as elevation are favorable. nevertheless, only one-half of the extreme altitude contemplated by sir isaac newton has yet been attained. can the greater heights be reached and permanently occupied? geographically and astronomically the most favorably located mountain for a great observatory is mount chimborazo in ecuador. its elevation is , feet, and it was ascended by edward whymper in . situated very nearly on the earth's equator, almost the entire sidereal heavens are visible from this single station, and all the planets are favored by circumzenith conditions when passing the meridian. no other mountain in the world approaches chimborazo in this respect. but the summit is perpetually snow-capped, exceedingly inaccessible, and the defect of barometric pressure would make life impossible up there in the open. only one method of occupation appears to be feasible. the permanent snow line is at about , feet, where excellent water power is available. by tunneling into the mountain at this point, and diagonally upward to the summit, permanent occupation could be accomplished, at a cost not to exceed one million dollars. the rooms of the summit observatory would need to be built as steel caissons, and supplied with compressed air at sea-level tension. the practicability of this plan was demonstrated by the writer in september, , at cerro de pasco, peru. a steel caisson was carried up to an elevation exceeding , feet. patients suffering acutely with mountain sickness were placed inside this caisson, and on restoring the atmospheric pressure within it artificially all unfavorable symptoms--headache, high respiration and accelerated pulse--disappeared. there was every indication that if persons liable to this uncomfortable complaint were brought up to this elevation, or indeed any attainable elevation, under unreduced pressure, the symptoms of mountain sickness would be unknown. comfortable occupation of the highest mountain summits was thereby assured. the working of astronomical instruments from within air-tight compartments does not present any insurmountable difficulties, either mechanical or physical. since the time these experiments were made, the guayaquil-quito railway has been constructed over a saddle of chimborazo, at an elevation of , feet; and only six miles of railway would need to be built from this station to the point where the tunnel would enter the mountain. only by the execution of some such plan as this can astronomers hope to overcome the baleful effects of an ever mobile atmosphere, and secure the advantages contemplated by sir isaac newton in that tranquillity of atmosphere, which he conceived as perpetually surrounding the summits of the highest mountains. in russell's theory of the progressive development of the stars, from the giant class to the dwarf, an element of verification from observation is lacking, because hitherto no certain method of measuring the very minute angular diameters of the stars has been successfully applied. the apparent surface brightness corresponding to each spectral type is pretty well known, and by dividing it into the total apparent brightness, we have the angular area subtended by the star, quite independent of the star's distance. this makes it easy to estimate the angular diameter of a star, and betelgeuse is the one which has the greatest angular diameter of all whose distances we know. antares is next in order of angular diameter, ". , aldebaran ". , arcturus ". , pollux ". , and sirius only ". . can these theoretical estimates be verified by observation? clearly it is of the utmost importance and the exceedingly difficult inquiry has been undertaken with the -inch reflector on mount wilson, employing the method of the interferometer developed by michelson and described later on, an instrument undoubtedly capable of measuring much smaller angles than can be measured by any other known method. unquestionably the interference of atmospheric waves, or in other words what astronomers call "poor seeing," will ultimately set the limit to what can be accomplished. "but even if," says eddington, "we have to send special expeditions to the top of one of the highest mountains in the world, the attack on this far-reaching problem must not be allowed to languish." chapter xxiii the program of a great observatory the mount wilson observatory has now been in operation about fifteen years. the novelty in construction of its instruments, the investigations undertaken with them and the discoveries made, the interpretation of celestial phenomena by laboratory experiment, and the recent addition to its equipment of a telescope inches in diameter, surpassing all others in power, directs especial attention to the extensive activities of this institution, whose budget now exceeds a million dollars annually. results are only achieved by a carefully elaborated program, such as the following, for which the reader is mainly indebted to dr. hale, the director of the observatory, who gives a very clear idea of the trend of present-day research on the magnetic nature of the sun, and the structure and evolution of the sidereal universe. the purpose of the observatory, as defined at its inception, was to undertake a general study of stellar evolution, laying especial emphasis upon the study of the sun, considered as a typical star; physical researches on stars and nebulæ; and the interpretation of solar and stellar phenomena by laboratory experiments. recognizing that the development of new instruments and methods afforded the most promising means of progress, well-equipped machine shops and optical shops were provided with this end in view. the original program of the observatory has been much modified and extended by the independent and striking discovery by campbell and kapteyn of an important relationship between stellar speed and spectral type; the demonstration by hertzsprung and russell of the existence of giant and dwarf stars; the successful application of the -inch reflector by van maanen to the measurement of minute parallaxes of stars and nebulæ; the important developments of shapley's investigation of globular star clusters; the possibilities of research resulting from seares's studies in stellar photometry; and the remarkable means of attack developed by adams through the method of spectroscopic parallaxes. by this method the absolute magnitude, and hence the distance of a star is accurately determined from estimates of the relative intensities of certain lines in stellar spectra. attention was first directed toward lines of this character in , when it was inferred that the weakening of some lines in the spectra of sun spots and the strengthening of others was the result of reduced temperature of the spot vapors. on testing this hypothesis by laboratory experiments, it was fully verified. subsequently adams, who had thus become familiar with these lines and their variability, studied them extensively in the spectra of other stars. in this way was discovered the dependence of their relative intensities on the star's absolute magnitude, so providing the powerful method of spectroscopic parallaxes. this method, giving the absolute magnitude as well as the distance of every star (excepting those of the earliest type) whose spectrum is photographed, is no less important from the evolutional than from the structural point of view. investigations in solar physics which formerly held chief place in the research program have developed along unexpected lines. it could not be foreseen at the outset that solar magnetic phenomena might become a subject of inquiry, demanding special instrumental facilities, and throwing light on the complex question of the nature of the sun spots and other solar problems of long standing. it is obvious that these researches, together with those on the solar rotation and the motions of the solar atmosphere, developed by adams and st. john, must be carried to their logical conclusion, if they are to be utilized to the fullest in interpreting stellar and nebular phenomena. the discovery of solar magnetism, like many other mount wilson results, was the direct outcome of a long series of instrumental developments. the progressive improvement and advance in size of the tools of research was absolutely necessary. hale's first spectroheliograph at kenwood in was attached to a -inch refractor, and the solar image was but two inches in diameter. it was soon found that a larger solar image was essential, and a spectrograph of much greater linear dispersion; in fact, the spectrograph must be made the prime element in the combination, and the telescope so designed as to serve as a necessary auxiliary. accordingly, successive steps have led through spectrographs of and feet dimension to a vertical spectrograph feet in focal length. the telescope is the feet tower telescope, giving a solar image of . inches in diameter. its spectrograph is massive in construction, and by extending deep into the earth, it enjoys the stability and constancy of temperature required for the most exacting work. another direct outgrowth of the work of sun-spot spectra is a study of the spectra of red stars, where the chemistry of these coolest regions of the sun is partially duplicated. the combination of titanium and oxygen, and the significant changes of line intensity already observed in both instances, and also in the electric furnace at reduced temperatures, give indication of what may be expected to result from an attack on the spectra of the red stars with more powerful instrumental means, which is now provided by the -inch telescope and its large stellar spectrograph. other elements in the design of the -inch hooker telescope have the same general object in view--that of developing and applying in astronomical practice the effective research methods suggested by recent advances in physics. fresh possibilities of progress are constantly arising, and these are utilized as rapidly as circumstances permit. the policy of undertaking the interpretations of celestial phenomena by laboratory experiments, an important element in the initial organization of mount wilson, has certainly been justified by its results. indeed, the development of many of the chief solar investigations would have been impossible without the aid of special laboratory studies, going hand in hand with the astronomical observations. so indispensable are such researches, and so great is the promise of their extension, that the time has now come for advancing the laboratory work from an accessory feature to full equality with the major factors in the work of the observatory. accordingly a new instrument now under installation is an extremely powerful electro-magnet, designed by anderson for the extension of researches on the zeeman effect, and for other related investigations. within the large and uniform field of this magnet, which is built in the form of a solenoid, a special electric furnace, designed for this purpose by king, is used for the study of the inverse zeeman effect at various angles with the lines of force. this will provide the means of interpreting certain remarkable anomalies in the magnetic phenomena of sun spots. the -inch telescope is now in regular use. all the tests so far applied show that it greatly surpasses the -inch telescope in every class of work. for many months most of the observations and photographs have been made with the cassegrain combination of mirrors, giving an equivalent focal length of feet and involving three reflections of light. the -inch telescope is found to give nearly . times as much light as the -inch telescope, and therefore extends the scope of the instrument to all the stars an entire magnitude fainter. this is a very important gain for research on the faint globular clusters, as well as the small and faint spiral and planetary nebulæ, providing a much larger scale for these objects and sufficient light at the same time. photographs of the moon and many other less critical tests have been made with very satisfactory results. those of the moon appear to be decidedly superior in definition to any previously taken with other instruments. another investigation is of great importance in the light of recent advances in theoretical dynamics. darwin, in his fundamental researches on the dynamics of rotating masses, dealt with incompressible matter, which assumes the well-known pear-shaped figure, and may ultimately separate into two bodies. roche on the other hand discussed the evolution of a highly compressible mass, which finally acquires a lens-shaped form and ejects matter at its periphery. both of these are extreme cases. jeans has recently dealt with intermediate cases, such as are actually encountered in stars and nebulæ. he finds that when the density is less than about one-fourth that of water, a lens-shaped figure will be produced with sharp edges, as depicted by roche. matter thrown off at opposite points on the periphery, under the influence of small tidal forces from neighboring masses, may take the form of two symmetric filaments, though it is not yet entirely clear how these may attain the characteristic configuration of spiral nebulæ. the preliminary results of van maanen indicate motion outward along the arms, in harmony with jeans's views. jeans further discusses the evolution of the arms, which will break up into nuclei (of the order of mass of the sun) if they are sufficiently massive, but will diffuse away if their gravitational attraction is small. the mass of our solar system is apparently not great enough, according to jeans, to account for its formation in this way. as is apparent, these investigations lead to conclusions very different from those derived by chamberlin and moulton from the planetesimal hypothesis. this is a critical study of spiral nebulæ for which the -inch telescope is of all instruments in existence the best suited. the spectra of the spirals must be studied, as well as the motions of the matter composing the arms. their parallaxes, too, must be ascertained. a photographic campaign including spiral nebulæ of various types will settle the question of internal motions. the large scale of the spiral nebulæ at the principal focus of the hooker telescope, and the experience gained in the measurement of nebular nuclei for parallax determination, will help greatly in this research. a multiple-slit spectrograph, already applied at mount wilson, will be employed, not only on spiral nebulæ whose plane is directed toward us, but also on those whose plane lies at an angle sufficient to permit both components of motion to be measured by the two methods. in dealing with problems of structure and motion in the galactic system, the -inch telescope offers especial advantages, because of its vast light-gathering power. studies of radial velocities of the stars have hitherto been necessarily confined to the brighter stars, for the most part even to those visible to the naked eye. while some of these are very distant, most of the stars whose radial velocities are known belong to a very limited group, perhaps constituting a distinct cluster of which the sun is a member, but in any event of insignificant proportions when contrasted with the galaxy. current spectrographic work with the -inch telescope includes stars of the eighth magnitude, and some even fainter. but while the -inch has enabled adams to measure the distances of many remote stars by his new spectroscopic method, and to double the known extent (so far as spectroscopic evidence is concerned) of the star streams of kapteyn, a much greater advance into space is necessary to find out the community of motion among the stars comprising the galactic system. the hooker telescope will enable us to determine accurate radial velocities to stars of the eleventh magnitude, which doubtless truly represent the galaxy. in order to secure a maximum return within a reasonable period of time, the stars in the selected areas of kapteyn will be given the preference, because of the vast amount of work already done, relating to their positions, proper motions, and visual and photographic magnitudes. such consideration as spectral type, the known directions of star-streaming, and the position of the chosen regions with reference to the plane of the galaxy are given adequate weight, and it is of fundamental importance that the method of spectroscopic parallaxes will permit dwarf stars to be distinguished from stars that are in the giant class, but rendered faint by their much greater distance. in addition to these problems, the stellar spectrograms will provide rich material for study of the relationship between stellar mass and speed, and the nature of giant stars and dwarf stars. shapley's recent studies of globular clusters have indicated the significance of these objects in both evolutional and structural problems, and the possibility of determining their parallaxes by a number of independent methods is of prime importance, both in its bearing on the structure of the universe and because it permits a host of apparent magnitudes to be at once transformed into absolute magnitudes. here the advantage of the hooker telescope is two-fold: at its -foot focus the increased scale of the crowded clusters makes it possible to select separate stars for spectrum photography (which could not be done with the -inch where the images were commingled); and the great gain in light is such that the spectra of stars to the th magnitude have been photographed in less than an hour. faint globular clusters, then, will comprise a large part of the early program with the -inch telescope: the faintest possible stars in them must be detected and their magnitudes and colors measured; spectral types must be determined, and the radial velocities of individual stars and of clusters as a whole; spectroscopic evidence of possible axial rotation of globular clusters must be searched for; and the method of spectroscopic parallaxes, as well as other methods, must be applied to ascertaining the distances of these clusters. the possibility of dealing with many problems relating to the distribution and evolution of the faintest stars depends upon the establishment of photographic and photovisual magnitude scales. below the twelfth magnitude, the only existing scale of standard visual or photovisual magnitudes is the mount wilson sequence, already extended by seares to magnitude . with the -inch telescope. extension of this scale to even fainter magnitudes, and its application to the faintest stars within its range is an important task for this great telescope, as it will doubtless bring within range hundreds of millions of stars that are beyond the reach of the -inch. the giants among them will form for us the outer boundary of the galactic system, while the dwarfs will be of almost equal interest from the evolutional standpoint. the photometric program of the -inch, then, will deal with such questions as the condensation of the fainter stars toward the galactic plane, the color of the most distant stars, and the final settlement of the long inquiry regarding the possible absorption of light in space. [illustration: great sun-spot group, august , . the disk in the lower left corner represents the comparative size of the earth. (_photo, mt. wilson solar observatory._)] [illustration: the sun's disk. the view shows the "rice grain" structure of the photosphere and brilliant calcium flocculi. (_photo, yerkes observatory._)] [illustration: the lunar surface visible during a total eclipse of the moon, february , . (_photo, yerkes observatory._)] another research of exceptional promise will be undertaken, which is of great importance in a general study of stellar evolution; and that is the determination of the spectral-energy curves of stars of various classes, for the purpose of measuring their surface temperatures. a very few of the nebulæ are found to be variable, and their peculiarities need investigation, also special problems of variable stars and temporary stars, and the spectra of the components of close double stars which are beyond the power of all other instruments to photograph. such a program of research conveys an excellent idea of many of the great problems that are under investigation by astronomers to-day, and gives some notion of the instrumental means requisite in executing comprehensive plans of this character. it will not escape notice that the climax of instrumental development attained at mount wilson has only been made possible by an unbroken chain of progress, link by link, each antecedent link being necessary to the successful forging of its following one. in very large part, and certainly indispensable to these instrumental advances, has the art of working in glass and metals been the mainstay of research. as we review the history of astronomical progress, from galileo's time to our own, the consummate genius of the artisan and his deft handiwork compel our admiration almost equally with the keen intelligence of the astronomer who uses these powerful engines of his own devising to wrest the secrets of nature from the heavens. chapter xxiv our solar system now let us go upward in imagination, far, far beyond the tops of the highest mountains, beyond the moon and sun, and outward in space until we reach a point in the northern heavens millions and millions of miles away, directly above and equally distant from all points in the ecliptic, or path in which our earth travels yearly round the sun. then we should have that sort of comprehensive view of the solar system which is necessary if we are to visualize as a whole the working of the vast machine, and the motions, sizes, and distances of all the bodies that comprise it. of such stupendous mechanism our earth is part. or in lieu of this, let us attempt to get in mind a picture of the solar system by means of sir william herschel's apt illustration: "choose any well-leveled field. on it place a globe two feet in diameter. this will represent the sun; mercury will be represented by a grain of mustard seed on the circumference of a circle feet in diameter for its orbit; venus, a pea on a circle of feet in diameter; the earth also a pea, on a circle of feet; mars a rather larger pin's head on a circle of feet; the asteroids, grains of sand in orbits of , to , feet; jupiter, a moderate sized orange in a circle of nearly half a mile across; saturn, a small orange on a circle of four-fifths of a mile; uranus, a full-sized cherry or small plum upon the circumference of a circle more than a mile and a half; and finally neptune, a good-sized plum on a circle about two miles and a half in diameter.... to imitate the motions of the planets in the above mentioned orbits, mercury must describe its own diameter in seconds; venus in minutes, seconds; the earth in minutes; mars in minutes seconds; jupiter in minutes seconds; saturn in minutes seconds; uranus in minutes seconds; and neptune in minutes seconds." now, let us look earthward from our imaginary station near the north pole of the ecliptic. all these planetary bodies would be seen to be traveling eastward round the sun, that is, in a counter-clockwise direction, or contrary to the motions of the hands of a timepiece. their orbits or paths of motion are very nearly circular, and the sun is practically at the center of all of them except mercury and mars; of venus and neptune, almost at the absolute center. the planes of all their orbits are very nearly the same as that of the ecliptic, or plane in which the earth moves. these and many other resemblances and characteristics suggest a uniformity of origin which comports with the idea of a family, and so the whole is spoken of as the solar system, or the sun and his family of planets. in addition to the nine bodies already specified, the solar system comprises a great variety of other and lesser bodies; no less than twenty-six moons or satellites tributary to the planets and traveling round them in various periods as the moon does round our earth. then between the orbits of mars and jupiter are many thousands of asteroids, so called, or minor planets (about , of them have actually been discovered, and their paths accurately calculated). and at all sorts of angles with the planetary orbits are the paths of hundreds of comets, delicate filmy bodies of a wholly different constitution from the planets, and which now and then blaze forth in the sky, their tails appearing much like the beam of a searchlight, and compelling for the time the attention of everybody. connected with the comets and doubtless originally parts of them are uncounted millions of millions of meteors, which for the time become a part of the solar system, their minute masses being attracted to the planets, upon which they fall, those hitting the earth being visible to us as familiar shooting stars. we next follow the story of astronomy through the solar system, beginning with the sun itself and proceeding outward through his family of planets, now much more numerous and vastly more extended than it was to the ancient world, or indeed till within a century and a half of our own day. chapter xxv the sun and observing it as lord of day, king of the heavens, mankind in the ancient world adored the sun. by their researches into the epoch of the assyrians, hittites, phoenicians and other early peoples now passed from earth, archæologists have unearthed many monuments that evidence the veneration in which the early peoples who inhabited egypt and asia minor many thousand years ago held the sun. a striking example is found in the architecture of early egyptian temples, on the lintels of which are carved representations of the winged globe or the winged solar disk, and there is a bare possibility that the wings of the globe were suggested by a type of the solar corona as glimpsed by the ancients. little knew they about the distance and size of the sun; but the effects of his light and heat upon all vegetal and animal life were obvious to them. doubtless this formed the basis for their worship of the sun. occasional huge spots must have been visible to the naked eye, and the sun's corona was seen at rare intervals. plutarch and philostratus describe it very much as we see it to-day. how completely dependent mankind is upon the sun and its powerful radiations, only the science of the present day can tell us. by means of the sun's heat the forests of early geologic ages were enabled to wrest carbon from the atmosphere and store it in forms later converted by nature's chemistry into peat and coal. through processes but imperfectly understood, the varying forms of vegetable life are empowered to conserve, from air and soil, nitrogen and other substances suitable for and essential to the life maintenance of animal creatures. breezes that bring rain and purify the air; the energy of water held under storage in stream and dam and fall; trade winds facilitating commerce between the continents; oceanic currents modifying coastal climates; the violence of tornado, typhoon and water-spout, together with other manifestations of natural forces--all can be traced back to their origin in the tremendous heating power of the solar rays. in everything material the sun is our constant and bountiful benefactor. if his light and heat were withdrawn, practically every form of human activity on this planet would come to an early end. how far away is the sun? what is the size of the sun? these are questions that astronomers of the present day can answer with accuracy. so closely do they know the sun's distance that it is employed as their yardstick of the sky, or unit of celestial measurement. many methods have been utilized in ascertaining the distance of the sun, and the remarkable agreement among them all is very extraordinary. some of them depend upon pure geometry, and the basic measure which we make from the earth is not the distance of the sun directly; but we find out how far away venus is during a transit of venus, for example, or how far away mars is or some of the asteroids are at their closer oppositions. then it is possible to calculate how far away the sun is, because one measurement of distance in the solar system affords us the scale on which the whole structure is built. but perhaps the simplest method of getting the sun's distance is by the velocity of light, , miles a second. from eclipses of jupiter's moons we know that light takes minutes seconds to pass from sun to earth. so that the sun's distance is the simple product of the two, or millions of miles. once this fundamental unit is established, we have a firm basis on which to build up our knowledge of the distances, the sizes and motions of the heavenly bodies, especially those that comprise the solar system. we can at once ascertain the size of the sun, which we do by measuring the angle which it fills, that is, the sun's apparent diameter. finding this to be something over a half a degree in arc, the processes of elementary trigonometry tell us that the sun's globe is , miles in diameter. for nearly a century this has been accurately measured with the greatest care, and diameters taken in every direction are found to be equal and invariably the same. so we conclude that the sun is a perfect sphere, and so far as our instruments can inform us, its actual diameter is not subject to appreciable change. the vastness of the sun's volume commands our attention. as his diameter is times that of the earth, his mere size or volume is × × or , thousand times that of the earth, because the volumes of spheres are in proportion as the cubes of their diameters. if the materials that compose the sun were as heavy as those that make up the earth, it would take , thousand earths to weigh as much as the sun does. but by a method which we need not detail here, the sun's actual weight or mass is found to be only thousand (more nearly , ), times greater than the earth's. so we must infer that, bulk for bulk, the component materials of the sun are about one-fourth lighter than those of the earth, that is, about one and one-half times as dense as water. to look at this in another way: it is known that a body falling freely toward the earth from outer space would acquire a speed of seven miles a second, whereas if it were to fall toward the sun instead, the velocity would be miles a second on reaching his surface. if all the other bodies of the solar system, that is, the earth and moon, all the planets and their satellites, the comets and all were to be fused together in a single globe, it would weigh only one-seven hundred and fiftieth as much as the sun does. at the surface, however, the disproportion of gravity is not so great, because of the sun's vast size: it is only about twenty-eight times greater on the sun than on the earth; and instead of a body falling feet the first second as here, it would fall feet there. pendulums of clocks on the sun would swing five times for every tick here, and an athlete's running high jump would be scaled down to three inches. let us next inquire into the amount of the sun's light and heat, and the enormously high temperature of a body whose heat is so intense even at the vast distance at which we are from it. the intensity of its brightness is such that we have no artificial source of light that we can readily compare it with. in the sky the next object in brightness is the full moon, but that gives less than the half-millionth part as much light as the sun. the standard candle used in physics gives so little light in comparison that we have to use an enormous number to express the quantity of light that the sun gives. a sperm candle burning grains hourly is the standard, and if we compare this with the sun when overhead, and allow for the light absorbed by the atmosphere, we get the number with twenty-four ciphers following it, to express the candlepower of the sun's light. if we interpose the intense calcium light or an electric arc light between the eye and the sun, these artificial sources will look like black spots on the disk. indeed, the sun is nearly four times brighter than the "crater," or brightest part of the electric arc. the late professor langley at a steel works in pennsylvania once compared direct sunlight with the dazzling stream of molten metal from a bessemer converter; but bright as it was, sunlight was found to be five thousand times brighter. equally enormous is the heat of the sun. our intensest sources of artificial heat do not exceed , degrees fahrenheit, but the temperature at the sun's surface is probably not less than , degrees f. one square meter of his surface radiates enough heat to generate , horsepower continuously. at our vast distance of millions of miles, the sun's heat received by the earth is still powerful enough to melt annually a layer of ice on the earth more than a hundred feet in thickness. if the solar heat that strikes the deck of a tropical steamship could be fully utilized in propelling it, the speed would reach at least ten knots. many attempts have been made in tropical and sub-tropical climates to utilize the sun's heat directly for power, and ericsson in sweden, mouchot in france, and shuman in egypt have built successful and efficient solar engines. necessary intermission of their power at night, as well as on cloudy days, will preclude their industrial introduction until present fuels have advanced very greatly in cost. all regions of the sun's disk radiate heat uniformly, and the sun's own atmosphere absorbs so much that we should receive . times more heat if it were removed. so far as is known, solar light and heat are radiated equally in all directions, so that only a very minute fraction of the total amount ever reaches the earth, that is, millionth part of the whole. indeed all the planets and other bodies of the solar system together receive only one one hundred millionth part; the vast remainder is, so far as we know, effectively wasted. it is transformed, but what becomes of it, and whether it ever reappears in any other form, we cannot say. how is this inconceivably vast output of energy maintained practically invariable throughout the centuries? many theories have been advanced, but only one has received nearly universal assent, that of secular contraction of the sun's huge mass upon itself. shrinkage means evolution of heat; and it is found by calculation that if the sun were to contract its diameter by shrinking only two-hundred and fifty feet per year, the entire output of solar heat might thus be accounted for. so distant is the sun and so slow this rate of contraction that centuries must elapse before we could verify the theory by actual measurements. meanwhile, the progress of physical research on the structure and elemental properties of matter has brought to light the existence of highly active internal forces which are doubtless intimately concerned in the enormous output of radiant energy, though the mechanism of its maintenance is as yet known only in part. abbot, from many years' observations of the solar constant, at washington, on mount wilson, and in algeria, finds certain evidence of fluctuation in the solar heat received by the earth. it cannot be a local phenomenon due to disturbances in our atmosphere, but must originate in causes entirely extraneous to the earth. interposition of meteoric dust might conceivably account for it, but there is sufficient evidence to show that the changes must be attributed to the sun itself. the sun, then, is a variable star; and it has not only a period connected with the periodicity of the sun spots, but also an irregular, nonperiodic variation during a cycle of a week or ten days, though sometimes longer, and occasioning irregular fluctuations of two to ten per cent of the total radiation. radiation is found to increase with the spottedness. attempts have been made on the basis of the contraction theory to find out the past history of the sun and to predict its future. probably to millions of years in the past represents the life of the sun much as it is at present; and if solar radiation in the future is maintained substantially as now, the sun will have shrunk to one-half its present diameter in the next five million years. so far then as heat and light from the sun are concerned, the sun may continue to support life on the earth not to exceed ten million years in the future. but the sun's own existence, independently of the orbs of the system dependent upon it, might continue for indefinite millions of aeons before it would ever become a cold dead globe; indeed, in the present state of science, we cannot be sure that it is destined to reach that condition within calculable time. a few words on observing the sun, an object much neglected by amateurs. on account of the intense light, a very slight degree of optical power is sufficient. indeed a piece of window glass, smoked in a candle flame with uniform graduation from end to end, will be found worth while in a beginner's daily observation of the sun. the glass should be smoked densely enough at one end so that the sunlight as seen through it will not dazzle the eye on the clearest days. at the other end of the glass, the degree of smoke film should not be quite so dense, so that the sun can be examined on hazy, foggy or partly cloudy days. an occasional naked-eye spot will reward the patient observer. if a small spyglass, opera glass or field glass is at hand, excellent views of the sun may be had by mounting the glass so that it can be held steadily pointed on the sun, and then viewing the disk by projection on a white card or sheet of paper. care must be taken to get a good focus on the projected image, and then the faculæ, or whitish spots, or mottling nearer the sun's edge will usually be well seen. by moving the card farther away from the eyepiece, a larger disk may be obtained, in effect a higher degree of magnification. but care must be used not to increase it too much. keep direct sunlight outside the tube from falling on the card where the image is being examined. this is conveniently done by cutting a large hole, the size of the brass cell of the object glass, through a sheet of corrugated strawboard, and slipping this on over the cell. in this way the spots on the sun can be examined with ease and safety to the eye. for large instruments a special type of eyepiece is provided known as a helioscope, which disposes of the intense heat rays that are harmful to the eye. frequent examination of the eyepiece should be made and the eyepiece cooled if necessary. that part of the sun's surface under observation is known as the photosphere, that is, the part which radiates light. if the atmosphere admits the use of high magnifying powers, the structure of the photosphere will be found more and more interesting the higher the power employed. it is an irregularly mottled surface showing a species of rice-grain structure under fairly high magnification. these grains are grouped irregularly and are about miles across. under fine conditions of vision they may be subdivided into granules. the faculæ, or white spots, are sometimes elevations above the general solar level; they have occasionally been seen projecting outside the limb, or edge of the disk. chapter xxvi sun spots and prominences dark spots of a deep bluish black will often be seen on the photosphere of the sun. sometimes single, though generally in groups, the larger ones will have a dark center, called the umbra, surrounded by the very irregular penumbra which is darker near its outer edge and much brighter apparently on its inner edge where it joins on the umbra. the penumbra often shows a species of thatch-work structure, and systematic sketches of sun spots by observers skilled in drawing are greatly to be desired, because photography has not yet reached the stage where it is possible to compete with visual observation in the matter of fine detail. the spots themselves nearly always appear like depressions in the photosphere, and on repeated occasions they have been seen as actual notches when on the edge of the sun. many spots, however, are not depressions: some appear to be actual elevations, with the umbra perhaps a central depression, like the crater in the general elevation of a volcano. spots are sometimes of enormous size. the largest on record was seen in ; it was nearly , miles in breadth, and covered a considerable proportion of the whole visible hemisphere of the sun. a spot must be nearly , miles across in order to be seen with the naked eye. in their beginning, development, and end, each spot or group of spots appears to be a law unto itself. sometimes in a few hours they will form, though generally it is a question of days and even weeks. very soon after their formation is complete, tonguelike encroachments of the penumbra appear to force their way across the umbra, and this splitting up of the central spot usually goes on quite rapidly. sun spots in violent disturbance are rarely observed. as the sun turns round on his axis, the spots will often be carried across the disk from the center to the edge, when they become very much foreshortened. the sun's period of rotation is days, so that if a spot lasts more than two weeks without breaking up, it may reappear on the eastern limb of the sun after having disappeared at the western edge. two or three months is an average duration for a spot; the longest on record lasted through months in - . the position of the sun's axis is well known, its equator being tilted about degrees to the ecliptic, and the spots are distributed in zones north and south of the equator, extending as far as degrees of solar latitude. in very high latitudes spots are never seen; they are most abundant in about latitude degrees both north and south, and rather more numerous in the northern than in the southern hemisphere of the sun. recent research at mount wilson makes the sun a great magnet; and its magnetic axis is inclined at an angle of degrees to the axis of rotation, around which it revolves in days. there is a most interesting periodicity of the spots on the sun, for months will sometimes elapse with spots in abundance and visible every day, while at other periods, days and even weeks will elapse without a single spot being seen. there is a well recognized period of eleven and one-tenth years, the reason underlying which is not, however, known. after passing through the minimum of spottedness, they begin to break out again first in latitudes of degrees- degrees, rather suddenly, and on both sides of the equator, and they move toward the equator as their number and individual size decrease. the last observed epoch of maximum spot activity on the sun was passed in . many attempts have been made to ascertain the cause of the periodicity of sun spots, but the real cause is not yet known. if the spots are eruptional in character, the forces held in check during seasons of few spots may well break out in period. the brighter streaks and mottlings known as faculæ are probably elevations above the general photosphere, and seem to be crusts of luminous matter, often incandescent calcium, protruding through from the lower levels. generally the faculæ are numerous around the dark spots, and absorption of the sun's light by his own atmosphere affords a darker background for them, with better visibility nearer the rim of the solar disk. the spectroheliograph reveals vast zones of faculæ otherwise invisible, related to the sun-spot zones proper on both sides of the equator. in some intimate way the magnetism of sun and earth are so related that outbreaks of solar spots are accompanied with disturbances of electrical and other instruments on the earth; also the aurora borealis is seen with greater frequency during periods when many spots are visible. within very recent years the discovery of a magnetic field in sun spots has been made by hale with powerful instruments of his own design. sun spots had never been investigated before with adequate instrumental means. he recognized the necessity of having a spectroscope that would record the widened lines of sun-spot spectra, and the strengthened and weakened lines on a large scale. certain changes in relative intensity were traced to a reduced temperature of the spot vapors by comparison with photographs of the spectrum of iron and other metallic vapors in an electric arc at different temperatures. here the work of the laboratory was essential. sun spots were thus found to be regions of reduced temperature in the solar atmosphere. chemical unions were thus possible, and thousands of faint lines in spot-spectra were measured and identified as band lines due to chemical compounds. thus the chemical changes at work in sun-spot vapors were recognized. then followed the highly significant investigations of solar vortices and magnetic fields. improvements in photographic methods had revealed immense vortices surrounding sun spots in the higher part of the hydrogen atmosphere; and this led to the hypothesis that a sun spot is a solar storm, resembling a terrestrial tornado, and in which the hot vapors whirling at high velocity are cooled by expansion. this would account for the observed intensity changes of the spectrum lines and the presence of chemical compounds. the vortex hypothesis suggested an explanation of the widening of many spot lines, and the doubling or trebling of some of them. as it is known that electrons are emitted by hot bodies, they must be present in vast numbers in the sun; and positive or negative electrons, if caught and whirled in a vortex, would produce a magnetic field. zeeman in had discovered that the lines in the spectrum of a luminous vapor in a magnetic field are widened, or even split into several components if the field is strong enough. characteristic effects of polarization appear also. the new apparatus of the observatory in conjunction with experiments in the laboratory immediately provided evidence that proved the existence of magnetic fields in sun spots, and strengthened the view that the spots are caused by electric vortices. extended investigations have led hale to the conclusion that the sun itself is a magnet, with its poles situated at or near the poles of rotation. in this respect the sun resembles the earth, which has long been known to be a magnet. the sun's axial rotation permits investigation of the magnetic phenomena of all parts of its surface, so that ultimately the exact position of the sun's magnetic poles and the intensity of the field at different levels in the solar atmosphere will be ascertained. schuster is of the opinion that not only the sun and earth, but every star, and perhaps every rotating body, becomes a magnet by virtue of its rotation. hale is confident that the -inch reflector will permit the test for magnetism to be applied to a few of the stars. the sun can be observed at mount wilson on at least nine-tenths of all the days in the year, and a daily record of the polarities of all spots with the -foot tower telescope is a part of the routine. a method has been devised for classifying sun spots on the basis of their magnetic properties, and more than a thousand spots have already been so classified. about per cent of all sun spots are found to be binary groups, the single or multiple members of which are of opposite magnetic polarity. unipolar spots are very seldom observed without some indication of the characteristics of bipolar groups. these are usually exhibited in the form of flocculi following the spot. the bipolar spot seems to be the dominant type, and the unipolar type a variant of it. although devised for quite another purpose, that of photographing the hydrogen prominences on the limb of the sun, the spectroheliograph has contributed very effectively to many departments of solar research. the prominences are dull reddish cloudlets that were first seen during total eclipses of the sun. probably vassenius, a swedish astronomer, during the total eclipse of , made the earliest record of them, as pinkish clouds quite detached from the edge of the moon; and in that day, when it had not yet been proved that the moon was without atmosphere, he naturally thought they belonged to the moon, not the sun. undoubtedly ulloa, a spanish admiral, also saw the prominences in observing the total eclipse of ; but they seem to have attracted little attention till , when a very important total eclipse was central throughout europe, and observed with great care by many of the eminent astronomers of all countries. so different did the prominences appear to different eyes, and so many were the theories as to what they were, that no general consensus of opinion was reached, and some thought them no part of either sun or moon, but a mere mirage or optical illusion. but at the return of this eclipse in , photography was employed so as to demonstrate beyond a shadow of doubt the real existence and true solar character of the prominences. by the slow progress of the moon across the sun and the prominences on the edge, a unique series of photographs by de la rue showed the moon's edge gradually cutting off the prominences piecemeal on one side of the sun, and equally gradually uncovering them on the opposite side. the prominences, then, were known to be real phenomena of the sun, some of them disconnectedly floating in his atmosphere, as if clouds. their forms did not vary rapidly, they were very abundant, and their light was so rich in rays of great photographic intensity that many were caught on the plate which the eye failed to see; they appeared at every part of the sun's limb and their height above it indicated that they must be many thousand miles in actual dimension. what they were, however, remained an entire mystery, and no one even thought it possible to find out what their chemical constitution might be or to measure the speed with which they moved. a few years later came the great indian eclipse (august , ), at that date the longest total eclipse ever observed. janssen of france and many others went out to india to witness it. fortunately the prominences were very brilliant and this led janssen to believe it would be possible for him to see them the day after the eclipse was over. by modifying the adjustment of his apparatus suitably and changing its relation to the sun's edge, he found that hydrogen is the main constituent in the light of the prominences. in addition to this he was able to trace out the shapes of the prominences, and even measure their dimensions. his station in india was at guntoor, many weeks by post from home; so that his account of this important discovery reached the paris academy of sciences for communication with another from the late sir norman lockyer of england, announcing a like discovery, wholly independently. the principle is simply this, and admirably stated by young: "under ordinary circumstances the prominences are invisible, for the same reason as the stars in the daytime: they are hidden by the intense light reflected from the particles of our own atmosphere near the sun's place in the sky; and if we could only sufficiently weaken this aerial illumination, without at the same time weakening _their_ light, the end would be gained. and the spectroscope accomplishes this very thing. since the air-light is reflected sunshine, it of course presents the same spectrum as sunlight, a continuous band of color crossed by dark lines. now, this sort of spectrum is greatly weakened by every increase of dispersive power, because the light is spread out into a longer ribbon and made to cover a more extended area. on the other hand, a spectrum of bright lines undergoes no such weakening by an increase in the dispersive power of the spectroscope. the bright lines are only more widely separated--not in the least diffused or shorn of their brightness." simultaneous announcement of this great discovery, by astronomers of different nations, working in widely separate regions of the earth, led to the striking of a gold medal by the french government in honor of both astronomers and bearing their united effigies. ever since the famous indian eclipse of , it has not been necessary to wait for a total eclipse in order to observe the solar prominences, but every observer provided with suitable apparatus has been able to observe them in full sunlight whenever desired, and the charting of them is part of the daily routine at several observatories in different parts of the world. so vast has been the accumulation of data about them that we know their numbers to fluctuate with the spots on the sun; and their distribution over the sun's surface resembles in a way that of the spots. while the spots and protuberances are most numerous around solar latitude degrees both north and south, the prominences do not disappear above latitude to degrees, as the spots do, but from latitude degrees they increase in number to about degrees, and are occasionally observed even at the sun's poles. faculæ and prominences are more closely related than the sun spots and prominences. there are wide variations in both magnitude and type of the prominences. heights above the sun's limb of a few thousand miles are very common, and they rarely reach elevations as great as , miles, though a very occasional one reaches even greater heights. classification of the prominences divides them into two broad types, the quiescent and the eruptive. the former are for the most part hydrogen, and the latter metallic. the quiescent prominences resemble closely the stratus and cirrus type of terrestrial clouds, and are frequently of enormous extent along the sun's edge. they are relatively long-lived, persisting sometimes for days without much change. the eruptive prominences are more brilliant, changing their form and brightness rapidly. often they appear as brilliant spikes or jets, reaching altitudes that average about , miles. rarely seen near the sun's poles, they are much more numerous nearer the sun spots. speed of motion of their filaments sometimes exceeds one hundred miles a second, and the changing variety of shapes of the eruptive prominences is most interesting. oftentimes they change so rapidly that only photography can do them justice. prominence photography began with young a half century ago, who obtained the first successful impression on a microscope slide with a sensitized film of collodion; as was necessary in the earlier wet-plate process of photography, which required exposures so long that little progress was effected for about twenty years. then it was taken up by deslandres of paris and hale of chicago independently, both of whom succeeded in devising a complex type of apparatus known as the spectroheliograph, by which all the prominences surrounding the entire limb of the sun can be photographed at any time by light of a single wave-length, together with the disk of the sun on the same negative. the prominences appear to be intimately connected with a gaseous envelope surrounding the solar photosphere, in which sodium and magnesium are present as well as hydrogen. the depth of the chromosphere is usually between , and , miles, and its existence was first made out during the total solar eclipses of and , when it appeared as an irregular rose-tinted fringe, though not at the time recognized as belonging to the sun. the constitution of the sun and its envelopes are still under discussion, and no complete theory of the sun has yet been advanced which commands the widest acceptance. of the interior of the sun we can only surmise that it is composed of gases which, because of intense heat and compression, are in a state unfamiliar on earth and impossible to reproduce in our laboratories. their consistency may be that of melted pitch or tar. surrounding the main body of the sun are a series of layers, shells, or atmospheres. outside of all and very irregular in structure, indeed probably not a solar atmosphere at all, is the solar corona, parts of which behave much as if it were an atmosphere, but it appears to be bound up in some way with the sun's radiation. it has streamers that vary with the sun-spot period, but its constitution and function are very imperfectly known, because it has never been seen or photographed except at rare intervals on occasion of total eclipses of the sun. beneath the corona we meet the projecting prominences, to which parts of the corona are certainly related, and beneath them the first true layer or atmosphere of the sun known as the chromosphere, its average depth being about one-hundredth part of the sun's diameter. beneath the chromosphere is the layer of the sun from which emanates the light by which we see it, called the photosphere. it appears to be composed of filaments due to the condensation of metallic vapors, and it is the outer extremities of these filaments which are seen as the granular structures everywhere covering the disk of the sun. their light shines through the chromosphere and the spots are ruptures in this envelope. between photosphere and chromosphere is a very thin envelope, probably not over miles in thickness, called the reversing layer. it is this relatively thin shell that is responsible for the absorption which produces the dark lines in the spectrum of the sun. under normal conditions the filaments of the photosphere are radial, that is vertical on the sun; but whenever eruptions take place, as during the occurrence of spots, the adjacent filaments are violently swept out of their normal vertical lines and these displaced columns then form what we view as the spot's penumbra. from the outer surface of the sun's chromosphere rise in eruptive columns vapors of hydrogen and the various metals of which the sun is composed. these and the spots would naturally occur in periods just as we see them. we have said that the sun is composed of a mass of highly heated or incandescent vapors or gases, whose compression on account of gravity must render their physical condition quite different from any gaseous forms known on the earth or which we can reproduce here. as the result of more than half a century of studious observation of the sun and mapping of its spectrum in every part, and diligent comparison with the spectra of all known chemical elements on the earth, we find that the sun contains no elements not already found here, but that a great preponderance of elements known to earth are found in the sun. the intensity of their spectral lines is one prominent indication of the presence of elements in the sun, and the number of coincidences of spectral lines is another. iron, nickel, calcium, manganese, sodium, cobalt, and carbon are among the elements most strongly identified. a few of the rarer terrestrial elements are of doubtful existence in the sun, and a very few, as gold, bismuth, antimony, and sulphur are not found there, and the existence of oxygen in the sun is regarded by some experts as doubtful. but if the whole earth were vaporized by heat, probably its spectrum would resemble that of the sun very closely. what are the effects of the sun, and sun spots in particular, on our weather? is the influence of their periodicity potent or negligible? if we investigate conditions pertaining to terrestrial magnetism, as fluctuations of the magnetic needle, and the frequency of auroræ, there is no occasion for doubt of the sun's direct influence, although we are not able to say just how that influence becomes potent. if, however, we look into questions of temperature, barometric pressure, rainfall, cyclones, crops, and consequent financial conditions, we find fully as much evidence against solar influence as for it. the slight variations of the sun's light and heat due to the presence or absence of sun spots can scarcely be sensible, and much longer periods of closer observation are necessary before such questions can be finally decided. the slighter such influences are, if they actually exist, and the more veiled they are by other influences more or less powerful, the more difficult it is to discover their effects with certainty. the importance of solar radiation in the prediction of terrestrial weather has long been recognized, but until very recently no practical application has been made. the smithsonian astrophysical observatory at washington, under the direction of dr. abbot, has for many years carried on at a number of stations a series of determinations of the constant of solar radiation by the spectro-bolometric method originated by langley. a new station in calama, chile, has recently been inaugurated, at which the solar constant is worked out each day, and telegraphed to the argentine weather service, where it is employed in forecasting for the day. abbot's new method of solar constant determination is based on the fact that atmospheric transparency varies oppositely to the variations of brightness of the sky. increase of haziness presents more reflecting surface to scatter the solar rays indirectly to the earth. of course it presents also additional surface to obstruct the direct rays from the sun. by measuring the brightness of the sky near the sun, it becomes possible to infer the coefficients of atmospheric transmission at all wave lengths. the direct observations and the complete deduction of the solar constant for the day can all be completed within two or three hours. clayton of buenos aires has now employed these results in the argentine weather predictions for two years, and the introduction of this new element in forecasting has brought about a pronounced gain in the value of the predictions. its adoption by the weather bureaus of other nations will doubtless come in due time, and the new method take a firmly established rank in practical meteorology. abbot's observations many years ago first called attention to the variability of the solar constant through a range of several per cent both from year to year, and in irregular short periods of weeks or even days. abbot considers this the more likely explanation than that atmospheric changes should take place simultaneously all over the earth. the sun is but a star, the stars that are irregularly variable in light and heat are numerous, and the sun itself appears to be one of these. especially important to the agricultural and vineyard interests of argentina is the question of precipitation, and clayton finds this very dependent on solar radiation. at epochs of practically stationary solar intensity, there is little or no precipitation; but quite generally he finds that great decrease of solar radiation is followed in from three to five days by heavy precipitation. direct temperature effects are also traced in buenos aires and other south american cities, lagging from two to three days behind the observed solar fluctuations. the station at calama yields about determinations of the solar constant each year, and the mount wilson station about half that number. they are the only stations of this character at present in existence, and others should be established in widely separated and cloudless regions, as egypt, southern california and australia. uniformity in the methods of observing would be highly desirable, and the smithsonian institution has perfected the details of common control of such stations which it is expected may be established at an early day. chapter xxvii the inner planets vulcan about the middle of the last century, le verrier, a great french astronomer, having added the planet neptune beyond the outside confines of the solar system, sought evidence of a lesser planet traveling round the sun within the orbit of mercury. for many years close watch was kept on the sun in the hope of discovering such a body in the act of passing across the disk, or in transit, as it is technically termed. lescarbault, a french physician, announced that he had actually seen such a planet, vulcan it was called, passing over the sun in . total eclipses of the sun would afford the best opportunity for seeing such a body, and on several such occasions astronomers thought they had found it. but the signal advantages of photography have been applied so often to this search, and always unsuccessfully, that the existence of vulcan, or the intramercurian planet, is now regarded as mythical. mercury this planet is an elusive body that very few, even astronomers, have ever seen. it is not very bright, has a rapid motion and never retreats far from the sun, so that it was a puzzle to the ancients who saw it, sometimes in the twilight after sunset and again in the twilight of dawn. when following the sun down in the west, in march or april, mercury is likely to be best seen; twinkling rather violently and nearly as bright as a star of the first magnitude. very little is to be seen on the minute disk of this planet, except that it goes through all the phases of the moon--crescent, gibbous, full, gibbous, crescent. whether mercury turns round on its axis or not, cannot be said to be known, because the markings that are suspected on its surface are too indefinite to permit exact observation. more than likely the planet presents always the same side or face to the sun, so that it turns round on its axis once, while traveling once around the sun in its orbit. mercury's day and year would therefore be equal in length. nor have we much evidence on the question of an atmosphere surrounding mercury; probably it is very thin, if indeed there is any at all. when mercury comes directly between us and the sun, crossing in transit, the edge of the planet as projected against the sun is very sharply defined, and this would indicate an absence of atmosphere on mercury. transits of mercury can occur in may and november only: there was one on november , , and there will be one on may , . the latter will be nearly eight hours in length, which is almost the limit. mercury's distance from the sun averages million miles, the diameter of the planet is , miles, and his orbital speed is miles per second, the swiftest of all the planets. no moon of mercury is known to exist, although many times diligently searched for, especially during transits of the planet. venus brightest of all the planets, and the most beautiful of all is venus. its path is next outside the orbit of mercury, but within that of the earth, so that it partakes of all the phases of the moon. like mercury it sometimes passes exactly between us and the sun, a rare phenomenon which is known as a transit of venus. being without telescopes, the ancients knew nothing about these occurrences, but they were puzzled for centuries over the appearance of the planet in the west after sunset, when they called it hesperus, and in early dawn in the east when they gave it the name phosphorus. venus is known to be girdled with an atmosphere denser than ours, and it seems to be always filled with dense clouds. it is the reflection of sunlight from this perpetually cloudy exterior which gives venus her singular radiance. so brilliant is she that even full daylight is not strong enough to overpower her rays; and she may often be seen glistening in the clear blue daytime sky, if one knows pretty nearly in what direction to look for her. venus is million miles from the sun, and as our own distance is million miles, this planet can come within million miles of the earth. it is therefore at times our nearest known neighbor in space, excepting only the moon and eros, one of the erratic little planets that travel round the sun between mars and jupiter. also possibly a comet might come much nearer. astronomers always take advantage of this nearness of venus to us, if a transit across the sun takes place; because it affords an excellent method of finding out what the distance of the sun is from the earth. a pair of these transits happens about once a century, there were transits in and , and the next pair occur in and . in actual size, venus is almost as large a planet as our own, being , miles in diameter, as compared with , for the earth. her velocity in her orbit is twenty-two miles per second, and she travels all the way round the sun in seven and one half months or days. venus from her striking brilliancy always leads the novice to expect to see great things on applying the telescope. but aside from a brilliant disk, now a slender crescent, now half full like the moon at quarter, and again gibbous as the moon is between quarter and full, the telescope reveals but little. there is pretty good evidence that the markings thought to have been seen on the planet's surface are illusory, and so it is wholly uncertain in what direction the planet's axis lies; also there is great uncertainty about the length of the day on venus, or the period of turning round on its axis. probably it is the same in length as the planet's year. once when venus passed very close to the sun, just barely escaping a transit, lyman of yale university caught sight of it by hiding the sun behind a tall building or church spire. the dark side of venus was turned toward us and he could not of course see that. but the planet was clearly there, completely encircled by a narrow delicate luminous ring, which was due to sunlight shining through the atmosphere that surrounds the planet. similar ring effects were seen by observers of the transits of venus in and ; and from all their observations it is concluded that venus has an atmosphere probably at least twice as dense and extensive as that which encircles the earth. spurious satellites of venus are many, but no real moon is known to attend this planet. [illustration: the surface of the moon in the region of copernicus. photograph made with the hooker -inch reflecting telescope. (_photo, mt. wilson solar observatory._)] [illustration: a view of the south central portion of the moon at last quarter. (_photo, mt. wilson solar observatory._)] chapter xxviii the moon and her surface as the sun has always reigned as king of day, so is the moon queen of night. observation of her phases, now waxing, now waning, with her stately motion always eastward among the stars, began with the earliest ages. often when near the full she must have been seen herself eclipsed, and much more rarely the occurrence of total eclipses of the sun are certain to have suggested the moon's intervention between earth and sun, shutting off the sunlight completely, because these eclipses never took place except when the moon was in the same part of the sky with the sun. if we watch the nightly march of the moon, we shall find that she travels over her own breadth in about an hour's time. by using a telescope on the stars just eastward or to the left of her, she will now and then be seen to pass between us and a star--on very rare occasions a planet--extinguishing its light with great suddenness, the most nearly instantaneous of all phenomena in nature. draw a line connecting the cusps, or horns of the lunar crescent, and then a line eastward at right angles to this, and it will show the direction of the moon's own motion in its orbit round the earth quite accurately. as the phase advances, note the inside edge of the advancing crescent: this will be quite rough and jagged, compared to the outside edge which is the moon's real contour and relatively very smooth. the position of the inside curve will change from night to night, and it marks the line of sunrise on the moon during the fortnight elapsing between new moon and full; while from full through last quarter and back to new moon, this advancing line marks the region of sunset on the moon. the general shape of this line is never a circle but always elliptical, and astronomers call it the terminator. all along the terminator, sunlight strikes the lunar surface at a small angle, whether near sunrise or sunset; so that owing to the mountains and other high masses of the moon's surface, the terminator is always a more or less jagged and irregular line. onward from new moon toward full the horns of the crescent are always turned upward or eastward. when the general line of the terminator becomes a straight line from cusp to cusp, the moon is said to have reached first quarter or quadrature. onward toward full the terminator will be seen to bend the other way, and in about a week's time it will have merged itself with the moon's limb. the moon is then said to be full. afterward the phase phenomena recur in the reverse order, with third quarter midway between full and new moon again; the phase of the moon being called gibbous all the way from first quarter to third quarter, except when exactly full. as we know that the moon is, like the earth, a nonluminous body, and shines only by virtue of the sunlight falling upon it, clearly an entire half of the moon's globe must be perpetually illumined by sunlight. the varying phases then are due simply to that part of the illuminated hemisphere which is turned toward us. new moon is entirely invisible because the sunward hemisphere is turned wholly away from us, while at full moon we see the lunar disk complete because we are on the same side of the moon that the sun is and practically in line with both sun and moon. if we could visit the moon, we should see the earth in exactly complementary phase. at new moon here we should be enjoying full earth there, and full moon here would be coincident with new or dark earth there. the narrow crescent of new moon here would be the period of gibbous earth there; and it is the reflection of sunlight from this gibbous earth which illuminates the part of the moon but faintly seen at this time, popularly known as the "old moon in the new moon's arms." its greater visibility at some times than at others is due to greater prevalence of clouded area in the reflecting regions of the earth turned toward the moon, and the higher reflective power of clouds than that possessed by mere land and water. as the moon goes all the way round the sky every month, the same as the sun does in a year, and travels in nearly the same path, clearly it must also go north and south every month as the sun does. so in midsummer when the sun runs high upon the meridian, we expect to find full moons running low, and likewise in midwinter the full moon always runs high, as almost everyone has sometimes or other noticed. this eastward or true orbital motion of the moon is responsible for another relation which soon comes to light when we begin to observe the moon; and that is the later hour of rising or setting each night. our clock time is regulated by the sun, which also is moving eastward about ° daily, or twice its own breadth. so the moon's eastward gain on the sun amounts to about degrees daily, and one degree being equal to minutes, the retarded time of moonrise or moonset each day amounts to very nearly minutes on the average; though sometimes the delay will be less than a half hour and at other times it will exceed an hour and a quarter. the season of least retardation of rising of the full moon is in the autumn, and so the moon that falls in late september or october is known as the harvest moon, and the next succeeding full moon is called the hunter's moon. lunation is a term sometimes given to the moon's period from any definite phase round to the same phase again. its length is the true period of the moon's revolution once around the earth, from the sun all the way round till it overtakes the sun again. the synodic period is another name for lunation, and its true length is and one-half days, or very accurately d. h. m. . s. as calculated by astronomers with great exactness from many thousand revolutions of the moon. but if we want the true period of the moon round the earth as referred to a star, it is much shorter than this, amounting to only days and nearly one-third. this is called the moon's sidereal period of revolution, because it is the time elapsed while she is traveling eastward from a given star around to coincidence with the same star again. if we study the moon's path in the sky more critically, we shall find that it does not quite follow the ecliptic, or the sun's path, but that twice each month she deviates from the ecliptic, once to the north and once to the south of it, by roughly ten times her own breadth. more accurately this angle is ° ' ", an almost invariable quantity, and it is therefore known as an astronomical constant, or the inclination of the moon's orbit to the ecliptic. so the moon's orbit must intersect the ecliptic, and as both are great circles in the sky, the points of intersection are known as the moon's nodes, one ascending and the other descending, and the nodes are degrees apart. the figure of the moon's orbit is not circular, although it deviates only slightly from that form. but like the paths of all other satellites round their primary planets, and of the planets themselves round the sun, the moon's orbit is also an ellipse. the distance of the moon's center from the earth's center is therefore perpetually changing; the point of nearest approach is called perigee, and that of farthest recession, apogee. the moon's distance from the earth is easier and simpler to be ascertained than that of any other heavenly body, because it is the nearest. an outline of the method of finding this distance is not difficult to present; and it resembles in every particular the method a surveyor uses to find the distance of some inaccessible point which he cannot measure directly. up and down a stream, for example, he measures the length of a line, and from each end of it he measures the angle between the other end of the line and the object on the opposite side of the stream whose distance he wishes to find out. then he applies the science of trigonometry to these three measures, two of angles and one the length of the side or base included between them, and a few minutes' calculation gives the distance of the inaccessible object from either end of the base line. now in like manner, to transfer the process to the sky, let the two ends of the base be represented by two astronomical observatories, for example, greenwich in the northern hemisphere and cape town in the southern. the base line is the chord or straight line through the earth connecting the two observatories, and we know the length of this line pretty accurately, because we know the size of the earth. the angles measured are somewhat different from those in the terrestrial example, but the process amounts to the same thing because the astronomers at the two observatories measure the angular distance of the center of the moon from the zenith, each using his own zenith at the same time; and the same science of trigonometry enables them to figure out the length of any side of the triangles involved. the side which belongs to both triangles is the distance from the center of the earth to the center of the moon, and the average of many hundred measures of this gives , miles, or about ten times the distance round the equator of the earth. we have said that the orbit in which the moon travels round the earth is practically a circle, but the earth's center is found not at the center of this orbit, but set to one side, or eccentrically, so that the distance spanning the centers of the two bodies is sometimes as small as , miles at perigee, and , miles at apogee. the moon's speed in this orbit averages rather more than half a mile every second of time--more accurately , feet a second, or , miles per hour. once the moon's distance is known, its size or diameter is easy to ascertain. an angular measure is necessary, of course, that of the angle which the disk of the moon fills as seen from the earth. there are many types of astronomical instruments with which this angle can be measured, and its value is something more than half a degree ( ' "). the moon's actual diameter figures out from this , miles; and it would therefore require nearly fifty moons merged in one to make a ball the size of the earth. still, no other planet has a satellite as large in proportion to its primary as the moon is in relation to the earth. but the materials that compose the moon have less than two-thirds the average density of those that make up the earth, so that eighty-one moons fused together would be necessary to equal the mass or weight of the earth. if we figure out the force of attraction of the moon for bodies on its surface, we find it equals about one-sixth that of the earth. athletes could perform some astounding feats there--miracles of high jump and hammer-throw. our interest in the moon's physical characteristics never wanes. her nearness to us has always fascinated astronomer and layman alike. early users of the telescope were readily led into error regarding the general characteristics of the lunar surface; and it is easy to see why they thought the smooth level planes must be seas, and gave them names to that effect which persist to-day, as mare crisium, mare serenitatis and so on. we may be sure that no water exists on the moon's surface, although some astronomers think that solid water, as ice or snow, may still exist there at a temperature too low for appreciable evaporation. perhaps water, seas, and oceans were once there, but their secular dissemination and loss as vapor have gone on through the millions of millions of years till even the moon's atmosphere appears to have vanished completely. at least there is much better evidence of absence of atmosphere on the moon than of its presence--not enough at any rate to equal a thousandth part of the barometric pressure that we have at the earth's surface. frequent observations of stars passing behind the moon in occultation have satisfied astronomers on this point. we often say of the brilliant full moon, it is as bright as day. the photometer or instrument for accurate comparison of lights, their amount and intensity, tells a different story. indeed, if the entire dome of the sky were filled with full moons, we should be receiving only one-eighth of the light the sun gives us, and it would require more than , average full moons to equal the light radiation of the sun. heat from the moon, however, is quite different. early attempts to measure it detected none at all, but with modern instruments there is little trouble in detecting heat from the moon, though measurement of it is not easy. much of the moon's heat is sun heat, directly reflected from the moon, as sunlight is, but most of it is due to radiation of solar heat previously absorbed by the materials of the lunar surface. the actual temperature of the moon's surface suffers great variation. a fortnight's perpetual shining of the sun upon the lunar rocks would certainly heat them above the temperature of boiling water, if the moon had an atmosphere to conserve and store this heat; but the entire absence of such an air blanket probably permits the sun's heat to be radiated away nearly as fast as it is received, leaving the temperature at the surface always very low. what physical influences the moon really has upon the earth must be very slight, barring the tides. but there is little hope of getting people generally to take that view, because the moon appears to be the planet of the people, and opinion that the moon controls the weather, for instance, amounts with them to practical certainty. more than likely all these notions are but legitimate survivals of superstition and astrology. in addition to the tides, our magnetic observatories reveal slight disturbances with the swinging of the moon from apogee to perigee and back; but long series of weather observations have been faithfully interrogated, with negative or contradictory results. if one believes that the moon's changes affect the weather, it is easy to remember coincidences, and pass over the many times when no change has taken place. the moon changes pretty frequently anyhow. as young well puts it: "a change of the moon necessarily occurs about once a week.... _all_ changes, of the weather for instance, must therefore occur within three and three-fourth days of a change of the moon, and fifty per cent of them ought to occur within forty-six hours of a change, even if there were no causal connection whatever." when we turn to the strongly diversified surface of the moon itself, we find much to rivet the attention, even with slender optical aid. everyone wants to know how near the telescope, the biggest possible telescope, brings the moon to us. that will depend on many things, first of all on the magnifying power of the eyepiece employed on the telescope, and eyepieces are changed on telescopes just as they are on microscopes, though not for the same reasons. the theoretical limit of the power of a telescope is usually considered as for each inch of diameter or aperture of the object glass. a -inch telescope, as that of the yerkes observatory, the largest refracting telescope in existence, should bear a magnifying power not to exceed , . but this limit is practically never reached, one-half of it or fifty to the inch of aperture being a good working limit of power, even under exceptional conditions of steadiness of atmosphere. if we reduce the effective distance of the moon from , miles to miles, that is about the utmost that can be expected. but even at that distance we can make out only landscape details, nothing whatever like buildings or the works of intelligence. the larger relations of light and shade, so obvious to the naked eye on the moon, vanish on looking at it with the telescope, but we are at once captivated by the novel character of the surface and the seemingly great variety of detail that is clearly visible. as soon as the new moon comes out in the west, one may begin to gaze with interest and watch the terminator or sunrise line gradually steal over the roughened surface, bringing new and striking craters into view each night. around the time of quarter moon, or a little past it, is one of the best times for telescopic views of the moon, because the huge craters, tycho and copernicus, are then in fine illumination. close to the phase of full moon is never a good time, because there are no shadows of the rough surface then, and its entire structure seems to be quite flat and uninteresting, except for the streaks or rills which radiate from tycho in every direction, and are the only lunar features that are best seen near full. in a broad, general way, the moon's surface, if compared with the earth's, differs in having no water. our extensive oceans are replaced there by smooth, level plains which were at first thought to be seas and so named. there are ten or twelve of them in all. then we find mountain ranges, so numerous on the earth, relatively few on the moon. those that exist are named, in part, for terrestrial mountain ranges, as the alps, caucasus, and the apennines. but the nearly circular crater, a relatively rare formation on the earth, is seen dotted all over the moon in every size, from a fraction of a mile in diameter up to sixty, seventy, and in extreme cases a hundred miles. no mere description of plains and mountains and craters affords an adequate idea of the moon's surface as it actually is; a telescopic view is necessary, or some of the modern photographs which give an even better notion of the moon than any telescopic view. many of the lunar craters are without doubt volcanic in origin, others seem to be ruins of molten lakes. many thousands of the smaller ones appear as if formed by a violent pelting of the surface when semi-plastic, perhaps by enormous showers of meteoric matter. more than , craters cover the half of the lunar surface visible from the earth, and hundreds of them are named for philosophers and astronomers. measurement of the height of lunar mountains has been made in numerous instances, especially when their shadows fall on plains or surfaces that are nearly level, so that the length of the shadow can be measured. in general, the height of lunar peaks is greater than that of terrestrial peaks, owing probably to the lesser surface gravity on the moon. about forty lunar peaks are higher than mont blanc. most astronomers regard it as certain that no changes ever take place on the moon; probably no very conspicuous changes ever do. some, however, have made out a fair case for comparatively recent changes in surface detail. extreme caution is necessary in drawing conclusions, because the varying changes of illumination from one phase to another are themselves sufficient to cause the appearance of change. at intervals of a double lunation, equal to fifty-nine days, one and one-half hours, the terminator goes very nearly through the same objects, so that the circumstances of illumination are comparable. in mare serenitatis the little crater named linné was announced to have disappeared about a half century ago; subsequently it became visible again and other minor changes were reported, perhaps due to falling in of the walls of the crater. if one were to visit the moon, he must needs take air and water along with him, as well as other sustenance. no atmosphere means no diffused light; we could see nothing unless the sun's direct rays were shining upon it. anyone stepping into the shadow of a lunar crag would become wholly invisible. no sound, however loud, could be heard; sound in fact would become impossible. a rock might roll down the wall of a lunar crater, but there would be no noise; though we should know what had happened by the tremor produced. so slight is gravity there that a good ball player might bat a baseball half a mile or more. looking upward, all the stars would be appreciably brighter than here, and visible perpetually in the daytime as well as at night. if one were to go to the opposite side of the moon, he would lose sight of the earth until he came back to the side which is always turned toward the earth. even then the earth would never rise and set at any given place, as the moon does to us, but would remain all the time at about the same height above the lunar horizon. the earth would go through all the phases that the moon shows to us here, full earth occurring there when it is new moon here. our globe would appear to be nearly four times broader than the moon seems to us. its white polar caps of ice and snow, its dark oceans, and the vast cloud areas would be very conspicuous. faint stars, the zodiacal light, and the filmy solar corona would be visible, probably even close up to the sun's edge; but although his rays might shine upon the lunar rocks without intermission for a fortnight, probably they would still be too cold to touch with safety. on the side of the moon turned away from the sun, the temperature of the moon's surface would fall to that of space, or many hundred degrees below zero. chapter xxix eclipses of the moon of all the weird happenings of the nighttime sky, eclipses of the moon are the most impressive. rarely is there a year without one. what is the cause? simply the earth getting in between sun and moon, and thereby shutting off the sunlight which at all other times enables us to see the moon. as the earth is a dark body it must cast a black shadow on the side away from the sun, and it is the moon's passing into this shadow or some part of it that causes a lunar eclipse. sun and earth being so different in size, the earth's shadow must stretch away from it into space, growing smaller and smaller, until at length it comes to an end--the apex of a cone , miles long. if we cut off this shadow at the moon's distance from the earth, we find it about , miles in diameter at that point; and this accounts for the fact that the curvature on the side of the moon, when the eclipse is coming on and where it is dropping into the shadow, is always much less rapid than the curvature of the moon's own disk is. when an eclipse is approaching, the eastern limb will be duskily darkened for half an hour or more, because the moon must first pass through the outer penumbra, or half-shadow which everywhere surrounds the true shadow itself. if the moon hits only the upper or lower part of the shadow, the eclipse will be only partial, and during the progress of the eclipse it will seem as if the uneclipsed part had swung or twisted around in the sky, from the western limb of the moon to the eastern. but when the moon passes through the middle regions of the shadow, the eclipse is always total, and direct sunlight is wholly cut off from every part of the moon's face, for a greater or less length of time, according to the part of the shadow through which it passes. when passing centrally through the shadow, the total eclipse will last about two hours, as the moon's diameter is about one-third of the breadth of the shadow; and the eclipse will be partial about two hours longer, an hour at beginning and an hour at the end, because the moon moves over her own breadth in about an hour. while the moon is wholly immersed in the shadow, her body is nevertheless visible, as a dull tarnished copper disk; and this is caused by the reddish sunlight which grazes the earth all around and is refracted or bent by our atmosphere into the shadow itself. if this belt or ring of terrestrial atmosphere happens to be everywhere filled with dense clouds, as was the case in , even the familiar copper moon of a total lunar eclipse disappears completely in the black sky. quite different from a solar eclipse, all the phases of a lunar eclipse are visible at the same time on the earth wherever the moon is above the horizon. eclipses of the moon are therefore seen with great frequency at any given place as compared with solar eclipses, which are restricted to relatively narrow areas of the earth's surface. nor are lunar eclipses of very much significance to the astronomer, mainly because of the slowness and indefiniteness of the phenomena. it is a good time to observe occultations of faint stars at the moon's edge or limb, and several such programs have been carried out by cooperation of observatories in widely separate regions of the world: the object being improvement in our knowledge of the distance of the moon, and in the accuracy of the mathematical tables of her motion. search by photography for a possible satellite, or moon of the moon, has been made on several occasions, though without success. a lunar eclipse was first observed and photographed from an aeroplane, may , . at the request of the writer, two aviators of the united states navy ascended to a height of , feet above rockaway, and secured many advantages accruing from great elevation in viewing a celestial phenomenon of this character. chapter xxx total eclipses of the sun primitive peoples indulged in every variety of explanation of mysterious happenings in the sky. to the chinese and all through india, a total eclipse of the sun is caused by "a certain dragon with very black claws," who, except for their frightening him away by every conceivable sort of hideous noise, would most certainly "eat up the sun." the eclipse always goes off, the sun has never been eaten yet. can you convince a chinaman that rahu, the dragon, wouldn't have eaten up the sun, if his unearthly din hadn't frightened him away? in japan the eclipse drops poison from the sky into wells, so the japanese cover them up. fontenelle relates that in the middle of the seventeenth century a multitude of people shut themselves up in cellars in paris during a total eclipse. in the shu-king, an ancient chinese work, occurs the earliest record of a total eclipse of the sun, in the year b. c. . the nineveh eclipse of b. c. is perhaps the first of the ancient eclipses of which we possess a really clear description on the assyrian eponym tablets in the british museum. it is the eclipse possibly referred to in the book of amos, viii. but of all the ancient eclipses none perhaps exceeds in interest the famous eclipse of thales, b. c. , may . it is the first eclipse to have been predicted, probably by means of the saros, or -year period of eclipses, which is useful as an approximate method even at the present day. but the accident of a war between the lydians and the medes has added greatly to the historic interest, because the combatants were so terrified by the sudden turning of day into night that they at once concluded a peace cemented by two marriages. very many of the ancient eclipses have been of great use to the historian in verifying dates, and mathematical astronomers have employed them in correcting the lunar tables, or intricate mathematical data by which the motion of the moon is predicted. coming down to the middle of the sixth century, we find the first eclipse recorded in england, in the "saxon chronicle," a. d. . during the epoch of the arabian nights several eclipses were witnessed at bagdad, a. d. to , and many a century later by ibu-jounis, court astronomer of hakem, the caliph of egypt. nothing is more interesting than to search the quaint records of these ancient eclipses. one occurring in , when tycho brahe was but fourteen, had much to do with turning his permanent interest toward mathematics and astronomy. the eclipse of was the first "seen through a tube," the telescope having been invented only a few years before. "paradise lost" was completed about , and the censorship was still in existence; and it is matter of record that the oft-quoted passage, "as when the sun, new risen, looks through the horizontal misty air, shorn of his beams; or from behind the moon, in dim eclipse, disastrous twilight sheds on half the nations, and with fear of change perplexes monarchs." _p. l._, i. was strongly urged as sufficient reason for suppressing the entire epic. london was favored with the outflashing corona, may , , and a pamphlet was issued in prediction, entitled "the black day, or a prospect of doomsday." the first american eclipse expedition was on occasion of the totality of oct. , , sent out by harvard college and the american academy of arts and sciences under professor samuel williams to penobscot. there was a fine total eclipse from albany to boston on june , , and many important observations of it were made in this country. but it was not till the european eclipse of that research got fully under way, because the germ of the new astronomy, particularly as applied to the sun, had begun its development; and the significance of the corona was obvious, if it could be proved a true appendage of the sun. photography had not long been discovered, and the corona of was the first to be automatically registered on a daguerreotype. in it was proved that prominences and corona both belong to the sun and not to the moon. the great indian eclipse of brought the important discovery that the prominences can be observed at any time without an eclipse by means of the spectroscope. in bright lines were found in the spectrum of the corona, one line in the green indicating the presence of an element not then known on the earth and hence called coronium. in the reversing layer or stratum of the sun was discovered. in a vast ecliptic extension of the streams of the corona many millions of miles both east and west of the sun was first seen. this is now known to be the type of corona characteristic of minimum spots on the sun. in the spectrum of the corona was first photographed and in excellent detail photographs of the corona were taken. in it was shown that the corona quite certainly rotates bodily with the sun. in actual spectrum photographs of the reversing layer established its existence beyond doubt--"flash spectrum" it is often called. in the long ecliptic streamers of the corona were successfully photographed for the first time. in the depth of the reversing layer was found to average miles, the heat of the corona was first measured by the bolometer, and many observations showed that the coronal streamers, in part at least, partake of the nature of electric discharges. all subsequent total eclipses have been carefully observed, in whatever part of the world they may happen, and each has added new results of significance to our theories of the corona and its relation to the radiant energy of the sun. in very recent eclipses the cinematograph has been brought into action as an efficient adjunct of observation; in the first successful "movie" of the eclipse was secured in sweden, and in frost of the yerkes observatory first applied the cinematograph to registry of the "flash spectrum," and stebbins tested out his photo-electric cell on the corona, making the brightness . that of the full moon. in (russia) and again in (on the atlantic) the obvious advantages of the aeroplane in ecliptic observation and photography were sought by the writer, though unsuccessfully. the photographic tests, however, conducted in preparation for these expeditions proved the entire practicability of securing eclipse results of much value, independently of clouds below. eclipses in the near future will be total in australia about six minutes on september , ; in california and mexico about four minutes on september , ; and along a line from toronto to nantucket about two minutes on the morning of january , . to all spectators, savage or civilized, scientist or layman, a total eclipse is wonderful and impressive. langley said: "the spectacle is one of which, though the man of science may prosaically state the facts, perhaps only the poet could render the impression." very gradually the moon steals its way across the face of the sun, the lessened light is hardly noticed. if one is near a tree through whose foliage the sunlight filters, an extraordinary sight is seen; the ground all about is covered with luminous crescents, instead of the overlapping disks which were there before the eclipse came on; in both cases they are images of the disk of the sun at the time, and the narrowing crescents will be watched with interest as totality approaches. then the shadow bands may be seen flitting across the landscape, like "visible wind." they are probably related to our atmosphere and the very slender crescent from which true sunlight still comes. then for a few seconds the moon's actual shadow may be caught in its approach, very suddenly the darkness steals over the landscape and--totality is on. how lucky if there are no clouds! every eye is riveted on "the incomparable corona, a silvery, soft, unearthly light, with radiant streamers, stretching at times millions of uncomprehended miles into space, while the rosy flaming protuberances skirt the black rim of the moon in ethereal splendor." then it is now or never with observer and photographer. months of diligent preparations at home followed by weeks of tedious journey abroad, with days of strenuous preparation and rehearsals at the station--all go for naught unless the whole is tuned up to perfect operation the instant totality begins. it may last but a minute, or even less; in , however, total eclipse will last minutes seconds, the longest ever observed, and within half a minute of the longest possible. all is over as suddenly as it came on. the first thing is to complete records, develop plates, and see if everything worked perfectly. there is great utility back of all eclipse research, on account of its wide bearing on meteorology and terrestrial physics, and possibly the direct use of solar energy for industrial purposes. with this purpose in view the astronomer devotes himself unsparingly to the acquisition of every possible fact about the sun and his corona. considering the earth as a whole, the number of total eclipses will average nearly seventy to the century. but at any given place, one may count himself very fortunate if he sees a single total eclipse, although he may see several partial ones without going from home. then, too, there are annular or ring eclipses, averaging seven in eight years. but had one been born in boston or new york in the latter part of the eighteenth century, he might have lived through the entire nineteenth century and a long way into the twentieth without seeing more than one total eclipse of the sun. in london in no total eclipse had been visible for six centuries. however, taking general averages, and recalling the comparatively narrow belt of total eclipse, every part of the earth is likely to come within range of the moon's shadow once in about three and a half centuries. the longest total eclipses always occur near the equator; this is because an observer on the equator is carried eastward by the earth's rotation at a velocity of about , miles per hour, so that he remains longer in the moon's shadow which is passing over him in the same direction with a velocity about twice as great. the general circumstances of total eclipses are readily foretold by means of the ancient chaldean period of eclipses known as the saros. it is years and or days in length (according to the number of leap years intervening). in one complete saros, forty-one solar eclipses will generally happen, but only about one-fourth of them will be total. the saros is a period at the end of which the centers of sun and moon return very nearly to their relative positions at the beginning of the cycle. so, in general, the eclipse of any year will be a repetition of one which took place years before, and another very similar in circumstances will happen years in the future. three periods of the saros, or years and month, will usually bring about a return of any given eclipse to any particular part of the earth, so far as longitude is concerned, though the returning track will lie about miles to the north or south of the one years earlier. paths of total eclipses frequently intersect, if large areas like an entire country are considered; spain, for instance, where total eclipses have occurred in , , , and . besides crossing spain, the tracks of totality on may , , and august , , were unique in intersecting exactly over a large city--tripoli in barbary, on both of which occasions the writer's expeditions to that city were rewarded with perfect observing conditions in that now italian province on the edge of the great desert. kepler was the first astronomer to calculate eclipses with some approach to scientific form, as exemplified in his rudolphine tables. his method was of course geometrical. but la grange, who applied the methods of more refined analysis to the problem, was the first to develop a method by which an eclipse and all its circumstances could be accurately predicted for any part of the earth. to many minds, the prediction of an eclipse affords the best illustration of the superior knowledge of the astronomer: it seems little short of the marvelous. but recalling that the motion of the moon follows the law of gravitation, and that its position in the sky is predictable for years in advance with a high degree of precision, it will readily be seen how the arrival of the moon's shadow, and hence the total eclipses of the sun, can be foretold for any place over which the shadow passes. all these data derived by the mathematician are known as the elements of the eclipse, and they are prepared many years in advance and published in the nautical almanacs and astronomical ephemerides issued by the leading nations. buchanan's "treatise on eclipses" will supply all the technical information regarding the prediction of eclipses that anyone desirous of inquiring into this phase of the problem may desire. so important are total eclipses in the scheme of modern solar research, and so necessary are clear skies in order that expeditions may be favored with success, that every effort is now made to ascertain the weather chances at particular stations along the line of eclipse many years in advance. this method of securing preliminary cloud observations for a series of years has proved especially useful for the eclipses of , , , and ; and had it been employed in russia for totality of , many well-equipped expeditions might have been spared disaster. the california and mexico totality of does not require this forethought, as the regions visited are quite likely to be free from cloud; but observations are now in process of accumulation for the total eclipse of . the out-look for clear skies on that occasion, the total eclipse nearest new york for more than a century, is not very promising. the path of totality passes over marquette, michigan, rochester and poughkeepsie, new york, newport, rhode island, and nantucket about nine in the morning. everyone who saw it will remember the last total eclipse in this part of the world--on june , , visible from oregon to florida. many will recall the last total eclipse that was visible before that in the eastern part of the united states, on may , , visible in a narrow path from new orleans to norfolk. one's father or grandfather will perhaps remember the total eclipse of july , , which passed over the united states from pike's peak to texas (it was the writer's maiden eclipse), and another on august , , which passed southeasterly over iowa and kentucky. on all these occasions the paths of total eclipse were dotted with numerous observing parties, many of them equipped with elaborate apparatus for studying and photographing the solar corona and prominences, together with a multitude of other phenomena which are seen only when total eclipses take place. looking forward rather than backward, a striking series, or family, of eclipses happens in the future: it is the series of may, and , recurring again on june , (over the pacific ocean), june , (through india, siam, and luzon), and june , (visible in sahara, abyssinia, and somali). already in this totality was minutes seconds in duration; in , as already mentioned, it will be minutes seconds, and at the subsequent returns even longer yet, approaching the estimated maximum of minutes seconds which has never been observed. this remarkable series of total eclipses is longer in duration than any others during a thousand years. its next subsequent return is in , occurring with the eclipsed sun practically at noon in the zenith of mount popocatepetl in mexico. whatever may be the progress of solar research during the intervening years, it is impossible to imagine the alert astronomer of that remote day without incentive for further investigation of the sun's corona, in which are concealed no doubt many secrets of the sun's evolution from nebula to star. chapter xxxi the solar corona "and what is the sun's corona?" mildly asked a college professor of a student who might better have answered "not prepared." "i did know, professor, but i have forgotten," was his reply. "what an incalculable loss to science," returned the professor with a twinkle. "the only man who ever knew what the sun's corona is, and he has forgotten!" only in part has the mystery of the corona been cleared by the research of the present day. our knowledge proceeds but slowly, because the corona has never been seen except during total eclipses of the sun; and astronomers, as a matter of fact, have never had a fair chance at it. two total eclipses happen on the average of every three years; their average duration is only two or three minutes; totality can be seen only in a narrow path about a hundred miles wide, though it may be several thousand miles long; there is usually about equal chance of cloud with clear skies; and fully three-fourths of the totality areas of the globe are unavailable because covered by water. so that even if we imagine the tracks of eclipses quite thickly populated with astronomers and telescopes, at least one every hundred miles, how much solid watching of the corona would this permit? only a little more than one week's time in a whole century. the true corona is at least a triple phenomenon and a very complex one. the photographs reveal it much as the eye sees it, with all its complexity of interlacing streamers projected into a flat, or plane, surrounding the disk of the dark moon which hides the true sun completely. but we must keep in mind the fact that the sun is a globe, not a disk, and that the streamers of the corona radiate more or less from all parts of the surface of the solar sphere, much as quills from a porcupine. from the sun's magnetic poles branch out the polar rays, nearly straight throughout their visible extent. gradually as the coronal rays originate at points around the solar disk farther and farther removed from the poles, they are more and more curved. very probably they extend into the equatorial regions, but it is not easy to trace them there because they are projected upon and confused with the filaments having their origin remote from the poles. then there is the inner equatorial corona, apparently connected intimately with truly solar phenomena, quite as the polar rays are. the third element in the composite is the outer ecliptic corona, for the most part made up of long streamers. this is most fully developed at the time of the fewest spots on the sun. it is traceable much farther against the black sky with the naked eye than by photography. without any doubt it is a solar appendage and possibly it may merge into the zodiacal light. naturally this superb spectacle must have been an amazing sight to the beholders of antiquity who were fortunate enough to see it. historical references are rare: perhaps the earliest was by plutarch about a. d. , who wrote of it, "a radiance shone round the rim, and would not suffer darkness to become deep and intense." philostratus a century later mentions the death of the emperor domitian at ephesus as "announced" by a total eclipse. kepler thought the corona was evidence of a lunar atmosphere; indeed, it was not until the middle of the th century that its lack of relation to the moon was finally demonstrated. later observers, wyberd in and ulloa, got the impression that the corona turned round the disk catherine-wheel fashion, "like an ignited wheel in fireworks, turning on its center." but no later observer has reported anything of the sort. quite the contrary, there it stands against the black sky in motionless magnificence a colorless pearly mass of wisps and streamers for the most part nebulous and ill-defined, fading out very irregularly into the black sky beyond, but with a complex interlacing of filaments, sometimes very sharply defined near the solar poles. it defies the skill of artist and draughtsman to sketch it before it is gone. photograph it? yes, but there are troubles. of course the camera work is superior to sketches by hand. as langley used to say, "the camera has no nerves, and what it sets down we may rely on." foremost among the photographic difficulties is the wide variation in intensity of the coronal light in different regions of the corona. if a plate is exposed long enough to get the outer corona, the exceeding brightness of the inner corona overexposes and burns out that part of the plate or film. if the exposure is short, we get certain regions of the inner corona excellently, but the outer regions are a blank because they can be caught only by a long exposure. so the only way is to take a series of pictures with a wide range of exposures, and then by careful and artistic handwork, combine them all into a single drawing. wesley of london has succeeded eminently in work of this character, and his drawings of the sun's corona, visible at total eclipses from onward, in possession of the royal astronomical society, are the finest in existence. they give a vastly better idea of the corona, as the eye sees it, than any single photograph possibly can. the early observers apparently never thought of the corona as being connected with the sun. it was a halo merely, and so drawn. its real structure was neither known, depicted, or investigated. sketches were structureless, as any aureola formed by stray sunlight grazing the moon might naturally be. that the rays are curved and far from radial round the sun was shown for the first time in the sketches of , and in sir francis galton observed that the long arms or streamers "do not radiate strictly from the center." the inner corona had first been recorded photographically on a daguerreotype plate during the eclipse of , but the lens belonged to a heliometer, and was of course uncorrected for the photographic rays. the wet collodion plates of the eclipse of , by de la rue, proved that not only the prominences but the corona were truly solar, because his series of technically perfect pictures revealed the steady and unchanged character of these phenomena while the moon's disk was passing over them as totality progressed. and at the eclipse of , young put the solar theory of the corona beyond the shadow of any further doubt by examination of its light with the spectroscope and discovering a green line in the spectrum due to incandescent vapor of a substance not then identified with anything terrestrial, and therefore called coronium. the total brilliance of the corona was very differently estimated by the earlier observers, though pretty carefully measured at later eclipses. the standard full moon is used for reference, and at one eclipse the corona falls short of, while at another it will exceed the full moon in brightness. variations in brilliancy are quite marked: at one eclipse it was nearly four times as bright as the full moon. much evidence has already accumulated on this question; but whether the observed variations are real, or due mainly to the varying relative sizes of sun and moon at different eclipses, is not yet known. the coronal light is largely bluish in tint, and this is the region of the spectrum most powerfully absorbed by our atmosphere. eclipses are observed by different expeditions located at stations where the eclipsed sun stands at very different altitudes above the horizon; besides this the localities of observation are at varied elevations above sea level; so that the varying amount of absorption of the coronal light renders the problem one of much difficulty. the long ecliptic streamers of the corona were first seen by newcomb and langley during the totality of . on one side of the sun there was a stupendous extension of at least twelve solar diameters, or nearly millions of miles. langley observed from the summit of pike's peak, over , feet high, and was sure that he was witnessing a "real phenomenon heretofore undescribed." the vast advantage of elevation was apparent also from the fact that he held the corona for more than four minutes after true totality had ended. these streamers are characteristic of the epoch of minimum spots on the sun, as ranyard first suggested. it was found that this type of corona had been recorded also in ; and it has reappeared in , and , and will doubtless be visible again in . how rapidly the streamers of the corona vary is not known. occasionally an observer reports having seen the filaments vibrate rapidly as in the aurora borealis, but this is not verified by others who saw the same corona perfectly unmoving. comparisons of photographs taken at widely separate stations during the same eclipse have shown that at least the corona remained stationary for hours at a time. whether it may be unchanged at the end of a day, or a week, or a month, is not known; because no two total eclipses can ever happen nearer each other than within an interval of days, or one-half of the eclipse year. and usually the interval between total eclipses is twice or three times this period. theories of what the solar corona may be are very numerous. the extreme inner corona is perhaps in part a sort of gaseous atmosphere of the sun, due to matter ejected from the sun, and kept in motion by forces of ejection, gravity, and repulsion of some sort. meteoric matter is likely concerned in it, and huggins suggested the débris of disintegrating comets. schuster was in agreement with huggins that the brighter filaments of the corona might be due to electric discharges, but it seems very unlikely that any single hypothesis can completely account for the intricate tracery of so complex a phenomenon. [illustration: solar corona and prominences. photographed during a total eclipse of the sun, june , . (_courtesy, american museum of natural history._)] [illustration: venus, showing crescent phase of the planet. venus is the earth's nearest neighbor on the side toward the sun. (_photo, yerkes observatory._)] [illustration: mars, the planet next beyond the earth. the photograph shows one of the white polar caps. the caps are thought to be snow or ice and may indicate the existence of atmosphere. (_photo, yerkes observatory._)] elaborate spectroscopic programs have been carried out at recent eclipses, affording evidence that certain regions are due to incandescent matter of lower temperature than the sun's surface. a small part of the light of the corona is sunlight reflected from dark particles possibly meteoric, but more likely dust particles or fog of some sort. this accounts for the weakened solar spectrum with fraunhofer absorption lines, and this part of the light is polarized. many have been the attempts to see, or photograph, the corona without an eclipse. none of them has, however, succeeded as yet. huggins got very promising results nearly forty years ago, and success was thought to have been reached; but subsequent experiments on the riffelberg in and later convinced him that his results related only to a spurious corona. in the writer made an unsuccessful attempt to visualize the corona from the summit of fujiyama, and hale tried both optical and photographic methods on pike's peak in without success. he devised later a promising method by which the heat of the corona in different regions can be measured by the bolometer, and an outline corona afterward sketched from these results. still another method of attacking the problem occurred to the writer in , which has not yet been carried out. it would take advantage of recent advances in aeronautics, and contemplates an artificial eclipse in the upper air by means of a black spherical balloon. this would be sent up to an altitude of perhaps , feet, where it would partake of the motion of the air current in which it came to equilibrium. then a snapshot camera would be mounted on an aeroplane, in which the aviator would ascend to such a height that the balloon just covered the sun, as the moon does in a total eclipse. with the center of the balloon in line with the sun's center, he would photograph the regions of the sky immediately surrounding the sun, against which the corona is projected. as the entire apparatus would be above more than an entire half of the earth's atmosphere, the experiment would be well worth the attempt, as pretty much everything else has been tried and found wanting. needless to say, the importance of seeing the corona at regular intervals whenever desired, without waiting for eclipses of the sun, remains as insistent as ever. chapter xxxii the ruddy planet mars is a planet next in order beyond the earth, and its distance from the sun averages - / million miles. it has a relatively rapid motion among the stars, its color is reddish, and, when nearest to us, it is perhaps the most conspicuous object in the sky. mars appeared to the ancients just as it does to us to-day. aristotle recorded an observation of mars, b. c., when the moon passed over the planet, or occulted it, as our expression is. galileo made the first observations of mars with a telescope in , and his little instrument was powerful enough to enable him to discover that the planet had phases, though it did not pass through all the phases that mercury and venus do. this was obvious from the fact that mars is always at a greater distance from the sun than we are, and the phase can only be gibbous, or about like the moon when midway between full and quarter. many observers in the seventeenth century followed up the planet with such feeble optical power as the telescopes of that epoch provided: fontana (who made the first sketch), riccioli and bianchini in italy, cassini in france, huygens in holland, and later sir william herschel in england. it was cassini who first made out the whitish spots or polar caps of mars in , but not until after huygens had noted the fact that mars turned round on an axis in a period but little longer than the earth's. cassini followed it up later with a more accurate value; and observations in our own day, when combined with these early ones, enable us to say that the martian day is equal to hours minutes . seconds, accurate probably to the hundredth part of a second. when we know that a planet turns round on an axis, we know that it has a day. when we know the direction of the axis in space or in relation to the plane of its path round the sun, we know that it has seasons: we can tell their length and when they begin and end. it did not take many years of observation to prove that the axis round which mars turns is tilted to the plane of its path round the sun by an angle practically the same as that at which the earth's axis is tilted. so there is the immediate inference that on mars the order and perhaps the character of the seasons is much the same as here on the earth. at least two things, however, tend to modify them. first, the year of mars is not days like ours, but days. each of the four seasons on mars, therefore, is proportionally longer than our seasons are. then comes the question of atmosphere--how much of an atmosphere does mars really possess in proportion to ours, and how would its lesser amount modify the blending of the seasons into one another? all discussion of mars and the problems of existence of life upon that planet hinge upon the character and extent of martian atmosphere. the planet seems never to be covered, as the earth usually is, with extensive areas of cloud which to an observer in space would completely mask its oceans and continents. nearly all the time mars in his equatorial and temperate zones is quite clear of clouds. a few whitish spots are occasionally seen to change their form and position in both northern and southern latitudes, and they vary with the progress of the day on mars, as clouds naturally would. but schiaparelli, perhaps the best of all observers, thought them to be not low-lying clouds of the nimbus type that would produce rains, but rather a veil of fog, or perhaps a temporary condensation of vapor, as dew or hoar frost. but the strongest argument for an atmosphere is based on the temporary darkening or obscuration of well known and permanent markings on the surface of mars. these are more or less frequently observed and clouds afford the best explanation of their occurrence. so much for evidence supplied by the telescope alone. when, however, we employ the spectroscope in conjunction with the telescope, another sort of evidence is at hand. several astronomers have reached the conclusion that watery vapor exists in the atmosphere of mars, while other astronomers equipped with equal or superior apparatus, and under equally favorable or even better conditions, have reached the remarkable conclusion that the spectra of mars and the moon are identical in every particular. from this we should be led to infer that mars has perhaps no more atmosphere than the moon has, that is to say, none whatever that present instruments and methods of investigation have enabled us to detect. what then, shall we conclude? simply that the atmosphere of mars is neither very dense nor extensive. probably its lower strata close to the planet's surface are about as dense as the earth's atmosphere is at the summits of our highest mountains. this conclusion is not unwelcome, if we keep a few fundamental facts in clear and constant view. mars is a planet of intermediate size between the earth and the moon: twice the moon's diameter ( , miles) very nearly equals the diameter of mars ( , miles), and twice the diameter of mars does not greatly exceed the earth's diameter ( , miles). as to the weights or masses of these bodies, mars is about one-ninth, and the moon one-eightieth of the earth. the atmospheric envelope of the earth is abundant, the moon has none as far as we can ascertain; so it seems safe to infer that mars has an atmosphere of slight density: not dense enough to be detected by spectroscopic methods, but yet dense enough to enable us to explain the varying telescopic phenomena of the planet's disk which we should not know how to account for, if there were no atmosphere whatever. one astronomer has, indeed, gone so far as to calculate that in comparison with our planet mars is entitled to one-twentieth as much atmosphere as we have, and that the mercurial barometer at "sea level" would run about five and a half inches, as against thirty inches on the earth. in general, then, the climate of mars is probably very much like that of a clear season on a very high terrestrial table land or mountain--a climate of wide extremes, with great changes of temperature from day to night. the inequality of martian seasons is such that in his northern hemisphere the winter lasts days and the summer only days. now, the polar caps of mars, which are reasonably assumed to be due to snow or hoar frost, attain their maximum three or four months after the winter solstice, and their minimum about the same length of time after the summer solstice. this lagging should be interpreted as an argument for a martian atmosphere with heat-storing qualities, similar to that possessed by the earth. upon this characteristic, indeed, depends the climate at the surface of mars: whether it is at all similar to our own, and whether fluid water is a possibility on mars or not. while the cosmic relations of the planet in its orbit are quite the same as ours, nevertheless the greater distance of mars diminishes his supply of direct solar heat to about half what we receive. on the other hand, his distance from the sun during his year of motion around it varies much more widely than ours, so that he receives when nearest the sun about one-half more of solar heat than he does when farthest away. southern summers on mars, therefore, must be much hotter, and southern winters colder than the corresponding seasons of his northern hemisphere. indeed, the length of the southern summer, nearly twice that of the terrestrial season, sometimes amply suffices to melt all the polar ice and snow, as in october, , when the southern polar cap of mars dwindled rapidly and finally vanished completely. very interesting in this connection are the researches of stoney on the general conditions affecting planetary atmospheres and their composition. according to the kinetic theory, if the molecules of gases which are continually in motion travel outward from the center of a planet, as they frequently must, and with velocities surpassing the limit that a planet's gravity is capable of controlling, these molecules will effect a permanent escape from the planet, and travel through space in orbits of their own. so the moon is wholly without atmosphere because the moon's gravity is not powerful enough to retain the molecules of its component gases. so also the earth's atmosphere contains no helium or free hydrogen. so, too, mars is possessed of insufficient force of gravity to retain water vapor, and the martian atmosphere may therefore consist mainly of nitrogen, argon, and carbon dioxide. as everyone knows, the axis of the earth if extended to the northern heavens would pass very near the north polar star, which on that account is known as polaris. in a similar manner the axis of mars pierces the northern heavens about midway between the two bright stars alpha cephei and alpha cygni (deneb). the direction of this axis is pretty accurately known, because the measurement of the polar caps of the planet as they turn round from night to night, year in and year out, has enabled astronomers to assign the inclination of the axis with great precision. these caps are a brilliant white, and they are generally supposed to be snow and ice. they wax and wane alternately with the seasons on mars, being largest at the end of the martian winter and smallest near the end of summer. the existence of the polar caps together with their seasonal fluctuations afford a most convincing argument for the reality of a martian atmosphere, sufficiently dense to be capable of diffusing and transporting vapor. the northern cap is centered on the pole almost with geometric exactness, and as far as the th parallel of latitude. on the other hand, the south polar cap is centered about miles from the true pole, and this distance has been observed to vary from one season to another. no suggestion has been made to account for this singular variation. on one occasion it stretched down to martian latitude degrees and was over , miles in diameter. pickering watched the changing conditions of shrinking of the south polar cap in with a large telescope located in the andes of peru. mars was faithfully followed on every night but one from july to september , and the apparent alterations in this cap were very marked, even from night to night. as the snows began to decrease, a long dark line made its appearance near the middle of the cap, and gradually grew until it cut the cap in two. this white polar area (and probably also the northern one in similar fashion) becomes notched on the edge with the progress of its summer season; dark interior spots and fissures form, isolated patches separate from the principal mass, and later seem to dissolve and disappear. possibly if one were located on mars and viewing our earth with a big telescope, the seasonal variation of our north and south polar caps might present somewhat similar phenomena. all the recent oppositions of mars have been critically observed by pickering from an excellent station in jamaica. quite obviously the fluctuations of the polar caps are the key to the physiographic situation on mars, and they are made the subject of the closest scrutiny at every recurring opposition of the planet. several observers, lowell in particular, record a bluish line or a sort of retreating polar sea, following up the diminishing polar cap as it shrinks with the advance of summer. it is said that no such line is visible during the formation of the polar cap with the approach of winter. all such results of critical observation, just on the limit of visibility, have to be repeated over and over again before they become part of the body of accepted scientific fact. and in many instances the only sure way is to fall back on the photographic record, which all astronomers, whether prejudiced or not, may have the opportunity to examine and draw their individual conclusions. already the approaching opposition of , the most favorable since the invention of the telescope, is beginning to attract attention, and preparations are in progress, of new and more powerful instruments, with new and more sensitive photographic processes, by means of which many of the present riddles of mars may be solved. chapter xxxiii the canals of mars then there are the so-called canals of mars, about which so much is written and relatively little known. faint markings which resemble them in character were first drawn in and later in , but schiaparelli, the famous italian astronomer, is probably their original discoverer, when mars was at its least distance from the earth in . he made the first accurate detailed map of mars at this time, and most of the important or more conspicuous canals (_canali_, he called them in italian, that is, channels merely, without any reference whatever to their being watercourses) were accurately charted by him. at all the subsequent close approaches of mars, the canals have been critically studied by a wide range of astronomical observers, and their conclusions as to the nature and visibility of the canals have been equally wide and varied. the most favorable oppositions have occurred in and , also in and . on these occasions a close minimum distance of mars was reached, that is, about millions of miles; but in the planet makes the closest approach in a period of nearly a thousand years. its distance will not much exceed millions of miles. but although this is a minimum distance for mars, it must not be forgotten that it is a really vast distance, absolutely speaking; it is something like times greater than the distance of the moon. with no telescopic power at our command could we possibly see anything on the moon of the size of the largest buildings or other works of human intelligence; so that we seem forever barred from detecting anything of the sort on mars. nevertheless, the closest scrutiny of the ruddy planet by observers of great enthusiasm and intelligence, coupled with imagination and persistence, have built up a system of canals on mars, covering the surface of the planet like spider webs over a printed page, crossing each other at intersecting spots known as "lakes," and embodying a wealth of detail which challenges criticism and explanation. to see the canals at all requires a favorable presentation of mars, a steady atmosphere and a perfect telescope, with a trained eye behind it. not even then are they sure to be visible. the training of the eye has no doubt much to do with it. so photography has been called in, and very excellent pictures of mars have already been taken, some nearly half as large as a dime, showing plainly the lights and shades of the grander divisions of the martian surface, but only in a few instances revealing the actual canals more unmistakably than they are seen at the eyepiece. the appearance and degree of visibility of the canals are variable: possibly clouds temporarily obscure them. but there is a certain capriciousness about their visibility that is little understood. in consequence of the changing physical aspects, as to season, on mars and his orbital position with reference to the earth, some of the canals remain for a long time invisible, adding to the intricacy of the puzzle. for the most part the canals are straight in their course and do not swerve much from a great circle on the planet. but their lengths are very different, some as short as miles, some as long as , miles; and they often join one another like spokes in the hub of a wheel, though at various angles. as depicted by lowell and his corps of observers at flagstaff, arizona, the canal system is a truly marvelous network of fine darkish stripes. their color is represented as a bluish green. each marking maintains its own breadth throughout its entire length, but the breadth of all the canals is by no means the same: the narrowest are perhaps fifteen to twenty miles wide, and the broadest probably ten times that. at least that must be the breadth of the nilosyrtis, which is generally regarded as the most conspicuous of all the canals. the lowell observatory has outstripped all others in the number of canals seen and charted, now about . what may be the true significance of this remarkable system of markings it is impossible to conclude at present. schiaparelli from his long and critical study of them, their changes of width and color, was led to think that they may be a veritable hydrographic system for distributing the liquid from the melting polar snows. in this case it would be difficult to escape the conviction that the canals have, at least in part, been designed and executed with a definite end in view. lowell went even farther and built upon their behavior an elaborate theory of life on the planet, with intelligent beings constructing and opening new canals on mars at the present epoch. pickering propounded the theory that the canals are not water-bearing channels at all, but that they are due to vegetation, starting in the spring when first seen and vitalized by the progress of the season poleward, the intensity of color of the vegetation coinciding with the progress of the season as we observe it. extensive irrigation schemes for conducting agricultural operations on a large scale seem a very plausible explanation of the canals, especially if we regard mars as a world farther advanced in its life history than our own. erosion may have worn the continents down to their minimum elevation, rendering artificial waterways not difficult to build; while with the vanishing martian atmosphere and absence of rains, the necessity of water for the support of animal and vegetal life could only be met by conducting it in artificial channels from one region of the planet to another. interesting as this speculative interpretation is, however, we cannot pass by the fact that many competent astronomers with excellent instruments finely located have been unable to see the canals, and therefore think the astronomers who do see them are deceived in some way. also many other astronomers, perhaps on insufficient grounds, deny their existence _in toto_. many patient years of labor would be required to consult all the literature of investigation of the planet mars, but much of the detail has been critically embodied in maps at different epochs, by kayser, proctor, green, and dreyer. and flammarion in two classic volumes on mars has presented all the observations from the earliest time, together with his own interpretation of them. areography is a term sometimes applied to a description of the surface of mars, and it is scarcely an exaggeration to say that areography is now better known than the geography of immense tracts of the earth. for some reason well recognized, though not at all well understood, mars although the nearest of all the planets, venus alone excepted, is an object by no means easy to observe with the telescope. possibly its unusual tint has something to do with this. with an ordinary opera glass examine the moon very closely, and try to settle precise markings, colors, and the nature of objects on her surface; mars under the best conditions, scrutinized with our largest and best telescope, presents a problem of about the same order of difficulty. there are delicate and changing local colors that add much uncertainty. nevertheless, the planet's leading features are well made out, and their stability since the time of the earliest observers leaves no room to doubt their reality as parts of a permanent planetary crust. the border of the martian disk is brighter than the interior, but this brightness is far from uniform. variations in the color of the markings often depend on the planet's turning round on its axis, and the relation of the surface to our angle of vision. if we keep in mind these obstacles to perfect vision in our own day, it is easy to see why the early users of very imperfect telescopes failed to see very much, and were misled by much that they thought they saw. then, too, they had to contend, as we do, with unsteadiness of atmosphere, which is least troublesome near the zenith. as their telescopes were all located in the northern hemisphere, the northern hemisphere of mars is the one best circumstanced for their investigation; because at the remote oppositions of mars, which always happen in our northern winter with the planet in high north declination, it is always the north pole of mars which is presented to our view. whereas the close oppositions of the planet always come in our northern midsummer, with mars in south declination and therefore passing through the zenith of places in corresponding south latitude. with mars near opposition, high up from the horizon, a fairly steady atmosphere, and a magnifying power of at least diameters, even the most casual observer could not fail to notice the striking difference in brightness of the two hemispheres: the northern chiefly bright and the southern markedly dark. formerly this was thought to indicate that the southern hemisphere of mars was chiefly water and the northern land, much as is the case on the earth: with this difference, however, that water and land on the earth are proportioned about as eleven to four. but mars in its general topography presents no analogy with the present relation of land and water on the earth. there seems no reason to doubt that the northern regions with their prevailing orange tint, in some places a dark red and in others fading to yellow and white, are really continental in character. other vast regions of the martian surface are possibly marshy, the varying depth of water causing the diversity of color. if we could ever catch a reflection of sunlight from any part of the surface of mars, we might conclude that deep water exists on the planet; but the farther research progresses, the more complete becomes the evidence that permanent water areas on mars, if they exist at all, are extremely limited. since mars has been known to possess two satellites, which were discovered in august of that year by hall at washington. moons of this planet had long been suspected to exist and on one or two previous occasions critically looked for, though without success. in the writings of dean swift there is a fanciful allusion to the two moons of mars; and if astronomers had chanced to give serious attention to this, phobos and deimos, as hall named them, might have been discovered long before. they are very small bodies, not only faint in the telescope, but actually of only ten or twenty miles diameter; and from the strange relation that phobos, the inner moon, moves round mars three times while the planet itself is turning round only once on its axis, some astronomers incline to the hypothesis that this moon at least was never part of mars itself, but that it was originally an inner or very eccentric member of the asteroid group, which ventured within the sphere of gravitation of mars, was captured by that planet, and has ever since been tributary to it as a secondary body or satellite. chapter xxxiv life in other worlds popular interest in astronomy is exceedingly wide, but it is very largely confined to the idea of resemblances and differences between our earth and the bodies of the sky. the question most frequently asked the astronomer is, "have any of the stars got people on them?" or more specifically, "is mars inhabited?" the average questioner will not readily be turned off with yes or no for an answer. he may or may not know that it is quite impossible for astronomers to ascertain anything definite in this matter, most interesting as it is. what he wants to find out is the view of the individual astronomer on this absorbing and ever recurring inquiry. we ought first to understand what is meant by the manifestation here on the earth called life, and agree concerning the conditions that render it possible. apparently they are very simple. we may or may not agree that a counterpart of life, or life of a wholly different type from ours, may exist on other planets under conditions wholly diverse from those recognized as essential to its existence here. the problem of the origin of life is, in the present state of knowledge, highly speculative and hardly within the domain of science. here on earth, life is intimately associated with certain chemical compounds, in which carbon is the common element without which life would not exist. also hydrogen, oxygen, and nitrogen are present, with iron, sulphur, phosphorus, magnesium and a few less important elements besides. but carbon is the only substance absolutely essential. protoplasm cannot be built without it, and protoplasm makes up the most of the living cell. closely related to carbon is silica also, as a substitution in certain organic compounds. protoplasm is able to stand very low temperatures, but its properties as a living cell cease when the temperature reaches fahrenheit. animal life as it exists on the earth to-day appears to have been here many million years. the palæontologists agree that all life originated in the waters of the earth. it has passed through evolutionary stages from the lowest to the highest. throughout this vast period the astronomer is able to say that the conditions of the earth which appear to be essential to the maintenance of life have been pretty constantly what they are to-day. the higher the type of life, the narrower the range of conditions under which it thrives. man can exist at the frigid poles even if the temperature is degrees below fahrenheit zero; and in the deserts and the tropics, he swelters under temperatures of degrees, but he still lives. at these extremes, however, he can scarcely be said to thrive. we have, then, a relatively narrow range of temperatures which seems to be essential to his comfortable existence and development: we may call it degrees in extent. had not the surface temperature of the earth been maintained within this range for indefinite ages, in the regions where the human race has developed, quite certainly man would not be here. how this equability of temperature has been maintained does not now matter. clearly the earth must have existed through indefinite ages in the process of cooling down from temperatures of at least , degrees. during this stage the temperature of the surface was earth-controlled. then this period merged very gradually into the stage where life became possible, and the temperature of the surface became, as it now is, sun-controlled. how many years are embraced in this span of periods, or ages, we have no means of knowing. but of the sequence of periods and the secular diminution of temperature, we may be certain. then there is the equally important consideration of water necessary for the origination, support, and development of life. we cannot conceive of life existing without it. on the earth water is superabundant, and has been for indefinite ages in the past. there is little evidence that the oceans are drying up; although the commonly accepted view is that the waters of the earth will very gradually disappear. water can exist in the fluid state, which is essential to life, at all temperatures between degrees and degrees f. air to breathe is essential to life also. the atmosphere which envelops the earth is at least miles in depth, and its own weight compresses it to a tension of nearly pounds to the square inch at sea level. this atmosphere and its physical properties have had everything to do with the development of animal life on the planet. without it and its remarkable property of selective absorption, which imprisons and diffuses the solar heat, it is inconceivable that the necessary equability of surface temperature could be maintained. this appears to be quite independent of the chemical constituents of the atmosphere, and is perhaps the most important single consideration affecting the existence of life on a planet. if the surface of a planet is partly covered with water, it will possess also an atmosphere containing aqueous vapor. heat, water, and air: these three essentials determine whether there is life on a planet or not. of course there must be nutrition suitable to the organism; mineral for the vegetal, and vegetal for the animal. but the narrow range of variation appears to be the striking thing: relatively but a few degrees of temperature, and a narrow margin of atmospheric pressure. if this pressure is doubled or trebled, as in submarine caissons, life becomes insupportable. if, on the other hand, it is reduced even one-third, as on mountains even , feet high, the human mechanism fails to function, partly from lack of oxygen necessary in vitalizing the blood, but mainly because of simple reduction of mechanical pressure. if, then, we conceive of life in other worlds and it is agreed that life there must manifest itself much as it does here, our answer to the question of habitability of the planets must follow upon an investigation of what we know, or can reasonably surmise, about the surface temperatures of these bodies, whether they have water, and what are the probable physical characteristics of their atmospheres. we may inquire about each planet, then, concerning each of these details. the case of mercury is not difficult. at an average distance of only million miles from the sun, and with a large eccentricity of orbit which brings it a fifth part nearer, conditions of temperature alone must be such as to forbid the existence of life. the solar heat received is seven times greater than at the earth, and this is perhaps sufficient reason for a minimum of atmosphere, as indicated by observation. if no air, then quite certainly no water, as evaporation would supply a slight atmosphere. but according to the kinetic theory of gases, the mass of mercury, only a very small fraction of that of the sun, is inadequate to retain an atmospheric envelope. if, however, the planet's day and year are equal, so that it turns a constant face to the sun, surface conditions would be greatly complicated, so that we cannot regard the planet as absolutely uninhabitable on the hemisphere that is always turned away from the sun. venus at millions of miles from the sun presents conditions that are quite different. she receives double the solar heat that we do, but possessing an atmosphere perhaps threefold denser than ours, as reliably indicated by observations of transits of venus, the intensity of the heat and its diffusion may be greatly modified. what the selective absorption of the atmosphere of venus may be, we do not know. nor is the rotation time of the planet definitely ascertained: if equal to her year, as many observations show and as indicated by the theory of tidal evolution, there may well be certain regions on the hemisphere perpetually turned away from the sun where temperature conditions are identical with those on the tropical earth, and where every condition for the origin and development of life is more fully met than anywhere else in the solar system. whether venus has water distributed as on the earth we do not know, as her surface is never seen, owing to dense clouds under which she is always enshrouded. her cloudy condition possibly indicates an overplus of water. is the moon inhabited? quite certainly not: no appreciable air, no water, and a surface temperature unmodified by atmosphere--rising perhaps to degrees f. during the day, which is a fortnight in length, and falling at night to degrees below zero, if not lower. is mars inhabited? the probable surface temperature is much lower than the earth's, because mars receives only half as much solar heat as we do; and more important still, the atmosphere of mars is neither so dense nor so extensive as our own. seasons on mars are established, much the same as here, except that they are nearly twice as long as ours; and alternate shrinking and enlarging of the polar caps keeps even pace with the seasons, thereby indicating a certainty of atmosphere whose equatorial and polar circulation transports the moisture poleward to form the snow and ice of which the polar caps no doubt consist. there is a variety of evidence pointing to an atmosphere on mars of one-third to one-half the density of our own: an atmosphere in which free hydrogen could not exist, although other gases might. the spectroscopic evidence of water vapor in the martian atmosphere is not very strong. it is very doubtful whether water exists on mars in large bodies: quite certainly not as oceans, though the evidence of many small "lakes" is pretty well made out. with very little water, a thin atmosphere and a zero temperature, is mars likely to be inhabited at the present time? the chances are rather against it. if, however, the past development of the planet has progressed in the way usually considered as probable, we may be practically certain that mars has been inhabited in the past, when water was more abundant, and the atmosphere more dense so as to retain and diffuse the solar heat. biologists tell me that they hardly know enough regarding the extreme adaptability of organisms to environment to enable them to say whether life on such a planet as mars would or would not keep on functioning with secular changes of moisture and temperature. the survival of a race might be insured against extremely low temperatures by dwelling in sub-martian caves, and sufficient water might be preserved by conceivable engineering and mechanical schemes; but the secular reduction of the quantity and pressure of atmosphere--it is not easy to see how a race even more advanced than ourselves could maintain itself alive under serious lack of an element so vital to existence. both wallace, the great biologist, and arrhenius, the eminent chemist (but biologist, astronomer, and physicist as well), both reject the habitation theory of mars, regarding the so-called canals as quite like the luminous streaks on the moon; that is, cracks in the volcanic crust caused by internal strains due to the heated interior. wallace, indeed, argues that the planet is absolutely uninhabitable. the asteroids, or minor planets? we may dismiss them with the simple consideration that their individual masses are so insignificant and their gravity so slight that no atmosphere can possibly surround them. their temperatures must be exceedingly low, and water, if present at all, can only exist in the form of ice. jupiter, the giant planet, presents the opposite extreme. his mass is nearly a thousandth part of the sun's, and is sufficient to retain a very high temperature, probably approximating to the condition we call red-hot. this precludes the possibility of life at the outset, although the indications of a very dense atmosphere many thousand miles in depth are unmistakable. of saturn, one thirty-five hundredth the mass of the sun, practically the same may be said. proctor thought it quite likely that saturn might be habitable for living creatures of some sort, but he regarded the planet as on many accounts unsuitable as a habitation for beings constituted like ourselves. mere consideration of surface temperature precludes the possibility of life in the present stage of saturn's development; but the consensus of opinion is to the effect that life may make its appearance on these great planets at some inconceivably remote epoch in the future when the surface temperature is sufficiently reduced for life processes to begin. discoveries of algæ flourishing in hot springs approaching degrees fahrenheit make it possible that these beginnings may take place earlier and at much higher temperatures than have hitherto been thought possible. a century ago, when the ring of saturn was believed to be a continuous plane, this was a favorite corner of the solar system for speculation as to habitability; but now that we know the true constitution of the rings, no one would for a moment consider any such possibility. conditions may, however, be quite different with saturn's huge satellite titan, the giant moon of the solar system. its diameter makes it approximately the size of the planet mars; and although it is much farther removed from the sun, its relative nearness to the highly heated globe of saturn may provide that equability of temperature which is essential to life processes. also the three inner galilean moons of jupiter, especially iii which is about the size of titan, are excellently placed for life possibilities, as far as probable temperature is concerned, but we have of course no basis for surmising what their conditions may be as to air and water, except that their small mass would indicate a probable deficiency of those elements. uranus and neptune are planets so remote, and their apparent disks are so small, that very little is known about their physical condition. they are each about one-third the diameter of jupiter, and the spectrum of uranus shows broad diffused bands, indicating strong absorption by a dense atmosphere very different from that of the earth. indications are that neptune has a similar atmosphere. it is possible that the denser atmospheres of these remote planets may be so conditioned as to selective absorption that the relatively slender supply of solar heat may be conserved, and thus insure a relatively high surface temperature when the sun comes into control. if our theories of origin of the planets are to be trusted, we may rather suppose that uranus and neptune are still in a highly heated condition; that life has not yet made its appearance on them, but that it will begin its development ages before saturn and jupiter have cooled to the requisite temperature. comets? in his _lettres cosmologiques_ ( ) lambert considers the question of habitability of the comets, naturally enough in his day, because he thought them solid bodies surrounded by atmosphere, and related to the planets. the extremes of temperature at perihelia and aphelia to which comets are subjected did not bother him particularly. after calculating that the comet of , "being times nearer to the sun than we are ourselves, must have been subjected to a degree of heat , times as great as we are," lambert goes on to say: "whether this comet was of a more compact substance than our globe, or was protected in some other way, it made its perihelion passage in safety, and we may suppose all its inhabitants also passed safely. no doubt they would have to be of a more vigorous temperament and of a constitution very different from our own. but why should all living beings necessarily be constituted like ourselves? is it not infinitely more probable that amongst the different globes of the universe a variety of organizations exist, adapted to the wants of the people who inhabit them, and fitting them for the places in which they dwell, and the temperatures to which they will be subjected? is man the only inhabitant of the earth itself? and if we had never seen either bird or fish, should we not believe that the air and water were uninhabitable? are we sure that fire has not its invisible inhabitants, whose bodies, made of asbestos, are impenetrable to flame? let us admit that the nature of the beings who inhabit comets is unknown to us; but let us not deny their existence, and still less the possibility of it." little enough is really known about the physical nature of comets even now, but what we do know indicates incessant transformation and instability of conditions that would render life of any type exceedingly difficult of maintenance. a word about sir william herschel's theory of the sun and its habitability. he thought the core of the sun a dark, solid body, quite cold, and surrounded by a double layer, the inner one of which he conceived to act as a sort of fire screen to shield the sun proper against the intense heat of the outer layer, or photosphere by which we see it. viewed in this light, the sun, he says, "appears to be nothing else than a very eminent, large and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system.... it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe." but physics and biology were undeveloped sciences in herschel's days. herschel knew, however, that the stars are all suns, so that he must have conceived that they are inhabited also, quite independently of the question whether they possess retinues of planets, after the manner of our solar system. this again is a question to which the astronomer of the present day can give no certain answer. so immensely distant are even the nearest of these multitudinous bodies that no telescope can ever be built large enough or powerful enough to reveal a dark planet as large as jupiter, alongside even the nearest fixed star. whatever may be the process of stellar evolution, there doubtless is an era of many hundreds of millions of years in the life of a star when it is passing through a planet-maintaining stage. this would likely depend upon spectral type, or to be indicated by it; and as about half of the stars are of the solar type, it would be a reasonable inference that at least half of the stars may have planets tributary to them. in such a case, the chances must be overwhelmingly in favor of vast numbers of the planets of other stellar systems being favorably circumstanced as to heat and moisture for the maintenance of life at the present time. that is, they are habitable, and if habitable, then thousands of them are no doubt inhabited now. but astronomers know absolutely nothing about this question, nor are they able to conceive at present any way that may lead them to any definite knowledge of it. there is, indeed, one piece of quasi-evidence which might reasonably be interpreted as implying that it is more likely that the stars are not attended by families of planets than that they are. chapter xxxv the little planets along toward the end of the eighteenth century and the beginning of the nineteenth, astronomers were leading a quiet unexcited life. sir william herschel had been knighted by king george for his discovery of the outer planet uranus, and practically everything seemed to be known and discovered in the solar system with a single exception. between mars and jupiter there existed an obvious gap in the planetary brotherhood. could it be possible that some time in the remote cosmic past a planet had actually existed there, and that some celestial cataclysm had blown it to fragments? if so, would they still be traveling round the sun as individual small planets? and might it not be possible to discover some of them among the faint stars that make up the belt of the zodiac in which all the other planets travel? so interesting was this question that the first international association of astronomers banded themselves together to carry on a systematic search round the entire zodiacal heavens in the faint hope of detecting possible fragments of the original planet of mere hypothesis. the astronomers of that day placed much reliance on what is known as bode's law--not a law at all, but a mere arithmetical succession of numbers which represented very well the relative distances of all the planets from the sun. and the distance of the newly found uranus fitted in so well with this law that the utter absence of a planet in the gap between mars and jupiter became very strongly marked. quite by accident a discovery of one of the guessed-at small planetary bodies was made, on january , , in palermo, sicily, by piazzi, who was regularly occupied in making an extensive catalogue of the stars. his observations soon showed that the new object he had seen could not be a fixed star, because it moved from night to night among the stars. he concluded that it was a planet, and named it ceres ( ), for the tutelary goddess of sicily. other astronomers kept up the search, and another companion planet, pallas ( ) was found in the following year. juno ( ) was found in , and vesta ( ), the largest and brightest of all the minor planets, in . vesta is sometimes bright enough when nearest the earth to be seen with the naked eye; but it was the last of the brighter ones, and no more discoveries of the kind were made till the fifth was found in . since then discoveries have been made in great abundance, more and more with every year till the number of little planets at present known is very near , . the early asteroid hunters found the search rather tedious, and the labor increased as it became necessary to examine the increasing thousands of fainter and fainter stars that must be observed in order to detect the undiscovered planets, which naturally grow fainter and fainter as the chase is prolonged. first a chart of the ecliptic sky had to be prepared containing all the stars that the telescope employed in the search would show. some of the most detailed charts of the sky in existence were prepared in connection with this work, particularly by the late dr. peters of hamilton college. once such charts are complete, they are compared with the sky, night after night when the moon is absent. thousands upon thousands of tedious hours are spent in this comparison, with no result whatever except that chart and sky are found to correspond exactly. but now and then the planet hunter is rewarded by finding a new object in the sky that does not appear on his chart. almost certainly this is a small planet, and only a few night's observation will be necessary to enable the discoverer to find out approximately the orbit it is traveling in, and whether it is out-and-out a new planet or only one that had been previously recognized, and then lost track of. nearly all the minor planets so far found have had names assigned to them principally legendary and mythological, and a nearly complete catalogue of them, containing the elements of their orbits (that is, all the mathematical data that tell us about their distance from the sun and the circumstances of their motion around him) is published each year in the "annuaire du bureau des longitudes" at paris. but these little planets require a great deal of care and attention, for some astronomers must accurately observe them every few years, and other astronomers must conduct intricate mathematical computations based on these observations; otherwise they get lost and have to be discovered all over again. professor watson, of the university of michigan and later of the university of wisconsin, endowed the asteroids of his own discovery, leaving to the national academy of sciences a fund for prosecuting this work perpetually, and leuschner is now ably conducting it. [illustration: jupiter, largest of the planets. the irregular belts change their mutual relation and shapes because they do not represent land, but are part of the atmosphere. (_photo, yerkes observatory._)] [illustration: the planet neptune and its satellite. the photograph required an exposure of the plate for one hour. (_photo, yerkes observatory._)] [illustration: saturn, as seen through the -inch refractor, at the time when only the edge of the rings is visible, showing condensations. (_photo, yerkes observatory._)] [illustration: saturn, photographed through the -inch refractor. the rings appear opened to the fullest extent they can be seen from the earth. the picture was made july , . (_photo, yerkes observatory._)] while the number of the asteroids is gratifyingly large, their individual size is so small and their total mass so slight that, even if there are a hundred thousand of them (as is wholly possible), they would not be comparable in magnitude with any one of the great planets. vesta, the largest, is perhaps miles in diameter, and if composed of substances similar to those which make up the earth, its mass may be perhaps one twenty-thousandth of the earth's mass. if we calculate the surface gravity on such a body, we find it about one-thirtieth of what it is here; so that a rifle ball, if fired on vesta with a muzzle velocity of only , feet a second, might overmaster the gravity of the little planet entirely and be projected in space never to return. if, as is likely, some of the smallest asteroids are not more than ten miles in diameter, their gravity must be so feeble a force that it might be overcome by a stone thrown from the hand. there is no reliable evidence that any of the asteroids are surrounded by atmospheric gases of any sort. probably they are for the most part spherical in form, although there is very reliable evidence that a few of the asteroids, being variable in the amount of sunlight that they reflect, are irregular in form, mere angular masses perhaps. the network of orbits of the asteroids is inconceivable complicated. nevertheless, there is a wide variation in their average distance from the sun, and their periods of traveling round him vary in a similar manner, the shortest being only about three years. while the longest is nearly nine years in duration, the average of all their periods is a little over four years. the gap in the zone of asteroids, at a distance from the sun equal to about five-eighths that of jupiter, is due to the excessive disturbing action of jupiter, whose periodic time is just twice as long as that of a theoretical planet at this distance. the average inclination of their orbits to the plane of the ecliptic is not far from degrees. but the orbit of pallas, for example, is inclined degrees, and the eccentricities of the asteroid orbits are equally erratic and excessive. both eccentricity and inclination of orbit at times suggest a possible relation to cometary orbits, but nothing has ever been definitely made out connecting asteroids and comets in a related origin. no comprehensive theory of the origin of the asteroid group has yet been propounded that has met with universal acceptance. according to the nebular hypothesis the original gaseous material, which should have been so concentrated as to form a planet of ordinary type, has in the case of the asteroids collected into a multitude of small masses instead of simply one. that there is a sound physical reason for this can hardly be denied. according to the laplacian hypothesis, the nearness of the huge planetary mass of jupiter just beyond their orbits produced violent perturbations which caused the original ring of gaseous material to collect into fragmentary masses instead of one considerable planet. the theory of a century ago that an original great planet was shattered by internal explosive forces is no longer regarded as tenable. to astronomers engaged upon investigation of distances in the solar system, the asteroid group has proved very useful. the late sir david gill employed a number of them in a geometrical research for finding the sun's distance, and more recently the discovery of eros ( ) has made it possible to apply a similar method for a like purpose when it approaches nearest to the earth in and . then the distance of eros will be less than half that of mars or even venus at their nearest. when the total number of asteroids discovered has reached , , with accurate determination of all their orbits, we shall have sufficient material for a statistical investigation of the group which ought to elucidate the question of its origin, and bear on other problems of the cosmogony yet unsolved. present methods of discovery of the asteroids by photography replace entirely the old method by visual observation alone, with the result that discoveries are made with relatively great ease and rapidity. chapter xxxvi the giant planet i can never forget as a young boy my first glimpse of the planet jupiter and his moons; it was through a bit of a telescope that i had put together with my own hands; a tube of pasteboard, and a pair of old spectacle lenses that chanced to be lying about the house. in the field of view i saw five objects; four of them looking quite alike, and as if they were stars merely (they were jupiter's moons), while the fifth was vastly larger and brighter. it was circular in shape, and i thought i could see a faint darkish line across the middle of it. this experience encouraged me immensely, and i availed myself eagerly of the first chance to see jupiter through a bigger and better glass. then i saw at once that i had observed nothing wrongly, but that i had seen only the merest fraction of what there was to see. in the first place, the planet's disk was not perfectly circular, but slightly oval. inquiring into the cause of this, we must remember that jupiter is actually not a flat disk but a huge ball or globe, more than ten times the diameter of the earth, which turns swiftly round on its axis once every ten hours as against the earth's turning round in twenty-four hours. then it is easy to see how the centrifugal force bulges outward the equatorial regions of jupiter, so that the polar regions are correspondingly drawn inward, thereby making the polar diameter shorter than the equatorial one, which is in line with the moons or satellites. the difference between the two diameters is very marked, as much as one part in fifteen. all the planets are slightly flattened in this way, but jupiter is the most so of all except saturn. the little darkish line across the planet's middle region or equator was found to be replaced by several such lines or irregular belts and spots, often seen highly colored, especially with reflecting telescopes; and they are perpetually changing their mutual relation and shapes, because they are not solid territory or land on jupiter, but merely the outer shapes of atmospheric strata, blown and torn and twisted by atmospheric circulation on this planet, quite the same as clouds in the atmosphere on the earth are. besides this the axial turning of jupiter brings an entirely different part of the planet into view every two or three hours; so that in making a map or chart of the planet, an arbitrary meridian must be selected. even then the process is not an easy one, and it is found that spots on jupiter's equator turn round in hours minutes, while other regions take a few minutes longer, the nearer the poles are approached. the great red spot, about , miles long and a quarter as much in breadth has been visible for about half a century. bolton, an english observer, has made interesting studies of it very recently. the four moons, or satellites, which a small telescope reveals, are exceedingly interesting on many accounts. they were the first heavenly bodies seen by the aid of the telescope, galileo having discovered them in . they travel round jupiter much the same as the moon does round the earth, but faster, the innermost moon about four times per week, the second moon about twice a week, the third or largest moon (larger than the planet mercury) once a week, and the outermost in about sixteen days. the innermost is about , miles from jupiter, and the outermost more than a million miles. from their nearness to the huge and excessively hot globe of jupiter, some astronomers, proctor especially, have inclined to the view that these little bodies may be inhabited. jupiter has other moons; a very small one, close to the planet, which goes round in less than twelve hours, discovered by barnard in . four others are known, very small and faint and remote from the planet, which travel slowly round it in orbits of great magnitude. the ninth, or outermost, is at a distance of fifteen and one-half million miles from jupiter, and requires nearly three years in going round the planet. it was discovered by nicholson at the lick observatory in . the eighth was discovered by melotte at greenwich in , and is peculiar in the great angle of degrees, at which its orbit is inclined to the equator of jupiter. the sixth and seventh satellites revolve round jupiter inside the eighth satellite, but outside the orbit of iv; and they were discovered by photography at the lick observatory in by perrine, now director of the argentine national observatory at cordoba. the ever-changing positions of the medicean moons, as galileo called the four satellites that he discovered--their passing into the shadow in eclipse, their transit in front of the disk, and their occultation behind it--form a succession of phenomena which the telescopist always views with delight. the times when all these events take place are predicted in the "nautical almanac," many thousand of them each year, and the predictions cover two or three years in advance. jupiter, as the naked eye sees him high up in the midnight sky, is the brightest of all the planets except venus; indeed, he is five times brighter than sirius, the brightest of all the fixed stars. his stately motion among the stars will usually be visible by close observation from day to day, and his distance from the earth, at times when he is best seen, is usually about million miles. jupiter travels all the way round the sun in twelve years; his motion in orbit is about eight miles a second. the eclipses of jupiter's moons, caused by passing into the shadow of the planet, would take place at almost perfectly regular intervals, if our distance from jupiter were invariable. but it was early found out that while the earth is approaching jupiter the eclipses take place earlier and earlier, but later and later when the earth is moving away. the acceleration of the earliest eclipse added to the retardation of the latest makes , seconds, which is the time that light takes in crossing a diameter of the earth's orbit round the sun. now the velocity of light is well known to be , miles per second, so we calculate at once and very simply that the sun's distance from the earth, which is half the diameter of the orbit, equals times , , or , , miles. chapter xxxvii the ringed planet saturn is the most remote of all the planets that the ancient peoples knew anything about. these anciently known planets are sometimes called the lucid or naked-eye planets--five in number: mercury, venus, mars, jupiter, and saturn. saturn shines as a first-magnitude star, with a steady straw-colored light, and is at a distance of about million miles from the earth when best seen. saturn travels completely round the sun in a little short of thirty years, and the telescope, when turned to saturn, reveals a unique and astonishing object; a vast globe somewhat similar to jupiter, but surrounded by a system of rings wholly unlike anything else in the universe, as far as at present known; the whole encircled by a family of ten moons or satellites. the saturnian system, therefore, is regarded by many as the most wonderful and most interesting of all the objects that the telescope reveals. at first the flattening of the disk of saturn is not easily made out, but every fifteen years (as and ) the earth comes into a position where we look directly at the thin edge of the rings, causing them to completely disappear. then the remarkable flattening of the poles of saturn is strikingly visible, amounting to as much as one-tenth of the entire diameter. the atmospheric belt system is also best seen at these times. but the rings of saturn are easily the most fascinating features of the system. they can never be seen as if we were directly above or beneath the planet so they never appear circular, as they really are in space, but always oval or elliptical in shape. the minor axis or greatest breadth is about one-half the major axis or length. the latter is the outer ring's actual diameter, and it amounts to , miles, or two and one-half times the diameter of saturn's globe. there are in fact no less than four rings; an outer ring, sometimes seen to be divided near its middle; an inner, broader and brighter ring; and an innermost dusky, or crape ring, as it is often called. this comes within about , miles of the planet itself. after the form and size of the rings were well made out, their thickness, or rather lack of thickness, was a great puzzle. if a model about a foot in diameter were cut out of tissue paper, the relative proportion of size and thickness would be about right. in space the thickness is very nearly miles, so that, when we look at the ring system edge-on, it becomes all but invisible except in very large telescopes. clearly a ring so thin cannot be a continuous solid object and recent observations have proved beyond a doubt that saturn's rings are made up of millions of separate particles moving round the planet, each as if it were an individual satellite. ever since the true theory of the constitution of the saturnian ring has been recognized on theoretic grounds, because clerke-maxwell founded the dynamical demonstration that the rings could be neither fluid nor solid, so that they must be made up of a vast multitude of particles traveling round the planet independently. but the physical demonstration that absolutely verified this conclusion did not come until , when, as we have said in a preceding chapter, keeler, by radial velocity measures on different regions of the ring by means of the spectroscope, proved that the inner parts of the ring travel more swiftly round the planet than the outer regions do. and he further showed that the rates of revolution in different parts of the ring exactly correspond to the periods of revolution which satellites of saturn would have, if at the same distance from the center of the planet. the innermost particles of the dusky ring, for example, travel round saturn in about five hours, while the outermost particles of the outer bright ring take hours to make their revolution. for many years it was thought that the saturnian ring system was a new satellite in process of formation, but this view is no longer entertained; and the system is regarded as a permanent feature of the planet, although astronomers are not in entire agreement as to the evolutionary process by which it came into existence--whether by some cosmic cataclysm, or by gradual development throughout indefinite aeons, as the rest of the solar system is thought to have come to its present state of existence. possibly the planetesimal hypothesis of chamberlin and moulton affords the true explanation, as the result of a rupture due to excessive tidal strain. chapter xxxviii the farthest planets on the th of march, , between and p. m., as sir william herschel was sweeping the constellation gemini with one of his great reflecting telescopes, one star among all that passed through the field of view attracted his attention. removing the eyepiece and applying another with a higher magnifying power, he found that, unlike all the other stars, this one had a small disk and was not a mere point of light, as all the fixed stars seem to be. a few nights' observation showed that the stranger was moving among the stars, so he thought it must be a comet; but a week's observation following showed that he had discovered a new member of the planetary system, far out beyond saturn, which from time immemorial had been assumed to be the outermost planet of all. this, then, was the first real discovery of a planet, as the finding of the satellites of jupiter had been the first of all astronomical discoveries. herschel's discovery occasioned great excitement, and he named the new planet georgium sidus or the georgian, after his king. the king created him a knight and gave him a pension, besides providing the means for building a huge telescope, feet long, with which he subsequently made many other astronomical discoveries. the planet that herschel discovered is now called uranus. uranus is an object not wholly impossible to see with the naked eye, if the sky background is clear and black, and one knows exactly where to look for it. its brightness is about that of a sixth magnitude star or a little fainter. its average distance from the sun is about , million miles and it takes eighty-four years to complete its journey round the sun, traveling only a little more than four miles a second. when we examine uranus closely with a large telescope, we find a small disk slightly greenish in tint, very slightly flattened, and at times faint bands or belts are apparently seen. uranus is about , miles in diameter, and is probably surrounded by a dense atmosphere. its rotation time is h. m. uranus is attended by four moons or satellites, named ariel, umbriel, titania, and oberon, the last being the most remote from the planet. this system of satellites has a remarkable peculiarity: the plane of the orbits in which they travel round uranus is inclined about degrees to the plane of the ecliptic, so that the satellites travel backward, or in a retrograde direction; or we might regard their motion as forward, or direct, if we considered the planes of their orbits inclined at degrees. for many years after the discovery of uranus it was thought that all the great bodies of the solar system had surely been found. least of all was any planet suspected beyond uranus until the mathematical tables of the motion of uranus, although built up and revised with the greatest care and thoroughness, began to show that some outside influence was disturbing it in accordance with newton's law of gravitation. the attraction of a still more distant planet would account for the disturbance, and since no such planet was visible anywhere a mathematical search for it was begun. neptune wholly independently of each other, two young astronomers, adams of england and le verrier of france, undertook to solve the unique problem of finding out the position in the sky where a planet might be found that would exactly account for the irregular motion of uranus. both reached practically identical results. adams was first in point of time, and his announcement led to the earliest observation, without recognition of the new planet (july , ), although it was le verrier's work that led directly to the new planet's being first seen and recognized as such (september , ). figuring backward, it was found that the planet had been accidentally observed in paris in , but its planetary character had been overlooked. neptune is the name finally assigned to this historical planet. it is thirty times farther from the sun than the earth, or , million miles; its velocity in orbit is a little over three miles per second, and it consumes years in going once completely round the sun. so faint is it that a telescope of large size is necessary to show it plainly. the brightness equals that of a star of the eighth magnitude, and with a telescope of sufficient magnifying power, the tiny disk can be seen and measured. the planet is about , miles in diameter, and is not known to possess more than one moon or satellite. if there are others, they are probably too faint to be seen by any telescope at present in existence. chapter xxxix the trans-neptunian planet investigation of the question of a possible trans-neptunian planet was undertaken by the writer in . as neptune requires years to travel completely round the sun, and the period during which it has been carefully observed embraces only half that interval, clearly its orbit cannot be regarded as very well known. any possible deviations from the mathematical orbit could not therefore be traced to the action of a possible unknown planet outside. but the case was different with uranus, which showed very slight disturbances, and these were assumed to be due to a possible planet exterior to both uranus and neptune. as a position for this body in the heavens was indicated by the writer's investigation, that region of the sky was searched by him with great care in - with the twenty-six-inch telescope at washington; and photographs of the same region were afterward taken by others, though only with negative results. in , forbes of edinburgh published his investigation of the problem from an entirely independent angle. families of comets have long been recognized whose aphelion distances correspond so nearly with the distances of the planets that these comet families are now recognized as having been created by the several planets, which have reduced the high original velocities possessed by the comets on first entering the solar system. their orbits have ever since been ellipses with their aphelia in groups corresponding to the distances of the planets concerned. jupiter has a large group of such comets, also saturn. uranus and neptune likewise have their families of comets, and forbes found two groups with average distances far outside of neptune; from which he drew the inference that there are two trans-neptunian planets. the position he assigned to the inner one agreed fairly well with the writer's planet as indicated by unexplained deviations of uranus. the theoretical problem of a trans-neptunian planet has since been taken up by gaillot and lau of paris, the late percival lowell, and w. h. pickering of harvard. the photographic method of search will, it is expected, ultimately lead to its discovery. on account of the probable faintness of the planet, at least the twelfth or thirteenth magnitude, metcalf's method of search is well adapted to this practical problem. when near its opposition the motion of neptune retrograding among the stars amounts to five seconds of arc in an hour; while the trans-neptunian planet would move but three seconds. by shifting the plate this amount hourly during exposure, the suspected object would readily be detected on the photographic plate as a minute and nearly circular disk, all the adjacent stars being represented by short trails. interest in a possible planet or planets outside the orbit of neptune is likely to increase rather than diminish. to the ancients seven was the perfect number, there were seven heavenly bodies already known, so there could be no use whatever in looking for an eighth. the discovery of uranus in proved the futility of such logic, and neptune followed in with further demonstration, if need be. the cosmogony of the present day sets no outer limit to the solar system, and some astronomers advocate the existence of many trans-neptunian planets. chapter xl comets--the hairy stars comets--hairy stars, as the origin of the name would indicate--are the freaks of the heavens. of great variety in shape, some with heads and some without, some with tails and some without, moving very slowly at one time and with exceedingly high velocity at another, in orbits at all possible angles of inclination to the general plane of the planetary paths round the sun, their antics and irregularities were the wonder and terror of the ancient world, and they are keenly dreaded by superstitious people even to the present day. down through the middle ages the advent of a comet was regarded as: threatening the world with famine, plague and war; to princes, death; to kingdoms, many curses; to all estates, inevitable losses; to herdsmen, rot; to plowmen, hapless seasons; to sailors, storms; to cities, civil treasons. comets appeared to be marvelous objects, as well as sinister, chiefly because they bid apparent defiance to all law. kepler had shown that the moon and the planets travel in regular paths--slightly elliptical to be sure, but nevertheless unvarying. none of the comets were known to follow regular paths till the time of halley late in the seventeenth century, when, as we have before told, a fine comet made its appearance, and halley calculated its orbit with much precision. comparing this with the orbits of comets that had previously been seen, he found its path about the sun practically identical with that of at least two comets previously observed in and . so halley ventured to think all these comets were one and the same body, and that it traveled round the sun in a long ellipse in a period of about seventy-five or seventy-six years. we have seen how his prediction of its return in was verified in every particular. on the comet's return in , crowell and crommelin of greenwich made a thorough mathematical investigation of the orbit, indicating that the year will witness its next return to the sun. there is a class of astronomers known as comet-hunters, and they pass hours upon hours of clear, sparkling, moonless nights in search for comets. they are equipped with a peculiar sort of telescope called a comet-seeker, which has an object glass usually about four or five inches in diameter, and a relatively short length of focus, so that a larger field of view may be included. regions near the poles of the heavens are perhaps the most fruitful fields for search, and thence toward the sun till its light renders the sky too bright for the finding of such a faint object as a new comet usually is at the time of discovery. generally when first seen it resembles a small circular patch of faint luminous cloud. when a suspect is found, the first thing to do is to observe its position accurately with relation to the surrounding stars. then, if on the next occasion when it is seen the object has moved, the chances are that it is a comet; and a few days' observation will provide material from which the path of the comet in space can be calculated. by comparing this with the complete lists of comets, now about in number, it is possible to tell whether the comet is a new one, or an old one returning. the total number of comets in the heavens must be very great, and thousands are doubtless passing continually undetected, because their light is wholly overpowered by that of the sun. of those that are known, perhaps one in twelve develops into a naked-eye comet, and in some years six or seven will be discovered. with sufficiently powerful telescopes, there are as a rule not many weeks in the year when no comet is visible. brilliant naked-eye comets are, however, infrequent. comets, except halley's, generally bear the name of their discoverer, as donati ( ), and pons-brooks ( ). pons was a very active discoverer of comets in france early in the nineteenth century: he was a doorkeeper at the observatory of marseilles, and his name is now more famous in astronomy than that of thulis, then the director of the observatory, who taught and encouraged him. messier was another very successful discoverer of comets in france, and in america we have had many: swift, brooks, and barnard the most successful. how bright a comet will be and how long it will be visible depends upon many conditions. so the comets vary much in these respects. the first comet of was under observation for nearly a year and a half, the longest on record till halley's in . in case a comet eludes discovery and observation until it has passed its perihelion, or nearest point to the sun, its period of visibility may be reduced to a few weeks only. the brightest comets on record were visible in and : so brilliant were they that even the effulgence of full daylight did not overpower them. in particular the comet of was not only excessively bright, but at its nearest approach to the earth its tail swept all the way across the sky from one horizon to the other. it must have looked very much like the straight beam of an enormous searchlight, though very much brighter. the tails of comets are to the naked eye the most compelling thing about them, and to the ancient peoples they were naturally most terrifying. their tails are not only curved, but sometimes curved with varying degrees of curvature, and this circumstance adds to their weirdness of appearance. if we examine the tail of a comet with a telescope, it vanishes as if there were nothing to it: as indeed one may almost say there is not. ordinarily, only the head of the comet is of interest in the telescope. when first seen there is usually nothing but the head visible, and that is made up of portions which develop more or less rapidly, presenting a succession of phenomena quite different in different comets. when first discovered a comet is usually at a great distance from the sun, about the distance of jupiter; and we see it, not as we do the planets, by sunlight reflected from them, but by the comet's own light. this is at that time very faint, and nearly all comets at such a distance look alike: small roundish hazy patches of faint, cloudlike light, with very often a concentration toward the center called the nucleus, on the average about , miles in diameter. approach toward the sun brightens up the comet more and more, and the nucleus usually becomes very much brighter and more starlike. then on the sunward side of the nucleus, jetlike streamers or envelopes appear to be thrown off, often as if in parallel curved strata, or concentrically. as they expand and move outward from the nucleus, these envelopes grow fainter and are finally merged in the general nebulosity known as the comet's head, which is anywhere from , to , miles in diameter. as a rule, this is an orderly development which can be watched in the telescope from hour to hour and from night to night; but occasionally a cometary visitor is quite a law to itself in development, presenting a fascinating succession of unpredictable surprises. then follows the development of the comet's tail, perhaps more striking than anything that has preceded it. here a genuine repulsion from the sun appears to come into play. it may be an electrical repulsion. much of the material projected from the comet's nucleus, seems to be driven backward or repelled by the sun, and it is this that goes to form the tail. the particles which form the tail then travel in modified paths which nevertheless can be calculated. the tail is made up of these luminous particles and it expands in space much in the form of a hollow, horn-shaped cone, the nucleus being near the tip of the horn. some comets possess multiple tails with different degrees of curvature, donati's for example. usually there is a nearly straight central dark space, marking the axis of the comet, and following the nucleus. but occasionally this is replaced by a thin light streak very much less in breadth than the diameter of the head. cometary tails are sometimes million miles in length. three different types of cometary tails are recognized. first, the long straight ones, apparently made up of matter repelled by the sun twelve to fifteen times more powerfully than gravitation attracts it. such particles must be brushed away from the comet's head with a velocity of perhaps five miles a second, and their speed is continually increasing. probably these straight tails are due to hydrogen. the second type tails are somewhat curved, or plume-like, and they form the most common type of cometary tail. in them the sun's repulsion is perhaps twice its gravitational attraction, and hydrocarbons in some form appear to be responsible for tails of this character. then there is a third type, much less often seen, short and quickly curving, probably due to heavier vapors, as of chlorine, or iron, or sodium, in which the repulsive force is only a small fraction of that of gravitation. many features of this theory of cometary tails are borne out by examination of their light with the spectroscope, although the investigation is as yet fragmentary. it is evident that the tail of a comet is formed at the expense of the substance of the nucleus and head; so that the matter repelled is forever dissipated through the regions of space which the comet has traveled. comets must lose much of their original substance every time they return to perihelion. comets actually age, therefore, and grow less and less in magnitude of material as well as brightness, until they are at last opaque, nonluminous bodies which it becomes impossible to follow with the telescope. chapter xli where do comets come from? where do comets come from? the answer to this question is not yet fully made out. most likely they have not all had a similar origin, and theories are abundant. apparently they come into the solar system from outer space, from any direction whatsoever. the depths of interstellar space seem to be responsible for most, if not all, of the new ones. whether they have come from other stars or stellar systems we cannot say. while comets are tremendous in size or volume, their mass or the amount of real substance in them is relatively very slight. we know this by the effect they produce on planets that they pass near, or rather by the effect that they fail to produce. the earth's atmosphere weighs about one two hundred and fifty thousandth as much as the earth itself, but a comet's entire mass must be vastly less than this. even if a comet were to collide with the earth head on, there is little reason to believe that dire catastrophe would ensue. at least twice the earth is known to have passed through the tail of a comet, and the only effect noticed was upon the comet itself; its orbit had been modified somewhat by the attraction of the earth. if the comet were a small one, collision with any of the planets would result in absorption and dissipation of the comet into vapor. the whole of a large comet has perhaps as much mass or weight as a sphere of iron a hundred miles in diameter. even this could not wreck the earth, but the effect would depend upon what part of the earth was hit. a comet is very thin and tenuous, because its relatively small mass is distributed through a volume so enormous. so it is probable that the earth's atmosphere could scatter and burn up the invading comet, and we should have only a shower of meteors on an unprecedented scale. diffusion of noxious gases through the atmosphere might vitiate it to some extent, though probably not enough to cause the extinction of animal life. every comet has an interesting history of its own, almost indeed unique. one of the smallest comets and the briefest in its period round the sun is known as encke's comet. it is a telescopic comet with a very short tail, its time of revolution is about three and a half years, and it exhibits a remarkable contraction of volume on approach to the sun. biela's comet has a period about twice as long. at one time it passed within about million miles of the earth, and somewhere about the year this comet divided into two distinct comets, which traveled for months side by side, but later separated and both have since completely disappeared. perhaps the most beautiful of all comets is that discovered by donati of florence in . its coma presented the development of jets and envelopes in remarkable perfection, and its tail was of the secondary or hydrocarbon type, but accompanied by two faint streamer tails, nearly tangential to the main tail and of the hydrogen type. donati's comet moves in an ellipse of extraordinary length, and it will not return to the sun for nearly , years. the most brilliant comet of the last half century is known as the great comet of . in a clear sky it could readily be seen at midday. on september it passed across the disk of the sun and was practically as bright as the surface of the sun itself. the comet had a multiple nucleus and a hydrocarbon tail of the second type, nearly a hundred million miles in length. doubtless this great comet is a member of what is known as a cometary group, which consists of comets having the same orbit and traveling tandem round the sun. the comets of , , , and belong to this particular group, and they all pass within , miles of the sun's surface, at a maximum velocity exceeding miles a second. they must therefore invade the regions of the solar corona, the inference being that the corona as well as the comet is composed of exceedingly rare matter. photography of comets has developed remarkably within recent years, especially under the deft manipulation of barnard, whose plates, in particular during his residence at the lick observatory on mount hamilton, california, show the features of cometary heads and tails in excellent definition. halley's comet, at the apparition, was particularly well photographed at many observatories. the question is often asked, when will the next comet come? if a large bright comet is meant, astronomers cannot tell. at almost any time one may blaze into prominence within only a few days. during the latter half of the last century, bright comets appeared at perihelion at intervals of eight years on the average. several of the lesser and fainter periodic comets return nearly every year, but they are mostly telescopic, and are rarely seen except by astronomers who are particularly interested in observing them. chapter xlii meteors and shooting stars "falling stars," or "shooting stars," have been familiar sights in all ages of the world, but the ancient philosophers thought them scarcely worthy of notice. according to aristotle they were mere nothings of the upper atmosphere, of no more account than the general happenings of the weather. but about the end of the eighteenth century and the beginning of the nineteenth the insufficiency of this view began to be fully recognized, and interplanetary space was conceived as tenanted by shoals of moving bodies exceedingly small in mass and dimension as compared with the planets. millions of these bodies are all the time in collision with the outlying regions of our atmosphere; and by their impact upon it and their friction in passing swiftly through it, they become heated to incandescence, thus creating the luminous appearances commonly known as shooting stars. for the most part they are consumed or dissipated in vapor before reaching the solid surface of the earth; but occasionally a luminous cloud or streak is left glowing in the wake of a large meteor, which sometimes remains visible for half an hour after the passage of the meteor itself. these mistlike clouds projected upon the dark sky have been especially studied by trowbridge of columbia university. many more meteors are seen during the morning hours, say from four to six, than at any other nightly period of equal length, because the visible sky is at that time nearly centered around the general direction toward which the earth is moving in its orbit round the sun; so that the number of meteors that would fall upon the earth if at rest is increased by those which the earth overtakes by its own motion. also from january to july while the earth is traveling from perihelion to aphelion, fewer meteors are seen than in the last half of the year; but this is chiefly because of the rich showers encountered in august and november. although the descent of meteoric bodies from the sky was pretty generally discredited until early in the nineteenth century, such falls had nevertheless been recorded from very early times. they were usually regarded as prodigies or miracles, and such stones were commonly objects of worship among ancient peoples. for example, the phrygian stone, known as the "diana of the ephesians which fell down from jupiter," was a famous stone built into the kaaba at mecca, and even to-day it is revered by mohammedans as a holy relic. perhaps the earliest known meteoric fall is that historically recorded in the parian chronicle as having occurred in the island of crete, b. c. . also in the imperial museum of petrograd is the pallas or krasnoiarsk iron, perhaps three-quarters of a ton in weight, found in by pallas, the famous traveler, at krasnoiarsk, siberia. but a fall of meteoric stones that chanced upon the department of orne, france, in , led to a critical investigation by biot, the distinguished physicist and academician. according to his report a violent explosion in the neighborhood of l'aigle had been heard for a distance of seventy-five miles around, and lasting five or six minutes, about p. m. on tuesday, april . from several adjoining towns a rapidly moving fireball had been seen in a sky generally clear, and there was absolutely no room for doubt that on the same day many stones fell in the neighborhood of l'aigle. biot estimated their number between two and three thousand, and they were scattered over an elliptical area more than six miles long, and two and a half miles broad. thenceforward the descent of meteoric matter from outer space upon the earth has been recognized as an unquestioned fact. the origin of these bodies being cosmic, meteors may be expected to fall upon the earth without reference to latitude, or season, or day and night, or weather. on entering our upper atmosphere their temperature must be that of space, many hundred degrees below zero; and their velocities range from ten miles per second upward. but atmospheric resistance to their flight is so great that their velocity is quickly reduced: at ground impact it does not exceed a few hundred feet per second. on january , , several meteoric stones fell on ice only a few inches thick in sweden, rebounding without either breaking through the ice or being themselves fractured. naturally the flight of a meteor through the atmosphere will be only a few seconds in duration, and owing to the sudden reduction of velocity, it will continue to be luminous throughout only the upper part of its course. visibility generally begins at an elevation of about seventy miles, and ends at perhaps half that altitude. what is the origin of meteors? theories there are in great abundance: that they come from the sun, that they come from the moon, that they come from the earth in past ages as a result of volcanic action, and so on. but there are many difficulties in the way of acceptance of these and several other theories. that all meteors were originally parts of cometary masses is however a theory that may be accepted without much hesitation. comets have been known to disintegrate. biela's comet even disappeared entirely, so that during a shower of biela meteors in november, , an actual fragment of the lost comet fell upon the earth, at mazapil, mexico. and as the bielid meteors encounter the earth with the relatively low velocity of ten miles a second, we may expect to capture other fragments in the future. numerous observers saw the weird disintegration of the nucleus of the great comet of , well recognized as a member of the family of the comet of . as these comets are fellow voyagers through space along the same orbit, probably all five members of the family, with perhaps others, were originally a single comet of unparalleled magnitude. the brooks comet of affords another instance of fragmentary nucleus. the oft-repeated action of solar forces tending to disrupt the mass of a comet more and more, and scatter its material throughout space, the secular dismemberment of all comets becomes an obvious conclusion. during the hundreds of millions of years that these forces are known to have been operant, the original comets have been broken up in great numbers, so that elliptical rings of opaque meteoric bodies now travel round the sun in place of the comets. these bodies in vast numbers are everywhere through space, each too small to reflect an appreciable amount of sunlight, and becoming visible only when they come into collision with our outer atmosphere. the practical identity of several such meteor streams and cometary orbits has already been established, and there is every reason for assigning a similar origin to all meteoric bodies. meteors, then, were originally parts of comets, which have trailed themselves out to such extent that particles of the primal masses are liable to be picked up anywhere along the original cometary paths. the historic records of all countries contain trustworthy accounts of meteoric showers. making due allowances for the flowery imagery of the oriental, it is evident that all have at one time or another seen much the same thing. in a. d. , for instance, the constantinople sky was reported alive with flying stars. in october, , "stars appeared like waves upon the sky; and they flew about like grasshoppers." during the reign of king william ii occurred a very remarkable shower in which "stars seemed to fall like rain from heaven." but the showers of november, and , are easily the most striking of all. the sky was filled with innumerable fiery trails and there was not a space in the heavens a few times the size of the moon that was not ablaze with celestial fireworks. frequently huge meteors blended their dazzling brilliancy with the long and seemingly phosphorescent trails of the shooting stars. the interval of thirty-four years between and appeared to indicate the possibility of a return of the shower in november of or , and all the people of that day were aroused on this subject and made every preparation to witness the spectacle. extemporized observatories were established, watchmen were everywhere on the lookout, and bells were to be rung the minute the shower began. the newspapers of the day did little to allay the fears of the multitude, but the critical days of november, , passed with disappointment in america. in europe, however, a fine shower was seen, though it was not equal to that of . the astronomers at greenwich counted many thousand meteors. in november of , however, american astronomers were gratified by a grand display, which, although failing to match the general expectation, nevertheless was a most striking spectacle, and the careful preparation for observing it afforded data of observation which were of the greatest scientific value. the actual orbits of these bodies in space became known with great exactitude, and it was found that their general path was identical with that of the first comet of , which travels outward somewhat beyond the planet uranus. when the visible paths of these meteors are traced backward, all appear as if they originated from the constellation leo. so they are known as leonids, and a return of the shower was confidently predicted for november, - , which for unknown reasons failed to appear. [illustration: two views of halley's comet. taken with the same camera from the same position, one on may , and the other on may , . (_photo, mt. wilson solar observatory._)] [illustration: swift's comet of . this comet showed extraordinary and rapid transformations, one day having a dozen streamers in its tail, another only two. (_photo by prof. e. e. barnard._)] [illustration: a large meteor trail in the field with fine nebulÆ. (_photo, yerkes observatory._)] during the last half century meteors have been pretty systematically observed, especially by the astronomers of italy and denning of england, so that several hundred distinct showers are now known, their radiant points fall in every part of the heavens, and there is scarcely a clear moonless night when careful watching for meteors will be unrewarded. besides november, the months of august (perseids), april (lyrids), and december (geminids) are favorable. following in tabular form is a fairly comprehensive list of the meteoric showers of the year, with the positions of the radiant points and the epochs of the showers according to denning: radiant point ============================================================ name of shower | r. a. | decl. | date of shower -----------------------+---------+--------+----------------- quadrantids | ° | + ° | jan. - zeta cepheids | ° | + ° | jan. alpha leonids | ° | + ° | feb. -march tau leonids | ° | + ° | march - beta ursids | ° | + ° | march - lyrids | ° | + ° | april - gamma aquarids | ° | - ° | may - zeta herculids | ° | + ° | may - eta pegasids | ° | + ° | may -june theta boötids | ° | + ° | june - alpha capricornids | ° | - ° | july - delta aquarids | ° | - ° | july - perseids | ° | + ° | aug. - omicron draconids | ° | + ° | aug. - zeta draconids | ° | + ° | aug. -sept. piscids | ° | + ° | sept. - alpha andromedids | ° | + ° | sept. epsilon arietids | ° | + ° | oct. - orionids | ° | + ° | oct. - epsilon perseids | ° | + ° | nov. leonids | ° | + ° | nov. - epsilon taurids | ° | + ° | nov. - andromedids | ° | + ° | nov. - beta geminids | ° | + ° | dec. - geminids | ° | + ° | dec. - alpha ursæ majorids | ° | + ° | dec. - kappa draconids | ° | + ° | dec. - ------------------------------------------------------------ the year was exceptional in providing an abundant and previously unknown shower on june , and its stream has nearly the same orbit as that of the pons-winnecke periodic comet. useful observations of meteors are not difficult to make, and they are of service to professional astronomers investigating the orbits of these bodies, among whom are mitchell and olivier of the university of virginia. chapter xliii meteorites meteorites, the name for meteors which have actually gone all the way through our atmosphere, are never regular in form or spherical. as a rule the iron meteorites are covered with pittings or thumb marks, due probably to the resistance and impact of the little columns of air which impede its progress, together with the unequal condition and fusibility of their surface material. the work done by the atmosphere in suddenly checking the meteor's velocity appears in considerable part as heat, fusing the exterior to incandescence. this thin liquid shell is quickly brushed off, making oftentimes a luminous train. but notwithstanding the exceedingly high temperature of the exterior, enforced upon it for the brief time of transit through the atmosphere, it is probable that all large meteorites, if they could be reached at once on striking the earth, would be found to be cold, because the smooth, black, varnishlike crust which always incases them as a result of intense heat is never thick. on one occasion a meteor which was seen to fall in india was dug out of the ground as quickly as possible, and found to be, not hot as was expected, but coated thickly over with ice frozen on it from the moisture in the surrounding soil. as to the composition of shooting stars, and their probable mass, and its effect upon the earth, our data are quite insufficient. the lines of sodium and magnesium have been hurriedly caught in the spectroscope, and, estimating on the basis of the light emitted by them, the largest meteors must weigh ounces rather than pounds. nevertheless, it is interesting to inquire what addition the continual fall of many millions daily upon the earth makes to its weight: somewhere between thirty and fifty thousand tons annually is perhaps a conservative estimate, but even this would not accumulate a layer one inch in thickness over the entire surface of the earth in less than a thousand million years. many hundreds of the meteors actually seen to fall, together with those picked up accidentally, are recovered and prized as specimens of great value in our collections, the richest of which are now in new york, paris, and london. the detailed investigation of them is rather the province of the chemist, the crystallographer and the mineralogist than of the astronomer whose interest is more keen in their life history before they reach the earth. to distinguish a stony meteorite from terrestrial rock substances is not always easy, but there is usually little difficulty in pronouncing upon an iron meteorite. these are most frequently found in deserts, because the dryness of the climate renders their oxidation and gradual disappearance very slow. the surface of a suspected iron meteorite is polished to a high luster and nitric acid is poured upon it. if it quickly becomes etched with a characteristic series of lines, or a sort of cross-hatching, it is almost certain to be a meteorite. occasionally carbon has been found in meteorites, and the existence of diamond has been suspected. the minerals composing meteorites are not unlike terrestrial materials of volcanic origin, though many of them are peculiar to meteorites only. more than one-third of all the known chemical elements have been found by analysis in meteorites, but not any new ones. meteoric iron is a rich alloy containing about ten per cent of nickel, also cobalt, tin, and copper in much smaller amount. calcium, chlorine, sodium, and sulphur likewise are found in meteoric irons. at very high temperatures iron will absorb gases and retain them until again heated to red heat. carbonic oxide, helium, hydrogen, and nitrogen are thus imprisoned, or occluded, in meteoric irons in very small quantities; and in , during a london lecture by graham, a room in the royal institution was for a brief space illuminated by gas brought to earth in a meteorite from interplanetary space. meteorites, too, have been most critically investigated by the biologist, but no trace of germs of organic life of any type has so far been found. farrington of chicago has published a full descriptive catalogue of all the north american meteorites. recent investigations of the radioactivity of meteorites show that the average stone meteorite is much less radioactive than the average rock, and probably less than one-fourth as radioactive as in average granite. the metallic meteorites examined were found about wholly free from radioactivity. from shooting stars, perhaps the chips of the celestial workshop, or more possibly related to the planetesimals which the processes of growth of the universe have swept up into the vastly greater bodies of the universe, transition is natural to the stars themselves, the most numerous of the heavenly bodies, all shining by their own light, and all inconceivably remote from the solar system, which nevertheless appears to be not far removed from the center of the stellar universe. chapter xliv the universe of stars our consideration of the solar system hitherto has kept us quite at home in the universe. the outer known planets, uranus and neptune, are indeed far removed from the sun, and a few of the comets that belong to our family travel to even greater distances before they begin to retrace their steps sunward. when we come to consider the vast majority of the glistening points on the celestial sphere--all in fact except the five great planets, mercury, venus, mars, jupiter, and saturn--we are dealing with bodies that are self-luminous like the sun, but that vary in size quite as the bodies of the solar system do, some stars being smaller than the sun and others many hundred fold larger than he is; some being "giants," and others "dwarfs." but the overwhelming remoteness of all these bodies arrests our attention and even taxes our credulity regarding the methods that astronomers have depended on to ascertain their distances from us. their seeming countlessness, too, is as bewildering as are the distances; though, if we make actual counts of those visible to the naked eye within a certain area, in the body of the "great bear," for example, the great surprise will be that there are so few. and if the entire dome of the sky is counted, at any one time, a clear, moonless sky would reveal perhaps , , so that in the entire sky, northern and southern, we might expect to find , to , lucid stars, or stars visible to the naked eye. but when the telescope is applied, every accession of power increases the myriads of fainter and fainter stars, until the number within optical reach of present instruments is somewhere between and millions. but if we were to push the -inch reflector on mount wilson to its limit by photography with plates of the highest sensitiveness, millions upon millions of excessively faint stars would be plainly visible on the plates which the human eye can never hope to see directly with any telescope present or future, and which would doubtless swell the total number of stars to a thousand millions. recent counts of stars by chapman and melotte of greenwich tend to substantiate this estimate. what have astronomers done to classify or catalogue this vast array of bodies in the sky? even before making any attempt to estimate their number, there is a system of classification simply by the amount of light they send us, or by their apparent stellar magnitudes--not their actual magnitudes, for of those we know as yet very little. we speak of stars of the "first magnitude," of which there are about , sirius being the brightest and regulus the faintest. then there are about of the second, or next fainter, magnitude, stars like polaris, for example, which give an amount of light two and a half times less than the average first magnitude star. stars of the third magnitude are fainter than those of the second in the same ratio, but their number increases to ; fourth magnitude, ; fifth magnitude, , ; sixth magnitude, , , and these are so faint that they are just visible on the best nights without telescopic aid. decimals express all intermediate graduations of magnitude. astronomers carry the telescopic magnitudes much farther, till a magnitude beyond the twentieth is reached, preserving in every case the ratio of two and one-half for each magnitude in relation to that numerically next to it. even jupiter and venus, and the sun and moon, are sometimes calculated on this scale of stellar magnitude, numerically negative, of course, venus sometimes being as bright as magnitude - . , and the sun - . . knowing thus the relation of sun, moon, and stars, and the number of the stars of different magnitudes, it is possible to estimate the total light from the stars. this interesting relation comes out this way: that the stars we cannot see with the naked eye give a greater total of light than those we can because of their vastly greater numbers. and if we calculate the total light of all the brighter stars down to magnitude nine and one-half, we find it equal to / th of the light of the average full moon. many stars show marked differences in color, and strictly speaking the stars are now classified by their colors. the atmosphere affects star colors very considerably, low altitudes, or greater thickness of air, absorbing the bluish rays more strongly and making the stars appear redder than they really are. aldebaran, betelgeuse and antares are well-known red stars, capella and alpha ceti yellowish, vega and sirius blue, and procyon and polaris white. among the telescopic stars are many of a deep blood-red tint, variable stars being numerous among them. double stars, too, are often complementary in color. there is evidence indicating change of color of a very few stars in long periods of time; sirius, for example, two thousand years ago was a red star, now it is blue or bluish white. but the meaning of color, or change of color in a star is as yet only incompletely ascertained. it may be connected with the radiative intensity of the star, or its age, or both. the late professor edward c. pickering was famous for his life-long study and determination of the magnitudes of the stars. standards of comparison have been many, and have led to much unnecessary work. pickering chose polaris as a standard and devised the meridian photometer, an ingenious instrument of high accuracy, in which the light of a star is compared directly with that of the pole star by reflection. all the bright stars of both the northern and the southern skies are worked into a standard system of magnitudes known as hp, or the harvard photometry. astronomers make use of several different kinds of magnitude for the stars: the apparent magnitude, as the eye sees it, often called the visual magnitude; the photographic magnitude, as the photographic plate records it, and these are now determined with the highest accuracy; the photovisual magnitude, quite the same as the visual, but determined photographically on an isochromatic plate with a yellow screen or filter, so that the intensity is nearly the same as it appears to the eye. the difference between the star's visual or photovisual magnitude and its photographic magnitude is called its color-index, and is often used as a measure of the star's color. light of the shorter wave lengths, as blue and violet, affects the photographic plate more rapidly than the reds and yellows of longer wave length by which the eye mainly sees; so that red stars will appear much fainter and blue stars much brighter on the ordinary photographic plate than the eye sees them. so great are the differences of color in the stars that well-known asterisms, with which the eye is perfectly familiar, are sometimes quite unrecognizable on the photographic plate, except by relative positions of the stars composing them. white stars affect the eye and the plate about equally, so that their visual or photovisual and photographic magnitudes are about equal. the studies of the colors of the stars, the different methods of determining them, and the relations of color to constitution have been made the subject of especial investigation by seares of mount wilson and many other astronomers. centuries of the work of astronomers have been faithfully devoted to mapping or charting the stars and cataloguing them. just as we have geographical maps of countries, so the heavens are parceled out in sections, and the stars set down in their true relative positions just as cities are on the map. recent years have added photographic charts, especially of detailed regions of the sky; but owing to spectral differences of the stars, their photographic magnitudes are often quite different from their visual magnitudes. from these maps and charts the positions of the stars can be found with much precision; but if we want the utmost accuracy, we must go to the star catalogues--huge volumes oftentimes, with stellar positions set down therein with the last degree of precision. first there will be the star's name, and in the next column its magnitude, and in a third the star's right ascension. this is its angular distance eastward around the celestial sphere starting from the vernal equinox, and it corresponds quite closely to the longitude of a place which we should get from a gazetteer, if we wished to locate it on the earth. then another column of the catalogue will give the star's declination, north or south of the equator, just as the gazetteer will locate a city by its north or south latitude. chapter xlv star charts and catalogues who made the first star chart or catalogue? there is little doubt that eudoxus (b. c. ) was the first to set down the positions of all the brighter stars on a celestial globe, and he did this from observations with a gnomon and an armillary sphere. later hipparchus (b. c. ) constructed the first known catalogue of stars, so that astronomers of a later day might discover what changes are in progress among the stars, either in their relative positions or caused by old stars disappearing or new stars appearing at times in the heavens. hipparchus was an accurate observer, and he discovered an apparent and perpetual shifting of the vernal equinox westward, by which the right ascensions of the stars are all the time increasing. he determined the amount of it pretty accurately, too. his catalogue contained , stars, and is printed in the "almagest" of ptolemy. centuries elapsed before a second star catalogue was made, by ulugh-beg, an arabian astronomer, a. d. , who was a son of tamerlane, the tartar monarch of samarcand, where the observations for the catalogue were made. the stars were mainly those of ptolemy, and much the same stars were reobserved by tycho brahe (a. d. ) with his greatly improved instruments, thus forming the third and last star catalogue of importance before the invention of the telescope. from the end of the seventeenth century onward, the application of the telescope to all the types of instruments for making observations of star places has increased the accuracy many-fold. the entire heavens has been covered by argelander in the northern hemisphere, and gould in the southern--over , stars in all. many government observatories are still at work cataloguing the stars. the carnegie institution of washington maintains a department of astrometry under boss of albany, which has already issued a preliminary catalogue of more than , stars, and has a great general catalogue in progress, together with investigations of stellar motions and parallaxes. this catalogue of star positions will include proper motions of stars to the seventh magnitude. in on proposal of the late sir david gill, an international congress of astronomers met at paris and arranged for the construction of a photographic chart of the entire heavens, allotting the work to eighteen observatories, equipped with photographic telescopes essentially alike. the total number of plates exceeds , . stars of the fourteenth magnitude are recorded, but only those including the eleventh magnitude will be catalogued, perhaps , , in all. the expense of this comprehensive map of the stars has already exceeded $ , , , and the work is now nearly complete. turner of oxford has conducted many special investigations that have greatly enhanced the progress of this international enterprise. other great photographic star charts have been carried through by the harvard observatory, with the annex at arequipa, peru, employing the bruce photographic telescope, a doublet with -inch lenses; also kapteyn of groningen has catalogued about , stars on plates taken at cape town. charting and cataloguing the stars, both visually and photographically, is a work that will never be entirely finished. improvements in processes will be such that it can be better done in the future than it is now, and the detection of changes in the fainter stars and investigation of their motions will necessitate repetition of the entire work from century to century. the origin of the names of individual stars is a question of much interest. the constellation figures form the basis of the method, and the earliest names were given according to location in the especial figure; as for instance, cor scorpii, the heart of the scorpion, later known as antares or alpha scorpii. the arabians adopted many star names from the greeks, and gave about a hundred special names to other stars. some of these are in common use to-day, by navigators, observers of meteors and of variable stars. sirius, vega, arcturus, and a few other first magnitude stars, are instances. but this method is quite insufficient for the fainter stars whose numbers increase so rapidly. bayer, a contemporary of galileo, originated our present system, which also employs the names of the constellations, the latin genitive in each case, prefixed by the small letters of the greek alphabet, from alpha to omega, in order of decreasing brightness; and followed by the roman letters when the greek alphabet is exhausted. if there were still stars left in a constellation unnamed, numbers were used, first by flamsteed, astronomer royal; and numbers in the order of right ascension in various catalogues are used to designate hundreds of other stars. the vast bulk of the stars are, however, nameless; but about one million are identifiable by their positions (right ascension and declination) on the celestial sphere. chapter xlvi the sun's motion toward lyra if hipparchus or galileo should return to earth to-night and look at the stars and constellations as we see them, there would be no change whatever discernible in either the brightness of the stars or in their relative positions. so the name fixed stars would appear to have been well chosen. halley in the seventeenth century was the first to detect that slow relative change of position of a few stars which is known as proper motion, and all the modern catalogues give the proper motions in both right ascension and declination. these are simply the small annual changes in position athwart the line of vision; and, as a whole, the proper motions of the brighter stars exceed the corresponding motions of the fainter ones because they are nearer to us. the average proper motion of the brightest stars is ". , and of stars of the sixth magnitude only one-sixth as great. a few extreme cases of proper motion have been detected, one as large as ", of an orange yellow star of the eighth magnitude in the southern constellation pictor, and barnard has recently discovered a star with a proper motion exceeding "; several determinations of its parallax give ". , corresponding to a distance of . light years. nevertheless, two centuries would elapse before these stars would be displaced as much as the breadth of the moon among their neighbors in the sky. the proper motions of stars are along perfectly straight lines, so far as yet observed. ultimately we may find a few moving in curved paths or orbits, but this is hardly likely. as for a central sun hypothesis, that pointing out alcyone in particular, there is no reliable evidence whatever. analysis of the proper motions of stars in considerable numbers, first by sir william herschel, showed that they were moving radially from the constellation hercules, and in great numbers also toward the opposite side of the stellar sphere. later investigation places this point, called the sun's goal, or apex of the sun's way, over in the adjacent constellation lyra; and the opposite point, or the sun's quit, is about halfway between sirius and canopus. by means of the radial velocities of stars in these antipodal regions of the sky, it is found that the sun's motion toward lyra, carrying all his planetary family along with him, is taking place at the rate of about miles in every second. while the right ascensions of the solar apex as given by the different investigations have been pretty uniform, the declination of this point has shown a rather wide variation not yet explained. for example, there is a difference of nearly ten degrees between the declination (+ °. ) of the apex as determined by boss from the proper motions of more than , stars, and the declination (+ °. ) found by campbell from the radial velocities of nearly , stars. several investigations tend to show that the fainter the stars are, the greater is the declination of the solar apex. more remarkable is the evidence that this declination varies with the spectral type of the stars, the later types, especially g and k, giving much more northerly values. on the whole the great amount of research that has been devoted to the solar motion relative to the system of the stars for the past hundred years may be said to indicate a point in right ascension h. ( °) and declination ° n. as the direction toward which the sun is moving. this is not very far from the bright star alpha lyræ, and the antipodal point from which the sun is traveling is quite near to beta columbæ. so swift is this motion (nearly twenty kilometers per second) that it has provided a base line of exceptional length, and very great service in determining the average distance of stars in groups or classes. after thousands of years the sun's own motion combined with the proper motions of the stars will displace many stars appreciably from their familiar places. the constellations as we know them will suffer slight distortions, particularly orion, cassiopeia and ursa major. identity or otherwise of spectra often indicates what stars are associated together in groups, and their community of motion is known as star drift. recent investigation of vast numbers of stars by both these methods have led to the epochal discovery of star streaming, which indicates that the stars of our system are drifting by, or rather through, each other, in two stately and interpenetrating streams. the grand primary cause underlying this motion is as yet only surmised. chapter xlvii stars and their spectral type when in dr. henry draper placed a very small wet plate in the camera of his spectroscope and, by careful following, on account of the necessarily long exposure, secured the first photographic spectrum of a star ever taken, he could hardly have anticipated the wealth of the new field of research which he was opening. his wife, anna palmer draper, was his enthusiastic assistant in both laboratory and observatory, and on his death in , she began to devote her resources very considerably to the amplification of stellar spectrum photography. at first with the cooperation of professor young of princeton, and later through extension of the facilities of harvard college observatory, whose director, the late professor edward c. pickering, devoted his energies in very large part to this matter, all the preliminaries of the great enterprise were worked out, and a comprehensive program was embarked upon, which culminated in the "henry draper memorial," a catalogue and classification of the spectra of all the stars brighter than the ninth magnitude, in both the northern and southern hemispheres. one very remarkable result from the investigation of large numbers of stars according to their type is the close correlation between a star's luminosity and its spectral type. but even more remarkable is the connection between spectral type and speed of motion. as early as monck of dublin, later kapteyn, and still later dyson, directed attention to the fact that stars of the secchi type ii had on the average larger proper motions than those of type i. in frost and adams brought out the exceptional character of the orion stars, the radial velocities of twenty of which averaged only seven kilometers per second. soon after, with the introduction of the two-stream hypothesis, a wider generalization was reached by campbell and kapteyn, whose radial velocities showed that the average linear velocity increases continually through the entire series b, a, f, g, k, m, from the earliest types of evolution to the latest. the younger stars of early type have velocities of perhaps five or six kilometers per second, while the older stars of later type have velocities nearly fourfold greater. the great question that occurs at once is: how do the individual stars get their motions? the farther back we go in a star's life history, the smaller we find its velocity to be. when a star reaches the orion stage of development, its velocity is only one-third of what it may be expected to have finally. apparently, then, the stars at birth have no motion, but gradually acquire it in passing through their several types or stages of development. more striking still is the motion of the planetary nebulæ, in excess of kilometers per second, while type a stars move kilometers, type g kilometers, and type m kilometers per second. can the law connecting speed of motion and spectral type be so general that the planetary nebula is to be regarded as the final evolutionary stage? stars have been seen to become nebulæ, and one astronomer at least is strongly of the opinion that a single such instance ought to outweigh all speculation to the contrary, as that stars originate from nebulæ. in his discussion of stellar proper motions, boss has reached a striking confirmation of the relation of speed to type, finding for the cross linear motion of the different types a series of velocities closely paralleling those of kapteyn and campbell. concerning the marked relation of the luminosities of the stars to their spectral types, there is a pronounced tendency toward equality of brightness among stars of a given type; also the brightness diminishes very markedly with advance in the stage of evolution. there has been much discussion as to the order of evolution as related to the type of spectrum, and russell of princeton has put forward the hypothesis of giant stars and dwarf stars, each spectral type having these two divisions, though not closely related. one class embraces intensely luminous stars, the other stars only feebly luminous. when a star is in process of contraction from a diffused gaseous mass, its temperature rises, according to lane's law, until that density is reached where the loss of heat by radiation exceeds the rise in temperature due to conversion of gravitational energy into heat. then the star begins to cool again. so that if the spectrum of a star depends mainly on the effective temperature of the body, clearly the classification of the draper catalogue would group stars together which are nearly alike in temperature, taking no note as to whether their present temperature is rising or falling. another classification of stars by lockyer divides them according to ascending and descending temperatures. russell's theory would assign the succession of evolutionary types in the order, m_{ }, k_{ }, g_{ }, f_{ }, a_{ }, b, a_{ }, f_{ }, g_{ }, k_{ }, m_{ }, the subscript referring to the "giants," and to the dwarf stars. in large part the weight of evidence would appear to favor the order of the harvard classification, independently confirmed as it is by studies of stellar velocities, galactic distribution, and periods of binary stars both spectroscopic and visual, where campbell and aiken find a marked increase in length of period with advance in spectral type. at the same time, a vast amount of evidence is accumulating in support of russell's theory. investigations in progress will doubtless reveal the ground on which both may be harmonized. the publication of the new henry draper catalogue of stellar spectra is in progress, a work of vast magnitude. the great catalogue of thirty years ago embraced the spectra of more than ten thousand stars, and was a huge work for that day; but the new catalogue utterly dwarfs it, with a classification much more detailed than in the earlier work, and with the number of stars increased more than twenty-fold. this work, projected by the late director of the harvard observatory, has been brought to a conclusion by the energy and enthusiasm of miss annie j. cannon through six years of close application, aided by many assistants. the catalogue ranges over the stars of both hemispheres, and is a monument to masterly organization and completed execution which will be of the highest importance and usefulness in all future researches on the bodies of the stellar universe. chapter xlviii star distances so vast are the distances of the stars that all attempts of the early astronomers to ascertain them necessarily proved futile. this led many astronomers after copernicus to reject his doctrine of the earth's motion round the sun, so that they clung rather to the ptolemaic view that the earth was without motion and was the center about which all the celestial motions took place. the geometry of stellar distances was perfectly understood, and many were the attempts made to find the parallaxes and distances of the stars; but the art of instrument making had not yet advanced to a stage where astronomers had the mechanisms that were absolutely necessary to measure very small angles. about , bessel undertook the work of determining stellar parallax in earnest. his instrument was the heliometer, originally designed for measuring the sun's diameter; but as modified for parallax work it is the most accurate of all angle-measuring instruments that the astronomers employ. the star that he selected was cygni, not a bright star, of the sixth magnitude only, but its large proper motion suggested that it might be one of those nearest to us. he measured with the heliometer, at opposite seasons of the year, the distance of cygni from another and very small star in the same field of view, and thus determined the relative parallax of the two stars. the assumption was made that the very faint star was very much more distant than the bright one, and this assumption will usually turn out to be sound. bessel got ". for his parallax of cygni, and struve by applying the same method to alpha lyræ, about the same time, got ". for the parallax of that star. these classic researches of bessel and struve are the most important in the history of star distances, because they were the first to prove that stellar parallax, although minute, could nevertheless be actually measured. about the same time success was achieved in another quarter, and henderson, the british astronomer at the cape of good hope, found a parallax of nearly a whole second for the bright star alpha centauri. although the parallaxes of many hundreds of stars have been measured since, and the parallaxes of other thousands of stars estimated, the measured parallax of alpha centauri, as later investigated by elkin and sir david gill, and found to be ". , is the largest known parallax, and therefore alpha centauri is our nearest neighbor among the stars, so far as we yet know. this star is a binary system and the light of the two components together is about the same as that of capella (alpha aurigæ). but it is never visible from this part of the world, being in degrees of south declination: one might just glimpse it near the southern horizon from key west. how the distances of the stars are found is not difficult to explain, although the method of doing it involves a good deal of complication, interesting to the practical astronomer only. recall the method of getting the moon's distance from the earth: it was done by measuring her displacement among the stars as seen from two widely separated observatories, as near the ends of a diameter of the earth as convenient. this is the base line, and the angle which a radius of the earth as seen from the center of the moon fills, or subtends, is the moon's parallax. so near is the moon that this angle is almost an entire degree, and therefore not at all difficult to measure. but if we go to the distance of even alpha centauri, the nearest of the stars, our earth shrinks to invisibility; so that we must seek a longer base line. fortunately there is one, but although its length is , times the earth's diameter, it is only just long enough to make the star distances measurable. we found that the sun's distance from the earth was million miles; the diameter of the earth's orbit is therefore double that amount. now conceive the diameter of the earth replaced by the diameter of the earth's orbit: by our motion round the sun we are transported from one extremity of this diameter to the opposite one in six month's time; so we may measure the displacement of a star from these two extremities, and half this displacement will be the star's parallax, often called the annual parallax because a year is consumed in traversing its period. and it is this very minute angle which bessel and struve were the first to measure with certainty, and which henderson found to be in the case of alpha centauri the largest yet known. evidently the earth by its motion round the sun makes every star describe, a little parallactic ellipse; the nearer the star is the larger this ellipse will be, and the farther the star the smaller: if the star were at an infinite distance, its ellipse would become a point, that is, if we imagine ourselves occupying the position of the star, even the vast orbit of the earth, million miles across, would shrink to invisibility or become a mathematical point. measurement of stellar parallax is one of many problems of exceeding difficulty that confront the practical astronomer. but the actual research nowadays is greatly simplified by photography, which enables the astronomer to select times when the air is not only clear, but very steady for making the exposures. development and measurement of the plates can then be done at any time. pritchard of oxford, england, was among the earliest to appreciate the advantages of photography in parallax work, and schlesinger, mitchell, miller, slocum and van maanen, with many others in this country, have zealously prosecuted it. how shall we intelligently express the vast distances at which the stars are removed from us? of course we can use miles, and pile up the millions upon millions by adding on ciphers, but that fails to give much notion of the star's distance. let us try with alpha centauri: its parallax of ". means that it is , times farther from the sun than the earth is. multiplying this out, we get trillion miles, that is, millions of million miles--an inconceivable number, and an unthinkable distance. suppose the entire solar system to shrink so that the orbit of neptune, sixty times million miles in diameter, would be a circle the size of the dot over this letter i. on the same scale the sun itself, although nearly a million miles in diameter, could not be seen with the most powerful microscope in existence; and on the same scale also we should have to have a circle ten feet in diameter, if the solar system were imagined at its center and alpha centauri in its circumference. so astronomers do not often use the mile as a yardstick of stellar distance, any more than we state the distance from london to san francisco in feet or inches. by convention of astronomers, the average distance between the centers of sun and earth, or million miles, is the accepted unit of measure in the solar system. so the adopted unit of stellar distance is the distance traveled by a wave of light in a year's time: and this unit is technically called the light-year. this unit of distance, or stellar yardstick, as we may call it, is nearly millions of million miles in length. alpha centauri, then, is four and one-third light-years distant, and cygni seven and one-fifth light-years away. for convenience in their calculations most astronomers now use a longer unit called the parsec, first suggested by turner. its length is equal to the distance of a star whose parallax is one second of arc; that is, one parsec is equal to about three and a quarter light-years. or the light-year is equal to . parsec. also the parsec is equal to , astronomical units, or about millions of million miles. we have, then four distinct methods of stating the distance of a star: sirius, for example, has a parallax of ". or its distance is two and two-thirds parsecs, or eight and a half light-years, or millions of million miles. it is the angle of parallax which is always found first by actual measurement and from this the three other estimates of distance are calculated. so difficult and delicate is the determination of a stellar distance that only a few hundred parallaxes have been ascertained in the past century. the distance of the same star has been many times measured by different astronomers, with much seeming duplication of effort. comprehensive campaigns for determining star parallaxes in large numbers have been undertaken in a few instances, particularly at the suggestion of kapteyn, the eminent astronomer of groningen, holland. his catalogue of star parallaxes is the most complete and accurate yet published, and is the standard in all statistical investigations of the stars. that we find relatively large parallaxes for some of the fainter stars, and almost no measurable parallax for some of the very bright stars is one of the riddles of the stellar universe. we may instance arcturus, in the northern hemisphere and canopus in the southern; the latter almost as bright as sirius. dr. elkin and the late sir david gill determined exhaustively the parallax of canopus, and found it very minute, only ". , making its distance in excess of a hundred light-years. the stupendous brilliancy of this star is apparent if we remember that the intensity of its light must vary inversely as the square of the distance; so that if canopus were to be brought as near us as even cygni is, it would be a hundredfold brighter than sirius, the brightest of all the stars of the firmament. in researches upon the distribution of the more distant stars, the method of measuring parallaxes of individual stars fails completely, and the secular parallax, or parallactic motion of the stars is employed instead. by parallactic motion is meant the apparent displacement in consequence of the solar motion which is now known with great accuracy, and amounts to . kilometers per second. even in a single year, then, the sun's motion is twice the diameter of the earth's orbit, so that in a hundred or more years, a much longer base line is available than in the usual type of observations for stellar parallax. if we ascertain the parallactic motion of a group of stars, then we can find their average distance. it is found, for example, that the mean parallax of stars of the sixth magnitude is ". . also the mean distances of stars thrown into classes according to their spectral type have been investigated by boss, kapteyn, campbell and others. the complete intermingling of the two great star streams has been proved, too, by using the magnitude of the proper motions to measure the average distances of both streams. these come out essentially the same, so that the streaming cannot be due to mere chance relation in the line of sight. most unexpected and highly important is the discovery that the peculiar behavior of certain lines in the spectrum leads to a fixed relation between a star's spectrum and its absolute magnitude, which provides a new and very effective method of ascertaining stellar distances. by absolute magnitudes are meant the magnitudes the stars would appear to have if they were all at the same standard distance from the earth. very satisfactory estimates of the distance of exceedingly remote objects have been made within recent years by this indirect method, which is especially applicable to spiral nebulæ and globular clusters. the absolute magnitude of a star is inferred from the relative intensities of certain lines in its spectrum, so that the observed apparent magnitude at once enables us to calculate the distance of the star. adams and joy have recently determined the luminosities and parallaxes of stars by this spectroscopic method. of these stars have had their parallaxes previously measured; and the average difference between the spectroscopic and the trigonometric values of the parallax is only the very small angle ". , a highly satisfactory verification. an indirect method, but a very simple one, and of the greatest value because it provides the key to stellar distances with the least possible calculation, and we can ascertain also the distances of whole classes of stars too remote to be ascertained in any other way at present known. the problem of spectroscopic determinations of luminosity and parallax has been investigated at mount wilson with great thoroughness from all sides, the separate investigations checking each other. a definitive scale for the spectroscopic determination of absolute magnitudes has now been established, and the parallaxes and absolute magnitudes have already been derived for about , stars. chapter xlix the nearest stars of especial interest are the few stars that we know are the nearest to us, and the following table includes all those whose parallax is ". or greater. there are nineteen in all and nearly half of them are binary systems. the radial motions given are relative to the sun. the transverse velocities are formed by using the measured parallaxes to transform proper motions into linear measures. they are given by eddington in his "stellar movements": column key ==================== a) magnitude b) parallax in seconds of arc c) proper motion in seconds of arc d) linear velocity km. per sec. e) radial velocity km. per sec. f) spectral type g) luminosity (sun= ) h) star stream ==================================================================== star's name | a | b | c | d | e | f | g | h ---------------+-----+------+------+-----+-----+------+-------+----- groombridge | . | . | . | | .. | ma | . | i eta cassiop | . | . | . | | + | f | . | i tau ceti | . | . | . | | - | k | . | ii epsilon erid | . | . | . | | + | k | . | ii cz h | . | . | . | |+ | g-k | . | ii sirius |- . | . | . | | - | a | . | ii procyon | . | . | . | | - | f | . | i ? lal. | . | . | . | | .. | ma | . | ii lal. | . | . | . | | .. | ma | . | i oa (n) | . | . | . | | .. | .. | . | i alpha centauri | . | . | . | | - | g,k |{ . | i | | | | | | |{ . | oa (n) | . | . | . | | .. | f | . | ii pos. med. | . | . | . | | .. | k | . | i sigma draco | . | . | . | | + | k | . | ii alpha aquilæ | . | . | . | | - | a | . | i cygni | . | . | . | | - | k | . | i epsilon indi | . | . | . | | - | k | . | i krüger | . | . | . | | .. | .. | . | ii lacaille | . | . | . | | + | ma | . | i -------------------------------------------------------------------- these stars are distant less than five parsecs (about light-years) from the sun, so they make up the closest fringe of the stellar universe immediately surrounding our system. the large number of binary systems is quite remarkable. why some stars are single and others double is not yet known. by the spectroscopic method the proportion is not so large; campbell finding that about one quarter of , stars examined are spectroscopic binaries, and frost two-fifths to a half. the exceptional number of large velocities is very remarkable; the average transverse motion of the nineteen stars is fifty kilometers per second, whereas thirty is about what would have been expected. as to star streams to which these nearest stars belong, eleven are in stream i and eight in stream ii, in close accord with the ratio : given by the , stars of boss's catalogue. "we are not able," says eddington, "to detect any significant difference between the luminosities, spectra, or speeds of the stars constituting the two streams. the thorough interpenetration of the two star streams is well illustrated, since we find even in this small volume of space that members of both streams are mingled together in just about the average proportion." [illustration: the ring nebula in _lyra_. this is the best example of the annular and elliptic nebulæ, which are not very abundant. (_photo, mt. wilson solar observatory._)] [illustration: the dumb-bell nebula of _vulpecula_. to take the photograph required an exposure of five hours. (_photo, mt. wilson solar observatory._)] chapter l actual dimensions of the stars we have seen that the distances of the stars from the solar system are immense beyond conception, and millions upon millions of them are probably forever beyond our power of ascertaining by direct measurement what their distance really is. after we had found the sun's distance and measured the angle filled by his disk, it was easy to calculate his actual size. this direct method, however, fails when we try to apply it to the stars, because their distances are so vast that no star's disk fills an angle of any appreciable size; and even if we try to get a disk with the highest magnifying powers of a great telescope our efforts end only in failure. there is, indeed, no instrumentally appreciable angle to measure. how then shall we ascertain the actual dimensions of the vast spheres which we know the stars actually are, as they exist in the remotest regions of space? clearly by indirect methods only, and it must be said that astronomers have as yet no general method that yields very satisfactory results for stellar dimensions. the actual magnitude of the variable system of algol, beta persei, is among the best known of all the stars, because the spectroscope measures the rate of approach and recession of algol when its invisible satellite is in opposite parts of the orbit; the law of gravitation gives the mass of the star and the size of its orbit, and so the length of the eclipse gives the actual size of the dark, eclipsing body. this figures out to be practically the same size as that of our sun, while algol's own diameter is rather larger, exceeding a million miles. if we try to estimate sizes of stars by their brightness merely, we are soon astray. differences of brightness are due to difference of dimensions, of course, or of light-giving area; but differences of distance also affect the brightness, inversely as the squares of the distances, while differences of temperature and constitution affect, in very marked degree, the intrinsic brilliance of the light-emitting surface of the star. there are big stars and little stars, stars relatively near to us and stars exceedingly remote, and stars highly incandescent as well as others feebly glowing. we have already shown how the angular diameters subtended by many of the stars have been estimated, through the relation of surface brightness and spectral type. antares and betelgeuse appear to be the most inviting for investigation, because their estimated angular diameters are about one-twentieth of a second of arc. this is the way in which their direct measurement is being attempted. as early as , michelson of chicago suggested the application of interference methods to the accurate measurement of very small angles, such as the diameters of the minor planets, and the satellites of jupiter and saturn, as well as the arc distance between the components of double stars. two portions of the object glass are used, as far apart as possible on the same diameter, and the interference fringes produced at the focus of the objective are then the subject of observation. these fringes form a series of equidistant interference bands, and are most distinct when the light comes from a source subtending an infinitesimal angle. if the object presents an appreciable angle, the visibility is less and may even become zero. michelson tested this method on the satellites of jupiter at the lick observatory in , and showed its accuracy and practicability. nevertheless, the method has not been taken up by astronomers, until very recently at the mount wilson observatory, where anderson has applied it to the measurement of close double stars. it is found that, contrary to general expectation, the method gives excellent results, even if the "seeing" is not the best-- on a scale of , for instance. to simplify the manipulation of the interferometer, a small plate with two apertures in it is placed in the converging beam of light coming from the telescope objective or mirror. the interference fringes formed in the focal plane are then viewed with an eyepiece of very high power, many thousand diameters. the resolving power of the interferometer is found to be somewhat more than double that of a telescope of the same aperture. by applying the interferometer method to capella, arc distances of much less than one-twentieth of a second of arc were measured. more recently the method has been applied to the great star betelgeuse in orion, whose angular diameter was found to be ". , corresponding to an actual diameter of , , miles, if the star's parallax is as small as it appears to be. chapter li the variable stars spectacular as they are to the layman, novæ, or temporary stars, are to the astronomers simply a class among many thousands of stars which they call variables, or variable stars. there are a few objects classified as irregular variables, one of which is very remarkable. we refer to eta argus, an erratic variable in the southern constellation argo and surrounded by a well-known nebula. there is a pretty complete record of this star. halley in when observing at saint helena recorded eta argus as of the fourth magnitude. during the th century, it fluctuated between the fourth magnitude and the second. early in the th it rapidly waxed in brightness, fluctuating between the first and second magnitudes from to . but two years later its light tripled, rivaling all the fixed stars except canopus and sirius. in it was even brighter for a few months, but since then it has declined fairly steadily, reaching a minimum at magnitude seven and a half in , with a slight increase in brightness more recently. a period of half a century has been suggested, but it is very doubtful if eta argus has any regular period of variation. another very interesting class of variables is known as the omicron ceti type. nearly all the time they are very faint, but quite suddenly they brighten through several magnitudes, and then fade away, more or less slowly, to their normal condition of faintness. but the extraordinary thing is that most of these variables go through their fluctuations in regular periods: from six months to two years in length. the type star, omicron ceti, or mira, is the oldest known variable, having been discovered by fabricius in . most of the time it is a relatively faint star of the th magnitude; but once in rather less than a year its brightness runs up to the fourth, third and sometimes even the second magnitude, where it remains for a week or ten days, and afterward it recedes more slowly to its usual faintness, the entire rise and decline in brightness usually requiring about days. the spectrum of omicron ceti contains many very bright lines, and a large proportion of the variable stars are of this type. another class of variables is designated as the beta lyræ type. their periods are quite regular, but there are two or more maxima and minima of light in each period, as if the variation were caused by superposed relations in some way. their spectra show a complexity of helium and hydrogen bands. no wholly satisfactory explanation has yet been offered. probably they are double stars revolving in very small orbits compared with their dimensions, their plane of motion passing nearly through the earth. but the most interesting of all the variables are those of the algol type, their light curves being just the reverse of the omicron ceti type; that is, they are at their maximum brightness most of the time, and then suffer a partial eclipse for a relatively brief interval. algol goes through its variations so frequently that its period is very accurately known; it is d. h. m. . s. for most of this period algol is an easy second magnitude star; then in about four and a half hours it loses nearly five-sixths of its light, receding to the fourth magnitude. here at minimum it remains for fifteen or twenty minutes, and then in the next three and a half hours it regains its full normal brilliancy of the second magnitude. during these fluctuations the star's spectrum undergoes no marked changes. the spectra of all the algol variables are of the first or sirian type. to explain the variation of the algol type of variables is easy: a dark, eclipsing body, somewhat smaller than the primary is supposed to be traveling round it in an orbit lying nearly edgewise to our line of sight. the gravitation of this dark companion displaces algol itself alternately toward and from the earth, because the two bodies revolve round their common center of gravity. with the spectroscope this alternate motion of algol, now advancing and now receding at the rate of miles per second, has been demonstrated; and the period of this motion synchronizes exactly with the period of the star's variability. russell and shapley have made extended studies of the eclipsing binaries, and developed the formulæ by which the investigations of their orbits are conducted. heretofore, visual binaries and spectroscopic binaries afforded the only means of deriving data regarding double systems, but it is now possible to obtain from the orbits of eclipsing variables fully as much information relating to binary systems in general and their bearing on stellar evolution. after an orbit has been determined from the photometric data of the light curve, the addition of spectroscopic data often permits the calculation of the masses, dimensions and densities in terms of the sun. shapley's original investigation included the orbits of ninety eclipsing variables, and with the aid of hypothetical parallaxes, he computed the approximate position of each system in space. the relation to the milky way is interesting, the condensation into the galactic plane being very marked; only thirteen of the ninety systems being found at galactic latitudes exceeding degrees. if we can suppose the variable stars covered with vast areas of spots, perhaps similar to the spots on the sun, and then combine the variation of these spot areas with rotation of the star on its axis, there is a possibility of explanation of many of the observed phenomena, especially where the range of variation is small. but for the omicron ceti type, no better explanation offers than that afforded by sir norman lockyer's collision theory. first he assumes that these stars are not condensed bodies, but still in the condition of meteoric swarms, and the revolution of lesser swarms around larger aggregations, in elliptic orbits of greater or less eccentricity, must produce vast multitudes of collisions; and these collisions, taking place at pretty regular periods, produce the variable maximum light by raising hosts of meteoric particles to a state of incandescence simultaneously. the catalogues of variable stars now contain many thousands of these objects. they are often designated by the letters r, s, t, and so on, followed by the genitive form of the name of the constellation wherein they are found. most of the recently found variables have a range of less than one magnitude. they are so distributed as to be most numerous in a zone inclined about degrees to the celestial equator, and split in two near where the cleft in the galaxy is located. nearly all the temporary stars are in this duplex region. bailey of harvard a quarter century ago began the investigation of variables in close star clusters, where they are very abundant, with marked changes of magnitude within only a few hours. many amateur astronomers afford very great assistance to the professional investigator of variable stars by their cooperation in observing these interesting bodies, in particular the american association of observers of variable stars, organized and directed by william tyler olcott. for a high degree of accuracy in determining stellar magnitudes the photo-electric cell is unsurpassed. stebbins of urbana has been very successful in its application and he discovered the secondary minimum of algol with the selenium cell. his most recent work was done with a potassium cell with walls of fused quartz, perfected after many trial attempts. the stars he has recently investigated are lambda tauri, and pi five orionis. combining results with those reached by the spectroscope, the masses of the two component stars of the former are . and . that of the sun, and the radii are . and . times the sun's. russell of princeton thinks it probable that similar causes are at work in all these variables. in the case of the typical novæ there is evidence that when the outburst takes place a shell of incandescent gas is actually ejected by the star at a very high velocity. what may be the forces that cause such an explosion can only be guessed. repeated outbursts have not, in the case of t pyxidis, destroyed the star, because it has gone through this process three times in the past thirty years. russell inclines to regard it as a standard process occurring somewhere in the stellar universe probably as often as once a year. novæ, then, cannot be due to collisions between two stars, for even if we suppose the stars to be a thousand millions in number, no two should collide except at average intervals of many million years. the idea is gaining ground that the stars are vast storehouses of energy which they are gradually transforming into heat and radiating into space. "under ordinary circumstances, it is probable that the rate of generation of heat is automatically regulated to balance the loss by radiation. but it is quite conceivable that some sudden disturbance in the substance of the star, near the surface, might cause an abrupt liberation of a great amount of energy, sufficient to heat the surface excessively, and drive the hot material off into infinite space, in much the form of a shell of gas, as seems to have been observed in the case of nova aquilæ.... with the rapid advance of our knowledge of the properties of the stars on one hand, and of the very nuclei of atoms on the other, we may, perhaps before many years have passed, find ourselves nearer a solution of the problem." the cepheid variables increase very rapidly in brightness from their least light to their maximum, and then fade out much more slowly, with certain irregularities or roughnesses of their light-curves when declining. their spectral lines also shift in period with their variations of light. in the case of these variables, whose regular fluctuation of light cannot be due to eclipse, and is as a rule embraced within a few days, there is a fluctuation in color also between maximum and minimum, as if there were a periodic change in the star's physical condition. eddington and shapley advocate the theory of a mechanical pulsation of the star as most plausible. knowledge of the internal conditions of the stars make it possible to predict the period of pulsation within narrow limits; and for delta cephei this theoretical period is between four and ten days. its observed period is five and one-third days, and corresponding agreement is found in all the cepheids so far tested. shapley of mount wilson finds that the cepheid variables with periods exceeding a day in length all lie close to the galactic lane. so greatly have the studies of these objects progressed that, as before remarked, when we know the star's period, we can get its absolute magnitude, and from this the star's distance. on all sides of the sun, the distances of the cepheids range up to , parsecs. so they indicate the existence of a galactic system far greater in extent than any previously dealt with. chapter lii the novÆ, or new stars new stars, or temporary stars, we have already mentioned in connection with variables. they are, next to comets, the most dramatic objects in the heavens. they may be variable stars which, in a brief period, increase enormously in brightness, and then slowly wane and disappear entirely, or remain of a very faint stellar magnitude. in the ancient historical records are found accounts of several such stars. for instance, in the chinese annals there is an allusion to such a stellar outburst in the constellation of scorpio, b. c. . this was observed also by hipparchus and, no doubt, it was the immediate incentive which led to his construction of the first known catalogue of stars, so that similar happenings might be detected in the future. in november, , tycho brahe observed the most famous of all new stars, which blazed out in the constellation cassiopeia. in something over a year it had completely disappeared. in - a new star of equal brightness was seen in ophiuchus by kepler; it also faded out to invisibility in . kepler and tycho printed very complete records of these remarkable objects. the eighteenth century passed without any new stars being seen or recorded. there was one of the fifth magnitude in , and another of the seventh magnitude in ; and in may, , a star of the second magnitude suddenly made its appearance in corona borealis; and one of the third magnitude in cygnus in november, . the latter was fully observed by schmidt of athens and became a faint telescopic star within a few weeks. it is now of the fifteenth magnitude. in astronomers were surprised to find suddenly a new star of the sixth magnitude very close to the brightest part of the great nebula in andromeda; it ran its course in about six months, fading with many fluctuations in brightness, and no star is now visible in its position even with the telescope. stars of this class are known to astronomers as novæ, usually with the genitive of the constellation name, as nova andromedæ. in - nova aurigæ made its spectacular appearance and yielded a distinctly double and complex spectrum for more than a month. many pairs of lines indicated a community of origin as to substance, and accurate measurement showed a large displacement with a relative velocity of more than miles per second. for each bright hydrogen line displaced toward the red there was a dark companion line or band about equally displaced toward the violet much as if the weird light of nova aurigæ originated in a solid globe moving swiftly away from us and plunging into an irregular nebulous mass as swiftly approaching us. parallax observations of nova aurigæ made it immensely remote, perhaps within the galaxy, and it still exists as a faint nebulous star. in february, , in the constellation perseus appeared the most brilliant nova of recent years. it was first discovered by dr. anderson, an amateur of glasgow, and at maximum on february it outshone capella. there were many unusual fluctuations in its waning brightness. its spectrum closely resembled that of nova aurigæ, with calcium, helium, and hydrogen lines. in august, , an enveloping nebula was discovered, and a month later certain wisps of this nebulosity appeared to have moved bodily, at a speed seventy-fold greater than ever previously observed in the stellar universe. according to sir norman lockyer's meteoritic hypothesis, a vast nebulous region was invaded, not by one but by many meteor swarms, under conditions such that the effects of collision varied greatly in intensity. the most violent of these collisions gave birth to nova persei itself, and the least violent occurred subsequently in other parts of the disturbed nebula, perhaps immeasurably removed. this explanation would avoid the necessity of supposing actual motion of matter through space at velocities heretofore unobserved and inconceivably high. a recent photograph of nova persei, by ritchey, reveals a nebulous ring of regular structure surrounding the star. the great power of the -inch has made it possible to photograph even the spectra of many of the novæ of years ago which are now very faint. after the lapse of years the characteristic lines of the nebular spectrum generally vanish, as if the star had passed out of the nebula--a plunge into which is generally thought to be the cause of the great and sudden outburst of light. many novæ have recently been found in the spiral nebulæ, especially in the great nebula of andromeda. chapter liii the double stars examining individual stars of the heavens more in detail, thousands of them are found to be double; not the stars that appear double to the naked eye, as theta tauri, mizar, epsilon lyræ, and others; but pairs of stars much closer together, and requiring the power of the telescope to divide or separate them. only a very few seconds apart they are or, in many cases, only the merest fraction of a second of arc. some of them, called binaries, are found to be revolving around a common center, sometimes in only a few years, sometimes in stately periods of hundreds of years. many such binary systems are now known, and the number is constantly increasing. castor is one, gamma virginis another, sirius also is one of these binaries, and a most interesting one, having a period of revolution of about years. aitken, of the lick observatory, in his work on binary stars, directs special attention to the correlation between the elements of known binary orbits and the star's spectral type, and presents a statistical study of the distribution of , visual double stars, of which the spectra of are known. that the masses of binary systems average about twice that of the sun's mass has long been known, and this fact can be employed with confidence in estimates of the probable parallax of these systems. aitken applies the test to fourteen visual systems for which the necessary data are available, and deduces for them a mean mass of . times that of the sun. for the spectroscopic binaries the masses are much greater. triple, quadruple and multiple stars are less frequent; but many exceedingly interesting objects of this class exist. epsilon lyræ is one, a double-double, or four stars as seen with slender telescopic power, and six or seven stars with larger instruments. sigma orionis and lyncis, also theta cancri and mu bootis are good examples of triple stars. chapter liv the star clusters from multiple stars the transition is natural to star clusters although the gap between these types of stellar objects is very broad. the familiar group of the winter sky known as the pleiades is a loose cluster, showing relatively very few stars even in telescopes or on photographic plates. the "beehive," or cluster known as praesepe in cancer, and a double group in the sword-handle of perseus, both just visible to the naked eye, are excellent examples of star clusters of the average type. when the moon is absent, they are easily recognized without a telescope as little patches of nebulous light; but every increase of optical power adds to their magnificence. then we come in regular succession to the truly marvelous globular clusters, that for instance in hercules. messier , a recent photograph of which, taken by ritchey with the -inch reflector on mount wilson, reveals an aggregation of more than , stars. but the finest specimens are in the southern hemisphere. sir john herschel spent much time investigating them nearly a century ago at the cape of good hope. his description of the cluster in the constellation of centaurus is as follows: "the noble globular cluster omega centauri is beyond all comparison the richest and largest object of the kind in the heavens. the stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the fifth or fourth to fifth magnitude, the minuteness of each star may be imagined." others of these clusters are so remote that the separate stars are not distinguishable, especially at the center, and their distances are entirely beyond our present powers of direct measurement, although methods of estimating them are in process of development. if gravitation is regnant among the uncounted components of stellar clusters, as doubtless it is, these stars must be in rapid motion, although our photographs of measurements have been made too recently for us to detect even the slightest motion in any of the component stars of a cluster. the only variations are changes of apparent magnitude, of a type first detected in a large number of stars in omega centauri, by bailey of harvard, who by comparison of photographs of the globular clusters was the first to find variable stars quite numerous in these objects. their unexplained variations of magnitude take place with great rapidity, often within a few hours. there are about a hundred of these globular clusters, and the radial velocities of ten of them have been measured by slipher and found to range from a recession of to an approach of kilometers per second. these excessive velocities are comparable with those found for the spiral nebulæ. shapley has estimated the distances of many of these bodies, which contain a large number of variable stars of the cepheid type. by assuming their absolute magnitudes equal to those of similar cepheids at known distances, he finds their distance represented by the inconceivably minute parallax of ". , corresponding to , light-years. this research also places the globular clusters far outside and independent of our galactic system of stars. the distribution of the globular clusters has also been investigated, and these interesting objects are found almost exclusively in but one hemisphere of the sky. its center lies in the rich star clouds of scorpio and sagittarius. success in finding the distances of these objects has made it possible to form a general idea of their distribution in three-dimensional space. the numerous variable stars in any one cluster are remarkable for their uniformity. accepting variables of this type as a constant standard of absolute brightness, and assuming that the differences of average magnitude of the variables in different clusters are entirely due to differences of distance, the relative distances of many clusters were ascertained with considerable accuracy. then it was found that the average absolute magnitude of the twenty-five brightest stars in a cluster is also a uniform standard, or about . magnitudes brighter than the mean magnitude of the variables. this new standard was employed in ascertaining the distances of other clusters not containing many variables. shapley further shows that the linear dimensions of the clusters are nearly uniform, and the proper relative positions in space are charted for sixty-nine of these objects. we can determine the scale of the charts, if we know the absolute brightness of our primary standard--the variable stars; and this is deduced from a knowledge of the distances of variables of the same type in our immediate stellar system. the most striking of all the globular clusters, omega centauri, comes out the nearest; nevertheless it is distant . kiloparsecs. a kiloparsec is a thousand parsecs, and is the equivalent of , light-years. at the inconceivable distance of sixty-seven kiloparsecs, or more than , light-years, is the most remote of the globular clusters, known to astronomers as n.g.c. , from its number in the catalogue which records its position in the sky, the new general catalogue of nebulæ by dreyer of armagh. the clusters are widely scattered, and their center of diffusion is about twenty kiloparsecs on the galactic plane toward the region of scorpio-sagittarius. marked symmetry with reference to this plane makes it evident that the entire system of globular clusters is associated with the galaxy itself. but to conceive of this it is necessary to extend our ideas of the actual dimensions of the galactic system. almost on the circumference of the great system of globular clusters our local stellar system is found, and it contains probably all the naked-eye stars, with millions of fainter ones. its size seems almost diminutive, only about one kiloparsec in diameter. the relative location of our local stellar system shows why the globular clusters appear to be crowded into one hemisphere only. shapley suggests that globular clusters can exist only in empty space, and that when they enter the regions of space tenanted by stars, they dissolve into the well-known loose clusters and the star clouds of the milky way. strangely the radial velocities of the clusters already observed show that most of them are traveling toward this region, and that some will enter the stellar regions within a period of the order of a hundred million years. the actual dimensions of globular clusters are not easy to determine, because the outer stars are much scattered. to a typical cluster, messier , shapley assigns a diameter of parsecs, which makes it comparable with the size of the stellar cluster to which the sun belongs. also on certain likely assumptions, he finds that the diameter of the great cluster in hercules, the finest one in our northern sky, is about parsecs, and its distance no less than , parsecs; in other words, the staggering distance that light would require , , years to travel over. while these distances can never be verified by direct measurement, it lends great weight to the three methods of indirect measurement, or estimation, ( ) from the diameter of the image of the clusters, ( ) from the mean magnitude of the twenty-five brightest stars, and ( ) from the mean magnitude of the short period variables, that they are in excellent agreement. chapter lv moving clusters recent researches on the proper motions of stars have brought to light many groups of stars whose individual members have equal and parallel velocities. eddington calls these moving clusters. the component stars are not exceptionally near to each other, and it often happens that other stars not belonging to the group are actually interspersed among them. they may be likened to double stars which are permanent neighbors, with some orbital motion, though exceedingly slow. the connection is rather one of origin; occurring in the same region of space, perhaps, from a single nebula. they set out with the same motion, and have "shared all the accidents of the journey together." their equality of motion is intact because any possible deflections by the gravitative pull of the stellar system is the same for both. mutual attraction may tend to keep the stars together, but their community of motion persists chiefly because no forces tend to interfere with it. in this way physically connected pairs may be separated by very great distances. so with the moving clusters: their component stars may be widely separate on the celestial sphere, but equality of their motions affords a clue to their association in groups. the hyades, a loose cluster in taurus, is a group of thirty-nine stars, within an area of about degrees square, which has been pretty fully investigated, especially by the late professor lewis boss; and no doubt many fainter stars in the same region will ultimately be found to belong to the same group. if we draw arrows on a chart representing the amount and direction of the proper motions of these stars, these arrows must all converge toward a point. this shows that their motions are parallel in space. it is a relatively compact group, and the close convergence shows that their individual velocities must agree within a small fraction of a kilometer per second. radial velocity measures of six of the component stars are in very satisfactory accord, giving . kilometers per second for the entire group. we can get the transverse velocity, and therefrom the distances of the stars, which are among the best known in the heavens, because the proper motions are very accurately known. the mean parallax of the group by this indirect method comes out ". , agreeing almost exactly with the direct determination by photography, ". , by kapteyn, de sitter, and others. eddington concludes that this taurus group is a globular cluster with a slight central condensation. its entire diameter is about ten parsecs, and its known motion enables us to trace its past and future history. it was nearest the sun , years ago, when it was at about half its present distance. boss calculated that in million years, if the present motion is maintained, this group will have receded so far as to appear like an ordinary globular cluster ' in diameter, its stars ranging from the ninth to the twelfth apparent magnitude. we may infer that the motion will likely continue undisturbed, because there are interspersed among the group many stars not belonging to it, and these have neither scattered its members nor sensibly interfered with the parallelism of their motion. another moving cluster, the similarity of proper motion of whose component stars was first pointed out by proctor, is known as the ursa major system, which embraces primarily beta, gamma, delta, epsilon, and zeta ursæ majoris, or five of the seven stars that mark the familiar dipper. but as many as eight other stars widely scattered are thought to belong to the same system, including sirius and alpha coronæ borealis. the absolute motion amounts to . kilometers per second, and is approximately parallel to the galaxy. turner has made a model of the cluster, which has the form of a flat disk. among stars of the orion type of spectrum are several examples of moving clusters. the pleiades together with many fainter stars form another moving cluster; as also do the brighter stars of orion, together with the faint cloudlike extensions of the great nebula in orion, whose radial velocity agrees with that of the stars in the constellation. still another very remarkable moving cluster is in perseus, first detected by eddington, and embracing eighteen stars, the brightest of which is alpha persei. the further discovery of moving clusters is most important in the future development of stellar astronomy, because with their aid we can find out the relative distribution, luminosity, and distance of very remote stars. so far the stars found associated in groups are of early types of spectrum; but the taurus cluster embraces several members equally advanced in evolution with the sun, and in the more scattered system of ursæ major there are three stars of type f. "some of these systems," eddington concludes, "would thus appear to have existed for a time comparable with the lifetime of an average star. they are wandering through a part of space in which are scattered stars not belonging to their system--interlopers penetrating right among the cluster stars. nevertheless, the equality of motion has not been seriously disturbed. it is scarcely possible to avoid the conclusion that the chance attractions of stars passing in the vicinity have no appreciable effect on stellar motions; and that if the motions change in course of time (as it appears they must do) this change is due, not to the passage of individual stars, but to the central attraction of the whole stellar universe, which is sensibly constant over the volume of space occupied by a moving cluster." chapter lvi the two star streams consider the ships on the atlantic voyaging between europe and america: at any one time there may be a hundred or more, all bound either east or west, some moving in interpenetrating groups, individuals frequently passing each other, but rarely or never colliding. we might say, there are two great streams of ships, one moving east and the other west. now in place of each ship, imagine a hundred ships, and magnify their distances from each other to the vast distances that the stars are from each other, and all in motion in two great streams as before. this will convey some idea of the relatively recent discovery, called by astronomers "star-streaming." early in this century the investigation of moving clusters began to reveal the fact that the motions of the stars were not at random throughout the universe, and about kapteyn was the first to show that the stellar motions considered in great groups are very far from being haphazard, but that the stars tend to travel in two great streams, or favored directions. this was ascertained by analyzing the proper motions of stars in the sky, many thousands of them, and correcting all for the effect which the known motion of the sun would have upon them. the corrected motion, or part that is left over, is known as the star's own motion, or _motus peculiaris_. this important investigation was very greatly facilitated by the general catalogue of , stars well distributed over the entire sky, the work of the late professor boss. it was published by the carnegie institution of washington, and includes all stars down to the sixth magnitude. boss was very critical in the matter of stellar positions and proper motions and his work is the most accurate at present available. excluding stars of the orion type and the known members of moving clusters, kapteyn's investigation was based on , stars, which he divided into seventeen regions of the sky, each northern region having an antipodal one in the southern hemisphere. mathematical analysis of these regions showed them all in substantial agreement, with one exception, and enabled kapteyn to draw the conclusion that the stars of one stream, called drift i, move with a speed of thirty-two kilometers per second, while those of the other, drift ii, travel with a speed of eighteen kilometers per second. their directions are not, like those of east and west bound ships, degrees from each other, but are inclined at an angle of degrees. drift i embraces about three-fifths of the stars, and drift ii the remaining two-fifths. quite as remarkable as the drifts themselves is the fact that the relative motion of the two is very closely parallel to the plane of the milky way. this epochal research has very great significance in all investigations of stellar motions, and it has been verified in various ways, particularly by the astronomer royal, sir frank dyson, who limited the stars under consideration to , in number, but all having very large proper motions. in this way the two streams are even more characteristically marked. but radial velocity determinations afford the ultimate and most satisfactory test, and campbell has this investigation in hand, classifying the stars in their streaming according to the type. type a stars are so far found to be confirmatory. turning to the question of physical differences between the stars of the two streams, eddington inquires into the average magnitude of the stars in both drifts, and their spectral type. also whether they are distributed at the same distance from the sun, and in the same proportion in all parts of the sky. his conclusion is that there is no important difference in the magnitudes of the stars constituting the two drifts. regarding their spectra, stars of early and late types are found in both streams, with a somewhat higher proportion of late types among the stars of drift ii than those of drift i. campbell and moore of the lick observatory have investigated seventy-three planetary nebulæ which exhibit the phenomena of star-streaming, and have motions which are characteristic of the stars. dealing with the very important question whether the two streams are actually intermingled in space, eddington finds them nearly at the same mean distance and thoroughly intermingled, and there is no possible hypothesis of drifts i and ii passing one behind the other in the same line of sight. a third drift, to which all the orion stars belong, is under investigation, together with comprehensive analysis of the drifts according to the spectral type of all the stars included. the farther research on star-streaming is pushed, the more it becomes evident that a third stream, called drift o, is necessary, especially to include b-type stars. the farther we recede from the sun, the more this drift is in evidence. at the average distances of b-type stars, the observed motions are almost completely represented by drift o alone. halm of cape town concludes from recent investigations that the double-drift phenomena (drifts i and ii) is of a distinctly _local_ character, and concerns chiefly the stars in the vicinity of the solar system; while stars at the greatest distances from the sun belong preeminently to drift o. the -inch reflector on mount wilson gathers sufficient light so that the spectra of very faint stars can be photographed, and a discussion of velocities derived in this manner has shown that kapteyn's two star streams extend into space much farther than it was possible to trace them with the nearer stars. star-streaming, then, may be a phenomenon of the widest significance in reference to the entire universe. as to the fundamental causes for the two opposite and nearly equal star streams, it is early perhaps to even theorize upon the subject. eddington, however, finds a possible explanation in the spiral nebulæ, which are so numerous as to indicate the certainty of an almost universal law compelling matter to flow in these forms. why it does so, we cannot be said to know; but obviously matter is either flowing into the nucleus from the branches of the spiral, or it is flowing out from the nucleus into the branches. which of the two directions does not matter, because in either case there would be currents of matter in opposite directions at the points where the arms merge in the central aggregation. the currents continue through the center, because the stars do not interfere with one another's paths. as eddington concludes: "there then we have an explanation of the prevalence of motions to and fro in a particular straight line; it is the line from which the spiral branches start out. the two star streams and the double-branched spirals arise from the same cause." chapter lvii the galaxy or milky way grandest of all the problems that have occupied the mind of man is the distribution of the stars throughout space. to the earliest astronomers who knew nothing about the distances of the stars, it was not much of a problem because they thought all the fixed stars were attached to a revolving sphere, and therefore all at essentially the same distance; a very moderate distance, too. even kepler held the idea that the distances of individual stars from each other are much less than their distances from our sun. thomas wright, of durham, england, seems to have been the first to suggest the modern theory of the structure of the stellar universe, about the middle of the eighteenth century. his idea was taken up by kant who elaborated it more fully. it is founded on the galaxy, the basal plane of stellar distribution, just as the ecliptic is the fundamental circle of reference in the solar system. what is the galaxy or milky way? here is a great poet's view of the most poetic object in all nature: a broad and ample road, whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powder'd with stars. _milton, p. l._ vii, . were the earth transparent as crystal, so that we could see downward through it and outward in all directions to the celestial sphere, the galaxy or milky way would appear as a belt or zone of cloudlike luminosity extending all the way round the heavens. as the horizon cuts the celestial sphere in two, we see at anyone time only one-half of the milky way, spanning the dome of the sky as a cloudlike arch. as the general plane of the galaxy makes a large angle with our equator, the milky way is continually changing its angle with the horizon, so that it rises at different elevations. one-half of the milky way will always be below our horizon, and a small region of it lies so near the south pole of the heavens that it can never be seen from medium northern latitudes. galileo was the first to explain the fundamental mystery of this belt, when he turned his telescope upon it and found that it was not a continuous sheet of faint light, as it seemed to be, but was made up of countless numbers of stars, individually too faint to be visible to the naked eye, but whose vast number, taken in the aggregate, gave the well-known effect which we see in the sky. in some regions, as perseus, the stars are more numerous than in others, and they are gathered in close clusters. the larger the telescope we employ, the greater the number of stars that are seen as we approach the galaxy on either side; and the farther we recede from the galaxy and approach either of its poles fewer and fewer stars are found. indeed, if all the stars visible in a -inch telescope could be conceived as blotted out, nearly all the stars that are left would be found in the galaxy itself. the naked eye readily notes the variations in breadth and brightness of the galactic zone. nearly a third of it, from scorpio to cygnus, is split into two divisions nearly parallel. in many regions its light is interrupted, especially in centaurus, where a dark starless region exists, known as the "coal sack." sir john herschel, who followed up the stellar researches of his father, sir william, in great detail, places the north pole of the galactic plane in declination degrees n., and right ascension h. m. this makes the plane of the milky way lie at an angle of about degrees with the ecliptic, which it intersects not far from the solstices. now kant, in view of the two great facts about the galaxy known in his time, ( ) that it wholly encircles the heavens, and ( ) that it is composed of countless stars too faint to be individually visible to the naked eye, drew the safe conclusions that the system of the stars must extend much farther in the direction of the milky way than in other directions. this theory of kant was next investigated from an observational standpoint by sir william herschel, the ultimate goal of whose researches was always a knowledge of the construction of the heavens. the present conclusion is that we may regard the stellar bodies of the sidereal universe as scattered, without much regard to uniformity, throughout a vast space having in general the shape of a thick watch, its thickness being perhaps one-tenth its diameter. on both sides of this disk of stars, and clustered about the poles of the sidereal system are the regions occupied by vast numbers of nebulæ. the entire visible universe, then, would be spheroidal in general shape. the plane of the milky way passes through the middle of this aggregation of stars and nebulæ, and the solar system is near the center of the milky way. throughout the watch-form space the stars are clustered irregularly, in varied and sometimes fantastic forms, but without approach to order or system. if we except some of the star groups and star clusters and consider only the naked-eye stars, we find them scattered with fair approach to uniformity. [illustration: star clouds and black holes in sagittarius. the dark rifts and lanes resemble those in the nearby milky way. (_photo, yerkes observatory._)] [illustration: the great nebula of andromeda, largest (apparently) of all the spiral nebulÆ. this nebula can be seen very faintly with the naked eye, but no telescope has yet resolved it into separate stars. (_photo, yerkes observatory._)] the watch-shaped disk is not to be understood as representing the actual form of the stellar system, but only in general the limits within which it is for the most part contained. a vigorous attack on the problem of the evolution and structure of the stellar universe as a whole is now being conducted by cooperation of many observatories in both hemispheres. it is known as the kapteyn "plan of selected areas," embracing regions which are distributed regularly over the entire sky. besides this a special plan includes forty-six additional regions, either very rich or extremely poor in stars, or to which other interest attaches. of all investigators kapteyn has gone into the question of our precise location in the milky way most thoroughly, concluding that the solar system lies, not at the center in the exact plane, but somewhat to the north of the galaxy. discussing the sirian stars he finds that if stars of equal brightness are compared, the sirians average nearly three times more distance from the sun than those of the solar type. so, probably, the sirians far exceed the solars in intrinsic brightness. farther, kapteyn concludes that the galaxy has no connection with our solar system, and is composed of a vast encircling annulus or ring of stars, far exceeding in number the stars of the great central solar cluster, and everywhere exceedingly remote from these stars, as well as differing from them in physical type and constitution. so it would be mainly the mere element of distance that makes them appear so faint and crowded thickly together into that gauzy girdle which we call the galaxy. the milky way reveals irregularities of stellar density and star clustering on a large scale, with deep rifts between great clouds of stars. modern photographs, particularly those of barnard in sagittarius, make this very apparent. within the milky way, nearly in its plane and almost central, is what eddington terms the inner stellar system, near the center of which is the sun. surrounding it and near its plane are the masses of star clouds which make up the milky way. whether these star clouds are isolated from the inner system or continuous with it, is not yet ascertained. the vast masses of the milky way stars are very faint, and we know nothing yet as to their proper motions, their radial motions, or their spectra. probably a few stars as bright as the sixth magnitude are actually located in the midst of the milky way clusters, the fainter ninth magnitude stars certainly begin the milky way proper, while the stars of the twelfth or thirteenth magnitude carry us into the very depths of the galaxy. it is now pretty generally believed that many of the dark regions of the milky way are due not to actual absence of stars so much as to the absorption of light by intervening tracts of nebulous matter on the hither side of the galactic aggregations and, probably in fact, within the oblate inner stellar system itself. easton has made many hundred counts of stars in galactic regions of cygnus and aquila where the range of intensity of the light is very marked; in fact, the star density of the bright patches of the galaxy is so far in excess of the density adjacent and just outside the milky way, that the conclusion is inevitable that this excess is due to the star clouds. of the distance of the milky way we have very little knowledge. it is certainly not less than , parsecs, and more likely , parsecs, a distance over which light would travel in about , years. quite certainly all parts of the galaxy are not at the same distance, and probably there are branches in some regions that lie behind one another. while the general regions of the nebulæ are remote from the galactic plane, the large irregular nebulæ, as the trifid, the keyhole, and the omega nebulæ, are found chiefly in the milky way. in addition to the irregular nebulæ many types of stellar objects appear to be strongly condensed toward the milky way, but this may be due to the inner stellar system, rather than a real relation to galactic formations. quite different are the magellanic clouds, which contain many gaseous nebulæ and are unique objects of the sky, having no resemblance to the true spiral nebulæ which, as a rule, avoid the galactic regions. worthy of note also is the theory of easton that the milky way has itself the form of a double-branched spiral, which explains the visible features quite well, but is incapable of either disproof or verification. the central nucleus he locates in the rich galactic region of cygnus, with the sun well outside the nucleus itself. by combining the available photographs of the galaxy, he has produced a chart which indicates in a general way how the stellar aggregations might all be arrayed so as to give the effect of the galaxy as we see it. shapley, at mount wilson, has studied the structure of the galactic system, in which he has been aided by mrs. shapley. an interesting part of this work relates to the distribution of the spiral nebulæ, and to certain properties of their systematic recessional motion, suggesting that the entire galactic system may be rapidly moving through space. apparently the spiral nebulæ are not distant stellar organizations or "island universes," but truly nebular structures of vast volume which in general are actively repelled from stellar systems. a tentative cosmogonic hypothesis has been formulated to account for the motions, distribution, and observed structure of clusters and spiral nebulæ. an additional great problem of the galaxy is a purely dynamical one. doubtless it is in some sort of equilibrium, according to eddington, that is to say, the individual stars do not oscillate to and fro across the stellar system in a period of million years, but remain concentrated in clusters as at present. poincaré has considered the entire milky way as in stately rotation, and on the assumption that the total mass of the inner stellar system is , , , times the sun's mass, and that the distance of the milky way is , parsecs, the angular velocity for equilibrium comes out ". per century. that is to say, a complete revolution would take place in about million years. chapter lviii star clouds and nebulÆ from star clusters to nebulæ, only a century ago, the transition was thought to be easy and immediate. accuracy in determining the distances of stars was just beginning to be reached, the clusters were obviously of all degrees of closeness following to the verge of irresolvability, and it was but natural to jump to the conclusion that the mystery of the nebulæ consisted in nothing but their vaster distance than that of clusters, and it was believed that all nebulæ would prove resolvable into stars whenever telescopes of sufficiently great power could be constructed. but the development of the spectroscope soon showed the error of this hypothesis, by revealing bright lines in the nebular spectra showing that many nebulæ emit light that comes from glowing incandescent gas, not from an infinitude of small stars. in pre-telescope days nothing was known about the nebulæ. the great nebula in andromeda, and possibly the great nebula in orion, are alone visible to the naked eye, but as thus seen they are the merest wisps of light, the same as the larger clusters are. galileo, huygens and other early users of the telescope made observations of nebulæ, but long-focus telescopes were not well adapted to this work. simon mayer has left us the first drawing of a nebula, the orion nebula as he saw it in . the vast light-gathering power of the reflectors built by sir william herschel first afforded glimpses of the structure of the nebulæ, and if his drawings are critically compared with modern ones, no case of motion with reference to the stars or of change in the filaments of the nebulæ themselves has been satisfactorily made out. only very recently has the distance of a nebula been determined, and the few that have been measured seem to indicate that the nebulæ are at distances comparable with the stars. of all celestial objects the nebulæ fill the greatest angles, so that we are forced to conclude, with regard to the actual size of the greater nebulæ as they exist in space, that they far surpass all other objects in bulk. photography invaded the realm of the nebulæ in , when dr. henry draper secured the first photograph of the nebula of orion. theoretically photography ought to help greatly in the study of the nebulæ, and enable us in the lapse of centuries to ascertain the exact nature of the changes which must be going on. the differences of photographic processes, of plates, of exposure and development produce in the finished photograph vastly greater differences than any actual changes that might be going on, so that we must rely rather on optical drawings made with the telescope, or on drawings made by expert artists from photographs with many lengths of exposure on the same object. the great work on nebulæ and star clusters recently concluded by bigourdan of the paris observatory and published in five volumes received the award of the gold medal of the royal astronomical society. while d'arrest measured about , nebulæ, and sir john herschel about double that number in both hemispheres, bigourdan has measured about , . his work forms an invaluable lexicon of information concerning the nebulæ. classification of the nebulæ is not very satisfactory, if made by their shapes alone. there are perhaps fifteen thousand nebulæ in all that have been catalogued, described, and photographed. dreyer's new general catalogue (n.g.c.) is the best and most useful. many of the nebulæ, especially the large ones, can only be classified as irregular nebulæ. the orion nebula is the principal one of this class, revealing an enormous amount of complicated detail, with exceptional brilliancy of many regions and filaments. an extraordinary multiple star, theta orionis, occupies a very prominent position in the nebula, and photographs by pickering have brought to light curved filaments, very faint and optically invisible, in the outlying regions which give the orion nebula in part a spiral character. but the delicate optical wisps of this nebula are well seen, even in very small telescopes. its spectrum yields hydrogen, helium and nitrogen. the orion nebula is receding from the earth about eleven miles in every second. keeler and campbell have shown that nearly every line of the nebular spectrum is a counterpart of a prominent dark line in the spectrum of the brighter stars of the constellation of orion. a recent investigator of the distribution of luminosity in the great nebula of orion finds that radiations from nebulium are confined chiefly to the huygenian region of the nebula and its immediate neighborhood. photography has revealed another extraordinary nebula or group of nebulæ surrounding the stars in the pleiades, which the deft manipulation of barnard has brought to light. all the stars and the nebula are so interrelated that they are obviously bound together physically, as the common proper motion of the stars also appears to show. also in the constellation cygnus, barnard has discovered very extensive nebulosities of a delicate filmy cloudlike nature which are wholly invisible with telescopes, but very obvious on highly sensitive plates with long exposures. another class of these objects are the annular and elliptic nebulæ which are not very abundant. the southern constellation grus, the crane, contains a fine one, but by far the best example is in the constellation lyra. it is a nearly perfect ring, elliptic in figure, exceedingly faint in small telescopes; but large instruments reveal many stars within the annulus, one near the center which, although very faint to the eye, is always an easy object on the photographic plate, because it is rich in blue and violet rays. the parallax of the ring nebula in lyra comes out only one-sixth of that of the planetary nebulæ, and the least greatest diameters of this huge continuous ring are and times the orbit of neptune. planetary nebulæ and nebulous stars are yet another class of nebulæ, for the most part faint and small, resembling in some measure a planetary disk or a star with nebulous outline. practically all are gaseous in composition, and have large radial velocities. probably they are located within our own stellar system. the parallaxes of several of them have been measured by van maanen: one of the very small angle ". , which enables us to calculate the diameter of this faint but interesting object as equal to nineteen times the orbit of neptune. chapter lix the spiral nebulÆ last and most important of all are the spiral nebulæ. the finest example is in the constellation canes venatici, and its spiral configuration was first noted by lord rosse, an epoch-making discovery. the convolutions of its spiral are filled with numerous starlike condensations, themselves engulfed in nebulosity. photography possesses a vast advantage over the eye in revealing the marvelous character of this object, an inconceivably vast celestial whirlpool. naturally the central regions of the whorl would revolve most swiftly, but no comparison of drawings and photographs, separated by intervals of many years, has yet revealed even a trace of any such motion. the number of large spiral nebulæ is not very great; the largest of all is the great nebula of andromeda, whose length stretches over an arc of seven times the breadth of the moon, and its width about half as great. this nebula is a naked-eye object near eta andromedæ, and it is often mistaken for a comet. optically it was always a puzzle, but photographs by roberts of england first revealed the true spiral, with ringlike formations partially distinct, and knots of condensing nebulosity as of companion stars in the making. while its spectrum shows the nongaseous constitution of this nebula, no telescope has yet resolved it into component stars. systematic search for spiral nebulæ by keeler, and later continued by perrine, at the lick observatory, with the -inch crossley reflector, disclosed the existence of vast numbers of these objects, in fact many hundreds of thousands by estimation; so that, next to the stars, the spiral nebulæ are by far the most abundant of all objects in the sky. they present every phase according to the angle of their plane with the line of sight, and the convolutions of the open ones are very perfectly marked. many are filled with stars in all degrees of condensation, and the appearance is strongly as if stars are here caught in every step of the process of making. the vast multitude of the spiral nebulæ indicates clearly their importance in the theory of the cosmogony, or science of the development of the material universe. curtis of the lick observatory has lately extended the estimated number of these objects to , . he has also photographed with the crossley reflector many nebulæ with lanes or dark streaks crossing them longitudinally through or near the center. these remarkable streaks appear as if due to opaque matter between us and the luminous matter of the nebula beyond. perhaps a dark ring of absorptive or occulting matter encircles the nebula in nearly the same plane with the luminous whorls. duncan has employed the -inch mount wilson reflector in photographing bright nebulæ and star clusters in the very interesting regions of sagittarius. one of these shows unmistakable dark rifts or lanes in all parts of the nebula, resembling the dark regions of the neighboring milky way. pease of mount wilson has recently employed the -inch and the -inch reflectors of the mount wilson observatory to good advantage in photographing several hundred of the fainter nebulæ. many of these are spirals, and others present very intricate and irregular forms. a search was made for additional spirals among the smaller nebulæ along the galaxy, but without success. several of the supposedly variable nebulæ are found to be unchanging. many nights in each month when the moon is absent are devoted to a systematic survey of the smaller nebulæ and their spectra by photography. the visible spiral figure of all these objects is a double-branched curve, its two arms joining on the nucleus in opposing points, and coiling round in the same geometrical direction. the spiral nebulæ, as to their distribution, are remote from the galaxy, and the north galactic polar region contains a greater aggregation than the south. the distances of the spiral nebulæ are exceedingly great. they lie far beyond the planetary and irregular gaseous nebulæ, like that of orion, which are closely related to the stars forming part of our own system. possibly the spiral nebulæ are exterior or separate "island universes." if so, they must be inconceivably vast in size, and would develop, not into solar systems, but into stellar clusters. the enormous radial velocities of the spiral nebulæ, averaging to kilometers per second, or twenty-fold that of the stars, tend to sustain the view that they may be "island universes," each comparable in extent with the universe of stars to which our sun belongs. recent spectroscopic observations of the nebulæ applying the principle of doppler have revealed high velocities of rotation. slipher of the lowell observatory made the first discovery of this sort and van maanen of mount wilson has detected in the great ursa major spiral, no. in messier's catalogue, a speed of rotation at five minutes of arc from the center that would correspond to a complete period in , years. as was to be expected, the nebula does not rotate as a rigid body, but the nearer the center the greater the angular velocity, and van maanen finds evidence of motion along the arms and away from the center. these great velocities appear to belong to the spiral nebulæ as a class, and not to other nebulæ. thirteen nebulæ investigated by keeler are as a whole almost at rest relatively to our system, as are the large irregular objects in orion, and the trifid nebula. this would seem to indicate that the spiral nebulæ form systems outside our own and independent of it. quite different from the spirals in their distribution through space are the planetary nebulæ. the spirals follow the early general law of nebulæ arrangement, that is, they are concentrated toward the poles of the galaxy; but the planetary nebulæ, on the other hand, are very few near the poles and show a marked frequency toward the galactic plane. campbell and moore have found spectroscopic evidence of internal rotatory motion in a large proportion of the planetary nebulæ. the distribution of the nebulæ throughout space, like that of the stars, is still under critical investigation, but the location of vast numbers of the more compact nebulæ on the celestial sphere is very extraordinary. the milky way appears to be the determining plane in both cases; the nearer we approach it the more numerous the stars become, whereas this is the general region of fewest nebulæ and they increase in number outward in both directions from the galaxy, and toward both poles of the galactic circle. obviously this relation, or contra-relation of stars and nebulæ on such a vast scale is not accidental, and it also must be duly accounted for in the true theory of the cosmogony. the nebulæ which are found principally in and near the milky way are the large irregular nebulæ, and vast nebulous backgrounds, like those photographed by barnard in scorpio, taurus and elsewhere, as well as the keyhole, omega, and trifid nebulæ. allied to these backgrounds are doubtless some of the dark galactic spaces, radiating little or no intrinsic light, and absorbing the light of the fainter stars beyond them. a peculiar veiled or tinted appearance has been remarked in some cases visually, and examination of the photographs strongly confirms the existence of absorbing nebulosity. the spiral nebulæ are so abundant, and so much attention is now being given to them, both by observers and mathematicians, that their precise relation to the stellar systems must soon be known; that is, whether they are comparatively small objects belonging to the stellar system, or independent systems on the borders of the stellar system, or as seems more likely, vast and exceedingly remote galaxies comparable with that of the milky way itself. our knowledge of the motions of the spirals, both radial and angular, is increasing rapidly, and must soon permit accurate general conclusions to be drawn. chapter lx cosmogony down to the middle of the last century and later, it was commonly believed that in the beginning the cosmos came into being by divine fiat substantially as it is. previously the earth had been "without form and void," as in the scripture. had it not been for the growth and gradual acceptance of the doctrine of evolution, and its reactionary effect upon human thought, it is conceivable that the early view might have persisted to the present day; but now it is universally held that everything in the heavens above and the earth beneath is subject more or less to secular change, and is the result of an orderly development throughout indefinite past ages, a progressive evolution which will continue through indefinite aeons of the future. in the writings of the greek philosophers, and down through the middle ages we find the idea of an original "chaos" prevailing, with no indication whatever of the modern view of the process by which the cosmos came to be what they saw it and as it is to-day. if we go still farther back, there is no glimmer of any ideas that will bear investigation by scientific method, however interesting they may be as purely philosophical conceptions. many ancient philosophers, among them anaxagoras, democritus, and anaximenes, regarded the earth as the product of diffused matter in a state of the original chaos having fallen together haphazard, and they even presumed to predict its future career and ultimate destiny. in anaximander and anaximenes alone do we find any conception of possible progress; their thought was that as the world had taken time to become what it is, so in time it would pass, and as the entire universe had undergone alternate renewal and destruction in the past, that would be its history in the future. aristotle, ptolemy, and others appear to have held the curious notion that although everything terrestrial is evanescent, nevertheless the cosmos beyond the orbit of the moon is imperishable and eternal. by tracing the history of the intellectual development of europe we may find why it was that scientific speculation on the cosmogony was delayed until the th century, and then undertaken quite independently by three philosophers in three different countries. swedenborg, the theologian, set down in due form many of the principles that underlie the modern nebular hypothesis. thomas wright of durham whose early theory of the arrangement of stars in the galaxy we have already mentioned, speculated also on the origin and development of the universe, and his writings were known to kant, who is now regarded as the author of the modern nebular hypothesis. this presents a definite mechanical explanation of the development and formation of the heavenly bodies, and in particular those composing the solar system. kant was illustrious as a metaphysician, but he was a great physicist or natural philosopher as well, and he set down his ideas regarding the cosmogony with precision. learned in the philosophy of the ancients, he did not follow their speculative conceptions, but merely assumed that all the materials from which the bodies of the solar system have been fashioned were resolved into their original elements at the beginning, and filled all that part of space in which they now move. true, this is pretty near the chaos of the greeks, but kant knew of the operation of the newtonian law of gravitation, which the greeks did not. as a natural result of gravitative processes, kant inferred that the denser portions of the original mass would draw upon themselves the less dense portions, whirling motions would be everywhere set up, and the process would continue until many spherical bodies, each with a gaseous exterior in process of condensation, had taken the place of the original elements which filled space. in this manner kant would explain the sameness in direction of motion, both orbital and axial, of all the planets and satellites of our system. but many philosophers are of the opinion that kant's hypothesis would result, not in the formation of such a collection of bodies as the solar system is, but rather in a single central sun formed by common gravitation toward a single center. from quite another viewpoint the work of the elder herschel is important here. no one knew the nebulæ from actual observation better than he did; but, while his ideas about their composition were wrong, he nevertheless conceived of them as gradually condensing into stars or clusters of stars. and it was this speculative aspect of the nebulæ, not as a possible means of accounting for the birth and development of the solar system, which constitutes herschel's chief contribution to the nebular hypothesis. classifying the nebulæ which he had carefully studied with his great telescopes, it seemed obvious to him that they were actually in all the different stages of condensation, and subsequent research has strongly tended to substantiate the herschelian view. then came laplace, who took up the great hypothesis where kant and herschel had left it, added new and important conceptions in the light of his mature labors as mathematician and astronomer, and put the theory in definitive form, such that it has ever since been known under the name of laplacian nebular hypothesis. for reasons like those that prevailed with kant, he began the evolution of the solar system with the sun already formed as the center, but surrounded by a vast incandescent atmosphere that filled all the space which the sun's family of planets now occupy. this entire mass, sun, atmosphere, and all, he conceived to have a stately rotation about its axis. with rotation of the mass and slow reduction of temperature in its outer regions, there would be contraction toward the solar center, and an increase in velocity of rotation until the whole mass had been much reduced in diameter at its poles and proportionately expanded at its equator. when the centrifugal force of the outer equatorial masses finally became equal to the gravitational forces of the central mass, then these conjoined outer portions would be left behind as a ring, still revolving at the velocity it had acquired when detached. the revolution of the entire inner mass goes on, its velocity accelerating until a similar equilibration of forces is again reached, when a second rotating ring is left behind. laplace conceived the process as repeated until as many rings had been detached as there are individual planets, all central about the sun, or nearly so. in all, then, we should have nine gaseous rings; the outer ones preceding the inner in formation, but not all existing as rings at the same time. radiation from the ring on all sides would lead to rapid contraction of its mass, so that many nuclei of condensation would form, of various sizes, all revolving round the central sun in practically the same period. laplace conceived the evolution of the ring to proceed still farther till the largest aggregation in it had drawn to itself all the other separate nuclei in the ring. this, then, was the planet in embryo, in effect a diminutive sun, a secondary incandescent mass endowed with axial rotation in the same direction as the parent nebula. with reduction of temperature by radiation, polar contraction and equatorial expansion go on, and planetary rings are detached from this secondary mass in exactly the same way as from the original sun nebula. and these planetary rings are, in the laplacian hypothesis, the embryo moons or planetary satellites, all revolving round their several planets in the same direction that the planets revolve about the sun. in the case of one of the planetary rings, its formation was so nearly homogeneous throughout that no aggregation into a single satellite was possible; all portions of the ring being of equal density, there was no denser region to attract the less dense regions, and in this manner the rings of saturn were formed, in lieu of condensation into a separate satellite. similarly in the case of the primal solar ring that was detached next after the jovian ring; there was such a nice balancing of masses and densities that, instead of a single major planet, we have the well-known asteroidal ring, composed of innumerable discrete minor planets. this, then, in bare outline, is the laplacian nebular hypothesis, and it accounted very well for the solar system as known in his day; the fairly regular progression of planetary distances; their orbits round the sun all nearly circular and approximately in a single plane; the planetary and satellite revolutions in orbit all in the same direction; the axial rotations of planets in the same direction as their orbital revolutions; and the plane of orbital revolution of the satellites practically coinciding with the plane of the planet's axial rotation. but the principle of conservation of energy was, of course, unknown to laplace, nor had the mechanical equivalence of heat with other forms of energy been established in his day. in , lane of washington first demonstrated the remarkable law that a gaseous sphere, in process of losing heat by radiation and contraction because of its own gravity, actually grows hotter instead of cooler, as long as it continues to be gaseous, and not liquid or solid. so there is no need of postulating with laplace an excessively high temperature of the original nebula. the chief objection to laplace's hypothesis by modern theorists is that the detachment of rings, though possible, would likely be a rare occurrence; protuberances or lumps on the equatorial exterior of a swiftly revolving mass would be more likely, and it is much easier to see how such masses would ultimately become planets than it is to follow the disruption of a possible ring and the necessary steps of the process by which it would condense into a final planet. the continued progress of research in many departments of astronomy has had important bearing on the nebular hypothesis, and we may rest assured that this hypothesis in somewhat modified form can hardly fail of ultimate acceptance, though not in every essential as its great originator left it. lord rosse's discovery of spiral nebulæ, followed up by keeler's photographic search for these bodies, revealing their actual existence in the heavens by the hundreds of thousands, has led to another criticism of the laplacian theory. could laplace have known of the existence of these objects in such vast numbers, his hypothesis would no doubt have been suitably modified to account for their formation and development. it is generally considered that the ring of saturn suggested to laplace the ring feature in his scheme of origin of planets and satellites; so far as we know, the saturnian ring is unique, the only object of its kind in the heavens. whereas, next to the star itself, the spiral nebula is the type object which occurs most frequently. a theory, therefore, which will satisfactorily account for the origin and development of spiral nebulæ must command recognition as of great importance in the cosmogony. such a theory has been set forth by chamberlin and moulton in their planetesimal hypothesis, according to which the genesis of spiral nebulæ happens when two giant suns approach each other so closely that tide-producing effects take place on a vast scale. these suns need not be luminous; they may perhaps belong to the class of dark or extinguished suns. the evidences of the existence of such in vast numbers throughout the universe is thought to be well established. now, on close approach, what happens? there will be huge tides, and the nearer the bodies come to each other, the vaster the scale on which tides will be formed. if the bodies are liquid or gaseous, they will be distorted by the force of gravitation, and the figure of both bodies will become ellipsoidal; and at last under greater stress, the restraining shell of both bodies will burst asunder on opposite sides in streams of matter from the interior. in this manner the arms of the spiral are formed. as chamberlin puts it: "if, with these potent forces thus nearly balanced, the sun closely approaches another sun, or body of like magnitude ... the gravity which restrains this enormous elastic power will be reduced along the line of mutual attraction. at the same time the pressure transverse to this line of relief will be increased. such localized relief and intensified pressure must bring into action corresponding portions of the sun's elastic potency, resulting in protuberances of corresponding mass and high velocity." only a fraction of one per cent of the sun's mass ejected in this fashion would be sufficient to generate the entire planetary system. nuclei or knots in the arms of the spiral gradually grew by accretion, the four interior knots forming mercury, venus, the earth, and mars. the earth knot was a double one, which developed into the earth-moon system. the absence of a dominating nucleus beyond mars accounts for the zone of the asteroids remaining in some sense in the original planetesimal condition. the vaster nuclei beyond mars gradually condensed into jupiter, saturn, uranus, and neptune; and lesser nuclei related to the larger ones form the systems of moons or satellites. the orbits of the planetesimals and the planetary and satellite nuclei would be very eccentric, forming a confusion of ellipses with frequently crossing paths. collisions would occur, and the nuclei would inevitably grow by accretion. each planet, then, would clear up the planetesimals of its zone; and moulton shows that this process would give rise to axial revolution of the planet in the same direction as its orbital revolution. the eccentricities would finally disappear, and the entire mass would revolve in a nearly circular orbit. rotation twists the streams into the spiral form, and the huge amounts of wreckage from the near-collision are thrown into eddies. the fragments or particles (planetesimals) which have given the name to the theory, begin their motion round their central sun in elliptical paths as required by gravitation. the form of the spiral is preserved by the orbital motion of its particles. there is a gradual gathering together of the planetesimals at points or nodes of intersection, and these become aggregations of matter, nuclei that will perhaps become planets, though more likely other stars. the appulse or near approach is but one of the methods by which the spiral nebulæ may have come into existence. the planetesimal hypothesis would seem to account for the formation of many of these objects as we see them in the sky, though perhaps it is hardly competent to replace entirely the laplacian hypothesis of the formation of the solar system, which would appear to be a special case by itself. it will be observed that while the laplacian hypothesis is concerned in the main with the progressive development of the solar system, and systems of a like order surrounding other stellar centers, whose existence is highly probable, the origin and development of the stellar universe is a vaster problem which can only be undertaken and completed in its broadest bearings when the structure of the stellar universe has been ascertained. darwin's important investigations in - on tidal friction may be here related. before his day acceptance of the ring-theory of development of the moon from the earth had scarcely been questioned; but his recondite mathematical researches on the tidal reaction between a central yielding mass and a body revolving round it brought to light the unsuspected effect of tides raised upon both bodies by their mutual attraction. the type of tides here meant is not the usual rise and fall of the waters of the ocean, but primeval tides in the plastic material of which the earth in its early history was composed. the newtonian law of gravitation afforded a complete explanation of the rise and fall of the waters of the oceans, but as applied to the motions of planets and satellites by the lagrangian formulæ, it presupposed that all these bodies are rigid and unyielding. however, mutual tides of phenomenal height in their early plastic substances must have been a necessary consequence of the action of the newtonian law, and they gradually drew upon the earth's rotational moment of momentum. in its very early history, before there was any moon to produce tides, the earth rotated much more rapidly, that is, the day was very much shorter than now, probably about five or six hours long. and with the rapid whirling, it was not a laplacian ring that was detached, but a huge globular mass was separated from the plastic earth's equator. darwin shows that the gravitative interaction of the two bodies immediately began to raise tides of extraordinary height in both, therefore tending to slow down the rotational periods of both bodies. action and reaction being equal, the reaction at once began driving the moon away from the earth and thereby lengthening its period of revolution. so small was the mass of the moon and so near was it to the earth, that its relative rotational energy was in time completely used up, and the moon has ever since turned her constant face toward us. tides of sun and moon in the plastic earth, acting through the ages, slowed down the earth's rotation to its present period, or the length of the day. moulton, however, has investigated the tidal theory of the origin of the moon in the light of the planetesimal hypothesis, concluding that the moon never was part of the earth and separated therefrom by too rapid rotation of the earth, but that the distance of the two bodies has always been the same as now. the more massive earth has in its development throughout time robbed the less massive moon in the gradual process of accretion. so the moon has never acquired either an ocean or atmosphere, and this view is acceptable to geologists who have studied the sheer lunar surface, shaler of harvard among the first, and laid the foundations for a separate science of selenology. tidal friction has also been operant in producing sun-raised tides upon the early plastic substances which composed the planets: more powerfully in the case of planets nearer the sun; less rapidly if the planet's mass is large; also less completely if the planet has solidified earlier on account of its small dimensions. so darwin would account for the present rotation periods of all the planets: both mercury and venus powerfully acted on by the sun on account of their nearness to him, and their rotational energy completely exhausted, so that they now and for all time turn a constant face toward him, as the moon does to the earth; earth and possibly mars even yet undergoing a very slight lengthening of their day; jupiter and saturn, also uranus and probably neptune, still exhibiting relatively swift axial rotation, because of their great mass and great original moment of momentum, and also by reason of their vast distances from the central tide-raising body, the sun. by applying to stellar systems the principles developed by darwin, see accounted for the fact, to which he was the first to direct attention, that the great eccentricity of the binary orbits is a necessary result of the secular action of tidal friction. the double stars, then, were double nebulæ, originally single, but separated by a process allied to that known as "fission" in protozoans. indeed, poincaré proved mathematically that a swiftly revolving nebula, in consequence of contraction, first undergoes distortion into a pear-shaped or hour-glass figure, the two masses ultimately separating entirely; and the observations of the herschels, lord rosse and others, with the recent photographic plates at the lick and mount wilson observatories, afford immediate confirmation in a multitude of double nebulæ, widely scattered throughout the nebular regions of the heavens. jeans of cambridge, england, among the most recent of mathematical investigators of the cosmogony, balances the advantages and disadvantages of the differing cosmogonic systems as follows, in his "problems of cosmogony and stellar dynamics": "some hundreds of millions of years ago all the stars within our galactic universe formed a single mass of excessively tenuous gas in slow rotation. as imagined by laplace, this mass contracted owing to loss of energy by radiation, and so increased its angular velocity until it assumed a lenticular shape.... after this, further contraction was a sheer mathematical impossibility and the system had to expand. the mechanism of expansion was provided by matter being thrown off from the sharp edge of the lenticular figure, the lenticular center now forming the nucleus, and the thrown-off matter forming the arms, of a spiral nebula of the normal type. the long filaments of matter which constituted the arms, being gravitationally unstable, first formed into chains of condensation about nuclei, and ultimately formed detached masses of gas. with continued shrinkage, the temperature of these masses increased until they attained to incandescence, and shone as luminous stars. at the same time their velocity of rotation increased until a large proportion of them broke up by fission into binary systems. the majority of the stars broke away from their neighbors and so formed a cluster of irregularly moving stars--our present galactic universe, in which the flattened shape of the original nebula may still be traced in the concentration about the galactic plane, while the original motion along the nebular arms still persists in the form of 'star-streaming.' in some cases a pair or small group of stars failed to get clear of one another's gravitational attractions and remain describing orbits about one another as wide binaries or multiple stars. the stars which were formed last, the present b-type stars, have been unusually immune from disturbance by their neighbors, partly because they were born when adjacent stars had almost ceased to interfere with one another, partly because their exceptionally large mass minimized the effect of such interference as may have occurred; consequently they remain moving in the plane in which they were formed, many of them still constituting closely associated groups of stars--the moving star clusters. "at intervals it must have happened that two stars passed relatively near to one another in their motion through the universe. we conjecture that something like million years ago our sun experienced an encounter of this kind, a large star passing within a distance of about the sun's diameter from its surface. the effect of this, as we have seen, would be the ejection of a stream of gas toward the passing star. at this epoch the sun is supposed to have been dark and cold, its density being so low that its radius was perhaps comparable with the present radius of neptune's orbit. the ejected stream of matter, becoming still colder by radiation, may have condensed into liquid near its ends and perhaps partially also near its middle. such a jet of matter would be longitudinally unstable and would condense into detached nuclei which would ultimately form planets." chapter lxi cosmogony in transition we have seen how wright in initiated a theory of evolution, not only of the solar system, but of all the stars and nebulæ as well; how kant in by elaborating this theory sought to develop the details of evolution of the solar system on the basis of the newtonian law, though weakened, as we know, by serious errors in applying physical laws; how laplace in put forward his nebular hypothesis of origin and development of the solar system, by contraction from an original gaseous nebula in accord with the newtonian law; how sir william herschel in saw in all nebulæ merely the stuff that stars are made of; how lord rosse in discovered spiral nebulæ; how helmholtz in put forward his contraction theory of maintenance of the solar heat, seemingly reinforcing the laplacian theory; how lane in proved that a contracting gaseous star might rise in temperature; how roche in in attempting to modify the laplacian hypothesis, pointed out the conditions under which a satellite would be broken up by tidal strains; how darwin in showed that the theory of tidal evolution of non-rigid bodies might account for the formation of the moon, and binary stars might originate by fission; how keeler in discovered the vast numbers of spiral nebulæ; how chamberlin and moulton in put forward the planetesimal hypothesis of formation of the spiral nebulæ, showing also how that hypothesis might account for the evolution of the solar system; and how jeans in advocated the median ground in evolution of the arms of the spiral nebulæ, showing that they will break up into nuclei, if sufficiently massive. in all these theories, truth and error, or lack of complete knowledge, appear to be intermingled in varying proportions. is it not early yet to say, either that any one of them must be abandoned as totally wrong, or on the other hand that any one of them, or indeed any single hypothesis, can explain all the evolutionary processes of the universe? clearly the great problems cannot all be solved by the kinetic theory of gases and the law of gravitation alone. recent physical researches into sub-atomic energy and the structure and properties of matter, appear to point in the direction where we must next look for more light on such questions as the origin and maintenance of the sun's heat, the complex phenomena of variable stars and the progressive evolution of the myriad bodies of the stellar universe. because we have actually seen one star turn into a nebula we should not jump to the conclusion that all nebulæ are formed from stars, even if this might seem a direct inference from the high radial velocities of planetary nebulæ. quite as obviously many of the spiral nebulæ are in a stage of transition into local universes of stars--even more obvious from the marvelous photographs in our day than the evolution of stars from nebulæ of all types was to herschel in his day. the physicist must further investigate such questions as the building up of heavy atomic elements by gravitative condensation of such lighter ones as compose the nebulæ; and laboratory investigation must elucidate further the process of development of energy from atomic disintegration under very high pressures. this leads to a reclassification of the stars on a temperature basis. equally important is the inquiry into the mechanism of radiative equilibrium in sun and stars. not impossibly the process of the earth's upper atmosphere in maintaining a terrestrial equilibrium may afford some clue. what this physical mechanism may be is very incompletely known, but it is now open to further research through recent progress of aeronautics, which will afford the investigator a "ceiling" of , feet and probably more. beneath this level, perhaps even below , feet, lie all the strata, including the inversion layer, where the sun's heat is conserved and an equilibrium maintained. even ten years ago, had an astronomer been asked about the physical condition of the interior of the stars, he would have replied that information of this character could only be had on visiting the stars themselves--and perhaps not even then. but at the cardiff meeting of the british association in , eddington, the president of section a, delivered an address on the internal constitution of the stars. he cites the recent investigations of russell and others on truly gaseous stars, like aldebaran, arcturus, antares and canopus, which are in a diffuse state and are the most powerful light-givers, and thus are to be distinguished from the denser stars like our sun. the term _giants_ is applied to the former, and _dwarfs_ to the latter, in accord with russell's theory. as density increases through contraction, these terms represent the progressive stages, from earlier to later, in a star's history. a red or m-type star begins its history as a giant of comparatively low temperature. contracting, according to lane's law, its temperature must rise until its density becomes such that it no longer behaves as a perfect gas. much depends on the star's mass; but after its maximum temperature is attained, the star, which has shrunk to the proportions of a dwarf, goes on cooling and contracts still further. each temperature-level is reached and passed twice, once during the ascending stage and once again in descending--once as a giant, and once as a dwarf. thus there are vast differences in luminosity: the huge giant, having a far larger surface than the shrunken dwarf, radiates an amount of light correspondingly greater. the physicist recognizes heat in two forms--the energy of motion of material atoms, and the energy of ether waves. in hot bodies with which we are familiar, the second form is quite insignificant; but in the giant stars, the two forms are present in about equal proportions. the super-heated conditions of the interior of the stars can only be estimated in millions of degrees; and the problem is not one of convection currents, as formerly thought, bringing hot masses to the surface from the highly heated interior, but how can the heat of the interior be barred against leakage and reduced to the relatively small radiation emitted by the stars. "smaller stars have to manufacture the radiant heat which they emit, living from hand to mouth; the giant stars merely leak radiant heat from their store." so a radioactive type of equilibrium must be established, rather than a convective one. laboratory investigations of the very short waves are now in progress, bearing on the transparency of stellar material to the radiation traversing it; and the penetrating power of the star's radiation is much like that of x-rays. the opacity is remarkably high, explaining why the star is so nearly "heat-tight." opacity being constant, the total radiation of a giant star depends on its mass only, and is quite independent of its temperature or state of diffuseness. so that the total radiation of a star which is measured roughly by its luminosity, may readily remain constant during the entire 'giant' stage of its history. as russell originally pointed out, giant stars of every spectral type have nearly the same luminosity. from the range of luminosity of the giant stars, then, we may infer their range of masses: they come out much alike, agreeing well with results obtained by double-star investigation. these studies of radiation and internal condition of the stars again bring up the question of the original source of that supply of radiant energy continually squandered by all self-luminous bodies. the giant stars are especially prodigal, and radiate at least a hundredfold faster than the sun. "a star is drawing on some vast reservoir of energy," says eddington, "by means unknown to us. this reservoir can scarcely be other than the sub-atomic energy which, it is known, exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. the store is well-nigh inexhaustible, if only it could be tapped. there is sufficient in the sun to maintain its output of heat for fifteen billion years." * * * * * transcriber's notes: obvious punctuation errors repaired. hyphenation and spelling was standardized by using the most prevalent form. the oe ligature was converted to the letters "oe". whole and fractional parts of numbers are displayed as follows: - / . page correction ==== =================== aa => aya ulugh begh => ulugh beg instaurata mecanica => instauratæ mechanica oscillatorium horologium => horologium oscillatorium seceded => succeeded areoplane => aeroplane plate - vulpeculæ => vulpecula text emphasis _text_ - italic =text= - bold astronomy with an opera-glass a popular introduction to the study of the starry heavens with the simplest of optical instruments with maps and directions to facilitate the recognition of the constellations and the principal stars visible to the naked eye by garrett p. serviss "known are their laws; in harmony unroll the nineteen-orbed cycles of the moon. and all the signs through which night whirls her car from belted orion back to orion and his dauntless hound, and all poseidon's, all high zeus' stars bear on their beams true messages to man." poste's aratus. _third edition_ new york d. appleton and company london: caxton house, paternoster square copyright, , by d appleton and company. to the reader in the pages that follow, the author has endeavored to encourage the study of the heavenly bodies by pointing out some of the interesting and marvelous phenomena of the universe that are visible with little or no assistance from optical instruments, and indicating means of becoming acquainted with the constellations and the planets. knowing that an opera-glass is capable of revealing some of the most beautiful sights in the starry dome, and believing that many persons would be glad to learn the fact, he set to work with such an instrument and surveyed all the constellations visible in the latitude of new york, carefully noting everything that it seemed might interest amateur star-gazers. all the objects thus observed have not been included in this book, lest the multiplicity of details should deter or discourage the very readers for whom it was specially written. on the other hand, there is nothing described as visible with an opera-glass or a field-glass which the author has not seen with an instrument of that description, and which any person possessing eye-sight of average quality and a competent glass should not be able to discern. but, in order to lend due interest to the subject, and place it before the reader in a proper light and true perspective, many facts have been stated concerning the objects described, the ascertainment of which has required the aid of powerful telescopes, and to observers with such instruments is reserved the noble pleasure of confirming with their own eyes those wonderful discoveries which the looker with an opera-glass can not hope to behold unless, happily, he should be spurred on to the possession of a telescope. yet even to glimpse dimly these distant wonders, knowing what a closer view would reveal, is a source of no mean satisfaction, while the celestial phenomena that lie easily within reach of an opera-glass are sufficient to furnish delight and instruction for many an evening. it should be said that the division of the stars used in this book into the "stars of spring," "stars of summer," "stars of autumn," and "stars of winter," is purely arbitrary, and intended only to indicate the seasons when certain constellations are best situated for observation or most conspicuous. the greater part of the matter composing this volume appeared originally in a series of articles contributed by the author to "the popular science monthly" in -' . the reception that those articles met with encouraged him to revise and enlarge them for publication in the more permanent form of a book. g. p. s. brooklyn, n. y., _september, ._ contents. page introduction popular interest in the phenomena of the heavens. the opera-glass as an instrument of observation for beginners in star-study. testing an opera-glass. chapter i. the stars of spring _description of the constellations_--auriga, the charioteer; berenice's hair; cancer, the crab [the manger]; canis minor, the lesser dog; corvus, the crow; crateris, the cup; gemini, the twins; hydra, the water-serpent; leo, the lion; ursa major, the greater bear [the great dipper]; ursa minor, the lesser bear [the pole-star]. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter ii. the stars of summer _description of the constellations_--aquila, the eagle; boötes, the herdsman, or bear-diver; canes venatici, the hunting-dogs; cygnus, the swan [the northern cross]; delphinus, the dolphin; draco, the dragon; hercules [the great sun-swarm, m]; libra, the balance; lyra, the harp; the northern crown; ophiuchus et serpens, the serpent-bearer and the serpent; sagitta, the arrow; sagittarius, the archer; scorpio, the scorpion; sobieski's shield; taurus poniatowskii, poniatowsky's bull; virgo, the virgin [the field of the nebulæ]; vulpecula, the little fox. a circular index-map, maps, on a larger scale, of the constellations described, and pictures of remarkable objects. chapter iii. the stars of autumn _description of the constellations_--andromeda [the great nebula]; aquarius, the water-bearer; aries, the ram; capricornus, the goat; cassiopeia; cepheus; cetus, the whale [mira, the wonderful variable star]; pegasus, the winged horse. perseus [algol, the demon-star]; pisces, the fishes; piscis australis, the southern fish; the triangles. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter iv. the stars of winter _description of the constellations_--argo, jason's ship; canis major, the great dog [sirius]; eridanus, the river po; lepus, the hare; monoceros, the unicorn; orion [the great nebula]; taurus, the bull [the pleiades and hyades]. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter v. the moon, the planets, and the sun description of lunar "seas," mountains, and "craters," with a map of the moon, and cuts showing its appearance with a field-glass. _opera-glass observation of_--the sun (one cut), mercury, venus, mars, jupiter and his satellites (one cut), saturn, uranus (three cuts). astronomy with an opera-glass. introduction. star-gazing was never more popular than it is now. in every civilized country many excellent telescopes are owned and used, often to very good purpose, by persons who are not practical astronomers, but who wish to see for themselves the marvels of the sky, and who occasionally stumble upon something that is new even to professional star-gazers. yet, notwithstanding this activity in the cultivation of astronomical studies, it is probably safe to assert that hardly one person in a hundred knows the chief stars by name, or can even recognize the principal constellations, much less distinguish the planets from the fixed stars. and of course they know nothing of the intellectual pleasure that accompanies a knowledge of the stars. modern astronomy is so rapidly and wonderfully linking the earth and the sun together, with all the orbs of space, in the bonds of close physical relationship, that a person of education and general intelligence can offer no valid excuse for not knowing where to look for sirius or aldebaran, or the orion nebula, or the planet jupiter. as australia and new zealand and the islands of the sea are made a part of the civilized world through the expanding influence of commerce and cultivation, so the suns and planets around us are, in a certain sense, falling under the dominion of the restless and resistless mind of man. we have come to possess vested intellectual interests in mars and saturn, and in the sun and all his multitude of fellows, which nobody can afford to ignore. a singular proof of popular ignorance of the starry heavens, as well as of popular curiosity concerning any uncommon celestial phenomenon, is furnished by the curious notions prevailing about the planet venus. when venus began to attract general attention in the western sky in the early evenings of the spring of , speculation quickly became rife about it, particularly on the great brooklyn bridge. as the planet hung dazzlingly bright over the new jersey horizon, some people appeared to think it was the light of liberty's torch, mistaking the bronze goddess's real flambeau for a part of the electric-light system of the metropolis. finally (to judge from the letters written to the newspapers, and the questions asked of individuals supposed to know something about the secrets of the sky), the conviction seems to have become pretty widely distributed that the strange light in the west was no less than an electrically illuminated balloon, nightly sent skyward by mr. edison, for no other conceivable reason than a wizardly desire to mystify his fellow-men. i have positive information that this ridiculous notion has been actually entertained by more than one person of intelligence. and as venus glowed with increasing splendor in the serene evenings of june, she continued to be mistaken for some petty artificial light instead of the magnificent world that she was, sparkling out there in the sunshine like a globe of burnished silver. yet venus as an evening star is not so rare a phenomenon that people of intelligence should be surprised at it. once in every days she reappears at the same place in the sunset sky-- "gem of the crimson-colored even, companion of retiring day." no eye can fail to note her, and as the nearest and most beautiful of the earth's sisters it would seem that everybody should be as familiar with her appearance as with the face of a friend. but the popular ignorance of venus, and the other members of the planetary family to which our mother, the earth, belongs, is only an index of the denser ignorance concerning the stars--the brothers of our great father, the sun. i believe this ignorance is largely due to mere indifference, which, in its turn, arises from a false and pedantic method of presenting astronomy as a creature of mathematical formulæ, and a humble handmaiden of the art of navigation. i do not, of course, mean to cast doubt upon the scientific value of technical work in astronomy. the science could not exist without it. those who have made the spectroscope reveal the composition of the sun and stars, and who are now making photography picture the heavens as they are, and even reveal phenomena which lie beyond the range of human vision, are the men who have taken astronomy out of its swaddling-clothes, and set it on its feet as a progressive science. but when one sees the depressing and repellent effect that has evidently been produced upon the popular mind by the ordinary methods of presenting astronomy, one can not resist the temptation to utter a vigorous protest, and to declare that this glorious science is not the grinning mathematical skeleton that it has been represented to be. perhaps one reason why the average educated man or woman knows so little of the starry heavens is because it is popularly supposed that only the most powerful telescopes and costly instruments of the observatory are capable of dealing with them. no greater mistake could be made. it does not require an optical instrument of any kind, nor much labor, as compared with that expended in the acquirement of some polished accomplishments regarded as indispensable, to give one an acquaintance with the stars and planets which will be not only pleasurable but useful. and with the aid of an opera-glass most interesting, gratifying, and, in some instances, scientifically valuable observations may be made in the heavens. i have more than once heard persons who knew nothing about the stars, and probably cared less, utter exclamations of surprise and delight when persuaded to look at certain parts of the sky with a good glass, and thereafter manifest an interest in astronomy of which they would formerly have believed themselves incapable. being convinced that whoever will survey the heavens with a good opera-glass will feel repaid many fold for his time and labor, i have undertaken to point out some of the objects most worthy of attention, and some of the means of making acquaintance with the stars. first, a word about the instrument to be used. galileo made his famous discoveries with what was, in principle of construction, simply an opera-glass. this form of telescope was afterward abandoned because very high magnifying powers could not be employed with it, and the field of view was restricted. but, on account of its brilliant illumination of objects looked at, and its convenience of form, the opera-glass is still a valuable and, in some respects, unrivaled instrument of observation. in choosing an opera-glass, see first that the object-glasses are achromatic, although this caution is hardly necessary, for all modern opera-glasses, worthy of the name, are made with achromatic objectives. but there are great differences in the quality of the work. if a glass shows a colored fringe around a bright object, reject it. let the diameter of the object-glasses, which are the large lenses in the end farthest from the eye, be not less than an inch and a half. the magnifying power should be at least three or four diameters. a familiar way of estimating the magnifying power is by looking at a brick wall through one barrel of the opera-glass with one eye, while the other eye sees the wall without the intervention of the glass. then notice how many bricks seen by the naked eye are required to equal in thickness one brick seen through the glass. that number represents the magnifying power. the instrument used by the writer in making most of the observations for this book has object-glasses . inch in diameter, and a magnifying power of about . times. see that the fields of view given by the two barrels of the opera-glass coincide, or blend perfectly together. if one appears to partially overlap the other when looking at a distant object, the effect is very annoying. this fault arises from the barrels of the opera-glass being placed too far apart, so that their optical centers do not coincide with the centers of the observer's eyes. [illustration: a very bad field.] occasionally, on account of faulty centering of the lenses, a double image is given of objects looked at, as illustrated in the accompanying cut. in such a case the glass is worthless; but if the effect is simply the addition of a small, crescent-shaped extension on one side of the field of view without any reduplication, the fault may be overlooked, though it is far better to select a glass that gives a perfectly round field. some glasses have an arrangement for adjusting the distance between the barrels to suit the eyes of different persons, and it would be well if all were made adjustable in the same way. don't buy a cheap glass, but don't waste your money on fancy mountings. what the rev. t. w. webb says of telescopes is equally true of opera-glasses: "inferior articles may be showily got up, and the outside must go for nothing." there are a few makers whose names, stamped upon the instrument, may generally be regarded as a guarantee of excellence. but the best test is that of actual performance. i have a field-glass which i found in a pawn-shop, that has no maker's name upon it, but in some respects is quite capable of bearing comparison with the work of the best advertised opticians. and this leads me to say that, by the exercise of good judgment, one may occasionally purchase superior glasses at very reasonable prices in the pawn-shops. ask to be shown the old and well-tried articles; you may find among them a second-hand glass of fine optical properties. if the lenses are not injured, one need not trouble one's self about the worn appearance of the outside of the instrument; so much the more evidence that somebody has found it well worth using. a good field or marine glass is in some respects better than an opera-glass for celestial observations. it possesses a much higher magnifying power, and this gives sometimes a decided advantage. but, on the other hand, its field of view is smaller, rendering it more difficult to find and hold objects. besides, it does not present as brilliant views of scattered star-clusters as an opera-glass does. for the benefit of those who possess field-glasses, however, i have included in this brief survey certain objects that lie just beyond the reach of opera-glasses, but can be seen with the larger instruments. i have thought it advisable in the descriptions of the constellations which follow to give some account of their mythological origin, both because of the historical interest which attaches to it, and because, while astronomers have long since banished the constellation figures from their maps, the names which the constellations continue to bear require some explanation, and they possess a literary and romantic interest which can not be altogether disregarded in a work that is not intended for purely scientific readers. chapter i. the stars of spring. having selected your glass, the next thing is to find the stars. of course, one could sweep over the heavens at random on a starry night and see many interesting things, but he would soon tire of such aimless occupation. the observer must know what he is looking at in order to derive any real pleasure or satisfaction from the sight. it really makes no difference at what time of the year such observations are begun, but for convenience i will suppose that they are begun in the spring. we can then follow the revolution of the heavens through a year, at the end of which the diligent observer will have acquired a competent knowledge of the constellations. the circular map, no. , represents the appearance of the heavens at midnight on the st of march, at eleven o'clock on the th of march, at ten o'clock on the st of april, at nine o'clock on the th of april, and at eight o'clock on the st of may. the reason why a single map can thus be made to show the places of the stars at different hours in different months will be plain upon a little reflection. in consequence of the earth's annual journey around the sun, the whole heavens make one apparent revolution in a year. this revolution, it is clear, must be at the rate of ° in a month, since the complete circuit comprises °. but, in addition to the annual revolution, there is a diurnal revolution of the heavens which is caused by the earth's daily rotation upon its axis, and this revolution must, for a similar reason, be performed at the rate of ° for each of the twenty-four hours. it follows that in two hours of the daily revolution the stars will change their places to the same extent as in one month of the annual revolution. it follows also that, if one could watch the heavens throughout the whole twenty-four hours, and not be interrupted by daylight, he would behold the complete circuit of the stars just as he would do if, for a year, he should look at the heavens at a particular hour every night. suppose that at nine o'clock on the st of june we see the star spica on the meridian; in consequence of the rotation of the earth, two hours later, or at eleven o'clock, spica will be ° west of the meridian. but that is just the position which spica would occupy at nine o'clock on the st of july, for in one month (supposing a month to be accurately the twelfth part of a year) the stars shift their places ° toward the west. if, then, we should make a map of the stars for nine o'clock on the st of july, it would answer just as well for eleven o'clock on the st of june, or for seven o'clock on the st of august. [illustration: map .] the center of the map is the zenith, or point overhead. the reader must now exercise his imagination a little, for it is impossible to represent the true appearance of the concave of the heavens on flat paper. holding the map over your head, with the points marked east, west, north, and south in their proper places, conceive of it as shaped like the inside of an open umbrella, the edge all around extending clear down to the horizon. suppose you are facing the south, then you will see, up near the zenith, the constellation of leo, which can be readily recognized on the map by six stars that mark out the figure of a sickle standing upright on its handle. the large star in the bottom of the handle is regulus. having fixed the appearance and situation of this constellation in your mind, go out-of-doors, face the south, and try to find the constellation in the sky. with a little application you will be sure to succeed. using leo as a basis of operations, your conquest of the sky will now proceed more rapidly. by reference to the map you will be able to recognize the twin stars of gemini, southwest of the zenith and high up; the brilliant lone star, procyon, south of gemini; the dazzling sirius, flashing low down in the southwest; orion, with all his brilliants, blazing in the west; red aldebaran and the pleiades off to his right; and capella, bright as a diamond, high up above orion, toward the north. in the southeast you will recognize the quadrilateral of corvus, with the remarkably white star spica glittering east of it. next face the north. if you are not just sure where north is, try a pocket-compass. this advice is by no means unnecessary, for there are many intelligent persons who are unable to indicate true north within many degrees, though standing on their own doorstep. having found the north point as near as you can, look upward about forty degrees from the horizon, and you will see the lone twinkler called the north or pole star. forty degrees is a little less than half-way from the horizon to the zenith. by the aid of the map, again, you will be able to find, high up in the northeast, near the zenith, the large dipper-shaped figure in ursa major, and, when you have once noticed that the two stars in the outer edge of the bowl of the dipper point almost directly to the pole-star, you will have an unfailing means of picking out the latter star hereafter, when in doubt.[a] continuing the curve of the dipper-handle, in the northeast, your eye will be led to a bright reddish star, which is arcturus, in the constellation boötes. [a] let the reader remember that the distance between the two stars in the brim of the bowl of the dipper is about ten degrees, and he will have a measuring-stick that he can apply in estimating other distances in the heavens. in the same way you will be able to find the constellations cassiopeia, cepheus, draco, and perseus. don't expect to accomplish it all in an hour. you may have to devote two or three evenings to such observation, and make many trips indoors to consult the map, before you have mastered the subject; but when you have done it you will feel amply repaid for your exertions, and you will have made for yourself silent friends in the heavens that will beam kindly upon you, like old neighbors, on whatever side of the world you may wander. having fixed the general outlines and location of the constellations in your mind, and learned to recognize the chief stars, take your opera-glass and begin with the constellation leo and the star regulus. contrive to have some convenient rest for your arms in holding the glass, and thus obtain not only comfort but steadiness of vision. a lazy-back chair makes a capital observing-seat. be very particular, too, to get a sharp focus. remember that no two persons' eyes are alike, and that even the eyes of the same observer occasionally require a change. in looking for a difficult object, i have sometimes suddenly brought the sought-for phenomenon into view by a slight turn of the focusing-screw. you will at once be gratified by the increased brilliancy of the star as seen by the glass. if the night is clear, it will glow like a diamond. yet regulus, although ranked as a first-magnitude star, and of great repute among the ancient astrologers, is far inferior in brilliancy to such stars as capella and arcturus, to say nothing of sirius. by consulting map no. you will next be able to find the celebrated star bearing the name of the greek letter gamma ([gamma]). if you had a telescope, you would see this star as a close and beautiful double, of contrasted colors. but it is optically double, even with an opera-glass. you can not fail to see a small star near it, looking quite close if the magnifying power of your glass is less than three times. you will be struck by the surprising change of color in turning from regulus to gamma--the former is white and the latter deep yellow. it will be well to look first at one and then at the other, several times, for this is a good instance of what you will meet with many times in your future surveys of the heavens--a striking contrast of color in neighboring stars. one can thus comprehend that there is more than one sense in which to understand the scriptural declaration that "one star differeth from another in glory." the radiant point of the famous november meteors, which, in and , filled the sky with fiery showers, is near gamma. turn next to the star in leo marked zeta ([zeta]). if your glass is a pretty large and good one, and your eye keen, you will easily see three minute companion stars keeping company with zeta, two on the southeast, and one, much closer, toward the north. the nearest of the two on the south is faint, being only between the eighth and ninth magnitude, and will probably severely test your powers of vision. next look at epsilon ([epsilon]), and you will find near it two seventh-magnitude companions, making a beautiful little triangle. [illustration: map .] away at the eastern end of the constellation, in the tail of the imaginary lion, upon whose breast shines regulus, is the star beta ([beta]) leonis, also called denebola. it is almost as bright as its leader, regulus, and you will probably be able to catch a tinge of blue in its rays. south of denebola, at a distance of nineteen minutes of arc, or somewhat more than half the apparent diameter of the moon, you will see a little star of the sixth magnitude, which is one of the several "companions" for which denebola is celebrated. there is another star of the eighth magnitude in the same direction from denebola, but at a distance of less than five minutes, and this you may be able to glimpse with a powerful field-glass, under favorable conditions. i have seen it well with a field-glass of . -inch aperture, and a magnifying power of seven times. but it requires an experienced eye and steady vision to catch this shy twinkler. when looking for a faint and difficult object, the plan pursued by telescopists is to avert the eye from the precise point upon which the attention is fixed, in order to bring a more sensitive part of the retina into play than that usually employed. look toward the edge of the field of view, while the object you are seeking is in the center, and then, if it can be seen at all with your glass, you will catch sight of it, as it were, out of the corner of your eye. the effect of seeing a faint star in this way, in the neighborhood of a large one, whose rays hide it from direct vision, is sometimes very amusing. the little star seems to dart out into view as through a curtain, perfectly distinct, though as immeasurably minute as the point of a needle. but the instant you direct your eyes straight at it, presto! it is gone. and so it will dodge in and out of sight as often as you turn your eyes. if you will sweep carefully over the whole extent of leo, whose chief stars are marked with their greek-letter names on our little map, you will be impressed with the power of your glass to bring into sight many faint stars in regions that seem barren to the naked eye. an opera-glass of . aperture will show ten times as many stars as the naked eye can see. a word about the "lion" which this constellation is supposed to represent. it requires a vivid imagination to perceive the outlines of the celestial king of beasts among the stars, and yet somebody taught the people of ancient india and the old egyptians to see him there, and there he has remained since the dawn of history. modern astronomers strike him out of their charts, together with all the picturesque multitude of beasts and birds and men and women that bear him company, but they can not altogether banish him, or any of his congeners, for the old names, and, practically, the old outlines of the constellations are retained, and always will be retained. the lion is the most conspicuous figure in the celebrated zodiac of dendera; and, indeed, there is evidence that before the story of hercules and his labors was told this lion was already imagined shining among the stars. it was characteristic of the greeks that they seized him for their own, and tried to rob him of his real antiquity by pretending that jupiter had placed him among the stars in commemoration of hercules's victory over the nemæan lion. in the hebrew zodiac leo represented the lion of judah. it was thus always a lion that the ancients thought they saw in this constellation. in the old star-maps the lion is represented as in the act of springing upon his prey. his face is to the west, and the star regulus is in his heart. the sickle-shaped figure covers his breast and head, gamma being in the shoulder, zeta in the mane of the neck, mu and epsilon in the cheek, and lambda in the jaws. the fore-paws are drawn up to the breast and represented by the stars zi and omicron. denebola is in the tuft of the tail. the hind-legs are extended downward at full length, in the act of springing. starting from the star delta in the hip, the row consisting of theta, iota, tau, and upsilon, shows the line of the hind-legs. leo had an unsavory reputation among the ancients because of his supposed influence upon the weather. the greatest heat of summer was felt when the sun was in this constellation: "most scorching is the chariot of the sun, and waving spikes no longer hide the furrows when he begins to travel with the lion." looking now westwardly from the sickle of leo, at a distance about equal to twice the length of the sickle, your eye will be caught by a small silvery spot in the sky lying nearly between two rather faint stars. this is the famous præsepe, or manger, in the center of the constellation cancer. the two stars on either side of it are called the aselli, or the ass's colts, and the imagination of the ancients pictured them feeding from their silver manger. turn your glass upon the manger and you will see that it consists of a crowd of little stars, so small and numerous that you will probably not undertake to count them, unless you are using a large field-glass. galileo has left a delightful description of his surprise and gratification when he aimed his telescope at this curious cluster and other similar aggregations of stars and discovered what they really were. using his best instrument, he was able to count thirty-six stars in the manger. the manger was a famous weather-sign in olden times, and aratus, in his "diosemia," advises his readers to-- "... watch the manger: like a little mist far north in cancer's territory it floats. its confines are two faintly glimmering stars; these are two asses that a manger parts, which suddenly, when all the sky is clear, sometimes quite vanishes, and the two stars seem to have closer moved their sundered orbs. no feeble tempest then will soak the leas; a murky manger with both stars shining unaltered is a sign of rain." like other old weather-saws, this probably possesses a gleam of sense, for it is only when the atmosphere is perfectly transparent that the manger can be clearly seen; when the air is thick with mist, the harbinger of coming storm, it fades from sight. the constellation cancer, or the crab, was represented by the egyptians under the figure of a scarabæus. the observer will probably think that it is as easy to see a beetle as a crab there. cancer, like leo, is one of the twelve constellations of the zodiac, the name applied to the imaginary zone ° degrees wide and extending completely around the heavens, the center of which is the ecliptic or annual path of the sun. the names of these zodiacal constellations, in their order, beginning at the west and counting round the circle, are: aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius, and pisces. cancer has given its name to the circle called the tropic of cancer, which indicates the greatest northerly declination of the sun in summer, and which he attains on the st or d of june. but, in consequence of the precession of the equinoxes, all of the zodiacal constellations are continually shifting toward the east, and cancer has passed away from the place of the summer solstice, which is now to be found in gemini. below the manger, a little way toward the south, your eye will be caught by a group of four or five stars of about the same brightness as the aselli. this marks the head of hydra, and the glass will show a striking and beautiful geometrical arrangement of the stars composing it. hydra is a very long constellation, and trending southward and eastward from the head it passes underneath leo, and, sweeping pretty close down to the horizon, winds away under corvus, the tail reaching to the eastern horizon. the length of this skyey serpent is about °. its stars are all faint, except alphard, or the hydra's heart, a second-magnitude star, remarkable for its lonely situation, southwest of regulus. a line from gamma leonis through regulus points it out. it is worth looking at with the glass on account of its rich orange-tint. hydra is fabled to be the hundred-headed monster that was slain by hercules. it must be confessed that there is nothing very monstrous about it now except its length. the most timid can look upon it without suspecting its grisly origin. coming back to the manger as a starting-point, look well up to the north and west, and at a distance somewhat less than that between regulus and the manger you will see a pair of first-magnitude stars, which you will hardly need to be informed are the celebrated twins, from which the constellation gemini takes its name. the star marked [alpha] in the map is castor, and the star marked [beta] is pollux. no classical reader needs to be reminded of the romantic origin of these names. a sharp contrast in the color of castor and pollux comes out as soon as the glass is turned upon them. castor is white, with occasionally, perhaps, a suspicion of a green ray in its light. pollux is deep yellow. castor is a celebrated double star, but its components are far too close to be separated with an opera-glass, or even the most powerful field-glass. you will be at once interested by the singular _cortége_ of small stars by which both castor and pollux are surrounded. these little attendant stars, for such they seem, are arrayed in symmetrical groups--pairs, triangles, and other figures--which, it seems difficult to believe, could be unintentional, although it would be still more difficult to suggest any reason why they should be arranged in that way. [illustration: map .] our map will show you the position of the principal stars of the constellation. castor and pollux are in the heads of the twins, while the row of stars shown in the map xi ([xi]), gamma ([gamma]), nu ([nu]), mu ([mu]), and eta ([eta]), marks their feet, which are dipped in the edge of the milky-way. one can spend a profitable and pleasurable half-hour in exploring the wonders of gemini. the whole constellation, from head to foot, is gemmed with stars which escape the naked eye, but it sparkles like a bead-spangled garment when viewed with the glass. owing to the presence of the milky-way, the spectacle around the feet of the twins is particularly magnificent. and here the possessor of a good opera-glass can get a fine view of a celebrated star-cluster known in the catalogues as m. it is situated a little distance northwest of the star eta, and is visible to the naked eye, on a clear, moonless night, as a nebulous speck. with a good glass you will see two wonderful streams of little stars starting, one from eta and the other from mu, and running parallel toward the northwest; m is situated between these star-streams. the stars in the cluster are so closely aggregated that you will be able to clearly separate only the outlying ones. the general aspect is like that of a piece of frosted silver over which a twinkling light is playing. a field-glass brings out more of the component stars. the splendor of this starry congregation, viewed with a powerful telescope, may be guessed at from admiral smyth's picturesque description: "it presents a gorgeous field of stars, from the ninth to the sixteenth magnitude, but with the center of the mass less rich than the rest. from the small stars being inclined to form curves of three or four, and often with a large one at the root of the curve, it somewhat reminds one of the bursting of a sky-rocket." and webb adds that there is an "elegant festoon near the center, starting with a reddish star." no one can gaze upon this marvelous phenomenon, even with the comparatively low powers of an opera-glass, and reflect that all these swarming dots of light are really suns, without a stunning sense of the immensity of the material universe. it is an interesting fact that the summer solstice, or the point which the sun occupies when it attains its greatest northerly declination, on the longest day of the year, is close by this great cluster in gemini. in the glare of the sunshine those swarming stars are then concealed from our sight, but with the mind's eye we can look past and beyond our sun, across the incomprehensible chasm of space, and behold them still shining, their commingled rays making our great god of day seem but a lonely wanderer in the expanse of the universe. it was only a short distance southwest of this cluster that one of the most celebrated discoveries in astronomy was made. there, on the evening of march , , william herschel observed a star whose singular aspect led him to put a higher magnifying power on his telescope. the higher power showed that the object was not a star but a planet, or a comet, as herschel at first supposed. it was the planet uranus, whose discovery "at one stroke doubled the breadth of the sun's dominions." the constellation of gemini, as the names of its two chief stars indicate, had its origin in the classic story of the twin sons of jupiter and leda: "fair leda's twins, in time to stars decreed, one fought on foot, one curbed the fiery steed." castor and pollux were regarded by both the greeks and the romans as the patrons of navigation, and this fact crops out very curiously in the adventures of st. paul. after his disastrous shipwreck on the island of melita he embarked again on a more prosperous voyage in a ship bearing the name of these very brothers. "and after three months," writes the celebrated apostle (acts xxviii, ) "we departed in a ship of alexandria, which had wintered in the isle, whose sign was castor and pollux." we may be certain that paul was acquainted with the constellation of gemini, not only because he was skilled in the learning of his times, but because, in his speech on mars hill, he quoted a line from the opening stanzas of aratus's "phenomena," a poem in which the constellations are described. the map will enable you next to find procyon, or the little dog-star, more than twenty degrees south of castor and pollux, and almost directly below the manger. this star will interest you by its golden-yellow color and its brightness, although it is far inferior in the latter respect to sirius, or the great dog-star, which you will see flashing splendidly far down beneath procyon in the southwest. about four degrees northwest of procyon is a third-magnitude star, called gomelza, and the glass will show you two small stars which make a right-angled triangle with it, the nearer one being remarkable for its ruddy color. procyon is especially interesting because it is attended by an invisible star, which, while it has escaped all efforts to detect it with powerful telescopes, nevertheless reveals its presence by the effect of its attraction upon procyon. it is a curious fact that both of the so-called dog-stars are thus attended by obscure or dusky companion-stars, which, notwithstanding their lack of luminosity, are of great magnitude. in the case of sirius, the improvement in telescopes has brought the mysterious attendant into view, but procyon's mate remains hidden from our eyes. but it can not escape the ken of the mathematician, whose penetrating mental vision has, in more than one instance, outstripped the discoveries of the telescope. almost half a century ago the famous bessel announced his conclusion--in the light of later developments it may well be called discovery--that both sirius and procyon were binary systems, consisting each of a visible and an invisible star. he calculated the probable period of revolution, and found it to be, in each case, approximately fifty years. sixteen years after bessel's death, one of alvan clark's unrivaled telescopes at last revealed the strange companion of sirius, a huge body, half as massive as the giant dog-star itself, but ten thousand times less brilliant, and more recent observations have shown that its period of revolution is within six or seven months of the fifty years assigned by bessel. if some of the enormous telescopes that have been constructed in the past few years should succeed in rendering procyon's companion visible also, it is highly probable that bessel's prediction would receive another substantial fulfillment. the mythological history of canis minor is somewhat obscure. according to various accounts it represents one of diana's hunting-dogs, one of orion's hounds, the egyptian dog-headed god anubis, and one of the dogs that devoured their master actæon after diana had turned him into a stag. the mystical dr. seiss leaves all the ancient myth-makers far in the rear, and advances a very curious theory of his own about this constellation, in his "gospel in the stars," which is worth quoting as an example of the grotesque fancies that even in our day sometimes possess the minds of men when they venture beyond the safe confines of this terraqueous globe. after summarizing the various myths we have mentioned, he proceeds to identify procyon, putting the name of the chief star for the constellation, "as the starry symbol of those heavenly armies which came forth along with the king of kings and lord of lords to the battle of the great day of god almighty, to make an end of misrule and usurpation on earth, and clear it of all the wild beasts which have been devastating it for these many ages." the reader will wonder all the more at this rhapsody after he has succeeded in picking out the modest little dog in the sky. sirius, orion, aldebaran, and the pleiades, all of which you will perceive in the west and southwest, are generally too much involved in the mists of the horizon to be seen to the best advantage at this season, although it will pay you to take a look through the glass at sirius. but the splendid star capella, in the constellation auriga, may claim a moment's attention. you will find it high up in the northwest, half-way between orion and the pole-star, and to the right of the twins. it has no rival near, and its creamy-white light makes it one of the most beautiful as well as one of the most brilliant stars in the heavens. its constitution, as revealed by the spectroscope, resembles that of our sun, but the sun would make but a sorry figure if removed to the side of this giant star. about seven and a half degrees above capella, and a little to the left, you will see a second-magnitude star called menkalina. two and a half times as far to the left, or south, in the direction of orion, is another star of equal brightness to menkalina. this is el nath, and marks the place where the foot of auriga, or the charioteer, rests upon the point of the horn of taurus. capella, menkalina, and el nath make a long triangle which covers the central part of auriga. the naked eye shows two or three misty-looking spots within this triangle, one to the right of el nath, one in the upper or eastern part of the constellation, near the third-magnitude star theta ([theta]), and another on a line drawn from capella to el nath, but much nearer to capella. turn your glass upon these spots, and you will be delighted by the beauty of the little stars to whose united rays they are due. el nath has around it some very remarkable rows of small stars, and the whole constellation of auriga, like that of gemini, glitters with star-dust, for the milky-way runs directly through it. with a powerful field-glass you may try a glimpse at the rich star-clusters marked m, m, and ^ . [illustration: map .] the mythology of auriga is not clear, but the ancients seem to have been of one mind in regarding the constellation as representing the figure of a man carrying a goat and her two kids in his arms. auriga was also looked upon as a beneficent constellation, and the goat and kids were believed to be on the watch to rescue shipwrecked sailors. as capella, which represents the fabled goat, shines nearly overhead in winter, and would ordinarily be the first bright star to beam down through the breaking clouds of a storm at that season, it is not difficult to imagine how it got its reputation as the seaman's friend. dr. seiss has so spirited a description of the imaginary figure contained in this constellation that i can not refrain from quoting it: "the figure itself is that of a mighty man seated on the milky-way, holding a band or ribbon in his right hand, and with his left arm holding up on his shoulder a she-goat which clings to his neck and looks out in astonishment upon the terrible bull; while in his lap are two frightened little kids which he supports with his great hand." it is scarcely necessary to add that dr. seiss insists that auriga, as a constellation, was invented long before the time of the greeks, and was intended prophetically to represent that good shepherd who was to come and rescue the sinful world. if any reader wishes to exercise his fancy by trying to trace the outlines of this figure, he will find the head of auriga marked by the star delta ([delta]) and the little group near it. capella, in the heart of the goat, is just below his left shoulder, and menkalina marks his right shoulder. el nath is in his right foot, and iota ([iota]) in his left foot. the stars epsilon ([epsilon]), zeta ([zeta]), eta ([eta]), and lambda ([lambda]) shine in the kids which lie in auriga's lap. the faint stars scattered over the eastern part of the constellation are sometimes represented as forming a whip with many lashes, which the giant flourishes with his right hand. let us turn back to denebola in the lion's tail. now glance from it down into the southeast, and you will see a brilliant star flashing well above the horizon. this is spica, the chief twinkler of virgo, and it is marked on our circular map. then look into the northwest, and at about the same distance from denebola, but higher above the horizon than spica, you will catch the sparkling of a large, reddish star. it is arcturus in boötes. the three, denebola, spica, and arcturus, mark the corners of a great equilateral triangle. nearly on a line between denebola and arcturus, and somewhat nearer to the former, you will perceive a curious twinkling, as if gossamers spangled with dew-drops were entangled there. one might think the old woman of the nursery rhyme who went to sweep the cobwebs out of the sky had skipped this corner, or else that its delicate beauty had preserved it even from her housewifely instincts. this is the little constellation called berenice's hair. your opera-glass will enable you to count twenty or thirty of the largest stars composing this cluster, which are arranged, as so often happens, with a striking appearance of geometrical design. the constellation has a very romantic history. it is related that the young queen berenice, when her husband was called away to the wars, vowed to sacrifice her beautiful tresses to venus if he returned victorious over his enemies. he did return home in triumph, and berenice, true to her vow, cut off her hair and bore it to the temple of venus. but the same night it disappeared. the king was furious, and the queen wept bitterly over the loss. there is no telling what might have happened to the guardians of the temple, had not a celebrated astronomer named conon led the young king and queen aside in the evening and showed them the missing locks shining transfigured in the sky. he assured them that venus had placed berenice's lustrous ringlets among the stars, and, as they were not skilled in celestial lore, they were quite ready to believe that the silvery swarm they saw near arcturus had never been there before. and so for centuries the world has recognized the constellation of berenice's hair. look next at corvus and crater, the crow and the cup, two little constellations which you will discover on the circular map, and of which we give a separate representation in map . you will find that the stars delta ([delta]) and eta ([eta]), in the upper left-hand corner of the quadrilateral figure of corvus, make a striking appearance. the little star zeta ([zeta]) is a very pretty double for an opera-glass. there is a very faint pair of stars close below and to the right of beta ([beta]). this forms a severe test. only a good opera-glass will show these two stars as a single faint point of light. a field-glass, however, will show both, one being considerably fainter than the other. crater is worth sweeping over for the pretty combinations of stars to be found in it. you will observe that the interminable hydra extends his lengthening coils along under both of the constellations. in fact, both the cup and the crow are represented as standing upon the huge serpent. the outlines of a cup are tolerably well indicated by the stars included under the name crater, but the constellation of the crow might as well have borne any other name so far as any traceable likeness is concerned. one of the legends concerning corvus avers that it is the daughter of the king of phocis, who was transformed into a crow to escape the pursuit of neptune. she is certainly safe in her present guise. arcturus and spica, and their companions, may be left for observation to a more convenient season, when, having risen higher, they can be studied to better advantage. it will be well, however, to merely glance at them with the glass in order to note the great difference of color--spica being brilliantly white and arcturus almost red. [illustration: map .] we will now turn to the north. you have already been told how to find the pole-star. look at it with your glass. the pole-star is a famous double, but its minute companion can only be seen with a telescope. as so often happens, however, it has another companion for the opera-glass, and this latter is sufficiently close and small to make an interesting test for an inexperienced observer armed with a glass of small power. it must be looked for pretty close to the rays of the large star, with such a glass. it is of the seventh magnitude. with a large field-glass several smaller companions may be seen, and a very excellent glass may show an . -magnitude star almost hidden in the rays of the seventh-magnitude companion. with the aid of map no. find in ursa minor, which is the constellation to which the pole-star belongs, the star beta ([beta]), which is also called kochab (the star marked [alpha] in the map is the pole-star). kochab has a pair of faint stars nearly north of it, about one degree distant. with a small glass these may appear as a single star, but a stronger glass will show them separately. [illustration: map .] and now for ursa major and the great dipper--draco, cepheus, cassiopeia, and the other constellations represented on the circular map, being rather too near the horizon for effective observation at this time of the year. first, as the easiest object, look at the star in the middle of the handle of the dipper (this handle forms the tail of ursa major), and a little attention will show you, without the aid of a glass, if your eye-sight is good, that the star is double. a smaller star seems to be almost in contact with it. the larger of these two stars is called mizar and the smaller alcor--the horse and his rider the arabs said. your glass will, of course, greatly increase the distance between alcor and mizar, and will also bring out a clear difference of color distinguishing them. now, if you have a very powerful glass, you may be able to see the sidus ludovicianum, a minute star which a german astronomer discovered more than a hundred and fifty years ago, and, strangely enough, taking it for a planet, named it after a german prince. the position of the sidus ludovicianum, with reference to mizar and alcor, is represented in the accompanying sketch. you must look very sharply if you expect to see it, and your opera-glass will have to be a large and strong one. a field-glass, however, can not fail to show it. sweep along the whole length of the dipper's handle, and you will discover many fine fields of stars. then look at the star alpha ([alpha]) in the outer edge of the bowl nearest to the pole-star. there is a faint star, of about the eighth magnitude, near it, in the direction of beta ([beta]). this will prove a very difficult test. you will have to try it with averted vision. if you have a field-glass, catch it first with that, and, having thus fixed its position in your mind, try to find it with the opera-glass. its distance is a little over half that between mizar and alcor. it is of a reddish color. you will notice nearly overhead three pairs of pretty bright stars in a long, bending row, about half-way between leo and the dipper. these mark three of ursa major's feet, and each of the pairs is well worth looking at with a glass, as they are beautifully grouped with stars invisible to the naked eye. the letters used to designate the stars forming these pairs will be found upon our map of ursa major. the scattered group of faint stars beyond the bowl of the dipper forms the bear's head, and you will find that also a field worth a few minutes' exploration. [illustration: mizar, alcor, and the sidus ludovicianum.] the two bears, ursa major and ursa minor, swinging around the pole of the heavens, have been conspicuous in the star-lore of all ages. according to fable, they represent the nymph calisto, with whom jupiter was in love, and her son arcas, who were both turned into bears by juno, whereupon jupiter, being unable to restore their form, did the next best thing he could by placing them among the stars. ursa major is calisto, or helica, as the greeks called the constellation. the greek name of ursa minor was cynosura. the use of the pole-star in navigation dates back at least to the time of the phoenicians. the observer will note the uncomfortable position of ursa minor, attached to the pole by the end of its long tail. but, after all, no one can expect to derive from such studies as these any genuine pleasure or satisfaction unless he is mindful of the real meaning of what he sees. the actual truth seems almost too stupendous for belief. the mind must be brought into an attitude of profound contemplation in order to appreciate it. from this globe we can look out in every direction into the open and boundless universe. blinded and dazzled during the day by the blaze of that star, of which the earth is a near and humble dependent, we are shut in as by a curtain. but at night, when our own star is hidden, our vision ranges into the depths of creation, and we behold them sparkling with a multitude of other suns. with so simple an aid as that of an opera-glass we penetrate still deeper into the profundities of space, and thousands more of these strange, far-away suns come into sight. they are arranged in pairs, sets, rows, streams, clusters--here they gleam alone in distant splendor, there they glow and flash in mighty swarms. this is a look into heaven more splendid than the imagination of bunyan pictured; here is a celestial city whose temples are suns, and whose streets are the pathways of light. chapter ii. the stars of summer. let us now suppose that the earth has advanced for three months in its orbit since we studied the stars of spring, and that, in consequence, the heavens have made one quarter of an apparent revolution. then we shall find that the stars which in spring shone above the western horizon have been carried down out of sight, while the constellations that were then in the east have now climbed to the zenith, or passed over to the west, and a fresh set of stars has taken their place in the east. in the present chapter we shall deal with what may be called the stars of summer; and, in order to furnish occupation for the observer with an opera-glass throughout the summer months, i have endeavored to so choose the constellations in which our explorations will be made, that some of them shall be favorably situated in each of the months of june, july, and august. the circular map represents the heavens at midnight on the st of june; at eleven o'clock, on the th of june; at ten o'clock, on the st of july; at nine o'clock, on the th of july; and at eight o'clock, on the st of august. remembering that the center of the map is the point over his head, and that the edge of it represents the circle of the horizon, the reader, by a little attention and comparison with the sky, will be able to fix in his mind the relative situation of the various constellations. the maps that follow will show him these constellations on a larger scale, and give him the names of their chief stars. [illustration: map .] the observer need not wait until midnight on the st of june in order to find some of the constellations included in our map. earlier in the evening, at about that date, say at nine o'clock, he will be able to see many of these constellations, but he must look for them farther toward the east than they are represented in the map. the bright stars in boötes and virgo, for instance, instead of being over in the southwest, as in the map, will be near the meridian; while lyra, instead of shining high overhead, will be found climbing up out of the northeast. it would be well to begin at nine o'clock, about the st of june, and watch the motions of the heavens for two or three hours. at the commencement of the observations you will find the stars in boötes, virgo, and lyra in the positions i have just mentioned, while half-way down the western sky will be seen the sickle of leo. the brilliant procyon and capella will be found almost ready to set in the west and northwest, respectively. between procyon and capella, and higher above the horizon, shine the twin stars in gemini. in an hour procyon, capella, and the twins will be setting, and spica will be well past the meridian. in another hour the observer will perceive that the constellations are approaching the places given to them in our map, and at midnight he will find them all in their assigned positions. a single evening spent in observations of this sort will teach him more about the places of the stars than he could learn from a dozen books. taking, now, the largest opera-glass you can get (i have before said that the diameter of the object-glasses should not be less than . inch, and, i may add, the larger they are the better), find the constellation scorpio, and its chief star antares. the map shows you where to look for it at midnight on the st of june. if you prefer to begin at nine o'clock at that date, then, instead of looking directly in the south for scorpio, you must expect to see it just rising in the southeast. you will recognize antares by its fiery color, as well as by the striking arrangement of its surrounding stars. there are few constellations which bear so close a resemblance to the objects they are named after as scorpio. it does not require a very violent exercise of the imagination to see in this long, winding trail of stars a gigantic scorpion, with its head to the west, and flourishing its upraised sting that glitters with a pair of twin stars, as if ready to strike. readers of the old story of phaeton's disastrous attempt to drive the chariot of the sun for a day will remember it was the sight of this threatening monster that so terrified the ambitious youth as he dashed along the zodiac, that he lost control of apollo's horses, and came near burning the earth up by running the sun into it. antares rather gains in redness when viewed with a glass. its color is very remarkable, and it is a curious circumstance that with powerful telescopes a small, bright-green star is seen apparently almost touching it. antares belongs to secchi's third type of suns, that in which the spectroscopic appearances suggest the existence of a powerfully absorptive atmosphere, and which are believed on various grounds to be, as lockyer has said, "in the last visible stage of cooling"; in other words, almost extinct. this great, red star probably in actual size exceeds our sun, and no one can help feeling the sublime nature of those studies which give us reason to think that here we can actually behold almost the expiring throes of a giant brother of our giant sun. only, the lifetime of a sun is many millions of years, and its gradual extinction, even after it has reached a stage as advanced as that of antares is supposed to be, may occupy a longer time than the whole duration of the human race. a little close inspection with the naked eye will show three fifth- or sixth-magnitude stars above antares and sigma ([sigma]), which form, with those stars, the figure of an irregular pentagon. an opera-glass shows this figure very plainly. the nearest of these stars to antares, the one directly above it, is known by the number , and belongs to scorpio, while the farthest away, which marks the northernmost corner of the pentagon, is rho in ophiuchus. try a powerful field-glass upon the two stars just named. take first. you will without much difficulty perceive that it has a little star under its wing, below and to the right, and more than twice as far away above it there is another faint star. then turn to rho. look sharp and you will catch sight of two companion stars, one close to rho on the right and a little below, and the other still closer and directly above rho. the latter is quite difficult to be seen distinctly, but the sight is a very pretty one. the opera-glass will show a number of faint stars scattered around antares. turn now to beta ([beta]) in scorpio, with the glass. a very pretty pair of stars will be seen hanging below [beta]. sweeping downward from this point to the horizon you will find many beautiful star-fields. the star marked nu ([nu]) is a double which you will be able to separate with a powerful field-glass, the distance between its components being ". [illustration: map .] and next let us look at a star-cluster. you will see on map no. an object marked m, near antares. its designation means that it is no. in messier's catalogue of nebulæ. it is not a true nebula, but a closely compacted cluster of stars. with the opera-glass, if you are looking in a clear and moonless night, you will see it as a curious nebulous speck. with a field-glass its real nature is more apparent, and it is seen to blaze brighter toward the center. it is, in fact, one of those universes within the universe where thousands of suns are associated together by some unknown law of aggregation into assemblages of whose splendor the slight view that we can get gives us but the faintest conception. the object above and to the right of antares, marked in the map m., is a nebula, and although the nebula itself is too small to be seen with an opera-glass (a field-glass shows it as a mere wisp of light), yet there is a pretty array of small stars in its neighborhood worth looking at. besides, this nebula is of special interest, because in a star suddenly took its place. at least, that is what seemed to have happened. what really did occur, probably, was that a variable or temporary star, situated between us and the nebula, and ordinarily too faint to be perceived, received a sudden and enormous accession of light, and blazed up so brightly as to blot out of sight the faint nebula behind it. if this star should make its appearance again, it could easily be seen with an opera-glass, and so it will not be useless for the reader to know where to look for it. the quarter of the heavens with which we are now dealing is famous for these celestial conflagrations, if so they may be called. the first temporary star of which there is any record appeared in the constellation of the scorpion, near the head, years before christ. it must have been a most extraordinary phenomenon, for it attracted attention all over the world, and both greek and chinese annals contain descriptions of it. in a. d. a temporary star shone out in the tail of scorpio. in a. d. arabian astronomers, under the caliph al-mamoun, the son of haroun-al-raschid, who broke into the great pyramid, observed a temporary star, that shone for four months in the constellation of the scorpion. in there was a temporary star, of a bluish color, in the tail of scorpio, and in another in the head of the constellation. besides these there are records of the appearance of four temporary stars in the neighboring constellation of ophiuchus, one of which, that of , is very famous, and will be described later on. it is conceivable that these strange outbursts in and near scorpio may have had some effect in causing this constellation to be regarded by the ancients as malign in its influence. we shall presently see some examples of star-clusters and nebulæ with which the instruments we are using are better capable of dealing than with the one described above. in the mean time, let us follow the bending row of stars from antares toward the south and east. when you reach the star mu ([mu]), you are not unlikely to stop with an exclamation of admiration, for the glass will separate it into two stars that, shining side by side, seem trying to rival each other in brightness. but the next star below [mu], marked zeta ([zeta]), is even more beautiful. it also separates into two stars, one being reddish and the other bluish in color. the contrast in a clear night is very pleasing. but this is not all. above the two stars you will notice a curious nebulous speck. now, if you have a powerful field-glass, here is an opportunity to view one of the prettiest sights in the heavens. the field-glass not only makes the two stars appear brighter, and their colors more pronounced, but it shows a third, fainter star below them, making a small triangle, and brings other still fainter stars into sight, while the nebulous speck above turns into a charmingly beautiful little star-cluster, whose components are so close that their rays are inextricably mingled in a maze of light. this little cut is an attempt to represent the scene, but no engraving can reproduce the life and sparkle of it. [illustration: zeta scorpionis.] following the bend of the scorpion's tail upward, we come to the pair of stars in the sting. these, of course, are thrown wide apart by the opera-glass. then let us sweep off to the eastward a little way and find the cluster known as m. you will see it marked on the map. above it, and near enough to be included in the same field of view, is m., a smaller cluster. both of these have a sparkling appearance with an opera-glass, and by close attention some of the separate stars in m. may be detected. with a field-glass these clusters become much more striking and starry looking, and the curious radiated structure of m. comes out. in looking at such objects we can not too often recall to our minds the significance of what we see--that these glimmering specks are the lights in the windows of the universe which carry to us, across inconceivable tracts of space, the assurance that we and our little system are not alone in the heavens; that all around us, and even on the very confines of immensity, nature is busy, as she is here, and the laws of light, heat, gravitation (and why not of life?), are in full activity. the clusters we have just been looking at lie on the borders of scorpio and sagittarius. let us cross over into the latter constellation, which commemorates the centaur chiron. we are now in another, and even a richer, region of wonders. the milky-way, streaming down out of the northeast, pours, in a luminous flood, through sagittarius, inundating that whole region of the heavens with seeming deeps and shallows, and finally bursting the barriers of the horizon disappears, only to glow with redoubled splendor in the southern hemisphere. the stars zeta ([zeta]), tau ([tau]), sigma ([sigma]), phi ([phi]), lambda ([lambda]), and mu ([mu]) indicate the outlines of a figure sometimes called the milk-dipper, which is very evident when the eye has once recognized it. on either side of the upturned handle of this dipper-like figure lie some of the most interesting objects in the sky. let us take the star [mu] for a starting-point. sweep downward and to the right a little way, and you will be startled by a most singular phenomenon that has suddenly made its appearance in the field of view of your glass. you may, perhaps, be tempted to congratulate yourself on having got ahead of all the astronomers, and discovered a comet. it is really a combination of a star-cluster with a nebula, and is known as m. sir john herschel has described the "nebulous folds and masses" and dark oval gaps which he saw in this nebula with his large telescope at the cape of good hope. but no telescope is needed to make it appear a wonderful object; an opera-glass suffices for that, and a field-glass reveals still more of its marvelous structure. the reader will recollect that we found the summer solstice close to a wonderful star-swarm in the feet of gemini. singularly enough the winter solstice is also near a star-cluster. it is to be found near a line drawn from m. to the star [mu] sagittarii, and about one third of the way from the cluster to the star. there is another less conspicuous star-cluster still closer to the solstitial point here, for this part of the heavens teems with such aggregations. on the opposite side of the star [mu]--that is to say, above and a little to the left--is an entirely different but almost equally attractive spectacle, the swarm of stars called m. here, again, the field-glass easily shows its superiority over the opera-glass, for magnifying power is needed to bring out the innumerable little twinklers of which the cluster is composed. but, whether you use an opera-glass or a field-glass, do not fail to gaze long and steadily at this island of stars, for much of its beauty becomes evident only after the eye has accustomed itself to disentangle the glimmering rays with which the whole field of view is filled. try the method of averted vision, and hundreds of the finest conceivable points of light will seem to spring into view out of the depths of the sky. the necessity of a perfectly clear night, and the absence of moonlight, can not be too much insisted upon for observations such as these. everybody knows how the moonlight blots out the smaller stars. a slight haziness, or smoke, in the air produces a similar effect. it is as important to the observer with an opera-glass to have a transparent atmosphere as it is to one who would use a telescope; but, fortunately, the work of the former is not so much interfered with by currents of air. always avoid the neighborhood of any bright light. electric lights in particular are an abomination to star-gazers. the cloud of stars we have just been looking at is in a very rich region of the milky-way, in the little modern constellation called "sobieski's shield," which we have not named upon our map. sweeping slowly upward from m. a little way with the field-glass, we will pass in succession over three nebulous-looking spots. the second of these, counting upward, is the famous horseshoe nebula. its wonders are beyond the reach of our instrument, but its place may be recognized. look carefully all around this region, and you will perceive that the old gods, who traveled this road (the milky-way was sometimes called the pathway of the gods), trod upon golden sands. off a little way to the east you will find the rich cluster called m. but do not imagine the thousands of stars that your opera-glass or field-glass reveals comprise all the riches of this golconda of the heavens. you might ply the powers of the greatest telescope in a vain attempt to exhaust its wealth. as a hint of the wonders that lie hidden here, let me quote father secchi's description of a starry spot in this same neighborhood, viewed with the great telescope at rome. after telling of "beds of stars superposed upon one another," and of the wonderful geometrical arrangement of the larger stars visible in the field, he adds: "the greater number are arranged in spiral arcs, in which one can count as many as ten or twelve stars of the ninth to the tenth magnitude following one another in a curve, like beads upon a string. sometimes they form rays which seem to diverge from a common focus, and, what is very singular, one usually finds, either at the center of the rays, or at the beginning of the curve, a more brilliant star of a red color, which seems to lead the march. it is impossible to believe that such an arrangement can be accidental." the reader will recall the somewhat similar description that admiral smyth and mr. webb have given of a star-cluster in gemini (see chapter i). the milky look of the background of the galaxy is, of course, caused by the intermingled radiations of inconceivably minute and inconceivably numerous stars, thousands of which become separately visible, the number thus distinguishable varying with the size of the instrument. but the most powerful telescope yet placed in human hands can not sound these starry deeps to the bottom. the evidence given by prof. holden, the director of the lick observatory, on this point is very interesting. speaking of the performance of the gigantic telescope on mount hamilton, thirty-six inches in aperture, he says: "the milky-way is a wonderful sight, and i have been much interested to see that there is, even with our superlative power, no final resolution of its finer parts into stars. there is always the background of unresolved nebulosity on which hundreds and thousands of stars are studded--each a bright, sharp, separate point." the groups of stars forming the eastern half of the constellation of sagittarius are worth sweeping over with the glass, as a number of pretty pairs may be found there. sagittarius stands in the old star-maps as a centaur, half-horse-half-man, facing the west, with drawn bow, and arrow pointed at the scorpion. [illustration: map .] next let us pass to the double constellation adjoining scorpio and sagittarius on the north--ophiuchus and the serpent. these constellations, as our map shows, are curiously intermixed. the imagination of the old star-gazers, who named them, saw here the figure of a giant grasping a writhing serpent with his hands. the head of the serpent is under the northern crown, and its tail ends over the star-gemmed region that we have just described, called "sobieski's shield." ophiuchus stands, as figured in flamsteed's "atlas," upon the back of the scorpion, holding the serpent with one hand below the neck, this hand being indicated by the pair of stars marked epsilon ([epsilon]) and delta ([delta]), and with the other near the tail. the stars tau ([tau]) and nu ([nu]) indicate the second hand. the giant's face is toward the observer, and the star alpha ([alpha]), also called ras alhague, shines in his forehead, while beta ([beta]) and gamma ([gamma]) mark his right shoulder. ophiuchus has been held to represent the famous physician Æsculapius. one may well repress the tendency to smile at these fanciful legends when he reflects upon their antiquity. there is no doubt that this double constellation is at least three thousand years old--that is to say, for thirty centuries the imagination of men has continued to shape these stars into the figures of a gigantic man struggling with a huge serpent. if it possesses no other interest, then it at least has that which attaches to all things ancient. like many other of the constellations it has proved longer-lived than the mightiest nations. while greece flourished and decayed, while rome rose and fell, while the scepter of civilization has passed from race to race, these starry creations of fancy have shone on unchanged. the mind that would ignore them now deserves compassion. the reader will observe a little circle in the map, and near it the figures . this indicates the spot where one of the most famous temporary stars on record appeared in the year . at first it was far brighter than any other star in the heavens; but it quickly faded, and in a little over a year disappeared. it is particularly interesting, because kepler--the quaintest, and not far from the greatest, figure in astronomical history--wrote a curious book about it. some of the philosophers of the day argued that the sudden outburst of the wonderful star was caused by the chance meeting of atoms. kepler's reply was characteristic, as well as amusing: "i will tell those disputants, my opponents, not my own opinion, but my wife's. yesterday, when i was weary with writing, my mind being quite dusty with considering these atoms, i was called to supper, and a salad i had asked for was set before me. 'it seems, then,' said i, aloud, 'that if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar and oil, and slices of egg, had been flying about in the air from all eternity, it might at last happen by chance that there would come a salad.' 'yes,' says my wife, 'but not so nice and well-dressed as this of mine is.'" while there are no objects of special interest for the observer with an opera-glass in ophiuchus, he will find it worth while to sweep over it for what he may pick up, and, in particular, he should look at the group of stars southeast of [beta] and [gamma]. these stars have been shaped into a little modern asterism called taurus poniatowskii, and it will be noticed that five of them mark the outlines of a letter v, resembling the well-known figure of the hyades. also look at the stars in the head of serpens, several of which form a figure like a letter [x]. a little west of theta ([theta]) in the tail of serpens, is a beautiful swarm of little stars, upon which a field-glass may be used with advantage. the star [theta] is itself a charming double, just within the separating power of a very powerful field-glass under favorable circumstances, the component stars being only about one third of a minute apart. do not fail to notice the remarkable subdivisions of the milky-way in this neighborhood. its current seems divided into numerous channels and bays, interspersed with gaps that might be likened to islands, and the star [theta] appears to be situated upon one of these islands of the galaxy. this complicated structure of the milky-way extends downward to the horizon, and upward through the constellation cygnus, and of its phenomenal appearance in that region we shall have more to say further on. directly north of ophiuchus is the constellation hercules, interesting as occupying that part of the heavens toward which the proper motion of the sun is bearing the earth and its fellow-planets, at the rate, probably, of not less than , , miles in a year--a stupendous voyage through space, of whose destination we are as ignorant as the crew of a ship sailing under sealed orders, and, like whom, we must depend upon such inferences as we can draw from courses and distances, for no other information comes to us from the flagship of our squadron. [illustration: map .] in the accompanying map we have represented the beautiful constellations lyra and the northern crown, lying on either side of hercules. the reader should note that the point overhead in this map is not far from the star eta ([eta]) in hercules. the bottom of the map is toward the south, the right-hand side is west, and the left-hand side east. it is important to keep these directions in mind, in comparing the map with the sky. for instance, the observer must not expect to look into the south and see hercules half-way up the sky, with lyra a little east of it; he must look for hercules nearly overhead, and lyra a little east of the zenith. the same precautions are not necessary in using the maps of scorpio, sagittarius, and ophiuchus, because those constellations are nearer the horizon, and so the observer does not have to imagine the map as being suspended over his head. the name hercules sufficiently indicates the mythological origin of the constellation, and yet the greeks did not know it by that name, for aratus calls it "the phantom whose name none can tell." the northern crown, according to fable, was the celebrated crown of ariadne, and lyra was the harp of orpheus himself, with whose sweet music he charmed the hosts of hades, and persuaded pluto to yield up to him his lost eurydice. with the aid of the map you will be able to recognize the principal stars and star-groups in hercules, and will find many interesting combinations of stars for yourself. an object of special interest is the celebrated star-cluster m. you will find it on the map between the stars eta ([eta]) and zeta ([zeta]). while an opera-glass will only show it as a faint and minute speck, lying nearly between two little stars, it is nevertheless well worth looking for, on account of the great renown of this wonderful congregation of stars. sir william herschel computed the number of stars contained in it as about fourteen thousand. it is roughly spherical in shape, though there are many straggling stars around it evidently connected with the cluster. in short, it is _a ball of suns_. the reader should not mistake what that implies, however. these suns, though truly solar bodies, are probably very much smaller than our sun. mr. gore has computed their average diameter to be forty-five thousand miles, and the distance separating each from the next to be , , , miles. it may not be uninteresting to inquire what would be the appearance of the sky to dwellers within such a system of suns. adopting mr. gore's estimates, and supposing , , , miles to be very nearly the uniform distance apart of the stars in the cluster, and forty-five thousand miles their uniform diameter, then, starting with a single star in the center, their arrangement might be approximately in concentric spherical shells, situated about , , , miles apart. the first shell, counting outward from the center, would contain a dozen stars, each of which, as seen by an observer stationed upon a planet at the center of the cluster, would shine eleven hundred times as bright as sirius appears to us. the number of the stars in each shell would increase as they receded from the center in proportion to the squares of the radii of the successive shells, while their luminosity, as seen from the center, would vary inversely as those squares. still, the outermost stars--the total number being limited to fourteen or fifteen thousand--would appear to our observer at the center of the system about five times as brilliant as sirius. it is clear, then, that he would be dwelling in a sort of perpetual daylight. his planet might receive from the particular sun around which it revolved as brilliant a daylight as our sun gives to us, but let us see what would be the illumination of its night side. adopting zöllner's estimate of the light of the sun as , times as great as that of the full moon, and choosing among the various estimates of the light of sirius as compared with the sun / as probably the nearest the truth, we find that the moon sends us about sixty-five hundred times as much light as sirius does. now, since the dozen stars nearest the center of the cluster would each appear to our observer eleven hundred times as bright as sirius, all of them together would give a little more than twice as much light as the full moon sheds upon the earth. but as only half the stars in the cluster would be above the horizon at once we must diminish this estimate by one half, in order to obtain the amount of light that our supposititious planet would receive on its night side from the nearest stars in the cluster. and since the number of these stars increases with their distance from the center in the same ratio as their light diminishes, it follows that the total light received from the cluster would exceed that received from the dozen nearest stars as many times as there were spherical shells in the cluster. this would be about fifteen times, and accordingly all the stars together would shed, at the center, some thirty times as much light as that of the moon. dividing this again by two, because only half of the stars could be seen at once, we find that the night side of our observer's planet would be illuminated with fifteen times as much light as the full moon sheds upon the earth. it is evident, too, that our observer would enjoy the spectacle of a starry firmament incomparably more splendid than that which we behold. only about three thousand stars are visible to our unassisted eyes at once on any clear night, and of those only a few are conspicuous, and two thirds are so faint that they require some attention in order to be distinguished. but the spectator at the center of the hercules cluster would behold some seven thousand stars at once, the faintest of which would be five times as brilliant as the brightest star in our sky, while the brighter ones would blaze like nearing suns. one effect of this flood of starlight would be to shut out from our observer's eyes all the stars of the outside universe. they would be effaced in the blaze of his sky, and he would be, in a manner, shut up within his own little star-system, knowing nothing of the greater universe beyond, in which we behold his multitude of luminaries, diminished and blended by distance into a faintly shining speck, floating like a silvery mote in a sunbeam. if our observer's planet, instead of being situated in the center of the cluster, circled around one of the stars at the outer edge of it, the appearance of his sky would be, in some respects, still more wonderful, the precise phenomena depending upon the position of the planet's orbit and the station of the observer. less than half of his sky would be filled, at any time, by the stars of the cluster, the other half opening upon outer space and appearing by comparison almost starless--a vast, cavernous expanse, with a few faint glimmerings out of its gloomy depths. the plane of the orbit of his planet being supposed to pass through the center of the spherical system, our observer would, during his year, behold the night at one season blazing with the splendors of the clustered suns, and at another emptied of brilliant orbs and faintly lighted with the soft glow of the milky-way and the feeble flickering of distant stars, scattered over the dark vault. the position of the orbit, and the inclination of the planet's axis might be such that the glories of the cluster would not be visible from one of its hemispheres, necessitating a journey to the other side of the globe to behold them.[b] [b] a similar calculation of the internal appearances of the hercules cluster, which i made, was published in in the "new york sun." of course, it is not to be assumed that the arrangement of the stars in the cluster actually is exactly that which we have imagined. still, whatever the arrangement, so long as the cluster is practically spherical, and the stars composing it are of nearly uniform size and situated at nearly uniform distances, the phenomena we have described would fairly represent the appearances presented to inhabitants of worlds situated in such a system. as to the possibility of the existence of such worlds and inhabitants, everybody must draw his own conclusions. astronomy, as a science, is silent upon that question. but there shine the congregated stars, mingling their rays in a message of light, that comes to us across the gulf, proclaiming their brotherhood with our own glorious sun. mathematicians can not unravel the interlocking intricacies of their orbits, and some would, perhaps _a priori_, have said that such a system was impossible, but the telescope has revealed them, and there they are! what purposes they subserve in the economy of the universe, who shall declare? if you have a field-glass, by all means try it upon m. it will give you a more satisfactory view than an opera-glass is capable of doing, and will magnify the cluster so that there can be no possibility of mistaking it for a star. compare this compact cluster, which only a powerful telescope can partially resolve into its component stars, with m. and m., described before, in order to comprehend the wide variety in the structure of these aggregations of stars. the northern crown, although a strikingly beautiful constellation to the naked eye, offers few attractions to the opera-glass. let us turn, then, to lyra. i have never been able to make up my mind which of three great stars is entitled to precedence--vega, the leading brilliant of lyra, arcturus in boötes, or capella in auriga. they are the three leaders of the northern firmament, but which of them should be called the chief, is very hard to say. at any rate, vega would probably be generally regarded as the most beautiful, on account of the delicate bluish tinge in its light, especially when viewed with a glass. there is no possibility of mistaking this star because of its surpassing brilliancy. two faint stars close to vega on the east make a beautiful little triangle with it, and thus form a further means of recognition, if any were needed. your opera-glass will show that the floor of heaven is powdered with stars, fine as the dust of a diamond, all around the neighborhood of vega, and the longer you gaze the more of these diminutive twinklers you will discover. [illustration: map .] now direct your glass to the northernmost of the two little stars near vega, the one marked epsilon ([epsilon]) in the map. you will perceive that it is composed of two stars of almost equal magnitude. if you had a telescope of considerable power, you would find that each of these stars is in turn double. in other words, this wonderful star which appears single to the unassisted eye, is in reality quadruple, and there is reason to think that the four stars composing it are connected in pairs, the members of each pair revolving around their common center while the two pairs in turn circle around a center common to all. with a field-glass you will be able to see that the other star near vega, zeta ([zeta]), is also double, the distance between its components being three quarters of a minute, while the two stars in [epsilon] are a little less than ½' apart. the star beta ([beta]) is remarkably variable in brightness. you may watch these variations, which run through a regular period of about days, ¾ hours, for yourself. between beta and gamma ([gamma]) lies the beautiful ring nebula, but it is hopelessly beyond the reach of the optical means we are employing. let us turn next to the stars in the west. in consulting the accompanying map of virgo and boötes (map no. ), the observer is supposed to face the southwest, at the hours and dates mentioned above as those to which the circular map corresponds. he will then see the bright star spica in virgo not far above the horizon, while arcturus will be half-way up the sky, and the northern crown will be near the zenith. the constellation virgo is an interesting one in mythological story. aratus tells us that the virgin's home was once on earth, where she bore the name of justice, and in the golden age all men obeyed her. in the silver age her visits to men became less frequent, "no longer finding the spirits of former days"; and, finally, when the brazen age came with the clangor of war: "justice, loathing that race of men, winged her flight to heaven; and fixed her station in that region where still by night is seen the virgin goddess near to bright boötes." the chief star of virgo, spica, is remarkable for its pure white light. to my eye there is no conspicuous star in the sky equal to it in this respect, and it gains in beauty when viewed with a glass. with the aid of the map the reader will find the celebrated binary star gamma ([gamma]) virginis, although he will not be able to separate its components without a telescope. it is a curious fact that the star epsilon ([epsilon]) in virgo has for many ages been known as the grape-gatherer. it has borne this name in greek, in latin, in persian, and in arabic, the origin of the appellation undoubtedly being that it was observed to rise just before the sun in the season of the vintage. it will be observed that the stars [epsilon], [delta], [gamma], [eta], and [beta], mark two sides of a quadrilateral figure of which the opposite corner is indicated by denebola in the tail of leo. within this quadrilateral lies the marvelous field of the nebulæ, a region where with adequate optical power one may find hundreds of these strange objects thronging together, a very storehouse of the germs of suns and worlds. unfortunately, these nebulæ are far beyond the reach of an opera-glass, but it is worth while to know where this curious region is, even if we can not behold the wonders it contains. the stars omicron ([omicron]), pi ([pi]), etc., forming a little group, mark the head of virgo. the autumnal equinox, or the place where the sun crosses the equator of the heavens on his southerly journey about the st of september, is situated nearly between the stars [eta] and [beta] virginis, a little below the line joining them, and somewhat nearer to [eta]. both [eta] and [zeta] virginis are almost exactly upon the equator of the heavens. the constellation libra, lying between virgo and scorpio, does not contain much to attract our attention. its two chief stars, [alpha] and [beta], may be readily recognized west of and above the head of scorpio. the upper one of the two, [beta], has a singular greenish tint, and the lower one, [alpha], is a very pretty double for an opera-glass. the constellation of libra appears to have been of later date than the other eleven members of the zodiacal circle. its two chief stars at one time marked the extended claws of scorpio, which were afterward cut off (perhaps the monster proved too horrible even for its inventors) to form libra. as its name signifies, libra represents a balance, and this fact seems to refer the invention of the constellation back to at least three hundred years before christ, when the autumnal equinox occurred at the moment when the sun was just crossing the western border of the constellation. the equality of the days and nights at that season readily suggests the idea of a balance. milton, in "paradise lost," suggests another origin for the constellation of the balance in the account of gabriel's discovery of satan in paradise: "... now dreadful deeds might have ensued, nor only paradise in this commotion, but the starry cope of heaven, perhaps, or all the elements at least had gone to wrack, disturbed and torn with violence of this conflict, had not soon the eternal, to prevent such horrid fray, hung forth in heaven his golden scales, yet seen betwixt astrea and the scorpion sign." just north of virgo's head will be seen the glimmering of berenice's hair. this little constellation was included among those described in the chapter on "the stars of spring," but it is worth looking at again in the early summer, on moonless nights, when the singular arrangement of the brighter members of the cluster at once strikes the eye. [illustration: berenice's hair.] boötes, whose leading brilliant, arcturus, occupies the center of our map, also possesses a curious mythical history. it is called by the greeks the bear-driver, because it seems continually to chase ursa major, the great bear, in his path around the pole. the story is that boötes was the son of the nymph calisto, whom juno, in one of her customary fits of jealousy, turned into a bear. boötes, who had become a famous hunter, one day roused a bear from her lair, and, not knowing that it was his mother, was about to kill her, when jupiter came to the rescue and snatched them both up into the sky, where they have shone ever since. lucan refers to this story when, describing brutus's visit to cato at night, he fixes the time by the position of these constellations in the heavens: "'twas when the solemn dead of night came on, when bright calisto, with her shining son, now half the circle round the pole had run." boötes is not specially interesting for our purposes, except for the splendor of arcturus. this star has possessed a peculiar charm for me ever since boyhood, when, having read a description of it in an old treatise on uranography, i felt an eager desire to see it. as my search for it chanced to begin at a season when arcturus did not rise till after a boy's bed-time, i was for a long time disappointed, and i shall never forget the start of surprise and almost of awe with which i finally caught sight of it, one spring evening, shooting its flaming rays through the boughs of an apple-orchard, like a star on fire. when near the horizon, arcturus has a remarkably reddish color; but, after it has attained a high elevation in the sky, it appears rather a deep yellow than red. there is a scattered cluster of small stars surrounding arcturus, forming an admirable spectacle with an opera-glass on a clear night. to see these stars well, the glass should be slowly moved about. many of them are hidden by the glare of arcturus. the little group of stars near the end of the handle of the great dipper, or, what is the same thing, the tail of the great bear, marks the upraised hand of boötes. between berenice's hair and the tail of the bear you will see a small constellation called canes venatici, the hunting-dogs. on the old star-maps boötes is represented as holding these dogs with a leash, while they are straining in chase of the bear. you will find some pretty groupings of stars in this constellation. and now we will turn to the east. our next map shows cygnus, a constellation especially remarkable for the large and striking figure that it contains, called the northern cross, aquila the eagle, the dolphin, and the little asterisms sagitta and vulpecula. in consulting the map, the observer is supposed to face toward the east. in aquila the curious arrangement of two stars on either side of the chief star of the constellation, called altair, at once attracts the eye. within a circle including the two attendants of altair you will probably be able to see with the naked eye only two or three stars in addition to the three large ones. now turn your glass upon the same spot, and you will see eight or ten times as many stars, and with a field-glass still more can be seen. watch the star marked eta ([eta]), and you will find that its light is variable, being sometimes more than twice as bright as at other times. its changes are periodical, and occupy a little over a week. the eagle is fabled to have been the bird that jupiter kept beside his throne. a constellation called antinous, invented by tycho brahe, is represented on some maps as occupying the lower portion of the space given to aquila. the dolphin is an interesting little constellation, and the ancients said it represented the very animal on whose back the famous musician arion rode through the sea after his escape from the sailors who tried to murder him. but some modern has dubbed it with the less romantic name of job's coffin, by which it is sometimes called. it presents a very pretty sight to the opera-glass. cygnus, the swan, is a constellation whose mythological history is not specially interesting, although, as remarked above, it contains one of the most clearly marked figures to be found among the stars, the famous northern cross. the outlines of this cross are marked with great distinctness by the stars alpha ([alpha]), epsilon ([epsilon]), gamma ([gamma]), delta ([delta]), and beta ([beta]), together with some fainter stars lying along the main beam of the cross between [beta] and [gamma]. the star [beta], also called albireo, is one of the most beautiful double stars in the heavens. the components are sharply contrasted in color, the larger star being golden-yellow, while the smaller one is a deep, rich blue. with a field-glass of . -inch aperture and magnifying seven times i have sometimes been able to divide this pair, and to recognize the blue color of the smaller star. it will be found a severe test for such a glass. [illustration: map .] about half-way from albireo to the two stars [zeta] and [epsilon] in aquila is a very curious little group, consisting of six or seven stars in a straight row, with a garland of other stars hanging from the center. to see it best, take a field-glass, although an opera-glass shows it. i have indicated the place of the celebrated star cygni in the map, because of the interest attaching to it as the nearest to us, so far as we know, of all the stars in the northern hemisphere, and with one exception the nearest star in all the heavens. yet it is very faint, and the fact that so inconspicuous a star should be nearer than such brilliants as vega and arcturus shows how wide is the range of magnitude among the suns that light the universe. the actual distance of cygni is something like , times as great as the distance from the earth to the sun. the star omicron ([omicron]) is very interesting with an opera-glass. the naked eye sees a little star near it. the glass throws them wide apart, and divides [omicron] itself into two stars. now, a field-glass, if of sufficient power, will divide the larger of these stars again into two--a fine test. sweep around [alpha] and [gamma] for the splendid star-fields that abound in this neighborhood; also around the upper part of the figure of the cross. we are here in one of the richest parts of the milky-way. between the stars [alpha], [gamma], [epsilon], is the strange dark gap in the galaxy called the coal-sack, a sort of hole in the starry heavens. although it is not entirely empty of stars, its blackness is striking in contrast with the brilliancy of the milky-way in this neighborhood. the divergent streams of the great river of light in this region present a very remarkable appearance. [illustration: map .] finally, we come to the great dragon of the sky. in using the map of draco and the neighboring constellations, the reader is supposed to face the north. the center of the upper edge of the map is directly over the observer's head. one of the stories told of this large constellation is that it represents a dragon that had the temerity to war against minerva. the goddess "seized it in her hand, and hurled it, twisted as it was, into the heavens round the axis of the world, before it had time to unwind its contortions." others say it is the dragon that guarded the golden apples in the garden of the hesperides, and that was slain by the redoubtable hercules. at any rate, it is plainly a monster of the first magnitude. the stars [beta], [gamma], [xi], [nu], and [mu] represent its head, while its body runs trailing along, first sweeping in a long curve toward cepheus, and then bending around and passing between the two bears. try [nu] with your opera-glass, and if you succeed in seeing it double you may congratulate yourself on your keen sight. the distance between the stars is about '. notice the contrasted colors of [gamma] and [beta], the former being a rich orange and the latter white. as you sweep along the winding way that draco follows, you will run across many striking fields of stars, although the heavens are not as rich here as in the splendid regions that we have just left. you will also find that cepheus, although not an attractive constellation to the naked eye, is worth some attention with an opera-glass. the head and upper part of the body of cepheus are plunged in the stream of the milky way, while his feet are directed toward the pole of the heavens, upon which he is pictured as standing. cepheus, however, sinks into insignificance in comparison with its neighbor cassiopeia, but that constellation belongs rather to the autumn sky, and we shall pass it by here. chapter iii. the stars of autumn. in the "fifth evening" of that delightful, old, out-of-date book of fontenelle's, on the "plurality of worlds," the astronomer and the marchioness, who have been making a wonderful pilgrimage through the heavens during their evening strolls in the park, come at last to the starry systems beyond the "solar vortex," and the marchioness experiences a lively impatience to know what the fixed stars will turn out to be, for the astronomer has sharpened her appetite for marvels. "tell me," says she, eagerly, "are they, too, inhabited like the planets, or are they not peopled? in short, what can we make of them?" the astronomer answers his charming questioner, as we should do to-day, that the fixed stars are so many suns. and he adds to this information a great deal of entertaining talk about the planets that may be supposed to circle around these distant suns, interspersing his conversation with explanations of "vortexes," and many quaint conceits, in which he is helped out by the ready wit of the marchioness. finally, the impressionable mind of the lady is overwhelmed by the grandeur of the scenes that the astronomer opens to her view, her head swims, infinity oppresses her, and she cries for mercy. "you show me," she exclaims, "a perspective so interminably long that the eye can not see the end of it. i see plainly the inhabitants of the earth; then you cause me to perceive those of the moon and of the other planets belonging to our vortex (system), quite clearly, yet not so distinctly as those of the earth. after them come the inhabitants of planets in the other vortexes. i confess, they seem to me hidden deep in the background, and, however hard i try, i can barely glimpse them at all. in truth, are they not almost annihilated by the very expression which you are obliged to use in speaking of them? you have to call them inhabitants of one of the planets contained in one out of the infinity of vortexes. surely we ourselves, to whom the same expression applies, are almost lost among so many millions of worlds. for my part, the earth begins to appear so frightfully little to me that henceforth i shall hardly consider any object worthy of eager pursuit. assuredly, people who seek so earnestly their own aggrandizement, who lay schemes upon schemes, and give themselves so much trouble, know nothing of the vortexes! i am sure my increase of knowledge will redound to the credit of my idleness, and when people reproach me with indolence i shall reply: 'ah! if you but knew the history of the fixed stars!'" it is certainly true that a contemplation of the unthinkable vastness of the universe, in the midst of which we dwell upon a speck illuminated by a spark, is calculated to make all terrestrial affairs appear contemptibly insignificant. we can not wonder that men for ages regarded the earth as the center, and the heavens with their lights as tributary to it, for to have thought otherwise, in those times, would have been to see things from the point of view of a superior intelligence. it has taken a vast amount of experience and knowledge to convince men of the parvitude of themselves and their belongings. so, in all ages they have applied a terrestrial measure to the universe, and imagined they could behold human affairs reflected in the heavens and human interests setting the gods together by the ears. [illustration: map. .] this is clearly shown in the story of the constellations. the tremendous truth that on a starry night we look, in every direction, into an almost endless vista of suns beyond suns and systems upon systems, was too overwhelming for comprehension by the inventors of the constellations. so they amused themselves, like imaginative children, as they were, by tracing the outlines of men and beasts formed by those pretty lights, the stars. they turned the starry heavens into a scroll filled with pictured stories of mythology. four of the constellations with which we are going to deal in this chapter are particularly interesting on this account. they preserve in the stars, more lasting than parchment or stone, one of the oldest and most pleasing of all the romantic stories that have amused and inspired the minds of men--the story of perseus and andromeda--a better story than any that modern novelists have invented. the four constellations to which i refer bear the names of andromeda, perseus, cassiopeia, and cepheus, and are sometimes called, collectively, the royal family. in the autumn they occupy a conspicuous position in the sky, forming a group that remains unrivaled until the rising of orion with his imperial _cortége_. the reader will find them in map no. , occupying the northeastern quarter of the heavens. this map represents the visible heavens at about midnight on september st, ten o'clock p. m. on october st, and eight o'clock p. m. on november st. at this time the constellations that were near the meridian in summer will be found sinking in the west, hercules being low in the northwest, with the brilliant lyra and the head of draco suspended above it; aquila, "the eagle of the winds," soars high in the southwest; while the cross of cygnus is just west of the zenith; and sagittarius, with its wealth of star-dust, is disappearing under the horizon in the southwest. far down in the south the observer catches the gleam of a bright lone star of the first magnitude, though not one of the largest of that class. it is fomalhaut, in the mouth of the southern fish, piscis australis. a slight reddish tint will be perceived in the light of this beautiful star, whose brilliance is enhanced by the fact that it shines without a rival in that region of the sky. fomalhaut is one of the important "nautical stars," and its position was long ago carefully computed for the benefit of mariners. the constellation of piscis australis, which will be found in our second map, does not possess much to interest us except its splendid leading star. in consulting map , the observer is supposed to be facing south, or slightly west of south, and he must remember that the upper part of the map reaches nearly to the zenith, while at the bottom it extends down to the horizon. [illustration: map .] to the right, or west, of fomalhaut, and higher up, is the constellation of capricornus, very interesting on many accounts, though by no means a striking constellation to the unassisted eye. the stars alpha ([alpha]), called giedi, and beta ([beta]), called dabih, will be readily recognized, and a keen eye will perceive that alpha really consists of two stars. they are about six minutes of arc apart, and are of the third and the fourth magnitude respectively. these stars, which to the naked eye appear almost blended into one, really have no physical connection with each other, and are slowly drifting apart. the ancient astronomers make no mention of giedi being composed of two stars, and the reason is plain, when it is known that in the time of hipparchus, as flammarion has pointed out, their distance apart was not more than two thirds as great as it is at present, so that the naked eye could not have detected the fact that there were two of them; and it was not until the seventeenth century that they got far enough asunder to begin to be separated by eyes of unusual power. with an ordinary opera-glass they are thrown well apart, and present a very pretty sight. considering the manner in which these stars are separating, the fact that both of them have several faint companions, which our powerful telescopes reveal, becomes all the more interesting. a suggestion of sir john herschel, concerning one of these faint companions, that it shines by reflected light, adds to the interest, for if the suggestion is well founded the little star must, of course, be actually a planet, and granting that, then some of the other faint points of light seen there are probably planets too. it must be said that the probabilities are against herschel's suggestion. the faint stars more likely shine with their own light. even so, however, these two systems, which apparently have met and are passing one another, at a distance small as compared with the space that separates them from us, possess a peculiar interest, like two celestial fleets that have spoken one another in the midst of the ocean of space. the star beta, or dabih, is also a double star. the companion is of a beautiful blue color, generally described as "sky-blue." it is of the seventh magnitude, while the larger star is of magnitude three and a half. the latter is golden-yellow. the blue of the small star can be seen with either an opera- or a field-glass, but it requires careful looking and a clear and steady atmosphere. i recollect discovering the color of this star with a field-glass, and exclaiming to myself, "why, the little one is as blue as a bluebell!" before i knew that that was its hue as seen with a telescope. trying my opera-glass upon it i found that the color was even more distinct, although the small star was then more or less enveloped in the yellow rays of the large one. the distance between the two stars in dabih is nearly the same as that between the components of [epsilon] lyræ, and the comparative difficulty of separating them is an instructive example of the effect of a large star in concealing a small one close beside it. the two stars in [epsilon] lyræ are of nearly equal brightness, and are very easily separated and distinguished, but in [beta] capricorni, or dabih, one star is about twenty times as bright as the other, and consequently the fainter star is almost concealed in the glare of its more brilliant neighbor. with the most powerful glass at your disposal, sweep from the star zeta ([zeta]) eastward a distance somewhat greater than that separating alpha and beta, and you will find a fifth-magnitude star beside a little nebulous spot. this is the cluster known as m, one of those sun-swarms that overwhelm the mind of the contemplative observer with astonishment, and especially remarkable in this case for the apparent vacancy of the heavens immediately surrounding the cluster, as if all the stars in that neighborhood had been drawn into the great assemblage, leaving a void around it. of course, with the instrument that our observer is supposed to be using, merely the _existence_ of this solar throng can be detected; but, if he sees that it is there, he may be led to provide himself with a telescope capable of revealing its glories. admiral smyth remarks that, "although capricorn is not a striking object, it has been the very pet of all constellations with astrologers," and he quotes from an old almanac of the year , that "whoso is borne in capcorn schal be ryche and wel lufyd." the mythological account of the constellation is that it represents the goat into which pan was turned in order to escape from the giant typhon, who once on a time scared all the gods out of their wits, and caused them to change themselves into animals, even jupiter assuming the form of a ram. according to some authorities, piscis australis represents the fish into which venus changed herself on that interesting occasion. directly above piscis australis, and to the east or left of capricorn, the map shows the constellation of aquarius, or the water-bearer. some say this commemorates ganymede, the cup-bearer of the gods. it is represented in old star-maps by the figure of a young man pouring water from an urn. the star alpha ([alpha]) marks his right shoulder, and beta ([beta]) his left, and gamma ([gamma]), zeta ([zeta]), eta ([eta]), and pi ([pi]) indicate his right hand and the urn. from this group a current of small stars will be recognized, sweeping downward with a curve toward the east, and ending at fomalhaut; this represents the water poured from the urn, which the southern fish appears to be drinking. in fact, according to the pictures in the old maps, the fish succeeds in swallowing the stream completely, and it vanishes from the sky in the act of entering his distended mouth! it is worthy of remark that in greek, latin, and arabic this constellation bears names all of which signify "a man pouring water." the ancient egyptians imagined that the setting of aquarius caused the rising of the nile, as he sank his huge urn in the river to fill it. alpha aquarii was called by the arabs sadalmelik, which is interpreted to mean the "king's lucky star," but whether it proved itself a lucky star in war or in love, and what particular king enjoyed its benign influence and recorded his gratitude in its name, we are not informed. thus, at every step, we find how shreds of history and bits of superstition are entangled among the stars. surely, humanity has been reflected in the heavens as lastingly as it has impressed itself upon the earth. starting from the group of stars just described as forming the water-bearer's urn, follow with a glass the winding stream of small stars that represent the water. several very pretty and striking assemblages of stars will be encountered in its course. the star tau ([tau]) is double and presents a beautiful contrast of color, one star being white and the other reddish-orange--two solar systems, it may be, apparently neighbors as seen from the earth, in one of which daylight is white and in the other red! point a good glass upon the star marked nu ([nu]), and you will see, somewhat less than a degree and a half to the west of it, what appears to be a faint star of between the seventh and eighth magnitudes. you will have to look sharp to see it. it is with your mind's eye that you must gaze, in order to perceive the wonder here hidden in the depths of space. that faint speck is a nebula, unrivaled for interest by many of the larger and more conspicuous objects of that kind. lord rosse's great telescope has shown that in form it resembles the planet saturn; in other words, that it consists apparently of a ball surrounded by a ring. but the spectroscope proves that it is a gaseous mass, and the micrometer--supposing its distance to be equal to that of the stars, and we have no reason to think it less--that it must be large enough to fill the whole space included within the orbit of neptune! here, then, as has been said, we seem to behold a genesis in the heavens. if laplace's nebular hypothesis, or any of the modifications of that hypothesis, represents the process of formation of a solar system, then we may fairly conclude that such a process is now actually in operation in this nebula in aquarius, where a vast ring of nebulous matter appears to have separated off from the spherical mass within it. this may not be the true explanation of what we see there, but, whatever the explanation is, there can be no question of the high significance of this nebula, whose shape proclaims unmistakably the operation of great metamorphic forces there. of course, with his insignificant optical means, our observer can see nothing of the strange form of this object, the detection of which requires the aid of the most powerful telescopes, but it is much to know where that unfinished creation lies, and to see it, even though diminished by distance to a mere speck of light. turn your glass upon the star shown in the map just above mu ([mu]) and epsilon ([epsilon]). you will find an attractive arrangement of small stars in its neighborhood. the star marked is double to the naked eye, and the row of stars below it is well worth looking at. the star delta ([delta]) indicates the place where, in , tobias mayer narrowly escaped making a discovery that would have anticipated that which a quarter of a century later made the name of sir william herschel world-renowned. the planet uranus passed near delta in , and tobias mayer saw it, but it moved so slowly that he took it for a fixed star, never suspecting that his eyes had rested upon a member of the solar system whose existence was, up to that time, unknown to the inhabitants of adam's planet. above aquarius you will find the constellation pegasus. it is conspicuously marked by four stars of about the second magnitude, which shine at the corners of a large square, called the great square of pegasus. this figure is some fifteen degrees square, and at once attracts the eye, there being few stars visible within the quadrilateral, and no large ones in the immediate neighborhood to distract attention from it. one of the four stars, however, as will be seen by consulting map , does not belong to pegasus, but to the constellation andromeda. mythologically, this constellation represents the celebrated winged horse of antiquity: "now heaven his further wandering flight confines, where, splendid with his numerous stars, he shines." the star alpha ([alpha]) is called markab; beta ([beta]) is scheat, and gamma ([gamma]) is algenib; the fourth star in the square, belonging to andromeda, is called alpheratz. although pegasus presents a striking appearance to the unassisted eye, on account of its great square, it contains little to attract the observer with an opera-glass. it will prove interesting, however, to sweep with the glass carefully over the space within the square, which is comparatively barren to the naked eye, but in which many small stars will be revealed, of whose existence the naked-eye observer would be unaware. the star marked pi ([pi]) is an interesting double, which can be separated by a good eye without artificial aid, and which, with an opera-glass, presents a fine appearance. and now we come to map no. , representing the constellations cetus, pisces, aries, and the triangles. in consulting it the observer is supposed to face the southeast. cetus is a very large constellation, and from the peculiar conformation of its principal stars it can be readily recognized. the head is to the east, the star alpha ([alpha]), called menkar, being in the nose of this imaginary inhabitant of the sky-depths. the constellation is supposed to represent the monster that, according to fable, was sent by neptune to devour the fair andromeda, but whose bloodthirsty design was happily and gallantly frustrated by perseus, as we shall learn from starry mythology further on. although bearing the name cetus, the whale, the pictures of the constellation in the old maps do not present us with the form of a whale, but that of a most extraordinary scaly creature with enormous jaws filled with large teeth, a forked tongue, fore-paws armed with gigantic claws, and a long, crooked, and dangerous-looking tail. indeed, aratus does not call it a "whale," but a "sea-monster," and dr. seiss would have us believe that it was intended to represent the leviathan, whose terrible prowess is celebrated in the book of job. [illustration: map .] by far the most interesting object in cetus is the star mira. this is a famous variable--a sun that sometimes shines a thousand-fold more brilliantly than at others! it changes from the second magnitude to the ninth or tenth, its period from maximum to maximum being about eleven months. during about five months of that time it is completely invisible to the naked eye; then it begins to appear again, slowly increasing in brightness for some three months, until it shines as a star of the second magnitude, being then as bright as, if not brighter than, the most brilliant stars in the constellation. it retains this brilliance for about two weeks, and then begins to fade again, and, within three months, once more disappears. there are various irregularities in its changes, which render its exact period somewhat uncertain, and it does not always attain the same degree of brightness at its maximum. for instance, in , mira was almost equal in brilliance to a first-magnitude star, but frequently at its greatest brightness it is hardly equal to an ordinary star of the second magnitude. by the aid of our little map you will readily be able to find it. you will perceive that it has a slightly reddish tint. watch it from one of its maxima, and you will see it gradually fade from sight until, at last, only the blackness of the empty sky appears where, a few months before, a conspicuous star was visible. keep watch of that spot, and in due course you will perceive mira shining there again--a mere speck, but slowly brightening--and in three months more the wonderful star will blaze again with renewed splendor. knowing that our own sun is a variable star--though variable only to a slight degree, its variability being due to the spots that appear upon its surface in a period of about eleven years--we possess some light that may be cast upon the mystery of mira's variations. it seems not improbable that, in the case of mira, the surface of the star at the maximum of spottedness is covered to an enormously greater extent than occurs during our own sun-spot maxima, so that the light of the star, instead of being merely dimmed to an almost imperceptible extent, as with our sun, is almost blotted out. when the star blazes with unwonted splendor, as in , we may fairly assume that the pent-up forces of this perishing sun have burst forth, as in a desperate struggle against extinction. but nothing can prevail against the slow, remorseless, unswerving progress of that obscuration, which comes from the leaking away of the solar heat, and which constitutes what we may call the death of a sun. and that word seems peculiarly appropriate to describe the end of a body which, during its period of visible existence, not only presents the highest type of physical activity, but is the parent and supporter of all forms of life upon the planets that surround it. we might even go so far as to say that possibly mira presents to us an example of what our sun will be in the course of time, as the dead and barren moon shows us, as in a magician's glass, the approaching fate of the earth. fortunately, human life is a mere span in comparison with the æons of cosmic existence, and so we need have no fear that either we or our descendants for thousands of generations shall have to play the tragic _rôle_ of campbell's "last man," and endeavor to keep up a stout heart amid the crash of time by meanly boasting to the perishing sun, whose rays have nurtured us, that, though his proud race is ended, we have confident anticipations of immortality. i trust that, when man makes his exit from this terrestrial stage, it will not be in the contemptible act of kicking a fallen benefactor. there are several other variable stars in cetus, but none possessing much interest for us. the observer should look at the group of stars in the head, where he will find some interesting combinations, and also at chi, which is the little star shown in the map near zeta ([zeta]). this is a double that will serve as a very good test of eye and instrument, the smaller companion-star being of only seven and a half magnitude. directly above cetus is the long, straggling constellation of pisces, the fishes. the northern fish is represented by the group of stars near andromeda and the triangles. a long band or ribbon, supposed to bind the fish together, trends thence first southeast and then west until it joins a group of stars under pegasus, which represents the western fish, not to be confounded with the southern fish described near the beginning of this chapter, which is a separate constellation. fable has, however, somewhat confounded these fishes; for while, as i have remarked above, the southern fish is said to represent venus after she had turned herself into a fish to escape from the giant typhon, the two fishes of the constellation we are now dealing with are also fabled to represent venus and her interesting son cupid under the same disguise assumed on precisely the same occasion. if typhon, however, was so great a brute that even cupid's arrows were of no avail against him, we should, perhaps, excuse mythology for duplicating the record of so wondrous an event. you will find it very interesting to take your glass and, beginning with the attractive little group in the northern fish, follow the windings of the ribbon, with its wealth of tiny stars, to the western fish. when you have arrived at that point, sweep well over the sky in that neighborhood, and particularly around and under the stars iota ([iota]), theta ([theta]), lambda ([lambda]), and kappa ([kappa]). if you are using a powerful glass, you will be surprised and delighted by what you see. below the star omega ([omega]), and to the left of lambda, is the place which the sun occupies at the time of the spring equinox--in other words, one of the two crossing-places of the equinoctial or the equator of the heavens, and the ecliptic, or the sun's path. the prime meridian of the heavens passes through this point. you can trace out this great circle, from which astronomical longitudes are reckoned, by drawing an imaginary line from the equinoctial point just indicated through [alpha] in andromeda and [beta] in cassiopeia to the pole-star. to the left of pisces, and above the head of cetus, is the constellation aries, or the ram. two pretty bright stars, four degrees apart, one of which has a fainter star near it, mark it out plainly to the eye. these stars are in the head of the ram. the brightest one, alpha ([alpha]), is called hamal; beta ([beta]) is named sheratan; and its fainter neighbor is mesarthim. according to fable, this constellation represents the ram that wore the golden fleece, which was the object of the celebrated expedition of the argonauts. there is not much in the constellation to interest us, except its historical importance, as it was more than two thousand years ago the leading constellation of the zodiac, and still stands first in the list of the zodiacal signs. owing to the precession of the equinoxes, however, the vernal equinoctial point, which was formerly in this constellation, has now advanced into the constellation pisces, as we saw above. gamma ([gamma]), arietis, is interesting as the first telescopic double star ever discovered. its duplicity was detected by dr. hooke while watching the passage of a comet near the star in . singularly enough, the brightest star in the constellation, now bearing the letter [alpha], originally did not belong to the constellation. tycho brahe finally placed it in the head of aries. the little constellation of the triangles, just above aries, is worth only a passing notice. insignificant as it appears, this little group is a very ancient constellation. it received its name, deltoton, from the greek letter [delta]. [illustration: map .] the reader must now be introduced to the "royal family." although the story of perseus and andromeda is, of course, well known to nearly all readers, yet, on account of the great beauty and brilliancy of the group of constellations that perpetuate the memory of it among the stars, it is worth recalling here. it will be remembered that, as perseus was returning through the air from his conquest of the gorgon medusa, he saw the beautiful andromeda chained to a rock on the sea-coast, waiting to be devoured by a sea-monster. the poor girl's only offense was that her mother, cassiopeia, had boasted for her that she was fairer than the sea-beauty, atergatis, and for this neptune had decreed that all the land of the ethiopians should be drowned and destroyed unless andromeda was delivered up as a sacrifice to the dreadful sea-monster. when perseus, dropping down to learn why this maiden was chained to the rocks, heard from andromeda's lips the story of her woes, he laughed with joy. here was an adventure just to his liking, and besides, unlike his previous adventures, it involved the fate of a beautiful woman with whom he was already in love. could he save her? well, wouldn't he! the sea-monster might frighten a kingdom full of ethiops, but it could not shake the nerves of a hero from greece. he whispered words of encouragement to andromeda, who could scarce believe the good news that a champion had come to defend her after all her friends and royal relations had deserted her. neither could she feel much confidence in her young champion's powers when suddenly her horrified gaze met the awful leviathan of the deep advancing to his feast! but perseus, with a warning to andromeda not to look at what he was about to do, sprang with his winged sandals up into the air. and then, as charles kingsley has so beautifully told the story-- "on came the great sea-monster, coasting along like a huge black galley, lazily breasting the ripple, and stopping at times by creek or headland to watch for the laughter of girls at their bleaching, or cattle pawing on the sand-hills, or boys bathing on the beach. his great sides were fringed with clustering shells and sea-weeds, and the water gurgled in and out of his wide jaws as he rolled along, dripping and glistening in the beams of the morning sun. at last he saw andromeda, and shot forward to take his prey, while the waves foamed white behind him, and before him the fish fled leaping. "then down from the height of the air fell perseus like a shooting-star--down to the crest of the waves, while andromeda hid her face as he shouted. and then there was silence for a while. "at last she looked up trembling, and saw perseus springing toward her; and, instead of the monster, a long, black rock, with the sea rippling quietly round it." perseus had turned the monster into stone by holding the blood-freezing head of medusa before his eyes; and it was fear lest andromeda herself might see the gorgon's head, and suffer the fate of all who looked upon it, that had led him to forbid her watching him when he attacked her enemy. afterward he married her, and cassiopeia, andromeda's mother, and cepheus, her father, gave their daughter's rescuer a royal welcome, and all the ethiops rose up and blessed him for ridding the land of the monster. and now, if we choose, we can, any fair night, see the principal characters of this old romance shining in starry garb in the sky. aratus saw them there in his day, more than two hundred years before christ, and has left this description in his "skies," as translated by poste: "nor shall blank silence whelm the harassed house of cepheus; the high heavens know their name, for zeus is in their line at few removes. cepheus himself by she-bear cynosure, iasid king stands with uplifted arms. from his belt thou castest not a glance to see the first spire of the mighty dragon. "eastward from him, heaven-troubled queen, with scanty stars but lustrous in the full-mooned night, sits cassiopeia. not numerous nor double-rowed the gems that deck her form, but like a key which through an inward-fastened folding-door men thrust to knock aside the bolts, they shine in single zigzag row. she, too, o'er narrow shoulders stretching uplifted hands, seems wailing for her child. "for there, a woful statue-form, is seen andromeda, parted from her mother's side. long i trow thou wilt not seek her in the nightly sky, so bright her head, so bright her shoulders, feet, and girdle. yet even there she has her arms extended, and shackled even in heaven; uplifted, outspread eternally are those fair hands. "her feet point to her bridegroom perseus, on whose shoulder they rest. he in the north-wind stands gigantic, his right hand stretched toward the throne where sits the mother of his bride. as one bent on some high deed, dust-stained he strides over the floor of heaven." the makers of old star-maps seem to have vied in the effort to represent with effect the figures of andromeda, perseus, and cassiopeia among the stars, and it must be admitted that some of them succeeded in giving no small degree of life and spirit to their sketches. the starry riches of these constellations are well matched with their high mythological repute. lying in and near the milky-way, they are particularly interesting to the observer with an opera-glass. besides, they include several of the most celebrated wonders of the firmament. in consulting map no. , the observer is supposed to face the east and northeast. we will begin our survey with andromeda. the three chief stars of this constellation are of the second magnitude, and lie in a long, bending row, beginning with alpha ([alpha]), or alpheratz, in the head, which, as we have seen, marks one corner of the great square of pegasus. beta ([beta]), or mirach, with the smaller stars mu ([mu]) and nu ([nu]), form the girdle. the third of the chief stars is gamma ([gamma]), or almaach, situated in the left foot. the little group of stars designated lambda ([lambda]), kappa ([kappa]), and iota ([iota]), mark the extended right hand chained to the rock, and zeta ([zeta]) and some smaller stars southwest of it show the left arm and hand, also stretched forth and shackled. in searching for picturesque objects in andromeda, begin with alpheratz and the groups forming the hands. below the girdle will be seen a rather remarkable arrangement of small stars in the mouth of the northern fish. now follow up the line of the girdle to the star nu ([nu]). if your glass has a pretty wide field, your eye will immediately catch the glimmer of the great nebula of andromeda in the same field with the star. this is the oldest or earliest discovered of the nebulæ, and, with the exception of that in orion, is the grandest visible in this hemisphere. of course, not much can be expected of an opera-glass in viewing such an object; and yet a good glass, in clear weather and the absence of the moon, makes a very attractive spectacle of it. [illustration: the great andromeda nebula.] by turning the eyes aside, the nebula can be seen, extended as a faint, wispy light, much elongated on either side of the brighter nucleus. the cut here given shows, approximately, the appearance of the nebula, together with some of the small stars in its neighborhood, as seen with a field-glass. with large telescopes it appears both larger and broader, expanding to a truly enormous extent, and in bond's celebrated picture of it we behold gigantic rifts running lengthwise, while the whole field of sky in which it is contained appears sprinkled over with minute stars apparently between us and the nebula. it was in, or, probably more properly speaking, in line with, this nebula that a new star suddenly shone out in , and, after flickering and fading for a few months, disappeared. that the outburst of light in this star had any real connection with the nebula is exceedingly improbable. although it appeared to be close beside the bright nucleus of the nebula, it is likely that it was really hundreds or thousands of millions of miles either this side or the other side of it. why it should suddenly have blazed into visibility, and then in so short a time have disappeared, is a question as difficult as it is interesting. the easiest way to account for it, if not the most satisfactory, is to assume that it is a variable star of long period, and possessing a very wide range of variability. one significant fact that would seem to point to some connection between star and the nebula, after all, is that a similar occurrence was noticed in the constellation scorpio in , and to which i have previously referred (see chapter ii). in that case a faint star projected against the background of a nebula, suddenly flamed into comparatively great brilliance, and then faded again. the chances against the accidental superposition of a variable star of such extreme variability upon a known nebula occurring twice are so great that, for that reason alone, we might be justified in thinking some mysterious causal relation must in each case exist between the nebula and the star. the temptation to indulge in speculation is very great here, but it is better to wait for more light, and confess that for the present these things are inexplicable. it will be found very interesting to sweep with the glass slowly from side to side over andromeda, gradually approaching toward cassiopeia or perseus. the increase in the richness of the stratum of faint stars that apparently forms the background of the sky will be clearly discernible as you approach the milky-way, which passes directly through cassiopeia and perseus. it may be remarked that the milky-way itself, in that splendidly rich region about sagittarius (described in the "stars of summer"), is not nearly so effective an object with an opera-glass as it is above cygnus and in the region with which we are now dealing. this seems to be owing to the smaller magnitude of its component stars in the southern part of the stream. there the background appears more truly "milky," while in the northern region the little stars shine distinct, like diamond-specks, on a black background. the star nu, which serves as a pointer to the great nebula, is itself worth some attention with a pretty strong glass on account of a pair of small stars near it. the star gamma ([gamma]) is interesting, not only as one of the most beautiful triples in the heavens (an opera-glass is far too feeble an instrument to reveal its companions), but because it serves to indicate the radiant point of the biela meteors. there was once a comet well known to astronomers by the name of its discoverer, biela. it repeated its visits to the neighborhood of the sun once in every six or seven years. in this comet astonished all observers by splitting into two comets, which continued to run side by side, like two equal racers, in their course around the sun. each developed a tail of its own. in , when the twin comets were due again, the astronomical world was on the _qui vive_, and they did not disappoint expectation, for back they came out of the depths of space, still racing, but much farther apart than they had been before, alternating in brightness as if the long struggle had nearly exhausted them, and finally, like spent runners, growing faint and disappearing. they have never been seen since. in , when the comets should have been visible, if they still existed, a very startling thing happened. out of the northern heavens, along the track of the missing comets, where the earth crossed it, on the night of the th of november came glistening and dashing the fiery spray of a storm of meteors. it was the dust and fragments of the lost comet of biela, which, after being split in two in , had evidently continued the process of disintegration until its cometary character was completely lost. it seems to have made a truly ghostly exit, for right after the meteor swarm of a mysterious cometary body was seen, which was supposed at the time to be the missing comet itself, and which, it is not altogether improbable, may have been a fragment of it. three days after the meteors burst over europe, it occurred to professor klinkerfues, of berlin, that if they came from biela's comet the comet itself ought to be seen in the southern hemisphere retreating from its encounter with the earth. on november th he sent his now historical telegram to mr. pogson, an astronomer at madras; "biela touched earth november th. search near theta centauri." for thirty-six hours after the receipt of this extraordinary request mr. pogson was prevented by clouds from scanning the heavens with his telescope. when the sky cleared at last, behold there was a comet in the place indicated in the telegram! it was glimpsed again the next night, and then clouds intervened, and not a trace of it was ever seen afterward. but every year, on the th of november, when the earth crosses the orbit of the lost comet, meteoric fragments come plunging into our atmosphere, burning as they fly. ordinarily their number is small, but when, as in , a swarm of the meteors is in that part of their orbit which the earth crosses, there is a brilliant spectacle. in this occurred, and the world was treated to one of the most splendid meteoric displays on record. [illustration: the attendants of alpha persei.] next let us turn to perseus. the bending row of stars marking the center of this constellation is very striking and brilliant. the brightest star in the constellation is alpha, or algenib, in the center of the row. the head of perseus is toward cassiopeia, and in his left hand he grasps the head of medusa, which hangs down in such a way that its principal star beta, or algol, forms a right angle with algenib and almaach in andromeda. this star algol, or the demon, as the arabs call it, is in some respects the most wonderful and interesting in all the heavens. it is as famous for the variability of its light as mira, but it differs widely from that star both in its period, which is very short, and in the extent of the changes it undergoes. during about two days and a half, algol is equal in brilliance to algenib, which is a second-magnitude star; then it begins to fade, and in the course of about four and a half hours it sinks to the fourth magnitude, being then about equal to the faint stars near it. it remains thus obscured for only a few minutes, and then begins to brighten again, and in about four and a half hours more resumes its former brilliance. this phenomenon is very easily observed, for, as will be seen by consulting our little map, algol can be readily found, and its changes are so rapid that under favorable circumstances it can be seen in the course of a single night to run through the whole gamut. of course, no optical instrument whatever is needed to enable one to see these changes of algol, for it is plainly visible to the naked eye throughout, but it will be found interesting to watch the star with an opera-glass. its periodic time from minimum to minimum is two days, twenty hours, and forty-nine minutes, lacking a few seconds. any one can calculate future minima for himself by adding the periodic time above given to the time of any observed minimum. while spots upon its surface may be the cause of the variations in the light of mira, it is believed that the more rapid changes of algol may be due to another cause; namely, the existence of a huge, dark body revolving swiftly around it at close quarters in an orbit whose plane is directed edgewise toward the earth, so that at regular intervals this dark body causes a partial eclipse of algol. notwithstanding the attacks that have been made upon this theory, it seems to hold its ground, and it will probably continue to find favor as a working hypothesis until some fresh light is cast upon the problem. it hardly needs to be said that the dark body in question, if it exists, must be of enormous size, bearing no such insignificant proportion to the size of algol as the earth does to the sun, but being rather the rival in bulk of its shining brother--a blind companion, an extinguished sun. there was certainly great fitness in the selection of the little group of stars of which this mysterious algol forms the most conspicuous member, to represent the awful head of the gorgon carried by the victorious perseus for the confusion of his enemies. in a darker age than ours the winking of this demon-star must have seemed a prodigy of sinister import. turn now to the bright star algenib, or alpha persei. you will find with the glass an exceedingly attractive spectacle there. in my note-book i find this entry, made while sweeping over perseus for materials for this chapter: "the field about alpha is one of the finest in the sky for an opera-glass. stars conspicuously ranged in curving lines and streams. a host follows alpha from the east and south." the picture on page will give the reader some notion of the exceeding beauty of this field of stars, and of the singular manner in which they are grouped, as it were, behind their leader. a field-glass increases the beauty of the scene. the reader will find a starry cluster marked on map as the "great cluster." this object can be easily detected by the naked eye, resembling a wisp of luminous cloud. it marks the hand in which perseus clasps his diamond sword, and, with a telescope of medium power, it is one of the most marvelously beautiful objects in the sky--a double swarm of stars, bright enough to be clearly distinguished from one another, and yet so numerous as to dazzle the eye with their lively beams. an opera-glass does not possess sufficient power to "resolve" this cluster, but it gives a startling suggestion of its half-hidden magnificence, and the observer will be likely to turn to it again and again with increasing admiration. sweep from this to alpha persei and beyond to get an idea of the procession of suns in the milky-way. the nebulous-looking cluster marked m appears with an opera-glass like a faint comet. about a thousand years ago the theologians undertook to reconstruct the constellation figures, and to give them a religious significance. they divided the zodiac up among the twelve apostles, st. peter taking the place of aries, with the triangles for his mitre. in this reconstruction perseus was transmogrified into st. paul, armed with a sword in one hand and a book in the other; cassiopeia became mary magdalene; while poor andromeda, stripped of all her beauty and romance, was turned into a sepulchre! next look at cassiopeia, which is distinctly marked out by the zigzag row of stars so well described by aratus. here the milky-way is so rich that the observer hardly needs any guidance; he is sure to stumble upon interesting sights for himself. the five brightest stars are generally represented as indicating the outlines of the chair or throne in which the queen sits, the star zeta ([zeta]) being in her head. look at zeta with a good field-glass, and you will see a singular and brilliant array of stars near it in a broken half-circle, which may suggest the notion of a crown. near the little star kappa ([kappa]) in the map will be seen a small circle and the figures . this shows the spot where the famous temporary star, which has of late been frequently referred to as the "star of bethlehem," appeared. it was seen in , and carefully observed by the famous astronomer tycho brahe. it seems to have suddenly burst forth with a brilliance that outshone every other star in the heavens, not excepting sirius itself. but its supremacy was short-lived. in a few months it had sunk to the second magnitude. it continued to grow fainter, exhibiting some remarkable changes of color in the mean time, and in less than a year and a half it disappeared. it has never been seen since. but in , and again in , a star is said to have suddenly blazed out near that point in the heavens. there is no certainty about these earlier apparitions, but, assuming that they are not apocryphal, they might possibly indicate that the star seen by tycho was a periodical one, its period considerably exceeding three hundred years. carrying this supposed period back, it was found that an apparition of this star might have occurred about the time of the birth of christ. it did not require a very prolific imagination to suggest its identity with the so-called star of the magi, and hence the legend of the star of bethlehem and its impending reappearance, of which we have heard so much of late. it will be observed, from the dates given above, that, even supposing them to be correct, no definite period is indicated for the reappearance of the star. in one case the interval is three hundred and eight years, and in the other three hundred and nineteen years. in short, there are too many suppositions and assumptions involved to allow of any credence being given to the theory of the periodicity of tycho's wonderful star. at the same time, nobody can say it is impossible that the star should appear again, and so it may be interesting for the reader to know where to look for it. many of the most beautiful sights of this splendid constellation are beyond the reach of an opera-glass, and reserved for the grander powers of the telescope. we will pause but briefly with cepheus, for the old king's constellation is comparatively dim in the heavens, as his part in the dramatic story of andromeda was contemptible, and he seems to have got among the stars only by virtue of his relationship to more interesting persons. he does possess one gem of singular beauty--the star mu, which may be found about two and a half degrees south of the star nu ([nu]). it is the so-called "garnet star," thus named by william herschel, who advises the observer, in order to appreciate its color, to glance from it to alpha cephei, which is a white star. mu is variable, changing from the fourth to the sixth magnitude in a long period of five or six years. its color is changeable, like its light. sometimes it is of a deep garnet hue, and at other times it is orange-colored. upon the whole, it appears of a deeper red than any other star visible to the naked eye. if you have a good field-glass, try its powers upon the star delta ([delta]) cephei. this is a double star, the components being about forty-one seconds of arc apart, the larger of four and one half magnitude, and the smaller of the seventh magnitude. the latter is of a beautiful blue color, while the larger star is yellow or orange. with a good eye, a steady hand, and a clear glass, magnifying not less than six diameters, you can separate them, and catch the contrasted tints of their light. besides being a double star, delta is variable. chapter iv. the stars of winter. i have never beheld the first indications of the rising of orion without a peculiar feeling of awakened expectation, like that of one who sees the curtain rise upon a drama of absorbing interest. and certainly the magnificent company of the winter constellations, of which orion is the chief, make their entrance upon the scene in a manner that may be described as almost dramatic. first in the east come the world-renowned pleiades. at about the same time capella, one of the most beautiful of stars, is seen flashing above the northeastern horizon. these are the sparkling ushers to the coming spectacle. in an hour the fiery gleam of aldebaran appears at the edge of the dome below the pleiades, a star noticeable among a thousand for its color alone, besides being one of the brightest of the heavenly host. the observer familiar with the constellations knows, when he sees this red star which marks the eye of the angry bull, taurus, that just behind the horizon stands orion with starry shield and upraised club to meet the charge of his gigantic enemy. with aldebaran rises the beautiful v-shaped group of the hyades. presently the star-streams of eridanus begin to appear in the east and southeast, the immediate precursors of the rising of orion: "and now the river-flood's first winding reach the becalmed mariner may see in heaven, as he watches for orion to espy if he hath aught to say of the night's measure or the slumbering winds." the first glimpse we get of the hero of the sky is the long bending row of little stars that glitter in the lion's skin which, according to mythology, serves him for a shield. the great constellation then advances majestically into sight. first of its principal stars appears bellatrix in the left shoulder; then the little group forming the head, followed closely by the splendid betelgeuse, "the martial star," flashing like a decoration upon the hero's right shoulder. then come into view the equally beautiful rigel in the left foot, and the striking row of three bright stars forming the belt. below these hangs another starry pendant marking the famous sword of orion, and last of all appears saiph in the right knee. there is no other constellation containing so many bright stars. it has two of the first magnitude, betelgeuse and rigel; the three stars in the belt, and bellatrix in the left shoulder, are all of the second magnitude; and besides these there are three stars of the third magnitude, more than a dozen of the fourth, and innumerable twinklers of smaller magnitudes, whose commingled scintillations form a celestial illumination of singular splendor. "thus graced and armed he leads the starry host." by the time orion has chased the bull half-way up the eastern slope of the firmament, the peerless dog-star, sirius, is flaming at the edge of the horizon, while farther north glitters procyon, the little dog-star, and still higher are seen the twin stars in gemini. when these constellations have advanced well toward the meridian, as shown in our circular map, their united radiance forms a scene never to be forgotten. counting one of the stars in gemini as of the first rank, there are no less than seven first-magnitude stars ranged around one another in a way that can not fail to attract the attention and the admiration of the most careless observer. aldebaran, capella, the twins, procyon, sirius, and rigel mark the angles of a huge hexagon, while betelgeuse shines with ruddy beauty not far from the center of the figure. the heavens contain no other naked-eye view comparable with this great array, not even the glorious celestial region where the southern cross shines supreme, being equal to it in splendor. as an offset to the discomforts of winter observations of the stars, the observer finds that the softer skies of summer have no such marvelous brilliants to dazzle his eyes as those that illumine the hyemal heavens. to comprehend the real glories of the celestial sphere in the depth of winter one should spend a few clear nights in the rural districts of new york or new england, when the hills, clad with sparkling blankets of crusted snow, reflect the glitter of the living sky. in the pure frosty air the stars seem splintered and multiplied indefinitely, and the brighter ones shine with a splendor of light and color unknown to the denizen of the smoky city, whose eyes are dulled and blinded by the glare of streetlights. there one may detect the delicate shade of green that lurks in the imperial blaze of sirius, the beautiful rose-red light of aldebaran, the rich orange hue of betelgeuse, the blue-white radiance of rigel, and the pearly luster of capella. if you have never seen the starry heavens except as they appear from city streets and squares, then, i had almost said, you have never seen them at all, and especially in the winter is this true. i wish i could describe to you the impression that they can make upon the opening mind of a country boy, who, knowing as yet nothing of the little great world around him, stands in the yawning silence of night and beholds the illimitably great world above him, looking deeper than thought can go into the shining vistas of the universe, and overwhelmed with the wonder of those marshaled suns. [illustration: map. .] looking now at map , we see the heavens as they appear at midnight on the st of december, at o'clock p. m. on the st of january, and at o'clock p. m. on the st of february. in the western half of the sky we recognize andromeda, pegasus, pisces, cetus, aries, cassiopeia, and other constellations that we studied in the "stars of autumn." far over in the east we see rising leo, cancer, and hydra, which we included among the "stars of spring." occupying most of the southern and eastern heavens are the constellations which we are now to describe under the name of the "stars of winter," because in that season they are seen under the most favorable circumstances. i have already referred to the admirable way in which the principal stars of some of these constellations are ranged round one another. by the aid of the map the observer can perceive the relative position of the different constellations, and, having fixed this in his mind, he will be prepared to study them in detail. [illustration: map .] let us now begin with map no. , which shows us the constellations of eridanus, lepus, orion, and taurus. eridanus is a large though not very conspicuous constellation, which is generally supposed to represent the celebrated river now known as the po. it has had different names among different peoples, but the idea of a river, suggested by its long, winding streams of stars, has always been preserved. according to fable, it is the river into which phaeton fell after his disastrous attempt to drive the chariot of the sun for his father phoebus, and in which hare-brained adventure he narrowly missed burning the world up. the imaginary river starts from the brilliant star rigel, in the left foot of orion, and flows in a broad upward bend toward the west; then it turns in a southerly direction until it reaches the bright star gamma ([gamma]), where it bends sharply to the north, and then quickly sweeps off to the west once more, until it meets the group of stars marking the head of cetus. thence it runs south, gradually turning eastward, until it flows back more than half-way to orion. finally it curves south again and disappears beneath the horizon. throughout the whole distance of more than ° the course of the stream is marked by rows of stars, and can be recognized without difficulty by the amateur observer. the first thing to do with your opera-glass, after you have fixed the general outlines of the constellation in your mind by naked-eye observations, is to sweep slowly over the whole course of the stream, beginning at rigel, and following its various wanderings. eridanus ends in the southern hemisphere near a first-magnitude star called achernar, which is situated in the stream, but can not be seen from our latitudes. along the stream you will find many interesting groupings of the stars. in the map see the pair of stars below and to the right of nu ([nu]). these are the two omicrons, the upper one being [omicron]¹ and the lower one [omicron]². the latter is of an orange hue, and is remarkable for the speed with which it is flying through space. there are only one or two stars whose proper motion, as it is called, is more rapid than that of [omicron]² in eridanus. it changes its place nearly seven minutes of arc in a century. the records of the earliest observations we possess show that near the beginning of the christian era it was about half-way between [omicron]¹ and [nu]. its companion [omicron]¹, on the contrary, seems to be almost stationary, so that [omicron]² will gradually draw away from it, passing on toward the southwest until, in the course of centuries, it will become invisible from our latitudes. this flying star is accompanied by two minute companions, which in themselves form a close and very delicate double star. these two little stars, of only . and . magnitude, respectively, are, of course beyond the ken of the observer with an opera-glass. the system of which they form a part, however, is intensely interesting, since the appearances indicate that they belong, in the manner of satellites, to [omicron]², and are fellow-voyagers of that wonderful star. [illustration: the "golden horns" of taurus.] having admired the star-groups of eridanus, one of the prettiest of which is to be seen around beta ([beta]), let us turn next to taurus, just above or north of eridanus. two remarkable clusters at once attract the eye, the hyades, which are shaped somewhat like the letter [v], with aldebaran in the upper end of the left-hand branch, and the pleiades, whose silvery glittering has made them celebrated in all ages. the pleiades are in the shoulder and the hyades in the face of taurus, aldebaran most appropriately representing one of his blazing eyes as he hurls himself against orion. the constellation-makers did not trouble themselves to make a complete bull, and only the head and fore-quarters of the animal are represented. if taurus had been completed on the scale on which he was begun, there would have been no room in the sky for aries; one of the fishes would have had to abandon his celestial swimming-place, and even the fair andromeda would have found herself uncomfortably situated. but, as if to make amends for neglecting to furnish their heavenly bull with hind-quarters, the ancients gave him a most prodigious and beautiful pair of horns, which make the beholder feel alarm for the safety of orion. starting out of the head above the hyades, as illustrated in our cut, the horns curve upward and to the east, each being tipped by a bright star. along and between the horns runs a scattered and broken stream of minute stars which seem to be gathered into knots just beyond the end of the horns, where they dip into the edge of the milky-way. many of these stars can be seen, on a dark night, with an ordinary opera-glass, but, to see them well, one should use as large a field-glass as he can obtain. with such a glass their appearance almost makes one suspect that virgil had a poetic prevision of the wonders yet to be revealed by the telescope when he wrote, as rendered by dryden, of the season-- "when with his _golden horns_ in full career the bull beats down the barriers of the year." below the tips of the horns, and over orion's head, there are also rich clusters of stars, as if the bull were flaunting shreds of sparkling raiment torn from some celestial victim of his fury. with an ordinary glass, however, the observer will not find this star-sprinkled region around the horns of taurus as brilliant a spectacle as that presented by the hyades and the group of stars just above them in the bull's ear. the two stars in the tips of the horns are both interesting, each in a different way. the upper and brighter one of the two, marked beta ([beta]) in map no. , is called el nath. it is common to the left horn of taurus and the right foot of auriga, who is represented standing just above. it is a singularly white star. this quality of its light becomes conspicuous when it is looked at with a glass. the most inexperienced observer will hardly fail to be impressed by the pure whiteness of el nath, in comparison with which he will find that many of the stars he had supposed to be white show a decided tinge of color. the star in the tip of the right or southern horn, zeta ([zeta]), is remarkable, not on its own account, but because it serves as a pointer to a famous nebula, the discovery of which led messier to form his catalogue of nebulæ. this is sometimes called the "crab nebula," from the long sprays of nebulous matter which were seen surrounding it with lord rosse's great telescope. our little sketch is simply intended to enable the observer to locate this strange object. if he wishes to study its appearance, he must use a powerful telescope. but with a first-rate field-glass he can see it as a speck of light in the position shown in the cut, where the large star is zeta and the smaller ones are faint stars, the relative position of which will enable the observer to find the nebula, if he keeps in mind that the top of the cut is toward the north. it is noteworthy that this nebula for a time deceived several of the watchers who were on the lookout for the predicted return of halley's comet in . [illustration: the crab nebula.] and now let us look at the hyades, an assemblage of stars not less beautiful than their more celebrated sisters the pleiades. the leader of the hyades is aldebaran, or alpha tauri, and his followers are worthy of their leader. the inexperienced observer is certain to be surprised by the display of stars which an opera-glass brings to view in the hyades. our illustration will give some notion of their appearance with a large field-glass. the "brackish poet," of whose rhymes admiral smyth was so fond, thus describes the hyades: "in lustrous dignity aloft see alpha tauri shine, the splendid zone he decorates attests the power divine: for mark around what glitt'ring orbs attract the wandering eye, you'll soon confess no other star has such attendants nigh." the redness of the light of aldebaran is a very interesting phenomenon. careful observation detects a decided difference between its color and that of betelgeuse, or alpha orionis, which is also a red star. it differs, too, from the brilliant red star of summer, antares. aldebaran has a trace of rose-color in its light, while betelgeuse is of a very deep orange, and antares may be described as fire-red. these shades of color can easily be detected by the naked eye after a little practice. first compare aldebaran and betelgeuse, and glance from each to the brilliant white, or bluish-white, star rigel in orion's foot. upon turning the eye back from rigel to aldebaran the peculiar color of the latter is readily perceived. spectroscopic analysis has revealed the presence in aldebaran of hydrogen, sodium, magnesium, calcium, iron, bismuth, tellurium, antimony, and mercury. and so modern discoveries, while they have pushed back the stars to distances of which the ancients could not conceive, have, at the same time, and equally, widened the recognized boundaries of the physical universe and abolished forever the ancient distinction between the heavens and the earth. it is a plain road from the earth to the stars, though mortal feet can not tread it. [illustration: the hyades.] keeping in mind that in our little picture of the hyades the top is north, the right hand west, and the left hand east, the reader will be able to identify the principal stars in the group. aldebaran is readily recognized, because it is the largest of all. the bright star near the upper edge of the picture is epsilon tauri, and its sister star, forming the point of the [v], is gamma tauri. the three brightest stars between epsilon and gamma, forming a little group, are the deltas, while the pair of stars surrounded by many smaller ones, half-way between aldebaran and gamma, are the thetas. these stars present a very pretty appearance, viewed with a good glass, the effect being heightened by a contrast of color in the two thetas. the little pair southeast of aldebaran, called the sigmas, is also a beautiful object. the distance apart of these stars is about seven minutes of arc, while the distance between the two thetas is about five and a half minutes of arc. these measures may be useful to the reader in estimating the distances between other stars that he may observe. it will also be found an interesting test of the eye-sight to endeavor to see these stars as doubles without the aid of a glass. persons having keen eyes will be able to accomplish this. north of the star epsilon will be seen a little group in the ear of the bull (see cut, "the golden horns of taurus"), which presents a brilliant appearance with a small glass. the southernmost pair in the group are the kappas, whose distance apart is very nearly the same as that of the thetas, described above; but i think it improbable that anybody could separate them with the naked eye, as there is a full magnitude between them in brightness, and the smaller star is only of magnitude . , while sixth-magnitude stars are generally reckoned as the smallest that can be seen by the naked eye. above the kappas, and in the same group in the ear, are the two upsilons, forming a wider pair. next we come to the pleiades: "though small their size and pale their light, wide is their fame." in every age and in every country the pleiades have been watched, admired, and wondered at, for they are visible from every inhabited land on the globe. to many they are popularly known as the seven stars, although few persons can see more than six stars in the group with the unaided eye. it is a singular fact that many of the earliest writers declare that only six pleiades can be seen, although they all assert that they are seven in number. these seven were the fabled daughters of atlas, or the atlantides, whose names were merope, alcyone, celæno, electra, taygeta, asterope, and maia. one of the stories connected with them is that merope married a mortal, whereupon her star grew dim among her sisters. another fable assures us that electra, unable to endure the sight of the burning of troy, hid her face in her hands, and so blotted her star from the sky. while we may smile at these stories, we can not entirely disregard them, for they are intermingled with some of the richest literary treasures of the world, and they come to us, like some old keepsake, perfumed with the memory of a past age. the mythological history of the pleiades is intensely interesting, too, because it is world-wide. they have impressed their mark, in one way or another, upon the habits, customs, traditions, language, and history of probably every nation. this is true of savage tribes as well as of great empires. the pleiades furnish one of the principal links that appear to connect the beginnings of human history with that wonderful prehistoric past, where, as through a gulf of mist, we seem to perceive faintly the glow of a golden age beyond. the connection of the pleiades with traditions of the flood is most remarkable. in almost every part of the world, and in various ages, the celebration of a feast or festival of the dead, dimly connected by traditions with some great calamity to the human race in the past, has been found to be directly related to the pleiades. this festival or rite, which has been discovered in various forms among the ancient hindoos, egyptians, persians, peruvians, mexicans, druids, etc., occurs always in the month of november, and is regulated by the culmination of the pleiades. the egyptians directly connected this celebration with a deluge, and the mexicans, at the time of the spanish conquest, had a tradition that the world had once been destroyed at the time of the midnight culmination of the pleiades. among the savages inhabiting australia and the pacific island groups a similar rite has been discovered. it has also been suggested that the japanese feast of lanterns is not improbably related to this world-wide observance of the pleiades, as commemorating some calamitous event in the far past which involved the whole race of man in its effects. the pleiades also have a supposed connection with that mystery of mysteries, the great pyramid of cheops. it has been found that about the year b. c., when the beginning of spring coincided with the culmination of the pleiades at midnight, that wonderful group of stars was visible, just at midnight, through the mysterious southward-pointing passage of the pyramid. at the same date the then pole-star, alpha draconis, was visible through the northward-pointing passage of the pyramid. another curious myth involving the pleiades as a part of the constellation taurus is that which represents this constellation as the bull into which jupiter changed himself when he carried the fair europa away from phoenicia to the continent that now bears her name. in this story the fact that only the head and fore-quarters of the bull are visible in the sky is accounted for on the ground that the remainder of his body is beneath the water through which he is swimming. here, then, is another apparent link with the legends of the flood, with which the pleiades have been so strangely connected, as by common consent among many nations, and in the most widely separated parts of the earth. with the most powerful field-glass you may be able to see all of the stars represented in our picture of the pleiades. with an ordinary opera-glass the fainter ones will not be visible; yet even with such a glass the scene is a remarkable one. not only all of the "seven sisters," but many other stars, can be seen twinkling among them. the superiority of alcyone to the others, which is not so clear to the naked eye, becomes very apparent. alcyone is the large star below the middle of the picture with a triangle of little stars beside it. to the left or east of alcyone the two most conspicuous stars are atlas and pleione. the latter--which is the uppermost one--is represented too large in the picture. it requires a sharp eye to see pleione without a glass, while atlas is plainly visible to the unaided vision, and is always counted among the naked-eye pleiades, although it does not bear the name of one of the mythological sisters, but that of their father. the bright star below and to the right of alcyone is merope; the one near the right-hand edge of the picture, about on a level with alcyone, is electra. above, or to the north of electra, are two bright stars lying in a line pointing toward alcyone; the upper one of these, or the one farthest from alcyone, is taygeta, and the other is maia. above taygeta and maia, and forming a little triangle with them, is a pair of stars which bears the name of asterope. about half-way between taygeta and electra, and directly above the latter, is celæno. [illustration: the pleiades.] the naked-eye observer will probably find it difficult to decide which he can detect the more easily, celæno or pleione, while he will discover that asterope, although composed of two stars, as seen with a glass, is so faint as to be much more difficult than either celæno or pleione. unless, as is not improbable, the names have become interchanged in the course of centuries, the brightness of these stars would seem to have undergone remarkable changes. the star of merope, it will be remembered, was said to have become indistinct, or disappeared, because she married a mortal. at present merope is one of those that can be plainly seen with the naked-eye, while the star of asterope, who was said to have had the god mars for her spouse, has faded away until only a glass can show it. it would appear, then, that notwithstanding an occasional temporary eclipse, it is, in the long run, better to marry a plain mortal than a god. electra, too, who hid her eyes at the sight of burning troy, seems to have recovered from her fright, and is at present, next to alcyone, the brightest star in the cluster. but, however we may regard those changes in the brightness of the pleiades which are based upon tradition, there is no doubt that well-attested changes have taken place in the comparative brilliancy of stars in this cluster since astronomy became an exact science. observations of the proper motions of the pleiades have shown that there is an actual physical connection between them; that they are, literally speaking, a flight of suns. their common motion is toward the southwest, under the impulse of forces that remain as yet beyond the grasp of human knowledge. alcyone was selected by mädler as the central sun around which the whole starry system revolved, but later investigations have shown that his speculation was not well founded, and that, so far as we can determine, the proper motions of the stars are not such as to indicate the existence of any common center. they appear to be flying with different velocities in every direction, although--as in the case of the pleiades--we often find groups of them associated together in a common direction of flight. still another curious fact about the pleiades is the existence of some rather mysterious nebulous masses in the cluster. in temple discovered an extensive nebula, of a broad oval form, with the star merope immersed in one end of it. subsequent observations showed that this strange phenomenon was variable. sometimes it could not be seen; at other times it was very plain and large. in jeaurat's chart of the pleiades, made in , a vast nebulous mass is represented near the stars atlas and pleione. this has since been identified by goldschmidt as part of a huge, ill-defined nebula, which he thought he could perceive enveloping the whole group of the pleiades. many observers, however, could never see these nebulous masses, and were inclined to doubt their actual existence. within the past few years astronomical photography, having made astonishing progress, has thrown new light upon this mysterious subject. the sensitized plate of the camera, when applied at the focus of a properly constructed telescope, has proved more effective than the human retina, and has, so to speak, enabled us to see beyond the reach of vision by means of the pictures it makes of objects which escape the eye. in november, , paul and prosper henry turned their great photographing telescope upon the pleiades, and with it discovered a nebula apparently attached to the star maia. the most powerful telescopes in the world had never revealed this to the eye. yet of its actual existence there can be no question. their photograph also showed the merope nebula, although much smaller, and of a different form from that represented by its discoverer and others. there evidently yet remains much to be discovered in this singular group, and the mingling of nebulous matter with its stars makes tennyson's picturesque description of the pleiades appear all the more life-like: "many a night i saw the pleiads, rising through the mellow shade, glitter like _a swarm of fire-flies tangled in a silver braid_." the reader should not expect to be able to see the nebulæ in the pleiades with an opera-glass. i have thought it proper to mention these singular objects only in order that he might be in possession of the principal and most curious facts about those interesting stars.[c] [footnote c: the henry brothers have continued the photographic work described above, and their later achievements are even more interesting and wonderful. they have found that there are many nebulous masses involved in the group of the pleiades, and have photographed them. one of the most amazing phenomena in their great photograph of the pleiades is a long wisp or streak of nebulous matter, along which eight or nine stars are strung in a manner which irresistibly suggests an intimate connection between the stars and the nebula. this recalls the recent (august, ) discovery made by prof. holden, with the great lick telescope, concerning the structure of the celebrated ring nebula in lyra, which, it appears, is composed of concentric ovals of stars and nebulous stuff, so arranged that we must believe they are intimately associated in a most wonderful community.] orion will next command our attention. you will find the constellation in map no. : "eastward beyond the region of the bull stands great orion; whoso kens not him in cloudless night gleaming aloft, shall cast his eyes in vain to find a brighter sign in all the heaven." to the naked eye, to the opera-glass, and to the telescope, orion is alike a mine of wonders. this great constellation embraces almost every variety of interesting phenomena that the heavens contain. here we have the grandest of the nebulæ, some of the largest and most beautifully colored stars, star-streams, star-clusters, nebulous stars, variable stars. i have already mentioned the positions of the principal stars in the imaginary figure of the great hunter. i may add that his upraised arm and club are represented by the stars seen in the map above alpha ([alpha]) or betelgeuse, one of which is marked nu ([nu]), and another, in the knob of the club, chi ([chi]). i have also, in speaking of aldebaran, described the contrast in the colors of betelgeuse and beta ([beta]) or rigel. betelgeuse, it may be remarked, is slightly variable. sometimes it appears brighter than rigel, and sometimes less brilliant. it is interesting to note that, according to secchi's division of the stars into types, based upon their spectra, betelgeuse falls into the third order, which seems to represent a type of suns in which the process of cooling, and the formation of an absorptive envelope or shell, have gone on so far that we may regard them as approaching the point of extinction. rigel, on the other hand, belongs to the first order or type which represents suns that are probably both hotter and younger in the order of development. so, then, we may look upon the two chief stars of this great constellation as representing two stages of cosmical existence. betelgeuse shows us a sun that has almost run its course, that has passed into its decline, and that already begins to faint and flicker and grow dim before the on-coming and inevitable fate of extinction; but in rigel we see a sun blazing with the fires of youth, splendid in the first glow of its solar energies, and holding the promise of the future yet before it. rigel belongs to a new generation of the universe; betelgeuse to the universe that is passing. we may pursue this comparison one step farther back and see in the great nebula, which glows dimly in the middle of the constellation, between rigel triumphant and betelgeuse languishing, a still earlier cosmical condition--the germ of suns whose infant rays may illuminate space when rigel itself is growing dim. [illustration: the sword of orion and the great nebula.] turn your glass upon the three stars forming the belt. you will not be likely to undertake to count all the twinkling lights that you will see, especially as many of them appear and disappear as you turn your attention to different parts of the field. sweep all around the belt and also between the belt and gamma ([gamma]) or bellatrix. according to the old astrologers, women born under the influence of the star bellatrix were lucky, and provided with good tongues. of course, this was fortunate for their husbands too! below the belt will be seen a short row of stars hanging downward and representing the sword. in the middle of this row is the great orion nebula. the star theta ([theta]) involved in the nebula is multiple, and the position of this little cluster of suns is such that, as has been said, they seem to be feeding upon the substance of the nebula surrounding them. other stars are seen scattered in different parts of the nebula. this phenomenon can be plainly seen with an opera-glass. our picture of the sword of orion shows its appearance with a good field-glass. with such a glass several fine test-objects will be found in the sword. one of the best of these is formed by the two five-pointed stars seen in the picture close together above the nebula. no difficulty will be encountered in separating these stars with a field-glass, but it will require a little sharp watching to detect the small star between the two and just above the line joining them. so, the bending row of faint stars above and to the right of the group just described will be found rather elusive as individuals, though easily glimpsed as a whole. of the great nebula itself not much detail can be seen. yet by averting the eyes the extension of the nebulous light in every direction from the center can be detected and traced, under favorable circumstances, to a considerable distance. the changes that this nebula certainly has undergone in the brilliancy, if not in the form, of different parts of it, are perhaps indications of the operation of forces, which we know must prevail there, and whose tendency can only be in the direction of condensation, and the ultimate formation of future suns and worlds. yet, as the appearance of the nebula in great telescopes shows, we can not expect that the processes of creation will here produce a homologue of our solar system. the curdled appearance of the nebula indicates the formation of various centers of condensation, the final result of which will doubtless be a group of stars like some of those which we see in the heavens, and whose common motion shows that they are bound together in the chains of reciprocal gravitation. the pleiades are an example of such a group. do not fail to look for a little star just west of rigel, which, with a good opera-glass, appears to be almost hidden in the flashing rays of its brilliant companion. if you have also a field-glass, after you have detected this shy little twinkler with your opera-glass, try the larger glass upon it. you will find then that the little star originally seen is not the only one there. a still smaller star, which had before been completely hidden, will now be perceived. i may add that, with telescopes, rigel is one of the most beautiful double stars in the sky, having a little blue companion close under its wing. run your glass along the line of little stars forming the lion's skin or shield that orion opposes to the onset of taurus. here you will find some interesting combinations, and the star marked on the map [pi]^ will especially attract your eye, because it is accompanied, about fifteen minutes to the northwest, by a seventh-magnitude star of a rich orange hue. look next at the little group of three stars forming the head of orion. although there is no nebula here, yet these stars, as seen with the naked eye, have a remarkably nebulous look, and ptolemy regarded the group as a nebulous star. the largest star is called lambda ([lambda]); the others are phi ([phi]) one and two. an opera-glass will show another star above ([lambda]), and a fifth star below [phi]^ which is the farthest of the two phis from lambda. it will also reveal a faint twinkling between [lambda] and [phi]^ . a field-glass shows that this twinkling is produced by a pretty little row of three stars of the eighth and ninth magnitudes. in fact, orion is such a striking object in the sky that more than one attempt has been made to steal away its name and substitute that of some modern hero. the university of leipsic, in , formally resolved that the stars forming the belt and sword of orion should henceforth be known as the constellation of napoleon. as if to offset this, an englishman proposed to rename orion for the british naval bull-dog nelson. but "orion armed" has successfully maintained his name and place against all comers. as becomes the splendor of his constellation, orion is a tremendous hero of antiquity, although it must be confessed that his history is somewhat shadowy and uncertain, even for a mythological story. all accounts agree, however, that he was the mightiest hunter ever known, and the hebrews claimed that he was no less a person than nimrod himself. [illustration: map .] the little constellations of lepus and columba, below orion, need not detain us long. you will find in them some pretty combinations of stars. in lepus is the celebrated "crimson star," which has been described as resembling a drop of blood in color--a truly marvelous hue for a sun--but, as it is never brighter than the sixth magnitude, and from that varies down to the ninth, we could hardly hope to see its color well with an opera-glass. besides, the observer would have difficulty in finding it. we will now turn to the constellation of canis major, represented in map no. . although, as a constellation, it is not to be compared with the brilliant orion, yet, on account of the unrivaled magnificence of its chief star, canis major presents almost as attractive a scene as its more extensive rival. everybody has heard of sirius, or the dog-star, and everybody must have seen it flashing and scintillating so splendidly in the winter heavens, that to call it a first-magnitude star does it injustice, since no other star of that magnitude is at all comparable with it. sirius, in fact, stands in a class by itself as the brightest star in the sky. its light is white, with a shade of green, which requires close watching to be detected. when it is near the horizon, or when the atmosphere is very unsteady, sirius flashes prismatic colors like a great diamond. the question has been much discussed, as to whether sirius was formerly a red star. it is described as red by several ancient authors, but it seems to be pretty well established that these descriptions are most of them due to a blunder made by cicero in his translation of the astronomical poem of aratus. it is not impossible, though it is highly improbable, that sirius has changed color. so intimately was sirius connected in the minds of the ancient egyptians with the annual rising of the nile, that it was called the nile-star. when it appeared in the morning sky, just before sunrise, the season of the overflowing of the great river was about to begin, and so the appearance of this star was regarded as foretelling the coming of the floods. the dog-days got their name from sirius, as they occur at the time when that star rises with the sun. your eyes will be fairly dazzled when you turn your glass upon this splendid star. by close attention you will be able to perceive a number of faint stars, mere points by comparison, in the immediate neighborhood of sirius. there are many interesting objects in the constellation. the star marked nu ([nu]) in the map is really triple, as the smallest glass will show. look next at the star-group m. the cloud of minute stars of which it is composed can be very well seen with a field-glass or a powerful opera-glass. the star is of a very ruddy color that contrasts beautifully with the light of epsilon ([epsilon]), which can be seen in the same field of view with an opera-glass. between the stars delta ([delta]) and [omicron]¹ and [omicron]² there is a remarkable array of minute stars, as shown in the accompanying cut. one never sees stars arranged in streams or rows, like these, without an irresistible impression that the arrangement can not be accidental; that some law must have been in operation which associated them together in the forms which we see. yet, when we reflect that these are all suns, how far do we seem to be from understanding the meaning of the universe! [illustration: delta canis majoris and its neighbors.] the extraordinary size and brilliancy of sirius might naturally enough lead one to suppose that it is the nearest of the stars, and such it was once believed to be. observations of stellar parallax, however, show that this was a mistake. the distance of sirius is so great that no satisfactory determination of it has yet been made. we may safely say, though, that that distance is, at the least calculation, , , , , miles. in other words, sirius is about , times as far from the earth as the sun is. then, since light diminishes as the square of the distance increases, the sun, if placed as far from us as sirius is, would send us, in round numbers, , , , times less light than we now receive from it. but sirius actually sends us only about , , , times less light than the sun does; consequently sirius must shine , , , / , , , = times as brilliantly as the sun. if we adopt wollaston's estimate of the light of sirius, as compared with that of the sun, viz., / , , , , we shall still find that the actual brilliancy of that grand star is more than fourteen times as great as that of our sun. but as observations on the companion of sirius show that sirius's mass is fully twenty times the sun's, and since the character of sirius's spectrum indicates that its intrinsic brightness, surface for surface, is much superior to the sun's, it is probable that our estimate of the star's actual brilliancy, as compared with what the sun would possess at the same distance, viz., seventy-two times, is much nearer the truth. it is evident that life would be insupportable upon the earth if it were placed as near to sirius as it is to the sun. if the earth were a planet belonging to the system of sirius, in order to enjoy the same amount of heat and light it now receives, it would have to be removed to a distance of nearly , , miles, or eight and a half times its distance from the sun. its time of revolution around sirius would then be nearly five and a half years, or, in other words, the year would be lengthened five and a half times. but, as i have said, the estimate of sirius's distance used in these calculations is the smallest that can be accepted. good authorities regard the distance as being not less than , , , , miles; in which case the star's brilliancy must be as much as times greater than that of the sun! and yet even sirius is probably not the greatest sun belonging to the visible universe. there can be little doubt that canopus, in the southern hemisphere, is a grander sun than sirius. to our eyes, canopus is only about half as bright as sirius, and it ranks as the second star in the heavens in the order of brightness. but while sirius's distance is measurable, that of canopus is so unthinkably immense that astronomers can get no grip upon it. if it were only twice as remote as sirius, it would be equal to two of the latter, but in all probability its distance is much greater than that. and possibly even canopus is not the greatest gem in the coronet of creation. sirius, as we saw when talking of procyon (see chapter i), is a double star. for many years after bessel had declared his belief that the dog-star was subjected to the attraction of an invisible companion, telescopes failed to reveal the accompanying star.[d] finally, in , a new telescope that alvan clark had just finished and was testing, brought the hidden star into view. the suggestion that it may shine by reflected light from sirius has been made. in that case it must, of course, be a planet, but a planet of such stupendous magnitude that the imagination can scarcely grasp it; a planet probably as large as our sun, perhaps larger; a planet equal in size to more than a million earths! but, as was remarked of the faint stars in alpha capricornis, it is probable that the hypothesis of reflected light is not the true one. more probably the companion of sirius shines with light of its own, though its excessive faintness in comparison with its bulk indicates that its condition must be very different from that of an ordinary star. [d] the following extract from a letter by bessel to humboldt, written in (see "cosmos," vol. iii, p. ), is interesting, in view of the discoveries made since then: "at all events i continue in the belief that procyon and sirius are true double stars, consisting of a visible and an invisible star. no reason exists for considering luminosity an essential property of these bodies. the fact that numberless stars are visible is evidently no proof against the existence of an equally incalculable number of invisible ones. the physical difficulty of a change in the proper motion is satisfactorily set aside by the hypothesis of dark stars." readers of voltaire will remember that the hero of his extraordinary story of "micromegas" came from an imaginary planet circling around sirius. inasmuch as voltaire, together with dean swift, ascribed two moons to mars many years before they were discovered (probably suggested by a curiously mistaken interpretation by kepler of an anagram in which galileo had concealed his discovery of the ring of saturn), it is all the more interesting that the great infidel should have imagined an enormous planet circling around the dog-star. but voltaire went far astray when he ascribed a gigantic stature to his "sirian." he makes micromegas, whose world was , , times larger in circumference than the earth, more than twenty miles tall, so that when he visited our little planet he was able to wade through the oceans and step over the mountains without inconvenience, and, when he had scooped up some of the inhabitants on his thumb-nail, was obliged to use a powerful microscope in order to see them. voltaire should rather have gone to some of the most minute of the asteroids for his giant, for under the tremendous gravitation of such a world as he has described micromegas himself would have been a fit subject for microscopic examination. but, however much we may doubt the stature of voltaire's visitor from sirius, we can not doubt the soundness of the conclusion at which he arrived, after having, by an ingenious arrangement, succeeded in holding a conversation with some earthly philosophers under his microscope, namely, that these infinitely little creatures possessed a pride that was almost infinitely great. east and south of canis major, which, by-the-way, is said to represent one of orion's hounds, is part of the constellation argo, which stands for the ship in which jason sailed in search of the golden fleece. the observer will find many objects of interest here, although some of them are so close to the horizon in our latitudes that much of their brilliancy is lost. note the two stars [zeta] and [pi] near the lower edge of the map, then sweep slowly over the space lying between them. about half-way your attention will be arrested by a remarkable stellar arrangement, in which a beautiful half-circle of small stars curving above a larger star, which is reddish in color, is conspicuous. this neighborhood will be found rich in stars that the naked eye can not see. just below the star [eta], in canis major, is another fine group. the star [pi], which is deep yellow or orange, has three little stars above it, two of which form a pretty pair. the star [xi] has a companion, which forms a fine test for an opera-glass, and is well worth looking for. look also at the cluster m, just above and to the west of [xi]. the stars [mu] and [kappa] are seen double with an opera-glass. the two neighboring clusters, m and ^ , are very interesting objects. to see them well, use a powerful field-glass. a "fiery fifth-magnitude star," as webb calls it, can be seen in the field at the same time. the presence of the milky-way is manifest by the sprinkling of stars all about this region. in fact, the attentive observer will before this have noticed that the majority of the most brilliant constellations lie either in the milky-way or along its borders. cassiopeia, as we saw, sits athwart the galaxy whose silvery current winds in and out among the stars of her "chair"; perseus is aglow with its sheen as it wraps him about like a mantle of stars; taurus has the tips of his horns dipped in the great stream; it flows between the shining feet of gemini and the head and shoulders of orion as between starry banks; the peerless sirius hangs like a gem pendent from the celestial girdle. in the southern hemisphere we should find the beautiful constellation of the ship argo, containing canopus, sailing along the milky-way, blown by the breath of old romance on an endless voyage; the southern cross glitters in the very center of the galaxy; and the bright stars of the centaur might be likened to the heads of golden nails pinning this wondrous scarf, woven of the beams of millions of tiny stars, against the dome of the sky. passing back into the northern hemisphere we find scorpio, sagittarius, aquila, the dolphin, cygnus, and resplendent lyra, all strung along the course of the milky-way. turning now to the constellation monoceros, we shall find a few objects worthy of attention. this constellation is of comparatively modern origin, having been formed by bartschius, whose chief title to distinction is that he married the daughter of john kepler. the region around the stars , , and will be found particularly rich, and the cluster ^ shows well with a strong glass. look also at the cluster m, and compare its appearance with that of the clusters in argo. with these constellations we finish our review of the stellar wonders that lie within the reach of so humble an instrument as an opera-or field-glass. we have made the circuit of the sky, and the hosts that illumine the vernal heavens are now seen advancing from the east, and pressing close upon the brighter squadrons of winter. their familiar figures resemble the faces of old friends whom we are glad to welcome. these starry acquaintances never grow wearisome. their interest for us is as fathomless as the deeps of space in which they shine. the man never yet lived whose mind could comprehend the full meaning of the wondrous messages that they flash to us upon the wings of light. as we watch them in their courses, the true music of the spheres comes to our listening ears, the chorus of creation--faint with distance, for it is by slow approaches that man draws near to it--chanting the grandest of epics, the poem of the universe; and the theme that runs through it all is the reign of law. do not be afraid to become a star-gazer. the human mind can find no higher exercise. he who studies the stars will discover-- "an endless fountain of immortal drink pouring unto us from heaven's brink." chapter v. the moon, the planets, and the sun. "it is a most beautiful and delightful sight," exclaims galileo, in describing the discoveries he had made with his telescope, "to behold the body of the moon, which is distant from us nearly sixty semi-diameters of the earth, as near as if it was at a distance of only two of the same measures.... and, consequently, any one may know with the certainty that is due to the use of our senses that the moon assuredly does not possess a smooth and polished surface, but one rough and uneven, and, just like the face of the earth itself, is everywhere full of vast protuberances, deep chasms, and sinuosities." there was, perhaps, nothing in the long series of discoveries with which galileo astonished the world after he had constructed his telescope, which, as he expresses it, "was devised by me through god's grace first enlightening my mind," that had a greater charm for him than his lunar observations. certainly there was nothing which he has described with greater enthusiasm and eloquence. and this could hardly have been otherwise, for the moon was the first celestial object to which galileo turned his telescope, and then for the first time human eyes may be said to have actually looked into another world than the earth, though his discoveries and those of his successors have not realized all the poetic fancies about the moon contained in the verses that are ascribed to orpheus: "and he another wandering world has made which gods selene name, and men the moon. it mountains, cities has, and temples grand." yet galileo's observations at once upset the theory, for which apollonius was responsible, and which seems to have been widely prevalent up to his time, that the moon was a smooth body, polished like a mirror, and presenting in its light and dark spots reflections of the continents and oceans of the earth. he also demonstrated that its surface was covered with plains and mountains, but the "cities and temples" of the moon have remained to our time only within the ken of romance. galileo's telescope, as i have before remarked, was, in the principle of its construction, simply an opera-glass of one tube. he succeeded in making a glass of this kind that magnified thirty diameters, a very much higher power than is given to the opera-and field-glasses of to-day. yet he had to contend with the disadvantages of single lenses, achromatic combinations of glass for optical purposes not being contrived until nearly a hundred years after his death, and so his telescope did not possess quite as decided a superiority over a modern field-glass as the difference in magnifying power would imply. in fact, if the reader will view the moon with a first-rate field-glass, he will perceive that the true nature of the surface of the lunar globe can be readily discerned with such an instrument. even a small opera-glass will reveal much to the attentive observer of the moon; but for these observations the reader should, if possible, make use of a field-glass, and the higher its power the better. the illustrations accompanying this chapter were made by the author with the aid of a glass magnifying seven diameters. of course, the first thing the observer will wish to see will be the mountains of the moon, for everybody has heard of them, and the most sluggish imagination is stirred by the thought that one can look off into the sky and behold "the eternal hills" of another planet as solid and substantial as our own. but the chances are that, if left to their own guidance, ninety-nine persons out of a hundred would choose exactly the wrong time to see these mountains. at any rate, that is my experience with people who have come to look at the moon through my telescope. unless warned beforehand, they invariably wait until full moon, when the flood of sunshine poured perpendicularly upon the face of our satellite conceals its rugged features as effectually as if a veil had been drawn over them. begin your observations with the appearance of the narrowest crescent of the new moon, and follow it as it gradually fills, and then you will see how beautifully the advancing line of lunar sunrise reveals the mountains, over whose slopes and peaks it is climbing, by its ragged and sinuous outline. the observer must keep in mind the fact that he is looking straight down upon the tops of the lunar mountains. it is like a view from a balloon, only at a vastly greater height than any balloon has ever attained. even with a powerful telescope the observer sees the moon at an apparent distance of several hundred miles, while with a field-glass, magnifying seven diameters, the moon appears as if thirty-five thousand miles off. the apparent distance with galileo's telescope was eight thousand miles. recollect how when seen from a great height the rugosities of the earth's surface flatten out and disappear, and then try to imagine how the highest mountains on the earth would look if you were suspended thirty-five thousand miles above them, and you will, perhaps, rather wonder at the fact that the moon's mountains can be seen at all. it is the contrast of lights and shadows that not only reveals them to us, but enables us to measure their height. on the moon shadows are very much darker than upon the earth, because of the extreme rarity of the moon's atmosphere, if indeed it has any atmosphere at all. by stepping around the corner of a rock there, one might pass abruptly from dazzling noonday into the blackness of midnight. the surface of the moon is extraordinarily rough and uneven. it possesses broad plains, which are probably the bottoms of ancient seas that have now dried up, but these cover only about two fifths of the surface visible to us, and most of the remaining three fifths are exceedingly rugged and mountainous. many of the mountains of the moon are, foot for foot, as lofty as the highest mountains on the earth, while all of them, in proportion to the size of the moon's globe, are much larger than the earth's mountains. it is obvious, then, that the sunshine, as it creeps over these alpine landscapes in the moon, casting the black shadows of the peaks and craters many miles across the plains, and capping the summits of lofty mountains with light, while the lower regions far around them are yet buried in night, must clearly reveal the character of the lunar surface. mountains that can not be seen at all when the light falls perpendicularly upon them, or, at the most, appear then merely as shining points, picture themselves by their shadows in startling silhouettes when illuminated laterally by the rising sun. but at full moon, while the mountains hide themselves in light, the old sea-beds are seen spread out among the shining table-lands with great distinctness. even the naked eye readily detects these as ill-defined, dark patches upon the face of the moon, and to their presence are due the popular notions that have prevailed in all quarters of the world about the "man in the moon," the "woman in the moon," "jacob in the moon," the "hare in the moon," the "toad in the moon," and so on. but, however clearly one may imagine that he discerns a man in the moon while recalling the nursery-rhymes about him, an opera-glass instantly puts the specter to flight, and shows the round lunar disk diversified and shaded like a map.[e] [e] i should, perhaps, qualify the statement in the text slightly in favor of a lunar lady to whom mr. henry m. parkhurst first called my attention. about nine days after new moon a rather pretty and decidedly feminine face appears on the western half of the disk. it is formed by the mountains and table-lands embraced by the sea of serenity, the sea of tranquillity, the sea of vapors, etc., and is best seen with the aid of an opera-glass of low power. the face is readily distinguishable on rutherfurd's celebrated photograph of the full moon. it is necessary for this purpose to turn the photograph upside down, since it is a telescopic picture, and consequently reversed. the crater tycho forms a breastpin for the lady, and menelaus glitters like a diamond ornament in her hair, while the range of the apennines resembles a sort of coronet resting on her forehead. this same woman in the moon, it appears, was described by dr. james thompson years ago, and, for aught i know, she may be the diana to whom herrick sang: "queen and huntress chaste and fair, seated in thy silver chair, now the sun is laid to sleep, state in wonted manner keep. hesperus entreats thy light, goddess excellently bright." a feature of the full moon's surface that instantly attracts attention is the remarkable brightness of the southern part of the disk, and the brilliant streaks radiating from a bright point near the lower edge. the same simile almost invariably comes to the lips of every person who sees this phenomenon for the first time--"it looks like a peeled orange." the bright point, which is the great crater-mountain tycho, looks exactly like the pip of the orange, and the light-streaks radiating from it in all directions bear an equally striking resemblance to the streaks that one sees upon an orange after the outer rind has been removed. i shall have something more to say about these curious streaks further on; in the mean time, let us glance at our little sketch-map of the moon. [illustration: map of the moon.] the so-called seas are marked on the map, for the purpose of reference, by the letters which they ordinarily bear in lunar maps. the numerals indicate craters, or ring-plains, and mountain-ranges. the following key-list will enable the reader to identify all the objects that are lettered or numbered upon the map. i have given english translations of the latin names which the old astronomers bestowed upon the seas: _seas, gulfs, and marshes._ a. the crisian sea. b. humboldt sea. c. the sea of cold. d. the lake of death. e. the lake of dreams. f. the marsh of sleep. g. the sea of tranquillity. h. the sea of serenity. i. the marsh of mists. k. the marsh of putrefaction. l. the sea of vapors. m. the central gulf. n. the gulf of heats. o. the sea of showers. p. the bay of rainbows. q. the ocean of storms. r. the bay of dew. s. the sea of clouds. t. the sea of humors. v. the sea of nectar. x. the sea of fertility. z. the south sea. _mountains and crater rings._ . grimaldi. . letronne. . gassendi. . euclides. . bullialdus. . pitatus. . schickhard. . longomontanus. . tycho. . maginus. . clavius. . newton. . maurolycus. . stöfler. . walter. . regiomontanus. . purbach. . arzachel. . alphonsus. . ptolemaus. . hipparchus. . albategnius. . theophilus. . cyrillus. . catharina. . the altai mts. . piccolomini. . petavius. . langrenus. . proclus. . cleomedes. . atlas. . hercules. . posidonius. . plinius. . menelaus. . manilius. . the caucasus mts. . eudoxus. . aristotle. . the alps. . plato. . archimedes. . the apennines. . eratosthenes. . copernicus. . the carpathian mts. . timocharis. . lambert. . euler. . aristarchus. . kepler. . flamsteed. the early selenographers certainly must have been men of vivid imagination, and the romantic names they gave to the lunar landscapes, and particularly to the "seas," add a charm of their own to the study of the moon. who would not wish to see the "bay of rainbows," or the "lake of dreams," or the "sea of tranquillity," if for no other reason than a curiosity to know what could have induced men to give to these regions in the moon such captivating titles? or who would not desire to visit them if he could? though no doubt we should find them, like the "delectable mountains" in the "pilgrim's progress," most charming when seen from afar. the limited scale of our map, of course, renders it impossible to represent upon it more than a comparatively small number of the lunar mountains that have received names. in selecting those to be put in the map i have endeavored to choose such as, on account of their size, their situation, or some striking peculiarity, would be most likely to attract the attention of a novice. the observer must not expect to see them all at once, however. the lunar features change their appearance to a surprising extent, in accordance with the direction of their illumination. some great mountain-masses and ring-plains, or craters, which present scenes of magnificence when the sun is rising or setting upon them, disappear under a perpendicular light, such as they receive at full moon. the great crater-plain, known as maginus, numbered in our map, is one of these. the broken mountain-wall surrounding this vast depressed plain rises in some places to a height of over fourteen thousand feet above the valley within, and the spectacle of sunrise upon maginus, seen with a powerful telescope, is a most impressive sight, and even with a field-glass is very interesting. yet, a few days later, maginus vanishes, as if it had been swallowed up, and as beer and mädler have expressed it, "the full moon knows no maginus." the still grander formation of mountain, plain, and crater, called clavius ( in the map), disappears almost as completely as maginus at full moon, yet, under the proper illumination, it presents a splendid pageant of light and shadow. on the other hand, some of the lunar mountains shine vividly at full moon, and can be well seen then, though, of course, only as light spots, since at that time they cast no shadows. menelaus ( in the map), aristarchus ( ), proclus ( ), copernicus ( ), and kepler ( ), are among these shining mountains. aristarchus is the most celebrated of them all, being the brightest point on the moon. it can even be seen glimmering on the dark side of the moon--that is to say, when no light reaches it except that which is reflected from the earth. with a large telescope, aristarchus is so dazzlingly bright under a high sun, that the eye is partly blinded in gazing at it. it consists of a mountain-ring surrounding a circular valley, about twenty-eight miles in diameter. the flanks of these mountains, especially on their inner slopes, and the floor of the valley within, are very bright, while a peak in the center of the valley, about as high as storm-king mountain on the hudson, shines with piercing brilliancy. sir william herschel mistook it for a volcano in action. it certainly is not an active volcano, but just what makes it so dazzling no one knows. the material of which this mountain is formed would seem to possess a higher reflective power than that of any other portion of the moon's surface. one is irresistibly reminded of the crystallized mountains described in the celebrated "moon hoax" of richard adams locke. with an opera-glass you can readily recognize aristarchus as a bright point at full moon. with a field-glass it is better seen, and some of the short, light rays surrounding it are perceived, while, when the sun is rising upon it, about four days after first quarter, its crateriform shape can be detected with such a glass. the visibility of aristarchus on the dark side of the moon leads us to a brief consideration of the illumination by the earth of that portion of the moon's surface which is not touched directly by sunlight at new and old moon. this phenomenon is shown in the accompanying illustration. not only can the outlines of the dark part of the moon be seen under such circumstances, but even the distinction in color between the dusky "seas" and the more brilliant table-lands and mountain-regions can be perceived, and with powerful telescopes many minor features come into sight. a little consideration must convince any one, as it convinced galileo more than two hundred and seventy-five years ago, that the light reflected from the earth upon the moon is sufficient to produce this faint illumination of the lunar landscapes. we have only to recall the splendors of a night that is lighted by a full moon, and then to recollect that at new or old moon the earth is "full" as seen from our satellite, and that a full earth must give some fourteen times as much light as a full moon, in order to realize the brilliancy of an earth-lit night upon the moon. as the moon waxes to us, the earth wanes to the moon, and _vice versa_, and so the phenomenon of earth-shine on the lunar surface must be looked for before the first quarter and after the last quarter of the moon. [illustration: sunrise on the sea of serenity, and theophilus and other craters.] the reader will find it an attractive occupation to identify, by means of the map, the various "seas," "lakes," and "marshes," for not only are they interesting on account of the singularity of their names, but they present many remarkable differences of appearance, which may be perceived with the instrument he is supposed to be using. the oval form of the crisian sea (a), which is the first of the "seas" to come into sight at new moon, makes it a very striking object. with good telescopes, and under favorable illumination, a decidedly green tint is perceived in the crisian sea. it measures about two hundred and eighty by three hundred and fifty-five miles in extent, and is, perhaps, the deepest of all the old sea-beds visible on the moon. it is surrounded by mountains, which can be readily seen when the sun strikes athwart them a few days after new or full moon. on the southwestern border a stupendous mountain-promontory, called cape agarum, projects into the crisian sea fifty or sixty miles, the highest part rising precipitously eleven thousand feet above the floor of the sea. i have seen cape agarum very clearly defined with a field-glass. near the eastern border is the crater-mountain proclus, which i have already mentioned as possessing great brilliancy under a high sun, being in this respect second only to aristarchus. from the foot of proclus spreads away the somewhat triangular region called the marsh of sleep (f). the term "golden-brown," which has been applied to it, perhaps describes its hue well enough. with a telescope it is a most interesting region, but with less powerful instruments one must be content with recognizing its outline and color. the broad, dark-gray expanse of the sea of tranquillity (g) will be readily recognized by the observer, and he will be interested in the mottled aspect which it presents in certain regions, caused by ridges and elevations, which, when this sea-bottom was covered with water, may have formed shoals and islands. the sea of fertility (x) is remarkable for its irregular surface, and the long, crooked bays into which its southern extremity is divided. the sea of nectar (v) is connected with the sea of tranquillity by a broad strait (one would naturally anticipate from their names that there must be some connection between them), while between it and the sea of fertility runs the range of the pyrenees mountains, twelve thousand feet high, flanked by many huge volcanic mountain-rings. the sea of serenity (h), lying northeast of the sea of tranquillity, is about four hundred and twenty miles broad by four hundred and thirty miles long, being very nearly of the same area as our caspian sea. it is deeper than the sea of tranquillity, and a greenish hue is sometimes detected in its central parts. it deepens toward the middle. three quarters of its shore-line are bordered by high mountains, and many isolated elevations and peaks are scattered over its surface. in looking at these dried-up seas of the moon, one is forcibly reminded of the undulating and in some places mountainous character of terrestrial sea-bottoms, as shown by soundings and the existence of small islands in the deep sea, like the bermudas, the azores and st. helena. the sea of serenity is divided nearly through the center by a narrow, bright streak, apparently starting from the crater-mountain menelaus ( in the map), but really taking its rise at tycho far in the south. this curious streak can be readily detected even with a small opera-glass. just what it is no one is prepared to say, and so the author of the "moon hoax" was fairly entitled to take advantage of the romancer's license, and declare that "its edge throughout its whole length of three hundred and forty miles is an acute angle of solid quartz-crystal, brilliant as a piece of derbyshire spar just brought from the mine, and containing scarcely a fracture or a chasm from end to end!" along the southern shore, on either side of menelaus, extends the high range of the hæmus mountains. south and southeast of the sea of serenity are the sea of vapors (l), the central gulf (m), and the gulf of heats (n). the observer will notice at full moon three or four curious dark spots in the region occupied by these flat expanses. on the north and northwest of the sea of serenity are the lake of death (d), and the lake of dreams (e), chiefly remarkable for their names. the sea of showers (o) is a very interesting region, not only in itself, but on account of its surroundings. its level is very much broken by low, winding ridges, and it is variegated by numerous light-streaks. at its western end it blends into the marsh of mists (i) and the marsh of putrefaction (k). on its northeast border is the celebrated sinus iridum, or bay of rainbows (p), upon which selenographers have exhausted the adjectives of admiration. the bay is semicircular in form, one hundred and thirty-five miles long and eighty-four miles broad. its surface is dark and level. at either end a splendid cape extends into the sea of showers, the eastern one being called cape heraclides, and the western cape laplace. they are both crowned by high peaks. along the whole shore of the bay runs a chain of gigantic mountains, forming the southern border of a wild and lofty plateau, called the sinus iridum highlands. of course, a telescope is required to see the details of this "most magnificent of all lunar landscapes," and yet much can be done with a good field-glass. with such an instrument i have seen the capes at the ends of the bay projecting boldly into the dark, level expanse surrounding them, and the high lights of the bordering mountains sharply contrasted with the dusky semicircle at their feet, and have been able to detect the presence of the low ridges that cross the front of the bay like shoals, separating it from the "sea" outside. two or three days after first quarter, the shadows of the peaks about the bay of rainbows may be seen. the bay of dew (r) above the bay of rainbows, and the sea of cold (c), are the northernmost of the dark levels visible. it was in keeping with the supposed character of this region of the moon that riccioli named two portions of it the land of hoar frost and the land of drought. extending along the eastern side of the disk is the great ocean of storms (q), while between the ocean of storms and the middle of the moon lies the sea of clouds (s). both of these are very irregular in outline, and much broken by ridges and mountains. the sea of humors (t), although comparatively small, is one of the most easily seen of all the lunar plains. to the naked eye it looks like a dark, oval patch on the moon. with a telescope it is seen, under favorable conditions, to possess a decided green tint. humboldt sea (b) and the south sea (z) belong principally to that part of the moon which is always turned away from the earth, and only their edges project into the visible hemisphere, although, under favorable librations, their farther borders, lined as usual with mountain-peaks, may be detected. for our purposes they possess little interest. let us now glance at some of the mountains and "craters." the dark oval called grimaldi ( ) can be detected by the naked eye, or at least it has been thus seen, although it requires a sharp eye; and perhaps a shade or a pair of eye-glasses of london smoke-glass, to take off the glare of the moon, should be used in looking for it.[f] it is simply a plain, containing some fourteen thousand square miles, remarkable for its dark color, and surrounded by mountains. schickhard ( ) is another similar plain, nearly as large, but not possessing the same dark tint in the interior. the huge mountains around schickhard make a fine spectacle when the sun is rising upon them shortly before full moon. [f] there are other uses to which such eye-glasses may be put by sky-gazers. i habitually carry a pair for studying clouds. it is wonderful how much the effect of great cloud-masses is heightened by them, especially when seen in a bright light. delicate curls and striæ of cirrus, which escape the uncovered eye in the glare of sunlight, can be readily detected and studied by the use of neutral-tinted eye-glasses or spectacles. tycho ( ) is the most famous of the crater-mountains, though not the largest. it is about fifty-four miles across and three miles deep. in its center is a peak five or six thousand feet high. tycho is the radial point of the great light-streaks that, as i have already remarked, cause the southern half of the moon to be likened to a peeled orange. it is a tough problem in selenography to account for these streaks. they are best seen at full moon. they can not be seen at all until the sun has risen to a certain elevation above them, ° according to neison; but, when they once become visible, they dominate everything. they turn aside for neither mountains nor plains, but pass straight on their courses over the ruggedest regions of the moon, retaining their brilliancy undiminished, and pouring back such a flood of reflected light that they completely conceal some of the most stupendous mountain-masses across which they lie. they clearly consist of different material from that of which the most of the moon's surface is composed--a material possessing a higher reflective power. in this respect they resemble aristarchus and other lunar craters that are remarkable for their brilliancy under a high illumination. tycho itself, the center or hub, from which these streaks radiate like spokes, is very brilliant in the full moon. but immediately around tycho there is a dark rim some twenty-five miles broad. beyond this rim the surface becomes bright, and the bright region extends about ninety miles farther. out of it spring the great rays or streaks, which vary from ten to twenty miles in width, and many of which are several hundred miles long--one, which we have already mentioned as extending across the sea of serenity, being upward of two thousand miles in length. it has been truly said that we have nothing like these streaks upon the earth, and so there is no analogy to go by in trying to determine their nature. it has been suggested that if the moon had been split or shattered from within by some tremendous force, and molten matter from the interior had been thrust up into the cracks thus formed, and had cooled there into broad seams of rock, possessing a higher reflective power than the surrounding surface of the moon, then the appearances presented would not be unlike what we actually see. but there are serious objections to such a view, which we have not space to discuss here. it is enough to say that the nature of these streaks is still a question awaiting solution, and here is an opportunity for an important discovery, but not one to be achieved with an opera-glass. i may add an interesting suggestion as to the nature of these streaks made by the rev. mr. grensted. he holds that the air and water of the moon were chemically, and not mechanically, absorbed in the process of oxidation which went on at the time when her surface temperature was above a red heat. having a much larger surface in proportion to her bulk than the earth, the oxidation of the moon has, he thinks, extended much deeper than that of the earth, and her atmosphere and oceans have been exhausted in the process. both the earth and the moon, he maintains, have metallic nuclei, and the streaks about tycho and copernicus, and some other lunar craters, may be dikes of pure and shining metal, which have escaped oxidation owing to the comparatively small supply of lunar oxygen. upon this theory aristarchus must be a metallic mountain. [illustration: sunrise on clavius, tycho, plato, etc.] clavius ( ) is one of the most impressive of all the lunar formations. there probably does not exist anywhere upon the earth so wild a scene upon a corresponding scale of grandeur. of course, its details are far beyond the reach of the instrument we are supposed to be using, and yet, even with a field-glass, or a powerful opera-glass, some of its main features are visible. it is represented in our picture of the half-moon, being the lowest and largest of the ring-like forms seen at the inner edge of the illuminated half of the disk; the rays of the rising sun touching the summits of some of the peaks in its interior have brought them into sight as a point of light, and at the same time, reaching across the gulf within, have lighted up the higher slopes of the great mountain-wall on the farther or eastern side of the crater-valley, making it resemble a semicircle of light projecting into the blackness of the still unilluminated plains around it. i should advise every reader to take advantage of any opportunity that may be presented to him to see clavius with a powerful telescope when the sun is either rising or setting upon it. neison has given a spirited description of the scene, as follows: the sunrise on clavius commences with the illumination of a few peaks on the western wall, but soon rapidly extends along the whole wall of clavius, which then presents the appearance of a great double bay of the dark night-side of the moon penetrating so deep into the illuminated portion as to perceptibly blunt the southern horn to the naked eye. within the dark bay some small, bright points soon appear--the summits of the great ring-plains within--followed shortly by similar light-points near the center, due to peaks on the walls of the smaller ring-plains, these light-islands gradually widening and forming delicate rings of light in the dark mass of shadow still enveloping the floor of clavius. far in the east then dimly appear a few scarcely perceptible points, rapidly widening into a thin bright line, the crest of the great southeastern wall of clavius, the end being still lost far within the night-side of the moon. by the period the extreme summit of the lofty wall of clavius on the east becomes distinct, fine streaks of light begin to extend across the dark mass of shadow on the interior of clavius, from the light breaking through some of the passes on the west wall and illuminating the interior; and these streaks widen near the center and form illuminated spots on the floor, when both east and west it still lies deeply immersed in shadow, strongly contrasting with the now brightly illuminated crest of the lofty east wall and the great circular broad rings of light formed by the small ring-plains within clavius. the illumination of the interior of clavius now proceeds rapidly, and forms a magnificent spectacle: the great, brightly illuminated ring-plains on the interior, with their floors still totally immersed in shadow; the immense steep line of cliffs on the east and southeast are now brilliantly illuminated, though the entire surface at their base is still immersed in the shades of night; and the great peaks on the west towering above the floor are thrown strongly into relief against the dark shadow beyond them. newton ( ) is the deepest of the great crateriform chasms on the moon. some of the peaks on its walls rise twenty-four thousand feet above the interior gulf. its shadow, and those of its gigantic neighbors--for the moon is here crowded with colossal walls, peaks, and craters--may be seen breaking the line of sunlight below clavius, in our illustration. i have just spoken of these great lunar formations as chasms. the word describes very well the appearance which some of them present when the line separating day and night on the moon falls across them, but the reader should not be led by it into an erroneous idea of their real character. such formations as newton, which is one hundred and forty miles long by seventy broad, may more accurately be described as vast depressed plains, generally containing peaks and craters, which are surrounded by a ring of steep mountains, or mountain-walls, that rise by successive ridges and terraces to a stupendous height. the double chain of great crater-plains reaching half across the center of the moon contains some of the grandest of these strange configurations of conjoined mountain, plain, and crater. the names of the principal ones can be learned from the map, and the reader will find it very interesting to watch them coming into sight about first quarter, and passing out of sight about third quarter. at such times, with a field-glass, some of them look like enormous round holes in the inner edge of the illuminated half of the moon. theophilus ( ), cyrillus ( ), and catharina ( ), are three of the finest walled plains on the moon--theophilus, in particular, being a splendid specimen of such formations. this chain of craters may be seen rapidly coming into sunlight at the edge of the sea of nectar, in our picture of "sunrise on the sea of serenity," etc. the altai mountains ( ) are a line of lofty cliffs, two hundred and eighty miles in length, surmounting a high table-land. the caucasus mountains ( ) are a mass of highlands and peaks, which introduce us to a series of formations resembling those of the mountainous regions of the earth. the highest peak in this range is about nineteen thousand feet. between the caucasus and the apennines ( ) lies a level pass, or strait, connecting the sea of serenity with the sea of showers. the apennines are the greatest of the lunar mountain-chains, extending some four hundred and sixty miles in length, and containing one peak twenty-one thousand feet high, and many varying from twelve thousand to nearly twenty thousand. it will thus be seen that the apennines of the earth sink into insignificance in comparison with their gigantic namesakes on the moon. as this range runs at a considerable angle to the line of sunrise, its high peaks are seen tipped with sunlight for a long distance beyond the generally illuminated edge about the time of first quarter. even with the naked eye the sun-touched summits of the lunar apennines may at that time be detected as a tongue of light projecting into the dark side of the moon. the alps ( ) are another mountain-mass of great elevation, whose highest peak is a good match for the mont blanc of the earth, after which it has been named. plato ( ) is a very celebrated dark and level plain, surrounded by a mountain-ring, and presenting in its interior many puzzling and apparently changeable phenomena which have given rise to much speculation, but which, of course, lie far beyond the reach of opera-glasses. plato is seen in the picture of "sunrise on clavius," etc., on page , being the second ring from the top. if ariosto had had a telescope, we might have suspected that it was this curious plain that he had in mind when he described that strange valley in the moon, in which was to be found everything that was lost from the earth, including lost wits; and where the redoubtable knight astolpho, having been sent in search of the missing wit of the great orlando, was astonished to find what he sought carefully preserved in a vial along with other similar vials belonging to many supposedly wise people of the earth, whom nobody suspected of keeping a good part of their sapience in the moon. copernicus ( ) is the last of the lunar formations that we shall describe. it bears a general resemblance to tycho, and is slightly greater in diameter; it is, however, not quite so deep. it has a cluster of peaks in the center, whose tops may be detected with a field-glass, as a speck of light when the rays of the morning sun, slanting across the valley, illuminate them while their environs are yet buried in night. copernicus is the center of a system of light-streaks somewhat resembling those of tycho, but very much shorter. we must not dismiss the moon without a few words as to its probable condition. it was but natural, after men had seen the surface of the moon diversified with hills and valleys like another earth, that the opinion should find ready acceptance that beings not unlike ourselves might dwell upon it. nothing could possibly have been more interesting than the realization of such a fancy by the actual discovery of the lunar inhabitants, or at least of unmistakable evidence of their existence. the moon is so near to the earth, as astronomical distances go, and the earth and the moon are so intimately connected in the companionship of their yearly journey around the sun, and their greater journey together with the sun and all his family, through the realms of space, that we should have looked upon the lunar inhabitants, if any had existed, as our neighbors over the way--dwelling, to be sure, upon a somewhat more restricted domain than ours, vassals of the earth in one sense, yet upon the whole very respectable and interesting people, with whom one would be glad to have a closer acquaintance. but, alas! as the powers of the telescope increased, the vision of a moon crowded with life faded, until at last the cold fact struck home that the moon is, in all probability, a frozen and dried-up globe, a mere planetary skeleton, which could no more support life than the humboldt glacier could grow roses. and yet this opinion may go too far. there is reason for thinking that the moon is not absolutely airless, and, while it has no visible bodies of water, its soil may, after all, not be entirely arid and desiccated. there are observations which hint at visible changes in certain spots that could possibly be caused by vegetation, and there are other observations which suggest the display of electric luminosity in a rarefied atmosphere covering the moon. to declare that no possible form of life can exist under the conditions prevailing upon the lunar surface would be saying too much, for human intelligence can not set bounds to creative power. yet, within the limits of life, such as we know them, it is probably safe to assert that the moon is a dead and deserted world. in other words, if a race of beings resembling ourselves, or resembling any of our contemporaries in terrestrial life, ever existed upon the moon, they must long since have perished. that such beings may have existed, is possible, particularly if it be true, as generally believed, that the moon once had a comparatively dense atmosphere and water upon its surface, which have now, in the process of cooling of the lunar globe, been withdrawn into its interior. it certainly does not detract from the interest with which we study the rugged and beautiful scenery of the moon to reflect that if we could visit those ancient sea-bottoms, or explore those glittering mountains, we might, perchance, find there some remains or mementos of a race that flourished, and perhaps was all gathered again to its fathers, before man appeared upon the earth. that slight physical changes, such as the downfall of mountain-walls or crater-cones, still occasionally occur upon the moon, is an opinion entertained by some selenographers, and apparently justified by observation. the enormous changes of temperature, from burning heat under a cloudless sun to the freezing cold of space at night with no atmospheric blanket to retain heat (which has generally been assumed to be the condition of things on the moon), would naturally exert a disintegrating effect upon the lunar rocks. but the question is now in dispute whether the surface of the moon ever rises above the freezing-point of water, even under a midday sun. mankind has always been a little piqued by the impossibility of seeing the other side of the moon, and all sorts of odd fancies have been indulged in regard to it. among the most curious is the ancient belief that the souls of the good who die on earth are transported to that side of the moon which is turned away from the earth; while the souls of the wicked sojourn on this side, in full view of the scene of their evil deeds. the visible side of the moon--with its tremendous craters, its yawning chasms, its frightful contrasts of burning sunshine and cimmerian darkness, its airless and arid plains and dried-up sea-bottoms exposed to the pitiless cold of open space, and heated, if heated at all, by scorching sunbeams as fierce as naked flame--would certainly appear to be in a proper condition to serve as a purgatory. but we have no reason to think that the other side is any better off in these respects. in fact, the glimpses that we get of it around the corners, so to speak, indicate that the whole round globe of the moon is as ragged, barren, and terrible as that portion of it which is turned to our view. the planets.--in attempting to view the planets with an opera-glass, too much must not be expected; and yet interesting views can sometimes be obtained. the features of their surfaces, of course, can not be detected even with a powerful field-glass, but the difference between the appearance of a large planet and that of the stars will at once strike the observer. mercury, which, on account of its nearness to the sun and its rapid changes of place, comparatively few persons ever see, can perhaps hardly be called an interesting object for an opera-glass, and yet the beauty of the planet is greatly increased when viewed with such aid. mercury is brilliant enough to be readily distinguishable, even while the twilight is still pretty bright; and i have had most charming views of the shy planet, glittering like a globule of shining metal through the fading curtain of a winter sunset. venus is, under favorable circumstances, a very interesting planet for opera-glass observations. the crescent phase can be seen with a powerful glass near inferior conjunction, and, even when the form of the planet can not be discerned, its exceeding brilliancy makes it an attractive object. the flood of light which venus pours forth, and which is so dazzling that it baffles the best telescopes, to a greater or less extent, in any effort to descry the features of that resplendent disk, is evidently reflected from a cloud-burdened atmosphere. while these clouds render the planet surprisingly lustrous to our eyes, they must, of course, keep the globe beneath them most of the time in shadow. it is a source of keen regret that the surface of venus can not be seen as clearly as that of mars, for, _a priori_, there is rather more reason to regard venus as possibly an inhabited world than any other of the earth's sister planets, not excepting mars. still, even if we could plainly make out the presence of oceans and continents on venus, that fact would hardly be any better indication of the possibility of life there than is furnished by the phenomena of its atmosphere. it is an interesting reflection that in admiring the brilliancy of this splendid planet the light that produces so striking an effect upon our eyes has but a few minutes before traversed the atmosphere of a distant world, which, like our own air, may furnish the breath of life to millions of intelligent creatures, and vibrate with the music of tongues speaking languages as expressive as those of the earth. mars, being both more distant and smaller than venus, does not present so splendid a scene, and yet when it is at or near opposition it is a superb object even for an opera-glass, its deep reddish-yellow color presenting a fine contrast to that of most of the stars. it can often be seen in conjunction with, or near to, the moon and stars, and the beauty of these phenomena is in some cases greatly enhanced by the use of a glass. to find mars (and the same remark applies to the other planets), take its right ascension and declination for the required date from the nautical almanac, and then mark its place upon a planisphere or any good star-map. this planet is at the present time ( ) slowly drawing nearer to the earth at each opposition, and in it will be closer to us than at any time since , when its two minute satellites were discovered. it will consequently grow brighter every year until then. how splendidly it shines when at its nearest approach to the earth may be inferred from the fact that in it was so brilliant as actually to cause a panic. this was doubtless owing to its peculiar redness. i well remember the almost startling appearance which the planet presented in the autumn of . mars is especially interesting because of the apparently growing belief that it may be an inhabited world, and because of certain curious markings on its surface that can only be seen under favorable conditions. the recent completion of the great lick telescope and other large glasses, and the approach of the planet to a favorable opposition, give reason to hope that within the next few years a great deal of light will be cast upon some of the enigmatical features of mars's surface. [illustration: jupiter and his moons. (seen with a field-glass; seven diameters.)] jupiter, although much more distant than mars, is ordinarily a far more conspicuous phenomenon in the sky on account of his vast bulk. his interest to observers with an opera-glass depends mainly upon his four moons, which, as they circle about him, present a miniature of the solar system. with a strong opera-glass one or two of jupiter's little family of moons may occasionally be caught sight of as excessively minute dots of light half-hidden in the glare of the planet. if you succeed under favorable circumstances in seeing one of these moons with your glass, you will be all the more astonished to learn that there are several apparently well-authenticated instances of one of the moons of jupiter having been seen with the naked eye. with a field-glass, however, you will have no difficulty in seeing all of the moons when they are properly situated. if you miss one or more of them, you may know that it is either between you and the planet, or behind the planet, or buried in the planet's shadow, or else so close to the planet as to be concealed by its radiance. it will be best for the observer to take out of the nautical almanac the "configurations of jupiter's satellites" for the evenings on which he intends to make his observations, recollecting that the position of the whole system, as there given, is reversed, or presented as seen with an astronomical telescope, which inverts objects looked at, as an opera-glass does not. in order to bring the satellites into the positions in which he will see them, our observer has only to turn the page in the nautical almanac showing their configurations upside down. of course, since the motions of the satellites, particularly of the inner ones, are very rapid, their positions are continually changing, and their configurations are different every night. if the observer has any doubt about his identification of them, or thinks they may be little stars, he has only to carefully note their position and then look at them again the next evening. he may even notice their motion in the course of a single evening, if he begins early and follows them for three or four hours. it is impossible to describe the peculiar attractions of the scene presented by the great planet and his four little moons on a serene evening to an observer armed with a powerful glass. probably much of the impressiveness of the spectacle is owing to the knowledge that those little points of light, shining now in a row and now in a cluster, are actually, at every instant, under the government of their giant neighbor and master, and that as we look upon them, obediently making their circuits about him, never venturing beyond a certain distance away, we behold a type of that gravitational mastery to which our own little planet is subject as it revolves around its still greater ruler, the sun, to whose control even jupiter in his turn must submit. the beautiful planet saturn requires for the observation of its rings magnifying powers far beyond those of the instruments with which our readers are supposed to be armed. it would be well, however, for the observer to trace its slow motion among the stars with the aid of the nautical almanac, and he should be able with a good field-glass to see, under favorable circumstances, the largest of its eight moons, titan. this is equal in brilliancy to an . magnitude star. its position with respect to saturn on any given date can be learned from the ephemeris. it may appear somewhat presumptuous to place uranus, a planet which it required the telescope and the eye of a herschel to discover, in a list of objects for the opera-glass. but it must not be forgotten that uranus was seen certainly several, and probably many, times before herschel's discovery, being simply mistaken, on account of the slowness of its motion, for a fixed star. when near opposition, uranus looks as bright as a sixth-magnitude star, and can be easily detected with the naked eye when its position is known. with an opera-glass (and still more readily with a field-glass) this distant planet can be watched as it moves deliberately onward in its gigantic orbit. its passage by neighboring stars is an exceedingly interesting phenomenon, and it is in this way that you may recognize the planet. on the evening of may , , i knew, from the co-ordinates given in the nautical almanac, that uranus was to be found a short distance east of mars, which was then only a few degrees from the well-known star gamma virginis. accordingly, i turned my opera-glass upon mars, and at once saw a star in the expected position, which i knew was uranus. but there were other small stars in the field, and, supposing i had not been certain which was uranus, how could i have recognized it? the answer is plain: simply by watching for a night or two to see which star moved. that star would, of course, be uranus. the accompanying cuts will show the motions of mars and uranus with respect to neighboring stars at that time, and will serve as an example of the method of distinguishing a planet from the fixed stars by its change of place. in the first cut we have the two planets and three neighboring stars as they appeared on may th. these stars were best seen with a field-glass, although an opera-glass readily showed them. [illustration: mars and uranus, may , .] [illustration: mars and uranus, june , .] [illustration: mars and uranus, june , .] on june st the relative positions of the planets and stars were as shown in the second cut. a glance suffices to show that not only mars but uranus also has shifted its position with respect to the three immovable stars. this change of place alone would have sufficed to indicate the identity of uranus. to make sure, the inexperienced observer had only to continue his observations a few nights longer. on june th mars and uranus were in conjunction, and their position, as well as that of the same set of three stars, is shown in the third cut. it will be seen that while mars had changed its place very much more than uranus, yet that the latter planet had now moved so far from its original position on may th, that there could be no possibility that the merest tyro in star-gazing would fail to notice the change. whenever the observer sees an object which he suspects to be a planet, he can satisfy himself of its identity by making a series of little sketches like the above, showing the position of the suspected object on successive evenings, with respect to neighboring stars. the same plan suffices to identify the larger planets, in the case of which no glass is necessary. the observer can simply make a careful estimate by the naked eye of the supposed planet's distance and bearing from large stars near it, and compare them with similar observations made on subsequent evenings. the sun.--that spots upon the sun may be seen with no greater optical aid than that of an opera-glass is perhaps well known to many of my readers, for during the past ten years public attention has been drawn to sun-spots in an especial manner, on account of their supposed connection with meteorology, and in that time there have been many spots upon the solar disk which could not only be seen with an opera-glass, but even with the unassisted eye. at present ( ) we are near a minimum period of sun-spots, and the number to be seen even with a telescope is comparatively very small, yet only a few days before this page was written there was a spot on the sun large enough to be conspicuous with the aid of a field-glass. during the time of a spot-maximum the sun is occasionally a wonderful object, no matter how small the power of the instrument used in viewing it may be. strings of spots of every variety of shape sometimes extend completely across the disk. our illustration shows the appearance of the sun, as drawn by the author on the st of september, . every one of the spots and spot-groups there represented could be seen with a good field-glass, and nearly all of them with an opera-glass. [illustration: the sun, september , .] as in all such cases, our interest in the phenomena increases in proportion to our understanding of their significance and their true scale of magnitude. in glancing from side to side of the sun's disk, the eye ranges over a distance of more than , miles--not a mere ideal distance, or an expanse of empty space, but a distance filled by an actual and, so to speak, tangible body, whose diameter is of that stupendous magnitude. one sees at a glance, then, the enormous scale on which these spots are formed. the earth placed beside them would be but a speck, and yet they are mere pits in the surface of the sun, filled perhaps with partially cooled metallic vapors, which have been cast up from the interior, and are settling back again. it is worth anybody's while to get a glimpse at a sun-spot if he can, for, although he may see it merely as a black dot on the shining disk, yet it represents the play of physical forces whose might and power are there exercised on a scale really beyond human comprehension. the imagination of milton or dante would have beheld the mouth of hell yawning in a sun-spot. in order to view the sun it is, of course, necessary to contrive some protection for the eyes. this may be constructed by taking two strips of glass four or five inches long and an inch wide, and smoking one of them until you can without discomfort look at the sun through it. then place the two strips together, with the smoked surface inside--taking care to separate them slightly by pieces of cardboard placed between the ends--and fasten the edges together with strips of paper gummed on. then, by means of a rubber band, fasten the dark glass thus prepared over the eye-end of your opera-glass in such a way that both of the lenses are completely covered by it. it will require a little practice to enable you to get the sun into the field of view and keep it there, and for this purpose you should assume a posture--sitting, if possible--which will enable you to hold the glass very steady. then point the glass nearly in the direction of the sun, and move it slowly about until the disk comes in sight. it is best to carefully focus your instrument on some distant object before trying to look at the sun with it. as there is some danger of the shade-glass being cracked by the heat, especially if the object-glasses of the instrument are pretty large, it would be well to get the strips of glass for the shade large enough to cover the object-end of the instrument instead of the eye-end. at a little expense an optician will furnish you with strips of glass of complementary tints, which, when fastened together, give a very pleasing view of the sun without discoloring the disk. dark red with dark blue or green answer very well; but the color must be very deep. the same arrangement, of course, will serve for viewing an eclipse of the sun. a word, finally, about the messenger which brings to us all the knowledge we possess of the contents and marvels of space--light. without the all-pervading luminiferous ether, narrow indeed would be our acquaintance with the physical creation. this is a sympathetic bond by which we may conceive that intelligent creatures throughout the universe are united. light tells us of the existence of suns and systems so remote that the mind shrinks from the attempt to conceive their distance; and light bears back again to them a similar message in the feeble glimmering of our own sun. and can any one believe that there are no eyes out yonder to receive, and no intelligence to interpret that message? sir humphry davy has beautifully expressed a similar thought in one of his philosophical romances: in jupiter you would see creatures similar to those in saturn, but with different powers of locomotion; in mars and venus you would find races of created forms more analogous to those belonging to the earth; but in every part of the planetary system you would find one character peculiar to all intelligent natures, a sense of receiving impressions from light by various organs of vision, and toward this result you can not but perceive that all the arrangements and motions of the planetary bodies, their satellites and atmospheres, are subservient. the spiritual natures, therefore, that pass from system to system in progression toward power and knowledge preserve at least this one invariable character, and their intellectual life may be said to depend more or less upon the influence of light.[g] [g] see "consolations in travel, or, the last days of a philosopher"; dialogue i. light is a result, and an expression, of the energy of cosmical life. the universe lives while light exists. but when the throbbing energies of all the suns are exhausted, and space is filled with universal gloom, the light of intelligence must vanish too. one can not read the wonderful messages of light--one can not study the sun, the moon, and the stars in any manner--without perceiving that the physical universe is enormously greater than he had thought, and that the creation, of which the earth is an infinitesimal part, is almost infinitely more magnificent in actual magnitude than the imaginary domain which men of old times pictured as the dwelling-place of the all-controlling gods; without feeling that he has risen to a higher plane, and that his intellectual life has taken a nobler aim and a broader scope. index. achernar, . albireo ([beta] cygni), . alcor, . alcyone, . mädler's "central sun," . aldebaran, , , , , , . algenib ([alpha] persei), , . algol, the demon-star, . probable cause of variation of, . al-mamoun, the caliph, observation of a temporary star, . almaach ([gamma] andromedæ), , . alphard, . alpha andromedæ, . agnarii (sadalmelik), . arietis (hamal), . capricorni (giedi), . ceti (menkar), . draconis, formerly the pole-star, . libræ, . ophiuchi (ras alhague), . orionis (betelgeuse), , , . pegasi (markab), . ursæ majoris, . alpheratz ([alpha] andromedæ), . alps, the lunar, . altai mountains, . altair, . andromedæ, map of, . mythology of, . antares, , , . antinous, . apennines, the lunar, . apollonius, regarded the moon as a mirror, . aquarius, map of, . mythology of, . aquila, map of, . mythology of, . aratus, description of the manger, . the "diosemia" of, . the phenomena of, . story of virgo, . description of the "royal family," . description of cetus, . arcturus, , , , , . argo, map of, . mythology of, . aries, map of, . mythology of, . ariosto, story of a trip to the moon, . aristarchus, the shining mountain, . aselli, . asterope, . atlas, . auriga, map of, . mythology of, . star swarms in, . autumn, map of the stars of, . bartschius invents monoceros, . bay of dew, . bay of rainbows, . bear's head, stars forming the, . bellatrix, , . belt, orion's, , . berenice's hair, the constellation of, . picture of, . bessel, studies of sirius and procyon, . letter about "dark stars," . beta andromedæ (mirach), . arietis (sheratan), . capricorni (dabih), . cassiopeia, . beta corvi, . cygni (albireo), . libræ, . leonis (denebola), . lyræ, . pegasi, . scorpionis, . ursæ minoris (kochab), . betelgeuse ([alpha] orionis), , , . bethlehem, the so-called star of, . biela's comet, it breaks up, . biela meteors, radiant point of the, . boötes, map of, . mythology of, . calisto, another name of ursa major, . cancer, map of, . mythology of, . canes venatici, . canis major, map of, . mythology of, . canis minor, map of, . mythology of, . canopus, . capella, , , , , . cape heraclides, . laplace, . capricornus, map of, . mythology of, . cassiopeia, map of, . mythology of, . castor, . catharina, . caucasus mountains, . celæno, . central gulf, . "central sun," mädler's ideas about a, . cepheus, map of, , . cetus, map of, . mythology of, . chi ceti, . clavius, , , . coal-sack, . comet, biela's, . comet, halley's, the crab nebula mistaken for, . constellations, origin of, , , . along the milky-way, . the zodiacal, . constellations, st. paul's knowledge of, . copernicus, . corvus, map of, . mythology of, . "crimson star," . crisian sea, . cynosura, a name of ursa minor, . cygnus, map of, . cyrillus, . dabih ([beta] capricorni), . dark stars, bessel's suggestion about, . davy, humphry, on life in other worlds, . delta canis majoris, . cephei, . tauri, . deltoton, . denebola ([beta] leonis), , , . dipper, the great, , . dog-days, origin of the, . dog-star, . dolphin, map of the, . mythology of the, . draco, map of, . mythology of, . el nath, , . epsilon leonis, . lyræ, . tauri, . virginis, . equinox, autumnal, . vernal, . eridanus, map of, . eta aquilæ, . field-glass, . field of the nebulæ, . flammarion, on [alpha] capricorni, . flood traditions connected with the pleiades, , . focus, importance of a sharp, . fomalhaut, . fontenelle, "plurality of worlds," . galileo, his telescope an opera-glass, . his description of præsepe, . his description of the moon, . power of his telescope, . gamma andromedæ, , . leonis, . pegasi, . tauri, . virginis, . "garnet star" (mu cephei), . gemini, map of, . mythology of, . genesis, a celestial, . giedi ([alpha] capricorni), . glass, use of smoked or colored, , . goldschmidt sees a nebula in the pleiades, . gomelza, . gore, estimate of the stars in m, . "grape-gatherer" ([epsilon] virginis), . grensted, rev. mr., suggestion about lunar rays, . grimaldi, . halley's comet and crab nebula, . hamal ([alpha] arietis), . hæmus mountains, . henry, paul and prosper, photographs of the pleiades, . hercules, map of, . mythology of, . motion of solar system toward, . herschel, william, discovers uranus, . computation of stars in m, . advice about seeing star-colors, . thinks he sees lunar volcano, . john, description of m, . suggestion about [alpha] capricorni, . holden, prof., on the milky-way, . structure of ring nebula, . hooke, discovers first telescopic double star, . hyades, , , , . hydra, map of part of, . mythology of, . hydra's heart (alphard), . humboldt sea, . jeaurat, chart of the pleiades, . job's coffin, . jupiter, . satellites of, . kappa argus, . tauri, . kepler observes the star of , . kingsley, story of andromeda, . "king's lucky star," . kochab (beta ursæ minoris), . lake of death, . of dreams, . land of drought, . of hoar frost, . leo, map of, . mythology of, . sickle-shaped figure in, , . lepus, map of, . lick telescope, views of milky-way, . views of ring nebula, . light, the messenger of the universe, . in a star-cluster, . libra, description and mythology of, . life, does it exist beyond the earth? , , , , , . locke, richard adams, author of the "moon hoax," . lyra, map of, . mythology of, . mädler, on the "central sun," . maginus, . maia, , . man in the moon, . manger (præsepe), . marine glass, . markab ([alpha] pegasi), . marsh of mists, . of putrefaction, . of sleep, . mars, . medusa, the head of, . menelaus, . menkalina, . menkar ([alpha] ceti), . mercury, . merope, , . mesarthim, . meteors, radiant point of november, . radiant point of biela, . micromegas, the story of, . milk-dipper, . milky-way, , , , , , , , , . mira ([omicron] ceti), . probable cause of its variations, . milton, account of libra, . mirach ([beta] andromedæ), . mizar, . moon, mountains of the, . shadows on the, . map of the, . list of mountains, "seas," etc., . inhabitableness of the, . the other side of the, . "moon hoax," , . monoceros, map of, . mu argus, . scorpionis, . nebulæ (and star-clusters): m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . ^ , . ^ , . ^ , . andromeda, great nebula in, , . aquarius, nebula in, . crab nebula, , . field of the nebulæ, . horseshoe nebula, . orion, great nebula in, . perseus, great cluster in, . pleiades, nebulæ in the, . ring nebula in lyra, . nebular hypothesis, . neison, description of sunrise on clavius, . newton, . "nile-star," . northern cross, , . northern crown, map of the, . northern fish, , . nu andromedæ, , . aquarii, a pointer to a nebula, . canis majoris, . draconis, . scorpionis, . ocean of storms, . omicron ceti (mira), , . cygni, . omicron two eridani, a flying-star, . opera-glass, views of the stars with, . how to choose a good, . magnifying power of, . defects of, . ophiuchus and serpens, map of, . mythology of, . orion, map of, . mythology of, . great array of stars around, . riches of, . spectacle of the rising of, . orpheus, fancies about the moon, . pegasus, map of, . mythology of, . perseus, map of, . mythology of, . great cluster in, . phantom, another name of hercules, . photography, astronomical, , . pi argus, . five orionis, . pegasi, . pisces, map of, . mythology of, . piscis australis, . plato, . pleiades, , , , . names of the, . mythology of, . and the flood, , . and the great pyramid, . picture of the, . common motion of the, . pleione, , . pole-star, , . pollux, . præsepe (the manger), . prime meridian, . proclus, . procyon, , . pyramid of cheops and the pleiades, . pyrenees mountains, . ras alhague ([alpha] ophiuchi), . rays of the moon, . regulus, , . revolution of the heavens, , . rho ophiuchi, . rigel, , , , . ring nebula, . "royal family," , . rutherford, photograph of the moon, . sadalmelik ([alpha] aquarii), . sagitta, map of, . sagittarius, map of, . mythology of, . saiph, . saturn, . scorpio, map of, . mythology of, . pair of stars in sting of, . schickhard, . sea of clouds, . sea of cold, . sea of fertility, . sea of humors, . sea of nectar, . sea of serenity, . sea of showers, . sea of tranquillity, . sea of vapors, . secchi, father, types of the stars, . description of a star-swarm, . seiss, rev. dr., on canis minor, . description of auriga, . sheratan ([beta] arietis), . sidus ludovicianum, . sirius, , , . color of, . size and distance of, . the companion of, , . its light compared with the sun's, . sigma tauri, . sixty-one cygni, . smyth, admiral, on capricorn, . description of aldebaran, . description of m, . solstice, summer, , . winter, . sobieski's shield, . solar system, voyaging of, in space, . southern cross, , . south sea, . spectroscopic analysis, , . spica, , , , . spring, map of the stars of, . square of pegasus, . st. paul, acquainted with the constellations, . star-clusters (see nebulæ, etc.). star-cluster, light in a, . summer, map of the stars of, . sun, opera-glass observations of the, . the, a variable star, . sword of orion, . taurus, map of, . mythology of, . the "golden horns" of, . poniatowskii, . tau aquarii, . taygeta, . temporary stars: b. c. the first on record, . a. d., . , . , . , tycho's star, . , . , , . , , . , . temple, discovers a nebula in the pleiades, . tennyson, describes the pleiades, . theophilus, . theta orionis, . serpentis, . tauri, . tobias mayer, sees the planet neptune, . triangles, map of the, . mythology of, . twenty-two canis majoris, . scorpii, . tycho brahe, invents antinous, . places hamal in aries, . studies the star of , . tycho, , . upsilon tauri, . uranus, discovery of, . how to find, . ursa major, map of, . mythology of, . stars in the feet of, . ursa minor, map of, . mythology of, . vega, . venus, mistaken for artificial light, . opera-glass observation of, . virgil, description of taurus, . virgo, map of, . mythology of, . vision, seeing with averted, . voltaire, story of "micromegas," . vulpecula, map of, . webb, rev. t. w., on telescopes, . on m, . western fish, . winter, brilliancy of the heavens in, . map of the stars of, . woman in the moon, . zeta corvi, . cassiopeia, . leonis, . lyræ, . scorpionis, . tauri, a pointer to the crab nebula, . zi argus, . zodiac, . zodiac, divided among the twelve apostles, . of dendera, . zöllner, estimate of sirius's light, . the end. [illustration path of biela's comet.] letters on astronomy, in which the elements of the science are familiarly explained in connection with biographical sketches of the most eminent astronomers. with numerous engravings. by denison olmsted, ll.d., professor of natural philosophy and astronomy in yale college revised edition. including the latest discoveries. new york: harper & brothers, publishers, & pearl street, franklin square. . entered according to act of congress, in the year , by marsh, capen, lyon, and webb, in the clerk's office of the district court of massachusetts. advertisement to the revised edition. since the first publication of these letters, in , the work has passed through numerous editions, and received many tokens of public favor, both as a class-book for schools and as a reading-book for the family circle. the valuable discoveries made in the science within a few years have suggested an additional letter, which is accordingly annexed to the series in the present revised form, giving a brief but comprehensive notice of all the leading contributions with which astronomy has of late been enriched. the form of _letters_ was chosen on account of the greater freedom it admits, both of matter and of style, than a dress more purely scientific. thus the technical portion of the work, it was hoped, might be relieved, and the whole rendered attractive to the youthful reader of either sex by interspersing sketches of the master-builders who, in successive ages, have reared the great temple of astronomy, composing, as they do, some of the most remarkable and interesting specimens of the human race. the work was addressed to a female friend (now no more), who was a distinguished ornament of her sex, and whose superior intellect and refined taste required that the work should be free from every thing superficial in matter or negligent in style; and it was deemed by the writer no ordinary privilege that, in the composition of the work, an image at once so exalted and so pure was continually present to his mental vision. yale college, _january_, . contents. preface, letter i. introductory observations, letter ii. doctrine of the sphere, letter iii. astronomical instruments.--telescope, letter iv. telescope continued, letter v. observatories, letter vi. time and the calendar, letter vii. figure of the earth, letter viii. diurnal revolution, letter ix. parallax and refraction, letter x. the sun, letter xi. annual revolution.--seasons, letter xii. laws of motion, letter xiii. terrestrial gravity, letter xiv. sir isaac newton.--universal gravitation.--figure of the earth's orbit.--precession of the equinoxes, letter xv. the moon, letter xvi. the moon.--phases.--harvest moon.--librations, letter xvii. moon's orbit.--her irregularities, letter xviii. eclipses, letter xix. longitude.--tides, letter xx. planets.--mercury and venus, letter xxi. superior planets: mars, jupiter, saturn, and uranus, letter xxii. copernicus.--galileo, letter xxiii. saturn.--uranus.--asteroids, letter xxiv. the planetary motions.--kepler's laws.--kepler, letter xxv. comets, letter xxvi. comets, letter xxvii. meteoric showers, letter xxviii. fixed stars, letter xxix. fixed stars, letter xxx. system of the world, letter xxxi. natural theology, letter xxxii. recent discoveries, index, letters on astronomy. letter . introductory observations. "ye sacred muses, with whose beauty fired, my soul is ravished, and my brain inspired, whose priest i am, whose holy fillets wear; would you your poet's first petition hear, give me the ways of wandering stars to know, the depths of heaven above, and earth below; teach me the various labors of the moon, and whence proceed th' eclipses of the sun; why flowing tides prevail upon the main, and in what dark recess they shrink again; what shakes the solid earth, what cause delays the summer nights, and shortens winter days." _dryden's virgil_ to mrs. c---- m----. dear madam,--in the conversation we recently held on the study of astronomy, you expressed a strong desire to become better acquainted with this noble science, but said you had always been repelled by the air of severity which it exhibits, arrayed as it is in so many technical terms, and such abstruse mathematical processes: or, if you had taken up some smaller treatise, with the hope of avoiding these perplexities, you had always found it so meager and superficial, as to afford you very little satisfaction. you asked, if a work might not be prepared, which would convey to the general reader some clear and adequate knowledge of the great discoveries in astronomy, and yet require for its perusal no greater preparation, than may be presumed of every well-educated english scholar of either sex. you were pleased to add the request, that i would write such a work,--a work which should combine, with a luminous exposition of the leading truths of the science, some account of the interesting historical facts with which it is said the records of astronomical discovery abound. having, moreover, heard much of the grand discoveries which, within the last fifty years, have been made among the _fixed stars_, you expressed a strong desire to learn more respecting these sublime researches. finally, you desired to see the argument for the existence and natural attributes of the deity, as furnished by astronomy, more fully and clearly exhibited, than is done in any work which you have hitherto perused. in the preparation of the proposed treatise, you urged me to supply, either in the text or in notes, every _elementary principle_ which would be essential to a perfect understanding of the work; for although, while at school, you had paid some attention to geometry and natural philosophy, yet so much time had since elapsed, that your memory required to be refreshed on the most simple principles of these elementary studies, and you preferred that i should consider you as altogether unacquainted with them. although, to satisfy a mind, so cultivated and inquisitive as yours, may require a greater variety of powers and attainments than i possess, yet, as you were pleased to urge me to the trial, i have resolved to make the attempt, and will see how far i may be able to lead you into the interior of this beautiful temple, without obliging you to force your way through the "jargon of the schools." astronomy, however, is a very difficult or a comparatively easy study, according to the view we take of it. the investigation of the great laws which govern the motions of the heavenly bodies has commanded the highest efforts of the human mind; but profound truths, which it required the mightiest efforts of the intellect to disclose, are often, when once discovered, simple in their complexion, and may be expressed in very simple terms. thus, the creation of that element, on whose mysterious agency depend all the forms of beauty and loveliness, is enunciated in these few monosyllables, "and god said, let there be light, and there was light;" and the doctrine of universal gravitation, which is the key that unlocks the mysteries of the universe, is simply this,--that every portion of matter in the universe tends towards every other. the three great laws of motion, also, are, when stated, so plain, that they seem hardly to assert any thing but what we knew before. that all bodies, if at rest, will continue so, as is declared by the first law of motion, until some force moves them; or, if in motion, will continue so, until some force stops them, appears so much a matter of course, that we can at first hardly see any good reason why it should be dignified with the title of the first great law of motion; and yet it contains a truth which it required profound sagacity to discover and expound. it is, therefore, a pleasing consideration to those who have not either the leisure of the ability to follow the astronomer through the intricate and laborious processes, which conducted him to his great discoveries, that they may fully avail themselves of the _results_ of this vast toil, and easily understand truths which it required ages of the severest labor to unfold. the descriptive parts of astronomy, or what may be called the natural history of the heavens, is still more easily understood than the laws of the celestial motions. the revelations of the telescope, and the wonders it has disclosed in the sun, in the moon, in the planets, and especially in the fixed stars, are facts not difficult to be understood, although they may affect the mind with astonishment. the great practical purpose of astronomy to the world is, enabling us safely to navigate the ocean. there are indeed many other benefits which it confers on man; but this is the most important. if, however, you ask, what advantages the study of astronomy promises, as a branch of education, i answer, that few subjects promise to the mind so much profit and entertainment. it is agreed by writers on the human mind, that the intellectual powers are enlarged and strengthened by the habitual contemplation of great objects, while they are contracted and weakened by being constantly employed upon little or trifling subjects. the former elevate, the latter depress, the mind, to their own level. now, every thing in astronomy is great. the magnitudes, distances, and motions, of the heavenly bodies; the amplitude of the firmament itself; and the magnificence of the orbs with which it is lighted, supply exhaustless materials for contemplation, and stimulate the mind to its noblest efforts. the emotion felt by the astronomer is not that sudden excitement or ecstasy, which wears out life, but it is a continued glow of exalted feeling, which gives the sensation of breathing in a purer atmosphere than others enjoy. we should at first imagine, that a study which calls upon its votaries for the severest efforts of the human intellect, which demands the undivided toil of years, and which robs the night of its accustomed hours of repose, would abridge the period of life; but it is a singular fact, that distinguished astronomers, as a class, have been remarkable for longevity. i know not how to account for this fact, unless we suppose that the study of astronomy itself has something inherent in it, which sustains its votaries by a peculiar aliment. it is the privilege of the student of this department of nature, that his cabinet is already collected, and is ever before him; and he is exempted from the toil of collecting his materials of study and illustration, by traversing land and sea, or by penetrating into the depths of the earth. nor are they in their nature frail and perishable. no sooner is the veil of clouds removed, that occasionally conceals the firmament by night, than his specimens are displayed to view, bright and changeless. the renewed pleasure which he feels, at every new survey of the constellations, grows into an affection for objects which have so often ministered to his happiness. his imagination aids him in giving them a personification, like that which the ancients gave to the constellations; (as is evident from the names which they have transmitted to us;) and he walks abroad, beneath the evening canopy, with the conscious satisfaction and delight of being in the presence of old friends. this emotion becomes stronger when he wanders far from home. other objects of his attachment desert him; the face of society changes; the earth presents new features; but the same sun illumines the day, the same moon adorns the night, and the same bright stars still attend him. when, moreover, the student of the heavens can command the aid of telescopes, of higher and higher powers, new acquaintances are made every evening. the sight of each new member of the starry train, that the telescope successively reveals to him, inspires a peculiar emotion of pleasure; and he at length finds himself, whenever he sweeps his telescope over the firmament, greeted by smiles, unperceived and unknown to his fellow-mortals. the same personification is given to these objects as to the constellations, and he seems to himself, at times, when he has penetrated into the remotest depths of ether, to enjoy the high prerogative of holding converse with the celestials. it is no small encouragement, to one who wishes to acquire a knowledge of the heavens, that the subject is embarrassed with far less that is technical than most other branches of natural history. having first learned a few definitions, and the principal circles into which, for convenience, the sphere is divided, and receiving the great laws of astronomy on the authority of the eminent persons who have investigated them, you will find few hard terms, or technical distinctions, to repel or perplex you; and you will, i hope, find that nothing but an intelligent mind and fixed attention are requisite for perusing the letters which i propose to address to you. i shall indeed be greatly disappointed, if the perusal does not inspire you with some portion of that pleasure, which i have described as enjoyed by the astronomer himself. the dignity of the study of the heavenly bodies, and its suitableness to the most refined and cultivated mind, has been recognised in all ages. virgil celebrates it in the beautiful strains with which i have headed this letter, and similar sentiments have ever been cherished by the greatest minds. as, in the course of these letters, i propose to trace an outline of the history of astronomy, from the earliest ages to the present time, you may think this the most suitable place for introducing it; but the successive discoveries in the science cannot be fully understood and appreciated, until after an acquaintance has been formed with the science itself. we must therefore reserve the details of this subject for a future opportunity; but it may be stated, here, that astronomy was cultivated the earliest of all the sciences; that great attention was paid to it by several very ancient nations, as the egyptians and chaldeans, and the people of india and china, before it took its rise in greece. more than six hundred years before the christian era, however, it began to be studied in this latter country. thales and pythagoras were particularly distinguished for their devotion to this science; and the celebrated school of alexandria, in egypt, which took its rise about three hundred years before the christian era, and flourished for several hundred years, numbered among its disciples a succession of eminent astronomers, among whom were hipparchus, eratosthenes, and ptolemy. the last of these composed a great work on astronomy, called the 'almagest,' in which is transmitted to us an account of all that was known of the science by the alexandrian school. the 'almagest' was the principal text-book in astronomy, for many centuries afterwards, and comparatively few improvements were made until the age of copernicus. copernicus was born at thorn, in prussia, in . previous to his time, the doctrine was held, that the earth is at rest in the centre of the universe, and that the sun, moon, and stars, revolve about it, every day, from east to west; in short, that the _apparent_ motions of the heavenly bodies are the same with their _real_ motions. but copernicus expounded what is now known to be the true theory of the celestial motions, in which the sun is placed in the centre of the solar system, and the earth and all the planets are made to revolve around him, from west to east, while the apparent diurnal motion of the heavenly bodies, from east to west, is explained by the revolution of the earth on its axis, in the same time, from west to east; a motion of which we are unconscious, and which we erroneously ascribe to external objects, as we imagine the shore is receding from us, when we are unconscious of the motion of the ship that carries us from it. although many of the appearances, presented by the motions of the heavenly bodies, may be explained on the former erroneous hypothesis, yet, like other hypotheses founded in error, it was continually leading its votaries into difficulties, and blinding their minds to the perception of truth. they had advanced nearly as far as it was practicable to go in the wrong road; and the great and sublime discoveries of modern times are owing, in no small degree, to the fact, that, since the days of copernicus, astronomers have been pursuing the plain and simple path of truth, instead of threading their way through the mazes of error. near the close of the sixteenth century, tycho brahe, a native of sweden, but a resident of denmark, carried astronomical observations (which constitute the basis of all that is valuable in astronomy) to a far greater degree of perfection than had ever been done before. kepler, a native of germany, one of the greatest geniuses the world has ever seen, was contemporary with tycho brahe, and was associated with him in a part of his labors. galileo, an italian astronomer of great eminence, flourished only a little later than tycho brahe. he invented the telescope, and, both by his discoveries and reasonings, contributed greatly to establish the true system of the world. soon after the commencement of the seventeenth century, ( ,) lord bacon, a celebrated english philosopher, pointed out the true method of conducting all inquiries into the phenomena of nature, and introduced the _inductive method of philosophizing_. according to the inductive method, we are to begin our inquiries into the causes of any events by first examining and classifying all the _facts_ that relate to it, and, from the comparison of these, to deduce our conclusions. but the greatest single discovery, that has ever been made in astronomy, was the law of universal gravitation, a discovery made by sir isaac newton, in the latter part of the seventeenth century. the discovery of this law made us acquainted with the hidden forces that move the great machinery of the universe. it furnished the key which unlocks the inner temple of nature; and from this time we may regard astronomy as fixed on a sure and immovable basis. i shall hereafter endeavor to explain to you the leading principles of universal gravitation, when we come to the proper place for inquiring into the causes of the celestial motions, as exemplified in the motion of the earth around the sun. letter ii. doctrine of the sphere. "all are but parts of one stupendous whole, whose body nature is, and god the soul."--_pope._ let us now consider what astronomy is, and into what great divisions it is distributed; and then we will take a cursory view of the doctrine of the sphere. this subject will probably be less interesting to you than many that are to follow; but still, permit me to urge upon you the necessity of studying it with attention, and reflecting upon each definition, until you fully understand it; for, unless you fully and clearly comprehend the circles of the sphere, and the use that is made of them in astronomy, a mist will hang over every subsequent portion of the science. i beg you, therefore, to pause upon every paragraph of this letter; and if there is any point in the whole which you cannot clearly understand, i would advise you to mark it, and to recur to it repeatedly; and, if you finally cannot obtain a clear idea of it yourself, i would recommend to you to apply for aid to some of your friends, who may be able to assist you. _astronomy is that science which treats of the heavenly bodies._ more particularly, its object is to teach what is known respecting the sun, moon, planets, comets, and fixed stars; and also to explain the methods by which this knowledge is acquired. astronomy is sometimes divided into descriptive, physical, and practical. descriptive astronomy respects _facts_; physical astronomy, _causes_; practical astronomy, the _means of investigating the facts_, whether by instruments or by calculation. it is the province of descriptive astronomy to observe, classify, and record, all the phenomena of the heavenly bodies, whether pertaining to those bodies individually, or resulting from their motions and mutual relations. it is the part of physical astronomy to explain the causes of these phenomena, by investigating the general laws on which they depend; especially, by tracing out all the consequences of the law of universal gravitation. practical astronomy lends its aid to both the other departments. the definitions of the different lines, points, and circles, which are used in astronomy, and the propositions founded upon them, compose the _doctrine of the sphere_. before these definitions are given, i must recall to your recollection a few particulars respecting the method of measuring angles. (see fig. , page .) a line drawn from the centre to the circumference of a circle is called a _radius_, as c d, c b, or c k. any part of the circumference of a circle is called an _arc_, as a b, or b d. an angle is measured by an arc included between two radii. thus, in fig. , the angle contained between the two radii, c a and c b, that is, the angle a c b, is measured by the arc a b. every circle, it will be recollected, is divided into three hundred and sixty equal parts, called degrees; and any arc, as a b, contains a certain number of degrees, according to its length. thus, if the arc a b contains forty degrees, then the opposite angle a c b is said to be an angle of forty degrees, and to be measured by a b. but this arc is the same part of the smaller circle that e f is of the greater. the arc a b, therefore, contains the same number of degrees as the arc e f, and either may be taken as the measure of the angle a c b. as the whole circle contains three hundred and sixty degrees, it is evident, that the quarter of a circle, or _quadrant_, contains ninety degrees, and that the semicircle a b d g contains one hundred and eighty degrees. [illustration fig. .] the _complement_ of an arc, or angle, is what it wants of ninety degrees. thus, since a d is an arc of ninety degrees, b d is the complement of a b, and a b is the complement of b d. if a b denotes a certain number of degrees of latitude, b d will be the complement of the latitude, or the colatitude, as it is commonly written. the _supplement_ of an arc, or angle, is what it wants of one hundred and eighty degrees. thus, b a is the supplement of g d b, and g d b is the supplement of b a. if b a were twenty degrees of longitude, g d b, its supplement, would be one hundred and sixty degrees. an angle is said to be _subtended_ by the side which is opposite to it. thus, in the triangle a c k, the angle at c is subtended by the side a k, the angle at a by c k, and the angle at k by c a. in like manner, a side is said to be subtended by an angle, as a k by the angle at c. let us now proceed with the doctrine of the sphere. a section of a sphere, by a plane cutting it in any manner, is a circle. _great circles_ are those which pass through the centre of the sphere, and divide it into two equal hemispheres. _small circles_ are such as do not pass through the centre, but divide the sphere into two unequal parts. the _axis_ of a circle is a straight line passing through its centre at right angles to its plane. the _pole_ of a great circle is the point on the sphere where its axis cuts through the sphere. every great circle has two poles, each of which is every where ninety degrees from the great circle. all great circles of the sphere cut each other in two points diametrically opposite, and consequently their points of section are one hundred and eighty degrees apart. a great circle, which passes through the pole of another great circle, cuts the latter at right angles. the great circle which passes through the pole of another great circle, and is at right angles to it, is called a _secondary_ to that circle. the angle made by two great circles on the surface of the sphere is measured by an arc of another great circle, of which the angular point is the pole, being the arc of that great circle intercepted between those two circles. in order to fix the position of any place, either on the surface of the earth or in the heavens, both the earth and the heavens are conceived to be divided into separate portions, by circles, which are imagined to cut through them, in various ways. the earth thus intersected is called the _terrestrial_, and the heavens the _celestial_, sphere. we must bear in mind, that these circles have no existence in nature, but are mere landmarks, artificially contrived for convenience of reference. on account of the immense distances of the heavenly bodies, they appear to us, wherever we are placed, to be fixed in the same concave surface, or celestial vault. the great circles of the globe, extended every way to meet the concave sphere of the heavens, become circles of the celestial sphere. the _horizon_ is the great circle which divides the earth into upper and lower hemispheres, and separates the visible heavens from the invisible. this is the _rational_ horizon. the _sensible_ horizon is a circle touching the earth at the place of the spectator, and is bounded by the line in which the earth and skies seem to meet. the sensible horizon is parallel to the rational, but is distant from it by the semidiameter of the earth, or nearly four thousand miles. still, so vast is the distance of the starry sphere, that both these planes appear to cut the sphere in the same line; so that we see the same hemisphere of stars that we should see, if the upper half of the earth were removed, and we stood on the rational horizon. the poles of the horizon are the zenith and nadir. the _zenith_ is the point directly over our heads; and the _nadir_, that directly under our feet. the plumb-line (such as is formed by suspending a bullet by a string) is in the axis of the horizon, and consequently directed towards its poles. every place on the surface of the earth has its own horizon; and the traveller has a new horizon at every step, always extending ninety degrees from him, in all directions. _vertical circles_ are those which pass through the poles of the horizon, (the zenith and nadir,) perpendicular to it. the _meridian_ is that vertical circle which passes through the north and south points. the _prime vertical_ is that vertical circle which passes through the east and west points. the _altitude_ of a body is its elevation above the horizon, measured on a vertical circle. the _azimuth_ of a body is its distance, measured on the horizon, from the meridian to a vertical circle passing through that body. the _amplitude_ of a body is its distance, on the horizon, from the prime vertical to a vertical circle passing through the body. azimuth is reckoned ninety degrees from either the north or south point; and amplitude ninety degrees from either the east or west point. azimuth and amplitude are mutually complements of each other, for one makes up what the other wants of ninety degrees. when a point is _on_ the horizon, it is only necessary to count the number of degrees of the horizon between that point and the meridian, in order to find its azimuth; but if the point is _above_ the horizon, then its azimuth is estimated by passing a vertical circle through it, and reckoning the azimuth from the point where this circle cuts the horizon. the _zenith distance_ of a body is measured on a vertical circle passing through that body. it is the complement of the altitude. the _axis of the earth_ is the diameter on which the earth is conceived to turn in its diurnal revolution. the same line, continued until it meets the starry concave, constitutes the _axis of the celestial sphere_. the _poles of the earth_ are the extremities of the earth's axis: the _poles of the heavens_, the extremities of the celestial axis. the _equator_ is a great circle cutting the axis of the earth at right angles. hence, the axis of the earth is the axis of the equator, and its poles are the poles of the equator. the intersection of the plane of the equator with the surface of the earth constitutes the _terrestrial_, and its intersection with the concave sphere of the heavens, the _celestial_, equator. the latter, by way of distinction, is sometimes denominated the _equinoctial_. the secondaries to the equator,--that is, the great circles passing through the poles of the equator,--are called _meridians_, because that secondary which passes through the zenith of any place is the meridian of that place, and is at right angles both to the equator and the horizon, passing, as it does, through the poles of both. these secondaries are also called _hour circles_ because the arcs of the equator intercepted between them are used as measures of time. the _latitude_ of a place on the earth is its distance from the equator north or south. the _polar distance_, or angular distance from the nearest pole, is the complement of the latitude. the _longitude_ of a place is its distance from some standard meridian, either east or west, measured on the equator. the meridian, usually taken as the standard, is that of the observatory of greenwich, in london. if a place is directly _on_ the equator, we have only to inquire, how many degrees of the equator there are between that place and the point where the meridian of greenwich cuts the equator. if the place is north or south of the equator, then its longitude is the arc of the equator intercepted between the meridian which passes through the place and the meridian of greenwich. the _ecliptic_ is a great circle, in which the earth performs its annual revolutions around the sun. it passes through the centre of the earth and the centre of the sun. it is found, by observation, that the earth does not lie with its axis at right angles to the plane of the ecliptic, so as to make the equator coincide with it, but that it is turned about twenty-three and a half degrees out of a perpendicular direction, making an angle with the plane itself of sixty-six and a half degrees. the equator, therefore, must be turned the same distance out of a coincidence with the ecliptic, the two circles making an angle with each other of twenty-three and a half degrees. it is particularly important that we should form correct ideas of the ecliptic, and of its relations to the equator, since to these two circles a great number of astronomical measurements and phenomena are referred. the _equinoctial points_, or _equinoxes_, are the intersections of the ecliptic and equator. the time when the sun crosses the equator, in going northward, is called the _vernal_, and in returning southward, the _autumnal_, equinox. the vernal equinox occurs about the twenty-first of march, and the autumnal, about the twenty-second of september. the _solstitial points_ are the two points of the ecliptic most distant from the equator. the times when the sun comes to them are called _solstices_. the summer solstice occurs about the twenty-second of june, and the winter solstice about the twenty-second of december. the ecliptic is divided into twelve equal parts, of thirty degrees each, called _signs_, which, beginning at the vernal equinox, succeed each other, in the following order: . aries, [zodiac: aries] . taurus, [zodiac: taurus] . gemini, [zodiac: gemini] . cancer, [zodiac: cancer] . leo, [zodiac: leo] . virgo, [zodiac: virgo] . libra, [zodiac: libra] . scorpio, [zodiac: scorpio] . sagittarius, [zodiac: sagittarius] . capricornus, [zodiac: capricornus] . aquarius, [zodiac: aquarius] . pisces. [zodiac: pisces] the mode of reckoning on the ecliptic is by signs, degrees, minutes, and seconds. the sign is denoted either by its name or its number. thus, one hundred degrees may be expressed either as the tenth degree of cancer, or as s °. it will be found an advantage to repeat the signs in their proper order, until they are well fixed in the memory, and to be able to recognise each sign by its appropriate character. of the various meridians, two are distinguished by the name of _colures_. the _equinoctial colure_ is the meridian which passes through the equinoctial points. from this meridian, right ascension and celestial longitude are reckoned, as longitude on the earth is reckoned from the meridian of greenwich. the _solstitial colure_ is the meridian which passes through the solstitial points. the position of a celestial body is referred to the equator by its right ascension and declination. _right ascension_ is the angular distance from the vernal equinox measured on the equator. if a star is situated _on_ the equator, then its right ascension is the number of degrees of the equator between the star and the vernal equinox. but if the star is north or south of the equator, then its right ascension is the number of degrees of the equator, intercepted between the vernal equinox and that secondary to the equator which passes through the star. _declination_ is the distance of a body from the equator measured on a secondary to the latter. therefore, right ascension and declination correspond to terrestrial longitude and latitude,--right ascension being reckoned from the equinoctial colure, in the same manner as longitude is reckoned from the meridian of greenwich. on the other hand, celestial longitude and latitude are referred, not to the equator, but to the ecliptic. _celestial longitude_ is the distance of a body from the vernal equinox measured on the ecliptic. _celestial latitude_ is the distance from the ecliptic measured on a secondary to the latter. or, more briefly, longitude is distance _on_ the ecliptic: latitude, distance _from_ the ecliptic. the _north polar distance_ of a star is the complement of its declination. _parallels of latitude_ are small circles parallel to the equator. they constantly diminish in size, as we go from the equator to the pole. the _tropics_ are the parallels of latitude which pass through the solstices. the northern tropic is called the tropic of cancer; the southern, the tropic of capricorn. the _polar circles_ are the parallels of latitude that pass through the poles of the ecliptic, at the distance of twenty-three and a half degrees from the poles of the earth. the _elevation of the pole_ of the heavens above the horizon of any place is always equal to the latitude of the place. thus, in forty degrees of north latitude we see the north star forty degrees above the northern horizon; whereas, if we should travel southward, its elevation would grow less and less, until we reached the equator, where it would appear _in_ the horizon. or, if we should travel northwards, the north star would rise continually higher and higher, until, if we could reach the pole of the earth, that star would appear directly over head. the _elevation of the equator_ above the horizon of any place is equal to the complement of the latitude. thus, at the latitude of forty degrees north, the equator is elevated fifty degrees above the southern horizon. the earth is divided into five zones. that portion of the earth which lies between the tropics is called the _torrid zone_; that between the tropics and the polar circles, the _temperate zones_; and that between the polar circles and the poles, the _frigid zones_. the _zodiac_ is the part of the celestial sphere which lies about eight degrees on each side of the ecliptic. this portion of the heavens is thus marked off by itself, because all the planets move within it. after endeavoring to form, from the definitions, as clear an idea as we can of the various circles of the sphere, we may next resort to an artificial globe, and see how they are severally represented there. i do not advise to _begin_ learning the definitions from the globe; the mind is more improved, and a power of conceiving clearly how things are in nature is more effectually acquired, by referring every thing, at first, to the grand sphere of nature itself, and afterwards resorting to artificial representations to aid our conceptions. we can get but a very imperfect idea of a man from a profile cut in paper, unless we know the original. if we are acquainted with the individual, the profile will assist us to recall his appearance more distinctly than we can do without it. in like manner, orreries, globes, and other artificial aids, will be found very useful, in assisting us to form distinct conceptions of the relations existing between the different circles of the sphere, and of the arrangements of the heavenly bodies; but, unless we have already acquired some correct ideas of these things, by contemplating them as they are in nature, artificial globes, and especially orreries, will be apt to mislead us. i trust you will be able to obtain the use of a globe,[ ] to aid you in the study of the foregoing definitions, or doctrine of the sphere; but if not, i would recommend the following easy device. to represent the earth, select a large _apple_, (a melon, when in season, will be found still better.) the eye and the stem of the apple will indicate the position of the two poles of the earth. applying the thumb and finger of the left hand to the poles, and holding the apple so that the poles may be in a north and south line, turn this globe from west to east, and its motion will correspond to the diurnal movement of the earth. pass a wire or a knitting needle through the poles, and it will represent the _axis_ of the sphere. a circle cut around the apple, half way between the poles, will be the _equator_; and several other circles cut between the equator and the poles, parallel to the equator, will represent _parallels of latitude_; of which, two, drawn twenty-three and a half degrees from the equator, will be the _tropics_, and two others, at the same distance from the poles, will be the _polar circles_. a great circle cut through the poles, in a north and south direction, will form the _meridian_, and several other great circles drawn through the poles, and of course perpendicularly to the equator, will be secondaries to the equator, constituting meridians, or _hour circles_. a great circle cut through the centre of the earth, from one tropic to the other, would represent the _plane_ of the ecliptic; and consequently a line cut round the apple where such a section meets the surface, will be the terrestrial _ecliptic_. the points where this circle meets the tropics indicate the position of the _solstices_; and its intersection with the equator, that of the _equinoctial points_. the _horizon_ is best represented by a circular piece of pasteboard, cut so as to fit closely to the apple, being movable upon it. when this horizon is passed through the poles, it becomes the horizon of the equator; when it is so placed as to coincide with the earth's equator, it becomes the horizon of the poles; and in every other situation it represents the horizon of a place on the globe ninety degrees every way from it. suppose we are in latitude forty degrees; then let us place our movable paper parallel to our own horizon, and elevate the pole forty degrees above it, as near as we can judge by the eye. if we cut a circle around the apple, passing through its highest part, and through the east and west points, it will represent the _prime vertical_. simple as the foregoing device is, if you will take the trouble to construct one for yourself, it will lead you to more correct views of the doctrine of the sphere, than you would be apt to obtain from the most expensive artificial globes, although there are many other useful purposes which such globes serve, for which the apple would be inadequate. when you have thus made a sphere for yourself, or, with an artificial globe before you, if you have access to one, proceed to point out on it the various arcs of azimuth and altitude, right ascension and declination, terrestrial and celestial latitude and longitude,--these last being referred to the equator on the earth, and to the ecliptic in the heavens. when the circles of the sphere are well learned, we may advantageously employ projections of them in various illustrations. by the _projection of the sphere_ is meant a representation of all its parts on a plane. the plane itself is called the plane of projection. let us take any circular ring, as a wire bent into a circle, and hold it in different positions before the eye. if we hold it parallel to the face, with the whole breadth opposite to the eye, we see it as an entire circle. if we turn it a little sideways, it appears oval, or as an ellipse; and, as we continue to turn it more and more round, the ellipse grows narrower and narrower, until, when the edge is presented to the eye, we see nothing but a line. now imagine the ring to be near a perpendicular wall, and the eye to be removed at such a distance from it, as not to distinguish any interval between the ring and the wall; then the several figures under which the ring is seen will appear to be inscribed on the wall, and we shall see the ring as a circle, when perpendicular to a straight line joining the centre of the ring and the eye, or as an ellipse, when oblique to this line, or as a straight line, when its edge is towards us. [illustration: fig. .] it is in this manner that the circles of the sphere are projected, as represented in the following diagram, fig. . here, various circles are represented as projected on the meridian, which is supposed to be situated directly before the eye, at some distance from it. the horizon h o, being perpendicular to the meridian, is seen edgewise, and consequently is projected into a straight line. the same is the case with the prime vertical z n, with the equator e q, and the several small circles parallel to the equator, which represent the two tropics and the two polar circles. in fact, all circles whatsoever, which are perpendicular to the plane of projection, will be represented by straight lines. but every circle which is perpendicular to the horizon, except the prime vertical, being seen obliquely, as z m n, will be projected into an ellipse, one half only of which is seen,--the other half being on the other side of the plane of projection. in the same manner, p r p, an hour circle, is represented by an ellipse on the plane of projection. footnote: [ ] a small pair of globes, that will answer every purpose required by the readers of these letters, may be had of the publishers of this work, at a price not exceeding ten dollars; or half that sum for a celestial globe, which will serve alone for studying astronomy. letter iii. astronomical instruments.----telescope. "here truths sublime, and sacred science charm, creative arts new faculties supply, mechanic powers give more than giant's arm, and piercing optics more than eagle's eye; eyes that explore creation's wondrous laws, and teach us to adore the great designing cause."--_beattie_. if, as i trust, you have gained a clear and familiar knowledge of the circles and divisions of the sphere, and of the mode of estimating the position of a heavenly body by its azimuth and altitude, or by its right ascension and declination, or by its longitude and latitude, you will now enter with advantage upon an account of those _instruments_, by means of which our knowledge of astronomy has been greatly promoted and perfected. the most ancient astronomers employed no instruments of observation, but acquired their knowledge of the heavenly bodies by long-continued and most attentive inspection with the naked eye. instruments for measuring angles were first used in the alexandrian school, about three hundred years before the christian era. wherever we are situated on the earth, we appear to be in the centre of a vast sphere, on the concave surface of which all celestial objects are inscribed. if we take any two points on the surface of the sphere, as two stars, for example, and imagine straight lines to be drawn to them from the eye, the angle included between these lines will be measured by the arc of the sky contained between the two points. thus, if d b h, fig. , page , represents the concave surface of the sphere, a, b, two points on it, as two stars, and c a, c b, straight lines drawn from the spectator to those points, then the angular distance between them is measured by the arc a b, or the angle a c b. but this angle may be measured on a much smaller circle, having the same centre, as g f k, since the arc e f will have the same number of degrees as the arc a b. the simplest mode of taking an angle between two stars is by means of an arm opening at a joint like the blade of a penknife, the end of the arm moving like c e upon the graduated circle k f g. in fact, an instrument constructed on this principle, resembling a carpenter's rule with a folding joint, with a semicircle attached, constituted the first rude apparatus for measuring the angular distance between two points on the celestial sphere. thus the sun's elevation above the horizon might be ascertained, by placing one arm of the rule on a level with the horizon, and bringing the edge of the other into a line with the sun's centre. [illustration fig. .] the common surveyor's compass affords a simple example of angular measurement. here, the needle lies in a north and south line, while the circular rim of the compass, when the instrument is level, corresponds to the horizon. hence the compass shows the azimuth of an object, or how many degrees it lies east or west of the meridian. it is obvious, that the larger the graduated circle is, the more minutely its limb may be divided. if the circle is one foot in diameter, each degree will occupy one tenth of an inch. if the circle is twenty feet in diameter, a degree will occupy the space of two inches, and could be easily divided into minutes, since each minute would cover a space one thirtieth of an inch. refined astronomical circles are now divided with very great skill and accuracy, the spaces between the divisions being, when read off, magnified by a microscope; but in former times, astronomers had no mode of measuring small angles but by employing very large circles. but the telescope and microscope enable us at present to measure celestial arcs much more accurately than was done by the older astronomers. in the best instruments, the measurements extend to a single second of space, or one thirty-six hundredth part of a degree,--a space, on a circle twelve feet in diameter, no greater than one fifty-seven hundredth part of an inch. to divide, or _graduate_, astronomical instruments, to such a degree of nicety, requires the highest efforts of mechanical skill. indeed, the whole art of instrument-making is regarded as the most difficult and refined of all the mechanical arts; and a few eminent artists, who have produced instruments of peculiar power and accuracy, take rank with astronomers of the highest celebrity. i will endeavor to make you acquainted with several of the principal instruments employed in astronomical observations, but especially with the telescope, which is the most important and interesting of them all. i think i shall consult your wishes, as well as your improvement, by giving you a clear insight into the principles of this prince of instruments, and by reciting a few particulars, at least, respecting its invention and subsequent history. the _telescope_, as its name implies, is an instrument employed for viewing distant objects.[ ] it aids the eye in two ways; first, by enlarging the visual angle under which objects are seen, and, secondly, by collecting and conveying to the eye a much larger amount of the light that emanates from the object, than would enter the naked pupil. a complete knowledge of the telescope cannot be acquired, without an acquaintance with the science of optics; but one unacquainted with that science may obtain some idea of the leading principles of this noble instrument. its main principle is as follows: _by means of the telescope, we first form an image of a distant object,--as the moon, for example,--and then magnify that image by a microscope._ [illustration fig. .] let us first see how the image is formed. this may be done either by a convex lens, or by a concave mirror. a convex lens is a flat piece of glass, having its two faces convex, or spherical, as is seen in a common sun-glass, or a pair of spectacles. every one who has seen a sun-glass, knows, that, when held towards the sun, it collects the solar rays into a small bright circle in the focus. this is in fact a small _image_ of the sun. in the same manner, the image of any distant object, as a star, may be formed, as is represented in the following diagram. let a b c d, fig. , represent the tube of the telescope. at the front end, or at the end which is directed towards the object, (which we will suppose to be the moon,) is inserted a convex lens, l, which receives the rays of light from the moon, and collects them into the focus at _a_, forming an image of the moon. this image is viewed by a magnifier attached to the end b c. the lens, l, is called the _object-glass_, and the microscope in b c, the _eyeglass_. we apply a microscope to this image just as we would to any object; and, by greatly enlarging its dimensions, we may render its various parts far more distinct than they would otherwise be; while, at the same time, the lens collects and conveys to the eye a much greater quantity of light than would proceed directly from the body under examination. a very few rays of light only, from a distant object, as a star, can enter the eye directly; but a lens one foot in diameter will collect a beam of light of the same dimensions, and convey it to the eye. by these means, many obscure celestial objects become distinctly visible, which would otherwise be either too minute, or not sufficiently luminous, to be seen by us. but the image may also be formed by means of a _concave mirror_, which, as well as the concave lens, has the property of collecting the rays of light which proceed from any luminous body, and of forming an image of that body. the image formed by a concave mirror is magnified by a microscope, in the same manner as when formed by the concave lens. when the lens is used to form an image, the instrument is called a _refracting telescope_; when a concave mirror is used, it is called a _reflecting telescope_. the office of the object-glass is simply _to collect_ the light, and to form an _image_ of the object, but not to magnify it: the magnifying power is wholly in the eyeglass. hence the principle of the telescope is as follows: _by means of the object-glass_, (in the refracting telescope,) _or by the concave mirror_, (in the reflecting telescope,) _we form an image of the object_, _and magnify that image by a microscope_. the invention of this noble instrument is generally ascribed to the great philosopher of florence, galileo. he had heard that a spectacle maker of holland had accidentally hit upon a discovery, by which distant objects might be brought apparently nearer; and, without further information, he pursued the inquiry, in order to ascertain what forms and combinations of glasses would produce such a result. by a very philosophical process of reasoning, he was led to the discovery of that peculiar form of the telescope which bears his name. although the telescopes made by galileo were no larger than a common spyglass of the kind now used on board of ships, yet, as they gave new views of the heavenly bodies, revealing the mountains and valleys of the moon, the satellites of jupiter, and multitudes of stars which are invisible to the naked eye, it was regarded with infinite delight and astonishment. _reflecting_ telescopes were first constructed by sir isaac newton, although the use of a concave reflector, instead of an object-glass, to form the image, had been previously suggested by gregory, an eminent scotch astronomer. the first telescope made by newton was only six inches long. its reflector, too, was only a little more than an inch. notwithstanding its small dimensions, it performed so well, as to encourage further efforts; and this illustrious philosopher afterwards constructed much larger instruments, one of which, made with his own hands, was presented to the royal society of london, and is now carefully preserved in their library. newton was induced to undertake the construction of reflecting telescopes, from the belief that refracting telescopes were necessarily limited to a very small size, with only moderate illuminating powers, whereas the dimensions and powers of the former admitted of being indefinitely increased. considerable _magnifying_ powers might, indeed, be obtained from refractors, by making them very long; but the _brightness_ with which telescopic objects are seen, depends greatly on the dimensions of the beam of light which is collected by the object-glass, or by the mirror, and conveyed to the eye; and therefore, small object-glasses cannot have a very high illuminating power. now, the experiments of newton on colors led him to believe, that it would be impossible to employ large lenses in the construction of telescopes, since such glasses would give to the images, they formed, the colors of the rainbow. but later opticians have found means of correcting these imperfections, so that we are now able to use object-glasses a foot or more in diameter, which give very clear and bright images. such instruments are called _achromatic_ telescopes,--a name implying the absence of prismatic or rainbow colors in the image. it is, however, far more difficult to construct large achromatic than large reflecting telescopes. very large pieces of glass can seldom be found, that are sufficiently pure for the purpose; since every inequality in the glass, such as waves, tears, threads, and the like, spoils it for optical purposes, as it distorts the light, and produces nothing but confused images. the achromatic telescope (that is, the refracting telescope, having such an object-glass as to give a colorless image) was invented by dollond, a distinguished english artist, about the year . he had in his possession a quantity of glass of a remarkably fine quality, which enabled him to carry his invention at once to a high degree of perfection. it has ever since been, with the manufacturers of telescopes, a matter of the greatest difficulty to find pieces of glass, of a suitable quality for object-glasses, more than two or three inches in diameter. hence, large achromatic telescopes are very expensive, being valued in proportion to the _cubes_ of their diameters; that is, if a telescope whose aperture (as the breadth of the object-glass is technically called) is two inches, cost one hundred dollars, one whose aperture is eight inches would cost six thousand four hundred dollars. since it is so much easier to make large reflecting than large refracting telescopes, you may ask, why the latter are ever attempted, and why reflectors are not exclusively employed? i answer, that the achromatic telescope, when large and well constructed, is a more perfect and more durable instrument than the reflecting telescope. much more of the light that falls on the mirror is absorbed than is lost in passing through the object-glass of a refractor; and hence the larger achromatic telescopes afford a stronger light than the reflecting, unless the latter are made of an enormous and unwieldy size. moreover, the mirror is very liable to tarnish, and will never retain its full lustre for many years together; and it is no easy matter to restore the lustre, when once impaired. in my next letter, i will give you an account of some of the most celebrated telescopes that have ever been constructed, and point out the method of using this excellent instrument, so as to obtain with it the finest views of the heavenly bodies. footnote: [ ] from two greek words, =têle=, (_tele_,) _far_, and =schopeô=, (_skopeo_,) _to see_. letter iv telescope continued. ----"the broad circumference hung on his shoulders like the moon, whose orb through _optic glass_ the tuscan artist views at evening, from the top of fesolé or in valdarno, to descry new lands, rivers or mountains, in her spotted globe."--_milton._ the two most celebrated telescopes, hitherto made, are herschel's _forty-feet reflector_, and the _great dorpat refractor_. herschel was a hanoverian by birth, but settled in england in the younger part of his life. as early as , he began to make telescopes for his own use; and, during his life, he made more than four hundred, of various sizes and powers. under the patronage of george the third, he completed, in , his great telescope, having a tube of iron, forty feet long, and a speculum, forty-nine and a half inches or more than four feet in diameter. let us endeavor to form a just conception of this gigantic instrument, which we can do only by dwelling on its dimensions, and comparing them with those of other objects with which we are familiar, as the length or height of a house, and the breadth of a hogshead or cistern, of known dimensions. the reflector alone weighed nearly a ton. so large and ponderous an instrument must require a vast deal of machinery to work it, and to keep it steady; and, accordingly, the framework surrounding it was formed of heavy timbers, and resembled the frame of a large building. when one of the largest of the fixed stars, as sirius, is entering the field of this telescope, its approach is announced by a bright dawn, like that which precedes the rising sun; and when the star itself enters the field, the light is insupportable to the naked eye. the planets are expanded into brilliant luminaries, like the moon; and innumerable multitudes of stars are scattered like glittering dust over the celestial vault. the great dorpat telescope is of more recent construction. it was made by fraunhofer, a german optician of the greatest eminence, at munich, in bavaria, and takes its name from its being attached to the observatory at dorpat, in russia. it is of much smaller dimensions than the great telescope of herschel. its object-glass is nine and a half inches in diameter, and its length, fourteen feet. although the price of this instrument was nearly five thousand dollars, yet it is said that this sum barely covered the actual expenses. it weighs five thousand pounds, and yet is turned with the finger. in facility of management, it has greatly the advantage of herschel's telescope. moreover, the sky of england is so much of the time unfavorable for astronomical observation, that _one hundred_ good hours (or those in which the higher powers can be used) are all that can be obtained in a whole year. on this account, and on account of the difficulty of shifting the position of the instrument, herschel estimated that it would take about six hundred years to obtain with it even a momentary glimpse of every part of the heavens. this remark shows that such great telescopes are unsuited to the common purposes of astronomical observation. indeed, most of herschel's discoveries were made with his small telescopes; and although, for certain rare purposes, powers were applied which magnified seven thousand times, yet, in most of his observations, powers magnifying only two or three hundred times were employed. the highest power of the dorpat telescope is only seven hundred, and yet the director of this instrument, professor struve, is of the opinion, that it is nearly or quite equal in quality, all things considered, to herschel's forty-feet reflector. it is not generally understood in what way greatness of size in a telescope increases its powers; and it conveys but an imperfect idea of the excellence of a telescope, to tell how much it magnifies. in the same instrument, an increase of magnifying power is always attended with a diminution of the light and of the field of view. hence, the lower powers generally afford the most agreeable views, because they give the clearest light, and take in the largest space. the several circumstances which influence the qualities of a telescope are, illuminating power, distinctness, field of view, and magnifying power. large mirrors and large object-glasses are superior to smaller ones, because they collect a larger beam of light, and transmit it to the eye. stars which are invisible to the naked eye are rendered visible by the telescope, because this instrument collects and conveys to the eye a large beam of the few rays which emanate from the stars; whereas a beam of these rays of only the diameter of the pupil of the eye, would afford too little light for distinct vision. in this particular, large telescopes have great advantages over small ones. the great mirror of herschel's forty-feet reflector collects and conveys to the eye a beam more than four feet in diameter. the dorpat telescope also transmits to the eye a beam nine and one half inches in diameter. this seems small, in comparison with the reflector; but much less of the light is lost on passing through the glass than is absorbed by the mirror, and the mirror is very liable to be clouded or tarnished; so that there is not so great a difference in the two instruments, in regard to illuminating power, as might be supposed from the difference of size. _distinctness of view_ is all-important to the performance of an instrument. the object may be sufficiently bright, yet, if the image is distorted, or ill-defined, the illumination is of little consequence. this property depends mainly on the skill with which all the imperfections of figure and color in the glass or mirror are corrected, and can exist in perfection only when the image is rendered completely achromatic, and when all the rays that proceed from each point in the object are collected into corresponding points of the image, unaccompanied by any other rays. distinctness is very much affected by the _steadiness_ of the instrument. every one knows how indistinct a page becomes, when a book is passed rapidly backwards and forwards before the eyes, and how difficult it is to read in a carriage in rapid motion on a rough road. _field of view_ is another important consideration. the finest instruments exhibit the moon, for example, not only bright and distinct, in all its parts, but they take in the whole disk at once; whereas, the inferior instruments, when the higher powers, especially, are applied, permit us to see only a small part of the moon at once. i hope, my friend, that, when you have perused these letters, or rather, while you are perusing them, you will have frequent opportunities of looking through a good telescope. i even anticipate that you will acquire such a taste for viewing the heavenly bodies with the aid of a good glass, that you will deem a telescope a most suitable appendage to your library, and as certainly not less an ornament to it than the more expensive statues with which some people of fortune adorn theirs. i will therefore, before concluding this letter, offer you a few _directions for using the telescope_. some states of weather, even when the sky is clear, are far more favorable for astronomical observation than others. after sudden changes of temperature in the atmosphere, the medium is usually very unsteady. if the sun shines out warm after a cloudy season, the ground first becomes heated, and the air that is nearest to it is expanded, and rises, while the colder air descends, and thus ascending and descending currents of air, mingling together, create a confused and wavy medium. the same cause operates when a current of hot air rises from a chimney; and hence the state of the atmosphere in cities and large towns is very unfavorable to the astronomer, on this account, as well as on account of the smoky condition in which it is usually found. after a long season of dry weather, also, the air becomes smoky, and unfit for observation. indeed, foggy, misty, or smoky, air is so prevalent in some countries, that only a very few times in the whole year can be found, which are entirely suited to observation, especially with the higher powers; for we must recollect, that these inequalities and imperfections are magnified by telescopes, as well as the objects themselves. thus, as i have already mentioned, not more than one hundred good hours in a year could be obtained for observation with herschel's great telescope. by _good_ hours, herschel means that the sky must be very clear, the moon absent, no twilight, no haziness, no violent wind, and no sudden change of temperature. as a general fact, the warmer climates enjoy a much finer sky for the astronomer than the colder, having many more clear evenings, a short twilight, and less change of temperature. the watery vapor of the atmosphere, also, is more perfectly dissolved in hot than in cold air, and the more water air contains, provided it is in a state of perfect solution, the clearer it is. a _certain preparation of the observer himself_ is also requisite for the nicest observations with the telescope. he must be free from all agitation, and the eye must not recently have been exposed to a strong light, which contracts the pupil of the eye. indeed, for delicate observations, the observer should remain for some time beforehand in a dark room, to let the pupil of the eye dilate. by this means, it will be enabled to admit a larger number of the rays of light. in ascending the stairs of an observatory, visitors frequently get out of breath, and having perhaps recently emerged from a strongly-lighted apartment, the eye is not in a favorable state for observation. under these disadvantages, they take a hasty look into the telescope, and it is no wonder that disappointment usually follows. want of steadiness is a great difficulty attending the use of the highest magnifiers; for the motions of the instrument are magnified as well as the object. hence, in the structure of observatories, the greatest pains is requisite, to avoid all tremor, and to give to the instruments all possible steadiness; and the same care is to be exercised by observers. in the more refined observations, only one or two persons ought to be near the instrument. in general, _low powers_ afford better views of the heavenly bodies than very high magnifiers. it may be thought absurd, to recommend the use of low powers, in respect to large instruments especially, since it is commonly supposed that the advantage of large instruments is, that they will bear high magnifying powers. but this is not their only, nor even their principal, advantage. a good light and large field are qualities, for most purposes, more important than great magnifying power; and it must be borne in mind, that, as we increase the magnifying power in a given instrument, we diminish both the illumination and the field of view. still, different objects require different magnifying powers; and a telescope is usually furnished with several varieties of powers, one of which is best fitted for viewing the moon, another for jupiter, and a still higher power for saturn. comets require only the lowest magnifiers; for here, our object is to command as much light, and as large a field, as possible, while it avails little to increase the dimensions of the object. on the other hand, for certain double stars, (stars which appear single to the naked eye, but double to the telescope,) we require very high magnifiers, in order to separate these minute objects so far from each other, that the interval can be distinctly seen. whenever we exhibit celestial objects to inexperienced observers, it is useful to precede the view with good _drawings_ of the objects, accompanied by an explanation of what each appearance, exhibited in the telescope, indicates. the novice is told, that mountains and valleys can be seen in the moon by the aid of the telescope; but, on looking, he sees a confused mass of light and shade, and nothing which looks to him like either mountains or valleys. had his attention been previously directed to a plain drawing of the moon, and each particular appearance interpreted to him, he would then have looked through the telescope with intelligence and satisfaction. letter v. observatories. "we, though from heaven remote, to heaven will move, with strength of mind, and tread the abyss above; and penetrate, with an interior light, those upper depths which nature hid from sight. pleased we will be, to walk along the sphere of shining stars, and travel with the year."--_ovid._ an observatory is a structure fitted up expressly for astronomical observations, and furnished with suitable instruments for that purpose. the two most celebrated observatories, hitherto built, are that of tycho brahe, and that of greenwich, near london. the observatory of tycho brahe, fig. , was constructed at the expense of the king of denmark, in a style of royal magnificence, and cost no less than two hundred thousand crowns. it was situated on the island of huenna, at the entrance of the baltic, and was called uraniburg, or the palace of the skies. before i give you an account of tycho's observatory, i will recite a few particulars respecting this great astronomer himself. tycho brahe was of swedish descent, and of noble family; but having received his education at the university of copenhagen, and spent a large part of his life in denmark, he is usually considered as a dane, and quoted as a danish astronomer. he was born in the year . when he was about fourteen years old, there happened a great eclipse of the sun, which awakened in him a high interest, especially when he saw how [illustration fig. .] accurately all the circumstances of it answered to the prediction with which he had been before made acquainted. he was immediately seized with an irresistible passion to acquire a knowledge of the science which could so successfully lift the veil of futurity. his friends had destined him for the profession of law, and, from the superior talents of which he gave early promise, and with the advantage of powerful family connexions, they had marked out for him a distinguished career in public life. they therefore endeavored to discourage him from pursuing a path which they deemed so much less glorious than that, and vainly sought, by various means, to extinguish the zeal for astronomy which was kindled in his youthful bosom. despising all the attractions of a court, he contracted an alliance with a peasant girl, and, in the peaceful retirement of domestic life, desired no happier lot than to peruse the grand volume which the nocturnal heavens displayed to his enthusiastic imagination. he soon established his fame as one of the greatest astronomers of the age, and monarchs did homage to his genius. the king of denmark became his munificent patron, and james the first, king of england, when he went to denmark to complete his marriage with a danish princess, passed eight days with tycho in his observatory, and, at his departure, addressed to the astronomer a latin ode, accompanied with a magnificent present. he gave him also his royal license to print his works in england, and added to it the following complimentary letter: "nor am i acquainted with these things on the relation of others, or from a mere perusal of your works, but i have seen them with my own eyes, and heard them with my own ears, in your residence at uraniburg, during the various learned and agreeable conversations which i there held with you, which even now affect my mind to such a degree, that it is difficult to decide, whether i recollect them with greater pleasure or admiration." admiring disciples also crowded to this sanctuary of the sciences, to acquire a knowledge of the heavens. the observatory consisted of a main building, which was square, each side being sixty feet, and of large wings in the form of round towers. the whole was executed in a style of great magnificence, and tycho, who was a nobleman by descent, gratified his taste for splendor and ornament, by giving to every part of the structure an air of the most finished elegance. nor were the instruments with which it was furnished less magnificent than the buildings. they were vastly larger than had before been employed in the survey of the heavens, and many of them were adorned with costly ornaments. the cut on page , fig. , represents one of tycho's large and splendid instruments, (an astronomical quadrant,) on one side of which was figured a representation of the astronomer and his assistants, in the midst of their instruments, and intently engaged in making and recording observations. it conveys to us a striking idea of the magnificence of his arrangements, and of the extent of his operations. here tycho sat in state, clad in the robes of nobility, and supported throughout his establishment the etiquette due to his rank. his observations were more numerous than all that had ever been made before, and they were carried to a degree of accuracy that is astonishing, when we consider that they were made without the use of the telescope, which was not yet invented. tycho carried on his observations at uraniburg for about twenty years, during which time he accumulated an immense store of accurate and valuable _facts_, which afforded the groundwork of the discovery of the great laws of the solar system established by kepler, of whom i shall tell you more hereafter. but the high marks of distinction which tycho enjoyed, not only from his own sovereign, but also from foreign potentates, provoked the envy of the courtiers of his royal patron. they did not indeed venture to make their attacks upon him while his generous patron was living; but the king was no sooner dead, and succeeded by a young monarch, who did not feel the same [illustration fig. .] interest in protecting and encouraging this great ornament of the kingdom, than his envious foes carried into execution their long-meditated plot for his ruin. they represented to the young king, that the treasury was exhausted, and that it was necessary to retrench a number of pensions, which had been granted for useless purposes, and in particular that of tycho, which, they maintained, ought to be conferred upon some person capable of rendering greater services to the state. by these means, they succeeded in depriving him of his support, and he was compelled to retreat under the hospitable mansion of a friend in germany. here he became known to the emperor, who invited him to prague, where, with an ample stipend, he resumed his labors. but, though surrounded with affectionate friends and admiring disciples, he was still an exile in a foreign land. although his country had been base in its ingratitude, it was yet the land which he loved; the scene of his earliest affection; the theatre of his scientific glory. these feelings continually preyed upon his mind, and his unsettled spirit was ever hovering among his native mountains. in this condition he was attacked by a disease of the most painful kind, and, though its agonizing paroxysms had lengthened intermissions, yet he saw that death was approaching. he implored his pupils to persevere in their scientific labors; he conversed with kepler on some of the profoundest points of astronomy; and with these secular occupations he mingled frequent acts of piety and devotion. in this happy condition he expired, without pain, at the age of fifty-five.[ ] the observatory at greenwich was not built until a hundred years after that of tycho brahe, namely, in . the great interests of the british nation, which are involved in navigation, constituted the ruling motive with the government to lend their aid in erecting and maintaining this observatory. the site of the observatory at greenwich is on a commanding eminence facing the river thames, five miles east of the central parts of london. being part of a royal park, the neighboring grounds are in no danger of being occupied by buildings, so as to obstruct the view. it is also in full view of the shipping on the thames; and, according to a standing regulation of the observatory, at the instant of one o'clock, every day, a huge ball is dropped from over the house, as a signal to the commanders of vessels for regulating their chronometers. the buildings comprise a series of rooms, of sufficient number and extent to accommodate the different instruments, the inmates of the establishment, and the library; and on the top is a celebrated camera obscura, exhibiting a most distinct and perfect picture of the grand and unrivalled scenery which this eminence commands. this establishment, by the accuracy and extent of its observations, has contributed more than all other institutions to perfect the science of astronomy. to preside over and direct this great institution, a man of the highest eminence in the science is appointed by the government, with the title of _astronomer royal_. he is paid an ample salary, with the understanding that he is to devote himself exclusively to the business of the observatory. the astronomers royal of the greenwich observatory, from the time of its first establishment, in , to the present time, have constituted a series of the proudest names of which british science can boast. a more detailed sketch of their interesting history will be given towards the close of these letters. six assistants, besides inferior laborers, are constantly in attendance; and the business of making and recording observations is conducted with the utmost system and order. the great objects to be attained in the construction of an observatory are, a commanding and unobstructed view of the heavens; freedom from causes that affect the transparency and uniform state of the atmosphere, such as fires, smoke, or marshy grounds; mechanical facilities for the management of instruments, and, especially, every precaution that is necessary to secure perfect steadiness. this last consideration is one of the greatest importance, particularly in the use of very large magnifiers; for we must recollect, that any motion in the instrument is magnified by the full power of the glass, and gives a proportional unsteadiness to the object. a situation is therefore selected as remote as possible from public roads, (for even the passing of carriages would give a tremulous motion to the ground, which would be sensible in large instruments,) and structures of solid masonry are commenced deep enough in the ground to be unaffected by frost, and built up to the height required, without any connexion with the other parts of the building. many observatories are furnished with a movable dome for a roof, capable of revolving on rollers, so that instruments penetrating through the roof may be easily brought to bear upon any point at or near the zenith. you will not perhaps desire me to go into a minute description of all the various instruments that are used in a well-constructed observatory. nor is this necessary, since a very large proportion of all astronomical observations are taken on the meridian, by means of the transit instrument and clock. when a body, in its diurnal revolution, comes to the meridian, it is at its highest point above the horizon, and is then least affected by refraction and parallax. this, then, is the most favorable position for taking observations upon it. moreover, it is peculiarly easy to take observations on a body when in this situation. hence the transit instrument and clock are the most important members of an astronomical observatory. you will, therefore, expect me to give you some account of these instruments. [illustration fig. .] the _transit instrument_ is a telescope which is fixed permanently in the meridian, and moves only in that plane. the accompanying diagram, fig. , represents a side view of a portable transit instrument, exhibiting the telescope supported on a firm horizontal axis, on which it turns in the plane of the meridian, from the south point of the horizon through the zenith to the north point. it can therefore be so directed as to observe the passage of a star across the meridian at any altitude. the accompanying graduated circle enables the observer to set the instrument at any required altitude, corresponding to the known altitude at which the body to be observed crosses the meridian. or it may be used to measure the altitude of a body, or its zenith distance, at the time of its meridian passage. near the circle may be seen a spirit-level, which serves to show when the axis is exactly on a level with the horizon. the framework is made of solid metal, (usually brass,) every thing being arranged with reference to keeping the instrument perfectly steady. it stands on screws, which not only afford a steady support, but are useful for adjusting the instrument to a perfect level. the transit instrument is sometimes fixed immovably to a solid foundation, as a pillar of stone, which is built up from a depth in the ground below the reach of frost. when enclosed in a building, as in an observatory, the stone pillar is carried up separate from the walls and floors of the building, so as to be entirely free from the agitations to which they are liable. the use of the transit instrument is to show the precise instant when a heavenly body is on the meridian, or to measure the time it occupies in crossing the meridian. the _astronomical clock_ is the constant companion of the transit instrument. this clock is so regulated as to keep exact pace with the stars, and of course with the revolution of the earth on its axis; that is, it is regulated to _sidereal_ time. it measures the progress of a star, indicating an hour for every fifteen degrees, and twenty-four hours for the whole period of the revolution of the star. sidereal time commences when the vernal equinox is on the meridian, just as solar time commences when the sun is on the meridian. hence the hour by the sidereal clock has no correspondence with the hour of the day, but simply indicates how long it is since the equinoctial point crossed the meridian. for example, the clock of an observatory points to three hours and twenty minutes; this may be in the morning, at noon, or any other time of the day,--for it merely shows that it is three hours and twenty minutes since the equinox was on the meridian. hence, when a star is on the meridian, the clock itself shows its right ascension, which you will recollect is the angular distance measured on the equinoctial, from the point of intersection of the ecliptic and equinoctial, called the vernal equinox, reckoning fifteen degrees for every hour, and a proportional number of degrees and minutes for a less period. i have before remarked, that a very large portion of all astronomical observations are taken when the bodies are on the meridian, by means of the transit instrument and clock. having now described these instruments, i will next explain the manner of using them for different observations. any thing becomes a measure of time, which divides duration equally. the equinoctial, therefore, is peculiarly adapted to this purpose, since, in the daily revolution of the heavens, equal portions of the equinoctial pass under the meridian in equal times. the only difficulty is, to ascertain the amount of these portions for given intervals. now, the clock shows us exactly this amount; for, when regulated to sidereal time, (as it easily may be,) the hour-hand keeps exact pace with the equator, revolving once on the dial-plate of the clock while the equator turns once by the revolution of the earth. the same is true, also, of all the small circles of diurnal revolution; they all turn exactly at the same rate as the equinoctial, and a star situated any where between the equator and the pole will move in its diurnal circle along with the clock, in the same manner as though it were in the equinoctial. hence, if we note the interval of time between the passage of any two stars, as shown by the clock, we have a measure of the number of degrees by which they are distant from each other in right ascension. hence we see how easy it is to take arcs of right ascension: the transit instrument shows us when a body is on the meridian; the clock indicates how long it is since the vernal equinox passed it, which is the right ascension itself; or it tells us the difference of right ascension between any two bodies, simply by indicating the difference in time between their periods of passing the meridian. again, it is easy to take the _declination_ of a body when on the meridian. by declination, you will recollect, is meant the distance of a heavenly body from the equinoctial; the same, indeed, as latitude on the earth. when a star is passing the meridian, if, on the instant of crossing the meridian wire of the telescope, we take its distance from the north pole, (which may readily be done, because the position of the pole is always known, being equal to the latitude of the place,) and subtract this distance from ninety degrees, the remainder will be the distance from the equator, which is the declination. you will ask, why we take this indirect method of finding the declination? why we do not rather take the distance of the star from the equinoctial, at once? i answer, that it is easy to point an instrument to the north pole, and to ascertain its exact position, and of course to measure any distance from it on the meridian, while, as there is nothing to mark the exact situation of the equinoctial, it is not so easy to take direct measurements from it. when we have thus determined the situation of a heavenly body, with respect to two great circles at right angles with each other, as in the present case, the distance of a body from the equator and from the equinoctial colure, or that meridian which passes though the vernal equinox, we know its relative position in the heavens; and when we have thus determined the relative positions of all the stars, we may lay them down on a map or a globe, exactly as we do places on the earth, by means of their latitude and longitude. the foregoing is only a _specimen_ of the various uses of the transit instrument, in finding the relative places of the heavenly bodies. another use of this excellent instrument is, to regulate our clocks and watches. by an observation with the transit instrument, we find when the sun's centre is on the meridian. this is the exact time of _apparent_ noon. but watches and clocks usually keep _mean_ time, and therefore, in order to set our timepiece by the transit instrument, we must apply to the apparent time of noon the equation of time, as will be explained in my next letter. a _noon-mark_ may easily be made by the aid of the transit instrument. a window sill is frequently selected as a suitable place for the mark, advantage being taken of the shadow projected upon it by the perpendicular casing of the window. let an assistant stand, with a rule laid on the line of shadow, and with a knife ready to make the mark, the instant when the observer at the transit instrument announces that the centre of the sun is on the meridian. by a concerted signal, as the stroke of a bell, the inhabitants of a town may all fix a noon-mark from the same observation. if the signal be given on one of the days when apparent time and mean time become equal to each other, as on the twenty-fourth of december, no equation of time is required. as a noon-mark is convenient for regulating timepieces, i will point out a method of making one, which may be practised without the aid of the telescope. upon a smooth, level plane, freely exposed to the sun, with a pair of compasses describe a circle. in the centre, where the leg of the compasses stood, erect a perpendicular wire of such a length, that the termination of its shadow shall fall upon the circumference of the circle at some hour before noon, as about ten o'clock. make a small dot at the point where the end of the shadow falls upon the circle, and do the same where it falls upon it again in the afternoon. take a point half-way between these two points, and from it draw a line to the centre, and it will be a true meridian line. the direction of this line would be the same, whether it were made in the summer or in the winter; but it is expedient to draw it about the fifteenth of june, for then the shadow alters its length most rapidly, and the moment of its crossing the wire will be more definite, than in the winter. at this time of year, also, the sun and clock agree, or are together, as will be more fully explained in my next letter; whereas, at other times of the year, the time of noon, as indicated by a common clock, would not agree with that indicated by the sun. if the upper end of the wire is flattened, and a small hole is made in it, through which the sun may shine, the instant when this bright spot falls upon the circle will be better defined than the termination of the shadow. another important instrument of the observatory is the _mural circle_. it is a graduated circle, usually of very large size, fixed permanently in the plane of the meridian, and attached firmly to a perpendicular wall; and on its centre is a telescope, which revolves along with it, and is easily brought to bear on any object in any point in the meridian. it is made of large size, sometimes twenty feet in diameter, in order that very small angles may be measured on its limb; for it is obvious that a small angle, as one second, will be a larger space on the limb of an instrument, in proportion as the instrument itself is larger. the vertical circle usually connected with the transit instrument, as in fig. , may indeed be employed for the same purposes as the mural circle, namely, to measure arcs of the meridian, as meridian altitudes, zenith distances, north polar distances, and declinations; but as that circle must necessarily be small, and therefore incapable of measuring very minute angles, the mural circle is particularly useful in measuring these important arcs. it is very difficult to keep so large an instrument perfectly steady; and therefore it is attached to a massive wall of solid masonry, and is hence called a _mural_ circle, from a latin word, (_murus_,) which signifies a wall. the diagram, fig. , page , represents a mural circle fixed to its wall, and ready for observations. it will be seen, that every expedient is employed to give the instrument firmness of parts and steadiness of position. the circle is of solid metal, usually of brass, and it is strengthened by numerous radii, which keep it from warping or bending; and these are made in the form of hollow cones, because that is the figure which unites in the highest degree lightness and strength. on the rim of the instrument, at a, you may observe a microscope. this is attached to a micrometer,--a delicate piece of apparatus, used for reading the minute subdivisions of angles; for, after dividing the limb of the instrument as minutely as possible, it will then be necessary to magnify those divisions with the microscope, and subdivide each of these parts with the micrometer. thus, if we have a mural circle twenty feet in diameter, and of course nearly sixty-three feet in circumference, since there are twenty-one thousand and six hundred minutes in the whole circle, we shall find, by calculation, that one minute would occupy, on the limb of such an instrument, only about one thirtieth of an inch, and a second, only one eighteen hundredth of an inch. we could not, therefore, hope to carry the actual divisions to a greater degree of minuteness than minutes; but each of these spaces may again be subdivided into seconds by the micrometer. [illustration fig. .] from these statements, you will acquire some faint idea of the extreme difficulty of making perfect astronomical instruments, especially where they are intended to measure such minute angles as one second. indeed, the art of constructing astronomical instruments is one which requires such refined mechanical genius,--so superior a mind to devise, and so delicate a hand to execute,--that the most celebrated instrument-makers take rank with the most distinguished astronomers; supplying, as they do, the means by which only the latter are enabled to make these great discoveries. astronomers have sometimes made their own telescopes; but they have seldom, if ever, possessed the refined manual skill which is requisite for graduating delicate instruments. the _sextant_ is also one of the most valuable instruments for taking celestial arcs, or the distance between any two points on the celestial sphere, being applicable to a much greater number of purposes than the instruments already described. it is particularly valuable for measuring celestial arcs at sea, because it is not, like most astronomical instruments, affected by the motion of the ship. the principle of the sextant may be briefly described, as follows: it gives the angular distance between any two bodies on the celestial sphere, by reflecting the image of one of the bodies so as to coincide with the other body, as seen directly. the arc through which the reflector is turned, to bring the reflected body to coincide with the other body, becomes a measure of the angular distance between them. by keeping this principle in view, you will be able to understand the use of the several parts of the instrument, as they are exhibited in the diagram, fig. , page . it is, you observe, of a triangular shape, and it is made strong and firm by metallic cross-bars. it has two reflectors, i and h, called, respectively, the index glass and the horizon glass, both of which are firmly fixed perpendicular to the plane of the instrument. the index glass is attached to the movable arm, id, and turns as this is moved along the graduated limb, ef. this arm also carries a _vernier_, at d, a contrivance which, like the micrometer, enables us to take off minute parts of the spaces into which the limb is divided. the horizon glass, h, consists of two parts; the upper part being transparent or open, so that the eye, looking through the telescope, t, can see through it a distant body, as a star at s, while the lower part is a reflector. [illustration fig. .] suppose it were required to measure the angular distance between the moon and a certain star,--the moon being at m, and the star at s. the instrument is held firmly in the hand, so that the eye, looking through the telescope, sees the star, s, through the transparent part of the horizon glass. then the movable arm, id, is moved from f towards e, until the image of m is reflected down to s, when the number of degrees and parts of a degree reckoned on the limb, from f to the index at d, will show the angular distance between the two bodies. footnote: [ ] brewster's life of newton letter vi. time and the calendar. "from old eternity's mysterious orb was time cut off, and cast beneath the skies."--_young._ having hitherto been conversant only with the many fine and sentimental things which the poets have sung respecting old time, perhaps you will find some difficulty in bringing down your mind to the calmer consideration of what time really is, and according to what different standards it is measured for different purposes. you will not, however, i think, find the subject even in our matter-of-fact and unpoetical way of treating it, altogether uninteresting. what, then, is time? _time is a measured portion of indefinite duration._ it consists of equal portions cut off from eternity, as a line on the surface of the earth is separated from its contiguous portions that constitute a great circle of the sphere, by applying to it a two-foot scale; or as a few yards of cloth are measured off from a piece of unknown or indefinite extent. any thing, or any event which takes place at equal intervals, may become a measure of time. thus, the pulsations of the wrist, the flowing of a given quantity of sand from one vessel to another, as in the hourglass, the beating of a pendulum, and the revolution of a star, have been severally employed as measures of time. but the great standard of time is the period of the revolution of the earth on its axis, which, by the most exact observations, is found to be always the same. i have anticipated a little of this subject, in giving an account of the transit instrument and clock, but i propose, in this letter, to enter into it more at large. the time of the earth's revolution on its axis, as already explained, is called a sidereal day, and is determined by the revolution of a star in the heavens. this interval is divided into twenty-four _sidereal_ hours. observations taken on numerous stars, in different ages of the world, show that they all perform their diurnal revolution in the same time, and that their motion, during any part of the revolution, is always uniform. here, then, we have an exact measure of time, probably more exact than any thing which can be devised by art. _solar time_ is reckoned by the apparent revolution of the sun from the meridian round to the meridian again. were the sun stationary in the heavens, like a fixed star, the time of its apparent revolution would be equal to the revolution of the earth on its axis, and the solar and the sidereal days would be equal. but, since the sun passes from west to east, through three hundred and sixty degrees, in three hundred and sixty-five and one fourth days, it moves eastward nearly one degree a day. while, therefore, the earth is turning round on its axis, the sun is moving in the same direction, so that, when we have come round under the same celestial meridian from which we started, we do not find the sun there, but he has moved eastward nearly a degree, and the earth must perform so much more than one complete revolution, before we come under the sun again. now, since we move, in the diurnal revolution, fifteen degrees in sixty minutes, we must pass over one degree in four minutes. it takes, therefore, four minutes for us to _catch up_ with the sun, after we have made one complete revolution. hence the solar day is about four minutes longer than the sidereal; and if we were to reckon the sidereal day twenty-four hours, we should reckon the solar day twenty-four hours four minutes. to suit the purposes of society at large, however, it is found more convenient to reckon the solar days twenty-four hours, and throw the fraction into the sidereal day. then, h. m. : h. :: h. : h. m. s. that is, when we reduce twenty-four hours and four minutes to twenty-four hours, the same proportion will require that we reduce the sidereal day from twenty-four hours to twenty-three hours fifty-six minutes four seconds; or, in other words, a sidereal day is such a part of a solar day. the solar days, however, do not always differ from the sidereal by precisely the same fraction, since they are not constantly of the same length. time, as measured by the sun, is called _apparent time_, and a clock so regulated as always to keep exactly with the sun, is said to keep apparent time. _mean time_ is time reckoned by the _average_ length of all the solar days throughout the year. this is the period which constitutes the _civil_ day of twenty-four hours, beginning when the sun is on the lower meridian, namely, at twelve o'clock at night, and counted by twelve hours from the lower to the upper meridian, and from the upper to the lower. the _astronomical_ day is the apparent solar day counted through the whole twenty-four hours, (instead of by periods of twelve hours each, as in the civil day,) and begins at noon. thus it is now the tenth of june, at nine o'clock, a.m., according to civil time; but we have not yet reached the tenth of june by astronomical time, nor shall we, until noon to-day; consequently, it is now june ninth, twenty-first hour of astronomical time. astronomers, since so many of their observations are taken on the meridian, are always supposed to look towards the south. geographers, having formerly been conversant only with the northern hemisphere, are always understood to be looking towards the north. hence, left and right, when applied to the astronomer, mean east and west, respectively; but to the geographer the right is east, and the left, west. clocks are usually regulated so as to indicate mean solar time; yet, as this is an artificial period not marked off, like the sidereal day, by any natural event, it is necessary to know how much is to be added to, or subtracted from, the apparent solar time, in order to give the corresponding mean time. the interval, by which apparent time differs from mean time, is called the _equation of time_. if one clock is so constructed as to keep exactly with the sun, going faster or slower, according as the lengths of the solar days vary, and another clock is regulated to mean time, then the difference of the two clocks, at any period, would be the equation of time for that moment. if the apparent clock were _faster_ than the mean, then the equation of time must be subtracted; but if the apparent clock were slower than the mean, then the equation of time must be added, to give the mean time. the two clocks would differ most about the third of november, when the apparent time is sixteen and one fourth minutes greater than the mean. but since apparent time is sometimes greater and sometimes less than mean time, the two must obviously be sometimes equal to each other. this is, in fact, the case four times a year, namely, april fifteenth, june fifteenth, september first, and december twenty-fourth. astronomical clocks are made of the best workmanship, with every advantage that can promote their regularity. although they are brought to an astonishing degree of accuracy, yet they are not as regular in their movements as the stars are, and their accuracy requires to be frequently tested. the transit instrument itself, when once accurately placed in the meridian, affords the means of testing the correctness of the clock, since one revolution of a star, from the meridian to the meridian again, ought to correspond exactly to twenty-four hours by the clock, and to continue the same, from day to day; and the right ascensions of various stars, as they cross the meridian, ought to be such by the clock, as they are given in the tables, where they are stated according to the accurate determinations of astronomers. or, by taking the difference of any two stars, on successive days, it will be seen whether the going of the clock is uniform for that part of the day; and by taking the right ascensions of different pairs of stars, we may learn the rate of the clock at various parts of the day. we thus learn, not only whether the clock accurately measures the length of the sidereal day, but also whether it goes uniformly from hour to hour. although astronomical clocks have been brought to a great degree of perfection, so as hardly to vary a second for many months, yet none are absolutely perfect, and most are so far from it, as to require to be corrected by means of the transit instrument, every few days. indeed, for the nicest observations, it is usual not to attempt to bring the clock to a state of absolute correctness, but, after bringing it as near to such a state as can conveniently be done, to ascertain how much it gains or loses in a day; that is, to ascertain the _rate_ of its going, and to make allowance accordingly. having considered the manner in which the smaller divisions of time are measured, let us now take a hasty glance at the larger periods which compose the calendar. as a _day_ is the period of the revolution of the earth on its axis, so a _year_ is the period of the revolution of the earth around the sun. this time, which constitutes the _astronomical year_, has been ascertained with great exactness, and found to be three hundred and sixty-five days five hours forty-eight minutes and fifty-one seconds. the most ancient nations determined the number of days in the year by means of the _stylus_, a perpendicular rod which casts its shadow on a smooth plane bearing a meridian line. the time when the shadow was shortest, would indicate the day of the summer solstice; and the number of days which elapsed, until the shadow returned to the same length again, would show the number of days in the year. this was found to be three hundred and sixty-five whole days, and accordingly, this period was adopted for the civil year. such a difference, however, between the civil and astronomical years, at length threw all dates into confusion. for if, at first, the summer solstice happened on the twenty-first of june, at the end of four years, the sun would not have reached the solstice until the twenty-second of june; that is, it would have been behind its time. at the end of the next four years, the solstice would fall on the twenty-third; and in process of time, it would fall successively on every day of the year. the same would be true of any other fixed date. julius cæsar, who was distinguished alike for the variety and extent of his knowledge, and his skill in arms, first attempted to make the calendar conform to the motions of the sun. "amidst the hurry of tumultuous war, the stars, the gods, the heavens, were still his care." aided by sosigenes, an egyptian astronomer, he made the first correction of the calendar, by introducing an additional day every fourth year, making february to consist of twenty-nine instead of twenty-eight days, and of course the whole year to consist of three hundred and sixty-six days. this fourth year was denominated _bissextile_, because the sixth day before the kalends of march was reckoned twice. it is also called leap year. the julian year was introduced into all the civilized nations that submitted to the roman power, and continued in general use until the year . but the true correction was not six hours, but five hours forty-nine minutes; hence the addition was too great by eleven minutes. this small fraction would amount in one hundred years to three fourths of a day, and in one thousand years to more than seven days. from the year to the year , it had, in fact, amounted to more than ten days; for it was known that, in , the vernal equinox fell on the twenty-first of march, whereas, in , it fell on the eleventh. it was ordered by the council of nice, a celebrated ecclesiastical council, held in the year , that easter should be celebrated upon the first sunday after the first full moon, next following the vernal equinox; and as certain other festivals of the romish church were appointed at particular seasons of the year, confusion would result from such a want of constancy between any fixed date and a particular season of the year. suppose, for example, a festival accompanied by numerous religious ceremonies, was decreed by the church to be held at the time when the sun crossed the equator in the spring, (an event hailed with great joy, as the harbinger of the return of summer,) and that, in the year , march twenty-first was designated as the time for holding the festival, since, at that period, it was on the twenty-first of march when the sun reached the equinox; the next year, the sun would reach the equinox a little sooner than the twenty-first of march, only eleven minutes, indeed, but still amounting in twelve hundred years to ten days; that is, in , the sun reached the equinox on the eleventh of march. if, therefore, they should continue to observe the twenty-first as a religious festival in honor of this event, they would commit the absurdity of celebrating it ten days after it had passed by. pope gregory the thirteenth, who was then at the head of the roman see, was a man of science, and undertook to reform the calendar, so that fixed dates would always correspond to the same seasons of the year. he first decreed, that the year should be brought forward ten days, by reckoning the fifth of october the fifteenth; and, in order to prevent the calendar from falling into confusion afterwards, he prescribed the following rule: _every year whose number is not divisible by four, without a remainder, consists of three hundred and sixty-five days; every year which is so divisible, but is not divisible by one hundred, of three hundred and sixty-six; every year divisible by one hundred, but not by four hundred, again, of three hundred and sixty-five; and every year divisible by four hundred, of three hundred and sixty-six._ thus the year , not being divisible by four, contains three hundred and sixty-five days, while and are leap years. yet, to make every fourth year consist of three hundred and sixty-six days would increase it too much, by about three fourths of a day in a century; therefore every hundredth year has only three hundred and sixty-five days. thus , although divisible by four, was not a leap year, but a common year. but we have allowed a _whole_ day in a hundred years, whereas we ought to have allowed only _three fourths_ of a day. hence, in four hundred years, we should allow a day too much, and therefore, we let the four hundredth remain a leap year. this rule involves an error of less than a day in four thousand two hundred and thirty-seven years. the pope, who, you will recollect, at that age assumed authority over all secular princes, issued his decree to the reigning sovereigns of christendom, commanding the observance of the calendar as reformed by him. the decree met with great opposition among the protestant states, as they recognised in it a new exercise of ecclesiastical tyranny; and some of them, when they received it, made it expressly understood, that their acquiescence should not be construed as a submission to the papal authority. in , the gregorian year, or _new style_, was established in great britain by act of parliament; and the dates of all deeds, and other legal papers, were to be made according to it. as above a century had then passed since the first introduction of the new style, eleven days were suppressed, the third of september being called the fourteenth. by the same act, the beginning of the year was changed from march twenty-fifth to january first. a few persons born previously to have come down to our day, and we frequently see inscriptions on tombstones of those whose time of birth is recorded in old style. in order to make this correspond to our present mode of reckoning, we must add eleven days to the date. thus the same event would be june twelfth of old style, or june twenty-third of new style; and if an event occurred between january first and march twenty-fifth, the date of the year would be advanced one, since february st, , o.s. would be february st, , n.s. thus, general washington was born february th, , o.s., or february d, , n.s. if we inquire how any present event may be made to correspond in date to the old style, we must subtract twelve days, and put the year back one, if the event lies between january first and march twenty-fifth. thus, june tenth, n.s. corresponds to may twenty-ninth, o.s.; and march th, , to march th, . france, being a roman catholic country, adopted the new style soon after it was decreed by the pope; but protestant countries, as we have seen, were much slower in adopting it; and russia, and the greek church generally, still adhere to the old style. in order, therefore, to make the russian dates correspond to ours, we must add to them twelve days. it may seem to you very remarkable, that so much pains should have been bestowed upon this subject; but without a correct and uniform standard of time, the dates of deeds, commissions, and all legal papers; of fasts and festivals, appointed by ecclesiastical authority; the returns of seasons, and the records of history,--must all fall into inextricable confusion. to change the observance of certain religious feasts, which have been long fixed to particular days, is looked upon as an impious innovation; and though the times of the events, upon which these ceremonies depend, are utterly unknown, it is still insisted upon by certain classes in england, that the glastenbury thorn blooms on christmas day. although the ancient grecian calendar was extremely defective, yet the common people were entirely averse to its reformation. their superstitious adherence to these errors was satirized by aristophanes, in his comedy of the clouds. an actor, who had just come from athens, recounts that he met with diana, or the moon, and found her extremely incensed, that they did not regulate her course better. she complained, that the order of nature was changed, and every thing turned topsyturvy. the gods no longer knew what belonged to them; but, after paying their visits on certain feast-days, and expecting to meet with good cheer, as usual, they were under the disagreeable necessity of returning back to heaven without their suppers. among the greeks, and other ancient nations, the length of the year was generally regulated by the course of the moon. this planet, on account of the different appearances which she exhibits at her full, change, and quarters, was considered by them as best adapted of any of the celestial bodies for this purpose. as one lunation, or revolution of the moon around the earth, was found to be completed in about twenty-nine and one half days, and twelve of these periods being supposed equal to one revolution of the sun, their months were made to consist of twenty-nine and thirty days alternately, and their year of three hundred and fifty-four days. but this disagreed with the annual revolution of the sun, which must evidently govern the seasons of the year, more than eleven days. the irregularities, which such a mode of reckoning would occasion, must have been too obvious not to have been observed. for, supposing it to have been settled, at any particular time, that the beginning of the year should be in the spring; in about sixteen years afterwards, the beginning would have been in autumn; and in thirty-three or thirty-four years, it would have gone backwards through all the seasons, to spring again. this defect they attempted to rectify, by introducing a number of days, at certain times, into the calendar, as occasion required, and putting the beginning of the year forwards, in order to make it agree with the course of the sun. but as these additions, or _intercalations_, as they were called, were generally consigned to the care of the priests, who, from motives of interest or superstition, frequently omitted them, the year was made long or short at pleasure. the _week_ is another division of time, of the highest antiquity, which, in almost all countries, has been made to consist of seven days; a period supposed by some to have been traditionally derived from the creation of the world; while others imagine it to have been regulated by the phases of the moon. the names, saturday, sunday, and monday, are obviously derived from saturn, the sun, and the moon; while tuesday, wednesday, thursday, and friday, are the days of tuisco, woden, thor, and friga, which are saxon names for mars, mercury, jupiter, and venus.[ ] the common year begins and ends on the same day of the week; but leap year ends one day later than it began. fifty-two weeks contain three hundred and sixty-four days; if, therefore, the year begins on tuesday, for example, we should complete fifty-two weeks on monday, leaving one day, (tuesday,) to complete the year, and the following year would begin on wednesday. hence, any day of the month is one day later in the week, than the corresponding day of the preceding year. thus, if the sixteenth of november, , falls on friday, the sixteenth of november, , fell on thursday, and will fall, in , on saturday. but if leap year begins on sunday, it ends on monday, and the following year begins on tuesday; while any given day of the month is two days later in the week than the corresponding date of the preceding year. footnote: [ ] bonnycastle's astronomy. letter vii. figure of the earth. "he took the golden compasses, prepared in god's eternal store, to circumscribe this universe, and all created things; one foot he centred, and the other turned round through the vast profundity obscure, and said, 'thus far extend, thus far thy bounds, this be thy just circumference, o world!'"--_milton._ in the earliest ages, the earth was regarded as one continued plane; but, at a comparatively remote period, as five hundred years before the christian era, astronomers began to entertain the opinion that the earth is round. we are able now to adduce various arguments which severally prove this truth. first, when a ship is coming in from sea, we first observe only the very highest parts of the ship, while the lower portions come successively into view. were the earth a continued plane, the lower parts of the ship would be visible as soon as the higher, as is evident from fig. , page . [illustration fig. .] [illustration fig. .] since light comes to the eye in straight lines, by which objects become visible, it is evident, that no reason exists why the parts of the ship near the water should not be seen as soon as the upper parts. but if the earth be a sphere, then the line of sight would pass above the deck of the ship, as is represented in fig. ; and as the ship drew nearer to land, the lower parts would successively rise above this line and come into view exactly in the manner known to observation. secondly, in a lunar eclipse, which is occasioned by the moon's passing through the earth's shadow, the figure of the shadow is seen to be spherical, which could not be the case unless the earth itself were round. thirdly, navigators, by steering continually in one direction, as east or west, have in fact come round to the point from which they started, and thus confirmed the fact of the earth's rotundity beyond all question. one may also reach a given place on the earth, by taking directly opposite courses. thus, he may reach canton in china, by a westerly route around cape horn, or by an easterly route around the cape of good hope. all these arguments severally prove that the earth is round. but i propose, in this letter, to give you some account of the unwearied labors which have been performed to ascertain the _exact_ figure of the earth; for although the earth is properly described in general language as round, yet it is not an exact sphere. were it so, all its diameters would be equal; but it is known that a diameter drawn through the equator exceeds one drawn from pole to pole, giving to the earth the form of a _spheroid_,--a figure resembling an orange, where the ends are flattened a little and the central parts are swelled out. although it would be a matter of very rational curiosity, to investigate the precise shape of the planet on which heaven has fixed our abode, yet the immense pains which has been bestowed on this subject has not all arisen from mere curiosity. no accurate measurements can be taken of the distances and magnitudes of the heavenly bodies, nor any exact determinations made of their motions, without a knowledge of the exact figure of the earth; and hence is derived a powerful motive for ascertaining this element with all possible precision. the first satisfactory evidence that was obtained of the exact figure of the earth was derived from reasoning on the effects of the earth's _centrifugal force_, occasioned by its rapid revolution on its own axis. when water is whirled in a pail, we see it recede from the centre and accumulate upon the sides of the vessel; and when a millstone is whirled rapidly, since the portions of the stone furthest from the centre revolve much more rapidly than those near to it, their greater tendency to recede sometimes makes them fly off with a violent explosion. a case, which comes still nearer to that of the earth, is exhibited by a mass of clay revolving on a potter's wheel, as seen in the process of making earthen vessels. the mass swells out in the middle, in consequence of the centrifugal force exerted upon it by a rapid motion. now, in the diurnal revolution, the equatorial parts of the earth move at the rate of about one thousand miles per hour, while the poles do not move at all; and since, as we take points at successive distances from the equator towards the pole, the rate at which these points move grows constantly less and less; and since, in revolving bodies, the centrifugal force is proportioned to the velocity, consequently, those parts which move with the greatest rapidity will be more affected by this force than those which move more slowly. hence, the equatorial regions must be higher from the centre than the polar regions; for, were not this the case, the waters on the surface of the earth would be thrown towards the equator, and be piled up there, just as water is accumulated on the sides of a pail when made to revolve rapidly. huyghens, an eminent astronomer of holland, who investigated the laws of centrifugal forces, was the first to infer that such must be the actual shape of the earth; but to sir isaac newton we owe the full developement of this doctrine. by combining the reasoning derived from the known laws of the centrifugal force with arguments derived from the principles of universal gravitation, he concluded that the distance through the earth, in the direction of the equator, is greater than that in the direction of the poles. he estimated the difference to be about thirty-four miles. but it was soon afterwards determined by the astronomers of france, to ascertain the figure of the earth by actual measurements, specially instituted for that purpose. let us see how this could be effected. if we set out at the equator and travel towards the pole, it is easy to see when we have advanced one degree of latitude, for this will be indicated by the rising of the north star, which appears in the horizon when the spectator stands on the equator, but rises in the same proportion as he recedes from the equator, until, on reaching the pole, the north star would be seen directly over head. now, were the earth a perfect sphere, the meridian of the earth would be a perfect circle, and the distance between any two places, differing one degree in latitude, would be exactly equal to the distance between any other two places, differing in latitude to the same amount. but if the earth be a spheroid, flattened at the poles, then a line encompassing the earth from north to south, constituting the terrestrial meridian, would not be a perfect circle, but an ellipse or oval, having its longer diameter through the equator, and its shorter through the poles. the part of this curve included between two radii, drawn from the centre of the earth to the celestial meridian, at angles one degree asunder, would be greater in the polar than in the equatorial region; that is, the degrees of the meridian would lengthen towards the poles. the french astronomers, therefore, undertook to ascertain by actual measurements of arcs of the meridian, in different latitudes, whether the degrees of the meridian are of uniform length, or, if not, in what manner they differ from each other. after several indecisive measurements of an arc of the meridian in france, it was determined to effect simultaneous measurements of arcs of the meridian near the equator, and as near as possible to the north pole, presuming that if degrees of the meridian, in different latitudes, are really of different lengths, they will differ most in points most distant from each other. accordingly, in , the french academy, aided by the government, sent out two expeditions, one to peru and the other to lapland. three distinguished mathematicians, bouguer, la condamine, and godin, were despatched to the former place, and four others, maupertius, camus, clairault, and lemonier, were sent to the part of swedish lapland which lies at the head of the gulf of tornea, the northern arm of the baltic. this commission completed its operations several years sooner than the other, which met with greater difficulties in the way of their enterprise. still, the northern detachment had great obstacles to contend with, arising particularly from the extreme length and severity of their winters. the measurements, however, were conducted with care and skill, and the result, when compared with that obtained for the length of a degree in france, plainly indicated, by its greater amount, a compression of the earth towards the poles. mean-while, bouguer and his party were prosecuting a similar work in peru, under extraordinary difficulties. these were caused, partly by the localities, and partly by the ill-will and indolence of the inhabitants. the place selected for their operations was in an elevated valley between two principal chains of the andes. the lowest point of their arc was at an elevation of a mile and a half above the level of the sea; and, in some instances, the heights of two neighboring signals differed more than a mile. encamped upon lofty mountains, they had to struggle against storms, cold, and privations of every description, while the invincible indifference of the indians, they were forced to employ, was not to be shaken by the fear of punishment or the hope of reward. yet, by patience and ingenuity, they overcame all obstacles, and executed with great accuracy one of the most important operations, of this nature, ever undertaken. to accomplish this, however, took them nine years; of which, three were occupied in determining the latitudes alone.[ ] i have recited the foregoing facts, in order to give you some idea of the unwearied pains which astronomers have taken to ascertain the exact figure of the earth. you will find, indeed, that all their labors are characterized by the same love of accuracy. years of toilsome watchings, and incredible labor of computation, have been undergone, for the sake of arriving only a few seconds nearer to the truth. the length of a degree of the meridian, as measured in peru, was less than that before determined in france, and of course less than that of lapland; so that the spheroidal figure of the earth appeared now to be ascertained by actual measurement. still, these measures were too few in number, and covered too small a portion of the whole quadrant from the equator to the pole, to enable astronomers to ascertain the exact law of curvature of the meridian, and therefore similar measurements have since been prosecuted with great zeal by different nations, particularly by the french and english. in , two english mathematicians of great eminence, mason and dixon, undertook the measurement of an arc in pennsylvania, extending more than one hundred miles. [illustration fig. .] [illustration fig. .] these operations are carried on by what is called a system of _triangulation_. without some knowledge of trigonometry, you will not be able fully to understand this process; but, as it is in its nature somewhat curious, and is applied to various other geographical measurements, as well as to the determination of arcs of the meridian, i am desirous that you should understand its general principles. let us reflect, then, that it must be a matter of the greatest difficulty, to execute with exactness the measurement of a line of any great length in one continued direction on the earth's surface. even if we select a level and open country, more or less inequalities of surface will occur; rivers must be crossed, morasses must be traversed, thickets must be penetrated, and innumerable other obstacles must be surmounted; and finally, every time we apply an artificial measure, as a rod, for example, we obtain a result not absolutely perfect. each error may indeed be very small, but small errors, often repeated, may produce a formidable aggregate. now, one unacquainted with trigonometry can easily understand the fact, that, when we know certain parts of a triangle, we can find the other parts by calculation; as, in the rule of three in arithmetic, we can obtain the fourth term of a proportion, from having the first three terms given. thus, in the triangle a b c, fig. , if we know the side a b, and the angles at a and b, we can find by computation, the other sides, a c and b c, and the remaining angle at c. suppose, then, that in measuring an arc of the meridian through any country, the line were to pass directly through a b, but the ground was so obstructed between a and b, that we could not possibly carry our measurement through it. we might then measure another line, as a c, which was accessible, and with a compass take the bearing of b from the points a and c, by which means we should learn the value of the angles at a and c. from these data we might calculate, by the rules of trigonometry, the exact length of the line a b. perhaps the ground might be so situated, that we could not reach the point b, by any route; still, if it could be seen from a and c, it would be all we should want. thus, in conducting a trigonometrical survey of any country, conspicuous signals are placed on elevated points, and the bearings of these are taken from the extremities of a known line, called the base, and thus the relative situation of various places is accurately determined. were we to undertake to run an exact north and south line through any country, as new england, we should select, near one extremity, a spot of ground favorable for actual measurement, as a level, unobstructed plain; we should provide a measure whose length in feet and inches was determined with the greatest possible precision, and should apply it with the utmost care. we should thus obtain a _base line_. from the extremities of this line, we should take (with some appropriate instrument) the bearing of some signal at a greater or less distance, and thus we should obtain one side and two angles of a triangle, from which we could find, by the rules of trigonometry, either of the unknown sides. taking this as a new base, we might take the bearing of another signal, still further on our way, and thus proceed to run the required north and south line, without actually measuring any thing more than the first, or base line. thus, in fig. , we wish to measure the distance between the two points a and o, which are both on the same meridian, as is known by their having the same longitude; but, on account of various obstacles, it would be found very inconvenient to measure this line directly, with a rod or chain, and even if we could do it, we could not by this method obtain nearly so accurate a result, as we could by a series of triangles, where, after the base line was measured, we should have nothing else to measure except angles, which can be determined, by observation, to a greater degree of exactness, than lines. we therefore, in the first place, measure the base line, a b, with the utmost precision. then, taking the bearing of some signal at c from a and b, we obtain the means of calculating the side b c, as has been already explained. taking b c as a new base, we proceed, in like manner, to determine successively the sides c d, d e, and e f, and also a c, and c e. although a c is not in the direction of the meridian, but considerably to the east of it, yet it is easy to find the corresponding distance on the meridian, a m; and in the same manner we can find the portions of the meridian m n and n o, corresponding respectively to c e and e f. adding these several parts of the meridian together, we obtain the length of the arc from a to o, in miles; and by observations on the north star, at each extremity of the arc, namely, at a and at o, we could determine the difference of latitude between these two points. suppose, for example, that the distance between a and o is exactly five degrees, and that the length of the intervening line is three hundred and forty-seven miles; then, dividing the latter by the former number, we find the length of a degree to be sixty-nine miles and four tenths. to take, however, a few of the results actually obtained, they are as follows: places of observation. latitude. length of a deg. in miles. peru, ° ' " . pennsylvania, . france, . england, - / . sweden, . this comparison shows, that the length of a degree gradually increases, as we proceed from the equator towards the pole. combining the results of various estimates, the dimensions of the terrestrial spheroid are found to be as follows: equatorial diameter, . miles. polar diameter, . " average diameter, . " the difference between the greatest and the least is about twenty-six and one half miles, which is about one two hundred and ninety-ninth part of the greatest. this fraction is denominated the _ellipticity_ of the earth,--being the excess of the equatorial over the polar diameter. the operations, undertaken for the purpose of determining the figure of the earth, have been conducted with the most refined exactness. at any stage of the process, the length of the last side, as obtained by calculation, may be actually measured in the same manner as the base from which the series of triangles commenced. when thus measured, it is called the _base of verification_. in some surveys, the base of verification, when taken at a distance of four hundred miles from the starting point, has not differed more than one foot from the same line, as determined by calculation. another method of arriving at the exact figure of the earth is, by observations with the _pendulum_. if a pendulum, like that of a clock, be suspended, and the number of its vibrations per hour be counted, they will be found to be different in different latitudes. a pendulum that vibrates thirty-six hundred times per hour, at the equator, will vibrate thirty-six hundred and five and two thirds times, at london, and a still greater number of times nearer the north pole. now, the vibrations of the pendulum are produced by the force of gravity. hence their comparative number at different places is a measure of the relative forces of gravity at those places. but when we know the relative forces of gravity at different places, we know their relative distances from the centre of the earth; because the nearer a place is to the centre of the earth, the greater is the force of gravity. suppose, for example, we should count the number of vibrations of a pendulum at the equator, and then carry it to the north pole, and count the number of vibrations made there in the same time,--we should be able, from these two observations, to estimate the relative forces of gravity at these two points; and, having the relative forces of gravity, we can thence deduce their relative distances from the centre of the earth, and thus obtain the polar and equatorial diameters. observations of this kind have been taken with the greatest accuracy, in many places on the surface of the earth, at various distances from each other, and they lead to the same conclusions respecting the figure of the earth, as those derived from measuring arcs of the meridian. it is pleasing thus to see a great truth, and one apparently beyond the pale of human investigation, reached by two routes entirely independent of each other. nor, indeed, are these the only proofs which have been discovered of the spheroidal figure of the earth. in consequence of the accumulation of matter above the equatorial regions of the earth, a body weighs less there than towards the poles, being further removed from the centre of the earth. the same accumulation of matter, by the force of attraction which it exerts, causes slight inequalities in the motions of the moon; and since the amount of these becomes a measure of the force which produces them, astronomers are able, from these inequalities, to calculate the exact quantity of the matter thus accumulated, and hence to determine the figure of the earth. the result is not essentially different from that obtained by the other methods. finally, the shape of the earth's shadow is altered, by its spheroidal figure,--a circumstance which affects the time and duration of a lunar eclipse. all these different and independent phenomena afford a pleasing example of the harmony of truth. the known effects of the centrifugal force upon a body revolving on its axis, like the earth, lead us to infer that the earth is of a spheroidal figure; but if this be the fact, the pendulum ought to vibrate faster near the pole than at the equator, because it would there be nearer the centre of the earth. on trial, such is found to be the case. if, again, there be such an accumulation of matter about the equatorial regions, its effects ought to be visible in the motions of the moon, which it would influence by its gravity; and there, also, its effects are traced. at length, we apply our measures to the surface of the earth itself, and find the same fact, which had thus been searched out among the hidden things of nature, here palpably exhibited before our eyes. finally, on estimating from these different sources, what the exact amount of the compression at the poles must be, all bring out nearly one and the same result. this truth, so harmonious in itself, takes along with it, and establishes, a thousand other truths on which it rests. footnote: [ ] library of useful knowledge: history of astronomy, page . letter viii. diurnal revolutions. "to some she taught the fabric of the sphere, the changeful moon, the circuit of the stars, the golden zones of heaven."--_akenside._ with the elementary knowledge already acquired, you will now be able to enter with pleasure and profit on the various interesting phenomena dependent on the revolution of the earth on its axis and around the sun. the apparent diurnal revolution of the heavenly bodies, from east to west, is owing to the actual revolution of the earth on its own axis, from west to east. if we conceive of a radius of the earth's equator extended until it meets the concave sphere of the heavens, then, as the earth revolves, the extremity of this line would trace out a curve on the face of the sky; namely, the celestial equator. in curves parallel to this, called the _circles of diurnal revolution_, the heavenly bodies actually _appear_ to move, every star having its own peculiar circle. after you have first rendered familiar the real motion of the earth from west to east, you may then, without danger of misapprehension, adopt the common language, that all the heavenly bodies revolve around the earth once a day, from east to west, in circles parallel to the equator and to each other. i must remind you, that the time occupied by a star, in passing from any point in the meridian until it comes round to the same point again, is called a _sidereal day_, and measures the period of the earth's revolution on its axis. if we watch the returns of the same star from day to day, we shall find the intervals exactly equal to each other; that is, _the sidereal days are all equal_. whatever star we select for the observation, the same result will be obtained. the stars, therefore, always keep the same relative position, and have a common movement round the earth,--a consequence that naturally flows from the hypothesis that their _apparent_ motion is all produced by a single _real_ motion; namely, that of the earth. the sun, moon, and planets, as well as the fixed stars, revolve in like manner; but their returns to the meridian are not, like those of the fixed stars, at exactly equal intervals. the _appearances_ of the diurnal motions of the heavenly bodies are different in different parts of the earth,--since every place has its own horizon, and different horizons are variously inclined to each other. nothing in astronomy is more apt to mislead us, than the obstinate habit of considering the horizon as a fixed and immutable plane, and of referring every thing to it. we should contemplate the earth as a huge globe, occupying a small portion of space, and encircled on all sides, at an immense distance, by the starry sphere. we should free our minds from their habitual proneness to consider one part of space as naturally _up_ and another _down_, and view ourselves as subject to a force (gravity) which binds us to the earth as truly as though we were fastened to it by some invisible cords or wires, as the needle attaches itself to all sides of a spherical loadstone. we should dwell on this point, until it appears to us as truly up, in the direction b b, c c, d d, when one is at b, c, d, respectively, as in the direction a a, when he is at a, fig. . let us now suppose the spectator viewing the diurnal revolutions from several different positions on the earth. on the _equator_, his horizon would pass through both poles; for the horizon cuts the celestial vault at ninety degrees in every direction from the zenith of the spectator; but the pole is likewise ninety degrees from his zenith, when he stands on the equator; and consequently, the pole must be in the horizon. here, also, the celestial equator would coincide with the prime vertical, being a great circle passing through the east and west points. since all the diurnal circles are parallel to the equator, consequently, they would all, like the equator be perpendicular to the horizon. such a view of the heavenly bodies is called a right sphere, which may be thus defined: _a right sphere is one in which all the daily revolutions of the stars are in circles perpendicular to the horizon_. [illustration fig. .] a right sphere is seen only at the equator. any star situated in the celestial equator would appear to rise directly in the east, at midnight to be in the zenith of the spectator, and to set directly in the west. in proportion as stars are at a greater distance from the equator towards the pole, they describe smaller and smaller circles, until, near the pole, their motion is hardly perceptible. if the spectator advances one degree from the equator towards the north pole, his horizon reaches one degree beyond the pole of the earth, and cuts the starry sphere one degree below the pole of the heavens, or below the north star, if that be taken as the place of the pole. as he moves onward towards the pole, his horizon continually reaches further and further beyond it, until, when he comes to the pole of the earth, and under the pole of the heavens, his horizon reaches on all sides to the equator, and coincides with it. moreover, since all the circles of daily motion are parallel to the equator, they become, to the spectator at the pole, parallel to the horizon. or, _a parallel sphere is that in which all the circles of daily motion are parallel to the horizon_. to render this view of the heavens familiar, i would advise you to follow round in mind a number of separate stars, in their diurnal revolution, one near the horizon, one a few degrees above it, and a third near the zenith. to one who stood upon the north pole, the stars of the northern hemisphere would all be perpetually in view when not obscured by clouds, or lost in the sun's light, and none of those of the southern hemisphere would ever be seen. the sun would be constantly above the horizon for six months in the year, and the remaining six continually out of sight. that is, at the pole, the days and nights are each six months long. the appearances at the south pole are similar to those at the north. a perfect parallel sphere can never be seen, except at one of the poles,--a point which has never been actually reached by man; yet the british discovery ships penetrated within a few degrees of the north pole, and of course enjoyed the view of a sphere nearly parallel. as the circles of daily motion are parallel to the horizon of the pole, and perpendicular to that of the equator, so at all places between the two, the diurnal motions are oblique to the horizon. this aspect of the heavens constitutes an oblique sphere, which is thus defined: _an oblique sphere is that in which the circles of daily motion are oblique to the horizon_. suppose, for example, that the spectator is at the latitude of fifty degrees. his horizon reaches fifty degrees beyond the pole of the earth, and gives the same apparent elevation to the pole of the heavens. it cuts the equator and all the circles of daily motion, at an angle of forty degrees,--being always equal to what the altitude of the pole lacks of ninety degrees: that is, it is always equal to the co-altitude of the pole. thus, let h o, fig. , represent the horizon, e q the equator, and p p´ the axis of the earth. also, _l l, m m, n n_, parallels of latitude. then the horizon of a spectator at z, in latitude fifty degrees, reaches to fifty degrees beyond the pole; and the angle e c h, which the equator makes with the horizon, is forty degrees,--the complement of the latitude. as we advance still further north, the elevation of the diurnal circle above the horizon grows less and less, and consequently, the motions of the heavenly bodies more and more oblique to the horizon, until finally, at the pole, where the latitude is ninety degrees, the angle of elevation of the equator vanishes, and the horizon and the equator coincide with each other, as before stated. [illustration fig. .] _the circle of perpetual apparition is the boundary of that space around the elevated pole, where the stars never set._ its distance from the pole is equal to the latitude of the place. for, since the altitude of the pole is equal to the latitude, a star, whose polar distance is just equal to the latitude, will, when at its lowest point, only just reach the horizon; and all the stars nearer the pole than this will evidently not descend so far as the horizon. thus _m m_, fig. , is the circle of perpetual apparition, between which and the north pole, the stars never set, and its distance from the pole, o p, is evidently equal to the elevation of the pole, and of course to the latitude. in the opposite hemisphere, a similar part of the sphere adjacent to the depressed pole never rises. hence, _the circle of perpetual occultation is the boundary of that space around the depressed pole, within which the stars never rise._ thus _m´ m´_, fig. , is the circle of perpetual occultation, between which and the south pole, the stars never rise. in an oblique sphere, the horizon cuts the circles of daily motion unequally. towards the elevated pole, more than half the circle is above the horizon, and a greater and greater portion, as the distance from the equator is increased, until finally, within the circle of perpetual apparition, the whole circle is above the horizon. just the opposite takes place in the hemisphere next the depressed pole. accordingly, when the sun is in the equator, as the equator and horizon, like all other great circles of the sphere, bisect each other, the days and nights are equal all over the globe. but when the sun is north of the equator, the days become longer than the nights, but shorter, when the sun is south of the equator. moreover, the higher the latitude, the greater is the inequality in the lengths of the days and nights. by examining fig. , you will easily see how each of these cases must hold good. most of the appearances of the diurnal revolution can be explained, either on the supposition that the celestial sphere actually turns around the earth once in twenty-four hours, or that this motion of the heavens is merely apparent, arising from the revolution of the earth on its axis, in the opposite direction,--a motion of which we are insensible, as we sometimes lose the consciousness of our own motion in a ship or steam-boat, and observe all external objects to be receding from us, with a common motion. proofs, entirely conclusive and satisfactory, establish the fact, that it is the earth, and not the celestial sphere, that turns; but these proofs are drawn from various sources, and one is not prepared to appreciate their value, or even to understand some of them, until he has made considerable proficiency in the study of astronomy, and become familiar with a great variety of astronomical phenomena. to such a period we will therefore postpone the discussion of the earth's rotation on its axis. while we retain the same place on the earth, the diurnal revolution occasions no change in our horizon, but our horizon goes round, as well as ourselves. let us first take our station on the equator, at sunrise; our horizon now passes through both the poles and through the sun, which we are to conceive of as at a great distance from the earth, and therefore as cut, not by the terrestrial, but by the celestial, horizon. as the earth turns, the horizon dips more and more below the sun, at the rate of fifteen degrees for every hour; and, as in the case of the polar star, the sun appears to rise at the same rate. in six hours, therefore, it is depressed ninety degrees below the sun, bringing us directly under the sun, which, for our present purpose, we may consider as having all the while maintained the same fixed position in space. the earth continues to turn, and in six hours more, it completely reverses the position of our horizon, so that the western part of the horizon, which at sunrise was diametrically opposite to the sun, now cuts the sun, and soon afterwards it rises above the level of the sun, and the sun sets. during the next twelve hours, the sun continues on the invisible side of the sphere, until the horizon returns to the position from which it set out, and a new day begins. let us next contemplate the similar phenomena at the _poles_. here the horizon, coinciding, as it does, with the equator, would cut the sun through its centre and the sun would appear to revolve along the surface of the sea, one half above and the other half below the horizon. this supposes the sun in its annual revolution to be at one of the equinoxes. when the sun is north of the equator, it revolves continually round in a circle, which, during a single revolution, appears parallel to the equator, and it is constantly day; and when the sun is south of the equator, it is, for the same reason, continual night. when we have gained a clear idea of the appearances of the diurnal revolutions, as exhibited to a spectator at the equator and at the pole, that is, in a right and in a parallel sphere, there will be little difficulty in imagining how they must be in the intermediate latitudes, which have an oblique sphere. the appearances of the sun and stars, presented to the inhabitants of different countries, are such as correspond to the sphere in which they live. thus, in the fervid climates of india, africa, and south america, the sun mounts up to the highest regions of the heavens, and descends directly downwards, suddenly plunging beneath the horizon. his rays, darting almost vertically upon the heads of the inhabitants, strike with a force unknown to the people of the colder climates; while in places remote from the equator, as in the north of europe, the sun, in summer, rises very far in the north, takes a long circuit towards the south, and sets as far northward in the west as the point where it rose on the other side of the meridian. as we go still further north, to the northern parts of norway and sweden, for example, to the confines of the frigid zone, the summer's sun just grazes the northern horizon, and at noon appears only twenty-three and one half degrees above the southern. on the other hand, in mid-winter, in the north of europe, as at st. petersburgh, the day dwindles almost to nothing,--lasting only while the sun describes a very short arc in the extreme south. in some parts of siberia and iceland, the only day consists of a little glimmering of the sun on the verge of the southern horizon, at noon. letter ix. parallax and refraction. "go, wondrous creature! mount where science guides, go measure earth, weigh air, and state the tides; instruct the planets in what orbs to run, correct old time, and regulate the sun."--_pope._ i think you must have felt some astonishment, that astronomers are able to calculate the exact distances and magnitudes of the sun, moon, and planets. we should, at the first thought, imagine that such knowledge as this must be beyond the reach of the human faculties, and we might be inclined to suspect that astronomers practise some deception in this matter, for the purpose of exciting the admiration of the unlearned. i will therefore, in the present letter, endeavor to give you some clear and correct views respecting the manner in which astronomers acquire this knowledge. in our childhood, we all probably adopt the notion that the sky is a real dome of definite surface, in which the heavenly bodies are fixed. when any objects are beyond a certain distance from the eye, we lose all power of distinguishing, by our sight alone, between different distances, and cannot tell whether a given object is one million or a thousand millions of miles off. although the bodies seen in the sky are in fact at distances extremely various,--some, as the clouds, only a few miles off; others, as the moon, but a few thousand miles; and others, as the fixed stars, innumerable millions of miles from us,--yet, as our eye cannot distinguish these different distances, we acquire the habit of referring all objects beyond a moderate height to one and the same surface, namely, an imaginary spherical surface, denominated the celestial vault. thus, the various objects represented in the diagram on next page, though differing very much in shape and diameter, would all be _projected_ upon the sky alike, and compose a part, indeed, of the imaginary vault itself. the place which each object occupies is determined by lines drawn from the eye of the spectator through the extremities of the body, to meet the imaginary concave sphere. thus, to a spectator at o, fig , the several lines a b, c d, and e f, would all be projected into arches on the face of the sky, and be seen as parts of the sky itself, as represented by the lines a´ b´, c´ d´, and e´ f´. and were a body actually to move in the several directions indicated by these lines, they would appear to the spectator to describe portions of the celestial vault. thus, even when moving through the crooked line, from _a_ to _b_, a body would appear to be moving along the face of the sky, and of course in a regular curve line, from _c_ to _d_. [illustration fig. .] but, although all objects, beyond a certain moderate height, are projected on the imaginary surface of the sky, yet different spectators will project the same object on _different parts_ of the sky. thus, a spectator at a, fig. , would see a body, c, at m, while a spectator at b would see the same body at n. this change of place in a body, as seen from different points, is called parallax, which is thus defined: _parallax is the apparent change of place which bodies undergo by being viewed from different points_. [illustration fig. .] the arc m n is called the _parallactic arc_, and the angle a c b, the _parallactic angle_. it is plain, from the figure, that near objects are much more affected by parallax than distant ones. thus, the body c, fig. , makes a much greater parallax than the more distant body d,--the former being measured by the arc m n, and the latter by the arc o p. we may easily imagine bodies to be so distant, that they would appear projected at very nearly the same point of the heavens, when viewed from places very remote from each other. indeed, the fixed stars, as we shall see more fully hereafter, are so distant, that spectators, a hundred millions of miles apart, see each star in one and the same place in the heavens. it is by means of parallax, that astronomers find the distances and magnitudes of the heavenly bodies. in order fully to understand this subject, one requires to know something of trigonometry, which science enables us to find certain unknown parts of a triangle from certain other parts which are known. although you may not be acquainted with the principles of trigonometry, yet you will readily understand, from your knowledge of arithmetic, that from certain things given in a problem others may be found. every triangle has of course three sides and three angles; and, if we know two of the angles and one of the sides, we can find all the other parts, namely, the remaining angle and the two unknown sides. thus, in the triangle a b c, fig. , if we know the length of the side a b, and how many degrees each of the angles a b c and b c a contains, we can find the length of the side b c, or of the side a c, and the remaining angle at a. now, let us apply these principles to the measurements of some of the heavenly bodies. [illustration fig. .] [illustration fig. .] in fig. , let a represent the earth, c h the horizon, and h z a quadrant of a great circle of the heavens, extending from the horizon to the zenith; and let e, f, g, o, be successive positions of the moon, at different elevations, from the horizon to the meridian. now, a spectator on the surface of the earth, at a, would refer the moon, when at e, to _h_, on the face of the sky, whereas, if seen from the centre of the earth, it would appear at h. so, when the moon was at f, a spectator at a would see it at _p_, while, if seen from the centre, it would have appeared at p. the parallactic arcs, h _h_, p _p_, r _r_, grow continually smaller and smaller, as a body is situated higher above the horizon; and when the body is in the zenith, then the parallax vanishes altogether, for at o the moon would be seen at z, whether viewed from a or c. since, then, a heavenly body is liable to be referred to different points on the celestial vault, when seen from different parts of the earth, and thus some confusion be occasioned in the determination of points on the celestial sphere, astronomers have agreed to consider the true place of a celestial object to be that where it would appear, if seen from the centre of the earth; and the doctrine of parallax teaches how to reduce observations made at any place on the surface of the earth, to such as they would be, if made from the centre. when the moon, or any heavenly body, is seen in the horizon, as at e, the change of place is called the horizontal parallax. thus, the angle a e c, measures the horizontal parallax of the moon. were a spectator to view the earth from the centre of the moon, he would see the semidiameter of the earth under this same angle; hence, _the horizontal parallax of any body is the angle subtended by the semidiameter of the earth, as seen from the body_. please to remember this fact. it is evident from the figure, that the effect of parallax upon the place of a celestial body is to _depress_ it. thus, in consequence of parallax, e is depressed by the arc h _h_; f, by the arc p _p_; g, by the arc r _r_; while o sustains no change. hence, in all calculations respecting the altitude of the sun, moon, or planets, the amount of parallax is to be added: the stars, as we shall see hereafter, have no sensible parallax. it is now very easy to see how, when the parallax of a body is known, we may find its distance from the centre of the earth. thus, in the triangle a c e, fig. , the side a c is known, being the semidiameter of the earth; the angle c a e, being a right angle, is also known; and the parallactic angle, a e c, is found from observation; and it is a well-known principle of trigonometry, that when we have any two angles of a triangle, we may find the remaining angle by subtracting the sum of these two from one hundred and eighty degrees. consequently, in the triangle a e c, we know all the angles and one side, namely, the side a c; hence, we have the means of finding the side c e, which is the distance from the centre of the earth to the centre of the moon. [illustration fig. .] when the distance of a heavenly body is known, and we can measure, with instruments, its angular breadth, we can easily determine its _magnitude_. thus, if we have the distance of the moon, e s, fig. , and half the breadth of its disk s c, (which is measured by the angle s e c,) we can find the length of the line, s c, in miles. twice this line is the diameter of the body; and when we know the diameter of a sphere, we can, by well-known rules, find the contents of the surface, and its solidity. you will perhaps be curious to know, _how the moon's horizontal parallax is found_; for it must have been previously ascertained, before we could apply this method to finding the distance of the moon from the earth. suppose that two astronomers take their stations on the same meridian, but one south of the equator, as at the cape of good hope, and another north of the equator, as at berlin, in prussia, which two places lie nearly on the same meridian. the observers would severally refer the moon to different points on the face of the sky,--the southern observer carrying it further north, and the northern observer further south, than its true place, as seen from the centre of the earth. this will be plain from the diagram, fig. . if a and b represent the positions of the spectators, m the moon, and c d an arc of the sky, then it is evident, that c d would be the parallactic arc. [illustration fig. .] these observations furnish materials for calculating, by the aid of trigonometry, the moon's horizontal parallax, and we have before seen how, when we know the parallax of a heavenly body, we can find both its distance from the earth and its magnitude. beside the change of place which these heavenly bodies undergo, in consequence of parallax, there is another, of an opposite kind, arising from the effect of the atmosphere, called _refraction_. refraction elevates the apparent place of a body, while parallax depresses it. it affects alike the most distant as well as nearer bodies. in order to understand the nature of refraction, we must consider, that an object always appears in the direction in which the _last_ ray of light comes to the eye. if the light which comes from a star were bent into fifty directions before it reached the eye, the star would nevertheless appear in the line described by the ray nearest the eye. the operation of this principle is seen when an oar, or any stick, is thrust into water. as the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance,--the part below the water having apparently a different direction from the part above. thus, in fig. , page , if s _a x_ be the oar, s _a b_ will be the bent appearance, as affected by refraction. the transparent substance through which any ray of light passes is called a _medium_. it is a general fact in optics, that, when light passes out of a rarer into a denser medium, as out of air into water, or out of space into air, it is turned _towards_ a perpendicular to the surface of the medium; and when it passes out of a denser into a rarer medium, as out of water into air, it is turned _from_ the perpendicular. in the above case, the light, passing out of space into air, is turned towards the radius of the earth, this being perpendicular to the surface of the atmosphere; and it is turned more and more towards that radius the nearer it approaches to the earth, because the density of the air rapidly increases near the earth. [illustration fig. .] let us now conceive of the atmosphere as made up of a great number of parallel strata, as a a, b b, c c, and d d, increasing rapidly in density (as is known to be the fact) in approaching near to the surface of the earth. let s be a star, from which a ray of light, s _a_, enters the atmosphere at _a_, where, being much turned towards the radius of the convex surface, it would change its direction into the line _a b_, and again into _b c_, and _c_ o, reaching the eye at o. now, since an object always appears in the direction in which the light finally strikes the eye, the star would be seen in the direction o _c_, and, consequently, the star would apparently change its place, by refraction, from s to s´, being elevated out of its true position. moreover, since, on account of the continual increase of density in descending through the atmosphere, the light would be continually turned out of its course more and more, it would therefore move, not in the polygon represented in the figure, but in a corresponding curve line, whose curvature is rapidly increased near the surface of the earth. when a body is in the zenith, since a ray of light from it enters the atmosphere at right angles to the refracting medium, it suffers no refraction. consequently, the position of the heavenly bodies, when in the zenith, is not changed by refraction, while, near the horizon, where a ray of light strikes the medium very obliquely, and traverses the atmosphere through its densest part, the refraction is greatest. the whole amount of refraction, when a body is in the horizon, is thirty-four minutes; while, at only an elevation of one degree, the refraction is but twenty-four minutes; and at forty-five degrees, it is scarcely a single minute. hence it is always important to make our observations on the heavenly bodies when they are at as great an elevation as possible above the horizon, being then less affected by refraction than at lower altitudes. since the whole amount of refraction near the horizon exceeds thirty-three minutes, and the diameters of the sun and moon are severally less than this, these luminaries are in view both before they have actually risen and after they have set. the rapid increase of refraction near the horizon is strikingly evinced by the _oval_ figure which the sun assumes when near the horizon, and which is seen to the greatest advantage when light clouds enable us to view the solar disk. were all parts of the sun equally raised by refraction, there would be no change of figure; but, since the lower side is more refracted than the upper, the effect is to shorten the vertical diameter, and thus to give the disk an oval form. this effect is particularly remarkable when the sun, at his rising or setting, is observed from the top of a mountain, or at an elevation near the seashore; for in such situations, the rays of light make a greater angle than ordinary with a perpendicular to the refracting medium, and the amount of refraction is proportionally greater. in some cases of this kind, the shortening of the vertical diameter of the sun has been observed to amount to six minutes, or about one fifth of the whole. the apparent enlargement of the sun and moon, when near the horizon, arises from an optical illusion. these bodies, in fact, are not seen under so great an angle when in the horizon as when on the meridian, for they are nearer to us in the latter case than in the former. the distance of the sun, indeed, is so great, that it makes very little difference in his apparent diameter whether he is viewed in the horizon or on the meridian; but with the moon, the case is otherwise; its angular diameter, when measured with instruments, is perceptibly larger when at its culmination, or highest elevation above the horizon. why, then, do the sun and moon appear so much larger when near the horizon? it is owing to a habit of the mind, by which we judge of the magnitudes of distant objects, not merely by the angle they subtend at the eye, but also by our impressions respecting their distance, allowing, under a given angle, a greater magnitude as we imagine the distance of a body to be greater. now, on account of the numerous objects usually in sight between us and the sun, when he is near the horizon, he appears much further removed from us than when on the meridian; and we unconsciously assign to him a proportionally greater magnitude. if we view the sun, in the two positions, through a smoked glass, no such difference of size is observed; for here no objects are seen but the sun himself. _twilight_ is another phenomenon depending on the agency of the earth's atmosphere. it is that illumination of the sky which takes place just before sunrise and which continues after sunset. it is owing partly to refraction, and partly to reflection, but mostly to the latter. while the sun is within eighteen degrees of the horizon, before it rises or after it sets, some portion of its light is conveyed to us, by means of numerous reflections from the atmosphere. at the equator, where the circles of daily motion are perpendicular to the horizon, the sun descends through eighteen degrees in an hour and twelve minutes. the light of day, therefore, declines rapidly, and as rapidly advances after daybreak in the morning. at the pole, a constant twilight is enjoyed while the sun is within eighteen degrees of the horizon, occupying nearly two thirds of the half year when the direct light of the sun is withdrawn, so that the progress from continual day to constant night is exceedingly gradual. to an inhabitant of an oblique sphere, the twilight is longer in proportion as the place is nearer the elevated pole. were it not for the power the atmosphere has of dispersing the solar light, and scattering it in various directions, no objects would be visible to us out of direct sunshine; every shadow of a passing cloud would involve us in midnight darkness; the stars would be visible all day; and every apartment into which the sun had not direct admission would be involved in the obscurity of night. this scattering action of the atmosphere on the solar light is greatly increased by the irregularity of temperature caused by the sun, which throws the atmosphere into a constant state of undulation; and by thus bringing together masses of air of different temperatures, produces partial reflections and refractions at their common boundaries, by which means much light is turned aside from a direct course, and diverted to the purposes of general illumination.[ ] in the upper regions of the atmosphere, as on the tops of very high mountains, where the air is too much rarefied to reflect much light, the sky assumes a black appearance, and stars become visible in the day time. although the atmosphere is usually so transparent, that it is invisible to us, yet we as truly move and live in a fluid as fishes that swim in the sea. considered in comparison with the whole earth, the atmosphere is to be regarded as a thin layer investing the surface, like a film of water covering the surface of an orange. its actual height, however, is over a hundred miles, though we cannot assign its precise boundaries. being perfectly elastic, the lower portions, bearing as they do, the weight of all the mass above them, are greatly compressed, while the upper portions having little to oppose the natural tendency of air to expand, diffuse themselves widely. the consequence is, that the atmosphere undergoes a rapid diminution of density, as we ascend from the earth, and soon becomes exceedingly rare. at so moderate a height as seven miles, it is four times rarer than at the surface, and continues to grow rare in the same proportion, namely, being four times less for every seven miles of ascent. it is only, therefore, within a few miles of the earth, that the atmosphere is sufficiently dense to sustain clouds and vapors, which seldom rise so high as eight miles, and are usually much nearer to the earth than this. so rare does the air become on the top of mount chimborazo, in south america, that it is incompetent to support most of the birds that fly near the level of the sea. the condor, a bird which has remarkably long wings, and a light body, is the only bird seen towering above this lofty summit. the transparency of the atmosphere,--a quality so essential to fine views of the starry heavens,--is much increased by containing a large proportion of water, provided it is perfectly dissolved, or in a state of invisible vapor. a country at once hot and humid, like some portions of the torrid zone, presents a much brighter and more beautiful view of the moon and stars, than is seen in cold climates. before a copious rain, especially in hot weather, when the atmosphere is unusually humid, we sometimes observe the sky to be remarkably resplendent, even in our own latitude. accordingly, this unusual clearness of the sky, when the stars shine with unwonted brilliancy, is regarded as a sign of approaching rain; and when, after the rain is apparently over, the air is remarkably transparent, and distant objects on the earth are seen with uncommon distinctness, while the sky exhibits an unusually deep azure, we may conclude that the serenity is only temporary, and that the rain will probably soon return. footnote: [ ] sir j. herschel. letter x. the sun. "great source of day! best image here below of thy creator, ever pouring wide, from world to world, the vital ocean round, on nature write, with every beam, his praise."--_thomson._ the subjects which have occupied the preceding letters are by no means the most interesting parts of our science. they constitute, indeed, little more than an introduction to our main subject, but comprise such matters as are very necessary to be clearly understood, before one is prepared to enter with profit and delight upon the more sublime and interesting field which now opens before us. we will begin our survey of the heavenly bodies with the sun, which first claims our homage, as the natural monarch of the skies. the moon will next occupy our attention; then the other bodies which compose the solar system, namely, the planets and comets; and, finally, we shall leave behind this little province in the great empire of nature, and wing a bolder flight to the fixed stars. the _distance_ of the sun from the earth is about ninety-five millions of miles. it may perhaps seem incredible to you, that astronomers should be able to determine this fact with any degree of certainty. some, indeed, not so well informed as yourself, have looked upon the marvellous things that are told respecting the distances, magnitudes, and velocities, of the heavenly bodies, as attempts of astronomers to impose on the credulity of the world at large; but the certainty and exactness with which the predictions of astronomers are fulfilled, as an eclipse, for example, ought to inspire full confidence in their statements. i can assure you, my dear friend, that the evidence on which these statements are founded is perfectly satisfactory to those whose attainments in the sciences qualify them to understand them; and, so far are astronomers from wishing to impose on the unlearned, by announcing such wonderful discoveries as they have made among the heavenly bodies, no class of men have ever shown a stricter regard and zeal than they for the exact truth, wherever it is attainable. ninety-five millions of miles is indeed a vast distance. no human mind is adequate to comprehend it fully; but the nearest approaches we can make towards it are gained by successive efforts of the mind to conceive of great distances, beginning with such as are clearly within our grasp. let us, then, first take so small a distance as that of the breadth of the atlantic ocean, and follow, in mind, a ship, as she leaves the port of new york, and, after twenty days' steady sail, reaches liverpool. having formed the best idea we are able of this distance, we may then reflect, that it would take a ship, moving constantly at the rate of ten miles per hour, more than a thousand years to reach the sun. the diameter of the sun is towards a million of miles; or, more exactly, it is eight hundred and eighty-five thousand miles. one hundred and twelve bodies as large as the earth, lying side by side, would be required to reach across the solar disk; and our ship, sailing at the same rate as before, would be ten years in passing over the same space. immense as is the sun, we can readily understand why it appears no larger than it does, when we reflect, that its distance is still more vast. even large objects on the earth, when seen on a distant eminence, or over a wide expanse of water, dwindle almost to a point. could we approach nearer and nearer to the sun, it would constantly expand its volume, until finally it would fill the whole vault of heaven. we could, however, approach but little nearer to the sun without being consumed by the intensity of his heat. whenever we come nearer to any fire, the heat rapidly increases, being four times as great at half the distance, and one hundred times as great at one tenth the distance. this fact is expressed by saying, that the heat increases as the square of the distance decreases. our globe is situated at such a distance from the sun, as exactly suits the animal and vegetable kingdoms. were it either much nearer or much more remote, they could not exist, constituted as they are. the intensity of the solar light also follows the same law. consequently, were we nearer to the sun than we are, its blaze would be insufferable; or, were we much further off, the light would be too dim to serve all the purposes of vision. the sun is one million four hundred thousand times as large as the earth; but its matter is not more than about one fourth as dense as that of the earth, being only a little heavier than water, while the average density of the earth is more than five times that of water. still, on account of the immense magnitude of the sun, its entire quantity of matter is three hundred and fifty thousand times as great as that of the earth. now, the force of gravity in a body is greater, in proportion as its quantity of matter is greater. consequently, we might suppose, that the weight of a body (weight being nothing else than the measure of the force of gravity) would be increased in the same proportion; or, that a body, which weighs only one pound at the surface of the earth, would weigh three hundred and fifty thousand pounds at the sun. but we must consider, that the attraction exerted by any body is the same as though all the matter were concentrated in the centre. thus, the attraction exerted by the earth and by the sun is the same as though the entire matter of each body were in its centre. hence, on account of the vast dimensions of the sun, its surface is one hundred and twelve times further from its centre than the surface of the earth is from its centre; and, since the force of gravity diminishes as the square of the distance increases, that of the sun, exerted on bodies at its surface, is (so far as this cause operates) the square of one hundred and twelve, or twelve thousand five hundred and forty-four times less than that of the earth. if, therefore, we increase the weight of a body three hundred and fifty-four thousand times, in consequence of the greater amount of matter in the sun, and diminish it twelve thousand five hundred and forty-four times, in consequence of the force acting at a greater distance from the body, we shall find that the body would weigh about twenty-eight times more on the sun than on the earth. hence, a man weighing three hundred pounds would, if conveyed to the surface of the sun, weigh eight thousand four hundred pounds, or nearly three tons and three quarters. a limb of our bodies, weighing forty pounds, would require to lift it a force of one thousand one hundred and twenty pounds, which would be beyond the ordinary power of the muscles. at the surface of the earth, a body falls from rest by the force of gravity, in one second, sixteen and one twelfth feet; but at the surface of the sun, a body would, in the same time, fall through four hundred and forty-eight and seven tenths feet. the sun turns on his own axis once in a little more than twenty-five days. this fact is known by observing certain dark places seen by the telescope on the sun's disk, called _solar spots_. these are very variable in size and number. sometimes, the sun's disk, when viewed with a telescope, is quite free from spots, while at other times we may see a dozen or more distinct clusters, each containing a great number of spots, some large and some very minute. occasionally, a single spot is so large as to be visible to the naked eye, especially when the sun is near the horizon, and the glare of his light is taken off. one measured by dr. herschel was no less than fifty thousand miles in diameter. a solar spot usually consists of two parts, the _nucleus_ and the _umbra_. the nucleus is black, of a very irregular shape, and is subject to great and sudden changes, both in form and size. spots have sometimes seemed to burst asunder, and to project fragments in different directions. the umbra is a wide margin, of lighter shade, and is often of greater extent than the nucleus. the spots are usually confined to a zone extending across the central regions of the sun, not exceeding sixty degrees in breadth. fig. exhibits the appearance of the solar spots. as these spots have all a common motion from day to day, across the sun's disk; as they go off at one limb, and, after a certain interval, sometimes come on again on the opposite limb, it is inferred that this apparent motion is imparted to them by an actual revolution of the sun on his own axis. we know that the spots must be in contact, or very nearly so, at least, with the body of the sun, and cannot be bodies revolving around it, which are projected on the solar disk when they are between us and the sun; for, in that case, they would not be so long in view as out of view, as will be evident from inspecting the following diagram. let s, fig. , page , represent the sun, and _b_ a body revolving round it in the orbit _a b c_; and let e represent the earth, where, of course, the spectator is situated. the body would be seen projected on the sun only while passing from _b_ to _c_, while, throughout the remainder of its orbit, it would be out of view, whereas no such inequality exists in respect to the two periods. [illustration fig. .] [illustration fig. .] if you ask, what is the _cause_ of the solar spots, i can only tell you what different astronomers have supposed respecting them. they attracted the notice of galileo soon after the invention of the telescope, and he formed an hypothesis respecting their nature. supposing the sun to consist of a solid body embosomed in a sea of liquid fire, he believed that the spots are composed of black cinders, formed in the interior of the sun by volcanic action, which rise and float on the surface of the fiery sea. the chief objections to this hypothesis are, first, the _vast extent_ of some of the spots, covering, as they do, two thousand millions of square miles, or more,--a space which it is incredible should be filled by lava in so short a time as that in which the spots are sometimes formed; and, secondly, the _sudden disappearance_ which the spots sometimes undergo, a fact which can hardly be accounted for by supposing, as galileo did, that such a vast accumulation of matter all at once sunk beneath the fiery flood. moreover, we have many reasons for believing that the spots are _depressions_ below the general surface. la lande, an eminent french astronomer of the last century, held that the sun is a solid, opaque body, having its exterior diversified with high mountains and deep valleys, and covered all over with a burning sea of liquid matter. the spots he supposed to be produced by the flux and reflux of this fiery sea, retreating occasionally from the mountains, and exposing to view a portion of the dark body of the sun. but it is inconsistent with the nature of fluids, that a liquid, like the sea supposed, should depart so far from its equilibrium and remain so long fixed, as to lay bare the immense spaces occupied by some of the solar spots. dr. herschel's views respecting the nature and constitution of the sun, embracing an explanation of the solar spots, have, of late years, been very generally received by the astronomical world. this great astronomer, after attentively viewing the surface of the sun, for a long time, with his large telescopes, came to the following conclusions respecting the nature of this luminary. he supposes the sun to be a planetary body like our earth, diversified with mountains and valleys, to which, on account of the magnitude of the sun, he assigns a prodigious extent, some of the mountains being six hundred miles high, and the valleys proportionally deep. he employs in his explanation no volcanic fires, but supposes two separate regions of dense clouds floating in the solar atmosphere, at different distances from the sun. the exterior stratum of clouds he considers as the depository of the sun's light and heat, while the inferior stratum serves as an awning or screen to the body of the sun itself, which thus becomes fitted to be the residence of animals. the proofs offered in support of this hypothesis are chiefly the following: first, that the appearances, as presented to the telescope, are such as accord better with the idea that the fluctuations arise from the motions of clouds, than that they are owing to the agitations of a liquid, which could not depart far enough from its general level to enable us to see its waves at so great a distance, where a line forty miles in length would subtend an angle at the eye of only the tenth part of a second; secondly, that, since clouds are easily dispersed to any extent, the great dimensions which the solar spots occasionally exhibit are more consistent with this than with any other hypothesis; and, finally, that a lower stratum of clouds, similar to those of our atmosphere, was frequently seen by the doctor, far below the luminous clouds which are the fountains of light and heat. such are the views of one who had, it must be acknowledged, great powers of observation, and means of observation in higher perfection than have ever been enjoyed by any other individual; but, with all deference to such authority, i am compelled to think that the hypothesis is encumbered with very serious objections. clouds analogous to those of our atmosphere (and the doctor expressly asserts that his lower stratum of clouds are analogous to ours, and reasons respecting the upper stratum according to the same analogy) cannot exist in hot air; they are tenants only of cold regions. how can they be supposed to exist in the immediate vicinity of a fire so intense, that they are even dissipated by it at the distance of ninety-five millions of miles? much less can they be supposed to be the depositories of such devouring fire, when any thing in the form of clouds, floating in our atmosphere, is at once scattered and dissolved by the accession of only a few degrees of heat. nothing, moreover, can be imagined more unfavorable for radiating heat to such a distance, than the light, inconstant matter of which clouds are composed, floating loosely in the solar atmosphere. there is a logical difficulty in the case: it is ascribing to things properties which they are not known to possess; nay, more, which they are known not to possess. from variations of light and shade in objects seen at moderate distances on the earth, we are often deceived in regard to their appearances; and we must distrust the power of an astronomer to decide upon the nature of matter seen at the distance of ninety-five millions of miles. i think, therefore, we must confess our ignorance of the nature and constitution of the sun; nor can we, as astronomers, obtain much more satisfactory knowledge respecting it than the common apprehension, namely, that it is an immense globe of fire. we have not yet learned what causes are in operation to maintain its undecaying fires; but our confidence in the divine wisdom and goodness leads us to believe, that those causes are such as will preserve those fires from extinction, and at a nearly uniform degree of intensity. any material change in this respect would jeopardize the safety of the animal and vegetable kingdoms, which could not exist without the enlivening influence of the solar heat, nor, indeed, were that heat any more or less intense than it is at present. if we inquire whether the surface of the sun is in a state of actual combustion, like burning fuel, or merely in a state of intense ignition, like a stone heated to redness in a furnace, we shall find it most reasonable to conclude that it is in a state of ignition. if the body of the sun were composed of combustible matter and were actually on fire, the material of the sun would be continually wasting away, while the products of combustion would fill all the vast surrounding regions, and obscure the solar light. but solid bodies may attain a very intense state of ignition, and glow with the most fervent heat, while none of their material is consumed, and no clouds or fumes rise to obscure their brightness, or to impede their further emission of heat. an ignited surface, moreover, is far better adapted than flame to the radiation of heat. flame, when made to act in contact with the surfaces of solid bodies, heats them rapidly and powerfully; but it sends forth, or _radiates_, very little heat, compared with solid matter in a high state of ignition. these various considerations render it highly probable to my mind, that the body of the sun is not in a state of actual combustion, but merely in a state of high ignition. the solar beam consists of a mixture of several different sorts of rays. first, there are the _calorific_ rays, which afford heat, and are entirely distinct from those which afford light, and may be separated from them. secondly, there are the _colorific_ rays, which give light, consisting of rays of seven distinct colors, namely, violet, indigo, blue, green, yellow, orange, red. these, when separated, as they may be by a glass prism, compose the _prismatic spectrum_. they appear also in the rainbow. when united again, in due proportions, they constitute white light, as seen in the light of the sun. thirdly, there are found in the solar beam a class of rays which afford neither heat nor light, but which produce chemical changes in certain bodies exposed to their influence, and hence are called _chemical_ rays. fourthly, there is still another class, called _magnetizing_ rays, because they are capable of imparting magnetic properties to steel. these different sorts of rays are sent forth from the sun, to the remotest regions of the planetary worlds, invigorating all things by their life-giving influence, and dispelling the darkness that naturally fills all space. but it was not alone to give heat and light, that the sun was placed in the firmament. by his power of attraction, also, he serves as the great regulator of the planetary motions, bending them continually from the straight line in which they tend to move, and compelling them to circulate around him, each at nearly a uniform distance, and all in perfect harmony. i will hereafter explain to you the manner in which the gravity of the sun thus acts, to control the planetary motions. for the present, let us content ourselves with reflecting upon the wonderful force which the sun must put forth, in order to bend out of their courses, into circular orbits, such a number of planets, some of which are more than a thousand times as large as the earth. were a ship of war under full sail, and it should be required to turn her aside from her course by a rope attached to her bow, we can easily imagine that it would take a great force to do it, especially were it required that the force should remain stationary and the ship be so constantly diverted from her course, as to be made to go round the force as round a centre. somewhat similar to this is the action which the sun exerts on each of the planets by some invisible influence, called gravitation. the bodies which he thus turns out of their course, and bends into a circular orbit around himself, are, however, many millions of times as ponderous as the ship, and are moving many thousand times as swiftly. letter xi. annual revolution.--seasons "these, as they change, almighty father, these are but the varied god. the rolling year is full of thee."--_thomson._ we have seen that the apparent revolution of the heavenly bodies, from east to west, every twenty-four hours, is owing to a real revolution of the earth on its own axis, in the opposite direction. this motion is very easily understood, resembling, as it does, the spinning of a top. we must, however, conceive of the top as turning without any visible support, and not as resting in the usual manner on a plane. the annual motion of the earth around the sun, which gives rise to an apparent motion of the sun around the earth once a year, and occasions the change of seasons, is somewhat more difficult to understand; and it may cost you some reflection, before you will settle all the points respecting the changes of the seasons clearly in your mind. we sometimes see these two motions exemplified in a top. when, as the string is pulled, the top is thrown forwards on the floor, we may see it move forward (sometimes in a circle) at the same time that it spins on its axis. let a candle be placed on a table, to represent the sun, and let these two motions be imagined to be given to a top around it, and we shall have a case somewhat resembling the actual motions of the earth around the sun. when bodies are at such a distance from each other as the earth and the sun, a spectator on either would project the other body upon the concave sphere of the heavens, always seeing it on the opposite side of a great circle one hundred and eighty degrees from himself. recollect that the path in which the earth moves round the sun is called the ecliptic. we are not to conceive of this, or of any other celestial circle, as having any real, palpable existence, any more than the path of a bird through the sky. you will perhaps think it quite superfluous for me to remind you of this; but, from the habit of seeing the orbits of the heavenly bodies represented in diagrams and orreries, by palpable lines and circles, we are apt inadvertently to acquire the notion, that the orbits of the planets, and other representations of the artificial sphere, have a real, palpable existence in nature; whereas, they denote the places where mere geometrical or imaginary lines run. you might have expected to see an orrery, exhibiting a view of the sun and planets, with their various motions, particularly described in my letter on astronomical instruments and apparatus. i must acknowledge, that i entertain a very low opinion of the utility of even the best orreries, and i cannot recommend them as auxiliaries in the study of astronomy. the numerous appendages usually connected with them, some to support them in a proper position, and some to communicate to them the requisite motions, enter into the ideas which the learner forms respecting the machinery of the heavens; and it costs much labor afterwards to divest the mind of such erroneous impressions. astronomy can be exhibited much more clearly and beautifully to the mental eye than to the visual organ. it is much easier to conceive of the sun existing in boundless space, and of the earth as moving around him at a great distance, the mind having nothing in view but simply these two bodies, than it is, in an orrery, to contemplate the motion of a ball representing the earth, carried by a complicated apparatus of wheels around another ball, supported by a cylinder or wire, to represent the sun. i would advise you, whenever it is practicable, to think how things are in nature, rather than how they are represented by art. the machinery of the heavens is much simpler than that of an orrery. in endeavoring to obtain a clear idea of the revolution of the earth around the sun, imagine to yourself a plane (a geometrical plane, having merely length and breadth, but no thickness) passing through the centres of the sun and the earth, and extended far beyond the earth till it reaches the firmament of stars. although, indeed, no such dome actually exists as that under which we figure to ourselves the vault of the sky, yet, as the fixed stars appear to be set in such a dome, we may imagine that the circles of the sphere, when indefinitely enlarged, finally reach such an imaginary vault. all that is essential is, that we should imagine this to exist far beyond the bounds of the solar system, the various bodies that compose the latter being situated close around the sun, at the centre. along the line where this great circle meets the starry vault, are situated a series of constellations,--aries, taurus, gemini, &c.,--which occupy successively this portion of the heavens. when bodies are at such a distance from each other as the sun and the earth, i have said that a spectator on either would project the other body upon the concave sphere of the heavens, always seeing it on the opposite side of a great circle one hundred and eighty degrees from himself. the place of a body, when viewed from any point, is denoted by the position it occupies among the stars. thus, in the diagram, fig. , page , when the earth arrives at e, it is said to be in aries, because, if viewed from the sun, it would be projected on that part of the heavens; and, for the same reason, to a spectator at e, the sun would be in libra. when the earth shifts its position from aries to taurus, as we are unconscious of our own motion, the sun it is that appears to move from libra to scorpio, in the opposite part of the heavens. hence, as we go forward, in the order of the signs, on one side of the ecliptic, the sun seems to be moving forward at the same rate on the opposite side of the same great circle; and therefore, although we are unconscious of our own motion, we can read it, from day to day, in the motions of the sun. if we could see the stars at the same time with the sun, we could actually observe, from day to day, the sun's progress through them, as we observe the progress of the moon at night; only the sun's rate of motion would be nearly fourteen times slower than that of the moon. although we do not see the stars when the sun is present, we can observe that it makes daily progress eastward, as is apparent from the constellations of the zodiac occupying, successively, the western sky immediately after sunset, proving that either all the stars have a common motion westward, independent of their diurnal motion, or that the sun has a motion past them from west to east. we shall see, hereafter, abundant evidence to prove, that this change in the relative position of the sun and stars, is owing to a change in the apparent place of the sun, and not to any change in the stars. [illustration fig. .] to form a clear idea of the two motions of the earth, imagine yourself standing on a circular platform which turns slowly round its centre. while you are carried slowly round the entire of the circuit of the heavens, along with the platform, you may turn round upon your heel the same way three hundred and sixty-five times. the former is analogous to our annual motion with the earth around the sun; the latter, to our diurnal revolution in common with the earth around its own axis. although the apparent revolution of the sun is in a direction opposite to the real motion of the earth, as regards absolute space, yet both are nevertheless from west to east, since these terms do not refer to any directions in absolute space, but to the order in which certain constellations (the constellations of the zodiac) succeed one another. the earth itself, on opposite sides of its orbit, does in fact move towards directly opposite points of space; but it is all the while pursuing its course in the order of the signs. in the same manner, although the earth turns on its axis from west to east, yet any place on the surface of the earth is moving in a direction in space exactly opposite to its direction twelve hours before. if the sun left a visible trace on the face of the sky, the ecliptic would of course be distinctly marked on the celestial sphere, as it is on an artificial globe; and were the equator delineated in a similar manner, we should then see, at a glance, the relative position of these two circles,--the points where they intersect one another, constituting the equinoxes; the points where they are at the greatest distance asunder, that is, the solstices; and various other particulars, which, for want of such visible traces, we are now obliged to search for by indirect and circuitous methods. it will aid you, to have constantly before your mental vision an imaginary delineation of these two important circles on the face of the sky. the equator makes an angle with the ecliptic of twenty-three degrees and twenty-eight minutes. this is called the obliquity of the ecliptic. as the sun and earth are both always in the ecliptic, and as the motion of the earth in one part of it makes the sun appear to move in the opposite part, at the same rate, the sun apparently descends, in winter, twenty-three degrees and twenty-eight minutes to the south of the equator, and ascends, in summer, the same number of degrees north of it. we must keep in mind, that the celestial equator and celestial ecliptic are here understood, and we may imagine them to be two great circles delineated on the face of the sky. on comparing observations made at different periods, for more than two thousand years, it is found, that the obliquity of the ecliptic is not constant, but that it undergoes a slight diminution, from age to age, amounting to fifty-two seconds in a century, or about half a second annually. we might apprehend that, by successive approaches to each other, the equator and ecliptic would finally coincide; but astronomers have discovered, by a most profound investigation, based on the principles of universal gravitation, that this irregularity is confined within certain narrow limits; and that the obliquity, after diminishing for some thousands of years, will then increase for a similar period, and will thus vibrate forever about a mean value. as the earth traverses every part of her orbit in the course of a year, she will be once at each solstice, and once at each equinox. the best way of obtaining a correct idea of her two motions is, to conceive of her as standing still for a single day, at some point in her orbit, until she has turned once on her axis, then moving about a degree, and halting again, until another diurnal revolution is completed. let us suppose the earth at the autumnal equinox, the sun, of course, being at the vernal equinox,--for we must always think of these two bodies as diametrically opposite to each other. suppose the earth to stand still in its orbit for twenty-four hours. the revolution of the earth on its axis, from west to east, will make the sun appear to describe a great circle of the heavens from east to west, coinciding with the equator. at the end of this period, suppose the sun to move northward one degree, and to remain there for twenty-four hours; in which time, the revolution of the earth, will make the sun appear to describe another circle, from east to west, parallel to the equator, but one degree north of it. thus, we may conceive of the sun as moving one degree north, every day, for about three months, when it will reach the point of the ecliptic furthest from the equator, which point is called the _tropic_, from a greek word, signifying _to turn_; because, after the sun has passed this point, his motion in his orbit carries him continually towards the equator, and therefore he seems to turn about. the same point is also called the _solstice_, from a latin word, signifying to _stand still_; since, when the sun has reached its greatest northern or southern limit, while its declination is at the point where it ceases to increase, but begins to decrease, there the sun seems for a short time stationary, with regard to the equator, appearing for several days to describe the same parallel of latitude. when the sun is at the northern tropic, which happens about the twenty-first of june, his elevation above the southern horizon at noon is the greatest in the year; and when he is at the southern tropic, about the twenty-first of december, his elevation at noon is the least in the year. the difference between these two meridian altitudes will give the whole distance from one tropic to the other, and consequently, twice the distance from each tropic to the equator. by this means, we find how far the tropic is from the equator, and that gives us the angle which the equator and ecliptic make with each other; for the greatest distance between any two great circles on the sphere is always equal to the angle which they make with each other. thus, the ancient astronomers were able to determine the obliquity of the ecliptic with a great degree of accuracy. it was easy to find the situation of the zenith, because the direction of a plumb-line shows us where that is; and it was easy to find the distances from the zenith where the sun was at the greatest and least distances; respectively. the difference of these two arcs is the angular distance from one tropic to the other; and half this arc is the distance of either tropic from the equator, and of course, equal to the obliquity of the ecliptic. all this will be very easily understood from the annexed diagram, fig. . let z be the zenith of a spectator situated at c; z _n_ the least, and z _s_ the greatest distance of the sun from the zenith. from z _s_ subtract z _n_, and then _s n_, the difference, divided by two, will give the obliquity of the ecliptic. [illustration fig. .] the motion of the earth in its orbit is nearly seventy times as great as its greatest motion around its axis. in its revolution around the sun, the earth moves no less than one million six hundred and forty thousand miles per day, sixty-eight thousand miles per hour, eleven hundred miles per minute, and nearly nineteen miles every second; a velocity nearly sixty times as great as the greatest velocity of a cannon ball. places on the earth turn with very different degrees of velocity in different latitudes. those near the equator are carried round on the circumference of a large circle; those towards the poles, on the circumference of a small circle; while one standing on the pole itself would not turn at all. those who live on the equator are carried about one thousand miles an hour. in our latitude, (forty-one degrees and eighteen minutes,) the diurnal velocity is about seven hundred and fifty miles per hour. it would seem, at first view, quite incredible, that we should be whirled round at so rapid a rate, and yet be entirely insensible of any motion; and much more, that we could be going so swiftly through space, in our circuit around the sun, while all things, when unaffected by local causes, appear to be in such a state of quiescence. yet we have the most unquestionable evidence of the fact; nor is it difficult to account for it, in consistency with the general state of repose among bodies on the earth, when we reflect that their relative motions, with respect to each other, are not in the least disturbed by any motions which they may have in common. when we are on board a steam-boat, we move about in the same manner when the boat is in rapid motion, as when it is lying still; and such would be the case, if it moved steadily a hundred times faster than it does. were the earth, however, suddenly to stop its diurnal revolution, all movable bodies on its surface would be thrown off in tangents to the surface with velocities proportional to that of their diurnal motion; and were the earth suddenly to halt in its orbit, we should be hurled forward into space with inconceivable rapidity. i will next endeavor to explain to you the phenomena of the _seasons_. these depend on two causes; first, the inclination of the earth's axis to the plane of its orbit; and, secondly, to the circumstance, that the axis always remains parallel to itself. imagine to yourself a candle placed in the centre of a ring, to represent the sun in the centre of the earth's orbit, and an apple with a knittingneedle running through it in the direction of the stem. run a knife around the central part of the apple, to mark the situation of the equator. the circumference of the ring represents the earth's orbit in the plane of the ecliptic. place the apple so that the equator shall coincide with the wire; then the axis will lie directly across the plane of the ecliptic; that is, at right angles to it. let the apple be carried quite round the ring, constantly preserving the axis parallel to itself, and the equator all the while coinciding with the wire that represents the orbit. now, since the sun enlightens half the globe at once, so the candle, which here represents the sun, will shine on the half of the apple that is turned towards it; and the circle which divides the enlightened from the unenlightened side of the apple, called the _terminator_, will pass through both the poles. if the apple be turned slowly round on its axis, the terminator will successively pass over all places on the earth, giving the appearance of sunrise to places at which it arrives, and of sunset to places from which it departs. if, therefore, the equator had coincided with the ecliptic, as would have been the case, had the earth's axis been perpendicular to the plane of its orbit, the diurnal motion of the sun would always have been in the equator, and the days and nights would have been equal all over the globe. to the inhabitants of the equatorial parts of the earth, the sun would always have appeared to move in the prime vertical, rising directly in the east, passing through the zenith at noon, and setting in the west. in the polar regions, the sun would always have appeared to revolve in the horizon; while, at any place between the equator and the pole, the course of the sun would have been oblique to the horizon, but always oblique in the same degree. there would have been nothing of those agreeable vicissitudes of the seasons which we now enjoy; but some regions of the earth would have been crowned with perpetual spring, others would have been scorched with the unremitting fervor of a vertical sun, while extensive regions towards either pole would have been consigned to everlasting frost and sterility. to understand, then, clearly, the causes of the change of seasons, use the same apparatus as before; but, instead of placing the axis of the earth at right angles to the plane of its orbit, turn it out of a perpendicular position a little, (twenty-three degrees and twenty-eight minutes,) then the equator will be turned just the same number of degrees out of a coincidence with the ecliptic. let the apple be carried around the ring, always holding the axis inclined at the same angle to the plane of the ring, and always parallel to itself. you will find that there will be two points in the circuit where the plane of the equator, that you had marked around the centre of the apple, will pass through the centre of the sun; these will be the points where the celestial equator and the ecliptic cut one another, or the equinoxes. when the earth is at either of these points, the sun shines on both poles alike; and, if we conceive of the earth, while in this situation, as turning once round on its axis, the apparent diurnal motion of the sun will be the same as it would be, were the earth's axis perpendicular to the plane of the equator. for that day, the sun would revolve in the equator, and the days and nights would be equal all over the globe. if the apple were carried round in the manner supposed, then, at the distance of ninety degrees from the equinoxes, the same pole would be turned from the sun on one side, just as much as it was turned towards him on the other. in the former case, the sun's light would fall short of the pole twenty-three and one half degrees, and in the other case, it would reach beyond it the same number of degrees. i would recommend to you to obtain as clear an idea as you can of the cause of the change of seasons, by thinking over the foregoing illustration. you may then clear up any remaining difficulties, by studying the diagram, fig. , on page . [illustration fig. .] let a b c d represent the earth's place in different parts of its orbit, having the sun in the centre. let a, c, be the positions of the earth at the equinoxes, and b, d, its positions at the tropics,--the axis _n s_ being always parallel to itself. it is difficult to represent things of this kind correctly, all on the same plane; but you will readily see, that the figure of the earth, here, answers to the apple in the former illustration; that the hemisphere towards _n_ is above, and that towards _s_ is below, the plane of the paper. when the earth is at a and c, the vernal and autumnal equinoxes, the sun, you will perceive, shines on both the poles _n_ and _s_; and, if you conceive of the globe, while in this position, as turned round on its axis, as it is in the diurnal revolution, you will readily understand, that the sun would describe the celestial equator. this may not at first appear so obvious, by inspecting the figure; but if you consider the point _n_ as raised above the plane of the paper, and the point _s_ as depressed below it, you will readily see how the plane of the equator would pass through the centre of the sun. again, at b, when the earth is at the southern tropic, the sun shines twenty-three and a half degrees beyond the north pole, _n_, and falls the same distance short of the south pole, _s_. the case is exactly reversed when the earth is at the northern tropic, and the sun at the southern. while the earth is at one of the tropics, at b, for example, let us conceive of it as turning on its axis, and we shall readily see, that all that part of the earth which lies within the north polar circle will enjoy continual day, while that within the south polar circle will have continual night; and that all other places will have their days longer as they are nearer to the enlightened pole, and shorter as they are nearer to the unenlightened pole. this figure likewise shows the successive positions of the earth, at different periods of the year, with respect to the signs, and what months correspond to particular signs. thus, the earth enters libra, and the sun aries, on the twenty-first of march, and on the twenty-first of june, the earth is just entering capricorn, and the sun, cancer. you will call to mind what is meant by this phraseology,--that by saying the earth enters libra, we mean that a spectator placed on the sun would see the earth in that part of the celestial ecliptic, which is occupied by the sign libra; and that a spectator on the earth sees the sun at the same time projected on the opposite part of the heavens, occupied by the sign cancer. had the axis of the earth been perpendicular to the plane of the ecliptic, then the sun would always have appeared to move in the equator, the days would every where have been equal to the nights, and there could have been no change of seasons. on the other hand, had the inclination of the ecliptic to the equator been much greater than it is, the vicissitudes of the seasons would have been proportionally greater, than at present. suppose, for instance, the equator had been at right angles to the ecliptic, in which case, the poles of the earth would have been situated in the ecliptic itself; then, in different parts of the earth, the appearances would have been as follows: to a spectator on the _equator_, (where all the circles of diurnal revolution are perpendicular to the horizon,) the sun, as he left the vernal equinox, would every day perform his diurnal revolution in a smaller and smaller circle, until he reached the north pole, when he would halt for a moment, and then wheel about and return to the equator, in a reverse order. the progress of the sun through the southern signs, to the south pole, would be similar to that already described. such would be the appearances to an inhabitant of the equatorial regions. to a spectator living in an _oblique_ sphere, in our own latitude, for example, the sun, while north of the equator, would advance continually northward, making his diurnal circuit in parallels further and further distant from the equator, until he reached the circle of perpetual apparition; after which, he would climb, by a spiral course, to the north star, and then as rapidly return to the equator. by a similar progress southward, the sun would at length pass the circle of perpetual occultation, and for some time (which would be longer or shorter, according to the latitude of the place of observation) there would be continual night. to a spectator on the _pole_ of the earth and under the pole of the heaven, during the long day of six months, the sun would wind its way to a point directly over head, pouring down upon the earth beneath not merely the heat of the torrid zone, but the heat of a torrid noon, accumulating without intermission. the great vicissitudes of heat and cold, which would attend these several movements of the sun, would be wholly incompatible with the existence of either the animal or the vegetable kingdom, and all terrestrial nature would be doomed to perpetual sterility and desolation. the happy provision which the creator has made against such extreme vicissitudes, by confining the changes of the seasons within such narrow bounds, conspires with many other express arrangements in the economy of nature, to secure the safety and comfort of the human race. perhaps you have never reflected upon all the reasons, why the several changes of position, with respect to the horizon, which the sun undergoes in the course of the year, occasion such a difference in the amount of heat received from him. two causes contribute to increase the heat of summer and the cold of winter. the higher the sun ascends above the horizon, the more directly his rays fall upon the earth; and their heating power is rapidly augmented, as they approach a perpendicular direction. when the sun is nearly over head, his rays strike us with far greater force than when they meet us obliquely; and the earth absorbs a far greater number of those rays of heat which strike it perpendicularly, than of those which meet it in a slanting direction. when the sun is near the horizon, his rays merely glance along the ground, and many of them, before they reach it, are absorbed and dispersed in passing through the atmosphere. those who have felt only the oblique solar rays, as they fall upon objects in the high latitudes, have a very inadequate idea of the power of a vertical, noonday sun, as felt in the region of the equator. the increased length of the day in summer is another cause of the heat of this season of the year. this cause more sensibly affects places far removed from the equator, because at such places the days are longer and the nights shorter than in the torrid zone. by the operation of this cause, the solar heat accumulates there so much, during the longest days of summer, that the temperature rises to a higher degree than is often known in the torrid climates. but the temperature of a place is influenced very much by several other causes, as well as by the force and duration of the sun's heat. first, the _elevation_ of a country above the level of the sea has a great influence upon its climate. elevated districts of country, even in the torrid zone, often enjoy the most agreeable climate in the world. the cold of the upper regions of the atmosphere modifies and tempers the solar heat, so as to give a most delightful softness, while the uniformity of temperature excludes those sudden and excessive changes which are often experienced in less favored climes. in ascending certain high mountains situated within the torrid zone, the traveller passes, in a short time, through every variety of climate, from the most oppressive and sultry heat, to the soft and balmy air of spring, which again is succeeded by the cooler breezes of autumn, and then by the severest frosts of winter. a corresponding difference is seen in the products of the vegetable kingdom. while winter reigns on the summit of the mountain, its central regions may be encircled with the verdure of spring, and its base with the flowers and fruits of summer. secondly, the proximity of the _ocean_ also has a great effect to equalize the temperature of a place. as the ocean changes its temperature during the year much less than the land, it becomes a source of warmth to contiguous countries in winter, and a fountain of cool breezes in summer. thirdly, the relative _humidity_ or _dryness_ of the atmosphere of a place is of great importance, in regard to its effects on the animal system. a dry air of ninety degrees is not so insupportable as a humid air of eighty degrees; and it may be asserted as a general principle, that a hot and humid atmosphere is unhealthy, although a hot air, when dry, may be very salubrious. in a warm atmosphere which is dry, the evaporation of moisture from the surface of the body is rapid, and its cooling influence affords a most striking relief to an intense heat without; but when the surrounding atmosphere is already filled with moisture, no such evaporation takes place from the surface of the skin, and no such refreshing effects are experienced from this cause. moisture collects on the skin; a sultry, oppressive sensation is felt; and chills and fevers are usually in the train. letter xii. laws of motion. "what though in solemn silence, all move round this dark, terrestrial ball! in reason's ear they all rejoice, and utter forth a glorious voice; for ever singing, as they shine, 'the hand that made us is divine.'"--_addison._ however incredible it may seem, no fact is more certain, than that the earth is constantly on the wing, flying around the sun with a velocity so prodigious, that, for every breath we draw, we advance on our way forty or fifty miles. if, when passing across the waters in a steam-boat, we can wake, after a night's repose, and find ourselves conducted on our voyage a hundred miles, we exult in the triumphs of art, which could have moved so ponderous a body as a steam-ship over such a space in so short a time, and so quietly, too, as not to disturb our slumbers; but, with a motion vastly more quiet and uniform, we have, in the same interval, been carried along with the earth in its orbit more than half a million of miles. in the case of the steam-ship, however perfect the machinery may be, we still, in our waking hours at least, are made sensible of the action of the forces by which the motion is maintained,--as the roaring of the fire, the beating of the piston, and the dashing of the paddle-wheels; but in the more perfect machinery which carries the earth forward on her grander voyage, no sound is heard, nor the least intimation afforded of the stupendous forces by which this motion is achieved. to the pious observer of nature it might seem sufficient, without any inquiry into second causes, to ascribe the motions of the spheres to the direct agency of the supreme being. if, however, we can succeed in finding the secret springs and cords, by which the motions of the heavenly bodies are immediately produced and controlled, it will detract nothing from our just admiration of the great first cause of all things. we may therefore now enter upon the inquiry into the nature or laws of the forces by which the earth is made to revolve on her axis and in her orbit; and having learned what it is, that causes and maintains the motions of the earth, you will then acquire, at the same time, a knowledge of all the celestial machinery. the subject will involve an explanation of the laws of motion, and of the principles of universal gravitation. it was once supposed, that we could never reason respecting the laws that govern the heavenly bodies from what we observe in bodies around us, but that motion is one thing on the earth and quite another thing in the skies; and hence, that it is impossible for us, by any inquiries into the laws of terrestrial nature, to ascertain how things take place among the heavenly bodies. galileo and newton, however, proceeded on the contrary supposition, that nature is uniform in all her works; that the same almighty arm rules over all; and that he works by the same fixed laws through all parts of his boundless realm. the certainty with which all the predictions of astronomers, made on these suppositions, are fulfilled, attests the soundness of the hypothesis. accordingly, those laws, which all experience, endlessly multiplied and varied, proves to be the laws of terrestrial motion, are held to be the laws that govern also the motions of the most distant planets and stars, and to prevail throughout the universe of matter. let us, then, briefly review these great laws of motion, which are three in number. the first law is as follows: _every body perseveres in a state of rest, or of uniform motion in a straight line, unless compelled by some force to change its state_. by _force_ is meant any thing which produces motion. the foregoing law has been fully established by experiment, and is conformable to all experience. it embraces several particulars. first, a body, when at rest, remains so, unless some force puts it in motion; and hence it is inferred, when a body is found in motion, that some force must have been applied to it sufficient to have caused its motion. thus, the fact, that the earth is in motion around the sun and around its own axis, is to be accounted for by assigning to each of these motions a force adequate, both in quantity and direction, to produce these motions, respectively. secondly, when a body is once in motion, it will continue to move for ever, unless something stops it. when a ball is struck on the surface of the earth, the friction of the earth and the resistance of the air soon stop its motion; when struck on smooth ice, it will go much further before it comes to a state of rest, because the ice opposes much less resistance than the ground; and, were there no impediment to its motion, it would, when once set in motion, continue to move without end. the heavenly bodies are actually in this condition: they continue to move, not because any new forces are applied to them; but, having been once set in motion, they continue in motion because there is nothing to stop them. this property in bodies to persevere in the state they are actually in,--if at rest, to remain at rest, or, if in motion, to continue in motion,--is called _inertia_. the inertia of a body (which is measured by the force required to overcome it) is proportioned to the quantity of matter it contains. a steam-boat manifests its inertia, on first starting it, by the enormous expenditure of force required to bring it to a given rate of motion; and it again manifests its inertia, when in rapid motion, by the great difficulty of stopping it. the heavenly bodies, having been once put in motion, and meeting with nothing to stop them, move on by their own inertia. a top affords a beautiful illustration of inertia, continuing, as it does, to spin after the moving force is withdrawn. thirdly, the motion to which a body naturally tends is _uniform_; that is, the body moves just as far the second minute as it did the first, and as far the third as the second; and passes over equal spaces in equal times. i do not assert that the motion of all moving bodies is _in fact_ uniform, but that such is their _tendency_. if it is otherwise than uniform, there is some cause operating to disturb the uniformity to which it is naturally prone. fourthly, a body in motion will move in a _straight line_, unless diverted out of that line by some external force; and the body will resume its straight-forward motion, whenever the force that turns it aside is withdrawn. every body that is revolving in an orbit, like the moon around the earth, or the earth around the sun, _tends_ to move in a straight line which is a tangent[ ] to its orbit. thus, if a b c, fig. , represents the orbit of the moon around the earth, were it not for the constant action of some force that draws her towards the earth, she would move off in a straight line. if the force that carries her towards the earth were suspended at a, she would immediately desert the circular motion, and proceed in the direction a d. in the same manner, a boy whirls a stone around his head in a sling, and then letting go one of the strings, and releasing the force that binds it to the circle, it flies off in a straight line which is a tangent to that part of the circle where it was released. this tendency which a body revolving in an orbit exhibits, to recede from the centre and to fly off in a tangent, is called the _centrifugal force_. we see it manifested when a pail of water is whirled. the water rises on the sides of the vessel, leaving a hollow in the central parts. we see an example of the effects of centrifugal action, when a horse turns swiftly round a corner, and the rider is thrown outwards; also, when a wheel passes rapidly through a small collection of water, and portions of the water are thrown off from the top of the wheel in straight lines which are tangents to the wheel. [illustration fig. .] the centrifugal force is increased as the velocity is increased. thus, the parts of a millstone most remote from the centre sometimes acquire a centrifugal force so much greater than the central parts, which move much slower, that the stone is divided, and the exterior portions are projected with great violence. in like manner, as the equatorial parts of the earth, in the diurnal revolution, revolve much faster than the parts towards the poles, so the centrifugal force is felt most at the equator, and becomes strikingly manifest by the diminished weight of bodies, since it acts in opposition to the force of gravity. although the foregoing law of motion, when first presented to the mind, appears to convey no new truth, but only to enunciate in a formal manner what we knew before; yet a just understanding of this law, in all its bearings, leads us to a clear comprehension of no small share of all the phenomena of motion. the second and third laws may be explained in fewer terms. the second law of motion is as follows: _motion is proportioned to the force impressed, and in the direction of that force_. the meaning of this law is, that every force that is applied to a body produces its full effect, proportioned to its intensity, either in causing or in preventing motion. let there be ever so many blows applied at once to a ball, each will produce its own effect in its own direction, and the ball will move off, not indeed in the zigzag, complex lines corresponding to the directions of the several forces, but in a single line expressing the united effect of all. if you place a ball at the corner of a table, and give it an impulse, at the same instant, with the thumb and finger of each hand, one impelling it in the direction of one side of the table, and the other in the direction of the other side, the ball will move diagonally across the table. if the blows be exactly proportioned each to the length of the side of the table on which it is directed, the ball will run exactly from corner to corner, and in the same time that it would have passed over each side by the blow given in the direction of that side. this principle is expressed by saying, that a body impelled by two forces, acting respectively in the directions of the two sides of a parallelogram, and proportioned in intensity to the lengths of the sides, will describe the diagonal of the parallelogram in the same time in which it would have described the sides by the forces acting separately. the converse of this proposition is also true, namely, that any single motion may be considered as the _resultant_ of two others,--the motion itself being represented by the diagonal, while the two _components_ are represented by the sides, of a parallelogram. this reduction of a motion to the individual motions that produce it, is called the _resolution of motion_, or the _resolution of forces_. nor can a given motion be resolved into _two_ components, merely. these, again, may be resolved into others, varying indefinitely, in direction and intensity, from all which the given motion may be considered as having resulted. this composition and resolution of motion or forces is often of great use, in inquiries into the motions of the heavenly bodies. the composition often enables us to substitute a single force for a great number of others, whose individual operations would be too complicated to be followed. by this means, the investigation is greatly simplified. on the other hand, it is frequently very convenient to resolve a given motion into two or more others, some of which may be thrown out of the account, as not influencing the particular point which we are inquiring about, while others are far more easily understood and managed than the single force would have been. it is characteristic of great minds, to simplify these inquiries. they gain an insight into complicated and difficult subjects, not so much by any extraordinary faculty of seeing in the dark, as by the power of removing from the object all incidental causes of obscurity, until it shines in its own clear and simple light. if every force, when applied to a body, produces its full and legitimate effect, how many other forces soever may act upon it, impelling it different ways, then it must follow, that the smallest force ought to move the largest body; and such is in fact the case. a snap of a finger upon a seventy-four under full sail, if applied in the direction of its motion, would actually increase its speed, although the effect might be too small to be visible. still it is something, and may be truly expressed by a fraction. thus, suppose a globe, weighing a million of pounds, were suspended from the ceiling by a string, and we should apply to it the snap of a finger,--it is granted that the motion would be quite insensible. let us then divide the body into a million equal parts, each weighing one pound; then the same impulse, applied to each one separately, would produce a sensible effect, moving it, say one inch. it will be found, on trial, that the same impulse given to a mass of two pounds will move it half an inch; and hence it is inferred, that, if applied to a mass weighing a million of pounds, it would move it the millionth part of an inch. it is one of the curious results of the second law of motion, that an unlimited number of motions may exist together in the same body. thus, at the same moment, we may be walking around a post in the cabin of a steam-boat, accompanying the boat in its passage around an island, revolving with the earth on its axis, flying through space in our annual circuit around the sun, and possibly wheeling, along with the sun and his whole retinue of planets, around some centre in common with the starry worlds. the third law of motion is this: _action and reaction are equal, and in contrary directions_. whenever i give a blow, the body struck exerts an equal force on the striking body. if i strike the water with an oar, the water communicates an equal impulse to the oar, which, being communicated to the boat, drives it forward in the opposite direction. if a magnet attracts a piece of iron, the iron attracts the magnet just as much, in the opposite direction; and, in short, every portion of matter in the universe attracts and is attracted by every other, equally, in an opposite direction. this brings us to the doctrine of universal gravitation, which is the very key that unlocks all the secrets of the skies. this will form the subject of my next letter. footnote: [ ] a tangent is a straight line touching a circle, as a d, in fig. letter xiii. terrestrial gravity. "to him no high, no low, no great, no small, he fills, he bounds, connects, and equals all."--_pope._ we discover in nature a tendency of every portion of matter towards every other. this tendency is called _gravitation_. in obedience to this power, a stone falls to the ground, and a planet revolves around the sun. we may contemplate this subject as it relates either to phenomena that take place near the surface of the earth, or in the celestial regions. the former, _gravity_, is exemplified by falling bodies; the latter, _universal gravitation_, by the motions of the heavenly bodies. the laws of terrestrial gravity were first investigated by galileo; those of universal gravitation, by sir isaac newton. terrestrial gravity is only an individual example of universal gravitation; being the tendency of bodies towards the centre of the earth. we are so much accustomed, from our earliest years, to see bodies fall to the earth, that we imagine bodies must of necessity fall "downwards;" but when we reflect that the earth is round, and that bodies fall towards the centre on all sides of it, and that of course bodies on opposite sides of the earth fall in precisely opposite directions, and towards each other, we perceive that there must be some force acting to produce this effect; nor is it enough to say, as the ancients did, that bodies "naturally" fall to the earth. every motion implies some force which produces it; and the fact that bodies fall towards the earth, on all sides of it, leads us to infer that that force, whatever it is, resides in the earth itself. we therefore call it _attraction_. we do not, however, say what attraction _is_, but what it _does_. we must bear in mind, also, that, according to the third law of motion, this attraction is mutual; that when a stone falls towards the earth, it exerts the same force on the earth that the earth exerts on the stone; but the motion of the earth towards the stone is as much less than that of the stone towards the earth, as its quantity of matter is greater; and therefore its motion is quite insensible. but although we are compelled to acknowledge the _existence_ of such a force as gravity, causing a tendency in all bodies towards each other, yet we know nothing of its _nature_, nor can we conceive by what medium bodies at such a distance as the moon and the earth exercise this influence on each other. still, we may trace the modes in which this force acts; that is, its _laws_; for the laws of nature are nothing else than the modes in which the powers of nature act. we owe chiefly to the great galileo the first investigation of the laws of terrestrial gravity, as exemplified in falling bodies; and i will avail myself of this opportunity to make you better acquainted with one of the most interesting of men and greatest of philosophers. galileo was born at pisa, in italy, in the year . he was the son of a florentine nobleman, and was destined by his father for the medical profession, and to this his earlier studies were devoted. but a fondness and a genius for mechanical inventions had developed itself, at a very early age, in the construction of his toys, and a love of drawing; and as he grew older, a passion for mathematics, and for experimental research, predominated over his zeal for the study of medicine, and he fortunately abandoned that for the more congenial pursuits of natural philosophy and astronomy. in the twenty-fifth year of his age, he was appointed, by the grand duke of tuscany, professor of mathematics in the university of pisa. at that period, there prevailed in all the schools a most extraordinary reverence for the writings of aristotle, the preceptor of alexander the great,--a philosopher who flourished in greece, about three hundred years before the christian era. aristotle, by his great genius and learning, gained a wonderful ascendency over the minds of men, and became the oracle of the whole reading world for twenty centuries. it was held, on the one hand, that all truths worth knowing were contained in the writings of aristotle; and, on the other, that an assertion which contradicted any thing in aristotle could not be true. but galileo had a greatness of mind which soared above the prejudices of the age in which he lived, and dared to interrogate nature by the two great and only successful methods of discovering her secrets,--experiment and observation. galileo was indeed the first philosopher that ever fully employed experiments as the means of learning the laws of nature, by imitating on a small what she performs on a great scale, and thus detecting her modes of operation. archimedes, the great sicilian philosopher, had in ancient times introduced mathematical or geometrical reasoning into natural philosophy; but it was reserved for galileo to unite the advantages of both mathematical and experimental reasonings in the study of nature,--both sure and the only sure guides to truth, in this department of knowledge, at least. experiment and observation furnish materials upon which geometry builds her reasonings, and from which she derives many truths that either lie for ever hidden from the eye of observation, or which it would require ages to unfold. this method, of interrogating nature by experiment and observation, was matured into a system by lord bacon, a celebrated english philosopher, early in the seventeenth century,--indeed, during the life of galileo. previous to that time, the inquirers into nature did not open their eyes to see how the facts really _are_; but, by metaphysical processes, in imitation of aristotle, determined how they _ought to be_, and hastily concluded that they were so. thus, they did not study into the laws of motion, by observing how motion actually takes place, under various circumstances, but first, in their closets, constructed a definition of motion, and thence inferred all its properties. the system of reasoning respecting the phenomena of nature, introduced by lord bacon, was this: in the first place, to examine all the facts of the case, and then from these to determine the laws of nature. to derive general conclusions from the comparison of a great number of individual instances constitutes the peculiarity of the baconian philosophy. it is called the _inductive_ system, because its conclusions were built on the induction, or comparison, of a great many single facts. previous to the time of lord bacon, hardly any insight had been gained into the causes of natural phenomena, and hardly one of the laws of nature had been clearly established, because all the inquirers into nature were upon a wrong road, groping their way through the labyrinth of error. bacon pointed out to them the true path, and held before them the torch-light of experiment and observation, under whose guidance all successful students of nature have since walked, and by whose illumination they have gained so wonderful an insight into the mysteries of the natural world. it is a remarkable fact, that two such characters as bacon and galileo should appear on the stage at the same time, who, without any communication with each other, or perhaps without any personal knowledge of each other's existence, should have each developed the true method of investigating the laws of nature. galileo practised what bacon only taught; and some, therefore, with much reason, consider galileo as a greater philosopher than bacon. "bacon," says hume, "pointed out, at a great distance, the road to philosophy; galileo both pointed it out to others, and made, himself, considerable advances in it. the englishman was ignorant of geometry; the florentine revived that science, excelled in it, and was the first who applied it, together with experiment, to natural philosophy. the former rejected, with the most positive disdain, the system of copernicus; the latter fortified it with new proofs, derived both from reason and the senses." when we reflect that geometry is a science built upon self-evident truths, and that all its conclusions are the result of pure demonstration, and can admit of no controversy; when we further reflect, that experimental evidence rests on the testimony of the senses, and we infer a thing to be true because we actually see it to be so; it shows us the extreme bigotry, the darkness visible, that beclouded the human intellect, when it not only refused to admit conclusions first established by pure geometrical reasoning, and afterwards confirmed by experiments exhibited in the light of day, but instituted the most cruel persecutions against the great philosopher who first proclaimed these truths. galileo was hated and persecuted by two distinct bodies of men, both possessing great influence in their respective spheres,--the one consisting of the learned doctors of philosophy, who did nothing more, from age to age, than reiterate the doctrines of aristotle, and were consequently alarmed at the promulgation of principles subversive of those doctrines; the other consisting of the romish priesthood, comprising the terrible inquisition, who denounced the truths taught by galileo, as inconsistent with certain declarations of the holy scriptures. we shall see, as we advance, what a fearful warfare he had to wage against these combined powers of darkness. aristotle had asserted, that, if two different weights of the same material were let fall from the same height, the heavier one would reach the ground sooner than the other, in proportion as it was more weighty. for example: if a ten-pound leaden weight and a one-pound were let fall from a given height at the same instant, the former would reach the ground ten times as soon as the latter. no one thought of making the trial, but it was deemed sufficient that aristotle had said so; and accordingly this assertion had long been received as an axiom in the science of motion. galileo ventured to appeal from the authority of aristotle to that of his own senses, and maintained, that both weights would fall in the same time. the learned doctors ridiculed the idea. galileo tried the experiment in their presence, by letting fall, at the same instant, large and small weights from the top of the celebrated leaning tower of pisa. yet, with the sound of the two weights clicking upon the pavement at the same moment, they still maintained that the ten-pound weight would reach the ground in one tenth part of the time of the other, because they could quote the chapter and verse of aristotle where the fact was asserted. wearied and disgusted with the malice and folly of these aristotelian philosophers, galileo, at the age of twenty-eight, resigned his situation in the university of pisa, and removed to padua, in the university of which place he was elected professor of mathematics. up to this period, galileo had devoted himself chiefly to the studies of the laws of motion, and the other branches of mechanical philosophy. soon afterwards, he began to publish his writings, in rapid succession, and became at once among the most conspicuous of his age,--a rank which he afterwards well sustained and greatly exalted, by the invention of the telescope, and by his numerous astronomical discoveries. i will reserve an account of these great achievements until we come to that part of astronomy to which they were more immediately related, and proceed, now, to explain to you the leading principles of _terrestrial gravity_, as exemplified in falling bodies. first, _all bodies near the earth's surface fall in straight lines towards the centre of the earth_. we are not to infer from this fact, that there resides at the centre any peculiar force, as a great loadstone, for example, which attracts bodies towards itself; but bodies fall towards the centre of the sphere, because the combined attractions of all the particles of matter in the earth, each exerting its proper force upon the body, would carry it towards the centre. this may be easily illustrated by a diagram. let b, fig. , page , be the centre of the earth, and a a body without it. every portion of matter in the earth exerts some force on a, to draw it down to the earth. but since there is just as much matter on one side of the line a b, as on the other side, each half exerts an equal force to draw the body towards itself; therefore it falls in the direction of the diagonal between the two forces. thus, if we compare the effects of any two particles of matter at equal distances from the line a b, but on opposite sides of it, as _a_, _b_, while the force of the particle at _a_ would tend to draw a in the direction of a _a_, that of _b_ would draw it in the direction of a _b_, and it would fall in the line a b, half way between the two. the same would hold true of any other two corresponding particles of matter on different sides of the earth, in respect to a body situated in any place without it. [illustration fig. .] secondly, _all bodies fall towards the earth, from the same height, with equal velocities_. a musket-ball, and the finest particle of down, if let fall from a certain height towards the earth, tend to descend towards it at the same rate, and would proceed with equal speed, were it not for the resistance of the air, which retards the down more than it does the ball, and finally stops it. if, however, the air be removed out of the way, as it may be by means of the air-pump, the two bodies keep side by side in falling from the greatest height at which we can try the experiment. thirdly, _bodies, in falling towards the earth, have their rate of motion continually accelerated_. suppose we let fall a musket-ball from the top of a high tower, and watch its progress, disregarding the resistance of the air: the first second, it will pass over sixteen feet and one inch, but its speed will be constantly increased, being all the while urged onward by the same force, and retaining all that it has already acquired; so that the longer it is in falling, the swifter its motion becomes. consequently, when bodies fall from a great height, they acquire an immense velocity before they reach the earth. thus, a man falling from a balloon, or from the mast-head of a ship, is broken in pieces; and those meteoric stones, which sometimes fall from the sky, bury themselves deep in the earth. on measuring the spaces through which a body falls, it is found, that it will fall four times as far in two seconds as in one, and one hundred times as far in ten seconds as in one; and universally, the space described by a falling body is proportioned to the time multiplied into itself; that is, to the square of the time. fourthly, _gravity is proportioned to the quantity of matter_. a body which has twice as much matter as another exerts a force of attraction twice as great, and also receives twice as much from the same body as it would do, if it were only just as heavy as that body. thus the earth, containing, as it does, forty times as much matter as the moon, exerts upon the moon forty times as much force as it would do, were its mass the same with that of the moon; but it is also capable of _receiving_ forty times as much gravity from the moon as it would do, were its mass the same as the moon's; so that the power of attracting and that of being attracted are reciprocal; and it is therefore correct to say, that the moon attracts the earth _just as much_ as the earth attracts the moon; and the same may be said of any two bodies, however different in quantity of matter. fifthly, _gravity, when acting at a distance from the earth, is not as intense as it is near the earth_. at such a distance as we are accustomed to ascend above the general level of the earth, no great difference is observed. on the tops of high mountains, we find bodies falling towards the earth, with nearly the same speed as they do from the smallest elevations. it is found, nevertheless, that there is a real difference; so that, in fact, the weight of a body (which is nothing more than the measure of its force of gravity) is not quite so great on the tops of high mountains as at the general level of the sea. thus, a thousand pounds' weight, on the top of a mountain half a mile high, would weigh a quarter of a pound less than at the level of the sea; and if elevated four thousand miles above the earth,--that is, _twice_ as far from the centre of the earth as the surface is from the centre,--it would weigh only one fourth as much as before; if _three times_ as far, it would weigh only one ninth as much. so that the force of gravity decreases, as we recede from the earth, in the same proportion as the square of the distance increases. this fact is generalized by saying, that _the force of gravity, at different distances from the earth, is inversely as the square of the distance_. were a body to fall from a great distance,--suppose a thousand times that of the radius of the earth,--the force of gravity being one million times less than that at the surface of the earth, the motion of the body would be exceedingly slow, carrying it over only the sixth part of an inch in a day. it would be a long time, therefore, in making any sensible approaches towards the earth; but at length, as it drew near to the earth it would acquire a very great velocity, and would finally rush towards it with prodigious violence. falling so far, and being continually accelerated on the way, we might suppose that it would at length attain a velocity infinitely great; but it can be demonstrated, that, if a body were to fall from an infinite distance, attracted to the earth only by gravity, it could never acquire a velocity greater than about seven miles per second. this, however, is a speed inconceivably great, being about eighteen times the greatest velocity that can be given to a cannon-ball, and more than twenty-five thousand miles per hour. but the phenomena of falling bodies must have long been observed, and their laws had been fully investigated by galileo and others, before the cause of their falling was understood, or any such principle as gravity, inherent in the earth and in all bodies, was applied to them. the developement of this great principle was the work of sir isaac newton; and i will give you, in my next letter, some particulars respecting the life and discoveries of this wonderful man. letter xiv. sir isaac newton.--universal gravitation.--figure of the earth's orbit.--precession of the equinoxes. "the heavens are all his own; from the wild rule of whirling vortices, and circling spheres, to their first great simplicity restored. the schools astonished stood; but found it vain to combat long with demonstration clear, and, unawakened, dream beneath the blaze of truth. at once their pleasing visions fled, with the light shadows of the morning mixed, when newton rose, our philosophic sun."--_thomson's elegy._ sir isaac newton was born in lincolnshire, england, in , just one year after the death of galileo. his father died before he was born, and he was a helpless infant, of a diminutive size, and so feeble a frame, that his attendants hardly expected his life for a single hour. the family dwelling was of humble architecture, situated in a retired but beautiful valley, and was surrounded by a small farm, which afforded but a scanty living to the widowed mother and her precious charge. the cut on page , fig , represents the modest mansion, and the emblems of rustic life that first met the eyes of this pride of the british nation, and ornament of human nature. it will probably be found, that genius has oftener emanated from the cottage than from the palace. [illustration fig. .] the boyhood of newton was distinguished chiefly for his ingenious mechanical contrivances. among other pieces of mechanism, he constructed a windmill so curious and complete in its workmanship, as to excite universal admiration. after carrying it a while by the force of the wind, he resolved to substitute animal power, and for this purpose he inclosed in it a mouse, which he called the miller, and which kept the mill a-going by acting on a tread-wheel. the power of the mouse was brought into action by unavailing attempts to reach a portion of corn placed above the wheel. a water-clock, a four-wheeled carriage propelled by the rider himself, and kites of superior workmanship, were among the productions of the mechanical genius of this gifted boy. at a little later period, he began to turn his attention to the motions of the heavenly bodies, and constructed several sun-dials on the walls of the house where he lived. all this was before he had reached his fifteenth year. at this age, he was sent by his mother, in company with an old family servant, to a neighboring market-town, to dispose of products of their farm, and to buy articles of merchandise for their family use; but the young philosopher left all these negotiations to his worthy partner, occupying himself, mean-while, with a collection of old books, which he had found in a garret. at other times, he stopped on the road, and took shelter with his book under a hedge, until the servant returned. they endeavored to educate him as a farmer; but the perusal of a book, the construction of a water-mill, or some other mechanical or scientific amusement, absorbed all his thoughts, when the sheep were going astray, and the cattle were devouring or treading down the corn. one of his uncles having found him one day under a hedge, with a book in his hand, and entirely absorbed in meditation, took it from him, and found that it was a mathematical problem which so engrossed his attention. his friends, therefore, wisely resolved to favor the bent of his genius, and removed him from the farm to the school, to prepare for the university. in the eighteenth year of his age, newton was admitted into trinity college, cambridge. he made rapid and extraordinary advances in the mathematics, and soon afforded unequivocal presages of that greatness which afterwards placed him at the head of the human intellect. in , at the age of twenty-seven, he became professor of mathematics at cambridge, a post which he occupied for many years afterwards. during the four or five years previous to this he had, in fact, made most of those great discoveries which have immortalized his name. we are at present chiefly interested in one of these, namely, that of _universal gravitation_; and let us see by what steps he was conducted to this greatest of scientific discoveries. in the year , when newton was about twenty-four years of age, the plague was prevailing at cambridge, and he retired into the country. one day, while he sat in a garden, musing on the phenomena of nature around him, an apple chanced to fall to the ground. reflecting on the mysterious power that makes all bodies near the earth fall towards its centre, and considering that this power remains unimpaired at considerable heights above the earth, as on the tops of trees and mountains, he asked himself,--"may not the same force extend its influence to a great distance from the earth, even as far as the moon? indeed, may not this be the very reason, why the moon is drawn away continually from the straight line in which every body tends to move, and is thus made to circulate around the earth?" you will recollect that it was mentioned, in my letter which contained an account of the first law of motion, that if a body is put in motion by any force, it will always move forward in a straight line, unless some other force compels it to turn aside from such a direction; and that, when we see a body moving in a curve, as a circular orbit, we are authorized to conclude that there is some force existing within the circle, which continually draws the body away from the direction in which it tends to move. accordingly, it was a very natural suggestion, to one so well acquainted with the laws of motion as newton, that the moon should constantly bend towards the earth, from a tendency to fall towards it, as any other heavy body would do, if carried to such a distance from the earth. newton had already proved, that if such a power as gravity extends from the earth to distant bodies, it must decrease, as the square of the distance from the centre of the earth increases; that is, at double the distance, it would be four times less; at ten times the distance, one hundred times less; and so on. now, it was known that the moon is about sixty times as far from the centre of the earth as the surface of the earth is from the centre, and consequently, the force of attraction at the moon must be the square of sixty, or thirty-six hundred times less than it is at the earth; so that a body at the distance of the moon would fall towards the earth very slowly, only one thirty-six hundredth part as far in a given time, as at the earth. does the moon actually fall towards the earth at this rate; or, what is the same thing, does she depart at this rate continually from the straight line in which she tends to move, and in which she would move, if no external force diverted her from it? on making the calculation, such was found to be the fact. hence gravity, and no other force than gravity, acts upon the moon, and compels her to revolve around the earth. by reasonings equally conclusive, it was afterwards proved, that a similar force compels all the planets to circulate around the sun; and now, we may ascend from the contemplation of this force, as we have seen it exemplified in falling bodies, to that of a universal power whose influence extends to all the material creation. it is in this sense that we recognise the principle of universal gravitation, the law of which may be thus enunciated; _all bodies in the universe, whether great or small, attract each other, with forces proportioned to their respective quantities of matter, and inversely as the squares of their distances from each other_. this law asserts, first, that attraction reigns throughout the material world, affecting alike the smallest particle of matter and the greatest body; secondly, that it acts upon every mass of matter, precisely in proportion to its quantity; and, thirdly, that its intensity is diminished as the square of the distance is increased. observation has fully confirmed the prevalence of this law throughout the solar system; and recent discoveries among the fixed stars, to be more fully detailed hereafter, indicate that the same law prevails there. the law of universal gravitation is therefore held to be the grand principle which governs all the celestial motions. not only is it consistent with all the observed motions of the heavenly bodies, even the most irregular of those motions, but, when followed out into all its consequences, it would be competent to assert that such irregularities must take place, even if they had never been observed. newton first published the doctrine of universal gravitation in the 'principia,' in . the name implies that the work contains the fundamental principles of natural philosophy and astronomy. being founded upon the immutable basis of mathematics, its conclusions must of course be true and unalterable, and thenceforth we may regard the great laws of the universe as traced to their remotest principle. the greatest astronomers and mathematicians have since occupied themselves in following out the plan which newton began, by applying the principles of universal gravitation to all the subordinate as well as to the grand movements of the spheres. this great labor has been especially achieved by la place, a french mathematician of the highest eminence, in his profound work, the 'mecanique celeste.' of this work, our distinguished countryman, dr. bowditch, has given a magnificent translation, and accompanied it with a commentary, which both illustrates the original, and adds a great amount of matter hardly less profound than that. [illustration fig. .] we have thus far taken the earth's orbit around the sun as a great circle, such being its projection on the sphere constituting the celestial ecliptic. the real path of the earth around the sun is learned, as i before explained to you, by the apparent path of the sun around the earth once a year. now, when a body revolves about the earth at a great distance from us, as is the case with the sun and moon, we cannot certainly infer that it moves in a circle because it appears to describe a circle on the face of the sky, for such might be the appearance of its orbit, were it ever so irregular a curve. thus, if e, fig. , represents the earth, and acb, the irregular path of a body revolving about it, since we should refer the body continually to some place on the celestial sphere, xyz, determined by lines drawn from the eye to the concave sphere through the body, the body, while moving from a to b through c, would appear to move from x to z, through y. hence, we must determine from other circumstances than the actual appearance, what is the true figure of the orbit. [illustration fig. .] were the earth's path a circle, having the sun in the centre, the sun would always appear to be at the same distance from us; that is, the radius of the orbit, or _radius vector_, (the name given to a line drawn from the centre of the sun to the orbit of any planet,) would always be of the same length. but the earth's distance from the sun is constantly varying, which shows that its orbit is not a circle. we learn the true figure of the orbit, by ascertaining the _relative distances_ of the earth from the sun, at various periods of the year. these distances all being laid down in a diagram, according to their respective lengths, the extremities, on being connected, give us our first idea of the shape of the orbit, which appears of an oval form, and at least resembles an ellipse; and, on further trial, we find that it has the properties of an ellipse. thus, let e, fig. , be the place of the earth, and _a_, _b_, _c_, &c., successive positions of the sun; the _relative_ lengths of the lines e _a_, e _b_, &c., being known, on connecting the points _a_, _b_, _c_, &c., the resulting figure indicates the true figure of the earth's orbit. these relative distances are found in two different ways; first, _by changes in the sun's apparent diameter_, and, secondly, _by variations in his angular velocity_. the same object appears to us smaller in proportion as it is more distant; and if we see a heavenly body varying in size, at different times, we infer that it is at different distances from us; that when largest, it is nearest to us, and when smallest, furthest off. now, when the sun's diameter is accurately measured by instruments, it is found to vary from day to day; being, when greatest, more than thirty-two minutes and a half, and when smallest, only thirty-one minutes and a half,--differing, in all, about seventy-five seconds. when the diameter is greatest, which happens in january, we know that the sun is nearest to us; and when the diameter is least, which occurs in july, we infer that the sun is at the greatest distance from us. the point where the earth, or any planet, in its revolution, is nearest the sun, is called its _perihelion_; the point where it is furthest from the sun, its _aphelion_. suppose, then, that, about the first of january, when the diameter of the sun is greatest, we draw a line, e _a_, fig. , to represent it, and afterwards, every ten days, draw other lines, e _b_, e _c_, &c.; increasing in the same ratio as the apparent diameters of the sun decrease. these lines must be drawn at such a distance from each other, that the triangles, e _a b_, e _b c_, &c., shall be all equal to each other, for a reason that will be explained hereafter. on connecting the extremities of these lines, we shall obtain the figure of the earth's orbit. similar conclusions may be drawn from observations on the sun's _angular velocity_. a body appears to move most rapidly when nearest to us. indeed, the apparent velocity increases rapidly, as it approaches us, and as rapidly diminishes, when it recedes from us. if it comes twice as near as before, it appears to move not merely twice as swiftly, but four times as swiftly; if it comes ten times nearer, its apparent velocity is one hundred times as great as before. we say, therefore, that the velocity varies inversely as the square of the distance; for, as the distance is diminished ten times, the velocity is increased the square of ten; that is, one hundred times. now, by noting the time it takes the sun, from day to day, to cross the central wire of the transit-instrument, we learn the comparative velocities with which it moves at different times; and from these we derive the comparative distances of the sun at the corresponding times; and laying down these relative distances in a diagram, as before, we get our first notions of the actual figure of the earth's orbit, or the path which it describes in its annual revolution around the sun. having now learned the fact, that the earth moves around the sun, not in a circular but in an elliptical orbit, you will desire to know by what forces it is impelled, to make it describe this figure, with such uniformity and constancy, from age to age. it is commonly said, that gravity causes the earth and the planets to circulate around the sun; and it is true that it is gravity which turns them aside from the straight line in which, by the first law of motion, they tend to move, and thus causes them to revolve around the sun. but what force is that which gave to them this original impulse, and impressed upon them such a tendency to move forward in a straight line? the name _projectile_ force is given to it, because it is the same _as though_ the earth were originally projected into space, when first created; and therefore its motion is the result of two forces, the projectile force, which would cause it to move forward in a straight line which is a tangent to its orbit, and gravitation, which bends it towards the sun. but before you can clearly understand the nature of this motion, and the action of the two forces that produce it, i must explain to you a few elementary principles upon which this and all the other planetary motions depend. you have already learned, that when a body is acted on by two forces, in different directions, it moves in the direction of neither, but in some direction between them. if i throw a stone horizontally, the attraction of the earth will continually draw it downward, out of the line of direction in which it was thrown, and make it descend to the earth in a curve. the particular form of the curve will depend on the velocity with which it is thrown. it will always _begin_ to move in the line of direction in which it is projected; but it will soon be turned from that line towards the earth. it will, however, continue nearer to the line of projection in proportion as the velocity of projection is greater. thus, let a c, fig. , be perpendicular to the horizon, and a b parallel to it, and let a stone be thrown from a, in the direction of a b. it will, in every case, commence its motion in the line a b, which will therefore be a tangent to the curve it describes; but, if it is thrown with a small velocity, it will soon depart from the tangent, describing the line a d; with a greater velocity, it will describe a curve nearer the tangent, as a e; and with a still greater velocity, it will describe the curve a f. [illustration fig. .] as an example of a body revolving in an orbit under the influence of two forces, suppose a body placed at any point, p, fig. , above the surface of the earth, and let p a be the direction of the earth's centre; that is, a line perpendicular to the horizon. if the body were allowed to move, without receiving any impulse, it would descend to the earth in the direction p a with an accelerated motion. but suppose that, at the moment of its departure from p, it receives a blow in the direction p b, which would carry it to b in the time the body would fall from p to a; then, under the influence of both forces, it would descend along the curve p d. if a stronger blow were given to it in the direction p b, it would describe a larger curve, p e; or, finally, if the impulse were sufficiently strong, it would circulate quite around the earth, and return again to p, describing the circle p f g. with a velocity of projection still greater, it would describe an ellipse, p i k; and if the velocity be increased to a certain degree, the figure becomes a parabola, l p m,--a curve which never returns into itself. [illustration fig. .] in fig. , page , suppose the planet to have passed the point c, at the aphelion, with so small a velocity, that the attraction of the sun bends its path very much, and causes it immediately to begin to approach towards the sun. the sun's attraction will increase its velocity, as it moves through d, e, and f, for the sun's attractive force on the planet, when at d, is acting in the direction d s; and, on account of the small angle made between d e and d s, the force acting in the line d s helps the planet forward in the path d e, and thus increases its velocity. in like manner, the velocity of the planet will be continually increasing as it passes through d, e, and f; and though the attractive force, on account of the planet's nearness, is so much increased, and tends, therefore, to make the orbit more curved, yet the velocity is also so much increased, that the orbit is not more curved than before; for the same increase of velocity, occasioned by the planet's approach to the sun, produces a greater increase of centrifugal force, which carries it off again. we may see, also, the reason why, when the planet has reached the most distant parts of its orbit, it does not entirely fly off, and never return to the sun; for, when the planet passes along h, k, a, the sun's attraction retards the planet, just as gravity retards a ball rolled up hill; and when it has reached c, its velocity is very small, and the attraction to the centre of force causes a great deflection from the tangent, sufficient to give its orbit a great curvature, and the planet wheels about, returns to the sun, and goes over the same orbit again. as the planet recedes from the sun, its centrifugal force diminishes faster than the force of gravity, so that the latter finally preponderates. [illustration fig. .] i shall conclude what i have to say at present, respecting the motion of the earth around the sun, by adding a few words respecting the precession of the equinoxes. the _precession of the equinoxes_ is a slow but continual shifting of the equinoctial points, from east to west. suppose that we mark the exact place in the heavens where, during the present year, the sun crosses the equator, and that this point is close to a certain star; next year, the sun will cross the equator a little way westward of that star, and so every year, a little further westward, until, in a long course of ages, the place of the equinox will occupy successively every part of the ecliptic, until we come round to the same star again. as, therefore, the sun revolving from west to east, in his apparent orbit, comes round to the point where it left the equinox, it meets the equinox before it reaches that point. the appearance is as though the equinox _goes forward_ to meet the sun, and hence the phenomenon is called the _precession_ of the equinoxes; and the fact is expressed by saying, that the equinoxes retrograde on the ecliptic, until the line of the equinoxes (a straight line drawn from one equinox to the other) makes a complete revolution, from east to west. this is of course a retrograde motion, since it is contrary to the order of the signs. the equator is conceived as _sliding_ westward on the ecliptic, always preserving the same inclination to it, as a ring, placed at a small angle with another of nearly the same size which remains fixed, may be slid quite around it, giving a corresponding motion to the two points of intersection. it must be observed, however, that this mode of conceiving of the precession of the equinoxes is purely imaginary, and is employed merely for the convenience of representation. the amount of precession annually is fifty seconds and one tenth; whence, since there are thirty-six hundred seconds in a degree, and three hundred and sixty degrees in the whole circumference of the ecliptic, and consequently one million two hundred and ninety-six thousand seconds, this sum, divided by fifty seconds and one tenth, gives twenty-five thousand eight hundred and sixty-eight years for the period of a complete revolution of the equinoxes. suppose we now fix to the centre of each of the two rings, before mentioned, a wire representing its axis, one corresponding to the axis of the ecliptic, the other to that of the equator, the extremity of each being the pole of its circle. as the ring denoting the equator turns round on the ecliptic, which, with its axis, remains fixed, it is easy to conceive that the axis of the equator revolves around that of the ecliptic, and the pole of the equator around the pole of the ecliptic, and constantly at a distance equal to the inclination of the two circles. to transfer our conceptions to the celestial sphere, we may easily see that the axis of the diurnal sphere (that of the earth produced) would not have its pole constantly in the same place among the stars, but that this pole would perform a slow revolution around the pole of the ecliptic, from east to west, completing the circuit in about twenty-six thousand years. hence the star which we now call the pole-star has not always enjoyed that distinction, nor will it always enjoy it, hereafter. when the earliest catalogues of the stars were made, this star was twelve degrees from the pole. it is now one degree twenty-four minutes, and will approach still nearer; or, to speak more accurately, the pole will come still nearer to this star, after which it will leave it, and successively pass by others. in about thirteen thousand years, the bright star lyra (which lies near the circle in which the pole of the equator revolves about the pole of the ecliptic, on the side opposite to the present pole-star) will be within five degrees of the pole, and will constitute the pole-star. as lyra now passes near our zenith, you might suppose that the change of position of the pole among the stars would be attended with a change of altitude of the north pole above the horizon. this mistaken idea is one of the many misapprehensions which result from the habit of considering the horizon as a fixed circle in space. however the pole might shift its position in space, we should still be at the same distance from it, and our horizon would always reach the same distance beyond it. the time occupied by the sun, in passing from the equinoctial point round to the same point again, is called the _tropical year_. as the sun does not perform a complete revolution in this interval, but falls short of it fifty seconds and one tenth, the tropical year is shorter than the sidereal by twenty minutes and twenty seconds, in mean solar time, this being the time of describing an arc of fifty seconds and one tenth, in the annual revolution. the changes produced by the precession of the equinoxes, in the apparent places of the circumpolar stars, have led to some interesting results in _chronology_. in consequence of the retrograde motion of the equinoctial points, the _signs_ of the ecliptic do not correspond, at present, to the _constellations_ which bear the same names, but lie about one sign, or thirty degrees, westward of them. thus, that division of the ecliptic which is called the sign taurus lies in the constellation aries, and the sign gemini, in the constellation taurus. undoubtedly, however, when the ecliptic was thus first divided, and the divisions named, the several constellations lay in the respective divisions which bear their names. letter xv. the moon. "soon as the evening shades prevail the moon takes up the wondrous tale, and nightly to the listening earth repeats the story of her birth."--_addison._ having now learned so much of astronomy as relates to the earth and the sun, and the mutual relations which exist between them, you are prepared to enter with advantage upon the survey of the other bodies that compose the solar system. this being done, we shall then have still before us the boundless range of the fixed stars. the moon, which next claims our notice, has been studied by astronomers with greater attention than any other of the heavenly bodies, since her comparative nearness to the earth brings her peculiarly within the range of our telescopes, and her periodical changes and very irregular motions, afford curious subjects, both for observation and speculation. the mild light of the moon also invites our gaze, while her varying aspects serve barbarous tribes, especially, for a kind of dial-plate inscribed on the face of the sky, for weeks, and months, and times, and seasons. the moon is distant from the earth about two hundred and forty thousand miles; or, more exactly, two hundred and thirty-eight thousand five hundred and forty-five miles. her angular or apparent diameter is about half a degree, and her real diameter, two thousand one hundred and sixty miles. she is a companion, or satellite, to the earth, revolving around it every month, and accompanying us in our annual revolution around the sun. although her nearness to us makes her appear as a large and conspicuous object in the heavens, yet, in comparison with most of the other celestial bodies, she is in fact very small, being only one forty-ninth part as large as the earth, and only about one seventy millionth part as large as the sun. the moon shines by light borrowed from the sun, being itself an opaque body, like the earth. when the disk, or any portion of it, is illuminated, we can plainly discern, even with the naked eye, varieties of light and shade, indicating inequalities of surface which we imagine to be land and water. i believe it is the common impression, that the darker portions are land and the lighter portions water; but if either part is water, it must be the darker regions. a smooth polished surface, like water, would reflect the sun's light like a mirror. it would, like a convex mirror, form a diminished image of the sun, but would not itself appear luminous like an uneven surface, which multiplies the light by numerous reflections within itself. thus, from this cause, high broken mountainous districts appear more luminous than extensive plains. [illustration figures , . telescopic views of the moon.] by the aid of the telescope, we may see undoubted indications of mountains and valleys. indeed, with a good glass, we can discover the most decisive evidence that the surface of the moon is exceedingly varied,--one part ascending in lofty peaks, another clustering in huge mountain groups, or long ranges, and another bearing all the marks of deep caverns or valleys. you will not, indeed, at the first sight of the moon through a telescope, recognise all these different objects. if you look at the moon when half her disk is enlightened, (which is the best time for seeing her varieties of surface,) you will, at the first glance, observe a motley appearance, particularly along the line called the _terminator_, which separates the enlightened from the unenlightened part of the disk. (fig. .) on one side of the terminator, within the dark part of the disk, you will see illuminated points, and short, crooked lines, like rude characters marked with chalk on a black ground. on the other side of the terminator you will see a succession of little circular groups, appearing like numerous bubbles of oil on the surface of water. the further you carry your eye from the terminator, on the same side of it, the more indistinctly formed these bubbles appear, until towards the edge of the moon they assume quite a different aspect. some persons, when they look into a telescope for the first time, having heard that mountains and valleys are to be seen, and discovering nothing but these unmeaning figures, break off in disappointment, and have their faith in these things rather diminished than increased. i would advise you, therefore, before you take even your first view of the moon through a telescope, to form as clear an idea as you can, how mountains, and valleys, and caverns, situated at such a distance from the eye, ought to look, and by what marks they may be recognised. seize, if possible, the most favorable period, (about the time of the first quarter,) and previously learn from drawings and explanations, how to interpret every thing you see. what, then, ought to be the respective appearances of mountains, valleys, and deep craters, or caverns, in the moon? the sun shines on the moon in the same way as it shines on the earth; and let, us reflect, then, upon the manner in which it strikes similar objects here. one half the globe is constantly enlightened; and, by the revolution of the earth on its axis, the terminator, or the line which separates the enlightened from the unenlightened part of the earth, travels along from east to west, over different places, as we see the moon's terminator travel over her disk from new to full moon; although, in the case of the earth, the motion is more rapid, and depends on a different cause. in the morning, the sun's light first strikes upon the tops of the mountains, and, if they are very high, they may be brightly illuminated while it is yet night in the valleys below. by degrees, as the sun rises, the circle of illumination travels down the mountain, until at length it reaches the bottom of the valleys; and these in turn enjoy the full light of day. again, a mountain casts a shadow opposite to the sun, which is very long when the sun first rises, and shortens continually as the sun ascends, its length at a given time, however, being proportioned to the height of the mountain; so that, if the shadow be still very long when the sun is far above the horizon, we infer that the mountain is very lofty. we may, moreover, form some judgment of the shape of a mountain, by observing that of its shadow. now, the moon is so distant that we could not easily distinguish places simply by their elevations, since they would be projected into the same imaginary plane which constitutes the apparent disk of the moon; but the foregoing considerations would enable us to infer their existence. thus, when you view the moon at any time within her first quarter, but better near the end of that period, you will observe, on the side of the terminator within the dark part of the disk, the tops of mountains which the light of the sun is just striking, as the morning sun strikes the tops of mountains on the earth. these you will recognise by those white specks and little crooked lines, before mentioned, as is represented in fig. . these bright points and lines you will see altering their figure, every hour, as they come more and more into the sun's light; and, mean-while, other bright points, very minute at first, will start into view, which also in turn grow larger as the terminator approaches them, until they fall into the enlightened part of the disk. as they fall further and further within this part, you will have additional proofs that they are mountains, from the shadows which they cast on the plain, always in a direction opposite to the sun. the mountain itself may entirely disappear, or become confounded with the other enlightened portions of the surface; but its position and its shape may still be recognised by the dark line which it projects on the plane. this line will correspond in shape to that of the mountain, presenting at one time a long serpentine stripe of black, denoting that the mountain is a continued range; at another time exhibiting a conical figure tapering to a point, or a series of such sharp points; or a serrated, uneven termination, indicating, in each case respectively, a conical mountain, or a group of peaks, or a range with lofty cliffs. all these appearances will indeed be seen in miniature; but a little familiarity with them will enable you to give them, in imagination, their proper dimensions, as you give to the pictures of known animals their due sizes, although drawn on a scale far below that of real life. in the next place, let us see how valleys and deep craters in the moon might be expected to appear. we could not expect to see depressions any more than elevations, since both would alike be projected on the same imaginary disk. but we may recognise such depressions, from the manner in which the light of the sun shines into them. when we hold a china tea-cup at some distance from a candle, in the night, the candle being elevated but little above the level of the top of the cup, a luminous crescent will be formed on the side of the cup opposite to the candle, while the side next to the candle will be covered by a deep shadow. as we gradually elevate the candle, the crescent enlarges and travels down the side of the cup, until finally the whole interior becomes illuminated. we observe similar appearances in the moon, which we recognise as deep depressions. they are those circular spots near the terminator before spoken of, which look like bubbles of oil floating on water. they are nothing else than circular craters or deep valleys. when they are so situated that the light of the sun is just beginning to shine into them, you may see, as in the tea-cup, a luminous crescent around the side furthest from the sun, while a deep black shadow is cast on the side next to the sun. as the cavity is turned more and more towards the light, the crescent enlarges, until at length the whole interior is illuminated. if the tea-cup be placed on a table, and a candle be held at some distance from it, nearly on a level with the top, but a little above it, the cup itself will cast a shadow on the table, like any other elevated object. in like manner, many of these circular spots on the moon cast deep shadows behind them, indicating that the tops of the craters are elevated far above the general level of the moon. the regularity of some of these circular spots is very remarkable. the circle, in some instances, appears as well formed as could be described by a pair of compasses, while in the centre there not unfrequently is seen a conical mountain casting its pointed shadow on the bottom of the crater. i hope you will enjoy repeated opportunities to view the moon through a telescope. allow me to recommend to you, not to rest satisfied with a hasty or even with a single view, but to verify the preceding remarks by repeated and careful inspection of the lunar disk, at different ages of the moon. the various places on the moon's disk have received appropriate names. the dusky regions being formerly supposed to be seas, were named accordingly; and other remarkable places have each two names, one derived from some well-known spot on the earth, and the other from some distinguished personage. thus, the same bright spot on the surface of the moon is called _mount sinai_ or _tycho_, and another, _mount etna_ or _copernicus_. the names of individuals, however, are more used than the others. the diagram, fig. , (see page ,) represents rudely, the telescopic appearance of the full moon. the reality is far more beautiful. a few of the most remarkable points have the following names corresponding to the numbers and letters on the map. . tycho, . eratosthenes, . kepler, . plato, . copernicus, . archimedes, . aristarchus, . eudoxus, . helicon, . aristotle. a. mare humorum, _sea of humors_, b. mare nubium, _sea of clouds_, c. mare imbrium, _sea of rains_, d. mare nectaris, _sea of nectar_, e. mare tranquillitatis, _sea of tranquillity_, f. mare serenitatis, _sea of serenity_, g. mare fecunditatis, _sea of plenty_, h. mare crisium, _crisian sea_. the heights of the lunar mountains, and the depths of the valleys, can be estimated with a considerable degree of accuracy. some of the mountains are as high as five miles, and the valleys, in some instances, are four miles deep. hence it is inferred, that the surface of the moon is more broken and irregular than that of the earth, its mountains being higher and its valleys deeper, in proportion to its magnitude, than those of the earth. the varieties of surface in the moon, as seen by the aid of large telescopes, have been well described by dr. dick, in his 'celestial scenery,' and i cannot give you a better idea of them, than to add a few extracts from his work. the lunar mountains in general exhibit an arrangement and an aspect very different from the mountain scenery of our globe. they may be arranged under the four following varieties: first, _insulated mountains_, which rise from plains nearly level, shaped like a sugar loaf, which may be supposed to present an appearance somewhat similar to mount etna, or the peak of teneriffe. the shadows of these mountains, in certain phases of the moon, are as distinctly perceived as the shadow of an upright staff, when placed opposite to the sun; and these heights can be calculated from the length of their shadows. some of these mountains being elevated in the midst of extensive plains, would present to a spectator on their summits magnificent views of the surrounding regions. secondly, _mountain ranges_, extending in length two or three hundred miles. these ranges bear a distant resemblance to our alps, apennines, and andes; but they are much less in extent. some of them appear very rugged and precipitous; and the highest ranges are in some places more than four miles in perpendicular altitude. in some instances, they are nearly in a straight line from northeast to southwest, as in the range called the _apennines_; in other cases, they assume the form of a semicircle, or crescent. thirdly, _circular ranges_, which appear on almost every part of the moon's surface, particularly in its southern regions. this is one grand peculiarity of the lunar ranges, to which we have nothing similar on the earth. a plain, and sometimes a large cavity, is surrounded with a circular ridge of mountains, which encompasses it like a mighty rampart. these annular ridges and plains are of all dimensions, from a mile to forty or fifty miles in diameter, and are to be seen in great numbers over every region of the moon's surface; they are most conspicuous, however, near the upper and lower limbs, about the time of the half moon. the mountains which form these circular ridges are of different elevations, from one fifth of a mile to three miles and a half, and their shadows cover one half of the plain at the base. these plains are sometimes on a level with the general surface of the moon, and in other cases they are sunk a mile or more below the level of the ground which surrounds the exterior circle of the mountains. fourthly, _central mountains_, or those which are placed in the middle of circular plains. in many of the plains and cavities surrounded by circular ranges of mountains there stands a single insulated mountain, which rises from the centre of the plain, and whose shadow sometimes extends, in the form of a pyramid, half across the plain to the opposite ridges. these central mountains are generally from half a mile to a mile and a half in perpendicular altitude. in some instances, they have two, and sometimes three, different tops, whose shadows can be easily distinguished from each other. sometimes they are situated towards one side of the plain, or cavity; but in the great majority of instances their position is nearly or exactly central. the lengths of their bases vary from five to about fifteen or sixteen miles. the _lunar caverns_ form a very peculiar and prominent feature of the moon's surface, and are to be seen throughout almost every region, but are most numerous in the southwest part of the moon. nearly a hundred of them, great and small, may be distinguished in that quarter. they are all nearly of a circular shape, and appear like a very shallow egg-cup. the smaller cavities appear, within, almost like a hollow cone, with the sides tapering towards the centre; but the larger ones have, for the most part, flat bottoms, from the centre of which there frequently rises a small, steep, conical hill, which gives them a resemblance to the circular ridges and central mountains before described. in some instances, their margins are level with the general surface of the moon; but, in most cases, they are encircled with a high annular ridge of mountains, marked with lofty peaks. some of the larger of these cavities contain smaller cavities of the same kind and form, particularly in their sides. the mountainous ridges which surround these cavities reflect the greatest quantity of light; and hence that region of the moon in which they abound appears brighter than any other. from their lying in every possible direction, they appear, at and near the time of full moon, like a number of brilliant streaks, or radiations. these radiations appear to converge towards a large brilliant spot, surrounded by a faint shade, near the lower part of the moon, which is named tycho,--a spot easily distinguished even by a small telescope. the spots named kepler and copernicus are each composed of a central spot with luminous radiations.[ ] the broken surface and apparent geological structure of the moon has suggested the opinion, that the moon has been subject to powerful _volcanic_ action. this opinion receives support from certain actual appearances of volcanic fires, which have at different times been observed. in a total eclipse of the sun, the moon comes directly between us and that luminary, and presents her dark side towards us under circumstances very favorable for observation. at such times, several astronomers, at different periods, have noticed bright spots, which they took to be volcanoes. it must evidently require a large fire to be visible at all, at such a distance; and even a burning spark, or point but just visible in a large telescope, might be in fact a volcano raging like etna or vesuvius. still, as fires might be supposed to exist in the moon from different causes, we should require some marks peculiar to volcanic fires, to assure us that such was their origin in a given case. dr. herschel examined this point with great attention, and with better means of observation than any of his predecessors enjoyed, and fully embraced the opinion that what he saw were volcanoes. in april, , he records his observations as follows: "i perceive three volcanoes in different places in the dark part of the moon. two of them are already nearly extinct, or otherwise in a state of going to break out; the third shows an eruption of fire or luminous matter." on the next night, he says: "the volcano burns with greater violence than last night; its diameter cannot be less than three seconds; and hence the shining or burning matter must be above three miles in diameter. the appearance resembles a small piece of burning charcoal, when it is covered with a very thin coat of white ashes; and it has a degree of brightness about as strong as that with which such a coal would be seen to glow in faint daylight." that these were really volcanic fires, he considered further evident from the fact, that where a fire, supposed to have been volcanic, had been burning, there was seen, after its extinction, an accumulation of matter, such as would arise from the production of a great quantity of lava, sufficient to form a mountain. it is probable that the moon has an _atmosphere_, although it is difficult to obtain perfectly satisfactory evidence of its existence; for granting the existence of an atmosphere bearing the same proportion to that planet as our atmosphere bears to the earth, its dimensions and its density would be so small, that we could detect its presence only by the most refined observations. as our twilight is owing to the agency of our atmosphere, so, could we discern any appearance of twilight in the moon, we should regard that fact as indicating that she is surrounded by an atmosphere. or, when the moon covers the sun in a solar eclipse, could we see around her circumference a faint luminous ring, indicating that the sunlight shone through an aerial medium, we might likewise infer the existence of such a medium. such a faint ring of light has sometimes, as is supposed, been observed. schroeter, a german astronomer, distinguished for the acuteness of his vision and his powers of observation in general, was very confident of having obtained, from different sources, clear evidence of a lunar atmosphere. he concluded, that the inferior or more dense part of the moon's atmosphere is not more than fifteen hundred feet high, and that the entire height, at least to the limit where it would be too rare to produce any of the phenomena which are relied on as proofs of its existence, is not more than a mile. it has been a question, much agitated among astronomers, whether there is _water_ in the moon. analogy strongly inclines us to reply in the affirmative. but the analogy between the earth and the moon, as derived from all the particulars in which we can compare the two bodies, is too feeble to warrant such a conclusion, and we must have recourse to other evidence, before we can decide the point. in the first place, then, there is no positive evidence in favor of the existence of water in the moon. those extensive level regions, before spoken of, and denominated seas in the geography of this planet, have no other signs of being water, except that they are level and dark. but both these particulars would characterize an earthly plain, like the deserts of arabia and africa. in the second place, were those dark regions composed of water, the terminator would be entirely smooth where it passed over these oceans or seas. it is indeed indented by few inequalities, compared with those which it exhibits where it passes over the mountainous regions; but still, the inequalities are too considerable to permit the conclusion, that these level spots are such perfect levels as water would form. they do not appear to be more perfect levels than many plain countries on the globe. the deep caverns, moreover, seen in those dusky spots which were supposed to be seas, are unfavorable to the supposition that those regions are covered by water. in the third place, the face of the moon, when illuminated by the sun and not obscured by the state of our own atmosphere, is always serene, and therefore free from clouds. clouds are objects of great extent; they frequently intercept light, like solid bodies; and did they exist about the moon, we should certainly see them, and should lose sight of certain parts of the lunar disk which they covered. but neither position is true; we neither see any clouds about the moon, with our best telescopes, nor do we, by the intervention of clouds, ever lose sight of any portion of the moon when our own atmosphere is clear. but the want of clouds in the lunar atmosphere almost necessarily implies the absence of water in the moon. this planet is at the same distance from the sun as our own, and has, in this respect, an equal opportunity to feel the influence of his rays. its days are also twenty-seven times as long as ours, a circumstance which would augment the solar heat. when the pressure of the atmosphere is diminished on the surface of water, its tendency to pass into the state of vapor is increased. were the whole pressure of the atmosphere removed from the surface of a lake, in a summer's day, when the temperature was no higher than seventy-two degrees, the water would begin to boil. now it is well ascertained, that if there be any atmosphere about the moon, it is much lighter than ours, and presses on the surface of that body with a proportionally small force. this circumstance, therefore, would conspire with the other causes mentioned, to convert all the water of the moon into vapor, if we could suppose it to have existed at any given time. but those, who are anxious to furnish the moon and other planets with all the accommodations which they find in our own, have a subterfuge in readiness, to which they invariably resort in all cases like the foregoing. "there may be," say they, "some means, unknown to us, provided for retaining water on the surface of the moon, and for preventing its being wasted by evaporation: perhaps it remains unaltered in quantity, imparting to the lunar regions perpetual verdure and fertility." to this i reply, that the bare possibility of a thing is but slight evidence of its reality; nor is such a condition possible, except by miracle. if they grant that the laws of nature are the same in the moon as in the earth, then, according to the foregoing reasoning, there cannot be water in the moon; but if they say that the laws of nature are not the same there as here, then we cannot reason at all respecting them. one who resorts to a subterfuge of this kind ruins his own cause. he argues the existence of water in the moon, from the analogy of that planet to this. but if the laws of nature are not the same there as here, what becomes of his analogy? a liquid substance which would not evaporate by such a degree of solar heat as falls on the moon, which would not evaporate the faster, in consequence of the diminished atmospheric pressure which prevails there, could not be water, for it would not have the properties of water, and things are known by their properties. whenever we desert the cardinal principle of the newtonian philosophy,--that the laws of nature are uniform throughout all her realms,--we wander in a labyrinth; all analogies are made void; all physical reasonings cease; and imaginary possibilities or direct miracles take the place of legitimate natural causes. on the supposition that the moon is inhabited, the question has often been raised, whether we may hope that our telescopes will ever be so much improved, and our other means of observation so much augmented, that we shall be able to discover either the lunar inhabitants or any of their works. the improbability of our ever identifying _artificial structures_ in the moon may be inferred from the fact, that a space a mile in diameter is the least space that could be distinctly seen. extensive works of art, as large cities, or the clearing up of large tracts of country for settlement or tillage, might indeed afford some varieties of surface; but they would be merely varieties of light and shade, and the individual objects that occasioned them would probably never be recognised by their distinctive characters. thus, a building equal to the great pyramid of egypt, which covers a space less than the fifth of a mile in diameter, would not be distinguished by its figure; indeed, it would be a mere point. still less is it probable that we shall ever discover any inhabitants in the moon. were we to view the moon with a telescope that magnifies ten thousand times, it would bring the moon apparently ten thousand times nearer, and present it to the eye like a body twenty-four miles off. but even this is a distance too great for us to see the works of man with distinctness. moreover, from the nature of the telescope itself, we can never hope to apply a magnifying power so high as that here supposed. as i explained to you, when speaking of the telescope, whenever we increase the magnifying power of this instrument we diminish its field of view, so that with very high magnifiers we can see nothing but a point, such as a fixed star. we at the same time, also, magnify the vapors and smoke of the atmosphere, and all the imperfections of the medium, which greatly obscures the object, and prevents our seeing it distinctly. hence it is generally most satisfactory to view the moon with low powers, which afford a large field of view and give a clear light. with clark's telescope, belonging to yale college, we seldom gain any thing by applying to the moon a higher power than one hundred and eighty, although the instrument admits of magnifiers as high as four hundred and fifty. some writers, however, suppose that possibly we may trace indications of lunar inhabitants in their works, and that they may in like manner recognise the existence of the inhabitants of our planet. an author, who has reflected much on subjects of this kind, reasons as follows: "a navigator who approaches within a certain distance of a small island, although he perceives no human being upon it, can judge with certainty that it is inhabited, if he perceives human habitations, villages, corn-fields, or other traces of cultivation. in like manner, if we could perceive changes or operations in the moon, which could be traced to the agency of intelligent beings, we should then obtain satisfactory evidence that such beings exist on that planet; and it is thought possible that such operations may be traced. a telescope which magnifies twelve hundred times will enable us to perceive, as a visible point on the surface of the moon, an object whose diameter is only about three hundred feet. such an object is not larger than many of our public edifices; and therefore, were any such edifices rearing in the moon, or were a town or city extending its boundaries, or were operations of this description carrying on, in a district where no such edifices had previously been erected, such objects and operations might probably be detected by a minute inspection. were a multitude of living creatures moving from place to place, in a body, or were they even encamping in an extensive plain, like a large army, or like a tribe of arabs in the desert, and afterwards removing, it is possible such changes might be traced by the difference of shade or color, which such movements would produce. in order to detect such minute objects and operations, it would be requisite that the surface of the moon should be distributed among at least a hundred astronomers, each having a spot or two allotted to him, as the object of his more particular investigation, and that the observations be continued for a period of at least thirty or forty years, during which time certain changes would probably be perceived, arising either from physical causes, or from the operations of living agents."[ ] footnote: [ ] dick's 'celestial scenery,' chapter iv letter xvi. the moon.--phases.--harvest moon.--librations. "first to the neighboring moon this mighty key of nature he applied. behold! it turned the secret wards, it opened wide the course and various aspects of the queen of night: whether she wanes into a scanty orb, or, waxing broad, with her pale shadowy light, in a soft deluge overflows the sky."--_thomson's elegy._ let us now inquire into the revolutions of the moon around the earth, and the various changes she undergoes every month, called her _phases_, which depend on the different positions she assumes, with respect to the earth and the sun, in the course of her revolution. the moon revolves about the earth from west to east. her apparent orbit, as traced out on the face of the sky, is a great circle; but this fact would not certainly prove that the orbit is really a circle, since, if it were an ellipse, or even a more irregular curve, the projection of it on the face of the sky would be a circle, as explained to you before. (see page .) the moon is comparatively so near to the earth, that her apparent movements are very rapid, so that, by attentively watching her progress in a clear night, we may see her move from star to star, changing her place perceptibly, every few hours. the interval during which she goes through the entire circuit of the heavens, from any star until she comes round to the same star again, is called a _sidereal month_, and consists of about twenty-seven and one fourth days. the time which intervenes between one new moon and another is called a _synodical month_, and consists of nearly twenty-nine and a half days. a new moon occurs when the sun and moon meet in the same part of the heavens; but the sun as well as the moon is apparently travelling eastward, and nearly at the rate of one degree a day, and consequently, during the twenty-seven days while the moon has been going round the earth, the sun has been going forward about the same number of degrees in the same direction. hence, when the moon comes round to the part of the heavens where she passed the sun last, she does not find him there, but must go on more than two days, before she comes up with him again. the moon does not pursue precisely the same track around the earth as the sun does, in his apparent annual motion, though she never deviates far from that track. the inclination of her orbit to the ecliptic is only about five degrees, and of course the moon is never seen further from the ecliptic than about that distance, and she is commonly much nearer to the ecliptic than five degrees. we may therefore see nearly what is the situation of the ecliptic in our evening sky at any particular time of year, just by watching the path which the moon pursues, from night to night, from new to full moon. the two points where the moon's orbit crosses the ecliptic are called her _nodes_. they are the intersections of the lunar and solar orbits, as the equinoxes are the intersections of the equinoctial and ecliptic, and, like the latter, are one hundred and eighty degrees apart. the changes of the moon, commonly called her _phases_, arise from different portions of her illuminated side being turned towards the earth at different times. when the moon is first seen after the setting sun, her form is that of a bright crescent, on the side of the disk next to the sun, while the other portions of the disk shine with a feeble light, reflected to the moon from the earth. every night, we observe the moon to be further and further eastward of the sun, until, when she has reached an elongation from the sun of ninety degrees, half her visible disk is enlightened, and she is said to be in her _first quarter_. the terminator, or line which separates the illuminated from the dark part of the moon, is convex towards the sun from the new to the first quarter, and the moon is said to be _horned_. the extremities of the crescent are called _cusps_. at the first quarter, the terminator becomes a straight line, coinciding with the diameter of the disk; but after passing this point, the terminator becomes concave towards the sun, bounding that side of the moon by an elliptical curve, when the moon is said to be _gibbous_. when the moon arrives at the distance of one hundred and eighty degrees from the sun, the entire circle is illuminated, and the moon is _full_. she is then _in opposition_ to the sun, rising about the time the sun sets. for a week after the full, the moon appears gibbous again, until, having arrived within ninety degrees of the sun, she resumes the same form as at the first quarter, being then at her _third quarter_. from this time until new moon, she exhibits again the form of a crescent before the rising sun, until, approaching her _conjunction_ with the sun, her narrow thread of light is lost in the solar blaze; and finally, at the moment of passing the sun, the dark side is wholly turned towards us, and for some time we lose sight of the moon. by inspecting fig. , (where t represents the earth, a, b, c, &c., the moon in her orbit, and _a_, _b_, _c_, &c., her phases, as seen in the heavens,) we shall easily see how all these changes occur. [illustration fig. .] you have doubtless observed, that the moon appears much further in the south at one time than at another, when of the same age. this is owing to the fact that the ecliptic, and of course the moon's path, which is always very near it, is differently situated with respect to the _horizon_, at a given time of night, at different seasons of the year. this you will see at once, by turning to an artificial globe, and observing how the ecliptic stands with respect to the horizon, at different periods of the revolution. thus, if we place the two equinoctial points in the eastern and western horizon, libra being in the west, it will represent the position of the ecliptic at sunset in the month of september, when the sun is crossing the equator; and at that season of the year, the moon's path through our evening sky, one evening after another, from new to full, will be nearly along the same route, crossing the meridian nearly at right angles. but if we place the winter solstice, or first degree of capricorn, in the western horizon, and the first degree of cancer in the eastern, then the position of the ecliptic will be very oblique to the meridian, the winter solstice being very far in the southwest, and the summer solstice very far in the northeast; and the course of the moon from new to full will be nearly along this track. keeping these things in mind, we may easily see why the moon runs sometimes high and sometimes low. recollect, also, that the new moon is always in the same part of the heavens with the sun, and that the full moon is in the opposite part of the heavens from the sun. now, when the sun is at the winter solstice, it sets far in the southwest, and accordingly the new moon runs very low; but the full moon, being in the opposite tropic, which rises far in the northeast, runs very high, as is known to be the case in mid-winter. but now take the position of the ecliptic in mid-summer. then, at sunset, the tropic of cancer is in the northwest, and the tropic of capricorn in the southeast; consequently, the new moons run high and the full moons low. it is a natural consequence of this arrangement, to render the moon's light the most beneficial to us, by giving it to us in greatest abundance, when we have least of the sun's light, and giving it to us most sparingly, when the sun's light is greatest. thus, during the long nights of winter, the full moon runs high, and continues a very long time above the horizon; while in mid-summer, the full moon runs low, and is above the horizon for a much shorter period. this arrangement operates very favorably to the inhabitants of the polar regions. at the season when the sun is absent, and they have constant night, then the moon, during the second and third quarters, embracing the season of full moon, is continually above the horizon, compensating in no small degree for the absence of the sun; while, during the summer months, when the sun is constantly above the horizon, and the light of the moon is not needed, then she is above the horizon during the first and last quarters, when her light is least, affording at that time her greatest light to the inhabitants of the other hemisphere, from whom the sun is withdrawn. about the time of the autumnal equinox, the moon, when near her full, rises about sunset a number of nights in succession. this occasions a remarkable number of brilliant moonlight evenings; and as this is, in england, the period of harvest, the phenomenon is called the _harvest moon_. its return is celebrated, particularly among the peasantry, by festive dances, and kept as a festival, called the _harvest home_,--an occasion often alluded to by the british poets. thus henry kirke white: "moon of harvest, herald mild of plenty, rustic labor's child, hail, o hail! i greet thy beam, as soft it trembles o'er the stream, and gilds the straw-thatch'd hamlet wide, where innocence and peace reside; 'tis thou that glad'st with joy the rustic throng, promptest the tripping dance, th' exhilarating song." to understand the reason of the harvest moon, we will, as before, consider the moon's orbit as coinciding with the ecliptic, because we may then take the ecliptic, as it is drawn on the artificial globe, to represent that orbit. we will also bear in mind, (what has been fully illustrated under the last head,) that, since the ecliptic cuts the meridian obliquely, while all the circles of diurnal revolution cut it perpendicularly, different portions of the ecliptic will cut the horizon at different angles. thus, when the equinoxes are in the horizon, the ecliptic makes a very small angle with the horizon; whereas, when the solstitial points are in the horizon, the same angle is far greater. in the former case, a body moving eastward in the ecliptic, and being at the eastern horizon at sunset, would descend but a little way below the horizon in moving over many degrees of the ecliptic. now, this is just the case of the moon at the time of the harvest home, about the time of the autumnal equinox. the sun being then in libra, and the moon, when full, being of course opposite to the sun, or in aries; and moving eastward, in or near the ecliptic, at the rate of about thirteen degrees per day, would descend but a small distance below the horizon for five or six days in succession; that is for two or three days before, and the same number of days after, the full; and would consequently rise during all these evenings nearly at the same time, namely, a little before, or a little after, sunset, so as to afford a remarkable succession of fine moonlight evenings. the moon _turns on her axis_ in the same time in which she revolves around the earth. this is known by the moon's always keeping nearly the same face towards us, as is indicated by the telescope, which could not happen unless her revolution on her axis kept pace with her motion in her orbit. take an apple, to represent the moon; stick a knittingneedle through it, in the direction of the stem, to represent the axis, in which case the two eyes of the apple will aptly represent the poles. through the poles cut a line around the apple, dividing it into two hemispheres, and mark them, so as to be readily distinguished from each other. now place a candle on the table, to represent the earth, and holding the apple by the knittingneedle, carry it round the candle, and you will see that, unless you make the apple turn round on the axis as you carry it about the candle, it will present different sides towards the candle; and that, in order to make it always present the same side, it will be necessary to make it revolve exactly once on its axis, while it is going round the circle,--the revolution on its axis always keeping exact pace with the motion in its orbit. the same thing will be observed, if you walk around a tree, always keeping your face towards the tree. if you have your face towards the tree when you set out, and walk round without turning, when you have reached the opposite side of the tree, your back will be towards it, and you will find that, in order to keep your face constantly towards the tree, it will be necessary to turn yourself round on your heel at the same rate as you go forward. since, however, the motion of the moon on its axis is uniform, while the motion in its orbit is unequal, the moon does in fact reveal to us a little sometimes of one side and sometimes of the other. thus if, while carrying the apple round the candle, you carry it forward a little faster than the rate at which it turns on its axis, a portion of the hemisphere usually out of sight is brought into view on one side; or if the apple is moved forward slower than it is turned on its axis, a portion of the same hemisphere comes into view on the other side. these appearances are called the moon's _librations in longitude_. the moon has also a _libration in latitude_;--so called, because in one part of her revolution more of the region around one of the poles comes into view, and, in another part of the revolution, more of the region around the other pole, which gives the appearance of a tilting motion to the moon's axis. this is owing to the fact, that the moon's axis is inclined to the plane of her orbit. if, in the experiment with the apple, you hold the knittingneedle parallel to the candle, (in which case the axis will be perpendicular to the plane of revolution,) the candle will shine upon both poles during the whole circuit, and an eye situated where the candle is would constantly see both poles; but now incline the needle towards the plane of revolution, and carry it round, always keeping it parallel to itself, and you will observe that the two poles will be alternately in and out of sight. the moon exhibits another appearance of this kind, called her _diurnal libration_, depending on the daily rotation of the spectator. she turns the same face towards the _centre_ of the earth only, whereas we view her from the surface. when she is on the meridian, we view her disk nearly as though we viewed it from the centre of the earth, and hence, in this situation, it is subject to little change; but when she is near the horizon, our circle of vision takes in more of the upper limb than would be presented to a spectator at the centre of the earth. hence, from this cause, we see a portion of one limb while the moon is rising, which is gradually lost sight of, and we see a portion of the opposite limb, as the moon declines to the west. you will remark that neither of the foregoing changes implies any actual motion in the moon, but that each arises from a change of position in the spectator. since the succession of day and night depends on the revolution of a planet on its own axis, and it takes the moon twenty-nine and a half days to perform this revolution, so that the sun shall go from the meridian of any place and return to the same meridian again, of course the lunar day occupies this long period. so protracted an exposure to the sun's rays, especially in the equatorial regions of the moon, must occasion an excessive accumulation of heat; and so long an absence of the sun must occasion a corresponding degree of cold. a spectator on the side of the moon which is opposite to us would never see the earth, but one on the side next to us would see the earth constantly in his firmament, undergoing a gradual succession of changes, corresponding to those which the moon exhibits to the earth, but in the reverse order. thus, when it is full moon to us, the earth, as seen from the moon, is then in conjunction with the sun, and of course presents her dark side to the moon. soon after this, an inhabitant of the moon would see a crescent, resembling our new moon, which would in like manner increase and go through all the changes, from new to full, and from full to new, as we see them in the moon. there are, however, in the two cases, several striking points of difference. in the first place, instead of twenty-nine and a half days, all these changes occur in one lunar day and night. during the first and last quarters, the changes would occur in the day-time; but during the second and third quarters, during the night. by this arrangement, the lunarians would enjoy the greatest possible benefit from the light afforded by the earth, since in the half of her revolution where she appears to them as full, she would be present while the sun was absent, and would afford her least light while the sun was present. in the second place, the earth would appear thirteen times as large to a spectator on the moon as the moon appears to us, and would afford nearly the same proportion of light, so that their long nights must be continually cheered by an extraordinary degree of light derived from this source; and if the full moon is hailed by our poets as "refulgent lamp of night,"[ ] with how much more reason might a lunarian exult thus, in view of the splendid orb that adorns his nocturnal sky! in the third place, the earth, as viewed from any particular place on the moon, would occupy invariably the same part of the heavens. for while the rotation of the moon on her axis from west to east would appear to make the earth (as the moon does to us) revolve from east to west, the corresponding progress of the moon in her orbit would make the earth appear to revolve from west to east; and as these two motions are equal, their united effect would be to keep the moon apparently stationary in the sky. thus, a spectator at e, fig. , page , in the middle of the disk that is turned towards the earth, would have the earth constantly on his meridian, and at e, the conjunction of the earth and sun would occur at mid-day; but when the moon arrived at g, the same place would be on the margin of the circle of illumination, and will have the sun in the horizon; but the earth would still be on his meridian and in quadrature. in like manner, a place situated on the margin of the circle of illumination, when the moon is at e, would have the earth in the horizon; and the same place would always see the earth in the horizon, except the slight variations that would occur from the librations of the moon. in the fourth place, the earth would present to a spectator on the moon none of that uniformity of aspect which the moon presents to us, but would exhibit an appearance exceedingly diversified. the comparatively rapid rotation of the earth, repeated fifteen times during a lunar night, would present, in rapid succession, a view of our seas, oceans, continents, and mountains, all diversified by our clouds, storms, and volcanoes. footnotes: [ ] dick's 'celestial scenery.' [ ] "as when the moon, refulgent lamp of night, o'er heaven's clear azure sheds her sacred light, when not a breath disturbs the deep serene, and not a cloud o'ercasts the solemn scene, around her throne the vivid planets roll, and stars unnumbered gild the glowing pole; o'er the dark trees a yellower verdure shed, and tip with silver every mountain's head; then shine the vales, the rocks in prospect rise, a flood of glory bursts from all the skies; the conscious swains, rejoicing in the sight, eye the blue vault, and bless the useful light." _pope's homer._ letter xvii. moon's orbit.--her irregularities. "some say the zodiac constellations have long since left their antique stations, above a sign, and prove the same in taurus now, once in the ram; that in twelve hundred years and odd, the sun has left his ancient road, and nearer to the earth is come, 'bove fifty thousand miles from home."--_hudibras._ we have thus far contemplated the revolution of the moon around the earth as though the earth were at rest. but in order to have just ideas respecting the moon's motions, we must recollect that the moon likewise revolves along with the earth around the sun. it is sometimes said that the earth _carries_ the moon along with her, in her annual revolution. this language may convey an erroneous idea; for the moon, as well as the earth, revolves around the sun under the influence of two forces, which are independent of the earth, and would continue her motion around the sun, were the earth removed out of the way. indeed, the moon is attracted towards the sun two and one fifth times more than towards the earth, and would abandon the earth, were not the latter also carried along with her by the same forces. so far as the sun acts equally on both bodies, the motion with respect to each other would not be disturbed. because the gravity of the moon towards the sun is found to be greater, at the conjunction, than her gravity towards the earth, some have apprehended that, if the doctrine of universal gravitation is true, the moon ought necessarily to abandon the earth. in order to understand the reason why it does not do thus, we must reflect, that, when a body is revolving in its orbit under the influence of the projectile force and gravity, whatever diminishes the force of gravity, while that of projection remains the same, causes the body to approach nearer to the tangent of her orbit, and of course to recede from the centre; and whatever increases the amount of gravity, carries the body towards the centre. thus, in fig. , page , if, with a certain force of projection acting in the direction a b, and of attraction, in the direction a c, the attraction which caused a body to move in the line a d were diminished, it would move nearer to the tangent, as in a e, or a f. now, when the moon is in conjunction, her gravity towards the earth acts in opposition to that towards the sun, (see fig. , page ,) while her velocity remains too great to carry her with what force remains, in a circle about the sun, and she therefore recedes from the sun, and commences her revolution around the earth. on arriving at the opposition, the gravity of the earth conspires with that of the sun, and the moon's projectile force being less than that required to make her revolve in a circular orbit, when attracted towards the sun by the sum of these forces, she accordingly begins to approach the sun, and descends again to the conjunction. the attraction of the sun, however, being every where greater than that of the earth, the actual path of the moon around the sun is every where concave towards the latter. still, the elliptical path of the moon around the earth is to be conceived of, in the same way as though both bodies were at rest with respect to the sun. thus, while a steam-boat is passing _swiftly_ around an island, and a man is walking _slowly_ around a post in the cabin, the line which he describes in space between the forward motion of the boat and his circular motion around the post, may be every where concave towards the island, while his path around the post will still be the same as though both were at rest. a nail in the rim of a coach-wheel will turn around the axis of the wheel, when the coach has a forward motion, in the same manner as when the coach is at rest, although the line actually described by the nail will be the resultant of both motions, and very different from either. we have hitherto regarded the moon as describing a great circle on the face of the sky, such being the visible orbit, as seen by projection. but, on a more exact investigation, it is found that her orbit is not a circle, and that her motions are subject to very numerous irregularities. these will be best understood in connexion with the causes on which they depend. the law of universal gravitation has been applied with wonderful success to their developement, and its results have conspired with those of long-continued observation, to furnish the means of ascertaining with great exactness the place of the moon in the heavens, at any given instant of time, past or future, and thus to enable astronomers to determine longitudes, to calculate eclipses, and to solve other problems of the highest interest. the whole number of irregularities to which the moon is subject is not less than sixty, but the greater part are so small as to be hardly deserving of attention; but as many as thirty require to be estimated and allowed for, before we can ascertain the exact place of the moon at any given time. you will be able to understand something of the cause of these irregularities, if you first gain a distinct idea of the mutual actions of the sun, the moon, and the earth. the irregularities in the moon's motions are due chiefly to the disturbing influence of the sun, which operates in two ways; first, by acting unequally on the earth and moon; and secondly, by acting obliquely on the moon, on account of the inclination of her orbit to the ecliptic. if the sun acted equally on the earth and moon, and always in parallel lines, this action would serve only to restrain them in their annual motions around the sun, and would not affect their actions on each other, or their motions about their common centre of gravity. in that case, if they were allowed to fall towards the sun, they would fall equally, and their respective situations would not be affected by their descending equally towards it. but, because the moon is nearer the sun in one half of her orbit than the earth is, and in the other half of her orbit is at a greater distance than the earth from the sun, while the power of gravity is always greater at a less distance; it follows, that in one half of her orbit the moon is more attracted than the earth towards the sun, and, in the other half, less attracted than the earth. to see the effects of this process, let us suppose that the projectile motions of the earth and moon were destroyed, and that they were allowed to fall freely towards the sun. (see fig. , page .) if the moon was in conjunction with the sun, or in that part of her orbit which is nearest to him, the moon would be more attracted than the earth, and fall with greater velocity towards the sun; so that the distance of the moon from the earth would be increased by the fall. if the moon was in opposition, or in the part of her orbit which is furthest from the sun, she would be less attracted than the earth by the sun, and would fall with a less velocity, and be left behind; so that the distance of the moon from the earth would be increased in this case, also. if the moon was in one of the quarters, then the earth and the moon being both attracted towards the centre of the sun, they would both descend directly towards that centre, and, by approaching it, they would necessarily at the same time approach each other, and in this case their distance from each other would be diminished. now, whenever the action of the sun would increase their distance, if they were allowed to fall towards the sun, then the sun's action, by endeavoring to separate them, diminishes their gravity to each other; whenever the sun's action would diminish the distance, then it increases their mutual gravitation. hence, in the conjunction and opposition, their gravity towards each other is diminished by the action of the sun, while in the quadratures it is increased. but it must be remembered, that it is not the total action of the sun on them that disturbs their motions, but only that part of it which tends at one time to separate them, and at another time to bring them nearer together. the other and far greater part has no other effect than to retain them in their annual course around the sun. the cause of the lunar irregularities was first investigated by sir isaac newton, in conformity with his doctrine of universal gravitation, and the explanation was first published in the 'principia;' but, as it was given in a mathematical dress, there were at that age very few persons capable of reading or understanding it. several eminent individuals, therefore, undertook to give a popular explanation of these difficult points. among newton's contemporaries, the best commentator was m'laurin, a scottish astronomer, who published a large work entitled 'm'laurin's account of sir isaac newton's discoveries.' no writer of his own day, and, in my opinion, no later commentator, has equalled m'laurin, in reducing to common apprehension the leading principles of the doctrine of gravitation, and the explanation it affords of the motions of the heavenly bodies. to this writer i am indebted for the preceding easy explanation of the irregularities of the moon's motions, as well as for several other illustrations of the same sublime doctrine. the figure of the moon's orbit is an ellipse. we have before seen, that the earth's orbit around the sun is of the same figure; and we shall hereafter see this to be true of all the planetary orbits. the path of the earth, however, departs very little from a circle; that of the moon differs materially from a circle, being considerably longer one way than the other. were the orbit a circle having the earth in the centre, then the radius vector, or line drawn from the centre of the moon to the centre of the earth, would always be of the same length; but it is found that the length of the radius vector is only fifty-six times the radius of the earth when the moon is nearest to us, while it is sixty-four times that radius when the moon is furthest from us. the point in the moon's orbit nearest the earth is called her _perigee_; the point furthest from the earth, her _apogee_. we always know when the moon is at one of these points, by her apparent diameter or apparent velocity; for, when at the perigee, her diameter is greater than at any time, and her motion most rapid; and, on the other hand, her diameter is least, and her motion slowest, when she is at her apogee. the moon's nodes constantly shift their positions in the ecliptic, from east to west, at the rate of about nineteen and a half degrees every year, returning to the same points once in eighteen and a half years. in order to understand what is meant by this backward motion of the nodes, you must have very distinctly in mind the meaning of the terms themselves; and if, at any time, you should be at a loss about the signification of any word that is used in expressing an astronomical proposition, i would advise you to turn back to the previous definition of that term, and revive its meaning clearly in the mind, before you proceed any further. in the present case, you will recollect that the moon's nodes are the two points where her orbit cuts the plane of the ecliptic. suppose the great circle of the ecliptic marked out on the face of the sky in a distinct line, and let us observe, at any given time, the exact moment when the moon crosses this line, which we will suppose to be close to a certain star; then, on its next return to that part of the heavens, we shall find that it crosses the ecliptic sensibly to the westward of that star, and so on, further and further to the westward, every time it crosses the ecliptic at either node. this fact is expressed by saying that _the nodes retrograde on the ecliptic_; since any motion from east to west, being contrary to the order of the signs, is called retrograde. the line which joins these two points, or the line of the nodes, is also said to have a retrograde motion, or to revolve from east to west once in eighteen and a half years. the _line of the apsides_ of the moon's orbit revolves from west to east, through her whole course, in about nine years. you will recollect that the apsides of an elliptical orbit are the two extremities of the longer axis of the ellipse; corresponding to the perihelion and aphelion of bodies revolving about the sun, or to the perigee and apogee of a body revolving about the earth. if, in any revolution of the moon, we should accurately mark the place in the heavens where the moon is nearest the earth, (which may be known by the moon's apparent diameter being then greatest,) we should find that, at the next revolution, it would come to its perigee a little further eastward than before, and so on, at every revolution, until, after nine years, it would come to its perigee nearly at the same point as at first. this fact is expressed by saying, that the perigee, and of course the apogee, revolves, and that the line which joins these two points, or the line of the apsides, also revolves. these are only a few of the irregularities that attend the motions of the moon. these and a few others were first discovered by actual observation and have been long known; but a far greater number of lunar irregularities have been made known by following out all the consequences of the law of universal gravitation. the moon may be regarded as a body endeavoring to make its way around the earth, but as subject to be continually impeded, or diverted from its main course, by the action of the sun and of the earth; sometimes acting in concert and sometimes in opposition to each other. now, by exactly estimating the amount of these respective forces, and ascertaining their resultant or combined effect, in any given case, the direction and velocity of the moon's motion may be accurately determined. but to do this has required the highest powers of the human mind, aided by all the wonderful resources of mathematics. yet, so consistent is truth with itself, that, where some minute inequality in the moon's motions is developed at the end of a long and intricate mathematical process, it invariably happens, that, on pointing the telescope to the moon, and watching its progress through the skies, we may actually see her commit the same irregularities, unless (as is the case with many of them) they are too minute to be matters of observation, being beyond the powers of our vision, even when aided by the best telescopes. but the truth of the law of gravitation, and of the results it gives, when followed out by a chain of mathematical reasoning, is fully confirmed, even in these minutest matters, by the fact that the moon's place in the heavens, when thus determined, always corresponds, with wonderful exactness, to the place which she is actually observed to occupy at that time. the mind, that was first able to elicit from the operations of nature the law of universal gravitation, and afterwards to apply it to the complete explanation of all the irregular wanderings of the moon, must have given evidence of intellectual powers far elevated above those of the majority of the human race. we need not wonder, therefore, that such homage is now paid to the genius of newton,--an admiration which has been continually increasing, as new discoveries have been made by tracing out new consequences of the law of universal gravitation. the chief object of astronomical _tables_ is to give the amount of all the irregularities that attend the motions of the heavenly bodies, by estimating the separate value of each, under all the different circumstances in which a body can be placed. thus, with respect to the moon, before we can determine accurately the distance of the moon from the vernal equinox, that is, her longitude at any given moment, we must be able to make exact allowances for all her irregularities which would affect her longitude. these are in all no less than sixty, though most of them are so exceedingly minute, that it is not common to take into the account more than twenty-eight or thirty. the values of these are all given in the lunar tables; and in finding the moon's place, at any given time, we proceed as follows: we first find what her place would be on the supposition that she moves uniformly in a circle. this gives her _mean_ place. we next apply the various corrections for her irregular motions; that is, we apply the _equations_, subtracting some and adding others, and thus we find her _true_ place. the astronomical tables have been carried to such an astonishing degree of accuracy, that it is said, by the highest authority, that an astronomer could now predict, for a thousand years to come, the precise moment of the passage of any one of the stars over the meridian wire of the telescope of his transit-instrument, with such a degree of accuracy, that the error would not be so great as to remove the object through an angular space corresponding to the semidiameter of the finest wire that could be made; and a body which, by the tables, ought to appear in the transit-instrument in the middle of that wire, would in no case be removed to its outer edge. the astronomer, the mathematician, and the artist, have united their powers to produce this great result. the astronomer has collected the data, by long-continued and most accurate observations on the actual motions of the heavenly bodies, from night to night, and from year to year; the mathematician has taken these data, and applied to them the boundless resources of geometry and the calculus; and, finally, the instrument-maker has furnished the means, not only of verifying these conclusions, but of discovering new truths, as the foundation of future reasonings. since the points where the moon crosses the ecliptic, or the moon's nodes, constantly shift their positions about nineteen and a half degrees to the westward, every year, the sun, in his annual progress in the ecliptic, will go from the node round to the same node again in less time than a year, since the node goes to meet him nineteen and a half degrees to the west of the point where they met before. it would have taken the sun about nineteen days to have passed over this arc; and consequently, the interval between two successive conjunctions between the sun and the moon's node is about nineteen days shorter than the solar year of three hundred and sixty-five days; that is, it is about three hundred and forty-six days; or, more exactly, it is . days. the time from one new moon to another is . days. now, nineteen of the former periods are almost exactly equal to two hundred and twenty-three of the latter: for . Ã� = . days= y. d. and . Ã� = . " = " " " " hence, if the sun and moon were to leave the moon's node together, after the sun had been round to the same node nineteen times, the moon would have made very nearly two hundred and twenty-three conjunctions with the sun. if, therefore, she was in conjunction with the sun at the beginning of this period, she would be in conjunction again at the end of it; and all things relating to the sun, the moon, and the node, would be restored to the same relative situation as before, and the sun and moon would start again, to repeat the same phenomena, arising out of these relations, as occurred in the preceding period, and in the same order. now, when the sun and moon meet at the moon's node, an eclipse of the sun happens; and during the entire period of eighteen and a half years eclipses will happen, nearly in the same manner as they did at corresponding times in the preceding period. thus, if there was a great eclipse of the sun on the fifth year of one of these periods, a similar eclipse (usually differing somewhat in magnitude) might be expected on the fifth year of the next period. hence this period, consisting of about eighteen years and ten days, under the name of the _saros_, was used by the chaldeans, and other ancient nations, in predicting eclipses. it was probably by this means that thales, a grecian astronomer who flourished six hundred years before the christian era, predicted an eclipse of the sun. herodotus, the old historian of greece, relates that the day was suddenly changed into night, and that thales of miletus had foretold that a great eclipse was to happen _this year_. it was therefore, at that age, considered as a distinguished feat to predict even the year in which an eclipse was to happen. this eclipse is memorable in ancient history, from its having terminated the war between the lydians and the medes, both parties being smitten with such indications of the wrath of the gods. the _metonic cycle_ has sometimes been confounded with the saros, but it is not the same with it, nor was the period used, like the saros, for foretelling eclipses, but for ascertaining the _age_ of the moon at any given period. it consisted of nineteen tropical years, during which time there are exactly two hundred and thirty-five new moons; so that, at the end of this period, the new moons will recur at seasons of the year corresponding exactly to those of the preceding cycle. if, for example, a new moon fell at the time of the vernal equinox, in one cycle, nineteen years afterwards it would occur again at the same equinox; or, if it had happened ten days after the equinox, in one cycle, it would also happen ten days after the equinox, nineteen years afterwards. by registering, therefore, the exact days of any cycle at which the new or full moons occurred, such a calendar would show on what days these events would occur in any other cycle; and, since the regulation of games, feasts, and fasts, has been made very extensively, both in ancient and modern times, according to new or full moons, such a calendar becomes very convenient for finding the day on which the new or full moon required takes place. suppose, for example, it were decreed that a festival should be held on the day of the first full moon after the vernal equinox. then, to find on what day that would happen, in any given year, we have only to see what year it is of the lunar cycle; for the day will be the same as it was in the corresponding year of the calendar which records all the full moons of the cycle for each year, and the respective days on which they happen. the athenians adopted the metonic cycle four hundred and thirty-three years before the christian era, for the regulation of their calendars, and had it inscribed in letters of gold on the walls of the temple of minerva. hence the term _golden number_, still found in our almanacs, which denotes the year of the lunar cycle. thus, fourteen was the golden number for , being the fourteenth year of the lunar cycle. the inequalities of the moon's motions are divided into periodical and secular. _periodical_ inequalities are those which are completed in comparatively short periods. _secular_ inequalities are those which are completed only in very long periods, such as centuries or ages. hence the corresponding terms _periodical equations_ and _secular equations_. as an example of a secular inequality, we may mention the acceleration of the _moon's mean motion_. it is discovered that the moon actually revolves around the earth in a less period now than she did in ancient times. the difference, however, is exceedingly small, being only about ten seconds in a century. in a lunar eclipse, the moon's longitude differs from that of the sun, at the middle of the eclipse, by exactly one hundred and eighty degrees; and since the sun's longitude at any given time of the year is known, if we can learn the day and hour when an eclipse occurred at any period of the world, we of course know the longitude of the sun and moon at that period. now, in the year , before the christian era, ptolemy records a lunar eclipse to have happened, and to have been observed by the chaldeans. the moon's longitude, therefore, for that time, is known; and as we know the mean motions of the moon, at present, starting from that epoch, and computing, as may easily be done, the place which the moon ought to occupy at present, at any given time, she is found to be actually nearly a degree and a half in advance of that place. moreover, the same conclusion is derived from a comparison of the chaldean observations with those made by an arabian astronomer of the tenth century. this phenomenon at first led astronomers to apprehend that the moon encountered a resisting medium, which, by destroying at every revolution a small portion of her projectile force, would have the effect to bring her nearer and nearer to the earth, and thus to augment her velocity. but, in , la place demonstrated that this acceleration is one of the legitimate effects of the sun's disturbing force, and is so connected with changes in the eccentricity of the earth's orbit, that the moon will continue to be accelerated while that eccentricity diminishes; but when the eccentricity has reached its minimum, or lowest point, (as it will do, after many ages,) and begins to increase, then the moon's motions will begin to be retarded, and thus her mean motions will oscillate for ever about a mean value. letter xviii. eclipses. ----"as when the sun, new risen, looks through the horizontal misty air, shorn of his beams, or from behind the moon, in dim eclipse, disastrous twilight sheds on half the nations, and with fear of change perplexes monarchs: darkened so, yet shone, above them all, the archangel."--_milton._ having now learned various particulars respecting the earth, the sun, and the moon, you are prepared to understand the explanation of solar and lunar eclipses, which have in all ages excited a high degree of interest. indeed, what is more admirable, than that astronomers should be able to tell us, years beforehand, the exact instant of the commencement and termination of an eclipse, and describe all the attendant circumstances with the greatest fidelity. you have doubtless, my dear friend, participated in this admiration, and felt a strong desire to learn how it is that astronomers are able to look so far into futurity. i will endeavor, in this letter, to explain to you the leading principles of the calculation of eclipses, with as much plainness as possible. an _eclipse of the moon_ happens when the moon, in its revolution around the earth, falls into the earth's shadow. an _eclipse of the sun_ happens when the moon, coming between the earth and the sun, covers either a part or the whole of the solar disk. the earth and the moon being both opaque, globular bodies, exposed to the sun's light, they cast shadows opposite to the sun, like any other bodies on which the sun shines. were the sun of the same size with the earth and the moon, then the lines drawn touching the surface of the sun and the surface of the earth or moon (which lines form the boundaries of the shadow) would be parallel to each other, and the shadow would be a cylinder infinite in length; and were the sun less than the earth or the moon, the shadow would be an increasing cone, its narrower end resting on the earth; but as the sun is vastly greater than either of these bodies, the shadow of each is a cone whose base rests on the body itself, and which comes to a point, or vertex, at a certain distance behind the body. these several cases are represented in the following diagrams, figs. , , . [illustration figs. , , .] it is found, by calculation, that the length of the moon's shadow, on an average, is just about sufficient to reach to the earth; but the moon is sometimes further from the earth than at others, and when she is nearer than usual, the shadow reaches considerably beyond the surface of the earth. also, the moon, as well as the earth, is at different distances from the sun at different times, and its shadow is longest when it is furthest from the sun. now, when both these circumstances conspire, that is, when the moon is in her perigee and along with the earth in her aphelion, her shadow extends nearly fifteen thousand miles beyond the centre of the earth, and covers a space on the surface one hundred and seventy miles broad. the earth's shadow is nearly a million of miles in length, and consequently more than three and a half times as long as the distance of the earth from the moon; and it is also, at the distance of the moon, three times as broad as the moon itself. an eclipse of the sun can take place only at new moon, when the sun and moon meet in the same part of the heavens, for then only can the moon come between us and the sun; and an eclipse of the moon can occur only when the sun and moon are in opposite parts of the heavens, or at full moon; for then only can the moon fall into the shadow of the earth. [illustration fig. .] the nature of eclipses will be clearly understood from the following representation. the diagram, fig. , exhibits the relative position of the sun, the earth, and the moon, both in a solar and in a lunar eclipse. here, the moon is first represented, while revolving round the earth, as passing between the earth and the sun, and casting its shadow on the earth. as the moon is here supposed to be at her average distance from the earth, the shadow but just reaches the earth's surface. were the moon (as is sometimes the case) nearer the earth her shadow would not terminate in a point, as is represented in the figure, but at a greater or less distance nearer the base of the cone, so as to cover a considerable space, which, as i have already mentioned, sometimes extends to one hundred and seventy miles in breadth, but is commonly much less than this. on the other side of the earth, the moon is represented as traversing the earth's shadow, as is the case in a lunar eclipse. as the moon is sometimes nearer the earth and sometimes further off, it is evident that it will traverse the shadow at a broader or a narrower part, accordingly. the figure, however, represents the moon as passing the shadow further from the earth than is ever actually the case, since the distance from the earth is never so much as one third of the whole length of the shadow. it is evident from the figure, that if a spectator were situated where the moon's shadow strikes the earth, the moon would cut off from him the view of the sun, or the sun would be totally eclipsed. or, if he were within a certain distance of the shadow on either side, the moon would be partly between him and the sun, and would intercept from him more or less of the sun's light, according as he was nearer to the shadow or further from it. if he were at _c_ or _d_, he would just see the moon entering upon the sun's disk; if he were nearer the shadow than either of these points, he would have a portion of this light cut off from his view, and more, in proportion as he drew nearer the shadow; and the moment he entered the shadow, he would lose sight of the sun. to all places between _a_ or _b_ and the shadow, the sun would cast a partial shadow of the moon, growing deeper and deeper, as it approached the true shadow. this partial shadow is called the moon's _penumbra_. in like manner, as the moon approaches the earth's shadow, in a lunar eclipse, as soon as she arrives at _a_, the earth begins to intercept from her a portion of the sun's light, or she falls in the earth's penumbra. she continues to lose more and more of the sun's light, as she draws near to the shadow, and hence her disk becomes gradually obscured, until it enters the shadow, when the sun's light is entirely lost. as the sun and earth are both situated in the plane of the ecliptic, if the moon also revolved around the earth in this plane, we should have a solar eclipse at every new moon, and a lunar eclipse at every full moon; for, in the former case, the moon would come directly between us and the sun, and in the latter case, the earth would come directly between the sun and the moon. but the moon is inclined to the ecliptic about five degrees, and the centre of the moon may be all this distance from the centre of the sun at new moon, and the same distance from the centre of the earth's shadow at full moon. it is true, the moon extends across her path, one half her breadth lying on each side of it, and the sun likewise reaches from the ecliptic a distance equal to half his breadth. but these luminaries together make but little more than a degree, and consequently, their two semidiameters would occupy only about half a degree of the five degrees from one orbit to the other where they are furthest apart. also, the earth's shadow, where the moon crosses it, extends from the ecliptic less than three fourths of a degree, so that the semidiameter of the moon and of the earth's shadow would together reach but little way across the space that may, in certain cases, separate the two luminaries from each other when they are in opposition. thus, suppose we could take hold of the circle in the figure that represents the moon's orbit, (fig. , page ,) and lift the moon up five degrees above the plane of the paper, it is evident that the moon, as seen from the earth, would appear in the heavens five degrees above the sun, and of course would cut off none of his light; and it is also plain that the moon, at the full, would pass the shadow of the earth five degrees below it, and would suffer no eclipse. but in the course of the sun's apparent revolution round the earth once a year he is successively in every part of the ecliptic; consequently, the conjunctions and oppositions of the sun and moon may occur at any part of the ecliptic, and of course at the two points where the moon's orbit crosses the ecliptic,--that is, at the nodes; for the sun must necessarily come to each of these nodes once a year. if, then, the moon overtakes the sun just as she is crossing his path, she will hide more or less of his disk from us. since, also, the earth's shadow is always directly opposite to the sun, if the sun is at one of the nodes, the shadow must extend in the direction of the other node, so as to lie directly across the moon's path; and if the moon overtakes it there, she will pass through it, and be eclipsed. thus, in fig. , let bn represent the sun's path, and an, the moon's,--n being the place of the node; then it is evident, that if the two luminaries at new moon be so far from the node, that the distances between their centres is greater than their semidiameters, no eclipse can happen; but if that distance is less than this sum, as at e, f, then an eclipse will take place; but if the position be as at c, d, the two bodies will just touch one another. if a denotes the earth's shadow, instead of the sun, the same illustration will apply to an eclipse of the moon. [illustration fig. .] since bodies are defined to be in conjunction when they are in the _same_ part of the heavens, and to be in opposition when they are in _opposite_ parts of the heavens, it may not appear how the sun and moon can be in conjunction, as at a and b, when they are still at some distance from each other. but it must be recollected that bodies are in conjunction when they have the same longitude, in which case they are situated in the same great circle perpendicular to the ecliptic,--that is, in the same secondary to the ecliptic. one of these bodies may be much further from the ecliptic than the other; still, if the same secondary to the ecliptic passes through them both, they will be in conjunction or opposition. in a total eclipse of the moon, its disk is still visible, shining with a dull, red light. this light cannot be derived directly from the sun, since the view of the sun is completely hidden from the moon; nor by reflection from the earth, since the illuminated side of the earth is wholly turned from the moon; but it is owing to refraction from the earth's atmosphere, by which a few scattered rays of the sun are bent round into the earth's shadow and conveyed to the moon, sufficient in number to afford the feeble light in question. it is impossible fully to understand the _method of calculating eclipses_, without a knowledge of trigonometry; still it is not difficult to form some general notion of the process. it may be readily conceived that, by long-continued observations on the sun and moon, the laws of their revolution may be so well understood, that the exact places which they will occupy in the heavens at any future times may be foreseen and laid down in tables of the sun and moon's motions; that we may thus ascertain, by inspecting the tables, the instant when these two bodies will be together in the heavens, or be in conjunction, and when they will be one hundred and eighty degrees apart, or in opposition. moreover, since the exact place of the moon's node among the stars at any particular time is known to astronomers, it cannot be difficult to determine when the new or full moon occurs in the same part of the heavens as that where the node is projected, as seen from the earth. in short, as astronomers can easily determine what will be the relative position of the sun, the moon, and the moon's nodes, for any given time, they can tell when these luminaries will meet so near the node as to produce an eclipse of the sun, or when they will be in opposition so near the node as to produce an eclipse of the moon. a little reflection will enable you to form a clear idea of the situation of the sun, the moon, and the earth, at the time of a solar eclipse. first, suppose the conjunction to take place at the node; that is, imagine the moon to come _directly_ between the earth and the sun, as she will of course do, if she comes between the earth and the sun the moment she is crossing the ecliptic; for then the three bodies will all lie in one and the same straight line. but when the moon is in the ecliptic, her shadow, or at least the axis, or central line, of the shadow, must coincide with the line that joins the centres of the sun and earth, and reach along the plane of the ecliptic towards the earth. the moon's shadow, at her average distance from the earth, is just about long enough to reach the surface of the earth; but when the moon, at the new, is in her apogee, or at her greatest distance from the earth, the shadow is not long enough to reach the earth. on the contrary, when the moon is nearer to us than her average distance, her shadow is long enough to reach beyond the earth, extending, when the moon is in her perigee, more than fourteen thousand miles beyond the centre of the earth. now, as during the eclipse the moon moves nearly in the plane of the ecliptic, her shadow which accompanies her must also move nearly in the same plane, and must therefore traverse the earth across its central regions, along the terrestrial ecliptic, since this is nothing more than the intersection of the plane of the celestial ecliptic with the earth's surface. the motion of the earth, too, on its axis, in the same direction, will carry a place along with the shadow, though with a less velocity by more than one half; so that the actual velocity of the shadow, in respect to places over which it passes on the earth, will only equal the difference between its own rate and that of the places, as they are carried forward in the diurnal revolution. we have thus far supposed that the moon comes to her conjunction precisely at the node, or at the moment when she is crossing the ecliptic. but, secondly, suppose she is on the north side of the ecliptic at the time of conjunction, and moving towards her descending node, and that the conjunction takes place as far from the node as an eclipse can happen. the shadow will not fall in the plane of the ecliptic, but a little northward of it, so as just to graze the earth near the pole of the ecliptic. the nearer the conjunction comes to the node, the further the shadow will fall from the polar towards the equatorial regions. in a solar eclipse, the shadow of the moon travels over a portion of the earth, as the shadow of a small cloud, seen from an eminence in a clear day, rides along over hills and plains. let us imagine ourselves standing on the moon; then we shall see the earth partially eclipsed by the moon's shadow, in the same manner as we now see the moon eclipsed by the shadow of the earth; and we might calculate the various circumstances of the eclipse,--its commencement, duration, and quantity,--in the same manner as we calculate these elements in an eclipse of the moon, as seen from the earth. but although the general characters of a solar eclipse might be investigated on these principles, so far as respects the earth at large, yet, as the appearances of the same eclipse of the sun are very different at different places on the earth's surface, it is necessary to calculate its peculiar aspects for each place separately, a circumstance which makes the calculation of a solar eclipse much more complicated and tedious than that of an eclipse of the moon. the moon, when she enters the shadow of the earth, is deprived of the light of the part immersed, and the effect upon its appearance is the same as though that part were painted black, in which case it would be black alike to all places where the moon was above the horizon. but it not so with a solar eclipse. we do not see this by the shadow cast on the earth, as we should do, if we stood on the moon, but by the interposition of the moon between us and the sun; and the sun may be hidden from one observer, while he is in full view of another only a few miles distant. thus, a small insulated cloud sailing in a clear sky will, for a few moments, hide the sun from us, and from a certain space near us, while all the region around is illuminated. but although the analogy between the motions of the shadow of a small cloud and of the moon in a solar eclipse holds good in many particulars, yet the velocity of the lunar shadow is far greater than that of the cloud, being no less than two thousand two hundred and eighty miles per hour. the moon's shadow can never cover a space on the earth more than one hundred and seventy miles broad, and the space actually covered commonly falls much short of that. the portion of the earth's surface ever covered by the moon's penumbra is about four thousand three hundred and ninety-three miles. the apparent diameter of the moon varies materially at different times, being greatest when the moon is nearest to us, and least when she is furthest off; while the sun's apparent dimensions remain nearly the same. when the moon is at her average distance from the earth, she is just about large enough to cover the sun's disk; consequently, if, in a central eclipse of the sun, the moon is at her mean distance, she covers the sun but for an instant, producing only a momentary eclipse. if she is nearer than her average distance, then the eclipse may continue total some time, though never more than eight minutes, and seldom so long as that; but if she is further off than usual, or towards her apogee, then she is not large enough to cover the whole solar disk, but we see a ring of the sun encircling the moon, constituting an _annular eclipse_, as seen in fig. . even the elevation of the moon above the horizon will sometimes sensibly affect the dimensions of the eclipse. you will recollect that the moon is nearer to us when on the meridian than when in the horizon by nearly four thousand miles, or by nearly the radius of the earth; and consequently, her apparent diameter is largest when on the meridian. the difference is so considerable, that the same eclipse will appear total to a spectator who views it near his meridian, while, at the same moment, it appears annular to one who has the moon near his horizon. an annular eclipse may last, at most, twelve minutes and twenty-four seconds. [illustration fig. .] eclipses of the sun are more frequent than those of the moon. yet lunar eclipses being visible to every part of the terrestrial hemisphere opposite to the sun, while those of the sun are visible only to a small portion of the hemisphere on which the moon's shadow falls, it happens that, for any particular place on the earth, lunar eclipses are more frequently visible than solar. in any year, the number of eclipses of both luminaries cannot be less than two nor more than seven: the most usual number is four, and it is very rare to have more than six. a total eclipse of the moon frequently happens at the next full moon after an eclipse of the sun. for since, in a solar eclipse, the sun is at or near one of the moon's nodes,--that is, is projected to the place in the sky where the moon crosses the ecliptic,--the earth's shadow, which is of course directly opposite to the sun, must be at or near the other node, and may not have passed too far from the node before the moon comes round to the opposition and overtakes it. in total eclipses of the sun, there has sometimes been observed a remarkable radiation of light from the margin of the sun, which is thought to be owing to the zodiacal light, which is of such dimensions as to extend far beyond the solar orb. a striking appearance of this kind was exhibited in the total eclipse of the sun which occurred in june, . a total eclipse of the sun is one of the most sublime and impressive phenomena of nature. among barbarous tribes it is ever contemplated with fear and astonishment, and as strongly indicative of the displeasure of the gods. two ancient nations, the lydians and medes, alluded to before, who were engaged in a bloody war, about six hundred years before christ, were smitten with such awe, on the appearance of a total eclipse of the sun, just on the eve of a battle, that they threw down their arms, and made peace. when columbus first discovered america, and was in danger of hostility from the natives, he awed them into submission by telling them that the sun would be darkened on a certain day, in token of the anger of the gods at them, for their treatment of him. among cultivated nations, a total eclipse of the sun is recognised, from the exactness with which the time of occurrence and the various appearances answer to the prediction, as affording one of the proudest triumphs of astronomy. by astronomers themselves, it is of course viewed with the highest interest, not only as verifying their calculations, but as contributing to establish, beyond all doubt, the certainty of those grand laws, the truth of which is involved in the result. i had the good fortune to witness the total eclipse of the sun of june, , which was one of the most remarkable on record. to the wondering gaze of childhood it presented a spectacle that can never be forgotten. a bright and beautiful morning inspired universal joy, for the sky was entirely cloudless. every one was busily occupied in preparing smoked glass, in readiness for the great sight, which was to be first seen about ten o'clock. a thrill of mingled wonder and delight struck every mind when, at the appointed moment, a little black indentation appeared on the limb of the sun. this gradually expanded, covering more and more of the solar disk, until an increasing gloom was spread over the face of nature; and when the sun was wholly lost, near mid-day, a feeling of horror pervaded almost every beholder. the darkness was wholly unlike that of twilight or night. a thick curtain, very different from clouds, hung upon the face of the sky, producing a strange and indescribably gloomy appearance, which was reflected from all things on the earth, in hues equally strange and unnatural. some of the planets, and the largest of the fixed stars, shone out through the gloom, yet with their usual brightness. the temperature of the air rapidly declined, and so sudden a chill came over the earth, that many persons caught severe colds from their exposure. even the animal tribes exhibited tokens of fear and agitation. birds, especially, fluttered and flew swiftly about, and domestic fowls went to rest. indeed, the word _eclipse_ is derived from a greek word, (= ekleipsis=, _ekleipsis_,) which signifies to fail, to faint or swoon away; since the moon, at the period of her greatest brightness, falling into the shadow of the earth, was imagined by the ancients to sicken and swoon, as if she were going to die. by some very ancient nations she was supposed, at such times, to be in pain; and, in order to relieve her fancied distress, they lifted torches high in the atmosphere, blew horns and trumpets, beat upon brazen vessels, and even, after the eclipse was over, they offered sacrifices to the moon. the opinion also extensively prevailed, that it was in the power of witches, by their spells and charms, not only to darken the moon, but to bring her down from her orbit, and to compel her to shed her baleful influences upon the earth. in solar eclipses, also, especially when total, the sun was supposed to turn away his face in abhorrence of some atrocious crime, that either had been perpetrated or was about to be perpetrated, and to threaten mankind with everlasting night, and the destruction of the world. to such superstitions milton alludes, in the passage which i have taken for the motto of this letter. the chinese, who, from a very high period of antiquity, have been great observers of eclipses, although they did not take much notice of those of the moon, regarded eclipses of the sun in general as unfortunate, but especially such as occurred on the first day of the year. these were thought to forebode the greatest calamities to the emperor, who on such occasions did not receive the usual compliments of the season. when, from the predictions of their astronomers, an eclipse of the sun was expected, they made great preparation at court for observing it; and as soon as it commenced, a blind man beat a drum, a great concourse assembled, and the mandarins, or nobility, appeared in state. letter xix. longitude.--tides. "first in his east, the glorious lamp was seen, regent of day, and all the horizon round invested with bright rays, jocund to run his _longitude_ through heaven's high road; the gray dawn and the pleiades before him danced, shedding sweet influence."--_milton._ the ancients studied astronomy chiefly as subsidiary to astrology, with the vain hope of thus penetrating the veil of futurity, and reading their destinies among the stars. the moderns, on the other hand, have in view, as the great practical object of this study, the perfecting of the art of navigation. when we reflect on the vast interests embarked on the ocean, both of property and life, and upon the immense benefits that accrue to society from a safe and speedy intercourse between the different nations of the earth, we cannot but see that whatever tends to enable the mariner to find his way on the pathless ocean, and to secure him against its multiplied dangers, must confer a signal benefit on society. in ancient times, to venture out of sight of land was deemed an act of extreme audacity; and horace, the roman poet, pronounces him who first ventured to trust his frail bark to the stormy ocean, endued with a heart of oak, and girt with triple folds of brass. but now, the navigator who fully avails himself of all the resources of science, and especially of astronomy, may launch fearlessly on the deep, and almost bid defiance to rocks and tempests. by enabling the navigator to find his place on the ocean with almost absolute precision, however he may have been driven about by the winds, and however long he may have been out of sight of land, astronomers must be held as great benefactors to all who commit either their lives or their fortunes to the sea. nor have they secured to the art of navigation such benefits without incredible study and toil, in watching the motions of the heavenly bodies, in investigating the laws by which their movements are governed, and in reducing all their discoveries to a form easily available to the navigator, so that, by some simple observation on one or two of the heavenly bodies, with instruments which the astronomer has invented, and prepared for his use, and by looking out a few numbers in tables which have been compiled for him, with immense labor, he may ascertain the exact place he occupies on the surface of the globe, thousands of miles from land. the situation of any place is known by its latitude and longitude. as charts of every ocean and sea are furnished to the sailor, in which are laid down the latitudes and longitudes of every point of land, whether on the shores of islands or the main, he has, therefore, only to ascertain his latitude and longitude at any particular place on the ocean, in order to find where he is, with respect to the nearest point of land, although this may be, and may always have been, entirely out of sight to him. to determine the _latitude_ of a place is comparatively an easy matter, whenever we can see either the sun or the stars. the distance of the sun from the zenith, when on the meridian, on a given day of the year, (which distance we may easily take with the sextant,) enables us, with the aid of the tables, to find the latitude of the place; or, by taking the altitude of the north star, we at once obtain the latitude. the _longitude_ of a place may be found by any method, by which we may ascertain how much its time of day differs from that of greenwich at the same moment. a place that lies eastward of another comes to the meridian an hour earlier for every fifteen degrees of longitude, and of course has the hour of the day so much in advance of the other, so that it counts one o'clock when the other place counts twelve. on the other hand, a place lying westward of another comes to the meridian later by one hour for every fifteen degrees, so that it counts only eleven o'clock when the other place counts twelve. keeping these principles in view, it is easy to see that a comparison of the difference of time between two places at the same moment, allowing fifteen degrees for an hour, sixty minutes for every four minutes of time, and sixty seconds for every four seconds of time, affords us an accurate mode of finding the difference of longitude between the two places. this comparison may be made by means of a chronometer, or from solar or lunar eclipses, or by what is called the lunar method of finding the longitude. _chronometers_ are distinguished from clocks, by being regulated by means of a balance-wheel instead of a pendulum. a watch, therefore, comes under the general definition of a chronometer; but the name is more commonly applied to larger timepieces, too large to be carried about the person, and constructed with the greatest possible accuracy, with special reference to finding the longitude. suppose, then, we are furnished with a chronometer set to greenwich time. we arrive at new york, for example, and compare it with the time there. we find it is five hours in advance of the new-york time, indicating five o'clock, p.m., when it is noon at new york. hence we find that the longitude of new york is Ã� = degrees.[ ] the time at new york, or any individual place, can be known by observations with the transit-instrument, which gives us the precise moment when the sun is on the meridian. it would not be necessary to resort to greenwich, for the purpose of setting our chronometer to greenwich time, as it might be set at any place whose longitude is known, having been previously determined. thus, if we know that the longitude of a certain place is exactly sixty degrees east of greenwich, we have only to set our chronometer four hours behind the time at that place, and it will be regulated to greenwich time. hence it is a matter of the greatest importance to navigation, that the longitude of numerous ports, in different parts of the earth, should be accurately determined, so that when a ship arrives at any such port, it may have the means of setting or verifying its chronometer. this method of taking the longitude seems so easy, that you will perhaps ask, why it is not sufficient for all purposes, and accordingly, why it does not supersede the move complicated and laborious methods? why every sailor does not provide himself with a chronometer, instead of finding his longitude at sea by tedious and oft-repeated calculations, as he is in the habit of doing? i answer, it is only in a few extraordinary cases that chronometers have been constructed of such accuracy as to afford results as exact as those obtained by the other methods, to be described shortly; and instruments of such perfection are too expensive for general use among sailors. indeed, the more common chronometers cost too much to come within the means of a great majority of sea-faring men. moreover, by being transported from place to place, chronometers are liable to change their _rate_. by the rate of any timepiece is meant its deviation from perfect accuracy. thus, if a clock should gain one second per day, one day with another, and we should find it impossible to bring it nearer to the truth, we may reckon this as its rate, and allow for it in our estimate of the time of any particular observation. if the error was not uniform, but sometimes greater and sometimes less than one second per day, then the amount of such deviation is called its "variation from its mean rate." i introduce these minute statements, (which are more precise than i usually deem necessary,) to show you to what an astonishing degree of accuracy chronometers have in some instances been brought. they have been carried from london to baffin's bay, and brought back, after a three years' voyage, and found to have varied from their mean rate, during the whole time, only a second or two, while the extreme variation of several chronometers, tried at the royal observatory at greenwich, never exceeded a second and a half. could chronometers always be depended on to such a degree of accuracy as this, we should hardly desire any thing better for determining the longitude of different places on the earth. a recent determination of the longitude of the city hall in new york, by means of three chronometers, sent out from london expressly for that purpose, did not differ from the longitude as found by a solar eclipse (which is one of the best methods) but a second and a quarter. _eclipses of the sun and moon_ furnish the means of ascertaining the longitude of a place, because the entrance of the moon into the earth's shadow in a lunar eclipse, and the entrance of the moon upon the disk of the sun in a solar eclipse, are severally examples of one of those instantaneous occurrences in the heavens, which afford the means of comparing the times of different places, and of thus determining their differences of longitude. thus, if the commencement of a lunar eclipse was seen at one place an hour sooner than at another, the two places would be fifteen degrees apart, in longitude; and if the longitude of one of the places was known, that of the other would become known also. the exact instant of the moon's entering into the shadow of the earth, however, cannot be determined with very great precision, since the moon, in passing through the earth's penumbra, loses its light gradually, so that the moment when it leaves the penumbra and enters into the shadow cannot be very accurately defined. the first contact of the moon with the sun's disk, in a solar eclipse, or the moment of leaving it,--that is, the beginning and end of the eclipse,--are instants that can be determined with much precision, and accordingly they are much relied on for an accurate determination of the longitude. but, on account of the complicated and laborious nature of the calculation of the longitude from an eclipse of the sun, (since the beginning and end are not seen at different places, at the same moment,) this method of finding the longitude is not adapted to common use, nor available at sea. it is useful, however, for determining the longitude of fixed observatories. the _lunar method of finding the longitude_ is the most refined and accurate of all the modes practised at sea. the motion of the moon through the heavens is so rapid, that she perceptibly alters her distance from any star every minute; consequently, the moment when that distance is a certain number of degrees and minutes is one of those instantaneous events, which may be taken advantage of for comparing the times of different places, and thus determining their difference of longitude. now, in a work called the 'nautical almanac,' printed in london, annually, for the use of navigators, the distance of the moon from the sun by day, or from known fixed stars by night, for every day and night in the year, is calculated beforehand. if, therefore, a sailor wishes to ascertain his longitude, he may take with his sextant the distance of the moon from one of these stars at any time,--suppose nine o'clock, at night,--and then turn to the 'nautical almanac,' and see _what time it was at greenwich_ when the distance between the moon and that star was the same. let it be twelve o'clock, or three hours in advance of his time: his longitude, of course, is forty-five degrees west. this method requires more skill and accuracy than are possessed by the majority of seafaring men; but, when practised with the requisite degree of skill, its results are very satisfactory. captain basil hall, one of the most scientific commanders in the british navy, relates the following incident, to show the excellence of this method. he sailed from san blas, on the west coast of mexico, and, after a voyage of eight thousand miles, occupying eighty-nine days, arrived off rio de janeiro, having, in this interval, passed through the pacific ocean, rounded cape horn, and crossed the south atlantic, without making any land, or even seeing a single sail, with the exception of an american whaler off cape horn. when within a week's sail of rio, he set seriously about determining, by lunar observations, the precise line of the ship's course, and its situation at a determinate moment; and having ascertained this within from five to ten miles, ran the rest of the way by those more ready and compendious methods, known to navigators, which can be safely employed for short trips between one known point and another, but which cannot be trusted in long voyages, where the moon is the only sure guide. they steered towards rio janeiro for some days after taking the lunars, and, having arrived within fifteen or twenty miles of the coast, they hove to, at four in the morning, till the day should break, and then bore up, proceeding cautiously, on account of a thick fog which enveloped them. as this cleared away, they had the satisfaction of seeing the great sugar-loaf rock, which stands on one side of the harbor's mouth, so nearly right ahead, that they had not to alter their course above a point, in order to hit the entrance of the harbor. this was the first land they had seen for three months, after crossing so many seas, and being set backwards and forwards by innumerable currents and foul winds. the effect on all on board was electric; and the admiration of the sailors was unbounded. indeed, what could be more admirable than that a man on the deck of a vessel, by measuring the distance between the moon and a star, with a little instrument which he held in his hand, could determine his exact place on the earth's surface in the midst of a vast ocean, after having traversed it in all directions, for three months, crossing his track many times, and all the while out of sight of land? the lunar method of finding the longitude could never have been susceptible of sufficient accuracy, had not the motions of the moon, with all their irregularities, been studied and investigated by the most laborious and profound researches. hence newton, while wrapt in those meditations which, to superficial minds, would perhaps have appeared rather curious than useful, inasmuch as they respected distant bodies of the universe which seemed to have little connexion with the affairs of this world, was laboring night and day for the benefit of the sailor and the merchant. he was guiding the vessel of the one, and securing the merchandise of the other; and thus he contributed a large share to promote the happiness of his fellow-men, not only in exalting the powers of the human intellect, but also in preserving the lives and fortunes of those engaged in navigation and commerce. principles in science are rules in art; and the philosopher who is engaged in the investigation of these principles, although his pursuits may be thought less practically useful than those of the artisan who carries out those principles into real life, yet, without the knowledge of the principles, the rules would have never been known. studies, therefore, the most abstruse, are, when viewed as furnishing rules to act, often productive of the highest practical utility. since the _tides_ are occasioned by the influence of the sun and moon, i will conclude this letter with a few remarks on this curious phenomenon. by the tides are meant the alternate rising and falling of the waters of the ocean. its greatest and least elevations are called _high and low water_; its rising and falling are called _flood and ebb_; and the extraordinary high and low tides that occur twice every month are called _spring and neap tides_. it is high or low tide on opposite sides of the globe at the same time. if, for example, we have high water at noon, it is also high water to those who live on the meridian below us, where it is midnight. in like manner, low water occurs simultaneously on opposite sides of the meridian. the average amount of the tides for the whole globe is about two and a half feet; but their actual height at different places is very various, sometimes being scarcely perceptible, and sometimes rising to sixty or seventy feet. at the same place, also, the phenomena of the tides are very different at different times. in the bay of fundy, where the tide rises seventy feet, it comes in a mighty wave, seen thirty miles off, and roaring with a loud noise. at the mouth of the severn, in england, the flood comes up in one head about ten feet high, bringing certain destruction to any small craft that has been unfortunately left by the ebbing waters on the flats and as it passes the mouth of the avon, it sends up that small river a vast body of water, rising, at bristol, forty or fifty feet. tides are caused by the unequal attractions of the sun and moon upon different parts of the earth. suppose the projectile force by which the earth is carried forward in her orbit to be suspended, and the earth to fall towards one of these bodies,--the moon, for example,--in consequence of their mutual attraction. then, if all parts of the earth fell equally towards the moon, no derangement of its different parts would result, any more than of the particles of a drop of water, in its descent to the ground. but if one part fell faster than another, the different portions would evidently be separated from each other. now, this is precisely what takes place with respect to the earth, in its fall towards the moon. the portions of the earth in the hemisphere next to the moon, on account of being nearer to the centre of attraction, fall faster than those in the opposite hemisphere, and consequently leave them behind. the solid earth, on account of its cohesion, cannot obey this impulse, since all its different portions constitute one mass, which is acted on in the same manner as though it were all collected in the centre; but the waters on the surface, moving freely under this impulse, endeavor to desert the solid mass and fall towards the moon. for a similar reason, the waters in the opposite hemisphere, falling less towards the moon than the solid earth does, are left behind, or appear to rise. [illustration fig. .] but if the moon draws the waters of the earth into an oval form towards herself, raising them simultaneously on the opposite sides of the earth, they must obviously be drawn away from the intermediate parts of the earth, where it must at the same time be low water. thus, in fig. , the moon, m, raises the waters beneath itself at z and n, at which places it is high water, but at the same time depresses the waters at h and r, at which places it is low water. hence, the interval between the high and low tide, on successive days, is about fifty minutes, corresponding to the progress of the moon in her orbit from west to east, which causes her to come to the meridian about fifty minutes later every day. there occurs, however, an intermediate tide, when the moon is on the lower meridian, so that the interval between two high tides is about twelve hours, and twenty-five minutes. were it not for the impediments which prevent the force from producing its full effects, we might expect to see the great tide-wave, as the elevated crest is called, always directly beneath the moon, attending it regularly around the globe. but the inertia of the waters prevents their instantly obeying the moon's attraction, and the friction of the waters on the bottom of the ocean still further retards its progress. it is not, therefore, until several hours (differing at different places) after the moon has passed the meridian of a place, that it is high tide at that place. the _sun_ has an action similar to that of the moon, but only _one third_ as great. on account of the great mass of the sun, compared with that of the moon, we might suppose that his action in raising the tides would be greater than the moon's; but the nearness of the moon to the earth more than compensates for the sun's greater quantity of matter. as, however, wrong views are frequently entertained on this subject, let us endeavor to form a correct idea of the advantage which the moon derives from her proximity. it is not that her actual amount of attraction is thus rendered greater than that of the sun; but it is that her attraction for the _different parts_ of the earth is very unequal, while that of the sun is nearly uniform. it is the _inequality_ of this action, and not the absolute force, that produces the tides. the sun being ninety-five millions of miles from the earth, while the diameter of the earth is only one twelve thousandth part of this distance, the effects of the sun's attraction will be nearly the same on all parts of the earth, and therefore will not, as was explained of the moon, tend to separate the waters from the earth on the nearest side, or the earth from the waters on the remotest side, but in a degree proportionally smaller. but the diameter of the earth is one thirtieth the distance of the moon, and therefore the moon acts with considerably greater power on one part of the earth than on another. as the sun and moon both contribute to produce the tides, and as they sometimes act together and sometimes in opposition to each other, so corresponding variations occur in the height of the tide. the _spring tides_, or those which rise to an unusual height twice a month, are produced by the sun and moon's acting together; and the _neap tides_, or those which are unusually low twice a month, are produced by the sun and moon's acting in opposition to each other. the spring tides occur at the syzygies: the neap tides at the quadratures. at the time of new moon, the sun and moon both being on the same side of the earth, and acting upon it in the same line, their actions conspire, and the sun may be considered as adding so much to the force of the moon. we have already seen how the moon contributes to raise a tide on the opposite side of the earth. but the sun, as well as the moon, raises its own tide-wave, which at new moon coincides with the lunar tide-wave. this will be plain on inspecting the diagram, fig. , on page , where s represents the sun, c, the moon in conjunction, o, the moon in opposition, and z, n, the tide-wave. since the sun and moon severally raise a tide-wave, and the two here coincide, it is evident that a peculiarly high tide must occur when the two bodies are in conjunction, or at new moon. at full moon, also, the two luminaries conspire in the same way to raise the tide; for we must recollect that each body contributes to raise a tide on the opposite side. thus, when the sun is at s and the moon at o, the sun draws the waters on the side next to it away from the earth, and the moon draws the earth away from the waters on that side; their united actions, therefore, conspire, and an unusually high tide is the result. on the side next to o, the two forces likewise conspire: for while the moon draws the waters away from the earth, the sun draws the earth away from the waters. in both cases an unusually low tide is produced; for the more the water is elevated at z and n, the more it will be depressed at h and r, the places of low tide. [illustration fig. .] twice a month, also, namely, at the quadratures of the moon, the tides neither rise so high nor fall so low as at other times, because then the sun and moon act against each other. thus, in fig. , while f tends to raise the water at z, s tends to depress it, and consequently the high tide is less than usual. again, while f tends to depress the water at r, s tends to elevate it, and therefore the low tide is less than usual. hence the difference between high and low water is only half as great at neap as at spring tide. in the diagrams, the elevation and depression of the waters is represented, for the sake of illustration, as far greater than it really is; for you must recollect that the average height of the tides for the whole globe is only about two and a half feet, a quantity so small, in comparison with the diameter of the earth, that were the due proportions preserved in the figures, the effect would be wholly insensible. [illustration fig. .] the variations of distance in the sun are not great enough to influence the tides very materially, but the variations in the moon's distances have a striking effect. the tides which happen, when the moon is in perigee, are considerably greater than when she is in apogee; and if she happens to be in perigee at the time of the syzygies, the spring tides are unusually high. the motion of the tide-wave is not a _progressive_ motion, but a mere undulation, and is to be carefully distinguished from the currents to which it gives rise. if the ocean completely covered the earth, the sun and moon being in the equator, the tide-wave would travel at the same rate as the earth revolves on its axis. indeed, the correct way of conceiving of the tide-wave, is to consider the moon at rest, and the earth, in its rotation from west to east, as bringing successive portions of water under the moon, which portions being elevated successively, at the same rate as the earth revolves on its axis, have a relative motion westward, at the same rate. the tides of rivers, narrow bays, and shores far from the main body of the ocean, are not produced in those places by the direct action of the sun and moon, but are subordinate waves propagated from the great tide-wave, and are called _derivative_ tides, while those raised directly by the sun and moon are called _primitive_ tides. [illustration fig. .] the velocity with which the tide moves will depend on various circumstances, but principally on the depth, and probably on the regularity, of the channel. if the depth is nearly uniform the tides will be regular; but if some parts of the channel are deep while others are shallow, the waters will be detained by the greater friction of the shallow places, and the tides will be irregular. the direction, also, of the derivative tide may be totally different from that of the primitive. thus, in fig. , if the great tide-wave, moving from east to west, is represented by the lines , , , , the derivative tide, which is propagated up a river or bay, will be represented by the lines , , , , . advancing faster in the channel than next the bank, the tides will lag behind towards the shores, and the tide-wave will take the form of curves, as represented in the diagram. on account of the retarding influence of shoals, and an uneven, indented coast, the tide-wave travels more slowly along the shores of an island than in the neighboring sea, assuming convex figures at a little distance from the island, and on opposite sides of it. these convex lines sometimes meet, and become blended in such a way, as to create singular anomalies in a sea much broken by islands, as well as on coasts indented with numerous bays and rivers. peculiar phenomena are also produced, when the tide flows in at opposite extremities of a reef or island, as into the two opposite ends of long-island sound. in certain cases, a tide-wave is forced into a narrow arm of the sea, and produces very remarkable tides. the tides of the bay of fundy (the highest in the world) are ascribed to this cause. the tides on the coast of north america are derived from the great tide-wave of the south atlantic, which runs steadily northward along the coast to the mouth of the bay of fundy, where it meets the northern tide-wave flowing in the opposite direction. this accumulated wave being forced into the narrow space occupied by the bay, produces the remarkable tide of that place. the largest lakes and inland seas have no perceptible tides. this is asserted by all writers respecting the caspian and euxine; and the same is found to be true of the largest of the north-american lakes, lake superior. although these several tracts of water appear large, when taken by themselves, yet they occupy but small portions of the surface of the globe, as will appear evident from the delineation of them on the artificial globe. now, we must recollect that the primitive tides are produced by the _unequal_ action of the sun and moon upon the different parts of the earth; and that it is only at points whose distance from each other bears a considerable ratio to the whole distance of the sun or moon, that the inequality of action becomes manifest. the space required to make the effect sensible is larger than either of these tracts of water. it is obvious, also, that they have no opportunity to be subject to a derivative tide. although all must admit that the tides have _some connexion_ with the sun and the moon, yet there are so many seeming anomalies, which at first appear irreconcilable with the theory of gravitation, that some are unwilling to admit the explanation given by this theory. thus, the height of the tide is so various, that at some places on the earth there is scarcely any tide at all, while at other places it rises to seventy feet. the time of occurrence is also at many places wholly unconformable to the motions of the moon, as is required by the theory, being low water where it should be high water; or, instead of appearing just beneath the moon, as the theory would lead us to expect, following it at the distance of six, ten, or even fifteen, hours; and finally, the moon sometimes appears to have no part at all in producing the tide, but it happens uniformly at noon and midnight, (as is said to be the case at the society islands,) and therefore seems wholly dependent on the sun. notwithstanding these seeming inconsistencies with the law of universal gravitation, to which the explanation of the tides is referred, the correspondence of the tides to the motions of the sun and moon, in obedience to the law of attraction, is in general such as to warrant the application of that law to them, while in a great majority of the cases which appear to be exceptions to the operation of that law, local causes and impediments have been discovered, which modified or overruled the uniform operation of the law of gravitation. thus it does not disprove the reality of the existence of a force which carries bodies near the surface of the earth towards its centre, that we see them sometimes compelled, by the operation of local causes, to move in the opposite direction. a ball shot from a cannon is still subject to the law of gravitation, although, for a certain time, in obedience to the impulse given it, it may proceed in a line contrary to that in which gravity alone would carry it. the fact that water may be made to run up hill does not disprove the fact that it usually runs down hill by the force of gravity, or that it is still subject to this force, although, from the action of modifying or superior forces, it may be proceeding in a direction contrary to that given by gravity. indeed, those who have studied the doctrine of the tides most profoundly consider them as affording a striking and palpable exhibition of the truth of the doctrine of universal gravitation. footnote: [ ] the exact longitude of the city hall, in the city of new york, is h. m. . s. letter xx. planets.--mercury and venus. "first, mercury, amidst full tides of light, rolls next the sun, through his small circle bright; our earth would blaze beneath so fierce a ray, and all its marble mountains melt away. fair venus next fulfils her larger round, with softer beams, and milder glory crowned; friend to mankind, she glitters from afar, now the bright evening, now the morning, star."--_baker._ there is no study in which more is to be hoped for from a lucid arrangement, than in the study of astronomy. some subjects involved in this study appear very difficult and perplexing to the learner, before he has fully learned the doctrine of the sphere, and gained a certain familiarity with astronomical doctrines, which would seem very easy to him after he had made such attainments. such an order ought to be observed, as shall bring out the facts and doctrines of the science just in the place where the mind of the learner is prepared to receive them. some writers on astronomy introduce their readers at once to the most perplexing part of the whole subject,--the planetary motions. i have thought a different course advisable, and have therefore commenced these letters with an account of those bodies which are most familiarly known to us, the earth, the sun, and the moon. in connexion with the earth, we are able to acquire a good knowledge of the artificial divisions and points of reference that are established on the earth and in the heavens, constituting the doctrine of the sphere. you thus became familiar with many terms and definitions which are used in astronomy. these ought to be always very clearly borne in mind; and if you now meet with any term, the definition of which you have either partially or wholly forgotten, let me strongly recommend to you, to turn back and review it, until it becomes as familiar to you as household words. indeed, you will find it much to your advantage to go back frequently, and reiterate the earlier parts of the subject, before you advance to subjects of a more intricate nature. if this process should appear to you a little tedious, still you will find yourself fully compensated by the clear light in which all the succeeding subjects will appear. this clear and distinct perception of the ground we have been over shows us just where we are on our journey, and helps us to find the remainder of the way with far greater ease than we could otherwise do. i do not, however, propose by any devices to relieve you from the trouble of thinking. those who are not willing to incur this trouble can never learn much of astronomy. in introducing you to the planets, (which next claim our attention,) i will, in the first place, endeavor to convey to you some clear views of these bodies individually, and afterwards help you to form as correct a notion as possible of their motions and mutual relations. the name _planet_ is derived from a greek word, (= planêtês=, _planetes_,) which signifies a _wanderer_, and is applied to this class of bodies, because they shift their positions in the heavens, whereas the fixed stars constantly maintain the same places with respect to each other. the planets known from a high antiquity are, mercury, venus, earth, mars, jupiter, and saturn. to these, in , was added uranus, (or _herschel_, as it is sometimes called, from the name of its discoverer;) and, as late as the commencement of the present century, four more were added, namely, ceres, pallas, juno, and vesta. these bodies are designated by the following characters: . mercury, [planet: mercury] . venus, [planet: venus] . earth, [planet: earth] . mars, [planet: mars] . vesta, [planet: vesta] . juno, [planet: juno] . ceres, [planet: ceres] . pallas, [planet: pallas] . jupiter, [planet: jupiter] . saturn, [planet: saturn] . uranus, [planet: uranus] the foregoing are called the _primary_ planets. several of these have one or more attendants, or satellites, which revolve around them as they revolve around the sun. the earth has one satellite, namely, the moon; jupiter has four; saturn, seven; and uranus, six. these bodies are also planets, but, in distinction from the others, they are called _secondary_ planets. hence, the whole number of planets are twenty-nine, namely, eleven primary, and eighteen secondary, planets. you need never look for a planet either very far in the north or very far in the south, since they are always near the ecliptic. mercury, which deviates furthest from that great circle, never is seen more than seven degrees from it; and you will hardly ever see one of the planets so far from it as this, but they all pursue nearly the same great route through the skies, in their revolutions around the sun. the new planets, however, make wider excursions from the plane of the ecliptic, amounting, in the case of pallas, to thirty-four and a half degrees. mercury and venus are called _inferior_ planets, because they have their orbits nearer to the sun than that of the earth; while all the others, being more distant from the sun than the earth, are called _superior_ planets. the planets present great diversities among themselves, in respect to distance from the sun, magnitude, time of revolution, and density. they differ, also, in regard to satellites, of which, as we have seen, three have respectively four, six, and seven, while more than half have none at all. it will aid the memory, and render our view of the planetary system more clear and comprehensive, if we classify, as far as possible, the various particulars comprehended under the foregoing heads. as you have had an opportunity, in preceding letters, of learning something respecting the means which astronomers have of ascertaining the distances and magnitudes of these bodies, you will not doubt that they are really as great as they are represented; but when you attempt to conceive of spaces so vast, you will find the mind wholly inadequate to the task. it is indeed but a comparatively small space that we can fully comprehend at one grasp. still, by continual and repeated efforts, we may, from time to time, somewhat enlarge the boundaries of our mental vision. let us begin with some known and familiar space, as the distance between two places we are accustomed to traverse. suppose this to be one hundred miles. taking this as our measure, let us apply it to some greater distance, as that across the atlantic ocean,--say three thousand miles. from this step we may advance to some faint conception of the diameter of the earth; and taking that as a new measure, we may apply it to such greater spaces as the distance of the planets from the sun. i hope you will make trial of this method on the following comparative statements respecting the planets. _distances from the sun, in miles._ . mercury, , , . venus, , , . earth, , , . mars, , , . vesta, , , . juno, } . ceres, } , , . pallas, } . jupiter, , , . saturn, , , . uranus, or herschel, , , the _dimensions_ of the planetary system are seen from this table to be vast, comprehending a circular space thirty-six hundred millions of miles in diameter. a rail-way car, travelling constantly at the rate of twenty miles an hour, would require more than twenty thousand years to cross the orbit of uranus. _magnitudes._ diam. in miles. . mercury, . venus, . earth, . mars, . ceres, . jupiter, , . saturn, , . uranus, , we remark here a great diversity in regard to magnitude,--a diversity which does not appear to be subject to any definite law. while venus, an inferior planet, is nine tenths as large as the earth, mars, a superior planet, is only one seventh, while jupiter is twelve hundred and eighty-one times as large. although several of the planets, when nearest to us, appear brilliant and large, when compared with most of the fixed stars, yet the angle which they subtend is very small,--that of venus, the greatest of all, never exceeding about one minute, which is less than one thirtieth the apparent diameter of the sun or moon. jupiter, also, by his superior brightness, sometimes makes a striking figure among the stars; yet his greatest apparent diameter is less than one fortieth that of the sun. _periodic times_. mercury revolves around the sun in nearly months. venus, " " " " - / " earth, " " " " year. mars, " " " " years. ceres, " " " " - / " jupiter, " " " " " saturn, " " " " " uranus, " " " " " from this view, it appears that the planets nearest the sun move most rapidly. thus, mercury performs nearly three hundred and fifty revolutions while uranus performs one. the apparent progress of the most distant planets around the sun is exceedingly slow. uranus advances only a little more than four degrees in a whole year; so that we find this planet occupying the same sign, and of course remaining nearly in the same part of the heavens, for several years in succession. after this comparative view of the planets in general, let us now look at them individually; and first, of the inferior planets, mercury and venus. mercury and venus, having their orbits so far within that of the earth, appear to us as attendants upon the sun. mercury never appears further from the sun than twenty-nine degrees, and seldom so far; and venus, never more than about forty-seven degrees. both planets, therefore, appear either in the west soon after sunset, or in the east a little before sunrise. in high latitudes, where the twilight is long, mercury can seldom be seen with the naked eye, and then only when its angular distance from the sun is greatest. copernicus, the great prussian astronomer, (who first distinctly established the order of the solar system, as at present received,) lamented, on his death-bed, that he had never been able to obtain a sight of mercury; and delambre, a distinguished astronomer of france, saw it but twice. in our latitude, however, we may see this planet for several evenings and mornings, if we will watch the time (as usually given in the almanac) when it is at its greatest elongations from the sun. it will not, however, remain long for our gaze, but will soon run back to the sun. the reason of this will be readily understood from the following diagram, fig. . let s represent the sun, e, the earth, and m, n, mercury at its greatest elongations from the sun, and o z p, a portion of the sky. then, since we refer all distant bodies to the same concave sphere of the heavens, it is evident that we should see the sun at z, and mercury at o, when at its greatest eastern elongation, and at p, when at its greatest western elongation; and while passing from m to n through q, it would appear to describe the arc o p; and while passing from n to m through r, it would appear to run back across the sun on the same arc. it is further evident that it would be visible only when at or near one of its greatest elongations; being at all other times so near the sun as to be lost in his light. [illustration fig. .] a planet is said to be in _conjunction_ with the sun when it is seen in the same part of the heavens with the sun. mercury and venus have each two conjunctions, the inferior and the superior conjunction. the _inferior conjunction_ is its position when in conjunction on the same side of the sun with the earth, as at q, in the figure; the _superior conjunction_ is its position when on the side of the sun most distant from the earth, as at r. the time which a planet occupies in making one entire circuit of the heavens, from any star, until it comes round to the same star again, is called its _sidereal revolution_. the period occupied by a planet between two successive conjunctions with the earth is called its _synodical revolution_. both the planet and the earth being in motion, the time of the synodical revolution of mercury or venus exceeds that of the sidereal; for when the planet comes round to the place where it before overtook the earth, it does not find the earth at that point, but far in advance of it. thus, let mercury come into inferior conjunction with the earth at c, fig. . in about eighty-eight days, the planet will come round to the same point again; but, mean-while, the earth has moved forward through the arc e e´, and will continue to move while the planet is moving more rapidly to overtake her; the case being analogous to that of the hour and minute hand of a clock. [illustration fig. .] the synodical period of mercury is one hundred and sixteen days, and that of venus five hundred and eighty-four days. the former is increased twenty-eight days, and the latter, three hundred and sixty days, by the motion of the earth; so that venus, after being in conjunction with the earth, goes more than twice round the sun before she comes into conjunction again. for, since the earth is likewise in motion, and moves more than half as fast as venus, by the time the latter has gone round and returned to the place where the two bodies were together, the earth is more than half way round, and continues moving, so that it will be a long time before venus comes up with it. the motion of an inferior planet is _direct_ in passing through its superior conjunction, and _retrograde_ in passing through its inferior conjunction. you will recollect that the motion of a heavenly body is said to be direct when it is in the order of the signs from west to east, and retrograde when it is contrary to the order of the signs, or from east to west. now venus, while going from b through d to a, (fig. ,) moves from west to east, and would appear to traverse the celestial vault b´ s´ a´, from right to left; but in passing from a through c to b, her course would be retrograde, returning on the same arc from left to right. if the earth were at rest, therefore, (and the sun, of course, at rest,) the inferior planets would appear to oscillate backwards and forwards across the sun. but it must be recollected that the earth is moving in the same direction with the planet, as respects the signs, but with a slower motion. this modifies the motions of the planet, accelerating it in the superior, and retarding it in the inferior, conjunction. thus, in fig. , venus, while moving through b d a, would seem to move in the heavens from b´ to a´, were the earth at rest; but, mean-while, the earth changes its position from e to e´, on which account the planet is not seen at a´, but at a´´, being accelerated by the arc a´ a´´, in consequence of the earth's motion. on the other hand, when the planet is passing through its inferior conjunction a c b, it appears to move backwards in the heavens from a´ to b´, if the earth is at rest, but from a´ to b´´, if the earth has in the mean time moved from e to e´, being retarded by the arc b´ b´´. although the motions of the earth have the effect to accelerate the planet in the superior conjunction, and to retard it in the inferior, yet, on account of the greater distance, the apparent motion of the planet is much slower in the superior than in the inferior conjunction, venus being the whole breadth of her orbit, or one hundred and thirty-six millions of miles further from us when at her greatest, than when at her least, distance, as is evident from fig. . when passing from the superior to the inferior conjunction, or from the inferior to the superior, through the greatest elongations, the inferior planets are _stationary_. thus, (fig. ,) when the planet is at a, the earth being at e, as the planet's motion is directly towards the spectator, he would constantly project it at the same point in the heavens, namely, a´; consequently, it would appear to stand still. or, when at its greatest elongation on the other side, at b, as its motion would be directly from the spectator, it would be seen constantly at b´. if the earth were at rest, the stationary points would be at the greatest elongations, as at a and b; but the earth itself is moving nearly at right angles to the planet's motion, which makes the planet appear to move in the opposite direction. its direct motion will therefore continue longer on the one side, and its retrograde motion longer on the other side, than would be the case, were it not for the motion of the earth. mercury, whose greatest angular distance from the sun is nearly twenty-nine degrees, is stationary at an elongation of from fifteen to twenty degrees; and venus, at about twenty-nine degrees, although her greatest elongation is about forty-seven degrees. mercury and venus exhibit to the telescope _phases_ similar to those of the moon. when on the side of their inferior conjunction, as from b to c through d, fig. , less than half their enlightened disk is turned towards us, and they appear horned, like the moon in her first and last quarters; and when on the side of the superior conjunction, as from c to b through a, more than half the enlightened disk is turned towards us, and they appear gibbous. at the moment of superior conjunction, the whole enlightened orb of the planet is turned towards the earth, and the appearance would be that of the full moon; but the planet is too near the sun to be commonly visible. [illustration fig. .] we should at first thought expect, that each of these planets would be largest and brightest near their inferior conjunction, being then so much nearer to us than at other times; but we must recollect that, when in this situation, only a small part of the enlightened disk is turned toward us. still, the period of greatest brilliancy cannot be when most of the illuminated side is turned towards us, for then, being at the superior conjunction, its light will be diminished, both by its great distance, and by its being so near the sun as to be partially lost in the twilight. hence, when venus is a little within her place of greatest elongation, about forty degrees from the sun, although less than half her disk is enlightened, yet, being comparatively near to us, and shining at a considerable altitude after the evening or before the morning twilight, she then appears in greatest splendor, and presents an object admired for its beauty in all ages. thus milton, "fairest of stars, last in the train of night, if better thou belong not to the dawn, sure pledge of day that crown'st the smiling morn with thy bright circlet." mercury and venus both _revolve on their axes_ in nearly the same time with the earth. the diurnal period of mercury is a little greater, and that of venus a little less, than twenty-four hours. these revolutions have been determined by means of some spot or mark seen by the telescope, as the revolution of the sun on his axis is ascertained by means of his spots. mercury owes most of its peculiarities to its proximity to the sun. its light and heat, derived from the sun, are estimated to be neatly seven times as great as on the earth, and the apparent magnitude of the sun to a spectator on mercury would be seven times greater than to us. hence the sun would present to an inhabitant of that planet, with eyes like ours, an object of insufferable brightness; and all objects on the surface would be arrayed in a light more glorious than we can well imagine. (see fig. .) the average heat on the greater portion of this planet would exceed that of boiling water, and therefore be incompatible with the existence both of an animal and a vegetable kingdom constituted like ours. the motion of mercury, in his revolution round the sun, is swifter than that of any other planet, being more than one hundred thousand miles every hour; whereas that of the earth is less than seventy thousand. eighteen hundred miles every minute,--crossing the atlantic ocean in less than two minutes,--this is a velocity of which we can form but a very inadequate conception, although, as we shall see hereafter, it is far less than comets sometimes exhibit. venus is regarded as the most beautiful of the planets, and is well known as the _morning and evening star_. the most ancient nations, indeed, did not recognise the morning and evening star as one and the same body, but supposed they were different planets, and accordingly gave them different names, calling the morning star lucifer, and the evening star hesperus. at her period of greatest splendor, venus casts a shadow, and is sometimes visible in broad daylight. her light is then estimated as equal to that of twenty stars of the first magnitude. in the equatorial regions of the earth, where the twilight is short, and venus, at her greatest elongation, appears very high above the horizon, her splendors are said to be far more conspicuous than in our latitude. [illustration fig. . apparent magnitudes of the sun, as seen from the different planets.] [illustration figures , , . venus and mars.] every eight years, venus forms her conjunction with the sun in the same part of the heavens. whatever appearances, therefore, arise from her position with respect to the earth and the sun, they are repeated every eight years, in nearly the same form. thus, every eight years, venus is remarkably conspicuous, so as to be visible in the day-time, being then most favorably situated, on several accounts; namely, being nearest the earth, and at the point in her orbit where she gives her greatest brilliancy, that is, a little within the place of greatest elongation. this is the period for obtaining fine telescopic views of venus, when she is seen with spots on her disk. thus two figures of the annexed diagram (fig. ) represent venus as seen near her inferior conjunction, and at the period of maximum brilliancy. the former situation is favorable for viewing her inequalities of surface, as indicated by the roughness of the line which separates the enlightened from the unenlightened part, (the _terminator_.) according to schroeter, a german astronomer, venus has mountains twenty-two miles high. her mountains, however, are much more difficult to be seen than those of the moon. the sun would appear, as seen from venus, twice as large as on the earth, and its light and heat would be augmented in the same proportion. (see fig. .) in many respects, however, the phenomena of this planet are similar to those of our own; and the general likeness between venus and the earth, in regard to dimensions, revolutions, and seasons, is greater than exists between any other two bodies of the system. i will only add to the present letter a few words on the _transits_ of the inferior planets. the transit of mercury or venus is its passage across the sun's disk, as the moon passes over it in a solar eclipse. the planet is seen projected on the sun's disk in a small, black, round spot, moving slowly over the face of the sun. as the transit takes place only when the planet is in inferior conjunction, at which time her motion is retrograde, it is always from left to right; and, on account of its motion being retarded by the motion of the earth, (as was explained by fig. , page ,) it remains sometimes a long time on the solar disk. mercury, when it makes its transit across the sun's centre, may remain on the sun from five to seven hours. you may ask, why we do not observe this appearance every time one of the inferior planets comes into inferior conjunction, for then, of course, it passes between us and the sun. it must, indeed, at this time, cross the meridian at the same time with the sun; but, because its orbit is inclined to that of the sun, it may cross it (and generally does) a little above or a little below the sun. it is only when the conjunction takes place at or very near the point where the two orbits cross one another, that is, near the _node_, that a transit can occur. thus, if the orbit of mercury, n m r, fig. , (page ,) were in the same plane with the earth's orbit, (and of course with the sun's apparent orbit,) then, when the planet was at q, in its inferior conjunction, the earth being at e, it would always be projected on the sun's disk at z, on the concave sphere of the heavens, and a transit would happen at every inferior conjunction. but now let us take hold of the point r, and lift the circle which represents the orbit of mercury upwards seven degrees, letting it turn upon the diameter _d b_; then, we may easily see that a spectator at e would project the planet higher in the heavens than the sun; and such would always be the case, except when the conjunction takes place at the node. then the point of intersection of the two orbits being in one and the same plane, both bodies would be referred to the same point on the celestial sphere. as the sun, in his apparent revolution around the earth every year, passes through every point in the ecliptic, of course he must every year be at each of the points where the orbit of mercury or venus crosses the ecliptic, that is, at each of the nodes of one of these planets;[ ] and as these nodes are on opposite sides of the ecliptic, consequently, the sun will pass through them at opposite seasons of the year, as in january and july, february and august. now, should mercury or venus happen to come between us and the sun, just as the sun is passing one of the planet's nodes, a transit would happen. hence the transits of mercury take place in may and november, and those of venus, in june and december. transits of mercury occur more frequently than those of venus. the periodic times of mercury and the earth are so adjusted to each other, that mercury performs nearly twenty-nine revolutions while the earth performs seven. if, therefore, the two bodies meet at the node in any given year, seven years afterwards they will meet nearly at the same node, and a transit may take place, accordingly, at intervals of seven years. but fifty-four revolutions of mercury correspond still nearer to thirteen revolutions of the earth; and therefore a transit is still more probable after intervals of thirteen years. at intervals of thirty-three years, transits of mercury are exceedingly probable, because in that time mercury makes almost exactly one hundred and thirty-seven revolutions. intermediate transits, however, may occur at the other node. thus, transits of mercury happened at the ascending node in , and , at intervals of seven years; and at the descending node in , which will return in , after thirteen years. transits of venus are events of very unfrequent occurrence. eight revolutions of the earth are completed in nearly the same time as thirteen revolutions of venus; and hence two transits of venus may occur after an interval of eight years, as was the case at the last return of the phenomenon, one transit having occurred in , and another in . but if a transit does not happen after eight years, it will not happen at the same node, until an interval of two hundred and thirty-five years: but intermediate transits may occur at the other node. the next transit of venus will take place in , being two hundred and thirty-five years after the first that was ever _observed_, which occurred in . this was seen, for the first time by mortal eyes, by two youthful english astronomers, horrox and crabtree. horrox was a young man of extraordinary promise, and indicated early talents for practical astronomy, which augured the highest eminence; but he died in the twenty-third year of his age. he was only twenty when the transit appeared, and he had made the calculations and observations, by which he was enabled to anticipate its arrival several years before. at the approach of the desired time for observing the transit, he received the sun's image through a telescope in a dark room upon a white piece of paper, and after waiting many hours with great impatience, (as his calculation did not lead him to a knowledge of the precise time of the occurrence,) at last, on the twenty-fourth of november, , old style, at three and a quarter hours past twelve, just as he returned from church, he had the pleasure to find a large round spot near the limb of the sun's image. it moved slowly across the sun's disk, but had not entirely left it when the sun set. the great interest attached by astronomers to a transit of venus arises from its furnishing the most accurate means in our power of determining the _sun's horizontal parallax_,--an element of great importance, since it leads us to a knowledge of the distance of the earth from the sun, which again affords the means of estimating the distances of all the other planets, and possibly, of the fixed stars. hence, in , great efforts were made throughout the civilized world, under the patronage of different governments, to observe this phenomenon under circumstances the most favorable for determining the parallax of the sun. the common methods of finding the parallax of a heavenly body cannot be relied on to a greater degree of accuracy than four seconds. in the case of the moon, whose greatest parallax amounts to about one degree, this deviation from absolute accuracy is not very material; but it amounts to nearly half the entire parallax of the sun. if the sun and venus were equally distant from us, they would be equally affected by parallax, as viewed by spectators in different parts of the earth, and hence their _relative_ situation would not be altered by it; but since venus, at the inferior conjunction, is only about one third as far off as the sun, her parallax is proportionally greater, and therefore spectators at distant points will see venus projected on different parts of the solar disk, as the planet traverses the disk. astronomers avail themselves of this circumstance to ascertain the sun's horizontal parallax, which they are enabled to do by comparing it with that of venus, in a manner which, without a knowledge of trigonometry, you will not fully understand. in order to make the difference in the apparent places of venus on the sun's disk as great as possible, very distant places are selected for observation. thus, in the transits of and , several of the european governments fitted out expensive expeditions to parts of the earth remote from each other. for this purpose, the celebrated captain cook, in , went to the south pacific ocean, and observed the transit at the island of otaheite, while others went to lapland, for the same purpose, and others still, to many other parts of the globe. thus, suppose two observers took their stations on opposite sides of the earth, as at a, and b, fig. , page ; at a, the planet v would be seen on the sun's disk at _a_, while at b, it would be seen at _b_. the appearance of venus on the sun's disk being that of a well-defined black spot, and the exactness with which the moment of external or internal contact may be determined, are circumstances favorable to the exactness of the result; and astronomers repose so much confidence in the estimation of the sun's horizontal parallax, as derived from observations on the transit of , that this important element is thought to be ascertained within one tenth of a second. the general result of all these observations gives the sun's horizontal parallax eight seconds and six tenths,--a result which shows at once that the sun must be a great way off, since the semidiameter of the earth, a line nearly four thousand miles in length, would appear at the sun under an angle less than one four hundredth of a degree. during the transits of venus over the sun's disk, in and , a sort of penumbral light was observed around the planet, by several astronomers, which was thought to indicate an _atmosphere_. this appearance was particularly observable while the planet was coming on or going off the solar disk. the total immersion and emersion were not instantaneous; but as two drops of water, when about to separate, form a ligament between them, so there was a dark shade stretched out between venus and the sun; and when the ligament broke, the planet seemed to have got about an eighth part of her diameter from the limb of the sun. the various accounts of the two transits abound with remarks like these, which indicate the existence of an atmosphere about venus of nearly the density and extent of the earth's atmosphere. similar proofs of the existence of an atmosphere around this planet are derived from appearances of twilight. [illustration fig. .] the elder astronomers imagined that they had discovered a _satellite_ accompanying venus in her transit. if venus had in reality any satellite, the fact would be obvious at her transits, as, in some of them at least, it is probable that the satellite would be projected near the primary on the sun's disk; but later astronomers have searched in vain for any appearances of the kind, and the inference is, that former astronomers were deceived by some optical illusion. footnote: [ ] you will recollect that the sun is said to be at the node, when the places of the node and the sun are both projected, by a spectator on the earth, upon the same part of the heavens. letter xxi. superior planets: mars, jupiter, saturn, and uranus. "with what an awful, world-revolving power, were first the unwieldy planets launched along the illimitable void! there to remain amidst the flux of many thousand years, that oft has swept the toiling race of men, and all their labored monuments, away."--_thomson._ mercury and venus, as we have seen, are always observed near the sun, and from this circumstance, as well as from the changes of magnitude and form which they undergo, we know that they have their orbits within that of the earth, and hence we call them _inferior_ planets. on the other hand, mars, jupiter, saturn, and uranus, exhibit such appearances, at different times, as show that they revolve around the sun at a greater distance than the earth, and hence we denominate them _superior_ planets. we know that they never come between us and the sun, because they never undergo those changes which mercury and venus, as well as the moon, sustain, in consequence of their coming into such a position. they, however, wander to the greatest angular distance from the sun, being sometimes seen one hundred and eighty degrees from him, so as to rise when the sun sets. all these different appearances must naturally result from their orbits' being exterior to that of the earth, as will be evident from the following representation. let e, fig. , page , be the earth, and m, one of the superior planets, mars, for example, each body being seen in its path around the sun. at m, the planet would be in opposition to the sun, like the moon at the full; at q and q´, it would be seen ninety degrees off, or in quadrature; and at m´, in conjunction. we know, however, that this must be a superior and not an inferior conjunction, for the illuminated disk is still turned towards us; whereas, if it came between us and the sun, like mercury, or venus, in its inferior conjunction, its dark side would be presented to us. [illustration fig. .] the superior planets do not exhibit to the telescope different phases, but, with a single exception, they always present the side that is turned towards the earth fully enlightened. this is owing to their great distance from the earth; for were the spectator to stand upon the sun, he would of course always have the illuminated side of each of the planets turned towards him; but so distant are all the superior planets, except mars, that they are viewed by us very nearly, in the same manner as they would be if we actually stood on the sun. mars, however, is sufficiently near to appear somewhat gibbous when at or near one of its quadratures. thus, when the planet is at q, it is plain that, of the hemisphere that is turned towards the earth, a small part is unilluminated. mars is a small planet, his diameter being only about half that of the earth, or four thousand two hundred miles. he also, at times, comes nearer to the earth than any other planet, except venus. his _mean_ distance from the sun is one hundred and forty-two millions of miles; but his orbit is so elliptical, that his distance varies much in different parts of his revolution. mars is always very near the ecliptic, never varying from it more than two degrees. he is distinguished from all the planets by his deep red color, and fiery aspect; but his brightness and apparent magnitude vary much, at different times, being sometimes nearer to us than at others by the whole diameter of the earth's orbit; that is, by about one hundred and ninety millions of miles. when mars is on the same side of the sun with the earth, or at his opposition, he comes within forty-seven millions of miles of the earth, and, rising about the time the sun sets, surprises us by his magnitude and splendor; but when he passes to the other side of the sun, to his superior conjunction, he dwindles to the appearance of a small star, being then two hundred and thirty-seven millions of miles from us. thus, let m, fig, , represent mars in opposition, and m´, in the superior conjunction, while e represents the earth. it is obvious that, in the former situation, the planet must be nearer to the earth than in the latter, by the whole diameter of the earth's orbit. when viewed with a powerful telescope, the surface of mars appears diversified with numerous varieties of light and shade. the region around the poles is marked by white spots, (see fig. , page ,) which vary their appearances with the changes of seasons in the planet. hence dr. herschel conjectured that they were owing to ice and snow, which alternately accumulate and melt away, according as it is winter or summer, in that region. they are greatest and most conspicuous when that part of the planet has just emerged from a long winter, and they gradually waste away, as they are exposed to the solar heat. fig. , represents the planet, as exhibited, under the most favorable circumstances, to a powerful telescope, at the time when its gibbous form is strikingly obvious. it has been common to ascribe the ruddy light of mars to an extensive and dense atmosphere, which was said to be distinctly indicated by the gradual diminution of light observed in a star, as it approaches very near to the planet, in undergoing an occultation; but more recent observations afford no such evidence of an atmosphere. by observations on the spots, we learn that mars revolves on his axis in very nearly the same time with the earth, (twenty-four hours thirty-nine minutes twenty-one seconds and three tenths,) and that the inclination of his axis to that of his orbit is also nearly the same, being thirty degrees eighteen minutes ten seconds and eight tenths. hence the changes of day and night must be nearly the same there as here, and the seasons also very similar to ours. since, however, the distance of mars from the sun is one hundred and forty-two while that of the earth is only ninety-five millions of miles, the sun will appear more than twice as small on that planet as on ours, (see fig. , page ,) and its light and heat will be diminished in the same proportion. only the equatorial regions, therefore, will be suitable for the existence of animals and vegetables. the earth will be seen from mars as an inferior planet, always near the sun, presenting appearances similar, in many respects, to those which venus presents to us. it will be to that planet the evening and morning star, sung by their poets (if poets they have) with a like enthusiasm. the moon will attend the earth as a little star, being never seen further from her side than about the diameter under which we view the moon. to the telescope, the earth will exhibit phases similar to those of venus; and, finally, she will, at long intervals, make her transits over the solar disk. mean-while, venus will stand to mars in a relation similar to that of mercury [illustration figures , . jupiter and saturn.] to us, revealing herself only when at the periods of her greatest elongation, and at all other times hiding herself within the solar blaze. mercury will never be visible to an inhabitant of mars. jupiter is distinguished from all the other planets by his great _magnitude_. his diameter is eighty-nine thousand miles, and his volume one thousand two hundred and eighty times that of the earth. his figure is strikingly spheroidal, the equatorial being more than six thousand miles longer than the polar diameter. such a figure might naturally be expected from the rapidity of his diurnal rotation, which is accomplished in about ten hours. a place on the equator of jupiter must turn twenty-seven times as fast as on the terrestrial equator. the distance of jupiter from the sun is nearly four hundred and ninety millions of miles, and his revolution around the sun occupies nearly twelve years. every thing appertaining to jupiter is on a grand scale. a world in itself, equal in dimensions to twelve hundred and eighty of ours; the whole firmament rolling round it in the short space of ten hours, a movement so rapid that the eye could probably perceive the heavenly bodies to change their places every moment; its year dragging out a length of more than four thousand days, and more than ten thousand of its own days, while its nocturnal skies are lighted up with four brilliant moons;--these are some of the peculiarities which characterize this magnificent planet. the view of jupiter through a good telescope is one of the most splendid and interesting spectacles in astronomy. the disk expands into a large and bright orb, like the full moon; the spheroidal figure which theory assigns to revolving spheres, especially to those which turn with great velocity, is here palpably exhibited to the eye; across the disk, arranged in parallel stripes, are discerned several dusky bands, called _belts_; and four bright satellites, always in attendance, and ever varying their positions, compose a splendid retinue. indeed, astronomers gaze with peculiar interest on jupiter and his moons, as affording a miniature representation of the whole solar system, repeating, on a smaller scale, the same revolutions, and exemplifying more within the compass of our observation, the same laws as regulate the entire assemblage of sun and planets. figure , facing page , gives a correct view of jupiter, as exhibited to a powerful telescope in a clear evening. you will remark his flattened or spheroidal figure, the belts which appear in parallel stripes across his disk, and the four satellites, that are seen like little stars in a straight line with the equator of the planet. the _belts of jupiter_ are variable in their number and dimensions. with the smaller telescopes only one or two are seen, and those across the equatorial regions; but with more powerful instruments, the number is increased, covering a large part of the entire disk. different opinions have been entertained by astronomers respecting the cause of these belts; but they have generally been regarded as clouds formed in the atmosphere of the planet, agitated by winds, as is indicated by their frequent changes, and made to assume the form of belts parallel to the equator, like currents that circulate around our globe. sir john herschel supposes that the belts are not ranges of clouds, but portions of the planet itself, brought into view by the removal of clouds and mists, that exist in the atmosphere of the planet, through which are openings made by currents circulating around jupiter. the _satellites of jupiter_ may be seen with a telescope of very moderate powers. even a common spyglass will enable us to discern them. indeed, one or two of them have been occasionally seen with the naked eye. in the largest telescopes they severally appear as bright as sirius. with such an instrument, the view of jupiter, with his moons and belts, is truly a magnificent spectacle. as the orbits of the satellites do not deviate far from the plane of the ecliptic, and but little from the equator of the planet, they are usually seen in nearly a straight line with each other, extending across the central part of the disk. (see fig. , facing page .) jupiter and his satellites exhibit in miniature all the phenomena of the solar system. the satellites perform, around their primary, revolutions very analogous to those which the planets perform around the sun, having, in like manner, motions alternately direct, stationary, and retrograde. they are all, with one exception, a little larger than the moon; and the second satellite, which is the smallest, is nearly as large as the moon, being two thousand and sixty-eight miles in diameter. they are all very small compared with the primary, the largest being only one twenty-sixth part of the primary. the outermost satellite extends to the distance from the planet of fourteen times his diameter. the whole system, therefore, occupies a region of space more than one million miles in breadth. rapidity of motion, as well as greatness of dimensions, is characteristic of the system of jupiter. i have already mentioned that the planet itself has a motion on its own axis much swifter than that of the earth, and the motions of the satellites are also much more rapid than that of the moon. the innermost, which is a little further off than the moon is from the earth, goes round its primary in about a day and three quarters; and the outermost occupies less than seventeen days. the orbits of the satellites are nearly or quite circular, and deviate but little from the plane of the planet's equator, and of course are but slightly inclined to the plane of his orbit. they are therefore in a similar situation with respect to jupiter, as the moon would be with respect to the earth, if her orbit nearly coincided with the ecliptic, in which case, she would undergo an eclipse at every opposition. the eclipses of jupiter's satellites, in their general circumstances, are perfectly analogous to those of the moon, but in their details they differ in several particulars. owing to the much greater distance of jupiter from the sun, and its greater magnitude, the cone of its shadow is much longer and larger than that of the earth. on this account, as well as on account of the little inclination of their orbit to that of the primary, the three inner satellites of jupiter pass through his shadow, and are totally eclipsed, at every revolution. the fourth satellite, owing to the greater inclination of its orbit, sometimes, though rarely, escapes eclipse, and sometimes merely grazes the limits of the shadow, or suffers a partial eclipse. these eclipses, moreover, are not seen, as is the case with those of the moon, from the centre of their motion, but from a remote station, and one whose situation with respect to the line of the shadow is variable. this makes no difference in the _times_ of the eclipses, but it makes a very great one in their visibility, and in their apparent situations with respect to the planet at the moment of their entering or quitting the shadow. [illustration fig. .] the eclipses of jupiter's satellites present some curious phenomena, which you will easily understand by studying the following diagram. let a, b, c, d, fig. , represent the earth in different parts of its orbit; j, jupiter, in his orbit, surrounded by his four satellites, the orbits of which are marked , , , . at _a_, the first satellite enters the shadow of the planet, emerges from it at _b_, and advances to its greatest elongation at _c_. the other satellites traverse the shadow in a similar manner. the apparent place, with respect to the planet, at which these eclipses will be seen to occur, will be altered by the position the earth happens at that moment to have in its orbit; but their appearances for any given night, as exhibited at greenwich, are calculated and accurately laid down in the nautical almanac. when one of the satellites is passing between jupiter and the sun, it casts its shadow on the primary, as the moon casts its shadow on the earth in a solar eclipse. we see with the telescope the shadow traversing the disk. sometimes, the satellite itself is seen projected on the disk; but, being illuminated as well as the primary, it is not so easily distinguished as venus or mercury, when seen on the sun's disk in one of their transits, since these bodies have their dark sides turned towards us; but the satellite is illuminated by the sun, as well as the primary, and therefore is not easily distinguishable from it. the eclipses of jupiter's satellites have been studied with great attention by astronomers, on account of their affording one of the easiest methods of determining the _longitude_. on this subject, sir john herschel remarks: "the discovery of jupiter's satellites by galileo, which was one of the first fruits of the invention of the telescope, forms one of the most memorable epochs in the history of astronomy. the first astronomical solution of the problem of 'the longitude,'--the most important problem for the interests of mankind that has ever been brought under the dominion of strict scientific principles,--dates immediately from this discovery. the final and conclusive establishment of the copernican system of astronomy may also be considered as referable to the discovery and study of this exquisite miniature system, in which the laws of the planetary motions, as ascertained by kepler, and especially that which connects their periods and distances, were speedily traced, and found to be satisfactorily maintained." the entrance of one of jupiter's satellites into the shadow of the primary, being seen like the entrance of the moon into the earth's shadow at the same moment of absolute time, at all places where the planet is visible, and being wholly independent of parallax, that is, presenting the same phenomenon to places remote from each other; being, moreover, predicted beforehand, with great accuracy, for the instant of its occurrence at greenwich, and given in the nautical almanac; this would seem to be one of those events which are peculiarly adapted for finding the longitude. for you will recollect, that "any instantaneous appearance in the heavens, visible at the same moment of absolute time at any two places, may be employed for determining the difference of longitude between those places; for the difference in their local times, as indicated by clocks or chronometers, allowing fifteen degrees for every hour, will show their difference of longitude." with respect to the method by the eclipses of jupiter's satellites, it must be remarked, that the extinction of light in the satellite, at its immersion, and the recovery of its light at its emersion, are not instantaneous, but gradual; for the satellite, like the moon, occupies some time in entering into the shadow, or in emerging from it, which occasions a progressive diminution or increase of light. two observers in the same room, observing with different telescopes the same eclipse, will frequently disagree, in noting its time, to the amount of fifteen or twenty seconds. better methods, therefore, of finding the longitude, are now employed, although the facility with which the necessary observations can be made, and the little calculation required, still render this method eligible in many cases where extreme accuracy is not important. as a telescope is essential for observing an eclipse of one of the satellites, it is obvious that this method cannot be practised at sea, since the telescope cannot be used on board of ship, for want of the requisite steadiness. the grand discovery of the _progressive motion of light_ was first made by observations on the eclipses of jupiter's satellites. in the year , it was remarked by roemer, a danish astronomer, on comparing together observations of these eclipses during many successive years, that they take place sooner by about sixteen minutes, when the earth is on the same side of the sun with the planet, than when she is on the opposite side. the difference he ascribes to the progressive motion of light, which takes that time to pass through the diameter of the earth's orbit, making the velocity of light about one hundred and ninety-two thousand miles per second. so great a velocity startled astronomers at first, and produced some degree of distrust of this explanation of the phenomenon; but the subsequent discovery of what is called the aberration of light, led to an independent estimation of the velocity of light, with almost precisely the same result. few greater feats have ever been performed by the human mind, than to measure the speed of light,--a speed so great, as would carry it across the atlantic ocean in the sixty-fourth part of a second, and around the globe in less than the seventh part of a second! thus has man applied his scale to the motions of an element, that literally leaps from world to world in the twinkling of an eye. this is one example of the great power which the invention of the telescope conferred on man. could we plant ourselves on the surface of this vast planet, we should see the same starry firmament expanding over our heads as we see now; and the same would be true if we could fly from one planetary world to another, until we made the circuit of them all; but the sun and the planetary system would present themselves to us under new and strange aspects. the sun himself would dwindle to one twenty-seventh of his present surface, (fig. , facing page ,) and afford a degree of light and heat proportionally diminished; mercury, venus, and even the earth, would all disappear, being too near the sun to be visible; mars would be as seldom seen as mercury is by us, and constitute the only inferior planet. on the other hand, saturn would shine with greatly augmented size and splendor. when in opposition to the sun, (at which time it comes nearest to jupiter,) it would be a grand object, appearing larger than either venus or jupiter does to us. when, however, passing to the other side of the sun, through its superior conjunction, it would gradually diminish in size and brightness, and at length become much less than it ever appears to us, since it would then be four hundred millions of miles further from jupiter than it ever is from us. although jupiter comes four hundred millions of miles nearer to uranus than the earth does, yet it is still thirteen hundred millions of miles distant from that planet. hence the augmentation of the magnitude and light of uranus would be barely sufficient to render it distinguishable by the naked eye. it appears, therefore, that saturn is the peculiar ornament of the firmament of jupiter, and would present to the telescope most interesting and sublime phenomena. as we owe the revelation of the system of jupiter and his attendant worlds wholly to the telescope, and as the discovery and observation of them constituted a large portion of the glory of galileo, i am now forcibly reminded of his labors, and will recur to his history, and finish the sketch which i commenced in a previous letter. letter xxii. copernicus.--galileo. "they leave at length the nether gloom, and stand before the portals of a better land; to happier plains they come, and fairer groves, the seats of those whom heaven, benignant, loves; a brighter day, a bluer ether, spreads its lucid depths above their favored heads; and, purged from mists that veil our earthly skies, shine suns and stars unseen by mortal eyes."--_virgil._ in order to appreciate the value of the contributions which galileo made to astronomy, soon after the invention of the telescope, it is necessary to glance at the state of the science when he commenced his discoveries for many centuries, during the middle ages, a dark night had hung over astronomy, through which hardly a ray of light penetrated, when, in the eastern part of civilized europe, a luminary appeared, that proved the harbinger of a bright and glorious day. this was copernicus, a native of thorn, in prussia. he was born in . though destined for the profession of medicine, from his earliest years he displayed a great fondness and genius for mathematical studies, and pursued them with distinguished success in the university of cracow. at the age of twenty-five years, he resorted to italy, for the purpose of studying astronomy, where he resided a number of years. thus prepared, he returned to his native country, and, having acquired an ecclesiastical living that was adequate to his support in his frugal mode of life, he established himself at frauenberg, a small town near the mouth of the vistula, where he spent nearly forty years in observing the heavens, and meditating on the celestial motions. he occupied the upper part of a humble farm-house, through the roof of which he could find access to an unobstructed sky, and there he carried on his observations. his instruments, however, were few and imperfect, and it does not appear that he added any thing to the art of practical astronomy. this was reserved for tycho brahe, who came a half a century after him. nor did copernicus enrich the science with any important discoveries. it was not so much his genius or taste to search for new bodies, or new phenomena among the stars, as it was to explain the reasons of the most obvious and well-known appearances and motions of the heavenly bodies. with this view, he gave his mind to long-continued and profound meditation. copernicus tells us that he was first led to think that the apparent motions of the heavenly bodies, in their diurnal revolution, were owing to the real motion of the earth in the opposite direction, from observing instances of the same kind among terrestrial objects; as when the shore seems to the mariner to recede, as he rapidly sails from it; and as trees and other objects seem to glide by us, when, on riding swiftly past them, we lose the consciousness of our own motion. he was also smitten with the _simplicity_ prevalent in all the works and operations of nature, which is more and more conspicuous the more they are understood; and he hence concluded that the planets do not move in the complicated paths which most preceding astronomers assigned to them. i shall explain to you, hereafter, the details of his system. i need only at present remind you that the hypothesis which he espoused and defended, (being substantially the same as that proposed by pythagoras, five hundred years before the christian era,) supposes, first, that the apparent movements of the sun by day, and of the moon and stars by night, from east to west, result from the actual revolution of the earth on its own axis from west to east; and, secondly, that the earth and all the planets revolve about the sun in circular orbits. this hypothesis, when he first assumed it, was with him, as it had been with pythagoras, little more than mere conjecture. the arguments by which its truth was to be finally established were not yet developed, and could not be, without the aid of the telescope, which was not yet invented. upon this hypothesis, however, he set out to explain all the phenomena of the visible heavens,--as the diurnal revolutions of the sun, moon, and stars, the slow progress of the planets through the signs of the zodiac, and the numerous irregularities to which the planetary motions are subject. these last are apparently so capricious,--being for some time forward, then stationary, then backward, then stationary again, and finally direct, a second time, in the order of the signs, and constantly varying in the velocity of their movements,--that nothing but long-continued and severe meditation could have solved all these appearances, in conformity with the idea that each planet is pursuing its simple way all the while in a circle around the sun. although, therefore, pythagoras fathomed the profound doctrine that the sun is the centre around which the earth and all the planets revolve, yet we have no evidence that he ever solved the irregular motions of the planets in conformity with his hypothesis, although the explanation of the diurnal revolution of the heavens, by that hypothesis, involved no difficulty. ignorant as copernicus was of the principle of gravitation, and of most of the laws of motion, he could go but little way in following out the consequences of his own hypothesis; and all that can be claimed for him is, that he solved, by means of it, most of the common phenomena of the celestial motions. he indeed got upon the road to truth, and advanced some way in its sure path; but he was able to adduce but few independent proofs, to show that it was the truth. it was only quite at the close of his life that he published his system to the world, and that only at the urgent request of his friends; anticipating, perhaps, the opposition of a bigoted priesthood, whose fury was afterwards poured upon the head of galileo, for maintaining the same doctrines. although, therefore, the system of copernicus afforded an explanation of the celestial motions, far more simple and rational than the previous systems which made the earth the centre of those motions, yet this fact alone was not sufficient to compel the assent of astronomers; for the greater part, to say the least, of the same phenomena, could be explained on either hypothesis. with the old doctrine astronomers were already familiar, a circumstance which made it seem easier; while the new doctrines would seem more difficult, from their being imperfectly understood. accordingly, for nearly a century after the publication of the system of copernicus, he gained few disciples. tycho brahe rejected it, and proposed one of his own, of which i shall hereafter give you some account; and it would probably have fallen quite into oblivion, had not the observations of galileo, with his newly-invented telescope, brought to light innumerable proofs of its truth, far more cogent than any which copernicus himself had been able to devise. galileo no sooner had completed his telescope, and directed it to the heavens, than a world of wonders suddenly burst upon his enraptured sight. pointing it to the moon, he was presented with a sight of her mottled disk, and of her mountains and valleys. the sun exhibited his spots; venus, her phases; and jupiter, his expanded orb, and his retinue of moons. these last he named, in honor of his patron, cosmo d'medici, _medicean stars_. so great was this honor deemed of associating one's name with the stars, that express application was made to galileo, by the court of france, to award this distinction to the reigning monarch, henry the fourth, with plain intimations, that by so doing he would render himself and his family rich and powerful for ever. galileo published the result of his discoveries in a paper, denominated '_nuncius sidereus_,' the 'messenger of the stars.' in that ignorant and marvellous age, this publication produced a wonderful excitement. "many doubted, many positively refused to believe, so novel an announcement; all were struck with the greatest astonishment, according to their respective opinions, either at the new view of the universe thus offered to them, or at the high audacity of galileo, in inventing such fables." even kepler, the great german astronomer, of whom i shall tell you more by and by, wrote to galileo, and desired him to supply him with arguments, by which he might answer the objections to these pretended discoveries with which he was continually assailed. galileo answered him as follows: "in the first place, i return you my thanks that you first, and almost alone, before the question had been sifted, (such is your candor, and the loftiness of your mind,) put faith in my assertions. you tell me you have some telescopes, but not sufficiently good to magnify distant objects with clearness, and that you anxiously expect a sight of mine, which magnifies images more than a thousand times. it is mine no longer, for the grand duke of tuscany has asked it of me, and intends to lay it up in his museum, among his most rare and precious curiosities, in eternal remembrance of the invention. "you ask, my dear kepler, for other testimonies. i produce, for one, the grand duke, who, after observing the medicean planets several times with me at pisa, during the last months, made me a present, at parting, of more than a thousand florins, and has now invited me to attach myself to him, with the annual salary of one thousand florins, and with the title of 'philosopher and principal mathematician to his highness;' without the duties of any office to perform, but with the most complete leisure. i produce, for another witness, myself, who, although already endowed in this college with the noble salary of one thousand florins, such as no professor of mathematics ever before received, and which i might securely enjoy during my life, even if these planets should deceive me and should disappear, yet quit this situation, and take me where want and disgrace will be my punishment, should i prove to have been mistaken." the learned professors in the universities, who, in those days, were unaccustomed to employ their senses in inquiring into the phenomena of nature, but satisfied themselves with the authority of aristotle, on all subjects, were among the most incredulous with respect to the discoveries of galileo. "oh, my dear kepler," says galileo, "how i wish that we could have one hearty laugh together. here, at padua, is the principal professor of philosophy, whom i have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do. why are you not here? what shouts of laughter we should have at this glorious folly, and to hear the professor of philosophy at pisa laboring before the grand duke, with logical arguments, as if with magical incantations, to charm the new planets out of the sky." the following argument by sizzi, a contemporary astronomer of some note, to prove that there can be only seven planets, is a specimen of the logic with which galileo was assailed. "there are seven windows given to animals in the domicile of the head, through which the air is admitted to the tabernacle of the body, to enlighten, to warm, and to nourish it; which windows are the principal parts of the microcosm, or little world,--two nostrils, two eyes, two ears, and one mouth. so in the heavens, as in a macrocosm, or great world, there are two favorable stars, jupiter and venus; two unpropitious, mars and saturn; two luminaries, the sun and moon; and mercury alone, undecided and indifferent. from which, and from many other phenomena of nature, such as the seven metals, &c., which it were tedious to enumerate, we gather that the number of planets is necessarily seven. moreover, the satellites are invisible to the naked eye, and therefore can exercise no influence over the earth, and therefore would be useless, and therefore do not exist. besides, as well the jews and other ancient nations, as modern europeans, have adopted the division of the week into seven days, and have named them from the seven planets. now, if we increase the number of planets, this whole system falls to the ground." when, at length, the astronomers of the schools found it useless to deny the fact that jupiter is attended by smaller bodies, which revolve around him, they shifted their ground of warfare, and asserted that galileo had not told the whole truth; that there were not merely _four_ satellites, but a still greater number; one said five; another, nine; and another, twelve; but, in a little time, jupiter moved forward in his orbit, and left all behind him, save the four medicean stars. it had been objected to the copernican system, that were venus a body which revolved around the sun in an orbit interior to that of the earth, she would undergo changes similar to those of the moon. as no such changes could be detected by the naked eye, no satisfactory answer could be given to this objection; but the telescope set all right, by showing, in fact, the phases of venus. the same instrument, disclosed, also, in the system of jupiter and his moons, a miniature exhibition of the solar system itself. it showed the actual existence of the motion of a number of bodies around one central orb, exactly similar to that which was predicated of the sun and planets. every one, therefore, of these new and interesting discoveries, helped to confirm the truth of the system of copernicus. but a fearful cloud was now rising over galileo, which spread itself, and grew darker every hour. the church of rome had taken alarm at the new doctrines respecting the earth's motion, as contrary to the declarations of the bible, and a formidable difficulty presented itself, namely, how to publish and defend these doctrines, without invoking the terrible punishments inflicted by the inquisition on heretics. no work could be printed without license from the court of rome; and any opinions supposed to be held and much more known to be taught by any one, which, by an ignorant and superstitious priesthood, could be interpreted as contrary to scripture, would expose the offender to the severest punishments, even to imprisonment, scourging, and death. we, who live in an age so distinguished for freedom of thought and opinion, can form but a very inadequate conception of the bondage in which the minds of men were held by the chains of the inquisition. it was necessary, therefore, for galileo to proceed with the greatest caution in promulgating truths which his own discoveries had confirmed. he did not, like the christian martyrs, proclaim the truth in the face of persecutions and tortures; but while he sought to give currency to the copernican doctrines, he labored, at the same time, by cunning artifices, to blind the ecclesiastics to his real designs, and thus to escape the effects of their hostility. before galileo published his doctrines in form, he had expressed himself so freely, as to have excited much alarm among the ecclesiastics. one of them preached publicly against him, taking for his text, the passage, "ye men of galilee, why stand ye here gazing up into heaven?" he therefore thought it prudent to resort to rome, and confront his enemies face to face. a contemporary describes his appearance there in the following terms, in a letter addressed to a romish cardinal: "your eminence would be delighted with galileo, if you heard him holding forth, as he often does, in the midst of fifteen or twenty, all violently attacking him, sometimes in one house, sometimes in another. but he is armed after such fashion, that he laughs all of them to scorn; and even if the novelty of his opinions prevents entire persuasion, at least he convicts of emptiness most of the arguments with which his adversaries endeavor to overwhelm him." in , galileo, as he himself states, had a most gracious audience of the pope, paul the fifth, which lasted for nearly an hour, at the end of which his holiness assured him, that the congregation were no longer in a humor to listen lightly to calumnies against him, and that so long as he occupied the papal chair, galileo might think himself out of all danger. nevertheless, he was not allowed to return home, without receiving formal notice not to teach the opinions of copernicus, "that the sun is in the centre of the system, and that the earth moves about it," from that time forward, in any manner. galileo had a most sarcastic vein, and often rallied his persecutors with the keenest irony. this he exhibited, some time after quitting rome, in an epistle which he sent to the arch duke leopold, accompanying his 'theory of the tides.' "this theory," says he, "occurred to me when in rome, whilst the theologians were debating on the prohibition of copernicus's book, and of the opinion maintained in it of the motion of the earth, which i at that time believed; until it pleased those gentlemen to suspend the book, and to declare the opinion false and repugnant to the holy scriptures. now, as i know how well it becomes me to obey and believe the decisions of my superiors, which proceed out of more profound knowledge than the weakness of my intellect can attain to, this theory, which i send you, which is founded on the motion of the earth, i now look upon as a fiction and a dream, and beg your highness to receive it as such. but, as poets often learn to prize the creations of their fancy, so, in like manner, do i set some value on this absurdity of mine. it is true, that when i sketched this little work, i did hope that copernicus would not, after eighty years, be convicted of error; and i had intended to develope and amplify it further; but a voice from heaven suddenly awakened me, and at once annihilated all my confused and entangled fancies." it is difficult, however, sometimes to decide whether the language of galileo is ironical, or whether he uses it with subtlety, with the hope of evading the anathemas of the inquisition. thus he ends one of his writings with the following passage: "in conclusion, since the motion attributed to the earth, which i, as a pious and catholic person, consider most false, and not to exist, accommodates itself so well to explain so many and such different phenomena, i shall not feel sure that, false as it is, it may not just as deludingly correspond with the phenomena of comets." in the year , soon after the accession of urban the eighth to the pontifical chair, galileo went to rome again, to offer his congratulations to the new pope, as well as to propitiate his favor. he seems to have been received with unexpected cordiality; and, on his departure, the pope commended him to the good graces of ferdinand, grand duke of tuscany, in the following terms: "we find in him not only literary distinction, but also the love of piety, and he is strong in those qualities by which pontifical good-will is easily obtained. and now, when he has been brought to this city, to congratulate us on our elevation, we have lovingly embraced him; nor can we suffer him to return to the country whither your liberality recalls him, without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honorable testimonial of virtue and piety. and we further signify, that every benefit which you shall confer upon him will conduce to our gratification." in the year , galileo finished a great work, on which he had been long engaged, entitled, 'the dialogue on the ptolemaic and copernican systems.' from the notion which prevailed, that he still countenanced the copernican doctrine of the earth's motion, which had been condemned as heretical, it was some time before he could obtain permission from the inquisitors (whose license was necessary to every book) to publish it. this he was able to do, only by employing again that duplicity or artifice which would throw dust in the eyes of the vain and superstitious priesthood. in , the work appeared under the following title: 'a dialogue, by galileo galilei, extraordinary mathematician of the university of pisa, and principal philosopher and mathematician of the most serene grand duke of tuscany; in which, in a conversation of four days, are discussed the two principal systems of the world, the ptolemaic and copernican, indeterminately proposing the philosophical arguments as well on one side as on the other.' the subtle disguise which he wore, may be seen from the following extract from his 'introduction,' addressed 'to the discreet reader.' "some years ago, a salutary edict was promulgated at rome, which, in order to obviate the perilous scandals of the present age, enjoined an opportune silence on the pythagorean opinion of the earth's motion. some were not wanting, who rashly asserted that this decree originated, not in a judicious examination, but in ill-informed passion; and complaints were heard, that counsellors totally inexperienced in astronomical observations ought not, by hasty prohibitions, to clip the wings of speculative minds. my zeal could not keep silence when i heard these rash lamentations, and i thought it proper, as being fully informed with regard to that most prudent determination, to appear publicly on the theatre of the world, as a witness of the actual truth. i happened at that time to be in rome: i was admitted to the audiences, and enjoyed the approbation, of the most eminent prelates of that court; nor did the publication of that decree occur without my receiving some prior intimation of it. wherefore, it is my intention, in this present work, to show to foreign nations, that as much is known of this matter in italy, and particularly in rome, as ultramontane diligence can ever have formed any notion of, and collecting together all my own speculations on the copernican system, to give them to understand that the knowledge of all these preceded the roman censures; and that from this country proceed not only dogmas for the salvation of the soul, but also ingenious discoveries for the gratification of the understanding. with this object, i have taken up in the 'dialogue' the copernican side of the question, treating it as a pure mathematical hypothesis; and endeavoring, in every artificial manner, to represent it as having the advantage, not over the opinion of the stability of the earth absolutely, but according to the manner in which that opinion is defended by some, who indeed profess to be aristotelians, but retain only the name, and are contented, without improvement, to worship shadows, not philosophizing with their own reason, but only from the recollection of the four principles imperfectly understood." although the pope himself, as well as the inquisitors, had examined galileo's manuscript, and, not having the sagacity to detect the real motives of the author, had consented to its publication, yet, when the book was out, the enemies of galileo found means to alarm the court of rome, and galileo was summoned to appear before the inquisition. the philosopher was then seventy years old, and very infirm, and it was with great difficulty that he performed the journey. his unequalled dignity and celebrity, however, commanded the involuntary respect of the tribunal before which he was summoned, which they manifested by permitting him to reside at the palace of his friend, the tuscan ambassador; and when it became necessary, in the course of the inquiry, to examine him in person, although his removal to the holy office was then insisted upon, yet he was not, like other heretics, committed to close and solitary confinement. on the contrary, he was lodged in the apartments of the head of the inquisition, where he was allowed the attendance of his own servant, who was also permitted to sleep in an adjoining room, and to come and go at pleasure. these were deemed extraordinary indulgences in an age when the punishment of heretics usually began before their trial. about four months after galileo's arrival in rome, he was summoned to the holy office. he was detained there during the whole of that day; and on the next day was conducted, in a penitential dress, to the convent of minerva, where the cardinals and prelates, his judges, were assembled for the purpose of passing judgement upon him, by which this venerable old man was solemnly called upon to renounce and abjure, as impious and heretical, the opinions which his whole existence had been consecrated to form and strengthen. probably there is not a more curious document in the history of science, than that which contains the sentence of the inquisition on galileo, and his consequent abjuration. it teaches us so much, both of the darkness and bigotry of the terrible inquisition, and of the sufferings encountered by those early martyrs of science, that i will transcribe for your perusal, from the excellent 'life of galileo' in the 'library of useful knowledge,' (from which i have borrowed much already,) the entire record of this transaction. the sentence of the inquisition is as follows: "we, the undersigned, by the grace of god, cardinals of the holy roman church, inquisitors general throughout the whole christian republic, special deputies of the holy apostolical chair against heretical depravity: "whereas, you, galileo, son of the late vincenzo galilei of florence, aged seventy years, were denounced in , to this holy office, for holding as true a false doctrine taught by many, namely, that the sun is immovable in the centre of the world, and that the earth moves, and also with a diurnal motion; also, for having pupils which you instructed in the same opinions; also, for maintaining a correspondence on the same with some german mathematicians; also, for publishing certain letters on the solar spots, in which you developed the same doctrine as true; also, for answering the objections which were continually produced from the holy scriptures, by glozing the said scriptures, according to your own meaning; and whereas, thereupon was produced the copy of a writing, in form of a letter, professedly written by you to a person formerly your pupil, in which, following the hypothesis of copernicus, you include several propositions contrary to the true sense and authority of the holy scriptures: therefore, this holy tribunal, being desirous of providing against the disorder and mischief which was thence proceeding and increasing, to the detriment of the holy faith, by the desire of his holiness, and of the most eminent lords cardinals of this supreme and universal inquisition, the two propositions of the stability of the sun, and motion of the earth, were _qualified_ by the _theological qualifiers_, as follows: " . the proposition that the sun is in the centre of the world, and immovable from its place, is absurd, philosophically false, and formally heretical; because it is expressly contrary to the holy scriptures. " . the proposition that the earth is not the centre of the world, nor immovable, but that it moves, and also with a diurnal motion, is also absurd, philosophically false, and, theologically considered, equally erroneous in faith. "but whereas, being pleased at that time to deal mildly with you, it was decreed in the holy congregation, held before his holiness on the twenty-fifth day of february, , that his eminence the lord cardinal bellarmine should enjoin you to give up altogether the said false doctrine; if you should refuse, that you should be ordered by the commissary of the holy office to relinquish it, not to teach it to others, nor to defend it, and in default of the acquiescence, that you should be imprisoned; and in execution of this decree, on the following day, at the palace, in presence of his eminence the said lord cardinal bellarmine, after you had been mildly admonished by the said lord cardinal, you were commanded by the acting commissary of the holy office, before a notary and witnesses, to relinquish altogether the said false opinion, and in future neither to defend nor teach it in any manner, neither verbally nor in writing, and upon your promising obedience, you were dismissed. "and, in order that so pernicious a doctrine might be altogether rooted out, nor insinuate itself further to the heavy detriment of the catholic truth, a decree emanated from the holy congregation of the index, prohibiting the books which treat of this doctrine; and it was declared false, and altogether contrary to the holy and divine scripture. "and whereas, a book has since appeared, published at florence last year, the title of which showed that you were the author, which title is, '_the dialogue of galileo galilei, on the two principal systems of the world, the ptolemaic and copernican_;' and whereas, the holy congregation has heard that, in consequence of printing the said book, the false opinion of the earth's motion and stability of the sun is daily gaining ground; the said book has been taken into careful consideration, and in it has been detected a glaring violation of the said order, which had been intimated to you; inasmuch as in this book you have defended the said opinion, already, and in your presence, condemned; although in the said book you labor, with many circumlocutions, to induce the belief that it is left by you undecided, and in express terms probable; which is equally a very grave error, since an opinion can in no way be probable which has been already declared and finally determined contrary to the divine scripture. therefore, by our order, you have been cited to this holy office, where, on your examination upon oath, you have acknowledged the said book as written and printed by you. you also confessed that you began to write the said book ten or twelve years ago, after the order aforesaid had been given. also, that you demanded license to publish it, but without signifying to those who granted you this permission, that you had been commanded not to hold, defend, or teach, the said doctrine in any manner. you also confessed, that the style of said book was, in many places, so composed, that the reader might think the arguments adduced on the false side to be so worded, as more effectually to entangle the understanding than to be easily solved, alleging, in excuse, that you have thus run into an error, foreign (as you say) to your intention, from writing in the form of a dialogue, and in consequence of the natural complacency which every one feels with regard to his own subtilties, and in showing himself more skilful than the generality of mankind in contriving, even in favor of false propositions, ingenious and apparently probable arguments. "and, upon a convenient time being given you for making your defence, you produced a certificate in the handwriting of his eminence, the lord cardinal bellarmine, procured, as you said, by yourself, that you might defend yourself against the calumnies of your enemies, who reported that you had abjured your opinions, and had been punished by the holy office; in which certificate it is declared, that you had not abjured, nor had been punished, but merely that the declaration made by his holiness, and promulgated by the holy congregation of the index, had been announced to you, which declares that the opinion of the motion of the earth, and stability of the sun, is contrary to the holy scriptures, and therefore cannot be held or defended. wherefore, since no mention is there made of two articles of the order, to wit, the order 'not to teach,' and 'in any manner,' you argued that we ought to believe that, in the lapse of fourteen or sixteen years, they had escaped your memory, and that this was also the reason why you were silent as to the order, when you sought permission to publish your book, and that this is said by you, not to excuse your error, but that it may be attributed to vain-glorious ambition rather than to malice. but this very certificate, produced on your behalf, has greatly aggravated your offence, since it is therein declared, that the said opinion is contrary to the holy scriptures, and yet you have dared to treat of it, and to argue that it is probable; nor is there any extenuation in the license artfully and cunningly extorted by you, since you did not intimate the command imposed upon you. but whereas, it appeared to us that you had not disclosed the whole truth with regard to your intentions, we thought it necessary to proceed to the rigorous examination of you, in which (without any prejudice to what you had confessed, and which is above detailed against you, with regard to your said intention) you answered like a good catholic. "therefore, having seen and maturely considered the merits of your cause, with your said confessions and excuses, and every thing else which ought to be seen and considered, we have come to the underwritten final sentence against you: "invoking, therefore, the most holy name of our lord jesus christ, and of his most glorious virgin mother, mary, by this our final sentence, which, sitting in council and judgement for the tribunal of the reverend masters of sacred theology, and doctors of both laws, our assessors, we put forth in this writing touching the matters and controversies before us, between the magnificent charles sincerus, doctor of both laws, fiscal proctor of this holy office, of the one part, and you, galileo galilei, an examined and confessed criminal from this present writing now in progress, as above, of the other part, we pronounce, judge, and declare, that you, the said galileo, by reason of these things which have been detailed in the course of this writing, and which, as above, you have confessed, have rendered yourself vehemently suspected, by this holy office, of heresy; that is to say, that you believe and hold the false doctrine, and contrary to the holy and divine scriptures, namely, that the sun is the centre of the world, and that it does not move from east to west, and that the earth does move, and is not the centre of the world; also, that an opinion can be held and supported, as probable, after it has been declared and finally decreed contrary to the holy scripture, and consequently, that you have incurred all the censures and penalties enjoined and promulgated in the sacred canons, and other general and particular constitutions against delinquents of this description. from which it is our pleasure that you be absolved, provided that, with a sincere heart and unfeigned faith, in our presence, you abjure, curse, and detest, the said errors and heresies, and every other error and heresy, contrary to the catholic and apostolic church of rome, in the form now shown to you. "but that your grievous and pernicious error and transgression may not go altogether unpunished, and that you may be made more cautious in future, and may be a warning to others to abstain from delinquencies of this sort, we decree, that the book of the dialogues of galileo galilei be prohibited by a public edict, and we condemn you to the formal prison of this holy office for a period determinable at our pleasure; and, by way of salutary penance, we order you, during the next three years, to recite, once a week, the seven penitential psalms, reserving to ourselves the power of moderating, commuting, or taking off the whole or part of the said punishment, or penance. "and so we say, pronounce, and by our sentence declare, decree, and reserve, in this and in every other better form and manner, which lawfully we may and can use. so we, the subscribing cardinals, pronounce." [subscribed by seven cardinals.] in conformity with the foregoing sentence, galileo was made to kneel before the inquisition, and make the following _abjuration_: "i, galileo galilei, son of the late vincenzo galilei, of florence, aged seventy years, being brought personally to judgement, and kneeling before you, most eminent and most reverend lords cardinals, general inquisitors of the universal christian republic against heretical depravity, having before my eyes the holy gospels, which i touch with my own hands, swear, that i have always believed, and with the help of god will in future believe, every article which the holy catholic and apostolic church of rome holds, teaches, and preaches. but because i had been enjoined, by this holy office, altogether to abandon the false opinion which maintains that the sun is the centre and immovable, and forbidden to hold, defend, or teach, the said false doctrine, in any manner: and after it had been signified to me that the said doctrine is repugnant to the holy scripture, i have written and printed a book, in which i treat of the same doctrine now condemned, and adduce reasons with great force in support of the same, without giving any solution, and therefore have been judged grievously suspected of heresy; that is to say, that i held and believed that the sun is the centre of the world and immovable, and that the earth is not the centre and movable; willing, therefore, to remove from the minds of your eminences, and of every catholic christian, this vehement suspicion rightfully entertained towards me, with a sincere heart and unfeigned faith, i abjure, curse, and defeat, the said errors and heresies, and generally every other error and sect contrary to the said holy church; and i swear, that i will never more in future say or assert any thing, verbally or in writing, which may give rise to a similar suspicion of me: but if i shall know any heretic, or any one suspected of heresy, that i will denounce him to this holy office, or to the inquisitor and ordinary of the place in which i may be. i swear, moreover, and promise, that i will fulfil and observe fully, all the penances which have been or shall be laid on me by this holy office. but if it shall happen that i violate any of my said promises, oaths, and protestations, (which god avert!) i subject myself to all the pains and punishments which have been decreed and promulgated by the sacred canons, and other general and particular constitutions, against delinquents of this description. so may god help me, and his holy gospels, which i touch with my own hands. i, the above-named galileo galilei, have abjured, sworn, promised, and bound myself, as above; and in witness thereof, with my own hand have subscribed this present writing of my abjuration, which i have recited, word for word. "at rome, in the convent of minerva, twenty-second june, , i, galileo galilei, have abjured as above, with my own hand." from the court galileo was conducted to prison, to be immured for life in one of the dungeons of the inquisition. his sentence was afterwards mitigated, and he was permitted to return to florence; but the humiliation to which he had been subjected pressed heavily on his spirits, beset as he was with infirmities, and totally blind, and he never more talked or wrote on the subject of astronomy. there was enough in the character of galileo to command a high admiration. there was much, also, in his sufferings in the cause of science, to excite the deepest sympathy, and even compassion. he is moreover universally represented to have been a man of great equanimity, and of a noble and generous disposition. no scientific character of the age, or perhaps of any age, forms a structure of finer proportions, or wears in a higher degree the grace of symmetry. still, we cannot approve of his employing artifice in the promulgation of truth; and we are compelled to lament that his lofty spirit bowed in the final conflict. how far, therefore, he sinks below the dignity of the christian martyr! "at the age of seventy," says dr. brewster, in his life of sir isaac newton, "on his bended knees, and with his right hand resting on the holy evangelists, did this patriarch of science avow his present and past belief in the dogmas of the romish church, abandon as false and heretical the doctrine of the earth's motion and of the sun's immobility, and pledge himself to denounce to the inquisition any other person who was even suspected of heresy. he abjured, cursed, and detested, those eternal and immutable truths which the almighty had permitted him to be the first to establish. had galileo but added the courage of the martyr to the wisdom of the sage; had he carried the glance of his indignant eye round the circle of his judges; had he lifted his hands to heaven, and called the living god to witness the truth and immutability of his opinions; the bigotry of his enemies would have been disarmed, and science would have enjoyed a memorable triumph." letter xxiii. saturn.--uranus.--asteroids. "into the heaven of heavens i have presumed, an earthly guest, and drawn empyreal air."--_milton._ the consideration of the system of jupiter and his satellites led us to review the interesting history of galileo, who first, by means of the telescope, disclosed the knowledge of that system to the world. i will now proceed with the other superior planets. saturn, as well as jupiter, has within itself a system on a scale of great magnificence. in size it is next to jupiter the largest of the planets, being seventy-nine thousand miles in diameter, or about one thousand times as large as the earth. it has likewise belts on its surface, and is attended by seven satellites. but a still more wonderful appendage is its _ring_, a broad wheel, encompassing the planet at a great distance from it. as saturn is nine hundred millions of miles from us, we require a more powerful telescope to see his glories, in all their magnificence, than we do to enjoy a full view of the system of jupiter. when we are privileged with a view of saturn, in his most favorable positions, through a telescope of the larger class, the mechanism appears more wonderful than even that of jupiter. saturn's ring, when viewed with telescopes of a high power, is found to consist of two concentred rings, separated from each other by a dark space. although this division of the rings appears to us, on account of our immense distance, as only a fine line, yet it is, in reality, an interval of not less than eighteen hundred miles. the dimensions of the whole system are, in round numbers, as follows: miles. diameter of the planet, , from the surface of the planet to the inner ring, , breadth of the inner ring, , interval between the rings, , breadth of the outer ring, , extreme dimensions from outside to outside, , figure , facing page , represents saturn, as it appears to a powerful telescope, surrounded by its rings, and having its body striped with dark belts, somewhat similar, but broader and less strongly marked than those of jupiter. in telescopes of inferior power, but still sufficient to see the ring distinctly, we should scarcely discern the belts at all. we might, however, observe the shadow cast upon the ring by the planet, (as seen in the figure on the right, on the upper side;) and, in favorable situations of the planet, we might discern glimpses of the shadow of the ring on the body of the planet, on the lower side beneath the ring. to see the division of the ring and the satellites requires a better telescope than is in possession of most observers. with smaller telescopes, we may discover an oval figure of peculiar appearance, which it would be difficult to interpret. galileo, who first saw it in the year , recognised this peculiarity, but did not know what it meant. seeing something in the centre with two projecting arms, one on each side, he concluded that the planet was triple-shaped. this was, at the time, all he could learn respecting it, as the telescopes he possessed were very humble, compared with those now used by astronomers. the first constructed by him magnified but three times; his second, eight times; and his best, only thirty times, which is no better than a common ship-glass. it was the practice of the astronomers of those days to give the first intimation of a new discovery in a latin verse, the letters of which were transposed. this would enable them to claim priority, in case any other person should contest the honor of the discovery, and at the same time would afford opportunity to complete their observations, before they published a full account of them. accordingly, galileo announced the discovery of the singular appearance of saturn under this disguise, in a line which, when the transposed letters were restored to their proper places, signified that he had observed, "that the most distant planet is triple-formed."[ ] he shortly afterwards, at the request of his patron, the emperor rodolph, gave the solution, and added, "i have, with great admiration, observed that saturn is not a single star, but three together, which, as it were, touch each other; they have no relative motion, and are constituted of this form, ooo, the middle one being somewhat larger than the two lateral ones. if we examine them with an eyeglass which magnifies the surface less than one thousand times, the three stars do not appear very distinctly, but saturn has an oblong appearance, like that of an olive, thus, {oblong symbol}. now, i have discovered a court for jupiter, (alluding to his satellites,) and two servants for this old man, (saturn,) who aid his steps, and never quit his side." it was by this mystic light that galileo groped his way through an organization which, under the more powerful glasses of his successors, was to expand into a mighty orb, encompassed by splendid rings of vast dimensions, the whole attended by seven bright satellites. this system was first fully developed by huyghens, a dutch astronomer, about forty years afterwards.[ ] it requires a superior telescope to see it to advantage; but, when seen through such a telescope, it is one of the most charming spectacles afforded to that instrument. to give some idea of the properties of a telescope suited to such observations, i annex an extract from an account, that was published a few years since, of a telescope constructed by mr. tully, a distinguished english artist. "the length of the instrument was twelve feet, but was easily adjusted, and was perfectly steady. the magnifying powers ranged from two hundred to seven hundred and eighty times; but the great excellence of the telescope consisted more in the superior distinctness and brilliancy with which objects were seen through it, than in its magnifying powers. with a power of two hundred and forty, the light of jupiter was almost more than the eye could bear, and his satellites appeared as bright as sirius, but with a clear and steady light; and the belts and spots on the face of the planet were most distinctly defined. with a power of nearly four hundred, saturn appeared large and well defined, and was one of the most beautiful objects that can well be conceived." that the ring is a solid opaque substance, is shown by its throwing its shadow on the body of the planet on the side nearest the sun, and on the other side receiving that of the body. the ring encompasses the equatorial regions of the planet, and the planet revolves on an axis which is perpendicular to the plane of the ring in about ten and a half hours. this is known by observing the rotation of certain dusky spots, which sometimes appear on its surface. this motion is nearly the same with the diurnal motion of jupiter, subjecting places on the equator of the planet to a very swift revolution, and occasioning a high degree of compression at the poles, the equatorial being to the polar diameter in the high ratio of eleven to ten. saturn's ring, in its revolution around the sun, _always remains parallel to itself_. if we hold opposite to the eye a circular ring or disk, like a piece of coin, it will appear as a complete circle only when it is at right angles to the axis of vision. when it is oblique to that axis, it will be projected into an ellipse more and more flattened, as its obliquity is increased, until, when its plane coincides with the axis of vision, it is projected into a straight line. please to take some circle, as a flat plate, (whose rim may well represent the ring of saturn,) and hold it in these different positions before the eye. now, place on the table a lamp to represent the sun, and holding the ring at a certain distance, inclined a little towards the lamp, carry it round the lamp, always keeping it parallel to itself. during its revolution, it will twice present its edge to the lamp at opposite points; and twice, at places ninety degrees distant from those points, it will present its broadest face towards the lamp. at intermediate points, it will exhibit an ellipse more or less open, according as it is nearer one or the other of the preceding positions. it will be seen, also, that in one half of the revolution, the lamp shines on one side of the ring, and in the other half of the revolution, on the other side. such would be the successive appearances of saturn's ring to a spectator on the sun; and since the earth is, in respect to so distant a body as saturn, very near the sun, these appearances are presented to us nearly in the same manner as though we viewed them from the sun. accordingly, we sometimes see saturn's ring under the form of a broad ellipse, which grows continually more and more acute, until it passes into a line, and we either lose sight of it, altogether, or, by the aid of the most powerful telescopes, we see it as a fine thread of light drawn across the disk, and projecting out from it on each side. as the whole revolution occupies thirty years, and the edge is presented to the sun twice in the revolution, this last phenomenon, namely, the disappearance of the ring, takes place every fifteen years. [illustration fig. .] you may perhaps gain a still clearer idea of the foregoing appearances from the following diagram, fig. . let a, b, c, &c., represent successive positions of saturn and his ring, in different parts of his orbit, while _a b_ represents the orbit of the earth. please to remark, that these orbits are drawn so elliptical, not to represent the eccentricity of either the earth's or saturn's orbit, but merely as the projection of circles seen very obliquely. also, imagine one half of the body of the planet and of the ring to be above the plane of the paper, and the other half below it. were the ring, when at c and g, perpendicular to c g, it would be seen by a spectator situated at _a_ or _b_ as a perfect circle; but being inclined to the line of vision twenty-eight degrees four minutes, it is projected into an ellipse. this ellipse contracts in breadth as the ring passes towards its nodes at a and e, where it dwindles into a straight line. from e to g the ring opens again, becomes broadest at g, and again contracts, till it becomes a straight line at a, and from this point expands, till it recovers its original breadth at c. these successive appearances are all exhibited to a telescope of moderate powers. the ring is extremely _thin_, since the smallest satellite, when projected on it, more than covers it. the thickness is estimated at only one hundred miles. saturn's ring shines wholly by _reflected light_ derived from the sun. this is evident from the fact that that side only which is turned towards the sun is enlightened; and it is remarkable, that the illumination of the ring is greater than that of the planet itself, but the outer ring is less bright than the inner. although we view saturn's ring nearly as though we saw it from the sun, yet the plane of the ring produced may pass between the earth and the sun, in which case, also, the ring becomes invisible, the illuminated side being wholly turned from us. thus, when the ring is approaching its node at e, a spectator at _a_ would have the dark side of the ring presented to him. the ring was invisible in , and will be invisible again in . the northern side of the ring will be in sight until , when the southern side will come into view. it appears, therefore, that there are three causes for the disappearance of saturn's ring: first, when the edge of the ring is presented to the sun; secondly, when the edge is presented to the earth; and thirdly, when the unilluminated side is towards the earth. saturn's ring _revolves_ in its own plane in about ten and a half hours. la place inferred this from the doctrine of universal gravitation. he proved that such a rotation was necessary; otherwise, the matter of which the ring is composed would be precipitated upon its primary. he showed that, in order to sustain itself, its period of rotation must be equal to the time of revolution of a satellite, circulating around saturn at a distance from it equal to that of the middle of the ring, which period would be about ten and a half hours. by means of spots in the ring, dr. herschel followed the ring in its rotation, and actually found its period to be the same as assigned by la place,--a coincidence which beautifully exemplifies the harmony of truth. although the rings have very nearly the same centre with the planet itself, yet, recent measurements of extreme delicacy have demonstrated, that the coincidence is not mathematically exact, but that the centre of gravity of the rings describes around that of the body a very minute orbit. "this fact," says sir j. herschel, "unimportant as it may seem, is of the utmost consequence to the stability of the system of rings. supposing them mathematically perfect in their circular form, and exactly concentric with the planet, it is demonstrable that they would form (in spite of their centrifugal force) a system in a state of unstable equilibrium, which the slightest external power would subvert, not by causing a rupture in the substance of the rings, but by precipitating them unbroken upon the surface of the planet." the ring may be supposed of an unequal breadth in its different parts, and as consisting of irregular solids, whose common centre of gravity does not coincide with the centre of the figure. were it not for this distribution of matter, its equilibrium would be destroyed by the slightest force, such as the attraction of a satellite, and the ring would finally precipitate itself upon the planet. sir j. herschel further observes, that, "as the smallest difference of velocity between the planet and its rings must infallibly precipitate the rings upon the planet, never more to separate, it follows, either that their motions in their common orbit round the sun must have been adjusted to each other by an external power, with the minutest precision, or that the rings must have been formed about the planet while subject to their common orbitual motion, and under the full and free influence of all the acting forces. "the rings of saturn must present a magnificent spectacle from those regions of the planet which lie on their enlightened sides, appearing as vast arches spanning the sky from horizon to horizon, and holding an invariable situation among the stars. on the other hand, in the region beneath the dark side, a solar eclipse of fifteen years in duration, under their shadow, must afford (to our ideas) an inhospitable abode to animated beings, but ill compensated by the full light of its satellites. but we shall do wrong to judge of the fitness or unfitness of their condition, from what we see around us, when, perhaps, the very combinations which convey to our minds only images of horror, may be in reality theatres of the most striking and glorious displays of beneficent contrivance." saturn is attended by _seven satellites_. although they are bodies of considerable size, their great distance prevents their being visible to any telescope but such as afford a strong light and high magnifying powers. the outermost satellite is distant from the planet more than thirty times the planet's diameter, and is by far the largest of the whole. it exhibits, like the satellites of jupiter, periodic variations of light, which prove its revolution on its axis in the time of a sidereal revolution about saturn, as is the case with our moon, while performing its circuit about the earth. the next satellite in order, proceeding inwards, is tolerably conspicuous; the three next are very minute, and require powerful telescopes to see them; while the two interior satellites, which just skirt the edge of the ring, and move exactly in its plane, have never been discovered but with the most powerful telescopes which human art has yet constructed, and then only under peculiar circumstances. at the time of the disappearance of the rings, (to ordinary telescopes,) they were seen by sir william herschel, with his great telescope, projected along the edge of the ring, and threading, like beads, the thin fibre of light to which the ring is then reduced. owing to the obliquity of the ring, and of the orbits of the satellites, to that of their primary, there are no eclipses of the satellites, the two interior ones excepted, until near the time when the ring is seen edgewise. "the firmament of saturn will unquestionably present to view a more magnificent and diversified scene of celestial phenomena than that of any other planet in our system. it is placed nearly in the middle of that space which intervenes between the sun and the orbit of the remotest planet. including its rings and satellites, it may be considered as the largest body or system of bodies within the limits of the solar system; and it excels them all in the sublime and diversified apparatus with which it is accompanied. in these respects, saturn may justly be considered as the sovereign among the planetary hosts. the prominent parts of its celestial scenery may be considered as belonging to its own system of rings and satellites, and the views which will occasionally be opened of the firmament of the fixed stars; for few of the other planets will make their appearance in its sky. jupiter will appear alternately as a morning and an evening star, with about the same degree of brilliancy it exhibits to us; but it will seldom be conspicuous, except near the period of its greatest elongation; and it will never appear to remove from the sun further than thirty-seven degrees, and consequently will not appear so conspicuous, nor for such a length of time, as venus does to us. uranus is the only other planet which will be seen from saturn, and it will there be distinctly perceptible, like a star of the third magnitude, when near the time of its opposition to the sun. but near the time of its conjunction it will be completely invisible, being then eighteen hundred millions of miles more distant than at the opposition, and eight hundred millions of miles more distant from saturn than it ever is from the earth at any period."[ ] uranus.--uranus is the remotest planet belonging to our system, and is rarely visible, except to the telescope. although his diameter is more than four times that of the earth, being thirty-five thousand one hundred and twelve miles, yet his distance from the sun is likewise nineteen times as great as the earth's distance, or about eighteen hundred millions of miles. his revolution around the sun occupies nearly eighty-four years, so that his position in the heavens, for several years in succession, is nearly stationary. his path lies very nearly in the ecliptic, being inclined to it less than one degree. the sun himself, when seen from uranus dwindles almost to a star, subtending, as it does, an angle of only one minute and forty seconds; so that the surface of the sun would appear there four hundred times less than it does to us. this planet was discovered by sir william herschel on the thirteenth of march, . his attention was attracted to it by the largeness of its disk in the telescope; and finding that it shifted its place among the stars, he at first took it for a comet, but soon perceived that its orbit was not eccentric, like the orbits of comets, but nearly circular, like those of the planets. it was then recognised as a new member of the planetary system, a conclusion which has been justified by all succeeding observations. it was named by the discoverer the _george star_, (georgium sidus,) after his munificent patron, george the third; in the united states, and in some other countries, it was called _herschel_; but the name _uranus_, from a greek word, (= ouranos=, _ouranos_,) signifying the oldest of the gods, has finally prevailed. so distant is uranus from the sun, that light itself, which moves nearly twelve millions of miles every minute, would require more than two hours and a half to pass to it from the sun. and now, having contemplated all the planets separately, just cast your eyes on the diagram facing page , fig. , and you will see a comparative view of the various magnitudes of the sun, as seen from each of the planets. uranus is attended by _six satellites_. so minute objects are they, that they can be seen only by powerful telescopes. indeed, the existence of more than two is still considered as somewhat doubtful. these two, however, offer remarkable and indeed quite unexpected and unexampled peculiarities. contrary to the unbroken analogy of the whole planetary system, _the planes of their orbits are nearly perpendicular to the ecliptic_, and in these orbits their motions are retrograde; that is, instead of advancing from west to east around their primary, as is the case with all the other planets and satellites, they move in the opposite direction. with this exception, all the motions of the planets, whether around their own axes, or around the sun, are from west to east. the sun himself turns on his axis from west to east; all the primary planets revolve around the sun from west to east; their revolutions on their own axes are also in the same direction; all the secondaries, with the single exception above mentioned, move about their primaries from west to east; and, finally, such of the secondaries as have been discovered to have a diurnal revolution, follow the same course. such uniformity among so many motions could have resulted only from forces impressed upon them by the same omnipotent hand; and few things in the creation more distinctly proclaim that god made the world. retiring now to this furthest verge of the solar system, let us for a moment glance at the aspect of the firmament by night. notwithstanding we have taken a flight of eighteen hundred millions of miles, the same starry canopy bends over our heads; sirius still shines with exactly the same splendor as here; orion, the scorpion, the great and the little bear, all occupy the same stations; and the galaxy spans the sky with the same soft and mysterious light. the planets, however, with the exception of saturn, are all lost to the view, being too near the sun ever to be seen; and saturn himself is visible only at distant intervals, at periods of fifteen years, when at its greatest elongations from the sun, and is then too near the sun to permit a clear view of his rings, much less of the satellites that unite with the rings to compose his gorgeous retinue. comets, moving slowly as they do when so distant from the sun, will linger much longer in the firmament of uranus than in ours. although the sun sheds by day a dim and cheerless light, yet the six satellites that enlighten and diversify the nocturnal sky present interesting aspects. "let us suppose one satellite presenting a surface in the sky eight or ten times larger than our moon; a second, five or six times larger; a third, three times larger; a fourth, twice as large; a fifth, about the same size as the moon; a sixth, somewhat smaller; and, perhaps, three or four others of different apparent dimensions: let us suppose two or three of those, of different phases, moving along the concave of the sky, at one period four or five of them dispersed through the heavens, one rising above the horizon, one setting, one on the meridian, one towards the north, and another towards the south; at another period, five or six of them displaying their lustre in the form of a half moon, or a crescent, in one quarter of the heavens; and, at another time, the whole of these moons shining, with full enlightened hemispheres, in one glorious assemblage, and we shall have a faint idea of the beauty, variety, and sublimity of the firmament of uranus."[ ] _the new planets,--ceres, pallas, juno, and vesta._--the commencement of the present century was rendered memorable in the annals of astronomy, by the discovery of four new planets, occupying the long vacant tract between mars and jupiter. kepler, from some analogy which he found to subsist among the distances of the planets from the sun, had long before suspected the existence of one at this distance; and his conjecture was rendered more probable by the discovery of uranus, which follows the analogy of the other planets. so strongly, indeed, were astronomers impressed with the idea that a planet would be found between mars and jupiter, that, in the hope of discovering it, an association was formed on the continent of europe, of twenty-four observers, who divided the sky into as many zones, one of which was allotted to each member of the association. the discovery of the first of these bodies was, however, made accidentally by piazzi, an astronomer of palermo, on the first of january, . it was shortly afterwards lost sight of on account of its proximity to the sun, and was not seen again until the close of the year, when it was re-discovered in germany. piazzi called it _ceres_, in honor of the tutelary goddess of sicily, and her emblem, the sickle, ([planet: ceres]) has been adopted as its appropriate symbol. the difficulty of finding ceres induced dr. olbers, of bremen, to examine with particular care all the small stars that lie near her path, as seen from the earth; and, while prosecuting these observations, in march, , he discovered another similar body, very nearly at the same distance from the sun, and resembling the former in many other particulars. the discoverer gave to this second planet the name of _pallas_, choosing for its symbol the lance, ([planet: pallas]) the characteristic of minerva. the most surprising circumstance connected with the discovery of _pallas_ was the existence of two planets at nearly the same distance from the sun, and apparently crossing the ecliptic in the same part of the heavens, or having the same node. on account of this singularity, dr. olbers was led to conjecture that ceres and pallas are only fragments of a larger planet, which had formerly circulated at the same distance, and been shattered by some internal convulsion. the hypothesis suggested the probability that there might be other fragments, whose orbits might be expected to cross the ecliptic at a common point, or to have the same node. dr. olbers, therefore, proposed to examine carefully, every month, the two opposite parts of the heavens in which the orbits of ceres and pallas intersect one another, with a view to the discovery of other planets, which might be sought for in those parts with a greater chance of success, than in a wider zone, embracing the entire limits of these orbits. accordingly, in , near one of the nodes of ceres and pallas, a third planet was discovered. this was called _juno_, and the character ([planet: juno]) was adopted for its symbol, representing the starry sceptre of the queen of olympus. pursuing the same researches, in a fourth planet was discovered, to which was given the name of _vesta_, and for its symbol the character ([planet: vesta]) was chosen,--an altar surmounted with a censer holding the sacred fire. the _average distance_ of these bodies from the sun is two hundred and sixty-one millions of miles; and it is remarkable that their orbits are very near together. taking the distance of the earth from the sun for unity, their respective distances are . , . , . , . . their _times_ of revolution around the sun are nearly equal, averaging about four and a half years. in respect to the _inclination of their orbits_ to the ecliptic, there is also considerable diversity. the orbit of vesta is inclined only about seven degrees, while that of pallas is more than thirty-four degrees. they all, therefore, have a higher inclination than the orbits of the old planets, and of course make excursions from the ecliptic beyond the limits of the zodiac. hence they have been called the _ultra-zodiacal planets_. when first discovered, before their place in the system was fully ascertained it was also proposed to call them _asteroids_, a name implying that they were planets under the form of stars. their title, however, to take their rank among the primary planets, is now generally conceded. the _eccentricity of their orbits_ is also, in general, greater than that of the old planets. you will recollect that this language denotes that their orbits are more elliptical, or depart further from the circular form. the eccentricities of the orbits of pallas and juno exceed that of the orbit of mercury. the asteroids differ so much, however, in eccentricity, that their orbits may cross each other. the orbits of the old planets are so nearly circular, and at such a great distance apart, that there is no danger of their interfering with each other. the earth, for example, when at its nearest distance from the sun, will never come so near it as venus is when at its greatest distance, and therefore can never cross the orbit of venus. but since the average distance of ceres and pallas from the sun is about the same, while the eccentricity of the orbit of pallas is much greater than that of ceres, consequently, pallas may come so near to the sun at its perihelion, as to cross the orbit of ceres. the _small size_ of the asteroids constitutes one of their most remarkable peculiarities. the difficulty of estimating the apparent diameter of bodies at once so very small and so far off, would lead us to expect different results in the actual estimates. accordingly, while dr. herschel estimates the diameter of pallas at only eighty miles, schroeter places it as high as two thousand miles, or about the diameter of the moon. the volume of vesta is estimated at only one fifteen thousandth part of the earth's, and her surface is only about equal to that of the kingdom of spain. these little bodies are surrounded by _atmospheres_ of great extent, some of which are uncommonly luminous, and others appear to consist of nebulous matter, like that of comets. these planets shine with a more vivid light than might be expected, from their great distance and diminutive size; but a good telescope is essential for obtaining a distinct view of their phenomena. although the great chasm which occurs between mars and jupiter,--a chasm of more than three hundred millions of miles,--suggested long ago the idea of other planetary bodies occupying that part of the solar system, yet the discovery of the asteroids does not entirely satisfy expectation since they are bodies so dissimilar to the other members of the series in size, in appearance, and in the form and inclinations of their orbits. hence, dr. olbers, the discoverer of three of these bodies, held that they were fragments of a single large planet, which once occupied that place in the system, and which exploded in different directions by some internal violence. of the fragments thus projected into space, some would be propelled in such directions and with such velocities, as, under the force of projection and that of the solar attraction would make them revolve in regular orbits around the sun. others would be so projected among the other bodies in the system, as to be thrown in very irregular orbits, apparently wandering lawless through the skies. the larger fragments would receive the least impetus from the explosive force, and would therefore circulate in an orbit deviating less than any other of the fragments from the original path of the large planet; while the lesser fragments, being thrown off with greater velocity, would revolve in orbits more eccentric, and more inclined to the ecliptic. dr. brewster, editor of the 'edinburgh encyclopedia,' and the well-known author of various philosophical works, espoused this hypothesis with much zeal; and, after summing up the evidence in favor of it, he remarks as follows: "these singular resemblances in the motions of the greater fragments, and in those of the lesser fragments, and the striking coincidences between theory and observation in the eccentricity of their orbits, in their inclination to the ecliptic, in the position of their nodes, and in the places of their perihelia, are phenomena which could not possibly result from chance, and which concur to prove, with an evidence amounting almost to demonstration, that the four new planets have diverged from one common node, and have therefore composed a single planet." the same distinguished writer supposes that some of the smallest fragments might even have come within reach of the earth's attraction, and by the combined effects of their projectile forces and the attraction of the earth, be made to revolve around this body as the larger fragments are carried around the sun; and that these are in fact the bodies which afford those _meteoric stones_ which are occasionally precipitated to the earth. it is now a well-ascertained fact, a fact which has been many times verified in our own country, that large meteors, in the shape of fire-balls, traversing the atmosphere, sometimes project to the earth masses of stony or ferruginous matter. such were the meteoric stones which fell at weston, in connecticut, in , of which a full and interesting account was published, after a minute examination of the facts, by professors silliman and kingsley, of yale college. various accounts of similar occurrences may be found in different volumes of the american journal of science. it is for the production of these wonderful phenomena that dr. brewster supposes the explosion of the planet, which, according to dr. olbers, produced the asteroids, accounts. others, however, as sir john herschel, have been disposed to ascribe very little weight to the doctrine of olbers. footnotes: [ ] altissimum planetam tergeminum observavi. or, as transposed, smaismrmilme poeta leumi bvne nugttaviras. [ ] in imitation of galileo, huyghens announced his discovery in this form: a a a a a a a c c c c c d e e e e e g h i i i i i i i l l l l m m n n n n n n n n n o o o o p p q r r s t t t t t u u u u u; which he afterwards recomposed into this sentence: _annulo cingitur, tenui, plano, nusquam cohærente, ad eclipticam inclinato._ [ ] dick's 'celestial scenery.' [ ] dick's 'celestial scenery.' letter xxiv. the planetary motions.----kepler's laws.----kepler. "god of the rolling orbs above! thy name is written clearly bright in the warm day's unvarying blaze, or evening's golden shower of light; for every fire that fronts the sun, and every spark that walks alone around the utmost verge of heaven, was kindled at thy burning throne."--_peabody._ if we could stand upon the sun and view the planetary motions, they would appear to us as simple as the motions of equestrians riding with different degrees of speed around a large ring, of which we occupied the centre. we should see all the planets coursing each other from west to east, through the same great highway, (the zodiac,) no one of them, with the exception of the asteroids, deviating more than seven degrees from the path pursued by the earth. most of them, indeed, would always be seen moving much nearer than that to the ecliptic. we should see the planets moving on their way with various degrees of speed. mercury would make the entire circuit in about three months, hurrying on his course with a speed about one third as great as that by which the moon revolves around the earth. the most distant planets, on the other hand, move at so slow a pace, that we should see mercury, venus, the earth, and mars, severally overtaking them a great many times, before they had completed their revolutions. but though the movements of some were comparatively rapid, and of others extremely slow, yet they would not seem to differ materially, in other respects: each would be making a steady and nearly uniform march along the celestial vault. such would be the simple and harmonious motions of the planets, as they would be seen from the sun, the centre of their motions; and such they are, in fact. but two circumstances conspire to make them appear exceedingly different from these, and vastly more complicated; one is, that we view them out of the centre of their motions; the other, that we are ourselves in motion. i have already explained to you the effect which these two causes produce on the apparent motions of the inferior planets, mercury and venus. let us now see how they effect those of the superior planets, mars, jupiter, saturn, and uranus. orreries, or machines intended to exhibit a model of the solar system, are sometimes employed to give a popular view of the planetary motions; but they oftener mislead than give correct ideas. they may assist reflection, but they can never supply its place. the impossibility of representing things in their just proportions will be evident, when we reflect that, to do this, if in an orrery we make mercury as large as a cherry, we should have to represent the sun six feet in diameter. if we preserve the same proportions, in regard to distance, we must place mercury two hundred and fifty feet, and uranus twelve thousand five hundred feet, or more than two miles from the sun. the mind of the student of astronomy must, therefore, raise itself from such imperfect representations of celestial phenomena, as are afforded by artificial mechanism, and, transferring his contemplations to the celestial regions themselves, he must conceive of the sun and planets as bodies that bear an insignificant ratio to the immense spaces in which they circulate, resembling more a few little birds flying in the open sky, than they do the crowded machinery of an orrery. the _real_ motions of the planets, indeed, or such as orreries usually exhibit, are very easily conceived of, as before explained; but the _apparent_ motions are, for the most part, entirely different from these. the apparent motions of the inferior planets have been already explained. you will recollect that mercury and venus move backwards and forwards across the sun, the former never being seen further than twenty-nine degrees, and the latter never more than about forty-seven degrees, from that luminary; that, while passing from the greatest elongation on one side, to the greatest elongation on the other side, through the superior conjunction, the apparent motions of these planets are accelerated by the motion of the earth; but that, while moving through the inferior conjunction, at which time their motions are retrograde, they are apparently retarded by the earth's motion. let us now see what are the apparent motions of the superior planets. let a, b, c, fig. , page , represent the earth in different positions in its orbit, m, a superior planet, as mars, and n r, an arc of the concave sphere of the heavens. first, suppose the planet to remain at rest in m, and let us see what apparent motions it will receive from the real motions of the earth. when the earth is at b, it will see the planet in the heavens at n; and as the earth moves successively through c, d, e, f, the planet will appear to move through o, p, q, r. b and f are the two points of greatest elongation of the earth from the sun, as seen from the planet; hence, between these two points, while passing through its orbit most remote from the planet, (when the planet is seen in superior conjunction,) the earth, by its own motion, gives an apparent motion to the planet in the order of the signs; that is, the _apparent_ motion given by the _real_ motion of the earth is _direct_. but in passing from f to b through a, when the planet is seen in opposition, the apparent motion given to the planet by the earth's motion is from r to n, and is therefore _retrograde_. as the arc described by the earth, when the motion is direct, is much greater than when the motion is retrograde, while the apparent arc of the heavens described by the planet from n to r, in the one case, and from r to n, in the other, is the same in both cases, the retrograde motion is much swifter than the direct, being performed in much less time. [illustration fig. .] but the superior planets are not in fact at rest, as we have supposed, but are all the while moving eastward, though with a slower motion than the earth. indeed, with respect to the remotest planets, as saturn and uranus, the forward motion is so exceedingly slow, that the above representation is nearly true for a single year. still, the effect of the real motions of all the superior planets, eastward, is to increase the direct apparent motion communicated by the earth, and to diminish the retrograde motion. this will be evident from inspecting the figure; for if the planet _actually_ moves eastward while it is _apparently_ carried eastward by the earth's motion, the whole motion eastward will be equal to the sum of the two; and if, while it is really moving eastward, it is apparently carried westward still more by the earth's motion, the retrograde movement will equal the difference of the two. if mars stood still while the earth went round the sun, then a second opposition, as at a, would occur at the end of one year from the first; but, while the earth is performing this circuit, mars is also moving the same way, more than half as fast; so that, when the earth returns to a, the planet has already performed more than half the same circuit, and will have completed its whole revolution before the earth comes up with it. indeed mars, after having been seen once in opposition, does not come into opposition again until after two years and fifty days. and since the planet is then comparatively near to us, as at m, while the earth is at a, and appears very large and bright, rising unexpectedly about the time the sun sets, he surprises the world as though it were some new celestial body. but on account of the slow progress of saturn and uranus, we find, after having performed one circuit around the sun, that they are but little advanced beyond where we left them at the last opposition. the time between one opposition of saturn and another is only a year and thirteen days. it appears, therefore, that the superior planets steadily pursue their course around the sun, but that their apparent retrograde motion, when in opposition, is occasioned by our passing by them with a swifter motion, of which we are unconscious, like the apparent backward motion of a vessel, when we overtake it and pass by it rapidly in a steam-boat. such are the real and the apparent motions of the planets. let us now turn our attention to the _laws of the planetary orbits_. there are three great principles, according to which the motions of the earth and all the planets around the sun are regulated, called kepler's laws, having been first discovered by the astronomer whose name they bear. they may appear to you, at first, dry and obscure; yet they will be easily understood from the explanations which follow; and so important have they proved in astronomical inquiries, that they have acquired for their renowned discoverer the appellation of the '_legislator of the skies_.' we will consider each of these laws separately; and, for the sake of rendering the explanation clear and intelligible, i shall perhaps repeat some things that have been briefly mentioned before. [illustration fig. .] first law.--_the orbits of the earth and all the planets are ellipses, having the sun in the common focus._ in a circle, all the diameters are equal to one another; but if we take a metallic wire or hoop, and draw it out on opposite sides, we elongate it into an ellipse, of which the different diameters are very unequal. that which connects the points most distant from each other is called the _transverse_, and that which is at right angles to this is called the _conjugate_, axis. thus, a b, fig. , is the transverse axis, and c d, the conjugate of the ellipse a b c. by such a process of elongating the circle into an ellipse, the centre of the circle may be conceived of as drawn opposite ways to e and f, each of which becomes a _focus_, and both together are called the _foci_ of the ellipse. the distance g e, or g f, of the focus from the centre is called the _eccentricity_ of the ellipse; and the ellipse is said to be more or less eccentric, as the distance of the focus from the centre is greater or less. figure represents such a collection of ellipses around the common focus f, the innermost, a g d, having a small eccentricity, or varying little from a circle, while the outermost, a c b, is an eccentric ellipse. the orbits of all the bodies that revolve about the sun, both planets and comets, have, in like manner, a common focus, in which the sun is situated, but they differ in eccentricity. most of the planets have orbits of very little eccentricity, differing little from circles, but comets move in very eccentric ellipses. the earth's path around the sun varies so little from a circle, that a diagram representing it truly would scarcely be distinguished from a perfect circle; yet, when the comparative distances of the sun from the earth are taken at different seasons of the year, we find that the difference between their greatest and least distances is no less than three millions of miles. [illustration fig. .] second law.--_the radius vector of the earth, or of any planet, describes equal areas in equal times._ you will recollect that the radius vector is a line drawn from the centre of the sun to a planet revolving about the sun. this definition i have somewhere given you before, and perhaps it may appear to you like needless repetition to state it again. in a book designed for systematic instruction, where all the articles are distinctly numbered, it is commonly sufficient to make a reference back to the article where the point in question is explained; but i think, in letters like these, you will bear with a little repetition, rather than be at the trouble of turning to the index and hunting up a definition long since given. [illustration fig. . ] in figure , _e a_, _e b_, _e c_, &c., are successive representations of the radius vector. now, if a planet sets out from _a_, and travels round the sun in the direction of _a b c_, it will move faster when nearer the sun, as at _a_, than when more remote from it, as at _m_; yet, if _a b_ and _m n_ be arcs described in equal times, then, according to the foregoing law, the space _e a b_ will be equal to the space _e m n_; and the same is true of all the other spaces described in equal times. although the figure _e a b_ is much shorter than _e m n_, yet its greater breadth exactly counterbalances the greater length of those figures which are described by the radius vector where it is longer. third law.--_the squares of the periodical times are as the cubes of the mean distances from the sun._ the periodical time of a body is the time it takes to complete its orbit, in its revolution about the sun. thus the earth's periodic time is one year, and that of the planet jupiter about twelve years. as jupiter takes so much longer time to travel round the sun than the earth does, we might suspect that his orbit is larger than that of the earth, and of course, that he is at a greater distance from the sun; and our first thought might be, that he is probably twelve times as far off; but kepler discovered that the distance does not increase as fast as the times increase, but that the planets which are more distant from the sun actually move slower than those which are nearer. after trying a great many proportions, he at length found that, if we take the squares of the periodic times of two planets, the greater square contains the less, just as often as the cube of the distance of the greater contains that of the less. this fact is expressed by saying, that the squares of the periodic times are to one another as the cubes of the distances. this law is of great use in determining the distance of the planets from the sun. suppose, for example, that we wish to find the distance of jupiter. we can easily determine, from observation, what is jupiter's periodical time, for we can actually see how long it takes for jupiter, after leaving a certain part of the heavens to come round to the same part again. let this period be twelve years. the earth's period is of course one year; and the distance of the earth, as determined from the sun's horizontal parallax, as already explained, is about ninety-five millions of miles. now, we have here three terms of a proportion to find the fourth, and therefore the solution is merely a simple case of the rule of three. thus:--the square of year : square of years :: cube of , , : cube of jupiter's distance. the three first terms being known, we have only to multiply together the second and third and divide by the first, to obtain the fourth term, which will give us the cube of jupiter's distance from the sun; and by extracting the cube root of this sum, we obtain the distance itself. in the same manner we may obtain the respective distances of all the other planets. so truly is this a law of the solar system, that it holds good in respect to the new planets, which have been discovered since kepler's time, as well as in the case of the old planets. it also holds good in respect to comets, and to all bodies belonging to the solar system, which revolve around the sun as their centre of motion. hence, it is justly regarded as one of the most interesting and important principles yet developed in astronomy. but who was this kepler, that gained such an extraordinary insight into the laws of the planetary system, as to be called the 'legislator of the skies?' john kepler was one of the most remarkable of the human race, and i think i cannot gratify or instruct you more, than by occupying the remainder of this letter with some particulars of his history. kepler was a native of germany. he was born in the duchy of wurtemberg, in . as copernicus, tycho brahe, galileo, kepler, and newton, are names that are much associated in the history of astronomy, let us see how they stood related to each other in point of time. copernicus was born in ; tycho, in ; galileo, in ; kepler, in ; and newton, in . hence, copernicus was seventy-three years before tycho, and tycho ninety-six years before newton. they all lived to an advanced age, so that tycho, galileo, and kepler, were contemporary for many years; and newton, as i mentioned in the sketch i gave you of his life, was born the year that galileo died. kepler was born of parents who were then in humble circumstances, although of noble descent. their misfortunes, which had reduced them to poverty, seem to have been aggravated by their own unhappy dispositions; for his biographer informs us, that "his mother was treated with a degree of barbarity by her husband and brother-in-law, that was hardly exceeded by her own perverseness." it is fortunate, therefore, that kepler, in his childhood, was removed from the immediate society and example of his parents, and educated at a public school at the expense of the duke of wurtemberg. he early imbibed a taste for natural philosophy, but had conceived a strong prejudice against astronomy, and even a contempt for it, inspired, probably, by the arrogant and ridiculous pretensions of the astrologers, who constituted the principal astronomers of his country. a vacant post, however, of teacher of astronomy, occurred when he was of a suitable age to fill it, and he was compelled to take it by the authority of his tutors, though with many protestations, on his part, wishing to be provided for in some other more brilliant profession. happy is genius, when it lights on a profession entirely consonant to its powers, where the objects successively presented to it are so exactly suited to its nature, that it clings to them as the loadstone to its kindred metal among piles of foreign ores. nothing could have been more congenial to the very mental constitution of kepler, than the study of astronomy,--a science where the most capacious understanding may find scope in unison with the most fervid imagination. much as has been said against hypotheses in philosophy, it is nevertheless a fact, that some of the greatest truths have been discovered in the pursuit of hypotheses, in themselves entirely false; truths, moreover, far more important than those assumed by the hypotheses; as columbus, in searching for a northwest passage to india, discovered a new world. thus kepler groped his way through many false and absurd suppositions, to some of the most sublime discoveries ever made by man. the fundamental principle which guided him was not, however, either false or absurd. it was, that god, who made the world, had established, throughout all his works, fixed laws,--laws that are often so definite as to be capable of expression in exact numerical terms. in accordance with these views, he sought for numerical relations in the disposition and arrangement of the planets, in respect to their number, the times of their revolution, and their distances from one another. many, indeed, of the subordinate suppositions which he made, were extremely fanciful; but he tried his own hypotheses by a rigorous mathematical test, wherever he could apply it; and as soon as he discovered that a supposition would not abide this test, he abandoned it without the least hesitation, and adopted others, which he submitted to the same severe trial, to share, perhaps, the same fate. "after many failures," he says, "i was comforted by observing that the motions, in every case, seemed to be connected with the distances; and that, when there was a great gap between the orbits, there was the same between the motions. and i reasoned that, if god had adapted motions to the orbits in some relation to the distances, he had also arranged the distances themselves in relation to something else." in two years after he commenced the study of astronomy, he published a book, called the '_mysterium cosmographicum_,' a name which implies an explanation of the mysteries involved in the construction of the universe. this work was full of the wildest speculations and most extravagant hypotheses, the most remarkable of which was, that the distances of the planets from the sun are regulated by the relations which subsist between the five regular solids. it is well known to geometers, that there are and can be only five _regular solids_. these are, first, the _tetraedron_, a four-sided figure, all whose sides are equal and similar triangles; secondly, the _cube_, contained by six equal squares; thirdly, an _octaedron_, an eight-sided figure, consisting of two four-sided pyramids joined at their bases; fourthly, a _dodecaedron_, having twelve five-sided or pentagonal faces; and, fifthly, an _icosaedron_, contained by twenty equal and similar triangles. you will be much at a loss, i think, to imagine what relation kepler could trace between these strange figures and the distances of the several planets from the sun. he thought he discovered a connexion between those distances and the spaces which figures of this kind would occupy, if interposed in certain ways between them. thus, he says the earth is a circle, the measure of all; round it describe a dodecaedron, and the circle including this will be the orbit of mars. round this circle describe a tetraedron, and the circle including this will be the orbit of jupiter. describe a cube round this, and the circle including it will be the orbit of saturn. now, inscribe in the earth an icosaedron, and the circle included in this will give the orbit of venus. in this inscribe an octaedron, and the circle included in this will be the orbit of mercury. on this supposed discovery kepler exults in the most enthusiastic expressions. "the intense pleasure i have received from this discovery never can be told in words. i regretted no more time wasted; i tired of no labor; i shunned no toil of reckoning; days and nights i spent in calculations, until i could see whether this opinion would agree with the orbits of copernicus, or whether my joy was to vanish into air. i willingly subjoin that sentiment of archytas, as given by cicero; 'if i could mount up into heaven, and thoroughly perceive the nature of the world and the beauty of the stars, that admiration would be without a charm for me, unless i had some one like you, reader, candid, attentive, and eager for knowledge, to whom to describe it.' if you acknowledge this feeling, and are candid, you will refrain from blame, such as, not without cause, i anticipate; but if, leaving that to itself, you fear, lest these things be not ascertained, and that i have shouted triumph before victory, at least approach these pages, and learn the matter in consideration: you will not find, as just now, new and unknown planets interposed; that boldness of mine is not approved; but those old ones very little loosened, and so furnished by the interposition (however absurd you may think it) of rectilinear figures, that in future you may give a reason to the rustics, when they ask for the hooks which keep the skies from falling." when tycho brahe, who had then retired from his famous uraniburg, and was settled in prague, met with this work of kepler's, he immediately recognised under this fantastic garb the lineaments of a great astronomer. he needed such an unwearied and patient calculator as he perceived kepler to be, to aid him in his labors, in order that he might devote himself more unreservedly to the taking of observations,--an employment in which he delighted, and in which, as i mentioned, in giving you a sketch of his history, he excelled all men of that and preceding ages. kepler, therefore, at the express invitation of tycho, went to prague, and joined him in the capacity of assistant. had tycho been of a nature less truly noble, he might have looked with contempt on one who had made so few observations, and indulged so much in wild speculation; or he might have been jealous of a rising genius, in which he descried so many signs of future eminence as an astronomer; but, superior to all the baser motives, he extends to the young aspirant the hand of encouragement, in the following kind invitation: "come not as a stranger, but as a very welcome friend; come, and share in my observations, with such instruments as i have with me." several years previous to this, kepler, after one or two unsuccessful trials, had found him a wife, from whom he expected a considerable fortune; but in this he was disappointed; and so poor was he, that, when on his journey to prague, in company with his wife, being taken sick, he was unable to defray the expenses of the journey, and was forced to cast himself on the bounty of tycho. in the course of the following year, while absent from prague, he fancied that tycho had injured him, and accordingly addressed to the noble dane a letter full of insults and reproaches. a mild reply from tycho opened the eyes of kepler to his own ingratitude. his better feelings soon returned, and he sent to his great patron this humble apology: "most noble tycho! how shall i enumerate, or rightly estimate, your benefits conferred on me! for two months you have liberally and gratuitously maintained me, and my whole family; you have provided for all my wishes; you have done me every possible kindness; you have communicated to me every thing you hold most dear; no one, by word or deed, has intentionally injured me in any thing; in short, not to your own children, your wife, or yourself, have you shown more indulgence than to me. this being so, as i am anxious to put upon record, i cannot reflect, without consternation, that i should have been so given up by god to my own intemperance, as to shut my eyes on all these benefits; that, instead of modest and respectful gratitude, i should indulge for three weeks in continual moroseness towards all your family, and in headlong passion and the utmost insolence towards yourself, who possess so many claims on my veneration, from your noble family, your extraordinary learning, and distinguished reputation. whatever i have said or written against the person, the fame, the honor, and the learning, of your excellency; or whatever, in any other way, i have injuriously spoken or written, (if they admit no other more favorable interpretation,) as to my grief i have spoken and written many things, and more than i can remember; all and every thing i recant, and freely and honestly declare and profess to be groundless, false, and incapable of proof." this was ample satisfaction to the generous tycho. "to err is human: to forgive, divine." on kepler's return to prague, he was presented to the emperor by tycho, and honored with the title of imperial mathematician. this was in , when he was thirty years of age. tycho died shortly after, and kepler succeeded him as principal mathematician to the emperor; but his salary was badly paid, and he suffered much from pecuniary embarrassments. although he held the astrologers, or those who told fortunes by the stars, in great contempt, yet he entertained notions of his own, on the same subject, quite as extravagant, and practised the art of casting nativities, to eke out a support for his family. when galileo began to observe with his telescope, and announced, in rapid succession, his wonderful discoveries, kepler entered into them with his characteristic enthusiasm, although they subverted many of his favorite hypotheses. but such was his love of truth, that he was among the first to congratulate galileo, and a most engaging correspondence was carried on between these master-spirits. the first planet, which occupied the particular attention of kepler, was mars, the long and assiduous study of whose motions conducted him at length to the discovery of those great principles called 'kepler's laws.' rarely do we meet with so remarkable a union of a vivid fancy with a profound intellect. the hasty and extravagant suggestions of the former were submitted to the most laborious calculations, some of which, that were of great length, he repeated seventy times. this exuberance of fancy frequently appears in his style of writing, which occasionally assumes a tone ludicrously figurative. he seems constantly to contemplate mars as a valiant hero, who had hitherto proved invincible, and who would often elude his own efforts to conquer him, "while thus triumphing over mars, and preparing for him, as for one altogether vanquished, tabular prisons, and equated, eccentric fetters, it is buzzed here and there, that the victory is vain, and that the war is raging anew as violently as before. for the enemy, left at home a despised captive, has burst all the chains of the equation, and broken forth of the prisons of the tables. skirmishes routed my forces of physical causes, and, shaking off the yoke, regained their liberty. and now, there was little to prevent the fugitive enemy from effecting a junction with his own rebellious supporters, and reducing me to despair, had i not suddenly sent into the field a reserve of new physical reasonings, on the rout and dispersion of the veterans, and diligently followed, without allowing the slightest respite, in the direction in which he had broken out." but he pursued this warfare with the planet until he gained a full conquest, by the discovery of the first two of his laws, namely, that _he revolves in an elliptical orbit_, and that _his radius vector passes over equal spaces in equal times_. domestic troubles, however, involved him in the deepest affliction. poverty, the loss of a promising and favorite son, the death of his wife, after a long illness;--these were some of the misfortunes that clustered around him. although his first marriage had been an unhappy one, it was not consonant to his genius to surrender any thing with only a single trial. accordingly, it was not long before he endeavored to repair his loss by a second alliance. he commissioned a number of his friends to look out for him, and he soon obtained a tabular list of eleven ladies, among whom his affections wavered. the progress of his courtship is thus narrated in the interesting 'life' contained in the 'library of useful knowledge.' it furnishes so fine a specimen of his eccentricities, that i cannot deny myself the pleasure of transcribing the passage for your perusal. it is taken from an account which kepler himself gave in a letter to a friend. "the first on the list was a widow, an intimate friend of his first wife and who, on many accounts, appeared a most eligible match. at first, she seemed favorably inclined to the proposal: it is certain that she took time to consider it, but at last she very quietly excused herself. finding her afterwards less agreeable in person than he had anticipated, he considered it a fortunate escape, mentioning, among other objections, that she had two marriageable daughters, whom, by the way, he had got on his list for examination. he was much troubled to reconcile his astrology with the fact of his having taken so much pains about a negotiation not destined to succeed. he examined the case professionally. 'have the stars,' says he, 'exercised any influence here? for, just about this time, the direction of the mid-heaven is in hot opposition to mars, and the passage of saturn through the ascending point of the zodiac, in the scheme of my nativity, will happen again next november and december. but, if these are the causes, how do they act? is that explanation the true one, which i have elsewhere given? for i can never think of handing over to the stars the office of deities, to produce effects. let us, therefore, suppose it accounted for by the stars, that at this season i am violent in my temper and affections, in rashness of belief, in a show of pitiful tender-heartedness, in catching at reputation by new and paradoxical notions, and the singularity of my actions; in busily inquiring into, and weighing, and discussing, various reasons; in the uneasiness of my mind, with respect to my choice. i thank god, that that did not happen which might have happened; that this marriage did not take place. now for the others.' of these, one was too old; another, in bad health; another, too proud of her birth and quarterings; a fourth had learned nothing but showy accomplishments, not at all suitable to the kind of life she would have to lead with him. another grew impatient, and married a more decided admirer while he was hesitating. 'the mischief,' says he, 'in all these attachments was, that, whilst i was delaying, comparing, and balancing, conflicting reasons, every day saw me inflamed with a new passion.' by the time he reached no. , of his list, he found his match in this respect. 'fortune has avenged herself at length on my doubtful inclinations. at first, she was quite complying, and her friends also. presently, whether she did or did not consent, not only i, but she herself, did not know. after the lapse of a few days, came a renewed promise, which, however, had to be confirmed a third time: and, four days after that, she again repented her conformation, and begged to be excused from it. upon this, i gave her up, and this time all my counsellors were of one opinion.' this was the longest courtship in the list, having lasted three whole months; and, quite disheartened by its bad success, kepler's next attempt was of a more timid complexion. his advances to no. were made by confiding to her the whole story of his recent disappointment, prudently determining to be guided in his behavior, by observing whether the treatment he experienced met with a proper degree of sympathy. apparently, the experiment did not succeed; and, when almost reduced to despair, kepler betook himself to the advice of a friend, who had for some time past complained that she was not consulted in this difficult negotiation. when she produced no. , and the first visit was paid, the report upon her was as follows: 'she has, undoubtedly, a good fortune, is of good family, and of economical habits: but her physiognomy is most horribly ugly; she would be stared at in the streets, not to mention the striking disproportion in our figures. i am lank, lean, and spare; she is short and thick. in a family notorious for fatness, she is considered superfluously fat.' the only objection to no. seems to have been, her excessive youth; and when this treaty was broken off, on that account, kepler turned his back upon all his advisers, and chose for himself one who had figured as no. , in his list, to whom he professes to have felt attached throughout, but from whom the representations of his friends had hitherto detained him, probably on account of her humble station." having thus settled his domestic affairs, kepler now betook himself, with his usual industry, to his astronomical studies, and brought before the world the most celebrated of his publications, entitled 'harmonics.' in the fifth book of this work he announced his _third law_,--that the squares of the periodical times of the planets are as the cubes of the distances. kepler's rapture on detecting it was unbounded. "what," says he, "i prophesied two-and-twenty years ago, as soon as i discovered the five solids among the heavenly orbits; what i firmly believed long before i had seen ptolemy's harmonics; what i had promised my friends in the title of this book, which i named before i was sure of my discovery; what, sixteen years ago, i urged as a thing to be sought; that for which i joined tycho brahe, for which i settled in prague, for which i have devoted the best part of my life to astronomical contemplations;--at length i have brought to light, and have recognised its truth beyond my most sanguine expectations. it is now eighteen months since i got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze on, burst out upon me. nothing holds me: i will indulge in my sacred fury; i will triumph over mankind by the honest confession, that i have stolen the golden vases of the egyptians to build up a tabernacle for my god, far from the confines of egypt. if you forgive me, i rejoice: if you are angry, i can bear it; the die is cast, the book is written, to be read either now or by posterity,--i care not which. i may well wait a century for a reader, as god has waited six thousand years for an observer." in accordance with the notion he entertained respecting the "music of the spheres," he made saturn and jupiter take the bass, mars the tenor, the earth and venus the counter, and mercury the treble. "the misery in which kepler lived," says sir david brewster, in his 'life of newton,' "forms a painful contrast with the services which he performed for science. the pension on which he subsisted was always in arrears; and though the three emperors, whose reigns he adorned, directed their ministers to be more punctual in its payment, the disobedience of their commands was a source of continual vexation to kepler. when he retired to silesia, to spend the remainder of his days, his pecuniary difficulties became still more harassing. necessity at length compelled him to apply personally for the arrears which were due; and he accordingly set out, in , when nearly sixty years of age, for ratisbon; but, in consequence of the great fatigue which so long a journey on horseback produced, he was seized with a fever, which put an end to his life." professor whewell (in his interesting work on astronomy and general physics considered with reference to natural theology) expresses the opinion that kepler, notwithstanding his constitutional oddities, was a man of strong and lively piety. his 'commentaries on the motions of mars' he opens with the following passage: "i beseech my reader, that, not unmindful of the divine goodness bestowed on man, he do with me praise and celebrate the wisdom and greatness of the creator, which i open to him from a more inward explication of the form of the world, from a searching of causes, from a detection of the errors of vision; and that thus, not only in the firmness and stability of the earth, he perceive with gratitude the preservation of all living things in nature as the gift of god, but also that in its motion, so recondite, so admirable, he acknowledge the wisdom of the creator. but him who is too dull to receive this science, or too weak to believe the copernican system without harm to his piety,--him, i say, i advise that, leaving the school of astronomy, and condemning, if he please, any doctrines of the philosophers, he follow his own path, and desist from this wandering through the universe; and, lifting up his natural eyes, with which he alone can see, pour himself out in his own heart, in praise of god the creator; being certain that he gives no less worship to god than the astronomer, to whom god has given to see more clearly with his inward eye, and who, for what he has himself discovered, both can and will glorify god." in a life of kepler, very recently published in his native country, founded on manuscripts of his which have lately been brought to light, there are given numerous other examples of a similar devotional spirit. kepler thus concludes his harmonics: "i give thee thanks, lord and creator, that thou has given me joy through thy creation; for i have been ravished with the work of thy hands. i have revealed unto mankind the glory of thy works, as far as my limited spirit could conceive their infinitude. should i have brought forward any thing that is unworthy of thee, or should i have sought my own fame, be graciously pleased to forgive me." as galileo experienced the most bitter persecutions from the church of rome, so kepler met with much violent opposition and calumny from the protestant clergy of his own country, particularly for adopting, in an almanac which, as astronomer royal, he annually published, the reformed calendar, as given by the pope of rome. his opinions respecting religious liberty, also, appear to have been greatly in advance of the times in which he lived. in answer to certain calumnies with which he was assailed, for his boldness in reasoning from the light of nature, he uttered these memorable words: "the day will soon break, when pious simplicity will be ashamed of its blind superstition; when men will recognise truth in the book of nature as well as in the holy scriptures, and rejoice in the two revelations." letter xxv. comets. ----"fancy now no more wantons on fickle pinions through the skies, but, fixed in aim, and conscious of her power, sublime from cause to cause exults to rise, creation's blended stores arranging as she flies."--_beattie._ nothing in astronomy is more truly admirable, than the knowledge which astronomers have acquired of the motions of comets, and the power they have gained of predicting their return. indeed, every thing appertaining to this class of bodies is so wonderful, as to seem rather a tale of romance than a simple recital of facts. comets are truly the knights-errant of astronomy. appearing suddenly in the nocturnal sky, and often dragging after them a train of terrific aspect, they were, in the earlier ages of the world, and indeed until a recent period, considered as peculiarly ominous of the wrath of heaven, and as harbingers of wars and famines, of the dethronement of monarchs, and the dissolution of empires. science has, it is true, disarmed them of their terrors, and demonstrated that they are under the guidance of the same hand, that directs in their courses the other members of the solar system; but she has, at the same time, arrayed them in a garb of majesty peculiarly her own. although the ancients paid little attention to the ordinary phenomena of nature, hardly deeming them worthy of a reason, yet, when a comet blazed forth, fear and astonishment conspired to make it an object of the most attentive observation. hence the aspects of remarkable comets, that have appeared at various times, have been handed down to us, often with circumstantial minuteness, by the historians of different ages. the comet which appeared in the year , before the christian era, at the birth of mithridates, is said to have had a disk equal in magnitude to that of the sun. ten years before this, one was seen, which, according to justin, occupied a fourth part of the sky, that is, extended over forty-five degrees, and surpassed the sun in splendor. in the year , one was seen which resembled a sword in shape, and extended from the zenith to the horizon. such are some of the accounts of comets of past ages; but it is probable we must allow much for the exaggerations naturally accompanying the descriptions of objects in themselves so truly wonderful. a comet, when perfectly formed, consists of three parts, the nucleus, the envelope, and the tail. the nucleus, or body of the comet, is generally distinguished by its forming a bright point in the centre of the head, conveying the idea of a solid, or at least of a very dense, portion of matter. though it is usually exceedingly small, when compared with the other parts of the comet, and is sometimes wanting altogether, yet it occasionally subtends an angle capable of being measured by the telescope. the envelope (sometimes called the _coma_, from a latin word signifying hair, in allusion to its hairy appearance) is a dense nebulous covering, which frequently renders the edge of the nucleus so indistinct, that it is extremely difficult to ascertain its diameter with any degree of precision. many comets have no nucleus, but present only a nebulous mass, exceedingly attenuated on the confines, but gradually increasing in density towards the centre. indeed, there is a regular gradation of comets, from such as are composed merely of a gaseous or vapory medium, to those which have a well-defined nucleus. in some instances on record, astronomers have detected with their telescopes small stars through the densest part of a comet. the tail is regarded as an expansion or prolongation of the coma; and presenting, as it sometimes does, a train of appalling magnitude, and of a pale, portentous light, it confers on this class of bodies their peculiar celebrity. these several parts are exhibited in fig. , which [illustration figures , . comets of and .] represents the appearance of the comet of . fig. also exhibits that of the comet of . the _number_ of comets belonging to the solar system, is probably very great. many no doubt escape observation, by being above the horizon in the day-time. seneca mentions, that during a total eclipse of the sun, which happened sixty years before the christian era, a large and splendid comet suddenly made its appearance, being very near the sun. the leading particulars of at least one hundred and thirty have been computed, and arranged in a table, for future comparison. of these, _six_ are particularly remarkable; namely, the comets of , , and ; and those which bear the names of halley, biela, and encke. the comet of was remarkable, not only for its astonishing size and splendor, and its near approach to the sun, but is celebrated for having submitted itself to the observations of sir isaac newton, and for having enjoyed the signal honor of being the first comet whose elements were determined on the sure basis of mathematics. the comet of is memorable for the changes its orbit has undergone by the action of jupiter, as i shall explain to you more particularly hereafter. the comet of was the most remarkable in its appearance of all that have been seen in the present century. it had scarcely any perceptible nucleus, but its train was very long and broad, as is represented in fig. . halley's comet (the same which reappeared in ) is distinguished as that whose return was first successfully predicted, and whose orbit is best determined; and biela's and encke's comets are well known for their short periods of revolution, which subject them frequently to the view of astronomers. in _magnitude and brightness_, comets exhibit great diversity. history informs us of comets so bright, as to be distinctly visible in the day-time, even at noon, and in the brightest sunshine. such was the comet seen at rome a little before the assassination of julius cæsar. the comet of covered an arc of the heavens of ninety-seven degrees, and its length was estimated at one hundred and twenty-three millions of miles. that of had a nucleus of only four hundred and twenty-eight miles in diameter, but a tail one hundred and thirty-two millions of miles long. had it been coiled around the earth like a serpent, it would have reached round more than five thousand times. other comets are exceedingly small, the nucleus being in one case estimated at only twenty-five miles; and some, which are destitute of any perceptible nucleus, appear to the largest telescopes, even when nearest to us, only as a small speck of fog, or as a tuft of down. the majority of comets can be seen only by the aid of the telescope. indeed, the same comet has very different aspects, at its different returns. halley's comet, in , was described by the historians of that age as the comet of terrific magnitude; (_cometa horrendæ magnitudinis_;) in its tail reached from the horizon to the zenith, and inspired such terror, that, by a decree of the pope of rome, public prayers were offered up at noonday in all the catholic churches, to deprecate the wrath of heaven; while in its tail was only thirty degrees in length; and in it was visible only to the telescope until after it had passed its perihelion. at its recent return, in , the greatest length of the tail was about twelve degrees. these changes in the appearance of the same comet are partly owing to the different positions of the earth with respect to them, being sometimes much nearer to them when they cross its track than at others; also, one spectator, so situated as to see the comet at a higher angle of elevation, or in a purer sky, than another, will see the train longer than it appears to another less favorably situated; but the extent of the changes are such as indicate also a real change in magnitude and brightness. the _periods_ of comets in their revolutions around the sun are equally various. encke's comet, which has the shortest known period, completes its revolution in three and one third years; or, more accurately, in twelve hundred and eight days; while that of is estimated to have a period of thirty-three hundred and eighty three years. the _distances_ to which different comets recede from the sun are equally various. while encke's comet performs its entire revolution within the orbit of jupiter, halley's comet recedes from the sun to twice the distance of uranus; or nearly thirty-six hundred millions of miles. some comets, indeed, are thought to go a much greater distance from the sun than this, while some are supposed to pass into curves which do not, like the ellipse, return into themselves; and in this case they never come back to the sun. (see fig. , page .) comets shine _by reflecting the light of the sun_. in one or two instances, they have been thought to exhibit distinct _phases_, like the moon, although the nebulous matter with which the nucleus is surrounded would commonly prevent such phases from being distinctly visible, even when they would otherwise be apparent. moreover, certain qualities of _polarized_ light,--an affection by which a ray of light seems to have different properties on different sides,--enable opticians to decide whether the light of a given body is direct or reflected; and m. arago, of paris, by experiments of this kind on the light of the comet of , ascertained it to be reflected light. the tail of a comet usually increases very much as it approaches the sun; and it frequently does not reach its maximum until after the perihelion passage. in receding from the sun, the tail again contracts, and nearly or quite disappears before the body of the comet is entirely out of sight. the tail is frequently divided into two portions, the central parts, in the direction of the axis, being less bright than the marginal parts. in a comet appeared which had six tails spread out like a fan. the tails of comets extend in a direct line from the sun, although more or less curved, like a long quill or feather, being convex on the side next to the direction in which they are moving,--a figure which may result from the less velocity of the portion most remote from the sun. expansions of the envelope have also been at times observed on the side next the sun; but these seldom attain any considerable length. the _quantity of matter_ in comets is exceedingly small. their tails consist of matter of such tenuity, that the smallest stars are visible through them. they can only be regarded as masses of thin vapor, susceptible of being penetrated through their whole substance by the sunbeams, and reflecting them alike from their interior parts and from their surfaces. it appears perhaps incredible, that so thin a substance should be visible by reflected light, and some astronomers have held that the matter of comets is self-luminous; but it requires but very little light to render an object visible in the night, and a light vapor may be visible when illuminated throughout an immense stratum, which could not be seen if spread over the face of the sky like a thin cloud. "the highest clouds that float in our atmosphere," says sir john herschel, "must be looked upon as dense and massive bodies, compared with the filmy and all but spiritual texture of a comet." the small quantity of matter in comets is proved by the fact, that they have at times passed very near to some of the planets, without disturbing their motions in any appreciable degree. thus the comet of , in its way to the sun, got entangled among the satellites of jupiter, and remained near them four months; yet it did not perceptibly change their motions. the same comet, also, came very near the earth; so that, had its quantity of matter been equal to that of the earth, it would, by its attraction, have caused the earth to revolve in an orbit so much larger than at present, as to have increased the length of the year two hours and forty-seven minutes. yet it produced no sensible effect on the length of the year, and therefore its mass, as is shown by la place, could not have exceeded / of that of the earth, and might have been less than this to any extent. it may indeed be asked, what proof we have that comets have any matter, and are not mere reflections of light. the answer is, that, although they are not able by their own force of attraction to disturb the motions of the planets, yet they are themselves exceedingly disturbed by the action of the planets, and in exact conformity with the laws of universal gravitation. a delicate compass may be greatly agitated by the vicinity of a mass of iron, while the iron is not sensibly affected by the attraction of the needle. by approaching very near to a large planet, a comet may have its orbit entirely changed. this fact is strikingly exemplified in the history of the comet of . at its appearance in , its orbit was found to be an ellipse, requiring for a complete revolution only five and a half years; and the wonder was, that it had not been seen before, since it was a very large and bright comet. astronomers suspected that its path had been changed, and that it had been recently compelled to move in this short ellipse, by the disturbing force of jupiter and his satellites. the french institute, therefore, offered a high prize for the most complete investigation of the elements of this comet, taking into account any circumstances which could possibly have produced an alteration in its course. by tracing back the movements of this comet, for some years previous to , it was found that, at the beginning of , it had entered considerably within the sphere of jupiter's attraction. calculating the amount of this attraction from the known proximity of the two bodies, it was found what must have been its orbit previous to the time when it became subject to the disturbing action of jupiter. it was therefore evident why, as long as it continued to circulate in an orbit so far from the centre of the system, it was never visible from the earth. in january, , jupiter and the comet happened to be very near to one another, and as both were moving in the same direction, and nearly in the same plane, they remained in the neighborhood of each other for several months, the planet being between the comet and the sun. the consequence was, that the comet's orbit was changed into a smaller ellipse, in which its revolution was accomplished in five and a half years. but as it approached the sun, in , it happened again to fall in with jupiter. it was in the month of june that the attraction of the planet began to have a sensible effect; and it was not until the month of october following, that they were finally separated. at the time of their nearest approach, in august, jupiter was distant from the comet only / of its distance from the sun, and exerted an attraction upon it two hundred and twenty-five times greater than that of the sun. by reason of this powerful attraction, jupiter being further from the sun than the comet, the latter was drawn out into a new orbit, which even at its perihelion came no nearer to the sun than the planet ceres. in this third orbit, the comet requires about twenty years to accomplish its revolution; and being at so great a distance from the earth, it is invisible, and will for ever remain so unless, in the course of ages, it may undergo new perturbations, and move again in some smaller orbit, as before. with the foregoing leading facts respecting comets in view, i will now explain to you a few things equally remarkable respecting their _motions_. the paths of the planets around the sun being nearly circular, we are able to see a planet in every part of its orbit. but the case is very different with comets. for the greater part of their course, they are wholly out of sight, and come into view only while just in the neighborhood of the sun. this you will readily see must be the case, by inspecting the frontispiece, which represents the orbit of biela's comet, in . sometimes, the orbit is so eccentric, that the place of the focus occupied by the sun appears almost at the extremity of the orbit. this was the case with the orbit of the comet of . indeed, this comet, at its perihelion, came in fact nearer to the sun than the sixth part of the sun's diameter, being only one hundred and forty-six thousand miles from the surface of the sun, which, you will remark, is only a little more than half the distance of the moon from the earth; while, at its aphelion, it was estimated to be thirteen thousand millions of miles from the sun,--more than eleven thousand millions of miles beyond the planet uranus. its _velocity_, when nearest the sun, exceeded a million of miles an hour. to describe such an orbit as was assigned to it by sir isaac newton, would require five hundred and seventy-five years. during all this period, it was entirely out of view to the inhabitants of the earth, except the few months, while it was running down to the sun from such a distance as the orbit of jupiter and back. the velocity of bodies moving in such eccentric orbits differs widely in different parts of their orbits. in the remotest parts it is so slow, that years would be required to pass over a space equal to that which it would run over in a single day, when near the sun. the appearances of the same comet at different periods of its return are so various, that we can never pronounce a given comet to be the same with one that has appeared before, from any peculiarities in its physical aspect, as from its color, magnitude, or shape; since, in all these respects, it is very different at different returns; but it is judged to be the same if its _path_ through the heavens, as traced among the stars, is the same. the comet whose history is the most interesting, and which both of us have been privileged to see, is halley's. just before its latest visit, in , its return was anticipated with so much expectation, not only by astronomers, but by all classes of the community, that a great and laudable eagerness universally prevailed, to learn the particulars of its history. the best summary of these, which i met with, was given in the edinburgh review for april, . i might content myself with barely referring you to that well-written article; but, as you may not have the work at hand, and would, moreover, probably not desire to read the whole article, i will abridge it for your perusal, interspersing some remarks of my own. i have desired to give you, in the course of these letters, some specimen of the labors of astronomers, and shall probably never be able to find a better one. it is believed that the first recorded appearance of halley's comet was that which was supposed to signalize the birth of mithridates, one hundred and thirty years before the birth of christ. it is said to have appeared for twenty-four days; its light is said to have surpassed that of the sun; its magnitude to have extended over a fourth part of the firmament; and it is stated to have occupied, consequently, about four hours in rising and setting. in the year , a comet appeared in the sign virgo. another, according to the historians of the lower empire, appeared in the year , seventy-six years after the last, at an interval corresponding to that of halley's comet. the interval between the birth of mithridates and the year was four hundred and fifty-three years, which would be equivalent to six periods of seventy-five and a half years. thus it would seem, that in the interim there were five returns of this comet unobserved, or at least unrecorded. the appearance in the year was attended with extraordinary circumstances. it was described in the old writers as a "comet of monstrous size and appalling aspect, its tail seeming to reach down to the ground." the next recorded appearance of a comet agreeing with the ascertained period marks the taking of rome, in the year ,--an interval of one hundred and fifty-one years, or two periods of seventy-five and a half years having elapsed. one unrecorded return must, therefore, have taken place in the interim. the next appearance of a comet, coinciding with the assigned period, is three hundred and eighty years afterwards; namely, in the year ,--five revolutions having been completed in the interval. the next appearance is recorded in the year , after an interval of a single period of seventy-five years. three revolutions would now seem to have passed unrecorded, when the comet again makes its appearance in . in this, as well as in former appearances, it is proper to state, that the sole test of identity of these cornets with that of halley is the coincidence of the times, as near as historical records enable us to ascertain, with the epochs at which the comet of halley might be expected to appear. that such evidence, however, is very imperfect, must be evident, if the frequency of cometary appearances be considered, and if it be remembered, that hitherto we find no recorded observations, which could enable us to trace, even with the rudest degree of approximation, the paths of those comets, the times of whose appearances raise a presumption of their identity with that of halley. we now, however, descend to times in which more satisfactory evidence may be expected. in the year , a year in which the return of halley's comet might have been expected, there is recorded a comet of remarkable character: "a comet of terrific dimensions made its appearance about the time of the feast of the passover, which was followed by a great plague." had the terrific appearance of this body alone been recorded, this description might have passed without the charge of great exaggeration; but when we find the great plague connected with it as a consequence, it is impossible not to conclude, that the comet was seen by its historians through the magnifying medium of the calamity which followed it. another appearance is recorded in the year , unaccompanied by any other circumstance than its mere date. this, however, is in strict accordance with the ascertained period of halley's comet. we now arrive at the first appearance at which observations were taken, possessing sufficient accuracy to enable subsequent investigators to determine the path of the comet; and this is accordingly the first comet the identity of which with the comet of halley can be said to be conclusively established. in the year , a comet is stated to have appeared "of unheard of magnitude;" it was accompanied by a tail of extraordinary length, which extended over sixty degrees, (a third part of the heavens,) and continued to be seen during the whole month of june. the influence which was attributed to this appearance renders it probable, that in the record there is more or less of exaggeration. it was considered as the celestial indication of the rapid success of mohammed the second, who had taken constantinople, and struck terror into the whole christian world. pope calixtus the second levelled the thunders of the church against the enemies of his faith, terrestrial and celestial; and in the same bull excommunicated the turks and the comet; and, in order that the memory of this manifestation of his power should be for ever preserved, he ordained that the bells of all the churches should be rung at mid-day,--a custom which is preserved in those countries to our times. the extraordinary length and brilliancy which was ascribed to the tail, upon this occasion, have led astronomers to investigate the circumstances under which its brightness and magnitude would be the greatest possible; and upon tracing back the motion of the comet to the year , it has been found that it was then actually in the position, with respect to the earth and sun, most favorable to magnitude and splendor. so far, therefore, the result of astronomical calculation corroborates the records of history. the next return took place in . pierre appian, who first ascertained the fact that the tails of comets are usually turned from the sun, examined this comet with a view to verify his statement, and to ascertain the true direction of its tail. he made, accordingly, numerous observations upon its position, which, although rude, compared with the present standard of accuracy, were still sufficiently exact to enable halley to identify this comet with that observed by himself. the next return took place in , when the comet was observed by kepler. this astronomer first saw it on the evening of the twenty-sixth of september, when it had the appearance of a star of the first magnitude, and, to his vision, was without a tail; but the friends who accompanied him had better sight, and distinguished the tail. before three o'clock the following morning the tail had become clearly visible, and had acquired great magnitude. two days afterwards, the comet was observed by longomontanus, a distinguished philosopher of the time. he describes its appearance, to the naked eye, to be like jupiter, but of a paler and more obscured light; that its tail was of considerable length, of a paler light than that of the head, and more dense than the tails of ordinary comets. the next appearance, and that which was observed by halley himself, took place in , a little before the publication of the '_principia_.' in the interval between and , practical astronomy had made great advances; instruments of observation had been brought to a state of comparative perfection; numerous observatories had been established, and the management of them had been confided to the most eminent men in europe. in , the scientific world was therefore prepared to examine the visitor of our system with a degree of care and accuracy before unknown. in the year , about four years afterwards, newton published his '_principia_,' in which he applied to the comet of the general principles of physical investigation first promulgated in that work. he explained the method of determining, by geometrical construction, the visible portion of the path of a body of this kind, and invited astronomers to apply these principles to the various recorded comets,--to discover whether some among them might not have appeared at different epochs, the future returns of which might consequently be predicted. such was the effect of the force of analogy upon the mind of newton, that, without awaiting the discovery of a periodic comet, he boldly assumed these bodies to be analogous to planets in their revolution round the sun. extraordinary as these conjectures must have appeared at the time, they were soon strictly realized. halley, who was then a young man, but possessed one of the best minds in england, undertook the labor of examining the circumstances attending all the comets previously recorded, with a view to discover whether any, and which of them, appeared to follow the same path. antecedently to the year , four hundred and twenty-five of these bodies had been recorded in history; but those which had appeared before the fourteenth century had not been submitted to any observations by which their paths could be ascertained,--at least, not with a sufficient degree of precision, to afford any hope of identifying them with those of other comets. subsequently to the year , however, halley found twenty-four comets on which observations had been made and recorded, with a degree of precision sufficient to enable him to calculate the actual paths which these bodies followed while they were visible. he examined, with the most elaborate care, the _courses_ of each of these twenty-four bodies; he found the exact points at which each one of them crossed the ecliptic, or their _nodes_; also the angle which the direction of their motion made with that plane,--that is, the _inclination of their orbits_; he also calculated the nearest distance at which each of them approached the sun, or their _perihelion distance_; and the exact place of the body when at that nearest point,--that is, the _longitude of the perihelion_. these particulars are called the _elements_ of a comet, because, when ascertained, they afford sufficient data for determining a comet's path. on comparing these paths, halley found that one, which had appeared in , followed nearly the same path as one which had appeared in . supposing, then, these to be two successive appearances of the same comet, it would follow, that its period would be one hundred and twenty-nine years, reckoning from . had this conjecture been well founded, the comet must have appeared about the year . no comet, however, appeared at or near that time, following a similar path. in his second conjecture, halley was more fortunate, as indeed might be expected, since it was formed upon more conclusive grounds. he found that the paths of comets which had appeared in and were nearly identical, and that they were in fact the same as the path followed by the comet observed by himself in . he suspected, therefore, that the appearances at these three epochs were produced by three successive returns of the same comet, and that, consequently, its period in its orbit must be about seventy-five and a half years. the probability of this conclusion is strikingly exhibited to the eye, by presenting the elements in a tabular form, from which it will at once be seen how nearly they correspond at these regular intervals. ===================================================================== time.|inclination of|long. of the |long. per.|per. dist. |course. |the orbit. |node. | | | ===================================================================== | ° ´ | ° ´ | ° ´ | ° ´ |retrograde. | | | | | " | | | | | " | | | | | " ===================================================================== so little was the scientific world, at this time, prepared for such an announcement, that halley himself only ventured at first to express his opinion in the form of conjecture; but, after some further investigation of the circumstances of the recorded comets, he found three which, at least in point of time, agreed with the period assigned to the comet of . collecting confidence from these circumstances, he announced his discovery as the result of observation and calculation combined, and entitled to as much confidence as any other consequence of an established physical law. there were, nevertheless, two circumstances which might be supposed to offer some difficulty. first, the intervals between the supposed successive returns were not precisely equal; and, secondly, the inclination of the comet's path to the plane of the earth's orbit was not exactly the same in each case. halley, however, with a degree of sagacity which, considering the state of knowledge at the time, cannot fail to excite unqualified admiration, observed, that it was natural to suppose that the same causes which disturbed the planetary motions must likewise act upon comets; and that their influence would be so much the more sensible upon these bodies, because of their great distances from the sun. thus, as the attraction of jupiter for saturn was known to affect the velocity of the latter planet, sometimes retarding and sometimes accelerating it, according to their relative position, so as to affect its period to the extent of thirteen days, it might well be supposed, that the comet might suffer by a similar attraction an effect sufficiently great, to account for the inequality observed in the interval between its successive returns: and also for the variation to which the direction of its path upon the plane of the ecliptic was found to be subject. he observed, in fine, that, as in the interval between and , the comet passed so near jupiter that its velocity must have been augmented, and consequently its period shortened, by the action of that planet, this period, therefore, having been only seventy-five years, he inferred that the following period would probably be seventy-six years, or upwards; and consequently, that the comet ought not to be expected to appear until the end of , or the beginning of . it is impossible to imagine any quality of mind more enviable than that which, in the existing state of mathematical physics, could have led to such a prediction. the imperfect state of mathematical science rendered it impossible for halley to offer to the world a demonstration of the event which he foretold. the theory of gravitation, which was in its infancy in the time of halley's investigations, had grown to comparative maturity before the period at which his prediction could be fulfilled. the exigencies of that theory gave birth to new and more powerful instruments of mathematical inquiry: the differential and integral calculus, or the science of fluxions, as it is sometimes called,--a branch of the mathematics, expressed by algebraic symbols, but capable of a much higher reach, as an instrument of investigation, than either algebra or geometry,--was its first and greatest offspring. this branch of science was cultivated with an ardor and success by which it was enabled to answer all the demands of physics, and it contributed largely to the advancement of mechanical science itself, building upon the laws of motion a structure which has since been denominated 'celestial mechanics.' newton's discoveries having obtained reception throughout the scientific world, his inquiries and his theories were followed up; and the consequences of the great principle of universal gravitation were rapidly developed. since, according to this doctrine, _every body in nature attracts and is attracted by every other body_, it follows, that the comet was liable to be acted on by each of the planets, as well as by the sun,--a circumstance which rendered its movements much more difficult to follow, than would be the case were it subject merely to the projectile force and to the solar attraction. to estimate the time it would take for a ship to cross the atlantic would be an easy task, were she subject to only one constant wind; but to estimate, beforehand, the exact influence which all other winds and the tides might have upon her passage, some accelerating and some retarding her course, would present a problem of the greatest difficulty. clairaut, however, a celebrated french mathematician, undertook to estimate the effects that would be produced on halley's comet by the attractions of all the planets. his aim was to investigate _general rules_, by which the computation could be made arithmetically, and hand them over to the practical calculator, to make the actual computations. lalande, a practical astronomer, no less eminent in his own department, and who indeed first urged clairaut to this inquiry, undertook the management of the astronomical and arithmetical part of the calculation. in this prodigious labor (for it was one of most appalling magnitude) he was assisted by the wife of an eminent watchmaker in paris, named lepaute, whose exertions on this occasion have deservedly registered her name in astronomical history. it is difficult to convey to one who is not conversant with such investigations, an adequate notion of the labor which such an inquiry involved. the calculation of the influence of any one _planet_ of the system upon any other is itself a problem of some complexity and difficulty; but still, one general computation, depending upon the calculation of the terms of a certain series, is sufficient for its solution. this comparative simplicity arises entirely from two circumstances which characterize the planetary orbits. these are, that, though they are ellipses, they differ very slightly from circles; and though the planets do not move in the plane of the ecliptic, yet none of them deviate considerably from that plane. but these characters do not belong to the orbits of comets, which, on the contrary, are highly eccentric, and make all possible angles with the ecliptic. the consequence of this is, that the calculation of the disturbances produced in the cometary orbits by the action of the planets must be conducted not like the planets, in one general calculation applicable to the whole orbits, but in a vast number of separate calculations; in which the orbit is considered, as it were, bit by bit, each bit requiring a calculation similar to the whole orbit of the planet. now, when it is considered that the period of halley's comet is about seventy-five years, and that every portion of its course, for two successive periods, was necessary to be calculated separately in this way, some notion may be formed of the labor encountered by lalande and madame lepaute. "during six months," says lalande, "we calculated from morning till night, sometimes even at meals; the consequence of which was, that i contracted an illness which changed my constitution for the remainder of my life. the assistance rendered by madame lepaute was such, that, without her, we never could have dared to undertake this enormous labor, in which it was necessary to calculate the distance of each of the two planets, jupiter and saturn, from the comet, and their attraction upon that body, separately, for every successive degree, and for one hundred and fifty years." the attraction of a body is proportioned to its quantity of matter. therefore, before the attraction exerted upon the comet by the several planets within whose influence it might fall, could be correctly estimated, it was necessary to know the mass of each planet; and though the planets had severally been weighed by methods supplied by newton's 'principia,' yet the estimate had not then attained the same measure of accuracy as it has now reached; nor was it certain that there was not (as it has since appeared that there actually was) one or more planets beyond saturn, whose attractions might likewise influence the motions of the comet. clairaut, making the best estimate he was able, under all these disadvantages, of the disturbing influence of the planets, fixed the return of the comet to the place of its nearest distance from the sun on the fourth of april, . in the successive appearances of the comet, subsequently to , it was found to have gradually decreased in magnitude and splendor. while in it reached across one third part of the firmament, and spread terror over europe, in , its appearance, when observed by kepler and longomontanus, was that of a star of the first magnitude; and so trifling was its tail that, kepler himself, when he first saw it, doubted whether it had any. in , it excited little attention, except among astronomers. supposing this decrease of magnitude and brilliancy to be progressive, lalande entertained serious apprehensions that on its expected return it might be so inconsiderable, as to escape the observation even of astronomers; and thus, that this splendid example of the power of science, and unanswerable proof of the principle of gravitation, would be lost to the world. it is not uninteresting to observe the misgivings of this distinguished astronomer with respect to the appearance of the body, mixed up with his unshaken faith in the result of the astronomical inquiry. "we cannot doubt," says he, "that it will return; and even if astronomers cannot see it, they will not therefore be the less convinced of its presence. they know that the faintness of its light, its great distance, and perhaps even bad weather, may keep it from our view. but the world will find it difficult to believe us; they will place this discovery, which has done so much honor to modern philosophy, among the number of chance predictions. we shall see discussions spring up again in colleges, contempt among the ignorant, terror among the people; and seventy-six years will roll away, before there will be another opportunity of removing all doubt." fortunately for science, the arrival of the expected visitor did not take place under such untoward circumstances. as the commencement of the year approached, "astronomers," says voltaire, "hardly went to bed at all." the honor, however, of the first glimpse of the stranger was not reserved for the possessors of scientific rank, nor for the members of academies or universities. on the night of christmas-day, , george palitzch, of politz, near dresden,--"a peasant," says sir john herchel, "by station, an astronomer by nature," first saw the comet. an astronomer of leipzic found it soon after; but, with the mean jealousy of a miser, he concealed his treasure, while his contemporaries throughout europe were vainly directing their anxious search after it to other quarters of the heavens. at this time, delisle, a french astronomer, and his assistant, messier, who, from his unweared assiduity in the pursuit of comets, was called the _comet-hunter_, had been constantly engaged, for eighteen months, in watching for the return of halley's comet. messier passed his life in search of comets. it is related of him, that when he was in expectation of discovering a comet, his wife was taken ill and died. while attending on her, being withdrawn from his observatory, another astronomer anticipated him in the discovery. messier was in despair. a friend, visiting him, began to offer some consolation for the recent affliction he had suffered. messier, thinking only of the comet, exclaimed, "i had discovered twelve: alas, that i should be robbed of the thirteenth by montague!"--and his eyes filled with tears. then, remembering that it was necessary to mourn for his wife, whose remains were still in the house, he exclaimed, "ah! this poor woman!" (_ah! cette pauvre femme_,) and again wept for his comet. we can easily imagine how eagerly such an enthusiast would watch for halley's comet; and we could almost wish that it had been his good fortune to be the first to announce its arrival: but, being misled by a chart which directed his attention to the wrong part of the firmament, a whole month elapsed after its discovery by palitzch, before he enjoyed the delightful spectacle. the comet arrived at its perihelion on the thirteenth of march, only twenty-three days from the time assigned by clairaut. it appeared very round, with a brilliant nucleus, well distinguished from the surrounding nebulosity. it had, however, no appearance of a tail. it became lost in the sun, as it approached its perihelion, and emerged again, on the other side of the sun, on the first of april. its exhibiting an appearance, so inferior to what it presented on some of its previous returns, is partly accounted for by its being seen by the european astronomers under peculiarly disadvantageous circumstances, being almost always within the twilight, and in the most unfavorable situations. in the southern hemisphere, however, the circumstances for observing it were more favorable, and there it exhibited a tail varying from ten to forty-seven degrees in length. in my next letter i will give you some particulars respecting the late return of halley's comet. letter xxvi. comets, continued. "incensed with indignation, satan stood unterrified, and like a comet burned, that fires the length of ophiucus huge in the arctic sky, and from his horrid train shakes pestilence and war."--_milton._ among other great results which have marked the history of halley's comet, it has itself been a criterion of the existing state of the mathematical and astronomical sciences. we have just seen how far the knowledge of the great laws of physical astronomy, and of the higher mathematics, enabled the astronomers of and , respectively, to deal with this wonderful body; and let us now see what higher advantages were possessed by the astronomers of . during this last interval of seventy-six years, the science of mathematics, in its most profound and refined branches, has made prodigious advances, more especially in its application to the laws of the celestial motions, as exemplified in the 'mecanique celeste' of la place. the methods of investigation have acquired greater simplicity, and have likewise become more general and comprehensive; and mechanical science, in the largest sense of that term, now embraces in its formularies the most complicated motions, and the most minute effects of the mutual influences of the various members of our system. you will probably find it difficult to comprehend, how such hidden facts can be disclosed by formularies, consisting of _a_'s and _b_'s, and _x_'s and _y_'s, and other algebraic symbols; nor will it be easy to give you a clear idea of this subject, without a more extensive acquaintance than you have formed with algebraic investigations; but you can easily understand that even an equation expressed in numbers may be so changed in its form, by adding, subtracting, multiplying and dividing, as to express some new truth at every transformation. some idea of this may be formed by the simplest example. take the following: + = . this equation expresses the fact, that three added to four is equal to seven. by multiplying all the terms by , we obtain a new equation, in which + = . this expresses a new truth; and by varying the form, by similar operations, an indefinite number of separate truths may be elicited from the simple fundamental expression. i will add another illustration, which involves a little more algebra, but not, i think, more than you can understand; or, if it does, you will please pass over it to the next paragraph. according to a rule of arithmetical progression, _the sum of all the terms is equal to half the sum of the extremes multiplied into the number of terms_. calling the sum of the terms _s_, the first term _a_, the last _h_, and the number of terms _n_, and we have _( / )n(a+h)=s_; or _n(a+h)= s_; or _a+h= s/n_; or _a=( s/n)-h_; or _h=( s/n)-a_. these are only a few of the changes which may be made in the original expression, still preserving the equality between the quantities on the left hand and those on the right; yet each of these transformations expresses a new truth, indicating distinct and (as might be the case) before unknown relations between the several quantities of which the whole expression is composed. the last, for example, shows us that the last term in an arithmetical series is always equal to twice the sum of the whole series divided by the number of terms and diminished by the first term. in analytical formularies, as expressions of this kind are called, the value of a single unknown quantity is sometimes given in a very complicated expression, consisting of known quantities; but before we can ascertain their united value, we must reduce them, by actually performing all the additions, subtractions, multiplications, divisions, raising to powers, and extracting roots, which are denoted by the symbols. this makes the actual calculations derived from such formularies immensely laborious. we have already had an instance of this in the calculations made by lalande and madame lepaute, from formularies furnished by clairaut. the analytical formularies, contained in such works as la place's 'mecanique celeste,' exhibit to the eye of the mathematician a record of all the evolutions of the bodies of the solar system in ages past, and of all the changes they must undergo in ages to come. such has been the result of the combination of transcendent mathematical genius and unexampled labor and perseverance, for the last century. the learned societies established in various centres of civilization have more especially directed their attention to the advancement of physical astronomy, and have stimulated the spirit of inquiry by a succession of prizes, offered for the solutions of problems arising out of the difficulties which were progressively developed by the advancement of astronomical knowledge. among these questions, the determination of the return of comets, and the disturbances which they experience in their course, by the action of the planets near which they happen to pass, hold a prominent place. in , the french institute offered a prize for the determination of the exact time of the return of halley's comet to its perihelion in . m. pontecoulant aspired to the honor. "after calculations," says he, "of which those alone who have engaged in such researches can estimate the extent and appreciate the fastidious monotony, i arrived at a result which satisfied all the conditions proposed by the institute. i determined the perturbations of halley's comet, by taking into account the simultaneous actions of jupiter, saturn, uranus, and the earth, and i then fixed its return to its perihelion for the seventh of november." subsequently to this, however, m. pontecoulant made some further researches, which led him to correct the former result; and he afterwards altered the time to november fourteenth. it actually came to its perihelion on the sixteenth, within two days of the time assigned. nothing can convince us more fully of the complete mastery which astronomers have at last acquired over these erratic bodies, than to read in the edinburgh review for april, , the paragraph containing the final results of all the labors and anticipations of astronomers, matured as they were, in readiness for the approaching visitant, and then to compare the prediction with the event, as we saw it fulfilled a few months afterwards. the paragraph was as follows: "on the whole, it may be considered as tolerably certain, that the comet will become visible in every part of europe about the latter end of august, or beginning of september, next. it will most probably be distinguishable by the naked eye, like a star of the first magnitude, but with a duller light than that of a planet, and surrounded with a pale nebulosity, which will slightly impair its splendor. on the night of the seventh of october, the comet will approach the well-known constellation of the great bear; and between that and the eleventh, it will pass directly through the seven conspicuous stars of that constellation, (the dipper.) towards the end of november, the comet will plunge among the rays of the sun, and disappear, and will not issue from them, on the other side, until the end of december." let us now see how far the actual appearances corresponded to these predictions. the comet was first discovered from the observatory at rome, on the morning of the fifth of august; by professor struve, at dorpat, on the twentieth; in england and france, on the twenty-third; and at yale college, by professor loomis and myself, on the thirty-first. on the morning of that day, between two and three o'clock, in obedience to the directions which the great minds that had marked out its path among the stars had prescribed, we directed clarke's telescope (a noble instrument, belonging to yale college) towards the northeastern quarter of the heavens, and lo! there was the wanderer so long foretold,--a dim speck of fog on the confines of creation. it came on slowly, from night to night, increasing constantly in magnitude and brightness, but did not become distinctly visible to the naked eye until the twenty-second of september. for a month, therefore, astronomers enjoyed this interesting spectacle before it exhibited itself to the world at large. from this time it moved rapidly along the northern sky, until, about the tenth of october, it traversed the constellation of the great bear, passing a little above, instead of "through" the seven conspicuous stars constituting the dipper. at this time it had a lengthened train, and became, as you doubtless remember, an object of universal interest. early in november, the comet ran down to the sun, and was lost in his beams; but on the morning of december thirty-first, i again obtained, through clarke's telescope, a distinct view of it on the other side of the sun, a moment before the morning dawn. this return of halley's comet was an astronomical event of transcendent importance. it was the chronicler of ages, and carried us, by a few steps, up to the origin of time. if a gallant ship, which has sailed round the globe, and commanded successively the admiration of many great cities, diverse in language and customs, is invested with a peculiar interest, what interest must attach to one that has made the circuit of the solar system, and fixed the gaze of successive worlds! so intimate, moreover, is the bond which binds together all truths in one indissoluble chain, that the establishment of one great truth often confirms a multitude of others, equally important. thus the return of halley's comet, in exact conformity with the predictions of astronomers, established the truth of all those principles by which those predictions were made. it afforded most triumphant proof of the doctrine of universal gravitation, and of course of the received laws of physical astronomy; it inspired new confidence in the power and accuracy of that instrument (the calculus) by means of which its elements had been investigated; and it proved that the different planets, which exerted upon it severally a disturbing force proportioned to their quantity of matter, had been correctly weighed, as in a balance. i must now leave this wonderful body to pursue its sublime march far beyond the confines of uranus, (a distance it has long since reached,) and take a hasty notice of two other comets, whose periodic returns have also been ascertained; namely, those of biela and encke. biela's comet has a period of six years and three quarters. it has its perihelion near the orbit of the earth, and its aphelion a little beyond that of jupiter. its orbit, therefore, is far less eccentric than that of halley's comet; (see frontispiece;) it neither approaches so near the sun, nor departs so far from it, as most other known comets: some, indeed, never come nearer to the sun than the orbit of jupiter, while they recede to an incomprehensible distance beyond the remotest planet. we might even imagine that they would get beyond the limits of the sun's attraction; nor is this impossible, although, according to la place, the solar attraction is sensible throughout a sphere whose radius is a hundred millions of times greater than the distance of the earth from the sun, or nearly ten thousand billions of miles. some months before the expected return of biela's comet, in , it was announced by astronomers, who had calculated its path, that it would cross the plane of the earth's orbit very near to the earth's path, so that, should the earth happen at the time to be at that point of her revolution, a collision might take place. this announcement excited so much alarm among the ignorant classes in france, that it was deemed expedient by the french academy, that one of their number should prepare and publish an article on the subject, with the express view of allaying popular apprehension. this task was executed by m. arago. he admitted that the earth would in fact pass so near the point where the comet crossed the plane of its orbit, that, should they chance to meet there, the earth would be enveloped in the nebulous atmosphere of the comet. he, however, showed that the earth would not be near that point at the same time with the comet, but fifty millions of miles from it. the comet came at the appointed time, but was so exceedingly faint and small, that it was visible only to the largest telescopes. in one respect, its diminutive size and feeble light enhanced the interest with which it was contemplated; for it was a sublime spectacle to see a body, which, as projected on the celestial vault, even when magnified a thousand times, seemed but a dim speck of fog, still pursuing its way, in obedience to the laws of universal gravitation, with the same regularity as jupiter and saturn. we are apt to imagine that a body, consisting of such light materials that it can be compared only to the thinnest fog, would be dissipated and lost in the boundless regions of space; but so far is this from the truth, that, when subjected to the action of the same forces of projection and solar attraction, it will move through the void regions of space, and will describe its own orbit about the sun with the same unerring certainty, as the densest bodies of the system. encke's comet, by its frequent returns, (once in three and a third years,) affords peculiar facilities for ascertaining the laws of its revolution; and it has kept the appointments made for it with great exactness. on its return in , it exhibited to the telescope a globular mass of nebulous matter, resembling fog, and moved towards its perihelion with great rapidity. it makes its entire excursions within the orbit of jupiter. but what has made encke's comet particularly famous, is its having first revealed to us the existence of a _resisting medium_ in the planetary spaces. it has long been a question, whether the earth and planets revolve in a perfect void, or whether a fluid of extreme rarity may not be diffused through space. a perfect vacuum was deemed most probable, because no such effects on the motions of the planets could be detected as indicated that they encountered a resisting medium. but a feather, or a lock of cotton, propelled with great velocity, might render obvious the resistance of a medium which would not be perceptible in the motions of a cannon ball. accordingly, encke's comet is thought to have plainly suffered a retardation from encountering a resisting medium in the planetary regions. the effect of this resistance, from the first discovery of the comet to the present time, has been to diminish the time of its revolution about two days. such a resistance, by destroying a part of the projectile force, would cause the comet to approach nearer to the sun, and thus to have its periodic time shortened. the ultimate effect of this cause will be to bring the comet nearer to the sun, at every revolution, until it finally falls into that luminary, although many thousand years will be required to produce this catastrophe. it is conceivable, indeed, that the effects of such a resistance may be counteracted by the attraction of one or more of the planets, near which it may pass in its successive returns to the sun. still, it is not probable that this cause will exactly counterbalance the other; so that, if there is such an elastic medium diffused through the planetary regions, it must follow that, in the lapse of ages, every comet will fall into the sun. newton conjectured that this would be the case, although he did not found his opinion upon the existence of such a resisting medium as is now detected. to such an opinion he adhered to the end of life. at the age of eighty-three, in a conversation with his nephew, he expressed himself thus: "i cannot say when the comet of will fall into the sun; possibly after five or six revolutions; but whenever that time shall arrive, the heat of the sun will be raised by it to such a point, that our globe will be burned, and all the animals upon it will perish." of the _physical nature_ of comets little is understood. the greater part of them are evidently mere masses of vapor, since they permit very small stars to be seen through them. in september, , sir john herschel, when observing biela's comet, saw that body pass directly between his eye and a small cluster of minute telescopic stars of the sixteenth or seventeenth magnitude. this little constellation occupied a space in the heavens, the breadth of which was not the twentieth part of that of the moon; yet the whole of the cluster was distinctly visible through the comet. "a more striking proof," says sir john herschel, "could not have been afforded, of the extreme transparency of the matter of which this comet consists. the most trifling fog would have entirely effaced this group of stars, yet they continued visible through a thickness of the comet which, calculating on its distance and apparent diameter, must have exceeded fifty thousand miles, at least towards its central parts." from this and similar observations, it is inferred, that the nebulous matter of comets is vastly more rare than that of the air we breathe, and hence, that, were more or less of it to be mingled with the earth's atmosphere, it would not be perceived, although it might possibly render the air unwholesome for respiration. m. arago, however, is of the opinion, that some comets, at least, have a solid nucleus. it is difficult, on any other supposition, to account for the strong light which some of them have exhibited,--a light sufficiently intense to render them visible in the day-time, during the presence of the sun. the intense heat to which comets are subject, in approaching so near the sun as some of them do, is alleged as a sufficient reason for the great expansion of the thin vapory atmospheres which form their tails; and the inconceivable cold to which they are subject, in receding to such a distance from the sun, is supposed to account for the condensation of the same matter until it returns to its original dimensions. thus the great comet of , at its perihelion, approached within one hundred and forty-six thousand miles of the surface of the sun, a distance of only one sixth part of the sun's diameter. the heat which it must have received was estimated to be equal to twenty-eight thousand times that which the earth receives in the same time, and two thousand times hotter than red-hot iron. this temperature would be sufficient to volatilize the most obdurate substances, and to expand the vapor to vast dimensions; and the opposite effects of the extreme cold to which it would be subject in the regions remote from the sun would be adequate to condense it into its former volume. this explanation, however, does not account for the direction of the tail, extending, as it usually does, only in a line opposite to the sun. some writers, therefore, suppose that the nebulous matter of the comet, after being expanded to such a volume that the particles are no longer attracted to the nucleus, unless by the slightest conceivable force, are carried off in a direction from the sun, by the impulse of the solar rays themselves. but to assign such a power to the sun's rays, while they have never been proved to have any momentum, is unphilosophical; and we are compelled to place the phenomena of comets' tails among the points of astronomy yet to be explained. since comets which approach very near the sun, like the comet of , cross the orbits of all the planets, the possibility that one of them may strike the earth has frequently been suggested. still it may quiet our apprehensions on this subject, to reflect on the vast amplitude of the planetary spaces, in which these bodies are not crowded together, as we see them erroneously represented in orreries and diagrams, but are sparsely scattered at immense distances from each other. they are like insects flying, singly, in the expanse of heaven. if a comet's tail lay with its axis in the plane of the ecliptic when it was near the sun, we can imagine that the tail might sweep over the earth; but the tail may be situated at any angle with the ecliptic, as well as in the same plane with it, and the chances that it will not be in the same plane are almost infinite. it is also extremely improbable that a comet will cross the plane of the ecliptic precisely at the earth's path in that plane, since it may as probably cross it at any other point nearer or more remote from the sun. a french writer of some eminence (du sejour) has discussed this subject with ability, and arrived at the following conclusions: that of all the comets whose paths had been ascertained, none _could pass_ nearer to the earth than about twice the moon's distance; and that none ever _did pass_ nearer to the earth than nine times the moon's distance. the comet of , already mentioned, which became entangled among the satellites of jupiter, came within this limit. some have taken alarm at the idea that a comet, by approaching very near to the earth, might raise so high a _tide_, as to endanger the safety of maritime countries especially: but this writer shows, that the comet could not possibly remain more than two hours so near the earth as a fourth part of the moon's distance; and it could not remain even so long, unless it passed the earth under very peculiar circumstances. for example, if its orbit were nearly perpendicular to that of the earth, it could not remain more than half an hour in such a position. under such circumstances, the production of a tide would be impossible. eleven hours, at least, would be necessary to enable a comet to produce an effect on the waters of the earth, from which the injurious effects so much dreaded would follow. the final conclusion at which he arrives is, that although, in strict geometrical rigor, it is not physically impossible that a comet should encounter the earth, yet the probability of such an event is absolutely nothing. m. arago, also, has investigated the probability of such a collision on the mathematical doctrine of chances, and remarks as follows: "suppose, now, a comet, of which we know nothing but that, at its perihelion, it will be nearer the sun than we are, and that its diameter is equal to one fourth that of the earth; the doctrine of chances shows that, out of two hundred and eighty-one millions of cases, there is but one against us; but one, in which the two bodies could meet." la place has assigned the consequences that would result from a direct collision between the earth and a comet. "it is easy," says he, "to represent the effects of the shock produced by the earth's encountering a comet. the axis and the motion of rotation changed; the waters abandoning their former position to precipitate themselves towards the new equator; a great part of men and animals whelmed in a universal deluge, or destroyed by the violent shock imparted to the terrestrial globe; entire species annihilated; all the monuments of human industry overthrown;--such are the disasters which the shock of a comet would necessarily produce." la place, nevertheless, expresses a decided opinion that the orbits of the planets have never yet been disturbed by the influence of comets. comets, moreover, have been, and are still to some degree, supposed to exercise much influence in the affairs of this world, affecting the weather, the crops, the public health, and a great variety of atmospheric commotions. even halley, finding that his comet must have been near the earth at the time of the deluge, suggested the possibility that the comet caused that event,--an idea which was taken up by whiston, and formed into a regular theory. in gregory's astronomy, an able work, published at oxford in , the author remarks, that among all nations and in all ages, it has been observed, that the appearance of a comet has always been followed by great calamities; and he adds, "it does not become philosophers lightly to set down these things as fables." among the various things ascribed to comets by a late english writer, are hot and cold seasons, tempests, hurricanes, violent hail-storms, great falls of snow, heavy rains, inundations, droughts, famines, thick fogs, flies, grasshoppers, plague, dysentery, contagious diseases among animals, sickness among cats, volcanic eruptions, and meteors, or shooting stars. these notions are too ridiculous to require a distinct refutation; and i will only add, that we have no evidence that comets have hitherto ever exercised the least influence upon the affairs of this world; and we still remain in darkness, with respect to their physical nature, and the purposes for which they were created. letter xxvii. meteoric showers. "oft shalt thou see, ere brooding storms arise, star after star glide headlong down the skies, and, where they shot, long trails of lingering light sweep far behind, and gild the shades of night."--_virgil._ few subjects of astronomy have excited a more general interest, for several years past, than those extraordinary exhibitions of shooting stars, which have acquired the name of meteoric showers. my reason for introducing the subject to your notice, in this place, is, that these small bodies are, as i believe, derived from nebulous or cometary bodies, which belong to the solar system, and which, therefore, ought to be considered, before we take our leave of this department of creation, and naturally come next in order to comets. the attention of astronomers was particularly directed to this subject by the extraordinary shower of meteors which occurred on the morning of the thirteenth of november, . i had the good fortune to witness these grand celestial fire-works, and felt a strong desire that a phenomenon, which, as it afterwards appeared, was confined chiefly to north america, should here command that diligent inquiry into its causes, which so sublime a spectacle might justly claim. as i think you were not so happy as to witness this magnificent display, i will endeavor to give you some faint idea of it, as it appeared to me a little before daybreak. imagine a constant succession of fire-balls, resembling sky-rockets, radiating in all directions from a point in the heavens a few degrees southeast of the zenith, and following the arch of the sky towards the horizon. they commenced their progress at different distances from the radiating point; but their directions were uniformly such, that the lines they described, if produced upwards, would all have met in the same part of the heavens. around this point, or imaginary radiant, was a circular space of several degrees, within which no meteors were observed. the balls, as they travelled down the vault, usually left after them a vivid streak of light; and, just before they disappeared, exploded, or suddenly resolved themselves into smoke. no report of any kind was observed, although we listened attentively. beside the foregoing distinct concretions, or individual bodies, the atmosphere exhibited _phosphoric lines_, following in the train of minute points, that shot off in the greatest abundance in a northwesterly direction. these did not so fully copy the figure of the sky, but moved in paths more nearly rectilinear, and appeared to be much nearer the spectator than the fire-balls. the light of their trains was also of a paler hue, not unlike that produced by writing with a stick of phosphorus on the walls of a dark room. the number of these luminous trains increased and diminished alternately, now and then crossing the field of view, like snow drifted before the wind, although, in fact, their course was towards the wind. from these two varieties, we were presented with meteors of various sizes and degrees of splendor: some were mere points, while others were larger and brighter than jupiter or venus; and one, seen by a credible witness, at an earlier hour, was judged to be nearly as large as the moon. the flashes of light, although less intense than lightning, were so bright, as to awaken people in their beds. one ball that shot off in the northwest direction, and exploded a little northward of the star capella, left, just behind the place of explosion, a phosphorescent train of peculiar beauty. this train was at first nearly straight, but it shortly began to contract in length, to dilate in breadth, and to assume the figure of a serpent drawing itself up, until it appeared like a small luminous cloud of vapor. this cloud was borne eastward, (by the wind, as was supposed, which was blowing gently in that direction,) opposite to the direction in which the meteor itself had moved, remaining in sight several minutes. the point from which the meteors seemed to radiate kept a fixed position among the stars, being constantly near a star in leo, called gamma leonis. such is a brief description of this grand and beautiful display, as i saw it at new haven. the newspapers shortly brought us intelligence of similar appearances in all parts of the united states, and many minute descriptions were published by various observers; from which it appeared, that the exhibition had been marked by very nearly the same characteristics wherever it had been seen. probably no celestial phenomenon has ever occurred in this country, since its first settlement, which was viewed with so much admiration and delight by one class of spectators, or with so much astonishment and fear by another class. it strikingly evinced the progress of knowledge and civilization, that the latter class was comparatively so small, although it afforded some few examples of the dismay with which, in barbarous ages of the world, such spectacles as this were wont to be regarded. one or two instances were reported, of persons who died with terror; many others thought the last great day had come; and the untutored black population of the south gave expression to their fears in cries and shrieks. after collecting and collating the accounts given in all the periodicals of the country, and also in numerous letters addressed either to my scientific friends or to myself, the following appeared to be the _leading facts_ attending the phenomenon. the shower pervaded nearly the whole of north america, having appeared in nearly equal splendor from the british possessions on the north to the west-india islands and mexico on the south, and from sixty-one degrees of longitude east of the american coast, quite to the pacific ocean on the west. throughout this immense region, the duration was nearly the same. the meteors began to attract attention by their unusual frequency and brilliancy, from _nine to twelve_ o'clock in the evening; were most striking in their appearance from _two to five;_ arrived at their maximum, in many places, about _four_ o'clock; and continued until rendered invisible by the light of day. the meteors moved either in right lines, or in such apparent curves, as, upon optical principles, can be resolved into right lines. their general tendency was towards the northwest, although, by the effect of perspective, they appeared to move in various directions. such were the leading phenomena of the great meteoric shower of november , . for a fuller detail of the facts, as well as of the reasonings that were built on them, i must beg leave to refer you to some papers of mine in the twenty-fifth and twenty-sixth volumes of the american journal of science. soon after this wonderful occurrence, it was ascertained that a similar meteoric shower had appeared in , and, what was remarkable, almost at exactly the same time of year, namely, on the morning of the twelfth of november; and we were again surprised as well as delighted, at receiving successive accounts from different parts of the world of the phenomenon, as having occurred on the morning of the same thirteenth of november, in , , and . hence this was evidently an event independent of the casual changes of the atmosphere; for, having a periodical return, it was undoubtedly to be referred to astronomical causes, and its recurrence, at a certain definite period of the year, plainly indicated _some_ relation to the revolution of the earth around the sun. it remained, however, to develope the nature of this relation, by investigating, if possible, the origin of the meteors. the views to which i was led on this subject suggested the probability that the same phenomenon would recur on the corresponding seasons of the year, for at least several years afterwards; and such proved to be the fact, although the appearances, at every succeeding return, were less and less striking, until , when, so far as i have heard, they ceased altogether. mean-while, two other distinct periods of meteoric showers have, as already intimated, been determined; namely, about the ninth of august, and seventh of december. the facts relative to the history of these periods have been collected with great industry by mr. edward c. herrick; and several of the most ingenious and most useful conclusions, respecting the laws that regulate these singular exhibitions, have been deduced by professor twining. several of the most distinguished astronomers of the old world, also, have engaged in these investigations with great zeal, as messrs. arago and biot, of paris; doctor olbers, of bremen; m. wartmann, of geneva; and m. quetelet, of brussels. but you will be desirous to learn what are the _conclusions_ which have been drawn respecting these new and extraordinary phenomena of the heavens. as the inferences to which i was led, as explained in the twenty-sixth volume of the 'american journal of science,' have, at least in their most important points, been sanctioned by astronomers of the highest respectability, i will venture to give you a brief abstract of them, with such modifications as the progress of investigation since that period has rendered necessary. the principal questions involved in the inquiry were the following:--was the _origin_ of the meteors within the atmosphere, or beyond it? what was the _height_ of the place above the surface of the earth? by what _force_ were the meteors drawn or impelled towards the earth? in what _directions_ did they move? with what _velocity_? what was the cause of their _light_ and _heat_? of what _size_ were the larger varieties? at what height above the earth did they _disappear_? what was the nature of the _luminous trains_ which sometimes remained behind? what _sort of bodies_ were the meteors themselves; of what _kind of matter_ constituted; and in what manner did they exist _before they fell to the earth_? finally, what _relations_ did the source from which they emanated sustain to our earth? in the first place, _the meteors had their origin beyond the limits of our atmosphere_. we know whether a given appearance in the sky is within the atmosphere or beyond it, by this circumstance: all bodies near the earth, including the atmosphere itself, have a common motion with the earth around its axis from west to east. when we see a celestial object moving regularly from west to east, at the same rate as the earth moves, leaving the stars behind, we know it is near the earth, and partakes, in common with the atmosphere, of its diurnal rotation: but when the earth leaves the object behind; or, in other words, when the object moves westward along with the stars, then we know that it is so distant as not to participate in the diurnal revolution of the earth, and of course to be beyond the atmosphere. the source from which the meteors emanated thus kept pace with the stars, and hence was beyond the atmosphere. in the second place, _the height of the place whence the meteors proceeded was very great, but it has not yet been accurately determined_. regarding the body whence the meteors emanated after the similitude of a cloud, it seemed possible to obtain its height in the same manner as we measure the height of a cloud, or indeed the height of the moon. although we could not see the body itself, yet the part of the heavens whence the meteors came would indicate its position. this point we called the _radiant_; and the question was, whether the radiant was projected by distant observers on different parts of the sky; that is, whether it had any _parallax_. i took much pains to ascertain the truth of this matter, by corresponding with various observers in different parts of the united states, who had accurately noted the position of the radiant among the fixed stars, and supposed i had obtained such materials as would enable us to determine the parallax, at least approximately; although such discordances existed in the evidence as reasonably to create some distrust of its validity. putting together, however, the best materials i could obtain, i made the height of the radiant above the surface of the earth _twenty-two hundred and thirty-eight miles_. when, however, i afterwards obtained, as i supposed, some insight into the celestial origin of the meteors, i at once saw that the meteoric body must be much further off than this distance; and my present impression is, that we have not the means of determining what its height really is. we may safely place it at many thousand miles. in the third place, with respect to the _force_ by which the meteors were _drawn_ or impelled towards the earth, my first impression was, that they fell merely by the force of _gravity_; but the velocity which, on careful investigation by professor twining and others, has been ascribed to them, is greater than can possibly result from gravity, since a body can never acquire, by gravity alone, a velocity greater than about seven miles per second. some other cause, beside gravity, must therefore act, in order to give the meteors so great an apparent velocity. in the fourth place, _the meteors fell towards the earth in straight lines, and in directions which, within considerable distances, were nearly parallel with each other_. the courses are inferred to have been in _straight lines_, because no others could have appeared to spectators in different situations to have described arcs of great circles. in order to be projected into the arc of a great circle, the line of descent must be in a plane passing through the eye of the spectator; and the intersection of such planes, passing through the eyes of different spectators, must be straight lines. the lines of direction are inferred to have been _parallel_, on account of their apparent radiation from one point, that being the vanishing point of parallel lines. this may appear to you a little paradoxical, to infer that lines are parallel, because they _diverge_ from one and the same point; but it is a well-known principle of perspective, that parallel lines, when continued to a great distance from the eye, appear to converge towards the remoter end. you may observe this in two long rows of trees, or of street lamps. [illustration fig. .] some idea of the manner in which the meteors fell, and of the reason of their apparent radiation from a common point, may be gathered from the annexed diagram. let a b c, fig. , represent the vault of the sky, the centre of which, d, being the place of the spectator. let , , , &c., represent parallel lines directed towards the earth. a luminous body descending through ' , coinciding with the line d e, coincident with the axis of vision, (or the line drawn from the meteoric body to the eye,) would appear stationary all the while at ´, because distant bodies always appear stationary when they are moving either directly towards us or directly from us. a body descending through , would seem to describe the short arc ' ', appearing to move on the concave of the sky between the lines drawn from the eye to the two extremities of its line of motion; and, for a similar reason, a body descending through , would appear to describe the larger arc ' '. hence, those meteors which fell nearer to the axis of vision, would describe shorter arcs, and move slower, while those which were further from the axis and nearer the horizon would appear to describe longer arcs, and to move with greater velocity; the meteors would all seem to radiate from a common centre, namely, the point where the axis of vision met the celestial vault; and if any meteor chanced to move directly in the line of vision, it would be seen as a luminous body, stationary, for a few seconds, at the centre of radiation. to see how exactly the facts, as observed, corresponded to these inferences, derived from the supposition that the meteors moved in _parallel lines_, take the following description, as given immediately after the occurrence, by professor twining. "in the vicinity of the radiant point, a few star-like bodies were observed, possessing very little motion, and leaving very little length of trace. further off, the motions were more rapid and the traces longer; and most rapid of all, and longest in their traces, were those which originated but a few degrees above the horizon, and descended down to it." in the fifth place, had the meteors come from a point twenty-two hundred and thirty-eight miles from the earth, and derived their apparent velocity from gravity alone, then it would be found, by a very easy calculation, that their actual velocity was about four miles per second; but, as already intimated, the velocity observed was estimated much greater than could be accounted for on these principles; not less, indeed, than fourteen miles per second, and, in some instances, much greater even than this. the motion of the earth in its orbit is about nineteen miles per second; and the most reasonable supposition we can make, at present, to account for the great velocity of the meteors, is, that they derived a relative motion from the earth's passing rapidly by them,--a supposition which is countenanced by the fact that they generally tended _westward_ contrary to the earth's motion in its orbit. in the sixth place, _the meteors consisted of combustible matter, and took fire, and were consumed, in traversing the atmosphere_. that these bodies underwent combustion, we had the direct evidence of the senses, inasmuch as we saw them burn. that they took fire in the _atmosphere_, was inferred from the fact that they were not luminous in their original situations in space, otherwise, we should have seen the body from which they emanated; and had they been luminous before reaching the atmosphere, we should have seen them for a much longer period than they were in sight, as they must have occupied a considerable time in descending towards the earth from so great a distance, even at the rapid rate at which they travelled. the immediate consequence of the prodigious velocity with which the meteors fell into the atmosphere must be a powerful condensation of the air before them, retarding their progress, and producing, by a sudden compression of the air, a great evolution of heat. there is a little instrument called the _air-match_, consisting of a piston and cylinder, like a syringe, in which we strike a light by suddenly forcing down the piston upon the air below. as the air cannot escape, it is suddenly compressed, and gives a spark sufficient to light a piece of tinder at the bottom of the cylinder. indeed, it is a well-known fact, that, whenever air is suddenly and forcibly compressed, heat is elicited; and, if by such a compression as may be given by the hand in the air-match, heat is evolved sufficient to fire tinder, what must be the heat evolved by the motion of a large body in the atmosphere, with a velocity so immense. it is common to resort to electricity as the agent which produces the heat and light of shooting stars; but even were electricity competent to produce this effect, its presence, in the case before us, is not proved; and its agency is unnecessary, since so swift a motion of the meteors themselves, suddenly condensing the air before them, is both a known and adequate cause of an intense light and heat. a combustible body falling into the atmosphere, under such circumstances, would become speedily ignited, but could not burn freely, until it became enveloped in air of greater density; but, on reaching the lower portions of the atmosphere, it would burn with great rapidity. in the seventh place, _some of the larger meteors must have been bodies of great size_. according to the testimony of various individuals, in different parts of the united states, a few fire-balls appeared as large as the full moon. dr. smith, (then of north carolina, but since surgeon-general of the texian army,) who was travelling all night on professional business, describes one which he saw in the following terms: "in size it appeared somewhat larger than the full moon rising. i was startled by the splendid light in which the surrounding scene was exhibited, rendering even small objects quite visible; but i heard no noise, although every sense seemed to be suddenly aroused, in sympathy with the violent impression on the sight." this description implies not only that the body was very large, but that it was at a considerable distance from the spectator. its actual size will depend upon the distance; for, as it appeared under the same angle as the moon, its diameter will bear the same ratio to the moon's, as its distance bears to the moon's distance. we could, therefore, easily ascertain how large it was, provided we could find how far it was from the observer. if it was one hundred and ten miles distant, its diameter was one mile, and in the same proportion for a greater or less distance; and, if only at the distance of one mile, its diameter was forty-eight feet. for a moderate estimate, we will suppose it to have been twenty-two miles off; then its diameter was eleven hundred and fifty-six feet. upon every view of the case, therefore, it must be admitted, that these were bodies of great size, compared with other objects which traverse the atmosphere. we may further infer the great magnitude of some of the meteors, from the dimensions of the trains, or clouds, which resulted from their destruction. these often extended over several degrees, and at length were borne along in the direction of the wind, exactly in the manner of a small cloud. it was an interesting problem to ascertain, if possible, the height above the earth at which these fire-balls exploded, or resolved themselves into a cloud of smoke. this would be an easy task, provided we could be certain that two or more distant observers could be sure that both saw the same meteor; for as each would refer the place of explosion, or the position of the cloud that resulted from it, to a different point of the sky, a parallax would thus be obtained, from which the height might be determined. the large meteor which is mentioned in my account of the shower, (see page ,) as having exploded near the star capella, was so peculiar in its appearance, and in the form and motions of the small cloud which resulted from its combustion, that it was noticed and distinguished by a number of observers in distant parts of the country. all described the meteor as exhibiting, substantially, the same peculiarities of appearance; all agreed very nearly in the time of its occurrence; and, on drawing lines, to represent the course and direction of the place where it exploded to the view of each of the observers respectively, these lines met in nearly one and the same point, and that was over the place where it was seen in the zenith. little doubt, therefore, could remain, that all saw the same body; and on ascertaining, from a comparison of their observations, the amount of parallax, and thence deducing its height,--a task which was ably executed by professor twining,--the following results were obtained: that this meteor, and probably all the meteors, entered the atmosphere with a velocity not less, but perhaps greater, than _fourteen miles in a second_; that they became luminous many miles from the earth,--in this case, over _eighty miles_; and became extinct high above the surface,--in this case, nearly _thirty miles_. in the eighth place, _the meteors were combustible bodies, and were constituted of light and transparent materials_. the fact that they burned is sufficient proof that they belonged to the class of _combustible_ bodies; and they must have been composed of very _light materials_, otherwise their momentum would have been sufficient to enable them to make their way through the atmosphere to the surface of the earth. to compare great things with small, we may liken them to a wad discharged from a piece of artillery, its velocity being supposed to be increased (as it may be) to such a degree, that it shall take fire as it moves through the air. although it would force its way to a great distance from the gun, yet, if not consumed too soon, it would at length be stopped by the resistance of the air. although it is supposed that the meteors did in fact slightly disturb the atmospheric equilibrium, yet, had they been constituted of dense matter, like meteoric stones, they would doubtless have disturbed it vastly more. their own momentum would be lost only as it was imparted to the air; and had such a number of bodies,--some of them quite large, perhaps a mile in diameter, and entering the atmosphere with a velocity more than forty times the greatest velocity of a cannon ball,--had they been composed of dense, ponderous matter, we should have had appalling evidence of this fact, not only in the violent winds which they would have produced in the atmosphere, but in the calamities they would have occasioned on the surface of the earth. the meteors were _transparent_ bodies; otherwise, we cannot conceive why the body from which they emanated was not distinctly visible, at least by reflecting the light of the sun. if only the meteors which were known to fall towards the earth had been collected and restored to their original connexion in space, they would have composed a body of great extent; and we cannot imagine a body of such dimensions, under such circumstances, which would not be visible, unless formed of highly transparent materials. by these unavoidable inferences respecting the kind of matter of which the meteors were composed, we are unexpectedly led to recognise a body bearing, in its constitution, a strong analogy to comets, which are also composed of exceedingly light and transparent, and, as there is much reason to believe, of combustible matter. we now arrive at the final inquiry, _what relations did the body which afforded the meteoric shower sustain to the earth_? was it of the nature of a satellite, or terrestrial comet, that revolves around the earth as its centre of motion? was it a collection of nebulous, or cometary matter, which the earth encountered in its annual progress? or was it a comet, which chanced at this time to be pursuing its path along with the earth, around their common centre of motion? it could not have been of the nature of a satellite to the earth, (or one of those bodies which are held by some to afford the meteoric stones, which sometimes fall to the earth from huge meteors that traverse the atmosphere,) because it remained so long stationary with respect to the earth. a body so near the earth as meteors of this class are known to be, could not remain apparently stationary among the stars for a moment; whereas the body in question occupied the same position, with hardly any perceptible variation, for at least two hours. nor can we suppose that the earth, in its annual progress, came into the vicinity of a _nebula_, which was either stationary, or wandering lawless through space. such a collection of matter could not remain stationary within the solar system, in an insulated state, for, if not prevented by a motion of its own, or by the attraction of some nearer body, it would have proceeded directly towards the sun; and had it been in motion in any other direction than that in which the earth was moving, it would soon have been separated from the earth; since, during the eight hours, while the meteoric shower was visible, the earth moved in its orbit through the space of nearly five hundred and fifty thousand miles. the foregoing considerations conduct us to the following train of reasoning. first, if all the meteors which fell on the morning of november , , had been collected and restored to their original connexion in space, they would of themselves have constituted a nebulous body of great extent; but we have reason to suppose that they, in fact, composed but a small part of the mass from which they emanated, since, after the loss of so much matter as proceeded from it in the great meteoric shower of , and in the several repetitions of it that preceded the year , it was still capable of affording so copious a shower on that year; and similar showers, more limited in extent, were repeated for at least five years afterwards. we are therefore to regard the part that descended only as _the extreme portions of a body or collection of meteors, of unknown extent, existing in the planetary spaces_. secondly, since the earth fell in with this body in the same part of its orbit, for several years in succession, it must either have remained there while the earth was performing its whole revolution around the sun, or it must itself have had a revolution, as well as the earth. but i have already shown that it could not have remained stationary in that part of space; therefore, _it must have had a revolution around the sun_. thirdly, its period of revolution must have either been greater than the earth's, equal to it, or less. it could not have been greater, for then the two bodies could not have been together again at the end of the year, since the meteoric body would not have completed its revolution in a year. its period might obviously be the same as the earth's, for then they might easily come together again after one revolution of each; although their orbits might differ so much in shape as to prevent their being together at any intermediate point. but the period of the body might also be less than that of the earth, provided it were some _aliquot part of a year_, so as to revolve just twice, or three times, for example, while the earth revolves once. let us suppose that the period is one third of a year. then, since we have given the periodic times of the two bodies, and the major axis of the orbit of one of them, namely, of the earth, we can, by kepler's law, find the major axis of the other orbit; for the square of the earth's periodic time ^ is to the square of the body's time ( / )^ as the cube of the major axis of the earth's orbit is to the cube of the major axis of the orbit in question. now, the three first terms of this proportion are known, and consequently, it is only to solve a case in the simple rule of three, to find the term required. on making the calculation, it is found, that the supposition of a periodic time of only one third of a year gives an orbit of insufficient length; the whole major axis would not reach from the sun to the earth; and consequently, a body revolving in it could never come near to the earth. on making trial of six months, we obtain an orbit which satisfies the conditions, being such as is represented by the diagram on page , fig. ', where the outer circle denotes the earth's orbit, the sun being in the centre, and the inner ellipse denotes the path of the meteoric body. the two bodies are together at the top of the figure, being the place of the meteoric body's aphelion on the thirteenth of november, and the figures , , &c., denote the relative positions of the earth and the body for every ten days, for a period of six months, in which time the body would have returned to its aphelion. [illustration fig. '.] such would be the relation of the body that affords the meteoric shower of november, provided its revolution is accomplished in six months; but it is still somewhat uncertain whether the period be half a year or a year; it must be one or the other. if we inquire, now, why the meteors always appear to radiate from a point in the constellation leo, recollecting that this is the point to which the body is projected among the stars, the answer is, that this is the very point towards which the earth is moving in her orbit at that time; so that if, as we have proved, the earth passed through or near a nebulous body on the thirteenth of november, that body must necessarily have been projected into the constellation leo, else it could not have lain directly in her path. i consider it therefore as established by satisfactory proof, that the meteors of november thirteenth emanate from a nebulous or cometary body, revolving around the sun, and coming so near the earth at that time that the earth passes through its _skirts_, or extreme portions, and thus attracts to itself some portions of its matter, giving to the meteors a greater velocity than could be imparted by gravity alone, in consequence of passing rapidly by them. all these conclusions were made out by a process of reasoning strictly inductive, without supposing that the meteoric body itself had ever been seen. but there are some reasons for believing that we do actually see it, and that it is no other than that mysterious appearance long known under the name of the _zodiacal light_. this is a faint light, which at certain seasons of the year appears in the west after evening twilight, and at certain other seasons appears in the east before the dawn, following or preceding the track of the sun in a triangular figure, with its broad base next to the sun, and its vertex reaching to a greater or less distance, sometimes more than ninety degrees from that luminary. you may obtain a good view of it in february or march, in the west, or in october, in the morning sky. the various changes which this light undergoes at different seasons of the year are such as to render it probable, to my mind, that this is the very body which affords the meteoric showers; its extremity coming, in november, within the sphere of the earth's attraction. but, as the arguments for the existence of a body in the planetary regions, which affords these showers, were drawn without the least reference to the zodiacal light, and are good, should it finally be proved that this light has no connexion with them, i will not occupy your attention with the discussion of this point, to the exclusion of topics which will probably interest you more. it is perhaps most probable, that the meteoric showers of august and december emanate from the same body. i know of nothing repugnant to this conclusion, although it has not yet been distinctly made out. had the periods of the earth and of the meteoric body been so adjusted to each other that the latter was contained an exact even number of times in the former; that is, had it been _exactly_ either a year or half a year; then we might expect a similar recurrence of the meteoric shower every year; but only a slight variation in such a proportion between the two periods would occasion the repetition of the shower for a few years in succession, and then an intermission of them, for an unknown length of time, until the two bodies were brought into the same relative situation as before. disturbances, also, occasioned by the action of venus and mercury, might wholly subvert this numerical relation, and increase or diminish the probability of a repetition of the phenomenon. accordingly, from the year , when the meteoric shower of november was first observed, until , there was a regular increase of the exhibition; in , it came to its maximum; and after that time it was repeated upon a constantly diminishing scale, until , since which time it has not been observed. perhaps ages may roll away before the world will be again surprised and delighted with a display of celestial fire-works equal to that of the morning of november , . letter xxviii. fixed stars. ----"o, majestic night! nature's great ancestor! day's elder born, and fated to survive the transient sun! by mortals and immortals seen with awe! a starry crown thy raven brow adorns, an azure zone thy waist; clouds, in heaven's loom wrought, through varieties of shape and shade, in ample folds of drapery divine, thy flowing mantle form; and heaven throughout voluminously pour thy pompous train."--_young._ since the solar system is but one among a myriad of worlds which astronomy unfolds, it may appear to you that i have dwelt too long on so diminutive a part of creation, and reserved too little space for the other systems of the universe. but however humble a province our sun and planets compose, in the vast empire of jehovah, yet it is that which most concerns us; and it is by the study of the laws by which this part of creation is governed, that we learn the secrets of the skies. until recently, the observation and study of the phenomena of the solar system almost exclusively occupied the labors of astronomers. but sir william herschel gave his chief attention to the _sidereal heavens_, and opened new and wonderful fields of discovery, as well as of speculation. the same subject, has been prosecuted with similar zeal and success by his son, sir john herschel, and sir james south, in england, and by professor struve, of dorpat, until more has been actually achieved than preceding astronomers had ventured to conjecture. a limited sketch of these wonderful discoveries is all that i propose to offer you. the fixed stars are so called, because, to common observation, they always maintain the same situations with respect to one another. the stars are classed by their apparent _magnitudes_. the whole number of magnitudes recorded are _sixteen_, of which the first six only are visible to the naked eye; the rest are _telescopic stars_. these magnitudes are not determined by any very definite scale, but are merely ranked according to their relative degrees of brightness, and this is left in a great measure to the decision of the eye alone. the brightest stars, to the number of fifteen or twenty, are considered as stars of the first magnitude; the fifty or sixty next brightest, of the second magnitude; the next two hundred, of the third magnitude; and thus the number of each class increases rapidly, as we descend the scale, so that no less than fifteen or twenty thousand are included within the first seven magnitudes. the stars have been grouped in _constellations_ from the most remote antiquity; a few, as orion, bootes, and ursa major, are mentioned in the most ancient writings, under the same names as they bear at present. the names of the constellations are sometimes founded on a supposed resemblance to the objects to which they belong; as the swan and the scorpion were evidently so denominated from their likeness to those animals; but in most cases, it is impossible for us to find any reason for designating a constellation by the figure of the animal or hero which is employed to represent it. these representations were probably once blended with the fables of pagan mythology. the same figures, absurd as they appear, are still retained for the convenience of reference; since it is easy to find any particular star, by specifying the part of the figure to which it belongs; as when we say, a star is in the neck of taurus, in the knee of hercules, or in the tail of the great bear. this method furnishes a general clue to its position; but the stars belonging to any constellation are distinguished according to their apparent magnitudes, as follows: first, by the greek letters, alpha, beta, gamma, &c. thus, _alpha orionis_ denotes the largest star in orion; _beta andromedæ_ the second star in andromeda; and _gamma leonis_, the third brightest star in the lion. when the number of the greek letters is insufficient to include all the stars in a constellation, recourse is had to the letters of the roman alphabet, a, b, c, &c.; and in all cases where these are exhausted the final resort is to numbers. this is evidently necessary, since the largest constellations contain many hundreds or even thousands of stars. _catalogues_ of particular stars have also been published, by different astronomers, each author numbering the individual stars embraced in his list according to the places they respectively occupy in the catalogue. these references to particular catalogues are sometimes entered on large celestial globes. thus we meet with a star marked h., meaning that this is its number in herschel's catalogue; or m., denoting the place the star occupies in the catalogue of mayer. the earliest catalogue of the stars was made by hipparchus, of the alexandrian school, about one hundred and forty years before the christian era. a new star appearing in the firmament, he was induced to count the stars, and to record their positions, in order that posterity might be able to judge of the permanency of the constellations. his catalogue contains all that were conspicuous to the naked eye in the latitude of alexandria, being one thousand and twenty-two. most persons, unacquainted with the actual number of the stars which compose the visible firmament, would suppose it to be much greater than this; but it is found that the catalogue of hipparchus embraces nearly all that can now be seen in the same latitude; and that on the equator, where the spectator has both the northern and southern hemispheres in view, the number of stars that can be counted does not exceed three thousand. a careless view of the firmament in a clear night gives us the impression of an infinite number of stars; but when we begin to count them, they appear much more sparsely distributed than we supposed, and large portions of the sky appear almost destitute of stars. by the aid of the telescope, new fields of stars present themselves, of boundless extent; the number continually augmenting, as the powers of the telescope are increased. lalande, in his 'histoire celeste,' has registered the positions of no less than fifty thousand; and the whole number visible in the largest telescopes amounts to many millions. when you look at the firmament on a clear autumnal or winter evening, it appears so thickly studded with stars, that you would perhaps imagine that the task of learning even the brightest of them would be almost hopeless. let me assure you, this is all a mistake. on the contrary, it is a very easy task to become acquainted with the names and positions of the stars of the first magnitude, and of the leading constellations. if you will give a few evenings to the study, you will be surprised to find, both how rapidly you can form these new acquaintances, and how deeply you will become interested in them. i would advise you, at first, to obtain, for an evening or two, the assistance of some friend who is familiar with the stars, just to point out a few of the most conspicuous constellations. this will put you on the track, and you will afterwards experience no difficulty in finding all the constellations and stars that are particularly worth knowing; especially if you have before you a map of the stars, or, what is much better, a celestial globe. it is a pleasant evening recreation for a small company of young astronomers to go out together, and learn one or two constellations every favorable evening, until the whole are mastered. if you have a celestial globe, _rectify_ it for the evening; that is, place it in such a position, that the constellations shall be seen on it in the same position with respect to the horizon, that they have at that moment in the sky itself. to do this, i first elevate the north pole until the number of degrees on the brass meridian from the pole to the horizon corresponds to my latitude, (forty-one degrees and eighteen minutes.) i then find the sun's place in the ecliptic, by looking for the day of the month on the broad horizon, and against it noting the corresponding sign and degree. i now find the same sign and degree on the ecliptic itself, and bring that point to the brass meridian. as that will be the position of the sun at noon, i set the hour-index at twelve, and then turn the globe westward, until the index points to the given hour of the evening. if i now inspect the figures of the constellations, and then look upward at the firmament, i shall see that the latter are spread over the sky in the same manner as the pictures of them are painted on the globe. i will point out a few marks by which the leading constellations may be recognised; this will aid you in finding them, and you can afterwards learn the individual stars of a constellation, to any extent you please, by means of the globes or maps. let us begin with the _constellations of the zodiac_, which, succeeding each other, as they do, in a known order, are most easily found. _aries_ (_the ram_) is a small constellation, known by two bright stars which form his head, _alpha_ and _beta arietis_. these two stars are about four degrees apart; and directly south of beta, at the distance of one degree, is a smaller star, _gamma arietis_. it has been already intimated that the vernal equinox probably was near the head of aries, when the signs of the zodiac received their present names. _taurus_ (_the bull_) will be readily found by the seven stars, or _pleiades_, which lie in his neck. the largest star in taurus is _aldebaran_, in the bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet mars. aldebaran and four other stars, close together in the face of taurus, compose the _hyades_. _gemini_ (_the twins_) is known by two very bright stars, _castor and pollux_, five degrees asunder. castor (the northern) is of the first, and pollux of the second, magnitude. _cancer_ (_the crab_.) there are no large stars in this constellation, and it is regarded as less remarkable than any other in the zodiac. it contains, however, an interesting group of small stars, called _præsepe_, or the nebula of cancer, which resembles a comet, and is often mistaken for one, by persons unacquainted with the stars. with a telescope of very moderate powers this nebula is converted into a beautiful assemblage of exceedingly bright stars. _leo_ (_the lion_) is a very large constellation, and has many interesting members. _regulus_ (_alpha leonis_) is a star of the first magnitude, which lies directly in the ecliptic, and is much used in astronomical observations. north of regulus, lies a semicircle of bright stars, forming a _sickle_, of which regulus is the handle. _denebola_, a star of the second magnitude, is in the lion's tail, twenty-five degrees northeast of regulus. _virgo_ (_the virgin_) extends a considerable way from west to east, but contains only a few bright stars. _spica_, however, is a star of the first magnitude, and lies a little east of the place of the autumnal equinox. eighteen degrees eastward of denebola, and twenty degrees north of spica, is _vindemiatrix_, in the arm of virgo, a star of the third magnitude. _libra_ (_the balance_) is distinguished by three large stars, of which the two brightest constitute the beam of the balance, and the smallest forms the top or handle. _scorpio_ (_the scorpion_) is one of the finest of the constellations. his head is formed of five bright stars, arranged in the arc of a circle, which is crossed in the centre by the ecliptic nearly at right angles, near the brightest of the five, _beta scorpionis_. nine degrees southeast of this is a remarkable star of the first magnitude, of a reddish color, called _cor scorpionis_, or _antares_. south of this, a succession of bright stars sweep round towards the east, terminating in several small stars, forming the tail of the scorpion. _sagittarius_ (_the archer_.) northeast of the tail of the scorpion are three stars in the arc of a circle, which constitute the _bow_ of the archer, the central star being the brightest, directly west of which is a bright star which forms the _arrow_. _capricornus_ (_the goat_) lies northeast of sagittarius, and is known by two bright stars, three degrees apart, which form the head. _aquarius_ (_the water-bearer_) is recognised by two stars in a line with _alpha capricorni_, forming the shoulders of the figure. these two stars are ten degrees apart; and three degrees southeast is a third star, which, together with the other two, make an acute triangle, of which the westernmost is the vertex. _pisces_ (_the fishes_) lie between aquarius and aries. they are not distinguished by any large stars, but are connected by a series of small stars, that form a crooked line between them. _piscis australia_, the southern fish, lies directly below aquarius, and is known by a single bright star far in the south, having a declination of thirty degrees. the name of this star is _fomalhaut_, and it is much used in astronomical measurements. the constellations of the zodiac, being first well learned, so as to be readily recognised, will facilitate the learning of others that lie north and south of them. let us, therefore, next review the principal _northern constellations_, beginning north of aries, and proceeding from west to east. _andromeda_ is characterized by three stars of the second magnitude, situated in a straight line, extending from west to east. the middle star is about seventeen degrees north of beta arietis. it is in the girdle of andromeda, and is named _mirach_. the other two lie at about equal distances, fourteen degrees west and east of mirach. the western star, in the head of andromeda, lies in the equinoctial colure. the eastern star, _alamak_, is situated in the foot. _perseus_ lies directly north of the pleiades, and contains several bright stars. about eighteen degrees from the pleiades is _algol_, a star of the second magnitude, in the head of medusa, which forms a part of the figure; and nine degrees northeast of algol is _algenib_, of the same magnitude, in the back of perseus. between algenib and the pleiades are three bright stars, at nearly equal intervals, which compose the right leg of perseus. _auriga_ (_the wagoner_) lies directly east of perseus, and extends nearly parallel to that constellation, from north to south. _capella_, a very white and beautiful star of the first magnitude, distinguishes this constellation. the feet of auriga are near the bull's horns. the _lynx_ comes next, but presents nothing particularly interesting, containing no stars above the fourth magnitude. _leo minor_ consists of a collection of small stars north of the sickle in leo, and south of the great bear. its largest star is only of the third magnitude. _coma berenices_ is a cluster of small stars, north of denebola, in the tail of the lion, and of the head of virgo. about twelve degrees directly north of berenice's hair, is a single bright star, called _cor caroli_, or charles's heart. _bootes_, which comes next, is easily found by means of _arcturus_, a star of the first magnitude, of a reddish color, which is situated near the knee of the figure. arcturus is accompanied by three small stars, forming a triangle a little to the southwest. two bright stars, _gamma_ and _delta bootis_, form the shoulders, and _beta_, of the third magnitude, is in the head, of the figure. _corona borealis_, (_the crown_,) which is situated east of bootes, is very easily recognised, composed as it is of a semicircle of bright stars. in the centre of the bright crown is a star of the second magnitude, called _gemma_: the remaining stars are all much smaller. _hercules_, lying between the crown on the west and the lyre on the east, is very thickly set with stars, most of which are quite small. this constellation covers a great extent of the sky, especially from north to south, the head terminating within fifteen degrees of the equator, and marked by a star of the third magnitude, called _ras algethi_, which is the largest in the constellation. _ophiucus_ is situated directly south of hercules, extending some distance on both sides of the equator, the feet resting on the scorpion. the head terminates near the head of hercules, and, like that, is marked by a bright star within five degrees of _alpha herculis_ ophiucus is represented as holding in his hands the _serpent_, the head of which, consisting of three bright stars, is situated a little south of the crown. the folds of the serpent will be easily followed by a succession of bright stars, which extend a great way to the east. _aquila_ (_the eagle_) is conspicuous for three bright stars in its neck, of which the central one, _altair_, is a very brilliant white star of the first magnitude. _antinous_ lies directly south of the eagle, and north of the head of capricornus. _delphinus_ (_the dolphin_) is a small but beautiful constellation, a few degrees east of the eagle, and is characterized by four bright stars near to one another, forming a small rhombic square. another star of the same magnitude, five degrees south, makes the tail. _pegasus_ lies between aquarius on the southwest and andromeda on the northeast. it contains but few large stars. a very regular square of bright stars is composed of _alpha andromedæ_ and the three largest stars in pegasus; namely, _scheat_, _markab_, and _algenib_. the sides composing this square are each about fifteen degrees. algenib is situated in the equinoctial colure. we may now review the _constellations which surround the north pole_, within the circle of perpetual apparition. _ursa minor_ (_the little bear_) lies nearest the pole. the pole-star, _polaris_, is in the extremity of the tail, and is of the third magnitude. three stars in a straight line, four degrees or five degrees apart, commencing with the pole-star, lead to a trapezium of four stars, and the whole seven form together a _dipper_,--the trapezium being the body and the three stars the handle. _ursa major_ (_the great bear_) is situated between the pole and the lesser lion, and is usually recognised by the figure of a larger and more perfect dipper which constitutes the hinder part of the animal. this has also seven stars, four in the body of the dipper and three in the handle. all these are stars of much celebrity. the two in the western side of the dipper, alpha and beta, are called _pointers_, on account of their always being in a right line with the pole-star, and therefore affording an easy mode of finding that. the first star in the tail, next the body, is named _alioth_, and the second, _mizar_. the head of the great bear lies far to the westward of the pointers, and is composed of numerous small stars; and the feet are severally composed of two small stars very near to each other. _draco_ (_the dragon_) winds round between the great and the little bear; and, commencing with the tail, between the pointers and the pole-star, it is easily traced by a succession of bright stars extending from west to east. passing under ursa minor, it returns westward, and terminates in a triangle which forms the head of draco, near the feet of hercules, northwest of lyra. _cepheus_ lies eastward of the breast of the dragon, but has no stars above the third magnitude. _cassiopeia_ is known by the figure of a _chair_, composed of four stars which form the legs, and two which form the back. this constellation lies between perseus and cepheus, in the milky way. _cygnus_ (_the swan_) is situated also in the milky way, some distance southwest of cassiopeia, towards the eagle. three bright stars, which lie along the milky way, form the body and neck of the swan, and two others, in a line with the middle one of the three, one above and one below, constitute the wings. this constellation is among the few that exhibit some resemblance to the animals whose names they bear. _lyra_ (_the lyre_) is directly west of the swan, and is easily distinguished by a beautiful white star of the first magnitude, _alpha lyræ_. the _southern constellations_ are comparatively few in number. i shall notice only the whale, orion, the greater and lesser dog, hydra, and the crow. _cetus_ (_the whale_) is distinguished rather for its extent than its brilliancy, reaching as it does through forty degrees of longitude, while none of its stars, except one, are above the third magnitude. _menkar_ (_alpha ceti_) in the mouth, is a star of the second magnitude; and several other bright stars, directly south of aries, make the head and neck of the whale. _mira_, (_omicron ceti_,) in the neck of the whale, is a variable star. _orion_ is one of the largest and most beautiful of the constellations, lying southeast of taurus. a cluster of small stars forms the head; two large stars, _betalgeus_ of the first and _bellatrix_ of the second magnitude, make the shoulders; three more bright stars compose the buckler, and three the sword; and _rigel_, another star of the first magnitude, makes one of the feet. in this constellation there are seventy stars plainly visible to the naked eye, including two of the first magnitude, four of the second, and three of the third. _canis major_ lies southeast of orion, and is distinguished chiefly by its containing the largest of the fixed stars, _sirius_. _canis minor_, a little north of the equator, between canis major and gemini, is a small constellation, consisting chiefly of two stars, of which, _procyon_ is of the first magnitude. _hydra_ has its head near procyon, consisting of a number of stars of ordinary brightness. about fifteen degrees southeast of the head is a star of the second magnitude, forming the heart, (_cor hydræ_;) and eastward of this is a long succession of stars of the fourth and fifth magnitudes, composing the body and tail, and reaching a few degrees south of spica virginis. _corvus_ (_the crow_) is represented as standing on the tail of hydra. it consists of small stars, only three of which are as large as the third magnitude. in assigning the places of individual stars, i have not aimed at great precision; but such a knowledge as you will acquire of the constellations and larger stars, by nothing more even than you can obtain from the foregoing sketch, will not only add greatly to the interest with which you will ever afterwards look at the starry heavens, but it will enable you to locate any phenomenon that may present itself in the nocturnal sky, and to understand the position of any object that may be described, by assigning its true place among the stars; although i hope you will go much further than this mere outline, in cultivating an actual acquaintance with the stars. leaving, now, these great divisions of the bodies of the firmament, let us ascend to the next order of stars, composing clusters. in various parts of the nocturnal heavens are seen large groups which, either by the naked eye, or by the aid of the smallest telescope, are perceived to consist of a great number of small stars. such are the pleiades, coma berenices, and præsepe, or the bee-hive, in cancer. the _pleiades_, or seven stars, as they are called, in the neck of taurus, is the most conspicuous cluster. when we look _directly_ at this group, we cannot distinguish more than six stars; but by turning the eye _sideways_ upon it, we discover that there are many more; for it is a remarkable fact that indirect vision is far more delicate than direct. thus we can see the zodiacal light or a comet's tail much more distinctly and better defined, if we fix one eye on a part of the heavens at some distance and turn the other eye obliquely upon the object, than we can by looking directly towards it. telescopes show the pleiades to contain fifty or sixty stars, crowded together, and apparently insulated from the other parts of the heavens. _coma berenices_ has fewer stars, but they are of a larger class than those which compose the pleiades. the _bee-hive_, or nebula of cancer, as it is called, is one of the finest objects of this kind for a small telescope, being by its aid converted into a rich congeries of shining points. the head of orion affords an example of another cluster, though less remarkable than those already mentioned. these clusters are pleasing objects to the telescope; and since a common spyglass will serve to give a distinct view of most of them, every one may have the power of taking the view. but we pass, now, to the third order of stars, which present themselves much more obscurely to the gaze of the astronomer, and require large instruments for the full developement of their wonderful organization. these are the nebulÃ�. [illustration figures , , , . clusters of stars and nebulÃ�.] nebulæ are faint misty appearances which are dimly seen among the stars, resembling comets, or a speck of fog. they are usually resolved by the telescope into myriads of small stars; though in some instances, no powers of the telescope have been found sufficient thus to resolve them. the _galaxy_ or milky way, presents a continued succession of large nebulas. the telescope reveals to us innumerable objects of this kind. sir william herschel has given catalogues of two thousand nebulæ, and has shown that the nebulous matter is distributed through the immensity of space in quantities inconceivably great, and in separate parcels, of all shapes and sizes, and of all degrees of brightness between a mere milky appearance and the condensed light of a fixed star. in fact, more distinct nebulæ have been hunted out by the aid of telescopes than the whole number of stars visible to the naked eye in a clear winter's night. their appearances are extremely diversified. in many of them we can easily distinguish the individual stars; in those apparently more remote, the interval between the stars diminishes, until it becomes quite imperceptible; and in their faintest aspect they dwindle to points so minute, as to be appropriately denominated _star-dust_. beyond this, no stars are distinctly visible, but only streaks or patches of milky light. the diagram facing page represents a magnificent nebula in the galaxy. in objects so distant as the fixed stars, any apparent interval must denote an immense space; and just imagine yourself situated any where within the grand assemblage of stars, and a firmament would expand itself over your head like that of our evening sky, only a thousand times more rich and splendid. many of the nebulæ exhibit a tendency towards a globular form, and indicate a rapid condensation towards the centre. this characteristic is exhibited in the forms represented in figs. and . we have here two specimens of nebulæ of the nearer class, where the stars are easily discriminated. in figs. and we have examples of two others of the remoter kind, one of which is of the variety called _star-dust_. these wonderful objects, however, are not confined to the spherical form, but exhibit great varieties of figure. sometimes they appear as ovals; sometimes they are shaped like a fan; and the unresolvable kind often affect the most fantastic forms. the opposite diagram, fig. , as well as the preceding, affords a specimen of these varieties, as given in professor nichols's 'architecture of the heavens,' where they are faithfully copied from the papers of herschel, in the 'philosophical transactions.' [illustration figure . various forms of nebulÃ�.] sir john herschel has recently returned from a residence of five years at the cape of good hope, with the express view of exploring the hidden treasures of the southern hemisphere. the kinds of nebulæ are in general similar to those of the northern hemisphere, and the forms are equally various and singular. the _magellan clouds_, two remarkable objects seen among the stars of that hemisphere, and celebrated among navigators, appeared to the great telescope of herschel (as we are informed by professor nichols) no longer as simple milky spots, or permanent light flocculi of cloud, as they appear to the unassisted eye, but shone with inconceivable splendor. the _nubecula major_, as the larger object is called, is a congeries of clusters of stars, of irregular form, globular clusters and nebulæ of various magnitudes and degrees of condensation, among which is interspersed a large portion of irresolvable nebulous matter, which may be, and probably is, star-dust, but which the power of the twenty-feet telescope shows only as a general illumination of the field of view, forming a bright ground on which the other objects are scattered. the _nubecula minor_ (the lesser cloud) exhibited appearances similar, though inferior in degree. [illustration figure . a nebula in the milky way.] it is a grand idea, first conceived by sir william herschel, and generally adopted by astronomers, that the whole galaxy, or milky way, is nothing else than a nebula, and appears so extended, merely because it happens to be that particular nebula to which we belong. according to this view, our sun, with his attendant planets and comets, constitutes but a single star of the galaxy, and our firmament of stars, or visible heavens, is composed of the stars of _our_ nebula alone. an inhabitant of any of the other nebulæ would see spreading over him a firmament equally spacious, and in some cases inconceivably more brilliant. it is an exalted spectacle to travel over the galaxy in a clear night, with a powerful telescope, with the heart full of the idea that every star is a world. sir william herschel, by counting the stars in a single field of his telescope, estimated that fifty thousand had passed under his review in a zone two degrees in breadth, during a single hour's observation. notwithstanding the apparent contiguity of the stars which crowd the galaxy, it is certain that their mutual distances must be inconceivably great. it is with some reluctance that i leave, for the present, this fairy land of astronomy; but i must not omit, before bringing these letters to a conclusion, to tell you something respecting other curious and interesting objects to be found among the stars. variable stars are those which undergo a periodical change of brightness. one of the most remarkable is the star _mira_, in the whale, (_omicron ceti_.) it appears once in eleven months, remains at its greatest brightness about a fortnight, being then, on some occasions, equal to a star of the second magnitude. it then decreases about three months, until it becomes completely invisible, and remains so about five months, when it again becomes visible, and continues increasing during the remaining three months of its period. another very remarkable variable star is _algol_, (_beta persei_.) it is usually visible as a star of the second magnitude, and continues such for two days and fourteen hours, when it suddenly begins to diminish in splendor, and in about three and a half hours is reduced to the fourth magnitude. it then begins again to increase, and in three and a half hours more is restored to its usual brightness, going through all its changes in less than three days. this remarkable law of variation appears strongly to suggest the revolution round it of some opaque body, which, when interposed between us and algol, cuts off a large portion of its light. "it is," says sir j. herschel, "an indication of a high degree of activity in regions where, but for such evidences, we might conclude all lifeless. our sun requires almost nine times this period to perform a revolution on its axis. on the other hand, the periodic time of an opaque revolving body, sufficiently large, which would produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours." the duration of these periods is extremely various. while that of beta persei, above mentioned, is less than three days, others are more than a year; and others, many years. temporary stars are new stars, which have appeared suddenly in the firmament, and, after a certain interval, as suddenly disappeared, and returned no more. it was the appearance of a new star of this kind, one hundred and twenty-five years before the christian era, that prompted hipparchus to draw up a catalogue of the stars, the first on record. such, also, was the star which suddenly shone out, a.d. , in the eagle, as bright as venus, and, after remaining three weeks, disappeared entirely. at other periods, at distant intervals, similar phenomena have presented themselves. thus the appearance of a star in was so sudden, that tycho brahe, returning home one day, was surprised to find a collection of country people gazing at a star which he was sure did not exist half an hour before. it was then as bright as sirius, and continued to increase until it surpassed jupiter when brightest, and was visible at mid-day. in a month it began to diminish; and, in three months afterwards, it had entirely disappeared. it has been supposed by some that, in a few instances, the same star has returned, constituting one of the periodical or variable stars of a long period. moreover, on a careful reexamination of the heavens, and a comparison of catalogues, many stars are now discovered to be missing. double stars are those which appear single to the naked eye, but are resolved into two by the telescope; or, if not visible to the naked eye, are seen in the telescope so close together as to be recognised as objects of this class. sometimes, three or more stars are found in this near connexion, constituting triple, or multiple stars. castor, for example, when seen by the naked eye, appears as a single star, but in a telescope even of moderate powers, it is resolved into two stars, of between the third and fourth magnitudes, within five seconds of each other. these two stars are nearly of equal size; but more commonly, one is exceedingly small in comparison with the other, resembling a satellite near its primary, although in distance, in light, and in other characteristics, each has all the attributes of a star, and the combination, therefore, cannot be that of a planet with a satellite. in most instances, also, the distance between these objects is much less than five seconds; and, in many cases, it is less than one second. the extreme closeness, together with the exceeding minuteness, of most of the double stars, requires the best telescopes united with the most acute powers of observation. indeed, certain of these objects are regarded as the severest _tests_ both of the excellence of the instruments and of the skill of the observer. the diagram on page , fig. , represents four double stars, as seen with appropriate magnifiers. no. , exhibits epsilon bootis with a power of three hundred and fifty; no. , rigel, with a power of one hundred and thirty; no. , the pole-star, with a power of one hundred; and no. , castor, with a power of three hundred. our knowledge of the double stars almost commenced with sir william herschel, about the year . at the time he began his search for them, he was acquainted with only _four_. within five years he discovered nearly _seven hundred_ double stars, and during his life, he observed no less than twenty-four hundred. in his memoirs, published in the philosophical transactions, he gave most accurate measurements of the distances between the two stars, and of the angle which a line joining the two formed with a circle parallel to the equator. these data would enable him, or at least posterity, to judge whether these minute bodies ever change their position with respect to each other. since , these researches have been prosecuted, with great zeal and industry, by sir james south and sir john herschel, in england; while professor struve, of dorpat, with the celebrated telescope of fraunhofer, has published, from his own observations, a catalogue of three thousand double stars, the determination of which involved the distinct and most minute inspection of at least one hundred and twenty thousand stars. sir john herschel, in his recent survey of the southern hemisphere, is said to have added to the catalogue of double stars nearly three thousand more. [illustration fig. .] two circumstances add a high degree of interest to the phenomena of double stars: the first is, that a few of them, at least, are found to have a revolution around each other; the second, that they are supposed to afford the means of ascertaining the parallax of the fixed stars. but i must defer these topics till my next letter. letter xxix. fixed stars continued. "o how canst thou renounce the boundless store of charms that nature to her votary yields? the warbling woodland, the resounding shore, the pomp of groves, and garniture of fields; all that the genial ray of morning yields, and all that echoes to the song of even, all that the mountain's sheltering bosom shields, and all the dread magnificence of heaven,-- o how canst thou renounce, and hope to be forgiven!"--_beattie._ in , sir william herschel first determined and announced to the world, that there exist among the stars separate systems, composed of two stars revolving about each other in regular orbits. these he denominated _binary stars_, to distinguish them from other double stars where no such motion is detected, and whose proximity to each other may possibly arise from casual juxtaposition, or from one being in the range of the other. between fifty and sixty instances of changes, to a greater or less amount, of the relative positions of double stars, are mentioned by sir william herschel; and a few of them had changed their places so much, within twenty-five years, and in such order, as to lead him to the conclusion that they performed revolutions, one around the other, in regular orbits. these conclusions have been fully confirmed by later observers; so that it is now considered as fully established, that there exist among the fixed stars binary systems, in which two stars perform to each other the office of sun and planet, and that the periods of revolution of more than one such pair have been ascertained with some degree of exactness. immersions and emersions of stars behind each other have been observed, and real motions among them detected, rapid enough to become sensible and measurable in very short intervals of time. the periods of the double stars are very various, ranging, in the case of those already ascertained, from forty-three years to one thousand. their orbits are very small ellipses, only a few seconds in the longest direction, and more eccentric than those of the planets. a double star in the northern crown (_eta coronæ_) has made a complete revolution since its first discovery, and is now far advanced in its second period; while a star in the lion (_gamma leonis_) requires twelve hundred years to complete its circuit. you may not at once see the reason why these revolutions of one member of a double star around the other, should be deemed facts of such extraordinary interest; to you they may appear rather in the light of astronomical curiosities. but remark, that the revolutions of the binary stars have assured us of this most interesting fact, that _the law of gravitation extends to the fixed stars_. before these discoveries, we could not decide, except by a feeble analogy, that this law transcended the bounds of the solar system. indeed, our belief of the fact rested more upon our idea of unity of design in the works of the creator, than upon any certain proof; but the revolution of one star around another, in obedience to forces which are proved to be similar to those which govern the solar system, establishes the grand conclusion, that the law of gravitation is truly the law of the material universe. "we have the same evidence," says sir john herschel, "of the revolutions of the binary stars about each other, that we have of those of saturn and uranus about the sun; and the correspondence between their calculated and observed places, in such elongated ellipses, must be admitted to carry with it a proof of the prevalence of the newtonian law of gravity in their systems, of the very same nature and cogency as that of the calculated and observed places of comets round the centre of our own system. but it is not with the revolution of bodies of a cometary or planetary nature round a solar centre, that we are now concerned; it is with that of sun around sun, each, perhaps, accompanied with its train of planets and their satellites, closely shrouded from our view by the splendor of their respective suns, and crowded into a space, bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself." many of the double stars are of different colors; and sir john herschel is of the opinion that there exist in nature suns of different colors. "it may," says he, "be easier suggested in words than conceived in imagination, what variety of illumination two suns, a red and a green, or a yellow and a blue one, must afford to a planet circulating about either; and what charming contrasts and 'grateful vicissitudes' a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon. insulated stars of a red color, almost as deep as that of blood, occur in many parts of the heavens; but no green or blue star, of any decided hue, has ever been noticed unassociated with a companion brighter than itself." beside these revolutions of the binary stars, _some of the fixed stars appear to have a real motion in space_. there are several _apparent_ changes of place among the stars, arising from real changes in the earth, which, as we are not conscious of them, we refer to the stars; but there are other motions among the stars which cannot result from any changes in the earth, but must arise from changes in the stars themselves. such motions are called the _proper motions_ of the stars. nearly two thousand years ago, hipparchus and ptolemy made the most accurate determinations in their power of the relative situations of the stars, and their observations have been transmitted to us in ptolemy's 'almagest;' from which it appears that the stars retain at least _very nearly_ the same places now as they did at that period. still, the more accurate methods of modern astronomers have brought to light minute changes in the places of certain stars, which force upon us the conclusion, _either that our solar system causes an apparent displacement of certain stars, by a motion of its own in space, or that they have themselves a proper motion_. possibly, indeed, both these causes may operate. if the sun, and of course the earth which accompanies him, is actually in motion, the fact may become manifest from the apparent approach of the stars in the region which he is leaving, and the recession of those which lie in the part of the heavens towards which he is travelling. were two groves of trees situated on a plain at some distance apart, and we should go from one to the other, the trees before us would gradually appear further and further asunder, while those we left behind would appear to approach each other. some years since, sir william herschel supposed he had detected changes of this kind among two sets of stars in opposite points of the heavens, and announced that the solar system was in motion towards a point in the constellation hercules; but other astronomers have not found the changes in question such as would correspond to this motion, or to any motion of the sun; and, while it is a matter of general belief that the sun has a motion in space, the fact is not considered as yet entirely proved. in most cases, where a proper motion in certain stars has been suspected, its annual amount has been so small, that many years are required to assure us, that the effect is not owing to some other cause than a real progressive motion in the stars themselves; but in a few instances the fact is too obvious to admit of any doubt. thus, the two stars, cygni, which are nearly equal, have remained constantly at the same or nearly at the same distance of fifteen seconds, for at least fifty years past. mean-while, they have shifted their local situation in the heavens four minutes twenty-three seconds, the annual proper motion of each star being five seconds and three tenths, by which quantity this system is every year carried along in some unknown path, by a motion which for many centuries must be regarded as uniform and rectilinear. a greater proportion of the double stars than of any other indicate proper motions, especially the binary stars, or those which have a revolution around each other. among stars not double, and no way differing from the rest in any other obvious particular, a star in the constellation cassiopeia, (_mu cassiopeiæ_) has the greatest proper motion of any yet ascertained, amounting to nearly four seconds annually. you have doubtless heard much respecting the "immeasurable _distances_" of the fixed stars, and will desire to learn what is known to astronomers respecting this interesting subject. we cannot ascertain the actual distance of any of the fixed stars, but we can certainly determine that the nearest star is more than twenty millions of millions of miles from the earth, ( , , , , .) for all measurements relating to the distances of the _sun and planets_, the radius of the earth furnishes the base line. the length of this line being known, and the horizontal parallax of the sun or any planet, we have the means of calculating the distance of the body from us, by methods explained in a previous letter. but any star, viewed from the opposite sides of the earth, would appear from both stations to occupy precisely the same situation in the celestial sphere, and of course it would exhibit no horizontal parallax. but astronomers have endeavored to find a parallax in some of the fixed stars, by taking the _diameter of the earth's orbit_ as a base line. yet even a change of position amounting to one hundred and ninety millions of miles proved, until very recently, insufficient to alter the place of a single star, so far as to be capable of detection by very refined observations; from which it was concluded that the stars have not even any _annual parallax_; that is, the angle subtended by the semidiameter of the earth's orbit, at the nearest fixed star, is insensible. the errors to which instrumental measurements are subject, arising from the defects of instruments themselves, from refraction, and from various other sources of inaccuracy, are such, that the angular determinations of arcs of the heavens cannot be relied on to less than one second, and therefore cannot be appreciated by direct measurement. it follows, that, when viewed from the nearest star, the diameter of the earth's orbit would be insensible; the spider-line of the telescope would more than cover it. taking, however, the annual parallax of a fixed star at one second, it can be demonstrated, that the distance of the nearest fixed star _must exceed_ Ã� = Ã� , or one hundred thousand times one hundred and ninety millions of miles. of a distance so vast we can form no adequate conceptions, and even seek to measure it only by the time that light (which moves more than one hundred and ninety-two thousand miles per second, and passes from the sun to the earth in eight minutes and seven seconds) would take to traverse it, which is found to be more than three and a half years. if these conclusions are drawn with respect to the largest of the fixed stars, which we suppose to be vastly nearer to us than those of the smallest magnitude, the idea of distance swells upon us when we attempt to estimate the remoteness of the latter. as it is uncertain, however, whether the difference in the apparent magnitudes of the stars is owing to a real difference, or merely to their being at various distances from the eye, more or less uncertainty must attend all efforts to determine the relative distances of the stars; but astronomers generally believe, that the lower orders of stars are vastly more distant from us than the higher. of some stars it is said, that thousands of years would be required for their light to travel down to us. i have said that the stars have always been held, until recently, to have no annual parallax; yet it may be observed that astronomers were not exactly agreed on this point. dr. brinkley, a late eminent irish astronomer, supposed that he had detected an annual parallax in alpha lyræ, amounting to one second and thirteen hundreths, and in alpha aquilæ, of one second and forty-two hundreths. these results were controverted by mr. pond, of the royal observatory of greenwich; and mr. struve, of dorpat, has shown that, in a number of cases, the supposed parallax is in a direction opposite to that which would arise from the motion of the earth. hence it is considered doubtful whether, in all cases of an apparent parallax, the effect is not wholly due to errors of observation. but as if nothing was to be hidden from our times, the long sought for parallax among the fixed stars has at length been found, and consequently the distance of some of these bodies, at least, is no longer veiled in mystery. in the year , professor bessel, of köningsberg, announced the discovery of a parallax in one of the stars of the swan, ( _cygni_,) amounting to about _one third of a second_. this seems, indeed, so small an angle, that we might have reason to suspect the reality of the determination; but the most competent judges who have thoroughly examined the process by which the discovery was made, assent to its validity. what, then, do astronomers understand, when they say that a parallax has been discovered in one of the fixed stars, amounting to one third of a second? they mean that the star in question apparently shifts its place in the heavens, to that amount, when viewed at opposite extremities of the earth's orbit, namely, at points in space distant from each other one hundred and ninety millions of miles. on calculating the distance of the star from us from these data, it is found to be six hundred and fifty-seven thousand seven hundred times ninety-five millions of miles,--a distance which it would take light more than ten years to traverse. indirect methods have been proposed, for ascertaining the parallax of the fixed stars, by means of observations on the _double stars_. if the two stars composing a double star are at different distances from us, parallax would affect them unequally, and change their relative positions with respect to each other; and since the ordinary sources of error arising from the imperfection of instruments, from precession, and from refraction, would be avoided, (as they would affect both objects alike, and therefore would not disturb their relative positions,) measurements taken with the micrometer of changes much less than one second may be relied on. sir john herschel proposed a method, by which changes may be determined that amount to only one fortieth of a second. the immense distance of the fixed stars is inferred also from the fact, that the largest telescopes do not increase their apparent magnitude. they are still points, when viewed with glasses that magnify five thousand times. with respect to the nature of the stars, it would seem fruitless to inquire into the nature of bodies so distant, and which reveal themselves to us only as shining points in space. still, there are a few very satisfactory inferences that can be made out respecting them. first, _the fixed stars are bodies greater than our earth_. if this were not the case, they would not be visible at such an immense distance. dr. wollaston, a distinguished english philosopher, attempted to estimate the magnitudes of certain of the fixed stars from the light which they afford. by means of an accurate photometer, (an instrument for measuring the relative intensities of light,) he compared the light of sirius with that of the sun. he next inquired how far the sun must be removed from us, in order to appear no brighter than sirius. he found the distance to be one hundred and forty-one thousand times its present distance. but sirius is more than two hundred thousand times as far off as the sun; hence he inferred that, upon the lowest computation, it must actually give out twice as much light as the sun; or that, in point of splendor, sirius must be at least equal to two suns. indeed, he has rendered it probable, that its light is equal to that of fourteen suns. there is reason, however, to believe that the stars are actually of various magnitudes, and that their apparent difference is not owing merely to their different distances. bessel estimates the quantity of matter in the two members of a double star in the swan, as less than half that of the sun. secondly, _the fixed stars are suns_. we have already seen that they are large bodies; that they are immensely further off than the furthest planet; that they shine by their own light; in short, that their appearance is, in all respects, the same as the sun would exhibit if removed to the region of the stars. hence we infer that they are bodies of the same kind with the sun. we are justified, therefore, by a sound analogy, in concluding that the stars were made for the same end as the sun, namely, as the centres of attraction to other planetary worlds, to which they severally dispense light and heat. although the starry heavens present, in a clear night, a spectacle of unrivalled grandeur and beauty, yet it must be admitted that the chief purpose of the stars could not have been to adorn the night, since by far the greater part of them are invisible to the naked eye; nor as landmarks to the navigator, for only a very small proportion of them are adapted to this purpose; nor, finally, to influence the earth by their attractions, since their distance renders such an effect entirely insensible. if they are suns, and if they exert no important agencies upon our world, but are bodies evidently adapted to the same purpose as our sun, then it is as rational to suppose that they were made to give light and heat, as that the eye was made for seeing and the ear for hearing. it is obvious to inquire, next, to what they dispense these gifts, if not to planetary worlds; and why to planetary worlds, if not for the use of percipient beings? we are thus led, almost inevitably, to the idea of a _plurality of worlds_; and the conclusion is forced upon us, that the spot which the creator has assigned to us is but a humble province in his boundless empire. letter xxx. system of the world "o how unlike the complex works of man, heaven's easy, artless, unincumbered, plan."--_cowper._ having now explained to you, as far as i am able to do it in so short a space, the leading phenomena of the heavenly bodies, it only remains to inform you of the different systems of the world which have prevailed in different ages,--a subject which will necessarily involve a sketch of the history of astronomy. by a system of the world, i understand an explanation of _the arrangement of all the bodies that compose the material universe, and of their relations to each other_. it is otherwise called the 'mechanism of the heavens;' and indeed, in the system of the world, we figure to ourselves a machine, all parts of which have a mutual dependence, and conspire to one great end. "the machines that were first invented," says adam smith, "to perform any particular movement, are always the most complex; and succeeding artists generally discover that, with fewer wheels, and with fewer principles of motion, than had originally been employed, the same effects may be more easily produced. the first systems, in the same manner, are always the most complex; and a particular connecting chain or principle is generally thought necessary, to unite every two seemingly disjointed appearances; but it often happens, that _one great connecting principle_ is afterwards found to be sufficient to bind together all the discordant phenomena that occur in a whole species of things!" this remark is strikingly applicable to the origin and progress of systems of astronomy. it is a remarkable fact in the history of the human mind, that astronomy is the oldest of the sciences, having been cultivated, with no small success, long before any attention was paid to the causes of the common terrestrial phenomena. the opinion has always prevailed among those who were unenlightened by science, that very extraordinary appearances in the sky, as comets, fiery meteors, and eclipses, are omens of the wrath of heaven. they have, therefore, in all ages, been watched with the greatest attention: and their appearances have been minutely recorded by the historians of the times. the idea, moreover, that the aspects of the stars are connected with the destinies of individuals and of empires, has been remarkably prevalent from the earliest records of history down to a very late period, and, indeed, still lingers among the uneducated and credulous. this notion gave rise to astrology,--an art which professed to be able, by a knowledge of the varying aspects of the planets and stars, to penetrate the veil of futurity, and to foretel approaching irregularities of nature herself, and the fortunes of kingdoms and of individuals. that department of astrology which took cognizance of extraordinary occurrences in the natural world, as tempests, earthquakes, eclipses, and volcanoes, both to predict their approach and to interpret their meaning, was called _natural astrology_: that which related to the fortunes of men and of empires, _judicial astrology_. among many ancient nations, astrologers were held in the highest estimation, and were kept near the persons of monarchs; and the practice of the art constituted a lucrative profession throughout the middle ages. nor were the ignorant and uneducated portions of society alone the dupes of its pretensions. hippocrates, the 'father of medicine,' ranks astrology among the most important branches of knowledge to the physician; and tycho brahe, and lord bacon, were firm believers in its mysteries. astrology, fallacious as it was, must be acknowledged to have rendered the greatest services to astronomy, by leading to the accurate observation and diligent study of the stars. at a period of very remote antiquity, astronomy was cultivated in china, india, chaldea, and egypt. the chaldeans were particularly distinguished for the accuracy and extent of their astronomical observations. calisthenes, the greek philosopher who accompanied alexander the great in his eastern conquests, transmitted to aristotle a series of observations made at babylon nineteen centuries before the capture of that city by alexander; and the wise men of babylon and the chaldean astrologers are referred to in the sacred writings. they enjoyed a clear sky and a mild climate, and their pursuits as shepherds favored long-continued observations; while the admiration and respect accorded to the profession, rendered it an object of still higher ambition. in the seventh century before the christian era, astronomy began to be cultivated in greece; and there arose successively three celebrated astronomical schools,--the school of miletus, the school of crotona, and the school of alexandria. the first was established by thales, six hundred and forty years before christ; the second, by pythagoras, one hundred and forty years afterwards; and the third, by the ptolemies of egypt, about three hundred years before the christian era. as egypt and babylon were renowned among the most ancient nations, for their knowledge of the sciences, long before they were cultivated in greece, it was the practice of the greeks, when they aspired to the character of philosophers and sages, to resort to these countries to imbibe wisdom at its fountains. thales, after extensive travels in crete and egypt, returned to his native place, miletus, a town on the coast of asia minor, where he established the first school of astronomy in greece. although the minds of these ancient astronomers were beclouded with much error, yet thales taught a few truths which do honor to his sagacity. he held that the stars are formed of fire; that the moon receives her light from the sun, and is invisible at her conjunctions because she is hid in the sun's rays. he taught the sphericity of the earth, but adopted the common error of placing it in the centre of the world. he introduced the division of the sphere into five zones, and taught the obliquity of the ecliptic. he was acquainted with the saros, or sacred period of the chaldeans, (see page ,) and employed it in calculating eclipses. it was thales that predicted the famous eclipse of the sun which terminated the war between the lydians and the medes, as mentioned in a former letter. indeed, thales is universally regarded as a bright but solitary star, glimmering through mists on the distant horizon. to thales succeeded, in the school of miletus, two other astronomers of much celebrity, anaximander and anaxagoras. among many absurd things held by anaximander, he first taught the sublime doctrine that the planets are inhabited, and that the stars are suns of other systems. anaxagoras attempted to explain all the secrets of the skies by natural causes. his reasonings, indeed, were alloyed with many absurd notions; but still he alone, among the astronomers, maintained the existence of one god. his doctrines alarmed his countrymen, by their audacity and impiety to their gods, whose prerogatives he was thought to invade; and, to deprecate their wrath, sentence of death was pronounced on the philosopher and all his family,--a sentence which was commuted only for the sad alternative of perpetual banishment. the very genius of the heathen mythology was at war with the truth. false in itself, it trained the mind to the love of what was false in the interpretation of nature; it arrayed itself against the simplicity of truth, and persecuted and put to death its most ardent votaries. the religion of the bible, on the other hand, lends all its aid to truth in nature as well as in morals and religion. in its very genius it inculcates and inspires the love of truth; it suggests, by its analogies, the existence of established laws in the system of the world; and holds out the moon and the stars, which the creator has ordained, as fit objects to give us exalted views of his glory and wisdom. pythagoras was the founder of the celebrated school of crotona. he was a native of samos, an island in the Ã�gean sea, and flourished about five hundred years before the christian era. after travelling more than thirty years in egypt and chaldea, and spending several years more at sparta, to learn the laws and institutions of lycurgus, he returned to his native island to dispense the riches he had acquired to his countrymen. but they, probably fearful of incurring the displeasure of the gods by the freedom with which he inquired into the secrets of the skies, gave him so unwelcome a reception, that he retired from them, in disgust, and established his school at crotona, on the southeastern coast of italy. hither, as to an oracle, the fame of his wisdom attracted hundreds of admiring pupils, whom he instructed in every species of knowledge. from the visionary notions which are generally understood to have been entertained on the subject of astronomy, by the ancients, we are apt to imagine that they knew less than they actually did of the truths of this science. but pythagoras was acquainted with many important facts in astronomy, and entertained many opinions respecting the system of the world, which are now held to be true. among other things well known to pythagoras, either derived from his own investigations, or received from his predecessors, were the following; and we may note them as a synopsis of the state of astronomical knowledge at that age of the world. first, the principal _constellations_. these had begun to be formed in the earliest ages of the world. several of them, bearing the same name as at present, are mentioned in the writings of hesiod and homer; and the "sweet influences of the pleiades," and the "bands of orion," are beautifully alluded to in the book of job. secondly, _eclipses_. pythagoras knew both the causes of eclipses and how to predict them; not, indeed, in the accurate manner now practised, but by means of the saros. thirdly, pythagoras had divined the true _system of the world_, holding that the sun, and not the earth, (as was generally held by the ancients, even for many ages after pythagoras,) is the centre around which all the planets revolve; and that the stars are so many suns, each the centre of a system like our own. among lesser things, he knew that the earth is round; that its surface is naturally divided into five zones; and that the ecliptic is inclined to the equator. he also held that the earth revolves daily on its axis, and yearly around the sun; that the galaxy is an assemblage of small stars; and that it is the same luminary, namely, venus, that constitutes both the morning and evening star; whereas all the ancients before him had supposed that each was a separate planet, and accordingly the morning star was called lucifer, and the evening star, hesperus. he held, also, that the planets were inhabited, and even went so far as to calculate the size of some of the animals in the moon. pythagoras was also so great an enthusiast in music, that he not only assigned to it a conspicuous place in his system of education, but even supposed that the heavenly bodies themselves were arranged at distances corresponding to the intervals of the diatonic scale, and imagined them to pursue their sublime march to notes created by their own harmonious movements, called the 'music of the spheres;' but he maintained that this celestial concert, though loud and grand, is not audible to the feeble organs of man, but only to the gods. with few exceptions, however, the opinions of pythagoras on the system of the world were founded in truth. yet they were rejected by aristotle, and by most succeeding astronomers, down to the time of copernicus; and in their place was substituted the doctrine of _crystalline spheres_, first taught by eudoxus, who lived about three hundred and seventy years before christ. according to this system, the heavenly bodies are set like gems in hollow solid orbs, composed of crystal so transparent, that no anterior orb obstructs in the least the view of any of the orbs that lie behind it. the sun and the planets have each its separate orb; but the fixed stars are all set in the same grand orb; and beyond this is another still, the _primum mobile_, which revolves daily, from east to west, and carries along with it all the other orbs. above the whole spreads the _grand empyrean_, or third heavens, the abode of perpetual serenity. to account for the planetary motions, it was supposed that each of the planetary orbs, as well as that of the sun, has a motion of its own, eastward, while it partakes of the common diurnal motion of the starry sphere. aristotle taught that these motions are effected by a tutelary genius of each planet, residing in it, and directing its motions, as the mind of man directs his movements. two hundred years after pythagoras, arose the famous school of alexandria, under the ptolemies. these were a succession of egyptian kings, and are not to be confounded with ptolemy, the astronomer. by the munificent patronage of this enlightened family, for the space of three hundred years, beginning at the death of alexander the great, from whom the eldest of the ptolemies had received his kingdom, the school of alexandria concentrated in its vast library and princely halls, erected for the accommodation of the philosophers, nearly all the science and learning of the world. in wandering over the immense territories of ignorance and barbarism which covered, at that time, almost the entire face of the earth, the eye reposes upon this little spot, as upon a verdant island in the midst of the desert. among the choice fruits that grew in this garden of astronomy were several of the most distinguished ornaments of ancient science, of whom the most eminent were hipparchus and ptolemy. hipparchus is justly considered as the newton of antiquity. he sought his knowledge of the heavenly bodies not in the illusory suggestions of a fervid imagination, but in the vigorous application of an intellect of the first order. previous to this period, celestial observations were made chiefly with the naked eye: but hipparchus was in possession of instruments for measuring angles, and knew how to resolve spherical triangles. these were great steps beyond all his predecessors. he ascertained the length of the year within six minutes of the truth. he discovered the eccentricity, or elliptical figure, of the solar orbit, although he supposed the sun actually to move uniformly in a circle, but the earth to be placed out of the centre. he also determined the positions of the points among the stars where the earth is nearest to the sun, and where it is most remote from it. he formed very accurate estimates of the obliquity of the ecliptic and of the precession of the equinoxes. he computed the exact period of the synodic revolution of the moon, and the inclination of the lunar orbit; discovered the backward motion of her node and of her line of apsides; and made the first attempts to ascertain the horizontal parallaxes of the sun and moon. upon the appearance of a new star in the firmament, he undertook, as already mentioned, to number the stars, and to assign to each its true place in the heavens, in order that posterity might have the means of judging what changes, if any, were going forward among these apparently unalterable bodies. although hipparchus is generally considered as belonging to the alexandrian school, yet he lived at rhodes, and there made his astronomical observations, about one hundred and forty years before the christian era. one of his treatises has come down to us; but his principal discoveries have been transmitted through the 'almagest' of ptolemy. ptolemy flourished at alexandria nearly three centuries after hipparchus, in the second century after christ. his great work, the 'almagest,' which has conveyed to us most that we know respecting the astronomical knowledge of the ancients, was the universal text-book of astronomers for fourteen centuries. [illustration fig. .] the name of this celebrated astronomer has also descended to us, associated with the system of the world which prevailed from ptolemy to copernicus, called the _ptolemaic system_. the doctrines of the ptolemaic system did not originate with ptolemy, but, being digested by him out of materials furnished by various hands, it has come down to us under the sanction of his name. according to this system, the earth is the centre of the universe, and all the heavenly bodies daily revolve around it, from east to west. but although this hypothesis would account for the apparent diurnal motion of the firmament, yet it would not account for the apparent annual motion of the sun, nor for the slow motions of the planets from west to east. in order to explain these phenomena, recourse was had to _deferents_ and _epicycles_,--an explanation devised by apollonius, one of the greatest geometers of antiquity. he conceived that, in the circumference of a circle, having the earth for its centre, there moves the centre of a smaller circle in the circumference of which the planet revolves. the circle surrounding the earth was called the deferent, while the smaller circle, whose centre was always in the circumference of the deferent, was called the epicycle. thus, if e, fig. , represents the earth, abc will be the deferent, and dfg, the epicycle; and it is obvious that the motion of a body from west to east, in this small circle, would be alternately direct, stationary, and retrograde, as was explained, in a previous letter, to be actually the case with the apparent motions of the planets. the hypothesis, however, is inconsistent with the _phases_ of mercury and venus, which, being between us and the sun, on both sides of the epicycle, would present their dark sides towards us at both conjunctions with the sun, whereas, at one of the conjunctions, it is known that they exhibit their disks illuminated. it is, moreover, absurd to speak of a geometrical centre, which has no bodily existence, moving round the earth on the circumference of another circle. in addition to these absurdities, the whole ptolemaic system is encumbered with the following difficulties: first, it is a mere hypothesis, having no evidence in its favor except that it explains the phenomena. this evidence is insufficient of itself, since it frequently happens that each of two hypotheses, which are directly opposite to each other, will explain all the known phenomena. but the ptolemaic system does not even do this, as it is inconsistent with the phases of mercury and venus, as already observed. secondly, now that we are acquainted with the distances of the remoter planets, and especially the fixed stars, the swiftness of motion, implied in a daily revolution of the starry firmament around the earth, renders such a motion wholly incredible. thirdly, the centrifugal force which would be generated in these bodies, especially in the sun, renders it impossible that they can continue to revolve around the earth as a centre. absurd, however, as the system of ptolemy was, for many centuries no great philosophic genius appeared to expose its fallacies, and it therefore guided the faith of astronomers of all countries down to the time of copernicus. after the age of ptolemy, the science made little progress. with the decline of grecian liberty, the arts and sciences declined also; and the romans, then masters of the world, were ever more ambitious to gain conquests over man than over matter; and they accordingly never produced a single great astronomer. during the middle ages, the arabians were almost the only astronomers, and they cultivated this noble study chiefly as subsidiary to astrology. at length, in the fifteenth century, copernicus arose, and after forty years of intense study and meditation, divined the true system of the world. you will recollect that the copernican system maintains, . that the _apparent_ diurnal motions of the heavenly bodies, from east to west, is owing to the _real_ revolution of the earth on its own axis from west to east; and, . that the sun is the centre around which the earth and planets all revolve from west to east. it rests on the following arguments: in the first place, _the earth revolves on its own axis_. first, because this supposition is vastly more _simple_. secondly, it is agreeable to _analogy_, since all the other planets that afford any means of determining the question, are seen to revolve on their axes. thirdly, the _spheroidal figure_ of the earth is the figure of equilibrium, that results from a revolution on its axis. fourthly, the _diminished weight_ of bodies at the equator indicates a centrifugal force arising from such a revolution. fifthly, bodies let fall from a high eminence, fall _eastward of their base_, indicating that when further from the centre of the earth they were subject to a greater velocity, which, in consequence of their inertia, they do not entirely lose in descending to the lower level. in the second place, _the planets, including the earth, revolve about the sun_. first, the _phases_ of mercury and venus are precisely such, as would result from their circulating around the sun in orbits within that of the earth; but they are never seen in opposition, as they would be, if they circulate around the earth. secondly, the superior planets do indeed revolve around the earth; but they also revolve around the sun, as is evident from their phases, and from the known dimensions of their orbits; and that the sun, and not the earth, is the _centre_ of their motions, is inferred from the greater symmetry of their motions, as referred to the sun, than as referred to the earth; and especially from the laws of gravitation, which forbid our supposing that bodies so much larger than the earth, as some of these bodies are, can circulate permanently around the earth, the latter remaining all the while at rest. in the third place, the annual motion of _the earth_ itself is indicated also by the most conclusive arguments. for, first, since all the planets, with their satellites and the comets, revolve about the sun, analogy leads us to infer the same respecting the earth and its satellite, as those of jupiter and saturn, and indicates that it is a law of the solar system that the smaller bodies revolve about the larger. secondly, on the supposition that the earth performs an annual revolution around the sun, it is embraced along with the planets, in kepler's law, that the squares of the times are as the cubes of the distances; otherwise, it forms an exception, and the only known exception, to this law. such are the leading arguments upon which rests the copernican system of astronomy. they were, however, only very partially known to copernicus himself, as the state both of mechanical science, and of astronomical observation, was not then sufficiently matured to show him the strength of his own doctrine, since he knew nothing of the telescope, and nothing of the principle of universal gravitation. the evidence of this beautiful system being left by copernicus in so imperfect a state, and indeed his own reasonings in support of it being tinctured with some errors, we need not so much wonder that tycho brahe, who immediately followed copernicus, did not give it his assent, but, influenced by certain passages of scripture, he still maintained, with ptolemy, that the earth is in the centre of the universe; and he accounted for the diurnal motions in the same manner as ptolemy had done, namely, by an actual revolution of the whole host of heaven around the earth every twenty-four hours. but he rejected the scheme of deferents and epicycles, and held that the moon revolves about the earth as the centre of her motions; but that the sun and not the earth is the centre of the planetary motions; and that the sun, accompanied by the planets, moves around the earth once a year, somewhat in the manner in which we now conceive of jupiter and his satellites as revolving around the sun. this system is liable to most of the objections that lie against the ptolemaic system, with the disadvantage of being more complex. kepler and galileo, however, as appeared in the sketch of their lives, embraced the theory of copernicus with great avidity, and all their labors contributed to swell the evidence of its truth. when we see with what immense labor and difficulty the disciples of ptolemy sought to reconcile every new phenomenon of the heavens with their system, and then see how easily and naturally all the successive discoveries of galileo and kepler fall in with the theory of copernicus, we feel the full force of those beautiful lines of cowper which i have chosen for the motto of this letter. newton received the torch of truth from galileo, and transmitted it to his successors, with its light enlarged and purified; and since that period, every new discovery, whether the fruit of refined instrumental observation or of profound mathematical analysis, has only added lustre to the glory of copernicus. with newton commenced a new and wonderful era in astronomy, distinguished above all others, not merely for the production of the greatest of men, but also for the establishment of those most important auxiliaries to our science, the royal society of london, the academy of sciences at paris, and the observatory of greenwich. i may add the commencement of the transactions of the royal society, and the memoirs of the academy of sciences, which have been continued to the present time,--both precious storehouses of astronomical riches. the observatory of greenwich, moreover, has been under the direction of an extraordinary succession of great astronomers. their names are flamstead, halley, bradley, maskeleyne, pond, and airy,--the last being still at his post, and worthy of continuing a line so truly illustrious. the observations accumulated at this celebrated observatory are so numerous, and so much superior to those of any other institution in the world, that it has been said that astronomy would suffer little, if all other contemporary observations of the same kind were annihilated. sir william herschel, however, labored chiefly in a different sphere. the astronomers royal devoted themselves not so much to the discovery of new objects among the heavenly bodies, as to the exact determination of the places of the bodies already known, and to the developement of new laws or facts among the celestial motions. but herschel, having constructed telescopes of far greater reach than any ever used before, employed them to sound new and untried depths in the profundities of space. we have already seen what interesting and amazing discoveries he made of double stars, clusters, and nebulæ. the english have done most for astronomy in observation and discovery; but the french and germans, in developing, by the most profound mathematical investigation, the great laws of physical astronomy. it only remains to inquire, whether the copernican system is now to be regarded as a full exposition of the 'mechanism of the heavens,' or whether there subsist higher orders of relations between the fixed stars themselves. the revolutions of the _binary stars_ afford conclusive evidence of at least subordinate systems of suns, governed by the same laws as those which regulate the motions of the solar system. the _nebulæ_ also compose peculiar systems, in which the members are evidently bound together by some common relation. in these marks of organization,--of stars associated together in clusters; of sun revolving around sun; and of nebulæ disposed in regular figures,--we recognise different members of some grand system, links in one great chain that binds together all parts of the universe; as we see jupiter and his satellites combined in one subordinate system, and saturn and his satellites in another,--each a vast kingdom, and both uniting with a number of other individual parts, to compose an empire still more vast. this fact being now established, that the stars are immense bodies, like the sun, and that they are subject to the laws of gravitation, we cannot conceive how they can be preserved from falling into final disorder and ruin, unless they move in harmonious concert, like the members of the solar system. otherwise, those that are situated on the confines of creation, being retained by no forces from without, while they are subject to the attraction of all the bodies within, must leave their stations, and move inward with accelerated velocity; and thus all the bodies in the universe would at length fall together in the common centre of gravity. the immense distance at which the stars are placed from each other would indeed delay such a catastrophe; but this must be the ultimate tendency of the material world, unless sustained in one harmonious system by nicely-adjusted motions. to leave entirely out of view our confidence in the wisdom and preserving goodness of the creator, and reasoning merely from what we know of the stability of the solar system, we should be justified in inferring, that other worlds are not subject to forces which operate only to hasten their decay, and to involve them in final ruin. we conclude, therefore, that the material universe is one great system; that the combination of planets with their satellites constitutes the first or lowest order of worlds; that next to these, planets are linked to suns; that these are bound to other suns, composing a still higher order in the scale of being; and finally, that all the different systems of worlds move around their common centre of gravity. letter xxxi. natural theology. ----"philosophy, baptized in the pure fountain of eternal love, has eyes indeed; and, viewing all she sees as meant to indicate a god to man, gives him the praise, and forfeits not her own."--_cowper._ i intended, my dear friend, to comply with your request "that i would discuss the arguments which astronomy affords to natural theology;" but these letters have been already extended so much further than i anticipated, that i shall conclude with suggesting a few of those moral and religious reflections, which ought always to follow in the train of such a survey of the heavenly bodies as we have now taken. although there is evidence enough in the structure, arrangement, and laws, which prevail among the heavenly bodies, to prove the _existence_ of god, yet i think there are many subordinate parts of his works far better adapted to this purpose than these, being more fully within our comprehension. it was intended, no doubt, that the evidence of his being should be accessible to all his creatures, and should not depend on a kind of knowledge possessed by comparatively few. the mechanism of the eye is probably not more perfect than that of the universe; but we can analyze it better, and more fully understand the design of each part. but the existence of god being once proved, and it being admitted that he is the creator and governor of the world, then the discoveries of astronomy are admirably adapted to perform just that office in relation to the great first cause, which is assigned to them in the bible, namely, "to declare the glory of god, and to show his handiwork." in other words, the discoveries of astronomy are peculiarly fitted,--more so, perhaps, than any other department of creation,--to exhibit the unity, power, and wisdom, of the creator. the most modern discoveries have multiplied the proofs of the _unity_ of god. it has usually been offered as sufficient evidence of the truth of this doctrine, that the laws of nature are found to be uniform when applied to the utmost bounds of the _solar system_; that the law of gravitation controls alike the motions of mercury, and those of uranus; and that its operation is one and the same upon the moon and upon the satellites of saturn. it was, however, impossible, until recently, to predicate the same uniformity in the great laws of the universe respecting the starry worlds, except by a feeble analogy. however improbable, it was still possible, that in these distant worlds other laws might prevail, and other lords exercise dominion. but the discovery of the revolutions of the binary stars, in exact accordance with the law of gravitation, not merely in a single instance, but in many instances, in all cases, indeed, wherever those revolutions have advanced so far as to determine their law of action, gives us demonstration, instead of analogy, of the prevalence of the same law among the other systems as that which rules in ours. the marks of a still higher organization in the structure of clusters and nebulæ, all bearing that same characteristic union of resemblance and variety which belongs to all the other works of creation that fall under our notice, speak loudly of one, and only one, grand design. every new discovery of the telescope, therefore, has added new proofs to the great truth that god is one: nor, so far as i know, has a single fact appeared, that is not entirely consonant with it. light, moreover, which brings us intelligence, and, in most cases, the only intelligence we have, of these remote orbs, testifies to the same truth, being similar in its properties and uniform in its motions, from whatever star it emanates. in displays of the _power_ of jehovah, nothing can compare with the starry heavens. the magnitudes, distances, and velocities, of the heavenly bodies are so much beyond every thing of this kind which belongs to things around us, from which we borrowed our first ideas of these qualities, that we can scarcely avoid looking with incredulity at the numerical results to which the unerring principles of mathematics have conducted us. and when we attempt to apply our measures to the fixed stars, and especially to the nebulæ, the result is absolutely overwhelming: the mind refuses its aid in our attempts to grasp the great ideas. nor less conspicuous, among the phenomena of the heavenly bodies, is the _wisdom_ of the creator. in the first place, this attribute is every where exhibited _in the happy adaptation of means to their ends_. no principle can be imagined more simple, and at the same time more effectual to answer the purposes which it serves, than gravitation. no position can be given to the sun and planets so fitted, as far as we can judge, to fulfil their mutual relations, as that which the creator has given them. i say, as far as we can judge; for we find this to be the case in respect to our own planet and its attendant satellite, and hence have reason to infer that the same is the case in the other planets, evidently holding, as they do, a similar relation to the sun. thus the position of the earth at just such a distance from the sun as suits the nature of its animal and vegetable kingdoms, and confining the range of solar heat, vast as it might easily become, within such narrow bounds; the inclination of the earth's axis to the plane of its orbit, so as to produce the agreeable vicissitudes of the seasons, and increase the varieties of animal and vegetable life, still confining the degree of inclination so exactly within the bounds of safety, that, were it much to transcend its present limits, the changes of temperature of the different seasons would be too sudden and violent for the existence of either animals or vegetables; the revolution of the earth on its axis, so happily dividing time into hours of business and of repose; the adaptation of the moon to the earth, so as to afford to us her greatest amount of light just at the times when it is needed most, and giving to the moon just such a quantity of matter, and placing her at just such a distance from the earth, as serves to raise a tide productive of every conceivable advantage, without the evils which would result from a stagnation of the waters on the one hand, or from their overflow on the other;--these are a few examples of the wisdom displayed in the mutual relations instituted between the sun, the earth, and the moon. in the second place, similar marks of wisdom are exhibited in _the many useful and important purposes_ _which the same thing is made to serve_. thus the sun is at once the great regulator of the planetary motions, and the fountain of light and heat. the moon both gives light by night and raises the tides. or, if we would follow out this principle where its operations are more within our comprehension, we may instance the _atmosphere_. when man constructs an instrument, he deems it sufficient if it fulfils one single purpose as the watch, to tell the hour of the day, or the telescope, to enable him to see distant objects; and had a being like ourselves made the atmosphere, he would have thought it enough to have created a medium so essential to animal life, that to live is to breathe, and to cease to breathe is to die. but beside this, the atmosphere has manifold uses, each entirely distinct from all the others. it conveys to plants, as well as animals, their nourishment and life; it tempers the heat of summer with its breezes; it binds down all fluids, and prevents their passing into the state of vapor; it supports the clouds, distils the dew, and waters the earth with showers; it multiplies the light of the sun, and diffuses it over earth and sky; it feeds our fires, turns our machines, wafts our ships, and conveys to the ear all the sentiments of language, and all the melodies of music. in the third place, the wisdom of the creator is strikingly manifested in the provision he has made for the _stability of the universe_. the perturbations occasioned by the motions of the planets, from their action on each other, are very numerous, since every body in the system exerts an attraction on every other, in conformity with the law of universal gravitation. venus and mercury, approaching, as they do at times, comparatively near to the earth, sensibly disturb its motions; and the satellites of the remoter planets greatly disturb each other's movements. nor was it possible to endow this principle with the properties it has, and make it operate as it does in regulating the motions of the world, without involving such an incident. on this subject, professor whewell, in his excellent work composing one of the bridgewater treatises, remarks: "the derangement which the planets produce in the motion of one of their number will be very small, in the course of one revolution; but this gives us no security that the derangement may not become very large, in the course of many revolutions. the cause acts perpetually, and it has the whole extent of time to work in. is it not easily conceivable, then, that, in the lapse of ages, the derangements of the motions of the planets may accumulate, the orbits may change their form, and their mutual distances may be much increased or diminished? is it not possible that these changes may go on without limit, and end in the complete subversion and ruin of the system? if, for instance, the result of this mutual gravitation should be to increase considerably the eccentricity of the earth's orbit, or to make the moon approach continually nearer and nearer to the earth, at every revolution, it is easy to see that, in the one case, our year would change its character, producing a far greater irregularity in the distribution of the solar heat; in the other, our satellite must fall to the earth, occasioning a dreadful catastrophe. if the positions of the planetary orbits, with respect to that of the earth, were to change much, the planets might sometimes come very near us, and thus increase the effect of their attraction beyond calculable limits. under such circumstances, 'we might have years of unequal length, and seasons of capricious temperature; planets and moons, of portentous size and aspect, glaring and disappearing at uncertain intervals; tides, like deluges, sweeping over whole continents; and perhaps the collision of two of the planets, and the consequent destruction of all organization on both of them.' the fact really is, that changes are taking place in the motions of the heavenly bodies, which have gone on progressively, from the first dawn of science. the eccentricity of the earth's orbit has been diminishing from the earliest observations to our times. the moon has been moving quicker from the time of the first recorded eclipses, and is now in advance, by about four times her own breadth, of what her own place would have been, if it had not been affected by this acceleration. the obliquity of the ecliptic, also, is in a state of diminution, and is now about two fifths of a degree less than it was in the time of aristotle." but amid so many seeming causes of irregularity and ruin, it is worthy of a grateful notice, that effectual provision is made for the _stability of the solar system_. the full confirmation of this fact is among the grand results of physical astronomy. "newton did not undertake to demonstrate either the stability or instability of the system. the decision of this point required a great number of preparatory steps and simplifications, and such progress in the invention and improvement of mathematical methods, as occupied the best mathematicians of europe for the greater part of the last century. towards the end of that time, it was shown by la grange and la place, that the arrangements of the solar system are stable; that, in the long run, the orbits and motions remain unchanged; and that the changes in the orbits, which take place in shorter periods, never transgress certain very moderate limits. each orbit undergoes deviations on this side and on that side of its average state; but these deviations are never very great, and it finally recovers from them, so that the average is preserved. the planets produce perpetual perturbations in each other's motions; but these perturbations are not indefinitely progressive, but periodical, reaching a maximum value, and then diminishing. the periods which this restoration requires are, for the most part, enormous,--not less than thousands, and in some instances, millions, of years. indeed, some of these apparent derangements have been going on in the same direction from the creation of the world. but the restoration is in the sequel as complete as the derangement; and in the mean time the disturbance never attains a sufficient amount seriously to affect the stability of the system. 'i have succeeded in demonstrating,' says la place, 'that, whatever be the masses of the planets, in consequence of the fact that they all move in the same direction, in orbits of small eccentricity, and but slightly inclined to each other, their secular irregularities are periodical, and included within narrow limits; so that the planetary system will only oscillate about a mean state, and will never deviate from it, except by a very small quantity. the ellipses of the planets have been and always will be nearly circular. the ecliptic will never coincide with the equator; and the entire extent of the variation, in its inclination, cannot exceed three degrees.'" to these observations of la place, professor whewell adds the following, on the importance, to the stability of the solar system, of the fact that those planets which have _great masses_ have orbits of _small eccentricity_. "the planets mercury and mars, which have much the largest eccentricity among the old planets, are those of which the masses are much the smallest. the mass of jupiter is more than two thousand times that of either of these planets. if the orbit of jupiter were as eccentric as that of mercury, all the security for the stability of the system, which analysis has yet pointed out, would disappear. the earth and the smaller planets might, by the near approach of jupiter at his perihelion, change their nearly circular orbits into very long ellipses, and thus might fall into the sun, or fly off into remoter space. it is further remarkable, that in the newly-discovered planets, of which the orbits are still more eccentric than that of mercury, the masses are still smaller, so that the same provision is established in this case, also." with this hasty glance at the unity, power, and wisdom, of the creator, as manifested in the greatest of his works, i close. i hope enough has been said to vindicate the sentiment that called 'devotion, daughter of astronomy!' i do not pretend that this, or any other science, is adequate of itself to purify the heart, or to raise it to its maker; but i fully believe that, when the heart is already under the power of religion, there is something in the frequent and habitual contemplation of the heavenly bodies under all the lights of modern astronomy, very favorable to devotional feelings, inspiring, as it does, humility, in unison with an exalted sentiment of grateful adoration. letter xxxii. recent discoveries. "all are but parts of one stupendous whole."--_pope._ within a few years, astronomy has been enriched with a number of valuable discoveries, of which i will endeavor to give you a summary account in this letter. the heavens have been explored with far more powerful telescopes than before; instrumental measurements have been carried to an astonishing degree of accuracy; numerous additions have been made to the list of small planets or asteroids; a comet has appeared of extraordinary splendor, remarkable, above all others, for its near approach to the sun; the distances of several of the fixed stars, an element long sought for in vain, have been determined; a large planet, composing in itself a magnificent world, has been added to the solar system, at such a distance from the central luminary as nearly to double the supposed dimensions of that system; various nebulæ, before held to be irresolvable, have been resolved into stars; and a new satellite has been added to saturn. improvements in the telescope.--herschel's forty-feet telescope, of which i gave an account in my fourth letter (see page ), remained for half a century unequalled in magnitude and power; but in , lord rosse, an irish nobleman, commenced a telescope on a scale still more gigantic. like herschel's, it was a _reflector_, the image being formed by a concave mirror. this was six feet in diameter, and weighed three tons; and the tube was fifty feet in length. the entire cost of the instrument was sixty thousand dollars. its reflecting surface is nearly twice as great as the great herschelian, and consequently it greatly exceeds all instruments hitherto constructed in the _amount of light_ which it collects and transmits to the eye; and this adapts it peculiarly to viewing those objects, as nebulæ, whose light is exceedingly faint. accordingly, it has revealed to us new wonders in this curious department of astronomy. some idea of the great dimensions of the _leviathan_ telescope (as this instrument has been called) may be formed when it is said that the dean of ely, a full-sized man, walked through the tube from one end to the other, with an umbrella over his head. but still greater advances have been made in refracting than in reflecting telescopes. such was the difficulty of obtaining large pieces of glass which are free from impurities, and such the liability of large lenses to form obscure and colored images, that it was formerly supposed impossible to make a refracting telescope larger in diameter than five or six inches; but their size has been increased from one step to another, until they are now made more than fifteen inches in diameter; and so completely have all the difficulties arising from the imperfections of glass, and from optical defects inherent in lenses, been surmounted, that the great telescopes of pulkova, at st. petersburgh, and of harvard university (the two finest refractors in the world) are considered among the most perfect productions of the arts. a lens of only inches in diameter seems, indeed, diminutive when compared with a concave reflector of six feet; but for most purposes of the astronomer, the pulkova and cambridge instruments are more useful than such great reflectors as those of herschel and rosse. if there is any particular in which these are more effective, it is in observations on the faintest nebulæ, where it is necessary to collect and convey to the eye the greatest possible beam of light. instrumental measurements.--when astronomical instruments were first employed to measure the angular distance between two points on the celestial sphere, it was not attempted to measure spaces smaller than ten minutes--a space equal to the third part of the breadth of the full moon. tycho brahe, however, carried his measures to sixty times that degree of minuteness, having devised means of determining angles no larger than ten seconds, or the one hundred and eightieth part of the breadth of the lunar disk. for many years past, astronomers have carried these measures to single seconds, or have determined spaces no greater than the eighteen hundredth part of the diameter of the moon. this is considered the smallest arc which can be accurately measured directly on the limb of an instrument; but _differences_ between spaces may be estimated to a far greater degree of accuracy than this, even to the hundredth part of a second--a space less than that intercepted by a spider's web held before the eye. discovery of new planets.--in my twenty-third letter (see page ), i gave an account of the small planets called asteroids, which lie between the orbits of mars and jupiter. when that letter was written, no longer ago than , only four of those bodies had been discovered, namely, ceres, pallas, juno, and vesta. within a few years past, nineteen more have been added, making the number of the asteroids known at present twenty-three, and every year adds one or more to the list.[ ] the idea first suggested by olbers, one of the earliest discoverers of asteroids, that they are fragments of a large single planet once revolving between mars and jupiter, has gained credit since the discovery of so many additional bodies of the same class, all, like the former, exceedingly small and irregular in their motions, although there are still great difficulties in tracing them to a common origin. great comet of .--this is the most wonderful body that has appeared in the heavens in modern times; first, on account of its appearing, when first seen, in the broad light of noonday; and, secondly, on account of its approaching so near the sun as almost to graze his surface. it was first discovered, in new england, on the th of february, a little eastward of the sun, shining like a white cloud illuminated by the solar rays. it arrested the attention of many individuals from half past seven in the morning until three o'clock in the afternoon, when the sky became obscured by clouds. in mexico, it was observed from nine in the morning until sunset. at a single station in south america, it was said to have been seen on the th of february, almost in contact with the sun. early in march, it had receded so far to the eastward of that body as to be visible in the southwest after sunset, throwing upward a long train, which increased in length from night to night until it covered a space of degrees. its position may be seen on a celestial globe adjusted to the latitude of new haven ( ° ´) for the th of march, by tracing a line, or, rather, a broad band proceeding from the place of the sun towards the bright star sirius, in the south, between the ears of the hare and the feet of orion. the comet passed its perihelion on the th of february, at which time it almost came in contact with the sun. to prevent its falling into the sun it was endued with a prodigious velocity; a velocity so great that, had it continued at the same rate as at the instant of perihelion passage, it would have whirled round the sun in two hours and a half. it did, in fact, complete more than half its revolution around the sun in that short period, and it made more than three quarters of its circuit around the sun in one day. its velocity, when nearest the sun, exceeded a million of miles per hour, and its tail, at its greatest elongation, was one hundred and eight millions of miles; a length more than sufficient to have reached from the sun to the earth. its heat was estimated to be , times greater than that received by the earth from a vertical sun, and consequently it was more intense than that produced by the most powerful blowpipes, and sufficient to melt like wax the most infusible bodies. no doubt, when in the vicinity of the sun, the solid matter of the comet was first melted and then converted into vapor, which itself became red hot, or, more properly speaking, _white hot_. much discussion has arisen among astronomers respecting the periodic time of this comet. its most probable period is about years. distances of the stars.--i have already mentioned (page ) that the distance of at least one of the fixed stars has at length been determined, although at so great a distance that its annual parallax is only about one third of a second, implying a distance from the sun of nearly sixty millions of millions of miles. of a distance so immense the mind can form no adequate conception. the most successful effort towards it is made by gradual and successive approximations. let us, therefore, take the motion of a rail-way car as the most rapid with which we are familiar, and apply it first to the planetary spaces, and then to the vast interval that separates these nether worlds from the fixed stars. a rail-way car, travelling constantly night and day at the rate of twenty miles per hour, would make miles per day. at this rate, to travel around the earth on a great circle would require about days, and days to reach the moon. if we took our departure from the sun, and journeyed night and day, we should reach mercury in a little more than years, venus in nearly , and the earth in years; but to reach neptune, the outermost planet, would require , years. great as appear the dimensions of the solar system, when we imagine ourselves thus borne along from world to world, yet this space is small compared with that which separates us from the fixed stars; for to reach cygni it would take , , years. but this is believed, for certain satisfactory reasons, to be one of the nearest of the stars. several other stars whose parallax has been determined are at a much greater distance than cygni. the pole star is five times as far off; and the greater part of the stars are at distances inconceivably more remote. such, especially, are those which compose the faintest nebulæ. discovery of the planet neptune.--from the earliest ages down to the year , the solar system was supposed to terminate with the planet saturn, at the distance of nine hundred millions of miles from the sun; but the discovery of uranus added another world, and doubled the dimensions of the solar system. it seemed improbable that any more planets should exist at a distance still more remote, since such a body could hardly receive any of the vivifying influences of the central luminary. still, certain irregularities to which the uranus was subject, led to the suspicion that there exists a planet beyond it, which, by its attractions, caused these irregularities. impressed with this belief, two young astronomers of great genius, le verrier, of france, and adams, of england, applied themselves to the task of finding the hidden planet. the direction in which the disturbed body was moved afforded some clue to the part of the heavens where the disturbing body lay concealed; the kind of action it excited at different times indicated that it was beyond uranus, and not this side of that planet; and the magnitude of the forces it exerted gave some intimation of its size and mass. the law of distances from the sun which the superior planets observe (saturn being nearly twice the distance of jupiter, and uranus twice that of saturn), led both these astronomers to assume that the body sought was nearly double the distance of uranus from the sun. with these few and imperfect data, as so many leading-strings proceeding from the planet uranus, they felt their way into the abysses of space by the aid of two sure guides--the law of gravitation and the higher geometry. both astronomers arrived at nearly the same results, although they wrought independently of each other, and each, indeed, without the knowledge of the other. le verrier was the first to make public his conclusions, which he communicated to the french academy at their sitting, august , . they saw that there existed, at nearly double the distance of uranus from the sun, a planet larger than that body; that it lay near a certain star seen at that season in the southwest, in the evening sky; that, on account of its immense distance, it was invisible to the naked eye, and could be distinctly seen with a perceptible disk only by the most powerful telescopes; being no brighter than a star of the ninth magnitude, and subtending an angle of only three seconds. le verrier communicated these results to dr. galle, of berlin, with the request that he would search for the stranger with his powerful telescope, pointing out the exact spot in the heavens where it would be found. on the same evening, dr. galle directed his instrument to that part of the heavens, and immediately the planet presented itself to view, within one degree of the very spot assigned to it by le verrier. subsequent investigations have shown that its apparent size is within half a second of that which the same sagacious mind foresaw, and that its diameter is nearly equal to that of uranus, being , , while uranus is , miles.[ ] the distance from the sun is less than was predicted, being only about , instead of millions of miles; and its periodic time is - / , instead of years, as was supposed by le verrier. one satellite only has yet been discovered, and this was first seen by professor bond with the great telescope of harvard university. recent telescopic discoveries.--the great reflecting telescope of lord rosse, and the powerful refracting telescopes of pulkova and cambridge, have opened new fields of discovery to the delighted astronomer. a new satellite has been added to saturn, first revealed to the cambridge instrument, making the entire number of moons that adorn the nocturnal sky of that remarkable planet no less than eight. still more wonderful things have been disclosed among the remotest _nebulæ_. a number of these objects before placed among the irresolvable nebulæ, and supposed to consist not of stars, but of mere nebulous matter, have been resolved into stars; others, of which we before saw only a part, have revealed themselves under new and strange forms, one resembling an animal with huge branching arms, and hence called the _crab_ nebula; another imitating a scroll or vortex, and called the _whirlpool_ nebula; and other figures, which to ordinary telescopes appear only as dim specks on the confines of creation, are presented to these wonderful instruments as glorious firmaments of stars. in the year , sir john herschel left england for the cape of good hope, furnished with powerful instruments for observing the stars and nebulæ of the southern hemisphere, which had never been examined in a manner suited to disclose their full glories. this great astronomer and benefactor to science devoted five years of the most assiduous toil in observing and delineating the astronomical objects of that portion of the heavens. he had before extended the catalogue of nebulæ begun by his illustrious father, sir william herschel, to the number of ; and beginning at that point, he swelled the number, by his labors at the cape of good hope, to . he extended also the list of double stars from to , and showed that the luminous spots near the south pole, known to sailors by the name of the "magellan clouds," consist of an assemblage of several hundred brilliant nebulæ. the united states have contributed their full share to the recent progress of astronomy. powerful telescopes have been imported, made by the first european artists, and numerous others, of scarcely inferior workmanship and power, have been produced by artists of our own. the american astronomers have also been the first to bring the electric telegraph into use in astronomical observations; electric clocks have been so constructed as to beat simultaneously at places distant many hundred miles from each other, and thus to furnish means of determining the difference of longitude between places with an astonishing degree of accuracy; and facilities for recording observations on the stars have been devised which render the work vastly more rapid as well as more accurate than before. indeed, the inventive genius for which americans have been distinguished in all the useful arts seems now destined to be equally conspicuous in promoting the researches of science. footnotes: [ ] the names of all the asteroids known at present are as follows: . ceres. . metis. . psyche. . pallas. . hygeia. . melpomene. . juno. . parthenope. . fortuna. . vesta. . victoria. . massalia. . astræa. . egeria. . lutetia. . hebe. . irene. . calliope. . iris. . eunomia. . un-named. . flora. . thetis. [ ] sir john herschel, however, states its diameter at , miles index. a. alamak, aldebaran, alexandrian school, algenib, algol, alioth, almagest, altair, altitude, amplitude, anaxagoras, anaximander, andromeda, antares, antinous, apogee, apsides, aquarius, aquila, archimedes, arcturus, aries, aristotle, astrology, astronomers royal, , astronomical clock, astronomical tables, astronomy, history of, , atmosphere, , attraction, auriga, axis of the earth, azimuth, b. bacon, , base line, base of verification, bellatrix, betalgeus, bissextile, bootes, bouguer, bowditch, brahean system, c. cæsar, julius, calendar, grecian, gregorian, cancer, canis major, canis minor, capella, capricorn, cassiopeia, catalogues of the stars, central forces, cepheus, ceres, cetus, chronology, chronometers, circles, great and small, of diurnal revolution, of perpetual apparition, of perpetual occultation, vertical, clusters, colures, coma berenices, comet, biela's, encke's, halley's, comets, brightness of, comets, distances of, light of, magnitude of, mass of, motions of, number of, periods of, perturbations of, structure of, tails of, complement, conjunction, constellations, copernican system, , copernicus, , cor caroli, cor hydræ, corona borealis, corvus, crotona, crystalline spheres, cygnus, d. day, astronomical, sidereal, solar, days of the week, declination, deferents, denebola, distances of the heavenly bodies, how measured, distances of the stars, dolphin, double stars, draco, e. earth, diameter of the, ellipticity of the, figure of the, motion of the, orbit of the, eclipses, annular, calculation of, of the moon, of the sun, ecliptic, epicycles, equation of time, equations, periodical, secular, tabular, equator, equinoxes, precession of the, eudoxus, f. fomalhaut, fraunhofer, g. galaxy, galileo, abjuration of, condemnation of, life of, persecutions of, gemini, gemma, globes, artificial, gravitation, universal, gravity, terrestrial, h. hercules, herschel, sir wm., , , hesperus, hipparchus, horizon, rational, sensible, hour-circles, huyghens, i. inductive system, inquisition, instruments, astronomical, j. juno, jupiter, belts of, diameter of, distance of, eclipses of, magnitude of, satellites of, scenery of, telescopic view of, k. kepler, kepler's laws, l. latitude, how found, laws of motion, terrestrial gravity, leap year, leo, leo minor, libra, librations of the moon, light, velocity of, how measured, longitude, celestial, terrestrial, its importance, how found, by chronometers, by eclipses, by jupiter's satellites, by lunar method, lucifer, lynx, m. magnitudes, how measured, magellan clouds, mars, changes of, distance of, revolutions of, mecanique celeste, mercury, conjunctions of, diurnal revolution of, phases of, sidereal revolut'n of, synodical revolut'n of, transits of, meridian, meteoric showers, origin of, meteoric stones, metonic cycle, miletus, school of, milky way, mira, mirach, mizar, month, sidereal, synodical, moon, atmosphere of the, cusps of the, diameter of the, distance of the, eclipses of the, harvest, irregularities of the, librations of the, light of the, mountains in the, nodes of the, phases of the, revolutions of the, - scenery of the, telescopic appearance of the, volcanoes in the, volume of the, motion, laws of, motions of the planets, mural circle, n. nadir, nature of the stars, nebulæ, new planets, distances of, origin of, periods of, size of, new style, newton, , o. oblique sphere, obliquity of the ecliptic, effect of, on the seasons, how found, observatory, greenwich, - tycho's, old style, ophiucus, opposition, orion, orreries, , p. pallas, parallactic arc, parallax, , annual, horizontal, how found, parallel sphere, parallels of latitude, pegasus, pendulum, perigee, periodical inequalities, perseus, pisces, piscis australis, planets, distances of, inferior, magnitudes of, periods, superior, pleiades, pointers, polar distance, polaris, pole, of the earth, pollux, power of the deity, præsepe, precession, prime vertical, primum mobile, principia, procyon, projection of the sphere, proper motions of the stars, ptolemaic system, ptolemy, pythagoras, q. quadrant, r. radius, refraction, regulus, resolution of motion, resultant, revolution, annual, diurnal, rigel, right ascension, right sphere, s. sagittarius, saros, saturn, diameter of, ring of, satellites of, scenery of, scorpio, seasons, secondary, secular inequalities, serpent, sextant, sidereal day, month, signs, sirius, solstices, sphere, celestial, doctrine of the, oblique, parallel, right, terrestrial, spica, spots on the sun, cause of, dimensions of, number of, stability of the universe, stars, fixed, stylus, sun, attraction of the, density of the, diameter of the, distance of the, mass of the, nature and constitution of the, revolutions of the, sun, spots on the, volume of the, supplement, system of the world, - brahean, copernican, ptolemaic, t. tangent, taurus, telescope, the, achromatic, directions for using, dorpat, herschelian, history of, reflecting, temperature, changes of, temporary stars, terminator, , thales, tides, cause of, spring and neap, time, apparent, equation of, mean, sidereal, transits, triangulation, tropic, twilight, u. unity of the deity, uranus, diameter of, distance of, history of, period of, satellites of, scenery of, ursa major, ursa minor, v. variable stars, venus, conjunctions of, mountains of, phases of, revolutions of, transits of, vesta, vindemiatrix, virgo, y. year, astronomical, tropical, z. zenith, zenith distance, zodiac, zodiacal light, zones, recent discoveries. improvements in the telescope, rosse's leviathan telescope, pulkova and cambridge telescopes, improvements in instrumental measurements, new planets and asteroids, great comet of , distances of the stars, discovery of neptune, recent telescopic discoveries, longitude by the electric telegraph, * * * * * transcriber's notes obvious punctuation and spelling errors repaired. greek transliterations are inclosed by equals signs. inconsistent hyphenation has been repaired. characters that could not be fully expressed are "unpacked" and shown within braces, e.g. {oblong symbol}. in ambiguous cases, the text has been left as it appears in the original book. in particular many mismatched quotation marks, have not been changed. page , "knittingneedle" changed to "knitting needle". page , "trignometry" changed to "trigonometry". page , "dedecaedron" changed to "dodecaedron". page , "generrally" changed to "generally". the science of the stars by e. walter maunder, f.r.a.s. of the royal observatory, greenwich author of "astronomy without a telescope" "the astronomy of the bible," etc. london: t. c. & e. c. jack long acre, w.c., and edinburgh new york: dodge publishing co. {vii} contents chap. i. astronomy before history ii. astronomy before the telescope iii. the law of gravitation iv. astronomical measurements v. the members of the solar system vi. the system of the stars index { } the science of the stars chapter i astronomy before history the plan of the present series requires each volume to be complete in about eighty small pages. but no adequate account of the achievements of astronomy can possibly be given within limits so narrow, for so small a space would not suffice for a mere catalogue of the results which have been obtained; and in most cases the result alone would be almost meaningless unless some explanation were offered of the way in which it had been reached. all, therefore, that can be done in a work of the present size is to take the student to the starting-point of astronomy, show him the various roads of research which have opened out from it, and give a brief indication of the character and general direction of each. that which distinguishes astronomy from all the other sciences is this: it deals with objects that we cannot touch. the heavenly bodies are beyond our reach; we cannot tamper with them, or subject them to any form of experiment; we cannot bring them into our laboratories to analyse or dissect them. we can only watch them and wait for such indications as their { } own movements may supply. but we are confined to this earth of ours, and they are so remote; we are so short-lived, and they are so long-enduring; that the difficulty of finding out much about them might well seem insuperable. yet these difficulties have been so far overcome that astronomy is the most advanced of all the sciences, the one in which our knowledge is the most definite and certain. all science rests on sight and thought, on ordered observation and reasoned deduction; but both sight and thought were earlier trained to the service of astronomy than of the other physical sciences. it is here that the highest value of astronomy lies; in the discipline that it has afforded to man's powers of observation and reflection; and the real triumphs which it has achieved are not the bringing to light of the beauties or the sensational dimensions and distances of the heavenly bodies, but the vanquishing of difficulties which might well have seemed superhuman. the true spirit of the science can be far better exemplified by the presentation of some of these difficulties, and of the methods by which they have been overcome, than by many volumes of picturesque description or of eloquent rhapsody. there was a time when men knew nothing of astronomy; like every other science it began from zero. but it is not possible to suppose that such a state of things lasted long, we know that there was a time when men had noticed that there were two great lights in the sky--a greater light that shone by day, a lesser light that shone by night--and there were the stars also. and this, the earliest observation of primitive astronomy, is preserved for us, expressed in the simplest possible language, in the first chapter of the first book { } of the sacred writings handed down to us by the hebrews. this observation, that there are bodies above us giving light, and that they are not all equally bright, is so simple, so inevitable, that men must have made it as soon as they possessed any mental power at all. but, once made, a number of questions must have intruded themselves: "what are these lights? where are they? how far are they off?" many different answers were early given to these questions. some were foolish; some, though intelligent, were mistaken; some, though wrong, led eventually to the discovery of the truth. many myths, many legends, some full of beauty and interest, were invented. but in so small a book as this it is only possible to glance at those lines of thought which eventually led to the true solution. as the greater light, the lesser light, and the stars were carefully watched, it was seen not only that they shone, but that they appeared to move; slowly, steadily, and without ceasing. the stars all moved together like a column of soldiers on the march, not altering their positions relative to each other. the lesser light, the moon, moved with the stars, and yet at the same time among them. the greater light, the sun, was not seen with the stars; the brightness of his presence made the day, his absence brought the night, and it was only during his absence that the stars were seen; they faded out of the sky before he came up in the morning, and did not reappear again until after he passed out of sight in the evening. but there came a time when it was realised that there were stars shining in the sky all day long as well as at night, and this discovery was one of the greatest and most important ever made, { } because it was the earliest discovery of something quite unseen. men laid hold of this fact, not from the direct and immediate evidence of their senses, but from reflection and reasoning. we do not know who made this discovery, nor how long ago it was made, but from that time onward the eyes with which men looked upon nature were not only the eyes of the body, but also the eyes of the mind. it followed from this that the sun, like the moon, not only moved with the general host of the stars, but also among them. if an observer looks out from any fixed station and watches the rising of some bright star, night after night, he will notice that it always appears to rise in the same place; so too with its setting. from any given observing station the direction in which any particular star is observed to rise or set is invariable. not so with the sun. we are accustomed to say that the sun rises in the east and sets in the west. but the direction in which the sun rises in midwinter lies far to the south of the east point; the direction in which he rises in midsummer lies as far to the north. the sun is therefore not only moving with the stars, but among them. this gradual change in the position of the sun in the sky was noticed in many ancient nations at an early time. it is referred to in job xxxviii. : "hast thou commanded the morning since thy days; and caused the dayspring to know his place?" and the apparent path of the sun on one day is always parallel to its path on the days preceding and following. when, therefore, the sun rises far to the south of east, he sets correspondingly far to the south of west, and at noon he is low down in the south. his course during the day is a short one, and the daylight { } is much shorter than the night, and the sun at noon, being low down in the sky, has not his full power. the cold and darkness of winter, therefore, follows directly upon this position of the sun. these conditions are reversed when the sun rises in the north-east. the night is short, the daylight prolonged, and the sun, being high in the heavens at noon, his heat is felt to the full. thus the movements of the sun are directly connected with the changes of season upon the earth. but the stars also are connected with those seasons; for if we look out immediately after it has become dark after sunset, we shall notice that the stars seen in the night of winter are only in part those seen in the nights of summer. in the northern part of the sky there are a number of stars which are always visible whenever we look out, no matter at what time of the night nor what part of the year. if we watch throughout the whole night, we see that the whole heavens appear to be slowly turning--turning, as if all were in a single piece--and the pivot about which it is turning is high up in the northern sky. the stars, therefore, are divided into two classes. those near this invisible pivot--the "pole" of the heavens, as we term it--move round it in complete circles; they never pass out of sight, but even when lowest they clear the horizon. the other stars move round the same pivot in curved paths, which are evidently parts of circles, but circles of which we do not see the whole. these stars rise on the eastern side of the heavens and set on the western, and for a greater or less space of time are lost to sight below the horizon. and some of these stars are visible at one time of the year, others at another; some being seen during the { } whole of the long nights of winter, others throughout the short nights of summer. this distinction again, and its connection with the change of the seasons on the earth, was observed many ages ago. it is alluded to in job xxxviii. : "canst thou lead forth the signs of the zodiac in their season, or canst thou guide the bear with her train?" (r.v., margin). the signs of the zodiac are taken as representing the stars which rise and set, and therefore have each their season for being "led forth," while the northern stars, which are always visible, appearing to be "guided" in their continual movement round the pole of the sky in perfect circles, are represented by "the bear with her train." the changes in position of the sun, the greater light, must have attracted attention in the very earliest ages, because these changes are so closely connected with the changes of the seasons upon the earth, which affect men directly. the moon, the lesser light, goes through changes of position like the sun, but these are not of the same direct consequence to men, and probably much less notice was taken of them. but there were changes of the moon which men could not help noticing--her changes of shape and brightness. one evening she may be seen soon after the sun has set, as a thin arch of light, low down in the sunset sky. on the following evenings she is seen higher and higher in the sky, and the bow of light increases, until by the fourteenth day it is a perfect round. then the moon begins to diminish and to disappear, until, on the twenty-ninth or thirtieth day after the first observation, she is again seen in the west after sunset as a narrow crescent. this succession of changes gave men an important measure of time, and, in an age when artificial means of light were difficult to procure, moonlight was of the greatest { } value, and the return of the moonlit portion of the month was eagerly looked for. these early astronomical observations were simple and obvious, and of great practical value. the day, month, and year were convenient measures of time, and the power of determining, from the observation of the sun and of the stars, how far the year had progressed was most important to farmers, as an indication when they should plough and sow their land. such observations had probably been made independently by many men and in many nations, but in one place a greater advance had been made. the sun and moon are both unmistakable, but one star is very like another, and, for the most part, individual stars can only be recognised by their positions relative to others. the stars were therefore grouped together into +constellations+ and associated with certain fancied designs, and twelve of these designs were arranged in a belt round the sky to mark the apparent path of the sun in the course of the year, these twelve being known as the "+signs of the zodiac+"--the ram, bull, twins, crab, lion, virgin, balance, scorpion, archer, goat, water-pourer, and fishes. in the rest of the sky some thirty to thirty-six other groups, or constellations, were formed, the bear being the largest and brightest of the constellations of the northern heavens. but these ancient constellations do not cover the entire heavens; a large area in the south is untouched by them. and this fact affords an indication both of the time when and the place where the old stellar groups were designed, for the region left untouched was the region below the horizon of ° north latitude, about years ago. it is probable, therefore, that the ancient astronomers who carried out this great work { } lived about b.c., and in north latitude ° or °. the indication is only rough, but the amount of uncertainty is not very large; the constellations must be at least years old, they cannot be more than . all this was done by prehistoric astronomers; though no record of the actual carrying out of the work and no names of the men who did it have come down to us. but it is clear from the fact that the signs of the zodiac are arranged so as to mark out the annual path of the sun, and that they are twelve in number--there being twelve months in the year--that those who designed the constellations already knew that there are stars shining near the sun in full daylight, and that they had worked out some means for determining what stars the sun is near at any given time. another great discovery of which the date and the maker are equally unknown is referred to in only one of the ancient records available to us. it was seen that all along the eastern horizon, from north to south, stars rise, and all along the western horizon, from north to south, stars set. that is what was seen; it was the fact observed. there is no hindrance anywhere to the movement of the stars--they have a free passage under the earth; the earth is unsupported in space. that is what was _thought_; it was the inference drawn. or, as it is written in job xxvi. , "he (god) stretcheth out the north over empty space, and hangeth the earth upon nothing." the earth therefore floats unsupported in the centre of an immense star-spangled sphere. and what is the shape of the earth? the natural and correct inference is that it is spherical, and we find in some of the early greek writers the arguments which establish this inference as clearly set forth as they would be to-day. { } the same inference followed, moreover, from the observation of a simple fact, namely, that the stars as observed from any particular place all make the same angle with the horizon as they rise in the east, and all set at the same angle with it in the west; but if we go northward, we find that angle steadily decreasing; if we go southward, we find it increasing. but if the earth is round like a globe, then it must have a definite size, and that size can be measured. the discoveries noted above were made by men whose names have been lost, but the name of the first person whom we know to have measured the size of the earth was eratosthenes. he found that the sun was directly overhead at noon at midsummer at syene (the modern assouan), in egypt, but was ° south of the "zenith"--the point overhead--at alexandria, and from this he computed the earth to be , stadia (a stadium = feet) in circumference. another consequence of the careful watch upon the stars was the discovery that five of them were planets; "wandering" stars; they did not move all in one piece with the rest of the celestial host. in this they resemble the sun and moon, and they further resemble the moon in that, though too small for any change of shape to be detected, they change in brightness from time to time. but their movements are more complicated than those of the other heavenly bodies. the sun moves a little slower than the stars, and so seems to travel amongst them from west to east; the moon moves much slower than the stars, so her motion from west to east is more pronounced than that of the sun. but the five planets sometimes move slower than the stars, sometimes quicker, and sometimes at the same rate. two of the five, which we now know as mercury { } and venus, never move far from the sun, sometimes being seen in the east before he rises in the morning, and sometimes in the west after he has set in the evening. mercury is the closer to the sun, and moves more quickly; venus goes through much the greater changes of brightness. jupiter and saturn move nearly at the same average rate as the stars, saturn taking about thirteen days more than a year to come again to the point of the sky opposite to the sun, and jupiter about thirty-four days. mars, the fifth planet, takes two years and fifty days to accomplish the same journey. these planetary movements were not, like those of the sun and moon and stars, of great and obvious consequence to men. it was important to men to know when they would have moonlight nights, to know when the successive seasons of the year would return. but it was no help to men to know when venus was at her brightest more than when she was invisible. she gave them no useful light, and she and her companion planets returned at no definite seasons. nevertheless, men began to make ordered observations of the planets--observations that required much more patience and perseverance than those of the other celestial lights. and they set themselves with the greatest ingenuity to unravel the secret of their complicated and seemingly capricious movements. this was a yet higher development than anything that had gone before, for men were devoting time, trouble, and patient thought, for long series of years, to an inquiry which did not promise to bring them any profit or advantage. yet the profit which it actually did bring was of the highest order. it developed men's mental powers; it led to the devising of { } instruments of precision for the observations; it led to the foundation of mathematics, and thus lay at the root of all our modern mechanical progress. it brought out, in a higher degree, ordered observation and ordered thought. { } chapter ii astronomy before the telescope there was thus a real science of astronomy before we have any history of it. some important discoveries had been made, and the first step had been taken towards cataloguing the fixed stars. it was certainly known to some of the students of the heavens, though perhaps only to a few, that the earth was a sphere, freely suspended in space, and surrounded on all sides by the starry heavens, amongst which moved the sun, moon, and the five planets. the general character of the sun's movement was also known; namely, that he not only moved day by day from east to west, as the stars do, but also had a second motion inclined at an angle to the first, and in the opposite direction, which he accomplished in the course of a year. to this sum of knowledge, no doubt, several nations had contributed. we do not know to what race we owe the constellations, but there are evidences of an elementary acquaintance with astronomy on the part of the chinese, the babylonians, the egyptians, and the jews. but in the second stage of the development of the science the entire credit for the progress made belongs to the greeks. the greeks, as a race, appear to have been very little apt at originating ideas, but they possessed, beyond all other races, the power of developing and perfecting crude ideas which they had obtained from other sources, { } and when once their attention was drawn to the movements of the heavenly bodies, they devoted themselves with striking ingenuity and success to devising theories to account for the appearances presented, to working out methods of computation, and, last, to devising instruments for observing the places of the luminaries in which they were interested. in the brief space available it is only possible to refer to two or three of the men whose commanding intellects did so much to help on the development of the science. eudoxus of knidus, in asia minor ( - b.c.), was, so far as we know, the first to attempt to represent the movements of the heavenly bodies by a simple mathematical process. his root idea was something like this. the earth was in the centre of the universe, and it was surrounded, at a great distance from us, by a number of invisible transparent shells, or spheres. each of these spheres rotated with perfect uniformity, though the speed of rotation differed for different spheres. one sphere carried the stars, and rotated from east to west in about h. m. the sun was carried by another sphere, which rotated from west to east in a year, but the pivots, or poles, of this sphere were carried by a second, rotating exactly like the sphere of the stars. this explained how it is that the ecliptic--that is to say, the apparent path of the sun amongst the stars--is inclined -½° to the equator of the sky, so that the sun is -½° north of the equator at midsummer and -½° south of the equator at midwinter, for the poles of the sphere peculiar to the sun were supposed to be -½° from the poles of the sphere peculiar to the stars. then the moon had three spheres; that which actually carried the moon having its poles ° from the poles of the sphere peculiar to the { } sun. these poles were carried by a sphere placed like the sphere of the sun, but rotating in days; and this, again, had its poles in the sphere of the stars. the sphere carrying the moon afforded the explanation of the wavy motion of the moon to and fro across the ecliptic in the course of a month, for at one time in the month the moon is ° north of the ecliptic, at another time ° south. the motions of the planets were more difficult to represent, because they not only have a general daily motion from east to west, like the stars, and a general motion from west to east along the ecliptic, like the sun and moon, but from time to time they turn back on their course in the ecliptic, and "retrograde." but the introduction of a third and fourth sphere enabled the motions of most of the planets to be fairly represented. there were thus twenty-seven spheres in all--four for each of the five planets, three for the moon, three for the sun (including one not mentioned in the foregoing summary), and one for the stars. these spheres were not, however, supposed to be solid structures really existing; the theory was simply a means for representing the observed motions of the heavenly bodies by computations based upon a series of uniform movements in concentric circles. but this assumption that each heavenly body moves in its path at a uniform rate was soon seen to be contrary to fact. a reference to the almanac will show at once that the sun's movement is not uniform. thus for the year - the solstices and equinoxes fell as given on the next page: { } _epoch time interval_ winter solstice dec. d. h. m. p.m. d. h. m. spring equinox mar. " " " p.m. " " " summer solstice l june " " " p.m. " " " autumn equinox sept. " " " a.m. " " " winter solstice dec. " " " p.m. so that the winter half of the year is shorter than the summer half; the sun moves more quickly over the half of its orbit which is south of the equator than over the half which is north of it. the motion of the moon is more irregular still, as we can see by taking out from the almanac the times of new and full moon: _new moon interval to full moon_ dec. d. h. . m. p.m. d. h. . m. " " " " . " p.m. " " . " jan. " " . " a.m. " " . " march " " " . " a.m. " " . " " " " " . " p.m. " " . " april " " " . " p.m. " " . " may " " " . " a.m. " " . " june " " " . " p.m. " " . " july " " " . " p.m. " " . " aug. " " " . " a.m. " " . " sept. " " " . " p.m. " " . " oct. " " " . " a.m. " " . " nov. " " " . " p.m. " " . " dec. " " " . " p.m. " " . " { } _full moon interval to new moon_ dec. d h. . m. a.m. d. h. . m. jan. " " . " p.m. " " . " feb. " " " . " a.m. " " . " march " " " . " p.m. " " . " april " " " . " p.m. " " . " may " " " . " a.m. " " . " june " " " . " p.m. " " . " july " " " . " p.m. " " . " aug. " " " . " a.m. " " . " sept. " " " . " p.m. " " . " oct. " " " . " a.m. " " . " nov. " " " . " p.m. " " . " dec. " " " . " a.m. " " . " jan. " " . " p.m. " " . " the astronomer who dealt with this difficulty was hipparchus (about - b.c.), who was born at nicæa, in bithynia, but made most of his astronomical observations in rhodes. he attempted to explain these irregularities in the motions of the sun and moon by supposing that though they really moved uniformly in their orbits, yet the centre of their orbits was not the centre of the earth, but was situated a little distance from it. this point was called "+the excentric+," and the line from the excentric to the earth was called "+the line of apsides+." but when he tried to deal with the movements of the planets, he found that there were not enough good observations available for him to build up any satisfactory theory. he therefore devoted himself to the work of making systematic determinations of the places of the planets that he might put his successors in a better position to deal with the problem than he was. his great successor was claudius ptolemy of { } alexandria, who carried the work of astronomical observation from about a.d. to . he was, however, much greater as a mathematician than as an observer, and he worked out a very elaborate scheme, by which he was able to represent the motions of the planets with considerable accuracy. the system was an extremely complex one, but its principle may be represented as follows: if we suppose that a planet is moving round the earth in a circle at a uniform rate, and we tried to compute the place of the planet on this assumption for regular intervals of time, we should find that the planet gradually got further and further away from the predicted place. then after a certain time the error would reach a maximum, and begin to diminish, until the error vanished and the planet was in the predicted place at the proper time. the error would then begin to fall in the opposite direction, and would increase as before to a maximum, subsequently diminishing again to zero. this state of things might be met by supposing that the planet was not itself carried by the circle round the earth, but by an +epicycle+--_i.e._ a circle travelling upon the first circle--and by judiciously choosing the size of the epicycle and the time of revolution the bulk of the errors in the planet's place might be represented. but still there would be smaller errors going through their own period, and these, again, would have to be met by imagining that the first epicycle carried a second, and it might be that the second carried a third, and so on. the ptolemaic system was more complicated than this brief summary would suggest, but it is not possible here to do more than indicate the general principles upon which it was founded, and the numerous other systems or modifications of them produced in the { } five centuries from eudoxus to ptolemy must be left unnoticed. the point to be borne in mind is that one fundamental assumption underlay them all, an assumption fundamental to all science--the assumption that like causes must always produce like effects. it was apparent to the ancient astronomers that the stars--that is to say, the great majority of the heavenly bodies--do move round the earth in circles, and with a perfect uniformity of motion, and it seemed inevitable that, if one body moved round another, it should thus move. for if the revolving body came nearer to the centre at one time and receded at another, if it moved faster at one time and slower at another, then, the cause remaining the same, the effect seemed to be different. any complexity introduced by superposing one epicycle upon another seemed preferable to abandoning this great fundamental principle of the perfect uniformity of the actings of nature. for more than years the ptolemaic system remained without serious challenge, and the next great name that it is necessary to notice is that of copernicus ( - ). copernicus was a canon of frauenburg, and led the quiet, retired life of a student. the great work which made him immortal, _de revolutionibus_, was the result of many years' meditation and work, and was not printed until he was on his deathbed. in this work copernicus showed that he was one of those great thinkers who are able to look beyond the mere appearance of things and to grasp the reality of the unseen. copernicus realised that the appearance would be just the same whether the whole starry vault rotated every twenty-four hours round an immovable earth from east to west or the earth rotated from west to east in the midst of the starry sphere; and, as the { } stars are at an immeasurable distance, the latter conception was much the simpler. extending the idea of the earth's motion further, the supposition that, instead of the sun revolving round a fixed earth in a year, the earth revolved round a fixed sun, made at once an immense simplification in the planetary motions. the reason became obvious why mercury and venus were seen first on one side of the sun and then on the other, and why neither of them could move very far from the sun; their orbits were within the orbit of the earth. the stationary points and retrogressions of the planets were also explained; for, as the earth was a planet, and as the planets moved in orbits of different sizes, the outer planets taking a longer time to complete a revolution than the inner, it followed, of necessity, that the earth in her motion would from time to time be passed by the two inner planets, and would overtake the three outer. the chief of the ptolemaic epicycles were done away with, and all the planets moved continuously in the same direction round the sun. but no planet's motion could be represented by uniform motion in a single circle, and copernicus had still to make use of systems of epicycles to account for the deviations from regularity in the planetary motions round the sun. the earth having been abandoned as the centre of the universe, a further sacrifice had to be made: the principle of uniform motion in a circle, which had seemed so necessary and inevitable, had also to be given up. for the time came when the instruments for measuring the positions of the stars and planets had been much improved, largely due to tycho brahe ( - ), a dane of noble birth, who was the keenest and most careful observer that astronomy had yet produced. { } his observations enabled his friend and pupil, johann kepler, ( - ), to subject the planetary movements to a far more searching examination than had yet been attempted, and he discovered that the sun is in the plane of the orbit of each of the planets, and also in its +line of apsides+--that is to say, the line joining the two points of the orbit which are respectively nearest and furthest from the sun. copernicus had not been aware of either of these two relations, but their discovery greatly strengthened the copernican theory. then for many years kepler tried one expedient after another in order to find a combination of circular motions which would satisfy the problem before him, until at length he was led to discard the circle and try a different curve--the oval or ellipse. now the property of a circle is that every point of it is situated at the same distance from the centre, but in an ellipse there are two points within it, the "foci," and the sum of the distances of any point on the circumference from these two foci is constant. if the two foci are at a great distance from each other, then the ellipse is very long and narrow; if the foci are close together, the ellipse differs very little from a circle; and if we imagine that the two foci actually coincide, the ellipse becomes a circle. when kepler tried motion in an ellipse instead of motion in a circle, he found that it represented correctly the motions of all the planets without any need for epicycles, and that in each case the sun occupied one of the foci. and though the planet did not move at a uniform speed in the ellipse, yet its motion was governed by a uniform law, for the straight line joining the planet to the sun, the "+radius vector+," passed over equal areas of space in equal periods of time. { } these two discoveries are known as kepler's first and second laws. his third law connects all the planets together. it was known that the outer planets not only take longer to revolve round the sun than the inner, but that their actual motion in space is slower, and kepler found that this actual speed of motion is inversely as the square root of its distance from the sun; or, if the square of the speed of a planet be multiplied by its distance from the sun, we get the same result in each case. this is usually expressed by saying that the cube of the distance is proportional to the square of the time of revolution. thus the varying rate of motion of each planet in its orbit is not only subject to a single law, but the very different speeds of the different planets are also all subject to a law that is the same for all. thus the whole of the complicated machinery of ptolemy had been reduced to three simple laws, which at the same time represented the facts of observation much better than any possible development of the ptolemaic mechanism. on his discovery of his third law kepler had written: "the book is written to be read either now or by posterity--i care not which; it may well wait a century for a reader, as god has waited years for an observer." twelve years after his death, on christmas day (old style), near grantham, in lincolnshire, the predestined "reader" was born. the inner meaning of kepler's three laws was brought to light by isaac newton. { } chapter iii the law of gravitation the fundamental thought which, recognised or not, had lain at the root of the ptolemaic system, as indeed it lies at the root of all science, was that "like causes must always produce like effects." upon this principle there seemed to the ancient astronomers no escape from the inference that each planet must move at a uniform speed in a circle round its centre of motion. for, if there be any force tending to alter the distance of the planet from that centre, it seemed inevitable that sooner or later it should either reach that centre or be indefinitely removed from it. if there be no such force, then the planet's distance from that centre must remain invariable, and if it move at all, it must move in a circle; move uniformly, because there is no force either to hasten or retard it. uniform motion in a circle seemed a necessity of nature. but all this system, logical and inevitable as it had once seemed, had gone down before the assault of observed facts. the great example of uniform circular motion had been the daily revolution of the star sphere; but this was now seen to be only apparent, the result of the rotation of the earth. the planets revolved round the sun, but the sun was not in the centre of their motion; they moved, not in circles, but in ellipses; not at a uniform speed, but at a speed which diminished with the increase of their distance from { } the sun. there was need, therefore, for an entire revision of the principles upon which motion was supposed to take place. the mistake of the ancients had been that they supposed that continued motion demanded fresh applications of force. they noticed that a ball, set rolling, sooner or later came to a stop; that a pendulum, set swinging, might swing for a good time, but eventually came to rest; and, as the forces that were checking the motion--that is to say, the friction exercised by the ground, the atmosphere, and the like--did not obtrude themselves, they were overlooked. newton brought out into clear statement the true conditions of motion. a body once moving, if acted upon by no force whatsoever, must continue to move forward in a straight line at exactly the same speed, and that for ever. it does not require any maintaining force to keep it going. if any change in its speed or in its direction takes place, that change must be due to the introduction of some further force. this principle, that, if no force acts on a body in motion, it will continue to move uniformly in a straight line, is newton's first law of motion. his second lays it down that, if force acts on a body, it produces a change of motion proportionate to the force applied, and in the same direction. and the third law states that when one body exerts force upon another, that second body reacts with equal force upon the first. the problem of the motions of the planets was, therefore, not what kept them moving, but what made them deviate from motion in a straight line, and deviate by different amounts. it was quite clear, from the work of kepler, that the force deflecting the planets from uniform motion in a { } straight line lay in the sun. the facts that the sun lay in the plane of the orbits of all the planets, that the sun was in one of the foci of each of the planetary ellipses, that the straight line joining the sun and planet moved for each planet over equal areas in equal periods of time, established this fact clearly. but the amount of deflection was very different for different planets. thus the orbit of mercury is much smaller than that of the earth, and is travelled over in a much shorter time, so that the distance by which mercury is deflected in a course of an hour from movement in a straight line is much greater than that by which the earth is deflected in the same time, mercury falling towards the sun by about miles, whilst the fall of the earth is only about . miles. the force drawing mercury towards the sun is therefore . times that drawing the earth, but . is the square of . , and the earth is . times as far from the sun as mercury. similarly, the fall in an hour of jupiter towards the sun is about . miles, so that the force drawing the earth is times that drawing jupiter towards the sun. but is the square of . , and jupiter is . times as far from the sun as the earth. similarly with the other planets. the force, therefore, which deflects the planets from motion in a straight line, and compels them to move round the sun, is one which varies inversely as the square of the distance. but the sun is not the only attracting body of which we know. the old ptolemaic system was correct to a small extent; the earth is the centre of motion for the moon, which revolves round it at a mean distance of , miles, and in a period of d. h. m. hence the circumference of her orbit is , , miles, and the length of the straight line which she would travel { } in one second of time, if not deflected by the earth, is feet. in this distance the deviation of a circle from a straight line is one inch divided by . . but we know from experiment that a stone let fall from a height of inches above the earth's surface will reach the ground in exactly one second of time. the force drawing the stone to the earth, therefore, is x . ; _i.e._ times as great as that drawing the moon. but the stone is only / of a mile from the earth's surface, while the moon is , miles away--more than million times as far. the force, therefore, would seem not to be diminished in the proportion that the distance is increased--much less in the proportion of its square. but newton proved that a sphere of uniform density, or made up of any number of concentric shells of uniform density, attracted a body outside itself, just as if its entire mass was concentrated at its centre. the distance of the stone from the earth must therefore be measured, not from the earth's surface, but from its centre; in other words, we must consider the stone as being distant from the earth, not some feet, but miles. this is very nearly one-sixtieth of the moon's distance, and the square of is . the earth's pull upon the moon, therefore, is almost exactly in the inverse square of the distance as compared with its pull on the stone. kepler's book had found its "reader." his three laws were but three particular aspects of newton's great discovery that the planets moved under the influence of a force, lodged in the sun, which varied inversely as the square of their distances from it. but newton's work went far beyond this, for he showed that the same law governed the motion of the moon round the { } earth and the motions of the satellites revolving round the different planets, and also governed the fall of bodies upon the earth itself. it was universal throughout the solar system. the law, therefore, is stated as of universal application. "every particle of matter in the universe attracts every other particle with a force varying inversely as the square of the distance between them, and directly as the product of the masses of the two particles." and newton further proved that if a body, projected in free space and moving with any velocity, became subject to a central force acting, like gravitation, inversely as the square of the distance, it must revolve in an ellipse, or in a closely allied curve. these curves are what are known as the "+conic sections+"--that is, they are the curves found when a cone is cut across in different directions. their relation to each other may be illustrated thus. if we have a very powerful light emerging from a minute hole, then, if we place a screen in the path of the beam of light, and exactly at right angles to its axis, the light falling on the screen will fill an exact circle. if we turn the screen so as to be inclined to the axis of the beam, the circle will lengthen out in one direction, and will become an ellipse. if we turn the screen still further, the ellipse will lengthen and lengthen, until at last, when the screen has become parallel to one of the edges of the beam of light, the ellipse will only have one end; the other will be lost. for it is clear that that edge of the beam of light which is parallel to the screen can never meet it. the curve now shown on the screen is called a +parabola+, and if the screen is turned further yet, the boundaries of the light falling upon it become divergent, and we have a fourth curve, the +hyperbola+. bodies moving under the influence of { } gravitation can move in any of these curves, but only the circle and ellipse are closed orbits. a particle moving in a parabola or hyperbola can only make one approach to its attracting body; after such approach it continually recedes from it. as the circle and parabola are only the two extreme forms of an ellipse, the two foci being at the same point for the circle and at an infinite distance apart for the parabola, we may regard all orbits under gravitation as being ellipses of one form or another. from his great demonstration of the law of gravitation, newton went on to apply it in many directions. he showed that the earth could not be truly spherical in shape, but that there must be a flattening of its poles. he showed also that the moon, which is exposed to the attractions both of the earth and of the sun, and, to a sensible extent, of some of the other planets, must show irregularities in her motion, which at that time had not been noticed. the moon's orbit is inclined to that of the earth, cutting its plane in two opposite points, called the "+nodes+." it had long been observed that the position of the nodes travelled round the ecliptic once in about nineteen years. newton was able to show that this was a consequence of the sun's attraction upon the moon. and he further made a particular application of the principle thus brought out, for, the earth not being a true sphere, but flattened at the poles and bulging at the equator, the equatorial belt might be regarded as a compact ring of satellites revolving round the earth's equator. this, therefore, would tend to retrograde precisely as the nodes of a single satellite would, so that the axis of the equatorial belt of the earth--in other words, the axis of the earth--must revolve round the pole of the ecliptic. { } consequently the pole of the heavens appears to move amongst the stars, and the point where the celestial equator crosses the equator necessarily moves with it. this is what we know as the "+precession of the equinoxes+," and it is from our knowledge of the fact and the amount of precession that we are able to determine roughly the date when the first great work of astronomical observation was accomplished, namely, the grouping of the stars into constellations by the astronomers of the prehistoric age. the publication of newton's great work, the _principia_ (_the mathematical principles of natural philosophy_), in which he developed the laws of motion, the significance of kepler's three planetary laws, and the law of universal gravitation, took place in , and was due to his friend edmund halley, to whom he had confided many of his results. that he was the means of securing the publication of the _principia_ is halley's highest claim to the gratitude of posterity, but his own work in the field which newton had opened was of great importance. newton had treated +comets+ as moving in parabolic orbits, and halley, collecting all the observations of comets that were available to him, worked out the particulars of their orbits on this assumption, and found that the elements of three were very closely similar, and that the interval between their appearances was nearly the same, the comets having been seen in , , and . on further consulting old records he found that comets had been observed in , , and . he concluded that these were different appearances of the same object, and predicted that it would be seen again in , or, according to a later and more careful computation, in . as the time for its return drew near, clairaut { } computed with the utmost care the retardation which would be caused to the comet by the attractions of jupiter and saturn. the comet made its predicted nearest approach to the sun on march , , just one month earlier than clairaut had computed. but in its next return, in , the computations effected by pontÉcoulant were only two days in error, so carefully had the comet been followed during its unseen journey to the confines of the solar system and back again, during a period of seventy-five years. pontécoulant's exploit was outdone at the next return by drs. cowell and crommelin, of greenwich observatory, who not only computed the time of its perihelion passage--that is to say, its nearest approach to the sun--for april , , but followed the comet back in its wanderings during all its returns to the year b.c. halley's comet, therefore, was the first comet that was known to travel in a closed orbit and to return to the neighbourhood of the sun. not a few small or telescopic comets are now known to be "periodic," but halley's is the only one which has made a figure to the naked eye. notices of it occur not a few times in history; it was the comet "like a flaming sword" which josephus described as having been seen over jerusalem not very long before the destruction by titus. it was also the comet seen in the spring of the year when william the conqueror invaded england, and was skilfully used by that leader as an omen of his coming victory. the law of gravitation had therefore enabled men to recognise in halley's comet an addition to the number of the primary bodies in the solar system--the first addition that had been made since prehistoric times. on march , , sir william herschel { } detected a new object, which he at first supposed to be a comet, but afterwards recognised as a planet far beyond the orbit of saturn. this planet, to which the name of uranus was finally given, had a mean distance from the sun nineteen times that of the earth, and a diameter four times as great. this was a second addition to the solar system, but it was a discovery by sight, not by deduction. the first day of the nineteenth century, january , , was signalised by the discovery of a small planet by piazzi. the new object was lost for a time, but it was redetected on december of the same year. this planet lay between the orbits of mars and jupiter--a region in which many hundreds of other small bodies have since been found. the first of these "+minor planets+" was called ceres; the next three to be discovered are known as pallas, juno, and vesta. beside these four, two others are of special interest: one, eros, which comes nearer the sun than the orbit of mars--indeed at some oppositions it approaches the earth within , , miles, and is therefore, next to the moon, our nearest neighbour in space; the other, achilles, moves at a distance from the sun equal to that of jupiter. ceres is much the largest of all the minor planets; indeed is larger than all the others put together. yet the earth exceeds ceres times in volume, and times in mass, and the entire swarm of minor planets, all put together, would not equal in total volume one-fiftieth part of the moon. the search for these small bodies rendered it necessary that much fuller and more accurate maps of the stars should be made than had hitherto been attempted, and this had an important bearing on the next great event in the development of gravitational astronomy. { } the movements of uranus soon gave rise to difficulties. it was found impossible, satisfactorily, to reconcile the earlier and later observations, and in the tables of uranus, published by bouvard in , the earlier observations were rejected. but the discrepancies between the observed and calculated places for the planet soon began to reappear and quickly increase, and the suggestion was made that these discrepancies were due to an attraction exercised by some planet as yet unknown. thus mrs. somerville in a little book on the connection of the physical sciences, published in , wrote, "possibly it (that is, uranus) may be subject to disturbances from some unseen planet revolving about the sun beyond the present boundaries of our system. if, after the lapse of years, the tables formed from a combination of numerous observations should still be inadequate to represent the motions of uranus, the discrepancies may reveal the existence, nay, even the mass and orbit of a body placed for ever beyond the sphere of vision." in john c. adams, who had just graduated as senior wrangler at cambridge, proceeded to attack the problem of determining the position, orbit, and mass of the unknown body by which on this assumption uranus was disturbed, from the irregularities evident in the motion of that planet. the problem was one of extraordinary intricacy, but by september adams had obtained a first solution, which, he submitted to airy, the astronomer royal. as, however, he neglected to reply to some inquiries made by airy, no search for the new planet was instituted in england until the results of a new and independent worker had been published. the same problem had been attacked by a well-known and very gifted french mathematician, u. j. j. leverrier, and { } in june he published his position for the unseen planet, which proved to be in close accord with that which adams had furnished to airy nine months before. on this airy stirred up challis, the director of the cambridge observatory, which then possessed the most powerful telescope in england, to search for the planet, and challis commenced to make charts, which included more than stars, in order to make sure that the stranger should not escape his net. leverrier, on the other hand, communicated his result to the berlin observatory, where they had just received some of the star charts prepared by dr. bremiker in connection with the search for minor planets. the berlin observer, dr. galle, had therefore nothing to do but to compare the stars in the field, upon which he turned his telescope, with those shown on the chart; a star not in the chart would probably be the desired stranger. he found it, therefore, on the very first evening, september , , within less than four diameters of the moon of the predicted place. the same object had been observed by challis at cambridge on august and , but he was deferring the reduction of his observations until he had completed his scrutiny of the zone, and hence had not recognised it as different from an ordinary star. this discovery of the planet now known as neptune, which had been disturbing the movement of uranus, has rightly been regarded as the most brilliant triumph of gravitational astronomy. it was the legitimate crown of that long intellectual struggle which had commenced more than years earlier, when the first greek astronomers set themselves to unravel the apparently aimless wanderings of the planets in the assured faith that they would find them obedient unto law. { } but of what use was all this effort? what is the good of astronomy? the question is often asked, but it is the question of ignorance. the use of astronomy is the development which it has given to the intellectual powers of man. directly the problem of the planetary motions was first attempted, it became necessary to initiate mathematical processes in order to deal with it, and the necessity for the continued development of mathematics has been felt in the same connection right down to the present day. when the greek astronomers first began their inquiries into the planetary movements they hoped for no material gain, and they received none. they laboured; we have entered into their labours. but the whole of our vast advances in mechanical and engineering science--advances which more than anything else differentiate this our present age from all those which have preceded it--are built upon our command of mathematics and our knowledge of the laws of motion--a command and a knowledge which we owe directly to their persevering attempts to advance the science of astronomy, and to follow after knowledge, not for any material rewards which she had to offer, but for her own sake. { } chapter iv astronomical measurements the old proverb has it that "science is measurement," and of none of the sciences is this so true as of the science of astronomy. indeed the measurement of time by observation of the movements of the heavenly bodies was the beginning of astronomy. the movement of the sun gave the day, which was reckoned to begin either at sunrise or at sunset. the changes of the moon gave the month, and in many languages the root meaning of the word for _moon_ is "measurer." the apparent movement of the sun amongst the stars gave a yet longer division of time, the year, which could be determined in a number of different ways, either from the sun alone, or from the sun together with the stars. a very simple and ancient form of instrument for measuring this movement of the sun was the obelisk, a pillar with a pointed top set up on a level pavement. such obelisks were common in egypt, and one of the most celebrated, known as cleopatra's needle, now stands on the thames embankment. as the sun moved in the sky, the shadow of the pillar moved on the pavement, and midday, or noon, was marked when the shadow was shortest. the length of the shadow at noon varied from day to day; it was shortest at mid-summer, and longest at midwinter, _i.e._ at the summer and winter solstices. twice in the year the shadow of the pillar pointed due west at sunrise, and due east at { } sunset--that is to say, the shadow at the beginning of the day was in the same straight line as at its end. these two days marked the two equinoxes of spring and autumn. the obelisk was a simple means of measuring the height and position of the sun, but it had its drawbacks. the length of the shadow and its direction did not vary by equal amounts in equal times, and if the pavement upon which the shadow fell was divided by marks corresponding to equal intervals of time for one day of the year, the marks did not serve for all other days. but if for the pillar a triangular wall was substituted--a wall rising from the pavement at the south and sloping up towards the north at such an angle that it seemed to point to the invisible pivot of the heavens, round which all the stars appeared to revolve--then the shadow of the wall moved on the pavement in the same manner every day, and the pavement if marked to show the hours for one day would show them for any day. the sundials still often found in the gardens of country houses or in churchyards are miniatures of such an instrument. but the greek astronomers devised other and better methods for determining the positions of the heavenly bodies. obelisks or dials were of use only with the sun and moon which cast shadows. to determine the position of a star, "sights" like those of a rifle were employed, and these were fixed to circles which were carefully divided, generally into "degrees." as there are days in a year, and as the sun makes a complete circuit of the zodiac in this time, it moves very nearly a degree in a day. the twelve signs of the zodiac are therefore each ° in length, and each { } takes on the average a double-hour to rise or set. while the sun and moon are each about half a degree in diameter, _i.e._ about one-sixtieth of the length of a sign, and therefore take a double-minute to rise or set. each degree of a circle is therefore divided into minutes, and each minute may be divided into seconds. as the sun or moon are each about half a degree, or, more exactly, minutes in diameter, it is clear that, so long as astronomical observations were made by the unaided sight, a minute of arc (written ') was the smallest division of the circle that could be used. a cord or wire can indeed be detected when seen projected against a moderately bright background if its thickness is a second of arc (written ")--a sixtieth of a minute--but the wire is merely perceived, not properly defined. tycho brahe had achieved the utmost that could be done by the naked eye, and it was the certainty that he could not have made a mistake in an observation in the place of the planet mars amounting to as much as minutes of arc--that is to say, of a quarter the apparent diameter of the moon--that made kepler finally give up all attempts to explain the planetary movements on the doctrine of circular orbits and to try movements in an ellipse. but a contemporary of kepler, as gifted as he was himself, but in a different direction, was the means of increasing the observing power of the astronomer. galileo galilei ( - ), of a noble florentine family, was appointed lecturer in mathematics at the university of pisa. here he soon distinguished himself by his originality of thought, and the ingenuity and decisiveness of his experiments. up to that time it had been taught that of { } two bodies the heavier would fall to the ground more quickly than the lighter. galileo let fall a -lb. weight and a -lb. weight from the top of the leaning tower, and both weights reached the pavement together. by this and other ingenious experiments he laid a firm foundation for the science of mechanics, and he discovered the laws of motion which newton afterwards formulated. he heard that an instrument had been invented in holland which seemed to bring distant objects nearer, and, having himself a considerable knowledge of optics, it was not long before he made himself a little telescope. he fixed two spectacle glasses, one for long and one for short sight, in a little old organ-pipe, and thus made for himself a telescope which magnified three times. before long he had made another which magnified thirty times, and, turning it towards the heavenly bodies, he discovered dark moving spots upon the sun, mountains and valleys on the moon, and four small satellites revolving round jupiter. he also perceived that venus showed "+phases+"--that is to say, she changed her apparent shape just as the moon does--and he found the milky way to be composed of an immense number of small stars. these discoveries were made in the years - . a telescope consists in principle of two parts--an +object-glass+, to form an image of the distant object, and an +eye-piece+, to magnify it. the rays of light from the heavenly body fall on the object-glass, and are so bent out of their course by it as to be brought together in a point called the focus. the "light-gathering power" of the telescope, therefore, depends upon the size of the object-glass, and is proportional to its area. but the size of the image depends upon the focal length of the telescope, _i.e._ upon the distance that the focus { } is from the object-glass. thus a small disc, an inch in diameter--such as a halfpenny--will exactly cover the full moon if held up nine feet away from the eye; and necessarily the image of the full moon made by an object-glass of nine-feet focus will be an inch in diameter. the eye-piece is a magnifying-glass or small microscope applied to this image, and by it the image can be magnified to any desired amount which the quality of the object-glass and the steadiness of the atmosphere may permit. this little image of the moon, planet, or group of stars lent itself to measurement. a young english gentleman, gascoigne, who afterwards fell at the battle of marston moor, devised the "micrometer" for this purpose. the micrometer usually has two frames, each carrying one or more very thin threads--usually spider's threads--and the frames can be moved by very fine screws, the number of turns or parts of a turn of each screw being read off on suitable scales. by placing one thread on the image of one star, and the other on the image of another, the apparent separation of the two can be readily and precisely measured. within the last thirty years photography has immensely increased the ease with which astronomical measurements can be made. the sensitive photographic plate is placed in the focus of the telescope, and the light of sun, moon, or stars, according to the object to which the telescope is directed, makes a permanent impression on the plate. thus a picture is obtained, which can be examined and measured in detail at any convenient time afterwards; a portion of the heavens is, as it were, brought actually down to the astronomer's study. it was long before this great advance was effected. { } the first telescopes were very imperfect, for the rays of different colour proceeding from any planet or star came to different foci, so that the image was coloured, diffused, and ill-defined. the first method by which this difficulty was dealt with was by making telescopes of enormously long focal length; , , or feet were not uncommon, but these were at once cumbersome and unsteady. sir isaac newton therefore discarded the use of object-glasses, and used curved mirrors in order to form the image in the focus, and succeeded in making two telescopes on this principle of reflection. others followed in the same direction, and a century later sir william herschel was most skilful and successful in making "+reflectors+," his largest being feet in focal length, and thus giving an image of the moon in its focus of nearly -½ inches diameter. but in chester moor hall found that by combining two suitable lenses together in the object-glass he could get over most of the colour difficulty, and in the optician dollond began to make object-glasses that were almost free from the colour defect. from that time onward the manufacture of "+refractors+," as object-glass telescopes are called, has improved; the glass has been made more transparent and more perfect in quality, and larger in size, and the figure of the lens improved. the largest refractor now in use is that of the yerkes observatory, wisconsin, u.s.a., and is inches in aperture, with a focal length of feet, so that the image of the moon in its focus has a diameter of more than inches. at present this seems to mark the limit of size for refractors, and the difficulty of getting good enough glass for so large a lens is very great indeed. reflectors have therefore come again into favour, as mirrors can be made larger { } than any object-glass. thus lord rosse's great telescope was feet in diameter; and the most powerful telescope now in action is the great -foot mirror of the mt. wilson observatory, california, with a focal length, as sometimes used, of feet. thus its light-gathering power is about , times that of the unaided eye, and the full moon in its focus is inches in diameter; such is the enormous increase to man's power of sight, and consequently to his power of learning about the heavenly bodies, which the development of the telescope has afforded to him. the measurement of time was the first purpose for which men watched the heavenly bodies; a second purpose was the measurement of the size of the earth. if at one place a star was observed to pass exactly overhead, and if at another, due south of it, the same star was observed to pass the meridian one degree north of the zenith, then by measuring the distance between the two places the circumference of the whole earth would be known, for it would be times that amount. in this way the size of the earth was roughly ascertained years before the invention of the telescope. but with the telescope measures of much greater precision could be made, and hence far more difficult problems could be attacked. one great practical problem was that of finding out the position of a ship when out of sight of land. the ancient phoenician and greek navigators had mostly confined themselves to coasting voyages along the shores of the mediterranean sea, and therefore the quick recognition of landmarks was the first requisite for a good sailor. but when, in , columbus had brought a new continent to light, and long voyages were freely taken across the great oceans, it became an urgent { } necessity for the navigator to find out his position when he had been out of sight of any landmark for weeks. this necessity was especially felt by the nations of western europe, the countries facing the atlantic with the new world on its far-distant other shore. spain, france, england, and holland, all were eager competitors for a grasp on the new lands, and therefore were earnest in seeking a solution of the problem of navigation. the latitude of the ship could be found out by observing the height of the sun at noon, or of the pole star at night, or in several other ways. but the longitude was more difficult. as the earth turns on its axis, different portions of its surface are brought in succession under the sun, and if we take the moment when the sun is on the meridian of any place as its noon, as twelve o'clock for that place, then the difference of longitude between any two places is essentially the difference in their local times. it was possible for the sailor to find out when it was local noon for him, but how could he possibly find out what time it was at that moment at the port from which he had sailed, perhaps several weeks before? the moon and stars supplied eventually the means for giving this information. for the moon moves amongst the stars, as the hand of a clock moves amongst the figures of a dial, and it became possible at length to predict for long in advance exactly where amongst the stars the moon would be, for any given time, of any selected place. when this method was first suggested, however, neither the motion of the moon nor the places of the principal stars were known with sufficient accuracy, and it was to remedy this defect, and put navigation upon { } a sound basis, that charles ii. founded greenwich observatory in the year , and appointed flamsteed the first astronomer royal. in the year maskelyne, the fifth astronomer royal, brought out the first volume of the _nautical almanac_, in which the positions of the moon relative to certain stars were given for regular intervals of greenwich time. much about the same period the problem was solved in another way by the invention of the chronometer, by john harrison, a yorkshire carpenter. the +chronometer+ was a large watch, so constructed that its rate was not greatly altered by heat or cold, so that the navigator had greenwich time with him wherever he went. the new method in the hands of captain cook and other great navigators led to a rapid development of navigation and the discovery of australia and new zealand, and a number of islands in the pacific. the building up of the vast oceanic commerce of great britain and of her great colonial empire, both in north america and in the southern oceans, has arisen out of the work of the royal observatory, greenwich, and has had a real and intimate connection with it. to observe the motions of the moon, sun, and planets, and to determine with the greatest possible precision the places of the stars have been the programme of greenwich observatory from its foundation to the present time. other great national observatories have been copenhagen, founded in ; paris, in ; berlin, in ; st. petersburg, in , superseded by that of pulkowa, in ; and washington, in ; while not a few of the great universities have also efficient observatories connected with them. of the directly practical results of astronomy, the { } promotion of navigation stands in the first rank. but the science has never been limited to merely utilitarian inquiries, and the problem of measuring celestial distances has followed on inevitably from the measurement of the earth. the first distance to be attacked was that of the nearest companion to the earth, _i.e._ the moon. it often happens on our own planet that it is required to find the distance of an object beyond our reach. thus a general on the march may come to a river and need to know exactly how broad it is, that he may prepare the means for bridging it. such problems are usually solved on the following principle. let a be the distant object. then if the direction of a be observed from each of two stations, b and c, and the distance of b from c be measured, it is possible to calculate the distances of a from b and from c. the application of this principle to the measurement of the moon's distance was made by the establishment of an observatory at the cape of good hope, to co-operate with that of greenwich. it is, of course, not possible to see greenwich observatory from the cape, or vice versa, but the stars, being at an almost infinite distance, lie in the same direction from both observatories. what is required then is to measure the apparent distance of the moon from the same stars as seen from greenwich and as seen from the cape, and, the distance apart of the two observatories being known, the distance of the moon can be calculated. this was a comparatively easy problem. the next step in celestial measurement was far harder; it was to find the distance of the sun. the sun is times as far off as the moon, and therefore it seems to be practically in the same direction as seen from each of { } the two observatories, and, being so bright, stars cannot be seen near it in the telescope. but by carefully watching the apparent movements of the planets their _relative_ distances from the sun can be ascertained, and were known long before it was thought possible that we should ever know their real distances. thus venus never appears to travel more than ° ' from the sun. this means that her distance from the sun is a little more than seven-tenths of that of the earth. if, therefore, the distance of one planet from the sun can be measured, or the distance of one planet from the earth, the actual distances of all the planets will follow. we know the proportions of the parts of the solar system, and, if we can fix the scale of one of the parts, we fix the scale of all. it has been found possible to determine the distance of mars, of several of the "minor planets," and especially of eros, a very small minor planet that sometimes comes within , , miles of the earth, or seven times nearer to us than is the sun. from the measures of eros, we have learned that the sun is separated from us by very nearly , , miles--an unimaginable distance. perhaps the nearest way of getting some conception of this vast interval is by remembering that there are only , , seconds of time in a year. if, therefore, an express train, travelling miles an hour--a mile a minute--set out for the sun, and travelled day and night without cease, it would take more than years to accomplish the journey. but this astronomical measure has led on to one more daring still. the earth is on one side of the sun in january, on the other in july. at these two dates, therefore, we are occupying stations , , miles { } apart, and can ascertain the apparent difference in direction of the stars as viewed from the two points but the astonishing result is that this enormous change in the position of the earth makes not the slightest observable difference in the position of most of the stars. a few, a very few, do show a very slight difference. the nearest star to us is about , times as far from us as the sun; this is alpha centauri, the brightest star in the constellation of the centaur and the third brightest star in the sky. sirius, the brightest star, is twice this distance. some forty or fifty stars have had their distances roughly determined; but the stars in general far transcend all our attempts to plumb their distances. but, from certain indirect hints, it is generally supposed that the mass of stars in the milky way are something like , , times as far from us as we are from our sun. thus far, then, astronomy has led us in the direction or measurement. it has enabled us to measure the size of the earth upon which we live, and to find out the position of a ship in the midst of the trackless ocean. it has also enabled us to cast a sounding-line into space, to show how remote and solitary the earth moves through the void, and to what unimaginable lengths the great stellar universe, of which it forms a secluded atom, stretches out towards infinity. { } chapter v the members of the solar system astronomical measurement has not only given us the distances of the various planets from the sun; it has also furnished us, as in the annexed table, with their real diameters, and, as a consequence of the law of gravitation, with their densities and weights, and the force of gravity at their surfaces. and these numerical details are of the first importance in directing us as to the inferences that we ought to draw as to their present physical conditions. the theory of copernicus deprived the earth of its special position as the immovable centre of the universe, but raised it to the rank of a planet. it is therefore a heavenly body, yet needs no telescope to bring it within our ken; bad weather does not hide it from us, but rather shows it to us under new conditions. we find it to be a globe of land and water, covered by an atmosphere in which float changing clouds; we have mapped it, and we find that the land and water are always there, but their relations are not quite fixed; there is give and take between them. we have found of what elements the land and water consist, and how these elements combine with each other or dissociate. in a word, the earth is the heavenly body that we know the best, and with it we must compare and contrast all the others. before the invention of the telescope there were but { } two other heavenly bodies--the sun and the moon--that appeared as orbs showing visible discs, and even in their cases nothing could be satisfactorily made out as to their conditions. now each of the five planets known to the ancients reveals to us in the telescope a measurable disc, and we can detect significant details on their surfaces. the moon is the one object in the heavens which does not disappoint a novice when he first sees it in the telescope. every detail is hard, clear-cut, and sharp; it is manifest that we are looking at a globe, a very rough globe, with hills and mountains, plains and valleys, the whole in such distinct relief that it seems as if it might be touched. no clouds ever conceal its details, no mist ever softens its outlines; there are no half-lights, its shadows are dead black, its high lights are molten silver. certain changes of illumination go on with the advancing age of the moon, as the crescent broadens out to the half, the half to the full, and the full, in its turn, wanes away; but the lunar day is nearly thirty times as long as that of the earth, and these changes proceed but slowly. the full moon, as seen by the naked eye, shows several vague dark spots, which most people agree to fancy as like the eyes, nose, and mouth of a broad, sorrowful face. the ordinary astronomical telescope inverts the image, so the "eyes" of the moon are seen in the lower part of the field of the telescope as a series of dusky plains stretching right across the disc. but in the upper part, near the left-hand corner of the underlip, there is a bright, round spot, from which a number of bright streaks radiate--suggesting a peeled orange with its stalk, and the lines marking the sections radiating from it. this bright spot has been called after the great { } mean distance from sun. period velocity class. name. earth's in millions of revolution. in orbit. eccentricity. distance of miles. in years. miles per = . sec. terrestrial mercury . . . . . planets venus . . . . . earth . . . . . mars . . . . . minor eros . . . . . planets ceres . . . . . achilles . . . . . major jupiter . . . . . planets saturn . . . . . uranus . . . . . neptune . . . . . { } mean diameter. surface. volume. mass. name. symbol. in miles. [earth]= . [earth]= . [earth]= . [earth]= . sun [sun] . . . . moon [moon] . . . . mercury [mercury] . . . . venus [venus] . . . . earth [earth] . . . . mars [mars] . . . . jupiter [jupiter] . . . . saturn [saturn] . . . . uranus [uranus] . . . . neptune [neptune] . . . . { } light gravity. and heat albedo; density. fall in received _i.e._ re- [earth] water [earth] feet per from sun. time of rotation flecting name. = . = . = . sec. [earth]= . on axis. power. d. h. m. sun . . . . ... ± ... moon . . . . . . d. h. m. s. mercury . . . . . (?) . venus . . . . . (?) . earth . . . . . . (?) mars . . . . . . h. m. jupiter . . . . . ± . saturn . . . . . ± . uranus . . . . . (?) . neptune . . . . . (?) . { } danish astronomer, "tycho," and is one of the most conspicuous objects of the full moon. the contrasts of the moon are much more pronounced when she is only partly lit up. then the mountains and valleys stand out in the strongest relief, and it becomes clear that the general type of formation on the moon is that of rings--rings of every conceivable size, from the smallest point that the telescope can detect up to some of the great dusky plains themselves, hundreds of miles in diameter. these rings are so numerous that galileo described the moon as looking as full of "eyes" as a peacock's tail. the "right eye" of the moonface, as we see it in the sky, is formed by a vast dusky plain, nearly as large as france and germany put together, to which has been given the name of the "sea of rains" (_mare imbrium_), and just below this (as seen in the telescope) is one of the most perfect and beautiful of all the lunar rings--a great ring-plain, miles in diameter, called after the thinker who revolutionised men's ideas of the solar system, "copernicus." "copernicus," like "tycho," is the centre of a set of bright streaks; and a neighbouring but smaller ring, bearing the great name of "kepler," stands in a like relation to another set. the most elevated region of the moon is immediately in the neighbourhood of the great "stalk of the orange," "tycho." here the rings are crowded together as closely as they can be packed; more closely in many places, for they intrude upon and overlap each other in the most intricate manner. a long chain of fine rings stretches from this disturbed region nearly to the centre of the disc, where the great alexandrian astronomer is commemorated by a vast walled plain, { } considerably larger than the whole of wales, and known as "ptolemæus." the distinctness of the lunar features shows at once that the moon is in an altogether different condition from that of the earth. here the sky is continually being hidden by cloud, and hence the details of the surface of the earth as viewed from any other planet must often be invisible, and even when actual cloud is absent there is a more permanent veil of dust, which must greatly soften and confuse terrestrial outlines. the clearness, therefore, with which we perceive the lunar formations proves that there is little or no atmosphere there. nor is there any sign upon it of water, either as seas or lakes or running streams. yet the moon shows clearly that in the past it has gone through great and violent changes. the gradation is so complete from the little craterlets, which resemble closely, in form and size, volcanic craters on the earth, up to the great ring-plains, like "copernicus" or "tycho," or formations larger still, that it seems natural to infer not only that the smaller craters were formed by volcanic eruption, like the similar objects with which we are acquainted on our own earth, but that the others, despite their greater sizes, had a like origin. in consequence of the feebler force of gravity on the moon, the same explosive force there would carry the material of an eruption much further than on the earth. the darker, low-lying districts of the moon give token of changes of a different order. it is manifest that the material of which the floors of these plains is composed has invaded, broken down, and almost submerged many of the ring-formations. sometimes half { } of a ring has been washed away; sometimes just the outline of a ring can still be traced upon the floor of the sea; sometimes only a slight breach has been made in the wall. so it is clear that the moon was once richer in the great crater-like formations than it is to-day, but a lava-flood has overflowed at least one-third of its area. more recent still are the bright streaks, or rays, which radiate in all directions from "tycho," and from some of the other ring-plains. it is evident from these different types of structure on the moon, and from the relations which they bear to each other, that the lunar surface has passed through several successive stages, and that its changes tended, on the whole, to diminish in violence as time went on; the minute crater pits with which the surface is stippled having been probably the last to form. but the years during which the moon has been watched with the telescope have afforded no trace of any continuance of these changes. she has had a stormy and fiery past; but nothing like the events of those bygone ages disturbs her serenity to-day. and yet we must believe that change does take place on the moon even now, because during the hours of its long day the sun beats down with full force on the unprotected surface, and during the equally long night that surface is exposed to the cold of outer space. every part of the surface must be exposed in turn to an extreme range of temperature, and must be cracked, torn, and riven by alternate expansion and contraction. apart from this slow, wearing process, and a very few rather doubtful cases in which a minute alteration of some surface detail has been suspected, our sister planet, the moon, shows herself as changeless and inert, without any appreciable trace of air or water or any sign { } of life--a dead world, with all its changes and activities in the past. mars, after the moon, is the planet whose surface we can study to best advantage. its orbit lies outside that of the earth, so that when it is nearest to us it turns the same side to both the sun and earth, and we see it fully illuminated. mercury and venus, on the contrary, when nearest us are between us and the sun, and turn their dark sides to us. when fully illuminated they are at their greatest distance, and appear very small, and, being near the sun, are observed with difficulty. these three are intermediate in size between the moon and the earth. in early telescopic days it was seen that mars was an orange-coloured globe with certain dusky markings upon it, and that these markings slowly changed their place--that, in short, it was a world rotating upon its axis, and in a period not very different from that of the earth. the rotation period of mars has indeed been fixed to the one-hundredth part of a second of time; it is h. m. . s. and this has been possible because some of the dusky spots observed in the seventeenth century can be identified now in the twentieth. some of the markings on mars, like our own continents and seas, and like the craters on the moon, are permanent features; and many charts of the planet have been constructed. other markings are variable. since the planet rotates on its axis, the positions of its poles and equator are known, its equator being inclined to its orbit at an angle of ° ', while the angle in the case of the earth is ° '. the times when its seasons begin and end are therefore known; and it is found that the spring of its northern hemisphere lasts of our { } days, the summer , the autumn , and the winter . round the pole in winter a broad white cap forms, which begins to shrink as spring comes on, and may entirely disappear in summer. no corresponding changes have been observed on the moon, but it is easy to find an analogy to them on the earth. round both our poles a great cap of ice and snow is spread--a cap which increases in size as winter comes on, and diminishes with the advance of summer--and it seems a reasonable inference to suppose that the white polar caps of mars are, like our own, composed of ice and snow. from time to time indications have been observed of the presence on mars of a certain amount of cloud. familiar dark markings have, for a short time, been interrupted, or been entirely hidden, by white bands, and have recovered their ordinary appearance later. with rotation on its axis and succession of seasons, with atmosphere and cloud, with land and water, with ice and snow, mars would seem to be a world very similar to our own. this was the general opinion up to the year , when schiaparelli announced that he had discovered a number of very narrow, straight, dark lines on the planet--lines to which he gave the name of "canali"--that is, "channels." this word was unfortunately rendered into english by the word "+canals+," and, as a canal means a waterway artificially made, this mistranslation gave the idea that mars was inhabited by intelligent beings, who had dug out the surface of the planet into a network of canals of stupendous length and breadth. the chief advocate of this theory is lowell, an american observer, who has given very great attention { } to the study of the planet during the last seventeen years. his argument is that the straight lines, the canals, which he sees on the planet, and the round dots, the "+oases+," which he finds at their intersections, form a system so obviously _un_natural, that it must be the work of design--of intelligent beings. the canals are to him absolutely regular and straight, like lines drawn with ruler and pen-and-ink, and the oases are exactly round. but, on the one hand, the best observers, armed with the most powerful telescopes, have often been able to perceive that markings were really full of irregular detail, which lowell has represented as mere hard straight lines and circular dots, and, on the other hand, the straight line and the round dot are the two geometric forms which all very minute objects must approach in appearance. that we cannot see irregularities in very small and distant objects is no proof at all that irregularities do not exist in them, and it has often happened that a marking which appeared a typical "canal" when mars was at a great distance lost that appearance when the planet was nearer. astronomers, therefore, are almost unanimous that there is no reason for supposing that any of the details that we see on the surface of mars are artificial in their origin. and indeed the numerical facts that we know about the planet render it almost impossible that there should be any life upon it. if we turn to the table, we see that in size, volume, density, and force of gravity at its surface, mars lies between the moon and the earth, but is nearer the moon. this has an important bearing as to the question of the planet's atmosphere. on the earth we pass through half the atmosphere by ascending a mountain { } that is three and a third miles in height; on mars we should have to ascend nearly nine miles. if the atmospheric pressure at the surface of mars were as great as it is at the surface of the earth, his atmosphere would be far deeper than ours and would veil the planet more effectively. but we see the surface of mars with remarkable distinctness, almost as clearly, when its greater distance is allowed for, as we see the moon. it is therefore accepted that the atmospheric pressure at the surface of mars must be very slight, probably much less than at the top of our very highest mountains, where there is eternal snow, and life is completely absent. but mars compares badly with the earth in another respect. it receives less light and heat from the sun in the proportion of three to seven. this we may express by saying that mars, on the whole, is almost as much worse off than the earth as a point on the arctic circle is worse off than a point on the equator. the mean temperature of the earth is taken as about ° of the fahrenheit thermometer (say, ° cent.); the mean temperature of mars must certainly be considerably below freezing-point, probably near ° f. here on our earth the boiling-point of water is °, and, since the mean temperature is ° and water freezes at °, it is normally in the liquid state. on mars it must normally be in the solid state--ice, snow, or frost, or the like. but with so rare an atmosphere water will boil at a low temperature, and it is not impossible that under the direct rays of the sun--that is to say, at midday of the torrid zone of mars--ice may not only melt, but water boil by day, condensing and freezing again during the night. newcomb, the foremost astronomer of his day, concluded "that during { } the night of mars, even in the equatorial regions, the surface of the planet probably falls to a lower temperature than any we ever experienced on our globe. if any water exists, it must not only be frozen, but the temperature of the ice must be far below the freezing point.... the most careful calculation shows that if there are any considerable bodies of water on our neighbouring planet, they exist in the form of ice, and can never be liquid to a depth of more than one or two inches, and that only within the torrid zone and during a few hours each day." with regard to the snow caps of mars, newcomb thought it not possible that any considerable fall of snow could ever take place. he regarded the white caps as simply due to a thin deposit of hoar frost, and it cannot be deemed wonderful that such should gradually disappear, when it is remembered that each of the two poles of mars is in turn presented to the sun for more than consecutive days. newcomb's conclusion was: "thus we have a kind of martian meteorological changes, very slight indeed, and seemingly very different from those of our earth, but yet following similar lines on their small scale. for snowfall substitute frostfall; instead of (the barometer reading) feet or inches say fractions of a millimetre, and instead of storms or wind substitute little motions of an air thinner than that on the top of the himalayas, and we shall have a general description of martian meteorology." we conclude, then, that mars is not so inert a world as the moon, but, though some slight changes of climate or weather take place upon it, it is quite unfitted for the nourishment and development of the different forms of organic life. of mercury we know very little. it is smaller than mars but larger than the moon, but it differs from them { } both in that it is much nearer the sun, and receives, therefore, many times the light and heat, surface for surface. we should expect, therefore, that water on mercury would exist in the gaseous state instead of in the solid state as on mars. the little planet reflects the sunlight only feebly, and shows no evidence of cloud. a few markings have been made out on its surface, and the best observers agree that it appears to turn the same face always to the sun. this would imply that the one hemisphere is in perpetual darkness and cold, the other, exposed to an unimaginable fiery heat. venus is nearly of the same size as the earth, and the conditions as to the arrangement of its atmosphere, the force of gravity at its surface, must be nearly the same as on our own world. but we know almost nothing of the details of its surface; the planet is very bright, reflecting fully seven-tenths of the sunlight that falls upon it. it would seem that, in general, we see nothing of the actual details of the planet, but only the upper surface of a very cloudy atmosphere. owing to the fact that venus shows no fixed definite marking that we can watch, it is still a matter of controversy as to the time in which it rotates upon its axis. schiaparelli and some other observers consider that it rotates in the same time as it revolves round the sun. others believe that it rotates in a little less than twenty-four hours. if this be so, and there is any body in the solar system other than the earth, which is adapted to be the home of life, then the planet venus is that one. the sun, like the moon, presents a visible surface to the naked eye, but one that shows no details. in the telescope the contrast between it and the moon is very great, and still greater is the contrast which is brought { } out by the measurements of its size, volume, and weight. but the really significant difference is that the sun is a body giving out light and heat, not merely reflecting them. without doubt this last difference is connected most closely with the difference in size. the moon is cold, dead, unchanging, because it is a small world; the sun is bright, fervent, and undergoes the most violent change, because it is an exceedingly large world. the two bodies--the sun and moon--appear to the eye as being about the same size, but since the sun is times as far off as the moon it must be times the diameter. that means that it has times , or , times the surface and times times , or , , times the volume. the sun and moon, therefore, stand at the very extremes of the scale. the heat of the sun is so great that there is some difficulty in observing it in the telescope. it is not sufficient to use a dark glass in order to protect the eye, unless the telescope be quite a small one. some means have to be employed to get rid of the greater part of the heat and light. the simplest method of observing is to fix a screen behind the eyepiece of a telescope and let the image of the sun be projected upon the screen, or the sensitive plate may be substituted for the screen, and a photograph obtained, which can be examined at leisure afterwards. as generally seen, the surface of the sun appears to be mottled all over by a fine irregular stippling. this stippling, though everywhere present, is not very strongly marked, and a first hasty glance might overlook it. from time to time, however, dark spots are seen, of ever-changing form and size. by watching these, galileo proved that the sun rotated on its axis in a little more than twenty-five days, and in the { } nineteenth century schwabe proved that the sunspots were not equally large and numerous at all times, but that there was a kind of cycle of a little more than eleven years in average length. at one time the sun would be free from spots; then a few small ones would appear; these would gradually become larger and more numerous; then a decline would follow, and another spotless period would succeed about eleven years after the first. as a rule, the increase in the spots takes place more quickly than the decline. most of the spot-groups last only a very few days, but about one group in four lasts long enough to be brought round by the rotation of the sun a second time; in other words, it continues for about a month. in a very few cases spots have endured for half a year. an ordinary form for a group of spots is a long stream drawn out parallel to the sun's equator, the leading spot being the largest and best defined. it is followed by a number of very small irregular and ill-developed spots, and the train is brought up by a large spot, sometimes even larger than the leader, but by no means so regular in form or so well defined. the leading spot for a short time moves forward much faster than its followers, at a speed of about miles per day. the small middle spots then gradually die out, or rather seem to be overflowed by the bright material of the solar surface, the "+photosphere+," as it is called; the spot in the rear breaks up a little later, and the leader, which is now almost circular, is left alone, and may last in this condition for some weeks. finally, it slowly contracts or breaks up, and the disturbance comes to an end. this is the course of development of many long-lived spot-groups, but all do not conform to the same type. { } the very largest spots are indeed usually quite different in their appearance and history. in size, sunspots vary from the smallest dot that can be discovered in the telescope up to huge rents with areas that are to be counted by thousands of millions of square miles; the great group of february had an area of , , , square miles, a thousand times the area of europe. closely associated with the _maculæ_, as the spots were called by the first observers, are the "+faculæ+"--long, branching lines of bright white light, bright as seen even against the dazzling background of the sun itself, and looking like the long lines of foam of an incoming tide. these are often associated with the spots; the spots are formed between their ridges, and after a spot-group has disappeared the broken waves of faculæ will sometimes persist in the same region for quite a long time. the faculæ clearly rise above the ordinary solar surface; the spots as clearly are depressed a little below it; because from time to time we see the bright material of the surface pour over spots, across them, and sometimes into them. but there is no reason to believe that the spots are deep, in proportion either to the sun itself or even to their own extent. sunspots are not seen in all regions of the sun. it is very seldom that they are noted in a higher solar latitude than °, the great majority of spots lying in the two zones between ° and ° latitude on either side of the equator. faculæ, on the other hand, though most frequent in the spot zones, are observed much nearer the two poles. it is very hard to find analogies on our earth for sunspots and for their peculiarities of behaviour. some { } of the earlier astronomers thought they were like terrestrial volcanoes, or rather like the eruptions from them. but if there were a solid nucleus to the sun, and the spots were eruptions from definite areas of the nucleus, they would all give the same period of rotation. but sunspots move about freely on the solar surface, and the different zones of that surface rotate in different times, the region of the equator rotating the most quickly. this alone is enough to show that the sun is essentially not a solid body. yet far down below the photosphere something approaching to a definite structure must already be forming. for there is a well-marked progression in the zones of sunspots during the eleven-year cycle. at a time when spots are few and small, known as +the sunspot minimum+, they begin to be seen in fairly high latitudes. as they get more numerous, and many of them larger, they frequent the medium zones. when the sun is at its greatest activity, known as +the sunspot maximum+, they are found from the highest zone right down to the equator. then the decline sets in, but it sets in first in the highest zones, and when the time of minimum has come again the spots are close to the equator. before these have all died away, a few small spots, the heralds of a new cycle of activity, begin to appear in high latitudes. this law, called after spÖrer, its discoverer, indicates that the origin and source of sunspot activity lie within the sun. at one time it was thought that sunspots were due to some action of jupiter--for jupiter moves round the sun in . years, a period not very different from the sunspot cycle--or to some meteoric stream. but spörer's law could not be imposed by some influence from without. still sunspots, once formed, may be influenced by the earth, and perhaps by other { } planets also, for mrs. walter maunder has shown that the numbers and areas of spots tend to be smaller on the western half of the disc, as seen from the earth, than on the eastern, while considerably more groups come into view at the east edge of the sun than pass out of view at the west edge, so that it would appear as if the earth had a damping effect upon the spots exposed to it. but the sun is far greater than it ordinarily appears to us. twice every year, and sometimes oftener, the moon, when new, comes between the earth and the sun, and we have an +eclipse of the sun+, the dark body of the moon hiding part, or all, of the greater light. the sun and moon are so nearly of the same apparent size that an eclipse of the sun is total only for a very narrow belt of the earth's surface, and, as the moon moves more quickly than the sun, the eclipse only remains total for a very short time--seven minutes at the outside, more usually only two or three. north or south of that belt the moon is projected, so as to leave uncovered a part of the sun north or south of the moon. a total eclipse, therefore, is rare at any particular place, and if a man were able to put himself in the best possible position on each occasion, it would cost him thirty years to secure an hour's accumulated duration. eclipses of the moon are visible over half the world at one time, for there is a real loss to the moon of her light. her eclipses are brought about when, in her orbit, she passes behind the earth, and the earth, being between the sun and the moon, cuts off from the latter most of the light falling upon her; not quite all; a small portion reaches her after passing through the thickest part of the earth's atmosphere, so that the { } moon in an eclipse looks a deep copper colour, much as she does when rising on a foggy evening. total eclipses of the sun have well repaid all the efforts made to observe them. it is a wonderful sight to watch the blackness of darkness slowly creeping over the very fountain of light until it is wholly and entirely hidden; to watch the colours fade away from the landscape and a deathlike, leaden hue pervade all nature, and then to see a silvery, star-like halo, flecked with bright little rose-coloured flames, flash out round the black disc that has taken the place of the sun. these rose-coloured flames are the solar "+prominences+," and the halo is the "+corona+," and it is to watch these that astronomers have made so many expeditions hither and thither during the last seventy years. the "prominences," or red flames, can be observed, without an eclipse, by means of the spectroscope, but, as the work of the spectroscope is to form the subject of another volume of this series, it is sufficient to add here that the prominences are composed of various glowing gases, chiefly of hydrogen, calcium, and helium. these and other gases form a shell round the sun, about miles in depth, to which the name "+chromosphere+" has been given. it is out of the chromosphere that the prominences arise as vast irregular jets and clouds. ordinarily they do not exceed or thousand miles in height, but occasionally they extend for or even thousand miles from the sun. their changes are as remarkable as their dimensions; huge jets of or thousand miles have been seen to form, rise, and disappear within an hour or less, and movements have been chronicled of or miles in a single second of time. prominences are largest and most frequent when { } sunspots and faculæ are most frequent, and fewest when those are fewest. the corona, too, varies with the sunspots. at the time of maximum the corona sends forth rays and streamers in all directions, and looks like the conventional figure of a star on a gigantic scale. at minimum the corona is simpler in form, and shows two great wings, east and west, in the direction of the sun's equator, and round both of his poles a number of small, beautiful jets like a crest of feathers. some of the streamers or wings of the corona have been traced to an enormous distance from the sun. mrs. walter maunder photographed one ray of the corona of to a distance of millions of miles. langley, in the clear air of pike's peak, traced the wings of the corona of with the naked eye to nearly double this distance. but the rapid changes of sunspots and the violence of some of the prominence eruptions are but feeble indications of the most wonderful fact concerning the sun, _i.e._ the enormous amount of light and heat which it is continually giving off. here we can only put together figures which by their vastness escape our understanding. sunlight is to moonlight as , is to , so that if the entire sky were filled up with full moons, they would not give us a quarter as much light as we derive from the sun. the intensity of sunlight exceeds by far any artificial light; it is times as bright as the calcium light, and three or four times as bright as the brightest part of the electric arc light. the amount of heat radiated by the sun has been expressed in a variety of different ways; c. a. young very graphically by saying that if the sun were encased in a shell of ice feet deep, its heat would melt the shell in one minute, and that if a bridge of ice could be { } formed from the earth to the sun, -½ miles square in section and millions of miles long, and the entire solar radiation concentrated upon it, in one second the ice would be melted, in seven more dissipated into vapour. the earth derives from the sun not merely light and heat, but, by transformation of these, almost every form of energy manifest upon it; the energy of the growth of plants, the vital energy of animals, are only the energy received from the sun, changed in its expression. the question naturally arises, "if the sun, to which the earth is indebted for nearly everything, passes through a change in its activity every eleven years or so, how is the earth affected by it?" it would seem at first sight that the effect should be great and manifest. a sunspot, like that of february , one thousand times as large as europe, into which worlds as large as our earth might be poured, like peas into a saucer, must mean, one might think, an immense falling off of the solar heat. yet it is not so. for even this great sunspot was but small as compared with the sun as a whole. had it been dead black, it would have stopped out much less than per cent. of the sun's heat; and even the darkest sunspot is really very bright. and the more spots there are, the more numerous and brighter are the faculæ; so that we do not know certainly which of the two phases, maximum or minimum, means the greater radiation. if the weather on the earth answers to the sunspot cycle, the connection is not a simple one; as yet no connection has been proved. thus two of the worst and coldest summers experienced in england fell the one in , the other in , _i.e._ at { } maximum and minimum respectively. so, too, the hot summers of and were also, the one at maximum and the other at minimum; and ordinary average years have fallen at both the phases just the same. yet there is an answer on the part of the earth to these solar changes. the earth itself is a kind of magnet, possessing a magnetism of which the intensity and direction is always changing. to watch these changes, very sensitive magnets are set up, and a slight daily to-and-fro swing is noticed in them; this swing is more marked in summer than in winter, but it is also more marked at times of the sunspot maximum than at minimum, showing a dependence upon the solar activity. yet more, from time to time the magnetic needle undergoes more or less violent disturbance; in extreme cases the electric telegraph communication has been disturbed all over the world, as on september , , when the submarine cables ceased to carry messages for several hours. in most cases when such a "magnetic storm" occurs, there is an unusually large or active spot on the sun. the writer was able in to further prove that such "storms" have a marked tendency to recur when the same longitude of the sun is presented again towards the earth. thus in february , when a very large spot was on the sun, a violent magnetic storm broke out. the spot passed out of sight and the storm ceased, but in the following month, when the spot reached exactly the same apparent place on the sun's disc, the storm broke out again. such magnetic disturbances are therefore due to streams of particles driven off from limited areas of the sun, probably in the same way that the long, { } straight rays of the corona are driven off. such streams of particles, shot out into space, do not spread out equally in all directions, like the rays of light and heat, but are limited in direction, and from time to time they overtake the earth in its orbit, and, striking it, cause a magnetic storm, which is felt all over the earth at practically the same moment. jupiter is, after the sun, much the largest member of the solar system, and it is a peculiarly beautiful object in the telescope. even a small instrument shows the little disc striped with many delicately coloured bands or belts, broken by white clouds and dark streaks, like a "windy sky" at sunset. and it changes while being watched, for, though , , miles away from us, it rotates so fast upon its axis that its central markings can actually be seen to move. this rapid rotation, in less than ten hours, is the most significant fact about jupiter. for different spots have different rotation periods, even in the same latitude, proving that we are looking down not upon any solid surface of jupiter, but upon its cloud envelope--an envelope swept by its rapid rotation and by its winds into a vast system of parallel currents. one object on jupiter, the great "+red spot+," has been under observation since , and possibly for years before that. it is a large, oval object fitted in a frame of the same shape. the spot itself has often faded and been lost since , but the frame has remained. the spot is in size and position relative to jupiter much as australia is to the earth, but while australia moves solidly with the rest of the earth in the daily rotation, neither gaining on south america nor losing on africa, the red spot on jupiter sees many other spots and clouds pass it by, and does not even { } retain the same rate of motion itself from one year to another. no other marking on jupiter is so permanent as this. from time to time great round white clouds form in a long series as if shot up from some eruption below, and then drawn into the equatorial current. from time to time the belts themselves change in breadth, in colour, and complexity. jupiter is emphatically the planet of change. and such change means energy, especially energy in the form of heat. if jupiter possessed no heat but that it derived from the sun, it would be colder than mars, and therefore an absolutely frozen globe. but these rushing winds and hurrying clouds are evidences of heat and activity--a native heat much above that of our earth. while mars is probably nearer to the moon than to the earth in its condition, jupiter has probably more analogies with the sun. the one unrivalled distinction of saturn is its ring. nothing like this exists elsewhere in the solar system. everywhere else we see spherical globes; this is a flat disc, but without its central portion. it surrounds the planet, lying in the plane of its equator, but touches it nowhere, a gap of miles intervening. it appears to be circular, and is , miles in breadth. yet it is not, as it appears to be, a flat continuous surface. it is in reality made up of an infinite number of tiny satellites, mere dust or pebbles for the most part, but so numerous as to look from our distance like a continuous ring, or rather like three or four concentric rings, for certain divisions have been noticed in it--an inner broad division called after its discoverer, cassini, and an outer, fainter, narrower one discovered by encke. the innermost part of the ring is dusky, fainter { } than the planet or the rest of the ring, and is known as the "crape-ring." of saturn itself we know little; it is further off and fainter than jupiter, and its details are not so pronounced, but in general they resemble those of jupiter. the planet rotates quickly--in h. m.--its markings run into parallel belts, and are diversified by spots of the same character as on jupiter. saturn is probably possessed of no small amount of native heat. uranus and neptune are much smaller bodies than jupiter and saturn, though far larger than the earth. but their distance from the earth and sun makes their discs small and faint, and they show little in the telescope beyond a hint of "belts" like those of jupiter; so that, as with that planet, the surfaces that they show are almost certainly the upper surfaces of a shell of cloud. in general, therefore, the rule appears to hold good throughout the solar system that a very large body is intensely hot and in a condition of violent activity and rapid change; that smaller bodies are less hot and less active, until we come down to the smallest, which are cold, inert, and dead. our own earth, midway in the series, is itself cold, but is placed at such a distance from the sun as to receive from it a sufficient but not excessive supply of light and heat, and the changes of the earth are such as not to prohibit but to nourish and support the growth and development of the various forms of life. the smallest members of the solar system are known as meteors. these are often no more than pebbles or particles of dust, moving together in associated orbits round the sun. they are too small and too scattered to be seen in open space, and become visible to us only { } when their orbits intersect that of the earth, and the earth actually encounters them. they then rush into our atmosphere at a great speed, and become highly heated and luminous as they compress the air before them; so highly heated that most are vapourised and dissipated, but a few reach the ground. as they are actually moving in parallel paths at the time of one of these encounters, they appear from the effect of perspective to diverge from a point, hence called the "+radiant+." some showers occur on the same date of every year; thus a radiant in the constellation lyra is active about april , giving us meteors, known as the "lyrids"; and another in perseus in august, gives us the "perseids." other radiants are active at intervals of several years; the most famous of all meteoric showers, that of the "leonids," from a radiant in leo, was active for many centuries every thirty-third year; and another falling in the same month, november, came from a radiant in andromeda every thirteen years. in these four cases and in some others the meteors have been found to be travelling along the same path as a comet. it is therefore considered that meteoric swarms are due to the gradual break up of comets; indeed the comet of the andromeda shower, known from one of its observers as "biela's," was actually seen to divide into two in december , and has not been observed as a comet since , though the showers connected with it, giving us the meteors known as the "andromedes," have continued to be frequent and rich. meteors, therefore, are the smallest, most insignificant, of all the celestial bodies; and the shining out of a meteor is the last stage of its history--its death; after death it simply goes to add an infinitesimal trifle to the dust of the earth. { } chapter vi the system of the stars the first step towards our knowledge of the starry heavens was made when the unknown and forgotten astronomers of b.c. arranged the stars into constellations, for it was the first step towards distinguishing one star from another. when one star began to be known as "the star in the eye of the bull," and another as "the star in the shoulder of the giant," the heavens ceased to display an indiscriminate crowd of twinkling lights; each star began to possess individuality. the next step was taken when hipparchus made his catalogue of stars ( b.c.), not only giving its name to each star, but measuring and fixing its place--a catalogue represented to us by that of claudius ptolemy (a.d. ). the third step was taken when bradley, the third astronomer royal, made, at greenwich, a catalogue of more than star-places determined with the telescope. a century later argelander made the great bonn zone catalogue of , stars, and now a great photographic catalogue and chart of the entire heavens have been arranged between eighteen observatories of different countries. this great chart when complete will probably present millions of stars in position and brightness. { } the question naturally arises, "why so many stars? what conceivable use can be served by catalogues of millions or even of stars?" and so far as strictly practical purposes are concerned, the answer must be that there is none. thus maskelyne, the fifth astronomer royal, restricted his observations to some thirty-six stars, which were all that he needed for his _nautical almanac_, and these, with perhaps a few additions, would be sufficient for all purely practical ends. but there is in man a restless, resistless passion for knowledge--for knowledge for its own sake--that is always compelling him to answer the challenge of the unknown. the secret hid behind the hills, or across the seas, has drawn the explorer in all ages; and the secret hid behind the stars has been a magnet not less powerful. so catalogues of stars have been made, and made again, and enlarged and repeated; instruments of ever-increasing delicacy have been built in order to determine the positions of stars, and observations have been made with ever-increasing care and refinement. it is knowledge for its own sake that is longed for, knowledge that can only be won by infinite patience and care. the chief instrument used in making a star catalogue is called a transit circle; two great stone pillars are set up, each carrying one end of an axis, and the axis carries a telescope. the telescope can turn round like a wheel, in one direction only; it points due north or due south. a circle carefully divided into degrees and fractions of a degree is attached to the telescope. in the course of the twenty-four hours every star above the horizon of the observatory must come at least once within the range of this telescope, and at that moment the observer points the telescope to the { } star, and notes the time by his clock when the star crossed the spider's threads, which are fitted in the focus of his eye-piece. he also notes the angle at which the telescope was inclined to the horizon by reading the divisions of his circle. for by these two--the time when the star passed before the telescope and the angle at which the telescope was inclined--he is able to fix the position of the star. "but why should catalogues be repeated? when once the position of a star has been observed, why trouble to observe it again? will not the record serve in perpetuity?" the answers to these questions have been given by star catalogues themselves, or have come out in the process of making them. the earth rotates on its axis and revolves round the sun. but that axis also has a rolling motion of its own, and gives rise to an apparent motion of the stars called +precession+. hipparchus discovered this effect while at work on his catalogue, and our knowledge of the amount of precession enables us to fix the date when the constellations were designed. similarly, bradley discovered two further apparent motions of the stars--+aberration+ and +nutation+. of these, the first arises from the fact that the light coming from the stars moves with an inconceivable speed, but does not cross from star to earth instantly; it takes an appreciable, even a long, time to make the journey. but the earth is travelling round the sun, and therefore continually changing its direction of motion, and in consequence there is an apparent change in the direction in which the star is seen. the change is very small, for though the earth moves -½ miles in a second, light travels , times as fast. stars therefore are deflected from their true positions by aberration, by { } an extreme amount of . " of arc, that being the angle shown by an object that is slightly more distant than , times its diameter. the axis of the earth not only rolls on itself, but it does so with a slight staggering, nodding motion, due to the attractions of the sun and moon, known as +nutation+. and the axis does not remain fixed in the solid substance of the earth, but moves about irregularly in an area of about feet in diameter. the positions of the north and south poles are therefore not precisely fixed, but move, producing what is known as the +variation of latitude+. then star-places have to be corrected for the effect of our own atmosphere, _i.e._ refraction, and for errors of the instruments by which their places are determined. and when all these have been allowed for, the result stands out that different stars have real movement of their own--their +proper motions+. no stars are really "fixed"; the name "+fixed stars+" is a tradition of a time when observation was too rough to detect that any of the heavenly bodies other than the planets were in motion. but nothing is fixed. the earth on which we stand has many different motions; the stars are all in headlong flight. and from this motion of the stars it has been learned that the sun too moves. when copernicus overthrew the ptolemaic theory and showed that the earth moves round the sun, it was natural that men should be satisfied to take this as the centre of all things, fixed and immutable. it is not so. just as a traveller driving through a wood sees the trees in front apparently open out and drift rapidly past him on either hand, and then slowly close together behind him, so sir william herschel showed that the stars in one { } part of the heavens appear to be opening out, or slowly moving apart, while in the opposite part there seems to be a slight tendency for them to come together, and in a belt midway between the two the tendency is for a somewhat quicker motion toward the second point. and the explanation is the same in the one case as in the other--the real movement is with the observer. the sun with all its planets and smaller attendants is rushing onward, onward, towards a point near the borders of the constellations lyra and hercules, at the rate of about twelve miles per second. part of the proper motions of the stars are thus only apparent, being due to the actual motion of the sun--the "+sun's way+," as it is called--but part of the proper motions belong to the stars themselves; they are really in motion, and this not in a haphazard, random manner. for recently kapteyn and other workers in the same field have brought to light the fact of +star-drift+, _i.e._ that many of the stars are travelling in associated companies. this may be illustrated by the seven bright stars that make up the well-known group of the "plough," or "charles's wain," as country people call it. for the two stars of the seven that are furthest apart in the sky are moving together in one direction, and the other five in another. another result of the close study of the heavens involved in the making of star catalogues has been the detection of double stars--stars that not only appear to be near together but are really so. quite a distinct and important department of astronomy has arisen dealing with the continual observation and measurement of these objects. for many double stars are in motion round each other in obedience to the law of gravitation, and their orbits have been computed. { } some of these systems contain three or even four members. but in every case the smaller body shines by its own light; we have no instance in these double stars of a sun attended by a planet; in each case it is a sun with a companion sun. the first double star to be observed as such was one of the seven stars of the plough. it is the middle star in the plough handle, and has a faint star near it that is visible to any ordinarily good sight. star catalogues and the work of preparing them have brought out another class--variable stars. as the places of stars are not fixed, so neither are their brightnesses, and some change their brightness quickly, even as seen by the naked eye. one of these is called +algol+, _i.e._ the demon star, and is in the constellation perseus. the ancient greeks divided all stars visible to the naked eye into six classes, or "+magnitudes+," according to their brightness, the brightest stars being said to be of the first magnitude, those not quite so bright of the second, and so on. algol is then usually classed as a star of the second magnitude, and for two days and a half it retains its brightness unchanged. then it begins to fade, and for four and a half hours its brightness declines, until two-thirds of it has gone. no further change takes place for about twenty minutes, after which the light begins to increase again, and in another four and a half hours it is as bright as ever, to go through the same changes again after another interval of two days and a half. algol is a double star, but, unlike those stars that we know under that name, the companion is dark, but is nearly as large as its sun, and is very close to it, moving round it in a little less than three days. at one point of its orbit it comes between algol and the earth, { } and algol suffers, from our point of view, a partial eclipse. there are many other cases of variable stars of this kind in which the variation is caused by a dark companion moving round the bright star, and eclipsing it once in each revolution; and the diameters and distances of some of these have been computed, showing that in some cases the two stars are almost in contact. in some instances the companion is a dull but not a dark star; it gives a certain amount of light. when this is the case there is a fall of light twice in the period--once when the fainter star partly eclipses the brighter, once when the brighter star partly eclipses the fainter. but not all variable stars are of this kind. there is a star in the constellation cetus which is sometimes of the second magnitude, at which brightness it may remain for about a fortnight. then it will gradually diminish in brightness for nine or ten weeks, until it is lost to the unassisted sight, and after six months of invisibility it reappears and increases during another nine or ten weeks to another maximum. "mira," _i.e._ wonderful star, as this variable is called, is about times as bright at maximum as at minimum, but some maxima are fainter than others; neither is the period of variation always the same. it is clear that variation of this kind cannot be caused by an eclipse, and though many theories have been suggested, the "+long-period variables+," of which mira is the type, as yet remain without a complete explanation. more remarkable still are the "new stars"--stars that suddenly burst out into view, and then quickly fade away, as if a beacon out in the stellar depths had suddenly been fired. one of these suggested to hipparchus the need for a catalogue of the { } stars; another, the so-called "pilgrim star," in the year was the means of fixing the attention of tycho brahe upon astronomy; a third in was observed and fully described by kepler. the real meaning of these "new," or "temporary," stars was not understood until the spectroscope was applied to astronomy. they will therefore be treated in the volume of this series to be devoted to that subject. it need only be mentioned here that their appearance is evidently due to some kind of collision between celestial bodies, producing an enormous and instantaneous development of light and heat. these new stars do not occur in all parts of the heavens. even a hasty glance at the sky will show that the stars are not equally scattered, but that a broad belt apparently made up of an immense number of very small stars divides them into two parts. the milky way, or galaxy, as this belt is called, bridges the heavens at midnight, early in october, like an enormous arch, resting one foot on the horizon in the east, and the other in the west, and passing through the "+zenith+," _i.e._ the point overhead. it is on this belt of small stars--on the milky way--that new stars are most apt to break out. the region of the milky way is richer in stars than are the heavens in general. but it varies itself also in richness in a remarkable degree. in some places the stars, as seen on some of the wonderful photographs taken by e. e. barnard, seem almost to form a continuous wall; in other places, close at hand, barren spots appear that look inky black by contrast. and the +star clusters+, stars evidently crowded together, are frequent in the milky way. and yet again beside the stars the telescope reveals { } to us the nebulÆ. some of these are the irregular nebulæ--wide-stretching, cloudy, diffused masses of filmy light, like the great nebula in orion. others are faint but more defined objects, some of them with small circular discs, and looking like a very dim uranus, or even like saturn--that is to say, like a planet with a ring round its equator. this class are therefore known as "+planetary nebulæ+," and, when bright enough to show traces of colour, appear green or greenish blue. these are, however, comparatively rare. other of these faint, filmy objects are known as the "+white nebulæ+," and are now counted by thousands. they affect the spiral form. sometimes the spiral is seen fully presented; sometimes it is seen edgewise; sometimes more or less foreshortened, but in general the spiral character can be detected. and these white nebulæ appear to shun the galaxy as much as the planetary nebula; and star clusters prefer it; indeed the part of the northern heavens most remote from the milky way is simply crowded with them. it can be by no accident or chance that in the vast edifice of the heavens objects of certain classes should crowd into the belt of the milky way, and other classes avoid it; it points to the whole forming a single growth, an essential unity. for there is but one belt in the heavens, like the milky way, a belt in which small stars, new stars, and planetary nebulæ find their favourite home; and that belt encircles the entire heavens; and similarly that belt is the only region from which the white nebulæ appear to be repelled. the milky way forms the foundation, the strong and buttressed wall of the celestial building; the white nebulæ close in the roof of its dome. { } and how vast may that structure be--how far is it from wall to wall? that, as yet, we can only guess. but the stars whose distances we can measure, the stars whose drifting we can watch, almost infinitely distant as they are, carry us but a small part of the way. still, from little hints gathered here and there, we are able to guess that, though the nearest star to us is nearly , times as far as the sun, yet we must overpass the distance of that star times before we shall have reached the further confines of the galaxy. nor is the end in sight even there. this is, in briefest outline, the story of astronomy. it has led us from a time when men were acquainted with only a few square miles of the earth, and knew nothing of its size and shape, or of its relation to the moving lights which shone down from above, on to our present conception of our place in a universe of suns of which the vastness, glory, and complexity surpass our utmost powers of expression. the science began in the desire to use sun, moon, and stars as timekeepers, but as the exercise of ordered sight and ordered thought brought knowledge, knowledge began to be desired, not for any advantage it might bring, but for its own sake. and the pursuit itself has brought its own reward in that it has increased men's powers, and made them keener in observation, clearer in reasoning, surer in inference. the pursuit indeed knows no ending; the questions to be answered that lie before us are now more numerous than ever they have been, and the call of the heavens grows more insistent: "lift up your eyes on high." { } books to read popular general descriptions:-- sir r. s. ball.--_star-land_. (cassell.) agnes giberne.---sun, moon and stars_. (seeley.) w. t. lynn.--_celestial motions_. (stanford.) a. & w. maunder.---the heavens and their story_. (culley.) simon newcomb.--_astronomy for everybody_. (isbister.) for beginners in observation:-- w. f. denning.--_telescopic work for starlight evenings_. (taylor & francis.) e. w. maunder.--_astronomy without a telescope_. (thacker.) arthur p. norton.--_a star atlas and telescopic handbook_. (gall & inglis.) garrett p. serviss.--_astronomy with an opera-glass_. (appleton.) star-atlases:-- rev. j. gall--_an easy guide to the constellations_. (gall and inglis.) e. m'clure and h. j. klein.--_star-atlas_. (society for promoting christian knowledge.) r. a. proctor.--_new star atlas_. (longmans.) astronomical instruments and methods:-- sir g. b. airy.--_popular astronomy; lectures delivered at ipswich_. (macmillan.) e. w. maunder.--_royal observatory, greenwich; its history and work_. (religious tract society.) { } general text-books:-- clerke, fowler & gore.--concise astronomy. (hutchinson.) simon newcomb.--popular astronomy. (macmillan.) c. a. young.--manual of astronomy. (ginn.) special subjects:-- rev. e. ledger.--_the sun; its planets and satellites_. (stanford.) c. a. young.--_the sun_. (kegan paul.) mrs. todd.--_total eclipses_. (sampson low.) nasmyth and carpenter.--_the moon_. (john murray.) percival lowell.--_mars_. (longmans.) ellen m. clerke.--_jupiter_. (stanford.) e. a. proctor.--_saturn and its system_. (longmans.) w. t. lynn.--_remarkable comets_. (stanford.) e. w. maunder.--_the astronomy of the bible_. (hodder and stoughton.) historical:-- w. w. bryant.--_history of astronomy_. (methuen.) agnes m. clerke.--_history of astronomy in the nineteenth century_. (a. & c. black.) george forbes.--_history of astronomy_. (watts.) biographical:-- sir e. s. ball.--_great astronomers_. (isbister.) agnes m. clerke.--_the herschels and modern astronomy_. (cassell.) sir o. lodge.--_pioneers of science_. (macmillan.) { } index aberration, "achilles" (minor planet), adams, john c., airy, "algol," "andromedes" (meteors), apsides, , argelander, barnard, e. e., "bear," the, biela's comet, bouvard, bradley, , bremiker, catalogues (star), - centauri, alpha, "ceres" (minor planet), challis, charles ii., chromosphere, chronometer, clairaut, columbus, comets, comet, halley's, ---- biela's, conic sections, constellations, the, ---- date of, cook, capt., copernicus, , , "copernicus" (lunar crater), , corona, cowell, crommelin, degrees, dollond, double stars, earth, form of, ---- size of, , eclipses, ecliptic, ellipse, epicycle, eratosthenes, "eros" (minor planet), , eudoxus, excentric, eye-piece, faculÆ, flamsteed, galileo, galle, gascoigne, gravitation, law of, hall, chester moor, halley, halley's comet, harrison, john, herschel, sir w., , , hipparchus, , , , hyperbola, job, book of, , "juno" (minor planet), jupiter, , , - kapteyn, kepler, , , kepler's laws, "kepler" (lunar crater), langley, latitude, variation of, "leonids" (meteors), leverrier, lowell, , "lyrids" (meteors), magnetic storm, magnetism, earth's, magnitudes of stars, "mare imbrium," mars, , , - ---- canals of, maskelyne, , maunder, mrs. walter, , mercury, , , , , - meteors, , micrometer, milky way, , minor planets, , minutes of arc, "mira," moon, , , , , , , - ---- distance of, "_nautical almanac_," , navigation, nebulæ, neptune, , newcomb, new stars, newton, , , newton's laws of motion, nodes, nutation, , "oases of mars," obelisks, object glass, observatories, berlin, ---- copenhagen, ---- greenwich, ---- mt. wilson, ---- paris, ---- pulkowa, ---- st. petersburg, ---- washington, ---- yerkes, "pallas" (minor planet), parabola, "perseids" (meteors), photography, photosphere, "pilgrim" star, piazzi, planets, pole of the heavens, pontécoulant, precession of the equinoxes, , "_principia_," prominences, "ptolemæus" (lunar crater), ptolemy, , radiant points, radius vector, reflectors, refractors, saturn, , - schiaparelli, schwabe, seconds of arc, sirius, solar system, tables of, - somerville, mrs., spheres, planetary, spörer, spörer's law, star catalogues, - ---- clusters, ---- drift, stars, fixed, ---- proper motions of, sun, , , , , , - ---- distance of, ---- dials, sun spots, ---- spot maximum, ---- ---- minimum, "sun's way," telescope, invention of, transit circle, tycho brahe, , , "tycho" (lunar crater), , , uranus, , variable stars, ---- ----, long period, venus, , , "vesta" (minor planet), young, c. a., zenith, , zodiac, signs of, , , , printed by ballantyne, hanson & co. edinburgh & london * * * * * "we have nothing but the highest praise for these little books, and no one who examines them will have anything else."--_westminster gazette_, nd june . the people's books the first ninety volumes the volumes issued are marked with an asterisk science . the foundations of science . . . by w. c. d. whetham, m.a., f.r.s. . embryology--the beginnings of life . . . by prof. gerald leighton, m.d. . biology . . . by prof. w. d. henderson, m.a. . zoology: the study of animal life . . . by prof. e. w. macbride, m.a., f.r.s. . botany; the modern study of plants . . . by m. c. stopes, d.sc., ph.d., f.l.s. . bacteriology . . . by w. e. carnegie dickson, m.d. . the structure of the earth . . . by prof. t. g. bonney, f.r.s. . evolution . . . by e. s. goodrich, m.a., f.r.s. . darwin . . . by prof. w. garstang, m.a., d.sc. . heredity . . . by j. a. s. watson, b.sc. . inorganic chemistry . . . by prof. e. c. c. baly, f.r.s. . organic chemistry . . . by prof. j. b. cohen, b.sc., f.r.s. . the principles of electricity . . . by norman k. campbell, m.a. . radiation . . . by p. phillips, d.sc. . the science of the stars . . . by e. w. maunder, f.r.a.s. . the science of light . . . by p. phillips, d.sc. . weather science . . . by r. g. k. lempfert, m.a. . hypnotism and self-education . . . by a. m. hutchison, m.d. . the baby: a mother's book . . . by a university woman. . youth and sex--dangers and safeguards for boys and girls . . . by mary scharlieb, m.d., m.s., and f. arthur sibly, m.a., ll.d. . marriage and motherhood . . . by h. s. davidson, m.b., f.r.c.s.e. . lord kelvin . . . by a. russell, m.a., d.sc., m.i.e.e. . huxley . . . by professor g. leighton, m.d. . sir william huggins and spectroscopic astronomy . . . by e. w. maunder, f.r.a.s., of the royal observatory, greenwich. . practical astronomy . . . by h. macpherson, jr., f.r.a.s. . aviation . . . by sydney f. walker, r.n. . navigation . . . by william hall, r.n., b.a. . pond life . . . by e. c. ash, m.r.a.c. . dietetics . . . by alex. bryce, m.d., d.p.h. philosophy and religion . the meaning of philosophy . . . by prof. a. e. taylor, m.a., f.b.a. . henri bergson . . . by h. wildon carr, litt.d. . psychology . . . by h. j. watt, m.a., ph.d., d.phil. . ethics . . . by canon rashdall, d.litt., f.b.a. . kant's philosophy . . . by a. d. lindsay, m.a. . the teaching of plato . . . by a. d. lindsay, m.a. . aristotle . . . by prof. a. e. taylor, m.a., f.b.a. . friedrich nietzsche . . . by m. a. mügge. . eucken: a philosophy of life . . . by a. j. jones, m.a., b.sc., ph.d. . the experimental psychology of beauty . . . by c. w. valentine, b.a., d.phil. . the problem of truth . . . by h. wildon carr, litt.d. . buddhism . . . by prof. t. w. rhys davids, m.a., f.b.a. . roman catholicism . . . by h. b. coxon. preface, mgr. r. h. benson. . the oxford movement . . . by wilfrid ward. . the bible and criticism . . . by w. h. bennett, d.d., litt.p., and w. f. adeney, d.d. . cardinal newman . . . by wilfrid meynell. . the church of england . . . by rev. canon masterman. . anglo-catholicism . . . by a. e. manning foster. . the free churches . . . by rev. edward shillito, m.a. . judaism . . . by ephraim levine, m.a. . theosophy . . . by annie besant. history . the growth of freedom . . . by h. w. nevinson. . bismarck and the origin of the german empire . . . by professor f. m. powicke. . oliver cromwell . . . by hilda johnstone, m.a. . mary queen of scots . . . by e. o'neill, m.a. . cecil john rhodes, - 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[transcriber's note: italicized text is indicated with _underscores_. bold text is indicated with +plus signs+. numbers inside curly braces, e.g. { } are page numbers.] transciber's note supercripts are denoted with a carat (^). whole and fractional parts are displayed as - / . italic text is displayed as _text_. new theories in astronomy by william stirling civil engineer [illustration] london: e. & f. n. spon, limited, haymarket new york: spon & chamberlain, liberty street to the reader. mr. william stirling, civil engineer, who devoted the last years of his life to writing this work, was born in kilmarnock, scotland, his father being the rev. robert stirling, d.d., of that city, and his brothers, the late mr. patrick stirling and mr. james stirling, the well known engineers and designers of locomotive engines for the great northern and south eastern railways respectively. after completing his studies in scotland he settled in south america, and was engaged as manager and constructing engineer in important railway enterprises on the west coast, besides other concerns both in peru and chile; his last work being the designing and construction of the railway from the port of tocopilla on the pacific ocean to the nitrate fields of toco in the interior, the property of the anglo-chilian and nitrate railway company. he died in lima, peru, on the th october, , much esteemed and respected, leaving the ms. of the present work behind him, which is now published as a tribute to his memory, and wish to put before those who are interested in the science of astronomy his theories to which he devoted so much thought. contents. page introduction. chapter i. the bases of modern astronomy. their late formation instruments and measures used by ancient astronomers weights and measures sought out by modern astronomers means employed to discover the density of the earth. measuring by means of plummets not sufficiently exact measurements with torsion and chemical balances more accurate sir george b. airy's theory, and experiments at the harton colliery results of experiments not reliable. theory contrary to the law of attraction proof by arithmetical calculation of its error difficulties in comparing beats of pendulums at top and bottom of a mine the theory upheld by text-books without proper examination of a particle of matter within the shell of a hollow sphere. not exempt from the law of attraction a particle so situated confronted with the law of the inverse square ofdistance from an attracting body. remarks thereon it is not true that the attraction of a spherical shell is "zero" for a particle of matter within it chapter ii. the moon cannot have even an imaginary rotation on its axis, but is generally believed to have. quotations to prove this proofs that there can be no rotation. the most confused assertion that there is rotation shown to be without foundations a gin horse does not rotate on its axis in its revolution a gin horse, or a substitute, driven instead of being a driver results of the wooden horse being driven by the mill the same results produced by the revolution of the moon. centrifugal force sufficient to drive air and water away from our side of the moon that force not sufficient to drive them away from its other side no one seems ever to have thought of centrifugal force in connection with air and water on the moon near approach made by hansen to this notion far-fetched reasons given for the non-appearance of air and water the moon must have both on the far-off hemisphere proofs of this deduced from its appearance at change where the evidences of this may be seen if looked for at the right place. the centrifugal force shown to be insufficient to drive off even air, and less water, altogether from the moon the moon must have rotated on its axis at one period of its existence the want of polar compression no proof to the contrary want of proper study gives rise to extravagant conceptions, jumping at conclusions, and formation of "curious theories" chapter iii. remarks on some of the principal cosmogonies. ancient notions the nebular hypothesis of laplace. early opinions on it. received into favour. again condemned as erroneous defects attributed to it as fatal. new cosmogonies advanced dr. croll's collision, or impact, theory discussed dr. braun's cosmogony examined m. faye's "origine du monde" defined shown to be without proper foundation, confused, and in some parts contradictory reference to other hypotheses not noticed. all more or less only variations on the nebular hypothesis necessity for more particular examination into it chapter iv. preliminaries to analysis of the nebular hypothesis definition of the hypothesis elements of solar system. tables of dimensions and masses explanation of tables and density of saturn volume, density and mass of saturn's rings, general remarks about them, and satellites to be made from them future of saturn's rings notions about saturn's satellites and their masses nature of rings seemingly not well understood masses given to the satellites of uranus and neptune. explanations of volumes of the members of the solar system at density of water chapter v. analysis of the nebular hypothesis. separation from the nebula of the rings for the separate planets, etc. excessive heat attributed to the nebula erroneous and impossible centigrade thermometer to be used for temperatures temperature of the nebula not far from absolute zero erroneous ideas about glowing gases produced by collisions of their atoms, or particles of cosmic matter in the form of vapours separation of ring for neptune. it could not have been thrown off in one mass, but in a sheet of cosmic matter thickness and dimensions of the ring uranian ring abandoned, and its dimensions saturnian ring do. do. jovian ring do. do. asteroidal ring do. do. martian ring do. do. earth ring do. do. venus ring do. do. mercurian ring do. do. residual mass. condensation of solar nebula to various diameters, and relative temperatures and densities unaccountable confusion in the mode of counting absolute temperature examined and explained. negative degrees of heat only equal degrees of absolute temperature the centigrade thermometric scale no better than any other, and cannot be made decimal the sun's account current with the nebula drawn up and represented by table iii. chapter vi. analysis continued. excessive heat of nebula involved condensation only at the surface. proof that this was laplace's idea noteworthy that some astronomers still believe in excessive heat interdependence of temperature and pressure in gases and vapours. collisions of atoms the source of heat conditions on which a nebula can be incandescent. sir robert ball no proper explanation yet given of incandescent or glowing gas how matter was thrown off, or abandoned by the jovian nebula division into rings of matter thrown off determined during contraction how direct rotary motion was determined by friction and collisions of particles saturn's rings going through the same process. left to show process form gradually assumed by nebulæ. cause of saturn's square-shouldered appearance a lens-shaped nebula could not be formed by surface condensation retrograde rotary motion of neptune and uranus, and revolution of their satellites recognised by laplace as possible satellites of mars. rapid revolution of inner one may be accounted for laplace's proportion of millions not reduced but enormously increased by discoveries of this century chapter vii. analysis continued. no contingent of heat could be imparted to any planet by the parent nebula only one degree of heat added to the nebula from the beginning till it had contracted to the density of / th of an atmosphere increase in temperature from ° to possible average of ° when condensed to , , miles in diameter time when the sun could begin to act as sustainer of life and light anywhere. temperature of space the ether devised as carrier of light, heat, etc. what effect it might have on the nebula first measure of its density, as far as we know the estimate _too_ high. may be many times less return to the solar nebula at , , miles in diameter plausible reason for the position of neptune not conforming to bode's law. the ring being very wide had separated into two rings bode's law reversed. ideas suggested by it rates of acceleration of revolution from one planet to another little possibility of there being a planet in the position assigned to vulcan densities of planets compared. seem to point to differences in the mass of matter abandoned by the nebula at different periods giving rise to the continuous sheet of matter separating into different masses. probably the rings had to arrive at a certain stage of density before contracting circumferentially possible average temperature of the sun at the present day. central heat probably very much greater churning of matter going on in the interior of the sun, caused by unequal rotation between the equator and the poles chapter viii. inquiry into the interior construction of the earth. what is really known of the exterior or surface what is known of the interior little to be learned from geology, which reaches very few miles down various notions of the interior what is learnt from earthquake and volcanoes. igno-aqueous fusion, liquid magma. generally believed that the earth consists of solid matter to the centre. mean density. surface density more detailed estimate of densities near the surface causes of increased surface density after the crust was formed calculations of densities for miles deep, and from there to the centre forming table iv. reflections on the results of the calculations notion that the centre is composed of the heaviest metals. "sorting-out" theory absurd considerations as to how solid matter got to the centre gravitation might carry it there, but attraction could not how the earth could be made out of cosmic matter, meteorites or meteors chapter ix. inquiry into the interior construction of the earth--_continued_ the earth gasiform at one period. density including the moon may have been / , th that of air. must have been a hollow body. proofs given division of the mass of the earth alone into two parts division of the two masses at miles from surface reasons why the earth cannot be solid to the centre gasiform matter condensing in a cone leaves apex empty proportions of the matter in a cone calculations of the densities of the outer half of the hollow shell of the earth. remarks upon the condensation calculations of inner half of the hollow shell remarks upon position of inner surface of the shell calculations of the same chapter x. inquiry into the interior construction of the earth--_continued_ density of · times that of water still too high for the possible compression of the component matter of the earth as known to us reasons for this conclusion drawn from crushing strains of materials a limit to density shown thereby the greatest density need not exceed · of water gases shut up in the hollow centre. their weight must so far diminish the conceded maximum of · density of inner half of earth at miles diameter. greatest density may be less than · of water supposed pressure of inclosed gases very moderate meaning of heat limit to density. temperature of interior half of shell and inclosed gases must be equal state of the hollow interior results of the whole inquiry chapter xi. the earth. the idea entertained by some celebrated men, and others difficulties of forming a sphere out of a lens-shaped nebula various studies of the earth's interior made for special purposes. difficulty some people find in conceiving how the average density of little over · can be possible, the earth being a hollow sphere what is gained by its being a hollow shell geological theories of the interior discussed. volcanoes and earthquakes in relation to the interior liquid matter on the interior surface of the shell, and gases in the hollow, better means for eruptions than magma layers focal depths of earthquakes within reach of water, but not of lavas minute vesicles in granite filled with gases, oxygen and hydrogen, but not water the moon. a small edition of the earth rotation stopped. convulsions and cataclysms caused thereby. air, water, vapour driven off thereby to far-off hemisphere. liquid matter in hollow interior would gravitate to the inside of the nearest hemisphere form and dimensions during rotation. altered form after it stopped agreeing very closely with hansen's "curious theory" chapter xii. some of the results arising from the sun's being a hollow sphere repetition of the effects of condensation on the temperature of the nebula ideas called up by the apparently anomalous increase of temperature how heat is carried from the sun to the earth the sun supposed to radiate heat only to bodies that can receive and hold it, and not to all space. the heat of the sun accumulated in a hot box to considerably beyond the boiling point of water the heat accumulated in this way supposed to be due to a peculiar function of the ether, as it is a fact that heat can be radiated from a cold to a hot body the sun must be gaseous, or rather gasiform, throughout. no matter in it solid or even liquid. divisions and densities of shell the hollow centre filled with gases, whose mass naturally diminishes the mean density of the whole body the amount of this reduction so far defined. the presence of gases or vapours in the hollow a natural result of condensation the hollow centre filled with gases not incompatible with the sun's being a hollow sphere. the temperature at the centre may be anything, not depending on any law of gases further exposition of hollow-sphere theory put off till after further development of the construction of the sun chapter xiii. the ether. its nature considered. behaves like a gas can be pumped out of a receive light and heat do not pass through a tube _in vacuo_. laboratory experiments examined light and darkness in a partial vacuum, though high electricity not a carrying agent why there are light and dark strata in a high vacuum the real carrying agent through a high vacuum is the residue of ether left in it. digression to consider the aurora how air may be carried to extraordinary heights. zones of air carried up are made luminous by electricity comparison of this method with experiments quoted experiment suggested to prove whether light passes freely through a vacuum tube the ether does not pervade all bodies freely it must be renounced altogether or acknowledged to be a material body, subject to expansion, condensation, heating or cooling how light and heat pass through glass temperature of the ether variable. zodiacal light, cause of chapter xiv. the ether considered and its nature explained. further proofs given by dr. crookes's work, of its material substance highest vacuum yet produced. absorbents cannot absorb the ether dr. crookes's definition of a gas. not satisfactory. why a fluid required to pump matter out of a vessel gas as described by dr. crookes would not suit the ether the only elastic fluid we have. the only real gas, if it is a gas a possible measure of the density of the ether causes of dark and light zones in high vacua the real conductor of light in a high vacuum how a vacuum tube glows, when electricity passes through it conclusions arrived at through foregoing discussions some exhibitions of light explained gases can be put in motion, but cannot move even themselves the ether shown to be attraction. and primitive matter also all chemical elements evolved from it. its nature stated action at a distance explained by the ether and attraction being one and the same chapter xv. construction of the solar system. matter out of which it was formed domains of the sun out of which the matter was collected stars nearest to the sun. table vii. showing distances remarks on binary stars. table viii. showing spheres of attraction between the sun and a very few sirius actually our nearest neighbour. form of the sun's domains of a very jagged nature creation of matter for the nebulæ, out of which the whole universe was elaborated. beginning of construction the law of attraction begins to operate through the agency of evolution form of the primitive solar nebula. the jagged peaks probably soon left behind in contraction how the nebula contracted. two views of the form it might take. comparison of the two forms, solid or hollow the hollow centre form adopted. the jagged peaks left behind the nebula assuming a spherical form. shreds, masses, crescents separated from one side probable form of interior of nebula. compared with envelopes in heads of some comets reflections on the nebula being hollow. opinions of others quoted the matter of a sphere solid to the centre must be inert there further proofs of the nebula being hollow how rotary motion was instituted such a nebula might take one of two forms the form depending on the class of nebula. planetary in the case of the solar system. a similar conception of how rotary motion could be instituted chapter xvi. the sun's neighbours still exercise their attraction over him regions of greatest density in the nebulæ dealt with; compared with the orbits of the planets made from them results of comparison favourable to the theory differences of size in the planets have arisen from variations in the quantity of matter accumulating on the nebulæ causes of the retrograde motions in neptune, uranus, and their satellites probable causes of the anomalous position of neptune rises and falls in the densities and dimensions of the planets explained the form of the nebulæ must have resembled a dumb-bell more about rises and falls in densities reason why the asteroid nebula was the least dense of the system not necessary to revise the dimensions given to the nebulæ causes of the anomalies in the dimensions, densities, etc., of the earth and venus the strictly spherical form of the sun accounted for. but it may yet be varied repetition that a spherical body could not be made from a lens-shaped nebula by attraction and condensation chapter xvii. former compromises taken up and begun to be fulfilled estimates of the heat-power of the sun made only from gravitation hitherto contraction and condensation of a nebula solid to the centre. heat produced from attraction as well as by gravitation what quantity of heat is produced by a stone falling upon the earth showing again that there is a difference between attraction and gravitation contraction and condensation of a hollow-sphere nebula, in the same manner as the solid one differences of rotation would be greater in a hollow nebula; because a great deal of the matter would be almost motionless in a solid sphere in neither case could matter be brought to rest, but only retarded in motion. different periods of rotation accounted for table of different rates explained heat produced by gravitation, attraction and churning, not all constituents of the heat-power of the sun there can be no matter in the sun so dense as water the hollow part of the sun acting as a reservoir of gases, heat and pressure the behaviour of heat produced in the nebula, and its power how sun-spots are produced cyclonic motions observed in sun-spots. why not all in certain directions, and why only observed in a very few cyclonic motions in prominences treated of many other things might be explained, on some of which we do not dare to venture. concluding observations chapter xviii. return to the peaks abandoned by the original nebula. an idea of their number example of their dimensions. what was made out of them what could be made from one of them how it could be divided into comets and meteor swarms an example given. how a comet may rotate on its axis. and what might be explained thereby. orbits and periods of revolution not ejected from planets. their true origin study of the velocities in orbit of comets, and results thereof how far comets may wander from the sun and return again no reason why comets should wander from one sun to another. confirmatory of the description, in chapter xv. of the sun's domains of the eternal evolution and involution of matter. the atmosphere and corona of the sun partial analogy between it and the earth's atmosphere the density of it near the sun's surface cannot be normally less than atmospheres, but might be so partially and accidentally probable causes of the enormous height of its atmosphere the mass taken into account, but cannot be valued most probably no matter in the sun exceeds half the density of water. the unknown line in the spectrum of the corona belongs to the ether new theories in astronomy. introduction. that a little knowledge is a dangerous thing to the possessor, has been pointed out often enough, probably with the idea of keeping him quiet, but it is very certain that the warning has not always had the desired effect; and in some respects it is perhaps much better that it has not, for it is sometimes the case that a little knowledge exhibited on an inappropriate occasion, or even wrongly applied, throws light upon some subject that was previously not very well understood. it sometimes happens that unconscious error leads to the discovery of what is right. the fact is, all knowledge is at first little, so that if the first possessor of it is kept quiet there is little chance of its ever increasing. on the other hand, much knowledge seems to be quite as ready to become dangerous on occasion, for it has sometimes led its possessor to fall into errors that can be easily pointed out, even by the possessor of little, if it is combined with ordinary intelligence. the possessor of much knowledge is apt to forget, in his keen desire to acquire more, that he has not examined with sufficient care all the steps by which he has attained to what he has got, and that by placing reliance on one false step he has erected for himself a structure that cannot stand; or, what is worse perhaps, has prevented those who have followed him in implicit dependence on his attainments and fame from finding out the truth. if, then, both of these classes are liable to fall into error, there appears to be no good reason why one belonging to the first mentioned of them should absolutely refrain from making his ideas known, especially as he may thus induce someone of the second to re-examine the foundations on which he has built up his knowledge. these reflections are in greater or lesser degree applicable to all knowledge and science of all kinds, even theological, in all their individual branches, and can be very easily shown to be both reasonable and true. and it may be added, or rather it is necessary to add, that every one of all the branches of all of them has a very manifest tendency towards despotism; to impose its sway and way of thinking upon the whole world. at various intervals during the present century speculation has been indulged in, and more or less lively discussion has taken place about the great benefit it would confer on universal humanity, were all the weights and measures of the whole earth arranged on the same standard. the universal standard proposed has been, of course, the metrical system, which had been elaborated by french _savants_ who most probably thought they had arrived at such a state of knowledge that they were able to establish the foundations of all science of all kinds and for all time, upon the most sure and most durable principles. these periods of metrical fever, so to speak, seem to come on without any apparent immediately exciting cause, and some people succumb to the disease, others do not, just the same as in the cases of cholera, influenza, plague, etc. whether some species of inoculation for it may be discovered, or whether it will be found that an unlimited attack is really perfect health, will most probably be found out in the course of time, although it may be some centuries hence. what is of interest to understand at the present time is, what are the benefits to be derived from the proposed universal standard of weights and measures, and how they are to be attained. the principal and most imposing reason for its adoption is that it would be of immense service to scientific men all over the world, who would thus be able to understand the discourses, writings, discoveries, etc. of each other without the necessity of having to enter into calculations of any kind in order to be able to comprehend the arithmetical part of what they have listened to or read. another argument brought forward in favour is, that it would greatly facilitate commercial transactions with foreign countries; and it has been lately advanced that great loss is suffered by one country selling its goods, manufactured according to its own measures, in countries where the metrical system has been adopted. yet another advantage held out is the convenience it would be to travellers in money matters; but as this argument cannot be admitted without taking into consideration the necessity for one universal language all over the world, it has practically no place in any discussion on the subject, until the evil caused by the building of the tower of babel has been remedied. not long after one of the periodical attacks of metric fever we came upon an essay written by j. j. jeans on "england's supremacy," and published in new york by harper and brothers, in , in which we found the following:-- numerical relation of occupations in england and wales in : professional · per cent. domestic · " agricultural · " commercial · " industrial · " in all · " this statement shows that per cent. of the whole population are occupied in some business or work of some kind, and leads us reasonably to suppose that the remaining per cent. consist of women, children, and people who--to put it short--are non-producers; the whole of whom can hardly be considered as much interested in the making of any alterations in the weights and measures of their country, rather the contrary, for they cannot expect to be much benefited by any change. the professional class naturally comprehends theology, law, medicine, and science generally, so that the · per cent. ascribed to it would be seriously reduced, if the advantage derived from the desired change were reckoned by the number really benefited by it. a similar reduction would have to be made on the · per cent. stated to be occupied in commerce, as it is not to be supposed that the whole of the number are engaged in foreign trade. thus the number of people in these two classes who might really reap some advantage from the change, may be reduced by at least one half; and if we consider that one person in ten of those occupied in the agricultural and industrial classes is a scientist--we may pardon the domestic class--a very liberal allowance indeed, we arrive at the conclusion that per cent. of the whole population might find, some more, some less, interest in the introduction into our country of the french metric system. the above statement refers only to england and wales, but if scotland and ireland are added to them, the per cent. proportion could not be very greatly altered: perhaps it would be less favourable to the change. thus per cent., or something like millions, of the whole population of the united kingdom would be called upon to change their whole system of weights and measures, in order that per cent., or somewhere between and - / millions, should find some little alleviation in a part of their labours; and surely to - / millions of scientists and merchants engaged in foreign trade is a very liberal allowance for the population of our country. if this does not show a tendency towards despotism, it would be hard to tell what it does show. of course, it would not be fair to assume that the whole of the per cent. would desire to see the proposed change carried into effect. in all likelihood, a very considerable portion of the number would be disposed to count the cost of erecting such a structure before actually laying its foundations, and would refrain from beginning the work on considering by what means it was to be brought to a conclusion; even without going so far as to find out that per cent. of it at least would have to be done by forced labour. they might even go the length of speculating on how long it would take to coerce the per cent. into furnishing the forced labour, and on the hopelessness of the task. on the other hand, they might think it more natural to lay hold of the alternative of adopting a special system of weights and measures for the use of science and foreign commerce alone, and leave the per cent. to follow their own national and natural customs, which they would be very likely to do whatever might be determined, if we may judge by the progress made in france a century after the system was thought to be established. very little opposition could be made to such a course, and if the best possible system were not adopted, the scientists would be the only parties put to inconvenience. they could improve and reform it, should they find it not to be perfect, without the necessity of coercing the per cent. into furnishing another contingent of forced labour. but little is to be gained by saying any more about it. should the metrical system be adopted some day by act of parliament, science will have obtained what it has so long coveted, will be quite satisfied, and will trouble itself very little about how it affects the rest of the population. it will perhaps never even think of how india will be brought to buy and sell through the medium of the french metrical system. and now we have only one step to take on this subject. we may say that the project of establishing one standard of weights and measures for the whole world has a most unpleasant resemblance to the object proposed by the builders of the tower of babel; the only thing that can be said in its favour being that it points towards an endeavour to do away with the bad results produced by that enterprise and to bring matters back to the state the world was in before the foundations of that celebrated edifice were laid. the foregoing is only one instance of the many that could be cited where science has schemed projects for universal progress without due thought, and has come to the conclusion that they could be easily carried out. there are as many examples of this jumping at conclusions as would fill many books, which of course it is not our purpose to do; but there is one that it is necessary to have brought forward for examination, because of its having, through a most incomprehensible want of thought, a tendency to establish natural religion on the very bases upon which the christian religion is established. the one referred to is that by which some of the most eminent scientists of the present century, following up what was done in former times, have been able by deep study and experiment, unfortunately coupled with unaccountable blindness or preconceived erroneous ideas, to formulate processes by which the whole universe may have elaborated itself from protyle and protoplasm, or some such substances which, without any foundation to build upon, they suppose to have existed from all eternity. this advance in science has been called the theory of evolution, and has been very generally considered to be new, or of comparatively very recent conception; but it is only a piece of the evidence of a very general propensity in those who come to acquire a little more knowledge, to flatter themselves that they have power to seize hold of the unknown. the theory may be _new_, but evolution most assuredly is not, as any one may convince himself who will take the trouble to read the first chapter of the book of genesis _and to think_. there he will find it stated that the earth and all things in it and on it were created and made in six days, or periods of time, showing him distinctly, if he does not shut his eyes wilfully, that two operations were employed in the process, one of creation and the other of making, which last can mean nothing but _evolution_. it does not matter a straw whether the latter operation was carried on personally by the creator and maker, or under the power of laws ordained by him for the purpose; it was evolution all the same, and just the kind of evolution the scientists above alluded to would have us believe to be new, not far from years after the account of the creation and making of the world was written by moses. it will do no harm to take special notice of the work that was done in each of the six periods, as it will help to fix attention on the subject during examination and judgment; and may even tend to open the eyes of any one who had made up his mind to keep them shut. in the first period the heavens and the earth were created, but the earth was without form and void, _inanis et vacuus_, according to _the vulgate_--(does that mean empty and hollow?)--and darkness was upon the face of the deep; but light was _let_ shine upon the earth to alternate with darkness, and between the two to establish day and night. it is therefore evident that after the earth was created it had to be reduced to something like its present form, a globe of some kind, and to rotate on an axis, otherwise there could have been no alternations of light and darkness, of day and night. where did the light come from? some people seem to think that moses should have included a treatise on the creation and evolution of the universe, in his account of the work done in the first period of creation. for all that can be truly said to the contrary, he seems to have been quite as able to do so as any scientist of the present day; but it is evident he thought it best to limit himself to writing only of the earth, as being of most interest to its inhabitants, and enough for them as a first lesson. the literature of science, however, of the present day, will tell them that long ages after the earth was _evolved_ into a globe, it must have been in a molten, liquid state, surrounded by an atmosphere of vapours of some of the chemical elements so dense that no light from without could shine through it, and could only be penetrated by light after the cooling of the earth had dispelled a sufficient portion of that dense atmosphere. with this explanation, which they had at hand for the looking for, they might have been so far satisfied, and have left moses to tell his story in his own way. in passing, it may not be out of place to say that, after the cooling of the earth had proceeded so far that the vapours of matter had been condensed and precipitated on its surface, all boiling of water whether in the seas or on its surface must soon have ceased, so that no inconceivably enormous volumes of steam could be thrown upwards to maintain an atmosphere impenetrable to light; and that when dense volumes of steam ceased to be thrown up, the condensation of what was already in the atmosphere would be so rapid, and its density so soon reduced sufficiently to admit of the passage of light through it, that one can almost fancy himself present on the occasion and appreciate the sublimity of the language. "and god said, let there be light, and there was light"; more especially if he had ever stood by the side of the cylinder of a large steam engine, and understood what he heard when the steam rushed from it into the condenser, and noted how instantaneous it seemed to be. any one who has watched a pot of water boiling on the fire and emitting clouds of steam, will have noticed how immediately the boiling ceased whenever the pot was removed from the fire; but he will also have noticed that the water still continued to emit a considerable quantity of vapour, and will be able to understand how it was that the cloudy atmosphere of the earth, at the time we are dealing with, could allow light to pass through it but still keep the source of light from being visible. he experiences daily how thin a cloud will hide the sun from his sight. but there is more to be said about this when the time comes for taking note of the actual appearance on the scene of the sun, moon, and stars. to obtain some rude idea of the time to be disposed of for evolution during the first period, let it be supposed that the whole of the time consumed in the creation and development of the earth was million years, as demanded by some geologists, the first period of the six would naturally be somewhere about millions of years, a period which would allow, probably, very liberal time for evolution, but could never have been consumed in creation, seeing that creation has always been looked upon as an almost instantaneous act. and if anyone is still capable of exacting that the period was a day of twenty-four hours, he has to acknowledge that at least twenty-three of them were dedicated to the work of evolution. the second period was evidently one solely of evolution, as all that was done during it was confined to _making_ the firmament which divides the waters from the waters; an operation which could never be confounded with creation, being probably brought about solely by the cooling of the earth, which was the only means by which a separation between the waters covering the earth, and those held in suspension above it by the atmosphere, could be brought about, and must have been purely the work of evolution. the third period was begun by collecting the waters under the firmament into one place and letting the dry land appear; which, it may be well to note, gives it to be understood that the surface of the solid part of the earth had come to be uneven either by the elevation or depression, perhaps both, of some parts of it, and next the earth was _let_ bring forth grass and trees, and in general vegetation of all kinds. these cannot be considered otherwise than as operations of evolution: there was no creation going on beyond what may have been necessary to help evolution, and of that not a word is said. here it is well to notice that until the waters were gathered together into one place and the dry land appeared there could be no alluvial deposits made in the sea, and that till well on into this third period, that is well on for million years from the beginning, there could be no geological strata deposited in it containing vegetable matter, for the very good reason that although rains and rivers may have swept earthy matter into the sea, the rivers could not carry along in their flow any vegetable matter until it had time to grow. should evolutionists think they have discovered something new in spontaneous generation, we refer them to the th verse of the chapter, where they will see--"and god said, let the earth bring forth grass, the herb yielding seed, and the fruit-tree yielding fruit after his kind, whose seed is in itself, upon the earth." the conclusion of this passage asserts plainly that the seed was already in the earth, somehow or other, ready to germinate and sprout when the necessary accompanying conditions were prepared. the words are very few, and they can have no other meaning. in the first period "god made two great lights: the greater light to rule the day and the lesser light to rule the night; he made the stars also." this passage has been "a stumbling block and rock of offence" to some people possessed of much knowledge and to some possessed of little; the one party professing to disbelieve all because the sun was _made_ four days after there was light, and the other party, supposing that there might have been light proceeding from some other source during the first four days. both parties seem to have forgotten that the earth was created without form and void, and that being so the same would naturally be the case with the sun and the moon; all of them had to be made into form after their creation. by what means? by evolution, of course, or whatever else anyone chooses to call it; that will make no difference. as far as it can penetrate into the mysteries of creation, physical astronomy has endeavoured to show how the solar system may have been formed out of a mass of nebulous matter. furthermore, as has already been adduced in evidence, that at one time the earth must have been a molten, liquid globe surrounded by vapours of metals, metalloids, gases, and finally by water; and even goes the length of supposing that the planets were evolved to something approaching their present state, long before the sun attained its present form. following up this hypothesis, it is more than probable that the sun had not attained that form when this fourth period began, and, although capable of emitting light early in the first period, still required a vast amount of evolution to reduce it to the brilliant globe now seen in the heavens. everybody knows that plants grow without sunshine, and it is generally believed that the primary forests of the earth grew most rapidly in a moist, stifling atmosphere, which neither admitted of animal life, nor could be penetrated by sunshine. thus physical astronomy cannot say that the sun could not have been made into its present state until near the end of this fourth period. it _may_ have been as bright as it is now, though very probably not, as we shall see in due time; but it could not _shine_ upon the earth, neither could the earth, nor anything thereon, see it. it is not necessary to say anything about the moon, as it only reflects sunlight, and the reflection could not reach the earth if the light could not. in the fifth period the waters were _let_ "bring forth the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven." here again spontaneous generation may have been provided for beforehand, the same as in the case of vegetation. also it is said "god created great whales," and it is to be observed that this is only the second time that creation has been mentioned in the book, and would seem to teach that _making_, or evolution, was the most active agent at work in the construction of the earth--and, we may add, of the universe. the sixth period was one almost exclusively of evolution, unless it should be considered that spontaneous generation is a different, and newly discovered process. in it god _made_ the beast of the earth, cattle, and everything that creepeth upon the earth, after his kind. last of all: "god said, let us make man in our image, after our likeness." thus it appears that the only work of creation done in this period was that of creating man, and even that _after_ some length of time and work had been expended in _making_ or _evolution_, which may have extended over a very considerable portion of the fifty millions of years corresponding to it. we have supposed the work of creation to have extended over three hundred million years to satisfy some geologists, but our arguments would not be affected in any way by the time being reduced to the limit given by lord kelvin to the heat-giving power of the sun in the past, which he has made out to be between fifteen and twenty million years. that would only limit our periods of evolution to two and a half or three million years each; each of them quite long enough to be totally inconsistent with our ideas of creation, which conceive of this as an instantaneous act. but although lord kelvin has in rather strong terms placed this limit, he at the same time says that it could by no means exceed four hundred million years, which is one-third more than we have calculated upon. neither can our arguments be affected in any serious way by our dividing the periods into fifty million years each; these may have varied much in length, but whatever was taken from one would have to be added to the others. furthermore, we may be allowed to say that fifteen to twenty millions of years of the sun's heat at the rate it is now being expended, can be no reliable measure of the time required for the operations of geology, for the reason that its heat must have been emitted in proportion to the quantity it possessed at any time. when it was created without form and void as no doubt it was, the same as the earth, it would have no heat to emit, but that does not mean that it possessed no heat until it was formed into the brilliant globe that we cannot now bear to turn our eyes upon. even when it became hot enough to show light sufficient to penetrate the "darkness that was upon the face of the deep," it may still have been an almost shapeless mass, and have continued more or less so until it was formed into the body of the fourth period, which may even then have been very different from what it is now. thus geology would have not far from one hundred and fifty million years in which a very small fractional part of the sun's emission of heat would suffice for its operations. but we shall have more to say on this subject when the time comes. it being, therefore, a matter beyond all question--to people possessed of the faculty of thinking, and of candour to confess that they cannot help seeing what has been set plainly before their sight and understanding--that the opening chapter of the book of genesis plainly teaches that making--evolution--had a very large and active part to perform in the creation of the universe and--much more within our grasp--of the earth; we can come to the conclusion that the theory of evolution, instead of being new and wonderful, comes to be almost infinitely older than the everlasting hills, without losing any of its power of inspiring inexpressible wonder. looking back over the examination into the first chapter of the book of genesis we have just concluded, we cannot conceive how it could ever have entered into the thoughts of man, that the state of vegetable and animal life on the earth, at the present day, must have been brought about by continual and unceasing acts of creation, when creation has been mentioned only on three occasions during the whole process described in the chapter we have analysed, that is, out of verses; and while the other processes which we have brought forward--making and spontaneous generation--have never been alluded to, perhaps not even thought of. we have no desire, neither are we qualified, to follow up this subject any further, but we have still one or two things to bring into remembrance. one of the most illustrious of the founders of the theory of evolution has based his dissertations on the descent of man, on the variation of animals and plants under domestication, and on their _wonderful plasticity under the care of man_. here there is an explicit acknowledgment of the necessity for the direction of an intelligent guiding power to produce such variations; these never having any useful or progressive results except under such care. if, then, there is a necessity of such directing and guiding power in the case of variations of such inferior importance, the superintendence of some similar power must have assuredly been much more necessary for the creation and evolution of matter, of life, and of man himself. this is what, one would think, common sense and reason would point, and what the theory of evolution seems to think--evidently without studying the subject far enough; but all that it has been able to do has been to substitute nature for the creator to whom moses has ascribed not only _creation_ but the _making--evolution_--of the universe. this naturally leads us to speculate on what evolutionists consider nature to be, and as none of them--nor anyone else--as far as we know, has ever thought it necessary to define nature, we have to endeavour to draw from their writings what, in some measure and some way, they would like us to believe it to be. we find, then, that the base of their operations seems to be natural selection, which can hardly be interpreted in any other way than by calling it the selection of nature. thus, then, they apparently want us to look upon nature as the _first cause_. but, if nature can select, it must be a being, an entity, a something, that can distinguish one particle of matter from another, and be able to choose such pieces of it, be they protyle or protoplasm, and to make them unite, so as to form some special body, organic or inorganic. it is plain, also, that selection can only be performed by such a being, or something, such as just so far described, that can distinguish, choose, and arrange the particles of matter destined to form the very smallest body or the universe. thus we see that in whatever way the basis of the theory of evolution is looked upon--_even for its own evolution_--there is required a being of some kind that has knowledge and power to evolve or make all things that are "in heaven above, or in the earth beneath, or in the waters under the earth." so we see that, if the theory of evolution dethrones the creator and evolver of the first chapter of genesis, it has to enthrone another god which it calls nature; and has to get rid of that god, and any number of others, before it can be what it pretends to be. we are all very voluble in talking of nature, and enthusiastic in admiring its beauties, wonders, and wisdom, but it seldom occurs to us that we are really doing so without thinking of whence come the beauty, wonders, and wisdom. we must, therefore, not be too hard on evolutionists, as they have only done what we all do every day of our lives; but if the theory of evolution is to be looked upon as a branch of science, we would recommend its students to open their eyes and think of it as a process which has been in existence from the beginning of things at least, and not as one of their invention or discovery. they may be able some day, through more accurate study and more convincing argumentation than they generally use, to lay claim to having discovered, as far as it is possible for man to do, the _modus operandi_ of evolution, but that is all, and we would also warn some of them to think that, when we see them in their highest flights of science, genius, and self-sufficiency, we can "conceive the bard the hero of the story." we have read a good deal of what has been called the war of science, without having been able to see that there ever was any cause for such a war, with the exception of ignorance. if theology had been able, or rather had taken the trouble, to study thoroughly the first chapter of genesis, and thus to comprehend that, if the earth was created without form and void, a great deal of work had to be done, after creation, in forming it into its present condition, there was no call upon it to find fault with copernicus or persecute galileus, because they said the earth revolved round the sun; more especially as they do not appear to have ever said anything against religion or revelation. neither was there any necessity for opposing the so-called new science of evolution, because it (theology) ought to have seen that the work expended in reducing the earth into form could hardly be conceived of otherwise than as a process of evolution; and would thus have been in a position to tell the authors of the _new_ science that they had only discovered what had existed before the beginning of time. on the other hand, there was no occasion for science to take up the war. if it, in its turn, had taken the trouble to study and understand the first chapter of genesis, it could have shown theology that _it_ did not comprehend, and could not give a true account of what religion and revelation are; whereas it (science) seems to have had a strong tendency to demonstrate that religion and revelation are altogether false, and that the great work it has to perform is to dethrone theology, and set itself up it in its stead. it is not worth while even to think of who or which was the aggressor, seeing that the war originated from ignorance caused by want of thought and study on both sides. all that has to be said on the subject reduces itself to the fact that both religion and science have been coming, and are at present going, through the process of evolution. can anyone say that science has been truly scientific, without ever incurring in error, from the beginning of history up to the present day? will any one venture to maintain that there has been no evolution, no progress, no softening of the spirit of religion, from the institution of christianity up to the end of the nineteenth century? if such there be, let the one look back to the time of aristotle, and the other to the establishment of the church under constantine. there has been for long an opinion, which goes on increasing in strength, that science will ultimately reform theology and put religion in its right place; but if such is to be the case, science has to begin by reforming itself and putting an end to error it has been, in many cases, teaching for generations; and by ceasing to formulate new theories, or bases of progress, which can be in many cases exploded by suppressing some of the error just alluded to. little advance is made in science by forming hypotheses and theories, however brilliant they may appear, unless they are carefully studied and thought out to the very uttermost; because, if published abroad on the authority of some celebrated or even well-known name, they have a tendency to stop further investigation, and prevent students from exercising their own judgment and perhaps discovering what they might possibly find out were they to study them to the very end for their own satisfaction. this is in some measure the case even with respect to the solar system. we believe it can be shown that a more complete knowledge and comprehension of it, and even of the universe, has been kept back by the unquestioning acceptation by successive astronomers of the ideas and conceptions of their predecessors. we have to acknowledge, at the same time, that astronomy could not start into perfection at once, any more than any other science, and it is not to be wondered at that in times past ideas relating to it should have been formed without being properly thought out; even ideas that could not be properly thought out to the end for want of the requisite knowledge. but it is much to be regretted that such ideas should continue to be published at the present day as trustworthy instruction for readers who may look upon it as strictly correct. among those who read text-books even on astronomy, there must be a very considerable number who are rather surprised when they see statements made which do not agree with what they were taught at school, or with what they have practised in other sciences in their own professions or trades. it may be said that any person of ordinary intelligence will easily be able to correct such errors, but the evil does not stop here. if he can really correct them he will most probably find as well, that his instructors have been led into more serious errors, perhaps in more important matters, founded on the ideas which they had not fully studied out before giving them a place in their books. he may also find sometimes, in his reading, such ideas brought forward to substantiate some theory, just as far as they are required and then dropped, while a step or two further forward in the examination of these same ideas, would have exploded the theory altogether; because, although founded to a certain extent on one law of nature, they are in contradiction with what is laid down in some other law. the above will be looked upon as an unwarrantably bold assertion; but a careful study of, or attention to, what is taught in the most advanced works on the solar system, even in science generally, will show it to be perfectly true. it is not only true, but the consequences of its being true have been much more serious than will be readily believed. in our own endeavours to understand what we had been reading, we have seen that some of the notions presented to us were only half formed, and that they have led to theories being founded which could never have been entertained at all had they been thoroughly studied out. more than that, they have prevented the truth from being arrived at in the fundamental conceptions of the construction of the earth, and, as a natural consequence, of the whole solar system, perhaps even of the whole universe. there are probably many, even a great many, people who have arrived at the same conclusions as we have, but as far as it has been in our power to search into the matter, we have met with no attempt from any quarter to put an end to this defect in the literature of science; perhaps because the work has the appearance of being too great to be readily undertaken, and also because it may be thought that there is little to be gained by it--as all is sure to be set right through time. but, as we believe that it will be beneficial immediately, in the case of the earth and solar system at least, we shall first attempt to show what are some of the defects alluded to, and then what knowledge may be acquired through their removal. chapter i. page the bases of modern astronomy. their late formation instruments and measures used by ancient astronomers weights and measures sought out by modern astronomers means employed to discover the density of the earth. measuring by means of plummets not sufficiently exact measurements with torsion and chemical balances more accurate sir george b. airy's theory, and experiments at the harton colliery results of experiments not reliable. theory contrary to the law of attraction proof by arithmetical calculation of its error difficulties in comparing beats of pendulums at top and bottom of a mine the theory upheld by text-books without proper examination of a particle of matter within the shell of a hollow sphere. not exempt from the law of attraction a particle so situated confronted with the law of the inverse square ofdistance from an attracting body. remarks thereon it is not true that the attraction of a spherical shell is "zero" for a particle of matter within it before astronomers could begin to determine the relative distances from each other, and the relative dimensions and masses of the various members of the solar system, they had to establish scales of measurements appropriate to their undertaking. this entailed upon them, of course, the necessity of determining the form, the different circumferences and diameters, and the weight of the whole earth, as any other scales derived from the only available source, the earth, would have been too small to give even an approximate value of the measures and masses to be sought for. history tells us that at least one attempt had been made, over two thousand years ago, to find the circumference and necessarily the diameter of the earth, but it says nothing of any to ascertain its weight. there may have been many to determine both diameter and mass, but we know nothing of them; and when we think seriously about this, we cannot help feeling somewhat surprised that no attempt had been made to find out the density and mass till more than a century after sir isaac newton's discovery of the law of attraction, or gravitation, as it is more usually called. but perhaps this is an idea that could only occur to one who has been _spoilt_ by witnessing, in great measure, the immense strides in advance that have been made during the nineteenth century in science of all kinds, and does not duly take into account the immense labour, and the incessant meeting with almost insurmountable difficulties, that astronomers have had to encounter and overcome between the birth of modern astronomy and the end of the eighteenth century. indeed, the difficulties can hardly be looked upon as altogether overcome even yet, as efforts are still being made to find out the exact distance of the sun, and it is not impossible that some small difference may be found, plus or minus, in the density at present adopted for the earth of · times the weight of water. the geometer who, more than two thousand years ago, set himself the task of measuring the circumference of the earth, is supposed to have made use of very much the same kind of implements as those employed by modern astronomers. he must have had a very fair instrument for measuring angles, and have known very well how to use it, seeing he was able to determine a value for the obliquity of the ecliptic which agrees so well with that established by modern science, its variations being, for what we know, taken into account; and for length or distance he would doubtless have some implement analogous to the metre, chain, foot-rule, or something called by other name that would, in those days, present facilities for selling a yard of calico. his operations would probably be as plain and simple as those applied to the measuring of a village green--for we are not told that he had any idea of there being any difference between the length of a degree of the meridian at the equator and one nearer either of the poles--and involved no hypotheses or theories, any more than modern operations have done. when the time came for making efforts to ascertain the density of the earth, science seems to have employed the very simplest means it had at its disposal for attaining its object, and to have gone on refining its implements and operations in conformity with the lessons it went on learning while pursuing its self-imposed task. every one who, even for recreation, has read a fair amount of the multitude of works and writings that have been published on popular astronomy--not to speak of text-books--knows that the first attempts were made by measuring the attraction of steep, or precipitous, mountains for plummets suspended in appropriate positions in their neighbourhood; then--evidently from knowledge acquired during these operations--by the attraction for each other of large and small leaden balls suspended on frames and torsion balances, which go under the name of the cavendish experiment; and afterwards by a refinement on this in using the chemical balance, where only one large and one small ball of metal are required. all these operations and their results are to be found described in works of various kinds, and are generally reduced to something like the following tubular form, which we reproduce in order to make more intelligible what we have just said, and that we may make a few remarks upon them. there is no hypothesis, no theory, connected with any of the operations, unless it was the supposition that a plummet--which was naturally believed to point to the centre of the earth--should be pulled to one side by the attraction for it of a mountain in its neighbourhood, and that was found to be a fact. methods employed for finding the density of the earth, and their results. ( ) _deviation of plummet by the attraction of mountains_:-- experiments made. by whom, and date. mean density found. at schiehallien maskelyne · at arthur's seat sir h. james · ( ) _torsion balance experiments_:-- cavendish · at freyberg, saxony reich · at manchester francis baily - · ( ) _chemical balance experiments_:-- j. h. pointing · in the case of the plummet deviating from its absolutely straight direction towards the centre of the earth, caused by their attraction, not only the mountains themselves had to be measured and virtually weighed as far as they were measurable, but the weight of the wedge or pyramid between that measurable point, in each case, and the centre of the earth had to be estimated in some way; then the centre of gravity of the whole of this mass had to be ascertained, as well as the respective distances from the centre of the earth of this centre of gravity and that of the plummet, and only after all this and a deep study of the mutual attractions of this mass and the plummet could an estimate be formed of the mass of the earth. it will thus be seen that such measurements and estimates could never be looked upon as very exact and reliable; and nevertheless they have come very near the density of · finally adopted for the earth. in the case of the torsion balance experiments a very considerable advance was made in consequence, most undoubtedly, of the knowledge acquired from what had been done by maskelyne. when it was found that the attraction of schiehallien for the plummets was such a measurable quantity, cavendish evidently saw that the attraction of manageable leaden balls for each other would be measurable also, and that as no calculations of any kind whatever were necessary to find the masses of the balls, the mutual attraction of large and small balls would furnish a more exact means of measuring the density of the earth, than the roundabout way of having to calculate the weight of a mountain as a beginning; and with the requisite ingenuity, invention, and labour, he found the means of applying the torsion balance, to make the experiments. after these experiments were revised by reich and baily--and the density of · adopted, we believe--still another set were undertaken by j. h. pointing, with the chemical balance, in which only two metal balls, one large and one small were required, which gave a density of · as shown opposite, and from its extreme simplicity may perhaps have been the most exact of all. we have said, we think with truth, that there is no hypothesis or theory involved in any of these experiments, but only the simplest form of--we might almost say--arithmetical calculation. but there is a theory built up on hypothesis which has no foundation whatever, and about which most people, who take the trouble to study it out to the very end, will come to the conclusion that "the less said the better." this, at all events, is our opinion, and we would not have taken any notice whatever of it had it not been that up to the present day, it is published in many works on popular astronomy, and even in some text-books, and is looked upon in them, apparently, as an example of the transcendent height to which human science can reach. we allude, of course, to the theory that the deeper we go down into the earth--at least to an undefined and undefinable depth--the greater is its attraction for the bob of a pendulum at that depth, and the greater the number of vibrations the pendulum is caused to make in a given time. the explanation of the theory is, that were the earth homogeneous throughout its whole volume, the pendulum ought to make the fewer vibrations, the deeper down in the earth it is placed; but as the earth is not homogeneous, it actually makes a greater number of vibrations in a given time, because the attractive force of the earth increases--up to the undefined and undefinable depth--on account of the denser matter beneath the pendulum bob more than overbalancing the loss of attraction from the lighter matter left above it. the author of the theory was the late astronomer royal, sir george b. airy, who from it endeavoured to calculate the mean density of the earth, and with that view made two experiments which are thus described by professor c. piazzi smythe in his work on the great pyramid:-- "another species of experiment... was tried in by mr. (now sir) george b. airy, astronomer royal, dr. whewell, and the rev. richard sheepshanks, by means of pendulum observations at the top and bottom of a deep mine in cornwall; but the proceedings at that time failed. subsequently, in , the case was taken up again by sir george b. airy and his greenwich assistants, in a mine near newcastle. they were reinforced by the new invention of sympathetic electric control between clocks at the top and bottom of a mine, and had much better, though still unexpectedly large results--the mean density of the earth coming out, for them, · ." from other sources we have also found that the pit, or mine, was at the harton colliery and feet deep, that the pendulum at the bottom of it gained - / seconds on the similar one at the top, in hours; and that the surrounding country had to be extensively surveyed, the strata had to be studied, and their specific gravities ascertained. a little unbiassed thought bestowed on this theory will at once show that it begins by violating the law of attraction discovered by newton, when he showed _that the mutually attractive forces of several bodies are the same as if they were resident in the centres of gravity of the bodies_. in the case in point this means, that the attraction of the earth for the bob of the pendulum at the top of the mine was the same as if all its force was collected at its (the earth's) centre. in that position the force of the earth's attraction comprehended, most undeniably, the whole of its attractive power, including whatever might be imagined to be derived from the non-homogeneity of the earth, due to its density increasing towards the centre; and we are called upon to believe that when, virtually, the same pendulum was removed to the bottom of the mine, and a segment feet thick, at the centre as good as cut off from the earth and--as far as the pendulum was concerned--hung up on a peg in a laboratory, the diminished quantity of its matter had a greater attractive force, a very little beyond the centre--non-homogeneity again included--than the whole when the sphere was intact. this we cannot do, because all that we can see in the placing of the pendulum at the bottom of the mine, is that the position of the bob has divided the earth into two sections, one of which has a tendency to pull it up towards the surface, and the other to pull it down towards its centre of gravity; and because the mass of the smaller segment is so insignificant that its entire removal to the laboratory peg, not only could not produce the reverse action, on which the theory is based, but could not be measured by any stretch of human invention or ingenuity; it is far beyond the reach of mathematics and human comprehension of quantity. the difficulty of belief is increased when we reflect that, were the pendulum taken down towards the centre of the earth, the number of its vibrations in a given time ought gradually to decrease as it approached the centre, and would cease altogether when that point was reached. and we feel confident that no mathematician could calculate where the theoretical acceleration of the vibrations would cease, and the inevitable retardation commence; where the theory would come to an end and the law of attraction begin to assert its rights, simply because he does not know how the non-homogeneity is distributed in the earth. no man can tell, even yet, how the mean density of · is made up throughout the earth, and without that any theory founded on its non-homogeneity is out of place. but to follow up our assertion of non-commensurability. taking the diameter of the earth at miles, and its mean specific gravity at · , its mass would be represented by , , , , cubic miles of water. on the other hand, supposing the earth to be a true sphere, the volume of a segment of it cut off from one side, at one quarter of a mile deep--not , but feet--would be · cubic miles in volume, and if we suppose its specific gravity to be · --greater most probably than the average of all the strata in the neighbourhood of the harton colliery--its mass would be represented by · cubic miles of water. then, if we divide the mass of the section below the pendulum, that is, , , , , minus the mass of the one above it, · , viz. , , , , · by the mass of · just mentioned, we find that the proportion they bear to each other is as to , , . this being so, we are asked to believe that by removing / , , th part of the mass of the earth from one side of it, its force of attraction at the centre will not only not be decreased, but will be so increased that it will cause a pendulum, suspended at the centre of the flat left by the removal of the segment, to vibrate , · times in twenty-four hours instead of , times as it did when suspended at the surface before the segment was removed; that is, that the vibrations will be increased by / , th part. again we cannot do so. had we been asked to believe that the removal of so small a fraction as / , , th had decreased the earth's attraction at its centre, so much as to produce a diminution of / , th part in the number of vibrations of the pendulum, we could not have done so; how much less then can we believe that the central attractive force had increased so much as to produce an augmentation of the vibrations in the same proportions? but more in this strain presently. we have no doubt whatever that sir george b. airy and his assistants satisfied themselves that the pendulum at the bottom of the mine gained - / seconds in twenty-four hours over the one at the top, but they may have been deceived by their over-enthusiastic adoption of what seemed to be a very grandly scientific theory, or by some unperceived changes in the temperature in the pendulums, caused by varying ventilation in the mine or the varying weather outside of it, or by the insidious manifestations of the "sympathetic electric control between clocks at the top and bottom of a mine," called in to assist at the experiments. an error of / , th part of the time the sympathetic electricity would take to travel from the top to the bottom of the shaft would be sufficient to make the experiments of no value whatever; not to speak of the small errors that may have been made in surveying the surrounding country, calculating the specific gravities of the strata--for we are told that all this had to be done-and applying the elements thus obtained to the solution of the problem they had in hand. we have read of the difficulties met with by mr. francis baily when he began to revise the cavendish experiment--some twelve or fifteen years before the final harton colliery experiments were made, and suppose it possible that they met with similar difficulties without being aware of it. and / , th part is such a very small fractional difference in the vibrations in twenty-four hours, of the pendulums of the two separate clocks, that--taking into consideration the circumstances under which it was found--it would hardly be looked upon as reliable at the present day, when the clocks of astronomical observatories are placed in the deepest cellars or even caves available, so as to free them as much as possible from variations of temperature. having referred to the difficulties met with by mr. baily, we believe it worth while to transcribe professor c. piazzi smythe's account of them, given in his work already referred to at page ; because it not only has a very direct bearing on what we have been saying of changes of temperature, but is exceedingly interesting, and probably very rarely to be met with in other works. it is as follows:-- "nearly forty years after cavendish's great work, his experiment was repeated by professor reich of freyberg, in saxony, with a result of · ; and then came the grander repetition of the late mr. francis baily, representing therein the royal astronomical society, and, in fact, the british government and the british nation. "with exquisite care did that well-versed and methodical observer proceed to his task, and yet his observations did not prosper. "week after week, and month after month, unceasing measures were recorded; but only to show that some disturbing element was at work, overpowering the attraction of the larger on the smaller balls. "what could it be? "professor reich was applied to, and requested to state how he had continued to get the much greater degree of accordance with each other, that his published observations showed. "'ah!' he explained, 'he had to reject all his earlier observations until he had guarded against variations of _temperature_ by putting the whole apparatus into a cellar, and only looking at it with a telescope through a small hole in the door.' "then it was remembered that a very similar plan had been adopted by cavendish, who had furthermore left this note behind him for his successor's attention--'that even still or after all the precautions which he did take, minute variations and small changes of _temperature_ between the large and small balls were the chief obstacles to full accuracy.' "mr. baily therefore adopted yet further, and very peculiar, means to prevent sudden changes of temperature in his observing room, and then only did the anomalies vanish and the real observations begin. "the full history of them, and all the particulars of every numerical entry, and the whole of the steps of calculation, are to be found in the memoirs of the royal astronomical society, and constitute one of the most interesting volumes (the fourteenth) of that important series; and its final result for the earth's mean density was announced as · , probable error ± · ." after reading this story of baily's experiments with care, one cannot help feeling something stronger than want of confidence in those made at the harton colliery, especially after what has been shown of the smallness of the fraction of the earth that was dealt with, and due consideration is given to the insignificant difference of effect that the non-homogeneity of the earth could produce on the remainder after the supposed removal of such a small fraction; and here we might let the theory drop. perhaps it may be thought that now there is nothing to be gained by spending time and work in showing it to be more truly erroneous than we have yet made it out to be; but if there is error, it cannot be too clearly exposed, and the sooner it is put an end to, the better; more especially as it has been accepted as true by some authors of text-books, and by some competent astronomers who, in trying to explain the anomaly of the increase instead of decrease in the force of attraction at the bottom of a mine compared with the top, have used arguments which are not consistent with the law of gravitation, or rather attraction. messrs. newcomb and holden in their work, entitled "astronomy for high schools and colleges," sixth edition, , apparently accept the theory, and proceed to explain and support it by showing what would be the action of a hollow spherical shell of any substance on a particle of it, say the bob of a pendulum, placed on the outside and also on the inside of the shell; and give us two theorems which are supposed to comprehend both cases. these are:-- ( ) "if the particle be outside of the shell, it will be attracted as if the whole mass of the shell were concentrated at its centre." ( ) "if it be inside the shell, the opposite attractions in every direction will neutralise each other, no matter whereabouts in the interior the particles may be, and the resultant attraction of the shell will therefore be zero." to the first theorem no objection can be made: the particle on the outside of the shell will undoubtedly be attracted by every particle in the shell, with the same force as if the attractive power of all the particles composing it were concentrated in the centre. not so with the second theorem: for it can be objected that it altogether ignores the law of attraction laid down by sir isaac newton, where it asserts that the resultant attraction of the shell for the particle will be zero, when it is placed anywhere on the inside. in fact the theorem supposes a case impossible for the harton colliery experiments, in order to demonstrate their accuracy; for it makes use of the bob of the pendulum--a particle of matter--as if it were transferable to any part of the interior of the earth instead of being confined within the bounds of its swing. that the attraction of the shell-- feet thick all round the earth--on the pendulum bob inside of it continues in all its force, and is only divided into two opposing parts, is made plain by fig. . supposing o to represent the bob of the pendulum at the bottom of the mine, and the space between the two circles the shell of the earth. then the line b c will show where the attraction of the shell for the bob is divided into two parts acting in opposite directions. supposing these two parts to be separated from each other, only far enough to admit the bob--a particle to all intents and purposes--between them; the part b a c will attract the bob as if its whole attractive force were collected at its centre of gravity, and the part b d c as if the whole of its attractive force were collected, not at the centre b of the shell, but at its centre of gravity, a very little distance from b in the direction towards d. this is an incontrovertible fact, because it is in strict accordance with newton's law of attraction, which is: _every particle of matter in the universe attracts every other particle with a force directly as their masses, and inversely as the square of the distance which separates them._ [illustration: fig. .] if we now suppose the interior of the shell to be filled up solid, that will make no difference, because the mass of the part b d c will only be increased vastly thereby, while the mass of a b c will remain the same; the two parts only increasing their proportion to each other, and thus coming to be for the earth--in the harton colliery experiments--what we represented them to be at page ; and we can now proceed to find the attractive force of each of the two masses for the bob of the pendulum which is as the inverse square of their distances from it. these distances may be taken, without any very great stretch of conscience, as one-tenth of a mile and · miles; because the centre of gravity of the segment a b c will be about that distance from o, and that of b d c cannot be adequately represented by a greater sum than · , always supposing the diameter of the earth to be miles. thus the squares of these two distances will be · and , , miles respectively, and the relative force of attraction for the pendulum of the two segments a b c and b d c will be as × · and , , , and , , × , , ; that is as is to , , , , , , . here then we get confirmed the unbelief in the theory we expressed at pages and . surely no one will be bold enough to assert that by decreasing the total attractive force of the earth by a little less than a - / trillionth part cut off from one side of it, the want of homogeneity in what remains will not only not decrease its attractive force at the centre, but increase it so as to make a pendulum be lessened by / , th part of its time in beating one second. this fraction of time is quite small enough to inspire doubt of any theory founded upon it; and if there ever is a quantity in mathematics that can be called negligible, the fraction of attractive force found above ought to be included in the same category. we may therefore assert that no human measurements could find a true difference between the beats of a seconds pendulum at the top and bottom of the pit at the harton colliery. if all the people who have puzzled themselves with this theory had spent an hour or two in making the above calculations before they began them, there would have been no experiments made, and the theory would have died almost ere it was born. those who believed in it may have looked upon a particle as a negligible quantity, but as the whole earth is made up of particles a little thought would have put an end to such a notion. what puzzles us is how such a theory could be formed by people who knew nothing whatever of the nature of the interior of the earth at a depth of even one mile, and how they could speculate on its want of homogeneity without knowing anything of how the density of · is made up in it? to suppose that the earth is made up of strata of different densities, and that each is in some degree elliptical--the ellipticity of one stratum being different from another, as the french mathematician clairaut did--is all very allowable; but to build up any theory on any such suppositions is to build upon shifting sands without examining the foundations. for anything that is known up to the present time, the density of the earth may go on increasing gradually from the surface to the centre, or it may attain nearly its greatest density at a few miles from the surface, and continue homogeneous or nearly so from there to the centre. to go further now: it is not true that the attraction of a hollow shell of a sphere for any particle within it, is the same "no matter whereabouts in the interior the particle may be." the only place where the attraction will be the same is when the particle is at the centre. in that position a particle would be in a state of very unstable equilibrium, and a little greater thickness of the shell on one side than the others, would pull it a little, perhaps a great, distance from the centre towards that side; and if we extend our ideas to a plurality of particles within the shell of a sphere, we are led to speculate on how they would be distributed, and to see the possibility of there not being any at all at the centre. this is a point which has never been mooted, as far as we have been able to learn, and we shall have to return to it when the proper time comes. it is difficult to understand how any man could conceive the notion that a shell of a sphere, such as that shown at fig. , could have no attraction for each separate one of all the particles which make up the mass of the whole solid sphere within it; for that is the truth of the matter if properly looked into, when it is asserted, as has been done by messrs. newcomb and holden, that "the resultant attraction of the shell will therefore be zero." if such a notion could be carried out in a supposed formation of the earth, an infinity of particles would carry off the whole of the interior, and leave the earth as only a shell of feet thick, as per the hartley colliery experiment; only we are told, or left to understand, that that process could not go on for ever, but would have to come to an end somehow and somewhere; and then we are left to speculate on how the unattracted particles could come back to take part in the composition of the earth. left to ourselves we can only liken the process to that followed by a man who peels off the outer layer of an onion, eats the interior part, and when he is satisfied throws down the outer layer and thinks no more of it; not even that he might be asked what had become of the interior part. curiously enough, there is a way of explaining how, or rather why, the notion was formed--not unlike the one just given--to be found in the third of sir george b. airy's lectures on popular astronomy, delivered at ipswich several years before the final experiments were made at the harton colliery. in that lecture, while describing how the greek astronomers accounted for the motions of the sun and planets round the stationary earth, he says, "it does appear strange that any reasonable man could entertain such a theory as this. it is, however, certain that they did entertain such a notion; and there is one thing which seems to me to give something of a clue to it. in speaking to-day and yesterday of the faults of education, i said that we take things for granted without evidence; mankind in general adopts things instilled into them in early youth as truths, without sufficient examination; and i now add that philosophers are much influenced by the common belief of the common people." we can agree with sir george b. airy in his ideas about education, and now conclude by saying that he has given us a very clear and notable example of a theory being accepted very generally, without being thoroughly examined to the very end, and of how easy it is for such theories to be handed down to future generations for their admiration. chapter ii. page the moon cannot have even an imaginary rotation on its axis, but is generally believed to have. quotations to prove this. proofs that there can be no rotation. the most confused assertion that there is rotation shown to be without foundations. a gin horse does not rotate on its axis in its revolution. a gin horse, or a substitute, driven instead of being a driver. results of the wooden horse being driven by the mill. the same results produced by the revolution of the moon. centrifugal force sufficient to drive air and water away from our side of the moon. that force not sufficient to drive them away from its other side. no one seems ever to have thought of centrifugal force in connection with air and water on the moon. near approach made by hansen to this notion. far-fetched reasons given for the non-appearance of air and water. the moon must have both on the far-off hemisphere. proofs of this deduced from its appearance at change. where the evidences of this may be seen if looked for at the right place. the centrifugal force shown to be insufficient to drive off even air, and less water, altogether from the moon. the moon must have rotated on its axis at one period of its existence. the want of polar compression no proof to the contrary. want of proper study gives rise to extravagant conceptions, jumping at conclusions, and formation of "curious theories." a good deal of theorising has been expended in accounting for the absence of all but traces of an atmosphere and water on the moon, which might have been avoided had astronomers not caught up the notion, and stuck to it, that it rotates on its axis once for every revolution that it makes round the earth. it might be difficult to find out with whom the notion originated; but perhaps it was first conceived to be the case by some celebrated astronomer, and has been accepted by almost all his successors without being properly looked into. any one who chose to take the trouble to study the matter thoroughly, would have easily discovered that the moon can have no rotation of any kind on its axis, and immediately afterwards have found out the reason why nothing beyond traces of air and water were to be seen on the side of it constantly turned towards the earth. this is another example we can give of erroneous ideas leading to erroneous and impossible conclusions, and preventing the truth from being discovered. that the rotation of the moon on its axis is stated to be a fact, by recognised and celebrated astronomers, will be seen from the following quotations. ( ) sir john herschel, in his "treatise on astronomy," new edition of , says at page : "the lunar summer and winter arise, in fact, from the rotation of the moon on its own axis, the period of which rotation is exactly equal to its sidereal revolution about the earth, and is performed in a plane ° ´ ´´ inclined to the ecliptic, and therefore nearly coincident with her own orbit. this is the cause why we always see the same face of the moon, and have no knowledge of the other side." ( ) in his "poetry of astronomy," page , mr. proctor says: "for my own part, though i cannot doubt that the substance of the moon once formed a ring around the earth, i think there is good reason for believing that when the earth's vaporous mass, receding, left the moon's mass behind, this mass must have been already gathered up into a single vaporous globe. my chief reason for thinking this is, that i cannot on any other supposition find a sufficient explanation of one of the most singular characteristics of our satellite--her revolution on her axis in the same mean time, exactly, as she circuits around the earth." ( ) professor newcomb, in his "popular astronomy," th edition, , at page , has what follows: "the most remarkable feature in the motion of the moon is, that she makes one revolution on her axis in the same time that she revolves around the earth, and so always presents the same face to us. in consequence, the other side of the moon must remain for ever invisible to human eyes. the reason for this peculiarity is to be found in the ellipticity of her globe." then he enlarges upon and confirms the fact of her rotation. ( ) mr. george f. chambers, in his "handbook of astronomy," th edition, , says at page , vol. i.: "in order that the same hemisphere should be continually turned towards us, it would be necessary not only that the time of the moon's rotation on its axis should be precisely equal to the time of the revolution in its orbit, but that the angular velocity in its orbit should, in every part of its course, exactly equal its angular velocity on its axis." it may be necessary, to avoid misconception, to note that angular velocity on its axis confirms rotation; and what is more extraordinary, that chambers must have thought that its angular velocity on its axis must have increased and diminished in order to agree with its increased and diminished velocities in its elliptic orbit at its perigee, apogee, and quadratures. a rather strange notion in mechanics where there is no provision made for acceleration or retardation of rotation. ( ) dr. samuel kinns, in "moses and geology," twelfth thousand, , says at page , "the same side of its (the moon's) sphere is always towards us. this could only happen by its having an axial rotation equal in period to its orbital revolution, which is d. h. m. s." ( ) in the "story of the heavens," sir robert s. ball informs us, in the fifteenth thousand, , page , "that the moon should bend the same face to the earth depends immediately on the condition that the moon should rotate on its axis in precisely the same period as that which it requires to revolve around the earth. the tides are a regulating power of the most unremitting efficiency to ensure that this condition should be observed." ( ) and finally we have what follows from messrs. newcomb and holden, at page of their work already referred to at page , "the moon rotates on her axis in the same time and in the same direction in which she moves around the earth. in consequence, she always presents very nearly the same face to the earth." and in a footnote to this consequence, add: "this conclusion is often a _pons asinorum_ to some who conceive that, if the same face of the moon is always presented to the earth, she cannot rotate at all. the difficulty arises from a misunderstanding of the difference between a relative and an absolute rotation. it is true that she does not rotate relatively to a line drawn from the earth to her centre, but she must rotate relative to a fixed line, or a line drawn to a fixed star." in six of the above cases it is distinctly maintained that the moon rotates once on its axis in the same time that it makes one revolution round the earth, and that it is in consequence of this rotation that it always presents the same side to the earth. thus we feel authorised to conclude that their authors did either believe that it does so rotate, or that they entertained some confused idea on the subject, which they did not take the trouble to examine properly, but accepted as a dogma, because some predecessor, with a great name, had stated that such rotation was necessary in order that its same side should be always turned towards the earth. in the seventh case the authors, while actually making the same assertion, try to persuade those who they acknowledge can see that the moon does not rotate on its axis in any sense, that their difficulty in comprehending what is meant by rotation, arises from the misunderstanding of the difference between an absolute rotation and one relative to a line drawn to a fixed star. but they do not attempt to show how this relative rotation has anything to do with or has any effect in causing the moon to present always the same side to the earth; and leave the story in the same confused state, out of which nobody can draw any satisfactory conclusion. also, though they distinctly recognise that it does not rotate relatively to a line drawn from the surface of the earth to its centre, they do not include in their general description of the moon anything in any way connected with what would be the consequences of its not really rotating on its axis relatively to the earth. so they leave us the problem in much the same state as they found it, and it is still necessary to show that there can be no actual rotation of any kind on its axis; and the worst of it is that it is a thing that will have to be done in such very plain language that it will compel people to think of the absurdity of the idea so generally accepted. to begin, it is very difficult to comprehend what the authors, above alluded to, meant by saying that the moon "must rotate relative to a fixed line, or a line drawn to a fixed star." it may mean relative to the line itself or to the star to which it is drawn. if it is to the line itself we cannot form any notion of what direction the rotation will have, direct, retrograde, or otherwise; and if it is relative to the star itself, then we can see that the relative rotation must depend on what is the position of the star. should it be placed in the "milky way," we can understand how the moon could show every side it has--almost, not quite--to the star during every revolution it makes round the earth, and how they may look upon it as a relative rotation. but if we draw the line to the pole star we cannot see how the moon can show every side it has to it in every revolution round the earth, so there can be no relative rotation in that case--and the "almost, not quite," applies to every star between the pole and the ecliptic. the moon shows only the northern hemisphere, or a little more due to libration of its own kind, to that star, and would have to remove its poles to the equator, and make a new departure, in order to show the whole of its surface to that star in every revolution round the earth. thus it is clear that the explanation given us of the relative rotation, is evidently one of the kind not properly thought out to the end. no one has ever said, or perhaps even thought, that a gin-horse makes one rotation on his vertical axis, in the same time as he makes a circuit round his ring, but, all the same, he keeps his same side always towards the gin, or mill, he is giving motion to. the proof that he does not make any such rotation is easy--no proof is really required. but, suppose he is giving motion to a whim for raising ores from a mine, and that his motion is what is called direct. when the cage containing the ore is brought to bank, is emptied, and has to be lowered into the mine again, the horse has then to reverse his motion to retrograde, in doing which he has to make a half rotation on his vertical axis, and turn his other side to the whim. when again the cage has to be raised to bank, he has to resume his direct motion, for which he has to make another half rotation on his vertical axis, but it is this time in the opposite direction. thus it is shown that he can only make half rotations, under any circumstances, on his axis, and these in opposite directions, when he changes his motion from direct to retrograde, or _vice versâ_; and that, when he moves in only one direction he cannot make even one rotation on his vertical axis, however long he may travel round the mill. in the same manner the moon which never turns back in its orbit can never make even one half rotation on its axis, which is all that we have had to prove. it is hardly necessary to observe that its axis is nearly parallel to the earth's, just the same as the horse's is to that of the whim. neither could any one say that the relative rotation of the horse to a star, or tower, or, say, a bridge, outside of his ring, could have any effect on his revolution round the mill, or his always keeping his same side to it, there being no mechanical connection between them, nor any law of attraction; and the same is the case between the moon and a fixed star. now, we may begin to consider what effects must be produced by the moon not rotating on its axis, and we can do so most easily by continuing to work with our gin horse, or some equivalent substitute. it would not cost a great deal of ingenuity to plant a steam engine in the centre of the mill he is supposed to be driving, and to drive with it not only the mill but the horse also at the end of his lever. there might be some dissipation--professor tate would call it degradation--of energy in such an experiment, but we could get over that by making _divina palladis arte_ a wooden horse. we might arrange the steam-engine so as to cause the mill to make - / revolutions for one made by our wooden horse, and so have a sort of a model of the earth and moon performing their most important relative motions. then, having got our model ready for action, instead of filling it _armato milite_ we might fill it half full of water. we fill it only half full, because the armed soldiers could not lie on the top of each other in the _other_ horse, and there would be a vacant space above them for air, thus making the resemblance between the two the more similar; and also because it suits our purpose better, as will soon be seen. we have still to propose that a lot of holes should be supposed to be made in the sides of _our_ horse all round, just a little higher than between wind and water. _pallas_ did not order any holes to be made in _hers_ as far as we know, even for ventilation, though we think it would have been an advantage; but that will not spoil the experiment we are now prepared for. let the steam-engine be started now and we shall soon see what will happen to the water. as the speed increases it will not be long till it begins to be thrown out, not from the side turned towards the mill but from the one furthest from it; and if it is increased sufficiently the whole of it will be very soon thrown out. if we could now close up the holes on the side of the horse turned towards the mill, it would so happen that a good deal of the air would be expelled also; and if the speed of the horse were brought up so as to equal that of the moon in its orbit, there would be nothing more, at the most, than traces of air left even in it. the expelling agent in this experiment would, of course, be centrifugal force, and we do not need to exercise our mental faculties very greatly, to comprehend that it is the same force that has driven both air and water away from the side of the moon always turned towards the earth. all the difficulty we have to contend with will be to make sure that the orbital velocity of the moon is sufficient to produce the force required. that the force is exceedingly greater than what is required is proved by the fact, that the velocity with which the moon travels in its orbit is a little more than miles per minute, whereas the velocity of the circumference of a centrifugal machine, used for clarifying sugar, drying clothes, or any other similar industrial purpose, does not require a greater velocity than about _one_ mile per minute, in order to throw everything in the form of water out of the material to be dried, and out of the centrifugal machine itself; and we know that air would be expelled more easily than water, were none re-admitted to supply the place of what was expelled. here the idea very naturally occurs to any one, that so great a velocity would drive both air and water away, even from the far off side of the moon, into space, but in order to do so the velocity would have to be , not , miles per minute. our authority for this statement will be found in "the nineteenth century," for august , in an article written by prince kropotkin, in which he says: "but it appears from dr. johnstone stoney's investigations that even if the moon was surrounded at some time of its existence with a gaseous envelope consisting of oxygen, nitrogen and water vapour, it would not have retained much of it. the gases, as is known, consist of molecules rushing in all directions at immense speeds; and the moment that the speed of a molecule which moves near the outer boundary of the atmosphere exceeds a certain limit (which would be about , feet in a second for the moon) it can escape from the sphere of attraction of the planet. molecule by molecule the gas must wander off into interplanetary space; and the smaller the mass of the molecule of a given gas, the feebler the planet's attraction, and this is why no free hydrogen could be retained in the earth's atmosphere, and why the moon could retain no air or water vapour." a velocity of , feet per second is as near miles per minute as there is any use for, which is more than three times as great as the velocity of the moon in its orbit, so there is no possibility whatever of air and water having been swept away from the far off side of it by centrifugal force; more especially as it ought to be well known that that force is always counteracted by the attractive force of the satellite for these or any other elements. we do not want to discuss the point of whether the mutual collisions of the molecules of a gas could get up such a velocity as would enable them to free themselves from the attraction of the moon, for it looks to us too much like one of those notions that are got up to account for something that does not exist; but we do want to state our dissent to the conclusion--evidently jumped at--that because there are hardly any signs of there being air or water on our side of the moon, there can be none on the other. no astronomer, physicist, scientist of any kind, can prove that there is none, simply because he has never been round there to see or make experiments to prove it; and if there is any one bold enough to make such an assertion, it is only an example of how stupendous a jump to a conclusion can be made. when we first read, many years ago, some of the reasons given for there being no water visible on the side of the moon constantly turned to the earth, one of which was that if there ever had been any it must have been absorbed into its body during the process of cooling and consolidation; and when we had convinced ourselves, by placing two oranges on two ends of a wire and revolving the one round the other, that the moon did not rotate on its axis in any sense whatever, we came to the conclusion that both water and air could be removed to the far off hemisphere by centrifugal force. we thought this so simple, so self-evident, and so indisputable an explanation, that every one who had read what we had read must have come to the same conclusion; so that we were not a little surprised when we saw it stated by "the times" of september , , in its first report of the meeting of the british association for that year, that sir robert ball had suggested, some time previously, that the "absence of any atmosphere investing the moon is a simple and necessary consequence of the kinetic theory of gases." this at once made us suspect that the theory--our theory--must have been new, but we could not altogether believe it. it seemed to us passing strange that it should not have occurred to astronomers, from the moment they discovered that they could not find any, or hardly any, traces of air or water on the only hemisphere they could examine; but it would appear from sir robert ball's suggestion, being even discussed at that meeting, that the notion of their having been removed simply by centrifugal force to the unseen hemisphere, had never been entertained by, to say the least, any one who was present at that discussion. not satisfied with this conclusion, we proceeded to examine all the books, journals, magazines, and _papers_ we could get hold of, to see whether we could find any indication of such a conception having been published previously, and the nearest approach to anything of the kind having been conceived of by anyone, we found in chambers's work--already referred to--at page , vol. i., where we read, "professor hansen has recently started a curious theory from which he concludes that the hemisphere of the moon which is turned away from the earth may possess an atmosphere. having discovered certain irregularities in the moon's motion, which he was unable to reconcile with theory, he was led to suspect that they might arise from the centre of gravity of the moon not coinciding with the centre of figure. pursuing this idea, he found upon actual investigation that the irregularities could be almost wholly accounted for by supposing the centre of gravity to be at a distance of - / miles _beyond_ the centre of figure. assuming this hypothesis to be well founded, professor hansen remarks that the hemisphere of the moon, which is turned towards the earth, is in the condition of a high mountain, and that consequently we need not be surprised that (little or) no trace of an atmosphere exists; but that on the opposite hemisphere, the surface of which is situated _beneath_ the mean level, we have no reason to suppose that there may not exist an atmosphere and consequently both animal and vegetable life. professor newcomb has disputed these conclusions of hansen, which it is obvious must be very difficult of either proof or disproof." what professor newcomb's objections to the conclusions of hansen were we do not know, but we do know that mr. proctor also objected to the "curious theory," as it is called by mr. chambers. in his "poetry on astronomy," he discusses pretty fully the withdrawal of water from the surface of the moon during the process of cooling and condensation, ascribing the conception of it to four independent authors, namely, seeman, a german geologist, frankland in england, stanislas mennier in france, and sterry hunt in america; and in a footnote, at page , says of hansen's theory: "the idea was that the moon, though nearly spherical, is sometimes egg-shaped, the smaller end of the egg-shaped figure being directed towards the earth. now, while it is perfectly clear that on this supposition the greater part of the moon's visible half would be of the nature of a gigantic elevation above the mean level, and would, therefore, be denuded (or might be denuded) of its seas and denser parts of the air covering it, yet it is equally clear that all around the base of this monstrous lunar elevation, the seas would be gathered together, and the air would be at its densest. but it is precisely round the base of this part of the moon or, in other words, round the border of the lunar hemisphere, that we should have the best chance of perceiving the effects of air and seas, if any really existed; and it is because of the absolute absence of all evidence of the kind, that astronomers regard the moon as having no seas and very little air." had the idea of centrifugal force ever occurred to mr. proctor, he could not have written this last sentence; for he could not have failed to see that "the border of the visible lunar hemisphere" would be the very place, from which it could most easily remove air and water, after they had got so far down the monstrous elevation; because there it--the centrifugal force--would be acting at right angles to the moon's attraction, instead of having to contend against it, as it would have to do in a constantly increasing degree until it arrived at its maximum, just in proportion to the distance the air and water got down to the similar monstrous _depression_ on the other hemisphere, down which the gradient would start off under the most favourable circumstances possible. from what has been said, it is very evident that neither hansen, chambers, proctor, nor any of those whose names have been mentioned by the last, in connexion with the withdrawal of water into the body of the moon by absorption, while cooling and condensing, had ever thought of the possibility of air and water having been removed by centrifugal force from the side of the moon turned towards the earth. that it should not have occurred to hansen seems passing strange, seeing that he had conceived the idea of their possible existence on the hemisphere turned away from the earth, which could hardly fail to make him think of how they got there, and could exist only there; and the only explanation of his not having perceived the true cause seems to be, that his thoughts were hampered by a sort of confused notion that the moon actually rotates on its axis once for every revolution it makes around the earth, that being, as it were, one of the dogmas of astronomic belief, handed down from some great authority of times past, and never properly inquired into. we do not want to question the suggestion, that the absence of any atmosphere investing the moon is a simple and necessary consequence of the kinetic theory of gases--though we see that a good deal could be argued against it--as we do not consider it to be necessary--neither the questioning nor the theory. we have demonstrated clearly, how both air and water could be removed from the side of the moon constantly shown to us, and that is sufficient for our purpose both now and later on; besides it would appear that the moon really has some sort of an atmosphere somewhere. following up the quotation, made at page , from prince kropotkin's article in the "nineteenth century" as being the latest information we have on the subject, we are told that "a feeble twilight is seen on our satellite, and twilight is due, as is known, to the reflection of light within the gaseous envelope; besides it has been remarked long since at greenwich that the stars which are covered by the moon during its movements in its orbit remain visible for a couple of seconds longer than they ought to be visible if their rays were not slightly broken as they pass near the moon's surface. consequently it was concluded that the moon must have an atmosphere" ... and: "the observations made at lick, paris, and arequipa, fully confirm this view. a twilight is decidedly visible at the cusps of the crescent-moon, especially near the first and last quarters. it prolongs the cusps as a faint glow over the dark shadowed part, for a distance of about miles ( "), and this indicates the existence of an atmosphere having on the surface of the moon the same density as our atmosphere has at a height of about forty miles." what is of interest for us to know is where that "feeble twilight," or, "reflection of light within the gaseous envelope," is seen. whether it is at what mr. proctor calls "the border of the visible lunar hemisphere," on this side of it, or beyond it. it cannot be a difficult matter to decide. it must be beyond it, for the following reasons: if the atmosphere has been driven away to the far-off hemisphere of the moon by centrifugal force, its natural tendency would be to spread out immediately after it had passed the visible border where we have said the centrifugal force would be acting most effectively. also, if all the air at one time belonging to our side of the moon has been driven away to the other, that side must have a double allowance of atmosphere, which, though it does not increase its density at the surface, on account of the centrifugal force, will double its volume, and enable it to extend to a greater proportionate distance in all directions from the border and from the far-off hemisphere. in this way there must be a considerable wedge of atmosphere illuminated by the sun, and visible past the edge of the moon's disc, to reflect a feeble twilight--perhaps something stronger--towards the earth, and to intercept the light of a star before its edge and that of the moon come into actual apparent contact. but before the wedge becomes thick enough to reflect that light, the reflecting part must be far beyond the edge of the moon's disc. perhaps the feeble light might be seen more clearly when looked for in the proper place; quite possibly hundreds of miles beyond the disc. in order to make more clear the truth of what we have said about water and air--and more especially the latter--being thrown away to the far-off side of the moon by centrifugal force, we may add the following details: if the force of gravity at its surface is one-sixth part of what it is at the surface of the earth, the pressure of an atmosphere there would be · lb. per square inch, if it rotated on its axis; but as it does not so rotate and is subjected to centrifugal force, the pressure of an atmosphere will vary according to the part of it over which it exists. on the nearest part of the side turned towards the earth, gravity, which we have just seen must be equal to · lb., would be acting in the same direction as centrifugal force, which in its turn is equal to · lb. or thereby, and the whole would be · lb. per square inch tending to drive off air and water to the far-off hemisphere. but from that place, gravity would gradually diminish its aid till it came to be nil at the disc separating the two hemispheres, where it would have no effect whatever as it would be acting at right angles to centrifugal force, and this would be reduced to · lb. per square inch. then, from the edges of the disc forward, on the far-off hemisphere, gravity would begin to act against centrifugal force, or rather _vice versâ_, until it, gravity, got reduced to · lb. per square inch. also, as that hemisphere must have a double portion of air or atmosphere on it, and as its pressure on any part of it cannot be greater than the · lb. just mentioned, we can imagine that the double quantity will hang closer to the surface than if there was only one portion. such being the case the atmosphere would spread out much more rapidly than would be represented by the extension of a triangle starting from the earth and reaching beyond the moon's disc to the farthest limit of the atmosphere; and thus the wedge, which we have supposed to be visible beyond the edges of the disc may come to have a very considerable thickness. what that thickness may be, and up to what distance beyond the disc the density of the wedge would be sufficient to reflect the light of the sun, it would be very difficult to calculate, but we think it might possibly extend even as far as one-fourth of the radius of the moon--because at that point the force of gravity pulling it towards the centre, or the axis, would be very small, and its distance from the axis would be little less than the radius, not over miles--and cause it to project over the edges as far, to appearance, as the miles ( ") that have been observed at greenwich. this reflected light must be all round the moon--not at the cusps only of the crescent-moon--and it has occurred to us that it may, most probably does, account for the appearance of what we call "the old moon in the young moon's arms." we know what effect the "earth-shine" has upon the moon at its change, and the brighter _ring-shine_ just outside of it, may very well be caused by the sunlight reflected from the atmosphere far beyond the visible limit of the hemisphere turned to us. in support of this suggestion we may refer to professor c. a. young's description, in his "sun," p. , of one particular feature observed at the time of a total eclipse of the sun. he says:--"on such an occasion, if the sky is clear, the moon appears of almost inky darkness, with just a sufficient illumination at the edge of the disc to bring out its rotundity in a striking manner. it looks not like a flat screen, but like a huge black ball, as it really is. from behind it stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre." there can be little doubt, we think, from what is said here, that professor young looks upon this "illumination of the edge of the disc" as pertaining to the moon, and upon the "radiant filaments, beams," etc. behind it as belonging to the sun. and in that case the illumination can only be caused by the light of the sun, refracted by the atmosphere belonging to the hemisphere of the moon that is never seen from the earth. we have taken it for granted in what we have been doing, that the moon has really rotated on its axis, and to some purpose, at some former period of its existence. some people think otherwise, or that there is at least a doubt about it; we cannot see even the shadow of a doubt. all that we need to say in support of our opinion is, that there is no other conceivable way of accounting for its perfectly circular form. all the planets are circular, or spheroidal--to speak more correctly--in form, admittedly in consequence of rotation on their axes; and if one or two of jupiter's satellites are not completely circular or spheroidal, it does not stretch our conscience very much to suppose that it is because they have not yet been rotated into form. saturn apparently has satellites still in the form of rings, and there can be nothing out of the way in supposing that all of jupiter's are not yet licked into shape. the fact that there is no appearance of compression on the moon makes us think of why there is none, and the only explanation that occurs to us is, that, as its rotation must have come to an end gradually, the compression it must have had when rotating must have disappeared gradually also, by reason of the differences of force in the equatorial and polar attractions, drawing in the bulged out, and thus forcing out the compressed parts. this is a notion that will be scoffed at by those who have always thought, and maintained, that the earth acquired its present form when in a liquid state; but they have not thought this supposition--for it is nothing else--out to the very end. several reasons could easily be given against their opinion, among others the variations in rate of rotation we so frequently see used in favour of other notions; but we shall content ourselves with the best one of all, which is this: the pressures in the interior of the earth must be so enormous that they are quite sufficient to compress steel, or adamant if that is supposed to be more resistant, into any shape whatever, almost as if it were dough, and there can be no doubt--mathematics notwithstanding--that the earth has the form, to-day, due to its present rate of rotation. we shall have to return to this subject some time hence, if we live to complete what we have taken in hand. how many things there are, in what is considered to be astronomical science, that have not been properly thought out to the end, and to what strange notions they have given rise! this one of the rotation of the moon which we have been discussing, has evidently given occasion for the conception of the theory that the absence of atmosphere and seas from the moon is the natural consequence of the kinetic theory of gases; and the author of the theory, and its supporters, have never, apparently, taken the trouble to think whether their absence from the near hemisphere is a satisfactory and convincing proof of there not being any air or water on the far-off one. in what we have proposed to write many similar examples of want of study will be met with, but we do not intend to call special attention to them, unless it be in cases where we consider it to be of some importance to do so. in fact we have already been working on that plan. chapter iii. page remarks on some of the principal cosmogonies. ancient notions. the nebular hypothesis of laplace. early opinions on it. received into favour. again condemned as erroneous. defects attributed to it as fatal. new cosmogonies advanced. dr. croll's collision, or impact, theory discussed. dr. braun's cosmogony examined. m. faye's "origine du monde" defined. shown to be without proper foundation, confused, and in some parts contradictory. reference to other hypotheses not noticed. all more or less only variations on the nebular hypothesis. necessity for more particular examination into it. we have thought it worth while to dedicate this chapter to some remarks on cosmogonies in general, and examination into a very few conceived by eminent men; these forming in our opinion the most attractive matter for those readers who do not pretend to make a study of astronomy, but are very desirous to have some knowledge of the most plausible ideas which have been conceived by astronomers, of how the universe, and more particularly the solar system, were brought into existence; while, at the same time, they are the subjects on which more crude conceptions, more limited study, and more fanciful unexamined thought have been expended, than any others we have met with. some readers will, no doubt, be able to reject what is erroneous, to speak mildly, but there will be, equally surely, some who cannot do so; and it must be confessed there are a good many to whom the most complicated conceptions, and the most difficult of comprehension, are the most attractive. a great many centuries ago, astronomers and philosophers had already conceived the idea that the sun and stars had been formed into spherical bodies by the condensation of celestial vapours; but when the telescope was invented, and the nature of nebulæ in some measure understood, it was not long till it came to be thought that the matter, out of which the sun and stars were formed, must have been much more substantial in its nature than celestial vapours. being visible, they were naturally considered to be self-luminous, and consequently endowed with great heat, because the self-luminous sun was felt to be so endowed, though perhaps not with the same degree. accordingly, astronomers began to form theories, or hypotheses, on the construction of the solar system out of a nebula, which, like everything else, went on each one improving on its predecessor as, through continued observation and study, more knowledge was acquired of the nature of nebulæ. the most notable of these cosmogonists were descartes, newton, kant, and laplace, each of whom contributed valuable contingents to the general work; which may be said to have culminated about a century ago in the nebular hypothesis of the last-named; for the many attempts that have been made to improve upon it, or to supplant it altogether, have been very far from successful. the hypothesis is about a century old, as we have said, and there may still be many people who can remember having heard it denounced as a profane, impious, atheistic speculation, for it is not over half a century since the ban begun to be taken off it. sir david brewster, in his "life of newton," said of it, "that the nebular hypothesis, that dull and dangerous heresy of the age, is incompatible with the established laws of the material universe, and that an omnipotent arm was required to give the planets their positions and motions in space, and a presiding intelligence to assign to them the different functions they had to perform." with others, its chief defect was that the time required to form even the earth in the manner prescribed by it, must have been infinitely greater than six days of twenty-four hours each. in the meantime, geologists had also discovered that, for the formation of the strata of the earth, which they had been examining and studying, the time required for their being deposited must have been, not days of twenty-four hours, but periods of many millions of years each; and the evidence adduced by them that such must have been the case was so overwhelming, that theology had to acknowledge its force, and gradually to recognise that the days must have been periods of undefinable length. thus relieved from the charge of heresy, the hypothesis rose rapidly into favour, and came to be generally accepted by the most eminent astronomers, subject always to certain modifications, which modifications have never been clearly defined, if at all. it was not, however, allowed to enjoy long the exalted station to which it had attained. astronomers had begun to consider from whence the sun had acquired the enormous quantity of heat it had been expending ever since the world began, and, after long discussion, had come to the conclusion that by far the greatest source must have been the condensation from the nebulous state of the matter of which it is composed. having settled this point, it was calculated that the amount of heat derived from that and all other sources could not have kept up its expenditure, at the present rate of consumption, for more than twenty million years, and could not maintain it for more than from six to eight million years in the time to come. owing in good part to this great difference between the calculations of astronomers and geologists about the age of the earth, the hypothesis began again to suffer in repute, and then all its faults and shortcomings were sought out and arrayed against it. the chief defects attributed to it were: the retrograde motion of rotation of uranus and neptune and revolution of their satellites--that fault in the former having been noted by sir john herschel, in his treatise on astronomy already cited; the discovery of the satellites of mars which exposed the facts, that the inner one revolves round the planet in less than one-third of the time that it ought to, and that the outer one is too small to have been thrown off by mars, in accordance with the terms of the hypothesis; the exclusion from it of comets, some of which at least have been proved, in the most irrefutable manner, to form part of the solar system; and what can only be called _speculations_, on the formation of a lens-shaped nebula brought about by the acceleration of rotation--caused by condensation according to the areolar theory--which it is supposed would be enormously in excess of the actual revolution of the inner planets, and of the rotation of the sun. here we must protest against retrograde motion of rotation in any of the members of the solar system being considered as militating against the theory, because laplace states distinctly, while explaining his hypothesis, that the rotation of the earth might just as well have been retrograde as direct: a fact that some eminent astronomers have not noticed, simply because they have not paid proper attention to what they were reading. we shall have to return to this statement again, and to present the proof of its being true. an idea of how far the hypothesis had fallen into disrepute may be formed from the following extract, from "nature" of august , , of a review of a "new cosmogony," by a. m. clerke, in which it is said: "but now the reiterated blows of objectors may fairly be said to have shattered the symmetrical mould in which laplace cast his ideas. what remains of it is summed up in the statement that the solar system did originate somehow, by the condensation of a primitive nebula. the rest is irrecoverably gone, and the field is open for ingenious theorising. it has not been wanting.... the newer cosmogonists are divided into two schools by the more or less radical tendencies of the reforms they propose. some seek wholly to abolish, others merely to renovate the kant laplace scheme. the first class is best represented by m. faye, the second by mr. wolfe and dr. braun"--the author of the "new cosmogony." we cannot pass this quotation without remarking "how glibly some people can write!" more we do not want to say about it, except that it gave us the notion to examine closely some of the new cosmogonies, _which have not been wanting_, to see whether they are better than laplace's. we have not had the opportunity of knowing what are mr. wolfe's amendments, but the review, just cited, gives us a pretty good notion of those of dr. braun, and we have been able to study carefully m. faye's "origine du monde," in which he considers the solar system to have been evolved from cosmic matter partially endowed with motion in the form of eddies, whirlwinds, vortices, or _tourbillons_, which last may comprehend all of them, and even more. we have also studied, with some surprise, in "climate and cosmology" dr. croll's impact, or collision, theory, and will confine our examination to the three of which we know something, beginning with dr. croll's, which we believe to be the oldest of the three. we understand that dr. croll accepts the nebular hypothesis in all its main features, including the intense heat in which the original nebula is supposed to have existed from the beginning; and has only invented the collision theory in order to increase its quantity, to suit the demands of geologists for unlimited time, by showing how an unlimited supply of both heat and time may be obtained. but he has incurred an oversight in not taking into consideration the kind of matter in which that unlimited supply of heat was to be stored up--whether it would hold it. he wrote in times when something was really known about heat, and we cannot suppose him to have believed that heat could exist independent of matter, or that a gas or vapour could be heated to a high temperature except under corresponding pressure; but he has evidently overlooked this point, his thoughts recurring to old notions; and he has fallen, probably for the same reason, into other oversights equally as grave. when showing how a supply of fifty millions of years of sun-heat could be produced from the collision of two half-suns colliding with velocities of miles per second, dr. croll says in his "discussions on climate and cosmology," of , at page : "the whole mass would be converted into an incandescent gas" (the handmaid of the period), "with a temperature of which we can have no adequate conception. if we assume the specific heat of the gaseous mass to be equal to that of air (viz. · ), the mass would have a temperature of about , , ° c., or more than , times that of the voltaic arc." now, let us suppose the whole mass of the whole solar system to be converted into a gas, or vapour, at the pressure of our atmosphere, and temperature of ° c., its volume would be equal to that of a sphere of not quite , , miles in diameter. suppose, then, this volume to be heated to , , ° c. in a close vessel, as would necessarily have to be the case, the pressure corresponding to that temperature would be , , atmospheres, according to the theory on which the absolute zero of temperature is founded. without stopping to consider whether air or any gas could be heated to the temperature mentioned; or the strength of the vessel , , miles in diameter required to retain it at the equivalent pressure; if we increase the diameter of the containing sphere to a little more than that of the orbit of neptune, or, say , , , miles, and allow the air or gas or vapour to expand into it; then, as the volume of the new sphere will be greater than the former one in the proportion of , , cubed to , , , cubed, or as is to , , , the pressure of the gas will be reduced to , , divided by , , , that is just over the th part of an atmosphere; which, in its turn would correspond to a temperature of a very little more than - °, or what is considered to be[a] ° c. above absolute zero of temperature; or, at all events, to the temperature of space, whatever that may be. [a] this temperature is altogether erroneous, as we shall show in due time; at present our proof would not be accepted without a demonstration, for which we have not sufficient data. dr. croll goes on to say at page : "it may be objected that enormous as would be such a temperature, it would nevertheless be insufficient to expand the mass against gravity so as to occupy the entire space included within the orbit of neptune. to this objection it might be replied, that if the temperature in question were not sufficient to produce the required expansion, it might readily have been so if the two bodies before encounter be assumed to possess a higher velocity, which of course might have been the case. but without making any such assumption, the necessary expansion of the mass can be accounted for on very simple principles. it follows in fact from the theory, that the expansion of the gaseous mass must have been far greater than could have resulted simply from the temperature produced by the concussion. this will be obvious by considering what must take place immediately after the encounter of the two bodies, and before the mass has had sufficient time to pass completely into the gaseous condition. the two bodies coming into collision with such enormous velocities would not rebound like two elastic balls, neither would they instantly be converted into vapour by the encounter. the first effect of the blow would be to shiver them into fragments, small indeed as compared with the size of the bodies themselves, but still into what might be called in ordinary language immense blocks. before the motion of the two bodies could be stopped, they would undoubtedly interpenetrate each other; and this of course would break them up into fragments. but this would only be the work of a few minutes. here then we should have all the energy of the lost motion existing in the blocks as heat (molecular motion), while they were still in the solid state; for as yet they would not have had time to assume the gaseous condition. it is obvious, however, that the greater part of the heat would exist on the surface of the blocks (the place receiving the greatest concussion), and would continue there while the blocks retained their solid condition. it is difficult in imagination to realize what the temperature of the surfaces would be at this moment. for supposing the heat were uniformly distributed through the entire mass, each pound, as we have already seen, would possess , , , foot-pounds of heat. but, as the greater part of the heat would at this instant be concentrated on the outer layers of the blocks, these layers would be at once transformed into the gaseous condition, thus enveloping the blocks and filling up the interstices. the temperature of the incandescent gas, owing to this enormous concentration of heat, would be excessive, and its expansive force inconceivably great. as a consequence the blocks would be separated from each other, and driven in all directions with a velocity far more than sufficient to carry them to an infinite distance against the force of gravity were no opposing obstacle in the way. the blocks, by their mutual impact, would be shivered into small fragments, each of which would consequently become enveloped in incandescent gas. these smaller fragments would in a similar manner break up into smaller pieces, and so on until the whole came to assume the gaseous state. the general effect of the explosion would be to disperse the blocks in all directions, radiating from the centre of the mass. those towards the circumference of the mass, meeting with little or no obstruction to their outward progress, would pass outwards into space to indefinite distances, leaving in this manner a free path for the layers of blocks behind them to follow in their track. thus eventually a space, perhaps twice or even thrice that included within the orbit of neptune, might be filled with fragments by the time the whole had assumed the gaseous condition. "it would be the suddenness and almost instantaneity with which the mass would receive the entire store of energy before it had time even to assume the molten, far less the gaseous condition, which would lead to such fearful explosions and dispersion of the materials. if the heat had been gradually applied, no explosions, and consequently no dispersion of the materials would have taken place. there would first have been a gradual melting; and then the mass would pass by slow degrees in vapour, after which the vapour would rise in temperature as the heat continued, until it became possessed of the entire amount. but the space thus occupied by the gaseous mass would necessarily be very much smaller than in the case we have been considering, where the shattered materials were first dispersed in space before the gaseous condition could be assumed." we have made this very long quotation; first, because we have not been able to condense it without running the risk of not placing sufficiently clearly the whole of the argumentations employed in it; secondly, because the purport of the whole explanation set forth is evidently to demonstrate that, by means of the explosions of gases produced by the collision, the matter of the whole mass would be more extensively distributed into space--bearing heat along with it--than were it gradually melted and converted into vapour; and thirdly, because every argument advanced in favour of the theory of explosions, if carefully looked into, brings along with it its testimony that it has not been studied thoroughly out to the end. thus the quotation in a great measure saves us that labour. dr. croll seems sometimes to demand more from the laws of nature than they can give. he says, at p. of the work cited, that the expansion of the gaseous mass, produced by the collision of the two bodies, must have been far greater than could have resulted simply from the temperature produced by the concussion; and goes on to show how it--the expansion--might be caused by explosions of gases blowing out blocks of matter in all directions to indefinite distances. but he forgets that these explosions of gases would consume a great part of the heat they contained, that is, turn it into motion of the blocks, and so diminish the quantity produced by the collision, just in proportion to the velocities given to the masses of all the blocks blown out; so that what was gained in expansion would be lost in heat, and the object aimed at--of producing heat for the expenditure of the sun--so far lost. also, that, were the thing feasible, the blocks could not carry with them any of the heat of the exploded gases that might not be used up, and that the heat contained in them derived from the concussion would have time in their flight--about two hours at miles per second--to melt the matter composing them and turn it into vapour, long before even the orbit of neptune was reached. the heat produced by the explosion of powder in a cannon gives the projectile all the impulse it can, and disappears; it is converted into motion. it does not cluster round the projectile, nor follow it up in its flight, nor push it through an armour plate when it pierces one. we cannot admit--for this reason--the possibility of a block of matter flying off into space, with a mass of heat clustering round it, like bees when swarming round a branch of a tree. thermodynamics does not teach us anything about a mass of heat sticking to the surface of a block of matter of any kind. if the heat were, at a given moment--that is, when motion was stopped--brought into existence uniformly throughout the entire mass, which, according to the law of conversion of motion into heat and _vice versâ_, would most assuredly be the case, and each pound of the mass possessed , , , foot-pounds of heat, it could not be heaped up on the outer layers of the blocks--it matters not whether this means the layers of the outside of the whole mass, or at the outsides of the blocks--for the energy of lost motion, converted into heat, must have existed at the centres of the blocks or masses just in as great force as it did at the surfaces when motion was stopped. if each pound of matter carried along with it , , , foot-pounds of heat, that given out by one pound at the centre of a block would be as great as that given out by one pound at its surface; and the pounds at the surface could not acquire any greater heat from a neighbouring pound, because its neighbour could have no greater quantity to give it. pounds of matter would be melted and vaporized, or converted into gas, just as readily at the centre of the mass or block as at its surface; and storing up of heat in the interstices of the blocks is rather a strange notion, because we are not at liberty to stow away heat in a vacuum. besides, it is impossible to conceive how anything in the shape of a block could exist in any part of the whole mass, long enough for it to be blown out into space as a block. but supposing that a block could exist, it would most notoriously be in a state of _unstable equilibrium_; and were it then to receive from an explosion of gas, an impulse sufficient to drive it off to the verge of the sun's power of attraction--or rather to a distance equal to what that is--which would imply a velocity of not less than miles per second, the shock would be quite sufficient to blow it into its constituent atoms. moreover, as already stated, the heat of the explosion of the gas required to give the impulse would be immediately converted into motion, and disappear; so that out of the heat produced by the stoppage of a motion of miles per second, that required to produce a motion of miles per second, in each one of the blocks blown out to the distance above mentioned, would be entirely lost to the stock of heat schemed for so boldly. of course, the less the distance from the centre the blocks were blown the less would be the loss, but the fact remains that there would be a loss instead of a gain of heat, in dispersing the matter of two half suns into space by explosions of gas. in fine, a given amount of heat will raise the temperature of a given amount of matter to an easily calculable degree, and no more; and if part of that is expended in expanding the volume of the matter, the whole stock of heat will be diminished by exactly the quantity required to produce the expansions. so that we come back to what we have said at page , viz., that when the matter and the heat of the collision of the two half suns were dispersed, under the most favourable circumstances, into a sphere of , , , miles in diameter, the mean density of the matter would be equal to about / th part of an atmosphere, and its temperature--what is called-- ° c. of absolute temperature, always considering the quantity of the heat to have been , , ° c. dr. croll says that if a velocity of miles per second were not sufficient to produce the quantity of heat required, any other necessary velocity might be supposed, but when we consider that his supply of , , ° c. would have to be increased to , , , ° c., in order to add ° c. of heat to the matter dispersed through a sphere of , , , miles in diameter, it seems unnecessary to pursue the subject any farther. we may now take a look at dr. braun's impact cosmogony, of which we know nothing beyond what is set forth in the review in "nature" already alluded to, but that is enough for our purpose. we understand that he extends his operations to the whole universe, which he conceives to have been formed out of almost unlimited, and almost imponderable, nebulous matter, not homogeneous, but with local irregularities in it, which "would lead to the breaking up of the nebula into a vast number of separate fragments." out of one of these fragments he supposes the solar system to have been formed. this fragment would contain local irregularities also, which through condensation would lead to the formation of separate bodies, and these bodies are supposed to have been driven into their present forms, and gyrating movements of all kinds, by centric and eccentric collisions among themselves, caused by their mutual attractions. of course anything can be supposed, but in a construction of this kind the idea is forced upon us of the necessity of the active superintendence of the creator, to create in the proper places and bring in the matter at the exact moment required, and to see that the collisions were directed with the proper degree of energy and eccentricity, to construct the kind of machine that was proposed. to this idea we have no objections whatever, but we would like to see the necessity for it acknowledged. perhaps dr. braun does acknowledge it, but the cosmogony is given to us, it would seem, to show what most probably was the original scheme of construction, and implying that no continual supervision and direction were required during the process. if dr. braun could show us some method of attraction, and suspension and variation of attraction, by which some of the separate bodies could be drawn towards each other so as to form a central mass, nebula, or sun, and to give it, by their impacts of collision, a rotary motion; and how others of the separate bodies could be formed and held in appropriate places, so as to be set in motion at the right moment; and how they were to be so set in motion without the direct action of the constructor, to revolve as planets around the central mass, we might be able to recognise that a mechanism such as that of the solar system might be brought into existence; but when we are left to discover all these requisites, and their _modus operandi_, we find that we might be as well employed in designing a cosmogony of our own. dr. braun indulges in somewhat startling numbers in temperature and pressure. he considers that the temperature of the sun, at the surface, may be from , ° to , ° c., and that it may reach to from ten to thirty million degrees at the centre. in this he may be right for anything we know to the contrary. when riding over a sandy desert, under an unclouded vertical sun, we could easily have believed anything of the central heat of such a fire, especially when we considered that it was at a distance of ninety-three millions of miles from us. but when he tells us that in the depths of the sun's interior the pressure reaches a maximum of two thousand millions of atmospheres, we "pull in resolution and begin to doubt." air at that pressure would have a density , , times that of water, or , times the mean density of the earth, and we should have a species of matter to ponder over, of which no physicist has ever as yet dreamt. we have been able to study m. faye's cosmogony in his work on "l'origine du monde," second edition of , and can give a better account of it than of dr. braun's. ( ) he repudiates almost all existence of heat in the cosmic matter he is about to deal with, recognising that its temperature must have been very near the point of absolute zero, and also that its tenuity must have been almost inconceivable; so tenuous that a cubic miriamètre of it would not contain more perhaps than · grammes in weight. and very properly, we think, he looks upon the solar systems as having, at one time, formed a part of the whole universe, all of which was brought into existence, created, more or less, about the same time. in this universe, he considers that the stars have been formed, as well as the sun, by the progressive concentration of primitive materials disseminated in space, which conception gives rise to a totally new notion of the most positive character: viz. that each star owes to its mode of formation a provision of heat essentially limited; that it is not permissible, as laplace thought he could do, to endow a sun with an indefinite amount of heat; and that what it has expended and what it still possesses, depend upon its volume and actual mass. and also that the primitive materials of the solar system were, at the beginning, part of a universal chaos from which they were afterwards separated, in virtue of movements previously impressed on the whole of the matter; and sums up his first ideas in the following manner or theorem: _"at the beginning the universe consisted of a general chaos, of extreme tenuity, formed of all the elements of chemistry more or less mixed and confounded together. these materials under the force of their mutual attractions were, from the beginning, endowed with diverse movements which brought about their separation into masses or clouds. these still retained their movements of rapid translation, and very gentle interior gyrations. these myriads of chaotic fragments have given birth, by means of progressive condensations, to the diverse worlds of the universe."_ ( ) so much for the formation of the universe, including, of course, the solar system, for which he acknowledges the necessity for the intervention of a creating power, because it is impossible to account for it simply by the laws of nature; and adds: it is unnecessary to say that the universe is an indefinite series of transformations, that what we see results logically from a previous condition, and thus necessary in the past as in the future; we cannot see how a previous condition could tend towards the immense diffusion of matter, to the chaos out of which the actual condition has arisen; and that it is, therefore, necessary to begin with a hypothesis, and postulate of god, as descartes did, the disseminated matter and the forces which govern it. ( ) from dealing with the universe, m. faye comes to the formation of an isolated star, and begins with an entirely ideal case, that of a spherical homogeneous mass, without interior movement of any kind, and concludes that the molecules would fall in straight lines towards the centre; that the mass would condense regularly without losing its homogeneity, and would end in producing an incandescent sphere perfectly immovable; and that that would be a star, but a star without satellites, without rotation, without proper movement. this not being what was wanted, he goes on to show how, previous to its separation and complete isolation from the universal chaos, such a mass would possess, and carry with it when separated, a considerable velocity of rotation, and would still retain the internal movements it had acquired from the attraction of the other masses with which it had been previously in contact; and how the molecules, drawn towards the centre in obedience to gravitation, would not fall in straight lines but in concentric ellipses. ( ) from this state of affairs, two very different results might arise. one, that the molecules might resolve themselves into a multitude of small masses without the centre acquiring a preponderating increase. the other, that the central condensation might greatly exceed the others, and there would be formed a central star accompanied by a crowd of small dark bodies. m. faye accepts the second result, in which case the ellipses described by the small bodies, now become satellites, would, as the central mass increased in preponderance, have one of their centres at the centre of the preponderating mass, and their times of revolution would vary from one to another in conformity to the third law of kepler. ( ) for the formation of the solar system m. faye finds that it is of little importance whether the movements of bodies around the sun be very eccentric or almost circular; the first cause is always the same. they arise from the eddies, _tourbillonnements_, they have brought with them from their rectilinear movements in the primitive chaos. but the circle is such a particular case of the ellipse, that we ought not to expect to see it realized in any system. it is therefore necessary that, among the initial conditions of the chaotic mass, one should be found which would prevent the gyrations, eddies, from degenerating into elliptical movements, and which has at first made right, and afterwards firmly preserved, the form, more or less circular, in all its changes. ( ) for the formation of circular rings he gives us the following conceptions: in order that a star should have companions, great or small, circulating round the centre of gravity of the system, it is necessary that the partial chaos from whence it proceeded should have possessed, from the beginning, a gentle eddying movement affecting a part of its materials. besides, if the partial chaos has been really round and homogeneous, we shall see that these gyrations must have taken up, and to some extent preserved, the circular form. he then requests the reader not to lose sight of the feeble density of the medium, in which a succession of mechanical changes are to be brought about; and not to conclude that that density was such that a cubic miriamètre of the space occupied by it might not contain grammes of matter, as he stated in the preceding chapter (we think he said grammes), but that it might contain only grammes or even less. and adds that in such a medium, the small agglomerations of matter which would be formed all through it, would move as if they were in an absolute vacuum, and any changes in them would be produced extremely slowly. ( ) then he goes on to say that the gyrating movements belonging to the chaotic mass, would have very little difficulty in transforming a part of a motion of that kind into a veritable rotation, if this last were compatible with the law of the internal gravitation; that it is the nature of that kind of masses to only permit, to the bodies moving in them, revolutions, elliptic or circular, concentric and of the same duration; that therefore notable portions of the gyrating matter could take the form and movements of a flat ring, turning around the centre with the same angular velocity, exactly as if this nebulous ring were a solid body; that all the particles which have the proper velocity in the plane of the gyrations, will arrange themselves under the influence of gravitation in a flat ring with a veritable rotation around the centre; that any other parts having velocities too great or too small, will move in the same plane, describing ellipses concentric to the ring; that if the ellipses are very elongated the materials composing them will approach the centre, where they will produce a progressive condensation, communicating to the central globe formed there a rotation in the same plane with the primitive gyrations; and finishes off the whole scheme by specifying the first results to be: ( ) the formation of concentric rings turning in one piece, in the manner of a solid body, around a centre almost empty (_d'abord vide_); and ( ) a rotation in the same direction, communicated to the condensation which would be produced, little by little, by means of matter coming in, partly, from regions affected by the internal eddyings (_tourbillonnements_). ( ) it is unnecessary to go any farther, and take note of his method of the formation of planets and satellites from rings, as it is much the same as what we have seen described by others who have written on the same subject; only interpreted by him in a way to suit his own purposes, and in which interpretation he does not do full justice to laplace, through not having paid sufficient attention to his explanation of how planets could be formed out of rings. except in so far as to note that all along he has considered that rings were formed, and even those nearest to the centre condensed into globes, long before the central condensation had attained any magnitude of importance, or assumed any distinctive shape, and that afterwards all the disposable matter of the rings and also all the exterior matter that had not formed part of what was separated from the original universal chaos, had fallen in towards the small central mass, and so completed the formation of the sun last of all. we shall now proceed to make a few remarks with respect to this condensation of m. faye's cosmogony, which we think we have made without adding to or omitting anything of importance that we have met with in his work, for which purpose we have numbered the paragraphs containing it, in the last six pages, in order to do away with the necessity of repeating the parts to which we refer. no. . all those who believe that "the solar system did originate somehow, by the condensation of a primitive nebula," agree with m. faye in considering that the density of the nebulous matter must have been extremely low, and some of them seem almost to vie with each other in showing how great must have been the degree of its tenuity; but m. faye is one of the few who, paying due respect to the law of the interdependence of temperature and pressure in a gas or vapour, maintain that it must have been almost devoid of temperature, and we have to acknowledge that he is in the right. then we believe that his assumption, that the whole universe of stars, including the sun, was created, humanly speaking, about the same time, is shared by the great majority of those who have thought at all seriously on the subject. also, we agree with him firmly in his statement that each star--and we add planet, satellite, etc.--was originally supplied with an extremely limited quantity of heat, and that what it has expended and what it still retains has been derived entirely from the condensation of the original cosmic matter out of which it was made. with regard to his theorem: we cannot follow him in his statement that the diverse movements caused by the mutual attractions of parts of the original universal mass of cosmic matter, have brought about its separation into myriads of fragments; nor how these fragments could carry with them a rapid movement of translation, unless the whole universal mass was endowed with a rapid movement of translation through space, in which case we think that such a motion would have had no greater particular effect in producing new forms of motion in the fragments, than if the whole had been created in a state of rest. stray movements of translation might give rise to collisions among the multitude of fragments, and perhaps that was one of the modes of formation into suns through which they had to pass; but we cannot follow it out. neither can we see clearly how translation could be effected of one mass into the space occupied by another mass--unless empty spaces were reserved for that purpose from the beginning. without that, translation could not exist: it would be collision. no. . we have nothing to object to what is said in this paragraph; except that a rotating sphere might have been postulated at once, in imitation of laplace, instead of trying like descartes to join fragments together, endowed with movements so adjusted that, among the whole of them, they would produce in the whole mass, when united, the kind of movement that was wanted. no. . to the ideal case of the formation of an isolated sun from a homogeneous mass without interior movement of any kind, we cannot agree in any way. the molecules of matter would not, could not, fall in towards the centre in straight lines. their mutual collisions would drive them generally in curved lines in all directions as they fell in, which would create new internal movements; and these movements would prevent the possibility of the formation of an immovable incandescent sphere such as is described. there could be no immobility in the interior of a sun, as long as its temperature was sufficient to keep the surface incandescent. but we cannot give our reasons here for this assertion--to most people they will, we think, occur at once--because we have a long road to travel before we can do so. when m. faye abandons the isolated case, he leaves us without giving us any help, to conceive for ourselves how the mass would possess and carry with it a considerable velocity of rotation, and still retain the internal movements it had acquired from the attraction of the other masses--of the universal chaos--with which it had been in contact; and also how the molecules drawn towards the centre would not fall in straight lines but in concentric ellipses. and this last we have to do without his giving us any reason why the molecules should fall in towards the centre at all; or rather in spite of the fact that one of his principal ideas would lead us to expect exactly the contrary, as we shall see presently. no. . here he places before us again, two cases in one of which the molecules might resolve themselves into a multitude of small masses, without the centre acquiring any preponderating increase; and the other where the central condensation might greatly exceed the others, and there would be formed a central star accompanied by a crowd of small dark bodies, now become satellites, describing ellipses around the central preponderating mass. this second case he seems, for the time being, to accept as the most probable; but it is strangely at variance with what he sets forth afterwards. he does not give us the least hint as to why or how the satellites acquired their various times of revolution, but only assumes that they did so; and we are very sure that it was not the third law of kepler that was the agent in the case, however much it might suit his purpose. no. . although this part of his exposition is dedicated to the formation of the solar system, all that m. faye says is that it is of little importance whether the movements of bodies around the sun be very eccentric or almost circular; and that among the initial conditions of the chaotic mass, all that we require is that one should be found which would prevent the gyrations from degenerating into elliptic movements, and which had first put right and afterwards firmly preserved the form, more or less circular, in all its changes. but he does not make any attempt to show what that one condition is, and allows us to find it out for ourselves. no. . what m. faye says about the formation of circular rings is more or less a repetition of what he has adduced, to explain all the other movements which he has derived from the universal chaos; and which he seems to think sufficient to account for such movements being nearly circular. for our part we do not think they are sufficient, and he does not show us how they influence each other to bring about the final movements he wants to present to us. we duly take note of the tenuity of the cosmic matter on which he operates, which at grammes in weight to cubic miriamètre would correspond to one grain in weight to , , , cubic feet of space, or grain to a cube of feet--more than yards--to the side. we do this in order to remind him of what he says at page of his work, when dealing with the rotation of the kant-laplace nebula--namely, that it is impossible to comprehend how an immense chaos, of almost inconceivable tenuity, could possess such a rotation from the beginning, and that for want of that inadmissible supposition nothing remains to fall back upon but the _mouvements tourbillonnaires_ of descartes. thus he wants us to believe that his _tourbillons_ could move in straight or curved lines, have motions of translation, could attract, restrain, and drive each other into all sorts of movements with the tenuity he has indicated; but that laplace's nebula, with a density of grain to a cube of feet--or at most feet--to the side, could not be conceived to have the single movement of rotation. and lastly, we repeat that if the centre of the chaos was almost empty, we do not see what induced the cosmic matter to fall into it in elliptic orbits. nos. and . in these paragraphs, the main features are repetitions of the simple assertions made in all the others, that certain movements possessed by matter in one state would produce other movements in another state, without attempting to show how they all came to so far coincide with each other and form one harmonious whole, with movements in almost one single direction. it is clear that one side of the separated chaos might have acquired motion in one direction from the universal chaos with which it had been in contact, and that the opposite side might have acquired motion in exactly the opposite direction from the original chaos with which it had been in contact; and we are left to find out how these came to agree with each other in the end. and, going back to the beginning, we are left to find out where the mass, out of which he constructs his solar system, was stowed away, after it was separated from the original universal chaos. we can conceive of its being separated by condensation, in obedience to the law of attraction, from the surrounding chaos, in which case it might fall towards a centre, or that some parts of it might come to revolve round each other, and that finally the whole of these parts might come to rotate about a common centre; but that is evidently very different from the mode of formation of the solar system which m. faye has advocated. it comes to be by far too like the nebula which laplace supposed to be endowed with rotary motion from the beginning, probably because he did not see, or did not take the trouble to see, how such a motion could be produced. in any case, laplace did not consider that the primary motion of rotation was the most important part of his hypothesis; neither was it, as it seems to have been in the case we have been considering. and he did not go much further than m. faye in postulating primary motion, only he did it in a more effectual and business-like manner. he drew on the bank at once for all the funds he required, instead of having to draw afresh every time he found himself in difficulties, as has been the lot of his critic and successor. finally, m. faye tries to show that after all his rings, flat or otherwise, converted or not converted into globes, had been formed according to his ideas, the greater mass by far of the chaos had fallen into the centre, and had formed the sun there last of all. now, if the preponderating mass of the chaos had been outside of the field of his operations, up to the period when all his planets, satellites, etc. were formed, or at least laid out, it is more natural to suppose that the matter inside of his structure, if there was any, would be drawn outwards by the attraction of the greatly preponderating mass outside, than that any portion of it should have fallen in, in elongated ellipses, towards the insignificant mass that he supposes to have been inside his structure. this, of course, would be nearly exactly the reverse of the mode of formation he was trying to demonstrate, and clearly shows that he was working on unsound principles from the beginning to the end of his cosmogony. it had never occurred to him that matter could be attracted outwards as well as inwards, most probably because it would seem to him ridiculous to imagine that anything in the universe could _gravitate_ upwards. there are other theories of the formation of the solar system from meteorites and meteors, giving us the idea of its being made out of manufactured articles instead of originally created raw material, which does not in any way simplify the process. in some of them, the inrush of meteor swarms is invoked as the cause of gyratory motion, which places them in much the same category as impact theories. we know that broadcloth is made out of woollen yarn, but we also know how the yarn is made out of wool, and how it is woven into the cloth, whereas we are not told by what process, or even out of what the meteors and meteorites are made, although some of them are said to have thumb-marks upon them. all these theories and cosmogonies may be very appropriately classified as variations of the nebula hypothesis, and like variations in another science, may be very brilliant, scientific, imaginative, grand, but after all the flights of fancy exhibited by them are set before us, we feel in a measure relieved when a return is made to the original air. they all assume original motion, varied, accidental, opportune, more dependent upon the will of the cosmogonist than on the laws of nature, which tend to confound rather than enlighten any one who tries to understand and bring them, mentally, into actual operation. laplace assumed rotary motion for the whole of his nebula, and was thus able to account at once for the relation which exists among the planets in respect of distance from, and period of revolution around the sun--arising from the original rotation of the whole mass in one piece--a result which, in any impact theory, has to be accounted for separately, and, in plain truth, empirically in each case, and at each step. seeing, then, that we have not been able to find any cosmogony, or speculation, that gives us a more plausible idea of how the solar system has been formed, we shall try whether from the original nebula as imagined by laplace, it is possible to separate the various members, and form the system in the manner described in his celebrated hypothesis. in other words, we shall endeavour to analyse the hypothesis. chapter iv. page preliminaries to analysis of the nebular hypothesis. definition of the hypothesis. elements of solar system. tables of dimensions and masses. explanation of tables and density of saturn. volume, density and mass of saturn's rings, general remarks about them, and satellites to be made from them. future of saturn's rings. notions about saturn's satellites and their masses. nature of rings seemingly not well understood. masses given to the satellites of uranus and neptune, explanations of. volumes of the members of the solar system at density of water. preliminaries to analysis of the nebular hypothesis. it may be thought that there is little benefit to be derived from analysing an hypothesis which has been declared, by very eminent authorities in the matter treated of, to be erroneous in some points of very serious importance; but hypotheses are somewhat of the nature of inventions, and we know that it has often happened that many parties, aiming at the same invention, have altogether failed, while some other person using almost exactly the same means as his predecessors, has been entirely successful in his pursuit. how many times has it been pointed out to us, that if such a person had only gone one step further in the process he was following, or had only studied more deeply the matter he had in hand, he would have anticipated by many years one of the greatest discoveries of the age! in some cases the failure to take that one step was occasioned through want of knowledge acquired long years afterwards; whereas we think that in the case we have in hand, it can be shown that the want of knowledge acquired many years after he had formulated his hypothesis, or if otherwise, the want of faith in what he knew, enabled laplace to construct an edifice which otherwise he could hardly have convinced himself could be built up in a practical form. we think also that if he had made the proper use of the knowledge he must have had of the law of attraction, he would have seen that no nebula could ever have existed such as the one he assumed, extending far beyond the orbit of the remotest planet. furthermore, we think it can be shown that if he had thoroughly considered what must have been the interior construction of his nebula, he would have found one that would have suited his hypothesis in the main point, viz. condensation at the surface, at least equally as well as endowing it with excessive heat. but to be able to show these things our first step must be to analyse the hypothesis, to examine into it as minutely and deeply as lies in our power. for this purpose it will be necessary to define what the hypothesis is. many definitions have been given, more or less clear, and it would be only a waste of time to try to set forth laplace's own exposition of it, with all its details, which he had no doubt studied very carefully. but in those definitions that have come under our observation, several of the conditions he has specified are wanting, or not made sufficiently prominent; so instead of adopting any one of them we will make a sort of condensation of the whole, adding the conditions that have been left out; because the want of them, has been the cause of mistaken conceptions of the evolution of the system having been formed by very eminent astronomers. our definition will therefore be as follows:-- i. it is supposed that before the solar system was formed the portion of space in which its planets and other bodies now perform their revolutions and other movements, was occupied by an immense nebula of cosmic matter in its most simple condition--of molecules or atoms--somewhat of a spherical form, extending far beyond its present utmost limits, and that it was endowed with excessive heat and a slow rotary motion round its centre; which means that while it made one revolution at the circumference it also made one at the centre. the excessive heat, by counteracting in a certain measure the force of gravitation, kept the molecules of matter apart from each other; but as the heat was gradually radiated into space, gravitation became more effective, and then began to condense and contract more rapidly, by which process its rotary motion was, in accordance with the areolar law, gradually increased at the surface, _in the atmosphere of the sun_, where the cooling took place, and condensation was most active; and the increase of rotation was propagated from there towards the centre. ( ) as the contraction and rotation increased a time or times arrived, when the centrifugal force produced by the rotation came to balance the force of gravitation, and a series of zones or rings were separated from the nebula, each one of them continuing to rotate--revolve now--around the central mass, with the same velocities they had at the times of their separation; until at last the nebula became so contracted that it could not abandon any more rings, and what of it remained condensed and contracted into a central mass which ultimately assumed the form of the actual sun. ( ) in the meantime, or following afterwards, each one of the rings which were abandoned by the nebula, acquired, through the friction of its molecules with each other, an equal movement of revolution throughout its entire mass, so that the real velocities of the molecules furthest removed from the centre of the nebula were greater than those of the molecules nearest to its centre, and the ring revolved as if it were in one solid piece. arrived at this stage the rings broke up and formed themselves into smaller nebulæ, each of which condensed into a globe or planet, and continued to revolve around the central mass in the same time as its mass had done when in the form of a ring. and some of these sub-nebulæ, imitating the example of their common parent more perfectly than others, abandoned in space in their turn smaller rings which in the same manner condensed, broke up, and formed themselves into smaller globes or satellites; all, as far as we know, except the rings of saturn, which have not as yet been converted into satellites. table i. elements and other data of the solar system employed in this analysis. part i.--sun and planets. --------+-------------+----------+-----------------------+-----------+ |mean distance|equatorial|volume in cubic miles. | density. | name. | from sun |diameter | |(water = )| | in miles. |in miles. | | | --------+-------------+----------+-----------------------+-----------+ sun | -- | , | , , , , , | · | mercury | , , | , | , , , | · | venus | , , | , | , , , | · | earth | , , | , | , , , | · | mars | , , | , | , , , | · | supposed| | | | | planet| , , | -- | -- | -- | jupiter | , , | , | , , , , | · | saturn | , , | , | , , , , | · | uranus | , , , | , | , , , , | · | neptune | , , , | , | , , , , | · | --------+-------------+----------+-----------------------+-----------+ | volume at density of | time of revolution | name. | water in cubic miles. | round sun in days. | | | | --------+------------------------+-----------------------+ sun | , , , , , | -- | mercury | , , , | · | venus | , , , , | · | earth | , , , , | · | mars | , , , | · | supposed| | | planet| , , , | , · | jupiter | , , , , | , · | saturn | , , , , | , · | uranus | , , , , | , · | neptune | , , , , | , · | --------+------------------------+-----------------------+ part ii.--satellites of planets. ----------+--------------+-------------+-----------------+ | mean distance| equatorial | volume | names. | from primary | diameter | in cubic miles. | | in miles. | in miles. | | ----------+--------------+-------------+-----------------+ | _of the earth._ | moon | , , , , | | | | _of jupiter._ | jo | , , , , | europa | , , , , | ganymede | , , , , | callisto | , , , , , | | | | _of saturn._ | mimas | , , , | enceladus | , ? , , | tethys | , , , | dione | , , , | rhea | , , , | titan | , , , , | hyperon | , , ? , , , | | | japetus | , , , , , | | | | _of uranus._ | ariel | , } total mass taken at | umbriel | , } / , th of primary | titania | , } , , , | oberon | , } | | | | _of neptune._ | ----- | , } mass taken at | | } / , th of primary | | , , | ----------+----------+-----------------+-----------------+ | density. |volume at density| total volume at | names. |(water= .)| of water in |density of water | | | cubic miles. | in cubic miles.| ----------+----------+-----------------+-----------------+ | | | _of the earth._ | moon | · .. , , , | | | | _of jupiter._ | jo | · , , , | europa | · , , , | ganymede | · , , , | callisto | · , , , | | ----------------- | | , , , | | | | _of saturn._ | mimas | ... | enceladus | ... | tethys | ... | dione | ... | rhea | ... | titan | ... total volume | hyperon | ... in cubic miles. | | | japetus | · , , , , , , | -----------+----------------------------------------------+ part iii.--rings of saturn. ---------+-----------------------------+--------------------------+ | | | rings. | diameters of rings | areas of rings | | in miles. | in square miles. | ---------+-----------------------------+--------------------------+ outer { | outer | , | } , , , | { | inner | , | } | | | | | middle { | outer | , | } , , , | { | inner | , | } | | | | | dark { | outer | , | } , , , | { | inner | , | } | | | | | | | total | , , , | ---------+----------+------------------+---------+----------------+ |thickness | | | volume at | rings. | of rings | volume of rings |density. |density of water| | in miles.| in cubic miles. |(water= )|in cubic miles. | ---------+----------+------------------+---------+----------------+ outer { | | | | | { | | | | | | | | | | middle { | | | | | { | | | | | | | | | | dark { | | | | | { | | | | | | | | | | | | , , , , | · | , , | ---------+----------+------------------+---------+----------------+ ( ) all of these bodies, planets, satellites, and rings were supposed to revolve around their primaries, and to rotate on their axes, in the same direction viz., from right to left, in the opposite direction to the hands of a watch. in addition to the above definition it is necessary to give some sort of description of the various parts of the machine or system which has to be made out of the nebula, with their positions, dimensions, and details. this we believe will be made plain enough, in the simplest manner, by table no. i., taken and calculated from the elements of the solar system given in almost all astronomical works, from which we have selected what we believe to be the most modern data. the construction of this table requires some explanation on account of its being made to show complete results from incomplete data. there has been no difficulty with the sun, the major planets, and the satellites of the earth and jupiter, but for the minor planets, the satellites of the three outer planets, and the rings of saturn, we have been obliged to exercise our judgment as best we could. there being almost no data whatever of the dimensions and densities of the minor planets, to be found, we have been driven in order to assign some mass to them, to imagine the existence of one planet to represent the whole of them (in fact olbers's planet before it exploded), which we have supposed to be placed at the mean distance of , , miles from the centre of the sun; and we have given to it a mass equal to one-fourth of the mass of the earth, that being, in the opinion of some astronomers, the greatest mass which the whole of them put together could have. this assumption we shall explain more fully at a more suitable time. in the case of saturn the diameters of two of the satellites are wanting which we have assumed to be the same as those of the smallest of those nearest to them, and thus have been able to compute the volumes of the whole of them; but we have not been able to find any statement anywhere of their densities, and to get over this difficulty we have reasoned in the following manner. the density of the moon is very little over two-thirds of that of the earth, while that of the satellites of jupiter varies from a little more than the same to a little more than twice as much as the density of their primary. why this difference? to account for it we appeal to the very general opinion of astronomers, that the four inner planets are in a more advanced stage of their development, or existence, than the four outer ones. in this way it is easy to conceive that the earth has arrived at the stage of being more dense than its satellite; while in the case of jupiter, his satellites being of so very much less volume than their primary, have already arrived at a higher degree of development. carrying this motion forward to saturn, we have supposed that from his being considerably less dense than any other of the outer planets--quite possibly from having been formed out of material comparatively (perhaps not actually) less dense than the others--his satellites may not have condensed to a greater degree than his own mass, and we have, therefore assumed their density, that is the density of the volume of the whole of them, to be the same as that of their primary. to determine some mass for the rings of saturn, is a much more intricate matter than for his satellites, and presents to us some ideas--facts rather--which had never before crossed our imagination. the most natural way to look upon these rings is to suppose that they are destined to become satellites at some future time. all the modern cosmogonies that have come under our notice are founded upon the idea that rings are the seed, as it were, of planets and satellites, and if those of saturn have been left, as it has been said, to show how the solar system has been evolved, it cannot be said that the supposition is not well founded. in this way we are led to speculate upon how many satellites are to be made out of the rings before us. considering, then, that the nearest satellite is , miles from the centre of saturn, leaving only , miles between his surface and that of mimas, and also that the distances between satellites diminish rapidly as they come to be nearer to their primaries, there is not room to stow away a great number of satellites. on the other hand, seeing that there are at least three distinct rings, we cannot reasonably do less than conclude that three satellites are intended to be made out of them. but let the number be what it may, all that we have to do with them for our present purpose is to assign some mass to them. with this view, we have given, arbitrarily, to each one of the three we have supposed, a volume equal to that of one of the satellites of miles in diameter, that is, about , , cubic miles, and we have supposed their density to be the same as that of water, instead of that of the planet. thus, in the table, we have assigned to the three a mass of , , cubic miles at density of water, which would be more than sufficient to make four other satellites for the system of miles in diameter each, and of the same density as the planet. for the table referred to we have calculated the areas of the three rings to be , , , square miles, and we have assumed the thickness as miles, that is about two-thirds of that estimated by chambers in his handbook of astronomy, but almost the same as that given by edmund dubois; nevertheless their total volume comes up to , , , , cubic miles, which reduces their average density to · that of water, to make up the mass of , , cubic miles at the density of water, which we have adopted for the three. this density corresponds to very nearly one-tenth of that of air, which, however strange it may appear to us, may be considered to be a very full allowance, seeing that we shall find, later on, that the planet itself was formed out of matter whose density could not have been more than one twenty-six millionth part of that of air. all the same, it is hardly matter that we could liken to brickbats. after being driven to this low estimate of density, which startled us, we referred to an article in "nature" of nov. , , on ten years' progress in astronomy, where we find what follows:--"he (newcomb) finds the mass of titan to be about / , that of saturn. it may be noted, too, that hall's observations of the motions of mimas and enceladus indicate for the rings a mass less than / that deduced by bessel; instead of being / as large as the planet, they cannot be more than / , and are probably less than / , ." (we make them / ). thinking over the numbers herein given we cannot help being surprised by them. if titan be / of the mass of saturn, we cannot conceive how the mass of his rings can be so much greater than that of titan. we cannot pretend to fit even one satellite of that size, mechanically, into a space of , miles wide, while titan revels in an ample domain with a width of , miles. but we shall not pursue this part of our speculations any further. astronomers may be able to demonstrate that the rings are of a totally different nature to those out of which the planets and their satellites are supposed to have been made, or that the nebular hypothesis or anything resembling it is no better than a foolish dream. all that we have pretended to do has been to give them their due place in the hypothesis we are attempting to analyze, and to look upon them in a practical and mechanical light, as an unfinished part of the solar system. to determine masses for the satellites of the two outer planets, we have to be more empirical even than we have yet been. a little trouble will show that the whole mass of all the satellites and rings of saturn put together is about / th of the mass of the planet, and we shall avail ourselves of this proportion to assign masses for the satellites of the remaining planets, the numbers and names of which are the only data we have been able to find. considering then, that uranus has only four satellites and no rings, we think if we give them / , th of the mass of their primary, it will be a very fair allowance; and with the same empiricism we have adopted for the solitary satellite of neptune / , th of the mass of its primary. however rude and crude these approximations may be, we have the satisfaction of thinking that the masses obtained by their means, can have no appreciable effect upon the operations into which they are to be introduced, whilst they enable us to deal with a complete system or machine. but for these we have another table no. ii. to present, a _résumé_ of the foregoing one, for greater facility of reference. table ii.--volumes of the various members of the solar system at the density of water. ---------+------------+-------------------+-----------------------+ | | volume in | total volume in | name. |designation.| cubic miles at | cubic miles at | | | density of water. | density of water. | ---------+------------+-------------------+-----------------------+ sun | | | , , , , , | | | |-----------------------| mercury | planet | | , , , | venus | " | | , , , , | earth | " | , , , , | | moon | satellite | , , , | , , , , | | |-------------------| | mars | planet | | , , , | ---- | asteroids |one fourth of earth| , , , | jupiter | planet | , , , , | | " | satellites| , , , | , , , , | | |-------------------| | saturn | planet | , , , , | | " | satellites| , , , | | " | rings | , , | , , , , | | |-------------------| | uranus | planet | , , , , | | " | satellites| , , , | , , , , | | |-------------------| | neptune | planet | , , , , | | " | satellite | , , | , , , , | ---------+------------+-------------------+-----------------------+ total of planets, satellites and rings | , , , , | ------------------------------------------+-----------------------+ dividing , , , , , by , , , , makes the mass ofthe whole of the members to be / · th part of the mass of the sun, instead of / th as generally stated by astronomers. chapter v. page analysis of the nebular hypothesis. separation from the nebula of the rings for the separate planets, etc. excessive heat attributed to the nebula erroneous and impossible. centigrade thermometer to be used for temperatures. temperature of the nebula not far from absolute zero. erroneous ideas about glowing gases produced by collisions of their atoms, or particles of cosmic matter in the form of vapours. separation of ring for neptune. it could not have been thrown off in one mass, but in a sheet of cosmic matter. thickness and dimensions of the ring. uranian ring abandoned, and its dimensions. saturnian ring do. do. jovian ring do. do. asteroidal ring do. do. martian ring do. do. earth ring do. do. venus ring do. do. mercurian ring do. do. residual mass. condensation of solar nebula to various diameters, and relative temperatures and densities. unaccountable confusion in the mode of counting absolute temperature examined and explained. negative degrees of heat only equal degrees of absolute temperature. the centigrade thermometric scale no better than any other, and cannot be made decimal. the sun's account current with the nebula drawn up and represented by table iii. analysis of the nebular hypothesis. we may now proceed to take the original nebula to pieces, by separating from it all the members of the solar system, in performing which operation we shall suppose the divisions between the nebula and each successive ring to have taken place at a little more or less than the half distances between the orbits of two neighbouring planets, because we have no other data to guide us in determining the proper places. these divisions have manifestly been brought about in obedience to some law, as is proved in great measure by what is called bode's law; although no one has as yet been able to explain the action of that law. it is no doubt certain that a division must have taken place much nearer to the outer than the inner planet in each case, if we think of what would be the limit to the sphere of attraction between the nebula and a ring just detached from it--for the attraction of the abandoned ring, and even of all those that were outside of it, would have very little influence in determining the line where gravitation and centrifugal force came to balance each other--but the data necessary for calculating what these would be are wanting. even if they existed the calculations would become too complicated for our powers as the number of rings increased; and for our purpose it is really of very little importance where the divisions took place. the breadths of the rings would be practically the same, whether they were divided at the half distances between, or much nearer to, the outermost of two neighbouring planets; and although the extreme diameters of the consecutive residuary nebulæ would be somewhat greater, their densities and temperatures would not materially differ from those we shall find for them as we proceed in our operations. their masses would be the same in all cases, which is the principal thing in which we are interested. this premised, we shall first examine into the excessive heat attributed to the nebula, that being the first condition mentioned in our definition of the hypothesis. the diameter of the sun being , miles, his volume is , , , , , cubic miles, and his density being · times that of water, his volume reduced to the density of water would be , , , , , cubic miles. now, astronomers tell us that the whole of the planets, with their satellites and rings, do not form a mass of more than / th part of the mass of the sun. if, then, we add / th part to the above volume, we get a total volume, for the whole of the system, of , , , , , cubic miles at the density of water, which corresponds to a sphere of about , miles in diameter. on the other hand, the diameter of the orbit of neptune being , , , miles, if we increase that diameter to , , , miles, so that the extreme boundary of the supposed nebula may be as far beyond his orbit, as half the distance between him and uranus is within it, we shall still be far within the limit at which the process of separation from the nebula, of the matter out of which neptune was made, must have begun. from these data we can form a very correct calculation of what the density--tenuity rather--of the nebula must have been. for, as the volumes of spheres are to each other as the cubes of their diameters, the cube , is easily found to be to the cube of , , , , as is to , , , , or in other words, the density of the nebula turns out to have been / , , , th part of density of the whole solar system reduced to that of water. carrying the comparison a little further, we find that as water is · times more dense than air, and , · times more dense than hydrogen, the density of the nebula could not have been more than / , , th part that of air, and / , , th that of hydrogen. but, confining the comparison to air, as it suits our purpose better, we see that it would take , , cubic feet of the nebula to be equal in mass to cubic foot of air at atmospheric pressure; and that were we to expand this cubic foot of air to this number of times its volume, the space occupied by it would be as nearly in the state of absolute vacuum as could be imagined, far beyond what could be produced by any human means. now, were heat a material, imponderable substance, as it was at one time supposed to be, we could conceive of its being piled up in any place in space in any desired quantity; but it has been demonstrated not only not to be a substance at all, but that its very existence cannot be detected or made manifest, unless it is introduced by some known means--friction, hammering, combustion--into a real material substance. therefore, we must conclude that if it existed at all in the nebula, it must have been in a degree corresponding to the tenuity of the medium, and the air thermometer will tell us what the temperature must have been if we only choose to apply it. applying, then, this theory of the air thermometer, if we divide[b] ° by , , --the number of times the density of the nebula was less than that of air--we get · °, as the absolute temperature of the nebula, something very different to excessive heat, incandescence, firemist, or any other name that has been given to its supposed state. furthermore, as a cubic foot of air weighs · grains, , , divided by · , which is equal to , , would be the number of cubic feet of the space occupied by the nebula, corresponding to each grain of matter in the whole solar system, which would be equal to a cube of very nearly feet to the side. and as the only means by which the nebula could acquire heat would be by collision with each other of the particles of matter of which it was composed; to conceive that two particles weighing grain each, butting each other from an average distance of feet, could not only bring themselves, but all the space corresponding to both of them--which would be , , cubic feet, _of what_?--up to the heat of incandescence, or excessive heat of any kind, is a thing which passes the wit of man. consequently, neither by primitive piling up, nor by collisions among the particles, could there be any heat in the nebula at the dimensions we have specified, beyond what we have measured above. [b] here we beg to state that in all our coming operations, we will use the centigrade scale for temperatures without adding c to each number specified, unless a different scale has to be referred to, in which case the distinctive of the scale shall be given in the usual way. this we do because it is the fashion, not because we think it possesses any advantage over any other scale, but rather the contrary. perhaps we may have something more to say about scales after we have handled the centigrade a little more than it has been our lot to do hitherto. some people believe, at least they seem to say so, that meteors or meteorites colliding would knock gas out of each other, sufficient to fill up the empty space around them, and become incandescent, and so pile up heat in nebulæ sufficient to supply suns for any number of millions of years of expenditure. but they forget that gas is not a _nothing_. it possesses substance, matter, of some kind, however tenuous. therefore, if the meteors knock matter out of each other in the form of gas, they must end by becoming gas themselves, and we come back to what we have said above; we have two grains, in weight, of gas abutting each other at an average distance of yards, instead of two grains of granite or anything else, and things are not much improved thereby. and if we compare yards with m. faye's , where are we? the next thing to deal with is the formation of the planets. separation of ring for neptune. when the nebula was , , , miles in diameter its volume would be , , , ^{ }[c] cubic miles, and we have just seen that its density must have been , , , times less than that of water, or , , less than air, and its temperature · ° above absolute zero. on the other hand, we find from table ii. that the volume of neptune and his satellite is , , , , cubic miles at the density of water. multiplying, therefore, this volume by , , , we get , , ^{ } cubic miles as the volume of the ring for the formation of neptune's system at the same density as the nebula. then, subtracting this volume from , , , ^{ }, there remain , , , ^{ } cubic miles as the volume to which the nebula was reduced by the abandonment of the ring out of which neptune and his satellite were formed. [c] the exponent in , , , ^{ } means that cyphers have to be added to complete the number. the same is the case with any other number and exponent of large quantities. then the mean diameter of the orbit of neptune being , , , miles, its circumference or length will be , , , miles, and if we divide the volume of his system as stated above, by this length, we get , , , , square miles as the area of the cross section of the ring, which is equal to the area of a square of , , miles to the side. again, if we divide the circumference of the orbit by this length of side, we find that it is / · th part of it, and therefore about minutes of arc. also if we divide the diameter of the orbit by an arc of , , miles in length, we find that it bears the proportion of to to the diameter of the orbit. thus the cross section of the ring would bear the same ratio to its diameter that a ring of foot square would bear to a globe of feet in diameter. here we find it difficult to believe that by rotating a ball of feet in diameter of cosmic matter, meteorites, or brickbats, we could detach from it, mechanically, by centrifugal force a ring of foot square, and the same difficulty presents itself to us with respect to the nebula. we cannot conceive how a ring of that form could be separated by centrifugal force from a rotating nebula, and have therefore to suppose it to have had some different form, and to apply for that to the example of saturn's rings--just the same as laplace no doubt did. we cannot tell how the idea originated that the ring should be of the form we were looking for--perhaps it was naturally--but it seems to have been very general, and in some cases to have led to misconceptions. it is not difficult to show how a saturnian or flat ring could be formed, but we shall have a better opportunity hereafter of doing so. we must try, nevertheless, to form some notion, however crude it may be, of what might be the thickness of a flat ring of the cross section and volume we have found for neptune. let us suppose that the final separation of the ring took place somewhere near the half-distance between his orbit and that of uranus, say, , , , miles from the centre of the nebula, the breadth of the ring would be the difference between the radius of the original nebula, i.e. , , , miles and the above sum, which is , , , miles. then if we divide the area of the cross section of the ring by this breadth, that is, , , , , by , , , , we find that the thickness would be , miles; provided the ring did not contract from its outer edge inwards during the process of separation. this could not, of course, be the case, but, as we have no means of finding how much it would contract in that direction, we cannot assign any other breadth for it; and we shall proceed in the same manner in calculating the thicknesses of the rings for all the other planets as we go along. we can, however, make one small approach to greater accuracy. we shall see presently that the density of the ring would be increased threefold at its inner edge as compared with the outer during the process of separation, which would reduce its average thickness to somewhere about , miles at density of water, of course. the nebula remaining after neptune's ring we may now call the uranian nebula. the volume of the nebula after abandoning the ring for the system of neptune was found to be , , , ^{ } cubic miles at its original density, but during the separation it has been condensed into a sphere of , , , miles in diameter, whose volume would be , , , ^{ } cubic miles; so that if we divide the larger of these two volumes by the smaller, we find that the density of the uranian nebula would be increased · times, and therefore it would then be , , , divided by · , equal to , , , times less dense than water. furthermore, if we compare it to the density of air, which we can do by dividing this last quantity by · , we find it to have been , , times less than that density; and if we apply the air thermometer to it, we shall find that its absolute temperature must have been divided by , , = · ° or - · .° we can now separate the ring for the system of uranus from the uranian nebula, reduced as we have seen to , , , miles in diameter, volume of , , , ^{ } cubic miles, and density of , , , times less than water. referring to table ii., we find the volume of the whole system of uranus to have been , , , , cubic miles at the density of water, but we have to multiply this volume by the new density of , , , times less than water in order to bring it to the same density as the nebula, which will make the volume of his system to be , , , ^{ } cubic miles at that density. then, subtracting this volume from , , , ^{ }, we find that the nebula has been reduced to , , , , ^{ } cubic miles in volume. then the diameter of the orbit of uranus being , , , miles, its circumference will be , , , miles, so that dividing the volume , , , ^{ } of his system by this length of circumference, the area of the cross section of the ring would be , , , , square miles. if we now suppose the diameter of the nebula, after abandoning the ring for the whole system of uranus, to have been , , , miles--dimension derived from nearly the half-distance between the orbits of uranus and saturn--we find that the breadth of the ring would be , , miles, which would be the difference between the radii of the uranian and saturnian nebulæ, respectively , , , miles, and , , , miles; so that if we divide the area of cross section of uranus' ring or , , , , square miles by this breadth we find the thickness of the ring to have been , miles. but the density of the inner edge of the ring would be · times more dense than the outer edge, for the same reason as in the case of the neptunian ring, which would make the average thickness to have been about , miles. saturnian nebula. we have seen that the volume of the nebula after the separation of the ring for uranus' system would be , , , , ^{ } cubic miles, but as we have reduced the diameter of the saturnian nebula to , , , miles, its volume would also be reduced, or condensed to , , ^{ } cubic miles, so that dividing the larger volume by the smaller we find that its density must have been increased · fold. then dividing , , , by · , we see that the density would be reduced, or increased rather, to , , , times less than that of water. this can be easily found to be , , times less than the density of air, and the air-thermometer would show that the absolute temperature of the saturnian nebula must have been · ° or - · °. we have just seen that the saturnian nebula has been condensed to , , , miles in diameter, to volume of , , ^{ } cubic miles, and density of , , , times less than that of water. then from table ii. we get the volume of the whole of the system of saturn as , , , , cubic miles at the density of water, and multiplying this by , , , will give , , , ^{ } as its volume at the same density as the nebula; and subtracting this from , , ^{ } we find that the volume of the nebula had been reduced to , , , , ^{ } cubic miles. then the diameter of the orbit of saturn being , , , miles its circumference would be , , , miles in length, and if we divide the volume of his system, viz. , , , ^{ } cubic miles, by this length, we find the area of the cross section of the ring to have been , , , , square miles. now, supposing the diameter of the nebula, after abandoning the ring, to have contracted to , , , miles and radius consequently of , , miles, the breadth of the ring would be , , , less , , or , , miles; and if we divide the area of the cross section of the ring, that is, , , , , square miles, by this breadth, we get , miles for its thickness. but in the same way as before, the inner edge of the ring would be · times more dense than the outer edge, which would reduce its average thickness to , miles. jovian nebula. the volume of the nebula after separation of the ring for saturn's system having been , , , , ^{ } cubic miles, this volume has to be condensed into the volume of the jovian nebula of , , , miles in diameter, which would be , , , , ^{ } cubic miles. then if we divide the first of these two volumes by the second, we find the density of the jovian nebula to have been increased · fold over the previous one. but the density of the saturnian nebula was , , , times less than water, dividing which by · makes the jovian nebula to have been , , , times less dense than water. dividing this by · we get a density for it of , , times less than that of air, which corresponds to the absolute temperature of · ° or - · °. from the jovian nebula of , , , miles in diameter, volume of , , , , ^{ } cubic miles, and density of , , , times less than water, we have now to deduct the whole of the system of jupiter, which, by table no. ii., is , , , , cubic miles at density of water. multiplying this by , , , we get the volume of , , , ^{ } cubic miles for his system at the same density as the nebula; therefore, substracting this amount from , , , , ^{ } we get , , , , ^{ } cubic miles as the volume to be condensed into the succeeding nebula which we shall call asteroidal, the dimensions of which we can determine in the following manner, although only very approximately. according to the nebula hypothesis, there must have been a ring detached from the nebula for the formation of the asteroids, as well as the formation of the other planets. so, in order to be able to assign elements for that ring, corresponding to those we have found for the others, we shall suppose the whole of them to have been collected into one representative planet, at the mean distance from the centre of the nebula of , , miles, more or less in the position denoted by the number in bode's law; also its mass to have been one-fourth of that of the earth, or , , , cubic miles at density of water, which, in the opinion of probably most astronomers, is a considerably greater mass than would be made up by the whole of them put together--discovered and not yet discovered. with the above distance from the centre of the nebula, the divisionary line between the jovian and the asteroidal nebulæ would be , , miles from the said centre, and the diameter of the latter , , miles in consequence. we know that some of the asteroids move in their orbits beyond this supposed divisionary line, and it may be that when we come to determine the divisionary line between the supposed asteroidal and the martian nebulæ, some of them may revolve in their orbits nearer to mars than that line, but that will not interfere in any way with our operations, because we are only dealing with the whole of them collected into one representative. for finding the dimensions of the ring for jupiter's system, we have the mean diameter of his orbit as , , miles, which makes its circumference to be , , , miles in length. therefore, dividing the volume of the ring as found above, viz. , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , , square miles, which divided in turn by the breadth of , , --the difference between the radii of the jovian and asteroidal nebulæ, or , , less , , --makes the thickness of the ring to have been , , miles. but, as before, the inner edge of the ring had become · times more dense than the outer edge, so that the average thickness would be only , miles. asteroidal nebula. the volume of the nebula after the separation of the ring for the system of jupiter having been , , , , ^{ } cubic miles, this volume has to be condensed into the volume of the asteroidal nebula of , , miles in diameter and consequently of volume of , , , , , ^{ } cubic miles. then if we divide the first of these volumes by the second, we find the density to have been increased · fold, as used above for the average thickness of jupiter's ring. but the density of the jovian nebula was , , , times less than water, dividing which by · makes the asteroidal nebula to have been , , times less dense than water. this again divided by · makes it , times less dense than air, which will give us · ° as its absolute temperature--or the same as - · °. next, from the asteroidal nebula , , miles in diameter, volume of , , , , , ^{ } cubic miles, and density , , times less than water, we have to deduct the volume of the whole of the system which in table no. ii. we have supposed to have been , , , cubic miles at density of water. multiplying this by , , we get the volume to have been , , , ^{ } cubic miles for the ring at the same density as the nebula; so, deducting this quantity from , , , , ^{ }, we get , , , , , ^{ } cubic miles as the volume to which the nebula had been reduced by the separation of the ring. for the dimensions of the ring we have the mean diameter of the orbit of the representative asteroid as , , miles, that is twice its distance from the centre of the nebula, which makes its circumference to be , , , miles in length. dividing then the volume of the ring, which we found to have been , , , ^ cubic miles by this length, the area of its cross-section must have been , , , square miles, which divided by the breadth of , , miles--the difference between the radii of the asteroidal and martian nebula, namely , , less , , --makes the thickness of the ring to have been miles. but the inner having been · times more than the outer edge, as we shall see presently, the average thickness would be miles. martian nebula. the volume of the last nebula after the separation of the ring for the asteroids was found to have been , , , , , ^{ } cubic miles, which had to be condensed into the volume of the martian nebula of , , miles in diameter, which would give a volume of , , , , , ^{ } cubic miles. dividing then, the larger of these volumes by the smaller, we find that the density of the martian nebula had been increased · times by the condensation. but we found the density of the asteroidal nebula to have been , , times less dense than water, dividing which by · makes the martian nebula to have been , , times less dense than water. this divided again by · makes it , times less dense than air, and consequently its absolute temperature to have been · ° or - · °. from the martian nebula of , , miles in diameter, volume , , , , , ^{ } cubic miles, and density , , times less than water, we have to deduct the volume of his ring, which by table ii., was estimated at , , , cubic miles at density of water. multiplying this by , , we find its volume to be , , , ^{ } cubic miles at the same density as the nebula, deducting which from its whole volume we get , , , , , ^{ } cubic miles as the volume after the separation of the ring. for finding the dimensions of the ring we have , , miles as the mean diameter of the orbit of mars, which makes its circumference , , miles in length. then dividing the volume of the ring , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , square miles, which, divided by the breadth of , , miles--that is one-half of the difference between the diameters of the martian and earth nebula, respectively , , and , , miles--makes the thickness of the ring to have been miles. but as before, the inner having become through condensation, · times more dense than the outer edge, the average thickness would be miles. earth nebula. as the volume of the nebula was , , , , , ^{ } cubic miles after the separation of the ring for mars, we have to condense it into the volume of the earth nebula, which at , , miles in diameter would be , , , , , ^{ } cubic miles. dividing the larger of these volumes by the smaller we find that the density of the nebula has been increased · times, as employed above. but we found the density of the martian nebula to have been , , times less than that of water, dividing which by · makes the earth nebula to have been , , times less dense than water. dividing this again by · we find it to have been , times less dense than air, and ° divided by this density of air--the same as in all the respective cases--gives · ° as the absolute temperature of the nebula and corresponds to - · °. from the earth nebula , , miles in diameter, , , , , , ^{ } cubic miles in volume, and , , times less dense than water, we have to subtract the volume of the ring of the earth's system, which, in table ii., appears as , , , , cubic miles at density of water. multiplying this by , , we find it to have been , , , ^{ } cubic miles at the same density as the nebula. and subtracting this quantity from , , , , , ^ , we get , , , , , ^ cubic miles for the volume of the previous nebula after the separation of the ring for the system of the earth. for finding the dimensions of the ring we have , , miles for the mean diameter of the earth's orbit, which makes the circumference , , miles in length, and dividing the volume of the ring for the system, which was found to be , , , ^ cubic miles, by this length, the area of its cross section comes to be , , , square miles, which divided by the breadth of , , miles--that is one-half of the difference between the diameters of the earth and venus nebulæ, respectively , , and , , miles--makes the thickness of the ring to have been miles. but the inner will presently be seen to have been · times more dense than the outer edge when its separation was completed, so that the average thickness would be miles. venus nebula. as the volume of the nebula was , , , , , ^ cubic miles after the separation of the ring for the system of the earth, we have to condense it into the volume of the venus nebula, which at , , miles in diameter would be , , , , , ^ cubic miles. then dividing the larger of these two volumes by the smaller, we find that the density of the venus nebula had been increased to · times what that of the earth nebula was. but we found the density of that nebula to have been , , times less than that of water, dividing which by · makes the venus nebula to have been , , times less dense than water. dividing this again by · we find it to have been , times less dense than air, which would make its absolute temperature to have been · °, which corresponds to - · °. from the venus nebula of , , miles in diameter, volume , , , , , , ^{ } cubic miles, and density , , times less than that of water, we have now to deduct the volume of her ring, which by table ii. is , , , , cubic miles at the density of water. multiplying this volume by , , we find the volume of the ring to have been , , , , ^{ } cubic miles at the same density as the nebula, and subtracting this amount from , , , , , , ^{ } we get , , , , , ^{ } cubic miles for the volume to be condensed into the nebula following. to find the dimensions of the ring we have , , miles for the diameter of the orbit of venus, which makes its circumference , , miles in length. then dividing the volume of the ring, i.e. , , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , square miles, which, divided by the breadth of , , miles--that is one-half of the difference between the diameters of the venus and mercurian nebulæ, respectively , , and , , miles--makes the thickness of the ring to have been miles. but the inner edge having become, in the process of separation, · times more dense than the outer one (see below) the average thickness would be reduced to miles. mercurian nebula. as the volume of the nebula was , , , , , , ^{ } cubic miles after the separation of the ring for venus, we have to condense it into the volume of the mercurian nebula, which at , , miles in diameter would be , , , , , ^{ } cubic miles. then, dividing the larger of these two volumes by the smaller, we find that the density of the mercurian nebula must have been increased · fold over that of its predecessor. but we find the density of the venus nebula to have been , , times less than water, dividing which by · makes the mercurian nebula to have been , , times less dense than water. dividing again this density by · we find it to have been times less than air, and ° divided by this air density gives · ° as its absolute temperature, which corresponds to - · °. from the mercurian nebula , , miles in diameter, volume of , , , , , ^{ } cubic miles, and density of , , times less than water, we have to deduct the volume of his ring, which by table ii. is , , , cubic miles at density of water. multiplying this volume by , , makes the ring to have been , , , ^{ } cubic miles in volume at the density of the nebula, and subtracting this amount from , , , , , ^{ }, we get , , , , , ^{ } cubic miles for the volume to be condensed into the nebula following. to find the dimensions of the ring we have , , miles for the mean diameter of the orbit of mercury, which makes its circumference , , miles in length. then dividing the volume of his ring, i.e. , , , ^{ } cubic miles, as above, by this length, the area of its cross-section comes to be , , square miles. here we have to determine the breadth of the ring in a new way, that is empirically. seeing that the breadth of the ring for the earth's system was , , and of that for venus , , miles, we shall assume , , miles for the breadth of the ring for mercury. this will make the residuary, now the solar nebula, to have been , , miles in radius and , , miles in diameter. returning now to the area of the cross-section of the ring, that is, , , square miles, and dividing it by the assumed breadth , , miles, makes the thickness of the ring to have been miles. but, as before, its inner edge having become · times more dense than the outer one during the process of separation (see below) the average thickness must have been only miles. solar nebula. lastly, as the volume of the nebula was , , , , , ^{ } cubic miles after the separation of the ring for mercury, we have to condense it into the volume of the solar nebula, which at , , miles in diameter would be , , , , , ^ cubic miles. then dividing the first of these two volumes by the second, we find that its density must have been increased · fold. but we found that the density of the mercurian nebula was , , times less than that of water, dividing which by · makes the solar nebula to have been , times less dense than water. dividing this in turn by · shows it to have been times less dense than air, and, still further, dividing ° by this air density makes its absolute temperature to have been · ° equal to - · °. we might conclude our analysis here, but it will be more convenient to carry our calculations a few steps further, to save the additional trouble that might be occasioned by having to return to them later on. first we shall condense the solar nebula to , times less dense than water, and therefore times less dense than air, which we may note will increase its density · times. this supposed to be done, its diameter would be , , miles, its volume , , , ^{ } cubic miles, and its density / th of an atmosphere--about _one-ninth_ inch of mercury--which would, in consequence, make its absolute mean heat equal to _one degree_ of the ordinary centigrade scale, or, in another way of expressing it, equal to - °. second. let us condense this same nebula to · times less dense than water, and consequently to the density of air at atmospheric pressure, then its diameter will be , , miles, volume , , , ^ cubic miles, and the mean heat °, or the heat of freezing water--which by some unexplained process of thought has hitherto been considered to be ° of absolute _temperature_. third. by again condensing the solar nebula to the density of water, corresponding to a pressure of more than atmospheres, its diameter becomes , miles, its volume , ^{ } cubic miles, and mean heat °, including the ° acquired in condensing it to the pressure of atmosphere, as is plainly shown in table iii. before going any further we must enter into a digression to examine into the process of thought by which the absolute zero of heat has come to be called the absolute zero of temperature, and absolute temperature to be so many degrees of negative--less than ° or nothing--heat counted from the lower or wrong end, to be called positive absolute temperature; thus making heat and temperature appear to be two very different things, without giving any explanation of what is the difference between them. science has, as it were, gone down a stair of steps carrying along with it the laws of gases, and has found, most legitimately, with their assistance the total absence of even negative heat at the bottom of it; and, leaving these laws there, has jumped up to the top of the stair, thinking that it carried along with it ° of absolute heat, which it now calls temperature; instead of bringing the said laws up with it and verifying, if not at every step at least at intervals, how much it brought up with it of what it had taken down. had it done so it would have found that at the top of the stair it had got what was equal to only ° of positive heat as measured by the centigrade scale, as has been shown above, which might be called temperature, but that would not mend matters. science seems to have forgotten, for the time being at least, all about the laws of gases; it had got something which it thought would enable it to mount much higher, and was satisfied. it will not be difficult to do away with the confusion of thought that is thus shown to have occurred. the laws of gases are founded upon the fact that in gases there is a necessary interdependence between heat and pressure, and the starting points adopted by science for calculating this interdependence in them are ° of heat and atmosphere of pressure at ° of heat. obeying these laws, we have argued, from the beginning of our operations, that heat requires something to hold it in, and that the nebula from which the solar system was formed--if it was so formed--could only contain heat in proportion to its density; that is being a gas, or vapour in the form of a gas, it could not contain, i.e. hold in it, more than ° of positive heat when its density was equal to the pressure belonging to atmosphere of a gas; all as shown in the most irrefragable manner in this chapter and in the accompanying table iii. a gas can be easily compressed in a close vessel to a pressure of atmospheres, which would enable it to hold ° of heat due to that compression; in fact, were it compressed to that degree by a piston in a cylinder, without any loss of heat, it would be raised to that heat by that act alone, but that would raise it to only ° instead of ° of what is called absolute temperature according to present usage; because as a gas it could not hold any more heat at that pressure. it is, therefore, evident that this _usage_ has not been derived from the laws of gases. neither has it been derived from the other two states of liquid and solid to which all gases can be reduced, as can be very easily demonstrated. to cool steam at atmospheric pressure from its gaseous to its liquid state ° of heat of one kind and another--as measured by the centigrade thermometer--have to be abstracted from it, which leaves the liquid at its boiling point of °--a quantity that has been arbitrarily adopted to mark the difference between the freezing and boiling points of this liquid. in order, after this, to reduce the liquid, now water, to the freezing, or what is called ° of heat, these degrees of heat have to be extracted from it, which is not very difficult to do because the heat put into it arbitrarily can be extracted from it; but if it is now wanted to change the steam from its liquid to its solid state, the work, or operation assumes a very different character, because heat cannot be extracted from a substance which contains none at all. it is well known that ° of heat are required to change one pound of ice at ° into a pound of water also at ° of heat; but it is equally well known that ° of heat cannot be taken out of the pound of water which has none in it; how then, is the water to be changed into ice? even in cooling water to ° it has to be put into a bath of some kind, either of cold water or some cold mixture of other substances at least as cold; because, otherwise, extraneous heat from any source might find its way into it, and prevent it from cooling down to zero of heat. in the same manner, to change the water into its solid state of ice it has to be put into a similar bath, not to extract heat from it, because it has not any to extract, but to prevent extraneous heat from getting into it. this being the case, it is evident that if water is put into a bath at what is called - ° of heat, or even a fraction of that amount, it will be converted into ice though very gradually, by keeping extraneous heat from getting to it to sustain the collisions, or vibrations, of its constituent atoms necessary to maintain it in its liquid state. all for the very same reason why a stone, a piece of metal, or of anything assumes the same degree of heat, or absence of heat, as the medium by which it is surrounded; be it derived from sun-heat, earth-heat, or heat produced chemically or mechanically, and is not cooled down to a lower degree than the surrounding bath, be it what it may. the heat required to change a solid into a liquid is called _latent heat_, which in the case of ice and water may be a fraction of - ° or - °, or _minus_ almost anything according to the time it is necessary for it to act; so that no quantity of what is called absolute temperature can be ascribed to ice without the element time being involved in it. the absolute temperature of water and ice, just changing from freezing to frozen, might be counted as the same, seeing that a fraction of a degree of heat may make all the difference between them; but no fixed absolute temperature can be applied to ice, as it, in conjunction with all solid bodies, may have any degree of absolute temperature between its melting point and the absolute zero of heat, as far as is at present known. the same, of course, must be the case with any gas or vapour, or nebulous matter changed into its liquid and then solid state; and this fact enables us to go a little further. we have seen that what, according to present usage, is called the absolute temperature of solid hydrogen may be anything between - ° and - ° of heat, that is, between the absolute temperature of ° and °, which, of course, is no measure at all; and, therefore, absolute temperature can only be looked upon as a conventional term, which, when added to positive centigrade, or other, heat, conveys no clear idea to the mind, as it must always be mixed up with the concomitant idea of latent heat and its time of action. this leads us to think of what remains in the vessel, in which pure hydrogen has been changed into its liquid and then solid state, after these operations have been performed; and our first conclusion comes to be that there can be nothing in it but a small piece of solid hydrogen; but from the limited accounts we have seen of these operations, there does appear to be something remaining, because it seems that by it the degree of negative heat in the vessel can be measured. what that remaining something may be can hardly be anything but a matter of conjecture. the first and most probable idea that occurs is that it may be some lighter gas mixed with the pure (?) hydrogen that was put into the vessel; the next is that it may be the vapour of solid hydrogen; and the last refuge for speculation is that it may be radiant matter, whatever that may turn out to be. at one time it was supposed to be impurities mixed with the gases operated upon, which in the case of common air, were found to be removed to a certain extent by means of absorbents; but the numerous components of common air discovered since that time, have gone far to throw light upon that supposition, and we are thus led to think of what a true gas really is. but we are not yet prepared to follow up this thought. this is not an inappropriate place to say that when we adopted the centigrade scale for our work, we thought that a special thermometer, decimal throughout and consequently more handy, might be arranged for science alone, leaving every man the free use of whatever scale he liked best; but our experience acquired in this chapter put an end to that thought, and has left us totally unable to see how any decimal scale can be contrived, which will start from absolute zero of heat and will admit of any combination with any existing scale, or will assist humanity in any of its operations in connection with heat and temperature, whichever science may choose to call it. we therefore see that no known thermometer scale is superior to another, and end where we began by saying that the centigrade is the fashionable one at the present time. it is decimal as far as boiling water and resulting steam are concerned, but all the world is not boiling water; even steam has to be complicated with latent heat. table iii.--abstract of measurements, etc., resulting from the calculations made in chapter v. -------------+--------------+-------------------+---------------+----- | | | | | | volume of the | |incr. nebulæ | | mass of each | times less | of ---- |explanations. | separate system at| dense than |dens- diameter | | density of water | water. | ity in miles. | | in cubic miles. | | in | | | |times -------------+--------------+-------------------+---------------+----- original or | | | | neptunian | | | | , , , | volume of | | , , , | | neptune's | | | | ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+----- uranian |condensed from| | | , , , | neptunian | | | . | nebula | | | | | | | | volume of | | | | uranus' ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ saturnian |condensed from| | | , , , | uranian | | | . | nebula | | | | | | | | volume of | | | |saturn's ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ jovian |condensed from| | | , , , | saturnian | | | . | nebula | | | | | | | | volume of | | | |jupiter's ring| , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ asteroidal |condensed from| | | , , | jovian nebula| | | . | | | | | volume of | | | | asteroidal | | | | ring | , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ martian |condensed from| | | , , | asteroid | | | . | nebula | | | | | | | | volume of | | | | martian ring | , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ earth |condensed from| | | , , |martian nebula| | | . | | | | | volume of | | | | earth ring | , , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ venus |condensed from| | | , , | earth nebula | | | . | | | | | volume of | | | | venus ring | , , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ mercurian |condensed from| | | , , | venus nebula | | | . | | | | | volume of | | | |mercurian ring| , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ solar |condensed from| | | , , | mercurian | | | | nebula | | , | . | | | | |volume @ / | | | , , | of atm. | | , | . | | | | |vol. @ density| | | , , | of atm. | | | . | | | | |vol. @ density| | | , | of water | | | . -------------+--------------+-------------------+---------------+------ -------+---------------------------------------+-----------+----------- | | | | | | | | | | volumes at densities | times | absolute | of respective nebulæ in | less dense| tempera- | cubic miles. | than air. | ture. | | | | | | (degrees.) | | | | | | -------+---------------------------------------+-----------+----------- | | | neptun-| , , , , , , , , , | , , | · ian | | | | , , , , , , , , | .. | .. -------+---------------------------------------| | | , , , , , , , , , | | uranian| | | | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | saturn-| | | ian | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | jovian| | | | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | aster- | | | oidal | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | martian| | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | earth | | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | venus | | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | mercur-| | | ian | , , , , , , , | | | | | | , , , , , | , | · -------+---------------------------------------| | | , , , , , , , | | solar | | | | , , , , , , , | | · | | | | , , , , , , , | | · | | | | , , , , , , | | · -------+-+---------------------------------+---+-----------+------+--- | at density of water. | at air density. | +---------------------------------+---------------+------+ | dimensions of rings. |space to grain | | | | of matter. | | +--------------+------------------+-------+-------+ i | | | | aver- | | side | n | | | thick- | age | | of | c | | breadth | ness |thick- | cubic | cube | h | | in miles. |in miles.| ness | feet. | in | e | | | |in miles| | feet. | s | ---------+--------------+---------+--------+-------+-------+------| | | | | | | | neptunian| ... | ... | ... | , | · | | | | | | | | | | | | | | | | | , , , | , | , | | | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | uranian | | | | | | | | | | | | | | | | | | | | | | , , | , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | saturnian| | | | | | | | | | | | | | | | | | | | | | , , | , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | jovian | | | | | | | | | | | | | | | | | | | | | | , , | , , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | aster- | | | | | | | oidal | | | | | | | | | | | | | | | , , | | | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | martian | | | | | | | | | | | | | | | | | | | | | | , , | | | | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | earth | | | | | | | | | | | | | | | | | | | | | | , , | | | | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | venus | | | | | | | | | | | | | | | | | | | | | | , , | | | · | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | mercurian| | | | | | | | | | | | | | | | | | | | | | , , | | | · | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | solar | | | | | | | | ... | ... | ... | · | · | · | | | | | | | | | ... | ... | ... | · | · | · | | | | | | | | | ... | ... | ... | · | · | · | ---------+--------------+---------+--------+-------+-------+------+ returning now to page , we see that the volume of the sun alone was considered to be , ^{ } cubic miles, which corresponds to a diameter of , miles. comparing this with the volume , ^{ } cubic miles, see page , left after all the members of the solar system have been separated from the original nebula, we find that there is a remainder of , , , , cubic miles _less_ than we ought to have. but it will be remembered that we added only / th part to the mass of the sun for the mass of the whole solar system, whereas it will be seen, by referring to table ii., that we ought to have added / · th part. had we done so the sphere containing the whole solar system at the density of water would have been , · miles in diameter with volume of , , ^{ } cubic miles, which would have added , , , , cubic miles to the volume we started with, and would have left us with , , , , cubic miles _more_ than we ought to have had. besides, for the sake of round numbers, we made the diameter of the nebula containing the whole solar system, at the density of water, to be , instead of , · miles, and thereby really added more to the original volume than we should have; so that the defects in accuracy at the beginning of our work partially counterbalanced each other, which accounts so far for the difference noted at the end not being much more than half of what it should have been. taking all this into consideration, and the really insignificant magnitudes of the differences that would result from the corrections that could be made, we have not thought it necessary to reform the whole of our calculations. besides, the data we have been working upon are not so absolutely exact as to insure us that we should get nearer to the truth by making the revision. the whole error would be much more than obliterated were we to apply · instead of · for the mean density of the earth to the debit side of the sun's account. to simply describe arithmetical operations conveys no really satisfactory meaning to the mind; of working them out in full there is no end; and to partially represent them as we have done in these pages, although showing how the results are arrived at, still leaves them so mixed up together that it is difficult to compare them with each other, and to note the sequences from the beginning to the end of the whole operation. for these reasons we have compiled table iii., where the whole of the principal and most important data, and results from them, may be followed out and examined. we may now say that we have taken our nebula to pieces, with the exception of the parts belonging to the satellites of those planets which have them; which would only be a tiresome repetition of what we have done for each principal member of the system, provided we had the necessary data, which we have not; and have thus acquired a certain amount of knowledge of the primitive conditions of each one of them. but we have still to examine into and draw conclusions from what we have seen and learned during the operation; which in some points, differ very much from our notions, formed from what we had previously read on the subject. chapter vi. page analysis continued. excessive heat of nebula involved condensation only at the surface. proof that this was laplace's idea. noteworthy that some astronomers still believe in excessive heat. interdependence of temperature and pressure in gases and vapours. collisions of atoms the source of heat. conditions on which a nebula can be incandescent. sir robert ball. no proper explanation yet given of incandescent or glowing gas. how matter was thrown off, or abandoned by the jovian nebula. division into rings of matter thrown off determined during contraction. how direct rotary motion was determined by friction and collisions of particles. saturn's rings going through the same process. left to show process. form gradually assumed by nebulæ. cause of saturn's square-shouldered appearance. a lens-shaped nebula could not be formed by surface condensation. retrograde rotary motion of neptune and uranus, and revolution of their satellites recognised by laplace as possible. satellites of mars. rapid revolution of inner one may be accounted for. laplace's proportion of millions not reduced but enormously increased by discoveries of this century. analysis of the nebular hypothesis--_continued_. when laplace elaborated his hypothesis, heat was considered to be an imponderable material substance, and continued to be thought of as such--though perhaps not altogether believed to be so--for somewhere about half a century afterwards; so that it cannot be wondered at that he thought the nebula could have been endowed with excessive heat, more especially as it was looked upon as imponderable, and could in no way have any effect on the mass of the nebula. he only accepted the idea that was common to almost all astronomers of his time, that nebulæ were masses of cosmic matter of extreme tenuity but self-luminous, and consequently possessed of intense heat; they saw the sun gave light and felt its heat, and very naturally thought the nebula must be hot also. without this idea he could not have formed the hypothesis at all, because he could not have conceived that the condensation of the nebula could only take place at its surface, or, as he terms it, "in the atmosphere of the sun," as most assuredly would be the case with an excessively hot body. and in order that there may be no doubt about this being his idea, we quote his own words as guaranteed by m. faye in "l'origine du monde": "la considération des mouvements planétaires nous conduit donc à penser qu'en vertu d'une chaleur excessive l'atmosphère du soleil s'est primitivement étendu au delà des orbes de toutes les planètes, et qu'elle s'est reserrée successivement jusqu'à ses limites actuelles." and again: "mais comment l'atmosphère solaire a-t-elle déterminé les mouvements de rotation et de révolution des planètes et des satellites? si ces corps avaient pénétré profondément dans cette atmosphère, sa résistance les aurait fait tomber sur le soleil. on peut donc conjecturer que les planètes ont été formées à ses limites successives par la condensation des zones de vapeurs qu'elle à dû, en se refroidissant, abandonner dans le plan de son équateur." proceeding on these ideas laplace was quite in order and logical in conceiving that successive rings could be abandoned by the hot nebula, through the centrifugal force of rotation, for the formation of planets, more or less just in the way we have separated them. having obtained his end quite legitimately, as he thought, in this way, he had no occasion to look any deeper into the affair, and consequently was not under the necessity of taking any thought of what the interior construction of the nebula might be, any more than so many others have not done since his day. that he should have conceived the nebula to have been endowed with intense heat was, as we have already said, a natural consequence of the mistaken notions of the nature of heat at that period; but that so many astronomers should, up to the present day, think that the nebula must have been intensely hot, even to the degree required to dissociate the meteorites of which they conceive it to have consisted, seems to us to be almost inconceivable. we believe we have shown abundantly plainly, that there could have been almost no heat in the primitive nebula, because there was hardly any cosmic matter to hold it in. we have given as proof of this the laws of gases recognised and accepted by every scientist, according to which a gas cannot contain a stated amount of heat except it be at a pressure corresponding to that temperature, that is, unless it is subjected to conditions foreign to its natural state. therefore we must either persist in maintaining that there was almost no heat in the original nebula, or we must throw the laws of gases to the winds, for they all depend one upon another. there may be nebulæ possessed of very high temperature, that of incandescence for example, but certainly the nebula out of which the solar system was made, could not have contained more heat than what we have shown it had at the various stages through which we have carried it. if there be nebulæ at the temperature of incandescence, they must be possessed of densities, or pressures, corresponding to that temperature. a few pages back we have spoken of the impossibility of two grains of matter feet apart, raising, by mutual collisions, their temperature and that of the space occupied by each to the temperature of incandescence, and if we now substitute for them meteorites of a pound weight each, the space occupied by each of them will be a cube of feet to the side, which does not help us in any way to believe that the spaces occupied by them could be heated up by their collisions, so as to shine with the temperature of incandescence. so we get no help from meteorites. some people evidently seem to think that nebulæ can be incandescent and give the spectrum of incandescent gas, without their density or pressure being increased to the corresponding degree. sir robert ball seems to be one of them, though at the same time he appears to be not altogether sure of it. when discussing the self-luminosity of the nebula in orion, in his "story of the heavens," ed. , p. , he says: "we have, fortunately, one or two very interesting observations on this point. on a particularly fine night, when the speculum of the great six-foot telescope of parsonstown was in its finest order, the skilled eye of the late earl of rosse and of his assistant, mr. stoney, detected in the densest part of the nebula myriads of minute stars, which had never before been recognised by human eye. unquestionably the commingled rays of these stars contribute not a little to the brilliancy of the nebula, but there still remains the question as to whether the entire luminosity of the great nebula can be explained, or whether the light thereof may not partly arise from some other source. the question is one which must necessarily be forced on the attention of any observer who has ever enjoyed the privilege of viewing the great nebula through a telescope of power really adequate to render justice to its beauty. it seems impossible to believe that the bluish light of such delicately graduated shades has really arisen merely from stellar points. the object is so soft and so continuous--might we not almost say ghost-like?--that it is impossible not to believe that we are really looking at some gaseous matter." here we see that his own belief about the matter is not very firm. he admits that the stars contribute not a little to the brilliancy of the nebula, and the most he can say in favour of its shining with its own light is, that it seems impossible to believe that the light has arisen merely from stellar points. he then goes on to show how the self-luminosity may be explained, as follows:-- "but here a difficulty may be suggested. the nebula is a luminous body, but ordinary gas is invisible. we do not see the gases which surround us and form the atmosphere in which we live. how, then, if the nebula consisted merely of gaseous matter, would we see it shining on the far distant heavens? a well-known experiment will at once explain this difficulty. we take a tube containing a very small quantity of some gas: for example hydrogen; this gas is usually invisible; no one could tell that there is any gas in the tube, or still less could the kind of gas be known; but pour a stream of electricity through the tube, and instantly the gas begins to glow with a violet light. what has the electricity done for us in this experiment? its sole effect has been to heat the gas. it is, indeed, merely a convenient means of heating the gas and making it glow. it is not the electricity which we see, it is rather the gas heated by the electricity. we infer, then, that if the gas be heated it becomes luminous. the gas does not burn in the ordinary sense of the word; no chemical change has taken place. the tube contains exactly the same amount of hydrogen after the experiment that it did before. it glows with the heat just as red-hot iron glows. if, then, we could believe that in the great nebula of orion there were vast volumes of rarefied gas in the same physical condition as the gas in the tube while the electricity was passing, then we should expect to find that this gas would actually glow." there is a great deal to be said about this explanation. we presume that a very small quantity of hydrogen gas means that it was considerably below atmospheric pressure. even so we admit that by introducing sufficient heat into the tube by means of electricity or otherwise, the gas could be raised to the temperature of incandescence, but its pressure would, at the same time, be increased to the corresponding force measured in atmospheres; and we also admit that when the gas was allowed to cool down to its original temperature, the same quantity of hydrogen would be found in the tube; but how about the tube? when the gas came to be at the temperature of incandescence the tube would be the same, or very soon raised to it, and being made of glass would be sufficiently plastic to be distorted, or even burst by the pressure within, probably even before the gas reached the temperature of incandescence. we must not forget that the first appearance of incandescence begins with red heat whose temperature is not far from ° in daylight, and that white heat rises to above °. if the experiment was made in an almost capillary tube, sufficiently thick to prevent accidents, then it might appear to prove a foregone conclusion, but nothing else; it might keep the idea of pressure out of sight, but it could not prove that the gas inside was in a rarefied state when incandescent. that the gas glowed the same as a red-hot bar of iron has not been shown. the gas had to be shut up in a tube to make it glow, but the bar of iron could glow outside of the tube. could a streak of hydrogen be put into a furnace along with a bar of iron and heated to incandescence by its side, there might be some fair comparison between them, as long as they were in the furnace together, but the moment they were taken out the glow would disappear from the gas, whereas the iron would glow for some time. on the other hand we might _say_ that a stream of incandescent gas might be made to heat a bar of iron in an oven to its own temperature, but the moment the stream of gas and the iron bar were removed from the oven, the former would disappear at once and the latter would continue to glow, simply because it was dense enough to contain a very considerable supply of heat compared to what the gas could, or rather, because the pressure of the gas, even did it correspond to the temperature, would disappear at once and the heat with it. so it is not always safe to _say_ things. but it is quite safe to say that no gas--or substance such as we are accustomed to look upon as gas--can abide in a state of incandescence, and merely glow, unless its pressure, or density, corresponds to the temperature of incandescence; which for red heat (in the dark) would be ° = · atmospheres, and for white heat at ° = · atmospheres, above absolute zero of pressure in both cases. and also, that if the self-luminosity of a nebula arises from incandescent gas, the pressure in the gas of that nebula must be somewhere between and atmospheres above absolute zero of pressure. now we have shown, at page , that the density and pressure in the solar nebula, at the stage there specified, could not have been more than the millionth part of those of our atmosphere, and consequently were justified in asserting that in it there could be almost no heat whatever. we have just been speaking of a streak of gas and a bar of iron being heated in an oven to a red or white heat side by side, but everybody knows that this could not be done; but everybody has not thought of why it could not be done, otherwise sir robert ball would not have favoured us with his laboratory experiment of a streak, or remnant, of hydrogen in a glass tube. we know that a plate, or bar, of iron can be heated up to the temperature of incandescence in an oven, but it has never occurred to anyone, who has seen the thing done, that the gas, air, or vapour which heats them must be at a pressure corresponding to that temperature. multitudes of people may have thought of how the thing is done, but apparently very few have thought that it is not the gaseous part of the current of heated matter introduced into the oven, that heats it and the metal in it, but the solid part which is the distinctive and most important part of the constituents of the current. the solid part of the matter--let it be gas or any other element--is heated to incandescence in some furnace and carried along by the gaseous part--that is the _stuff_ that fills the empty spaces between the solid molecules--to give it out to the oven and iron. we are not sure that the gaseous part even glows. we see plainly enough that the walls of the oven glow, but with respect to the gas, or carrying agent, we are inclined to think that it rather dims the glow of the oven and iron than otherwise. in passing, we say it is not unreasonable to suppose that the solid matter which contained the heat till it was given out, consisted of the elements which were put into the furnace to raise the heat, and of those which were drawn in by the draught--in a word, the elements of combustion--but about the carrying constituent there is a great deal to be said after we know more about it. it seems to us from all this that the hydrogen gas in sir robert ball's tube was not made to glow by heating up to the temperature of incandescence, but somehow by the electricity passing through it, if it did pass. we, therefore, come to the conclusion that the light of nebulæ does not come from gas--or what we call gas--heated up to be incandescent merely to make it glow, and that it might be as cold as the light that comes from the aurora, or as that of a glow-worm. sir robert ball refers to stellar points seen through the nebula, and acknowledges that part of the glow may be due to them, which shows that the nebula must have been excessively tenuous; for we know how thin a cloud will hide sirius from us, and we think that nobody will assert that two grains of matter dispersed in , , cubic feet of space, as we have seen at page , would hide sirius from us. therefore, we must acknowledge that the glow of nebula in orion, observed by sir robert ball, was caused either by the stellar points, or by some other thing that most assuredly could not be gas heated to the temperature of incandescence, or in part from both. for we believe that the glowing of nebulæ, fluorescence, phosphorescence, will-o'-the-wisp, auroras, fire-flies, fire-on-the-wave, etc., etc., all, all proceed from the same cause. we may now proceed to say a few words about the separation of the rings for the planets, brought about by the rotation of the nebula on its axis, and the centrifugal force produced throughout it thereby. we have shown, at page , that a ring could not be detached from the nebula at once in one large annular mass, as it seems to have been the common notion was the mode of separation; and we shall now try to show with some detail what the process must have been, notwithstanding that it has been in a general way described by others; because, like everything else, there is something to be learnt from it. for this purpose we shall select what we have called the jovian nebula, because we can suppose, for the present, it must have been more nearly in the form of a sphere than either the original or any of the exterior nebulæ, which may not have been properly licked into shape, as it were; and also because we have found that the thickness and mass of the ring for his, jupiter's, system were vastly greater than those for any other one of the planets. we have made the jovian nebula to have been , , , miles in diameter, and the greatest thickness of the ring detatched from it to have been , , miles. now in a circle of that diameter, a chord of the length of that thickness would subtend an arc of very little more than minutes, one half of which we shall suppose to be measured on each side of the equatorial diameter of the nebula at right angles to the diameter; then, the middle ordinate of a chord of , , miles long, would be miles long. this length would be a very small fraction of the radius of the circle which would be , , miles long, but in a rotating sphere of the same dimension, we must acknowledge that the centrifugal force at the middle of the arc would be greater--however small the difference--than at its ends, and would sooner come to balance the force of gravitation; therefore we must admit that the process of separation would begin there by abandoning a thin layer of matter, convex on the outer side and in a measure concave on the inner side, for the reason just given, much the same as a layer that could be peeled off from the equator of an orange--the poles and equator of an orange are easily distinguished. as the velocity of rotation increased another layer would be abandoned following the first, so far curved on both sides, i.e. convex and concave, and the same process would continue on and on, according as the centrifugal force continued to balance that of gravitation, till the whole of the matter for all the attendants of the sun was abandoned; so that in the process itself no such division of rings as we have been following could have taken place, but one continuous sheet, as it were, would be formed from first to last. whether the thickness of the ring for jupiter's system, or any other system or planet, was limited to the length of the chord we have been dealing with, or came to be many times greater or even less, makes no difference on our explanation. after being abandoned in a sheet, as we have shown it would be, the centrifugal force they had acquired would, for a time at least, keep the particles of the sheet near the radial positions they then occupied, and their mutual attraction would go on diminishing its thickness, till finally the radial attractions among the particles divided the sheet into entirely separate rings after the manner of those of saturn; which would in due course break up and form themselves into the smaller nebulæ from which the planets were supposed to have been made. m. faye has made it a great point against the nebula hypothesis that when these rings broke up, the rotary motions of the planets resulting from them would be retrograde, because the outer parts of them would be travelling at a slower rate than the inner ones, and has taken the trouble to construct a diagram to show how this would be the case; but he himself has told us, in "l'origine du monde," that laplace had duly considered this point, and had shown how the friction of the particles of the flat rings among themselves would, through course of time, retard and accelerate each other, so that a ring would come to revolve as if it were one solid piece, and consequently that the outer edge of the ring would come to be travelling faster than the inner one, which according to his (m. faye's) own showing would produce, on breaking up, a planet with direct motion of rotation. laplace's words, as cited by him, are:--"le frottement mutuel des molécules de chaque anneau a dû accélérer les unes et retarder les autres jusqu'à ce qu'elles aient acquis une même mouvement angulaire. ainsi les vitesses réelles des molécules éloignées du centre de l'astre out été plus grandes. la cause suivante a dû contribuer encore à cette différence de vitesse: les molécules les plus distantes du soleil et qui, par les effets du refroidissement et de la condensation, s'en sont rapprochées pour former la partie supérieure de l'anneau out toujours décrit les aires proportionnelles aux temps, puisque la force centrale dont elles étaient animées a été constamment dirigée vers cet astre; or cette constance des airs exige un accroissement de vitesse à mesure qu'elles s'en sont rapprochées. on voit que la même cause a dû diminuer la vitesse des molécules qui se sont élevées vers l'anneau pour former sa partie inférieure." in his method of bringing all the molecules of matter in a ring, to revolve round the centre as if they formed one sole piece, laplace does not appeal to any accommodating force among them except friction, while he might have called in that of the collisions of the molecules amongst themselves. it is not to be supposed that each molecule would remain fixed in the position it occupied when separated from the nebulæ, and only went on rubbing against--and creating friction with--its neighbours, and only creeping closer to the centre or farther from it, as it was acted upon by the attraction of the other parts of the ring. the molecules would be rushing against each other in all directions, in spite of, although in the main obedient to, the law of attraction; and we could conceive the possibility of molecules gradually working their way from the extreme outer edge to the extreme inner edge of a ring, or _vice versâ_, which would be a much more effectual means of bringing about one period of revolution throughout the whole ring, than the simple force of rubbing against each other. when physicists get a gas shut up in a close vessel, they grant to its molecules the power of committing exactly the same kind of freaks; and a planetary ring is, to all intents and purposes, a closed vessel to our molecules; because they have been placed in it by the laws of attraction and centrifugal force, and there is no other force acting upon them sufficiently powerful to liberate them from it. therefore there is no reason why a molecule in a ring should be always wedged up in one place, especially after we have shown that each molecule of matter, in any of the rings we have been dealing with, must have had a much greater free path to move about in, than a molecule of gas shut up in any of the vessels used by physicists. we have no reason to look upon the rings of saturn otherwise than as in process of being converted into one or more satellites, most probably more than one; because if the matter they are composed of has been separated from the planet in the form of a sheet, the same as we have seen must have been the case with the matter separated from the original nebula for the planets, the sheet has been already divided into at least three distinct parts, and surely that cannot have been done without some object. if these rings have been left, as has been said, in order to show us how the solar system has been formed, that does not authorise us to conclude that they will always remain in the form they have. there is no reason why the lesson should not be carried out to the very end, through the breaking up of the rings, formation of spherical nebiculæ, and finally satellites. it would be rash to assert that the matter of which any one of them is composed--be it atoms, molecules, meteorites, or brickbats--cannot, through friction and collisions of its particles among themselves, come to revolve around saturn as if it were one solid piece. but should anyone do so, and adopt m. faye's condemnation of laplace's mode of forming rings, he must confess that when saturn's rings are converted into satellites, their rotations must be retrograde; and it might be, for him, an interesting inquiry to find out whether the rotations of the existing satellites are direct or retrograde. astronomers have learnt the lesson as far as it has gone, have noted and registered the state of affairs as it is at present, and their successors will no doubt do the same as changes succeed each other. the day may be inconceivably remote, but it will inevitably come for the rings to be changed into satellites, unless they are disposed of in some other way. it has been said that were the rings to break up, in consequence of their being in a state of unstable equilibrium, they would fall back upon the planet, but that would depend on circumstances. if the motion of their revolution were stopped altogether, they would certainly fall back upon the planet; but if it were not stopped then each molecule would retain its centrifugal force, and would revolve around the primary on its own account, just as, according to very general opinion, it does at present. we do not see why, or for what purpose, these rings could have been separated from saturn merely to fall back upon him again. it would be rather a strange way of giving a lesson if it were stopped, by a cataclysm of some kind, just when the most interesting part of it was in a fair way of being exhibited. such a proceeding would assuredly not suit the ideas of those who believe that the solar system has been self-formed by a simple process of evolution. during the whole process of separation of rings from the original nebula, the nebulous matter would be abandoned in what we may call the form of thin hoop-shaped rings, so that the equatorial region of the nebula would be flat--as we have shown at p. --and when the nebula came to be so much reduced that it could abandon no more matter through centrifugal force, its form would be, in some measure, like that of a rotating cylinder terminating at each end in a cap in the form of a segment of a sphere. when explaining the formation of planetary rings, we have seen that in the jovian nebula the length of the flat part would have come very soon to be nearly , , miles, and that it would increase rapidly. but, remembering that the flattening of the equatorial part must have begun on the original nebula, we see that the flat part must have increased vastly in length before it reached jupiter, and that by the time the residuary, or solar, nebula was reached--which we made to be only a little over , , miles in diameter--the cylindrical part of it would bear no small proportion to that diameter. taking this form of the nebula into consideration, and also the fact that the separation of matter from it by centrifugal force could not always be absolutely equal all around it, the swaying in its rotary motion produced by the all but inevitable inequality of mass, at the two ends of the cylindrical part, and at the sides of the segmental caps, may have been the cause of the differences in the inclinations of the orbits of the planets to the ecliptic; and especially of why the difference came to be so much greater in the case of mercury than in any of the others. in connection with this very reasonable conclusion as to the form of the nebula almost from the beginning, we may add that, when it ceased to throw off rings, it would be very much in the same condition as saturn is at the present day. therefore we may conclude with very great safety, that the present form of saturn is that of a cylinder with segments of spheres forming the ends; and in this manner can account for his square-shouldered appearance, which has puzzled more than one astronomer. the idea has been very general that in condensing and contracting, the nebula would gradually come to assume the form of a lens of a very pronounced character, from the circumference of which the rings would be abandoned one after the other; but when thoroughly looked into, it is difficult to see how this could be the case. in a sphere of cosmic matter contracting equally all round towards the centre through the force of attraction, it is more natural to suppose that the separation of matter from its equator through centrifugal force, would have a tendency to diminish the equatorial more rapidly than the polar diameter, as we have been trying to show above, more especially as the attraction of the matter in the rings as they were abandoned one after the other would, in a constantly increasing degree, assist the centrifugal force in facilitating the separation by drawing the matter outwards. matter falling in from the polar regions would afterwards require to have its motion turned off at right angles before it could be sent off by centrifugal force to the equator, an operation which would be more easily effected in the equatorial regions, where the gravitating motion had only to be retarded; and as very unequal amounts of density could not be created in the interior parts of such a sphere by gravitation, so as to cause pressure outwards, it is difficult to show how the polar diameter could be more rapidly reduced than the equatorial diameter, which was being continually shorn of its length. it may be said that all that we have been writing in the last few pages is absurd, because we have been proceeding on the supposition that the condensation of the nebula was effected at or near its surface. laplace procured this condition by piling up imponderable heat in his nebula, but he might have got it otherwise. given a nebula such as the one we are dealing with of , , , miles in diameter, where would condensation be most active? most undoubtedly where there was the greatest mass of matter. compare, then, the mass of , , miles in diameter at the surface with the mass of the same diameter at the centre, and we cannot hesitate for a moment in concluding that the most active condensation would not be very far from the surface. not only so, but the same would continue to be the case, at least until the last ring was abandoned. thus by working upon what may have appeared to be an absurd foundation, i.e. condensation at the surface due to the intense heat of the nebula, we have been able to acquire more correct ideas than we had before, of how the solar system was elaborated. but we shall have much more to say on the same subject hereafter. there has been a great outcry raised about the rotation of the planets neptune and uranus being retrograde, as is correctly concluded to be the case from the revolution of their satellites being retrograde, but we do not see that there has been any good reason for it. laplace, no doubt, concluded, wrongly, that the motions of all the bodies of the solar system--as known to him--were direct, and therefore used that conclusion in showing that there were milliards against in favour of his hypothesis being right; but at the same time it cannot be concluded that he thought that it would be destroyed by the motion of rotation of one or even several of the forty-three bodies turning out to be retrograde; because, when discussing the hypothesis of buffon, he states, most distinctly, that it is not necessary that the rotation of a planet should be in the same sense as that of its revolution, and that the earth might revolve from east to west, and at the same time the absolute movement of each of its molecules might be directed from west to east. his words as cited by m. faye in "l'origine du monde," at page , are: "a la verité, le mouvement absolu des molécules d'une planète doit être alors dirigé dans le sens du mouvement de son centre de gravité, mais il ne s'ensuit point que le mouvement de rotation de la planète soit dirigé dans le même sens; ainsi la terre pouvait tourner d'orient en occident, et cependant le mouvement absolu de chacune de ses molécules serait dirigé d'occident en orient, ce qui doit s'appliquer au mouvement de révolution des satellites, dont la direction, dans l'hypothèse dont il s'agit, n'est pas nécessairement la même que celle de la projection des planètes." he seems to say, "this would suit buffon's hypothesis, but i do not require it for mine." even were this not so, it would not be very difficult to account for the retrograde rotation of these two planets, but we are not yet prepared to show, in a convincing manner, how these motions were produced. we have to show first how the nebula itself was brought to the dimensions at which we took it up, and there is a great deal to be done before we can show that. should our belief in being able to explain how the retrograde rotations of uranus and neptune were brought about turn out to be unacceptable, we would not condemn the nebular hypothesis, because, as m. faye himself says, if we add the asteroids to laplace's we should have somewhere about bodies, all with direct motion, agreeing with the hypothesis, against that do not, that is about to instead of to , which was all laplace could claim. moreover, we have not been able to see that m. faye's objections to it are well founded, rather the contrary; nor can we agree with him when he says that when one point in a hypothesis is found to be erroneous it ought to be abandoned altogether, and something better sought for. is his something any better? all acquired knowledge has been built up from ideas collected from all sides, and from errors reformed. what would a grammarian say were we to return to him his grammar as useless, because we had found one exception to one of his rules against cases in which we had found it to be right? perhaps it would put him in mind of the name of a tree. and grammar is not the only case in which we say that the exception confirms the rule. in taking the nebula to pieces, we have taken no notice of the satellites of mars, not only because they are so small that they would have had no sensible effect on our calculations, but because we cannot conceive that they could have been abandoned by the planet, when in a nebulous state, in the same manner as the planetary rings are supposed to have been by the parent nebula; and we might simply refer to the dimensions, especially the thinness, we have found for the ring out of which mercury was formed, for proof of our assertion; but for more satisfactory corroboration, we will go a little deeper into the affair. let us take the diameters of mars and of the orbits of his satellites, as they are stated in text-books of astronomy; that is , , and , miles respectively, and suppose the diameters of what--in the method we have applied to the planets--we would call the deimos and phobos nebulæ to have been , and , miles also, respectively; then these two diameters would make the breadth of the ring for the formation of deimos to have been miles. with these data, if we go through a series of calculations with respect to this outer satellite, in all respects similar to those we have made for each of the rings of the planets, we shall find that the ring for deimos would have been only · _inches_ thick, without taking into account its condensation during the process of separation. this, of course, points out at once the impossibility of any such operation going on in nature. we can imagine the possibility of a ring of even millions of miles broad, and of very great tenuity, holding together provided it be hundreds of thousands of _miles_ thick, but to think of one , miles broad and less than _inches_ thick holding together is another affair altogether. with respect to phobos, it is only necessary to say that he revolves round mars in considerably less than one-third of the time that he ought to, and is therefore not a legitimate production of the nebular hypothesis any more than deimos can be. here, then, we have come upon two bodies, one of which has not been formed in the way, and the other has not the proper motion, prescribed in the hypothesis; but we do not think ourselves justified in declaring it to be worthy of condemnation on that account, seeing that we have found no other difficulty in working out the solar system from it. moreover, it is not impossible, nor do we think it at all improbable, that through the course of time astronomers may discover that phobos is a captured asteroid--perhaps deimos also--gradually working its way into final annexation. and who can tell how many of these erratic bodies jupiter and mars may have captured already? in the dark as it were, for they may have been too small to be noticed when they were being run in. neither of these two worthies has ever been very much celebrated in song or history for respect for his neighbour's property. jupiter is credited with sorting out the asteroids and arranging them in bands, and perhaps he has been human enough to exact a commission for his labour; and it might be more in his line, and certainly much more easy for mars, to take forcible possession of as many of them as came within his reach. chapter vii. page analysis continued. no contingent of heat could be imparted to any planet by the parent nebula. only one degree of heat added to the nebula from the beginning till it had contracted to the density of / th of an atmosphere. increase in temperature from ° to possible average of ° when condensed to , , miles in diameter. time when the sun could begin to act as sustainer of life and light anywhere. temperature of space. the ether devised as carrier of light, heat, etc. etc. what effect it might have on the nebula. first measure of its density, as far as we know. the estimate _too_ high. may be many times less. return to the solar nebula at , , miles in diameter. plausible reason for the position of neptune not conforming to bode's law. the ring being very wide had separated into two rings. bode's law reversed. ideas suggested by it. rates of acceleration of revolution from one planet to another. little possibility of there being a planet in the position assigned to vulcan. densities of planets compared. seem to point to differences in the mass of matter abandoned by the nebula at different periods. giving rise to the continuous sheet of matter separating into different masses. probably the rings had to arrive at a certain stage of density before contracting circumferentially. possible average temperature of the sun at the present day. central heat probably very much greater. churning of matter going on in the interior of the sun, caused by unequal rotation between the equator and the poles. coming back to the period when we reduced the residuary nebula to the density of our atmosphere with temperature of °, or freezing water, we can with confidence affirm that none of the rings abandoned by it for the formation of planets, could have carried with them any contingent of heat to help them in their formation--any beyond the temperature of space--for even if they did it would very soon be reduced to that. each one of them in condensing, breaking up, rejoining the broken fragments, converting itself into a minor nebula, and finally constituting itself as a planet, must have accumulated in the process its own heat requisite to convert it into a molten liquid globe--a stage of existence through which they are all, that is, the major planets, acknowledged to have passed, or have to pass. during that process its primitive annular form, and the multitude of fragments into which each one of them broke up, would present sufficient radiating surface, not only to dispose of all the heat it could have brought with it from the nebula, but a considerable part of the little it could create for itself while contracting and condensing. we may even go farther and assert that no one of them would have any necessity for being supplied with extraneous heat until it had, in a great measure, exhausted the stock it had produced for itself, or so far as to cool down from the molten liquid to the solid state, and to the stage when vegetable and animal life could exist upon its surface. we have no reason for supposing that an enormous supply of extraneous heat was crammed into each nebula, merely to be radiated into space before condensation could take place, and thus retard the execution of the work in hand. if there are astronomers or physicists who believe that the sun could not acquire by gravitation, all the heat he must have expended during geological time, they must look for it in some other source than that of useless and impossible cramming. hitherto we have said nothing of heat being radiated into space by the nebula during our operations, because there could be almost absolutely none to radiate from it at ° of temperature. no doubt there is a large range between this and the absolute zero of temperature which is - °; but we have seen, at page , that when the nebula was condensed from , , to times less dense than air, only _one degree_ was added to its temperature--that is, it was raised from - ° to - °--and that these - ° of absolute temperature were added to it in its condensation from being only times less dense than air to atmospheric pressure, when its temperature became ° of the ordinary centigrade scale. therefore the only period when there could be any measurable radiation of heat into space would be between the times when the diameter of the nebula was (see table iii.) between , , miles and , , miles. even when the end of this period came, the temperature, after a contraction of , , miles in diameter, would be only - ° raised to °--in other words - ° raised to °--and that would not furnish much positive heat--heat such as we are accustomed to deal with--to be radiated into space, whose temperature is without doubt somewhat warmer, so to speak, than - °. and let us repeat, and fix it in our memory, that this - ° was equal to only ° of positive heat. if we now suppose the nebula to be condensed to one-tenth of its volume, with consequent density of atmospheres, and corresponding diameter of about , , miles, its temperature would be ° of the ordinary centigrade scale--according to our mode of calculating hitherto--provided no heat had been radiated from it into space in the meantime. of course this could not be the case, but we have no means of calculating what the amount of radiation would be, and it will not make much difference on our operations to take no notice of it. however, it is here necessary to take into consideration that ° would be the average temperature of the nebula; consequently, if condensation was most active where the greatest mass was, which certainly could not be at the centre or even near it, there also heat would be produced most rapidly, from whence it would spread towards the centre and surface. from the centre it would have no outlet, and would accumulate there as condensation advanced; whereas from the surface it would be radiated into space, and would tend to decrease in amount, so that we may conclude that the surface must have been considerably colder than the centre. if to this we add the fact that, in order to get to the surface, heat would have to be conducted, or conveyed by currents; over from one to two millions of miles, it becomes all the more certain that the central heat would be very much greater than that of the surface. how much less it would be at the surface we cannot pretend to calculate, but we may suppose it to have been from one-fifth to one-third of the average, or rather, somewhere between ° and °, which we have taken, at page , to be the temperatures of red-heat and white-heat. and thus we come to find that the nebula, which was supposed to be endowed with excessive heat when it extended far beyond the orbit of neptune, could not have radiated either heat or light into space to much purpose, until it had been condensed into not much more than , , miles in diameter. this then we must acknowledge to be the earliest period at which the sun began to act as the life sustainer of his system; because, even were it to be found that there are other planets revolving within the orbit of mercury, which we do not think very probable, we have seen that he could have no light or heat with sufficient vivifying power to radiate to them, till his diameter was reduced to not far from what we have shown above. even then the sun would most likely be very much less brilliant than he is now, but the light may have been sufficient to promote vegetation on mars--or the earth, if it was sufficiently cooled down from its molten state--and not much heat would be required by him, as there would probably be a remnant of his own interior heat, still sensible at the surface, sufficient for vegetation at least. we have had occasion to refer several times to the temperature of space, and, though we cannot pretend to determine what it is, our operations enable us to show that it must be very much less than any estimate of it that has ever come under our notice. the nearest approach made to absolute zero by m. olzewski, in his experiments on the liquefaction of gases, as reported in the "scientific american" of june , , was - °, or so-called ° of absolute temperature,[d] which would correspond to a density of · of an atmosphere. this could not be the density of space, because it can be easily shown that our nebula, when at the same density, must have had a diameter of about , , miles, and we must admit that were a globe of this diameter rotating in a medium of its own density, the friction between the two would have been so great as to put a stop to the rotation before very long. we may even say that distinct rotation could never have been imparted to it. following the same reasoning, we must acknowledge that the density of space must be much lower than that of our original nebula, if that could be, and therefore we can assert with confidence that the temperature of space must be far below - °. [d] from the same source, date june , , we learn that the greatest cold probably ever reached was - · ° or · ° of so-called absolute temperature, but that will have very little effect on our calculations, and so it is not worth while altering them all to suit. here our operations put us in mind that we have said nothing yet about the ether, or what effect it might have on our nebula and the bodies formed out of it. we have not done so for the simple reason that, with one exception, it has never been taken into account in any scientific work that has come into our hands, except so far as its being called upon to perform the offices of a dog that has been taught to carry and fetch, and we have not known how to deal with it. but as we have come along, we have seen that it must have had something to do with the density, and consequent temperature, of all the bodies we have been dealing with, and that, if properly studied, it may enable us to account for some things that we have never seen, to our mind, properly explained. we know that it was devised, or conceived of--somewhere between thousands of years ago and the birth of modern astronomy--as a medium for carrying light, heat, and anything that was hard to move, through space, or to where it was wanted to be moved, by its vibrations or undulations, in the same way that sound is conveyed by wave motion, or vibration, through air, water, and a multitude of bodies; and we understand that some time during that long period it began to be looked upon as a material substance. we are told that it is supposed to pervade all bodies of all classes, but we think this idea must be taken in a limited sense, because, whether it is combined with electricity, as some suppose, or is only a carrier of electricity, a good conductor must have a larger supply of it than a bad one, and an absolute non-conductor, if there be such a substance, must contain none at all, always provided the ether is the conducting or carrying power. we are told also, that it is neither of the nature of a gas nor a liquid, but may be of the nature of a jelly, and of its nature we shall have more to say hereafter. it was natural that it should be conceived to be a material substance, because if light and heat were to be carried from one place to another by wave motion, as sound is by water and air, then the medium for carrying it must be of the same nature as air and water--or any other carrier of sound--that is, it must be a material substance and, in consequence, possessed of some density or specific gravity. the only place where we have seen any density assigned to it has been, in a series of articles on the "origin of motion," published in "engineering" of , where it is estimated to be / , , th[e] of the density of air. how this estimate was formed is explained in the number for december , , page , from which we make the following very long quotation, because we look upon it as of great importance. [e] years after this was written we have seen it stated that the density of the ether has been calculated from the energy with which light from the sun strikes the earth, and that to represent it there are twenty-seven cyphers after the decimal point before the figures begin. but as this gives something like one thousand quadrillionth part of the density of water, we refuse to accept or even think of it. "steel of the best quality in the form of fine wire has been known to bear a tensile strain represented by not less than tons per square inch before breaking, and even this cannot be said to be the limit to the tensile strength of steel, since the tenacity increases as the diameter of the wire is reduced. rejecting 'action at a distance,' therefore, these molecules of the wire must be controlled by some external agent, and therefore, the pressure of the external agent must _at least_ equal the static value of the strain. the pressure of the ether therefore cannot be less than tons per square inch. now, since it is a known fact that the strain required to separate molecules in 'chemical union' would be very much greater than in a mere case of 'cohesion,' it follows that the ether pressure must be greater than the above figure. if we suppose the strain required to separate the molecules of oxygen and hydrogen combined in the state of water (one of the most powerful cases of chemical union) to be only three times greater than in the case of the molecules of steel, then this would give tons per square inch as the effective ether pressure. it may be taken as certain that the strain required would be greater than this, as it has not been found possible by any ordinary mechanical means to separate molecules in chemical union. however, as it is only our object to fix a limiting value for the ether pressure, or a value that is less than the actual fact, we will therefore take in round numbers tons per square inch as the total ether pressure, having thus valid grounds for inferring that this estimate is within the facts as they actually exist. the existence of such a pressure as this might well be sufficient to strike one with astonishment and legitimately excite incredulity, if it were not kept in mind that this pressure is exercised against _molecules_ of matter, a perfect equilibrium of pressure existing, so that it may be deduced with certainty beforehand, that, however great this pressure might be, it could not make itself apparent to the senses. the air exercises a pressure of some tons on the human body without such pressure being detected, how much more cause, therefore, is there for the perfect concealment of the ether pressure, which is exercised against the molecules of matter themselves. this great pressure is the absolutely essential mechanical condition to enable the ether to control forcibly the molecules of matter in stable equilibrium, and to produce forcible molecular movements when the equilibrium of pressure is disturbed (as exemplified in the molecular movements of 'chemical action,' etc.). "it is generally admitted that the ether must have a very low density, one reason being the almost imperceptible resistance opposed by it to the passage of cosmical bodies (the planets, etc.) at high speed through its substance. the pressure of an aëriform body constituted according to the theory of joule and clausius, being less as its density is less, it will therefore be necessary to show that the ether can exert so great a pressure as the above, consistent with a very low density. from the known principles belonging to gases, the pressure exerted by an aëriform medium is as the _square_ of the velocity of its component particles, and as the density. we will, in the first place, consider what the density of the ether would be, if it only gave a pressure equal to that of the atmosphere ( lb. per square inch). from the above principles, therefore, it follows that for the ether to give a pressure equal to that of the atmosphere, the ether density will require to be as much less than that of the atmosphere, as the _square_ of the velocity of the other particles is greater than the square of the velocity of the air molecules. the velocity of the air molecules giving a measure of lb. per square inch is known to amount to feet per second. taking, therefore, the square of the velocity of the ether particles in feet per second, and the square of the velocity of the air molecules and dividing the one by the other, we have the number of times the ether density must be less than that of the atmosphere, in order for the ether to give a pressure of lb. per square inch, or we have ( , × )^{ }/ = , , , . this result shows therefore that the density of ether, if it only gave a pressure equal to that of the atmosphere, would be upwards of , , , times less than the density of the atmosphere. this result expresses such an infinitesimal amount of almost vanishing quantity, that the ether density might be well much greater than this. we will now, therefore, consider what the ether density would be to give a pressure of tons per square inch. pressure and density being proportional to each other, it follows that for the ether to give a pressure of tons per square inch, the ether density would require to be as much greater than the above value, as tons is greater than lb. multiplying, therefore, the above value for the density by this ratio, we have / , , , × ( × )/ = / , , ; or this shows that the density of the ether to give a pressure of tons per square inch would be only / , , th of the density of the atmosphere. this value representing a density less than that of the best gaseous _vacua_ is therefore quite consistent with the known fact of the extremely low density of the ether. it follows, therefore, as a mathematical certainty dependent on the recognised principles belonging to gaseous bodies, that the ether could exert a pressure of not less than tons per square inch consistent with such an extremely low density as to harmonize with observation." if the ether is possessed of a density equal to that shown above, then the density of our original nebula must have been greater than what we have shown it to be. the density we found for it was / , , th that of air, or · of an atmosphere, and / , , th is equal to · of an atmosphere; if then we add these two together we get · of an atmosphere as the density of our nebula. this comes to be very slightly greater than the density of the ether, and shows that the estimate in the foregoing quotation is too high; unless it is asserted that the ether can exert no frictional action at all, which, we believe, no one has ever done; while the absolute temperature of the nebula at the new density would be · °, which would be a very small addition indeed to the · °, we found for it at first. on the other hand, when the nebula was reduced to , , miles in diameter the density of the ether would have increased its density from · , which we showed it then to have, only to · of an atmosphere, which would make no appreciable difference on its temperature, and would be so immensely greater than the · of an atmosphere of the ether that it could hardly be supposed to have any effect in retarding the rotation of so much heavier a body. and should it be found that the density of the ether is / , / , or / less, or even a great deal more, than that shown in the above quotation, it would only have proportionately less effect on our nebula, in every sense, than what we have just shown. we may, therefore, conclude that the introduction of the element ether has not vitiated our operations in any way up till now, and we shall leave it until we have acquired more knowledge of its nature and effects. although we have already condensed our nebula to somewhere about , , miles in diameter, where we have shown it might begin to radiate light--radiation of heat may have begun when the diameter was ten times as great, or even before that--we propose to return to the period when it had just abandoned the ring for the formation of mercury and was , , miles in diameter, and became what we have called the solar nebula; because there is a good deal to be learned from a careful study of our operations up to that period, and of what must have taken place during further condensation up till the final establishment of the sun such as it is at the present time. when the planet neptune was discovered, bode's law fell into disrepute for a time, because the new planet was found to be much nearer to the sun than, according to it, it should have been. all the other planets occupied the places assigned to them within per cent. of the exact appointed distance from the sun, but neptune turned out to be · per cent. out of his exact place, and hence the discredit thrown upon the law. it was hard treatment for a servant that had helped so unmistakably--as we know to have been the case--to the discovery of the first four asteroids, which has afterwards been followed by the discovery of a whole host of them, and that had been pressed into the service for the discovery of the very planet which was the cause of its discredit--but such is the world. however, first offences against the law are generally looked upon with merciful eyes, and the series of titius seems to have been so far received into favour again that, some astronomers are said to have been looking out for another planet farther off than neptune, being convinced that there must be some reason why a law that has shown itself to be right in eight cases should be altogether wrong in the ninth. here, we think that the most likely explanation that can be given is, that the ring out of which neptune was formed divided itself, after breaking up, into two planets instead of one, and that this is the reason why, bode's law could not point out the true position of either of them. it is hard enough to believe that the ring out of which uranus was made--which we have seen may have been , , miles broad, and over , , , miles in extreme diameter--could have united its fragments, after breaking up, into one planet, and the difficulty of belief becomes greater the greater the diameter comes to be. we have, in our work, considered the breadth of neptune's ring to have been , , , miles, but then we limited the diameter of the nebula to , , , miles--we had to draw the line somewhere--whereas it may have been a thousand million miles greater, which would very greatly increase the probability of two planets, perhaps even more, having been formed out of the ring. if it has been so, the law could not apply to the case. a new act was required. besides, it is not a law, never has been, but only a register of facts; and we know that truths are often discovered from similar registers. it registers, and at the same time shows, that there is a nearly fixed inter-relation, even proportion, in the distances of the planets from the centre of the sun as far out as uranus; and were we to make a similar register, beginning at the (present) outside of the planetary system, and registering the number of revolutions, beginning with for neptune, rates of acceleration of revolution in number of days, and densities of the planets, we may draw from it some useful knowledge. but we shall first extend bode's law to embrace neptune, and show the discrepancies between the actual positions of the planets and those pointed out by the law. here we see that, with the exception of the first step from neptune to uranus which is only · , we have an average gradation of acceleration of · times, from one planet to another, from the outermost as far in as mars; and that had neptune had the period of revolution sought for by leverrier in his discovery of that planet, viz. · years, or , · days, the average rate of acceleration would have been · times, from planet to planet, as far in as mars. this, we think, is pretty strong evidence that one law of acceleration was in force from the beginning of the separation of rings from the nebula up to the time when the ring for mars was separated--the departure from it in the case of neptune, notwithstanding--and goes far to prove that part of the nebular hypothesis which implies that each of the planets is now revolving round the sun in the orbit, and with the velocity, belonging to the centre of gyration of the ring out of which it was formed. from mars to venus the law--the areolar law, of course--had changed to a variable decreasing law, as seen from the foregoing register, which then again changed into an increasing one, till at mercury the rate of acceleration rose again to · times from venus, or very nearly the same rate of increase that existed from uranus to mars. the causes of these changes may or may not be able to be accounted for--we shall have to return to them hereafter, in the cases of neptune, the earth and venus--but there is one thing of some importance that is deducible from the register, which we shall endeavour to make clear. bode's law extended. --------------+----------+----------+----------+-----------+----------- ---- | mercury. | venus. | earth. | mars. | asteroids. --------------+----------+----------+----------+-----------+----------- numbers in }| | | | | progression }| | | | | | | | | | add to each}| | | | | for distance }| | | | | from sun, }| | | | | earth's = }| | | | | | | | | | distance from}| | | | | sun according}| , , | , , | , , | , , | , , to the law, }| | | | | (miles) }| | | | | | | | | | actual | , , | , , | , , | , , | .. distance | | | | | | | | | | percentage }| | | | | of distance }| .. | . | .. | .. | .. beyond law }| | | | | | | | | | percentage }| | | | | of distance }| . | .. | .. | . | .. within law }| | | | | --------------+------------------------------------------------------- ---- | jupiter. | saturn. | uranus. | neptune. --------------+-----------+-----------+-------------+----------------- numbers in }| | | | progression }| | | | | | | | add to each}| | | | for distance }| | | | from sun, }| | | | earth's = }| | | | | | | | distance from}| | | | sun according}| , , | , , | , , , | , , , to the law, }| | | | (miles) }| | | | | | | | actual | , , | , , | , , , | , , , distance | | | | | | | | percentage of}| | | | distance }| . | .. | . | .. beyond law }| | | | | | | | percentage of}| | | | distance }| .. | . | .. | . within law }| | | | --------------+-----------+-----------+-------------+----------------- our register as specified above will be the following:-- -----------+--------+-------+---------------------+-----------------+ planet. | rev. of planet,|accel. of revolution,|density of planet| |(in solar days) |(neptune taken as ) | | -----------+----------------+---------------------+-----------------+ neptune. | , . | . | . | uranus. | , . | . | . | saturn. | , . | . | . | jupiter. | , . | . | . | asteroids.| , . | . | ... | mars. | . | . | . | earth. | . | . | . | venus. | . | . | . | mercury. | . | . | . | -----------+----------------+---------------------+-----------------+ a good deal has been written about planets or other bodies existing between mercury and the sun, especially about vulcan whose existence seemed to be so certain, that his distance from the sun and period of revolution were calculated to be about , , miles and days respectively. now, with what we have seen about the rate of acceleration of planets as their orbits approach the sun, we may endeavour to form some notion of where any within the orbit of mercury may be found. if we take the same rate of acceleration we have found between venus and mercury--that is · , which may be looked upon as almost the general rate for all the planets--we find that there might be a planet revolving round the sun in · days; but here we must stop, because, though we could make no objection to the existence of a planet with the period of revolution just shown, were we to take another equal step towards the centre of the nebula, the same acceleration of rotation would give us a planet, or ring for a planet, revolving round the sun in · days; not much more than one-half the average of his rotation round his axis at the present day, which would knock on the head most completely the theory that each planet was detached from the nebula at the time that it was rotating with the velocity of the planet's orbit, or we should have to conclude that the nebula had passed, by a long way, its power to abandon matter through centrifugal force. no one could suppose that a ring for a planet could be formed within the body of the nebula and abandoned, or thrown out, afterwards, because centrifugal force could not throw out the ring and at the same time retain the surrounding matter. turning our thoughts now to the supposed planet vulcan, which was calculated to revolve round the sun in about days, we have either to conclude that it was formed in the body of the nebula and come to the same breakdown of the nebular hypothesis, or we have to acknowledge that the sun is now rotating much more slowly on its axis than the nebula did at the time the ring for vulcan was abandoned. if we now direct our attention to the densities of the several planets, we shall find some suggestive matter in their study. a general look shows us at once that there are four periods of rise and fall in their densities. there is one rise and fall (referring to our register) from neptune to uranus and on to saturn; then another rise to jupiter and fall to, we suppose, the asteroids, because we are told that the quantity of matter in the region where the asteroids travel is less than in any other zone of the solar system, and the general density must in consequence have been less there than anywhere else; still another rise from the asteroids to the earth, and fall to venus; and then a final rise to mercury accompanied, without doubt, by a fall after the planet was abandoned, because the centrifugal force of the rotating nebula must have been decreasing, at the least, preparatory to its ceasing to have the power to throw off more matter. the first rise and fall would seem to indicate that there had been a much closer mutual relation in the births of neptune, uranus and saturn than is indicated in any way in the nebular hypothesis. we could imagine that at one time they formed one flat ring, which afterwards divided itself into three, following the same law as we see dividing the rings of saturn at the present day. with respect to jupiter, his enormous size is sufficient to entitle us to believe that his ring was separated from the nebula independently of any of the others, and to account for there having been the rise and fall in the density that we have noted between saturn and the asteroids. then the rise and fall from mars to venus, or further on towards mercury as it would be, may indicate one ring divided into three in the same manner as we have supposed for the three outer planets. and the final rise to mercury and subsequent fall to the sun or to the solar nebula might be either due to one operation or to complication with other unknown bodies that may be travelling between mercury and the sun. in support of the foregoing ideas, we may also refer to our having said on a previous occasion, that the whole of the matter separated from the nebula in the form of thin hoop-shaped rings, would condense into one continuous sheet, perhaps even up to the time when centrifugal force could not throw off any more matter against the force of gravitation. in that case we can conceive that the radial attraction, outwards and inwards, of the particles of the matter forming the sheet would gradually establish lines of separation, dividing off the matter into distinctly separate rings, preparatory to their transformation into planets; but we cannot explain how these separate rings came to be more dense in one place than another. we must leave that for future discovery. meanwhile the idea of one continuous sheet of matter extending from the sun out to neptune, suggests the possibility of all the rings having been in existence as rings, more or less advanced in their evolution, at the same time; and if not so much as that, makes it more easy for us to see how the four inner planets, being made out of more condensed cosmic matter, and being of so much smaller volume, have arrived at a much more advanced stage of their being than the four outer ones. going a little further, we can see how the cosmic matter of the rings condensing from both sides in the direction of their thickness, and falling in impeded, so to speak, the tendency to contract in length, or circularly, until they arrived at a certain stage of density, when they began to contract in their orbital direction, to break up into pieces, each one of which would form itself into a small, probably shapeless, nebula with a tendency to direct rotation, as explained and shown by m. faye in "l'origine du monde," chapter xiii., page , entitled "formation de l'universe et du monde solaire"--an explanation which must have occurred to everyone who has taken the trouble to think seriously, of how nebulous spheres could be formed out of a flat nebulous ring endowed with a motion of revolution. we have seen at page that when the nebula was condensed to a little over , , miles in diameter, its average temperature might have been °, provided no heat had been radiated into space. in like manner, we can see that the sun being now condensed to · times the density of water, or times the density of air, in other words, that number of atmospheres, its present average temperature might be about , °--as each atmosphere corresponds to °--provided no radiation of heat into space had been going on. but this way of estimating could not in any way apply to the nebula after it had ceased to throw off planetary matter; because from that time, or at all events from the time when it came to be of a density equal to one atmosphere and temperature of °, or freezing point of water, that would be accumulated within it, owing to the difficulty of carrying to the surface, to be radiated into space, what was produced by condensation in the interior, as we have shown before. both heat and pressure would increase from the surface towards the centre, the former rising, in spite of surface radiation, to something far beyond what we have stated above that it might be, aided by the increase of pressure which near the centre must be enormously greater than the average of atmospheres, seeing that the pressure at the surface of the sun is estimated to be not far from atmospheres. the first cause of the increase of pressure would be the condensation produced by gravitation, which according to the areolar law would increase the rotary velocity of the nebula in proportion as the centre was approached; and as this would begin long before it had given up abandoning rings, or rather from the very beginning of its rotation; from that time, there would be different rates of rotation at different distances between the surface and the centre, which would cause friction among the particles of its matter, in other words a churning of the matter shut up in the interior of the nebula, and thus produce heat over and above that produced by the condensation of gravitation alone. if two particles of matter would produce a given quantity of heat, in falling from the surface of the nebula to any point nearer to the centre, they would surely produce more if they were rubbed against each other by churning action during their fall. reflecting on what we have written up till now, we see that the analysis of the nebular hypothesis we have made, which at first may have appeared to be unnecessary or even useless, has shown us and made us think over many details, of which we had only a vague notion previously. it has shown us that without condensation at or near the surface of the nebula--which we have pointed out must have been caused by its greatest mass being near that region, and which laplace procured by endowing it with excessive heat--the various members of the solar system could not have been evolved from it in terms of the hypothesis. from it we have been able to learn, by means of the register of the acceleration of revolution from one planet to another, when, and for what reason, the nebula ceased to be able to throw off any planet nearer to the sun than the supposed vulcan, or almost even so near. finally, and not to go into greater detail, it has so far given us some ideas, that we had not before, of the internal structure of the sun, and has made us believe that a great deal may be learnt by attempting to find out what that structure really is. for this purpose, it appears to us that a careful examination into, and study of, the interior of the earth might be a great help, and to this we shall appeal, as we cannot think of any other process by which our object can be attained. this, therefore, we shall endeavour to do in the following chapters. chapter viii. page inquiry into the interior construction of the earth. what is really known of the exterior or surface. what is known of the interior. little to be learned from geology, which reaches very few miles down. various notions of the interior. what is learnt from earthquake and volcanoes. igno-aqueous fusion, liquid magma. generally believed that the earth consists of solid matter to the centre. mean density. surface density. more detailed estimate of densities near the surface. causes of increased surface density after the crust was formed. calculations of densities for miles deep, and from there to the centre forming table iv. reflections on the results of the calculations. notion that the centre is composed of the heaviest metals. "sorting-out" theory absurd. considerations as to how solid matter got to the centre. gravitation might carry it there, but attraction could not. how the earth could be made out of cosmic matter, meteorites or meteors. the interior of the earth and its density. before attempting to inquire into the nature and structure of the interior of the earth, it will be convenient to specify the bases on which the inquiry is to be made, in other words, the data we have to proceed with; which data should be denuded of everything whatever having the semblance of a hypothesis or theory, and should consist of simple facts. anything founded upon theory must come to an end should the theory be afterwards found to be erroneous, and all the labour would be lost. what we really know of the earth in this way may be stated as follows:-- of the exterior or surface we know that it is of a spherical form, surrounded by an atmosphere of probably miles or even more, in height, consisting of common air mixed with vapour of water in more or less degree; that, of its surface, nearly three-fourths are covered by water, and the remaining fourth consists of dry land, intersected in all directions by rivers; that on the dry land there are elevated tablelands and ranges of mountains from two to three miles high, with occasional ridges and peaks rising up to altitudes of from five to near six miles, and that in the part covered by water or sea, there are depressions or furrows with depths in them probably exceeding the heights of the highest mountains; that the sea does not remain constantly at the same level but rises and falls twice in every twenty-four hours, or thereby, in obedience to the attraction of the moon and sun, forming what are called tides; and that its polar regions are enveloped in dense masses of snow and ice, which the persevering energy of man has not been able to penetrate in centuries of continued and determined effort. what we know of the interior of the earth is found in great measure from the exterior, that is, from the construction of the rocks as seen in deep ravines, in precipices, and on the sides of hills or mountains; and also from what we have been able to learn from the exploration of mines and from deep wells, the deepest of which have penetrated it very little beyond one mile in depth; all of which knowledge may be summarised as follows: that the substances which compose the earth are manifold and of manifold nature--or, more appropriately speaking, simply the elements of chemistry--varying in density, or specific gravity, from the same as that of water, or in some cases much less, to three or four times as much in some kinds of rock and earths (disintegrated rock), to more than twenty times in the heaviest metals; that from a depth great enough not to be affected by the changes of seasons, the heat of the earth increases in descending towards the centre, by one degree of fahrenheit's thermometer for every fifty to sixty feet in depth--that is about thirty metres for each degree of the centigrade scale--as far down as we have been able to penetrate; that at the greatest of these depths abundant supplies of water are found, which shows that it must exist at much greater depths than any that have yet been reached; and that at unknown depths, as shown by the eruptions of volcanoes, there are masses of matter in a molten liquid state, or that, owing to their great heat, can be suddenly liquefied by diminution of pressure. over and above what has been stated, little can be learnt from geology, because the earth must have been formed and fashioned almost to its present condition before geology could begin to exist, and all its teachings are confined to a very few miles from its surface. its first lesson could only begin when the earth was so far cooled down that a crust could be formed on its surface, and that crust could be deluged by copious falls of rain on it. some help or guidance may be obtained however, from the ideas which astronomers and physicists have formed on its interior, and it may be useful to have the principal of these ideas specified, as they may help to strengthen arguments that may be advanced, or conclusions that may be drawn. when it was discovered that the temperature of the earth increases, as we go downwards, at what may be considered a rapid rate, it was calculated that at a depth of from twenty-five to thirty miles, the heat would be great enough to melt any substances that have been found near the surface; and it was immediately concluded that from that depth to the centre the whole of the interior was a molten liquid mass, whose temperature far exceeded any heat that could be produced upon the surface. even up to the present day, the belief in a liquid interior has not disappeared. many years afterwards, the supposed liquid state of the interior of the earth was taken advantage of, to frame a theory that earthquakes and eruptions of volcanoes are caused by the attraction of the moon on the liquid interior producing tides, in the same manner as it produces tides in the sea, which in their turn act upon the crust, cracking and rending it to produce the one, and forcing the liquid matter out through the rents, or up through the vents of volcanoes to produce the other, in some way that it is more easy to imagine than to explain mechanically. also when the effect of the attraction of the moon on the liquid internal matter came to be duly considered, it was concluded that the crust, with only to miles in thickness, could not be rigid enough to resist the pressure brought upon it by the movements of the interior tides; and it began to be thought that, owing to the pressure of the superincumbent strata, the density of the matter at that depth might be so great that it would become solid at a much higher temperature than it does at the surface; and some physicists went the length of supposing that the earth has a solid crust and solid nucleus with liquid matter between them. on the other hand sir william thomson, lord kelvin, looking more as it would appear to the effects of the moon's attraction on the crust than on the liquid interior, concluded that the earth must be a solid globe, contracting through gravitation in the interior, and cooling at the surface, because a crust so thin as to miles, or even miles, would be continually rent and broken up by the tidal action of the moon; but professor clerk maxwell and others have thought that the elasticity of the crust would be great enough to admit of its accommodating itself to all the changes of form that would be caused by the action of those tides. notwithstanding that they agree with lord kelvin in the main, in his objections to the existence of a liquid interior, many scientific men suppose that, through the effects of pressure, the liquid interior of the earth may have been changed into a viscous state, as it went on contracting through gravitation, which would, according to the degree of viscosity, either annul, or almost annul, the tidal action on it of the moon. to which it may be added that that action would not raise such high waves in even perfectly liquid molten matter as it would upon water; because it would be easier for the moon to lift a cubic mile of water three or four feet high, than to lift a cubic mile of melted rock or metal to the same height. other parties look upon the earth as mainly solid to the centre, but with large reservoirs of liquid matter in various parts of it near the surface, which furnish all the material for volcanic eruptions and are the causes of earthquakes. there are others also who, believing the earth to be altogether solid, consider that when any part of the intensely heated and dense interior is relieved suddenly from pressure, as, for example, by the convulsive action of an earthquake, it will immediately assume the liquid state and become material for volcanic eruptions; a theory which they consider to be substantiated by the fact of these two phenomena generally accompanying each other. and mr. mallet seems to have demonstrated that earthquake-shocks proceed from centres not far from the surface, which would seem to point out that if a liquid interior did exist at to miles from the surface, it could have no part in causing earthquakes. there are others still who consider earthquakes and volcanic eruptions to be caused by water penetrating deeply into the interior, but it is difficult to understand how water could penetrate into the interior to a greater depth than where it would be converted into steam, that is to a greater depth than from three to four miles. many other notions about the interior state and conditions of the earth have been formed, more or less entertainable, more or less fanciful, to provide liquid matter for volcanic eruptions. one of these, referred to in "nature" of december , , takes for granted "that granite has consolidated from a state of igneo-aqueous fusion, and that the liquid magma from which all granitic intrusions have proceeded contains water-substance," and proceeds, "it is, therefore, only a further step to assume that this water-substance is an essential constituent of the liquid substratum (assumed by the author), and to suppose that it has been there since the consolidation of the earth." this mixture of water, fire, and molten granite is one that does not agree with what we have been taught of the nature of any of the three components, and we cannot accept it. why we refer to it more particularly than to the other ideas we have cited, is because it so far comprehends some of them, and that we shall have to return to it hereafter, when we think it will be seen that it has not been properly thought out. bearing in mind all these ideas we have cited, and working with the data we have considered as actual facts, we may now proceed with our inquiry. the belief that the earth is a mass of matter increasing, whether liquid or solid, or part of both, in density from the surface to the centre is so general that we shall look at it in that light first, and endeavour to find out what must be its density at any place between its surface and its centre. astronomers and geologists concur in telling us that the mean density of the earth is very near to · times that of water: knowledge that has been acquired by measuring the attraction of high and precipitous mountains for plummets; by the attraction of masses of metals for each other, measured by the torsion balance; and by the acceleration or retardation of the vibrations of pendulums, as observed in the depths of mines and on the tops of mountains, compared with each other. they also tell us that the average density of the matter and rocks of which the crust is composed is about - / times that of water; and then, in a general way, that the average density of the crust, taking into consideration that so much of its surface is covered by the sea, is not much more than - / times that of water. this estimate is manifestly incorrect, for it implies that the whole of the crust of twenty-five to thirty miles is affected by the presence of water, when we know that the depth of the sea at any place does not exceed one-fourth of that thickness. therefore, we shall endeavour to obtain some more accurate computation, as it is the only datum we have to go upon, and has a greater effect upon the result, and upon all things relating to the interior, than might at first sight be supposed. we find in "nature," of january , , that mr. john murray has calculated that if the whole solid land of the earth were reduced to one level under the sea, its surface would be covered by an ocean with a uniform depth of about miles. here we have a very good beginning for our calculations. without taking into consideration the increase of density in water at miles deep, at that depth we may suppose we have come to solid matter, the specific gravity of which could not be less than twice that of water, on account of the pressure of that depth of water upon it. if we now go down - / miles further we shall have the solid matter subjected to a pressure proportioned to that depth; and if we take its weight per cubic foot at an average between granite (at lb.) and earth (at lb.), or lb., the pressure at - / miles deep of solid matter alone will be about tons per square foot, or just about the crushing strain of our strongest granites, and therefore, the density of the matter under it must be equal to that of granite, or · times that of water. we do not add the pressure of the water, at present, because that may be looked upon by some people as of the same nature as of the atmosphere upon a human body, which neither increases the pressure upon it nor adds to its weight; but we see that at that depth the solid matter must have a density equal to the average between water on its surface and · --that of granite; and if we choose to take the average between miles of water and - / miles of solid matter, we shall have · as the average density of the outer - / miles in thickness of the crust of the earth. for our purposes, however, and for obvious reasons, we shall consider the average density of the - / miles alone of solid matter to be · times that of water. we shall now go down to miles deep, because the diameter of miles we have adopted for the earth will there be reduced to miles, which will be convenient for our further operations. at that depth we shall have a superincumbent pressure at the very least as follows:-- tons. at miles deep, miles of sea at tons per mile - / " - / " solid matter at spec. grav. · equal to · tons per mile " - / " of rock at lb. per cubic foot ---- total pressure per square foot or just about times the crushing strain of our best granites. then, as when crushing takes place compression begins, it will, we believe, be far below the mark to estimate the general specific gravity of the earth at miles deep to be times that of water. we have now added the pressure of the miles of water, because there could be no water at the depth of miles; for the critical temperature of water is known to be °, beyond which temperature water cannot be maintained in its liquid state by any amount of pressure, however great; and miles would give ° temperature at ° for each metres. at that depth there might be steam, although it is difficult to see how it could penetrate so far, because the only force to help it to penetrate would be gravitation, and that would have to act against the increasing repulsion of heat. there is another circumstance to be considered which would tend to increase the density of the outer portion of the crust, if there be a crust, and if not, of the outer portion of the earth itself. when the earth was in the molten liquid state, it is generally supposed to have been surrounded by vapours of a great proportion of the metals and of some of the metalloids, in addition to the vapour of water, air, and other gases, which floated above them higher up in the atmosphere. in that case when the crust began to be formed through cooling, these vapours would be precipitated on the surface and mixed with the half-liquid half-solid matter there, but the proportion of condensed vapours would be very small compared with what they fell upon, and the specific gravity of the mixture would not be great enough to cause it to sink much below the surface, because it would soon meet with matter as dense as itself; consequently we must consider that all these metals would remain near the surface--most likely much nearer to it than the miles which we have as yet descended to--and whatever may have been the proportion of their density it ought to be added to the weights and pressures that have been taken into account above. we believe that it will be shown later on that this estimate of a density of three times that of water at miles deep in the earth is very much lower than it should be; because, when the pressure upon the matter there came to be greater than its crushing strain, compression would go on more rapidly than shortly afterwards, and it might so be that with a strain of very much less than four times that of crushing, compression would be reduced to its utmost limit. but more of this hereafter. having determined densities for the matter composing the earth at , - / , and miles below the surface, that is, to where the mean diameter comes to be miles, if we divide that diameter into layers of miles each in thickness, compute the volume of each layer or shell, increase the density of each layer as we descend in direct proportion from --the density we have fixed for miles deep--to · times the density of water, at the centre, and multiply the volume of each layer from the surface downwards by its respective average density, we shall find a mass nearly equal to the mass of the earth at the density of water--always taking its mean diameter at miles, and mean density at · times that of water, as already premised. these calculations have been carefully carried out, and are represented in detail in table iv. for future reference. they terminate in a deficiency of over , , of cubic miles, a deficiency which would be more than made up by making the central density · instead of · . thus we see that if the density of the earth increases regularly from the surface to the centre, and if the densities we have given to the layers between the surface and miles in depth are not greater than those adopted, the central density must be exceedingly near - / times that of water. of course, if the three surface densities are in reality _less_ than those we have adopted, the central density must be greater than - / times that of water. the whole being a result to our calculations which leads us to speculate on what kind of matter there is at the centre of the earth. we are acquainted with various kinds of rocks, stones and other solid matter that have densities (specific gravities) of - / to times that of water, and we have to conceive that a cubic foot of one of these would have to be compressed into a height of - / or - / inches in order to have the density of - / required at the centre, a result which presents us with a substance which it is difficult to imagine or to believe to exist. it may be that the centre of the earth is occupied by the heaviest metals we know, arranged in layers proportioned in thickness to the masses required of them, and that they are laid one over the other according to their densities, or mixed together until a distance from the centre is attained, at which ordinary rocks compressed as highly as their nature would admit of, may exist; but we do not derive much knowledge or satisfaction from such a supposition. an examination of our table of calculations will show that miles in diameter of the central part might be filled up with platinum, the few other rarer and heavier metals, and gold amalgamated with mercury in due proportions. then there might be a mixture of mercury and lead to miles in diameter, followed by a mixture of lead and silver to miles. after that might come a compound of silver, copper, tin, and zinc to miles, and some compounds of iron might finish the filling process up to miles in diameter, or thereby; where the known rocks, compressed to half their volume at first, but gradually allowed to expand, might complete the whole mass of the earth. it will be seen, also, that by the time compressed rocks could be used for this filling process, more than per cent. of the whole volume of the earth would be occupied exclusively by pure metals mixed by rule and measure. it would appear then that the "sorting-out theory"--about which a good deal has been written--whereby, in suns and planets, the metals on account of being heavier fall more rapidly to the centre, and the lighter metalloids remain near the surface--a theory probably got up to get over the difficulty we are in--is not a very happy one, as too much metal would be required for the process, at least for the earth. no doubt it might be applied differently to what we have done by mixing metals with rocks, stones, earth, etc., forming metallic ores--very rich they would doubtless have to be--from the centre outwards; but however disposed it would seem that very much the same quantity would be required to furnish the desired densities up to miles in diameter, where we have supposed compressed granites, etc., might come into play. besides, such an arrangement would do away with the whole beauty of the theory; there would be no law to invoke; it would be all pick-and-shovel work. the sorting-out theory is one of these notions that occur to humanity and are accepted at once, without consideration of what the consequences may be. if it is made to account for the four inferior planets being so much more dense, and of coming so much sooner to maturity--so to speak--than the four superior ones, it is hard to understand why the sun up to the present day almost ranks in low density with the large planets. if that theory holds good, it would be most natural to suppose that the mean density of the sun should be very much greater than that of mercury. but it appears to be only carried as far as it suits the theorist, and to be there dropped, or rather ignored. having been defeated in our attempt to build up or construct an earth solid to the centre by appealing to the metals to make up the weight or density required for the foundation layers, and that even to somewhere about three-fourths of the diameter of the whole structure, we are forced to fall back upon our known rocks, earths, etc., in order to compound out of them the dense material we require, and of course we feel that we have in hand a more hopeless task than we had with the metals. how are we to compress the everlasting hills into one-fourth or one-fifth of their volume? some solution of the difficulty, or mystery, must be found somewhere; but at the same time the mountains of gold, silver, and less precious metals required have shown us how absurd, even laughable, it is to appeal to them. let us suppose that we have a cubic foot of matter of any kind of - / times the density of water, and that we place it in one of the scales of a balance at the centre of the earth; we shall find that it does not depress the scale one hair-breadth, for the very good reason that it has nowhere to depress it to; it would be already at what may be called the end of gravitation or tendency to fall lower. as it could not get any lower it would have a tendency to fly off anywhere--provided it was free to do so--and drag the scale and balance along with it, in obedience to its own attractive power and the attraction of all the matter of the earth surrounding it, except that the attraction might be so equally distributed all around it that it would not move in any direction. it would, however, be in a state of very unstable equilibrium, and if by some means the attraction were increased a little on one side more than the others, and it were at liberty to do so, it would abandon the centre and fly off in that direction never to return. now, this being the case, we are forced to consider how a cubic foot of matter, such as the one we are dealing with, could ever have found its way to the centre of the earth; and the law of gravitation, or rather of attraction, does not in any way help us out of the difficulty. we know that we put our cubic foot of extremely dense matter there for an experiment, but we do not know of any process of nature that could place there any equal mass of matter of that density. gravitation and attraction are generally used as synonymous terms, more especially gravitation--somewhat after the manner of the likeness between the two negroes, cæsar and pompey, the latter being most especial in the likeness--but there is a very appreciable distinction between them, if we want to use each of them in its proper and strict sense. gravitation implies the conception of a weight of some kind falling to a fixed centre, while attraction gives the idea of two weights, or masses, drawing each other to a common centre, which when properly looked at is a different thing; because the centre may be anywhere between the two, depending entirely on the difference, if any, in the weights of the masses. the confounding of the two, or rather the almost universal adoption of the less correct term, name, expression--whichever it may be called--has been the cause of wrong conceptions being formed of the construction of almost all--probably all--celestial bodies, and of that most absurd expression, _attraction of gravitation_, used by all our most eminent physicists. the _gravitation_ of _attraction_ might be excused, but putting cause for effect is hardly scientific. a name is nothing as long as what is meant by it is understood and taken into consideration, but that is not always the case, as we shall proceed to show. the term gravitation may be applied with almost, but not absolutely, perfect strictness to the attraction between the sun and the planets, because the common centres of their attractions and the centre of the sun are so near each other that they may be looked upon as one and the same thing, or point; but it is not so with the attractions of the planets for each other where there is no common fixed centre, or if there is something approaching to it in a far off way, it is constantly varying, so that the term gravitation cannot be strictly applied to them, nor even to the sun, to speak truly. planets sometimes _gravitate away_ from each other and from the sun, otherwise adams and leverrier could not have discovered neptune from the perturbations of uranus. neither can it be properly applied to the different masses of matter in the sun or in the earth--although it was no doubt notions connected with the earth that gave rise to the term, from all ponderable matter falling upon it--because _per se_ they could have no tendency to fall to the centre, for _at the centre_ there is no sufficient attractive force to draw them towards it. gravitation was a known term long before the days of newton, who had the glory of enlightening the world by showing that attraction was the cause of it; and, perhaps unfortunately, the name was continued to represent what it in reality does not. let us suppose that we have an empty earth to fill up; if we place one mass of matter at london and another at calcutta, they could have no tendency of themselves to fall to the centre, but if left alone would go for each other in a straight line and meet half-way between the two, provided they were equal in mass, and attraction, not gravitation, would be the proper term to apply to them. but supposing that two equal masses were placed at their antipodes and the four were left to themselves, they would gravitate towards and meet at the centre in the usual meaning of the word, but the force that drew them there would be really that of attraction. we could, however, place four similar and equal masses at the centre, and give the outer ones just and good reason for gravitating or falling down to it, because those at the centre being equally attracted in the four directions might remain stationary there, but would be in a state of unstable equilibrium. we may now suppose that when the masses had just left london and calcutta to meet the others, a goodly number of other equal masses were added to those at these two places and began to attract the two bound towards the centre, they would prevent the two from proceeding, or at least retard them on their journey inwards. moreover, the larger numbers at these two places would attract the four masses at the centre with more force than would the two at the antipodes, and would draw the whole of the four away from the centre and outwards towards themselves; but we might also suppose that at the same moment an equal number of equal masses were added to those at the antipodes, which would again equalize the attractions at the four outer posts, and things would continue as they were at the first; with this difference, that the four at the centre would not be able to balance the attractions at the four outer posts, and the consequence would be--seeing that the forces at the four outer stations were equal to each other, and far superior to the four at the centre--that each one of the four at the centre would be drawn away from it towards one of the outer stations--provided the law of attraction acted impartially--and so the centre would be left without any of the masses at it, that is empty. no doubt when the four outgoing masses met the larger ones coming in, they would all then move towards the centre; but the four places where they met would be immensely nearer the places occupied at first by the outer masses than half-way between them and the centre--proportioned, in exact conformance with the law of attraction, to the excess of the numbers of the masses at the outer stations over those at the centre--and they would be moving, all of them together, to a remote and void space. we may now increase the four outer stations to thousands or millions, with the security that the mode of proceeding would be the same with the whole of them; that is, that the first tendency of the masses at each one of the millions of stations would be to draw away the filling we were pouring into the hollow earth--provided we did it equally and impartially all over the hollow--from the centre, and to leave a void there. we are accustomed to look upon the earth as a solid body in which there are no acting and counteracting forces, no movements of matter from one place to another, similar to those we have been calling into play, and as if there was only one force acting upon its whole mass and driving it to the centre; we have, in our ideas, got the whole mass so compressed and wedged in that it cannot move, and never has been able to move in any direction except towards the centre, and this is no doubt the case at the present day. we never stop to think with sufficient care how this compression and wedging-in were brought about, and we only accept what we have been accustomed to believe to be facts, and trouble ourselves no more about it; but there must have been a time, according to any cosmogony we may choose to adopt--even to the vague one that the solar system was somehow made out of a nebula of some kind--when the matter of the earth was neither compressed nor wedged in, nor prevented from moving in any direction towards which it was most powerfully attracted--before superincumbent matter came, so to speak, to have any wedging-in force--and we must go back to that period and study it deeply, if we want to acquire an accurate knowledge of the construction of the earth. table iv.--calculations of the volumes and densities of the earth between the diameter specified, reduced to the density of water. -----+------+---------------+-------+-----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+-------------+ | | | | |{total volume| | | , , , | · | , , , , |{of the earth| | |---------------+-------+-----------------|{at density | | | | | |{ of water. | | | | | | | | | | | |{density at | | | | | |{ miles | | · | , , | · | , , |{in diameter.| | | | | |{the miles | | · | , , | · | , , , |{above being | | | | | |{at density | | | | | |{of water. | | · | , , | · | , , , | | | |---------------+-------+-----------------|{volume to | | | , , , | | , , , |{ miles deep| | | | | |{at density | | | | | |{of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , | | -----+------+---------------+-------+-----------------+-------------+ -----+------+---------------+-------+----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+------+---------------+-------+----------------+-------------+ | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |----------------| | half volume}| | | |{ · of | of earth }| , , , | | , , , |{whole volume| | | | | |{of the earth| | | | | |{at density | | | | | |{of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | -----+------+---------------+-------+----------------+-------------+ -----+------+---------------+-------+----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+------+---------------+-------+----------------+-------------+ | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | - / | , , | · | , , , | | | |---------------| |----------------| | · } | | | |{half mass of| of whole } | , , , | | , , , |{whole earth | volume of} | | | |{at density | the earth} | | | |{of water | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | -----+------+---------------+-------+----------------+-------------+ -----+------+---------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+------------+ | | | | | about | | · | , , , | · | , , , | density | | | | | | of iron | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | . | , , , | . | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+------+---------------+-------+-----------------+------------+ -----+------+---------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+------------+ | | | | | about | | · | , , , | · | , , , , | density | | | | | | of copper | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+------+---------------+-------+-----------------+------------+ -----+------+----------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+----------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | |{ about | | · | , , | · | , , , |{ density | | | | | |{of silver. | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+-------+---------------+-------+-----------------+------------+ -----+-------+---------------+-------+-----------------+------------+ diam.|densi- | volumes | avgs. | volumes at |observations| in | ties. | in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+-------+---------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , |} about | | | | | |} density | | · | , , | · | , , , |} of lead | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+-------+---------------+-------+-----------------+------------+ -----+-------+---------------+-------+-----------------+------------+ diam.|densi- | volumes | avgs. | volumes at |observations| in | ties. | in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+-------+---------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | |{ about | | . | , | . | , , |{ density | | | | | |{of mercury.| | · | , | · | , | | | |---------------| |-----------------| | | | , , , | | , , , , | | | | | | | | |true volume at density of water| , , , , | | | |-----------------|{ about | | deficiency | , , |{ / , th | | | |{ part. | -----+-------------------------------+-----------------+------------+ chapter ix. page inquiry into the interior construction of the earth--_continued_. the earth gasiform at one period. density including the moon may have been / , th that of air. must have been a hollow body. proofs given. division of the mass of the earth alone into two parts. division of the two masses at miles from surface. reasons why the earth cannot be solid to the centre. gasiform matter condensing in a cone leaves apex empty. proportions of the matter in a cone. calculations of the densities of the outer half of the hollow shell of the earth. remarks upon the condensation. calculations of inner half of the hollow shell. remarks upon position of inner surface of the shell. calculations of the same. the interior of the earth and its density--_continued._ when, according to the nebular hypothesis, the ring for the formation of the earth and moon had been thrown off by the nebula, and had broken up and formed itself into one isolated mass--rotating or not on an axis, as the case may have been--it must have been in a gasiform state. what was its density, more or less, may be so far deduced from table iii., where it will be seen that when it had condensed to about one-half of its volume, it must have had a density of only / th part of our atmosphere, and in which each grain of matter would have for its habitat cubic feet of space, or a cube of · feet to the side. so that, with an average distance from its neighbours of - / feet, a grain of matter could not be looked upon as wedged-in in any way, and would be free to move anywhere. now, supposing this earth-moon nebula to have been in the form of even an almost shapeless mass, and that it was nearly homogeneous--as it could hardly be otherwise after the tumbling about it had in condensing from a flat ring--its molecules would attract each other in all directions, and as the mass--without having arrived perhaps at the stage of having any well defined centre--would have an exterior as well as an interior, the individual molecules at the exterior would draw those of the interior out towards them, just as much as those at the interior would attract those of the exterior in towards them; but as the number of those at the exterior would--owing to the much greater space there, being able to contain an immensely greater number--be almost infinitely greater than of those nearer to the central part, the latter would be more effectually attracted, or drawn, outwards than the former would be inwards, and there would be none left at the interior after condensation had fairly begun. the mass would speedily become a hollow body, the hollow part gradually increasing in diameter. but let us go deeper into the matter. let us suppose that the whole mass had assumed nearly the form of a sphere. we have already shown that, although the general force of attraction would cause all the component particles of the sphere to mutually draw each other in towards the centre, yet the more powerful tendency of the particles at the exterior--due to their greatly superior number--would at first be to draw the particles near the centre outwards towards them, and that there would consequently be a void at the centre, for a time at least. of course it is to be understood that each part of the exterior surface would draw out to it the particles on its own side of the centre, just in the same manner as the four masses we placed at the centre were shown to be drawn out by those at london, calcutta, and their antipodes. now we must try to find out what would be the ultimate result of this action; whether it would be to form a sphere solid to the centre, or whether the void at first established there would be permanent. in order to show how the heat of the sun is maintained by the condensation and contraction of that luminary, lord kelvin--in his lecture delivered at the royal institution, on friday, january , --described an ideal churn which he supposed to be placed in a pit excavated in the body of the sun, with the dimension of one metre square at the surface, and tapering inwards to nothing at the centre. in imitation of him, we shall suppose a similar pit of the same dimensions to be dug in the spherical mass, out of which we have supposed the earth to have been formed; only we shall call it a pyramid instead of a pit. this we shall suppose to be filled with cosmic matter, and try to determine what form it would assume were it condensed into solid matter, in conformity with the law of attraction. the apex of our imaginary pyramid would, mathematically speaking, have no dimension at all, but we shall assume that it had space enough to contain one molecule of the cosmic matter of which the sphere was formed. this being so arranged, we have to imagine how many similar molecules would be contained in one layer at the base of the pyramid at the surface of the sphere, and we may be sure that when brought under the influence of attraction, the great multitude of them would have far more power to draw away the solitary molecule from the apex, than the single one there would have to draw the whole of those in the layer at the base in to the centre of the sphere. a molecule of the size of a cubic millimetre would be an enormously large one, nevertheless one of that size placed at the apex of the pyramid would give us one million for the first layer at the base, and shows us what chance there would be of the solitary one maintaining its place at the apex. at the distance of one-twentieth of the radius of the sphere from the centre, the dimension of the base of the pyramid would be one-twentieth of a square metre, and the proportion of preponderance of a layer of molecules there would be as to , so that the molecule at the centre would be drawn out almost to touch those of that layer; at one-tenth of the radius from the centre, the preponderance of a layer over the solitary central molecule would be as , to ; and so on progressively to , , to , as we have already said. following up this fact, if we divide the pyramid into any number of frusta, the action of attraction will be the same in each of them; the molecules in the larger end of each will have more power to draw outwards those of the small end, than they will have to draw inwards those of the larger end; and then the condensed frusta will act upon each other in the same manner as the molecules did, the greater mass of those at the larger end, or base, drawing down, or out--whichever way it may seem best to express it--a greater number of the frusta at the smaller end of the pyramid, until, in the whole of it, a point would be reached where the number of molecules in the various frusta drawn down from the apex would be equal to those drawn up from the base, leaving a part of the pyramid void at each end, because we are dealing with attraction, not gravitation, and there would be no falling to the base or apex, but concurrence to the point, just hinted at, where the outwards and inwards attractions of the masses would balance each other. this point of meeting of the two equal portions of cosmic matter may be called the plane of attraction in the pyramid. the whole pyramid would thus be reduced to the frustum of a pyramid, whose height would be as much more than double the distance from the plane of attraction to its base, as would be required to make the upper part above the plane of attraction equal in volume, or rather in number of molecules, to the lower part. it would be impossible for us to explain how, in a pyramid such as the one we have before us, the action of attraction could condense, and at the same time cram, the whole of the molecules contained in it into the apex end. we must not, however, forget that there are two sides to a sphere, as well as to a question, and that we must place on the opposite side to the one we are dealing with, another equal pyramid with apex at the centre and base at the surface, at a place diametrically opposite to the first one, and that the tendency of the whole of this new pyramid would be to draw the whole of the first one in towards the centre of the sphere. but in the second, the law of attraction would have the same action as in the first; the molecules of the matter contained in it near the base would far exceed, in attractive force, those near the apex, and would draw them outwards till the whole were concentrated in a frustum of a pyramid, exactly the same as the one in the first pyramid. and while the whole masses of matter in the two pyramids were attracting each other at an average distance, say, for simplicity's sake, of one-half the diameter of the sphere, the molecules in each of them would be attracting each other from an average distance of one-quarter the diameter of the sphere; their action would consequently be four times more active, and they would concentrate into the frusta as we have shown, before the two pyramids had time to draw each other in to the centre. there would be then two frusta of pyramids attracting each other _towards_ the centre with an empty space between them. here then we have two elements of a hollow sphere, one on each side of the centre, and if we suppose the whole sphere to have been composed of the requisite number of similar pyramids, set in pairs diametrically opposite to each other, we see that the whole mass of the matter out of which the earth was formed must have--by the mutual attractions of its molecules--formed itself into a hollow sphere. all that has been said must apply equally well whether we consider the earth to have been in a gasiform state, or when by condensation and consequent increase of temperature it had been brought into a molten liquid condition. for up to that time it must have been a hollow sphere, and we must either consider it to be so still, or conceive that the opposite sides have continued to draw each other inwards till the hollow was closed up; in which case, the greatest density would not be at the centre, but at a distance therefrom corresponding to what has been called the plane of attraction of the pyramid. that the opposite sides have not yet met will be abundantly demonstrated by facts that will meet us, if we try to find out what is the greatest density of the earth at the region of greatest mass or attraction, wherever that may be. seeing that the foregoing reasoning forces us to look upon the earth as a hollow sphere, or shell, in which the whole of the matter composing it is divided into two equal parts, attracted outwards and inwards by each other to a common plane, or region of meeting, we shall divide its whole volume into two equal parts radially, that is, one comprising a half from the surface inwards, and the other a half from the centre outwards--that is to say, each one containing one-half of the whole volume of the earth. referring now to our calculations, table iv., we find that the actual half volume of the earth is comprised in very nearly miles from the surface, where the diameter is miles, because the total volume at miles in diameter is , , , cubic miles. this being the case, we cannot avoid coming to the conclusion, after what has just been demonstrated by the pyramids that if one-half of the whole volume is comprehended in that distance from the surface, so also must be one-half of the mass. but for further substantiation of this conclusion let us return to the table of calculations. there we find that from the surface to the depth of miles--where the diameter would be miles--which comprehends one-half of the volume--the mass at the density of water is shown to be only , , , miles instead of , , , cubic miles, which is the half of the whole mass of the earth reduced to the density of water. that is, the outer half of the volume gives only · per cent. of half the mass, while the inner half of the volume gives not only one-half of the mass but · per cent. more; or, to put it more clearly, the mass of the inner half-volume is · times, nearly twice as great, as the mass of the outer half-volume. on the other hand, we have to notice that the line of division of the mass into two halves falls at · miles from the surface, where the diameter is · miles; so that on the outer half of the earth, measured by mass, · per cent. of the whole volume of the earth contains only one-half of the mass, whereas on the inner portion, measured in the same way, · per cent. of the same whole contains the other half. all these results must be looked upon as unsatisfactory, or we must believe that two volumes of cosmic matter which at one time were not far from equal, had been so acted upon by their mutual attractions that the one has come to be not far from double the mass of the other; that the vastly greater amount of cosmic matter at the outer part of a nebula has only one-half of the attractive force of the vastly inferior quantity at the centre. this we cannot believe if the original cosmic, or nebulous, matter was homogeneous; and if it was not homogeneous we have, in order to bring about such result, to conceive that the earth was built up, like any other mound of matter, under the direction of some superintendent who pointed out where the heavier and where the lighter matter was to be placed. we shall now proceed to find out what would be the internal form, and greatest density of the earth, under the supposition that it is a hollow sphere divided into two equal volumes and masses--exterior and interior--meeting at miles from the surface; but before entering upon this subject we have something to say about the notion of the earth being solid to the centre. we are forced to believe that, according to the theory of a nucleus being formed at the centre as the first act, the matter collected there must have remained stationary ever since, because we cannot see what force there would be to uniform the nucleus just formed; gravitation, weight falling to a centre, would only tend to increase, condense, and wedge in the nucleus more thoroughly. attraction, as we have shown, would not allow the matter to get to the centre at all. convection currents, or currents of any kind, could not be established in matter that was being wedged in constantly. moreover, when in a gasiform state, it would be colder than when condensed by gravitation to, or nearly to, a liquid or solid state, and heat would be produced in it in proportion to its condensation, that is, gradually increasing from the surface to the centre in the same manner as density, which, when the cooling stage came, would be conducted back to the surface to be radiated into space, but could not be carried--by convection currents--because the matter being heavier there than any placed above it, and being acted upon by gravitation all the time, would have no force tending to move it upwards; and above all, when solidification began at the surface, it is absurd to suppose that the first formed pieces of crust could sink down to the centre through matter more dense than themselves; unless it was that by solidification they were at once converted into matter of the specific gravity of · . even so the solid matter would not be very long in being made liquid again by meeting with matter not only hotter than itself, but constantly increasing in heat through continual condensation, which would act very effectively in preventing any convection current being formed to any appreciable depth, certainly never to any depth nearly approaching to the centre. if solidification began first at the centre--as some parties have thought might be the case--owing to the enormous pressure it would be subjected to there, before it began at the surface, then, without doubt, the central matter must have remained where it was placed at first, up to the present day. this would suit the sorting-out theory very well, as all the metals would find their way to the centre and there remain; but judged under a human point of view, it would be considered very bad engineering on the part of the supreme architect to bury all the most valuable part of his structure where they could never be availed of; or that he was not sufficiently fertile in resources to be able to construct his edifice in a way that did not involve the sacrifice of all the most precious materials in it. man uses granite for foundations--following the good example he has actually given we believe, and are trying to show--and employs the metals in superstructures; but some people may also think that it was better to keep the root of all evil as far out of man's reach as possible. what a grand prospectus for a joint stock company might be drawn up, on the basis of a sphere of a couple of thousand miles in diameter of the most precious metals, could only some inventive genius discover a way to get at them! returning to our pyramids. we know that the centre of gravity of a pyramid is at one-fourth of its height, or distance from the base, and if we lay one of miles long (the radius of the earth) over a fulcrum, so that - / miles of its length be on one side of it and - / miles on the other, it will be in a state of equilibrium. this does not mean, however, that there are equal masses of matter on each side of the fulcrum, for we know that the mass of the base part must be considerably greater than that of the apex part, and that it must be counterbalanced by the greater leverage of the apex part, due to its greater distance from the point of support. this being so, in the case of a pyramid consisting of gasiform, liquid, or solid matter, the attractive power of the - / miles of the base part would be greater than that of the - / miles of the apex part, and the plane of equal attraction of the two parts would be less than miles from the base of the pyramid. this is virtually the same argument we have used before repeated, but it is placed in a simpler and more practical light, and shows that the plane of attraction in a pyramid will not be at its centre of gravity but nearer to its base, and that it must be at or near its centre of volume. thus the plane of attraction in one of the pyramids we have been considering of miles in length, and consequently the radial distance of the region of maximum attraction of the earth, would not be at miles from the base or surface, but at some lesser distance. now, if we take a pyramid, such as those we have been dealing with, whose base is square and height , its volume would be the square of the base multiplied by one-third of the height, that is ^{ } × / = · , the half of which is · . again, if we take the plane of division of the volume of the pyramid into two equal parts to be · in length on each side, and consequently (from equal triangles) the distance from the plane to the apex to be · the total height of , which is · ; then, as we have divided it into a frustum and a now smaller pyramid, if we multiply the square of the base of this new pyramid by one-third of the height we have · ^{ } × · / , or · × · = · , which is equal to the half-volume of the whole pyramid as shown above. thus we get less · = · miles as the distance from the base of the plane of division of the pyramid into two equal parts, which naturally agrees with the division of the earth into the two equal volumes that we have extracted from the table of calculations, where we have supposed the earth to be made up of the requisite number of such pyramids. so that it would seem that we are justified in considering that the greatest density of the earth must be at the meeting of the two half-volumes, outer and inner, into which we have divided it. considering, then, that one-half of the volume and mass of the earth is contained within miles in depth from the surface, this half must have an average density of · times that of water, the same as the whole is estimated to have. also, as we have seen already, that, taking its mean diameter at miles, its mass will be equivalent to , , , , cubic miles, one-half of this quantity, or , , , cubic miles will represent the half-volume of the earth reduced to the density of water. with these data let us find out what must be the greatest density where the two half-volumes meet, supposing the densities at the surface and for miles down to remain the same as in the calculations we have already made, ending with specific gravity of at miles in diameter. following the same system as before when treating of the earth as solid to the centre, and using the same table of calculations for the volumes of the layers: if we adopt a direct proportional increase between densities at miles and · at · miles in diameter, multiply the volumes by their respective densities, and add about per cent. of the following layer, taken at the same density as the previous or last one of the number, we shall find a mass (see table v.) of , , , cubic miles at the density of water, which is as near the half mass , , , cubic miles as is necessary for our purpose. it would thus appear that if the earth is a hollow sphere, its greatest density in any part need not be more than · times that of water, instead of · times, if we consider it to be solid to the centre. let us now try to find out something about the inner half-mass of the earth, and the first thing we have got to bear in mind is, that where it comes in contact with it, its density must be the same as that of the outer half-mass at the same place, and continue to be so for a considerable distance, varying much the same as the other varies in receding from that place, and diminishing at the same rate as it diminishes. this being the case--and we cannot see how it can be otherwise--if we attempt to distribute the inner half-mass over the whole of the inner half-volume, and suppose that its density decreases from its contact with the outer half--where it was found to be · times that of water--to zero at the centre, in direct proportion to the distance; then, it is clear that at half the distance between that place and the centre, the density must be just · times that of water. now, if we divide the outer moiety of the inner half-mass of the earth--that is, the distance between the diameters of · miles and · miles--into layers of miles thick each, take their volumes from table iv., and multiply each of them by a corresponding density, decreasing from · to · , we shall obtain a mass far in excess of the whole mass corresponding to the inner half of the earth. this shows that a region of no density would not be at the centre but would begin at a distance very considerably removed from it. it is another notice to us that the earth must be a hollow sphere. but why should there be a zero point or place of no density? and what would a zero of no density be? it would represent something less than the density of the nebulous matter out of which the earth was formed; and all that we have contended for, as yet, is that there is a space at the centre where there is no greater density than that corresponding to the earth nebula; but we must now go farther. if the earth is a hollow sphere, it must have an internal as well as an external surface. but how are we to find out what is the distance between these two surfaces? let us, to begin, take a look at the hollow part of the sphere. from the time of arago it began to be supposed that there is a continual deposit of cosmic matter upon the earth going on, and since then it has been proved that there is a constant and enormous shower of meteors and meteorites falling upon it. but although this is the case on the exterior surface, it may be safely asserted that on the interior surface, where the supply of cosmic matter must have been limited from the beginning, there can be no continual deposit of such matter going on now; nor can there have been from, at least, the time when the earth changed from the form of vapour to a liquid state. we may, therefore, be sure that there is no undeposited _cosmic_ matter of any kind in the hollow of the sphere, and that, as far as it is concerned, there is an absolute vacuum. table v.--calculations of the volumes and densities of the outer half of the earth--taken as a hollow sphere--at the diameters specified, and reduced to the density of water. with mean diameter of miles. diameter of half-volume at · miles, and density there of · times that of water. -----+-------+---------------+-------+----------------+--------------+ diam.|densi- | volumes | avgs. | volumes at | observations.| in | ties. | in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+-------+---------------+-------+----------------+--------------+ | | | | |{ half-volumes| | | | | |{ of the earth| | ... | , , , | ... | , , , |{ actual and | | |---------------| |----------------|{ at density | | | | | |{ of water. | | | | | | | | | | | |{ density at | | | | | |{ miles | | | | | |{ in diameter.| | · | , , | · | , , |{ the miles | | | | | |{ above being | | | | | |{ at density | | | | | |{ of water. | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |----------------| | | | | | |{ volume to | | | , , , | | , , , |{ miles deep| | | | | |{ at density | | | | | |{ of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | ½| · | , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | | | | | | | true half-volume at density of water | , , , | | -------------------------------------+----------------+--------------| deficiency | , , | | -------------------------------------+----------------+--------------+ as to how far the internal surface is from the centre, it may be possible to designate a position, or region, from which it cannot be very far distant, although we can never expect to be able to point out exactly where it is. going back to the time when the whole earth was in a molten liquid state, and just before the outer surface began to become solid, it is certain that the interior surface must have been in the same liquid condition, whatever may have been the condition of the mass of matter between the two surfaces, owing to the pressure of superincumbent matter; nay, we may be sure that whatever may be its state now, it continued liquid long after the other became solid, because it had no outlet by which to get rid of its melting heat by radiation, nor weight of superincumbent matter to consolidate it; and it would always be much hotter than the outer surface. at that time we have every reason to believe that the outer surface was at least as dense as it is now, there being no water upon it to lower its average density, as is the case at the present day; and we have equal reason to consider that the density at the inner surface, whether liquid or solid, is now at least equal to what the outer surface was then. duly considering, therefore, the absence of water from the interior surface, we shall suppose that the first layer of miles thick upon it will have an average density of - / times that of water, terminating at times, which is the density we have taken for the outer surface at miles deep. but there is another contingency, which it will be necessary to take into consideration before going any farther. it has been understood--as it is certainly the truth--in the calculations made with respect to the outer half of the mass of the earth, that the increase of density in descending was due to the pressure of the superincumbent matter, caused by the attraction for it of the inner half, as well as that of the whole of both the outer and inner halves on the other side of the hollow interior. in the case of the inner half we have now to consider that the attraction of the outer half alone would be the effective agent, and that the superincumbent pressure--that is, of course, the pressure acting from the centre outwards--would be interfered with, or perturbed, by the attraction of the mass on the other side of the hollow interior, so that it would not exert its full power in that direction. but that does not mean that the density would be in any way diminished. the attractions of the planets for each other perturb them in their revolutions around the sun, accelerating or retarding each other, but do not increase or diminish their density or mass; only it will lead us to expect that the same depth of miles will not produce the same amount of pressure outwards at the meeting of the two halves as it does inwards, and that to obtain an equal pressure a greater depth will be required. we believe that an expert mathematician, taking as bases two opposite pyramids in a sphere, similar to those we have used in a former part of our work, could point out, with very approximate accuracy, what ought to be the distance of the inner surface of the shell from the centre--provided a maximum density were determined for the earth--but that goes beyond our powers, and we shall limit ourselves to the use of our own implements; which will cause us to depart from the statement we have made, that the density of the inner half must decrease from the place of meeting of the two halves, at the same rate as the outer half had increased. it must decrease much more rapidly than the other increased. all this premised, and having established a density of for the interior surface, we may proceed to calculate where that surface ought to be, so as to give for the interior half of the earth a mass equal to , , , cubic miles of water. if we begin our operations with a density of · times that of water at the meeting of the two halves of the shell, and diminish it for any considerable distance at the same rate as it increased when we were finding the mass of the outer half, that is · for each layer, we soon find that before we could make up the whole mass of the inner half of the shell, the density would be decreased to at least that of water, which cannot be, as there can be no liquid or solid matter of any kind of so low density anywhere in the interior half of the shell. furthermore, if we decrease it at the same rate as the volumes of the different layers of the earth decrease as they approach the centre, it involves a mass of calculation that serves no useful purpose, as such calculations bring no contingent of satisfaction with them; because all the densities with which we are dealing have to be brought to a rational form before we can frame a proper approximate idea of what the interior construction of the earth is, as will be seen hereafter; and because it takes no account of the perturbation--above alluded to--produced by the attraction of the matter on the opposite side of the hollow. but, in order to get such a result as we can with our limited powers, if we begin with a density of · at the diameter of · miles and fix the density of --which we have adopted above--at the diameter of miles, we shall get a mass somewhat less than one-half of the earth; and with a density of · at miles diameter we get a mass of , , , cubic miles of water, which is rather greater than one-half of the mass required (see operations of table v.). this density of · reduced to · , as we mentioned, might be done when we were fixing the number , would make very little difference on the resulting mass, compared with what we have been in quest of. here we may state that we found that, had the calculations been made with documents of density proportioned to the decrease of the volumes of the layers of the earth as they approached the centre, the density would have been reduced to · at miles in diameter; which tends to show that should that process be considered to be more accurate, it would not have made any great difference on the result. with all, we may consider that it has been demonstrated, that the greatest density of the earth is not necessarily greater at any part of its interior than · times that of water. table vi.--calculations of the volumes and densities of the inner half of the earth, on the same data as those for the outer half. ------+---------------+---------+----------------+--------------+ diam. | volumes |densities| volumes at | observations.| in | in cubic | |density of water| | miles | miles. | | in cubic miles.| | ------+---------------+---------+----------------+--------------+ ½| , , , | | , , , |}half-volumes | |---------------| |----------------|}of the earth.| | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | -------+---------------+---------+----------------+--------------+ | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | -------+---------------+---------+----------------+--------------+ | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | | | | |{ true | | | | , , , |{ half-volume | | | |----------------| | | | | , , | excess | -------+---------------+---------+----------------+--------------+ chapter x. page inquiry into the interior construction of the earth--_continued_. density of · times that of water still too high for the possible compression of the component matter of the earth as known to us. reasons for this conclusion drawn from crushing strains of materials. a limit to density shown thereby. the greatest density need not exceed · of water. gases shut up in the hollow centre. their weight must so far diminish the conceded maximum of · . density of inner half of earth at miles diameter. greatest density may be less than · of water. supposed pressure of inclosed gases very moderate. meaning of heat limit to density. temperature of interior half of shell and inclosed gases must be equal. state of the hollow interior. results of the whole inquiry. inquiry into the interior construction of the earth--_continued_. it may be well to revert here to the experiment we made of putting a cubic foot of rock, of specific gravity · in the scale of a balance at the centre of the earth, where we saw that it could not depress the scale one hair-breadth, and make the same experiment by placing a cubic foot of rock of · specific gravity in the same scale, at what we have called the region of greatest density of the earth, that is, at miles from its surface. here, also, we shall find that the scale is not depressed for the very same reason as in the former case, that is because it had nowhere to be depressed to; and it might be argued that for the same reasons advanced formerly there can be no matter at that place, but the cases are entirely different. in the first case, there is nearly the whole mass of the earth drawing the matter away from the centre were it at liberty to move; whereas, in the second case, the meeting of the two halves of the shell, at the region where there is the greatest mass of matter, is also the meeting place of the action of attraction in its greatest force; the place to which matter is attracted from all sides, remains stationary, and it is held there both by attraction and weight of superincumbent matter or gravitation. the attraction of the whole earth acts as if it were concentrated at its centre, but that is for external bodies. that kind of attraction on the inner half of the shell would be far inferior to that outwards of the outer half, owing to its greater distance and conflicting nature, and would perturb, as we have said, but not do away with it. the same could not occur at the centre, because it is not the centre of the mass, that is, it is not the place where the greatest quantity of matter existed originally, or is now to be found, and consequently never was, nor can now ever be, the actual centre of interior attraction. it has been said when treating of the earth as being solid to the centre, that it is not easy to comprehend what may be the nature of the rocks we are acquainted with, when compressed to one-fourth or one-fifth of their volume, and we do not find ourselves much better off when we contemplate them as reduced to one-third or one-fourth of their bulk, that is, when a cube of one foot is reduced to three or four inches in height, as would be the case with it at a maximum density of · times that of water when placed at a depth of miles from the surface of the earth. we find, therefore, the idea thrust upon us that there may be a limit to density, perhaps not an absolute limit, but a practical one; in which case, the greatest density of the earth may not greatly exceed · times that of water. for, if we conceive that it increases to its maximum at miles from the surface, and continues nearly uniform thereafter, a little calculation will show that the greatest density of the outer half of the shell need not much exceed times that of water; and, of course, the same will be the case with the inner half should its density be almost uniform till miles from the inner surface is reached. it might even so happen that at a depth of to miles the practical limit might be reached; for a column of granite of one foot square and miles high would weigh, and exert a pressure upon its base of , tons, a pressure equal to nearly fifteen times what would be sufficient to crush it into powder; in which case the greatest density of the earth might not much exceed the · that we are accustomed to think of--without thinking. it may be deemed absurd to think that there is even a practical limit to the density of matter, but on the other hand it is much more absurd to suppose that there is not an absolute limit to it. we cannot conceive of density being other than the result of compression, and we cannot believe that matter can be compressed more and more continually for ever. there must be some end to compression. perhaps it was the difficulty in conceiving of rock being compressed to so small a fraction of its volume as would enable it to take its place at the centre of the earth--where it has been said that, "it must weigh like lead"--that originated the idea of its centre being occupied by the metals, arranged as they would be in a rack in a store, the heaviest pieces at the bottom of the rack, and the lighter ones higher up. when fairly looked at, density would really seem to have a limit, except in so far as it may be combined with heat. we know that water is compressed · part of its volume for every atmosphere of pressure to which it is subjected. but · for round numbers, is in fractional numbers / , ; therefore a pressure of , atmospheres would compress a cubic foot of water into / , of a foot in height, or practically into nothing. we know, also, that as a column of water · feet high balances one atmosphere, one mile in height will be equal to · atmospheres, and , atmospheres will produce a pressure equal to a column of water miles high; therefore, a cubic foot of water, subjected to such a pressure, would be compressed into virtually nothing. again, supposing that we have a column of liquid rock, of - / times the density of water, of the same height of miles, we should have a pressure of - / times that of the column of water; and as we have no reason to believe that granite in a liquid state has to obey a different law of compression to the one obeyed by liquid ice; then a column of granite miles high would be sufficient to squeeze its own base, not only off the face of the earth but out of the bowels thereof. it will be seen, therefore, that at miles deep from the surface, the density of the earth might well be equal to not only · times the density of water but to a great deal more; and that our estimate of times the density of water, at miles deep, was far within the mark. the authors of text-books on the strength of materials tell us that "the modulus of elasticity of any material, is the force that would lengthen a bar of that material of inch square to double its length, or compress it till its length became zero; supposing it possible to stretch or compress the bar to this extent before breaking." this is neither more nor less than a counterpart of the law of gases, upon which the air thermometer is constructed, applied to solid matter, and may be used in the same manner. but we can never produce a perfect vacuum, and so annihilate a gas and temperature; neither can we annihilate matter, nor easily reduce it to one half of its volume. now, we have seen, a little way back, that a column of granite miles high would exert a pressure at its base times as great as would crush it to pieces; so that a column of ÷ , or · miles high would destroy the elasticity of the material, because, when crushing takes place, all elasticity is gone. we cannot, therefore, get much satisfaction out of any calculations made upon the theory of the strength of materials; still, by them, we can make more plain the absurdity of any notion of the indefinite compressibility of matter. but if, in the face of contravening its conditions, we follow the reasoning used for the formation of the theory, and take the modulus of elasticity for granite as , , feet, then the same modulus would compress a bar of granite of inch square in section till its height became zero. and as that length is equal to miles, at that depth from the surface of the earth, granite or any other rock or stone of a similar nature would be compressed out of existence by the weight of the superincumbent matter. thus we have arrived at two measures of force which would compress to zero the rocks that are known upon the earth. one where rocks are looked upon as in a molten, liquid state, and analogous to water, where the force is equal to that exerted by a column of the material miles high; and the other where the column requires to be miles high. in either case the same method of calculation will show that columns one-half of these heights, will compress the material into at least one-half of its volume--that is half-way between what it is at the surface and would be at the specified depths--and consequently into double its density. so we find in the one case that the density of the earth ought to be about · times that of water at a depth of - / miles; and, in the other, at somewhere less than miles deep. but, before proceeding to use and reason upon these depths, we must recall to mind that the calculations from which we have derived them, in the second case, have been made in violation of the theory that was adduced for the purpose, and that in consequence the latter depth must be excessive. for, were we to erect a structure of any kind, calculating the stresses it would have to bear, under the same violation of the theory, we should inevitably find that the structure would give way under the strains that would be brought upon it; that is the columns - / and miles high would compress the same kind of matter composing them into very far below one-half of its volume. this premised, let us go back to our layers of miles thick with their respective volumes. nine of them counted from the diameter of miles inwards, will be equal to miles and will bring us to miles deep, which at the same time that it leaves us the same volume and mass that we have always retained for the first miles in depth, will facilitate our calculations considerably without making any appreciable difference in them. we shall then have to find for the layers corresponding densities increasing from to · , and if we multiply these together respectively, and add the numbers of the volumes and masses of the outer miles in depth, we shall get, at the diameter of miles, a simple volume of , , , cubic miles, and mass volume of , , , cubic miles. deducting this latter sum from , , , cubic miles, which represents the half mass of the earth at the density of water, we have a remainder of , , , cubic miles. on the other hand we find that the simple volume of the earth comprehended between the diameters of and · miles is , , , cubic miles; so that if we divide , , , by this sum, we find that a density of · times that of water over the whole intervening space--between the two diameters just cited--will make up the whole half-volume, at the density of water, from the surface of the earth to the diameter of · miles. then, for the inner half-mass:--if we multiply the simple volume between the diameters of · miles, and miles, which is , , , cubic miles by · , we get , , , cubic miles at density of water; and if from there we run down the density to at miles in diameter we get , , , cubic miles, which added to the last mentioned amount gives , , , cubic miles, somewhat in excess of the inner half-mass of the earth at density of water. thus we see that in order that the average density of the earth of · may be made up, there is no necessity for appealing to matter of any kind with a density of more than · times of water. and there is still something else of importance to be taken into consideration before we can bind ourselves to a density even so great as that. we have said, a few pages back, that there can now be no undeposited cosmic matter in the interior of the hollow earth, and that as far as such matter is concerned the hollow part may be a perfect vacuum. this is not absolutely true, for gases may be cosmic matter, just the same as any others of the elements out of which the earth is formed, but what is generally meant by cosmic matter is solid--at least, we have always looked upon it in that light--and all solid matter must have been deposited upon the interior surface at an immeasurably long period of time before the nebula forming the earth came to have even the density of water; certainly before it came to be in a molten liquid state; and we did not want to introduce any posterior evolutions in order not to complicate our calculations, and also to obtain some tangible bases to which the consequences of these evolutions might be applied. but as we have now both form and density to work upon we may take them into account, and it will be found that neither of these two bases will be very materially altered by them. when the earth was in a molten liquid state, it is believed--as we have said on a former occasion--to have been surrounded by a dense atmosphere, composed of gases and vapours of metals, metalloids, and water, and we have no reason to doubt that the hollow of the sphere was filled with a similar atmosphere, only the vapour of water would, most probably, be dissociated into its elements of oxygen and hydrogen. also we have every reason to believe that even at the present day gases are being produced in the interior, one part of which find their way to the surface and are dissipated into the atmosphere in the same manner as the gases from the chimney of a furnace; and another part into the interior, where they could not escape but would be stored up in the hollow. thus at the present day there may be an atmosphere there, composed near the surface of vapours of the elements with gases above them, so to speak, at a very high degree of pressure. these gases could not have gone on accumulating always, but must have found an exit in some particular place, or places, when the pressure exceeded the resistance, or when this was diminished by some convulsion such as an earthquake; but we do not want to define too much, or make more suppositions on this point than what present themselves to us in a reasonable way. all that we need say is, that the resisting power of some thousands of miles of solid, or even viscous, matter must be enormous, and the pressure necessary to force its way through it must have been equal to many thousands of atmospheres. we know that a pressure of · atmospheres condenses air to the density of water, and it must be the same with any similar gas; so we have only to suppose that the pressure is atmospheres--which is equal to · multiplied by · --in order to bring the whole of the gases, and vapours of elements, in the hollow to the same density of · times that of water, which we have shown need not be exceeded in any part of the earth. and such being the case, we can place the division between solid and gasiform matter in any point of the radius that may seem to us reasonable, only we must always have as much solid matter in the inner as in the outer half-mass of the earth. following nearly the result we have obtained in another way, by placing the division of the hollow part at miles in diameter, the volume of which is , , , cubic miles, and multiplying this by · , we get a mass equal to , , , cubic miles at density of water, composed of vaporous and gaseous matter in the hollow centre, and consequently much greater than is required to make up the total mass of the earth at the density of water; which shows that the density of the mass between the diameters of and miles must be less than · times that of water. how much less is very easily found, by dividing the surplus of , , , cubic miles over the whole volume between miles in diameter and the centre, because in this way we shall include the whole mass arising from both solid and gasiform matter. this whole volume--that of a globe miles in diameter--is , , , cubic miles, which, divided by the surplus gives the amount · as the density to be deducted from · on its account, and therefore the greatest density of any part of the earth need not be over · times that of water. this result derived from our operations will be acknowledged, we doubt not, to be much more satisfactory, we might say, more comprehensible, than to have to believe that our known rocks and stones could be compressed till they were · or even · times heavier than water. at first sight , say , atmospheres or , lb. on the square inch, appears to be an enormous pressure, but it is nearly almost as nothing compared to the pressures we have been dealing with. a column of granite mile high would exert a pressure upon its base of lb. per square inch, and one of miles high of , lb., or double the number of atmospheres we have applied to the gases in the hollow of the earth. if we take a column miles high, such as we considered to be the least that would be necessary to compress granite into one-half of its volume, we get , , lb. per square inch, or over , atmospheres of pressure; and if we go into thinking of columns of and miles--this last being the depth from the surface of the division of the matter of the earth into two equal portions--we could have gases compressed to , and , atmospheres or, dividing the numbers by · , and times the density of water; so there is no cause to stumble over high pressure. with even , atmospheres, more than double the number assumed, we should have gases as heavy as the material we found at the centre of the earth, when we were looking upon it as solid to the centre--which was · times the density of water--and so get rid of burying the precious metals where they would be "matter in the wrong place," and according to d'israeli's definition, justly entitled to the epithet applied to them, sometimes, by people who have never been blessed with a superabundant supply of them. at the same time, we find out what we knew before, viz. that we may have gases heavier than the heaviest metals and as rigid as steel, if we can only find a vessel strong enough to compress them in, along with the means of doing it; and also that the thousands of miles of highly compressed matter, between the hollow centre and the surface of the earth, are far more than sufficient to imprison gases of far, very far, greater elasticity than our modest measure of atmospheres. and we hope to be able to show presently good reason for believing that the gases compressed in the hollow, at what may really be considered as very high pressures, have had, and may probably still have, a very important part to play in the evolution of the earth. we have just seen that the pressure produced by a column of granite mile high would be lb. per square inch, consequently one of double the height, or miles, would exert a pressure of , lb. per square inch at its base, equal to the crushing strain of the very strongest granite we know, while at the same time that strain would not amount to one-sixth of atmospheres; so that if the gases in the hollow of the earth were at a pressure of only atmospheres, their pressures would be able to crush granite of that class to pieces, and therefore the estimate of specific gravity of for the density of the interior surface--which we made at the beginning of our calculations for the hollow sphere--cannot be looked upon as by any means exaggerated. we might now reform our calculations of the two halves of the interior of the earth, giving a more rational and curve-like form to the densities, under the supposition that at much less distance than miles from the surface, matter might be compressed to its utmost limit; but as, according to our demonstration, the solid matter of the earth must have been divided into two equal parts at the place where the greatest mass was, long before it could have been condensed into a state to compress gases; and as the total mass of solid matter must, in order to make up the total mass of the earth, depend to some extent on the mass of imprisoned gases; we are unable to make any reform much different to what our calculations show. besides, as the difference between average densities of · and · makes a difference of , , , cubic miles on the mass of the earth reduced to the density of water, very approximate accuracy cannot be attained in any calculations. what is meant by a limit to density except in so far as it is combined with heat, is that whatever density may be given to matter by compression when it is in a heated state, a greater density will be found in it when it is deprived of that heat; that whatever may be the density of any part of the interior of the earth in its present state, that density will be increased when the earth becomes cooled down to the temperature derived from the heat of the sun, or to absolute zero of temperature, if such there be, on account of shrinking in cooling; and that therefore there can be no absolute limit to density as long as there is any heat in matter. it may not be unnecessary for us to recognise now that the weight of a column of granite would decrease as the depth increased, for the force of gravitation would be diminished by having a part of the attraction of the earth above instead of below it; but at miles in depth the diminution would be only about one-eighth--if distance is taken into account--of the miles down to the plane of greatest density, and / th part if the mass left above is considered; differences that would make extremely little alteration on our calculations. it will not be out of place either to take a look at what may be the temperature of the interior of the shell, and of the gases shut up in the hollow part of the earth; and we have not much to say on the subject, because we shall not depart from the system we have followed up till now, with considerable strictness, of not theorising or speculating on what may be; but will restrict our observations to theories that have been very generally adopted by astronomers, geologists, and scientists in general. the air thermometer will be of no use to us, for whatever may have been the temperature when the earth was in the process of formation, it must have diminished very greatly during the cooling process it has undergone since, and we know that gases heated in a closed vessel in such manner that pressure and temperature will agree to the theory on which the air thermometer is constructed, may be cooled down afterwards to almost any degree required, and the relation between temperature and pressure destroyed thereby. at one time it was thought that the earth had only a solid crust, and that, under it, the whole of the interior was in a molten liquid state. then some physicists thought that, through pressure of superincumbent matter, solidification must have begun at the centre; others that it began almost simultaneously at the surface and centre, and that there may still be a liquid mass between the two solidifications--this is repeating what we have said before, but it is done only to bring it to mind. we, at present at least, do not want to have anything to do with any of these theories, only we believe that we have shown in an indisputable manner that there could be no solidification at the centre, because there could be no matter there capable of being solidified--gases could not be solidified under such pressure, and at all events heat, as there must have been there. we believe at the same time that no one will deny that the heat of the earth increases as the centre is approached, and that the temperature of the interior may be very great. the crust of the earth was at one time supposed to be only to miles thick, because the increase of heat at that depth would be sufficient to melt any of the substances we are acquainted with on the surface--repetition again; but for many years past it has been deemed necessary to increase the thickness to even hundreds of miles, for reasons some of which will be alluded to in due time; and if, even at these depths, the increase of heat were only sufficient to fuse all the substances we know, it is very certain that at the interior surface of the shell it must be very much greater, as heat from there could only be _conducted_ outwards, and the difference required to cause conduction, of any considerable degree of activity, through more than miles must be enormous, according to the experiments made by various physicists upon metals, which have a very much higher conducting power than rocks, and especially strata, of any kind. therefore there can be no doubt, we think, that the inner surface of the shell must be at a very much higher temperature than what would preserve it in its liquid state, and that the matter composing it is liquid to a depth where it might be solidified by the pressure of superincumbent matter. we do not see how convection currents could be instituted, much less kept up, in melted matter, under the viscosity, and, at least quasi-solidity, sure to be produced by pressure of tens of thousands of pounds on the square inch, and therefore we do not take them into account. any way, whatever may be the temperature of the interior surface of the shell, the same must be that of the imprisoned gases, because there convection currents could and must exist--were they even only created by the rotation of the earth and attraction of the moon--and cannot fail to keep the whole of the hollow part at the same temperature. it would be absurd to suppose that these gases could be at a lower temperature than the upper layers, counted from the region of greatest density, of the interior surface of the shell. this section of our work may now be brought to a close by stating the conclusions at which we have arrived, leaving the results involved by them to be discussed separately, which we shall proceed to do immediately without binding ourselves so strictly, as we have done hitherto, to the avoidance of anything that may be looked upon as theorising or speculating. we believe we have conducted our operations in the most strict conformity to the law of attraction, and have no doubts whatever about the form of the interior of the earth resulting from them; but there may be some room for small variations in the details of the various densities, and the position of the interior surface of the shell, arising from the pressure of the gases in the hollow centre, and the weight they will, in consequence, add to the general mass of the earth. the conclusions are as follows:-- ( ) that the earth is not solid to the centre, nor is it possible that it could be, according to the law of attraction, but is a hollow sphere. ( ) that its greatest density must be at the region where the greatest mass of matter is to be found--as must have been always the case from the time it was a globe revolving on its axis, whether gasiform, liquid, or solid--which is now at miles deep from the surface; and that the greatest density may not be much more than the mean of · times that of water ascribed to it by astronomers. ( ) that the inner surface of the shell of the hollow globe cannot be much over or under to miles from the outer surface. ( ) that the hollow part of the globe must be filled by an atmosphere consisting possibly in part of vapours of the chemical elements, and by gases at a very high degree of pressure. ( ) that the region of greatest density, and the position of the interior surface of the shell, may be expressed with very approximate accuracy as follows:--the former must be at · of the mean radius of the earth, and the latter at · of the same; both counted from the centre. ( ) that if the earth is a hollow sphere, the same must be the case with all the major planets and their satellites, the sun, and all the suns, or stars, that are seen in the heavens; and that their interior proportions and form must be in much the same ratios to their radii as those we have found for the earth. chapter xi. page the earth. the idea entertained by some celebrated men, and others. difficulties of forming a sphere out of a lens-shaped nebula. various studies of the earth's interior made for special purposes. difficulty some people find in conceiving how the average density of little over · can be possible, the earth being a hollow sphere. what is gained by its being a hollow shell. geological theories of the interior discussed. volcanoes and earthquakes in relation to the interior. liquid matter on the interior surface of the shell, and gases in the hollow, better means for eruptions than magma layers. focal depths of earthquakes within reach of water, but not of lavas. minute vesicles in granite filled with gases, oxygen and hydrogen, but not water. the moon. a small edition of the earth. rotation stopped. convulsions and cataclysms caused thereby. air, water, vapour driven off thereby to far off hemisphere. liquid matter in hollow interior would gravitate to the inside of the nearest hemisphere. form and dimensions during rotation. altered form after it stopped. agreeing very closely with hansen's "curious theory." consequences of the earth and moon being hollow bodies. _the earth._--the idea that bodies such as those of the solar system, even of the whole universe, have their greatest density where the greatest mass is and are hollow spheres, is so natural and logical, more especially if it is supposed that they have all been formed out of some kind of nebulæ, that it seems strange it has never been brought forward prominently before. we say prominently because we know that the earth has been considered to be a hollow sphere by very eminent men, such as kepler, halley, sir john leslie, and by others of less name long after them. in support of this last remark, we shall make a few extracts--with comment on them--from an article on the "interior of the earth" in "chambers's journal" for february , which have some interest in connection with our work. . "the great astronomer kepler, for instance, in seeking to account for the ebb and flow of the ocean tides, depicted the earth as a living monster, the _earth animal_, whose whalelike mode of breathing occasioned the rise and fall of the ocean in recurring periods of sleeping and waking, dependent on solar time. he even, in his flights of fancy, attributed to the earth animal the possession of a soul having the faculties of memory and imagination." if it could be believed that kepler had any idea of the earth being formed out of a nebula, whether hollow, or solid to the centre, the idea of a breathing animal was almost a consequence, because the attraction--a thing he is supposed to have known nothing about--of the original nebula for the earth one, on matter so light as nebulous matter, would raise enormous tides and make the earth, in its then state, not far from like an enormous primitive bellows made out of goatskins. no one knows what dreams may have passed through his brain. the last part of his notion was altogether fanciful. . "halley was opposed to the idea of the globe being solid, 'regarding it as more worthy of the creator that the earth, like a house of several storeys, should be inhabited both without and within.' for light, too, in the hollow sphere, he thought provision might in some measure be contrived." this notion appears to be altogether fanciful, the fruit of an enthusiastic, exuberant imagination, leaving no trace of scientific thought upon the subject. . "sir john leslie, like halley, conceived the nucleus of the world to be a hollow sphere, but thought it filled, not with inhabitants, but with an assumed 'imponderable matter having an enormous force of expansion.'" it would be interesting to know on what bases he formed his ideas, as the filling of the hollow with imponderable matter seems to show more method than the former cases, but we have never seen any allusion made to his theory anywhere, except in the article we are quoting from. there may have been some reasons given for such a supposition in his "natural philosophy," but when we began to read that work in times long past, a more modern one was recommended to us, and we lost the chance, never to return. there are other theories referred to in the article, but we shall take notice of one more only. . "a certain captain symmes, who lived in the present century, was strongly convinced of the truth of leslie's theory. he held that near the north pole, whence the polar light emanates, was an enormous opening, through which a descent might be made into the hollow sphere, and sent frequent and pressing invitations to a. von humboldt and sir humphrey davy to undertake this subterranean expedition! but these imaginative conceptions must one and all be set aside, and the subject treated on more prosaic, though not less interesting, lines." this conception of captain symmes will probably be looked upon as the most absurd of the whole lot, but to us it seems to give evidence of more thought than any one of them. one would think that he must have formed some notion of how a hollow sphere, with an opening out to the surface at each one of its two poles, could be formed. we must note that he lived in, possibly after, the time of laplace. we doubt whether anyone has ever studied out thoroughly how even a solid sphere could be ultimately elaborated from a nebula. it has always been a very general idea that a condensing and contracting nebula would, under the areolar law, assume the form of a lens rather than of a sphere. if this be so in reality, we may ask: how can the law of attraction produce a sphere out of a lens-shaped mass of rotating vaporous or liquid matter? it seems evident that to bring about such a result attraction must cease to act altogether in the polar directions, and only continue to draw in the matter from the equatorial directions of the lens, till the desired sphere was formed; and, how were the action and inaction of the law of attraction to be regulated meanwhile? or, when the time came that a sphere of a pre-arranged diameter could be formed, a goodly part of the lens must have been cut off and abandoned; in which case we have again to ask: what was done with the surplus, the cuttings? no doubt they could be used up in meteor swarms, comets, or something; but captain symmes's theory has opened up a field for a good deal of thought, and our present knowledge of polar matters prevents us from being sure that strange discoveries may not be made as to the condition of the earth at the poles, although there may not actually be holes into the hollow interior. with regard to the last sentence of the quotation, we fully agree and are doing our best to comply with it. and in so doing, we shall have to return to the formation of globes out of nebulæ, elaborated into something more advanced than even lens-shaped discs. there is no doubt that the reasons assigned by most, if not all, of the authors of the notions above cited are very fanciful, but one can hardly believe that the true reason--why the earth must be hollow--has not occurred to some of them; and that they did not follow it out because it involved too much work, and they did not feel inclined to undertake it, or had not time. on the other hand, modern astronomers and physicists have been so fascinated by the discoveries they have made, and in following them up, that the temptation to go on in the same course has been too great to allow them to spend time on the investigation of sublunary and subterranean affairs. some of them have indeed studied the interior of the earth for special purposes, such as the thickness of the crust, solidity or liquidity, stability, precession of the equinoxes, the action of volcanoes, etc., etc.; but they never, apparently, examined into any of these features to the very end, otherwise, we believe, they would have come long ago to the same conclusion as we have. and withal it seems wonderful how near some of them have come to it. to most people it would appear absurd to think that any part of the earth of any great magnitude can be hollow, if in order to make up its mass its average specific gravity must be · --more especially, if we tell them that the greatest specific gravity at any place need hardly exceed · --forgetting that weight or mass can be taken from the interior where the volume per mile in diameter is small, and be distributed near the exterior where the volume per mile in diameter is comparatively immensely greater. but in whatever light we look upon the conclusions we have arrived at, a change in the construction of the bodies in space from solid to hollow spheres must produce changes in our ideas of them, and have consequences of great importance, too numerous to be all taken account of; we shall, therefore, only take notice of the most prominent. looking at the earth as a hollow sphere, we get rid of the difficulty of conceiving that matter can be compressed to three or four times less than the volume it has as known to us; and also of the misplacement of metals to the incredible degree we have shown to be necessary to make up its whole mass according to the sorting-out theory. and if we can only be bold enough to look upon gases as ponderable matter that can be compressed to great density, and so added to the weight of the whole mass, we may not be under the necessity of compressing the known matter composing it to even the half of its volume. somewhere in the first quarter of this century (see "edinburgh review," january ) mr. hopkins argued that the solid crust of the earth must be at least to miles thick, in order to account for the precession of the equinoxes and nutation, but about a quarter of a century afterwards m. delaunay demonstrated before the french academy by actual experiment that the thickness of the crust had no bearing whatever on the problem. and about the same time lord kelvin inferred from the same thickness of crust that "no continuous liquid vesicle at all approaching to the dimensions of a spheroid miles in diameter could possibly exist in the earth's interior without rendering the phenomena of precession and nutation sensibly different from what they are"; and that the earth, as a whole, must be far more rigid than glass and probably more rigid than steel, "while the interior must be on the whole more rigid, probably many times more rigid, than the upper crust." with the theory of a hollow shell, a better foundation is given for mr. hopkins's argument than a solid crust at about the same depth as he assumed, while at the same time the liquid vesicle of miles in diameter is removed, which lord kelvin showed would change the phenomena of precession and nutation. we have seen that imprisoned gases may have a high degree of density, and consequently rigidity, and may in some measure supply what was required by lord kelvin, who knows, also, very well that a structure with some degree of elasticity in it is stronger than one that is absolutely rigid. moreover, the shell of the earth, composed of solid materials at a very high temperature, and consequently so far plastic, could not fail to accommodate itself to any variation of centrifugal force that could take place. variations in rotation of the earth could only have come on extremely slowly, and even the most rigid matter we know will gradually yield to extreme pressure long continued. but this subject of the plasticity of the most solid part of the interior was discussed and, it may be said, demonstrated during the meeting of the british association of , as reported in "nature" from july to september of that year. any way, the possibility of plasticity is most patently shown by the hollow-sphere construction of the earth. we do not know what were m. delaunay's proofs that the thickness of the crust has no bearing whatever on precession and nutation, but if they were complicated with the fluidity, or even viscosity, of a liquid interior beyond a depth of to miles, they must be entirely changed under the notion of a hollow sphere where there could be no really liquid molten matter, except near the inner surface. one thing we may be certain of, and that is, there must be something to account for precession and nutation, and we believe that the hollow shell, with the greatest density where the mass is greatest, is a much more rational cause for these phenomena than the bulging out of the earth to the extent of miles or so at the equator. it is very difficult to find out what geologists consider to be the nature of the interior of the earth in its details, but for our purpose no particular knowledge is required. however, it is necessary to allude to the principal features of their theories in order to note and remark how far they will agree to, or be facilitated, or the reverse, when applied to a hollow sphere. it would seem that almost all geologists are agreed that the central part is solid, and possibly extremely rigid owing to the enormous pressure of superincumbent matter; that it has a solid crust of several hundreds of miles in thickness; and that under this there is a sub-crust divided into two or more layers of different densities, partially liquid or at all events plastic, extending all over the solid interior matter; the chief purpose for which it is required being apparently to supply matter for volcanic action and surface movements. under the theory we are advocating, the place of greatest density of the interior is calculated to be at miles from the surface, and its greatest approach to solidity will be there also; consequently, if geologists consider that it will have sufficient plasticity there to provide matter for volcanic eruptions, they will be at one with us so far. but should they consider that they require, for volcanoes, matter more liquid than is likely to be found at that depth, they will have to place their magma layers either much deeper or somewhere between that depth and the surface, in which case they will encroach on the requirements of astronomers, without liberating themselves from a difficulty in which they must find themselves involved under their present ideas. they say that these plastic layers exist under the solid crust all round the interior of the earth, so that if one of the duties they have to perform is to keep the various chains of volcanoes in communication with each other, their lateral movements must extend to some hundreds of miles in the cases of the enormous volumes of matter that are sometimes thrown out in even modern eruptions, and they have to provide the means for procuring that lateral motion. shrinkage from cooling, or falling in of part of the solid crust, might bring about these enormous outbursts of lava, but they would be more likely to produce simple overflows than the explosive ejection of such masses as are now being recorded from time to time. we have brought into remembrance, page , that water cannot penetrate into the interior of the earth to a greater depth than miles, more or less, as water, and that beyond that depth it can only exist in the form of steam, or dissociated into its elements of hydrogen and oxygen. as long as it continued in the form of water it could be suddenly flashed into steam, of not far from two thousand times its volume, by relief from pressure or sudden application of heat, and thus be converted into a violent explosive almost instantaneously; but when it came to have the form of a gas, it could only be heated gradually the same as any other gas. it is clear, therefore, that water cannot be looked to for producing the force, explosive or otherwise, that is required to raise even molten matter from depths of hundreds of miles to overflow from the summits or outlets of volcanoes. a pressure of atmospheres would be required to balance a column of average rock of _one mile_ high. a mass of water, through shrinkage of the crust, might get introduced to the vent of a volcano, or some cavity connected with it, a few miles under the surface of the earth and cause an earthquake--it might be introduced by an earthquake--or eruption or both, abundantly formidable and destructive, no doubt, but only comparatively superficial, such as those of naples and charleston, where the extreme depth was calculated to be only a few miles; but it seems to us to be totally inadequate to produce those outpours that last for days and weeks, covering leagues of land, and filling up bays of the sea, with floods of lavas. it may be the principal agent or ally in producing the horrors and devastation of a grand eruption that has invaded the regions of water, but it is not to be conceived as possible that it can be the prime cause. the volumes of steam, water, and mud thrown out on such occasions, only tend to distract our attention from looking deeper for the true cause of the eruption. geologists are therefore thrown back upon their magma layers to look for the motive power for producing these grand eruptions, and they cannot get water down deep enough to do it. tides produced by the sun and moon cannot be appealed to, otherwise the eruptions would be more or less uniform in their periods of occurrence. sudden evolution of gases in the magma layers could not be accounted for in any way known to us, and accumulation of gases would involve the idea of immense cavities, to serve as reservoirs to be gradually filled till the pressure was sufficient to force a way out, and would imply a formation of the interior in compartments specially adapted for particular purposes, and altogether too fanciful to be entertained. where could such enormous masses of matter, as those thrown out, come from at only a few miles from the surface? the great eruption at the sandwich islands, of about a century ago, after flowing over a distance of many miles of land, on which it left enormous quantities of lava, filled up a bay of the sea twenty miles long, and ran out a promontory of three or four miles into the sea; and we cannot conceive it to be possible that such a quantity of matter could be blown out from something less than miles deep by water suddenly flashed into steam. the critical temperature of water--that temperature at which it changes into steam under any pressure however great--being °, its pressure in the state of steam will be somewhere about lb. per square inch, let us say atmospheres; then, if atmospheres are required to balance mile in depth of average rock, as we have stated above, the pressure of steam just cited would balance only - / miles of rock. we can, therefore, see how inadequate it would be to force a column of lava up from even the depth of miles. at that depth atmospheres of pressure are required to balance a column of lava, and there are only available. it has been said that the downward pressure of steam would force up the lava through the vent of a volcano, but an arrangement of that kind would require a downcast shaft as well as the upcast one of the vent like as there are in collieries; but the downcast would have to go very deep to compress the steam--a gas now--to the required number of atmospheres. far more likely that the steam itself would put an end to any increase of water, by driving it back through the channels by which it was descending; for if they are supposed to exist under a solid crust of miles thick the pressure required would be , atmospheres, and with a crust of only miles thick , would be required. the only way, therefore, in which volcanic eruptions can be produced in the earth, if solid or liquid, or partially solid and partially liquid, to the centre--in other words, from magma layers--is by the shrinking of the crust squeezing out the lava. with a hollow earth and shell of more or less miles in thickness, liquid to some depth on the interior surface the difficulty becomes very much less. the communication between the vents of volcanoes would be complete and simple, without any lateral forcing of the lava through magma layers made expressly for the purpose; it would be an open and natural flow from one place to another. that there are such volcano vents connected with each other has been very generally believed, and even almost proved by observation of eruptions taking place in two or more almost simultaneously, or at the least showing signs of violent agitation, the motive forces for which would be the gases which we have concluded must be imprisoned in the hollow centre. when their pressure came to be sufficient to blow or force out the liquid, or semiliquid matter, bubbling and boiling in the vents in constant activity, there would be an eruption, during and after which the gases would escape till their pressure was greatly reduced, when the volcanoes would return to their semi-active state. the gases would naturally be those of the many kinds that are found in eruptions, by reason of their being generated in the earth, mixed with steam and water in the manner we have already shown. let it not be supposed that the gases would require to have force enough to raise lavas from depths of over miles from the surface. according to our arguments for a hollow earth, at miles from the surface the two halves--outer and inner--of the matter composing it meet and balance each other, so that all the pressure required would be what is necessary to overcome the inertia, viscosity, or cohesion of the matter in the vents. what that would be we do not pretend to be able to calculate, but we believe that it would be very much inferior to that required to balance a column of lava of even miles high. we have seen that gas compressed to atmospheres would be - / times more dense than water, and of equal specific gravity to the heaviest matter required in any part of the earth to make up its average density to · that of water, and we cannot assume any greater pressure than this, without diminishing that maximum. if that, or any lesser degree of compression, would supply the necessary force, then all difficulty is removed further than pointing out the means of keeping the volcano vents open or openable; and the quality of openable may be facilitated by the contraction of the interior from cooling. if a greater pressure be necessary, we need not be afraid of greatly increasing it, for the only consequence would be to diminish the maximum density of solid matter required in any part of the earth, to make up the general average to · , which means less compression of the matter. if the idea of the accumulation of gases in the hollow centre, or of the hollow centre itself, is inadmissible, then scientists in general can continue as before with their magma layers--aqueo-igneous if they like--but they must abandon the notion of lavas being expelled from them by steam pressure. we repeat that steam could never get down in the form of steam to the depths they require. the temperature there would be more than sufficient to resolve it into its elements of oxygen and hydrogen, and it would behave very much like the gases we have supposed to be in the hollow; there might be accumulation, but there could be no sudden flashing into existence like steam from water. in support of our observation--if it needs support--that water as water cannot penetrate into the earth to a greater depth than where it meets a temperature of ° we may refer to reports on earthquakes of comparatively recent occurrence. we learn from the "london quarterly review" of january , that in the neapolitan earthquake of , mr. mallett found the greatest focal depth to have been - / geographical, or · statute miles, which agrees very well with the depth to which water could penetrate and be suddenly flashed into steam. (we say nothing, for the present at least, about how the water and the heat managed to meet so instantaneously.) the shock of the instantaneous generation of steam might be felt much lower, but it would tend to interrupt, not to produce, the eruption of lavas. in speaking of the pressure on the walls of the cavity, where the shock was produced, being , millions of tons, the reviewer says, "it may have been greater because the steam might be supposed to have acquired the temperature of the lava," and that is °f.; but that could not well be. in order to meet lava of that temperature the steam would have to descend to from to miles deep; on the other hand, if the lava is assumed to have entered the cavity, it could only do so at a comparatively low velocity and would not reach more than a fraction of the steam at a time, and even for that reason there could be no flashing, as steam is only a gas, and cannot be heated otherwise than as a gas. here the spirit of facilitating the meeting of the lava and the steam, is as apparent as in bringing about the meeting of the water and the lava noticed above. on the whole, therefore, we think that we were right in saying that steam or water cannot be the cause of volcanic eruptions, but that the invasion of the domains of water by the lavas may be the cause, in the main, of the explosive part of eruptions, and of the most disastrous effects of earthquakes. moreover, the focus of the neapolitan earthquake was miles distant from vesuvius, and therefore far removed from anything like direct connection with the vent of the volcano, so that water from it in any form could have no effect upon the magmas of scientists. "the scientific american" of july , , tells us that captain e. d. dalton has calculated that the depth of the charleston earthquake was miles; statute miles, it is to be supposed, as nothing is said to the contrary. to reach the temperature of ° this would give an increase of ° in metres in depth, which is a considerably greater depth than what we have estimated, but does not invalidate our reasoning, as it has always been known that the gradient of increase of heat varies considerably from one place to another. besides, and more especially, charleston being a seaport, and, consequently, not far from the level of the sea, it is to be supposed that, owing to the presence of water, the cooling of the earth has penetrated to a greater depth there than in the heart of italy. the same authority states that in the formidable yokohama earthquake of , the mean depth was only - / miles. the mention of mean depth here makes us notice that the miles may have been the extreme depth to which the earthquake, or shock, was felt at charleston, and that the focal depth may have been considerably higher up than that. be that as it may, there is no proof existing that water or even steam can penetrate into the earth more than a very few miles, much less to hundreds of miles. having referred pretty freely to the aqueo-igneous magmas, supposed by some scientists to exist deep down in the interior of the earth, it is but fair to give our reasons for refusing to believe that there can be any such mixture in any part of it, or anywhere else. in order to do so, we shall first cite some of the bases upon which such ideas have been founded. in "nature" of december , , we find what follows:-- "let us now consider the alternative theory suggested by mr. fisher. he claims that geologists furnish him with a certain amount of positive evidence for the idea that water is an essential constituent of the liquid magma from which the igneous rocks have been derived. passing over the proofs of the existence of water in the crystals of volcanic rocks, and in the materials of deep-seated dykes, let us come at once to the granite, a rock which can only have been formed at great depths and under great pressures, and which often forms large tracts that are supposed to have been subterranean lakes or cisterns of liquid matter in direct communication with still deeper reservoirs. now, all granites contain crystals of quartz, and these crystals include numerous minute cavities which contain water and other liquids; and the quartz of some granites is so full of water-vesicles that mr. clifton ward has said: 'a thousand millions might easily be contained within a cubic inch of quartz, and sometimes the contained water must make up at least per cent. of the whole volume of the containing quartz.' this amount only represents the water that has been as it were, accidentally shut up in the granite, for some was doubtlessly given off in the form of steam which made its way through the surrounding rocks." we cannot follow mr. fisher in "passing over the proofs of the existence of water in the crystals of volcanic rocks and in the materials of deep-seated dykes"; because the presence of water in these crystals when examined in a laboratory is no proof that water was present in them when they were liquid, and before they put on the form of crystals. there is no analogy between them and general wade's read. any crystals that a man can pick up anywhere, even from the mouth of a volcano, are quite capable of absorbing vapour of water from the atmosphere before he can carry them to his laboratory. all matter is supposed to be pervaded, more or less, by the ether, and there is always an open road for it, i.e. the vapour of water to enter by. nature dives more rapidly into a piece of rock than a man can walk or drive down from the summit of a volcano, so that getting water out of it when he is in his laboratory, is no proof that the water was there when the piece of rock was at the bottom, not the mouth, of the volcano. the minute so-called water-vesicles in granite have only served the purpose of a snare to facilitate his deceiving himself, by the help of mr. clifton ward, to further his speculations. for we think it would have been far more natural for him to have supposed that these vesicles were originally filled with the all-pervading ether. or, are we to prohibit the ether from being present anywhere, except where it suits us? even the dimensions given to the vesicles of a thousand millions of them being contained in a cubic inch makes us at once think of something more ethereal than water. and the whole object of mr. fisher's argument is to show how the depth of the ocean may be increased by water expelled from such magmas. a hollow planet, with compressed gases in the centre, raises the idea of the possibility of explosion. it would have furnished olbers, or any follower of his, with the bursting force to shatter into fragments the planet, out of which he supposed the asteroids to have been made. it need not cause any alarm with respect to the earth, whose shell is very much thicker than that of the exploded planet, seeing that its whole mass has been estimated not to have exceeded one-fourth of that of the earth (see table i.). the atmospheres of pressure we have spoken of could have no such effect on so thick a shell as the earth's; and we cannot increase the number without diminishing its average density, as we have shown. when we see mars blown up, whose diameter, and consequent thickness of shell, are not much more than half those of the earth, we may begin to think of getting out of the way. the moon. this satellite is supposed, according to the nebular hypothesis, to have been at one time neither more nor less than a smaller edition of the earth itself, endowed with atmosphere, plains, mountains, volcanoes, rivers, seas, rotary motion, etc.; previous to which it had passed through the same stages of gasiform, molten-liquid, and solid as its parent had done. one would think that its almost perfectly round form proves to demonstration that it must have rotated rapidly on its, or an, axis at one time; but there are some astronomers who think that it has never rotated at all, an opinion in which we cannot concur by any means. when it arrived at the stage of having seas, the tides raised in them by the attraction of the earth must have acted like a brake on its rotation--in the same manner as its attraction is supposed to be now doing on the earth--and gradually reduced it until it ceased altogether; from which time forward it must have always presented the same side to the earth. it has been thought that the tides raised in it by the earth would be so tremendous that they would prevent anything like rotation having ever existed; but everything requires to be accounted for, and the only way to account for its perfectly circular form is by its having rotated. considering, then, the moon as having been dispossessed, absolutely, of rotation and reduced to the single motion of revolution round the earth--as far as we are at present concerned, at least--we can go back to the period when this change came over it, and consider what would happen about the time, and immediately after the rotation came to an end. when a fly-wheel is made to revolve rapidly and is then allowed to run until it stops, it very seldom comes to rest all at once, and generally swings backwards and forwards something like a pendulum, until it finally stops; because it is always a little heavier on one side than the opposite, even should the difference of weight be only that of the handle by which it was set in motion; so we may suppose it would be with the moon when at last it failed to turn the centre, as it is called--the tides, the retarding cause, giving origin to the difference of weight on opposite sides--and we can conceive what commotions would be created on its surface by the wobbles it would make. we can imagine how the seas would rush backwards and forwards over the lower land and hills, levelling them down to the flat plains that are seen spread abroad among the innumerable volcanoes which cover the side turned towards the earth, until it finally came to rest. when the commotions ceased and the centrifugal force of the moon's revolutionary motion round the earth--which is over miles per minute-came to act freely, we know that the atmosphere and seas, being the mobile parts of it, would be pretty nearly all driven off very quickly to the side farthest from the earth, perhaps even before it came to the final state of comparative rest, whose translation would involve mighty rushings of waters there as well. also, that all the liquid matter in its interior, being so much heavier and more difficult to be moved by centrifugal force, would gravitate towards the side nearest the earth, whose attractive force would soon put an end to anything in the form of interior tides of molten matter, which very probably existed up till that period. if the moon came to a stop without any wobbling, then the transference of atmosphere and seas to the farthest off hemisphere, and the gravitation of the liquid matter of the interior to the side nearest to us, might be more gradual but would finally and certainly come to pass. and here we must specially note that if it made one rotation for each revolution, or one rotation in any length of time or under any circumstances whatever, these transferences of matter from one hemisphere to the other could not have taken place, because there would be no stationary region to which they could be transferred by centrifugal force, as each part of its circumference would in its turn occupy that region. and above all--be it specially marked--because the moon would not, in that case, always present its same side to the earth. looking upon the moon as a hollow sphere of somewhat the same proportions as we have made out for the earth, the region of greatest density would be at about miles deep from the outer surface, the interior surface of the shell at the depth of miles, and the hollow centre miles in diameter, as long as it continued to rotate upon its axis. when that motion ceased and the seas were transferred to the hemisphere farthest off from the earth, and the liquid matter in the interior had gravitated towards the nearest, as we have just said above, its conditions would be very materially altered. lest it should be supposed that with a very thin crust, nearly its whole mass would gravitate to the side nearest to the earth, let us always bear in mind that the moon would be virtually solid to not far from the inner surface of the shell, through the pressure of superincumbent matter, both from without and from within, in the same manner as we have considered the earth to be. whatever water had been absorbed by the crust when it was still rotating on its axis--which, at most, could have penetrated only a few miles--and even whatever lakes or inland seas might have been left on the surface always seen by us, would be soon evaporated by the internal heat, and the heat radiated by the sun--which sir john herschel has calculated to be greater than boiling water--and driven off in the shape of vapour in the same manner as the atmosphere had been. these transferences would lead to two consequences, each one of its own nature, which we must not fail to notice particularly, as in great measure they explain to us the constitution, or rather the construction, of the moon. ( ) all air and vaporous matter being translated to the unseen hemisphere would tend to cool it more rapidly and deeply than the other, not only on account of the cooling powers of the water, but from the atmosphere and vapours preventing the heat of the sun from acting so powerfully upon it. ( ) on the other hand, owing to the accumulation of melted, or liquid, matter in the interior of the side now turned permanently towards the earth, the formerly solid part of that side would tend to increase in temperature, which, joined to the heat from the sun not intercepted by any atmosphere, and continuing without interruption for a fortnight at a time, would produce a great difference in the temperatures of the two hemispheres. thus it is natural to suppose that the thicker and cooler solid shell on the one side would tend to weaken and drive down the volcanic forces to a greater depth; while the greater temperature and thinner solid shell on the other, the down side--the one next to the earth--would have an exactly opposite tendency and would bring them nearer to the surface. in this manner we seem to find a very plausible reason for the great exuberance of the volcanic forces displayed on the surface of the moon always presented to us. both the interior construction and exterior form of the moon, as modified by losing its rotary motion, would no doubt be very different to that of a hollow sphere rotating on its axis; but hansen's "curious theory" has prepared us for this, by showing that some anomaly in its construction had been noted and commented upon, although the existence of the anomaly was not attributed to the atmosphere on its having been driven away to the far-off hemisphere. but with this subject we have dealt pretty fully already in chapter ii., which may be referred to for further explanation if required. chapter xii. page some of the results arising from the sun's being a hollow sphere. repetition of the effects of condensation on the temperature of the nebula. ideas called up by the apparently anomalous increase of temperature. how heat is carried from the sun to the earth. the sun supposed to radiate heat only to bodies that can receive and hold it, and not to all space. the heat of the sun accumulated in a hot box to considerably beyond the boiling point of water. the heat accumulated in this way supposed to be due to a peculiar function of the ether, as it is a fact that heat can be radiated from a cold to a hot body. the sun must be gaseous, or rather gasiform, throughout. no matter in it solid or even liquid. divisions and densities of shell. the hollow centre filled with gases, whose mass naturally diminishes the mean density of the whole body. the amount of this reduction so far defined. the presence of gases or vapours in the hollow a natural result of condensation. the hollow centre filled with gases not incompatible with the sun's being a hollow sphere. the temperature at the centre may be anything, not depending on any law of gases. further exposition of hollow-sphere theory put off till after further development of the construction of the sun. in the last chapter we have endeavoured to point out how much our knowledge of the interior construction of the earth and moon has been increased, and how many difficulties in the comprehension of their construction are overcome by the fact demonstrated in previous parts of our work that they are hollow bodies; and we now proceed to show some part of what may be learned from studying the sun under the same conception of its being a hollow body. we say part of what may be learned, because the whole seems to us to be so great that it would take much more time and space, not to speak of knowledge, than we can devote to the subject to make even a proper beginning to such a study. to our sight it takes away the necessity for guessing in the dark at what the construction may be, which is all that has hitherto been done; and furnishes the means of discovering, with intelligent study and investigation, what most probably is the actual constitution of the sun. in chapters v. and vii. we have followed up the contraction and condensation of the residue of the original nebula, after it had thrown off all the known planets; first, to the diameter of , , miles, with density of / th of an atmosphere and temperature of - °, or _one degree_ of absolute temperature; second, to about , , miles diameter, with density equal to air at atmospheric pressure, and temperature represented by zero of the centigrade scale, or what has been hitherto called ° of absolute temperature; third, to , , miles diameter, with density equal to ten atmospheres and temperature of ° of actual, or ° of absolute temperature; and fourth, to , miles diameter, with density equal to water and temperature which we do not venture to express. all these stated densities and temperatures are understood to be average, the temperatures being those the various stages would have had, had no heat been radiated into space by them. here, then, we might go on to set forth what might be the interior dimensions, various densities, and conditions of each one of the four stages, under the conception of their being all hollow spheres, and afterwards carry on a _résumé_ of the whole of them and apply it to the sun as it is at the present day; but this, in addition to involving an immense deal of difficult work, subject to errors and omissions in operation, would not do much towards enabling us to explain in a more simple way what may be, most probably is, its interior construction. we shall, therefore, look upon the four stages as represented by a model having the diameter and other known measurements of the sun in its present state. to begin what we propose to do we believe it is necessary to repeat, as a thing that has to be borne in mind, that when we had contracted the original nebula from , , , miles in diameter to , , miles, its density was only equal to a barometric pressure of _one-ninth_ of an inch of mercury, and its mean temperature had been increased only _one degree_, that is, from - ° to - °; and we can add that, although we had given the original nebula ten times that diameter, the result both in density and temperature would have been the same when it was condensed to , , miles in diameter. then, again, we believe it necessary to repeat that by contracting the nebula from , , to , , miles in diameter its mean density was raised from / th to full atmospheric pressure, and its mean temperature from - ° to zero of the ordinary centigrade scale, i.e. to the temperature of freezing water. these two results strike us, at first sight, as somewhat remarkable, seeing that what looks like almost unlimited condensation to , , miles diameter produced only one degree of temperature, while the comparatively insignificant condensation of from , , to , , miles in diameter produced ° of heat, _in the way we are accustomed to measure heat_. following up these two facts gives rise to ideas that have been borne in upon us ever since we stumbled upon them when making the analysis of the nebular hypothesis. one of these notions was that, were it practicable, the most effectual mode of liquefying gases would be by putting any one of them into a sealed vessel, and confining it in another vessel in which a vacuum of / th part of an atmosphere could be produced; no difficult matter as far as the vacuum is concerned, for a good exhausting air-pump would be all that is required. but the practicability? the vessel in which the vacuum is produced would have to be protected so that no extraneous heat could be conveyed or conducted into it in any way whatever. how this could be, or is, done without cutting off every possibility of manipulating the enclosed vessel, we do not see; but it seems evident that some method is available because something presenting the same difficulties has been actually done, as everybody knows. the only degree of vacuum of any use in the exterior vessel would be about one-ninth of an inch of mercury, because that would as we have just said, furnish a temperature of - °. there would be no necessity for applying pressure to the gas experimented upon. in fact pressure would be an obstacle to the experiment, according to the theory of the air thermometer; and could only be of use by furnishing a larger quantity of liquid to be handled and examined. another idea is that there can be no such condition as absolute zero of temperature of what we are accustomed to think of as a gas, as far as science is concerned; as on arriving at that condition, perhaps long before, any gas would slip out of its hands altogether. but there is a much more rational reason than this, which we have brought forward on a former occasion. we are taught that heat is a mode of motion, which means that as long as there is heat there will be motion to account for it, so that motion would have to be annihilated on the earth before absolute zero of temperature could be reached. we have, then, to come back to what we said when treating of the heat of space, and look upon the temperature of the vibration of the ether as being the lowest that can be measured by science. we said then that it must be far below - °. since then a temperature has been reached of within ° or ° of absolute zero, according as that condition is measured by or . this, of course, leads us to think of the ether as a carrier of light, heat, etc., and of how it can carry heat to the earth without becoming heated itself, as there can be no doubt about its being a material substance. how it can bring what may be called considerable heat to the earth and still have little or no heat in itself; even should it turn out, which we do not believe possible, that the estimates of the heat of space of - ° and - °, made about the beginning of this century by sir john herschel and pouillet, turn out to be near the truth. we have seen, in "nature" of july , , a monograph by captain ericsson, in which he shows that the heat radiated by the sun to where his rays strike our atmosphere is somewhere about °f., and it is not easy to see how radiated heat can be transmitted through million miles of space at a temperature of much lower than - °, and reach the confines of our atmosphere with the heat of °f. there is one supposition that occurs to us under which this can happen, and that is, that the sun only radiates heat to bodies which can receive it, and does not radiate it into all space where there is nothing but the ether to hold it. this, of course, implies that the ether acts the same part--the part for which it was really invented--with respect to heat that a telegraph wire does with respect to electricity; in which case, we could imagine that it starts from the sun with the maximum heat radiated by him, and that this goes on decreasing in the ratio of the square of the distance it travels through, the same as is understood to be the case with all radiated heat; and that the part of space not occupied, for the time necessary, by these connexions might be supposed to form the return current which we believe must exist, just the same as the earth does for electricity. for that there is a return current is demonstrated by the fact that the earth radiates heat into space when the sun is not shining upon it. again, even in this case, we have another difficulty thrown upon us, over and above that cited by captain ericsson, of the heat delivered at the bounds of our atmosphere being about °f., by our being informed in "engineering" of december , , that: "a hot box, contrived to observe the temperature which could be attained by the unconcentrated solar rays, was used on mount whitney, , feet above the sea"--well within the limits of our atmosphere--"and that the enclosed thermometer rose to · °f. on september , p.m., , the shade thermometer then reading · °f." how are we to comprehend these two facts? we have seen a way of getting over part of the first fact as far as to the boundary of our atmosphere, but from there we have to carry °f. to the top of mount whitney, through the atmosphere there and present it along with the other lot in the hot box at · °f. we may get the beginning of what may be an explanation of all the facts from another part of captain ericsson's monograph, where he says: "engineers of great experience in the application of heat for the production of motive power and other purposes deny that the temperature of a body can be increased by the application of heat of a lower degree than that of the body whose temperature we desire to augment." the soundness of their reasoning is apparently incontrovertible, yet the temperature of the mercury in the instrument just described raised to °f. by means of the parabolic reflector, increases at once when solar heat is admitted through the circular apertures, although the sun's radiant intensity at the time may not reach one-tenth of the stated temperature. it should be mentioned that the trial of this new pyrheliometer has not been concluded, owing to very unfavourable atmospheric conditions since its completion. for our present purpose the great fact established by the illustrated instrument is sufficient, namely, that the previous temperature of a body exposed to the sun's radiant heat is immaterial. the augmentation of temperature resulting from exposure to the sun, the pyrheliometer shows, depends upon the intensity of the sun's rays. a little study shows us that the steam engineers are perfectly right in their doctrine. the heat of steam can only be called a variety of the temperature of water. at lb. pressure per square inch the heat of steam is · °f., while at lb. pressure it is only · °f., and therefore the steam engineer has good reason to say that steam at the lower pressure--or derived from heat that can only produce that pressure--can add no heat to the higher; on the contrary, the only possible means of applying the heat of the lower to that of the other would be by mixing them, and we know what the result of that would be. this brings before us the fact that the steam engineer's heat is very limited, and can only be communicated in certain ways, while the sun's heat is comparatively unlimited, and can only be communicated to anything through the medium of the ether. but it probably teaches more than that. were the engineer's heat unlimited in quantity at low pressure it can easily be believed that it could be transmitted to another body at any temperature by radiation, the same as it is radiated from the sun to a hot box; but it is not, and we thus seem to find that radiation is a mode, possessed by the ether alone, of conveying heat from one body to another. it has nothing whatever to do with mixing, conduction, convection, or anything, except in so far as the ether is mixed in a more or less limited quantity with all matter. in support of this idea we can refer to professor tait's treatise on heat, where we find it stated that "heat does pass (though on an infinitesimal scale) from colder to hotter bodies"; and we can easily understand that the infinitesimal quantity so passed is due to the comparatively infinitesimal quantity of ether there is in either of the two bodies to perform the work of transference. professor tait has not told us how heat is carried from a cold to a hot body, but there can be no doubt about its being a function of the ether which can only be found out by a careful and analytical study of that agent. such a study we propose to undertake presently without much expectation of being successful, but still with the hope of helping in some measure to find out how the ether operates. meanwhile we shall return to what we had begun to say about the sun being a hollow sphere, and to our proposal to treat of the nebula contracted from million miles to its present diameter, as if it were a model representing a _résumé_ of all the effects produced on the nebula by that amount of condensation. we know from all our work that the sun must be a gasiform body, which means that all the cosmic matter contained in it must be in the form of vapour, even although its consistence should outrival a london fog--notwithstanding that some physicists have supposed that it may be solid at the centre through extreme pressure--and it is not altogether correct to compare its construction to that of a solid body such as the earth; but as we have no other we shall begin to make a comparison with it, which, it will be found, can lead us into no appreciable error. considering then the sun to be , miles in diameter, with mean density of · that of water, the hollow part being still completely empty, and applying to it the same proportion we have deduced for the earth, we find that the region of greatest density would be at · of the radius of the sphere--a proportion really derived from the line of division into two equal parts of the volume of a sphere--from the centre, or , miles from the surface; and the inner surface of the shell at · --a proportion derived from our calculations for the earth--of the radius of , miles, or , miles from the centre; which in turn makes the shell to be , miles thick, and the hollow centre to be , miles in diameter. on the other hand, still following the proportions derived from the earth, we find that the density at the surface might be one-third of the mean density or · ; that it might be one-fifth greater than the mean, or · at the region of greatest density and one-half, or · at the inner surface of the shell--all of these three densities being in terms of water. now, the hollow centre of , miles in diameter would have a volume of one-sixth of the whole volume of the sun, which, filled with gases, would diminish all these densities just in proportion to what may be considered the degree of compression and condensation the gases might be subjected to. that there should be gases in the interior hardly requires to be more than stated, as there can be no doubt that the degree of heat to which the shell had arrived by the time it came to have the dimensions above mentioned, would be amply sufficient to excite chemical action among the elements of which the sun is composed; and the gases or vapours produced by that action would flow as naturally towards the interior of the hollow centre as towards the space beyond the outer surface of the shell, until they were stopped by increase of pressure, which of course would mean increase of density in this case. we see then that if the hollow centre has a volume of one-sixth of the whole volume of the sun and we multiply this volume by , we have a mass equal to the whole mass of the sun, were its mean density only the same as that of water. consequently, if we multiply the said volume by and by · , that is by · , we get a mass equal to the whole mass of the sun at its known mean density. again, were we to suppose the hollow centre to be filled with gases of the same specific gravity of air, condensed to a pressure of atmospheres--which would correspond in density to · times the density of water--we should have in the hollow centre alone a mass equal to another sun, in addition to the one made up by the dimensions and densities stated above. we see then that if we fill the hollow centre with gases at the pressure, and with the density just stated, we have a sun of twice the mass it should be. but if we leave the specified gases in the hollow with one-half of the above density, and deduct the equivalent mass of the other half density of the gases from the shell, as estimated for the hollow centre, we should have a sun of the mass required by astronomy. in this way we should have the three specified densities reduced from · , · and · to · , · and · , for the outer surface, the region of greatest density, and the inner surface of the shell, respectively; and the pressure and density of the gases in the hollow centre reduced to atmospheres. thus, from what has just been shown, which at first sight may be thought very irrelevant matter, we discover that it is not necessary that there should be any matter in the sun even so dense as water. and still we have to think of what an insignificant pressure three or four thousand atmospheres would be in the centre of the sun. no one will pretend to allege that no gases can be produced in the shell of the sun, or to say anything against those formed in the inner half of it finding their way to the hollow centre, and going on increasing there till they were able to force their way out through the shell; that is, until their pressure was equal to the resistance offered by the gaseous body of the sun, or against their temperature increasing until it came to correspond to their density and most probably rising to a much higher degree. such, then, must even now be the construction of the sun, as reduced to its present diameter and density. that is, a hollow sphere consisting of cosmic matter combined with gases and having a hollow centre filled with chemically formed gases or vapours. here it may be argued that the sun ceases to be a hollow sphere, but that is not so. the most that can be said about it is that it is a hollow sphere with the empty part filled up. it would only be in much the same condition as a hollow globe of iron filled with melted antimony or bismuth. its construction would be in no way changed by the empty hollow being filled up, so long as its condition remained gaseous--not changed to liquid or solid. the only difference in our sphere would be that its density would virtually be the same from what we have called the region of greatest density to the centre, which would not only involve a greater distance of that region from the surface of the sphere, but another reduction of the above mentioned densities of the sun; for we cannot in any way imagine that the pressure in its interior can be less than many thousands of atmospheres. whatever may be the relative densities of the shell and the gases in the hollow, they will have no necessary effect upon the temperature of the latter, because, let the densities be what they may, the gases might be cooled down to absolute zero of temperature, or raised to any imaginary degree without any change being made in their weight as long as their volume was maintained the same. this has been proved by laboratory experiments almost as far as possible. gases at very high degrees of pressure and consequent densities have been cooled down to not far from the absolute zero of temperature, while others under very low pressures have been heated up to nearly as great heat as the enclosing vessel would bear, without their weight being altered in either case; but in the sun there is a larger laboratory in which we can place no limit to pressure or temperature. we know, however, that pressures are required sufficiently great to blow out jet prominences with velocities of , miles per second or more, to heights , and even , miles above the photosphere; and if we knew what these pressures are we might be able to learn something about the minimum temperatures of the gases. to obtain these pressures we have--in the construction we are advocating--a real containing receptacle with sides , miles thick, in the outer half of which we have the compressing force, due to the gravitation of the whole mass of the sun acting at the centre, and over and above, both in it and the inner half, we have the cohesive force of the matter of which it is composed. in fact we have a sun whose construction we can understand, in which we have gases shut up without their expansive forces being impaired in any way, ready to be exerted with full energy whenever they are relieved from compression by any commotions in any part of the whole body, and taking their part in keeping the whole of the matter composing it in constant motion. how these commotions are produced it is not difficult to explain to a very considerable extent at least, but this we must leave over until we have reconstructed the original nebula, and shown how the solar system could be elaborated from it, almost exactly in the way conceived by laplace in his nebular hypothesis. we shall then also be able to extend our exposition of what is to be learnt from our mode of construction, and to still further reduce our estimate of the mean density of the sun. meanwhile we have to go into another long digression, with the view of trying to find out something about what the nature of the ether is or may be, which we think to be quite necessary before we go any farther. chapter xiii. page the ether. its nature considered. behaves like a gas. can be pumped out of a receiver. light and heat do not pass through a tube _in vacuo_. laboratory experiments examined. light and darkness in a partial vacuum, though high. electricity not a carrying agent. why there are light and dark strata in a high vacuum. the real carrying agent through a high vacuum is the residue of ether left in it. digression to consider the aurora. how air may be carried to extraordinary heights. zones of air carried up are made luminous by electricity. comparison of this method with experiments quoted. experiment suggested to prove whether light passes freely through a vacuum tube. the ether does not pervade all bodies freely. it must be renounced altogether or acknowledged to be a material body, subject to expansion, condensation, heating or cooling. how light and heat pass through glass. temperature of the ether variable. zodiacal light, cause of. the ether a material substance, proved by its behaviour. we have said in a former part of this work, pages and following, that if the ether is capable of performing all the functions that are attributed to it, it must have some consistence or substance of some kind; that it must be matter of some kind in some form, and consequently must have density in some degree however low; and we might, for the same reasons, suppose that it must have some temperature; but as long as we believe that without motion there can be no heat, we cannot conceive it to have any temperature. no doubt we might suppose it to be in a constant state of vibration, and to have the temperature corresponding to that state, whatever that may be; but this, in addition to leaving us just where we were, would only entail upon us the task of supplying temperature as well as density to a body of whose existence no positive proof has hitherto been given, whatever we may believe about it. at the same time, the evident necessity of taking its temperature into consideration, seems to supply another reason for concluding that it is a material substance, over and above those we have cited now and before. the general belief regarding the ether has been, ever since it was invented, that it is a substance of some kind (imponderable and impalpable?) which fills and pervades all space and matter; but a little consideration will show that this belief requires to be modified. the ether is supposed to be the connecting link of the universe, and the agent for carrying light, heat, electricity, and magnetism from the sun to the earth and planets, and all over space; but it has been found that electricity will not pass through a vacuum, such as has been produced by experimenters, unless it be with a very powerful current. this, of course, would seem to prove that there must be almost no ether in such a vacuum; because if there was ether in it, of the same density as there is in space, electricity would pass through it with the same ease as it does from one body to another on the earth or in space; it would seem, also, to justify us in inferring that electricity would not pass through an absolute vacuum at all, however powerful the current might be, because there would be absolutely no ether to carry it; and, likewise, that the quantity of ether remaining in the experimenter's receiver had as much to do with the passing of a very powerful current of electricity through it--perhaps a great deal more--as the small quantity of air, or gas, or dust not altogether exhausted from it, to which the experimenters attribute its passage. moreover, it would appear that when air or any gas is pumped out of a receiver, the ether mixed with it is pumped out along with it; consequently it must be a material, tangible substance, possessing density in some degree, however low it may be. here, then, we have, it would appear, proof positive that there is such a carrying substance as the ether has been supposed to be. it is a thing which we have not to conceive of, fabricate, or build up in our minds. it is a thing we can pump out of a tube, and is as much a material substance, in that respect, as air or any other gas that is as invisible as itself--yet nevertheless in the tube until it is pumped out. against this idea of the nature of the ether, and what may be done with it, it may be argued that light and heat pass freely through a tube or receiver _in vacuo_, when electricity refuses to pass; but are we sure that they do pass? it would be a much more difficult matter to prove that they do, than to prove that electricity does not, because our eyesight gives us evidence in the latter case. besides, there are facts which, when thoroughly looked into, induce us to believe that light actually does disappear gradually from a vacuum as it is being formed. in an article on "the northern lights," in "science for all," vol. ii., reference is made to a well-known laboratory experiment in the following words: "we take a glass cylinder, covered at the ends with brass caps, one of which is fitted with a stop-cock, which we can screw to the plate of an air-pump. to the brass caps we now attach the terminals of a powerful induction coil, but as yet we perceive no result. we now begin to exhaust the air from the cylinder, and as the exhaustion goes on we soon see a soft tremulous light beginning to play about the ends of the cylinder; and this, when the air is sufficiently rarefied, gradually extends right through the cylinder. as we continue the exhaustion these phenomena will be reversed, the light gradually dying away as the exhaustion increases. we shall at once perceive how very much this resembles an aurora on a small scale, and so we have electricity suggested to us as the agent which produces the aurora." farther on in the same article we find that: "aurora displays usually take place at a great height--sometimes so high as miles--while their average height is over miles. at such heights the air must be extremely rarefied, and we should be disposed to expect that the electric discharge could not take place through it." now, at the beginning of this experiment, it must be granted that light was passing freely through the glass cylinder from side to side, and also that, when the electric current was turned on, the electricity was passing freely through the air in the cylinder though it was not visible. it could not pass through the glass on account of its being a non-conductor. then, when the air had been partially exhausted from the cylinder, and the "soft tremulous light" began to appear about its ends, it is clear that some interference with, or change in, the free passage of light through it must have been produced, both transversely and longitudinally, which occasioned the difference in the appearance of the light and caused its tremulous motion. and as change in the appearance of the light extended through the length of the cylinder as the exhaustion increased, and finally died away--light, change and all--when it approached more nearly that of an absolute vacuum, we cannot help concluding that the light disappeared because there was no medium left in the cylinder, of sufficient density at least, through which it could pass; which, of course, means that light cannot pass through a vacuum any more than electricity can. the experiment we have cited above may be considered antiquated, but similar results are presented to us in professor balfour stewart's "elementary physics," where he says at page of the reprint of : "another peculiarity of the current is the stratification of the light which is given out when it traverses a gas or vapour of very small pressure. we have a series of zones alternately light and dark, which occasionally present a display of colours. these stratifications have been much studied by gassiot and others, and are found to depend upon the nature of the substance in the tube." [the ether?] "if, however, the vacuum be a perfect one, gassiot has found that the most powerful current is unable to pass through any considerable length of such a tube." [in passing, we take the opportunity to assert, with confidence, that there can be no perfect vacuum on the earth.] here we see the gas or vapour in the tube divided into zones alternately light and dark, which occasionally present a display of colours, and are led to infer, from the colours depending upon the nature of the substance in the tube, that they disappear altogether when the exhaustion is sufficiently great; and are finally told that the most powerful current is unable to pass through such a tube of any considerable length. in this case also, we can say with perfect confidence that there can be no ether left in the tube, in sufficient quantity, or else it would be able to carry the electricity through it much more easily than from the sun to the earth, or from one part of the earth to another. if we refuse to acknowledge that the ether has been removed from the tube or cylinder, we are forced to conclude that it is not the carrying agent, for which alone it has been called into existence by the imagination of scientists; and we have to invent new theories, new methods for explaining what we have been accustomed to think we thoroughly understood. we have to look for a new dog to carry and fetch. furthermore, all that has been said about electricity is equally applicable to light, whether we can prove it or not. if light could pass freely through the experimental cylinder from side to side, as it was certainly doing before the exhaustion was begun, we cannot understand why there should be, first tremulous light which finally disappeared, and why dark strata were displayed in it by the forced passage of electricity; unless it was that the carrier of the light was removed, and then we naturally think of why there should be dark strata in the tube. we can understand electricity lighting up darkness, but not its darkening light--it lightens up midday--and we must conclude that both the one and the other were driven through the cylinder, or similarly conducted through it, by the same force, or were left behind. following up the quotations we have already made from "science for all," vol. ii., we now add another for further illustration of what we have been saying, to wit: "let us now return to the laboratory, and see whether we can make any experiment which will throw light upon this difficulty. if we send the electric discharge through one of the so-called vacuum tubes--choosing one which consists, through part of its length, of tube which is much narrower than the main portion--we find that when the discharge is passing the pressure is greater in the narrow part of the tube, showing that in some way gas is being carried along by means of the current, and professor a. s. herschel suggests that in some similar way air may be electrically carried up to these great heights." this quotation, of course, refers to the northern lights, but it serves to illustrate what we are seeking to show with respect to the ether. in this experiment, the explanation of the pressure being greater in the narrow part of the tube, is exactly the same as that for water passing through a conduit which is narrower at one place than another. the same quantity of water has to pass through the narrow as through the wide part, consequently the velocity and pressure (head) have to be greater than in the wide part--the water arranges that for itself; and the seeming difficulty of explanation arose from the idea "that in some way gas is (was) being carried along by the current," when it was only the gas that was being lighted up more vividly by the electricity passing through it, because the same amount of electricity had to be carried through the narrow part as the wide one. no portion of the gas could be carried along with the electricity, else it would very soon have been all accumulated at one end of the tube, or a reverse current must have been set up to restore the balance, which would speedily have shown itself. had the said tube been filled with copper instead of gas, the experimenter must have known that the electricity, in passing through it, would have spread itself all through the wide part, and contracted itself to pass through the narrow part, spreading itself out again through the other wide part, thus giving rise to differences of pressures and velocities at the different widths of the tube; but, of course, he would not have been able to see this, because the electricity could hardly be in sufficient quantity to light up the copper, or to impart to it sufficient heat to make it visible. neither would the electricity carry with it part, or the whole, of the copper when passing through the narrow part. it would be the gas lighted up more vividly, not set in motion, by the electricity that the operator saw in the experiment under discussion, and, no doubt, if the tube had been sufficiently exhausted of gas, the light would have disappeared the same as in the first quoted experiment, and the electricity would have ceased to pass because there was nothing, in sufficient quantity at least, to carry it along, not even the universally commissioned monopolist the ether. let us ask here: does not all this seem to prove that electricity is a carried, not a carrying, agent? in the quotation made, at page , from "elementary physics," we are told that when electricity passes through a gas or vapour of very small pressure, "we have a series of zones alternately light and dark." now we ask, why should part of these zones be dark? and the only answer to be given is--simply because there is no light in them, nothing in them to carry or hold light. otherwise, we cannot understand why they should appear to be dark. we cannot imagine a glass tube with light and dark zones in it longitudinally--we have understood the zone to be longitudinal; transverse sections would not be zones--at the same time that light is passing freely through it transversely, i.e. from side to side, unless it is that in the dark zones there is nothing, not even the all-pervading ether, to carry or hold light in; therefore, we conclude again that there is no light where there is no ether. for an explanation of the existence of light and dark zones in the almost exhausted cylinder or tube, we refer to professor tait's treatise on "heat," where he says, in section , "what happens at exceedingly small pressures is not certainly known. in fact, if the kinetic gas theory be true, a gas whose volume is immensely increased, cannot in any strict sense be said to have one definite pressure throughout. at any instant there would be here and there isolated impacts on widely different portions of the walls of the containing vessel, instead of that close and continuous bombardment which (to our coarse senses) appears as uniform and constant pressure." admitting the truth of the kinetic theory of gases, we can see that in a vacuum so rare that only electricity at a very high pressure could be forced (carried?) through it, we have the prescribed conditions in which there cannot be "one definite pressure throughout" the whole tube; in other words, we shall have some places in a vacuum tube where there is no gas at all, or perhaps even ether, and others where the gas is so rare that it takes a powerful stream of electricity to light it up in passing through, whether the lighted-up zones be composed of gas, or of ether, or part of both. if it did not pass, there would be no light-streak even. and further, we have to notice that the light and dark streaks would be changing places constantly, owing to the collisions of the small number of atoms or molecules of the gas, still not exhausted from the tube, driving each other from place to place. all this makes us think of what is the real carrier of electricity through a partial vacuum, through a gas, or through a substance of any kind whatever, and we can only imagine it to be the ether. in that case the conductivity of any substance would depend upon the quantity of ether contained in it, and we can give no other reason for there being conductors and non-conductors of electricity. all matter has been thought to be pervaded by the ether, but we have said before that this must be the case in a limited sense only. it can be shown that glass is permeable to ether, and is therefore not an absolute non-conductor. metals are supposed to consist of atoms bombarding and revolving around each other under the control of ether. intermediate conductors may have the quantity in them of ether corresponding to their conductivity; and the compressibility of water, or any liquid, may depend upon the quantity of the ether mixed with its ultimate atoms. although we consider it to be going rather beyond the course we had laid out for ourselves, we cannot help returning to the article on the "northern lights" in "science for all," quoted above in connection with electricity in the presence of a vacuum; because it helps to illustrate the subject we are dealing with. in the regions where these lights are seen, we know that there can be no want of ether, because it is supposed to pervade all space; but we know that there must be a very great want of air, or vapour of any kind, due to the height above the earth at which they are seen. here, then, we have a great field for differences of pressures being caused all through it, by the collisions among themselves of the molecules or atoms of the extremely attenuated air; we have the higher or lower pressed zones of the laboratory experiment spread out before us, and if we suppose currents of electricity to be passed through them, we have an aurora in the high heavens, a counterpart of what was seen in the vacuum tube. the bombardment of the molecules continually shifting their positions and creating zones of different pressures, when lighted up by electricity, would easily account for the flashes, coruscations, and changes of the aurora; but, how does the air get up so high as is stated in the quotation at page ? we cannot accept the supposition of professor a. s. herschel that the air is carried up to the height of from to miles by electricity. we must believe, till evidence is given to the contrary, that electricity is a carried, not a carrying, power. conductors of sound are all material substances; sound is not. it seems logical, therefore, to conclude that the ether is a material substance, because it conducts light, heat, etc. etc., which are not material substances. proof is therefore required that electricity is a material substance, before it can be called a carrier. that air does somehow get up so high there can be little doubt, as is satisfactorily proved by the burning of meteorites when they come into our atmosphere at heights said to be more than miles. how it does mount up so high is not so wonderful as it seems, when we take into consideration the causes of the trade winds, which are: the upward currents of the air created by the heat of the sun; the centrifugal force inherent in it at the time of leaving the earth; and its angular motion, which may be, at a guess, from to miles per minute, seeing that the equator has an angular velocity of over miles per hour. then, from the time it leaves the earth, the air must begin to lose its angular velocity, the impelling power being cut off, and form a bank higher up, opposing the motion forward of all the air following it, so that immediately above the tropics there must be forward motion and obstruction, producing whirlwinds of which we can see or know really nothing, though they must exist, and which may carry air or vapours up to very great heights, carrying with them densities far beyond what would correspond to the simple attraction of the earth. at these heights this attraction would be very much diminished, and almost the only way in which the density of the whirlwinds could be diminished would be by expansion, which would not be very active in bodies already very considerably attenuated, as the whirlwinds would naturally be. their movement towards the poles would be the same as that of the trade winds has always been supposed to be; and we can now see how there can be air at great heights in the aurora regions, not carried up by electricity. in fact, the air may, or rather must, have carried the electricity up with it, as we shall, we believe, presently see. we have not supposed that all the air, raised from the earth by the heat of the sun, is carried up to such altitudes and to its polar regions, but only a very small part of it; and we have to add that there is perhaps not always electricity present in sufficient quantity to illuminate the air when it is carried up, which would, from the nature of its ascent, be undoubtedly divided into zones, streams, or belts at different degrees of tenuity. we do not doubt, or rather we believe, that electricity is always present in the atmosphere; but we are not sure that it is always so in sufficient force to make itself manifest. a very homely example of this is: stroke a cat's back in ordinary circumstances, and it will only arch it up in recognition of the caress; but stroke it on a frosty night and it will emit sparks of electricity. the cat's hair does not shine--perhaps fortunately for the cat--because the electricity in it is not present in sufficient force, and only shows itself when the hand acting like a brush collects it into sparks. this shows not only that electricity is more abundant in the air at one time than at another, but that it is more so in cold and dry than in warm and moist air. it also shows one of the reasons why auroras of great brilliancy and extent are not continually in play in their own special regions, which is the want of a sufficient supply of electricity; another reason being, the absence of the requisite zones, or masses of air in cyclonic motion at different pressure and in sufficient quantity. we understand from what we have read that the glow of the aurora is seldom awanting in clear weather in the far north, and can imagine that there is always a sufficient supply of electricity and attenuated air to maintain the glow constantly; and also that the brilliant displays are only made when there is a sufficient influx of whirlwinds of air at low and varying pressures, and of electricity in sufficient force to light them up. we should suppose that the bright flashes would take place where the pressure was greatest, and the illuminated darkness, so to speak, where it was least. electricity does not carry up air to these heights, neither does magnetism bring it down from the sun; still a magnetic storm produces brilliant auroras. confronting these reflections with the laboratory experiment we have cited at page , we see that they are very fully confirmed by it; perhaps it would be more true to say that they were originated by it. when the current of electricity was first turned into the glass cylinder, no result was perceived. this must undoubtedly be construed into showing that the light in the cylinder, passing through it from side to side, was more powerful than the diffused light of the electricity passing through it from end to end; which was the reason why there was no result. by diffused, we mean that the electricity, turned into the cylinder through a thin wire, would immediately spread out over the whole of its width (or cross section) and thus very much weaken its light-giving power. when exhaustion had proceeded to a sufficient extent to produce the soft tremulous light, we can only conceive that the transverse light had decreased so far that the diffused light of the electricity, passing longitudinally, had begun to balance it, which caused the tremulous appearance on account of the one beginning to disappear and the other to take its place. and when the light extended through the whole length of the cylinder and the phenomena were reversed; and when the light died away altogether, when the vacuum became sufficiently pronounced; we can only believe that there was no light at all in it; neither natural light passing through it transversely, nor light of electricity passing longitudinally. should any one object to this demonstration, as we may call it, we refer him to the quotation, made at page , from professor balfour stewart's "elementary physics," and ask him, how could there be dark zones in a tube, through which light ought to pass freely from side to side? the thing appears to be tremendously absurd. there were dark streaks in the tube and other streaks of gas, or vapour of some kind at very low pressures (see also quotation from professor tait at page ) that were lighted up to some extent by the current of electricity, but even these died away. we do not pretend to impugn the idea that the stratification of light and dark zones depended upon the nature of the substances in the tube, we only want to insist that the substances left in it were so extremely rare that electricity could not pass freely through it longitudinally, nor daylight transversely, else there could have been no dark zones in it; and that even the ether was in such small force that it could not perform the carrying duties assigned to it. we have often wondered whether any experiments have ever been made to ascertain whether any changes, as far as the presence of light is concerned alone, have been brought about by producing a vacuum in a tube. the gradual dying away of light, and its final disappearance, are certainly suggestive of changes, and may have excited curiosity to know what actually happens. that there are changes cannot be denied, and it would be satisfactory to know what they are. it appears to us that one simple and easily made experiment would give a good deal of information on the subject. let a glass tube of cylindrical form--one of those prepared for vacuum experiments--be placed in a slit in the window-shutter of a dark room, so that absolutely no light can pass into the room except through the hollow part of the tube; which might be effectually managed by burying two opposite sixth parts of its circumference in the wood of the shutter, and there would still be left one-third of its diameter for the free passage of light from side to side. when so arranged, and when still full of air, let a spectrum be taken of sunlight passing through it, to serve for comparison. then let a high vacuum be produced in the tube, and another spectrum taken and compared with the first. this will at once show whether any change has been produced or not. should the difference we expect be found, the experiment might be extended by spectra being taken at different degrees of exhaustion, from which some useful information might be derived. we have said, at page , that the ether does not pervade all bodies of all classes, and such must be the case in some measure at least, otherwise there would be no non-conductors of electricity, no insulators for our electric telegraphs and deep sea cables. were glass, for instance, pervaded freely by the ether, and the ether is in reality the carrier of electricity, then electricity could pass freely through glass, but it does not; therefore, there can be no, or at all events very little, ether in glass or any other insulator. we can see, then, the possibility of the ether being removed from a glass tube, provided it is a material substance, by shutting up one end of it with a stopper of glass and passing a perfectly-fitting glass piston through it to the other end. suppose this done, it would be quite safe to say that electricity could not pass through the tube, because there would be nothing--absolutely nothing--to carry it, not even the piston-rod, for we could have that not only made of glass but on the outside of the piston. in this case the result would be exactly the same as when the contents of the tube were pumped out of it, and the residue left, if any, would be the same, that is, an immeasurably small quantity of the ether which had filtered through the glass. it may be argued that it would be impossible to make such an experiment as we have proposed, but that does not damage in the slightest degree the correctness of the consequences deduced from it; any more than the impossibility of constructing a perfect heat engine destroys the deductions drawn by sadi carnot, from the study of such an ideal machine. we can grant that glass being not an absolute non-conductor, the ether might, in course of time, ooze through it and fill the tube again, while gas, air, or dust could not so ooze through it, and thus re-establish the current of electricity that was stopped for want of it; but we cannot grant that there was any very perceptible quantity of ether in the tube, when the electric current could not pass through it without dismissing the ether altogether, and dropping back into the difficulties out of which it has in many cases lifted us. the evident fact that the ether cannot pass through glass freely, and therefore cannot carry electricity with it, may be disputed by referring to the free passage of light, and also of heat, through glass and other substances, in virtue of transparency and diathermancy, two terms that have the same meaning, at least, as nearly as that light and heat mean the same thing; but we believe that this free passage, instead of invalidating our reasoning, only tends to prove that the ether is a material substance; because, if it is not, it might pass through transparent bodies just as easily as light and heat do. of course, this belief obliges us to show how light and heat do pass through a transparent body such as glass, and the mode is exactly the same as of heat passing through any other body that is a conductor of heat. glass is a substance that is known to be a bad conductor, but it is also known that it is not an absolute non-conductor of heat; therefore, there is no difficulty in supposing that it, and its companion light, can be conducted through glass with velocity proportioned to its thickness. we know that in the case of a pane of glass in a window it is practically instantaneous, but that does not mean that it is absolutely so. we know also, that in passing through, both are refracted, and that comparatively little heat is imparted to the glass, even under bright sunshine, which may be very well accounted for by the ether on the other side of the window pane carrying them (light and heat) off, in the same direction they were going, quite as fast as they could be conducted through the glass. but, supposing there was no ether in the room to which the window gave light, or gas, or elementary matter of any kind--a condition which could be obtained by making the room of glass and pumping out its contents as was done with the vacuum tube--what would be the result? there would be no wave motion to carry on light and heat into the room, and it would be in the same state as the exhausted tube, except that there would be no electricity in the room--no current being passed through it--nor anything in sufficient quantity to be lighted up if there was; the light would be stopped and reflected back from the glass, and nothing inside the room could be seen; not even that it was dark, because there would be no electricity to make dark zones visible. the window, or rather the whole room, would become a many-sided mirror, for reasons almost identical with those that account for a sheet of glass being made into a mirror. we confess that all these deductions have startled us, but we can see no flaw in the reasonings which have led to them. if it is not for want of ether--in sufficient quantity at least--and the admission of variable quantity is to admit that it is a material substance, that electricity will not pass through a highly exhausted tube, we cannot imagine what can be the reason why it does not; simply accepting it as a fact is by no means satisfactory. in the dilemma between renouncing the ether altogether or acknowledging its disappearance--effective at least--it occurred to us that it might be for want of heat, and that in terms of the inter-dependency of temperature and pressure in a gas, heat disappeared in proportion to the decrease of pressure in the air or gas that was being exhausted from the tube, or from cold being applied to it from without; but that notion has already been disposed of by our own work, when we have seen that a gas in a close vessel can be heated or cooled to any degree, altogether independently of pressure. when, acknowledging that the ether ought to have some temperature as well as density, we have said that it might have the temperature of vibration whatever that might be, thereby admitting that we could not pretend to determine what it is; nevertheless, we may take a look at it from a distance, and at least see what it cannot be, anywhere within the limits of our system. we have shown, at page , that when the original nebula was about , , miles in diameter, its density must have been · that of air at atmospheric pressure, and its temperature - °, and that these could be neither the density nor temperature of space. with this temperature, then, it is evident that there was still heat enough and to spare in the ether--considering it to be a material substance--to cause it to vibrate and perform its assigned offices; and, therefore, it could not be for want of heat that neither it, nor light, nor electricity could be carried through the vacuum tube, but for want of the ether in due quantity; consequently, the temperature of vibration cannot be so great as - °. turning back now to page , we find the density of the ether estimated at / , , th of an atmosphere, which corresponds to an absolute temperature of · ° or - · °; but on the following page we expressed our opinion--well founded, we believe--that the estimate was too high, i.e. too dense, and that it might be , , or times, or more, too great. be this as it may, we can see that if the ether alone occupies space--beyond a comparatively very limited distance from any body belonging to the solar system--it must be almost absolutely free from temperature of any degree, for the difference between - · ° and - ° is virtually nothing; or it must have a special temperature derived from the collisions of its own atoms, or from the sun. we have said more than once that the temperature of space cannot be so high as _minus_ °, and now we cannot believe that it can be so low as absolute zero, because the ether in it is credited with the motion of vibration, which must be either the cause or effect of heat. what then shall we say? we can only speculate. we can suppose that when the chemical elements were created, or evolved by some process, and began to attract each other, they had the ether to carry them into collision and produce heat; and that it, being also a material substance, became heated to the same degree as the other matter, always increasing in proportion to its state of condensation, the ether mixed with the other matter being also, of course, condensed. then, following up this supposition, we can see that when the sun came to be condensed to its present state, the ether must have had the same degree of heat as itself at its surface, and be of the same density as it would in our air at the earth's surface condensed to the pressure of nearly atmospheres; knowing as we do that the attraction of the sun at the surface of its photosphere is almost times greater than that of the earth at its surface. under this supposition, therefore, the ether might emit light just as surely as any other matter that may exist, or can be seen, in the corona or atmosphere of the sun, and might be the cause of the zodiacal light, probably more naturally than any other cause that has been imagined for it. mr. proctor, in his "sun," has given us a most elaborate description of how the zodiacal light could be produced by the swarms of meteorites and meteors, that are generally supposed to be floating around the sun and continually showering in upon it, and we confess that his reasoning is very plausible; but it, along with other similar hypotheses, has one very serious defect which it is hard to get over, under our existing ideas about matter and its origin. if there is a constant rain of meteorites and meteors falling into the sun now, and the same has been going on during the multitude of millions of years that it is supposed to have existed, we have to acknowledge that it must either come to an end some day, or that there is going on a constant creation or evolution of matter to keep up the supply. it will not suffice to accept the hypothesis that the supply comes from other suns, or any idea of that kind, because each one of them would finally find itself alone with its planets, etc., if it has any, in its domains the same as our sun. neither would it suit the ideas of those who consider that matter has existed from all eternity and has _made itself_ into all sorts of bodies or systems to suit them. without continued creation, or evolution, matter must end in condensation into one mass. there can be no self-evolution to keep up the supply of matter. it would require another and exactly opposite power to unmake the final mass, and another change to original matter to start anew on the old course. but we are speculating too soon. it may be said that if the zodiacal light is caused by the ether, and if the ether is a material substance, it must be exhausted sooner or later, just the same as all other matter and the whole universe to one mass the same as before; and also that we have no authority for supposing that the ether can be heated and cooled or condensed and expanded. but we think that with what we have done in this chapter, and what we will be able to show in the following one, we shall be able to get over all these difficulties, and also show how the universe might be dissolved and renewed by the ordained process of evolution. chapter xiv. page the ether considered and its nature explained. further proofs given by dr. crookes's work, of its material substance. highest vacuum yet produced. absorbents cannot absorb the ether. dr. crookes's definition of a gas. not satisfactory. why. a fluid required to pump matter out of a vessel. gas as described by dr. crookes would not suit. the ether the only elastic fluid we have. the only real gas, if it is a gas. a possible measure of the density of the ether. causes of dark and light zones in high vacua. the real conductor of light in a high vacuum. how a vacuum tube glows, when electricity passes through it. conclusions arrived at through foregoing discussions. some exhibitions of light explained. gases can be put in motion, but cannot move even themselves. the ether shown to be attraction. and primitive matter also. all chemical elements evolved from it. its nature stated. action at a distance explained by the ether and attraction being one and the same. the idea that the ether can be pumped out of a tube of any kind, along with the air or gas that has been shut up with it therein, will very probably be declared to be absurd, by reference to dr. crookes's experiments with his radiometer, and investigations into the nature of radiant matter; but when duly considered his work seems to confirm it, and our reasonings in support of it, in a very convincing manner. radiant heat, or light, is shown, no doubt, to penetrate into an exhausted bulb and to cause a radiometer to revolve, but we have to consider what is the state of exhaustion at which its force is shown to be greatest, and why that force decreases rapidly when the exhaustion is progressively increased beyond a certain point; for a certain amount of exhaustion is required first of all to diminish the resistance of the air or gas to the vanes of the radiometer, before the radiant heat gathers force enough to make them revolve at all. its greatest power to produce revolution is shown to be when the exhaustion is at from to millionths of an atmosphere, according to the gas or medium in the bulb--see "engineering," vol. xxv., page --and decreases from that point, often rapidly, as the exhaustion is increased, till at last it ceases altogether. everybody who has taken any interest in the subject, knows that dr. crookes has exhausted radiometers to such a degree that they could not be influenced by the radiation of a candle placed a few inches from the bulb. we are not told at what degree of exhaustion this took place, nor at what degree repulsion, by radiation of heat, is supposed to have ceased altogether, but that does not matter, even though it should only cease when the vacuum comes to be absolute--most probably a stage to which it is impossible to attain. what concerns us is the fact that repulsion by radiation does reach a maximum at a certain degree of exhaustion, and then falls off as the exhaustion is increased; and what we have got to consider is what is the cause of the falling off. we are told it is caused by the attenuation of the matter, gaseous or material, contained in the bulb, and we are satisfied with the explanation. but in order to be thoroughly so, we must insist on believing that it is part of the whole of the matter that has been operated on; not only of the gas and other matter to the exclusion of the ether, but of the whole, ether and all. if the ether is left behind intact, it must perform the offices it was created for by the imagination of man, or man must discard it altogether. if it ceases to carry light and heat through a vacuum, it is of no more use than we found it to be in the case of electricity, and man is bound to dismiss it as a useless operative, who will strike work for no reason whatever. some people have supposed the ether to be an absolute non-conductor of electricity, because it does not convey that agent through a vacuum. will they also declare it to be a non-conductor of light and heat? if they will not, then they--and we presume everyone else--must admit that it can be pumped out of a bulb, in the same way as a gas or any other fluid matter. here we are led into another consideration, viz., whether the ether is exhausted from a receiver by pumping alone, or by the help of absorption. in his lecture, "on radiant matter," delivered at the british association, at sheffield, august , , dr. crookes said: "by introducing into the tubes appropriate absorbents of residual gas, i can see that the chemical attraction goes on long after the attenuation has reached the best stage for showing the phenomena now under illustration, and i am able by this means to carry the exhaustion to much higher degrees than i get by mere pumping;" and that when working with absorbents: "the highest vacuum i have succeeded in obtaining has been / , , th of an atmosphere, a degree which may be better understood if i say that it corresponds to about the hundredth of an inch in a barometer column three miles high." (we quote from "engineering," vol. xxviii., page .) now, what are we to think? are we to suppose that the ether was in part removed by the absorbents? we think we are justified in saying that the absorbents had not anything to do with the exhaustion of the ether, because dr. crookes used different kinds of absorbents for the different kinds of gases he dealt with, and it is hard to believe that all the _media_ he used were equally effective in absorbing the ether as they were with the gases. on the other hand, if we consider that the pumping was the only agent in removing the ether, we ought to acknowledge that it must have been more effective with regard to it than to the gases before absorption was resorted to with them; or that a stage had been reached at which the pump could not extract any more ether from the bulb. we shall have more to say of this presently. it is a difficult matter to determine, but there is one thing we can see clearly; when the exhaustion of the bulb was raised to / , , th of an atmosphere, the density of the ether--of itself--must have been at a lower degree than that. consequently if we assume its normal density to be / , , th of an atmosphere, in terms of the estimate we quoted from "engineering," it must have been diminished to less than one-fourth of that when the above high vacuum was obtained; because it must have been the density of the residual gas, or matter, and of the ether, added together which amounted to / , , th; the same as we have argued with regard to the solar nebula when at , , , and , , miles in diameter. one thing leads to another, and we have again to repeat our question--what is a gas? and all the answers we have been able to get to it hitherto have been far from satisfactory. a little earlier in the same lecture, referred to a few pages back, dr. crookes, after telling us, very elaborately, what would have been the definition of a gas at the beginning of this century, goes on to say: "modern research, however, has greatly enlarged and modified our views on the construction of these elastic fluids. gases are now considered to be composed of an almost infinite number of small particles or molecules, which are constantly moving in every direction with velocities of all conceivable magnitudes. as these molecules are exceedingly numerous, it follows that no molecule can move far in any direction without coming in contact with some other molecule. but if we exhaust the air or gas contained in a close vessel, the number of molecules becomes diminished, and the distance through which any one of them can move without coming in contact with another is increased, the length of the mean free path being inversely proportional to the number of molecules present. the farther this process is carried, the longer becomes the average distance a molecule can travel before entering into collision; or, in other words, the longer its mean free path, the more the physical properties of the gas or air are modified." of course, what we have looked upon as dr. crookes's definition of a gas, ends with the second sentence of the above quotation, and is far from being sufficiently complete to be satisfactory; but we have continued to quote from the lecture, because it contains matter which demands consideration, and helps very powerfully to support the conclusions we have been arriving at. why the definition is not satisfactory, is that it does not tell us what there is in the spaces between the molecules of what is called the gas. if there is room for them to move in every direction there must be spaces between them, and these spaces must either be absolutely empty, or filled with something. if they are supposed to be empty, then the molecules being actually small pellets, like diminutive marbles, or snipe-shot, we immediately begin to think why gravitation does not make them, being ponderable bodies, fall down to the bottom of the bulb; and seeing that, by the definition, they are evidently considered not to do so, we think of what can keep them from falling, and of how they can be pumped out of a bulb or any sort of vessel. if we fill a vessel with marbles, snipe-shot, wheat flour, or dust, and set a pump to work on it, we shall find that we make very little progress in pumping them out of it. at first we might extract a puff or two of flour or dust--marbles or snipe-shot by no means--carried into the pump by any air there might be mixed with them, but that would very soon come to an end; besides, there would be air, gas, something, in the interstices--if any--of the flour or dust to drive them into the pump when a vacuum was formed in it, and the puffs would cease when the air, which would be in exceedingly small quantity, was all extracted. but independently of all this, we have supposed the spaces between the gas pellets or molecules to be absolutely empty, and there would be nothing to push them into the vacuum created in the pump. there is no possibility of pumping marbles, sand, flour, or dust out of a vessel without the assistance of a fluid agent of some kind, water, gas, or air; and even then it would be done with much difficulty. let us, then, suppose there is some such agent filling the spaces between the atoms of the gas and think of what it must be. were we to ask the question we have a strong suspicion the first impulse of many people would be to reply--with gas of course. but this reply could not satisfy us. we should immediately be led to think of that gas also consisting of atoms with vacant spaces between them filled with something--some more gas; and were we to follow up that thought through a sufficient number of stages, it is easy to see that in the end the whole space occupied by any gas would come to be filled up with its own solid atoms, without any empty spaces between them through which they could move; and so rendered quite incapable of pushing each other into a vacuum formed by any pump that might be applied to extract them from any vessel of any kind; or we must suppose that each particle would fly of its own good will into the vacuum made by the pump--as it were on the wings of the morning. but we recall to memory that the wings of the morning do not always carry us to rest, and we see that filling the spaces with gas would only end in choking up the vessel altogether. it might be said: nobody imagines that the molecules of the gas in the spaces would be sufficient to fill them up altogether; and then we have only to ask, what then would there be in the spaces between the molecules of these successive gases to prevent the whole of them from gravitating to the bottom of the vessel? and to add that there would still be empty spaces left, absolutely empty, that would have nothing in them to help in any way to force the molecules or atoms of any gas or vapour into a vacuum anywhere. it is clear then that a gas, such as dr. crookes has described a gas to be, could only end in filling the spaces left between its molecules or atoms. it would be an obstruction to their collisions and bombardments which form an essential part of the description or definition. we must, therefore, have recourse to something else for filling up the spaces between the molecules of a gas, and the only thing we can lay hold of is our _limited liability_ agent the ether, which we allow to do all we want it to do and nothing more. vapours of solid or liquid matter would be of no use, for they would only condense into solid or liquid matter; unless always maintained at their temperatures of evaporation or ebullition, and that would at the best be only another form of a gas--nobody would use a liquid to assist in pumping air out of a vessel--and, besides, we should still have to show what keeps their particles apart, what fills the spaces between them, which would force us to appeal to the ether as the only source, just as before. if there are no spaces between the particles there can be no vapours. if by pumping air out of a close vessel the number of its particles is diminished, and we acknowledge that the ether pervades all space and matter, in a greater or smaller degree, then we must either recognise that a pump is able to separate the particles of the air from the ether which pervades it in the vessel, and extract them alone; or we must acknowledge that along with the particles of the air, the pump extracts a corresponding portion of the ether. which of these two consequences of the pumping we have to choose cannot for a moment be doubtful. it would be as reasonable to suppose that we could pump the colouring matter out of a pond of muddy water, or the mud itself, and leave the clear water behind, as to suppose that the molecules of air, or of a gas, could be extracted from a close vessel, by a pump, and the ether left behind in it. we have called attention two or three pages back to the fact that a fluid agent of some kind is required, in order to be able to pump matter of any description out of any kind of vessel. for solid matter a non-elastic fluid will suit, but for gaseous or vaporous bodies an elastic fluid is required; but we have just seen that what have hitherto been considered to be elastic fluids, that is, gases and vapours, have no elasticity whatever of their own, but are undoubtedly and in reality solid matter; and that in order to become elastic fluids they have to be mixed with the ether, or something that has yet to be discovered, invented, or imagined. if, then, until such a body is found we take the ether as a substitute, we have to acknowledge that it must be not only an elastic fluid but a material substance, capable of being compressed and expanded, and heated and cooled; for nobody could conceive clearly the existence of an elastic fluid that is not subject to these conditions. he could not understand how the molecules of a gas could be contracted, expanded, heated and cooled in a vessel, while the elastic fluid which gave them liberty to move or to be moved, remained constantly at one density and temperature. furthermore, until such a substitute is found, we have to acknowledge that it is the only thing we have any idea of corresponding to a gas as described by dr. crookes; that is, a multitude of molecules colliding with and bombarding each other or their prison walls. but even beyond this we can uphold it to be the only real, independent gas there is; because, being an elastic fluid, there is no necessity for there being empty spaces between its molecules, or even having molecules in the common acceptation of the term. we have no reason to think that there are empty spaces between the molecules or particles of indiarubber; and if there are, the ether is the only substance we can properly conceive them to be filled with. the law of avogadro is, that "equal volumes of gases and vapours contain the same numbers of molecules, and consequently that the relative weights of these molecules are proportioned to the densities." therefore we must always bear in mind that it is the _weights_, not the _volumes_, which are equal, and that the volumes may be very different. on this earth of ours, then, we may say with certainty that an atmosphere of gas is composed of a definite number of its special kind of molecules, mixed with a definite quantity of the ether, in such proportion that the sum of their densities shall be equal to the density of the air, at atmospheric pressure at sea level, and at ° of temperature. holding this belief, we can see that each molecule, or rather atom, of each gas must have its own amount of displacement to enable it to float in the ether with which it is mixed. this would account in the most satisfactory manner for the diffusion of gases, whereby any molecule, or atom, may float wherever it is driven by collisions with its neighbours, be it above, or below, or on a level with, a molecule of a lighter or heavier gas. therefore, were it possible to determine with sufficient accuracy the dimensions of the atoms of all gases, perhaps even of a limited number of them, it would be possible to calculate the real density, or specific gravity, of the ether. we have not forgotten that when, by pumping, the ether was reduced to at least one-fourth of its normal density, its buoyant power would be reduced in the same proportion, nor that, when in a state of rest, the displacement of a molecule, which enabled it to float in the ether, would not be sufficient to make it float at one-fourth of that density; but it might be supposed that when so far relieved from pressure, the molecule could expand in proportion to the relief, especially if its form were that of a vortex ring, or of a hollow sphere. however, should this supposition not be admissible, we shall see presently that it is not necessary. we know that as long as any degree of heat remains in a gas collisions of its molecules will continue, dependent on their attraction for each other, which may drive them to any part of the containing vessel; and that it can only be when they are cooled down to the absolute zero of temperature that they can come to be at rest. but as we believe that the ether can never be reduced to this absolute absence of temperature, nor completely extracted from any vessel, we cannot acknowledge that the molecules of any gas, left along with it in the vessel, could ever come to be absolutely at rest, even although the molecules did not increase in volume with the diminution of pressure. and we think this conclusion will agree with the opinion of professor tait, expressed in the quotation, made at page , from his work on "heat," where he says: "in fact, if the kinetic gas theory be true, a gas whose volume is immensely increased, cannot in any strict sense be said to have one definite pressure throughout." this, of course, is tantamount to saying that the diffusion of gases cannot continue to be always exactly regular at extremely low pressures, and must vary as the vacuum is increased; so that the volumes of the atoms and consequent displacements may continue always the same under all pressures. we see, then, from this quotation, that in all probability the molecules of a gas are not always equally buoyed up by the ether in a high vacuum; which very likely is the reason why there are dark streaks in it; streaks without any visible molecules of gas in them, because the ether was not dense enough to keep them afloat. we have still something to add in support of what we said, at page , of glass not being pervaded by the ether, in the common acceptation of the word, and of our acknowledging that the ether might, in the course of time, ooze through it and fill up the bulb again, while air, gas and dust could not so ooze through it--nor even the larger particles of the ether; should we be forced to acknowledge that it consists of particles. in one of a series of articles in "engineering," vol. xxv., on repulsion from radiation, we find, at page , what follows: "with the same apparatus, mr. crookes conducted a long series of experiments for determining the conductivity of the residual gas to a spark from the induction coil. in air he found, at a pressure of millionths ( / , th) of an atmosphere, which will be seen from the diagram, is the pressure at which the force of repulsion is at a maximum, that a spark whose striking distance at the normal pressure of the atmosphere is half an inch will illuminate a tube whose terminals are millimetres apart. by pushing the exhaustion farther, the half-inch spark ceases to pass, but a one-inch spark will illuminate the tube, and as a vacuum is approached more electromotive-force is required to force the spark to cross the space separating the terminals within the tube, until at still higher exhaustion a coil capable of giving a -inch spark in air at the pressure of the atmosphere is required to show any indication of conductivity in the residual air. it was found, however, in experimenting with so powerful a spark that occasionally the glass was perforated by the discharge taking place through the bulb; but it is a remarkable fact that the perforation in such cases was so excessively small that several days were occupied before equilibrium of pressure was established between the inside and outside of the bulb." here we notice first--and it was the reason why we have made the first and longest part of the quotation--that the spark whose striking distance was half an inch at the normal pressure of the atmosphere, fell to under one-fourth of its power in a vacuum of only / , th of that pressure; that when a one-inch spark was required to illuminate the tube, it must have decreased to one-eighth in a vacuum of / , th; and, if it be admissible to follow the same proportion, the -inch spark must have been exhibited in a vacuum of / , th an atmosphere at least. perhaps all this experiment was carried on in _vacua_ produced by pumping alone, and the final vacuum may have reached a greater height than that which we have just mentioned; but the most interesting part of it is the perforation of the bulb by the -inch spark. in it we have to consider what was the conveyer which carried the electric spark through the glass of the bulb, instead of to the other terminal of the coil so close at hand, and it is a very difficult problem to solve. we naturally recur for some solution to the stratification of light given out when an electric current traverses a gas at very low pressure and gives rise to zones alternately light and dark as noted in the reference we made, at page , to professor balfour stewart's experiments. we cannot think it unreasonable to suppose that the dark zones contained no matter at all that could be lighted up, and that it was the lighted zones alone which contained carrying matter for the electricity. if so, we can easily imagine one of these zones or strata carrying the perforating spark from the induction terminal to the nearest part of the glass of the bulb, for it was as possible for it to lie in that direction as in the direction of the other terminal, and the difference of distance between the first terminal and the glass, and between the two terminals, would not be so great as it appears to be on simply reading the accounts of the experiments; but we have still to think of how it managed to force itself through the glass of the bulb. to get over this difficulty, we can refer to what we have said, that is, that glass may be thoroughly pervaded by the ether in an almost infinitesimal degree, and suppose that the electricity may have discovered, or rather been led to, the ether contained in the glass tube or bulb, and so found its way to one of the oozing holes we have said might exist in the glass; even the oozing hole may not have passed quite through the glass, and there might remain a very thin film to be burst open before perforation was complete. also we may note that the zone which performed the office of carrier to the side of the bulb was much more probably composed of residual ether than residual air or gas, or at the least formed a preponderating part of the carrying element. the fact of the hole being so minute "that several days were occupied before equilibrium of pressure was established between the inside and outside of the bulb" on such occasions, goes far to prove that the carrying agent through the glass must have been the natural carrier of electricity, light, and heat. we cannot conceive that an eruptive force could open such a small passage through the glass of the bulb, but we can conceive that it should be able to force itself through a very minute passage already open, and even join two or more such passages into one. this conception makes us think of the many oozing passages there may be through a glass bulb; passages so minute that the ether might pass through them, but nothing so gross as any of our known gases; in fine, so minute that glass, for all the compact look it presents to us, may be only as a very fine sponge in respect to the ether. however, that the perforations related in the above quotation were large enough for air to pass through them there can be no doubt, otherwise the equilibrium between the pressures on the inside and outside of the bulb could not have been re-established even after many days; for there still remains the idea that the oozing holes might be so small that nothing but the ether could pass through them. should the glass of a vacuum tube or bulb be pervaded by the ether in the manner we have supposed it to be, and we believe there can be no doubt that it is so, it is obvious that its glowing when a current of electricity is passed through it must be caused by the electricity and consequently of its light, being carried into the body of the glass by means of the ether imbedded in, and forming a constituent part of, it. in connection with this we have to remember that the air in the tube does not glow when it is at full atmospheric pressure, but only when a certain degree of vacuum has been produced in it; and therefore it is equally obvious that it is only when the ether enclosed in the tube is reduced to the same degree of tenuity as that imbedded in the glass forming the tube, that the light of the electricity can be carried by it into the glass and make it glow. but to show this more clearly, it is necessary to refer to the steps by which we believe we have made very plain what must undoubtedly be the nature of the ether. ( ) first of all we have shown that, if there be such a thing as the ether, it can be pumped out of a close vessel of any kind; which proves that it must be a material substance, and in consequence can be expanded, or rarefied, and compressed the same as any other material substance; and that if there is no such thing, something else, having these qualities, has to be invented to take its place. ( ) in showing this it has been made abundantly clear by the example of the hair of a cat in variable weather, to which we may add the exhibition of lightning in daylight, that it cannot make electricity visible, or illuminate any matter, unless the quantity of electricity it has to carry bears some certain proportion to the density of the ether in the matter that is illuminated. ( ) in proof of this we have shown how, through its carrying power it can convey electricity of adequate force up to very great heights, so as to illuminate very rarefied air and cause auroras; the conveying being done either directly from the earth or by means of the ether mixed with the air carried up by whirlwinds to those great heights; and ( ) how electricity is carried into the body of a tube of glass and makes it glow. with these examples we can extend our ideas to other exhibitions of light, which, otherwise, we could hardly avoid looking upon as mysterious. we can see how marsh gas, rising up from boggy ground, becomes mixed with common air till it reaches a certain density, and forms the will-o'-the-wisp when there is sufficient electricity in the air to make the diffused marsh gas visible, through the medium of the ether always mixed with it; or, perhaps, rather when the density of the diffused gas corresponds to the density of the ether. then we have the phenomena of films of matter on the surfaces of certain liquids glowing with appropriate colours; which films must be pervaded by the ether in proportion to their conducting powers, the same as we have seen must be the case with all kinds of matter, the light given off corresponding as is natural to the composition of the films; and of course this same reasoning, or exposition, applies to the films formed on, or near, the surface of the sea which produce what sailors call "fire-on-the-wave." lastly, and akin to the glowing caused in a vacuum tube, we cite the case of the glow-worm, the radiation from which must of necessity contain a certain amount of the ether in it, and may either glow constantly or intermittently according to its capacity for carrying electricity or light of any kind, constant or inconstant. or if there is no radiation from it, its skin may possess the properties of a film on the surface of a liquid. we have seen in the "times" of september , , in its report for that day of the meeting of the british association at liverpool, that in experimenting with glow-worms dr. dawson turner had found some difficulty in getting them to glow when he wanted, but found they gave off the radiation whether glowing or not. perhaps his interference with them destroyed the balance of force between the electricity present and the density of the ether in it without stopping the radiation. hitherto the light given out by a nebula, and any light of the kind not easily accounted for, has been attributed to incandescent gas not burning or being consumed, but only glowing. now it is time to look upon it as belonging, at least in part, to the ether, and to look upon the bright line in the spectrum of a nebula as the _ether line_. we shall have to return to this later on. we said, at page , that a fluid of some kind, elastic or not elastic, is necessary to enable us to pump solid matter out of a vessel of any kind, and went on to show that a gas as described by dr. crookes, or that can be described, in its own independent state of existence, by anybody, could not supply the want; because it consists of particles, molecules, atoms--any name that can be given to them--which have no power in themselves to move or to give motion to anything; they can be moved but cannot impart motion to anything, even to one another, until they are first set in motion by attraction. this in its turn led us to see that the only elastic fluid we have is the ether, and our work since then has taught us that we were wrong in saying at page that a non-elastic fluid would suit for pumping solid matter out of a vessel; for we now see that what we have been in the habit of looking upon as non-elastic fluids, must owe their fluidity, such as it is, to the ether, which, in proportionate degree, pervades them the same as it does all other matter. in this way we are run down to the only conclusion we can come to, namely, that the ether is the only connecting medium and carrying agent of matter that we have, or even initiator of motion, except attraction; and being matter of the nature of an elastic fluid, there is no reason why we should not at once consider it to be attraction itself. it has been looked upon, for no one can exactly tell how long, as the connecting mechanism of the universe, thus having, in reality, assigned to it the attributes of the law of attraction, and all that we have to do is to put it in its right place. we are, in a manner, taught to look with suspicion on two agents being required to do one kind of work, or even two kinds of work that are so closely allied that we cannot separate them in a way that satisfies us; and this is precisely a case in which we can have one agent that can connect matter, and at the same time carry immaterial elements from one place to another. having got this length we have still to go one step farther. we cannot now doubt that the ether is a material substance, and if it is, there is nothing to prevent us from considering it to be the primitive matter; in fact it would be absurd to look upon it in any other light. we cannot conceive of anything having been created before the ether, or ordained before the law of attraction, and thus we have the two coeval and one. it is long years since physicists, chemists especially perhaps, began to think that the great number of chemical elements cannot all have existed from the beginning of things, and that it is far more probable that they have all been evolved from one primitive substance, and this idea must now be gathering more strength from day to day in view of the new elements that are being constantly discovered; the unknown is being made known, and the air we breathe instead of being one in four elements, as in former times it was considered to be, is now not far from double that number in one. adopting this notion, then, the ether is much more likely to have been the primitive element than any other material substance that can be thought of. if it has never been thought of in this light, it has come to be very remarkably near it, as may be seen by referring to the long quotation we made in chapter vii., beginning at page , where the idea of the ether being the connecting _medium_ of matter is made use of to compute its density. little thought we of this when we made the quotation, but there was the idea whether the author saw or not all that was implied in it. having broached the notion of the ether being the primitive element of the universe, or at all events, of the solar system, we might be expected to show how all the other elements were formed from it; but that has been done for us in a very much more able manner than we could have done it. anyone who chooses to refer to "nature" of september , , will find--in dr. crookes's opening address, on chemical science in section b, at the meeting of the british association for that year--a very detailed explanation of how all the chemical elements might have been elaborated from one that he called protyle; in which explanation he will only have to change this word into ether to comprehend the process much more easily than by any exposition we could pretend to draw up. to quote the whole address would be altogether out of place, and besides, our notes of it are only fragmentary. but for present satisfaction of those who cannot immediately refer to "nature," we may say that in the same report it is clearly stated that sir george b. airy was of opinion that all bodies may not be subject to the law of gravitation; and have no cause to think it strange we do not see that, were the ether and attraction one and the same, the whole universe would be finally collected into one mass, itself included. they will have better authority than ours for believing that the ether may connect matter evolved from itself, without being materially confounded with it. at the same time we acknowledge the necessity for expressing our idea of what we consider to be its nature, and in compliance with this obligation we say we have conceived it to be of the nature of indiarubber, not an elastic fluid as we have called it before, but rather an elastic substance like a jelly, as some people have conceived it to be; not a gas, because it does not require any medium to connect its particles. looking upon it in this light, action at a distance can be accounted for in a very natural manner. when a stretched indiarubber band is relieved from strain, the relief must be felt instantly throughout every part of its length; for, although the band may take time to contract, no time is required for the relief from strain being felt. in like manner an alteration in strain between the sun and the earth--and these alterations of strain are taking place every instant--connected by an indiarubber ether will be felt instantly in both bodies; and should anyone stand out for time being required to convey the attraction, let him remember that the difference of its power would be felt first at the two ends of the connecting medium, for the very good reason that even attraction itself could not prefer one extreme to the other. and that is all that is meant by action at a distance. here are some other things that could be explained more easily than they can be at present, through the ether and attraction being considered to be one and the same, than under any other conception we can form; but although we have a dim vision of such explanations in some cases, our knowledge of the sciences involved in them is not sufficient to warrant us in letting our dim conceptions see the light. therefore all that remains for us to add is, that some things we have said of the ether may have to be so far modified now, but as they have had their part in leading us to the conclusions we have arrived at, they cannot be altogether suppressed. chapter xv. page construction of the solar system. matter out of which it was formed. domains of the sun out of which the matter was collected. stars nearest to the sun. table vii. showing distances. remarks on binary stars. table viii. showing spheres of attraction between the sun and a very few. sirius actually our nearest neighbour. form of the sun's domains of a very jagged nature. creation of matter for the nebulæ, out of which the whole universe was elaborated. beginning of construction. the law of attraction begins to operate through the agency of evolution. form of the primitive solar nebula. the jagged peaks probably soon left behind in contraction. how the nebula contracted. two views of the form it might take. comparison of the two forms, solid or hollow. the hollow centre form adopted. the jagged peaks left behind. the nebula assuming a spherical form. shreds, masses, crescents separated from one side. probable form of interior of nebula. compared with envelopes in heads of some comets. reflections on the nebula being hollow. opinions of others quoted. the matter of a sphere solid to the centre must be inert there. further proofs of the nebula being hollow. how rotary motion was instituted. such a nebula might take one of two forms. the form depending on the class of nebula. planetary in the case of the solar system. a similar conception of how rotary motion could be instituted. in this chapter we proceed to consider how the original nebula was formed, and whether the solar system could be evolved therefrom in the manner shown in the analysis of chapter v. the usual way of treating the solar system has been to suppose it to have been formed out of a nebula extending far beyond the planet neptune, generally in a vague way; although some writers have specified a limit to the distance, in order to give some definite idea of what must have been the density of the nebula at some particular period of its existence. in the first part of our work we have adopted the same plan and we mean to follow it out, because it gives us a greater degree of facility for expressing our ideas, and making them more intelligible, than by adopting a new method. but we shall previously endeavour to show where the nebula itself came from and how it was formed, which seems to us to be as necessary as to show how it was transformed into the solar system. we understand laplace to have supposed the nebula to have been formed out of cosmic matter in its simplest condition, and in its most primitive atomic state, collected from enormously distant regions of space by the power or law of attraction. in this we shall follow him, because we do not see the necessity for matter having to be created in the form of meteorites or meteors, or any other form, to be afterwards dissociated and reduced to the atomic state, by heat produced by collisions amongst the dissociated atoms. surely it would show more prescience, more simplicity of work, and economy of labour, to create matter in this primitive state, than in one which required it to be passed through a mill of some kind, as it were, before it was manufactured into nebulous matter; in fact, to make brickbats in order that they should be afterwards ground down--dissociated--into impalpable powder, to render them fit to be worked up into bricks. but our first effort will be to attempt to define the collecting grounds of this cosmic matter, somewhat more particularly than has been done hitherto, as we believe that even a superficial study of them will assist us greatly in forming a more comprehensive idea of the whole solar system than anything we have met with in any of the books which we have had the opportunity of applying to for information. the collecting grounds, then, are clearly the whole region of space to which the attractive power of the sun extends, or what astronomers would call within the sphere of his attraction. these domains, like those of any other proprietor, are limited by the domains of his neighbours. at first sight, it would seem that his neighbours are infinite in number, but a little thought will show that the number may be very limited indeed. on this small earth of ours, it is a very common thing for a landed proprietor to be able to look over the domains of his neighbours, and see those of proprietors more remote; even to look over the domains of his neighbours' neighbours, and see properties so remote that he does not even know to whom they belong nor how they are named. with much more reason, the same must be the case with the sun, more especially as he, from his own mansion-house, sees nothing of the domains, but only the mansion-houses of others, there being no landmarks, hills, fences or woods to cut off his view, as there are upon the earth; the only interruption possible to his view being that another mansion-house should come to be exactly between his and that of a farther-off neighbour. for our purposes, we will assume that his nearest neighbours are those the distances of whose mansion-houses have been measured, and will adopt the following list of them, taken from mr. george chambers's "hand-book of astronomy," part , page , th edition, , and forming table vii. all that we can learn from this table is that the boundary between the sun and any one of the stars mentioned in it must be somewhere on a straight line connecting the two, but that does not furnish us with any information as to the extent of the sun's domains, although it does help to give us some idea of their form. for some knowledge of their extent, we require to know how far the lordship of each one of the proprietors extends from his mansion-house; which, very much the same as it does upon the earth, depends upon the power he has to take and keep it; it depends on the mass of each neighbour who actually marches with the sun when compared with his own mass. the list referred to does not help us in any way to determine this, as we have just said, but we have found in professor charles a. young's "lessons in astronomy," of , page , the masses of six binary stars whose distances, calculated from the parallaxes given in it, furnish us with data from which we can calculate the distance from the sun of the boundary between him and any one of them. the number is very small, but still from them we can gain some notion of what was the form of the domains from which the original nebula was collected; that is, always under the supposition that the sun and his system were evolved from a nebula. from these data, table viii. has been drawn up, which shows the distances of the six stars from the sun, and the limits of his sphere of attraction in relation to them expressed in terms of radii of the earth's orbit, and also in radii of neptune's orbit, which gives numbers more easily comprehended by us. table vii.--list of stars whose distances from the sun have been measured, and which are assumed to be his nearest neighbours. --------------------+-----+------+----+------------------+-----------+ | m | p | | distance. | | | a | r | p |---------+--------| | star. | g | o | a | sun's |time of | observers | | n | p m | r |distance | its | | | i | e o | a | = . | light | | | t | r t | l | |reaching| | | u | i | l | | earth. | | | d | o | a | | | | | e | n | x | | | | | | (") | (")| |(years) | | --------------------+-----+------+----+---------+--------+-----------+ [greek: a] centauri | | · | · | , | · |gill. | | | | | | | | cygni | | · | · | , | · |o. struve. | | | | | | | | lalande | - / | · | · | , | · |winnecke. | | | | | | | | sirius | | · | · | , | · |gill. | | | | | | | | [greek: m] cassiopeiæ| | | · | , | · |o. struve. | | | | | | | | groombridge | | · | · | , | · |auwers. | | | | | | | | lacaille | - / | · | · | , | · |gill. | | | | | | | | lalande | - / | · | · | , | · |krüger. | | | | | | | | Ö arg. | | · | · | , | · |krüger. | | | | | | | | [greek: s] draconis | | · | · | , | · |brunnow. | | | | | | | | [greek: e] indi | - / | · | · | , | · |gill. | | | | | | | | [greek: a] lyræ | | · | · | , , | · | | | | | | | | | o^ eridani | - / | · | · | , , | · |gill. | | | | | | | | [greek: r] ophiuchi | - / | · | · | , , | · |krüger. | | | | | | | | [greek: e] eridani | - / | · | · | , , | · |elkin. | | | | | | | | [greek: i] ursæ majoris | · | · | , , | · |c.a.f. | | | | | | | peters.| [greek: a] boötis | | · | · | , , | · |c.a.f. | | | | | | | peters.| [greek: g] draconis | | · | · | , , | · | | | | | | | | | groombridge | | · | · | , , | · |brunnow. | | | | | | | | polaris | | | · | , , | · |c.a.f. | | | | | | | peters.| bradley | | · | · | , , | · |brunnow. | | | | | | | | [greek: s] foucani | | · | · | , , | · |elkin. | | | | | | | | pegasi | | · | · | , , | · |brunnow. | | | | | | | | [greek: a] aurigæ | | · | · | , , | · |c.a.f. | | | | | | | peters.| canopus | | | · | , , | · |elkin. | --------------------+-----+------+----+---------+--------+-----------+ table viii.--masses of a few binary stars showing the limit of the sun's sphere of attraction with respect to them, in radii of the earth's orbit, and distances of their boundaries with the sun in the same measure, and also in neptune distances. -----------+-----+------+-------------+------------+---------------+ | p | |distance of |distance of | | | a | mass |star from sun| limit of | distance of | | r | |in radii of |sun's sphere|limit in radii | name of | a |sun's |the earth's |of attract'n| of neptune's | star. | l | mass | orbit |in radii of | orbit | | l | = . | | earth's | | | a | |( , , | miles. | | | x | | miles.) | |= , , , | | (") | | | orbit = . | | -----------+-----+------+-------------+------------+---------------+ [greek: a] | | | | | | centauri | · | · | , | , | , | | | | | | | cygni | · | · | , | , | , | | | | | | | sirius | · | · | , | , | , | | | | | | | [greek: a] | | | | | | geminorum | · | · | , , | , | , | | | | | | | ophiuchi | · | · | , , | , | , | | | | | | | [greek: ê] | | | | | | cassiopeiæ | · | · | , , | , | , | ------------+-----+------+-------------+------------+---------------+ but there is still something to be said with respect to the binary stars of table viii., and any others whose masses may be met with later on. if those forming a pair revolve around each other, or a common centre, in orbits, it must happen that they will be sometimes more or less in conjunction, opposition, and quadrature with regard to the sun; also the angles of the planes of their orbits to direct lines between them and the sun, whatever these angles may be, will cause variations in the separate and combined forces of attraction they exercise in the domains of the sun, at different periods of their revolutions; so that these powers of attraction will be constantly increasing and diminishing, and causing the boundaries of their domains to approach and recede from the sun; thus introducing between their domains and those of the sun a debatable land, which will reduce celestial to be very much like terrestrial affairs, where each proprietor, or power, takes the pull when an opportunity presents itself. no doubt all such invasions, or claims, between proprietors will be settled by the law of attraction, without lawsuit, arbitration or conflict; but as law gives right, and might is right--most emphatically in this case--we come back to the old seesaw of earthly matters. well, therefore, many astronomers teach that the whole universe is formed out of the same kind of materials, and governed by the same laws that we are having good reason to know something about on this earth of ours. accustomed to look upon [greek: a] centauri as the star nearest to us, on account of its light-distance being so much smaller than any other noted in our text-books, we were not prepared to find that, when measured by his sphere-of-attraction distance, sirius is actually a rather nearer neighbour to the sun than it; nor that his, apparently, next nearest neighbour, when measured in the same way, is twice as far away as either of them; and thus we have the conviction thrust upon us that they must have made deep hollows in the solar nebula when it was being formed. on the other hand, when we think of three of the other stars mentioned in the list of six, being practically from three to six times farther off than either of them, we come to the conclusion that the form of the nebula, when in its most primitive state, must have been of a very jagged character; a conclusion which is very considerably strengthened when we look at table vii., and see that the stars noted in it run up to from twice to not far from thirty times more distant from the sun than [greek: a] centauri. and now, having got a somewhat definite idea of the form of the sun's domains, we may attempt the construction in them, first of a nebula and afterwards of a solar system, such as our text-books describe to us; introducing into the construction, as a matter of course, the variations from existing theories which, we believe, we have demonstrated to be necessary. perhaps we ought to confine our operations to these domains, and so we will almost exclusively; but the sun has been so long considered as one of many millions of stars, and as part of what is now looked upon as our universe, that we cannot help looking upon the whole as having been the result of one act of creation; more especially as we have no reason whatever for supposing it to have been built up piece by piece; and whatever ideas we may form of our own little part of it, we are bound to apply them to the whole. we may, therefore, lay the foundations of our undertaking in the following manner. by creation we mean only creation of nebulæ. we shall suppose all space--if we can comprehend what that means--to have been filled with the ether, and the law of attraction to have been in force previous to the time when our operations are supposed to have commenced. these we may consider to have been the first acts of creation, or to have existed from all eternity. then, in that part of space occupied by our universe--even though it should extend infinitely beyond the reach of our most powerful telescopes--we shall suppose the work of creation to have begun by filling the whole of that space with what are known as the chemical elements, reduced to their atomic state. we do not want to have molecules or particles of matter, or meteorites or meteors; because they involve the idea of previous manipulation or agglomeration, but matter in its very simplest form, if there is any more simple than the atomic. at this stage the most natural idea is to suppose that the whole of this matter was at rest, without motion of any kind, because we cannot understand how motion could be an object of creation, but can very easily see how it might be of evolution; and because, under the law of attraction, matter had the elements of motion in itself. under that law it is quite possible for us to comprehend that all the suns of our universe could have been formed just as they are, with all their movements of rotation, revolution in the cases of multiple stars, and translation or what is called proper motion. and it is within the bounds of possibility that future astronomers may be able to show how these movements have been brought about, should it ever be possible for them to find out and define with sufficient accuracy what the translatory, or proper, motions are. then, as for the temperature of this newly created matter, we have no resource left but to suppose that it must have been that of space, whatever that may have been then, even as we have been obliged to say before. once created, the atoms of the cosmic matter would immediately begin to attract each other in all directions, and form themselves into groups. at first thought it might be supposed that these groups, and suns formed from them, ought to have been all of the same size, being formed from the same material under the same conditions, but nature, or evolution, seems never to be disposed to produce the same results in its manipulations of matter, whatever they may be. when the water is drawn off from a pond, and the mud left in the bottom of it allowed to dry in the sun, it breaks up into cakes of very various shapes and sizes. no doubt there are physical causes for this being the case, but, though perhaps not altogether impossible, it would be a hard task to find them out. much more so would it be with originally created matter, and we have only to accept the fact. moreover, there can be little doubt but that the universe was formed, evolved, according to some design--not at hap-hazard--and that the cosmic matter was created with the distribution necessary to carry out the plan. that the stars differ from each other in magnitude is the best proof of design; for no one can believe that inert matter could determine into what shapes and sizes it could arrange itself. but we have now nothing more to do with the universe, and will confine our operations to the domains of the sun. notwithstanding the vagueness and dimness of the description we have been able to give of the part of space to which our work is now to be confined, we can conceive it to resemble in some degree--not a comparatively flat but--a round starfish, with arms more unequal in length, and irregular in position than the kind we are accustomed to see. in such an allotment of space we can easily conceive that the work of attraction and condensation, of the newly created cosmic matter, in forming itself into a nebula, would be most active in the main body; that in the arms, or projecting peaks as they may be called, it would go on more slowly in the direction towards the centre, the quantity being smaller; and that on account of the greater distance in each from the centre of attraction, and of its being more under the influence of the still existing counter-attraction of the matter in the domains of the sun's neighbours, they might become almost, or rather altogether, detached from the more rapidly contracting main body. we shall, then, suppose that all this has taken place in our incipient nebula. the centre of attraction would at first be the centre of gravity of the whole region occupied by the cosmic matter, which would be ruled in due measure by the projecting peaks, and the indentations or hollows produced in it by the attractive force of the most powerful neighbours; which hollows would gradually disappear as the process of condensation went on, and the main mass would assume the figure of a nebula of some shape. from this stage we may reasonably conclude that, as it was contracting towards the common centre of gravity of all its parts, it would gradually assume a somewhat globular form, and we may now suppose it to have contracted to, say three times the diameter of the orbit of neptune. here, then, we may take into consideration what was the interior construction of the main mass which we may now look upon as a nebula; and we have only two states in which we can conceive it to have been. either that the whole was condensing to the common centre of gravity, in which case its greatest density would be at the centre; or that it was condensing towards the region of greatest mass, in which case its greatest density would be at that region, and its least density at the exterior of the nebula, and also at, or at some distance from, its centre; that is, that the nebula was hollow and without any cosmic matter at all at its centre. in the first case we must recognise that, from that period of time at least, the cosmic matter that was at, or even near, the centre of gravity then, must be there still all but inert, and being gradually compressed to a greater and greater degree of density. there would, no doubt, be attraction and collisions going on amongst the particles, with condensation towards the centre and production of heat--as long as the particles retained the gasiform condition--which might be increased in activity by the pressure, or superincumbent weight, of the whole exterior mass, but there would be no tendency in them to move outwards--provided their gravitation was always towards the centre; and any motion amongst them would be of the same kind as the vibration of the particles of air shut up in a cylinder and gradually compressed by a piston forced in upon them, and not allowed to escape owing to the sides of the cylinder exerting upon them a pressure increasing exactly in the same proportion as the pressure on the piston was increased. and if this was the case with the matter at or near the centre, it would be the same with that of the whole mass, with the exception, perhaps, of the outer layer, which might act the part of the piston in the cylinder. there could be no motions among the particles, except those of collisions and of falling down towards the centre. the outward impacts of collisions would be less strong than those inwards, on account of gravitation acting against them, and the general tendency of all matter would be to move towards the centre. even were we to assume that the whole mass was endowed with a rotary motion, the result would be much the same, that is, increasing stagnation of the matter as it approached to the centre. the areolar law teaches us, however, that the increase of condensation at the centre would increase the rotation there; but in that case we have to acknowledge that this increase of rotation would have to be propagated from near the centre to the circumference, which would be by far the most difficult mode of propagation, and we are forced to think of what would be the rate of rotation at the centre, of a nebulous globe, of some sixteen thousand million miles in diameter, required to produce a rotation at the circumference of even once in four or five hundred years; and from that to think of what must be the speed of rotation at the centre of the sun, at the present day, to produce one rotation at the circumference of twenty-five to twenty-seven days. we should also have to think seriously of how the rotary motion was instituted, and we could only appeal either to simple assumption, or to the impact theory, which, applied to a mass of the dimensions of the one we are dealing with, would require more explanation than the whole formation of the nebula itself. in the second case, that is, looking upon the nebula as a hollow sphere--when it was of the dimensions we have just supposed it to be--we get rid of all the difficulties, and we may add impossibilities, that we encountered in the first case. in such a formation there could be no particle of matter in a state approaching to inertness, not one that could not work its way, through force of attraction and collisions, from the outer to the inner surface of the hollow shell, or _vice versâ_, or all through and round it and from pole to pole--if it had poles then; it might increase or decrease in density, according to the density of the particles with which it came into collision, as it moved from one place to another, but it would find no spot where it could stand still or be imprisoned. even arrived at the region of greatest density, it could change places with its neighbours and move all over that region, if it were condemned to remain with one density once it had acquired it; if not, by acquiring or loosing a little density--_i.e._ by being compressed or allowed to expand a little--it could work its way outwards or inwards, as we have just said, and be as free as the law of attraction would admit of, and as active as that law would oblige it to be. it must be borne in mind that gravitation would act in two opposite directions depending on whether it was acting on the outside or inside of the region of greatest density. we do not go the length of supposing that it could escape altogether from the nebula were its progress outwards; because, as it approached the border, it would meet with plenty of other particles coming in, which would reduce its velocity and prevent its escape. besides, the law of attraction would take good care to prevent it from passing over to a neighbour nebula or sun. it may be argued that in the first case--_i.e._ condensation to the centre--the particles would have the same facilities for changing place, in so far as moving all round the interior of the nebula, or across it, on their way to quasi stagnation, as their densities and the superincumbent weight concentrated and increased; but there could be no motion outwards because the _attraction of gravitation_ would not permit it; nothing could _fall upwards_, all must _gravitate_ to the centre. thus the power of motion in the particles would be limited to very much less than half what they would have in the case of the hollow sphere. it will not do to argue that the increasing heat at the centre would create an upward current. it might create repulsion and prevent the farther-out particles from so soon reaching their final resting or vibrating place, but it could not create an upward convection current of any magnitude; because the colder particles falling down to replace those rising up--that is, if the warmer ones did rise up--being greater in number because occupying greater space, would soon cool down the centre and put an end to the upward current, that is, if it ever came to be set in motion. the greater weight of the greater number would be sure to keep the lesser number in their prison. if any one should say that those nearest the centre would be the heaviest, let him remember that the heaviest liquid or fluid does not rise to the surface. there could be no furnace at the centre to heat the cold particles as they came down to replace those that had just risen up; and if there was, it would be gradually cooled and extinguished. in fact, the centre region would become colder than that immediately outside of it, and so on until the greatest heat would be at the surface of the nebula. should it be argued that the vastly greater number of particles in the outer regions would help those at the centre to rise up, we agree; but it would be because the attraction would be greater outwards than inwards, as we have shown all along, and not because the pressure forced them out--against itself. but, it must be added, this means that if there was still a plenum at the centre the particles that had once left the centre could never come back again, nor any others to replace them, and that no convection current could ever be formed for carrying heat or matter from the centre, or its immediate neighbourhood, outwards. in view of the above comparison of the two cases--added as a complement to what we believe we have demonstrated in a former part of our work--we shall adopt the second as being most in harmony with the laws of attraction, and of nature in general, and shall endeavour to describe in some detail, the construction of the nebula out of the matter belonging to the domains of the sun, as we have marked them out. we have already said that on account of being at the greatest distance from the main body, and at the same time nearer than all other parts of it, to the attractive force in the domains of the neighbouring stars or nebulæ--which attraction continues to be exerted upon the solar system up to the present day--the matter in the high peaks which we have shown would form part of the sun's domains, would come to be completely separated from the rest of the nebulous matter. we shall now assume this to have come about, the detached pieces, somewhat in the shape of cones, occupying positions distant from the main body, in some sort of proportion to their altitudes and masses. this separation would naturally make some alteration on the centre of gravity of the remaining mass. it would come to be nearer to the deep hollows, made in the mass by the attraction of the most powerful of the nearest neighbouring stars; and as we have seen that the hollows made by sirius and [greek: a] centauri would be the deepest, and also for greater simplicity in description, we shall suppose that the centre of gravity would come to be nearer to these hollows than it had been before. then, as the condensation and contraction proceeded, the tendency would be to fill up these hollows, and, as a consequence, the matter at the opposite side of the nebula would at the same time tend to lag behind in approaching the centre--for the same reasons we have given in the case of the peaks--and might easily come to be detached from the main body altogether, first in the form of shreds, then in larger masses, and afterwards in concave segments of hollow spheres, as contraction advanced; and the whole seen from a sufficient distance, would have the appearance of a nebula with crescents, perhaps almost rings, of nebulous matter and detached masses on one side of it; all very much like what we know to be the figures presented by some nebulæ. when contraction had continued till the hollows caused by sirius and [greek: a] centauri were filled up, we might suppose that the nebula had come to be somewhat of a spherical form, although far from being very pronounced, and we have now to consider what its internal structure might be and most probably was. in describing the construction of the earth-nebula we showed that particles of matter placed at different parts of its interior, even not very far from the surface, would be drawn out, in the first place by the greater number coming in from a greater distance from the centre, and that when they met they would all be drawn in towards the centre by the conjoint attraction of the whole mass; and now we can apply this fact to the larger solar nebula, and consider what might be the result. let us fix upon a certain number of equidistant zones in a sphere of cosmic matter, extending from the centre at _a_ to _b_, _c_, _d_ and _e_, at the surface. we know that, according to our former reasoning on particles, and the law of attraction, part of the matter of the zone at _a_ will be drawn outwards by that at _b_, while part of that at _b_ will be drawn inwards by that at _a_, and that the same will take place with all the other zones out to the surface at _e_; and thus there might come to be congested layers between these equidistant places, and there might even be formed hollow spheres within hollow spheres, independent of each other, all through the nebula from near the centre to the surface. this idea is by no means fanciful, as is witnessed by the accounts given in chambers's "handbook of astronomy," already referred to, vol. i., and the figs. , and , showing the form and appearance of the remarkable comets of and . if different, almost concentric, zones or layers of cosmic matter can be constituted in the hemisphere forming the head of a comet, there is no reason why concentric layers of the same matter should not be formed in a nearly spherical nebula. in fact, we can appeal to what is seen in the heads of the two comets cited, donati's also represented in the same work, figs. - , as convincing proof of the correctness of our contention and demonstration that all satellites, planets, suns, and stars are hollow bodies. even the tails of comets, at least of the larger ones, are acknowledged to be hollow bodies. when steadily looked into we find the notion that all fluid bodies are hollow to be much more common than is perhaps generally believed. beginning with the smallest, we find what follows in the rev. dr. samuel kinn's work, entitled "moses and geology," edition , page : "a mist, whether in the form of a cloud or fog, is composed of small bodies of water obeying the laws of universal gravitation by forming themselves into spherules, which halley and other eminent philosophers thought to be hollow. as water is heavier than air, scientists were for a long time seeking for a good reason to account for clouds floating. it may be that kratzenstein has somewhat solved the problem. he was examining in the sunshine some of the vesicles of steam through a magnifying glass when he observed upon their surface coloured rings like those of soap-bubbles, and some of the rays of light were reflected by the outside surface, others penetrated through and were reflected by the inner surface; he concluded, therefore, that the envelope of the sphere must be excessively thin to admit of this taking place. we may, therefore, suppose that these vesicles are filled in some way with rarefied air, and are so many little balloons whose height in the atmosphere varies in proportion to the density of the air they contain. how this enclosed air should become rarefied on the formation of the tiny globule is a problem still to be solved." dr. kinn says nothing of _how_ the spherules of cloud or fog were formed by the laws of universal gravitation, nor _why_ halley and the other eminent philosophers thought them to be hollow, and only states the fact that kratzenstein found the vesicles of steam to be hollow; and only one cause can be assigned for such being the case, namely, the manner in which we have shown how hollow spheres can alone be formed. that the vesicles of steam examined in the sunshine were hollow it would seem there can be no doubt; and if so, there can be as little that halley and the others were right in thinking the spherules of clouds to be hollow. the steam vesicles could not come into existence at once in the air, in form large enough to be examined through a magnifying glass, but must have been built up out of a multitude of the very smallest atoms of water turned into vapour; and would follow the same law as the atoms of cosmic matter and so form the little balloons. in their formation the hollow space would be filled with air, which would expand when heated and contract when cooled, and so regulate their height in the atmosphere. and thus the problem of the last sentence of the quotation is solved. we shall now go to the opposite extreme of matter, and see what mr. proctor says when treating of the formation of a stellar system; but we must state that it is not very clear to us, whether he is exposing mädler's ideas or his own, although we think they are his own or, at least, adopted. he says in "the universe of stars" at page : "he (mädler) argues that if a galaxy has a centre within the range of the visible stars, a certain peculiarity must mark the motions of the stars which lie nearer to the centre than our sun does. as has already been mentioned, the neighbourhood of the centre of a stellar system is a scene of comparative rest. in the solar system we see the planets travelling faster and faster, the nearer they are to the great ruling centre of the scheme; and the reason is obvious. _a._ the nearer a body is to a great centre of attraction like the sun, the greater is the attraction to which it is subject, and the more rapid must its motion be to enable it to maintain itself, so to speak, against the increased attraction; but in a vast scheme of stars tolerably uniform in magnitude and distribution, _the outside of the scheme is the region of greatest attraction, for there the mass of all the stars is operative in one general direction_. (the italics are ours.) as we leave the outskirts of the scheme, the attraction towards the centre becomes counterbalanced by the attractions towards the circumference; and at the centre there is a perfect balance of force, so that a body placed there would remain in absolute rest. it is clear, then, that the nearer a body is to the centre, the more slowly will it move." (compare this last sentence with the one beginning at _a_ above.) here we have recognised, the principle that in a star system the immensely greater number of stars at the outside of the scheme would produce a perfect balance of force, and that a body placed at the centre would remain in absolute rest. this agrees wonderfully well with what we have been arguing, a few pages back, with respect to a sun solid to the centre. matter at the centre would be at absolute rest, _dead_, that nearest to it would be nearest to dead, and so on through a sun or planet, gradually coming to life as it came nearer to the surface; exactly as we have shown it would be, having in it little more than rotary motion. when once acknowledging the immense superiority of attractive force of the stars at the outskirts of the system, over the very few there could be at its centre, mr. proctor seems to have stopped short with the idea and to have contented himself with one body at the centre in absolute rest. had he gone one step further he must have seen that one, or even a very few, could not maintain themselves near the centre with such an immense number pulling them away in every direction. there could be no perfect balance of force. and had he applied the same idea to the earth, and followed it out to the end, he could not have written as he has done, in "the poetry of astronomy," at page , "that the frame of the earth is demonstrably not the hollow shell formerly imagined, but even denser at its core than near the surface." he would have found some difficulty in fixing his first dead particle at the centre, when there were such infinite hosts of near and far-off neighbours endeavouring to annex it. he would have found that the absolute rest was neither more nor less than absolute vacuum. it is utterly impossible to show how any body could be built up out of a nebula of cosmic matter, or even meteorites, from a solid centre, under the law of attraction. we repeat that any foundation laid there would be in a state of unstable equilibrium, and would be hauled away out of its place never to return; unless the cosmic matter around it were so perfectly arranged on all sides that its attraction on the foundation would be absolutely equal in all directions; a condition which cannot be imagined by any one who takes the trouble to think of it. and we think we may add, that no body could be established at the centre of a system of any kind unless it were of sufficient magnitude to control the whole matter within range of it, exactly as we see in the solar system; and that the central body could be no other than a hollow sphere. thus we have either to look upon the sun with his planets and their satellites as hollow bodies or to conclude that the solar system was not formed out of a nebula. coming back to our nebula after the hollows in it, caused by the attraction of sirius and [greek: a] centauri, were filled up, and when we showed that it might have had the interior form of a series of hollow spheres one within the other, and also might be accompanied by crescents and shreds of cosmic matter on the opposite side to the hollows--a supposition we put forward more in explanation of what is to be seen in some nebulæ and comets, than as in any way necessary for our purposes--then, even although it had been separated interiorly into different layers or concentric shells of spheres, these layers continuing to attract each other, would finally come to form one hollow sphere with its greatest density at the region where the inwards and outwards attractions came to balance each other. long previous to this stage--even from the very beginning--the atoms gradually coalescing into larger bodies, would be attracting, colliding with, repelling and revolving around each other, sometimes increasing in dimensions, at others knocking each other to atoms again; but there would be a tendency in them to combine into larger masses as they approached the region of greater density, where the attraction was greatest. now, if the collisions and encounters amongst the masses, great and small, always exactly balanced each other, the whole mass of the nebula would gradually contract towards the region of greatest density, and the whole would ever remain without any other kind of motion in it than what can be seen in a mity cheese--a kind of congeries of particles heaving in every, and at the same time in no, direction. but as an absolute balance of collisions could not be maintained for ever, especially where they would be constantly varying in force and direction, a time would come when movements of translation, as well as of collision, would be instituted on a large scale, in many directions, which, if they also did not manage to balance each other--an affair equally as impossible as in the other case--would ultimately resolve themselves into motion in one predominating direction through the whole nebula. we do not forget that we are dealing with the shell of a hollow sphere, not with a ring, or section of a cylinder, and we can conceive that there would be, from the first, partial motions of translation in multitudes of directions, radial, angular, transverse, etc. etc., constantly changing, even being sometimes reversed, but also constantly combining with each other, and gradually leading on to decided, though partial, uniformity in one direction. as a matter of course this motion of translation would be controlled by its own constituent parts attracting each other to some extent, and thus a rotary motion would be established in the interior of the nebula in the region of greatest density. we can also conceive that when the motions of translation had become nearly uniform, the plane of that uniform motion might be in any direction through the whole mass of the nebula, and might be continually varying until final uniformity was attained, when the greater part of the mass was moving in combination, and the rotation was thereby firmly established in one direction, though still not embracing the whole. we have to take into account also that when the rotary movement had settled down into one plane, it would be most active at the distance of the region of greatest density of the nebula from its centre; in fact it would be instituted at that region and be, therefore, most active there; and then the most active part of the matter would be in the form of a rotating ring, still surrounded by an immense mass of nebulous matter, both inwards and outwards, to which it would gradually communicate its own motion, until the whole mass would rotate, in one direction, on an axis. but it is evident that in the whole rotating mass there would be different degrees of velocity of rotation at different places, decreasing from the supposed ring inwards towards the centre, and outwards to the surface at what would thus become the equatorial region; and also decreasing from the equatorial plane to the poles. following up this idea, we have a more reasonable manner of accounting for the different velocities of rotation observed on the surface of the sun, between the equator and the poles, than we have seen suggested in any speculations on the cause that have come under our observation. until rotation was fully instituted, the areolar law could have no power over the multitudinous movements going on in the nebula, but from that time it would begin to act, and condensation would increase it at the region where it began; and as all increase had to be propagated from there, inwards, outwards, and in all directions, the differences in velocity of rotation throughout the sun must endure as long as he continues to contract. in this we find an immense field for producing heat in the sun, from the eternal churning which must be going on in the interior. a rotary motion produced in this way might have two different results: in one case the rotation might be continued until the matter at the polar regions had all fallen in towards the centre, and had been thrown out afterwards by centrifugal force and the whole mass converted into a nebular ring, in the form of the annular nebula in lyra. in the other case we could conceive that, in a smaller nebula, the centrifugal force of rotation caused zones to be abandoned at the equatorial surface, in the manner set forth by laplace in his hypothesis, and that the matter from the polar regions fell in more or less rapidly for the formation of the different members of a system like the sun's; and that the dimensions of the planets would be determined by the rapidity with which the matter fell in as the process went on. such a conception would help to account for the outer planets of the solar system being so much larger than the inner ones, because there would be more matter falling in; and make us think that the nebula in lyra is destined to form a system of multiple stars. some years after this mode of instituting rotary motion in a nebula was thought and written out, and also an extension of it to which we may refer later on, we came upon a kind of confirmation of the correctness of our views in an article in "science gossip" of january , on the nebular hypothesis, where it is said: "we have established, then, the existence of irregular nebulæ which are variable--that is, the various parts of which are in motion.... now, with the parts of the nebula in motion, whether the motion is in the form of currents determined hither and thither according to local circumstances, or in any other conceivable way, such motions bearing some reference to a common centre, unless the currents exactly balanced each other--a supposition against which the chances are as infinity to one--one set must eventually prevail over the other, and the mass must at last inevitably assume the form peculiar to rotating bodies in which the particles move freely upon each other. it must have become an oblate spheroid flattened at the poles and bulging at the equator, rotating faster and faster as it contracted. in some such manner has our solar system acquired its definite rotation from west to east." the writer in "science gossip" has taken the irregular motions in the nebula as made to his hand, and has come to the same conclusion as we have, namely, that they would all resolve themselves into motion in one direction only, always subject to the general attraction towards the centre of gravity of the nebula, which means motion round a centre, perhaps not necessarily rotary motion. however, the only difference between his ideas and ours is that we deal with a hollow nebular shell, in which, it will be acknowledged, it would be much more easy for the law of attraction to produce marked and distinct motions of any kind, and which would lead to one motion in one direction throughout, than in a nebula homogeneous, or nearly so, from the surface to the centre. whether it would lead to the formation of an oblate spheroid is another question, as that might depend on a variety of circumstances, one or more of which we shall have to touch later on; in fact, we have already shown how the very reverse might be the case. chapter xvi. page the sun's neighbours still exercise their attraction over him. regions of greatest density in the nebulæ dealt with; compared with the orbits of the planets made from them. results of comparison favourable to the theory. differences of size in the planets have arisen from variations in the quantity of matter accumulating on the nebulæ. causes of the retrograde motions in neptune, uranus, and their satellites. probable causes of the anomalous position of neptune. rises and falls in the densities and dimensions of the planets explained. the form of the nebulæ must have resembled a dumb-bell. more about rises and falls in densities. reason why the asteroid nebula was the least dense of the system. not necessary to revise the dimensions given to the nebulæ. causes of the anomalies in the dimensions, densities, etc., of the earth and venus. the strictly spherical form of the sun accounted for. but it may yet be varied. repetition that a spherical body could not be made from a lens-shaped nebula by attraction and condensation. testing the practicability of the hollow sphere theory. retrograde motions, positions, densities, masses, etc. etc., considered. before going any farther it will be convenient to try to find out whether the solar system could have been constructed from a hollow nebula such as we have been describing gradually contracting as the matter for the formation of one planet after another was abandoned until--as we have put it--the nebula could abandon no more matter, and finally resolved itself into the sun. for this purpose we may suppose it to have been condensed and contracted until its extreme diameter was , , , miles; the same as we supposed it to have been, when we began the analysis of the nebular hypothesis. we will not now, however, suppose it then to have contained the whole of the cosmic matter out of which the system was formed, as we did before; because we have seen as we have come along that a very considerable part of that matter must have been left behind, almost from the moment that contraction commenced. we have already given the reasons for this in describing the domains of the sun; and, leaving the peaks out of account altogether for the present, we will only deal with the regions of what we have called the main body. although we have fixed a limit beyond which the neighbouring stars could not draw off any cosmic matter from the domains of the sun, that does not mean to say that their attractive powers would cease at that limit; because we have had to acknowledge that each one of them continues, even now, to exert its attractive power up to the very centre of the sun. they would still have power to counteract, in some measure, the sun's attraction of the matter of the nebula towards his centre, and the result would follow that there would be one or more, even many, fragments of the main body which would be left more or less behind, and in varied forms, when the more central part had contracted to the dimensions to which we have now reduced the nebula--all much the same as we have already said a few pages back. when the nebula was , , , miles in diameter its volume would be , ^{ } cubic miles--as we have seen at page --the half of which is , ^{ } cubic miles, corresponding to a diameter of , , , miles, or radius of , , , miles. now, according to our theory, it would be at this distance from the centre that the greatest density and activity of the nebulous matter would be, where we have just been showing how a movement of rotation could be generated, and where, in consequence, its motive power, so to speak, originated and existed. here we find by dividing , , , by , , , that the region of greatest density in such a nebula would be at · of its diameter. in our calculations about the earth, as it is, the proportion was found to be · , but the densities of the outer layers were empirically arranged by us; and, besides, almost the whole of the mass was supposed to be solid matter, so that no accurate result could be expected from that operation. there also we found that the inner surface of the hollow shell was at · of the whole diameter, which we may adopt for the nebula we are about to deal with, as that dimension may be varied considerably--so may the other also--without in any way vitiating our theory. having found these proportions, which can only be considered as distantly approximate, let us go back to the nebulæ--excluding the final solar one--into which we supposed the original nebula to have been divided--in the analysis just alluded to--and see how the regions of greatest density in them would correspond to the orbits of the planets formed out of them. this examination requires a good deal of calculation and accompanying description, which it might be found tiresome to follow, and would really answer no good end were it written out; so we shall suppose it to be made and the results obtained from the calculations to be represented in the form of table ix., where they can be seen at a glance almost, and compared without much trouble. this arrangement will also furnish a readier means of reference for the remarks we shall have to make on, and the information obtained from, the examination. and we have still to add that the extreme diameters of the nebulæ are the same as those we used for the analysis; as also, that we make use of only the first of the proportions just cited, viz., · , it being the only one required for determining the positions of the regions of greatest density in the nebulæ. table ix.--dimensions of the nine nebulÆ, with their diameters and regions of greatest density compared with the diameters of the orbits of the planets formed from them. ----------+--------------+---------------------------+ | nebula. |region of greatest density.| | | | name of |--------------+-------------+-------------+ planet. |outer diameter| diameter | radius | | in miles. | in miles. | in miles. | ----------+--------------+-------------+-------------+ | | | | neptune | , , , | , , , | , , , | | | | | uranus | , , , | , , , | , , , | | | | | saturn | , , , | , , , | , , , | | | | | jupiter | , , , | , , , | , , | | | | | asteroids | , , | , , | , , | | | | | mars | , , | , , | , , | | | | | earth | , , | , , | , , | | | | | venus | , , | , , | , , | | | | | mercury | , , | , , | , , | +------------------------------------------+ | had the position of neptune been normal, | | the above data for him and uranus would | | have been as under. more or less. | +------------------------------------------+ neptune | , , , | , , , | , , , | | | | | uranus | , , , | , , , | , , , | ----------+--------------+-------------+-------------+ ---------+---------------------------+------------------------------+ | orbit of planet. | region of greatest density | | | compared with orbit. | name of |-------------+-------------+-----------+-----------+------+ planet. | diameter | radius | within, | without, | per | | in miles. | in miles. | (miles.) | (miles.) | cent.| ---------+-------------+-------------+-----------+-----------+------+ | | | | | | neptune | , , , | , , , | , , | | · | | | | | | | uranus | , , , | , , , | | , , | · | | | | | | | saturn | , , , | , , | | , , | · | | | | | | | jupiter | , , | , , | | , , | · | | | | | | | asteroids| , , | , , | | , , | · | | | | | | | mars | , , | , , | | , , | · | | | | | | | earth | , , | , , | | , | · | | | | | | | venus | , , | , , | , , | | · | | | | | | | mercury | , , | , , | | , , | · | +----------------------------------------------------------+ |had the position of neptune been normal, the above data | |for him and uranus would have been as under. more or less.| +----------------------------------------------------------+ neptune | , , , | , , , | | , , | · | | | | | | | uranus | , , , | , , , | | , , | · | ---------+-------------+-------------+-----------+-----------+------+ from the table we see that the region of greatest density of our original nebula was at · per cent. _within_ the distance of neptune's orbit from the sun, a state of matters which precludes the idea of condensation during, at least, a great part of the act of abandoning the ring for the formation of that planet. but it will be remembered that we gave it the diameter of , , , miles without assigning any adequate reason for doing so, and, we can say with truth, with the idea, more than anything else, of not increasing the almost unimaginable tenuity of the matter composing the nebula; and the position of neptune in the system is so peculiar compared with the other planets, that it cannot be properly used as a standard for any kind of inquiry. the result obtained above can therefore be of no use for the investigation we have undertaken. not only so, but the almost similar result in the case of uranus is also rendered useless from the same cause, in which we find that the region of greatest density of the nebula is only · per cent. beyond the orbit of the planet. if the mean distance from the sun of neptune's orbit had been what was used by leverrier in the calculations which led to his discovery, namely, · radii of the earth's orbit, the region of greatest density of the uranian nebula would have been · per cent. beyond his orbit, as may be seen from the addition to table ix., in finding which we have used the same system as in all our work. in the next four nebulæ of the table--including the one we introduced to represent the asteroids--we see that their regions of greatest density are respectively · , · , · and · per cent. farther out from the centre of the sun than the orbits of the planets formed from them. here, then, we see a very apparent approach of uniformity, and can say with much reason that planets could certainly be formed out of the matter abandoned, through centrifugal force, by hollow nebulæ similar in construction to what we have demonstrated that of the original nebula to have been; each of them occupying the position corresponding to its orbit. following these come the earth and venus nebulæ. in the former, the region of greatest density almost coincides with the orbit of the planet, being only · per cent. beyond it, instead of something like per cent. as it ought to be to conform with the four preceding cases; and in the latter it is · per cent. within the orbit of the planet to be made from it. but in this case we have to note that the orbit of venus is · per cent. beyond the position pointed out for it by bode's law, and that it is the only one of the whole number of planets whose orbit is farther removed from the sun than the distance assigned to it by that law. also we see from our reversal of bode's law, that the rates of acceleration of rotation for these two planets are · for the earth and · for venus, instead of the average of · of the four preceding planets; that the density of venus is less than that of the earth, instead of being greater as it is successively in all the other planets from saturn inwards; and we may add that the diameters are nearly equal. all showing that influences had been at work in the formation of these two planets, different to those in the preceding four; and that until we know what these influences have been, we cannot account for any anomalies produced by them. neither are we called upon to consider that our theory is destroyed by these anomalies, any more than it can be by the anomaly in the case of neptune's position. lastly, we have in mercury the region of greatest density of his nebula at · per cent. beyond his orbit, and the rate of acceleration of revolution over venus · times, both of which conform fairly well with the same noted facts; in relation to mars, the asteroids, jupiter, saturn, and, we may add, uranus. but, in justice, we must not omit to add that there may be some error in the excess of · per cent. in the distance from the sun beyond his (mercury's) orbit, arising from the fact that there may have been some difference from what we made it to be, in the line of separation between his nebula and that of venus; and also that we had to guess at the line of separation between his and the residuary nebula. moreover, it has to be taken into account that his orbit is · per cent. within the position assigned to it by bode's law. from the table ix., and an examination of it, we learn that out of the nebulæ into which we divided the original one, in the analysis of the nebular hypothesis, we have five--four of which are consecutive--which may have been almost of the same construction, and not far from the same proportions; that the original nebula cannot, for reasons assigned, be looked upon as either similar, or the reverse, to the five just classed; that one, the uranian, is practically similar to the five, and might be exactly similar could the anomaly in the position of neptune be explained; and that the remaining two, the earth and venus nebulæ, seem to show that they have been abandoned in a manner different from the others. perhaps we may be able, later on, and in a different way, to give a reasonable explanation of the anomalies in the positions occupied by neptune, the earth, and venus, and also of the peculiarities of their dimensions. so far, we believe we are justified in concluding that out of the nebulæ, may really be considered as supporting our theory, and the remaining as, in all probability, capable of being shown to be, at least, not opposed to it. to this we may add that on several occasions we have stated our opinion, that the divisions between the nebulæ we have established, could not have taken place at the half-distance between the orbits of any two planets, but much nearer to the outer one. it is evident, then, that if we had made the divisions at any distance farther out, say at three-fourths of that distance from the inner orbit, the extreme diameter of each one of the nebulæ would have been just so much greater, the region of greatest density farther out from the centre of the sun, and even that of neptune would have been beyond his orbit. all this could be done, yet but it would serve no good purpose, as will be seen presently; and we might be accused of cooking our data in order to produce a result favourable to our theory. we have made the foregoing examination because, when we began our work, the general idea was that, according to the nebular hypothesis, the material for the formation of each planet was abandoned by the ideal nebula in a distinct and separate mass from any other--we are not at all sure, however, that this was laplace's idea. this, we found out, could not be the case when we attempted to give some sort of separate or distinct form to the matter out of which neptune was supposed to have been formed; and when we became convinced that all the matter abandoned by the nebula, from first to last, must have been thrown off in one continuous and, most probably, uninterrupted sheet. this, of course, makes us think of how the division of the sheet into separate rings was brought about, for there must have been absolute separation between them, otherwise separate planets could not have been made out of the sheet; and the only explanation that can be given is, that it must have depended on the quantity of matter that was abandoned, in nearly equal times, at different periods of the operation; for the areolar law precludes the idea of there having been very rapid changes in the rate of rotation of the nebula, and certainly of its decrease at any period as long as condensation and contraction went on. whereas, although the sheet thrown off may have been continuous, we have no reason to suppose that it was of constant volume or density from beginning to end of the operation; in fact, we have already seen that its density was constantly increasing, and have suggested, in the reversal of bode's law, that the differences in dimensions and densities of the planets have arisen, from irregularity in the quantities of matter abandoned from time to time. this irregularity could only arise from the mode of construction of the nebula, and from the forms it assumed during condensation, as we shall attempt to show in due time. meanwhile we can conclude that the region of greatest density in any of our nebulæ had no influence whatever on the position of the orbit of the planet that was formed out of it. we have shown, very clearly we believe, at page , from quotations--at second hand--from his own exposition of his hypothesis, that laplace considered that condensation could only take place at the surface, or in the atmosphere as he called it, of his nebula, on account of its being possible only after radiation into space of part of its excessive heat; and that consequently there could be no acceleration of rotation in the nebula, due to the areolar law, except where there was condensation. on the other hand, in our cold hollow-sphere nebula, condensation could only take place at the region of greatest density, or greatest mass, which must be always very much nearer to the surface than to the centre; so that in both cases, equally, the abandoning of matter under the influence of centrifugal force would be virtually the same, and no further remarks are called for, on our part, on that head. neither is it necessary for us to show how planets could be formed out of the rings abandoned by their respective nebulæ, for everybody seems to agree that when they broke up, the fragments could not do otherwise than form themselves into small nebulæ, which in the course of time condensed into planets. m. faye's explanations are good for that. with respect to their motions of rotation being direct or retrograde, we have seen, at page , and following, that laplace's description of how the former motion could be brought about is mechanically correct; and, at page , that he did not consider that the direction of revolution of a ring necessarily demands that the rotation of a planet formed from it should be in the same direction. as already said, he has shown how direct rotation could be produced, and we have no doubt that he could have shown how retrograde rotation could also be produced, had he found it to be at all necessary. be that as it may, however, it is a very simple matter to show how, following our method of construction of the primitive nebula, the retrograde rotation of uranus and neptune could, or rather must, have been determined. it will be remembered that when we were "getting up" the original nebula in the domains of the sun, whose form we described as well as our limited means would admit of, we said that when the cosmic matter contained in them began to contract, not only the parts contained in the peaks and promontories would soon be left behind, and come in at a slower rate, but also large masses of the outer part of the main body, especially of what was on the sides opposite to the deep hollows made in the domains by the most powerful of the sun's neighbours, in the form of fragments, crescents, and parts of hollow segments. let us now, then, suppose the operation of planet-making to have advanced so far that the whole nebula was rotating on its axis, and abandoning matter through centrifugal force, from its equatorial regions in a continuous sheet, as we have said several times that it must have done, and that the matter destined for neptune and uranus has not only been abandoned, but divided into two distinct rings--a supposition made in this case only for facility of description. then some of the matter which had been left behind, but still being gradually drawn in, would be almost totally intercepted in the equatorial regions of the nebula by these two rings, and would fall in greater quantity upon their outer edges than anywhere else, more especially in the case of the outer one. these adventitious additions would come in without any angular, or tangential, movement whatever, because rotary motion was not yet established in them, and would retard the revolutionary movement of the rings--in decreasing degree from their outer to their inner edges--while acquiring angular motion themselves; and would also intensify the original difference in revolutionary motion already existing at these edges. at the same time these additions of extraneous matter would seriously impede the contraction of the rings in the radial direction on account of their volume, but would have little or no effect on contraction in the circumferential direction; the consequence of which would be that they would break up before friction, and the mutual collisions of their particles, had time to produce a uniform revolving motion throughout their whole breadth; that is, while their inner edges would be still revolving with more rapid velocities than the outer ones; and the rotary motions of the planets derived from them would be retrograde, according to m. faye's demonstration--or that of any other who has taken the trouble to think over the matter. and we may add that this mode of reasoning, applied with a little more detail, will very fully account for the rotation of neptune being more decidedly retrograde than that of uranus, because the quantity of matter so deposited on the outer flat ring in this process would unquestionably be greater than on the inner one, and consequently the difference of velocity between the outer and inner edges of the two rings also greatest on the outer one. we take it to be unnecessary even to say that, the revolution of the satellites of these two planets being retrograde and anomalous, the rotation of their principals must be retrograde and anomalous also. before going any farther we have something to say about the anomalous position of the orbit of neptune, which is certainly not the position sought for by m. leverrier; in fact, the elements employed by him in his calculations to discover a perturbing planet--whose existence may be said to have been known--are so different from the elements of the one actually discovered, that there would be nothing out of reason in saying that neptune is not the perturber that was sought for, but only an instalment of the perturbing force. it may raise a storm in some quarters to say so, but the fact remains the same, or it must be confessed that mathematics is a more elastic science than it professes to be. he has not the power of attraction required to produce the perturbations in the movements of uranus which gave rise to the search for an outer planet. m. leverrier made his calculations under the belief that a planet of / th part of the mass of the sun was required to produce the perturbations that had been observed in the orbital motion of uranus; whereas the planet discovered has only / , th of that mass--not one-half of what was required. on the other hand, the semi-axis major of the orbit of the planet discovered is found to be · instead of · (bode's law measures) used for the search; which greater proximity to the sun, it is true, increases its power of attraction · times, but as its mass is only · per cent. of what was expected, the attractive force would amount to less than · per cent. of what was required. then the question comes to be, where did the wanting · per cent. of attractive force come from? and the answer is that some astronomers have been searching for another planet to make up the weight, with more or less diligence, ever since the deficiency came to be recognised. but all that we want to have to do with the question is to suggest a very plausible reason for the anomalous position of the orbit of neptune. if there is another planet beyond neptune, the ring (perhaps the rings) out of which he and the others were made, must have been much greater in breadth than what we have assigned to it at page , viz. , , , miles; perhaps even one-half more, as may be deduced from the addition made to table ix., and what we have said in connection with the semi-axis major adopted for the sought-for planet, by m. leverrier in his calculations. now, that a ring of such enormous breadth should have held together in one piece, until it finally broke up through condensation and contraction, requires an extraordinary effort of imagination, after seeing what has taken place with the rings of saturn; even the breadth of , , miles appropriated to the uranian ring (see page ) demands an elastic imagination to conceive its holding together; so that the outer ring of the system may very well have been divided into two, as we have said at page , and two not very unequal planets made out of it--one into neptune, and the other into one as far beyond m. leverrier's adopted distance of · , and of such mass as would make up the missing · per cent. of deficient attractive power. no doubt the outer ring may have broken up into several planets, or even into a swarm of asteroids, but we prefer to think of only two planets; because it seems to us that to draw uranus into the position he occupied when neptune was discovered, the two planets must have been operating in conjunction; an idea that is not so easily entertained when there are several planets, or a host of asteroids, to be taken into account. we have already discussed, at page , the mode of formation of the sheet of matter abandoned by the nebula, its posterior division into separate rings, and how the part of these rings from saturn inwards could revolve themselves into planets having direct motion, so it is not necessary to go over the same ground again, merely because we are dealing with a hollow nebula instead of one full of cosmic matter to the centre. we have also shown, at page , that the nebula must have been somewhat in the form of a cylinder terminated at each end by what may be looked upon as a segment of a sphere, although it would more probably be an almost shapeless mass of cosmic matter, because the greater part of it would be very slowly brought under the influence of centrifugal force as it fell in from the polar directions; and again, a few pages back, that almost all the matter coming in from its equatorial regions--even what might be called its tropical regions--would be intercepted before it could reach the saturnian nebula. likewise, at page , when examining bode's law reversed, we have seen a limit set to the acceleration of the movement of revolution in the planets of the system as they approached the centre, because any acceleration beyond a certain limit, clearly marked out, would of necessity be within the nebula itself, and the rate of revolution would be less than that of the sun on its axis at the present day. this may be used as an argument against the nebular hypothesis, but we think we have shown in the same chapter vii. that this is not the case. but we have still to try to account for the repeated rises and falls in density in the planets from neptune to mercury, or even farther; which operation causes us to bring forward, first of all, a new idea as to what the form of the nebula would come to be. [illustration: fig. .] the accompanying rough sketch (fig. ), drawn to a scale of one-quarter inch to , , , miles shows that, supposing the saturnian nebula to have been a perfect sphere, and to have abandoned matter till the velocity of rotation came to be equal in a region corresponding to the tropical region of the earth, the cylindrical part of it would present a straight side of more than , , , miles in length; provided always that the general diameter of the nebula did not decrease through condensation and contraction during the operation; but as this could not be the case the length of the cylindrical part would be considerably less than that. how much less we have no means of calculating. on the other hand we have seen, when discussing, in the case of jupiter, how matter must have been abandoned by any nebula, that from the time the original nebula began to abandon matter through centrifugal force, it must have gone on acquiring a constantly increasing length of straight side as it contracted. thus the saturnian nebula would begin work with the accumulated cylindrical length it had inherited from neptune and uranus, so that the straight side may have been very much longer than that shown by the sketch; a simple look at it is enough to make one believe that this would be the case. but this idea naturally leads us to another digression. looking again at fig. , we see that acceleration of rotation in the nebula would originate where condensation was greatest, that is at the region of greatest density, and have to be propagated from there to its periphery so that it would reach the middle of the cylindrical part sooner than the ends; and as the nebulous matter at the ends of the cylindrical part could not be abandoned until it had acquired the centrifugal force necessary to overcome gravitation, it would lag behind and overhang, as it were, the middle of the cylindrical part; which means that instead of continuing to be straight, the line of separation between the nebula and the abandoned matter would come to be concave; and in this manner the nebula would soon assume the form of a dumb-bell, gradually becoming more and more pronounced as condensation proceeded. one can hardly help concluding that this must have been the way in which the dumb-bell nebula near star vulpeculæ was formed. the representations of it given by chambers, vol. iii., page , figs. and , as seen by smyth and sir john herschel are most confirming of this idea; notwithstanding the changes of appearance shown by lord rosse's reflectors of feet and feet diameter, figs. and , which are not difficult to account for. it is easy to imagine how fig. could be converted into fig. when observed with a much more powerful telescope. we can conceive the roundest end of it being reduced into the sort of compact segmental form on the left hand side of the figure, and the spread-out part of it into the more diffused segment on the other side; but the form of the whole figure forces us into another conception. mr. chambers says the general outline resembles a chemical retort, but to our eyes it is infinitely more like one half of a dumb-bell broken off from the other. so like it that we feel inclined to ask what has become of the other half. this again makes us think of an enormous dumb-bell nebula dividing itself into two parts, one of which has disappeared or broken up in some manner without leaving any distinguishable traces of its existence, and the other, either forming itself into a double star, assuming in the process the form of a dumb-bell, or actually of one rotating in a direction almost at right angles to that of the original one; more probably the former of the two. perhaps we have allowed our ideas, or fancy, to run on too far; nevertheless, looking over the forms of nebulæ represented in chambers's classical work, and duly considering how inconceivably strange some of them are, there is nothing impossible in all we have said. returning to the repeated changes of density in the solar planets, we know that the matter first abandoned by the original nebula, through centrifugal force, would be at the lowest stage of density, and that what followed would go on gradually increasing in density as it contracted to the saturnian nebula. but, as we have shown that immense quantities of matter belonging, so to speak, to the sun, though actually separated from the original nebula, must have fallen in upon the sheet after being abandoned, it is not difficult to see that the part of the sheet out of which neptune and uranus were made, might be more dense than the saturnian nebula, on account of this matter being added to it; and that, as the greater portion of it must, at the more advanced stage of the process of condensation, have fallen upon the uranian part of the ring, because the space from which it fell would be higher, the density of that would be greater than the neptunian part of the sheet; both of them exceeding the density of the saturnian nebula. again, we have supposed, very naturally we think, that all extraneous matter coming in from the equatorial direction would be intercepted by the rings destined for neptune and uranus, so that the density of the ring for saturn would be only what had been acquired through condensation, and the planet formed out of it would be less dense than those made out of matter accumulated in a different way. it may be argued against this deduction, that density would depend on the degree of contraction, but it is natural to think that lighter would take longer time than heavier matter to condense to the same degree; besides saturn is of necessity the youngest of the three planets, and may in due time come to be as dense as either of the other two, but his diameter will decrease proportionately. coming now to the jovian nebula, whose diameter we have made to be , , , miles, we have seen, at page , that--had it been a perfect sphere--by the time it had contracted one thousand miles in diameter, it must have had a flat side of more than , , miles in length? then if we add to that length all that the nebula had inherited from neptune, uranus, and saturn, the cylindrical part of it must have been many millions of miles in length, and the polar very much greater than the equatorial diameter of the nebula. in other words we have to deal with a body having the form of a very long cylinder terminating in spherical caps. to this we have to add that the density of the jovian was more than times greater than that of the original nebula. still farther we have to take into account that the whole of the matter abandoned by that nebula must have been thrown off in less than one-half of the space in which the ring for even saturn had been abandoned, the breadth of the two rings, as shown by us, see table iii., having been , , , and , , miles respectively. all these things considered, it is clear that the thickness of the ring for jupiter's system must have been very much greater than what we have given it in the table; which, coupled with its matter being over six times more dense than that of the preceding ring, is sufficient to account for the rise in density, the immense size, and mass of jupiter. next, we have the means of accounting for the fact that, the space occupied by the asteroids is, and has always been, the least dense of any portion of space occupied by the solar system. it is easy to understand that the enormous mass of matter abandoned by the nebula for the formation of the jovian ring--more especially towards the end of the process--would have a very appreciable effect, by its attractive power, in helping centrifugal force in freeing matter from the power of gravitation; the consequence of which would be, that the matter thrown off for the formation of the asteroidal ring would be considerably less dense than it would otherwise have been. in this way, then, we have the decrease of density, as well as the quantity of matter, in that space very plausibly accounted for. then, as the nebula continued to contract, the attractive power of jupiter's ring would decrease proportionally to the square of the distance of the receding mass, ceasing in doing so to lend so great assistance to centrifugal force in the nebula, and so letting it subside into its normal state; so that the matter abandoned would increase in density in comparison to the space over which it was distributed, thus accounting for the rise in density towards mars and the earth. with regard to the fall towards venus and final rise towards mercury, we have to take into consideration the anomalies--already taken notice of--in the dimensions, densities, etc. etc., of the two planets earth and venus; it being, we may confidently say, certain that the whole of them have arisen from the same causes. following up the idea of a dumb-bell nebula--as we might have done in the case of jupiter also--as the breadth of space for receiving matter abandoned by the nebula went on rapidly decreasing, the thickness of the ring left behind would go on increasing, and the overhanging matter of the dumb-bell would be deposited always in greater quantity on the outer than the inner part of the ring as it broadened; we can conceive that the whole extent of the sheet of matter allotted to the earth and venus would be thicker at the outer than the inner part. hence, when this part of the sheet came to be divided into two parts for the formation of two planets, the outer would naturally be the greater and denser of the two, and thus occasion the rise in density from mars to the earth, and the fall to venus. finally the rise in density to mercury would be only the beginning of the gradual, and final, rise to the sun as it is at present. if the idea of a nebula in the form of a cylinder with hemispherical ends is admitted as possible, or somewhat like a dumb-bell, the extreme diameters of the successive nebulæ we have dealt with would be considerably different in their equatorial directions to what we have given them, although their polar diameters might continue to be not far from the same; but that would have very little effect on the operations we have gone through, seeing we have shown that there could be no actual divisions between them such as we have adopted; and that the division of the sheet of matter abandoned into separate rings must have been brought about by some means which we cannot explain; a process, nevertheless, which has been subject to some law, or laws, operating evidently in a regular and steady manner throughout the whole time, during which the matter was being abandoned, as is proved by the general uniformity, or harmony, in the distances of the planets from the sun. should anyone come to be able to account for the division of this sheet of matter into distinct and separate rings, he will also be able to account for the acceleration of rate of revolution from one planet to another, and for the anomalous rates in the cases of the earth and venus. in a former part of our work we have followed up, at different stages, the condensation of the original nebula until it attained the dimensions, appearance, and some of the features of the sun as it is, but we have still something to add as to how the condensation could produce a body so strictly spherical as the sun is represented to be. all the other bodies of the solar system, as far as astronomers have been able to measure them, are spheroids more or less oblate, and it seems strange that the principal should be the only one that does not conform to the general figure. it is rather hard on the notion that the original nebula gradually assumed the form of a lens, for it would require a special mode of manipulation of a very mechanical kind, rather than the steady, imperceptible self-action of the law of attraction, to transform a lens into even an oblate spheroid; to transform it into a perfect sphere would be absolutely impossible. for, if at the end of the process it was found that there was too much material to form a sphere, it would be hard to get rid of the superabundance, unless it was converted into meteorites--evidently another hand process. on the other hand, should a hole remain to be filled up, the material would have to be lugged in somehow from some of the errant masses, or lambeaux, which impact-theorists find it so easy to have at hand when required. let us then think of why and how it came to pass that the sun is an almost perfect sphere. if we suppose that, when cosmic matter ceased to be thrown off by it, the form of the nebula was that of a cylinder terminating in semi-spherical caps at the ends, it requires no great stretch of imagination to conceive that, between attraction and centrifugal force, the whole mass should be converted through time, first into a prolate spheroid, and then into a perfect sphere. and very possibly time only is required for the sun to become an oblate spheroid, the same as his dependent planets. should this form of nebula not be admissible--and we can see no mechanical reason why it should not--and we are thrown back on a lens-shaped nebula, the only resource left us is to suppose that through continued action of attraction, and of centrifugal force, or rather revolution constantly increasing, the latter gaining the victory over attraction, finally converted the lens into an actual ring, something of the nature of the ring in lyra; and that that ring, no longer increasing in revolution, would have to yield to the law of attraction, and would condense and contract and close up into an oblate spheroid, and then into a sphere. it is a roundabout, rather fanciful, process, but any other way of converting a lens-shaped nebula into a sphere, under the law of attraction, is absolutely impossible. chapter xvii. page former compromises taken up and begun to be fulfilled. estimates of the heat-power of the sun made only from gravitation hitherto. contraction and condensation of a nebula solid to the centre. heat produced from attraction as well as by gravitation. what quantity of heat is produced by a stone falling upon the earth. showing again that there is a difference between attraction and gravitation. contraction and condensation of a hollow-sphere nebula, in the same manner as the solid one. differences of rotation would be greater in a hollow nebula; because a great deal of the matter would be almost motionless in a solid sphere. in neither case could matter be brought to rest, but only retarded in motion. different periods of rotation accounted for. table of different rates explained. heat produced by gravitation, attraction, and churning, not all constituents of the heat-power of the sun. there can be no matter in the sun so dense as water. the hollow part of the sun acting as a reservoir of gases, heat and pressure. the behaviour of heat produced in the nebula, and its power. how sun-spots are produced. cyclonic motions observed in sun-spots. why not all in certain directions, and why only observed in a very few. cyclonic motions in prominences treated of. many other things might be explained, on some of which we do not dare to venture. concluding observations. at the end of chapter vii., when making some remarks on the heat of the sun produced by gravitation, we said that according to the areolar law the condensation produced thereby would originate difference of rates of rotation in the nebula--provided it did rotate as laplace assumed--depending on its degree of contraction and consequent density increasing as the centre was approached; and that these differences of velocity of rotation would give rise to a churning action in its interior which, owing to the friction caused thereby amongst the particles of its matter, would produce heat over and above what was produced by gravitation alone. again, at the end of chapter xii., we said it would not be difficult to show what tremendous commotions throughout the whole nebula would be produced by these differences of rotation; but that this could not be properly done until we had reconstructed the original nebula, and had shown how from it the solar system might be constructed. now, therefore, that we have set forth, as fully as we can, our ideas of the formation of a hollow nebula and the construction from it of the solar system, we shall proceed to show how heat was, and must still be, produced by the churning action, over and above the definite quantity that could possibly be produced by simple gravitation. and also to show how our notions of the interior of the nebula first, and afterwards of the sun, are simplified and made more natural by looking upon it as a hollow sphere. we will begin by considering, first, what would take place during the contraction and condensation of a rotating nebula solid to the centre--i.e. filled with cosmic matter to the centre--as that is the condition under which such a body has been studied hitherto--as far as we know at least.... not to weary humanity--our own included--by repeating, what almost every one knows, who the parties were and how they came to the conclusion, that by far the greatest part--almost the whole--of the heat expended by the sun, ever since it had any to expend, has been produced by condensation caused by gravitation; we shall for the time being accept this as the general, almost universal, opinion at the present day. if any proof of this being the case is considered necessary, we have only to appeal to sir william thomson's lecture, delivered at the royal institution on january , , in which he showed how a cone of matter, similar to that of which the sun is made, with base at the surface and apex near the centre, falling into a similar hollow cone excavated in his body, would, in descending a certain distance, generate as much heat as would maintain a proportional part of his expenditure for a year; and in which, beyond stating that a very small part might be produced by the fall of meteoric matter on his surface, he makes no mention whatever of any heat-producing power except gravitation pure and simple. the weight of the cone falling into the conical pit alone, produced almost the whole of the desired supply. that this manner of calculation is one of those modes which, as we have said from the very beginning of our work, could never have been adopted had a little more thought been expended on them, can be easily demonstrated even in the case we are now considering. this we say with all due deference to so great an authority; more especially as we know how difficult it is, how seeming unnecessarily laborious, to examine everything to the very bottom; and how pleasant and satisfying it is to feel contented, when we have obtained what suits our purpose. when we began to consider, in chapter xv., what would be the interior construction of the nebula, we supposed, at page , that it had assumed a somewhat globular form when its diameter came to be three times that of the orbit of neptune, which would be , , , miles; and we will return to that supposition to set forth our conception of how heat would be produced in a nebula of that diameter solid to the centre--that is full to the centre of cosmic matter. in that case a particle of matter starting from the surface, under the power of gravitation, would have to travel , , , miles before it reached the centre, and would carry with it a constantly increasing power of producing heat, derived solely from the action of gravitation. next, we have to consider what would stop it when it reached the centre and enable it to give out its heat--for until it was stopped it could give out no heat at all--and the most easily conceived means of stoppage would be to suppose that an equal and similar particle coming in from exactly the opposite side of the nebula met it there. if it was not that it would be something equivalent and much more difficult to describe, while the result would be the same. the result would be that, as each particle came in with equal power of producing heat, the the amount produced when the two met and stopped each other would be just double what each of them brought with it; that is our way of looking at it at least, considering that the velocity with which they met would be just double what each brought with it, and the force of the shock would be double what it would have been had only one of them been stopped in some other way; that other way would have had to give or furnish its half of the shock, and would therefore be able to give out as much heat as the stopped particle. whether two of sir william thomson's cones meeting at the bottom of his pit, from exactly opposite sides of the sun, would have the same effect as we have found for the two particles, may perhaps give rise to the discussion; but we do not see why the result should be in any way different. when a stone falls from a height upon the earth it gives out, in the form of heat, all the heat-producing power it had accumulated in its fall, but we are apt to forget, perhaps have never thought at all of, the why and the how it gives it out, especially of the latter. the why is because it is stopped, and the how is by the earth coming to meet it, and these two ways have an inseparable relation to each other. and if the earth comes to meet it, which it most undoubtedly does, though we cannot measure how far it travels, it must bring along with it an amount of heat-producing power equal to that possessed by the stone, when it in its turn is stopped by the stone; thus the amount of heat arising from the fall of a stone to the earth is, apparently, just double what it is usually estimated to be. this fact comes under the category of splitting hairs or, more truly speaking, of negligible quantities; but the whole mass of the sun falling to the centre cannot enter into that category, and whether we will or no we have to take it all into account. we have conducted two particles of matter from exactly opposite points of the surface of the nebula to its centre, and shown that by simple gravitation a certain amount of heat would be produced by them when they met there and stopped each other; now, we propose to conduct two particles, not far from each other, from one side only of the nebula to the centre, and point out what would happen to them on their voyage thither. the road is long, as we have seen, and during their voyage there would be time enough for a good many things to happen, but we shall only take notice of two for the present, namely, gravitation--of which we have already almost disposed--and attraction; for as far as their journey is concerned there is a very marked difference in the meaning of the two words. gravitation--that is, the action of a ponderable body falling--acts only in a straight line from any point to a centre of attraction, while attraction acts in every possible or imaginable direction. we have already seen what happened to the first particle despatched to the centre under the power of gravitation alone, and have only to say, that the same would happen, under that power, to the two we have now in hand; but attraction--actually the father or mother of gravitation--would have a good deal to do with their journey. from the moment they started--very likely they were practising before they left--they would rush at and continue to bombard each other during the whole voyage. at each encounter or collision, however caused, a certain amount of heat would be produced in each of them which they would carry along with them, and would augment the gravitational quantity they would have to give out when stopped in their fall, in the way we have pointed out would be the only one that could bring them to rest. it may be said that that heat would be left behind in space on the way but space cannot absorb heat unless it contains something to hold it in, and that something could only be similar particles of matter on the same voyage, also creating heat and having as much to dispose of, no doubt, as the two we are conducting. this lateral attraction, so to speak, is really what instituted rotary motion in the nebula, and produced the differences of rotation and the churning action in it with which we shall have to deal presently. having passed under examination the quantity of heat produced by the contraction and condensation of the solar nebula into a globe solid to the centre, we have now to do the same for the case of its being a hollow sphere, and we may say that our work has already almost come to an end; for we have only to vary to a small extent what we have just set forth. beginning then as before, with one particle of matter falling, or rather being attracted, from the surface of a hollow-sphere nebula, we find that it would not reach the centre at all, but would be stopped by another drawn out from the centre by its own attraction, which would meet it--say for brevity--half-way between the starting points of the two, each bringing along with it its own heat-producing power and giving it out to its opponent, there being nothing else to give it to; so that if each brought with it _x_ heat-power they would have _x_ heat-power between them, just as we have said would happen in the first case, and the heat of each one of them would consequently be doubled. in this case we have to observe, though it is really unnecessary, that as yet we have spoken of attraction as acting in one direction only, that is, in doing only the work of gravitation; so we have still to consider the voyage of two particles of matter proceeding from the surface and meeting two coming from the centre, and have only to say that their mutual collisions caused by lateral attraction on the way, would enable them to bring along with them certain quantities of heat produced by these collisions, which would be over and above what they acquired in their straight-line imaginary voyage. if any one doubts that additional heat would be produced by this lateral attraction and bombarding, let him take two hammers and strike the one against the other as rapidly as he can for some time, and he will be able, by touch, to convince himself that heat can be produced by this lateral attraction as well as by the _attraction of gravitation_; and, if he could measure it afterwards, he would find that if he dropped the hammers on the ground, they would not give out any of that heat but only what they had derived from gravitation in falling from his hands to the ground, unless the ground was colder than they, and if the ground was not colder, the heat it had would be augmented from this source also. if the heat produced in both of the cases we have been examining caused differences of rotation in the nebula--as we have said on a former occasion--increasing in velocity as the region was approached where the stopping process came into action, it is clear that these differences would be greater near that region in a hollow-sphere nebula than near the centre of a solid sphere; for the reason that the particles of matter would there have more freedom, that is, more room to act in. we have shown that in the solid sphere the particles would come to be more or less inert, in proportion as they approached the centre; and also that in a hollow-sphere nebula no particle could ever come to be near to a state of rest, but that each could be freely driven by the collisions produced by lateral, angular, universal attraction over every part of the hollow shell--an effect that could by no means be produced in a nebula solid to the centre. we, therefore, think that there would be more life-power in a hollow-sphere sun than in the kind of sun from which all calculations of length of life have hitherto been made--at least, as far as we know. it will be understood that we have spoken of particles of matter being stopped, or stopping each other, before they could give out their heat, only for facility of explanation; for no particle of matter can ever be brought to absolute rest, until all its heat and heat-producing power, _i.e._ motion, could be taken out of it, and that can only be when it is reduced to absolute zero of temperature. cosmic matter could be reduced to the state of rock or steel, but its particles would not be at rest then, or else our ideas of the nature and construction of rock and steel are very erroneous; but it must be acknowledged that it would be much more easily reduced to the state of rock in a body solid to the centre than in the shell of a hollow sphere. in fact it is difficult to conceive how matter could exist at the centre of the sun at the present day without being as solid as rock, considering the enormous pressure it must be subjected to there, if its whole mass is condensing to the centre. but although the particles of the nebula could not be absolutely stopped, they might be so far retarded in their velocities derived from attraction that they would give out heat to each other, and wherever a collision took place there heat would be made evident, and condensation might take place. particles of matter would not have to fall to a centre, but only to a a meeting place, in order to condense and create heat, and might form layers of condensation anywhere between the centre and the surface, either in a solid or hollow sphere, which would ultimately, even in the former case, form a hollow shell, as we have supposed, at page , might be the case. for even a small sphere formed around the centre in that way would be hollow, and would be undone when the different concentric layers approached each other, under proportionate forces of attraction, and formed into one hollow sphere. thus we again come to the conclusion that the formation out of cosmic matter acted upon by the law of attraction, of a sphere full of that matter to the centre would be a mechanical impossibility. in either case the total quantity of heat produced by the contraction and condensation of the nebula would include, not only what has hitherto been looked upon as belonging to gravitation alone, but that other part derived from attraction in all other directions. so the age and duration of the sun still remain to be estimated. we have not said, but we have not forgotten that it may be said that, if in lord kelvin's estimate of the sun's heat, a cone of matter falling in from one side of it was stopped by a similar cone falling in from the exactly opposite side, one half of the sun's mass stopping the other could only produce the amount of heat calculated by him. neither do we deny that the same may be said of the two half-volumes of the sun meeting at the region of greatest density in a hollow sphere, and that the amount of heat produced by gravitation alone would be the same in both cases. all that we have wanted to show is that, in addition to the quantity so produced, the quantity produced by lateral attraction, so to speak, has to be taken into account, in order to estimate the total quantity ever possessed by the sun. referring now to what we have said towards the end of chapter xv., of rotary motion being instituted at the region of greatest density of the nebula, and being propagated from there to all parts both outwards and inwards, we can at once account for the different periods of rotation observed on different parts of the surface of the sun; and not only that, we can assert that these differences of rotation must exist throughout the whole volume and mass of its body up to the present day. we have no need to appeal for producing them to showers of meteors falling on its equatorial regions; neither do we pretend to say that such showers have no part in producing them; but we do say that the part they play in the affair, and the depth to which they can penetrate into the sun's body, must be altogether insignificant compared to what we have pointed out as the true and indisputable cause. we may now proceed to consider what would result from the commotions produced by these differences of rotation in the interior of the sun, and we shall begin by observing that an enormous amount of heat would be produced thereby. the churning action, as we have called it, must be of a very formidable character, for, supposing the whole of the interior to be in a gaseous or gasiform state, it must be effected under a pressure of not less than atmospheres at the surface, and at what pressures as the centre is approached no one can tell; and if the matter in the interior is in a viscous condition, the friction caused by the churning will only be the greater. but let us try to form an idea of what the force, or rather violence, of that churning action must be in the sun if constructed in the manner we are advocating; for which purpose we have to form some definite notion of what is the difference of velocity of rotation at different parts of its circumference, which can hardly be better shown than by table x., in as far as these rotations have been approximately measured. the first thing to be observed in the table is that the rate of rotation at the equator is · miles per minute, and that at lat. ° it is only · miles, giving a difference of · miles per minute in one-fourth part of the sun's circumference, which is a velocity times greater than our fastest express trains. and the next is to note, in the last column, how these · miles of difference, when divided over spaces of ° each, show decreases in velocity of from · at lat. ° to · miles between degrees and . a little thought bestowed on these two points will show what commotions must be produced at the surface by this enormous variation of rotation and make us speculate on how much greater it must be near the poles than at the half distance from the equator. then, if we look upon the sun as a hollow sphere we have to consider that, according to the theory that the condensation of a nebula increases its rotation in proportion to its approach to the region of greatest density, of the velocities of all the rotations expressed in the table, the greatest must be at that region, the others diminishing from there outwards to those of the surface, and inwards to almost nothing at the centre; for we have seen that there must be gases enclosed in the hollow, and that motion must be communicated to them, through friction, down to the very centre. taking all these things into consideration, it is certain that the churning must be very much greater than anything we have thought of up to the present moment, the commotions created more tumultuous, and the heat produced by friction incalculable. table x.--showing the differences in velocity of rotation of the surface of the sun, at distances of ° from each other, from the equator to ° of latitude. -------+----------+--------+--------+--------+--------+------------+ |circumfr. | | | | |retardation | lati- |at each °|time of |rotation|rotation|rotation|in miles per| tude |latitude |rotation|per day |per hour|per min.| minute for | degrees| from | (days) | (miles)| (miles)| (miles)| each | | ° to °.| | | | | degrees. | -------+----------+--------+--------+--------+--------+------------+ | | | | | | | | , , | · | , | | · | ... | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | -------+----------+--------+--------+--------+--------+------------+ note.--the times of rotation are taken from messrs. newcomb and holden's "astronomy," p. . lest we should have been misunderstood in what we have said a few pages back, and it be thought we consider that all the heat produced by this churning action ought to be added to that produced by gravitation alone, when attempts have been made to compute the total quantity ever possibly possessed by the sun, we have to insist that the idea of gravitation in itself--that is, of matter falling to a centre--is altogether erroneous in connection with the construction of the sun from a nebula, and that it is in truth utterly misleading. we know perfectly well that in the construction of the sun, heat could only be produced, in the main, by bodies colliding with, or rubbing against, each other, and that a large part of that produced by universal attraction must have been expended in producing rotary motion; but we also know that in its construction no particle of matter can ever, as yet, have been brought to the state of rest of solid matter even, that it has still the power of colliding with its neighbours and of producing heat, and that it will continue to preserve that power until it is bound up into a solid state along with its neighbours. even then it will not be absolutely at rest, but will have lost its heat-producing power, and will begin to lose the quantity it then possesses when it gets permission from its neighbours. it is a fallacy, therefore, to suppose that the matter of which the sun is composed has no other heat-producing power than what is derived from its fall, through gravitation alone, from the potential position it held to the centre of the incipient nebula. the only end to heat-producing power is fixed position. if science chooses to fix that position at the centre of the sun, or as near to it as successive particles can reach, there must be any quantity of it in a solid state even now in that neighbourhood, if due consideration is given to the pressure it must be subjected to there. if it chooses to entertain the idea of the sun's being a hollow sphere, somewhat in the form we have described, there can be nothing in its whole body so dense as even water up to the present time. in the first case it has to remember what we have done our best to prove: that _gravitation_ ceases to act when a body falls to a fixed centre or position and can fall no farther. from there it cannot rise except through upper or exterior attraction, and in that case it would leave a hollow space in the place it had occupied. it is altogether illusory to dream of convection currents where no means or force of any other kind than attraction could give rise to them, in which case we should have attraction and gravitation working against each other, two things that have been confounded into one turning out to be antagonistic, as no doubt they sometimes actually are--as we have shown when treating of the discovery of neptune--but when they are so, they never can produce convection currents. in the second case in which, as we have seen, there can be no matter at all near to the solid state or fixed position up to the present day, we can conclude that the life of the sun, measured by heat-producing power, must be very much longer than in the first case, in which a very large part of the matter of which it is composed must have lost that power ages ago. we have still to bring to mind what we have said in chapter xv. of the region of greatest density of the nebula being the region of greatest activity and greatest heat; and to add now, that the whole space between that region and the centre must have been acting as a reservoir--partly material, partly gasiform--of heat, ever since the nebula began to contract and condense, quite independently of its carrying before it the minus or plus sign. from that time that region would be the regulator of the radiation of heat into space, or to wherever it was radiated; because no heat produced on the inner side could escape into space without passing through and acquiring the temperature of that region, or first giving out to the outer side any greater heat that it might have produced and accumulated; facts which involve the necessity of the whole of the interior space, or volume being heated up or lowered down to the same degree before any of it could be transmitted outwards. thus, in addition to all we have said of the means of lengthening the sun's life, we have to take into consideration that all heat radiated from the surface must be conducted, or carried somehow, through a distance somewhere between about , , and , miles, before it could escape into space or elsewhere, according to when it began to be radiated at all. and we have also to take into consideration the probability that the heat produced and accumulated in the inner half of the volume would, by its repulsive force, retard the condensation of the nebula, and thus prolong its heat-giving life. looking back on our description of the construction of the sun, how rotary motion was established in it, and how that motion has produced the different velocities of rotation, not only on the surface where they have been observed and measured, but which must penetrate to the very centre; we may now proceed at the expense of some repetition--in which we have already somewhat indulged--to show how our mode of construction and development enables us to understand a great many things that have been observed in it, much better than we have been able to do from any explanations that have hitherto been available. it gives the most satisfactory reason possible for the sun-spots occupying principally two zones at marked distances from the equator. there is one belt round the equator of ° to ° wide on which we know, from table x., that the differences of velocity of its edges and of those of the contiguous zones, one on either side, hardly exceed mile per minute. towards the poles there are two segments measuring from ° to ° broad, at the borders of which the rotary velocity is slower by · miles per minute than it is at the equator, and · miles per minute slower than at ° less latitude, as also shown by the table. and between the central belt and these segments there are two belts or zones, each ° to ° wide, in which sun-spots are almost only to be found. in these two zones the churning of the interior would be in all its vigour, most probably more active at their centres than where they meet the central belt and the polar segments; where our knowledge of the diminished velocity ceases, but where we have no reason to suppose that it actually stops. were the period of rotation the same throughout the whole body of the sun--with the exception of what has hitherto been considered to be a mere surface difference produced by external causes--we could conceive that the heat produced solely by condensation would find its way to the surface equally in all directions, even bubble up all round like steam rising from the surface of the water in a boiler, in this way forming what is called the sierra; and that there would be neither sun-spots nor eruptive prominences, hardly any of the violent movements recorded in works on astronomy. but the churning action we have been exhibiting, extending to the deepest recesses of the sun, must produce commotions quite adequate to give birth to the most violent phenomena that have been recorded. viscous gases and vapours, gasiform vapours, ground against each other at depths of hundreds of thousands of miles, under pressures of hundreds, much more likely of many thousands, of atmospheres, and confined by superincumbent strata, so to speak, would acquire a dynamitical explosive force that could be conceived to be powerful enough to rend the sun into fragments, were it composed of anything comparable to solid matter. on the other hand, the friction of the solar matter operated under the pressure of atmospheres at the surface, and up to the unknowable number at the greatest depth, converted into heat, would have explosive energy enough to give rise to all the phenomena that have been observed; from the veiled spot to professor young's prominence, which was thrown up to the height of , miles above the photosphere. a veiled spot seems to be one that has broken through the photosphere, perhaps not even entirely, but not through the light or white clouds which float immediately over it; which, in consequence, goes a long way to prove that sun-spots have their origin in up-rushes of heated vapours from beneath; for a downfall of cooled metallic or other vapours would break through the light clouds first of all; and which is confirmed, as far as anything in solar physics can be confirmed, by what we are exposing. that there is a down-rush also, goes without saying, because there is no other way of giving account of what becomes of the vapours of metals and other elementary substances brought up by the outpours of heat, after they are cooled in the solar atmosphere. that they should fall down into the same opening they had made in rising up, is the most natural supposition that can be made; for, otherwise, they would have to be carried beyond, or outside of, the spot before falling. moreover, a sun-spot is said to be generally surrounded by prominences which bring up vapours of elementary substances, that we must believe to be much heavier than those from eruptions of sun-spots, because they issue much more violently, showing that they must have been expelled by much greater force, which must form a sort of wall all round the spot through which the matter, thrown out by it, would have to be carried before it could be deposited; and outside these walls there are no visible signs of where it falls, so that we are forced to believe that all the substances, those from prominences as well as those from sun-spots, fall into the same general receptacle. surely it could not be argued that there can be no eruptions from a sun-spot, seeing that the force required to drive matter through it must be less than when it is expelled from depths very much greater than the depths of the spots. thus we have both up-rush and down-rush in sun-spots accounted for very plainly; and they are always large enough for both operations being carried on at the same time. besides, they have been credited by eminent astronomers with the faculty of sucking in the cooled vapours from the surrounding prominences into the common pit. in some sun-spots, said to be about per cent. of those observed, cyclonic motions have been observed in the umbræ and penumbræ, which under the churning process might be expected to be universal in all of them, but it is not necessarily so; even leaving out the consideration of the difficulty of detecting them. we see in a deep smooth-flowing river eddies revolving in all directions, caused by currents of different velocities approaching each other, quite independent of the form of the banks of the river or obstructions in the places where we see them, but without doubt derived from sources of that kind higher up in the river; and so it may be with cyclonic motions in the sun-spots. the velocity and direction might be given to the vaporous matter by the churning action before issuing into the spot, which would cause eddies in it in all directions, the same as those in the water of the river. it would be absurd to think that in a space so immense as the bottom of a sun-spot, there should be only one orifice of emission of vaporous matter: there might be any number; consequently, there may be times when the out-flowing currents annul each other and none at all are seen, or when there are partial currents in any direction; others when they may be all so uniform as to produce a cyclonic motion all round a spot, or nearly all round it, or two or more in opposite directions, all as has been recorded on more than one occasion. neither could it be supposed that any cyclonic motion, caused by the churning, could depend on which side of the equator the spot was formed in. there must be little churning going on under the surface at the equatorial belt, hence the paucity of spots there; but between the surface and the centre there must be some point of meeting of the motions that are produced on each side of the equator which, even were there no special reason for it, would destroy all chance of uniformity, or distinctive direction, in the upheaved matter when it arrived at the surface, let it reach that place on whichever side of the equator it might. the original salient motion at the bottom of a sun-spot might be to right or left, or according as the material from which it proceeded had been tumbled about, and the issuing motion might also be controlled greatly by the form and position of the orifice, or rather tunnel, through which it escaped. common churning, we know, could not drive all the milk in one direction, even were the paddles of the churn solid; and in our case, the paddles have to be looked upon as even more divided, magnitude for magnitude, than they are in an ordinary churn, for the matter itself forms the paddles. the cyclonic motions observed in prominences must come from the same causes, and ought to be more general in them, seeing that they must proceed from apertures much fewer in number than in the sun-spots, and very probably from one orifice in the case of jet prominences. one would expect also that these cyclonic motions would be more regular in the prominences, from being generated deeper down in the interior than those of the sun-spots, and less affected by the motions they encountered on their way out, owing to the great original energy required to force them through the superincumbent mass of matter, and might even have--in jet prominences especially--the motion to be expected according to the hemisphere from which they proceeded. but we have already said that, deep in the interior, the churning motion may be in any direction whatever. it is natural to suppose that the highest prominences are ejected from the greatest depths, because they require the greatest ejective force to throw them to such immense heights, and because the greatest ejective force must be where the heat and pressure are greatest, that is, at the densest and most active depths. and probably the reason why prominences generally surround sun-spots is that they have had their exits facilitated by the relief from pressure, brought about by the discharge of churned matter into them (the sun-spots), and thus, as it were, attracting the eruptions of the prominences towards them. we had almost omitted to say that the churning theory would very well account for almost every sun-spot having more or less proper motion of its own independent of all others, and for all of them drifting towards the central belt, or towards the polar segments when they begin to dissolve and disappear. there are many other things in connection with the sun that could be explained through our mode of construction, some of which are so evident that they will occur to anyone, and others that lead into depths too profound for us to enter. to conclude. the construction of the sun we have set forth would be of great service towards the completion of either of what professor a. c. young calls the competing theories of m. faye and fr. secchi, in which the former would find the origin of the solar storms, to which he appeals for producing sun-spots in particular zones, and a better way of accounting for the differences in velocity of rotation between the equator and the poles than in the depths of the strata between these regions; and the latter the means of forming the dense clouds of eruption which he assumes to form sun-spots by settling down into the photosphere. but theorists seem to be partially right by a divination, and to have only failed through their not having found out the sources of the powers they called into existence, in order to have some foundation to build their theories upon. chapter xviii. page return to the peaks abandoned by the original nebula. an idea of their number. example of their dimensions. what was made out of them. what could be made from one of them. how it could be divided into comets and meteor swarms. an example given. how a comet may rotate on its axis. and what might be explained thereby. orbits and periods of revolution. not ejected from planets. their true origin. study of the velocities in orbit of comets, and results thereof. how far comets may wander from the sun and return again. no reason why comets should wander from one sun to another. confirmatory of the description, in chapter xv. of the sun's domains. of the eternal evolution and involution of matter. the atmosphere and corona of the sun. partial analogy between it and the earth's atmosphere. the density of it near the sun's surface cannot be normally less than atmospheres, but might be so partially and accidentally. probable causes of the enormous height of its atmosphere. the mass taken into account, but cannot be valued. most probably no matter in the sun exceeds half the density of water. the unknown line in the spectrum of the corona belongs to the ether. when we were attempting to describe in some measure the region of space from which the sun obtained the nebulous matter out of which it was formed, we found that it would produce a nebula somewhat resembling a most gigantic starfish, with arms or legs stretching out from it in every direction, which might be likened to mountain-peaks rising from a tableland or range of mountains; and when we began to condense the nebula we concluded that these peaks would very soon, comparatively, be left behind the main condensation, owing to their being more under the influence of the attraction of surrounding suns. and we might then have added less under the attraction of the main body, on account of its gradually increasing distance arising from its greater rapidity of contraction. now, we propose to return to these portions of the sun's property so long left out in the cold, to think of what in all probability became of them, seeing that they must all have had somehow a part of some kind to take in the formation of the solar system. first of all, we have to form some idea, however vague, of their number, which may be divined to a very limited extent from the following considerations: we see, from table viii., that the sun's sphere of attraction extends to more than neptune distances in the direction of [greek: a] centauri, the star nearest to the earth, which corresponds to billions of miles. then, although we have said, in chapter xv., that instead of there being a peak on the nebula in that direction there would be a deep hollow in it, we shall proceed to find out what might be the diameter of the base of a peak at that distance supposing it to be somewhat in the form of a cone. we know that the moon does more than eclipse the sun, which is , miles in diameter; so, for facility of calculation, we may suppose that it eclipses a portion of space at its distance of , , miles in diameter. consequently, the base of a peak such as we are measuring would be eclipsed were it , millions of miles in diameter, and then only. moreover, we have deduced the diameter of the base of such a peak from _one_ diameter of the moon; so that wherever we see two stars only one breadth of the moon from each other, there we have room for at least one peak with a base of the above diameter. last of all, when we come to think that there are as many as six to seven thousand stars visible to the naked eye, and of the intervening spaces between them, we have to conclude that the number of peaks surrounding the original nebula before they began to be left behind, or cut off, must have been almost beyond our conception; more especially if we look at table vii., where we see that the star canopus is times farther from the sun than [greek: a] centauri. we are accustomed to look with wonder on the volcanic peaks of the moon, but they can do nothing more than give us an exceedingly faint representation of the original nebula seen from an appropriate distance outside, when it had begun to contract more rapidly than the peaks could follow it; seeing that we are comparing a diameter of , miles with one really almost infinitely greater. finding ourselves, then, with an innumerable host of peaks, or cones, of cosmic matter on our hands, we have to think of what can be done with them, and we begin by saying that the use to be made of them was suggested to us when we discovered the jagged nature of the domains of the sun. some of them have been most probably swallowed up in the formation of the sun, and could we believe in the plenum of meteorites in all space, that has been fancied to exist by some physicists, we might derive its origin from a part of these peaks; but if there can be such a plenum in space, its origin might be much more naturally derived from a suggestion made in a former chapter, at page , to which we shall refer presently. in the meantime, looking upon the multitude of comets, meteor-swarms, etc., which revolve around the sun, or are supposed to exist somehow in its neighbourhood, it is very natural to entertain the belief that they have been made out of some of the most important peaks--or the refuse from them--that must have formed part of the original nebula. to deal with all of them when we cannot number them, or even with the six of table viii., about which we actually know something, is out of the question, so we shall only try to show what could be made out of one of them. confining ourselves, then, to the peak of [greek: a] geminorum, whose collecting ground had originally reached to , neptune distances, or billions of miles--this being the point of space where the attractions of the sun and that star balance each other--if we suppose it to have been contracted till its base was of the same diameter, and its distance the same from the sun, as that of the base of the peak we measured not many minutes ago, , million miles, and billions of miles, respectively, we can easily conceive that its height may have been times as great as the diameter of the base, or more than - / billions of miles. here then we have in the direction of only one star a mass of cosmic matter out of which something more than a comet, even of the grandest known to modern astronomy, could be made. of its tenuity, all that we have any necessity to think is, that it would be much less--i.e. more dense--than that of the original nebula. beginning then with the dimensions we have just stated, we know that the attraction of the nebula would draw the matter of the base-end of the peak more rapidly towards itself than that of the apex-end; we know also that there would be different rates of contraction going on in different parts of the length of the peak--for the same reason we have given for the peaks being cut off from the nebula; so that the condensation throughout its whole height, or length, would be proceeding at different rates at different places, which would certainly divide the peak into several parts, perhaps into many. if now we suppose that the leading part of it--the one nearest to the nebula or sun--or even the whole of it, formed itself into a comet, it is not difficult to see that it might have a tail infinitely longer than any comet the length of whose tail has been measured. there can be no doubt that in the whole length of the peak the action of attraction would be exactly the same as we have found it to be in the nebula itself; that is to say there is no reason why it should not come to be a hollow cone--comets are reported to be hollow in most cases--condensed into layers, and to revolve on their axes throughout a great part at least of where their diameters are greatest. this mode of formation seems to throw light on some of the phenomena that have been observed in comets. we have just said that our peak would be divided into several parts, so if we suppose the leading part of it to have been made into a comet, we can see why its tail should have the appearance of a hollow cylinder; and there might be no reason why the second division, or even the third, should not become a comet also. then for further divisions, where the diameter came to be too small to make a comet, its matter might have formed itself into a meteor-swarm, and account for the fact of some comets and meteor-swarms revolving round the sun in the same orbits; perhaps even for some of the observed meteor-swarms being denser at one part than another, owing to two or more of the sections of the peak following each other at some distance. we have to notice, after what we have just said, that it is quite possible that if the different sections of our peak did come to revolve round the sun, their perihelion distances might be so different that it would be impossible to trace any connection between them and the peak from which they were derived. but if we were to attempt to set forth all the explanations of the phenomena of comets and meteor-swarms that have occurred to us, there would be no end to our labour. passing now from one to the whole host of peaks, we have seen that at one time they projected from all sides of the nebula; it is clear, therefore, that the bodies formed from them must have fallen in towards the sun from all directions, which is exactly what they have been found to do. then, if we think of the multitude of them there would be, we have also to think that there would most certainly be collisions among them, which would smash them to atoms, and thus help to make the plenum, or host of independent meteorites that are supposed to exist, or would be swallowed up by the sun in mouthfuls. others might coalesce, which they could only do through coming in from slightly different directions and with nearly similar velocities; and they would thus account to us for comets with a plurality of tails. again, looking back to what we have just said of the form that might be assumed by the leading end of the peak [greek: a] geminorum, which was suggested by donati's comet, we could imagine another, the same in almost all respects, coalescing with it, and between the two showing us how coggia's comet was formed. furthermore, with respect to one of the gigantic comets with endless tails: if we suppose it to rotate on its axis, and to be not so smooth on its outside as a cone formed in a turning lathe, we could account for the light from the sun reflected from it having an appearance of flickering; and, were the outside very rough, for the reflected light flashing from millions of miles of its length in a few seconds. all this about nebular peaks, comets, etc. formed from them, will, far more than likely, be looked upon as imagination or speculation run mad; but if it is looked into properly, it will be found that no part of it is based on assumption; farther than that, the universe has been formed out of cosmic matter of some kind. there is no step in the whole process, from cosmic matter to the sun--even myriads of suns--that does not conform to what are generally called the laws of nature; whereas it is not difficult to show that some other speculations on the same subject have never been carried beyond the stage of conception. when thinking of how comets might be formed, we could not help thinking of their orbits and periods of revolution. it was easy to see that their orbits depended on where, and how far, they came from; that the where might be from any and every direction, and that the how far would be the principal element in their greater or lesser ellipticity, which could only be determined by measurement; but their periods of revolution, as far as we can see, could only be determined by observation, which would involve the study of several revolutions. on these points the data we have been able to collect are not very satisfying, neither are they given to us as very reliable, except as to those whose orbits have been often observed and measured; and even among these the orbits are said to vary, and some of the comets to disappear altogether. again, some of them are said to have a disposition to become associated with particular planets; and yet again, some people have gone the length of supposing that they have been ejected from some of the planets. to us it seems much more rational to suppose that the known periodical comets have been made out of part of the multitude of peaks which must have surrounded the nebula at one time, if the sun was formed out of nebulous matter, subject to the attraction of similar matter surrounding it on all sides. it seems to be only a way of getting out of a difficulty to suppose that matter ejected from, say the earth, with a velocity of miles per second would be freed from its attraction, that it would be involved somehow in the sun's attraction, and that it would revolve thenceforth round the sun like any other wanderer; because we cannot see what would stop its progress upwards, so to speak, from the earth after getting beyond its control, or communicate to it at the right height and time, the exact velocity required to make it revolve for ever afterwards round the sun; nor, supposing the sun would have nothing to do with it, where it would go. when it left the earth, it might have a direct motion of near one-third of a mile per second derived from its rotation, and also one of miles per second due to the revolution of the earth round the sun. it might also be ejected in a direction exactly away from, or directly towards the sun; so we should have two very different cases to reconcile in order to set up the theory of ejection of comets from planets, and of their being involved somehow in the sun's attraction. it presents us with a very strong case for calling for either the immediate intervention of some power other than what we conceive attraction to be, and of which we know nothing physically, or we have to trust in _man_ipulation of which we have no very exalted idea. we prefer to look upon the formation of all comets as derived from the peaks we have been treating of, or, if that is inadmissible, from shreds and patches of the original nebula; where no immediate intervention, or instant application, of supernatural power is required, but only the even and tranquil operation of original design. for comets larger, and which travel to greater distances, than those alluded to above, it is very difficult to get data on which we can form satisfactory calculations of the lengths of their orbits and mean velocities of revolution, for there is almost always awanting some one or more of their elements, or totally different statements given of their value; but we think we have found a few from which we can collect data sufficiently accurate to enable us to show that there is no necessity for going beyond the domains of the sun, as described by us in a former chapter, to account for any one of the comets which have been taken notice of in astronomical history; and still less necessity to suppose that any of them have wandered, or been shot forth, from some neighbouring star into the solar system. from the data we have been able to collect it would appear that when a comet comes to have a period of over years, it is either too far removed from the sun at its aphelion passage, or its mass is too great for it to be perturbed by the attraction of any of the planets. for instance, we have halley's comet, which has been observed for not far from years, whose period has averaged very close upon years during the whole of that time, showing that it has not been perturbed to any appreciable extent when near its perihelion passage. no doubt years is a very small period of time to judge from, and its aphelion distance being only , , , miles, it might be influenced to some extent by some planet, so we can hardly count upon its being permanently exempt from perturbation. indeed, halley himself supposed that its velocity of revolution had been considerably increased when it was in the neighbourhood of jupiter in the interval between and ; but if it was so, there must be some counter-perturbation which restores the balance so as to make the average period of years. looking over the register of its appearances, we find that in its re-appearances of the years and , the period was about years, and that in those of and it was years; so that if there are perturbations, we must claim that there are also compensations. seeing, then, that we can find no evidence to the contrary, we may suppose that when the periods of comets, and, perhaps more especially, when their aphelion distances reach to beyond--and the farther the more so--the orbit of the most distant planet, they may be looked upon as not being liable to be seriously perturbed by any of the members of the solar system, until something to the contrary had been proved. following this idea, then, it occurs to us that something may be learnt from their mean velocities in their orbits, as will be seen from the following very small list of those we have been able to submit to calculation, which form the accompanying table xi.--showing the mean velocities in orbit of several comets. ----------------------+-----------------+----------+---------------+ | | period of| mean velocity | designation of comet. |aphelion distance|revolution| in orbit. | | (miles) | (years) | (miles/sec) | ----------------------+-----------------+----------+---------------+ halley's comet | , , , | | · | | | | | comet of & | , , , | | · | | | | | donati's comet | , , , | , | · | | | | | comet of | , , , | , | · | | | | | comet of | , , , | , | · | ----------------------+-----------------+----------+---------------+ these orbital mean velocities per second have been calculated from aphelion distances as diameters and from circular orbits, which probably give results rather lower than would be derived from elliptical orbits--were they known--but on the other hand, the perihelion distances have not been taken into account in fixing the diameters--because they were unknown--so the error will be so far compensated, if not altogether. we know that the mean velocities in orbit of the planets decrease as their distances from the sun increase, and our table, as far as it goes, leads us to believe that the same holds good with comets whose aphelion distances are comparable to those of the planets, in being measured by hundreds of years or less of revolution; but with those whose periods are measured by thousands of years, the same rule seems to fail. one thing, however, that we seem entitled to believe is that, generally speaking, the greater the period of revolution of a comet is, the less will be its mean velocity per second in its orbit. it will be observed that the average mean velocity of the three remote comets in the table is only · mile per second, and it is by no means unreasonable to suppose that the average mean velocity per second of any number of comets whose aphelion distances are greater than the highest of those in the table, is not likely to be so great as the average of the three; on this understanding, then, let us take, or suppose, one whose mean velocity in orbit per second is only one mile, and look into what may be learnt from it. going back to the peak of [greek: a] geminorum which we supposed, at page , to be condensed to , million miles in diameter of base, its height - / billion miles, and distance from the sun billion miles, we may take a comet formed from it as an example. if, then, we suppose the leading part of it to have been formed into a comet with that aphelion distance-- billion miles--and other dimensions suitable to its new condition; taking its mean velocity in orbit at mile per second, we find that its period of revolution might be , , years, or three times greater than that of the comet of , namely , years, mentioned by mr. chambers as being not very reliable, probably because its angles in orbit could not be measured with sufficient accuracy. then, when we think that the sphere of the sun's attraction in that direction--of [greek: a] geminorum--extends to billions of miles, and that there are stars more than times farther off, e.g. canopus, see table vii., we see that a supposed comet might have an aphelion distance equal to that; and were we further to consider that were its major axis billion miles long, including aphelion and perihelion distances, and that it went straight from the one end of it to the other and back again, its period of revolution, if it could be so called, would be , , years; that is times greater than mr. chambers's doubtful , years for the comet of . there seems, therefore, to be no necessity for the solar system sending its cometary produce to a foreign market; and our mechanical imagination is not sufficiently vivid to allow us to conceive what kind of potential energy even jupiter can have to give an impetus to a comet, great enough to send it flying to so great a distance. what velocity would it have when it left the sun? and what would remain in it to carry it over the debatable land between the sun and a distant neighbour? or are we to believe that all the solar system's produce of that kind is only sent over the channel, as it were, to our nearest neighbour, [greek: a] centauri? conceptions of that kind are too elevated for us, and we must leave them alone. mr. chambers expresses doubts as to the determination of whether the orbit of a comet is elliptical or parabolic when its period of revolution is measured by hundreds of thousands of years, and we think we are safe in following him until actual proofs are presented. if the comet of never comes back, we may then believe it has gone elsewhere. having used up all the nebulous matter in the sun's domains, as described at the beginning of chapter xv., or at least shown how it may have been, or may yet be, used up, we have now only to make a few remarks to prove that our description of the said domains is not by any means fanciful. it matters very little whether the solar system was begun to be brought into existence at the same time as the surrounding systems or before or after them. what is certain is that the sun's sphere of attraction among its neighbours is bounded, at the present time, just in the way we have taken to describe its domains. how they were filled with cosmic matter may be disputed, but filled they must have been somehow, if the solar system was formed out of a nebula; and the way adopted by us was the only one that occurred to us when we began to reconstruct the original nebula. since then we have had time to reflect on our work, and to see how it points out the simplest way that can be conceived, which may be expressed in the few following words. we may suppose that the ether was the primitive matter, as we have done at page , and that the whole material universe has been formed from it and through it. this idea will assist physicists in forming their theory of a plenum of meteorites or meteoric matter, if such they choose to call it. it will also enable us to complete the circle of our notions with respect to matter. we believe that we can neither destroy nor produce the smallest portion of it, although we can change its form. thus, looking upon the ether as primitive matter, we can understand how the solar system could be elaborated from it; and how, after having accomplished the purposes for which it was brought into existence, it may again be resolved into the primitive element out of which it was made, ready to take its part in the evolution of some other system with, perhaps, a new earth "without form and void." we have now to direct our thoughts, as far as we can, to the mass, which furnishes the really effective power of the sun as the ruler of the system; and, first of all, we have to think of what are the real active elements which form that mass. hitherto we have looked upon them as all included within a diameter of , miles, but now we have to take notice of the clouds of meteoric matter which have been supposed by some astronomers and physicists to be revolving round the sun and continually raining into it; and of the enormous atmosphere which surrounds it. with regard to the former of these two elements, we shall compound our ignorance by looking upon it as a merchant does on his account of bills receivable, as not being available in the case of a sudden demand for cash, and therefore as not forming a part of the mass, any more than as the attraction of the earth aids the sun in its management of the planet neptune; the same as the bills receivable strengthen the credit of the merchant. but with regard to the second element of the two, we must recognise that it forms part of the mass and power over the whole of the system, and from all that is known about it we are not authorised to look upon it as a negligible quantity. it so happens that the only thing we have to which we can compare it is the atmosphere of the earth, and we immediately find that there is absolutely nothing to be learnt from such a comparison. we know that one-half of the weight or mass of the earth's atmosphere is contained in a belt of - / miles high above its surface, so that double the volume of that belt estimated at atmospheric pressure gives us the true measure of its mass. this mass, when reduced to the density of water, and compared to that of the earth as we have dealt with it all along, turns out to be about / , th part of it; and were we now to add that to the earth's mass we have been using, its mean density would be · instead · times that of water. now, let us suppose the sun to have an atmosphere of the same kind as the earth's: seeing that the force of gravity at its surface is about times greater than it is at the surface of the earth, a belt around it which would contain one-half of its mass would be × - / = miles, or say miles thick. dealing then with this dimension in the same manner as we have done in the case of the earth, we find that its supposed atmosphere would be / , th part of its mass, which, if added to the mass we have used for it, would make its mean density · instead of · times that of water. then again, if we suppose the earth's atmosphere to extend to or miles above its surface, the supposed atmosphere of the sun would extend to or miles above its surface, according to which of the above heights on the earth is adopted; whereas the highest of our authorities say that the corona, or apparent atmosphere, extends to at least , miles from its surface. it would appear then that there is no analogy whatever between the atmospheres of the sun and the earth; but there must be some analogy, because the law of attraction cannot be suppressed at the surface of the sun; neither can any vaporous matter near it cease to be attracted in the same proportion as it is at the surface. our atmosphere causes a pressure of - / inches of mercury at the earth's surface, and the attraction of the sun at its surface must cause a pressure equal to nearly times that without fail, i.e. lb. per square inch instead of the lb. of the earth. we know that some spectroscopists believe that the pressure at the surface of the sun is sometimes as low as it is at the surface of the earth, even lower; but we require an explanation of why it is so. at the surface of the sun one second of arc corresponds to a height of miles above its surface, and mr. proctor states in his "sun," page , that if even "two or three hundred miles separated the lower limit of chromatosphere from the photosphere, no telescopes we possess could suffice (when supplied with suitable spectroscopic appliances) to reveal any trace of this space. a width of two hundred miles at the sun's distance subtends an arc of less than half a second; and telescopists, who know the difficulty of separating a double star whose components lie so close as this, will readily understand that a corresponding arc upon the sun would be altogether unrecognisable." we can understand this, and perhaps find an explanation for ourselves. according to our supposition that the sun may have an atmosphere similar to the earth's, at one hundred miles in height it would be reduced in pressure to atmospheres, and, extending the analogy, at miles high the pressure would still be equal to one-eighth of atmospheres, or equal to something less than lb. per square inch at the surface of the earth; so that if spectroscopists have measured the sun's atmosphere at the disk, and found it to be lower than the earth's at its surface, their results must have been caused by some fortuitous circumstance which they did not notice at the time; because the force of attraction at the surface of the sun can never be overcome except by some counteracting force, which, if in the form of a vapour, or what we call a gas, issuing from its interior, would increase rather than diminish the pressure. we know that in the heart of a cyclone on the earth there is sometimes a vacuum sufficient to explode (pull out the walls of) houses near which it passes; and, at the same time, we know, more or less, what heat the sun sheds upon the outer atmosphere of the earth, and also the rate of rotation of the earth in the regions where the fiercest of these cyclones occur, the only two causes which can produce them. now, if we compare these causes in the two bodies, that is, the earth's rotation of about miles per minute and the sun's of, say, to miles per minute, and the temperatures of the sun and the earth at their respective surfaces, we can imagine that in the heart of a cyclone on the sun there may be a vacuum much nearer absolute zero than there can be in any one on the surface of the earth. if then the spectroscopists, without knowing it, have caught the spectra of the hearts of cyclones, we can conceive them to be right, otherwise no. again, we know that when big guns are fired off partial vacuums are formed near them, sufficient to cause disaster to windows, doors, and even walls of houses too near them, but whatever we may have said of force sufficient to produce explosions in the sun, we have never believed that matter is ejected from the sun by explosions. we have supposed the sierra, or chromosphere, to have oozed out through its pores, sometimes to less, sometimes to greater heights, like steam from an open boiler, and the prominences to be eruptive, neither of which modes could produce anything approaching to vacua in their neighbourhoods. there can be no resemblance between the ejection of matter or gas from the sun and from a cannon, but there is between the ejection of vapours and the escape of steam from the safety-valve of a closed steam boiler; both of them continue to pour out their vapours till the pressure within falls down till it is equal to the resistance to their escape; there is no explosion, therefore no vacuum, appreciable at least, in the neighbourhood. there may be surrounding matter drawn up by the velocity of the outward current, but that is all. notwithstanding all this, we see no reason why the sun should not have an atmosphere of exactly the same kind as the earth's, composed of exactly the same kinds of gases, including vapour of water in some part of it, though, perhaps, far removed from the photosphere. every other element found on the earth can be found in the sun, and so it is not unreasonable to suppose that the same kind of atmosphere may exist upon it; we have only to acknowledge that its conditions must be somewhat varied, all the difference being that the atmosphere of the sun must be heated up to the temperature of the photosphere where it comes in contact with it, while that of the earth is only of the temperature of the earth at its surface. in the case of the earth, if this were at a white heat, one-half of the weight of its atmosphere would not be comprehended in a belt around it of - / miles thick. that balance of mass might take place at a height of even hundreds of miles--we have no means of calculating how high--and still its pressure at the surface would be the same as now, as long as the earth's attraction remained the same; so must it be with the sun. instead of limiting its height to miles at the utmost as we have done above, it would be no stretch of imagination to suppose that it might extend to ten, twenty, or more times that height. in addition to this we have to take into consideration that the sun's atmosphere must be swept up to something far beyond miles high by the whirlwinds created by the velocity of rotation at its surface, the same as we saw the earth's might be when we were explaining how an aurora could be made to glow at heights far beyond what we were accustomed to believe its atmosphere could reach. adding, then, together these two motive forces for elevating the atmosphere of the sun, it would be a bold assertion to say that it cannot have one exactly similar to the earth's, reaching up to the height of , miles mentioned a few pages back. and now, having got this length, we may venture to assert that the corona of the sun is made up of this atmosphere, and of the vapours of the elements thrown out from its interior, somewhat in the manner we have described in last chapter; to which we have only to add that the bubbling up of vapours all around the sun, which produces the sierra or chromosphere, would not be interfered with in any way by the tremendous commotions which we have shown must be produced between the surfaces of the sun-spot zones and the centre; and that the projection of the high prominences would assist in elevating the aeriform atmosphere. if then the sun has a compound atmosphere of this kind, it must be considerably more dense, proportionately, than that of the earth, and will consequently form a greater addition to its mass than we have found would be made by its airlike atmosphere. but, whatever density has to be added to it on that account has to be subtracted from the interior having been ejected from thence; because, in whatever manner its mass has been calculated in respect of the other members of the system, the total amount must turn out to be always the same. we have always estimated its mass from a diameter of , miles, which gave us a volume of , , ^{ } cubic miles, so that if we now include in the diameter the , miles height of the atmosphere, we get a volume of , , ^{ } cubic miles, which is as near as possible six times the volume in which we had to distribute the volume of the sun. how to do this, we know not. we cannot fix the region of greatest density in the same manner we have done at page , but we know that it must be considerably nearer to the surface of the photosphere than we have there placed it; and of one thing we are sure, and that is, that the densities we have named for that region and the outer and inner surfaces of the shell, at page , must be less than those there expressed; how much we cannot calculate, but we have certainly found that the limits must be lower, and that most probably there is no matter in the sun exceeding the half of the density of water. whatever the composition of the sun's atmosphere, or corona if that name be preferred, may be, spectroscopists have found in it a _spectral_ line derived from some substance totally unknown to science. now, looking back on our work from almost the very beginning, it seems to have been gradually borne in upon us that this unknown substance is the ether. that it is a material substance we were hardly ever in doubt, and our studies of it have substantiated and confirmed our belief. in our analysis of the nebular hypothesis in chapter vi., after combating the notion that the light of nebulæ is occasioned by incandescent gas, we showed, by the example of an air furnace, that an incandescent gas is composed of two elements, one consisting of solid matter which takes up and gives out heat and has all the properties of a heated solid or liquid substance, and the other of gaseous matter which, being the element that fills up the empty spaces between the solid atoms of a gas or vapour, only performs the office of carrying the solid part into the furnace. this forced upon us the idea of the gaseous part being a carrying agent, and very naturally to think of its being really the ether, that being the only acknowledged agent for the carriage of light, heat, and electricity, two of which are easily seen and felt, and the third cannot be awanting, in an air furnace. again, when treating in chapter vii. of what effect the ether might have on the density of the original nebula, we concluded that its density must be much lower than what we then knew it had been estimated to be, and also that its temperature in space must be lower than - °; which two circumstances combined showed us that if it is a gaseous substance it must be very different to any gas that had been liquefied up to that time. this we repeated in great part in chapter xii., calling attention to the peculiarity of its being able to carry a higher temperature than its own--to all appearance--into a "hot box." then we have dedicated two chapters, xiii. and xiv., almost exclusively to the study of the ether, and have been led from one stage to another to look upon it as the only substance that agrees with the definition of a gas as given by science; true gas there is; as the primitive and sole element in the formation of all matter and in the evolution of the universe; and what is something more than an unfounded guess, as the mysterious and incomprehensible agent attraction, unfortunately almost universally spoken of as gravitation. and now to conclude: from what we have been able to learn, very slight differences have been found in various spectra of the position of the line representing the unknown substance, but this can cause very little doubt of its always being the same, as spectra often contain several lines of hydrogen, owing most probably to combinations with other substances; and if the ether is the primitive chemical element, there may be slight differences in the position of its line, as shown in all the phases in which we seem to have found it, but they must be slight as compared with the hydrogen lines, because even these must be in some measure, perhaps even great, influenced by the unfailing and inevitable mixture of the ether in their composition. london: printed by william clowes and sons, limited, great windmill street, w., and duke street, stamford street, s.e. transcriber note several tables were reformatted to fit a maximum -column width. generously made available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/heavensabovepopu gillrich transcriber's note: text enclosed by underscores is in italics (_italics_). superscripts, such as p to the second power, are shown by the caret character "^" before the superscript, such as p^ . subscripts are similarly shown by an underscore before the subscript which is wrapped in curly braces, such as m_{ }. [illustration: spectra of various sources of light.] the heavens above: a popular handbook of astronomy the heavens above: a popular handbook of astronomy. by j. a. gillet, professor of physics in the normal college of the city of new york, and w. j. rolfe, formerly head master of the high school, cambridge, mass. with six lithographic plates and four hundred and sixty wood engravings. potter, ainsworth, & co., new york and chicago. . copyright by j. a. gillet and w. j. rolfe, . franklin press: rand, avery, and company, boston. preface. it has been the aim of the authors to give in this little book a brief, simple, and accurate account of the heavens as they are known to astronomers of the present day. it is believed that there is nothing in the book beyond the comprehension of readers of ordinary intelligence, and that it contains all the information on the subject of astronomy that is needful to a person of ordinary culture. the authors have carefully avoided dry and abstruse mathematical calculations, yet they have sought to make clear the methods by which astronomers have gained their knowledge of the heavens. the various kinds of telescopes and spectroscopes have been described, and their use in the study of the heavens has been fully explained. the cuts with which the book is illustrated have been drawn from all available sources; and it is believed that they excel in number, freshness, beauty, and accuracy those to be found in any similar work. the lithographic plates are, with a single exception, reductions of the plates prepared at the observatory at cambridge, mass. the remaining lithographic plate is a reduced copy of professor langley's celebrated sun-spot engraving. many of the views of the moon are from drawings made from the photographs in carpenter and nasmyth's work on the moon. the majority of the cuts illustrating the solar system are copied from the french edition of guillemin's "heavens." most of the remainder are from lockyer's "solar physics," young's "sun," and other recent authorities. the cuts illustrating comets, meteors, and nebulæ, are nearly all taken from the french editions of guillemin's "comets" and guillemin's "heavens." contents. i. the celestial sphere ii. the solar system i. theory of the solar system the ptolemaic system the copernican system tycho brahe's system kepler's system the newtonian system ii. the sun and planets i. the earth form and size day and night the seasons tides the day and time the year weight of the earth and precession ii. the moon distance, size, and motions the atmosphere of the moon the surface of the moon iii. inferior and superior planets inferior planets superior planets iv. the sun i. magnitude and distance of the sun ii. physical and chemical condition of the sun physical condition of the sun the spectroscope spectra chemical constitution of the sun motion at the surface of the sun iii. the photosphere and sun-spots the photosphere sun-spots iv. the chromosphere and prominences v. the corona v. eclipses vi. the three groups of planets i. general characteristics of the groups ii. the inner group of planets mercury venus mars iii. the asteroids iv. outer group of planets jupiter the satellites of jupiter saturn the planet and his moons the rings of saturn uranus neptune vii. comets and meteors i. comets general phenomena of comets motion and origin of comets remarkable comets connection between meteors and comets, physical and chemical constitution of comets ii. the zodiacal light iii. the stellar universe i. general aspect of the heavens ii. the stars the constellations clusters double and multiple stars new and variable stars distance of the stars proper motion of the stars chemical and physical constitution of the stars iii. nebulæ classification of nebulæ irregular nebulæ spiral nebulæ the nebular hypothesis iv. the structure of the stellar universe i. the celestial sphere. i. _the sphere._--a _sphere_ is a solid figure bounded by a surface which curves equally in all directions at every point. the rate at which the surface curves is called the _curvature_ of the sphere. the smaller the sphere, the greater is its curvature. every point on the surface of a sphere is equally distant from a point within, called the _centre_ of the sphere. the _circumference_ of a sphere is the distance around its centre. the _diameter_ of a sphere is the distance through its centre. the _radius_ of a sphere is the distance from the surface to the centre. the surfaces of two spheres are to each other as the squares of their radii or diameters; and the volumes of two spheres are to each other as the cubes of their radii or diameters. distances on the surface of a sphere are usually denoted in _degrees_. a degree is / of the circumference of the sphere. the larger a sphere, the longer are the degrees on it. a curve described about any point on the surface of a sphere, with a radius of uniform length, will be a circle. as the radius of a circle described on a sphere is a curved line, its length is usually denoted in degrees. the circle described on the surface of a sphere increases with the length of the radius, until the radius becomes °, in which case the circle is the largest that can possibly be described on the sphere. the largest circles that can be described on the surface of a sphere are called _great circles_, and all other circles _small circles_. any number of great circles may be described on the surface of a sphere, since any point on the sphere may be used for the centre of the circle. the plane of every great circle passes through the centre of the sphere, while the planes of all the small circles pass through the sphere away from the centre. all great circles on the same sphere are of the same size, while the small circles differ in size according to the distance of their planes from the centre of the sphere. the farther the plane of a circle is from the centre of the sphere, the smaller is the circle. by a _section_ of a sphere we usually mean the figure of the surface formed by the cutting; by a _plane section_ we mean one whose surface is plane. every plane section of a sphere is a circle. when the section passes through the centre of the sphere, it is a great circle; in every other case the section is a small circle. thus, _an_ and _sb_ (fig. ) are small circles, and _mm'_ and _sn_ are large circles. [illustration: fig. .] in a diagram representing a sphere in section, all the circles whose planes cut the section are represented by straight lines. thus, in fig. , we have a diagram representing in section the sphere of fig. . the straight lines _an_, _sb_, _mm'_, and _sn_, represent the corresponding circles of fig. . the _axis_ of a sphere is the diameter on which it rotates. the _poles_ of a sphere are the ends of its axis. thus, supposing the spheres of figs. and to rotate on the diameter _pp'_, this line would be called the axis of the sphere, and the points _p_ and _p'_ the poles of the sphere. a great circle, mm', situated half way between the poles of a sphere, is called the _equator_ of the sphere. every great circle of a sphere has two poles. these are the two points on the surface of the sphere which lie ° away from the circle. the poles of a sphere are the poles of its equator. [illustration: fig. .] . _the celestial sphere._--the heavens appear to have the form of a sphere, whose centre is at the eye of the observer; and all the stars seem to lie on the surface of this sphere. this form of the heavens is a mere matter of perspective. the stars are really at very unequal distances from us; but they are all seen projected upon the celestial sphere in the direction in which they happen to lie. thus, suppose an observer situated at _c_ (fig. ), stars situated at _a_, _b_, _d_, _e_, _f_, and _g_, would be projected upon the sphere at _a_, _b_, _d_, _e_, _f_, and _g_, and would appear to lie on the surface of the heavens. [illustration: fig. .] . _the horizon._--only half of the celestial sphere is visible at a time. the plane that separates the visible from the invisible portion is called the _horizon_. this plane is tangent to the earth at the point of observation, and extends indefinitely into space in every direction. in fig. , _e_ represents the earth, _o_ the point of observation, and _sn_ the horizon. the points on the celestial sphere directly above and below the observer are the poles of the horizon. they are called respectively the _zenith_ and the _nadir_. no two observers in different parts of the earth have the same horizon; and as a person moves over the earth he carries his horizon with him. [illustration: fig. .] the dome of the heavens appears to rest on the earth, as shown in fig. . this is because distant objects on the earth appear projected against the heavens in the direction of the horizon. [illustration: fig. .] the _sensible_ horizon is a plane tangent to the earth at the point of observation. the _rational_ horizon is a plane parallel with the sensible horizon, and passing through the centre of the earth. as it cuts the celestial sphere through the centre, it forms a great circle. _sn_ (fig. ) represents the sensible horizon, and _s'n'_ the rational horizon. although these two horizons are really four thousand miles apart, they appear to meet at the distance of the celestial sphere; a line four thousand miles long at the distance of the celestial sphere becoming a mere point, far too small to be detected with the most powerful telescope. [illustration: fig. .] [illustration: fig. .] . _rotation of the celestial sphere._--it is well known that the sun and the majority of the stars rise in the east, and set in the west. in our latitude there are certain stars in the north which never disappear below the horizon. these stars are called the _circumpolar_ stars. a close watch, however, reveals the fact that these all appear to revolve around one of their number called the _pole star_, in the direction indicated by the arrows in fig. . in a word, the whole heavens appear to rotate once a day, from east to west, about an axis, which is the prolongation of the axis of the earth. the ends of this axis are called the _poles_ of the heavens; and the great circle of the heavens, midway between these poles, is called the _celestial equator_, or the _equinoctial_. this rotation of the heavens is apparent only, being due to the rotation of the earth from west to east. . _diurnal circles._--in this rotation of the heavens, the stars appear to describe circles which are perpendicular to the celestial axis, and parallel with the celestial equator. these circles are called _diurnal circles_. the position of the poles in the heavens and the direction of the diurnal circles with reference to the horizon, change with the position of the observer on the earth. this is owing to the fact that the horizon changes with the position of the observer. [illustration: fig. .] when the observer is north of the equator, the north pole of the heavens is _elevated_ above the horizon, and the south pole is _depressed_ below it, and the diurnal circles are _oblique_ to the horizon, leaning to the south. this case is represented in fig. , in which _pp'_ represents the celestial axis, _eq_ the celestial equator, _sn_ the horizon, and _ab_, _cn_, _de_, _fg_, _sh_, _kl_, diurnal circles. _o_ is the point of observation, _z_ the zenith, and _z'_ the nadir. [illustration: fig. .] when the observer is south of the equator, as at _o_ in fig. , the south pole is _elevated_, the north pole _depressed_, and the diurnal circles are _oblique_ to the horizon, leaning to the north. when the diurnal circles are oblique to the horizon, as in figs. and , the celestial sphere is called an _oblique sphere_. when the observer is at the equator, as in fig. , the poles of the heavens are on the horizon, and the diurnal circles are _perpendicular_ to the horizon. when the observer is at one of the poles, as in fig. , the poles of the heavens are in the zenith and the nadir, and the diurnal circles are _parallel_ with the horizon. [illustration: fig. .] [illustration: fig. .] . _elevation of the pole and of the equinoctial._--at the equator the poles of the heavens lie on the horizon, and the celestial equator passes through the zenith. as a person moves north from the equator, his zenith moves north from the celestial equator, and his horizon moves down from the north pole, and up from the south pole. the distance of the zenith from the equinoctial, and of the horizon from the celestial poles, will always be equal to the distance of the observer from the equator. in other words, the elevation of the pole is equal to the latitude of the place. in fig. , _o_ is the point of observation, _z_ the zenith, and _sn_ the horizon. _np_, the elevation of the pole, is equal to _ze_, the distance of the zenith from the equinoctial, and to the distance of _o_ from the equator, or the latitude of the place. two angles, or two arcs, which together equal °, are said to be _complements_ of each other. _ze_ and _es_ in fig. are together equal to °: hence they are complements of each other. _ze_ is equal to the latitude of the place, and _es_ is the _elevation_ of the equinoctial above the horizon: hence the elevation of the equinoctial is equal to the complement of the latitude of the place. [illustration: fig. .] were the observer south of the equator, the zenith would be south of the equinoctial, and the south pole of the heavens would be the elevated pole. [illustration: fig. .] _ . four sets of stars._--at most points of observation there are four sets of stars. these four sets are shown in fig. . ( ) the stars in the neighborhood of the elevated pole _never set_. it will be seen from fig. , that if the distance of a star from the elevated pole does not exceed the elevation of the pole, or the latitude of the place, its diurnal circle will be wholly above the horizon. as the observer approaches the equator, the elevation of the pole becomes less and less, and the belt of circumpolar stars becomes narrower and narrower: at the equator it disappears entirely. as the observer approaches the pole, the elevation of the pole increases, and the belt of circumpolar stars becomes broader and broader, until at the pole it includes half of the heavens. at the poles, no stars rise or set, and only half of the stars are ever seen at all. ( ) the stars in the neighborhood of the depressed pole _never rise_. the breadth of this belt also increases as the observer approaches the pole, and decreases as he approaches the equator, to vanish entirely when he reaches the equator. the distance from the depressed pole to the margin of this belt is always equal to the latitude of the place. ( ) the stars in the neighborhood of the equinoctial, on the side of the elevated pole, _set, but are above the horizon longer than they are below it_. this belt of stars extends from the equinoctial to a point whose distance from the elevated pole is equal to the latitude of the place: in other words, the breadth of this third belt of stars is equal to the complement of the latitude of the place. hence this belt of stars becomes broader and broader as the observer approaches the equator, and narrower and narrower as he approaches the pole. however, as the observer approaches the equator, the horizon comes nearer and nearer the celestial axis, and the time a star is below the horizon becomes more nearly equal to the time it is above it. as the observer approaches the pole, the horizon moves farther and farther from the axis, and the time any star of this belt is below the horizon becomes more and more unequal to the time it is above it. the farther any star of this belt is from the equinoctial, the longer the time it is above the horizon, and the shorter the time it is below it. ( ) the stars which are in the neighborhood of the equinoctial, on the side of the depressed pole, _rise, but are below the horizon longer than they are above it_. the width of this belt is also equal to the complement of the latitude of the place. the farther any star of this belt is from the equinoctial, the longer time it is below the horizon, and the shorter time it is above it; and, the farther the place from the equator, the longer every star of this belt is below the horizon, and the shorter the time it is above it. at the equator every star is above the horizon just half of the time; and any star on the equinoctial is above the horizon just half of the time in every part of the earth, since the equinoctial and horizon, being great circles, bisect each other. . _vertical circles._--great circles perpendicular to the horizon are called _vertical circles_. all vertical circles pass through the zenith and nadir. a number of these circles are shown in fig. , in which _senw_ represents the horizon, and _z_ the zenith. [illustration: fig. .] the vertical circle which passes through the north and south points of the horizon is called the _meridian_; and the one which passes through the east and west points, the _prime vertical_. these two circles are shown in fig. ; _szn_ being the meridian, and _ezw_ the prime vertical. these two circles are at right angles to each other, or ° apart; and consequently they divide the horizon into four quadrants. [illustration: fig. .] . _altitude and zenith distance._--the _altitude_ of a heavenly body is its distance above the horizon, and its _zenith distance_ is its distance from the zenith. both the altitude and the zenith distance of a body are measured on the vertical circle which passes through the body. the altitude and zenith distance of a heavenly body are complements of each other. . _azimuth and amplitude.--azimuth_ is distance measured east or west from the meridian. when a heavenly body lies north of the prime vertical, its azimuth is measured from the meridian on the north; and, when it lies south of the prime vertical, its azimuth is measured from the meridian on the south. the azimuth of a body can, therefore, never exceed °. the azimuth of a body is the angle which the plane of the vertical circle passing through it makes with that of the meridian. the _amplitude_ of a body is its distance measured north or south from the prime vertical. the amplitude and azimuth of a body are complements of each other. . _alt-azimuth instrument._--an instrument for measuring the altitude and azimuth of a heavenly body is called an _alt-azimuth_ instrument. one form of this instrument is shown in fig. . it consists essentially of a telescope mounted on a vertical circle, and capable of turning on a horizontal axis, which, in turn, is mounted on the vertical axis of a horizontal circle. both the horizontal and the vertical circles are graduated, and the horizontal circle is placed exactly parallel with the plane of the horizon. when the instrument is properly adjusted, the axis of the telescope will describe a vertical circle when the telescope is turned on the horizontal axis, no matter to what part of the heavens it has been pointed. the horizontal and vertical axes carry each a pointer. these pointers move over the graduated circles, and mark how far each axis turns. to find the _azimuth_ of a star, the instrument is turned on its vertical axis till its vertical circle is brought into the plane of the meridian, and the reading of the horizontal circle noted. the telescope is then directed to the star by turning it on both its vertical and horizontal axes. the reading of the horizontal circle is again noted. the difference between these two readings of the horizontal circle will be the azimuth of the star. [illustration: fig. .] to find the _altitude_ of a star, the reading of the vertical circle is first ascertained when the telescope is pointed horizontally, and again when the telescope is pointed at the star. the difference between these two readings of the vertical circle will be the altitude of the star. . _the vernier._--to enable the observer to read the fractions of the divisions on the circles, a device called a _vernier_ is often employed. it consists of a short, graduated arc, attached to the end of an arm _c_ (fig. ), which is carried by the axis, and turns with the telescope. this arc is of the length of _nine_ divisions on the circle, and it is divided into _ten_ equal parts. if of the vernier coincides with any division, say , of the circle, of the vernier will be / of a division to the left of , will be / of a division to the left of , will be / , of a division to the left of , etc. hence, when coincides with , will be at - / ; when coincides with , will be at - / ; when coincides with , will be at - / , etc. [illustration: fig. .] to ascertain the reading of the circle by means of the vernier, we first notice the zero line. if it exactly coincides with any division of the circle, the number of that division will be the reading of the circle. if there is not an exact coincidence of the zero line with any division of the circle, we run the eye along the vernier, and note which of its divisions does coincide with a division of the circle. the reading of the circle will then be the number of the first division on the circle behind the of the vernier, and a number of tenths equal to the number of the division of the vernier, which coincides with a division of the circle. for instance, suppose of the vernier beyond of the circle, and of the vernier to coincide with of the circle. the reading of the circle will then be - / . . _hour circles._--great circles perpendicular to the celestial equator are called _hour circles_. these circles all pass through the poles of the heavens, as shown in fig. . _eq_ is the celestial equator, and _p_ and _p'_ are the poles of the heavens. the point _a_ on the equinoctial (fig. ) is called the _vernal equinox_, or the _first point of aries_. the hour circle, _app'_, which passes through it, is called the _equinoctial colure_. [illustration: fig. .] . _declination and right ascension._--the _declination_ of a heavenly body is its distance north or south of the celestial equator. the _polar distance_ of a heavenly body is its distance from the nearer pole. declination and polar distance are measured on hour circles, and for the same heavenly body they are complements of each other. [illustration: fig. .] the _right ascension_ of a heavenly body is its distance eastward from the first point of aries, measured from the equinoctial colure. it is equal to the arc of the celestial equator included between the first point of aries and the hour circle which passes through the heavenly body. as right ascension is measured eastward entirely around the celestial sphere, it may have any value from ° up to °. right ascension corresponds to longitude on the earth, and declination to latitude. . _the meridian circle._--the right ascension and declination of a heavenly body are ascertained by means of an instrument called the _meridian circle_, or _transit instrument_. a side-view of this instrument is shown in fig. . [illustration: fig. .] it consists essentially of a telescope mounted between two piers, so as to turn in the plane of the meridian, and carrying a graduated circle. the readings of this circle are ascertained by means of fixed microscopes, under which it turns. a heavenly body can be observed with this instrument, only when it is crossing the meridian. for this reason it is often called the _transit circle_. to find the declination of a star with this instrument, we first ascertain the reading of the circle when the telescope is pointed to the pole, and then the reading of the circle when pointed to the star on its passage across the meridian. the difference between these two readings will be the polar distance of the star, and the complement of them the declination of the star. to ascertain the reading of the circle when the telescope is pointed to the pole, we must select one of the circumpolar stars near the pole, and then point the telescope to it when it crosses the meridian, both above and below the pole, and note the reading of the circle in each case. the mean of these two readings will be the reading of the circle when the telescope is pointed to the pole. . _astronomical clock._--an _astronomical clock_, or _sidereal clock_ as it is often called, is a clock arranged so as to mark hours from to , instead of from to , as in the case of an ordinary clock, and so adjusted as to mark when the vernal equinox, or first point of aries, is on the meridian. as the first point of aries makes a complete circuit of the heavens in twenty-four hours, it must move at the rate of ° an hour, or of ° in four minutes: hence, when the astronomical clock marks , the first point of aries must be ° west of the meridian, and when it marks , ° west of the meridian, etc. that is to say, by observing an accurate astronomical clock, one can always tell how far the meridian at any time is from the first point of aries. . _how to find right ascension with the meridian circle._--to find the right ascension of a heavenly body, we have merely to ascertain the exact time, by the astronomical clock, at which the body crosses the meridian. if a star crosses the meridian at hour minutes by the astronomical clock, its right ascension must be °; if at hours, its right ascension must be °. to enable the observer to ascertain with great exactness the time at which a star crosses the meridian, a number of equidistant and parallel spider-lines are stretched across the focus of the telescope, as shown in fig. . the observer notes the time when the star crosses each spider-line; and the mean of all of these times will be the time when the star crosses the meridian. the mean of several observations is likely to be more nearly exact than any single observation. [illustration: fig. .] [illustration: fig. .] . _the equatorial telescope._--the _equatorial_ telescope is mounted on two axes,--one parallel with the axis of the earth, and the other at right angles to this, and therefore parallel with the plane of the earth's equator. the former is called the _polar axis_, and the latter the _declination axis_. each axis carries a graduated circle. these circles are called respectively the _hour circle_ and the _declination circle_. the telescope is attached directly to the declination axis. when the telescope is fixed in any declination, and then turned on its polar axis, the line of sight will describe a diurnal circle; so that, when the tube is once directed to a star, it can be made to follow the star by simply turning the telescope on its polar axis. in the case of large instruments of this class, the polar axis is usually turned by clock-work at the rate at which the heavens rotate; so that, when the telescope has once been pointed to a planet or other heavenly body, it will continue to follow the body and keep it steadily in the field of view without further trouble on the part of the observer. the great washington equatorial is shown in fig. . its object-glass is inches in diameter, and its focal length is - / feet. it was constructed by alvan clark & sons of cambridge, mass. it is one of the three largest refracting telescopes at present in use. the newall refractor at gateshead, eng., has an objective inches in diameter, and a focal length of feet. the great refractor at vienna has an objective inches in diameter. there are several large refractors now in process of construction. [illustration: fig. .] . _the wire micrometer._--large arcs in the heavens are measured by means of the graduated circles attached to the axes of the telescopes; but small arcs within the field of view of the telescope are measured by means of instruments called _micrometers_, mounted in the focus of the telescope. one of the most convenient of these micrometers is that known as the _wire micrometer_, and shown in fig. . the frame _aa_ covers two slides, _c_ and _d_. these slides are moved by the screws _f_ and _g_. the wires _e_ and _b_ are stretched across the ends of the slides so as to be parallel to each other. on turning the screws _f_ and _g_ one way, these wires are carried apart; and on turning them the other way they are brought together again. sometimes two parallel wires, _x_ and _y_, shown in the diagram at the right, are stretched across the frame at right angles to the wires _e_, _b_. we may call the wires _x_ and _y_ the _longitudinal_ wires of the micrometer, and _e_ and _b_ the _transverse_ wires. many instruments have only one longitudinal wire, which is stretched across the middle of the focus. the longitudinal wires are just in front of the transverse wires, but do not touch them. to find the distance between any two points in the field of view with a micrometer, with a single longitudinal wire, turn the frame till the longitudinal wire passes through the two points; then set the wires _e_ and _b_ one on each point, turn one of the screws, known as the _micrometer screw_, till the two wires are brought together, and note the number of times the screw is turned. having previously ascertained over what arc one turn of the screw will move the wire, the number of turns will enable us to find the length of the arc between the two points. the threads of the micrometer screw are cut with great accuracy; and the screw is provided with a large head, which is divided into a hundred or more equal parts. these divisions, by means of a fixed pointer, enable us to ascertain what fraction of a turn the screw has made over and above its complete revolutions. . _reflecting telescopes._--it is possible to construct mirrors of much larger size than lenses: hence reflecting telescopes have an advantage over refracting telescopes as regards size of aperture and of light-gathering power. they are, however, inferior as regards definition; and, in order to prevent flexure, it is necessary to give the speculum, or mirror, a massiveness which makes the telescope unwieldy. it is also necessary frequently to repolish the speculum; and this is an operation of great delicacy, as the slightest change in the form of the surface impairs the definition of the image. these defects have been remedied, to a certain extent, by the introduction of silver-on-glass mirrors; that is, glass mirrors covered in front with a thin coating of silver. glass is only one-third as heavy as speculum-metal, and silver is much superior to that metal in reflecting power; and when the silver becomes tarnished, it can be removed and renewed without danger of changing the form of the glass. _the herschelian reflector._--in this form of telescope the mirror is slightly tipped, so that the image, instead of being formed in the centre of the tube, is formed near one side of it, as in fig. . the observer can then view it without putting his head inside the tube, and therefore without cutting off any material portion of the light. in observation, he must stand at the upper or outer end of the tube, and look into it, his back being turned towards the object. from his looking directly into the mirror, it is also sometimes called the _front-view_ telescope. the great disadvantage of this arrangement is, that the rays cannot be brought to an exact focus when they are thrown so far to one side of the axis, and the injury to the definition is so great that the front-view plan is now entirely abandoned. [illustration: fig. .] _the newtonian reflector._--the plan proposed by sir isaac newton was to place a small plane mirror just inside the focus, inclined to the telescope at an angle of °, so as to throw the rays to the side of the tube, where they come to a focus, and form the image. an opening is made in the side of the tube, just below where the image is formed; and in this opening the eye-piece is inserted. the small mirror cuts off some of the light, but not enough to be a serious defect. an improvement which lessens this defect has been made by professor henry draper. the inclined mirror is replaced by a small rectangular prism (fig. ), by reflection from which the image is formed very near the prism. a pair of lenses are then inserted in the course of the rays, by which a second image is formed at the opening in the side of the tube; and this second image is viewed by an ordinary eye-piece. [illustration: fig. .] _the gregorian reflector._--this is a form proposed by james gregory, who probably preceded newton as an inventor of the reflecting telescope. behind the focus, _f_ (fig. ), a small concave mirror, _r_, is placed, by which the light is reflected back again down the tube. the larger mirror, _m_, has an opening through its centre; and the small mirror, _r_, is so adjusted as to form a second image of the object in this opening. this image is then viewed by an eye-piece which is screwed into the opening. [illustration: fig. .] _the cassegrainian reflector._--in principle this is the same with the gregorian; but the small mirror, _r_, is convex, and is placed inside the focus, _f_, so that the rays are reflected from it before reaching the focus, and no image is formed until they reach the opening in the large mirror. this form has an advantage over the gregorian, in that the telescope may be made shorter, and the small mirror can be more easily shaped to the required figure. it has, therefore, entirely superseded the original gregorian form. [illustration: fig. .] optically these forms of telescope are inferior to the newtonian; but the latter is subject to the inconvenience, that the observer must be stationed at the upper end of the telescope, where he looks into an eye-piece screwed into the side of the tube. on the other hand, the cassegrainian telescope is pointed directly at the object to be viewed, like a refractor; and the observer stands at the lower end, and looks in at the opening through the large mirror. this is, therefore, the most convenient form of all in management. [illustration: fig. .] the largest reflecting telescope yet constructed is that of lord rosse, at parsonstown, ireland. its speculum is feet in diameter, and its focal length feet. it is commonly used as a newtonian. this telescope is shown in fig. . the great telescope of the melbourne observatory, australia, is a cassegranian reflector. its speculum is feet in diameter, and its focal length is feet. it is shown in fig. . [illustration: fig. .] the great reflector of the paris observatory is a newtonian reflector. its mirror of silvered glass is feet in diameter, and its focal length is feet. this telescope is shown in fig. . . _the sun's motion among the stars._--if we notice the stars at the same hour night after night, we shall find that the constellations are steadily advancing towards the west. new constellations are continually appearing in the east, and old ones disappearing in the west. this continual advancing of the heavens towards the west is due to the fact that the sun's place among the stars is _continually moving towards the east_. the sun completes the circuit of the heavens in a year, and is therefore moving eastward at the rate of about a degree a day. [illustration: fig. .] this motion of the sun's place among the stars is due to the revolution of the earth around the sun, and not to any real motion of the sun. in fig. suppose the inner circle to represent the orbit of the earth around the sun, and the outer circle to represent the celestial sphere. when the earth is at _e_, the sun's place on the celestial sphere is at _s'_. as the earth moves in the direction _ef_, the sun's place on the celestial sphere must move in the direction _s't_: hence the revolution of the earth around the sun would cause the sun's place among the stars to move around the heavens in the same direction that the earth is moving around the sun. . _the ecliptic._--the circle described by the sun in its apparent motion around the heavens is called the _ecliptic_. the plane of this circle passes through the centre of the earth, and therefore through the centre of the celestial sphere; the earth being so small, compared with the celestial sphere, that it practically makes no difference whether we consider a point on its surface, or one at its centre, as the centre of the celestial sphere. the ecliptic is, therefore, a great circle. the earth's orbit lies in the plane of the ecliptic; but it extends only an inappreciable distance from the sun towards the celestial sphere. [illustration: fig. .] . _the obliquity of the ecliptic._--the ecliptic is inclined to the celestial equator by an angle of about - / °. this inclination is called the _obliquity of the ecliptic_. the obliquity of the ecliptic is due to the deviation of the earth's axis from a perpendicular to the plane of its orbit. the axis of a rotating body tends to maintain the same direction; and, as the earth revolves around the sun, its axis points all the time in nearly the same direction. the earth's axis deviates about - / ° from the perpendicular to its orbit; and, as the earth's equator is at right angles to its axis, it will deviate about - / ° from the plane of the ecliptic. the celestial equator has the same direction as the terrestrial equator, since the axis of the heavens has the same direction as the axis of the earth. [illustration: fig. .] suppose the globe at the centre of the tub (fig. ) to represent the sun, and the smaller globes to represent the earth in various positions in its orbit. the surface of the water will then represent the plane of the ecliptic, and the rod projecting from the top of the earth will represent the earth's axis, which is seen to point all the time in the same direction, or to lean the same way. the leaning of the axis from the perpendicular to the surface of the water would cause the earth's equator to be inclined the same amount to the surface of the water, half of the equator being above, and half of it below, the surface. were the axis of the earth perpendicular to the surface of the water, the earth's equator would coincide with the surface, as is evident from fig. . [illustration: fig. .] . _the equinoxes and solstices._--the ecliptic and celestial equator, being great circles, bisect each other. half of the ecliptic is north, and half of it is south, of the equator. the points at which the two circles cross are called the _equinoxes_. the one at which the sun crosses the equator from south to north is called the _vernal_ equinox, and the one at which it crosses from north to south the _autumnal_ equinox. the points on the ecliptic midway between the equinoxes are called the _solstices_. the one north of the equator is called the _summer_ solstice, and the one south of the equator the _winter_ solstice. in fig. , _eq_ is the celestial equator, _ece'c'_ the ecliptic, _v_ the vernal equinox, a the autumnal equinox, ec the winter solstice, and _e'c'_ the summer solstice. [illustration: fig. .] . _the inclination of the ecliptic to the horizon._--since the celestial equator is perpendicular to the axis of the heavens, it makes the same angle with it on every side: hence, at any place, the equator makes always the same angle with the horizon, whatever part of it is above the horizon. but, as the ecliptic is oblique to the equator, it makes different angles with the celestial axis on different sides; and hence, at any place, the angle which the ecliptic makes with the horizon varies according to the part which is above the horizon. the two extreme angles for a place more than - / ° north of the equator are shown in figs. and . the least angle is formed when the vernal equinox is on the eastern horizon, the autumnal on the western horizon, and the winter solstice on the meridian, as in fig. . the angle which the ecliptic then makes with the horizon is equal to the elevation of the equinoctial _minus_ - / °. in the latitude of new york this angle = ° - - / ° = - / °. [illustration: fig. .] the greatest angle is formed when the autumnal equinox is on the eastern horizon, the vernal on the western horizon, and the summer solstice is on the meridian (fig. ). the angle between the ecliptic and the horizon is then equal to the elevation of the equinoctial _plus_ - / °. in the latitude of new york this angle = ° + - / ° = - / °. of course the equinoxes, the solstices, and all other points on the ecliptic, describe diurnal circles, like every other point in the heavens: hence, in our latitude, these points rise and set every day. . _celestial latitude and longitude._--_celestial latitude_ is distance measured north or south from the ecliptic; and _celestial longitude_ is distance measured on the ecliptic eastward from the vernal equinox, or the first point of aries. great circles perpendicular to the ecliptic are called _celestial meridians_. these circles all pass through the poles of the ecliptic, which are some - / ° from the poles of the equinoctial. the latitude of a heavenly body is measured by the arc of a celestial meridian included between the body and the ecliptic. the longitude of a heavenly body is measured by the arc of the ecliptic included between the first point of aries and the meridian which passes through the body. there are, of course, always two arcs included between the first point of aries and the meridian,--one on the east, and the other on the west, of the first point of aries. the one on the _east_ is always taken as the measure of the longitude. . _the precession of the equinoxes._--the equinoctial points have a slow westward motion along the ecliptic. this motion is at the rate of about '' a year, and would cause the equinoxes to make a complete circuit of the heavens in a period of about twenty-six thousand years. it is called the _precession of the equinoxes_. this westward motion of the equinoxes is due to the fact that the axis of the earth has a slow gyratory motion, like the handle of a spinning-top which has begun to wabble a little. this gyratory motion causes the axis of the heavens to describe a cone in about twenty-six thousand years, and the pole of the heavens to describe a circle about the pole of the ecliptic in the same time. the radius of this circle is - / °. [illustration: fig. .] . _illustration of precession._--the precession of the equinoxes may be illustrated by means of the apparatus shown in fig. . the horizontal and stationary ring _ec_ represents the ecliptic; the oblique ring _e'q_ represents the equator; _v_ and _a_ represent the equinoctial point, and _e_ and _c_ the solstitial points; _b_ represents the pole of the ecliptic, _p_ the pole of the equator, and _po_ the celestial axis. the ring _e'q_ is supported on a pivot at _o_; and the rod _bp_, which connects _b_ and _p_, is jointed at each end so as to admit of the movement of _p_ and _b_. on carrying _p_ around _b_, we shall see that _e'q_ will always preserve the same obliquity to _ec_, and that the points _v_ and _a_ will move around the circle _ec_. the same will also be true of the points _e_ and _c_. . _effects of precession._--one effect of precession, as has already been stated, is the revolution of the pole of the heavens around the pole of the ecliptic in a period of about twenty-six thousand years. the circle described by the pole of the heavens, and the position of the pole at various dates, are shown in fig. , where o indicates the position of the pole at the birth of christ. the numbers round the circle to the left of o are dates a.d., and those to the right of o are dates b.c. it will be seen that the star at the end of the little bear's tail, which is now near the north pole, will be exactly at the pole about the year . it will then recede farther and farther from the pole till the year a.d., when it will be about forty-seven degrees away from the pole. it will be noticed that one of the stars of the dragon was the pole star about years b.c. there are reasons to suppose that this was about the time of the building of the great pyramid. a second effect of precession is the shifting of the signs along the zodiac. the _zodiac_ is a belt of the heavens along the ecliptic, extending eight degrees from it on each side. this belt is occupied by twelve constellations, known as the _zodiacal constellations_. they are _aries_, _taurus_, _gemini_, _cancer_, _leo_, _virgo_, _libra_, _scorpio_, _sagittarius_, _capricornus_, _aquarius_, and _pisces_. the zodiac is also divided into twelve equal parts of thirty degrees each, called _signs_. these signs have the same names as the twelve zodiacal constellations, and when they were first named, each sign occupied the same part of the zodiac as the corresponding constellation; that is to say, the sign aries was in the constellation aries, and the sign taurus in the constellation taurus, etc. now the signs are always reckoned as beginning at the vernal equinox, which is continually shifting along the ecliptic; so that the signs are continually moving along the zodiac, while the constellations remain stationary: hence it has come about that the _first point of aries_ (the _sign_) is no longer in the _constellation_ aries, but in pisces. [illustration: fig. .] fig. shows the position of the vernal equinox b.c. it was then in taurus, just south of the pleiades. it has since moved from taurus, through aries, and into pisces, as shown in fig. . [illustration: fig. .] [illustration: fig. .] since celestial longitude and right ascension are both measured from the first point of aries, the longitude and right ascension of the stars are slowly changing from year to year. it will be seen, from figs. and , that the declination is also slowly changing. . _nutation._--the gyratory motion of the earth's axis is not perfectly regular and uniform. the earth's axis has a slight tremulous motion, oscillating to and fro through a short distance once in about nineteen years. this tremulous motion of the axis causes the pole of the heavens to describe an undulating curve, as shown in fig. , and gives a slight unevenness to the motion of the equinoxes along the ecliptic. this nodding motion of the axis is called _nutation_. [illustration: fig. .] . _refraction._--when a ray of light from one of the heavenly bodies enters the earth's atmosphere obliquely, it will be bent towards a perpendicular to the surface of the atmosphere, since it will be entering a denser medium. as the ray traverses the atmosphere, it will be continually passing into denser and denser layers, and will therefore be bent more and more towards the perpendicular. this bending of the ray is shown in fig. . a ray which started from _a_ would enter the eye at _c_, as if it came from _i_: hence a star at _a_ would appear to be at _i_. [illustration: fig. .] atmospheric refraction displaces all the heavenly bodies from the horizon towards the zenith. this is evident from fig. . _od_ is the horizon, and _z_ the zenith, of an observer at _o_. refraction would make a star at _q_ appear at _p_: in other words, it would displace it towards the zenith. a star in the zenith is not displaced by refraction, since the rays which reach the eye from it traverse the atmosphere vertically. the farther a star is from the zenith, the more it is displaced by refraction, since the greater is the obliquity with which the rays from it enter the atmosphere. [illustration: fig. .] at the horizon the displacement by refraction is about half a degree; but it varies considerably with the state of the atmosphere. refraction causes a heavenly body to appear above the horizon longer than it really is above it, since it makes it appear to be on the horizon when it is really half a degree below it. the increase of refraction towards the horizon often makes the sun, when near the horizon, appear distorted, the lower limb of the sun being raised more than the upper limb. this distortion is shown in fig. . the vertical diameter of the sun appears to be considerably less than the horizontal diameter. [illustration: fig. .] . _parallax._--_parallax_ is the displacement of an object caused by a change in the point of view from which it is seen. thus in fig. , the top of the tower _s_ would be seen projected against the sky at _a_ by an observer at _a_, and at _b_ by an observer at _b_. in passing from _a_ to _b_, the top of the tower is displaced from _a_ to _b_, or by the angle _asb_. this angle is called the parallax of _s_, as seen from _b_ instead of _a_. [illustration: fig. .] the _geocentric parallax_ of a heavenly body is its displacement caused by its being seen from the surface of the earth, instead of from the centre of the earth. in fig. , _r_ is the centre of the earth, and _o_ the point of observation on the surface of the earth. stars at _s_, _s'_, and _s''_, would, from the centre of the earth, appear at _q_, _q'_, and _q''_; while from the point _o_ on the surface of the earth, these same stars would appear at _p_, _p'_ and _p''_, being displaced from their position, as seen from the centre of the earth, by the angles _qsp_, _q's'p'_, and _q''s''p''_. it will be seen that parallax displaces a body from the zenith towards the horizon, and that the parallax of a body is greatest when it is on the horizon. the parallax of a heavenly body when on the horizon is called its _horizontal parallax_. a body in the zenith is not displaced by parallax, since it would be seen in the same direction from both the centre and the surface of the earth. [illustration: fig. .] the parallax of a body at _s'''_ is _q'''s'''p_, which is seen to be greater than _qsp_; that is to say, the parallax of a heavenly body increases with its nearness to the earth. the distance and parallax of a body are so related, that, either being known, the other may be computed. . _aberration._--_aberration_ is a slight displacement of a star, owing to an apparent change in the direction of the rays of light which proceed from it, caused by the motion of the earth in its orbit. if we walk rapidly in any direction in the rain, when the drops are falling vertically, they will appear to come into our faces from the direction in which we are walking. our own motion has apparently changed the direction in which the drops are falling. [illustration: fig. .] in fig. let _a_ be a gun of a battery, from which a shot is fired at a ship, _de_, that is passing. let _abc_ be the course of the shot. the shot enters the ship's side at _b_, and passes out at the other side at _c_; but in the mean time the ship has moved from _e_ to _e_, and the part _b_, where the shot entered, has been carried to _b_. if a person on board the ship could see the ball as it crossed the ship, he would see it cross in the diagonal line _bc_; and he would at once say that the cannon was in the direction of _cb_. if the ship were moving in the opposite direction, he would say that the cannon was just as far the other side of its true position. now, we see a star in the direction in which the light coming from the star appears to be moving. when we examine a star with a telescope, we are in the same condition as the person who on shipboard saw the cannon-ball cross the ship. the telescope is carried along by the earth at the rate of eighteen miles a second: hence the light will appear to pass through the tube in a slightly different direction from that in which it is really moving: just as the cannon-ball appears to pass through the ship in a different direction from that in which it is really moving. thus in fig. , a ray of light coming in the direction _sot_ would appear to traverse the tube _ot_ of a telescope, moving in the direction of the arrow, as if it were coming in the direction _s'o_. [illustration: fig. .] as light moves with enormous velocity, it passes through the tube so quickly, that it is apparently changed from its true direction only by a very slight angle: but it is sufficient to displace the star. this apparent change in the direction of light caused by the motion of the earth is called _aberration of light_. . _the planets._--on watching the stars attentively night after night, it will be found, that while the majority of them appear _fixed_ on the surface of the celestial sphere, so as to maintain their relative positions, there are a few that _wander_ about among the stars, alternately advancing towards the east, halting, and retrograding towards the west. these wandering stars are called _planets_. their motions appear quite irregular; but, on the whole, their eastward motion is in excess of their westward, and in a longer or shorter time they all complete the circuit of the heavens. in almost every instance, their paths are found to lie wholly in the belt of the zodiac. [illustration: fig. .] fig. shows a portion of the apparent path of one of the planets. ii. the solar system. i. theory of the solar system. . _members of the solar system._--the solar system is composed of the _sun_, _planets_, _moons_, _comets_, and _meteors_. five planets, besides the earth, are readily distinguished by the naked eye, and were known to the ancients: these are _mercury_, _venus_, _mars_, _jupiter_, and _saturn_. these, with the _sun_ and _moon_, made up the _seven planets_ of the ancients, from which the seven days of the week were named. the ptolemaic system. . _the crystalline spheres._--we have seen that all the heavenly bodies appear to be situated on the surface of the celestial sphere. the ancients assumed that the stars were really fixed on the surface of a crystalline sphere, and that they were carried around the earth daily by the rotation of this sphere. they had, however, learned to distinguish the planets from the stars, and they had come to the conclusion that some of the planets were nearer the earth than others, and that all of them were nearer the earth than the stars are. this led them to imagine that the heavens were composed of a number of crystalline spheres, one above another, each carrying one of the planets, and all revolving around the earth from east to west, but at different rates. this structure of the heavens is shown in section in fig. . [illustration: fig. .] . _cycles and epicycles._--the ancients had also noticed that, while all the planets move around the heavens from west to east, their motion is not one of uniform advancement. mercury and venus appear to oscillate to and fro across the sun, while jupiter and saturn appear to oscillate to and fro across a centre which is moving around the earth, so as to describe a series of loops, as shown in fig. . [illustration: fig. .] the ancients assumed that the planets moved in exact circles, and, in fact, that all motion in the heavens was circular, the circle being the simplest and most perfect curve. to account for the loops described by the planets, they imagined that each planet revolved in a circle around a centre, which, in turn, revolved in a circle around the earth. the circle described by this centre around the earth they called the _cycle_, and the circle described by the planet around this centre they called the _epicycle_. . _the eccentric._--the ancients assumed that the planets moved at a uniform rate in describing the epicycle, and also the centre in describing the cycle. they had, however, discovered that the planets advance eastward more rapidly in some parts of their orbits than in others. to account for this they assumed that the cycles described by the centre, around which the planets revolved, were _eccentric_; that is to say, that the earth was not at the centre of the cycle, but some distance away from it, as shown in fig. . _e_ is the position of the earth, and _c_ is the centre of the cycle. the lines from _e_ are drawn so as to intercept equal arcs of the cycle. it will be seen at once that the angle between any pair of lines is greatest at _p_, and least at _a_; so that, were a planet moving at the same rate at _p_ and _a_, it would seem to be moving much faster at _p_. the point _p_ of the planet's cycle was called its _perigee_, and the point a its _apogee_. [illustration: fig. .] as the apparent motion of the planets became more accurately known, it was found necessary to make the system of cycles, epicycles, and eccentrics exceedingly complicated to represent that motion. the copernican system. . _copernicus._--copernicus simplified the ptolemaic system greatly by assuming that the earth and all the planets revolved about the sun as a centre. he, however, still maintained that all motion in the heavens was circular, and hence he could not rid his system entirely of cycles and epicycles. tycho brahe's system. . _tycho brahe._--tycho brahe was the greatest of the early astronomical observers. he, however, rejected the system of copernicus, and adopted one of his own, which was much more complicated. he held that all the planets but the earth revolved around the sun, while the sun and moon revolved around the earth. this system is shown in fig. . [illustration: fig. .] kepler's system. . _kepler._--while tycho brahe devoted his life to the observation of the planets. kepler gave his to the study of tycho's observations, for the purpose of discovering the true laws of planetary motion. he banished the complicated system of cycles, epicycles, and eccentrics forever from the heavens, and discovered the three laws of planetary motion which have rendered his name immortal. . _the ellipse._--an _ellipse_ is a closed curve which has two points within it, the sum of whose distances from every point on the curve is the same. these two points are called the _foci_ of the ellipse. [illustration: fig. .] one method of describing an ellipse is shown in fig. . two tacks, _f_ and _f'_, are stuck into a piece of paper, and to these are fastened the two ends of a string which is longer than the distance between the tacks. a pencil is then placed against the string, and carried around, as shown in the figure. the curve described by the pencil is an ellipse. the two points _f_ and _f'_ are the foci of the ellipse: the sum of the distances of these two points from every point on the curve is equal to the length of the string. when half of the ellipse has been described, the pencil must be held against the other side of the string in the same way, and carried around as before. the point _o_, half way between _f_ and _f'_, is called the _centre_ of the ellipse; _aa'_ is the _major axis_ of the ellipse, and _cd_ is the _minor axis_. . _the eccentricity of the ellipse._--the ratio of the distance between the two foci to the major axis of the ellipse is called the _eccentricity_ of the ellipse. the greater the distance between the two foci, compared with the major axis of the ellipse, the greater is the eccentricity of the ellipse; and the less the distance between the foci, compared with the length of the major axis, the less the eccentricity of the ellipse. the ellipse of fig. has an eccentricity of / . this ellipse scarcely differs in appearance from a circle. the ellipse of fig. has an eccentricity of / , and that of fig. an eccentricity of / . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _kepler's first law._--kepler first discovered that _all the planets move from west to east in ellipses which have the sun as a common focus_. this law of planetary motion is known as _kepler's first law_. the planets appear to describe loops, because we view them from a moving point. the ellipses described by the planets differ in eccentricity; and, though they all have one focus at the sun, their major axes have different directions. the eccentricity of the planetary orbits is comparatively small. the ellipse of fig. has seven times the eccentricity of the earth's orbit, and twice that of the orbit of any of the larger planets except mercury; and its eccentricity is more than half of that of the orbit of mercury. owing to their small eccentricity, the orbits of the planets are usually represented by circles in astronomical diagrams. [illustration: fig. .] . _kepler's second law._--kepler next discovered that a planet's rate of motion in the various parts of its orbit is such that _a line drawn from the planet to the sun would always sweep over equal areas in equal times_. thus, in fig. , suppose the planet would move from _p_ to _p^ _ in the same time that it would move from _p^ _ to _p^ _, or from _p^ _ to _p^ _; then the dark spaces, which would be swept over by a line joining the sun and the planet, in these equal times, would all be equal. a line drawn from the sun to a planet is called the _radius vector_ of the planet. the radius vector of a planet is shortest when the planet is nearest the sun, or at _perihelion_, and longest when the planet is farthest from the sun, or at _aphelion_: hence, in order to have the areas equal, it follows that a planet must move fastest when at perihelion, and slowest at aphelion. _kepler's second law_ of planetary motion is usually stated as follows: _the radius vector of a planet describes equal areas in equal times in every part of the planet's orbit_. . _kepler's third law._--kepler finally discovered that the periodic times of the planets bear the following relation to the distances of the planets from the sun: _the squares of the periodic times of the planets are to each other as the cubes of their mean distances from the sun_. this is known as _kepler's third law_ of planetary motion. by _periodic time_ is meant the time it takes a planet to revolve around the sun. these three laws of kepler's are the foundation of modern physical astronomy. the newtonian system. . _newton's discovery._--newton followed kepler, and by means of his three laws of planetary motion made his own immortal discovery of the _law of gravitation_. this law is as follows: _every portion of matter in the universe attracts every other portion with a force varying directly as the product of the masses acted upon, and inversely as the square of the distances between them._ . _the conic sections._--the _conic sections_ are the figures formed by the various plane sections of a right cone. there are four classes of figures formed by these sections, according to the angle which the plane of the section makes with the axis of the cone. _opq_, fig. , is a right cone, and _on_ is its axis. any section, _ab_, of this cone, whose plane is perpendicular to the axis of the cone, is a _circle_. [illustration: fig. .] any section, _cd_, of this cone, whose plane is oblique to the axis, but forms with it an angle greater than _nop_, is an _ellipse_. the less the angle which the plane of the section makes with the axis, the more elongated is the ellipse. any section, _ef_, of this cone, whose plane makes with the axis an angle equal to _nop_, is a _parabola_. it will be seen, that, by changing the obliquity of the plane _cd_ to the axis _no_, we may pass uninterruptedly from the circle through ellipses of greater and greater elongation to the parabola. any section, _gh_, of this cone, whose plane makes with the axis _on_ an angle less than _nop_, is a _hyperbola_. [illustration: fig. .] it will be seen from fig. , in which comparative views of the four conic sections are given, that the circle and the ellipse are _closed_ curves, or curves which return into themselves. the parabola and the hyperbola are, on the contrary, _open_ curves, or curves which do not return into themselves. . _a revolving body is continually falling towards its centre of revolution._--in fig. let _m_ represent the moon, and _e_ the earth around which the moon is revolving in the direction _mn_. it will be seen that the moon, in moving from m to n, falls towards the earth a distance equal to _mn_. it is kept from falling into the earth by its orbital motion. [illustration: fig. .] the fact that a body might be projected forward fast enough to keep it from falling into the earth is evident from fig. . _ab_ represents the level surface of the ocean, _c_ a mountain from the summit of which a cannon-ball is supposed to be fired in the direction _ce_. _ad_ is a line parallel with _ce_; _db_ is a line equal to the distance between the two parallel lines _ad_ and _ce_. this distance is equal to that over which gravity would pull a ball towards the centre of the earth in a minute. no matter, then, with what velocity the ball _c_ is fired, at the end of a minute it will be somewhere on the line _ad_. suppose it were fired fast enough to reach the point _d_ in a minute: it would be on the line _ad_ at the end of the minute, but still just as far from the surface of the water as when it started. it will be seen, that, although it has all the while been falling towards the earth, it has all the while kept at exactly the same distance from the surface. the same thing would of course be true during each succeeding minute, till the ball came round to _c_ again, and the ball would continue to revolve in a circle around the earth. [illustration: fig. .] . _the form of a body's orbit depends upon the rate of its forward motion._--if the ball _c_ were fired fast enough to reach the line _ad_ beyond the point _d_, it would be farther from the surface at the end of the second than when it started. its orbit would no longer be circular, but _elliptical_. if the speed of projection were gradually augmented, the orbit would become a more and more elongated ellipse. at a certain rate of projection, the orbit would become a _parabola_; at a still greater rate, a _hyperbola_. . _the moon held in her orbit by gravity._--newton compared the distance _mn_ that the moon is drawn to the earth in a given time, with the distance a body near the surface of the earth would be pulled toward the earth in the same time; and he found that these distances are to each other inversely as the squares of the distances of the two bodies from the centre of the earth. he therefore concluded that _the moon is drawn to the earth by gravity_, and that the _intensity of gravity decreases as the square of the distance increases_. [illustration: fig. .] . _any body whose orbit is a conic section, and which moves according to kepler's second law, is acted upon by a force varying inversely as the square of the distance._--newton compared the distance which any body, moving in an ellipse, according to kepler's second law, is drawn towards the sun in the same time in different parts of its orbit. he found these distances in all cases to vary inversely as the square of the distance of the planet from the sun. thus, in fig. , suppose a planet would move from _k_ to _b_ in the same time that it would move from _k_ to _b_ in another part of its orbit. in the first instance the planet would be drawn towards the sun the distance _ab_, and in the second instance the distance _ab_. newton found that _ab : ab = (sk)^ : (sk)^ _. he also found that the same would be true when the body moved in a parabola or a hyperbola: hence he concluded that _every body that moves around the sun in an ellipse, a parabola, or a hyperbola, is moving under the influence of gravity_. [transcriber's note: in newton's equation above, (sk)^ means to group s and k together and square their product. in the original book, instead of using parentheses, there was a vinculum, a horizontal bar, drawn over the s and the k to express the same grouping.] [illustration: fig. .] . _the force that draws the different planets to the sun varies inversely as the squares of the distances of the planets from the sun._--newton compared the distances _jk_ and _ef_, over which two planets are drawn towards the sun in the same time, and found these distances to vary inversely as the squares of the distances of the planets from the sun: hence he concluded that _all the planets are held in their orbits by gravity_. he also showed that this would be true of any two bodies that were revolving around the sun's centre, according to kepler's third law. . _the copernican system._--the theory of the solar system which originated with copernicus, and which was developed and completed by kepler and newton, is commonly known as the _copernican system_. this system is shown in fig. . [illustration: fig. .] ii. the sun and planets. i. the earth. form and size. . _form of the earth._--in ordinary language the term _horizon_ denotes the line that bounds the portion of the earth's surface that is visible at any point. ( ) it is well known that the horizon of a plain presents the form of a circle surrounding the observer. if the latter moves, the circle moves also; but its form remains the same, and is modified only when mountains or other obstacles limit the view. out at sea, the circular form of the horizon is still more decided, and changes only near the coasts, the outline of which breaks the regularity. here, then, we obtain a first notion of the rotundity of the earth, since a sphere is the only body which is presented always to us under the form of a circle, from whatever point on its surface it is viewed. ( ) moreover, it cannot be maintained that the horizon is the vanishing point of distinct vision, and that it is this which causes the appearance of a circular boundary, because the horizon is enlarged when we mount above the surface of the plain. this will be evident from fig. , in which a mountain is depicted in the middle of a plain, whose uniform curvature is that of a sphere. from the foot of the mountain the spectator will have but a very limited horizon. let him ascend half way, his visual radius extends, is inclined below the first horizon, and reveals a more extended circular area. at the summit of the mountain the horizon still increases; and, if the atmosphere is pure, the spectator will see numerous objects where from the lower stations the sky alone was visible. [illustration: fig. .] this extension of the horizon would be inexplicable if the earth had the form of an extended plane. ( ) the curvature of the surface of the sea manifests itself in a still more striking manner. if we are on the coast at the summit of a hill, and a vessel appears on the horizon (fig. ), we see only the tops of the masts and the highest sails; the lower sails and the hull are invisible. as the vessel approaches, its lower part comes into view above the horizon, and soon it appears entire. [illustration: fig. .] in the same manner the sailors from the ship see the different parts of objects on the land appear successively, beginning with the highest. the reason of this will be evident from fig. , where the course of a vessel, seen in profile, is figured on the convex surface of the sea. [illustration: fig. .] as the curvature of the ocean is the same in every direction, it follows that the surface of the ocean is _spherical_. the same is true of the surface of the land, allowance being made for the various inequalities of the surface. from these and various other indications, we conclude that _the earth is a sphere_. . _size of the earth._--the size of the earth is ascertained by measuring the length of a degree of a meridian, and multiplying this by three hundred and sixty. this gives the circumference of the earth as about twenty-five thousand miles, and its diameter as about eight thousand miles. we know that the two stations between which we measure are one degree apart when the elevation of the pole at one station is one degree greater than at the other. . _the earth flattened at the poles._--degrees on the meridian have been measured in various parts of the earth, and it has been found that they invariably increase in length as we proceed from the equator towards the pole: hence the earth must curve less and less rapidly as we approach the poles; for the less the curvature of a circle, the larger the degrees on it. [illustration: fig. .] . _the earth in space._--in fig. we have a view of the earth suspended in space. the side of the earth turned towards the sun is illumined, and the other side is in darkness. as the planet rotates on its axis, successive portions of it will be turned towards the sun. as viewed from a point in space between it and the sun, it will present light and dark portions, which will assume different forms according to the portion which is illumined. these different appearances are shown in fig. . [illustration: fig. .] day and night. . _day and night._--the succession of day and night is due to _the rotation of the earth on its axis_, by which a place on the surface of the earth is carried alternately into the sunshine and out of it. as the sun moves around the heavens on the ecliptic, it will be on the celestial equator when at the equinoxes, and - / ° north of the equator when at the summer solstice, and - / ° south of the equator when at the winter solstice. . _day and night when the sun is at the equinoxes._--when the sun is at either equinox, the diurnal circle described by the sun will coincide with the celestial equator; and therefore half of this diurnal circle will be above the horizon at every point on the surface of the globe. at these times _day and night will be equal in every part of the earth_. [illustration: fig. .] [illustration: fig. .] the equality of days and nights when the sun is on the celestial equator is also evident from the following considerations: one-half of the earth is in sunshine all of the time; when the sun is on the celestial equator, it is directly over the equator of the earth, and the illumination extends from pole to pole, as is evident from figs. and , in the former of which the sun is represented as on the eastern horizon at a place along the central line of the figure, and in the latter as on the meridian along the same line. in each diagram it is seen that the illumination extends from pole to pole: hence, as the earth rotates on its axis, every place on the surface will be in the sunshine and out of it just half of the time. . _day and night when the sun is at the summer solstice._--when the sun is at the summer solstice, it will be - / ° north of the celestial equator. the diurnal circle described by the sun will then be - / ° north of the celestial equator; and more than half of this diurnal circle will be above the horizon at all places north of the equator, and less than half of it at places south of the equator: hence _the days will be longer than the nights at places north of the equator, and shorter than the nights at places south of the equator_. at places within - / ° of the north pole, the entire diurnal circle described by the sun will be above the horizon, so that the sun will not set. at places within - / ° of the south pole of the earth, the entire diurnal circle will be below the horizon, so that the sun will not rise. [illustration: fig. .] [illustration: fig. .] the illumination of the earth at this time is shown in figs. and . in fig. the sun is represented as on the western horizon along the middle line of the figure, and in fig. as on the meridian. it is seen at once that the illumination extends - / ° beyond the north pole, and falls - / ° short of the south pole. as the earth rotates on its axis, places near the north pole will be in the sunshine all the time, while places near the south pole will be out of the sunshine all the time. all places north of the equator will be in the sunshine longer than they are out of it, while all places south of the equator will be out of the sunshine longer than they are in it. . _day and night when the sun is at the winter solstice._--when the sun is at the winter solstice, it is - / ° south of the celestial equator. the diurnal circle described by the sun is then - / ° south of the celestial equator. more than half of this diurnal circle will therefore be above the horizon at all places south of the equator, and less than half of it at all places north of the equator: hence _the days will be longer than the nights south of the equator, and shorter than the nights at places north of the equator_. at places within - / ° of the south pole, the diurnal circle described by the sun will be entirely above the horizon, and the sun will therefore not set. at places within - / ° of the north pole, the diurnal circle described by the sun will be wholly below the horizon, and therefore the sun will not rise. the illumination of the earth at this time is shown in figs. and , and is seen to be the reverse of that shown in figs. and . [illustration: fig. .] [illustration: fig. .] . _variation in the length of day and night._--as long as the sun is north of the equinoctial, the nights will be longer than the days south of the equator, and shorter than the days north of the equator. it is just the reverse when the sun is south of the equator. the farther the sun is from the equator, the greater is the inequality of the days and nights. the farther the place is from the equator, the greater the inequality of its days and nights. when the distance of a place from the _north_ pole is less than the distance of the sun north of the equinoctial, it will have _continuous day without night_, since the whole of the sun's diurnal circle will be above the horizon. a place within the same distance of the _south_ pole will have _continuous night_. when the distance of a place from the _north_ pole is less than the distance of the sun south of the equinoctial, it will have _continuous night_, since the whole of the sun's diurnal circle will then be below the horizon. a place within the same distance of the _south_ pole will then have _continuous day_. at the _equator_ the _days and nights are always equal_; since, no matter where the sun is in the heavens, half of all the diurnal circles described by it will be above the horizon, and half of them below it. . _the zones._--it will be seen, from what has been stated above, that the sun will at some time during the year be directly overhead at every place within - / ° of the equator on either side. this belt of the earth is called the _torrid zone_. the torrid zone is bounded by circles called the _tropics_; that of _cancer_ on the north, and that of _capricorn_ on the south. it will also be seen, that, at every place within - / ° of either pole, there will be, some time during the year, a day during which the sun will not rise, or on which it will not set. these two belts of the earth's surface are called the _frigid zones_. these zones are bounded by the _arctic_ circles. the nearer a place is to the poles, the greater the number of days on which the sun does not rise or set. between the frigid zones and the torrid zones, there are two belts on the earth which are called the _temperate zones_. the sun is never overhead at any place in these two zones, but it rises and sets every day at every place within their limits. . _the width of the zones._--the distance the frigid zones extend from the poles, and the torrid zones from the equator, is exactly equal to _the obliquity of the ecliptic_, or the deviation of the axis of the earth from the perpendicular to the plane of its orbit. were this deviation forty-five degrees, the obliquity of the ecliptic would be forty-five degrees, the torrid zone would extend forty-five degrees from the equator, and the frigid zones forty-five degrees from the poles. in this case there would be no temperate zones. were this deviation fifty degrees, the torrid and frigid zones would overlap ten degrees, and there would be two belts of ten degrees on the earth, which would experience alternately during the year a torrid and a frigid climate. were the axis of the earth perpendicular to the plane of the earth's orbit, there would be no zones on the earth, and no variation in the length of day and night. . _twilight._--were it not for the atmosphere, the darkness of midnight would begin the moment the sun sank below the horizon, and would continue till he rose again above the horizon in the east, when the darkness of the night would be suddenly succeeded by the full light of day. the gradual transition from the light of day to the darkness of the night, and from the darkness of the night to the light of day, is called _twilight_, and is due to the _diffusion of light from the upper layers of the atmosphere_ after the sun has ceased to shine on the lower layers at night, or before it has begun to shine on them in the morning. [illustration: fig. .] let _abcd_ (fig. ) represent a portion of the earth, _a_ a point on its surface where the sun _s_ is setting; and let _sah_ be a ray of light just grazing the earth at _a_, and leaving the atmosphere at the point _h_. the point _a_ is illuminated by the whole reflective atmosphere _hgfe_. the point _b_, to which the sun has set, receives no direct solar light, nor any reflected from that part of the atmosphere which is below _alh_; but it receives a twilight from the portion _hlf_, which lies above the visible horizon _bf_. the point _c_ receives a twilight only from the small portion of the atmosphere; while at _d_ the twilight has ceased altogether. . _duration of twilight._--the astronomical limit of twilight is generally understood to be the instant when stars of the sixth magnitude begin to be visible in the zenith at evening, or disappear in the morning. twilight is usually reckoned to last until the sun's depression below the horizon amounts to eighteen degrees: this, however, varies; in the tropics a depression of sixteen or seventeen degrees being sufficient to put an end to the phenomenon, while in england a depression of seventeen to twenty-one degrees is required. the duration of twilight differs in different latitudes; it varies also in the same latitude at different seasons of the year, and depends, in some measure, on the meteorological condition of the atmosphere. when the sky is of a pale color, indicating the presence of an unusual amount of condensed vapor, twilight is of longer duration. this happens habitually in the polar regions. on the contrary, within the tropics, where the air is pure and dry, twilight sometimes lasts only fifteen minutes. strictly speaking, in the latitude of greenwich there is no true night from may to july , but constant twilight from sunset to sunrise. twilight reaches its minimum three weeks before the vernal equinox, and three weeks after the autumnal equinox, when its duration is an hour and fifty minutes. at midwinter it is longer by about seventeen minutes; but the augmentation is frequently not perceptible, owing to the greater prevalence of clouds and haze at that season of the year, which intercept the light, and hinder it from reaching the earth. the duration is least at the equator (an hour and twelve minutes), and increases as we approach the poles; for at the former there are two twilights every twenty-four hours, but at the latter only two in a year, each lasting about fifty days. at the north pole the sun is below the horizon for six months, but from jan. to the vernal equinox, and from the autumnal equinox to nov. , the sun is less than eighteen degrees below the horizon; so that there is twilight during the whole of these intervals, and thus the length of the actual night is reduced to two months and a half. the length of the day in these regions is about six months, during the whole of which time the sun is constantly above the horizon. the general rule is, _that to the inhabitants of an oblique sphere the twilight is longer in proportion as the place is nearer the elevated pole, and the sun is farther from the equator on the side of the elevated pole_. the seasons. . _the seasons._--while the sun is north of the celestial equator, places north of the equator are receiving heat from the sun by day longer than they are losing it by radiation at night, while places south of the equator are losing heat by radiation at night longer than they are receiving it from the sun by day. when, therefore, the sun passes north of the equator, the temperature begins to rise at places north of the equator, and to fall at places south of it. the rise of temperature is most rapid north of the equator when the sun is at the summer solstice; but, for some time after this, the earth continues to receive more heat by day than it loses by night, and therefore the temperature continues to rise. for this reason, the heat is more excessive after the sun passes the summer solstice than before it reaches it. . _the duration of the seasons._--summer is counted as beginning in june, when the sun is at the summer solstice, and as continuing until the sun reaches the autumnal equinox, in september. autumn then begins, and continues until the sun is at the winter solstice, in december. winter follows, continuing until the sun comes to the vernal equinox, in march, when spring begins, and continues to the summer solstice. in popular reckoning the seasons begin with the first day of june, september, december, and march. the reason why winter is counted as occurring after the winter solstice is similar to the reason why the summer is placed after the summer solstice. the earth north of the equator is losing heat most rapidly at the time of the winter solstice; but for some time after this it loses more heat by night than it receives by day: hence for some time the temperature continues to fall, and the cold is more intense after the winter solstice than before it. [illustration: fig. .] of course, when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and the reverse. fig. shows the portion of the earth's orbit included in each season. it will be seen that the earth is at perihelion in the winter season for places north of the equator, and at aphelion in the summer season. this tends to mitigate somewhat the extreme temperatures of our winters and summers. [illustration: fig. .] . _the illumination of the earth at the different seasons._--fig. shows the earth as it would appear to an observer at the sun during each of the four seasons; that is to say, the portion of the earth that is receiving the sun's rays. figs. , , , and are enlarged views of the earth, as seen from the sun at the time of the summer solstice, of the autumnal equinox, of the winter solstice, and of the vernal equinox. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. is, so to speak, a side view of the earth, showing the limit of sunshine on the earth when the sun is at the summer solstice; and fig. , showing the limit of sunshine when the sun is at the autumnal equinox. [illustration: fig. .] . _cause of the change of seasons._--variety in the length of day and night, and diversity in the seasons, depend upon _the obliquity of the ecliptic_. were there no obliquity of the ecliptic, there would be no inequality in the length of day and night, and but slight diversity of seasons. the greater the obliquity of the ecliptic, the greater would be the variation in the length of the days and nights, and the more extreme the changes of the seasons. tides. . _tides._--the alternate rise and fall of the surface of the sea twice in the course of a lunar day, or of twenty-four hours and fifty-one minutes, is known as the _tides_. when the water is rising, it is said to be _flood_ tide; and when it is falling, _ebb_ tide. when the water is at its greatest height, it is said to be _high_ water; and when at its least height, _low_ water. . _cause of the tides._--it has been known to seafaring nations from a remote antiquity that there is a singular connection between the ebb and flow of the tides and the diurnal motion of the moon. [illustration: fig. .] this tidal movement in seeming obedience to the moon was a mystery until the study of the law of gravitation showed it to be due to _the attraction of the moon on the waters of the ocean_. the reason why there are two tides a day will appear from fig. . let _m_ be the moon, _e_ the earth, and _em_ the line joining their centres. now, strictly speaking, the moon does not revolve around the earth any more than the earth around the moon; but the centre of each body moves around the common centre of gravity of the two bodies. the earth being eighty times as heavy as the moon, this centre is situated within the former, about three-quarters of the way from its centre to its surface, at the point _g_. the body of the earth itself being solid, every part of it, in consequence of the moon's attraction, may be considered as describing a circle once in a month, with a radius equal to _eg_. the centrifugal force caused by this rotation is just balanced by the mean attraction of the moon upon the earth. if this attraction were the same on every part of the earth, there would be everywhere an exact balance between it and the centrifugal force. but as we pass from _e_ to _d_ the attraction of the moon diminishes, owing to the increased distance: hence at _d_ the centrifugal force predominates, and the water therefore tends to move away from the centre _e_. as we pass from _e_ towards _c_, the attraction of the moon increases, and therefore exceeds the centrifugal force: consequently at _c_ there is a tendency to draw the water towards the moon, but still away from the centre _e_. at _a_ and _b_ the attraction of the moon increases the gravity of the water, owing to the convergence of the lines _bm_ and _am_, along which it acts: hence the action of the moon tends to make the waters rise at _d_ and _c_, and to fall at _a_ and _b_, causing two tides to each apparent diurnal revolution of the moon. . _the lagging of the tides._--if the waters everywhere yielded immediately to the attractive force of the moon, it would always be high water when the moon was on the meridian, low water when she was rising or setting, and high water again when she was on the meridian below the horizon. but, owing to the inertia of the water, some time is necessary for so slight a force to set it in motion; and, once in motion, it continues so after the force has ceased, and until it has acted some time in the opposite direction. therefore, if the motion of the water were unimpeded, it would not be high water until some hours after the moon had passed the meridian. the free motion of the water is also impeded by the islands and continents. these deflect the tidal wave from its course in such a way that it may, in some cases, be many hours, or even a whole day, behind its time. sometimes two waves meet each other, and raise a very high tide. in some places the tides run up a long bay, where the motion of a large mass of water will cause an enormous tide to be raised. in the bay of fundy both of these causes are combined. a tidal wave coming up the atlantic coast meets the ocean wave from the east, and, entering the bay with their combined force, they raise the water at the head of it to the height of sixty or seventy feet. . _spring-tides and neap-tides._--the sun produces a tide as well as the moon; but the tide-producing force of the sun is only about four-tenths of that of the moon. at new and full moon the two bodies unite their forces, the ebb and flow become greater than the average, and we have the _spring-tides_. when the moon is in her first or third quarter, the two forces act against each other; the tide-producing force is the difference of the two; the ebb and flow are less than the average; and we have the _neap-tides_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. shows the tide that would be produced by the moon alone; fig. , the tide produced by the combined action of the sun and moon; and fig. , by the sun and moon acting at right angles to each other. the tide is affected by the distance of the moon from the earth, being highest near the time when the moon is in perigee, and lowest near the time when she is in apogee. when the moon is in perigee, at or near the time of a new or full moon, unusually high tides occur. . _diurnal inequality of tides._--the height of the tide at a given place is influenced by the declination of the moon. when the moon has no declination, the highest tides should occur along the equator, and the heights should diminish from thence toward the north and south; but the two daily tides at any place should have the same height. when the moon has north declination, as shown in fig. , the highest tides on the side of the earth next the moon will be at places having a corresponding north latitude, as at _b_, and on the opposite side at those which have an equal south latitude. of the two daily tides at any place, that which occurs when the moon is nearest the zenith should be the greatest: hence, when the moon's declination is north, the height of the tide at a place in north latitude should be greater when the moon is above the horizon than when she is below it. at the same time, places south of the equator have the highest tides when the moon is below the horizon, and the least when she is above it. this is called the _diurnal inequality_, because its cycle is one day; but it varies greatly in amount at different places. [illustration: fig. .] . _height of tides._--at small islands in mid-ocean the tides never rise to a great height, sometimes even less than one foot; and the average height of the tides for the islands of the atlantic and pacific oceans is only three feet and a half. upon approaching an extensive coast where the water is shallow, the height of the tide is increased; so that, while in mid-ocean the average height does not exceed three feet and a half, the average in the neighborhood of continents is not less than four or five feet. the day and time. . _the day._--by the term _day_ we sometimes denote the period of sunshine as contrasted with that of the absence of sunshine, which we call _night_, and sometimes the period of the earth's rotation on its axis. it is with the latter signification that the term is used in this section. as the earth rotates on its axis, it carries the meridian of a place with it; so that, during each complete rotation of the earth, the portion of the meridian which passes overhead from pole to pole sweeps past every star in the heavens from west to east. the _interval between two successive passages of this portion of the meridian across the same star_ is the exact period of the complete rotation of the earth. this period is called a _sidereal day_. the sidereal day may also be defined as _the interval between two successive passages of the same star across the meridian_; the passage of the meridian across the star, and the passage or _transit_ of the star across the meridian, being the same thing looked at from a different point of view. the interval _between two successive passages of the meridian across the sun_, or _of the sun across the meridian_, is called a _solar day_. . _length of the solar day._--the solar day is a little longer than the sidereal day. this is owing to the sun's eastward motion among the stars. we have already seen that the sun's apparent position among the stars is continually shifting towards the east at a rate which causes it to make a complete circuit of the heavens in a year, or three hundred and sixty-five days. this is at the rate of about one degree a day: hence, were the sun and a star on the meridian together to-day, when the meridian again came around to the star, the sun would appear about one degree to the eastward: hence the meridian must be carried about one degree farther in order to come up to the sun. the solar day must therefore be _about four minutes longer_ than the sidereal day. [illustration: fig. .] [illustration: fig. .] the fact that the earth must make more than a complete rotation is also evident from figs. and . in fig. , _ba_ represents the plane of the meridian, and the small arrows indicate the direction the earth is rotating on its axis, and revolving in its orbit. when the earth is at , the sun is on the meridian at _a_. when the earth has moved to , it has made a complete rotation, as is shown by the fact that the plane of the meridian is parallel with its position at ; but it is evident that the meridian has not yet come up with the sun. in fig. , _oa_ represents the plane of the meridian, and _os_ the direction of the sun. the small arrows indicate the direction of the rotation and revolution of the earth. in passing from the first position to the second the earth makes a complete rotation, but the meridian is not brought up to the sun. . _inequality in the length of solar days._--the sidereal days are all of the same length; but the solar days differ somewhat in length. this difference is due to the fact that the sun's apparent position moves eastward, or _away from the meridian_, at a variable rate. there are three reasons why this rate is variable:-- ( ) the sun's eastward motion is due to the revolution of the earth in its orbit. now, the earth's orbital motion is _not uniform_, being fastest when the earth is at perihelion, and slowest when the earth is at aphelion: hence, other things being equal, solar days will be longest when the earth is at perihelion, and shortest when the earth is at aphelion. [illustration: fig. .] [illustration: fig. .] ( ) the sun's eastward motion is along the ecliptic. now, from figs. and , it will be seen, that, when the sun is at one of the equinoxes, it will be moving away from the meridian _obliquely_; and, from figs. and , that, when the sun is at one of the solstices, it will be moving away from the meridian _perpendicularly_: hence, other things being equal, the sun would move away from the meridian _fastest_, and the days be _longest_, when the sun is at the _solstices_; while it would move away from the meridian _slowest_, and the days be _shortest_, when the sun is at the _equinoxes_. that a body moving along the ecliptic must be moving at a variable angle to the meridian becomes very evident on turning a celestial globe so as to bring each portion of the ecliptic under the meridian in turn. [illustration: fig. .] [illustration: fig. .] ( ) the sun, moving along the ecliptic, always moves _in a great circle_, while the point of the meridian which is to overtake the sun moves in a diurnal circle, which is _sometimes a great circle_ and _sometimes a small circle_. when the sun is at the equinoxes, the point of the meridian which is to overtake it moves in a great circle. as the sun passes from the equinoxes to the solstices, the point of the meridian which is to overtake it moves on a smaller and smaller circle: hence, as we pass away from the celestial equator, the points of the meridian move slower and slower. therefore, other things being equal, the meridian will gain upon the sun _most rapidly_, and the days be _shortest_, when the sun is at the _equinoxes_; while it will gain on the sun _least rapidly_, and the days will be _longest_, when the sun is at the _solstices_. the ordinary or _civil day_ is the mean of all the solar days in a year. . _sun time and clock time._--it is noon by the sun when the sun is on the meridian, and by the clock at the middle of the civil day. now, as the civil days are all of the same length, while solar days are of variable length, it seldom happens that the middles of these two days coincide, or that sun time and clock time agree. the difference between sun time and clock time, or what is often called _apparent solar time_ and _mean solar time_, is called the _equation of time_. the sun is said to be _slow_ when it crosses the meridian after noon by the clock, and _fast_ when it crosses the meridian before noon by the clock. sun time and clock time coincide four times a year; during two intermediate seasons the clock time is ahead, and during two it is behind. * * * * * the following are the dates of coincidence and of maximum deviation, which vary but slightly from year to year:-- february true sun fifteen minutes slow. april true sun correct. may true sun four minutes fast. june true sun correct. july true sun six minutes slow. august true sun correct. november true sun sixteen minutes fast. december true sun correct. one of the effects of the equation of time which is frequently misunderstood is, that the interval from sunrise until noon, as given in the almanacs, is not the same as that between noon and sunset. the forenoon could not be longer or shorter than the afternoon, if by "noon" we meant the passage of the sun across the meridian; but the noon of our clocks being sometimes fifteen minutes before or after noon by the sun, the former may be half an hour nearer to sunrise than to sunset, or _vice versa_. the year. . _the year._--the _year_ is the time it takes the earth to revolve around the sun, or, what amounts to the same thing, _the time it takes the sun to pass around the ecliptic_. ( ) the time it takes the sun to pass from a star around to the same star again is called a _sidereal year_. this is, of course, the exact time it takes the earth to make a complete revolution around the sun. [illustration: fig. .] ( ) the time it takes the sun to pass around from the vernal equinox, or the _first point of aries_, to the vernal equinox again, is called the _tropical_ year. this is a little shorter than the sidereal year, owing to the precession of the equinoxes. this will be evident from fig. . the circle represents the ecliptic, _s_ the sun, and _e_ the vernal equinox. the sun moves around the ecliptic _eastward_, as indicated by the long arrow, while the equinox moves slowly _westward_, as indicated by the short arrow. the sun will therefore meet the equinox before it has quite completed the circuit of the heavens. the exact lengths of these respective years are:-- sidereal year . = days hours min sec tropical year . = days hours min sec since the recurrence of the seasons depends on the tropical year, the latter is the one to be used in forming the calendar and for the purposes of civil life generally. its true length is eleven minutes and fourteen seconds less than three hundred and sixty-five days and a fourth. it will be seen that the tropical year is about twenty minutes shorter than the sidereal year. ( ) the time it takes the earth to pass from its perihelion point around to the perihelion point again is called the _anomalistic year_. this year is about four minutes longer than the sidereal year. this is owing to the fact that the major axis of the earth's orbit is slowly moving around to the east at the rate of about ten seconds a year. this causes the perihelion point _p_ (fig. ) to move _eastward_ at that rate, as indicated by the short arrow. the earth _e_ is also moving eastward, as indicated by the long arrow. hence the earth, on starting at the perihelion, has to make a little more than a complete circuit to reach the perihelion point again. [illustration: fig. .] . _the calendar._--the _solar year_, or the interval between two successive passages of the same equinox by the sun, is days, hours, minutes, seconds. if, then, we reckon only days to a common or _civil year_, the sun will come to the equinox hours, minutes, seconds, or nearly a quarter of a day, later each year; so that, if the sun entered aries on the th of march one year, he would enter it on the st four years after, on the d eight years after, and so on. thus in a comparatively short time the spring months would come in the winter, and the summer months in the spring. among different ancient nations different methods of computing the year were in use. some reckoned it by the revolution of the moon, some by that of the sun; but none, so far as we know, made proper allowances for deficiencies and excesses. twelve moons fell short of the true year, thirteen exceeded it; days were not enough, were too many. to prevent the confusion resulting from these errors, julius cæsar reformed the calendar by making the year consist of days, hours (which is hence called a _julian_ year), and made every fourth year consist of days. this method of reckoning is called _old style_. but as this made the year somewhat too long, and the error in amounted to ten days, pope gregory xiii., in order to bring the vernal equinox back to the st of march again, ordered ten days to be struck out of that year, calling the next day after the th of october the th; and, to prevent similar confusion in the future, he decreed that three leap-years should be omitted in the course of every four hundred years. this way of reckoning time is called _new style_. it was immediately adopted by most of the european nations, but was not accepted by the english until the year . the error then amounted to eleven days, which were taken from the month of september by calling the d of that month the th. the old style is still retained by russia. according to the gregorian calendar, _every year whose number is divisible by four_ is a _leap-year_, except, that, _in the case of the years whose numbers are exact hundreds, those only are leap-years which are divisible by four after cutting off the last two figures_. thus the years , , , etc., are leap-years; , , , , , etc., are not. the error will not amount to a day in over three thousand years. . _the dominical letter._--the _dominical letter_ for any year is that which we often see placed against sunday in the almanacs, and is always one of the first seven in the alphabet. since a common year consists of days, if this number is divided by seven (the number of days in a week), there will be a remainder of one: hence a year commonly begins one day later in the week than the preceding one did. if a year of days begins on sunday, the next will begin on monday; if it begins on thursday, the next will begin on friday; and so on. if sunday falls on the st of january, the _first_ letter of the alphabet, or _a_, is the _dominical letter_. if sunday falls on the th of january (as it will the next year, unless the first is leap-year), the _seventh_ letter, _g_, is the dominical letter. if sunday falls on the th of january (as it will the third year, unless the first or second is leap-year), the _sixth_ letter, _f_, will be the dominical letter. thus, if there were no leap-years, the dominical letters would regularly follow a retrograde order, _g_, _f_, _e_, _d_, _c_, _b_, _a_. but _leap_-years have days, which, divided by seven, leaves two remainder: hence the years following leap-years will begin two days later in the week than the leap-years did. to prevent the interruption which would hence occur in the order of the dominical letters, leap-years have _two_ dominical letters, one indicating sunday till the th of february, and the other for the rest of the year. by _table i._ below, the dominical letter for any year (new style) for four thousand years from the beginning of the christian era may be found; and it will be readily seen how the table could be extended indefinitely by continuing the centuries at the top in the same order. to find the dominical letter by this table, _look for the hundreds of years at the top, and for the years below a hundred, at the left hand_. thus the letter for will be opposite the number , and in the column having at the top; that is, it will be _a_. in the same way, the letters for , which is a leap-year, will be found to be _fe_. having the dominical letter of any year, _table ii._ shows what days of every month of the year will be _sundays_. to find the sundays of any month in the year by this table, _look in the column, under the dominical letter, opposite the name of the month given at the left_. from the sundays the date of any other day of the week can be readily found. thus, if we wish to know on what day of the week christmas falls in , we look opposite december, under the letter _f_ (which we have found to be the dominical letter for the year), and find that the d of the month is a sunday; the th, or christmas, will then be wednesday. in the same way we may find the day of the week corresponding to any date (new style) in history. for instance, the th of june, , the day of the fight at bunker hill, is found to have been a _saturday_. these two tables then serve as a _perpetual almanac_. table i. --- --- --- ---- c e g ba b d f g a c e f g b d e fe ag cb dc d f a b c e g a b d f g ag cb ed fe f a c d e g b c d f a b cb ed gf ag a c e f g b d e f a c d .. ed gf ba cb .. c e g a .. b d f g .. a c e f .. gf ba dc ed .. e g b c .. d f a b .. c e g a .. ba dc fe gf .. g b d e .. f a c d .. e g b c .. dc fe ag ba table ii. a b c d e f g jan. . oct. . .. .. .. .. feb. - . .. .. .. march . nov. . .. .. .. .. .. .. .. april . july .. .. .. .. .. .. .. aug. . .. .. .. .. .. .. .. sept. . dec. . .. .. .. .. .. .. .. may. . .. .. .. .. .. .. .. june . .. weight of the earth and precession. . _the weight of the earth._--there are several methods of ascertaining the weight and mass of the earth. the simplest, and perhaps the most trustworthy method is to compare the pull of the earth upon a ball of lead with that of a known mass of lead upon it. the pull of a known mass of lead upon the ball may be measured by means of a torsion balance. one form of the balance employed for this purpose is shown in figs. and . two small balls of lead, _b_ and _b_, are fastened to the ends of a light rod _e_, which is suspended from the point _f_ by means of the thread _fe_. two large balls of lead, _w_ and _w_, are placed on a turn-table, so that one of them shall be just in front of one of the small balls, and the other just behind the other small ball. the pull of the large balls turns the rod around a little so as to bring the small balls nearer the large ones. the small balls move towards the large ones till they are stopped by the torsion of the thread, which is then equal to the pull of the large balls. the deflection of the rod is carefully measured. the table is then turned into the position indicated by the dotted lines in fig. , so as to reverse the position of the large balls with reference to the small ones. the rod is now deflected in the opposite direction, and the amount of deflection is again carefully measured. the second measurement is made as a check upon the accuracy of the first. the force required to twist the thread as much as it was twisted by the deflection of the rod is ascertained by measurement. this gives the pull of the two large balls upon the two small ones. we next calculate what this pull would be were the balls as far apart as the small balls are from the centre of the earth. we can then form the following proportion: the pull of the large balls upon the small ones is to the pull of the earth upon the small ones as the mass of the large balls is to the mass of the earth, or as the weight of the large balls is to the weight of the earth. of course, the pull of the earth upon the small balls is the weight of the small balls. in this way it has been ascertained that the mass of the earth is about . times that of a globe of water of the same size. in other words, the _mean density_ of the earth is about . . [illustration: fig. .] [illustration: fig. .] the weight of the earth in pounds may be found by multiplying the number of cubic feet in it by - / (the weight, in pounds, of one cubic foot of water), and this product by . . [illustration: fig. .] . _cause of precession._--we have seen that the earth is flattened at the poles: in other words, the earth has the form of a sphere, with a protuberant ring around its equator. this equatorial ring is inclined to the plane of the ecliptic at an angle of about - / °. in fig. this ring is represented as detached from the enclosed sphere. _s_ represents the sun, and _sc_ the ecliptic. as the point _a_ of the ring is nearer the sun than the point _b_ is, the sun's pull upon _a_ is greater than upon _b_: hence the sun tends to pull the ring over into the plane of the ecliptic; but the rotation of the earth tends to keep the ring in the same plane. the struggle between these two tendencies causes the earth, to which the ring is attached, to wabble like a spinning-top, whose rotation tends to keep it erect, while gravity tends to pull it over. the handle of the top has a gyratory motion, which causes it to describe a curve. the axis of the heavens corresponds to the handle of the top. ii. the moon. distance, size, and motions. . _the distance of the moon._--the moon is the nearest of the heavenly bodies. its distance from the centre of the earth is only about sixty times the radius of the earth, or, in round numbers, two hundred and forty thousand miles. the ordinary method of finding the distance of one of the nearer heavenly bodies is first to ascertain its horizontal parallax. this enables us to form a right-angled triangle, the lengths of whose sides are easily computed, and the length of whose hypothenuse is the distance of the body from the centre of the earth. [illustration: fig. .] horizontal parallax has already been defined ( ) as the displacement of a heavenly body when on the horizon, caused by its being seen from the surface, instead of the centre, of the earth. this displacement is due to the fact that the body is seen in a different direction from the surface of the earth from that in which it would be seen from the centre. horizontal parallax might be defined as the difference in the directions in which a body on the horizon would be seen from the surface and from the centre of the earth. thus, in fig. , _c_ is the centre of the earth, _a_ a point on the surface, and _b_ a body on the horizon of _a_. _ab_ is the direction in which the body would be seen from _a_, and _cb_ the direction in which it would be seen from _c_. the difference of these directions, or the angle _abc_, is the parallax of the body. the triangle _bac_ is right-angled at _a_; the side _ac_ is the radius of the earth, and the hypothenuse is the distance of the body from the centre of the earth. when the parallax _abc_ is known, the length of _cb_ can easily by found by trigonometrical computation. we have seen ( ) that the parallax of a heavenly body grows less and less as the body passes from the horizon towards the zenith. the parallax of a body and its altitude are, however, so related, that, when we know the parallax at any altitude, we can readily compute the horizontal parallax. the usual method of finding the parallax of one of the nearer heavenly bodies is first to find its parallax when on the meridian, as seen from two places on the earth which differ considerably in latitude: then to calculate what would be the parallax of the body as seen from one of these places and the centre of the earth: and then finally to calculate what would be the parallax were the body on the horizon. [illustration: fig. .] thus, we should ascertain the parallax of the body _b_ (fig. ) as seen from _a_ and _d_, or the angle _abd_. we should then calculate its parallax as seen from _a_ and _c_, or the angle _abc_. finally we should calculate what its parallax would be were the body on the horizon, or the angle _ab'c_. the simplest method of finding the parallax of a body _b_ (fig. ) as seen from the two points _a_ and _d_ is to compare its direction at each point with that of the same fixed star near the body. the star is so distant, that it will be seen in the same direction from both points: hence, if the direction of the body differs from that of the star ° as seen from one point, and ° ' as seen from the other point, the two lines _ab_ and _db_ must differ in direction by '; in other words, the angle _abd_ would be '. the method just described is the usual method of finding the parallax of the moon. . _the apparent size of the moon._--the _apparent size_ of a body is the visual angle subtended by it; that is, the angle formed by two lines drawn from the eye to two opposite points on the outline of the body. the apparent size of a body depends upon both its _magnitude_ and its _distance_. the apparent size, or _angular diameter_, of the moon is about thirty-one minutes. this is ascertained by means of the wire micrometer already described ( ). the instrument is adjusted so that its longitudinal wire shall pass through the centre of the moon, and its transverse wires shall be tangent to the limbs of the moon on each side, at the point where they are cut by the longitudinal wire. the micrometer screw is then turned till the wires are brought together. the number of turns of the screw needed to accomplish this will indicate the arc between the wires, or the angular diameter of the moon. [illustration: fig. .] in order to be certain that the longitudinal wire shall pass through the centre of the moon, it is best to take the moon when its disc is in the form of a crescent, and to place the longitudinal wire against the points, or _cusps_, of the crescent, as shown in fig. . [illustration: fig. .] . _the real size of the moon._--the real diameter of the moon is a little over one-fourth of that of the earth, or a little more than two thousand miles. the comparative sizes of the earth and moon are shown in fig. . [illustration: fig. .] the distance and apparent size of the moon being known, her real diameter is found by means of a triangle formed as shown in fig. . _c_ represents the centre of the moon, _cb_ the distance of the moon from the earth, and _ca_ the radius of the moon. _bac_ is a triangle, right-angled at _a_. the angle _abc_ is half the apparent diameter of the moon. with the angles _a_ and _b_, and the side _cb_ known, it is easy to find the length of _ac_ by trigonometrical computation. twice _ac_ will be the diameter of the moon. the volume of the moon is about one-fiftieth of that of the earth. . _apparent size of the moon on the horizon and in the zenith._.--the moon is nearly four thousand miles farther from the observer when she is on the horizon than when she is in the zenith. this is evident from fig. . _c_ is the centre of the earth, _m_ the moon on the horizon, _m'_ the moon in the zenith, and _o_ the point of observation. _om_ is the distance of the moon when she is on the horizon, and _om'_ the distance of the moon from the observer when she is in the zenith. _cm_ is equal to _cm'_, and _om_ is about the length of _cm_; but _om'_ is about four thousand miles shorter than _cm'_: hence _om'_ is about four thousand miles shorter than _om_. [illustration: fig. .] notwithstanding the moon is much nearer when at the zenith than at the horizon, it seems to us much larger at the horizon. this is a pure illusion, as we become convinced when we measure the disc with accurate instruments, so as to make the result independent of our ordinary way of judging. when the moon is near the horizon, it seems placed beyond all the objects on the surface of the earth in that direction, and therefore farther off than at the zenith, where no intervening objects enable us to judge of its distance. in any case, an object which keeps the same apparent magnitude seems to us, through the instinctive habits of the eye, the larger in proportion as we judge it to be more distant. . _the apparent size of the moon increased by irradiation._--in the case of the moon, the word _apparent_ means much more than it does in the case of other celestial bodies. indeed, its brightness causes our eyes to play us false. as is well known, the crescent of the new moon seems part of a much larger sphere than that which it has been said, time out of mind, to "hold in its arms." the bright portion of the moon as seen with our measuring instruments, as well as when seen with the naked eye, covers a larger space in the field of the telescope than it would if it were not so bright. this effect of _irradiation_, as it is called, must be allowed for in exact measurements of the diameter of the moon. [illustration: fig. .] . _apparent size of the moon in different parts of her orbit._--owing to the eccentricity of the moon's orbit, her distance from the earth varies somewhat from time to time. this variation causes a corresponding variation in her apparent size, which is illustrated in fig. . . _the mass of the moon._--the moon is considerably less dense than the earth, its mass being only about one-eightieth of that of the earth; that is, while it would take only about fifty moons to make the bulk of the earth, it would take about eighty to make the mass of the earth. one method of finding the mass of the moon is to compare her effect in producing the _tides_ with that of the sun. we first calculate what would be the moon's effect in producing the tides, were she as far off as the sun. we then form the following proportion: as the sun's effect in producing the tides is to the moon's effect at the same distance, so is the mass of the sun to the mass of the moon. the method of finding the mass of the sun will be given farther on. . _the orbital motion of the moon._--if we watch the moon from night to night, we see that she moves eastward quite rapidly among the stars. when the new moon is first visible, it appears near the horizon in the west, just after sunset. a week later the moon will be on the meridian at the same hour, and about a week later still on the eastern horizon. the moon completes the circuit of the heavens in a period of about thirty days, moving eastward at the rate of about twelve degrees a day. this eastward motion of the moon is due to the fact that she is revolving around the earth from west to east. [illustration: fig. .] . _the aspects of the moon._--as the moon revolves around the earth, she comes into different positions with reference to the earth and sun. these different positions of the moon are called the _aspects_ of the moon. the four chief aspects of the moon are shown in fig. . when the moon is at _m_, she appears in the opposite part of the heavens to the sun, and is said to be in _opposition_; when at _m'_ and at _m'''_, she appears ninety degrees away from the sun, and is said to be in _quadrature_; when at _m''_, she appears in the same part of the heavens as the sun, and is said to be in _conjunction_. . _the sidereal and synodical periods of the moon._--the _sidereal period_ of the moon is the time it takes her to pass around from a star to that star again, or the time it takes her to _make a complete revolution around the earth_. this is a period of about twenty-seven days and a third. it is sometimes called the _sidereal month_. the _synodical period_ of the moon is the time that it takes the moon to _pass from one aspect around to the same aspect again_. this is a period of about twenty-nine days and a half, and it is sometimes called the _synodical month_. [illustration: fig. .] the reason why the synodical period is longer than the sidereal period will appear from fig. . _s_ represents the position of the sun, _e_ that of the earth, and the small circle the orbit of the moon around the earth. the arrow in the small circle represents the direction the moon is revolving around the earth, and the arrow in the arc between _e_ and _e'_ indicates the direction of the earth's motion in its orbit. when the moon is at _m_{ }_, she is in conjunction. as the moon revolves around the earth, the earth moves forward in its orbit. when the moon has come round to _m_{ }_, so that _m_{ }m_{ }_ is parallel with _m_{ }m_{ }_, she will have made a complete or _sidereal_ revolution around the earth; but she will not be in conjunction again till she has come round to _m_, so as again to be between the earth and sun. that is to say, the moon must make more than a complete revolution in a synodical period. [illustration: fig. .] the greater length of the synodical period is also evident from fig. . _t_ represents the earth, and _l_ the moon. the arrows indicate the direction in which each is moving. when the earth is at _t_, and the moon at _l_, the latter is in conjunction. when the earth has reached _t'_, and the moon _l'_, the latter has made a sidereal revolution; but she will not be in conjunction again till the earth has reached _t''_, and the moon _l''_. . _the phases of the moon._--when the new moon appears in the west, it has the form of a _crescent_, with its convex side towards the sun, and its horns towards the east. as the moon advances towards quadrature, the crescent grows thicker and thicker, till it becomes a _half-circle_ at first quarter. when it passes quadrature, it begins to become convex also on the side away from the sun, or _gibbous_ in form. as it approaches opposition, it becomes more and more nearly circular, until at opposition it is a _full_ circle. from full moon to last quarter it is again gibbous, and at last quarter a half-circle. from last quarter to new moon it is again crescent; but the horns of the crescent are now turned towards the west. the successive phases of the moon are shown in fig. . [illustration: fig. .] . _cause of the phases of the moon._--take a globe, half of which is colored white and the other half black in such a way that the line which separates the white and black portions shall be a great circle which passes through the poles of the globe, and rotate the globe slowly, so as to bring the white half gradually into view. when the white part first comes into view, the line of separation between it and the black part, which we may call the _terminator_, appears concave, and its projection on a plane perpendicular to the line of vision is a concave line. as more and more of the white portion comes into view, the projection of the terminator becomes less and less concave. when half of the white portion comes into view, the terminator is projected as a straight line. when more than half of the white portion comes into view, the terminator begins to appear as a convex line, and this line becomes more and more convex till the whole of the white half comes into view, when the terminator becomes circular. [illustration: fig. .] the moon is of itself a dark, opaque globe; but the half that is towards the sun is always bright, as shown in fig. . this bright half of the moon corresponds to the white half of the globe in the preceding illustration. as the moon revolves around the earth, different portions of this illumined half are turned towards the earth. at new moon, when the moon is in conjunction, the bright half is turned entirely away from the earth, and the disc of the moon is black and invisible. between new moon and first quarter, less than half of the illumined side is turned towards the earth, and we see this illumined portion projected as a crescent. at first quarter, just half of the illumined side is turned towards the earth, and we see this half projected as a half-circle. between first quarter and full, more than half of the illumined side is turned towards the earth, and we see it as gibbous. at full, the whole of the illumined side is turned towards us, and we see it as a full circle. from full to new moon again, the phases occur in the reverse order. . _the form of the moon's orbit._--the orbit of the moon around the earth is an ellipse of slight eccentricity. the form of this ellipse is shown in fig. . _c_ is the centre of the ellipse, and _e_ the position of the earth at one of its foci. the eccentricity of the ellipse is only about one-eighteenth. it is impossible for the eye to distinguish such an ellipse from a circle. [illustration: fig. .] . _the inclination of the moon's orbit._--the plane of the moon's orbit is inclined to the ecliptic by an angle of about five degrees. the two points where the moon's orbit cuts the ecliptic are called her _nodes_. the moon's nodes have a westward motion corresponding to that of the equinoxes, but much more rapid. they complete the circuit of the ecliptic in about nineteen years. the moon's latitude ranges from ° north to ° south; and since, owing to the motion of her nodes, the moon is, during a period of nineteen years, ° north and ° south of every part of the ecliptic, her declination will range from - / ° + ° = - / ° north to - / ° + ° = - / ° south. . _the meridian altitude of the moon._--the _meridian altitude_ of any body is its altitude when on the meridian. in our latitude, the meridian altitude of any point on the equinoctial is forty-nine degrees. the meridian altitude of the summer solstice is ° + - / ° = - / °, and that of the winter solstice is ° - - / ° = - / °. the greatest meridian altitude of the moon is - / ° + ° = - / °, and its least meridian altitude, - / ° - ° = - / °. when the moon's meridian altitude is greater than the elevation of the equinoctial, it is said to run _high_, and when less, to run _low_. the full moon runs high when the sun is south of the equinoctial, and low when the sun is north of the equinoctial. this is because the full moon is always in the opposite part of the heavens to the sun. . _wet and dry moon._--at the time of new moon, the cusps of the crescent sometimes lie in a line which is nearly perpendicular with the horizon, and sometimes in a line which is nearly parallel with the horizon. in the former case the moon is popularly described as a _wet_ moon, and in the latter case as a _dry_ moon. [illustration: fig. .] the great circle which passes through the centre of the sun and moon will pass through the centre of the crescent, and be perpendicular to the line joining the cusps. now the ecliptic makes the least angle with the horizon when the vernal equinox is on the eastern horizon and the autumnal equinox is on the western. in our latitude, as we have seen, this angle is - / °: hence in our latitude, if the moon were at new on the ecliptic when the sun is at the autumnal equinox, as shown at _m_{ }_ (fig. ), the great circle passing through the centre of the sun and moon would be the ecliptic, and at new york would be inclined to the horizon at an angle of - / °. if the moon happened to be ° south of the ecliptic at this time, as at _m_{ }_, the great circle passing through the centre of the sun and moon would make an angle of only - / ° with the horizon. in either of these cases the line joining the cusps would be nearly perpendicular to the horizon. [illustration: fig. .] if the moon were at new on the ecliptic when the sun is near the vernal equinox, as shown at _m_{ }_ (fig. ), the great circle passing through the centres of the sun and moon would make an angle of - / ° with the horizon at new york; and were the moon ° north of the ecliptic at that time, as shown at _m_{ }_, this great circle would make an angle of - / ° with the horizon. in either of these cases, the line joining the cusps would be nearly parallel with the horizon. at different times, the line joining the cusps may have every possible inclination to the horizon between the extreme cases shown in figs. and . . _daily retardation of the moon's rising._--the moon rises, on the average, about fifty minutes later each day. this is owing to her eastward motion. as the moon makes a complete revolution around the earth in about twenty-seven days, she moves eastward at the rate of about thirteen degrees a day, or about twelve degrees a day faster than the sun. were the moon, therefore, on the horizon at any hour to-day, she would be some twelve degrees below the horizon at the same hour to-morrow. now, as the horizon moves at the rate of one degree in four minutes, it would take it some fifty minutes to come up to the moon so as to bring her upon the horizon. hence the daily retardation of the moon's rising is about fifty minutes; but it varies considerably in different parts of her orbit. there are two reasons for this variation in the daily retardation:-- ( ) the moon moves at a _varying rate in her orbit_; her speed being greatest at perigee, and least at apogee: hence, other things being equal, the retardation is greatest when the moon is at perigee, and least when she is at apogee. [illustration: fig. .] [illustration: fig. .] ( ) the moon moves at a _varying angle to the horizon_. the moon moves nearly in the plane of the ecliptic, and of course she passes both equinoxes every lunation. when she is near the autumnal equinox, her path makes the greatest angle with the eastern horizon, and when she is near the vernal equinox, the least angle: hence the moon moves away from the horizon fastest when she is near the autumnal equinox, and slowest when she is near the vernal equinox. this will be evident from figs. and . in each figure, _sn_ represents a portion of the eastern horizon, and _ec_, _e'c'_, a portion of the ecliptic. _ae_, in fig. , represents the autumnal equinox, and _aem_ the daily motion of the moon. _ve_, in fig. , represents the vernal equinox, and _vem'_ the motion of the moon for one day. in the first case this motion would carry the moon away from the horizon the distance _am_, and in the second case the distance _a'm'_. now, it is evident that _am_ is greater than _a'm'_: hence, other things being equal, the greatest retardation of the moon's rising will be when the moon is near the autumnal equinox, and the least retardation when the moon is near the vernal equinox. the least retardation at new york is twenty-three minutes, and the greatest an hour and seventeen minutes. the greatest and least retardations vary somewhat from month to month; since they depend not only upon the position of the moon in her orbit with reference to the equinoxes, but also upon the latitude of the moon, and upon her nearness to the earth. [illustration: fig. .] the direction of the moon's motion with reference to the ecliptic is shown in fig. , which shows the moon's motion for one day in july, . . _the harvest moon_--the long and short retardations in the rising of the moon, though they occur every month, are not likely to attract attention unless they occur at the time of full moon. the long retardations for full moon occur when the moon is near the autumnal equinox at full. as the full moon is always opposite to the sun, the sun must in this case be near the vernal equinox: hence the long retardations for full moon occur in the spring, the greatest retardation being in march. the least retardations for full moon occur when the moon is near the vernal equinox at full: the sun must then be near the autumnal equinox. hence the least retardations for full moon occur in the months of august, september, and october. the retardation is, of course, least for september; and the full moon of this month rises night after night less than half an hour later than the previous night. the full moon of september is called the "harvest moon," and that of october the "hunter's moon." . _the rotation of the moon._--a careful examination of the spots on the disc of the moon reveals the fact that she always presents the same side to the earth. in order to do this, she must rotate on her axis while making a revolution around the earth, or in about twenty-seven days. . _librations of the moon._--the moon appears to rock slowly to and fro, so as to allow us to see alternately a little farther around to the right and the left, or above and below, than we otherwise could. this apparent rocking of the moon is called _libration_. the moon has three librations:-- ( ) _libration in latitude._--this libration enables us to see alternately a little way around on the northern and southern limbs of the moon. this libration is due to the fact that the axis of the moon is not quite perpendicular to the plane of her orbit. the deviation from the perpendicular is six degrees and a half. as the axis of the moon, like that of the earth, maintains the same direction, the poles of the moon will be turned alternately six degrees and a half toward and from the earth. ( ) _libration in longitude._--this libration enables us to see alternately a little farther around on the eastern and western limbs of the moon. [illustration: fig. .] it is due to the fact that the moon's axial motion is uniform, while her orbital motion is not. at perigee her orbital motion will be in advance of her axial motion, while at apogee the axial motion will be in advance of the orbital. in fig. , _e_ represents the earth, _m_ the moon, the large arrow the direction of the moon's motion in her orbit, and the small arrow the direction of her motion of rotation. when the moon is at _m_, the line _ab_, drawn perpendicular to _em_, represents the circle which divides the visible from the invisible portion of the moon. while the moon is passing from _m_ to _m'_, the moon performs less than a quarter of a rotation, so that _ab_ is no longer perpendicular to _em'_. an observer on the earth can now see somewhat beyond _a_ on the western limb of the moon, and not quite up to _b_ on the eastern limb. while the moon is passing from _m'_ to _m''_, her axial motion again overtakes her orbital motion, so that the line _ab_ again becomes perpendicular to the line joining the centre of the moon to the centre of the earth. exactly the same side is now turned towards the earth as when the moon was at _m_. while the moon passes from _m''_ to _m'''_, her axial motion gets in advance of her orbital motion, so that _ab_ is again inclined to the line joining the centres of the earth and moon. a portion of the eastern limb of the moon beyond _b_ is now brought into view to the earth, and a portion of the western limb at _a_ is carried out of view. while the moon is passing from _m'''_ to _m_, the orbital motion again overtakes the axial motion, and _ab_ is again perpendicular to _me_. ( ) _parallactic libration._--while an observer at the centre of the earth would get the same view of the moon, whether she were on the eastern horizon, in the zenith, or on the western horizon, an observer on the surface of the earth does not get exactly the same view in these three cases. when the moon is on the eastern horizon, an observer on the surface of the earth would see a little farther around on the western limb of the moon than when she is in the zenith, and not quite so far around on the eastern limb. on the contrary, when the moon is on the western horizon, an observer on the surface of the earth sees a little farther around on the eastern limb of the moon than when she is in the zenith, and not quite so far around on her western limb. [illustration: fig. .] this will be evident from fig. . _e_ is the centre of the earth, and _o_ a point on its surface. _ab_ is a line drawn through the centre of the moon, perpendicular to a line joining the centres of the moon and the earth. this line marks off the part of the moon turned towards the centre of the earth, and remains essentially the same during the day. _cd_ is a line drawn through the centre of the moon perpendicular to a line joining the centre of the moon and the point of observation. this line marks off the part of the moon turned towards _o_. when the moon is in the zenith, _cd_ coincides with _ab_; but, when the moon is on the horizon, _cd_ is inclined to _ab_. when the moon is on the eastern horizon, an observer at _o_ sees a little beyond _b_, and not quite to _a_; and, when she is on the western horizon, he sees a little beyond _a_, and not quite to _b_. _b_ is on the western limb of the moon, and _a_ on her eastern limb. since this libration is due to the point from which the moon is viewed, it is called _parallactic_ libration; and, since it occurs daily, it is called _diurnal_ libration. [illustration: fig. .] . _portion of the lunar surface brought into view by libration._--the area brought into view by the first two librations is between one-twelfth and one-thirteenth of the whole lunar surface, or nearly one-sixth of the hemisphere of the moon which is turned away from the earth when the moon is at her state of mean libration. of course a precisely equal portion of the hemisphere turned towards us during mean libration is carried out of view by the lunar librations. if we add to each of these areas a fringe about one degree wide, due to the diurnal libration, and which we may call the _parallactic_ fringe, we shall find that the total area brought into view is almost exactly one-eleventh part of the whole surface of the moon. a similar area is carried out of view; so that the whole region thus swayed out of and into view amounts to two-elevenths of the moon's surface. this area is shown in fig. , which is a side view of the moon. [illustration: fig. .] . _the moon's path through space._--were the earth stationary, the moon would describe an ellipse around it similar to that of fig. ; but, as the earth moves forward in her orbit at the same time that the moon revolves around it, the moon is made to describe a sinuous path, as shown by the continuous line in fig. . this feature of the moon's path is greatly exaggerated in the upper portion of the diagram. the form of her path is given with a greater degree of accuracy in the lower part of the figure (the broken line represents the path of the earth); but even here there is considerable exaggeration. the complete serpentine path of the moon around the sun is shown, greatly exaggerated, in fig. , the broken line being the path of the earth. [illustration: fig. .] the path described by the moon through space is much the same as that described by a point on the circumference of a wheel which is rolled over another wheel. if we place a circular disk against the wall, and carefully roll along its edge another circular disk (to which a piece of lead pencil has been fastened so as to mark upon the wall), the curve described will somewhat resemble that described by the moon. this curve is called an _epicycloid_, and it will be seen that at every point it is concave towards the centre of the larger disk. in the same way the moon's orbit is _at every point concave towards the sun_. [illustration: fig. .] the exaggeration of the sinuosity in fig. will be more evident when it is stated, that, on the scale of fig. , the whole of the serpentine curve would lie _within the breadth_ of the fine circular line _mm'_. . _the lunar day._--the lunar day is twenty-nine times and a half as long as the terrestrial day. near the moon's equator the sun shines without intermission nearly fifteen of our days, and is absent for the same length of time. consequently, the vicissitudes of temperature to which the surface is exposed must be very great. during the long lunar night the temperature of a body on the moon's surface would probably fall lower than is ever known on the earth, while during the day it must rise higher than anywhere on our planet. [illustration: fig. .] it might seem, that, since the moon rotates on her axis in about twenty-seven days, the lunar day ought to be twenty-seven days long, instead of twenty-nine. there is, however, a solar, as well as a sidereal, day at the moon, as on the earth; and the solar day at the moon is longer than the sidereal day, for the same reason as on the earth. during the solar day the moon must make both a _synodical rotation_ and a _synodical revolution_. this will be evident from fig. , in which is shown the path of the moon during one complete lunation. _e_, _e'_, _e''_, etc., are the successive positions of the earth; and , , , , , the successive positions of the moon. the small arrows indicate the direction of the moon's rotation. the moon is full at and . at , _a_, at the centre of the moon's disk, will have the sun, which lies in the direction _as_, upon the meridian. before _a_ will again have the sun on the meridian, the moon must have made a synodical revolution; and, as will be seen by the dotted lines, she must have made more than a complete rotation. the rotation which brings the point _a_ into the same relation to the earth and sun is called a _synodical_ rotation. it will also be evident from this diagram that the moon must make a synodical rotation during a synodical revolution, in order always to present the same side to the earth. . _the earth as seen from the moon._--to an observer on the moon, the earth would be an immense moon, going through the same phases that the moon does to us; but, instead of rising and setting, it would only oscillate to and fro through a few degrees. on the other side of the moon it would never be seen at all. the peculiarities of the moon's motions which cause the librations, and make a spot on the moon's disk seem to an observer on the earth to oscillate to and fro, would cause the earth as a whole to appear to a lunar observer to oscillate to and fro in the heavens in a similar manner. it is a well-known fact, that, at the time of new moon, the dark part of the moon's surface is partially illumined, so that it becomes visible to the naked eye. this must be due to the light reflected to the moon from the earth. since at new moon the moon is between the earth and sun, it follows, that, when it is new moon at the earth, it must be _full earth_ at the moon: hence, while the bright crescent is enjoying full sunlight, the dark part of its surface is enjoying the light of the full _earth_. fig. represents the full earth as seen from the moon. [illustration: fig. .] the atmosphere of the moon. . _the moon has no appreciable atmosphere._--there are several reasons for believing that the moon has little or no atmosphere. ( ) had the moon an atmosphere, it would be indicated at the time of a solar eclipse, when the moon passes over the disk of the sun. if the atmosphere were of any considerable density, it would absorb a part of the sun's rays, so as to produce a dusky border in front of the moon's disk, as shown in fig. . in reality no such dusky border is ever seen; but the limb of the moon appears sharp, and clearly defined, as in fig. . [illustration: fig. .] [illustration: fig. .] if the atmosphere were not dense enough to produce this dusky border, its refraction would be sufficient to distort the delicate cusps of the sun's crescent in the manner shown at the top of fig. ; but no such distortion is ever observed. the cusps always appear clear and sharp, as shown at the bottom of the figure: hence it would seem that there can be no atmosphere of appreciable density at the moon. ( ) the absence of an atmosphere from the moon is also shown by the absence of twilight and of diffused daylight. upon the earth, twilight continues until the sun is eighteen degrees below the horizon; that is, day and night are separated by a belt twelve hundred miles in breadth, in which the transition from light to darkness is gradual. we have seen ( ) that this twilight results from the refraction and reflection of light by our atmosphere; and, if the moon had an atmosphere, we should notice a similar gradual transition from the bright to the dark portions of her surface. such, however, is not the case. the boundary between the light and darkness, though irregular, is sharply defined. close to this boundary the unillumined portion of the moon appears just as dark as at any distance from it. the shadows on the moon are also pitchy black, without a trace of diffused daylight. [illustration: fig. .] ( ) the absence of an atmosphere is also proved by the absence of refraction when the moon passes between us and the stars. let _ab_ (fig. ) represent the disk of the moon, and _cd_ an atmosphere supposed to surround it. let _sae_ represent a straight line from the earth, touching the moon at _a_, and let _s_ be a star situated in the direction of this line. if the moon had no atmosphere, this star would appear to touch the edge of the moon at _a_; but, if the moon had an atmosphere, a star behind the edge of the moon, at _s'_, would be visible at the earth; for the ray _s'a_ would be bent by the atmosphere into the direction _ae'_. so, also, on the opposite side of the moon, a star might be seen at the earth, although really behind the edge of the moon: hence, if the moon had an atmosphere, the time during which a star would be concealed by the moon would be less than if it had no atmosphere, and the amount of this effect must be proportional to the density of the atmosphere. the moon, in her orbital course across the heavens, is continually passing before, or _occulting_, some of the stars that so thickly stud her apparent path; and when we see a star thus pass behind the lunar disk on one side, and come out again on the other side, we are virtually observing the setting and rising of that star upon the moon. the moon's apparent diameter has been measured over and over again, and is known with great accuracy; the rate of her motion across the sky is also known with perfect accuracy: hence it is easy to calculate how long the moon will take to travel across a part of the sky exactly equal in length to her own diameter. supposing, then, that we observe a star pass behind the moon, and out again, it is clear, that, if there is no atmosphere, the interval of time during which it remains occulted ought to be exactly equal to the computed time which the moon would take to pass over the star. if, however, from the existence of a lunar atmosphere, the star disappears too late, and re-appears too soon, as we have seen it would, these two intervals will not agree; the computed time will be greater than the observed time, and the difference will represent the amount of refraction the star's light has sustained or suffered, and hence the extent of atmosphere it has had to pass through. comparisons of these two intervals of time have been repeatedly made, the most extensive being executed under the direction of the astronomer royal of england, several years ago, and based upon no less than two hundred and ninety-six occultation observations. in this determination the measured or telescopic diameter of the moon was compared with the diameter deduced from the occultations; and it was found that the telescopic diameter was greater than the occultation diameter by two seconds of angular measurement, or by about a thousandth part of the whole diameter of the moon. this discrepancy is probably due, in part at least, to _irradiation_ ( ), which augments the apparent size of the moon, as seen in the telescope as well as with the naked eye; but, if the whole two seconds were caused by atmospheric refraction, this would imply a horizontal refraction of one second, which is only one two-thousandth of the earth's horizontal refraction. it is possible that an atmosphere competent to produce this refraction would not make itself visible in any other way. but an atmosphere two thousand times rarer than our air can scarcely be regarded as an atmosphere at all. the contents of an air-pump receiver can seldom be rarefied to a greater extent than to about a thousandth of the density of air at the earth's surface; and the lunar atmosphere, if it exists at all, is thus proved to be twice as attenuated as what we commonly call a vacuum. the surface of the moon. [illustration: fig. .] . _dusky patches on the disk of the moon._--with the naked eye, large dusky patches are seen on the moon, in which popular fancy has detected a resemblance to a human face. with a telescope of low power, these dark patches appear as smooth as water, and they were once supposed to be seas. this theory was the origin of the name _mare_ (latin for _sea_), which is still applied to the larger of these plains; but, if there were water on the surface of the moon, it could not fail to manifest its presence by its vapor, which would form an appreciable atmosphere. moreover, with a high telescopic power, these plains present a more or less uneven surface; and, as the elevations and depressions are found to be permanent, they cannot, of course, belong to the surface of water. the chief of these plains are shown in fig. . they are _mare crisium_, _mare foecunditatis_, _mare nectaris_, _mare tranquillitatis_, _mare serenitatis_, _mare imbrium_, _mare frigoris_, and _oceanus procellarum_. all these plains can easily be recognized on the surface of the full moon with the unaided eye. . _the terminator of the moon._--the terminator of the moon is the line which separates the bright and dark portions of its disk. when viewed with a telescope of even moderate power, the terminator is seen to be very irregular and uneven. many bright points are seen just outside of the terminator in the dark portion of the disk, while all along in the neighborhood of the terminator are bright patches and dense shadows. these appearances are shown in figs. and , which represent the moon near the first and last quarters. they indicate that the surface of the moon is very rough and uneven. [illustration: fig. .] [illustration: fig. .] as it is always either sunrise or sunset along the terminator, the bright spots outside of it are clearly the tops of mountains, which catch the rays of the sun while their bases are in the shade. the bright patches in the neighborhood of the terminator are the sides of hills and mountains which are receiving the full light of the sun, while the dense shadows near by are cast by these elevations. . _height of the lunar mountains._--there are two methods of finding the height of lunar mountains:-- ( ) we may measure the length of the shadows, and then calculate the height of the mountains that would cast such shadows with the sun at the required height above the horizon. the length of a shadow may be obtained by the following method: the longitudinal wire of the micrometer ( ) is adjusted so as to pass through the shadow whose length is to be measured, and the transverse wires are placed one at each end of the shadow, as shown in fig. . the micrometer screw is then turned till the wires are brought together, so as to ascertain the length of the arc between them. we may then form the proportion: the number of seconds in the semi-diameter of the moon is to the number of seconds in the length of the shadow, as the length of the moon's radius in miles to the length of the shadow in miles. [illustration: fig. .] the height of the sun above the horizon is ascertained by measuring the angular distance of the mountain from the terminator. ( ) we may measure the distance of a bright point from the terminator, and then construct a right-angled triangle, as shown in fig. . a solution of this triangle will enable us to ascertain the height of the mountain whose top is just catching the level rays of the sun. [illustration: fig. .] _b_ is the centre of the moon, _m_ the top of the mountain, and _sam_ a ray of sunlight which just grazes the terminator at _a_, and then strikes the top of the mountain at _m_. the triangle _bam_ is right-angled at _a_. _ba_ is the radius of the moon, and _am_ is known by measurement; _bm_, the hypothenuse, may then be found by computation. _bm_ is evidently equal to the radius of the moon _plus_ the height of the mountain. by one or the other of these methods, the heights of the lunar mountains have been found with a great degree of accuracy. it is claimed that the heights of the lunar mountains are more accurately known than those of the mountains on the earth. compared with the size of the moon, lunar mountains attain a greater height than those on the earth. . _general aspect of the lunar surface._--a cursory examination of the moon with a low power is sufficient to show the prevalence of crater-like inequalities and the general tendency to _circular_ shape which is apparent in nearly all the surface markings; for even the large "seas" and the smaller patches of the same character repeat in their outlines the round form of the craters. it is along the terminator that we see these crater-like spots to the best advantage; as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations into bold relief. they vary greatly in size; some being so large as to bear a sensible proportion to the moon's diameter, while the smallest are so minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them. [illustration: fig. .] the prevalence of ring-shaped mountains and plains willbe evident from fig. , which is from a photograph of a model of the moon constructed by nasmyth. this same feature is nearly as marked in figs. and , which are copies of rutherfurd's photographs of the moon. . _lunar craters._--the smaller saucer-shaped formations on the surface of the moon are called _craters_. they are of all sizes, from a mile to a hundred and fifty miles in diameter; and they are supposed to be of volcanic origin. a high telescopic power shows that these craters vary remarkably, not only in size, but also in structure and arrangement. some are considerably elevated above the surrounding surface, others are basins hollowed out of that surface, and with low surrounding ramparts; some are like walled plains, while the majority have their lowest depression considerably below the surrounding surface; some are isolated upon the plains, others are thickly crowded together, overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while others, again, contain several minute craters instead; some have their ramparts whole and perfect, others have them broken or deformed, and many have them divided into terraces, especially on their inner sides. a typical lunar crater is shown in fig. . [illustration: fig. .] it is not generally believed that any active volcanoes exist on the moon at the present time, though some observers have thought they discerned indications of such volcanoes. [illustration: fig. .] . _copernicus._--this is one of the grandest of lunar craters (fig. ). although its diameter (forty-six miles) is exceeded by others, yet, taken as a whole, it forms one of the most impressive and interesting objects of its class. its situation, near the centre of the lunar disk, renders all its wonderful details conspicuous, as well as those of objects immediately surrounding it. its vast rampart rises to upwards of twelve thousand feet above the level of the plateau, nearly in the centre of which stands a magnificent group of cones, three of which attain a height of more than twenty-four hundred feet. many ridges, or spurs, may be observed leading away from the outer banks of the great rampart. around the crater, extending to a distance of more than a hundred miles on every side, there is a complex network of bright streaks, which diverge in all directions. these streaks do not appear in the figure, nor are they seen upon the moon, except at and near the full phase. they show conspicuously, however, by their united lustre on the full moon. this crater is seen just to the south-west of the large dusky plain in the upper part of fig. . this plain is _mare imbrium_, and the mountain-chain seen a little to the right of copernicus is named the _apennines_. copernicus is also seen in fig. , a little to the left of the same range. under circumstances specially favorable, myriads of comparatively minute but perfectly formed craters may be observed for more than seventy miles on all sides around copernicus. the district on the south-east side is specially rich in these thickly scattered craters, which we have reason to suppose stand over or upon the bright streaks. . _dark chasms._--dark cracks, or chasms, have been observed on various parts of the moon's surface. they sometimes occur singly, and sometimes in groups. they are often seen to radiate from some central cone, and they appear to be of volcanic origin. they have been called _canals_ and _rills_. [illustration: fig. .] one of the most remarkable groups of these chasms is that to the west of the crater named _triesneker_. the crater and the chasms are shown in fig. . several of these great cracks obviously diverge from a small crater near the west bank of the great one, and they subdivide as they extend from the apparent point of divergence, while they are crossed by others. these cracks, or chasms, are nearly a mile broad at the widest part, and, after extending full a hundred miles, taper away till they become invisible. [illustration: fig. .] . _mountain-ranges._--there are comparatively few mountain-ranges on the moon. the three most conspicuous are those which partially enclose mare imbrium; namely, the _apennines_ on the south, and the _caucasus_ and the _alps_ on the east and north-east. the apennines are the most extended of these, having a length of about four hundred and fifty miles. they rise gradually, from a comparatively level surface towards the south-west, in the form of innumerable small elevations, which increase in number and height towards the north-east, where they culminate in a range of peaks whose altitude and rugged aspect must form one of the most terribly grand and romantic scenes which imagination can conceive. the north-east face of the range terminates abruptly in an almost vertical precipice; while over the plain beneath, intensely black spire-like shadows are cast, some of which at sunrise extend full ninety miles, till they lose themselves in the general shading due to the curvature of the lunar surface. many of the peaks rise to heights of from eighteen thousand to twenty thousand feet above the plain at their north-east base (fig. ). [illustration: fig. .] fig. represents an ideal lunar landscape near the base of such a lunar range. owing to the absence of an atmosphere, the stars will be visible in full daylight. [illustration: fig. .] . _the valley of the alps._--the range of the _alps_ is shown in fig. . the great crater at the north end of this range is named _plato_. it is seventy miles in diameter. the most remarkable feature of the alps is the valley near the centre of the range. it is more than seventy-five miles long, and about six miles wide at the broadest part. when examined under favorable circumstances, with a high magnifying power, it is seen to be a vast flat-bottomed valley, bordered by gigantic mountains, some of which attain heights of ten thousand feet or more. [illustration: fig. .] . _isolated peaks._--there are comparatively few isolated peaks to be found on the surface of the moon. one of the most remarkable of these is that known as _pico_, and shown in fig. . its height exceeds eight thousand feet, and it is about three times as long at the base as it is broad. the summit is cleft into three peaks, as is shown by the three-peaked shadow it casts on the plain. . _bright rays._--about the time of full moon, with a telescope of moderate power, a number of bright lines may be seen radiating from several of the lunar craters, extending often to the distance of hundreds of miles. these streaks do not arise from any perceptible difference of level of the surface, they have no very definite outline, and they do not present any sloping sides to catch more sunlight, and thus shine brighter, than the general surface. indeed, one great peculiarity of them is, that they come out most forcibly when the sun is shining perpendicularly upon them: hence they are best seen when the moon is at full, and they are not visible at all at those regions upon which the sun is rising or setting. they are not diverted by elevations in their path, but traverse in their course craters, mountains, and plains alike, giving a slight additional brightness to all objects over which they pass, but producing no other effect upon them. "they look as if, after the whole surface of the moon had assumed its final configuration, a vast brush charged with a whitish pigment had been drawn over the globe in straight lines, radiating from a central point, leaving its trail upon every thing it touched, but obscuring nothing." [illustration: fig. .] the three most conspicuous craters from which these lines radiate are _tycho_, _copernicus_, and _kepler_. tycho is seen at the bottom of figs. and . kepler is a little to the left of copernicus in the same figures. it has been thought that these bright streaks are chasms which have been filled with molten lava, which, on cooling, would afford a smooth reflecting surface on the top. . _tycho._--this crater is fifty-four miles in diameter, and about sixteen thousand feet deep, from the highest ridge of the rampart to the surface of the plateau, whence rises a central cone five thousand feet high. it is one of the most conspicuous of all the lunar craters; not so much on account of its dimensions as from its being the centre from whence diverge those remarkable bright streaks, many of which may be traced over a thousand miles of the moon's surface (fig. ). tycho appears to be an instance of a vast disruptive action which rent the solid crust of the moon into radiating fissures, which were subsequently filled with molten matter, whose superior luminosity marks the course of the cracks in all directions from the crater as their common centre. so numerous are these bright streaks when examined by the aid of the telescope, and they give to this region of the moon's surface such increased luminosity, that, when viewed as a whole, the locality can be distinctly seen at full moon by the unassisted eye, as a bright patch of light on the southern portion of the disk. iii. inferior and superior planets. inferior planets. . _the inferior planets._--the _inferior planets_ are those which lie between the earth and the sun, and whose orbits are included by that of the earth. they are _mercury_ and _venus_. [illustration: fig. .] . _aspects of an inferior planet._--the four chief _aspects_ of an inferior planet as seen from the earth are shown in fig. , in which _s_ represents the sun, _p_ the planet, and _e_ the earth. when the planet is between the earth and the sun, as at _p_, it is said to be in _inferior conjunction_. when it is in the same direction as the sun, but beyond it, as at _p''_, it is said to be in _superior conjunction_. when the planet is at such a point in its orbit that a line drawn from the earth to it would be tangent to the orbit, as at _p'_ and _p'''_, it is said to be at its _greatest elongation_. [illustration: fig. .] . _apparent motion of an inferior planet._--when the planet is at _p_, if it could be seen at all, it would appear in the heavens at _a_. as it moves from _p_ to _p'_, it will appear to move in the heavens from _a_ to _b_. then, as it moves from _p'_ to _p''_, it will appear to move back again from _b_ to _a_. while it moves from _p''_ to _p'''_, it will appear to move from _a_ to _c_; and, while moving from _p'''_ to _p_, it will appear to move back again from _c_ to _a_. thus the planet will appear to oscillate to and fro across the sun from _b_ to _c_, never getting farther from the sun than _b_ on the west, or _c_ on the east: hence, when at these points, it is said to be at its _greatest western_ and _eastern elongations_. this oscillating motion of an inferior planet across the sun, combined with the sun's motion among the stars, causes the planet to describe a path among the stars similar to that shown in fig. . [illustration: fig. .] . _phases of an inferior planet._--an inferior planet, when viewed with a telescope, is found to present a succession of phases similar to those of the moon. the reason of this is evident from fig. . as an inferior planet passes around the sun, it presents sometimes more and sometimes less of its bright hemisphere to the earth. when the earth is at _t_, and venus at superior conjunction, the planet turns the whole of its bright hemisphere towards the earth, and appears _full_; it then becomes _gibbous_, _half_, and _crescent_. when it comes into _inferior conjunction_, it turns its dark hemisphere towards the earth: it then becomes _crescent_, _half_, _gibbous_, and _full_ again. . _the sidereal and synodical periods of an inferior planet._--the time it takes a planet to make a complete revolution around the sun is called the _sidereal period_ of the planet; and the time it takes it to pass from one aspect around to the same aspect again, its _synodical period_. [illustration: fig. .] the synodical period of an inferior planet is longer than its sidereal period. this will be evident from an examination of fig. . _s_ is the position of the sun, _e_ that of the earth, and _p_ that of the planet at inferior conjunction. before the planet can be in inferior conjunction again, it must pass entirely around its orbit, and overtake the earth, which has in the mean time passed on in its orbit to _e'_. while the earth is passing from _e_ to _e'_, the planet passes entirely around its orbit, and from _p_ to _p'_ in addition. now the arc _pp'_ is just equal to the arc _ee'_: hence the planet has to pass over the same arc that the earth does, and ° more. in other words, the planet has to gain ° on the earth. the synodical period of the planet is found by direct observation. . _the length of the sidereal period._--the length of the sidereal period of an inferior planet may be found by the following computation:-- let _a_ denote the synodical period of the planet, let _b_ denote the sidereal period of the earth, let _x_ denote the sidereal period of the planet. then _ °/b_ = the daily motion of the earth, and _ °/x_ = the daily motion of the planet, and _ °/x - °/b_ = the daily gain of the planet: also _ °/a_ = the daily gain of the planet: hence _ °/x - °/b = °/a_. dividing by °, we have _ /x - /b = /a_; clearing of fractions, we have _ab - ax = bx_: transposing and collecting, we have _(a + b)x = ab_: therefore _x = ab/a+b_. . _the relative distance of an inferior planet._--by the _relative distance_ of a planet, we mean its distance from the sun compared with the earth's distance from the sun. the relative distance of an inferior planet may be found by the following method:-- [illustration: fig. .] let _v_, in fig. , represent the position of venus at its greatest elongation from the sun, _s_ the position of the sun, and _e_ that of the earth. the line _ev_ will evidently be tangent to a circle described about the sun with a radius equal to the distance of venus from the sun at the time of this greatest elongation. draw the radius _sv_ and the line _se_. since _sv_ is a radius, the angle at _v_ is a right angle. the angle at _e_ is known by measurement, and the angle at _s_ is equal to °- the angle _e_. in the right-angled triangle _evs_, we then know the three angles, and we wish to find the ratio of the side _sv_ to the side _se_. the ratio of these lines may be found by trigonometrical computation as follows:-- _vs : es = sin sev : ._ substitute the value of the sine of sev, and we have _vs : es = . : ._ hence the relative distances of venus and of the earth from the sun are . and . superior planets. . _the superior planets._--the _superior planets_ are those which lie beyond the earth. they are _mars_, the _asteroids_, _jupiter_, _saturn_, _uranus_, and _neptune_. [illustration: fig. .] . _apparent motion of a superior planet._--in order to deduce the apparent motion of a superior planet from the real motions of the earth and planet, let _s_ (fig. ) be the place of the sun; , , , etc., the orbit of the earth; _a_, _b_, _c_, etc., the orbit of mars; and _cgl_ a part of the starry firmament. let the orbit of the earth be divided into twelve equal parts, each described in one month; and let _ab_, _bc_, _cd_, etc., be the spaces described by mars in the same time. suppose the earth to be at the point when mars is at the point _a_, mars will then appear in the heavens in the direction of _a_. when the earth is at , and mars at _c_, he will appear in the heavens at _c_. when the earth arrives at , mars will arrive at _d_, and will appear in the heavens at _d_. while the earth moves from to and from to , mars will appear to have advanced among the stars from _d_ to _e_ and from _e_ to _f_, in the direction from west to east. during the motion of the earth from to and from to , mars will appear to go backward from _f_ to _g_ and from _g_ to _h_, in the direction from east to west. during the motion of the earth from to and from to , mars will appear to advance from _h_ to _i_ and from _i_ to _k_, in the direction from west to east, and the motion will continue in the same direction until near the succeeding opposition. the apparent motion of a superior planet projected on the heavens is thus seen to be similar to that of an inferior planet, except that, in the latter case, the retrogression takes place near inferior conjunction, and in the former it takes place near opposition. [illustration: fig. .] . _aspects of a superior planet._--the four aspects of a superior planet are shown in fig. , in which _s_ is the position of the sun, _e_ that of the earth, and _p_ that of the planet. when the planet is on the opposite side of the earth to the sun, as at _p_, it is said to be in _opposition_. the sun and the planet will then appear in opposite parts of the heavens, the sun appearing at _c_, and the planet at _a_. when the planet is on the opposite side of the sun to the earth, as at _p''_, it is said to be in _superior conjunction_. it will then appear in the same part of the heavens as the sun, both appearing at _c_. when the planet is at _p'_ and _p'''_, so that a line drawn from the earth through the planet will make a right angle with a line drawn from the earth to the sun, it is said to be in _quadrature_. at _p'_ it is in its western quadrature, and at _p'''_ in its eastern quadrature. [illustration: fig. .] . _phases of a superior planet._--mars is the only one of the superior planets that has appreciable phases. at quadrature, as will appear from fig. , mars does not present quite the same side to the earth as to the sun: hence, near these parts of its orbit, the planet appears slightly gibbous. elsewhere in its orbit, the planet appears full. all the other superior planets are so far away from the sun and earth, that the sides which they turn towards the sun and the earth in every part of their orbit are so nearly the same, that no change in the form of their disks can be detected. . _the synodical period of a superior planet._--during a synodical period of a superior planet the earth must gain one revolution, or °, on the planet, as will be evident from an examination of fig. , in which _s_ represents the sun, _e_ the earth, and _p_ the planet at opposition. before the planet can be in opposition again, the earth must make a complete revolution, and overtake the planet, which has in the mean time passed on from _p_ to _p'_. [illustration: fig. .] in the case of most of the superior planets the synodical period is shorter than the sidereal period; but in the case of mars it is longer, since mars makes more than a complete revolution before the earth overtakes it. the synodical period of a superior planet is found by direct observation. . _the sidereal period of a superior planet._--the sidereal period of a superior planet is found by a method of computation similar to that for finding the sidereal period of an inferior planet:-- let _a_ denote the synodical period of the planet, let _b_ denote the sidereal period of the earth, let _x_ denote the sidereal period of the planet. then will _ °/b_ = daily motion of the earth, and _ °/x_ = daily motion of the planet; also _ °/b - °/x_ = daily gain of the earth. but _ °/a_ = daily gain of the earth: hence _ °/b - °/x = °/a_ _ /b - /x = /a_ _ax - ab = bx_ _(a-b)x = ab_ _x = ab/(a-b)_. [illustration: fig. .] . _the relative distance of a superior planet._--let _s_, _e_, and _m_, in fig. , represent the relative positions of the sun, the earth, and mars, when the latter planet is in opposition. let _e_ and _m_ represent the relative positions of the earth and mars the day after opposition. at the first observation mars will be seen in the direction _ema_, and at the second observation in the direction _ema_. but the fixed stars are so distant, that if a line, _ea_, were drawn to a fixed star at the first observation, and a line, _eb_, drawn from the earth to the same fixed star at the second observation, these two lines would be sensibly parallel; that is, the fixed star would be seen in the direction of the line _ea_ at the first observation, and in the direction of the line _eb_, parallel to _ea_, at the second observation. but if mars were seen in the direction of the fixed star at the first observation, it would appear back, or west, of that star at the second observation by the angular distance _bea_; that is, the planet would have retrograded that angular distance. now, this retrogression of mars during one day, at the time of opposition, can be measured directly by observation. this measurement gives us the value of the angle _bea_; but we know the rate at which both the earth and mars are moving in their orbits, and from this we can easily find the angular distance passed over by each in one day. this gives us the angles _esa_ and _msa_. we can now find the relative length of the lines _ms_ and _es_ (which represent the distances of mars and of the earth from the sun), both by construction and by trigonometrical computation. since _eb_ and _ea_ are parallel, the angle _eas_ is equal to _bea_. _sea = ° - (esa + eas)_ _esm = esa - msa_ _ems = ° - (sea + esm)_. we have then _ms : es = sin sea : sin ems._ substituting the values of the sines, and reducing the ratio to its lowest terms, we have _ms : es = . : ._ thus we find that the relative distances of mars and the earth from the sun are . and . by the simple observation of its greatest elongation, we are able to determine the relative distances of an inferior planet and the earth from the sun; and, by the equally simple observation of the daily retrogression of a superior planet, we can find the relative distances of such a planet and the earth from the sun. iv. the sun. i. magnitude and distance of the sun. [illustration: fig. .] . _the volume of the sun._--the apparent diameter of the sun is about ', being a little greater than that of the moon. the real diameter of the sun is , miles, or about a hundred and nine times that of the earth. as the diameter of the moon's orbit is only about , miles, or some sixty times the diameter of the earth, it follows that the diameter of the sun is nearly double that of the moon's orbit: hence, were the centre of the sun placed at the centre of the earth, the sun would completely fill the moon's orbit, and reach nearly as far beyond it in every direction as it is from the earth to the moon. the circumference of the sun as compared with the moon's orbit is shown in fig. . the volume of the sun is , , times that of the earth. . _the mass of the sun._--the sun is much less dense than the earth. the mass of the sun is only , times that of the earth, and its density only about a fourth that of the earth. to find the mass of the sun, we first ascertain the distance the earth would draw the moon towards itself in a given time, were the moon at the distance of the sun, and then form the proportion: as the distance the earth would draw the moon towards itself is to the distance that the sun draws the earth towards itself in the same time, so is the mass of the earth to the mass of the sun. although the mass of the sun is over three hundred thousand times that of the earth, the pull of gravity at the surface of the sun is only about twenty-eight times as great as at the surface of the earth. this is because the distance from the surface of the sun to its centre is much greater than from the surface to the centre of the earth. [illustration: fig. .] . _size of the sun compared with that of the planets._--the size of the sun compared with that of the larger planets is shown in fig. . the mass of the sun is more than seven hundred and fifty times that of all of the planets and moons in the solar system. in fig. is shown the apparent size of the sun as seen from the different planets. the apparent diameter of the sun decreases as the distance from it increases, and the disk of the sun decreases as the square of the distance from it increases. [illustration: fig. .] . _the distance of the sun._--the mean distance of the sun from the earth is about , , miles. owing to the eccentricity of the earth's orbit, the distance of the sun varies somewhat; being about , , miles less in january, when the earth is at perihelion, than in june, when the earth is at aphelion. "but, though the distance of the sun can easily be stated in figures, it is not possible to give any real idea of a space so enormous: it is quite beyond our power of conception. if one were to try to walk such a distance, supposing that he could walk four miles an hour, and keep it up for ten hours every day, it would take sixty-eight years and a half to make a single million of miles, and more than sixty-three hundred years to traverse the whole. "if some celestial railway could be imagined, the journey to the sun, even if our trains ran sixty miles an hour day and night and without a stop, would require over a hundred and seventy-five years. sensation, even, would not travel so far in a human lifetime. to borrow the curious illustration of professor mendenhall, if we could imagine an infant with an arm long enough to enable him to touch the sun and burn himself, he would die of old age before the pain could reach him; since, according to the experiments of helmholtz and others, a nervous shock is communicated only at the rate of about a hundred feet per second, or , miles a day, and would need more than a hundred and fifty years to make the journey. sound would do it in about fourteen years, if it could be transmitted through celestial space; and a cannon-ball in about nine, if it were to move uniformly with the same speed as when it left the muzzle of the gun. if the earth could be suddenly stopped in her orbit, and allowed to fall unobstructed toward the sun, under the accelerating influence of his attraction, she would reach the centre in about four months. i have said if she could be stopped; but such is the compass of her orbit, that, to make its circuit in a year, she has to move nearly nineteen miles a second, or more than fifty times faster than the swiftest rifle-ball; and, in moving twenty miles, her path deviates from perfect straightness by less than an eighth of an inch. and yet, over all the circumference of this tremendous orbit, the sun exercises his dominion, and every pulsation of his surface receives its response from the subject earth." (professor c. a. young: the sun.) . _method of finding the sun's distance._--there are several methods of finding the sun's distance. the simplest method is that of finding the actual distance of one of the nearer planets by observing its displacement in the sky as seen from widely separated points on the earth. as the _relative_ distances of the planets from each other and from the sun are well known, we can easily deduce the actual distance of the sun if we can find that of any of the planets. the two planets usually chosen for this method are mars and venus. ( ) the displacement of mars in the sky, as seen from two observatories which differ considerably in latitude, is, of course, greatest when mars is nearest the earth. now, it is evident than mars will be nearer the earth when in opposition than when in any other part of its orbit; and the planet will be least distant from the earth when it is at its perihelion point, and the earth is at its aphelion point, at the time of opposition. this method, then, can be used to the best advantage, when, at the time of opposition, mars is near its perihelion, and the earth near its aphelion. these favorable oppositions occur about once in fifteen years, and the last one was in . [illustration: fig. .] suppose two observers situated at _n'_ and _s'_ (fig. ), near the poles of the earth. the one at _n'_ would see mars in the sky at _n_, and the one at _s'_ would see it at _s_. the displacement would be the angle _nms_. each observer measures carefully the distance of mars from the same fixed star near it. the difference of these distances gives the displacement of the planet, or the angle _nms_. these observations were made with the greatest care in . ( ) venus is nearest the earth at the time of inferior conjunction; but it can then be seen only in the daytime. it is, therefore, impossible to ascertain the displacement of venus, as seen from different stations, by comparing her distances from a fixed star. occasionally, at the time of inferior conjunction, venus passes directly across the sun's disk. the last of these _transits_ of venus occurred in , and the next will occur in . it will then be over a hundred years before another will occur. [illustration: fig. .] suppose two observers, _a_ and _b_ (fig. ), near the poles of the earth at the time of a transit of venus. the observer at _a_ would see venus crossing the sun at _v_{ }_, and the one at _b_ would see it crossing at _v_{ }_. any observation made upon venus, which would give the distance and direction of venus from the centre of the sun, as seen from each station, would enable us to calculate the angular distance between the two chords described across the sun. this, of course, would give the displacement of venus on the sun's disk. this method was first employed at the last transits of venus which occurred before ; namely, those of and . there are three methods of observation employed to ascertain the apparent direction and distance of venus from the centre of the sun, called respectively the _contact method_, the _micrometric method_, and the _photographic method_. (_a_) in the _contact_ method, the observation consists in noting the exact time when venus crosses the sun's limb. to ascertain this it is necessary to observe the exact time of external and internal contact. this observation, though apparently simple, is really very difficult. with reference to this method professor young says,-- "the difficulties depend in part upon the imperfections of optical instruments and the human eye, partly upon the essential nature of light leading to what is known as diffraction, and partly upon the action of the planet's atmosphere. the two first-named causes produce what is called irradiation, and operate to make the apparent diameter of the planet, as seen on the solar disk, smaller than it really is; smaller, too, by an amount which varies with the size of the telescope, the perfection of its lenses, and the tint and brightness of the sun's image. the edge of the planet's image is also rendered slightly hazy and indistinct. [illustration: fig. .] "the planet's atmosphere also causes its disk to be surrounded by a narrow ring of light, which becomes visible long before the planet touches the sun, and, at the moment of internal contact, produces an appearance, of which the accompanying figure is intended to give an idea, though on an exaggerated scale. the planet moves so slowly as to occupy more than twenty minutes in crossing the sun's limb; so that even if the planet's edge were perfectly sharp and definite, and the sun's limb undistorted, it would be very difficult to determine the precise second at which contact occurs. but, as things are, observers with precisely similar telescopes, and side by side, often differ from each other five or six seconds; and, where the telescopes are not similar, the differences and uncertainties are much greater.... astronomers, therefore, at present are pretty much agreed that such observations can be of little value in removing the remaining uncertainty of the parallax, and are disposed to put more reliance upon the micrometric and photographic methods, which are free from these peculiar difficulties, though, of course, beset with others, which, however, it is hoped will prove less formidable." (_b_) of the _micrometric_ method, as employed at the last transit, professor young speaks as follows:-- "the micrometric method requires the use of a heliometer,--an instrument common only in germany, and requiring much skill and practice in its use in order to obtain with it accurate measures. at the late transit, a single english party, two or three of the russian parties, and all five of the german, were equipped with these instruments; and at some of the stations extensive series of measures were made. none of the results, however, have appeared as yet; so that it is impossible to say how greatly, if at all, this method will have the advantage in precision over the contact observations." (_c_) the following observations, with reference to the _photographic_ method, are also taken from professor young:-- "the americans and french placed their main reliance upon the photographic method, while the english and germans also provided for its use to a certain extent. the great advantage of this method is, that it makes it possible to perform the necessary measurements (upon whose accuracy every thing depends) at leisure after the transit, without hurry, and with all possible precautions. the field-work consists merely in obtaining as many and as good pictures as possible. a principal objection to the method lies in the difficulty of obtaining good pictures, i.e., pictures free from distortion, and so distinct and sharp as to bear high magnifying power in the microscopic apparatus used for their measurement. the most serious difficulty, however, is involved in the accurate determination of the scale of the picture; that is, of the number of seconds of arc corresponding to a linear inch upon the plate. besides this, we must know the exact greenwich time at which each picture is taken, and it is also extremely desirable that the _orientation_ of the picture should be accurately determined; that is, the north and south, the east and west points of the solar image on the finished plate. there has been a good deal of anxiety lest the image, however accurate and sharp when first produced, should alter, in course of time, through the contraction of the collodion film on the glass plate; but the experiments of rutherfurd, huggins, and paschen, seem to show that this danger is imaginary.... the americans placed the photographic telescope exactly in line with a meridian instrument, and so determined, with the extremest precision, the direction in which it was pointed. knowing this and the time at which any picture was taken, it becomes possible, with the help of the plumb-line image, to determine precisely the orientation of the picture,--an advantage possessed by the american pictures alone, and making their value nearly twice as great as otherwise it would have been. "the figure below is a representation of one of the american photographs reduced about one-half. _v_ is the image of venus, which, on the actual plate, is about a seventh of an inch in diameter; _aa'_ is the image of the plumb-line. the centre of the reticle is marked with a cross." [illustration: fig. .] the english photographs proved to be of little value, and the results of the measurements and calculations upon the american pictures have not yet been published. there is a growing apprehension that no photographic method can be relied upon. the most recent determinations by various methods indicate that the sun's distance is such that his parallax is about eighty-eight seconds. this would make the linear value of a second at the surface of the sun about four hundred and fifty miles. [illustration: plate .] ii. physical and chemical condition of the sun. physical condition of the sun. . _the sun composed mainly of gas._--it is now generally believed that the sun is mainly a ball of gas, or vapor, powerfully condensed at the centre by the weight of the superincumbent mass, but kept from liquefying by its exceedingly high temperature. the gaseous interior of the sun is surrounded by a layer of luminous clouds, which constitutes its visible surface, and which is called its _photosphere_. here and there in the photosphere are seen dark _spots_, which often attain an immense magnitude. these clouds float in the _solar atmosphere_, which extends some distance beyond them. the luminous surface of the sun is surrounded by a _rose-colored_ stratum of gaseous matter, called the _chromosphere_. here and there great masses of this chromospheric matter rise high above the general level. these masses are called _prominences_. outside of the chromosphere is the _corona_, an irregular halo of faint, pearly light, mainly composed of filaments and streamers, which radiate from the sun to enormous distances, often more than a million of miles. in fig. is shown a section of the sun, according to professor young. the accompanying lithographic plate gives a general view of the photosphere with its spots, and of the chromosphere and its prominences. . _the temperature of the sun._--those who have investigated the subject of the temperature of the sun have come to very different conclusions; some placing it as high as four million degrees fahrenheit, and others as low as ten thousand degrees. professor young thinks that rosetti's estimate of eighteen thousand degrees as the _effective temperature_ of the sun's surface is probably not far from correct. by this is meant the temperature that a uniform surface of lampblack of the size of the sun must have in order to radiate as much heat as the sun does. the most intense artificial heat does not exceed four thousand degrees fahrenheit. [illustration: fig. .] . _the amount of heat radiated by the sun._--a unit of heat is the amount of heat required to raise a pound of water one degree in temperature. it takes about a hundred and forty-three units of heat to melt a pound of ice without changing its temperature. a cubic foot of ice weighs about fifty-seven pounds. according to sir william herschel, were all the heat radiated by the sun concentrated on a cylinder of ice forty-five miles in diameter, it would melt it off at the rate of about a hundred and ninety thousand miles a second. professor young gives the following illustration of the energy of solar radiation: "if we could build up a solid column of ice from the earth to the sun, two miles and a quarter in diameter, spanning the inconceivable abyss of ninety-three million miles, and if then the sun should concentrate his power upon it, it would dissolve and melt, not in an hour, nor a minute, but in a single second. one swing of the pendulum, and it would be water; seven more, and it would be dissipated in vapor." [illustration: fig. .] this heat would be sufficient to melt a layer of ice nearly fifty feet thick all around the sun in a minute. to develop this heat would require the hourly consumption of a layer of anthracite coal, more than sixteen feet thick, over the entire surface of the sun; and the _mechanical equivalent_ of this heat is about ten thousand horse-power on every square foot of the sun's surface. . _the brightness of the sun's surface._--the sun's surface is a hundred and ninety thousand times as bright as a candle-flame, a hundred and forty-six times as bright as the calcium-light, and about three times and a half as bright as the voltaic arc. the sun's disk is much less bright near the margin than near the centre, a point on the limb of the sun being only about a fourth as bright as one near the centre of the disk. this diminution of brightness towards the margin of the disk is due to the increase in the absorption of the solar atmosphere as we pass from the centre towards the margin of the sun's disk; and this increased absorption is due to the fact, that the rays which reach us from near the margin have to traverse a much greater thickness of the solar atmosphere than those which reach us from the centre of the disk. this will be evident from fig. , in which the arrows mark the paths of rays from different parts of the solar disk. the spectroscope. [illustration: fig. .] . _the spectroscope as an astronomical instrument._--the _spectroscope_ is now continually employed in the study of the physical condition and chemical constitution of the sun and of the other heavenly bodies. it has become almost as indispensable to the astronomer as the telescope. . _the dispersion spectroscope._--the essential parts of the _dispersion_ spectroscope are shown in fig. . these are the _collimator tube_, the _prism_, and the _telescope_. the collimator tube has a narrow slit at one end, through which the light to be examined is admitted, and somewhere within the tube a lens for condensing the light. the light is dispersed on passing through the prism: it then passes through the objective of the telescope, and forms within the tube an image of the spectrum, which is examined by means of the eye-piece. the power of the spectroscope is increased by increasing the number of prisms, which are arranged so that the light shall pass through one after another in succession. such an arrangement of prisms is shown in fig. . one end of the collimator tube is seen at the left, and one end of the telescope at the right. sometimes the prisms are made long, and the light is sent twice through the same train of prisms, once through the lower, and once through the upper, half of the prisms. this is accomplished by placing a rectangular prism against the last prism of the train, as shown in fig. . [illustration: fig. .] [illustration: fig. .] . _the micrometer scale._--various devices are employed to obtain an image of a micrometer scale in the tube of the telescope beside that of the spectrum. [illustration: fig. .] one of the simplest of these methods is shown in fig. . _a_ is the telescope, _b_ the collimator, and _c_ the micrometer tube. the opening at the outer end of _c_ contains a piece of glass which has a micrometer scale marked upon it. the light from the candle shines through this glass, falls upon the surface of the prism _p_, and is thence reflected into the telescope, where it forms an enlarged image of the micrometer scale alongside the image of the spectrum. [illustration: fig. .] . _the comparison of spectra._--in order to compare two spectra, it is desirable to be able to see them side by side in the telescope. the images of two spectra may be obtained side by side in the telescope tube by the use of a little rectangular prism, which covers one-half of the slit of the collimator tube, as shown in fig. . the light from one source is admitted directly through the uncovered half of the slit, while the light from the other source is sent through the covered portion of the slit by reflection from the surface of the rectangular prism. this arrangement and its action will be readily understood from fig. . [illustration: fig. .] . _direct-vision spectroscope._--a beam of light may be dispersed, without any ultimate deflection from its course, by combining prisms of crown and flint glass with equal refractive, but unequal dispersive powers. such a combination of prisms is called a _direct-vision_ combination. one of three prisms is shown in fig. , and one of five prisms in fig. . [illustration: fig. .] [illustration: fig. .] a _direct-vision spectroscope_ (fig. ) is one in which a direct-vision combination of prisms is employed. _c_ is the collimator tube, _p_ the train of prisms, _f_ the telescope, and _r_ the comparison prism. [illustration: fig. .] . _the telespectroscope._--the spectroscope, when used for astronomical work, is usually combined with a telescope. the compound instrument is called a _telespectroscope_. the spectroscope is mounted at the end of the telescope in such a way that the image formed by the object-glass of the telescope falls upon the slit at the end of the collimator tube. a telespectroscope of small dispersive power is shown in fig. ; _a_ being the object-glass of the telescope, _cc_ the tube of the telescope, and _e_ the comparison prism at the end of the collimator tube. a more powerful instrument is shown in fig. . _a_ is the telescope, _c_ the collimator tube of the spectroscope, _p_ the train of prisms, and _e_ the telescope tube. fig. shows a still more powerful spectroscope attached to the great newall refractor ( ). [illustration: fig. .] [illustration: fig. .] . _the diffraction spectroscope._--a _diffraction_ spectroscope is one in which the spectrum is produced by reflection of the light from a finely ruled surface, or _grating_, as it is called, instead of by dispersion in passing through a prism. the essential parts of this instrument are shown in fig . this spectroscope may be attached to the telescope in the same manner as the dispersion spectroscope. when the spectroscope is thus used, the eye-piece of the telescope is removed. [illustration: fig. .] spectra. . _continuous spectra._--light from an incandescent solid or liquid which has suffered no absorption in the medium which it has traversed gives a spectrum consisting of a continuous colored band, in which the colors, from the red to the violet, pass gradually and imperceptibly into one another. the spectrum is entirely free from either light or dark lines, and is called a _continuous spectrum_. . _bright-lined spectra._--light from a luminous gas or vapor gives a spectrum composed of bright lines separated by dark spaces, and known as a _bright-lined spectrum_. it has been found that the lines in the spectrum of a substance in the state of a gas or vapor are the most characteristic thing about the substance, since no two vapors give exactly the same lines: hence, when we have once become acquainted with the bright-lined spectrum of any substance, we can ever after recognize that substance by the spectrum of its luminous vapor. even when several substances are mixed, they may all be recognized by the bright-lined spectrum of the mixture, since the lines of all the substances will be present in the spectrum of the mixture. this method of identifying substances by their spectra is called _spectrum analysis_. the bright-lined spectra of several substances are given in the frontispiece. the number of lines in the spectra of the elements varies greatly. the spectrum of sodium is one of the simplest, while that of iron is one of the most complex. the latter contains over six hundred lines. though no two vapors give identical spectra, there are many cases in which one or more of the spectral lines of one element coincide in position with lines of other elements. . _methods of rendering gases and vapors luminous._--in order to study the spectra of vapors and gases it is necessary to have some means of converting solids and liquids into vapor, and also of rendering the vapors and gases luminous. there are four methods of obtaining luminous vapors and gases in common use. [illustration: fig. .] ( ) _by means of the bunsen flame._--this is a very hot but an almost non-luminous flame. if any readily volatilized substance, such as the compounds of sodium, calcium, strontium, etc., is introduced into this flame on a fine platinum wire, it is volatilized in the flame, and its vapor is rendered luminous, giving the flame its own peculiar color. the flame thus colored may be examined by the spectroscope. the arrangement of the flame is shown in fig. . [illustration: fig. .] ( ) _by means of the voltaic arc._--an electric lamp is shown in fig. . when this lamp is to be used for obtaining luminous vapors, the lower carbon is made larger than the upper one, and hollowed out at the top into a little cup. the substance to be volatilized is placed in this cup, and the current is allowed to pass. the heat of the voltaic arc is much more intense than that of the bunsen flame: hence substances that cannot be volatilized in the flame are readily volatilized in the arc, and the vapor formed is raised to a very high temperature. ( ) _by means of the spark from an induction coil._--the arrangement of the coil for obtaining luminous vapors is shown in fig. . [illustration: fig. .] the terminals of the coil between which the spark is to pass are brought quite close together. when we wish to vaporize any metal, as iron, the terminals are made of iron. on the passage of the spark, a little of the iron at the ends of the terminals is evaporated; and the vapor is rendered luminous in the space traversed by the spark. a condenser is usually placed in the circuit. with the coil, the temperature may be varied at pleasure; and the vapor may be raised even to a higher temperature than with the electric lamp. to obtain a low temperature, the coil is used without the condenser. by using a larger and larger condenser, the temperature may be raised higher and higher. by means of the induction coil we may also heat gases to incandescence. it is only necessary to allow the spark to pass through a space filled with the gas. [illustration: fig. .] ( ) _by means of a vacuum tube._--the form of the vacuum tube commonly used for this purpose is shown in fig. . the gas to be examined, and which is contained in the tube, has very slight density: but upon the passage of the discharge from an induction coil or a holtz machine, through the tube, the gas in the capillary part of the tube becomes heated to a high temperature, and is then quite brilliant. . _reversed spectra._--if the light from an incandescent cylinder of lime, or from the incandescent point of an electric lamp, is allowed to pass through luminous sodium vapor, and is then examined with a spectroscope, the spectrum will be found to be a bright spectrum crossed by a single _dark_ line in the position of the yellow line of the sodium vapor. the spectrum of sodium vapor is _reversed_, its bright lines becoming dark and its dark spaces bright. with a spectroscope of any considerable power, the yellow line of sodium vapor is resolved into a double line. with a spectroscope of the same power, the dark sodium line of the reversed spectrum is seen to be a double line. it is found to be generally true, that the spectrum of the light from an incandescent solid or liquid which has passed _through a luminous vapor_ on its way to the spectroscope is made up of a bright ground crossed by dark lines; there being a dark line for every bright line that the vapor alone would give. . _explanation of reversed spectra._--it has been found that gases absorb and quench rays of the same degree of refrangibility as those which they themselves emit, and no others. when a solid is shining through a luminous vapor, this absorbs and quenches those rays from the solid which have the same degrees of refrangibility as those which it is itself emitting: hence the lines of the spectrum receive light from the vapor alone, while the spaces between the lines receive light from the solid. now, solids and liquids, when heated to incandescence, give a very much brighter light than vapors and gases at the same temperature: hence the lines of a reversed spectrum, though receiving light from the vapor or gas, appear dark by contrast. . _effect of increasing the power of the spectroscope upon the brilliancy of a spectrum._--an increase in the power of a spectroscope diminishes the brilliancy of a _continuous_ spectrum, since it makes the colored band longer, and therefore spreads the light out over a greater extent of surface; but, in the case of a _bright-lined_ spectrum, an increase of power in the spectroscope produces scarcely any alteration in the brilliancy of the lines, since it merely separates the lines farther without making the lines themselves any wider. in the case of a _reversed_ spectrum, an increase of power in the spectroscope dilutes the light in the spaces between the lines without diluting that of the lines: hence lines which appear dark in a spectroscope of slight dispersive power may appear bright in an instrument of great dispersive power. . _change of the spectrum with the density of the luminous vapor._--it has been found, that, as the density of a luminous vapor is diminished, the lines in its spectrum become fewer and fewer, till they are finally reduced to one. on the other hand, an increase of density causes new lines to appear in the spectrum, and the old lines to become thicker. . _change of the spectrum with the temperature of the luminous vapor._--it has also been found that the appearance of a bright-lined spectrum changes considerably with the temperature of the luminous vapor. in some cases, an increase of temperature changes the relative intensities of the lines; in other cases, it causes new lines to appear, and old lines to disappear. in the case of a compound vapor, an increase of temperature causes the colored bands (which are peculiar to the spectrum of the compound) to disappear, and to be replaced by the spectral lines of the elements of which the compound is made up. the heat appears to _dissociate_ the compound; that is, to resolve it into its constituent elements. in this case, each elementary vapor would give its own spectral lines. as the compound is not completely dissociated at once, it is possible, of course, for one or more of the spectral lines of the elementary vapors to co-exist in the spectrum with the bands of the compound. it has been found, that, in some cases, the spectra of the elementary gases change with the temperature of the gas; and lockyer thinks he has discovered conclusive evidence, in the spectra of the sun and stars, that many of the substances regarded as elementary are really resolved into simpler substances by the intense heat of the sun; in other words, that our so-called elements are really compounds. chemical constitution of the sun. . _the solar spectrum._--the solar spectrum is crossed transversely by a great number of fine dark lines, and hence it belongs to the class of _reversed_ spectra. these lines were first studied and mapped by fraunhofer, and from him they have been called _fraunhofer's lines_. [illustration: fig. .] a reduced copy of fraunhofer's map is shown in fig. . a few of the most prominent of the dark solar lines are designated by the letters of the alphabet. the other lines are usually designated by the numbers at which they are found on the scale which accompanies the map. this scale is usually drawn at the top of the map, as will be seen in some of the following diagrams. the two most elaborate maps of the solar spectrum are those of kirchhoff and angström. the scale on kirchhoff's map is an arbitrary one, while that of angström is based upon the wave-lengths of the rays of light which would fall upon the lines in the spectrum. [illustration: fig. .] the appearance of the spectrum varies greatly with the power of the spectroscope employed. fig. shows a portion of the spectrum as it appears in a spectroscope of a single prism: while fig. shows the _b_ group of lines alone, as they appear in a powerful diffraction spectroscope. [illustration: fig. .] . _the telluric lines._--there are many lines of the solar spectrum which vary considerably in intensity as the sun passes from the horizon to the meridian, being most intense when the sun is nearest the horizon, and when his rays are obliged to pass through the greatest depth of the earth's atmosphere. these lines are of atmospheric origin, and are due to the absorption of the aqueous vapor in our atmosphere. they are the same lines that are obtained when a candle or other artificial light is examined with a spectroscope through a long tube filled with steam. since these lines are due to the absorption of our own atmosphere, they are called _telluric lines_. a map of these lines is shown in fig. . [illustration: fig. .] . _the solar lines._--after deducting the telluric lines, the remaining lines of the solar spectrum are of solar origin. they must be due to absorption which takes place in the sun's atmosphere. they are, in fact, the reversed spectra of the elements which exist in the solar atmosphere in the state of vapor: hence we conclude that the luminous surface of the sun is surrounded with an atmosphere of luminous vapors. the temperature of this atmosphere, at least near the surface of the sun, must be sufficient to enable all the elements known on the earth to exist in it as vapors. [illustration: fig. .] . _chemical constitution of the sun's atmosphere._--to find whether any element which exists on the earth is present in the solar atmosphere, we have merely to ascertain whether the bright lines of its gaseous spectrum are matched by dark lines in the solar spectrum when the two spectra are placed side by side. in fig. , we have in no. a portion of the red end of the solar spectra, and in no. the spectrum of sodium vapor, both as obtained in the same spectroscope by means of the comparison prism. it will be seen that the double sodium line is exactly matched by a double dark line of the solar spectrum: hence we conclude that sodium vapor is present in the sun's atmosphere. fig. shows the matching of a great number of the bright lines of iron vapor by dark lines in the solar spectrum. this matching of the iron lines establishes the fact that iron vapor is present in the solar atmosphere. [illustration: fig. .] the following table (given by professor young) contains a list of all the elements which have, up to the present time, been detected with certainty in the sun's atmosphere. it also gives the number of bright lines in the spectrum of each element, and the number of those lines which have been matched by dark lines in the solar spectrum:-- elements. bright lines reversed. observer. lines. . iron kirchhoff. . titanium thalen. . calcium kirchhoff. . manganese angström. . nickel kirchhoff. . cobalt thalen. . chromium kirchhoff. . barium kirchhoff. . sodium kirchhoff. . magnesium kirchhoff. . copper? ? kirchhoff. . hydrogen angström. . palladium lockyer. . vanadium lockyer. . molybdenum lockyer. . strontium lockyer. . lead lockyer. . uranium lockyer. . aluminium angström. . cerium lockyer. . cadmium lockyer. . oxygen a ± bright h. draper. oxygen b ? schuster. in addition to the above elements, it is probable that several other elements are present in the sun's atmosphere; since at least one of their bright lines has been found to coincide with dark lines of the solar spectrum. there are, however, a large number of elements, no traces of which have yet been detected; and, in the cases of the elements whose presence in the solar atmosphere has been established, the matching of the lines is far from complete in the majority of the cases, as will be seen from the above table. this want of complete coincidence of the lines is undoubtedly due to the very high temperature of the solar atmosphere. we have already seen that the lines of the spectrum change with the temperature; and, as the temperature of the sun is far higher than any that we can produce by artificial means, we might reasonably expect that it would cause the disappearance from the spectrum of many lines which we find to be present at our highest temperature. lockyer maintains that the reason why no trace of the spectral lines of certain of our so-called elements is found in the solar atmosphere is, that these substances are not really elementary, and that the intense heat of the sun resolves them into simpler constituents. motion at the surface of the sun. . _change of pitch caused by motion of sounding body._--when a sounding body is moving rapidly towards us, the pitch of its note becomes somewhat higher than when the body is stationary; and, when such a body is moving rapidly from us, the pitch of its note is lowered somewhat. we have a good illustration of this change of pitch at a country railway station on the passage of an express-train. the pitch of the locomotive whistle is considerably higher when the train is approaching the station than when it is leaving it. . _explanation of the change of pitch produced by motion._--the pitch of sound depends upon the rapidity with which the pulsations of sound beat upon the drum of the ear. the more rapidly the pulsations follow each other, the higher is the pitch: hence the shorter the sound-waves (provided the sound is all the while travelling at the same rate), the higher the pitch of the sound. any thing, then, which tends to shorten the waves of sound tends also to raise its pitch, and any thing which tends to lengthen these waves tends to lower its pitch. when a sounding body is moving rapidly forward, the sound-waves are crowded together a little, and therefore shortened; when it is moving backward, the sound-waves are drawn out, or lengthened a little. the effect of the motion of a sounding body upon the length of its sonorous waves will be readily seen from the following illustration: suppose a number of persons stationed at equal intervals in a line on a long platform capable of moving backward and forward. suppose the men are four feet apart, and all walking forward at the same rate, and that the platform is stationary, and that, as the men leave the platform, they keep on walking at the same rate: the men will evidently be four feet apart in the line in front of the platform, as well as on it. suppose next, that the platform is moving forward at the rate of one foot in the interval between two men's leaving the platform, and that the men continue to walk as before: it is evident that the men will then be three feet apart in the line after they have left the platform. the forward motion of the platform has the effect of crowding the men together a little. were the platform moving backward at the same rate, the men would be five feet apart after they had left the platform. the backward motion of the platform has the effect of separating the men from one another. the distance between the men in this illustration corresponds to the length of the sound-wave, or the distance between its two ends. were a person to stand beside the line, and count the men that passed him in the three cases given above, he would find that more persons would pass him in the same time when the platform is moving forward than when it is stationary, and fewer persons would pass him in the same time when the platform is moving backward than when it is stationary. in the same way, when a sounding body is moving rapidly forward, the sound-waves beat more rapidly upon the ear of a person who is standing still than when the body is at rest, and less rapidly when the sounding body is moving rapidly backward. were the platform stationary, and were the person who is counting the men to be walking along the line, either towards or away from the platform, the effect upon the number of men passing him in a given time would be precisely the same as it would be were the person stationary, and the platform moving either towards or away from him at the same rate. so the change in the rapidity with which pulsations of sound beat upon the ear is precisely the same whether the ear is stationary and the sounding body moving, or the sounding body is stationary and the ear moving. . _change of refrangibility due to the motion of a luminous body._--refrangibility in light corresponds to pitch in sound, and depends upon the length of the luminous waves. the shorter the luminous waves, the greater the refrangibility of the waves. very rapid motion of a luminous body has the same effect upon the length of the luminous waves that motion of a sounding body has upon the length of the sonorous waves. when a luminous body is moving very rapidly towards us, its luminous waves are shortened a little, and its light becomes a little more refrangible; when the luminous body is moving rapidly from us, its luminous waves are lengthened a little, and its light becomes a little less refrangible. [illustration: fig. .] . _displacement of spectral lines._--in examining the spectra of the stars, we often find that certain of the dark lines are _displaced_ somewhat, either towards the red or the violet end of the spectrum. as the dark lines are in the same position as the bright lines of the absorbing vapor would be, a displacement of the lines towards the red end of the spectrum indicates a lowering of the refrangibility of the rays, due to a motion of the luminous vapor away from us; and a displacement of the lines towards the violet end of the spectrum indicates an increase of refrangibility, due to a motion of the luminous vapor towards us. from the amount of the displacement of the lines, it is possible to calculate the velocity at which the luminous gas is moving. in fig. is shown the displacement of the _f_ line in the spectrum of sirius. this is one of the hydrogen lines. _rv_ is the spectrum, _r_ being the red, and _v_ the violet end. the long vertical line is the bright _f_ line of hydrogen, and the short dark line to the left of it is the position of the _f_ line in the spectrum of sirius. it is seen that this line is displaced somewhat towards the red end of the spectrum. this indicates that sirius must be moving from us; and the amount of the displacement indicates that the star must be moving at the rate of some twenty-five or thirty miles a second. [illustration: fig. .] . _contortion of lines on the disk of the sun._--certain of the dark lines seen on the centre of the sun's disk often appear more or less distorted, as shown in fig. , which represents the contortion of the hydrogen line as seen at various times. and indicate a rapid motion of hydrogen away from us, or a _down-rush_ at the sun; and (in which the line at the centre is dark on one side, and bent towards the red end of the spectrum, and bright on the other side with a distortion towards the violet end of the spectrum) indicate a _down-rush_ of _cool_ hydrogen side by side with an _up-rush_ of _hot and bright_ hydrogen; indicates local _down-rushes_ associated with _quiescent_ hydrogen. the contorted lines, which indicate a violently agitated state of the sun's atmosphere, appear in the midst of other lines which indicate a quiescent state. this is owing to the fact that the absorption which produces the dark lines takes place at various depths in the solar atmosphere. there may be violent commotion in the lower layers of the sun's atmosphere, and comparative quiet in the upper layers. in this case, the lines which are due to absorption in the lower layers would indicate this disturbance by their contortions; while the lines produced by absorption in the upper layers would be free from contortion. it often happens, too, that the contortions are confined to one set of lines of an element, while other lines of the same element are entirely free from contortions. this is undoubtedly due to the fact that different layers of the solar atmosphere differ greatly in temperature; so that the same element would give one set of lines at one depth, and another set at another depth: hence commotion in the solar atmosphere at any particular depth would be indicated by the contortion of those lines of the element only which are produced by the temperature at that particular depth. a remarkable case of contortion witnessed by professor young is shown in fig. . three successive appearances of the _c_ line are shown. the second view was taken three minutes after the first, and the third five minutes after the second. the contortion in this case indicated a velocity ranging from two hundred to three hundred miles a second. [illustration: fig. .] . _contortion of lines on the sun's limb._--when the spectroscope is directed to the centre of the sun's disk, the distortion of the lines indicates only vertical motion in the sun's atmosphere; but, when the spectroscope is directed to the limb of the sun, displacements of the lines indicate horizontal motions in the sun's atmosphere. when a powerful spectroscope is directed to the margin of the sun's disk, so that the slit of the collimator tube shall be perpendicular to the sun's limb, one or more of the dark lines on the disk are seen to be prolonged by a bright line, as shown in fig. . but this prolongation, instead of being straight and narrow, as shown in the figure, is often widened and distorted in various ways, as shown in fig. . in the left-hand portion of the diagram, the line is deflected towards the red end of the spectrum; this indicates a violent wind on the sun's surface blowing away from us. in the right-hand portion of the diagram, the line is deflected towards the violet end of the spectrum; this indicates a violent wind blowing towards us. in the middle portion of the figure, the line is seen to be bent both ways; this indicates a cyclone, on one side of which the wind would be blowing from us, and on the other side towards us. [illustration: fig. .] [illustration: fig. .] the distortions of the solar lines indicate that the wind at the surface of the sun often blows with a velocity of _from one hundred to three hundred miles a second_. the most violent wind known on the earth has velocity of a hundred miles an hour. iii. the photosphere and sun spots. the photosphere. [illustration: fig. .] . _the granulation of the photosphere._--when the surface of the sun is examined with a good telescope under favorable atmospheric conditions, it is seen to be composed of minute grains of intense brilliancy and of irregular form, floating in a darker medium, and arranged in streaks and groups, as shown in fig. . with a rather low power, the general effect of the surface is much like that of rough drawing-paper, or of curdled milk seen from a little distance. with a high power and excellent atmospheric conditions, the _grains_ are seen to be irregular, rounded masses, some hundreds of miles in diameter, sprinkled upon a less brilliant background, and appearing somewhat like snow-flakes sparsely scattered over a grayish cloth. fig. is a representation of these grains according to secchi. [illustration: fig. .] with a very powerful telescope and the very best atmospheric conditions, the grains themselves are resolved into _granules_, or little luminous dots, not more than a hundred miles or so in diameter, which, by their aggregation, make up the grains, just as they, in their turn, make up the coarser masses of the solar surface. professor langley estimates that these granules constitute about one-fifth of the sun's surface, while they emit at least three-fourths of its light. . _shape of the grains._--the grains differ considerably in shape at different times and on different parts of the sun's surface. nasmyth, in , described them as _willow-leaves_ in shape, several thousand miles in length, but narrow and with pointed ends. he figured the surface of the sun as a sort of basket-work formed by the interweaving of such filaments. to others they have appeared to have the form of _rice-grains_. on portions of the sun's disk the elementary structure is often composed of long, narrow, blunt-ended filaments, not so much like willow-leaves as like bits of straw lying roughly parallel to each other,--a _thatch-straw_ formation, as it has been called. this is specially common in the immediate neighborhood of the spots. . _nature of the grains._--the grains are, undoubtedly, incandescent _clouds_ floating in the sun's atmosphere, and composed of partially condensed metallic vapors, just as the clouds of our atmosphere are composed of partially condensed aqueous vapor. rain on the sun is composed of white-hot drops of molten iron and other metals; and these drops are often driven with the wind with a velocity of over a hundred miles a second. as to the forms of the grains, professor young says, "if one were to speculate as to the explanation of the grains and thatch-straws, it might be that the grains are the upper ends of long filaments of luminous cloud, which, over most of the sun's surface, stand approximately vertical, but in the neighborhood of a spot are inclined so as to lie nearly horizontal. this is not certain, though: it may be that the cloud-masses over the more quiet portions of the solar surface are really, as they seem, nearly globular, while near the spots they are drawn out into filamentary forms by atmospheric currents." . _faculæ._--the _faculæ_ are irregular streaks of greater brightness than the general surface, looking much like the flecks of foam on the surface of a stream below a waterfall. they are sometimes from five to twenty thousand miles in length, covering areas immensely larger than a terrestrial continent. these faculæ are _elevated regions_ of the solar surface, ridges and crests of luminous matter, which rise above the general level of the sun's surface, and protrude through the denser portions of the solar atmosphere. when one of these passes over the edge of the sun's disk, it can be seen to project, like a little tooth. any elevation on the sun to be perceptible at all must measure at least half a second of an arc, or two hundred and twenty-five miles. the faculæ are most numerous in the neighborhood of the spots, and much more conspicuous near the limb of the sun than near the centre of the disk. fig. gives the general appearance of the faculæ, and the darkening of the limb of the sun. near the spots, the faculæ often undergo very rapid change of form, while elsewhere on the disk they change rather slowly, sometimes undergoing little apparent alteration for several days. [illustration: fig. .] . _why the faculæ are most conspicuous near the limb of the sun._--the reason why the faculæ are most conspicuous near the limb of the sun is this: the luminous surface of the sun is covered with an atmosphere, which, though not very thick compared with the diameter of the sun, is still sufficient to absorb a good deal of light. light coming from the centre of the sun's disk penetrates this atmosphere under the most favorable conditions, and is but slightly reduced in amount. the edges of the disk, on the other hand, are seen through a much greater thickness of atmosphere; and the light is reduced by absorption some seventy-five per cent. suppose, now, a facula were sufficiently elevated to penetrate quite through this atmosphere. its light would be undimmed by absorption on any part of the sun's disk; but at the centre of the disk it would be seen against a background nearly as bright as itself, while at the margin it would be seen against one only a quarter as bright. it is evident that the light of any facula, owing to the elevation, would be reduced less rapidly as we approach the edge of the disk than that of the general surface of the sun, which lies at a lower level. sun-spots. . _general appearance of sun-spots._--the general appearance of a well-formed sun-spot is shown in fig. . the spot consists of a very dark central portion of irregular shape, called the _umbra_, which is surrounded by a less dark fringe, called the _penumbra_. the penumbra is made up, for the most part, of filaments directed radially inward. [illustration: fig. .] there is great variety in the details of form in different sun-spots; but they are generally nearly circular during the middle period of their existence. during the period of their development and of their disappearance they are much more irregular in form. there is nothing like a gradual shading-off of the penumbra, either towards the umbra on the one side, or towards the photosphere on the other. the penumbra is separated from both the umbra and the photosphere by a sharp line of demarcation. the umbra is much brighter on the inner than on the outer edge, and frequently the photosphere is excessively bright at the margin of the penumbra. the brightness of the inner penumbra seems to be due to the crowding together of the penumbral filaments where they overhang the edge of the umbra. there is a general antithesis between the irregularities of the outer and inner edges of the penumbra. where an angle of the penumbral matter crowds in upon the umbra, it is generally matched by a corresponding outward extension into the photosphere, and _vice versa_. the umbra of the spot is far from being uniformly dark. many of the penumbral filaments terminate in little detached grains of luminous matter; and there are also fainter veils of a substance less brilliant, but sometimes rose-colored, which seem to float above the umbra. the umbra itself is made up of masses of clouds which are really intensely brilliant, and which appear dark only by contrast with the intenser brightness of the solar surface. among these clouds are often seen one or more minute circular spots much darker than the rest of the umbra. these darker portions are called _nuclei_. they seem to be the mouths of tubular orifices penetrating to unknown depths. the faint veils mentioned above continually melt away, and are replaced by others in some different position. the bright granules at the tips of the penumbral filaments seem to sink and dissolve, while fresh portions break off to replace them. there is a continual indraught of luminous matter over the whole extent of the penumbra. at times, though very rarely, patches of intense brightness suddenly break out, remain visible for a few minutes, and move over the spot with velocities as great as a hundred miles _a second_. the spots change their form and size quite perceptibly from day to day, and sometimes even from hour to hour. . _duration of sun-spots._--the average life of a sun-spot is two or three months: the longest on record is that of a spot observed in and , which lasted eighteen months. there are cases, however, where the disappearance of a spot is very soon followed by the appearance of another at the same point; and sometimes this alternate disappearance and re-appearance is several times repeated. while some spots are thus long-lived, others endure only a day or two, and sometimes only a few hours. . _groups of spots._--the spots usually appear not singly, but in groups. a large spot is often followed by a train of smaller ones to the east of it, many of which are apt to be irregular in form and very imperfect in structure, sometimes with no umbra at all, often with a penumbra only on one side. in such cases, when any considerable change of form or structure shows itself in the principal spot, it seems to rush westward over the solar surface, leaving its attendants trailing behind. when a large spot divides into two or more, as often happens, the parts usually seem to repel each other, and fly apart with great velocity. . _size of the spots._--the spots are sometimes of enormous size. groups have often been observed covering areas of more than a hundred thousand miles square, and single spots occasionally measure from forty to fifty thousand miles in diameter, the umbra being twenty-five or thirty thousand miles across. a spot, however, measuring thirty thousand miles over all, may be considered a large one. such a spot can easily be seen without a telescope when the brightness of the sun's surface is reduced by clouds or nearness to the horizon, or by the use of colored glass. during the years and spots were visible to the naked eye for a considerable portion of the time. the largest spot yet recorded was observed in . it had a breadth of more than a hundred and forty-three thousand miles, or nearly eighteen times the diameter of the earth, and covered about a thirty-sixth of the whole surface of the sun. [illustration: fig. .] fig. represents a group of sun-spots observed by professor langley, and drawn on the same scale as the small circle in the upper left-hand corner, which represents the surface of half of our globe. [illustration: fig. .] . _the penumbral filaments._--not unfrequently the penumbral filaments are curved spirally, indicating a cyclonic action, as shown in fig. . in such cases the whole spot usually turns slowly around, sometimes completing an entire revolution in a few days. more frequently, however, the spiral motion lasts but a short time; and occasionally, after continuing for a while in one direction, the motion is reversed. very often in large spots we observe opposite spiral movements in different portions of the umbra, as shown in figs. and . [illustration: fig. .] neighboring spots show no tendency to rotate in the same direction. the number of spots in which a decided cyclonic motion (like that shown in fig. ) appears is comparatively small, not exceeding two or three per cent of the whole. [illustration: fig. .] [illustration: fig. .] [illustration: plate .] plate ii. represents a typical sun-spot as delineated by professor langley. at the left-hand and upper portions of this great spot the filaments present the ordinary appearance, while at the lower edge, and upon the great overhanging branch, they are arranged very differently. the feathery brush below the branch, closely resembling a frost-crystal on a window-pane, is as rare as it is curious, and has not been satisfactorily explained. [illustration: fig. .] . _birth and decay of sun-spots._--the formation of a spot is sometimes gradual, requiring days or even weeks for its full development; and sometimes a single day suffices. generally, for some time before its appearance, there is an evident disturbance of the solar surface, indicated especially by the presence of many brilliant faculæ, among which _pores_, or minute black dots, are scattered. these enlarge, and between them appear grayish patches, in which the photospheric structure is unusually evident, as if they were caused by a dark mass lying below a thin veil of luminous filaments. this veil seems to grow gradually thinner, and finally breaks open, giving us at last the complete spot with its penumbra. some of the pores coalesce with the principal spot, some disappear, and others form the attendant train before described ( ). the spot when once formed usually assumes a circular form, and remains without striking change until it disappears. as its end approaches, the surrounding photosphere seems to crowd in, and overwhelm the penumbra. bridges of light (fig. ), often much brighter than the average of the solar surface, push across the umbra; the arrangement of the penumbra filaments becomes confused; and, as secchi expresses it, the luminous matter of the photosphere seems to tumble pell-mell into the chasm, which disappears, and leaves a disturbed surface marked with faculæ, which, in their turn, gradually subside. [illustration: fig. .] . _motion of sun-spots._--the spots have a regular motion across the disk of the sun from east to west, occupying about twelve days in the transit. a spot generally appears first on or near the east limb, and, after twelve or fourteen days, disappears at the west limb. at the end of another fourteen days, or more, it re-appears at the east limb, unless, in the mean time, it has vanished from sight entirely. this motion of the spots is indicated by the arrow in fig. . the interval between two successive appearances of the same spot on the eastern edge of the sun is about twenty-seven days. [illustration: fig. .] . _the rotation of the sun._--the spots are evidently carried around by the rotation of the sun on its axis. it is evident, from fig. , that the sun will need to make more than a complete rotation in order to bring a spot again upon the same part of the disk as seen from the earth. _s_ represents the sun, and _e_ the earth. the arrows indicate the direction of the sun's rotation. when the earth is at _e_, a spot at _a_ would be seen at the centre of the solar disk. while the sun is turning on its axis, the earth moves in its orbit from _e_ to _e'_: hence the sun must make a complete rotation, and turn from _a_ to _a'_ in addition, in order to bring the spot again to the centre of the disk. to carry the spot entirely around, and then on to _a'_, requires about twenty-seven days. from this _synodical period_ of the spot, as it might be called, it has been calculated that the sun must rotate on its axis in about twenty-five days. [illustration: fig. .] . _the inclination of the sun's axis._--the paths described by sun-spots across the solar disk vary with the position of the earth in its orbit, as shown in fig. . we therefore conclude that the sun's axis is not perpendicular to the plane of the earth's orbit. the sun rotates on its axis from west to east, and the axis leans about seven degrees from the perpendicular to the earth's orbit. . _the proper motion of the spots._--when the period of the sun's rotation is deduced from the motion of spots in different solar latitudes, there is found to be considerable variation in the results obtained. thus spots near the equator indicate that the sun rotates in about twenty-five days; while those in latitude ° indicate a period about eighteen hours longer; and those in latitude ° a period of twenty-seven days and a half. strictly speaking, the sun, as a whole, has no single period of rotation; but different portions of its surface perform their revolutions in different times. the equatorial regions not only move more rapidly in miles per hour than the rest of the solar surface, but they _complete the entire rotation in shorter time_. [illustration: fig. .] there appears to be a peculiar surface-drift in the equatorial regions of the sun, the cause of which is unknown, but which gives the spots a _proper_ motion; that is, a motion of their own, independent of the rotation of the sun. [illustration: fig. .] . _distribution of the sun-spots._--the sun-spots are not distributed uniformly over the sun's surface, but occur mainly in two zones on each side of the equator, and between the latitudes of ° and °, as shown in fig. . on and near the equator itself they are comparatively rare. there are still fewer beyond ° of latitude, and only a single spot has ever been recorded more than ° from the solar equator. fig. shows the distribution of the sun-spots observed by carrington during a period of eight years. the irregular line on the left-hand side of the figure indicates by its height the comparative frequency with which the spots occurred in different latitudes. in fig. the same thing is indicated by different degrees of darkness in the shading of the belts. [illustration: fig. .] . _the periodicity of the spots._--careful observations of the solar spots indicate a period of about eleven years in the spot-producing activity of the sun. during two or three years the spots increase in number and in size; then they begin to diminish, and reach a minimum five or six years after the maximum. another period of about six years brings the return of the maximum. the intervals are, however, somewhat irregular. [illustration: fig. .] fig. gives a graphic representation of the periodicity of the sun-spots. the height of the curve shows the frequency of the sun-spots in the years given at the bottom of the figure. it appears, from an examination of this sun-spot curve, that the average interval from a minimum to the next following maximum is only about four years and a half, while that from a maximum to the next following minimum is six years and six-tenths. the disturbance which produces the sun-spots is developed suddenly, but dies away gradually. . _connection between sun-spots and terrestrial magnetism._--the magnetic needle does not point steadily in the same direction, but is subject to various disturbances, some of which are regular, and others irregular. ( ) one of the most noticeable of the regular magnetic changes is the so-called _diurnal oscillation_. during the early part of the day the north pole of the needle moves toward the west in our latitude, returning to its mean position about ten p.m., and remaining nearly stationary during the night. the extent of this oscillation in the united states is about fifteen minutes of arc in summer, and not quite half as much in winter; but it differs very much in different localities and at different times, and the average diurnal oscillation in any locality increases and decreases pretty regularly during a period of about eleven years. the maximum and minimum of this period of magnetic disturbance are found to coincide with the maximum and minimum of the sun-spot period. this is shown in fig. , in which the dotted lines indicate the variations in the intensity of the magnetic disturbance. ( ) occasionally so-called _magnetic storms_ occur, during which the compass-needle is sometimes violently disturbed, oscillating five degrees, or even ten degrees, within an hour or two. these storms are generally accompanied by an aurora, and an aurora is _always_ accompanied by magnetic disturbance. a careful comparison of aurora observations with those of sun-spots shows an almost perfect parallelism between the curves of auroral and sun-spot frequency. ( ) a number of observations render it very probable that every intense disturbance of the solar surface is propagated to our terrestrial magnetism with the speed of light. [illustration: fig. .] fig. shows certain of the solar lines as they were observed by professor young on aug. , . the contortions of the _f_ line indicated an intense disturbance in the atmosphere of the sun. there were three especially notable paroxysms in this distortion, occurring at a quarter of nine, half-past ten, and ten minutes of twelve, a.m. [illustration: fig. .] fig. shows the curve of magnetic disturbance as traced at greenwich on the same day. it will be seen from the curve that it was a day of general magnetic disturbance. at the times of the three paroxysms, which are given at the bottom of the figure, it will be observed that there is a peculiar shivering of the magnetic curve. . _the spots are depressions in the photosphere._--this fact was first clearly brought out by dr. wilson of glasgow, in , from observations upon the penumbra of a spot in november of that year. he found, that when the spot appeared at the eastern limb, or edge of the sun, just moving into sight, the penumbra was well marked on the side of the spot nearest to the edge of the disk; while on the other edge of the spot, towards the centre of the sun, there was no penumbra visible at all, and the umbra itself was almost hidden, as if behind a bank. when the spot had moved a day's journey toward the centre of the disk, the whole of the umbra came into sight, and the penumbra on the inner edge of the spot began to be visible as a narrow line. after the spot was well advanced upon the disk, the penumbra was of the same width all around the spot. when the spot approached the sun's western limb, the same phenomena were repeated, but in the inverse order. the penumbra on the _inner_ edge of the spot narrowed much faster than that on the outer, disappeared entirely, and finally seemed to hide from sight much of the umbra nearly a whole day before the spot passed from view around the limb. this is precisely what would occur (as fig. clearly shows) if the spot were a saucer-shaped depression in the solar surface, the bottom of the saucer corresponding to the umbra, and the sloping sides to the penumbra. [illustration: fig. .] [illustration: fig. .] . _sun-spot spectrum._--when the image of a sun-spot is thrown upon the slit of the spectroscope, the spectrum is seen to be crossed longitudinally by a continuous dark band, showing an increased general absorption in the region of the sun-spot. many of the spectral lines are greatly thickened, as shown in fig. . this thickening of the lines shows that the absorption is taking place at a greater depth. new lines and shadings often appear, which indicate, that, in the cooler nucleus of the spot, certain compound vapors exist, which are dissociated elsewhere on the sun's surface. these lines and shadings are shown in fig. . [illustration: fig. .] it often happens that certain of the spectral lines are reversed in the spectrum of the spot, a thin bright line appearing over the centre of a thick dark one, as shown in fig. . these reversals are due to very bright vapors floating over the spot. [illustration: fig. .] at times, also, the spectrum of a spot indicates violent motion in the overlying gases by distortion and displacement of the lines. this phenomenon occurs oftener at points near the outer edge of the penumbra than over the centre of the spot; but occasionally the whole neighborhood is violently agitated. in such cases, lines in the spectrum side by side are often affected in entirely different ways, one being greatly displaced while its neighbor is not disturbed in the least, showing that the vapors which produce the lines are at different levels in the solar atmosphere, and moving independently of each other. [illustration: fig. .] . _the cause and nature of sun-spots._--according to professor young, the arrangement and relations of the photospheric clouds in the neighborhood of a spot are such as are represented in fig. . "over the sun's surface generally, these clouds probably have the form of vertical columns, as at _aa_. just outside the spot, the level of the photosphere is the most part, overtopped by eruptions of hydrogen and usually raised into faculæ, as at _bb_. these faculæ are, for metallic vapors, as indicated by the shaded clouds.... while the great clouds of hydrogen are found everywhere upon the sun, these spiky, vivid outbursts of metallic vapors seldom occur except just in the neighborhood of a spot, and then only during its season of rapid change. in the penumbra of the spot the photospheric filaments become more or less nearly horizontal, as at _pp_; in the umbra at _u_ it is quite uncertain what the true state of affairs may be. we have conjecturally represented the filaments there as vertical also, but depressed and carried down by a descending current. of course, the cavity is filled by the gases which overlie the photosphere; and it is easy to see, that, looked at from above, such a cavity and arrangement of the luminous filaments would present the appearances actually observed." professor young also suggests that the spots may be depressions in the photosphere caused "by the _diminution of upward pressure_ from below, in consequence of eruptions in the neighborhood; the spots thus being, so to speak, _sinks_ in the photosphere. undoubtedly the photosphere is not a strictly continuous shell or crust; but it is _heavy_ as compared with the uncondensed vapors in which it lies, just as a rain-cloud in our terrestrial atmosphere is heavier than the air; and it is probably continuous enough to have its upper level affected by any diminution of pressure below. the gaseous mass below the photosphere supports its weight and the weight of the products of condensation, which must always be descending in an inconceivable rain and snow of molten and crystallized material. to all intents and purposes, though nothing but a layer of clouds, the photosphere thus forms a constricting shell, and the gases beneath are imprisoned and compressed. moreover, at a high temperature the viscosity of gases is vastly increased, so that quite probably the matter of the solar nucleus resembles pitch or tar in its consistency more than what we usually think of as a gas. consequently, any sudden diminution of pressure would propagate itself slowly from the point where it occurred. putting these things together, it would seem, that, whenever a free outlet is obtained through the photosphere at any point, thus decreasing the inward pressure, the result would be the sinking of a portion of the photosphere somewhere in the immediate neighborhood, to restore the equilibrium; and, if the eruption were kept up for any length of time, the depression in the photosphere would continue till the eruption ceased. this depression, filled with the overlying gases, would constitute a spot. moreover, the line of fracture (if we may call it so) at the edges of the sink would be a region of weakness in the photosphere, so that we should expect a series of eruptions all around the spot. for a time the disturbance, therefore, would grow, and the spot would enlarge and deepen, until, in spite of the viscosity of the internal gases, the equilibrium of pressure was gradually restored beneath. so far as we know the spectroscopic and visual phenomena, none of them contradict this hypothesis. there is nothing in it, however, to account for the distribution of the spots in solar latitudes, nor for their periodicity." iv. the chromosphere and prominences. . _the sun's outer atmosphere._--what we see of the sun under ordinary circumstances is but a fraction of his total bulk. while by far the greater portion of the solar _mass_ is included within the photosphere, the larger portion of his _volume_ lies without, and constitutes a gaseous envelope whose diameter is at least double, and its bulk therefore sevenfold, that of the central globe. this outer envelope, though mainly gaseous, is not spherical, but has an exceedingly irregular and variable outline. it seems to be made up, not of regular strata of different density, like our atmosphere, but rather of flames, beams, and streamers, as transient and unstable as those of the aurora borealis. it is divided into two portions by a boundary as definite, though not so regular, as that which separates them both from the photosphere. the outer and far more extensive portion, which in texture and rarity seems to resemble the tails of comets, is known as the _coronal atmosphere_, since to it is chiefly due the _corona_, or glory, which surrounds the darkened sun during an eclipse. . _the chromosphere._--at the base of the coronal atmosphere, and in contact with the photosphere, is what resembles a sheet of scarlet fire. it appears as if countless jets of heated gas were issuing through vents over the whole surface, clothing it with flame, which heaves and tosses like the blaze of a conflagration. this is the _chromosphere_, or color-sphere. it owes its vivid redness to the predominance of hydrogen in the flames. the average depth of the chromosphere is not far from ten or twelve seconds, or five thousand or six thousand miles. . _the prominences._--here and there masses of this hydrogen, mixed with other substances, rise far above the general level into the coronal regions, where they float like clouds, or are torn to pieces by conflicting currents. these cloud-masses are known as solar _prominences_, or _protuberances_. . _magnitude and distribution of the prominences._--the prominences differ greatly in magnitude. of the , observed by secchi, , attained an altitude of eighteen thousand miles; , or nearly a fourth of the whole, reached a height of twenty-eight thousand miles; several exceeded eighty-four thousand miles. in rare instances they reach elevations as great as a hundred thousand miles. a few have been seen which exceeded a hundred and fifty thousand miles; and secchi has recorded one of three hundred thousand miles. [illustration: fig. .] the irregular lines on the right-hand side of fig. show the proportion of the prominences observed by secchi, that were seen in different parts of the sun's surface. the outer line shows the distribution of the smaller prominences, and the inner dotted line that of the larger prominences. by comparing these lines with those on the opposite side of the circle, which show the distribution of the spots, it will be seen, that, while the spots are confined mainly to two belts, the prominences are seen in all latitudes. . _the spectrum of the chromosphere._--the spectrum of the chromosphere is comparatively simple. there are eleven lines only which are always present; and six of these are lines of hydrogen, and the others, with a single exception, are of unknown elements. there are sixteen other lines which make their appearance very frequently. among these latter are lines of sodium, magnesium, and iron. where some special disturbance is going on, the spectrum at the base of the chromosphere is very complicated, consisting of hundreds of bright lines. "the majority of the lines, however, are seen only occasionally, for a few minutes at a time, when the gases and vapors, which generally lie low (mainly in the interstices of the clouds which constitute the photosphere), and below its upper surface, are elevated for the time being by some eruptive action. for the most part, the lines which appear only at such times are simply _reversals_ of the more prominent dark lines of the ordinary solar spectrum. but the selection of the lines seems most capricious: one is taken, and another left, though belonging to the same element, of equal intensity, and close beside the first." some of the main lines of the chromosphere and prominences are shown in fig. . [illustration: fig. .] . _method of studying the chromosphere and prominences._--until recently, the solar atmosphere could be seen only during a total eclipse of the sun; but now the spectroscope enables us to study the chromosphere and the prominences with nearly the same facility as the spots and faculæ. the protuberances are ordinarily invisible, for the same reason that the stars cannot be seen in the daytime; they are hidden by the intense light reflected from our own atmosphere. if we could only get rid of this aerial illumination, without at the same time weakening the light of the prominences, the latter would become visible. this the spectroscope enables us to accomplish. since the air-light is reflected sunshine, it of course presents the same spectrum as sunlight,--a continuous band of color crossed by dark lines. now, this sort of spectrum is weakened by increase of dispersive power ( ), because the light is spread out into a longer ribbon, and made to cover a greater area. on the other hand, the spectrum of the prominences, being composed of bright lines, undergoes no such diminution by increased dispersion. [illustration: fig. .] when the spectroscope is used as a means of examining the prominences, the slit is more or less widened. the telescope is directed so that the image of that portion of the solar limb which is to be examined shall be tangent to the opened slit, as in fig. , which represents the slit-plate of the spectroscope of its actual size, with the image of the sun in the proper position for observation. [illustration: fig. .] if, now, a prominence exists at this part of the solar limb, and if the spectroscope itself is so adjusted that the _c_ line falls in the centre of the field of view, then one will see something like fig. . "the red portion of the spectrum will stretch athwart the field of view like a scarlet ribbon with a darkish band across it; and in that band will appear the prominences, like scarlet clouds, so like our own terrestrial clouds, indeed, in form and texture, that the resemblance is quite startling. one might almost think he was looking out through a partly-opened door upon a sunset sky, except that there is no variety or contrast of color; all the cloudlets are of the same pure scarlet hue. along the edge of the opening is seen the chromosphere, more brilliant than the clouds which rise from it or float above it, and, for the most part, made up of minute tongues and filaments." . _quiescent prominences._--the prominences differ as widely in form and structure as in magnitude. the two principal classes are the _quiescent_, _cloud-formed_, or _hydrogenous_, and the _eruptive_, or _metallic_. [illustration: plate .] the _quiescent_ prominences resemble almost exactly our terrestrial clouds, and differ among themselves in the same manner. they are often of enormous dimensions, especially in horizontal extent, and are comparatively permanent, often undergoing little change for hours and days. near the poles they sometimes remain during a whole solar revolution of twenty-seven days. sometimes they appear to lie upon the limb of the sun, like a bank of clouds in the terrestrial horizon, probably because they are so far from the edge that only their upper portions are in sight. when fully seen, they are usually connected to the chromosphere by slender columns, generally smallest at the base, and often apparently made up of separate filaments closely intertwined, and expanding upward. sometimes the whole under surface is fringed with pendent filaments. sometimes they float entirely free from the chromosphere; and in most cases the larger clouds are attended by detached cloudlets. various forms of quiescent prominences are shown in plate iii. other forms are given in figs. and . [illustration: fig. .] their spectrum is usually very simple, consisting of the four lines of hydrogen and the orange _d_^ : hence the appellation _hydrogenous_. occasionally the sodium and magnesium lines also appear, even near the tops of the clouds. [illustration: fig. .] . _eruptive prominences._--the _eruptive_ prominences ordinarily consist of brilliant spikes or jets, which change very rapidly in form and brightness. as a rule, their altitude is not more than twenty thousand or thirty thousand miles; but occasionally they rise far higher than even the largest of the quiescent protuberances. their spectrum is very complicated, especially near their base, and often filled with bright lines. the most conspicuous lines are those of sodium, magnesium, barium, iron, and titanium: hence secchi calls them _metallic_ prominences. [illustration: fig. .] they usually appear in the immediate vicinity of a spot, never very near the solar poles. they change with such rapidity, that the motion can almost be seen with the eye. sometimes, in the course of fifteen or twenty minutes, a mass of these flames, fifty thousand miles high, will undergo a total transformation; and in some instances their complete development or disappearance takes no longer time. sometimes they consist of pointed rays, diverging in all directions, as represented in fig. . "sometimes they look like flames, sometimes like sheaves of grain, sometimes like whirling water-spouts capped with a great cloud; occasionally they present most exactly the appearance of jets of liquid fire, rising and falling in graceful parabolas; frequently they carry on their edges spirals like the volutes of an ionic column; and continually they detach filaments, which rise to a great elevation, gradually expanding and growing fainter as they ascend, until the eye loses them." [illustration: fig. .] . _change of form in prominences._--fig. represents a prominence as seen by professor young, sept. , . it was an immense quiescent cloud, a hundred thousand miles long and fifty-four thousand miles high. at _a_ there was a brilliant lump, somewhat in the form of a thunder-head. on returning to the spectroscope less than half an hour afterwards, he found that the cloud had been literally blown into shreds by some inconceivable uprush from beneath. the prominence then presented the form shown in fig. . the _débris_ of the cloud had already attained a height of a hundred thousand miles. while he was watching them for the next ten minutes, they rose, with a motion almost perceptible to the eye, till the uppermost reached an altitude of two hundred thousand miles. as the filaments rose, they gradually faded away like a dissolving cloud. [illustration: fig. .] meanwhile the little thunder-head had grown and developed into what appeared to be a mass of rolling and ever-changing flame. figs. and give the appearance of this portion of the prominence at intervals of fifteen minutes. other similar eruptions have been observed. [illustration: fig. .] [illustration: fig. .] v. the corona. . _general appearance of the corona._--at the time of a total eclipse of the sun, if the sky is clear, the moon appears as a huge black ball, the illumination at the edge of the disk being just sufficient to bring out its rotundity. "from behind it," to borrow professor young's vivid description, "stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre. the portion nearest the sun is of dazzling brightness, but still less brilliant than the prominences which blaze through it like carbuncles. generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance, and is far more irregular in form. usually there are several _rifts_, as they have been called, like narrow beams of darkness, extending from the very edge of the sun to the outer night, and much resembling the cloud-shadows which radiate from the sun before a thunder-shower; but the edges of these rifts are frequently curved, showing them to be something else than real shadows. sometimes there are narrow bright streamers, as long as the rifts, or longer. these are often inclined, occasionally are even nearly tangential to the solar surface, and frequently are curved. on the whole, the corona is usually less extensive and brilliant over the solar poles, and there is a recognizable tendency to accumulations above the middle latitudes, or spot-zones; so that, speaking roughly, the corona shows a disposition to assume the form of a quadrilateral or four-rayed star, though in almost every individual case this form is greatly modified by abnormal streamers at some point or other." [illustration: fig. .] . _the corona as seen at recent eclipses._--the corona can be seen only at the time of a total eclipse of the sun, and then for only a few minutes. its form varies considerably from one eclipse to another, and apparently also during the same eclipse. at least, different observers at different stations depict the same corona under very different forms. fig. represents the corona of as observed by liais. in this view the _petal-like_ forms, which have been noticed in the corona at other times, are especially prominent. [illustration: fig. .] fig. shows the corona of as it was observed by temple. [illustration: fig. .] fig. shows the corona of . this is interesting as being a corona at the time of sun-spot minimum. [illustration: fig. .] fig. represents the corona of . this is a larger and more irregular corona than usual. [illustration: fig. .] the corona of is shown in fig. . [illustration: fig. .] fig. is a view of the corona of as seen by capt. tupman. [illustration: fig. .] fig. shows the same corona as seen by foenander. [illustration: fig. .] fig. shows the same corona as photographed by davis. [illustration: fig. .] fig. shows the corona of made up from several views as combined by professor young. . _the spectrum of the corona._--the chief line in the spectrum of the corona is the one usually designated as , and now known as the _coronal_ line. it is seen as a dark line on the disk of the sun; and a spectroscope of great dispersive power shows this dark line to be closely double, the lower component being one of the iron lines, and the upper the coronal line. this dark line is shown at _x_, fig. . [illustration: fig. .] besides this bright line, the hydrogen lines appear faintly in the spectrum of the corona. the line has been sometimes traced with the spectroscope to an elevation of nearly twenty minutes above the moon's limb, and the hydrogen lines nearly as far; and the lines were just as strong _in the middle of a dark rift_ as anywhere else. the substance which produces the line is unknown as yet. it seems to be something with a vapor-density far below that of hydrogen, which is the lightest substance of which we have any knowledge. it can hardly be an "allotropic" form of any terrestrial element, as some scientists have suggested; for in the most violent disturbances in prominences and near sun-spots, when the lines of hydrogen, magnesium, and other metals, are contorted and shattered by the rush of the contending elements, this line alone remains fine, sharp, and straight, a little brightened, but not otherwise affected. for the present it remains, like a few other lines in the spectrum, an unexplained mystery. besides bright lines, the corona shows also a faint continuous spectrum, in which have been observed a few of the more prominent _dark_ lines of the solar spectrum. this shows, that, while the corona may be in the main composed of glowing gas (as indicated by the bright lines of its spectrum), it also contains considerable matter in such a state as to reflect the sunlight, probably in the form of dust or fog. v. eclipses. [illustration: fig. .] . _the shadows of the earth and moon._--the shadows cast by the earth and moon are shown in fig. . each shadow is seen to be made up of a dark portion called the _umbra_, and of a lighter portion called the _penumbra_. the light of the sun is completely excluded from the umbra, but only partially from the penumbra. the umbra is in the form of a cone, with its apex away from the sun; though in the case of the earth's shadow it tapers very slowly. the penumbra surrounds the umbra, and increases in size as we recede from the sun. the axis of the earth's shadow lies in the plane of the ecliptic, which in the figure is the surface of the page. as the moon's orbit is inclined five degrees to the plane of the ecliptic, the axis of the moon's shadow will sometimes lie above, and sometimes below, the ecliptic. it will lie on the ecliptic only when the moon is at one of her nodes. . _when there will be an eclipse of the moon._--the moon is eclipsed _whenever it passes into the umbra of the earth's shadow_. it will be seen from the figure that the moon can pass into the shadow of the earth only when she is in opposition, or _at full_. owing to the inclination of the moon's orbit to the ecliptic, the moon will pass either above or below the earth's shadow when she is at full, unless she happens to be near her node at this time: hence there is not an eclipse of the moon every month. when the moon simply passes into the penumbra of the earth's shadow, the light of the moon is somewhat dimmed, but not sufficiently to attract attention, or to be denominated an eclipse. [illustration: fig. .] . _the lunar ecliptic limits._--in fig. the line _ab_ represents the plane of the ecliptic, and the line _cd_ the moon's orbit. the large black circles on the line _ab_ represent sections of the umbra of the earth's shadow, and the smaller circles on _cd_ represent the moon at full. it will be seen, that, if the moon is full at _e_, she will just graze the umbra of the earth's shadow. in this case she will suffer no eclipse. were the moon full at any point nearer her node, as at _f_, she would pass into the umbra of the earth's shadow, and would be _partially_ eclipsed. were the moon full at _g_, she would pass through the centre of the earth's shadow, and be _totally_ eclipsed. it will be seen from the figure that full moon must occur when the moon is within a certain distance from her node, in order that there may be a lunar eclipse; and this space is called the _lunar ecliptic limits_. the farther the earth is from the sun, the less rapidly does its shadow taper, and therefore the greater its diameter at the distance of the moon; and, the nearer the moon to the earth, the greater the diameter of the earth's shadow at the distance of the moon. of course, the greater the diameter of the earth's shadow, the greater the ecliptic limits: hence the lunar ecliptic limits vary somewhat from time to time, according to the distance from the earth to the sun and from the earth to the moon. the limits within which an eclipse is inevitable under all circumstances are called the _minor ecliptic limits_; and those within which an eclipse is possible under some circumstances, the _major ecliptic limits_. [illustration: fig. .] . _lunar eclipses._--fig. shows the path of the moon through the earth's shadow in the case of a _partial eclipse_. the magnitude of such an eclipse depends upon the nearness of the moon to her nodes. the magnitude of an eclipse is usually denoted in _digits_, a digit being one-twelfth of the diameter of the moon. [illustration: fig. .] fig. shows the path of the moon through the earth's shadow in the case of a _total eclipse_. it will be seen from the figure that it is not necessary for the moon to pass through the centre of the earth's shadow in order to have a total eclipse. when the moon passes through the centre of the earth's shadow, the eclipse is both _total_ and _central_. at the time of a total eclipse, the moon is not entirely invisible, but shines with a faint copper-colored light. this light is refracted into the shadow by the earth's atmosphere, and its amount varies with the quantity of clouds and vapor in that portion of the atmosphere which the sunlight must graze in order to reach the moon. the duration of an eclipse varies between very wide limits, being, of course, greatest when the eclipse is central. a total eclipse of the moon may last nearly two hours, or, including the _partial_ portions of the eclipse, three or four hours. every eclipse of the moon, whether total or partial, is visible at the same time to the whole hemisphere of the earth which is turned towards the moon; and the eclipse will have exactly the same magnitude at every point of observation. . _when there will be an eclipse of the sun._--there will be an eclipse of the sun _whenever any portion of the moon's shadow is thrown on the earth_. it will be seen from fig. that this can occur only when the moon is in conjunction, or at _new_. it does not occur every month, because, owing to the inclination of the moon's orbit to the ecliptic, the moon's shadow is usually thrown either above or below the earth at the time of new moon. there can be an eclipse of the sun only when new moon occurs at or near one of the nodes of her orbit. . _solar ecliptic limits._--the distances from the moon's node within which a new moon would throw some portion of its shadow on the earth so as to produce an eclipse of the sun are called the _solar ecliptic limits_. as in the case of the moon, there are _major_ and _minor_ ecliptic limits; the former being the limits within which an eclipse of the sun is _possible_ under some circumstances, and the latter those under which an eclipse is _inevitable_ under all circumstances. the limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. this will be evident from an examination of fig. . were the moon in that figure just outside of the lines _ab_ and _cd_, it will be seen that the penumbra of her shadow would just graze the earth: hence the moon must be somewhere within the space bounded by these lines in order to cause an eclipse of the sun. now, these lines mark the prolongation to the sun of the cone of the umbra of the earth's shadow: hence, in order to produce an eclipse of the sun, new moon must occur somewhere within this prolongation of the umbra of the earth's shadow. now, it is evident that the diameter of this cone is greater on the side of the earth toward the sun than on the opposite side: hence the solar ecliptic limits are greater than the lunar ecliptic limits. . _solar eclipses._--an observer in the umbra of the moon's shadow would see a _total_ eclipse of the sun, while one in the penumbra would see only a _partial_ eclipse. the magnitude of this partial eclipse would depend upon the distance of the observer from the umbra of the moon's shadow. [illustration: fig. .] [illustration: fig. .] the umbra of the moon's shadow is just about long enough to reach the earth. sometimes the point of this shadow falls short of the earth's surface, as shown in fig. , and sometimes it falls upon the earth, as shown in fig. , according to the varying distance of the sun and moon from the earth. the diameter of the umbra at the surface of the earth is seldom more than a hundred miles: hence the belt of a total eclipse is, on the average, not more than a hundred miles wide; and a total eclipse seldom lasts more than five or six minutes, and sometimes only a few seconds. owing, however, to the rotation of the earth, the umbra of the moon's shadow may pass over a long reach of the earth's surface. fig. shows the track of the umbra of the moon's shadow over the earth in the total eclipse of . [illustration: fig. .] [illustration: fig. .] fig. shows the track of the total eclipse of across india and the adjacent seas. [illustration: fig. .] [illustration: fig. .] in a partial eclipse of the sun, more or less of one side of the sun's disk is usually concealed, as shown in fig. . occasionally, however, the centre of the sun's disk is covered, leaving a bright ring around the margin, as shown in fig. . such an eclipse is called an _annular_ eclipse. an eclipse can be annular only when the cone of the moon's shadow is too short to reach the earth, and then only to observers who are in the central portion of the penumbra. . _comparative frequency of solar and lunar eclipses._--there are more eclipses of the sun in the year than of the moon; and yet, at any one place, eclipses of the moon are more frequent than those of the sun. there are more lunar than solar eclipses, because, as we have seen, the limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. there are more eclipses of the moon visible at any one place than of the sun; because, as we have seen, an eclipse of the moon, whenever it does occur, is visible to a whole hemisphere at a time, while an eclipse of the sun is visible to only a portion of a hemisphere, and a total eclipse to only a very small portion of a hemisphere. a total eclipse of the sun is, therefore, a very rare occurrence at any one place. the greatest number of eclipses that can occur in a year is seven, and the least number, two. in the former case, five may be of the sun and two of the moon, or four of the sun and three of the moon. in the latter case, both must be of the sun. vi. the three groups of planets. i. general characteristics of the groups. . _the inner group._--the _inner group_ of planets is composed of _mercury_, _venus_, the _earth_, and _mars_; that is, of all the planets which lie between the asteroids and the sun. the planets of this group are comparatively small and dense. so far as known, they rotate on their axes in about twenty-four hours, and they are either entirely without moons, or are attended by comparatively few. the comparative sizes and eccentricities of the orbits of this group are shown in fig. . the dots round the orbits show the position of the planets at intervals of ten days. [illustration: fig. .] . _the outer group._--the _outer group_ of planets is composed of _jupiter_, _saturn_, _uranus_, and _neptune_. these planets are all very large and of slight density. so far as known, they rotate on their axes in about ten hours, and are accompanied with complicated systems of moons. fig. , which represents the comparative sizes of the planets, shows at a glance the immense difference between those of the inner and outer group. fig. shows the comparative sizes and eccentricities of the orbits of the outer planets. the dots round the orbits show the position of the planets at intervals of a thousand days. [illustration: fig. .] [illustration: fig. .] . _the asteroids._--between the inner and outer groups of planets there is a great number of very small planets known as the _minor planets_, or _asteroids_. over two hundred planets belonging to this group have already been discovered. their orbits are shown by the dotted lines in fig. . the sizes of the four largest of these planets, compared with the earth, are shown in fig. . [illustration: fig. .] the asteroids of this group are distinguished from the other planets, not only by their small size, but by the great eccentricities and inclinations of their orbits. if we except mercury, none of the larger planets has an eccentricity amounting to one-tenth the diameter of its orbit ( ), nor is any orbit inclined more than two or three degrees to the ecliptic; but the inclinations of many of the minor planets exceed ten degrees, and the eccentricities frequently amount to an eighth of the orbital diameter. the orbit of pallas is inclined thirty-four degrees to the ecliptic, while there are some planets of this group whose orbits nearly coincide with the plane of the ecliptic. [illustration: fig. .] fig. shows one of the most and one of the least eccentric of the orbits of this group as compared with that of the earth. [illustration: fig. .] the intricate complexity of the orbits of the asteroids is shown in fig. . ii. the inner group of planets. mercury. . _the orbit of mercury._--the orbit of mercury is more eccentric than that of any of the larger planets, and it has also a greater inclination to the ecliptic. its eccentricity ( ) is a little over a fifth, and its inclination to the ecliptic somewhat over seven degrees. the mean distance of mercury from the sun is about thirty-five million miles; but, owing to the great eccentricity of its orbit, its distance from the sun varies from about forty-three million miles at aphelion to about twenty-eight million at perihelion. [illustration: fig. .] . _distance of mercury from the earth._--it is evident, from fig. , that an inferior planet, like mercury, is the whole diameter of its orbit nearer the earth at inferior conjunction than at superior conjunction: hence mercury's distance from the earth varies considerably. owing to the great eccentricity of its orbit, its distance from the earth at inferior conjunction also varies considerably. mercury is nearest to the earth when its inferior conjunction occurs at its own aphelion and at the earth's perihelion. [illustration: fig. .] . _apparent size of mercury._--since mercury's distance from the earth is variable, the apparent size of the planet is also variable. fig. shows its apparent size at its extreme and mean distances from the earth. its apparent diameter varies from five seconds to twelve seconds. [illustration: fig. .] . _volume and density of mercury._--the real diameter of mercury is about three thousand miles. its size, compared with that of the earth, is shown in fig. . the earth is about sixteen times as large as mercury; but mercury is about one-fifth more dense than the earth. . _greatest elongation of mercury._--mercury, being an _inferior_ planet (or one within the orbit of the earth), appears to oscillate to and fro across the sun. its greatest apparent distance from the sun, or its _greatest elongation_, varies considerably. the farther mercury is from the sun, and the nearer the earth is to mercury, the greater is its angular distance from the sun at the time of its greatest elongation. under the most favorable circumstances, the greatest elongation amounts to about twenty-eight degrees, and under the least favorable to only sixteen or seventeen degrees. . _sidereal and synodical periods of mercury._--mercury accomplishes a complete revolution around the sun in about eighty-eight days; but it takes it a hundred and sixteen days to pass from its greatest elongation east to the same elongation again. the orbital motion of this planet is at the rate of nearly thirty miles a second. in fig. , _p'''_ represents elongation east of the sun, and _p'_ elongation west. it will be seen that it is much farther from _p'_ around to _p'''_ than from _p'''_ on to _p'_. mercury is only about forty-eight days in passing from greatest elongation east to greatest elongation west, while it is about sixty-eight days in passing back again. . _visibility of mercury._--mercury is too close to the sun for favorable observation. it is never seen long after sunset, or long before sunrise, and never far from the horizon. when visible at all, it must be sought for low down in the west shortly after sunset, or low in the east shortly before sunrise, according as the planet is at its east or west elongation. it is often visible to the naked eye in our latitude; but the illumination of the twilight sky, and the excess of vapor in our atmosphere near the horizon, combine to make the telescopic study of the planet difficult and unsatisfactory. [illustration: fig. .] . _the atmosphere and surface of mercury._--mercury seems to be surrounded by a dense atmosphere. one proof of the existence of such an atmosphere is furnished at the time of the planet's _transit_ across the disk of the sun, which occasionally happens. the planet is then seen surrounded by a border, as shown in fig. . a bright spot has also been observed on the dark disk of the planet during a transit, as shown in fig. . the border around the planet seems to be due to the action of the planet's atmosphere; but it is difficult to account for the bright spot. [illustration: fig. .] [illustration: fig. .] schröter, a celebrated german astronomer, at about the beginning of the present century, thought that he detected spots and shadings on the disk of the planet, which indicated both the presence of an atmosphere and of elevations. the shading along the terminator, which seemed to indicate the presence of a twilight, and therefore of an atmosphere, are shown in fig. . it also shows the blunted aspect of one of the cusps, which schröter noticed at times, and which he attributed to the shadow of a mountain, estimated to be ten or twelve miles high. fig. shows this mountain near the upper cusp, as schröter believed he saw it in the year . by watching certain marks upon the disk of mercury, schröter came to the conclusion that the planet rotates on its axis in about twenty-four hours. modern observers, with more powerful telescopes, have failed to verify schröter's observations as to the indications of an atmosphere and of elevations. nothing is known with certainty about the rotation of the planet. [illustration: fig. .] the border around mercury, and the bright spot on its disk at the time of the transit of the planet across the sun, have been seen since schröter's time, and the existence of these phenomena is now well established; but astronomers are far from being agreed as to their cause. . _intra-mercurial planets._--it has for some time been thought probable that there is a group of small planets between mercury and the sun; and at various times the discovery of such bodies has been announced. in a french observer believed that he had detected an intra-mercurial planet, to which the name of _vulcan_ was given, and for which careful search has since been made, but without success. during the total eclipse of professor watson observed two objects near the sun, which he thought to be planets; but this is still matter of controversy. venus. . _the orbit of venus._--the orbit of venus has but slight eccentricity, differing less from a circle than that of any other large planet. it is inclined to the ecliptic somewhat more than three degrees. the mean distance of the planet from the sun is about sixty-seven million miles. . _distance of venus from the earth._--the distance of venus from the earth varies within much wider limits than that of mercury. when venus is at inferior conjunction, her distance from the earth is ninety-two million miles _minus_ sixty-seven million miles, or twenty-five million miles; and when at superior conjunction it is ninety-two million miles _plus_ sixty-seven million miles, or a hundred and fifty-nine million miles. venus is considerably more than _six times_ as far off at superior conjunction as at inferior conjunction. [illustration: fig. .] . _apparent size of venus._--owing to the great variation in the distance of venus from the earth, her apparent diameter varies from about ten seconds to about sixty-six seconds. fig. shows the apparent size of venus at her extreme and mean distances from the earth. . _volume and density of venus._--the real size of venus is about the same as that of the earth, her diameter being only about three hundred miles less. the comparative sizes of the two planets are shown in fig. . the density of venus is a little less than that of the earth. [illustration: fig. .] . _the greatest elongation of venus._--venus, like mercury, appears to oscillate to and fro across the sun. the angular value of the greatest elongation of venus varies but slightly, its greatest value being about forty-seven degrees. . _sidereal and synodical periods of venus._--the _sidereal_ period of venus, or that of a complete revolution around the sun, is about two hundred and twenty-five days; her orbital motion being at the rate of nearly twenty-two miles a second. her _synodical_ period, or the time it takes her to pass around from her greatest eastern elongation to the same elongation again, is about five hundred and eighty-four days, or eighteen months. venus is a hundred and forty-six days, or nearly five months, in passing from her greatest elongation east through inferior conjunction to her greatest elongation west. . _venus as a morning and an evening star._--for a period of about nine months, while venus is passing from superior conjunction to her greatest eastern elongation, she will be east of the sun, and will therefore set after the sun. during this period she is the _evening star_, the _hesperus_ of the ancients. while passing from inferior conjunction to superior conjunction, venus is west of the sun, and therefore rises before the sun. during this period of nine months she is the _morning star_, the _phosphorus_, or _lucifer_, of the ancients. . _brilliancy of venus._--next to the sun and moon, venus is at times the most brilliant object in the heavens, being bright enough to be seen in daylight, and to cast a distinct shadow at night. her brightness, however, varies considerably, owing to her phases and to her varying distance from the earth. she does not appear brightest when at full, for she is then farthest from the earth, at superior conjunction; nor does she appear brightest when nearest the earth, at inferior conjunction, for her phase is then a thin crescent (see fig. ). she is most conspicuous while passing from her greatest eastern elongation to her greatest western elongation. after she has passed her eastern elongation, she becomes brighter and brighter till she is within about forty degrees of the sun. her phase at this point in her orbit is shown in fig. . her brilliancy then begins to wane, until she comes too near the sun to be visible. when she re-appears on the west of the sun, she again becomes more brilliant; and her brilliancy increases till she is about forty degrees from the sun, when she is again at her brightest. venus passes from her greatest brilliancy as an evening star to her greatest brilliancy as a morning star in about seventy-three days. she has the same phase, and is at the same distance from the earth, in both cases of maximum brilliancy. of course, the brilliancy of venus when at the maximum varies somewhat from time to time, owing to the eccentricities of the orbits of the earth and of venus, which cause her distance from the earth, at her phase of greatest brilliancy, to vary. she is most brilliant when the phase of her greatest brilliancy occurs when she is at her aphelion and the earth at its perihelion. [illustration: fig. .] . _the atmosphere and surface of venus._--schröter believed that he saw shadings and markings on venus similar to those on mercury, indicating the presence of an atmosphere and of elevations on the surface of the planet. fig. represents the surface of venus as it appeared to this astronomer. by watching certain markings on the disk of venus, schröter came to the conclusion that venus rotates on her axis in about twenty-four hours. [illustration: fig. .] it is now generally conceded that venus has a dense atmosphere; but schröter's observations of the spots on her disk have not been verified by modern astronomers, and we really know nothing certainly of her rotation. . _transits of venus._--when venus happens to be near one of the nodes of her orbit when she is in inferior conjunction, she makes a transit across the sun's disk. these transits occur in pairs, separated by an interval of over a hundred years. the two transits of each pair are separated by an interval of eight years, the dates of the most recent being and . venus, like mercury, appears surrounded with a border on passing across the sun's disk, as shown in fig. . [illustration: fig. .] mars. . _the orbit of mars._--the orbit of mars is more eccentric than that of any of the larger planets, except mercury; its eccentricity being about one-eleventh. the inclination of the orbit to the ecliptic is somewhat under two degrees. the mean distance of mars from the sun is about a hundred and forty million miles; but, owing to the eccentricity of his orbit, the distance varies from a hundred and fifty-three million miles to a hundred and twenty-seven million miles. [illustration: fig. .] . _distance of mars from the earth._--it will be seen, from fig. , that a _superior_ planet (or one outside the orbit of the earth), like mars, is nearer the earth, by the whole diameter of the earth's orbit, when in opposition than when in conjunction. the mean distance of mars from the earth, at the time of opposition, is a hundred and forty million miles _minus_ ninety-two million miles, or forty-eight million miles. owing to the eccentricity of the orbit of the earth and of mars, the distance of this planet when in opposition varies considerably. when the earth is in aphelion, and mars in perihelion, at the time of opposition, the distance of the planet from the earth is only about thirty-three million miles. on the other hand, when the earth is in perihelion, and mars in aphelion, at the time of opposition, the distance of the planet is over sixty-two million miles. the mean distance of mars from the earth when in conjunction is a hundred and forty million miles _plus_ ninety-two million miles, or two hundred and thirty-two million miles. it will therefore be seen that mars is nearly five times as far off at conjunction as at opposition. [illustration: fig. .] . _the apparent size of mars._--owing to the varying distance of mars from the earth, the apparent size of the planet varies almost as much as that of venus. fig. shows the apparent size of mars at its extreme and mean distances from the earth. the apparent diameter varies from about four seconds to about thirty seconds. [illustration: fig. .] . _the volume and density of mars._--among the larger planets mars is next in size to mercury. its real diameter is somewhat more than four thousand miles, and its bulk is about one-seventh of that of the earth. its size, compared with that of the earth, is shown in fig. . [illustration: plate .] the density of mars is only about three-fourths of that of the earth. . _sidereal and synodical periods of mars._--the _sidereal_ period of mars, or the time in which he makes a complete revolution around the sun, is about six hundred and eighty-seven days, or nearly twenty-three months; but he is about seven hundred and eighty days in passing from opposition to opposition again, or in performing a _synodical_ revolution. mars moves in his orbit at the rate of about fifteen miles a second. . _brilliancy of mars._--when near his opposition, mars is easily recognized with the naked eye by his fiery-red light. he is much more brilliant at some oppositions than at others, for reasons already explained ( ), but always shines brighter than an ordinary star of the first magnitude. . _telescopic appearance of mars._--when viewed with a good telescope (see plate iv.), mars is seen to be covered with dusky, dull-red patches, which are supposed to be continents, like those of our own globe. other portions, of a greenish hue, are believed to be tracts of water. the ruddy color, which overpowers the green, and makes the whole planet seem red to the naked eye, was believed by sir j. herschel to be due to an ochrey tinge in the general soil, like that of the red sandstone districts on the earth. in a telescope, mars appears less red, and the higher the power the less the intensity of the color. the disk, when well seen, is mapped out in a way which gives at once the impression of land and water. the bright part is red inclining to orange, sometimes dotted with brown and greenish points. the darker spaces, which vary greatly in depth of tone, are of a dull gray-green, having the aspect of a fluid which absorbs the solar rays. the proportion of land to water on the earth appears to be reversed on mars. on the earth every continent is an island; on mars all seas are lakes. long, narrow straits are more common than on the earth; and wide expanses of water, like our atlantic ocean, are rare. (see fig. .) [illustration: fig. .] [illustration: fig. .] fig. represents a chart of the surface of mars, which has been constructed from careful telescopic observation. the outlines, as seen in the telescope, are, however, much less distinct than they are represented here; and it is by no means certain that the light and dark portions are bodies of land and water. in the vicinity of the poles brilliant white spots may be noticed, which are considered by many astronomers to be masses of snow. this conjecture is favored by the fact that they appear to diminish under the sun's influence at the beginning of the martial summer, and to increase again on the approach of winter. . _rotation of mars._--on watching mars with a telescope, the spots on the disk are found to move (as shown in fig. ) in a manner which indicates that the planet rotates in about twenty-four hours on an axis inclined about twenty-eight degrees from a perpendicular to the plane of its orbit. the inclination of the axis is shown in fig. . it is evident from the figure that the variation in the length of day and night, and the change of seasons, are about the same on mars as on the earth. the changes will, of course, be somewhat greater, and the seasons will be about twice as long. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the satellites of mars._--in professor hall of the washington observatory discovered that mars is accompanied by two small moons, whose orbits are shown in fig. . the inner satellite has been named _phobos_, and the outer one _deimos_. it is estimated that the diameter of the outer moon is from five to ten miles, and that of the inner one from ten to forty miles. phobos is remarkable for its nearness to the planet and the rapidity of its revolution, which is performed in seven hours thirty-eight minutes. its distance from the centre of the planet is about six thousand miles, and from the surface less than four thousand. astronomers on mars, with telescopes and eyes like ours, could readily find out whether this satellite is inhabited, the distance being less than one-sixtieth of that of our moon. it will be seen that phobos makes about three revolutions to one rotation of the planet. it will, of course, rise in the west; though the sun, the stars, and the other satellite rise in the east. deimos makes a complete revolution in about thirty hours. iii. the asteroids. . _bode's law of planetary distances._--there is a very remarkable law connecting the distances of the planets from the sun, which is generally known by the name of _bode's law_. attention was drawn to it in by the astronomer bode, but he was not really its author. to express this law we write the following series of numbers:-- , , , , , , ; each number, with the exception of the first, being double the one which precedes it. if we add to each of these numbers, the series becomes-- , , , , , , ; which series was known to kepler. these numbers, with the exception of , are sensibly proportional to the distances of the principal planets from the sun, the actual distances being as follows:-- mercury. venus. earth. mars. ---- jupiter. saturn. · · · · · . _the first discovery of the asteroids._--the great gap between mars and jupiter led astronomers, from the time of kepler, to suspect the existence of an unknown planet in this region; but no such planet was discovered till the beginning of the present century. _ceres_ was discovered jan. , , _pallas_ in , _juno_ in , and _vesta_ in . then followed a long interval of thirty-eight years before _astræa_, the fifth of these minor planets, was discovered in . . _olbers's hypothesis._--after the discovery of pallas, olbers suggested his celebrated hypothesis, that the two bodies might be fragments of a single planet which had been shattered by some explosion. if such were the case, the orbits of all the fragments would at first intersect each other at the point where the explosion occurred. he therefore thought it likely that other fragments would be found, especially if a search were kept up near the intersection of the orbits of ceres and pallas. professor newcomb makes the following observations concerning this hypothesis:-- "the question whether these bodies could ever have formed a single one has now become one of cosmogony rather than of astronomy. if a planet were shattered, the orbit of each fragment would at first pass through the point at which the explosion occurred, however widely they might be separated through the rest of their course; but, owing to the secular changes produced by the attractions of the other planets, this coincidence would not continue. the orbits would slowly move away, and after the lapse of a few thousand years no trace of a common intersection would be seen. it is therefore curious that olbers and his contemporaries should have expected to find such a region of intersection, as it implied that the explosion had occurred within a few thousand years. the fact that the required conditions were not fulfilled was no argument against the hypothesis, because the explosion might have occurred millions of years ago; and in the mean time the perihelion and node of each orbit would have made many entire revolutions, so that the orbits would have been completely mixed up.... a different explanation of the group is given by the nebular hypothesis; so that olbers's hypothesis is no longer considered by astronomers." . _later discoveries of asteroids._--since over two hundred asteroids have been discovered. all these are so small, that it requires a very good telescope to see them; and even in very powerful telescopes they appear as mere points of light, which can be distinguished from the stars only by their motions. to facilitate the discovery of these bodies, very accurate maps have been constructed, including all the stars down to the thirteenth magnitude in the neighborhood of the ecliptic. a reduced copy of one of these maps is shown in fig. . [illustration: fig. .] furnished with a map of this kind, and with a telescope powerful enough to show all the stars marked on it, the observer who is searching for these small planets will place in the field of view of his telescope six spider-lines at right angles to each other, and at equal distances apart, in such a manner that several small squares will be formed, embracing just as much of the heavens as do those shown in the map. he will then direct his telescope to the region of the sky he wishes to examine, represented by the map, so as to be able to compare successively each square with the corresponding portion of the sky. fig. shows at the right hand the squares in the telescopic field of view, and at the left hand the corresponding squares of the map. [illustration: fig. .] he can then assure himself if the numbers and positions of the stars mapped, and of the stars observed, are identical. if he observes in the field of view a luminous point which is not marked in the map, it is evident that either the new body is a star of variable brightness which was not visible at the time the map was made, or it is a planet, or perhaps a comet. if the new body remains fixed at the same point, it is the former; but, if it changes its position with regard to the neighboring stars, it is the latter. the motion is generally so sensible, that in the course of one evening the change of position may be detected; and it can soon be determined, by the direction and rate of the motion, whether the body is a planet or a comet. iv. outer group of planets. jupiter. . _orbit of jupiter._--the orbit of jupiter is inclined only a little over one degree to the ecliptic; and its eccentricity is only about half of that of mars, being less than one-twentieth. the mean distance of jupiter from the sun is about four hundred and eighty million miles; but, owing to the eccentricity of his orbit, his actual distance from the sun ranges from four hundred and fifty-seven to five hundred and three million miles. . _distance of jupiter from the earth._--when jupiter is in opposition, his mean distance from the earth is four hundred and eighty million miles _minus_ ninety-two million miles, or three hundred and eighty-eight million miles, and, when he is in conjunction, four hundred and eighty million miles _plus_ ninety-two million miles, or five hundred and seventy-two million miles. it will be seen that he is less than twice as far off in conjunction as in opposition, and that the ratio of his greatest to his least distance is very much less than in the case of venus and mars. this is owing to his very much greater distance from the sun. owing to the eccentricities of the orbits of the earth and of jupiter, the greatest and least distances of jupiter from the earth vary somewhat from year to year. [illustration: fig. .] . _the brightness and apparent size of jupiter._--the apparent diameter of jupiter varies from about fifty seconds to about thirty seconds. his apparent size at his extreme and mean distances from the earth is shown in fig. . jupiter shines with a brilliant white light, which exceeds that of every other planet except venus. the planet is, of course, brightest when near opposition. . _the volume and density of jupiter._--jupiter is the "giant planet" of our system, his mass largely exceeding that of all the other planets combined. his mean diameter is about eighty-five thousand miles; but the equatorial exceeds the polar diameter by five thousand miles. in volume he exceeds our earth about thirteen hundred times, but in mass only about two hundred and thirteen times. his specific gravity is, therefore, far less than that of the earth, and even less than that of water. the comparative size of jupiter and the earth is shown in fig. . [illustration: fig. .] . _the sidereal and synodical periods of jupiter._--it takes jupiter nearly twelve years to make a _sidereal_ revolution, or a complete revolution around the sun, his orbital motion being at the rate of about eight miles a second. his _synodical_ period, or the time of his passage from opposition to opposition again, is three hundred and ninety-eight days. . _the telescopic aspect of jupiter._--there are no really permanent markings on the disk of jupiter; but his surface presents a very diversified appearance. the earlier telescopic observers descried dark belts across it, one north of the equator, and the other south of it. with the increase of telescopic power, it was seen that these bands were of a more complex structure than had been supposed, and consisted of stratified, cloud-like appearances, varying greatly in form and number. these change so rapidly, that the face of the planet rarely presents the same appearance on two successive nights. they are most strongly marked at some distance on each side of the planet's equator, and thus appear as two belts under a low magnifying power. both the outlines of the belts, and the color of portions of the planet, are subject to considerable changes. the equatorial regions, and the spaces between the belts generally, are often of a rosy tinge. this color is sometimes strongly marked, while at other times hardly a trace of it can be seen. a general telescopic view of jupiter is given in plate v. [illustration: plate .] . _the physical constitution of jupiter._--from the changeability of the belts, and of nearly all the visible features of jupiter, it is clear that what we see on that planet is not the solid nucleus, but cloud-like formations, which cover the entire surface to a great depth. the planet appears to be covered with a deep and dense atmosphere, filled with thick masses of clouds and vapor. until recently this cloud-laden atmosphere was supposed to be somewhat like that of our globe; but at present the physical constitution of jupiter is believed to resemble that of the sun rather than that of the earth. like the sun, he is brighter in the centre than near the edges, as is shown in the transits of the satellites over his disk. when the satellite first enters on the disk, it commonly seems like a bright spot on a dark background; but, as it approaches the centre, it appears like a dark spot on the bright surface of the planet. the centre is probably two or three times brighter than the edges. this may be, as in the case of the sun, because the light near the edge passes through a greater depth of atmosphere, and is diminished by absorption. it has also been suspected that jupiter shines partly by his own light, and not wholly by reflected sunlight. the planet cannot, however, emit any great amount of light; for, if it did, the satellites would shine by this light when they are in the shadow of the planet, whereas they totally disappear. it is possible that the brighter portions of the surface are from time to time slightly self-luminous. [illustration: fig. .] again: the interior of jupiter seems to be the seat of an activity so enormous that it can be ascribed only to intense heat. rapid movements are always occurring on his surface, often changing its aspect in a few hours. it is therefore probable that jupiter is not yet covered by a solid crust, and that the fiery interior, whether liquid or gaseous, is surrounded by the dense vapors which cease to be luminous on rising into the higher and cooler regions of the atmosphere. figs. and show the disk of jupiter as it appeared in december, . [illustration: fig. .] . _rotation of jupiter_.--spots are sometimes visible which are much more permanent than the ordinary markings on the belts. the most remarkable of these is "the great red spot," which was first observed in july, , and is still to be seen in february, . it is shown just above the centre of the disk in fig. . by watching these spots from day to day, the time of jupiter's axial rotation has been found to be about nine hours and fifty minutes. the axis of jupiter deviates but slightly from a perpendicular to the plane of its orbit, as is shown in fig. . [illustration: fig. .] the satellites of jupiter. [illustration: fig. .] . _jupiter's four moons._--jupiter is accompanied by four moons, as shown in fig. . the diameters of these moons range from about twenty-two hundred to thirty-seven hundred miles. the second from the planet is the smallest, and the third the largest. the smallest is about the size of our moon; the largest considerably exceeds mercury, and almost rivals mars, in bulk. the sizes of these moons, compared with those of the earth and its moon, are shown in fig. . [illustration: fig. .] the names of these satellites, in the order of their distance from the planet, are _io_, _europa_, _ganymede_, and _callisto_. their times of revolution range from about a day and three-fourths up to about sixteen days and a half. their orbits are shown in fig. . [illustration: fig. .] . _the variability of jupiter's satellites._--remarkable variations in the light of these moons have led to the supposition that violent changes are taking place on their surfaces. it was formerly believed, that, like our moon, they always present the same face to the planet, and that the changes in their brilliancy are due to differences in the luminosity of parts of their surface which are successively turned towards us during a revolution; but careful measurements of their light show that this hypothesis does not account for the changes, which are sometimes very sudden. the satellites are too distant for examination of their surfaces with the telescope: hence it is impossible to give any certain explanation of these phenomena. [illustration: fig. .] . _eclipses of jupiter's satellites._--jupiter, like the earth, casts a shadow away from the sun, as shown in fig. ; and, whenever one of his moons passes into this shadow, it becomes eclipsed. on the other hand, whenever one of the moons throws its shadow on jupiter, the sun is eclipsed to that part of the planet which lies within the shadow. to the inhabitants of jupiter (if there are any, and if they can see through the clouds) these eclipses must be very familiar affairs; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator, and the small inclination of the latter to the plane of jupiter's orbit, all the satellites, except the most distant one, are eclipsed in every revolution. a spectator on jupiter might therefore witness during the planetary year forty-five hundred eclipses of the moons, and about the same number of the sun. [illustration: fig. .] . _transits of jupiter's satellites._--whenever one of jupiter's moons passes in front of the planet, it is said to make a _transit_ across his disk. when a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from fig. : hence it is usually seen as a bright spot on the planet's disk; though sometimes, on the brighter central portions of the disk, it appears dark. [illustration: fig. .] it will be seen from fig. that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. the shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. fig. shows two moons of jupiter in transit. . _occultations of jupiter's satellites._--the eclipse of a moon of jupiter must be carefully distinguished from the _occultation_ of a moon by the planet. in the case of an eclipse, the moon ceases to be visible, because the mass of jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of jupiter is interposed between it and the earth, thus rendering the moon invisible to us. the third satellite, _m''_ (fig. ), is invisible from the earth _e_, having become _occulted_ when it passed behind the planet's disk; but it will not be _eclipsed_ until it passes into the shadow of jupiter. . _jupiter without satellites._--it occasionally happens that every one of jupiter's satellites will disappear at the same time, either by being eclipsed or occulted, or by being in transit. in this event, jupiter will appear without satellites. this occurred on the st of august, . the position of jupiter's satellites at this time is shown in fig. . [illustration: fig. .] saturn. the planet and his moons. . _the orbit of saturn._--the orbit of saturn is rather more eccentric than that of jupiter, its eccentricity being somewhat more than one-twentieth. its inclination to the ecliptic is about two degrees and a half. the mean distance of saturn from the sun is about eight hundred and eighty million miles. it is about a hundred million miles nearer the sun at perihelion than at aphelion. . _distance of saturn from the earth._--the mean distance of saturn from the earth at opposition is eight hundred and eighty million miles _minus_ ninety-two million miles, or seven hundred and eighty-eight million; and at conjunction, eight hundred and eighty million miles _plus_ ninety-two million, or nine hundred and seventy-two million. owing to the eccentricity of the orbit of saturn, his distance from the earth at opposition and at conjunction varies by about a hundred million miles at different times; but he is so immensely far away, that this is only a small fraction of his mean distance. . _apparent size and brightness of saturn._--the apparent diameter of saturn varies from about twenty seconds to about fourteen seconds. his apparent size at his extreme and mean distances from the earth is shown in fig. . [illustration: fig. .] the planet generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere. . _volume and density of saturn._--the real diameter of saturn is about seventy thousand miles, and its volume over seven hundred times that of the earth. the comparative size of the earth and saturn is shown in fig. . this planet is a little more than half as dense as jupiter. [illustration: fig. .] . _the sidereal and synodical periods of saturn._--saturn makes a complete revolution round the sun in a period of about twenty-nine years and a half, moving in his orbit at the rate of about six miles a second. the planet passes from opposition to opposition again in a period of three hundred and seventy-eight days, or thirteen days over a year. . _physical constitution of saturn._--the physical constitution of saturn seems to resemble that of jupiter; but, being twice as far away, the planet cannot be so well studied. the farther an object is from the sun, the less it is illuminated; and, the farther it is from the earth, the smaller it appears: hence there is a double difficulty in examining the more distant planets. under favorable circumstances, the surface of saturn is seen to be diversified with very faint markings; and, with high telescopic powers, two or more very faint streaks, or belts, may be discerned parallel to its equator. these belts, like those of jupiter, change their aspect from time to time; but they are so faint that the changes cannot be easily followed. it is only on rare occasions that the time of rotation can be determined from a study of the markings. . _rotation of saturn._--on the evening of dec. , , professor hall, who had been observing the satellites of saturn with the great washington telescope ( ), saw a brilliant white spot near the equator of the planet. it seemed as if an immense eruption of incandescent matter had suddenly burst up from the interior. the spot gradually spread itself out into a long light streak, of which the brightest point was near the western end. it remained visible until january, when it became faint and ill-defined, and the planet was lost in the rays of the sun. from all the observations on this spot, professor hall found the period of saturn to be ten hours fourteen minutes, reckoning by the brightest part of the streak. had the middle of the streak been taken, the time would have been less, because the bright matter seemed to be carried along in the direction of the planet's rotation. if this motion was due to a wind, the velocity of the current must have been between fifty and a hundred miles an hour. the axis of saturn is inclined twenty-seven degrees from the perpendicular to its orbit. [illustration: fig. .] . _the satellites of saturn._--saturn is accompanied by eight moons. seven of these are shown in fig. . the names of these satellites, in the order of their distances from the planet, are given in the accompanying table:-- number. name. distance sidereal discoverer. from period. planet mimas , . herschel enceladus , . herschel tethys , . cassini dione , . cassini rhea , . cassini titan , . huyghens hyperion , . bond japetus , , . cassini the apparent brightness or visibility of these satellites follows the order of their discovery. the smallest telescope will show titan, and one of very moderate size will show japetus in the western part of its orbit. an instrument of four or five inches aperture will show rhea, and perhaps tethys and dione; while seven or eight inches are required for enceladus, even at its greatest elongation from the planet. mimas can rarely be seen except at its greatest elongation, and then only with an aperture of twelve inches or more. hyperion can be detected only with the most powerful telescopes, on account of its faintness and the difficulty of distinguishing it from minute stars. _japetus_, the outermost satellite, is remarkable for the fact, that while, in one part of its orbit, it is the brightest of the satellites except titan, in the opposite part it is almost as faint as hyperion, and can be seen only in large telescopes. when west of the planet, it is bright; when east of it, faint. this peculiarity has been accounted for by supposing that the satellite, like our moon, always presents the same face to the planet, and that one side of it is white and the other intensely black; but it is doubtful whether any known substance is so black as one side of the satellite must be to account for such extraordinary changes of brilliancy. [illustration: fig. .] _titan_, the largest of these satellites, is about the size of the largest satellite of jupiter. the relative sizes of the satellites are shown in fig. , and their orbits in fig. . [illustration: fig. .] [illustration: fig. .] fig. shows the transit of one of the satellites, and of its shadow, across the disk of the planet. the rings of saturn. . _general appearance of the rings._--saturn is surrounded by a thin flat ring lying in the plane of its equator. this ring is probably less than a hundred miles thick. the part of it nearest saturn reflects little sunlight to us; so that it has a dusky appearance, and is not easily seen, although it is not quite so dark as the sky seen between it and the planet. the outer edge of this dusky portion of the ring is at a distance from saturn of between two and three times the earth's diameter. outside of this dusky part of the ring is a much brighter portion, and outside of this another, which is somewhat fainter, but still so much brighter than the dusky part as to be easily seen. the width of the brighter parts of the ring is over three times the earth's diameter. to distinguish the parts, the outer one is called ring _a_, the middle one ring _b_, and the dusky one ring _c_. between _a_ and _b_ is an apparently open space, nearly two thousand miles wide, which looks like a black line on the ring. other divisions in the ring have been noticed at times; but this is the only one always seen with good telescopes at times when either side of the ring is in view from the earth. the general telescopic appearance of the ring is shown in fig. . [illustration: fig. .] [illustration: fig. .] fig. shows the divisions of the rings as they were seen by bond. . _phases of saturn's ring._--the ring is inclined to the plane of the planet's orbit by an angle of twenty-seven degrees. the general aspect from the earth is nearly the same as from the sun. as the planet revolves around the sun, the axis and plane of the ring keep the same direction in space, just as the axis of the earth and the plane of the equator do. when the planet is in one part of its orbit, we see the upper or northern side of the ring at an inclination of twenty-seven degrees, the greatest angle at which the ring can ever be seen. this phase of the ring is shown in fig. . [illustration: fig. .] when the planet has moved through a quarter of a revolution, the edge of the ring is turned towards the sun and the earth; and, owing to its extreme thinness, it is visible only in the most powerful telescopes as a fine line of light, stretching out on each side of the planet. this phase of the ring is shown in fig. . [illustration: fig. .] all the satellites, except japetus, revolve very nearly in the plane of the ring: consequently, when the edge of the ring is turned towards the earth, the satellites seem to swing from one side of the planet to the other in a straight line, running along the thin edge of the ring like beads on a string. this phase affords the best opportunity of seeing the inner satellites, mimas and enceladus, which at other times are obscured by the brilliancy of the ring. [illustration: fig. .] fig. shows a phase of the ring intermediate between the last two. when the planet has moved ninety degrees farther, we again see the ring at an angle of twenty-seven degrees; but now it is the lower or southern side which is visible. when it has moved ninety degrees farther, the edge of the ring is again turned towards the earth and sun. [illustration: fig. .] the successive phases of saturn's ring during a complete revolution are shown in fig. . it will be seen that there are two opposite points of saturn's orbit in which the rings are turned edgewise to us, and two points half-way between the former in which the ring is seen at its maximum inclination of about twenty-seven degrees. since the planet performs a revolution in twenty-nine years and a half, these phases occur at average intervals of about seven years and four months. [illustration: fig. .] [illustration: fig. .] . _disappearance of saturn's ring._--it will be seen from fig. that the plane of the ring may not be turned towards the sun and the earth at exactly the same time, and also that the earth may sometimes come on one side of the plane of the ring while the sun is shining on the other. in the figure, _e_, _e'_, _e''_, and _e'''_ is the orbit of the earth. when saturn is at _s'_, or opposite, at _f_, the plane of the ring will pass through the sun, and then only the edge of the ring will be illumined. were saturn at _s_, and the earth at _e'_, the plane of the ring would pass through the earth. this would also be the case were the earth at _e'''_, and saturn at _s''_. were saturn at _s_ or at _s''_, and the earth farther to the left or to the right, the sun would be shining on one side of the ring while we should be looking on the other. in all these cases the ring will disappear entirely in a telescope of ordinary power. with very powerful telescopes the ring will appear, in the first two cases, as a thin line of light (fig. ). it will be seen that all these cases of disappearance must take place when saturn is in the parts of his orbit intercepted between the parallel lines _ac_ and _bd_. these lines are tangent to the earth's orbit, which they enclose, and are parallel to the plane of saturn's ring. as saturn passes away from these two lines on either side, the rings appear more and more open. when the dark side of the ring is in view, it appears as a black line crossing the planet; and on such occasions the sunlight reflected from the outer and inner edges of the rings _a_ and _b_ enables us to see traces of the ring on each side of saturn, at least in places where two such reflections come nearly together. fig. illustrates this reflection from the edges at the divisions of the rings. [illustration: fig. .] . _changes in saturn's ring._--the question whether changes are going on in the rings of saturn is still unsettled. some observers have believed that they saw additional divisions in the rings from time to time; but these may have been errors of vision, due partly to the shading which is known to exist on portions of the ring. professor newcomb says, "as seen with the great washington equatorial in the autumn of , there was no great or sudden contrast between the inner or dark edge of the bright ring and the outer edge of the dusky ring. there was some suspicion that the one shaded into the other by insensible gradations. no one could for a moment suppose, as some observers have, that there was a separation between these two rings. all these considerations give rise to the question whether the dusky ring may not be growing at the expense of the inner bright ring." struve, in , advanced the startling theory that the inner edge of the ring was gradually approaching the planet, the whole ring spreading inwards, and making the central opening smaller. the theory was based upon the descriptions and drawings of the rings by the astronomers of the seventeenth century, especially huyghens, and the measures made by later astronomers up to . this supposed change in the dimension of the ring is shown in fig. . [illustration: fig. .] . _constitution of saturn's ring._--the theory now generally held by astronomers is, that the ring is composed of a cloud of satellites too small to be separately seen in the telescope, and too close together to admit of visible intervals between them. the ring looks solid, because its parts are too small and too numerous to be seen singly. they are like the minute drops of water that make up clouds and fogs, which to our eyes seem like solid masses. in the dusky ring the particles may be so scattered that we can see through the cloud, the duskiness being due to the blending of light and darkness. some believe, however, that the duskiness is caused by the darker color of the particles rather than by their being farther apart. uranus. . _orbit and dimensions of uranus._--uranus, the smallest of the outer group of planets, has a diameter of nearly thirty-two thousand miles. it is a little less dense than jupiter, and its mean distance from the sun is about seventeen hundred and seventy millions of miles. its orbit has about the same eccentricity as that of jupiter, and is inclined less than a degree to the ecliptic. uranus makes a revolution around the sun in eighty-four years, moving at the rate of a little over four miles a second. it is visible to the naked eye as a star of the sixth magnitude. as seen in a large telescope, the planet has a decidedly sea-green color; but no markings have with certainty been detected on its disk, so that nothing is really known with regard to its rotation. fig. shows the comparative size of uranus and the earth. [illustration: fig. .] . _discovery of uranus._--this planet was discovered by sir william herschel in march, . he was engaged at the time in examining the small stars of the constellation _gemini_, or the twins. he noticed that this object which had attracted his attention had an appreciable disk, and therefore could not be a star. he also perceived by its motion that it could not be a nebula; he therefore concluded that it was a comet, and announced his discovery as such. on attempting to compute its orbit, it was soon found that its motions could be accounted for only on the supposition that it was moving in a circular orbit at about twice the distance of saturn from the sun. it was therefore recognized as a new planet, whose discovery nearly doubled the dimensions of the solar system as it was then known. . _the name of the planet._--herschel, out of compliment to his patron, george iii., proposed to call the new planet _georgium sidus_ (the georgian star); but this name found little favor. the name of _herschel_ was proposed, and continued in use in england for a time, but did not meet with general approval. various other names were suggested, and finally that of _uranus_ was adopted. [illustration: fig. .] . _the satellites of uranus._--uranus is accompanied by four satellites, whose orbits are shown in fig. . these satellites are remarkable for the great inclination of their orbits to the plane of the planet's orbit, amounting to about eighty degrees, and for their _retrograde_ motion; that is, they move _from east to west_, instead of from west to east, as in the case of all the planets and of all the satellites previously discovered. neptune. . _orbit and dimensions of neptune._--so far as known, neptune is the most remote member of the solar system, its mean distance from the sun being twenty-seven hundred and seventy-five million miles. this distance is considerably less than twice that of uranus. neptune revolves around the sun in a period of a little less than a hundred and sixty-five years. its orbit has but slight eccentricity, and is inclined less than two degrees to the ecliptic. this planet is considerably larger than uranus, its diameter being nearly thirty-five thousand miles. it is somewhat less dense than uranus. neptune is invisible to the naked eye, and no telescope has revealed any markings on its disk: hence nothing is certainly known as to its rotation. fig. shows the comparative size of neptune and the earth. [illustration: fig. .] . _the discovery of neptune._--the discovery of neptune was made in , and is justly regarded as one of the grandest triumphs of astronomy. soon after uranus was discovered, certain irregularities in its motion were observed, which could not be explained. it is well known that the planets are all the while disturbing each other's motions, so that none of them describe perfect ellipses. these mutual disturbances are called _perturbations_. in the case of uranus it was found, that, after making due allowance for the action of all the known planets, there were still certain perturbations in its course which had not been accounted for. this led astronomers to the suspicion that these might be caused by an unknown planet. leverrier in france, and adams in england, independently of each other, set themselves the difficult problem of computing the position and magnitude of a planet which would produce these perturbations. both, by a most laborious computation, showed that the perturbations were such as would be produced by a planet revolving about the sun at about twice the distance of uranus, and having a mass somewhat greater than that of this planet; and both pointed out the same part of the heavens as that in which the planet ought to be found at that time. almost immediately after they had announced the conclusion to which they had arrived, the planet was found with the telescope. the astronomer who was searching for the planet at the suggestion of leverrier was the first to recognize it: hence leverrier has obtained the chief credit of the discovery. the observed planet is proved to be nearer than the one predicted by leverrier and adams, and therefore of smaller magnitude. . _the observed planet not the predicted one._--professor peirce always maintained that the planet found by observation was not the one whose existence had been predicted by leverrier and adams, though its action would completely explain all the irregularities in the motion of uranus. his last statement on this point is as follows: "my position is, that there were _two possible planets_, either of which might have caused the observed irregular motions of uranus. each planet excluded the other; so that, if one was, the other was not. they coincided in direction from the earth at certain epochs, once in six hundred and fifty years. it was at one of these epochs that the prediction was made, and at no other time for six centuries could the prediction of the one planet have revealed the other. the observed planet was not the predicted one." . _bode's law disproved._--the following table gives the distances of the planets according to bode's law, their actual distances, and the error of the law in each case:-- planet. numbers of actual errors. bode. distances. mercury + = . . venus + = . . earth + = . . mars + = . . minor + = to planets jupiter + = . . saturn + = . . uranus + = . . neptune + = . . it will be seen, that, before the discovery of neptune, the agreement was so close as to indicate that this was an actual law of the distances; but the discovery of this planet completely disproved its existence. [illustration: fig. .] . _the satellite of neptune._--neptune is accompanied by at least one moon, whose orbit is shown in fig. . the orbit of this satellite is inclined about thirty degrees to the plane of the ecliptic, and the motion of the satellite is retrograde, or from east to west. vii. comets and meteors. i. comets. general phenomena of comets. . _general appearance of a bright comet._--comets bright enough to be seen with the naked eye are composed of three parts, which run into each other by insensible gradations. these are the _nucleus_, the _coma_, and the _tail_. the _nucleus_ is the bright centre of the comet, and appears to the eye as a star or planet. the _coma_ is a nebulous mass surrounding the nucleus on all sides. close to the nucleus it is almost as bright as the nucleus itself; but it gradually shades off in every direction. the nucleus and coma combined appear like a star shining through a small patch of fog; and these two together form what is called the _head_ of the comet. the _tail_ is a continuation of the coma, and consists of a stream of milky light, growing wider and fainter as it recedes from the head, till the eye is unable to trace it. [illustration: fig. .] the general appearance of one of the smaller of the brilliant comets is shown in fig. . [illustration: fig. .] [illustration: fig. .] . _general appearance of a telescopic comet._--the great majority of comets are too faint to be visible with the naked eye, and are called _telescopic_ comets. in these comets there seems to be a development of coma at the expense of nucleus and tail. in some cases the telescope fails to reveal any nucleus at all in one of these comets; at other times the nucleus is so faint and ill-defined as to be barely distinguishable. fig. shows a telescopic comet without any nucleus at all, and another with a slight condensation at the centre. in these comets it is generally impossible to distinguish the coma from the tail, the latter being either entirely invisible, as in fig. , or else only an elongation of the coma, as shown in fig. . many comets appear simply as patches of foggy light of more or less irregular form. [illustration: fig. .] . _the development of telescopic comets on their approach to the sun._--as a rule, all comets look nearly alike when they first come within the reach of the telescope. they appear at first as little foggy patches, without any tail, and often without any visible nucleus. as they approach the sun their peculiarities are rapidly developed. fig. shows such a comet as first seen, and the gradual development of its nucleus, head, and tail, as it approaches the sun. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] if the comet is only a small one, the tail developed is small; but these small appendages have a great variety of form in different comets. fig. shows the singular form into which _encke's_ comet was developed in . figs. and show other peculiar developments of telescopic comets. . _development of brilliant comets on their approach to the sun._--brilliant comets, as well as telescopic comets, appear nearly alike when they come into the view of the telescope; and it is only on their approach to the sun that their distinctive features are developed. not only do these comets, when they first come into view, resemble each other, but they also bear a close resemblance to telescopic comets. as the comet approaches the sun, bright vaporous jets, two or three in number, are emitted from the nucleus on the side of the sun and in the direction of the sun. these jets, though directed towards the sun, are soon more or less carried backward, as if repelled by the sun. fig. shows a succession of views of these jets as they were developed in the case of _halley's_ comet in . [illustration: fig. .] the jets in this case seemed to have an oscillatory motion. at and they seemed to be attracted towards the sun, and in to be repelled by him. in and they seemed to be again attracted, and in to be repelled, but in a reverse direction to that in . in they appeared to be again attracted. bessel likened this oscillation of the jets to the vibration of a magnetic needle when presented to the pole of a magnet. in the case of larger comets these luminous jets are surrounded by one or more envelops, which are thrown off in succession as the comet approaches the sun. the formation of these envelops was a conspicuous feature of _donati's_ comet of . a rough view of the jets and the surrounding envelops is given in fig. . fig. gives a view of the envelops without the jets. [illustration: fig. .] [illustration: fig. .] . _the tails of comets._--the _tails_ of brilliant comets are rapidly formed as the comet approaches the sun, their increase in length often being at the rate of several million miles a day. these appendages seem to be formed entirely out of the matter which is emitted from the nucleus in the luminous jets which are at first directed towards the sun. the tails of comets are, however, always directed away from the sun, as shown in fig. . [illustration: fig. .] it will be seen that the comet, as it approaches the sun, travels head foremost; but as it leaves the sun it goes tail foremost. the apparent length of the tail of a comet depends partly upon its real length, partly upon the distance of the comet, and partly upon the direction of the axis of the tail with reference to the line of vision. the longer the tail, the nearer the comet; and the more nearly at right angles to the line of vision is the axis of the tail, the greater is the apparent length of the tail. in the majority of cases the tails of comets measure only a few degrees; but, in the case of many comets recorded in history, the tail has extended half way across the heavens. the tail of a comet, when seen at all, is usually several million miles in length; and in some instances the tail is long enough to reach across the orbit of the earth, or twice as far as from the earth to the sun. the tails of comets are apparently hollow, and are sometimes a million of miles in diameter. so great, however, is the tenuity of the matter in them, that the faintest stars are seen through it without any apparent obscuration. see fig. , which is a view of the great comet of . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] the tails of comets are sometimes straight, as in fig. , but usually more or less curved, as in fig. , which is a view of _donati's_ comet as it appeared at one time. the tail of a comet is occasionally divided into a number of streamers, as in figs. and . fig. is a view of the great comet of , and fig. of the great comet of . no. , in fig. , is a view of the comet of ; no. , of the comet of ; and no. , of the comet of . [illustration: fig. .] fig. shows some of the forms which the imagination of a superstitious age saw depicted in comets, when these heavenly visitants were thought to be the forerunners of wars, pestilence, famine, and other dire calamities. . _visibility of comets._--even the brightest comets are visible only a short time near their perihelion passage. when near the sun, they sometimes become very brilliant, and on rare occasions have been visible even at mid-day. it is seldom that a comet can be seen, even with a powerful telescope, during its perihelion passage, unless its perihelion is either inside of the earth's orbit, or but little outside of it. motion and origin of comets. . _recognition of a telescopic comet._--it is impossible to distinguish telescopic comets by their appearance from another class of heavenly bodies known as _nebulæ_. such comets can be recognized only by their motion. thus, in fig. , the upper and lower bodies look exactly alike; but the upper one is found to remain stationary, while the lower one moves across the field of view. the upper one is thus shown to be a nebula, and the lower one a comet. [illustration: fig. .] . _orbits of comets._--all comets are found to move in _very eccentric ellipses_, in _parabolas_, or in _hyperbolas_. since an ellipse is a _closed_ curve ( ), all comets that move in ellipses, no matter how eccentric, are permanent members of the solar system, and will return to the sun at intervals of greater or less length, according to the size of the ellipses and the rate of the comet's motion. parabolas and hyperbolas being _open_ curves ( ), comets that move in either of these orbits are only temporary members of our solar system. after passing the sun, they move off into space, never to return, unless deflected hither by the action of some heavenly body which they pass in their journey. [illustration: fig. .] since a comet is visible only while it is near the sun, it is impossible to tell, by the form of the portion of the orbit which it describes during the period of its visibility, whether it is a part of a very elongated ellipse, a parabola, or a hyperbola. thus in fig. are shown two orbits, one of which is a very elongated ellipse, and the other a parabola. the part _ab_, in each case, is the portion of the orbit described by the comet during its visibility. while describing the dotted portions of the orbit, the comet is invisible. now it is impossible to distinguish the form of the visible portion in the two orbits. the same would be true were one of the orbits a hyperbola. whether a comet will describe an ellipse, a parabola, or a hyperbola, can be determined only by its _velocity_, taken in connection with its _distance from the sun_. were a comet ninety-two and a half million miles from the sun, moving away from the sun at the rate of twenty-six miles a second, it would have just the velocity necessary to describe a _parabola_. were it moving with a greater velocity, it would necessarily describe a _hyperbola_, and, with a less velocity, an _ellipse_. so, at any distance from the sun, there is a certain velocity which would cause a comet to describe a parabola; while a greater velocity would cause it to describe a hyperbola, and a less velocity to describe an ellipse. if the comet is moving in an ellipse, the less its velocity, the less the eccentricity of its orbit: hence, in order to determine the form of the orbit of any comet, it is only necessary to ascertain its distance from the sun, and its velocity at any given time. comets move in every direction in their orbits, and these orbits have every conceivable inclination to the ecliptic. . _periodic comets._--there are quite a number of comets which are known to be _periodic_, returning to the sun at regular intervals in elliptic orbits. some of these have been observed at several returns, so that their period has been determined with great certainty. in the case of others the periodicity is inferred from the fact that the velocity fell so far short of the parabolic limit that the comet must move in an ellipse. the number of known periodic comets is increasing every year, three having been added to the list in . the velocity of most comets is so near the parabolic limit that it is not possible to decide, from observations, whether it falls short of it, or exceeds it. in the case of a few comets the observations indicate a minute excess of velocity; but this cannot be confidently asserted. it is not, therefore, absolutely certain that any known comet revolves in a hyperbolic orbit; and thus it is possible that all comets belong to our system, and will ultimately return to it. it is, however, certain, that, in the majority of cases, the return will be delayed for many centuries, and perhaps for many thousand years. . _origin of comets._--it is now generally believed that the original home of the comets is in the stellar spaces outside of our solar system, and that they are drawn towards the sun, one by one, in the long lapse of ages. were the sun unaccompanied by planets, or were the planets immovable, a comet thus drawn in would whirl around the sun in a parabolic orbit, and leave it again never to return, unless its path were again deflected by its approach to some star. but, when a comet is moving in a parabola, the slightest _retardation_ would change its orbit to an ellipse, and the slightest _acceleration_ into a hyperbola. owing to the motion of the several planets in their orbits, the velocity of a comet would be changed on passing each of them. whether its velocity would be accelerated or retarded, would depend upon the way in which it passed. were the comet accelerated by the action of the planets, on its passage through our system, more than it was retarded by them, it would leave the system with a more than parabolic orbit, and would therefore move in a hyperbola. were it, on the contrary, retarded more than accelerated by the action of the planets, its velocity would be reduced, so that the comet would move in a more or less elongated ellipse, and thus become a permanent member of the solar system. in the majority of cases the retardation would be so slight that it could not be detected by the most delicate observation, and the comet would return to the sun only after the expiration of tens or hundreds of thousands of years; but, were the comet to pass very near one of the larger planets, the retardation might be sufficient to cause the comet to revolve in an elliptical orbit of quite a short period. the orbit of a comet thus captured by a planet would have its aphelion point near the orbit of the planet which captured it. now, it happens that each of the larger planets has a family of comets whose aphelia are about its own distance from the sun. it is therefore probable that these comets have been captured by the action of these planets. as might be expected from the gigantic size of jupiter, the jovian family of comets is the largest. the orbits of several of the comets of this group are shown in fig. . [illustration: fig. .] . _number of comets._--the number of comets recorded as visible to the naked eye since the birth of christ is about five hundred, while about two hundred telescopic comets have been observed since the invention of the telescope. the total number of comets observed since the christian era is therefore about seven hundred. it is certain, however, that only an insignificant fraction of all existing comets have ever been observed. since they can be seen only when near their perihelion, and since it is probable that the period of most of those which have been observed is reckoned by thousands of years (if, indeed, they ever return at all), our observations must be continued for many thousand years before we have seen all which come within range of our telescopes. besides, as already stated ( ), a comet can seldom be seen unless its perihelion is either inside the orbit of the earth, or but little outside of it; and it is probable that the perihelia of the great majority of comets are beyond this limit of visibility. remarkable comets. . _the comet of ._--the great comet of , shown in fig. , is one of the most celebrated on record. it was by his study of its motions that newton proved the orbit of a comet to be one of the conic sections, and therefore that these bodies move under the influence of gravity. this comet descended almost in a direct line to the sun, passing nearer to that luminary than any comet before known. newton estimated, that, at its perihelion point, it was exposed to a temperature two thousand times that of red-hot iron. during its perihelion passage it was exceedingly brilliant. halley suspected that this comet had a period of five hundred and seventy-five years, and that its first recorded appearance was in b.c., its third in , and its fourth in . if this is its real period, it will return in . the comet of b.c. made its appearance just after the assassination of julius cæsar. the romans called it the _julian star_, and regarded it as a celestial chariot sent to convey the soul of cæsar to the skies. it was seen two or three hours before sunset, and continued visible for eight successive days. the great comet of was described as an object of terrific splendor, and was visible in close proximity to the sun. the comet of has become celebrated, not only on account of its great brilliance, and on account of newton's investigation of its orbit, but also on account of the speculation of the theologian whiston in regard to it. he accepted five hundred and seventy-five years as its period, and calculated that one of its earlier apparitions must have occurred at the date of the flood, which he supposed to have been caused by its near approach to the earth; and he imagined that the earth is doomed to be destroyed by fire on some future encounter with this comet. [illustration: fig. .] . _the comet of ._--the great comet of , a view of which is given in fig. , is, perhaps, the most remarkable comet on record. it was visible for nearly seventeen months, and was very brilliant, although at its perihelion passage it was over a hundred million miles from the sun. its tail was a hundred and twenty million miles in length, and several million miles through. it has been calculated that its aphelion point is about two hundred times as far from the sun as its perihelion point, or some seven times the distance of neptune from the sun. its period is estimated at about three thousand years. it was an object of superstitious terror, especially in the east. the russians regarded it as presaging napoleon's great and fatal war with russia. [illustration: fig. .] [illustration: fig. .] . _halley's comet._--halley's comet has become one of the most celebrated of modern times. it is the first comet whose return was both predicted and observed. it made its appearance in . halley computed its orbit, and compared it with those of previous comets, whose orbits he also computed from recorded observations. he found that it coincided so exactly with that of the comet observed by kepler in , that there could be no doubt of the identity of the two orbits. so close were they together, that, were they both drawn in the heavens, the naked eye would almost see them joined into one line. there could therefore be no doubt that the comet of was the same that had appeared in , and that it moved in an elliptic orbit, with a period of about seventy-five years. he found that this comet had previously appeared in and in ; and he predicted that it would return about . its actual return was retarded somewhat by the action of the planets on it in its passage through the solar system. it, however, appeared again in , and a third time in . its next appearance will be about . the orbit of this comet is shown in fig. . fig. shows the comet as it appeared to the naked eye, and in a telescope of moderate power, in . this comet appears to be growing less brilliant. in it appeared as a comet of great splendor; and coming as it did in a very superstitious age, soon after the fall of constantinople, and during the threatened invasion of europe by the turks, it caused great alarm. fig. shows the changes undergone by the nucleus of this comet during its perihelion passage in . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _encke's comet._--this telescopic comet, two views of which are given in figs. and , appeared in . encke computed its orbit, and found it to lie wholly within the orbit of jupiter (fig. ), and the period to be about three years and a third. by comparing the intervals between the successive returns of this comet, it has been ascertained that its orbit is continually growing smaller and smaller. to account for the retardation of this comet, olbers announced his celebrated hypothesis, that the celestial spaces are filled with a subtile _resisting medium_. this hypothesis was adopted by encke, and has been accepted by certain other astronomers; but it has by no means gained universal assent. . _biela's comet._--this comet appeared in , and was found to have a period of about six years and two thirds. on its return in , it met with a singular, and as yet unexplained, accident, which has rendered the otherwise rather insignificant comet famous. in november and december of that year it was observed as usual, without any thing remarkable about it; but, in january of the following year, it was found to have been divided into two distinct parts, so as to appear as two comets instead of one. the two parts were at first of very unequal brightness; but, during the following month, the smaller of the two increased in brilliancy until it equalled its companion; it then grew fainter till it entirely disappeared, a month before its companion. the two parts were about two hundred thousand miles apart. fig. shows these two parts as they appeared on the th of february, and fig. as they appeared on the st of february. on its return in , the comets were found still to be double; but the two components were now about a million and a half miles apart. they are shown in fig. as they appeared at this time. sometimes one of the parts appeared the brighter, and sometimes the other; so that it was impossible to decide which was really the principal comet. the two portions passed out of view in september, and have not been seen since; although in the position of the comet would have been especially favorable for observation. the comet appears to have become completely broken up. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of ._--the great comet of , a view of which is given in fig. , was favorably situated for observation only in southern latitudes. it was exceedingly brilliant, and was easily seen in full daylight, in close proximity to the sun. the apparent length of its tail was sixty-five degrees, and its real length a hundred and fifty million miles, or nearly twice the distance from the earth to the sun. this comet is especially remarkable on account of its near approach to the sun. at the time of its perihelion passage the distance of the comet from the photosphere of the sun was less than one-fourteenth of the diameter of the sun. this distance was only one-half that of the comet of when at its perihelion. when at perihelion, this comet was plunging through the sun's outer atmosphere at the rate of one million, two hundred and eighty thousand miles an hour. it passed half way round the sun in the space of _two hours_, and its tail was whirled round through a hundred and eighty degrees in that brief time. as the tail extended almost double the earth's distance from the sun, the end of the tail must have traversed in two hours a space nearly equal to the circumference of the earth's orbit,--a distance which the earth, moving at the rate of about twenty miles a second, is a _whole year_ in passing. it is almost impossible to suppose that the matter forming this tail remained the same throughout this tremendous sweep. . _donati's comet._--the great comet of , known as _donati's_ comet, was one of the most magnificent of modern times. when at its brightest it was only about fifty million miles from the earth. its tail was then more than fifty million miles long. had the comet at this time been directly between the earth and sun, the earth must have passed through its tail; but this did not occur. the orbit of this comet was found to be decidedly elliptic, with a period of about two thousand years. this comet is especially celebrated on account of the careful telescopic observations of its nucleus and coma at the time of its perihelion passage. attention has already been called ( ) to the changes it underwent at that time. its tail was curved, and of a curious feather-like form, as shown in fig. . at times it developed lateral streamers, as shown in fig. . fig. shows the head of the comet as it was seen by bond of the harvard observatory, whose delineations of this comet have been justly celebrated. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of ._--the great comet of is remarkable for its great brilliancy, for its peculiar fan-shaped tail, and for the probable passage of the earth through its tail. sir john herschel declared that it far exceeded in brilliancy any comet he had ever seen, not excepting those of and . secchi found its tail to be a hundred and eighteen degrees in length, the largest but one on record. fig. shows this comet as it appeared at one time. fig. shows the position of the earth at _e_, in the tail of this comet, on the th of june, . fig. shows the probable passage of the earth through the tail of the comet on that date. as the tail of a comet doubtless consists of something much less dense than our atmosphere, it is not surprising that no noticeable effect was produced upon us by the encounter, if it occurred. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _coggia's comet._--this comet, which appeared in , looked very large, because it came very near the earth. it was not at all brilliant. its nucleus was carefully studied, and was found to develop a series of envelops similar to those of donati's comet. figs. and are two views of the head of this comet. fig. shows the system of envelops that were developed during its perihelion passage. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of june, ._--this comet, though far from being one of the largest of modern times, was still very brilliant. it will ever be memorable as the first brilliant comet which has admitted of careful examination with the spectroscope. connection between meteors and comets. . _shooting-stars._--on watching the heavens any clear night, we frequently see an appearance as of a star shooting rapidly through a short space in the sky, and then suddenly disappearing. three or four such _shooting-stars_ may, on the average, be observed in the course of an hour. they are usually seen only a second or two; but they sometimes move slowly, and are visible much longer. these stars begin to be visible at an average height of about seventy-five miles, and they disappear at an average height of about fifty miles. they are occasionally seen as high as a hundred and fifty miles, and continue to be visible till within thirty miles of the earth. their visible paths vary from ten to a hundred miles in length, though they are occasionally two hundred or three hundred miles long. their average velocity, relatively to the earth's surface, varies from ten to forty-five miles a second. the average number of shooting-stars visible to the naked eye at any one place is estimated at about _a thousand an hour_; and the average number large enough to be visible to the naked eye, that traverse the atmosphere daily, is estimated at _over eight millions_. the number of telescopic shooting-stars would of course be much greater. occasionally, shooting-stars leave behind them a trail of light which lasts for several seconds. these trails are sometimes straight, as shown in fig. , and sometimes curved, as in figs. and . they often disappear like trails of smoke, as shown in fig. . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] shooting-stars are seen to move in all directions through the heavens. their apparent paths are, however, generally inclined downward, though sometimes upward; and after midnight they come in the greatest numbers from that quarter of the heavens toward which the earth is moving in its journey around the sun. . _meteors._--occasionally these bodies are brilliant enough to illuminate the whole heavens. they are then called _meteors_, although this term is equally applicable to ordinary shooting-stars. such a meteor is shown in fig. . [illustration: fig. .] sometimes these brilliant meteors are seen to explode, as shown in fig. ; and the explosion is accompanied with a loud detonation, like the discharge of cannon. [illustration: fig. .] ordinary shooting-stars are not accompanied by any audible sound, though they are sometimes seen to break in pieces. meteors which explode with an audible sound are called _detonating meteors_. . _aerolites._--there is no certain evidence that any deposit from ordinary shooting-stars ever reaches the surface of the earth; though a peculiar dust has been found in certain localities, which has been supposed to be of meteoric origin, and which has been called _meteoric dust_. but solid bodies occasionally descend to the earth from beyond our atmosphere. these generally penetrate a foot or more into the earth, and, if picked up soon after their fall, are found to be warm, and sometimes even hot. these bodies are called _aerolites_. when they have a stony appearance, and contain but little iron, they are called _meteoric stones_; when they have a metallic appearance, and are composed largely of iron, they are called _meteoric iron_. there are eighteen well-authenticated cases in which aerolites have fallen in the united states during the last sixty years, and their aggregate weight is twelve hundred and fifty pounds. the entire number of known aerolites the date of whose fall is well determined is two hundred and sixty-one. there are also on record seventy-four cases of which the date is more or less uncertain. there have also been found eighty-six masses, which, from their peculiar composition, are believed to be aerolites, though their fall was not seen. the weight of these masses varies from a few pounds to several tons. the entire number of aerolites of which we have any knowledge is therefore about four hundred and twenty. aerolites are composed of the same elementary substances as occur in terrestrial minerals, not a single new element having been found in their analysis. of the sixty or more elements now recognized by chemists, about twenty have been found in aerolites. while aerolites contain no new elements, their appearance is quite peculiar; and the compounds found in them are so peculiar as to enable us by chemical analysis to distinguish an aerolite from any terrestrial substance. iron ores are very abundant in nature, but iron in the metallic state is exceedingly rare. now, aerolites invariably contain metallic iron, sometimes from ninety to ninety-six per cent. this iron is malleable, and may be readily worked into cutting instruments. it always contains eight or ten per cent of nickel, together with small quantities of cobalt, copper, tin, and chromium. this composition _has never been found in any terrestrial mineral_. aerolites also contain, usually in small amount, a compound of iron, nickel, and phosphorus, which has never been found elsewhere. meteorites often present the appearance of having been fused on the surface to a slight depth, and meteoric iron is found to have a peculiar crystalline structure. the external appearance of a piece of meteoric iron found near lockport, n.y., is shown in fig. . fig. shows the peculiar internal structure of meteoric iron. [illustration: fig. .] [illustration: fig. .] . _meteoroids._--astronomers now universally hold that shooting-stars, meteors, and aerolites are all minute bodies, revolving, like the comets, about the sun. they are moving in every possible direction through the celestial spaces. they may not average more than one in a million of cubic miles, and yet their total number exceeds all calculation. of the nature of the minuter bodies of this class nothing is certainly known. the earth is continually encountering them in its journey around the sun. they are burned by passing through the upper regions of our atmosphere, and the shooting-star is simply the light of that burning. these bodies, which are invisible till they plunge into the earth's atmosphere, are called _meteoroids_. . _origin of the light of meteors._--when one of these meteoroids enters our atmosphere, the resistance of the air arrests its motion to some extent, and so converts a portion of its energy of motion into that of heat. the heat thus developed is sufficient to raise the meteoroid and the air around it to incandescence, and in most cases either to cause the meteoroid to burn up, or to dissipate it as vapor. the luminous vapor thus formed constitutes the luminous train which occasionally accompanies a meteor, and often disappears as a puff of smoke. when a meteoroid is large enough and refractory enough to resist the heat to which it is exposed, its motion is sufficiently arrested, on entering the lower layers of our atmosphere, to cause it to fall to the earth. we then have an _aerolite_. a brilliant meteor differs from a shooting-star simply in magnitude. . _the intensity of the heat to which a meteoroid is exposed._--it has been ascertained by experiment that a body moving through the atmosphere at the rate of a hundred and twenty-five feet a second raises the temperature of the air immediately in front of it one degree, and that the temperature increases as the square of the velocity of the moving body; that is to say, that, with a velocity of two hundred and fifty feet, the temperature in front of the body would be raised four degrees; with a velocity of five hundred feet, sixteen degrees; and so on. to find, therefore, the temperature to which a meteoroid would be exposed in passing through our atmosphere, we have merely to divide its velocity in feet per second by a hundred and twenty-five, and square the quotient. with a velocity of forty-four miles a second in our atmosphere, a meteoroid would therefore be exposed to a temperature of between three and four million degrees. the air acts upon the body as if it were raised to this intense heat. at such a temperature small masses of the most refractory or incombustible substances known to us would flash into vapor with the evolution of intense light and heat. if one of these meteoric bodies is large enough to pass through the atmosphere and reach the earth, without being volatilized by the heat, we have an aerolite. as it is only a few seconds in making the passage, the heat has not time to penetrate far into its interior, but is expended in melting and vaporizing the outer portions. the resistance of the denser strata of the atmosphere to the motion of the aerolite sometimes becomes so enormous that the body is suddenly rent to pieces with a loud detonation. it seems like an explosion produced by some disruptive action within the mass; but there can be little doubt that it is due to the velocity--perhaps ten, twenty, or thirty miles a second--with which the body strikes the air. if, on the other hand, the meteoroid is so small as to be burned up or volatilized in the upper regions of the atmosphere, we have a common shooting-star, or a meteor of greater or less brilliancy. [illustration: fig. .] . _meteoric showers._--on ordinary nights only four or five shooting-stars are seen in an hour, and these move in every direction. their orbits lie in all possible positions, and are seemingly scattered at random. such meteors are called _sporadic_ meteors. on occasional nights, shooting-stars are more numerous, and all move in a common direction. such a display is called a _meteoric shower_. these showers differ greatly in brilliancy; but during any one shower the meteors all appear to radiate from some one point in the heavens. if we mark on a celestial globe the apparent paths of the meteors which fall during a shower, or if we trace them back on the celestial sphere, we shall find that they all meet in the same point, as shown in fig. . this point is called the _radiant point_. it always appears in the same position, wherever the observer is situated, and does not partake of the diurnal motion of the earth. as the stars move towards the west, the radiant point moves with them. the point in question is purely an effect of perspective, being the "vanishing point" of the parallel lines in which the meteors are actually moving. these lines are seen, not in their real direction in space, but as projected on the celestial sphere. if we look upwards, and watch snow falling through a calm atmosphere, the flakes which fall directly towards us do not seem to move at all, while the surrounding flakes seem to diverge from them on all sides. so, in a meteoric shower, a meteor coming directly towards the observer does not seem to move at all, and marks the point from which all the others seem to radiate. . _the august meteors._--a meteoric shower of no great brilliancy occurs annually about the th of august. the radiant point of this shower is in the constellation _perseus_, and hence these meteors are often called the _perseids_. the orbit of these meteoroids has been pretty accurately determined, and is shown in fig. . [illustration: fig. .] it will be seen that the perihelion point of this orbit is at about the distance of the earth from the sun; so that the earth encounters the meteors once a year, and this takes place in the month of august. the orbit is a very eccentric ellipse, reaching far beyond neptune. as the meteoric display is about equally brilliant every year, it seems probable that the meteoroids form a stream quite uniformly distributed throughout the whole orbit. it probably takes one of the meteoroids about a hundred and twenty-four years to pass around this orbit. [illustration: fig. .] . _the november meteors._--a somewhat brilliant meteoric shower also occurs annually, about the th of november. the radiant point of these meteors is in the constellation _leo_, and hence they are often called the _leonids_. their orbit has been determined with great accuracy, and is shown in fig. . while the november meteors are not usually very numerous or bright, a remarkably brilliant display of them has been seen once in about thirty-three or thirty-four years: hence we infer, that, while there are some meteoroids scattered throughout the whole extent of the orbit, the great majority are massed in a group which traverses the orbit in a little over thirty-three years. a conjectural form of this condensed group is shown in fig. . the group is so large that it takes it two or three years to pass the perihelion point: hence there may be a brilliant meteoric display two or three years in succession. [illustration: fig. .] the last brilliant display of these meteors was in the years and . the display was visible in this country only a short time before sunrise, and therefore did not attract general attention. the display of was remarkably brilliant in this country, and caused great consternation among the ignorant and superstitious. [illustration: fig. .] . _connection between meteors and comets._--it has been found that a comet which appeared in , and which is designated as , i., has exactly the same orbit and period as the november meteors, and that another comet, known as the , iii., has the same orbit as the august meteors. it has also been ascertained that a third comet, , i., has the same orbit as a stream of meteors which the earth encounters in april. furthermore, it was found, in , that there was a small stream of meteors following in the train of the lost comet of biela. these various orbits of comets and meteoric streams are shown in fig. . the coincidence of the orbits of comets and of meteoric streams indicates that these two classes of bodies are very closely related. they undoubtedly have a common origin. the fact that there is a stream of meteors in the train of biela's comet has led to the supposition that comets may become gradually disintegrated into meteoroids. physical and chemical constitution of comets. . _physical constitution of telescopic comets._--we have no certain knowledge of the physical constitution of telescopic comets. they are usually tens of thousands of miles in diameter, and yet of such tenuity that the smallest stars can readily be seen through them. it would seem that they must shine in part by reflected light; yet the spectroscope shows that their spectrum is composed of bright bands, which would indicate that they are composed, in part at least, of incandescent gases. it is, however, difficult to conceive how these gases become sufficiently heated to be luminous; and at the same time such gases would reflect no sunlight. it seems probable that these comets are really made up of a combination of small, solid particles in the form of minute meteoroids, and of gases which are, perhaps, rendered luminous by electric discharges of slight intensity. . _physical constitution of large comets._--in the case of large comets the nucleus is either a dense mass of solid matter several hundred miles in diameter, or a dense group of meteoroids. professor peirce estimated that the density of the nucleus is at least equal to that of iron. as such a comet approaches the sun, the nucleus is, to a slight extent, vaporized, and out of this vapor is formed the coma and the tail. that some evaporating process is going on from the nucleus of the comet is proved by the movements of the tail. it is evident that the tail cannot be an appendage carried along with the comet, as it seems to be. it is impossible that there should be any cohesion in matter of such tenuity that the smallest stars could be seen through a million of miles of it, and which is, moreover, continually changing its form. then, again, as a comet is passing its perihelion, the tail appears to be whirled from one side of the sun to another with a rapidity which would tear it to pieces if the movement were real. the tail seems to be, not something attached to the comet, and carried along with it, but a stream of vapor issuing from it, like smoke from a chimney. the matter of which it is composed is continually streaming outwards, and continually being replaced by fresh vapor from the nucleus. the vapor, as it emanates from the nucleus, is repelled by the sun with a force often two or three times as great as the ordinary solar attraction. the most probable explanation of this phenomenon is, that it is a case of electrical repulsion, the sun and the particles of the cometary mist being similarly electrified. with reference to this electrical theory of the formation of comets' tails, professor peirce makes the following observation: "in its approach to the sun, the surface of the nucleus is rapidly heated: it is melted and vaporized, and subjected to frequent explosions. the vapor rises in its atmosphere with a well-defined upper surface, which is known to observers as an _envelop_.... the electrification of the cometary mist is analogous to that of our own thunder-clouds. any portion of the coma which has received the opposite kind of electricity to the sun and to the repelled tail will be attracted. this gives a simple explanation of the negative tails which have been sometimes seen directed towards the sun. in cases of violent explosion, the whole nucleus might be broken to pieces, and the coma dashed around so as to give varieties of tail, and even multiple tails. there seems, indeed, to be no observed phenomenon of the tail or the coma which is not consistent with a reasonable modification of the theory." professor peirce regarded comets simply as the largest of the meteoroids. they appear to shine partly by reflected sunlight, and partly by their own proper light, which seems to be that of vapor rendered luminous by an electric discharge of slight intensity. [illustration: fig. .] . _collision of a comet and the earth._--it sometimes happens that the orbit of a comet intersects that of the earth, as is shown in fig. , which shows a portion of the orbit of biela's comet, with the positions of the comet and of the earth in . of course, were a comet and the earth both to reach the intersection of their orbits at the same time, a collision of the two bodies would be inevitable. with reference to the probable effect of such a collision, professor newcomb remarks,-- "the question is frequently asked, what would be the effect if a comet should strike the earth? this would depend upon what sort of a comet it was, and what part of the comet came in contact with our planet. the latter might pass through the tail of the largest comet without the slightest effect being produced; the tail being so thin and airy that a million miles thickness of it looks only like gauze in the sunlight. it is not at all unlikely that such a thing may have happened without ever being noticed. a passage through a telescopic comet would be accompanied by a brilliant meteoric shower, probably a far more brilliant one than has ever been recorded. no more serious danger would be encountered than that arising from a possible fall of meteorites; but a collision between the nucleus of a large comet and the earth might be a serious matter. if, as professor peirce supposes, the nucleus is a solid body of metallic density, many miles in diameter, the effect where the comet struck would be terrific beyond conception. at the first contact in the upper regions of the atmosphere, the whole heavens would be illuminated with a resplendence beyond that of a thousand suns, the sky radiating a light which would blind every eye that beheld it, and a heat which would melt the hardest rocks. a few seconds of this, while the huge body was passing through the atmosphere, and the collision at the earth's surface would in an instant reduce everything there existing to fiery vapor, and bury it miles deep in the solid earth. happily, the chances of such a calamity are so minute that they need not cause the slightest uneasiness. there is hardly a possible form of death which is not a thousand times more probable than this. so small is the earth in comparison with the celestial spaces, that if one should shut his eyes, and fire a gun at random in the air, the chance of bringing down a bird would be better than that of a comet of any kind striking the earth." [illustration: fig. .] [illustration: fig. .] . _the chemical constitution of comets._--fig. shows the bands of the spectrum of a telescopic comet of , as seen by two different observers. fig. shows the spectrum of the coma and tail of the comet of ; and the spectrum of the bright comet of showed the same three bands for the coma and tail. now, these three bands are those of certain hydrocarbon vapors: hence it would seem that the coma and tails of comets are composed chiefly of such vapors ( ). ii. the zodiacal light. . _the general appearance of the zodiacal light._--the phenomenon known as the _zodiacal light_ consists of a very faint luminosity, which may be seen rising from the western horizon after twilight on any clear winter or spring evening, also from the eastern horizon just before daybreak in the summer or autumn. it extends out on each side of the sun, and lies nearly in the plane of the ecliptic. it grows fainter the farther it is from the sun, and can generally be traced to about ninety degrees from that luminary, when it gradually fades away. in a very clear, tropical atmosphere, it has been traced all the way across the heavens from east to west, thus forming a complete ring. the general appearance of this column of light, as seen in the morning, in the latitude of europe, is shown in fig. . [illustration: fig. .] taking all these appearances together, they indicate that it is due to a lens-shaped appendage surrounding the sun, and extending a little beyond the earth's orbit. it lies nearly in the plane of the ecliptic; but its exact position is not easily determined. fig. shows the general form and position of this solar appendage, as seen in the west. [illustration: fig. .] . _the visibility of the zodiacal light._--the reason why the zodiacal light is more favorably seen in the evening during the winter and spring than in the summer and fall is evident from fig. , which shows the position of the ecliptic and the zodiacal light with reference to the western horizon at the time of sunset in march and in september. it will be seen that in september the axis of the light forms a small angle with the horizon, so that the phenomenon is visible only a short time after sunset and low down where it is difficult to distinguish it from the glimmer of the twilight; while in march, its axis being nearly perpendicular to the horizon, the light may be observed for some hours after sunset and well up in the sky. fig. gives the position of the ecliptic and of the zodiacal light with reference to the eastern horizon at the time of sunrise, and shows why the zodiacal light is seen to better advantage in the morning during the summer and fall than during the winter and spring. it will be observed that here the angle made by the axis of the light with the horizon is small in march, while it is large in september; the conditions represented in the preceding figure being thus reversed. [illustration: fig. .] [illustration: fig. .] . _nature of the zodiacal light._--various attempts have been made to explain the phenomena of the zodiacal light; but the most probable theory is, that it is due to an immense number of meteors which are revolving around the sun, and which lie mostly within the earth's orbit. each of these meteors reflects a sensible portion of sunlight, but is far too small to be separately visible. all of these meteors together would, by their combined reflection, produce a kind of pale, diffused light. iii. the stellar universe. i. general aspect of the heavens. . _the magnitude of the stars._--the stars that are visible to the naked eye are divided into six classes, according to their brightness. the brightest stars are called stars of the _first magnitude_; the next brightest, those of the _second magnitude_; and so on to the _sixth magnitude_. the last magnitude includes the faintest stars that are visible to the naked eye on the most favorable night. stars which are fainter than those of the sixth magnitude can be seen only with the telescope, and are called _telescopic stars_. telescopic stars are also divided into magnitudes; the division extending to the _sixteenth_ magnitude, or the faintest stars that can be seen with the most powerful telescopes. the classification of stars according to magnitudes has reference only to their brightness, and not at all to their actual size. a sixth magnitude star may actually be larger than a first magnitude star; its want of brilliancy being due to its greater distance, or to its inferior luminosity, or to both of these causes. none of the stars present any sensible disk, even in the most powerful telescope: they all appear as mere points of light. the larger the telescope, the greater is its power of revealing faint stars; not because it makes these stars appear larger, but because of its greater light-gathering power. this power increases with the size of the object-glass of the telescope, which plays the part of a gigantic pupil of the eye. the classification of the stars into magnitudes is not made in accordance with any very accurate estimate of their brightness. the stars which are classed together in the same magnitude are far from being equally bright. the stars of each lower magnitude are about two-fifths as bright as those of the magnitude above. the ratio of diminution is about a third from the higher magnitude down to the fifth. were the ratio two-fifths exact, it would take about - / stars of the d magnitude to make one of the st. stars of the d magnitude to make one of the st. stars of the th magnitude to make one of the st. stars of the th magnitude to make one of the st. stars of the th magnitude to make one of the st. , stars of the th magnitude to make one of the st. , , stars of the th magnitude to make one of the st. . _the number of the stars._--the total number of stars in the celestial sphere visible to the average naked eye is estimated, in round numbers, at five thousand; but the number varies much with the perfection and the training of the eye and with the atmospheric conditions. for every star visible to the naked eye, there are thousands too minute to be seen without telescopic aid. fig. shows a portion of the constellation of the twins as seen with the naked eye; and fig. shows the same region as seen in a powerful telescope. [illustration: fig. .] [illustration: fig. .] struve has estimated that the total number of stars visible with herschel's twenty-foot telescope was about twenty million. the number that can be seen with the great telescopes of modern times has not been carefully estimated, but is probably somewhere between thirty million and fifty million. the number of stars between the north pole and the circle thirty-five degrees south of the equator is about as follows:-- of the st magnitude about stars. of the d magnitude about stars. of the d magnitude about stars. of the th magnitude about stars. of the th magnitude about stars. of the th magnitude about stars. ---- total visible to naked eye stars. the number of stars of the several magnitudes is approximately in inverse proportion to that of their brightness, the ratio being a little greater in the higher magnitudes, and probably a little less in the lower ones. . _the division of the stars into constellations._--a glance at the heavens is sufficient to show that the stars are not distributed uniformly over the sky. the larger ones especially are collected into more or less irregular groups. the larger groups are called _constellations_. at a very early period a mythological figure was allotted to each constellation; and these figures were drawn in such a way as to include the principal stars of each constellation. the heavens thus became covered, as it were, with immense hieroglyphics. there is no historic record of the time when these figures were formed, or of the principle in accordance with which they were constructed. it is probable that the imagination of the earlier peoples may, in many instances, have discovered some fanciful resemblance in the configuration of the stars to the forms depicted. the names are still retained, although the figures no longer serve any astronomical purpose. the constellation hercules, for instance, no longer represents the figure of a man among the stars, but a certain portion of the heavens within which the ancients placed that figure. in star-maps intended for school and popular use it is still customary to give these figures; but they are not generally found on maps designed for astronomers. . _the naming of the stars._--the brighter stars have all proper names, as _sirius_, _procyon_, _arcturus_, _capella_, _aldebaran_, etc. this method of designating the stars was adopted by the arabs. most of these names have dropped entirely out of astronomical use, though many are popularly retained. the brighter stars are now generally designated by the letters of the greek alphabet,--_alpha_, _beta_, _gamma_, etc.,--to which is appended the genitive of the name of the constellation, the first letter of the alphabet being used for the brightest star, the second for the next brightest, and so on. thus _aldebaran_ would be designated as _alpha tauri_. in speaking of the stars of any one constellation, we simply designate them by the letters of the greek alphabet, without the addition of the name of the constellation, which answers to a person's surname, while the greek letter answers to his christian name. the names of the seven stars of the "dipper" are given in fig. . when the letters of the greek alphabet are exhausted, those of the roman alphabet are employed. the fainter stars in a constellation are usually designated by some system of numbers. [illustration: fig. .] . _the milky-way, or galaxy._--the milky-way is a faint luminous band, of irregular outline, which surrounds the heavens with a great circle, as shown in fig. . through a considerable portion of its course it is divided into two branches, and there are various vacant spaces at different points in this band; but at only one point in the southern hemisphere is it entirely interrupted. [illustration: fig. .] the telescope shows that the galaxy arises from the light of countless stars too minute to be separately visible with the naked eye. the telescopic stars, instead of being uniformly distributed over the celestial sphere, are mostly condensed in the region of the galaxy. they are fewest in the regions most distant from this belt, and become thicker as we approach it. the greater the telescopic power, the more marked is the condensation. with the naked eye the condensation is hardly noticeable; but with the aid of a very small telescope, we see a decided thickening of the stars in and near the galaxy, while the most powerful telescopes show that a large majority of the stars lie actually in the galaxy. if all the stars visible with a twelve-inch telescope were blotted out, we should find that the greater part of those remaining were in the galaxy. [illustration: fig. .] the increase in the number of the stars of all magnitudes as we approach the plane of the milky-way is shown in fig. . the curve _acb_ shows by its height the distribution of the stars above the ninth magnitude, and the curve _acb_ those of all magnitudes. . _star-clusters._--besides this gradual and regular condensation towards the galaxy, occasional aggregations of stars into _clusters_ may be seen. some of these are visible to the naked eye, sometimes as separate stars, like the "seven stars," or pleiades, but more commonly as patches of diffused light, the stars being too small to be seen separately. the number visible in powerful telescopes is, however, much greater. sometimes hundreds or even thousands of stars are visible in the field of view at once, and sometimes the number is so great that they cannot be counted. . _nebulæ._--another class of objects which are found in the celestial spaces are irregular masses of soft, cloudy light, known as _nebulæ_. many objects which look like nebulæ in small telescopes are shown by more powerful instruments to be really star-clusters. but many of these objects are not composed of stars at all, being immense masses of gaseous matter. [illustration: fig. .] the general distribution of nebulæ is the reverse of that of the stars. nebulæ are thickest where stars are thinnest. while stars are most numerous in the region of the milky-way, nebulæ are most abundant about the poles of the milky-way. this condensation of nebulæ about the poles of the milky-way is shown in figs. and , in which the points represent, not stars, but nebulæ. ii. the stars. the constellations. [illustration: fig. .] [illustration: fig. .] . _the great bear._--the great bear, or _ursa major_, is one of the circumpolar constellations ( ), and contains one of the most familiar _asterisms_, or groups of stars, in our sky; namely, the _great dipper_, or _charles's wain_. the positions and names of the seven prominent stars in it are shown in fig. . the two stars alpha and beta are called the _pointers_. this asterism is sometimes called the _butcher's cleaver_. the whole constellation is shown in fig. . a rather faint star marks the nose of the bear, and three equidistant pairs of faint stars mark his feet. . _the little bear, draco, and cassiopeia._--these are all circumpolar constellations. the most important star of the little bear, or _ursa minor_, is _polaris_, or the _pole star_. this star may be found by drawing a line from beta to alpha of the dipper, and prolonging it as shown in fig. . this explains why these stars are called the _pointers_. the pole star, with the six other chief stars of the little bear, form an asterism called the _little dipper_. these six stars are joined with polaris by a dotted line in fig. . [illustration: fig. .] the stars in a serpentine line between the two dippers are the chief stars of _draco_, or the _dragon_; the trapezium marking its head. fig. shows the constellations of ursa minor and draco as usually figured. [illustration: fig. .] to find _cassiopeia_, draw a line from delta of the dipper to polaris, and prolong it about an equal distance beyond, as shown in fig. . this line will pass near alpha of cassiopeia. the five principal stars of this constellation form an irregular _w_, opening towards the pole. between cassiopeia and draco are five rather faint stars, which form an irregular _k_. these are the principal stars of the constellation _cepheus_. these two constellations are shown in fig. . [illustration: fig. .] [illustration: fig. .] . _the lion, berenice's hair, and the hunting-dogs._--a line drawn from alpha to beta of the dipper, and prolonged as shown in fig. , will pass between the two stars _denebola_ and _regulus_ of _leo_, or the _lion_. regulus forms a _sickle_ with several other faint stars, and marks the heart of the lion. denebola is at the apex of a right-angled triangle, which it forms with two other stars, and marks the end of the lion's tail. this constellation is visible in the evening from february to july, and is figured in fig. . [illustration: fig. .] in a straight line between denebola and eta, at the end of the great bear's tail, are, at about equal distances, the two small constellations of _coma berenices_, or _berenice's hair_, and _canes venatici_, or the _hunting-dogs_. these are shown in fig. . the dogs are represented as pursuing the bear, urged on by the huntsman _boötes_. [illustration: fig. .] . _boötes, hercules, and the northern crown._--_arcturus_, the principal star of _boötes_, may be found by drawing a line from zeta to eta of the dipper, and then prolonging it with a slight bend, as shown in fig. . arcturus and polaris form a large isosceles triangle with a first-magnitude star called _vega_. this triangle encloses at one corner the principal stars of boötes, and the head of the dragon near the opposite side. the side running from arcturus to vega passes through _corona borealis_, or the _northern crown_, and the body of _hercules_, which is marked by a quadrilateral of four stars. [illustration: fig. .] _boötes_, who is often represented as a husbandman, _corona borealis_, and _hercules_, are delineated in fig. . these constellations are visible in the evening from may to september. [illustration: fig. .] [illustration: fig. .] . _the lyre, the swan, the eagle, and the dolphin._--_altair_, the principal star of _aquila_, or the _eagle_, lies on the opposite side of the milky-way from vega. altair is a first-magnitude star, and has a faint star on each side of it, as shown in fig. . vega, also of the first magnitude, is the principal star of _lyra_, or the _lyre_. between these two stars, and a little farther to the north, are several stars arranged in the form of an immense cross. the bright star at the head of this cross is called _deneb_. the cross lies in the milky-way, and contains the chief stars of the constellation _cygnus_, or the _swan_. a little to the north of altair are four stars in the form of a diamond. this asterism is popularly known as _job's coffin_. these four stars are the chief stars of _delphinus_, or the _dolphin_. these four constellations are shown together in fig. . the _swan_ is visible from june to december, in the evening. [illustration: fig. .] . _virgo._--a line drawn from alpha to gamma of the dipper, and prolonged with a slight bend at gamma, will reach to a first-magnitude star called _spica_ (fig. ). this is the chief star of the constellation _virgo_, or the _virgin_, and forms a large isosceles triangle with _arcturus_ and _denebola_. [illustration: fig. .] _virgo_ is represented in fig. . to the right of this constellation, as shown in the figure, there are four stars which form a trapezium, and mark the constellation _corvus_, or the _crow_. this bird is represented as standing on the body of _hydra_, or the _water-snake_. _virgo_ is visible in the evening, from april to august. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the twins._--a line drawn from delta to beta of the dipper, and prolonged as shown in fig. , passes between two bright stars called _castor_ and _pollux_. the latter of these is usually reckoned as a first-magnitude star. these are the principal stars of the constellation _gemini_, or the _twins_, which is shown in fig. . the constellation _canis minor_, or the _little dog_, is shown in the lower part of the figure. there are two conspicuous stars in this constellation, the brightest of which is of the first magnitude, and called _procyon_. the region to which we have now been brought is the richest of the northern sky, containing no less than seven first-magnitude stars. these are _sirius_, _procyon_, _pollux_, _capella_, _aldebaran_, _betelgeuse_, and _rigel_. they are shown in fig. . [illustration: fig. .] _betelgeuse_ and _rigel_ are in the constellation _orion_, being about equally distant to the north and south from the three stars forming the _belt_ of orion. betelgeuse is a red star. _sirius_ is the brightest star in the heavens, and belongs to the constellation _canis major_, or the _great dog_. it lies to the east of the belt of orion. _aldebaran_ lies at about the same distance to the west of the belt. it is a red star, and belongs to the constellation _taurus_, or the _bull_. _capella_ is in the constellation _auriga_, or the _wagoner_. these stars are visible in the evening, from about december to april. . _orion and his dogs, and taurus._--_orion_ and his _dogs_ are shown in fig. , and _orion_ and _taurus_ in fig. . _aldebaran_ marks one of the eyes of the bull, and is often called the _bull's eye_. the irregular _v_ in the face of the bull is called the _hyades_, and the cluster on the shoulder the _pleiades_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the wagoner._--the constellation _auriga_, or the _wagoner_ (sometimes called the _charioteer_), is shown in fig. . _capella_ marks the _goat_, which he is represented as carrying on his back, and the little right-angled triangle of stars near it the _kids_. the five chief stars of this constellation form a large, irregular pentagon. gamma of _auriga_ is also beta of _taurus_, and marks one of the horns of the _bull_. [illustration: fig. .] . _pegasus, andromeda, and perseus._--a line drawn from polaris near to beta of _cassiopeia_ will lead to a bright second-magnitude star at one corner of a large square (fig. ). alpha belongs both to the _square of pegasus_ and to _andromeda_. beta and gamma, which are connected with alpha in the figure by a dotted line, also belong to andromeda. _algol_, which forms, with the last-named stars and with the _square of pegasus_, an asterism similar in configuration to the _great dipper_, belongs to _perseus_. _algenib_, which is reached by bending the line at gamma in the opposite direction, is the principal star of _perseus_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] _pegasus_ is shown in fig. , and _andromeda_ in fig. . _cetus_, the _whale_, or the _sea monster_, shown in fig. , belongs to the same mythological group of constellations. [illustration: fig. .] . _scorpio, sagittarius, and ophiuchus._--during the summer months a brilliant constellation is visible, called _scorpio_, or the _scorpion_. the configuration of the chief stars of this constellation is shown in fig. . they bear some resemblance to a boy's kite. the brightest star is of the first magnitude, and called _antares_ (from _anti_, instead of, and _ares_, the greek name of mars), because it rivals mars in redness. the stars in the tail of the scorpion are visible in our latitude only under very favorable circumstances. this constellation is shown in fig. , together with _sagittarius_ and _ophiuchus_. _sagittarius_, or the _archer_, is to the east of _scorpio_. it contains no bright stars, but is easily recognized from the fact that five of its principal stars form the outline of an inverted dipper, which, from the fact of its being partly in the milky-way, is often called the _milk dipper_. [illustration: fig. .] _ophiuchus_, or the _serpent-bearer_, is a large constellation, filling all the space between the head of _hercules_ and _scorpio_. it is difficult to trace, since it contains no very brilliant stars. this constellation and _libra_, or the _balances_, which is the zodiacal constellation to the west of scorpio, are shown in fig. . [illustration: fig. .] [illustration: fig. .] . _capricornus, aquarius, and the southern fish._--the two zodiacal constellations to the east of sagittarius are _capricornus_ and _aquarius_. _capricornus_ contains three pairs of small stars, which mark the head, the tail, and the knees of the animal. _aquarius_ is marked by no conspicuous stars. an irregular line of minute stars marks the course of the stream of water which flows from the water-bearer's urn into the mouth of the _southern fish_. this mouth is marked by the first-magnitude star _fomalhaut_. these constellations are shown in fig. . [illustration: fig. .] . _pisces and aries._--the remaining zodiacal constellations are _pisces_, or the _fishes_, _aries_, or the _ram_ (fig. ), and _cancer_, or the _crab_. the _fishes_ lie under _pegasus_ and _andromeda_, but contain no bright stars. _aries_ (between _pisces_ and _taurus_) is marked by a pair of stars on the head,--one of the second, and one of the third magnitude. _cancer_ (between _leo_ and _gemini_) has no bright stars, but contains a remarkable cluster of small stars called _præsepe_, or the _beehive_. clusters. . _the hyades._--the _hyades_ are a very open cluster in the face of _taurus_ ( ). the three brightest stars of this cluster form a letter _v_, the point of the _v_ being on the nose, and the open ends at the eyes. this cluster is shown in fig. . the name, according to the most probable etymology, means _rainy_; and they are said to have been so called because their rising was associated with wet weather. they were usually considered the daughters of atlas, and sisters of the pleiades, though sometimes referred to as the nurses of bacchus. [illustration: fig. .] . _the pleiades._--the _pleiades_ constitute a celebrated group of stars, or a miniature constellation, on the shoulder of _taurus_. hesiod mentions them as "the seven virgins of atlas born," and milton calls them "the seven atlantic sisters." they are referred to in the book of job. the spaniards term them "the little nanny-goats;" and they are sometimes called "the hen and chickens." [illustration: fig. .] [illustration: fig. .] usually only six stars in this cluster can be seen with the naked eye, and this fact has given rise to the legend of the "lost pleiad." on a clear, moonless night, however, a good eye can discern seven or eight stars, and some observers have distinguished as many as eleven. fig. shows the _pleiades_ as they appear to the naked eye under the most favorable circumstances. fig. shows this cluster as it appears in a powerful telescope. with such an instrument more than five hundred stars are visible. . _cluster in the sword-handle of perseus._--this is a somewhat dense double cluster. it is visible to the naked eye, appearing as a hazy star. a line drawn from _algenib_, or _alpha_ of _perseus_ ( ), to _delta_ of _cassiopeia_ ( ), will pass through this cluster at about two-thirds the distance from the former. this double cluster is one of the most brilliant objects in the heavens, with a telescope of moderate power. [illustration: fig. .] . _cluster of hercules._--the celebrated globular cluster of _hercules_ can be seen only with a telescope of considerable power, and to resolve it into distinct stars (as shown in fig. ) requires an instrument of the very highest class. [illustration: fig. .] . _other clusters._--fig. shows a magnificent globular cluster in the constellation _aquarius_. herschel describes it as appearing like a heap of sand, being composed of thousands of stars of the fifteenth magnitude. [illustration: fig. .] fig. shows a cluster in the constellation _toucan_, which sir john herschel describes as a most glorious globular cluster, the stars of the fourteenth magnitude being immensely numerous. there is a marked condensation of light at the centre. [illustration: fig. .] [illustration: fig. .] fig. shows a cluster in the _centaur_, which, according to the same astronomer, is beyond comparison the richest and largest object of the kind in the heavens, the stars in it being literally innumerable. fig. shows a cluster in _scorpio_, remarkable for the peculiar arrangement of its component stars. star clusters are especially abundant in the region of the milky-way, the law of their distribution being the reverse of that of the nebulæ. double and multiple stars. . _double stars._--the telescope shows that many stars which appear single to the naked eye are really _double_, or composed of a pair of stars lying side by side. there are several pairs of stars in the heavens which lie so near together that they almost seem to touch when seen with the naked eye. [illustration: fig. .] [illustration: fig. .] pairs of stars are not considered double unless the components are so near together that they both appear in the field of view when examined with a telescope. in the majority of the pairs classed as double stars the distance between the components ranges from half a second to fifteen seconds. [illustration: fig. .] _epsilon lyræ_ is a good example of a pair of stars that can barely be separated with a good eye. figs. and show this pair as it appears in telescopes magnifying respectively four and fifteen times; and fig. shows it as seen in a more powerful telescope, in which each of the two components of the pair is seen to be a truly double star. [illustration: fig. .] [illustration: fig. .] . _multiple stars._--when a star is resolved into more than two components by a telescope, it is called a _multiple_ star. fig. shows a _triple_ star in _pegasus_. fig. shows a quadruple star in _taurus_. fig. shows a _sextuple_ star, and fig. a _septuple_ star. fig. shows the celebrated septuple star in _orion_, called _theta orionis_, or the _trapezium_ of orion. . _optically double and multiple stars._--two or more stars which are really very distant from each other, and which have no physical connection whatever, may appear to be near together, because they happen to lie in the same direction, one behind the other. such accidental combinations are called _optically_ double or multiple stars. [illustration: fig. .] [illustration: fig. .] . _physically double and multiple stars._--in the majority of cases the components of double and multiple stars are in reality comparatively near together, and are bound together by gravity into a physical system. such combinations are called _physically_ double and multiple stars. the components of these systems all revolve around their common centre of gravity. in many instances their orbits and periods of revolution have been ascertained by observation and calculation. fig. shows the orbit of one of the components of a double star in the constellation _hercules_. [illustration: fig. .] . _colors of double and multiple stars._--the components of double and multiple stars are often highly colored, and frequently the components of the same system are of different colors. sometimes one star of a binary system is _white_, and the other _red_; and sometimes a _white_ star is combined with a _blue_ one. other colors found in combination in these systems are _red_ and _blue_, _orange_ and _green_, _blue_ and _green_, _yellow_ and _blue_, _yellow_ and _red_, etc. [illustration: fig. .] if these double and multiple stars are accompanied by planets, these planets will sometimes have two or more suns in the sky at once. on alternate days they may have suns of different colors, and perhaps on the same day two suns of different colors. the effect of these changing colored lights on the landscape must be very remarkable. new and variable stars. . _variable stars._--there are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. these changes are in some cases apparently irregular, and in others _periodic_. all such stars are said to be _variable_, though the term is applied especially to those stars whose variability is _periodic_. [illustration: fig. .] . _algol._--_algol_, a star of _perseus_, whose position is shown in fig. , is a remarkable variable star of a short period. usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. these changes were first noticed some two centuries ago, but it was not till that they were accurately observed. the period is now known to be two days, twenty hours, forty-nine minutes. it takes about four hours and a half to fade away, and four hours more to recover its brilliancy. near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual. this variation of light was at first explained by supposing that a large dark planet was revolving round algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for. . _mira._--another remarkable variable star is _omicron ceti_, or _mira_ (that is, the _wonderful_ star). it is generally invisible to the naked eye; but at intervals of about eleven months it shines forth as a star of the second or third magnitude. it is about forty days from the time it becomes visible until it attains its greatest brightness, and is then about two months in fading to invisibility; so that its increase of brilliancy is more rapid than its waning. its period is quite irregular, ranging from ten to twelve months; so that the times of its appearance cannot be predicted with certainty. its maximum brightness is also variable, being sometimes of the second magnitude, and at others only of the third or fourth. [illustration: fig. .] . _eta argus._--perhaps the most extraordinary variable star in the heavens is _eta argus_, in the constellation _argo_, or the _ship_, in the southern hemisphere (fig. ). the first careful observations of its variability were made by sir john herschel while at the cape of good hope. he says, "it was on the th of december, , that, resuming the photometrical comparisons, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude in a part of the heavens where, being perfectly familiar with it, i was certain that no such brilliant object had before been seen. after a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configuration with other conspicuous stars in the neighborhood, i became satisfied of its identity with my old acquaintance, _eta argus_. its light was, however, nearly tripled. while yet low, it equalled rigel, and, when it attained some altitude, was decidedly greater." it continued to increase until jan. , , then faded a little till april following, though it was still as bright as aldebaran. in and it blazed up brighter than ever, and in march of the latter year was second only to _sirius_. during the twenty-five years following it slowly but steadily diminished. in it was barely visible to the naked eye; and the next year it vanished entirely from the unassisted view, and has not yet begun to recover its brightness. the curve in fig. shows the change in brightness of this remarkable star. the numbers at the bottom show the years of the century, and those at the side the brightness of the star. [illustration: fig. .] . _new stars._--in several cases stars have suddenly appeared, and even become very brilliant; then, after a longer or shorter time, they have faded away and disappeared. such stars are called _new_ or _temporary_ stars. for a time it was supposed that such stars were actually new. they are now, however, classified by astronomers among the variable stars, their changes being of a very irregular and fitful character. there is scarcely a doubt that they were all in the heavens as very small stars before they blazed forth in so extraordinary a manner, and that they are in the same places still. there is a wide difference between these irregular variations, or the breaking-forth of light on a single occasion in the course of centuries, and the regular and periodic changes in the case of a star like _algol_; but a long series of careful observation has resulted in the discovery of stars of nearly every degree of irregularity between these two extremes. some of them change gradually from one magnitude to another, in the course of years, without seeming to follow any law whatever; while in others some slight tendency to regularity can be traced. _eta argus_ may be regarded as a connecting link between new and variable stars. . _tycho brahe's star._--an apparently new star suddenly appeared in _cassiopeia_ in . it was first seen by tycho brahe, and is therefore associated with his name. its position in the constellation is shown in fig. . it was first seen on nov. , when it had already attained the first magnitude. it became rapidly brighter, soon rivalling venus in splendor, so that good eyes could discern it in full daylight. in december it began to wane, and gradually faded until the following may, when it disappeared entirely. [illustration: fig. .] a star showed itself in the same part of the heavens in and in . if these were three appearances of the same star, it must be reckoned as a periodic star with a period of a little more than three hundred years. . _kepler's star._--in a new star was seen in the constellation _ophiuchus_. it was first noticed in october of that year, when it was of the first magnitude. in the following winter it began to fade, but remained visible during the whole year . early in it disappeared entirely. a very full history of this star was written by kepler. one of the most remarkable things about this star was its brilliant scintillation. according to kepler, it displayed all the colors of the rainbow, or of a diamond cut with multiple facets, and exposed to the rays of the sun. it is thought that this star also appeared in , , and ; if so, it is a variable star with a period of a little over four hundred years. . _new star of ._--the most striking case of this kind in recent times was in may, , when a star of the second magnitude suddenly appeared in _corona borealis_. on the th and th of that month it was observed independently by at least five observers in europe and america. the fact that none of these new stars were noticed until they had nearly or quite attained their greatest brilliancy renders it probable that they all blazed up very suddenly. . _cause of the variability of stars._--the changes in the brightness of variable and temporary stars are probably due to operations similar to those which produce the spots and prominences in our sun. we have seen ( ) that the frequency of solar spots shows a period of eleven years, during one portion of which there are few or no spots to be seen, while during another portion they are numerous. if an observer so far away as to see our sun like a star could from time to time measure its light exactly, he would find it to be a variable star with a period of eleven years, the light being least when we see most spots, and greatest when few are visible. the variation would be slight, but it would nevertheless exist. now, we have reason to believe that the physical constitution of the sun and the stars is of the same general nature. it is therefore probable, that, if we could get a nearer view of the stars, we should see spots on their disks as we do on the sun. it is also likely that the varying physical constitution of the stars might give rise to great differences in the number and size of the spots; so that the light of some of these suns might vary to a far greater degree than that of our own sun does. if the variations had a regular period, as in the case of our sun, the appearances to a distant observer would be precisely what we see in the case of a periodic variable star. the spectrum of the new star of was found to be a continuous one, crossed by bright lines, which were apparently due to glowing hydrogen. the continuous spectrum was also crossed by dark lines, indicating that the light had passed through an atmosphere of comparatively cool gas. mr. huggins infers from this that there was a sudden and extraordinary outburst of hydrogen gas from the star, which by its own light, as well as by heating up the whole surface of the star, caused the extraordinary increase of brilliancy. now, the spectroscope shows that the red flames of the solar chromosphere ( ) are largely composed of hydrogen; and it is not unlikely that the blazing-forth of this star arose from an action similar to that which produces these flames, only on an immensely larger scale. distance of the stars. . _parallax of the stars._--such is the distance of the stars, that only in a comparatively few instances has any displacement of these bodies been detected when viewed from opposite parts of the earth's orbit, that is, from points a hundred and eighty-five million miles apart; and in no case can this displacement be detected except by the most careful and delicate measurement. half of the above displacement, or the displacement of the star as seen from the earth instead of the sun, is called the _parallax_ of the star. in no case has a parallax of one second as yet been detected. . _the distance of the stars._--the distance of a star whose parallax is one second would be , times the distance of the earth from the sun, or about nineteen million million miles. it is quite certain that no star is nearer than this to the earth. light has a velocity which would carry it seven times and a half around the earth in a second; but it would take it more than three years to reach us from that distance. were all the stars blotted out of existence to-night, it would be at least three years before we should miss a single one. _alpha centauri_, the brightest star in the constellation of the _centaur_, is, so far as we know, the nearest of the fixed stars. it is estimated that it would take its light about three years and a half to reach us. it has also been estimated that it would take light over sixteen years to reach us from _sirius_, about eighteen years to reach us from _vega_, about twenty-five years from _arcturus_, and over forty years from the _pole-star_. in many instances it is believed that it would take the light of stars hundreds of years to make the journey to our earth, and in some instances even thousands of years. proper motion of the stars. . _why the stars appear fixed._--the stars seem to retain their relative positions in the heavens from year to year, and from age to age; and hence they have come universally to be denominated as _fixed_. it is, however, now well known that the stars, instead of being really stationary, are moving at the rate of many miles a second; but their distance is so enormous, that, in the majority of cases, it would be thousands of years before this rate of motion would produce a sufficient displacement to be noticeable to the unaided eye. [illustration: fig. .] . _secular displacement of the stars._--though the proper motion of the stars is apparently slight, it will, in the course of many ages, produce a marked change in the configuration of the stars. thus, in fig. , the left-hand portion shows the present configuration of the stars of the great dipper. the small arrows attached to the stars show the direction and comparative magnitudes of their motion. the right-hand portion of the figure shows these stars as they will appear thirty-six thousand years from the present time. [illustration: fig. .] fig. shows in a similar way the present configuration and proper motion of the stars of _cassiopeia_, and also these stars as they will appear thirty-six thousand years hence. [illustration: fig. .] fig. shows the same for the constellation _orion_. . _the secular motion of the sun._--the stars in all parts of the heavens are found to move in all directions and with all sorts of velocities. when, however, the motions of the stars are averaged, there is found to be an apparent proper motion common to all the stars. the stars in the neighborhood of _hercules_ appear to be approaching us, and those in the opposite part of the heavens appear to be receding from us. in other words, all the stars appear to be moving away from hercules, and towards the opposite part of the heavens. [illustration: fig. .] this apparent motion common to all the stars is held by astronomers to be due to the real motion of the sun through space. the point in the heavens towards which our sun is moving at the present time is indicated by the small circle in the constellation hercules in fig. . as the sun moves, he carries the earth and all the planets along with him. fig. shows the direction of the sun's motion with reference to the ecliptic and to the axis of the earth. fig. shows the earth's path in space; and fig. shows the paths of the earth, the moon, mercury, venus, and mars in space. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] whether the sun is actually moving in a straight line, or around some distant centre, it is impossible to determine at the present time. it is estimated that the sun is moving along his path at the rate of about a hundred and fifty million miles a year. this is about five-sixths of the diameter of the earth's orbit. . _star-drift._--in several instances, groups of stars have a common proper motion entirely different from that of the stars around and among them. such groups probably form connected systems, in the motion of which all the stars are carried along together without any great change in their relative positions. the most remarkable case of this kind occurs in the constellation _taurus_. a large majority of the brighter stars in the region between _aldebaran_ and the _pleiades_ have a common proper motion of about ten seconds per century towards the east. proctor has shown that five out of the seven stars which form the great dipper have a common proper motion, as shown in fig. (see also fig. ). he proposes for this phenomenon the name of _star-drift_. [illustration: fig. .] . _motion of stars along the line of sight._--a motion of a star in the direction of the line of sight would produce no displacement of the star that could be detected with the telescope; but it would cause a change in the brightness of the star, which would become gradually fainter if moving from us, and brighter if approaching us. motion along the line of sight has, however, been detected by the use of the tele-spectroscope ( ), owing to the fact that it causes a displacement of the spectral lines. as has already been explained ( ), a displacement of a spectral line towards the red end of the spectrum indicates a motion away from us, and a displacement towards the violet end, a motion towards us. * * * * * by means of these displacements of the spectral lines, huggins has detected motion in the case of a large number of stars, and calculated its rate:-- stars receding from us. sirius miles per second. betelgeuse miles per second. rigel miles per second. castor miles per second. regulus miles per second. stars approaching us. arcturus miles per second. vega miles per second. deneb miles per second. pollux miles per second. alpha ursæ majoris miles per second. these results are confirmed by the fact that the amount of motion indicated is about what we should expect the stars to have, from their observed proper motions, combined with their probable distances. again: the stars in the neighborhood of hercules are mostly found to be approaching the earth, and those which lie in the opposite direction to be receding from it; which is exactly the effect which would result from the sun's motion through space. the five stars in the dipper, which have a common proper motion, are also found to have a common motion in the line of sight. but the displacement of the spectral lines is so slight, and its measurement so difficult, that the velocities in the above table are to be accepted as only an approximation to the true values. chemical and physical constitution of the stars. . _the constitution of the stars similar to that of the sun._--the stellar spectra bear a general resemblance to that of the sun, with characteristic differences. these spectra all show fraunhofer's lines, which indicate that their luminous surfaces are surrounded by atmospheres containing absorbent vapors, as in the case of the sun. the positions of these lines indicate that the stellar atmospheres contain elements which are also found in the sun's, and on the earth. [illustration: fig. .] . _four types of stellar spectra._--the spectra of the stars have been carefully observed by secchi and huggins. they have found that stellar spectra may be reduced to four types, which are shown in fig. . in the spectrum of _sirius_, a representative of _type i._, very few lines are represented; but the lines are very thick. next we have the solar spectrum, which is a representative of _type ii._, one in which more lines are represented. in _type iii._ fluted spaces begin to appear, and in _type iv._, which is that of the red stars, nothing but fluted spaces is visible; and this spectrum shows that something is at work in the atmosphere of those red stars different from what there is in the simpler atmosphere of _type i._ lockyer holds that these differences of spectra are due simply to differences of temperature. according to him, the red stars, which give the fluted spectra, are of the lowest temperature; and the temperature of the stars of the different types gradually rises till we reach the first type, in which the temperature is so high that the dissociation ( ) of the elements is nearly if not quite complete. iii. nebulÆ. classification of nebulæ. . _planetary nebulæ._--many nebulæ ( ) present a well-defined circular disk, like that of a planet, and are therefore called _planetary_ nebulæ. specimens of planetary nebulæ are shown in fig. . [illustration: fig. .] . _circular and elliptical nebulæ._--while many nebulæ are circular in form, others are elliptical. the former are called _circular_ nebulæ, and the latter _elliptical_ nebulæ. elliptical nebulæ have been discovered of every degree of eccentricity. examples of various circular and elliptical nebulæ are given in fig. . [illustration: fig. .] . _annular nebulæ._--occasionally ring-shaped nebulæ have been observed, sometimes with, and sometimes without, nebulous matter within the ring. they are called _annular_ nebulæ. they are both circular and elliptical in form. several specimens of this class of nebulæ are given in fig. . [illustration: fig. .] . _nebulous stars._--sometimes one or more minute stars are enveloped in a nebulous haze, and are hence called _nebulous stars_. several of these nebulæ are shown in fig. . [illustration: fig. .] . _spiral nebulæ._--very many nebulæ disclose a more or less spiral structure, and are known as _spiral_ nebulæ. they are illustrated in fig. . there are, however, a great variety of spiral forms. we shall have occasion to speak of these nebulæ again ( - ). [illustration: fig. .] . _double and multiple nebulæ._--many _double_ and _multiple_ nebulæ have been observed, some of which are represented in fig. . [illustration: fig. .] fig. shows what appears to be a double annular nebula. fig. gives two views of a double nebula. the change of position in the components of this double nebula indicates a motion of revolution similar to that of the components of double stars. [illustration: fig. .] [illustration: fig. .] irregular nebulæ. . _irregular forms._--besides the more or less regular forms of nebulæ which have been classified as indicated above, there are many of very irregular shapes, and some of these are the most remarkable nebulæ in the heavens. fig. shows a curiously shaped nebula, seen by sir john herschel in the southern heavens; and fig. , one in _taurus_, known as the _crab_ nebula. [illustration: fig. .] [illustration: fig. .] . _the great nebula of andromeda._--this is one of the few nebulæ that are visible to the naked eye. we see at a glance that it is not a star, but a mass of diffused light. indeed, it has sometimes been very naturally mistaken for a comet. it was first described by marius in , who compared its light to that of a candle shining through horn. this gives a very good idea of the impression it produces, which is that of a translucent object illuminated by a brilliant light behind it. with a small telescope it is easy to imagine it to be a solid like horn; but with a large one the effect is more like fog or mist with a bright body in its midst. unlike most of the nebulæ, its spectrum is a continuous one, similar to that from a heated solid, indicating that the light emanates, not from a glowing gas, but from matter in the solid or liquid state. this would suggest that it is really an immense star-cluster, so distant that the highest telescopic power cannot resolve it; yet in the largest telescopes it looks less resolvable, and more like a gas, than in those of moderate size. if it is really a gas, and if the spectrum is continuous throughout the whole extent of the nebula, either it must shine by reflected light, or the gas must be subjected to a great pressure almost to its outer limit, which is hardly possible. if the light is reflected, we cannot determine whether it comes from a single bright star, or a number of small ones scattered through the nebula. with a small telescope this nebula appears elliptical, as in fig. . fig. shows it as it appeared to bond, in the cambridge refractor. [illustration: fig. .] [illustration: fig. .] . _the great nebula of orion._--the nebula which, above all others, has occupied the attention of astronomers, and excited the wonder of observers, is the _great nebula of orion_, which surrounds the middle star of the three which form the sword of orion. a good eye will perceive that this star, instead of looking like a bright point, has a hazy appearance, due to the surrounding nebula. this object was first described by huyghens in , as follows:-- "there is one phenomenon among the fixed stars worthy of mention, which, so far as i know, has hitherto been noticed by no one, and indeed cannot be well observed except with large telescopes. in the sword of orion are three stars quite close together. in , as i chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). three of these almost touched each other, and with four others shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black; the effect being that of an opening in the sky, through which a brighter region was visible." [illustration: fig. .] the representation of this nebula in fig. is from a drawing made by bond. in brilliancy and variety of detail it exceeds any other nebula visible in the northern hemisphere. in its centre are four stars, easily distinguished by a small telescope with a magnifying power of forty or fifty, together with two smaller ones, requiring a nine-inch telescope to be well seen. besides these, the whole nebula is dotted with stars. in the winter of - the spectrum of this nebula was examined independently by secchi and huggins, who found that it consisted of three bright lines, and hence concluded that the nebula was composed, not of stars, but of glowing gas. the position of one of the lines was near that of a line of nitrogen, while another seemed to coincide with a hydrogen line. this would suggest that the nebula is a mixture of hydrogen and nitrogen gas; but of this we cannot be certain. [illustration: fig. .] . _the nebula in argus._--there is a nebula (fig. ) surrounding the variable star _eta argus_ ( ), which is remarkable as exhibiting variations of brightness and of outline. in many other nebulæ, changes have been suspected; but the indistinctness of outline which characterizes most of these objects, and the very different aspect they present in telescopes of different powers, render it difficult to prove a change beyond a doubt. . _the dumb-bell nebula._--this nebula was named from its peculiar shape. it is a good illustration of the change in the appearance of a nebula when viewed with different magnifying powers. fig. shows it as it appeared in herschel's telescope, and fig. as it appears in the great parsonstown reflector ( ). [illustration: fig. .] [illustration: fig. .] spiral nebulæ. . _the spiral nebula in canes venatici._--the great spiral nebula in the constellation _canes venatici_, or the _hunting-dogs_, is one of the most remarkable of its class. fig. shows this nebula as it appeared in herschel's telescope, and fig. shows it as it appears in the parsonstown reflector. [illustration: fig. .] [illustration: fig. .] . _condensation of nebulæ._--the appearance of the nebula just mentioned suggests a body rotating on its axis, and undergoing condensation at the same time. it is now a generally received theory that nebulæ are the material out of which stars are formed. according to this theory, the stars originally existed as nebulæ, and all nebulæ will ultimately become condensed into stars. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _other spiral nebulæ._--fig. represents a spiral nebula of the _great bear_. this nebula seems to have several centres of condensation. fig. is a view of a spiral nebula in _cepheus_, and fig. of a singular spiral nebula in the _triangle_. this also appears to have several points of condensation. figs. and represent oval and elliptical nebulæ having a spiral structure. [illustration: fig. .] [illustration: fig. .] _the magellanic clouds._ [illustration: fig. .] . _situation and general appearance of the magellanic clouds._--the _magellanic clouds_ are two nebulous-looking bodies near the southern pole of the heavens, as shown in the right-hand portion of fig. . in the appearance and brightness of their light they resemble portions of the milky-way. [illustration: fig. .] the larger of these clouds is called the _nubecula major_. it is visible to the naked eye in strong moonlight, and covers a space about two hundred times the surface of the moon. it is shown in fig. . the smaller cloud is called the _nubecula minor_. it has only about a fourth the extent of the larger cloud, and is considerably less brilliant. it is visible to the naked eye, but it disappears in full moonlight. this cloud is shown in fig. . the region around this cloud is singularly bare of stars; but the magnificent cluster of _toucan_, already described ( ), is near, and is shown a little to the right of the cloud in the figure. [illustration: fig. .] [illustration: fig. .] . _structure of the nubeculæ._--fig. shows the structure of these clouds as revealed by a powerful telescope. the general ground of both consists of large tracts and patches of nebulosity in every stage of resolution,--from that which is irresolvable with eighteen inches of reflecting aperture, up to perfectly separated stars, like the milky-way and clustering groups. there are also nebulæ in abundance, both regular and irregular, globular clusters in every state of condensation, and objects of a nebulous character quite peculiar, and unlike any thing in other regions of the heavens. in the area occupied by the _nubecula major_ two hundred and seventy-eight nebulæ and clusters have been enumerated, besides fifty or sixty outliers, which ought certainly to be reckoned as its appendages, being about six and a half per square degree; which very far exceeds the average of any other part of the nebulous heavens. in the _nubecula minor_ the concentration of such objects is less, though still very striking. the nubeculæ, then, combine, each within its own area, characters which in the rest of the heavens are no less strikingly separated; namely, those of the galactic and the nebular system. globular clusters (except in one region of small extent) and nebulæ of regular elliptic forms are comparatively rare in the milky-way, and are found congregated in the greatest abundance in a part of the heavens the most remote possible from that circle; whereas in the nubeculæ they are indiscriminately mixed with the general starry ground, and with irregular though small nebulæ. the nebular hypothesis. . _the basis of the nebular hypothesis._--we have seen that the planets all revolve around the sun from west to east in nearly the same plane, and that the sun rotates on his axis from west to east. the planets, so far as known, rotate on their axes from west to east; and all the moons, except those of uranus and neptune, revolve around their planets from west to east. these common features in the motion of the sun, moons, and planets, point to the conclusion that they are of a common origin. . _kant's hypothesis._--kant, the celebrated german philosopher, seems to have the best right to be regarded as the founder of the modern nebular hypothesis. his reasoning has been concisely stated thus: "examining the solar system, we find two remarkable features presented to our consideration. one is, that six planets and nine satellites [the entire number then known] move around the sun in circles, not only in the same direction in which the sun himself revolves on his axis, but very nearly in the same plane. this common feature of the motion of so many bodies could not by any reasonable possibility have been a result of chance: we are therefore forced to believe that it must be the result of some common cause originally acting on all the planets. "on the other hand, when we consider the spaces in which the planets move, we find them entirely void, or as good as void; for, if there is any matter in them, it is so rare as to be without effect on the planetary motions. there is, therefore, no material connection now existing between the planets through which they might have been forced to take up a common direction of motion. how, then, are we to reconcile this common motion with the absence of all material connection? the most natural way is to suppose that there was once some such connection, which brought about the uniformity of motion which we observe; that the materials of which the planets are formed once filled the whole space between them. there was no formation in this chaos, the formation of separate bodies by the mutual gravitation of parts of the mass being a later occurrence. but, naturally, some parts of the mass would be more dense than others, and would thus gather around them the rare matter which filled the intervening spaces. the larger collections thus formed would draw the smaller ones into them, and this process would continue until a few round bodies had taken the place of the original chaotic mass." kant, however, failed to account satisfactorily for the motion of the sun and planets. according to his system, all the bodies formed out of the original nebulous mass should have been drawn to a common centre so as to form one sun, instead of a system of revolving bodies like the solar system. . _herschel's hypothesis._--the idea of the gradual transmutation of nebulæ into stars seems to have been suggested to herschel, not by the study of the solar system, but by that of the nebulæ themselves. many of these bodies he believed to be immense masses of phosphorescent vapor; and he conceived that these must be gradually condensing, each around its own centre, or around the parts where it is most dense, until it should become a star, or a cluster of stars. on classifying the nebulæ, it seemed to him that he could see this process going on before his eyes. there were the large, faint, diffused nebulæ, in which the condensation had hardly begun; the smaller but brighter ones, which had become so far condensed that the central parts would soon begin to form into stars; yet others, in which stars had actually begun to form; and, finally, star-clusters in which the condensation was complete. the spectroscopic revelations of the gaseous nature of the true nebulæ tend to confirm the theory of herschel, that these masses will all, at some time, condense into stars. . _laplace's hypothesis._--laplace was led to the nebular hypothesis by considering the remarkable uniformity in the direction of the rotation of the planets. believing that this could not have been the result of chance, he sought to investigate its cause. this, he thought, could be nothing else than the atmosphere of the sun, which once extended so far out as to fill all the space now occupied by the planets. he begins with the sun, surrounded by this immense fiery atmosphere. since the sum total of rotary motion now seen in the planetary system must have been there from the beginning, he conceives the immense vaporous mass forming the sun and his atmosphere to have had a slow rotation on its axis. as the intensely hot mass gradually cooled, it would contract towards the centre. as it contracted, its velocity of rotation would, by the laws of mechanics, constantly increase; so that a time would arrive, when, at the outer boundary of the mass, the centrifugal force due to the rotation would counterbalance the attractive force of the central mass. then those outer portions would be left behind as a revolving ring, while the next inner portions would continue to contract until the centrifugal and attractive forces were again balanced, when a second ring would be left behind; and so on. thus, instead of a continuous atmosphere, the sun would be surrounded by a series of concentric revolving rings of vapor. as these rings cooled, their denser materials would condense first; and thus the ring would be composed of a mixed mass, partly solid and partly vaporous, the quantity of solid matter constantly increasing, and that of vapor diminishing. if the ring were perfectly uniform, this condensation would take place equally all around it, and the ring would thus be broken up into a group of small planets, like the asteroids. but if, as would more likely be the case, some portions of the ring were much denser than others, the denser portions would gradually attract the rarer portions, until, instead of a ring, there would be a single mass composed of a nearly solid centre, surrounded by an immense atmosphere of fiery vapor. this condensation of the ring of vapor around a single point would not change the amount of rotary motion that had existed in the ring. the planet with its atmosphere would therefore be in rotation; and would be, on a smaller scale, like the original solar mass surrounded by its atmosphere. in the same way that the latter formed itself first into rings, which afterwards condensed into planets, so the planetary atmospheres, if sufficiently extensive, would form themselves into rings, which would condense into satellites. in the case of saturn, however, one of the rings was so uniform throughout, that there was no denser portion to attract the rest around it; and thus the ring of saturn retained its annular form. [illustration: fig. .] such is the celebrated nebular hypothesis of laplace. it starts, not with a purely nebulous mass, but with the sun, surrounded by an immense atmosphere, out of which the planets were formed by gradual condensation. fig. represents the condensing mass according to this theory. . _the modern nebular hypothesis._--according to the nebular hypothesis as held at the present time, the sun, planets, and meteoroids originated from a purely nebulous mass. this nebula first condensed into a nebulous star, the star being the sun, and its surrounding nebulosity being the fiery atmosphere of laplace. the original nebula must have been put into rotation at the beginning. as it contracted and became condensed through the loss of heat by radiation into space, and under the combined attraction of gravity, cohesion, and affinity, its speed of rotation increased; and the nebulous envelop became, by the centrifugal force, flattened into a thin disk, which finally broke up into rings, out of which were formed the planets and their moons. according to laplace, the rings which were condensed into the planets were thrown off in succession from the equatorial region of the condensing nebula; and so the outer planets would be the older. according to the more modern idea, the nebulous mass was first flattened into a disk, and subsequently broken up into rings, in such a way that there would be no marked difference in the ages of the planets. the sun represents the central portion of the original nebula, and the comets and meteoroids its outlying portion. at the sun the condensation is still going on, and the meteoroids appear to be still gradually drawn in to the sun and planets. the whole store of energy with which the original solar nebula was endowed existed in it in the potential form. by the condensation and contraction this energy was gradually transformed into the kinetic energy of molar motion and of heat; and the heat became gradually dissipated by radiation into space. this transformation of potential energy into heat is still going on at the sun, the centre of the condensing mass, by the condensation of the sun itself, and by the impact of meteors as they fall into it. it has been calculated, that, by the shrinking of the sun to the density of the earth, the transformation of potential energy into heat would generate enough heat to maintain the sun's supply, at the present rate of dissipation, for seventeen million years. a shrinkage of the sun which would generate all the heat he has poured into space since the invention of the telescope could not be detected by the most powerful instruments yet constructed. the least velocity with which a meteoroid could strike the sun would be two hundred and eighty miles a second; and it is easy to calculate how much heat would be generated by the collision. it has been shown, that, were enough meteoroids to fall into the sun to develop its heat, they would not increase his mass appreciably during a period of two thousand years. the sun's heat is undoubtedly developed by contraction and the fall of meteoroids; that is to say, by the transformation of the potential energy of the original nebula into heat. it must be borne in mind that the nebular hypothesis is simply a supposition as to the way in which the present solar system may have been developed from a nebula endowed with a motion of rotation and with certain tendencies to condensation. of course nothing could have been developed out of the nebula, the germs of which had not been originally implanted in it by the creator. iv. the structure of the stellar universe. . _sir william herschel's view._--sir william herschel assumed that the stars are distributed with tolerable uniformity throughout the space occupied by our stellar system. he accounted for the increase in the number of stars in the field of view as he approached the plane of the milky-way, not by the supposition that the stars are really closer together in and about this plane, but by the supposition that our stellar system is in the form of a flat disk cloven at one side, and with our sun near its centre. a section of this disk is shown in fig. . [illustration: fig. .] an observer near _s_, with his telescope pointed in the direction of _s b_, would see comparatively few stars within the field of view, because looking through a comparatively thin stratum of stars. with his telescope pointed in the direction _s a_, he would see many more stars within his field of view, even though the stars were really no nearer together, because he would be looking through a thicker stratum of stars. as he directed his telescope more and more nearly in the direction _s f_, he would be looking through a thicker and thicker stratum of stars, and hence he would see a greater and greater number of them in the field of view, though they were everywhere in the disk distributed at uniform distances. he assumed, also, that the stars are all tolerably uniform in size, and that certain stars appear smaller than others, only because they are farther off. he supposed the faint stars of the milky-way to be merely the most distant stars of the stellar disk; that they are really as large as the other stars, but appear small owing to their great distance. the disk was assumed to be cloven on one side, to account for the division of the milky-way through nearly half of its course. this theory of the structure of the stellar universe is often referred to as the _cloven disk_ theory. [illustration: fig. .] . _the cloven ring theory._--according to mädler, the stars of the milky-way are entirely separated from the other stars of our system, belonging to an outlying ring, or system of rings. to account for the division of the milky-way, the ring is supposed to be cloven on one side: hence this theory is often referred to as the _cloven ring_ theory. according to this hypothesis, the stellar system viewed from without would present an appearance somewhat like that in fig. . the outlying ring cloven on one side would represent the stars of the milky-way; and the luminous mass at the centre, the remaining stars of the system. . _proctor's view._--according to proctor, the milky-way is composed of an irregular spiral stream of minute stars lying in and among the larger stars of our system, as represented in fig. . the spiral stream is shown in the inner circle as it really exists among the stars, and in the outer circle as it is seen projected upon the sky. according to this view, the stars of the milky-way appear faint, not because they are distant, but because they are really small. [illustration: fig. .] . _newcomb's view._--according to newcomb, the stars of our system are all situated in a comparatively thin zone lying in the plane of the milky-way, while there is a zone of nebulæ lying on each side of the stellar zone. he believes that so much is certain with reference to the structure of our stellar universe; but he considers that we are as yet comparatively ignorant of the internal structure of either the stellar or the nebular zones. the structure of the stellar universe, according to this view, is shown in fig. . [illustration: fig. .] index a. aberration of light, . aerolites, . aldebaran, star in taurus, , . algol, a variable star, , . almanac, perpetual, . alps, lunar mountains, . altair, star in aquila, . alt-azimuth instrument, . altitude, . andromeda (constellation), , . nebula in, . angström's map of spectrum, . antares, star in scorpio, . apennines, lunar mountains, , . aphelion, . apogee, . aquarius, or the water-bearer, . cluster in, . aquila, or the eagle, . arcturus, star in boötes, , , . argo, or the ship, . nebula in, . variable star in, . aries, or the ram, . asteroids, , . astræa, an asteroid, . auriga, or the wagoner, . azimuth, . b. betelgeuse, star in orion, , . berenice's hair (constellation), . bode's law, . disproved, . boötes (constellation), , . c. calendar, the, . callisto, moon of jupiter, . cancer, or the crab, . tropic of, . canes venatici, or the hunting-dogs, . canes venatici, nebula in, . canis major, or the great dog, . canis minor, or the little dog, . capella, star in auriga, , . capricorn, tropic of, . capricornus, or the goat, . cassiopeia (constellation), . new star in, . castor, star in gemini, , . caucasus, a lunar range, . centaurus, star-cluster in, . cepheus (constellation), . nebula in, . ceres, the planet, . cetus, or the whale, . variable star in, . charles's wain, . circles, great, . diurnal, . hour, . small, . vertical, . clock, astronomical, . time, . coma berenices, or berenice's hair, . comet, biela's, . and earth, collision of, . coggia's, . donati's, . encke's, . halley's, . of , . of , . of , . of , . of june, , . comets, appearance of, . and meteors, . bright, . chemical constitution of, . development of, . number of, . orbits of, . origin of, . periodic, . physical constitution of, . tails of, . telescopic, , . visibility of, . conic sections, . conjunction, . inferior, . superior, , . constellations, . zodiacal, . copernican system, the, , . copernicus, a lunar crater, , . corona borealis, or the northern crown, . corona borealis, new star in, . corvus, or the crow, . crystalline spheres, . cycles and epicycles, . cygnus, or the swan, . d. day and night, . civil, . lunar, . sidereal, . solar, . declination, . deimos, satellite of mars, . delphinus, or the dolphin, . deneb, star in cygnus, , . dione, satellite of saturn, . dipper, the great, , , , . the little, . the milk, . dissociation, . dominical letter, the, . draco, or the dragon, . e. earth, density of, . flattened at poles, . form of, . in space, . seen from moon, . size of, . weight of, . eccentric, the . eccentricity, . eclipses, . annular, . lunar, , . solar, . ecliptic, the, . obliquity of, . ellipse, the, , . elongation, of planet, . enceladus, moon of saturn, . epicycles, . epicycloid, . epsilon lyræ, a double star, . equator, the celestial, . equinoctial, the, . colure, . elevation of, . equinox, autumnal, . vernal, , . equinoxes, precession of, , . eta argus, a variable star, , . europa, moon of jupiter, . f. faculæ, solar, . fomalhaut, star in southern fish, . fraunhofer's lines, , . g. galaxy, the, . ganymede, moon of jupiter, . gemini, or the twins, . georgium sidus, . h. hercules (constellation), . cluster in, . orbit of double star in, . solar system moving towards, . herschel, the planet (see uranus). herschel's hypothesis, , . horizon, the, . hyades, the, , . hydra, or the water-snake, . hyperbola, the, . hyperion, moon of saturn, . i. io, moon of jupiter, . irradiation, , . j. japetus, moon of saturn, . job's coffin (asterism), . juno, the planet, . jupiter, apparent size of, . distance of, . great red spot of, . orbit of, . periods of, . physical constitution of, . rotation of, . satellites of, . eclipses of, . transits of, . volume of, . without satellites, . k. kant's hypothesis, . kepler, a lunar crater, . kepler's system, . laws, . star, . kirchhoff's map of spectrum, . l. laplace's hypothesis, . latitude, celestial, . leap year, . leo, or the lion, . leonids (meteors), . libra, or the balances, . libration, . longitude, celestial, . lyra, or the lyre, . double star in, . m. magellanic clouds, the, . magnetic storms, . magnetism and sun-spots, . mars, apparent size of, . brilliancy of, . distance of, . orbit of, . periods of, . rotation of, . satellites of, . volume of, . mercury, apparent size of, . atmosphere of, . distance of, . elongation of, . orbit of, . periods of, . volume of, . meridian, the, . meridian circle, . meridians, celestial, . meteoric iron, , . showers, . stones, . meteors, . august, . light of, . november, . sporadic, . meteoroids, . micrometers, , . milky-way, the, . mimas, moon of saturn, . mira, a variable star, . moon, apparent size of, , . aspects of, . atmosphere of, . chasms in, . craters in, . day of, . distance of, . eclipses of, . form of orbit, . harvest, . hunter's, . inclination of orbit, . kept in her path by gravity, . librations of, . mass of, . meridian altitude of, . mountains of, . orbital motion of, . phases of, . real size of, . rising of, . rotation of, . sidereal period of, . surface of, . synodical period of, . terminator of, . wet and dry, . n. nadir, the, . neap-tides, . nebula, in andromeda, . crab, . dumb-bell, . in argus, . in canes venatici, . in cepheus, . in orion, . in the triangle, . in ursa major, . nebulæ, , , . annular, . circular, . condensation of, . double, . elliptical, . irregular, . multiple, . spiral, , . nebular hypothesis, the, . neptune, discovery of, . orbit of, . satellite of, . new style, . newcomb's theory of the stellar universe, . newton's system, . nodes, . nubecula, major, . minor, . nutation, . o. olbers's hypothesis, . old style, . ophiuchus (constellation), . new star in, . opposition, , . orion, . nebula in, . the trapezium of, . p. pallas, the planet, . parabola, the, . parallax, . pegasus (constellation), , . triple star in, . perigee, . perihelion, . perseids (meteors), . perseus (constellation), . cluster in, . phobos, satellite of mars, . pico, a lunar mountain, . pisces, or the fishes, . piscis australis, or the southern fish, . planets, . inferior, . periods of, . phases of, . inner group of, . intra-mercurial, . minor, . outer group of, , . superior, . motion of, . periods of, . phases of, . three groups of, . pleiades, the, , , . pointers, the, . polar distance, . pole star, the, , , . poles, celestial, , . pollux, star in gemini, , . præsepe, or the beehive, . precession of equinoxes, , . prime vertical, the, . proctor's theory of the stellar universe, . procyon, star in canis minor, . ptolemaic system, the, . q. quadrature, , . r. radiant point (meteors), . radius vector, . refraction, . regulus, star in leo, , . rhea, moon of saturn, . rigel, star in orion, , . right ascension, . s. sagittarius, or the archer, . saturn, apparent size of, . distance of, . orbit of, . periods of, . physical constitution of, . ring of, . changes in, . constitution of, . phases of, . rotation of, . satellites of, . volume of, . scorpio, or the scorpion, . cluster in, . seasons, the, . sirius, the dog-star, , , , , . solar system, the, . solstices, , , . sound, effect of motion on, . spectra, bright-lined, . comparison of, . continuous, . displacement of lines in, . of comets, . reversed, . sun-spot, . types of stellar, . spectroscope, the, . diffraction, . direct-vision, . dispersion, . spectrum analysis, . solar, . sphere, defined, . the celestial, . rotation of, . spring-tides, . stars, circumpolar, . clusters of, , . color of, . constellations of, . constitution of, . distance of, . double, . drift of, . four sets of, . magnitude of, . motion of, in line of sight, . multiple, . names of, . nebulous, . new, . number of, . parallax of, . proper motion of, . secular displacement of, . temporary, . variable, . sun, atmosphere of, . brightness of, . chemical constitution of, . chromosphere of, , . corona of, , , . distance of, . faculæ of, . heat radiated by, . inclination of axis of, . mass of, . motion of, among the stars, . at surface of, . in atmosphere of, . secular, . photosphere of, , . prominences of, , . rotation of, . spectrum of, , . temperature of, . volume of, . winds on, . sun-spots, . and magnetism, . birth and decay of, . cause of, . cyclonic motion in, . distribution of, . duration of, . groups of, . periodicity of, . proper motion of, . size of, . spectrum of, . t. taurus, or the bull, . quadruple star in, . telescope, cassegrainian, . equatorial, . front-view, . gregorian, . herschelian, . lord rosse's, . melbourne, . newall, . newtonian, . paris, . reflecting, . washington, . vienna, . telespectroscope, the, . telluric lines of spectrum, . tethys, moon of saturn, . tides, . time, clock, . sun, . titan, moon of saturn, , . toucan, star cluster in, , . transit instrument, . transits of venus, . triesneker, lunar formation, . tropics, . twilight, . tycho brahe's star, . system, . tycho, a lunar crater, . u. universe, structure of the stellar, . uranus, discovery of, . name of, . orbit of, . satellites of, . ursa major, or the great bear, . nebula in, . ursa minor, or the little bear, . v. vega, star in lyra, , , . venus, apparent size of, . atmosphere of, . brilliancy of, . distance of, . elongation of, . orbit of, . periods of, . volume of, . transits of, , . vernier, the, . virgo, or the virgin, . vesta, the planet, . vulcan, the planet, . y. year, the, . anomalistic, . julian, . sidereal, . tropical, . z. zenith, the, . distance, . zodiac, the, . zodiacal constellations, . light, . zones, . * * * * * * transcriber's note: missing or obscured punctuation was corrected. typographical errors were silently corrected. [illustration: plate i. maps i.-iv.] half-hours with the telescope; being a popular guide to the use of the telescope as a means of amusement and instruction. by richard a. proctor, b.a., f.r.a.s., author of "saturn and its system," etc. with illustrations on stone and wood. * * * * * an undevout astronomer is mad: true, all things speak a god; but, in the small men trace out him: in great he seizes man. young. * * * * * new york: g.p. putnam's sons. . london: printed by william clowes and sons, stamford street and charing cross. preface. the object which the author and publisher of this little work have proposed to themselves, has been the production, at a moderate price, of a useful and reliable guide to the amateur telescopist. among the celestial phenomena described or figured in this treatise, by far the larger number may be profitably examined with small telescopes, and there are none which are beyond the range of a good -inch achromatic. the work also treats of the construction of telescopes, the nature and use of star-maps, and other subjects connected with the requirements of amateur observers. r.a.p. _january_, . contents. chapter i. page a half-hour on the structure of the telescope chapter ii. a half-hour with orion, lepus, taurus, etc. chapter iii. a half-hour with lyra, hercules, corvus, crater, etc. chapter iv. a half-hour with bootes, scorpio, ophiuchus, etc. chapter v. a half-hour with andromeda, cygnus, etc. chapter vi. half-hours with the planets chapter vii. half-hours with the sun and moon description of plates. plate i.--_frontispiece._ this plate presents the aspect of the heavens at the four seasons, dealt with in chapters ii., iii., iv., and v. in each map of this plate the central point represents the point vertically over the observer's head, and the circumference represents his horizon. the plan of each map is such that the direction of a star or constellation, as respects the compass-points, and its elevation, also, above the horizon, at the given season, can be at once determined. two illustrations of the use of the maps will serve to explain their nature better than any detailed description. suppose first, that--at one of the hours named under map i.--the observer wishes to find castor and pollux:--turning to map i. he sees that these stars lie in the lower left-hand quadrant, and very nearly towards the point marked s.e.; that is, they are to be looked for on the sky towards the south-east. also, it is seen that the two stars lie about one-fourth of the way from the centre towards the circumference. hence, on the sky, the stars will be found about one-fourth of the way from the zenith towards the horizon: castor will be seen immediately above pollux. next, suppose that at one of the hours named the observer wishes to learn what stars are visible towards the west and north-west:--turning the map until the portion of the circumference marked w ... n.w. is lowermost, he sees that in the direction named the square of pegasus lies not very high above the horizon, one diagonal of the square being vertical, the other nearly horizontal. above the square is andromeda, to the right of which lies cassiopeia, the stars [beta] and [epsilon] of this constellation lying directly towards the north-west, while the star [alpha] lies almost exactly midway between the zenith and the horizon. above andromeda, a little towards the left, lies perseus, algol being almost exactly towards the west and one-third of the way from the zenith towards the horizon (because one-third of the way from the centre towards the circumference of the map). almost exactly in the zenith is the star [delta] aurigæ. the four maps are miniatures of maps i., iv., vii., and x. of my 'constellation seasons,' fourth-magnitude stars, however, being omitted. plates ii., iii., iv., and v., illustrating chapters ii., iii., iv., and v. plates ii. and iv. contain four star-maps. they not only serve to indicate the configuration of certain important star-groups, but they illustrate the construction of maps, such as the observer should make for himself when he wishes to obtain an accurate knowledge of particular regions of the sky. they are all made to one scale, and on the conical projection--the simplest and best of all projections for maps of this sort. the way in which the meridians and parallels for this projection are laid down is described in my 'handbook of the stars.' with a little practice a few minutes will suffice for sweeping out the equidistant circular arcs which mark the parallels and ruling in the straight meridians. the dotted line across three of the maps represents a portion of the horizontal circle midway between the zenith and the horizon at the hour at which the map is supposed to be used. at other hours, of course, this line would be differently situated. plates iii. and v. represent fifty-two of the objects mentioned in the above-named chapters. as reference is made to these figures in the text, little comment is here required. it is to be remarked, however, that the circles, and especially the small circles, do not represent the whole of the telescope's field of view, only a small portion of it. the object of these figures is to enable the observer to know what to expect when he turns his telescope towards a difficult double star. many of the objects depicted are very easy doubles: these are given as objects of reference. the observer having seen the correspondence between an easy double and its picture, as respects the relation between the line joining the components and the apparent path of the double across the telescope's field of view, will know how to interpret the picture of a difficult double in this respect. and as all the small figures are drawn to one scale, he will also know how far apart he may expect to find the components of a difficult double. thus he will have an exact conception of the sort of duplicity he is to look for, and this is--_crede experto_--a great step towards the detection of the star's duplicity. plates vi. and vii., illustrating chapters vi. and vii. the views of mercury, venus, and mars in these plates (except the smaller view of jupiter in plate vii.) are supposed to be seen with the same "power." the observer must not expect to see the details presented in the views of mars with anything like the distinctness i have here given to them. if he place the plate at a distance of six or seven yards he will see the views more nearly as mars is likely to appear in a good three-inch aperture. the chart of mars is a reduction of one i have constructed from views by mr. dawes. i believe that nearly all the features included in the chart are permanent, though not always visible. i take this opportunity of noting that the eighteen orthographic pictures of mars presented with my shilling chart are to be looked on rather as maps than as representing telescopic views. they illustrate usefully the varying presentation of mars towards the earth. the observer can obtain other such illustrations for himself by filling in outlines, traced from those given at the foot of plate vi., with details from the chart. it is to be noted that mars varies in presentation, not only as respects the greater or less opening out of his equator towards the north or south, but as respects the apparent slope of his polar axis to the right or left. the four projections as shown, or inverted, or seen from the back of the plate (held up to the light) give presentations of mars towards the sun at twelve periods of the martial year,--viz., at the autumnal and vernal equinoxes, at the two solstices, and at intermediate periods corresponding to our terrestrial months. in fact, by means of these projections one might readily form a series of sun-views of mars resembling my 'sun-views of the earth.' in the first view of jupiter it is to be remarked that the three satellites outside the disc are supposed to be moving in directions appreciably parallel to the belts on the disc--the upper satellites from right to left, the lower one from left to right. in general the satellites, when so near to the disc, are not seen in a straight line, as the three shown in the figure happen to be. of the three spots on the disc, the faintest is a satellite, the neighbouring dark spot its shadow, the other dark spot the shadow of the satellite close to the planet's disc. half-hours with the telescope. chapter i. a half-hour on the structure of the telescope. there are few instruments which yield more pleasure and instruction than the telescope. even a small telescope--only an inch and a half or two inches, perhaps, in aperture--will serve to supply profitable amusement to those who know how to apply its powers. i have often seen with pleasure the surprise with which the performance even of an opera-glass, well steadied, and directed towards certain parts of the heavens, has been witnessed by those who have supposed that nothing but an expensive and colossal telescope could afford any views of interest. but a well-constructed achromatic of two or three inches in aperture will not merely supply amusement and instruction,--it may be made to do useful work. the student of astronomy is often deterred from telescopic observation by the thought that in a field wherein so many have laboured, with abilities and means perhaps far surpassing those he may possess, he is little likely to reap results of any utility. he argues that, since the planets, stars, and nebulæ have been scanned by herschel and rosse, with their gigantic mirrors, and at pulkova and greenwich with refractors whose construction has taxed to the utmost the ingenuity of the optician and mechanic, it must be utterly useless for an unpractised observer to direct a telescope of moderate power to the examination of these objects. now, passing over the consideration that a small telescope may afford its possessor much pleasure of an intellectual and elevated character, even if he is never able by its means to effect original discoveries, two arguments may be urged in favour of independent telescopic observation. in the first place, the student who wishes to appreciate the facts and theories of astronomy should familiarize himself with the nature of that instrument to which astronomers have been most largely indebted. in the second place, some of the most important discoveries in astronomy have been effected by means of telescopes of moderate power used skilfully and systematically. one instance may suffice to show what can be done in this way. the well-known telescopist goldschmidt (who commenced astronomical observation at the age of forty-eight, in ) added fourteen asteroids to the solar system, not to speak of important discoveries of nebulæ and variable stars, by means of a telescope only five feet in focal length, mounted on a movable tripod stand. the feeling experienced by those who look through a telescope for the first time,--especially if it is directed upon a planet or nebula--is commonly one of disappointment. they have been told that such and such powers will exhibit jupiter's belts, saturn's rings, and the continent-outlines on mars; yet, though perhaps a higher power is applied, they fail to detect these appearances, and can hardly believe that they are perfectly distinct to the practised eye. the expectations of the beginner are especially liable to disappointment in one particular. he forms an estimate of the view he is to obtain of a planet by multiplying the apparent diameter of the planet by the magnifying power of his telescope, and comparing the result with the apparent diameter of the sun or moon. let us suppose, for instance, that on the day of observation jupiter's apparent diameter is ", and that the telescopic power applied is , then in the telescope jupiter should appear to have a diameter of ", or half a degree, which is about the same as the moon's apparent diameter. but when the observer looks through the telescope he obtains a view--interesting, indeed, and instructive--but very different from what the above calculation would lead him to expect. he sees a disc apparently much smaller than the moon's, and not nearly so well-defined in outline; in a line with the disc's centre there appear three or four minute dots of light, the satellites of the planet; and, perhaps, if the weather is favourable and the observer watchful, he will be able to detect faint traces of belts across the planet's disc. yet in such a case the telescope is not in fault. the planet really appears of the estimated size. in fact, it is often possible to prove this in a very simple manner. if the observer wait until the planet and the moon are pretty near together, he will find that it is possible to view the planet with one eye through the telescope and the moon with the unaided eye, in such a manner that the two discs may coincide, and thus their relative apparent dimensions be at once recognised. nor should the indistinctness and incompleteness of the view be attributed to imperfection of the telescope; they are partly due to the nature of the observation and the low power employed, and partly to the inexperience of the beginner. it is to such a beginner that the following pages are specially addressed, with the hope of affording him aid and encouragement in the use of one of the most enchanting of scientific instruments,--an instrument that has created for astronomers a new sense, so to speak, by which, in the words of the ancient poet: subjecere oculis distantia sidera nostris, Ætheraque ingenio supposuere suo. in the first place, it is necessary that the beginner should rightly know what is the nature of the instrument he is to use. and this is the more necessary because, while it is perfectly easy to obtain such knowledge without any profound acquaintance with the science of optics, yet in many popular works on this subject the really important points are omitted, and even in scientific works such points are too often left to be gathered from a formula. when the observer has learnt what it is that his instrument is actually to do for him, he will know how to estimate its performance, and how to vary the application of its powers--whether illuminating or magnifying--according to the nature of the object to be observed. let us consider what it is that limits the range of _natural_ vision applied to distant objects. what causes an object to become invisible as its distance increases? two things are necessary that an object should be visible. it must be _large_ enough to be appreciated by the eye, and it must _send light_ enough. thus increase of distance may render an object invisible, either through diminution of its apparent size, or through diminution in the quantity of light it sends to the eye, or through both these causes combined. a telescope, therefore, or (as its name implies) an instrument to render distant objects visible, must be both a magnifying and an illuminating instrument. [illustration: _fig. ._] let ef, fig. , be an object, not near to ab as in the figure, but so far off that the bounding lines from a and b would meet at the point corresponding to the point p. then if a large convex glass ab (called an _object-glass_) be interposed between the object and the eye, all those rays which, proceeding from p, fall on ab, will be caused to converge nearly to a point _p_. the same is true for every point of the object emf, and thus a small image, _emf_, will be formed. this image will not lie exactly on a flat surface, but will be curved about the point midway between a and b as a centre. now if the lens ab is removed, and an eye is placed at _m_ to view the distant object emf, those rays only from each point of the object which fall on the pupil of the eye (whose diameter is about equal to _mp_ suppose) will serve to render the object visible. on the other hand, every point of the image _emf_ has received the whole of the light gathered up by the large glass ab. if then we can only make this light _available_, it is clear that we shall have acquired a large increase of _light_ from the distant object. now it will be noticed that the light which has converged to _p_, diverges from _p_ so that an eye, placed that this diverging pencil of rays may fall upon it, would be too small to receive the whole of the pencil. or, if it did receive the whole of this pencil, it clearly could not receive the whole of the pencils proceeding from other parts of the image _emf_. _something_ would be gained, though, even in this case, since it is clear that an eye thus placed at a distance of ten inches from _emf_ (which is about the average distance of distinct vision) would not only receive much more light from the image _emf_, than it would from the object emf, but see the image much larger than the object. it is in this way that a simple object-glass forms a telescope, a circumstance we shall presently have to notice more at length. but we want to gain the full benefit of the light which has been gathered up for us by our object-glass. we therefore interpose a small convex glass _ab_ (called an eye-glass) between the image and the eye, at such a distance from the image that the divergent pencil of rays is converted into a pencil of parallel or nearly parallel rays. call this an emergent pencil. then all the emergent pencils now converge to a point on the axial line _m_m (produced beyond _m_), and an eye suitably placed can take in all of them at once. thus the whole, or a large part, of the image is seen at once. but the image is seen inverted as shown. this is the telescope, as it was first discovered, and such an arrangement would now be called a _simple astronomical telescope_. let us clearly understand what each part of the astronomical telescope does for us:-- the object-glass ab gives us an illuminated image, the amount of illumination depending on the size of the object-glass. the eye-glass enables us to examine the image microscopically. we may apply eye-glasses of different focal length. it is clear that the shorter the focal length of _ab_, the nearer must _ab_ be placed to the image, and the smaller will the emergent pencils be, but the greater the magnifying power of the eye-glass. if the emergent pencils are severally larger than the pupil of the eye, light is wasted at the expense of magnifying power. therefore the eye-glass should never be of greater focal length than that which makes the emergent pencils about equal in diameter to the pupil of the eye. on the other hand, the eye-glass must not be of such small focal length that the image appears indistinct and contorted, or dull for want of light. [illustration: _fig. ._] let us compare with the arrangement exhibited in fig. that adopted by galileo. surprise is sometimes expressed that this instrument, which in the hands of the great florentine astronomer effected so much, should now be known as the _non-astronomical telescope_. i think this will be readily understood when we compare the two arrangements. in the galilean telescope a small concave eye-glass, _ab_ (fig. ), is placed between the object-glass and the image. in fact, no image is allowed to be formed in this arrangement, but the convergent pencils are intercepted by the concave eye-glass, and converted into parallel emergent pencils. now in fig. the concave eye-glass is so placed as to receive only a part of the convergent pencil a _p_ b, and this is the arrangement usually adopted. by using a concave glass of shorter focus, which would therefore be placed nearer to _m p_, the whole of the convergent pencil might be received in this as in the former case. but then the axis of the emergent pencil, instead of returning (as we see it in fig. ) _towards_ the axis of the telescope, would depart as much _from_ that axis. thus there would be no point on the axis at which the eye could be so placed as to receive emergent pencils showing any considerable part of the object. the difference may be compared to that between looking through the small end of a cone-shaped roll of paper and looking through the large end; in the former case the eye sees at once all that is to be seen through the roll (supposed fixed in position), in the latter the eye may be moved about so as to command the same range of view, but _at any instant_ sees over a much smaller range. to return to the arrangement actually employed, which is illustrated by the common opera-glass. we see that the full illuminating power of the telescope is not brought into play. but this is not the only objection to the galilean telescope. it is obvious that if the part c d of the object-glass were covered, the point p would not be visible, whereas, in the astronomical arrangement no other effect is produced on the visibility of an object, by covering part of the object-glass, than a small loss of illumination. in other words, the dimensions of the field of view of a galilean telescope depend on the size of the object-glass, whereas in the astronomical telescope the field of view is independent of the size of the object-glass. the difference may be readily tested. if we direct an opera-glass upon any object, we shall find that any covering placed over a part of the object-glass _becomes visible_ when we look through the instrument, interfering therefore _pro tanto_ with the range of view. a covering similarly placed on any part of the object-glass of an astronomical telescope does not become visible when we look through the instrument. the distinction has a very important bearing on the theory of telescopic vision. in considering the application of the telescope to practical observation, the circumstance that in the galilean telescope no real image is formed, is yet more important. a real image admits of measurement, linear or angular, while to a _virtual_ image (such an image, for instance, as is formed by a common looking-glass) no such process can be applied. in simple observation the only noticeable effect of this difference is that, whereas in the astronomical telescope a _stop_ or diaphragm can be inserted in the tube so as to cut off what is called the _ragged edge_ of the field of view (which includes all the part not reached by _full pencils of light_ from the object-glass), there is no means of remedying the corresponding defect in the galilean telescope. it would be a very annoying defect in a telescope intended for astronomical observation, since in general the edge of the field of view is not perceptible at night. the unpleasant nature of the defect may be seen by looking through an opera-glass, and noticing the gradual fading away of light round the circumference of the field of view. the properties of reflection as well as of refraction have been enlisted into the service of the astronomical observer. the formation of an image by means of a concave mirror is exhibited in fig. . as the observer's head would be placed between the object and the mirror, if the image, formed as in fig. , were to be microscopically examined, various devices are employed in the construction of reflecting telescopes to avoid the loss of light which would result--a loss which would be important even with the largest mirrors yet constructed. thus, in gregory's telescope, a small mirror, having its concavity towards the great one, is placed in the axis of the tube and forms an image which is viewed through an aperture in the middle of the great mirror. a similar plan is adopted in cassegrain's telescope, a small convex mirror replacing the concave one. in newton's telescope a small inclined-plane reflector is used, which sends the pencil of light off at right-angles to the axis of the tube. in herschel's telescope the great mirror is inclined so that the image is formed at a slight distance from the axis of the telescope. in the two first cases the object is viewed in the usual or direct way, the image being erect in gregory's and inverted in cassegrain's. in the third the observer looks through the side of the telescope, seeing an inverted image of the object. in the last the observer sees the object inverted, but not altered as respects right and left. the last-mentioned method of viewing objects is the only one in which the observer's back is turned towards the object, yet this method is called the _front view_--apparently _quasi lucus a non lucendo_. [illustration: _fig. ._] it appears, then, that in all astronomical telescopes, reflecting or refracting, a _real image_ of an object is submitted to microscopical examination. of this fact the possessor of a telescope may easily assure himself; for if the eye-glass be removed, and a small screen be placed at the focus of the object-glass, there will appear upon the screen a small picture of any object towards which the tube is turned. but the image may be viewed in another way which requires to be noticed. if the eye, placed at a distance of five or six inches from the image, be directed down the tube, the image will be seen as before; in fact, just as a single convex lens of short focus is the simplest microscope, so a simple convex lens of long focus is the simplest telescope.[ ] but a singular circumstance will immediately attract the observer's notice. a real picture, or the image formed on the screen as in the former case, can be viewed at varying distances; but when we view the image directly, it will be found that for distinct vision the eye must be placed almost exactly at a fixed distance from the image. this peculiarity is more important than it might be thought at first sight. in fact, it is essential that the observer who would rightly apply the powers of his telescope, or fairly test its performance, should understand in what respect an image formed by an object-glass or object-mirror differs from a real object. the peculiarities to be noted are the _curvature_, _indistinctness_, and _false colouring_ of the image. the curvature of the image is the least important of the three defects named--a fortunate circumstance, since this defect admits neither of remedy nor modification. the image of a distant object, instead of lying in a plane, that is, forming what is technically called a _flat field_, forms part of a spherical surface whose centre is at the centre of the object-glass. hence the centre of the field of view is somewhat nearer to the eye than are the outer parts of the field. the amount of curvature clearly depends on the extent of the field of view, and therefore is not great in powerful telescopes. thus, if we suppose that the angular extent of the field is about half a degree (a large or low-power field), the centre is nearer than the boundary of the field by about - th part only of the field's diameter. the indistinctness of the image is partly due to the obliquity of the pencils which form parts of the image, and partly to what is termed _spherical aberration_. the first cause cannot be modified by the optician's skill, and is not important when the field of view is small. spherical aberration causes those parts of a pencil which fall near the boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus than those which fall near the centre. this may be corrected by a proper selection of the forms of the two lenses which replace, in all modern telescopes, the single lens hitherto considered. the false colouring of the image is due to _chromatic aberration_. the pencil of light proceeding from a point, converges, not to one point, but to a short line of varying colour. thus a series of coloured images is formed, at different distances from the object-glass. so that, if a screen were placed to receive the mean image _in focus_, a coloured fringe due to the other images (_out of focus, and therefore too large_) would surround the mean image. newton supposed that it was impossible to get rid of this defect, and therefore turned his attention to the construction of reflectors. but the discovery that the _dispersive_ powers of different glasses are not proportional to their reflective powers, supplied opticians with the means of remedying the defect. let us clearly understand what is the discovery referred to. if with a glass prism of a certain form we produce a spectrum of the sun, this spectrum will be thrown a certain distance away from the point on which the sun's rays would fall if not interfered with. this distance depends on the _refractive_ power of the glass. the spectrum will have a certain length, depending on the _dispersive_ power of the glass. now, if we change our prism for another of exactly the same shape, but made of a different kind of glass, we shall find the spectrum thrown to a different spot. if it appeared that the length of the new spectrum was increased or diminished in exactly the same proportion as its distance from the line of the sun's direct light, it would have been hopeless to attempt to remedy chromatic aberration. newton took it for granted that this was so. but the experiments of hall and the dollonds showed that there is no such strict proportionality between the dispersive and refractive powers of different kinds of glass. it accordingly becomes possible to correct the chromatic aberration of one glass by superadding that of another. [illustration: _fig. ._] this is effected by combining, as shown in fig. , a convex lens of _crown_ glass with a concave lens of _flint_ glass, the convex lens being placed nearest to the object. a little colour still remains, but not enough to interfere seriously with the distinctness of the image. but even if the image formed by the object-glass were perfect, yet this image, viewed through a single convex lens of short focus placed as in fig. , would appear curved, indistinct, coloured, and also _distorted_, because viewed by pencils of light which do not pass through the centre of the eye-glass. these effects can be diminished (but not entirely removed _together_) by using an _eye-piece_ consisting of two lenses instead of a single eye-glass. the two forms of eye-piece most commonly employed are exhibited in figs. and . fig. is huyghens' eye-piece, called also the _negative_ eye-piece, because a real image is formed _behind_ the _field-glass_ (the lens which lies nearest to the object-glass). fig. represents ramsden's eye-piece, called also the _positive_ eye-piece, because the real image formed by the object-glass lies _in front of_ the field-glass. [illustration: _fig. ._] [illustration: _fig. ._] the course of a slightly oblique pencil through either eye-piece is exhibited in the figures. the lenses are usually plano-convex, the convexities being turned towards the object-glass in the negative eye-piece, and towards each other in the positive eye-piece. coddington has shown, however, that the best forms for the lenses of the negative eye-piece are those shown in fig. . the negative eye-piece, being achromatic, is commonly employed in all observations requiring distinct vision only. but as it is clearly unfit for observations requiring micrometrical measurement, or reference to fixed lines at the focus of the object-glass, the positive eye-piece is used for these purposes. for observing objects at great elevations the diagonal eye-tube is often convenient. its construction is shown in fig. . abc is a totally reflecting prism of glass. the rays from the object-glass fall on the face ab, are totally reflected on the face bc, and emerge through the face ac. in using this eye-piece, it must be remembered that it lengthens the sliding eye-tube, which must therefore be thrust further in, or the object will not be seen in focus. there is an arrangement by which the change of direction is made to take place between the two glasses of the eye-piece. with this arrangement (known as the _diagonal eye-piece_) no adjustment of the eye-tube is required. however, for amateurs' telescopes the more convenient arrangement is the diagonal eye-tube, since it enables the observer to apply any eye-piece he chooses, just as with the simple sliding eye-tube. [illustration: _fig. ._] we come next to the important question of the _mounting_ of our telescope. the best known, and, in some respects, the simplest method of mounting a telescope for general observation is that known as the _altitude-and-azimuth_ mounting. in this method the telescope is pointed towards an object by two motions,--one giving the tube the required _altitude_ (or elevation), the other giving it the required _azimuth_ (or direction as respects the compass points). for small alt-azimuths the ordinary pillar-and-claw stand is sufficiently steady. for larger instruments other arrangements are needed, both to give the telescope steadiness, and to supply slow movements in altitude and azimuth. the student will find no difficulty in understanding the arrangement of sliding-tubes and rack-work commonly adopted. this arrangement seems to me to be in many respects defective, however. the slow movement in altitude is not uniform, but varies in effect according to the elevation of the object observed. it is also limited in range; and quite a little series of operations has to be gone through when it is required to direct the telescope towards a new quarter of the heavens. however expert the observer may become by practice in effecting these operations, they necessarily take up some time (performed as they must be in the dark, or by the light of a small lantern), and during this time it often happens that a favourable opportunity for observation is lost. these disadvantages are obviated when the telescope is mounted in such a manner as is exhibited in fig. , which represents a telescope of my own construction. the slow movement in altitude is given by rotating the rod _he_, the endless screw in which turns the small wheel at _b_, whose axle in turn bears a pinion-wheel working in the teeth of the quadrant _a_. the slow movement in azimuth is given in like manner by rotating the rod _h'e'_, the lantern-wheel at the end of which turns a crown-wheel on whose axle is a pinion-wheel working in the teeth of the circle _c_. the casings at _e_ and _e'_, in which the rods _he_ and _h'e'_ respectively work, are so fastened by elastic cords that an upward pressure on the handle _h_, or a downward pressure on the handle _h'_, at once releases the endless screw or the crown-wheel respectively, so that the telescope can be swept at once through any desired angle in altitude or azimuth. this method of mounting has other advantages; the handles are conveniently situated and constant in position; also, as they do not work directly on the telescope, they can be turned without setting the tube in vibration. [illustration: _fig. ._] i do not recommend the mounting to be exactly as shown in fig. . that method is much too expensive for an alt-azimuth. but a simple arrangement of belted wheels in place of the toothed wheels _a_ and _c_ might very readily be prepared by the ingenious amateur telescopist; and i feel certain that the comfort and convenience of the arrangement would amply repay him for the labour it would cost him. my own telescope--though the large toothed-wheel and the quadrant were made inconveniently heavy (through a mistake of the workman who constructed the instrument)--worked as easily and almost as conveniently as an equatorial. still, it is well for the observer who wishes systematically to survey the heavens--and who can afford the expense--to obtain a well-mounted _equatorial_. in this method of mounting, the main axis is directed to the pole of the heavens; the other axis, at right angles to the first, carries the telescope-tube. one of the many methods adopted for mounting equatorials is that exhibited--with the omission of some minor details--in fig. . _a_ is the polar axis, _b_ is the axis (called the declination axis) which bears the telescope. the circles _c_ and _d_ serve to indicate, by means of verniers revolving with the axes, the motion of the telescope in right ascension and declination, respectively. the weight _w_ serves to counterpoise the telescope, and the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that the polar axis shall be in its proper position. the advantage gained by the equatorial method of mounting is that only one motion is required to follow a star. owing to the diurnal rotation of the earth, the stars appear to move uniformly in circles parallel to the celestial equator; and it is clear that a star so moving will be kept in the field of view, if the telescope, once directed to the star, be made to revolve uniformly and at a proper rate round the polar axis. [illustration: _fig. ._] the equatorial can be directed by means of the circles _c_ and _d_ to any celestial object whose right ascension and declination are known. on the other hand, to bring an object into the field of view of an alt-azimuth, it is necessary, either that the object itself should be visible to the naked eye, or else that the position of the object should be pretty accurately learned from star-maps, so that it may be picked up by the alt-azimuth after a little searching. a small telescope called a _finder_ is usually attached to all powerful telescopes intended for general observation. the finder has a large field of view, and is adjusted so as to have its axis parallel to that of the large telescope. thus a star brought to the centre of the large field of the finder (indicated by the intersection of two lines placed at the focus of the eye-glass) is at, or very near, the centre of the small field of the large telescope. if a telescope has no finder, it will be easy for the student to construct one for himself, and will be a useful exercise in optics. two convex lenses not very different in size from those shown in fig. , and placed as there shown--the distance between them being the sum of the focal lengths of the two glasses--in a small tube of card, wood, or tin, will serve the purpose of a finder for a small telescope. it can be attached by wires to the telescope-tube, and adjusted each night before commencing observation. the adjustment is thus managed:--a low power being applied to the telescope, the tube is turned towards a bright star; this is easily effected with a low power; then the finder is to be fixed, by means of its wires, in such a position that the star shall be in the centre of the field of the finder when also in the centre of the telescope's field. when this has been done, the finder will greatly help the observations of the evening; since with high powers much time would be wasted in bringing an object into the field of view of the telescope without the aid of a finder. yet more time would be wasted in the case of an object not visible to the naked eye, but whose position with reference to several visible stars is known; since, while it is easy to bring the point required to the centre of the _finder's_ field, in which the guiding stars are visible, it is very difficult to direct the _telescope's_ tube on a point of this sort. a card tube with wire fastenings, such as we have described, may appear a very insignificant contrivance to the regular observer, with his well-mounted equatorial and carefully-adjusted finder. but to the first attempts of the amateur observer it affords no insignificant assistance, as i can aver from my own experience. without it--a superior finder being wanting--our "half-hours" would soon be wasted away in that most wearisome and annoying of all employments, trying to "pick up" celestial objects. it behoves me at this point to speak of star-maps. such maps are of many different kinds. there are the observatory maps, in which the places of thousands of stars are recorded with an amazing accuracy. our beginner is not likely to make use of, or to want, such maps as these. then there are maps merely intended to give a good general idea of the appearance of the heavens at different hours and seasons. plate i. presents four maps of this sort; but a more complete series of eight maps has been published by messrs. walton and maberly in an octavo work; and my own 'constellation-seasons' give, at the same price, twelve quarto maps (of four of which those in plate i. are miniatures), showing the appearance of the sky at any hour from month to month, or on any night, at successive intervals of two hours. but maps intermediate in character to these and to observatory maps are required by the amateur observer. such are the society's six gnomonic maps, the set of six gnomonic maps in johnstone's 'atlas of astronomy,' and my own set of twelve gnomonic maps. the society's maps are a remarkably good set, containing on the scale of a ten-inch globe all the stars in the catalogue of the astronomical society (down to the fifth magnitude). the distortion, however, is necessarily enormous when the celestial sphere is presented in only six gnomonic maps. in my maps all the stars of the british association catalogue down to the fifth magnitude are included on the scale of a six-inch globe. the distortion is scarcely a fourth of that in the society's maps. the maps are so arranged that the relative positions of all the stars in each hemisphere can be readily gathered from a single view; and black duplicate-maps serve to show the appearance of the constellations. it is often convenient to make small maps of a part of the heavens we may wish to study closely. my 'handbook of the stars' has been prepared to aid the student in the construction of such maps. in selecting maps it is well to be able to recognise the amount of distortion and scale-variation. this may be done by examining the spaces included between successive parallels and meridians, near the edges and angles of the maps, and comparing these either with those in the centre of the map, or with the known figures and dimensions of the corresponding spaces on a globe. we may now proceed to discuss the different tests which the intending purchaser of a telescope should apply to the instrument. the excellence of an object-glass can be satisfactorily determined only by testing the performance of the telescope in the manner presently to be described. but it is well to examine the quality of the glass as respects transparency and uniformity of texture. bubbles, scratches, and other such defects, are not very important, since they do not affect the distinctness of the field as they would in a galilean telescope,--a little light is lost, and that is all. the same remark applies to dust upon the glass. the glass should be kept as free as possible from dirt, damp, or dust, but it is not advisable to remove every speck which, despite such precaution, may accidentally fall upon the object-glass. when it becomes necessary to clean the glass, it is to be noted that the substance used should be soft, perfectly dry, and free from dust. silk is often recommended, but some silk is exceedingly objectionable in texture,--old silk, perfectly soft to the touch, is perhaps as good as anything. if the dust which has fallen on the glass is at all gritty, the glass will suffer by the method of cleaning commonly adopted, in which the dust is _gathered up_ by pressure. the proper method is to clean a small space near the edge of the glass, and to _sweep_ from that space as centre. in this way the dust is _pushed before_ the silk or wash-leather, and does not cut the glass. it is well always to suspect the presence of gritty dust, and adopt this cautious method of cleaning. the two glasses should on no account be separated. in examining an eye-piece, the quality of the glass should be noted, and care taken that both glasses (but especially the field-glass) are free from the least speck, scratch, or blemish of any kind, for these defects will be exhibited in a magnified state in the field of view. hence the eye-pieces require to be as carefully preserved from damp and dust as the object-glass, and to be more frequently cleaned. the tube of the telescope should be light, but strong, and free from vibration. its quality in the last respect can be tested by lightly striking it when mounted; the sound given out should be dead or non-resonant. the inside of the tube must absorb extraneous light, and should therefore be coloured a dull black; and stops of varying radius should be placed along its length with the same object. sliding tubes, rack-work, etc., should work closely, yet easily. the telescope should be well balanced for vision with the small astronomical eye-pieces. but as there is often occasion to use appliances which disturb the balance, it is well to have the means of at once restoring equilibrium. a cord ring running round the tube (pretty tightly, so as to rest still when the tube is inclined), and bearing a small weight, will be all that is required for this purpose; it must be slipped along the tube until the tube is found to be perfectly balanced. nothing is more annoying than, after getting a star well in the field, to see the tube shift its position through defective balance, and thus to have to search again for the star. even with such an arrangement as is shown in fig. , though the tube cannot readily shift its position, it is better to have it well balanced. the quality of the stand has a very important influence on the performance of a telescope. in fact, a moderately good telescope, mounted on a steady stand, working easily and conveniently, will not only enable the observer to pass his time much more pleasantly, but will absolutely exhibit more difficult objects than a finer instrument on a rickety, ill-arranged stand. a good observing-chair is also a matter of some importance, the least constraint or awkwardness of position detracting considerably from the power of distinct vision. such, at least, is my own experience. but the mere examination of the glasses, tube, mounting, &c., is only the first step in the series of tests which should be applied to a telescope, since the excellence of the instrument depends, not on its size, the beauty of its mounting, or any extraneous circumstances, but on its performance. the observer should first determine whether the chromatic aberration is corrected. to ascertain this the telescope should be directed to the moon, or (better) to jupiter, and accurately focussed for distinct vision. if, then, on moving the eye-piece towards the object-glass, a ring of purple appears round the margin of the object, and on moving the eye-glass in the contrary direction a ring of green, the chromatic aberration is corrected, since these are the colours of the secondary spectrum. to determine whether the spherical aberration is corrected, the telescope should be directed towards a star of the third or fourth magnitude, and focussed for distinct vision. a cap with an aperture of about one-half its diameter should then be placed over the object-glass. if no new adjustment is required for distinct vision, the spherical aberration is corrected, since the mean focal length and the focal length of the central rays are equal. if, when the cap is on, the eye-piece has to be pulled out for distinct vision, the spherical aberration has not been fully corrected; if the eye-piece has to be pushed in, the aberration has been over-corrected. as a further test, we may cut off the central rays, by means of a circular card covering the middle of the object-glass, and compare the focal length for distinct vision with the focal length when the cap is applied. the extent of the spherical aberration may be thus determined; but if the first experiment gives a satisfactory result, no other is required. a star of the first magnitude should next be brought into the field of view. if an irradiation from one side is perceived, part of the object-glass has not the same refractive power as the rest; and the part which is defective can be determined by applying in different positions a cap which hides half the object-glass. if the irradiation is double, it will probably be found that the object-glass has been too tightly screwed, and the defect will disappear when the glass is freed from such undue pressure. if the object-glass is not quite at right angles to the axis of the tube, or if the eye-tube is at all inclined, a like irradiation will appear when a bright star is in the field. the former defect is not easily detected or remedied; nor is it commonly met with in the work of a careful optician. the latter defect may be detected by cutting out three circular cards of suitable size with a small aperture at the centre of each, and inserting one at each end of the eye-tube, and one over the object-glass. if the tube is rightly placed the apertures will of course lie in a right line, so that it will be possible to look through all three at once. if not, it will be easy to determine towards what part of the object-glass the eye-tube is directed, and to correct the position of the tube accordingly. the best tests for determining the defining power of a telescope are close double or multiple stars, the components of which are not very unequal. the illuminating power should be tested by directing the telescope towards double or multiple stars having one or more minute components. many of the nebulæ serve as tests both for illumination and defining power. as we proceed we shall meet with proper objects for testing different telescopes. for the present, let the following list suffice. it is selected from admiral smyth's tests, obtained by diminishing the aperture of a -in. telescope having a focal length of - / feet: a two-inch aperture, with powers of from to , should exhibit [alpha] piscium ( "· ). | [delta] cassiopeiæ ( "· ), | mag. ( and - / ) [gamma] leonis ( "· ). | polaris ( "· ), mag. ( - / | and - / ) a four-inch, powers to , should exhibit [xi] ursæ majoris ( "· ). | [sigma] cassiopeiæ ( "· ), | mag. ( and ). [gamma] ceti ( "· ). | [delta] geminorum ( "· ), | mag. ( and ). the tests in the first column are for definition, those in the second for illumination. it will be noticed that, though in the case of polaris the smaller aperture may be expected to show the small star of less than the th magnitude, a larger aperture is required to show the th magnitude component of [sigma] cassiopeiæ, on account of the greater closeness of this double. in favourable weather the following is a good general test of the performance of a telescope:--a star of the rd or th magnitude at a considerable elevation above the horizon should exhibit a small well defined disc, surrounded by two or three fine rings of light. a telescope should not be mounted within doors, if it can be conveniently erected on solid ground, as every movement in the house will cause the instrument to vibrate unpleasantly. further, if the telescope is placed in a warm room, currents of cold air from without will render observed objects hazy and indistinct. in fact, sir w. herschel considered that a telescope should not even be erected near a house or elevation of any kind round which currents of air are likely to be produced. if a telescope is used in a room, the temperature of the room should be made as nearly equal as possible to that of the outer air. when a telescope is used out of doors a 'dew-cap,' that is, a tube of tin or pasteboard, some ten or twelve inches long, should be placed on the end of the instrument, so as to project beyond the object-glass. for glass is a good radiator of heat, so that dew falls heavily upon it, unless the radiation is in some way checked. the dew-cap does this effectually. it should be blackened within, especially if made of metal. "after use," says old kitchener, "the telescope should be kept in a warm place long enough for any moisture on the object-glass to evaporate." if damp gets between the glasses it produces a fog (which opticians call a sweat) or even a seaweed-like vegetation, by which a valuable glass may be completely ruined. the observer should not leave to the precious hours of the night the study of the bearing and position of the objects he proposes to examine. this should be done by day--an arrangement which has a twofold advantage,--the time available for observation is lengthened, and the eyes are spared sudden changes from darkness to light, and _vice versâ_. besides, the eye is ill-fitted to examine difficult objects, after searching by candle-light amongst the minute details recorded in maps or globes. of the effect of rest to the eye we have an instance in sir j. herschel's rediscovery of the satellites of uranus, which he effected after keeping his eyes in darkness for a quarter of an hour. kitchener, indeed, goes so far as to recommend (with a _crede experto_) an _interval of sleep_ in the darkness of the observing-room before commencing operations. i have never tried the experiment, but i should expect it to have a bad rather than a good effect on the eyesight, as one commonly sees the eyes of a person who has been sleeping in his day-clothes look heavy and bloodshot. the object or the part of an object to be observed should be brought as nearly as possible to the centre of the field of view. when there is no apparatus for keeping the telescope pointed upon an object, the best plan is so to direct the telescope by means of the finder, that the object shall be just out of the field of view, and be brought (by the earth's motion) across the centre of the field. thus the vibrations which always follow the adjustment of the tube will have subsided before the object appears. the object should then be intently watched during the whole interval of its passage across the field of view. it is important that the student should recognise the fact that the highest powers do not necessarily give the best views of celestial objects. high powers in all cases increase the difficulty of observation, since they diminish the field of view and the illumination of the object, increase the motion with which (owing to the earth's motion) the image moves across the field, and magnify all defects due to instability of the stand, imperfection of the object-glass, or undulation of the atmosphere. a good object-glass of three inches aperture will in very favourable weather bear a power of about , when applied to the observation of close double or multiple stars, but for all other observations much lower powers should be used. nothing but failure and annoyance can follow the attempt to employ the highest powers on unsuitable objects or in unfavourable weather. the greatest care should be taken in focussing the telescope. when high powers are used this is a matter of some delicacy. it would be well if the eye-pieces intended for a telescope were so constructed that when the telescope is focussed for one, this might be replaced by any other without necessitating any use of the focussing rack-work. this could be readily effected by suitably placing the shoulder which limits the insertion of the eye-piece. it will be found that, even in the worst weather for observation, there are instants of distinct vision (with moderate powers) during which the careful observer may catch sight of important details; and, similarly, in the best observing weather, there are moments of unusually distinct vision well worth patient waiting for, since in such weather alone the full powers of the telescope can be employed. the telescopist should not be deterred from observation by the presence of fog or haze, since with a hazy sky definition is often singularly good. the observer must not expect distinct vision of objects near the horizon. objects near the eastern horizon during the time of morning twilight are especially confused by atmospheric undulations; in fact, early morning is a very unfavourable time for the observation of all objects. the same rules which we have been applying to refractors, serve for reflectors. the performance of a reflector will be found to differ in some respects, however, from that of a refractor. mr. dawes is, we believe, now engaged in testing reflectors, and his unequalled experience of refractors will enable him to pronounce decisively on the relative merits of the two classes of telescopes. we have little to say respecting the construction of telescopes. whether it is advisable or not for an amateur observer to attempt the construction of his own telescope is a question depending entirely on his mechanical ability and ingenuity. my own experience of telescope construction is confined to the conversion of a -feet into a - / -feet telescope. this operation involved some difficulties, since the aperture had to be increased by about an inch. i found a tubing made of alternate layers of card and calico well pasted together, to be both light and strong. but for the full length of tube i think a core of metal is wanted. a learned and ingenious friend, mr. sharp, fellow of st. john's college, informs me that a tube of tin, covered with layers of brown paper, well pasted and thicker near the middle of the tube, forms a light and strong telescope-tube, almost wholly free from vibration. suffer no inexperienced person to deal with your object-glass. i knew a valuable glass ruined by the proceedings of a workman who had been told to attach three pieces of brass round the cell of the double lens. what he had done remained unknown, but ever after a wretched glare of light surrounded all objects of any brilliancy. one word about the inversion of objects by the astronomical telescope. it is singular that any difficulty should be felt about so simple a matter, yet i have seen in the writings of more than one distinguished astronomer, wholly incorrect views as to the nature of the inversion. one tells us that to obtain the correct presentation from a picture taken with a telescope, the view should be inverted, held up to the light, and looked at from the back of the paper. another tells us to invert the picture and hold it opposite a looking-glass. neither method is correct. the simple correction wanted is to hold the picture upside down--the same change which brings the top to the bottom brings the right to the left, _i.e._, fully corrects the inversion. in the case, however, of a picture taken by an herschelian reflector, the inversion not being complete, a different method must be adopted. in fact, either of the above-named processes, incorrect for the ordinary astronomical, would be correct for the herschelian telescope. the latter inverts but does not reverse right and left; therefore after inverting our picture we must interchange right and left because they have been reversed by the inversion. this is effected either by looking at the picture from behind, or by holding it up to a mirror. [illustration: plate ii.] chapter ii. a half-hour with orion, lepus taurus, etc. any of the half-hours here assigned to the constellation-seasons may be taken first, and the rest in seasonal or cyclic order. the following introductory remarks are applicable to each:-- if we stand on an open space, on any clear night, we see above us the celestial dome spangled with stars, apparently fixed in position. but after a little time it becomes clear that these orbs are slowly shifting their position. those near the eastern horizon are rising, those near the western setting. careful and continuous observation would show that the stars are all moving in the same way, precisely, as they would if they were fixed to the concave surface of a vast hollow sphere, and this sphere rotated about an axis. this axis, in our latitude, is inclined about - / ° to the horizon. of course only one end of this imaginary axis can be above our horizon. this end lies very near a star which it will be well for us to become acquainted with at the beginning of our operations. it lies almost exactly towards the north, and is raised from ° to ° (according to the season and hour) above the horizon. there is an easy method of finding it. we must first find the greater bear. it will be seen from plate , that on a spring evening the seven conspicuous stars of this constellation are to be looked for towards the north-east, about half way between the horizon and the point overhead (or _zenith_), the length of the set of stars being vertical. on a summer's evening the great bear is nearly overhead. on an autumn evening he is towards the north-west, the length of the set of seven being somewhat inclined to the horizon. finally, on a winter's evening, he is low down towards the north, the length of the set of seven stars being nearly in a horizontal direction. having found the seven stars, we make use of the pointers [alpha] and [beta] (shown in plate ) to indicate the place of the pole-star, whose distance from the pointer [alpha] is rather more than three times the distance of [alpha] from [beta]. now stand facing the pole-star. then all the stars are travelling round that star _in a direction contrary to that in which the hands of a watch move_. thus the stars below the pole are moving _towards the right_, those above the pole _towards the left_, those to the right of the pole _upwards_, those to the left of the pole _downwards_. next face the south. then all the stars on our left, that is, towards the east, are rising slantingly towards the south; those due south are moving horizontally to the right, that is, towards the west; and those on our right are passing slantingly downwards towards the west. it is important to familiarise ourselves with these motions, because it is through them that objects pass out of the field of view of the telescope, and by moving the tube in a proper direction we can easily pick up an object that has thus passed away, whereas if we are not familiar with the varying motions in different parts of the celestial sphere, we may fail in the attempt to immediately recover an object, and waste time in the search for it. the consideration of the celestial motions shows how advantageous it is, when using an alt-azimuth, to observe objects as nearly as possible due south. of course in many cases this is impracticable, because a phenomenon we wish to watch may occur when an object is not situated near the meridian. but in examining double stars there is in general no reason for selecting objects inconveniently situated. we can wait till they come round to the meridian, and then observe them more comfortably. besides, most objects are higher, and therefore better seen, when due south. northern objects, and especially those within the circle of perpetual apparition, often culminate (that is, cross the meridian, or north and south line) at too great a height for comfortable vision. in this case we should observe them towards the east or west, and remember that in the first case they are rising, and in the latter they are setting, and that in both cases they have also a motion from left to right. if we allow an object to pass right across the field of view (the telescope being fixed), the apparent direction of its motion is the exact reverse of the true direction of the star's motion. this will serve as a guide in shifting the alt-azimuth after a star has passed out of the field of view. the following technical terms must be explained. that part of the field of view towards which the star appears to move is called the _preceding_ part of the field, the opposite being termed the _following_ part. the motion for all stars, except those lying in an oval space extending from the zenith to the pole of the heavens, is more or less from right to left (in the inverted field). now, if we suppose a star to move along a diameter of the field so as to divide the field into two semicircles, then in all cases in which this motion takes places from right to left, that semicircle which contains the lowest point (apparently) of the field is the _northern_ half, the other is the _southern_ half. over the oval space just mentioned the reverse holds. thus the field is divided into four quadrants, and these are termed _north following_ (_n.f._) and _south following_ (_s.f._); _north preceding_ (_n.p._), and _south preceding_ (_s.p._). the student can have no difficulty in interpreting these terms, since he knows which is the following and which the preceding _semicircle_, which the northern and which the southern. in the figures of plates and , the letters _n.f._, _n.p._, &c., are affixed to the proper quadrants. it is to be remembered that the quadrants thus indicated are measured either way from the point and feather of the diametral arrows. next, of the apparent annual motion of the stars. this takes place in exactly the same manner as the daily motion. if we view the sky at eight o'clock on any day, and again at the same hour one month later, we shall find that at the latter observation (as compared with the former) the heavens appear to have rotated by the _twelfth part_ of a complete circumference, and the appearance presented is precisely the same as we should have observed had we waited for two hours (the _twelfth part_ of a day) on the day of the first observation. * * * * * our survey of the heavens is supposed to be commenced during the first quarter of the year, at ten o'clock on the th of january, or at nine on the th of february, or at eight on the th of february, or at seven on the th of march, or at hours intermediate to these on intermediate days. we look first for the great bear towards the north-east, as already described, and thence find the pole-star; turning towards which we see, towards the right and downwards, the two guardians of the pole ([beta] and [gamma] ursæ minoris). immediately under the pole-star is the dragon's head, a conspicuous diamond of stars. just on the horizon is vega, scintillating brilliantly. overhead is the brilliant capella, near which the milky way is seen passing down to the horizon on either side towards the quarters s.s.e. and n.n.w. for the present our business is with the southern heavens, however. facing the south, we see a brilliant array of stars, sirius unmistakeably overshining the rest. orion is shining in full glory, his leading brilliant, betelgeuse[ ] being almost exactly on the meridian, and also almost exactly half way between the horizon and the zenith. in plate is given a map of this constellation and its neighbourhood. let us first turn the tube on sirius. it is easy to get him in the field without the aid of a finder. the search will serve to illustrate a method often useful when a telescope has no finder. having taking out the eye-piece--a low-power one, suppose--direct the tube nearly towards sirius. on looking through it, a glare of light will be seen within the tube. now, if the tube be slightly moved about, the light will be seen to wax and wane, according as the tube is more or less accurately directed. following these indications, it will be found easy to direct the tube, so that the object-glass shall appear _full of light_. when this is done, insert the eye-piece, and the star will be seen in the field. but the telescope is out of focus, therefore we must turn the small focussing screw. observe the charming chromatic changes--green, and red, and blue light, purer than the hues of the rainbow, scintillating and coruscating with wonderful brilliancy. as we get the focus, the excursions of these light flashes diminish until--if the weather is favourable--the star is seen, still scintillating, and much brighter than to the naked eye, but reduced to a small disc of light, surrounded (in the case of so bright a star as sirius) with a slight glare. if after obtaining the focus the focussing rack work be still turned, we see a coruscating image as before. in the case of a very brilliant star these coruscations are so charming that we may be excused for calling the observer's attention to them. the subject is not without interest and difficulty as an optical one. but the astronomer's object is to get rid of all these flames and sprays of coloured light, so that he has very little sympathy with the admiration which wordsworth is said to have expressed for out-of-focus views of the stars. we pass to more legitimate observations, noticing in passing that sirius is a double star, the companion being of the tenth magnitude, and distant about ten seconds from the primary. but our beginner is not likely to see the companion, which is a very difficult object, vowing to the overpowering brilliancy of the primary. orion affords the observer a splendid field of research. it is a constellation rich in double and multiple stars, clusters, and nebulæ. we will begin with an easy object. the star [delta] (plate ), or _mintaka_, the uppermost of the three stars forming the belt, is a wide double. the primary is of the second magnitude, the secondary of the seventh, both being white. the star [alpha] (_betelgeuse_) is an interesting object, on account of its colour and brilliance, and as one of the most remarkable variables in the heavens. it was first observed to be variable by sir john herschel in . at this period its variations were "most marked and striking." this continued until , when the changes became "much less conspicuous. in january, , they had recommenced, and on december th, , mr. fletcher observed [alpha] orionis brighter than capella, and actually the largest star in the northern hemisphere." that a star so conspicuous, and presumably so large, should present such remarkable variations, is a circumstance which adds an additional interest to the results which have rewarded the spectrum-analysis of this star by mr. huggins and professor miller. it appears that there is decisive evidence of the presence in this luminary of many elements known to exist in our own sun; amongst others are found sodium, magnesium, calcium, iron, and bismuth. hydrogen appears to be absent, or, more correctly, there are no lines in the star's spectrum corresponding to those of hydrogen in the solar spectrum. secchi considers that there is evidence of an actual change in the spectrum of the star, an opinion in which mr. huggins does not coincide. in the telescope betelgeuse appears as "a rich and brilliant gem," says lassell, "a rich topaz, in hue and brilliancy differing from any that i have seen." turn next to [beta] (rigel), the brightest star below the belt. this is a very noted double, and will severely test our observer's telescope, if small. the components are well separated (see plate ), compared with many easier doubles; the secondary is also of the seventh magnitude, so that neither as respects closeness nor smallness of the secondary, is rigel a difficult object. it is the combination of the two features which makes it a test-object. kitchener says a - / -inch object-glass should show rigel double; in earlier editions of his work he gave - / -inches as the necessary aperture. smyth mentions rigel as a test for a -inch aperture, with powers of from to . a -inch aperture, however, will certainly show the companion. rigel is an orange star, the companion blue. turn next to [lambda] the northernmost of the set of three stars in the head of orion. this is a triple star, though an aperture of inches will show it only as a double. the components are " apart, the colours pale white and violet. with the full powers of a - / -inch glass a faint companion may be seen above [lambda]. the star [zeta], the lowest in the belt, may be tried with a - / -inch glass. it is a close double, the components being nearly equal, and about - / " apart (see plate ). for a change we will now try our telescope on a nebula, selecting the great nebula in the sword. the place of this object is indicated in plate . there can be no difficulty in finding it since it is clearly visible to the naked eye on a moonless night--the only sort of night on which an observer would care to look at nebulæ. a low power should be employed. the nebula is shown in plate as i have seen it with a -inch aperture. we see nothing of those complex streams of light which are portrayed in the drawings of herschel, bond, and lassell, but enough to excite our interest and wonder. what is this marvellous light-cloud? one could almost imagine that there was a strange prophetic meaning in the words which have been translated "canst thou loose the bands of orion?" telescope after telescope had been turned on this wonderful object with the hope of resolving its light into stars. but it proved intractable to herschel's great reflector, to lassell's -feet reflector, to lord rosse's -feet reflector, and even partially to the great -feet reflector. then we hear of its supposed resolution into stars, lord rosse himself writing to professor nichol, in , "i may safely say there can be little, if any, doubt as to the resolvability of the nebula;--all about the trapezium is a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked." it was decided, therefore, that assuredly the great nebula is a congeries of stars, and not a mass of nebulous matter as had been surmised by sir w. herschel. and therefore astronomers were not a little surprised when it was proved by mr. huggins' spectrum-analysis that the nebula consists of gaseous matter. how widely extended this gaseous universe may be we cannot say. the general opinion is that the nebulæ are removed far beyond the fixed stars. if this were so, the dimensions of the orion nebula would be indeed enormous, far larger probably than those of the whole system whereof our sun is a member. i believe this view is founded on insufficient evidence, but this would not be the place to discuss the subject. i shall merely point out that the nebula occurs in a region rich in stars, and if it is not, like the great nebula in argo, clustered around a remarkable star, it is found associated in a manner which i cannot look upon as accidental with a set of small-magnitude stars, and notably with the trapezium which surrounds that very remarkable black gap within the nebula. the fact that the nebula shares the proper motion of the trapezium appears inexplicable if the nebula is really far out in space beyond the trapezium. a very small proper motion of the trapezium (alone) would long since have destroyed the remarkable agreement in the position of the dark gap and the trapezium which has been noticed for so many years. but whether belonging to our system or far beyond it, the great nebula must have enormous dimensions. a vast gaseous system it is, sustained by what arrangements or forces we cannot tell, nor can we know what purposes it subserves. mr. huggins' discovery that comets have gaseous nuclei, (so far as the two he has yet examined show) may suggest the speculation that in the orion nebula we see a vast system of comets travelling in extensive orbits around nuclear stars, and so slowly as to exhibit for long intervals of time an unchanged figure. "but of such speculations" we may say with sir j. herschel "there is no end." to return to our telescopic observations:--the trapezium affords a useful test for the light-gathering power of the telescope. large instruments exhibit nine stars. but our observer may be well satisfied with his instrument and his eye-sight if he can see five with a - / -inch aperture.[ ] a good -inch glass shows four distinctly. but with smaller apertures only three are visible. the whole neighbourhood of the great nebula will well repay research. the observer may sweep over it carefully on any dark night with profit. above the nebula is the star-cluster h. the star [iota] (double as shown in plate ) below the nebula is involved in a strong nebulosity. and in searching over this region we meet with delicate double, triple, and multiple stars, which make the survey interesting with almost any power that may be applied. above the nebula is the star [sigma], a multiple star. to an observer with a good - / -inch glass [sigma] appears as an octuple star. it is well seen, however, as a fine multiple star with a smaller aperture. some of the stars of this group appear to be variable. the star [rho] orionis is an unequal, easy double, the components being separated by nearly seven seconds. the primary is orange, the smaller star smalt-blue (see plate ). the middle star of the belt ([epsilon]) has a distant blue companion. this star, like [iota], is nebulous. in fact, the whole region within the triangle formed by stars [gamma], [kappa] and [beta] is full of nebulous double and multiple stars, whose aggregation in this region i do not consider wholly accidental. we have not explored half the wealth of orion, but leave much for future observation. we must turn, however, to other constellations. below orion is lepus, the hare, a small constellation containing some remarkable doubles. among these we may note [xi], a white star with a scarlet companion; [gamma], a yellow and garnet double; and [iota], a double star, white and pale violet, with a distant red companion. the star [kappa] leporis is a rather close double, white with a small green companion. the intensely red star r leporis (a variable) will be found in the position indicated in the map. still keeping within the boundary of our map, we may next turn to the fine cluster h (vii.) in monoceros. this cluster is visible to the naked eye, and will be easily found. the nebula h (iv.) is a remarkable one with a powerful telescope. the star monocerotis is a fine triple star described by the elder herschel as one of the finest sights in the heavens. our observer, however, will see it as a double (see plate ). [delta] monocerotis is an easy double, yellow and lavender. we may now leave the region covered by the map and take a survey of the heavens for some objects well seen at this season. towards the south-east, high up above the horizon, we see the twin-stars castor and pollux. the upper is castor, the finest double star visible in the northern heavens. the components are nearly equal and rather more than " apart (see plate ). both are white according to the best observers, but the smaller is thought by some to be slightly greenish. pollux is a coarse but fine triple star (in large instruments multiple). the components orange, grey, and lilac. there are many other fine objects in gemini, but we pass to cancer. the fine cluster præsepe in cancer may easily be found as it is distinctly visible to the naked eye in the position shown in plate , map i. in the telescope it is seen as shown in plate . the star [iota] cancri is a wide double, the colours orange and blue. procyon, the first-magnitude star between præsepe and sirius, is finely coloured--yellow with a distant orange companion, which appears to be variable. below the twins, almost in a line with them, is the star [alpha] hydræ, called al fard, or "the solitary one." it is a nd magnitude variable. i mention it, however, not on its own account, but as a guide to the fine double [epsilon] hydræ. this star is the middle one of a group of three, lying between pollux and al fard rather nearer the latter. the components of [epsilon] hydræ are separated by about - / " (see plate ). the primary is of the fourth, the companion of the eighth magnitude; the former is yellow, the latter a ruddy purple. the period of [epsilon] hydræ is about years. the constellation leo minor, now due east and about midway between the horizon and the zenith, is well worth sweeping over. it contains several fine fields. let us next turn to the western heavens. here there are some noteworthy objects. to begin with, there are the pleiades, showing to the naked eye only six or seven stars. in the telescope the pleiades appear as shown in plate . the hyades also show some fine fields with low powers. aldebaran, the principal star of the hyades, as also of the constellation taurus, is a noted red star. it is chiefly remarkable for the close spectroscopic analysis to which it has been subjected by messrs. huggins and miller. unlike betelgeuse, the spectrum of aldebaran exhibits the lines corresponding to hydrogen, and no less than eight metals--sodium, magnesium, calcium, iron, bismuth, tellurium, antimony, and mercury, are proved to exist in the constitution of this brilliant red star. on the right of aldebaran, in the position indicated in plate , map i., are the stars [zeta] and [beta] tauri. if with a low power the observer sweep from [zeta] towards [beta], he will soon find--not far from [zeta] (at a distance of about one-sixth of the distance separating [beta] from [zeta]), the celebrated crab nebula, known as m. this was the first nebula discovered by messier, and its discovery led to the formation of his catalogue of nebulæ. in a small telescope this object appears as a nebulous light of oval form, no traces being seen of the wisps and sprays of light presented in lord rosse's well known picture of the nebula. here i shall conclude the labours of our first half-hour among the stars, noticing that the examination of plate will show what other constellations besides those here considered are well situated for observation at this season. it will be remarked that many constellations well seen in the third half-hour (chapter iv.) are favourably seen in the first also, and _vice versâ_. for instance, the constellation ursa major well-placed towards the north-east in the first quarter of the year, is equally well-placed towards the north-west in the third, and similarly of the constellation cassiopeia. the same relation connects the second and fourth quarters of the year. [illustration: plate iii.] chapter iii. a half-hour with lyra, hercules, corvus, crater, etc. the observations now to be commenced are supposed to take place during the second quarter of the year,--at ten o'clock on the th of april, or at nine on the th of may, or at eight on the st of may, or at seven on the th of june, or at hours intermediate to these on intermediate days. we again look first for the great bear, now near the zenith, and thence find the pole-star. turning towards the north, we see cassiopeia between the pole-star and the horizon. towards the north-west is the brilliant capella, and towards the north-east the equally brilliant vega, beneath which, and somewhat northerly, is the cross in cygnus. the milky way passes from the eastern horizon towards the north (low down), and so round to the western horizon. in selecting a region for special observation, we shall adopt a different plan from that used in the preceding "half-hour." the region on the equator and towards the south is indeed particularly interesting, since it includes the nebular region in virgo. within this space nebulæ are clustered more closely than over any corresponding space in the heavens, save only the greater magellanic cloud. but to the observer with telescopes of moderate power these nebulæ present few features of special interest; and there are regions of the sky now well situated for observation, which, at most other epochs are either low down towards the horizon or inconveniently near to the zenith. we shall therefore select one of these, the region included in the second map of plate , and the neighbouring part of the celestial sphere. at any of the hours above named, the constellation hercules lies towards the east. a quadrant taken from the zenith to the eastern horizon passes close to the last star ([eta]) of the great bear's tail, through [beta], a star in bootes' head, near [beta] herculis, between the two "alphas" which mark the heads of hercules and ophiuchus, and so past [beta] ophiuchi, a third-magnitude star near the horizon. and here we may turn aside for a moment to notice the remarkable vertical row of six conspicuous stars towards the east-south-east; these are, counting them in order from the horizon, [zeta], [epsilon], and [delta] ophiuchi, [epsilon], [alpha], and [delta] serpentis. let the telescope first be directed towards vega. this orb presents a brilliant appearance in the telescope. its colour is a bluish-white. in an ordinary telescope vega appears as a single star, but with a large object-glass two distant small companions are seen. a nine-inch glass shows also two small companions within a few seconds of vega. in the great harvard refractor vega is seen with no less than thirty-five companions. i imagine that all these stars, and others which can be seen in neighbouring fields, indicate the association of vega with the neighbouring stream of the milky way. let our observer now direct his telescope to the star [epsilon] lyræ. or rather, let him first closely examine this star with the naked eye. the star is easily identified, since it lies to the left of vega, forming with [zeta] a small equilateral triangle. a careful scrutiny suffices to indicate a peculiarity in this star. if our observer possesses very good eye-sight, he will distinctly recognise it as a "naked-eye double"; but more probably he will only notice that it appears lengthened in a north and south direction.[ ] in the finder the star is easily divided. applying a low power to the telescope itself, we see [epsilon] lyræ as a wide double, the line joining the components lying nearly north and south. the southernmost component (the upper in the figure) is called [epsilon]^{ }, the other [epsilon]^{ }. seen as a double, both components appear white. now, if the observer's telescope is sufficiently powerful, each of the components may be seen to be itself double. first try [epsilon]^{ }, the northern pair. the line joining the components is directed as shown in plate . the distance between them is "· , their magnitudes and - / , and their colours yellow and ruddy. if the observer succeeds in seeing [epsilon]^{ } fairly divided, he will probably not fail in detecting the duplicity of [epsilon]^{ }, though this is a rather closer pair, the distance between the components being only "· . the magnitudes are and - / , both being white. between [epsilon]^{ } and [epsilon]^{ } are three faint stars, possibly forming with the quadruple a single system. let us next turn to the third star of the equilateral triangle mentioned above--viz. to the star [zeta] lyræ. this is a splendid but easy double. it is figured in plate , but it must be noticed that the figure of [zeta] and the other nine small figures are not drawn on the same scale as [epsilon] lyræ. the actual distance between the components of [zeta] lyra is ", or about one-fourth of the distance separating [epsilon]^{ } from [epsilon]^{ }. the components of [zeta] are very nearly equal in magnitude, in colour topaz and green, the topaz component being estimated as of the fifth magnitude, the green component intermediate between the fifth and sixth magnitudes. we may now turn to a star not figured in the map, but readily found. it will be noticed that the stars [epsilon], [alpha], [beta], and [gamma] form, with two small stars towards the left, a somewhat regular hexagonal figure--a hexagon, in fact, having three equal long sides and three nearly equal short sides alternating with the others. the star [eta] lyræ forms the angle next to [epsilon]. it is a wide and unequal double, as figured in plate . the larger component is azure blue; the smaller is violet, and, being only of the ninth magnitude, is somewhat difficult to catch with apertures under inches. the star [delta]^{ } lyræ is orange, [delta]^{ } blue. in the same field with these are seen many other stars. the stars [gamma]^{ } and [gamma]^{ } may also be seen in a single field, the distance between them being about half the moon's mean diameter. the former is quadruple, the components being yellow, bluish, pale blue, and blue. turn next to the stars [beta] and [gamma] lyræ, the former a multiple, the latter an unequal double star. it is not, however, in these respects that these stars are chiefly interesting, but for their variability. the variability of [gamma] has not indeed been fully established, though it is certain that, having once been less bright, [gamma] is now considerably brighter than [beta]. the change, however, may be due to the variation of [beta] alone. this star is one of the most remarkable variables known. its period is d. h. m. s. in this time it passes from a maximum brilliancy--that of a star of the · magnitude--to a minimum lustre equal to that of a star of the · magnitude, thence to the same maximum brilliancy as before, thence to another minimum of lustre--that of a star of the · magnitude--and so to its maximum lustre again, when the cycle of changes recommences. these remarkable changes seem to point to the existence of two unequal dark satellites, whose dimensions bear a much greater proportion to those of the bright components of [beta] lyræ than the dimensions of the members of the solar system bear to those of the sun. in this case, at any rate, the conjecture hazarded about algol, that the star revolves around a dark central orb, would be insufficient to account for the observed variation. nearly midway between [beta] and [gamma] lies the wonderful ring-nebula m, of which an imperfect idea will be conveyed by the last figure of plate . this nebula was discovered in , by darquier, at toulouse. it is seen as a ring of light with very moderate telescopic power. in a good - / -inch telescope the nebula exhibits a mottled appearance and a sparkling light. larger instruments exhibit a faint light within the ring; and in lord rosse's great telescope "wisps of stars" are seen within, and faint streaks of light stream from the outer border of the ring. this nebula has been subjected to spectrum-analysis by mr. huggins. it turns out to be a gaseous nebula! in fact, ring-nebulæ--of which only seven have been detected--seem to belong to the same class as the planetary nebulæ, all of which exhibit the line-spectrum indicative of gaseity. the brightest of the three lines seen in the spectrum of the ring-nebula in lyra presents a rather peculiar appearance, "since it consists," says mr. huggins, "of two bright dots, corresponding to sections of the ring, and between these there is not darkness, but an excessively faint line joining them. this observation makes it probable that the faint nebulous matter occupying the central portion is similar in constitution to that of the ring." the constellation hercules also contains many very interesting objects. let us first inspect a nebula presenting a remarkable contrast with that just described. i refer to the nebula m, known as halley's nebula (plate ). this nebula is visible to the naked eye, and in a good telescope it is a most wonderful object: "perhaps no one ever saw it for the first time without uttering a shout of wonder." it requires a very powerful telescope completely to resolve this fine nebula, but the outlying streamers may be resolved with a good -inch telescope. sir w. herschel considered that the number of the stars composing this wonderful object was at least , . the accepted views respecting nebulæ would place this and other clusters far beyond the limits of our sidereal system, and would make the component stars not very unequal (on the average) to our own sun. it seems to me far more probable, on the contrary, that the cluster belongs to our own system, and that its components are very much smaller than the average of separate stars. perhaps the whole mass of the cluster does not exceed that of an average first-magnitude star. the nebulæ m and h may be found, after a little searching, from the positions indicated in the map. they are both well worthy of study, the former being a very bright globular cluster, the latter a bright and large round nebula. the spectra of these, as of the great cluster, resemble the solar spectrum, being continuous, though, of course, very much fainter. the star [delta] herculis (seen at the bottom of the map) is a wide and easy double--a beautiful object. the components, situated as shown in plate , are of the fourth and eighth magnitude, and coloured respectively greenish-white and grape-red. the star [kappa] herculis is not shown in the map, but may be very readily found, lying between the two gammas, [gamma] herculis and [gamma] serpentis (_see_ frontispiece, map ), rather nearer the latter. it is a wide double, the components of fifth and seventh magnitude, the larger yellowish-white, the smaller ruddy yellow.[ ] ras algethi, or [alpha] herculis, is also beyond the limits of the map, but may be easily found by means of map , frontispiece. it is, properly speaking, a multiple star. considered as a double, the arrangement of the components is that shown in plate . the larger is of magnitude - / , the smaller of magnitude - / ; the former orange, the latter emerald. the companion stars are small, and require a good telescope to be well seen. ras algethi is a variable, changing from magnitude to magnitude - / in a period of - / days. the star [rho] herculis is a closer double. the components are "· apart, and situated as shown in plate . the larger is of magnitude , the smaller - / ; the former bluish-white, the latter pale emerald. there are other objects within the range of our map which are well worthy of study. such are [mu] draconis, a beautiful miniature of castor; [gamma]^{ } and [gamma]^{ } draconis, a wide double, the distance between the components being nearly " (both grey); and [gamma]^{ } and [gamma]^{ } coronæ, a naked-eye double, the components being ' apart, and each double with a good -inch telescope. we turn, however, to another region of the sky. low down, towards the south is seen the small constellation corvus, recognised by its irregular quadrilateral of stars. of the two upper stars, the left-hand one is algorab, a wide double, the components placed as in plate , "· apart, the larger of magnitude , the smaller - / , the colours pale yellow and purple. there is a red star in this neighbourhood which is well worth looking for. to the right of corvus is the constellation crater, easily recognised as forming a tolerably well-marked small group. the star alkes, or [alpha] crateris, must first be found. it is far from being the brightest star in the constellation, and may be assumed to have diminished considerably in brilliancy since it was entitled [alpha] by bayer. it will easily be found, however, by means of the observer's star maps. if now the telescope be directed to alkes, there will be found, following him at a distance of · s, and about one minute southerly, a small red star, r. crateris. like most red stars, this one is a variable. a somewhat smaller blue star may be seen in the same field. there is another red star which may be found pretty easily at this season. first find the stars [eta] and [omicron] leonis, the former forming with regulus (now lying towards the south-west, and almost exactly midway between the zenith and the horizon) the handle of the sickle in leo, the other farther off from regulus towards the right, but lower down. now sweep from [omicron] towards [eta] with a low power.[ ] there will be found a sixth-magnitude star about one-fourth of the way from [omicron] to [eta]. south, following this, will be found a group of four stars, of which one is crimson. this is the star r leonis. like r crateris and r leporis it is variable. next, let the observer turn towards the south again. above corvus, in the position shown in the frontispiece, there are to be seen five stars, forming a sort of wide v with somewhat bowed legs. at the angle is the star [gamma] virginis, a noted double. in the components were - / seconds apart. they gradually approached till, in , they could not be separated by the largest telescopes. since then they have been separating, and they are now - / seconds apart, situated as shown in plate . they are nearly equal in magnitude ( ), and both pale yellow. the star [gamma] leonis is a closer and more beautiful double. it will be found above regulus, and is the brightest star on the blade of the sickle. the components are separated by about - / seconds, the larger of the second, the smaller of the fourth magnitude; the former yellow-orange, the latter greenish-yellow. lastly, the star [iota] leonis may be tried. it will be a pretty severe test for our observer's telescope, the components being only "· apart, and the smaller scarcely exceeding the eighth magnitude. the brighter (fourth magnitude) is pale yellow, the other light blue. chapter iv. a half-hour with bootes, scorpio, ophiuchus, etc. we now commence a series of observations suited to the third quarter of the year, and to the following hours:--ten o'clock on the nd of july; nine on the th of august; eight on the rd of august; seven on the th of october; and intermediate hours on days intermediate to these. we look first for the great bear towards the north-west, and thence find the pole-star. turning towards the north we see capella and [beta] aurigæ low down and slightly towards the left of the exact north point. the milky way crosses the horizon towards the north-north-east and passes to the opposite point of the compass, attaining its highest point above the horizon towards east-south-east. this part of the milky way is well worth observing, being marked by singular variations of brilliancy. near arided (the principal star of cygnus, and now lying due east--some twenty-five degrees from the zenith) there is seen a straight dark rift, and near this space is another larger cavity, which has been termed the northern coal-sack. the space between [gamma], [delta], and [beta] cygni is covered by a large oval mass, exceedingly rich and brilliant. the neighbouring branch, extending from [epsilon] cygni, is far less conspicuous here, but near sagitta becomes brighter than the other, which in this neighbourhood suddenly loses its brilliancy and fading gradually beyond this point becomes invisible near [beta] ophiuchi. the continuous stream becomes patchy--in parts very brilliant--where it crosses aquila and clypeus. in this neighbourhood the other stream reappears, passing over a region very rich in stars. we see now the greatest extent of the milky way, towards this part of its length, ever visible in our latitudes--just as in spring we see its greatest extent towards monoceros and argo. [illustration: plate iv.] i may note here in passing that sir john herschel's delineation of the northern portion of the milky way, though a great improvement on the views given in former works, seems to require revision, and especially as respects the very remarkable patches and streaks which characterise the portion extending over cepheus and cygnus. it seems to me, also, that the evidence on which it has been urged that the stars composing the milky way are (on an average) comparable in magnitude to our own sun, or to stars of the leading magnitudes, is imperfect. i believe, for instance, that the brilliant oval of milky light in cygnus comes from stars intimately associated with the leading stars in that constellation, and not far removed in space (proportionately) beyond them. of course, if this be the case, the stars, whose combined light forms the patch of milky light, must be far smaller than the leading brilliants of cygnus. however, this is not the place to enter on speculations of this sort; i return therefore to the business we have more immediately in hand. towards the east is the square of pegasus low down towards the horizon. towards the south is scorpio, distinguished by the red and brilliant antares, and by a train of conspicuous stars. towards the west is bootes, his leading brilliant--the ruddy arcturus--lying somewhat nearer the horizon than the zenith, and slightly south of west. bootes as a constellation is easily found if we remember that he is delineated as chasing away the greater bear. thus at present he is seen in a slightly inclined position, his head (marked by the third-magnitude star [beta]) lying due west, some thirty degrees from the zenith. it has always appeared to me, by the way, that bootes originally had nobler proportions than astronomers now assign to him. it is known that canes venatici now occupy the place of an upraised arm of bootes, and i imagine that corona borealis, though undoubtedly a very ancient constellation, occupies the place of his other arm. giving to the constellation the extent thus implied, it exhibits (better than most constellations) the character assigned to it. one can readily picture to oneself the figure of a herdsman with upraised arms driving ursa major before him. this view is confirmed, i think, by the fact that the arabs called this constellation the vociferator. bootes contains many beautiful objects. partly on this account, and partly because this is a constellation with which the observer should be specially familiar, a map of it is given in plate . arcturus has a distant pale lilac companion, and is in other respects a remarkable and interesting object. it is of a ruddy yellow colour. schmidt, indeed, considers that the star has changed colour of late years, and that whereas it was once very red it is now a yellow star. this opinion does not seem well grounded, however. the star _may_ have been more ruddy once than now, though no other observer has noticed such a peculiarity; but it is certainly not a pure yellow star at present (at any rate as seen in our latitude). owing probably to the difference of colour between vega, capella and arcturus, photometricians have not been perfectly agreed as to the relative brilliancy of these objects. some consider vega the most brilliant star in the northern heavens, while others assign the superiority to capella. the majority, however, consider arcturus the leading northern brilliant, and in the whole heavens place three only before him, viz., sirius, canopus, and [alpha] centauri. arcturus is remarkable in other respects. his proper motion is very considerable, so great in fact that since the time of ptolemy the southerly motion (alone) of arcturus has carried him over a space nearly half as great again as the moon's apparent diameter. one might expect that so brilliant a star, apparently travelling at a rate so great compared with the average proper motions of the stars, must be comparatively near to us. this, however, has not been found to be the case. arcturus is, indeed, one of the stars whose distance it has been found possible to estimate roughly. but he is found to be some three times as far from us as the small star cygni, and more than seven times as far from us as [alpha] centauri. the star [delta] bootis is a wide and unequal double, the smaller component being only of the ninth magnitude. above alkaid the last star in the tail of the greater bear, there will be noticed three small stars. these are [theta], [iota], and [kappa] bootis, and are usually placed in star-maps near the upraised hand of the herdsman. the two which lie next to alkaid, [iota] and [kappa], are interesting doubles. the former is a wide double (see plate ), the magnitudes of components and , their colours yellow and white. the larger star of this pair is itself double. the star [kappa] bootis is not so wide a double (see plate ), the magnitudes of the components and , their colours white and faint blue--a beautiful object. the star [xi] bootis is an exceedingly interesting object. it is double, the colours of the components being orange-yellow and ruddy purple, their magnitudes - / and - / . when this star was first observed by herschel in the position of the components was quite different from that presented in plate . they were also much closer, being separated by a distance of less than - / seconds. since that time the smaller component has traversed nearly a full quadrant, its distance from its primary first increasing, till in the stars were nearly - / seconds apart, and thence slowly diminishing, so that at present the stars are less than seconds apart. the period usually assigned to the revolution of this binary system is years, and the period of peri-astral passage is said to be . it appears to me, however, that the period should be about years, the epoch of last peri-astral passage and of next peri-astral passage, therefore, . the angular motion of the secondary round the primary is now rapidly increasing, and the distance between the components is rapidly diminishing, so that in a few years a powerful telescope will be required to separate the pair. not far from [xi] is [pi] bootis, represented in plate as a somewhat closer double, but in reality--now at any rate--a slightly wider pair, since the distance between the components of [xi] has greatly diminished of late. both the components of [pi] are white, and their magnitudes are - / and . we shall next turn to an exceedingly beautiful and delicate object, the double star [epsilon] bootis, known also as mirac and, on account of its extreme beauty, called pulcherrima by admiral smyth. the components of this beautiful double are of the third and seventh magnitude, the primary orange, the secondary sea-green. the distance separating the components is about seconds, perhaps more; it appears to have been slowly increasing during the past ten or twelve years. smyth assigns to this system a period of revolution of years, but there can be little doubt that the true period is largely in excess of this estimate. observers in southern latitudes consider that the colours of the components are yellow and blue, not orange and green as most of our northern observers have estimated them. a little beyond the lower left-hand corner of the map is the star [delta] serpentis, in the position shown in the frontispiece, map . this is the star which at the hour and season depicted in map formed the uppermost of a vertical row of stars, which has now assumed an almost horizontal position. the components of [delta] serpentis are about - / seconds apart, their magnitudes and , both white. the stars [theta]^{ } and [theta]^{ } serpentis form a wide double, the distance between the components being - / seconds. they are nearly equal in magnitude, the primary being - / , the secondary . both are yellow, the primary being of a paler yellow colour than the smaller star. but the observer may not know where to look for [theta] serpentis, since it falls in a part of the constellation quite separated from that part in which [delta] serpentis lies. in fact [theta] lies on the extreme easterly verge of the eastern half of the constellation. it is to be looked for at about the same elevation as the brilliant altair, and (as to azimuth) about midway between altair and the south. the stars [alpha]^{ } and [alpha]^{ } libræ form a wide double, perhaps just separable by the naked eye in very favourable weather. the larger component is of the third, the smaller of the sixth magnitude, the former yellow the latter light grey. the star [beta] libræ is a beautiful light-green star to the naked eye; in the telescope a wide double, pale emerald and light blue. in scorpio there are several very beautiful objects:-- the star antares or cor scorpionis is one of the most beautiful of the red stars. it has been termed the sirius of red stars, a term better merited perhaps by aldebaran, save for this that, in our latitude, antares is, like sirius, always seen as a brilliant "scintillator" (because always low down), whereas aldebaran rises high above the horizon. antares is a double star, its companion being a minute green star. in southern latitudes the companion of antares may be seen with a good -inch, but in our latitudes a larger opening is wanted. mr. dawes once saw the companion of antares shining alone for seven seconds, the primary being hidden by the moon. he found that the colour of the secondary is not merely the effect of contrast, but that this small star is really a green sun. the star [beta] scorpionis is a fine double, the components "· apart, their magnitudes and - / , colours white and lilac. it has been supposed that this pair is only an optical double, but a long time must elapse before a decisive opinion can be pronounced on such a point. the star [sigma] scorpionis is a wider but much more difficult double, the smaller component being below the th magnitude. the colour of the primary ( ) is white, that of the secondary maroon. the star [xi] scorpionis is a neat double, the components "· apart, their magnitudes - / and - / , their colours white and grey. this star is really triple, a fifth-magnitude star lying close to the primary. in ophiuchus, a constellation covering a wide space immediately above scorpio, there are several fine doubles. among others-- ophiuchi, distance between components "· , their magnitudes - / and - / , their colours orange and blue. the star ophiuchi, a fourth-magnitude star on the right shoulder of ophiuchus, is a noted double. the distance between the components about - / ", their magnitudes - / and , the colours yellow and red. the pair form a system whose period of revolution is about years. ophiuchi (variable), distance "· , magnitudes - / and - / , colours red and yellow. [rho] opiuchi, distance ", colours yellow and blue, magnitudes and . between [alpha] and [beta] scorpionis the fine nebula m may be looked for. (or more closely thus:--below [beta] is the wide double [omega]^{ } and [omega]^{ } scorpionis; about as far to the right of antares is the star [sigma] scorpionis, and immediately above this is the fifth-magnitude star .) the nebula we seek lies between and [omega], nearer to (about two-fifths of the way towards [omega]). this nebula is described by sir w. herschel as "the richest and most condensed mass of stars which the firmament offers to the contemplation of astronomers." there are two other objects conveniently situated for observation, which the observer may now turn to. the first is the great cluster in the sword-hand of perseus (see plate ), now lying about ° above the horizon between n.e. and n.n.e. the stars [gamma] and [delta] cassiopeiæ (see map of frontispiece) point towards this cluster, which is rather farther from [delta] than [delta] from [gamma], and a little south of the produced line from these stars. the cluster is well seen with the naked eye, even in nearly full moonlight. in a telescope of moderate power this cluster is a magnificent object, and no telescope has yet revealed its full glory. the view in plate gives but the faintest conception of the glories of [chi] persei. sir w. herschel tried in vain to gauge the depths of this cluster with his most powerful telescope. he spoke of the most distant parts as sending light to us which must have started or years ago. but it appears improbable that the cluster has in reality so enormous a longitudinal extension compared with its transverse section as this view would imply. on the contrary, i think we may gather from the appearance of this cluster, that stars are far less uniform in size than has been commonly supposed, and that the mere irresolvability of a cluster is no proof of excessive distance. it is unlikely that the faintest component of the cluster is farther off than the brightest (a seventh-magnitude star) in the proportion of more than about to , while the ordinary estimate of star magnitudes, applied by herschel, gave a proportion of or to at least. i can no more believe that the components of this cluster are stars greatly varying in distance, but accidentally seen in nearly the same direction, (or that they form an _enormously long system_ turned by accident directly towards the earth), than i could look on the association of several thousand persons in the form of a procession as a fortuitous arrangement. next there is the great nebula in andromeda--known as "the transcendantly beautiful queen of the nebulæ." it will not be difficult to find this object. the stars [epsilon] and [delta] cassiopeiæ (map , frontispiece) point to the star [beta] andromedæ. almost in a vertical line above this star are two fourth-magnitude stars [mu] and [gamma], and close above [nu], a little to the right, is the object we seek--visible to the naked eye as a faint misty spot. to tell the truth, the transcendantly beautiful queen of the nebulæ is rather a disappointing object in an ordinary telescope. there is seen a long oval or lenticular spot of light, very bright near the centre, especially with low powers. but there is a want of the interest attaching to the strange figure of the great orion nebula. the andromeda nebula has been partially resolved by lord rosse's great reflector, and (it is said) more satisfactorily by the great refractor of harvard college. in the spectroscope, mr. huggins informs us, the spectrum is peculiar. continuous from the blue to the orange, the light there "appears to cease very abruptly;" there is no indication of gaseity. lastly, the observer may turn to the pair mizar and alcor, the former the middle star in the great bear's tail, the latter ' off. it seems quite clear, by the way, that alcor has increased in brilliancy of late, since among the arabians it was considered an evidence of very good eyesight to detect alcor, whereas this star may now be easily seen even in nearly full moonlight. mizar is a double star, and a fourth star is seen in the same field of view with the others (see plate ). the distance between mizar and its companion is "· ; the magnitude of mizar , of the companion ; their colours white and pale green, respectively. chapter v. a half-hour with andromeda, cygnus, etc. our last half-hour with the double stars, &c., must be a short one, as we have already nearly filled the space allotted to these objects. the observations now to be made are supposed to take place during the fourth quarter of the year,--at ten o'clock on october rd; or at nine on november th; or at eight on november nd; or at seven on december th; or at hours intermediate to these on intermediate days. we look first, as in former cases, for the great bear, now lying low down towards the north. towards the north-east, a few degrees easterly, are the twin-stars castor and pollux, in a vertical position, castor uppermost. above these, a little towards the right, we see the brilliant capella; and between capella and the zenith is seen the festoon of perseus. cassiopeia lies near the zenith, towards the north, and the milky way extends from the eastern horizon across the zenith to the western horizon. low down in the east is orion, half risen above horizon. turning to the south, we see high up above the horizon the square of pegasus. low down towards the south-south-west is fomalhaut, pointed to by [beta] and [alpha] pegasi. towards the west, about half-way between the zenith and the horizon, is the noble cross in cygnus; below which, towards the left, we see altair, and his companions [beta] and [gamma] aquilæ: while towards the right we see the brilliant vega. during this half-hour we shall not confine ourselves to any particular region of the heavens, but sweep the most conveniently situated constellations. [illustration: plate v.] first, however, we should recommend the observer to try and get a good view of the great nebula in andromeda, which is _not_ conveniently situated for observation, but is so high that after a little trouble the observer may expect a more distinct view than in the previous quarter. he will see [beta] andromedæ towards the south-east, about ° from the zenith, [mu] and [nu] nearly in a line towards the zenith, and the nebula about half-way between [beta] and the zenith. with a similar object it will be well to take another view of the great cluster in perseus, about ° from the zenith towards the east-north-east (_see_ the pointers [gamma] and [delta] cassiopeiæ in map , frontispiece), the cluster being between [delta] cassiopeiæ and [alpha] persei. not very far off is the wonderful variable algol, now due east, and about ° above the horizon. the variability of this celebrated object was doubtless discovered in very ancient times, since the name al-gol, or "the demon" seems to point to a knowledge of the peculiarity of this "slowly winking eye." to goodricke, however, is due the rediscovery of algol's variability. the period of variation is d. h. m.; during h. m. algol appears of the second magnitude; the remaining - / hours are occupied by the gradual decline of the star to the fourth magnitude, and its equally gradual return to the second. it will be found easy to watch the variations of this singular object, though, of course, many of the minima are attained in the daytime. the following may help the observer:-- on october th, , at about half-past eleven in the evening, i noticed that algol had reached its minimum of brilliancy. hence the next minimum was attained at about a quarter-past eight on the evening of october th; the next at about five on the evening of october th, and so on. now, if this process be carried on, it will be found that the next evening minimum occurred at about h. (_circiter_) on the evening of october st, the next at about h. m. on the evening of november th. thus at whatever hour any minimum occurs, another occurs _six weeks and a day later_, at about the same hour. this would be exact enough if the period of variation were _exactly_ d. m. s., but the period is nearly a minute greater, and as there are fifteen periods in six weeks and a day, it results that there is a difference of about m. in the time at which the successive recurrences at nearly the same hour take place. hence we are able to draw up the two following tables, which will suffice to give all the minima conveniently observable during the next two years. starting from a minimum at about h. m. on november th, , and noticing that the next -day period (with the m. added) gives us an observation at midnight on january nd, , and that successive periods would make the hour later yet, we take the minimum next after that of january nd, viz. that of january th, , h. m., and taking -day periods (with m. added to each), we get the series-- h. m. jan. , , p.m. feb. , ----, ---- mar. , ----, ---- may , ----, ---- june , ----, ---- aug. , ----, ---- sept. , ----, ---- nov. ----, ---- dec. , ----, ---- jan. , , ---- mar. , ----, ---- mar. , ----, ----[ ] apr. , ----, ---- june , ----, ---- july , ----, ---- sept. , ----, ---- oct. , ----, ---- nov. , ----, ---- jan. , , ---- feb. , ----, ---- from the minimum at about p.m. on october st, , we get in like manner the series-- h. m. dec. , , p.m. jan. , , ---- mar. , ----, ---- apr. , ----, ---- june , ----, ---- june , ----, ----[ ] july , ----, ---- aug. , ----, ---- oct. , ----, ---- nov. , ----, ---- jan. , , ---- feb. , ----, ---- apr. , ----, ---- may , ----, ---- june , ----, ---- aug. , ----, ---- sept. , ----, ---- nov. , ----, ---- dec. , ----, ---- jan. , , ---- from one or other of these tables every observable minimum can be obtained. thus, suppose the observer wants to look for a minimum during the last fortnight in august, . the first table gives him no information, the latter gives him a minimum at h. m. p.m. on august ; hence of course there is a minimum at h. m. p.m. on august ; and there are no other conveniently observable minima during the fortnight in question. the cause of the remarkable variation in this star's brilliancy has been assigned by some astronomers to the presence of an opaque secondary, which transits algol at regular intervals; others have adopted the view that algol is a luminous secondary, revolving around an opaque primary. of these views the former seems the most natural and satisfactory. it points to a secondary whose mass bears a far greater proportion to that of the primary, than the mass even of jupiter bears to the sun; the shortness of the period is also remarkable. it may be noticed that observation points to a gradual diminution in the period of algol's variation, and the diminution seems to be proceeding more and more rapidly. hence (assuming the existence of a dark secondary) we must suppose that either it travels in a resisting medium which is gradually destroying its motion, or that there are other dependent orbs whose attractions affect the period of this secondary. in the latter case the decrease in the period will attain a limit and be followed by an increase. however, interesting as the subject may be, it is a digression from telescopic work, to which we now return. within the confines of the second map in plate is seen the fine star [gamma] andromedæ. at the hour of our observations it lies high up towards e.s.e. it is seen as a double star with very moderate telescopic power, the distance between the components being upwards of "; their magnitudes and - / , their colours orange and green. perhaps there is no more interesting double visible with low powers. the smaller star is again double in first-class telescopes, the components being yellow and blue according to some observers, but according to others, both green. below [gamma] andromedæ lie the stars [beta] and [gamma] triangulorum, [gamma] a fine naked-eye triple (the companions being [delta] and [eta] triangulorum), a fine object with a very low power. to the right is [alpha] triangulorum, certainly less brilliant than [beta]. below [alpha] are the three stars [alpha], [beta], and [gamma] arietis, the first an unequal and difficult double, the companion being purple, and only just visible (under favourable circumstances) with a good -inch telescope; the last an easy double, interesting as being the first ever discovered (by hook, in ), the colours of components white and grey. immediately below [alpha] arietis is the star [alpha] ceti, towards the right of which (a little lower) is mira, a wonderful variable. this star has a period of - / days; during a fortnight it appears as a star of the nd magnitude,--on each side of this fortnight there is a period of three months during one of which the star is increasing, while during the other it is diminishing in brightness: during the remaining five months of the period the star is invisible to the naked eye. there are many peculiarities and changes in the variation of this star, into which space will not permit me to enter. immediately above mira is the star [alpha] piscium at the knot of the fishes' connecting band. this is a fine double, the distance between the components being about - / ", their magnitudes and , their colours pale green and blue (see plate ). close to [gamma] aquarii (see frontispiece, map ), above and to the left of it, is the interesting double [zeta] aquarii; the distance between the components is about - / ", their magnitudes and - / , both whitish yellow. the period of this binary seems to be about years. turning next towards the south-west we see the second-magnitude star [epsilon] pegasi, some ° above the horizon. this star is a wide but not easy double, the secondary being only of the ninth magnitude; its colour is lilac, that of the primary being yellow. towards the right of [epsilon] pegasi and lower down are seen the three fourth-magnitude stars which mark the constellation equuleus. of these the lowest is [alpha], to the right of which lies [epsilon] equulei, a fifth-magnitude star, really triple, but seen as a double star with ordinary telescopes (plate ). the distance between the components is nearly ", their colours white and blue, their magnitudes - / and - / . the primary is a very close double, which appears, however, to be opening out rather rapidly. immediately below equuleus are the stars [alpha]^{ } and [alpha]^ capricorni, seen as a naked-eye double to the right of and above [beta]. both [alpha]^ and [alpha]^ are yellow; [alpha]^ is of the rd, [alpha]^ of the th magnitude; in a good telescope five stars are seen, the other three being blue, ash-coloured, and lilac. the star [beta] capricorni is also a wide double, the components yellow and blue, with many telescopic companions. to the right of equuleus, towards the west-south-west is the constellation delphinus. the upper left-hand star of the rhombus of stars forming the head of the delphinus is the star [gamma] delphini, a rather easy double (see plate ), the components being nearly " apart, their magnitudes and , their colours golden yellow and flushed grey. turn we next to the charming double albireo, on the beak of cygnus, about ° above the horizon towards the west. the components are - / " apart, their magnitudes and , their colours orange-yellow, and blue. it has been supposed (perhaps on insufficient evidence) that this star is merely an optical double. it must always be remembered that a certain proportion of stars (amongst those separated by so considerable a distance) _must_ be optically combined only. the star [chi] cygni is a wide double (variable) star. the components are separated by nearly ", their magnitudes and , their colours yellow and light blue. [chi] may be found by noticing that there is a cluster of small stars in the middle of the triangle formed by the stars [gamma], [delta], and [beta] cygni (see map , frontispiece), and that [chi] is the nearest star _of the cluster_ to [beta]. the star [phi] cygni, which is just above and very close to [beta] (albireo), does not belong to the cluster. [chi] is about half as far again from [phi] as [phi] from albireo. but as [chi] descends to the th magnitude at its minimum the observer must not always expect to find it very easily. it has been known to be invisible at the epoch when it should have been most conspicuous. the period of this variable is days. the star cygni is an interesting one. so far as observation has yet extended, it would seem to be the nearest to us of all stars visible in the northern hemisphere. it is a fine double, the components nearly equal ( - / and ), both yellow, and nearly " apart. the period of this binary appears to be about years. to find cygni note that [epsilon] and [delta] cygni form the diameter of a semicircle divided into two quadrants by [alpha] cygni (arided). on this semicircle, on either side of [alpha], lie the stars [nu] and [alpha] cygni, [nu] towards [epsilon]. now a line from [alpha] to [nu] produced passes very near to cygni at a distance from [nu] somewhat greater than half the distance of [nu] from [alpha]. the star [mu] cygni lies in a corner of the constellation, rather farther from [zeta] than [zeta] from [epsilon] cygni. a line from [epsilon] to [zeta] produced meets [kappa] pegasi, a fourth-magnitude star; and [mu] cygni, a fifth-magnitude star, lies close above [kappa] pegasi. the distance between the components is about - / ", their magnitudes and , their colours white and pale blue. the star [psi] cygni may next be looked for, but for this a good map of cygnus will be wanted, as [psi] is not pointed to by any well-marked stars. a line from [alpha], parallel to the line joining [gamma] and [delta], and about one-third longer than that line, would about mark the position of [psi] cygni. the distance between the components of this double is about - / ", their magnitudes - / and , their colours white and lilac. lastly, the observer may turn to the stars [gamma]_{ } and [gamma]_{ } draconis towards the north-west about ° above the horizon (they are included in the second map of plate ). they form a wide double, having equal (fifth-magnitude) components, both grey. (see plate .) chapter vi. half-hours with the planets. in observing the stars, we can select a part of the heavens which may be conveniently observed; and in this way in the course of a year we can observe every part of the heavens visible in our northern hemisphere. but with the planets the case is not quite so simple. they come into view at no fixed season of the year: some of them can never be seen _by night_ on the meridian; and they all shift their place among the stars, so that we require some method of determining where to look for them on any particular night, and of recognising them from neighbouring fixed stars. the regular observer will of course make use of the 'nautical almanac'; but 'dietrichsen and hannay's almanac' will serve every purpose of the amateur telescopist. i will briefly describe those parts of the almanac which are useful to the observer. it will be found that three pages are assigned to each month, each page giving different information. if we call these pages i. ii. iii., then in order that page i. for each month may fall to the left of the open double page, and also that i. and ii. may be open together, the pages are arranged in the following order: i. ii. iii.; iii. i. ii.; i. ii. iii.; and so on. now page iii. for any month does not concern the amateur observer. it gives information concerning the moon's motions, which is valuable to the sailor, and interesting to the student of astronomy, but not applicable to amateur observation. [illustration: plate vi.] we have then only pages i. and ii. to consider:-- across the top of both pages the right ascension and declination of the planets venus, jupiter, mars, saturn, mercury, and uranus are given, accompanied by those of two conspicuous stars. this information is very valuable to the telescopist. in the first place, as we shall presently see, it shows him what planets are well situated for observation, and secondly it enables him to map down the path of any planet from day to day among the fixed stars. this is a very useful exercise, by the way, and also a very instructive one. the student may either make use of the regular maps and mark down the planet's path in pencil, taking a light curve through the points given by the data in his almanac, or he may lay down a set of meridians suited to the part of the heavens traversed by the planet, and then proceed to mark in the planet's path and the stars, taking the latter either from his maps or from a convenient list of stars.[ ] my 'handbook of the stars' has been constructed to aid the student in these processes. it must be noticed that old maps are not suited for the work, because, through precession, the stars are all out of place as respects r.a. and dec. even the society's maps, constructed so as to be right for , are beginning to be out of date. but a matter of or years either way is not important.[ ] my maps, handbook and zodiac-chart have been constructed for the year , so as to be serviceable for the next fifty years or so. next, below the table of the planets, we have a set of vertical columns. these are, in order, the days of the month, the calendar--in which are included some astronomical notices, amongst others the diameter of saturn on different dates, the hours at which the sun rises and sets, the sun's right ascension, declination, diameter, and longitude; then eight columns which do not concern the observer; after which come the hours at which the moon rises and sets, the moon's age; and lastly (so far as the observer is concerned) an important column about jupiter's system of satellites. next, we have, at the foot of the first page, the hours at which the planets rise, south, and set; and at the foot of the second page we have the dates of conjunctions, oppositions, and of other phenomena, the diameters of venus, jupiter, mars, and mercury, and finally a few words respecting the visibility of these four planets. after the thirty-six pages assigned to the months follow four (pp. - ) in which much important astronomical information is contained; but the points which most concern our observer are (i.) a small table showing the appearance of saturn's rings, and (ii.) a table giving the hours at which jupiter's satellites are occulted or eclipsed, re-appear, &c. we will now take the planets in the order of their distance from the sun: we shall see that the information given by the almanac is very important to the observer. mercury is so close to the sun as to be rarely seen with the naked eye, since he never sets much more than two hours and a few minutes after the sun, or rises by more than that interval before the sun. it must not be supposed that at each successive epoch of most favourable appearance mercury sets so long after the sun or rises so long before him. it would occupy too much of our space to enter into the circumstances which affect the length of these intervals. the question, in fact, is not a very simple one. all the necessary information is given in the almanac. we merely notice that the planet is most favourably seen as an evening star in spring, and as a morning star in autumn.[ ] the observer with an equatorial has of course no difficulty in finding mercury, since he can at once direct his telescope to the proper point of the heavens. but the observer with an alt-azimuth might fail for years together in obtaining a sight of this interesting planet, if he trusted to unaided naked-eye observations in looking for him. copernicus never saw mercury, though he often looked for him; and mr. hind tells me he has seen the planet but once with the naked eye--though this perhaps is not a very remarkable circumstance, since the systematic worker in an observatory seldom has occasion to observe objects with the unaided eye. by the following method the observer can easily pick up the planet. across two uprights (fig. ) nail a straight rod, so that when looked at from some fixed point of view the rod may correspond to the sun's path near the time of observation. the rod should be at right-angles to the line of sight to its centre. fasten another rod at right angles to the first. from the point at which the rods cross measure off and mark on both rods spaces each subtending a degree as seen from the point of view. thus, if the point of view is - / feet off, these spaces must each be inches long, and they must be proportionately less or greater as the eye is nearer or farther. [illustration: _fig. ._] now suppose the observer wishes to view mercury on some day, whereon mercury is an evening star. take, for instance, june th, . we find from 'dietrichsen' that on this day (at noon) mercury's r.a. is h. m. s.: and the sun's h. m. s. we need not trouble ourselves about the odd hours after noon, and thus we have mercury's r.a. greater than the sun's by h. m. s. now we will suppose that the observer has so fixed his uprights and the two rods, that the sun, seen from the fixed point of view, appears to pass the point of crossing of the rods at half-past seven, then mercury will pass the cross-rod at m. s. past nine. but where? to learn this we must take out mercury's declination, which is ° ' " n., and the sun's, which is ° ' " n. the difference, ° ' " n. gives us mercury's place, which it appears is rather less than - / degree north of the sun. thus, about h. m. after the sun has passed the cross-rod, mercury will pass it between the first and second divisions above the point of fastening. the sun will have set about an hour, and mercury will be easily found when the telescope is directed towards the place indicated. it will be noticed that this method does not require the time to be exactly known. all we have to do is to note the moment at which the sun passes the point of fastening of the two rods, and to take our h. m. from that moment. this method, it may be noticed in passing, may be applied to give naked-eye observations of mercury at proper seasons (given in the almanac). by a little ingenuity it may be applied as well to morning as to evening observations, the sun's passage of the cross-rod being taken on one morning and mercury's on the next, so many minutes _before_ the hour of the first observation. in this way several views of mercury may be obtained during the year. such methods may appear very insignificant to the systematic observer with the equatorial, but that they are effective i can assert from my own experience. similar methods may be applied to determine from the position of a known object, that of any neighbouring unknown object even at night. the cross-rod must be shifted (or else two cross-rods used) when the unknown _precedes_ the known object. if two cross-rods are used, account must be taken of the gradual diminution in the length of a degree of right ascension as we leave the equator. even simpler methods carefully applied may serve to give a view of mercury. to show this, i may describe how i obtained my first view of this planet. on june st, , i noticed, that at five minutes past seven the sun, as seen from my study window, appeared from behind the gable-end of mr. st. aubyn's house at stoke, devon. i estimated the effect of mercury's northerly declination (different of course for a vertical wall, than for the cross-rod in fig. , which, in fact, agrees with a declination-circle), and found that he would pass out opposite a particular point of the wall a certain time after the sun. i then turned the telescope towards that point, and focussed for distinct vision of distant objects, so that the outline of the house was seen out of focus. as the calculated time of apparition approached, i moved the telescope up and down so that the field swept the neighbourhood of the estimated point of apparition. i need hardly say that mercury did not appear exactly at the assigned point, nor did i see him make his first appearance; but i picked him up so soon after emergence that the outline of the house was in the field of view with him. he appeared as a half-disc. i followed him with the telescope until the sun had set, and soon after i was able to see him very distinctly with the naked eye. he shone with a peculiar brilliance on the still bright sky; but although perfectly distinct to the view when his place was indicated, he escaped detection by the undirected eye.[ ] mercury does not present any features of great interest in ordinary telescopes; though he usually appears better defined than venus, at least as the latter is seen on a dark sky. the phases are pleasingly seen (as shown in plate ) with a telescope of moderate power. for their proper observation, however, the planet must be looked for with the telescope in the manner above indicated, as he always shows a nearly semi-circular disc when he is visible to the naked eye. we come next to venus, the most splendid of all the planets to the eye. in the telescope venus disappoints the observer, however. her intense lustre brings out every defect of the instrument, and especially the chromatic aberration. a dark glass often improves the view, but not always. besides, an interposed glass has an unpleasant effect on the field of view. perhaps the best method of observing venus is to search for her when she is still high above the horizon, and when therefore the background of the sky is bright enough to take off the planet's glare. the method i have described for the observation of mercury will prove very useful in the search for venus when the sun is above the horizon or but just set. of course, when an object is to be looked for high above the horizon, the two rods which support the cross-rods must not be upright, but square to the line of view to that part of the sky. but the observer must not expect to see much during his observation of venus. in fact, he can scarcely do more than note her varying phases (see plate ) and the somewhat uneven boundary of the terminator. our leading observers have done so little with this fascinating but disappointing planet, that amateurs must not be surprised at their own failure. i suppose the question whether venus has a satellite, or at any rate whether the object supposed to have been seen by cassini and other old observers were a satellite, must be considered as decided in the negative. that cassini should have seen an object which dawes and webb have failed to see must be considered utterly improbable. leaving the inferior planets, we come to a series of important and interesting objects. first we have the planet mars, nearly the last in the scale of planetary magnitude, but far from being the least interesting of the planets. it is in fact quite certain that we obtain a better view of mars than of any object in the heavens, save the moon alone. he may present a less distinguished appearance than jupiter or saturn, but we see his surface on a larger scale than that of either of those giant orbs, even if we assume that we ever obtain a fair view of their real surface. nor need the moderately armed observer despair of obtaining interesting views of mars. the telescope with which beer and mädler made their celebrated series of views was only a -inch one, so that with a -inch or even a -inch aperture the attentive observer may expect interesting views. in fact, more depends on the observer than on the instrument. a patient and attentive scrutiny will reveal features which at the first view wholly escape notice. in plate i have given a series of views of mars much more distinct than an observer may expect to obtain with moderate powers. i add a chart of mars, a miniature of one i have prepared from a charming series of tracings supplied me by mr. dawes. the views taken by this celebrated observer in , , , , and , are far better than any others i have seen. the views by beer and mädler are good, as are some of secchi's (though they appear badly drawn), nasmyth's and phillips'; delarue's two views are also admirable; and lockyer has given a better set of views than any of the others. but there is an amount of detail in mr. dawes' views which renders them superior to any yet taken. i must confess i failed at a first view to see the full value of mr. dawes' tracings. faint marks appeared, which i supposed to be merely intended to represent shadings scarcely seen. a more careful study shewed me that every mark is to be taken as the representative of what mr. dawes actually saw. the consistency of the views is perfectly wonderful, when compared with the vagueness and inconsistency observable in nearly all other views. and this consistency is not shown by mere resemblance, which might have been an effect rather of memory (unconsciously exerted) than observation. the same feature changes so much in figure, as it appears on different parts of the disc, that it was sometimes only on a careful projection of different views that i could determine what certain features near the limb represented. but when this had been done, and the distortion through the effect of foreshortening corrected, the feature was found to be as true in shape as if it had been seen in the centre of the planet's disc. in examining mr. dawes' drawings it was necessary that the position of mars' axis should be known. the data for determining this were taken from dr. oudemann's determinations given in a valuable paper on mars issued from mr. bishop's observatory. but instead of calculating mars' presentation by the formulæ there given, i found it convenient rather to make use of geometrical constructions applied to my 'charts of the terrestrial planets.' taking mädler's start-point for martial longitudes, that is the longitude-line passing near dawes' forked bay, i found that my results agreed pretty fairly with those in prof. phillips' map, so far as the latter went; but there are many details in my charts not found in prof. phillips' nor in mädler's earlier charts. i have applied to the different features the names of those observers who have studied the physical peculiarities presented by mars. mr. dawes' name naturally occurs more frequently than others. indeed, if i had followed the rule of giving to each feature the name of its discoverer, mr. dawes' name would have occurred much more frequently than it actually does. on account of the eccentricity of his orbit, mars is seen much better in some oppositions than in others. when best seen the southern hemisphere is brought more into view than the northern because the summer of his northern hemisphere occurs when he is nearly in aphelion (as is the case with the earth by the way). the relative dimensions and presentation of mars, as seen in opposition in perihelion, and in opposition in aphelion, are shown in the two rows of figures. in and near quadrature mars is perceptibly gibbous. he is seen thus about two months before or after opposition. in the former case, he rises late and comes to the meridian six hours or so after midnight. in the latter case, he is well seen in the evening, coming to the meridian at six. his appearance and relative dimensions as he passes from opposition to quadrature are shown in the last three figures of the upper row. mars' polar caps may be seen with very moderate powers. i add four sets of meridians (plate ), by filling in which from the charts the observer may obtain any number of views of the planet as it appears at different times. passing over the asteroids, which are not very interesting objects to the amateur telescopist, we come to jupiter, the giant of the solar system, surpassing our earth more than times in volume, and overweighing all the planets taken together twice over. jupiter is one of the easiest of all objects of telescopic observation. no one can mistake this orb when it shines on a dark sky, and only venus can be mistaken for it when seen as a morning or evening star. sometimes both are seen together on the twilight sky, and then venus is generally the brighter. seen, however, at her brightest and at her greatest elongation from the sun, her splendour scarcely exceeds that with which jupiter shines when high above the southern horizon at midnight. jupiter's satellites may be seen with very low powers; indeed the outer ones have been seen with the naked eye, and all are visible in a good opera-glass. their dimensions relatively to the disc are shown in plate . their greatest elongations are compared with the disc in the low-power view. jupiter's belts may also be well seen with moderate telescopic power. the outer parts of his disc are perceptibly less bright than the centre. more difficult of observation are the transits of the satellites and of their shadows. still the attentive observer can see the shadows with an aperture of two inches, and the satellites themselves with an aperture of three inches. the minute at which the satellites enter on the disc, or pass off, is given in 'dietrichsen's almanac.' the 'nautical almanac' also gives the corresponding data for the shadows. the eclipses of the satellites in jupiter's shadow, and their occultations by his disc, are also given in 'dietrichsen's almanac.' in the inverting telescope the satellites move from right to left in the nearer parts of their orbit, and therefore transit jupiter's disc in that direction, and from left to right in the farther parts. also note that _before_ opposition, (i.) the shadows travel in front of the satellites in transiting the disc; (ii.) the satellites are eclipsed in jupiter's _shadow_; (iii.) they reappear from behind his _disc_. on the other hand, _after_ opposition, (i.) the shadows travel _behind_ the satellites in transiting the disc; (ii.) the satellites are occulted by the _disc_; (iii.) they reappear from eclipse in jupiter's _shadow_. conjunctions of the satellites are common phenomena, and may be waited for by the observer who sees the chance. an eclipse of one satellite by the shadow of another is not a common phenomenon; in fact, i have never heard of such an eclipse being seen. that a satellite should be quite extinguished by another's shadow is a phenomenon not absolutely impossible, but which cannot happen save at long intervals. the shadows are not _black spots_ as is erroneously stated in nearly all popular works on astronomy. the shadow of the fourth, for instance, is nearly all penumbra, the really black part being quite minute by comparison. the shadow of the third has a considerable penumbra, and even that of the first is not wholly black. these penumbras may not be perceptible, but they affect the appearance of the shadows. for instance, the shadow of the fourth is perceptibly larger but less black than that of the third, though the third is the larger satellite. in transit the first satellite moves fastest, the fourth slowest, the others in their order. the shadow moves just as fast (appreciably) as the satellite it belongs to. sometimes the shadow of the satellite may be seen to overtake (apparently) the disc of another. in such a case the shadow does not pass over the disc, but the disc conceals the shadow. this is explained by the fact that the shadow, if visible throughout its length, would be a line reaching slantwise from the satellite it belongs to, and the end of the shadow (that is, the point where it meets the disc) is _not_ the point where the shadow crosses the orbit of any inner satellite. thus the latter may be interposed between the end of the shadow--the only part of the shadow really visible--and the eye; but the end of the shadow _cannot_ be interposed between the satellite and the eye. if a satellite _on the disc_ were eclipsed by another satellite, the black spot thus formed would be in another place from the black spot on the planet's body. i mention all this because, simple as the question may seem, i have known careful observers to make mistakes on this subject. a shadow is seen crossing the disc and overtaking, apparently, a satellite in transit. it seems therefore, on a first view, that the shadow will hide the satellite, and observers have even said that they have _seen_ this happen. but they are deceived. it is obvious that _if one satellite eclipse another, the shadows of both must occupy the same point on jupiter's body_. thus it is the overtaking of one _shadow_ by another on the disc, and not the overtaking of a _satellite_ by a shadow, which determines the occurrence of that as yet unrecorded phenomenon, the eclipse of one satellite by another.[ ] the satellites when far from jupiter seem to lie in a straight line through his centre. but as a matter of fact they do not in general lie in an exact straight line. if their orbits could be seen as lines of light, they would appear, in general, as very long ellipses. the orbit of the fourth would frequently be seen to be _quite clear_ of jupiter's disc, and the orbit of the third might in some very exceptional instances pass _just_ clear of the disc. the satellites move most nearly in a straight line (apparently) when jupiter comes to opposition in the beginning of february or august, and they appear to depart most from rectilinear motion when opposition occurs in the beginning of may and november. at these epochs the fourth satellite may be seen to pass above and below jupiter's disc at a distance equal to about one-sixth of the disc's radius. the shadows do not travel in the same apparent paths as the satellites themselves across the disc, but (in an inverting telescope) _below_ from august to january, and _above_ from february to july. we come now to the most charming telescopic object in the heavens--the planet saturn. inferior only to jupiter in mass and volume, this planet surpasses him in the magnificence of his system. seen in a telescope of adequate power, saturn is an object of surpassing loveliness. he must be an unimaginative man who can see saturn for the first time in such a telescope, without a feeling of awe and amazement. if there is any object in the heavens--i except not even the sun--calculated to impress one with a sense of the wisdom and omnipotence of the creator it is this. "his fashioning hand" is indeed visible throughout space, but in saturn's system it is most impressively manifest. saturn, to be satisfactorily seen, requires a much more powerful telescope than jupiter. a good -inch telescope will do much, however, in exhibiting his rings and belts. i have never seen him satisfactorily myself with such an aperture, but mr. grover has not only seen the above-named features, but even a penumbra to the shadow on the rings with a -inch telescope. saturn revolving round the sun in a long period--nearly thirty years--presents slowly varying changes of appearance (see plate ). at one time the edge of his ring is turned nearly towards the earth; seven or eight years later his rings are as much open as they can ever be; then they gradually close up during a corresponding interval; open out again, exhibiting a different face; and finally close up as first seen. the last epoch of greatest opening occurred in , the next occurs in : the last epoch of disappearance occurred in - , the next occurs in . the successive views obtained are as in plate in order from right to left, then back to the right-hand figure (but sloped the other way); inverting the page we have this figure thus sloped, and the following changes are now indicated by the other figures in order back to the first (but sloped the other way and still inverted), thus returning to the right-hand figure as seen without inversion. the division in the ring can be seen in a good -inch aperture in favourable weather. the dark ring requires a good -inch and good weather. saturn's satellites do not, like jupiter's, form a system of nearly equal bodies. titan, the sixth, is probably larger than any of jupiter's satellites. the eighth also (japetus) is a large body, probably at least equal to jupiter's third satellite. but rhea, dione, and tethys are much less conspicuous, and the other three cannot be seen without more powerful telescopes than those we are here dealing with. so far as my own experience goes, i consider that the five larger satellites may be seen distinctly in good weather with a good - / -inch aperture. i have never seen them with such an aperture, but i judge from the distinctness with which these satellites may be seen with a -inch aperture. titan is generally to be looked for at a considerable distance from saturn--_always_ when the ring is widely open. japetus is to be looked for yet farther from the disc. in fact, when saturn comes to opposition in perihelion (in winter only this can happen) japetus may be as far from saturn as one-third of the apparent diameter of the moon. i believe that under these circumstances, or even under less favourable circumstances, japetus could be seen with a good opera-glass. so also might titan. transits, eclipses, and occulations of saturn's satellites can only be seen when the ring is turned nearly edgewise towards the earth. for the orbits of the seven inner satellites lying nearly in the plane of the rings would (if visible throughout their extent) then only appear as straight lines, or as long ellipses cutting the planet's disc. the belts on saturn are not very conspicuous. a good - / -inch is required (so far as my experience extends) to show them satisfactorily. the rings when turned edgewise either towards the earth or sun, are not visible in ordinary telescopes, neither can they be seen when the earth and sun are on opposite sides of the rings. in powerful telescopes the rings seem never entirely to disappear. the shadow of the planet on the rings may be well seen with a good -inch telescope, which will also show ball's division in the rings. the shadow of the rings on the planet is a somewhat more difficult feature. the shadow of the planet on the rings is best seen when the rings are well open and the planet is in or near quadrature. it is to be looked for to the left of the ball (in an inverting telescope) at quadrature preceding opposition, and to the right at quadrature following opposition. saturn is more likely to be studied at the latter than at the former quadrature, as in quadrature preceding opposition he is a morning star. the shadow of the rings on the planet is best seen when the rings are but moderately open, and saturn is in or near quadrature. when the shadow lies outside the rings it is best seen, as the dark ring takes off from the sharpness of the contrast when the shadow lies within the ring. it would take more space than i can spare here to show how it is to be determined (independently) whether the shadow lies within or without the ring. but the 'nautical almanac' gives the means of determining this point. when, in the table for assigning the appearance of the rings, _l_ is less than _l'_ the shadow lies outside the ring, when _l_ is greater than _l'_ the shadow lies within the ring. uranus is just visible to the naked eye when he is in opposition, and his place accurately known. but he presents no phenomena of interest. i have seen him under powers which made his disc nearly equal to that of the moon, yet could see nothing but a faint bluish disc. neptune also is easily found if his place be accurately noted on a map, and a good finder used. we have only to turn the telescope to a few stars seen in the finder nearly in the place marked in our map, and presently we shall recognise the one we want by the peculiarity of its light. what is the lowest power which will exhibit neptune as a disc i do not know, but i am certain no observer can mistake him for a fixed star with a -inch aperture and a few minutes' patient scrutiny in favourable weather. [illustration: plate vii.] chapter vii. half-hours with the sun and moon. the moon perhaps is the easiest of all objects of telescopic observation. a very moderate telescope will show her most striking features, while each increase of power is repaid by a view of new details. yet in one sense the moon is a disappointing object even to the possessor of a first-class instrument. for the most careful and persistent scrutiny, carried on for a long series of years, too often fails to reward the observer by any new discoveries of interest. our observer must therefore rather be prepared to enjoy the observation of recognised features than expect to add by his labours to our knowledge of the earth's nearest neighbour. although the moon is a pleasing and surprising telescopic object when full, the most interesting views of her features are obtained at other seasons. if we follow the moon as she waxes or wanes, we see the true nature of that rough and bleak mountain scenery, which when the moon is full is partially softened through the want of sharp contrasts of light and shadow. if we watch, even for half an hour only, the changing form of the ragged line separating light from darkness on the moon's disc, we cannot fail to be interested. "the outlying and isolated peak of some great mountain-chain becomes gradually larger, and is finally merged in the general luminous surface; great circular spaces, enclosed with rough and rocky walls many miles in diameter, become apparent; some with flat and perfectly smooth floors, variegated with streaks; others in which the flat floor is dotted with numerous pits or covered with broken fragments of rock. occasionally a regularly-formed and unusually symmetrical circular formation makes its appearance; the exterior surface of the wall bristling with terraces rising gradually from the plain, the interior one much more steep, and instead of a flat floor, the inner space is concave or cup-shaped, with a solitary peak rising in the centre. solitary peaks rise from the level plains and cast their long narrow shadows athwart the smooth surface. vast plains of a dusky tint become visible, not perfectly level, but covered with ripples, pits, and projections. circular wells, which have no surrounding wall dip below the plain, and are met with even in the interior of the circular mountains and on the tops of their walls. from some of the mountains great streams of a brilliant white radiate in all directions and can be traced for hundreds of miles. we see, again, great fissures, almost perfectly straight and of great length, although very narrow, which appear like the cracks in moist clayey soil when dried by the sun."[ ] but interesting as these views may be, it was not for such discoveries as these that astronomers examined the surface of the moon. the examination of mere peculiarities of physical condition is, after all, but barren labour, if it lead to no discovery of physical variation. the principal charm of astronomy, as indeed of all observational science, lies in the study of change--of progress, development, and decay, and specially of systematic variations taking place in regularly-recurring cycles. and it is in this relation that the moon has been so disappointing an object of astronomical observation. for two centuries and a half her face has been scanned with the closest possible scrutiny; her features have been portrayed in elaborate maps; many an astronomer has given a large portion of his life to the work of examining craters, plains, mountains, and valleys, for the signs of change; but until lately no certain evidence--or rather, no evidence save of the most doubtful character--has been afforded that the moon is other than "a dead and useless waste of extinct volcanoes." whether the examination of the remarkable spot called linné--where lately signs were supposed to have been seen of a process of volcanic eruption--will prove an exception to this rule, remains to be seen. the evidence seems to me strongly to favour the supposition of a change of some sort having taken place in this neighbourhood. the sort of scrutiny required for the discovery of changes, or for the determination of their extent, is far too close and laborious to be attractive to the general observer. yet the kind of observation which avails best for the purpose is perhaps also the most interesting which he can apply to the lunar details. the peculiarities presented by a spot upon the moon are to be observed from hour to hour (or from day to day, according to the size of the spot) as the sun's light gradually sweeps across it, until the spot is fully lighted; then as the moon wanes and the sun's light gradually passes from the spot, the series of observations is to be renewed. a comparison of them is likely--especially if the observer is a good artist and has executed several faithful delineations of the region under observation, to throw much light upon the real contour of the moon's surface at this point. in the two lunar views in plate some of the peculiarities i have described are illustrated. but the patient observer will easily be able to construct for himself a set of interesting views of different regions. it may be noticed that for observation of the waning moon there is no occasion to wait for those hours in which only the waning moon is visible _during the night_. of course for the observation of a particular region under a particular illumination, the observer has no choice as to hour. but for generally interesting observations of the waning moon he can wait till morning and observe by daylight. the moon is, of course, very easily found by the unaided eye (in the day time) when not very near to the sun; and the methods described in chapter v. will enable the observer to find the moon when she is so near to the sun as to present the narrowest possible sickle of light. one of the most interesting features of the moon, when she is observed with a good telescope, is the variety of colour presented by different parts of her surface. we see regions of the purest white--regions which one would be apt to speak of as _snow-covered_, if one could conceive the possibility that snow should have fallen where (now, at least) there is neither air nor water. then there are the so-called seas, large grey or neutral-tinted regions, differing from the former not merely in colour and in tone, but in the photographic quality of the light they reflect towards the earth. some of the seas exhibit a greenish tint, as the sea of serenity and the sea of humours. where there is a central mountain within a circular depression, the surrounding plain is generally of a bluish steel-grey colour. there is a region called the marsh of sleep, which exhibits a pale red tint, a colour seen also near the hyrcinian mountains, within a circumvallation called lichtenburg. the brightest portion of the whole lunar disc is aristarchus, the peaks of which shine often like stars, when the mountain is within the unillumined portion of the moon. the darkest regions are grimaldi and endymion and the great plain called plato by modern astronomers--but, by hevelius, the greater black lake. the sun.--observation of the sun is perhaps on the whole the most interesting work to which the possessor of a moderately good telescope can apply his instrument. those wonderful varieties in the appearance of the solar surface which have so long perplexed astronomers, not only supply in themselves interesting subjects of observation and examination, but gain an enhanced meaning from the consideration that they speak meaningly to us of the structure of an orb which is the source of light and heat enjoyed by a series of dependent worlds whereof our earth is--in size at least--a comparatively insignificant member. swayed by the attraction of this giant globe, jupiter and saturn, uranus and neptune, as well as the four minor planets, and the host of asteroids, sweep continuously in their appointed orbits, in ever new but ever safe and orderly relations amongst each other. if the sun's light and heat were lost, all life and work among the denizens of these orbs would at once cease; if his attractive energy were destroyed, these orbs would cease to form a _system_. the sun may be observed conveniently in many ways, some more suited to the general observer who has not time or opportunity for systematic observation; others more instructive, though involving more of preparation and arrangement. the simplest method of observing the sun is to use the telescope in the ordinary manner, protecting the eye by means of dark-green or neutral-tinted glasses. some of the most interesting views i have ever obtained of the sun, have resulted from the use of the ordinary terrestrial or erecting eye-piece, capped with a dark glass. the magnifying power of such an eye-piece is, in general, much lower than that available with astronomical eye-pieces. but vision is very pleasant and distinct when the sun is thus observed, and a patient scrutiny reveals almost every feature which the highest astronomical power applicable could exhibit. then, owing to the greater number of intervening lenses, there is not the same necessity for great darkness or thickness in the coloured glass, so that the colours of the solar features are seen much more satisfactorily than when astronomical eye-pieces are employed. in using astronomical eye-pieces it is convenient to have a rotating wheel attached, by which darkening glasses of different power may be brought into use as the varying illumination may require. those who wish to observe carefully and closely a minute portion of the solar disc, should employ dawes' eye-piece: in this a metallic screen placed in the focus keeps away all light but such as passes through a minute hole in the diaphragm. another convenient method of diminishing the light is to use a glass prism, light being partially reflected from one of the exterior surfaces, while the refracted portion is thrown out at another. very beautiful and interesting views may be obtained by using such a pyramidal box as is depicted in fig. . [illustration: _fig. ._] this box should be made of black cloth or calico fastened over a light framework of wire or cane. the base of the pyramid should be covered on the inside with a sheet of white glazed paper, or with some other uniform white surface. captain noble, i believe, makes use of a surface of plaster of paris, smoothed while wet with plate-glass. the door _b c_ enables the observer to "change power" without removing the box, while larger doors, _d e_ and _g f_, enable him to examine the image; a dark cloth, such as photographers use, being employed, if necessary, to keep out extraneous light. the image may also be examined from without, if the bottom of the pyramid be formed of a sheet of cut-glass or oiled tissue-paper. when making use of the method just described, it is very necessary that the telescope-tube should be well balanced. a method by which this may be conveniently accomplished has been already described in chapter i. but, undoubtedly, for the possessor of a moderately good telescope there is no way of viewing the sun's features comparable to that now to be described, which has been systematically and successfully applied for a long series of years by the rev. f. howlett. to use his own words: "any one possessing a good achromatic of not more than three inches' aperture, who has a little dexterity with his pencil, and a little time at his disposal (all the better if it be at a somewhat early hour of the morning)" may by this method "deliberately and satisfactorily view, measure, and (if skill suffice) delineate most of those interesting and grand solar phenomena of which he may have read, or which he may have seen depicted, in various works on physical astronomy."[ ] the method in question depends on the same property which is involved in the use of the pyramidal box just described, supplemented (where exact and systematic observation is required) by the fact that objects lying on or between the lenses of the eye-piece are to be seen faithfully projected on the white surface on which the sun's image is received. in place, however, of a box carried upon the telescope-tube, a darkened room (or true _camera obscura_) contains the receiving sheet. a chamber is to be selected, having a window looking towards the south--a little easterly, if possible, so as to admit of morning observation. all windows are to be completely darkened save one, through which the telescope is directed towards the sun. an arrangement is to be adopted for preventing all light from entering by this window except such light as passes down the tube of the telescope. this can readily be managed with a little ingenuity. mr. howlett describes an excellent method. the following, perhaps, will sufficiently serve the purposes of the general observer:--a plain frame (portable) is to be constructed to fit into the window: to the four sides of this frame triangular pieces of cloth (impervious to light) are to be attached, their shape being such that when their adjacent edges are sewn together and the flaps stretched out, they form a rectangular pyramid of which the frame is the base. through the vertex of this pyramid (near which, of course, the cloth flaps are not sewn together) the telescope tube is to be passed, and an elastic cord so placed round the ends of the flaps as to prevent any light from penetrating between them and the telescope. it will now be possible, without disturbing the screen (fixed in the window), to move the telescope so as to follow the sun during the time of observation. and the same arrangement will serve for all seasons, if so managed that the elastic cord is not far from the middle of the telescope-tube; for in this case the range of motion is small compared to the range of the tube's extremity. a large screen of good drawing-paper should next be prepared. this should be stretched on a light frame of wood, and placed on an easel, the legs of which should be furnished with holes and pegs that the screen may be set at any required height, and be brought square to the tube's axis. a large t-square of light wood will be useful to enable the observer to judge whether the screen is properly situated in the last respect. we wish now to direct the tube towards the sun, and this "without dazzling the eyes as by the ordinary method." this may be done in two ways. we may either, before commencing work--that is, before fastening our elastic cord so as to exclude all light--direct the tube so that its shadow shall be a perfect circle (when of course it is truly directed), then fasten the cord and afterwards we can easily keep the sun in the field by slightly shifting the tube as occasion requires. or (if the elastic cord has already been fastened) we may remove the eye-tube and shift the telescope-tube about--the direction in which the sun lies being roughly known--until we see the spot of light received down the telescope's axis grow brighter and brighter and finally become a _spot of sun-light_. if a card be held near the focus of the telescope there will be seen in fact an image of the sun. the telescope being now properly directed, the eye-tube may be slipped in again, and the sun may be kept in the field as before. there will now be seen upon the screen a picture of the sun very brilliant and pleasing, but perhaps a little out of focus. the focusing should therefore next be attended to, the increase of clearness in the image being the test of approach to the true focus. and again, it will be well to try the effect of slight changes of distance between the screen and the telescope's eye-piece. mr. howlett considers one yard as a convenient distance for producing an excellent effect with almost any eye-piece that the state of the atmosphere will admit of. of course, the image becomes more sharply defined if we bring the screen nearer to the telescope, while all the details are enlarged when we move the screen away. the enlargement has no limits save those depending on the amount of light in the image. but, of course, the observer must not expect enlargement to bring with it a view of new details, after a certain magnitude of image has been attained. still there is something instructive, i think, in occasionally getting a very magnified view of some remarkable spot. i have often looked with enhanced feelings of awe and wonder on the gigantic image of a solar spot thrown by means of the diagonal eye-piece upon the ceiling of the observing-room. blurred and indistinct through over-magnifying, yet with a new meaning to me, _there_ the vast abysm lies pictured; vague imaginings of the vast and incomprehensible agencies at work in the great centre of our system crowd unbidden into my mind; and i seem to _feel_--not merely think about--the stupendous grandeur of that life-emitting orb. to return, however, to observation:--by slightly shifting the tube, different parts of the solar disc can be brought successively upon the screen and scrutinized as readily as if they were drawn upon a chart. "with a power of--say about or linear--the most minute solar spot, properly so called, that is capable of formation" (mr. howlett believes "they are never less than three seconds in length or breadth) will be more readily detected than by any other method," see plate ; "as also will any faculæ, mottling, or in short, any other phenomena that may then be existing on the disc." "drifting clouds frequently sweep by, to vary the scene, and occasionally an aërial hail- or snow-storm." mr. howlett has more than once seen a distant flight of rooks pass slowly across the disc with wonderful distinctness, when the sun has been at a low altitude, and likewise, much more frequently, the rapid dash of starlings, which, very much closer at hand, frequent his church-tower." an eclipse of the sun, or a transit of an inferior planet, is also much better seen in this way than by any other method of observing the solar disc. in plate are presented several solar spots as they have appeared to mr. howlett, with an instrument of moderate power. the grotesque forms of some of these are remarkable; and the variations the spots undergo from day to day are particularly interesting to the thoughtful observer. a method of measuring the spots may now be described. it is not likely indeed that the ordinary observer will care to enter upon any systematic series of measurements. but even in his case, the means of forming a general comparison between the spots he sees at different times cannot fail to be valuable. also the knowledge--which a simple method of measurement supplies--of the actual dimensions of a spot in miles (roughly) is calculated to enhance our estimate of the importance of these features of the solar disc. i give mr. howlett's method in his own words:-- "cause your optician to rule for you on a circular piece of glass a number of fine graduations, the th part of an inch apart, each fifth and tenth line being of a different length in order to assist the eye in their enumeration. insert this between the anterior and posterior lenses of a huygenian eye-piece of moderate power, say linear. direct your telescope upon the sun, and having so arranged it that the whole disc of the sun may be projected on the screen, count carefully the number of graduations that are seen to exactly occupy the solar diameter.... it matters not in which direction you measure your diameter, provided only the sun has risen some ° or ° above the horizon, and so escaped the distortion occasioned by refraction.[ ] "next let us suppose that our observer has been observing the sun on any day of the year, say, if you choose, at the time of its mean apparent diameter, namely about the first of april or first of october, and has ascertained that" (as is the case with mr. howlett's instrument) "sixty-four graduations occupy the diameter of the projected image. now the semi-diameter of the sun, at the epochs above mentioned, according to the tables given for every day of the year in the 'nautical almanac' (the same as in dietrichsen and hannay's very useful compilation) is ' ", and consequently his mean total diameter is ' " or ". if now we divide " by " this will, of course, award as nearly as possible " as the value in celestial arc of each graduation, either as seen on the screen, or as applied directly to the sun or any heavenly body large enough to be measured by it." since the sun's diameter is about , miles, each graduation (in the case above specified) corresponds to one- th part of , miles--that is, to a length of , miles on the sun's surface. any other case can be treated in precisely the same manner. it will be found easy so to place the screen that the distance between successive graduations (as seen projected upon the screen) may correspond to any desired unit of linear measurement--say an inch. then if the observer use transparent tracing-paper ruled with faint lines forming squares half-an-inch in size, he can comfortably copy directly from the screen any solar phenomena he may be struck with. a variety of methods of drawing will suggest themselves. mr. howlett, in the paper i have quoted from above, describes a very satisfactory method, which those who are anxious to devote themselves seriously to solar observation will do well to study. it is necessary that the observer should be able to determine approximately where the sun's equator is situated at the time of any observation, in order that he may assign to any spot or set of spots its true position in relation to solar longitude and latitude. mr. howlett shows how this may be done by three observations of the sun made at any fixed hour on successive days. perhaps the following method will serve the purpose of the general observer sufficiently well:-- the hour at which the sun crosses the meridian must be taken for the special observation now to be described. this hour can always be learnt from 'dietrichsen's almanac'; but noon, civil time, is near enough for practical purposes. now it is necessary first to know the position of the ecliptic with reference to the celestial equator. of course, at noon a horizontal line across the sun's disc is parallel to the equator, but the position of that diameter of the sun which coincides with the ecliptic is not constant: at the summer and winter solstices this diameter coincides with the other, or is horizontal at noon; at the spring equinox the sun (which travels on the ecliptic) is passing towards the north of the equator, crossing that curve at an angle of - / °, so that the ecliptic coincides with that diameter of the sun which cuts the horizontal one at an angle of - / ° and has its _left_ end above the horizontal diameter; and at the autumn equinox the sun is descending and the same description applies, only that the diameter (inclined - / ° to the horizon) which has its _right_ end uppermost, now represents the ecliptic. for intermediate dates, use the following little table:-- -------------------------------------------------------------------------- date. |dec. |jan. |jan. |feb. |feb. |mar. |mar. (circiter.) | |june |may |may |apr. |apr. | -------------------+-------+------+-------+------+-------+-------+-------- inclination of |left |left |left |left |left |left |left ecliptical diameter| | | | | | | of sun to the | ° ' | ° ' | ° ' | ° ' | ° ' | ° ' | ° ' horizon.[ ] |right |right |right |right |right |right |right -------------------+-------+------+-------+------+-------+-------+-------- date. | |dec. |nov. |nov. |oct. |oct. | (circiter.) |jan. |july |july |aug. |aug. |sept. |sept. -------------------------------------------------------------------------- now if our observer describe a circle, and draw a diameter inclined according to above table, this diameter would represent the sun's equator if the axis of the sun were square to the ecliptic-plane. but this axis is slightly inclined, the effect of which is, that on or about june the sun is situated as shown in fig. with respect to the ecliptic _ab_; on or about september he is situated as shown in fig. ; on or about december as shown in fig. ; and on or about march as shown in fig. . the inclination of his equator to the ecliptic being so small, the student can find little difficulty in determining with sufficient approximation the relation of the sun's polar axis to the ecliptic on intermediate days, since the equator is never more _inclined_ than in figs. and , never more _opened out_ than in figs. and . having then drawn a line to represent the sun's ecliptical diameter inclined to the horizontal diameter as above described, and having (with this line to correspond to _ab_ in figs. - ) drawn in the sun's equator suitably inclined and opened out, he has the sun's actual presentation (at noon) as seen with an erecting eye-piece. holding his picture upside down, he has the sun's presentation as seen with an astronomical eye-piece--and, finally, looking at his picture from behind (without inverting it), he has the presentation seen when the sun is projected on the screen. hence, if he make a copy of this last view of his diagram upon the centre of his screen, and using a low power, bring the whole of the sun's image to coincide with the circle thus drawn (to a suitable scale) on the screen, he will at once see what is the true position of the different sun-spots. after a little practice the construction of a suitably sized and marked circle on the screen will not occupy more than a minute or two. [illustration: _fig. ._] [illustration: _fig. ._] [illustration: _fig. ._] [illustration: _fig. ._] it must be noticed that the sun's apparent diameter is not always the same. he is nearer to us in winter than in summer, and, of course, his apparent diameter is greater at the former than at the latter season. the variation of the apparent diameter corresponds (inversely) to the variation of distance. as the sun's greatest distance from the earth is , , miles (pretty nearly) and his least , , , his greatest, mean, and least apparent diameters are as , - / , and respectively; that is, as , , and respectively. mr. howlett considers that with a good -inch telescope, applied in the manner we have described, all the solar features may be seen, except the separate granules disclosed by first-class instruments in the hands of such observers as dawes, huggins, or secchi. faculæ may, of course, be well seen. they are to be looked for near spots which lie close to the sun's limb. when the sun's general surface is carefully scrutinised, it is found to present a mottled appearance. this is a somewhat delicate feature. it results, undoubtedly, from the combined effect of the granules separately seen in powerful instruments. sir john herschel has stated that he cannot recognise the marbled appearance of the sun with an achromatic. mr. webb, however, has seen this appearance with such a telescope, of moderate power, used with direct vision; and certainly i can corroborate mr. howlett in the statement that this appearance may be most distinctly seen when the image of the sun is received within a well-darkened room. my space will not permit me to enter here upon the discussion of any of those interesting speculations which have been broached concerning solar phenomena. we may hope that the great eclipse of august, , which promises to be the most favourable (for effective observation) that has ever taken place, will afford astronomers the opportunity of resolving some important questions. it seems as if we were on the verge of great discoveries,--and certainly, if persevering and well-directed labour would seem in any case to render such discoveries due as man's just reward, we may well say that he deserves shortly to reap a harvest of exact knowledge respecting solar phenomena. the end. footnotes: [footnote : such a telescope is most powerful with the shortest sight. it may be remarked that the use of a telescope often reveals a difference in the sight of the two eyes. in my own case, for instance, i have found that the left eye is very short-sighted, the sight of the right eye being of about the average range. accordingly with my left eye a - / -foot object-glass, alone, forms an effective telescope, with which i can see jupiter's moons quite distinctly, and under favourable circumstances even saturn's rings. i find that the full moon is too bright to be observed in this way without pain, except at low altitudes.] [footnote : betelgeuse--commonly interpreted the giant's shoulder--_ibt-al-jauza_. the words, however, really signify, "the armpit of the central one," orion being so named because he is divided centrally by the equator.] [footnote : i have never been able to see more than four with a - / -inch aperture. i give a view of the trapezium as seen with an -inch equatorial.] [footnote : sir w. herschel several times saw [epsilon] lyræ as a double. bessel also relates that when he was a lad of thirteen he could see this star double. i think persons having average eye-sight could see it double if they selected a suitable hour for observation. my own eye-sight is not good enough for this, but i can distinctly see this star wedged whenever the line joining the components is inclined about ° to the horizon, and also when lyra is near the zenith.] [footnote : they were so described by admiral smyth in . mr. main, in , describes them as straw-coloured and reddish, while mr. webb, in , saw them pale-yellow and _lilac_!] [footnote : or the observer may sweep from [omicron] towards [nu], looking for r about two-fifths of the way from [omicron] to [nu].] [footnote : here a single period only is taken, to get back to a convenient hour of the evening.] [footnote : here a single period only is taken, to get back to a convenient hour of the evening.] [footnote : i have constructed a zodiac-chart, which will enable the student to mark in the path of a planet, at any season of the year, from the recorded places in the almanacs.] [footnote : it is convenient to remember that through precession a star near the ecliptic shifts as respects the r.a. and dec. lines, through an arc of one degree--or nearly twice the moon's diameter--in about years, all other stars through a less arc.] [footnote : mercury is best seen when in quadrature to the sun, but _not_ (as i have seen stated) at those quadratures in which he attains his maximum elongation from the sun. this will appear singular, because the maximum elongation is about °, the minimum only about °. but it happens that in our northern latitudes mercury is always _south_ of the sun when he attains his maximum elongation, and this fact exercises a more important effect than the mere amount of elongation.] [footnote : it does not seem to me that the difficulty of detecting mercury is due to the difficulty "of identifying it amongst the surrounding stars, during the short time that it can be seen" (hind's 'introduction to astronomy'). there are few stars which are comparable with mercury in brilliancy, when seen under the same light.] [footnote : i may notice another error sometimes made. it is said that the shadow of a satellite _appears_ elliptical when near the edge of the disc. the shadow is _in reality_ elliptical when thus situated, but _appears_ circular. a moment's consideration will show that this should be so. the part of the disc concealed by a _satellite_ near the limb is also elliptical, but of course appears round.] [footnote : from a paper by mr. breen, in the 'popular science review,' october, .] [footnote : 'intellectual observer' for july, , to which magazine the reader is referred for full details of mr. howlett's method of observation, and for illustrations of the appliances he made use of, and of some of his results.] [footnote : as the sun does not attain such an altitude as ° during two months in the year, it is well to notice that the true length of the sun's apparent solar diameter is determinable even immediately after sun-rise, if the line of graduation is made to coincide with the _horizontal_ diameter of the picture on the screen--for refraction does not affect the length of this diameter.] [footnote : the words "left" and "right" indicate which end of the sun's ecliptical diameter is uppermost at the dates in upper or lower row respectively.] london: printed by w. clowes and sons, duke street, stamford street, and charing cross. astronomical discovery [illustration: astronomers royal.] astronomical discovery by herbert hall turner, d.sc., f.r.s. savilian professor of astronomy in the university of oxford _with plates_ london edward arnold & maddox street, w. (all rights reserved) to edward emerson barnard astronomical discoverer these pages are inscribed in memory of never-to-be-forgotten days spent with him at the yerkes observatory of the university of chicago preface the aim of the following pages is to illustrate, by the study of a few examples chosen almost at random, the variety in character of astronomical discoveries. an attempt has indeed been made to arrange the half-dozen examples, once selected, into a rough sequence according to the amount of "chance" associated with the discovery, though from this point of view chapter iv. should come first; but i do not lay much stress upon it. there is undoubtedly an element of "luck" in most discoveries. "the biggest strokes are all luck," writes a brother astronomer who had done me the honour to glance at a few pages, "but a man must not drop his catches. have you ever read montaigne's essay 'of glory'? it is worth reading. change war and glory to discovery and it is exactly the same theme. if you are looking for a motto you will find a score in it." indeed even in cases such as those in chapters v. and vi., where a discovery is made by turning over a heap of rubbish--declared such by experts and abandoned accordingly--we instinctively feel that the finding of something valuable was especially "fortunate." we should scarcely recommend such waste material as the best hunting ground for gems. the chapters correspond approximately to a series of six lectures delivered at the university of chicago in august , at the hospitable invitation of president harper. they afforded me the opportunity of seeing something of this wonderful university, only a dozen years old and yet so amazingly vigorous; and especially of its observatory (the yerkes observatory, situated eighty miles away on lake geneva), which is only eight years old and yet has taken its place in the foremost rank. for these opportunities i venture here to put on record my grateful thanks. in a portion of the first chapter it will be obvious that i am indebted to miss clerke's "history of astronomy in the nineteenth century"; in the second to professor r. a. sampson's memoir on the adams mss.; in the third to rigaud's "life of bradley." there are other debts which i hope are duly acknowledged in the text. my grateful thanks are due to mr. f. a. bellamy for the care with which he has read the proofs; and i am indebted for permission to publish illustrations to the royal astronomical society, the astronomer royal, the editors of _the observatory_, the cambridge university press, the harvard college observatory, the yerkes observatory, and the living representatives of two portraits. h. h. turner. university observatory, oxford, _november , _. contents page chapter i uranus and eros chapter ii the discovery of neptune chapter iii bradley's discoveries of the aberration of light and of the nutation of the earth's axis chapter iv accidental discoveries chapter v schwabe and the sun-spot period chapter vi the variation of latitude index list of plates plate i. portrait of j. c. adams _to face page_ ii. portrait of a. graham " " iii. portrait of u. j. le verrier " " iv. portrait of j. g. galle " " v. corner of the berlin map by the use of which galle found neptune " " vi. astronomers royal _frontispiece_ vii. great comet of nov. , _to face page_ viii. the oxford new star " " ix. nebulosity round nova persei " " x. sun-spots at greenwich, feb. and , " " xi. sun-spots at greenwich, feb. and , " " xii. number of sun-spots compared with daily range of magnetic declination and daily range of magnetic horizontal force " " xiii. greenwich magnetic curves, - " " xiv. greenwich magnetic curves, - " " xv. sun-spots and turns of vane " " errata page , line , _for_ " stars" _read_ " stars per hour." " , see note on page . " , bottom of page. this nebulosity was first discovered by dr. max wolf of heidelberg. see _astr. nachr._ . " , line , _for_ "observation" _read_ "aberration." astronomical discovery chapter i uranus and eros [sidenote: popular view of discovery.] discovery is expected from an astronomer. the lay mind scarcely thinks of a naturalist nowadays discovering new animals, or of a chemist as finding new elements save on rare occasions; but it does think of the astronomer as making discoveries. the popular imagination pictures him spending the whole night in watching the skies from a high tower through a long telescope, occasionally rewarded by the finding of something new, without much mental effort. i propose to compare with this romantic picture some of the actual facts, some of the ways in which discoveries are really made; and if we find that the image and the reality differ, i hope that the romance will nevertheless not be thereby destroyed, but may adapt itself to conditions more closely resembling the facts. [sidenote: keats' lines.] the popular conception finds expression in the lines of keats:-- then felt i like some watcher of the skies when a new planet swims into his ken. keats was born in , published his first volume of poems in , and died in . at the time when he wrote the discovery of planets was comparatively novel in human experience. uranus had been found by william herschel in , and in the years to followed the first four minor planets, a number destined to remain without additions for nearly forty years. it would be absurd to read any exact allusion into the words quoted, when we remember the whole circumstances under which they were written; but perhaps i may be forgiven if i compare them especially with the actual discovery of the planet uranus, for the reason that this was by far the largest of the five--far larger than any other planet known except jupiter and saturn, while the others were far smaller--and that keats is using throughout the poem metaphors drawn from the first glimpses of "vast expanses" of land or water. perhaps i may reproduce the whole sonnet. his friend c. c. clarke had put before him chapman's "paraphrase" of homer, and they sat up till daylight to read it, "keats shouting with delight as some passage of especial energy struck his imagination. at ten o'clock the next morning mr. clarke found the sonnet on his breakfast-table." sonnet xi _on first looking into chapman's "homer"_ much have i travell'd in the realms of gold, and many goodly states and kingdoms seen; round many western islands have i been which bards in fealty to apollo hold. oft of one wide expanse had i been told that deep-brow'd homer ruled as his demesne; yet did i never breathe its pure serene till i heard chapman speak out loud and bold: then felt i like some watcher of the skies when a new planet swims into his ken; or like stout cortez when with eagle eyes he star'd at the pacific--and all his men look'd at each other with a wild surmise-- silent, upon a peak in darien. [sidenote: comparison with discovery of uranus.] let us then, as our first example of the way in which astronomical discoveries are made, turn to the discovery of the planet uranus, and see how it corresponds with the popular conception as voiced by keats. in one respect his words are true to the life or the letter. if ever there was a "watcher of the skies," william herschel was entitled to the name. it was his custom to watch them the whole night through, from the earliest possible moment to daybreak; and the fruits of his labours were many and various almost beyond belief. but did the planet "swim into his ken"? let us turn to the original announcement of his discovery as given in the philosophical transactions for . philosophical transactions, xxxii.--account of a comet by mr. herschel, f.r.s. (communicated by dr. watson, jun., of bath, f.r.s.) _read april , _ [sidenote: original announcement.] "on tuesday the th of march, between ten and eleven in the evening, while i was examining the small stars in the neighbourhood of h geminorum, i perceived one that appeared visibly larger than the rest; being struck with its uncommon magnitude, i compared it to h geminorum and the small star in the quartile between auriga and gemini, and finding it to be so much larger than either of them, suspected it to be a comet. "i was then engaged in a series of observations on the parallax of the fixed stars, which i hope soon to have the honour of laying before the royal society; and those observations requiring very high powers, i had ready at hand the several magnifiers of , , , , , &c., all which i have successfully used upon that occasion. the power i had on when i first saw the comet was . from experience i knew that the diameters of the fixed stars are not proportionally magnified with higher powers as the planets are; therefore i now put on the powers of and , and found the diameter of the comet increased in proportion to the power, as it ought to be, on a supposition of its not being a fixed star, while the diameters of the stars to which i compared it were not increased in the same ratio. moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations i knew they would retain. the sequel has shown that my surmises were well founded, this proving to be the comet we have lately observed. "i have reduced all my observations upon this comet to the following tables. the first contains the measures of the gradual increase of the comet's diameter. the micrometers i used, when every circumstance is favourable, will measure extremely small angles, such as do not exceed a few seconds, true to , , or thirds at most; and in the worst situations true to or thirds; i have therefore given the measures of the comet's diameter in seconds and thirds. and the parts of my micrometer being thus reduced, i have also given all the rest of the measures in the same manner; though in large distances, such as one, two, or three minutes, so great an exactness, for several reasons, is not pretended to." [sidenote: called first a comet.] [sidenote: other observers would not have found it at all.] at first sight this seems to be the wrong reference, for it speaks of a new comet, not a new planet. but it is indeed of uranus that herschel is speaking; and so little did he realise the full magnitude of his discovery at once, that he announced it as that of a comet; and a comet the object was called for some months. attempts were made to calculate its orbit as a comet, and broke down; and it was only after much work of this kind had been done that the real nature of the object began to be suspected. but far more striking than this misconception is the display of skill necessary to detect any peculiarity in the object at all. among a number of stars one seemed somewhat exceptional in size, but the difference was only just sufficient to awaken suspicion in a keen-eyed herschel. would any other observer have noticed the difference at all? certainly several good observers had looked at the object before, and looked at it with the care necessary to record its position, without noting any peculiarity. their observations were recovered subsequently and used to fix the orbit of the new planet more accurately. i shall remind you in the next chapter that uranus had been observed in this way no less than seventeen times by first-rate observers without exciting their attention to anything remarkable. the first occasion was in , nearly a century before herschel's grand discovery, and these chance observations, which lay so long unnoticed as in some way erroneous, subsequently proved to be of the utmost value in fixing the orbit of the new planet. but there is even more striking testimony than this to the exceptional nature of herschel's achievement. it is a common experience in astronomy that an observer may fail to notice in a general scrutiny some phenomenon which he can see perfectly well when his attention is directed to it: when a man has made a discovery and others are told what to look for, they often see it so easily that they are filled with amazement and chagrin that they never saw it before. not so in the case of uranus. at least two great astronomers, lalande and messier, have left on record their astonishment that herschel could differentiate it from an ordinary star at all; for even when instructed where to look and what to look for, they had the greatest difficulty in finding it. i give a translation of messier's words, which herschel records in the paper already quoted announcing the discovery:-- "nothing was more difficult than to recognise it; and i cannot conceive how you have been able to return several times to this star or comet; for absolutely it has been necessary to observe it for several consecutive days to perceive that it was in motion." [sidenote: no "swimming into ken."] we cannot, therefore, fit the facts to keats' version of them. the planet did not majestically reveal itself to a merely passive observer: rather did it, assuming the disguise of an ordinary star, evade detection to the utmost of its power; so that the keenest eye, the most alert attention, the most determined following up of a mere hint, were all needed to unmask it. but is the romance necessarily gone? if another keats could arise and know the facts, could he not coin a newer and a truer phrase for us which would still sound as sweetly in our ears? [sidenote: though this may happen at times.] [sidenote: name of new planet.] i must guard against a possible misconception. i do not mean to convey that astronomical discoveries are not occasionally made somewhat in the manner so beautifully pictured by keats. three years ago a persistent "watcher of the skies," dr. anderson of edinburgh, suddenly caught sight of a brilliant new star in perseus; though here "flashed into his ken" would perhaps be a more suitable phrase than "swam." and comets have been detected by a mere glance at the heavens without sensible effort or care on the part of the discoverer. but these may be fairly called exceptions; in the vast majority of cases hard work and a keen eye are necessary to make the discovery. the relative importance of these two factors of course varies in different cases; for the detection of uranus perhaps the keen eye may be put in the first place, though we must not forget the diligent watching which gave it opportunity. other cases of planetary discovery may be attributed more completely to diligence alone, as we shall presently see. but before leaving uranus for them i should like to recall the circumstances attending the naming of the planet. herschel proposed to call it _georgium sidus_ in honour of his patron, king george iii., and as the best way of making his wishes known, wrote the following letter to the president of the royal society, which is printed at the beginning of the philosophical transactions for . _a letter from_ william herschel, esq., f.r.s., _to_ sir joseph banks, bart., p.r.s. "sir,--by the observations of the most eminent astronomers in europe it appears that the new star, which i had the honour of pointing out to them in march , is a primary planet of our solar system. a body so nearly related to us by its similar condition and situation in the unbounded expanse of the starry heavens, must often be the subject of conversation, not only of astronomers, but of every lover of science in general. this consideration then makes it necessary to give it a name whereby it may be distinguished from the rest of the planets and fixed stars. [sidenote: _georgium sidus._] "in the fabulous ages of ancient times, the appellations of mercury, venus, mars, jupiter, and saturn were given to the planets as being the names of their principal heroes and divinities. in the present more philosophical era, it would hardly be allowable to have recourse to the same method, and call on juno, pallas, apollo, or minerva for a name to our new heavenly body. the first consideration in any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, _when_ this last found planet was discovered? it would be a very satisfactory answer to say, 'in the reign of king george the third.' as a philosopher then, the name georgium sidus presents itself to me, as an appellation which will conveniently convey the information of the time and country where and when it was brought to view. but as a subject of the best of kings, who is the liberal protector of every art and science; as a native of the country from whence this illustrious family was called to the british throne; as a member of that society which flourishes by the distinguished liberality of its royal patron; and, last of all, as a person now more immediately under the protection of this excellent monarch, and owing everything to his unlimited bounty;--i cannot but wish to take this opportunity of expressing my sense of gratitude by giving the name _georgium sidus_, _georgium sidus ----jam nunc assuesce vocari,_ _virg. georg._ to a star which (with respect to us) first began to shine under his auspicious reign. "by addressing this letter to you, sir, as president of the royal society, i take the most effectual method of communicating that name to the literati of europe, which i hope they will receive with pleasure.--i have the honour to be, with the greatest respect, sir, your most humble and most obedient servant, w. herschel." [sidenote: herschel.] this letter reminds us how long it was since a new name had been required for a new planet,--to find a similar occasion herschel had to go to the almost prehistoric past, when the names of heroes and divinities were given to the planets. it is, perhaps, not unnatural that he should have considered an entirely new departure appropriate for a discovery separated by so great a length of time from the others; but his views were not generally accepted, especially on the continent. lalande courteously proposed the name of herschel for the new planet, in honour of the discoverer, and this name was used in france; but bode, on the other hand, was in favour of retaining the old practice simply, and calling the new planet uranus. all three names seem to have been used for many years. only the other day i was interested to see an old pack of cards, used for playing a parlour game of astronomy, in which the name herschel is used. the owner told me that they had belonged to his grandfather; and the date of publication was , and the place london, so that this name was in common use in england nearly half a century after the actual discovery; though in the "english nautical almanac" the name "the georgian" (apparently preferred to herschel's _georgium sidus_) was being used officially after , and did not disappear from that work until (published in .) [sidenote: uranus finally adopted.] it would appear to have been the discovery of neptune, with which we shall deal in the next chapter, which led to this official change; for in the volume for is included adams' account of his discovery with the title-- "on the perturbations of uranus," and there was thus a definite reason for avoiding two names for the same planet in the same work. but le verrier's paper on the same topic at the same date still uses the name "herschel" for the planet. [sidenote: bode's law.] the discovery of neptune, as we shall see, was totally different in character from that of uranus. the latter may be described as the finding of something by an observer who was looking for anything; neptune was the finding of something definitely sought for, and definitely pointed out by a most successful and brilliant piece of methodical work. but before that time several planets had been found, as the practical result of a definite search, although the guiding principle was such as cannot command our admiration to quite the same extent as in the case of neptune. to explain it i must say something of the relative sizes of the orbits in which planets move round the sun. these orbits are, as we know, ellipses; but they are very nearly circles, and, excluding refinements, we may consider them as circles, with the sun at the centre of each, so that we may talk of the distance of any planet from the sun as a constant quantity without serious error. now if we arrange the planetary distances in order, we shall notice a remarkable connection between the terms of the series. here is a table showing this connection. table of the distances of the planets from the sun, showing "bode's law." +----------------------------------------------------+ | name of | distance from | "bode's law" | | planet. | sun, taking | (originally formulated | | | that of earth | by titius, but brought | | | as . | into notice by bode). | |----------------------------------------------------| | mercury | | + = | | venus | | + = | | the earth | | + = | | mars | | + = | | ( ) | ( ) | + = | | jupiter | | + = | | saturn | | + = | | uranus | | + = | +----------------------------------------------------+ [sidenote: gap in the series suggesting unknown planet.] [sidenote: search for it.] [sidenote: accidental discovery.] if we write down a series of 's, and then add the numbers , , , and so on, each formed by doubling the last, we get numbers representing very nearly the planetary distances, which are shown approximately in the second column. but three points call for notice. firstly, the number before should be - / , and not zero, to agree with the rest. secondly, there is a gap, or rather was a gap, after the discovery of uranus, between mars and jupiter; and thirdly, we see that when uranus was discovered, and its distance from the sun determined, this distance was found to fall in satisfactorily with this law, which was first stated by titius of wittenberg. this third fact naturally attracted attention. no explanation of the so-called "law" was known at the time; nor is any known even yet, though we may be said to have some glimmerings of a possible cause; and in the absence of such explanation it must be regarded as merely a curious coincidence. but the chances that we are in the presence of a mere coincidence diminish very quickly with each new term added to the series, and when it was found that herschel's new planet fitted in so well at the end of the arrangement, the question arose whether the gap above noticed was real, or whether there was perhaps another planet which had hitherto escaped notice, revolving in an orbit represented by this blank term. this question had indeed been asked even before the discovery of uranus, by bode, a young astronomer of berlin; and for fifteen years he kept steadily in view this idea of finding a planet to fill the vacant interval. the search would be a very arduous one, involving a careful scrutiny, not perhaps of the whole heavens, but of a considerable portion of it along the zodiac; too great for one would-be discoverer single-handed; but in september bode succeeded in organising a band of six german astronomers (including himself) for the purpose of conducting this search. they divided the zodiac into twenty-four zones, and were assigning the zones to the different observers, when they were startled by the news that the missing planet had been accidentally found by piazzi in the constellation taurus. the discovery was made somewhat dramatically on the first evening of the nineteenth century (january , ). piazzi was not looking for a planet at all, but examining an error made by another astronomer; and in the course of this work he recorded the position of a star of the eighth magnitude. returning to it on the next night, it seemed to him that it had slightly moved westwards, and on the following night this suspicion was confirmed. remark that in this case no peculiar appearance in the star suggested that it might be a comet or planet, as in the case of the discovery of uranus. we are not unfair in ascribing the discovery to pure accident, although we must not forget that a careless observer might easily have missed it. piazzi was anything but careless, and watched the new object assiduously till february th, when he became dangerously ill; but he had written, on january rd, to oriani of milan, and to bode at berlin on the following day. these letters, however, did not reach the recipients (in those days of leisurely postal service) until april th and march th respectively; and we can imagine the mixed feelings with which bode heard that the discovery which he had contemplated for fifteen years, and for which he was just about to organise a diligent search, was thus curiously snatched from him. [sidenote: hegel's forecast.] more curious still must have seemed the intelligence to a young philosopher of jena named hegel, who has since become famous, but who had just imperilled his future reputation by publishing a dissertation proving conclusively that the number of the planets could not be greater than seven, and pouring scorn on the projected search of the half-dozen enthusiasts who were proposing to find a new planet merely to fill up a gap in a numerical series. [sidenote: the planet lost again.] the sensation caused by the news of the discovery was intensified by anxiety lest the new planet should already have been lost; for it had meanwhile travelled too close to the sun for further observation, and the only material available for calculating its orbit, and so predicting its place in the heavens at future dates, was afforded by the few observations made by piazzi. was it possible to calculate the orbit from such slender material? it would take too long to explain fully the enormous difficulty of this problem, but some notion of it may be obtained, by those unacquainted with mathematics, from a rough analogy. if we are given a portion of a circle, we can, with the help of a pair of compasses, complete the circle: we can find the centre from which the arc is struck, either by geometrical methods, or by a few experimental trials, and then fill in the rest of the circumference. if the arc given is large we can do this with certainty and accuracy; but if the arc is small it is difficult to make quite sure of the centre, and our drawing may not be quite accurate. now the arc which had been described by the tiny planet during piazzi's observations was only three degrees; and if any one will kindly take out his watch and look at the minute marks round the dial, three degrees is just _half_ a single minute space. if the rest of the dial were obliterated, and only this small arc left, would he feel much confidence in restoring the obliterated portion? this problem gives some idea of the difficulties to be encountered, but only even then a very imperfect one. [sidenote: gauss shows how to find it.] briefly, the solution demanded a new mathematical method in astronomy. but difficulties are sometimes the opportunities of great men, and this particular difficulty attracted to astronomy the great mathematician gauss, who set himself to make the best of the observation available, and produced his classical work, the _theoria motus_, which is the standard work for such calculations to the present day. may we look for a few moments at what he himself says in the preface to his great work? i venture to reproduce the following rough translation (the book being written in latin, according to the scientific usage of the time):-- extract from the preface to the _theoria motus_. [sidenote: the _theoria motus_.] "some ideas had occurred to me on this subject in september , at a time when i was occupied on something quite different; ideas which seemed to contribute to the solution of the great problem of which i have spoken. in such cases it often happens that, lest we be too much distracted from the attractive investigation on which we are engaged, we allow associations of ideas which, if more closely examined, might prove extraordinarily fruitful, to perish from neglect. perchance these same idea-lets of mine would have met with this fate, if they had not most fortunately lighted upon a time than which none could have been chosen more favourable for their preservation and development. for about the same time a rumour began to be spread abroad concerning a new planet which had been detected on january st of that year at the observatory of palermo; and shortly afterwards the actual observations which had been made between january st and february th by the renowned philosopher piazzi were published. nowhere in all the annals of astronomy do we find such an important occasion; and scarcely is it possible to imagine a more important opportunity for pointing out, as emphatically as possible, the importance of that problem, as at the moment when every hope of re-discovering, among the innumerable little stars of heaven, that mite of a planet which had been lost to sight for nearly a year, depended entirely on an approximate knowledge of its orbit, which must be deduced from those scanty observations. could i ever have had a better opportunity for trying whether those idea-lets of mine were of any practical value than if i then were to use them for the determination of the orbit of ceres, a planet which, in the course of those forty-one days, had described around the earth an arc of no more than three degrees? and, after a year had passed, required to be tracked out in a region of the sky far removed from its original position? the first application of this method was made in the month of october , and the first clear night, when the planet was looked for by the help of the ephemeris i had made, revealed the truant to the observer. three new planets found since then have supplied fresh opportunities for examining and proving the efficacy and universality of this method. "now a good many astronomers, immediately after the rediscovery of ceres, desired me to publish the methods which had been used in my calculations. there were, however, not a few objections which prevented me from gratifying at that moment these friendly solicitations, viz. other business, the desire of treating the matter more fully, and more especially the expectation that, by continuing to devote myself to this research, i should bring the different portions of the solution of the problem to a more perfect pitch of universality, simplicity, and elegance. as my hopes have been justified, i do not think there is any reason for repenting of my delay. for the methods which i had repeatedly applied from the beginning admitted of so many and such important variations, that scarcely a vestige of resemblance remains between the method by which formerly i had arrived at the orbit of ceres and the practice which i deal with in this work. although indeed it would be alien to my intention to write a complete history about all these researches which i have gradually brought to even greater perfection, yet on many occasions, especially whenever i was confronted by some particularly serious problem, i thought that the first methods which i employed ought not to be entirely suppressed. nay, rather, in addition to the solutions of the principal problems, i have in this work followed out many questions which presented themselves to me, in the course of a long study of the motions of the heavenly bodies in conic sections, as being particularly worthy of attention, whether on account of the neatness of the analysis, or more especially by reason of their practical utility. yet i have always given the greater care to subjects which i have made my own, merely noticing by the way well-known facts where connection of thought seemed to demand it." [sidenote: rediscovery of ceres.] [sidenote: another planet found.] these words do not explain in any way the methods introduced by gauss, but they give us some notion of the flavour of the work. aided by these brilliant researches, the little planet was found on the last day of the year by von zach at gotha, and on the next night, independently, by olbers at bremen. but, before this success, there had been an arduous search, which led to a curious consequence. olbers had made himself so familiar with all the small stars along the track which was being searched for the missing body, that he was at once struck by the appearance of a stranger near the spot where he had just identified ceres. at first he thought this must be some star which had blazed up to brightness; but he soon found that it also was moving, and, to the great bewilderment of the astronomical world, it proved to be another planet revolving round the sun at a distance nearly the same as the former. this was an extraordinary and totally unforeseen occurrence. the world had been prepared for _one_ planet; but here were _two_! [sidenote: hypothesis of many fragments.] the thought occurred to olbers that they were perhaps fragments of a single body which had been blown to pieces by some explosion, and that there might be more of the pieces; and he therefore suggested as a guide for finding others that, since by the known laws of gravitation, bodies which circle round the sun return periodically to their starting-point, therefore all these fragments would in due course return to the point in the heavens where the original planet had exploded. hence the search might be most profitably conducted in the neighbourhood of the spot where the two first fragments (which had been named ceres and pallas) had already been found. we now have good reason to believe that this view is a mistaken one, but nevertheless it was apparently confirmed by the discovery of two more bodies of the same kind, which were called juno and vesta; the second of these being found by olbers himself after three years' patient work in . hence, although the idea of searching for a more or less definitely imagined planet was not new, although bode had conceived it as early as , and organised a search on this plan, three planets were actually found before the first success attending a definite search. ceres, as already remarked, was found by a pure accident; and the same may be said of pallas and juno, though it may fairly be added that pallas was actually contrary to expectation. minor planets, to . +---------------------------------------+ |number| name. | discoverer. | date.| |---------------------------------------| | | ceres | piazzi | | | | pallas | olbers | | | | juno | harding | | | | vesta | olbers | | |------|-----------|-------------|------| | | astraea | hencke | | | | hebe | hencke | | | | iris | hind | | | | flora | hind | | | | metis | graham | | | | hygeia | de gasparis | | | | parthenope| de gasparis | | | | victoria | hind | | | | egeria | de gasparis | | +---------------------------------------+ [sidenote: hencke's long search.] here now is a table showing how other bodies were gradually added to this first list of four, but you will see that no addition was made for a long time. not that the search was immediately abandoned; but being rewarded by no success for some years, it was gradually dropped, and the belief gained ground that the number of the planets was at last complete. the discoverers of uranus and of these first four minor planets all died before any further addition was made; and it was not until the end of that astraea was found by an ex-postmaster of the prussian town of driessen, by name hencke, who, in spite of the general disbelief in the existence of any more planets, set himself diligently to search for them, and toiled for fifteen long years before at length reaping his reward. others then resumed the search; hind, the observer of an english amateur astronomer near london, found iris a few weeks after hencke had been rewarded by a second discovery in , and in the following year mr. graham at markree in ireland (who is still living, and has only just retired from active work at the cambridge observatory) found metis; and from that time new discoveries have been added year by year, until the number of planets now known exceeds , and is steadily increasing. [illustration: _by permission of messrs. macmillan & co._ i.--j. c. adams.] [illustration: ii.--a. graham. discoverer of the ninth minor planet (metis).] [sidenote: the photographic method.] you will see the great variety characterising these discoveries; some of them are the result of deliberate search, others have come accidentally, and some even contrary to expectation. of the great majority of the earlier ones it may be said that enormous diligence was required for each discovery; to identify a planet it is necessary to have either a good map of the stars or to know them thoroughly, so that the map practically exists in the brain. we need only remember hencke's fifteen years of search before success to recognise what vast stores of patience and diligence were required in carrying out the search. but of late years photography has effected a great revolution in this respect. it is no longer necessary to do more than set what sir robert ball has called a "star-trap," or rather planet-trap. if a photograph be taken of a region of the heavens, by the methods familiar to astronomers, so that each star makes a round dot on the photographic plate, any sufficiently bright object moving relatively to the stars will make a small line or trail, and thus betray its planetary character. in this way most of the recent discoveries have been made, and although diligence is still required in taking the photographs, and again in identifying the objects thus found (which are now very often the images of already known members of the system), the tedious scrutiny with the eye has become a thing of the past. table showing the number of minor planets discovered in each decade since . to --altogether discoveries. to -- " " to -- " " to -- " " to -- " " to -- " announcements in " " " " " " " " --- total [_n.b._--many of the more recent announcements turned out to refer to old discoveries.] [sidenote: scarcity of names.] the known number of these bodies has accordingly increased so rapidly as to become almost an embarrassment; and in one respect the embarrassment is definite, for it has become quite difficult to find _names_ for the new discoveries. we remember with amusement at the present time that for the early discoveries there was sometimes a controversy (of the same kind as in the case of uranus) about the exact name which a planet should have. thus when it was proposed to call no. (discovered in , in london, by mr. hind) "victoria," there was an outcry by foreign astronomers that by a subterfuge the name of a reigning monarch was again being proposed for a planet, and considerable opposition was manifested, especially in america. but it became clear, as other discoveries were added, that the list of goddesses, or even humbler mythological people, would not be large enough to go round if we were so severely critical, and must sooner or later be supplemented from sources hitherto considered unsuitable; so, ultimately, the opposition to the name victoria was withdrawn. later still the restriction to feminine names has been broken through; one planet has been named endymion, and another, of which we shall presently speak more particularly, has been called eros. but before passing to him you may care to look at some of the names selected for others:-- no. name. lameia bettina prymno libussa phaëtusa eduarda california ornamenta ninina ilmatar industria ingeborg eros photographica alleghenia eriphyla ocllo pittsburghia evelyn [sidenote: bettina.] [sidenote: the provisional letters.] in connection with no. there is an interesting little history. in the _observatory_ for , page , appeared the following advertisement:--"herr palisa being desirous to raise funds for his intended expedition to observe the total solar eclipse of august , will sell the right of naming the minor planet no. for £ ." the bright idea seems to have struck herr palisa, who had already discovered many planets and begun to find difficulties in assigning suitable names, that he might turn his difficulty into a source of profit in a good cause. the offer was not responded to immediately, nor until herr palisa had discovered two more planets, nos. and . he found names for two, leaving, however, the last discovered always open for a patron, and on page of the same magazine for the following note informs us how his patience was ultimately rewarded:--"minor planet no. has been named 'bettina' by baron albert de rothschild." i have not heard, however, that this precedent has been followed in other cases, and the ingenuity of discoverers was so much overtaxed towards the end of last century that the naming of their planets fell into arrears. recently a commission, which has been established to look after these small bodies generally, issued a notice that unless the naming was accomplished before a certain date it would be ruthlessly taken out of the hands of the negligent discoverers. perhaps we may notice, before passing on, the provisional system which was adopted to fill up the interval required for finding a suitable name, and required also for making sure that the planet was in fact a new one, and not merely an old one rediscovered. there was a system of _numbering_ in existence as well as of _naming_, but it was unadvisable to attach even a number to a planet until it was quite certain that the discovery was new, for otherwise there might be gaps created in what should be a continuous series by spurious discoveries being struck out. accordingly it was decided to attach at first to the object merely a _letter of the alphabet_, with the year of discovery, as a provisional name. the alphabet was, however, run through so quickly, and confusion was so likely to ensue if it was merely repeated, that on recommencing it the letter a was prefixed, and the symbols adopted were therefore aa, ab, ac, &c.; after completing the alphabet again, the letter b was prefixed, and so on; and astronomers began to fear that they had before them a monotonous prospect of continually adding new planets, varied by no incident more exciting than starting the alphabet over again after every score. [sidenote: eros.] fortunately, however, on running through it for the fifth time, an object of particular interest was discovered. most of these bodies revolve at a distance from the sun intermediate between that of mars and that of jupiter, but the little planet which took the symbol dq, and afterwards the name of eros, was found to have a mean distance actually less than that of mars, and this gave it an extraordinary importance with respect to the great problem of determining the sun's distance. to explain this importance we must make a small digression. [sidenote: transit of venus.] about the middle of the last century our knowledge of the sun's distance was very rough, as may be seen from the table on p. ; but there were in prospect two transits of venus, in and , and it was hoped that these would give opportunities of a special kind for the measurement of this important quantity, which lies at the root of all our knowledge of the exact masses and dimensions of not only the sun, but of the planets as well. [illustration: fig. .] [sidenote: the "black drop."] the method may be briefly summarised thus: an observer in one part of the earth would see venus cross the disc of the sun along a different path from that seen by another observer, as will be clear from the diagram. if the size of the earth, the distance of the sun, and the _relative_ distance of venus be known, it can be calculated what this difference in path will be. now the relative distance of venus _is_ known with great accuracy, from observing the time of her revolution round the sun; the size of the earth we can measure by a survey; there remains, therefore, only one unknown quantity, the sun's distance. and since from a knowledge of this we could calculate the difference in path, it is easy to invert the problem, and calculate the sun's distance from the knowledge of the observed difference in path. accordingly, observers were to be scattered, not merely to two, but to many stations over the face of the earth, to observe the exact path taken by venus in transit over the sun's disc as seen from their station; and especially to observe the exact times of beginning and ending of the transit; and, by comparison of their results, it was hoped to determine this very important quantity, the sun's distance. it was known from previous experience that there were certain difficulties in observing very exactly the beginning and end of the transit. there was an appearance called the "black drop," which had caused trouble on previous occasions; an appearance as though the round black spot which can be seen when venus has advanced some distance over the sun's disc was reluctant to make the entry and clung to the edge or "limb" of the sun as it is called, somewhat as a drop of ink clings to a pen which is slowly withdrawn from an inkpot. similarly, at the end of the transit or egress, instead of approaching the limb steadily the planet seems at the last moment to burst out towards it, rendering the estimation of the exact moment when the transit is over extremely doubtful. [sidenote: failure.] these difficulties, as already stated, were known to exist; but there is a long interval between transits of venus, or rather between every pair of such transits. after those of and there will be no more until and , so that we shall never see another; similarly, before that pair of the last century, there had not been any such occasion since and , and no one was alive who remembered at first hand the trouble which was known to exist. it was proposed to obviate the anticipated difficulties by careful practice beforehand; models were prepared to resemble as nearly as possible the expected appearances, and the times recorded by different observers were compared with the true time, which could, in this case of a model, be determined. in this way it was hoped that the habit of each observer, his "personal equation" as it is called, could be determined beforehand, and allowed for as a correction when he came to observe the actual transit. the result, however, was a great disappointment. the actual appearances were found to be totally different in character from those shown by the model; chiefly, perhaps, because it had been impossible to imitate with a model the effect of the atmosphere which surrounds the planet venus. observers trained beforehand, using similar instruments, and standing within a few feet of each other, were expected, after making due allowance for personal equation, to give the same instant for contact; but their observations when made were found to differ by nearly a minute of time, and after an exhaustive review of the whole material it was felt that all hope of determining accurately the sun's distance by this method must be given up. the following table will show how much was learned from the transits of venus, and how much remained to be settled. they left the result in doubt over a range of about two million miles. sun's distance, in millions of miles, as found by different observers =before the transits of venus= estimates varied between = = million miles (gilliss and gould, ) and = = million (winneche, ), a range of million miles. =the transits of and = gave results lying between = - / = million (airy, from british observations of ), = - / = million (stone, from british observations of ), and = - / = million (puiseux, from french observations), a range of - / millions. =gill's heliometer results= all lie very near = = millions. the observations of mars in give about , miles over this figure: but the observations of victoria, iris, and sappho, which are more trustworthy, all agree in giving about , miles _less_ than the millions. it became necessary, therefore, to look to other methods; and before the second transit of was observed, an energetic astronomer, dr. david gill, had already put into operation the method which may be now regarded as the standard one. [sidenote: modern method for sun's distance.] [sidenote: photography.] [sidenote: dr. gill's expedition to ascension.] we have said that the _relative_ distance of venus from the sun is accurately known from observations of the exact time of revolution. it is easy to see that these times of revolution can be measured accurately by mere accumulation. we may make an error of a few seconds in noting the time of return; but if the whole interval comprises revolutions, this error is divided by , if revolutions by , and so on; and by this time a great number of revolutions of all the planets (except those just discovered) have been recorded. hence we know their relative distances with great precision; and if we can find the distance in miles of any one of them, we can find that of the sun itself, or of any other planet, by a simple rule-of-three sum. by making use of this principle many of the difficulties attending the direct determination of the sun's distance can be avoided; for instance, since the sun's light overpowers that of the stars, it is not easy to directly observe the place of the sun among the stars; but this is not so for the planets. we can photograph a planet and the stars surrounding it on the same plate, and then by careful measurement determine its exact position among the stars; and since this position differs slightly according to the situation of the observer on the earth's surface, by comparing two photographs taken at stations a known distance apart we can find the distance of the planet from the earth; and hence, as above remarked, the distance of the sun and all the other members of the solar system. or, instead of taking photographs from two different stations, we can take from the same station two photographs at times separated by a known interval. for in that interval the station will have been carried by the earth's rotation some thousands of miles away from its former position, and becomes virtually a second station separated from the first by a distance which is known accurately when we know the elapsed time. again, instead of taking photographs, and from them measuring the position of the planet among the stars, we may make the measurements on the planet and stars in the sky itself; and since in , when dr. gill set out on his enterprise of determining the sun's distance, photography was in its infancy as applied to astronomy, he naturally made his observations on the sky with an instrument known as a heliometer. he made them in the little island of ascension, which is suitably situated for the purpose; because, being near the earth's equator, it is carried by the earth's rotation a longer distance in a given time than places nearer the poles, and in these observations for "parallax," as they are called, it is important to have the displacement of the station as large as possible. for a similar reason the object selected among the planets must be as near the earth as possible; and hence the planet mars, which at favourable times comes nearer to us than any other superior planet[ ] then known, was selected for observation with the heliometer. and now it will be seen why the discovery of the little planet eros was important, for mars was no longer the known planet capable of coming nearest to us; it had been replaced by this new arrival. [sidenote: victoria, iris, and sappho.] [sidenote: eros.] further, a small planet which is in appearance just like an ordinary star has, irrespective of this great proximity, some distinct advantages over a planet like mars, which appears as a round disc, and is, moreover, of a somewhat reddish colour. when the distance of an object of this kind from a point of line such as a star is measured with the heliometer it is found that a certain bias, somewhat difficult to allow for with certainty, is introduced into the measures; and our confidence in the final results suffers accordingly. after his observations of mars in , dr. david gill was sufficiently impressed with this source of error to make three new determinations of the sun's distance, using three of the minor planets instead of mars, in spite of the fact that they were sensibly farther away; and his choice was justified by finding that the results from these three different sets of observations agreed well among themselves, and differed slightly from that given by the observations of mars. hence it seems conclusively proved that one of these bodies is a better selection than mars in any case, and the discovery of eros, which offered the advantage of greater proximity in addition, was hailed as a new opportunity of a most welcome kind. it was seen by a little calculation that in the winter of - the planet would come very near the earth; not the nearest possible (for it was also realised that a still better opportunity had occurred in , though it was lost because the planet had not yet been discovered), but still the nearest approach which would occur for some thirty years; and extensive, though somewhat hasty, preparations were made to use it to the fullest advantage. photography had now become established as an accurate method of making measurements of the kind required; and all the photographic telescopes which could be spared were pressed into the service, and diligently photographed the planet and surrounding stars every fine night during the favourable period. the work of measuring and reducing these photographs involves an enormous amount of labour, and is even yet far from completed, but we know enough to expect a result of the greatest value. more than this we have not time to say here about this great problem, but it will have been made clear that just when astronomers were beginning to wonder whether it was worth while continuing the monotonous discovery of new minor planets by the handful, the rd discovery also turned out to be one of the greatest importance. to canons for the advantageous prosecution of research, if we care to make them, we may therefore add this--that there is no line of research, however apparently unimportant or monotonous, which we can afford to neglect. just when we are on the point of relinquishing it under the impression that the mine is exhausted, we may be about to find a nugget worth all our previous and future labour. this rule will not, perhaps, help us very much in choosing what to work at; indeed, it is no rule at all, for it leaves us the whole field of choice unlimited. but this negative result will recur again and again as we examine the lessons taught by discoveries: there seem to be no rules at all. whenever we seem to be able to deduce one from an experience, some other experience will flatly contradict it. thus we might think that the discovery of eros taught us to proceed patiently with a monotonous duty, and not turn aside to more novel and attractive work; yet it is often by leaving what is in hand and apparently has first claim on our attention that we shall do best, and we shall learn in the next chapter how a failure thus to turn flexibly aside was repented. chapter ii the discovery of neptune [sidenote: search for definite objects.] in the last chapter we saw that the circumstances under which planets were discovered varied considerably. sometimes the discoveries were not previously expected, occurring during a general examination of the heavens, or a search for other objects; and, on one occasion at least, the discovery may be said to have been even contrary to expectation, though, as the existence of a number of minor planets began to be realised, there have also been many cases where the discovery has been made as the result of a definite and deliberate search. but the search cannot be said to have been inspired by any very clear or certain principle: for the law of bode, successful though it has been in indicating the possible existence of new planets, cannot, as yet, be said to be founded upon a formulated law of nature. we now come, however, to a discovery made in direct interpretation of newton's great law of gravitation--the discovery of neptune from its observed disturbance of uranus. i will first briefly recall the main facts relating to the actual discovery. [sidenote: disturbance of uranus.] after uranus had been discovered and observed sufficiently long for its orbit to be calculated, it was found that the subsequent position of the planet did not always agree with this orbit; and, more serious than this, some early observations were found which could not be reconciled with the later ones at all. it is a wonderful testimony to the care and sagacity of sir william herschel, as was remarked in the last chapter, that uranus was found to have been observed, under the mistaken impression that it was an ordinary star, by flamsteed, lemonnier, bradley, and mayer, all observers of considerable ability. flamsteed's five observations dated as far back as , , and ; observations by others were in , , , , and so on up to , and the body of testimony was so considerable that there was no room for doubt as to the irreconcilability of the observations with the orbit, such as might have been the case had there been only one or two, possibly affected with some errors. [sidenote: suspicion of perturbing planet.] it is difficult to mention an exact date for the conversion into certainty of the suspicion that no single orbit could be found to satisfy all the observations; but we may certainly regard this fact as established in , when alexis bouvard published some tables of the planet, and showed fully in the introduction that when every correction for the disturbing action of other planets had been applied, it was still impossible to reconcile the old observations with the orbit calculated from the new ones. the idea accordingly grew up that there might be some other body or bodies attracting the planet and causing these discrepancies. here again it is not easy to say exactly when this notion arose, but it was certainly existent in , as the following letter to the astronomer royal will show. i take it from his well-known "account of some circumstances historically connected with the discovery of the planet exterior to uranus," which he gave to the royal astronomical society at its first meeting after that famous discovery (monthly notices of the r.a.s., vol. iii., and memoirs, vol. xvi.). no. .--_the_ rev. t. j. hussey _to_ g. b. airy. [_extract._] "'hayes, kent, _ th november _. "'with m. alexis bouvard i had some conversation upon a subject i had often meditated, which will probably interest you, and your opinion may determine mine. having taken great pains last year with some observations of _uranus_, i was led to examine closely bouvard's tables of that planet. the apparently inexplicable discrepancies between the ancient and modern observations suggested to me the possibility of some disturbing body beyond _uranus_, not taken into account because unknown. my first idea was to ascertain some approximate place of this supposed body empirically, and then with my large reflector set to work to examine all the minute stars thereabouts: but i found myself totally inadequate to the former part of the task. if i could have done it formerly, it was beyond me now, even supposing i had the time, which was not the case. i therefore relinquished the matter altogether; but subsequently, in conversation with bouvard, i inquired if the above might not be the case: his answer was, that, as might have been expected, it had occurred to him, and some correspondence had taken place between hansen and himself respecting it. hansen's opinion was, that one disturbing body would not satisfy the phenomena; but that he conjectured there were two planets beyond _uranus_. upon my speaking of obtaining the places empirically, and then sweeping closely for the bodies, he fully acquiesced in the propriety of it, intimating that the previous calculations would be more laborious than difficult; that if he had leisure he would undertake them and transmit the results to me, as the basis of a very close and accurate sweep. i have not heard from him since on the subject, and have been too ill to write. what is your opinion on the subject? if you consider the idea as possible, can you give me the limits, roughly, between which this body or those bodies may probably be found during the ensuing winter? as we might expect an eccentricity [inclination?] approaching rather to that of the old planets than of the new, the breadth of the zone to be examined will be comparatively inconsiderable. i may be wrong, but i am disposed to think that, such is the perfection of my equatoreal's object-glass, i could distinguish, almost at once, the difference of light of a small planet and a star. my plan of proceeding, however, would be very different: i should accurately map the whole space within the required limits, down to the minutest star i could discern; the interval of a single week would then enable me to ascertain any change. if the whole of this matter do not appear to you a chimæra, which, until my conversation with bouvard, i was afraid it might, i shall be very glad of any sort of hint respecting it.' "my answer was in the following terms:-- [sidenote: airy's scepticism.] no. .--g. b. airy _to the_ rev. t. j. hussey. [_extract._] "'observatory, cambridge, _ , nov. _. "'i have often thought of the irregularity of _uranus_, and since the receipt of your letter have looked more carefully to it. it is a puzzling subject, but i give it as my opinion, without hesitation, that it is not yet in such a state as to give the smallest hope of making out the nature of any external action on the planet ... if it were certain that there were any extraneous action, i doubt much the possibility of determining the place of a planet which produced it. i am sure it could not be done till the nature of the irregularity was well determined from several successive revolutions.'" [sidenote: le verrier's papers.] [sidenote: planet to be detected by disc.] [sidenote: galle's discovery of the planet.] although only a sentence or two have been selected from airy's reply (he was not yet astronomer royal), they are sufficient to show that the problem of finding the place of such a possible disturbing body was regarded at that time as one of extreme difficulty; and no one appears seriously to have contemplated embarking upon its solution. it was not until many years later that the solution was attempted. of the first attempt we shall speak presently, putting it aside for the moment because it had no actual bearing on the discovery of the planet, for reasons which form an extraordinary episode of this history. the attempt which led to success dates from november . the great french astronomer le verrier, on november , , read to the french academy a paper on the orbit of uranus, considering specially the disturbances produced by jupiter and saturn, and showing clearly that with no possible orbit could the observations be satisfied. on june , , followed a second paper by the same author, in which he considers all the possible explanations of the discordance, and concludes that none is admissible except that of a disturbing planet exterior to uranus. and assuming, in accordance with bode's law, that the distance of this new planet from the sun would be about double that of uranus (and it is important to note this assumption), he proceeds to investigate the orbit of such a planet, and to calculate the place where it must be looked for in the heavens. this was followed by a third paper on august st, giving a rather completer discussion, and arriving at the conclusion that the planet should be recognisable from its disc. this again is an important point. we remember that in the discovery of uranus it needed considerable skill on the part of sir william herschel to detect the disc, to see in fact any difference between it and surrounding stars; and that other observers, even when their attention had been called to the planet, found it difficult to see this difference. it might be expected, therefore, that with a planet twice as far away (as had been assumed for the new planet) the disc would be practically unrecognisable, and as we shall presently see, this assumption was made in some searches for the planet which had been commenced even before the publication of this third paper. le verrier's courageous announcement, which he deduced from a consideration of the mass of the planet, that the disc should be recognisable, led immediately to the discovery of the suspected body. he wrote to a german astronomer, dr. galle (still, i am glad to say, alive and well, though now a very old man), telling him the spot in the heavens to search, and stating that he might expect to detect the planet by its appearance in this way; and the same night dr. galle, by comparing a star map with the heavens, found the planet. [sidenote: adams' work publicly announced.] to two points to which i have specially called attention in this brief summary--namely, the preliminary assumption that the planet would be, according to bode's law, twice as far away as uranus; secondly, the confident assertion that it would have a visible disc--i will ask you to add, thirdly, that it was found by the aid of a star map, for this map played an important part in the further history to which we shall now proceed. it may naturally be supposed that the announcement of the finding of a planet in this way, the calculation of its place from a belief in the universal action of the great law of gravitation, the direction to an eminent observer to look in that place for a particular thing, and his immediate success,--this extraordinary combination of circumstances caused a profound sensation throughout not only the astronomical, but the whole world; and this sensation was greatly enhanced by the rumour which had begun to gather strength that, but for some unfortunate circumstances, the discovery might have been made even earlier and as a consequence of totally independent calculations made by a young cambridge mathematician, j. c. adams. some of you are doubtless already familiar with the story in its abridged form, for it has been scattered broadcast through literature. in england it generally takes the form of emphasising the wickedness or laziness of the astronomer royal who, when told where to look for a planet, neglected his obvious duty, so that in consequence another astronomer who made the calculation much later and gave a more virtuous observer the same directions where to look, obtained for france the glory of a discovery which ought to have been retained in england. there is no doubt that airy's conduct received a large amount of what he called "savage abuse." when the facts are clearly stated i think it will be evident that many of the harsh things said of him were scarcely just, though at the same time it is also difficult to understand his conduct at two or three points of the history, even as explained by himself. [sidenote: facts undoubted.] there is fortunately no doubt whatever about any of the _facts_. airy himself gave a very clear and straightforward account of them at the time, for which more credit is due to him than he commonly receives; and since the death of the chief actors in this sensational drama they have been naturally again ransacked, with the satisfactory result that there is practically no doubt about any of the facts. as to the proper interpretations of them there certainly may be wide differences of opinion, nor does this circumstance detract from their interest. it is almost impossible to make a perfectly colourless recital of them, nor is it perhaps necessary to do so. i will therefore ask you to remember in what i now say that there is almost necessarily an element of personal bias, and that another writer would probably give a different colouring. having said this, i hope i may speak quite freely as the matter appears in my personal estimation. [sidenote: airy's "account."] [sidenote: "a movement of the age."] airy's account was, as above stated, given to the royal astronomical society at their first meeting (after the startling announcement of the discovery of the new planet), on november , , and i have already quoted an extract from it. he opens with a tribute to the sensational character of the discovery, and then states that although clearly due to two individuals (namely, le verrier and galle), it might also be regarded as to some extent the consequence of a movement of the age. his actual words are these: "the principal steps in the theoretical investigations have been made by one individual, and the published discovery of the planet was necessarily made by one individual. to these persons the public attention has been principally directed; and well do they deserve the honours which they have received, and which they will continue to receive. yet we should do wrong if we considered that these two persons alone are to be regarded as the authors of the discovery of this planet. i am confident that it will be found that the discovery is a consequence of what may properly be called a movement of the age; that it has been urged by the feeling of the scientific world in general, and has been nearly perfected by the collateral, but independent labours, of various persons possessing the talents or powers best suited to the different parts of the researches." [sidenote: airy under-estimated adams' work.] i have quoted these words as the first point at which it is difficult to understand airy's conduct in excluding from them all specific mention of adams, knowing as he did the special claims which entitled him to such mention; claims indeed which he proceeded immediately to make clear. it seems almost certain that airy entirely under-estimated the value of adams' work throughout. but this will become clearer as we proceed. the "account" takes the form of the publication of a series of letters with occasional comments. airy was a most methodical person, and filed all his correspondence with great regularity. it was jestingly said of him once that if he wiped his pen on a piece of blotting-paper, he would date the blotting-paper and file it for reference. the letters reproduced in this "account" are still in the observatory at greenwich, pinned together just as airy left them; and in preparing his "account" it was necessary to do little else than to have them copied out and interpolate comments. from two of them i have already quoted to show how difficult the enterprise of finding an exterior planet from its action on uranus was considered in . to these may be added the following sentence from no. , dated . "if it be the effect of any unseen body," writes airy to bouvard, "it will be nearly impossible ever to find out its place." but the first letter which need concern us is no. , and it is only necessary to explain that professor challis was the professor of astronomy at cambridge, and in charge of the cambridge observatory, in which offices he had succeeded airy himself on his leaving cambridge for greenwich some eight years earlier. no. .--professor challis _to_ g. b. airy. [_extract._] "'cambridge observatory, _feb. , _. [sidenote: challis mentions adams to airy, and suggests adams' visit to greenwich.] "'a young friend of mine, mr. adams of st. john's college, is working at the theory of _uranus_, and is desirous of obtaining errors of the tabular geocentric longitudes of this planet, when near opposition, in the years - , with the factors for reducing them to errors of heliocentric longitude. are your reductions of the planetary observations so far advanced that you could furnish these data? and is the request one which you have any objection to comply with? if mr. adams may be favoured in this respect, he is further desirous of knowing, whether in the calculation of the tabular errors any alterations have been made in bouvard's _tables of uranus_ besides that of _jupiter's_ mass.' "my answer to him was as follows:-- no. .--g. b. airy _to_ professor challis. [_extract._] "'royal observatory, greenwich, _ , feb. _. "'i send all the results of the observations of _uranus_ made with both instruments (that is, the heliocentric errors of _uranus_ in longitude and latitude from to , for all those days on which there were observations, both of right ascension and of polar distance). no alteration is made in bouvard's _tables of uranus_ except in increasing the two equations which depend on _jupiter_ by / part. as constants have been added (in the printed tables) to make the equations positive, and as / part of the numbers in the tables has been added, / part of the constants has been subtracted from the final results.' "professor challis in acknowledging the receipt of these, used the following expressions:-- no. .--professor challis _to_ g. b. airy. [_extract._] "'cambridge observatory, _feb. , _. "'i am exceedingly obliged by your sending so complete a series of tabular errors of _uranus_.... the list you have sent will give mr. adams the means of carrying on in the most effective manner the inquiry in which he is engaged.' "the next letter shows that mr. adams has derived results from these errors. no. .--professor challis _to_ g. b. airy. "'cambridge observatory, _sept. , _. "'my friend mr. adams (who will probably deliver this note to you) has completed his calculations respecting the perturbation of the orbit of _uranus_ by a supposed ulterior planet, and has arrived at results which he would be glad to communicate to you personally, if you could spare him a few moments of your valuable time. his calculations are founded on the observations you were so good as to furnish him with some time ago; and from his character as a mathematician, and his practice in calculation, i should consider the deductions from his premises to be made in a trustworthy manner. if he should not have the good fortune to see you at greenwich, he hopes to be allowed to write to you on this subject.' "on the day on which this letter was dated, i was present at a meeting of the french institute. i acknowledged it by the following letter:-- no. .--g. b. airy _to_ professor challis. "'royal observatory, greenwich, _ , sept. _. "'i was, i suppose, on my way from france, when mr. adams called here; at all events, i had not reached home, and therefore, to my regret, i have not seen him. would you mention to mr. adams that i am very much interested with the subject of his investigations, and that i should be delighted to hear of them by letter from him?' "on one of the last days of october , mr. adams called at the royal observatory, greenwich, in my absence and left the following important paper:-- no. .--j. c. adams, esq., _to_ g. b. airy. [sidenote: adams' announcement of the new planet.] "'according to my calculations, the observed irregularities in the motion of _uranus_ may be accounted for by supposing the existence of an exterior planet, the mass and orbit of which are as follows:-- mean distance (assumed nearly in accordance with bode's law) . mean sidereal motion in . days ° '. mean longitude, st october longitude of perihelion eccentricity . . mass (that of the sun being unity) . . for the modern observations i have used the method of normal places, taking the mean of the tabular errors, as given by observations near three consecutive oppositions, to correspond with the mean of the times; and the greenwich observations have been used down to : since which, the cambridge and greenwich observations, and those given in the _astronomische nachrichten_, have been made use of. the following are the remaining errors of mean longitude:-- _observation--theory._ " + . - . - . + . - . + . - . - . + . - . + . - . - . - . + . + . + . - . - . - . + . the error for is concluded from that for given by observation, compared with those of four or five following years, and also with lemonnier's observations in and . "'for the ancient observations, the following are the remaining errors:-- _observation--theory._ " + . + . - . - . + . - . - . + . + . the errors are small, except for flamsteed's observation of . this being an isolated observation, very distant from the rest, i thought it best not to use it in forming the equations of condition. it is not improbable, however, that this error might be destroyed by a small change in the assumed mean motion of the planet.' "i acknowledged the receipt of this paper in the following terms:-- no. .--g. b. airy _to_ j. c. adams, esq. "'royal observatory, greenwich, _ , nov. _. [sidenote: airy's inquiry about the "radius vector."] "'i am very much obliged by the paper of results which you left here a few days since, showing the perturbations on the place of _uranus_ produced by a planet with certain assumed elements. the latter numbers are all extremely satisfactory: i am not enough acquainted with flamsteed's observations about to say whether they bear such an error, but i think it extremely probable. "'but i should be very glad to know whether this assumed perturbation will explain the error of the radius vector of _uranus_. this error is now very considerable, as you will be able to ascertain by comparing the normal equations, given in the greenwich observations for each year, for the times _before_ opposition with the times _after_ opposition.' "i have before stated that i considered the establishment of this error of the radius vector of _uranus_ to be a very important determination. i therefore considered that the trial, whether the error of radius vector would be explained by the same theory which explained the error of longitude, would be truly an _experimentum crucis_. and i waited with much anxiety for mr. adams' answer to my query. had it been in the affirmative, i should at once have exerted all the influence which i might possess, either directly, or indirectly through my friend professor challis, to procure the publication of mr. adams' theory. "from some cause with which i am unacquainted, probably an accidental one, i received no immediate answer to this inquiry. i regret this deeply, for many reasons." [sidenote: adams' silence.] here we may leave airy's "account" for a few moments to consider the reason why he received no answer. adams was a very shy and retiring young man, and very sensitive; though capable of a great resolution, and of enormous perseverance in carrying it out. we know (what is not indicated in the above account), how steadily he had kept in view the idea of solving this great problem. it was characteristic of him that as early as he had formed a resolution to undertake it, although at the time he was not able to enter upon its accomplishment. the following memorandum, which is still in existence, having been found among his papers after his death, records these facts: " , july . formed a design, in the beginning of this week, of investigating, as soon as possible after taking my degree, the irregularities in the motion of uranus, which were as yet unaccounted for: in order to find whether they may be attributed to the action of an undiscovered planet beyond it, and if possible thence to determine the elements of its orbit, &c., approximately, which would probably lead to its discovery." accordingly, "as soon as possible after taking his degree" he embarked upon the enterprise, and the first solution was made in the long vacation of , assuming the orbit of the unknown planet to be a circle with a radius equal to twice the mean distance of uranus from the sun (an assumption which, as we have seen, was also made by le verrier). having satisfied himself that there was a good general agreement between his results and the observations, adams began a more complete solution; indeed from first to last he made no less than six separate solutions, the one which he announced to airy in the above letter being the fourth. hence he had already done an enormous amount of work on the problem, and was in his own mind so justly convinced of the correctness and value of his results that he was liable to forget that others had not had the same opportunity of judging of their completeness; and he was grievously disappointed when his announcement was not received with full confidence. [sidenote: his disappointment at greenwich, and at airy's question.] but perhaps it should first be stated that by a series of mischances adams had been already much disappointed at the failure of his attempts to see the astronomer royal on his visits to greenwich. this does not seem to have been exactly airy's fault; he was, as may well be supposed, an extremely busy man, and was much occupied at the time on a question of great practical importance, at the direct request of the government, namely, the settling of the proper gauge for railways throughout the country. the first time adams called to see him, he was actually in london sitting on the committee which dealt with this question, and adams was asked to call later; when the visit was repeated, airy was unfortunately at dinner (and it may be added that his hours for dinner were somewhat peculiar), and the butler, acting somewhat in the manner of his kind, protected his master's dinner by sending away one whom he doubtless regarded as a troublesome visitor. there is, as i have said, little doubt about any of the facts, and it seems well established that airy himself did not learn of adams' visits until afterwards, and it would scarcely be just to blame him for a servant's oversight. but adams had left the paper above reproduced, and airy with his business-like habits ultimately proceeded to deal with it; he wrote the answer given above asking adams a definite question, filed a copy of it with the original letter, and then dismissed the matter from his thoughts until the reply from adams, which he confidently expected should again bring it under notice. this further disappointment was, however, too much for adams; he regarded the question put by airy as having so obvious an answer that it was intended as an evasion, though this was far from being the case. airy was thoroughly in earnest about his question, though it must be admitted that a more careful study of the problem would have shown him that it was unnecessary. later, when he learnt of le verrier's researches, he put the same question to him, and received a polite but very clear answer, showing that the suggested test was not an _experimentum crucis_ as he supposed. but adams did not feel equal to making this reply; he shrank into his shell and solaced himself only by commencing afresh another solution of the problem which had so engrossed his life at that time. [sidenote: the merits of airy's question.] [sidenote: the range of possibilities.] i have heard severe or contemptuous things said about this question by those who most blame airy. some of them have no hesitation in accusing him of intellectual incompetence: they say that it was the question of a stupid man. i think that in the first place they forget the difference between a deliberate error of judgement and a mere consequence of insufficient attention. but there is even more than this to be said in defence of the question. the "error of radius vector" came before airy in an entirely independent way, and as an entirely independent phenomenon, from the "error of longitude," and there was nothing unnatural in regarding it as requiring independent explanation. it is true that, _as the event proved_, a mere readjustment of the orbit of uranus got rid of this error of radius vector (this was substantially le verrier's answer to airy's question); but we must not judge of what was possible before the event in the light of what we now know. the original possibilities were far wider, though we have forgotten their former extent now that they have been narrowed down by the discovery. if a sentry during war time hears a noise in a certain direction, he may be compelled to make the assumption that it is the movement of an enemy; and if he fires in that direction and kills him, and thus saves his own army from destruction, he is deservedly applauded for the success which attends his action. but it does not follow that the assumption on which he acted was the only possible one. or, to take a more peaceful illustration, in playing whist it sometimes becomes apparent that the game can only be won if the cards lie in a certain way; and a good player will thereupon assume that this is the fact, and play accordingly. adams and le verrier played to win the game on the particular assumption that the disturbance of uranus was due to an external planet revolving at a distance from the sun about twice that of uranus; _and won it_; and we applaud them for doing so. but it is easy to imagine a rearrangement of the cards with which they would have lost it; and airy's question simply meant that he was alive to these wider possibilities, and did not see the need for attempting to win the game in that particular way. one such alternative possibility has already been mentioned. "hansen's opinion was, that one disturbing body would not satisfy the phenomena; but he conjectured that there were two planets beyond _uranus_." another conceivable alternative is that there was some change in the law of gravitation at the distance of uranus, which, it must be remembered, is twice as great as that of any planet previously known. or some wandering body might have passed close enough to uranus to change its orbit somewhat suddenly. we now know, for instance, that the swarm of meteorites which gives rise to the well-known "november meteors" must have passed very close to uranus in a.d. , assuming that neither the planet nor the swarm have been disturbed in any unknown manner in the meantime. it is to this encounter that we owe the introduction of this swarm to our solar system: wandering through space, they met uranus, and were swept by his attraction into an orbit round the sun. was there no reaction upon uranus himself? the probabilities are that the total mass of the swarm was so small as to affect the huge planet inappreciably; but who was to say that some other swarm of larger mass, or other body, might not have approached near uranus at some date between and , and been responsible at any rate in part for the observed errors? these are two or three suppositions from our familiar experience; and there are, of course, limitless possibilities beyond. which is the true scientific attitude, to be alive to them all, or to concentrate attention upon one? but we are perhaps wandering too far from the main theme. it is easy to do so in reviewing this extraordinary piece of history, for at almost every point new possibilities are suggested. [illustration: iii--u. j. le verrier. (_from a print in the possession of the royal astronomical society._)] [illustration: iv--j. g. galle. who first saw the planet neptune] [sidenote: airy receives le verrier's memoir.] we must return, however, to airy's "account." we reached the point where he had written to adams (on november , ), asking his question about the radius vector, and received no reply; and there the matter remained, so far as he was concerned, until the following june, when le verrier's memoir reached him; and we will let him give his own version of the result. "this memoir reached me about the rd or th of june. i cannot sufficiently express the feeling of delight and satisfaction which i received from it. the place which it assigned to the disturbing planet was the same, to one degree, as that given by mr. adams' calculations, which i had perused seven months earlier. to this time i had considered that there was still room for doubt of the accuracy of mr. adams' investigations; for i think that the results of algebraic and numerical computations, so long and so complicated as those of an inverse problem of perturbations, are liable to many risks of error in the details of the process: i know that there are important numerical errors in the _mécanique céleste_ of laplace; in the _théorie de la lune_ of plana; above all, in bouvard's first tables of _jupiter_ and _saturn_; and to express it in a word, i have always considered the correctness of a distant mathematical result to be a subject rather of moral than of mathematical evidence. but now i felt no doubt of the accuracy of both calculations, as applied to the perturbation in longitude. i was, however, still desirous, as before, of learning whether the perturbation in radius vector was fully explained. i therefore addressed to m. le verrier the following letter:-- no. .--g. b. airy _to_ m. le verrier. "'royal observatory, greenwich, _ , june _. [sidenote: he puts the "radius-vector" question to le verrier, but makes no mention of adams.] "'i have read, with very great interest, the account of your investigations on the probable place of a planet disturbing the motions of _uranus_, which is contained in the _compte rendu de l'académie_ of june ; and i now beg leave to trouble you with the following question. it appears, from all the later observations of _uranus_ made at greenwich (which are most completely reduced in the _greenwich observations_ of each year, so as to exhibit the effect of an error either in the tabular heliocentric longitude, or the tabular radius vector), that the tabular radius vector is considerably too small. and i wish to inquire of you whether this would be a consequence of the disturbance produced by an exterior planet, now in the position which you have indicated?'" there is more of the letter, but this will suffice to show that he wrote to le verrier in the same way as to adams, and, as already stated, received a reply dated three or four days later. but the rest of the letter contains no mention of adams, and thus arises a second difficulty in understanding airy's conduct. it seems extraordinary that when he wrote to le verrier he made no mention of the computations which he had previously received from adams; or that he should not have written to adams, and made some attempt to understand his long silence, now that, as he himself states, he "felt no doubt of the accuracy of both calculations." the omission may have been, and probably was, mere carelessness or forgetfulness; but he could hardly be surprised if others mistook it for deliberate action. [sidenote: airy announces the likelihood of a new planet, and suggests a search for it at cambridge not having suitable telescope at greenwich] however, attention had now been thoroughly attracted to the near possibility of finding the planet. on june , , there was a special meeting of the board of visitors of greenwich observatory, and airy incidentally mentioned to them this possibility. the impression produced must have been definite and deep; for sir john herschel, who was present, was bold enough to say on september th following to the british association assembled at southampton: "we see it (the probable new planet) as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to that of ocular demonstration." airy discussed the matter with professor challis (who, it will be remembered, had originally written to him on behalf of adams), suggesting that he should immediately commence a search for the supposed planet at cambridge. it may be asked why airy did not commence this search himself at greenwich, and the answer is that he had no telescope which he regarded as large enough for the purpose. the royal observatory at greenwich has always been, and is now, better equipped in some respects than any other observatory, as might be expected from its deservedly great reputation; but to possess the largest existing telescope has never been one of its ambitions. the instruments in which it takes most pride are remarkable for their steadiness and accuracy rather than for their size; and at that time the best telescope possessed by the observatory was not, in airy's opinion, large enough to detect the planet with certainty. in this opinion we now know that he was mistaken; but, again, we must not judge his conduct before the event in the light of what we have since discovered. it may be recalled here that it was not until le verrier's third paper, published on august , that he (le verrier) emphatically pointed out that the new planet might be of such a size as to have a sensible disc; and it was this remark which led immediately to its discovery. until this was so decisively stated, it must have seemed exceptionally improbable; for we saw in the last chapter how diligently the zodiac had been swept in the search for minor planets,--how, for instance, hencke had searched for fifteen years without success; and it might fairly be considered that if there were a fairly bright object (such as neptune has since been found to be) it would have been discovered earlier. hence airy not unreasonably considered it necessary to spread his net for very small objects. on july he wrote to professor challis as follows:-- no. .--g. b. airy _to_ professor challis. "the deanery, ely, _ , july _. "you know that i attach importance to the examination of that part of the heavens in which there is ... reason for suspecting the existence of a planet exterior to _uranus_. i have thought about the way of making such examination, but i am convinced that (for various reasons, of declination, latitude of place, feebleness of light, and regularity of superintendence) there is no prospect whatever of its being made with any chance of success, except with the northumberland telescope. "now, i should be glad to ask you, in the first place, whether you could make such an examination? "presuming that your answer would be in the negative, i would ask, secondly, whether, supposing that an assistant were supplied to you for this purpose, you would superintend the examination? "you will readily perceive that all this is in a most unformed state at present, and that i am asking these questions almost at a venture, in the hope of rescuing the matter from a state which is, without the assistance that you and your instruments can give, almost desperate. therefore i should be glad to have your answer, not only responding simply to my questions, but also entering into any other considerations which you think likely to bear on the matter. "the time for the said examination is approaching near." [sidenote: challis undertakes the search.] [sidenote: he finds too late that he had observed the planet.] professor challis did not require an assistant, but determined to undertake the work himself, and devised his own plan of procedure; but he also set out on the undertaking with the expectation of a long and arduous search. no such idea as that of finding the planet on the first night ever entered his head. for one thing, he had no map of the region to be examined, for although the map used by galle had been published, no copy of it had as yet reached cambridge, and professor challis had practically to construct a map for himself. in these days of photography to make such a map is a simple matter, but at that time the process was terribly laborious. "i get over the ground very slowly," he wrote on september nd to airy, "thinking it right to include all stars to - magnitude; and i find that to scrutinise thoroughly in this way the proposed portion of the heavens will require many more observations than i can take this year." with such a prospect, it is not surprising that one night's observations were not even compared with the next; there would be a certain economy in waiting until a large amount of material had been accumulated, and then making the comparisons all together, and this was the course adopted. but when le verrier's third paper, with the decided opinion that the planet would be bright enough to be seen by its disc, ultimately reached professor challis, it naturally gave him an entirely different view of the possibilities; he immediately began to compare the observations already made, and found that he had observed the planet early in august. but it was now too late to be first in the field, for galle had already made his announcement of discovery. writing to airy on october , challis could only lament that after four days' observing the planet was in his grasp, _if_ only he had examined or mapped the observations, and _if_ he had not delayed doing so until he had more observations to reduce, and _if_ he had not been very busy with some comet observations. oh! these terrible _ifs_ which come so often between a man and success! the third of them is a peculiarly distressing one, for it represents that eternal conflict between one duty and another, which is so constantly recurring in scientific work. shall we finish one piece of work now well under way, or shall we attend to something more novel and more attractive? challis thought his duty lay in steadily completing the comet observations already begun. we saw in the last lecture how the steady pursuit of the discovery of minor planets, a duty which had become tedious and apparently led nowhere, suddenly resulted in the important discovery of eros. but challis was not so fortunate in electing to plod along the beaten track; he would have done _better_ to leave it. there is no golden rule for the answer; we must be guided in each case by the special circumstances, and the dilemma is consequently a new one on every occasion, and perhaps the more trying with each repetition. [sidenote: sensation caused by the discovery.] [sidenote: not all _national_ jealousy.] such are briefly the events which led to the discovery of neptune, which was made in germany by direction from france, when it might have been made in cambridge alone. the incidents created a great stir at the time. the "account" of them, as read by airy to the royal astronomical society on november , , straightforward and interesting though it was, making clear where he had himself been at fault, nevertheless stirred up angry passions in many quarters, and chiefly directed against airy himself. cambridge was furious at airy's negligence, which it considered responsible for costing the university a great discovery; and others were equally irate at his attempting to claim for adams some of that glory which they considered should go wholly to le verrier. but it may be remarked that feeling was not purely national. some foreigners were cordial in their recognition of the work of adams, while some of those most eager to oppose his claims were found in this country. in their anxiety to show that they were free from national jealousy, scientific men went almost too far in the opposite direction. [sidenote: the position of cambridge in the matter.] [sidenote: challis the weakest point.] airy's conduct was certainly strange at several points, as has already been remarked. one cannot understand his writing to le verrier in june without any mention of adams. he could not even momentarily have forgotten adams' work; for he tells us himself how he noticed the close correspondence of his result with that of le verrier: and had he even casually mentioned this fact in writing to the latter, it would have prepared the way for his later statement. but we can easily understand the unfavourable impression produced by this statement after the discovery had been made, when there had been no previous hint on the subject at all. of those who abused him cambridge had the least excuse; for there is no doubt that with a reasonably competent professor of astronomy in cambridge, she need not have referred to airy at all. it would not seem to require any great amount of intelligence to undertake to look in a certain region for a strange object if one is in possession of a proper instrument. we have seen that challis had the instrument, and when urged to do so was equal to the task of finding the planet; but he was a man of no initiative, and the idea of doing so unless directed by some authority never entered his head. he had been accustomed for many years to lean rather helplessly upon airy, who had preceded him in office at cambridge. for instance, when appointed to succeed him, and confronted with the necessity of lecturing to students, he was so helpless that he wrote to implore airy to come back to cambridge and lecture for him; and this was actually done, airy obtaining leave from the government to leave his duties at greenwich for a time in order to return to cambridge, and show challis how to lecture. now it seems to me that this helplessness was the very root of all the mischief of which cambridge so bitterly complained. i claimed at the outset the privilege of stating my own views, with which others may not agree: and of all the mistakes and omissions made in this little piece of history, the most unpardonable and the one which had most serious consequences seems to me to be this: that challis never made the most casual inquiry as to the result of the visit to greenwich which he himself had directed adams to make. i am judging him to some extent by default; because i assume the facts from lack of evidence to the contrary: but it seems practically certain that after sending this young man to see airy on this important topic, challis thereupon washed his hands of all responsibility so completely that he never even took the trouble to inquire on his return, "well! how did you get on? what did the astronomer royal say?" had he put this simple question, which scarcely required the initiative of a machine, and learnt in consequence, as he must have done, that the sensitive young man thought airy's question trivial, and did not propose to answer it, i think we might have trusted events to right themselves. even challis might have been trusted to reply, "oh! but you must answer the astronomer royal's question: you may think it stupid, but you had better answer it politely, and show him that you know what you are about." it is unprofitable to pursue speculation further; this did _not_ happen, and something else did. but i have always felt that my old university made a scapegoat of the wrong man in venting its fury upon airy, when the real culprit was among themselves, and was the man they had themselves chosen to represent astronomy. he was presumably the best they had; but if they had no one better than this, they should not have been surprised, and must not complain, if things went wrong. if a university is ambitious of doing great things, it must take care to see that there are men of ability and initiative in the right places. this is a most difficult task in any case, and we require all possible incentives towards it. to blink the facts when a weak spot is mercilessly exposed by the loss of a great opportunity is to lose one kind of incentive, and perhaps not the least valuable. [sidenote: curious difference between actual and supposed planet.] [sidenote: professor peirce's contention that the discovery was a mere accident.] [sidenote: the explanation.] let us now turn to some curious circumstances attending this remarkable discovery of a planet by mathematical investigation, of which there are several. the first is, that although neptune was found so near the place where it was predicted, its orbit, after discovery, proved to be very different from that which adams and le verrier had supposed. you will remember that both calculators assumed the distance from the sun, in accordance with bode's law, to be nearly twice that of uranus. the actual planet was found to have a mean distance less than this by per cent., an enormous quantity in such a case. for instance, if the supposed planet and the real were started round the sun together, the real planet would soon be a long way ahead of the other, and the ultimate disturbing effect of the two on uranus would be very different. to explain the difference, we must first recall a curious property of such disturbances. when two planets are revolving, so that one takes just twice or three times, or any exact number of times, as long to revolve round the sun as the other, the usual mathematical expressions for the disturbing action of one planet on the other would assign an _infinite_ disturbance, which, translated into ordinary language, means that we must start with a fresh assumption, for this state of things cannot persist. if the period of one were a little _longer_ than this critical value, some of the mathematical expressions would be of contrary sign from those corresponding to a period a little _shorter_. now it is curious that the supposed planet and the real had orbits on opposite sides of a critical value of this kind, namely, that which would assign a period of revolution for neptune exactly half that of uranus; and it was pointed out in america by professor peirce that the effect of the planet imagined by adams and le verrier was thus totally different from that of neptune. he therefore declared that the mathematical work had not really led to the discovery at all; but that it had resulted from mere coincidence, and this opinion--somewhat paradoxical though it was--found considerable support. it was not replied to by adams until some thirty years later, when a short reply was printed in _liouville's journal_. the explanation is this: the expressions considered by professor peirce are those representing the action of the planet throughout an indefinite past, and did not enter into the problem, which would have been precisely the same if neptune had been suddenly created in ; while, on the other hand, if neptune had existed up till (the time when uranus was first observed, although unknowingly), and then had been destroyed, there would have been no means of tracing its previous existence. in past ages it had no doubt been perturbing the orbit of uranus, and had effected large changes in it; but if it had then been suddenly destroyed, we should have had no means of identifying these changes. there might have been instead of neptune another planet, such as that supposed by adams and le verrier; and its action in all past time would have been very different from that of neptune, as is properly represented in the mathematical expressions which professor peirce considered. in consequence the orbit of uranus in would have been very different from the orbit as it was actually found; but in either case the mathematicians adams and le verrier would have had to take it as they found it; and the disturbing action which they considered in their calculations was the comparatively small disturbance which began in and ended in . during this limited number of years the disturbance of the planet they imagined, although not precisely the same as that of neptune, was sufficiently like it to give them the approximate place of the planet. still it is somewhat bewildering to look at the mathematical expressions for the disturbances as used by adams and le verrier, when we can now compare with them the actual expressions to which they ought to correspond; and one may say frankly that there seems to be no sort of resemblance. recently a memorial of adams' work has been published by the royal astronomical society; they have reproduced in their memoirs a facsimile of adams' ms. containing the "first solution," which he made in in the long vacation after he had taken his degree, and which would have given the place of neptune at that time with an error of °. in an introduction describing the whole of the mss., written by professor r. a. sampson of durham, it is shown how different the actual expressions for neptune's influence are from those used by adams, and it is one of the curiosities of this remarkable piece of history that some of them seem to be actually _in the wrong direction_; and others are so little alike that it is only by fixing our attention resolutely on the considerations above mentioned that we can realise that the analytical work did indeed lead to the discovery of the planet. [sidenote: suggested elementary method for finding neptune illusory.] a second curiosity is that a mistaken idea should have been held by at least one eminent man (sir j. herschel), to the effect that it would have been possible to find the place of the planet by a much simpler mathematical calculation than that actually employed by adams or le verrier. in his famous "outlines of astronomy" sir john herschel describes a simple graphical method, which he declares would have indicated the place of the planet without much trouble. concerning it i will here merely quote professor sampson's words:-- "the conclusion is drawn that _uranus_ arrived at a conjunction with the disturbing planet about ; and this was the case. plausible as this argument may seem, it is entirely baseless. for the maximum of perturbations depending on the eccentricities has no relation to conjunction, and the others which depend upon the differences of the mean motions alone are of the nature of forced oscillations, and conjunction is not their maximum or stationary position, but their position of most rapid change." professor sampson goes on to show that a more elaborate discussion seems quite as unpromising; and he concludes that the refinements employed were not superfluous, although it seems _now_ clear that a different mode of procedure might have led more certainly to the required conclusion. [sidenote: the evil influence of bode's law.] for the third curious point is that both calculators should have adhered so closely to bode's law. if they had not had this guiding principle it seems almost certain that they would have made a better approximation to the place of the planet, for instead of helping them it really led them astray. we have already remarked that if two planets are at different distances from the sun, however slight, and if they are started in their revolution together, they must inevitably separate in course of time, and the amount of separation will ultimately become serious. thus by assuming a distance for the planet which was in error, however slight, the calculators immediately rendered it impossible for themselves to obtain a place for the planet which should be correct for more than a very brief period. professor sampson has given the following interesting lists of the dates at which adams' six solutions gave the true place of the planet and the intervals during which the error was within ° either way. i. ii. iii. iv. v. vi. correct within ± ° { { now the date at which it was most important to obtain the correct place was or thereabouts when it was proposed to look for the planet; but no special precaution seems to have been taken by either investigator to secure any advantage for this particular date. criticising the procedure after the event (and of course this is a very unsatisfactory method of criticism), we should say that it would have been better to make several assumptions as regards the distance instead of relying upon bode's law; but no one, so far as i know, has ever taken the trouble to write out a satisfactory solution of the problem as it might have been conducted. such a solution would be full of interest, though it could only have a small weight in forming our estimation of the skill with which the problem was solved in the first instance. [sidenote: le verrier's erroneous limits.] fourthly, we may notice a very curious point. le verrier went to some trouble not only to point out the most likely place for the planet, but to indicate limits outside which it was not necessary to look. this part of his work is specially commented upon with enthusiasm by airy, and i will reproduce what he says. it is rather technical perhaps, but those who cannot follow the mathematics will be able to appreciate the tone of admiration. [sidenote: the visible disc.] "m. le verrier then enters into a most ingenious computation of the limits between which the planet must be sought. the principle is this: assuming a time of revolution, all the other unknown quantities may be varied in such a manner that though the observations will not be so well represented as before, yet the errors of observation will be tolerable. at last, on continuing the variation of elements, one error of observation will be intolerably great. then, by varying the elements in another way, we may at length make another error of observation intolerably great; and so on. if we compute, for all these different varieties of elements, the place of the planet for , its _locus_ will evidently be a discontinuous curve or curvilinear polygon. if we do the same thing with different periodic times, we shall get different polygons; and the extreme periodic times that can be allowed will be indicated by the polygons becoming points. these extreme periodic times are and years. if now we draw one grand curve, circumscribing all the polygons, it is certain that the planet must be within that curve. in one direction, m. le verrier found no difficulty in assigning a limit; in the other he was obliged to restrict it, by assuming a limit to the eccentricity. thus he found that the longitude of the planet was certainly not less than °, and not greater than ° or °, according as we limit the eccentricity to . or . . and if we adopt . as the limit, then the mass will be included between the limits . and . ; either of which exceeds that of _uranus_. from this circumstance, combined with a probable hypothesis as to the density, m. le verrier concluded that the planet would have a visible disk, and sufficient light to make it conspicuous in ordinary telescopes. "m. le verrier then remarks, as one of the strong proofs of the correctness of the general theory, that the error of radius vector is explained as accurately as the error of longitude. and finally, he gives his opinion that the latitude of the disturbing planet must be small. "my analysis of this paper has necessarily been exceedingly imperfect, as regards the astronomical and mathematical parts of it; but i am sensible that, in regard to another part, it fails totally. i cannot attempt to convey to you the impression which was made on me by the author's undoubting confidence in the general truth of his theory, by the calmness and clearness with which he limited the field of observation, and by the firmness with which he proclaimed to observing astronomers, 'look in the place which i have indicated, and you will see the planet well.' since copernicus declared that, when means should be discovered for improving the vision, it would be found that _venus_ had phases like the moon, nothing (in my opinion) so bold, and so justifiably bold, has been uttered in astronomical prediction. it is here, if i mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher." [sidenote: peirce's views of the limits.] but now this process of limitation was faulty and actually misleading. let us compare what is said about it by professor peirce a little later. "guided by this principle, well established, and legitimate, if confined within proper limits, m. le verrier narrowed with consummate skill the field of research, and arrived at two fundamental propositions, namely:-- " st. that the mean distance of the planet cannot be less than or more than . . the corresponding limits of the time of sidereal revolution are about and years. " nd. 'that there is only one region in which the disturbing planet can be placed in order to account for the motions of uranus; that the mean longitude of this planet must have been, on january , , between ° and °.' "'neither of these propositions is of itself necessarily opposed to the observations which have been made upon neptune, but the two combined are decidedly inconsistent with observation. it is impossible to find an orbit, which, satisfying the observed distance and motion, is subject to them. if, for instance, a mean longitude and time of revolution are adopted according with the first, the corresponding mean longitude in must have been at least ° distant from the limits of the second proposition. and again, if the planet is assumed to have had in a mean longitude near the limits of the second proposition, the corresponding time of revolution with which its motions satisfy the present observations cannot exceed years, and must therefore be about years less than the limits of the first proposition.' "neptune cannot, then, be the planet of m. le verrier's theory, and cannot account for the observed perturbations of uranus under the form of the inequalities involved in his analysis"--(_proc. amer. acad. i._, - , _p._ ). [sidenote: newcomb's criticism.] at the time when professor peirce wrote, the orbit of neptune was not sufficiently well determined to decide whether one of the two limitations might not be correct, though he could see that they could not both be right, and we now know that they are _both wrong_. the mean distance of neptune is , which does _not_ lie between and . ; and the longitude in was °, which does _not_ lie between ° and °. the ingenious process which airy admired and which peirce himself calls "consummately skilful" was wrong in principle. as professor newcomb has said, "the error was the elementary one that, instead of considering all the elements simultaneously variable, le verrier took them one at a time, considering the others as fixed, and determining the limits between which each could be contained on this hypothesis. no solver of least square equations at the present day ought to make such a blunder. of course one trouble in le verrier's demonstration, had he attempted a rigorous one, would have been the impossibility of forming the simultaneous equations expressive of possible variations of all the elements." [sidenote: element of good fortune.] [sidenote: the map used by galle.] the account of le verrier's limits by professor peirce, though it exhibits the error with special clearness, is a little unfair to le verrier in one point. if, instead of taking the limits for the date , we take them for (when the search for neptune was actually made), we shall find that they do include the actual place of the planet, as airy found. the erroneous mean motion of le verrier's planet allowed of his being right at one time and wrong at another; and airy examined the limits under favourable conditions, which explains his enthusiasm. but we can scarcely wonder that professor peirce came to the conclusion that the planet discovered was not the one pointed out by le verrier, and had been found by mere accident. and all these circumstances inevitably contribute to a general impression that the calculators had a large element of good fortune to thank for their success. nor need we hesitate to make this admission, for there is an element of good fortune in all discoveries. to look no further than this--if a man had not been doing a particular thing at a particular time, as he might easily not have been, most discoveries would never have been made. if sir william herschel had not been looking at certain small stars for a totally different purpose he would never have found uranus; and no one need hesitate to admit the element of chance in the finding of neptune. it is well illustrated by a glance at the map which, as has been remarked, galle used to compare with the sky on the night when he made the actual discovery. the planet was found down near the bottom corner of the map, and since the limits assigned for its place might easily have varied a few degrees one way or the other, it might easily have been off the map; in which case, it is probable that the search would not have been successful, or at any rate that success would have been delayed. [illustration: v.--corner of the berlin map, by the use of which galle found neptune.] [sidenote: every one made mistakes.] thus, it is a most remarkable feature of the discovery of neptune that mistakes were made by almost every one concerned, however eminent. airy made a mistake in regarding the question of the radius vector as of fundamental importance; sir j. herschel was wrong in describing an elementary method which he considered might have found the planet; professor peirce was wrong in supposing that the actual and the supposed planet were essentially different in their action on uranus; le verrier was wrong in assigning limits outside which it was not necessary to look when the actual planet was outside them; adams was more or less wrong in thinking that the eccentricity of the new planet could be found from the material already at disposal of man. both adams and le verrier gave far too much importance to bode's law. to review a piece of history of this kind and note the mistakes of such men is certainly comforting, and need not in any way lessen our admiration. in the case of the investigators themselves, much may be set down to excitement in the presence of a possible discovery. professor sampson has provided us with a small but typical instance of this fact. when adams had carried through all his computations for finding neptune, and was approaching the actual place of the planet, he, "who could carry through fabulous computations without error," for the first time wrote down a wrong figure. the mistake was corrected upon the ms., "probably as soon as made," but no doubt betrays the excitement which the great worker could not repress at this critical moment. there is a tradition that, similarly, when the mighty newton was approaching the completion of his calculations to verify the law of gravitation, his excitement was so great that he was compelled to assign to a friend the task of finishing them. finally, we may remark how the history of the discovery of neptune again illustrates the difficulty of formulating any general principles for guiding scientific work. sometimes it is well to follow the slightest clue, however imperfectly understood; at other times we shall do better to refuse such guidance. bode's law pointed to the existence of minor planets, and might conceivably have helped in finding uranus: but by trusting to it in the case of neptune, the investigators were perilously near going astray. sometimes it is better to follow resolutely the work in hand whatever it may be, shutting one's ears to other calls; but airy and challis lost their opportunities by just this course of action. the history of science is full of such contradictory experiences; and the only safe conclusion seems to be that there are no general rules of conduct for discovery. chapter iii bradley's discoveries of the aberration of light and of the nutation of the earth's axis [sidenote: biographical method adopted.] in examining different types of astronomical discovery, we shall find certain advantages in varying to some extent the method of presentation. in the two previous chapters our opportunities for learning anything of the life and character of those who made the discoveries have been slight; but i propose to adopt a more directly biographical method in dealing with bradley's discoveries, which are so bound up with the simple earnestness of his character that we could scarcely appreciate their essential features properly without some biographical study. but the record of his life apart from his astronomical work is not in any way sensational; indeed it is singularly devoid of incident. he had not even a scientific quarrel. there was scarcely a man of science of that period who had not at least one violent quarrel with some one, save only bradley, whose gentle nature seems to have kept him clear of them all. judged by ordinary standards his life was uneventful: and yet it may be doubted whether, to him who lived it, that life contained one dull moment. incident came for him in his scientific work: in the preparation of apparatus, the making of observations, above all in the hard-thinking which he did to get at the clue which would explain them; and after reviewing his biography,[ ] i think we shall be inclined to admit that if ever there was a happy life, albeit one of unremitting toil, it was that of james bradley. [sidenote: bradley's birth and early life.] [sidenote: brief clerical career.] he was born at sherbourn, in gloucestershire, in . we know little of his boyhood except that he went to the grammar school at northleach, and that the memory of this fact was preserved at the school in when rigaud was writing his memoir. [the school is at present shut up for want of funds to carry it on; and all inquiries i have made have failed to elicit any trace of this memory.] similarly we know little of his undergraduate days at oxford, except that he entered as a commoner at balliol in , took his b.a. in the regular course in , and his m.a. in . as a career he chose the church, being ordained in , and presented to the vicarage of bridstow in monmouthshire; but he only discharged the duties of vicar for a couple of years, for in he returned to oxford as professor of astronomy, an appointment which involved the resignation of his livings; and so slight was this interruption to his career as an astronomer that we may almost disregard it, and consider him as an astronomer from the first. but to guard against a possible misconception, let me say that bradley entered on a clerical career in a thoroughly earnest spirit; to do otherwise would have been quite foreign to his nature. when vicar of bridstow he discharged his duties faithfully towards that tiny parish, and moreover was so active in his uncle's parish of wansted that he left the reputation of having been curate there, although he held no actual appointment. and thirty years later, when he was astronomer royal and resident at greenwich, and when the valuable vicarage of greenwich was offered to him by the chancellor of the exchequer, he honourably refused the preferment, "because the duty of a pastor was incompatible with his other studies and necessary engagements." [sidenote: learnt astronomy _not_ at oxford, but from his uncle, james pound.] [sidenote: pound a first-rate observer.] but now let us turn to bradley's astronomical education. i must admit, with deep regret, that we cannot allow any of the credit of it to oxford. there was a great astronomer in oxford when bradley was an undergraduate, for edmund halley had been appointed savilian professor of geometry in , and had immediately set to work to compute the orbits of comets, which led to his immortal discovery that some of these bodies return to us again and again, especially the one which bears his name--halley's comet--and returns every seventy-five years, being next expected about . but there is no record that bradley came under halley's teaching or influence as an undergraduate. in later years the two men knew each other well, and it was halley's one desire towards the close of his life that bradley should succeed him as astronomer royal at greenwich; a desire which was fulfilled in rather melancholy fashion, for halley died without any assurance that his wish would be gratified. but bradley got no astronomical teaching at oxford either from halley or others. the art of astronomical observation he learnt from his maternal uncle, the rev. james pound, rector of wansted, in essex. he is the man to whom we owe bradley's training and the great discoveries which came out of it. he was, i am glad to say, an oxford man too; very much an oxford man; for he seems to have spent some thirteen years there migrating from one hall to another. his record indeed was such as good tutors of colleges frown upon; for it was seven years before he managed to take a degree at all; and he could not settle to anything. after ten years at oxford he thought he would try medicine; after three years more he gave it up and went out in as chaplain to the east indies. but he seems to have been a thoroughly lovable man, for news was brought of him four years later that he had a mind to come home, but was dissuaded by the governor saying that "if dr. pound goes, i and the rest of the company will not stay behind." soon afterwards the settlement was attacked in an insurrection, and pound was one of the few who escaped with his life, losing however all the property he had gradually acquired. he returned to england in , and was presented to the living of wansted; married twice, and ended his days in peace and fair prosperity in . such are briefly the facts about bradley's uncle, james pound; but the most important of all remains to be told--that somehow or other he had learnt to make first-rate astronomical observations, how or when is not recorded; but in he was already so skilled that sir isaac newton made him a present of fifty guineas for some observations; and repeated the gift in the following year; and even three years before this we find halley writing to ask for certain observations from mr. pound. [sidenote: bradley worked with him.] with this excellent man bradley used frequently to stay. to his nephew he seems to have been more like a father than an uncle. when his nephew had smallpox in , he nursed him through it; and he supplemented from his own pocket the scanty allowance which was all that bradley's own father could afford. but what concerns us most is that he fostered, if he did not actually implant, a love of astronomical observation in his nephew. the two worked together, entering their observations one after the other on the same paper; and it was to the pair of them together, rather than to the uncle alone, that newton made his princely presents, and halley wrote for help in his observations. there seems to be no doubt that the uncle and nephew were about this time the best astronomical observers in the world. there was no rivalry between them, and therefore there is no need to discuss whether the partnership was one of equal merit on both sides; but it is interesting to note that it probably was. the ability of pound was undoubted; many were keenly desirous that he, and not his nephew, should be elected to the oxford chair in , but he felt unequal to the duties at his advanced age. on the other hand, when bradley lost his uncle's help, there was no trace of faltering in his steps to betray previous dependence on a supporting or guiding hand. he walked erect and firm, and trod paths where even his uncle might not have been able to follow. [sidenote: the work done by pound and bradley.] [sidenote: use of very long telescopes.] [sidenote: reason for great length.] a few instances will suffice to show the kind of observations made by this notable firm of pound and bradley. they observed the positions of the fixed stars and nebulæ: these being generally the results required by halley and newton. they also observed the places of the planets among the stars, and especially the planet mars, and determined its distance from the earth by the method of parallax, thus anticipating the modern standard method of finding the sun's distance; and though with their imperfect instruments they did not obtain a greater accuracy than in , still this was a great advance on what had been done before, and excited the wonder and admiration of halley. they also paid some attention to double stars, and did a great deal of work on jupiter's satellites. we might profitably linger over the records of these early years, which are full of interest, but we must press on to the time of the great discoveries, and we will dismiss them with brief illustrations of three points: bradley's assiduity, his skill in calculation, and his wonderful skill in the management of instruments. of his assiduity an example is afforded by his calculations of the orbits of two comets which are still extant. one of them fills thirty-two pages of foolscap, and the other sixty; and it must be remembered that the calculations themselves were quite novel at that time. of his _skill_ in calculation, apart from his assiduity, we have a proof in a paper communicated to the royal society rather later ( ), where he determines the longitudes of lisbon and new york from the eclipses of jupiter's satellites, using observations which were not simultaneous, and had therefore to be corrected by an ingenious process which bradley devised expressly for this purpose. and finally, his skill in the management of instruments is shown by his measuring the diameter of the planet venus with a telescope actually - / feet in length. it is difficult for us to realise in these days what this means; even the longest telescope of modern times does not exceed feet in length, and it is mounted so conveniently with all the resources of modern engineering, in the shape of rising floors, &c., that the management of it is no more difficult than that of a -foot telescope. but bradley had no engineering appliances beyond a pole to hold up one end of the telescope and his own clever fingers to work the other; and he managed to point the unwieldy weapon accurately to the planet, and measure the diameter with an exactness which would do credit to modern times. a few words of explanation may be given why such long telescopes were used at all. the reason lay in the difficulty of getting rid of coloured images, due to the composite character of white light. whenever we use a _single_ lens to form an image, coloured fringes appear. nowadays we know that by making two lenses of different kinds of glass and putting them together, we can practically get rid of these coloured fringes; but this discovery had not been made in bradley's time. the only known ways of dealing with the evil then were to use a reflecting telescope like newton and gregory, or if a lens was used, to make one of very great focal length; and hence the primary necessity for these very long telescopes. they had another advantage in producing a large image, or they would probably have given way to the reflector. this advantage is gradually bringing them back into use, and perhaps in the eclipse of we may use a telescope as long as bradley's; but we shall not use it as he did in any case. it will be laid comfortably flat on the ground, and the rays of light reflected into it by a coelostat. [sidenote: bradley appointed at oxford, but continues to work at wansted.] in bradley was appointed to the savilian professorship of astronomy at oxford, vacant by the death of dr. john keill. once it became clear that there was no chance of securing his uncle for this position, bradley himself was supported enthusiastically by all those whose support was worth having, especially by the earl of macclesfield, who was then lord chancellor; by martin foulkes, who was afterwards the president of the royal society; and by sir isaac newton himself. he was accordingly elected on october , , and forthwith resigned his livings. his resignation of the livings was necessitated by a definite statute of the university relating to the professorship, and not by the existence of any very onerous duties attaching to it; indeed such duties seem to have been conspicuously absent, and after bradley's election he passed more time than ever with his uncle in wansted, making the astronomical observations which both loved; for there was not the vestige of an observatory in oxford. his uncle's death in interrupted the continuity of these joint observations, and by an odd accident prepared the way for bradley's great discovery. he was fain to seek elsewhere that companionship in his work which had become so essential to him, and his new friend gave a new bent to his observations. [sidenote: samuel molyneux.] [sidenote: attempts to find stellar parallax.] samuel molyneux was a gentleman of fortune much attached to science, and particularly to astronomy, who was living about this time at kew. he was one of the few, moreover, who are not content merely to amuse themselves with a telescope, but had the ambition to do some real earnest work, and the courage to choose a problem which had baffled the human race for more than a century. the theory of copernicus, that the earth moved round the sun, necessitated a corresponding apparent change in the places of the stars, one relatively to another; and it was a standing difficulty in the way of accepting this theory that no such change could be detected. in the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no "parallax," as the astronomical term goes, could be found for the stars. the observations of galileo, and the measures of tycho brahé, as reduced to systematic laws by kepler, and finally by the great newton, made it clear that the copernican theory was _true_: but no one had succeeded in proving its truth in this particular way. samuel molyneux must have been a man of great courage to set himself to try to crack this hard nut; and we can understand the attraction which his enterprise must have had for bradley, who had just lost the beloved colleague of many courageous astronomical undertakings. his co-operation seems to have been welcomed from the first; his help was invited and freely given in setting up the instrument, and he fortunately had the leisure to spend considerable time at kew making the observations with molyneux, just as he had been wont to observe with his uncle. i must now briefly explain what these observations were. there is a bright star [gamma] draconis, which passes almost directly overhead in the latitude of london. its position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by london astronomers owing to this circumstance, that it passes directly overhead, and so its position is practically undisturbed by the refraction of our atmosphere. [sidenote: the instrument.] [sidenote: expected results.] it was therefore thought at the time that, there being no disturbance from refraction, the disturbance from precession being accurately known, and there being nothing else to disturb the position but "parallax" (the apparent shift due to the earth's motion which it was desirable to find), this star ought to be a specially favourable object for the determination of parallax. indeed it had been announced many years before by hooke that its parallax had been found; but his observations were not altogether satisfactory, and it was with a view of either confirming them or seeing what was wrong with them that molyneux and bradley started their search. they set up a much more delicate piece of apparatus than hooke had employed. it was a telescope feet long pointed upwards to the star, and firmly attached to a large stack of brick chimneys within the house. the telescope was not absolutely fixed, for the lower end could be moved by a screw so as to make it point accurately to the star, and a plumb-line showed how far it was from the vertical when so pointing. hence if the star changed its position, however slightly, the reading of this screw would show the change. now, before setting out on the observations, the observers knew what to expect if the star had a real parallax; that is to say, they knew that the star would seem to be farthest south in december, farthest north in june, and at intermediate positions in march and september; though they did not know _how much_ farther south it would appear in december than in june--this was exactly the point to be decided. [illustration: fig. .] [sidenote: unexpected results.] the reason of this will be clear from fig. . [remark, however, that this figure and the corresponding figure do not represent the case of bradley's star, [gamma] draconis: another star has been chosen which simplifies the diagram, though the principle is essentially the same.] let a b c d represent the earth's orbit, the earth being at a in june, at b in september, and so on, and let k represent the position of the star on the line d b. then in march and september it will be seen from the earth in the same direction, namely, d b k; but the directions in which it is seen in june and december, viz. a k and c k, are inclined in opposite ways to this line. the farther away the star is, the less will this inclination or "parallax" be; and the star is actually so far away that the inclination can only be detected with the utmost difficulty: the lines c k and a k are sensibly parallel to d b k. but bradley did not know this; it was just this point which he was to examine, and he expected the greatest inclination in one direction to be in december. accordingly when a few observations had been made on december , , , and it was thought that the star had been caught at its most southerly apparent position, and might be expected thereafter to move northwards, if at all. but when bradley repeated the observation on december , he found to his great surprise that the star was still moving southwards. here was something quite new and unexpected, and such a keen observer as bradley was at once on the alert. he soon found that the changes in the position of the star were of a totally unexpected character. instead of the extreme positions being occupied in june and december, they were occupied in march and september, just midway between these. and the range in position was quite large, about "--not a quantity which could have been detected in the days before telescopes, but one which was unmistakable with an instrument of the most moderate measuring capacity. [sidenote: tentative explanations.] what, then, was the cause of this quite unforeseen behaviour on the part of the star? the first thought of the observers was that something might be wrong with their instrument, and it was carefully examined, but without result. the next was that the apparent movement was in the plumb-line, the line of reference. if the whole earth, instead of carrying its axis round the sun in a constant direction, were to be executing an oscillation, then all our plumb-lines would oscillate, and when the direction of a star like [gamma] draconis was compared with that of the plumb-line it would seem to vary, owing actually to the variation in the plumb-line. the earth might have a motion of this kind in two ways, which it will be necessary for us to distinguish, and the adopted names for them are "nutation of the axis" and "variation of latitude" respectively. in the case of nutation the north pole remains in the same geographical position, but points to a different part of the heavens. the "variation of latitude," on the other hand, means that the north pole wanders about on the earth itself. we shall refer to the second phenomenon more particularly in the sixth chapter. [sidenote: nutation?] [sidenote: anomalous refraction.] but it was the first kind of change, the nutation, which bradley suspected; and very early in the series of observations he had already begun to test this hypothesis. if it was not the star, but the earth and the plumb-line, which were in motion, then other stars ought to be affected. the telescope had been deliberately restricted in its position to suit [gamma] draconis; but since the stars circle round the pole, if we draw a narrow belt in the heavens with the pole as centre, and including [gamma] draconis, the other stars included would make the same circuit, preceding or following [gamma] draconis by a constant interval. most of them would be too faint for observation with bradley's telescope; but there was one bright enough to be observed, which also came within its limited range, and it was promptly put under _surveillance_ when a nutation of the earth's axis was suspected. careful watching showed that it was not affected in the same way as [gamma] draconis, and hence the movement could not be in the plumb-line. was there, then, after all, some effect of the earth's atmosphere which had been overlooked? we have already remarked that since the star passes directly overhead there should be practically no refraction; and this assumption was made by molyneux and bradley in choosing this particular star for observation. it follows at once, if we assume that the atmosphere surrounds the earth in spherical layers. but perhaps this was not so? perhaps, on the contrary, the atmosphere was deformed by the motion of the earth, streaming out behind her like the smoke of a moving engine? no possibility must be overlooked if the explanation of this puzzling fact was to be got at. [illustration: fig. .] the way in which a deformation of the atmosphere might explain the phenomenon is best seen by a diagram. first, it must be remarked that rays of light are only bent by the earth's atmosphere, or "refracted," if they enter it obliquely. if the atmosphere were of the same density throughout, like a piece of glass, then a vertical ray of light, a b (see fig. ), entering the atmosphere at b would suffer no bending or refraction, and a star shining from the direction a b would be seen truly in that direction from c. but an oblique ray, d e, would be bent on entering the atmosphere at e along the path ef, and a star shining along d e would appear from f to be shining along the dotted line g e f. the atmosphere is not of the same density throughout, but thins out as we go upwards from the earth; and in consequence there is no clear-cut surface, b e, and no sudden bending of the rays as at e: they are gradually bent at an infinite succession of imaginary surfaces. but it still remains true that there is no bending at all for vertical rays; and of oblique rays those most oblique are most bent. [illustration: fig. .] now, suppose the atmosphere of the earth took up, owing to its revolution round the sun, an elongated shape like that indicated in diagram , and suppose the star to be at a great distance away to the right of the diagram. when the earth is in the position labelled "june," the light would fall vertically on the nose of the atmosphere at a, and there would be no refraction. similarly in "december" the light would fall at c on the stern, also vertically, and there would be no refraction. [the rays from the distant star in december are to be taken as sensibly parallel to those received in june, notwithstanding that the earth is on the opposite side of the sun, as was remarked on p. .] but in march and september the rays would strike obliquely on the sides of the supposed figure, and thus be bent in opposite directions, as indicated by the dotted lines; and the extreme positions would thus occur in march and september, as had been observed. the explanation thus far seems satisfactory enough. but we have assumed the star to lie in the plane of the earth's orbit; and the stars under observation by bradley did not lie in this plane, nor did they lie in directions equally inclined to it. making the proper allowance for their directions, it was found impossible to fit in the facts with this hypothesis, which had ultimately to be abandoned. [sidenote: delay in finding real explanation.] [sidenote: bradley sets up another instrument at wansted.] [sidenote: finds the right clue.] [sidenote: a wind-vane on a boat.] it is remarkable to find that two or three years went by before the real explanation of this new phenomenon occurred to bradley, and during this time he must have done some hard thinking. we have all had experience of the _kind_ of thinking if only in the guessing of conundrums. we know the apparent hopelessness of the quest at the outset: the racking of our brains for a clue, the too frequent despair and "giving it up," and the simplicity of the answer when once it is declared. but with scientific conundrums the expedient of "giving it up" is not available. we must find the answer for ourselves or remain in ignorance; and though we may feel sure that the answer when found will be as simple as that to the best conundrum, this expected simplicity does not seem to aid us in the search. bradley was not content with sitting down to think: he set to work to accumulate more facts. molyneux's instrument only allowed of the observation of two stars, [gamma] draconis and the small star above mentioned. bradley determined to have an instrument of his own which should command a wider range of stars; and by this time he was able to return to his uncle's house at wansted for this purpose. his uncle had been dead for two or three years, and the memory of the loss was becoming mellowed with time. his uncle's widow was only too glad to welcome back her nephew, though no longer to the old rectory, and she allowed him to set up a long telescope, even though he cut holes in her floor to pass it through. the object-glass end was out on the roof and the eye end down in the coal cellar; and accordingly in this coal cellar bradley made the observations which led to his immortal discovery. he had a list of seventy stars to observe, fifty of which he observed pretty regularly. it may seem odd that he did not set up this new instrument at oxford, but we find from an old memorandum that his professorship was not bringing him in quite £ a year, and probably he was glad to accept his aunt's hospitality for reasons of economy. by watching these different stars he gradually got a clear conception of the laws of aberration. the real solution of the problem, according to a well-authenticated account, occurred to him almost accidentally. we all know the story of the apple falling and setting newton to think about the causes of gravitation. it was a similarly trivial circumstance which suggested to bradley the explanation which he had been seeking for two or three years in vain. in his own words, "at last, when he despaired of being able to account for the phenomena which he had observed, a satisfactory explanation of them occurred to him all at once when he was not in search of it." he accompanied a pleasure party in a sail upon the river thames. the boat in which they were was provided with a mast which had a vane at the top of it. it blew a moderate wind, and the party sailed up and down the river for a considerable time. dr. bradley remarked that every time the boat put about the vane at the top of the boat's mast shifted a little, as if there had been a slight change in the direction of the wind. he observed this three or four times without speaking; at last he mentioned it to the sailors, and expressed his surprise that the wind should shift so regularly every time they put about. the sailors told him that the wind had not shifted, but that the apparent change was owing to the change in the direction of the boat, and assured him that the same thing invariably happened in all cases. this accidental observation led him to conclude that the phenomenon which had puzzled him so much was owing to the combined motion of light and of the earth. to explain exactly what is meant we must again have recourse to a diagram; and we may also make use of an illustration which has become classical. [illustration: fig. .] [sidenote: analogy of rain.] if rain is falling vertically, as represented by the direction a b; and if a pedestrian is walking horizontally in the direction c d, the rain will appear to him to be coming in an inclined direction, e f, and he will find it better to tilt his umbrella forwards. the quicker his pace the more he will find it advisable to tilt the umbrella. this analogy was stated by lalande before the days of umbrellas in the following words: "je suppose que, dans un temps calme, la pluie tombe perpendiculairement, et qu'on soit dans une voiture ouverte sur le devant; si la voiture est en repos, on ne reçoit pas la moindre goutte de pluie; si la voiture avance avec rapidité, la pluie entre sensiblement, comme si elle avoit pris une direction oblique." lalande's example, modified to suit modern conditions, has been generally adopted by teachers, and in examinations candidates produce graphic pictures of the stationary, the moderate-paced, and the flying, possessors of umbrellas. [sidenote: aberration.] applying it to the phenomenon which it is intended to illustrate, if light is being received from a star by an earth, travelling across the direction of the ray, the telescope (which in this case represents the umbrella) must be tilted forward to catch the light. now on reference to fig. it will be seen that the earth is travelling across the direction of rays from the star in march and september; and in opposite directions in the two cases. hence the telescope must be tilted a little, in opposite directions, to catch the light; or, in other words, the star will appear to be farthest south in march, farthest north in september. and so at last the puzzle was solved, and the solution was found, as so often happens, to be of the simplest kind; so simple when once we know, and so terribly hard to imagine when we don't! it may comfort us in our struggles with minor problems to reflect that bradley manfully stuck to his problem for two or three years. it was probably never out of his thoughts, waking or sleeping; when at work it was the chief object of his labours, and when on a pleasure party he was ready to catch at the slightest clue, in the motion of a wind-vane on a boat, which might help him to the solution. [sidenote: results of discovery.] the discovery of aberration made bradley famous at a bound. oxford might well be proud of her two savilian professors at this time, for they had both made world-famous discoveries--halley that of the periodicity of comets, and bradley of the aberration of light. how different their tastes were and how difficult it would have been for either to do the work of the other! bradley was no great mathematician, and though he was quite able to calculate the orbit of a comet, and carried on such work when halley left it, it was probably not congenial to him. halley, on the other hand, almost despised accurate observations as finicking. "be sure you are correct to a minute," he was wont to say, "and the fractions do not so much matter." with such a precept bradley would never have made his discoveries. no quantity was too small in his eyes, and no sooner was the explanation of aberration satisfactorily established than he perceived that though it would account for the main facts, it would not explain all. there was something left. this is often the case in the history of science. a few years ago it was thought that we knew the constitution of our air completely--oxygen, nitrogen, water vapour, and carbonic acid gas; but a great physicist, lord rayleigh, found that after extracting all the water and carbonic acid gas, all the oxygen and all the nitrogen, there was something left--a very minute residuum, which a careless experimenter would have overlooked or neglected, but which a true investigator like lord rayleigh saw the immense importance of. he kept his eye on that something left, and presently discovered a new gas which we now know as argon. had he repeated the process, extracting all the argon after the nitrogen, he might have found by a scrutiny much more accurate still yet another gas, helium, which we now know to exist in extremely minute quantities in the air. but meantime this discovery was made in another way. [sidenote: still something to be explained.] [sidenote: probably nutation.] [sidenote: his nineteen years' campaign.] when bradley had extracted all the aberration from his observations he found that there was something left, another problem to be solved and some more thinking to be done to solve it. but he was now able to profit by his previous labours, and the second step was made more easily than the first. the residuum was not the parallax of which he had originally been in search, for it did not complete a cycle within the year; it was rather a progressive change from year to year. but there was an important clue of another kind. he saw that the apparent movements of all stars were in this case the same; and he knew that a movement of this kind can be referred, not to the stars themselves, but to the plumb-line from which their directions are measured. he had thought out the possible causes of such a movement of the plumb-line or of the earth itself, and had realised that there might be a _nutation_ which would go through a cycle in about nineteen years, the period in which the moon's nodes revolve. he was not mathematician enough to work out the cause completely, but he saw clearly that to trace the whole effect he must continue the observations for nineteen years; and accordingly he entered on this long campaign without any hesitation. his instrument was still that in his aunt's house at wansted, where he continued to live and make the observations for a few years, but in he removed to oxford, as we shall see, and he must have made many journeys between wansted and oxford in the course of the remaining fifteen years during which he continued to trace out the effects of nutation. his aunt too left wansted to accompany bradley to oxford, and the house passed into other hands. it is to the lasting credit of the new occupant, mrs. elizabeth williams, that the great astronomer was allowed to go on and complete the valuable series of observations which he had commenced. bradley was not lodged in her house; he stayed with a friend close by on his visits to wansted, but came freely in and out of his aunt's old home to make his observations. how many of us are there who would cheerfully allow an astronomer to enter our house at any hour of the night to make observations in the coal-cellar! it says much, not only for bradley's fame, but for his personal attractiveness, that he should have secured this permission, and that there should be no record of any friction during these fifteen years. at the end of the whole series of nineteen years his conclusions were abundantly verified, and his second great discovery of nutation was established. honours were showered upon him, and no doubt the gentle heart of mrs. elizabeth williams was uplifted at the glorious outcome of her long forbearance. [sidenote: residence at oxford.] but we may now turn for a few moments from bradley's scientific work to his daily life. we have said that in , after holding his professorship for eleven years, he first went definitely to reside in oxford. he actually had not been able to afford it previously. his income was only £ a year, and the statutes prevented him from holding a living: so that he was fain to accept mrs. pound's hospitable shelter. but in an opportunity of adding to his income presented itself, by giving lectures in "experimental philosophy." the observations on nutation were not like those on aberration: he was not occupied day and night trying to find the solution: he had practically made up his mind about the solution, and the actual observations were to go on in a quiet methodical manner for nineteen years, so that he now had leisure to look about him for other employment. dr. keill, who had been professor of astronomy before bradley, had attracted large classes to lectures, not on astronomy, but on experimental philosophy: but had sold his apparatus and goodwill to mr. whiteside, of christ church, one of the candidates who were disappointed by bradley's election. in bradley purchased the apparatus from whiteside, and began to give lectures in experimental philosophy. his discovery of aberration had made him famous, so that his classes were large from the first, and paid him considerable fees. suddenly therefore he changed his poverty for a comfortable income, and he was able to live in oxford in one of two red brick houses in new college lane, which were in those days assigned to the savilian professors (now inhabited by new college undergraduates). his aunt, mrs. pound, to whom he was devotedly attached, came with him, and two of her nephews. in his time of prosperity bradley was thus able to return the hospitality which had been so generously afforded him in times of stress. [sidenote: astronomer royal at greenwich.] [sidenote: letter from earl of macclesfield.] before he completed his observations for nutation another great change in his fortunes took place. in he was elected to succeed halley as astronomer royal. it was halley's dying wish that bradley should succeed him, and it is said that he was even willing to resign in his favour, for his right hand had been attacked by paralysis, and the disease was gradually spreading. but he died without any positive assurance that his wish would be fulfilled. the chief difficulty in securing the appointment of bradley seems to have been that he was the obvious man for the post in universal opinion. "it is not only my friendship for mr. bradley that makes me so ardently wish to see him possessed of the position," wrote the earl of macclesfield to the lord chancellor; "it is my real concern for the honour of the nation with regard to science. for as our credit and reputation have hitherto not been inconsiderable amongst the astronomical part of the world, i should be extremely sorry we should forfeit it all at once by bestowing upon a man of inferior skill and abilities the most honourable, though not the most lucrative, post in the profession (a post so well filled by dr. halley and his predecessor), when at the same time we have amongst us a man known by all the foreign, as well as our own astronomers, not to be inferior to either of them, and one whom sir isaac newton was pleased to call the best astronomer in europe." and again, "as mr. bradley's abilities in astronomical learning are allowed and confessed by all, so his character in every respect is so well established, and so unblemished, that i may defy the worst of his enemies (if so good and worthy a man have any) to make even the lowest or most trifling objection to it." "after all," the letter goes on, "it may be said if mr. bradley's skill is so universally acknowledged, and his character so established, there is little danger of opposition, since no competitor can entertain the least hope of success against him. but, my lord, we live in an age when most men how little soever their merit may be, seem to think themselves fit for whatever they can get, and often meet with some people, who by their recommendations of them appear to entertain the same opinion of them, and it is for this reason that i am so pressing with your lordship not to lose any time." such recommendations had, however, their effect: the dreaded possibility of a miscarriage of justice was averted, and bradley became the third astronomer royal, though he did not resign his professorship at oxford. halley, bradley, and bliss, who were astronomers royal in succession, all held the appointment along with one of the savilian professorships at oxford; but since the death of bliss in , the appointment has always gone to a cambridge man. [sidenote: instruments very defective.] when bradley went to greenwich, in june , he was at first unable to do much from the wretched state in which he found the instruments. halley was not a good observer: his heart was not in the work, and he had not taken the trouble to set the instruments right when they went wrong. the counterpoises of that instrument which ought to have been the best in the world at the time rubbed against the roof so that the telescope could scarcely be moved in some positions: and some of the screws were broken. there was no proper means of illuminating the cross-wires, and so on. with care and patience bradley set all this right, and began observations. he had the good fortune to secure the help of his nephew, john bradley, as assistant, and the companionship seems to have been as happy as that previous one of james bradley and his uncle pound. john bradley was able to carry on the observations when his uncle was absent in oxford, and the work the two got through together in the first year is (in the words of bradley's biographer rigaud) "scarcely to be credited." the transit observations occupy folio pages, and no less than observations were taken on one night. and at the same time, it must be remembered, bradley was still carrying on his nutation observations at wansted, still lecturing at oxford, and not content with all this, began a course of experiments on the length of the seconds' pendulum. truly a giant for hard work! [sidenote: new instruments.] but, in spite of his care in setting them right, the instruments in the observatory were found to be hopelessly defective. the history of the instruments at the royal observatory is a curious one. when flamsteed was appointed the first astronomer royal he was given the magnificent salary of £ a year, and no instruments to observe with. he purchased some instruments with his own money, and at his death they were claimed by his executors. hence halley, the second astronomer royal, found the observatory totally unprovided in this respect. he managed to persuade the nation to furnish the funds for an equipment; but halley, though a man of great ability in other ways, did not know a good instrument from a bad one; so that bradley's first few years at the observatory were wasted owing to the imperfection of the equipment. when this was fully realised he asked for funds to buy new instruments, and such was the confidence felt in him that he got what he asked for without much difficulty. more than £ , a large sum for those days, was spent under his direction, the principal purchases being two quadrants for observation of the position of the stars, one to the north and the other to the south. with these quadrants, which represented the perfection of such apparatus at that time, bradley made that long and wonderful series of observations which is the starting-point of our knowledge of the movements of the stars. the instruments are still in the royal observatory, the more important of the two in its original position as bradley mounted it and left it. [sidenote: work at greenwich.] it seems needless to mention his work as astronomer royal, but i will give quite briefly a summary of what he accomplished, and then recall a particular incident, which shows how far ahead of his generation his genius for observation placed him. the summary may be given as follows. we owe to bradley-- . a better knowledge of the movements of jupiter's satellites. . the orbits of several comets calculated directly from his own observations, when such work was new and difficult. . experiments on the length of the pendulum. . the foundation of our knowledge of the refraction of our atmosphere. . considerable improvements in the tables of the moon, and the promotion of the method for finding the longitude by lunar distances. . the proper equipment of our national observatory with instruments, and the use of these to form the basis of our present knowledge of the positions and motions of the stars. many men would consider any one of these six achievements by itself a sufficient title to fame. bradley accomplished them all in addition to his great discoveries of aberration and nutation. [sidenote: might have found variation of latitude.] and with a little more opportunity he might have added another great discovery which has shed lustre on the work of the last decade. we said earlier in this chapter that the axis of the earth may move in one or two ways. either it may point to a different star, remaining fixed relatively to the earth, as in the nutation which bradley discovered; or it may actually change its position in the earth. this second kind of movement was believed until twenty years ago not to exist appreciably; but the work of küstner and chandler led to the discovery that it did exist, and its complexities have been unravelled, and will be considered in the sixth chapter. now a century and a half ago bradley was on the track of this "variation of latitude." his careful observations actually showed the motion of the pole, as mr. chandler has recently demonstrated; and, moreover, bradley himself noticed that there was something unexplained. once again there was a _residuum_ after (first) aberration and (next) nutation had been extracted from the observations; and with longer life he might have explained this residuum, and added a third great discovery to the previous two. or another coming after him might have found it; but after the giant came men who could not tread in his footsteps, and the world waited years before the discrepancy was explained. [sidenote: oxford's tardy recognition of bradley.] the attitude of our leading universities towards science and scientific men is of sufficient importance to justify another glance at the relations between bradley and oxford. we have seen that oxford's treatment of bradley was not altogether satisfactory. she left him to learn astronomy as he best could, and he owes no teaching to her. she made him professor of astronomy, but gave him no observatory and a beggarly income which he had to supplement by giving lectures on a different subject. but when he had disregarded these discouragements and made a name for himself, oxford took her share in recognition. he was created d.d. by diploma in ; and when his discovery of nutation was announced in , and produced distinctions and honours of all kinds from over the world, we are told that "amidst all these distinctions, wide as the range of modern science, and permanent as its history, there was one which probably came nearer his heart, and was still more gratifying to his feeling than all. lowth (afterwards bishop of london), a popular man, an elegant scholar, and possessed of considerable eloquence, had in to make his last speech in the sheldonian theatre at oxford as professor of poetry. in recording the benefits for which the university was indebted to its benefactors, he mentioned the names of those whom sir henry savile's foundation had established there: 'what men of learning! what mathematicians! we owe to savile, briggs, wallis, halley; to savile we owe greaves, ward, wren, gregory, keill, and one whom i will not name, for posterity will ever have his name on its lips.' bradley was himself present; there was no one in the crowded assembly on whom the allusion was lost, or who did not feel the truth and justice of it; all eyes were turned to him, while the walls rung with shouts of heartfelt affection and admiration; it was like the triumph of themistocles at the olympic games." [sidenote: the study of "residual phenomena."] these words of rigaud indicate the fame deservedly acquired by an earnest and simple-minded devotion to science: but can we learn anything from the study of bradley's work to guide us in further research? the chief lessons would seem to be that if we make a series of careful observations, we shall probably find some deviation from expectation: that we must follow up this clue until we have found some explanation which fits the facts, not being discouraged if we cannot hit upon the explanation at once, since bradley himself was puzzled for several years: that after finding one _vera causa_, and allowing for the effect of it, the observations may show traces of another which must again be patiently hunted, even though we spend nineteen years in the chase: and that again we may have to leave the complete rectification of the observations to posterity. but though we may admit the general helpfulness of these directions, and that this patient dealing with residual phenomena seems to be a method capable of frequent application, we cannot deduce any universal principle of procedure from them: witness the difficulty of dealing with meteorological observations, for instance. it is not always possible to find any orderly arrangement of the residuals which will give us a clue to start with. when such an arrangement is manifested, we must certainly follow up the clue; it would almost seem that no expense should be prohibitive, since it is impossible to foresee the importance of the result. chapter iv accidental discoveries [sidenote: the oxford new star found during work on astrographic chart.] in reviewing various types of astronomical discovery i have laid some stress upon the fact that they are, generally speaking, far from being accidental in character. a new planet does not "swim into our ken," at any rate not usually, but is found only after diligent search, and then only by an investigator of acute vision, or other special qualifications. but this is, of course, not always the case. some discoveries are made by the merest accident, as we have had occasion to remark incidentally in the case of the minor planets; and for the sake of completeness it is desirable to include among our types at least one case of such accidental discovery. as, however, the selection is a little invidious, i may perhaps be pardoned for taking the instance from my own experience, which happens to include a case where one of those remarkable objects called "new stars" walked deliberately into a net spread for totally different objects. there is the further reason for choosing this instance: that it will afford me the opportunity of saying something about the special research in which we were actually engaged, the work of mapping out the heavens by photography, or, as it has been called, the astrographic chart--a great scheme of international co-operation by which it is hoped to leave as a legacy for future centuries a record of the state of the sky in our age. such a record cannot be complete; for however faint the stars included, we know that there are fainter stars which might have been included had we given longer exposures to the plates. nor can it be in other ways final or perfect; however large the scale, for instance, on which the map is made, we can imagine the scale doubled or increased many-fold. but the map will be a great advance on anything that has hitherto been made, and some account of it will therefore no doubt be of interest. [sidenote: origin of the chart.] we may perhaps begin with a brief historical account of the enterprise. photographs of the stars were taken many years ago, but only by a few enthusiasts, and with no serious hope of competing with eye observations of the sky. the old wet-plate photography was, in fact, somewhat unsuited to astronomical purposes; to photograph faint objects a long exposure is necessary, and the wet plate may dry up before the exposure is concluded--nay, even before it is commenced, if the observer has to wait for passing clouds--and therefore it may be said that the successful application of photography to astronomy dates from the time when the dry plate was invented; when it became possible to expose a plate in the telescope for hours, or by accumulation even for days. the dry plate remains sensitive for a long period, and if it is desired to extend an exposure beyond the limits of one night, it is quite easy to close up the telescope and return to the operations again on the next fine night; and so on, if not perhaps indefinitely, at any rate so long as to transcend the limits of human patience up to the present. [illustration: vii.--great comet of nov. th, (_from a photograph taken at the royal observatory, cape of good hope._)] [sidenote: comet of .] [sidenote: stars shown on the pictures.] but to consider our particular project. we may assign, perhaps, the date as that in which it first began to take shape. in that year there was a magnificent bright comet, the last really large comet which we, in the northern hemisphere, have had the good fortune to see. some of us, of course, were not born at that time, and perhaps others who were alive may nevertheless not have seen that comet; for it kept somewhat uncomfortably early morning hours, and i can well remember myself feeling rather more resentment than gratitude to the man who waked me up about four o'clock to see it. many observations were of course made of this interesting visitor, and what specially concerns us is that at the cape of good hope some enterprising photographers tried to photograph it. they tried in the first instance with ordinary cameras, and soon found--what any astronomer could have told them--that the movement of the earth, causing an apparent movement of the comet and the stars in the opposite direction, frustrated their efforts. the difficulties of obtaining pictures of moving objects are familiar to all photographers. a "snap-shot" might have met the difficulty, but the comet was scarcely bright enough to affect the plate with a short exposure. ultimately dr. david gill, the astronomer at the cape observatory, invited one of the photographers to strap his camera to one of the telescopes at the observatory, a telescope which could be carried round by clockwork in the usual way, so as to counteract the earth's motion, and in effect to keep the comet steadily in view, as though it were at rest. as a consequence, some very beautiful and successful pictures of the comet were obtained, and on them a large number of stars were also shown. they were, as i have said, not by any means the first pictures of stars obtained by photography, but they represented in facility and in success so great an advance upon what had been formerly obtained that they attracted considerable attention. they were sent to europe and stimulated various workers to further experiments. [sidenote: the brothers henry begin work.] [sidenote: conference of .] the late dr. common in england, an amateur astronomer, began that magnificent pioneer work in astronomical photography which soon brought him the gold medal of the royal astronomical society for his photographs of nebulæ. but the most important result for our purpose was produced in france. there had been started many years before by the french astronomer chacornac a series of star maps round the zodiac similar in intention to the berlin maps which figured in the history of the discovery of neptune. chacornac died before his enterprise was very far advanced, and the work was taken up by two brothers, paul and prosper henry, who followed chacornac in adopting for the work the laborious method of eye observation of each individual star. they proceeded patiently with the work on these lines; but when they came to the region where the zodiac is crossed by the milky way, and the number of stars in a given area increases enormously, they found the labour so great as to be practically prohibitive, and were in doubt how to deal with the difficulty. it was at this critical moment that these comet photographs, showing the stars so beautifully, suggested the alternative of mapping the stars photographically. they immediately set to work with a trial lens, and obtained such encouraging results that they proceeded themselves to make a larger lens of the same type; this again was satisfactory, and the idea naturally arose of extending to the whole heavens the scheme which they had hitherto intended only for the zodiac, a mere belt of the heavens. but this rendered the enterprise too large for a single observatory. it became necessary to obtain the co-operation of other observatories, and with this end in view an international conference was summoned to meet in paris in to consider the whole project. there were delegates from, if not all nations, at any rate a considerable number:-- france british empire germany russia holland u.s. america austria sweden denmark belgium italy spain switzerland portugal brazil argentine republic [sidenote: choice of instrument.] [sidenote: expense of "doublet."] [sidenote: advantages of reflector.] [sidenote: refractor chosen.] the conference had a number of very important questions to discuss, for knowledge of the photographic method and its possibilities was at that time in its infancy. there was, for instance, the question whether all the instruments need be of the same pattern, and if so what that pattern should be. the first of these questions was settled in the affirmative, as we might expect; in the interests of uniformity it was desirable that the maps should be as nearly similar as possible. the second question was not so easy; there were at least three different types of instruments which might be used. first of all, there was the photographic lens, such as is familiar to all who have used an ordinary camera, consisting of two lenses with a space between; though since each of these lenses is itself made up of two, we should more correctly say four lenses in all. it was with a lens of this kind that the comet pictures had been taken at the cape of good hope, and it might seem the safest plan to adopt what had been shown to be capable of such good work. but there was this difficulty; the pictures of the comet were on a very small scale, and taken with a small lens; a much larger lens was required for the scheme now under contemplation, and when there are four separate lenses to be made, each with two surfaces to polish, and each requiring a perfectly sound clear piece of glass, it will be obvious that the difficulties of making a large compound lens of this kind are much greater, and the expense much more serious than in the case of a single lens, or even a pair. it was this question of expense which had led the brothers henry to experiment with a different kind of instrument, in which only one pair of lenses was used instead of two. their instrument was, in fact, very similar to the ordinary telescope, excepting that they were bound to make their lenses somewhat different in shape in order to bring to focus the rays of light suitable for photography, which are not the same as those suitable for eye observation with the ordinary telescope. dr. common, again, had used a third kind of instrument, mainly with the view of reducing the necessary expense still further, or, perhaps, of increasing the size of the instrument for the same expense. his telescope had no lens at all, but a curved mirror instead, the mirror being made of glass silvered on the face (not on the back as in the ordinary looking-glass). in this case there is only one surface to polish instead of four, as in the henrys' telescope, or eight, as in the case of the photographic doublet; and, moreover, since the rays of light are reflected from the surface of the glass, and do not pass _through_ it at all, the internal structure of the glass is not so strictly important as in the other cases. hence the reflector is a very cheap instrument, and it is, moreover, quite free from some difficulties attached to the other instruments. no correction for rays of light of different colours is required, since all rays of whatever colour come to the same focus automatically. these advantages of the reflector were so considerable as to almost outweigh one well-known disadvantage, which is, however, not very easily expressed in words. the reflector might be described as an instrument with a temper; sometimes it gives excellent results, but at others _something_ seems to be wrong, though the worried observer does not exactly know what. long experience and patience are requisite to humour the instrument and get the best results from it, and it was felt that this uncertainty was sufficient to disqualify the instrument for the serious piece of routine work contemplated in mapping the heavens. accordingly the handier and more amiable instrument with which the brothers henry had done such good work was selected as the pattern to be adopted. [sidenote: doublet would have been better.] it is curious that at the conference of nothing at all was said about the type of instrument first mentioned (the "doublet lens"), although a letter was written in its favour by professor pickering of harvard college observatory. since that time we have learnt much of its advantages, and it is probable that if the conference were to meet now they might arrive at a different decision; but at that time they were, to put it briefly, somewhat afraid of an instrument which seemed to promise, if anything, too well, especially in one respect. with the reflector and the refractor it had been found that the field of good images was strictly limited. the henrys' telescope would not photograph an area of the sky greater in extent than ° in diameter at any one time, and the reflector was more limited still; within this area the images of the stars were good, and it had been found that their places were accurately represented. now the "doublet" seemed to be able to show much larger areas than this with accuracy, but no one had been able to test the accuracy to see whether it was sufficient for astronomical purposes; and although no such feeling was openly expressed or is on record, i think there is no doubt that a feeling existed of general mistrust of an instrument which seemed to offer such specious promises. whatever the reason, its claims were passed over in silence at the conference, and the safer line (as it was then thought) of adopting as the type the henrys' instrument, was taken. [sidenote: the eighteen observatories.] this was perhaps the most important question settled at the conference, and the answers to many of the others naturally followed. the size of the plates, for instance, was settled automatically. the question down to what degree of faintness should stars be included, resolved itself into the equivalent question, what should be the length of time during which the plates were exposed? then, again, the question, what observatories should take part in the work? became simply this: what observatories could afford to acquire the instruments of this new pattern and get other funds for carrying out the work specified? it was ultimately found that eighteen observatories were able to obtain the apparatus and funds, though unfortunately three of the eighteen have since found it impossible to proceed. the following is the original list, and in brackets are added the names of three other observatories which in undertook to fill the places of the defaulters. observatories co-operating for the astrographic chart. +----------------------+------------+----------+ | observatory. | zones of | number | | |declination.|of plates.| +----------------------+------------+----------+ |greenwich |+ ° to + °| | |rome |+ ° " + °| | |catania |+ ° " + °| | |helsingfors |+ ° " + °| | |potsdam |+ ° " + °| | |oxford |+ ° " + °| | |paris |+ ° " + °| | |bordeaux |+ ° " + °| | |toulouse |+ ° " + °| | |algiers |+ ° " - °| | |san fernando |- ° " - °| | |tacubaya |- ° " - °| | |santiago (monte video)|- ° " - °| | |la plata (cordoba) |- ° " - °| | |rio (perth, australia)|- ° " - °| | |cape of good hope |- ° " - °| | |sydney |- ° " - °| | |melbourne |- ° " - °| | +----------------------+------------+----------+ [sidenote: sky covered twice.] in the list is also shown the total number of plates that were to be taken by each observatory. when once the size of the plates had been settled, it was a straightforward matter to divide up the sky into the proper number of regions necessary to cover it completely, not only without gaps between the plates, but with actually a small overlap of contiguous plates. and more than this, it was decided that the whole sky should be completely covered _twice over_. it was conceivable that a question might arise whether an apparent star image on a plate was, on the one hand, a dust speck, or, on the other hand, a planet, or perhaps a variable or new star. by taking two different plates at slightly different times, questions of this kind could be settled; and to make the check more independent it was decided that the plates should not be exactly repeated on the same portion of sky, but that in the second series the centre of a plate should occupy the point assigned to the corner of a plate in the first series. [sidenote: times of exposure.] then there came the important question of time of exposure, which involved a long debate between those who desired the most modest programme possible consistent with efficiency, and those enthusiasts who were anxious to strain the programme to the utmost limits attainable. ultimately it was resolved to take two series of plates; one series of long exposure which was set in the first instance at minutes, then became , then , then , and has by some enterprising observers been extended to - / hours; the other a series of short exposures which have been generally fixed at minutes. thus instead of covering the sky twice, it was decided to cover it in all four times, and the number of plates assigned to each observatory in the above list must be regarded as doubled by this new decision. and further still, on the series of short-exposure plates it was decided to add to the exposure of six minutes another one of three minutes, having slightly shifted the telescope between the two so that they should not be superimposed; and later still, a third exposure of twenty seconds was added to these. it would take too long to explain here the reasons for these details, but it will be clear that the general result of the discussion was to extend the original programme considerably, and render the work even more laborious than it had appeared at the outset. [sidenote: measurement of plates.] [sidenote: the réseau.] [sidenote: the microscope.] [sidenote: reversal of plates.] [sidenote: personal equation.] when all these plates have been taken, the work is by no means finished; indeed, it is only just commencing. there remains the task of measuring accurately on each of the short-exposure plates the positions of the stars which it represents, numbering on the average some or ; so that for instance at oxford the total number of stars measured on the twelve hundred plates is nearly half a million. these are not all separate stars; for the sky is represented twice over, and there is also the slight overlap of contiguous plates; but the number of actual separate stars measured at this one observatory is not far short of a quarter of a million, and it has taken nearly ten years to make the measurements, with the help of three or four measurers trained for the purpose. to render the measures easy, a network or réseau of cross lines is photographed on each plate by artificial light after it has been exposed to the stars, so that on development these cross lines and the stars both appear. we can see at a glance the approximate position of a star by counting the number of the space from left to right and from top to bottom in which it occurs; and we can also estimate the fraction of a space in addition to the whole number; but it is necessary for astronomical purposes to estimate this fraction with the greatest exactness. the whole numbers are already given with great exactness by the careful ruling of the cross lines, which can be spaced with extraordinary perfection. to measure the fraction, we place the plate under a microscope in the eye-piece of which there is a finally divided cross scale; the centre of the cross is placed over a star image, and then it is noted where the lines of the réseau cut the cross scale. in this way the position of the image of a star is read off with accuracy, and after a little practice with considerable rapidity. it has been found at oxford that under favourable conditions the places of nearly stars per hour can be recorded in this way by a single measurer, if he has some one to write down for him the numbers he calls out. this is only one form of measuring apparatus; there are others in which, instead of a scale in the eye-piece, micrometer screws are used to measure the fractions; but the general principle in all these instruments is much the same, and the rate of work is not very different; while to the minor advantages and disadvantages of the different types there seems no need here to refer. one particular point, however, is worth noting. after a plate has been measured, it is turned round completely, so that left is now right, and top is now bottom, and the measurements are repeated. this repetition has the advantage first of all of checking any mistakes. when a long piece of measuring or numerical work of any kind is undertaken there are invariably moments when the attention seems to wander, and some small error is the result. but there are also certain errors of a systematic character similar to those denoted by the term "personal equation," which has found its way into other walks of life. in the operation of placing a cross exactly over the image of a star, different observers would show slight differences of habit; one might place it a little more to the right than another. but when the plate is turned round the effect of this habit on the measure is exactly reversed, and hence if we take the mean of the two measures any personal habit of this kind is eliminated. it has been found by experience that such personal habits are much smaller for measures of this kind than for those to which we have long been accustomed in observations made by eye on the stars themselves. the troubles from "personal equation" have been much diminished by the photographic method, and certain peculiarities of the former method have been clearly exhibited by the comparison. for instance, it has gradually become clear that with eye observations personal equation is not a constant quantity, but is different for stars of different brightness. when observing the transit of a bright star the observer apparently records an instant definitely earlier than in recording the transit of a faint one; and this peculiarity seems to be common to the large majority of observers, which is perhaps the reason why it was not noticed earlier. but when positions of the stars determined in this way are compared with their positions measured on the photographic plates, the peculiarity is made clearly manifest. for example, at oxford, our first business after making measurements is to compare them with visual observations on a limited number of the brighter stars made at cambridge about twenty years ago. (about , stars were observed at cambridge, and we are dealing with ten times that number.) the comparison shows that the cambridge observations are affected with the following systematic errors:-- if stars of magnitude are observed correctly, then " " " . secs. too early " " " . " " " " . " " " " . " " " " . " [sidenote: main object of the work.] this may serve as an illustration of various incidental results which are already flowing from the enormous and laborious piece of work which, as far as the university observatory at oxford is concerned, we have just completed, though some of the other colleagues are not so far advanced. but the main results will not appear just yet. the work must be repeated, and the positions of the stars just obtained must be compared with those which they will be found to occupy at some future date, in order to see what kind of changes are going on in the heavens. whether this future date shall be one hundred years hence, or fifty, or ten, or whether we should begin immediately to repeat what has been done, is a matter not yet decided, and one which requires some little consideration. [sidenote: the concluding year.] i have said perhaps enough to give you a general idea of the work on which we have been engaged at oxford for the last ten years. ten years ago it seemed to stretch out in front of us rather hopelessly; the pace we were able to make seemed so slow in view of the distance to be covered. we felt rather like the schoolboy who has just returned to school and sees the next holidays as a very remote prospect, and we solaced ourselves much in the same way as he does, by making a diagram representing the total number of plates to be dealt with and crossing off each one as it was finished, just as he sometimes crosses off the days still remaining between him and the prospective holidays. it was pleasant to watch the growth of the number of crosses on this diagram, and by the end of the year we had the satisfaction of seeing very little blank space remaining. now, up to this point it had not much mattered whether any particular plate was secured in any particular year, or in a subsequent year, so long as there were always sufficient plates to keep us occupied in measuring them. but it then became a matter of importance to secure each plate at the proper time of year; for the sun, as we know, travels round the zodiac among the stars, obliterating by his radiance a large section of the sky for a period of some months, and in this way a particular region of the heavens is apt to "run into daylight," as the observatory phrase goes, and ceases to be available for photography during several months, until the sun is again far enough away to allow of the particular region being seen at night. [sidenote: a disappointment.] [sidenote: a curious plate.] [sidenote: a strange object.] [sidenote: a new star?] roughly speaking then, if a plate which should be taken in february is not secured in this month owing to bad weather, the proper time for taking it will not occur again until the following february; and when there was a fair prospect of finishing our work in , it became important to secure each plate at the proper time in that year. hence we were making special efforts to utilise to the full any fine night that providence sent in our way, and on such occasions it is clearly an economy, if not exactly to "make hay while the sun shines," at any rate to take plates vigorously while the sun is _not_ shining and the night is fine; leaving the development of them until the daytime. there is, of course, the risk that the whole night's work may in this way be lost owing to some fault in the plates, which might have been detected if some of them were immediately developed. perhaps in the early days of our work it would have been reckless or foolish to neglect this little precaution; but we had for years been accustomed to rely upon the excellence of the plates without finding our trust betrayed; and the sensitiveness of the plates had increased rather than diminished as time went on. hence it will be readily understood that when one fatal morning we developed a series of some thirty plates, and found that owing to some unexplained lack of sensitiveness they were all unsuitable for our purpose, it came as a most unwelcome and startling surprise. it was, of course, necessary to make certain that there was no oversight, that the developer was not at fault, and that the weather had not been treacherous. all such possibilities were carefully considered before communication with the makers of the plates, but it ultimately became clear that there had been some unfortunate failure in sensitiveness, and that it would be necessary to repeat the work with opportunities restricted by the intervening lapse of time. however, disappointments from this or similar causes are not unknown in astronomical work; and we set about this repetition with as little loss of time and cheerfulness as was possible. under the circumstances, however, it seemed desirable to examine carefully whether anything could be saved from the wreck--whether any of the plates could be admitted as _just_ coming up to the minimum requirements. and i devoted a morning to this inquiry. in the course of it i came across one plate which certainly seemed worth an inclusion among our series from the point of view of the number of stars shown upon it. it seemed quite rich in stars, perhaps even a little richer than might have been expected. on inquiry i was told that this was not one of the originally condemned plates, but one which had been taken since the failure in sensitiveness of the plates had been detected; was from a new and specially sensitive batch with which the courteous makers had supplied us; but though there were certainly a sufficient number of stars upon the plate, owing to some unexplained cause the telescope had been erroneously pointed, and the region taken did not correspond to the region required. to investigate the cause of the discrepancy i thereupon took down from our store of plates the other one of the same region which had been rejected for insufficiency of stars, and on comparing the two it was at once evident that there was a strange object on the plate taken later of the two, a bright star or other heavenly body, which was not on the former plate. i have explained that by repeating the exposure more than once, it is easily possible to recognise whether a mark upon the plate is really a celestial body or is an accidental blot or dust speck, and there was no doubt that this was the image of some strange celestial body. it might, of course, be a new planet, or even an old one which had wandered into the region; but a few measures soon showed that it was not in movement. the measures consisted in comparing the separation of the three exposures with the separation of the corresponding exposures of obvious stars, for the exposures were not, of course, simultaneous, and if the body were a planet and had moved in the interval between them, this would be made manifest on measuring the separations. no such movements could be detected; and the possibilities were thus restricted to two. so far as we knew the object was a star, but might be either a star of the class known as _variable_ or of that known as _new_. in the former case it would become bright and faint at more or less regular intervals, and might possibly have been already catalogued; for the number of these bodies already known amounts to some hundreds. search being made in the catalogues, no entry of it was found, though it still might be one of this class which had hitherto escaped detection. or it might be a "new star," one of those curious bodies which blaze up quite suddenly to brightness and then die away gradually until they become practically invisible. the most famous perhaps of these is the star which appeared in , and was so carefully observed by tycho brahé; but such apparitions are rare, and altogether we have not records as yet of a score altogether; so that in this latter case the discovery would be of much greater interest than in the former. in either event it was desirable to inform other observers as soon as possible of the existence of a strange body; already some time had elapsed since the plate had been taken, march th, for the examination of which i have spoken was not made until march th. accordingly, a telegram was at once despatched to the central office at kiel, which undertakes to distribute such information all over the world, and a few post-cards were sent to observers close at hand who might be able to observe the star the same night. certain observations with the spectroscope soon made it clear that the object was really a "new star." [sidenote: the discovery accidental.] [sidenote: mrs. fleming's discoveries.] this, therefore, is the discovery which we made at oxford: as you will see, in an entirely accidental manner, during the course of a piece of work in which it was certainly never contemplated. its purely accidental nature is sufficiently illustrated by the fact that if the plates originally supplied by the makers had been of the proper quality, the plate which led to the discovery would never have been taken. if the plates exposed in february had been satisfactory, we should have been content, and should not have repeated the exposure on march th. again i can testify personally how purely accidental it was that the examination was made on march th to see whether anything could be saved, as i have said, from the wreck. the idea came casually into my mind as i was walking through the room and saw the neat pile of rejected plates; and one may fairly call it an accidental impulse. this new star is not, however, the first of such objects to have been discovered "accidentally"; many of the others were found just as much by chance, though a notable exception must be made of those discovered at the harvard observatory, which are the result of a deliberate search for such bodies by the careful examination of photographic plates. mrs. fleming, who spends her life in such work, has had the good fortune to detect no less than six of these wonderful objects as the reward of her laborious scrutiny; and she is the _only_ person who has thus found new stars by photography until this accidental discovery at oxford. the following is a complete list of new stars discovered to date:-- list of new stars. +----------------------------------------------+ |ref. no.| constellation. | year.| discoverer. | +----------------------------------------------+ | | cassiopeia | | tycho brahé.| | | cygnus | | janson. | | | ophiuchus | | kepler. | | | vulpecula | | anthelm. | | | ophiuchus | | hind. | | | scorpio | | auwers. | | | corona borealis| | birmingham. | | | cygnus | | schmidt. | | | andromeda | | hartwig. | | | perseus | | fleming. | | | auriga | | anderson. | | | norma | | fleming. | | | carina | | fleming. | | | centaurus | | fleming. | | | sagittarius | | fleming. | | | aquila | | fleming. | | | perseus | | anderson. | | | gemini | | at oxford. | +----------------------------------------------+ [illustration: march , march , viii.--the oxford new star. a pair of photographs taken at the harvard college observatory before and after its appearance (_the arrow indicates the place of the new star. it will be seen that the left-hand picture though it shews fainter stars than the other, has not a trace of the new star._)] [sidenote: dr. anderson.] [sidenote: nova persei.] generally these stars have been noted by eye observation, as in the case of the two found by dr. anderson of edinburgh. in these cases also we may say that deliberate search was rewarded; for dr. anderson is probably the most assiduous "watcher of the skies" living, though he seldom uses a telescope; sometimes he uses an opera-glass, but usually the naked eye. he describes himself as an "astrophil" rather than as an astronomer. "i love the stars," he says; "and whenever they are shining, i must be looking." and so on every fine night he stands or sits at his open study window gazing at the heavens. i believe he was just about to leave them for his bed, near a.m. on the night of february , , when, throwing a last glance upward, he suddenly saw a brilliant star in the constellation perseus. his first feeling was actually one of disappointment, for he felt sure that this object must have been there for some time past without his knowing of it, and he grudged the time lost when he might have been regarding it. more in a spirit of complaint than of inquiry, he made his way to the royal observatory at edinburgh next day to hear what they had to say about it, though he found it difficult to approach the subject. he first talked about the weather, and the crops, and similar topics of general interest; and only after some time dared he venture a casual reference to the "new portent in the heavens." seeing his interlocutor look somewhat blank, he ventured a little farther, and made a direct reference to the new star in perseus; and then found to his astonishment, as also to his great delight, that he was the first to bring news of it. the news was soon communicated to other observers; all the telescopes of the world were soon trained upon it; and this wonderful "new star of the new century" has taught us more of the nature of these extraordinary bodies than all we knew before. [sidenote: records previous to discovery.] [sidenote: was nova geminorum previously shining faintly?] [sidenote: the suspicion negatived.] perhaps i may add a few remarks on one or two features of these bodies. firstly, let us note that professor pickering of harvard is now able to make a most important contribution to the _former_ history of these objects--that is to say, their history preceding their actual detection. we remember that, after uranus had been discovered, it was found that several observers had long before recorded its place unknowingly; and similarly professor pickering and his staff have usually photographed other new objects unknowingly. there are on the shelves at harvard vast stores of photographs, so many that they are unable to examine them when they have been taken; but once any object of interest has been discovered, it is easy to turn over the store and examine the particular plates which may possibly show it at an earlier date. in this way it was found that dr. anderson's new star had been visible only for a few days before its discovery, there being no trace of it on earlier plates. similarly, in the case of the new star found at oxford, plates taken on march st and th, fifteen days and ten days respectively before the discovery-plate of march th, showed the star. but, in this particular instance, greater interest attaches to two still earlier plates taken elsewhere, and with exposures much longer than any available at harvard. one had been obtained at heidelberg by dr. max wolf, and another at the yerkes observatory of chicago university, by mr. parkhurst; and on both there appeared to be a faint star of about the fourteenth or fifteenth magnitude, in the place subsequently occupied by the nova; and the question naturally arose, was this the object which ultimately blazed up and became the new star? to settle this point, it was necessary to measure its position, with reference to neighbouring stars, with extreme precision; and here it was unfortunate that the photographs did not help us as much as they might, for they were scarcely capable of being measured with the requisite precision. the point was an important one, because if the identity of the nova with this faint star could be established, it would be the second instance of the kind; but so far as they went, measurements of the photographs were distinctly against the identity. such was the conclusion of mr. parkhurst from his photograph alone; and it was confirmed by measures made at oxford on copies of both plates, which were kindly sent there for the purpose. the conclusion seemed to be that there was a faint star _very near_, but _not at_, the place of the new star; and it was therefore probable that, although this faint star was temporarily invisible from the brightness of the adjacent nova, as the latter became fainter (in the way with which we have become familiar in the case of new stars), it might be possible to see the two stars alongside each other. this critical observation was ultimately made by the sharp eyes of professor barnard, aided by the giant telescope of the yerkes observatory; and it seems clear therefore that the object which blazed up to become the nova of could not have previously been so bright as a faint star of the fourteenth magnitude. although this is merely a negative conclusion, it is an important one in the history of these bodies. [sidenote: nebula round nova persei.] [sidenote: its changes.] [sidenote: due to travelling illumination.] the second point to which i will draw your attention is from the history of the other nova just mentioned--dr. anderson's new star of . in this instance it is not the history previous to discovery, but what followed many months after discovery, that was of engrossing interest; and again yerkes observatory, with its magnificent equipment, played an important part in the drama. when, with its giant reflecting telescope, photographs were taken of the region of nova persei after it had become comparatively faint, it was found that there was an extraordinarily faint nebulosity surrounding the star. repeating the photographs at intervals, it was found that this nebulosity was rapidly changing in shape. "rapidly" is, of course, a relative term, and a casual inspection of two of the photographs might not convey any impression of rapidity; it is only when we come to consider the enormous distance at which the movements, or apparent movements, of the nebulæ must be taking place that it becomes clear how rapid the changes must be. it was not possible to determine this distance with any exactness, but limits to it could be set, and it seemed probable that the velocity of the movement was comparable with that of light. the conclusion suggested itself that the velocity might actually be identical with that of light, in which case what we saw was not the movement of actual matter, but merely that of illumination, travelling from point to point of matter already existing. [illustration: sept. , nov. , ix--nebulosity round nova persei (_from photographs taken at the yerkes observatory by g. w. ritchey._)] [sidenote: when did it all happen?] an analogy from the familiar case of sound may make clearer what is meant. if a loud noise is made in a large hall, we hear echoes from the walls. the sound travels with a velocity of about feet per second, reaches the walls, is reflected back from them, and returns to us with the same velocity. from the interval occupied in going and returning we could calculate the distance of the walls. the velocity of light is so enormous compared with that of sound that we are usually quite unable to observe any similar phenomenon in the case of light. if we strike a match in the largest hall, all parts of it are illuminated so immediately that we cannot possibly realise that there was really an interval between the striking of the match, the travelling of the light to the walls, and its return to our eyes. the scale of our terrestrial phenomenon is far too small to render this interval perceptible. but those who accept the theory above mentioned regarding the appearances round nova persei (although there are some who discredit it) believe that we have in this case an illustration of just this phenomenon of light echoes, on a scale large enough to be easily visible. they think that, surrounding the central star which blazed up so brightly in february , there was a vast dark nebula, of which we had no previous knowledge, because it was not shining with any light of its own. when the star blazed up, the illumination travelled from point to point of this dark nebula and lighted it up; but the size of the nebula was so vast that, although the light was travelling with the enormous velocity of , miles per second, it was not until months afterwards that it reached different portions of this nebula; and we accordingly got news of the existence of this nebula some months after the news reached us of the central conflagration, whatever it was. remark that all we can say is that the news of the nebula reached us _some months later_ than that of the outburst. the actual date when either of the actual things happened, we have as yet no means of knowing; it may have been hundreds or even thousands of years ago that the conflagration actually occurred of which we got news in february , the light having taken all that time to reach us from that distant part of space; and the light reflected from the nebula was following it on its way to us all these years at that same interval of a few months. [sidenote: an objection.] now, let me refer before leaving this point to the chief objection which has been urged against this theory. it has been maintained that the illumination would necessarily appear to travel outwards from the centre with an approach to uniformity, whereas the observed rate of travel is not uniform, and has been even towards the centre instead of away from it; which would seem as though portions of the nebula more distant from the centre were lighted up sooner than those closer to it. by a simple illustration from our solar system, we shall see that these curious anomalies may easily be explained. let us consider for simplicity two planets only, say the earth and saturn. we know that saturn travels round the sun in an orbit which is ten times larger than the orbit of the earth. suppose now that the sun were suddenly to be extinguished; light takes about eight minutes to travel from the sun to the earth, and consequently we should not get news of the extinction for some eight minutes; the sun would appear to us to still go on shining for eight minutes after he had really been extinguished. saturn being about ten times as far away from the sun, the news would take eighty minutes to reach saturn; and from the earth we should see saturn shining more[ ] than eighty minutes after the sun had been extinguished, although we ourselves should have lost the sun's light after eight minutes. i think we already begin to see possibilities of curious anomalies; but they can be made clearer than this. instead of imagining an observer on the earth, let us suppose him removed to a great distance away in the plane of the two orbits; and let us suppose that the sun is now lighted up again as suddenly as the new star blazed up in february . then such an observer would first see this blaze in the centre; eight minutes afterwards the illumination would reach the earth, a little speck of light near the sun would be illuminated, just as we saw a portion of the dark nebula round nova persei illuminated; eighty minutes later another speck, namely, saturn, would begin to shine. but now, would saturn necessarily appear to the distant observer to be farther away from the sun than the earth was? looking at the diagram, we can see that if saturn were at s{ } then it would present this natural appearance of being farther away from the sun than the earth; but it might be at s{ } or s{ }, in which case it would seem to be nearer the sun, and the illumination would seem to travel inwards towards the central body instead of outwards. without considering other cases in detail, it will be tolerably clear that almost any anomalous appearance might be explained by choosing a suitable arrangement of the nebulous matter which we suppose lighted up by the explosion of nova persei. another objection urged against the theory i have sketched is that the light reflected from such a nebula would be so feeble that it would not affect our photographic plates. this depends upon various assumptions which we have no time to notice here; but i think we may say that there is certainly room for the acceptance of the theory. [illustration: fig. .] [sidenote: did the nebula cause the outburst?] now, if this dark nebula was previously existing in this way all round the star which blazed up, the question naturally arises whether the nebula had anything to do with the conflagration. was there previously a star, either so cold or so distant as not to be shining with appreciable light, which, travelling through space, encountered this vast nebula, and by the friction of the encounter was suddenly rendered so luminous as to outshine a star of the first magnitude? the case of meteoric stones striking our own atmosphere seems to suggest such a possibility. these little stones are previously quite cold and invisible, and are travelling in some way through space, many of them probably circling round our sun. if they happen in their journey to encounter our earth, even the extremely tenuous atmosphere, so thin that it will scarcely bend the rays of light appreciably, even this is sufficient by its friction to raise the stones to a white heat, so that they blaze up into the falling stars with which we are familiar. this analogy is suggested, but we must be cautious in accepting it; for we know so very little of the nature of nebulæ such as that of which we have been speaking. but in any case, a totally new series of phenomena have been laid open to our study by those wonderful photographs taken at the yerkes observatory and the lick observatory in the few years which the present century has as yet run. [sidenote: importance of new stars] one thing is quite certain: we must lose no opportunity of studying such stars as may appear, and no diligence spent in discovering them at the earliest possible moment is thrown away. we have only known up to the present, as already stated, less than a score of them, and of these many have told us but little; partly because they were only discovered too late (after they had become faint), and partly because the earlier ones could not be observed with the spectroscope, which had not then been invented. it seems clear that in the future we must not allow accident to play so large a part in the discovery of these objects; more must be done in the way of deliberate search. although we know beforehand that this will involve a vast amount of apparently useless labour, that months and years must be spent in comparing photographic plates, or portions of the sky itself, with one another without detecting anything remarkable, it will not be the first time that years have been cheerfully spent in such searches without result. we need only recall hencke's fifteen years of fruitless search, before finding a minor planet, to realise this fact. [sidenote: superposition of plates.] [sidenote: the stereo-comparator.] one thing of importance may be done; we may improve our methods of making the search, so as to economise labour, and several successful attempts have already been made in this direction. the simplest plan is to superpose two photographs taken at different dates, so that the stars on one lie very close to those on the other; then if an image is seen to be unpaired we _may_ have found a new star, though of course the object may be merely a planet or a variable. the superposition of the plates may be either actual or virtual. a beautiful instrument has been devised on the principle of the stereoscope for examining two plates placed side by side, one with each eye. we know that in this way two photographs of the same object from different points of view will appear to coalesce, and at the same time to give an appearance of solidity to the object or landscape, portions of which will seem to stand out in front of the background. applying this principle to two photographs of stars, what happens is this: if the stars have all remained in the same positions exactly, the two pictures will seem to us to coalesce, and the images all to lie on a flat background; but if in the interval between the exposures of the two plates one of the stars has appreciably moved or disappeared, it will seem, when looked at with this instrument, to stand out in front of this background, and is accordingly detected with comparatively little trouble. this new instrument, to which the name stereo-comparator has been given, promises to be of immense value in dredging the sky for strange bodies in the future. i am glad to say that a generous friend has kindly presented the university observatory at oxford with one of these beautiful instruments, which have been constructed by messrs. zeiss of jena after the skilful designs of dr. pulfrich. whether we shall be able to repeat by deliberate search the success which mere accident threw in our way remains to be seen. chapter v schwabe and the sun-spot period [sidenote: discoveries contrary to expectation.] in preceding chapters we have reviewed discoveries, some of which have been made as a result of a deliberate search, and others accidentally in the course of work directed to a totally different end; but so far we have not considered a case in which the discoverer entered upon an enterprise from which he was positively dissuaded. [sidenote: nothing expected from spots.] in the next chapter we shall come across a very striking instance of this type; but even in the discovery that there was a periodicity in the solar spots, with which i propose to deal now, herr schwabe began his work in the face of deterrent opinions from eminent men. his definite announcement was first made in , though he had himself been convinced some years earlier. in the royal astronomical society awarded him their gold medal for the discovery; and in the address delivered on the occasion the president commenced by drawing attention to this very fact, that astronomers who had expressed any opinions on the subject had been uniformly and decidedly against the likelihood of there being anything profitable in the study of the solar spots. i will quote the exact words of the president, mr. manuel johnson, then radcliffe observer at oxford. "it was in that heinrich schwabe, a gentleman resident in dessau, entered upon those researches which are now to engage our attention. i am not aware of the motive that induced him--whether any particular views had suggested themselves to his own mind--or whether it was a general desire of investigating, more thoroughly than his predecessors had done, the laws of a remarkable phenomenon, which it had long been the fashion to neglect. he could hardly have anticipated the kind of result at which he has arrived; at the same time we cannot imagine a course of proceeding better calculated for its detection, even if his mind had been prepared for it, than that which he has pursued from the very commencement of his career. assuredly if he entertained such an idea, it was not borrowed from the authorities of the last century, to whom the solar spots were objects of more attention than they have been of late years. "'nulla constanti temporum lege apparent aut evanescunt,' says keill in .--_introduct. ad physic. astronom._, p. . "'il est manifest par ce que nous venons de rapporter qu'il n'y a point de règle certaine de leur formation, ni de leur nombre et de leur figure,' says cassini ii. in .--_elém d'astron._, vol. i. p. . "'il semble qu'elles ne suivent aucune loi dans leur apparitions,' says le monnier in .--_instit. astron._, p. . "'solar spots observe no regularity in their shape, magnitude, number, or in the time of their appearance or continuance,' says long in .--_astron._, vol. ii. p. . "'les apparitions des tâches du soleil n'ont rien de regulier,' says lalande in .--_astron._, vol. iii. § , nd edit. "and delambre's opinion may be inferred from a well-known passage in the third volume of his 'astronomy' (p. ), published in , where treating of the solar spots he says, 'il est vrai qu'elles sont plus curieuses que vraiment utiles.'"[ ] it will thus be evident that herr schwabe had the courage to enter upon a line of investigation which others had practically condemned as likely to lead nowhere, and that his discovery was quite contrary to expectation. it is a lesson to us that not even the most unlikely line of work is to be despised; for the outcome of schwabe's work was the first step in the whole series of discoveries which have gradually built up the modern science of solar physics, which occupies so deservedly large a part of the energies of, for instance, the great observatory attached to the university of chicago. [sidenote: schwabe's announcement.] it has been our practice to recall the actual words in which the discoverer himself stated his discovery, and i will give the original modest announcement of schwabe, though for convenience of those who do not read german i will attempt a rough translation. he had communicated year by year the results of his daily counting of the solar spots to the _astronomische nachrichten_, and after he had given ten years' results in this way he collected them together, but he made no remark on the curious sequence which they undoubtedly showed at that time. waiting patiently six years for further material, in he ventured to make his definite announcement as follows:--"from my earlier observations, which i have communicated annually to this journal, there was manifest already a certain periodicity of sun-spots; and the probability of this being really the case is confirmed by this year's results. although i gave in volume the total numbers of groups for the years - , nevertheless i will repeat here a complete series of all my observations of sun-spots, giving not only the number of groups, but also the number of days of observation, and further the days when the sun was free from spots. the number of groups alone will not in itself give sufficient accuracy for determination of a period, since i have convinced myself that when there are a large number of sun-spots the number will be reckoned somewhat too small, and when few sun-spots, the number somewhat too large; in the first case several groups are often counted together in one, and in the second it is easy to divide a group made up of two component parts into two separate groups. this must be my excuse for repeating the early catalogue, as follows:-- +---------------------------------------------+ | year.| number of | days free | days of | | | groups. | from spots.| observation.| |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | |( )| ( ) | ( ) | ( ) | +---------------------------------------------+ "if we now compare together the number of groups, and the days free from spots, we find that the sun-spots have a period of about ten years, and that for about five years they are so numerous that during this period few days, if any, are free from spots. the sequel must show whether this period is constant, whether the minimum activity of the sun in producing spots lasts for one or two years, and whether this activity increases more quickly than it decreases." [illustration: feb. , . feb. , . x.--photographs of the sun taken at the royal observatory, greenwich, shewing sunspots.] [sidenote: attracted little attention, until eight years later.] this brief announcement is all that the discoverer says upon the subject; and it is perhaps not remarkable that it attracted very little attention, especially when we remember that it related to a matter which the astronomical world had agreed to put aside as unprofitable and not worth attention. next year, in giving his usual paper on the spots for he recurs to the subject in the following sentence: "the periodicity of spots of about ten years which was indicated in my summary published last year, is confirmed by this year's observations." i have added in brackets to the table above reproduced the numbers for subsequently given, and it will be seen how nearly they might have been predicted. [sidenote: other phenomena sympathetic and others not.] still the subject attracted little attention. turning over the leaves of the journal at random, i came across the annual report of the astronomer royal of england, printed at length. but in it there is no reference to this discovery, which opened up a line of work now strongly represented in the annual programme of the royal observatory at greenwich. mr. johnson remarks that the only person who had taken it up was julius schmidt, who then resided near hamburg. but schwabe went on patiently accumulating facts; and in the great von humboldt in the third volume of his _cosmos_, drew attention to the discovery, which was accordingly for the first time brought into general notice. it will be seen that there are not many facts of general interest relating to the actual discovery beyond the courage with which the work was commenced in a totally unpromising direction, and the scant attention it received after being made for us. we may admit that interest centres chiefly in the tremendous consequences which flowed from it. we now recognise that many other phenomena are bound up with this waxing and waning of the solar spots. we might be prepared for a sympathy in phenomena obviously connected with the sun itself; but it was an unexpected and startling discovery that magnetic phenomena on the earth had also a sympathetic relation with the changes in sun-spots, and it is perhaps not surprising that when once this connection of solar and terrestrial phenomena was realised, various attempts have been made to extend it into regions where we cannot as yet allow that it has earned a legitimate right of entry. we have heard of the weather and of indian famines occurring in cycles identical with the sun-spot cycle; and it is obvious how tremendously important it would be for us if this were found to be actually the case. for we might in this way predict years of possible famine and guard against them; or if we could even partially foretell the kind of weather likely to occur some years hence, we might take agricultural measures accordingly. the importance of the connection, if only it could be established, is no doubt the reason which has misled investigators into laying undue stress on evidence which will not bear close scrutiny. for the present we must say decidedly that no case has been made out for paying serious attention to the influence of sun-spots on weather. nevertheless, putting all this aside, there is quite enough of first-rate importance in the sequel to schwabe's discovery. [sidenote: greenwich sun records.] [sidenote: the sun's rotation.] let us review the facts in order. most of us, though we may not have had the advantage of seeing an actual sun-spot through a telescope, have seen drawings or photographs of spots. there is a famous drawing made by james nasmyth (of steam-hammer fame), in july, , which is of particular interest, because at that time nasmyth was convinced--and he convinced many others with him--that the solar surface was made up of a miscellaneous heap of solid bodies in shape like willow leaves, or grains of rice, thrown together almost at random, and the drawing was made by him to illustrate this idea. comparing a modern photograph with it, we see that there is something to be said for nasmyth's view, which attracted much attention at the time and occasioned a somewhat heated controversy. but since the invention of the spectroscope it has become quite obsolete; it probably does not correspond in any way to the real facts. but instead of looking at pictures which have been enlarged to show the detailed structure in and near a spot, we will look at a series of pictures of the whole sun taken on successive days at greenwich in which the spots are necessarily much smaller, but which show the behaviour of the spots from day to day. (see plates x. and xi.) from the date at the foot of each it will be seen that they gradually cross the disc of the sun (a fact first discovered by galileo in ), showing that the sun rotates on an axis once in about every twenty-five days. there are many interesting facts connected with this rotation; especially that the sun does not rotate as a solid body, the parts near the (sun's) equator flowing quicker than those nearer the poles; but for the present we cannot stop to dwell upon them. what interests us particularly is the history, not from day to day, but from year to year, as schwabe has already given it for a series of years. [illustration: feb. , . feb. , . xi.--photographs of the sun taken at the royal observatory, greenwich, shewing sunspots] [sidenote: wolf's numbers.] [sidenote: greenwich areas.] [sidenote: magnetic fluctuations.] when it became generally established that this periodicity existed, rudolf wolf of zurich collected the facts about sun-spots from the earliest possible date, and represented this history by a series of numbers which are still called wolf's sun-spot numbers. you will see from the diagram the obvious rise and fall for eleven years,--not ten years, as schwabe thought, but just a little over eleven years. the facts are, however, given more completely by the work done at the royal observatory at greenwich. it is part of the regular daily work of that observatory to photograph the sun at least twice. many days are of course cloudy or wet, so that photographs cannot be obtained; but there are available photographs similarly taken in india or in mauritius, where the weather is more favourable, and from these the gaps are so well filled up that very few days, if any, during the whole year are left without some photograph of the sun's surface. on these photographs the positions and the areas of the spots are carefully measured under a microscope, and the results when submitted to certain necessary calculations are published year by year. it is clearly a more accurate estimate of the spottedness of the sun to take the total _area_ of all the spots rather than their mere _number_, for in the latter case a large spot and a small one count equally. hence the greenwich records will perhaps give us an even better idea of the periodicity than wolf's numbers. now, at the same observatory magnetic observations are also made continuously. if a magnet be suspended freely we are accustomed to say that it will point to the north pole; but this is only very roughly true. in the first place, it is probably well known to you that there is a considerable deviation from due north owing to the fact that the magnetic north pole is not the same as the geographical north pole; but this for the present need not concern us. what does concern us is, that if the needle is hung up and left long enough to come to rest, it does not then remain steadily at rest, but executes slow and small oscillations backwards and forwards, up and down, throughout the day; repeating nearly the same oscillations on the following day, but at the same time gradually changing its behaviour so as to oscillate differently in summer and winter. these changes are very small, and would pass unnoticed by the naked eye; but when carefully watched through a telescope, or better still, when photographed by some apparatus which will at the same time magnify them, they can be rendered easily visible. when the history of these changes is traced it is seen at once that there is a manifest connection with the cycle of sun-spot changes; for instance, if we measure the range of swing backwards and forwards during the day and take the average for all the days in the year, and then compare this with the average number of sun-spots, we shall see that the averages rise and fall together. similarly we may take the up and down swing, find the average amount of it throughout the year, and again we shall find that this corresponds very closely with the average number of sun-spots. [illustration: plate xii. number of sunspots (wolf) compared with daily range of magnetic declination & daily range of magnetic horzl. force (as observed at greenwich.)] [sidenote: daily curves.] [sidenote: difference between summer and winter, and between sun-spot maximum and minimum.] [sidenote: cause unknown.] but perhaps the most striking way to exhibit the sympathy is to combine different variations of the needle into one picture. and first we must remark that there is another important variation of the earth's magnetic action which we have not yet considered. we have mentioned the swing of the needle to and fro, and the swing up and down, and these correspond to changes in the _direction_ of the force of attraction on the needle. but there may be also changes in _intensity_ of this action; the pull may be a little stronger or a little weaker than before, and these variations are not represented by any actual movement of the needle, though they can be measured by proper experiments. we can, however, imagine them represented by a movement of the end of the needle if we suppose it made of elastic material, so that it would lengthen when the force was greater and contract slightly when the force was less. if a pencil were attached to the end of such an elastic needle so as to make a mark on a sheet of paper, and if for a moment we exclude the up and down movements, the pencil would describe during the day a curve on the paper, as the end of the needle swung backwards and forwards with the change in direction, and moved across the direction of swing with the change in intensity. now when curves of this kind are described for a day in each month of the year, there is a striking difference between the forms of them. during the summer months they are, generally speaking, comparatively large and open, and during the winter months they are small and close. this change in form is seen by a glance at plate xiii., which gives the curves throughout the whole of one year. let us now, instead of forming a curve of this kind for each month, form a single average curve for the whole year; and let us further do this for a series of years. (plate xiv.) we see that the curves change from year to year in a manner very similar to that in which they change from month to month in any particular year, and the law of change is such that in years when there are many sun-spots we get a large open curve similar to those found in the summer, while for years when there are few sun-spots we get small close curves very like those in the winter. hence we have two definite conclusions suggested: firstly, that the changes of force are sympathetic with the changes in the sun-spots; and secondly, that times of maximum sun-spots correspond to summer, and times of minimum to winter. and here i must admit that this is about as far as we have got at present in the investigation of this relationship. _why_ the needle behaves in this way we have as yet only the very vaguest ideas; suggestions of different kinds have certainly been put forward, but none of them as yet can be said to have much evidence in favour of its being the true one. for our present purpose, however, it is sufficient to note that there is this very real connection, and that consequently schwabe's sun-spot period may have a very real importance with regard to changes in our earth itself. [illustration: greenwich magnetic curves - plate xiii. greenwich magnetic curves for april - ] [sidenote: illustration of spurious connection.] but i may perhaps repeat the word of caution already uttered against extending without sufficient evidence this notion of the influence of sun-spots to other phenomena, such as weather. a simple illustration will perhaps serve better than a long argument to show both the way in which mistakes have been made and the way in which they can be seen to be mistakes. there is at the royal observatory at greenwich an instrument for noting the direction of the wind, the essential part being an ordinary wind-vane, the movements of which are automatically recorded on a sheet of paper. as the wind shifts from north to east the pencil moves in one direction, and when it shifts back again towards the north the pencil moves in the reverse way. but sometimes the wind shifts continuously from north to east, south, west, and back to north again, the vane making a complete revolution; and this causes the pencil to move continuously in one direction, until when the vane has come to north again, the pencil is far away from the convenient place of record; on such occasions it is often necessary to replace it by hand. then again, the vane may turn in the opposite direction, sending the pencil inconveniently to the other side of the record. during the year it is easy to count the number of complete changes of wind in either direction, and subtracting one number from the other, we get the excess of complete revolutions of the vane in one direction over that in the other. now if these rather arbitrary numbers are set down year by year, or plotted in the shape of a diagram, we get a curve which may be compared with the sun-spot curve, and during a period of no less than sixteen years--from to --there was a remarkable similarity between the two diagrams. from this evidence _alone_ it might fairly be inferred that the sun-spots had some curious effect upon the weather at greenwich, traceable in this extraordinary way in the changes of the wind. but the particular way in which these changes are recorded is so arbitrary that we should naturally feel surprise if there was a real connection between the two phenomena; and fortunately there were other records preceding these years and following them which enabled us to test the connection further, and it was found, as we might naturally expect, that it was not confirmed. on looking at diagrams (plate xv.) for the periods before and after, no similarity can be traced between the sun-spot curve and the wind-vane curve, and we infer that the similarity during the period first mentioned was entirely accidental. this shows that we must be cautious in accepting, from a limited amount of evidence, a connection between two phenomena as real and established; for it may be purely fortuitous. we may particularly remark that it is desirable to have repetitions through several complete periods instead of one alone. it is possible to reduce to mathematical laws the rules for caution in this matter; and much useful work has already been done in this direction by professor schuster of manchester and others, though as yet too little attention has been paid to their rules by investigators naturally eager to discover some hitherto unthought-of connection between phenomena. [sidenote: faculæ follow spots and the chromosphere.] with this example of the need for caution, we may return to phenomena of which we can certainly say that they vary sympathetically with the sun-spots. roughly speaking, the whole history of the sun seems to be bound up with them. besides these dark patches which we call spots (which, by the way, are not really dark but only less bright than the surrounding part of the disc), there are patches brighter than the rest which have been called faculæ. with ordinary telescopes, either visual or photographic, these can generally only be detected near the edge of the sun's disc; but even with this limitation it can easily be established that the faculæ vary in number and size from year to year much in the same way as the spots, and this conclusion is amply confirmed by the beautiful method of observing the faculæ with the new instrument designed by professor hale of the yerkes observatory. with this instrument, called a spectroheliograph, it is possible to photograph the faculæ in all parts of the sun's disc, and thus to obtain a much more complete history of them, and there is no doubt whatever of their variation sympathetically with the spots. nor is there any doubt about similar variations in other parts of the sun which we cannot see _at all_ with ordinary telescopes, except on the occasions when the sun is totally eclipsed. roughly speaking, these outlying portions of the sun consist of two kinds, the chromosphere and the corona, the former looking like an irregular close coating of the ordinary sun, and the latter like a pearly halo of light extending to many diameters of the sun's disc, but not with any very regular form. [illustration: plate xv. smoothed sunspot curve (wolf) compared with the number of turns made in each year by the osler anemometer vane of the royal observatory, greenwich (the excess of the direct turns (d) over the retrograde turns (r) or _vice versa_.) the upper curve is in each case the sunspot curve, the lower the vane curve. the break in in the vane curve is due to the omission of evidently accidental turns from that date.] the chromosphere, from which shoot out the prominences or "red flames," can now be observed without an eclipse if we employ the beautiful instrument above-mentioned, the spectroheliograph; and professor hale has succeeded in photographing spots, faculæ, and prominences all on the same plate. but although many have made the attempt (and professor hale, perhaps, a more determined attempt than any man living), no one has yet succeeded in obtaining any picture or evidence of the existence of the corona excepting on the occasion of a total solar eclipse. [sidenote: eclipses of sun.] [sidenote: total eclipses rare.] now these occasions are very rare. there are two or three eclipses of the sun every year, but they are generally of the kind known as partial; when the moon does indeed come between us and the sun to some extent, but only cuts off a portion of his light--a clean-cut black disc is seen to encroach more or less on the surface of the sun. most of us have had an opportunity of seeing a partial eclipse, probably more than once; but few have seen a total eclipse. for this the moon must come with great exactness centrally between us and the sun; and the spot where this condition is fulfilled completely only covers a few hundred miles of the earth's surface at one moment. as the earth turns round, and as the moon revolves in its orbit, this patch from which the sun is totally eclipsed travels over the earth's surface, marking out a track some thousands of miles in length possibly, but still not more than miles wide; and in order to see the sun totally eclipsed even on the rare occasions when it is possible at all (for, as already remarked, in the majority of cases the eclipse is only partial), we must occupy some station in this narrow belt or track, which often tantalisingly passes over either the ocean or some regions not easily accessible to civilised man. moreover, if we travel to such favoured spots the whole time during which the sun is totally eclipsed cannot exceed a few minutes, and hence observations are made under rather hurried and trying conditions. in these modern days of photography it is easier to take advantage of these precious moments than it used to be when there was only the eye and memory of an excited observer to rely upon. it is perhaps not surprising that some of the evidence collected on these earlier occasions was conflicting; but nowadays the observers, generally speaking, direct their energies in the first place to mounting accurately in position photographic apparatus of different kinds, each item of it specially designed to settle some particular problem in the most feasible way; secondly, to rehearsing very carefully the exact programme of exposures necessary during the critical few minutes; and finally, to securing these photographs with as few mistakes as possible when the precious moments actually arrive. even then the whole of their efforts are quite likely to be rendered unavailing by a passing cloud; and bitter is the disappointment when, after travelling thousands of miles, and spending months in preparation, the whole enterprise ends in nothing owing to some caprice of the weather. [sidenote: corona follows spots.] hence it will easily be imagined that our knowledge of the corona, the part of the sun which we can still only study on occasions of a total solar eclipse, advances but slowly. during the last twenty years there has been altogether scarcely half-an-hour available for this research, though it may fairly be said that the very best possible use has been made of that half-hour. and, what is of importance for our immediate purpose, it has gradually been established by comparing the photographs of one eclipse with those of another, that the corona itself undergoes distinct changes in form in the same period which governs the changes of sun-spots. when there are many sun-spots the corona spreads out in all directions from the edge of the sun's disc; when there are few sun-spots the corona extends very much further in the direction of the sun's equator, so that at sun-spot minimum there is an appearance of two huge wings. although the evidence is necessarily collected in a scrappy manner, by this time there is sufficient to remove this relationship out of the region of mere suspicion, and to give it a well-established place in our knowledge of the sun's surroundings. [sidenote: corona may influence magnets.] now the corona of the sun may be compared to some rare animal which we only see by paying a visit to some distant land, and may consider ourselves even then fortunate to get a glimpse of; and it might be thought that the habits of such an animal are not likely to be of any great importance in our everyday life. but so far from this being the case in regard to the corona, it is more than possible that the knowledge of its changes may be of vital interest to us. i have already said that, as yet, we have no satisfactory account of the reason why changes in sun-spots seem to influence changes in our magnets on the earth; but one of the theories put forward in explanation, and one by no means the least plausible, is that this influence may come, not from the sun-spots themselves, but from some other solar phenomenon which varies in sympathy with them; and in particular that it may come from the corona. these wings which reach out at sun-spot minimum can be seen to extend a considerable distance, and there is no reason to suppose that they actually cease at the point where they become too faint for us to detect them further; they may extend quite as far as the earth itself and even beyond; and they may be of such a nature as to influence our magnets. as the earth revolves round the sun it may sometime plunge into them, to emerge later and pass above or below them; as again the wings spread themselves at sun-spot minimum and seem to shrink at maximum, so our magnets may respond by sympathetic though very small vibrations. hence it is quite possible that the corona is directly influencing the magnetic changes on the earth. [sidenote: possible importance of corona.] but it may be urged that these changes are so slight as to be merely of scientific interest. that may be true to-day, but who will be bold enough to say that it will be true to-morrow? if we are thinking of practical utility alone, we may remember that two great forces of nature which we have chained into the service of man, steam and electricity, put forth originally the most feeble manifestations, which might readily have been despised as valueless; but by careful attention to proper conditions results of overwhelming practical importance have been obtained from these forces, which might have been, and for many centuries were, neglected as too trivial to be worth attention. recently the world has been startled by the discovery of new elements, such as radium, whose very existence was only detected by a triumph of scientific acuteness in investigation, and yet which promise to yield influences on our lives which may overwhelm in importance all that has gone before. and similarly it may be that these magnetic changes, when properly interpreted or developed, may become of an importance in the future out of all proportion to the attention which they have hitherto attracted. hence, although perhaps sufficient has already been established to show the immense consequences which flow from schwabe's remarkable discovery of the periodicity in solar spots, we may be as yet only on the threshold of its real value. from what little causes great events spring! how little can schwabe have realised, when he began to point his modest little telescope at the sun, and to count the number of spots--the despised spots which he had been assured were of no interest and exhibited no laws, and were generally unprofitable--that he was taking the first step in the invention of the great science of solar physics!--a science which is, i am glad to say, occupying at the present moment so much of the attention, not only of the great yerkes observatory, but of many other observatories scattered over the globe. chapter vi the variation of latitude if we should desire to classify discoveries in order of merit, we must undoubtedly give a high place to those which are made under direct discouragements. in the last chapter we saw that schwabe entered upon his work under conditions of this kind, it being the opinion of experienced astronomers who had looked at the facts that there was nothing of interest to be got by watching sun-spots. in the present chapter i propose to deal with a discovery made in the very teeth of the unanimous opinion of the astronomical world by an american amateur, mr. s. c. chandler of cambridge (massachusetts). it is my purpose to allow him to himself explain the steps of this discovery by giving extracts from the magnificent series of papers which he contributed to the _astronomical journal_ on the subject in the years - , but it may help in the understanding of these extracts if i give a brief summary of the facts. and i will first explain what is meant by the "variation of latitude." [sidenote: latitude.] [sidenote: precession.] we are all familiar with the existence of a certain star in the heavens called the pole star, and we know that at any particular place it is seen constantly in the north at a definite height above the horizon, which is the latitude of the place. when watched carefully with a telescope it is found to be not absolutely stationary, but to describe a small circle in the heavens day by day, or rather night by night. these simple facts are bound up with the phenomenon of the earth's rotation in this way: the axis about which it is rotating points to the centre of that little circle, and any change in the position of the axis can therefore be determined by observing these motions of the pole star. such changes may be of two kinds: firstly, we might find that the size of the circle increased or diminished, and this would mean that the earth's axis was pointing farther away from the pole star or nearer to it--pointing, that is to say, in a different direction in space. this actually happens (as has been known for some thousands of years) owing to the phenomenon called "precession"; the circle described by our pole star is at present getting a little smaller, but it will ultimately increase in size, and after thousands of years become so large that the pole star will entirely lose its character as a steady guide to the north. [sidenote: change of latitude.] [sidenote: twenty years ago disbelieved.] secondly (and this is what more immediately concerns us), the centre of the circle may alter its position and be no longer at the same height above the horizon of any given place. this would mean that the earth's axis was shifting _in the earth itself_--that the north pole which our explorers go to seek is not remaining in the same place. that it does not change appreciably in position we know from familiar experience; our climates, for instance, would suffer considerably if there were any large changes. but astronomers are concerned with minute changes which would not have any appreciable effect on climate, and the question has long been before them whether, putting aside large movements, there were any minute variations in position of the north pole. twenty years ago the answer to this question would have been given decidedly in the negative; it was considered as certain that the north pole did not move at all within the limits of our most refined astronomical observations. accepted theory seemed to indicate that any movements must in any case recur after a period of ten months, and careful discussion of the observations showed that there was no oscillation in such a period. now we know that the theory itself was wrong, or rather was founded upon a mistaken assumption; and that the facts when properly examined show clearly a distinct movement of the north pole, not a very large one, for all its movements take place within the area occupied by a moderate-sized room, but still a movement easily measurable by astronomical observations, and mr. chandler was the first to point out the law of these movements, and very possibly the first to suspect them. [sidenote: chandler's papers.] with these few words of explanation i will let mr. chandler tell his own story. his first paper appeared in the _astronomical journal_ in november , and is courageously headed, "on the variation of latitude"--i say courageously, because at that time it was believed that the latitude did _not_ vary, and mr. chandler himself was only in possession of a small portion of the facts. they unravelled themselves as he went forward; but he felt that he had firm hold of the end of the thread, and he faced the world confidently in that belief. he begins thus:-- [sidenote: first signs of change.] "in the determination of the latitude of cambridge[ ] with the almucantar, about six years and a half ago, it was shown that the observed values, arranged according to nights of observation, exhibited a decided and curious progression throughout the series, the earlier values being small, the later ones large, and the range from november to april being about four-tenths of a second. there was no known or imaginable instrumental or personal cause for this phenomenon, yet the only alternative seemed to be an inference that the latitude had actually changed. this seemed at the time too bold an inference to place upon record, and i therefore left the results to speak for themselves. the subsequent continuation of the series of observations to the end of june gave a maximum about may , while the discussion of the previous observations from may to november gave a minimum about september , indicating a range of ". within a half-period of about seven months." mr. chandler then gives some figures in support of these statements, presenting them with the clearness which is so well marked a feature of the whole series of papers, and concludes this introductory paper as follows:-- "it thus appears that the apparent change in the latitude of cambridge is verified by this discussion of more abundant material. the presumption that it is real, on this determination alone, would justify further inquiry. [sidenote: confirmed in europe.] "curiously enough dr. küstner, in his determination of the aberration from a series of observations coincident in time with those of the almucantar, came upon similar anomalies, and his results, published in , furnish a counterpart to those which i had pointed out in . the verification afforded by the recent parallel determinations at berlin, prague, potsdam, and pulkowa, which show a most surprising and satisfactory accordance, as to the character of the change, in range and periodicity, with the almucantar results, has led me to make further investigations on the subject. they seem to establish the nature of the law of those changes, and i will proceed to present them in due order." the second paper appeared on november , and opens with the following brief statement of his general results at that time:-- [sidenote: days' period.] "before entering upon the details of the investigations spoken of in the preceding number, it is convenient to say that the general result of a preliminary discussion is to show a revolution of the earth's pole in a period of days, from west to east, with a radius of thirty feet, measured at the earth's surface. assuming provisionally, for the purpose of statement, that this is a motion of the north pole of the principal axis of inertia about that of the axis of rotation, the direction of the former from the latter lay towards the greenwich meridian about the beginning of the year . this, with the period of days, will serve to fix approximately the relative positions of these axes at any other time, for any given meridian. it is not possible at this stage of the investigation to be more precise, as there are facts which appear to show that the rotation is not a perfectly uniform one, but is subject to secular change, and perhaps irregularities within brief spaces of time." [sidenote: contrary to received views.] it is almost impossible, now that we have become familiar with the ideas conveyed in this paragraph, to understand, or even fully to remember, the impression produced by them at the time; the sensation caused in some quarters, and the ridicule excited in others. they were in flat contradiction to all accepted views; and it was believed that these views were not only theoretically sound, but had been matured by a thorough examination of observational evidence. the only period in which the earth's pole could revolve was believed to be ten mouths; and here was mr. chandler proclaiming, apparently without any idea that he was contradicting the laws of dynamics, that it was revolving in fourteen months! the radius of its path had been found to be insensible by careful discussion of observations, and now he proclaimed a sensible radius o£ thirty feet. finally, he had the audacity to announce a _variable_ period, to which there was nothing at all corresponding in the mathematical possibilities. this was the bitterest pill of all. even after professor newcomb had shown us how to swallow the other two, he could not recommend any attempt at the third, as we shall presently see; and mr. chandler was fain ultimately to gild it a little before it could be gulped. [sidenote: pulkowa puzzle solved, also washington.] but this is anticipating, and it is our intention to follow patiently the evidence adduced in support of the above statements, made with such splendid confidence to a totally disbelieving world. mr. chandler first examines the observations of dr. küstner of berlin, quoted at the end of his last paper, and shows how well they are suited by the existence of a variation in the latitude of days; and that this new fact is added--when the cambridge (u.s.a.) latitudes were the smallest those of berlin were the largest, and _vice versâ_, as would clearly be the case if the phenomenon was due to a motion of the earth's pole; for if it moved nearer america it must move further from europe. he then examines a long series of observations made in the years - at pulkowa, near st. petersburg, and again finds satisfactory confirmation of his law of variation. now it had long been known that there was something curious about these observations, but no one could tell what it was. the key offered by mr. chandler fitted the lock exactly, and the anomalies which had been a puzzle were removed. this was in itself a great triumph; but there was another to come, which we may let mr. chandler describe in his own words:-- "in professor hubbard began a series of observations of [a] lyræ at the washington observatory with the prime vertical transit instrument, for the purpose of determining the constants of aberration and nutation and the parallax of the star. the methods of observation and reduction were conformed to those used with such success by w. struve. after hubbard's death the series was continued by professors newcomb, hall, and harkness until the beginning of . professor hall describes these observations as the most accurate determinations of declination ever made at the naval observatory. the probable error of a declination from a single transit was ± ". , and judging from the accidental errors, the series ought to give trustworthy results. upon reducing them, however, it was found that some abnormal source of error existed, which resulted in anomalous values of the aberration-constant in the different years, and a negative parallax in all. a careful verification of the processes of reduction failed to discover the cause of the trouble, and professor hall says that the results must stand as printed, and that probably some annual disturbance in the observations or the instrument occurred, which will never be explained, and which renders all deductions from them uncertain. the trouble could not be connected with personal equation, the anomalies remaining when the observations of the four observers who took part were separately treated. nor, as professor hall points out, will the theoretical ten-month period in the latitude furnish the explanation. "it is manifest, however, that if the -day period exists, its effect ought to appear distinctly in declination-measurements of such high degree of excellence as these presumably were, and, as i hope satisfactorily to show, actually are. when this variation is taken into account the observations will unquestionably vindicate the high expectations entertained with regard to them by the accomplished and skilful astronomers who designed and carried them out." [sidenote: direction of revolution of pole.] [sidenote: example of results.] from this general account i am excluding technical details and figures, and unfortunately a great deal is thereby lost. we lose the sense of conviction which the long rows of accordant figures force upon us, and we lose the opportunities of admiring both the astonishing amount of work done and the beautiful way in which the material is handled by a master. but i am tempted to give one very small illustration of the numerical results from near the end of the paper. after discussing the washington results, and amply fulfilling the promise made in the preceding extract, mr. chandler compares them with the pulkowa results, and shows that the earth's pole must be revolving from west to east, and not from east to west. and then he writes down a simple formula representing this motion, and compares his formula with the observations. he gives the results in seconds of arc, but for the benefit of those not familiar with astronomical measurements we may readily convert these into feet; and in the following tables are shown the distances of the earth's pole _in feet_ from its average position,[ ] as observed at washington and at pulkowa, and the same distances calculated according to the formula which mr. chandler was able to write down at this early stage. the signs + and - of course indicate opposite directions of displacement:-- washington. _deviation of pole._ +-------------------------------------+ | date. | observed.| formula. | |-------------------------------------| | , dec. | - feet | - feet | | , mar. | - " | - " | | " june | + " | + " | | " aug. | + " | + " | | " oct. | + " | + " | | " dec. | - " | - " | +-------------------------------------+ pulkowa. _deviation of pole._ +-------------------------------------+ | date. | observed.| formula. | |-------------------------------------| | , july | - feet | - feet | | " sept. | + " | + " | | " nov. | + " | + " | | , feb. | + " | + " | | " june | - " | - " | | " july | - " | - " | +-------------------------------------+ of course the figures are not exact in every case, but they are never many feet wrong; and it may well be imagined that it is a difficult thing to deduce, even from the most refined observations, the position of the earth's pole to within a foot. the difficulty is exactly the same as that of measuring the length of an object miles away to within an inch! mr. chandler winds up his second paper thus:-- "we thus find that the comparison of the simultaneous series at pulkowa and washington, - , leads to the same conclusion as that already drawn from the simultaneous series at berlin and cambridge, - . the direction of the polar motion may therefore be looked upon as established with a large degree of probability. "in the next paper i will present the results derived from peters, struve, bradley, and various other series of observations, after which the results of all will be brought to bear upon the determination of the best numerical values of the constants involved." [sidenote: bradley's observations.] [sidenote: latitude varied in twelve months then.] the results were not, however, presented in this order. in the next paper, which appeared on december , , mr. chandler begins, with the work of bradley, the very series of observations at kew and wansted which led to the discoveries of aberration and nutation, and which we considered in the third chapter. he first shows that, notwithstanding the obvious accuracy of the observations, there is some unexplained discordance. the very constant of aberration which bradley discovered from them differs by half-a-second of arc from our best modern determinations. attempts have been made to ascribe the discordance to changes in the instrument, but mr. chandler shows that such changes, setting aside the fact that bradley would almost certainly have discovered them, will not fit in with the facts. the facts, when analysed with the skill to which we have become accustomed, are that there is a periodic swing in the results _with a period of about a year_, and not fourteen months, as before, "a result so curious," as he admits, that "if we found no further support, it might lead us to distrust the above reasoning, and throw us back to the possibility that, after all, bradley's observations may have been vitiated by some kind of annual instrumental error. but it will abundantly appear, when i have had the opportunity to print the deductions from all the other series of observations down to the present time, that the inference of an increase in the period of polar revolution is firmly established by their concurrent testimony." we shall presently return to this curious result, which might well have dismayed a less determined researcher than mr. chandler, but which only led him on to renewed exertions. the results obtained from bradley's observations may be put in the form of a diagram thus:-- [illustration: fig. .] it will be seen that the maxima and minima fall in the spring and autumn, and this fact alone seemed to show that the effect could not be due to temperature, for we should expect the greatest effect in that case in winter and summer. it could not be due to the parallax of the stars for which bradley began his search, for stars in different quarters of the heavens would then be differently affected, and this was not the case. "there remains," concluded mr. chandler after full discussion, "the only natural conclusion of an actual displacement of the zenith, in other words, a change of latitude." and he concludes this paper with the following fine passage:-- "so far, then, as the results of this incomparable series of observations at kew and wansted, considered by themselves alone, can now be stated, the period of the polar rotation at that epoch appears to have been probably somewhat over a year, and certainly shorter by about two months than it is at the present time. the range of the variation was apparently in the neighbourhood of a second of arc, or considerably larger than that shown by the best modern observations. [sidenote: bradley's greatness.] "before taking leave of these observations for the present i cannot forbear to speak of the profound impression which a study of them leaves upon the mind, and the satisfaction which all astronomers must feel in recognising that, besides its first fruits of the phenomena of aberration and nutation, we now owe also our first knowledge of the polar motion to this same immortal work of bradley. its excellence, highly appreciated as it has been, has still been hitherto obscured by the presence of this unsuspected phenomenon. when divested of its effects, the wonderful accuracy of this work must appear in a finer light, and our admiration must be raised to higher pitch. going back to it after one hundred and sixty years seems indeed like advancing into an era of practical astronomy more refined than that from which we pass. and this leads to a suggestion worthy of serious practical consideration--whether we can do better in the future study of the polar rotation, than again to avail ourselves of bradley's method, without endangering its elegant simplicity and effectiveness by attempts at improvement, other than supplying certain means of instrumental control which would without doubt commend themselves to his sagacious mind. [sidenote: other puzzles explained.] "in the next article bradley's later observations at greenwich, the results of which are not so distinct, will be discussed; and also those of brinkley at dublin, - and - . this will bring again to the surface one of the most interesting episodes in astronomical history, the spirited and almost acrimonious dispute between brinkley and pond with regard to stellar parallaxes. i hope to show that the hitherto unsolved enigma of brinkley's singular results finds its easy solution in the fact of the polar motion. the period of his epoch appears to have been about a year, and its range more than a second. afterwards will follow various discussions already more or less advanced towards completion. these include bessel's observations at königsberg, - , with the reichenbach circle, and in - with the repsold circle; the latitudes derived from the polar-point determinations of struve and mädler with the dorpat circle, - ; struve's observations for the determination of the aberration; peters' observations of _polaris_, - , with the vertical-circle; the results obtained from the reflex zenith-tube at greenwich, - , whose singular anomalies can be referred in large part to our present phenomenon, complicated with instrumental error, to which until now they have been exclusively attributed; the greenwich transit-circle results, - , in which case, however, a similar complication and the large accidental errors of observation seem to frustrate efforts to get any pertinent results; the berlin prime-vertical observations of weyer and brünnow, - , in which i hope to show that the parallax of [beta] _draconis_ derived from them is simply a record of the change of latitude; the conflicting latitude determinations at cambridge, england; the washington observation of _polaris_ and other close polars, - , with the transit-circle; also those at melbourne, - , a portion of which have already been drawn upon in the last number of the _journal_, and some others. while the list is a considerable one, i shall be able to compress the statement of results for many of the series into a short space. [sidenote: provisional nature of results.] "in connection with this synopsis of the scope of the investigations, one or two particulars may be of interest, which at the present writing seem to foreshadow the probable outcome. i beg, however, that the statement will be regarded merely as a provisional one. first, while the period is manifestly subject to change, as has already once or twice been intimated, i have hitherto failed in tracing the variations to any regular law, expressible in a numerical formula. indeed, the general impression produced by a study of these changes in the length of the period is that the cause which produces them operates capriciously to a certain degree, although the average effect for a century has been to diminish the velocity of the revolution of the pole. how far this impression is due to the uncertainty of the observations, and to the complication of the phenomenon with other periodical changes of a purely instrumental kind, i cannot say. almost all of the series of any extent which have been examined, have the peculiarity that they manifest the periodicity quite uniformly and distinctly for a number of years, then for a while obscurely. in some cases, however, what at first appears to be an objective irregularity proves not to be so by comparison with overlapping series at other observatories. "another characteristic which has struck my attention, although somewhat vaguely, is that the variations in the length of the period seem to go hand in hand with simultaneous alterations in the amplitude of the rotation; the shorter periods being apparently associated with the larger coefficients for the latter. the verification of these surmises awaits a closer comparative scrutiny, the opportunity for which will come when the computations are in a more forward state. if confirmed, these observations will afford a valuable touchstone, in seeking for the cause of a phenomenon which now seems to be at variance with the accepted laws of terrestrial rotation." [sidenote: reception of discovery.] let us now for a few moments turn aside from the actual research to see how the announcement was received. it would be ungracious to reprint here any of the early statements of incredulity which found their way into print, especially in germany. but the first note of welcome came from simon newcomb, in the same number of the _astronomical journal_ as the paper just dealt with, and the following extract will indicate both the difficulties felt in receiving mr. chandler's results and the way in which newcomb struck at the root of them. [sidenote: newcomb's explanation.] "mr. chandler's remarkable discovery, that the apparent variations in terrestrial latitudes may be accounted for by supposing a revolution of the axis of rotation of the earth around that of figure, in a period of days, is in such disaccord with the received theory of the earth's rotation that at first i was disposed to doubt its possibility. but i am now able to point out a _vera causa_ which affords a complete explanation of this period. up to the present time the treatment of this subject has been this: the ratio of the moment of inertia of the earth around its principal axis to the mean of the other two principal moments, admits of very accurate determination from the amount of precession and nutation. this ratio involves what we might call, in a general way, the solid ellipticity of the earth, or the ellipticity of a homogeneous spheroid having the same moments of inertia as the earth. "when the differential equations of the earth's rotation are integrated, there appear two arbitrary constants, representing the position of any assigned epoch of the axis of rotation relative to that of figure. theory then shows that the axis of rotation will revolve round that of figure, in a period of days, and in a direction from west toward east. the attempts to determine the value of these constants have seemed to show that both are zero, or that the axes of rotation and figure are coincident. several years since, sir william thomson published the result of a brief computation from the washington prime-vertical observations of [alpha] lyrae which i made at his request and which showed a coefficient ". . this coefficient did not exceed the possible error of the result; i therefore regarded it as unreal. [sidenote: the forgotten assumption.] "the question now arises whether mr. chandler's result can be reconciled with dynamic theory. i answer that it can, because the theory which assigns days as the time of revolution is based on the hypothesis that the earth is an absolutely rigid body. but, as a matter of fact, the fluidity of the ocean plays an important part in the phenomenon, as does also the elasticity of the earth. the combined effect of this fluidity and elasticity is that if the axis of rotation is displaced by a certain amount, the axis of figure will, by the changed action of the centrifugal force, be moved toward coincidence with the new axis of rotation. the result is, that the motion of the latter will be diminished in a corresponding ratio, and thus the time of revolution will be lengthened. an exact computation of the effect is not possible without a knowledge of the earth's modulus of elasticity. but i think the result of investigation will be that the rigidity derived from mr. chandler's period is as great as that claimed by sir william thomson from the phenomena of the tides." [sidenote: but chandler's work still mistrusted.] this was very satisfactory. professor newcomb put his finger on the assumption which had been made so long ago that it had been forgotten: and the lesson is well worth taking to heart, for it is not the first time that mistaken confidence in a supposed fact has been traced to some forgotten preliminary assumption: and we must be ever ready to cast our eyes backward over all our assumptions, when some new fact seems to challenge our conclusions. it might further be expected that this discovery of the way in which theory had been defective would as a secondary consequence inspire confidence in the other conclusions which mr. chandler had arrived at in apparent contradiction to theory; or at least suggest the suspension of judgment. but professor newcomb did not feel that this was possible in respect of the _change_ of period, from about twelve months in bradley's time to fourteen months in ours. we have seen that mr. chandler himself regarded this as a "curious result" requiring confirmation: but since the confirmation was forthcoming, he stated it with full confidence, and drew the following remarks from professor newcomb in july , :-- "the fact of a periodic variation of terrestrial latitudes, and the general law of that variation, have been established beyond reasonable doubt by the observations collected by mr. chandler. but two of his minor conclusions, as enumerated in no. of this volume, do not seem to me well founded. they are-- " . that the period of the inequality is a variable quantity. " . that the amplitude of the inequality has remained constant for the last half century." professor newcomb proceeds to give his reasons for scepticism, which are too technical in character to reproduce here. but i will quote the following further sentence from his paper:-- "the question now arises how far we are entitled to assume that the period must be invariable. i reply that, perturbations aside, any variation of the period is in such direct conflict with the laws of dynamics that we are entitled to pronounce it impossible. but we know that there are perturbations, and i do not see how one can doubt that they have so acted as to increase the amplitude of the variation since ." [sidenote: chandler's reply.] in other words, while recognising that there may be a way of reconciling one of the "minor" conclusions with theory, professor newcomb considers that in this case the other must go. mr. chandler's answer will speak for itself. it was delayed a little in order that he might present an immense mass of evidence in support of his conclusions, and was ultimately printed on august , . "the material utilised in the foregoing forty-five series aggregates more than thirty-three thousand observations. of these more than one-third were made in the southern hemisphere, a fact which we owe principally to cordoba. it comprises the work of seventeen observatories (four of them in the southern hemisphere) with twenty-one different instruments, and by nine distinct methods of observation. only three of the series (xxi., xxv., and xxxv.), and these among the least precise intrinsically, give results contradictory of the general law developed in no. . this degree of general harmony is indeed surprising when the evanescent character of the phenomenon under investigation is considered. "the reader has now before him the means for independent scrutiny of the material on which the conclusions already drawn, and those which are to follow, are based. the space taken in the printing may seem unconscionable, but i hope this will be charged to the extent of the evidence collected, and not to diffuseness or the presentation of needless detail; for i have studiously sought to compress the form of statement without omitting anything essential for searching criticism. that it was important to do this is manifest, since the conclusions, if established, overthrow the existing theory of the earth's rotation, as i have pointed out on p. . i am neither surprised nor disconcerted, therefore, that professor newcomb should hesitate to accept some of these conclusions on the ground (_a. j._, no. ) that they are in such conflict with the laws of dynamics that we are entitled to pronounce them impossible. he has been so considerate and courteous in his treatment of my work thus far, that i am sure he will not deem presumptuous the following argument in rebuttal. [sidenote: he "put aside all teachings of theory," and "is not dismayed."] "it should be said, first, that in beginning these investigations last year, i deliberately put aside all teachings of theory, because it seemed to me high time that the facts should be examined by a purely inductive process; that the nugatory results of all attempts to detect the existence of the eulerian period probably arose from a defect of the theory itself; and that the entangled condition of the whole subject required that it should be examined afresh by processes unfettered by any preconceived notions whatever. the problem which i therefore proposed to myself was to see whether it would not be possible to lay the numerous ghosts--in the shape of numerous discordant residual phenomena pertaining to determinations of aberration, parallaxes, latitudes, and the like--which had heretofore flitted elusively about the astronomy of precision during the century; or to reduce them to tangible form by some simple consistent hypothesis. it was thought that if this could be done, a study of the nature of the forces, as thus indicated, by which the earth's rotation is influenced, might lead to a physical explanation of them. "naturally, then, i am not much dismayed by the argument of conflict with dynamic laws, since all that such a phrase means must refer merely to the existent state of the theory at any given time. when the -day period was propounded, it was as inconsistent with known dynamic law as the variation of it now appears to be. professor newcomb's own happy explanation has already set aside the first difficulty, as it would appear, and advanced the theory by an important step. are we so sure yet of a complete knowledge of all the forces at work as to exclude the chance of a _vera causa_ for the second?" [sidenote: faraday's words.] there is a splendid ring of resolution about these words. let us compare them with a notable utterance of faraday:-- "the philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. he should not be biassed by appearances; have no favourite hypothesis; be of no school; and in doctrine have no master. he should not be a respecter of persons, but of things. truth should be his primary object. if to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature." [sidenote: chandler's other work at this time.] [sidenote: his ultimate satisfactory solution.] [sidenote: interference of two waves.] tested by this severe standard, mr. chandler fails in no particular, least of all in that of industry. the amount of work he got through about this time was enormous, for besides the main line of investigation, of which we have only had after all a mere glimpse, he had been able to turn aside to discuss a subsidiary question with professor comstock; he had examined with great care some puzzling characteristics in the variability of stars; he computed some comet ephemerides; and he was preparing a new catalogue of variable stars--a piece of work involving the collection and arrangement of great masses of miscellaneous material. yet within a few months after replying as above to professor newcomb's criticism, he was able to announce that he had found the key to the new puzzle, and that "theory and observation were again brought into complete accord." we will as before listen to the account of this new step in his own words, but a slight preliminary explanation may help those unaccustomed to the terminology. the polar motion was found to be compounded of _two_ independent motions, both periodic, but having different periods. now, the general results of such a composition are well known in several different branches of physics, especially in the theory of sound. if two notes of nearly the same pitch be struck at the same time, we hear the resultant sound alternately swell and die away, because the vibrations caused by the two notes are sometimes going in the same direction, and after an interval are going exactly in opposite directions. diagrammatically we should represent the vibrations by two waves, as below; the upper wave goes through its period seven and a half times between a and d, the lower only six times; and it is easily seen that at a and c the waves are sympathetic, at b and d antipathetic. at a and c the compound vibration would be doubled; at b and d reduced to insensibility. the point is so important that perhaps a numerical illustration of it will not be superfluous. the waves are now represented by rows of figures as below. the first series recurs after every , the second after every . [illustration: fig. .] first wave second wave ------------------------------------------------------------- combined effect great disturbance. calm. ----------------------------------------------------------------------------- first wave second wave ------------------------------------------------------------- combined effect great disturbance. [sidenote: illustration from ocean travel.] adding the two rows together, the oscillations at first reinforce one another and we get numbers ranging from to instead of from to ; but one wave gains on the other, until it is rising when the other is falling, and the numbers add up to a steady series of 's. it will be seen that there are no less than seven consecutive 's, and all the variation seems to have disappeared. but presently the waves separate again, and the period of great disturbance recurs; it will be seen that in the "combined effect" the numbers repeat exactly after the nd term. now those unfamiliar with the subject may not be prepared for the addition of one physical wave to another, as though they were numbers, but the analogy is perfect. travellers by some of the fast twin-screw steamers have had unpleasant occasion to notice this phenomenon, when the engineer does not run the two screws precisely at the same speed; there come times when the ship vibrates violently, separated by periods of comparative stillness. instances from other walks of life may recur to the memory when once attention is called to the general facts; but enough has been said to explain the point numbered ( ) in the subjoined statement. to understand the rest, we must remember that if the two waves are not equal in "amplitude," _i.e._ if the backward and forward motion is not the same in both, they cannot annul one another, but the greater will always predominate. those interested in following the matter further should have no difficulty in constructing simple examples to illustrate such points. we will proceed to give mr. chandler's statements:-- [sidenote: chandler's final formulæ.] "we now come upon a new line of investigation. heretofore, as has been seen, the method has been to condense the results of each series of observations into the interval comprised by a single period, then to determine the mean epoch of minimum and the mean range for each series, and, finally, by a discussion of these quantities, to establish the general character of the law of the rotation of the pole. it is now requisite to analyse the observations in a different way, and discover whether the deviations from the general provisional law, in the last column of table ii., are real, and also in what manner the variation of the period is brought about. the outcome of this discussion, which is to be presented in the present paper, is extremely satisfactory. the real nature of the phenomenon is most distinctly revealed, and may be described as follows:-- " . the observed variation of the latitude is the resultant curve arising from two periodic fluctuations superposed upon each other. the first of these, and in general the more considerable, has a period of about days, and a semi-amplitude of about ". . the second has an annual period with a range variable between ". and ". during the last half-century. during the middle portion of this interval, roughly characterised as between and , the value represented by the lower limit has prevailed, but before and after those dates, the higher one. the minimum and maximum of this annual component of the variation occur at the meridian of greenwich, about ten days before the vernal and autumnal equinoxes respectively, and it becomes zero just before the solstices. " . as the resultant of these two motions, the effective variation of the latitude is subject to a systematic alternation in a cycle of seven years' duration, resulting from the commensurability of the two terms. according as they conspire or interfere, the total range varies between two-thirds of a second as a maximum, to but a few hundredths of a second, generally speaking, as a minimum. " . in consequence of the variability of the coefficient of the annual term above mentioned, the apparent average period between and approximated to or days; widely fluctuated from to ; from to about was very nearly days, with minor fluctuations; afterwards increased to near days, and very recently fell to somewhat below days. the general course of these fluctuations is quite faithfully represented by the law of eq. ( ), (no. ), and accurately, even down to the minor oscillations of individual periods, by the law of eq. ( ), hereafter given, and verbally interpreted above. this law also gives a similarly accurate account of the corresponding oscillations in the amplitude. the closeness of the accordance between observation and the numerical theory, in both particulars, places the reality of the law beyond reasonable doubt." those who cannot follow the details of the above statement will nevertheless catch the general purport--that the difficulties felt by professor newcomb have been surmounted; and this is made clearer by a later extract:-- "a very important conclusion necessarily follows from the agreement of the values of the -day term, deduced from the intervals between the consecutive values of t in table xii., namely, that there has been no discontinuity in the revolution, such as professor newcomb regarded as so probable that he doubted the possibility of drawing any conclusions from the comparison of observations before and after (_a. j._, , p. ). [sidenote: theory must go, if it will not fit observation.] "the present investigation demonstrates that the way out of the apparently irreconcilable contradiction of theory and observation in this matter does not lie in the direction of discrediting the observations, as he is inclined to do. on the contrary, the result is a beautiful vindication of the trustworthiness of the latter, and, at the same time, of the theory that demands an invariable rate of motion; providing a perfectly fitting key to the riddle by showing that another cause has intervened to produce the variability of the period. i feel confident that professor newcomb will agree with the reality of the explanation here set forth, and will reconsider his view that the perturbations in the position of the pole must be of the nature of chance accumulations of motion, a view which he then considered necessary to the maintenance of the constancy in the period of latitude-variation." [sidenote: the final paper.] the paper from which these words are taken appeared on november , . the next paper on the main theme did not appear till a year later, though much work was being done in the meantime on the constant of aberration and other matters arising immediately after the discovery. on november , , mr. chandler winds up the series of eight papers "on the variation of latitude," which he had commenced just two years before. his work was by no means done; rather was it only beginning, for the torch he had lit illuminated many dark corners. but he rightly regarded his discovery as now so firmly established that the series of papers dealing with it as still under consideration might be terminated. in this final paper he first devotes the most careful attention to one point of detail. he had shown earlier in the series that the north pole must be revolving from west to east, and not from east to west; but this was when the motion was supposed to be simple and not complex, and it was necessary to re-examine the question of direction for each of the components. after establishing conclusively that the original direction holds for each of the components, he almost apologises for the trouble he has taken, thus:-- "it is therefore proved beyond reasonable doubt that the directions of the rotations is from west to east in both elements; whence the general form of the equation for the variation of latitude adopted in _a. j._, , p. , eq. ( ). it may be thought that too much pains have been here bestowed upon a point which might be trusted to theory to decide. i cannot think so. one of the most salient results of these articles has been the proof of the fact that theory has been a blind guide with regard to the velocity of the polar rotation, obscuring truth and misleading investigators for a half a century. and even if we were certain, which we are not, that the fourteen months' term is the eulerian period in a modified form. it would still be necessary to settle by observation the direction of the annual motion, with regard to which theory is powerless to inform us. to save repetition of argument, i must refer to the statement in _a. j._, , pp. , , of the principles adopted in beginning these inquiries in ." finally, he answers one of the few objectors of eminence who still lingered, the great french physicist cornu:-- [sidenote: cornu answered.] "the ground is now cleared for examination of the only topic remaining to be covered, to establish, upon the foundation of fact, every point in the present theory of these remarkable movements of the earth's axis. this is the question of the possibility that these movements are not real, but merely misinterpretations of the observed phenomena; being in whole or in part an illusory effect of instrumental error due to the influence of temperature. such a possibility has been a nightmare in practical astronomy from the first, frightening us in every series of unexplained residuals, brought to light continually in nearly all attempts at delicate instrumental research. a source of danger so subtile could not fail to be ever present in the mind of every astronomer and physicist who has given even a superficial attention to the question of the latitude variations, and there is no doubt that some are even now thus deterred from accepting these variations as proved facts. perhaps the most explicit and forcible statement of the doubts that may arise on this subject has been given very recently by mr. cornu. the views of so distinguished a physicist, and of others who are inclined to agree with him, call for careful attention, and cannot be neglected in the present closing argument upon the theory presented in these articles. it is unnecessary, for the purpose of disposing of objections of the sort raised by cornu, to insist that it is not sufficient to show that the observed variations, attributed to the unsteadiness of the earth's pole, are near the limit of precision attainable in linear differential measures, and in the indication of the direction of gravity by means of the air bubble of the level; or to show that there are known variations in divided circles and in levels, dependent on temperature and seasons. nor need we require of objectors the difficult, although essential, task--which they have not distinctly attempted--of showing that these errors are not eliminated, as they appear to be, by the modes in which astronomers use their instruments. neither need we even urge the fact that a large portion of the data which have been utilised in the present researches on the latitude were derived by methods which dispense with levels, or with circles, a part of them indeed with both, and yet that the results of all are harmonious. on the contrary, let us admit, although merely for argument's sake, that all the known means of determining the direction of gravity--including the plumb-line, the level, and a fluid at rest, whether used for a reflecting surface or as a support for a floating instrument--are subject to a common law of periodical error which vitiates the result of astronomical observation, obtained by whatever methods, and in precisely the same manner. now, the observed law of latitude variation includes two terms, with periods of fourteen and twelve months respectively. since the phases of the first term are repeated at intervals of two months in successive years, and hence in a series of years come into all possible relations to conditions of temperature dependent on season, the argument against the reality of this term, on this ground, absolutely fails, and needs no further notice. as to the second, or annual term, while the phases, as observed in any given longitude, are indeed synchronical with the seasons, they are not so as regards different longitudes. if, therefore, the times of any given phase, as observed in the same latitude, but in successively increasing longitudes, occurred at the same date in all of them, there would be a fatal presumption against the existence of an annual period in the polar motion. if, on the contrary, they occur at times successively corresponding to the differences of longitude, the presumption is equally fatal to the hypothesis that they can possibly be due to temperature variation as affecting instrumental measurement. but the facts given in the foregoing section correspond most distinctly to the latter condition. therefore, unless additional facts can be brought to disprove successively these observed results, we may dismiss for ever the bugbear which has undoubtedly led many to distrust the reality of the annual component of the latitude-variation, while they admit the existence of the -day term." [sidenote: consequences of the discovery.] [sidenote: suspected observers acquitted.] at this point we must leave the fascinating account of the manner in which this great discovery was established, in the teeth of opposition such as might have dismayed and dissuaded a less clear-sighted or courageous man. it is my purpose to lay more stress upon the method of making the discovery than upon its results; but we may afford a brief glance at some of the consequences which have already begun to flow from this step in advance. some of them have indeed already come before us, especially that large class represented by the explanation of anomalies in series of observations which had been put aside as inexplicable. we have seen how the observations made in russia, or in washington, or at greenwich, in all of which there was some puzzling error, were immediately straightened out when chandler applied his new rule to them. we in england have special cause to be grateful to chandler; not only has he demonstrated more clearly than ever the greatness of bradley, but he has rehabilitated pond, the astronomer royal of the beginning of the nineteenth century; showing that his observations, which had been condemned as in some way erroneous, were really far more accurate than might have been expected; and further he has shown that the beautiful instrument designed by airy, and called the reflex zenith tube, which seemed to have unaccountably failed in the purpose for which it was designed, was really all the time accumulating observations of this new phenomenon, the variation of latitude. instead of airy having failed in his design, he had in chandler's words "builded better than he knew." [sidenote: constant of aberration improved.] secondly, there is the modifying influence of this new phenomenon on other phenomena already known, such, for instance, as that of "aberration." we saw in the third chapter how bradley discovered this effect of the velocity of light, and how the measure of it is obtained by comparing the velocity of light with that of the earth. this comparison can be effected in a variety of ways, and we should expect all the results to agree within certain limits; but this agreement was not obtained, and chandler has been able to show one reason why, and to remove some of the more troublesome differences. it is impossible to give here an idea of the far-reaching consequences which such work as this may have; so long as there are differences of this kind we cannot trust any part of the chain of evidence, and there is in prospect the enormous labour of examining each separate link until the error is found. the velocity of light, for instance, may be measured by a terrestrial experiment; was there anything wrong in the apparatus? the velocity of the earth in its journey round the sun depends directly upon the distance of the sun: have we measured this distance wrongly, and if so what was the error in the observations made? these are some of the questions which may arise so long as the values for the _constant of aberration_ are still conflicting; but it requires considerable knowledge of astronomy to appreciate them fully. [sidenote: latitude variation tide.] [sidenote: earthquakes.] another example will, perhaps, be of more general interest. if the axis of the earth is executing small oscillations of this kind, there should be an effect upon the tides; the liquid ocean should feel the wobble of the earth's axis in some way; and an examination of tidal registers showed that there was in fact a distinct effect. it may cause some amusement when i say that the rise and fall are only a few inches in any case; but they are unmistakable evidences that the earth is not spinning smoothly, but has this kind of unbalanced vibration, which i have compared to the vibrations felt by passengers on an imperfectly engineered twin-screw steamer. a more sensational effect is that apparently earthquakes are more numerous at the time when the vibration is greatest. we remarked that the vibration waxes and wanes, much as that of the steamer waxes and wanes if the twin-screws are not running quite together. now the passengers on the steamer would be prepared to find that breakages would be more numerous during the times of vigorous oscillation; and it seems probable that in a similar way the little cracks of the earth's skin which we call great earthquakes are more numerous when these unbalanced vibrations are at their maximum; that is to say, about once every seven years. this result is scarcely yet worthy of complete confidence, for our observations of earthquakes have only very recently been reduced to proper order; but if it should turn out to be true, it is scarcely necessary to add any words of mine to demonstrate the importance of this rather unexpected result of the latitude variation. [sidenote: the kimura phenomenon.] finally i will mention another phenomenon which seems to be at present more of a curiosity than anything else, but which may lead to some future great discovery. it is the outcome of observations which have been recently made to watch these motions of the pole; for although there seems good reason to accept mr. chandler's laws of variation as accurate, it is necessary to establish their accuracy and complete the details by making observations for some time yet to come; and there could be no better proof of this necessity than the discovery recently made by mr. kimura, one of those engaged in this watch of the pole in japan. perhaps i can give the best idea of it by mentioning one possible explanation, which, however, i must caution you may not be by any means the right one. we are accustomed to think of this great earth as being sufficiently constant in shape; if asked, for instance, whether its centre of gravity remains constantly in the same place inside it, we should almost certainly answer in the affirmative, just as only twenty years ago we thought that the north pole remained in the same place. but it seems possible that the centre of gravity moves a few feet backwards and forwards each year--this would at any rate explain certain curious features in the observations to which mr. kimura has drawn attention. whatever the explanation of them may be, or to settle whether this explanation is correct, we want more observations, especially observations in the southern hemisphere; and it is a project under consideration by astronomers at the present moment whether three stations can be established in the southern hemisphere for the further observation of this curious phenomenon. the question resolves itself chiefly into a question of money; indeed, most astronomical projects do ultimately resolve themselves into questions of money; and i fear the world looks upon scientific men as insatiable in this respect. one can only hope that on the whole the money is expended so as to give a satisfactory return. in this instance i have no hesitation in saying that an immediate return of value for a comparatively modest expenditure is practically certain, if only in some way we can get the means of making the observations. it would be natural, at the conclusion of this brief review of some types of astronomical discovery, to summarise the lessons indicated: but there is the important difficulty that there appear to be none. it has been pointed out as we proceeded that what seemed to be a safe deduction from one piece of history has been flatly contradicted by another; no sooner have we learnt that important results may be obtained by pursuing steadily a line of work in spite of the fact that it seems to have become tedious and unprofitable (as in the search for minor planets) than we are confronted with the possibility that by such simple devotion to the day's work we may be losing a great opportunity, as challis did. we can scarcely go wrong in following up the study of residual phenomena in the wake of bradley; but there is the important difficulty that we may be wholly unable to find a clue for the arrangement of our residuals, as is at present largely the case in meteorology. and, in general, human expectations are likely to be quite misleading, as has been shown in the last two chapters; the discoveries we desire may lie in the direction precisely opposite to that indicated by the best opinion at present available. there is no royal road to discovery, and though this statement may meet with such ready acceptance that it seems scarcely worth making, it is hoped that there may be sufficient of interest in the illustrations of its truth. the one positive conclusion which we may derive from the examples studied is that discoveries are seldom made without both hard work and conspicuous ability. a new planet, even as large as uranus, does not reveal itself to a passive observer: thirteen times it may appear to such a one without fear of detection, until at last it encounters an alert herschel, who suspects, tests, and verifies, and even then announces a comet--so little did he realise the whole truth. fifteen years of unrequited labour before astræa was found, nineteen years of observation before the discovery of nutation could be announced: how seldom do these years of toil present themselves to our imaginations when we glibly say that "bradley discovered nutation," or "hencke discovered astræa"! that the necessary labour is so often forgotten must be my excuse for recalling attention to it somewhat persistently in these examples. but beyond the fact that he must work hard, it would seem as though there were little of value to tell the would-be discoverer. the situation has been well summarised by jevons in his chapter on induction in the "principles of science;" and his words will form a fitting conclusion to these chapters:-- "it would seem as if the mind of the great discoverer must combine contradictory attributes. he must be fertile in theories and hypotheses, and yet full of facts and precise results of experience. he must entertain the feeblest analogies, and the merest guesses at truth, and yet he must hold them as worthless till they are verified in experiment. when there are any grounds of probability he must hold tenaciously to an old opinion, and yet he must be prepared at any moment to relinquish it when a clearly contradictory fact is encountered." index aberration, - , , , , , , , , , accidental discovery, , , - adams, , - ; resolution, airy, , - , algiers, alleghenia, almucantar, , alphabet used for planets, anderson, dr. t. c., , , , , anthelm, apollo, argon, ascension, assumption, forgotten, astræa, , , astrographic chart, , , _astronomical journal_, - _astronomische nachrichten_, , astrophil, auwers, ball, sir r., balliol college, banks, sir j., barnard, e. e., , berlin, , , , , berlin star-map, , , , bessel, bettina, , birmingham, "black drop" (in transit of venus), bliss, board of visitors of greenwich observatory, bode, , , , bode's law, , , , , , , , , , bourdeaux, bouvard, , , , , , , bradley, , - , - , , , , bradley, john, bremen, bridstow, , , briggs, brinkley, british association, brünnow, california, cambridge (mass.), , , cambridge observatory, , , , , , , , , cambridge university, - , cape observatory, , , cards, cassini ii., catania, ceres, - chacornac, challis, - , - , , , chandler, s. c., , - chapman's "homer," chicago, chromosphere, clarke, c. c., coelostat, columbus, comet, - , , , , , commission, planetary, common, a. a., , _compte rendu_, comstock, conference, astrographic, - copernicus, , cordoba, , cornu, - corona, - _cosmos_ (humboldt's), delambre, deviation of pole, disc of neptune, , , disc of uranus, - dorpat, doublet (photographic), - draconis, [gamma], - draconis, [beta], driessen, dry plate, dublin, earthquakes, earth's pole, - eccentricity, , eclipses, - edinburgh, eduarda, egeria, endymion, eriphyla, eros, , , , , , eulerian, , evelyn, exposure, times of, , faculæ, faraday, flamsteed, , , fleming, mrs., flora, foulkes, martin, french academy, , , galileo, , galle, , , , , , gasparis, gauge (railways), gauss, - geminorum, h., george iii., , "georgian," _georgium sidus_, , , gill, sir d., , , , gilliss, gotha, gould, graham, , gravitation, law of, , , , , greaves, greenwich observatory, - , , , - , , - , , , , , gregory, , hale, g. e., , hall, a., , halley, - , , - , hansen, , harkness, hartwig, harvard college observatory, , , , hebe, hegel, heidelberg, heliometer, , helium, helsingfors, hencke, , , , , henry brothers, - herschel, sir john, , , herschel, sir william, - , , , , herschel (uranus), , hind, , , , hooke, , hubbard, humboldt, hussey, rev. t. j., , hygeia, ilmata, industria, ingeborg, instruments at greenwich, - iris, , , , janson, jevons, johnson, m., , juno, , , jupiter, , , , , , ; satellites, , keats, - , , keill, , , , kelvin, lord, , kepler, , kew, , , , kiel, kimura, königsberg, küstner, , , lalande, , , , lameia, laplace, la plata, latitude variation, , , , , - lemonnier, , , le verrier, , - libussa, lick observatory, _liouville's journal_, lisbon, longitude of, london, , , long, longitude, , lowth, bishop, lyrae, [alpha], , macclesfield, earl of, , mädler, magnetic observations, , , magnitude equation, markree, mars, , , , , , mayer, measurement of plates, - _mécanique céleste_, melbourne, , memorandum (adams), mercury, messier, meteorites, meteors (november), metis, , micrometer, , milky way, minerva, minor planets, - minor planets tables, , , mistakes, - molyneux, samuel, - , , monte video, moon, tables of, names of minor planets, - nasmyth, "nautical almanac," nebula, , - neptune, , , - , new college lane, newcomb, simon, , , , - , , new stars, , - newton, , , - , , new york, longitude, ninina, northleach, northumberland, nova geminorum, , , nova persei, , - nutation, , , , , , , , _observatory_ (magazine), ocllo, olbers, - olympic games, oriani, ornamenta, oxford university, - , , - oxford university observatory, , , , , , , palermo, observatory of, palisa, pallas, , , parallax, , , - , , paris, parkhurst, j. a., parthenope, peirce, , - pendulum, perseus, , personal equation, , , , perth, perturbations of uranus, , , , , , , peters, , phaëtusa, philosopher, , _philosophical transactions_, , , photographica, photographic methods, , , , - ; lenses, , photographs of sun, , - piazzi, - , pickering, e. c., , pittsburghia, plana, planetary distances, ; commission, ; numbering, planets by photography, pole star (_polaris_), , , , pond, , potsdam, , pound, mrs., , - pound, rev. james, - , , prague, precession, , prymno, puiseux, pulfrich, pulkowa, - , quadrants at greenwich, radium, radius vector, - , - , , rayleigh, lord, records before discovery, reflector, , , reflex zenith tube, , refraction, , - , refractor, , réseau, residual phenomena, - , , , rigaud, s. p., , , rome, rothschild, royal astronomical society, , , , , , , royal society, , , , , sampson, r. a., - , san fernando, santiago, sappho, , saturn, , , , , savile, sir h., savilian professorship, - , - schmidt, julius, , schuster, a., schwabe, - , , sheldonian theatre, sherbourn, solar eclipse, , - spectro-heliograph, , star-maps, , , , "star-trap," stereo-comparator, stone, e. j., struve, , , sun's distance, - sun-spots, - sydney observatory, tacubaya observatory, telescopes, , - thames river, themistocles, _theoria motus_, theory and observation, thomson, sir w., , tides, titius, toulouse observatory, tycho brahé, , , uranus, - , , - , , variable stars, variation of latitude, , , , , - venus, , ; diameter of, ; transit of, - , vesta, , victoria, , , , von zach, wallace, wansted, - , , , , , ward, washington observatory, - , , , weather and sun-spots, , - weyer, whiteside, williams, mrs. e., , wind-vane, revolutions, - winnecke, wolf, dr. max, wolf, rudolf, wren, sir c., yerkes observatory, , , , , , zeiss, zodiac, , , the end printed by ballantyne, hanson & co. edinburgh & london mr. edward arnold's list of scientific and technical books. =lectures on diseases of children.= by robert hutchison, m.d. 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[ ] the facts were collected with great care and ability by s. p. rigaud, and published by the oxford university press in as "miscellaneous works and correspondence of the rev. james bradley." [ ] since the light must travel from the sun to saturn _and back again to the earth_, the interval would be more nearly minutes. [ ] monthly notices of the royal astronomical society, vol. xvii. p. . [ ] this should be cambridge, _mass._ [ ] the distances do not represent the _total_ displacement, but only the displacement towards washington in one case and towards pulkowa in the other. transcriber's notes: passages in italics are indicated by _italics_. passages in bold are indicated by =bold=. subscripted letters are indicated by {subscript}. the original text includes the greek a, b, and g. for this text version these letters are presented as [alpha], [beta], and [gamma]. all side notes belonging to a single paragraph have been moved to the beginning of the paragraph. sidenotes split across pages have been joined together. punctuation has been corrected without note. corrections in the "errata" have been made in this text version. the following misprints have been corrected: "hencke'" corrected to "hencke's" (page sidenote) "annouced" corrected to "announced" (page sidenote) "are are" corrected to "are" (page ) "konigsberg" corrected to "königsberg" (index) other than the corrections listed above, inconsistencies in spelling and hyphenation have been retained from the original. none note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) other worlds by garrett p. serviss. * * * * * * other worlds. their nature and possibilities in the light of the latest discoveries. illustrated. mo. cloth, $ . net; postage additional. no science has ever equaled astronomy in its appeal to the imagination, and recently popular interest in the wonders of the starry heavens has been stimulated by surprising discoveries and imaginary discoveries, as well as by a marked tendency of writers of fiction to include other worlds and their possible inhabitants within the field of romance. mr. serviss's new book on "other worlds, their nature and possibilities in the light of the latest discoveries," summarizes what is known. with helpful illustrations, the most interesting facts about the planets venus, mars, jupiter, saturn, etc., as well as about the nearest of all other worlds, the moon, are presented in a popular manner, and always from the point of view of human interest--a point that is too seldom taken by writers on science. astronomy with an opera-glass. a popular introduction to the study of the starry heavens with the simplest of optical instruments. illustrated. vo. cloth, $ . . "by its aid thousands of people who have resigned themselves to the ignorance in which they were left at school, by our wretched system of teaching by the book only, will thank mr. serviss for the suggestions he has so well carried out."--_new york times._ pleasures of the telescope. a descriptive guide to amateur astronomers and all lovers of the stars. illustrated. vo. cloth, $ . . "the volume will be found interesting by those for whom it is written, and will inspire many with a love for the study of astronomy, one of the most far-reaching of the sciences."--_milwaukee journal._ d. appleton and company, new york. * * * * * * [illustration: chart of mars. after schiaparelli.] other worlds their nature, possibilities and habitability in the light of the latest discoveries. by garrett p. serviss author of "astronomy with an opera-glass" and "pleasures of the telescope" with charts and illustrations "shall we measure the councils of heaven by the narrow impotence of human faculties, or conceive that silence and solitude reign throughout the mighty empire of nature?" --dr. thomas chalmers. new york d. appleton and company copyright, , by d. appleton and company. to the memory of william jay youmans. preface the point of view of this book is human interest in the other worlds around us. it presents the latest discoveries among the planets of the solar system, and shows their bearing upon the question of life in those planets. it points out the resemblances and the differences between the earth and the other worlds that share with it in the light of the sun. it shows what we should see and experience if we could visit those worlds. while basing itself upon facts, it does not exclude the discussion of interesting probabilities and theories that have commanded wide popular attention. it points out, for instance, what is to be thought of the idea of interplanetary communication. it indicates what must be the outlook of the possible inhabitants of some of the other planets toward the earth. as far as may be, it traces the origin and development of the other worlds of our system, and presents a graphic picture of their present condition as individuals, and of their wonderful contrasts as members of a common family. in short, the aim of the author has been to show how wide, and how rich, is the field of interest opened to the human mind by man's discoveries concerning worlds, which, though inaccessible to him in a physical sense, offer intellectual conquests of the noblest description. and, finally, in order to assist those who may wish to recognize for themselves these other worlds in the sky, this book presents a special series of charts to illustrate a method of finding the planets which requires no observatory and no instruments, and only such knowledge of the starry heavens as anybody can easily acquire. g.p.s. borough of brooklyn, new york city, _september, ._ contents chapter i _introductory_ remarkable popular interest in questions concerning other worlds and their inhabitants--theories of interplanetary communication--the plurality of worlds in literature--romances of foreign planets--scientific interest in the subject--opposing views based on telescopic and spectroscopic revelations--changes of opinion--desirability of a popular presentation of the latest facts--the natural tendency to regard other planets as habitable--some of the conditions and limitations of the problem--the solar system viewed from outer space--the resemblances and contrasts of its various planets--three planetary groups recognized--the family character of the solar system chapter ii _mercury, a world of two faces and many contrasts_ grotesqueness of mercury considered as a world--its dimensions, mass, and movements--the question of an atmosphere--mercury's visibility from the earth--its eccentric orbit, and rapid changes of distance from the sun--momentous consequences of these peculiarities--a virtual fall of fourteen million miles toward the sun in six weeks--the tremendous heat poured upon mercury and its great variations--the little planet's singular manner of rotation on its axis--schiaparelli's astonishing discovery--a day side and a night side--interesting effects of libration--the heavens as viewed from mercury--can it support life? chapter iii _venus, the twin of the earth_ a planet that matches ours in size--its beauty in the sky--remarkable circularity of its orbit--probable absence of seasons and stable conditions of temperature and weather on venus--its dense and abundant atmosphere--seeing the atmosphere of venus from the earth--is the real face of the planet hidden under an atmospheric veil?--conditions of habitability--all planetary life need not be of the terrestrial type--the limit fixed by destructive temperature--importance of air and water in the problem--reasons why venus may be a more agreeable abode than the earth--splendor of our globe as seen from venus--what astronomers on venus might learn about the earth--a serious question raised--does venus, like mercury, rotate but once in the course of a revolution about the sun?--reasons for and against that view chapter iv _mars, a world more advanced than ours_ resemblances between mars and the earth--its seasons and its white polar caps--peculiar surface markings--schiaparelli's discovery of the canals--his description of their appearance and of their duplication--influence of the seasons on the aspect of the canals--what are the canals?--mr. lowell's observations--the theory of irrigation--how the inhabitants of mars are supposed to have taken advantage of the annual accession of water supplied by the melting of the polar caps--wonderful details shown in charts of mars--curious effects that may follow from the small force of gravity on mars--imaginary giants--reasons for thinking that mars may be, in an evolutionary sense, older than the earth--speculations about interplanetary signals from mars, and their origin--mars's atmosphere--the question of water--the problem of temperature--eccentricities of mars's moons chapter v _the asteroids, a family of dwarf worlds_ only four asteroids large enough to be measured--remarkable differences in their brightness irrespective of size--their widely scattered and intermixed orbits--eccentric orbit of eros--the nearest celestial body to the earth except the moon--its existence recorded by photography before it was discovered--its great and rapid fluctuations in light, and the curious hypotheses based upon them--is it a fragment of an exploded planet?--the startling theory of olbers as to the origin of the asteroids revived--curious results of the slight force of gravity on an asteroid--an imaginary visit to a world only twelve miles in diameter chapter vi _jupiter, the greatest of known worlds_ jupiter compared with our globe--his swift rotation on his axis--remarkable lack of density--the force of gravity on jupiter--wonderful clouds--strange phenomena of the great belts--brilliant display of colors--the great red spot and the many theories it has given rise to--curious facts about the varying rates of rotation of the huge planet's surface--the theory of a hidden world in jupiter--when jupiter was a companion star to the sun--the miracle of world-making before our eyes--are jupiter's satellites habitable?--magnificent spectacles in the jovian system chapter vii _saturn, a prodigy among planets_ the wonder of the great rings--saturn's great distance and long year--the least dense of all the planets--it would float in water--what kind of a world is it?--sir humphry davy's imaginary inhabitants of saturn--facts about the rings, which are a phenomenon unparalleled in the visible universe--the surprising nature of the rings, as revealed by mathematics and the spectroscope--the question of their origin and ultimate fate--dr. dick's idea of their habitability--swedenborg's curious description of the appearance of the rings from saturn--is saturn a globe of vapor, or of dust?--the nine satellites and "roche's limit"--the play of spectacular shadows in the saturnian system--uranus and neptune--is there a yet undiscovered planet greater than jupiter? chapter viii _the moon, child of the earth and the sun_ the moon a favorite subject for intellectual speculation--its nearness to the earth graphically illustrated--ideas of the ancients--galileo's discoveries--what first raised a serious question as to its habitability--singularity of the moon's motions--appearance of its surface to the naked eye and with the telescope--the "seas" and the wonderful mountains and craters--a terrible abyss described--tycho's mysterious rays--difference between lunar and terrestrial volcanoes--mountain-ringed valleys--gigantic cracks in the lunar globe--slight force of gravity of the moon and some interesting deductions--the moon a world of giantism--what kind of atmospheric gases can the moon contain--the question of water and of former oceans--the great volcanic cataclysm in the moon's history--evidence of volcanic and other changes now occurring--is there vegetation on the moon?--lunar day and night--the earth as seen from the moon--discoveries yet to be made chapter ix _how to find the planets_ it is easy to make acquaintance with the planets and to follow them among the stars--the first step a knowledge of the constellations--how this is to be acquired--how to use the nautical almanac in connection with the charts in this book--the visibility of mercury and venus--the oppositions of mars, jupiter, and saturn index list of illustrations page chart of mars _frontispiece_ diagram showing causes of day and night on portions of mercury regions of day and night on mercury venus's atmosphere seen as a ring of light view of jupiter _facing_ three views of saturn _facing_ diagram showing the moon's path through space the lunar alps, apennines, and caucasus _facing_ the moon at first and last quarter _facing_ phases and rotation of the moon charts showing the zodiacal constellations: . from right ascension hours to hours . " " " " " . " " " " " . " " " " " . " " " " " . " " " " " other worlds chapter i introductory other worlds and their inhabitants are remarkably popular subjects of speculation at the present time. every day we hear people asking one another if it is true that we shall soon be able to communicate with some of the far-off globes, such as mars, that circle in company with our earth about the sun. one of the masters of practical electrical science in our time has suggested that the principle of wireless telegraphy may be extended to the transmission of messages across space from planet to planet. the existence of intelligent inhabitants in some of the other planets has become, with many, a matter of conviction, and for everybody it presents a question of fascinating interest, which has deeply stirred the popular imagination. the importance of this subject as an intellectual phenomenon of the opening century is clearly indicated by the extent to which it has entered into recent literature. poets feel its inspiration, and novelists and romancers freely select other planets as the scenes of their stories. one tells us of a visit paid by men to the moon, and of the wonderful things seen, and adventures had, there. lucian, it is true, did the same thing eighteen hundred years ago, but he had not the aid of hints from modern science to guide his speculations and lend verisimilitude to his narrative. another startles us from our sense of planetary security with a realistic account of the invasion of the earth by the terrible sons of warlike mars, seeking to extend their empire by the conquest of foreign globes. sometimes it is a trip from world to world, a kind of celestial pleasure yachting, with depictions of creatures more wonderful than-- "the anthropophagi and men whose heads do grow beneath their shoulders"-- that is presented to our imagination; and sometimes we are informed of the visions beheld by the temporarily disembodied spirits of trance mediums, or other modern thaumaturgists, flitting about among the planets. then, to vary the theme, we find charming inhabitants of other worlds represented as coming down to the earth and sojourning for a time on our dull planet, to the delight of susceptible successors of father adam, who become, henceforth, ready to follow their captivating visitors to the ends of the universe. in short, writers of fiction have already established interplanetary communication to their entire satisfaction, thus vastly and indefinitely enlarging the bounds of romance, and making us so familiar with the peculiarities of our remarkable brothers and sisters of mars, venus, and the moon, that we can not help feeling, notwithstanding the many divergences in the descriptions, that we should certainly recognize them on sight wherever we might meet them. but the subject is by no means abandoned to the tellers of tales and the dreamers of dreams. men of science, also, eagerly enter into the discussion of the possibilities of other worlds, and become warm over it. around mars, in particular, a lively war of opinions rages. not all astronomers have joined in the dispute--some have not imagination enough, and some are waiting for more light before choosing sides--but those who have entered the arena are divided between two opposed camps. one side holds that mars is not only a world capable of having inhabitants, but that it actually has them, and that they have given visual proof of their existence and their intelligence through the changes they have produced upon its surface. the other side maintains that mars is neither inhabited nor habitable, and that what are taken for vast public works and engineering marvels wrought by its industrious inhabitants, are nothing but illusions of the telescope, or delusions of the observer's mind. both adduce numerous observations, telescopic and spectroscopic, and many arguments, scientific and theoretic, to support their respective contentions, but neither side has yet been able to convince or silence the other, although both have made themselves and their views intensely interesting to the world at large, which would very much like to know what the truth really is. and not only mars, but venus--the beauteous twin sister of the earth, who, when she glows in the evening sky, makes everybody a lover of the stars--and even mercury, the moor among the planets, wearing "the shadowed livery of the burnished sun," to whom he is "a neighbor and near bred," and jupiter, saturn, and the moon itself--all these have their advocates, who refuse to believe that they are lifeless globes, mere reflectors of useless sunshine. the case of the moon is, in this respect, especially interesting, on account of the change that has occurred in the opinions held concerning its physical condition. for a very long time our satellite was confidently, and almost universally, regarded as an airless, waterless, lifeless desert, a completely "dead world," a bare, desiccated skull of rock, circling about the living earth. but within a few years there has been a reaction from this extreme view of the lifelessness of the moon. observers tell us of clouds suddenly appearing and then melting to invisibility over volcanic craters; of evidences of an atmosphere, rare as compared with ours, yet manifest in its effects; of variations of color witnessed in certain places as the sunlight drifts over them at changing angles of incidence; of what seem to be immense fields of vegetation covering level ground, and of appearances indicating the existence of clouds of ice crystals and deposits of snow among the mountainous lunar landscapes. thus, in a manner, the moon is rehabilitated, and we are invited to regard its silvery beams not as the reflections of the surface of a desert, but as sent back to our eyes from the face of a world that yet has some slight remnants of life to brighten it. the suggestion that there is an atmosphere lying close upon the shell of the lunar globe, filling the deep cavities that pit its face and penetrating to an unknown depth in its interior, recalls a speculation of the ingenious and entertaining fontenelle, in the seventeenth century--recently revived and enlarged upon by the author of one of our modern romances of adventure in the moon--to the effect that the lunar inhabitants dwell beneath the surface of their globe instead of on the top of it. now, because of this widespread and continually increasing interest in the subject of other worlds, and on account of the many curious revelations that we owe to modern telescopes and other improved means of investigation, it is certainly to be desired that the most important and interesting discoveries that have lately been made concerning the various globes which together with the earth constitute the sun's family, should be assembled in a convenient and popular form--and that is the object of this book. fact is admittedly often stranger and more wonderful than fiction, and there are no facts that appeal more powerfully to the imagination than do those of astronomy. technical books on astronomy usually either ignore the subject of the habitability of the planets, or dismiss it with scarcely any recognition of the overpowering human interest that it possesses. hence, a book written specially from the point of view of that subject would appear calculated to meet a popular want; and this the more, because, since mr. proctor wrote his other worlds than ours and m. flammarion his pluralité des mondes habités, many most important and significant discoveries have been made that, in several notable instances, have completely altered the aspect in which the planets present themselves for our judgment as to their conditions of habitability. no doubt the natural tendency of the mind is to regard all the planets as habitable worlds, for there seems to be deeply implanted in human nature a consciousness of the universality of life, giving rise to a conviction that one world, even in the material sense, is not enough for it, but that every planet must belong to its kingdom. we are apt to say to ourselves: "the earth is one of a number of planets, all similarly circumstanced; the earth is inhabited, why should not the others also be inhabited?" what has been learned of the unity in chemical constitution and mechanical operation prevailing throughout the solar system, together with the continually accumulating evidence of the common origin of its various members, and the identity of the evolutionary processes that have brought them into being, all tends to strengthen the _a priori_ hypothesis that life is a phenomenon general to the entire system, and only absent where its essential and fundamental conditions, for special and local, and perhaps temporary, reasons, do not exist. if we look for life in the sun, for instance, while accepting the prevalent conception of the sun as a center of intense thermal action, we must abandon all our ideas of the physical organization of life formed upon what we know of it from experimental evidence. we can not imagine any form of life that has ever been presented to our senses as existing in the sun. but this is not generally true of the planets. life, in our sense of it, is a planetary, not a solar, phenomenon, and while we may find reasons for believing that on some of the planets the conditions are such that creatures organized like ourselves could not survive, yet we can not positively say that every form of living organism must necessarily be excluded from a world whose environment would be unsuited for us and our contemporaries in terrestrial life. although our sole knowledge of animated nature is confined to what we learn by experience on the earth, yet it is a most entertaining, and by no means unedifying, occupation, to seek to apply to the exceedingly diversified conditions prevailing in the other planets, as astronomical observations reveal them to us, the principles, types, and limitations that govern the living creatures of our world, and to judge, as best we can, how far those types and limits may be modified or extended so that those other planets may reasonably be included among the probable abodes of life. in order to form such judgments each planet must be examined by itself, but first it is desirable to glance at the planetary system as a whole. to do this we may throw off, in imagination, the dominance of the sun, and suppose ourselves to be in the midst of open space, far removed both from the sun and the other stars. in this situation it is only by chance, or through foreknowledge, that we can distinguish our sun at all, for it is lost among the stars; and when we discover it we find that it is only one of the smaller and less conspicuous members of the sparkling host. we rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude, until now it enormously outshines all the other stars, and its rays begin to produce the effect of daylight upon the orbs that they reach. but we are in no danger of mistaking its apparent superiority to its fellow stars for a real one, because we clearly perceive that our nearness alone makes it seem so great and overpowering. and now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. it is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. although they vary in relative size, yet we observe a striking similarity among them. they are all globular bodies, they all turn upon their axes, they all travel about the sun in the same direction, and their paths all lie very nearly in one plane. some of them have one or more moons, or satellites, circling about them in imitation of their own revolution about the sun. their family relationship to one another and to the sun is so evident that it colors our judgment about them as individuals; and when we happen to find, upon closer approach, that one of them, the earth, is covered with vegetation and water and filled with thousands of species of animated creatures, we are disposed to believe, without further examination, that they are all alike in this respect, just as they are all alike in receiving light and heat from the sun. this preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail. one striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. the solar system occupies a disk-shaped, or flat circular, expanse, about , , , miles across and relatively very thin, the sun being in the center. from the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. but the vast majority of the stars are probably a hundred times yet more remote. in other words, if the solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles! or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the pacific ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it! from these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. besides the earth, the sun has seven other principal planets in attendance. these eight planets fall into two classes--the terrestrial planets and the major, or jovian, planets. the former class comprises mercury, venus, the earth, and mars, and the latter jupiter, saturn, uranus, and neptune. i have named them all in the order of their distance from the sun, beginning with the nearest. the terrestrial planets, taking their class name from _terra_, the earth, are relatively close to the sun and comparatively small. the major planets--or the jovian planets, if we give them a common title based upon the name of their chief, jupiter or jove--are relatively distant from the sun and are characterized both by great comparative size and slight mean density. the terrestrial planets are all included within a circle, having the sun for a center, about , , miles in radius. the space, or gap, between the outermost of them, mars, and the innermost of the jovian planets, jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. and not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. between jupiter and saturn is a gap , , miles across, and this becomes , , miles between saturn and uranus, and more than , , , miles between uranus and neptune. all of these distances are given in round numbers. finally, we come to some very extraordinary worlds--if we can call them worlds at all--the asteroids. they form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. they are situated in the gap between the terrestrial and the jovian planets, and about of them have been discovered, while there is reason to think that their real number may be many thousands. the largest of them is less than miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. what marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. but without regard to the question of habitability, the asteroids will be found extremely interesting. in the next chapter we proceed to take up the planets for study as individuals, beginning with mercury, the one nearest the sun. chapter ii mercury, a world of two faces and many contrasts mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of chinese ivory carving, so small is it for its kind and so finished in its eccentric details. in a little while we shall see how singular mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. its diameter, , miles, only exceeds the moon's by less than one half, while both jupiter and saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet mercury. but, insignificant though it be in size, it holds the place of honor, nearest to the sun. it was formerly thought that mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury. this led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet. but more recent, and probably more correct, computations place mercury third in the order of density among the members of the solar system, the earth ranking as first and venus as second. mercury's density is now believed to be less than the earth's in the ratio of to . accepting this estimate, we find that the force of gravity upon the surface of mercury is only one third as great as upon the surface of the earth--i.e., a body weighing pounds on the earth would weigh only pounds on mercury. this is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own. this alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization. one of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere. we should wish to know in advance if it had air and water, and in what proportions and quantities. however its own peculiar inhabitants might be supposed able to dispense with these things, to _us_ their presence would be essential, and if we did not find them, even a planet that blazed with gold and diamonds only waiting to be seized would remain perfectly safe from our invasion. now, in the case of mercury, some doubt on this point exists. messrs. huggins, vogel, and others have believed that they found spectroscopic proof of the existence of both air and the vapor of water on mercury. but the necessary observations are of a very delicate nature, and difficult to make, and some astronomers doubt whether we possess sufficient proof that mercury has an atmosphere. at any rate, its atmosphere is very rare as compared with the earth's, but we need not, on that account, conclude that mercury is lifeless. possibly, in view of certain other peculiarities soon to be explained, a rare atmosphere would be decidedly advantageous. being much nearer the sun than the earth is, mercury can be seen by us only in the same quarter of the sky where the sun itself appears. as it revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night. when seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it. when it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays. consequently, except with the instruments of an observatory, which are able to show it in broad day, mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun. the nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and mercury, being the nearest to the sun of all the planets, is by far the swiftest footed among them. but its velocity is subject to remarkable variation, owing to the peculiar form of the orbit in which the planet travels. this is more eccentric than the orbit of any other planet, except some of the asteroids. the sun being situated in one focus of the elliptical orbit, when mercury is at perihelion, or nearest to the sun, its distance from that body is , , miles, but when it is at aphelion, or farthest from the sun, its distance is , , miles. the difference is no less than , , miles! when nearest the sun mercury darts forward in its orbit at the rate of twenty-nine miles in a second, while when farthest from the sun the speed is reduced to twenty-three miles. now, let us return for a moment to the consideration of the wonderful variations in mercury's distance from the sun, for we shall find that their effects are absolutely startling, and that they alone suffice to mark a wide difference between mercury and the earth, considered as the abodes of sentient creatures. the total change of distance amounts, as already remarked, to , , miles, which is almost half the entire distance separating the planet from the sun at perihelion. this immense variation of distance is emphasized by the rapidity with which it takes place. mercury's periodic time, i.e., the period required for it to make a single revolution about the sun--or, in other words, the length of its year--is eighty-eight of our days. in just one half of that time, or in about six weeks, it passes from aphelion to perihelion; that is to say, in six weeks the whole change in its distance from the sun takes place. in six weeks mercury falls , , miles--for it _is_ a fall, though in a curve instead of a straight line--falls , , miles toward the sun! and, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede. at the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward. of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall! next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination. first, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance. the earth's distance from the sun being , , miles, while mercury's is only , , , it follows, to begin with, that mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does. that alone is enough to make it seem impossible that mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer's day increased six or seven fold! if there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam. still, notwithstanding the tremendous heat poured upon mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there. we might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays. but, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of mercury, how are we going to meet the conditions imposed by the rapid changes of distance--the swift fall of the planet toward the sun, followed by the equally swift rush away from it? for change of distance implies change of heat and temperature. it is true that we have a slight effect of this kind on the earth. between midsummer (of the northern hemisphere) and midwinter our planet draws , , miles nearer the sun, but the change occupies six months, and, at the earth's great average distance, the effect of this change is too slight to be ordinarily observable, and only the astronomer is aware of the consequent increase in the apparent size of the sun. it is not to this variation of the sun's distance, but rather to the changes of the seasons, depending on the inclination of the earth's axis, that we owe the differences of temperature that we experience. in other words, the total supply of heat from the sun is not far from uniform at all times of the year, and the variations of temperature depend upon the distribution of that supply between the northern and southern hemispheres, which are alternately inclined sunward. but on mercury the supply of solar heat is itself variable to an enormous extent. in six weeks, as we have seen, mercury diminishes its distance from the sun about one third, which is proportionally ten times as great a change of distance as the earth experiences in six months. the inhabitants of mercury in those six pregnant weeks see the sun expand in the sky to more than two and a half times its former magnitude, while the solar heat poured upon them swiftly augments from something more than four and a half times to above eleven times the amount received upon the earth! then, immediately, the retreat of the planet begins, the sun visibly shrinks, as a receding balloon becomes smaller in the eyes of its watchers, the heat falls off as rapidly as it had previously increased, until, the aphelion point being reached, the process is again reversed. and thus it goes on unceasingly, the sun growing and diminishing in the sky, and the heat increasing and decreasing by enormous amounts with astonishing rapidity. it is difficult to imagine any way in which atmospheric influences could equalize the effects of such violent changes, or any adjustments in the physical organization of living beings that could make such changes endurable. but we have only just begun the story of mercury's peculiarities. we come next to an even more remarkable contrast between that planet and our own. during the paris exposition of a little company of astronomers was assembled at the juvisy observatory of m. flammarion, near the french capital, listening to one of the most surprising disclosures of a secret of nature that any _savant_ ever confided to a few trustworthy friends while awaiting a suitable time to make it public. it was a secret as full of significance as that which galileo concealed for a time in his celebrated anagram, which, when at length he furnished the key, still remained a riddle, for then it read: "the mother of the loves imitates the shapes of cynthia," meaning that the planet venus, when viewed with a telescope, shows phases like those of the moon. the secret imparted in confidence to the knot of astronomers at juvisy came from a countryman of galileo's, signor g. v. schiaparelli, the director of the observatory of milan, and its purport was that the planet mercury always keeps the same face directed toward the sun. schiaparelli had satisfied himself, by a careful series of observations, of the truth of his strange announcement, but before giving it to the world he determined to make doubly sure. early in he withdrew the pledge of secrecy from his friends and published his discovery. no one can wonder that the statement was generally received with incredulity, for it was in direct contradiction to the conclusions of other astronomers, who had long believed that mercury rotated on its axis in a period closely corresponding with that of the earth's rotation--that is to say, once every twenty-four hours. schiaparelli's discovery, if it were received as correct, would put mercury, as a planet, in a class by itself, and would distinguish it by a peculiarity which had always been recognized as a special feature of the moon, viz., that of rotating on its axis in the same period of time required to perform a revolution in its orbit, and, while this seemed natural enough for a satellite, almost nobody was prepared for the ascription of such eccentric conduct to a planet. the italian astronomer based his discovery upon the observation that certain markings visible on the disk of mercury remained in such a position with reference to the direction of the sun as to prove that the planet's rotation was extremely slow, and he finally satisfied himself that there was but one rotation in the course of a revolution about the sun. that, of course, means that one side of mercury always faces toward the sun while the opposite side always faces away from it, and neither side experiences the alternation of day and night, one having perpetual day and the other perpetual night. the older observations, from which had been deduced the long accepted opinion that mercury rotated, like the earth, once in about twenty-four hours, had also been made upon the markings on the planet's disk, but these are not easily seen, and their appearances had evidently been misinterpreted. the very fact of the difficulty of seeing any details on mercury tended to prevent or delay corroboration of schiaparelli's discovery. but there were two circumstances that contributed to the final acceptance of his results. one of these was his well-known experience as an observer and the high reputation that he enjoyed among astronomers, and the other was the development by prof. george darwin of the theory of tidal friction in its application to planetary evolution, for this furnished a satisfactory explanation of the manner in which a body, situated as near the sun as mercury is, could have its axial rotation gradually reduced by the tidal attraction of the sun until it coincided in period with its orbital revolution. accepting the accuracy of schiaparelli's discovery, which was corroborated in every particular in by percival lowell in a special series of observations on mercury made with his -inch telescope at flagstaff, arizona, and which has also been corroborated by others, we see at once how important is its bearing on the habitability of the planet. it adds another difficulty to that offered by the remarkable changes of distance from the sun, and consequent variations of heat, which we have already discussed. in order to bring the situation home to our experience, let us, for a moment, imagine the earth fallen into mercury's dilemma. there would then be no succession of day and night, such as we at present enjoy, and upon which not alone our comfort but perhaps our very existence depends, but, instead, one side of our globe--it might be the asiatic or the american half--would be continually in the sunlight, and the other side would lie buried in endless night. and this condition, so suggestive of the play of pure imagination, this plight of being a two-faced world, like the god janus, one face light and the other face dark, must be the actual state of things on mercury. there is one interesting qualification. in the case just imagined for the earth, supposing it to retain the present inclination of its axis while parting with its differential rotation, there would be an interchange of day and night once a year in the polar regions. on mercury, whose axis appears to be perpendicular, a similar phenomenon, affecting not the polar regions but the eastern and western sides of the planet, is produced by the extraordinary eccentricity of its orbit. as the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun. but this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis. the latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun. the accompanying figure may assist the explanation. [illustration: diagram showing that, owing to the eccentricity of its orbit, and its varying velocity, mercury, although making but one turn on its axis in the course of a revolution about the sun, nevertheless experiences on parts of its surface the alternation of day and night.] let us start with mercury in perihelion at the point _a_. the little cross on the planet stands exactly under the sun and in the center of the illuminated hemisphere. the large arrows show the direction in which the planet travels in its revolution about the sun, and the small curved arrows the direction in which it rotates on its axis. now, in moving along its orbit from _a_ to _b_ the planet, partly because of its swifter motion when near the sun, and partly because of the elliptical nature of the orbit, traverses a greater angular interval with reference to the sun than the cross, moving with the uniform rotation of the planet on its axis, is able to traverse in the same time. as drawn in the diagram, the cross has moved through exactly ninety degrees, or one right angle, while the planet in its orbit has moved through considerably more than a right angle. in consequence of this gain of the angle of revolution upon the angle of rotation, the cross at _b_ is no longer exactly under the sun, nor in the center of the illuminated hemisphere. it appears to have shifted its position toward the west, while the hemispherical cap of sunshine has slipped eastward over the globe of the planet. in the next following section of the orbit the planet rotates through another right angle, but, owing to increased distance from the sun, the motion in the orbit now becomes slower until, when the planet arrives at aphelion, _c_, the angular difference disappears and the cross is once more just under the sun. on returning from aphelion to perihelion the same phenomena recur in reverse order and the line between day and night on the planet first shifts westward, attaining its limit in that respect at _d_, and then, at perihelion, returns to its original position. now, if we could stand on the sunward hemisphere of mercury what, to our eyes, would be the effect of this shifting of the sun's position with regard to a fixed point on the planet's surface? manifestly it would cause the sun to describe a great arc in the sky, swinging to and fro, in an east and west line, like a pendulum bob, the angular extent of the swing being a little more than forty-seven degrees, and the time required for the sun to pass from its extreme eastern to its extreme western position and back again being eighty-eight days. but, owing to the eccentricity of the orbit, the sun swings much faster toward the east than toward the west, the eastward motion occupying about thirty-seven days and the westward motion about fifty-one days. [illustration: the regions of perpetual day, perpetual night, and alternate day and night on mercury. in the left-hand view the observer looks at the planet in the plane of its equator; in the right-hand view he looks down on its north pole.] another effect of the libratory motion of the sun as seen from mercury is represented in the next figure, where we have a view of the planet showing both the day and the night hemisphere, and where we see that between the two there is a region upon which the sun rises and sets once every eighty-eight days. there are, in reality, two of these lune-shaped regions, one at the east and the other at the west, each between , and , miles broad at the equator. at the sunward edge of these regions, once in eighty-eight days, or once in a mercurial year, the sun rises to an elevation of forty-seven degrees, and then descends again straight to the horizon from which it rose; at the nightward edge, once in eighty-eight days, the sun peeps above the horizon and quickly sinks from sight again. the result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year--and on mercury a year is eighty-eight days. one half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night. and now again, what of life in such a world as that? on the night side, where no sunshine ever penetrates, the temperature must be extremely low, hardly greater than the fearful cold of open space, unless modifying influences beyond our ken exist. it is certain that if life flourishes there, it must be in such forms as can endure continual darkness and excessive cold. some heat would be carried around by atmospheric circulation from the sunward side, but not enough, it would seem, to keep water from being perpetually frozen, or the ground from being baked with unrelaxing frost. it is for the imagination to picture underground dwellings, artificial sources of heat, and living forms suited to unearthlike environment. what would be the mental effects of perpetual night upon a race of intelligent creatures doomed to that condition? perhaps not quite so grievous as we are apt to think. the constellations in all their splendor would circle before their eyes with the revolution of their planet about the sun, and with the exception of the sun itself--which they could see by making a journey to the opposite hemisphere--all the members of the solar system would pass in succession through their mid-heaven, and two of them would present themselves with a magnificence of planetary display unknown on the earth. venus, when in opposition under the most favorable circumstances, is scarcely more than , , miles from mercury, and, showing herself at such times with a fully illuminated disk--as, owing to her position within the orbit of the earth, she never can do when at her least distance from us--she must be a phenomenon of unparalleled beauty, at least four times brighter than we ever see her, and capable, of course, of casting a strong shadow. the earth, also, is a splendid star in the midnight sky of mercury, and the moon may be visible to the naked eye as a little attendant circling about its brilliant master. the outer planets are slightly less conspicuous than they are to us, owing to increase of distance. the revolution of the heavens as seen from the night side of mercury is quite different in period from that which we are accustomed to, although the apparent motion is in the same direction, viz., from east to west. the same constellations remain above the horizon for weeks at a time, slowly moving westward, with the planets drifting yet more slowly, but at different rates, among them; the nearer planets, venus and the earth, showing the most decided tendency to loiter behind the stars. on the side where eternal sunlight shines the sky of mercury contains no stars. forever the pitiless blaze of day; forever, "all in a hot and copper sky the bloody sun at noon." as it is difficult to understand how water can exist on the night hemisphere, except in the shape of perpetual snow and ice, so it is hard to imagine that on the day hemisphere water can ever be precipitated from the vaporous form. in truth, there can be very little water on mercury even in the form of vapor, else the spectroscope would have given unquestionable evidence of its presence. those who think that mercury is entirely waterless and almost, if not quite, airless may be right. in these respects it would then resemble the moon, and, according to some observers, it possesses another characteristic lunar feature in the roughening of its surface by what seem to be innumerable volcanic craters. but if we suppose mercury to possess an atmosphere much rarer than that of the earth, we may perceive therein a possible provision against the excessive solar heat to which it is subjected, since, as we see on high mountains, a light air permits a ready radiation of heat, which does not become stored up as in a denser atmosphere. as the sun pours its heat without cessation upon the day hemisphere the warmed air must rise and flow off on all sides into the night hemisphere, while cold air rushes in below, to take its place, from the region of frost and darkness. the intermediate areas, which see the sun part of the time, as explained above, are perhaps the scene of contending winds and tempests, where the moisture, if there be any, is precipitated, through the rapid cooling of the air, in whelming floods and wild snow-storms driven by hurrying blasts from the realm of endless night. enough seems now to have been said to indicate clearly the hopelessness of looking for any analogies between mercury and the earth which would warrant the conclusion that the former planet is capable of supporting inhabitants or forms of life resembling those that swarm upon the latter. if we would still believe that mercury is a habitable globe we must depend entirely upon the imagination for pictures of creatures able to endure its extremes of heat and cold, of light and darkness, of instability, swift vicissitude, and violent contrast. in the next chapter we shall study a more peaceful and even-going world, yet one of great brilliancy, which possesses some remarkable resemblances to the earth, as well as some surprising divergences from it. chapter iii venus, the twin of the earth we come now to a planet which seems, at the first glance, to afford a far more promising outlook than mercury does for the presence of organic life forms bearing some resemblance to those of the earth. one of the strongest arguments for regarding venus as a world much like ours is based upon its remarkable similarity to the earth in size and mass, because thus we are assured that the force of gravity is practically the same upon the two planets, and the force of gravity governs numberless physical phenomena of essential importance to both animal and vegetable life. the mean diameter of the earth is , miles; that of venus is , miles. the difference is so slight that if the two planets were suspended side by side in the sky, at such a distance that their disks resembled that of the full moon, the eye would notice no inequality between them. the mean density of venus is about nine tenths of that of the earth, and the force of gravity upon its surface is in the ratio of about to as compared to its force on the surface of the earth. a man removed to venus would, consequently, find himself perceptibly lighter than he was at home, and would be able to exert his strength with considerably greater effect than on his own planet. but the difference would amount only to an agreeable variation from accustomed conditions, and would not be productive of fundamental changes in the order of nature. being, like mercury, nearer to the sun than the earth is, venus also is visible to us only in the morning or the evening sky. but her distance from the sun, slightly exceeding , , miles, is nearly double that of mercury, so that, when favorably situated, she becomes a very conspicuous object, and, instead of being known almost exclusively by astronomers, she is, perhaps, the most popular and most admired of all the members of the planetary system, especially when she appears in the charming rôle of the "evening star." as she emerges periodically from the blinding glare of the sun's immediate neighborhood and begins to soar, bright as an electric balloon, in the twilight, she commands all eyes and calls forth exclamations of astonishment and admiration by her singular beauty. the intervals between her successive reappearances in the evening sky, measured by her synodic period of days, are sufficiently long to give an element of surprise and novelty to every return of so dazzling a phenomenon. even the light of the full moon silvering the tree tops does not exercise greater enchantment over the mind of the contemplative observer. in either of her rôles, as morning or as evening star, venus has no rival. no fixed star can for an instant bear comparison with her. what she lacks in vivacity of light--none of the planets twinkles, as do all of the true stars--is more than compensated by the imposing size of her gleaming disk and the striking beauty of her clear lamplike rays. her color is silvery or golden, according to the state of the atmosphere, while the distinction of her appearance in a dark sky is so great that no eye can resist its attraction, and i have known an unexpected glimpse of venus to put an end to an animated conversation and distract, for a long time, the attention of a party of ladies and gentlemen from the social occupation that had brought them together. as a telescopic object venus is exceedingly attractive, even when considered merely from the point of view of simple beauty. both mercury and venus, as they travel about the sun, exhibit phases like those of the moon, but venus, being much larger and much nearer to the earth than mercury, shows her successive phases more effectively, and when she shines as a thin crescent in the morning or evening twilight, only a very slight magnifying power is required to show the sickle form of her disk. a remarkable difference between venus and mercury comes out as soon as we examine the shape of the former's orbit. venus's mean distance from the sun is , , miles, and her orbit is so nearly a circle, much more nearly than that of any other planet, that in the course of a revolution her distance from the sun varies less than a million miles. the distance of the earth varies , , miles, and that of mercury , , . her period of revolution, or the length of her year, is of our days. when she comes between the sun and the earth she approaches us nearer than any other planet ever gets, except the asteroid eros, her distance at such times being , , miles, or about one hundred and ten times the distance of the moon. being nearer to the sun in the ratio of to , venus receives almost twice as much solar light and heat as we get, but less than one third as much as mercury gets. there is reason to believe that her axis, instead of being considerably inclined, like that of the earth, is perpendicular to the plane of her orbit. thus venus introduces to us another novelty in the economy of worlds, for with a perpendicular axis of rotation she can have no succession of seasons, no winter and summer flitting, one upon the other's heels, to and fro between the northern and southern hemispheres; but, on the contrary, her climatic conditions must be unchangeable, and, on any particular part of her surface, except for local causes of variation, the weather remains the same the year around. so, as far as temperature is concerned, venus may have two regions of perpetual winter, one around each pole; two belts of perpetual spring in the upper middle latitudes, one on each side of the equator; and one zone of perpetual summer occupying the equatorial portion of the planet. but, of course, these seasonal terms do not strictly apply to venus, in the sense in which we employ them on the earth, for with us spring is characterized rather by the change in the quantity of heat and other atmospheric conditions that it witnesses than by a certain fixed and invariable temperature. to some minds it may appear very undesirable, from the point of view of animate existences, that there should be no alternation of seasons on the surface of a planet, but, instead, fixed conditions of climate; yet it is not clear that such a state of affairs might not be preferable to that with which we are familiar. even on the earth, we find that tropical regions, where the seasonal changes are comparatively moderate, present many attractions and advantages in contrast with the violent and often destructive vicissitudes of the temperate zones, and nature has shown us, within the pale of our own planet, that she is capable of bringing forth harvests of fruit and grain without the stimulus of alternate frost and sunshine. even under the reign of perpetual summer the fields and trees find time and opportunity to rest and restore their productive forces. the circularity of venus's orbit, and the consequently insignificant change in the sun's distance and heating effect, are other elements to be considered in estimating the singular constancy in the operation of natural agencies upon that interesting planet, which, twin of the earth though it be in stature, is evidently not its twin in temperament. and next as to the all-important question of atmosphere. in what precedes, the presence of an atmosphere has been assumed, and, fortunately, there is very convincing evidence, both visual and spectroscopic, that venus is well and abundantly supplied with air, by which it is not meant that venus's air is precisely like the mixture of oxygen and nitrogen, with a few other gases, which we breathe and call by that name. in fact, there are excellent reasons for thinking that the atmosphere of venus differs from the earth's quite as much as some of her other characteristics differ from those of our planet. but, however it may vary from ours in constitution, the atmosphere of venus contains water vapor, and is exceedingly abundant. listen to professor young: "its [venus's] atmosphere is probably from one and a half to two times as extensive and as dense as our own, and the spectroscope shows evidence of the presence of water vapor in it." and prof. william c. pickering, basing his statement on the result of observations at the mountain observatory of arequipa, says: "we may feel reasonably certain that at the planet's [venus's] surface the density of its atmosphere is many times that of our own." we do not have to depend upon the spectroscope for evidence that venus has a dense atmosphere, for we can, in a manner, _see_ her atmosphere, in consequence of its refractive action upon the sunlight that strikes into it near the edge of the planet's globe. this illumination of venus's atmosphere is witnessed both when she is nearly between the sun and the earth, and when, being exactly between them, she appears in silhouette against the solar disk. during a transit of this kind, in , many observers, and the present writer was one, saw a bright atmospheric bow edging a part of the circumference of venus when the planet was moving upon the face of the sun--a most beautiful and impressive spectacle. even more curious is an observation made in by prof. c.s. lyman, of yale college, who, when venus was very near the sun, saw her atmosphere _in the form of a luminous ring_. a little fuller explanation of this appearance may be of interest. when approaching inferior conjunction--i.e., passing between the earth and sun--venus appears, with a telescope, in the shape of a very thin crescent. professor lyman watched this crescent, becoming narrower day after day as it approached the sun, and noticed that its extremities gradually extended themselves beyond the limits of a semicircle, bending to meet one another on the opposite side of the invisible disk of the planet, until, at length, they did meet, and he beheld a complete ring of silvery light, all that remained visible of the planet venus! the ring was, of course, the illuminated atmosphere of the planet refracting the sunlight on all sides around the opaque globe. in m. flammarion witnessed the same phenomenon in similar circumstances. one may well envy those who have had the good fortune to behold this spectacle--to actually see, as it were, the air that the inhabitants of another world are breathing and making resonant with all the multitudinous sounds and voices that accompany intelligent life. but perhaps some readers will prefer to think that even though an atmosphere is there, there is no one to breathe it. [illustration: venus's atmosphere seen as a ring of light.] as the visibility of venus's atmosphere is unparalleled elsewhere in the solar system, it may be worth while to give a graphic illustration of it. in the accompanying figure the planet is represented at three successive points in its advance toward inferior conjunction. as it approaches conjunction it slowly draws nearer the earth, and its apparent diameter consequently increases. at _a_ a large part of the luminous crescent is composed of the planet's surface reflecting the sunshine; at _b_ the ratio of the reflecting surface to the illuminated atmosphere has diminished, and the latter has extended, like the curved arms of a pair of calipers, far around the unilluminated side of the disk; at _c_ the atmosphere is illuminated all around by the sunlight coming through it from behind, while the surface of the planet has passed entirely out of the light--that is to say, venus has become an invisible globe embraced by a circle of refracted sunshine. we return to the question of life. with almost twice as much solar heat and light as we have, and with a deeper and denser atmosphere than ours, it is evident, without seeking other causes of variation, that the conditions of life upon venus are notably different from those with which we are acquainted. at first sight it would seem that a dense atmosphere, together with a more copious supply of heat, might render the surface temperature of venus unsuitable for organic life as we understand it. but so much depends upon the precise composition of the atmosphere and upon the relative quantities of its constituents, that it will not do to pronounce a positive judgment in such a case, because we lack information on too many essential points. experiment has shown that the temperature of the air varies with changes in the amount of carbonic acid and of water vapor that it contains. it has been suggested that in past geologic ages the earth's atmosphere was denser and more heavily charged with vapors than it is at present; yet even then forms of life suited to their environment existed, and from those forms the present inhabitants of our globe have been developed. there are several lines of reasoning which may be followed to the conclusion that venus, as a life-bearing world, is younger than the earth, and, according to that view, we are at liberty to imagine our beautiful sister planet as now passing through some such period in its history as that at which the earth had arrived in the age of the carboniferous forests, or the age of the gigantic reptiles who ruled both land and sea. but, without making any assumptions as to the phase of evolution which life may have attained on venus, it is also possible to think that the planet's thick shell of air, with its abundant vapors, may serve as a shield against the excessive solar radiation. venus is extraordinarily brilliant, its reflective power being greatly in excess of mercury's, and it has often been suggested that this may be due to the fact that a large share of the sunlight falling upon it is turned back before reaching the planet's surface, being reflected both from the atmosphere itself and from vast layers of clouds. even when viewed with the most powerful telescopes and in the most favoring circumstances, the features of venus's surface are difficult to see, and generally extremely difficult. they consist of faint shadowy markings, indefinite in outline, and so close to the limit of visibility that great uncertainty exists not only as to their shape and their precise location upon the planet, but even as to their actual existence. no two observers have represented them exactly alike in drawings of the planet, and, unfortunately, photography is as yet utterly unable to deal with them. mr. percival lowell, in his special studies of venus in , using a -inch telescope of great excellence, in the clear and steady air of arizona, found delicate spokelike streaks radiating from a rounded spot like a hub, and all of which, in his opinion, were genuine and definite markings on the planet's surface. but others, using larger telescopes, have failed to perceive the shapes and details depicted by mr. lowell, and some are disposed to ascribe their appearances to venus's atmosphere. mr. lowell himself noticed that the markings seemed to have a kind of obscuring veil over them. in short, all observers of venus agree in thinking that her atmosphere, to a greater or less extent, serves as a mask to conceal her real features, and the possibilities of so extensive an atmosphere with reference to an adjustment of the peculiar conditions of the planet to the requirements of life upon it, are almost unlimited. if we could accurately analyze that atmosphere we would have a basis for more exact conclusions concerning venus's habitability. but the mere existence of the atmosphere is, in itself, a strong argument for the habitability of the planet, and as to the temperature, we are really not compelled to imagine special adaptations by means of which it may be brought into accord with that prevailing upon the earth. as long as the temperature does not rise to the _destructive_ point, beyond which our experience teaches that no organic life can exist, it may very well attain an elevation that would mean extreme discomfort from our point of view, without precluding the existence of life even in its terrestrial sense. and would it not be unreasonable to assume that vital phenomena on other planets must be subject to exactly the same limitations that we find circumscribing them in our world? that kind of assumption has more than once led us far astray even in dealing with terrestrial conditions. it is not so long ago, for instance, since life in the depths of the sea was deemed to be demonstrably impossible. the bottom of the ocean, we were assured, was a region of eternal darkness and of frightful pressure, wherein no living creatures could exist. yet the first dip of the deep-sea trawl brought up animals of marvelous delicacy of organization, which, although curiously and wonderfully adapted to live in a compressed liquid, collapsed when lifted into a lighter medium, and which, despite the assumed perpetual darkness of their profound abode, were adorned with variegated colors and furnished with organs of phosphorescence whereby they could create for themselves all the light they needed. even the fixed animals of the sea, growing, like plants, fast to the rocks, are frequently vivid with living light, and there is a splendid suggestion of nature's powers of adaptation, which may not be entirely inapplicable to the problems of life on strange planets, in alexander agassiz's statement that species of sea animals, living below the depths to which sunlight penetrates, "may dwell in total darkness and be illuminated at times merely by the movements of abyssal fishes through the forests of phosphorescent alcyonarians." in attempting to judge the habitability of a planet such as venus we must first, as far as possible, generalize the conditions that govern life and restrict its boundaries. on the earth we find animated existence confined to the surface of the crust of the globe, to the lower and denser strata of the atmosphere, and to the film of water that constitutes the oceans. it does not exist in the heart of the rocks forming the body of the planet nor in the void of space surrounding it outside the atmosphere. as the earth condensed from the original nebula, and cooled and solidified, a certain quantity of matter remained at its surface in the form of free gases and unstable compounds, and, within the narrow precincts where these things were, lying like a thin shell between the huge inert globe of permanently combined elements below, and the equally unchanging realm of the ether above, life, a phenomenon depending upon ceaseless changes, combinations and recombinations of chemical elements in unstable and temporary union, made its appearance, and there only we find it at the present time. it is because air and water furnish the means for the continual transformations by which the bodies of animals and plants are built up and afterward disintegrated and dispersed, that we are compelled to regard their presence as prerequisites to the existence, on any planet, of life in any of the forms in which we are acquainted with it. but if we perceive that another world has an atmosphere, and that there is water vapor in its atmosphere--both of which conditions are fulfilled by venus--and if we find that that world is bathed in the same sunshine that stimulates the living forces of our planet, even though its quantity or intensity may be different, then it would seem that we are justified in averring that the burden of proof rests upon those who would deny the capability of such a world to support inhabitants. the generally accepted hypothesis of the origin of the solar system leads us to believe that venus has experienced the same process of evolution as that which brought the earth into its present condition, and we may fairly argue that upon the rocky shell of venus exists a region where chemical combinations and recombinations like those on the surface of the earth are taking place. it is surely not essential that the life-forming elements should exist in exactly the same states and proportions as upon the earth; it is enough if some of them are manifestly present. even on the earth these things have undergone much variation in the course of geological history, coincidently with the development of various species of life. just at present the earth appears to have reached a stage where everything contributes to the maintenance of a very high organization in both the animal and vegetable kingdoms. so each planet that has attained the habitable stage may have a typical adjustment of temperature and atmospheric constitution, rendering life possible within certain limits peculiar to that planet, and to the special conditions prevailing there. admitting, as there is reason for doing, that different planets may be at different stages of development in the geological and biological sense, we should, of course, not expect to find them inhabited by the same living species. and, since there is also reason to believe that no two planets upon arriving at the same stage of evolution as globes would possess identical gaseous surroundings, there would naturally be differences between their organic life forms notwithstanding the similarity of their common phase of development in other respects. thus a departure from the terrestrial type in the envelope of gases covering a planet, instead of precluding life, would only tend to vary its manifestations. after all, why should the intensity of the solar radiation upon venus be regarded as inimical to life? the sunbeams awaken life. it is not impossible that relative nearness to the sun may be an advantage to venus from the biologic point of view. she gets less than one third as much heat as mercury receives on the average, and she gets it with almost absolute uniformity. at aphelion mercury is about two and four tenths times hotter than venus; then it rushes sunward, and within forty-four days becomes six times hotter than venus. in the meantime the temperature of the latter, while high as compared with the earth's, remains practically unchanged. not only may mercury's temperature reach the destructive point, and thus be too high for organic life, but mercury gets nothing with either moderation or constancy. it is a world both of excessive heat and of violent contrasts of temperature. venus, on the other hand, presents an unparalleled instance of invariableness and uniformity. she may well be called the favorite of the sun, and, through the advantages of her situation, may be stimulated by him to more intense vitality than falls to the lot of the earth. it is open, at least to the writers of the interplanetary romances now so popular, to imagine that on venus, life, while encompassed with the serenity that results from the circular form of her orbit, and the unchangeableness of her climates, is richer, warmer, more passionate, more exquisite in its forms and more fascinating in its experiences, keener of sense, capable of more delicious joys, than is possible to it amid the manifold inclemencies of the colder earth. we have seen that there is excellent authority for saying that venus's atmosphere is from one and a half to two times as dense and as extensive as ours. here is an interesting suggestion of aerial possibilities for her inhabitants. if man could but fly, how would he take to himself wings and widen his horizons along with the birds! give him an atmosphere the double in density of that which now envelopes him, take off a little of his weight, thereby increasing the ratio of his strength and activity, put into his nervous system a more puissant stimulus from the life-giving sun, and perchance he _would_ fly. well, on venus, apparently, these very conditions actually exist. how, then, do intellectual creatures in the world of venus take wing when they choose? upon what spectacle of fluttering pinions afloat in iridescent air, like a raphael dream of heaven and its angels, might we not look down if we could get near enough to our brilliant evening star to behold the intimate splendors of its life? as venus herself would be the most brilliant member of the celestial host to an observer stationed on the night side of mercury, so the earth takes precedence in the midnight sky of venus. for the inhabitants of venus mercury is a splendid evening and morning star only, while the earth, being an outer planet, is visible at times in that part of the sky which is directly opposite to the place of the sun. the light reflected from our planet is probably less dazzling than that which venus sends to us, both because, at our greater distance, the sunlight is less intense, and because our rarer atmosphere reflects a smaller proportion of the rays incident upon it. but the earth is, after all, a more brilliant phenomenon seen from venus than the latter is seen from the earth, for the reason that the entire illuminated disk of the earth is presented toward our sister planet when the two are at their nearest point of approach, whereas, at that time, the larger part of the surface of venus that is turned earthward has no illumination, while the illuminated portion is a mere crescent. owing, again, to the comparative rarity of the terrestrial atmosphere, it is probable that the inhabitants of venus--assuming their existence--enjoy a superb view of the continents, oceans, polar snows, and passing clouds that color and variegate the face of the earth. our astronomers can study the full disk of venus only when she is at her greatest distance, and on the opposite side of the sun from us, where she is half concealed in the glare. the astronomers of venus, on the other hand, can study the earth under the most favorable conditions of observation--that is to say, when it is nearest to them and when, being in opposition to the sun, its whole disk is fully illuminated. in fact, there is no planet in the entire system which enjoys an outlook toward a sister world comparable with that which venus enjoys with regard to the earth. if there be astronomers upon venus, armed with telescopes, it is safe to guess that they possess a knowledge of the surface of the earth far exceeding in minuteness and accuracy the knowledge that we possess of the features of any heavenly body except the moon. they must long ago have been able to form definite conclusions concerning the meteorology and the probable habitability of our planet. it certainly tends to increase our interest in venus when, granting that she is inhabited, we reflect upon the penetrating scrutiny of which the earth may be the object whenever venus--as happens once every days--passes between us and the sun. the spectacle of our great planet, glowing in its fullest splendor in the midnight sky, pied and streaked with water, land, cloud, and snow, is one that might well excite among the astronomers of another world, so fortunately placed to observe it, an interest even greater than that which the recurrence of total solar eclipses occasions upon the earth. for the inhabitants of venus the study of the earth must be the most absorbing branch of observational astronomy, and the subject, we may imagine, of numberless volumes of learned memoirs, far exceeding in the definiteness of their conclusions the books that we have written about the physical characteristics of other members of the solar system. and, if we are to look for attempts on the part of the inhabitants of other worlds to communicate with us by signals across the ether, it would certainly seem that venus is the most likely source of such efforts, for from no other planet can those features of the earth that give evidence of its habitability be so clearly discerned. of one thing it would seem we may be certain: if venus has intellectual inhabitants they possess far more convincing evidence of our existence than we are likely ever to have of theirs. in referring to the view of the earth from mercury it was remarked that the moon is probably visible to the naked eye. from venus the moon is not only visible, but conspicuous, to the naked eye, circling about the earth, and appearing at times to recede from it to a distance of about half a degree--equal to the diameter of the full moon as we see it. the disk of the earth is not quite four times greater in diameter than that of the moon, and nowhere else in the solar system is there an instance in which two bodies, no more widely different in size than are the moon and the earth, are closely linked together. the moons of the other planets that possess satellites are relatively so small that they appear in the telescope as mere specks beside their primaries, but the moon is so large as compared with the earth that the two must appear, as viewed from venus, like a double planet. to the naked eye they may look like a very wide and brilliant double star, probably of contrasted colors, the moon being silvery white and the earth, perhaps, now of a golden or reddish tinge and now green or blue, according to the part of its surface turned toward venus, and according, also, to the season that chances to be reigning over that part. such a spectacle could not fail to be of absorbing interest, and we can not admit the possibility of intelligent inhabitants on venus without supposing them to watch the motions of the moon and the earth with the utmost intentness. the passage of the moon behind and in front of the earth, and its eclipses when it goes into the earth's shadow, could be seen without the aid of telescopes, while, with such instruments, these phenomena would possess the highest scientific interest and importance. because the earth has a satellite so easily observable, the astronomers of venus could not remain ignorant of the exact mass of our planet, and in that respect they would outstrip us in the race for knowledge, since, on account of the lack of a satellite attending venus, we have been able to do no more than make an approximate estimate of her mass. with telescopes, too, in the case of a solar eclipse occurring at the time of the earth's opposition, they could see the black spot formed by the shadow of the moon, where the end of its cone moved across the earth like the point of an invisible pencil, and could watch it traversing continents and oceans, or thrown out in bold contrast upon the white background of a great area of clouds. indeed, the phenomena which our globe and its satellite present to venus must be so varied and wonderful that one might well wish to visit that planet merely for the sake of beholding them. thus far we have found so much of brilliant promise in the earth's twin sister that i almost hesitate to approach another phase of the subject which may tend to weaken the faith of some readers in the habitability of venus. it may have been observed that heretofore nothing has been said as to the planet's rotation period, but, without specifically mentioning it, i have tacitly assumed the correctness of the generally accepted period of about twenty-four hours, determined by de vico and other observers. this period, closely accordant with the earth's, is, as far as it goes, another argument for the habitability of venus. but now it must be stated that no less eminent an authority than schiaparelli holds that venus, as well as mercury, makes but a single turn on its axis in the course of a revolution about the sun, and, consequently, is a two-faced world, one side staring eternally at the sun and the other side wearing the black mask of endless night. schiaparelli made this announcement concerning venus but a few weeks after publishing his discovery of mercury's peculiar rotation. he himself appears to be equally confident in both cases of the correctness of his conclusions and the certainty of his observation. as with mercury, several other observers have corroborated him, and particularly percival lowell in this country. mr. lowell, indeed, seems unwilling to admit that any doubt can be entertained. nevertheless, very grave doubt is entertained, and that by many, and probably by the majority, of the leading professional astronomers and observers. in fact, some observers of great ability, equipped with powerful instruments, have directly contradicted the results of schiaparelli and his supporters. the reader may ask: "why so readily accept schiaparelli's conclusions with regard to mercury while rejecting them in the case of venus?" the reply is twofold. in the first place the markings on venus, although mr. lowell sketched them with perfect confidence in , are, by the almost unanimous testimony of those who have searched for them with telescopes, both large and small, extremely difficult to see, indistinct in outline, and perhaps evanescent in character. the sketches of no two observers agree, and often they are remarkably unlike. the fact has already been mentioned that mr. lowell noticed a kind of veil partially obscuring the markings, and which he ascribed, no doubt correctly, to the planet's atmosphere. but he thinks that, notwithstanding the atmospheric veil, the markings noted by him were unquestionably permanent features of the planet's real surface. inasmuch, however, as his drawings represent things entirely different from what others have seen, there seems to be weight in the suggestion that the radiating bands and shadings noticed by him were in some manner illusory, and perhaps of atmospheric origin. if the markings were evidently of a permanent nature and attached to the solid shell of the planet, and if they were of sufficient distinctness to be seen in substantially the same form by all observers armed with competent instruments, then whatever conclusion was drawn from their apparent motion as to the period of the planet's rotation would have to be accepted. in the case of mercury the markings, while not easily seen, appear to be sufficiently distinct to afford confidence in the result of observations based upon them, but venus's markings have been represented in so many different ways that it seems advisable to await more light before accepting any extraordinary, and in itself improbable, conclusion based upon them. it should also be added that in spectroscopic observations by belopolski at pulkova gave evidence that venus really rotates rapidly on her axis, in a period probably approximating to the twenty-four hours of the earth's rotation, thus corroborating the older conclusions. belopolski's observation, it may be remarked, was based upon what is known as the doppler principle, which is employed in measuring the motion of stars in the line of sight, and in other cases of rapidly moving sources of light. according to this principle, when a source of light, either original or reflected, is approaching the observer, the characteristic lines in its spectrum are shifted toward the blue end, and when it is retreating from the observer the lines are shifted toward the red end. now, in the case of a planet rotating rapidly on its axis, it is clear that if the observer is situated in, or nearly in, the plane of the planet's equator, one edge of its disk will be approaching his eye while the opposite edge is retreating, and the lines in the spectrum of a beam of light from the advancing edge will be shifted toward the blue, while those in the spectrum of the light coming from the retreating edge will be shifted toward the red. and, by carefully noting the amount of the shifting, the velocity of the planet's rotation can be computed. this is what was done by belopolski in the case of venus, with the result above noted. secondly, the theory that venus rotates but once in the course of a revolution finds but slight support from the doctrine of tidal friction, as compared with that which it receives when applied to mercury. the effectiveness of the sun's attraction in slowing down the rotation of a planet through the braking action of the tides raised in the body of the planet while it is yet molten or plastic, varies inversely as the sixth power of the planet's distance. for mercury this effectiveness is nearly three hundred times as great as it is for the earth, while for venus it is only seven times as great. while we may admit, then, that mercury, being relatively close to the sun and subject to an enormous braking action, lost rotation until--as occurred for a similar reason to the moon under the tidal attraction of the earth--it ended by keeping one face always toward its master, we are not prepared to make the same admission in the case of venus, where the effective force concerned is comparatively so slight. it should be added, however, that no certain evidence of polar compression in the outline of venus's disk has ever been obtained, and this fact would favor the theory of a very slow rotation because a plastic globe in swift rotation has its equatorial diameter increased and its polar diameter diminished. if venus were as much flattened at the poles as the earth is, it would seem that the fact could not escape detection, yet the necessary observations are very difficult, and venus is so brilliant that her light increases the difficulty, while her transits across the sun, when she can be seen as a round black disk, are very rare phenomena, the latest having occurred in and , and the next not being due until . upon the whole, probably the best method of settling the question of venus's rotation is the spectroscopic method, and that, as we saw, has already given evidence for the short period. even if it were established that venus keeps always the same face to the sun, it might not be necessary to abandon altogether the belief that she is habitable, although, of course, the obstacles to that belief would be increased. venus's orbit being so nearly circular, and her orbital motion so nearly invariable, she has but a very slight libration with reference to the sun, and the east and west lunes on her surface, where day and night would alternate once in her year of days, would be so narrow as to be practically negligible. but, owing to her extensive atmosphere, there would be a very broad band of twilight on venus, running entirely around the planet at the inner edge of the light hemisphere. what the meteorological conditions within this zone would be is purely a matter of conjecture. as in the case of mercury, we should expect an interchange of atmospheric currents between the light and dark sides of the planet, the heated air rising under the influence of the unsetting sun in one hemisphere, and being replaced by an indraught of cold air from the other. the twilight band would probably be the scene of atmospheric conflicts and storms, and of immense precipitation, if there were oceans on the light hemisphere to charge the air with moisture. it has been suggested that ice and snow might be piled in a vast circle of glaciers, belting the planet along the line between perpetual day and night, and that where the sunbeams touched these icy deposits near the edge of the light hemisphere a marvelous spectacle of prismatic hills of crystal would be presented! it may be remarked that it would be the inhabitants of the dark hemisphere who would enjoy the beautiful scene of the earth and the moon in opposition. chapter iv mars, a world more advanced than ours mars is the fourth planet in the order of distance from the sun, and the outermost member of the terrestrial group. its mean distance is , , miles, variable, through the eccentricity of its orbit, to the extent of about , , miles. it will be observed that this is only a million miles less than the variation in mercury's distance from the sun, from which, in a previous chapter, were deduced most momentous consequences; but, in the case of mars, the ratio of the variation to the mean distance is far smaller than with mercury, so that the effect upon the temperature of the planet is relatively insignificant. mars gets a little less than half as much solar light and heat as the earth receives, its situation in this respect being just the opposite to that of venus. its period of orbital revolution, or the length of its year, is of our days. the diameter of mars is , miles, and its density is per cent of the earth's density. gravity on its surface is only per cent of terrestrial gravity--i.e., a one hundred-pound weight removed from the earth to mars would there weigh but thirty-eight pounds. mars evidently has an atmosphere, the details of which we shall discuss later. the poles of the planet are inclined from a perpendicular to the plane of its orbit at very nearly the same angle as that of the earth's poles, viz., ° ´. its rotation on its axis is also effected in almost the same period as the earth's, viz., hours, minutes. when in opposition to the sun, mars may be only about , , miles from the earth, but its average distance when in that position is more than , , miles, and may be more than , , . these differences arise from the eccentricities of the orbits of the two planets. when on the farther side of the sun--i.e., in conjunction with the sun as seen from the earth--mars's average distance from us is about , , miles. in consequence of these great changes in its distance, mars is sometimes a very conspicuous object in the sky, and at other times inconspicuous. the similarity in the inclination of the axis of the two planets results in a close resemblance between the seasons on mars and on the earth, although, owing to the greater length of its year, mars's seasons are much longer than ours. winter and summer visit in succession its northern and southern hemispheres just as occurs on the planet that we inhabit, and the torrid, temperate, and frigid zones on its surface have nearly the same angular width as on the earth. in this respect mars is the first of the foreign planets we have studied to resemble the earth. around each of its poles appears a circular white patch, which visibly expands when winter prevails upon it, and rapidly contracts, sometimes almost completely disappearing, under a summer sun. from the time of sir william herschel the almost universal belief among astronomers has been that these gleaming polar patches on mars are composed of snow and ice, like the similar glacial caps of the earth, and no one can look at them with a telescope and not feel the liveliest interest in the planet to which they belong, for they impart to it an appearance of likeness to our globe which at first glance is all but irresistible. to watch one of them apparently melting, becoming perceptibly smaller week after week, while the general surface of the corresponding hemisphere of the planet deepens in color, and displays a constantly increasing wealth of details as summer advances across it, is an experience of the most memorable kind, whose effect upon the mind of the observer is indescribable. early in the history of the telescope it became known that, in addition to the polar caps, mars presented a number of distinct surface features, and gradually, as instruments increased in power and observers in skill, charts of the planet were produced showing a surface diversified somewhat in the manner that characterizes the face of the earth, although the permanent forms do not closely resemble those of our planet. two principal colors exist on the disk of mars--dark, bluish gray or greenish gray, characterizing areas which have generally been regarded as seas, and light yellowish red, overspreading broad regions looked upon as continents. it was early observed that if the dark regions really are seas, the proportion of water to land upon mars is much smaller than upon the earth. for two especial reasons mars has generally been regarded as an older or more advanced planet than the earth. the first reason is that, accepting laplace's theory of the origin of the planetary system from a series of rings left off at the periphery of the contracting solar nebula, mars must have come into existence earlier than the earth, because, being more distant from the center of the system, the ring from which it was formed would have been separated sooner than the terrestrial ring. the second reason is that mars being smaller and less massive than the earth has run through its developments a cooling globe more rapidly. the bearing of these things upon the problems of life on mars will be considered hereafter. and now, once more, schiaparelli appears as the discoverer of surprising facts about one of the most interesting worlds of the solar system. during the exceptionally favorable opposition of mars in , when an american astronomer, asaph hall, discovered the planet's two minute satellites, and again during the opposition of , the italian observer caught sight of an astonishing network of narrow dark lines intersecting the so-called continental regions of the planet and crossing one another in every direction. schiaparelli did not see the little moons that hall discovered, and hall did not perceive the enigmatical lines that schiaparelli detected. hall had by far the larger and more powerful telescope; schiaparelli had much the more steady and favorable atmosphere for astronomical observation. yet these differences in equipment and circumstances do not clearly explain why each observer should have seen what the other did not. there may be a partial explanation in the fact that an observer having made a remarkable discovery is naturally inclined to confine his attention to it, to the neglect of other things. but it was soon found that schiaparelli's lines--to which he gave the name "canals," merely on account of their shape and appearance, and without any intention to define their real nature--were excessively difficult telescopic objects. eight or nine years elapsed before any other observer corroborated schiaparelli's observations, and notwithstanding the "sensation" which the discovery of the canals produced they were for many years regarded by the majority of astronomers as an illusion. but they were no illusion, and in schiaparelli added to the astonishment created by his original discovery, and furnished additional grounds for skepticism, by announcing that, at certain times, many of the canals geminated, or became double! he continued his observations at each subsequent opposition, adding to the number of the canals observed, and charting them with classical names upon a detailed map of the planet's surface. at length in perrotin, at nice, detected many of schiaparelli's canals, and later they were seen by others. in schiaparelli greatly extended his observations, and in and some of the canals were studied with the -inch telescope of the lick observatory, and in the last-named year a very elaborate series of observations upon them was made by percival lowell and his associates, prof. william c. pickering and mr. a.e. douglass, at flagstaff, arizona. mr. lowell's charts of the planet are the most complete yet produced, containing canals to which separate names have been given, besides more than a hundred other markings also designated by individual appellations. it should not be inferred from the fact that schiaparelli's discovery in excited so much surprise and incredulity that no glimpse of the peculiar canal-like markings on mars had been obtained earlier than that. at least as long ago as mr. dawes, in england, had seen and sketched half a dozen of the larger canals, or at least the broader parts of them, especially where they connect with the dark regions known as seas, but dawes did not see them in their full extent, did not recognize their peculiar character, and entirely failed to catch sight of the narrower and more numerous ones which constitute the wonderful network discovered by the italian astronomer. schiaparelli found no less than sixty canals during his first series of observations in . let us note some of the more striking facts about the canals which schiaparelli has described. we can not do better than quote his own words: "there are on this planet, traversing the continents, long dark lines which may be designated as _canals_, although we do not yet know what they are. these lines run from one to another of the somber spots that are regarded as seas, and form, over the lighter, or continental, regions a well-defined network. their arrangement appears to be invariable and permanent; at least, as far as i can judge from four and a half years of observation. nevertheless, their aspect and their degree of visibility are not always the same, and depend upon circumstances which the present state of our knowledge does not yet permit us to explain with certainty. in a great number were seen which were not visible in , and in all those which had been seen at former oppositions were found again, together with new ones. sometimes these canals present themselves in the form of shadowy and vague lines, while on other occasions they are clear and precise, like a trace drawn with a pen. in general they are traced upon the sphere like the lines of great circles; a few show a sensible lateral curvature. they cross one another obliquely, or at right angles. they have a breadth of two degrees, or kilometres [ miles], and several extend over a length of eighty degrees, or , kilometres [nearly , miles]. their tint is very nearly the same as that of the seas, usually a little lighter. every canal terminates at both its extremities in a sea, or in another canal; there is not a single example of one coming to an end in the midst of dry land. "this is not all. in certain seasons these canals become double. this phenomenon seems to appear at a determinate epoch, and to be produced simultaneously over the entire surface of the planet's continents. there was no indication of it in , during the weeks that preceded and followed the summer solstice of that world. a single isolated case presented itself in . on the th of december, this year--a little before the spring equinox, which occurred on mars on the st of january, --i noticed the doubling of the nile [a canal thus named] between the lakes of the moon and the ceraunic gulf. these two regular, equal, and parallel lines caused me, i confess, a profound surprise, the more so because a few days earlier, on the d and the th of december, i had carefully observed that very region without discovering anything of the kind. "i awaited with curiosity the return of the planet in , to see if an analogous phenomenon would present itself in the same place, and i saw the same thing reappear on the th of january, , one month after the spring equinox--which occurred on the th of december, . the duplication was still more evident at the end of february. on this same date, the th of january, another duplication had already taken place, that of the middle portion of the canal of the cyclops, adjoining elysium. [elysium is a part of one of the continental areas.] "yet greater was my astonishment when, on the th of january, i saw the canal jamuna, which was then in the center of the disk, formed very rigidly of two parallel straight lines, crossing the space which separates the niliac lake from the gulf of aurora. at first sight i believed it was an illusion, caused by fatigue of the eye and some new kind of strabismus, but i had to yield to the evidence. after the th of january i simply passed from wonder to wonder; successively the orontes, the euphrates, the phison, the ganges, and the larger part of the other canals, displayed themselves very clearly and indisputably duplicated. there were not less than twenty examples of duplication, of which seventeen were observed in the space of a month, from the th of january to the th of february. "in certain cases it was possible to observe precursory symptoms which are not lacking in interest. thus, on the th of january, a light, ill-defined shade extended alongside the ganges; on the th and the th one could only distinguish a series of white spots; on the th the shadow was still indecisive, but on the st the duplication was perfectly clear, such as i observed it until the d of february. the duplication of the euphrates, of the canal of the titans, and of the pyriphlegethon also began in an uncertain and nebulous form. "these duplications are not an optical effect depending on increase of visual power, as happens in the observation of double stars, and it is not the canal itself splitting in two longitudinally. here is what is seen: to the right or left of a pre-existing line, without any change in the course and position of that line, one sees another line produce itself, equal and parallel to the first, at a distance generally varying from six to twelve degrees--i.e., from to kilometres ( to miles); even closer ones seem to be produced, but the telescope is not powerful enough to distinguish them with certainty. their tint appears to be a quite deep reddish brown. the parallelism is sometimes rigorously exact. there is nothing analogous in terrestrial geography. everything indicates that here there is an organization special to the planet mars, probably connected with the course of its seasons."[ ] [footnote : l'astronomie, vol. i, , pp. _et seq._] schiaparelli adds that he took every precaution to avoid the least suspicion of illusion. "i am absolutely sure," he says, "of what i have observed." i have quoted his statement, especially about the duplication of the canals, at so much length, both on account of its intrinsic interest and because it has many times been argued that this particular phenomenon must be illusory even though the canals are real. one of the most significant facts that came out in the early observations was the evident connection between the appearance of the canals and the seasonal changes on mars. it was about the time of the spring equinox, when the white polar caps had begun to melt, that schiaparelli first noticed the phenomenon of duplication. as the season advanced the doubling of the canals increased in frequency and the lines became more distinct. in the meantime the polar caps were becoming smaller. broadly speaking, schiaparelli's observation showed that the doubling of the canals occurred principally a little after the spring equinox and a little before the autumn equinox; that the phenomenon disappeared in large part at the epoch of the winter solstice, and disappeared altogether at the epoch of the summer solstice. moreover, he observed that many of the canals, without regard to duplication, were invisible at times, and reappeared gradually; faint, scarcely visible lines and shadows, deepened and became more distinct until they were clearly and sharply defined, and these changes, likewise, were evidently seasonal. the invariable connection of the canals at their terminations with the regions called seas, the fact that as the polar caps disappeared the sealike expanses surrounding the polar regions deepened in color, and other similar considerations soon led to the suggestion that there existed on mars a wonderful system of water circulation, whereby the melting of the polar snows, as summer passed alternately from one hemisphere to the other, served to reenforce the supply of water in the seas, and, through the seas, in the canals traversing the broad expanses of dry land that occupy the equatorial regions of the planet. the thought naturally occurred that the canals might be of artificial origin, and might indicate the existence of a gigantic system of irrigation serving to maintain life upon the globe of mars. the geometrical perfection of the lines, their straightness, their absolute parallelism when doubled, their remarkable tendency to radiate from definite centers, lent strength to the hypothesis of an artificial origin. but their enormous size, length, and number tended to stagger belief in the ability of the inhabitants of any world to achieve a work so stupendous. after a time a change of view occurred concerning the nature of the expanses called seas, and mr. lowell, following his observations of , developed the theory of the water circulation and irrigation of mars in a new form. he and others observed that occasionally canals were visible cutting straight across some of the greenish, or bluish-gray, areas that had been regarded as seas. this fact suggested that, instead of seas, these dark expanses may rather be areas of marshy ground covered with vegetation which flourishes and dies away according as the supply of water alternately increases and diminishes, while the reddish areas known as continents are barren deserts, intersected by canals; and as the water released by the melting of the polar snows begins to fill the canals, vegetation springs up along their sides and becomes visible in the form of long narrow bands. according to this theory, the phenomena called canals are simply lines of vegetation, the real canals being individually too small to be detected. it may be supposed that from a central supply canal irrigation ditches are extended for a distance of twenty or thirty miles on each side, thus producing a strip of fertile soil from forty to sixty miles wide, and hundreds, or in some cases two or three thousands, of miles in length. the water supply being limited, the inhabitants can not undertake to irrigate the entire surface of the thirsty land, and convenience of circulation induces them to extend the irrigated areas in the form of long lines. the surface of mars, according to lowell's observation, is remarkably flat and level, so that no serious obstacle exists to the extension of the canal system in straight bands as undeviating as arcs of great circles. wherever two or more canals meet, or cross, a rounded dark spot from a hundred miles, or less, to three hundred miles in diameter, is seen. an astonishing number of these appear on mr. lowell's charts. occasionally, as occurs at the singular spot named lacus solis, several canals converging from all points of the compass meet at a central point like the spokes of a wheel; in other cases, as, for instance, that of the long canal named eumenides, with its continuation orcus, a single conspicuous line is seen threading a large number of round dark spots, which present the appearance of a row of beads upon a string. these circular spots, which some have regarded as lakes, mr. lowell believes are rather oases in the great deserts, and granting the correctness of his theory of the canals the aptness of this designation is apparent.[ ] [footnote : the reader can find many of these "canals" and "oases," as well as some of the other regions on mars that have received names, in the frontispiece.] wherever several canals, that is to say, several bands of vegetation or bands of life, meet, it is reasonable to assume that an irrigated and habitable area of considerable extent will be developed, and in such places the imagination may picture the location of the chief centers of population, perhaps in the form of large cities, or perhaps in groups of smaller towns and villages. the so-called lacus solis is one of these localities. so, likewise, it seems but natural that along the course of a broad, well-irrigated band a number of expansions should occur, driving back the bounds of the desert, forming rounded areas of vegetation, and thus affording a footing for population. wherever two bands cross such areas would be sure to exist, and in almost every instance of crossing the telescope actually shows them. as to the gemination or duplication of many of the lines which, at the beginning of the season, appear single, it may be suggested that, in the course of the development of the vast irrigation system of the planet parallel bands of cultivation have been established, one receiving its water supply from the canals of the other, and consequently lagging a little behind in visibility as the water slowly percolates through the soil and awakens the vegetation. or else, the character of the vegetation itself may differ as between two such parallel bands, one being supplied with plants that spring up and mature quickly when the soil about their roots is moistened, while the plants in the twin band respond more slowly to stimulation. objection has been made to the theory of the artificial origin of the canals of mars on the ground, already mentioned, that the work required to construct them would be beyond the capacity of any race of creatures resembling man. the reply that has been made to this is twofold. in the first place, it should be remembered that the theory, as mr. lowell presents it, does not assert that the visible lines are the actual canals, but only that they are strips of territory intersected, like holland or the center of the plain of lombardy, by innumerable irrigation canals and ditches. to construct such works is clearly not an impossible undertaking, although it does imply great industry and concentration of effort. in the second place, since the force of gravity on mars is in the ratio of only to compared with the earth's, it is evident that the diminished weight of all bodies to be handled would give the inhabitants of mars an advantage over those of the earth in the performance of manual labor, provided that they possess physical strength and activity as great as ours. but, in consequence of this very fact of the slighter force of gravity, a man upon mars could attain a much greater size, and consequently much greater muscular strength, than his fellows upon the earth possess without being oppressed by his own weight. in other words, as far as the force of gravity may be considered as the decisive factor, mars could be inhabited by giants fifteen feet tall, who would be relatively just as active, and just as little impeded in their movements by the weight of their bodies, as a six-footer is upon the earth. but they would possess far more physical strength than we do, while, in doing work, they would have much lighter materials to deal with. whether the theory that the canals of mars really are canals is true or not, at any rate there can now be no doubt as to the existence of the strange lines which bear that designation. the suggestion has been offered that their builders may no longer be in existence, mars having already passed the point in its history where life must cease upon its surface. this brings us to consider again the statement, made near the beginning of this chapter, that mars is, perhaps, at a more advanced stage of development than the earth. if we accept this view, then, provided there was originally some resemblance between mars's life forms and those of the earth, the inhabitants of that planet would, at every step, probably be in front of their terrestrial rivals, so that at the present time they should stand well in advance. mr. lowell has, perhaps, put this view of the relative advancement in evolution of mars and its inhabitants as picturesquely as anybody. "in mars," he says, "we have before us the spectacle of a world relatively well on in years, a world much older than the earth. to so much about his age mars bears witness on his face. he shows unmistakable signs of being old. advancing planetary years have left their mark legible there. his continents are all smoothed down; his oceans have all dried up.... mars being thus old himself, we know that evolution on his surface must be similarly advanced. this only informs us of its condition relative to the planet's capabilities. of its actual state our data are not definite enough to furnish much deduction. but from the fact that our own development has been comparatively a recent thing, and that a long time would be needed to bring even mars to his present geological condition, we may judge any life he may support to be not only relatively, but really older than our own. from the little we can see such appears to be the case. the evidence of handicraft, if such it be, points to a highly intelligent mind behind it. irrigation, unscientifically conducted, would not give us such truly wonderful mathematical fitness in the several parts to the whole as we there behold.... quite possibly such martian folk are possessed of inventions of which we have not dreamed, and with them electrophones and kinetoscopes are things of a bygone past, preserved with veneration in museums as relics of the clumsy contrivances of the simple childhood of the race. certainly what we see hints at the existence of beings who are in advance of, not behind us, in the journey of life."[ ] [footnote : mars, by percival lowell, p. _et seq._] granted the existence of such a race as is thus described, and to them it might not seem a too appalling enterprise, when their planet had become decrepit, with its atmosphere thinned out and its supply of water depleted, to grapple with the destroying hand of nature and to prolong the career of their world by feats of chemistry and engineering as yet beyond the compass of human knowledge. it is confidence, bred from considerations like these, in the superhuman powers of the supposed inhabitants of mars that has led to the popular idea that they are trying to communicate by signals with the earth. certain enigmatical spots of light, seen at the edge of the illuminated disk of mars, and projecting into the unilluminated part--for mars, although an outer planet, shows at particular times a gibbous phase resembling that of the moon just before or just after the period of full moon--have been interpreted by some, but without any scientific evidence, as of artificial origin. upon the assumption that these bright points, and others occasionally seen elsewhere on the planet's disk, are intended by the martians for signals to the earth, entertaining calculations have been made as to the quantity of light that would be required in the form of a "flash signal" to be visible across the distance separating the two planets. the results of the calculations have hardly been encouraging to possible investors in interplanetary telegraphy, since it appears that heliographic mirrors with reflecting surfaces measured by square miles, instead of square inches, would be required to send a visible beam from the earth to mars or _vice versa_. the projections of light on mars can be explained much more simply and reasonably. various suggestions have been made about them; among others, that they are masses of cloud reflecting the sunshine; that they are areas of snow; and that they are the summits of mountains crowned with ice and encircled with clouds. in fact, a huge mountain mass lying on the terminator, or the line between day and night, would produce the effect of a tongue of light projecting into the darkness without assuming that it was snow-covered or capped with clouds, as any one may convince himself by studying the moon with a telescope when the terminator lies across some of its most mountainous regions. to be sure, there is reason to think that the surface of mars is remarkably flat; yet even so the planet may have some mountains, and on a globe the greater part of whose shell is smooth any projections would be conspicuous, particularly where the sunlight fell at a low angle across them. another form in which the suggestion of interplanetary communication has been urged is plainly an outgrowth of the invention and surprising developments of wireless telegraphy. the human mind is so constituted that whenever it obtains any new glimpse into the arcana of nature it immediately imagines an indefinite and all but unlimited extension of its view in that direction. so to many it has not appeared unreasonable to assume that, since it is possible to transmit electric impulses for considerable distances over the earth's surface by the simple propagation of a series of waves, or undulations, without connecting wires, it may also be possible to send such impulses through the ether from planet to planet. the fact that the electric undulations employed in wireless telegraphy pass between stations connected by the crust of the earth itself, and immersed in a common atmospheric envelope, is not deemed by the supporters of the theory in question as a very serious objection, for, they contend, electric waves are a phenomenon of the ether, which extends throughout space, and, given sufficient energy, such waves could cross the gap between world and world. but nobody has shown how much energy would be needed for such a purpose, and much less has anybody indicated a way in which the required energy could be artificially developed, or cunningly filched from the stores of nature. it is, then, purely an assumption, an interesting figment of the mind, that certain curious disturbances in the electrical state of the air and the earth, affecting delicate electric instruments, possessing a marked periodicity in brief intervals of time, and not yet otherwise accounted for, are due to the throbbing, in the all-enveloping ether, of impulses transmitted from instruments controlled by the _savants_ of mars, whose insatiable thirst for knowledge, and presumably burning desire to learn whether there is not within reach some more fortunate world than their half-dried-up globe, has led them into a desperate attempt to "call up" the earth on their interplanetary telephone, with the hope that we are wise and skilful enough to understand and answer them. in what language they intend to converse no one has yet undertaken to tell, but the suggestion has sapiently been made that, mathematical facts being invariable, the eternal equality of two plus two with four might serve as a basis of understanding, and that a statement of that truth sent by electric taps across the ocean of ether would be a convincing assurance that the inhabitants of the planet from which the message came at least enjoyed the advantages of a common-school education. but, while speculation upon this subject rests on unverified, and at present unverifiable, assumptions, of course everybody would rejoice if such a thing were possible, for consider what zest and charm would be added to human life if messages, even of the simplest description, could be sent to and received from intelligent beings inhabiting other planets! it is because of this hold that it possesses upon the imagination, and the pleasing pictures that it conjures up, that the idea of interplanetary communication, once broached, has become so popular a topic, even though everybody sees that it should not be taken too seriously. the subject of the atmosphere of mars can not be dismissed without further consideration than we have yet given it, because those who think the planet uninhabitable base their opinion largely upon the assumed absence of sufficient air to support life. it was long ago recognized that, other things being equal, a planet of small mass must possess a less dense atmosphere than one of large mass. assuming that each planet originally drew from a common stock, and that the amount and density of its atmosphere is measured by its force of gravity, it can be shown that mars should have an atmosphere less than one fifth as dense as the earth's. dr. johnstone stoney has attacked the problem of planetary atmospheres in another way. knowing the force of gravity on a planet, it is easy to calculate the velocity with which a body, or a particle, would have to start radially from the planet in order to escape from its gravitational control. for the earth this critical velocity is about seven miles per second; for mars about three miles per second. estimating the velocity of the molecules of the various atmospheric gases, according to the kinetic theory, dr. stoney finds that some of the smaller planets, and the moon, are gravitationally incapable of retaining all of these gases in the form of an atmosphere. among the atmospheric constituents that, according to this view, mars would be unable permanently to retain is water vapor. indeed, he supposes that even the earth is slowly losing its water by evaporation into space, and on mars, owing to the slight force of gravity there, this process would go on much more rapidly, so that, in this way, we have a means of accounting for the apparent drying up of that planet, while we may be led to anticipate that at some time in the remote future the earth also will begin to suffer from lack of water, and that eventually the chasms of the sea will yawn empty and desolate under a cloudless sky. but it is not certain that the original supply of atmospheric elements was in every case proportional to the respective force of gravity of a planet. the fact that venus appears to have an atmosphere more extensive and denser than the earth's, although its force of gravity is a little less than that of our globe, indicates at once a variation as between these two planets in the amount of atmospheric material at their disposal. this may be a detail depending upon differences in the mode, or in the stage, of their evolution. thus, after all, dr. stoney's theory may be substantially correct and yet mars may retain sufficient water to form clouds, to be precipitated in snow, and to fill its canals after each annual melting of the polar caps, because the original supply was abundant, and its escape is a gradual process, only to be completed by age-long steps. even though the evidence of the spectroscope, as far as it goes, seems to lend support to the theory that there is no water vapor in the atmosphere of mars, we can not disregard the visual evidence that, nevertheless, water vapor exists there. what are the polar caps if they are not snow? frozen carbon dioxide, it has been suggested; but this is hardly satisfactory, for it offers no explanation of the fact that when the polar caps diminish, and in proportion as they diminish, the "seas" and the canals darken and expand, whereas a reasonable explanation of the correlation of these phenomena is offered if we accept the view that the polar caps consist of snow. then there are many observations on record indicating the existence of clouds in mars's atmosphere. sometimes a considerable area of its surface has been observed to be temporarily obscured, not by dense masses of cloud such as accompany the progress of great cyclonic storms across the continents and oceans of the earth, but by comparatively thin veils of vapor such as would be expected to form in an atmosphere so comparatively rare as that of mars. and these clouds, in some instances at least, appear, like the cirrus streaks and dapples in our own air, to float at a great elevation. mr. douglass, one of mr. lowell's associates in the observations of at flagstaff, arizona, observed what he believed to be a cloud over the unilluminated part of mars's disk, which, by micrometric measurement and estimate, was drifting at an elevation of about fifteen miles above the surface of the planet. this was seen on two successive days, november th and november th, and it underwent curious fluctuations in visibility, besides moving in a northerly direction at the rate of some thirteen miles an hour. but, upon the whole, as mr. lowell remarks, the atmosphere of mars is remarkably free of clouds. the reader will remember that mars gets a little less than half as much heat from the sun as the earth gets. this fact also has been used as an argument against the habitability of the planet. in truth, those who think that life in the solar system is confined to the earth alone insist upon an almost exact reproduction of terrestrial conditions as a _sine qua non_ to the habitability of any other planet. venus, they think, is too hot, and mars too cold, as if life were rather a happy accident than the result of the operation of general laws applicable under a wide variety of conditions. all that we are really justified in asserting is that venus may be too hot and mars too cold for _us_. of course, if we adopt the opinion held by some that the temperature on mars is constantly so low that water would remain perpetually frozen, it does throw the question of the kind of life that could be maintained there into the realm of pure conjecture. the argument in favor of an extremely low temperature on mars is based on the law of the diminution of radiant energy inversely as the square of the distance, together with the assumption that no qualifying circumstances, or no modification of that law, can enter into the problem. according to this view, it could be shown that the temperature on mars never rises above - ° f. but it is a view that seems to be directly opposed to the evidence of the telescope, for all who have studied mars under favorable conditions of observation have been impressed by the rapid and extensive changes that the appearance of its surface undergoes coincidently with the variation of the planet's seasons. it has its winter aspect and its summer aspect, perfectly distinct and recognizable, in each hemisphere by turns, and whether the polar caps be snow or carbon dioxide, at any rate they melt and disappear under a high sun, thus proving that an accumulation of heat takes place. professor young says: "as to the temperature of mars we have no certain knowledge. on the one hand, we know that on account of the planet's distance from the sun the intensity of solar radiation upon its surface must be less than here in the ratio of to ( . )^ --i.e., only about per cent as great as with us; its 'solar constant' must be less than calories against our . then, too, the low density of its atmosphere, probably less at the planet's surface than on the tops of our highest mountains, would naturally assist to keep down the temperature to a point far below the freezing-point of water. but, on the other hand, things certainly _look_ as if the polar caps were really masses of _snow_ and _ice_ deposited from vapor in the planet's atmosphere, and as if these actually melted during the martian summer, sending floods of water through the channels provided for them, and causing the growth of vegetation along their banks. we are driven, therefore, to suppose either that the planet has sources of heat internal or external which are not yet explained, or else, as long ago suggested, that the polar 'snow' may possibly be composed of something else than frozen _water_."[ ] [footnote : general astronomy, by charles a. young. revised edition, , p. .] even while granting the worst that can be said for the low temperature of mars, the persistent believer in its habitability could take refuge in the results of recent experiments which have proved that bacterial life is able to resist the utmost degree of cold that can be applied, microscopic organisms perfectly retaining their vitality--or at least their power to resume it--when subjected to the fearfully low temperature of liquid air. but then he would be open to the reply that the organisms thus treated are in a torpid condition and deprived of all activity until revived by the application of heat; and the picture of a world in a state of perpetual sleep is not particularly attractive, unless the fortunate prince who is destined to awake the slumbering beauty can also be introduced into the romance.[ ] [footnote : many of the present difficulties about temperatures on the various planets would be beautifully disposed of if we could accept the theory urged by mr. cope whitehouse, to the effect that the sun is not really a hot body at all, and that what we call solar light and heat are only local manifestations produced in our atmosphere by the transformation of some other form of energy transmitted from the sun; very much as the electric impulses carried by a wire from the transmitting to the receiving station on a telephone line are translated by the receiver into waves of sound. according to this theory, which is here mentioned only as an ingenuity and because something of the kind so frequently turns up in one form or another in popular semi-scientific literature, the amount of heat and light on a planet would depend mainly upon local causes.] to an extent which most of us, perhaps, do not fully appreciate, we are indebted for many of the pleasures and conveniences and some of the necessities of life on our planet to its faithful attendant, the moon. neither mercury nor venus has a moon, but mars has two moons. this statement, standing alone, might lead to the conclusion that, as far as the advantages a satellite can afford to the inhabitants of its master planet are concerned, the people of mars are doubly fortunate. so they would be, perhaps, if mars's moons were bodies comparable in size with our moon, but in fact they are hardly more than a pair of very entertaining astronomical toys. the larger of the two, phobos, is believed to be about seven miles in diameter; the smaller, deimos, only five or six miles. their dimensions thus resemble those of the more minute of the asteroids, and the suggestion has even been made that they may be captured asteroids which have fallen under the gravitational control of mars. the diameters just mentioned are professor pickering's estimates, based on the amount of light the little satellites reflect, for they are much too small to present measurable disks. deimos is , miles from the center of mars and , miles from its surface. phobos is , miles from the center of the planet and only , from the surface. deimos completes a revolution about the planet in thirty hours and eighteen minutes, and phobos in the astonishingly short period--although, of course, it is in strict accord with the law of gravitation and in that sense not astonishing--of seven hours and thirty-nine minutes. since mars takes twenty-four hours and thirty-seven minutes for one rotation on its axis, it is evident that phobos goes round the planet three times in the course of a single martian day and night, rising, contrary to the general motion of the heavens, in the west, running in a few hours through all the phases that our moon exhibits in the course of a month, and setting, where the sun and all the stars rise, in the east. deimos, on the other hand, has a period of revolution five or six hours longer than that of the planet's axial rotation, so that it rises, like the other heavenly bodies, in the east; but, because its motion is so nearly equal, in angular velocity, to that of mars's rotation, it shifts very slowly through the sky toward the west, and for two or three successive days and nights it remains above the horizon, the sun overtaking and passing it again and again, while, in the meantime, its protean face swiftly changes from full circle to half-moon, from half-moon to crescent, from crescent back to half, and from half to full, and so on without ceasing. and during this time phobos is rushing through the sky in the opposite direction, as if in defiance of the fundamental law of celestial revolution, making a complete circuit three times every twenty-four hours, and changing the shape of its disk four times as rapidly as deimos does! truly, if we were suddenly transported to mars, we might well believe that we had arrived in the mother world of lunatics, and that its two moons were bewitched. yet it must not be supposed that all the peculiarities just mentioned would be clearly seen from the surface of mars by eyes like ours. the phases of phobos would probably be discernible to the naked eye, but those of deimos would require a telescope in order to be seen, for, notwithstanding their nearness to the planet, mars's moons are inconspicuous phenomena even to the martians themselves. professor young's estimate is that phobos may shed upon mars one-sixtieth and deimos one-twelve-hundredth as much reflected moonlight as our moon sends to the earth. accordingly, a "moonlit night" on mars can have no such charm as we associate with the phrase. but it is surely a tribute to the power and perfection of our telescopes that we have been able to discover the existence of objects so minute and inconspicuous, situated at a distance of many millions of miles, and half concealed by the glaring light of the planet close around which they revolve. if mars's moons were as massive as our moon is they would raise tremendous tides upon mars, and would affect the circulation of water in the canals, but, in fact, their tidal effects are even more insignificant than their light-giving powers. but for astronomers on mars they would be objects of absorbing interest. upon quitting mars we pass to the second distinctive planetary group of the solar system, that of the asteroids. chapter v the asteroids, a family of dwarf worlds beyond mars, in the broad gap separating the terrestrial from the jovian planets, are the asteroids, of which nearly five hundred have been discovered and designated by individual names or numbers. but any statement concerning the known number of asteroids can remain valid for but a short time, because new ones are continually found, especially by the aid of photography. very few of the asteroids are of measurable size. among these are the four that were the first to be discovered--ceres, pallas, juno, and vesta. their diameters, according to the measurements of prof. e.e. barnard, of the yerkes observatory, are as follows: ceres, miles; pallas, miles; juno, miles; vesta, miles. it is only necessary to mention these diameters in order to indicate how wide is the difference between the asteroids and such planets as the earth, venus, or mars. the entire surface of the largest asteroid, ceres, does not equal the republic of mexico in area. but ceres itself is gigantic in comparison with the vast majority of the asteroids, many of which, it is believed, do not exceed twenty miles in diameter, while there may be hundreds or thousands of others still smaller--ten miles, five miles, or perhaps only a few rods, in diameter! curiously enough, the asteroid which appears brightest, and which it would naturally be inferred is the largest, really stands third in the order of measured size. this is vesta, whose diameter, according to barnard, is only miles. it is estimated that the surface of vesta possesses about four times greater light-reflecting power than the surface of ceres. some observations have also shown a variation in the intensity of the light from vesta, a most interesting fact, which becomes still more significant when considered in connection with the great variability of another most extraordinary member of the asteroidal family, eros, which is to be described presently. the orbits of the asteroids are scattered over a zone about , , miles broad. the mean distance from the sun of the nearest asteroid, eros, is , , miles, and that of the most distant, thule, , , miles. wide gaps exist in the asteroidal zone where few or no members of the group are to be found, and prof. daniel kirkwood long ago demonstrated the influence of jupiter in producing these gaps. almost no asteroids, as he showed, revolve at such a distance from the sun that their periods of revolution are exactly commensurable with that of jupiter. originally there may have been many thus situated, but the attraction of the great planet has, in the course of time, swept those zones clean. many of the asteroids have very eccentric orbits, and their orbits are curiously intermixed, varying widely among themselves, both in ellipticity and in inclination to the common plane of the solar system. considered with reference to the shape and position of its orbit, the most unique of these little worlds is eros, which was discovered in by de witt, at berlin, and which, on account of its occasional near approach to the earth, has lately been utilized in a fresh attempt to obtain a closer approximation to the true distance of the sun from the earth. the mean distance of eros from the sun is , , miles, its greatest distance is , , miles, and its least distance , , miles. it will thus be seen that, although all the other asteroids are situated beyond mars, eros, at its mean distance, is nearer to the sun than mars is. when in aphelion, or at its greatest distance, eros is outside of the orbit of mars, but when in perihelion it is so much inside of mars's orbit that it comes surprisingly near the earth. indeed, there are times when eros is nearer to the earth than any other celestial body ever gets except the moon--and, it might be added, except meteors and, by chance, a comet, or a comet's tail. its least possible distance from the earth is less than , , miles, and it was nearly as close as that, without anybody knowing or suspecting the fact, in , four years in advance of its discovery. yet the fact, strange as the statement may seem, had been recorded without being recognized. after de witt's discovery of eros in , at a time when it was by no means as near the earth as it had been some years before, prof. e.c. pickering ascertained that it had several times imprinted its image on the photographic plates of the harvard observatory, with which pictures of the sky are systematically taken, but had remained unnoticed, or had been taken for an ordinary star among the thousands of star images surrounding it. from these telltale plates it was ascertained that in it had been in perihelion very near the earth, and had shone with the brilliance of a seventh-magnitude star. it will, unfortunately, be a long time before eros comes quite as near us as it did on that occasion, when we failed to see it, for its close approaches to the earth are not frequent. prof. solon i. bailey selects the oppositions of eros in and as probably the most favorable that will occur during the first half of the twentieth century. we turn to the extraordinary fluctuations in the light of eros, and the equally extraordinary conclusions drawn from them. while the little asteroid, whose diameter is estimated to be in the neighborhood of twenty or twenty-five miles, was being assiduously watched and photographed during its opposition in the winter of - , several observers discovered that its light was variable to the extent of more than a whole magnitude; some said as much as two magnitudes. when it is remembered that an increase of one stellar magnitude means an accession of light in the ratio of . to , and an increase of two magnitudes an accession of . to , the significance of such variations as eros exhibited becomes immediately apparent. the shortness of the period within which the cycle of changes occurred, about two hours and a half, made the variation more noticeable, and at the same time suggested a ready explanation, viz., that the asteroid was rapidly turning on its axis, a thing, in itself, quite in accordance with the behavior of other celestial bodies and naturally to be expected. but careful observation showed that there were marked irregularities in the light fluctuations, indicating that eros either had a very strange distribution of light and dark areas covering its surface, or that instead of being a globular body it was of some extremely irregular shape, so that as it rotated it presented successively larger and smaller reflecting surfaces toward the sun and the earth. one interesting suggestion was that the little planet is in reality double, the two components revolving around their common center of gravity, like a close binary star, and mutually eclipsing one another. but this theory seems hardly competent to explain the very great fluctuation in light, and a better one, probably, is that suggested by prof. e.c. pickering, that eros is shaped something like a dumb-bell. we can picture such a mass, in imagination, tumbling end over end in its orbit so as to present at one moment the broad sides of both bells, together with their connecting neck, toward the sun, and, at the same time, toward the observer on the earth, and, at another moment, only the end of one of the bells, the other bell and the neck being concealed in shadow. in this way the successive gain and loss of sixfold in the amount of light might be accounted for. owing to the great distance the real form of the asteroid is imperceptible even with powerful telescopes, but the effect of a change in the amount of reflecting surface presented produces, necessarily, an alternate waxing and waning of the light. as far as the fluctuations are concerned, they might also be explained by supposing that the shape of the asteroid is that of a flat disk, rotating about one of its larger diameters so as to present, alternately, its edge and its broadside to the sun. and, perhaps, in order completely to account for all the observed eccentricities of the light of eros, the irregularity of form may have to be supplemented by certain assumptions as to the varying reflective capacity of different parts of the misshapen mass. the invaluable harvard photographs show that long before eros was recognized as an asteroid its light variations had been automatically registered on the plates. some of the plates, prof. e.c. pickering says, had had an exposure of an hour or more, and, owing to its motion, eros had formed a trail on each of these plates, which in some cases showed distinct variations in brightness. differences in the amount of variation at different times will largely depend upon the position of the earth with respect to the axis of rotation. another interesting deduction may be made from the changes that the light of eros undergoes. we have already remarked that one of the larger asteroids, and the one which appears to the eye as the most brilliant of all, vesta, has been suspected of variability, but not so extensive as that of eros. olbers, at the beginning of the last century, was of the opinion that vesta's variations were due to its being not a globe but an angular mass. so he was led by a similar phenomenon to precisely the same opinion about vesta that has lately been put forth concerning eros. the importance of this coincidence is that it tends to revive a remarkable theory of the origin of the asteroids which has long been in abeyance, and, in the minds of many, perhaps discredited. this theory, which is due to olbers, begins with the startling assumption that a planet, perhaps as large as mars, formerly revolving in an orbit situated between the orbits of mars and jupiter, was destroyed by an explosion! although, at first glance, such a catastrophe may appear too wildly improbable for belief, yet it was not the improbability of a world's blowing up that led to a temporary abandonment of olbers's bold theory. the great french mathematician lagrange investigated the explosive force "which would be necessary to detach a fragment of matter from a planet revolving at a given distance from the sun," and published the results in the connaissance des temps for . "applying his results to the earth, lagrange found that if the velocity of the detached fragment exceeded that of a cannon ball in the proportion of to the fragment would become a comet with a direct motion; but if the velocity rose in the proportion of to the motion of the comet would be retrograde. if the velocity was less than in either of these cases the fragment would revolve as a planet in an elliptic orbit. for any other planet besides the earth the velocity of explosion corresponding to the different cases would vary in the inverse ratio of the square root of the mean distance. it would therefore manifestly be less as the planet was more distant from the sun. in the case of each of the four smaller planets (only the four asteroids, ceres, pallas, juno, and vesta, were known at that time), the velocity of explosion indicated by their observed motion would be less than twenty times the velocity of a cannon ball."[ ] [footnote : grant's history of physical astronomy, p. .] instead, then, of being discredited by its assumption of so strange a catastrophe, olbers's theory fell into desuetude because of its apparent failure to account for the position of the orbits of many of the asteroids after a large number of those bodies had been discovered. he calculated that the orbits of all the fragments of his exploded planet would have nearly equal mean distances, and a common point of intersection in the heavens, through which every fragment of the original mass would necessarily pass in each revolution. at first the orbits of the asteroids discovered seemed to answer to these conditions, and olbers was even able to use his theory as a means of predicting the position of yet undetected asteroids. only ceres and pallas had been discovered when he put forth his theory, but when juno and vesta were found they fell in with his predictions so well that the theory was generally regarded as being virtually established; while the fluctuations in the light of vesta, as we have before remarked, led olbers to assert that that body was of a fragmental shape, thus strongly supporting his explosion hypothesis. afterward, when the orbits of many asteroids had been investigated, the soundness of olbers's theory began to be questioned. the fact that the orbits did not all intersect at a common point could easily be disposed of, as professor newcomb has pointed out, by simply placing the date of the explosion sufficiently far back, say millions of years ago, for the secular changes produced by the attraction of the larger planets would effectively mix up the orbits. but when the actual effects of these secular changes were calculated for particular asteroids the result seemed to show that "the orbits could never have intersected unless some of them have in the meantime been altered by the attraction of the small planets on each other. such an action is not impossible, but it is impossible to determine it, owing to the great number of these bodies and our ignorance of their masses."[ ] [footnote : popular astronomy, by simon newcomb, p. .] yet the theory has never been entirely thrown out, and now that the discovery of the light fluctuations of eros lends support to olbers's assertion of the irregular shape of some of the asteroids, it is very interesting to recall what so high an authority as professor young said on the subject before the discovery of eros: "it is true, as has often been urged, that this theory in its original form, as presented by olbers, can not be correct. no _single_ explosion of a planet could give rise to the present assemblage of orbits, nor is it possible that even the perturbations of jupiter could have converted a set of orbits originally all crossing at one point (the point of explosion) into the present tangle. the smaller orbits are so small that, however turned about, they lie wholly inside the larger and can not be made to intersect them. if, however, we admit a _series_ of explosions, this difficulty is removed; and if we grant an explosion at all, there seems to be nothing improbable in the hypothesis that the fragments formed by the bursting of the parent mass would carry away within themselves the same forces and reactions which caused the original bursting, so that they themselves would be likely enough to explode at some time in their later history."[ ] [footnote : general astronomy, by charles a. young. revised edition, , p. .] the rival theory of the origin of the asteroids is that which assumes that the planetary ring originally left off from the contracting solar nebula between the orbits of mars and jupiter was so violently perturbed by the attraction of the latter planet that, instead of being shaped into a single globe, it was broken up into many fragments. either hypothesis presents an attractive picture; but that which presupposes the bursting asunder of a large planet, which might at least have borne the germs of life, and the subsequent shattering of its parts into smaller fragments, like the secondary explosions of the pieces of a pyrotechnic bomb, certainly is by far the more impressive in its appeal to the imagination, and would seem to offer excellent material for some of the extra-terrestrial romances now so popular. it is a startling thought that a world can possibly carry within itself, like a dynamite cartridge, the means of its own disruption; but the idea does not appear so extremely improbable when we recall the evidence of collisions or explosions, happening on a tremendous scale, in the case of new or temporary stars.[ ] [footnote : "since the discovery of eros, the extraordinary position of its orbit has led to the suggestion that possibly mars itself, instead of being regarded as primarily a major planet, belonging to the terrestrial group, ought rather to be considered as the greatest of the asteroids, and a part of the original body from which the asteroidal system was formed."--j. bauschinger, astronomische nachrichten, no. .] coming to the question of life upon the asteroids, it seems clear that they must be excluded from the list of habitable worlds, whatever we may choose to think of the possible habitability of the original planet through whose destruction they may have come into existence. the largest of them possesses a force of gravity far too slight to enable it to retain any of the gases or vapors that are recognized as constituting an atmosphere. but they afford a captivating field for speculation, which need not be altogether avoided, for it offers some graphic illustrations of the law of gravitation. a few years ago i wrote, for the entertainment of an audience which preferred to meet science attired in a garb woven largely from the strands of fancy, an account of some of the peculiarities of such minute globes as the asteroids, which i reproduce here because it gives, perhaps, a livelier picture of those little bodies, from the point of view of ordinary human interest, than could be presented in any other way. a waif of space one night as i was waiting, watch in hand, for an occultation, and striving hard to keep awake, for it had been a hot and exhausting summer's day, while my wife--we were then in our honeymoon--sat sympathetically by my side, i suddenly found myself withdrawn from the telescope, and standing in a place that appeared entirely strange. it was a very smooth bit of ground, and, to my surprise, there was no horizon in sight; that is to say, the surface of the ground disappeared on all sides at a short distance off, and beyond nothing but sky was visible. i thought i must be on the top of a stupendous mountain, and yet i was puzzled to understand how the face of the earth could be so far withdrawn. presently i became aware that there was some one by me whom i could not see. "you are not on a mountain," my companion said, and as he spoke a cold shiver ran along my back-bone; "you are on an asteroid, one of those miniature planets, as you astronomers call them, and of which you have discovered several hundred revolving between the orbits of mars and jupiter. this is the little globe that you have glimpsed occasionally with your telescope, and that you, or some of your fellows, have been kind enough to name menippe." then i perceived that my companion, whose address had hardly been reassuring, was a gigantic inhabitant of the little planet, towering up to a height of three quarters of a mile. for a moment i was highly amused, standing by his foot, which swelled up like a hill, and straining my neck backward to get a look up along the precipice of his leg, which, curiously enough, i observed was clothed in rough homespun, the woolly knots of the cloth appearing of tremendous size, while it bagged at the knee like any terrestrial trousers' leg. his great head and face i could see far above me, as it were, in the clouds. yet i was not at all astonished. "this is all right," i said to myself. "of course on menippe the people must be as large as this, for the little planet is only a dozen miles in diameter, and the force of gravity is consequently so small that a man without loss of activity, or inconvenience, can grow three quarters of a mile tall." suddenly an idea occurred to me. "just to think what a jump i can make! why, only the other day i was figuring it out that a man could easily jump a thousand feet high from the surface of menippe, and now here i actually am on menippe. i'll jump." the sensation of that glorious rise skyward was delightful beyond expression. my legs seemed to have become as powerful as the engines of a transatlantic liner, and with one spring i rose smoothly and swiftly, and as straight as an arrow, surmounting the giant's foot, passing his knee and attaining nearly to the level of his hip. then i felt that the momentum of my leap was exhausted, and despite my efforts i slowly turned head downward, glancing in affright at the ground a quarter of a mile below me, on which i expected to be dashed to pieces. but a moment's thought convinced me that i should get no hurt, for with so slight a force of gravity it would be more like floating than falling. just then the menippean caught me with his monstrous hand and lifted me to the level of his face. "i should like to know," i said, "how you manage to live up here; you are so large and your planet is so little." "now, you are altogether too inquisitive," replied the giant. "you go!" he stooped down, placed me on the toe of his boot, and drew back his foot to kick me off. it flashed into my mind that my situation had now become very serious. i knew well what the effects of the small attractive force of these diminutive planets must be, for i had often amused myself with calculations about them. in this moment of peril i did not forget my mathematics. it was clear that if the giant propelled me with sufficient velocity i should be shot into space, never to return. how great would that velocity have to be? my mind worked like lightning on this problem. the diameter of menippe i knew did not exceed twelve miles. its mean density, as near as i could judge, was about the same as that of the earth. its attraction must therefore be as its radius, or nearly times less than that of the earth. a well-known formula enables us to compute the velocity a body would acquire in falling from an infinite distance to the earth or any other planet whose size and force of gravity are known. the same formula, taken in the opposite sense, of course, shows how fast a body must start from a planet in order that it may be freed from its control. the formula is v = square root of ( gr.), in which "g" is the acceleration of gravity, equal for the earth to feet in a second, and "r" is the radius of the attracting body. on menippe i knew "g" must equal about one twentieth of a foot, and "r" , feet. like a flash i applied the formula while the giant's muscles were yet tightening for the kick: , Ã� / Ã� = , , the square root of which is a fraction more than . fifty-six feet in a second, then, was the critical velocity with which i must be kicked off in order that i might never return. i perceived at once that the giant would be able to accomplish it. i turned and shouted up at him: "hold on, i have something to say to you!" i dimly saw his mountainous face puckered into mighty wrinkles, out of which his eyes glared fiercely, and the next moment i was sailing into space. i could no more have kept a balance than the earth can stand still upon its axis. i had become a small planet myself, and, like all planets, i rotated. yet the motion did not dizzy me, and soon i became intensely interested in the panorama of creation that was spread around me. for some time, whenever my face was turned toward the little globe of menippe, i saw the giant, partly in profile against the sky, with his back bent and his hands upon his knees, watching me with an occasional approving nod of his big head. he looked so funny standing there on his little seven-by-nine world, like a clown on a performing ball, that, despite my terrible situation, i shook my sides with laughter. there was no echo in the profundity of empty space. soon menippe dwindled to a point, and i saw her inhospitable inhabitant no more. then i watched the sun and the blazing firmament around, for there was at the same time broad day and midnight for me. the sunlight, being no longer diffused by an atmosphere, did not conceal the face of the sky, and i could see the stars shining close to the orb of day. i recognized the various planets much more easily than i had been accustomed to do, and, with a twinge at my heart, saw the earth traveling along in its distant orbit, splendid in the sunshine. i thought of my wife sitting alone by the telescope in the darkness and silence, wondering what had become of me. i asked myself, "how in the world can i ever get back there again?" then i smiled to think of the ridiculous figure i cut, out here in space, exposed to the eyes of the universe, a rotating, gyrating, circumambulating astronomer, an animated teetotum lost in the sky. i saw no reason to hope that i should not go on thus forever, revolving around the sun until my bones, whitening among the stars, might be revealed to the superlative powers of some future telescope, and become a subject of absorbing interest, the topic of many a learned paper for the astronomers of a future age. afterward i was comforted by the reflection that in airless space, although i might die and my body become desiccated, yet there could be no real decay; even my garments would probably last forever. the _savants_, after all, should never speculate on my bones. i saw the ruddy disk of mars, and the glinting of his icy poles, as the beautiful planet rolled far below me. "if i could only get there," i thought, "i should know what those canals of schiaparelli are, and even if i could never return to the earth, i should doubtless meet with a warm welcome among the martians. what a lion i should be!" i looked longingly at the distant planet, the outlines of whose continents and seas appeared most enticing, but when i tried to propel myself in that direction i only kicked against nothingness. i groaned in desperation. suddenly something darted by me flying sunward; then another and another. in a minute i was surrounded by strange projectiles. every instant i expected to be dashed in pieces by them. they sped with the velocity of lightning. hundreds, thousands of them were all about me. my chance of not being hit was not one in a million, and yet i escaped. the sweat of terror was upon me, but i did not lose my head. "a comet has met me," i said. "these missiles are the meteoric stones of which it is composed." and now i noticed that as they rushed along collisions took place, and flashes of electricity darted from one to another. a pale luminosity dimmed the stars. i did not doubt that, as seen from the earth, the comet was already flinging the splendors of its train upon the bosom of the night. while i was wondering at my immunity amid such a rain of death-threatening bolts, i became aware that their velocity was sensibly diminishing. this fact i explained by supposing that i was drawn along with them. notwithstanding the absence of any collision with my body, the overpowering attraction of the whole mass of meteors was overcoming my tangential force and bearing me in their direction. at first i rejoiced at this circumstance, for at any rate the comet would save me from the dreadful fate of becoming an asteroid. a little further reflection, however, showed me that i had gone from the frying-pan into the fire. the direction of my expulsion from menippe had been such that i had fallen into an orbit that would have carried me around the sun without passing very close to the solar body. now, being swept along by the comet, whose perihelion probably lay in the immediate neighborhood of the sun, i saw no way of escape from the frightful fate of being broiled alive. even where i was, the untempered rays of the sun scorched me, and i knew that within two or three hundred thousand miles of the solar surface the heat must be sufficient to melt the hardest rocks. i was aware that experiments with burning-glasses had sufficiently demonstrated that fact. but perforce i resigned myself to my fate. at any rate it would the sooner be all over. in fact, i almost forgot my awful situation in the interest awakened by the phenomena of the comet. i was in the midst of its very head. i was one of its component particles. i was a meteor among a million millions of others. if i could only get back to the earth, what news could i not carry to signor schiaparelli and mr. lockyer and dr. bredichin about the composition of comets! but, alas! the world could never know what i now saw. nobody on yonder gleaming earth, watching the magnificent advance of this "specter of the skies," would ever dream that there was a lost astronomer in its blazing head. i should be burned and rent to pieces amid the terrors of its perihelion passage, and my fragments would be strewn along the comet's orbit, to become, in course of time, particles in a swarm of aerolites. perchance, through the effects of some unforeseen perturbation, the earth might encounter that swarm. thus only could i ever return to the bosom of my mother planet. i took a positive pleasure in imagining that one of my calcined bones might eventually flash for a moment, a falling star, in the atmosphere of the earth, leaving its atoms to slowly settle through the air, until finally they rested in the soil from which they had sprung. from such reflections i was aroused by the approach of the crisis. the head of the comet had become an exceedingly uncomfortable place. the collisions among the meteors were constantly increasing in number and violence. how i escaped destruction i could not comprehend, but in fact i was unconscious of danger from that source. i had become in spirit an actual component of the clashing, roaring mass. tremendous sparks of electricity, veritable lightning strokes, darted about me in every direction, but i bore a charmed life. as the comet drew in nearer to the sun, under the terrible stress of the solar attraction, the meteors seemed to crowd closer, crashing and grinding together, while the whole mass swayed and shrieked with the uproar of a million tormented devils. the heat had become terrific. i saw stone and iron melted like snow and dissipated in steam. stupendous jets of white-hot vapor shot upward, and, driven off by the electrical repulsion of the sun, streamed backward into the tail. suddenly i myself became sensible of the awful heat. it seemed without warning to have penetrated my vitals. with a yell i jerked my feet from a boiling rock and flung my arms despairingly over my head. "you had better be careful," said my wife, "or you'll knock over the telescope." i rubbed my eyes, shook myself, and rose. "i must have been dreaming," i said. "i should think it was a very lively dream," she replied. i responded after the manner of a young man newly wed. at this moment the occultation began. chapter vi jupiter, the greatest of known worlds when we are thinking of worlds, and trying to exalt the imagination with them, it is well to turn to jupiter, for there is a planet worth pondering upon! a world thirteen hundred times as voluminous as the earth is a phenomenon calculated to make us feel somewhat as the inhabitant of a rural village does when his amazed vision ranges across the million roofs of a metropolis. jupiter is the first of the outer and greater planets, the major, or jovian, group. his mean diameter is , miles, and his average girth more than , miles. an inhabitant of jupiter, in making a trip around his planet, along any great circle of the sphere, would have to travel more than , miles farther than the distance between the earth and the moon. the polar compression of jupiter, owing to his rapid rotation, amounts in the aggregate to more than , miles, the equatorial diameter being , miles and the polar diameter , miles. jupiter's mean distance from the sun is , , miles, and the eccentricity of his orbit is sufficient to make this distance variable to the extent of , , miles; but, in view of his great average distance, the consequent variation in the amount of solar light and heat received by the planet is not of serious importance. when he is in opposition to the sun as seen from the earth jupiter's mean distance from us is about , , miles. his year, or period of revolution about the sun, is somewhat less than twelve of our years ( . years). his axis is very nearly upright to the plane of his orbit, so that, as upon venus, there is practically no variation of seasons. gigantic though he is in dimensions, jupiter is the swiftest of all the planets in axial rotation. while the earth requires twenty-four hours to make a complete turn, jupiter takes less than ten hours (nine hours fifty-five minutes), and a point on his equator moves, in consequence of axial rotation, between , and , miles in an hour. the density of the mighty planet is slight, only about one quarter of the mean density of the earth and virtually the same as that of the sun. this fact at once calls attention to a contrast between jupiter and our globe that is even more significant than their immense difference in size. the force of gravity upon jupiter's surface is more than two and a half times greater than upon the earth's surface (more accurately . times), so that a hundred-pound weight removed from the planet on which we live to jupiter would there weigh pounds, and an average man, similarly transported, would be oppressed with a weight of at least pounds. but, as a result of the rapid rotation of the great planet, and the ellipticity of its figure, the unfortunate visitor could find a perceptible relief from his troublesome weight by seeking the planet's equator, where the centrifugal tendency would remove about twenty pounds from every one hundred as compared with his weight at the poles. if we could go to the moon, or to mercury, venus, or mars, we may be certain that upon reaching any of those globes we should find ourselves upon a solid surface, probably composed of rock not unlike the rocky crust of the earth; but with jupiter the case would evidently be very different. as already remarked, the mean density of that planet is only one quarter of the earth's density, or only one third greater than the density of water. consequently the visitor, in attempting to set foot upon jupiter, might find no solid supporting surface, but would be in a situation as embarrassing as that of milton's satan when he undertook to cross the domain of chaos: "fluttering his pinions vain, plumb down he drops, ten thousand fathom deep, and to this hour down had been falling had not, by ill chance, the strong rebuff of some tumultuous cloud. instinct with fire and niter, hurried him as many miles aloft; that fury stayed, quenched in a boggy syrtis, neither sea nor good dry land, nigh foundered, as he fares, treading the crude consistence, half on foot, half flying." the probability that nothing resembling a solid crust, nor, perhaps, even a liquid shell, would be found at the visible surface of jupiter, is increased by considering that the surface density must be much less than the mean density of the planet taken as a whole, and since the latter but little exceeds the density of water, it is likely that at the surface everything is in a state resembling that of cloud or smoke. our imaginary visitor upon reaching jupiter would, under the influence of the planet's strong force of gravity, drop out of sight, with the speed of a shot, swallowed up in the vast atmosphere of probably hot, and perhaps partially incandescent, gases. when he had sunk--supposing his identity could be preserved--to a depth of thousands of miles he might not yet have found any solid part of the planet; and, perchance, there is no solid nucleus even at the very center. the cloudy aspect of jupiter immediately strikes the telescopic observer. the huge planet is filled with color, and with the animation of constant movement, but there is no appearance of markings, like those on mars, recalling the look of the earth. there are no white polar caps, and no shadings that suggest the outlines of continents and oceans. what every observer, even with the smallest telescope, perceives at once is a pair of strongly defined dark belts, one on either side of, and both parallel to, the planet's equator. these belts are dark compared with the equatorial band between them and with the general surface of the planet toward the north and the south, but they are not of a gray or neutral shade. on the contrary, they show decided, and, at times, brilliant colors, usually of a reddish tone. more delicate tints, sometimes a fine pink, salmon, or even light green, are occasionally to be seen about the equatorial zone, and the borders of the belts, while near the poles the surface is shadowed with bluish gray, imperceptibly deepening from the lighter hues of the equator. all this variety of tone and color makes of a telescopic view of jupiter a picture that will not quickly fade from the memory; while if an instrument of considerable power is used, so that the wonderful details of the belts, with their scalloped edges, their diagonal filaments, their many divisions, and their curious light and dark spots, are made plain, the observer is deeply impressed with the strangeness of the spectacle, and the more so as he reflects upon the enormous real magnitude of that which is spread before his eye. the whole earth flattened out would be but a small blotch on that gigantic disk! then, the visible rotation of the great jovian globe, whose effects become evident to a practised eye after but a few minutes' watching, heightens the impression. and the presence of the four satellites, whose motions in their orbits are also evident, through the change in their positions, during the course of a single not prolonged observation, adds its influence to the effectiveness of the scene. indeed, color and motion are so conspicuous in the immense spectacle presented by jupiter that they impart to it a powerful suggestion of life, which the mind does not readily divest itself of when compelled to face the evidence that jupiter is as widely different from the earth, and as diametrically opposed to lifelike conditions, as we comprehend them, as a planet possibly could be. the great belts lie in latitudes about corresponding to those in which the trade-winds blow upon the earth, and it has often been suggested that their existence indicates a similarity between the atmospheric circulation of jupiter and that of the world in which we live. no doubt there are times when the earth, seen with a telescope from a distant planet, would present a belted appearance somewhat resembling that of jupiter, but there would almost certainly be no similar display of colors in the clouds, and the latter would exhibit no such persistence in general form and position as characterizes those of jupiter. our clouds are formed by the action of the sun, producing evaporation of water; on jupiter, whose mean distance from the sun is more than five times as great as ours, the intensity of the solar rays is reduced to less than one twenty-fifth part of their intensity on the earth, so that the evaporation can not be equally active there, and the tendency to form aerial currents and great systems of winds must be proportionally slight. in brief, the clouds of jupiter are probably of an entirely different origin from that of terrestrial clouds, and rather resemble the chaotic masses of vapor that enveloped the earth when it was still in a seminebulous condition, and before its crust had formed. although the strongest features of the disk of jupiter are the great cloud belts, and the white or colored spots in the equatorial zone, yet the telescope shows many markings north and south of the belts, including a number of narrower and fainter belts, and small light or dark spots. none of them is absolutely fixed in position with reference to others. in other words, all of the spots, belts, and markings shift their places to a perceptible extent, the changes being generally very slow and regular, but occasionally quite rapid. the main belts never entirely disappear, and never depart very far from their mean positions with respect to the equator, but the smaller belts toward the north and south are more or less evanescent. round or oblong spots, as distinguished from belts, are still more variable and transient. the main belts themselves show great internal commotion, frequently splitting up, through a considerable part of their length, and sometimes apparently throwing out projections into the lighter equatorial zone, which occasionally resemble bridges, diagonally spanning the broad space between the belts. [illustration: jupiter as seen at the lick observatory in . the great red spot is visible, together with the indentation in the south belt.] perhaps the most puzzling phenomenon that has ever made its appearance on jupiter is the celebrated "great red spot," which was first noticed in , although it has since been shown to be probably identical with a similar spot seen in , and possibly with one noticed in . this spot, soon after its discovery in , became a clearly defined red oval, lying near the southern edge of the south belt in latitude about °. its length was nearly one third of the diameter of the disk and its width almost one quarter as great as its length. translated into terrestrial measure, it was about , miles long and , miles broad. in it seemed to deepen in color until it became a truly wonderful object, its redness of hue irresistibly suggesting the idea that it was something hot and glowing. during the following years it underwent various changes of appearance, now fading almost to invisibility and now brightening again, but without ever completely vanishing, and it is still ( ) faintly visible. nobody has yet suggested an altogether probable and acceptable theory as to its nature. some have said that it might be a part of the red-hot crust of the planet elevated above the level of the clouds; others that its appearance might be due to the clearing off of the clouds above a heated region of the globe beneath, rendering the latter visible through the opening; others that it was perhaps a mass of smoke and vapor ejected from a gigantic volcano, or from the vents covering a broad area of volcanic action; others that it might be a vast incandescent slag floating upon the molten globe of the planet and visible through, or above, the enveloping clouds; and others have thought that it could be nothing but a cloud among clouds, differing, for unknown reasons, in composition and cohesion from its surroundings. all of these hypotheses except the last imply the existence, just beneath the visible cloud shell, of a more or less stable and continuous surface, either solid or liquid. when the red spot began to lose distinctness a kind of veil seemed to be drawn over it, as if light clouds, floating at a superior elevation, had drifted across it. at times it has been reduced in this manner to a faint oval ring, the rim remaining visible after the central part has faded from sight. one of the most remarkable phenomena connected with the mysterious spot is a great bend, or scallop, in the southern edge of the south belt adjacent to the spot. this looks as if it were produced by the spot, or by the same cause to which the spot owes its existence. if the spot were an immense mountainous elevation, and the belt a current of liquid, or of clouds, flowing past its base, one would expect to see some such bend in the stream. the visual evidence that the belt is driven, or forced, away from the neighborhood of the spot seems complete. the appearance of repulsion between them is very striking, and even when the spot fades nearly to invisibility the curve remains equally distinct, so that in using a telescope too small to reveal the spot itself one may discover its location by observing the bow in the south belt. the suggestion of a resemblance to the flowing of a stream past the foot of an elevated promontory, or mountain, is strengthened by the fact, which was observed early in the history of the spot, that markings involved in the south belt have a quicker rate of rotation about the planet's axis than that of the red spot, so that such markings, first seen in the rear of the red spot, gradually overtake and pass it, and eventually leave it behind, as boats in a river drift past a rock lying in the midst of the current. this leads us to another significant fact concerning the peculiar condition of jupiter's surface. not only does the south belt move perceptibly faster than the red spot, but, generally speaking, the various markings on the surface of the planet move at different rates according as they are nearer to or farther from the equator. between the equator and latitude ° or ° there is a difference of six minutes in the rotation period--i.e., the equatorial parts turn round the axis so much faster than the parts north and south of them, that in one rotation they gain six minutes of time. in other words, the clouds over jupiter's equator flow past those in the middle latitudes with a relative velocity of miles per hour. but there are no sharp lines of separation between the different velocities; on the contrary, the swiftness of rotation gradually diminishes from the equator toward the poles, as it manifestly could not do if the visible surface of jupiter were solid. in this respect jupiter resembles the sun, whose surface also has different rates of rotation diminishing from the equator. measured by the motion of spots on or near the equator, jupiter's rotation period is about nine hours fifty minutes; measured by the motion of spots in the middle latitudes, it is about nine hours fifty-six minutes. the red spot completes a rotation in a little less than nine hours and fifty-six minutes, but its period can not be positively given for the singular reason that it is variable. the variation amounts to only a few seconds in the course of several years, but it is nevertheless certain. the phenomenon of variable motion is not, however, peculiar to the red spot. mr. w.f. denning, who has studied jupiter for a quarter of a century, says: "it is well known that in different latitudes of jupiter there are currents, forming the belts and zones, moving at various rates of speed. in many instances the velocity changes from year to year. and it is a singular circumstance that in the same current a uniform motion is not maintained in all parts of the circumference. certain spots move faster than others, so that if we would obtain a fair value for the rotation period of any current it is not sufficient to derive it from one marking alone; we must follow a number of objects distributed in different longitudes along the current and deduce a mean from the whole."[ ] [footnote : the observatory, no. , december, .] nor is this all. observation indicates that if we could look at a vertical section of jupiter's atmosphere we should behold an equally remarkable contrast and conflict of motions. there is evidence that some of the visible spots, or clouds, lie at a greater elevation than others, and it has been observed that the deeper ones move more rapidly. this fact has led some observers to conclude that the deep-lying spots may be a part of the actual surface of the planet. but if we could think that there is any solid nucleus, or core, in the body of jupiter, it would seem, on account of the slight mean density of the planet, that it can not lie so near the visible surface, but must be at a depth of thousands, perhaps tens of thousands, of miles. since the telescope is unable to penetrate the cloudy envelope we can only guess at the actual constitution of the interior of jupiter's globe. in a spirit of mere speculative curiosity it has been suggested that deep under the clouds of the great planet there may be a comparatively small solid globe, even a habitable world, closed round by a firmament all its own, whose vault, raised , or , miles above the surface of the imprisoned planet, appears only an unbroken dome, too distant to reveal its real nature to watchers below, except, perhaps, under telescopic scrutiny; enclosing, as in a shell, a transparent atmosphere, and deriving its illumination partly from the sunlight that may filter through, but mainly from some luminous source within. but is not jupiter almost equally fascinating to the imagination, if we dismiss all attempts to picture a humanly impossible world shut up within it, and turn rather to consider what its future may be, guided by the not unreasonable hypothesis that, because of its immense size and mass, it is still in a chaotic condition? mention has been made of the resemblance of jupiter to the sun by virtue of their similar manner of rotation. this is not the only reason for looking upon jupiter as being, in some respects, almost as much a solar as a planetary body. its exceptional brightness rather favors the view that a small part of the light by which it shines comes from its own incandescence. in size and mass it is half-way between the earth and the sun. jupiter is eleven times greater than the earth in diameter and thirteen hundred times greater in volume; the sun is ten times greater than jupiter in diameter and a thousand times greater in volume. the mean density of jupiter, as we have seen, is almost exactly the same as the sun's. now, the history of the solar system, according to the nebular hypothesis, is a history of cooling and condensation. the sun, a thousand times larger than jupiter, has not yet sufficiently cooled and contracted to become incrusted, except with a shell of incandescent metallic clouds; jupiter, a thousand times smaller than the sun, has cooled and contracted until it is but slightly, if at all, incandescent at its surface, while its thickening shell, although still composed of vapor and smoke, and still probably hot, has grown so dense that it entirely cuts off the luminous radiation from within; the earth, to carry the comparison one step further, being more than a thousand times smaller than jupiter, has progressed so far in the process of cooling that its original shell of vapor has given place to one of solid rock. a sudden outburst of light from jupiter, such as occurs occasionally in a star that is losing its radiance through the condensation of absorbing vapors around it, would furnish strong corroboration of the theory that jupiter is really an extinguished sun which is now on the way to become a planet in the terrestrial sense. not very long ago, as time is reckoned in astronomy, our sun, viewed from the distance of the nearer fixed stars, may have appeared as a binary star, the brighter component of the pair being the sun itself and the fainter one the body now called the planet jupiter. supposing the latter to have had the same intrinsic brilliance, surface for surface, as the sun, it would have radiated one hundred times less light than the sun. a difference of one hundredfold between the light of two stars means that they are six magnitudes apart; or, in other words, from a point in space where the sun appeared as bright as what we call a first-magnitude star, its companion, jupiter, would have shone as a sixth-magnitude star. many stars have companions proportionally much fainter than that. the companion of sirius, for instance, is at least ten thousand times less bright than its great comrade. looking at jupiter in this way, it interests us not as the probable abode of intelligent life, but as a world in the making, a world, moreover, which, when it is completed--if it ever shall be after the terrestrial pattern--will dwarf our globe into insignificance. that stupendous miracle of world-making which is dimly painted in the grand figures employed by the writers of genesis, and the composers of other cosmogonic legends, is here actually going on before our eyes. the telescope shows us in the cloudy face of jupiter the moving of the spirit upon the face of the great deep. what the final result will be we can not tell, but clearly the end of the grand processes there in operation has not yet been reached. the interesting suggestion was made and urged by mr. proctor that if jupiter itself is in no condition at present to bear life, its satellites may be, in that respect, more happily circumstanced. it can not be said that very much has been learned about the satellites of jupiter since proctor's day, and his suggestion is no less and no more probable now than it was when first offered. there has been cumulative evidence that jupiter's satellites obey the same law that governs the rotation of our moon, viz., that which compels them always to keep the same face turned toward their primary, and this would clearly affect, although it might not preclude, their habitability. with the exception of the minute fifth satellite discovered by barnard in , they are all of sufficient size to retain at least some traces of an atmosphere. in fact, one of them is larger than the planet mars, and another is of nearly the same size as that planet, while the smallest of the four principal ones is about equal to our moon. under the powerful attraction of jupiter they travel rapidly, and viewed from the surface of that planet they would offer a wonderful spectacle. they are continually causing solar eclipses and themselves undergoing eclipse in jupiter's shadow, and their swiftly changing aspects and groupings would be watched by an astronomer on jupiter with undying interest. but far more wonderful would be the spectacle presented by jupiter to inhabitants dwelling on his moons. from the nearer moon, in particular, which is situated less than , miles from jupiter's surface, the great planet would be an overwhelming phenomenon in the sky. its immense disk, hanging overhead, would cover a circle of the firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! it would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives. the maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us. here, again, the mind is carried back to long past ages in the history of the planet on which we dwell. it is believed by some that our moon may have contained inhabitants when the earth was still hot and glowing, as jupiter appears to be now, and that, as the earth cooled and became habitable, the moon gradually parted with its atmosphere and water so that its living races perished almost coincidently with the beginning of life on the earth. if we accept this view and apply it to the case of jupiter we may conclude that when that enormous globe has cooled and settled down to a possibly habitable condition, its four attendant moons will suffer the fate that overtook the earth's satellite, and in their turn become barren and death-stricken, while the great orb that once nurtured them with its light and heat receives the promethean fire and begins to bloom with life. chapter vii saturn, a prodigy among planets one of the first things that persons unaccustomed to astronomical observations ask to see when they have an opportunity to look through a telescope is the planet saturn. many telescopic views in the heavens disappoint the beginner, but that of saturn does not. even though the planet may not look as large as he expects to see it from what he has been told of the magnifying power employed, the untrained observer is sure to be greatly impressed by the wonderful rings, suspended around it as if by a miracle. no previous inspection of pictures of these rings can rob them of their effect upon the eye and the mind. they are overwhelming in their inimitable singularity, and they leave every spectator truly amazed. sir john herschel has remarked that they have the appearance of an "elaborately artificial mechanism." they have even been regarded as habitable bodies! what we are to think of that proposition we shall see when we come to consider their composition and probable origin. in the meantime let us recall the main facts of saturn's dimensions and situation in the solar system. saturn is the second of the major, or jovian, group of planets, and is situated at a mean distance from the sun of , , miles. we need not consider the eccentricity of its orbit, which, although relatively not very great, produces a variation of , , miles in its distance from the sun, because, at its immense mean distance, this change would not be of much importance with regard to the planet's habitability or non-habitability. under the most favorable conditions saturn can never be nearer than , , miles to the earth, or eight times the sun's distance from us. it receives from the sun about one ninetieth of the light and heat that we get. [illustration: saturn in its three principal phases as seen from the earth. from a drawing by bond.] saturn takes twenty-nine and a half years to complete a journey about the sun. like jupiter, it rotates very rapidly on its axis, the period being ten hours and fourteen minutes. its axis of rotation is inclined not far from the same angle as that of the earth's axis ( ° ´), so that its seasons should resemble ours, although their alternations are extremely slow in consequence of the enormous length of saturn's year. not including the rings in the calculation, saturn exceeds the earth in size times. the addition of the rings would not, however, greatly alter the result of the comparison, because, although the total surface of the rings, counting both faces, exceeds the earth's surface about times, their volume, owing to their surprising thinness, is only about six times the volume of the earth, and their mass, in consequence of their slight density, is very much less than the earth's, perhaps, indeed, inappreciable in comparison. saturn's mean diameter is , miles, and its polar compression is even greater than that of jupiter, a difference of , miles--almost comparable with the entire diameter of the earth--existing between its equatorial and its polar diameter, the former being , and the latter , miles. we found the density of jupiter astonishingly slight, but that of saturn is slighter still. jupiter would sink if thrown into water, but saturn would actually float, if not "like a cork," yet quite as buoyantly as many kinds of wood, for its mean density is only three quarters that of water, or one eighth of the earth's. in fact, there is no known planet whose density is so slight as saturn's. thus it happens that, notwithstanding its vast size and mass, the force of gravity upon saturn is nearly the same as upon our globe. upon visiting venus we should find ourselves weighing a little less than at home, and upon visiting saturn a little more, but in neither case would the difference be very important. if the relative weight of bodies on the surfaces of planets formed the sole test of their habitability, venus and saturn would both rank with the earth as suitable abodes for men. but the exceedingly slight density of saturn seems to be most reasonably accounted for on the supposition that, like jupiter, it is in a vaporous condition, still very hot within--although but slightly, if at all, incandescent at the surface--and, therefore, unsuited to contain life. it is hardly worth while to speculate about any solid nucleus within, because, even if such a thing were possible, or probable, it must lie forever hidden from our eyes. but if we accept the theory that saturn is in an early formative stage, and that, millions of years hence, it may become an incrusted and habitable globe, we shall, at least, follow the analogy of what we believe to have been the history of the earth, except that saturn's immense distance from the sun will always prevent it from receiving an amount of solar radiation consistent with our ideas of what is required by a living world. of course, since one can imagine what he chooses, it is possible to suppose inhabitants suited to existence in a world composed only of whirling clouds, and a poet with the imagination of a milton might give us very imposing and stirring images of such creatures and their chaotic surroundings, but fancies like these can have no basis in human experience, and consequently can make no claim upon scientific recognition. or, as an alternative, it might be assumed that saturn is composed of lighter elements and materials than those which constitute the earth and the other solid planets in the more immediate neighborhood of the sun. but such an assumption would put us entirely at sea as regards the forms of organic life that could exist upon a planet of that description, and, like sir humphry davy in the vision, that occupies the first chapter of his quaintly charming consolations in travel, or, the last days of a philosopher, we should be thrown entirely upon the resources of the imagination in representing to ourselves the nature and appearance of its inhabitants. yet minds of unquestioned power and sincerity have in all ages found pleasure and even profit in such exercises, and with every fresh discovery arises a new flight of fancies like butterflies from a roadside pool. as affording a glimpse into the mind of a remarkable man, as well as a proof of the fascination of such subjects, it will be interesting to quote from the book just mentioned davy's description of his imaginary inhabitants of saturn: "i saw below me a surface infinitely diversified, something like that of an immense glacier covered with large columnar masses, which appeared as if formed of glass, and from which were suspended rounded forms of various sizes which, if they had not been transparent, i might have supposed to be fruit. from what appeared to me to be analogous to bright-blue ice, streams of the richest tint of rose color or purple burst forth and flowed into basins, forming lakes or seas of the same color. looking through the atmosphere toward the heavens, i saw brilliant opaque clouds, of an azure color, that reflected the light of the sun, which had to my eyes an entirely new aspect and appeared smaller, as if seen through a dense blue mist. "i saw moving on the surface below me immense masses, the forms of which i find it impossible to describe. they had systems for locomotion similar to those of the morse, or sea-horse, but i saw, with great surprise, that they moved from place to place by six extremely thin membranes, which they used as wings. their colors were varied and beautiful, but principally azure and rose color. i saw numerous convolutions of tubes, more analogous to the trunk of the elephant than to anything else i can imagine, occupying what i supposed to be the upper parts of the body. it was with a species of terror that i saw one of them mounting upward, apparently flying toward those opaque clouds which i have before mentioned. "'i know what your feelings are,' said the genius; 'you want analogies, and all the elements of knowledge to comprehend the scene before you. you are in the same state in which a fly would be whose microscopic eye was changed for one similar to that of man, and you are wholly unable to associate what you now see with your former knowledge. but those beings who are before you, and who appear to you almost as imperfect in their functions as the zoophytes of the polar sea, to which they are not unlike in their apparent organization to your eyes, have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of your earth. each of those tubes, which appears like the trunk of an elephant, is an organ of peculiar motion or sensation. they have many modes of perception of which you are wholly ignorant, at the same time that their sphere of vision is infinitely more extended than yours, and their organs of touch far more perfect and exquisite.'" after descanting upon the advantages of saturn's position for surveying some of the phenomena of the solar system and of outer space, and the consequent immense advances that the saturnians have made in astronomical knowledge, the genius continues: "'if i were to show you the different parts of the surface of this planet you would see the marvelous results of the powers possessed by these highly intellectual beings, and of the wonderful manner in which they have applied and modified matter. those columnar masses, which seem to you as if rising out of a mass of ice below, are results of art, and processes are going on within them connected with the formation and perfection of their food. the brilliant-colored fluids are the results of such operations as on the earth would be performed in your laboratories, or more properly in your refined culinary apparatus, for they are connected with their system of nourishment. those opaque azure clouds, to which you saw a few minutes ago one of those beings directing his course, are works of art, and places in which they move through different regions of their atmosphere, and command the temperature and the quantity of light most fitted for their philosophical researches, or most convenient for the purposes of life.'"[ ] [footnote : davy, of course, was aware that, owing to increase of distance, the sun would appear to an inhabitant of saturn with a disk only one ninetieth as great in area as that which it presents to our eyes.] but, while saturn does not appear, with our present knowledge, to hold out any encouragement to those who would regard it as the abode of living creatures capable of being described in any terms except those of pure imagination, yet it is so unique a curiosity among the heavenly bodies that one returns again and again to the contemplation of its strange details. saturn has nine moons, but some of them are relatively small bodies--the ninth, discovered photographically by professor pickering in , being especially minute--and others are situated at great distances from the planet, and for these reasons, together with the fact that the sunlight is so feeble upon them that, surface for surface, they have only one ninetieth as much illumination as our moon receives, they can not make a very brilliant display in the saturnian sky. to astronomers on saturn they would, of course, be intensely interesting because of their perturbations and particularly the effect of their attraction on the rings. this brings us again to the consideration of those marvelous appendages, and to the statement of facts about them which we have not yet recalled. if the reader will take a ball three inches in diameter to represent the globe of saturn, and, out of the center of a circular piece of writing-paper seven inches in diameter, will cut a round hole three and three quarter inches across, and will then place the ball in the middle of the hole in the paper, he will have a very fair representation of the relative proportions of saturn and its rings. to represent the main gap or division in the rings he might draw, a little more than three eighths of an inch from the outer edge of the paper disk, a pencil line about a sixteenth of an inch broad. perhaps the most striking fact that becomes conspicuous in making such a model of the saturnian system is the exceeding thinness of the rings as compared with their enormous extent. they are about , miles across from outer edge to outer edge, and about , miles broad from outer edge to inner edge--including the gauze ring presently to be mentioned--yet their thickness probably does not surpass one hundred miles! in fact, the sheet of paper in our imaginary model is several times too thick to represent the true relative thickness of saturn's rings. several narrow gaps in the rings have been detected from time to time, but there is only one such gap that is always clearly to be seen, the one already mentioned, situated about , miles from the outer edge and about , miles in width. inside of this gap the broadest and brightest ring appears, having a width of about , miles. for some reason this great ring is most brilliant near the gap, and its brightness gradually falls off toward its inner side. at a distance of something less than , miles from the planet--or perhaps it would be more correct to say above the planet, for the rings hang directly over saturn's equator--the broad, bright ring merges into a mysterious gauzelike object, also in the form of a ring, which extends to within , or , miles of the planet's surface, and therefore itself has a width of say , miles. in consequence of the thinness of the rings they completely disappear from the range of vision of small telescopes when, as occurs once in every fifteen years, they are seen exactly edgewise from the earth. in a telescope powerful enough to reveal them when in that situation they resemble a thin, glowing needle run through the ball of the planet. the rings will be in this position in , and again in . the opacity of the rings is proved by the shadow which they cast upon the ball of the planet. this is particularly manifest at the time when they are edgewise to the earth, for the sun being situated slightly above or below the plane of the rings then throws their shadow across saturn close to its equator. when they are canted at a considerable angle to our line of sight their shadow is seen on the planet, bordering their outer edge where they cross the ball. the gauze ring, the detection of which as a faintly luminous phenomenon requires a powerful telescope, can be seen with slighter telescopic power in the form of a light shade projected against the planet at the inner edge of the broad bright ring. the explanation of the existence of this peculiar object depends upon the nature of the entire system, which, instead of being, as the earliest observers thought it, a solid ring or series of concentric rings, is composed of innumerable small bodies, like meteorites, perhaps, in size, circulating independently but in comparatively close juxtaposition to one another about saturn, and presenting to our eyes, because of their great number and of our enormous distance, the appearance of solid, uniform rings. so a flock of ducks may look from afar like a continuous black line or band, although if we were near them we should perceive that a considerable space separates each individual from his neighbors. the fact that this is the constitution of saturn's rings can be confidently stated because it has been mathematically proved that they could not exist if they were either solid or liquid bodies in a continuous form, and because the late prof. james e. keeler demonstrated with the spectroscope, by means of the doppler principle, already explained in the chapter on venus, that the rings circulate about the planet with varying velocities according to their distance from saturn's center, exactly as independent satellites would do. it might be said, then, that saturn, instead of having nine satellites only, has untold millions of them, traveling in orbits so closely contiguous that they form the appearance of a vast ring. as to their origin, it may be supposed that they are a relic of a ring of matter left in suspension during the contraction of the globe of saturn from a nebulous mass, just as the rings from which the various planets are supposed to have been formed were left off during the contraction of the main body of the original solar nebula. other similar rings originally surrounding saturn may have become satellites, but the matter composing the existing rings is so close to the planet that it falls within the critical distance known as "roche's limit," within which, owing to the tidal effect of the planet's attraction, no body so large as a true satellite could exist, and accordingly in the process of formation of the saturnian system this matter, instead of being aggregated into a single satellite, has remained spread out in the form of a ring, although its substance long ago passed from the vaporous and liquid to the solid form. we have spoken of the rings as being composed of meteorites, but perhaps their component particles may be so small as to answer more closely to the definition of dust. in these rings of dust, or meteorites, disturbances are produced by the attraction of the planet and that of the outer satellites, and it is yet a question whether they are a stable and permanent feature of saturn, or will, in the course of time, be destroyed.[ ] [footnote : for further details about saturn's rings, see the tides, by g.h. darwin, chap. xx.] it has been thought that the gauze ring is variable in brightness. this would tend to show that it is composed of bodies which have been drawn in toward the planet from the principal mass of the rings, and these bodies may end their career by falling upon the planet. this process, indefinitely continued, would result in the total disappearance of the rings--saturn would finally swallow them, as the old god from whom the planet gets its name is fabled to have swallowed his children. near the beginning of this chapter reference was made to the fact that saturn's rings have been regarded as habitable bodies. that, of course, was before the discovery that they were not solid. knowing what we now know about them, even dr. thomas dick, the great scotch popularizer of astronomy in the first half of the nineteenth century, would have been compelled to abandon his theory that saturn's rings were crowded with inhabitants. at the rate of to the square mile he reckoned that they could easily contain , , , , people. he even seems to have regarded their edges--in his time their actual thinness was already well known--as useful ground for the support of living creatures, for he carefully calculated the aggregate area of these edges and found that it considerably exceeded the area of the entire surface of the earth. indeed, dr. dick found room for more inhabitants on saturn's rings than on saturn itself, for, excluding the gauze ring, undiscovered in his day, the two surfaces of the rings are greater in area than the surface of the globe of the planet. he did not attack the problem of the weight of bodies on worlds in the form of broad, flat, thin, surfaces like saturn's rings, or indulge in any reflections on the interrelations of the inhabitants of the opposite sides, although he described the wonderful appearance of saturn and other celestial objects as viewed from the rings. but all these speculations fall to the ground in face of the simple fact that if we could reach saturn's rings we should find nothing to stand upon, except a cloud of swiftly flying dust or a swarm of meteors, swayed by contending attractions. and, indeed, it is likely that upon arriving in the immediate neighborhood of the rings they would virtually disappear! seen close at hand their component particles might be so widely separated that all appearance of connection between them would vanish, and it has been estimated that from saturn's surface the rings, instead of presenting a gorgeous arch spanning the heavens, may be visible only as a faintly gleaming band, like the milky way or the zodiacal light. in this respect the mystic swedenborg appears to have had a clearer conception of the true nature of saturn's rings than did dr. dick, for in his book on the earths in the universe he says--using the word "belt" to describe the phenomenon of the rings: "being questioned concerning that great belt which appears from our earth to rise above the horizon of that planet, and to vary its situations, they [the inhabitants of saturn] said that it does not appear to them as a belt, but only as somewhat whitish, like snow in the heaven, in various directions." in view of such observations as that of prof. e.e. barnard, in , showing that a satellite passing through the shadow of saturn's rings does not entirely disappear--a fact which proves that the rings are partially transparent to the sunlight--one might be tempted to ask whether saturn itself, considering its astonishing lack of density, is not composed, at least in its outer parts, of separate particles of matter revolving independently about their center of attraction, and presenting the appearance of a smooth, uniform shell reflecting the light of the sun. in other words, may not saturn be, exteriorly, a globe of dust instead of a globe of vapor? certainly the rings, incoherent and translucent though they be, reflect the sunlight to our eyes, at least from the brighter part of their surface, with a brilliance comparable with that of the globe of the planet itself. as bearing on the question of the interior condition of saturn and jupiter, it should, perhaps, be said that mathematical considerations, based on the figures of equilibrium of rotating liquid masses, lead to the conclusion that those planets are comparatively very dense within. professor darwin puts the statement very strongly, as follows: "in this way it is known with certainty that the central portions of the planets jupiter and saturn are much denser, compared to their superficial portions, than is the case with the earth."[ ] [footnote : the tides, by g.h. darwin, p. .] the globe and rings of saturn witness an imposing spectacle of gigantic moving shadows. the great ball stretches its vast shade across the full width of the rings at times, and the rings, as we have seen, throw their shadow in a belt, whose position slowly changes, across the ball, sweeping from the equator, now toward one pole and now toward the other. the sun shines alternately on each side of the rings for a space of nearly fifteen years--a day fifteen years long! and then, when that face of the ring is turned away from the sun, there ensues a night of fifteen years' duration also. whatever appearance the rings may present from the equator and the middle latitudes on saturn, from the polar regions they would be totally invisible. as one passed toward the north, or the south, pole he would see the upper part of the arch of the rings gradually sink toward the horizon until at length, somewhere in the neighborhood of the polar circle, it would finally disappear, hidden by the round shoulder of the great globe. uranus, neptune, and the suspected ultraneptunian planet what has been said of jupiter and saturn applies also to the remaining members of the jovian group of planets, uranus and neptune, viz., that their density is so small that it seems probable that they can not, at the present time, be in a habitable planetary condition. all four of these outer, larger planets have, in comparatively recent times, been solar orbs, small companions of the sun. the density of uranus is about one fifth greater than that of water, and slightly greater than that of neptune. uranus is , miles in diameter, and neptune , miles. curiously enough, the force of gravity upon each of these two large planets is a little less than upon the earth. this arises from the fact that in reckoning gravity on the surface of a planet not only the mass of the planet, but its diameter or radius, must be considered. gravity varies directly as the mass, but inversely as the square of the radius, and for this reason a large planet of small density may exercise a less force of gravity at its surface than does a small planet of great density. the mean distance of uranus from the sun is about , , , miles, and its period of revolution is eighty-four years; neptune's mean distance is about , , , miles, and its period of revolution is about years. uranus has four satellites, and neptune one. the remarkable thing about these satellites is that they revolve _backward_, or contrary to the direction in which all the other satellites belonging to the solar system revolve, and in which all the other planets rotate on their axis. in the case of uranus, the plane in which the satellites revolve is not far from a position at right angles to the plane of the ecliptic; but in the case of neptune, the plane of revolution of the satellites is tipped much farther backward. since in every other case the satellites of a planet are situated nearly in the plane of the planet's equator, it may be assumed that the same rule holds with uranus and neptune; and, that being so, we must conclude that those planets rotate backward on their axes. this has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of laplace's theory the peculiar behavior of uranus and neptune can be reconciled with it. very little is known of the surfaces of uranus and neptune. indications of the existence of belts resembling those of jupiter have been found in the case of both planets. there are similar belts on saturn, and as they seem to be characteristic of large, rapidly rotating bodies of small density, it was to be expected that they would be found on uranus and neptune. the very interesting opinion is entertained by some astronomers that there is at least one other great planet beyond neptune. the orbits of certain comets are relied upon as furnishing evidence of the existence of such a body. prof. george forbes has estimated that this, as yet undiscovered, planet may be even greater than jupiter in mass, and may be situated at a distance from the sun one hundred times as great as the earth's, where it revolves in an orbit a single circuit of which requires a thousand years. whether this planet, with a year a thousand of our years in length, will ever be seen with a telescope, or whether its existence will ever, in some other manner, be fully demonstrated, can not yet be told. it will be remembered that neptune was discovered by means of computations based upon its disturbing attraction on uranus before it had ever been recognized with the telescope. but when the astronomers in the observatories were told by their mathematical brethren where to look they found the planet within half an hour after the search began. so it is possible the suspected great planet beyond neptune may be within the range of telescopic vision, but may not be detected until elaborate calculations have deduced its place in the heavens. as a populous city is said to furnish the best hiding-place for a man who would escape the attention of his fellow beings, so the star-sprinkled sky is able to conceal among its multitudes worlds both great and small until the most painstaking detective methods bring them to recognition. chapter viii the moon, child of the earth and the sun very naturally the moon has always been a great favorite with those who, either in a scientific or in a literary spirit, have speculated about the plurality of inhabited worlds. the reasons for the preference accorded to the moon in this regard are evident. unless a comet should brush us--as a comet is suspected of having done already--no celestial body, of any pretensions to size, can ever approach as near to the earth as the moon is, at least while the solar system continues to obey the organic laws that now control it. it is only a step from the earth to the moon. what are , miles in comparison with the distances of the stars, or even with the distances of the planets? jupiter, driving between the earth and the moon, would occupy more than one third of the intervening space with the chariot of his mighty globe; saturn, with broad wings outspread, would span more than two thirds of the distance; and the sun, so far from being able to get through at all, would overlap the way more than , miles on each side. in consequence, of course, of its nearness, the moon is the only member of the planetary system whose principal features are visible to the naked eye. in truth, the naked eye perceives the larger configurations of the lunar surface more clearly than the most powerful telescope shows the details on the disk of mars. long before the time of galileo and the invention of the telescope, men had noticed that the face of the moon bears a resemblance to the appearance that the earth would present if viewed from afar off. in remote antiquity there were philosophers who thought that the moon was an inhabited world, and very early the romancers took up the theme. lucian, the voltaire of the second century of our era, mercilessly scourged the pretenders of the earth from an imaginary point of vantage on the moon, which enabled him to peer down into their secrets. lucian's description of the appearance of the earth from the moon shows how clearly defined in his day had become the conception of our globe as only an atom in space. "especially did it occur to me to laugh at the men who were quarreling about the boundaries of their land, and at those who were proud because they cultivated the sikyonian plain, or owned that part of marathon around oenoe, or held possession of a thousand acres at acharnæ. of the whole of greece, as it then appeared to me from above, being about the size of four fingers, i think attica was in proportion a mere speck. so that i wondered on what condition it was left to these rich men to be proud."[ ] [footnote : ikaromenippus; or, above the clouds. prof. d.c. brown's translation.] such scenes as lucian beheld, in imagination, upon the earth while looking from the moon, many would fain behold, with telescopic aid, upon the moon while looking from the earth. galileo believed that the details of the lunar surface revealed by his telescope closely resembled in their nature the features of the earth's surface, and for a long time, as the telescope continued to be improved, observers were impressed with the belief that the moon possessed not only mountains and plains, but seas and oceans also. it was the discovery that the moon has no perceptible atmosphere that first seriously undermined the theory of its habitability. yet, as was remarked in the introductory chapter, there has of late been some change of view concerning a lunar atmosphere; but the change has been not so much in the ascertained facts as in the way of looking at those facts. but before we discuss this matter, it will be well to state what is known beyond peradventure about the moon. its mean distance from the earth is usually called, for the sake of a round number, , miles, but more accurately stated it is , miles. this is variable to the extent of more than , miles, on account of the eccentricity of its orbit, and the eccentricity itself is variable, in consequence of the perturbing attractions of the earth and the sun, so that the distance of the moon from the earth is continually changing. it may be as far away as , miles and as near as , miles. although the orbit of the moon is generally represented, for convenience, as an ellipse about the earth, it is, in reality, a varying curve, having the sun for its real focus, and always concave toward the latter. this is a fact that can be more readily explained with the aid of a diagram. [illustration: the moon's path with respect to the sun and the earth.] in the accompanying cut, when the earth is at _a_ the moon is between it and the sun, in the phase called new moon. at this point the earth's orbit about the sun is more curved than the moon's, and the earth is moving relatively faster than the moon, so that when it arrives at _b_ it is ahead of the moon, and we see the latter to the right of the earth, in the phase called first quarter. the earth being at this time ahead of the moon, the effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its orbit about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at _c_, and appears on the side opposite the sun, in the phase called full moon. at this point the moon's orbit about the sun has a shorter radius of curvature than the earth's. in traveling from _c_ to _d_ the moon still moves more rapidly than the earth, and, having passed it, appears at _d_ to the left of the earth, in the phase called third quarter. now, the earth being behind the moon, the effect of its attraction combined with the sun's tends to retard the moon in its orbit about the sun, with the result that the moon moves again less rapidly than the earth, and the latter overtakes it, so that, upon reaching _e_, the two are once more in the same relative positions that they occupied at _a_, and it is again new moon. thus it will be seen that, although the real orbit of the moon has the sun for its center of revolution, nevertheless, in consequence of the attraction of the earth, combined in varying directions with that of the sun, the moon, once every month, makes a complete circuit of our globe. the above explanation should not be taken for a mathematical demonstration of the moon's motion, but simply for a graphical illustration of how the moon appears to revolve about the earth while really obeying the sun's attraction as completely as the earth does. there is no other planet that has a moon relatively as large as ours. the moon's diameter is , miles. its volume, compared with the earth's, is in the ratio of to , and its density is about six tenths of the earth's. this makes its mass to that of our globe about as to . in other words, it would take eighty-one moons to counterbalance the earth. before speaking of the force of gravity on the moon we will examine the character of the lunar surface. to the naked eye the moon's face appears variegated with dusky patches, while a few points of superior brilliance shine amid the brighter portions, especially in the southern and eastern quarters, where immense craters like tycho and copernicus are visible to a keen eye, gleaming like polished buttons. with a telescope, even of moderate power, the surface of the moon presents a scene of astonishing complexity, in which strangeness, beauty, and grandeur are all combined. the half of the moon turned earthward contains an area of , , square miles, a little greater than the area of south america and a little less than that of north america. of these , , square miles, about , , square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, _maria_--i.e., "seas." whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges and regions of depression, intersected occasionally by immense cracks, having the width and depth of our mightiest river cañons, and sprinkled with bright points and crater pits. the remaining , , square miles are mainly occupied by mountains of the most extraordinary character. owing partly to roughness of the surface and partly to more brilliant reflective power, the mountainous regions of the moon appear bright in comparison with the dull-colored plains. some of the lunar mountains lie in long, massive chains, with towering peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling precipices. looking at them with a powerful telescope, the observer might well fancy himself to be gazing down from an immense height into the heart of the untraveled himalayas. but these, imposing though they are, do not constitute the most wonderful feature of the mountain scenery of the moon. appearing sometimes on the shores of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters. frequently they have in the middle of their depressed interior floors a peak, or a cluster of peaks. their inner and outer walls are seamed with ridges, and what look like gigantic streams of frozen lava surround them. the resemblance that they bear to the craters of volcanoes is, at first sight, so striking that probably nobody would ever have thought of questioning the truth of the statement that they are such craters but for their incredible magnitude. many of them exceed fifty miles in diameter, and some of them sink two, three, four, and more miles below the loftiest points upon their walls! there is a chasm, miles long and broad, named newton, situated about miles from the south pole of the moon, whose floor lies , feet below the summit of a peak that towers just above it on the east! this abyss is so profound that the shadows of its enclosing precipices never entirely quit it, and the larger part of its bottom is buried in endless night. one can not but shudder at the thought of standing on the broken walls of newton, and gazing down into a cavity of such stupendous depth that if chimborazo were thrown into it, the head of the mighty andean peak would be thousands of feet beneath the observer. a different example of the crater mountains of the moon is the celebrated tycho, situated in latitude about ° south, corresponding with the latitude of southern new zealand on the earth. tycho is nearly circular and a little more than miles across. the highest point on its wall is about , feet above the interior. in the middle of its floor is a mountain , or , feet high. tycho is especially remarkable for the vast system of whitish streaks, or rays, which starting from its outer walls, spread in all directions over the face of the moon, many of them, running, without deviation, hundreds of miles across mountains, craters, and plains. these rays are among the greatest of lunar mysteries, and we shall have more to say of them. [illustration: the lunar alps, apennines, and caucasus. photographed with the lick telescope.] copernicus, a crater mountain situated about ° north of the equator, in the eastern hemisphere of the moon, is another wonderful object, miles in diameter, a polygon appearing, when not intently studied, as a circle, , or , feet deep, and having a group of relatively low peaks in the center of its floor. around copernicus an extensive area of the moon's surface is whitened with something resembling the rays of tycho, but more irregular in appearance. copernicus lies within the edge of the great plain named the _oceanus procellarum_, or "ocean of storms," and farther east, in the midst of the "ocean," is a smaller crater mountain, named kepler, which is also enveloped by a whitish area, covering the lunar surface as if it were the result of extensive outflows of light-colored lava. in one important particular the crater mountains of the moon differ from terrestrial volcanoes. this difference is clearly described by nasmyth and carpenter in their book on the moon: "while the terrestrial crater is generally a hollow on a mountain top, with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external height being frequently only a half or one third of the internal depth." it has been suggested that these gigantic rings are only "basal wrecks" of volcanic mountains, whose conical summits have been blown away, leaving vast crateriform hollows where the mighty peaks once stood; but the better opinion seems to be that which assumes that the rings were formed by volcanic action very much as we now see them. if such a crater as copernicus or the still larger one named theophilus, which is situated in the western hemisphere of the moon, on the shore of the "sea of nectar," ever had a conical mountain rising from its rim, the height attained by the peak, if the average slope were about °, would have been truly stupendous--fifteen or eighteen miles! there is a kind of ring mountains, found in many places on the moon, whose forms and surroundings do not, as the craters heretofore described do, suggest at first sight a volcanic origin. these are rather level plains of an oval or circular outline, enclosed by a wall of mountains. the finest example is, perhaps, the dark-gray plato, situated in ° of north latitude, near an immense mountain uplift named the lunar alps, and on the northern shore of the _mare imbrium_, or "sea of showers." plato appears as an oval plain, very smooth and level, about miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of , to , feet. enclosed plains, bearing more or less resemblance to plato--sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges--are to be found scattered over almost all parts of the moon. if our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle. it has been thought that they may once have been filled with water, forming lakes that recall the curious crater lake of oregon. [illustration: the moon at first and last quarter (western and eastern hemispheres). photographed with the lick telescope.] it is not my intention to give a complete description of the various lunar features, and i mention but one other--the "clefts" or "rills," which are to be seen running across the surface like cracks. one of the most remarkable of these is found in the _oceanus procellarum_, near the crater-mountain aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. the cleft, or crack, in question is very erratic in its course, and many miles in length, and it terminates in a ringed plain named herodotus not far east of aristarchus, breaking through the wall of the plain and entering the interior. many other similar chasms or cañons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. mr. thomas gwyn elger has this to say on the subject of the lunar clefts: "if, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now a 'changeless world,' that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for." mr. elger then proceeds to describe his discovery in , in the ring-plain mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[ ] [footnote : the moon, a full description and map of its principal features, by thomas gwyn elger, . those who desire to read detailed descriptions of lunar scenery may consult, in addition to mr. elger's book, the following: the moon, considered as a planet, a world, and a satellite, by james nasmyth and james carpenter, ; the moon, and the condition and configurations of its surface, by edmund neison, . see also annals of harvard college observatory, vol. xxxii, part ii, , for observations made by prof. william h. pickering at the arequipa observatory.] we now return to the question of the force of lunar gravity. this we find to be only one sixth as great as gravity on the surface of the earth. it is by far the smallest force of gravity that we have found anywhere except on the asteroids. employing the same method of comparison that was made in the case of mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of adam is on the earth. whether this furnishes a sound reason for assuming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an assumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. we have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. if these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. the same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. the giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a distance of fifteen miles, could send the same projectile ninety miles on the moon. an athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. in other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. in that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear--with the hat off. these things are not only amusing, but important. there can be no question that the force of gravity on the moon actually is as slight as it has just been described. so, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. it has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the sun until various perturbations have brought them down upon the earth. a body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity. another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. according to the theory of dr. johnstone stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. the escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmosphere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.[ ] [footnote : the discovery of free hydrogen in the earth's atmosphere, by professor dewar, , bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. in a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.] after it had been found that, to ordinary tests, the moon offered no evidence of the possession of an atmosphere, and before dr. stoney's theory was broached, it was supposed by many that the moon had lost its original supply of air by absorption into its interior. the oxygen was supposed to have entered into combination with the cooling rocks and minerals, thus being withdrawn from the atmosphere, and the nitrogen was imagined to have disappeared also within the lunar crust. for it seems to have always been tacitly assumed that the phenomenon to be accounted for was not so much the _absence_ of a lunar atmosphere as its _disappearance_. but disappearance, of course, implies previous existence. in like manner it has always been a commonly accepted view that the moon probably once had enough water to form lakes and seas. these, it has been calculated, could have been absorbed into the lunar globe as it cooled off. but johnstone stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open space. possibly both methods have been in operation, a portion of the constituents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them. but why, it may be asked, should it be assumed that the moon ever had things which it does not now possess? perhaps no entirely satisfactory reply can be made. some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the shores of the "seas." messrs. loewy and puiseux, of the paris observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject: "there exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans. in these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed space. in the same way the 'seas' of the moon present, generally at the edges, rather pronounced depressions. in one case, as in the other, we observe normal deformations of a shrinking globe shielded from the erosive action of rain, which tends, on the contrary, in all the abundantly watered parts of the earth to make the concave surfaces predominate. the explanation of this structure, such as is admitted at present by geologists, seems to us equally valid for the moon."[ ] [footnote : comptes rendus, june , july , .] it might be urged that there is evidence of former volcanic activity on the moon of such a nature that explosions of steam must have played a part in the phenomena, and if there was steam, of course there was water. but perhaps the most convincing argument tending to show that the moon once had a supply of water, of which some remnant may yet remain below the surface of the lunar globe, is based upon the probable similarity in composition of the earth and the moon. this similarity results almost equally whether we regard the moon as having originated in a ring of matter left off from the contracting mass that became the earth, or whether we accept the suggestion of prof. g.h. darwin, that the moon is the veritable offspring of the earth, brought into being by the assistance of the tidal influence of the sun. the latter hypothesis is the more picturesque of the two, and, at present, is probably the more generally favored. it depends upon the theory of tidal friction, which was referred to in chapter iii, as offering an explanation of the manner in which the rotation of the planet mercury has been slowed down until its rotary period coincides with that of its revolution. the gist of the hypothesis in question is that at a very early period in its history, when the earth was probably yet in a fluid condition, it rotated with extreme rapidity on its axis, and was, at the same time, greatly agitated by the tidal attraction of the sun, and finally huge masses were detached from the earth which, ultimately uniting, became the moon.[ ] [footnote : the tides, by g.h. darwin, chapter xvi.] born in this manner from the very substance of the earth, the moon would necessarily be composed, in the main, of the same elements as the globe on which we dwell, and is it conceivable that it should not have carried with it both air and water, or the gases from which they were to be formed? if the moon ever had enough of these prime requisites to enable it to support forms of life comparable with those of the earth, the disappearance of that life must have been a direct consequence of the gradual vanishing of the lunar air and water. the secular drying up of the oceans and wasting away of the atmosphere on our little neighbor world involved a vast, all-embracing tragedy, some of the earlier scenes of which, if theories be correct, are now reenacted on the half-desiccated planet mars--a planet, by the way, which in size, mass, and ability to retain vital gases stands about half-way between the earth and the moon. one of the most interesting facts about the moon is that its surface affords evidence of a cataclysm which has wiped out many, and perhaps nearly all, of the records of its earlier history, that were once written upon its face. even on the earth there have been geological catastrophes destroying or burying the accumulated results of ages of undisturbed progress, but on the moon these effects have been transcendent. the story of the tremendous disaster that overtook the moon is partly written in its giant volcanoes. although it may be true, as some maintain, that there is yet volcanic action going on upon the lunar surface, it is evident that such action must be insignificant in comparison with that which took place ages ago. there is a spot in the western hemisphere of the moon, on the border of a placid bay or "sea," that i can never look at without a feeling of awe and almost of shrinking. there, within a space about miles in length by in width, is an exhibition of the most terrifying effects of volcanic energy that the eye of man can anywhere behold. three immense craters--theophilus, miles across and - / miles deep; cyrillus, miles across and , feet deep; and catharina, miles across and from , to , feet deep--form an interlinked chain of mountain rings, ridges, precipices, chasms, and bottomless pits that take away one's breath. but when the first impression of astonishment and dismay produced by this overwhelming spectacle has somewhat abated, the thoughtful observer will note that here the moon is telling him a part of her wonderful story, depicted in characters so plain that he needs no instruction in order to decipher their meaning. he will observe that this ruin was not all wrought at once or simultaneously. theophilus, the crater-mountain at the northwestern end of the chain, whose bottom lies deepest of all, is the youngest of these giants, though the most imposing. for a distance of forty miles the lofty wall of theophilus has piled itself upon the ruins of the wall of cyrillus, and the circumference of the circle of its tremendous crater has been forcibly thrust within the original rim of the more ancient crater, which was thus rudely compelled to make room for its more vigorous rival and successor. the observer will also notice that catharina, the huge pit at the southeastern end of the chain, bears evidence of yet greater age. its original walls, fragments of which still stand in broken grandeur, towering to a height of , feet, have, throughout the greater part of their circuit, been riddled by the outbreak of smaller craters, and torn asunder and thrown down on all sides. in the vast enclosure that was originally the floor of the crater-mountain catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere etnas, cotopaxis, and kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity. this is only one example among hundreds, scattered all over the moon, which show how the surface of our satellite has suffered upheaval after upheaval. it is possible that some of the small craters, not included within the walls of the greater ones, may represent an early stage in the era of volcanic activity that wrecked the moon, but where larger and smaller are grouped together a certain progression can be seen, tending finally to extinction. the internal energies reached a maximum and then fell off in strength until they died out completely. it can hardly be supposed that the life-bearing phase of lunar history--if there ever was one--could survive the outbreak of the volcanic cataclysm. north america, or europe, if subjected to such an experience as the continental areas of the moon have passed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circumstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures. there are other interesting glimpses to be had of an older world in the moon than that whose scarred face is now beautified for us by distance. not far from theophilus and the other great crater-mountains just described, at the upper, or southern, end of the level expanse called the "sea of nectar," is a broad, semicircular bay whose shores are formed by the walls of a partially destroyed crater named fracastorius. it is evident that this bay, and the larger part of the "sea of nectar," have been created by an outwelling of liquid lavas, which formed a smooth floor over a portion of the pre-existing surface of the moon, and broke down and submerged a large part of the mountain ring of fracastorius, leaving the more ancient walls standing at the southern end, while, outlined by depressions and corrugations in the rocky blanket, are certain half-defined forms belonging to the buried world beneath. near copernicus, some years ago, as dr. edward s. holden pointed out, photographs made with the great lick telescope, then under his direction, showed, in skeleton outline, a huge ring buried beneath some vast outflow of molten matter and undiscerned by telescopic observers. and mr. elger, who was a most industrious observer and careful interpreter of lunar scenery, speaks of "the undoubted existence of the relics of an earlier lunar world beneath the smooth superficies of the _maria_." although, as already remarked, it seems necessary to assume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed. germs of the earlier life might have survived, despite the terrible nature of the catastrophe. but the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to assume that anything beyond the lowest and simplest organic forms--mainly, if not wholly, in the shape of vegetation--can exist there. the impression that even such life is possible rests upon the accumulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon's surface. prof. william h. pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other american astronomer, has recorded numberless instances of change in minute details of the lunar landscapes. he regards some of his observations made at arequipa as "pointing very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time." the mountain-ringed valley of plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley. professor pickering has thoroughly discussed the observations relating to a celebrated crater named linné in the _mare serenitatis_, and after reading his description of its changes of appearance one can hardly reject his conclusion that linné is an active volcanic vent, but variable in its manifestations. this is only one of a number of similar instances among the smaller craters of the moon. the giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity. nor should it be assumed that these relatively slight manifestations of volcanic action are really insignificant. as professor pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of plato, he says of its evidences of volcanic action: "it is, i believe, more active than any area of similar size upon the earth. there seems to be no evidences of lava, but the white streaks indicate apparently something analogous to snow or clouds. there must be a certain escape of gases, presumably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."[ ] [footnote : annals of harvard college observatory, vol. xxxii, part ii, .] to professor pickering we owe the suggestion that the wonderful rays emanating from tycho consist of some whitish substance blown by the wind, not from tycho itself, but from lines of little volcanic vents or craters lying along the course of the rays. this substance may be volcanic powder or snow, in the form of minute ice crystals. mr. elger remarks of this theory that the "confused network of streaks" around copernicus seems to respond to it more happily than the rays of tycho do, because of the lack of definiteness of direction so manifest in the case of the rays. as an encouragement to amateur observers who may be disposed to find out for themselves whether or not changes now take place in the moon, the following sentence from the introduction to professor pickering's chapter on plato in the harvard observatory annals, volume xxxii, will prove useful and interesting: "in reviewing the history of selenography, one must be impressed by the singular fact that, while most of the astronomers who have made a special study of the moon, such as schroeter, maedler, schmidt, webb, neison, and elger, have all believed that its surface was still subject to changes readily visible from the earth, the great majority of astronomers who have paid little attention to the subject have quite as strenuously denied the existence of such changes." in regard to the lunar atmosphere, it may be said, in a word, that even those who advocate the existence of vegetation and of clouds of dust or ice crystals on the moon do not predicate any greater amount, or greater density, of atmosphere than do those who consider the moon to be wholly dead and inert. professor pickering himself showed, from his observations, that the horizontal refraction of the lunar atmosphere, instead of being less than ´´, as formerly stated, was less than . ´´. yet he found visual evidence that on the sunlit side of the moon this rare atmosphere was filled to a height of four miles with some absorbing medium which was absent on the dark side, and which was apparently an emanation from the lunar crust, occurring after sunrise. and messrs. loewy and puiseux, of the paris observatory, say, after showing reasons for thinking that the great volcanic eruptions belong to a recent period in the history of the moon, that "the diffusion of cinders to great distances infers a gaseous envelope of a certain density.... the resistance of the atmosphere must have been sufficient to retard the fall of this dust [the reference is to the white trails, like those from tycho], during its transport over a distance of more than , kilometers [ miles]."[ ] [footnote : comptes rendus, june , july , .] we come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface. the moon keeps the same side forever turned toward the earth, behaving, in this respect, as mercury does with regard to the sun. the consequence is that the lunar globe makes but one rotation on its axis in the course of a month, or in the course of one revolution about the earth. some of the results of this practical identity of the periods of rotation and revolution are illustrated in the diagram on page . the moon really undergoes considerable libration, recalling the libration of mercury, which was explained in the chapter on that planet, and in consequence we are able to see a little way round into the opposite lunar hemisphere, now on this side and now on the other, but in the diagram this libration has been neglected. if it had been represented we should have found that, instead of only one half, about three fifths of the total superficies of the moon are visible from the earth at one time or another. [illustration: phases and rotation of the moon.] perhaps it should be remarked that in drawing the moon's orbit about the earth as a center we offer no contradiction to what was shown earlier in this chapter. the moon does travel around the earth, and its orbit about our globe may, for our present purpose, be treated independently of its motion about the sun. let the central globe, then, represent the earth, and let the sun be supposed to shine from the left-hand side of the diagram. a little cross is erected at a fixed spot on the globe of the moon. at _a_ the moon is between the earth and the sun, or in the phase of new moon. the lunar hemisphere facing the earth is now buried in night, except so far as the light reflected from the earth illuminates it, and this illumination, it is interesting to remember, is about fourteen times as great--reckoned by the relative areas of the reflecting surfaces--as that which the full moon sends to the earth. an inhabitant of the moon, standing beside the cross, sees the earth in the form of a huge full moon directly above his head, but, as far as the sun is concerned, it is midnight for him. in the course of about seven days the moon travels to _b_. in the meantime it has turned one quarter of the way around its axis, and the spot marked by the cross is still directly under the earth. for the lunar inhabitant standing on that spot the sun is now on the point of rising, and he sees the earth no longer in the shape of a full moon, but in that of a half-moon. the lunar globe itself appears, at the same time from the earth, as a half-moon, being in the position or phase that we call first quarter. seven more days elapse, and the moon arrives at _c_, opposite to the position of the sun, and with the earth between it and the solar orb. it is now high noon for our lunarian standing beside the cross, while the earth over his head appears, if he sees it at all, only as a black disk close to the sun, or--as would sometimes be the case--covering the sun, and encircled with a beautiful ring of light produced by the refraction of its atmosphere. (recall the similar phenomenon in the case of venus.) the moon seen from the earth is now in the phase called full moon. another lapse of seven days, and the moon is at _d_, in the phase called third quarter, while the earth, viewed from the cross on the moon, which is still pointed directly at it, appears again in the shape of a huge half-moon. during the next seven days the moon returns to its original position at _a_, and becomes once more new moon, with "full earth" shining upon it. now it is evident that in consequence of the peculiar law of the moon's rotation its days and nights are each about two of our weeks, or fourteen days, in length. that hemisphere of the moon which is in the full sunlight at _a_, for instance, is buried in the middle of night at _c_. the result is different than in the case of mercury, because the body toward which the moon always keeps the same face directed is not the luminous sun, but the non-luminous earth. it is believed that the moon acquired this manner of rotation in consequence of the tidal friction exercised upon it by the earth. the tidal attraction of the earth exceeds that of the sun upon the moon because the earth is so much nearer than the sun is, and tidal attraction varies inversely as the cube of the distance. in fact, the braking effect of tidal friction varies inversely as the sixth power of the distance, so that the ability of the earth to stop the rotation of the moon on its axis is immensely greater than that of the sun. this power was effectively applied while the moon was yet a molten mass, so that it is probable that the moon has rotated just as it does now for millions of years. as was remarked a little while ago, the moon traveling in an elliptical orbit about the earth has a libratory movement which, if represented in our picture, would cause the cross to swing now a little one way and now a little the other, and thus produce an apparent pendulum motion of the earth in the sky, similar to that of the sun as seen from mercury. but it is not necessary to go into the details of this phenomenon. the reader, if he chooses, can deduce them for himself. but we may inquire a little into the effects of the long days and nights of the moon. in consequence of the extreme rarity of the lunar atmosphere, it is believed that the heat of the sun falling upon it during a day two weeks in length, is radiated away so rapidly that the surface of the lunar rocks never rises above the freezing temperature of water. on the night side, with no warm atmospheric blanket such as the earth enjoys, the temperature may fall far toward absolute zero, the most merciful figure that has been suggested for it being ° below the zero of our ordinary thermometers! but there is much uncertainty about the actual temperature on the moon, and different experiments, in the attempt to make a direct measurement of it, have yielded discordant results. at one time, for instance, lord rosse believed he had demonstrated that at lunar noon the temperature of the rocks rose above the boiling-point of water. but afterward he changed his mind and favored the theory of a low temperature. in this and in other respects much remains to be discovered concerning our interesting satellite, and there is plenty of room, and an abundance of original occupation, for new observers of the lunar world. chapter ix how to find the planets there is no reason why everybody should not know the principal planets at sight nearly as well as everybody knows the moon. it only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. no telescope, or any other instrument whatever, is required for the purpose. there is but one preliminary requirement, just as every branch of human knowledge presupposes its a b c. this is an acquaintance with the constellations and the principal stars--not a difficult thing to obtain. almost everybody knows the "great dipper" from childhood's days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. some know orion when he shines gloriously in the winter heavens. many are able to point out the north star, or pole star, as everybody should be able to do. all this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens. if you are fortunate enough to number an astronomer among your acquaintance--an amateur will do as well as a professor--you may, with his aid, make a short cut to a knowledge of the stars. otherwise you must depend upon books and charts. my astronomy with an opera-glass was prepared for this very purpose. for simply learning the constellations and the chief stars you need no opera-glass or other instrument. with the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. you need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention. learn by observation at what seasons particular constellations are on, or near, the meridian--i.e., the north and south line through the middle of the heavens. make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius, and pisces. the importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[ ] how to tell which planet it may be, it is the object of this chapter to show you. as an indispensable aid--unless you happen already to possess a complete star atlas on a larger scale--i have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter. [footnote : in our latitudes, planets are never seen in the northern quarter of the sky. when on the meridian, they are always somewhere between the zenith and the southern horizon.] [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] having learned to recognize the constellations and their chief stars on sight, one other step, an extremely easy one, remains to be taken before beginning your search for the planets--buy the american ephemeris and nautical almanac for the current year. it is published under the direction of the united states naval observatory at washington, and can be purchased for one dollar. this book, which may appear to you rather bulky and formidable for an almanac, contains hundreds of pages and scores of tables to which you need pay no attention. they are for navigators and astronomers, and are much more innocent than they look. the plain citizen, seeking only an introduction to the planets, can return their stare and pass by, without feeling in the least humiliated. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] in the front part of the book, after the long calendar, and the tables relating to the sun and the moon, will be found about thirty pages of tables headed, in large black letters, with the names of the planets--mercury, venus, mars, jupiter, saturn, etc. two months are represented on each page, and opposite the number of each successive day of the month the position of the planet is given in hours, minutes, and seconds of right ascension, and degrees, minutes, and seconds of north and south declination, the sign + meaning north, and the sign - south. do not trouble yourself with the seconds in either column, and take the minutes only when the number is large. the hours of right ascension and the degrees of declination are the main things to be noticed. right ascension, by the way, expresses the distance of a celestial body, such as a star or a planet, east of the vernal equinox, or the first point of aries, which is an arbitrary point on the equator of the heavens, which serves, like the meridian of greenwich on the earth, as a starting-place for reckoning longitude. the entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering ° of space. if a planet then is in right ascension (usually printed for short r.a.) h. m. s., it is on the meridian of the vernal equinox, or the celestial greenwich; if it is in r.a. h., it will be found ° east of the vernal equinox, and so on. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] declination (printed d. or dec.) expresses the distance of a celestial body north or south of the equator of the heavens. with these explanations we may proceed to find a planet by the aid of the nautical almanac and our charts. i take, for example, the ephemeris for the year , and i look under the heading "jupiter" on page , for the month of july. opposite the th day of the month i find the right ascension to be h. m., neglecting the seconds. now minutes are so near to half an hour that, for our purposes, we may say jupiter is in r.a. h. m. i set this down on a slip of paper, and then examine the declination column, where i find that on july jupiter is in south declination (the sign - meaning south, as before explained) ° ´ ´´, which is almost ° ´, and, for our purposes, we may call this ° ´, which is what i set down on my slip. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] next, i turn to chart no. , in this chapter, where i find the meridian line of r.a. h. running through the center of the chart. i know that jupiter is to be looked for about m. east, or to the left, of that line. at the bottom and top of the chart, every twenty minutes of r.a. is indicated, so that it is easy, with the eye, or with the aid of a ruler, to place the vertical line at some point of which jupiter is to be found. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] then i consult my note of the declination of the planet. it is south ° ´. on the vertical borders of the chart i find the figures of the declination, and i observe that ° dec., which represents the equator of the heavens, is near the top of the chart, while each parallel horizontal line across the chart indicates ° north or south of its next neighbor. next to the bottom of the chart i find the parallel of °, and i see that every five degrees is indicated by the figures at the sides. by the eye, or with the aid of a ruler, i easily estimate where the horizontal line of ° would fall, and since ´ is the third of a degree i perceive that it is, for the rough purpose of merely finding a conspicuous planet, negligible, although it, too, can be included in the estimate, if thought desirable. having already found the vertical line on which jupiter is placed and having now found the horizontal line also, i have simply to regard their crossing point, which will be the situation of the planet among the stars. i note that it is in the constellation sagittarius in a certain position with reference to a familiar group of stars in that constellation, and when i look at the heavens, there, in the place thus indicated, jupiter stands revealed. [illustration: chart no. .--from right ascension hours to hours ( ii.); declination ° north to ° south.] the reader will readily perceive that, in a precisely similar manner, any planet can be located, at any time of the year, and at any point in its course about the heavens. but it may turn out that the place occupied by the planet is too near the sun to render it easily, or at all, visible. such a case can be recognized, either from a general knowledge of the location of the constellations at various seasons, or with the aid of the nautical almanac, where at the beginning of each set of monthly tables in the calendar the sun's right ascension and declination will be found. in locating the sun, if you find that its right ascension differs by less than an hour, one way or the other, from that of the planet sought, it is useless to look for the latter. if the planet is situated west of the sun--to the right on the chart--then it is to be looked for in the east before sunrise. but if it is east of the sun--to the left on the chart--then you must seek it in the west after sunset. for instance, i look for the planet mercury on october , . i find its r.a. to be h. m. and its dec. ° ´. looking at the sun's place for october th, i find it to be r.a. h. m. and dec. ° ´. placing them both on chart no. , i discover that mercury is well to the east, or left hand of the sun, and will consequently be visible in the western sky after sundown. additional guidance will be found by noting the following facts about the charts: the meridian (the north and south line) runs through the middle of chart no. between and o'clock p.m. on november st, between and o'clock p.m. on december st, and between and o'clock p.m. on january st. the meridian runs through the middle of chart no. between and o'clock p.m. on january st, between and o'clock p.m. on february st, and between and o'clock p.m. on march st. the meridian runs through the middle of chart no. between and o'clock p.m. on march st, between and o'clock p.m. on april st, and between and o'clock p.m. on may st. the meridian runs through the middle of chart no. between and o'clock p.m. on may st, between and o'clock p.m. on june st, and between and o'clock p.m. on july st. the meridian runs through the middle of chart no. between and o'clock p.m. on july st, between and o'clock p.m. on august st, and between and o'clock p.m. on september st. the meridian runs through the middle of chart no. between and o'clock p.m. on september st, between and o'clock p.m. on october st, and between and o'clock p.m. on november st. note well, also, these particulars about the charts: chart no. includes the first four hours of right ascension, from h. to h. inclusive; chart no. includes h. to h.; chart no. , h. to h.; chart no. , h. to h.; chart no. , h. to h.; and chart no. , h. to h., which completes the circuit. in the first three charts the line of °, or the equator, is found near the bottom, and in the last three near the top. this is a matter of convenience in arrangement, based upon the fact that the ecliptic, which, and not the equator, marks the center of the zodiac, indicates the position of the tracks of the planets among the stars; and the ecliptic, being inclined ° to the plane of the equator, lies half to the north and half to the south of the latter. those who, after all, may not care to consult the ephemeris in order to find the planets, may be able to locate them, simply from a knowledge of their situation among the constellations. some ordinary almanacs tell in what constellations the principal planets are to be found at various times of the year. having once found them in this way, it is comparatively easy to keep track of them thereafter through a general knowledge of their movements. jupiter, for instance, requiring a period of nearly twelve years to make a single journey around the sun, moves about ° eastward among the stars every year. the zodiacal constellations are roughly about ° in length, and as jupiter was in sagittarius in , he will be in capricornus in . saturn, requiring nearly thirty years for a revolution around the sun, moves only between ° and ° eastward every year, and, being in conjunction with jupiter in sagittarius in , does not get beyond the border of that constellation in . jupiter having been in opposition to the sun june , , will be similarly placed early in august, , the time from one opposition of jupiter to the next being days. saturn passes from one opposition to the next in days, so that having been in that position july , , it reaches it again about july , . mars requires about days to complete a revolution, and comes into conjunction with the earth, or opposition to the sun--the best position for observation--on the average once every days. mars was in opposition near the end of february, , and some of its future oppositions will be in march, ; may, ; july, ; and september, . the oppositions of and will be unusually favorable ones, for they will occur when the planet is comparatively near the earth. when a planet is in opposition to the sun it is on the meridian, the north and south line, at midnight. mercury and venus being nearer the sun than the earth is, can never be seen very far from the place of the sun itself. venus recedes much farther from the solar orb than mercury does, but both are visible only in the sunset or the sunrise sky. all almanacs tell at what times these planets play their respective rôles as morning or as evening stars. in the case of mercury about days on the average elapse between its reappearances; in the case of venus, about days. the latter, for instance, having become an evening star at the end of april, , will become an evening star again in december, . with the aid of the nautical almanac and the charts the amateur will find no difficulty, after a little practise, in keeping track of any of the planets. in the back part of the nautical almanac will be found two pages headed "phenomena: planetary configurations." with the aid of these the student can determine the position of the planets with respect to the sun and the moon, and with respect to one another. the meaning of the various symbols used in the tables will be found explained on a page facing the calendar at the beginning of the book. from these tables, among other things, the times of greatest elongation from the sun of the planets mercury and venus can be found. it may be added that only bright stars, and stars easily seen, are included in the charts, and there will be no danger of mistaking any of these stars for a planet, if the observer first carefully learns to recognize their configurations. neither mars, jupiter, nor saturn ever appears as faint as any of the stars, except those of the first magnitude, included in the charts. uranus and neptune being invisible to the naked eye--uranus can occasionally be just glimpsed by a keen eye--are too faint to be found without the aid of more effective appliances. index agassiz, alexander, on deep-sea animals, . asteroids, the, , . brightness of, . imaginary adventures on, . life on, . number of, known, . orbits of, . origin of, , . size of, . aristarchus, lunar crater, . atmosphere, importance of, . bailey, solon i., on oppositions of eros, . barnard, e.e., discovers fifth satellite of jupiter, . measures asteroids, . on saturn's rings, . belopolski, on rotation of venus, . ceres, an asteroid, , . clefts in the moon, . copernicus, lunar crater, , . darwin, george h., on jupiter and saturn, . on origin of moon, . theory of tidal friction, . davy, sir humphry, on saturn, . dawes sees canals on mars, . deimos, satellite of mars, . denning, w.f., description of jupiter, . de vico on rotation of venus, . dewar, james, discovers free hydrogen in air, . de witt discovers eros, . dick, thomas, on saturn, . douglass, a.e., sees mars's canals, . sees clouds in mars, . doppler's principle, , . earth and moon's orbit, . birth of moon from, . change of distance from sun, . less advanced than mars, . older than venus, . seen from mercury, . seen from venus, - , . seen from moon, . earth, similarity to venus, . supposed signals to and from mars, . elger, t.g., on cracks in moon, . on tycho's rays, . ephemeris, how to use, , . eros, an asteroid, - , , . flammarion, c., observes venus's atmosphere, . on plurality of worlds, . forbes, prof. george, on ultra-neptunian planet, . galileo on lunar world, . gravity, as affecting life on planets, , . hall, asaph, discovers mars's moons, . herodotus, lunar crater, . herschel, sir john, on saturn, . holden, e.s., on photograph of lunar crater, . huggins on mercury's atmosphere, . inhabitants of foreign planets, , , . interplanetary communication, , , , , . juno, an asteroid, . jupiter, cloudy aspect of, . density of, . distance of, . equatorial belts on, . future of, . gravity on, . great red spot on, . markings outside the belts, . and the nebular theory, . once a companion star, . polar compression of, . possibly yet incandescent, . question of a denser core, . resemblance of, to sun, . rotation of, , . satellites of, , . seen from satellites, . size of, . solar light and heat on, . south belt of, . surface conditions of, . theories about the red spot, . trade-winds and the belts of, . various rates of rotation of, . visibility of rotation of, . keeler, j.e., on saturn's rings, . kepler, lunar crater, . kinetic theory of gases, . kirkwood, daniel, on asteroids, . lagrange on olbers's theory, . lick observatory and mars's canals, . life, a planetary phenomenon, . in sea depths, . on planets, , . prime requisites of, . resisting extreme cold, . universality of, . loewy and puiseux, on lunar atmosphere, . on lunar "seas," . lowell, percival, description of mars, . on markings of venus, . on mercury's rotation, . on rotation of venus, . sees mars's canals, . theory of martian canals, . lucian, on appearance of earth from moon, . lyman, c.s., observes venus's atmosphere, . mars, age of, . atmosphere of, , , . bands of life on, . canals on, . described by schiaparelli, . gemination of, , . have builders of, disappeared? . and irrigation, . and lines of vegetation, . and seasonal changes, . and water circulation, . carbon dioxide on, . circular spots or "oases" on, . colors of, . dimensions of, . distance of, , . enigmatical lights on, . gravity on, . inclination of axis, . length of year, . lowell's theory of, . light and heat on, . moonlight on, . orbit of, . polar caps of, , . possible size of inhabitants, . satellites of, , , . seasons on, . supposed signals from, , . temperature of, , . water vapor on, . mercury, atmosphere of, , , , . day and night on, , , . dimensions, . earth seen from, . habitability of, , , . heavens seen from, , . heat and light on, , . holds place of honor, . length of year, . mass of, . moon visible from, . resemblances to moon, . rotation of, . shape of orbit, . sun as seen from, . velocity in orbit, . venus seen from, . virtual fall toward sun, . visibility of, . water on, . moon, the area of surface, . atmosphere of, , , , , . clouds on, , . constitution of, . craters, . day and night on, . distance of, , . density of, . former cataclysm on, . former life on, , . giantism on, , . gravity on, , , . libration of, . meteorites and, . mountains on, . the older world in, . origin of, . phases and motions of, . rotation of, . seas of, . size of, . snow on, . speculation about, . temperature of, . vegetation on, , , . visibility of features of, . nasmyth and carpenter on lunar craters, . neptune, description of, - . newcomb, simon, on olbers's theory, . newton, lunar crater, . olbers's theory of planetary explosion, . on vesta's light, . pallas, an asteroid, . perrotin sees canals on mars, . phobos, satellite of mars, . pickering, e.c., discovers ninth moon of saturn, . finds eros on harvard plates, . on shape of eros, . on light of eros, . pickering, w.h., on lunar atmosphere, . observes changes in moon, . sees mars's canals, . theory of tycho's rays, . on venus's atmosphere, . planets, classification of, . how to find, , . resemblances among, . plato, lunar ring plain, . plurality of worlds in literature, . subject ignored, . proctor, r.a., on jupiter's moons, . on other worlds, . roche's limit, . rosse, lord, on temperature of moon, . saturn, age of, . composition of, . density of, . distance of, . the gauze ring, - . gravity on, . inclination of axis, . interior of, . length of year, . popular telescopic object, . rings of, , . gaps in, . origin of, . periodic disappearance of, . seen from planet, . shadow of, . rotation of, . satellites of, . size of, . schiaparelli discovers canals on mars, . describes martian canals, . discovers mercury's rotation, , . on rotation of venus, . solar system, shape and size of, . unity of, . viewed from space, . stoney, johnstone, on atmospheres of planets, . on escape of gases from moon, . sun, the, isolation in space, . no life on, . resemblances with jupiter, . swedenborg, on saturn's rings, . tidal friction, , , , . tycho, lunar crater, . ultra-neptunian planet, . uranus, description of, - . venus, age of, . atmosphere of, , , , , . absence of seasons on, . density of, . distance of, , . gravity on, , . inclination of axis, . life on, , , , , , , , . light and heat on, - . orbit of, . phases of, . resemblances of, to earth, . rotation of, , , . size of, . twilight on, . visibility of, . vesta, an asteroid, , , . vogel on mercury's atmosphere, . wireless telegraphy, , . young, c.a., on olbers's theory of asteroids, . on temperature of mars, . on venus's atmosphere, . zodiac, the, . the end a new book by prof. groos. the play of man. by karl groos, professor of philosophy in the university of basel, and author of "the play of animals." translated, with the author's cooperation, by elizabeth l. baldwin, and edited, with a preface and appendix, by prof. j. mark baldwin, of princeton university. mo. cloth, $ . net; postage, cents additional. the results of professor groos's original and acute investigations are of peculiar value to those who are interested in psychology and sociology, and they are of great importance to educators. he presents the anthropological aspects of the subject treated in his psychological study of the play of animals, which has already become a classic. professor groos, who agrees with the followers of weismann, develops the great importance of the child's play as tending to strengthen his inheritance in the acquisition of adaptations to his environment. the influence of play on character, and its relation to education, are suggestively indicated. the playful manifestations affecting the child himself and those affecting his relations to others have been carefully classified, and the reader is led from the simpler exercises of the sensory apparatus through a variety of divisions to inner imitations and social play. the biological, æsthetic, ethical, and pedagogical 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"one can here get a clear conception of the relative condition of the stars and constellations, and of the existent universe so far as it is disclosed to view. the author presents his wonderful and at times bewildering facts in a bright and cheery spirit that makes the book doubly attractive."--_boston home journal._ d. appleton and company, new york. pleasures of the telescope an illustrated guide for amateur astronomers and a popular description of the chief wonders of the heavens for general readers by garrett p. serviss author of astronomy with an opera-glass "this being made, he yearned for worlds to make from other chaos out beyond our night-- for to create is still god's prime delight. the large moon, all alone, sailed her dark lake, and the first tides were moving to her might; then darkness trembled, and began to quake big with the birth of stars, and when he spake a million worlds leapt into radiant light." lloyd mifflin. _with many illustrations_ new york d. appleton and company copyright, , by d. appleton and company. preface by the introduction of a complete series of star maps, drawn specially for the use of the amateur and distributed through the body of the work, thus facilitating consultation, it is believed that this book makes a step in advance of its predecessors. the maps show all of the stars visible to the naked eye in the regions of sky represented, and, in addition, some stars that can only be seen with optical aid. the latter have been placed in the maps as guide posts in the telescopic field to assist those who are searching for faint and inconspicuous objects referred to in the text. as the book was not written for those who possess the equipment of an observatory, with telescopes driven by clockwork and provided with graduated circles, right ascensions and declinations are not given. all of the telescopic phenomena described are, however, represented in the maps. star clusters are indicated by a conventional symbol, and nebulæ by a little white circle; while a small cross serves to mark the places where notable new stars have appeared. the relative magnitudes of the stars are approximately shown by the dimensions of their symbols in the maps, the smaller stars being represented by white dots and the larger by star-shaped figures. in regard to binary stars, it should be remembered that, in many cases, their distances and angles of position change so rapidly that any statement concerning them remains valid only for a few years at the most. there is also much confusion among the measurements announced by different authorities. in general, the most recent measurements obtainable in are given in the text, but the observer who wishes to study close and rapid binaries will do well to revise his information about them as frequently as possible. an excellent list of double stars kept up to date, will be found in the annual companion to the observatory, published in london. in the lunar charts the plan of inserting the names of the principal formations has been preferred to that usually followed, of indicating them only by numbers, accompanied by a key list. even in the most detailed charts of the moon only a part of what is visible with telescopes can be shown, and the representation, at best, must be merely approximate. it is simply a question of what to include and what to omit; and in the present case the probable needs of the amateur observer have governed the selection--readiness and convenience of reference being the chief aim. it should, perhaps, be said here that the various chapters composing this book--like those of "astronomy with an opera-glass"--were, in their original form, with the single exception of chapter ix, published in appletons' popular science monthly. the author, it is needless to say, was much gratified by the expressed wish of many readers that these scattered papers should be revised and collected in a more permanent form. as bearing upon the general subject of the book, a chapter has been added, at the end, treating on the question of the existence of planets among the stars. this also first appeared in the periodical above mentioned. in conclusion, the author wishes for his readers as great a pleasure in the use of the telescope as he himself has enjoyed. g. p. s. borough of brooklyn, new york, _january, _. contents chapter i page the selection and testing of a glass how to get a good telescope--difference between reflectors and refractors--how a telescope is made achromatic--the way to test a telescope on stars. chapter ii in the starry heavens orion and its wonders, lepus, canis major, argo, monoceros, canis minor, and the head of hydra. chapter iii from gemini to leo and round about the zodiacal constellations gemini, cancer, and leo, and their neighbors auriga, the lynx, hydra, sextans, and coma berenices. chapter iv virgo and her neighbors crater and corvus, hydra, virgo, the "field of the nebulæ," libra, boötes, and the great arcturus, canes venatici, and corona borealis. chapter v in summer star-lands scorpio and its red-green gem, ophiuchus, sagittarius, scutum sobieskii, capricornus, serpens, hercules, draco, aquila, and delphinus. chapter vi from lyra to eridanus lyra and its brilliant vega, cygnus, vulpecula, aquarius, equuleus, pegasus, cetus, and eridanus. chapter vii pisces, aries, taurus, and the northern mars the first double star ever discovered, the pleiades and their photographic wonders, the royal family of the sky, andromeda, cassiopeia, perseus and cepheus, ursa major, camelopardalus, ursa minor, and the pole star. chapter viii scenes on the planets jupiter, its belts and its moons--saturn, the ringed planet--saturn's moons and roche's limit--mars and its white polar caps and so-called seas and continents--venus and her atmosphere--the peculiar rotations of venus and mercury. chapter ix the mountains and plains of the moon and the spectacles of the sun peculiarities of the lunar landscapes--the so-called seas, the craters, the ring mountains, the inclosed plains, the mountain ranges, tycho's mysterious streaks, and other lunar features described--how to view the sun and its spots. chapter x are there planets among the stars? significance of dr. see's observations--why our telescopes do not show planets circling around distant suns--reasons for thinking that such planets may exist--the bearing of stellar evolution on the question. pleasures of the telescope chapter i the selection and testing of a glass "o telescope, instrument of much knowledge, more precious than any scepter! is not he who holds thee in his hand made king and lord of the works of god?"--john kepler. if the pure and elevated pleasure to be derived from the possession and use of a good telescope of three, four, five, or six inches aperture were generally known, i am certain that no instrument of science would be more commonly found in the homes of intelligent people. the writer, when a boy, discovered unexpected powers in a pocket telescope not more than fourteen inches long when extended, and magnifying ten or twelve times. it became his dream, which was afterward realized, to possess a more powerful telescope, a real astronomical glass, with which he could see the beauties of the double stars, the craters of the moon, the spots on the sun, the belts and satellites of jupiter, the rings of saturn, the extraordinary shapes of the nebulæ, the crowds of stars in the milky way, and the great stellar clusters. and now he would do what he can to persuade others, who perhaps are not aware how near at hand it lies, to look for themselves into the wonder-world of the astronomers. there is only one way in which you can be sure of getting a good telescope. first, decide how large a glass you are to have, then go to a maker of established reputation, fix upon the price you are willing to pay--remembering that good work is never cheap--and finally see that the instrument furnished to you answers the proper tests for a telescope of its size. there are telescopes and telescopes. occasionally a rare combination of perfect homogeneity in the material, complete harmony between the two kinds of glass of which the objective is composed, and lens surfaces whose curves are absolutely right, produces a telescope whose owner would part with his last dollar sooner than with it. such treasures of the lens-maker's art can not, perhaps, be commanded at will, yet, they are turned out with increasing frequency, and the best artists are generally able, at all times, to approximate so closely to perfection that any shortcoming may be disregarded. in what is said above i refer, of course, to the refracting telescope, which is the form of instrument that i should recommend to all amateurs in preference to the reflector. but, before proceeding further, it may be well to recall briefly the principal points of difference between these two kinds of telescopes. the purpose of a telescope of either description is, first, to form an image of the object looked at by concentrating at a focus the rays of light proceeding from that object. the refractor achieves this by means of a carefully shaped lens, called the object glass, or objective. the reflector, on the other hand, forms the image at the focus of a concave mirror. [illustration: image at the focus of a lens.] a very pretty little experiment, which illustrates these two methods of forming an optical image, and, by way of corollary, exemplifies the essential difference between refracting and reflecting telescopes, may be performed by any one who possesses a reading glass and a magnifying hand mirror. in a room that is not too brightly illuminated pin a sheet of white paper on the wall opposite to a window that, by preference, should face the north, or away from the position of the sun. taking first the reading glass, hold it between the window and the wall parallel to the sheet of paper, and a foot or more distant from the latter. by moving it to and fro a little you will be able to find a distance, corresponding to the focal length of the lens, at which a picture of the window is formed on the paper. this picture, or image, will be upside down, because the rays of light cross at the focus. by moving the glass a little closer to the wall you will cause the picture of the window to become indistinct, while a beautiful image of the houses, trees, or other objects of the outdoor world beyond, will be formed upon the paper. we thus learn that the distance of the image from the lens varies with the distance of the object whose image is formed. in precisely a similar manner an image is formed at the focus of the object glass of a refracting telescope. [illustration: image at the focus of a concave mirror.] take next your magnifying or concave mirror, and detaching the sheet of paper from the wall, hold it nearly in front of the mirror between the latter and the window. when you have adjusted the distance to the focal length of the mirror, you will see an image of the window projected upon the paper, and by varying the distance, as before, you will be able to produce, at will, pictures of nearer or more remote objects. it is in this way that images are formed at the focus of the mirror of a reflecting telescope. now, you will have observed that the chief apparent difference between these two methods of forming an image of distant objects is that in the first case the rays of light, passing through the transparent lens, are brought to a focus on the side opposite to that where the real object is, while in the second case the rays, being reflected from the brilliant surface of the opaque mirror, come to a focus on the same side as that on which the object itself is. from this follows the most striking difference in the method of using refracting and reflecting telescopes. in the refractor the observer looks toward the object; in the reflector he looks away from it. sir william herschel made his great discoveries with his back to the sky. he used reflecting telescopes. this principle, again, can be readily illustrated by means of our simple experiment with a reading glass and a magnifying mirror. hold the reading glass between the eye and a distant object with one hand, and with the other hand place a smaller lens such as a pocket magnifier, near the eye, and in line with the reading glass. move the two carefully until they are at a distance apart equal to the sum of the focal lengths of the lenses, and you will see a magnified image of the distant object. in other words, you have constructed a simple refracting telescope. then take the magnifying mirror, and, turning your back to the object to be looked at, use the small lens as before--that is to say, hold it between your eye and the mirror, so that its distance from the latter is equal to the sum of the focal lengths of the mirror and the lens, and you will see again a magnified image of the distant object. this time it is a reflecting telescope that you hold in your hands. the magnification of the image reminds us of the second purpose which is subserved by a telescope. a telescope, whether refracting or reflecting, consists of two essential parts, the first being a lens, or a mirror, to form an image, and the second a microscope, called an eyepiece, to magnify the image. the same eyepieces will serve for either the reflector or the refractor. but in order that the magnification may be effective, and serve to reveal what could not be seen without it, the image itself must be as nearly perfect as possible; this requires that every ray of light that forms the image shall be brought to a point in the image precisely corresponding to that from which it emanates in the real object. in reflectors this is effected by giving a parabolic form to the concave surface of the mirror. in refractors there is a twofold difficulty to be overcome. in the first place, a lens with spherical surfaces does not bend all the rays that pass through it to a focus at precisely the same distance. the rays that pass near the outer edge of the lens have a shorter focus than that of the rays which pass near the center of the lens; this is called spherical aberration. a similar phenomenon occurs with a concave mirror whose surface is spherical. in that case, as we have seen, the difficulty is overcome by giving the mirror a parabolic instead of a spherical form. in an analogous way the spherical aberration of a lens can be corrected by altering its curves, but the second difficulty that arises with a lens is not so easily disposed of: this is what is called chromatic aberration. it is due to the fact that the rays belonging to different parts of the spectrum have different degrees of refrangibility, or, in other words, that they come to a focus at different distances from the lens; and this is independent of the form of the lens. the blue rays come to a focus first, then the yellow, and finally the red. it results from this scattering of the spectral rays along the axis of the lens that there is no single and exact focus where all meet, and that the image of a star, for instance, formed by an ordinary lens, even if the spherical aberration has been corrected, appears blurred and discolored. there is no such difficulty with a mirror, because there is in that case no refraction of the light, and consequently no splitting up of the elements of the spectrum. in order to get around the obstacle formed by chromatic aberration it is necessary to make the object glass of a refractor consist of two lenses, each composed of a different kind of glass. one of the most interesting facts in the history of the telescope is that sir isaac newton could see no hope that chromatic aberration would be overcome, and accordingly turned his attention to the improvement of the reflecting telescope and devised a form of that instrument which still goes under his name. and even after chester more hall in , and john dollond in , had shown that chromatic aberration could be nearly eliminated by the combination of a flint-glass lens with one of crown glass, william herschel, who began his observations in , devoted his skill entirely to the making of reflectors, seeing no prospect of much advance in the power of refractors. a refracting telescope which has been freed from the effects of chromatic aberration is called achromatic. the principle upon which its construction depends is that by combining lenses of different dispersive power the separation of the spectral colors in the image can be corrected while the convergence of the rays of light toward a focus is not destroyed. flint glass effects a greater dispersion than crown glass nearly in the ratio of three to two. the chromatic combination consists of a convex lens of crown backed by a concave, or plano-concave, lens of flint. when these two lenses are made of focal lengths which are directly proportional to their dispersions, they give a practically colorless image at their common focus. the skill of the telescope-maker and the excellence of his work depend upon the selection of the glasses to be combined and his manipulation of the curves of the lenses. [illustration: achromatic object glass. _a_, crown glass; _b_, flint glass.] now, the reader may ask, "since reflectors require no correction for color dispersion, while that correction is only approximately effected by the combination of two kinds of lenses and two kinds of glass in a refractor, why is not the reflector preferable to the refractor?" the answer is, that the refractor gives more light and better definition. it is superior in the first respect because a lens transmits more light than a mirror reflects. professor young has remarked that about eighty-two per cent of the light reaches the eye in a good refractor, while "in a newtonian reflector, in average condition, the percentage seldom exceeds fifty per cent, and more frequently is lower than higher." the superiority of the refractor in regard to definition arises from the fact that any distortion at the surface of a mirror affects the direction of a ray of light three times as much as the same distortion would do at the surface of a lens. and this applies equally both to permanent errors of curvature and to temporary distortions produced by strains and by inequality of temperature. the perfect achromatism of a reflector is, of course, a great advantage, but the chromatic aberration of refractors is now so well corrected that their inferiority in that respect may be disregarded. it must be admitted that reflectors are cheaper and easier to make, but, on the other hand, they require more care, and their mirrors frequently need resilvering, while an object glass with reasonable care never gets seriously out of order, and will last for many a lifetime. enough has now, perhaps, been said about the respective properties of object glasses and mirrors, but a word should be added concerning eyepieces. without a good eyepiece the best telescope will not perform well. the simplest of all eyepieces is a single double-convex lens. with such a lens the magnifying power of the telescope is measured by the ratio of the focal length of the objective to that of the eye lens. suppose the first is sixty inches and the latter half an inch; then the magnifying power will be a hundred and twenty diameters--i. e., the disk of a planet, for instance, will be enlarged a hundred and twenty times along each diameter, and its area will be enlarged the square of a hundred and twenty, or fourteen thousand four hundred times. but in reckoning magnifying power, diameter, not area, is always considered. for practical use an eyepiece composed of an ordinary single lens is seldom advantageous, because good definition can only be obtained in the center of the field. lenses made according to special formulæ, however, and called solid eyepieces, give excellent results, and for high powers are often to be preferred to any other. the eyepieces usually furnished with telescopes are, in their essential principles, compound microscopes, and they are of two descriptions, "positive" and "negative." the former generally goes under the name of its inventor, ramsden, and the latter is named after great dutch astronomer, huygens. the huygens eyepiece consists of two plano-convex lenses whose focal lengths are in the ratio of three to one. the smaller lens is placed next to the eye. both lenses have their convex surfaces toward the object glass, and their distance apart is equal to half the sum of their focal lengths. in this kind of eyepiece the image is formed between the two lenses, and if the work is properly done such an eyepiece is achromatic. it is therefore generally preferred for mere seeing purposes. in the ramsden eyepiece two plano-convex lenses are also used, but they are of equal focal length, are placed at a distance apart equal to two thirds of the focal length of either, and have their convex sides facing one another. with such an eyepiece the image viewed is beyond the farther or field lens instead of between the two lenses, and as this fact renders it easier to adjust wires or lines for measuring purposes in the focus of the eyepiece, the ramsden construction is used when a micrometer is to be employed. in order to ascertain the magnifying power which an eyepiece gives when applied to a telescope it is necessary to know the equivalent, or combined, focal length of the two lenses. two simple rules, easily remembered, supply the means of ascertaining this. the equivalent focal length of a negative or huygens eyepiece is equal to half the focal length of the larger or field lens. the equivalent focal length of a positive or ramsden eyepiece is equal to three fourths of the focal length of either of the lenses. having ascertained the equivalent focal length of the eyepiece, it is only necessary to divide it into the focal length of the object glass (or mirror) in order to know the magnifying power of your telescope when that particular eyepiece is in use. [illustration: negative eyepiece.] [illustration: positive eyepiece.] a first-class object glass (or mirror) will bear a magnifying power of one hundred to the inch of aperture when the air is in good condition--that is, if you are looking at stars. if you are viewing the moon, or a planet, better results will always be obtained with lower powers--say fifty to the inch at the most. and under ordinary atmospheric conditions a power of from fifty to seventy-five to the inch is far better for stars than a higher power. with a five-inch telescope that would mean from two hundred and fifty to three hundred and seventy-five diameters, and such powers should only be applied for the sake of separating very close double stars. as a general rule, the lowest power that will distinctly show what you desire to see gives the best results. the experienced observer never uses as high powers as the beginner does. the number of eyepieces purchased with a telescope should never be less than three--a very low power--say ten to the inch; a very high power, seventy-five or one hundred to the inch, for occasional use; and a medium power--say forty to the inch--for general use. if you can afford it, get a full battery of eyepieces--six or eight, or a dozen--for experience shows that different objects require different powers in order to be best seen, and, moreover, a slight change of power is frequently a great relief to the eye. there is one other thing of great importance to be considered in purchasing a telescope--the mounting. if your glass is not well mounted on a steady and easily managed stand, you might better have spent your money for something more useful. i have endured hours of torment while trying to see stars through a telescope that was shivering in the wind and dancing to every motion of the bystanders, to say nothing of the wriggling contortions caused by the application of my own fingers to the focusing screw. the best of all stands is a solid iron pillar firmly fastened into a brick or stone pier, sunk at least four feet in the ground, and surmounted by a well-made equatorial bearing whose polar axis has been carefully placed in the meridian. it can be readily protected from the weather by means of a wooden hood or a rubber sheet, while the tube of the telescope may be kept indoors, being carried out and placed on its bearing only when observations are to be made. with such a mounting you can laugh at the observatories with their cumbersome domes, for the best of all observatories is the open air. but if you dislike the labor of carrying and adjusting the tube every time it is used, and are both fond of and able to procure luxuries, then, after all, perhaps, you had better have the observatory, dome, draughts and all. the next best thing in the way of a mounting is a portable tripod stand. this may be furnished either with an equatorial bearing for the telescope, or an altazimuth arrangement which permits both up-and-down and horizontal motions. the latter is cheaper than the equatorial and proportionately inferior in usefulness and convenience. the essential principle of the equatorial bearing is motion about two axes placed at right angles to one another. when the polar axis is in the meridian, and inclined at an angle equal to the latitude of the place, the telescope can be moved about the two axes in such a way as to point to any quarter of the sky, and the motion of a star, arising from the earthy rotation, can be followed hour after hour without disturbing the instrument. when thus mounted, the telescope may be driven by clockwork, or by hand with the aid of a screw geared to a handle carrying a universal joint. and now for testing the telescope. it has already been remarked that the excellence of a telescope depends upon the perfection of the image formed at the focus of the objective. in what follows i have only a refractor in mind, although the same principles would apply to a reflector. with a little practice anybody who has a correct eye can form a fair judgment of the excellence of a telescopic image. suppose we have our telescope steadily mounted out of doors (if you value your peace of mind you will not try to use a telescope pointed out of a window, especially in winter), and suppose we begin our observations with the pole star, employing a magnifying power of sixty or seventy to the inch. our first object is to see if the optician has given us a good glass. if the air is not reasonably steady we had better postpone our experiment to another night, because we shall find that the star as seen in the telescope flickers and "boils," and behaves in so extraordinary a fashion that there is no more definition in the image than there is steadiness in a bluebottle buzzing on a window pane. but if the night is a fine one the star image will be quiescent, and then we may note the following particulars: the real image is a minute bright disk, about one second of arc in diameter if we are using a four-and-a-half or five-inch telescope, and surrounded by one very thin ring of light, and the fragments, so to speak, of one or possibly two similar rings a little farther from the disk, and visible, perhaps, only by glimpses. these "diffraction rings" arise from the undulatory nature of light, and their distance apart as well as the diameter of the central disk depend upon the length of the waves of light. if the telescope is a really good one, and both object glass and eyepiece are properly adjusted, the disk will be perfectly round, slightly softer at the edge, but otherwise equally bright throughout; and the ring or rings surrounding it will be exactly concentric, and not brighter on one side than on another. even if our telescope were only two inches or two inches and a half in aperture we should at once notice a little bluish star, the mere ghost of a star in a small telescope, hovering near the polar star. it is the celebrated "companion," but we shall see it again when we have more time to study it. now let us put the star out of focus by turning the focusing screw. suppose we turn it in such a way that the eyepiece moves slightly outside the focus, or away from the object glass. very beautiful phenomena immediately begin to make their appearance. a slight motion outward causes the little disk to expand perceptibly, and just as this expansion commences, a bright-red point appears at the precise center of the disk. but, the outward motion continuing, this red center disappears, and is replaced by a blue center, which gradually expands into a sort of flare over the middle of the disk. the disk itself has in the mean time enlarged into a series of concentric bright rings, graduated in luminosity with beautiful precision from center toward circumference. the outermost ring is considerably brighter, however, than it would be if the same gradation applied to it as applies to the inner rings, and it is surrounded, moreover, on its outer edge by a slight flare which tends to increase its apparent width. next let us return to the focus and then move the eyepiece gradually inside the focal point or plane. once more the star disk expands into a series of circles, and, if we except the color phenomena noticed outside the focus, these circles are precisely like those seen before in arrangement, in size, and in brightness. if they were not the same, we should pronounce the telescope to be imperfect. there is one other difference, however, besides the absence of the blue central flare, and that is a faint reddish edging around the outer ring when the expansion inside the focus is not carried very far. upon continuing to move the eyepiece inside or outside the focus we observe that the system of rings becomes larger, while the rings themselves rapidly increase in number, becoming at the same time individually thinner and fainter. [illustration: the star image.] by studying the appearance of the star disk when in focus and of the rings when out of focus on either side, an experienced eye can readily detect any fault that a telescope may have. the amateur, of course, can only learn to do this by considerable practice. any glaring and serious fault, however, will easily make itself manifest. suppose, for example, we observe that the image of a star instead of being perfectly round is oblong, and that a similar defect appears in the form of the rings when the eyepiece is put out of focus. we know at once that something is wrong; but the trouble may lie either in the object glass, in the eyepiece, in the eye of the observer himself, or in the adjustment of the lenses in the tube. a careful examination of the image and the out-of-focus circles will enable us to determine with which of these sources of error we have to deal. if the star image when in focus has a sort of wing on one side, and if the rings out of focus expand eccentrically, appearing wider and larger on one side than on the other, being at the same time brightest on the least expanded side, then the object glass is probably not at right angles to the axis of the tube and requires readjustment. that part of the object glass on the side where the rings appear most expanded and faintest needs to be pushed slightly inward. this can be effected by means of counterscrews placed for that purpose in or around the cell. but it, after we have got the object glass properly squared to the axis of the tube or the line of sight, the image and the ring system in and out of focus still appear oblong, the fault of astigmatism must exist either in the objective, the eyepiece, or the eye. the chances are very great that it is the eye itself that is at fault. we may be certain of this if we find, on turning the head so as to look into the telescope with the eye in different positions, that the oblong image turns with the head of the observer, keeping its major axis continually in the same relative position with respect to the eye. the remedy then is to consult an oculist and get a pair of cylindrical eyeglasses. if the oblong image does not turn round with the eye, but does turn when the eyepiece is twisted round, then the astigmatism is in the latter. if, finally, it does not follow either the eye or the eyepiece, it is the objective that is at fault. but instead of being oblong, the image and the rings may be misshapen in some other way. if they are three-cornered, it is probable that the object glass is subjected to undue pressure in its cell. this, if the telescope has been brought out on a cool night from a warm room, may arise from the unequal contraction of the metal work and the glass as they cool off. in fact, no good star image can be got while a telescope is assuming the temperature of the surrounding atmosphere. even the air inclosed in the tube is capable of making much trouble until its temperature has sunk to the level of that outside. half an hour at least is required for a telescope to adjust itself to out-of-door temperature, except in the summer time, and it is better to allow an hour or two for such adjustment in cold weather. any irregularity in the shape of the rings which persists after the lenses have been accurately adjusted and the telescope has properly cooled may be ascribed to imperfections, such as veins or spots of unequal density in the glass forming the objective. [illustration: the out-of-focus rings. , correct figure; and , spherical aberration.] the spherical aberration of an object glass may be undercorrected or overcorrected. in the former case the central rings inside the focus will appear faint and the outer ones unduly strong, while outside the focus the central rings will be too bright and the outer ones too feeble. but if the aberration is overcorrected the central rings will be overbright inside the focus and abnormally faint outside the focus. [illustration: two views of mars in . the smaller with a three-and-three-eighths-inch telescope; the larger with a nine-inch.] assuming that we have a telescope in which no obvious fault is discernible, the next thing is to test its powers in actual work. in what is to follow i shall endeavor to describe some of the principal objects in the heavens from which the amateur observer may expect to derive pleasure and instruction, and which may at the same time serve as tests of the excellence of his telescope. no one should be deterred or discouraged in the study of celestial objects by the apparent insignificance of his means of observation. the accompanying pictures of the planet mars may serve as an indication of the fact that a small telescope is frequently capable of doing work that appears by no means contemptible when placed side by side with that of the greater instruments of the observatories. chapter ii in the starry heavens "now constellations, muse, and signs rehearse; in order let them sparkle in thy verse."--manilius. let us imagine ourselves the happy possessors of three properly mounted telescopes of five, four, and three inches aperture, respectively. a fine midwinter evening has come along, the air is clear, cool, and steady, and the heavens, of that almost invisible violet which is reserved for the lovers of celestial scenery, are spangled with stars that hardly twinkle. we need not disturb our minds about a few thin clouds here and there floating lazily in the high air; they announce a change of weather, but they will not trouble us to-night. which way shall we look? our eyes will answer the question for us. however we may direct them, they instinctively return to the south, and are lifted to behold orion in his glory, now near the meridian and midway to the zenith, with taurus shaking the glittering pleiades before him, and canis major with the flaming dog star following at his heels. not only is orion the most brilliant of all constellations to the casual star-gazer, but it contains the richest mines that the delver for telescopic treasures can anywhere discover. we could not have made a better beginning, for here within a space of a few square degrees we have a wonderful variety of double stars and multiple stars, so close and delicate as to test the powers of the best telescopes, besides a profusion of star-clusters and nebulæ, including one of the supreme marvels of space, the great nebula in the sword. [illustration: map no. .] our star map no. will serve as a guide to the objects which we are about to inspect. let us begin operations with our smallest telescope, the three-inch. i may remark here that, just as the lowest magnifying power that will clearly reveal the object looked for gives ordinarily better results than a higher power, so the smallest telescope that is competent to show what one wishes to see is likely to yield more satisfaction, as far as that particular object is concerned, than a larger glass. the larger the object glass and the higher the power, the greater are the atmospheric difficulties. a small telescope will perform very well on a night when a large one is helpless. turn the glass upon beta (rigel), the white first-magnitude star in orion's left foot. observe whether the image with a high power is clear, sharp, and free from irregular wisps of stray light. look at the rings in and out of focus, and if you are satisfied with the performance, try for the companion. a good three-inch is certain to show it, except in a bad state of the atmosphere, and even then an expert can see it, at least by glimpses. the companion is of the ninth magnitude, some say the eighth, and the distance is about . ", angle of position (hereafter designated by p.) °.[ ] its color is blue, in decided contrast with the white light of its great primary. sir john herschel, however, saw the companion red, as others have done. these differences are doubtless due to imperfections of the eye or the telescope. in burnham believed he had discovered that the companion was an exceedingly close double star. no one except burnham himself succeeded in dividing it, and he could only do so at times. afterward, when he was at mount hamilton, he tried in vain to split it with the great thirty-six-inch telescope, in , , and . his want of success induced him to suggest that the component stars were in rapid motion, and so, although he admitted that it might not be double after all, he advised that it should be watched for a few years longer. his confidence was justified, for in aitken, with the lick telescope, saw and measured the distance of the extremely minute companion--distance . ", p. °. [ ] the angle of position measures the inclination to the meridian of a line drawn between the principal star and its companion; in other words, it shows in what direction from the primary we must look for the companion. it is reckoned from ° up to °, beginning at the north point and passing around by east through south and west to north again. thus, if the angle of position is ° or °, the companion is on the north side of the primary; if the angle is °, the companion is to the east; if °, to the south; if °, to the west, and so for intermediate angles. it must be remembered, however, that in the field of the telescope the top is south and the bottom north, unless a prism is used, when directions become complicated. east and west can be readily identified by noticing the motion of a star through the field; it moves toward the west and from the east. rigel has been suspected of a slight degree of variability. it is evidently a star of enormous actual magnitude, for its parallax escapes trustworthy measurement. it can only be ranked among the very first of the light-givers of the visible universe. spectroscopically it belongs to a peculiar type which has very few representatives among the bright stars, and which has been thus described: "spectra in which the hydrogen lines and the few metallic lines all appear to be of equal breadth and sharp definition." rigel shows a line which some believe to represent magnesium; but while it has iron lines in its spectrum, it exhibits no evidence of the existence of any such cloud of volatilized iron as that which helps to envelop the sun. for another test of what the three-inch will do turn to zeta, the lower, or left-hand, star in the belt. this is a triple, the magnitudes being second, sixth, and tenth. the sixth-magnitude star is about . " from the primary, p. °, and has a very peculiar color, hard to describe. it requires careful focusing to get a satisfactory view of this star with a three-inch telescope. use magnifying powers up to two hundred and fifty diameters. with our four-inch the star is much easier, and the five-inch shows it readily with a power of one hundred. the tenth-magnitude companion is distant ", p. °, and may be glimpsed with the three-inch. upon the whole, we shall find that we get more pleasing views of zeta orionis with the four-inch glass. just to the left of zeta, and in the same field of view with a very low power, is a remarkable nebula bearing the catalogue number . we must use our five-inch on this with a low power, but with zeta out of the field in order to avoid its glare. the nebula is exceedingly faint, and we can be satisfied if we see it simply as a hazy spot, although with much larger telescopes it has appeared at least half a degree broad. tempel saw several centers of condensation in it, and traced three or four broad nebulous streams, one of which decidedly suggested spiral motion. the upper star in the belt, delta, is double; distance, ", p. °; magnitudes, second and seventh very nearly; colors, white and green or blue. this, of course, is an easy object for the three-inch with a low magnifying power. it would be useless to look for the two fainter companions of delta, discovered by burnham, even with our five-inch glass. but we shall probably need the five-inch for our next attempt, and it will be well to put on a high power, say three hundred diameters. the star to be examined is the little brilliant dangling below the right-hand end of the belt, toward rigel. it appears on the map as eta. spare no pains in getting an accurate focus, for here is something worth looking at, and unless you have a trained eye you will not easily see it. the star is double, magnitudes third and sixth, and the distance from center to center barely exceeds ", p. °. a little tremulousness of the atmosphere for a moment conceals the smaller star, although its presence is manifest from the peculiar jutting of light on one side of the image of the primary. but in an instant the disturbing undulations pass, the air steadies, the image shrinks and sharpens, and two points of piercing brightness, almost touching one another, dart into sight, the more brilliant one being surrounded by an evanescent circle, a tiny ripple of light, which, as it runs round the star and then recedes, alternately embraces and releases the smaller companion. the wash of the light-waves in the atmosphere provokes many expressions of impatience from the astronomer, but it is often a beautiful phenomenon nevertheless. between eta and delta is a fifth-magnitude double star, sigma , which is worth a moment's attention. the primary, of a reddish color, has a very faint star, eleventh magnitude, at a distance of . ", p. °. still retaining the five-inch in use, we may next turn to the other end of the belt, where, just under zeta, we perceive the fourth-magnitude star sigma. he must be a person of indifferent mind who, after looking with unassisted eyes at the modest glimmering of this little star, can see it as the telescope reveals it without a thrill of wonder and a cry of pleasure. the glass, as by a touch of magic, changes it from one into eight or ten stars. there are two quadruple sets three and a half minutes of arc apart. the first set exhibits a variety of beautiful colors. the largest star, of fourth magnitude, is pale gray; the second in rank, seventh magnitude, distance ", p. °, presents a singular red, "grape-red" webb calls it; the third, eighth magnitude, distance ", p. °, is blue; and the fourth, eleventh magnitude, distance ", p. °, is apparently white. burnham has doubled the fourth-magnitude star, distance . ". the second group of four stars consists of three of the eighth to ninth magnitude, arranged in a minute triangle with a much fainter star near them. between the two quadruple sets careful gazing reveals two other very faint stars. while the five-inch gives a more satisfactory view of this wonderful multiple star than any smaller telescope can do, the four-inch and even the three-inch would have shown it to us as a very beautiful object. however we look at them, there is an appearance of association among these stars, shining with their contrasted colors and their various degrees of brilliance, which is significant of the diversity of conditions and circumstances under which the suns and worlds beyond the solar walk exist. from sigma let us drop down to see the wonders of orion's sword displayed just beneath. we can use with advantage any one of our three telescopes; but since we are going to look at a nebula, it is fortunate that we have a glass so large as five inches aperture. it will reveal interesting things that escape the smaller instruments, because it grasps more than one and a half times as much light as the four-inch, and nearly three times as much as the three-inch; and in dealing with nebulæ a plenty of light is the chief thing to be desired. the middle star in the sword is theta, and is surrounded by the celebrated nebula of orion. the telescope shows theta separated into four stars arranged at the corners of an irregular square, and shining in a black gap in the nebula. these four stars are collectively named the trapezium. the brightest is of the sixth magnitude, the others are of the seventh, seven and a half, and eighth magnitudes respectively. the radiant mist about them has a faint greenish tinge, while the four stars, together with three others at no great distance, which follow a fold of the nebula like a row of buttons on a coat, always appear to me to show an extraordinary liveliness of radiance, as if the strange haze served to set them off. [illustration: the trapezium with the fifth and sixth stars.] our three-inch would have shown the four stars of the trapezium perfectly well, and the four-inch would have revealed a fifth star, very faint, outside a line joining the smallest of the four and its nearest neighbor. but the five-inch goes a step farther and enables us, with steady gazing to see even a sixth star, of only the twelfth magnitude, just outside the trapezium, near the brightest member of the quartet. the lick telescope has disclosed one or two other minute points of light associated with the trapezium. but more interesting than the trapezium is the vast cloud, full of strange shapes, surrounding it. nowhere else in the heavens is the architecture of a nebula so clearly displayed. it is an unfinished temple whose gigantic dimensions, while exalting the imagination, proclaim the omnipotence of its builder. but though unfinished it is not abandoned. the work of creation is proceeding within its precincts. there are stars apparently completed, shining like gems just dropped from the hand of the polisher, and around them are masses, eddies, currents, and swirls of nebulous matter yet to be condensed, compacted, and constructed into suns. it is an education in the nebular theory of the universe merely to look at this spot with a good telescope. if we do not gaze at it long and wistfully, and return to it many times with unflagging interest, we may be certain that there is not the making of an astronomer in us. before quitting the orion nebula do not fail to notice an eighth-magnitude star, a short distance northeast of the great nebula, and nearly opposite the broad opening in the latter that leads in toward the gap occupied by the trapezium. this star is plainly enveloped in nebulosity, that is unquestionably connected with the larger mass of which it appears to form a satellite. at the lower end of the sword is the star iota, somewhat under the third magnitude. our three-inch will show that it has a bluish companion of seventh or eighth magnitude, at a little more than " distance, p. °, and the larger apertures will reveal a third star, of tenth magnitude, and reddish in color, distance ", p. °. close by iota we find the little double star sigma , whose components are of five and a half and six and a half magnitudes respectively, and separated ", p. °. above the uppermost star in the sword is a small star cluster, no. , which derives a special interest from the fact that it incloses a delicate double star, sigma , whose larger component is of the sixth magnitude, while the smaller is of the ninth, and the distance is only . ", p. °. we may try the four-inch on this object. having looked at alpha (betelgeuse), the great topaz star on orion's right shoulder, and admired the splendor of its color, we may turn the four-inch upon the star sigma , frequently referred to by its number as " orionis." it consists of one star of the sixth and another of sixth and a half magnitude, only . " apart, p. °. having separated them with a power of two hundred and fifty diameters on the four-inch, we may try them with a high power on the three-inch. we shall only succeed this time if our glass is of first-rate quality and the air is perfectly steady. the star lambda in orion's head presents an easy conquest for the three-inch, as it consists of a light-yellow star of magnitude three and a half and a reddish companion of the sixth magnitude; distance ", p. °. there is also a twelfth-magnitude star at ", p. °, and a tenth or eleventh magnitude one at ", p. °. these are tests for the five-inch, and we must not be disappointed if we do not succeed in seeing the smaller one even with that aperture. other objects in orion, to be found with the aid of our map, are: sigma , a double star, magnitude six and a half and seven, distance ", p. °; omicron sigma , otherwise named iota orionis, double, magnitude six and seven, distance ", p. °, requires five-inch glass; sigma , double, magnitudes six and a half and eight, distance . ", p. °; rho, double, magnitudes five and eight and a half, the latter blue, distance ", p. °, may be tried with a three-inch; tau, triple star, magnitudes four, ten and a half, and eleven, distances ", p. °, and ", p. °. burnham discovered that the ten-and-a-half magnitude star is again double, distance ", p. °. there is not much satisfaction in attempting tau orionis with telescopes of ordinary apertures; sigma otherwise _m_ orionis, double, magnitudes five and a half (greenish) and seven, distance . ", p. °, a pretty object; sigma , otherwise a , double, magnitudes five and seven, distance, . " or less, p. °, a rapid binary,[ ] which is at present too close for ordinary telescopes, although it was once within their reach; sigma , double, magnitudes six and eight, distance ", p. °, the smaller star pale blue--try it with a four-inch, but five-inch is better; sigma , double, magnitudes six and half and eight and a half, distance ", p. °; psi , double, magnitudes five and a half and eleven, distance ", or a little less, p. °; , star cluster, contains about twenty stars from the eighth to the eleventh magnitude; , nebula, faint, containing a triple star of the eighth magnitude, two of whose components are " apart, while the third is only . " from its companion, p. °; , star cluster, small and crowded; , star cluster, triangular shape, containing thirty stars, seventh to tenth magnitudes, one of which is a double, distance . ". [ ] the term "binary" is used to describe double stars which are in motion about their common center of gravity. let us now leave the inviting star-fields of orion and take a glance at the little constellation of lepus, crouching at the feet of the mythical giant. we may begin with a new kind of object, the celebrated red variable r leporis (map no. ). this star varies from the sixth or seventh magnitude to magnitude eight and a half in a period of four hundred and twenty-four days. hind's picturesque description of its color has frequently been quoted. he said it is "of the most intense crimson, resembling a blood-drop on the black ground of the sky." it is important to remember that this star is reddest when faintest, so that if we chance to see it near its maximum of brightness it will not impress us as being crimson at all, but rather a dull, coppery red. its spectrum indicates that it is smothered with absorbing vapors, a sun near extinction which, at intervals, experiences an accession of energy and bursts through its stifling envelope with explosive radiance, only to faint and sink once more. it is well to use our largest aperture in examining this star. we may also employ the five-inch for an inspection of the double star iota, whose chief component of the fifth magnitude is beautifully tinged with green. the smaller companion is very faint, eleventh magnitude, and the distance is about ", p. °. another fine double in lepus is kappa, to be found just below iota; the components are of the fifth and eighth magnitudes, pale yellow and blue respectively, distance . ", p. °; the third-magnitude star alpha has a tenth-magnitude companion at a distance of ", p. °, and its neighbor beta (map no. ), according to burnham, is attended by three eleventh-magnitude stars, two of which are at distances of ", p. °, and ", p. °, respectively, while the third is less than " from beta, p. °; the star gamma (map no. ) is a wide double, the distance being ", and the magnitudes four and eight. the star numbered is a remarkable multiple, but the components are too faint to possess much interest for those who are not armed with very powerful telescopes. [illustration: map no. .] from lepus we pass to canis major (map no. ). there is no hope of our being able to see the companion of alpha (sirius), at present ( ), even with our five-inch. discovered by alvan clark with an eighteen-inch telescope in , when its distance was " from the center of sirius, this ninth-magnitude star has since been swallowed up in the blaze of its great primary. at first, it slightly increased its distance, and from until most of the measures made by different observers considerably exceeded ". then it began to close up, and in the distance scarcely exceeded ". burnham was the last to catch sight of it with the lick telescope in that year. after that no human eye saw it until , when it was rediscovered at the lick observatory. since then the distance has gradually increased to nearly ". according to burnham, its periodic time is about fifty-three years, and its nearest approach to sirius should have taken place in the middle of . later calculations reduce the periodic time to forty-eight or forty-nine years. if we can not see the companion of the dog star with our instruments, we can at least, while admiring the splendor of that dazzling orb, reflect with profit upon the fact that although the companion is ten thousand times less bright than sirius, it is half as massive as its brilliant neighbor. imagine a subluminous body half as ponderous as the sun to be set revolving round it somewhere between uranus and neptune. remember that that body would possess one hundred and sixty-five thousand times the gravitating energy of the earth, and that five hundred and twenty jupiters would be required to equal its power of attraction, and then consider the consequences to our easy-going planets! plainly the solar system is not cut according to the sirian fashion. we shall hardly find a more remarkable coupling of celestial bodies until we come, on another evening, to a star that began, ages ago, to amaze the thoughtful and inspire the superstitious with dread--the wonderful algol in perseus. we may remark in passing that sirius is the brightest representative of the great spectroscopic type i, which includes more than half of all the stars yet studied, and which is characterized by a white or bluish-white color, and a spectrum possessing few or at best faint metallic lines, but remarkably broad, black, and intense lines of hydrogen. the inference is that sirius is surrounded by an enormous atmosphere of hydrogen, and that the intensity of its radiation is greater, surface for surface, than that of the sun. there is historical evidence to support the assertion, improbable in itself, that sirius, within eighteen hundred years, has changed color from red to white. with either of our telescopes we shall have a feast for the eye when we turn the glass upon the star cluster no. , some four degrees south of sirius. look for a red star near the center. observe the curving rows so suggestive of design, or rather of the process by which this cluster was evolved out of a pre-existing nebula. you will recall the winding streams in the great nebula of orion. another star cluster worth a moment's attention is no. , above and to the left of sirius. we had better use the five-inch for this, as many of the stars are very faint. not far away we find the double star , whose components are of the fifth and eighth magnitudes, distance . ", p. °. the small star is pale blue. cluster no. is a pleasing object with our largest aperture. in no. we have a faint nebula remarkable for the rows of minute stars in and near it. the star gamma is an irregular variable. in it is said to have almost disappeared, while at the beginning of the eighteenth century it was more than twice as bright as it is to-day. the reddish star delta is also probably variable. in my "astronomy with an opera glass" will be found a cut showing a singular array of small stars partly encircling delta. these are widely scattered by a telescope, even with the lowest power. eastward from canis major we find some of the stars of argo navis. sigma , of the sixth magnitude, has two minute companions at " distance, p. ° and °. the large star is itself double, but the distance, . ", p. °, places it beyond our reach. according to burnham, there is yet a fourth faint star at ", p. °. some three degrees and a half below and to the left of the star just examined is a beautiful star cluster, no. . nos. , , and are other star clusters well worth examination. a planetary nebula is included in . with very powerful telescopes this nebula has been seen ring-shaped. sigma , otherwise known as navis, is a pretty double, colors pale yellow and blue, magnitudes five and seven, distance . ", p. °. our three-inch will suffice for this. [illustration: map no. .] north of canis major and argo we find monoceros and canis minor (map no. ). the stars forming the western end of monoceros are depicted on map no. . we shall begin with these. the most interesting and beautiful is , a fine triple star, magnitudes five, six, and seven, distances . ", p. °, and . ", p. °. sir william herschel regarded this as one of the most beautiful sights in the heavens. it is a good object to try our three-inch on, although it should not be difficult for such an aperture. the star is also a triple, magnitudes six, ten, and eleven, distances . ", p. °, and ", p. °. we should glance at the star to admire its fine orange color. in we find a golden fifth-magnitude star, combined with a blue or lilac star of the seventh magnitude, distance ", p. °. sigma is a difficult double, magnitudes six and a half and twelve, distance ", p. °. sigma is double, magnitudes six and a half and eight, distance ", p. °. at the spot marked on the map we find an interesting cluster containing one star of the sixth magnitude. the remaining stars of monoceros will be found on map no. . the double and triple stars to be noted are s, or sigma (which is also a variable and involved in a faint nebula), magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes five and a half and eight, distance ", p. °; sigma , triple, magnitudes five and a half, ten, and nine, distances ", p. °, and ", p. °. the clusters are , which has a minute triple star near the center; , one member of whose swarm is red; , very small but rich; and , interesting for the great number of ninth-magnitude stars that it contains. we should use the five-inch for all of these. canis minor and the head of hydra are also contained on map no. . procyon, alpha of canis minor, has several minute stars in the same field of view. there is, besides, a companion which, although it was known to exist, no telescope was able to detect until november, . it must be of immense mass, since its attraction causes perceptible perturbations in the motion of procyon. its magnitude is eight and a half, distance . ", p. °. one of the small stars just referred to, the second one east of procyon, distant one third of the moon's diameter, is an interesting double. our four-inch may separate it, and the five-inch is certain to do so. the magnitudes are seven and seven and a half or eight, distance . ", p. °. this star is variously named sigma and can. min. bode. star no. is a wide triple, magnitudes six, seven, and eight, distances , p. °, and ", p. °. procyon and its neighbors. in the head of hydra we find sigma , a double of the sixth and seventh magnitudes, distance . ", p. °. the larger star shows a fine yellow. in epsilon we have a beautiful combination of a yellow with a blue star, magnitudes four and eight, distance . ", p. °. finally, let us look at theta for a light test with the five-inch. the two stars composing it are of the fourth and twelfth magnitudes, distance ", p. °. the brilliant constellations of gemini and taurus tempt us next, but warning clouds are gathering, and we shall do well to house our telescopes and warm our fingers by the winter fire. there will be other bright nights, and the stars are lasting. chapter iii from gemini to leo and round about "if thou wouldst gaze on starry charioteer, and hast heard legends of the wondrous goat, vast looming shalt thou find on the twins' left, his form bowed forward."--poste's aratus. [illustration: map no. .] the zodiacal constellations of gemini, cancer, and leo, together with their neighbors auriga, the lynx, hydra, sextans, and coma berenices, will furnish an abundance of occupation for our second night at the telescope. we shall begin, using our three-inch glass, with alpha, the chief star of gemini (map no. ). this is ordinarily known as castor. even an inexperienced eye perceives at once that it is not as bright as its neighbor pollux, beta. whether this fact is to be regarded as indicating that castor was brighter than pollux in , when bayer attached their greek letters, is still an unsettled question. castor may or may not be a variable, but it is, at any rate, one of the most beautiful double stars in the heavens. a power of one hundred is amply sufficient to separate its components, whose magnitudes are about two and three, the distance between them being ", p. °. a slight yet distinct tinge of green, recalling that of the orion nebula, gives a peculiar appearance to this couple. green is one of the rarest colors among the stars. castor belongs to the same general spectroscopic type in which sirius is found, but its lines of hydrogen are broader than those seen in the spectrum of the dog star. there is reason for thinking that it may be surrounded with a more extensive atmosphere of that gaseous metal called hydrogen than any other bright star possesses. there seems to be no doubt that the components of castor are in revolution around their common center of gravity, although the period is uncertain, varying in different estimates all the way from two hundred and fifty to one thousand years; the longer estimate is probably not far from the truth. there is a tenth-magnitude star, distance ", p. °, which may belong to the same system. from castor let us turn to pollux, at the same time exchanging our three-inch telescope for the four-inch, or, still better, the five-inch. pollux has five faint companions, of which we may expect to see three, as follows: tenth magnitude, distance ", p. °; nine and a half magnitude, distance ", p. °, and ninth magnitude, distance ", p. °. burnham has seen a star of thirteen and a half magnitude, distance ", p. °, and has divided the tenth-magnitude star into two components, only . " apart, the smaller being of the thirteenth magnitude, and situated at the angle °. a calculation based on dr. elkin's parallax of . " for pollux shows that that star may be a hundredfold more luminous than the sun, while its nearest companion may be a body smaller than our planet jupiter, but shining, of course, by its own light. its distance from pollux, however, exceeds that of jupiter from the sun in the ratio of about one hundred and thirty to one. in the double star pi we shall find a good light test for our three-inch aperture, the magnitudes being six and eleven, distance ", p. °. the four-inch will show that kappa is a double, magnitudes four and ten, distance ", p. °. the smaller star is of a delicate blue color, and it has been suspected of variability. that it may be variable is rendered the more probable by the fact that in the immediate neighborhood of kappa there are three undoubted variables, s, t, and u, and there appears to be some mysterious law of association which causes such stars to group themselves in certain regions. none of the variables just named ever become visible to the naked eye, although they all undergo great changes of brightness, sinking from the eighth or ninth magnitude down to the thirteenth or even lower. the variable r, which lies considerably farther west, is well worth attention because of the remarkable change of color which it sometimes exhibits. it has been seen blue, red, and yellow in succession. it varies from between the sixth and seventh magnitudes to less than the thirteenth in a period of about two hundred and forty-two days. not far away we find a still more curious variable zeta; this is also an interesting triple star, its principal component being a little under the third magnitude, while one of the companions is of the seventh magnitude, distance ", p. °, and the other is of the eleventh magnitude or less, distance ", p. °. we should hardly expect to see the fainter companion with the three-inch. the principal star varies from magnitude three and seven tenths down to magnitude four and a half in a period of a little more than ten days. [illustration: wonderful nebula in gemini ( ).] with the four-or five-inch we get a very pretty sight in delta, which appears split into a yellow and a purple star, magnitudes three and eight, distance ", p. °. near delta, toward the east, lies one of the strangest of all the nebulæ. (see the figures on the map.) our telescopes will show it to us only as a minute star surrounded with a nebulous atmosphere, but its appearance with instruments of the first magnitude is so astonishing and at the same time so beautiful that i can not refrain from giving a brief description of it as i saw it in with the great lick telescope. in the center glittered the star, and spread evenly around it was a circular nebulous disk, pale yet sparkling and conspicuous. this disk was sharply bordered by a narrow _black_ ring, and outside the ring the luminous haze of the nebula again appeared, gradually fading toward the edge to invisibility. the accompanying cut, which exaggerates the brightness of the nebula as compared with the star, gives but a faint idea of this most singular object. if its peculiarities were within the reach of ordinary telescopes, there are few scenes in the heavens that would be deemed equally admirable. in the star eta we have another long-period variable, which is also a double star; unfortunately the companion, being of only the tenth magnitude and distant less than " from its third-magnitude primary, is beyond the reach of our telescopes. but eta points the way to one of the finest star clusters in the sky, marked on the map. the naked eye perceives that there is something remarkable in that place, and the opera glass faintly reveals its distant splendors, but the telescope fairly carries us into its presence. its stars are innumerable, varying from the ninth magnitude downward to the last limit of visibility, and presenting a wonderful array of curves which are highly interesting from the point of view of the nebular origin of such clusters. looking backward in time, with that theory to guide us, we can see spiral lines of nebulous mist occupying the space that now glitters with interlacing rows of stars. it is certainly difficult to understand how such lines of nebula could become knotted with the nuclei of future stars, and then gradually be absorbed into those stars; and yet, if such a process does not occur, what is the meaning of that narrow nebulous streak in the pleiades along which five or six stars are strung like beads on a string? the surroundings of this cluster, , as one sweeps over them with the telescope gradually drawing toward the nucleus, have often reminded me of the approaches to such a city as london. thicker and closer the twinkling points become, until at last, as the observers eye follows the gorgeous lines of stars trending inward, he seems to be entering the streets of a brilliantly lighted metropolis. other objects in gemini that we can ill miss are: , double, magnitudes three and eleven, distance ", p. °, colors yellow and blue; , double, magnitudes six and eight, distance ", p. °; gamma, remarkable for array of small stars near it; , double, magnitudes six and eight, distance . ", p. °, colors yellow and blue (very pretty); lambda, double, magnitudes four and eleven, distance ", p. °, color of larger star blue--try with the five-inch; epsilon, double, magnitudes three and nine, distance ", p. °. from gemini we pass to cancer. this constellation has no large stars, but its great cluster præsepe ( on map no. ) is easily seen as a starry cloud with the naked eye. with the telescope it presents the most brilliant appearance with a very low power. it was one of the first objects that galileo turned to when he had completed his telescope, and he wonderingly counted its stars, of which he enumerated thirty-six, and made a diagram showing their positions. the most interesting star in cancer is zeta, a celebrated triple. the magnitudes of its components are six, seven, and seven and a half; distances . ", p. °, and . ", p. °. we must use our five-inch glass in order satisfactorily to separate the two nearest stars. the gravitational relationship of the three stars is very peculiar. the nearest pair revolve around their common center in about fifty-eight years, while the third star revolves with the other two, around a center common to all three, in a period of six or seven hundred years. but the movements of the third star are erratic, and inexplicable except upon the hypothesis advanced by seeliger, that there is an invisible, or dark, star near it by whose attraction its motion is perturbed. in endeavoring to picture the condition of things in zeta cancri we might imagine our sun to have a companion sun, a half or a third as large as itself, and situated within what may be called planetary distance, circling with it around their center of gravity; while a third sun, smaller than the second and several times as far away, and accompanied by a _black_ or non-luminous orb, swings with the first two around another center of motion. there you would have an entertaining complication for the inhabitants of a system of planets! other objects in cancer are: sigma , double star, magnitudes six and six and a half, distance ", p. °; sigma , double, magnitudes both six, distance . ", p. °--four-inch should split it; iota, double, magnitudes four and a half and six and a half, distance ", p. °; , double magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes both about the seventh, distance ", p. °; , star cluster, very beautiful with the five-inch glass. [illustration: map no. .] the constellation of auriga may next command our attention (map no. ). the calm beauty of its leading star capella awakens an admiration that is not diminished by the rivalry of orion's brilliants glittering to the south of it. although capella must be an enormously greater sun than ours, its spectrum bears so much resemblance to the solar spectrum that a further likeness of condition is suggested. no close telescopic companion to capella has been discovered. a ninth-magnitude companion, distant ", p. °, and two others, one of twelfth magnitude at ", p. °, the other of thirteenth magnitude at ", p. °, may be distant satellites of the great star, but not planets in the ordinary sense, since it is evident that they are self-luminous. it is a significant fact that most of the first-magnitude stars have faint companions which are not so distant as altogether to preclude the idea of physical relationship. but while capella has no visible companion, campbell, of the lick observatory, has lately discovered that it is a conspicuous example of a peculiar class of binary stars only detected within the closing decade of the nineteenth century. the nature of these stars, called spectroscopic binaries, may perhaps best be described while we turn our attention from capella to the second star in auriga beta (menkalina), which not only belongs to the same class, but was the first to be discovered. neither our telescopes, nor any telescope in existence, can directly reveal the duplicity of beta aurigæ to the eye--i. e., we can not see the two stars composing it, because they are so close that their light remains inextricably mingled after the highest practicable magnifying power has been applied in the effort to separate them. but the spectroscope shows that the star is double and that its components are in rapid revolution around one another, completing their orbital swing in the astonishingly short period of _four days_! the combined mass of the two stars is estimated to be two and a half times the mass of the sun, and the distance between them, from center to center, is about eight million miles. the manner in which the spectroscope revealed the existence of two stars in beta aurigæ is a beautiful illustration of the unexpected and, so to speak, automatic application of an old principle in the discovery of new facts not looked for. it was noticed at the harvard observatory that the lines in the photographed spectrum of beta aurigæ (and of a few other stars to be mentioned later) appeared single in some of the photographs and double in others. investigation proved that the lines were doubled at regular intervals of about two days, and that they appeared single in the interim. the explanation was not far to seek. it is known that all stars which are approaching us have their spectral lines shifted, by virtue of their motion of approach, toward the violet end of the spectrum, and that, for a similar reason, all stars which are receding have their lines shifted toward the red end of the spectrum. now, suppose two stars to be revolving around one another in a plane horizontal, or nearly so, to the line of sight. when they are at their greatest angular distance apart as seen from the earth one of them will evidently be approaching at the same moment that the other is receding. the spectral lines of the first will therefore be shifted toward the violet, and those of the second will be shifted toward the red. then if the stars, when at their greatest distance apart, are still so close that the telescope can not separate them, their light will be combined in the spectrum; but the spectral lines, being simultaneously shifted in opposite directions, will necessarily appear to be doubled. as the revolution of the stars continues, however, it is clear that their motion will soon cease to be performed in the line of sight, and will become more and more athwart that line, and as this occurs the spectral lines will gradually assume their normal position and appear single. this is the sequence of phenomena in beta aurigæ. and the same sequence is found in capella and in several other more or less conspicuous stars in various parts of the heavens. such facts, like those connecting rows and groups of stars with masses and spiral lines of nebula are obscure signboards, indicating the opening of a way which, starting in an unexpected direction, leads deep into the mysteries of the universe. southward from beta we find the star theta, which is a beautiful quadruple. we shall do best with our five-inch here, although in a fine condition of the atmosphere the four-inch might suffice. the primary is of the third magnitude; the first companion is of magnitude seven and a half, distance ", p. °; the second, of the tenth magnitude, distance ", p. °; and the third, of the tenth magnitude, distance ", p. °. we should look at the double sigma with one of our larger apertures in order to determine for ourselves what the colors of the components are. there is considerable diversity of opinion on this point. some say the larger star is pale red and the smaller light blue; others consider the color of the larger star to be greenish, and some have even called it white. the magnitudes are five and nine, distance ", p. °. auriga contains several noteworthy clusters which will be found on the map. the most beautiful of these is , in which about five hundred stars have been counted. the position of the new star of , known as nova aurigæ, is also indicated on the map. while this never made a brilliant appearance, it gave rise to a greater variety of speculative theories than any previous phenomenon of the kind. although not recognized until january , , this star, as photographic records prove, was in existence on december , . at its brightest it barely exceeded magnitude four and a half, and its maximum occurred within ten days after its first recognition. when discovered it was of the fifth magnitude. it was last seen in its original form with the lick telescope on april th, when it had sunk to the lowest limit of visibility. to everybody's astonishment it reappeared in the following august, and on the th of that month was seen shining with the light of a tenth-magnitude star, _but presenting the spectrum of a nebula!_ its visual appearance in the great telescope was now also that of a planetary nebula. its spectrum during the first period of its visibility had been carefully studied, so that the means existed for making a spectroscopic comparison of the phenomenon in its two phases. during the first period, when only a stellar spectrum was noticed, remarkable shiftings of the spectral lines occurred, indicating that two and perhaps three bodies were concerned in the production of the light of the new star, one of which was approaching the earth, while the other or the others receded with velocities of several hundred miles per second! on the revival in the form of a planetary nebula, while the character of the spectrum had entirely changed, evidences of rapid motion in the line of sight remained. but what was the meaning of all this? evidently a catastrophe of some kind had occurred out there in space. the idea of a collision involving the transformation of the energy of motion into that of light and heat suggests itself at once. but what were the circumstances of the collision? did an extinguished sun, flying blindly through space, plunge into a vast cloud of meteoric particles, and, under the lashing impact of so many myriads of missiles, break into superficial incandescence, while the cosmical wrack through which it had driven remained glowing with nebulous luminosity? such an explanation has been offered by seeliger. or was vogel right when he suggested that nova aurigæ could be accounted for by supposing that a wandering dark body had run into collision with a system of planets surrounding a decrepit sun (and therefore it is to be hoped uninhabited), and that those planets had been reduced to vapor and sent spinning by the encounter, the second outburst of light being caused by an outlying planet of the system falling a prey to the vagabond destroyer? or some may prefer the explanation, based on a theory of wilsing's, that _two_ great bodies, partially or wholly opaque and non-luminous at their surfaces, but liquid hot within, approached one another so closely that the tremendous strain of their tidal attraction burst their shells asunder so that their bowels of fire gushed briefly visible, amid a blaze of spouting vapors. and yet lockyer thinks that there was no solid or semisolid mass concerned in the phenomenon at all, but that what occurred was simply the clash of two immense swarms of meteors that had crossed one another's track. well, where nobody positively knows, everybody has free choice. in the meantime, look at the spot in the sky where that little star made its appearance and underwent its marvelous transformation, for, even if you can see no remains of it there, you will feel your interest in the problem it has presented, and in the whole subject of astronomy, greatly heightened and vivified, as the visitor to the field of waterloo becomes a lover of history on the spot. the remaining objects of special interest in auriga may be briefly mentioned: , triple star, magnitudes five, eight, and eleven, distances ", p. °, and ", p. °; , triple star, magnitudes five, seven and a half, and eleven, distances ", p. °, and . ", p. °, the last difficult for moderate apertures; lambda, double, magnitudes five and nine, distance ", p. °; epsilon, variable, generally of third magnitude, but has been seen of only four and a half magnitude; , double, magnitudes five and six, distance ", p. °; , , , and , clusters all well worth inspection, being especially beautiful. the inconspicuous lynx furnishes some fine telescopic objects, all grouped near the northwestern corner of the constellation. without a six-inch telescope it would be a waste of time to attack the double star , whose components are of sixth and eighth magnitudes, distance . ", p. °; but its neighbor, , a fine triple, is within our reach, the magnitudes being six, ten, and eight, distances ", p. °, and ", p. °. in lyncis we find one of the most attractive of triple stars, which in good seeing weather is not beyond the powers of a three-inch glass, although we shall have a far more satisfactory view of it with the four-inch. the components are of the sixth, seventh, and eighth magnitudes, distances . ", p. °, and . ", p. °. a magnifying power which just suffices clearly to separate the disks of the two nearer stars makes this a fine sight. a beautiful contrast of colors belongs to the double star , but unfortunately the star is at present very close, the distance between its sixth and seventh magnitude components not exceeding . ", position angle °. sigma is a pretty double, both stars being of the sixth magnitude, distance ", p. °. still finer is sigma , a double, whose stars are both a little above the seventh magnitude and nearly equal, distance ", p. °. a low power suffices to show the three stars in , their magnitudes being six and a half, seven and a half, and eight, distances ", p. °, and ", p. °. webb describes the two smaller stars as plum-colored. plum-colored suns! at the opposite end of the constellation are two fine doubles, sigma , magnitudes six and a half and seven, distance . ", p. °; and , magnitudes four and seven, distance . ", p. °. under the guidance of map no. we turn to leo, which contains one of the leading gems among the double stars, gamma, whose components, of the second and fourth magnitudes, are respectively yellow and green, the green star, according to some observers, having a peculiar tinge of red. their distance apart is . ", p. °, and they are undoubtedly in revolution about a common center, the probable period being about four hundred years. the three-inch glass should separate them easily when the air is steady, and a pleasing sight they are. the star iota is a closer double, and also very pretty, magnitudes four and eight, colors lemon and light blue, distance . ", p. °. other doubles are tau, magnitudes five and seven, distance ", p. °; , magnitudes seven and nine, distance ", p. °; , triple, magnitudes six, seven and a half, and ten, distance, . ", p. °, and ", p. °; , magnitudes four and a half and seven, distance . ", p. °; and , magnitudes six and nine, distance . ", p. °. leo contains a remarkable variable star, r, deep red in color, and varying in a space of a hundred and forty-four days from the fifth to the tenth magnitude. it has also several nebulæ, of which only one needs special mention, no. . this is spindle-shaped, and large telescopes show that it consists of three nebulæ. the observer with ordinary instruments finds little to see and little to interest him in these small, faint nebulæ. we may just glance at two double stars in the small constellation of sextans, situated under leo. these are: , magnitudes seven and eight, distance ", p. °; and , magnitudes six and seven, distance . ", p. °. [illustration: map no. .] coma berenices (map no. ) includes several interesting objects. let us begin with the star , a double, of magnitudes six and seven and a half, distance . ", p. °. the color of the smaller star is lilac. this hue, although not extremely uncommon among double stars elsewhere, recurs again and again, with singular persistence, in this little constellation. for instance, in the very next star that we look at, , we find a double whose smaller component is _lilac_. the magnitudes in are five and eight, distance ", p. °. so also the wide double , magnitudes five and a half and six, distance ", exhibits a tinge of _lilac_ in the smaller component; the triple , magnitudes five, eight, and nine, distances ", p. °, and . ", p. °, has four colors yellow, _lilac_, and blue, and the double , magnitudes five and six, distance ", p. °, combines an orange with a _lilac_ star, a very striking and beautiful contrast. we should make a mistake if we regarded this wonderful distribution of color among the double stars as accidental. it is manifestly expressive of their physical condition, although we can not yet decipher its exact meaning. the binary comæ berenicis is too close for ordinary telescopes, but it is highly interesting as an intermediate between those pairs which the telescope is able to separate and those--like beta aurigæ--which no magnifying power can divide, but which reveal the fact that they are double by the periodical splitting of their spectral lines. the orbit in comæ berenicis is a very small one, so that even when the components are at their greatest distance apart they can not be separated by a five-or six-inch glass. burnham, using the lick telescope, in made the distance . "; hall, using the washington telescope, in made it a trifle more than . ". he had measured it in as only . ". the period of revolution is believed to be about twenty-five years. in coma berenices there is an outlying field of the marvelous nebulous region of virgo, which we may explore on some future evening. but the nebulæ in coma are very faint, and, for an amateur, hardly worth the trouble required to pick them up. the two clusters included in the map, and , are bright enough to repay inspection with our largest aperture. [illustration: map no. .] although hydra is the largest constellation in the heavens, extending about seven hours, or °, in right ascension, it contains comparatively few objects of interest, and most of these are in the head or western end of the constellation, which we examined during our first night at the telescope. in the central portion of hydra, represented on map no. , we find its leading star alpha, sometimes called alphard, or cor hydræ, a bright second-magnitude star that has been suspected of variability. it has a decided orange tint, and is accompanied, at a distance of ", p. °, by a greenish tenth-magnitude star. bu. is a fine double, magnitudes eight and nine and a half, distance . ", p. °. the planetary nebula is about ' in diameter, pale blue in color, and worth looking at, because it is brighter than most objects of its class. tempel and secchi have given wonderful descriptions of it, both finding multitudes of stars intermingled with nebulous matter. for a last glimpse at celestial splendors for the night, let us turn to the rich cluster , in argo, just above the place where the stream of the milky way--here bright in mid-channel and shallowing toward the shores--separates into two or three currents before disappearing behind the horizon. it is by no means as brilliant as some of the star clusters we have seen, but it gains in beauty and impressiveness from the presence of one bright star that seems to captain a host of inferior luminaries. chapter iv virgo and her neighbors ... "that region where still by night is seen the virgin goddess near to bright boötes."--poste's aratus. [illustration: map no. .] following the order of right ascension, we come next to the little constellations crater and corvus, which may be described as standing on the curves of hydra (map no. ). beginning with crater, let us look first at alpha, a yellow fourth-magnitude star, near which is a celebrated red variable r. with a low power we can see both alpha and r in the same field of view, like a very wide double. there is a third star of ninth magnitude, and bluish in color, near r on the side toward alpha. r is variable both in color and light. when reddest, it has been described as "scarlet," "crimson," and "blood-colored"; when palest, it is a deep orange-red. its light variation has a period the precise length of which is not yet known. the cycle of change is included between the eighth and ninth magnitudes. while our three-inch telescope suffices to show r, it is better to use the five-inch, because of the faintness of the star. when the color is well seen, the contrast with alpha is very pleasing. there is hardly anything else in crater to interest us, and we pass over the border into corvus, and go at once to its chief attraction, the star delta. the components of this beautiful double are of magnitudes three and eight; distance ", p. °; colors yellow and purple. the night being dark and clear, we take the five-inch and turn it on the nebula , which the map shows just under the border of corvus in the edge of hydra. herschel believed he had resolved this into stars. it is a faint object and small, not exceeding one eighth of the moon's diameter. farther east in hydra, as indicated near the left-hand edge of map no. , is a somewhat remarkable variable, r hydræ. this star occasionally reaches magnitude three and a half, while at minimum it is not much above the tenth magnitude. its period is about four hundred and twenty-five days. [illustration: map no. .] while we have been examining these comparatively barren regions, glad to find one or two colored doubles to relieve the monotony of the search, a glittering white star has frequently drawn our eyes eastward and upward. it is spica, the great gem of virgo, and, yielding to its attraction, we now enter the richer constellation over which it presides (map no. ). except for its beauty, which every one must admire, spica, or alpha virginis, has no special claim upon our attention. some evidence has been obtained that, like beta aurigæ and capella, it revolves with an invisible companion of great mass in an orbit only six million miles in diameter. spica's spectrum resembles that of sirius. the faint star which our larger apertures show about ' northeast of spica is of the tenth magnitude. sweeping westward, we come upon sigma , a pretty little double with nearly equal components of about the sixth magnitude, distance . ", p. °. but our interest is not fully aroused until we reach gamma, a star with a history. the components of this celebrated binary are both nearly of the third magnitude, distance about . ", p. °. they revolve around their common center in something less than two hundred years. according to some authorities, the period is one hundred and seventy years, but it is not yet certainly ascertained. it was noticed about the beginning of the seventeenth century that gamma virginis was double. in the stars were so close together that no telescope then in existence was able to separate them, although it is said that the disk into which they had merged was elongated at pulkowa. in a few years they became easily separable once more. if the one-hundred-and-seventy-year period is correct, they should continue to get farther apart until about . according to asaph hall, their greatest apparent distance is . ", and their least apparent distance . "; consequently, they will never again close up beyond the separating power of existing telescopes. there is a great charm in watching this pair of stars even with a three-inch telescope--not so much on account of what is seen, although they are very beautiful, as on account of what we know they are doing. it is no slight thing to behold two distant stars obeying the law that makes a stone fall to the ground and compels the earth to swing round the sun. in theta we discover a fine triple, magnitudes four and a half, nine, and ten; distances ", p. °, and ", p. °. the ninth-magnitude star has been described as "violet," but such designations of color are often misleading when the star is very faint. on the other hand it should not be assumed that a certain color does not exist because the observer can not perceive it, for experience shows that there is a wide difference among observers in the power of the eye to distinguish color. i have known persons who could not perceive the difference of hue in some of the most beautifully contrasted colored doubles to be found in the sky. i am acquainted with an astronomer of long experience in the use of telescopes, whose eye is so deficient in color sense that he denies that there are any decided colors among the stars. such persons miss one of the finest pleasures of the telescope. in examining theta virginis we shall do best to use our largest aperture, viz., the five-inch. yet webb records that all three of the stars in this triple have been seen with a telescope of only three inches aperture. the amateur must remember in such cases how much depends upon practice as well as upon the condition of the atmosphere. there are lamentably few nights in a year when even the best telescope is ideally perfect in performance, but every night's observation increases the capacity of the eye, begetting a kind of critical judgment which renders it to some extent independent of atmospheric vagaries. it will also be found that the idiosyncrasies of the observer are reflected in his instrument, which seems to have its fits of excellence, its inspirations so to speak, while at other times it behaves as if all its wonderful powers had departed. another double that perhaps we had better not try with less than four inches aperture is virginis. the magnitudes are six and nine; distance, . ", p. °. colors yellow and blue. sigma is a fifth-magnitude star with a tenth-magnitude companion, distance only ", p. °. use the five-inch. and now we approach something that is truly marvelous, the "field of the nebulæ." this strange region, lying mostly in the constellation virgo, is roughly outlined by the stars beta, eta, gamma, delta, and epsilon, which form two sides of a square some ° across. it extends, however, for some distance into coma berenices, while outlying nebulæ belonging to it are also to be found in the eastern part of leo. unfortunately for those who expect only brilliant revelations when they look through a telescope, this throng of nebulæ consists of small and inconspicuous wisps as ill defined as bits of thistle-down floating high in the air. there are more than three hundred of them all told, but even the brightest are faint objects when seen with the largest of our telescopes. why do they congregate thus? that is the question which lends an interest to the assemblage that no individual member of it could alone command. it is a mystery, but beyond question it is explicable. the explanation, however, is yet to be discovered. the places of only three of the nebulæ are indicated on the map. no. has been described as resembling in shape a shuttle. its length is nearly one third of the moon's diameter. it is brightest near the center, and has several faint companions. no. is round, ' in diameter, and is accompanied by another round nebula in the same field of view toward the south. no. is double, and powerful telescopes show two more ghostly companions. there is an opportunity for good and useful work in a careful study of the little nebulæ that swim into view all over this part of virgo. celestial photography has triumphs in store for itself here. scattered over and around the region where the nebulæ are thickest we find eight or nine variable stars, three of the most remarkable of which, r, s, and u, may be found on the map. r is very irregular, sometimes attaining magnitude six and a half, while at other times its maximum brightness does not exceed that of an eighth-magnitude star. at minimum it sinks to the tenth or eleventh magnitude. its period is one hundred and forty-five days. u varies from magnitude seven or eight down to magnitude twelve or under and then regains its light, in a period of about two hundred and seven days. s is interesting for its brilliant red color. when brightest, it exceeds the sixth magnitude, but at some of its maxima the magnitude is hardly greater than the eighth. at minimum it goes below the twelfth magnitude. period, three hundred and seventy-six days. [illustration: map no. .] next east of virgo is libra, which contains a few notable objects (map no. ). the star alpha has a fifth-magnitude companion, distant about ", which can be easily seen with an opera glass. at the point marked a on the map is a curious multiple star, sometimes referred to by its number in piazzi's catalogues as follows: p. xiv. the two principal stars are easily seen, their magnitudes being six and seven and a half; distance ", p. °. burnham found four other faint companions, for which it would be useless for us to look. the remarkable thing is that these faint stars, the nearest of which is distant about " from the largest member of the group and the farthest about ", do not share, according to their discoverer, in the rapid proper motion of the two main stars. in iota we find a double a little difficult for our three-inch. the components are of magnitudes four and a half and nine, distance ", p. °. burnham discovered that the ninth-magnitude star consists of two of the tenth less than " apart, p. °. no astronomer who happens to be engaged in this part of the sky ever fails, unless his attention is absorbed by something of special interest, to glance at beta libræ, which is famous as the only naked-eye star having a decided green color. the hue is pale, but manifest.[ ] [ ] is the slight green tint perceptible in sirius variable? i am sometimes disposed to think it is. the star is a remarkable variable, belonging to what is called the algol type. its period, according to chandler, is days hours, minutes, . seconds. the time occupied by the actual changes is about twelve hours. at maximum the star is of magnitude five and at minimum of magnitude . . [illustration: map no. .] we may now conveniently turn northward from virgo in order to explore boötes, one of the most interesting of the constellations (map no. ). its leading star alpha, arcturus, is the brightest in the northern hemisphere. its precedence over its rivals vega and capella, long in dispute, has been settled by the harvard photometry. you notice that the color of arcturus, when it has not risen far above the horizon, is a yellowish red, but when the star is near mid-heaven the color fades to light yellow. the hue is possibly variable, for it is recorded that in arcturus appeared to have nearly lost its color. if it should eventually turn white, the fact would have an important bearing upon the question whether sirius was, as alleged, once a red or flame-colored star. but let us sit here in the starlight, for the night is balmy, and talk about arcturus, which is perhaps actually the greatest sun within the range of terrestrial vision. its parallax is so minute that the consideration of the tremendous size of this star is a thing that the imagination can not placidly approach. calculations, based on its assumed distance, which show that it _outshines the sun several thousand times_, may be no exaggeration of the truth! it is easy to make such a calculation. one of dr. elkin's parallaxes for arcturus is . ". that is to say, the displacement of arcturus due to the change in the observer's point of view when he looks at the star first from one side and then from the other side of the earth's orbit, , , miles across, amounts to only eighteen one-thousandths of a second of arc. we can appreciate how small that is when we reflect that it is about equal to the apparent distance between the heads of two pins placed an inch apart and viewed from a distance of a hundred and eighty miles! assuming this estimate of the parallax of arcturus, let us see how it will enable us to calculate the probable size or light-giving power of the star as compared with the sun. the first thing to do is to multiply the earth's distance from the sun, which may be taken at , , miles, by , , the number of seconds of arc in a radian, the base of circular measure, and then divide the product by the parallax of the star. performing the multiplication and division, we get the following: , , , , / . = , , , , , . the quotient represents miles! call it, in round numbers, a thousand millions of millions of miles. this is about , , times the distance from the earth to the sun. now for the second part of the calculation: the amount of light received on the earth from some of the brighter stars has been experimentally compared with the amount received from the sun. the results differ rather widely, but in the case of arcturus the ratio of the star's light to sunlight may be taken as about one twenty-five-thousand-millionth--i. e., , , , stars, each equal to arcturus, would together shed upon the earth as much light as the sun does. but we know that light varies inversely as the square of the distance; for instance, if the sun were twice as far away as it is, its light would be diminished for us to a quarter of its present amount. suppose, then, that we could remove the earth to a point midway between the sun and arcturus, we should then be , , times as far from the sun as we now are. in order to estimate how much light the sun would send us from that distance we must square the number , , and then take the result inversely, or as a fraction. we thus get / , , , , , representing the ratio of the sun's light at half the distance of arcturus to that at its real distance. but while receding from the sun we should be approaching arcturus. we should get, in fact, twice as near to that star as we were before, and therefore its light would be increased for us fourfold. now, if the amount of sunlight had not changed, it would exceed the light of arcturus only a quarter as much as it did before, or in the ratio of , , , / = , , , to . but, as we have seen, the sunlight would diminish through increase of distance to one , , , , th part of its original amount. hence its altered ratio to the light of arcturus would become , , , to , , , , , or to , . this means that if the earth were situated midway between the sun and arcturus, it would receive , times as much light from that star as it would from the sun! it is quite probable, moreover, that the heat of arcturus exceeds the solar heat in the same ratio, for the spectroscope shows that although arcturus is surrounded with a cloak of metallic vapors proportionately far more extensive than the sun's, yet, smothered as the great star seems in some respects to be, it rivals sirius itself in the intensity of its radiant energy. if we suppose the radiation of arcturus to be the same per unit of surface as the sun's, it follows that arcturus exceeds the sun about , times in volume, and that its diameter is no less than , , miles! imagine the earth and the other planets constituting the solar system removed to arcturus and set revolving around it in orbits of the same forms and sizes as those in which they circle about the sun. poor mercury! for that little planet it would indeed be a jump from the frying pan into the fire, because, as it rushed to perihelion, mercury would plunge more than , , miles beneath the surface of the giant star. venus and the earth would melt like snowflakes at the mouth of a furnace. even far-away neptune, the remotest member of the system, would swelter in torrid heat. but stop! look at the sky. observe how small and motionless the disks of the stars have become. back to the telescopes at once, for this is a token that the atmosphere is steady, and that "good seeing" may be expected. it is fortunate, for we have some delicate work before us. the very first double star we try in boötes, sigma , requires the use of the four-inch, and the five-inch shows it more satisfactorily. the magnitudes are sixth and ninth, distance ", p. °. on the other side of arcturus we find zeta, a star that we should have had no great difficulty in separating thirty years ago, but which has now closed up beyond the reach even of our five-inch. the magnitudes are both fourth, and the distance less than a quarter of a second; position angle changing. it is apparently a binary, and if so will some time widen again, but its period is unknown. the star , also known as sigma , near the southeastern edge of the constellation, is a pretty double, each component being of the seventh magnitude, distance ", p. °. just above zeta we come upon pi, an easy double for the three-inch, magnitudes four and six, distance " p. °. next is xi, a yellow and purple pair, whose magnitudes are respectively five and seven, distance less than ", p. °. this is undoubtedly a binary with a period of revolution of about a hundred and thirty years. its distance decreased about " between and . it was still decreasing in , when it had become . ". the orbital swing is also very apparent in the change of the position angle. the telescopic gem of boötes, and one of "the flowers of the sky," is epsilon, also known as mirac. when well seen, as we shall see it to-night, epsilon boötis is superb. the magnitudes of its two component stars are two and a half (according to hall, three) and six. the distance is about . ", p. °. the contrast of colors--bright orange yellow, set against brilliant emerald green--is magnificent. there are very few doubles that can be compared with it in this respect. the three-inch will separate it, but the five-inch enables us best to enjoy its beauty. it appears to be a binary, but the motion is very slow, and nothing certain is yet known of its period. in delta we have a very wide and easy double; magnitudes three and a half and eight and a half, distance ", p. °. the smaller star has a lilac hue. we can not hope with any of our instruments to see all of the three stars contained in , but two of them are easily seen; magnitudes four and seven, distance ", p. °. the smaller star is again double; magnitudes seven and eight, distance . ", p. °. it is clearly a binary, with a long period. a six-inch telescope that could separate this star at present would be indeed a treasure. sigma is another object rather beyond our powers, on account of the contrast of magnitudes. these are six and eight and a half; distance . ", p. °. other doubles are: (sigma ), magnitudes five and six, distance . ", p. °; (sigma ), magnitudes both nearly six, distance . ", p. °. smaller star light red; iota, magnitudes four and a half and seven and a half, distance ", p. °; kappa, magnitudes five and a half and eight, distance . ", p. °. some observers see a greenish tinge in the light of the larger star, the smaller one being blue. there are one or two interesting things to be seen in that part of canes venatici which is represented on map no. . the first of these is the star cluster . this will reward a good look with the five-inch. with large telescopes as many as one thousand stars have been discerned packed within its globular outlines. the star (sigma ) is a close binary with a period estimated at one hundred and twenty-five years. the magnitudes are six and seven or eight, distance about ", p. °. we may try for this with the five-inch, and if we do not succeed in separating the stars we may hope to do so some time, for the distance between them is increasing. although the nebula is a very wonderful object, we shall leave it for another evening. eastward from boötes shines the circlet of corona borealis, whose form is so strikingly marked out by the stars that the most careless eye perceives it at once. although a very small constellation, it abounds with interesting objects. we begin our attack with the five-inch on sigma , but not too confident that we shall come off victors, for this binary has been slowly closing for many years. the magnitudes are six and a half and seven, distance . ", p. °. not far distant is another binary, at present beyond our powers, eta. here the magnitudes are both six, distance . ", p. °. hall assigns a period of forty years to this star. the assemblage of close binaries in this neighborhood is very curious. only a few degrees away we find one that is still more remarkable, the star gamma. what has previously been said about comæ berenicis applies in a measure to this star also. it, too, has a comparatively small orbit, and its components are never seen widely separated. in their distance was . "; in they could not be split; in the distance had increased to . ", and in it had become . ", p. °. but in lewis made the distance only . ". the period has been estimated at one hundred years. while the group of double stars in the southern part of corona borealis consists, as we have seen, of remarkably close binaries, another group in the northern part of the same constellation comprises stars that are easily separated. let us first try zeta. the powers of the three-inch are amply sufficient in this case. the magnitudes are four and five, distance . ", p. °. colors, white or bluish-white and blue or green. next take sigma, whose magnitudes are five and six, distance ", p. °. with the five-inch we may look for a second companion of the tenth magnitude, distance ", p. °. it is thought highly probable that sigma is a binary, but its period has simply been guessed at. finally, we come to nu, which consists of two very widely separated stars, nu^ and nu^ , each of which has a faint companion. with the five-inch we may be able to see the companion of nu^ , the more southerly of the pair. the magnitude of the companion is variously given as tenth and twelfth, distance ", p. °. with the aid of the map we find the position of the new star of , which is famous as the first so-called temporary star to which spectroscopic analysis was applied. when first noticed, on may , , this star was of the second magnitude, fully equaling in brilliancy alpha, the brightest star of the constellation; but in about two weeks it fell to the ninth magnitude. huggins and miller eagerly studied the star with the spectroscope, and their results were received with deepest interest. they concluded that the light of the new star had two different sources, each giving a spectrum peculiar to itself. one of the spectra had dark lines and the other bright lines. it will be remembered that a similar peculiarity was exhibited by the new star in auriga in . but the star in corona did not disappear. it diminished to magnitude nine and a half or ten, and stopped there; and it is still visible. in fact, subsequent examination proved that it had been catalogued at bonn as a star of magnitude nine and a half in . consequently this "blaze star" of will bear watching in its decrepitude. nobody knows but that it may blaze again. perhaps it is a sun-like body; perhaps it bears little resemblance to a sun as we understand such a thing. but whatever it may be, it has proved itself capable of doing very extraordinary things. we have no reason to suspect the sun of any latent eccentricities, like those that have been displayed by "temporary" stars; yet, acting on the principle which led the old emperor-astrologer rudolph ii to torment his mind with self-made horoscopes of evil import, let us unscientifically imagine that the sun _could_ suddenly burst out with several hundred times its ordinary amount of heat and light, thereby putting us into a proper condition for spectroscopic examination by curious astronomers in distant worlds. but no, after all, it is far pleasanter to keep within the strict boundaries of science, and not imagine anything of the kind. chapter v in summer star-lands "i heard the trailing garments of the night sweep through her marble halls, i saw her sable skirts all fringed with light from the celestial walls."--h. w. longfellow. in the soft air of a summer night, when fireflies are flashing their lanterns over the fields, the stars do not sparkle and blaze like those that pierce the frosty skies of winter. the light of sirius, aldebaran, rigel, and other midwinter brilliants possesses a certain gemlike hardness and cutting quality, but antares and vega, the great summer stars, and arcturus, when he hangs westering in a july night, exhibit a milder radiance, harmonizing with the character of the season. this difference is, of course, atmospheric in origin, although it may be partly subjective, depending upon the mental influences of the mutations of nature. [illustration: map no. .] the constellation scorpio is nearly as striking in outline as orion, and its brightest star, the red antares (alpha in map no. ), carries concealed in its rays a green jewel which, to the eye of the enthusiast in telescopic recreation, appears more beautiful and inviting each time that he penetrates to its hiding place. we shall begin our night's work with this object, and the four-inch glass will serve our purpose, although the untrained observer would be more certain of success with the five-inch. a friend of mine has seen the companion of antares with a three-inch, but i have never tried the star with so small an aperture. when the air is steady and the companion can be well viewed, there is no finer sight among the double stars. the contrast of colors is beautifully distinct--fire-red and bright green. the little green star has been seen emerging from behind the moon, ahead of its ruddy companion. the magnitudes are one and seven and a half or eight, distance ", p. °. antares is probably a binary, although its binary character has not yet been established. a slight turn of the telescope tube brings us to the star sigma, a wide double, the smaller component of which is blue or plum-colored; magnitudes four and nine, distance ", p. °. from sigma we pass to beta, a very beautiful object, of which the three-inch gives us a splendid view. its two components are of magnitudes two and six, distance ", p. °; colors, white and bluish. it is interesting to know that the larger star is itself double, although none of the telescopes we are using can split it. burnham discovered that it has a tenth-magnitude companion; distance less than ", p. °. and now for a triple, which will probably require the use of our largest glass. up near the end of the northern prolongation of the constellation we perceive the star xi. the three-inch shows us that it is double; the five-inch divides the larger star again. the magnitudes are respectively five, five and a half, and seven and a half, distances . ", p. °, and ", p. °. a still more remarkable star, although one of its components is beyond our reach, is nu. with the slightest magnifying this object splits up into two stars, of magnitudes four and seven, situated rather more than " apart. a high power divides the seventh-magnitude companion into two, each of magnitude six and a half, distance . ", p. °. but (and this was another of burnham's discoveries) the fourth-magnitude star itself is double, distance . ", p. about °. the companion in this case is of magnitude five and a half. next we shall need a rather low-power eyepiece and our largest aperture in order to examine a star cluster, no. , which was especially admired by sir william herschel, who discovered that it was not, as messier had supposed, a circular nebula. herschel regarded it as the richest mass of stars in the firmament, but with a small telescope it appears merely as a filmy speck that has sometimes been mistaken for a comet. in a new star, between the sixth and seventh magnitude in brilliance, suddenly appeared directly in or upon the cluster, and the feeble radiance of the latter was almost extinguished by the superior light of the stranger. the latter disappeared in less than a month, and has not been seen again, although it is suspected to be a variable, and, as such, has been designated with the letter t. two other known variables, both very faint, exist in the immediate neighborhood. according to the opinion that was formerly looked upon with favor, the variable t, if it is a variable, simply lies in the line of sight between the earth and the star cluster, and has no actual connection with the latter. but this opinion may not, after all, be correct, for mr. bailey's observations show that variable stars sometimes exist in large numbers in clusters, although the variables thus observed are of short period. the cluster , just west of antares, is also worth a glance with the five-inch glass. it is dense, but its stars are very small, so that to enjoy its beauty we should have to employ a large telescope. yet there is a certain attraction in these far-away glimpses of starry swarms, for they give us some perception of the awful profundity of space. when the mind is rightly attuned for these revelations of the telescope, there are no words that can express its impressions of the overwhelming perspective of the universe. the southern part of the constellation ophiuchus is almost inextricably mingled with scorpio. we shall, therefore, look next at its attractions, beginning with the remarkable array of star clusters , , , and . all of these are small, ' or ' in diameter, and globular in shape. no. is the largest, and we can see some of the stars composing it. but these clusters, like those just described in scorpio, are more interesting for what they signify than for what they show; and the interest is not diminished by the fact that their meaning is more or less of a mystery. whether they are composed of pygmy suns or of great solar globes like that one which makes daylight for the earth, their association in spherical groups is equally suggestive. there are two other star clusters in ophiuchus, and within the limits of map no. , both of which are more extensive than those we have just been looking at. no. is ' or ' in diameter, also globular, brighter at the center, and surrounded by several comparatively conspicuous stars. no. is still larger, about half as broad as the moon, and many of its scattered stars are of not less than the ninth magnitude. with a low magnifying power the field of view surrounding the cluster appears powdered with stars. there are only two noteworthy doubles in that part of ophiuchus with which we are at present concerned: , whose magnitudes are five and seven, distance . ", p. °, colors yellow and red; and , magnitudes six and seven and a half, distance ", p. °, colors yellow or orange and blue. the first named is a binary whose period has not been definitely ascertained. the variable r has a period a little less than three hundred and three days. at its brightest it is of magnitude seven or eight, and at minimum it diminishes to about the twelfth magnitude. the spot where the new star of appeared is indicated on the map. this was, with the exception of tycho's star in , the brightest temporary star of which we possess a trustworthy account. it is frequently referred to as kepler's star, because kepler watched it with considerable attention, but unfortunately he was not as good an observer as tycho was. the star was first seen on october , , and was then brighter than jupiter. it did not, however, equal venus. it gradually faded and in march, , disappeared. about twelve degrees northwest of the place of the star of , and in that part of the constellation serpens which is included in map no. , we find the location of another temporary star, that of . it was first noticed by mr. hind on april th of that year, when its magnitude was not much above the seventh, and its color was red. it brightened rapidly, until on may d it was of magnitude three and a half. then it began to fade, but very slowly, and it has never entirely disappeared. it is now of the twelfth or thirteenth magnitude. in passing we may glance with a low power at nu serpentis, a wide double, magnitudes four and nine, distance ", p. °, colors contrasted but uncertain. sagittarius and its neighbor, the small but rich constellation scutum sobieskii, attract us next. we shall first deal with the western portions of these constellations which are represented on map no. . the star in sagittarius is a wide triple, magnitudes three and a half, nine and a half, and ten, distances ", p. °, and ", p. °. but the chief glory of sagittarius (and the same statement applies to scutum sobieskii) lies in its assemblage of star clusters. one of these, no. , also known as m , is plainly visible to the naked eye as a bright spot in the milky way. we turn our five-inch telescope, armed with a low magnifying power, upon this subject and enjoy a rare spectacle. as we allow it to drift through the field we see a group of three comparatively brilliant stars advancing at the front of a wonderful train of mingled star clusters and nebulous clouds. a little northwest of it appears the celebrated trifid nebula, no. on the map. there is some evidence that changes have occurred in this nebula since its discovery in the last century. barnard has made a beautiful photograph showing m and the trifid nebula on the same plate, and he remarks that the former is a far more remarkable object than its more famous neighbor. near the eastern border of the principal nebulous cloud there is a small and very black hole with a star poised on its eastern edge. this hole and the star are clearly shown in the photograph. cluster no. (m ) is usually described as resembling, to the naked eye, a protuberance on the edge of the milky way. it is nearly three times as broad as the moon, and is very rich in minute stars, which are at just such a degree of visibility that crowds of them continually appear and disappear while the eye wanders over the field, just as faces are seen and lost in a vast assemblage of people. this kind of luminous agitation is not peculiar to m , although that cluster exhibits it better than most others do on account of both the multitude and the minuteness of its stars. a slight sweep eastward brings us to yet another meeting place of stars, the cluster m , situated between the variables u and v. this is brilliant and easily resolved into its components, which include a number of double stars. the two neighboring variables just referred to are interesting. u has a period of about six days and three quarters, and its range of magnitude runs from the seventh down to below the eighth. v is a somewhat mysterious star. chandler removed it from his catalogue of variables because no change had been observed in its light by either himself, sawyer, or yendell. quirling, the discoverer of its variability, gave the range as between magnitudes . and . . it must, therefore, be exceedingly erratic in its changes, resembling rather the temporary stars than the true variables. in that part of scutum sobieskii contained in map no. we find an interesting double, sigma , whose magnitudes are six and nine, distance . ", p. °, colors white and orange. sigma is a triple, magnitudes seven, eight, and nine, distances ", p. °, and . ", p. °. the third star is, however, beyond our reach. the colors of the two larger are respectively yellow and violet. the star cluster is about one quarter as broad as the moon, and easily seen with our smallest aperture. [illustration: map no. .] passing near to the region covered by map no. , we find the remaining portions of the constellations sagittarius and scutum sobieskii. it will be advisable to finish with the latter first. glance at the clusters and . neither is large, but both are rich in stars. the nebula is a fine object of its kind. it brightens toward the center, and herschel thought he had resolved it into stars. the variable r is remarkable for its eccentricities. sometimes it attains nearly the fourth magnitude, although usually at maximum it is below the fifth, while at minimum it is occasionally of the sixth and at other times of the seventh or eighth magnitude. its period is irregular. turning back to sagittarius, we resume our search for interesting objects there, and the first that we discover is another star cluster, for the stars are wonderfully gregarious in this quarter of the heavens. the number our cluster bears on the map is , corresponding with m in messier's catalogue. it is very bright, containing many stars of the tenth and eleventh magnitudes, as well as a swarm of smaller ones. sir john herschel regarded the larger stars in this cluster as possessing a reddish tint. possibly there was some peculiarity in his eye that gave him this impression, for he has described a cluster in the constellation toucan in the southern hemisphere as containing a globular mass of rose-colored stars inclosed in a spherical shell of white stars. later observers have confirmed his description of the shape and richness of this cluster in toucan, but have been unable to perceive the red hue of the interior stars. the eastern expanse of sagittarius is a poor region compared with the western end of the constellation, where the wide stream of the milky way like a great river enriches its surroundings. the variables t and r are of little interest to us, for they never become bright enough to be seen without the aid of a telescope. in we find, however, an interesting double, which with larger telescopes than any of ours appears as a triple. the two stars that we see are of magnitudes six and seven and a half, distance ", p. °, colors yellow and blue. the third star, perhaps of thirteenth magnitude, is distant ", p. °. retaining map no. as our guide, we examine the western part of the constellation capricornus. its leader alpha is a naked-eye double, the two stars being a little more than ' apart. their magnitudes are three and four, and both have a yellowish hue. the western star is alpha^ , and is the fainter of the two. the other is designated as alpha^ . both are double. the components of alpha^ are of magnitudes four and eight and a half, distance ", p. °. with the washington twenty-six-inch telescope a third star of magnitude fourteen has been found at a distance of ", p. °. in alpha^ the magnitudes of the components are three and ten and a half, distance . ", p. °. the smaller star has a companion of the twelfth or thirteenth magnitude, distance . ", p. °. this, of course, is hopelessly beyond our reach. yet another star of magnitude nine, distance ", p. , we may see easily. dropping down to beta, we find it to be a most beautiful and easy double, possessing finely contrasted colors, gold and blue. the larger star is of magnitude three, and the smaller, the blue one, of magnitude six, distance ", p. °. between them there is a very faint star which larger telescopes than ours divide into two, each of magnitude eleven and a half; separated ", p. °. still farther south and nearly in a line drawn from alpha through beta we find a remarkable group of double stars, sigma, pi, rho, and omicron. the last three form a beautiful little triangle. we begin with sigma, the faintest of the four. the magnitudes of its components are six and nine, distance ", p. °. in pi the magnitudes are five and nine, distance . ", p. °; in rho, magnitudes five and eight, distance . ", p. ° (a third star of magnitude seven and a half is seen at a distance of ', p. °); in omicron, magnitudes six and seven, distance ", p. °. the star cluster is small, yet, on a moonless night, worth a glance with the five-inch. [illustration: map no. .] we now pass northward to the region covered by map no. , including the remainder of ophiuchus and serpens. beginning with the head of serpens, in the upper right-hand corner of the map, we find that beta, of magnitude three and a half, has a ninth-magnitude companion, distance ", p. °. the larger star is light blue and the smaller one yellowish. the little star nu is double, magnitudes five and nine, distance ", p. °, colors contrasted but uncertain. in delta we find a closer double, magnitudes three and four, distance . ", p. °. it is a beautiful object for the three-inch. the leader of the constellation, alpha, of magnitude two and a half, has a faint companion of only the twelfth magnitude, distance ", p. °. the small star is bluish. the variable r has a period about a week short of one year, and at maximum exceeds the sixth magnitude, although sinking at minimum to less than the eleventh. its color is ruddy. passing eastward, we turn again into ophiuchus, and find immediately the very interesting double, lambda, whose components are of magnitudes four and six, distance ", p. °. this is a long-period binary, and notwithstanding the closeness of its stars, our four-inch should separate them when the seeing is fine. we shall do better, however, to try with the five-inch. sigma consists of two stars of magnitudes six and seven and a half, distance ", p. °. sigma is a double of quite a different order. the magnitudes of its components are both six, the distance in . ", p. °. it is evidently a binary in rapid motion, as the distance changed from about a quarter of a second in to more than a second in . the star tau is a fine triple, magnitudes five, six, and nine, distances . ", p. °, and ", p. °. the close pair is a binary system with a long period of revolution, estimated at about two hundred years. we discover another group of remarkable doubles in , , and . in the first-named star the magnitudes are four and eight, distance ", p. °, colors finely contrasted, pale yellow and red. much more interesting, however, is , a binary whose components have completed a revolution since their discovery by sir william herschel, the period being ninety-five years. the magnitudes are four and six, or, according to hall, five and six, distance in . "; in , . ", according to maw. hall says the apparent distance when the stars are closest is about . ", and when they are widest . ". this star is one of those whose parallax has been calculated with a reasonable degree of accuracy. its distance from us is about , , times the distance of the sun, the average distance apart of the two stars is about , , , miles (equal to the distance of neptune from the sun), and their combined mass is three times that of the sun. hall has seen in the system of ophiuchi three stars of the thirteenth magnitude or less, at distances of about ", ", and " respectively. the star is also a close double, and beyond our reach. its magnitudes are six and seven, distance . ", p. °. it is, no doubt, a binary. three star clusters in ophiuchus remain to be examined. the first of these, no. , is partially resolved into stars by the five-inch. no. is globular, and has a striking environment of bystanding stars. it is about one quarter as broad as the full moon, and our largest aperture reveals the faint coruscation of its crowded components. no. is a coarser and more scattered star swarm--a fine sight! farther toward the east we encounter a part of serpens again, which contains just one object worth glancing at, the double theta, whose stars are of magnitudes four and four and a half, distance ", p. °. color, both yellow, the smaller star having the deeper hue. [illustration: map no. .] let us next, with the guidance of map no. , enter the rich star fields of hercules, and of the head and first coils of draco. according to argelander, hercules contains more stars visible to the naked eye than any other constellation, and he makes the number of them one hundred and fifty-five, nearly two thirds of which are only of the sixth magnitude. but heis, who saw more naked-eye stars than argelander, makes ursa major precisely equal to hercules in the number of stars, his enumeration showing two hundred and twenty-seven in each constellation, while, according to him, draco follows very closely after, with two hundred and twenty stars. yet, on account of the minuteness of the majority of their stars, neither of these constellations makes by any means as brilliant a display as does orion, to which argelander assigns only one hundred and fifteen naked-eye stars, and heis one hundred and thirty-six. we begin in hercules with the star kappa, a pretty little double of magnitudes five and a half and seven, distance ", p. °, colors yellow and red. not far away we find, in gamma, a larger star with a fainter companion, the magnitudes in this case being three and a half and nine, distance ", p. °, colors white and faint blue or lilac. one of the most beautiful of double stars is alpha herculis. the magnitudes are three and six, distance . ", p. °, colors orange and green, very distinct. variability has been ascribed to each of the stars in turn. it is not known that they constitute a binary system, because no certain evidence of motion has been obtained. another very beautiful and easily separated double is delta, magnitudes three and eight, distance ", p. °, colors pale green and purple. sweeping northwestward to zeta, we encounter a celebrated binary, to separate which at present requires the higher powers of a six-inch glass. the magnitudes are three and six and a half, distance in , . ", p. °; in , . ", p. °. the period of revolution is thirty-five years, and two complete revolutions have been observed. the apparent distance changes from . " to . ". they were at their extreme distance in . two pleasing little doubles are sigma , magnitudes six and nine, distance ", p. °, and sigma , magnitudes six and eight, distance ", p. °. at the northern end of the constellation is , a double that requires the light-grasping power of our largest glass. its magnitudes are six and twelve, distance ", p. °. in rho we discover another distinctly colored double, both stars being greenish or bluish, with a difference of tone. the magnitudes are four and five and a half, distance . ", p. °. but the double is yet more remarkable for the colors of its stars. their magnitudes are five and five and a half, distance ", p. °, colors, according to webb, "light apple-green and cherry-red." but other observers have noted different hues, one calling them both golden yellow. i think webb's description is more nearly correct. sigma is a very close double, requiring larger telescopes than those we are working with. its magnitudes are six and a half and eight, distance . ", p. °. it is probably a binary. sigma is also close, but our five-inch will separate it: magnitudes six and seven, distance . ", p. °. turning to , we have to deal with a triple, one of whose stars is at present beyond the reach of our instruments. the magnitudes of the two that we see are four and ten, distance ", p. °. the tenth-magnitude star is a binary of short period (probably less than fifty years), the distance of whose components was " in , " in , . " in , and . " in , when the position angle was °, and rapidly increasing. the distance is still much less than ". for a glance at a planetary nebula we may turn with the five-inch to no. . it is very small and faint, only " in diameter, and equal in brightness to an eighth-magnitude star. only close gazing shows that it is not sharply defined like a star, and that it possesses a bluish tint. its spectrum is gaseous. the chief attraction of hercules we have left for the last, the famous star cluster between eta and zeta, no. , more commonly known as m . on a still evening in the early summer, when the moon is absent and the quiet that the earth enjoys seems an influence descending from the brooding stars, the spectacle of this sun cluster in hercules, viewed with a telescope of not less than five-inches aperture, captivates the mind of the most uncontemplative observer. with the lick telescope i have watched it resolve into separate stars to its very center--a scene of marvelous beauty and impressiveness. but smaller instruments reveal only the in-running star streams and the sprinkling of stellar points over the main aggregation, which cause it to sparkle like a cloud of diamond dust transfused with sunbeams. the appearance of flocking together that those uncountable thousands of stars present calls up at once a picture of our lone sun separated from its nearest stellar neighbor by a distance probably a hundred times as great as the entire diameter of the spherical space within which that multitude is congregated. it is true that unless we assume what would seem an unreasonable remoteness for the hercules cluster, its component stars must be much smaller bodies than the sun; yet even that fact does not diminish the wonder of their swarming. here the imagination must bear science on its wings, else science can make no progress whatever. it is an easy step from hercules to draco. in the conspicuous diamond-shaped figure that serves as a guide-board to the head of the latter, the southernmost star belongs not to draco but to hercules. the brightest star in this figure is gamma, of magnitude two and a half, with an eleventh-magnitude companion, distant ", p. °. two stars of magnitude five compose nu, their distance apart being ", p. °. a more interesting double is , magnitudes five and five, distance . ", p. °. both stars are white, and they present a pretty appearance when the air is steady. they form a binary system of unknown period. sigma (also called draconis) is a triple, magnitudes six, six and a half, and six, distances . ", p. °, and ", p. °. sigma is an easy double, magnitudes six and a half and eight and a half, distance . ", p. °. the star eta is a very difficult double for even our largest aperture, on account of the faintness of one of its components. the magnitudes are two and a half and ten, distance . ", p. °. its near neighbor, sigma , may be a binary. its magnitudes are six and seven, distance ", p. °. in sigma we have another triple, magnitudes five, eight and a half, and seven, distances . ", p. °, and ", p. °, colors white, blue, and reddish. a fine double is epsilon, magnitudes five and eight, distance ", p. °. the nebula no. is of a planetary character, and interesting as occupying the pole of the ecliptic. a few years ago dr. holden, with the lick telescope, discovered that it is unique in its form. it consists of a double spiral, drawn out nearly in the line of sight, like the thread of a screw whose axis lies approximately endwise with respect to the observer. there is a central star, and another fainter star is involved in the outer spiral. the form of this object suggests strange ideas as to its origin. but the details mentioned are far beyond the reach of our instruments. we shall only see it as a hazy speck. no. is another nebula worth glancing at. it is tuttle's so-called variable nebula. [illustration: map no. .] there are three constellations represented on map no. to which we shall pay brief visits. first aquila demands attention. its doubles may be summarized as follows: , magnitudes five and nine, distance . ", p. °; pi, magnitudes six and seven, distance . ", p. °; , magnitudes six and ten, distance . ", p. °--requires the five-inch and good seeing; , magnitudes five and six, distance ", p. °; sigma , magnitudes six and eight, distance ", p. °; sigma , magnitudes six and seven, distance . ", p. °. the star eta is an interesting variable between magnitudes three and a half and . ; period, seven days, four hours, fourteen minutes. the small red variable r changes from magnitude six to magnitude seven and a half and back again in a period of three hundred and fifty-one days. star cluster no. is a striking object, its stars ranging from the ninth down to the twelfth magnitude. just north of aquila is the little constellation sagitta, containing several interesting doubles and many fine star fields, which may be discovered by sweeping over it with a low-power eyepiece. the star zeta is double, magnitudes five and nine, distance . ", p. °. the larger star is itself double, but far too close to be split, except with very large telescopes. in theta we find three components of magnitudes seven, nine, and eight respectively, distances . ", p. °, and ", p. °. a wide double is epsilon, magnitudes six and eight, distance ", p. °. nebula no. is planetary. turning to delphinus, we find a very beautiful double in gamma, magnitudes four and five, distance ", p. °, colors golden and emerald. the leader alpha, which is not as bright as its neighbor beta, and which is believed to be irregularly variable, is of magnitude four, and has a companion of nine and a half magnitude at the distance ", p. °. at a similar distance, ", p. °, beta has an eleventh-magnitude companion, and the main star is also double, but excessively close, and much beyond our reach. it is believed to be a swiftly moving binary, whose stars are never separated widely enough to be distinguished with common telescopes. chapter vi from lyra to eridanus "this orpheus struck when with his wondrous song he charmed the woods and drew the rocks along."--manilius. [illustration: map no. .] we resume our celestial explorations with the little constellation lyra, whose chief star, vega (alpha), has a very good claim to be regarded as the most beautiful in the sky. the position of this remarkable star is indicated in map no. . every eye not insensitive to delicate shades of color perceives at once that vega is not white, but blue-white. when the telescope is turned upon the star the color brightens splendidly. indeed, some glasses decidedly exaggerate the blueness of vega, but the effect is so beautiful that one can easily forgive the optical imperfection which produces it. with our four-inch we look for the well-known companion of vega, a tenth-magnitude star, also of a blue color deeper than the hue of its great neighbor. the distance is ", p. °. under the most favorable circumstances it might be glimpsed with the three-inch, but, upon the whole, i should regard it as too severe a test for so small an aperture. vega is one of those stars which evidently are not only enormously larger than the sun (one estimate makes the ratio in this case nine hundred to one), but whose physical condition, as far as the spectroscope reveals it, is very different from that of our ruling orb. like sirius, vega displays the lines of hydrogen most conspicuously, and it is probably a much hotter as well as a much more voluminous body than the sun. close by, toward the east, two fourth-magnitude stars form a little triangle with vega. both are interesting objects for the telescope, and the northern one, epsilon, has few rivals in this respect. let us first look at it with an opera glass. the slight magnifying power of such an instrument divides the star into two twinkling points. they are about two and a quarter minutes of arc apart, and exceptionally sharp-sighted persons are able to see them divided with the naked eye. now take the three-inch telescope and look at them, with a moderate power. each of the two stars revealed by the opera glass appears double, and a fifth star of the ninth magnitude is seen on one side of an imaginary line joining the two pairs. the northern-most pair is named epsilon_ , the magnitudes being fifth and sixth, distance ", p. °. the other pair is epsilon_ , magnitudes fifth and sixth, distance . ", p. °. each pair is apparently a binary; but the period of revolution is unknown. some have guessed a thousand years for one pair, and two thousand for the other. another guess gives epsilon_ a period of one thousand years, and epsilon_ a period of eight hundred years. hall, in his double-star observations, simply says of each, "a slow motion." purely by guesswork a period has also been assigned to the two pairs in a supposed revolution around their common center, the time named being about a million years. it is not known, however, that such a motion exists. manifestly it could not be ascertained within the brief period during which scientific observations of these stars have been made. the importance of the element of time in the study of stellar motions is frequently overlooked, though not, of course, by those who are engaged in such work. the sun, for instance, and many of the stars are known to be moving in what appear to be straight lines in space, but observations extending over thousands of years would probably show that these motions are in curved paths, and perhaps in closed orbits. if now in turn we take our four-inch glass, we shall see something else in this strange family group of epsilon lyræ. between epsilon_ and epsilon_ , and placed one on each side of the joining line, appear two exceedingly faint specks of light, which sir john herschel made famous under the name of the _debillissima_. they are of the twelfth or thirteenth magnitude, and possibly variable to a slight degree. if you can not see them at first, turn your eye toward one side of the field of view, and thus, by bringing their images upon a more sensitive part of the retina, you may glimpse them. the sight is not much, yet it will repay you, as every glance into the depths of the universe does. the other fourth-magnitude star near vega is zeta, a wide double, magnitudes fourth and sixth, distance ", p. °. below we find beta, another very interesting star, since it is both a multiple and an eccentric variable. it has four companions, three of which we can easily see with our three-inch; the fourth calls for the five-inch; the magnitudes are respectively four, seven or under, eight, eight and a half, and eleven; distances ", p. °; ", p. °; ", p. °; and ", p. °. the primary, beta, varies from about magnitude three and a half to magnitude four and a half, the period being twelve days, twenty-one hours, forty-six minutes, and fifty-eight seconds. two unequal maxima and minima occur within this period. in the spectrum of this star some of the hydrogen lines and the d_ line (the latter representing helium, a constituent of the sun and of some of the stars, which, until its recent discovery in a few rare minerals was not known to exist on the earth) are bright, but they vary in visibility. moreover, dark lines due to hydrogen also appear in its spectrum simultaneously with the bright lines of that element. then, too, the bright lines are sometimes seen double. professor pickering's explanation is that beta lyræ probably consists of two stars, which, like the two composing beta aurigæ, are too close to be separated with any telescope now existing, and that the body which gives the bright lines is revolving in a circle in a period of about twelve days and twenty-two hours around the body which gives the dark lines. he has also suggested that the appearances could be accounted for by supposing a body like our sun to be rotating in twelve days and twenty-two hours, and having attached to it an enormous protuberance extending over more than one hundred and eighty degrees of longitude, so that when one end of it was approaching us with the rotation of the star the other end would be receding, and a splitting of the spectral lines at certain periods would be the consequence. "the variation in light," he adds, "may be caused by the visibility of a larger or smaller portion of this protuberance." unfortunate star, doomed to carry its parasitical burden of hydrogen and helium, like sindbad in the clasp of the old man of the sea! surely, the human imagination is never so wonderful as when it bears an astronomer on its wings. yet it must be admitted that the facts in this case are well calculated to summon the genius of hypothesis. and the puzzle is hardly simplified by bélopolsky's observation that the body in beta lyræ giving dark hydrogen lines shows those lines also split at certain times. it has been calculated, from a study of the phenomena noted above, that the bright-line star in beta lyræ is situated at a distance of about fifteen million miles from the center of gravity of the curiously complicated system of which it forms a part. we have not yet exhausted the wonders of lyra. on a line from beta to gamma, and about one third of the distance from the former to the latter, is the celebrated ring nebula, indicated on the map by the number . we need all the light we can get to see this object well, and so, although the three-inch will show it, we shall use the five-inch. beginning with a power of one hundred diameters, which exhibits it as a minute elliptical ring, rather misty, very soft and delicate, and yet distinct, we increase the magnification first to two hundred and finally to three hundred, in order to distinguish a little better some of the details of its shape. upon the whole, however, we find that the lowest power that clearly brings out the ring gives the most satisfactory view. the circumference of the ring is greater than that of the planet jupiter. its ellipticity is conspicuous, the length of the longer axis being " and that of the shorter ". closely following the nebula as it moves through the field of view, our five-inch telescope reveals a faint star of the eleventh or twelfth magnitude, which is suspected of variability. the largest instruments, like the washington and the lick glasses, have shown perhaps a dozen other stars apparently connected with the nebula. a beautiful sparkling effect which the nebula presents was once thought to be an indication that it was really composed of a circle of stars, but the spectroscope shows that its constitution is gaseous. just in the middle of the open ring is a feeble star, a mere spark in the most powerful telescope. but when the ring nebula is photographed--and this is seen beautifully in the photographs made with the crossley reflector on mount hamilton by the late prof. j. e. keeler--this excessively faint star imprints its image boldly as a large bright blur, encircled by the nebulous ring, which itself appears to consist of a series of intertwisted spirals. not far away we find a difficult double star, , whose components are of magnitudes six and ten or eleven, distance . ", p. °. from lyra we pass to cygnus, which, lying in one of the richest parts of the milky way, is a very interesting constellation for the possessor of a telescope. its general outlines are plainly marked for the naked eye by the figure of a cross more than twenty degrees in length lying along the axis of the milky way. the foot of the cross is indicated by the star beta, also known as albireo, one of the most charming of all the double stars. the three-inch amply suffices to reveal the beauty of this object, whose components present as sharp a contrast of light yellow and deep blue as it would be possible to produce artificially with the purest pigments. the magnitudes are three and seven, distance . ", p. °. no motion has been detected indicating that these stars are connected in orbital revolution, yet no one can look at them without feeling that they are intimately related to one another. it is a sight to which one returns again and again, always with undiminished pleasure. the most inexperienced observer admires its beauty, and after an hour spent with doubtful results in trying to interest a tyro in double stars it is always with a sense of assured success that one turns the telescope to beta cygni. following up the beam of the imaginary cross along the current of the milky way, every square degree of which is here worth long gazing into, we come to a pair of stars which contend for the name-letter chi. on our map the letter is attached to the southernmost of the two, a variable of long period--four hundred and six days--whose changes of brilliance lie between magnitudes four and thirteen, but which exhibits much irregularity in its maxima. the other star, not named but easily recognized in the map, is sometimes called . it is an attractive double whose colors faintly reproduce those of beta. the magnitudes are five and eight, distance ", p. °. where the two arms of the cross meet is gamma, whose remarkable _cortége_ of small stars running in curved streams should not be missed. use the lowest magnifying power. at the extremity of the western arm of the cross is delta, a close double, difficult for telescopes of moderate aperture on account of the difference in the magnitudes of the components. we may succeed in dividing it with the five-inch. the magnitudes are three and eight, distance . ", p. °. it is regarded as a binary of long and as yet unascertained period. in omicron^ we find a star of magnitude four and orange in color, having two blue companions, the first of magnitude seven and a half, distance ", p. °, and the second of magnitude five and a half, distance ", p. °. farther north is psi, which presents to us the combination of a white five-and-a-half-magnitude star with a lilac star of magnitude seven and a half. the distance is ", p. °. a very pretty sight. we now pass to the extremity of the other arm of the cross, near which lies the beautiful little double , whose components are of magnitudes six and eight, distance . ", p. °. the colors are yellow and blue, conspicuous and finely contrasted. a neighboring double of similar hues is , in which the magnitudes are four and nine, distance ", p. °. sweeping a little way northward we come upon an interesting binary, lambda, which is unfortunately beyond the dividing power of our largest glass. a good seven-inch or seven-and-a-half-inch should split it under favorable circumstances. its magnitudes are six and seven, distance . ", p. °. the next step carries us to a very famous object, cygni, long known as the nearest star in the northern hemisphere of the heavens. it is a double which our three-inch will readily divide, the magnitudes being both six, distance ", p. °. the distance of cygni, according to hall's parallax of . ", is about , , , , miles. there is some question whether or not it is a binary, for, while the twin stars are both moving in the same direction in space with comparative rapidity, yet conclusive evidence of orbital motion is lacking. when one has noticed the contrast in apparent size between this comparatively near-by star, which the naked eye only detects with considerable difficulty, and some of its brilliant neighbors whose distance is so great as to be immeasurable with our present means, no better proof will be needed of the fact that the faintness of a star is not necessarily an indication of remoteness. we may prepare our eyes for a beautiful exhibition of contrasted colors once more in the star . this is really a quadruple, although only two of its components are close and conspicuous. the magnitudes are five, six, seven and a half, and twelve; distances . ", p. °; ", p. °; and ", p. °. the color of the largest star is white and that of its nearest companion blue; the star of magnitude seven and a half is also blue. the star cluster is a fine sight with our largest glass. in the map we find the place marked where the new star of made its appearance. this was first noticed on november , , when it shone with the brilliance of a star of magnitude three and a half. its spectrum was carefully studied, especially by vogel, and the very interesting changes that it underwent were noted. within a year the star had faded to less than the tenth magnitude, and its spectrum had completely changed in appearance, and had come to bear a close resemblance to that of a planetary nebula. this has been quoted as a possible instance of a celestial collision through whose effects the solid colliding masses were vaporized and expanded into a nebula. at present the star is very faint and can only be seen with the most powerful telescopes. compare with the case of nova aurigæ, previously discussed. underneath cygnus we notice the small constellation vulpecula. it contains a few objects worthy of attention, the first being the nebula , the "dumb-bell nebula" of lord rosse. with the four-inch, and better with the five-inch, we are able to perceive that it consists of two close-lying tufts of misty light. many stars surround it, and large telescopes show them scattered between the two main masses of the nebula. the lick photographs show that its structure is spiral. the star points out the place where a new star of the third magnitude appeared in . sigma is a close double, magnitudes six and eight, distance . ", p. °. [illustration: map no. .] we turn to map no. , and, beginning at the western end of the constellation aquarius, we find the variable t, which ranges between magnitudes seven and thirteen in a period of about two hundred and three days. its near neighbor sigma is a very close double, beyond the separating power of our five-inch, the magnitudes being six and seven, distance . ", p. °. sigma , also known as aquarii, is a good double for the three-inch. its magnitudes are six and eight, distance . ", p. °. in zeta we discover a beauty. it is a slow binary of magnitudes four and four, distance . ", p. °. according to some observers both stars have a greenish tinge. the star is a wider double, magnitudes six and eight, distance ", p. °, colors yellow and blue. the uncommon stellar contrast of white with light garnet is exhibited by tau, magnitudes six and nine, distance ", p. °. yellow and blue occur again conspicuously in psi, magnitudes four and a half and eight and a half, distance ", p. °. rose and emerald have been recorded as the colors exhibited in sigma , whose magnitudes are five and seven, distance . ", p. °. the variables s and r are both red. the former ranges between magnitudes eight and twelve, period two hundred and eighty days, and the latter between magnitudes six and eleven, period about three hundred and ninety days. the nebula is rosse's "saturn nebula," so called because with his great telescope it presented the appearance of a nebulous model of the planet saturn. with our five-inch we see it simply as a planetary nebula. we may also glance at another nebula, , which appears circular and is pinned with a little star at the edge. the small constellation equuleus contains a surprisingly large number of interesting objects. sigma is a rather close double, magnitudes six and eight, distance . ", p. °. sigma (the first star to the left of sigma , the name having accidentally been omitted from the map) is a beautiful triple, although the two closest stars, of magnitudes six and seven, can not be separated by our instruments. their distance in was . ", p. °, and they had then been closing rapidly since , when the distance was . ". the third star, of magnitude eight, is distant ", p. °. sigma consists of two stars, magnitudes six and seven, distance . ", p. . °. it is probably a binary. sigma is wider double, magnitudes both six, distance . ", p. °. another triple, one of whose components is beyond our reach, is gamma. here the magnitudes are fifth, twelfth, and sixth, distances ", p. ° and ". it would also be useless for us to try to separate delta, but it is interesting to remember that this is one of the closest of known double stars, the magnitudes being fourth and fifth, distance . ", p. °. these data are from hall's measurements in . the star is, no doubt, a binary. with the five-inch we may detect one and perhaps two of the companion stars in the quadruple beta. the magnitudes are five, ten, and two eleven, distances ", p. °; ", p. °; and . ", p. °. the close pair is comprised in the tenth-magnitude star. [illustration: map no. .] map no. introduces us to the constellation pegasus, which is comparatively barren to the naked eye, and by no means rich in telescopic phenomena. the star epsilon, of magnitude two and a half, has a blue companion of the eighth magnitude, distance ", p. °; colors yellow and violet. a curious experiment that may be tried with this star is described by webb, who ascribes the discovery of the phenomenon to sir john herschel. when near the meridian the small star in epsilon appears, in the telescope, underneath the large one. if now the tube of the telescope be slightly swung from side to side the small star will appear to describe a pendulumlike movement with respect to the large one. the explanation suggested is that the comparative faintness of the small star causes its light to affect the retina of the eye less quickly than does that of its brighter companion, and, in consequence, the reversal of its apparent motion with the swinging of the telescope is not perceived so soon. the third-magnitude star eta has a companion of magnitude ten and a half, distance ", p. °. the star beta, of the second magnitude, and reddish, is variable to the extent of half a magnitude in an irregular period, and gamma, of magnitude two and a half, has an eleventh-magnitude companion, distance ", p. °. [illustration: map no. .] our interest is revived on turning, with the guidance of map no. , from the comparative poverty of pegasus to the spacious constellation cetus. the first double star that we meet in this constellation is , whose components are of magnitudes six and nine, distance . ", p. °; colors, topaz and lilac. not far away is the closer double , composed of a sixth and a seventh magnitude star, distance . ", p. °. the four-inch is capable of splitting this star, but we shall do better to use the five-inch. in passing we may glance at the tenth-magnitude companion to eta, distance ", p. °. another wide pair is found in zeta, magnitudes three and nine, distance ", p. °. the next step brings us to the wonderful variable omicron, or mira, whose changes have been watched for three centuries, the first observer of the variability of the star having been david fabricius in . not only is the range of variability very great, but the period is remarkably irregular. in the time of hevelius, mira was once invisible for four years. when brightest, the star is of about the second magnitude, and when faintest, of the ninth magnitude, but at maximum it seldom exhibits the greatest brilliance that it has on a few occasions shown itself capable of attaining. ordinarily it begins to fade after reaching the fourth or fifth magnitude. the period averages about three hundred and thirty-one days, but is irregularly variable to the extent of twenty-five days. its color is red, and its spectrum shows bright lines, which it is believed disappear when the star sinks to a minimum. among the various theories proposed to account for such changes as these the most probable appears to be that which ascribes them to some cause analogous to that operating in the production of sun spots. the outburst of light, however, as pointed out by scheiner, should be regarded as corresponding to the maximum and not the minimum stage of sun-spot activity. according to this view, the star is to be regarded as possessing an extensive atmosphere of hydrogen, which, during the maximum, is upheaved into enormous prominences, and the brilliance of the light from these prominences suffices to swamp the photospheric light, so that in the spectrum the hydrogen lines appear bright instead of dark. it is not possible to suppose that mira can be the center of a system of habitable planets, no matter what we may think of the more constant stars in that regard, because its radiation manifestly increases more than six hundred fold, and then falls off again to an equal extent once in every ten or eleven months. i have met people who can not believe that the almighty would make a sun and then allow its energies "to go to waste," by not supplying it with a family of worlds. but i imagine that if they had to live within the precincts of mira ceti they would cry out for exemption from their own law of stellar utility. the most beautiful double star in cetus is gamma, magnitudes three and seven, distance ", p. °; hues, straw-color and blue. the leading star alpha, of magnitude two and a half, has a distant blue companion three magnitudes fainter, and between them are two minute stars, the southernmost of which is a double, magnitudes both eleven, distance ", p. °. the variable s ranges between magnitudes seven and twelve in a somewhat irregular period of about eleven months, while r ranges between the seventh and the thirteenth magnitudes in a period of one hundred and sixty-seven days. [illustration: map no. .] the constellation eridanus, represented in map no. , contains a few fine double stars, one of the most interesting of which is , a rather close binary. the magnitudes are four and eight, distance ", p. °. we shall take the five-inch for this, and a steady atmosphere and sharp seeing will be necessary on account of the wide difference in the brightness of the component stars. amateurs frequently fail to make due allowance for the effect of such difference. when the limit of separating power for a telescope of a particular aperture is set at " or ", as the case may be, it is assumed that the stars composing the doubles on which the test is made shall be of nearly the same magnitude, or at least that they shall not differ by more than one or two magnitudes at the most. the stray light surrounding a comparatively bright star tends to conceal a faint companion, although the telescope may perfectly separate them so far as the stellar disks are concerned. then, too, i have observed in my own experience that a very faint and close double is more difficult than a brighter pair not more widely separated, usually on account of the defect of light, and this is true even when the components of the faint double are of equal magnitude. sigma , otherwise known as eridani, is a superb object on account of the colors of its components, the larger star being a rich topaz and the smaller an ultramarine; while the difference in magnitude is not as great as in many of the colored doubles. the magnitudes are five and seven, distance . ", p. °. the star gamma, of magnitude two and a half, has a tenth-magnitude companion, distant ", p. °. sigma , also called eridani, consists of two stars of magnitudes six and nine, distance . ", p. °; colors, yellow and blue. the supposed binary character of this star has not yet been established. in omicron^ we come upon an interesting triple star, two of whose components at any rate we can easily see. the largest component is of the fourth magnitude. at a distance of ", p. °, we find a tenth-magnitude companion. this companion is itself double, the magnitudes of its components being ten and eleven, distance . ", p. °. hall says of these stars that they "form a remarkable system." he has also observed a fourth star of the twelfth magnitude, distant " from the largest star, p. °. this is apparently unconnected with the others, although it is only half as distant as the tenth-magnitude component is from the primary. sigma is interesting because of the similarity of its two components in size, both being of about the seventh magnitude, distance ", p. °. finally, we turn to the nebula . this is planetary in form and inconspicuous, but lassell has described it as presenting a most extraordinary appearance with his great reflector--a circular nebula lying upon another fainter and larger nebula of a similar shape, and having a star in its center. yet it may possibly be an immensely distant star cluster instead of a nebula, since its spectrum does not appear to be gaseous. chapter vii pisces, aries, taurus, and the northern stars "now sing we stormy skies when autumn weighs the year, and adds to nights and shortens days, and suns declining shine with feeble rays."--dryden's virgil. [illustration: map no. .] the eastern end of pisces, represented in map no. , includes most of the interesting telescopic objects that the constellation contains. we begin our exploration at the star numbered , a double that is very beautiful when viewed with the three-inch glass. the components are of magnitudes five and eight, distance . ", p. °. the larger star is yellow and the smaller deep blue. the star , while lacking the peculiar charm of contrasted colors so finely displayed in , possesses an attraction in the equality of its components which are both of the sixth magnitude and milk-white. the distance is . ", p. °. in we find a swift binary whose components are at present far too close for any except the largest telescopes. the distance in was only . ", p. °. the magnitudes are six and seven. in contrast with this excessively close double is psi, whose components are both of magnitude five and a half, distance ", p. °. dropping down to we come upon another very wide and pleasing double, magnitudes six and seven, distance ", p. °, colors white and lilac or pale blue. hardly less beautiful is zeta magnitudes five and six, distance ", p. °. finest of all is alpha, which exhibits a remarkable color contrast, the larger star being greenish and the smaller blue. the magnitudes are four and five, distance ", p. °. this star is a binary, but the motion is slow. the variable r ranges between magnitudes seven and thirteen, period three hundred and forty-four days. the constellation aries contains several beautiful doubles, all but one of which are easy for our smallest aperture. the most striking of these is gamma, which is historically interesting as the first double star discovered. the discovery was made by robert hooke in by accident, while he was following the comet of that year with his telescope. he expressed great surprise on noticing that the glass divided the star, and remarked that he had not met with a like instance in all the heavens. his observations could not have been very extensive or very carefully conducted, for there are many double stars much wider than gamma arietis which hooke could certainly have separated if he had examined them. the magnitudes of the components of gamma are four and four and a half, or, according to hall, both four; distance . ", p. °. a few degrees above gamma, passing by beta, is a wide double lambda, magnitudes five and eight, distance ", p. °, colors white and lilac or violet. three stars are to be seen in : magnitudes five and a half, ten, and nine, distances ", p. °, and ", p. °, colors white, blue, and lilac. the star is a very pretty double, magnitudes six and seven, distance . ", p. °. sigma consists of a topaz star combined with a sapphire, magnitudes six and nine, distance . ", p. °. the fourth-magnitude star has several faint companions. the magnitudes of two of these are eleven and nine, distances ", p. °, and ", p. °. we discover another triple in pi, magnitudes five, eight, and eleven, distances . ", p. °, and ", p. °. the double mentioned above as being too close for our three-inch glass is epsilon, which, however, can be divided with the four-inch, although the five-inch will serve us better. the magnitudes are five and a half and six, distance . ", p. °. the star has two companions, one of which is so close that our instruments can not separate it, while the other is too faint to be visible in the light of its brilliant neighbor without the aid of a very powerful telescope. [illustration: map no. .] we are now about to enter one of the most magnificent regions in the sky, which is hardly less attractive to the naked eye than orion, and which men must have admired from the beginning of their history on the earth, the constellation taurus (map no. ). two groups of stars especially distinguish taurus, the hyades and the pleiades, and both are exceedingly interesting when viewed with the lowest magnifying powers of our telescopes. we shall begin with a little star just west of the pleiades, sigma , also called tauri. this is a triple, but we can see it only as a double, the third star being exceedingly close to the primary. the magnitudes are six and a half, seven, and ten, distances . ", p. °, and ", p. °. in the pleiades we naturally turn to the brightest star eta, or alcyone, famous for having once been regarded as the central sun around which our sun and a multitude of other luminaries were supposed to revolve, and picturesque on account of the little triangle of small stars near it which the least telescopic assistance enables us to see. one may derive much pleasure from a study of the various groupings of stars in the pleiades. photography has demonstrated, what had long been suspected from occasional glimpses revealed by the telescope, that this celebrated cluster of stars is intermingled with curious forms of nebulæ. the nebulous matter appears in festoons, apparently attached to some of the larger stars, such as alcyone, merope, and maia, and in long, narrow, straight lines, the most remarkable of which, a faintly luminous thread starting midway between maia and alcyone and running eastward some ', is beaded with seven or eight stars. the width of this strange nebulous streak is, on an average, " or ", and there is, perhaps, no more wonderful phenomenon anywhere in celestial space. unfortunately, no telescope is able to show it, and all our knowledge about it is based upon photographs. it might be supposed that it was a nebulous disk seen edgewise, but for the fact that at the largest star involved in its course it bends sharply about ° out of its former direction, and for the additional fact that it seems to take its origin from a curved offshoot of the intricate nebulous mass surrounding maia. exactly at the point where this curve is transformed into a straight line shines a small star! in view of all the facts the idea does not seem to be very far-fetched that in the pleiades we behold an assemblage of suns, large and small, formed by the gradual condensation of a nebula, and in which evolution has gone on far beyond the stage represented by the orion nebula, where also a group of stars may be in process of formation out of nebulous matter. if we look a little farther along this line of development, we may perceive in such a stellar assemblage as the cluster in hercules, a still later phase wherein all the originally scattered material has, perhaps, been absorbed into the starry nuclei. [illustration: the chief stars in the pleiades.] the yellow star sigma has two companions: magnitudes six, nine, and nine and a half, distances ", p. °, and ", p. °. the star of the fifth magnitude has a companion of the ninth magnitude, distance ", p. °, colors emerald and purple, faint. an interesting variable, of the type of algol, is lambda, which at maximum is of magnitude three and four tenths and at minimum of magnitude four and two tenths. its period from one maximum to the next is about three days and twenty-three hours, but the actual changes occupy only about ten hours, and it loses light more swiftly than it regains it. a combination of red and blue is presented by phi (mistakenly marked on map no. as psi). the magnitudes are six and eight, distance ", p. °. a double of similar magnitudes is chi, distance ", p. °. between the two stars which the naked eye sees in kappa is a minute pair, each of less than the eleventh magnitude, distance ", p. °. another naked-eye double is formed by theta^ and theta^ , in the hyades. the magnitudes are five and five and a half, distance about ' ". the leading star of taurus, aldebaran (alpha), is celebrated for its reddish color. the precise hue is rather uncertain, but aldebaran is not orange as betelgeuse in orion is, and no correct eye can for an instant confuse the colors of these two stars, although many persons seem to be unable to detect the very plain difference between them in this respect. aldebaran has been called "rose-red," and it would be an interesting occupation for an amateur to determine, with the aid of some proper color scale, the precise hue of this star, and of the many other stars which exhibit chromatic idiosyncrasy. aldebaran is further interesting as being a standard first-magnitude star. with the four-inch glass we see without difficulty the tenth-magnitude companion following aldebaran at a distance of ", p. °. there is an almost inexplicable charm about these faint attendants of bright stars, which is quite different from the interest attaching to a close and nearly equal pair. the impression of physical relationship is never lacking though it may be deceptive, and this awakens a lively appreciation of the vast differences of magnitude that exist among the different suns of space. the actual size and might of this great red sun form an attractive subject for contemplation. as it appears to our eyes aldebaran gives one twenty-five-thousand-millionth as much light as the sun, but if we were placed midway between them the star would outshine the sun in the ratio of not less than to . and yet, gigantic as it is, aldebaran is possibly a pygmy in comparison with arcturus, whose possible dimensions were discussed in the chapter relating to boötes. although aldebaran is known to possess several of the metallic elements that exist in the sun, its spectrum differs widely from the solar spectrum in some respects, and more closely resembles that of arcturus. other interesting objects in taurus are sigma, divisible with the naked eye, magnitudes five and five and a half, distance '; sigma , double, magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes six and seven, distance ", p. °--a pleasing sight; tau, triple, magnitudes four, ten and a half, and eleven, distances ", p. °, and ", p. °--the ten-and-a-half-magnitude star is itself double, as discovered by burnham; star cluster no. , not quite as broad as the moon, and containing some stars as large as the eleventh magnitude; and nebula no. , the so-called "crab nebula" of lord rosse, which our glasses will show only as a misty patch of faint light, although large telescopes reveal in it a very curious structure. [illustration: map no. .] we now turn to the cluster of circumpolar constellations sometimes called the royal family, in allusion to the well-known story of the ethiopian king cepheus and his queen cassiopeia, whose daughter andromeda was exposed on the seashore to be devoured by a monster, but who was saved by the hero perseus. all these mythologic personages are represented in the constellations that we are about to study.[ ] we begin with andromeda (map no. ). the leading star alpha marks one corner of the great square of pegasus. the first star of telescopic interest that we find in andromeda is , a double difficult on account of the faintness of the smaller component. the magnitudes are four and eleven, distance ", p. °. a few degrees north of the naked eye detects a glimmering point where lies the great nebula in andromeda. this is indicated on the map by the number . with either of our three telescopes it is an interesting object, but of course it is advisable to use our largest glass in order to get as much light as possible. all that we can see is a long, shuttle-shaped nebulous object, having a brighter point near the center. many stars are scattered over the field in its neighborhood, but the nebula itself, although its spectrum is peculiar in resembling that of a faint star, is evidently a gaseous or at any rate a meteoritic mass, since photographs show it to be composed of a series of imperfectly separated spirals surrounding a vast central condensation. this peculiarity of the andromeda nebula, which is invisible with telescopes although conspicuous in the photographs, has, since its discovery a few years ago, given a great impetus to speculation concerning the transformation of nebulæ into stars and star clusters. no one can look at a good photograph of this wonderful phenomenon without noticing its resemblance to the ideal state of things which, according to the nebular hypothesis, must once have existed in the solar system. it is to be remembered, however, that there is probably sufficient material in the andromeda nebula to make a system many times, perhaps hundreds or thousands of times, as extensive as that of which our sun is the center. if one contemplates this nebula only long enough to get a clear perception of the fact that creation was not ended when, according to the mosaic history, god, having in six days finished "the heavens and the earth and all the host of them," rested from all his work, a good blow will have been dealt for the cause of truth. systems far vaster than ours are now in the bud, and long before they have bloomed, ambitious man, who once dreamed that all these things were created to serve him, will probably have vanished with the extinguishment of the little star whose radiant energy made his life and his achievements briefly possible. [ ] for further details on this subject see astronomy with an opera-glass. in august, , a new star of magnitude six and a half made its appearance suddenly near the center of the andromeda nebula. within one year it had disappeared, having gradually dwindled until the great washington telescope, then the largest in use, no longer showed it. that this was a phenomenon connected with the nebula is most probable, but just what occurred to produce it nobody knows. the observed appearances might have been produced by a collision, and no better hypothesis has yet been suggested to account for them. near the opposite end of the constellation from alpha we find the most interesting of triple stars in gamma. the two larger components of this beautiful star are of magnitudes three and six, distance ", colors golden yellow and deep blue. the three-inch shows them finely. the smaller star is itself double, its companion being of magnitude eight, distance when discovered in . ", color bluish green. a few years ago this third star got so close to its primary that it was invisible even with the highest powers of the great lick telescope, but at present it is widening again. in october, , i had the pleasure of looking at gamma andromedæ with the lick telescope, and at that time it was possible just to separate the third star. the angle seemed too small for certain measurement, but a single setting of the micrometer by mr. barnard, to whose kindness i was indebted for my view of the star, gave . " as the approximate distance. in the distance had increased to . ", p. °. the brilliance of color contrast between the two larger stars of gamma andromedæ is hardly inferior to that exhibited in beta cygni, so that this star may be regarded as one of the most picturesque of stellar objects for small telescopes. other pleasing objects in this constellation are the binary star , magnitudes six and six and a half, distance ", p. °--the two stars are slowly closing and the five-inch glass is required to separate them: the richly colored variable r, which fades from magnitude five and a half to invisibility, and then recovers its light in a period of about four hundred and five days; and the bright star cluster , which covers a space about equal to the area of the full moon. just south of the eastern end of andromeda is the small constellation triangulum, or the triangles, containing two interesting objects. one of these is the beautiful little double , magnitudes five and six, distance . ", p. °, colors yellow and blue; and the other, the nebula , which equals in extent the star cluster in andromeda described above, but nevertheless appears very faint with our largest glass. its faintness, however, is not an indication of insignificance, for to very powerful telescopes it exhibits a wonderful system of nuclei and spirals--another bit of chaos that is yielding by age-long steps to the influence of demiurgic forces. a richer constellation than andromeda, both for naked-eye and telescopic observation, is perseus, which is especially remarkable for its star clusters. two of these, and , constitute the celebrated double cluster, sometimes called the sword-hand of perseus, and also chi persei. to the smallest telescope this aggregation of stars, ranging in magnitude from six and a half to fourteen, and grouped about two neighboring centers, presents a marvelous appearance. as an educative object for those unaccustomed to celestial observations it may be compared among star clusters to beta cygni among double stars, for the most indifferent spectator is struck with wonder in viewing it. all the other clusters in perseus represented on the map are worth examining, although none of them calls for special mention, except perhaps , where we may distinguish at least a hundred separate stars within an area less than one quarter as expansive as the face of the moon. among the double stars of perseus we note first eta, whose components are of magnitudes four and eight, distance ", colors white and pale blue. the double epsilon is especially interesting on account of an alleged change of color from blue to red which the smaller star undergoes coincidently with a variation of brightness. the magnitudes are three and eight, distance ", p. °. an interesting multiple is zeta, two of whose stars at least we can see. the magnitudes are three, nine, ten, and ten, distances ", p. °, ", and ". the chief attraction in perseus is the changeful and wonderful beta, or algol, the great typical star among the short-period variables. during the greater part of its period this star is of magnitude two and two tenths, but for a very short time, following a rapid loss of light, it remains at magnitude three and seven tenths. the difference, one magnitude and a half, corresponds to an actual difference in brightness in the ratio of . to . the entire loss of light during the declension occupies only four hours and a half. the star remains at its faintest for a few minutes only before a perceptible gain of light occurs, and the return to maximum is as rapid as was the preceding decline. the period from one minimum to the next is two days twenty hours forty-eight minutes fifty-three seconds, with an irregularity amounting to a few seconds in a year. the arabs named the star algol, or the demon, on account of its eccentricity which did not escape their attention; and when goodricke, in , applied a scientific method of observation to it, the real cause of its variations was suggested by him, but his explanation failed of general acceptance until its truth was established by prof. e. c. pickering in . this explanation gives us a wonderful insight into stellar constitution. according to it, algol possesses a companion as large as the sun, but invisible, both because of its proximity to that star and because it yields no light, and revolving in a plane horizontal to our line of sight. the period of revolution is identical with the period of algol's cycle of variation, and the diminution of light is caused by the interposition of the dark body as it sweeps along that part of its orbit lying between our point of view and the disk of algol. in other words, once in every two days twenty hours and forty-nine minutes algol, as seen from the earth, undergoes a partial eclipse. in consequence of the great comparative mass of its dark companion, algol itself moves in an orbit around their common center with a velocity quite sufficient to be detected by the shifting of the lines in its spectrum. by means of data thus obtained the mass, size, and distance apart of algol and its singular comrade have been inferred. the diameter of algol is believed to be about , , miles, that of the dark body about , miles, and the mean distance from center to center , , miles. the density of both the light and the dark star is slight compared with that of the sun, so that their combined mass is only two thirds as great as the sun's. mention has been made of a slight irregularity in algol's period of variation. basing his calculations upon this inequality, dr. chandler has put forward the hypothesis that there is another invisible body connected with algol, and situated at a distance from it of about , , , miles, and that around this body, which is far more massive than the others, algol and its companions revolve in a period of one hundred and thirty years! dr. chandler has earned the right to have his hypotheses regarded with respect, even when they are as extraordinary as that which has just been described. it needs no indulgence of the imagination to lend interest to algol; the simple facts are sufficient. how did that bright star fall in with its black neighbors? or were they created together? [illustration: map no. .] passing to the region covered by map no. , our eyes are caught by the curious figure, formed by the five brightest stars of the constellation cassiopeia, somewhat resembling the letter w. like perseus, this is a rich constellation, both in star clusters and double stars. among the latter we select as our first example sigma, in which we find a combination of color that is at once very unusual and very striking--green and blue. the magnitudes are five and seven, distance ", p. °. another beautiful colored double is eta, whose magnitudes are four and seven and a half, distance ", p. °, colors white and purple. this is one of the comparatively small number of stars the measure of whose distance has been attempted, and a keen sense of the uncertainty of such measures is conveyed by the fact that authorities of apparently equal weight place eta cassiopeiæ at such discordant distances as , , , , miles, , , , , miles, and , , , , miles. it will be observed that the difference between the greatest and the least of these estimates is about double the entire distance given by the latter. the same thing is practically true of the various attempts to ascertain the distance of the other stars which have a perceptible parallax, even those which are evidently the nearest. in some cases the later measures increase the distance, in other cases they diminish it; in no case is there anything like a complete accord. yet of course we are not to infer that it is hopeless to learn anything about the distances of the stars. with all their uncertainties and disagreements the few parallaxes we possess have laid a good foundation for a knowledge of the dimensions of at least the nearer parts of the universe. we find an interesting triple in psi, the magnitudes of the larger components being four and a half and eight and a half, distance ". the smaller star has a nine-and-a-half-magnitude companion, distance ". a more beautiful triple is iota, magnitudes four, seven, and eight, distances ", p. °, and . ", p. °. cassiopeia contains many star clusters, three of which are indicated in the map. of these is perhaps the most interesting, as it includes stars of many magnitudes, among which are a red one of the eighth magnitude, and a ninth-magnitude double whose components are " apart. not far from the star kappa we find the spot where the most brilliant temporary star on record made its appearance on november , . tycho brahe studied this phenomenon during the entire period of its visibility, which lasted until march, . it burst out suddenly with overpowering splendor, far outshining every fixed star, and even equaling venus at her brightest. in a very short time it began to fade, regularly diminishing in brightness, and at the same time undergoing changes of color, ending in red, until it disappeared. it has never been seen since, and the suspicion once entertained that it was a variable with a period considerably exceeding three hundred years has not been confirmed. there is a tenth-magnitude star near the place given by tycho as that occupied by the stranger. many other faint stars are scattered about, however, and tycho's measures were not sufficiently exact to enable us to identify the precise position of his star. if the phenomenon was due to a collision, no reappearance of the star is to be expected. camelopardalus is a very inconspicuous constellation, yet it furnishes considerable occupation for the telescope. sigma , of magnitude five, has a companion of magnitude nine and a half, distance ", °. sigma , also of the fifth magnitude, has a ninth-magnitude companion, distance only . ", p. °. according to some observers, the larger star is yellow and the smaller white. the star is a very pretty double, magnitudes both six, distance . ". its neighbor of magnitude six has an eighth-magnitude companion, distance . ", p. °. the star of magnitude five is also double, the companion of magnitude eight being distant only . ". a glance at star cluster , which shows a slight central condensation, completes our work in camelopardalus, and we turn to ursa major, represented in map no. . here there are many interesting doubles and triples. beginning with iota we find at once occupation for our largest glass. the magnitudes are three and ten, distance ", p. °. in the double star the magnitudes are four and nine, distance ", p. °. a more pleasing object is sigma^ , a greenish fifth-magnitude star which has an eighth-magnitude companion, distance . ", p. °. a good double for our four-inch glass is xi, whose magnitudes are four and five, distance . ", p. °. this is a binary with a period of revolution of about sixty years, and is interesting as the first binary star whose orbit was determined. savary calculated it in . near by is nu, a difficult double, magnitudes four and ten and a half, distance ", p. °. in we find again an easy double magnitudes six and eight, distance . ", p. °. another similar double is , magnitudes six and eight, distance . ", p. °. a third star, magnitude seven, is seen at a distance of " from the primary. we come now to ursa major's principal attraction zeta, frequently called mizar. the naked eye perceives near it a smaller star, named alcor. with the three-inch glass and a medium power we divide mizar into two bright stars brilliantly contrasted in color, the larger being white and the smaller blue-green. beside alcor, several fainter stars are seen scattered over the field of view, and, taken all in all, there are very few equally beautiful sights in the starry heavens. the magnitudes of the double are three and four, distance . ", p. °. the large star is again double, although no telescope has been able to show it so, its duplicity being revealed, like that of beta aurigæ, by the periodical splitting of the lines in its spectrum. ursa major contains several nebulæ which may be glimpsed with telescopes of moderate dimensions. an interesting pair of these objects, both of which are included in one field of view, is formed by and . the first named is the brighter of the two, its nucleus resembling a faint star. the nebula presents itself to us in the form of a faint, hazy star, but with large telescopes its appearance is very singular. according to a picture made by lord rosse, it bears no little resemblance to a skull, there being two symmetrically placed holes in it, each of which contains a star. [illustration: map no. .] the portion of canes venatici, represented in map no. , contains two or three remarkable objects. sigma is a close double, magnitudes six and seven, distance ", p. °. it is a pretty sight with the five-inch. the double star is singular in that its larger component is red and its smaller blue; magnitudes six and eight, distance . ", p. °. still more beautiful is , commonly called cor caroli. this double is wide, and requires but a slight magnifying power. the magnitudes are three and six, distance ", colors white or light yellow and blue. the nebula , although we can see it only as a pair of misty specks, is in reality a very wonderful object. lord rosse's telescope has revealed in it a complicated spiral structure, recalling the photographs of the andromeda nebula, and indicating that stupendous changes must be in process within it, although our records of observation are necessarily too brief to bring out any perceptible alteration of figure. it would seem that the astronomer has, of all men, the best reasons for complaining of the brevity of human life. lastly, we turn to ursa minor and the pole star. the latter is a celebrated double, not difficult, except with a telescope of less than three inches aperture in the hands of an inexperienced observer. the magnitudes are two and nine, distance . ". the small star has a dull blue color. in it was discovered by spectroscopic evidence that the pole star is triple. in pi' we see a wide double, magnitudes six and seven, distance ", p. °. this completes our survey of the starry heavens. chapter viii scenes on the planets "these starry globes far surpassed the earth in grandeur, and the latter looked so diminutive that our empire, which appeared only as a point on its surface, awoke my pity."--cicero, the dream of scipio. although amateurs have played a conspicuous part in telescopic discovery among the heavenly bodies, yet every owner of a small telescope should not expect to attach his name to a star. but he certainly can do something perhaps more useful to himself and his friends; he can follow the discoveries that others, with better appliances and opportunities, have made, and can thus impart to those discoveries that sense of reality which only comes from seeing things with one's own eyes. there are hundreds of things continually referred to in books and writings on astronomy which have but a misty and uncertain significance for the mere reader, but which he can easily verify for himself with the aid of a telescope of four or five inches aperture, and which, when actually confronted by the senses, assume a meaning, a beauty, and an importance that would otherwise entirely have escaped him. henceforth every allusion to the objects he has seen is eloquent with intelligence and suggestion. take, for instance, the planets that have been the subject of so many observations and speculations of late years--mars, jupiter, saturn, venus. for the ordinary reader much that is said about them makes very little impression upon his mind, and is almost unintelligible. he reads of the "snow patches" on mars, but unless he has actually seen the whitened poles of that planet he can form no clear image in his mind of what is meant. so the "belts of jupiter" is a confusing and misleading phrase for almost everybody except the astronomer, and the rings of saturn are beyond comprehension unless they have actually been seen. it is true that pictures and photographs partially supply the place of observation, but by no means so successfully as many imagine. the most realistic drawings and the sharpest photographs in astronomy are those of the moon, yet i think nobody would maintain that any picture in existence is capable of imparting a really satisfactory visual impression of the appearance of the lunar globe. nobody who has not seen the moon with a telescope--it need not be a large one--can form a correct and definite idea of what the moon is like. the satisfaction of viewing with one's own eyes some of the things the astronomers write and talk about is very great, and the illumination that comes from such viewing is equally great. just as in foreign travel the actual seeing of a famous city, a great gallery filled with masterpieces, or a battlefield where decisive issues have been fought out illuminates, for the traveler's mind, the events of history, the criticisms of artists, and the occurrences of contemporary life in foreign lands, so an acquaintance with the sights of the heavens gives a grasp on astronomical problems that can not be acquired in any other way. the person who has been in rome, though he may be no archæologist, gets a far more vivid conception of a new discovery in the forum than does the reader who has never seen the city of the seven hills; and the amateur who has looked at jupiter with a telescope, though he may be no astronomer, finds that the announcement of some change among the wonderful belts of that cloudy planet has for him a meaning and an interest in which the ordinary reader can not share. [illustration: jupiter seen with a five-inch telescope. shadow of a satellite visible.] jupiter is perhaps the easiest of all the planets for the amateur observer. a three-inch telescope gives beautiful views of the great planet, although a four-inch or a five-inch is of course better. but there is no necessity for going beyond six inches' aperture in any case. for myself, i should care for nothing better than my byrne five-inch of fifty-two inches' focal distance. with such a glass more details are visible in the dark belts and along the bright equatorial girdle than can be correctly represented in a sketch before the rotation of the planet has altered their aspect, while the shadows of the satellites thrown upon the broad disk, and the satellites themselves when in transit, can be seen sometimes with exquisite clearness. the contrasting colors of various parts of the disk are also easily studied with a glass of four or five inches' aperture. there is a charm about the great planet when he rides high in a clear evening sky, lording it over the fixed stars with his serene, unflickering luminousness, which no possessor of a telescope can resist. you turn the glass upon him and he floats into the field of view, with his _cortége_ of satellites, like a yellow-and-red moon, attended by four miniatures of itself. you instantly comprehend jupiter's mastery over his satellites--their allegiance is evident. no one would for an instant mistake them for stars accidentally seen in the same field of view. although it requires a very large telescope to magnify their disks to measurable dimensions, yet the smallest glass differentiates them at once from the fixed stars. there is something almost startling in their appearance of companionship with the huge planet--this sudden verification to your eyes of the laws of gravitation and of central forces. it is easy, while looking at jupiter amid his family, to understand the consternation of the churchmen when galileo's telescope revealed that miniature of the solar system, and it is gratifying to gaze upon one of the first battle grounds whereon science gained a decisive victory for truth. the swift changing of place among the satellites, as well as the rapidity of jupiter's axial rotation, give the attraction of visible movement to the jovian spectacle. the planet rotates in four or five minutes less than ten hours--in other words, it makes two turns and four tenths of a third turn while the earth is rolling once upon its axis. a point on jupiter's equator moves about twenty-seven thousand miles, or considerably more than the entire circumference of the earth, in a single hour. the effect of this motion is clearly perceptible to the observer with a telescope on account of the diversified markings and colors of the moving disk, and to watch it is one of the greatest pleasures that the telescope affords. it would be possible, when the planet is favorably situated, to witness an entire rotation of jupiter in the course of one night, but the beginning and end of the observation would be more or less interfered with by the effects of low altitude, to say nothing of the tedium of so long a vigil. but by looking at the planet for an hour at a time in the course of a few nights every side of it will have been presented to view. suppose the first observation is made between nine and ten o'clock on any night which may have been selected. then on the following night between ten and eleven o'clock jupiter will have made two and a half turns upon his axis, and the side diametrically opposite to that seen on the first night will be visible. on the third night between eleven and twelve o'clock jupiter will have performed five complete rotations, and the side originally viewed will be visible again. owing to the rotundity of the planet, only the central part of the disk is sharply defined, and markings which can be easily seen when centrally located become indistinct or disappear altogether when near the limb. approach to the edge of the disk also causes a foreshortening which sometimes entirely alters the aspect of a marking. it is advisable, therefore, to confine the attention mainly to the middle of the disk. as time passes, clearly defined markings on or between the cloudy belts will be seen to approach the western edge of the disk, gradually losing their distinctness and altering their appearance, while from the region of indistinct definition near the eastern edge other markings slowly emerge and advance toward the center, becoming sharper in outline and more clearly defined in color as they swing into view. watching these changes, the observer is carried away by the reflection that he actually sees the turning of another distant world upon its axis of rotation, just as he might view the revolving earth from a standpoint on the moon. belts of reddish clouds, many thousands of miles across, are stretched along on each side of the equator of the great planet he is watching; the equatorial belt itself, brilliantly lemon-hued, or sometimes ruddy, is diversified with white globular and balloon-shaped masses, which almost recall the appearance of summer cloud domes hanging over a terrestrial landscape, while toward the poles shadowy expanses of gradually deepening blue or blue-gray suggest the comparative coolness of those regions which lie always under a low sun. [illustration: eclipses and transits of jupiter's satellites. satellite i and the shadow of iii are seen in transit. iv is about to be eclipsed.] after a few nights' observation even the veriest amateur finds himself recognizing certain shapes or appearances--a narrow dark belt running slopingly across the equator from one of the main cloud zones to the other, or a rift in one of the colored bands, or a rotund white mass apparently floating above the equator, or a broad scallop in the edge of a belt like that near the site of the celebrated "red spot," whose changes of color and aspect since its first appearance in , together with the light it has thrown on the constitution of jupiter's disk, have all but created a new jovian literature, so thoroughly and so frequently have they been discussed. and, having noticed these recurring features, the observer will begin to note their relations to one another, and will thus be led to observe that some of them gradually drift apart, while others drift nearer; and after a time, without any aid from books or hints from observatories, he will discover for himself that there is a law governing the movements on jupiter's disk. upon the whole he will find that the swiftest motions are near the equator, and the slowest near the poles, although, if he is persistent and has a good eye and a good instrument, he will note exceptions to this rule, probably arising, as professor hough suggests, from differences of altitude in jupiter's atmosphere. finally, he will conclude that the colossal globe before him is, exteriorly at least, a vast ball of clouds and vapors, subject to tremendous vicissitudes, possibly intensely heated, and altogether different in its physical constitution, although made up of similar elements, from the earth. then, if he chooses, he can sail off into the delightful cloud-land of astronomical speculation, and make of the striped and spotted sphere of jove just such a world as may please his fancy--for a world of some kind it certainly is. for many observers the satellites of jupiter possess even greater attractions than the gigantic ball itself. as i have already remarked, their movements are very noticeable and lend a wonderful animation to the scene. although they bear classical names, they are almost universally referred to by their roman numbers, beginning with the innermost, whose symbol is i, and running outward in regular order ii, iii, and iv.[ ] the minute satellite much nearer to the planet than any of the others, which mr. barnard discovered with the lick telescope in , is called the fifth, although in the order of distance it would be the first. in size and importance, however, it can not rank with its comparatively gigantic brothers. of course, no amateur's telescope can afford the faintest glimpse of it. [ ] their names, in the same order as their numbers, are io, europa, ganymede, and callisto. satellite i, situated at a mean distance of , miles from jupiter's center--about , miles farther than the moon is from the earth--is urged by its master's overpowering attraction to a speed of miles per minute, so that it performs a complete revolution in about forty-two hours and a half. the others, of course, move more slowly, but even the most distant performs its revolution in several hours less than sixteen days. the plane of their orbits is presented edgewise toward the earth, from which it follows that they appear to move back and forth nearly in straight lines, some apparently approaching the planet, while others are receding from it. the changes in their relative positions, which can be detected from hour to hour, are very striking night after night, and lead to a great variety of arrangements always pleasing to the eye. the most interesting phenomena that they present are their transits and those of their round, black shadows across the face of the planet; their eclipses by the planet's shadow, when they disappear and afterward reappear with astonishing suddenness; and their occultations by the globe of jupiter. upon the whole, the most interesting thing for the amateur to watch is the passage of the shadows across jupiter. the distinctness with which they can be seen when the air is steady is likely to surprise, as it is certain to delight, the observer. when it falls upon a light part of the disk the shadow of a satellite is as black and sharply outlined as a drop of ink; on a dark-colored belt it can not so easily be seen. it is more difficult to see the satellites themselves in transit. there appears to be some difference among them as to visibility in such circumstances. owing to their luminosity they are best seen when they have a dark belt for a background, and are least easily visible when they appear against a bright portion of the planet. every observer should provide himself with a copy of the american ephemeris for the current year, wherein he will find all the information needed to enable him to identify the various satellites and to predict, by turning washington mean time into his own local time, the various phenomena of the transits and eclipses. while a faithful study of the phenomena of jupiter is likely to lead the student to the conclusion that the greatest planet in our system is not a suitable abode for life, yet the problem of its future, always fascinating to the imagination, is open; and whosoever may be disposed to record his observations in a systematic manner may at least hope to render aid in the solution of that problem. saturn ranks next to jupiter in attractiveness for the observer with a telescope. the rings are almost as mystifying to-day as they were in the time of herschel. there is probably no single telescopic view that can compare in the power to excite wonder with that of saturn when the ring system is not so widely opened but that both poles of the planet project beyond it. one returns to it again and again with unflagging interest, and the beauty of the spectacle quite matches its singularity. when saturn is in view the owner of a telescope may become a recruiting officer for astronomy by simply inviting his friends to gaze at the wonderful planet. the silvery color of the ball, delicately chased with half-visible shadings, merging one into another from the bright equatorial band to the bluish polar caps; the grand arch of the rings, sweeping across the planet with a perceptible edging of shadow; their sudden disappearance close to the margin of the ball, where they go behind it and fall straightway into night; the manifest contrast of brightness, if not of color, between the two principal rings; the fine curve of the black line marking the , -mile gap between their edges--these are some of the elements of a picture that can never fade from the memory of any one who has once beheld it in its full glory. [illustration: saturn seen with a five-inch telescope.] saturn's moons are by no means so interesting to watch as are those of jupiter. even the effect of their surprising number (raised to nine by professor pickering's discovery in of a new one which is almost at the limit of visibility, and was found only with the aid of photography) is lost, because most of them are too faint to be seen with ordinary telescopes, or, if seen, to make any notable impression upon the eye. the two largest--titan and japetus--are easily found, and titan is conspicuous, but they give none of that sense of companionship and obedience to a central authority which strikes even the careless observer of jupiter's system. this is owing partly to their more deliberate movements and partly to the inclination of the plane of their orbits, which seldom lies edgewise toward the earth. [illustration: polar view of saturn's system. the orbits of the five nearest satellites are shown. the dotted line outside the rings shows roche's limit.] but the charm of the peerless rings is abiding, and the interest of the spectator is heightened by recalling what science has recently established as to their composition. it is marvelous to think, while looking upon their broad, level surfaces--as smooth, apparently, as polished steel, though thirty thousand miles across--that they are in reality vast circling currents of meteoritic particles or dust, through which run immense waves, condensation and rarefaction succeeding one another as in the undulations of sound. yet, with all their inferential tumult, they may actually be as soundless as the depths of interstellar space, for struve has shown that those spectacular rings possess no appreciable mass, and, viewed from saturn itself, their (to us) gorgeous seeming bow may appear only as a wreath of shimmering vapor spanning the sky and paled by the rivalry of the brighter stars. in view of the theory of tidal action disrupting a satellite within a critical distance from the center of its primary, the thoughtful observer of saturn will find himself wondering what may have been the origin of the rings. the critical distance referred to, and which is known as roche's limit, lies, according to the most trustworthy estimates, just outside the outermost edge of the rings. it follows that if the matter composing the rings were collected into a single body that body would inevitably be torn to pieces and scattered into rings; and so, too, if instead of one there were several or many bodies of considerable size occupying the place of the rings, all of these bodies would be disrupted and scattered. if one of the present moons of saturn--for instance, mimas, the innermost hitherto discovered--should wander within the magic circle of roche's limit it would suffer a similar fate, and its particles would be disseminated among the rings. one can hardly help wondering whether the rings have originated from the demolition of satellites--saturn devouring his children, as the ancient myths represent, and encircling himself, amid the fury of destruction, with the dust of his disintegrated victims. at any rate, the amateur student of saturn will find in the revelations of his telescope the inspirations of poetry as well as those of science, and the bent of his mind will determine which he shall follow. professor pickering's discovery of a ninth satellite of saturn, situated at the great distance of nearly eight million miles from the planet, serves to call attention to the vastness of the "sphere of activity" over which the ringed planet reigns. surprising as the distance of the new satellite appears when compared with that of our moon, it is yet far from the limit where saturn's control ceases and that of the sun becomes predominant. that limit, according to prof. asaph hall's calculation, is nearly , , miles from saturn's center, while if our moon were removed to a distance a little exceeding , miles the earth would be in danger of losing its satellite through the elopement of artemis with apollo. although, as already remarked, the satellites of saturn are not especially interesting to the amateur telescopist, yet it may be well to mention that, in addition to titan and japetus, the satellite named rhea, the fifth in order of distance from the planet, is not a difficult object for a three-or four-inch telescope, and two others considerably fainter than rhea--dione (the fourth) and tethys (the third)--may be seen in favorable circumstances. the others--mimas (the first), enceladus (the second), and hyperion (the seventh)--are beyond the reach of all but large telescopes. the ninth satellite, which has received the name of ph[oe]be, is much fainter than any of the others, its stellar magnitude being reckoned by its discoverer at about . . mars, the best advertised of all the planets, is nearly the least satisfactory to look at except during a favorable opposition, like those of and , when its comparative nearness to the earth renders some of its characteristic features visible in a small telescope. the next favorable opposition will occur in . when well seen with an ordinary telescope, say a four-or five-inch glass, mars shows three peculiarities that may be called fairly conspicuous--viz., its white polar cap, its general reddish, or orange-yellow, hue, and its dark markings, one of the clearest of which is the so-called syrtis major, or, as it was once named on account of its shape, "hourglass sea." other dark expanses in the southern hemisphere are not difficult to be seen, although their outlines are more or less misty and indistinct. the gradual diminution of the polar cap, which certainly behaves in this respect as a mass of snow and ice would do, is a most interesting spectacle. as summer advances in the southern hemisphere of mars, the white circular patch surrounding the pole becomes smaller, night after night, until it sometimes disappears entirely even from the ken of the largest telescopes. at the same time the dark expanses become more distinct, as if the melting of the polar snows had supplied them with a greater depth of water, or the advance of the season had darkened them with a heavier growth of vegetation. [illustration: mars seen with a five-inch telescope.] the phenomena mentioned above are about all that a small telescope will reveal. occasionally a dark streak, which large instruments show is connected with the mysterious system of "canals," can be detected, but the "canals" themselves are far beyond the reach of any telescope except a few of the giants handled by experienced observers. the conviction which seems to have forced its way into the minds even of some conservative astronomers, that on mars the conditions, to use the expression of professor young, "are more nearly earthlike than on any other of the heavenly bodies which we can see with our present telescopes," is sufficient to make the planet a center of undying interest notwithstanding the difficulties with which the amateur is confronted in his endeavors to see the details of its markings. the illumination of venus's atmosphere at the beginning of her transit across the sun. in venus "the fatal gift of beauty" may be said, as far as our observations are concerned, to be matched by the equally fatal gift of brilliance. whether it be due to atmospheric reflection alone or to the prevalence of clouds, venus is so bright that considerable doubt exists as to the actual visibility of any permanent markings on her surface. the detailed representations of the disk of venus by mr. percival lowell, showing in some respects a resemblance to the stripings of mars, can not yet be accepted as decisive. more experienced astronomers than mr. lowell have been unable to see at all things which he draws with a fearless and unhesitating pencil. that there are some shadowy features of the planet's surface to be seen in favourable circumstances is probable, but the time for drawing a "map of venus" has not yet come. the previous work of schiaparelli lends a certain degree of probability to mr. lowell's observations on the rotation of venus. this rotation, according to the original announcement of schiaparelli, is probably performed in the same period as the revolution around the sun. in other words, venus, if schiaparelli and lowell are right, always presents the same side to the sun, possessing, in consequence, a day hemisphere and a night hemisphere which never interchange places. this condition is so antagonistic to all our ideas of what constitutes habitability for a planet that one hesitates to accept it as proved, and almost hopes that it may turn out to have no real existence. venus, as the twin of the earth in size, is a planet which the imagination, warmed by its sunny aspect, would fain people with intelligent beings a little fairer than ourselves; but how can such ideas be reconciled with the picture of a world one half of which is subjected to the merciless rays of a never-setting sun, while the other half is buried in the fearful gloom and icy chill of unending night? any amateur observer who wishes to test his eyesight and his telescope in the search of shades or markings on the disk of venus by the aid of which the question of its rotation may finally be settled should do his work while the sun is still above the horizon. schiaparelli adopted that plan years ago, and others have followed him with advantage. the diffused light of day serves to take off the glare which is so serious an obstacle to the successful observation of venus when seen against a dark sky. knowing the location of venus in the sky, which can be ascertained from the ephemeris, the observer can find it by day. if his telescope is not permanently mounted and provided with "circles" this may not prove an easy thing to do, yet a little perseverance and ingenuity will effect it. one way is to find, with a star chart, some star whose declination is the same, or very nearly the same, as that of venus, and which crosses the meridian say twelve hours ahead of her. then set the telescope upon that star, when it is on the meridian at night, and leave it there, and the next day, twelve hours after the star crossed the meridian, look into your telescope and you will see venus, or, if not, a slight motion of the tube will bring her into view. for many amateurs the phases of venus will alone supply sufficient interest for telescopic observation. the changes in her form, from that of a round full moon when she is near superior conjunction to the gibbous, and finally the half-moon phase as she approaches her eastern elongation, followed by the gradually narrowing and lengthening crescent, until she is a mere silver sickle between the sun and the earth, form a succession of delightful pictures. not very much can be said for mercury as a telescopic object. the little planet presents phases like those of venus, and, according to schiaparelli and lowell, it resembles venus in its rotation, keeping always the same side to the sun. in fact, schiaparelli's discovery of this peculiarity in the case of mercury preceded the similar discovery in the case of venus. there are markings on mercury which have reminded some astronomers of the moon, and there are reasons for thinking that the planet can not be a suitable abode for living beings, at least for beings resembling the inhabitants of the earth. uranus and neptune are too far away to present any attraction for amateur observers. chapter ix the mountains and plains of the moon, and the spectacles of the sun "... the moon, whose orb the tuscan artist views through optic glass at evening from the top of fesolé, or in valdarno, to descry new lands, rivers or mountains in her spotty globe."--paradise lost. the moon is probably the most interesting of all telescopic objects. this arises from its comparative nearness to the earth. a telescope magnifying , diameters brings the moon within an apparent distance of less than miles. if telescopes are ever made with a magnifying power of , diameters, then, provided that atmospheric difficulties can be overcome, we shall see the moon as if it were only about twenty miles off, and a sensitive astronomer might be imagined to feel a little hesitation about gazing so closely at the moon--as if he were peering into a neighbor world's window. but a great telescope and a high magnifying power are not required to interest the amateur astronomer in the study of the moon. our three-inch telescope is amply sufficient to furnish us with entertainment for many an evening while the moon is running through its phases, and we shall find delight in frequently changing the magnifying power as we watch the lunar landscapes, because every change will present them in a different aspect. it should be remembered that a telescope, unless a terrestrial eyepiece or prism is employed, reverses such an object as the moon top for bottom. accordingly, if the moon is on or near the meridian when the observations are made, we shall see the north polar region at the bottom and the south polar region at the top. in other words, the face of the moon as presented in the telescope will be upside down, north and south interchanging places as compared with their positions in a geographical map. but east and west remain unaltered in position, as compared with such a map--i. e., the eastern hemisphere of the moon is seen on the right and the western hemisphere on the left. it is the moon's western edge that catches the first sunlight when "new moon" begins, and, as the phase increases, passing into "first quarter" and from that to "full moon," the illumination sweeps across the disk from west to east. [illustration: lunar chart no. , northwest quarter.] the narrow sickle of the new moon, hanging above the sunset, is a charming telescopic sight. use a low power, and observe the contrast between the bright, smooth round of the sunward edge, which has almost the polish of a golden rim, and the irregular and delicately shaded inner curve, where the adjacent mountains and plains picturesquely reflect or subdue the sunshine. while the crescent grows broader new objects are continually coming into view as the sun rises upon them, until at length one of the most conspicuous and remarkable of the lunar "seas," the _mare crisium_, or sea of crises, lies fully displayed amid its encircling peaks, precipices, and craters. the _mare crisium_ is all in the sunlight between the third and fourth day after "new moon." it is about by miles in extent, and if ever filled with water must have been a very deep sea, since its arid bed lies at a great but not precisely ascertained depth below the general level of the moon. there are a few small craters on the floor of the _mare crisium_, the largest bearing the name of picard, and its borders are rugged with mountains. on the southwestern side is a lofty promontory, , feet in height, called cape agarum. at the middle of the eastern side a kind of bay opens deep in the mountains, whose range here becomes very narrow. southeast of this bay lies a conspicuous bright point, the crater mountain proclus, on which the sun has fully risen in the fourth day of the moon, and which reflects the light with extraordinary liveliness. adjoining proclus on the east and south is a curious, lozenge-shaped flat, broken with short, low ridges, and possessing a most peculiar light-brown tint, easily distinguished from the general color tone of the lunar landscapes. it would be interesting to know what was passing in the mind of the old astronomer who named this singular region _palus somnii_. it is not the only spot on the moon which has been called a "marsh," and to which an unexplained connection with dreams has been ascribed. nearly on the same meridian with proclus, at a distance of about a hundred miles northward, lies a fine example of a ring mountain, rather more than forty miles in diameter, and with peak-tipped walls which in some places are , feet in height, as measured from the floor within. this is macrobius. there is an inconspicuous central mountain in the ring. north of the _mare crisium_, and northwest of macrobius, we find a much larger mountain ring, oblong in shape and nearly eighty miles in its greatest diameter. it is named cleomenes. the highest point on its wall is about , feet above the interior. near the northeast corner of the wall yawns a huge and very deep crater, tralles, while at the northern end is another oblong crater mountain called burckhardt. from cleomenes northward to the pole, or to the northern extremity of the crescent, if our observations are made during new moon, the ground appears broken with an immense number of ridges, craters, and mountain rings, among which we may telescopically wander at will. one of the more remarkable of these objects, which may be identified with the aid of lunar chart no. , is the vast ringed plain near the edge of the disk, named gauss. it is more than a hundred and ten miles in diameter. owing to its situation, so far down the side of the lunar globe, it is foreshortened into a long ellipse, although in reality it is nearly a circle. a chain of mountains runs north and south across the interior plain. geminus, berzelius, and messala are other rings well worth looking at. the remarkable pair called atlas and hercules demand more than passing attention. the former is fifty-five and the latter forty-six miles in diameter. each sinks , feet below the summit of the loftiest peak on its encircling wall. both are full of interesting detail sufficient to occupy the careful observer for many nights. the broad ring bearing the name of endymion is nearly eighty miles in diameter, and has one peak , feet high. the interior plain is flat and dark. beyond endymion on the edge of the disk is part of a gloomy plain called the _mare humboltianum_. after glancing at the crater-shaped mountains on the western and southern border of the _mare crisium_, alhazen, hansen, condorcet, firmicus, etc., we pass southward into the area covered in lunar chart no. . the long dark plain south of the _mare crisium_ is the _mare fecunditatis_, though why it should have been supposed to be particularly fecund, or fertile, is by no means clear. on the western border of this plain, about three hundred miles from the southern end of the _mare crisium_, is the mountain ring, or circumvallation, called langrenus, about ninety miles across and in places , feet high. there is a fine central mountain with a number of peaks. nearly a hundred miles farther south, on the same meridian, lies an equally extensive mountain ring named vendelinus. the broken and complicated appearance of its northern walls will command the observer's attention. another similar step southward, and still on the same meridian brings us to a yet finer mountain ring, slightly larger than the others, and still more complicated in its walls, peaks, and terraces, and in its surroundings of craters, gorges, and broken ridges. this is petavius. west of petavius, on the very edge of the disk, is a wonderful formation, a walled plain named humboldt, which is looked down upon at one point near its eastern edge by a peak , feet in height. about a hundred and forty miles south of petavius is the fourth great mountain ring lying on the same meridian. its name is furnerius. look particularly at the brilliantly shining crater on the northeast slope of the outer wall of furnerius. [illustration: lunar chart no. , southwest quarter.] suppose that our observations are now interrupted, to be resumed when the moon, about "seven days old," is in its first quarter. if we had time, it would be a most interesting thing to watch the advance of the lunar sunrise every night, for new beauties are displayed almost from hour to hour; but, for the purposes of our description it is necessary to curtail the observations. at first quarter one half of the lunar hemisphere which faces the earth is illuminated by the sun, and the line of sunrise runs across some of the most wonderful regions of the moon. we begin, referring once more to lunar chart no. , in the neighborhood of the north pole of the moon. here the line along which day and night meet is twisted and broken, owing to the roughness of the lunar surface. about fifteen degrees southwest of the pole lies a remarkable square-cornered, mountain-bordered plain, about forty miles in length, called barrow. very close to the pole is a ring mountain, about twenty-five miles in diameter, whose two loftiest peaks, , to , feet high, according to neison, must, from their situation, enjoy perpetual day. the long, narrow, dark plain, whose nearest edge is about thirty degrees south of the pole, is the _mare frigoris_, bordered on both sides by uplands and mountains. at its southern edge we find the magnificent aristoteles, a mountain ring, sixty miles across, whose immense wall is composed of terraces and ridges running up to lofty peaks, which rise nearly , feet above the floor of the valley. about a hundred miles south of aristoteles is eudoxus, another fine mountain ring, forty miles in diameter, and quite as deep as its northern neighbor. these two make a most striking spectacle. we are now in the neighborhood of the greatest mountain chains on the moon, the lunar alps lying to the east and the lunar caucasus to the south of aristoteles and eudoxus, while still farther south, separated from the caucasus by a strait not more than a hundred miles broad, begins the mighty range of the lunar apennines. we first turn the telescope on the alps. as the line of sunrise runs directly across their highest peaks, the effect is startling. the greatest elevations are about , feet. the observer's eye is instantly caught by a great valley, running like a furrow through the center of the mountain mass, and about eighty or ninety miles in length. the sealike expanse south and southeast of the alps is the _mare imbrium_, and it is along the coast of this so-called sea that the alps attain their greatest height. the valley, or gorge, above mentioned, appears to cut through the loftiest mountains and to reach the "coast," although it is so narrowed and broken among the greater peaks that its southern portion is almost lost before it actually reaches the _mare imbrium_. opening wider again as it enters the _mare_, it forms a deep bay among precipitous mountains. the caucasus mountains are not so lofty nor so precipitous as the alps, and consequently have less attraction for the observer. they border the dark, oval plain of the _mare serenitatis_ on its northeastern side. the great bay running out from the _mare_ toward the northwest, between the caucasus and the huge mountain ring of posidonius, bears the fanciful name of _lacus somniorum_. in the old days when the moon was supposed to be inhabited, those terrestrial godfathers, led by the astronomer riccioli, who were busy bestowing names upon the "seas" and mountains of our patient satellite, may have pleased their imagination by picturing this arm of the "serene sea" as a peculiarly romantic sheet of water, amid whose magical influences the lunar gentlefolk, drifting softly in their silver galleons and barges, and enjoying the splendors of "full earth" poured upon their delightful little world, were accustomed to fall into charming reveries, as even we hard-headed sons of adam occasionally do when the waters under the keel are calm and smooth and the balmy air of a moonlit night invokes the twin spirits of poetry and music. posidonius, the dominating feature of the shore line here, is an extraordinary example of the many formations on the moon which are so different from everything on the earth that astronomers do not find it easy to bestow upon them names that truly describe them. it may be called a ring mountain or a ringed plain, for it is both. its diameter exceeds sixty miles, and the interior plain lies about , feet below the outer surface of the lunar ground. the mountain wall surrounding the ring is by no means remarkable for elevation, its greatest height not exceeding , feet, but, owing to the broad sweep of the curved walls, the brightness of the plain they inclose, and the picturesque irregularity of the silhouette of shadow thrown upon the valley floor by the peaks encircling it, the effect produced upon the observer is very striking and attractive. having finished with posidonius and glanced across the broken region of the taurus mountains toward the west, we turn next to consider the _mare serenitatis_. this broad gray plain, which, with a slight magnifying power, certainly looks enough like a sea to justify the first telescopists in thinking that it might contain water, is about by miles in extent, its area being , square miles. running directly through its middle, nearly in a north and south line, is a light streak, which even a good opera glass shows. this streak is the largest and most wonderful of the many similar rays which extend on all sides from the great crater, or ring, of tycho in the southern hemisphere. the ray in question is more than , miles long, and, like its shorter congeners, it turns aside for nothing; neither "sea," nor peak, nor mountain range, nor crater ring, nor gorge, nor cañon, is able to divert it from its course. it ascends all heights and drops into all depths with perfect indifference, but its continuity is not broken. when the sun does not illuminate it at a proper angle, however, the mysterious ray vanishes. is it a metallic vein, or is it volcanic lava or ash? was the globe of the moon once split open along this line? the _mare serenitatis_ is encircled by mountain ranges to a greater extent than any of the other lunar "seas." on its eastern side the caucasus and the apennines shut it in, except for a strait a hundred miles broad, by means of which it is connected with the _mare imbrium_. on the south the range of the hæmus mountains borders it, on the north and northwest the caucasus and the taurus mountains confine it, while on the west, where again it connects itself by a narrow strait with another "sea," the _mare tranquilitatis_, it encounters the massive uplift of mount argæus. not far from the eastern strait is found the remarkable little crater named linné, not conspicuous on the gray floor of the _mare_, yet easily enough found, and very interesting because a considerable change of form seems to have come over this crater some time near the middle of the nineteenth century. in referring to it as a crater it must not be forgotten that it does not form an opening in the top of a mountain. in fact, the so-called craters on the moon, generally speaking, are simply cavities in the lunar surface, whose bottoms lie deep below the general level, instead of being elevated on the summit of mountains, and inclosed in a conical peak. in regard to the alleged change in linné, it has been suggested, not that a volcanic eruption brought it about, but that a downfall of steep walls, or of an unsupported rocky floor, was the cause. the possibility of such an occurrence, it must be admitted, adds to the interest of the observer who regularly studies the moon with a telescope. just on the southern border of the _mare_, the beautiful ring menelaus lies in the center of the chain of the hæmus mountains. the ring is about twenty miles across, and its central peak is composed of some highly reflecting material, so that it shines very bright. the streak or ray from tycho which crosses the _mare serenitatis_ passes through the walls of menelaus, and perhaps the central peak is composed of the same substance that forms the ray. something more than a hundred miles east-southeast from menelaus, in the midst of the dark _mare vaporum_, is another brilliant ring mountain which catches the eye, manilius. it exceeds menelaus in brightness as well as in size, its diameter being about twenty-five miles. there is something singular underlying the dark lunar surface here, for not only is manilius extraordinarily brilliant in contrast with the surrounding plain, but out of that plain, about forty miles toward the east, projects a small mountain which is also remarkable for its reflecting properties, as if the gray ground were underlain by a stratum of some material that flashes back the sunlight wherever it is exposed. the crater mountain, sulpicius gallus, on the border of the _mare_, north of manilius and east of menelaus, is another example of the strange shining quality of certain formations on the moon. follow next the hæmus range westward until the attention falls upon the great ring mountain plinius, more than thirty miles across, and bearing an unusual resemblance to a fortification. mr. t. g. elger, the celebrated english selenographer, says of plinius that, at sunrise, "it reminds one of a great fortress or redoubt erected to command the passage between the _mare tranquilitatis_ and the _mare serenitatis_." but, of course, the resemblance is purely fanciful. men, even though they dwelt in the moon, would not build a rampart , feet high! mount argæus, at the southwest corner of the _mare serenitatis_, is a very wonderful object when the sun has just risen upon it. this occurs five days after the new moon. returning to the eastern extremity of the _mare_, we glance, in passing, at the precipitous mount hadley, which rises more than , feet above the level of the _mare_ and forms the northern point of the apennine range. passing into the region of the _mare imbrium_, whose western end is divided into the _palus putredinis_ on the south and the _palus nebularum_ on the north, we notice three conspicuous ring mountains, cassini near the alps, and aristillus and autolycus, a beautiful pair, nearly opposite the strait connecting the two _maria_. cassini is thirty-six miles in diameter, aristillus thirty-four, and autolycus twenty-three. the first named is shallow, only , feet in depth from the highest point of its wall, while aristillus carries some peaks on its girdle , feet high. autolycus, like cassini, is of no very great depth. westward from the middle of an imaginary line joining aristillus and cassini is the much smaller crater theætetus. outside the walls of this are a number of craterlets, and a french astronomer, charbonneaux, of the meudon observatory, reported in december, , that he had repeatedly observed white clouds appearing and disappearing over one of these small craters. south of the _mare vaporum_ are found some of the most notable of those strange lunar features that are called "clefts" or "rills." two crater mountains, in particular, are connected with them, ariadæus at the eastern edge of the _mare tranquilitatis_ and hyginus on the southern border of the _mare vaporum_. these clefts appear to be broad and deep chasms, like the cañons cut by terrestrial rivers, but it can not be believed that the lunar cañons are the work of rivers. they are rather cracks in the lunar crust, although their bottoms are frequently visible. the principal cleft from ariadæus runs eastward and passes between two neighboring craters, the southern of which is named silberschlag, and is noteworthy for its brightness. the hyginus cleft is broader and runs directly through the crater ring of that name. the observer will find much to interest him in the great, irregular, and much-broken mountain ring called julius cæsar, as well as in the ring mountains, godin, agrippa, and triesnecker. the last named, besides presenting magnificent shadows when the sunlight falls aslant upon it, is the center of a complicated system of rills, some of which can be traced with our five-inch glass. we next take up lunar chart no. , and pay a telescopic visit to the southwestern quarter of the lunar world. the _mare tranquilitatis_ merges through straits into two southern extensions, the _mare fecunditatis_ and the _mare nectaris_. the great ring mountains or ringed plains, langrenus, vendelinus, petavius, and furnerius, all lying significantly along the same lunar meridian, have already been noticed. their linear arrangement and isolated position recall the row of huge volcanic peaks that runs parallel with the shore of the pacific ocean in oregon and washington--mount jefferson, mount hood, mount st. helen's, mount tacoma--but these terrestrial volcanoes, except in elevation, are mere pins' heads in the comparison. in the eastern part of the _mare fecunditatis_ lies a pair of relatively small craters named messier, which possess particular interest because it has been suspected, though not proved, that a change of form has occurred in one or other of the pair. mädler, in the first half of the nineteenth century, represented the two craters as exactly alike in all respects. in webb discovered that they are not alike in shape, and that the easternmost one is the larger, and every observer easily sees that webb's description is correct. messier is also remarkable for the light streak, often said to resemble a comet's tail, which extends from the larger crater eastward to the shore of the _mare fecunditatis_. goclenius and guttemberg, on the highland between the _mare fecunditatis_ and the _mare nectaris_, are intersected and surrounded by clefts, besides being remarkable for their broken and irregular though lofty walls. guttemberg is forty-five miles and goclenius twenty-eight miles in diameter. the short mountain range just east of guttemberg, and bordering a part of the _mare nectaris_ on the west, is called the pyrenees. the _mare nectaris_, though offering in its appearance no explanation of its toothsome name--perhaps it was regarded as the drinking cup of the olympian gods--is one of the most singular of the dark lunar plains in its outlines. at the south it ends in a vast semicircular bay, sixty miles across, which is evidently a half-submerged mountain ring. but submerged by what? not water, but perhaps a sea of lava which has now solidified and forms the floor of the _mare nectaris_. the name of this singular formation is fracastorius. elger has an interesting remark about it. "on the higher portion of the interior, near the center," he says, "is a curious object consisting apparently of four light spots, arranged in a square, with a craterlet in the middle, all of which undergo notable changes of aspect under different phases." other writers also call attention to the fine markings, minute craterlets, and apparently changeable spots on the floor of fracastorius. we go now to the eastern side of the _mare nectaris_, where we find one of the most stupendous formations in the lunar world, the great mountain ring of theophilus, noticeably regular in outline and perfect in the completeness of its lofty wall. the circular interior, which contains in the center a group of mountains, one of whose peaks is , feet high, sinks , feet below the general level of the moon outside the wall! one of the peaks on the western edge towers more than , feet above the floor within, while several other peaks attain elevations of , to , feet. the diameter of the immense ring, from crest to crest of the wall, is sixty-four miles. theophilus is especially wonderful on the fifth and sixth days of the moon, when the sun climbs its shining pinnacles and slowly discloses the tremendous chasm that lies within its circles of terrible precipices. on the southeast theophilus is connected by extensions of its walls with a shattered ring of vast extent called cyrillus; and south from cyrillus, and connected with the same system of broken walls, lies the still larger ring named catharina, whose half-ruined walls and numerous crater pits present a fascinating spectacle as the shadows retreat before the sunrise advancing across them. these three--theophilus, cyrillus, and catharina--constitute a scene of surpassing magnificence, a glimpse of wonders in another world sufficient to satisfy the most riotous imagination. south of the _mare nectaris_ the huge ring mountain of piccolomini attracts attention, its massive walls surrounding a floor nearly sixty miles across, and rising in some places to an altitude of nearly , feet. it should be understood that wherever the height of the mountain wall of such a ring is mentioned, the reference level is that of the interior plain or floor. the elevation, reckoned from the outer side, is always very much less. the entire region south and east of theophilus and its great neighbors is marvelously rough and broken. approaching the center of the moon, we find a system of ringed plains even greater in area than any of those we have yet seen. hipparchus is nearly a hundred miles long from north to south, and nearly ninety miles broad from east to west. but its walls have been destroyed to such an extent that, after all, it yields in grandeur to a formation like theophilus. albategnius is sixty-five miles across, with peaks from , to , feet in height. sacrobosco is a confused mass of broken and distorted walls. aliacensis is remarkable for having a peak on the eastern side of its wall which is more than , feet high. werner, forty-five miles in diameter, is interesting because under its northeastern wall mädler, some seventy years ago, saw a light spot of astonishing brightness, unmatched in that respect by anything on the moon except the peak of aristarchus, which we shall see later. this spot seems afterward to have lost brilliance, and the startling suggestion has been made that its original brightness might have been due to its then recent deposit from a little crater that lies in the midst of it. walter is of gigantic dimensions, about one hundred miles in diameter. unlike the majority of the ringed plains, it departs widely from a circle. stöfler is yet larger than walter; but most interesting of all these gigantic formations is maurolycus, whose diameter exceeds one hundred and fifty miles, and which has walls , or , feet high. yet, astonishing though it may seem, this vast and complicated mass of mountain walls, craters, and peaks, is virtually unseen at full moon, owing to the perpendicularity of the sunlight, which prevents the casting of shadows. we shall next suppose that another period of about seven days has elapsed, the moon in the meantime reaching its full phase. we refer for guidance to lunar chart no. . the peculiarity of the northeastern quadrant which immediately strikes the eye is the prevalence of the broad plains called _maria_, or "seas." the northern and central parts are occupied by the _mare imbrium_, the "sea of showers" or of "rains," with its dark bay the _sinus Æstuum_, while the eastern half is covered by the vast _oceanus procellarum_, the "ocean of storms" or of "tempests." toward the north a conspicuous oval, remarkably dark in hue, immediately attracts our attention. it is the celebrated ringed plain of plato, about sixty miles in diameter and surrounded by a saw-edged rampart, some of whose pinnacles are , or , feet high. plato is a favorite subject for study by selenographers because of the changes of color which its broad, flat floor undergoes as the sun rises upon it, and also because of the existence of enigmatical spots and streaks whose visibility changes. south of plato, in the _mare imbrium_, rises a precipitous, isolated peak called pico, , feet in height. its resemblance in situation to the conical mountain pico in the azores strikes the observer. [illustration: lunar chart no. , northeast quarter.] eastward of plato a line of highlands, separating the _mare imbrium_ from the _mare frigoris,_ carries the eye to the beautiful semicircular _sinus iridum_, or "bay of rainbows." the northwestern extremity of this remarkable bay is guarded by a steep and lofty promontory called cape laplace, while the southeastern extremity also has its towering guardian, cape heraclides. the latter is interesting for showing, between nine and ten days after full moon, a singularly perfect profile of a woman's face looking out across the _mare imbrium_. the winding lines, like submerged ridges, delicately marking the floor of the _sinus iridum_ and that of the _mare_ beyond, are beautiful telescopic objects. the "bay" is about one hundred and thirty-five miles long by eighty-four broad. the _mare imbrium_, covering , square miles, is sparingly dotted over with craters. all of the more conspicuous of them are indicated in the chart. the smaller ones, like caroline herschel, helicon, leverrier, délisle, etc., vary from eight to twelve miles in diameter. lambert is seventeen miles in diameter, and euler nineteen, while timocharis is twenty-three miles broad and , feet deep below its walls, which rise only , feet above the surface of the _mare_. toward the eastern border of the sea, south of the harbinger mountains, we find a most remarkable object, the mountain ring, or crater plain, called aristarchus. this ring is not quite thirty miles in diameter, but there is nothing on the moon that can compare with it in dazzling brilliance. the central peak, , or , feet high, gleams like a mountain of crusted snow, or as if it were composed of a mass of fresh-broken white metal, or of compacted crystals. part of the inner slope of the east wall is equally brilliant. in fact, so much light is poured out of the circumvallation that the eye is partially blinded, and unable distinctly to see the details of the interior. no satisfactory explanation of the extraordinary reflecting power of aristarchus has ever been offered. its neighbor toward the east, herodotus, is somewhat smaller and not remarkably bright, but it derives great interest from the fact that out of a breach in its northern wall issues a vast cleft, or chasm, which winds away for nearly a hundred miles across the floor of the _mare_, making an abrupt turn when it reaches the foot of the harbinger mountains. the comparatively small crater, lichtenberg, near the northeastern limb of the moon, is interesting because mädler used to see in its neighborhood a pale-red tint which has not been noticed since his day. returning to the western side of the quadrant represented in lunar chart no. , we see the broad and beautifully regular ringed plain of archimedes, fifty miles in diameter and , feet deep. a number of clefts extend between the mountainous neighborhood of archimedes and the feet of the gigantic apennine mountains on the southwest. the little double crater, beer, between archimedes and timocharis, is very bright. the apennines extend about four hundred and eighty miles in a northwesterly and southeasterly direction. one of their peaks near the southern end of the range, mount huygens, is at least , feet high, and the black silhouettes of their sharp-pointed shadows thrown upon the smooth floor of the _mare imbrium_ about the time of first quarter present a spectacle as beautiful as it is unique. the apennines end at the southeast in the ring mountain, eratosthenes, thirty-eight miles across and very deep, one of its encircling chain of peaks rising , feet above the floor, and about half that height above the level of the _mare imbrium_. the shadows cast by eratosthenes at sunrise are magnificent. and now we come to one of the supreme spectacles of the moon, the immense ring or crater mountain copernicus. this is generally regarded as the grandest object that the telescope reveals on the earth's satellite. it is about fifty-six miles across, and its interior falls to a depth of , feet below the _mare imbrium_. its broad wall, composed of circle within circle of ridges, terraces, and precipices, rises on the east about , feet above the floor. on the inner side the slopes are very steep, cliff falling below cliff, until the bottom of the fearful abyss is attained. to descend those precipices and reach the depressed floor of copernicus would be a memorable feat for a mountaineer. in the center of the floor rises a complicated mountain mass about , feet high. all around copernicus the surface of the moon is dotted with countless little crater pits, and splashed with whitish streaks. northward lie the carpathian mountains, terminating on the east in tobias mayer, a ring mountain more than twenty miles across. the mountain ring kepler, which is also the center of a great system of whitish streaks and splashes, is twenty-two miles in diameter, and notably brilliant. finally, we turn to the southeastern quadrant of the moon, represented in lunar chart no. . the broad, dark expanse extending from the north is the _mare nubium_ on the west and the _oceanus procellarum_ on the east. toward the southeast appears the notably dark, rounded area of the _mare humorum_ inclosed by highlands and rings. we begin with the range of vast inclosures running southward near the central meridian, and starting with ptolemæus, a walled plain one hundred and fifteen miles in its greatest diameter and covering an area considerably exceeding that of the state of massachusetts. its neighbor toward the south, alphonsus, is eighty-three miles across. next comes arzachel, more than sixty-five miles in diameter. thebit, more than thirty miles across, is very deep. east of thebit lies the celebrated "lunar railroad," a straight, isolated wall about five hundred feet high and sixty-five miles long, dividing at its southern end into a number of curious branches, forming the buttresses of a low mountain. purbach is sixty miles broad, and south of that comes a wonderful region where the ring mountains hell, ball, lexell, and others, more or less connected with walls, inclose an area even larger than ptolemæus, but which, not being so distinctly bordered as some of the other inclosed plains, bears no distinctive name. [illustration: lunar chart no. , southeast quarter.] the next conspicuous object toward the south ranks with copernicus among the grandest of all lunar phenomena--the ring, or crater, tycho. it is about fifty-four miles in diameter and some points on its wall rise , feet above the interior. in the center is a bright mountain peak , feet high. but wonderful as are the details of its mountain ring, the chief attraction of tycho is its manifest relation to the mysterious bright rays heretofore referred to, which extend far across the surface of the moon in all directions, and of which it is the center. the streaks about copernicus are short and confused, constituting rather a splash than a regular system of rays; but those emanating from tycho are very long, regular, comparatively narrow, and form arcs of great circles which stretch away for hundreds of miles, allowing no obstacle to interrupt their course. southwest of tycho lies the vast ringed plain of maginus, a hundred miles broad and very wonderful to look upon, with its labyrinth of formations, when the sun slopes across it, and yet, like maurolycus, invisible under a vertical illumination. "the full moon," to use mädler's picturesque expression, "knows no maginus." still larger and yet more splendid is clavius, which exceeds one hundred and forty miles in diameter and covers , square miles of ground within its fringing walls, which carry some of the loftiest peaks on the moon, several attaining , feet. the floor is deeply depressed, so that an inhabitant of this singular inclosure, larger than massachusetts, connecticut, and rhode island combined, would dwell in land sunk two miles below the general level of the world about him. in the neighborhood of the south pole lies the large walled plain of newton, whose interior is the deepest known depression on the moon. it is so deep that the sunshine never touches the larger part of the floor of the inner abyss, and a peak on its eastern wall rises , feet sheer above the tremendous pit. other enormous walled plains are longomontanus, wilhelm i, schiller, bailly, and schickard. the latter is one hundred and thirty-four miles long and bordered by a ring varying from , to , feet in height. wargentin, the oval close to the moon's southeast limb, beyond schickard, is a unique formation in that, instead of its interior being sunk below the general level, it is elevated above it. it has been suggested that this peculiarity is due to the fact that the floor of wargentin was formed by inflation from below, and that it has cooled and solidified in the shape of a gigantic dome arched over an immense cavity beneath. a dome of such dimensions, however, could not retain its form unless partly supported from beneath. hainzel is interesting from its curious outline; cichus for the huge yawning crater on its eastern wall; capuanus for a brilliant shining crater also on its eastern wall; and mercator for possessing bright craters on both its east and its west walls. vitello has a bright central mountain and gains conspicuousness from its position at the edge of the dark _mare humorum_. agatharchides is the broken remnant of a great ring mountain. gassendi, an extremely beautiful object, is about fifty-five miles across. it is encircled with broken walls, craters and bright points, and altogether presents a very splendid appearance about the eleventh day of the moon's age. letronne is a half-submerged ring, at the southern end of the _oceanus procellarum_, which recalls fracastorius in the western lunar hemisphere. it lies, however, ten degrees nearer the equator than fracastorius. billy is a mountain ring whose interior seems to have been submerged by the dark substance of the _oceanus procellarum_, although its walls have remained intact. mersenius is a very conspicuous ring, forty miles in diameter, east of the _mare humorum_. vieta, fifty miles across, is also a fine object. grimaldi, a huge dusky oval, is nearly one hundred and fifty miles in its greatest length. the ring mountain landsberg, on the equator, and near the center of the visible eastern hemisphere, is worth watching because elger noticed changes of color in its interior in . bullialdus, in the midst of the _mare nubium_, is a very conspicuous and beautiful ring mountain about thirty-eight miles in diameter, with walls , feet high above the interior. those who wish to see the lunar mountains in all their varying aspects will not content themselves with views obtained during the advance of the sunlight from west to east, between "new moon" and "full moon," but will continue their observations during the retreat of the sunlight from east to west, after the full phase is passed. it is evident that the hemisphere of the moon which is forever turned away from the earth is quite as marvelous in its features as the part that we see. it will be noticed that the entire circle of the moon's limb, with insignificant interruptions, is mountainous. possibly the invisible side of our satellite contains yet grander peaks and crater mountains than any that our telescopes can reach. this probability is increased by the fact that the loftiest known mountain on the moon is never seen except in silhouette. it is a member of a great chain that breaks the lunar limb west of the south pole, and that is called the leibnitz mountains. the particular peak referred to is said by some authorities to exceed , feet in height. other great ranges seen only in profile are the dörfel mountains on the limb behind the ring plain bailly, the cordilleras, east of eichstadt, and the d'alembert mountains beyond grimaldi. the profile of these great mountains is particularly fine when they are seen during an eclipse of the sun. then, with the disk of the sun for a background, they stand out with startling distinctness. the sun when the sun is covered with spots it becomes a most interesting object for telescopic study. every amateur's telescope should be provided with apparatus for viewing the sun. a dark shade glass is not sufficient and not safe. what is known as a solar prism, consisting of two solid prisms of glass, cemented together in a brass box which carries a short tube for the eyepiece, and reflecting an image of the sun from their plane of junction--while the major remnant of light and heat passes directly through them and escapes from an opening provided for the purpose--serves very well. better and more costly is an apparatus called a helioscope, constructed on the principle of polarization and provided with prisms and reflectors which enable the observer, by proper adjustment, to govern very exactly and delicately the amount of light that passes into the eyepiece. furnished with an apparatus of this description we can employ either a three-, four-, or five-inch glass upon the sun with much satisfaction. for the amateur's purposes the sun is only specially interesting when it is spotted. the first years of the twentieth century will behold a gradual growth in the number and size of the solar spots as those years happen to coincide with the increasing phase of the sun-spot period. large sun spots and groups of spots often present an immense amount of detail which tasks the skill of the draughtsman to represent it. but a little practice will enable one to produce very good representations of sun spots, as well as of the whitish patches called faculæ by which they are frequently surrounded. for the simple purpose of exhibiting the spotted face of the sun without much magnifying power, a telescope may be used to project the solar image on a white sheet or screen. if the experiment is tried in a room, a little ingenuity will enable the observer to arrange a curtain covering the window used, in such a way as to exclude all the light except that which comes through the telescope. then, by placing a sheet of paper or a drawing board before the eyepiece and focusing the image of the sun upon it, very good results may be obtained. if one has a permanent mounting and a driving clock, a small spectroscope may be attached, for solar observations, even to a telescope of only four or five inches aperture, and with its aid most interesting views may be obtained of the wonderful red hydrogen flames that frequently appear at the edge of the solar disk. chapter x are there planets among the stars? "... and if there should be worlds greater than thine own, inhabited by greater things, and they themselves far more in number than the dust of thy dull earth, what wouldst thou think?"--byron's cain. this always interesting question has lately been revived in a startling manner by discoveries that have seemed to reach almost deep enough to touch its solution. the following sentences, from the pen of dr. t. j. j. see, of the lowell observatory, are very significant from this point of view: "our observations during -' have certainly disclosed stars more difficult than any which astronomers had seen before. among these obscure objects about half a dozen are truly wonderful, in that they seem to be dark, almost black in color, and apparently are shining by a dull reflected light. it is unlikely that they will prove to be self-luminous. if they should turn out dark bodies in fact, shining only by the reflected light of the stars around which they revolve, we should have the first case of planets--dark bodies--noticed among the fixed stars." of course, dr. see has no reference in this statement to the immense dark bodies which, in recent years, have been discovered by spectroscopic methods revolving around some of the visible stars, although invisible themselves. the obscure objects that he describes belong to a different class, and might be likened, except perhaps in magnitude, to the companion of sirius, which, though a light-giving body, exhibits nevertheless a singular defect of luminosity in relation to its mass. sirius has only twice the mass, but ten thousand times the luminosity, of its strange companion! yet the latter is evidently rather a faint, or partially extinguished, sun than an opaque body shining only with light borrowed from its dazzling neighbor. the objects seen by dr. see, on the contrary, are "apparently shining by a dull reflected light." if, however (as he evidently thinks is probable), these objects should prove to be really non-luminous, it would not follow that they are to be regarded as more like the planets of the solar system than like the dark companions of certain other stars. a planet, in the sense which we attach to the word, can not be comparable in mass and size with the sun around which it revolves. the sun is a thousand times larger than the greatest of its attendant planets, jupiter, and more than a million times larger than the earth. it is extremely doubtful whether the relation of sun and planet could exist between two bodies of anything like equal size, or even if one exceeded the other many times in magnitude. it is only when the difference is so great that the smaller of the two bodies is insignificant in comparison with the larger, that the former could become a cool, life-bearing globe, nourished by the beneficent rays of its organic comrade and master. judged by our terrestrial experience, which is all we have to go by, the magnitude of a planet, if it is to bear life resembling that of the earth, is limited by other considerations. even jupiter, which, as far as our knowledge extends, represents the extreme limit of great planetary size, may be too large ever to become the abode of living beings of a high organization. the force of gravitation on the surface of jupiter exceeds that on the earth's surface as . to . considering the effects of this on the weight and motion of bodies, the density of the atmosphere, etc., it is evident that jupiter would, to say the very least, be an exceedingly uncomfortable place of abode for beings resembling ourselves. but jupiter, if it is ever to become a solid, rocky globe like ours, must shrink enormously in volume, since its density is only . as compared with the earth. now, the surface gravity of a planet depends on its mass and its radius, being directly as the former and inversely as the square of the latter. but in shrinking jupiter will lose none of its mass, although its radius will become much smaller. the force of gravity will consequently increase on its surface as the planet gets smaller and more dense. the present mean diameter of jupiter is , miles, while its mass exceeds that of the earth in the ratio of to . suppose jupiter shrunk to three quarters of its present diameter, or , miles, then its surface gravity would exceed the earth's nearly five times. with one half its present diameter the surface gravity would become more than ten times that of the earth. on such a planet a man's bones would snap beneath his weight, even granting that he could remain upright at all! it would seem, then, that, unless we are to abandon terrestrial analogies altogether and "go it blind," we must set an upper limit to the magnitude of a habitable planet, and that jupiter represents such upper limit, if, indeed, he does not transcend it. the question then becomes, can the faint objects seen by dr. see and his fellow-observers, in the near neighborhood of certain stars, be planets in the sense just described, or are they necessarily far greater in magnitude than the largest planet, in the accepted sense of that word, which can be admitted into the category--viz., the planet jupiter? this resolves itself into another question: at what distance would jupiter be visible with a powerful telescope, supposing it to receive from a neighboring star an amount of illumination not less than that which it gets from the sun? to be sure, we do not know how far away the faint objects described by dr. see are; but, at any rate, we can safely assume that they are at the distance of the nearest stars, say somewhere about three hundred thousand times the earth's distance from the sun. the sun itself removed to that distance would appear to our eyes only as a star of the first magnitude. but zöllner has shown that the sun exceeds jupiter in brilliancy , , , times. seen from equal distances, however, the ratio would be about , , to . this would be the ratio of their light if both sun and jupiter could be removed to about the distance of the nearest stars. since the sun would then be only as bright as one of the stars of the first magnitude, and since jupiter would be , , times less brilliant, it is evident that the latter would not be visible at all. the faintest stars that the most powerful telescopes are able to show probably do not fall below the sixteenth or, at the most, the seventeenth magnitude. but a seventeenth-magnitude star is only between two and three million times fainter than the sun would appear at the distance above supposed, while, as we have seen, jupiter would be more than two hundred million times fainter than the sun. to put it in another way: jupiter, at the distance of the nearest stars, would be not far from one hundred times less bright than the faintest star which the largest telescope is just able, under the most exquisite conditions, to glimpse. to see a star so faint as that would require an object-glass of a diameter half as great as the length of the tube of the lick telescope, or say thirty feet! of course, jupiter might be more brilliantly illuminated by a brighter star than the sun; but, granting that, it still would not be visible at such a distance, even if we neglect the well-known concealing or blinding effect of the rays of a bright star when the observer is trying to view a faint one close to it. clearly, then, the obscure objects seen by dr. see near some of the stars, if they really are bodies visible only by light reflected from their surfaces, must be enormously larger than the planet jupiter, and can not, accordingly, be admitted into the category of planets proper, whatever else they may be. perhaps they are extreme cases of what we see in the system of sirius--i.e., a brilliant star with a companion which has ceased to shine as a star while retaining its bulk. such bodies may be called planets in that they only shine by reflected light, and that they are attached to a brilliant sun; but the part that they play in their systems is not strictly planetary. owing to their great mass they bear such sway over their shining companions as none of our planets, nor all of them combined, can exercise; and for the same reason they can not, except in a dream, be imagined to possess that which, in our eyes, must always be the capital feature of a planet, rendering it in the highest degree interesting wherever it may be found--sentient life. it does not follow, however, that there are no real planetary bodies revolving around the stars. as dr. see himself remarks, such insignificant bodies as our planets could not be seen at the distance of the fixed stars, "even if the power of our telescopes were increased a hundredfold, and consequently no such systems are _known_." this brings me to another branch of the subject. in the same article from which i have already quoted (recent discoveries respecting the origin of the universe, atlantic monthly, vol. lxxx, pages - ), dr. see sets forth the main results of his well-known studies on the origin of the double and multiple star systems. he finds that the stellar systems differ from the solar system markedly in two respects, which he thus describes: " . the orbits are highly eccentric; on the average twelve times more elongated than those of the planets and satellites. " . the components of the stellar systems are frequently equal and always comparable in mass, whereas our satellites are insignificant compared to their planets, and the planets are equally small compared to the sun." these peculiarities of the star systems dr. see ascribes to the effect of "tidal friction," the double stars having had their birth through fission of original fluid masses (just as the moon, according to george darwin's theory, was born from the earth), and the reaction of tidal friction having not only driven them gradually farther apart but rendered their orbits more and more eccentric. this manner of evolution of a stellar system dr. see contrasts with laplace's hypothesis of the origin of the planetary system through the successive separation of rings from the periphery of the contracting solar nebula, and the gradual breaking up of those rings and their aggregation into spherical masses or planets. while not denying that the process imagined by laplace may have taken place in our system, he discovers no evidence of its occurrence among the double stars, and this leads him to the following statement, in which believers in the old theological doctrine that the earth is the sole center of mortal life and of divine care would have found much comfort: "it is very singular that no visible system yet discerned has any resemblance to the orderly and beautiful system in which we live; and one is thus led to think that probably our system is unique in its character. at least it is unique among all _known_ systems." if we grant that the solar system is the only one in which small planets exist revolving around their sun in nearly circular orbits, then indeed we seem to have closed all the outer universe against such beings as the inhabitants of the earth. beyond the sun's domain only whirling stars, coupled in eccentric orbits, dark stars, some of them, but no planets--in short a wilderness, full of all energies except those of sentient life! this is not a pleasing picture, and i do not think we are driven to contemplate it. beyond doubt, dr. see is right in concluding that double and multiple star systems, with their components all of magnitudes comparable among themselves, revolving in exceedingly eccentric orbits under the stress of mutual gravitation, bear no resemblance to the orderly system of our sun with its attendant worlds. and it is not easy to imagine that the respective members of such systems could themselves be the centers of minor systems of planets, on account of the perturbing influences to which the orbits of such minor systems would be subjected. but the double and multiple stars, numerous though they be, are outnumbered a hundred to one by the single stars which shine alone as our sun does. what reason can we have, then, for excluding these single stars, constituting as they do the vast majority of the celestial host, from a similarity to the sun in respect to the manner of their evolution from the original nebulous condition? these stars exhibit no companions, such planetary attendants as they may have lying, on account of their minuteness, far beyond the reach of our most powerful instruments. but since they vastly outnumber the binary and multiple systems, and since they resemble the sun in having no large attendants, should we be justified, after all, in regarding our system as "unique"? it is true we do not know, by visual evidence, that the single stars have planets, but we find planets attending the only representative of that class of stars that we are able to approach closely--the sun--and we know that the existence of those planets is no mere accident, but the result of the operation of physical laws which must hold good in every instance of nebular condensation. two different methods are presented in which a rotating and contracting nebula may shape itself into a stellar or planetary system. the first is that described by laplace, and generally accepted as the probable manner of origin of the solar system--viz., the separation of rings from the condensing mass, and the subsequent transformation of the rings into planets. the planet saturn is frequently referred to as an instance of the operation of this law, in which the evolution has been arrested after the separation of the rings, the latter having retained the ring form instead of breaking and collecting into globes, forming in this case rings of meteorites, and reminding us of the comparatively scattered rings of asteroids surrounding the sun between the orbits of mars and jupiter. this laplacean process dr. see regards as theoretically possible, but apparently he thinks that if it took place it was confined to our system. the other method is that of the separation of the original rotating mass into two nearly equal parts. the mechanical possibility of such a process has been proved, mathematically, by poincaré and darwin. this, dr. see thinks, is the method which has prevailed among the stars, and prevailed to such a degree as to make the solar system, formed by the ring method, probably a unique phenomenon in the universe. is it not more probable that both methods have been in operation, and that, in fact, the ring method has operated more frequently than the other? if not, why do the single stars so enormously outnumber the double ones? it is of the essence of the fission process that the resulting masses should be comparable in size. if, then, that process has prevailed in the stellar universe to the practical exclusion of the other, there should be very few single stars; whereas, as a matter of fact, the immense majority of the stars are single. and, remembering that the sun viewed from stellar distances would appear unattended by subsidiary bodies, are we not justified in concluding that its origin is a type of the origin of the other single stars? while it is, as i have remarked, of the essence of the fission process that the resulting parts of the divided mass should be comparable in magnitude, it is equally of the essence of the ring, or laplacean process, that the bodies separated from the original mass should be comparatively insignificant in magnitude. as to the coexistence of the two processes, we have, perhaps, an example in the solar system itself. darwin's demonstration of the possible birth of the moon from the earth, through fission and tidal friction, does not apply to the satellites attending the other planets. the moon is relatively a large body, comparable in that respect with the earth, while the satellites of jupiter and saturn, for instance, are relatively small. but in the case of saturn there is visible evidence that the ring process of satellite formation has prevailed. the existing rings have not broken up, but their very existence is a testimony of the origin of the satellites exterior to them from other rings which did break up. thus we need not go as far away as the stars in order to find instances illustrating both the methods of nebular evolution that we have been dealing with. the conclusion, then, seems to be that we are not justified in assuming that the solar system is unique simply because it differs widely from the double and multiple star systems; and that we should rather regard it as probable that the vast multitude of stars which do not appear, when viewed with the telescope, or studied by spectroscopic methods, to have any attendants comparable with themselves in magnitude, have originated in a manner resembling that of the sun's origin, and may be the centers of true planetary systems like ours. the argument, i think, goes further than to show the mere possibility of the existence of such planetary systems surrounding the single stars. if those stars did not originate in a manner quite unlike the origin of the sun, then the existence of planets in their neighborhood is almost a foregone conclusion, for the sun could hardly have passed through the process of formation out of a rotating nebula without evolving planets during its contraction. and so, notwithstanding the eccentricities of the double stars, we may still cherish the belief that there are eyes to see and minds to think out in celestial space. index note.--double, triple, multiple, and colored stars, star clusters, nebulæ, and temporary stars will be found arranged under the heads of their respective constellations. andromeda, map no. , . stars: alpha, . gamma, . , . , . temporary star: , . cluster: , . variable: r, . nebula: , . aquarius, map no. , . stars: zeta, . tau, . psi, . , . sigma , . sigma ( ), . sigma , . variables: r, . s, . t, . nebulæ: (rosse's "saturn"), . , . aquila, map no. , . stars: pi, . , . , . , . sigma , . sigma , . cluster: , . variables: eta, . r, . argo: map no. , ; map no. , . stars: sigma , . sigma ( ), . clusters: , . , . , . , . nebula: , . aries, map no. , . stars: gamma, . epsilon, . lambda, . pi, . , . , . , . , . sigma , . auriga, map no. , . stars: alpha (capella), . beta (menkalina), . epsilon, . theta, . lambda, . , . , . , . sigma , . temporary star: , . clusters: , . , . , . , . , . boÖtes, map no. , . stars: alpha (arcturus), . delta, . epsilon (mirac), . zeta, . iota, . kappa, . , . xi, . pi, . sigma , . sigma ( ), . sigma ( ), . sigma ( ), . sigma , . camelopardalus, map no. , . stars: , . , . , . sigma , . sigma , . cluster: , . canes venatici, map no. , ; map no. , . stars: , . (cor caroli), . sigma , . sigma ( ), . cluster: , . nebula: , . canis major, map no. , . stars: alpha (sirius), . delta, . , . clusters: , . , . , . variable: gamma, . nebula: , . canis minor, map no. , . stars: alpha (procyon), . , . sigma ( can. min. bode), . cancer, map no. , . stars: zeta, . iota, . , . sigma , . sigma , . sigma , . clusters: præsepe, . , . capricornus, map no. , ; map no. , . stars: alpha, . beta, . omicron, . pi, . rho, . sigma, . cluster: , . cassiopeia, map no. , . stars: eta, . iota, . sigma, . psi, . temporary star: (tycho's), . cluster: , . cepheus, map no. , . cetus, map no. , . stars: alpha, . gamma, . zeta, . eta, . , . , . variables: omicron (mira), . r, . s, . columba, map no. , . coma berenices, map no. , . stars: , . , . , . , . , . , . clusters: , . , . corona borealis, map no. , . stars: gamma, . zeta, . eta, . nu, . sigma, . sigma , . temporary star: , . corvus, map no. , . star: delta, . crater, map no. , . variable: r, . cygnus, map no. , . stars: beta (albireo), . delta, . lambda, . , . omicron^ , . chi ( ), . psi, . , . , . , . temporary star: , . cluster: , . variable: chi, . delphinus, map no. , . stars: alpha, . beta, . gamma, . draco, map no. , ; map no. , . stars: gamma, . epsilon, . eta, . , . nu, . sigma , . sigma , . sigma ( ), . sigma , . nebulæ: , . , . equuleus, map no. , . stars: beta, . gamma, . sigma , . sigma , . sigma , . sigma , . eridanus, map no. , . stars: gamma, . omicron^ , . , . sigma ( ), . sigma ( ), . sigma , . nebula: , . gemini, map no. , . stars: alpha (castor), . beta (pollux), . gamma, . delta, . epsilon, . zeta, . eta, . kappa, . lambda, . , . pi, . , . , . cluster: , . variables: zeta, . eta, . r, . s, . t, . u, . nebula: , . hercules, map no. , ; map no. , . stars: alpha, . gamma, . delta, . zeta, . kappa, . , . rho, . , . , . sigma , . sigma , . sigma , . sigma , . nebulæ: (m ), . , . hydra, map no. , ; map no. , ; map no. , . stars: alpha, . epsilon, . theta, . bu. , . sigma , . variable: r, . nebulæ: , . , . lacerta, map no. , . leo, map no. , . stars: gamma, . iota, . tau, . , . , . , . , . variable: r, . nebula: , . leo minor, map no. , . lepus, map no. , ; map no. , . stars: alpha, . beta, . gamma, . iota, . , . variable: r, . libra, map no. , . stars: a, . alpha, . beta, . iota, . variable: delta, . lynx, map no. , . stars: , . , . , . , . , . , . sigma , . sigma , . sigma , . lyra, map no. , . stars: alpha (vega), . beta, . epsilon, . zeta, . , . variable: beta, . nebula: (ring), . monoceros, map no. , ; map no. , . stars: , . , . , . sigma , . sigma , . sigma , . sigma , . sigma , . clusters: , . , . , . , . , . variable: s, . ophiuchus, map no. , ; map no. , . stars: lambda, . tau, . , . , . , . , . , . sigma , . sigma , . temporary star: , . clusters: , . , . , . , . , . , . , . , . , . variable: r, . orion, map no. , . stars: alpha (betelgeuse), . beta (rigel), . delta, . zeta, . eta, . theta (trapezium), . iota, . lambda, . rho, . sigma, . tau, . psi^ , . sigma , . sigma , . sigma , . sigma , . sigma (a ), . sigma , . sigma , . sigma , . sigma ( ), . sigma , . omicron sigma (i), . clusters: , . , . , . , . nebulæ: great orion nebula, . , . , . pegasus, map no. , . stars: beta, . gamma, . epsilon, . eta, . perseus, map no. , . stars: epsilon, . zeta, . eta, . clusters: , . , . variable: beta (algol), . pisces, map no. , ; map no. , ; map no. , . stars: alpha, . zeta, . psi, . , . , . , . , . variable: r, . sagitta, map no. , . stars: epsilon, . zeta, . theta, . nebula: , . sagittarius, map no. , ; map no. , . stars: , . , . clusters: m , . , . (m ), . (m ), . , . variables: r, . t, . u, . v, . scorpio, map no. , . stars: alpha (antares), . beta, . nu, . xi, . sigma, . temporary star: , . clusters: , . , . scutum sobieskii, map no. , ; map no. , . stars: sigma , . sigma , . clusters: , . , . , . variable: r, . nebula: , . serpens, map no. , ; map no. , . stars: alpha, . beta, . delta, . theta, . nu, . variable: r, . taurus, map no. , . stars: alpha (aldebaran), . eta (alcyone), . theta, . kappa, . sigma, . tau, . phi, . chi, . , . sigma ( ), . sigma , . sigma , . sigma , . clusters: hyades, . pleiades, . , . variable: lambda, . nebulæ: in pleiades, . (crab net), . triangulum, map no. , . star: , . nebula: , . ursa major, map no. , . stars: zeta (mizar), . iota, . nu, . xi, . sigma^ , . , . , . , . nebulæ: , . , . , . ursa minor, map no. , . stars: alpha (pole star), . pi, . virgo, map no. , . stars: alpha (spica), . gamma, . theta, . , . sigma , . sigma , . variables: r, . s, . u, . nebulæ: field of the, . , . , . , . vulpecula, map no. , . star: sigma , . temporary star: , . nebula: (dumb bell), . the moon, most interesting of telescopic objects, ; telescopic views of moon reversed, . craters, ring mountains, and ringed plains: agatharchides, . agrippa, . albategnius, . alhazen, . aliacensis, . alphonsus, . archimedes, . ariadæus, . aristarchus, . aristillus, . aristoteles, . arzachel, . atlas, . autolycus, . bailly, . ball, . barrow, . beer, . berzelius, . billy, . bullialdus, . burckhardt, . capuanus, . cassini, . catharina, , cichus, . clavius, . cleomenes, . condorcet, . copernicus, . cyrillus, . délisle, endymion, . eratosthenes, . eudoxus, . euler, . firmicus, . fracastorius, , . furnerius, . gassendi, . gauss, . geminus, . goclenius, . godin, . grimaldi, . guttemberg, . hainzel, . hansen, . helicon, . hell, . hercules, . herodotus, . herschel, caroline, . hipparchus, . humboldt, . hyginus, . julius cæsar, . kepler, . lambert, . landsberg, . langrenus, , . letronne, . leverrier, . lexell, . lichtenberg, . linné, . longomontanus, . macrobius, . maginus, . manilius, . maurolycus, . menelaus, . mercator, . mersenius, . messala, . messier, . newton, . petavius, , . picard, . piccolomini, . pico, . plato, . plinius, . posidonius, , . proclus, . ptolemæus, . purbach, . sacrobosco, . schickard, . schiller, . silberschlag, . stöfler, . sulpicius gallus, . theætetus, . thebit, . theophilus, . timocharis, . tobias mayer, . tralles, . triesnecker, . tycho, , . vendelinus, , . vieta, . vitello, . walter, . wargentin, . werner, . wilhelm i, . _maria_, or "seas": _lacus somniorum_, . _mare crisium_, , , . _mare fecunditatis_, , . _mare frigoris_, , . _mare humboldtianum_, . _mare humorum_, , . _mare imbrium_, , , . _mare nectaris_, . _mare nubium_, . _mare serenitatis_, , , . _mare tranquilitatis_, . _mare vaporum_, , . _oceanus procellarum_, , , . _palus nebularum_, . _palus putredinis_, . _palus somnii_, . _sinus Æstuum_, . _sinus iridum_, , . other formations: alps mountains, . apennine mountains, , , . cape agarum, . cape heraclides, . cape laplace, . carpathian mountains, . caucasus mountains, . cordilleras mountains, . d'alembert mountains, . dörfel mountains, . hæmus mountains, . harbinger mountains, . leibnitz mountains, . "lunar railroad," . mt. argæus, , . mt. hadley, . mt. huygens, . pyrenees mountains, . taurus mountains, . the planets: are there planets among the stars? . mars, two views of, . best advertised of planets, . favorable oppositions of, . seen with -inch telescope, . polar caps of, . color of, . dark markings on, . "canals," . earthlike condition of, . mercury, phases of, . peculiar rotation of, . markings on, . probably not habitable, . jupiter, easiest planet for amateurs, . seen with -inch glass, . satellites, swift motions of, . velocity of planet's equator, . how to see all sides of, , . watching rotation of, . eclipses and transits of satellites, , . belts and clouds of, . different rates of rotation, . names and numbers of satellites, . saturn, next to jupiter in attractiveness, . seen with -inch glass, . its moons and their orbits, , . polar view of system, . roche's limit, , . origin of the rings, . pickering's ninth satellite, . the satellites as telescopic objects, . venus, her wonderful brilliance, . her atmosphere seen, . lowell's observations, . schiaparelli's observations, . her peculiar rotation, . how to see, in daytime, . neptune and uranus, . the sun, . shade glasses for telescopes in viewing, . solar prism, . helioscope, . periodicity of spots, . to see, by projection, . spectroscope for solar observation, . the telescope: refractors and reflectors, , . eyepieces, , , . aberration (chromatic), ; (spherical), , . achromatic telescopes, how made, . object glass, . magnifying power, . mountings, . rules for testing, . image of star in, . image in and out of focus, , , . astigmatism, . the end [illustration] s. _pleasures of the telescope_ _garrett p. serviss_ this book says to the amateur, in effect:--"what if you have not all advantages of clockwork and observatory equipment. you may know something of the witchery of the heavens even with a little telescope of three to five inches aperture!" "pleasures of the telescope" is popular in style rather than technical. for setting forth "the chief attractions of the starry heavens," a complete set of star-maps is included, showing "all the stars visible to the naked eye in the regions of sky represented, and in addition some stars that can only be seen with optical aid." in six chapters these twenty-six maps are described so plainly that the amateur can readily find all the interesting star-groups, clusters, and nebulæ, and also the colored or double stars. in the three concluding chapters the moon and planets receive special consideration. in the opening chapter the amateur is told how to select and test a glass. _booklovers bulletin._ transcriber's note minor errors and inconsistencies in punctuation and hyphenation have been silently corrected. some illustrations have been relocated a short distance within the text. original page numbers have been retained in the index. greek letters, used to identify stars, are replaced with the full name of the greek letter, e.g. alpha. upper case greek letters are shown by capitalising the initial letter, e.g. sigma a caret (^) is used to represent superscripts, e.g. nu^ and nu^ the following minor corrections have also been made: p : "wil" has been corrected to "will". p : sigma is not shown on map no. . the location of _m_ orionis is marked as sigma . this inconsistency has not been corrected. p : "for colors" has been corrected to "four colors". p : " , , , , , " has been corrected to " , , , , , ". p - : "magnical" has been corrected to "magical". p : a repeated "and" has been removed. making of america collection of the digital & multimedia center, michigan state university libraries (http://digital.lib.msu.edu/) note: images of the original pages are available through the digital & multimedia center, michigan state university libraries. see http://www.hti.umich.edu/cgi/t/text/text-idx?c=moa;idno=aan . . the uses of astronomy. an oration delivered at albany, on the th of july, by edward everett, on the occasion of the inauguration of the dudley astronomical observatory, with a condensed report of the proceedings, and an account of the dedication of new york state geological hall. new york: published by ross & tousey, nassau street. . a note explanatory. the undersigned ventures to put forth this report of mr. everett's oration, in connection with a condensed account of the inauguration of the dudley observatory, and the dedication of the new state geological hall, at albany,--in the hope that the demand which has exhausted the newspaper editions, may exhaust this as speedily as possible; not that he is particularly tenacious of a reward for his own slight labors, but because he believes that the extensive circulation of the record of the two events so interesting and important to the cause of science will exercise a beneficial influence upon the public mind. the effort of the distinguished statesman who has invested astronomy with new beauties, is the latest and one of the most brilliant of his compositions, and is already wholly out of print, though scarcely a month has elapsed since the date of its delivery. the account of the proceedings at albany during the ceremonies of inauguration is necessarily brief, but accurate, and is respectfully submitted to the consideration of the reader. a. maverick. new york, _october , ._ two new institutions of science; and the scenes which attended their christening. in the month of august last, two events took place in the city of albany, which have more than an ephemeral interest. they occurred in close connection with the proceedings of a scientific convention, and the memory of them deserves to be cherished as a recollection of the easy way in which science may be popularized and be rendered so generally acceptable that the people will cry, like oliver twist, for more. it is the purpose of this small publication to embody, in a form more durable than that of the daily newspaper, the record of proceedings which have so near a relation to the progress of scientific research. a marked feature in the ceremonies was the magnificent oration of the hon. edward everett, inaugurating the dudley observatory of albany; and it is believed that the reissue of that speech in its present form will be acceptable to the admirers of that distinguished gentleman, not less than to the lovers of science, who hung with delight upon his words. the dedication of the geological hall. on wednesday, august , , the state geological hall of new york was dedicated with appropriate ceremonies. for the purpose of affording accommodation to the immense crowds of people who, it was confidently anticipated, would throng to this demonstration and that of the succeeding day, at which mr. everett spoke, a capacious tent was arranged with care in the center of academy park, on capitol hill; and under its shelter the ceremonies of the inauguration of both institutions were conducted without accident or confusion; attended on the first day by fully three thousand persons, and on the second by a number which may be safely computed at from five to seven thousand. the announcement that hon. wm. h. seward would be present at the dedication of the geological hall, excited great interest among the citizens; but the hope of his appearance proved fallacious. his place was occupied by seven picked men of the american association for the advancement of science, one of whom (prof. henry) declared his inability to compute the problem why seven men of science were to be considered equal to one statesman. the result justified the selections of the committee, and although the senator was not present, the seven commoners of science made the occasion a most notable one by the flow of wit, elegance of phrase, solidity and cogency of argument, and rare discernment of natural truths, with which their discourse was garnished. the members of the american association marched in procession to the tent, from their place of meeting in the state capitol. on the stage were assembled many distinguished gentlemen, and in the audience were hundreds of ladies. gov. clark and ex-governors hunt and seymour, of new york, sir wm. logan, of canada, hon. george bancroft, and others as well known as these, were among the number present. the tent was profusely decorated. small banners in tri-color were distributed over the entire area covered by the stage, and adorned the wings. the following inscriptions were placed over the front of the rostrum,--that in honor of "_the press_" occupying a central position: geology. the press. meteorology. mineralogy. metallurgy. ethnology. astronomy. the following were arranged in various positions on the right and left: chemistry. telegraph. physiology. letters. conchology. hydrology. palÆontology. zoology. microscopy. ichthyology. art. manufactures. steam. agriculture. commerce. physics. science. anatomy. navigation. botany. the proceedings of the day were opened with prayer by rev. geo. w. bethune, d.d., of brooklyn. hon. garrit y. lansing, of albany, then introduced professor louis agassiz, of cambridge, mass., who was the first of the "seven men of science" to entertain his audience, always with the aid of the inevitable black-board, without which the excellent professor would be as much at a loss as a chemist without a laboratory. professor agassiz spoke for an hour, giving his views of a new theory of animal development. he began by saying:-- we are here to inaugurate the geological hall, which has grown out of the geological survey of the state. to make the occasion memorable, a distinguished statesman of your own state, and mr. frank c. gray, were expected to be present and address you. the pressure of public duties has detained mr. seward, and severe sickness has detained mr. gray. i deeply lament that the occasion is lost to you to hear my friend mr. gray, who is a devotee to science, and as warm-hearted a friend as ever i knew. night before last i was requested to assist in taking their place--i, who am the most unfit of men for the post. i never made a speech. i have addressed learned bodies, but i lack that liberty of speech--the ability to present in finished style, and with that rich imagery which characterize the words of the orator, the thoughts fitting to such an occasion as this. he would limit himself, he continued, to presenting some motives why the community should patronize science, and foster such institutions as this. we scientific men regard this as an occasion of the highest interest, and thus do not hesitate to give the sanction of the highest learned body of the country as an indorsement of the liberality of this state. the geological survey of new york has given to the world a new nomenclature. no geologist can, hereafter, describe the several strata of the earth without referring to it. its results, as recorded in your published volumes, are treasured in the most valuable libraries of the world. they have made this city famous; and now, when the scientific geologist lands on your shore, his first question is, "which is the way to albany? i want to see your fossils." but paleontology is only one branch of the subject, and many others your survey has equally fostered. he next proceeded to show that organized beings were organized with reference to a plan, which the relations between different animals, and between different plants, and between animals and plants, everywhere exhibit;--drew sections of the body of a fish, and of the bird, and of man, and pointed out that in each there was the same central back-bone, the cavity above and the ribbed cavity below the flesh on each side, and the skin over all--showing that the maker of each possessed the same thought--followed the same plan of structure. and upon that plan he had made all the kinds of quadrupeds, , in number, all the kinds of birds, , in number, all of the reptiles, , to , in number, all the fish, , to , in number. all their forms may be derived as different expressions of the same formula. there are only four of these great types; or, said he, may i not call them the four tunes on which divinity has played the harmonies that have peopled, in living and beautiful reality, the whole world? professor hitchcock on reminiscences. erastus c. benedict, esq. of new york, introduced prof. hitchcock, of amherst, as a gentleman whose name was very familiar, who had laid aside, voluntarily, the charge of one of the largest colleges in new england, but who could never lay aside the honors he had earned in the literature and science of geology. after a few introductory observations, prof. hitchcock said:-- this, i believe, is the first example in which a state government in our country has erected a museum for the exhibition of its natural resources, its mineral and rock, its plants and animals, living and fossil. and this seems to me the most appropriate spot in the country for placing the first geological hall erected by the government; for the county of albany was the district where the first geological survey was undertaken, on this side of the atlantic, and, perhaps, the world. this was in , and ordered by that eminent philanthropist, stephen van rensselaer, who, three years later, appointed prof. eaton to survey, in like manner, the whole region traversed by the erie canal. this was the commencement of a work, which, during the last thirty years, has had a wonderful expansion, reaching a large part of the states of the union, as well as canada, nova scotia, and new brunswick, and, i might add, several european countries, where the magnificent surveys now in progress did not commence till after the survey of albany and rensselaer counties. how glad are we, therefore, to find on this spot the first museum of economical geology on this side of the atlantic! nay, embracing as it does all the department of natural history, i see in it more than a european museum of economical geology, splendid though they are. i fancy, rather, that i see here the germ of a cis-atlantic british museum, or garden of plants. north carolina was the first state that ordered a geological survey; and i have the pleasure of seeing before me the gentleman who executed it, and in - published a report of pages. i refer to professor olmstead, who, though he has since won brighter laurels in another department of science, will always be honored as the first commissioned state geologist in our land. of the new york state survey he said:-- this survey has developed the older fossiliferous rocks, with a fullness and distinctness unknown elsewhere. hence european savans study the new york reports with eagerness. in , as i entered the woodwardian museum, in the university of cambridge, in england, i found professor mccoy busy with a collection of silurian fossils before him, which he was studying with hall's first volume of paleontology as his guide; and in the splendid volumes, entitled _british paleozoric rocks and fossils_, which appeared last year as the result of those researches, i find professor hall denominated the great american paleontologist. i tell you, sir, that this survey has given new york a reputation throughout the learned world, of which she may well be proud. am i told that it will, probably, cost half a million? very well. the larger the sum, the higher will be the reputation of new york for liberality; and what other half million expended in our country, has developed so many new facts or thrown so much light upon the history of the globe, or won so world-wide and enviable a reputation? and of geological surveys in general:-- in regard to this matter of geological surveys, i can hardly avoid making a suggestion here. so large a portion of our country has now been examined, more or less thoroughly, by the several state governments, that it does seem to me the time has come when the national government should order a survey--geological, zoological, and botanical--of the whole country, on such a liberal and thorough plan as the surveys in great britain are now conducted; in the latter country it being understood that at least thirty years will be occupied in the work. could not the distinguished new york statesman who was to have addressed us to-day be induced, when the present great struggle in which he is engaged shall have been brought to a close, by a merciful providence, to introduce this subject, and urge it upon congress? and would it not be appropriate for the american association for the advancement of science to throw a petition before the government for such an object? or might it not, with the consent of the eminent gentleman who has charge of the coast survey, be connected therewith, as it is with the ordnance survey in great britain. the history of the american association was then given:-- prof. mather, i believe, through prof. emmons, first suggested to the new-york board of geologists in november, , in a letter proposing a number of points for their consideration. i quote from him the following paragraph relating to the meeting. as to the credit he has here given me of having personally suggested the subject, i can say only that i had been in the habit for several years of making this meeting of scientific men a sort of hobby in my correspondence with such. whether others did the same, i did not then, and do not now know. were this the proper place, i could go more into detail on this point; but i will merely quote prof. mather's language to the board:-- * * * * "would it not be well to suggest the propriety of a meeting of geologists and other scientific men of our country at some central point next fall,--say at new-york or philadelphia? there are many questions in our geology that will receive new light from friendly discussion and the combined observations of various individuals who have noted them in different parts of our country. such a meeting has been suggested by prof. hitchcock; and to me it seems desirable. it would undoubtedly be an advantage not only to science but to the several surveys that are now in progress and that may in future be authorized. it would tend to make known our scientific men to each other personally, give them more confidence in each other, and cause them to concentrate their observation on those questions that are of interest in either a scientific or economical point of view. more questions may be satisfactorily settled in a day by oral discussion in such a body, than a year by writing and publication."[a] [footnote a: in the letter alluded to, on examination, we discover another passage bearing on the point, which, owing to the professor's modesty we suspect, he did not read. prof. mather adds. "you, so far as i know, first suggested the matter of such an association. i laid the matter before the board of geologists of new-york, specifying some of the advantages that might be expected to result; and prof. vanuxem probably made the motion before the board in regard to it."] though the board adopted the plan of a meeting, various causes delayed the first over till april, , when we assembled in philadelphia, and spent a week in most profitable and pleasant discussion, and the presentation of papers. our number that year was only , because confined almost exclusively to the state geologists; but the next year, when we met again in philadelphia, and a more extended invitation was given, about eighty were present; and the members have been increasing to the present time. but, in fact, those first two meetings proved the type, in all things essential, of all that have followed. the principal changes have been those of expansion and the consequent introduction of many other branches of science with their eminent cultivators. in , we changed the name to that of the association of american geologists and naturalists; and in , to that of the american association for the advancement of science. i trust it has not yet reached its fullest development, as our country and its scientific men multiply, and new fields of discovery open. prof. h. said of this particular occasion:-- we may be quite sure that this hall will be a center of deep interest to coming generations. long after we shall have passed away will the men of new-york, as they survey these monuments, feel stimulated to engage in other noble enterprises by this work of their progenitors, and from many a distant part of the civilized world will men come here to solve their scientific questions, and to bring far-off regions into comparison with this. new-york, then, by her liberal patronage, has not only acquired an honorable name among those living in all civilized lands, but has secured the voice of history to transmit her fame to far-off generations. sir william logan asks "the way to albany." sir william e. logan, of canada, in a brief speech acknowledged the services rendered by the new-york survey to canada. he should manifest ingratitude if he declined to unite in the joyful occasion of inaugurating the museum which was to hold forever the evidence of the truth of its published results. the survey of canada had been ordered, and the commission of five years twice renewed; and the last time, the provision for it was more than doubled. it happened to him, as mr. agassiz had said: after crossing the ocean first, the first thing he asked was, "which is the way to albany?" and when he arrived here, he found that with the aid of prof. hall's discoveries, he had only to take up the different formations as he had left them on the boundary line, and follow them into canada. it was both a convenience and a necessity to adopt the new-york nomenclature, which was thus extended over an area six times as large as new-york. in paris he heard de vernier using the words trenton and niagara, as if they were household words. he was delighted to witness the impatience with which barron inquired when the remaining volumes of the paleontology of new-york would be published. your paleontological reputation, said he, has made new-york known, even among men not scientific, all over europe. i hope you will not stop here, but will go on and give us in equally thorough, full, and magnificent style, the character of the durassic and cretaceous formations. professor henry on dutchmen. professor henry was at a loss to know by what process they had arrived at the conclusion that seven men of science must be substituted to fill the place of one distinguished statesman whom they had expected to hear. he prided himself on his albany nativity. he was proud of the old dutch character, that was the substratum of the city. the dutch are hard to be moved, but when they do start their momentum is not as other men's in proportion to the velocity, but as the square of the velocity. so when the dutchman goes three times as fast, he has nine times the force of another man. the dutchman has an immense potentia agency, but it wants a small spark of yankee enterprise to touch it off. in this strain the professor continued, making his audience very merry, and giving them a fine chance to express themselves with repeated explosions of laughter. professor davies on the practical nature of science. prof. charles davies was introduced by ex-governor seymour, and spoke briefly, but humorously and very much to the point, in defense of the practical character of scientific researches. he said that to one accustomed to speak only on the abstract quantities of number and space, this was an unusual occasion, and this an unusual audience; and inquired how he could discuss the abstract forms of geometry, when he saw before him, in such profusion, the most beautiful real forms that providence has vouchsafed to the life of man. he proposed to introduce and develop but a single train of thought--the unchangeable connection between what in common language is called the theoretical and practical, but in more technical phraseology, the ideal and the actual. the actual, or true practical, consists in the uses of the forces of nature, according to the laws of nature; and here we must distinguish between it and the empirical, which uses, or attempts to use, those forces, without a knowledge of the laws. the true practical, therefore, is the result, or actual, of an antecedent ideal. the ideal, full and complete, must exist in the mind before the actual can be brought forth according to the laws of science. who, then, are the truly practical men of our age? are they not those who are engaged most laboriously and successfully in investigating the great laws? are they not those who are pressing out the boundaries of knowledge, and conducting the mind into new and unexplored regions, where there may yet be discovered a california of undeveloped thought? is not the gentleman from massachusetts (professor agassiz) the most practical man in our country in the department of natural history, not because he has collected the greatest number of specimens, but because he has laid open to us all the laws of the animal kingdom? are the formulas written on the black-board by the gentleman from cambridge (prof. pierce) of no practical value, because they cannot be read by the uninstructed eye? a single line may contain the elements of the motions of all the heavenly bodies; and the eye of science, taking its stand-point at the center of gravity of the system, will see in the equation the harmonious revolutions of all the bodies which circle the heavens. it is such labors and such generalizations that have rendered his name illustrious in the history of mathematical science. is it of no practical value that the chief of the coast survey (prof. bache), by a few characters written upon paper, at washington, has determined the exact time of high and low tide in the harbor of boston, and can determine, by a similar process, the exact times of high and low water at every point on the surface of the globe? are not these results, the highest efforts of science, also of the greatest practical utility? and may we not, then, conclude that _there is nothing truly practical which is not the consequence of an antecedent ideal_? science is to art what the great fly-wheel and governor of a steam-engine are to the working part of the machinery--it guides, regulates, and controls the whole. science and art are inseparably connected; like the siamese twins, they cannot be separated without producing the death of both. how, then, are we to regard the superb specimens of natural history, which the liberality, the munificence; and the wisdom of our state have collected at the capitol? they are the elements from which we can here determine all that belongs to the natural history of our state; and may we not indulge the hope, that science and genius will come here, and, striking them with a magic wand, cause the true practical to spring into immortal life? remarks were also uttered by prof. chester dewey, president anderson, and rev. dr. cox. and thus ended the inauguration of the state geological hall. we turn to the observatory, in regular order of succession. inauguration of dudley observatory. the inauguration of the dudley observatory took place under the same tent which was appropriated to the dedication of the geological hall, and on the day following that event. an immense audience was assembled, drawn by the announcement of mr. everett's oration. at a little past three o'clock the procession of _savans_ arrived from the assembly chamber, escorted by the burgesses corps. directly in front of the speaker's stand sat mrs. dudley, the venerable lady to whose munificence the world is indebted for this observatory. she was dressed in an antique, olive-colored silk, with a figure of a lighter color, a heavy, red broché shawl, and her bonnet, cap, &c., after the strictest style of the old school. her presence added a new point of interest. prayer having been uttered by rev. dr. sprague, of albany, thomas w. olcott, esq., introduced to the audience ex-governor washington hunt, who spoke briefly in honor of the memory of charles e. dudley, whose widow has founded and in part endowed this observatory with a liberality so remarkable. remarks were offered by dr. b. a. gould and prof. a. d. bache, and judge harris read the following letter from mrs. dudley, announcing another munificent donation in aid of the new observatory--$ , , in addition to the $ , which had been already expended in the construction of the building. the letter was received with shouts of applause, prof. agassiz rising and leading the vast assemblage in three vehement cheers in honor of mrs. dudley! albany, thursday, aug. , . _to the trustees of the dudley observatory:_ gentlemen,--i scarcely need refer in a letter to you to the modest beginning and gradual growth of the institution over which you preside, and of which you are the responsible guardians. but we have arrived at a period in its history when its inauguration gives to it and to you some degree of prominence, and which must stamp our past efforts with weakness and inconsideration, or exalt those of the future to the measure of liberality necessary to certain success. you have a building erected and instruments engaged of unrivaled excellence; and it now remains to carry out the suggestion of the astronomer royal of england in giving permanency to the establishment. the very distinguished professors bache, pierce, and gould, state in a letter, which i have been permitted to see, that to expand this institution to the wants of american science and the honors of a national character, will require an investment which will yield annually not less than $ , ; and these gentlemen say, in the letter referred to,-- "if the greatness of your giving can rise to this occasion, as it has to all our previous suggestions, with such unflinching magnanimity, we promise you our earnest and hearty coöperation, and stake our reputation that the scientific success shall fill up the measure of your hopes and anticipations." for the attainment of an object so rich in scientific reward and national glory, guaranteed by men with reputations as exalted and enduring as the skies upon which they are written, contributions should be general, and not confined to an individual or a place. for myself, i offer, as my part of the required endowment, the sum of $ , in addition to the advances which i have already made; and, trusting that the name which you have given to the observatory may not be regarded as an undeserved compliment, and that it will not diminish the public regard by giving to the institution a seemingly individual character, i remain, gentlemen, your obedient servant, blandina dudley. judge harris then introduced the orator of the occasion, hon. edward everett, whose speech is given verbatim in these pages. the instruments of the dudley observatory. during the sessions of the american association, the new astronomical instruments of dudley observatory were described in detail by dr. b. a. gould, who is the astronomer in charge. we condense his statements:-- the meridian circle and transit instrument were ordered from pistor & martins, the celebrated manufacturers of berlin, by whom the new instrument at ann arbor was made. a number of improvements have been introduced in the albany instruments, not perhaps all absolutely new, but an eclectic combination of late adaptations with new improvements. dr. gould made a distinction of modern astronomical instruments into two classes, the english and the german. the english is the massive type; the german, light and airy. the english instrument is the instrument of the engineer; the german, the instrument of the artist. in ordering the instruments for the albany observatory, the doctor preferred the german type and discarded the heavier english. he instanced, as a specimen of the latter, the new instrument at greenwich, recently erected under the superintendence of the astronomer royal. that instrument registers observations in single seconds; the dudley instrument will register to tenths of seconds. that has six or eight microscopes; this has four. that has a gas lamp, by the light of which the graduations are read off; the albany instrument has no lamp, and the doctor considered the lamp a hazardous experiment, affecting the integrity of the experiment, not only by its radiant heat but by the currents of heated air which it produces. the diameter of the object-glass of the albany instrument is - / french inches clear aperture, or english inches, and the length of the tube feet. he would have preferred an instrument in which the facilities of manipulation would have been greater, but was hampered by one proviso, upon which the trustees of the institution insisted--that this should be the biggest instrument of its kind; and the instruction was obeyed. the glass was made by chance, and ground by pistor himself. the eye-piece is fitted with two micrometers, for vertical and horizontal observations. another apparatus provides for the detection and measurement of the flexure of the tube. much trouble was experienced in securing a good casting for the steel axis of the instrument. three were found imperfect under the lathe, and the fourth was chosen; but even then the pivots were made in separate pieces, which were set in very deeply and welded. dr. gould said he had been requested by the gentlemen who had this enterprise in charge to suggest, as a mark of respect to a gentleman of albany who was a munificent patron of science, that this instrument be known as the olcott meridian circle. what the dudley observatory is. it stands a mile from the capitol, in the city of albany, upon the crest of a hill, so difficult of approach, as to be in reality a hill of science. there are two ways of getting to it. in both cases there are rail fences to be clambered over, and long grass to wade through, settlements to explore, and a clayey road to travel; but these are minor troubles. the elevation of the hill above tide-water is, perhaps, feet; its distance from the capitol about a mile and a half. the view for miles is unimpeded; and the observatory is belted about with woods and verdant lawns. there could not be a finer location or a purer air. the plateau contains some fifteen acres. the observatory is constructed in the form of a latin cross. its eastern arm is an apartment by feet, in which the meridian circle is to be placed. the western arm is a room of the same dimensions, intended for the transit instrument. from the north and south faces of both rooms are semi-circular apsides, projecting feet inches, containing the collimator piers and the vertical openings for observation. the entire length of each room is, therefore, feet. in the northern arm are placed the library, feet by feet; two computing rooms, feet by feet each; side entrance halls, staircases, &c. the southern arm contains the principal entrance, consisting of an arched colonnade of four tuscan columns, surrounded by a pediment. a broad flight of stone steps leads to this colonnade; and through the entrance door beneath it to the main central hall, feet square, in which are placed (in niches) the very beautiful electric clock and pendulum presented by erastus corning, esq. the center of this hall is occupied by a massive pier of stone, feet square, passing from the basement into the dome above, and intended for the support of the great heliometer. directly opposite the entrance door is a large niche, in which it is proposed to place the bust of the late mr. dudley. immediately above this hall is the equatorial room, a circular apartment, feet inches in diameter, and feet high, covered by a low conical roof, in which and in the walls are the usual observing slits. the drum, or cylindrical portion, of this room is divided into two parts--the lower one fixed, the upper, revolving on cast-iron balls moving in grooved metal plates, can command the entire horizon. the building is in two stories--the upper of brick, with freestone quoins, impost and window and door dressings, rests upon a rusticated basement of freestone, six feet high. the style adopted is the modern italian, of which it is a very excellent specimen. the building has been completed some time; but, in consequence of the size of the instruments now procured being greater than that originally contemplated, sundry alterations were required in the transit and meridian circle rooms. these consist of the semi-circular projections already mentioned, and which, by varying the outlines of the building, will add greatly to its beauty and picturesqueness. the piers for the meridian circle and transit have, after careful investigation, been procured from the lockport quarries. the great density and uniformity of the structure of the stone, and the facility with which such large masses as are required for this purpose can be procured there, have induced the selection of these quarries. the stones will weigh from six and a half to eight tons each. the main building was erected from the drawings of messrs. woollett and ogden, architects, albany; the additions and the machinery have been designed by mr. w. hodgins, civil engineer; and the latter is now being constructed under his superintendence, in a very superior manner, at the iron works of messrs. pruyn and lansing, albany. the entire building is a tasteful and elegant structure, much superior in architectural character to any other in america devoted to a similar purpose. oration. fellow citizens of albany:-- assembled as we are, under your auspices, in this ancient and hospitable city, for an object indicative of a highly-advanced stage of scientific culture, it is natural, in the first place, to cast a historical glance at the past. it seems almost to surpass belief, though an unquestioned fact, that more than a century should have passed away, after cabot had discovered the coast of north america for england, before any knowledge was gained of the noble river on which your city stands, and which was destined by providence to determine, in after times, the position of the commercial metropolis of the continent. it is true that verazzano, a bold and sagacious florentine navigator, in the service of france, had entered the narrows in , which he describes as a very large river, deep at its mouth, which forced its way through steep hills to the sea; but though he, like all the naval adventurers of that age, was sailing westward in search of a shorter passage to india, he left this part of the coast without any attempt to ascend the river; nor can it be gathered from his narrative that he believed it to penetrate far into the interior. voyage of hendrick hudson. near a hundred years elapsed before that great thought acquired substance and form. in the spring of , the heroic but unfortunate hudson, one of the brightest names in the history of english maritime adventure, but then in the employment of the dutch east india company, in a vessel of eighty tons, bearing the very astronomical name of the _half moon_, having been stopped by the ice in the polar sea, in the attempt to reach the east by the way of nova zembla, struck over to the coast of america in a high northern latitude. he then stretched down southwardly to the entrance of chesapeake bay (of which he had gained a knowledge from the charts and descriptions of his friend, captain smith), thence returning to the north, entered delaware bay, standing out again to sea, arrived on the second of september in sight of the "high hills" of neversink, pronouncing it "a good land to fall in with, and a pleasant land to see;" and, on the following morning, sending his boat before him to sound the way, passed sandy hook, and there came to anchor on the third of september, ; two hundred and forty-seven years ago next wednesday. what an event, my friends, in the history of american population, enterprise, commerce, intelligence, and power--the dropping of that anchor at sandy hook! discovery of the hudson river. here he lingered a week, in friendly intercourse with the natives of new jersey, while a boat's company explored the waters up to newark bay. and now the great question. shall he turn back, like verazzano, or ascend the stream? hudson was of a race not prone to turn back, by sea or by land. on the eleventh of september he raised the anchor of the _half moon_, passed through the narrows, beholding on both sides "as beautiful a land as one can tread on;" and floated cautiously and slowly up the noble stream--the first ship that ever rested on its bosom. he passed the palisades, nature's dark basaltic malakoff, forced the iron gateway of the highlands, anchored, on the fourteenth, near west point; swept onward and upward, the following day, by grassy meadows and tangled slopes, hereafter to be covered with smiling villages;--by elevated banks and woody heights, the destined site of towns and cities--of newburg, poughkeepsie, catskill;--on the evening of the fifteenth arrived opposite "the mountains which lie from the river side," where he found "a very loving people and very old men;" and the day following sailed by the spot hereafter to be honored by his own illustrious name. one more day wafts him up between schodac and castleton; and here he landed and passed a day with the natives,--greeted with all sorts of barbarous hospitality,--the land "the finest for cultivation he ever set foot on," the natives so kind and gentle, that when they found he would not remain with them over night, and feared that he left them--poor children of nature!--because he was afraid of their weapons,--he, whose quarter-deck was heavy with ordnance,--they "broke their arrows in pieces, and threw them in the fire." on the following morning, with the early flood-tide, on the th of september, , the _half moon_ "ran higher up, two leagues above the shoals," and came to anchor in deep water, near the site of the present city of albany. happy if he could have closed his gallant career on the banks of the stream which so justly bears his name, and thus have escaped the sorrowful and mysterious catastrophe which awaited him the next year! champlain's voyage and the growth of colonies. but the discovery of your great river and of the site of your ancient city, is not the only event which renders the year memorable in the annals of america and the world. it was one of those years in which a sort of sympathetic movement toward great results unconsciously pervades the races and the minds of men. while hudson discovered this mighty river and this vast region for the dutch east india company, champlain, in the same year, carried the lilies of france to the beautiful lake which bears his name on your northern limits; the languishing establishments of england in virginia were strengthened by the second charter granted to that colony; the little church of robinson removed from amsterdam to leyden, from which, in a few years, they went forth, to lay the foundations of new england on plymouth rock; the seven united provinces of the netherlands, after that terrific struggle of forty years (the commencement of which has just been embalmed in a record worthy of the great event by an american historian) wrested from spain the virtual acknowledgment of their independence, in the twelve years' truce; and james the first, in the same year, granted to the british east india company their first permanent charter,--corner-stone of an empire destined in two centuries to overshadow the east. galileo's discoveries one more incident is wanting to complete the list of the memorable occurrences which signalize the year , and one most worthy to be remembered by us on this occasion. cotemporaneously with the events which i have enumerated--eras of history, dates of empire, the starting-point in some of the greatest political, social, and moral revolutions in our annals, an italian astronomer, who had heard of the magnifying glasses which had been made in holland, by which distant objects could be brought seemingly near, caught at the idea, constructed a telescope, and pointed it to the heavens. yes, my friends, in the same year in which hudson discovered your river and the site of your ancient town, in which robinson made his melancholy hegira from amsterdam to leyden, galileo galilei, with a telescope, the work of his own hands, discovered the phases of venus and the satellites of jupiter; and now, after the lapse of less than two centuries and a half, on a spot then embosomed in the wilderness--the covert of the least civilized of all the races of men--we are assembled--descendants of the hollanders, descendants of the pilgrims, in this ancient and prosperous city, to inaugurate the establishment of a first-class astronomical observatory. early days of albany. one more glance at your early history. three years after the landing of the pilgrims at plymouth, fort orange was erected, in the center of what is now the business part of the city of albany; and, a few years later, the little hamlet of beverswyck began to nestle under its walls. two centuries ago, my albanian friends, this very year, and i believe this very month of august, your forefathers assembled, not to inaugurate an observatory, but to lay the foundations of a new church, in the place of the rude cabin which had hitherto served them in that capacity. it was built at the intersection of yonker's and handelaar's, better known to you as state and market streets. public and private liberality coöperated in the important work. the authorities at the fort gave fifteen hundred guilders; the patroon of that early day, with the liberality coëval with the name and the race, contributed a thousand; while the inhabitants, for whose benefit it was erected, whose numbers were small and their resources smaller, contributed twenty beavers "for the purchase of an oaken pulpit in holland." whether the largest part of this subscription was bestowed by some liberal benefactress, tradition has not informed us. new amsterdam nor is the year memorable in the annals of albany alone. in that same year your imperial metropolis, then numbering about three hundred inhabitants, was first laid out as a city, by the name of new amsterdam.[a] in eight years more, new netherland becomes new york; fort orange and its dependent hamlet assumes the name of albany. a century of various fortune succeeds; the scourge of french and indian war is rarely absent from the land; every shock of european policy vibrates with electric rapidity across the atlantic; but the year finds a population of , in your growing province. albany, however, may still be regarded almost as a frontier settlement. of the twelve counties into which the province was divided a hundred years ago, the county of albany comprehended all that lay north and west of the city; and the city itself contained but about three hundred and fifty houses. [footnote a: these historical notices are, for the most part, abridged from mr. brodhead's excellent history of new york.] two hundred years. one more century; another act in the great drama of empire; another french and indian war beneath the banners of england; a successful revolution, of which some of the most momentous events occurred within your limits; a union of states; a constitution of federal government; your population carried to the st. lawrence and the great lakes, and their waters poured into the hudson; your territory covered with a net-work of canals and railroads, filled with life and action, and power, with all the works of peaceful art and prosperous enterprise with all the institutions which constitute and advance the civilization of the age; its population exceeding that of the union at the date of the revolution; your own numbers twice as large as those of the largest city of that day, you have met together, my friends, just two hundred years since the erection of the little church of beverswyck, to dedicate a noble temple of science and to take a becoming public notice of the establishment of an institution, destined, as we trust, to exert a beneficial influence on the progress of useful knowledge at home and abroad, and through that on the general cause of civilization. scientific progress. you will observe that i am careful to say the progress of science "at home and abroad;" for the study of astronomy in this country has long since, i am happy to add, passed that point where it is content to repeat the observations and verify the results of european research. it has boldly and successfully entered the field of original investigation, discovery, and speculation; and there is not now a single department of the science in which the names of american observers and mathematicians are not cited by our brethren across the water, side by side with the most eminent of their european contemporaries. this state of things is certainly recent. during the colonial period and in the first generation after the revolution, no department of science was, for obvious causes, very extensively cultivated in america--astronomy perhaps as much as the kindred branches. the improvement in the quadrant, commonly known as hadley's, had already been made at philadelphia by godfrey, in the early part of the last century; and the beautiful invention of the collimating telescope was made at a later period by rittenhouse, an astronomer of distinguished repute. the transits of venus of and were observed, and orreries were constructed in different parts of the country; and some respectable scientific essays are contained and valuable observations are recorded in the early volumes of the transactions of the philosophical society, at philadelphia, and the american academy of arts and sciences at boston and cambridge. but in the absence of a numerous class of men of science to encourage and aid each other, without observatories and without valuable instruments, little of importance could be expected in the higher walks of astronomical life. american observations. the greater the credit due for the achievement of an enterprise commenced in the early part of the present century, and which would reflect honor on the science of any country and any age; i mean the translation and commentary on laplace's _mécanique celeste_, by bowditch; a work of whose merit i am myself wholly unable to form an opinion, but which i suppose places the learned translator and commentator on a level with the ablest astronomers and geometers of the day. this work may be considered as opening a new era in the history of american science. the country was still almost wholly deficient in instrumental power; but the want was generally felt by men of science, and the public mind in various parts of the country began to be turned towards the means of supplying it. in , president john quincy adams brought the subject of a national observatory before congress. political considerations prevented its being favorably entertained at that time; and it was not till , and as an incident of the exploring expedition, that an appropriation was made for a dépôt for the charts and instruments of the navy. on this modest basis has been reared the national observatory at washington; an institution which has already taken and fully sustains an honorable position among the scientific establishments of the age. besides the institution at washington, fifteen or twenty observatories have within the last few years, been established in different parts of the country, some of them on a modest scale, for the gratification of the scientific taste and zeal of individuals, others on a broad foundation of expense and usefulness. in these establishments, public and private, the means are provided for the highest order of astronomical observation, research, and instruction. there is already in the country an amount of instrumental power (to which addition is constantly making), and of mathematical skill on the part of our men of science, adequate to a manly competition with their european contemporaries. the fruits are already before the world, in the triangulation of several of the states, in the great work of the coast survey, in the numerous scientific surveys of the interior of the continent, in the astronomical department of the exploring expedition, in the scientific expedition to chili, in the brilliant hydrographical labors of the observatory at washington, in the published observations of washington and cambridge, in the journal conducted by the nestor of american science, now in its eighth lustrum; in the _sidereal messenger_, the _astronomical journal_, and the _national ephemeris_; in the great chronometrical expeditions to determine the longitude of cambridge, better ascertained than that of paris was till within the last year; in the prompt rectification of the errors in the predicted elements of neptune; in its identification with lalande's missing star, and in the calculation of its ephemeris; in the discovery of the satellite of neptune, of the eighth satellite of saturn, and of the innermost of its rings; in the establishment, both by observation and theory, of the non-solid character of saturn's rings; in the separation and measurement of many double and triple stars, amenable only to superior instrumental power, in the immense labor already performed in preparing star catalogues, and in numerous accurate observations of standard stars; in the diligent and successful observation of the meteoric showers; in an extensive series of magnetic observations; in the discovery of an asteroid and ten or twelve telescopic comets; in the resolution of nebulæ which had defied every thing in europe but lord rosse's great reflector; in the application of electricity to the measurement of differences in longitude; in the ascertainment of the velocity of the electro-magnetic fluid, and its truly wonderful uses in recording astronomical observations. these are but a portion of the achievements of american astronomical science within fifteen or twenty years, and fully justify the most sanguine anticipations of its further progress. how far our astronomers may be able to pursue their researches, will depend upon the resources of our public institutions, and the liberality of wealthy individuals in furnishing the requisite means. with the exception of the observatories at washington and west point, little can be done, or be expected to be done, by the government of the union or the states; but in this, as in every other department of liberal art and science, the great dependence,--and may i not add, the safe dependence?--as it ever has been, must continue to be upon the bounty of enlightened, liberal, and public-spirited individuals. the dudley observatory. it is by a signal exercise of this bounty, my friends, that we are called together to-day. the munificence of several citizens of this ancient city, among whom the first place is due to the generous lady whose name has with great propriety been given to the institution, has furnished the means for the foundation of the dudley observatory at albany. on a commanding elevation on the northern edge of the city, liberally given for that purpose by the head of a family in which the patronage of science is hereditary, a building of ample dimensions has been erected, upon a plan which combines all the requisites of solidity, convenience, and taste. a large portion of the expense of the structure has been defrayed by mrs. blandina dudley; to whose generosity, and that of several other public-spirited individuals, the institution is also indebted for the provision which has been made for an adequate supply of first-class instruments, to be executed by the most eminent makers in europe and america; and which, it is confidently expected, will yield to none of their class in any observatory in the world.[a] [footnote a: prof. loomis, in _harper's magazine_ for june, p. .] with a liberal supply of instrumental power; established in a community to whose intelligence and generosity its support may be safely confided, and whose educational institutions are rapidly realizing the conception of a university; countenanced by the gentleman who conducts the united states coast survey with such scientific skill and administrative energy; committed to the immediate supervision of an astronomer to whose distinguished talent had been added the advantage of a thorough scientific education in the most renowned universities of europe, and who, as the editor of the _american astronomical journal_, has shown himself to be fully qualified for the high trust;--under these favorable circumstances, the dudley observatory at albany takes its place among the scientific foundations of the country and the world. wonders of astronomy. it is no affected modesty which leads me to express the regret that this interesting occasion could not have taken place under somewhat different auspices. i feel that the duty of addressing this great and enlightened assembly, comprising so much of the intelligence of the community and of the science of the country, ought to have been elsewhere assigned; that it should have devolved upon some one of the eminent persons, many of whom i see before me, to whom you have been listening the past week, who, as observers and geometers, could have treated the subject with a master's power; astronomers, whose telescopes have penetrated the depths of the heavens, or mathematicians, whose analysis unthreads the maze of their wondrous mechanism. if, instead of commanding, as you easily could have done, qualifications of this kind, your choice has rather fallen on one making no pretensions to the honorable name of a man of science,--but whose delight it has always been to turn aside from the dusty paths of active life, for an interval of recreation in the green fields of sacred nature in all her kingdoms,--it is, i presume, because you have desired on an occasion of this kind, necessarily of a popular character, that those views of the subject should be presented which address themselves to the general intelligence of the community, and not to its select scientific circles. there is, perhaps, no branch of science which to the same extent as astronomy exhibits phenomena which, while they task the highest powers of philosophical research, are also well adapted to arrest the attention of minds barely tinctured with scientific culture, and even to teach the sensibilities of the wholly uninstructed observer. the profound investigations of the chemist into the ultimate constitution of material nature, the minute researches of the physiologist into the secrets of animal life, the transcendental logic of the geometer, clothed in a notation, the very sight of which terrifies the uninitiated,--are lost on the common understanding. but the unspeakable glories of the rising and the setting sun; the serene majesty of the moon, as she walks in full-orbed brightness through the heavens; the soft witchery of the morning and the evening star; the imperial splendors of the firmament on a bright, unclouded night; the comet, whose streaming banner floats over half the sky,--these are objects which charm and astonish alike the philosopher and the peasant, the mathematician who weighs the masses and defines the orbits of the heavenly bodies, and the untutored observer who sees nothing beyond the images painted upon the eye. what is an astronomical observatory? an astronomical observatory, in the general acceptation of the word, is a building erected for the reception and appropriate use of astronomical instruments, and the accommodation of the men of science employed in making and reducing observations of the heavenly bodies. these instruments are mainly of three classes, to which i believe all others of a strictly astronomical character may be referred. . the instruments by which the heavens are inspected, with a view to discover the existence of those celestial bodies which are not visible to the naked eye (beyond all comparison more numerous than those which are), and the magnitude, shapes, and other sensible qualities, both of those which are and those which are not thus visible to the unaided sight. the instruments of this class are designated by the general name of telescope, and are of two kinds,--the refracting telescope, which derives its magnifying power from a system of convex lenses; and the reflecting telescope, which receives the image of the heavenly body upon a concave mirror. d. the second class of instruments consists of those which are designed principally to measure the angular distances of the heavenly bodies from each other, and their time of passing the meridian. the transit instrument, the meridian circle, the mural circle, the heliometer, and the sextant, belong to this class. the brilliant discoveries of astronomy are, for the most part, made with the first class of instruments; its practical results wrought out by the second. d. the third class contains the clock, with its subsidiary apparatus, for measuring the time and making its subdivisions with the greatest possible accuracy; indispensable auxiliary of all the instruments, by which the positions and motions of the heavenly bodies are observed, and measured, and recorded. the telescope. the telescope may be likened to a wondrous cyclopean eye, endued with superhuman power, by which the astronomer extends the reach of his vision to the further heavens, and surveys galaxies and universes compared with which the solar system is but an atom floating in the air. the transit may be compared to the measuring rod which he lays from planet to planet, and from star to star, to ascertain and mark off the heavenly spaces, and transfer them to his note-book; the clock is that marvelous apparatus by which he equalizes and divides into nicely measured parts a portion of that unconceived infinity of duration, without beginning and without end, in which all existence floats as on a shoreless and bottomless sea. in the contrivance and the execution of these instruments, the utmost stretch of inventive skill and mechanical ingenuity has been put forth. to such perfection have they been carried, that a single second of magnitude or space is rendered a distinctly visible and appreciable quantity. "the arc of a circle," says sir j. herschell, "subtended by one second, is less than the , th part of the radius, so that on a circle of six feet in diameter, it would occupy no greater linear extent than - part of an inch, a quantity requiring a powerful microscope to be discerned at all."[a] the largest body in our system, the sun, whose real diameter is , miles, subtends, at a distance of , , miles, but an angle of little more than ; while so admirably are the best instruments constructed, that both in europe and america a satellite of neptune, an object of comparatively inconsiderable diameter, has been discovered at a distance of , millions of miles. [footnote a: _outlines_, § .] utility of astronomical observations. the object of an observatory, erected and supplied with instruments of this admirable construction, and at proportionate expense, is, as i have already intimated, to provide for an accurate and systematic survey of the heavenly bodies, with a view to a more correct and extensive acquaintance with those already known, and as instrumental power and skill in using it increase, to the discovery of bodies hitherto invisible, and in both classes to the determination of their distances, their relations to each other, and the laws which govern their movements. why should we wish to obtain this knowledge? what inducement is there to expend large sums of money in the erection of observatories, and in furnishing them with costly instruments, and in the support of the men of science employed in making, discussing, and recording, for successive generations, those minute observations of the heavenly bodies? in an exclusively scientific treatment of this subject, an inquiry into its utilitarian relations would be superfluous--even wearisome. but on an occasion like the present, you will not, perhaps, think it out of place if i briefly answer the question, what is the use of an observatory, and what benefit may be expected from the operations of such an establishment in a community like ours? . in the first place, then, we derive from the observations of the heavenly bodies which are made at an observatory, our only adequate measures of time, and our only means of comparing the time of one place with the time of another. our artificial time-keepers--clocks, watches, and chronometers--however ingeniously contrived and admirably fabricated, are but a transcript, so to say, of the celestial motions, and would be of no value without the means of regulating them by observation. it is impossible for them, under any circumstances, to escape the imperfection of all machinery the work of human hands; and the moment we remove with our time-keeper east or west, it fails us. it will keep home time alone, like the fond traveler who leaves his heart behind him. the artificial instrument is of incalculable utility, but must itself be regulated by the eternal clock-work of the skies. relations between natural phenomena and daily life. this single consideration is sufficient to show how completely the daily business of life is affected and controlled by the heavenly bodies. it is they--and not our main-springs, our expansion balances, and our compensation pendulums--which give us our time. to reverse the line of pope: "'tis with our watches as our judgments;--none go just alike, but each believes his own." but for all the kindreds and tribes and tongues of men--each upon their own meridian--from the arctic pole to the equator, from the equator to the antarctic pole, the eternal sun strikes twelve at noon, and the glorious constellations, far up in the everlasting belfries of the skies, chime twelve at midnight;--twelve for the pale student over his flickering lamp; twelve amid the flaming glories of orion's belt, if he crosses the meridian at that fated hour; twelve by the weary couch of languishing humanity; twelve in the star-paved courts of the empyrean; twelve for the heaving tides of the ocean; twelve for the weary arm of labor; twelve for the toiling brain; twelve for the watching, waking, broken heart; twelve for the meteor which blazes for a moment and expires; twelve for the comet whose period is measured by centuries; twelve for every substantial, for every imaginary thing, which exists in the sense, the intellect, or the fancy, and which the speech or thought of man, at the given meridian, refers to the lapse of time. not only do we resort to the observation of the heavenly bodies for the means of regulating and rectifying our clocks, but the great divisions of day and month and year are derived from the same source. by the constitution of our nature, the elements of our existence are closely connected with celestial times. partly by his physical organization, partly by the experience of the race from the dawn of creation, man as he is, and the times and seasons of the heavenly bodies, are part and parcel of one system. the first great division of time, the day-night (nychthemerum), for which we have no precise synonym in our language, with its primal alternation of waking and sleeping, of labor and rest, is a vital condition of the existence of such a creature as man. the revolution of the year, with its various incidents of summer and winter, and seed-time and harvest, is not less involved in our social, material, and moral progress. it is true that at the poles, and on the equator, the effects of these revolutions are variously modified or wholly disappear; but as the necessary consequence, human life is extinguished at the poles, and on the equator attains only a languid or feverish development. those latitudes only in which the great motions and cardinal positions of the earth exert a mean influence, exhibit man in the harmonious expansion of his powers. the lunar period, which lies at the foundation of the _month_, is less vitally connected with human existence and development; but is proved by the experience of every age and race to be eminently conducive to the progress of civilization and culture. but indispensable as are these heavenly measures of time to our life and progress, and obvious as are the phenomena on which they rest, yet owing to the circumstance that, in the economy of nature, the day, the month, and the year are not exactly commensurable, some of the most difficult questions in practical astronomy are those by which an accurate division of time, applicable to the various uses of life, is derived from the observation of the heavenly bodies. i have no doubt that, to the supreme intelligence which created and rules the universe, there is a harmony hidden to us in the numerical relation to each other of days, months, and years; but in our ignorance of that harmony, their practical adjustment to each other is a work of difficulty. the great embarrassment which attended the reformation of the calendar, after the error of the julian period had, in the lapse of centuries, reached ten (or rather twelve) days, sufficiently illustrates this remark. it is most true that scientific difficulties did not form the chief obstacle. having been proposed under the auspices of the roman pontiff, the protestant world, for a century and more, rejected the new style. it was in various places the subject of controversy, collision, and bloodshed.[a] it was not adopted in england till nearly two centuries after its introduction at rome; and in the country of struve and the pulkova equatorial, they persist at the present day in adding eleven minutes and twelve seconds to the length of the tropical year. [footnote a: stern's "_himmelskunde_," p. .] geographical science. . the second great practical use of an astronomical observatory is connected with the science of geography. the first page of the history of our continent declares this truth. profound meditation on the sphericity of the earth was one of the main reasons which led columbus to undertake his momentous voyage; and his thorough acquaintance with the astronomical science of that day was, in his own judgment, what enabled him to overcome the almost innumerable obstacles which attended its prosecution.[a] in return, i find that copernicus in the very commencement of his immortal work _de revolutionibus orbium coelestium_, fol. , appeals to the discovery of america as completing the demonstration of the sphericity of the earth. much of our knowledge of the figure, size, density, and position of the earth, as a member of the solar system, is derived from this science; and it furnishes us the means of performing the most important operations of practical geography. latitude and longitude, which lie at the basis of all descriptive geography, are determined by observation. no map deserves the name, on which the position of important points has not been astronomically determined. some even of our most important political and administrative arrangements depend upon the coöperation of this science. among these i may mention the land system of the united states, and the determination of the boundaries of the country. i believe that till it was done by the federal government, a uniform system of mathematical survey had never in any country been applied to an extensive territory. large grants and sales of public land took place before the revolution, and in the interval between the peace and the adoption of the constitution; but the limits of these grants and sales were ascertained by sensible objects, by trees, streams, rocks, hills, and by reference to adjacent portions of territory, previously surveyed. the uncertainty of boundaries thus defined, was a never-failing source of litigation. large tracts of land in the western country, granted by virginia under this old system of special and local survey, were covered with conflicting claims; and the controversies to which they gave rise formed no small part of the business of the federal court after its organization. but the adoption of the present land-system brought order out of chaos. the entire public domain is now scientifically surveyed before it is offered for sale; it is laid off into ranges, townships, sections, and smaller divisions, with unerring accuracy, resting on the foundation of base and meridian lines; and i have been informed that under this system, scarce a case of contested location and boundary has ever presented itself in court. the general land office contains maps and plans, in which every quarter-section of the public land is laid down with mathematical precision. the superficies of half a continent is thus transferred in miniature to the bureaus of washington; while the local land offices contain transcripts of these plans, copies of which are furnished to the individual purchaser. when we consider the tide of population annually flowing into the public domain, and the immense importance of its efficient and economical administration, the utility of this application of astronomy will be duly estimated. [footnote a: humboldt, _histotre de la geographie_, &c., tom. , page .] i will here venture to repeat an anecdote, which i heard lately from a son of the late hon. timothy pickering. mr. octavius pickering, on behalf of his father, had applied to mr. david putnam of marietta, to act as his legal adviser, with respect to certain land claims in the virginia military district, in the state of ohio. mr. putnam declined the agency. he had had much to do with business of that kind, and found it beset with endless litigation. "i have never," he added, "succeeded but in a single case, and that was a location and survey made by general washington before the revolution; and i am not acquainted with any surveys, except those made by him, but what have been litigated." at this moment, a most important survey of the coast of the united states is in progress, an operation of the utmost consequence, in reference to the commerce, navigation, and hydrography of the country. the entire work, i need scarce say, is one of practical astronomy. the scientific establishment which we this day inaugurate is looked to for important coöperation in this great undertaking, and will no doubt contribute efficiently to its prosecution. astronomical observation furnishes by far the best means of defining the boundaries of states, especially when the lines are of great length and run through unsettled countries. natural indications, like rivers and mountains, however indistinct in appearance, are in practice subject to unavoidable error. by the treaty of , a boundary was established between the united states and great britain, depending chiefly on the course of rivers and highlands dividing the waters which flow into the atlantic ocean from those which flow into the st. lawrence. it took twenty years to find out which river was the true st. croix, that being the starting point. england then having made the extraordinary discovery that the bay of fundy is not a part of the atlantic ocean, forty years more were passed in the unsuccessful attempt to re-create the highlands which this strange theory had annihilated; and just as the two countries were on the verge of a war, the controversy was settled by compromise. had the boundary been accurately described by lines of latitude and longitude, no dispute could have arisen. no dispute arose as to the boundary between the united states and spain, and her successor, mexico, where it runs through untrodden deserts and over pathless mountains along the d degree of latitude. the identity of rivers may be disputed, as in the case of the st. croix; the course of mountain chains is too broad for a dividing line; the division of streams, as experience has shown, is uncertain; but a degree of latitude is written on the heavenly sphere, and nothing but an observation is required to read the record. questions of boundary. but scientific elements, like sharp instruments, must be handled with scientific accuracy. a part of our boundary between the british provinces ran upon the forty-fifth degree of latitude; and about forty years ago, an expensive fortress was commenced by the government of the united states, at rouse's point, on lake champlain, on a spot intended to be just within our limits. when a line came to be more carefully surveyed, the fortress turned out to be on the wrong side of the line; we had been building an expensive fortification for our neighbor. but in the general compromises of the treaty of washington by the webster and ashburton treaty in , the fortification was left within our limits.[a] [footnote a: webster's works. vol. v., , .] errors still more serious had nearly resulted, a few years since, in a war with mexico. by the treaty of guadalupe hidalgo, in , the boundary line between the united states and that country was in part described by reference to the town of el paso, as laid down on a specified map of the united states, of which a copy was appended to the treaty. this boundary was to be surveyed and run by a joint commission of men of science. it soon appeared that errors of two or three degrees existed in the projection of the map. its lines of latitude and longitude did not conform to the topography of the region; so that it became impossible to execute the text of the treaty. the famous mesilla valley was a part of the debatable ground; and the sum of $ , , , paid to the mexican government for that and for an additional strip of territory on the southwest, was the smart-money which expiated the inaccuracy of the map--the necessary result, perhaps, of the want of good materials for its construction. it became my official duty in london, a few years ago, to apply to the british government for an authentic statement of their claim to jurisdiction over new zealand. the official _gazette_ for the d of october, , was sent me from the foreign office, as affording the desired information. this number of the _gazette_ contained the proclamations issued by the lieutenant governor of new zealand, "in pursuance of the instructions he received from the marquis of normanby, one of her majesty's principal secretaries of state," asserting the jurisdiction of his government over the islands of new zealand, and declaring them to extend "from ° ' north to ° ' south latitude." it is scarcely necessary to say that south latitude was intended in both instances. this error of ° of latitude, which would have extended the claim of british jurisdiction over the whole breadth of the pacific, had, apparently, escaped the notice of that government. commerce and navigation. it would be easy to multiply illustrations in proof of the great practical importance of accurate scientific designations, drawn from astronomical observations, in various relations connected with boundaries, surveys, and other geographical purposes; but i must hasten to . a third important department, in which the services rendered by astronomy are equally conspicuous. i refer to commerce and navigation. it is mainly owing to the results of astronomical observation, that modern commerce has attained such a vast expansion, compared with that of the ancient world. i have already reminded you that accurate ideas in this respect contributed materially to the conception in the mind of columbus of his immortal enterprise, and to the practical success with which it was conducted. it was mainly his skill in the use of astronomical instruments--imperfect as they were--which enabled him, in spite of the bewildering variation of the compass, to find his way across the ocean. with the progress of the true system of the universe toward general adoption, the problem of finding the longitude at sea presented itself. this was the avowed object of the foundation of the observatory at greenwich;[a] and no one subject has received more of the attention of astronomers, than those investigations of the lunar theory on which the requisite tables of the navigator are founded. the pathways of the ocean are marked out in the sky above. the eternal lights of the heavens are the only pharos whose beams never fail, which no tempest can shake from its foundation. within my recollection, it was deemed a necessary qualification for the master and the mate of a merchant-ship, and even for a prime hand, to be able to "work a lunar," as it was called. the improvements in the chronometer have in practice, to a great extent, superseded this laborious operation; but observation remains, and unquestionably will for ever remain, the only dependence for ascertaining the ship's time and deducting the longitude from the comparison of that time with the chronometer. [footnote a: grant's _physical astronomy_, p. .] it may, perhaps, be thought that astronomical science is brought already to such a state of perfection that nothing more is to be desired, or at least that nothing more is attainable, in reference to such practicable applications as i have described. this, however, is an idea which generous minds will reject, in this, as in every other department of human knowledge. in astronomy, as in every thing else, the discoveries already made, theoretical or practical, instead of exhausting the science, or putting a limit to its advancement, do but furnish the means and instruments of further progress. i have no doubt we live on the verge of discoveries and inventions, in every department, as brilliant as any that have ever been made; that there are new truths, new facts, ready to start into recognition on every side; and it seems to me there never was an age, since the dawn of time, when men ought to be less disposed to rest satisfied with the progress already made, than the age in which we live; for there never was an age more distinguished for ingenious research, for novel result, and bold generalization. that no further improvement is desirable in the means and methods of ascertaining the ship's place at sea, no one i think will from experience be disposed to assert. the last time i crossed the atlantic, i walked the quarter-deck with the officer in charge of the noble vessel, on one occasion, when we were driving along before a leading breeze and under a head of steam, beneath a starless sky at midnight, at the rate certainly of ten or eleven miles an hour. there is something sublime, but approaching the terrible, in such a scene;--the rayless gloom, the midnight chill,--the awful swell of the deep,--the dismal moan of the wind through the rigging, the all but volcanic fires within the hold of the ship. i scarce know an occasion in ordinary life in which a reflecting mind feels more keenly its hopeless dependence on irrational forces beyond its own control. i asked my companion how nearly he could determine his ship's place at sea under favorable circumstances. theoretically, he answered, i think, within a mile;--practically and usually within three or four. my next question was, how near do you think we may be to cape race;--that dangerous headland which pushes its iron-bound unlighted bastions from the shore of newfoundland far into the atlantic,--first landfall to the homeward-bound american vessel. we must, said he, by our last observations and reckoning, be within three or four miles of cape race. a comparison of these two remarks, under the circumstances in which we were placed at the moment, brought my mind to the conclusion, that it is greatly to be wished that the means should be discovered of finding the ship's place more accurately, or that navigators would give cape race a little wider berth. but i do not remember that one of the steam packets between england and america was ever lost on that formidable point. it appears to me by no means unlikely that, with the improvement of instrumental power, and of the means of ascertaining the ship's time with exactness, as great an advance beyond the present state of art and science in finding a ship's place at sea may take place, as was effected by the invention of the reflecting quadrant, the calculation of lunar tables, and the improved construction of chronometers. babbage's difference machine. in the wonderful versatility of the human mind, the improvement, when made, will very probably be made by paths where it is least expected. the great inducement to mr. babbage to attempt the construction of an engine by which astronomical tables could be calculated, and even printed, by mechanical means and with entire accuracy, was the errors in the requisite tables. nineteen such errors, in point of fact, were discovered in an edition of taylor's logarithms printed in ; some of which might have led to the most dangerous results in calculating a ship's place. these nineteen errors, (of which one only was an error of the press), were pointed out in the _nautical almanac_ for . in one of these _errata_ the seat of the error was stated to be in cosine of ° ' ". subsequent examination showed that there was an error of one second in this correction; and, accordingly, in the _nautical almanac_ of the next year a new correction was necessary. but in making the new correction of one second, a new error was committed of ten degrees. instead of cosine ° ' " the correction was printed cosine ° ' " making it still necessary, in some future edition of the _nautical almanac_, to insert an _erratum_ in an _erratum_ of the _errata_ in taylor's logarithms.[a] [footnote a: edinburgh review, vol. lix., .] in the hope of obviating the possibility of such errors, mr. babbage projected his calculating, or, as he prefers to call it, his difference machine. although this extraordinary undertaking has been arrested, in consequence of the enormous expense attending its execution, enough has been achieved to show the mechanical possibility of constructing an engine of this kind, and even one of far higher powers, of which mr. babbage has matured the conception, devised the notation, and executed the drawings--themselves an imperishable monument of the genius of the author. i happened on one occasion to be in company with this highly distinguished man of science, whose social qualities are as pleasing as his constructive talent is marvelous, when another eminent _savant_, count strzelecki, just returned from his oriental and australian tour, observed that he found among the chinese, a great desire to know something more of mr. babbage's calculating machine, and especially whether, like their own _swampan_, it could be made to go into the pocket. mr. babbage good-humouredly observed that, thus far, he had been very much out of pocket with it. increased command of instrumental power. whatever advances may be made in astronomical science, theoretical or applied, i am strongly inclined to think that they will be made in connection with an increased command of instrumental power. the natural order in which the human mind proceeds in the acquisition of astronomical knowledge is minute and accurate observation of the phenomena of the heavens, the skillful discussion and analysis of these observations, and sound philosophy in generalizing the results. in pursuing this course, however, a difficulty presented itself, which for ages proved insuperable--and which to the same extent has existed in no other science, viz.: that all the leading phenomena are in their appearance delusive. it is indeed true that in all sciences superficial observation can only lead, except by chance, to superficial knowledge; but i know of no branch in which, to the same degree as in astronomy, the great leading phenomena are the reverse of true; while they yet appeal so strongly to the senses, that men who could foretell eclipses, and who discovered the precession of the equinoxes, still believed that the earth was at rest in the center of the universe, and that all the host of heaven performed a daily revolution about it as a center. it usually happens in scientific progress, that when a great fact is at length discovered, it approves itself at once to all competent judges. it furnishes a solution to so many problems, and harmonizes with so many other facts,--that all the other _data_ as it were crystallize at once about it. in modern times, we have often witnessed such an impatience, so to say, of great truths, to be discovered, that it has frequently happened that they have been found out simultaneously by more than one individual; and a disputed question of priority is an event of very common occurrence. not so with the true theory of the heavens. so complete is the deception practiced on the senses, that it failed more than once to yield to the suggestion of the truth; and it was only when the visual organs were armed with an almost preternatural instrumental power, that the great fact found admission to the human mind. the copernican system. it is supposed that in the very dawn of science, pythagoras or his disciples explained the apparent motion of the heavenly bodies about the earth by the diurnal revolution of the earth on its axis. but this theory, though bearing so deeply impressed upon it the great seal of truth, _simplicity_, was in such glaring contrast with the evidence of the senses, that it failed of acceptance in antiquity or the middle ages. it found no favor with minds like those of aristotle, archimedes, hipparchus, ptolemy, or any of the acute and learned arabian or mediæval astronomers. all their ingenuity and all their mathematical skill were exhausted in the development of a wonderfully complicated and ingenious, but erroneous history. the great master truth, rejected for its simplicity, lay disregarded at their feet. at the second dawn of science, the great fact again beamed into the mind of copernicus. now, at least, in that glorious age which witnessed the invention of printing, the great mechanical engine of intellectual progress, and the discovery of america, we may expect that this long-hidden revelation, a second time proclaimed, will command the assent of mankind. but the sensible phenomena were still too strong for the theory; the glorious delusion of the rising and the setting sun could not be overcome. tycho de brahe furnished his observatory with instruments superior in number and quality to all that had been collected before; but the great instrument of discovery, which, by augmenting the optic power of the eye, enables it to penetrate beyond the apparent phenomena, and to discern the true constitution of the heavenly bodies, was wanting at uranienburg. the observations of tycho as discussed by kepler, conducted that most fervid, powerful, and sagacious mind to the discovery of some of the most important laws of the celestial motions; but it was not till galileo, at florence, had pointed his telescope to the sky, that the copernican system could be said to be firmly established in the scientific world. the home of galileo. on this great name, my friends, assembled as we are to dedicate a temple to instrumental astronomy, we may well pause for a moment. there is much, in every way, in the city of florence to excite the curiosity, to kindle the imagination, and to gratify the taste. sheltered on the north by the vine-clad hills of fiesoli, whose cyclopean walls carry back the antiquary to ages before the roman, before the etruscan power, the flowery city (fiorenza) covers the sunny banks of the arno with its stately palaces. dark and frowning piles of mediæval structure; a majestic dome, the prototype of st. peter's; basilicas which enshrine the ashes of some of the mightiest of the dead; the stone where dante stood to gaze on the campanile; the house of michael angelo, still occupied by a descendant of his lineage and name, his hammer, his chisel, his dividers, his manuscript poems, all as if he had left them but yesterday; airy bridges, which seem not so much to rest on the earth as to hover over the waters they span; the loveliest creations of ancient art, rescued from the grave of ages again to enchant the world; the breathing marbles of michael angelo, the glowing canvas of raphael and titian, museums filled with medals and coins of every age from cyrus the younger, and gems and amulets and vases from the sepulchers of egyptian pharaohs coëval with joseph, and etruscan lucumons that swayed italy before the romans,--libraries stored with the choicest texts of ancient literature,--gardens of rose and orange, and pomegranate, and myrtle,--the very air you breathe languid with music and perfume;--such is florence. but among all its fascinations, addressed to the sense, the memory, and the heart, there was none to which i more frequently gave a meditative hour during a year's residence, than to the spot where galileo galilei sleeps beneath the marble door of santa croce; no building on which i gazed with greater reverence, than i did upon the modest mansion at arcetri, villa at once and prison, in which that venerable sage, by command of the inquisition, passed the sad closing years of his life. the beloved daughter on whom he had depended to smooth his passage to the grave, laid there before him; the eyes with which he had discovered worlds before unknown, quenched in blindness: ahime! quegli occhi si son fatti oscuri, che vider più di tutti i tempi antichi, e luce fur dei secoli futuri. that was the house, "where," says milton (another of those of whom the world was not worthy), "i found and visited the famous galileo, grown old--a prisoner to the inquisition, for thinking on astronomy otherwise than as the dominican and franciscan licensers thought."[a] great heavens! what a tribunal, what a culprit, what a crime! let us thank god, my friends, that we live in the nineteenth century. of all the wonders of ancient and modern art, statues and paintings, and jewels and manuscripts,--the admiration and the delight of ages,--there was nothing which i beheld with more affectionate awe than that poor, rough tube, a few feet in length,--the work of his own hands,--that very "optic glass," through which the "tuscan artist" viewed the moon, "at evening, from the top of fesolé, or in valdarno, to descry new lands, rivers, or mountains, in her spotty globe." that poor little spy-glass (for it is scarcely more) through which the human eye first distinctly beheld the surface of the moon--first discovered the phases of venus, the satellites of jupiter, and the seeming handles of saturn--first penetrated the dusky depths of the heavens--first pierced the clouds of visual error, which, from the creation of the world, involved the system of the universe. [footnote a: prose works, vol. , p. .] there are occasions in life in which a great mind lives years of rapt enjoyment in a moment. i can fancy the emotions of galileo, when, first raising the newly-constructed telescope to the heavens, he saw fulfilled the grand prophecy of copernicus, and beheld the planet venus crescent like the moon. it was such another moment as that when the immortal printers of mentz and strasburg received the first copy of the bible into their hands, the work of their divine art; like that when columbus, through the gray dawn of the th of october, (copernicus, at the age of eighteen, was then a student at cracow), beheld the shores of san salvador; like that when the law of gravitation first revealed itself to the intellect of newton; like that when franklin saw by the stiffening fibers of the hempen cord of his kite, that he held the lightning in his grasp; like that when leverrier received back from berlin the tidings that the predicted planet was found. yes, noble galileo, thou art right, _e pur si muove._ "it does move." bigots may make thee recant it; but it moves, nevertheless. yes, the earth moves, and the planets move, and the mighty waters move, and the great sweeping tides of air move, and the empires of men move, and the world of thought moves, ever onward and upward to higher facts and bolder theories. the inquisition may seal thy lips, but they can no more stop the progress of the great truth propounded by copernicus, and demonstrated by thee, than they can stop the revolving earth. close now, venerable sage, that sightless, tearful eye; it has seen what man never before saw--it has seen enough. hang up that poor little spy-glass--it has done its work. not herschell nor rosse have, comparatively, done more. franciscans and dominicans deride thy discoveries now; but the time will come when, from two hundred observatories in europe and america, the glorious artillery of science shall nightly assault the skies, but they shall gain no conquests in those glittering fields before which thine shall be forgotten. rest in peace, great columbus of the heavens--like him scorned, persecuted, broken-hearted!--in other ages, in distant hemispheres, when the votaries of science, with solemn acts of consecration, shall dedicate their stately edifices to the cause of knowledge and truth, thy name shall be mentioned with honor. new periods in astronomical science. it is not my intention, in dwelling with such emphasis upon the invention of the telescope, to ascribe undue importance, in promoting the advancement of science, to the increase of instrumental power. too much, indeed, cannot be said of the service rendered by its first application in confirming and bringing into general repute the copernican system; but for a considerable time, little more was effected by the wondrous instrument than the gratification of curiosity and taste, by the inspection of the planetary phases, and the addition of the rings and satellites of saturn to the solar family. newton, prematurely despairing of any further improvement in the refracting telescope, applied the principle of reflection; and the nicer observations now made, no doubt, hastened the maturity of his great discovery of the law of gravitation; but that discovery was the work of his transcendent genius and consummate skill. with bradley, in , a new period commenced in instrumental astronomy, not so much of discovery as of measurement. the superior accuracy and minuteness with which the motions and distances of the heavenly bodies were now observed, resulted in the accumulation of a mass of new materials, both for tabular comparison and theoretical speculation. these materials formed the enlarged basis of astronomical science between newton and sir william herschell. his gigantic reflectors introduced the astronomer to regions of space before unvisited--extended beyond all previous conception the range of the observed phenomena, and with it proportionably enlarged the range of constructive theory. the discovery of a new primary planet and its attendant satellites was but the first step of his progress into the labyrinth of the heavens. cotemporaneously with his observations, the french astronomers, and especially la place, with a geometrical skill scarcely, if at all, inferior to that of its great author, resumed the whole system of newton, and brought every phenomenon observed since his time within his laws. difficulties of fact, with which he struggled in vain, gave way to more accurate observations; and problems that defied the power of his analysis, yielded to the modern improvements of the calculus. herschell's nebular theory. but there is no _ultima thule_ in the progress of science. with the recent augmentations of telescopic power, the details of the nebular theory, proposed by sir w. herschell with such courage and ingenuity, have been drawn in question. many--most--of those milky patches in which he beheld what he regarded as cosmical matter, as yet in an unformed state,--the rudimental material of worlds not yet condensed,--have been resolved into stars, as bright and distinct as any in the firmament. i well recall the glow of satisfaction with which, on the d of september, , being then connected with the university at cambridge, i received a letter from the venerable director of the observatory there, beginning with these memorable words:--"you will rejoice with me that the great nebula in orion has yielded to the powers of our incomparable telescope! * * * it should be borne in mind that this nebula, and that of andromeda [which has been also resolved at cambridge], are the last strongholds of the nebular theory."[a] [footnote a: _annals of the observatory of harvard college_, p. .] but if some of the adventurous speculations built by sir william herschell on the bewildering revelations of his telescope have been since questioned, the vast progress which has been made in sidereal astronomy, to which, as i understand, the dudley observatory will be particularly devoted, the discovery of the parallax of the fixed stars, the investigation of the interior relations of binary and triple systems of stars, the theories for the explanation of the extraordinary, not to say fantastic, shapes discerned in some of the nebulous systems--whirls and spirals radiating through spaces as vast as the orbit of neptune;[a] the glimpses at systems beyond that to which our sun belongs;--these are all splendid results, which may fairly be attributed to the school of herschell, and will for ever insure no secondary place to that name in the annals of science. [footnote a: see the remarkable memoir of professor alexander, "on the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulæ," (gould's _astronomical journal_, vol. iii, p. .)] relationship of the liberal arts. in the remarks which i have hitherto made, i have had mainly in view the direct connection of astronomical science with the uses of life and the service of man. but a generous philosophy contemplates the subject in higher relations. it is a remark as old, at least, as plato, and is repeated from him more than once by cicero, that all the liberal arts have a common bond and relationship.[a] the different sciences contemplate as their immediate object the different departments of animate and inanimate nature; but this great system itself is but one, and its parts are so interwoven with each other, that the most extraordinary relations and unexpected analogies are constantly presenting themselves; and arts and sciences seemingly the least connected, render to each other the most effective assistance. [footnote a: archias, i.; de oratore, iii., .] the history of electricity, galvanism, and magnetism, furnishes the most striking illustration of this remark. commencing with the meteorological phenomena of our own atmosphere, and terminating with the observation of the remotest heavens, it may well be adduced, on an occasion like the present. franklin demonstrated the identity of lightning and the electric fluid. this discovery gave a great impulse to electrical research, with little else in view but the means of protection from the thunder-cloud. a purely accidental circumstance led the physician galvani, at bologna, to trace the mysterious element, under conditions entirely novel, both of development and application. in this new form it became, in the hands of davy, the instrument of the most extraordinary chemical operations; and earths and alkalis, touched by the creative wire, started up into metals that float on water, and kindle in the air. at a later period, the closest affinities are observed between electricity and magnetism, on the one hand; while, on the other, the relations of polarity are detected between acids and alkalis. plating and gilding henceforth become electrical processes. in the last applications of the same subtle medium, it has become the messenger of intelligence across the land and beneath the sea; and is now employed by the astronomer to ascertain the difference of longitudes, to transfer the beats of the clock from one station to another, and to record the moment of his observations with automatic accuracy. how large a share has been borne by america in these magnificent discoveries and applications, among the most brilliant achievements of modern science, will sufficiently appear from the repetition of the names of franklin, henry, morse, walker, mitchell, lock, and bond. versatility of genius. it has sometimes happened, whether from the harmonious relations to each other of every department of science, or from rare felicity of individual genius, that the most extraordinary intellectual versatility has been manifested by the same person. although newton's transcendent talent did not blaze out in childhood, yet as a boy he discovered great aptitude for mechanical contrivance. his water-clock, self-moving vehicle, and mill, were the wonder of the village; the latter propelled by a living mouse. sir david brewster represents the accounts as differing, whether the mouse was made to advance "by a string attached to its tail," or by "its unavailing attempts to reach a portion of corn placed above the wheel." it seems more reasonable to conclude that the youthful discoverer of the law of gravitation intended by the combination of these opposite attractions to produce a balanced movement. it is consoling to the average mediocrity of the race to perceive in these sportive assays, that the mind of newton passed through the stage of boyhood. but emerging from boyhood, what a bound it made, as from earth to heaven! hardly commencing bachelor of arts, at the age of twenty-four, he untwisted the golden and silver threads of the solar spectrum, simultaneously or soon after conceived the method of fluxions, and arrived at the elemental idea of universal gravity before he had passed to his master's degree. master of arts indeed! that degree, if no other, was well bestowed. universities are unjustly accused of fixing science in stereotype. that diploma is enough of itself to redeem the honors of academical parchment from centuries of learned dullness and scholastic dogmatism. but the great object of all knowledge is to enlarge and purify the soul, to fill the mind with noble contemplations, to furnish a refined pleasure, and to lead our feeble reason from the works of nature up to its great author and sustainer. considering this as the ultimate end of science, no branch of it can surely claim precedence of astronomy. no other science furnishes such a palpable embodiment of the abstractions which lie at the foundation of our intellectual system; the great ideas of time, and space, and extension, and magnitude, and number, and motion, and power. how grand the conception of the ages on ages required for several of the secular equations of the solar system; of distances from which the light of a fixed star would not reach us in twenty millions of years, of magnitudes compared with which the earth is but a foot-ball; of starry hosts--suns like our own--numberless as the sands on the shore; of worlds and systems shooting through the infinite spaces, with a velocity compared with which the cannon-ball is a way-worn, heavy-paced traveler![a] [footnote a: nichol's _architecture of the heavens_, p. .] the spectacle of the heavens. much, however, as we are indebted to our observatories for elevating our conceptions of the heavenly bodies, they present, even to the unaided sight, scenes of glory which words are too feeble to describe. i had occasion, a few weeks since, to take the early train from providence to boston; and for this purpose rose at o'clock in the morning. every thing around was wrapped in darkness and hushed in silence, broken only by what seemed at that hour the unearthly clank and rush of the train. it was a mild, serene midsummer's night; the sky was without a cloud--the winds were whist. the moon, then in the last quarter, had just risen, and the stars shone with a spectral luster but little affected by her presence; jupiter, two hours high, was the herald of the day; the pleiades, just above the horizon, shed their sweet influence in the east; lyra sparkled near the zenith; andromeda veiled her newly discovered glories from the naked eye in the south; the steady pointers, far beneath the pole, looked meekly up from the depths of the north to their sovereign. such was the glorious spectacle as i entered the train. as we proceeded, the timid approach of twilight became more perceptible; the intense blue of the sky began to soften, the smaller stars, like little children, went first to rest; the sister-beams of the pleiades soon melted together; but the bright constellations of the west and north remained unchanged. steadily the wondrous transfiguration went on. hands of angels hidden from mortal eyes shifted the scenery of the heavens; the glories of night dissolved into the glories of the dawn. the blue sky now turned more softly gray; the great watch-stars shut up their holy eyes; the east began to kindle. faint streaks of purple soon blushed along the sky; the whole celestial concave was filled with the inflowing tides of the morning light, which came pouring down from above in one great ocean of radiance; till at length, as we reached the blue hills, a flash of purple fire blazed out from above the horizon, and turned the dewy teardrops of flower and leaf into rubies and diamonds. in a few seconds the everlasting gates of the morning were thrown wide open, and the lord of day, arrayed in glories too severe for the gaze of man, began his course. i do not wonder at the superstition of the ancient magians, who in the morning of the world went up to the hill-tops of central asia, and ignorant of the true god, adored the most glorious work of his hand. but i am filled with amazement, when i am told that in this enlightened age, and in the heart of the christian world, there are persons who can witness this daily manifestation of the power and wisdom of the creator, and yet say in their hearts, "there is no god." undiscovered bodies. numerous as are the heavenly bodies visible to the naked eye, and glorious as are their manifestations, it is probable that in our own system there are great numbers as yet undiscovered. just two hundred years ago this year, huyghens announced the discovery of one satellite of saturn, and expressed the opinion that the six planets and six satellites then known, and making up the perfect number of _twelve_, composed the whole of our planetary system. in an astronomical writer expressed the opinion that there might be other bodies in our system, but that the limit of telescopic power had been reached, and no further discoveries were likely to be made.[a] the orbit of one comet only had been definitively calculated. since that time the power of the telescope has been indefinitely increased; two primary planets of the first class, ten satellites, and forty-three small planets revolving between mars and jupiter, have been discovered, the orbits of six or seven hundred comets, some of brief period, have been ascertained;--and it has been computed, that hundreds of thousands of these mysterious bodies wander through our system. there is no reason to think that all the primary planets, which revolve about the sun, have been discovered. an indefinite increase in the number of asteroids may be anticipated; while outside of neptune, between our sun and the nearest fixed star, supposing the attraction of the sun to prevail through half the distance, there is room for ten more primary planets succeeding each other at distances increasing in a geometrical ratio. the first of these will, unquestionably, be discovered as soon as the perturbations of neptune shall have been accurately observed; and with maps of the heavens, on which the smallest telescopic stars are laid down, it may be discovered much sooner. [footnote a: _memoirs of a.a.s._, vol. iii, .] the vastness of creation. but it is when we turn our observation and our thoughts from our own system, to the systems which lie beyond it in the heavenly spaces, that we approach a more adequate conception of the vastness of creation. all analogy teaches us that the sun which gives light to us is but one of those countless stellar fires which deck the firmament, and that every glittering star in that shining host is the center of a system as vast and as full of subordinate luminaries as our own. of these suns--centers of planetary systems--thousands are visible to the naked eye, millions are discovered by the telescope. sir john herschell, in the account of his operations at the cape of good hope (p. ) calculates that about five and a half millions of stars are visible enough to be _distinctly counted_ in a twenty-foot reflector, in both hemispheres. he adds, that "the actual number is much greater, there can be little doubt." his illustrious father, estimated on one occasion that , stars passed through the field of his forty foot reflector in a quarter of an hour. this would give , , for the entire circuit of the heavens, in a single telescopic zone; and this estimate was made under the assumption that the nebulæ were masses of luminous matter not yet condensed into suns. these stupendous calculations, however, form but the first column of the inventory of the universe. faint white specks are visible, even to the naked eye of a practiced observer in different parts of the heavens. under high magnifying powers, several thousands of such spots are visible,--no longer however, faint, white specks, but many of them resolved by powerful telescopes into vast aggregations of stars, each of which may, with propriety, be compared with the milky way. many of these nebulæ, however, resisted the power of sir wm. herschell's great reflector, and were, accordingly, still regarded by him as masses of unformed matter, not yet condensed into suns. this, till a few years since, was, perhaps, the prevailing opinion; and the nebular theory filled a large space in modern astronomical science. but with the increase of instrumental power, especially under the mighty grasp of lord rosse's gigantic reflector, and the great refractors at pulkova and cambridge, the most irresolvable of these nebulæ have given way; and the better opinion now is, that every one of them is a galaxy, like our own milky way, composed of millions of suns. in other words, we are brought to the bewildering conclusion that thousands of these misty specks, the greater part of them too faint to be seen with the naked eye, are, not each a universe like our solar system, but each a "swarm" of universes of unappreciable magnitude.[a] the mind sinks, overpowered by the contemplation. we repeat the words, but they no longer convey distinct ideas to the understanding. [footnote a: humboldt's _cosmos_, iii. .] conceptions of the universe. but these conclusions, however vast their comprehension, carry us but another step forward in the realms of sidereal astronomy. a proper motion in space of our sun, and of the fixed stars as we call them, has long been believed to exist. their vast distances only prevent its being more apparent. the great improvement of instruments of measurement within the last generation has not only established the existence of this motion, but has pointed to the region in the starry vault around which our whole solar and stellar system, with its myriad of attendant planetary worlds, appears to be performing a mighty revolution. if, then, we assume that outside of the system to which we belong and in which our sun is but a star like aldebaran or sirius, the different nebulæ of which we have spoken,--thousands of which spot the heavens--constitute a distinct family of universes, we must, following the guide of analogy, attribute to each of them also, beyond all the revolutions of their individual attendant planetary systems, a great revolution, comprehending the whole; while the same course of analogical reasoning would lead us still further onward, and in the last analysis, require us to assume a transcendental connection between all these mighty systems--a universe of universes, circling round in the infinity of space, and preserving its equilibrium by the same laws of mutual attraction which bind the lower worlds together. it may be thought that conceptions like these are calculated rather to depress than to elevate us in the scale of being; that, banished as he is by these contemplations to a corner of creation, and there reduced to an atom, man sinks to nothingness in this infinity of worlds. but a second thought corrects the impression. these vast contemplations are well calculated to inspire awe, but not abasement. mind and matter are incommensurable. an immortal soul, even while clothed in "this muddy vesture of decay," is in the eye of god and reason, a purer essence than the brightest sun that lights the depths of heaven. the organized human eye, instinct with life and soul, which, gazing through the telescope, travels up to the cloudy speck in the handle of orion's sword, and bids it blaze forth into a galaxy as vast as ours, stands higher in the order of being than all that host of luminaries. the intellect of newton which discovered the law that holds the revolving worlds together, is a nobler work of god than a universe of universes of unthinking matter. if, still treading the loftiest paths of analogy, we adopt the supposition,--to me i own the grateful supposition,--that the countless planetary worlds which attend these countless suns, are the abodes of rational beings like man, instead of bringing back from this exalted conception a feeling of insignificance, as if the individuals of our race were but poor atoms in the infinity of being, i regard it, on the contrary, as a glory of our human nature, that it belongs to a family which no man can number of rational natures like itself. in the order of being they may stand beneath us, or they may stand above us; _he_ may well be content with his place, who is made "a little lower than the angels." contemplation of the heavens. finally, my friends, i believe there is no contemplation better adapted to awaken devout ideas than that of the heavenly bodies,--no branch of natural science which bears clearer testimony to the power and wisdom of god than that to which you this day consecrate a temple. the heart of the ancient world, with all the prevailing ignorance of the true nature and motions of the heavenly orbs, was religiously impressed by their survey. there is a passage in one of those admirable philosophical treatises of cicero composed in the decline of life, as a solace under domestic bereavement and patriotic concern at the impending convulsions of the state, in which, quoting from some lost work of aristotle, he treats the topic in a manner which almost puts to shame the teachings of christian wisdom. "præclare ergo aristoteles, 'si essent,' inquit, 'qui sub terra semper habitavissent, bonis et illustribus domiciliis quæ essent ornata signis atque picturis, instructaque rebus iis omnibus quibus abundant ii qui beati putantur, nec tamen exissent unquam supra terram; accepissent autem fama et auditione, esse quoddam numen et vim deorum,--deinde aliquo tempore patefactis terræ faucibus ex illis abditis sedibus evadere in hæc loca quæ nos incolimus, atque exire potuissent; cum repente terram et maria coelumque, vidissent; nubium magnitudinem ventorumque vim, cognovissent; aspexissentque solem, ejusque tum magnitudinem, pulchritudinemque; tum etiam efficientiam cognovissent, quod is diem efficeret, toto coelo luce diffusa; cum autem terras nox opacasset, tum coelum totum cernerent astris distinctum et ornatum, lunæque luminum varietatem tum crescentis tum senescentis, corumque omnium ortus et occasus atque in æternitate ratos immutabilesque cursus;--hæc cum viderent, profecto et esse deos, et hæc tanta opera deorum esse, arbitrarentur."[a] there is much by day to engage the attention of the observatory; the sun, his apparent motions, his dimensions, the spots on his disc (to us the faint indications of movements of unimagined grandeur in his luminous atmosphere), a solar eclipse, a transit of the inferior planets, the mysteries of the spectrum;--all phenomena of vast importance and interest. but night is the astronomer's accepted time; he goes to his delightful labors when the busy world goes to its rest. a dark pall spreads over the resorts of active life; terrestrial objects, hill and valley, and rock and stream, and the abodes of men disappear; but the curtain is drawn up which concealed the heavenly hosts. there they shine and there they move, as they moved and shone to the eyes of newton and galileo, of kepler and copernicus, of ptolemy and hipparchus; yes, as they moved and shone when the morning stars sang together, and all the sons of god shouted for joy. all has changed on earth; but the glorious heavens remain unchanged. the plow passes over the site of mighty cities,--the homes of powerful nations are desolate, the languages they spoke are forgotten; but the stars that shone for them are shining for us; the same eclipses run their steady cycle; the same equinoxes call out the flowers of spring, and send the husbandman to the harvest; the sun pauses at either tropic as he did when his course began; and sun and moon, and planet and satellite, and star and constellation and galaxy, still bear witness to the power, the wisdom, and the love, which placed them in the heavens and uphold them there. [footnote a: "nobly does aristotle observe, that if there were beings who had always lived under ground, in convenient, nay, in magnificent dwellings, adorned with statues and pictures, and every thing which belongs to prosperous life, but who had never come above ground; who had heard, however, by fame and report, of the being and power of the gods; if, at a certain time, the portals of the earth being thrown open, they had been able to emerge from those hidden abodes to the regions inhabited by us; when suddenly they had seen the earth, the sea, and the sky; had perceived the vastness of the clouds and the force of the winds; had contemplated the sun, his magnitude and his beauty, and still more his effectual power, that it is he who makes the day, by the diffusion of his light through the whole sky; and, when night had darkened the earth, should then behold the whole heavens studded and adorned with stars, and the various lights of the waxing and waning moon, the risings and the settings of all these heavenly bodies, and the courses fixed and immutable in all eternity; when, i say, they should see these things, truly they would believe that there were gods, and these so great things are their works."--cicero, _de natura deorum_ lib. ii., § .] the children's book of stars the children's book of stars mitton a.&c. black [illustration: the moon-child must keep on running round her. p. .] the children's book of stars +-------------------------------------------------+ | by the same author | | | | children's book of london | | | | containing full-page illustrations | | in colour by john williamson | | price = s.= | | | | 'the stories are told in a way that is bound | | to rivet attention, and the historical sketches| | will leave a lasting impression on the minds | | of young readers which will be very useful | | when their studies in history become more | | advanced.'--_scotsman._ | | | | | | animal autobiographies | | | | the dog | | | | with full-page illustrations in | | colour by j. williamson | | | | price = s.= | | | | 'a true life history, written "out of the | | fulness of first-hand knowledge" by an author | | who is thoroughly acquainted with all the | | ways of "the friend of man."'--_glasgow | | herald._ | | | | 'the story is admirably told in clear and | | fascinating language.'--_freeman's journal._ | | | | a. & c. black. soho square. london, w. | | | +-------------------------------------------------+ agents america the macmillan company & fifth avenue, new york canada the macmillan company of canada, ltd. richmond street west, toronto india macmillan & company, ltd. macmillan building, bombay bow bazaar street, calcutta the children's book of stars by g. e. mitton author of 'the children's book of london,' 'animal autobiographies: the dog,' etc. london adam and charles black _published september, _ preface it was the intention of the late agnes clerke to write the preface to this 'children's book of stars.' miss clerke took a warm and sympathetic interest in the authoress and her work, but her lamented death occurred before this kindly intention could be fulfilled. i cannot pretend to write adequately as her substitute, but i could not resist the appeal made to me by the author, in the name and for the sake of her dear friend and mine, to write a few words of introduction. i am in no way responsible either for the plan or for any portion of this work, but i can commend it as a book, written in a simple and pleasant style, calculated to awaken the interest of intelligent children, and to enable parents otherwise ignorant or astronomy to answer many of those puzzling questions which such children often put. david gill. author's note this little work is the outcome of many suggestions on the part of friends who were anxious to teach their small children something of the marvels of the heavens, but found it exceedingly difficult to get hold of a book wherein the intense fascination of the subject was not lost in conventional phraseology--a book in which the stupendous facts were stated in language simple enough to be read aloud to a child without paraphrase. whatever merit there may be in the present work is due entirely to my friend agnes clerke, the well-known writer on astronomy; the faults are all my own. she gave me the impetus to begin by her warm encouragement, and she helped me to continue by hearing every chapter read as it was written, and by discussing its successor and making suggestions for it. thus she heard the whole book in ms. a week after the last chapter had been read to her i started on a journey lasting many months, and while i was in the far east the news reached me of her death, by which the world is the poorer. for her sake, as he has stated, her friend sir david gill, k.c.b., kindly undertook to supply the missing preface. g. e. mitton. contents chapter i page the earth chapter ii hanging in space chapter iii the shining moon chapter iv the earth's brothers and sister chapter v four small worlds chapter vi four large worlds chapter vii the sun chapter viii shining visitors chapter ix shooting stars and fiery balls chapter x the glittering heavens chapter xi the constellations chapter xii what the stars are made of chapter xiii restless stars chapter xiv the colours of the stars chapter xv temporary and variable stars chapter xvi star clusters and nebulÆ illustrations printed in colour the moon-child must keep on running round her _frontispiece_ facing page the earth and moon hanging in space the english summer and winter jupiter and one of his moons the planet saturn and two of his moons flames from the sun the comet in the bayeux tapestry a stick thrust into the water appears crooked constellations near the pole star orion and his neighbours the spectrum of the sun and sirius illustrations in black and white page the moon _facing_ an eclipse of the moon an eclipse of the sun the moon raising the tides comparative sizes of the planets different phases of venus orbits of mars, the earth, venus, and mercury map of mars _facing_ orbits of the earth and mars jupiter and his principal moons sun-spots _facing_ a great comet " the great nebula in andromeda " the children's book of stars chapter i the earth it is a curious fact that when we are used to things, we often do not notice them, and things which we do every day cease to attract our attention. we find an instance of this in the curious change that comes over objects the further they are removed from us. they grow smaller and smaller, so that at a distance a grown-up person looks no larger than a doll; and a short stick planted in the ground only a few feet away appears as long as a much longer one at ten times the distance. this process is going on all round us every minute: houses, trees, buildings, animals, all seem larger or smaller in proportion to their distance from us. sometimes i have seen a row of raindrops hanging on a bar by the window. when the sun catches one of them, it shines so brilliantly that it is as dazzling as a star; but my sense tells me it is a raindrop, and not a star at all. it is only because it is so near it seems as bright and important as a mighty star very, very far away. we are so much accustomed to this fact that we get into a habit of judging the distance of things by their size. if we see two lights shining on a dark night, and one is much larger than the other, we think that the bright one must be nearer to us; yet it need not necessarily be so, for the two lights might possibly be at the same distance from us, and one be large and the other small. there is no way in which we can tell the truth by just looking at them. now, if we go out on any fine moonlight night and look up at the sky, we shall see one object there apparently much larger than any other, and that is the moon, so the question that occurs to us at once is, is the moon really very much larger than any of the stars, or does it only seem so because it is very much nearer to us? as a matter of fact, the moon is one of the smallest objects in view, only, as it is our nearest neighbour, it appears very conspicuous. having learned this, we shall probably look about to see what else there is to attract attention, and we may notice one star shining very brilliantly, almost like a little lamp, rather low down in the sky, in that part of it where the sun has lately set. it is so beautifully bright that it makes all the others look insignificant in comparison, yet it is not really large compared with the others, only, as it comes nearer to us than anything else in the sky except the moon, it looks larger than it has any right to do in comparison with the others. after this we might jump to the conclusion that all the bright large stars are really small and near to us, and all the faintly shining ones large and far away. but that would not be true at all, for some bright ones are very far away and some faint ones comparatively near, so that all we can do is to learn about them from the people who have studied them and found out about them, and then we shall know of our own knowledge which of them seem bright only because they are nearer than the others, and which are really very, very brilliant, and so still shine brightly, though set in space at an almost infinite distance from us. the sun, as we all know, appears to cross the sky every day; he gets up in the east and drops down in the west, and the moon does the same, only the moon is unlike the sun in this, that it changes its shape continually. we see a crescent moon growing every night larger and larger, until it becomes full and fat and round, and then it grows thinner and thinner, until it dies away; and after a little while it begins again, and goes through all the same changes once more. i will tell you why this is so further on, when we have a chapter all about the moon. if you watch the stars quietly for at least five minutes, you will see that they too are moving steadily on in the same way as the sun and moon. watch one bright star coming out from behind a chimney-pot, and after about five minutes you will see that it has changed its place. yet this is not true of all, for if we watch carefully we shall find that some, fairly high up in the sky, do not appear to move at all. the few which are moving so slowly that they seem to us to stand still are at a part of the sky close to the pole star, so called because it is always above the north pole of the earth. i will explain to you how to find it in the sky for yourselves later on, but now you can ask anyone to point it out. watch it. it appears to be fixed in one place, while the other stars are swinging round it in circles. in fact, it is as if we on the earth were inside a great hollow globe or ball, which continually turned round, with the pole star near the top of the globe; and you know that if you put your finger on the spot at the top of a spinning globe or ball, you can hold it there while all the rest of the ball runs round. now, if you had to explain things to yourself, you would naturally think: 'here is the great solid earth standing still, and the sun and moon go round it; the stars are all turning round it too, just as if they were fixed on to the inside of a hollow globe; we on the earth are in the middle looking up at them; and this great globe is slowly wheeling round us night by night.' in the childhood of the world men believed that this was really true--that the earth was the centre of the universe, that the sun and moon and all the hosts of heaven were there solely to light and benefit us; but as the world grew wiser the wonders of creation were fathomed little by little. some men devoted their whole lives to watching the heavens, and the real state of things was gradually revealed to them. the first great discovery was that of the daily movement of the earth, its rotation on its own axis, which makes it appear as if all these shining things went round it. it is indeed a very difficult matter to judge which of two objects is moving unless we can compare them both with something outside. you must have noticed this when you are sitting in a train at a station, and there is another train on the other side of yours. for if one of the trains moves gently, either yours or the other, you cannot tell which one it is unless you look at the station platform; and if your position remains the same in regard to that, you know that your train is still standing, while the other one beside it has begun to move. and i am quite sure that there is no one of us who has not, at one time or another, stood on a bridge and watched the water running away underneath until we felt quite dizzy, and it seemed as if the water were standing still and the bridge, with ourselves on it, was flying swiftly away backwards. it is only when we turn to the banks and find them standing still, that we realize the bridge is not moving, and that it is the running water that makes it seem to do so. these everyday instances show us how difficult it is to judge whether we are moving or an outside object unless we have something else to compare with it. and the marvellous truth is that, instead of the sun and moon and stars rolling round the earth, it is the earth that is spinning round day by day, while the sun and the stars are comparatively still; and, though the moon does move, yet when we see her get up in the east and go down in the west that is due to our own movement and not to hers. the earth turns completely round once in a day and night. if you take an orange and stick a knitting-needle through it, and hold it so that the needle is not quite straight up but a little slanting, and then twirl it round, you will get quite a good idea of the earth, though of course there is no great pole like a gigantic needle stuck through it, that is only to make it easy for you to hold it by. in spinning the orange you are turning it as the earth turns day by day, or, as astronomers express it, as it rotates on its axis. there is a story of a cruel eastern king who told a prisoner that he must die if he did not answer three questions correctly, and the questions were very difficult; this is one of them: 'how long would it take a man to go round the earth if he never stopped to eat or drink on the way?' and the prisoner answered promptly: 'if he rose with the sun and kept pace with it all day, and never stopped for a moment to eat or drink, he would take just twenty-four hours, your royal highness.' for in those days it was supposed that the sun went round the earth. everyone is so remarkably clever nowadays that i am sure there will be someone clever enough to object that, if what i have said is true, there would be a great draught, for the air would be rushing past us. but, as a matter of fact, the air goes with us too. if you are inside a railway carriage with the windows shut you do not feel the rush of air, because the air in the carriage travels with you; and it is the same thing on the earth. the air which surrounds the earth clings to it and goes round with it, so there is no continuous breeze from this cause. but the spinning round on its own axis is not the earth's only movement, for all the time it is also moving on round the sun, and once in a whole year it completes its journey and comes back to the place from whence it started. thus the turning round like a top or rotating on its axis makes the day and night, and the going in a great ring or revolving round the sun makes the years. our time is divided into other sections besides days and years. we have, for instance, weeks and months. the weeks have nothing to do with the earth's movements; they are only made by man to break up the months; but the months are really decided by something over which we have no control. they are due to the moon, and, as i have said already, the moon must have a chapter to herself, so we won't say any more about the months here. if any friend of ours goes to india or new zealand or america, we look upon him as a great traveller; yet every baby who has lived one year on the earth has travelled millions of miles without the slightest effort. every day of our lives we are all flung through space without knowing it or thinking of it. it is as if we were all shut up in a comfortable travelling car, and were provided with so many books and pictures and companions that we never cared to look out of the windows, so that hour by hour as we were carried along over miles of space we never gave them a thought. even the most wonderful car ever made by man rumbles and creaks and shakes, so that we cannot help knowing it is moving; but this beautiful travelling carriage of ours called the earth makes never a creak or groan as she spins in her age-long journey. it is always astonishing to me that so few people care to look out of the window as we fly along; most of them are far too much absorbed in their little petty daily concerns ever to lift their eyes from them. it is true that sometimes the blinds are down, for the sky is thickly covered with clouds, and we cannot see anything even if we want to. it is true also that we cannot see much of the scenery in the daytime, for the sun shining on the air makes a veil of blue glory, which hides the stars; but on clear nights we can see on every side numbers of stars quite as interesting and beautiful as any landscape; and yet millions of people never look up, never give a thought to the wonderful scenery through which their car is rushing. by reason of the onward rush of the earth in space we are carried over a distance of at least eighteen miles every second. think of it: as we draw a breath we are eighteen miles away in space from the point we were at before, and this goes on unceasingly day and night. these astonishing facts make us feel how small and feeble we are, but we can take comfort in the thought that though our bodies are insignificant, the brain of man, which has discovered these startling facts, must in itself be regarded as one of the most marvellous of all the mysteries amid which we live. well, we have arrived at some idea of our earth's position; we know that the earth is turning round day by day, and progressing round the sun year by year, and that all around lie the sentinel stars, scattered on a background of infinite space. if you take an older boy or girl and let him or her stand in the middle to represent the sun, then a smaller one would be the earth, and the smallest of all the moon; only in truth we could never get anyone large enough to represent the sun fairly, for the biggest giant that ever lived would be much too small in proportion. the one representing the sun must stand in the middle, and turn slowly round and round. then let the earth-child turn too, and all the time she is spinning like a top she must be also hastening on in a big ring round the sun; but she must not go too fast, for the little moon-child must keep on running round her all the time. and the moon-child must keep her face turned always to the earth, so that the earth never sees her back. that is an odd thing, isn't it? we have never seen the other side of the moon, which goes round us, always presenting the same face to us. the earth is not the only world going round the sun; she has many brothers and a sister; some are nearer to the sun than she is, and some are further away, but all circle round the great central light-giver in rings lying one outside the other. these worlds are called planets, and the earth is one of them, and one of the smaller ones, too, nothing so great and important as we might have imagined. chapter ii hanging in space if you are holding something in your hand and you let it go, what happens? it falls to the ground, of course. now, why should it do so? you will say: 'how could it do anything else?' but that is only because you are hampered by custom. try to shake yourself free, and think, why should it go down instead of up or any other way? the first man who was clever enough to find some sort of an answer to this question was the great philosopher sir isaac newton, though he was not quite the first to be puzzled by it. after years of study he discovered that every thing attracts every other thing in proportion to their masses (which is what you know as weight) and their distance from each other. in more scientific language, we should say every _body_ instead of every _thing_, for the word body does not only mean a living body, but every lump or mass of matter in the universe. the earth is a body in this sense, and so is the table or anything else you could name. now as the earth is immeasurably heavier than anything that is on it, it pulls everything toward itself with such force that the little pulls of other things upon each other are not noticed. the earth draws us all toward it. it is holding us down to it every minute of the day. if we want to move we have to exert another force in order to overcome this attraction of the earth, so we exert our own muscles and lift first one foot and then the other away from the earth, and the effort we make in doing this tires us. all the while you are walking or running you are exercising force to lift your feet away from the ground. the pull of the earth is called gravitation. just remember that, while we go on to something else which is almost as astonishing. we know that nothing here on earth continues to move for ever; everything has to be kept going. anything left to itself has a tendency to stop. why is this? this is because here in the world there is something that fights against the moving thing and tries to stop it, whether it be sent along the ground or thrown up in the air. you know what friction is, of course. if you rub your hands along any rough substance you will quickly feel it, but on a smooth substance you feel it less. that is why if you send a stone spinning along a carpet or a rough road it stops comparatively soon, whereas if you use the same amount of force and send it along a sheet of ice it goes on moving much longer. this kind of resistance, which we call friction, is one of the causes which is at work to bring things to a standstill; and another cause is the resistance of the air, which is friction in another form. it may be a perfectly still day, yet if you are bicycling you are breaking through the air all the time, just as you would be through water in swimming, only the resistance of the air is less than that of water. as the friction or the resistance of the air, or both combined, gradually lessens the pace of the stone you sent off with such force, the gravitation of the earth begins to be felt. when the stone first started the force you gave to it was enough to overcome the gravitation force, but as the stone moves more slowly the earth-pull asserts itself, and the stone drops down to the ground and lies still upon the surface. now, if there were no friction, and therefore no resistance, there would be no reason why anything once set moving should not go on moving for ever. the force you give to any object you throw is enough to overcome gravitation; and it is only when the first force has been diminished by friction that the earth asserts its authority and pulls the moving object toward it. if it were possible to get outside the air and out of reach of the pull of the earth, we might fling a ball off into space, and it would go on in a straight line until something pulled it to itself by the force of gravity. gravitation affects everything connected with the earth; even our air is held to the earth by gravitation. it grows thinner and thinner as we get further away from the earth. at the top of a high mountain the air is so thin that men have difficulty in breathing, and at a certain height they could not breathe at all. as they cannot breathe in very fine air, it is impossible for them to tell by personal experiment exactly where the air ends; but they have tried to find out in other ways, and though different men have come to different conclusions on the subject, it is safe to say that at about two hundred miles above the earth there is nothing that could be called air. thus we can now picture our spinning earth clothed in a garment of air that clings closely about her, and grows thinner and thinner until it melts away altogether, for there is no air in space. [illustration: the earth and moon hanging in space] now in the beginning god made the world, and set it off by a first impulse. we know nothing about the details, though further on you shall hear what is generally supposed to have taken place; we only know that, at some remote age, this world, probably very different from what it is now, together with the other planets, was sent spinning off into space on its age-long journey. these planets were not sent off at random, but must have had some particular connection with each other and with the sun, for they all belong to one system or family, and act and react on each other. now, if they had been at rest and not in movement, they would have fallen right into the sun, drawn by the force of gravitation; then they would have been burned up, and there would have been an end of them. but the first force had imparted to them the impulse to go on in a straight line, so when the sun pulled the result was a movement between the two: the planets did not continue to move in a straight line, neither did they fall on to the sun, but they went on a course between the two--that is, a circle--for the sun never let them get right away from him, but compelled them to move in circles round him. there is a very common instance of this kind of thing which we can see, or perhaps feel, every day. if you try to sit still on a bicycle you tumble off, because the earth pulls you down to itself; but if, by using the force of your own muscles, you give the bicycle a forward movement this resists the earth-pull, and the result is the bicycle runs along the ground. it does not get right away from the earth, not even two or three feet above ground; it is held to the earth, but still it goes forward and does not fall over, for the movement is made up of the earth-pull, which holds it to the ground, and the forward movement, which propels it along. then again, as another instance, if you tie a ball to a string and whirl it round you, so long as you keep on whirling it will not fall to the ground, but the moment you stop down it drops, for there is nothing to fight against the pull of gravitation. thus we can picture the earth and all the planets as if they were swinging round the sun, held by invisible strings. it is the combination of two forces that keeps them in their places--the first force and the sun's pull. it is very wonderful to think of. here we are swinging in space on a ball that seems only large to us because we are so much smaller ourselves; there is nothing above or below it but space, yet it travels on day by day and year by year, held by invisible forces that the brain of man has discovered and measured. of course, every planet gives a pull at every other planet too, but these pulls are so small compared with that of the sun that we need not at present notice them. then we come to another point. we said that every body pulled every other body in proportion to their weights and their distance. now, gravity acts much more strongly when things are near together than when they are far away from each other; so that if a smaller body is near to another somewhat larger than itself, it is pulled by it much more strongly than by a very much larger one at a considerably greater distance. we have an instance of this in the case of the earth and moon: as the earth responds to the pull of the sun, so the moon responds to the pull of the earth. the moon is so comparatively near to the earth that the earth-pull forces her to keep on going round and round, instead of leaving her free to circle round the sun by herself; and yet if you think of it the moon does go round the sun too. recall that game we had when the sun was in the middle, and the two smaller girls, representing the earth and moon, went round it. the moon-child turned round the earth-child, but all the while the earth-child was going round the sun, so that in a year's time the moon had been all round the sun too, only not in a straight line. the moon is something like a dog who keeps on dancing round and round you when you go for a walk. he does go for the walk too, but he does much more than that in the same time. thus we have further completed our idea of our world. we see it now hanging in space, with no visible support, held in its place by two mighty forces; spinning on year after year, attended by its satellite the moon, while we run, and walk, and cry, and laugh, and play about on its surface--little atoms who, except for the brain that god has given them, would never even have known that they are continually moving on through endless space. chapter iii the shining moon 'once upon a time,' long, long ago, the earth was not a compact, round, hard body such as she is now, but much larger and softer, and as she rotated a fragment broke off from her; it did not go right away from her, but still went on circling round with the motion it had inherited from her. as the ages passed on both the earth and this fragment, which had been very hot, cooled down, and in cooling became smaller, so that the distance between them was greater than it had been before they shrank. and there were other causes also that tended to thrust the two further from each other. yet, compared with the other heavenly bodies, they are still near, and by looking up into the sky at night you can generally see this mighty fragment, which is a quarter the diameter of the earth--that is to say, a quarter the width of the earth measured from side to side through the middle. it is--as, of course, you have guessed--the moon. the moon is the nearest body to us in all space, and so vast is the distance that separates us from the stars that we speak as if she were not very far off, yet compared with the size of the earth the space lying between us and her is very great. if you went right round the world at the thickest part--that is to say, in the region of the equator--and when you arrived at your starting-point went off once again, and so on until you had been round ten times, you would only then have travelled about as far as from the earth to the moon! the earth is not the only planet which has a moon, or as it is called, a satellite, in attendance. some of the larger planets have several, but there is not one to compare with our moon. which would you prefer if you had the choice, three or four small moons, some of them not much larger than a very big bright star, or an interesting large body like our own moon? i know which i should say. 'you say that the moon broke off from the earth, so perhaps there may be some people living on her,' i hear someone exclaim. if there is one thing we have found out certainly about the moon, it is that no life, as we know it, could exist there, for there is neither air nor water. whether she ever had any air or water, and if so, why they disappeared, are questions we cannot answer. we only know that now she is a dead world. bright and beautiful as she is, shedding on us a pale, pure light, in vivid contrast with the fiery yellow rays of the sun, yet she is dead and lifeless and still. we can examine her surface with the telescope, and see it all very plainly. even with a large opera-glass those markings which, to the naked eye, seem to be like a queer distorted face are changed, and show up as the shadows of great mountains. we can only see one side of the moon, because as i have said, she keeps always the same face turned to the earth; but as she sways slightly in her orbit, we catch a glimpse of sometimes a little more on one side and sometimes a little more on the other, and so we can judge that the unseen part is very much the same as that turned toward us. at first it is difficult to realize what it means to have no air. besides supporting life in every breath that is drawn by living creatures, the air does numerous other kind offices for us--for instance, it carries sound. supposing the most terrific volcano exploded in an airless world, it could not be heard. the air serves as a screen by day to keep off the burning heat of the sun's rays, and as a blanket by night to keep in the heat and not let it escape too quickly. if there were no air there could be no water, for all water would evaporate and vanish at once. imagine the world deprived of air; then the sun's rays would fall with such fierceness that even the strongest tropical sun we know would be as nothing in comparison with it, and every green thing would shrivel up and die; this scorching sun would shine out of a black sky in which the stars would all be visible in the daytime, not hidden by the soft blue veil of air, as they are now. at night the instant the sun disappeared below the horizon black darkness would set in, for our lingering twilight is due to the reflection of the sun in the upper layers of air, and a bitterness of deathly cold would fall upon the earth--cold fiercer than that of the arctic regions--and everything would be frozen solid. it would need but a short time to reduce the earth to the condition of the moon, where there is nothing to shrivel up, nothing to freeze. her surface is made up of barren, arid rocks, and her scenery consists of icy black shadows and scorching white plains. [illustration: _paris observatory._ the moon.] the black shadows define the mountains, and tremendous mountains they are. most of them have craters. a crater is like a cup, and generally has a little peak in the middle of it. this is the summit of a volcano, and when the volcano has burst up and vomited out floods of lava and débris, this has fallen down in a ring a little distance away from it, leaving a clear space next to the peak, so that, as the mountain ceases vomiting and the lava cools down, the ring hardens and forms a circular ridge. the craters on the moon are immense, not only in proportion to her size, but immense even according to our ideas on the earth. one of the largest craters in our own world is in japan, and this measures seven miles across, while in the moon craters of fifty, sixty, and even a hundred miles are by no means uncommon, though there are also hundreds and thousands of smaller ones. we can see the surface of the moon very plainly with the magnificent telescopes that have now been made, and with the best of these anything the size of a large town would be plainly visible. needless to say, no town ever has been or ever will be seen upon the moon! all these mountains and craters show that at one time the moon must have been convulsed with terrific disturbances, far worse than anything that we have any knowledge of on our earth; but this must have been ages ago, while the moon still probably had an atmosphere of its own. now it has long been quiet. nothing changes there; even the forces that are always at work on the earth--namely, damp and mould and water--altering the surface and breaking up the rocks, do not act there, where there is no moisture of any sort. so far as we can see, the purpose of the moon is to be the servant of the earth, to give her light by night and to raise the tides. beautiful light it is, soft and mysterious--light that children do not often have a chance of seeing, for they are generally in bed before the moon rises when she is at the full. we know that the moon has no heat of her own--she parted with all that long ago; she cannot give us glowing light from brilliant flames, as the sun does; she shines only by the reflection of the sun on her surface, and this is the reason why she appears to change her shape so constantly. she does not really change; the whole round moon is always there, only part of it is in shadow. sometimes you can see the dark part as well as the bright. when there is a crescent moon it looks as if it were encircling the rest; some people call it, 'seeing the old moon in the new moon's arms.' i don't know if you would guess why it is we can see the dark part then, or how it is lighted up. it is by reason of our own shining, for we give light to the moon, as she does to us. the sun's rays strike on the earth, and are reflected on to the moon, so that the moon is lighted by earthshine as we are lighted by moonshine, and it is these reflected earth-rays that light up the dark part of the moon and enable us to see it. what a journey these rays have had! they travel from the sun to the earth, and the earth to the moon, and then back to the earth again! from the moon the earth must appear a much bigger and more glorious spectacle than she does to us--four times wider across and probably brighter--for the sun's light strikes often on our clouds, which shine more brilliantly than her surface. once again we must use an illustration to explain the subject. set a lamp in the middle of a dark room, and let that be the sun, then take a small ball to represent the earth and a smaller one for the moon. place the moon-ball between the lamp and the earth-ball. you will see that the side turned to the earth-ball is dark, but if you move the moon to one side of the earth, then from the earth half of it appears light and half dark; if you put it right away from the lamp, on the outer side of the earth, it is all gloriously lit up, unless it happens to be exactly behind the earth, when the earth's shadow will darken it. this is the full explanation of all the changes of the moon. [illustration: an eclipse of the moon.] does it ever fall within the earth's shadow? yes, it does; for as it passes round the earth it is not always at the same level, but sometimes a little higher and sometimes a little lower, and when it chances to pass exactly behind it enters the shadow and disappears. that is what we call an eclipse of the moon. it is nothing more than the earth's shadow thrown on to the moon, and as the shadow is round that is one of the proofs that the earth is round too. but there is another kind of eclipse--the eclipse of the sun; and this is caused by the moon herself. for when she is nearest to the sun, at new moon--that is to say, when her dark side is toward us, and she happens to get exactly between us and the sun--she shuts out the face of the sun from us; for though she is tiny compared with him, she is so much nearer to us that she appears almost the same size, and can blot him right out. thus the eclipses of both sun and moon are not difficult to understand: that of the moon can only happen at full moon, when she is furthest from the sun, and it is caused by the earth's shadow falling upon the moon; and that of the sun at new moon, when she is nearest to him, and it is caused by the solid body of the moon coming between us and the sun. [illustration: an eclipse of the sun.] besides giving us light by night, the moon serves other important purposes, and the most important of all is the raising of the tides. without the rising of the sea twice in every day and night our coasts would become foul and unwholesome, for all the dead fish and rotting stuff lying on the beach would poison the air. the sea tides scour our coasts day by day with never-ceasing energy, and they send a great breath of freshness up our large rivers to delight many people far inland. the moon does most of this work, though she is a little helped by the sun. the reason of this is that the moon is so near to the earth that, though her pull is a comparatively small one, it is very strongly felt. she cannot displace the actual surface to any great extent, as it is so solid; but when it comes to the water she can and does displace that, so that the water rises up in answer to her pull, and as the earth turns round the raised-up water lags behind, reaching backward toward the moon, and is drawn up on the beach, and makes high tide. but it is stopped there, and meantime, by reason of the earth's movement, the moon is left far behind, and pulls the water to itself further on, when the first high tide relapses and falls down again. at length the moon gets round to quite the opposite side of the earth to that where she began, and there she makes a high tide too; but as she draws the water to herself she draws also the solid earth beneath the water to her in some degree, and so pulls it away from the place where the first high tide occurred, leaving the water there deeper than before, and so causing a secondary high tide. [illustration: the moon raising the tides.] the sun has some influence on the tides too, and when moon and sun are in the same line, as at full and new moon, then the tides are highest, and are called spring tides; but when they pull in different directions, as when it is half-moon, then the tides are lowest and are called neap tides. chapter iv the earth's brothers and sister the earth is not the only world that, poised in space, swings around the sun. it is one of a family called the solar system, which means the system controlled and governed by the sun. when we look up at the glorious sky, star-studded night by night, it might seem to us that the stars move only by reason of the earth's rotation; but when men first began to study the heavens attentively--and this is so long ago that the record of it is not to be found--they noticed that, while every shining object in the sky was apparently moving round us, there were a few which also had another movement, a proper motion of their own, like the moon. these curious stars, which appeared to wander about among the other stars, they called planets, or wanderers. and the reason, which was presently discovered, of our being able to see these movements was that these planets are very much nearer to us than any of the real stars, and in fact form part of our own solar system, while the stars are at immeasurable distances away. of all the objects in the heavens the planets are the most intensely interesting to us; for though removed from us by millions of miles, the far-reaching telescope brings some of them within such range that we can see their surfaces and discover their movements in a way quite impossible with the stars. and here, if anywhere, might we expect to find traces of other living beings like ourselves; for, after all the earth is but a planet, not a very large nor a very small one, and in no very striking position compared with the other planets; and thus, arguing by what seems common-sense, we say, if this one planet has living beings on its surface, may not the other planets prove to be homes for living beings also? counting our own earth, there are eight of these worlds in our solar system, and also a number of tiny planets, called asteroids; these likewise go round the sun, but are very much smaller than any of the first eight, and stand in a class by themselves, so that when the planets are mentioned it is generally the eight large well-known planets which are referred to. if we go back for a moment to the illustration of the large lamp representing our sun, we shall now be able to fill in the picture with much more detail. the orbits of the planets, as their paths round the sun are called, lie like great circles one outside another at various distances, and do not touch or cut each other. where do you suppose our own place to be? will it be the nearest to the sun or the furthest away from him? as a matter of fact, it is neither, we come third in order from the sun, for two smaller planets, one very small and the other nearly as large as the earth, circle round and round the sun in orbits lying inside ours. now if we want to place objects around our lamp-sun which will represent these planets in size, and to put them in places corresponding to their real positions, we should find no room large enough to give us the space we ought to have. we must take the lamp out into a great open field, where we shall not be limited by walls. then the smallest planet, named mercury, which lies nearest of all to the sun, would have to be represented by a pea comparatively close to the sun; venus, the next, would be a greengage plum, and would be about twice as far away; then would come the earth, a slightly larger plum, about half as far again as venus. after this there would be a lesser planet, called mars, like a marble. these are the first four, all comparatively small; beyond them there is a vast gap, in which we find the asteroids, and after this we come to four larger planets, mighty indeed as regards ourselves, for if our earth were a greengage plum, the first of these, jupiter, would have to be the size of a football at least, and the next, saturn, a smaller football, while uranus and neptune, the two furthest out, would be about the size of the toy balloons children play with. the outermost one, neptune, would be thirty times as far from the sun as we are. [illustration: comparative sizes of the planets.] this is the solar system, and in it the only thing that shines by its own light is the sun; all the rest, the planets and their moons, shine only because the rays of light from the sun strike on their surfaces and are reflected off again. our earth shines like that, and from the nearer planets must appear as a brilliant star. the little solar system is separated by distances beyond the realm of thought from the rest of the universe. vast as are the intervals between ourselves and our planetary neighbours, they are as nothing to the space that separates us from the nearest of the steady shining fixed stars. why, removed as far from us as the stars, the sun himself would have sunk to a point of light; and as for the planets, the largest of them, jupiter, could not possibly be seen. thus, when we look at those stars across the great gulf of space, we know that though we see them they cannot see us, and that to them our sun must seem only a star; consequently we argue that perhaps these stars themselves are suns with families of planets attached to them; and though there are reasons for thinking that this is not the case with all, it may be with some. now if, after learning this, we look again at the sky, we do so with very different eyes, for we realize that some of these shining bodies are like ourselves in many things, and are shining only with a light borrowed from the sun, while others are mighty glowing suns themselves, shining by their own light, some greater and brighter, some less than our sun. the next thing to do is to learn which are stars and which are planets. of the planets you will soon learn to pick out one or two, and will recognize them even if they do change their places--for instance, venus is at times very conspicuous, shining as an evening star in the west soon after the sun goes down, or us a morning star before he gets up, though you are not so likely to see her then; anyway, she is never found very far from the sun. jupiter is the only other planet that compares with her in brilliancy, and he shines most beautifully. he is, of course, much further away from us than venus, but so much larger that he rivals her in brightness. saturn can be quite easily seen as a conspicuous object, too, if you know where to look for him, and mars is sometimes very bright with a reddish glow. the others you would not be able to distinguish. it is to our earth's family of these eight large planets going steadily round the same sun that we must give our attention first, before going on to the distant stars. many of the planets are accompanied by satellites or moons, which circle round them. we may say that the sun is our parent--father, mother, what you will--and that the planets are the family of children, and that the moons are _their_ children. our earth, you see, has only one child, but that a very fine one, of which she may well be proud. when i say that the planets go round the sun in circles i am only speaking generally; as a matter of fact, the orbits of the planets are not perfect circles, though some are more circular than others. instead of this they are as a circle might look if it were pressed in from two sides, and this is called an ellipse. the path of our own earth round the sun is one of the most nearly circular of them all, and yet even in her orbit she is a good deal nearer to the sun at one time than another. would you be surprised to hear that she is nearer in our winter and further away in our summer? yet that is the case. and for the first moment it seems absurd; for what then makes the summer hotter than the winter? that is due to an altogether different cause; it depends on the position of the earth's axis. if that axis were quite straight up and down in reference to the earth's path round the sun we should have equal days and nights all the year round, but it is not; it leans over a little, so that at one time the north pole points towards the sun and at another time away from it, while the south pole is pointing first away from it and then toward it in exactly the reverse way. when the north pole points to the sun we in the northern hemisphere have our summer. to understand this you must look at the picture, which will make it much clearer than any words of mine can do. the dark part is the night, and the light part the day. when we are having summer any particular spot on the northern hemisphere has quite a long way to travel in the light, and only a very short bit in the dark, and the further north you go the longer the day and shorter the night, until right up near the north pole, within the arctic circle, it is daylight all the time. you have, perhaps, heard of the 'midnight sun' that people go to see in the north, and what the expression means is that at what should be midnight the sun is still there. he seems just to circle round the horizon, never very far above, but never dipping below it. when the sun is high overhead, his rays strike down with much more force than when he is low. it is, for instance, hotter at mid-day than in the evening. now, when the north pole is bowed toward the sun, the sun appears to us to be higher in the sky. in the british isles he never climbs quite to the zenith, as we call the point straight above our heads; he always keeps on the southern side of that, so that our shadows are thrown northward at mid-day, but yet he gets nearer to it than he does in winter. look at the picture of the earth as it is in winter. then we have long nights and short days, and the sun never appears to climb very high, because we are turned away from him. during the short days we do not receive a great deal of heat, and during the long night the heat we have received has time to evaporate to a great extent. these two reasons--the greater or less height of the sun in the sky and the length of the days--are quite enough to account for the difference between our summer and winter. there is one rather interesting point to remember, and that is that in the northern hemisphere, whether it is winter or summer, the sun is south at mid-day, so that you can always find the north then, for your shadow will point northwards. [illustration: the english summer (left) and winter (right).] new zealand and australia and other countries placed in the southern hemisphere, as we are in the northern, have their summer while we have winter, and winter while we have summer, and their summer is warmer than ours, because it comes when the earth in its journey is three million miles nearer to the sun than in our summer. all this seems to refer to the earth alone, and this chapter should be about the planets; but, after all, what applies to one planet applies to another in some degree, and we can turn to the others with much more interest now to see if their axes are bowed toward the sun as ours is. it is believed that in the case of mercury, in regard to its path round the sun, the axis is straight up and down; if it is the changes of the seasons must depend on the nearness of mercury to the sun and nothing else, and as he is a great deal nearer at one time than another, this might make a very considerable difference. some of the planets are like the earth in regard to the position of their axes, but the two outermost ones, uranus and neptune, are very peculiar, for one pole is turned right toward the sun and the other right away from it, so that in one hemisphere there is continuous day all the summer, in the other there is continuous night, and then the process is reversed. but these little peculiarities we shall have to note more particularly in the account of the planets separately. there is a curious fact in regard to the distances of the planets from the sun. each one, after the first, is, very roughly, about double the distance from the sun of the one inside it. this holds good for all the first four, then there is a great gap where we might expect to find another planet, after which follow the four large planets. now, this gap puzzled astronomers greatly; for though there seemed to be no reason why the planets should be at regular distances one outside the other, yet there the fact was, and that the series should be broken by a missing planet was annoying. so very careful search was made, and a thrill of excitement went all through the scientific world when it was known that a tiny planet had been discovered in the right place. but this was not the end of it, for within a few years three or four more tiny planets were observed not far from the first one, and, as years rolled on, one after another was discovered until now the number amounts to over six hundred and others are perpetually being added to the list! here was a new feature in the solar system, a band of tiny planets not one of which was to be compared in size with the least of those already known. the largest may be about as large as europe, and others perhaps about the size of wales, while there may be many that have only a few square miles of surface altogether, and are too small for us to see. to account for this strange discovery many theories were advanced. one was that there had been a planet--it might be about the size of mars--which had burst up in a great explosion, and that these were the pieces--a very interesting and exciting idea, but one which proved to be impossible. the explanation now generally accepted is a little complicated, and to understand it we must go back for a bit. when we were talking of the earth and the moon we realized that once long ago the moon must have been a part of the earth, at a time when the earth was much larger and softer than she now is; to put it in the correct way, we should say when she was less dense. there is no need to explain the word 'dense,' for in its ordinary sense we use it every day, but in an astronomical sense it does not mean exactly the same thing. everything is made up of minute particles or atoms, and when these atoms are not very close together the body they compose is loose in texture, while if they are closer together the body is firmer. for instance, air is less dense than water, and water than earth, and earth than steel. you see at once by this that the more density a thing has the heavier it is; for as a body is attracted to another body by every atom or particle in it, so if it has more particles it will be more strongly attracted. thus on the earth the denser things are really heavier. but 'weight' is only a word we use in connection with the earth; it means the earth's pulling power toward any particular thing at the surface, and if we were right out in space away from the earth, the pulling power of the earth would be less, and so the weight would be less; and as it would be impossible always to state just how far away a thing was from the earth, astronomers talk about density, which means the number of particles a body contains in proportion to other bodies. thus the planet jupiter is very much larger than the earth, but his density is less. that does not mean to say that if jupiter were in one scale and the earth in the other he would weigh less, because he is so very much bigger he would outweigh the earth still; his total _mass_ would be greater than that of the earth, but it means that a piece of jupiter the same size as a piece of the earth would weigh less under the same conditions. now, before there were any planets at all or any sun, in the place of our solar system was a vast gaseous cloud called a nebula, which slowly rotated, and this rotation was the first impulse or force which god gave it. it was not at all dense, and as it rotated a part broke off, and inheriting the first impulse, went on rotating too. the impulse would have sent it off in a straight line, but the pull of gravity from the nebula held it in place, and it circled round; then the nebula, as it rotated, contracted a little, and occupied less space and grew denser, and presently a second piece was thrown off, to become in time another planet. the same process was repeated with saturn, and then with the huge jupiter. the nebula was always rotating and always contracting. and as it behaved, so did the planets in their turn; they spun round and cooled and contracted, and the moons were flung off from them, just as they--the planets--had been flung off from the parent nebula. now, after the original nebula had parted with the mighty mass of jupiter, it never again made an effort so great, and for a long time the fragments that were detached were so small as hardly to be worth calling planets; they were the asteroids, little lumps and fragments that the nebula left behind. but as it still contracted in time there came mars; and having recovered a little, the nebula with more energy got rid of the earth, and next venus, and lastly little mercury, the smallest of the eight planets. then it contracted further, and perhaps you can guess what the remainder of it is--the sun; and by spinning in a plastic state the sun, like the earth, has become a globe, round and comparatively smooth; and its density is now too great to allow of its losing any more fragments, so, as far as we can see, the solar system is complete. this theory of the origin of the planets is called the nebula theory. we cannot prove it, but there are so many facts that can only be explained by it, we have strong reason for believing that something of the kind must have happened. when we come to speak of the starry heavens we shall see that there are many masses of glowing gas which are nebulæ of the same sort, and which form an object-lesson in our own history. we have spoken rather lightly of the nebula rotating and throwing off planets; but we must not think of all this as having happened in a short time. it is almost as impossible for the human mind to conceive the ages required for such slow changes as to grasp the great gulfs of space that separate us from the stars. we can only do it by comparison. you know what a second is, and how the seconds race past without ceasing day and night. it makes one giddy to picture the seconds there are in a year; yet if each one of those seconds was a year in itself, what then? that seems a stupendous time, but it is nothing compared with the time needed to form a nebula into a planetary system. if we had five thousand of such years, with every second in them a year, we should then only have counted one billion real years, and billions must have passed since the sun was a gaseous nebula filling the outermost bounds of our system! chapter v four small worlds what must the sun appear to mercury, who is so much nearer to him than we are? to understand that we should have to imagine our sun increased to eight or nine times his apparent size, and pouring out far greater heat and light than anything that we have here, even in the tropics. it was at first supposed that mercury must have an extra thick covering of clouds to protect him from this tremendous glare; but recent observations tend to prove that, far from this, he is singularly free from cloud. as this is so, no life as we know it could possibly exist on mercury. his year--the time he takes to go round the sun and come back to the same place again--is eighty-eight days, or about one-quarter of ours. as his orbit is much more like an ellipse than a circle, it follows that he is much nearer to the sun at one time than at another--in fact, when he is nearest, the size of the sun must seem three and a half times greater than when he is furthest away from it! even at the best mercury is very difficult to observe, and what we can learn about him is not much; but, as we have heard, his axis is supposed to be upright. if so his seasons cannot depend on the bend toward or away from the sun, but must be influenced solely by the changes in his distance from the sun, which are much greater than in our own ease. there is some reason to believe, too, that mercury's day and year are the same length. this means that as the planet circles round the sun he turns once. if this is so the sun will shine on one half of the planet, producing an accumulated heat terrific to think of; while the other side is plunged in blackness. the side which faces the sun must be heated to a pitch inconceivable to us during the nearer half of the orbit--a pitch at which every substance must be at boiling-point, and which no life as we know it could possibly endure. seen from our point of view, mercury goes through all the phases of the moon, as he shines by the reflected light of the sun; but this point we shall consider more particularly in regard to venus, as venus is nearer to us and easier to study. for a long time astronomers had a fancy that there might be another planet even nearer to the sun than mercury, perhaps hidden from us by the great glare of the sun. they even named this imaginary planet vulcan, and some thought they had seen it, but it is tolerably certain that vulcan existed only in imagination. mercury is the nearest planet to the sun, and also the smallest, of course excepting the asteroids. it is about three thousand miles in diameter, and as our moon is two thousand miles, it is not so much bigger than that. so far as we are concerned, it is improbable we shall ever know very much more about this little planet. but next we come to venus, our beautiful bright neighbour, who approaches nearer to us than any other heavenly body except the moon. alas! when she is nearest, she like mercury, turns her dark side toward us, coming in between us and the sun, so that we cannot observe her at all. everyone must have noticed venus, however carelessly they have looked at the sky; but it is likely that far more people have seen her as an evening than a morning star, for most people are in bed when the sun rises, and it is only before sunrise or after sunset we can see venus well. she is at her best from our point of view when she seems to us to be furthest from the sun, for then we can study her best, and at these times she appears like a half or three-quarter moon, as we only see a part of the side from which the sunlight is reflected. she shines like a little silver lamp, excelling every other planet, even jupiter, the largest of all. if we look at her even with the naked eye, we can see that she is elongated or drawn out, but her brilliance prevents us from seeing her shape exactly; to do this we must use a telescope. [illustration: different phases of venus.] it is a curious fact that some planets shine much more brightly than others, without regard to their size--that is to say, the surface on which the sun's rays strike is of greater reflecting power in some than in others. one of the brightest things in nature that we can imagine is a bank of snow in sunlight; it is so dazzling that we have to look away or wink hard at the sight; and the reflective power of the surface of venus is as dazzling as if she were made of snow. this is probably because the light strikes on the upper surface of the clouds which surround her. in great contrast to this is the surface of mercury, which reflects as dully as a mass of lead. our own moon has not a high reflecting power, as will be easily understood if we imagine what the world would be if condemned to perpetual moonlight only. it would, indeed, be a sad deprivation if the mournful cold light of the moon, welcome enough as a change from sunlight, were to take the place of sunlight in the daytime. for a very long time astronomers could not discover what time venus took in rotating on her own axis--that is to say, what the length of her day was. she is difficult to observe, and in order to find out the rotation it is necessary to note some fixed object on the surface which turns round with the planet and comes back to the same place again, so that the time it takes in its journey can be measured. but the surface of venus is always changing, so that it is impossible to judge at all certainly. opinions differ greatly, some astronomers holding that venus's day is not much longer than an earthly day, while others believe that the planet's day is equal to her year, just as in the case of mercury. venus's year is days, or about seven and a half of our months, and if, indeed, her day and year are the same length, very peculiar effects would follow. for instance, terrible heat would be absorbed by the side of the planet facing the sun in the perpetual summer; and the cold which would be felt in the dreary winter's night would far exceed our bitterest arctic climate. we cannot but fancy that any beings who might live on a planet of this kind must be different altogether from ourselves. then, there is another point: even here on earth very strong winds are caused by the heating of the tropics; the hot air, being lighter than the cold air, rises, and the colder air from the poles rushes in to supply its place. this causes wind, but the winds which would be raised on venus by the rush of air from the icy side of the planet to the hot one would be tornadoes such as we could but faintly dream of. it is, of course, useless to speculate when we know so little, but in a subject so intensely interesting we cannot help guessing a little. venus is only slightly smaller than the earth, and her density is not very unlike ours; therefore the pull of gravity must be pretty much there what it is here--that is to say, things will weigh at her surface about the same as they do here. her orbit is nearly a circle, so that her distance from the sun does not vary much, and the heat will not be much greater from this cause at one time of the year than another. as her orbit is tilted up a little she does not pass between us and the sun at each revolution, but occasionally she does so, and this passing is called a transit. many important facts have been learned by watching these transits. mercury also has transits across the sun, but as she is so much smaller than venus they are not of such great importance. it was by the close observation of venus during her transits that the distance from the earth to the sun was first measured. not until the year will another transit of venus occur. it is not difficult to imagine that the earth must appear a splendid spectacle from venus, whence she is seen to great advantage. when nearest to us she must see us like a little moon, with markings as the continents and seas rotate, and these will change as they are obscured by the clouds rolling over them. at the north and south poles will be glittering ice-caps, growing larger and smaller as they turn toward or away from the sun. a brilliant spectacle! [illustration: orbits of mars, the earth, venus, and mercury.] we might say with a sigh, 'if only we could see such a world!' well, we can see a world--not indeed, so large as venus, yet a world that comes almost as near to us as venus does, and which, unlike her, is outside us in order from the sun, so that when it is nearest to us the full sunlight is on it. this is mars, our neighbour on the other side, and of all the fascinating objects in the sky mars is the most fascinating, for there, if anywhere, should we be likely to discover beings like ourselves! mars takes rather more than half an hour longer to rotate than we do, and as he is so much smaller than the earth, this means that he moves round more slowly. his axis is bent at nearly the same angle as ours is. mars is much smaller than the earth, his diameter is about twice that of the moon, and his density is about three-quarters that of the earth, so that altogether, with his smaller size and less density, anything weighing a hundred pounds here would only weigh some forty pounds on mars; and if, by some miraculous agency, you were suddenly transported there, you would find yourself so light that you could jump enormous distances with little effort, and skip and hop as if you were on springs. [illustration: _memoirs of the british astronomical association._ map of mars.] look at the map of mars, in which the surface appears to be cut up into land and water, continents and oceans. the men who first observed mars with accuracy saw that some parts were of a reddish colour and others greenish, and arguing from our own world, they called the greenish parts seas and the reddish land. for a long while no one doubted that we actually looked on a world like our own, more especially as there was supposed to be a covering of atmosphere. the so-called land and water are much more cut up and mixed together than ours, it is true. here and there is a large sea, like that marked 'mare australe,' but otherwise the water and the land are strangely intermingled. the red colour of the part they named land puzzled astronomers a good deal, for our land seen at the same distance would not appear so red, and they came at last to the conclusion that vegetation on mars must be red instead of green! but after a while another disturbing fact turned up to upset their theories, and that was that they saw canals, or what they called canals, on mars. these were long, straight, dark markings, such as you see on the map. it is true that some people never saw these markings at all, and disbelieved in their existence; but others saw them clearly, and watched them change--first go fainter and then darker again. and quite recently a photograph has been obtained which shows them plainly, so they must have an existence, and cannot be only in the eye of the observer, as the most sceptical people were wont to suggest. but further than this, one astronomer announced that some of these lines appeared to be double, yet when he looked at them again they had grown single. it was like a conjuring trick. great excitement was aroused by this, for if the canals were altered so greatly it really did look as if there were intelligent beings on mars capable of working at them. in any case, if these are really canals, to make them would be a stupendous feat, and if they are artificial--that is, made by beings and not natural--they show a very high power of engineering. imagine anyone on earth making a canal many miles wide and two thousand miles long! it is inconceivable, but that is the feat attributed to the martians. the supposed doubling of the canals, as i say, caused a great deal of talk, and very few people could see that they were double at all. even now the fact is doubted, yet there seems every reason to believe it is true. they do not all appear to be double, and those that do are always the same ones, while others undoubtedly remain single all the time. but the canals do not exhaust the wonders of mars. at each pole there is an ice-cap resembling those found at our own poles, and this tells us pretty plainly something about the climate of mars, and that there is water there. this ice-cap melts when the pole which it surrounds is directed toward the sun, and sometimes in a hot summer it dwindles down almost to nothing, in a way that the ice-caps at the poles of the earth never do. a curious appearance has been noticed when it is melting: a dark shadow seems to grow underneath the edge of it and extends gradually, and as it extends the canals near it appear much darker and clearer than they did before, and then the canals further south undergo the same change. this looks as if the melting of the snow filled up the canals with water, and was a means of watering the planet by a system totally different from anything we know here, where our poles are surrounded by oceans, and the ice-caps do not in the least affect our water-supply. but, then, another strange fact had to be taken into consideration. these straight lines called canals ran out over the seas occasionally, and it was impossible to believe that if they were canals they could do that. other things began to be discussed, such as the fact that the green parts of mars did not always remain green. in what is the springtime of mars they are so, but afterwards they become yellow, and still later in the season parts near the pole turn brown. thus the idea that the greenish parts are seas had to be quite given up, though it appeared so attractive. the idea now generally believed is that the greenish parts are vegetation--trees and bushes and so on, and that the red parts are deserts of reddish sand, which require irrigation--that is to say, watering--before anything can be grown on them. the apparent doubling of the canals may be due to the green vegetation springing up along the banks. this might form two broad lines, while the canal itself would not be seen, and when the vegetation dies down, we should see only the trench of the canal, which would possibly appear faint and single. therefore the arrangements on mars appear to be a rich and a barren season on each hemisphere, the growth being caused by the melting of the polar ice-cap, which sends floods down even beyond the equator. if we could imagine the same thing on earth we should have to think of pieces of land lying drear and dry and dead in winter between straight canal-like ditches of vast size. a little water might remain in these ditches possibly, but not enough to water the surrounding land. then, as summer progressed, we should hear, 'the floods are coming,' and each deep, huge canal would be filled up with a tide of water, penetrating further and further. the water drawn up into the air would fall in dew or rain. vegetation would spring up, especially near the canal banks, and instead of dreary wastes rich growths would cover the land, gradually dying down again in the winter. so far mars seems in some important respects very different from the earth. he is also less favourably placed than we are, for being so much further from the sun, he receives very much less heat and light. his years are of our days, or one year and ten and a half months, and his atmosphere is not so dense as ours. with this greater distance from the sun and less air we might suppose the temperature would be very cold indeed, and that the surface would be frost-bound, not only at the poles, but far down towards the equator. instead of this being so, as we have seen, the polar caps melt more than those on the earth. we can only surmise there must be some compensation we do not know of that softens down the rigour of the seasons, and makes them milder than we should suppose possible. of course, the one absorbing question is, are there people on mars? to this it is at present impossible to reply. we can only say the planet seems in every way fitted to support life, even if it is a little different from our earth. it is most certainly a living world, not a dead one like the moon, and as our knowledge increases we may some day be able to answer the question which so thrills us. our opportunities for the observation of mars vary very greatly, for as the earth's orbit lies inside that of mars, we can best see him when we are between him and the sun. of course, it must be remembered that the earth and the other planets are so infinitely small in regard to the space between them that there is no possibility of any one of them getting in such a position that it would throw a shadow on any other or eclipse it. the planets are like specks in space, and could not interfere with one another in this way. when mars, therefore, is in a line with us and the sun we can see him best, but some of these times are better than others, for this reason--the earth's orbit is nearly a circle, and that of mars more of an ellipse. [illustration: orbits of the earth and mars.] look at the illustration and remember that mars' year is not quite two of ours--that is to say, every time we swing round our orbit we catch him up in a different place, for he will have progressed less than half his orbit while we go right round ours. sometimes when we overtake him he may be at that part which is furthest away from us, or he may be at that part which is nearest to us, and if he is in the latter position we can see him best. now at these, the most favourable times of all, he is still more than thirty-five millions of miles away--that is to say, one hundred and forty times as far as the moon, yet comparatively we can see him very well. he is coming nearer and nearer to us, and very soon will be nearer than he has been since , or fifteen years ago. then many telescopes will be directed on him, and much may be learned about him. for a long time it was supposed that mars had no moons, and when dean swift wrote 'gulliver's travels' he wanted to make the laputans do something very clever, so he described their discovery of two moons attending mars, and to make it quite absurd he said that when they observed these moons they found that one of them went round the planet in about ten hours. now, as mars takes more than twenty-four hours to rotate, this was considered ridiculous, for no moon known then took less time to go round its primary world than the primary world took to turn on its own axis. our own moon, of course, takes thirty times as long--that is a month contains thirty days. then one hundred and fifty years later this jest of dean swift's came true, for two moons were really discovered revolving round mars, and one of them does actually take less time to complete its orbit than the planet does to rotate--namely, a little more than seven hours! so the absurdity in 'gulliver's travels' was a kind of prophecy! these two moons are very small, the outer one perhaps five or six miles in diameter, and the inner one about seven; therefore from mars the outer one, deimos, cannot look much more than a brilliant star, and the inner one would be but a fifth part the apparent width of our own moon. so mars is not very well off, after all. still, there is great variety, for it must be odd to see the same moon appearing three times in the day, showing all the different phases as it goes from new to full, even though it is small! such wonderful discoveries have already been made that it is not too much to say that perhaps some day we may be able to establish some sort of communication with mars, and if it be inhabited by any intelligent beings, we may be able to signal to them; but it is almost impossible that any contrivance could bridge the gulf of airless space that separates us, and it is not likely that holiday trips to mars will ever become fashionable! chapter vi four large worlds i have told you about the four lesser worlds of which our earth is one, and you know that beyond mars, the last of them, there lies a vast space, in which are found the asteroids, those strange small planets circling near to each other, like a swarm of bees. after this there comes jupiter, who is so enormous, so superb in size compared with us, that he might well serve as the sun of a little system of his own. you remember that we represented him by a football, while the earth was only a greengage plum. but jupiter himself is far less in comparison with the sun than we are in comparison with him. he differs from the planets we have heard about up to the present in that he seems to glow with some heat that he does not receive from the sun. the illumination which makes him appear as a star to us is, of course, merely reflected sunlight, and what we see is the external covering, his envelope of cloud. there is every reason to believe that the great bulk of jupiter is still at a high temperature. we know that in the depths of the earth there is still plenty of heat, which every now and then makes its presence felt by bursting up through the vents we call volcanoes, the weak spots in the earth's crust; but our surface long ago cooled, for the outside of any body gets cool before the inside, as you may have found if ever you were trying to eat hot porridge, and circled round the edge of the plate with a spoon. a large body cools more slowly than a small one, and it is possible that jupiter, being so much larger than we are, has taken longer to cool. one reason we have for thinking this is that he is so very light compared with his size--in other words, his density is so small that it is not possible he could be made of materials such as the earth is made of. as i said, when we study him through telescopes we see just the exterior, the outer envelope of cloud, and as we should expect, this changes continually, and appears as a series of belts, owing to the rotation of the planet. jupiter's rotation is very rapid; though he is so much greater than the earth, he takes less than half the time the earth does to turn round--that is to say, only ten hours. his days and nights of five hours each seem short to us, accustomed to measure things by our own estimates. but we must remember that everything is relative; that is to say, there is really no such thing as fast or slow; it is all by comparison. a spider runs fast compared with a snail, but either is terribly slow compared with an express train; and the speed of an express train itself is nothing to the velocity of light. in the same way there is nothing absolutely great or small; it is all by comparison. we say how marvellous it is that a little insect has all the mechanism of life in its body when it is so tiny, but if we imagine that insect magnified by a powerful microscope until it appears quite large, the marvel ceases. again, imagine a man walking on the surface of the earth as seen from a great distance through a telescope: he would seem less than an insect, and we might ask how could the mechanism of life be compressed into anything so small? thus, when we say enormous or tiny we must always remember we are only speaking by the measurements of our own standards. there is nothing very striking about jupiter's orbit. he takes between eleven and twelve of our years to get round the sun, so you see, though his day is shorter, his year is longer than ours. and this is not only because his path is much larger, but because by the law of gravity the more distant a planet is from the sun the more slowly it travels, so that while the earth speeds over eighteen miles jupiter has only done eight. of course, we must be careful to remember the difference between rotation and revolution. jupiter rotates much quicker than the earth--that is to say, he turns round more quickly--but he actually gets over the ground more slowly. the sun appears much smaller to him than it does to us, and he receives considerably less light and heat. there are various spots on his surface, and one remarkable feature is a dark mark, which is called the 'great red spot.' if as we suppose what we see of the planet is merely the cloudy upper atmosphere, we should not expect to find anything permanent there, for the markings would change from day to day, and this they do with this exception--that this spot, dark red in colour, has been seen for many years, turning as the planet turned. it was first noticed in , and was supposed to be some great mountain or excrescence peeping up through the clouds. it grew stronger and darker for several years, and then seemed to fade, and was not so easily seen, and though still remaining it is now pale. but, most startling to say, it has shifted its position a little--that is, it takes a few seconds longer to get round the planet than it did at first. a few seconds, you will say, but that is nothing! it does not seem much, but it shows how marvellously accurate astronomers are. discoveries of vast importance have been made from observing a few seconds' discrepancy in the time the heavenly bodies take in their journeys, and the fact that this spot takes a little longer in its rotation than it did at first shows that it cannot be attached to the body of the planet. it is impossible for it to be the summit of a mountain or anything of that sort. what can it be? no one has yet answered that question. [illustration: jupiter and one of his moons] when we get to the chapter on the sun, we shall find curiosities respecting the spots there as well. jupiter has seven moons, and four of these are comparatively large. they have the honour of having been the first heavenly bodies ever actually discovered, for the six large planets nearest the sun have been known so long that there is no record of their first discovery, and of course our own moon has always been known. galileo, who invented the telescope, turned it on to the sky in , when our king charles i. was on the throne, and he saw these curious bodies which at first he could not believe to be moons. the four which he saw vary in size from two thousand one hundred miles in diameter to nearly three thousand six hundred. you remember our own moon is two thousand miles across, so even the smallest is larger than she. they go round at about the same level as the planet's equator, and therefore they cross right in front of him, and go behind him once in every revolution. since then the other three have been discovered in the band of jupiter's satellites--one a small moon closer to him than any of the first set, and two others further out. it was by observation of the first four, however, that very interesting results were obtained. mathematicians calculated the time that these satellites ought to disappear behind jupiter and reappear again, but they found that this did not happen exactly at the time predicted; sometimes the moons disappeared sooner than they should have done, and sometimes later. then this was discovered to have some relation to the distance of our earth from jupiter. when he was at the far side of his immense orbit he was much more distant from us than when he was on the nearer side--in fact, the difference may amount to more than three hundred millions of miles. and it occurred to some clever man that the irregularities in time we noticed in the eclipses of the satellites corresponded with the distance of jupiter from us. the further he drew away from us, the later were the eclipses, and as he came nearer they grew earlier. by a brilliant inspiration, this was attributed to the time light took to travel from them to us, and this was the first time anyone had been able to measure the velocity or speed of light. for all practical purposes, on the earth's surface we hold light to be instantaneous, and well we may, for light could travel more than eight times round the world in one second. it makes one's brain reel to think of such a thing. then think how far jupiter must be away from us at the furthest, when you hear that sometimes these eclipses were delayed seventeen minutes--minutes, not seconds--because it took that time for light to cross the gulf to us! [illustration: jupiter and his principal moons.] sound is very slow compared with light, and that is why, if you watch a man hammering at a distance, the stroke he gives the nail does not coincide with the bang that reaches you, for light gets to you practically at once, and the sound comes after it. no sound can travel without air, as we have heard, therefore no sound reaches us across space. if the moon were to blow up into a million pieces we should see the amazing spectacle, but should hear nothing of it. light travels everywhere throughout the universe, and by the use of this universal carrier we have learnt all that we know about the stars and planets. when the time that light takes to travel had been ascertained by means of jupiter's satellites, a still more important problem could be solved--that was our own distance from the sun, which before had only been known approximately, and this was calculated to be ninety-two millions seven hundred thousand miles, though sometimes we are a little nearer and sometimes a little further away. jupiter is marvellous, but beyond him lies the most wonderful body in the whole solar system. we have found curiosities on our way out: we have studied the problem of the asteroids, of the little moon that goes round mars in less time than mars himself rotates; we have considered the 'great red spot' on jupiter, which apparently moves independently of the planet; but nothing have we found as yet to compare with the rings of saturn. may you see this amazing sight through a telescope one day! look at the picture of this wonderful system, and think what it would be like if the earth were surrounded with similar rings! the first question which occurs to all of us is what must the sky look like from saturn? what must it be to look up overhead and see several great hoops or arches extending from one horizon to another, reflecting light in different degrees of intensity? it would be as if we saw several immense rainbows, far larger than any earthly rainbow, and of pure light, not split into colours, extending permanently across the sky, and now and then broken by the black shadow of the planet itself as it came between them and the sun. however, we must begin at the beginning, and find out about saturn himself before we puzzle ourselves over his rings. saturn is not a very great deal less than jupiter, though, so small are the other planets in comparison, that if saturn and all the rest were rolled together, they would not make one mass so bulky as jupiter! saturn is so light--in other words, his density is so small--that he is actually lighter than water. he is the lightest, in comparison with his size, of any of the planets. therefore he cannot be made largely of solid land, as our earth is, but must be to a great extent, composed of air and gaseous vapour, like his mighty neighbour. he approaches at times as near to jupiter as jupiter does to us, and on these occasions he must present a splendid spectacle to jupiter. he takes no less than twenty-nine and a half of our years to complete his stately march around the sun, and his axis is a little more bent than ours; but, of course, at his great distance from the sun, this cannot have the same effect on the seasons that it does with us. saturn turns fast on his axis, but not so fast as jupiter, and in turning his face, or what we call his surface, presents much the same appearance to us that we might expect, for it changes very frequently and looks like cloud belts. the marvellous feature about saturn is, of course, the rings. there are three of these, lying one within the other, and separated by a fine line from each other. the middle one is much the broadest, probably about ten thousand miles in width, and the inner one, which is the darkest, was not discovered until some time after the others. as the planet swings in his orbit the rings naturally appear very different to us at different times. sometimes we can only see them edgewise, and then even in the largest telescope they are only like a streak of light, and this shows that they cannot be more than fifty or sixty miles in thickness. the one which is nearest to saturn's surface does not approach him within ten thousand miles. saturn has no less than ten satellites, in addition to the rings, so that his midnight sky must present a magnificent spectacle. the rings, which do not shine by their own light but by reflected sunlight, are solid enough to throw a shadow on the body of the planet, and themselves receive his shadow. sometimes for days together a large part of saturn must suffer eclipse beneath the encircling rings, but at other times, at night, when the rings are clear of the planet's body, so that the light is not cut off from them, they must appear as radiant arches of glory spanning the sky. the subject of these rings is so complicated by the variety of their changes that it is difficult for us even to think about it. it is one of the most marvellous of all the features of our planetary system. what are these rings? what are they made of? it has been positively proved that they cannot be made of continuous matter, either liquid or solid, for the force of gravity acting on them from the planet would tear them to pieces. what, then, can they be? it is now pretty generally believed that they are composed of multitudes of tiny bodies, each separate, and circling separately round the great planet, as the asteroids circle round the sun. as each one is detached from its neighbour and obeys its own impulses, there is none of the strain and wrench there would be were they all connected. according to the laws which govern planetary bodies, those which are nearest to the planet will travel more quickly than those which are further away. of course, as we look at them from so great a distance, and as they are moving, they appear to us to be continuous. it is conjectured that the comparative darkness of the inside ring is caused by the fact that there are fewer of the bodies there to reflect the sunlight. then, in addition to the rings, enough themselves to distinguish him from all other planets, there are the ten moons of richly-endowed saturn to be considered. it is difficult to gather much about these moons, on account of our great distance from them. the largest is probably twice the diameter of our own moon. one of them seems to be much brighter--that is to say, of higher reflecting power--on one side than the other, and by distinguishing the sides and watching carefully, astronomers have come to the conclusion that it presents always the same face to saturn in the same way as our own moon does to us; in fact, there is reason to think that all the moons of large planets do this. [illustration: the planet saturn with two of his moons.] all the moons lie outside the rings, and some at a very great distance from saturn, so that they can only appear small as seen from him. yet at the worst they must be brighter than ordinary stars, and add greatly to the variations in the sky scenery of this beautiful planet. in connection with saturn's moons there is another of those astonishing facts that are continually cropping up to remind us that, however much we know, there is such a vast deal of which we are still ignorant. so far in dealing with all the planets and moons in the solar system we have made no remark on the way they rotate or revolve, because they all go in the same direction, and that direction is called counter-clockwise, which means that if you stand facing a clock and turn your hand slowly round the opposite direction to that in which the hands go, you will be turning it in the same way that the earth rotates on its axis and revolves in its orbit. it is, perhaps, just as well to give here a word of caution. rotating of course means a planet's turning on its own axis, revolving means its course in its orbit round the sun. mercury, venus, earth, mars, jupiter, and all their moons, as well as saturn himself, rotate on their axes in this one direction--counter-clockwise--and revolve in the same direction as they rotate. even the queer little moon of mars, which runs round him quicker than he rotates, obeys this same rule. nine of saturn's moons follow this example, but one independent little one, which has been named phoebe, and is far out from the planet, actually revolves in the opposite way. we cannot see how it rotates, but if, as we said just now, it turns the same face always to saturn, then of course it rotates the wrong way too. a theory has been suggested to account for this curious fact, but it could not be made intelligible to anyone who has not studied rather high mathematics, so there we must just leave it, and put it in the cabinet of curiosities we have already collected on our way out to saturn. for ages past men have known and watched the planets lying within the orbit of saturn, and they had made up their minds that this was the limit of our system. but in a great astronomer named herschel was watching the heavens through a telescope when he noticed one strange object that he was certain was no star. the vast distance of the stars prevents their having any definite outline, or what is called a disc. the rays dart out from them in all directions and there is no 'edge' to them, but in the case of the planets it is possible to see a disc with a telescope, and this object which attracted herschel's attention had certainly a disc. he did not imagine he had discovered a new planet, because at that time the asteroids had not been found, and no one thought that there could be any more planets. yet herschel knew that this was not a star, so he called it a comet! he was actually the first who discovered it, for he knew it was not a fixed star, but it was after his announcement of this fact that some one else, observing it carefully, found it to be a real planet with an orbit lying outside that of saturn, then the furthest boundary of the solar system. herschel suggested calling it georgius sidus, in honour of george iii., then king; but luckily this ponderous name was not adopted, and as the other planets had been called after the olympian deities, and uranus was the father of saturn, it was called uranus. it was subsequently found that this new planet had already been observed by other astronomers and catalogued as a star no less than seventeen times, but until herschel's clear sight had detected the difference between it and the fixed stars no one had paid any attention to it. uranus is very far away from the sun, and can only sometimes be seen as a small star by people who know exactly where to look for him. in fact, his distance from the sun is nineteen times that of the earth. yet to show at all he must be of great size, and that size has actually been found out by the most delicate experiments. if we go back to our former comparison, we shall remember that if the earth were like a greengage plum, then uranus would be in comparison about the size of one of those coloured balloons children play with; therefore he is much larger than the earth. in this far distant orbit the huge planet takes eighty-four of our years to complete one of his own. a man on the earth will have grown from babyhood to boyhood, from boyhood to the prime of life, and lived longer than most men, while uranus has only once circled in his path. but in dealing with uranus we come to another of those startling problems of which astronomy is full. so far we have dealt with planets which are more or less upright, which rotate with a rotation like that of a top. now take a top and lay it on one side on the table, with one of its poles pointing toward the great lamp we used for the sun and the other pointing away. that is the way uranus gets round his path, on his side! he rotates the wrong way round compared with the planets we have already spoken of, but he revolves the same way round the sun that all the others do. it seems wonderful that even so much can be found out about a body so far from us, but we know more: we have discovered that uranus is made of lighter material than the earth; his density is less. how can that be known? well, you remember every body attracts every other body in proportion to the atoms it contains. if, therefore, there were any bodies near to uranus, it could be calculated by his influence on them what was his own mass, which, as you remember, is the word we use to express what would be weight were it at the earth's surface; and far away as uranus is, the bodies from which such calculations may be made have been discovered, for he has no less than four satellites, or moons. considering now the peculiar position of the planet, we might expect to find these moons revolving in a very different way from others, and this is indeed the case. they turn round the planet at about its equator--that is to say, if you hold the top representing uranus as was suggested just now, these moons would go above and below the planet in passing round it. only we must remember there is really no such thing as above and below absolutely. we who are on one side of the world point up to the sky and down to the earth, while the people on the other side of the earth, say at new zealand, also point up to the sky and down to the earth, but their pointings are directly the opposite of ours. so when we speak of moons going above and below that is only because, for the moment, we are representing uranus as a top we hold in our hands, and so we speak of above and below as they are to us. it was herschel who discovered these satellites, as well as the planet, and for these great achievements he occupies one of the grandest places in the rôle of names of which england is proud. but he did much more than this: his improvements in the construction of telescopes, and his devotion to astronomy in many other ways, would have caused him to be remembered without anything else. of uranus's satellites one, the nearest, goes round in about two and a half days, and the one that is furthest away takes about thirteen and a half days, so both have a shorter period than our moon. the discovery of uranus filled the whole civilized world with wonder. the astronomers who had seen him, but missed finding out that he was a planet, must have felt bitterly mortified, and when he was discovered he was observed with the utmost accuracy and care. the calculations made to determine his path in the sky were the easier because he had been noted as a star in several catalogues previously, so that his position for some time past was known. everybody who worked at astronomy began to observe him. from these facts mathematicians set to work, and, by abstruse calculations, worked out exactly the orbit in which he ought to move; then his movements were again watched, and behold he followed the path predicted for him; but there was a small difference here and there: he did not follow it exactly. now, in the heavens there is a reason for everything, though we may not always be clever enough to find it out, and it was easily guessed that it was not by accident that uranus did not precisely follow the path calculated for him. the planets all act and react on one another, as we know, according to their mass and their distance, and in the calculations the pull of jupiter on saturn and of saturn on uranus were known and allowed for. but uranus was pulled by some unseen influence also. a young englishman named adams, by some abstruse and difficult mathematical work far beyond the power of ordinary brains, found out not only the fact that there must be another planet nearly as large as uranus in an orbit outside his, but actually predicted where such a planet might be seen if anyone would look for it. he gave his results to a professor of astronomy at cambridge. now, it seems an easy thing to say to anyone, 'look out for a planet in such and such a part of the sky,' but in reality, when the telescope is turned to that part of the sky, stars are seen in such numbers that, without very careful comparison with a star chart, it is impossible to say which are fixed stars and which, if any, is an intruder. there happened to be no star chart of this kind for the particular part of the sky wanted, and thus a long time elapsed and the planet was not identified. meantime a young frenchman named leverrier had also taken up the same investigation, and, without knowing anything of adams' work, had come to the same conclusion. he sent his results to the berlin observatory, where a star chart such as was wanted was actually just being made. by the use of this the berlin astronomers at once identified this new member of our system, and announced to the astonished world that another large planet, making eight altogether, had been discovered. then the english astronomers remembered that they too held in their hands the means for making this wonderful discovery, but, by having allowed so much time to elapse, they had let the honour go to france. however, the names of adams and leverrier will always be coupled together as the discoverers of the new planet, which was called neptune. the marvel is that by pure reasoning the mind of man could have achieved such results. if the observation of uranus is difficult, how much more that of neptune, which is still further plunged in space! yet by patience a few facts have been gleaned about him. he is not very different in size from uranus. he also is of very slight density. his year includes one hundred and sixty-five of ours, so that since his discovery in he has only had time to get round less than a third of his path. his axis is even more tilted over than that of uranus, so that if we compare uranus to a top held horizontally, neptune will be like a top with one end pointing downwards. he rotates in this extraordinary position, in the same manner as uranus--namely, the other way over from all the other planets, but he revolves, as they all do, counter-clockwise. seen from neptune the sun can only appear about as large as venus appears to us at her best, and the light and heat received are but one nine-hundreth part of what he sends us. yet so brilliant is sunshine that even then the light that falls on neptune must be very considerable, much more than that which we receive from venus, for the sun itself glows, and from venus the light is only reflected. the sun, small as it must appear, will shine with the radiance of a glowing electric light. to get some idea of the brilliance of sunlight, sit near a screen of leaves on some sunny day when the sun is high overhead, and note the intense radiance of even the tiny rays which shine through the small holes in the leaves. the scintillating light is more glorious than any diamond, shooting out coloured rays in all directions. a small sun the apparent size of venus would, therefore, give enough light for practical purposes to such a world as neptune, even though to us a world so illuminated would seem to be condemned to a perpetual twilight. chapter vii the sun so far we have referred to the sun just so much as was necessary to show the planets rotating round him, and to acknowledge him as the source of all our light and heat; but we have not examined in detail this marvellous furnace that nourishes all the life on our planet and burns on with undiminished splendour from year to year, without thought or effort on our part. to sustain a fire on the earth much time and care and expense are necessary; fuel has to be constantly supplied, and men have to stoke the fire to keep it burning. considering that the sun is not only vastly larger than all the fires on the earth put together, but also than the earth itself, the question very naturally occurs to us, who supplies the fuel, and who does the stoking on the sun? before we answer this we must try to get some idea of the size of this stupendous body. it is not the least use attempting to understand it by plain figures, for the figures would be too great to make any impression on us--they would be practically meaningless; we must turn to some other method. suppose, for instance, that the sun were a hollow ball; then, if the earth were set at the centre, the moon could revolve round her at the same distance she is now, and there would be as great a distance between the moon and the shell of the sun as there is between the moon and the earth. this gives us a little idea of the size of the sun. again, if we go back to that solar system in which we represented the planets by various objects from a pea to a football, and set a lamp in the centre to do duty for the sun, what size do you suppose that lamp would have to be really to represent the sun in proportion to the planets? well, if our greengage plum which did duty for the earth were about three-quarters of an inch in diameter we should want a lamp with a flame as tall as the tallest man you know, and even then it would not give a correct idea unless you imagined that man extending his arms widely, and you drew round him a circle and filled in all the circle with flame! if this glorious flame burnt clear and fair and bright, radiating beams of light all around, the little greengage plum would not have to be too near, or it would be shrivelled up as in the blast of a furnace. to place it at anything resembling the distance it is from the sun in reality you would have to walk away from the flaming light for about three hundred steps, and set it down there; then, after having done all this, you would have some little idea of the relative sizes of the sun and the earth, and of the distance between them. of course, all the other planets would have to be at corresponding distances. on this same scale, neptune, the furthest out, would be three miles from our artificial sun! it seems preposterous to think that some specks so small as to be quite invisible, specks that crawl about on that plum, have dared to weigh and measure the gigantic sun; but yet they have done it, and they have even decided what he is made of. the result of the experiments is that we know the sun to be a ball of glowing gas at a temperature so high that nothing we have on earth could even compare with it. of his radiating beams extending in all directions few indeed fall on our little plum, but those that do are the source of all life, whether animal or vegetable. if the sun's rays were cut off from us, we should die at once. even the coal we use to keep us warm is but sun's heat stored up ages ago, when the luxuriant tropical vegetation sprang up in the warmth and then fell down and was buried in the earth. at night we are still enjoying the benefit of the sun's rays--that is, of those which are retained by our atmosphere; for if none remained even the very air itself would freeze, and by the next morning not one inhabitant would be left alive to tell the awful tale. yet all this life and growth and heat we receive on the whole earth is but one part in two thousand two hundred millions of parts that go out in all directions into space. it has been calculated that the heat which falls on to all the planets together cannot be more than one part in one hundred millions and the other millions of parts seem to us to be simply wasted. for untold ages the sun has been pouring out this prodigal profusion of glory, and as we know that this cannot go on without some sort of compensation, we want to understand what keeps up the fires in the sun. it is true that the sun is so enormous that he might go on burning for a very long time without burning right away; but, then, even if he is huge, his expenditure is also huge. if he had been made of solid coal he would have been all used up in about six thousand years, burning at the pace he does. now, we know that the ancient egyptians kept careful note of the heavenly bodies, and if the sun were really burning away he must have been very much larger in their time; but we have no record of this; on the contrary, all records of the sun even to five thousand years ago show that he was much the same as at present. it is evident that we must search elsewhere for an explanation. it has been suggested that his furnace is supplied by the number of meteors that fall into him. meteors are small bodies of the same materials as the planets, and may be likened to the dust of the solar system. it is not difficult to calculate the amount of matter he would require on this assumption to keep him going, and the amount required is so great as to make it practically impossible that this is the source of his supply. we have seen that all matter influences all other matter, and the quantity of meteoric stuff that would be required to support the sun's expenditure would be enough to have a serious effect on mercury, an effect that would certainly have been noticed. there can, therefore, be no such mass of matter near the sun, and though there is no doubt a certain number of meteors do fall into his furnaces day by day, it is not nearly enough to account for his continuous radiation. it seems after this as if nothing else could be suggested; but yet an answer has been found, an answer so wonderful that it is more like a fairy tale than reality. to begin at the beginning, we must go back to the time when the sun was only a great gaseous nebula filling all the space included in the orbit of neptune. this nebula was not in itself hot, but as it rotated it contracted. now, heat is really only a form of energy, and energy and heat can be interchanged easily. this is a very startling thing when heard for the first time, but it is known as surely as we know anything and has been proved again and again. when a savage wants to make a fire he turns a piece of hard wood very very quickly between his palms--twiddles it, we should say expressively--into a hole in another piece of wood, until a spark bursts out. what is the spark? it is the energy of the savage's work turned to heat. when a horse strikes his iron-shod hoofs hard on the pavement you see sparks fly; that is caused by the energy of the horse's leg. when you pump hard at your bicycle you feel your pump getting quite hot, for part of the energy you are putting into your work is transformed into heat; and so on in numberless instances. no energetic action of any kind in this world takes place without some of the energy being turned into heat, though in many instances the amount is so small as to be unnoticeable. nothing falls to the ground without some heat being generated. now, when this great nebula first began its remarkable career, by the action of gravity all the particles in it were drawn toward the centre; little by little they fell in, and the nebula became smaller. we are not now concerned with the origin of the planets--we leave that aside; we are only contemplating the part of the nebula which remained to become the sun. now these particles being drawn inward each generated some heat, so as the nebula contracted its temperature rose. throughout the ages, over the space of millions and millions of miles, it contracted and grew hotter. it still remained gaseous, but at last it got to an immense temperature, and is the sun as we know it. what then keeps it shining? it is still contracting, but slowly, so slowly that it is quite imperceptible to our finest instruments. it has been calculated that if it contracts two hundred and fifty feet in diameter in a year, the energy thus gained and turned into heat is quite sufficient to account for its whole yearly output. this is indeed marvellous. in comparison with the sun's size two hundred and fifty feet is nothing. it would take nine thousand years at this rate before any diminution could be noticed by our finest instruments! here is a source of heat which can continue for countless ages without exhaustion. thus to all intents and purposes we may say the sun's shining is inexhaustible. yet we must follow out the train of reasoning, and see what will happen in the end, in eras and eras of time, if nothing intervenes. well, some gaseous bodies are far finer and more tenuous than others, and when a gaseous body contracts it is all the time getting denser; as it grows denser and denser it at last becomes liquid, and then solid, and then it ceases to contract, as of course the particles of a solid body cannot fall freely toward the centre, as those of a gaseous body can. our earth has long ago reached this stage. when solid the action ceases, and the heat is no more kept up by this source of energy, therefore the body begins to cool--surface first, and lastly the interior; it cools more quickly the smaller it is. our moon has parted with all her heat long ago, while the earth still retains some internally. in the sun, therefore, we have an object-lesson of the stages through which all the planets must have passed. they have all once been glowing hot, and some may be still hot even on the surface, as we have seen there is reason to believe is the case with jupiter. by this marvellous arrangement for the continued heat of the sun we can see that the warmth of our planets is assured for untold ages. there is no need to fear that the sun will wear out by burning. his brightness will continue for ages beyond the thoughts of man. besides this, a few other things have been discovered about him. he is, of course, exceptionally difficult to observe; for though he is so large, which should make it easy, he is so brilliant that anyone regarding him through a telescope without the precaution of prepared glasses to keep off a great part of the light would be blinded at once. one most remarkable fact about the sun is that his surface is flecked with spots, which appear sometimes in greater numbers and sometimes in less, and the reason and shape of these spots have greatly exercised men's minds. sometimes they are large enough to be seen without a telescope at all, merely by looking through a piece of smoked or coloured glass, which cuts off the most overpowering rays. when they are visible like this they are enormous, large enough to swallow many earths in their depths. at other times they may be observed by the telescope, then they may be about five thousand miles across. sometimes one spot can be followed by an astronomer as it passes all across the sun, disappears at the edge, and after a lapse of time comes back again round the other edge. this first showed men that the sun, like all the planets, rotated on his axis, and gave them the means of finding out how long he took in doing so. but the spots showed a most surprising result, for they took slightly different times in making their journey round the sun, times which differed according to their position. for instance, a spot near the equator of the sun took twenty-five days to make the circuit, while one higher up or lower down took twenty-six days, and one further out twenty-seven; so that if these spots are, as certainly believed, actually on the surface, the conclusion is that the sun does not rotate all in one piece, but that some parts go faster than others. no one can really explain how this could be, but it is certainly more easily understood in the case of a body of gas than of a solid body, when it would be simply impossible to conceive. the spots seem to keep principally a little north and a little south of the equator; there are very few actually at it, and none found near the poles, but no reason for this distribution has been discovered. it has been noted that about every eleven years the greatest number of spots appears, and that they become fewer again, mounting up in number to the next eleven years, and so on. all these curious facts show there is much yet to be solved about the sun. the spots were supposed for long to be eruptions bursting up above the surface, but now they are generally held to be deep depressions like saucers, probably caused by violent tempests, and it is thought that the inrush of cooler matter from above makes them look darker than the other parts of the sun's surface. but when we use the words 'cooler' and 'darker,' we mean only by comparison, for in reality the dark parts of the spots are brighter than electric light. [illustration: _royal observatory, greenwich._ sun-spots.] the fact that the spots are in reality depressions or holes is shown by their change of appearance as they pass over the face of the sun toward the edge; for the change of shape is exactly that which would be caused by foreshortening. it sounds odd to say that the best time for observing the sun is during a total eclipse, for then the sun's body is hidden by the moon. but yet to a certain extent this is true, and the reason is that the sun's own brilliance is our greatest hindrance in observing him, his rays are so dazzling that they light up our own atmosphere, which prevents us seeing the edges. now, during a total eclipse, when nearly all the rays are cut off, we can see marvellous things, which are invisible at other times. but total eclipses are few and far between, and so when one is approaching astronomers make great preparations beforehand. a total eclipse is not visible from all parts of the world, but only from that small part on which the shadow of the moon falls, and as the earth travels, this shadow, which is really a round spot, passes along, making a dark band. in this band astronomers choose the best observatories, and there they take up their stations. the dark body of the moon first appears to cut a little piece out of the side of the sun, and as it sails on, gradually blotting out more and more, eager telescopes follow it; at last it covers up the whole sun, and then a marvellous spectacle appears, for all round the edges of the black moon are seen glorious red streamers and arches and filaments of marvellous shapes, continually changing. these are thrown against a background of pale green light that surrounds the black moon and the hidden sun. in early days astronomers thought these wonderful coloured streamers belonged to the moon; but it was soon proved that they really are part of the sun, and are only invisible at ordinary times, because our atmosphere is too bright to allow them to be seen. an instrument has now been invented to cut off most of the light of the sun, and when this is attached to a telescope these prominences, as they are called, can be seen at any time, so that there is no need to wait for an eclipse. [illustration: the earth as it would appear in comparison with the flames shooting out from the sun.] what are these marvellous streamers and filaments? they are what they seem, eruptions of fiery matter discharged from the ever-palpitating sun thousands of miles into surrounding space. they are for ever shooting out and bursting and falling back, fireworks on a scale too enormous for us to conceive. some of these brilliant flames extend for three hundred thousand miles, so that in comparison with one of them the whole world would be but a tiny ball, and this is going on day and night without cessation. look at the picture where the artist has made a little black ball to represent the earth as she would appear if she could be seen in the midst of the flames shooting out from the sun. do not make a mistake and think the earth really could be in this position; she is only shown there so that you may see how tiny she is in comparison with the sun. all the time you have lived and your father, and grandfather, and right back to the beginnings of english history, and far, far further into the dim ages, this stupendous exhibition of energy and power has continued, and only of late years has anyone known anything about it; even now a mere handful of people do know, and the rest, who are warmed and fed and kept alive by the gracious beams of this great revolving glowing fireball, never give it a thought. i said just now a pale green halo surrounded the sun, extending far beyond the prominences; this is called the corona and can only be seen during an eclipse. it surrounds the sun in a kind of shell, and there is reason to believe that it too is made of luminous stuff ejected by the sun in its burning fury. it is composed of large streamers or filaments, which seem to shoot out in all directions; generally these are not much larger than the apparent width of the sun, but sometimes they extend much further. the puzzle is, this corona cannot be an atmosphere in any way resembling that of our earth; for the gravitational force of the sun, owing to its enormous size, is so great that it would make any such atmosphere cling to it much more densely near to the surface, while it would be thinner higher up, and the corona is not dense in any way, but thin and tenuous throughout. this makes it very difficult to explain; it is supposed that some kind of electrical force enters into the problem, but what it is exactly we are far from knowing yet. chapter viii shining visitors our solar system is set by itself in the midst of a great space, and so far as we have learnt about it in this book everything in it seems orderly: the planets go round the sun and the satellites go round the planets, in orbits more or less regular; there seems no place for anything else. but when we have considered the planets and the satellites, we have not exhausted all the bodies which own allegiance to the sun. there is another class, made up of strange and weird members, which flash in and out of the system, coming and going in all directions and at all times--sometimes appearing without warning, sometimes returning with a certain regularity, sometimes retiring to infinite depths of space, where no human eye will ever see them more. these strange visitors are called comets, and are of all shapes and sizes and never twice alike. even as we watch them they grow and change, and then diminish in splendour. some are so vast that men see them as flaming signs in the sky, and regard them with awe and wonder; some cannot be seen at all without the help of the telescope. from the very earliest ages those that were large enough to be seen without glasses have been regarded with astonishment. men used to think that they were signs from heaven foretelling great events in the world. timid people predicted that the end of the world would come by collision with one of them. others, again, fancifully likened them to fishes in that sea of space in which we swim--fishes gigantic and terrifying, endowed with sense and will. it is perhaps unnecessary to say that comets are no more alive than is our own earth, and as for causing the end of the world by collision, there is every reason to believe the earth has been more than once right through a comet's tail, and yet no one except scientific men even discovered it. these mysterious visitors from the outer regions of space were called comets from a greek word signifying hair, for they often leave a long luminous trail behind, which resembles the filaments of a woman's hair. it is not often that one appears large and bright enough to be seen by the naked eye, and when it does it is not likely to be soon forgotten. in the year such a comet is expected, a comet which at its former appearance compelled universal attention by its brilliancy and strangeness. at the time of the norman conquest of england a comet believed to be the very same one was stretching its glorious tail half across the sky, and the normans seeing it, took it as a good omen, fancying that it foretold their success. the history of the norman conquest was worked in tapestry--that is to say, in what we should call crewels on a strip of linen--and in this record the comet duly appears. look at him in the picture as the normans fancied him. he has a red head with blue flames starting from it, and several tails. the little group of men on the left are pointing and chattering about him. we can judge what an impression this comet must have made to be recorded in such an important piece of work. [illustration: the comet in the bayeux tapestry.] but we are getting on too fast. we have yet to learn how anyone can know that the comet which appeared at the time of the norman conquest is the same as that which has come back again at different times, and above all, how anyone can tell that it will come again in the year . all this involves a long story. before the invention of telescopes of course only those comets could be seen which were of great size and fine appearance. in those days men did not realize that our world was but one of a number and of no great importance except to ourselves, and they always took these blazing appearances in the heavens as a particular warning to the human race. but when astronomers, by the aid of the telescope, found that for one comet seen by the eye there were hundreds which no mortal eye unaided could see, this idea seemed, to say the least of it, unlikely. yet even then comets were looked upon as capricious visitors from outer space; odd creatures drawn into our system by the attraction of the sun, who disappeared, never to return. it was newton, the same genius who disclosed to us the laws of gravity, who first declared that comets moved in orbits, only that these orbits were far more erratic than any of those followed by the planets. so far we have supposed that the planets were all on what we should call a level--that is to say, we have regarded them as if they were floating in a sea of water around the sun; but this is only approximately correct, for the orbits of the planets are not all at one level. if you had a number of slender hoops or rings to represent the planetary orbits, you would have to tilt one a little this way and another a little that way, only never so far but that a line through the centre of the hoop from one side to another could pass through the sun. the way in which the planetary orbits are tilted is slight in comparison with that of the orbits of comets, for these are at all sorts of angles--some turned almost sideways, and others slanting, and all of them are ellipses long drawn out and much more irregular than the planetary orbits; but erratic as they are, in every case a line drawn through the sun and extended both ways would touch each side of the orbits. a great astronomer called halley, who was born in the time of the commonwealth, was lucky enough to see a very brilliant comet, and the sight interested him so much that he made all the calculations necessary to find out just in what direction it was travelling in the heavens. he found out that it followed an ellipse which brought it very near to the sun at one part of its journey, and carried it far beyond the orbit of the earth, right out to that of neptune, at the other. then he began to search the records for other comets which had been observed before his time. he found that two particularly bright ones had been carefully noted--one about seventy-five years before that which he had seen, and the other seventy-five years before that again. both these comets had been watched so scientifically that the paths in which they had travelled could be computed. a brilliant inspiration came to halley. he believed that instead of these three, his own and the other two, being different comets, they were the same one, which returned to the sun about every seventy-five years. this could be proved, for if this idea were correct, of course the comet would return again in another seventy-five years, unless something unforeseen occurred. but halley was in the prime of life: he could not hope to live to see his forecast verified. the only thing he could do was to note down exact particulars, by means of which others who lived after him might recognize his comet. and so when the time came for its return, though halley was in his grave, numbers of astronomers were watching eagerly to see the fulfilment of his prediction. the comet did indeed appear, and since then it has been seen once again, and now we expect it to come back in the year , when you and i may see it for ourselves. when the identity of the comet was fully established men began to search further back still, to compare the records of other previous brilliant comets, and found that this one had been noticed many times before, and once as i said, at the time of the norman conquest. halley's comet is peculiar in many ways. for instance, it is unusual that so large and interesting a comet should return within a comparatively limited time. it is the smaller comets, those that can only be seen telescopically, that usually run in small orbits. the smallest orbits take about three and a half years to traverse, and some of the largest orbits known require a period of one hundred and ten thousand years. between these two limits lies every possible variety of period. one comet, seen about the time napoleon was born, was calculated to take two thousand years to complete its journey, and another, a very brilliant one seen in , must journey for eight hundred years before it again comes near to the sun. but we never know what might happen, for at any moment a comet which has traversed a long solitary pathway in outer darkness may flash suddenly into our ken, and be for the first time noted and recorded, before flying off at an angle which must take it for ever further and further from the sun. everything connected with comets is mysterious and most fascinating. from out of the icy regions of space a body appears; what it is we know not, but it is seen at first as a hairy or softly-glowing star, and it was thus that herschel mistook uranus for a comet when he first discovered it. as it draws nearer the comet sends out some fan-like projections toward the sun, enclosing its nucleus in filmy wrappings like a cocoon of light, and it travels faster and faster. from its head shoots out a tail--it may be more than one--growing in splendour and width, and always pointing away from the sun. so enormous are some of these tails that when the comet's head is close to the sun the tail extends far beyond the orbit of the earth. faster still and faster flies the comet, for as we have seen it is a consequence of the law of gravitation that the nearer planets are to the sun the faster they move in their orbits, and the same rule applies to comets too. as the comet dashes up to the sun his pace becomes something indescribable; it has been reckoned for some comets at three hundred miles a second! but behold, as the head flies round the sun the tail is always projected outwards. the nucleus or head may be so near to the sun that the heat it receives would be sufficient to reduce molten iron to vapour; but this does not seem to affect it: only the tail expands. sometimes it becomes two or more tails, and as it sweeps round behind the head it has to cover a much greater space in the same time, and therefore it must travel even faster than the head. the pace is such that no calculations can account for it, if the tail is composed of matter in any sense as we know it. then when the sun is passed the comet sinks away again, and as it goes the tail dies down and finally disappears. the comet itself dwindles to a hairy star once more and goes--whither? into space so remote that we cannot even dream of it--far away into cold more appalling than anything we could measure, the cold of absolute space. more and more slowly it travels, always away and away, until the sun, a short time back a huge furnace covering all the sky, is now but a faint star. thus on its lonely journey unseen and unknown the comet goes. this comet which we have taken as an illustration is a typical one, but all are not the same. some have no tails at all, and never develop any; some change utterly even as they are watched. the same comet is so different at different times that the only possible way of identifying it is by knowing its path, and even this is not a certain method, for some comets appear to travel at intervals along the same path! now we come to the question that must have been in the mind of everyone from the beginning of this chapter, what are comets? this question no one can answer definitely, for there are many things so puzzling about these strange appearances that it is difficult even to suggest an explanation. yet a good deal is known. in the first place, we are certain that comets have very little density--that is to say, they are indescribably thin, thinner than the thinnest kind of gas; and air, which we always think so thin, would be almost like a blanket compared with the material of comets. this we judge because they exercise no sort of influence on any of the planetary bodies they draw near to, which they certainly would do if they were made of any kind of solid matter. they come sometimes very close to some of the planets. a comet was so near to jupiter that it was actually in among his moons. the comet was violently agitated; he was pulled in fact right out of his old path, and has been going on a new one ever since; but he did not exercise the smallest effect on jupiter, or even on the moons. and, as i said earlier in this chapter, we on the earth have been actually in the folds of a comet's tail. this astonishing fact happened in june, . one evening after the sun had set a golden-yellow disc, surrounded with filmy wrappings, appeared in the sky. the sun's light, diffused throughout our atmosphere, had prevented its being seen sooner. this was apparently the comet's head. it is described as 'though a number of light, hazy clouds were floating around a miniature full moon.' from this a cone of light extended far up into the sky, and when the head disappeared below the horizon this tail was seen to reach to the zenith. but that was not all. strange shafts of light seemed to hang right overhead, and could only be accounted for by supposing that they were caused by another tail hanging straight above us, so that we looked up at it foreshortened by perspective. the comet's head lay between the earth and the sun, and its tail, which extended over many millions of miles, stretched out behind in such a way that the earth must have gone right through it. the fact that the comet exercised no perceptible influence on the earth at all, and that there were not even any unaccountable magnetic storms or displays of electricity, may reassure us so that if ever we do again come in contact with one of these extremely fine, thin bodies, we need not be afraid. there is another way in which we can judge of the wonderful tenuity or thinness of comets--that is, that the smallest stars can be seen through their tails, even though those tails must be many thousands of miles in thickness. now, if the tails were anything approaching the density of our own atmosphere, the stars when seen through them would appear to be moved out of their places. this sounds odd, and requires a word of explanation. the fact is that anything seen through any transparent medium like water or air is what is called refracted--that is to say, the rays coming from it look bent. everyone is quite familiar with this in everyday life, though perhaps they may not have noticed it. you cannot thrust a stick into the water without seeing that it looks crooked. air being less dense than water has not quite so strong a refracting power, but still it has some. we cannot prove it in just the same way, because we are all inside the atmosphere ourselves, and there is no possibility of thrusting a stick into it from the outside! the only way we know it is by looking at something which is 'outside' already, and we find plenty of objects in the sky. as a matter of fact, the stars are all a little pulled out of their places by being seen through the air, and though of course we do not notice this, astronomers know it and have to make allowance for it. the effect is most noticeable in the case of the sun when he is going down, for the atmosphere bends his rays up, and though we see him a great glowing red ball on the horizon, and watch him, as we think, drop gradually out of sight, we are really looking at him for the last moment or two when he has already gone, for the rays are bent up by the air and his image lingers when the real sun has disappeared. [illustration: a stick thrust into the water appears crooked.] therefore in looking through the luminous stuff that forms a comet's tail astronomers might well expect to see the stars displaced, but not a sign of this appears. it is difficult to imagine, therefore, what the tail can be made of. the idea is that the sun exercises a sort of repulsive effect on certain elements found in the comet's head--that is to say, it pushes them away, and that as the head approaches the sun, these elements are driven out of it away from the sun in vapour. this action may have something to do with electricity, which is yet little understood; anyway, the effect is that, instead of attracting the matter toward itself, in which case we should see the comet's tails stretching toward the sun, the sun drives it away! in the chapter on the sun we had to imagine something of the same kind to account for the corona, and the corona and the comet's tails may be really akin to each other, and could perhaps be explained in the same way. now we come to a stranger fact still. some comets go right through the sun's corona, and yet do not seem to be influenced by it in the smallest degree. this may not seem very wonderful at first perhaps, but if you remember that a dash through anything so dense as our atmosphere, at a pace much less than that at which a comet goes, is enough to heat iron to a white heat, and then make it fly off in vapour, we get a glimpse of the extreme fineness of the materials which make the corona. here is herschel's account of a comet that went very near the sun: 'the comet's distance from the sun's centre was about the th part of our distance from it. all the heat we enjoy on this earth comes from the sun. imagine the heat we should have to endure if the sun were to approach us, or we the sun, to one th part of its present distance. it would not be merely as if suns were shining on us all at once, but, times , according to a rule which is well known to all who are conversant with such matters. now, that is , . only imagine a glare , times fiercer than that of the equatorial sunshine at noon day with the sun vertical. in such a heat there is no substance we know of which would not run like water, boil, and be converted into smoke or vapour. no wonder the comet gave evidence of violent excitement, coming from the cold region outside the planetary system torpid and ice-bound. already when arrived even in our temperate region it began to show signs of internal activity; the head had begun to develop, and the tail to elongate, till the comet was for a time lost sight of--not for days afterwards was it seen; and its tail, whose direction was reversed, and which could not possibly be the same tail it had before, had already lengthened to an extent of about ninety millions of miles, so that it must have been shot out with immense force in a direction away from the sun.' we remember that comets have sometimes more than one tail, and a theory has been advanced to account for this too. it is supposed that perhaps different elements are thrust away by the sun at different angles, and one tail may be due to one element and another to another. but if the comet goes on tail-making to a large extent every time it returns to the sun, what happens eventually? do the tails fall back again into the head when out of reach of the sun's action? such an idea is inconceivable; but if not, then every time a comet approaches the sun he loses something, and that something is made up of the elements which were formerly in the head and have been violently ejected. if this be so we may well expect to see comets which have returned many times to the sun without tails at all, for all the tail-making stuff that was in the head will have been used up, and as this is exactly what we do see, the theory is probably true. where do the comets come from? that also is a very large question. it used to be supposed they were merely wanderers in space who happened to have been attracted by our sun and drawn into his system, but there are facts which go very strongly against this, and astronomers now generally believe that comets really belong to the solar system, that their proper orbits are ellipses, and that in the case of those which fly off at such an angle that they can never return they must at some time have been pulled out of their original orbit by the influence of one of the planets. [illustration: _royal observatory, cape of good hope._ a great comet.] to get a good idea of a really fine comet, until we have the opportunity of seeing one for ourselves, we cannot do better than look at this picture of a comet photographed in at the cape of good hope. it is only comparatively recently that photography has been applied to comets. when halley's comet appeared last time such a thing was not thought of, but when he comes again numbers of cameras, fitted up with all the latest scientific appliances, will be waiting to get good impressions of him. chapter ix shooting stars and fiery balls all the substances which we are accustomed to see and handle in our daily lives belong to our world. there are vegetables which grow in the earth, minerals which are dug out of it, and elementary things, such as air and water, which have always made up a part of this planet since man knew it. these are obvious, but there are other things not quite so obvious which also help to form our world. among these we may class all the elements known to chemists, many of which have difficult names, such as oxygen and hydrogen. these two are the elements which make up water, and oxygen is an important element in air, which has nitrogen in it too. there are numbers and numbers of other elements perfectly familiar to chemists, of which many people never even hear the names. we live in the midst of these things, and we take them for granted and pay little attention to them; but when we begin to learn about other worlds we at once want to know if these substances and elements which enter so largely into our daily lives are to be found elsewhere in the universe or are quite peculiar to our own world. this question might be answered in several ways, but one of the most practical tests would be if we could get hold of something which had not been always on the earth, but had fallen upon it from space. then, if this body were made up of elements corresponding with those we find here, we might judge that these elements are very generally diffused throughout the bodies in the solar system. it sounds in the highest degree improbable that anything should come hurling through the air and alight on our little planet, which we know is a mere speck in a great ocean of space; but we must not forget that the power of gravity increases the chances greatly, for anything coming within a certain range of the earth, anything small enough, that is, and not travelling at too great a pace, is bound to fall on to it. and, however improbable it seems, it is undoubtedly true that masses of matter do crash down upon the earth from time to time, and these are called meteorites. when we think of the great expanse of the oceans, of the ice round the poles, and of the desert wastes, we know that for every one of such bodies seen to fall many more must have fallen unseen by any human being. meteors large enough to reach the earth are not very frequent, which is perhaps as well, and as yet there is no record of anyone's having been killed by them. most of them consist of masses of stone, and a few are of iron, while various substances resembling those that we know here have been found in them. chemists in analyzing them have also come across certain elements so far unknown upon earth, though of course there is no saying that these may not exist at depths to which man has not penetrated. a really large meteor is a grand sight. if it is seen at night it appears as a red star, growing rapidly bigger and leaving a trail of luminous vapour behind as it passes across the sky. in the daytime this vapour looks like a cloud. as the meteor hurls itself along there may be a deep continuous roar, ending in one supreme explosion, or perhaps in several explosions, and finally the meteor may come to the earth in one mass, with a force so great that it buries itself some feet deep in the soil, or it may burst into numbers of tiny fragments, which are scattered over a large area. when a meteor is found soon after its fall it is very hot, and all its surface has 'run,' having been fused by heat. the heat is caused by the friction of our atmosphere. the meteor gets entangled in the atmosphere, and, being drawn by the attraction of the earth, dashes through it. part of the energy of its motion is turned to heat, which grows greater and greater as the denser air nearer to the earth is encountered; so that in time all the surface of the meteor runs like liquid, and this liquid, rising to a still higher temperature, is blown off in vapour, leaving a new surface exposed. the vapour makes the trail of fire or cloud seen to follow the meteor. if the process went on for long the meteor would be all dissipated in vapour, and in any case it must reach the earth considerably reduced in size. numbers and numbers of comparatively small ones disappear, and for every one that manages to come to earth there must be hundreds seen only as shooting stars, which vanish and 'leave not a wrack behind.' when a meteor is seen to fall it is traced, and, whenever possible, it is found and placed in a museum. men have sometimes come across large masses of stone and iron with their surfaces fused with heat. these are in every way like the recognized meteorites, except that no eye has noted their advent. as there can be no reasonable doubt that they are of the same origin as the others, they too are collected and placed in museums, and in any large museum you would be able to see both kinds--those which have been seen to come to earth and those which have been found accidentally. the meteors which appear very brilliant in their course across the sky are sometimes called fire-balls, which is only another name for the same thing. some of these are brighter than the full moon, so bright that they cause objects on earth to cast a shadow. in a fiery ball was noticed above a small town in normandy; it burst and scattered stones far and wide, but luckily no one was hurt. the largest meteorites that have been found on the earth are a ton or more in weight; others are mere stones; and others again just dust that floats about in the atmosphere before gently settling. of course, meteors of this last kind could not be seen to fall like the larger ones, yet they do fall in such numbers that calculations have been made showing that the earth must catch about a hundred millions of meteors daily, having altogether a total weight of about a hundred tons. this sounds enormous, but compared with the weight of the earth it is very small indeed. now that we have arrived at the fact that strange bodies do come hurtling down upon us out of space, and that we can actually handle and examine them, the next question is, where do they come from? at one time it was thought that they were fragments which had been flung off by the earth herself when she was subject to violent explosions, and that they had been thrown far enough to resist the impulse to drop down upon her again, and had been circling round the sun ever since, until the earth came in contact with them again and they had fallen back upon her. it is not difficult to imagine a force which would be powerful enough to achieve the feat of speeding something off at such a velocity that it passed beyond the earth's power to pull it back, but nothing that we have on earth would be nearly strong enough to achieve such a feat. imaginative writers have pictured a projectile hurled from a cannon's mouth with such tremendous force that it not only passed beyond the range of the earth's power to pull it back, but so that it fell within the influence of the moon and was precipitated on to her surface! such things must remain achievements in imagination only; it is not possible for them to be carried out. other ideas as to the origin of meteors were that they had been expelled from the moon or from the sun. it would need a much less force to send a projectile away from the moon than from the earth on account of its smaller size and less density, but the distance from the earth to the moon is not very great, and any projectile hurled forth from the moon would cross it in a comparatively short time. therefore if the meteorites come from the moon, the moon must be expelling them still, and we might expect to see some evidence of it; but we know that the moon is a dead world, so this explanation is not possible. the sun, for its part, is torn by such gigantic disturbances that, notwithstanding its vast size, there is no doubt sufficient force there to send meteors even so far as the earth, but the chances of their encountering the earth would be small. both these theories are now discarded. it is believed that the meteors are merely lesser fragments of the same kind of materials as the planets, circling independently round the sun; and a proof of this is that far more meteorites fall on that part of the earth which is facing forward in its journey than on that behind, and this is what we should expect if the meteors were scattered independently through space and it was by reason of our movements that we came in contact with them. there is no need to explain this further. everyone knows that in cycling or driving along a road where there is a good deal of traffic both ways the people we meet are more in number than those who overtake us, and the same result would follow with the meteors; that is to say, in travelling through space where they were fairly evenly distributed we should meet more than we should be overtaken by. you remember that it was suggested the sun's fuel might be obtained from meteors, and this was proved to be not possible, even though there are no doubt unknown millions of these strange bodies circling throughout the solar system. there are so many names for these flashing bodies that we may get a little confused: when they are seen in the sky they are meteors, or fire-balls; when they reach the earth they are called meteorites, and also aerolites. then there is another class of the same bodies called shooting stars, and these are in reality only meteors on a smaller scale; but there ought to be no confusion in our thoughts, for all these objects are small bodies travelling round the sun, and caught by the earth's influence. when you watch the sky for some time on a clear night, you will seldom fail to see at least one star flash out suddenly in a path of thrilling light and disappear, and you cannot be certain whether that star had been shining in the sky a minute before, or if it had appeared suddenly only in order to go out. the last idea is right. we must get rid at once of the notion that it would be possible for any fixed star to behave in this manner. to begin with, the fixed stars are many of them actually travelling at a great velocity at present, yet so immeasurably distant are they that their movement makes no perceptible difference to us. for one of them to appear to dash across the heavens as a meteor does would mean a velocity entirely unknown to us, even comparing it with the speed of light. no, these shooting stars are not stars at all, though they were so named, long before the real motions of the fixed stars were even dimly guessed at. as we have seen, they belong to the same class as meteors. i remember being told by a clergyman, years ago, that one night in november he had gone up to bed very late, and as he pulled up his blind to look at the sky, to his amazement he saw a perfect hail of shooting stars, some appearing every minute, and all darting in vivid trails of light, longer or shorter, though all seemed to come from one point. so marvellous was the sight that he dashed across the village street, unlocked the church door, and himself pulled the bell with all his might. the people in that quiet country village had long been in bed, but they huddled on their clothes and ran out of their pretty thatched cottages, thinking there must be a great fire, and when they saw the wonder in the sky they were amazed and cried out that the world must be coming to an end. the clergyman knew better than that, and was able to reassure them, and tell them he had only taken the most effectual means of waking them so that they might not miss the display, for he was sure as long as they lived they would never see such another sight. a star shower of this kind is certainly well worth getting up to see, but though uncommon it is not unique. there are many records of such showers having occurred in times gone by, and when men put together and examined the records they found that the showers came at regular intervals. for instance, every year about the same time in november there is a star shower, not comparable, it is true, with the brilliant one the clergyman saw, but still noticeable, for more shooting stars are seen then than at other times, and once in every thirty-three years there is a specially fine one. it happened in fact to be one of these that the village people were wakened up to see. not all at once, but gradually, the mystery of these shower displays was solved. it was realized that the meteors need not necessarily come from one fixed place in the sky because they seemed to us to do so, for that was only an effect of perspective. if you were looking down a long, perfectly straight avenue of tree-trunks, the avenue would seem to close in, to get narrower and narrower at the far end until it became a point; but it would not really do so, for you would know that the trees at the far end were just the same distance from each other as those between which you were standing. now, two meteors starting from the same direction at a distance from each other, and keeping parallel, would seem to us to start from a point and to open out wider and wider as they approached, but they would not really do so; it would only be, as in the case of the avenue, an effect of perspective. if a great many meteors did the same thing, they would appear to us all to start from one point, whereas really they would be on parallel lines, only as they rushed to meet us or we rushed to meet them this effect would be produced. therefore the first discovery was that these meteors were thousands and thousands of little bodies travelling in lines parallel to each other, like a swarm of little planets. to judge that their path was not a straight line but a circle or ellipse was the next step, and this was found to be the case. from taking exact measurements of their paths in the sky an astronomer computed they were really travelling round the sun in a lengthened orbit, an ellipse more like a comet's orbit than that of a planet. but next came the puzzling question, why did the earth apparently hit them every year to some extent, and once in thirty-three years seem to run right into the middle of them? this also was answered. one has only to imagine a swarm of such meteors at first hastening busily along their orbit, a great cluster all together, then, by the near neighbourhood of some planet, or by some other disturbing causes, being drawn out, leaving stragglers lagging behind, until at last there might be some all round the path, but only thinly scattered, while the busy, important cluster that formed the nucleus was still much thicker than any other part. now, if the orbit that the meteors followed cut the orbit or path of the earth at one point, then every time the earth came to what we may call the level crossing she must run into some of the stragglers, and if the chief part of the swarm took thirty-three years to get round, then once in about thirty-three years the earth must strike right into it. this would account for the wonderful display. so long drawn-out is the thickest part of the swarm that it takes a year to pass the points at the level crossing. if the earth strikes it near the front one year, she may come right round in time to strike into the rear part of the swarm next year, so that we may get fine displays two years running about every thirty-three years. the last time we passed through the swarm was in , and then the show was very disappointing. here in england thick clouds prevented our seeing much, and there will not be another chance for us to see it at its best until . these november meteors are called leonids, because they _seem_ to come from a group of stars named leo, and though the most noticeable they are not the only ones. a shower of the same kind occurs in august too, but the august meteors, called perseids, because they seem to come from perseus, revolve in an orbit which takes a hundred and forty-two years to traverse! so that only every one hundred and forty-second year could we hope to see a good display. when all these facts had been gathered up, it seemed without doubt that certain groups of meteors travelled in company along an elliptical orbit. but there remained still something more--a bold and ingenious theory to be advanced. it was found that a comet, a small one, only to be seen with the telescope, revolved in exactly the same orbit as the november meteors, and another one, larger, in exactly the same orbit as the august ones; hence it could hardly be doubted that comets and meteors had some connection with each other, though what that connection is exactly no one knows. anyway, we can have no shadow of doubt when we find the comet following a marked path, and the meteors pursuing the same path in his wake, that the two have some mysterious affinity. there are other smaller showers besides these of november and august, and a remarkable fact is known about one of them. this particular stream was found to be connected with a comet named biela's comet, that had been many times observed, and which returned about every seven years to the sun. after it had been seen several times, this astonishing comet split in two and appeared as two comets, both of which returned at the end of the next seven years. but on the next two occasions when they were expected they never came at all, and the third time there came instead a fine display of shooting stars, so it really seemed as if these meteors must be the fragments of the lost comet. it is very curious and interesting to notice that in these star showers there is no certain record of any large meteorite reaching the earth; they seem to be made up of such small bodies that they are all dissipated in vapour as they traverse our air. chapter x the glittering heavens on a clear moonless night the stars appear uncountable. you see them twinkling through the leafless trees, and covering all the sky from the zenith, the highest point above your head, down to the horizon. it seems as if someone had taken a gigantic pepper-pot and scattered them far and wide so that some had fallen in all directions. if you were asked to make a guess as to how many you can see at one time, no doubt you would answer 'millions!' but you would be quite wrong, for the number of stars that can be seen at once without a telescope does not exceed two thousand, and this, after the large figures we have been dealing with, appears a mere trifle. with a telescope, even of small power, many more are revealed, and every increase in the size of the telescope shows more still; so that it might be supposed the universe is indeed illimitable, and that we are only prevented from seeing beyond a certain point by our limited resources. but in reality we know that this cannot be so. if the whole sky were one mass of stars, as it must be if the number of them were infinite, then, even though we could not distinguish the separate items, we should see it bright with a pervading and diffused light. as this is not so, we judge that the universe is not unending, though, with all our inventions, we may never be able to probe to the end of it. we need not, indeed, cry for infinity, for the distances of the fixed stars from us are so immeasurable that to atoms like ourselves they may well seem unlimited. our solar system is set by itself, like a little island in space, and far, far away on all sides are other great light-giving suns resembling our own more or less, but dwindled to the size of tiny stars, by reason of the great void of space lying between us and them. our sun is, indeed, just a star, and by no means large compared with the average of the stars either. but, then, he is our own; he is comparatively near to us, and so to us he appears magnificent and unique. judging from the solar system, we might expect to find that these other great suns which we call stars have also planets circling round them, looking to them for light and heat as we do to our sun. there is no reason to doubt that in some instances the conjecture is right, and that there may be other suns with attendant planets. it is however a great mistake to suppose that because our particular family in the solar system is built on certain lines, all the other families must be made on the same pattern. why, even in our own system we can see how very much the planets differ from each other: there are no two the same size; some have moons and some have not; saturn's rings are quite peculiar to himself, and uranus and neptune indulge in strange vagaries. so why should we expect other systems to be less varied? as science has advanced, the idea that these faraway suns must have planetary attendants as our sun has been discarded. the more we know the more is disclosed to us the infinite variety of the universe. for instance, so much accustomed are we to a yellow sun that we never think of the possibility of there being one of another colour. what would you say then to a ruby sun, or a blue one; or to two suns of different colours, perhaps red and green, circling round each other; or to two such suns each going round a dark companion? for there are dark bodies as well as shining bodies in the sky. these are some of the marvels of the starry sky, marvels quite as absorbing as anything we have found in the solar system. it requires great care and patience and infinite labour before the very delicate observations which alone can reveal to us anything of the nature of the fixed stars can be accomplished. it is only since the improvement in large telescopes that this kind of work has become possible, and so it is but recently men have begun to study the stars intimately, and even now they are baffled by indescribable difficulties. one of these is our inability to tell the distance of a thing by merely looking at it unless we also know its size. on earth we are used to seeing things appear smaller the further they are from us, and by long habit can generally tell the real size; but when we turn to the stars, which appear so much alike, how are we to judge how far off they are? two stars apparently the same size and close together in the sky may really be as far one from another as the earth is from the nearest; for if the further one were very much larger than the nearer, they would then appear the same size. at first it was natural enough to suppose that the big bright stars of what we call the first magnitude were the nearest to us, and the less bright the next nearest, and so on down to the tiny ones, only revealed by the telescope, which would be the furthest away of all; but research has shown that this is not correct. some of the brightest stars may be comparatively near, and some of the smallest may be near also. the size is no test of distance. so far as we have been able to discover, the star which seems nearest _is_ a first magnitude one, but some of the others which outshine it must be among the infinitely distant ones. thus we lie in the centre of a jewelled universe, and cannot tell even the size of the jewels which cover its radiant robe. i say 'lie,' but that is really not the correct word. so far as we have been able to find out, there is no such thing as absolute rest in the universe--in fact, it is impossible; for even supposing any body could be motionless at first, it would be drawn by the attraction of its nearest neighbours in space, and gradually gain a greater and greater velocity as it fell toward them. even the stars we call 'fixed' are all hurrying along at a great pace, and though their distance prevents us from seeing any change in their positions, it can be measured by suitable instruments. our sun is no exception to this universal rule. like all his compeers, he is hurrying busily along somewhere in obedience to some impulse of which we do not know the nature; and as he goes he carries with him his whole cortège of planets and their satellites, and even the comets. yes, we are racing through space with another motion, too, besides those of rotation and revolution, for our earth keeps up with its master attractor, the sun. it is difficult, no doubt, to follow this, but if you think for a moment you will remember that when you are in a railway-carriage everything in that carriage is really travelling along with it, though it does not appear to move. and the whole solar system may be looked at as if it were one block in movement. as in a carriage, the different bodies in it continue their own movements all the time, while sharing in the common movement. you can get up and change your seat in the train, and when you sit down again you have not only moved that little way of which you are conscious, but a great way of which you are not conscious unless you look out of the window. now in the case of the earth's own motion we found it necessary to look for something which does not share in that motion for purposes of comparison, and we found that something in the sun, who shows us very clearly we are turning on our axis. but in the case of the motion of the solar system the sun is moving himself, so we have to look beyond him again and turn to the stars for confirmation. then we find that the stars have motions of their own, so that it is very difficult to judge by them at all. it is as if you were bicycling swiftly towards a number of people all walking about in different directions on a wide lawn. they have their movements, but they all also have an apparent movement, really caused by you as you advance toward them; and what astronomers had to do was to separate the true movements of the stars from the false apparent movement made by the advance of the sun. this great problem was attacked and overcome, and it is now known with tolerable certainty that the sun is sweeping onward at a pace of about twelve miles a second toward a fixed point. it really matters very little to us where he is going, for the distances are so vast that hundreds of years must elapse before his movement makes the slightest difference in regard to the stars. but there is one thing which we can judge, and that is that though his course appears to be in a straight line, it is most probably only a part of a great curve so huge that the little bit we know seems straight. when we speak of the stars, we ought to keep quite clearly in our minds the fact that they lie at such an incredible distance from us that it is probable we shall never learn a great deal about them. why, men have not even yet been able to communicate with the planet mars, at its nearest only some thirty-five million miles from us, and this is a mere nothing in measuring the space between us and the stars. to express the distances of the stars in figures is really a waste of time, so astronomers have invented another way. you know that light can go round the world eight times in a second; that is a speed quite beyond our comprehension, but we just accept it. then think what a distance it could travel in an hour, in a day; and what about a year? the distance that light can travel in a year is taken as a convenient measure by astronomers for sounding the depths of space. measured in this way light takes four years and four months to reach us from the nearest star we know of, and there are others so much more distant that hundreds--nay, thousands--of years would have to be used to convey it. light which has been travelling along with a velocity quite beyond thought, silently, unresting, from the time when the britons lived and ran half naked on this island of ours, has only reached us now, and there is no limit to the time we may go back in our imaginings. we see the stars, not as they are, but as they were. if some gigantic conflagration had happened a hundred years ago in one of them situated a hundred light-years away from us, only now would that messenger, swifter than any messenger we know, have brought the news of it to us. to put the matter in figures, we are sure that no star can lie nearer to us than twenty-five billions of miles. a billion is a million millions, and is represented by a figure with twelve noughts behind it, so-- , , , , ; and twenty-five such billions is the least distance within which any star can lie. how much farther away stars may be we know not, but it is something to have found out even that. on the same scale as that we took in our first example, we might express it thus: if the earth were a greengage plum at a distance of about three hundred of your steps from the sun, and neptune were, on the same scale, about three miles away, the nearest fixed star could not be nearer than the distance measured round the whole earth at the equator! all this must provoke the question, how can anyone find out these things? well, for a long time the problem of the distances of the stars was thought to be too difficult for anyone to attempt to solve it, but at last an ingenious method was devised, a method which shows once more the triumph of man's mind over difficulties. in practice this method is extremely difficult to carry out, for it is complicated by so many other things which must be made allowance for; but in theory, roughly explained, it is not too hard for anyone to grasp. the way of it is this: if you hold up your finger so as to cover exactly some object a few feet distant from you, and shut first one eye and then the other, you will find that the finger has apparently shifted very considerably against the background. the finger has not really moved, but as seen from one eye or the other, it is thrown on a different part of the background, and so appears to jump; then if you draw two imaginary lines, one from each eye to the finger, and another between the two eyes, you will have made a triangle. now, all of you who have done a little euclid know that if you can ascertain the length of one side of a triangle, and the angles at each end of it, you can form the rest of the triangle; that is to say, you can tell the length of the other two sides. in this instance the base line, as it is called--that is to say the line lying between the two eyes--can easily be measured, and the angles at each end can be found by an instrument called a sextant, so that by simple calculation anyone could find out what distance the finger was from the eye. now, some ingenious man decided to apply this method to the stars. he knew that it is only objects quite near to us that will appear to shift with so small a base line as that between the eyes, and that the further away anything is the longer must the base line be before it makes any difference. but this clever man thought that if he could only get a base line long enough he could easily compute the distance of the stars from the amount that they appeared to shift against their background. he knew that the longest base line he could get on earth would be about eight thousand miles, as that is the diameter of the earth from one side to the other; so he carefully observed a star from one end of this immense base line and then from the other, quite confident that this plan would answer. but what happened? after careful observations he discovered that no star moved at all with this base line, and that it must be ever so much longer in order to make any impression. then indeed the case seemed hopeless, for here we are tied to the earth and we cannot get away into space. but the astronomer was nothing daunted. he knew that in its journey round the sun the earth travels in an orbit which measures about one hundred and eighty-five millions of miles across, so he resolved to take observations of the stars when the earth was at one side of this great circle, and again, six months later, when she had travelled to the other side. then indeed he would have a magnificent base line, one of one hundred and eighty-five millions of miles in length. what was the result? even with this mighty line the stars are found to be so distant that many do not move at all, not even when measured with the finest instruments, and others move, it may be, the breadth of a hair at a distance of several feet! but even this delicate measure, a hair's-breadth, tells its own tale; it lays down a limit of twenty-five billion miles within which no star can lie! this system which i have explained to you is called finding the star's parallax, and perhaps it is easier to understand when we put it the other way round and say that the hair's-breadth is what the whole orbit of the earth would appear to have shrunk to if it were seen from the distance of these stars! many, many stars have now been examined, and of them all our nearest neighbour seems to be a bright star seen in the southern hemisphere. it is in the constellation or star group called centaurus, and is the brightest star in it. in order to designate the stars when it is necessary to refer to them, astronomers have invented a system. to only the very brightest are proper names attached; others are noted according to the degree of their brightness, and called after the letters of the greek alphabet: alpha, beta, gamma, delta, etc. our own word 'alphabet' comes, you know, from the first two letters of this greek series. as this particular star is the brightest in the constellation centaurus, it is called alpha centauri; and if ever you travel into the southern hemisphere and see it, you may greet it as our nearest neighbour in the starry universe, so far as we know at present. chapter xi the constellations from the very earliest times men have watched the stars, felt their mysterious influence, tried to discover what they were, and noted their rising and setting. they classified them into groups, called constellations, and gave such groups the names of figures and animals, according to the positions of the stars composing them. some of these imaginary figures seem to us so wildly ridiculous that we cannot conceive how anyone could have gone so far out of their way as to invent them. but they have been long sanctioned by custom, so now, though we find it difficult to recognize in scattered groups of stars any likeness to a fish or a ram or a bear; we still call the constellations by their old names for convenience in referring to them. supposing the axis of the earth were quite upright, straight up and down in regard to the plane at which the earth goes round the sun, then we should always see the same set of stars from the northern and the same set of stars from the southern hemispheres all the year round. but as the axis is tilted slightly, we can, during our nights in the winter in the northern hemisphere, see more of the sky to the south than we can in the summer; and in the southern hemisphere just the reverse is the case, far more stars to the north can be seen in the winter than in the summer. but always, whether it is winter or summer, there is one fixed point in each hemisphere round which all the other stars seem to swing, and this is the point immediately over the north or the south poles. there is, luckily, a bright star almost at the point at which the north pole would seem to strike the sky were it infinitely lengthened. this is not one of the brightest stars in the sky, but quite bright enough to serve the purpose, and if we stand with our faces towards it, we can be sure we are looking due north. how can we discover this star for ourselves in the sky? go out on any starlight night when the sky is clear, and see if you can find a very conspicuous set of seven stars called the great bear. i shall not describe the great bear, because every child ought to know it already, and if they don't, they can ask the first grown-up person they meet, and they will certainly be told. (see map.) [illustration: constellations near the pole star.] having found the great bear, you have only to draw an imaginary line between the two last stars forming the square on the side away from the tail, and carry it on about three times as far as the distance between those two stars, and you will come straight to the pole star. the two stars in the great bear which help one to find it are called the pointers, because they point to it. the great bear is one of the constellations known from the oldest times; it is also sometimes called charles's wain, the dipper, or the plough. it is always easily seen in england, and seems to swing round the pole star as if held by an invisible rope tied to the pointers. besides the great bear there is, not far from it, the little bear, which is really very like it, only smaller and harder to find. the pole star is the last star in its tail; from it two small stars lead away parallel to the great bear, and they bring the eye to a small pair which form one side of a square just like that in the great bear. but the whole of the little bear is turned the opposite way from the great bear, and the tail points in the opposite direction. and when you come to think of it, it is very ridiculous to have called these groups bears at all, or to talk about tails, for bears have no tails! so it would have been better to have called them foxes or dogs, or almost any other animal rather than bears. now, if you look at the sky on the opposite side of the pole star from the great bear, you will see a clearly marked capital w made up of five or six bright stars. this is called cassiopeia, or the lady's chair. in looking at cassiopeia you cannot help noticing that there is a zone or broad band of very many stars, some exceedingly small, which apparently runs right across the sky like a ragged hoop, and cassiopeia seems to be set in or on it. this band is called the milky way, and crosses not only our northern sky, but the southern sky too, thus making a broad girdle round the whole universe. it is very wonderful, and no one has yet been able to explain it. the belt is not uniform and even, but it is here and there broken up into streamers and chips, having the same appearance as a piece of ribbon which has been snipped about by scissors in pure mischief; or it may be compared to a great river broken up into many channels by rocks and obstacles in its course. the milky way is mainly made up of thousands and thousands of small stars, and many more are revealed by the telescope; but, as we see in cassiopeia, there are large bright stars in it too, though, of course, these may be infinitely nearer to us, and may only appear to us to be in the milky way because they are between us and it. now, besides the few constellations that i have mentioned, there are numbers of others, some of which are difficult to discover, as they contain no bright stars. but there are certain constellations which every one should know, because in them may be found some of the brightest stars, those of the first magnitude. magnitude means size, and it is really absurd for us to say a star is of the first magnitude simply because it appears to us to be large, for, as i have explained already, a small star comparatively near to us might appear larger than a greater one further away. but the word 'magnitude' was used when men really thought stars were large or small according to their appearance, and so it is used to this day. they called the biggest and brightest first magnitude stars. of these there are not many, only some twenty, in all the sky. the next brightest--about the brightness of the pole star and the stars in the great bear--are of the second magnitude, and so on, each magnitude containing stars less and less bright. when we come to stars of the sixth magnitude we have reached the limit of our sight, for seventh magnitude stars can only be seen with a telescope. now that we understand what is meant by the magnitude, we can go back to the constellations and try to find some more. if you draw an imaginary line across the two stars forming the backbone of the bear, starting from the end nearest the tail, and continue it onward for a good distance, you will come to a very bright star called capella, which you will know, because near it are three little ones in a triangle. now, capella means a goat, so the small ones are called the kids. in winter capella gets high up into the sky, and then there is to be seen below her a little cluster called the pleiades. there is nothing else like this in the whole sky. it is formed of six stars, as it appears to persons of ordinary sight, and these stars are of the sixth magnitude, the lowest that can be seen by the naked eye. but though small, they are set so close together, and appear so brilliant, twinkling like diamonds, that they are one of the most noticeable objects in the heavens. a legend tells that there were once seven stars in the pleiades clearly visible, and that one has now disappeared. this is sometimes spoken of as 'the lost pleiad,' but there does not seem to be any foundation for the story. in old days people attached particular holiness or luck to the number seven, and possibly, when they found that there were only six stars in this wonderful group, they invented the story about the seventh. as the pleiades rise, a beautiful reddish star of the first magnitude rises beneath them. it is called aldebaran, and it, as well as the pleiades, forms a part of the constellation of taurus the bull. in england we can see in winter below aldebaran the whole of the constellation of orion, one of the finest of all the constellations, both for the number of the bright stars it contains and for the extent of the sky it covers. four bright stars at wide distances enclose an irregular four-sided space in which are set three others close together and slanting downwards. below these, again, are another three which seem to fall from them, but are not so bright. the figure of orion as drawn in the old representations of the constellations is a very magnificent one. the three bright stars form his belt, and the three smaller ones the hilt of his sword hanging from it. [illustration: orion and his neighbours.] if you draw an imaginary line through the stars forming the belt and prolong it downwards slantingly, you will see, in the very height of winter, the brightest star in all the sky, either in the northern or southern hemisphere. this is sirius, who stands in a class quite by himself, for he is many times brighter than any other first magnitude star. he never rises very high above the horizon here, but on crisp, frosty nights may be seen gleaming like a big diamond between the leafless twigs and boughs of the rime-encrusted trees. sirius is the dog star, and it is perhaps fortunate that, as he is placed, he can be seen sometimes in the southern and sometimes in the northern skies, so that many more people have a chance of looking at his wonderful brilliancy, than if he had been placed near the pole star. in speaking of the supreme brightness of sirius among the stars, we must remember that venus and jupiter, which outrival him, are not stars, but planets, and that they are much nearer to us. sirius is so distant that the measures for parallax make hardly any impression on him, but, by repeated experiments, it has now been proved that light takes more than eight years to travel from him to us. so that, if you are eight years old, you are looking at sirius as he was when you were a baby! not far from the pleiades, to the left as you face them, are to be found two bright stars nearly the same size; these are the heavenly twins, or gemini. returning now to the great bear, we find, if we draw a line through the middle and last stars of his tail, and carry it on for a little distance, we come fairly near to a cluster of stars in the form of a horseshoe; there is only one fairly bright one in it, and some of the others are quite small, but yet the horseshoe is distinct and very beautiful to look at. this is the northern crown. the very bright star not far from it is another first-class star called arcturus. to the left of the northern crown lies hercules, which is only mentioned because near it is the point to which the sun with all his system appears at present to be speeding. for other fascinating constellations, such as leo or the lion, andromeda and perseus, and the three bright stars by which we recognize aquila the eagle, you must wait awhile, unless you can get some one to point them out. those which you have noted already are enough to lead you on to search for more. perhaps some of you who live in towns and can see only a little strip of sky from the nursery or schoolroom windows have already found this chapter dull, and if so you may skip the rest of it and go on to the next. for the others, however, there is one more thing to know before leaving the subject, and that is the names of the string of constellations forming what is called the zodiac. you may have heard the rhyme: 'the ram, the bull, the heavenly twins, and next the crab, the lion shines, the virgin and the scales; the scorpion, archer, and he-goat, the man that holds the watering-pot, the fish with glittering tails.' this puts in a form easy to remember the signs of the constellations which lie in the zodiac, an imaginary belt across the whole heavens. it is very difficult to explain the zodiac, but i must try. imagine for a moment the earth moving round its orbit with the sun in the middle. now, as the earth moves the sun will be seen continually against a different background--that is to say, he will appear to us to move not only across our sky in a day by reason of our rotation, but also along the sky, changing his position among the stars by reason of our revolution. you will say at once that we cannot see the stars when the sun is there, and no more we can. but the stars are there all the same, and every month the sun seems to have moved on into a new constellation, according to astronomers' reckoning. if you count up the names of the constellations in the rhyme, you will find that there are just twelve, one for each month, and at the end of the year the sun has come round to the first one again. the first one is aries the ram, and the sun is seen projected or thrown against that part of the sky where aries is, in april, when we begin spring; this is the first month to astronomers, and not january, as you might suppose. perhaps you will learn to recognize all the constellations in the zodiac one day; a few of them, such as the bull and the heavenly twins, you know already if you have followed this chapter. chapter xii what the stars are made of how can we possibly tell what the stars are made of? if we think of the vast oceans of space lying between them and us, and realize that we can never cross those oceans, for in them there is no air, it would seem to be a hopeless task to find out anything about the stars at all. but even though we cannot traverse space ourselves, there is a messenger that can, a messenger that needs no air to sustain him, that moves more swiftly than our feeble minds can comprehend, and this messenger brings us tidings of the stars--his name is light. light tells us many marvellous things, and not the least marvellous is the news he gives us of the workings of another force, the force of gravitation. in some ways gravitation is perhaps more wonderful than light, for though light speeds across airless space, it is stopped at once by any opaque substance--that is to say, any substance not transparent, as you know very well by your own shadows, which are caused by your bodies stopping the light of the sun. light striking on one side of the earth does not penetrate through to the other, whereas gravitation does. you remember, of course, what the force of gravitation is, for we read about that very early in this book. it is a mysterious attraction existing between all matter. every atom pulls every other atom towards itself, more or less strongly according to distance. now, solid matter itself makes no difference to the force of gravitation, which acts through it as though it were not there. the sun is pulling the earth toward itself, and it pulls the atoms on the far side of the earth just as strongly as it would if there were nothing lying between it and them. therefore, unlike light, gravitation takes no heed of obstacles in the way, but acts in spite of them. the gravitation of the earth holds you down just the same, though you are on the upper floor of a house, with many layers of wood and plaster between you and it. it cannot pull you down, for the floor holds you up, but it is gravitation that keeps your feet on the ground all the same. a clever man made up a story about some one who invented a kind of stuff which stopped the force of gravitation going through it, just as a solid body stops light; when this stuff was made, of course, it went right away off into space, carrying with it anyone who stood on it, as there was nothing to hold it to the earth! that was only a story, and it is not likely anyone could invent such stuff, but it serves to make clear the working of gravitation. these two tireless forces, light and gravitation, run throughout the whole universe, and carry messages of tremendous importance for those who have minds to grasp them. without light we could know nothing of these distant worlds, and without understanding the laws of gravity we should not be able to interpret much that light tells us. to begin with light, what can we learn from it? we turn at once to our own great light-giver, the sun, to whom we owe not only all life, but also all the colour and beauty on earth. it is well known to men of science that colour lies in the light itself, and not in any particular object. that brilliant blue cloak of yours is not blue of itself, but because of the light that falls on it. if you cannot believe this, go into a room lighted only by gas, and hey, presto! the colour is changed as if it were a conjuring trick. you cannot tell now by looking at the cloak whether it is blue or green! therefore you must admit that as the colour changes with the change of light it must be due to light, and not to any quality belonging to the material of the cloak. but, you may protest, if the colour is solely due to light, and light falls on everything alike, why are there so many colours? that is a very fair question. if the light that comes from the sun were of only one colour--say blue or red--then everything would be blue or red all the world over. some doors in houses are made with a strip of red or blue glass running down the sides. if you have one in your house like that, go and look through it, and you will see an astonishing world made up of different tones of the same colour. everything is red or blue, according to the colour of the glass, and the only difference in the appearance of objects lies in the different shades, whether things are light or dark. this is a world as it might appear if the sun's rays were only blue or only red. but the sun's light is not of one colour only, fortunately for us; it is of all the colours mixed together, which, seen in a mass, make the effect of white light. now, objects on earth are only either seen by the reflected light of the sun or by some artificial light. they have no light of their own. put them in the dark and they do not shine at all; you cannot see them. it is the sun's light striking on them that makes them visible. but all objects do not reflect the light equally, and this is because they have the power of absorbing some of the rays that strike on them and not giving them back at all, and only those rays that are given back show to the eye. a white thing gives back all the rays, and so looks white, for we have the whole of the sun's light returned to us again. but how about a blue thing? it absorbs all the rays except the blue, so that the blue rays are the only ones that come back or rebound from it again to meet our eyes, and this makes us see the object blue; and this is the case with all the other colours. a red object retains all rays except the red, which it sends back to us; a yellow object gives back only the yellow rays, and so on. what an extraordinary and mysterious fact! imagine a brilliant flower-garden in autumn. here we have tall yellow sunflowers with velvety brown centres, clustering pink and crimson hollyhocks, deep red and bright yellow peonies, slender fairy-like japanese anemones, great bunches of mauve michaelmas daisies, and countless others, and mingled with all these are many shades of green. yet it is the light of the sun alone that falling on all these varied objects, makes that glorious blaze of colour; it seems incredible. it may be difficult to believe, but it is true beyond all doubt. each delicate velvety petal has some quality in it which causes it to absorb certain of the sun's rays and send back the others, and its colour is determined by those it sends back. well then how infinitely varied must be the colours hidden in the sun's light, colours which, mixed all together, make white light! yes, this is so, for all colours that we know are to be found there. in fact, the colours that make up sunlight are the colours to be seen in the rainbow, and they run in the same order. have you ever looked carefully at a rainbow? if not, do so at the next chance. you will see it begins by being dark blue at one end, and passes through all colours until it gets to red at the other. we cannot see a rainbow every day just when we want to, but we can see miniature rainbows which contain just the same colours as the real ones in a number of things any time the sun shines. for instance, in the cut-glass edge of an inkstand or a decanter, or in one of those old-fashioned hanging pieces of cut-glass that dangle from the chandelier or candle-brackets. of course you have often seen these colours reflected on the wall, and tried to get them to shine upon your face. or you have caught sight of a brilliant patch of colour on the wall and looked around to see what caused it, finally tracing it to some thick edge of shining glass standing in the sunlight. now, the cut-glass edge shows these colours to you because it breaks up the light that falls upon it into the colours it is made of, and lets each one come out separately, so that they form a band of bright colours instead of just one ray of white light. this is perhaps a little difficult to understand, but i will try to explain. when a ray of white light falls on such a piece of glass, which is known as a prism, it goes in as white light at one side, but the three-cornered shape of the glass breaks it up into the colours it is made of, and each colour comes out separately at the other side--namely, from blue to red--like a little rainbow, and instead of one ray of white light, we have a broad band of all the colours that light is made of. who would ever have thought a pretty plaything like this could have told us what we so much wanted to know--namely, what the sun and the stars are made of? it seems too marvellous to be true, yet true it is that for ages and ages light has been carrying its silent messages to our eyes, and only recently men have learnt to interpret them. it is as if some telegraph operator had been going steadily on, click, click, click, for years and years, and no one had noticed him until someone learnt the code of dot and dash in which he worked, and then all at once what he was saying became clear. the chief instrument in translating the message that the light brings is simply a prism, a three-cornered wedge of glass, just the same as those hanging lustres belonging to the chandeliers. when a piece of glass like this is fixed in a telescope in such a way that the sun's rays fall on it, then there is thrown on to a piece of paper or any other suitable background a broad coloured band of lovely light like a little rainbow, and this is called the sun's spectrum, and the instrument by which it is seen is called a spectroscope. but this in itself could tell us little; the message it brings lies in the fact that when it has passed through the telescope, so that it is magnified, it is crossed by hundreds of minute black lines, not placed evenly at all, but scattered up and down. there may be two so close together that they look like one, and then three far apart, and then some more at different distances. when this remarkable appearance was examined carefully it was found that in sunlight the lines that appeared were always exactly the same, in the same places, and this seemed so curious that men began to seek for an explanation. someone thought of an experiment which might teach us something about the matter, and instead of letting sunlight fall on the prism, he made an artificial light by burning some stuff called sodium, and then allowed the band of coloured light to pass through the telescope; when he examined the spectrum that resulted, he found that, though numbers of lines to be found in the sun's spectrum were missing, there were a few lines here exactly matching a few of the lines in the sun's spectrum; and this could not be the result of chance only, for the lines are so mathematically exact, and are in themselves so peculiarly distributed, that it could only mean that they were due to the same cause. what could this signify, then, but that away up there in the sun, among other things, stuff called sodium, very well known to chemists on earth, is burning? after this many other substances were heated white-hot so as to give out light, in order to discover if the lines to be seen in their spectra were also to be found in the sun's spectrum. one of these was iron, and, astonishing to say, all the many little thread-like lines that appeared in its spectrum were reproduced to a hair's-breadth, among others, in the sun's spectrum. so we have found out beyond all possibility of doubt some of the materials of which the sun is made. we know that iron, sodium, hydrogen, and numerous other substances and elements, are all burning away there in a terrific furnace, to which any furnace we have on earth is but as the flicker of a match. it was not, of course, much use applying this method to the planets, for we know that the light which comes from them to us is only reflected sunlight, and this, indeed, was proved by means of the spectroscope. but the stars shine by their own light, and this opened up a wide field for inquiry. the difficulty was, of course, to get the light of one star separated from all the rest, because the light of one star is very faint and feeble to cast a spectrum at all. yet by infinite patience difficulties were overcome. one star alone was allowed to throw its light into the telescope; the light passed through a prism, and showed a faint band of many colours, with the expected little black lines cutting across it more or less thickly. examinations have thus been made of hundreds of stars. in the course of them some substances as yet unknown to us on earth have been encountered, and in some stars one element--hydrogen--is much stronger than in others; but, on the whole, speaking broadly, it has been satisfactorily shown that the stars are made on the same principles as our own sun, so that the reasoning of astronomers which had argued them to be suns was proved. [illustration: the spectrum of the sun and sirius.] we have here in the picture the spectrum of the sun and the spectrum of arcturus. you can see that the lines which appear in the band of light belonging to sirius are also in the band of light belonging to the sun, together with many others. this means that the substances flaming out and sending us light from the far away star are also giving out light from our own sun, and that the sun and sirius both contain the same elements in their compositions. this, indeed, seems enough for the spectroscope to have accomplished; it has interpreted for us the message light brings from the stars, so that we know beyond all possibility of mistake that these glowing, twinkling points of light are brilliant suns in a state of intense heat, and that in them are burning elements with which we ourselves are quite familiar. but when the spectroscope had done that, its work was not finished, for it has not only told us what the stars are made of, but another thing which we could never have known without it--namely, if they are moving toward us or going away from us. chapter xiii restless stars you remember we have already remarked upon the difficulty of telling how far one star lies behind another, as we do not know their sizes. it is, to take another similar case, easy enough to tell if a star moves to one side or the other, but very difficult by ordinary observation to tell if it is advancing toward us or running away from us, for the only means we have of judging is if it gets larger or smaller, and at that enormous distance the fact whether it advances or recedes makes no difference in its size. now, the spectroscope has changed all this, and we can tell quite as certainly if a star is coming toward us as we can if it moves to one side. i will try to explain this. you know, perhaps, that sound is caused by vibration in the air. the noise, whatever it is, jars the air and the vibrations strike on our ears. it is rather the same thing as the result of throwing a stone into a pond: from the centre of the splash little wavelets run out in ever-widening circles; so through the air run ever-widening vibrations from every sound. the more vibrations there are in a second the shriller is the note they make. in a high note the air-vibrations follow one another fast, pouring into one's ear at a terrific speed, so that the apparatus in the ear which receives them itself vibrates fiercely and records a high note, while a lower note brings fewer and slower vibrations in a second, and the ear is not so much disturbed. have you ever noticed that if a railway engine is sweeping-toward you and screaming all the time, its note seems to get shriller and shriller? that is because the engine, in advancing, sends the vibrations out nearer to you, so more of them come in a second, and thus they are crowded up closer together, and are higher and higher. now, light is also caused by waves, but they are not the same as sound waves. light travels without air, whereas sound we know cannot travel without air, and is ever so much slower, and altogether a grosser, clumsier thing than light. but yet the waves or rays which make light correspond in some ways to the vibrations of sound. what corresponds to the treble on the piano is the blue end of the spectrum in light, and the bass is the red end. now, when we are looking at the spectrum of any body which is advancing swiftly toward us, something of the same effect is observed as in the case of the shrieking engine. take any star and imagine that that star is hastening toward us at a pace of three hundred miles a second, which is not at all an unusual rate for a star; then, if we examine the band of light, the spectrum, of such a star, we shall observe an extraordinary fact--all these little lines we have spoken of are shoved up toward the treble or blue end of the spectrum. they still remain just the same distances from each other, and are in twos and threes or single, so that the whole set of lines is unaltered as a set, but everyone of them is shifted a tiny fraction up toward the blue end of the spectrum, just a little displaced. now if, instead of advancing toward us, this same star had been rushing away from us at a similar pace, all these lines would have been moved a tiny bit toward the red or bass end of the spectrum. this is known to be certainly true, so that by means of the spectroscope we can tell that some of these great sun-stars are advancing toward us and some receding from us, according to whether the multitudes of little lines in the spectrum are shifted slightly to the blue or the red end. you remember that it has been surmised that the pace the sun moves with his system is about twelve miles a second. this seems fast enough to us, who think that one mile a minute is good time for an express train, but it is slow compared with the pace of many of the stars. as i have said, some are travelling at a rate of between two hundred and three hundred miles a second; and it is due to the spectroscope that we know not only whether a star is advancing toward us or receding from us, but also whether the pace is great or not; it even tells us what the pace is, up to about half a mile a second, which is very marvellous. it is a curious fact that many of the small stars show greater movement than the large ones, which mayor may not mean that they are nearer to us. it may be taken as established that there is no such thing as absolute rest in the universe: everything, stars and nebulæ alike, are moving somewhere; in an infinite variety of directions, with an infinite variety of speed they hasten this way and that. it would be impossible for any to remain still, for even supposing it had been so 'in the beginning,' the vast forces at work in the universe would not let it remain so. out of space would come the persistent call of gravitation: atoms would cry silently to atoms. there could be no perfect equality of pull on all sides; from one side or another the pull would be the stronger. slowly the inert mass would obey and begin falling toward it; it might be an inch at a time, but with rapid increase, until at last it also was hastening some whither in this universe which appears to us to be infinite. it must be remembered that these stars, even when moving at an enormous pace, do not change their places in the sky when regarded by ordinary observers. it would take thousands of years for any of the constellations to appear at all different from what they are now, even though the stars that compose them are moving in different directions with a great velocity, for a space of many millions of miles, at the distance of most of the stars, would be but as the breadth of a fine hair as seen by us on earth. so thousands of years ago men looked up at the great bear, and saw it apparently the same as we see it now; yet for all that length of time the stars composing it have been rushing in this direction and that at an enormous speed, but do not appear to us on the earth to alter their positions in regard to each other. i know of nothing that gives one a more overwhelming sense of the mightiness of the universe and the smallness of ourselves than this fact. from age to age men look on changeless heavens, yet this apparently stable universe is fuller of flux and reflux than is the restless ocean itself, and the very wavelets on the sea are not more numerous nor more restless than the stars that bestrew the sky. chapter xiv the colours of the stars has it ever occurred to you that the stars are not all of the same colour? it is true that, just glancing at them casually, you might say they are all white; but if you examine them more carefully you cannot help seeing that some shine with a steely blue light, while others are reddish or yellowish. these colours are not easy to distinguish with the naked eye, and might not attract any attention at all unless they were pointed out; yet when attention is drawn to the fact, it is impossible to deny the redness of some, such as aldebaran. but though we may admit this, we might add that the colours are so very faint and inconspicuous, that they might be, after all, only the result of imagination. to prove that the colours are constant and real we must use a telescope, and then we need have no further doubt of their reality, for instead of disappearing, the colours of some stars stand out quite vividly beyond the possibility of mistake. red stars are a bright red, and they are the most easily seen of all, though the other colours, blue and yellow and green, are seen very decidedly by some people. the red stars have been described by various observers as resembling 'a drop of blood on a black field,' 'most magnificent copper-red,' 'most intense blood-red,' and 'glowing like a live coal out of the darkness of space.' some people see them as a shining red, like that of a glowing cloud at sunset. therefore there can be no doubt that the colours are genuine enough, and are telling us some message. this message we are able to read, for we have begun to understand the language the stars speak to us by their light since the invention of the spectroscope. the spectroscope tells us that these colours indicate different stages in the development of the stars, or differences of constitution--that is to say, in the elements of which they are made. our own sun is a yellow star, and other yellow stars are akin to him; while red and blue and green stars contain different elements, or elements in different proportions. stars do not always remain the same colours for an indefinite time; one star may change slowly from yellow to white, and another from red to yellow; and there are instances of notable changes, such as that of the brilliant white sirius, who was stated in old times by many different observers to be a red star. all this makes us think, and year by year thought leads us on to knowledge, and knowledge about these distant suns increases. but though we know a good deal now, there are still many questions we should like to ask which we cannot expect to have answered for a long time yet, if ever. the star colours have some meanings which we cannot even guess; we can only notice the facts regarding them. for instance, blue stars are never known to be solitary--they always have a companion, but why this should be so passes our comprehension. what is it in the constitution of a blue star which holds or attracts another? whatever it may be, it is established by repeated instances that blue stars do not stand alone. in the constellation of cygnus there are two stars, a blue and a yellow one, which are near enough to each other to be seen in the same telescope at the same time, and yet in reality are separated by an almost incredible number of billions of miles. but as we know that a blue star is never seen alone, and that it has often as its companion a yellowish or reddish star, it is probable that these two, situated at an enormous distance from one another, are yet in some mysterious way dependent on each other, and are not merely seen together because they happen to fall in the same field of view. many double stars show most beautifully contrasted colours: among them are pairs of yellow and rose-red, golden and azure, orange and purple, orange and lilac, copper-colour and blue, apple-green and cherry-red, and so on. in the southern hemisphere there is a cluster containing so many stars of brilliant colours that sir john herschel named it 'the jewelled cluster.' i expect most of you have seen an advertisement of pear's soap, in which you are asked to stare at some red letters, and then look away to some white surface, such as a ceiling, when you will see the same letters in green. this is because green is the complementary or contrasting colour to red, and the same thing is the case with blue and yellow. when any one colour of either of these pairs is seen, it tends to make the other appear by reaction, and if the eye gazed hard at blue instead of red, it would next see yellow, and not green. now, many people to whom this curious fact is known argue that perhaps the colours of the double stars are not real, but the effect of contrast only; for instance, they say a red star near a companion white one would tend to make the companion appear green, and so, of course, it would. but this does not account for the star colours, which are really inherent in the stars themselves, as may be proved by cutting off the light of one star, and looking only at the other, when its colour still appears unchanged. another argument equally strong against the contrast theory is that the colours of stars in pairs are by no means always those which would appear if the effect was only due to complementary colours. it is not always blue and yellow or red and green pairs that we see, though these are frequent, but many others of various kinds, such as copper and blue, and ruddy and blue. we have therefore come to the conclusion that there are in this astonishing universe numbers of gloriously coloured suns, some of which apparently lie close together. what follows? why, we want to know, of course, if these stars are really pairs connected with each other, or if they only appear so by being in the same line of sight, though one is infinitely more distant than the other. and that question also has been answered. there are now known thousands of cases in which stars, hitherto regarded as single, have been separated into two, or even more, by the use of a telescope. of these thousands, some hundreds have been carefully investigated, and the result is that, though there are undoubtedly some in which the connexion is merely accidental, yet in by far the greater number of cases the two stars thus seen together have really some connexion which binds them to one another; they are dependent on one another. this has been made known to us by the working of the wonderful law of gravitation, which is obeyed throughout the whole universe. we know that by the operation of this law two mighty suns will be drawn toward each other with a certain pull, just as surely as we know that a stone let loose from the hand will fall upon the earth; so by noting the effect of two mighty suns upon each other many facts about them may be found out. by the most minute and careful measurements, by the use of the spectroscope, and by every resource known to science, astronomers have, indeed, actually found out with a near approach to exactness how far some of these great suns lie from each other, and how large they are in comparison with one another. the very first double star ever discovered was one which you have already seen, the middle one in the tail of the great bear. if you look at it you will be delighted to find that you can see a wee star close to it, and you will think you are looking at an example of a double star with your very own eyes; but you will be wrong, for that wee star is separated by untold distances from the large one to which it seems so near. in fact, any stars which can be seen to be separate by the naked eye must lie immeasurably far apart, however tiny seems the space between them. such stars may possibly have some connexion with each other, but, at any rate in this case, such a connexion has not been proved. no, the larger star itself is made up of two others, which can only be seen apart in a telescope. since this discovery double stars have been plentifully found in every part of the sky. the average space between such double stars as seen from our earth is--what do you think? it is the width of a single hair held up thirty-six feet from our eyes! this could not, of course, be seen without the use of a telescope or opera-glasses. it serves to give some impression of star distances when we think that the millions and millions of miles lying between those stars have shrunk to that hair's-breadth seen from our point of view. twin stars circle together round a common centre of gravity, and are bound by the laws of gravitation just as the planets are. our sun is a solitary star, with no companion, and therefore such a state of things seems to us to be incredible. fancy two gigantic suns, one topaz-yellow and the other azure-blue, circling around in endless movement! where in such a system would there be room for the planets? how could planets exist under the pull of two suns in opposite directions? still more wonders are unfolded as the inquiry proceeds. certain irregularities in the motions of some of these twin systems led astronomers to infer that they were acted upon by another body, though this other body was not discernible. in fact, though they could not see it, they knew it must be there, just as adams and leverrier knew of the existence of neptune, before ever they had seen him, by the irregularities in the movements of uranus. as the results showed, it was there, and was comparable in size to the twin suns it influenced, and yet they could not see it. so they concluded this third body must be dark, not light-giving like its companions. we are thus led to the strange conclusion that some of these systems are very complicated, and are formed not only of shining suns, but of huge dark bodies which cannot be called suns. what are they, then? can they be immense planets? is it possible that life may there exist? no fairy tale could stir the imagination so powerfully as the thought of such systems including a planetary body as large or larger than its sun or suns. if indeed life exists there, what a varied scene must be presented day by day! at one time both suns mingling their flashing rays may be together in the sky; at another time only one appears, a yellow or blue sun, as the case may be. the surface of such planets must undergo weird transformations, the foliage showing one day green, the next yellow, and the next blue; shadows of azure and orange will alternate! but fascinating as such thoughts are, we can get no further along that path. to turn from fancy to facts, we find that telescope and spectroscope have supplied us with quite enough matter for wonder without calling upon imagination. we have discovered that many of the stars which seem to shine with a pure single light are double, and many more consist not only of two stars, but of several, some of which may be dark bodies. the pole star was long known to be double, and is now discovered to have a third member in its system. these multiple systems vary from one another in almost every case. some are made up of a mighty star and a comparatively small one; others are composed of stars equal in light-giving power--twin suns. some progress swiftly round their orbits, some go slowly; indeed, so slowly that during the century they have been under observation only the very faintest sign of movement has been detected; and in other systems, which we are bound to suppose double, the stars are so slow in their movements that no progress seems to have been made at all. the star we know as the nearest to us in the heavens, alpha centauri, is composed of two very bright partners, which take about eighty-seven years to traverse their orbit. they sometimes come as near to each other as saturn is to the sun. in the case of sirius astronomers found out that he had a companion by reason of his irregularities of movement before they discovered that companion, which is apparently a very small star, only to be discerned with good telescopes. but here, again, it would be unwise to judge only by what we see. though the star appears small, we know by the influence it exercises on sirius that it is very nearly the same size as he is. thus we judge that it is poor in light-giving property; in fact, its shining power is much less than that of its companion, though its size is so nearly equal. this is not wonderful, for sirius's marvellous light-giving power is one of the wonders of the universe; he shines as brilliantly as twenty-nine or thirty of our suns! in some cases the dark body which we cannot see may even be larger than the shining one, through which alone we can know anything of it. here we have a new idea, a hint that in some of these systems there may be a mighty earth with a smaller sun going round it, as men imagined our sun went around the earth before the real truth was found out. so we see that, when we speak of the stars as suns comparable with our sun, we cannot think of them all as being exactly on the same model. there are endless varieties in the systems; there are solitary suns like ours which may have a number of small planets going round them, as in the solar system; but there are also double suns going round each other, suns with mighty dark bodies revolving round them which may be planets, and huge dark bodies with small suns too. every increase of knowledge opens up new wonders, and the world in which we live is but one kind of world amid an infinite number. in this chapter we have learnt an altogether new fact--the fact that the hosts of heaven comprise not only those shining stars we are accustomed to see, but also dark bodies equally massive, and probably equally numerous, which we cannot see. in fact, the regions of space may be strewn with such dark bodies, and we could have no possible means of discovering them unless they were near enough to some shining body to exert an influence upon it. it is not with his eyes alone, or with his senses, man knows of the existence of these great worlds, but often solely by the use of the powers of his mind. chapter xv temporary and variable stars it is a clear night, nearly all the world is asleep, when an astronomer crosses his lawn on his way to his observatory to spend the dark hours in making investigations into profound space. his brilliant mind, following the rays of light which shoot from the furthest star, will traverse immeasurable distances, while the body is forgotten. just before entering the observatory he pauses and looks up; his eye catches sight of something that arrests him, and he stops involuntarily. yet any stranger standing beside him, and gazing where he gazes, would see nothing unusual. there is no fiery comet with its tail stretching across from zenith to horizon, no flaming meteor dashing across the darkened sky. but that there is something unusual to be seen is evident, for the astronomer breathes quickly, and after another earnest scrutiny of the object which has attracted him, he rushes into the observatory, searches for a star-chart, and examines attentively that part of the sky at which he has been gazing. he runs his finger over the chart: here and there are the well-known stars that mark that constellation, but here? in that part there is no star marked, yet he knows, for his own eyes have told him but a few moments ago, that here there is actually blazing a star, not large, perhaps, but clear enough to be seen without a telescope--a star, maybe, which no eye but his has yet observed! he hurries to his telescope, and adjusts it so as to bring the stranger into the field of view. a new star! whence has it come? what does it mean? by the next day at the latest the news has flown over the wires, and all the scientific world is aware that a new star has been detected where no star ever was seen before. hundreds of telescopes are turned on to it; its spectrum is noted, and it stands revealed as being in a state of conflagration, having blazed up from obscurity to conspicuousness. night after night its brilliance grows, until it ranks with the brightest stars in heaven, and then it dies down and grows dim, gradually sinking--sinking into the obscurity from whence it emerged so briefly, and its place in the sky knows it no more. it may be there still, but so infinitely faint and far away that no power at our command can reveal it to us. and the amazing part of it is that this huge disaster, this mighty conflagration, is not actually happening as it is seen, but has happened many hundreds of years ago, though the message brought by the light carrier has but reached us now. there have not been a great many such outbursts recorded, though many may have taken place unrecorded, for even in these days, when trained observers are ceaselessly watching the sky, 'new' stars are not always noticed at once. in a new star appeared, and shone for two months before anyone noticed it. this particular one never rose to any very brilliant size. i twas situated in the constellation of auriga, and was noticed on february . it remained fairly bright until march , when it began to die down; but it has now sunk so low that it can only be seen in the very largest telescopes. photography has been most useful in recording these stars, for when one is noticed it has sometimes been found that it has been recorded on a photographic plate taken some time previously, and this shows us how long it has been visible. more and more photography becomes the useful handmaid of astronomers, for the photographic prepared plate is more sensitive to rays of light than the human eye, and, what is more useful still, such plates retain the rays that fall upon them, and fix the impression. also on a plate these rays are cumulative--that is to say, if a very faint star shines continuously on a plate, the longer the plate is exposed, within certain limits, the clearer will the image of that star become, for the light rays fall one on the top of the other, and tend to enforce each other, and so emphasize the impression, whereas with our eyes it is not the same thing at all, for if we do not see an object clearly because it is too faint, we do not see it any better, however much we may stare at the place where it ought to be. this is because each light ray that reaches our eye makes its own impression, and passes on; they do not become heaped on each other, as they do on a photographic plate. one variable star in perseus, discovered in , rose to such brilliancy that for one night it was queen of the northern hemisphere, outshining all the other first-class stars. it rose into prominence with wonderful quickness, and sank equally fast. at its height it outshone our sun eight thousand times! this star was so far from us that it was reckoned its light must take about three hundred years to reach us, consequently the great conflagration, or whatever caused the outburst, must have taken place in the reign of james i., though, as it was only seen here in , it was called the new star of the new century. when these new stars die down they sometimes continue to shine faintly for a long time, so that they are visible with a telescope, but in other cases they may die out altogether. we know very little about them, and have but small opportunity for observing them, and so it is not safe to hazard any theories to account for their peculiarities. at first men supposed that the great flame was made by a violent collision between two bodies coming together with great velocity so that both flared up, but this speculation has been shown by the spectroscope to be improbable, and now it is supposed by some people that two stars journeying through space may pass through a nebulous region, and thus may flare up, and such a theory is backed up by the fact that a very great number of such stars do seem to be mixed up in some strange way with a nebulous haze. all these new stars that we have been discussing so far have only blazed up once and then died down, but there is another class of stars quite as peculiar, and even more difficult to explain, and these are called variable stars. they get brighter and brighter up to a certain point, and then die down, only to become bright once more, and these changes occur with the utmost regularity, so that they are known and can be predicted beforehand. this is even more unaccountable than a sudden and unrepeated outburst, for one can understand a great flare-up, but that a star should flare and die down with regularity is almost beyond comprehension. clearly we must look further than before for an explanation. let us first examine the facts we know. variable stars differ greatly from each other. some are generally of a low magnitude, and only become bright for a short time, while others are bright most of the time and die down only for a short time. others become very bright, then sink a little bit, but not so low as at first; then they become bright again, and, lastly, go right down to the lowest point, and they keep on always through this regular cycle of changes. some go through the whole of these changes in three days, and others take much longer. the periods, as the intervals between the complete round of changes are called, vary, in fact, between three days and six hundred! it may seem impossible that changes covering so long as six hundred days could be known and followed, but there is nothing that the patience of astronomers will not compass. one very well-known variable star you can see for yourselves, and as an ounce of observation is worth a pound of hearsay, you might take a little trouble to find it. go out on any clear starlight night and look. not very far from cassiopeia (w.), to the left as you face it, are three bright stars running down in a great curve. these are in the constellation called perseus, and a little to the right of the middle and lowest one is the only variable star we can see in the sky without a telescope. this is algol. for the greater part of three days he is a bright star of about the second magnitude, then he begins to fade, and for four and a half hours grows steadily dimmer. at the dimmest he remains for about twenty minutes, and then rises again to his ordinary brightness in three and a half hours. how can we explain this? you may possibly be able to suggest a reason. what do you say to a dark body revolving round algol, or, rather, revolving with him round a common centre of gravity? if such a thing were indeed true, and if such a body happened to pass between us and algol at each revolution, the light of algol would be cut off or eclipsed in proportion to the size of such a body. if the dark body were the full size of algol and passed right between him and us, it would cut off all the light, but if it were not quite the same size, a little would still be seen. and this is really the explanation of the strange changes in the brightness of algol, for such a dark body as we are imagining does in reality exist. it is a large dark body, very nearly as large as algol himself, and if, as we may conjecture, it is a mighty planet, we have the extraordinary example of a planet and its sun being nearly the same size. we have seen that the eclipse happens every three days, and this means, of course, that the planetary body must go round its sun in that time, so as to return again to its position between us and him, but the thing is difficult to believe. why, the nearest of all our planets to the sun, the wee mercury, takes eighty-seven days to complete its orbit, and here is a mighty body hastening round its sun in three! to do this in the time the large dark planet must be very near to algol; indeed, astronomers have calculated that the surfaces of the two bodies are not more than about two million miles apart, and this is a trifle when we consider that we ourselves are more than forty-six times as far as that from the sun. at this distance algol, as observed from the planet, will fill half the sky, and the heat he gives out must be something stupendous. also the effects of gravitation must be queer indeed, acting on two such huge bodies so close together. if any beings live in such a strange world, the pull which draws them to their mighty sun must be very nearly equal to the pull which holds them to their own globe; the two together may counteract each other, but the effect must be strange! from irregularities in the movements of algol it has been judged that there may be also in the same system another dark body, but of it nothing has been definitely ascertained. but all variable stars need not necessarily be due to the light being intercepted by a dark body. there are cases where two bright stars in revolving round each other produce the same effect; for when seen side by side the two stars give twice as much light as when one is hidden behind the other, and as they are seen alternately side by side and in line, they seem to alter regularly in lustre. chapter xvi star clusters and nebulÆ could you point out any star cluster in the sky? you could if you would only think for a minute, for one has been mentioned already. this is the cluster known as the pleiades, and it is so peculiar and so different from anything else, that many people recognize the group and know where to look for it even before they know the great bear, the favourite constellation in the northern sky, itself. the pleiades is a real star cluster, and the chief stars in it are at such enormous distances from one another that they can be seen separately by the eye unaided, whereas in most clusters the stars appear to be so close together that without a telescope they make a mere blur of brightness. for a long time it was supposed that the stars composing the pleiades could not really be connected because of the great distances between them; for, as you know, even a hair's-breadth apparently between stars signifies in reality many millions of miles. light travelling from the pleiades to us, at that incomprehensible pace of which you already know, takes a hundred and ninety years to reach us! at this incredibly remote distance lies the main part of the cluster from us; but it is more marvellous still that we have every reason to believe that the outlying stars of this cluster are as far from the central ones as the nearest star we know of, alpha centauri, is from us! little wonder was it, then, that men hesitated to ascribe to the pleiades any real connection with each other, and supposed them to be merely an assemblage of stars which seemed to us to lie together. with the unaided eye we see comparatively few stars in the pleiades. six is the usual number to be counted, though people with very good sight have made out fourteen. viewed through the telescope, however, the scene changes: into this part of space stars are crowded in astonishing profusion; it is impossible to count them, and with every increase in the power of the telescope still more are revealed. well over a thousand in this small space seems no exaggerated estimate. now, it is impossible to say how many of these really belong to the group, and how many are seen there accidentally, but observations of the most prominent ones have shown that they are all moving in exactly the same direction at the same pace. it would be against probability to conceive that such a thing could be the result of mere chance, considering the infinite variety of star movements in general, and so we are bound to believe that this wonderful collection of stars is a real group, and not only an apparent one. so splendid are the great suns that illuminate this mighty system, that at least fifty or sixty of them far surpass our own sun in brilliancy. therefore when we look at that tiny sparkling group we must in imagination picture it as a vast cluster of mighty stars, all controlled and swayed by some dominant impulse, though separated by spaces enough to make the brain reel in thinking of them. if these suns possess also attendant planets, what a galaxy of worlds, what a universe within a universe is here! other star clusters there are, not so conspicuous as the pleiades, and most of these can only be seen through a telescope, so we may be thankful that we have one example so splendid within our own vision. there are some clusters so far and faintly shining that they were at first thought to be nebulæ, and not stars at all; but the telescope gradually revealed the fact that many of these are made up of stars, and so people began to think that all faint shining patches of nebulous light were really star clusters, which would be resolved into stars if only we had better telescopes. since the invention of the spectroscope, however, fresh light has been thrown on the matter, for the spectrum which is shown by some of the nebulous patches is not the same as that shown by stars, and we know that many of these strange appearances are not made up of infinitely distant stars. we are talking here quite freely about nebulæ because we have met one long ago when we discussed the gradual evolution of our own system, and we know quite well that a nebula is composed of luminous faintly-glowing gas of extreme fineness and thinness. we see in the sky at the present time what we may take to be object-lessons in our own history, for we see nebulæ of all sorts and sizes, and in some stars are mixed up, and in others stars are but dimly seen, so that it does not require a great stretch of the imagination to picture these stars as being born, emerging from the swaddling bands of filmy webs that have enwrapped them; and other nebulæ seem to be gas only, thin and glowing, with no stars at all to be found in it. we still know very little about these mysterious appearances, but the work of classifying and resolving them is going on apace. nebulæ are divided into several classes, but the easiest distinction to remember is that between white nebulæ and green nebulæ. this is not to say that we can see some coloured green, but that green appears in the spectrum of some of the nebulæ, while the spectrum of a white nebula is more like that of a star. it is fortunate for us that in the sky we can see without a telescope one instance of each of the several objects of interest that we have referred to. we have been able to see one very vivid example of a variable star; we have seen one very beautiful example of a star cluster; and it remains to look for one very good example of a white nebula. just as in finding algol you were doing a little bit of practical work, proving something of which you had read, so by seeing this nebula you will remember more about nebulæ in general than by reading many chapters on the subject. this particular nebula is in andromeda, and is not far from algol; and it is not difficult to find. it is the only one that can be well seen without a telescope, and was known to the ancients; it is believed to have been mentioned in a book of the tenth century! if you take an imaginary line down from the two left-hand stars of cassiopeia, and follow it carefully, you will come before long to a rather faint star, and close to it is the nebula. when you catch sight of it you will, perhaps, at first be disappointed, for all you will see is a soft blur of white, as if someone had laid a dab of luminous paint on the sky with a finger; but as you gaze at it night after night and realize its unchangeableness, realize also that it is a mass of glowing gas, an island in space, infinitely distant, unsupported and inexplicable, something of the wonder of it will creep over you. [illustration: _dr. max wolf._ the great nebula in andromeda.] thousands of telescopic nebulæ are now known, and have been examined, and they are of all shapes. roughly, they have been divided up into several classes--those that seem to us to be round and those that are long ovals, like this one in andromeda; but these may, of course, be only round ones seen edgewise by us; others are very irregular, and spread over an enormous part of the sky. the most remarkable of these is that in orion, and if you look very hard at the middle star in the sword-hilt of orion, you may be able to make out a faint mistiness. this, when seen through a telescope, becomes a wonderful and far-spreading nebula, with brighter and darker parts like gulfs in it, and dark channels. it has been sometimes called the fish-mouth nebula, from a fanciful idea as to its shape. indeed, so extraordinarily varied are these curious structures, that they have been compared with numbers of different objects. we have some like brushes, others resembling fans, rings, spindles, keyholes; others like animals--a fish, a crab, an owl, and so on; but these suggestions are imaginative, and have nothing to do with the real problem. in _the system of the stars_ miss clerke says: 'in regarding these singular structures we seem to see surges and spray-flakes of a nebulous ocean, bewitched into sudden immobility; or a rack of tempest-driven clouds hanging in the sky, momentarily awaiting the transforming violence of a fresh onset. sometimes continents of pale light are separated by narrow straits of comparative darkness; elsewhere obscure spaces are hemmed in by luminous inlets and channels.' one curious point about the orion nebula is that the star which seems to be in the midst of it resolves itself under the telescope into not one but six, of various sizes. nebulæ are in most cases too enormously remote from the earth for us to have any possible means of computing the distance; but we may take it that light must journey at least a thousand years to reach us from them, and in many cases much more. therefore, if at the time of the norman conquest a nebula had begun to grow dim and fade away, it would, for all intents and purposes, still be there for us, and for those that come after us for several generations, though all that existed of it in reality would be its pale image fleeting onward through space in all directions in ever-widening circles. that nebulæ do sometimes change we have evidence: there are cases in which some have grown indisputably brighter during the years they have been under observation, and some nebulæ that have been recorded by careful observers seem to have vanished. when we consider that these strange bodies fill many, many times the area of our whole solar system to the outermost bounds of neptune's orbit, it is difficult to imagine what force it is that acts on them to revive or quench their light. that that light is not the direct result of heat has long been known; it is probably some form of electric excitement causing luminosity, very much as it is caused in the comets. indeed, many people have been tempted to think of the nebulæ as the comets of the universe, and in some points there are, no doubt, strong resemblances between the two. both shine in the same way, both are so faint and thin that stars can be seen through them; but the spectroscope shows us that to carry the idea too far would be wrong, as there are many differences in constitution. we have seen that there are dark stars as well as light stars; if so, may there not be dark nebulæ as well as light ones? it may very well be so. we have seen that there are reasons for supposing our own system to have been at first a cool dark nebula rotating slowly. the heavens may be full of such bodies, but we could not discern them. their thinness would prevent their hiding any stars that happened to be behind them. no evidence of their existence could possibly be brought to us by any channel that we know. it is true that, besides the dark rifts in the bright nebulæ, which may themselves be caused by a darker and non-luminous gas, there are also strange rifts in the milky way, which at one time were conjectured to be due to a dark body intervening between us and the starry background. this idea is now quite discarded; whatever may cause them, it is not that. one of the most startling of these rifts is that called the coal-sack, in the southern hemisphere, and it occurs in a part of the sky otherwise so bright that it is the more noticeable. no possible explanation has yet been suggested to account for it. thus it may be seen that, though much has been discovered, much remains to be discovered. by the patient work of generations of astronomers we have gained a clear idea of our own position in the universe. here are we on a small globe, swinging round a far mightier and a self-luminous globe, in company with seven other planets, many of which, including ourselves, are attended by satellites or moons. between the orbits of these planets is a ring or zone of tiny bodies, also going round the sun. into this system flash every now and then strange luminous bodies--some coming but once, never to return; others returning again and again. far out in space lies this island of a system, and beyond the gulfs of space are other suns, with other systems: some may be akin to ours and some quite different. strewn about at infinite distances are star clusters, nebulæ, and other mysterious objects. the whole implies design, creation, and the working of a mighty intelligence; and yet there are small, weak creatures here on this little globe who refuse to believe in a god, or who, while acknowledging him, would believe themselves to know better than he. the end billing and sons, ltd., printers, guildford jill r. diffendal, barb grow pebareka@iexpress.net.au christine l. hall goleta, ca. usa pamela l. hall pamhall@www.edu great astronomers by sir robert s. ball d.sc. ll.d. f.r.s. lowndean professor of astronomy and geometry in the university of cambridge author of "in starry realms" "in the high heavens" etc. with numerous illustrations [plate: greenwich observatory.] preface. it has been my object in these pages to present the life of each astronomer in such detail as to enable the reader to realise in some degree the man's character and surroundings; and i have endeavoured to indicate as clearly as circumstances would permit the main features of the discoveries by which he has become known. there are many types of astronomers--from the stargazer who merely watches the heavens, to the abstract mathematician who merely works at his desk; it has, consequently, been necessary in the case of some lives to adopt a very different treatment from that which seemed suitable for others. while the work was in progress, some of the sketches appeared in "good words." the chapter on brinkley has been chiefly derived from an article on the "history of dunsink observatory," which was published on the occasion of the tercentenary celebration of the university of dublin in , and the life of sir william rowan hamilton is taken, with a few alterations and omissions, from an article contributed to the "quarterly review" on graves' life of the great mathematician. the remaining chapters now appear for the first time. for many of the facts contained in the sketch of the late professor adams, i am indebted to the obituary notice written by my friend dr. j. w. l. glaisher, for the royal astronomical society; while with regard to the late sir george airy, i have a similar acknowledgment to make to professor h. h. turner. to my friend dr. arthur a. rambaut i owe my hearty thanks for his kindness in aiding me in the revision of the work. r.s.b. the observatory, cambridge. october, contents. introduction. ptolemy. copernicus. tycho brahe. galileo. kepler. isaac newton. flamsteed. halley. bradley. william herschel. laplace. brinkley. john herschel. the earl of rosse. airy. hamilton. le verrier. adams. list of illustrations. the observatory, greenwich. ptolemy. ptolemy's planetary scheme. ptolemy's theory of the movement of mars. thorn, from an old print. copernicus. frauenburg, from an old print. explanation of planetary movements. tycho brahe. tycho's cross staff. tycho's "new star" sextant of . tycho's trigonic sextant. tycho's astronomic sextant. tycho's equatorial armillary. the great augsburg quadrant. tycho's "new scheme of the terrestrial system," . uraniborg and its grounds. ground-plan of the observatory. the observatory of uraniborg, island of hven. effigy on tycho's tomb at prague. by permission of messrs. a. & c. black. tycho's mural quadrant, uraniborg. galileo's pendulum. galileo. the villa arcetri. facsimile sketch of lunar surface by galileo. crest of galileo's family. kepler's system of regular solids. kepler. symbolical representation of the planetary system. the commemoration of the rudolphine tables. woolsthorpe manor. trinity college, cambridge. diagram of a sunbeam. isaac newton. sir isaac newton's little reflector. sir isaac newton's sun-dial. sir isaac newton's telescope. sir isaac newton's astrolabe. sir isaac newton's sun-dial in the royal society. flamsteed's house. flamsteed. halley. greenwich observatory in halley's time. , new king street, bath. from a photograph by john poole, bath. william herschel. caroline herschel. street view, herschel house, slough. from a photograph by hill & saunders, eton. garden view, herschel house, slough. from a photograph by hill & saunders, eton. observatory, herschel house, slough. from a photograph by hill & saunders, eton. the -foot telescope, herschel house, slough. from a photograph by hill & saunders, eton. laplace. the observatory, dunsink. from a photograph by w. lawrence, dublin. astronometer made by sir john herschel. sir john herschel. nebula in southern hemisphere. the cluster in the centaur. observatory at feldhausen. granite column at feldhausen. the earl of rosse. birr castle. from a photograph by w. lawrence, dublin. the mall, parsonstown. from a photograph by w. lawrence, dublin. lord rosse's telescope. from a photograph by w. lawrence, dublin. roman catholic church, parsonstown. from a photograph by w. lawrence, dublin. airy. from a photograph by e.p. adams, greenwich. hamilton. adams. the observatory, cambridge. introduction. of all the natural sciences there is not one which offers such sublime objects to the attention of the inquirer as does the science of astronomy. from the earliest ages the study of the stars has exercised the same fascination as it possesses at the present day. among the most primitive peoples, the movements of the sun, the moon, and the stars commanded attention from their supposed influence on human affairs. the practical utilities of astronomy were also obvious in primeval times. maxims of extreme antiquity show how the avocations of the husbandman are to be guided by the movements of the heavenly bodies. the positions of the stars indicated the time to plough, and the time to sow. to the mariner who was seeking a way across the trackless ocean, the heavenly bodies offered the only reliable marks by which his path could be guided. there was, accordingly, a stimulus both from intellectual curiosity and from practical necessity to follow the movements of the stars. thus began a search for the causes of the ever-varying phenomena which the heavens display. many of the earliest discoveries are indeed prehistoric. the great diurnal movement of the heavens, and the annual revolution of the sun, seem to have been known in times far more ancient than those to which any human monuments can be referred. the acuteness of the early observers enabled them to single out the more important of the wanderers which we now call planets. they saw that the star-like objects, jupiter, saturn, and mars, with the more conspicuous venus, constituted a class of bodies wholly distinct from the fixed stars among which their movements lay, and to which they bear such a superficial resemblance. but the penetration of the early astronomers went even further, for they recognized that mercury also belongs to the same group, though this particular object is seen so rarely. it would seem that eclipses and other phenomena were observed at babylon from a very remote period, while the most ancient records of celestial observations that we possess are to be found in the chinese annals. the study of astronomy, in the sense in which we understand the word, may be said to have commenced under the reign of the ptolemies at alexandria. the most famous name in the science of this period is that of hipparchus who lived and worked at rhodes about the year bc. it was his splendid investigations that first wrought the observed facts into a coherent branch of knowledge. he recognized the primary obligation which lies on the student of the heavens to compile as complete an inventory as possible of the objects which are there to be found. hipparchus accordingly commenced by undertaking, on a small scale, a task exactly similar to that on which modern astronomers, with all available appliances of meridian circles, and photographic telescopes, are constantly engaged at the present day. he compiled a catalogue of the principal fixed stars, which is of special value to astronomers, as being the earliest work of its kind which has been handed down. he also studied the movements of the sun and the moon, and framed theories to account for the incessant changes which he saw in progress. he found a much more difficult problem in his attempt to interpret satisfactorily the complicated movements of the planets. with the view of constructing a theory which should give some coherent account of the subject, he made many observations of the places of these wandering stars. how great were the advances which hipparchus accomplished may be appreciated if we reflect that, as a preliminary task to his more purely astronomical labours, he had to invent that branch of mathematical science by which alone the problems he proposed could be solved. it was for this purpose that he devised the indispensable method of calculation which we now know so well as trigonometry. without the aid rendered by this beautiful art it would have been impossible for any really important advance in astronomical calculation to have been effected. but the discovery which shows, beyond all others, that hipparchus possessed one of the master-minds of all time was the detection of that remarkable celestial movement known as the precession of the equinoxes. the inquiry which conducted to this discovery involved a most profound investigation, especially when it is remembered that in the days of hipparchus the means of observation of the heavenly bodies were only of the rudest description, and the available observations of earlier dates were extremely scanty. we can but look with astonishment on the genius of the man who, in spite of such difficulties, was able to detect such a phenomenon as the precession, and to exhibit its actual magnitude. i shall endeavour to explain the nature of this singular celestial movement, for it may be said to offer the first instance in the history of science in which we find that combination of accurate observation with skilful interpretation, of which, in the subsequent development of astronomy, we have so many splendid examples. the word equinox implies the condition that the night is equal to the day. to a resident on the equator the night is no doubt equal to the day at all times in the year, but to one who lives on any other part of the earth, in either hemisphere, the night and the day are not generally equal. there is, however, one occasion in spring, and another in autumn, on which the day and the night are each twelve hours at all places on the earth. when the night and day are equal in spring, the point which the sun occupies on the heavens is termed the vernal equinox. there is similarly another point in which the sun is situated at the time of the autumnal equinox. in any investigation of the celestial movements the positions of these two equinoxes on the heavens are of primary importance, and hipparchus, with the instinct of genius, perceived their significance, and commenced to study them. it will be understood that we can always define the position of a point on the sky with reference to the surrounding stars. no doubt we do not see the stars near the sun when the sun is shining, but they are there nevertheless. the ingenuity of hipparchus enabled him to determine the positions of each of the two equinoxes relatively to the stars which lie in its immediate vicinity. after examination of the celestial places of these points at different periods, he was led to the conclusion that each equinox was moving relatively to the stars, though that movement was so slow that twenty five thousand years would necessarily elapse before a complete circuit of the heavens was accomplished. hipparchus traced out this phenomenon, and established it on an impregnable basis, so that all astronomers have ever since recognised the precession of the equinoxes as one of the fundamental facts of astronomy. not until nearly two thousand years after hipparchus had made this splendid discovery was the explanation of its cause given by newton. from the days of hipparchus down to the present hour the science of astronomy has steadily grown. one great observer after another has appeared from time to time, to reveal some new phenomenon with regard to the celestial bodies or their movements, while from time to time one commanding intellect after another has arisen to explain the true import of the facts of observations. the history of astronomy thus becomes inseparable from the history of the great men to whose labours its development is due. in the ensuing chapters we have endeavoured to sketch the lives and the work of the great philosophers, by whose labours the science of astronomy has been created. we shall commence with ptolemy, who, after the foundations of the science had been laid by hipparchus, gave to astronomy the form in which it was taught throughout the middle ages. we shall next see the mighty revolution in our conceptions of the universe which are associated with the name of copernicus. we then pass to those periods illumined by the genius of galileo and newton, and afterwards we shall trace the careers of other more recent discoverers, by whose industry and genius the boundaries of human knowledge have been so greatly extended. our history will be brought down late enough to include some of the illustrious astronomers who laboured in the generation which has just passed away. ptolemy. [plate: ptolemy.] the career of the famous man whose name stands at the head of this chapter is one of the most remarkable in the history of human learning. there may have been other discoverers who have done more for science than ever ptolemy accomplished, but there never has been any other discoverer whose authority on the subject of the movements of the heavenly bodies has held sway over the minds of men for so long a period as the fourteen centuries during which his opinions reigned supreme. the doctrines he laid down in his famous book, "the almagest," prevailed throughout those ages. no substantial addition was made in all that time to the undoubted truths which this work contained. no important correction was made of the serious errors with which ptolemy's theories were contaminated. the authority of ptolemy as to all things in the heavens, and as to a good many things on the earth (for the same illustrious man was also a diligent geographer), was invariably final. though every child may now know more of the actual truths of the celestial motions than ever ptolemy knew, yet the fact that his work exercised such an astonishing effect on the human intellect for some sixty generations, shows that it must have been an extraordinary production. we must look into the career of this wonderful man to discover wherein lay the secret of that marvellous success which made him the unchallenged instructor of the human race for such a protracted period. unfortunately, we know very little as to the personal history of ptolemy. he was a native of egypt, and though it has been sometimes conjectured that he belonged to the royal families of the same name, yet there is nothing to support such a belief. the name, ptolemy, appears to have been a common one in egypt in those days. the time at which he lived is fixed by the fact that his first recorded observation was made in ad, and his last in ad. when we add that he seems to have lived in or near alexandria, or to use his own words, "on the parallel of alexandria," we have said everything that can be said so far as his individuality is concerned. ptolemy is, without doubt, the greatest figure in ancient astronomy. he gathered up the wisdom of the philosophers who had preceded him. he incorporated this with the results of his own observations, and illumined it with his theories. his speculations, even when they were, as we now know, quite erroneous, had such an astonishing verisimilitude to the actual facts of nature that they commanded universal assent. even in these modern days we not unfrequently find lovers of paradox who maintain that ptolemy's doctrines not only seem true, but actually are true. in the absence of any accurate knowledge of the science of mechanics, philosophers in early times were forced to fall back on certain principles of more or less validity, which they derived from their imagination as to what the natural fitness of things ought to be. there was no geometrical figure so simple and so symmetrical as a circle, and as it was apparent that the heavenly bodies pursued tracks which were not straight lines, the conclusion obviously followed that their movements ought to be circular. there was no argument in favour of this notion, other than the merely imaginary reflection that circular movement, and circular movement alone, was "perfect," whatever "perfect" may have meant. it was further believed to be impossible that the heavenly bodies could have any other movements save those which were perfect. assuming this, it followed, in ptolemy's opinion, and in that of those who came after him for fourteen centuries, that all the tracks of the heavenly bodies were in some way or other to be reduced to circles. ptolemy succeeded in devising a scheme by which the apparent changes that take place in the heavens could, so far as he knew them, be explained by certain combinations of circular movement. this seemed to reconcile so completely the scheme of things celestial with the geometrical instincts which pointed to the circle as the type of perfect movement, that we can hardly wonder ptolemy's theory met with the astonishing success that attended it. we shall, therefore, set forth with sufficient detail the various steps of this famous doctrine. ptolemy commences with laying down the undoubted truth that the shape of the earth is globular. the proofs which he gives of this fundamental fact are quite satisfactory; they are indeed the same proofs as we give today. there is, first of all, the well-known circumstance of which our books on geography remind us, that when an object is viewed at a distance across the sea, the lower part of the object appears cut off by the interposing curved mass of water. the sagacity of ptolemy enabled him to adduce another argument, which, though not quite so obvious as that just mentioned, demonstrates the curvature of the earth in a very impressive manner to anyone who will take the trouble to understand it. ptolemy mentions that travellers who went to the south reported, that, as they did so, the appearance of the heavens at night underwent a gradual change. stars that they were familiar with in the northern skies gradually sank lower in the heavens. the constellation of the great bear, which in our skies never sets during its revolution round the pole, did set and rise when a sufficient southern latitude had been attained. on the other hand, constellations new to the inhabitants of northern climes were seen to rise above the southern horizon. these circumstances would be quite incompatible with the supposition that the earth was a flat surface. had this been so, a little reflection will show that no such changes in the apparent movements of the stars would be the consequence of a voyage to the south. ptolemy set forth with much insight the significance of this reasoning, and even now, with the resources of modern discoveries to help us, we can hardly improve upon his arguments. ptolemy, like a true philosopher disclosing a new truth to the world, illustrated and enforced his subject by a variety of happy demonstrations. i must add one of them, not only on account of its striking nature, but also because it exemplifies ptolemy's acuteness. if the earth were flat, said this ingenious reasoner, sunset must necessarily take place at the same instant, no matter in what country the observer may happen to be placed. ptolemy, however, proved that the time of sunset did vary greatly as the observer's longitude was altered. to us, of course, this is quite obvious; everybody knows that the hour of sunset may have been reached in great britain while it is still noon on the western coast of america. ptolemy had, however, few of those sources of knowledge which are now accessible. how was he to show that the sun actually did set earlier at alexandria than it would in a city which lay a hundred miles to the west? there was no telegraph wire by which astronomers at the two places could communicate. there was no chronometer or watch which could be transported from place to place; there was not any other reliable contrivance for the keeping of time. ptolemy's ingenuity, however, pointed out a thoroughly satisfactory method by which the times of sunset at two places could be compared. he was acquainted with the fact, which must indeed have been known from the very earliest times, that the illumination of the moon is derived entirely from the sun. he knew that an eclipse of the moon was due to the interposition of the earth which cuts off the light of the sun. it was, therefore, plain that an eclipse of the moon must be a phenomenon which would begin at the same instant from whatever part of the earth the moon could be seen at the time. ptolemy, therefore, brought together from various quarters the local times at which different observers had recorded the beginning of a lunar eclipse. he found that the observers to the west made the time earlier and earlier the further away their stations were from alexandria. on the other hand, the eastern observers set down the hour as later than that at which the phenomenon appeared at alexandria. as these observers all recorded something which indeed appeared to them simultaneously, the only interpretation was, that the more easterly a place the later its time. suppose there were a number of observers along a parallel of latitude, and each noted the hour of sunset to be six o'clock, then, since the eastern times are earlier than western times, p.m. at one station a will correspond to p.m. at a station b sufficiently to the west. if, therefore, it is sunset to the observer at a, the hour of sunset will not yet be reached for the observer at b. this proves conclusively that the time of sunset is not the same all over the earth. we have, however, already seen that the apparent time of sunset would be the same from all stations if the earth were flat. when ptolemy, therefore, demonstrated that the time of sunset was not the same at various places, he showed conclusively that the earth was not flat. as the same arguments applied to all parts of the earth where ptolemy had either been himself, or from which he could gain the necessary information, it followed that the earth, instead of being the flat plain, girdled with an illimitable ocean, as was generally supposed, must be in reality globular. this led at once to a startling consequence. it was obvious that there could be no supports of any kind by which this globe was sustained; it therefore followed that the mighty object must be simply poised in space. this is indeed an astonishing doctrine to anyone who relies on what merely seems the evidence of the senses, without giving to that evidence its due intellectual interpretation. according to our ordinary experience, the very idea of an object poised without support in space, appears preposterous. would it not fall? we are immediately asked. yes, doubtless it could not remain poised in any way in which we try the experiment. we must, however, observe that there are no such ideas as upwards or downwards in relation to open space. to say that a body falls downwards, merely means that it tries to fall as nearly as possible towards the centre of the earth. there is no one direction along which a body will tend to move in space, in preference to any other. this may be illustrated by the fact that a stone let fall at new zealand will, in its approach towards the earth's centre, be actually moving upwards as far as any locality in our hemisphere is concerned. why, then, argued ptolemy, may not the earth remain poised in space, for as all directions are equally upward or equally downward, there seems no reason why the earth should require any support? by this reasoning he arrives at the fundamental conclusion that the earth is a globular body freely lying in space, and surrounded above, below, and on all sides by the glittering stars of heaven. the perception of this sublime truth marks a notable epoch in the history of the gradual development of the human intellect. no doubt, other philosophers, in groping after knowledge, may have set forth certain assertions that are more or less equivalent to this fundamental truth. it is to ptolemy we must give credit, however, not only for announcing this doctrine, but for demonstrating it by clear and logical argument. we cannot easily project our minds back to the conception of an intellectual state in which this truth was unfamiliar. it may, however, be well imagined that, to one who thought the earth was a flat plain of indefinite extent, it would be nothing less than an intellectual convulsion for him to be forced to believe that he stood upon a spherical earth, forming merely a particle relatively to the immense sphere of the heavens. what ptolemy saw in the movements of the stars led him to the conclusion that they were bright points attached to the inside of a tremendous globe. the movements of this globe which carried the stars were only compatible with the supposition that the earth occupied its centre. the imperceptible effect produced by a change in the locality of the observer on the apparent brightness of the stars made it plain that the dimensions of the terrestrial globe must be quite insignificant in comparison with those of the celestial sphere. the earth might, in fact, be regarded as a grain of sand while the stars lay upon a globe many yards in diameter. so tremendous was the revolution in human knowledge implied by this discovery, that we can well imagine how ptolemy, dazzled as it were by the fame which had so justly accrued to him, failed to make one further step. had he made that step, it would have emancipated the human intellect from the bondage of fourteen centuries of servitude to a wholly monstrous notion of this earth's importance in the scheme of the heavens. the obvious fact that the sun, the moon, and the stars rose day by day, moved across the sky in a glorious never-ending procession, and duly set when their appointed courses had been run, demanded some explanation. the circumstance that the fixed stars preserved their mutual distances from year to year, and from age to age, appeared to ptolemy to prove that the sphere which contained those stars, and on whose surface they were believed by him to be fixed, revolved completely around the earth once every day. he would thus account for all the phenomena of rising and setting consistently with the supposition that our globe was stationary. probably this supposition must have appeared monstrous, even to ptolemy. he knew that the earth was a gigantic object, but, large as it may have been, he knew that it was only a particle in comparison with the celestial sphere, yet he apparently believed, and certainly succeeded in persuading other men to believe, that the celestial sphere did actually perform these movements. ptolemy was an excellent geometer. he knew that the rising and the setting of the sun, the moon, and the myriad stars, could have been accounted for in a different way. if the earth turned round uniformly once a day while poised at the centre of the sphere of the heavens, all the phenomena of rising and setting could be completely explained. this is, indeed, obvious after a moment's reflection. consider yourself to be standing on the earth at the centre of the heavens. there are stars over your head, and half the contents of the heavens are visible, while the other half are below your horizon. as the earth turns round, the stars over your head will change, and unless it should happen that you have taken up your position at either of the poles, new stars will pass into your view, and others will disappear, for at no time can you have more than half of the whole sphere visible. the observer on the earth would, therefore, say that some stars were rising, and that some stars were setting. we have, therefore, two totally distinct methods, each of which would completely explain all the observed facts of the diurnal movement. one of these suppositions requires that the celestial sphere, bearing with it the stars and other celestial bodies, turns uniformly around an invisible axis, while the earth remains stationary at the centre. the other supposition would be, that it is the stupendous celestial sphere which remains stationary, while the earth at the centre rotates about the same axis as the celestial sphere did before, but in an opposite direction, and with a uniform velocity which would enable it to complete one turn in twenty-four hours. ptolemy was mathematician enough to know that either of these suppositions would suffice for the explanation of the observed facts. indeed, the phenomena of the movements of the stars, so far as he could observe them, could not be called upon to pronounce which of these views was true, and which was false. ptolemy had, therefore, to resort for guidance to indirect lines of reasoning. one of these suppositions must be true, and yet it appeared that the adoption of either was accompanied by a great difficulty. it is one of his chief merits to have demonstrated that the celestial sphere was so stupendous that the earth itself was absolutely insignificant in comparison therewith. if, then, this stupendous sphere rotated once in twenty-four hours, the speed with which the movement of some of the stars must be executed would be so portentous as to seem well-nigh impossible. it would, therefore, seem much simpler on this ground to adopt the other alternative, and to suppose the diurnal movements were due to the rotation of the earth. here ptolemy saw, or at all events fancied he saw, objections of the weightiest description. the evidence of the senses appeared directly to controvert the supposition that this earth is anything but stationary. ptolemy might, perhaps, have dismissed this objection on the ground that the testimony of the senses on such a matter should be entirely subordinated to the interpretation which our intelligence would place upon the facts to which the senses deposed. another objection, however, appeared to him to possess the gravest moment. it was argued that if the earth were rotating, there is nothing to make the air participate in this motion, mankind would therefore be swept from the earth by the furious blasts which would arise from the movement of the earth through an atmosphere at rest. even if we could imagine that the air were carried round with the earth, the same would not apply, so thought ptolemy, to any object suspended in the air. so long as a bird was perched on a tree, he might very well be carried onward by the moving earth, but the moment he took wing, the ground would slip from under him at a frightful pace, so that when he dropped down again he would find himself at a distance perhaps ten times as great as that which a carrier-pigeon or a swallow could have traversed in the same time. some vague delusion of this description seems even still to crop up occasionally. i remember hearing of a proposition for balloon travelling of a very remarkable kind. the voyager who wanted to reach any other place in the same latitude was simply to ascend in a balloon, and wait there till the rotation of the earth conveyed the locality which happened to be his destination directly beneath him, whereupon he was to let out the gas and drop down! ptolemy knew quite enough natural philosophy to be aware that such a proposal for locomotion would be an utter absurdity; he knew that there was no such relative shift between the air and the earth as this motion would imply. it appeared to him to be necessary that the air should lag behind, if the earth had been animated by a movement of rotation. in this he was, as we know, entirely wrong. there were, however, in his days no accurate notions on the subject of the laws of motion. assiduous as ptolemy may have been in the study of the heavenly bodies, it seems evident that he cannot have devoted much thought to the phenomena of motion of terrestrial objects. simple, indeed, are the experiments which might have convinced a philosopher much less acute than ptolemy, that, if the earth did revolve, the air must necessarily accompany it. if a rider galloping on horseback tosses a ball into the air, it drops again into his hand, just as it would have done had he been remaining at rest during the ball's flight; the ball in fact participates in the horizontal motion, so that though it really describes a curve as any passer-by would observe, yet it appears to the rider himself merely to move up and down in a straight line. this fact, and many others similar to it, demonstrate clearly that if the earth were endowed with a movement of rotation, the atmosphere surrounding it must participate in that movement. ptolemy did not know this, and consequently he came to the conclusion that the earth did not rotate, and that, therefore, notwithstanding the tremendous improbability of so mighty an object as the celestial sphere spinning round once in every twenty-four hours, there was no course open except to believe that this very improbable thing did really happen. thus it came to pass that ptolemy adopted as the cardinal doctrine of his system a stationary earth poised at the centre of the celestial sphere, which stretched around on all sides at a distance so vast that the diameter of the earth was an inappreciable point in comparison therewith. ptolemy having thus deliberately rejected the doctrine of the earth's rotation, had to make certain other entirely erroneous suppositions. it was easily seen that each star required exactly the same period for the performance of a complete revolution of the heavens. ptolemy knew that the stars were at enormous distances from the earth, though no doubt his notions on this point came very far short of what we know to be the reality. if the stars had been at very varied distances, then it would be so wildly improbable that they should all accomplish their revolutions in the same time, that ptolemy came to the conclusion that they must be all at the same distance, that is, that they must be all on the surface of a sphere. this view, however erroneous, was corroborated by the obvious fact that the stars in the constellations preserved their relative places unaltered for centuries. thus it was that ptolemy came to the conclusion that they were all fixed on one spherical surface, though we are not informed as to the material of this marvellous setting which sustained the stars like jewels. nor should we hastily pronounce this doctrine to be absurd. the stars do appear to lie on the surface of a sphere, of which the observer is at the centre; not only is this the aspect which the skies present to the untechnical observer, but it is the aspect in which the skies are presented to the most experienced astronomer of modern days. no doubt he knows well that the stars are at the most varied distances from him; he knows that certain stars are ten times, or a hundred times, or a thousand times, as far as other stars. nevertheless, to his eye the stars appear on the surface of the sphere, it is on that surface that his measurements of the relative places of the stars are made; indeed, it may be said that almost all the accurate observations in the observatory relate to the places of the stars, not as they really are, but as they appear to be projected on that celestial sphere whose conception we owe to the genius of ptolemy. this great philosopher shows very ingeniously that the earth must be at the centre of the sphere. he proves that, unless this were the case, each star would not appear to move with the absolute uniformity which does, as a matter of fact, characterise it. in all these reasonings we cannot but have the most profound admiration for the genius of ptolemy, even though he had made an error so enormous in the fundamental point of the stability of the earth. another error of a somewhat similar kind seemed to ptolemy to be demonstrated. he had shown that the earth was an isolated object in space, and being such was, of course, capable of movement. it could either be turned round, or it could be moved from one place to another. we know that ptolemy deliberately adopted the view that the earth did not turn round; he had then to investigate the other question, as to whether the earth was animated by any movement of translation. he came to the conclusion that to attribute any motion to the earth would be incompatible with the truths at which he had already arrived. the earth, argued ptolemy, lies at the centre of the celestial sphere. if the earth were to be endowed with movement, it would not lie always at this point, it must, therefore, shift to some other part of the sphere. the movements of the stars, however, preclude the possibility of this; and, therefore, the earth must be as devoid of any movement of translation as it is devoid of rotation. thus it was that ptolemy convinced himself that the stability of the earth, as it appeared to the ordinary senses, had a rational philosophical foundation. not unfrequently it is the lot of the philosophers to contend against the doctrines of the vulgar, but when it happens, as in the case of ptolemy's researches, that the doctrines of the vulgar are corroborated by philosophical investigation which bear the stamp of the highest authority, it is not to be wondered at that such doctrines should be deemed well-nigh impregnable. in this way we may, perhaps, account for the remarkable fact that the theories of ptolemy held unchallenged sway over the human intellect for the vast period already mentioned. up to the present we have been speaking only of those primary motions of the heavens, by which the whole sphere appeared to revolve once every twenty-four hours. we have now to discuss the remarkable theories by which ptolemy endeavoured to account for the monthly movement of the moon, for the annual movement of the sun, and for the periodic movements of the planets which had gained for them the titles of the wandering stars. possessed with the idea that these movements must be circular, or must be capable, directly or indirectly, of being explained by circular movements, it seemed obvious to ptolemy, as indeed it had done to previous astronomers, that the track of the moon through the stars was a circle of which the earth is the centre. a similar movement with a yearly period must also be attributed to the sun, for the changes in the positions of the constellations in accordance with the progress of the seasons, placed it beyond doubt that the sun made a circuit of the celestial sphere, even though the bright light of the sun prevented the stars in its vicinity, from being seen in daylight. thus the movements both of the sun and the moon, as well as the diurnal rotation of the celestial sphere, seemed to justify the notion that all celestial movements must be "perfect," that is to say, described uniformly in those circles which were the only perfect curves. the simplest observations, however, show that the movements of the planets cannot be explained in this simple fashion. here the geometrical genius of ptolemy shone forth, and he devised a scheme by which the apparent wanderings of the planets could be accounted for without the introduction of aught save "perfect" movements. to understand his reasoning, let us first set forth clearly those facts of observation which require to be explained. i shall take, in particular, two planets, venus and mars, as these illustrate, in the most striking manner, the peculiarities of the inner and the outer planets respectively. the simplest observations would show that venus did not move round the heavens in the same fashion as the sun or the moon. look at the evening star when brightest, as it appears in the west after sunset. instead of moving towards the east among the stars, like the sun or the moon, we find, week after week, that venus is drawing in towards the sun, until it is lost in the sunbeams. then the planet emerges on the other side, not to be seen as an evening star, but as a morning star. in fact, it was plain that in some ways venus accompanied the sun in its annual movement. now it is found advancing in front of the sun to a certain limited distance, and now it is lagging to an equal extent behind the sun. [fig. . ptolemy's planetary scheme.] these movements were wholly incompatible with the supposition that the journeys of venus were described by a single motion of the kind regarded as perfect. it was obvious that the movement was connected in some strange manner with the revolution of the sun, and here was the ingenious method by which ptolemy sought to render account of it. imagine a fixed arm to extend from the earth to the sun, as shown in the accompanying figure (fig. ), then this arm will move round uniformly, in consequence of the sun's movement. at a point p on this arm let a small circle be described. venus is supposed to revolve uniformly in this small circle, while the circle itself is carried round continuously by the movement of the sun. in this way it was possible to account for the chief peculiarities in the movement of venus. it will be seen that, in consequence of the revolution around p, the spectator on the earth will sometimes see venus on one side of the sun, and sometimes on the other side, so that the planet always remains in the sun's vicinity. by properly proportioning the movements, this little contrivance simulated the transitions from the morning star to the evening star. thus the changes of venus could be accounted for by a combination of the "perfect" movement of p in the circle which it described uniformly round the earth, combined with the "perfect" motion of venus in the circle which it described uniformly around the moving centre. in a precisely similar manner ptolemy rendered an explanation of the fitful apparitions of mercury. now just on one side of the sun, and now just on the other, this rarely-seen planet moved like venus on a circle whereof the centre was also carried by the line joining the sun and the earth. the circle, however, in which mercury actually revolved had to be smaller than that of venus, in order to account for the fact that mercury lies always much closer to the sun than the better-known planet. [fig. . ptolemy's theory of the movement of mars.] the explanation of the movement of an outer planet like mars could also be deduced from the joint effect of two perfect motions. the changes through which mars goes are, however, so different from the movements of venus that quite a different disposition of the circles is necessary. for consider the facts which characterise the movements of an outer planet such as mars. in the first place, mars accomplishes an entire circuit of the heaven. in this respect, no doubt, it may be said to resemble the sun or the moon. a little attention will, however, show that there are extraordinary irregularities in the movement of the planet. generally speaking, it speeds its way from west to east among the stars, but sometimes the attentive observer will note that the speed with which the planet advances is slackening, and then it will seem to become stationary. some days later the direction of the planet's movement will be reversed, and it will be found moving from the east towards the west. at first it proceeds slowly and then quickens its pace, until a certain speed is attained, which afterwards declines until a second stationary position is reached. after a due pause the original motion from west to east is resumed, and is continued until a similar cycle of changes again commences. such movements as these were obviously quite at variance with any perfect movement in a single circle round the earth. here, again, the geometrical sagacity of ptolemy provided him with the means of representing the apparent movements of mars, and, at the same time, restricting the explanation to those perfect movements which he deemed so essential. in fig. we exhibit ptolemy's theory as to the movement of mars. we have, as before, the earth at the centre, and the sun describing its circular orbit around that centre. the path of mars is to be taken as exterior to that of the sun. we are to suppose that at a point marked m there is a fictitious planet, which revolves around the earth uniformly, in a circle called the deferent. this point m, which is thus animated by a perfect movement, is the centre of a circle which is carried onwards with m, and around the circumference of which mars revolves uniformly. it is easy to show that the combined effect of these two perfect movements is to produce exactly that displacement of mars in the heavens which observation discloses. in the position represented in the figure, mars is obviously pursuing a course which will appear to the observer as a movement from west to east. when, however, the planet gets round to such a position as r, it is then moving from east to west in consequence of its revolution in the moving circle, as indicated by the arrow-head. on the other hand, the whole circle is carried forward in the opposite direction. if the latter movement be less rapid than the former, then we shall have the backward movement of mars on the heavens which it was desired to explain. by a proper adjustment of the relative lengths of these arms the movements of the planet as actually observed could be completely accounted for. the other outer planets with which ptolemy was acquainted, namely, jupiter and saturn, had movements of the same general character as those of mars. ptolemy was equally successful in explaining the movements they performed by the supposition that each planet had perfect rotation in a circle of its own, which circle itself had perfect movement around the earth in the centre. it is somewhat strange that ptolemy did not advance one step further, as by so doing he would have given great simplicity to his system. he might, for instance, have represented the movements of venus equally well by putting the centre of the moving circle at the sun itself, and correspondingly enlarging the circle in which venus revolved. he might, too, have arranged that the several circles which the outer planets traversed should also have had their centres at the sun. the planetary system would then have consisted of an earth fixed at the centre, of a sun revolving uniformly around it, and of a system of planets each describing its own circle around a moving centre placed in the sun. perhaps ptolemy had not thought of this, or perhaps he may have seen arguments against it. this important step was, however, taken by tycho. he considered that all the planets revolved around the sun in circles, and that the sun itself, bearing all these orbits, described a mighty circle around the earth. this point having been reached, only one more step would have been necessary to reach the glorious truths that revealed the structure of the solar system. that last step was taken by copernicus. copernicus [plate: thorn, from an old print.] the quaint town of thorn, on the vistula, was more than two centuries old when copernicus was born there on the th of february, . the situation of this town on the frontier between prussia and poland, with the commodious waterway offered by the river, made it a place of considerable trade. a view of the town, as it was at the time of the birth of copernicus, is here given. the walls, with their watch-towers, will be noted, and the strategic importance which the situation of thorn gave to it in the fifteenth century still belongs thereto, so much so that the german government recently constituted the town a fortress of the first class. copernicus, the astronomer, whose discoveries make him the great predecessor of kepler and newton, did not come from a noble family, as certain other early astronomers have done, for his father was a tradesman. chroniclers are, however, careful to tell us that one of his uncles was a bishop. we are not acquainted with any of those details of his childhood or youth which are often of such interest in other cases where men have risen to exalted fame. it would appear that the young nicolaus, for such was his christian name, received his education at home until such time as he was deemed sufficiently advanced to be sent to the university at cracow. the education that he there obtained must have been in those days of a very primitive description, but copernicus seems to have availed himself of it to the utmost. he devoted himself more particularly to the study of medicine, with the view of adopting its practice as the profession of his life. the tendencies of the future astronomer were, however, revealed in the fact that he worked hard at mathematics, and, like one of his illustrious successors, galileo, the practice of the art of painting had for him a very great interest, and in it he obtained some measure of success. by the time he was twenty-seven years old, it would seem that copernicus had given up the notion of becoming a medical practitioner, and had resolved to devote himself to science. he was engaged in teaching mathematics, and appears to have acquired some reputation. his growing fame attracted the notice of his uncle the bishop, at whose suggestion copernicus took holy orders, and he was presently appointed to a canonry in the cathedral of frauenburg, near the mouth of the vistula. to frauenburg, accordingly, this man of varied gifts retired. possessing somewhat of the ascetic spirit, he resolved to devote his life to work of the most serious description. he eschewed all ordinary society, restricting his intimacies to very grave and learned companions, and refusing to engage in conversation of any useless kind. it would seem as if his gifts for painting were condemned as frivolous; at all events, we do not learn that he continued to practise them. in addition to the discharge of his theological duties, his life was occupied partly in ministering medically to the wants of the poor, and partly with his researches in astronomy and mathematics. his equipment in the matter of instruments for the study of the heavens seems to have been of a very meagre description. he arranged apertures in the walls of his house at allenstein, so that he could observe in some fashion the passage of the stars across the meridian. that he possessed some talent for practical mechanics is proved by his construction of a contrivance for raising water from a stream, for the use of the inhabitants of frauenburg. relics of this machine are still to be seen. [plate: copernicus.] the intellectual slumber of the middle ages was destined to be awakened by the revolutionary doctrines of copernicus. it may be noted, as an interesting circumstance, that the time at which he discovered the scheme of the solar system has coincided with a remarkable epoch in the world's history. the great astronomer had just reached manhood at the time when columbus discovered the new world. before the publication of the researches of copernicus, the orthodox scientific creed averred that the earth was stationary, and that the apparent movements of the heavenly bodies were indeed real movements. ptolemy had laid down this doctrine , years before. in his theory this huge error was associated with so much important truth, and the whole presented such a coherent scheme for the explanation of the heavenly movements, that the ptolemaic theory was not seriously questioned until the great work of copernicus appeared. no doubt others, before copernicus, had from time to time in some vague fashion surmised, with more or less plausibility, that the sun, and not the earth, was the centre about which the system really revolved. it is, however, one thing to state a scientific fact; it is quite another thing to be in possession of the train of reasoning, founded on observation or experiment, by which that fact may be established. pythagoras, it appears, had indeed told his disciples that it was the sun, and not the earth, which was the centre of movement, but it does not seem at all certain that pythagoras had any grounds which science could recognise for the belief which is attributed to him. so far as information is available to us, it would seem that pythagoras associated his scheme of things celestial with a number of preposterous notions in natural philosophy. he may certainly have made a correct statement as to which was the most important body in the solar system, but he certainly did not provide any rational demonstration of the fact. copernicus, by a strict train of reasoning, convinced those who would listen to him that the sun was the centre of the system. it is useful for us to consider the arguments which he urged, and by which he effected that intellectual revolution which is always connected with his name. the first of the great discoveries which copernicus made relates to the rotation of the earth on its axis. that general diurnal movement, by which the stars and all other celestial bodies appear to be carried completely round the heavens once every twenty-four hours, had been accounted for by ptolemy on the supposition that the apparent movements were the real movements. as we have already seen, ptolemy himself felt the extraordinary difficulty involved in the supposition that so stupendous a fabric as the celestial sphere should spin in the way supposed. such movements required that many of the stars should travel with almost inconceivable velocity. copernicus also saw that the daily rising and setting of the heavenly bodies could be accounted for either by the supposition that the celestial sphere moved round and that the earth remained at rest, or by the supposition that the celestial sphere was at rest while the earth turned round in the opposite direction. he weighed the arguments on both sides as ptolemy had done, and, as the result of his deliberations, copernicus came to an opposite conclusion from ptolemy. to copernicus it appeared that the difficulties attending the supposition that the celestial sphere revolved, were vastly greater than those which appeared so weighty to ptolemy as to force him to deny the earth's rotation. copernicus shows clearly how the observed phenomena could be accounted for just as completely by a rotation of the earth as by a rotation of the heavens. he alludes to the fact that, to those on board a vessel which is moving through smooth water, the vessel itself appears to be at rest, while the objects on shore seem to be moving past. if, therefore, the earth were rotating uniformly, we dwellers upon the earth, oblivious of our own movement, would wrongly attribute to the stars the displacement which was actually the consequence of our own motion. copernicus saw the futility of the arguments by which ptolemy had endeavoured to demonstrate that a revolution of the earth was impossible. it was plain to him that there was nothing whatever to warrant refusal to believe in the rotation of the earth. in his clear-sightedness on this matter we have specially to admire the sagacity of copernicus as a natural philosopher. it had been urged that, if the earth moved round, its motion would not be imparted to the air, and that therefore the earth would be uninhabitable by the terrific winds which would be the result of our being carried through the air. copernicus convinced himself that this deduction was preposterous. he proved that the air must accompany the earth, just as his coat remains round him, notwithstanding the fact that he is walking down the street. in this way he was able to show that all a priori objections to the earth's movements were absurd, and therefore he was able to compare together the plausibilities of the two rival schemes for explaining the diurnal movement. [plate: frauenburg, from an old print.] once the issue had been placed in this form, the result could not be long in doubt. here is the question: which is it more likely--that the earth, like a grain of sand at the centre of a mighty globe, should turn round once in twenty-four hours, or that the whole of that vast globe should complete a rotation in the opposite direction in the same time? obviously, the former is far the more simple supposition. but the case is really much stronger than this. ptolemy had supposed that all the stars were attached to the surface of a sphere. he had no ground whatever for this supposition, except that otherwise it would have been well-nigh impossible to have devised a scheme by which the rotation of the heavens around a fixed earth could have been arranged. copernicus, however, with the just instinct of a philosopher, considered that the celestial sphere, however convenient from a geometrical point of view, as a means of representing apparent phenomena, could not actually have a material existence. in the first place, the existence of a material celestial sphere would require that all the myriad stars should be at exactly the same distances from the earth. of course, no one will say that this or any other arbitrary disposition of the stars is actually impossible, but as there was no conceivable physical reason why the distances of all the stars from the earth should be identical, it seemed in the very highest degree improbable that the stars should be so placed. doubtless, also, copernicus felt a considerable difficulty as to the nature of the materials from which ptolemy's wonderful sphere was to be constructed. nor could a philosopher of his penetration have failed to observe that, unless that sphere were infinitely large, there must have been space outside it, a consideration which would open up other difficult questions. whether infinite or not, it was obvious that the celestial sphere must have a diameter at least many thousands of times as great as that of the earth. from these considerations copernicus deduced the important fact that the stars and the other celestial bodies must all be vast objects. he was thus enabled to put the question in such a form that it could hardly receive any answer but the correct one. which is it more rational to suppose, that the earth should turn round on its axis once in twenty-four hours, or that thousands of mighty stars should circle round the earth in the same time, many of them having to describe circles many thousands of times greater in circumference than the circuit of the earth at the equator? the obvious answer pressed upon copernicus with so much force that he was compelled to reject ptolemy's theory of the stationary earth, and to attribute the diurnal rotation of the heavens to the revolution of the earth on its axis. once this tremendous step had been taken, the great difficulties which beset the monstrous conception of the celestial sphere vanished, for the stars need no longer be regarded as situated at equal distances from the earth. copernicus saw that they might lie at the most varied degrees of remoteness, some being hundreds or thousands of times farther away than others. the complicated structure of the celestial sphere as a material object disappeared altogether; it remained only as a geometrical conception, whereon we find it convenient to indicate the places of the stars. once the copernican doctrine had been fully set forth, it was impossible for anyone, who had both the inclination and the capacity to understand it, to withhold acceptance of its truth. the doctrine of a stationary earth had gone for ever. copernicus having established a theory of the celestial movements which deliberately set aside the stability of the earth, it seemed natural that he should inquire whether the doctrine of a moving earth might not remove the difficulties presented in other celestial phenomena. it had been universally admitted that the earth lay unsupported in space. copernicus had further shown that it possessed a movement of rotation. its want of stability being thus recognised, it seemed reasonable to suppose that the earth might also have some other kinds of movements as well. in this, copernicus essayed to solve a problem far more difficult than that which had hitherto occupied his attention. it was a comparatively easy task to show how the diurnal rising and setting could be accounted for by the rotation of the earth. it was a much more difficult undertaking to demonstrate that the planetary movements, which ptolemy had represented with so much success, could be completely explained by the supposition that each of those planets revolved uniformly round the sun, and that the earth was also a planet, accomplishing a complete circuit of the sun once in the course of a year. [plate: explanation of planetary movements.] it would be impossible in a sketch like the present to enter into any detail as to the geometrical propositions on which this beautiful investigation of copernicus depended. we can only mention a few of the leading principles. it may be laid down in general that, if an observer is in movement, he will, if unconscious of the fact, attribute to the fixed objects around him a movement equal and opposite to that which he actually possesses. a passenger on a canal-boat sees the objects on the banks apparently moving backward with a speed equal to that by which he is himself advancing forwards. by an application of this principle, we can account for all the phenomena of the movements of the planets, which ptolemy had so ingeniously represented by his circles. let us take, for instance, the most characteristic feature in the irregularities of the outer planets. we have already remarked that mars, though generally advancing from west to east among the stars, occasionally pauses, retraces his steps for awhile, again pauses, and then resumes his ordinary onward progress. copernicus showed clearly how this effect was produced by the real motion of the earth, combined with the real motion of mars. in the adjoining figure we represent a portion of the circular tracks in which the earth and mars move in accordance with the copernican doctrine. i show particularly the case where the earth comes directly between the planet and the sun, because it is on such occasions that the retrograde movement (for so this backward movement of mars is termed) is at its highest. mars is then advancing in the direction shown by the arrow-head, and the earth is also advancing in the same direction. we, on the earth, however, being unconscious of our own motion, attribute, by the principle i have already explained, an equal and opposite motion to mars. the visible effect upon the planet is, that mars has two movements, a real onward movement in one direction, and an apparent movement in the opposite direction. if it so happened that the earth was moving with the same speed as mars, then the apparent movement would exactly neutralise the real movement, and mars would seem to be at rest relatively to the surrounding stars. under the actual circumstances represented, however, the earth is moving faster than mars, and the consequence is, that the apparent movement of the planet backwards exceeds the real movement forwards, the net result being an apparent retrograde movement. with consummate skill, copernicus showed how the applications of the same principles could account for the characteristic movements of the planets. his reasoning in due time bore down all opposition. the supreme importance of the earth in the system vanished. it had now merely to take rank as one of the planets. the same great astronomer now, for the first time, rendered something like a rational account of the changes of the seasons. nor did certain of the more obscure astronomical phenomena escape his attention. he delayed publishing his wonderful discoveries to the world until he was quite an old man. he had a well-founded apprehension of the storm of opposition which they would arouse. however, he yielded at last to the entreaties of his friends, and his book was sent to the press. but ere it made its appearance to the world, copernicus was seized by mortal illness. a copy of the book was brought to him on may , . we are told that he was able to see it and to touch it, but no more, and he died a few hours afterwards. he was buried in that cathedral of frauenburg, with which his life had been so closely associated. tycho brahe. the most picturesque figure in the history of astronomy is undoubtedly that of the famous old danish astronomer whose name stands at the head of this chapter. tycho brahe was alike notable for his astronomical genius and for the extraordinary vehemence of a character which was by no means perfect. his romantic career as a philosopher, and his taste for splendour as a danish noble, his ardent friendships and his furious quarrels, make him an ideal subject for a biographer, while the magnificent astronomical work which he accomplished, has given him imperishable fame. the history of tycho brahe has been admirably told by dr. dreyer, the accomplished astronomer who now directs the observatory at armagh, though himself a countryman of tycho. every student of the career of the great dane must necessarily look on dr. dreyer's work as the chief authority on the subject. tycho sprang from an illustrious stock. his family had flourished for centuries, both in sweden and in denmark, where his descendants are to be met with at the present day. the astronomer's father was a privy councillor, and having filled important positions in the danish government, he was ultimately promoted to be governor of helsingborg castle, where he spent the last years of his life. his illustrious son tycho was born in , and was the second child and eldest boy in a family of ten. it appears that otto, the father of tycho, had a brother named george, who was childless. george, however, desired to adopt a boy on whom he could lavish his affection and to whom he could bequeath his wealth. a somewhat singular arrangement was accordingly entered into by the brothers at the time when otto was married. it was agreed that the first son who might be born to otto should be forthwith handed over by the parents to george to be reared and adopted by him. in due time little tycho appeared, and was immediately claimed by george in pursuance of the compact. but it was not unnatural that the parental instinct, which had been dormant when the agreement was made, should here interpose. tycho's father and mother receded from the bargain, and refused to part with their son. george thought he was badly treated. however, he took no violent steps until a year later, when a brother was born to tycho. the uncle then felt no scruple in asserting what he believed to be his rights by the simple process of stealing the first-born nephew, which the original bargain had promised him. after a little time it would seem that the parents acquiesced in the loss, and thus it was in uncle george's home that the future astronomer passed his childhood. when we read that tycho was no more than thirteen years old at the time he entered the university of copenhagen, it might be at first supposed that even in his boyish years he must have exhibited some of those remarkable talents with which he was afterwards to astonish the world. such an inference should not, however, be drawn. the fact is that in those days it was customary for students to enter the universities at a much earlier age than is now the case. not, indeed, that the boys of thirteen knew more then than the boys of thirteen know now. but the education imparted in the universities at that time was of a much more rudimentary kind than that which we understand by university education at present. in illustration of this dr. dreyer tells us how, in the university of wittenberg, one of the professors, in his opening address, was accustomed to point out that even the processes of multiplication and division in arithmetic might be learned by any student who possessed the necessary diligence. it was the wish and the intention of his uncle that tycho's education should be specially directed to those branches of rhetoric and philosophy which were then supposed to be a necessary preparation for the career of a statesman. tycho, however, speedily made it plain to his teachers that though he was an ardent student, yet the things which interested him were the movements of the heavenly bodies and not the subtleties of metaphysics. [plate: tycho brahe.] on the st october, , an eclipse of the sun occurred, which was partially visible at copenhagen. tycho, boy though he was, took the utmost interest in this event. his ardour and astonishment in connection with the circumstance were chiefly excited by the fact that the time of the occurrence of the phenomenon could be predicted with so much accuracy. urged by his desire to understand the matter thoroughly, tycho sought to procure some book which might explain what he so greatly wanted to know. in those days books of any kind were but few and scarce, and scientific books were especially unattainable. it so happened, however, that a latin version of ptolemy's astronomical works had appeared a few years before the eclipse took place, and tycho managed to buy a copy of this book, which was then the chief authority on celestial matters. young as the boy astronomer was, he studied hard, although perhaps not always successfully, to understand ptolemy, and to this day his copy of the great work, copiously annotated and marked by the schoolboy hand, is preserved as one of the chief treasures in the library of the university at prague. after tycho had studied for about three years at the university of copenhagen, his uncle thought it would be better to send him, as was usual in those days, to complete his education by a course of study in some foreign university. the uncle cherished the hope that in this way the attention of the young astronomer might be withdrawn from the study of the stars and directed in what appeared to him a more useful way. indeed, to the wise heads of those days, the pursuit of natural science seemed so much waste of good time which might otherwise be devoted to logic or rhetoric or some other branch of study more in vogue at that time. to assist in this attempt to wean tycho from his scientific tastes, his uncle chose as a tutor to accompany him an intelligent and upright young man named vedel, who was four years senior to his pupil, and accordingly, in , we find the pair taking up their abode at the university of leipzig. the tutor, however, soon found that he had undertaken a most hopeless task. he could not succeed in imbuing tycho with the slightest taste for the study of the law or the other branches of knowledge which were then thought so desirable. the stars, and nothing but the stars, engrossed the attention of his pupil. we are told that all the money he could obtain was spent secretly in buying astronomical books and instruments. he learned the name of the stars from a little globe, which he kept hidden from vedel, and only ventured to use during the latter's absence. no little friction was at first caused by all this, but in after years a fast and enduring friendship grew up between tycho and his tutor, each of whom learned to respect and to love the other. before tycho was seventeen he had commenced the difficult task of calculating the movements of the planets and the places which they occupied on the sky from time to time. he was not a little surprised to find that the actual positions of the planets differed very widely from those which were assigned to them by calculations from the best existing works of astronomers. with the insight of genius he saw that the only true method of investigating the movements of the heavenly bodies would be to carry on a protracted series of measurements of their places. this, which now seems to us so obvious, was then entirely new doctrine. tycho at once commenced regular observations in such fashion as he could. his first instrument was, indeed, a very primitive one, consisting of a simple pair of compasses, which he used in this way. he placed his eye at the hinge, and then opened the legs of the compass so that one leg pointed to one star and the other leg to the other star. the compass was then brought down to a divided circle, by which means the number of degrees in the apparent angular distance of the two stars was determined. his next advance in instrumental equipment was to provide himself with the contrivance known as the "cross-staff," which he used to observe the stars whenever opportunity offered. it must, of course, be remembered that in those days there were no telescopes. in the absence of optical aid, such as lenses afford the modern observers, astronomers had to rely on mechanical appliances alone to measure the places of the stars. of such appliances, perhaps the most ingenious was one known before tycho's time, which we have represented in the adjoining figure. [plate: tycho's cross staff.] let us suppose that it be desired to measure the angle between two stars, then if the angle be not too large it can be determined in the following manner. let the rod ab be divided into inches and parts of an inch, and let another rod, cd, slide up and down along ab in such a way that the two always remain perpendicular to each other. "sights," like those on a rifle, are placed at a and c, and there is a pin at d. it will easily be seen that, by sliding the movable bar along the fixed one, it must always be possible when the stars are not too far apart to bring the sights into such positions that one star can be seen along dc and the other along da. this having been accomplished, the length from a to the cross-bar is read off on the scale, and then, by means of a table previously prepared, the value of the required angular distance is obtained. if the angle between the two stars were greater than it would be possible to measure in the way already described, then there was a provision by which the pin at d might be moved along cd into some other position, so as to bring the angular distance of the stars within the range of the instrument. [plate: tycho's "new star" sextant of . (the arms, of walnut wood, are about / ft. long.)] no doubt the cross-staff is a very primitive contrivance, but when handled by one so skilful as tycho it afforded results of considerable accuracy. i would recommend any reader who may have a taste for such pursuits to construct a cross-staff for himself, and see what measurements he can accomplish with its aid. to employ this little instrument tycho had to evade the vigilance of his conscientious tutor, who felt it his duty to interdict all such occupations as being a frivolous waste of time. it was when vedel was asleep that tycho managed to escape with his cross staff and measure the places of the heavenly bodies. even at this early age tycho used to conduct his observations on those thoroughly sound principles which lie at the foundation of all accurate modern astronomy. recognising the inevitable errors of workmanship in his little instrument, he ascertained their amount and allowed for their influence on the results which he deduced. this principle, employed by the boy with his cross-staff in , is employed at the present day by the astronomer royal at greenwich with the most superb instruments that the skill of modern opticians has been able to construct. [plate: tycho's trigonic sextant. (the arms, ab and ac, are about / ft. long.)] after the death of his uncle, when tycho was nineteen years of age, it appears that the young philosopher was no longer interfered with in so far as the line which his studies were to take was concerned. always of a somewhat restless temperament, we now find that he shifted his abode to the university of rostock, where he speedily made himself notable in connection with an eclipse of the moon on th october, . like every other astronomer of those days, tycho had always associated astronomy with astrology. he considered that the phenomena of the heavenly bodies always had some significance in connection with human affairs. tycho was also a poet, and in the united capacity of poet, astrologer, and astronomer, he posted up some verses in the college at rostock announcing that the lunar eclipse was a prognostication of the death of the great turkish sultan, whose mighty deeds at that time filled men's minds. presently news did arrive of the death of the sultan, and tycho was accordingly triumphant; but a little later it appeared that the decease had taken place before the eclipse, a circumstance which caused many a laugh at tycho's expense. [plate: tycho's astronomic sextant. (made of steel: the arms, ab, ac, measure ft.) plate: tycho's equatorial armillary. (the meridian circle, e b c a d, made of solid steel, is nearly ft. in diameter.)] tycho being of a somewhat turbulent disposition, it appears that, while at the university of rostock, he had a serious quarrel with another danish nobleman. we are not told for certain what was the cause of the dispute. it does not, however, seem to have had any more romantic origin than a difference of opinion as to which of them knew the more mathematics. they fought, as perhaps it was becoming for two astronomers to fight, under the canopy of heaven in utter darkness at the dead of night, and the duel was honourably terminated when a slice was taken off tycho's nose by the insinuating sword of his antagonist. for the repair of this injury the ingenuity of the great instrument-maker was here again useful, and he made a substitute for his nose "with a composition of gold and silver." the imitation was so good that it is declared to have been quite equal to the original. dr. lodge, however, pointedly observes that it does not appear whether this remark was made by a friend or an enemy. [plate: the great augsburg quadrant. (built of heart of oak; the radii about ft.) plate: tycho's "new scheme of the terrestrial system," .] the next few years tycho spent in various places ardently pursuing somewhat varied branches of scientific study. at one time we hear of him assisting an astronomical alderman, in the ancient city of augsburg, to erect a tremendous wooden machine--a quadrant of -feet radius--to be used in observing the heavens. at another time we learn that the king of denmark had recognised the talents of his illustrious subject, and promised to confer on him a pleasant sinecure in the shape of a canonry, which would assist him with the means for indulging his scientific pursuits. again we are told that tycho is pursuing experiments in chemistry with the greatest energy, nor is this so incompatible as might at first be thought with his devotion to astronomy. in those early days of knowledge the different sciences seemed bound together by mysterious bonds. alchemists and astrologers taught that the several planets were correlated in some mysterious manner with the several metals. it was, therefore hardly surprising that tycho should have included a study of the properties of the metals in the programme of his astronomical work. [plate: uraniborg and its grounds. plate: ground-plan of the observatory.] an event, however, occurred in which stimulated tycho's astronomical labours, and started him on his life's work. on the th of november in that year, he was returning home to supper after a day's work in his laboratory, when he happened to lift his face to the sky, and there he beheld a brilliant new star. it was in the constellation of cassiopeia, and occupied a position in which there had certainly been no bright star visible when his attention had last been directed to that part of the heavens. such a phenomenon was so startling that he found it hard to trust the evidence of his senses. he thought he must be the subject of some hallucination. he therefore called to the servants who were accompanying him, and asked them whether they, too, could see a brilliant object in the direction in which he pointed. they certainly could, and thus he became convinced that this marvellous object was no mere creation of the fancy, but a veritable celestial body--a new star of surpassing splendour which had suddenly burst forth. in these days of careful scrutiny of the heavens, we are accustomed to the occasional outbreak of new stars. it is not, however, believed that any new star which has ever appeared has displayed the same phenomenal brilliance as was exhibited by the star of . this object has a value in astronomy far greater than it might at first appear. it is true, in one sense, that tycho discovered the new star, but it is equally true, in a different sense, that it was the new star which discovered tycho. had it not been for this opportune apparition, it is quite possible that tycho might have found a career in some direction less beneficial to science than that which he ultimately pursued. [plate: the observatory of uraniborg, island of hven.] when he reached his home on this memorable evening, tycho immediately applied his great quadrant to the measurement of the place of the new star. his observations were specially directed to the determination of the distance of the object. he rightly conjectured that if it were very much nearer to us than the stars in its vicinity, the distance of the brilliant body might be determined in a short time by the apparent changes in its distance from the surrounding points. it was speedily demonstrated that the new star could not be as near as the moon, by the simple fact that its apparent place, as compared with the stars in its neighbourhood, was not appreciably altered when it was observed below the pole, and again above the pole at an interval of twelve hours. such observations were possible, inasmuch as the star was bright enough to be seen in full daylight. tycho thus showed conclusively that the body was so remote that the diameter of the earth bore an insignificant ratio to the star's distance. his success in this respect is the more noteworthy when we find that many other observers, who studied the same object, came to the erroneous conclusion that the new star was quite as near as the moon, or even much nearer. in fact, it may be said, that with regard to this object tycho discovered everything which could possibly have been discovered in the days before telescopes were invented. he not only proved that the star's distance was too great for measurement, but he showed that it had no proper motion on the heavens. he recorded the successive changes in its brightness from week to week, as well as the fluctuations in hue with which the alterations in lustre were accompanied. it seems, nowadays, strange to find that such thoroughly scientific observations of the new star as those which tycho made, possessed, even in the eyes of the great astronomer himself, a profound astrological significance. we learn from dr. dreyer that, in tycho's opinion, "the star was at first like venus and jupiter, and its effects will therefore, first, be pleasant; but as it then became like mars, there will next come a period of wars, seditions, captivity, and death of princes, and destruction of cities, together with dryness and fiery meteors in the air, pestilence, and venomous snakes. lastly, the star became like saturn, and thus will finally come a time of want, death, imprisonment, and all kinds of sad things!" ideas of this kind were, however, universally entertained. it seemed, indeed, obvious to learned men of that period that such an apparition must forebode startling events. one of the chief theories then held was, that just as the star of bethlehem announced the first coming of christ, so the second coming, and the end of the world, was heralded by the new star of . the researches of tycho on this object were the occasion of his first appearance as an author. the publication of his book was however, for some time delayed by the urgent remonstrances of his friends, who thought it was beneath the dignity of a nobleman to condescend to write a book. happily, tycho determined to brave the opinion of his order; the book appeared, and was the first of a series of great astronomical productions from the same pen. [plate: effigy on tycho's tomb at prague.] the fame of the noble dane being now widespread, the king of denmark entreated him to return to his native country, and to deliver a course of lectures on astronomy in the university of copenhagen. with some reluctance he consented, and his introductory oration has been preserved. he dwells, in fervent language, upon the beauty and the interest of the celestial phenomena. he points out the imperative necessity of continuous and systematic observation of the heavenly bodies in order to extend our knowledge. he appeals to the practical utility of the science, for what civilised nation could exist without having the means of measuring time? he sets forth how the study of these beautiful objects "exalts the mind from earthly and trivial things to heavenly ones;" and then he winds up by assuring them that "a special use of astronomy is that it enables us to draw conclusions from the movements in the celestial regions as to human fate." an interesting event, which occurred in , distracted tycho's attention from astronomical matters. he fell in love. the young girl on whom his affections were set appears to have sprung from humble origin. here again his august family friends sought to dissuade him from a match they thought unsuitable for a nobleman. but tycho never gave way in anything. it is suggested that he did not seek a wife among the highborn dames of his own rank from the dread that the demands of a fashionable lady would make too great an inroad on the time that he wished to devote to science. at all events, tycho's union seems to have been a happy one, and he had a large family of children; none of whom, however, inherited their father's talents. [plate: tycho's mural quadrant picture, uraniborg.] tycho had many scientific friends in germany, among whom his work was held in high esteem. the treatment that he there met with seemed to him so much more encouraging than that which he received in denmark that he formed the notion of emigrating to basle and making it his permanent abode. a whisper of this intention was conveyed to the large-hearted king of denmark, frederick ii. he wisely realised how great would be the fame which would accrue to his realm if he could induce tycho to remain within danish territory and carry on there the great work of his life. a resolution to make a splendid proposal to tycho was immediately formed. a noble youth was forthwith despatched as a messenger, and ordered to travel day and night until he reached tycho, whom he was to summon to the king. the astronomer was in bed on the morning of th february, , when the message was delivered. tycho, of course, set off at once and had an audience of the king at copenhagen. the astronomer explained that what he wanted was the means to pursue his studies unmolested, whereupon the king offered him the island of hven, in the sound near elsinore. there he would enjoy all the seclusion that he could desire. the king further promised that he would provide the funds necessary for building a house and for founding the greatest observatory that had ever yet been reared for the study of the heavens. after due deliberation and consultation with his friends, tycho accepted the king's offer. he was forthwith granted a pension, and a deed was drawn up formally assigning the island of hven to his use all the days of his life. the foundation of the famous castle of uraniborg was laid on th august, . the ceremony was a formal and imposing one, in accordance with tycho's ideas of splendour. a party of scientific friends had assembled, and the time had been chosen so that the heavenly bodies were auspiciously placed. libations of costly wines were poured forth, and the stone was placed with due solemnity. the picturesque character of this wonderful temple for the study of the stars may be seen in the figures with which this chapter is illustrated. one of the most remarkable instruments that has ever been employed in studying the heavens was the mural quadrant which tycho erected in one of the apartments of uraniborg. by its means the altitudes of the celestial bodies could be observed with much greater accuracy than had been previously attainable. this wonderful contrivance is represented on the preceding page. it will be observed that the walls of the room are adorned by pictures with a lavishness of decoration not usually to be found in scientific establishments. a few years later, when the fame of the observatory at hven became more widely spread, a number of young men flocked to tycho to study under his direction. he therefore built another observatory for their use in which the instruments were placed in subterranean rooms of which only the roofs appeared above the ground. there was a wonderful poetical inscription over the entrance to this underground observatory, expressing the astonishment of urania at finding, even in the interior of the earth, a cavern devoted to the study of the heavens. tycho was indeed always fond of versifying, and he lost no opportunity of indulging this taste whenever an occasion presented itself. around the walls of the subterranean observatory were the pictures of eight astronomers, each with a suitable inscription--one of these of course represented tycho himself, and beneath were written words to the effect that posterity should judge of his work. the eighth picture depicted an astronomer who has not yet come into existence. tychonides was his name, and the inscription presses the modest hope that when he does appear he will be worthy of his great predecessor. the vast expenses incurred in the erection and the maintenance of this strange establishment were defrayed by a succession of grants from the royal purse. for twenty years tycho laboured hard at uraniborg in the pursuit of science. his work mainly consisted in the determination of the places of the moon, the planets, and the stars on the celestial sphere. the extraordinary pains taken by tycho to have his observations as accurate as his instruments would permit, have justly entitled him to the admiration of all succeeding astronomers. his island home provided the means of recreation as well as a place for work. he was surrounded by his family, troops of friends were not wanting, and a pet dwarf seems to have been an inmate of his curious residence. by way of change from his astronomical labours he used frequently to work with his students in his chemical laboratory. it is not indeed known what particular problems in chemistry occupied his attention. we are told, however, that he engaged largely in the production of medicines, and as these appear to have been dispensed gratuitously there was no lack of patients. tycho's imperious and grasping character frequently brought him into difficulties, which seem to have increased with his advancing years. he had ill-treated one of his tenants on hven, and an adverse decision by the courts seems to have greatly exasperated the astronomer. serious changes also took place in his relations to the court at copenhagen. when the young king was crowned in , he reversed the policy of his predecessor with reference to hven. the liberal allowances to tycho were one after another withdrawn, and finally even his pension was stopped. tycho accordingly abandoned hven in a tumult of rage and mortification. a few years later we find him in bohemia a prematurely aged man, and he died on the th october, . galileo. among the ranks of the great astronomers it would be difficult to find one whose life presents more interesting features and remarkable vicissitudes than does that of galileo. we may consider him as the patient investigator and brilliant discoverer. we may consider him in his private relations, especially to his daughter, sister maria celeste, a woman of very remarkable character; and we have also the pathetic drama at the close of galileo's life, when the philosopher drew down upon himself the thunders of the inquisition. the materials for the sketch of this astonishing man are sufficiently abundant. we make special use in this place of those charming letters which his daughter wrote to him from her convent home. more than a hundred of these have been preserved, and it may well be doubted whether any more beautiful and touching series of letters addressed to a parent by a dearly loved child have ever been written. an admirable account of this correspondence is contained in a little book entitled "the private life of galileo," published anonymously by messrs. macmillan in , and i have been much indebted to the author of that volume for many of the facts contained in this chapter. galileo was born at pisa, on th february, . he was the eldest son of vincenzo de' bonajuti de' galilei, a florentine noble. notwithstanding his illustrious birth and descent, it would seem that the home in which the great philosopher's childhood was spent was an impoverished one. it was obvious at least that the young galileo would have to be provided with some profession by which he might earn a livelihood. from his father he derived both by inheritance and by precept a keen taste for music, and it appears that he became an excellent performer on the lute. he was also endowed with considerable artistic power, which he cultivated diligently. indeed, it would seem that for some time the future astronomer entertained the idea of devoting himself to painting as a profession. his father, however, decided that he should study medicine. accordingly, we find that when galileo was seventeen years of age, and had added a knowledge of greek and latin to his acquaintance with the fine arts, he was duly entered at the university of pisa. here the young philosopher obtained some inkling of mathematics, whereupon he became so much interested in this branch of science, that he begged to be allowed to study geometry. in compliance with his request, his father permitted a tutor to be engaged for this purpose; but he did so with reluctance, fearing that the attention of the young student might thus be withdrawn from that medical work which was regarded as his primary occupation. the event speedily proved that these anxieties were not without some justification. the propositions of euclid proved so engrossing to galileo that it was thought wise to avoid further distraction by terminating the mathematical tutor's engagement. but it was too late for the desired end to be attained. galileo had now made such progress that he was able to continue his geometrical studies by himself. presently he advanced to that famous th proposition which won his lively admiration, and on he went until he had mastered the six books of euclid, which was a considerable achievement for those days. the diligence and brilliance of the young student at pisa did not, however, bring him much credit with the university authorities. in those days the doctrines of aristotle were regarded as the embodiment of all human wisdom in natural science as well as in everything else. it was regarded as the duty of every student to learn aristotle off by heart, and any disposition to doubt or even to question the doctrines of the venerated teacher was regarded as intolerable presumption. but young galileo had the audacity to think for himself about the laws of nature. he would not take any assertion of fact on the authority of aristotle when he had the means of questioning nature directly as to its truth or falsehood. his teachers thus came to regard him as a somewhat misguided youth, though they could not but respect the unflagging industry with which he amassed all the knowledge he could acquire. [plate: galileo's pendulum.] we are so accustomed to the use of pendulums in our clocks that perhaps we do not often realise that the introduction of this method of regulating time-pieces was really a notable invention worthy the fame of the great astronomer to whom it was due. it appears that sitting one day in the cathedral of pisa, galileo's attention became concentrated on the swinging of a chandelier which hung from the ceiling. it struck him as a significant point, that whether the arc through which the pendulum oscillated was a long one or a short one, the time occupied in each vibration was sensibly the same. this suggested to the thoughtful observer that a pendulum would afford the means by which a time-keeper might be controlled, and accordingly galileo constructed for the first time a clock on this principle. the immediate object sought in this apparatus was to provide a means of aiding physicians in counting the pulses of their patients. the talents of galileo having at length extorted due recognition from the authorities, he was appointed, at the age of twenty-five, professor of mathematics at the university of pisa. then came the time when he felt himself strong enough to throw down the gauntlet to the adherents of the old philosophy. as a necessary part of his doctrine on the movement of bodies aristotle had asserted that the time occupied by a stone in falling depends upon its weight, so that the heavier the stone the less time would it require to fall from a certain height to the earth. it might have been thought that a statement so easily confuted by the simplest experiments could never have maintained its position in any accepted scheme of philosophy. but aristotle had said it, and to anyone who ventured to express a doubt the ready sneer was forthcoming, "do you think yourself a cleverer man than aristotle?" galileo determined to demonstrate in the most emphatic manner the absurdity of a doctrine which had for centuries received the sanction of the learned. the summit of the leaning tower of pisa offered a highly dramatic site for the great experiment. the youthful professor let fall from the overhanging top a large heavy body and a small light body simultaneously. according to aristotle the large body ought to have reached the ground much sooner than the small one, but such was found not to be the case. in the sight of a large concourse of people the simple fact was demonstrated that the two bodies fell side by side, and reached the ground at the same time. thus the first great step was taken in the overthrow of that preposterous system of unquestioning adhesion to dogma, which had impeded the development of the knowledge of nature for nearly two thousand years. this revolutionary attitude towards the ancient beliefs was not calculated to render galileo's relations with the university authorities harmonious. he had also the misfortune to make enemies in other quarters. don giovanni de medici, who was then the governor of the port of leghorn, had designed some contrivance by which he proposed to pump out a dock. but galileo showed up the absurdity of this enterprise in such an aggressive manner that don giovanni took mortal offence, nor was he mollified when the truths of galileo's criticisms were abundantly verified by the total failure of his ridiculous invention. in various ways galileo was made to feel his position at pisa so unpleasant that he was at length compelled to abandon his chair in the university. the active exertions of his friends, of whom galileo was so fortunate as to have had throughout his life an abundant supply, then secured his election to the professorship of mathematics at padua, whither he went in . [plate: portrait of galileo.] it was in this new position that galileo entered on that marvellous career of investigation which was destined to revolutionize science. the zeal with which he discharged his professorial duties was indeed of the most unremitting character. he speedily drew such crowds to listen to his discourses on natural philosophy that his lecture-room was filled to overflowing. he also received many private pupils in his house for special instruction. every moment that could be spared from these labours was devoted to his private study and to his incessant experiments. like many another philosopher who has greatly extended our knowledge of nature, galileo had a remarkable aptitude for the invention of instruments designed for philosophical research. to facilitate his practical work, we find that in he had engaged a skilled workman who was to live in his house, and thus be constantly at hand to try the devices for ever springing from galileo's fertile brain. among the earliest of his inventions appears to have been the thermometer, which he constructed in . no doubt this apparatus in its primitive form differed in some respects from the contrivance we call by the same name. galileo at first employed water as the agent, by the expansion of which the temperature was to be measured. he afterwards saw the advantage of using spirits for the same purpose. it was not until about half a century later that mercury came to be recognised as the liquid most generally suitable for the thermometer. the time was now approaching when galileo was to make that mighty step in the advancement of human knowledge which followed on the application of the telescope to astronomy. as to how his idea of such an instrument originated, we had best let him tell us in his own words. the passage is given in a letter which he writes to his brother-in-law, landucci. "i write now because i have a piece of news for you, though whether you will be glad or sorry to hear it i cannot say; for i have now no hope of returning to my own country, though the occurrence which has destroyed that hope has had results both useful and honourable. you must know, then, that two months ago there was a report spread here that in flanders some one had presented to count maurice of nassau a glass manufactured in such a way as to make distant objects appear very near, so that a man at the distance of two miles could be clearly seen. this seemed to me so marvellous that i began to think about it. as it appeared to me to have a foundation in the theory of perspective, i set about contriving how to make it, and at length i found out, and have succeeded so well that the one i have made is far superior to the dutch telescope. it was reported in venice that i had made one, and a week since i was commanded to show it to his serenity and to all the members of the senate, to their infinite amazement. many gentlemen and senators, even the oldest, have ascended at various times the highest bell-towers in venice to spy out ships at sea making sail for the mouth of the harbour, and have seen them clearly, though without my telescope they would have been invisible for more than two hours. the effect of this instrument is to show an object at a distance of say fifty miles, as if it were but five miles." the remarkable properties of the telescope at once commanded universal attention among intellectual men. galileo received applications from several quarters for his new instrument, of which it would seem that he manufactured a large number to be distributed as gifts to various illustrious personages. but it was reserved for galileo himself to make that application of the instrument to the celestial bodies by which its peculiar powers were to inaugurate the new era in astronomy. the first discovery that was made in this direction appears to have been connected with the number of the stars. galileo saw to his amazement that through his little tube he could count ten times as many stars in the sky as his unaided eye could detect. here was, indeed, a surprise. we are now so familiar with the elementary facts of astronomy that it is not always easy to realise how the heavens were interpreted by the observers in those ages prior to the invention of the telescope. we can hardly, indeed, suppose that galileo, like the majority of those who ever thought of such matters, entertained the erroneous belief that the stars were on the surface of a sphere at equal distances from the observer. no one would be likely to have retained his belief in such a doctrine when he saw how the number of visible stars could be increased tenfold by means of galileo's telescope. it would have been almost impossible to refuse to draw the inference that the stars thus brought into view were still more remote objects which the telescope was able to reveal, just in the same way as it showed certain ships to the astonished venetians, when at the time these ships were beyond the reach of unaided vision. galileo's celestial discoveries now succeeded each other rapidly. that beautiful milky way, which has for ages been the object of admiration to all lovers of nature, never disclosed its true nature to the eye of man till the astronomer of padua turned on it his magic tube. the splendid zone of silvery light was then displayed as star-dust scattered over the black background of the sky. it was observed that though the individual stars were too small to be seen severally without optical aid, yet such was their incredible number that the celestial radiance produced that luminosity with which every stargazer was so familiar. but the greatest discovery made by the telescope in these early days, perhaps, indeed, the greatest discovery that the telescope has ever accomplished, was the detection of the system of four satellites revolving around the great planet jupiter. this phenomenon was so wholly unexpected by galileo that, at first, he could hardly believe his eyes. however, the reality of the existence of a system of four moons attending the great planet was soon established beyond all question. numbers of great personages crowded to galileo to see for themselves this beautiful miniature representing the sun with its system of revolving planets. of course there were, as usual, a few incredulous people who refused to believe the assertion that four more moving bodies had to be added to the planetary system. they scoffed at the notion; they said the satellites may have been in the telescope, but that they were not in the sky. one sceptical philosopher is reported to have affirmed, that even if he saw the moons of jupiter himself he would not believe in them, as their existence was contrary to the principles of common-sense! there can be no doubt that a special significance attached to the new discovery at this particular epoch in the history of science. it must be remembered that in those days the doctrine of copernicus, declaring that the sun, and not the earth, was the centre of the system, that the earth revolved on its axis once a day, and that it described a mighty circle round the sun once a year, had only recently been promulgated. this new view of the scheme of nature had been encountered with the most furious opposition. it may possibly have been that galileo himself had not felt quite confident in the soundness of the copernican theory, prior to the discovery of the satellites of jupiter. but when a picture was there exhibited in which a number of relatively small globes were shown to be revolving around a single large globe in the centre, it seemed impossible not to feel that the beautiful spectacle so displayed was an emblem of the relations of the planets to the sun. it was thus made manifest to galileo that the copernican theory of the planetary system must be the true one. the momentous import of this opinion upon the future welfare of the great philosopher will presently appear. it would seem that galileo regarded his residence at padua as a state of undesirable exile from his beloved tuscany. he had always a yearning to go back to his own country and at last the desired opportunity presented itself. for now that galileo's fame had become so great, the grand duke of tuscany desired to have the philosopher resident at florence, in the belief that he would shed lustre on the duke's dominions. overtures were accordingly made to galileo, and the consequence was that in we find him residing at florence, bearing the title of mathematician and philosopher to the grand duke. two daughters, polissena and virginia, and one son, vincenzo, had been born to galileo in padua. it was the custom in those days that as soon as the daughter of an italian gentleman had grown up, her future career was somewhat summarily decided. either a husband was to be forthwith sought out, or she was to enter the convent with the object of taking the veil as a professed nun. it was arranged that the two daughters of galileo, while still scarcely more than children, should both enter the franciscan convent of st. matthew, at arcetri. the elder daughter polissena, took the name of sister maria celeste, while virginia became sister arcangela. the latter seems to have been always delicate and subject to prolonged melancholy, and she is of but little account in the narrative of the life of galileo. but sister maria celeste, though never leaving the convent, managed to preserve a close intimacy with her beloved father. this was maintained only partly by galileo's visits, which were very irregular and were, indeed, often suspended for long intervals. but his letters to this daughter were evidently frequent and affectionate, especially in the latter part of his life. most unfortunately, however, all his letters have been lost. there are grounds for believing that they were deliberately destroyed when galileo was seized by the inquisition, lest they should have been used as evidence against him, or lest they should have compromised the convent where they were received. but sister maria celeste's letters to her father have happily been preserved, and most touching these letters are. we can hardly read them without thinking how the sweet and gentle nun would have shrunk from the idea of their publication. her loving little notes to her "dearest lord and father," as she used affectionately to call galileo, were almost invariably accompanied by some gift, trifling it may be, but always the best the poor nun had to bestow. the tender grace of these endearing communications was all the more precious to him from the fact that the rest of galileo's relatives were of quite a worthless description. he always acknowledged the ties of his kindred in the most generous way, but their follies and their vices, their selfishness and their importunities, were an incessant source of annoyance to him, almost to the last day of his life. on th december, , sister maria celeste writes:-- "i send two baked pears for these days of vigil. but as the greatest treat of all, i send you a rose, which ought to please you extremely, seeing what a rarity it is at this season; and with the rose you must accept its thorns, which represent the bitter passion of our lord, whilst the green leaves represent the hope we may entertain that through the same sacred passion we, having passed through the darkness of the short winter of our mortal life, may attain to the brightness and felicity of an eternal spring in heaven." when the wife and children of galileo's shiftless brother came to take up their abode in the philosopher's home, sister maria celeste feels glad to think that her father has now some one who, however imperfectly, may fulfil the duty of looking after him. a graceful note on christmas eve accompanies her little gifts. she hopes that-- "in these holy days the peace of god may rest on him and all the house. the largest collar and sleeves i mean for albertino, the other two for the two younger boys, the little dog for baby, and the cakes for everybody, except the spice-cakes, which are for you. accept the good-will which would readily do much more." the extraordinary forbearance with which galileo continually placed his time, his purse, and his influence at the service of those who had repeatedly proved themselves utterly unworthy of his countenance, is thus commented on by the good nun.-- "now it seems to me, dearest lord and father, that your lordship is walking in the right path, since you take hold of every occasion that presents itself to shower continual benefits on those who only repay you with ingratitude. this is an action which is all the more virtuous and perfect as it is the more difficult." when the plague was raging in the neighbourhood, the loving daughter's solicitude is thus shown:-- "i send you two pots of electuary as a preventive against the plague. the one without the label consists of dried figs, walnuts, rue, and salt, mixed together with honey. a piece of the size of a walnut to be taken in the morning, fasting, with a little greek wine." the plague increasing still more, sister maria celeste obtained with much difficulty, a small quantity of a renowned liqueur, made by abbess ursula, an exceptionally saintly nun. this she sends to her father with the words:-- "i pray your lordship to have faith in this remedy. for if you have so much faith in my poor miserable prayers, much more may you have in those of such a holy person; indeed, through her merits you may feel sure of escaping all danger from the plague." whether galileo took the remedy we do not know, but at all events he escaped the plague. [plate: the villa arcetri. galileo's residence, where milton visited him.] from galileo's new home in florence the telescope was again directed to the skies, and again did astounding discoveries reward the astronomer's labours. the great success which he had met with in studying jupiter naturally led galileo to look at saturn. here he saw a spectacle which was sufficiently amazing, though he failed to interpret it accurately. it was quite manifest that saturn did not exhibit a simple circular disc like jupiter, or like mars. it seemed to galileo as if the planet consisted of three bodies, a large globe in the centre, and a smaller one on each side. the enigmatical nature of the discovery led galileo to announce it in an enigmatical manner. he published a string of letters which, when duly transposed, made up a sentence which affirmed that the planet saturn was threefold. of course we now know that this remarkable appearance of the planet was due to the two projecting portions of the ring. with the feeble power of galileo's telescope, these seemed merely like small globes or appendages to the large central body. the last of galileo's great astronomical discoveries related to the libration of the moon. i think that the detection of this phenomenon shows his acuteness of observation more remarkably than does any one of his other achievements with the telescope. it is well known that the moon constantly keeps the same face turned towards the earth. when, however, careful measurements have been made with regard to the spots and marks on the lunar surface, it is found that there is a slight periodic variation which permits us to see now a little to the east or to the west, now a little to the north or to the south of the average lunar disc. but the circumstances which make the career of galileo so especially interesting from the biographer's point of view, are hardly so much the triumphs that he won as the sufferings that he endured. the sufferings and the triumphs were, however, closely connected, and it is fitting that we should give due consideration to what was perhaps the greatest drama in the history of science. on the appearance of the immortal work of copernicus, in which it was taught that the earth rotated on its axis, and that the earth, like the other planets, revolved round the sun, orthodoxy stood aghast. the holy roman church submitted this treatise, which bore the name "de revolutionibus orbium coelestium," to the congregation of the index. after due examination it was condemned as heretical in . galileo was suspected, on no doubt excellent grounds, of entertaining the objectionable views of copernicus. he was accordingly privately summoned before cardinal bellarmine on th february , and duly admonished that he was on no account to teach or to defend the obnoxious doctrines. galileo was much distressed by this intimation. he felt it a serious matter to be deprived of the privilege of discoursing with his friends about the copernican system, and of instructing his disciples in the principles of the great theory of whose truth he was perfectly convinced. it pained him, however, still more to think, devout catholic as he was, that such suspicions of his fervent allegiance to his church should ever have existed, as were implied by the words and monitions of cardinal bellarmine. in , galileo had an interview with pope paul v., who received the great astronomer very graciously, and walked up and down with him in conversation for three-quarters of an hour. galileo complained to his holiness of the attempts made by his enemies to embarrass him with the authorities of the church, but the pope bade him be comforted. his holiness had himself no doubts of galileo's orthodoxy, and he assured him that the congregation of the index should give galileo no further trouble so long as paul v. was in the chair of st. peter. on the death of paul v. in , maffeo barberini was elected pope, as urban viii. this new pope, while a cardinal, had been an intimate friend of galileo's, and had indeed written latin verses in praise of the great astronomer and his discoveries. it was therefore not unnatural for galileo to think that the time had arrived when, with the use of due circumspection, he might continue his studies and his writings, without fear of incurring the displeasure of the church. indeed, in , one of galileo's friends writing from rome, urges galileo to visit the city again, and added that-- "under the auspices of this most excellent, learned, and benignant pontiff, science must flourish. your arrival will be welcome to his holiness. he asked me if you were coming, and when, and in short, he seems to love and esteem you more than ever." the visit was duly paid, and when galileo returned to florence, the pope wrote a letter from which the following is an extract, commanding the philosopher to the good offices of the young ferdinand, who had shortly before succeeded his father in the grand duchy of tuscany. "we find in galileo not only literary distinction, but also the love of piety, and he is also strong in those qualities by which the pontifical good-will is easily obtained. and now, when he has been brought to this city to congratulate us on our elevation, we have very lovingly embraced him; nor can we suffer him to return to the country whither your liberality calls him, without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honourable testimonial of virtue and piety. and we further signify that every benefit which you shall confer upon him, imitating or even surpassing your father's liberality, will conduce to our gratification." the favourable reception which had been accorded to him by pope urban viii. seems to have led galileo to expect that there might be some corresponding change in the attitude of the papal authorities on the great question of the stability of the earth. he accordingly proceeded with the preparation of the chief work of his life, "the dialogue of the two systems." it was submitted for inspection by the constituted authorities. the pope himself thought that, if a few conditions which he laid down were duly complied with, there could be no objection to the publication of the work. in the first place, the title of the book was to be so carefully worded as to show plainly that the copernican doctrine was merely to be regarded as an hypothesis, and not as a scientific fact. galileo was also instructed to conclude the book with special arguments which had been supplied by the pope himself, and which appeared to his holiness to be quite conclusive against the new doctrine of copernicus. formal leave for the publication of the dialogue was then given to galileo by the inquisitor general, and it was accordingly sent to the press. it might be thought that the anxieties of the astronomer about his book would then have terminated. as a matter of fact, they had not yet seriously begun. riccardi, the master of the sacred palace, having suddenly had some further misgivings, sent to galileo for the manuscript while the work was at the printer's, in order that the doctrine it implied might be once again examined. apparently, riccardi had come to the conclusion that he had not given the matter sufficient attention, when the authority to go to press had been first and, perhaps, hastily given. considerable delay in the issue of the book was the result of these further deliberations. at last, however, in june, , galileo's great work, "the dialogue of the two systems," was produced for the instruction of the world, though the occasion was fraught with ruin to the immortal author. [plate: facsimile sketch of lunar surface by galileo.] the book, on its publication, was received and read with the greatest avidity. but presently the master of the sacred palace found reason to regret that he had given his consent to its appearance. he accordingly issued a peremptory order to sequestrate every copy in italy. this sudden change in the papal attitude towards galileo formed the subject of a strong remonstrance addressed to the roman authorities by the grand duke of tuscany. the pope himself seemed to have become impressed all at once with the belief that the work contained matter of an heretical description. the general interpretation put upon the book seems to have shown the authorities that they had mistaken its true tendency, notwithstanding the fact that it had been examined again and again by theologians deputed for the duty. to the communication from the grand duke the pope returned answer, that he had decided to submit the book to a congregation of "learned, grave, and saintly men," who would weigh every word in it. the views of his holiness personally on the subject were expressed in his belief that the dialogue contained the most perverse matter that could come into a reader's hands. the master of the sacred palace was greatly blamed by the authorities for having given his sanction to its issue. he pleaded that the book had not been printed in the precise terms of the original manuscript which had been submitted to him. it was also alleged that galileo had not adhered to his promise of inserting properly the arguments which the pope himself had given in support of the old and orthodox view. one of these had, no doubt, been introduced, but, so far from mending galileo's case, it had made matters really look worse for the poor philosopher. the pope's argument had been put into the mouth of one of the characters in the dialogue named "simplicio." galileo's enemies maintained that by adopting such a method for the expression of his holiness's opinion, galileo had intended to hold the pope himself up to ridicule. galileo's friends maintained that nothing could have been farther from his intention. it seems, however, highly probable that the suspicions thus aroused had something to say to the sudden change of front on the part of the papal authorities. on st october, , galileo received an order to appear before the inquisition at rome on the grave charge of heresy. galileo, of course, expressed his submission, but pleaded for a respite from compliance with the summons, on the ground of his advanced age and his failing health. the pope was, however, inexorable; he said that he had warned galileo of his danger while he was still his friend. the command could not be disobeyed. galileo might perform the journey as slowly as he pleased, but it was imperatively necessary for him to set forth and at once. on th january, , galileo started on his weary journey to rome, in compliance with this peremptory summons. on th february he was received as the guest of niccolini, the tuscan ambassador, who had acted as his wise and ever-kind friend throughout the whole affair. it seemed plain that the holy office were inclined to treat galileo with as much clemency and consideration as was consistent with the determination that the case against him should be proceeded with to the end. the pope intimated that in consequence of his respect for the grand duke of tuscany he should permit galileo to enjoy the privilege, quite unprecedented for a prisoner charged with heresy, of remaining as an inmate in the ambassador's house. he ought, strictly, to have been placed in the dungeons of the inquisition. when the examination of the accused had actually commenced, galileo was confined, not, indeed, in the dungeons, but in comfortable rooms at the holy office. by the judicious and conciliatory language of submission which niccolini had urged galileo to use before the inquisitors, they were so far satisfied that they interceded with the pope for his release. during the remainder of the trial galileo was accordingly permitted to go back to the ambassador's, where he was most heartily welcomed. sister maria celeste, evidently thinking this meant that the whole case was at an end, thus expresses herself:-- "the joy that your last dear letter brought me, and the having to read it over and over to the nuns, who made quite a jubilee on hearing its contents, put me into such an excited state that at last i got a severe attack of headache." in his defence galileo urged that he had already been acquitted in by cardinal bellarmine, when a charge of heresy was brought against him, and he contended that anything he might now have done, was no more than he had done on the preceding occasion, when the orthodoxy of his doctrines received solemn confirmation. the inquisition seemed certainly inclined to clemency, but the pope was not satisfied. galileo was accordingly summoned again on the st june. he was to be threatened with torture if he did not forthwith give satisfactory explanations as to the reasons which led him to write the dialogue. in this proceeding the pope assured the tuscan ambassador that he was treating galileo with the utmost consideration possible in consequence of his esteem and regard for the grand duke, whose servant galileo was. it was, however, necessary that some exemplary punishment be meted out to the astronomer, inasmuch as by the publication of the dialogue he had distinctly disobeyed the injunction of silence laid upon him by the decree of . nor was it admissible for galileo to plead that his book had been sanctioned by the master of the sacred college, to whose inspection it had been again and again submitted. it was held, that if the master of the sacred college had been unaware of the solemn warning the philosopher had already received sixteen years previously, it was the duty of galileo to have drawn his attention to that fact. on the nd june, , galileo was led to the great hall of the inquisition, and compelled to kneel before the cardinals there assembled and hear his sentence. in a long document, most elaborately drawn up, it is definitely charged against galileo that, in publishing the dialogue, he committed the essentially grave error of treating the doctrine of the earth's motion as open to discussion. galileo knew, so the document affirmed, that the church had emphatically pronounced this notion to be contrary to holy writ, and that for him to consider a doctrine so stigmatized as having any shadow of probability in its favour was an act of disrespect to the authority of the church which could not be overlooked. it was also charged against galileo that in his dialogue he has put the strongest arguments into the mouth, not of those who supported the orthodox doctrine, but of those who held the theory as to the earth's motion which the church had so deliberately condemned. after due consideration of the defence made by the prisoner, it was thereupon decreed that he had rendered himself vehemently suspected of heresy by the holy office, and in consequence had incurred all the censures and penalties of the sacred canons, and other decrees promulgated against such persons. the graver portion of these punishments would be remitted, if galileo would solemnly repudiate the heresies referred to by an abjuration to be pronounced by him in the terms laid down. at the same time it was necessary to mark, in some emphatic manner, the serious offence which had been committed, so that it might serve both as a punishment to galileo and as a warning to others. it was accordingly decreed that he should be condemned to imprisonment in the holy office during the pleasure of the papal authorities, and that he should recite once a week for three years the seven penitential psalms. then followed that ever-memorable scene in the great hall of the inquisition, in which the aged and infirm galileo, the inventor of the telescope and the famous astronomer, knelt down to abjure before the most eminent and reverend lords cardinal, inquisitors general throughout the christian republic against heretical depravity. with his hands on the gospels, galileo was made to curse and detest the false opinion that the sun was the centre of the universe and immovable, and that the earth was not the centre of the same, and that it moved. he swore that for the future he will never say nor write such things as may bring him under suspicion, and that if he does so he submits to all the pains and penalties of the sacred canons. this abjuration was subsequently read in florence before galileo's disciples, who had been specially summoned to attend. it has been noted that neither on the first occasion, in , nor on the second in , did the reigning pope sign the decrees concerning galileo. the contention has accordingly been made that paul v. and urban viii. are both alike vindicated from any technical responsibility for the attitude of the romish church towards the copernican doctrines. the significance of this circumstance has been commented on in connection with the doctrine of the infallibility of the pope. we can judge of the anxiety felt by sister maria celeste about her beloved father during these terrible trials. the wife of the ambassador niccolini, galileo's steadfast friend, most kindly wrote to give the nun whatever quieting assurances the case would permit. there is a renewed flow of these touching epistles from the daughter to her father. thus she sends word-- "the news of your fresh trouble has pierced my soul with grief all the more that it came quite unexpectedly." and again, on hearing that he had been permitted to leave rome, she writes-- "i wish i could describe the rejoicing of all the mothers and sisters on hearing of your happy arrival at siena. it was indeed most extraordinary. on hearing the news the mother abbess and many of the nuns ran to me, embracing me and weeping for joy and tenderness." the sentence of imprisonment was at first interpreted leniently by the pope. galileo was allowed to reside in qualified durance in the archbishop's house at siena. evidently the greatest pain that he endured arose from the forced separation from that daughter, whom he had at last learned to love with an affection almost comparable with that she bore to him. she had often told him that she never had any pleasure equal to that with which she rendered any service to her father. to her joy, she discovers that she can relieve him from the task of reciting the seven penitential psalms which had been imposed as a penance:-- "i began to do this a while ago," she writes, "and it gives me much pleasure. first, because i am persuaded that prayer in obedience to holy church must be efficacious; secondly, in order to save you the trouble of remembering it. if i had been able to do more, most willingly would i have entered a straiter prison than the one i live in now, if by so doing i could have set you at liberty." [plate: crest of galileo's family.] sister maria celeste was gradually failing in health, but the great privilege was accorded to her of being able once again to embrace her beloved lord and master. galileo had, in fact, been permitted to return to his old home; but on the very day when he heard of his daughter's death came the final decree directing him to remain in his own house in perpetual solitude. amid the advancing infirmities of age, the isolation from friends, and the loss of his daughter, galileo once again sought consolation in hard work. he commenced his famous dialogue on motion. gradually, however, his sight began to fail, and blindness was at last added to his other troubles. on january nd, , he writes to diodati:-- "alas, your dear friend and servant, galileo, has been for the last month perfectly blind, so that this heaven, this earth, this universe which i by my marvellous discoveries and clear demonstrations have enlarged a hundred thousand times beyond the belief of the wise men of bygone ages, henceforward is for me shrunk into such a small space as is filled by my own bodily sensations." but the end was approaching--the great philosopher, was attacked by low fever, from which he died on the th january, . kepler. while the illustrious astronomer, tycho brahe, lay on his death-bed, he had an interview which must ever rank as one of the important incidents in the history of science. the life of tycho had been passed, as we have seen, in the accumulation of vast stores of careful observations of the positions of the heavenly bodies. it was not given to him to deduce from his splendid work the results to which they were destined to lead. it was reserved for another astronomer to distil, so to speak, from the volumes in which tycho's figures were recorded, the great truths of the universe which those figures contained. tycho felt that his work required an interpreter, and he recognised in the genius of a young man with whom he was acquainted the agent by whom the world was to be taught some of the great truths of nature. to the bedside of the great danish astronomer the youthful philosopher was summoned, and with his last breath tycho besought of him to spare no labour in the performance of those calculations, by which alone the secrets of the movements of the heavens could be revealed. the solemn trust thus imposed was duly accepted, and the man who accepted it bore the immortal name of kepler. kepler was born on the th december, , at weil, in the duchy of wurtemberg. it would seem that the circumstances of his childhood must have been singularly unhappy. his father, sprung from a well-connected family, was but a shiftless and idle adventurer; nor was the great astronomer much more fortunate in his other parent. his mother was an ignorant and ill-tempered woman; indeed, the ill-assorted union came to an abrupt end through the desertion of the wife by her husband when their eldest son john, the hero of our present sketch, was eighteen years old. the childhood of this lad, destined for such fame, was still further embittered by the circumstance that when he was four years old he had a severe attack of small-pox. not only was his eyesight permanently injured, but even his constitution appears to have been much weakened by this terrible malady. it seems, however, that the bodily infirmities of young john kepler were the immediate cause of his attention being directed to the pursuit of knowledge. had the boy been fitted like other boys for ordinary manual work, there can be hardly any doubt that to manual work his life must have been devoted. but, though his body was feeble, he soon gave indications of the possession of considerable mental power. it was accordingly thought that a suitable sphere for his talents might be found in the church which, in those days, was almost the only profession that afforded an opening for an intellectual career. we thus find that by the time john kepler was seventeen years old he had attained a sufficient standard of knowledge to entitle him to admission on the foundation of the university at tubingen. in the course of his studies at this institution he seems to have divided his attention equally between astronomy and divinity. it not unfrequently happens that when a man has attained considerable proficiency in two branches of knowledge he is not able to see very clearly in which of the two pursuits his true vocation lies. his friends and onlookers are often able to judge more wisely than he himself can do as to which of the two lines it would be better for him to pursue. this incapacity for perceiving the path in which greatness awaited him, existed in the case of kepler. personally, he inclined to enter the ministry, in which a promising career seemed open to him. he yielded, however, to friends, who evidently knew him better than he knew himself, and accepted in , the important professorship of astronomy which had been offered to him in the university of gratz. it is difficult for us in these modern days to realise the somewhat extraordinary duties which were expected from an astronomical professor in the sixteenth century. he was, of course, required to employ his knowledge of the heavens in the prediction of eclipses, and of the movements of the heavenly bodies generally. this seems reasonable enough; but what we are not prepared to accept is the obligation which lay on the astronomers to predict the fates of nations and the destinies of individuals. it must be remembered that it was the almost universal belief in those days, that all the celestial spheres revolved in some mysterious fashion around the earth, which appeared by far the most important body in the universe. it was imagined that the sun, the moon, and the stars indicated, in the vicissitudes of their movements, the careers of nations and of individuals. such being the generally accepted notion, it seemed to follow that a professor who was charged with the duty of expounding the movements of the heavenly bodies must necessarily be looked to for the purpose of deciphering the celestial decrees regarding the fate of man which the heavenly luminaries were designed to announce. kepler threw himself with characteristic ardour into even this fantastic phase of the labours of the astronomical professor; he diligently studied the rules of astrology, which the fancies of antiquity had compiled. believing sincerely as he did in the connection between the aspect of the stars and the state of human affairs, he even thought that he perceived, in the events of his own life, a corroboration of the doctrine which affirmed the influence of the planets upon the fate of individuals. [plate: kepler's system of regular solids.] but quite independently of astrology there seem to have been many other delusions current among the philosophers of kepler's time. it is now almost incomprehensible how the ablest men of a few centuries ago should have entertained such preposterous notions, as they did, with respect to the system of the universe. as an instance of what is here referred to, we may cite the extraordinary notion which, under the designation of a discovery, first brought kepler into fame. geometers had long known that there were five, but no more than five, regular solid figures. there is, for instance, the cube with six sides, which is, of course, the most familiar of these solids. besides the cube there are other figures of four, eight, twelve, and twenty sides respectively. it also happened that there were five planets, but no more than five, known to the ancients, namely, mercury, venus, mars, jupiter, and saturn. to kepler's lively imaginations this coincidence suggested the idea that the five regular solids corresponded to the five planets, and a number of fancied numerical relations were adduced on the subject. the absurdity of this doctrine is obvious enough, especially when we observe that, as is now well known, there are two large planets, and a host of small planets, over and above the magical number of the regular solids. in kepler's time, however, this doctrine was so far from being regarded as absurd, that its announcement was hailed as a great intellectual triumph. kepler was at once regarded with favour. it seems, indeed, to have been the circumstance which brought him into correspondence with tycho brahe. by its means also he became known to galileo. the career of a scientific professor in those early days appears generally to have been marked by rather more striking vicissitudes than usually befall a professor in a modern university. kepler was a protestant, and as such he had been appointed to his professorship at gratz. a change, however, having taken place in the religious belief entertained by the ruling powers of the university, the protestant professors were expelled. it seems that special influence having been exerted in kepler's case on account of his exceptional eminence, he was recalled to gratz and reinstated in the tenure of his chair. but his pupils had vanished, so that the great astronomer was glad to accept a post offered him by tycho brahe in the observatory which the latter had recently established near prague. on tycho's death, which occurred soon after, an opening presented itself which gave kepler the opportunity his genius demanded. he was appointed to succeed tycho in the position of imperial mathematician. but a far more important point, both for kepler and for science, was that to him was confided the use of tycho's observations. it was, indeed, by the discussion of tycho's results that kepler was enabled to make the discoveries which form such an important part of astronomical history. kepler must also be remembered as one of the first great astronomers who ever had the privilege of viewing celestial bodies through a telescope. it was in that he first held in his hands one of those little instruments which had been so recently applied to the heavens by galileo. it should, however, be borne in mind that the epoch-making achievements of kepler did not arise from any telescopic observations that he made, or, indeed, that any one else made. they were all elaborately deduced from tycho's measurements of the positions of the planets, obtained with his great instruments, which were unprovided with telescopic assistance. to realise the tremendous advance which science received from kepler's great work, it is to be understood that all the astronomers who laboured before him at the difficult subject of the celestial motions, took it for granted that the planets must revolve in circles. if it did not appear that a planet moved in a fixed circle, then the ready answer was provided by ptolemy's theory that the circle in which the planet did move was itself in motion, so that its centre described another circle. when kepler had before him that wonderful series of observations of the planet, mars, which had been accumulated by the extraordinary skill of tycho, he proved, after much labour, that the movements of the planet refused to be represented in a circular form. nor would it do to suppose that mars revolved in one circle, the centre of which revolved in another circle. on no such supposition could the movements of the planets be made to tally with those which tycho had actually observed. this led to the astonishing discovery of the true form of a planet's orbit. for the first time in the history of astronomy the principle was laid down that the movement of a planet could not be represented by a circle, nor even by combinations of circles, but that it could be represented by an elliptic path. in this path the sun is situated at one of those two points in the ellipse which are known as its foci. [plate: kepler.] very simple apparatus is needed for the drawing of one of those ellipses which kepler has shown to possess such astonishing astronomical significance. two pins are stuck through a sheet of paper on a board, the point of a pencil is inserted in a loop of string which passes over the pins, and as the pencil is moved round in such a way as to keep the string stretched, that beautiful curve known as the ellipse is delineated, while the positions of the pins indicate the two foci of the curve. if the length of the loop of string is unchanged then the nearer the pins are together, the greater will be the resemblance between the ellipse and the circle, whereas the more the pins are separated the more elongated does the ellipse become. the orbit of a great planet is, in general, one of those ellipses which approaches a nearly circular form. it fortunately happens, however, that the orbit of mars makes a wider departure from the circular form than any of the other important planets. it is, doubtless, to this circumstance that we must attribute the astonishing success of kepler in detecting the true shape of a planetary orbit. tycho's observations would not have been sufficiently accurate to have exhibited the elliptic nature of a planetary orbit which, like that of venus, differed very little from a circle. the more we ponder on this memorable achievement the more striking will it appear. it must be remembered that in these days we know of the physical necessity which requires that a planet shall revolve in an ellipse and not in any other curve. but kepler had no such knowledge. even to the last hour of his life he remained in ignorance of the existence of any natural cause which ordained that planets should follow those particular curves which geometers know so well. kepler's assignment of the ellipse as the true form of the planetary orbit is to be regarded as a brilliant guess, the truth of which tycho's observations enabled him to verify. kepler also succeeded in pointing out the law according to which the velocity of a planet at different points of its path could be accurately specified. here, again, we have to admire the sagacity with which this marvellously acute astronomer guessed the deep truth of nature. in this case also he was quite unprovided with any reason for expecting from physical principles that such a law as he discovered must be obeyed. it is quite true that kepler had some slight knowledge of the existence of what we now know as gravitation. he had even enunciated the remarkable doctrine that the ebb and flow of the tide must be attributed to the attraction of the moon on the waters of the earth. he does not, however, appear to have had any anticipation of those wonderful discoveries which newton was destined to make a little later, in which he demonstrated that the laws detected by kepler's marvellous acumen were necessary consequences of the principle of universal gravitation. [plate: symbolical representation of the planetary system.] to appreciate the relations of kepler and tycho it is necessary to note the very different way in which these illustrious astronomers viewed the system of the heavens. it should be observed that copernicus had already expounded the true system, which located the sun at the centre of the planetary system. but in the days of tycho brahe this doctrine had not as yet commanded universal assent. in fact, the great observer himself did not accept the new views of copernicus. it appeared to tycho that the earth not only appeared to be the centre of things celestial, but that it actually was the centre. it is, indeed, not a little remarkable that a student of the heavens so accurate as tycho should have deliberately rejected the copernican doctrine in favour of the system which now seems so preposterous. throughout his great career, tycho steadily observed the places of the sun, the moon, and the planets, and as steadily maintained that all those bodies revolved around the earth fixed in the centre. kepler, however, had the advantage of belonging to the new school. he utilised the observations of tycho in developing the great copernican theory whose teaching tycho stoutly resisted. perhaps a chapter in modern science may illustrate the intellectual relation of these great men. the revolution produced by copernicus in the doctrine of the heavens has often been likened to the revolution which the darwinian theory produced in the views held by biologists as to life on this earth. the darwinian theory did not at first command universal assent even among those naturalists whose lives had been devoted with the greatest success to the study of organisms. take, for instance, that great naturalist, professor owen, by whose labours vast extension has been given to our knowledge of the fossil animals which dwelt on the earth in past ages. now, though owens researches were intimately connected with the great labours of darwin, and afforded the latter material for his epoch-making generalization, yet owen deliberately refused to accept the new doctrines. like tycho, he kept on rigidly accumulating his facts under the influence of a set of ideas as to the origin of living forms which are now universally admitted to be erroneous. if, therefore, we liken darwin to copernicus, and owen to tycho, we may liken the biologists of the present day to kepler, who interpreted the results of accurate observation upon sound theoretical principles. in reading the works of kepler in the light of our modern knowledge we are often struck by the extent to which his perception of the sublimest truths in nature was associated with the most extravagant errors and absurdities. but, of course, it must be remembered that he wrote in an age in which even the rudiments of science, as we now understand it, were almost entirely unknown. it may well be doubted whether any joy experienced by mortals is more genuine than that which rewards the successful searcher after natural truths. every science-worker, be his efforts ever so humble, will be able to sympathise with the enthusiastic delight of kepler when at last, after years of toil, the glorious light broke forth, and that which he considered to be the greatest of his astonishing laws first dawned upon him. kepler rightly judged that the number of days which a planet required to perform its voyage round the sun must be connected in some manner with the distance from the planet to the sun; that is to say, with the radius of the planet's orbit, inasmuch as we may for our present object regard the planet's orbit as circular. here, again, in his search for the unknown law, kepler had no accurate dynamical principles to guide his steps. of course, we now know not only what the connection between the planet's distance and the planet's periodic time actually is, but we also know that it is a necessary consequence of the law of universal gravitation. kepler, it is true, was not without certain surmises on the subject, but they were of the most fanciful description. his notions of the planets, accurate as they were in certain important respects, were mixed up with vague ideas as to the properties of metals and the geometrical relations of the regular solids. above all, his reasoning was penetrated by the supposed astrological influences of the stars and their significant relation to human fate. under the influence of such a farrago of notions, kepler resolved to make all sorts of trials in his search for the connection between the distance of a planet from the sun and the time in which the revolution of that planet was accomplished. it was quite easily demonstrated that the greater the distance of the planet from the sun the longer was the time required for its journey. it might have been thought that the time would be directly proportional to the distance. it was, however, easy to show that this supposition did not agree with the fact. finding that this simple relation would not do, kepler undertook a vast series of calculations to find out the true method of expressing the connection. at last, after many vain attempts, he found, to his indescribable joy, that the square of the time in which a planet revolves around the sun was proportional to the cube of the average distance of the planet from that body. the extraordinary way in which kepler's views on celestial matters were associated with the wildest speculations, is well illustrated in the work in which he propounded his splendid discovery just referred to. the announcement of the law connecting the distances of the planets from the sun with their periodic times, was then mixed up with a preposterous conception about the properties of the different planets. they were supposed to be associated with some profound music of the spheres inaudible to human ears, and performed only for the benefit of that being whose soul formed the animating spirit of the sun. kepler was also the first astronomer who ever ventured to predict the occurrence of that remarkable phenomenon, the transit of a planet in front of the sun's disc. he published, in , a notice to the curious in things celestial, in which he announced that both of the planets, mercury and venus, were to make a transit across the sun on specified days in the winter of . the transit of mercury was duly observed by gassendi, and the transit of venus also took place, though, as we now know, the circumstances were such that it was not possible for the phenomenon to be witnessed by any european astronomer. in addition to kepler's discoveries already mentioned, with which his name will be for ever associated, his claim on the gratitude of astronomers chiefly depends on the publication of his famous rudolphine tables. in this remarkable work means are provided for finding the places of the planets with far greater accuracy than had previously been attainable. kepler, it must be always remembered, was not an astronomical observer. it was his function to deal with the observations made by tycho, and, from close study and comparison of the results, to work out the movements of the heavenly bodies. it was, in fact, tycho who provided as it were the raw material, while it was the genius of kepler which wrought that material into a beautiful and serviceable form. for more than a century the rudolphine tables were regarded as a standard astronomical work. in these days we are accustomed to find the movements of the heavenly bodies set forth with all desirable exactitude in the nautical almanack, and the similar publication issued by foreign governments. let it be remembered that it was kepler who first imparted the proper impulse in this direction. [plate: the commemoration of the rudolphine tables.] when kepler was twenty-six he married an heiress from styria, who, though only twenty-three years old, had already had some experience in matrimony. her first husband had died; and it was after her second husband had divorced her that she received the addresses of kepler. it will not be surprising to hear that his domestic affairs do not appear to have been particularly happy, and his wife died in . two years later, undeterred by the want of success in his first venture, he sought a second partner, and he evidently determined not to make a mistake this time. indeed, the methodical manner in which he made his choice of the lady to whom he should propose has been duly set forth by him and preserved for our edification. with some self-assurance he asserts that there were no fewer than eleven spinsters desirous of sharing his joys and sorrows. he has carefully estimated and recorded the merits and demerits of each of these would-be brides. the result of his deliberations was that he awarded himself to an orphan girl, destitute even of a portion. success attended his choice, and his second marriage seems to have proved a much more suitable union than his first. he had five children by the first wife and seven by the second. the years of kepler's middle life were sorely distracted by a trouble which, though not uncommon in those days, is one which we find it difficult to realise at the present time. his mother, catherine kepler, had attained undesirable notoriety by the suspicion that she was guilty of witchcraft. years were spent in legal investigations, and it was only after unceasing exertions on the part of the astronomer for upwards of a twelve-month that he was finally able to procure her acquittal and release from prison. it is interesting for us to note that at one time there was a proposal that kepler should forsake his native country and adopt england as a home. it arose in this wise. the great man was distressed throughout the greater part of his life by pecuniary anxieties. finding him in a strait of this description, the english ambassador in venice, sir henry wotton, in the year , besought kepler to come over to england, where he assured him that he would obtain a favourable reception, and where, he was able to add, kepler's great scientific work was already highly esteemed. but his efforts were unavailing; kepler would not leave his own country. he was then forty-nine years of age, and doubtless a home in a foreign land, where people spoke a strange tongue, had not sufficient attraction for him, even when accompanied with the substantial inducements which the ambassador was able to offer. had kepler accepted this invitation, he would, in transferring his home to england, have anticipated the similar change which took place in the career of another great astronomer two centuries later. it will be remembered that herschel, in his younger days, did transfer himself to england, and thus gave to england the imperishable fame of association with his triumphs. the publication of the rudolphine tables of the celestial movements entailed much expense. a considerable part of this was defrayed by the government at venice but the balance occasioned no little trouble and anxiety to kepler. no doubt the authorities of those days were even less willing to spend money on scientific matters than are the governments of more recent times. for several years the imperial treasury was importuned to relieve him from his anxieties. the effects of so much worry, and of the long journeys which were involved, at last broke down kepler's health completely. as we have already mentioned, he had never been strong from infancy, and he finally succumbed to a fever in november, , at the age of fifty-nine. he was interred at st. peter's church at ratisbon. though kepler had not those personal characteristics which have made his great predecessor, tycho brahe, such a romantic figure, yet a picturesque element in kepler's character is not wanting. it was, however, of an intellectual kind. his imagination, as well as his reasoning faculties, always worked together. he was incessantly prompted by the most extraordinary speculations. the great majority of them were in a high degree wild and chimerical, but every now and then one of his fancies struck right to the heart of nature, and an immortal truth was brought to light. i remember visiting the observatory of one of our greatest modern astronomers, and in a large desk he showed me a multitude of photographs which he had attempted but which had not been successful, and then he showed me the few and rare pictures which had succeeded, and by which important truths had been revealed. with a felicity of expression which i have often since thought of, he alluded to the contents of the desk as the "chips." they were useless, but they were necessary incidents in the truly successful work. so it is in all great and good work. even the most skilful man of science pursues many a wrong scent. time after time he goes off on some track that plays him false. the greater the man's genius and intellectual resource, the more numerous will be the ventures which he makes, and the great majority of those ventures are certain to be fruitless. they are in fact, the "chips." in kepler's case the chips were numerous enough. they were of the most extraordinary variety and structure. but every now and then a sublime discovery was made of such a character as to make us regard even the most fantastic of kepler's chips with the greatest veneration and respect. isaac newton. it was just a year after the death of galileo, that an infant came into the world who was christened isaac newton. even the great fame of galileo himself must be relegated to a second place in comparison with that of the philosopher who first expounded the true theory of the universe. isaac newton was born on the th of december (old style), , at woolsthorpe, in lincolnshire, about a half-mile from colsterworth, and eight miles south of grantham. his father, mr. isaac newton, had died a few months after his marriage to harriet ayscough, the daughter of mr. james ayscough, of market overton, in rutlandshire. the little isaac was at first so excessively frail and weakly that his life was despaired of. the watchful mother, however, tended her delicate child with such success that he seems to have thriven better than might have been expected from the circumstances of his infancy, and he ultimately acquired a frame strong enough to outlast the ordinary span of human life. for three years they continued to live at woolsthorpe, the widow's means of livelihood being supplemented by the income from another small estate at sewstern, in a neighbouring part of leicestershire. [plate: woolsthorpe manor. showing solar dial made by newton when a boy.] in , mrs. newton took as a second husband the rev. barnabas smith, and on moving to her new home, about a mile from woolsthorpe, she entrusted little isaac to her mother, mrs. ayscough. in due time we find that the boy was sent to the public school at grantham, the name of the master being stokes. for the purpose of being near his work, the embryo philosopher was boarded at the house of mr. clark, an apothecary at grantham. we learn from newton himself that at first he had a very low place in the class lists of the school, and was by no means one of those model school-boys who find favour in the eyes of the school-master by attention to latin grammar. isaac's first incentive to diligent study seems to have been derived from the circumstance that he was severely kicked by one of the boys who was above him in the class. this indignity had the effect of stimulating young newton's activity to such an extent that he not only attained the desired object of passing over the head of the boy who had maltreated him, but continued to rise until he became the head of the school. the play-hours of the great philosopher were devoted to pursuits very different from those of most school-boys. his chief amusement was found in making mechanical toys and various ingenious contrivances. he watched day by day with great interest the workmen engaged in constructing a windmill in the neighbourhood of the school, the result of which was that the boy made a working model of the windmill and of its machinery, which seems to have been much admired, as indicating his aptitude for mechanics. we are told that isaac also indulged in somewhat higher flights of mechanical enterprise. he constructed a carriage, the wheels of which were to be driven by the hands of the occupant, while the first philosophical instrument he made was a clock, which was actuated by water. he also devoted much attention to the construction of paper kites, and his skill in this respect was highly appreciated by his school-fellows. like a true philosopher, even at this stage he experimented on the best methods of attaching the string, and on the proportions which the tail ought to have. he also made lanthorns of paper to provide himself with light as he walked to school in the dark winter mornings. the only love affair in newton's life appears to have commenced while he was still of tender years. the incidents are thus described in brewster's "life of newton," a work to which i am much indebted in this chapter. "in the house where he lodged there were some female inmates, in whose company he appears to have taken much pleasure. one of these, a miss storey, sister to dr. storey, a physician at buckminster, near colsterworth, was two or three years younger than newton and to great personal attractions she seems to have added more than the usual allotment of female talent. the society of this young lady and her companions was always preferred to that of his own school-fellows, and it was one of his most agreeable occupations to construct for them little tables and cupboards, and other utensils for holding their dolls and their trinkets. he had lived nearly six years in the same house with miss storey, and there is reason to believe that their youthful friendship gradually rose to a higher passion; but the smallness of her portion, and the inadequacy of his own fortune, appear to have prevented the consummation of their happiness. miss storey was afterwards twice married, and under the name of mrs. vincent, dr. stukeley visited her at grantham in , at the age of eighty-two, and obtained from her many particulars respecting the early history of our author. newton's esteem for her continued unabated during his life. he regularly visited her when he went to lincolnshire, and never failed to relieve her from little pecuniary difficulties which seem to have beset her family." the schoolboy at grantham was only fourteen years of age when his mother became a widow for the second time. she then returned to the old family home at woolsthorpe, bringing with her the three children of her second marriage. her means appear to have been somewhat scanty, and it was consequently thought necessary to recall isaac from the school. his recently-born industry had been such that he had already made good progress in his studies, and his mother hoped that he would now lay aside his books, and those silent meditations to which, even at this early age, he had become addicted. it was expected that, instead of such pursuits, which were deemed quite useless, the boy would enter busily into the duties of the farm and the details of a country life. but before long it became manifest that the study of nature and the pursuit of knowledge had such a fascination for the youth that he could give little attention to aught else. it was plain that he would make but an indifferent farmer. he greatly preferred experimenting on his water-wheels to looking after labourers, while he found that working at mathematics behind a hedge was much more interesting than chaffering about the price of bullocks in the market place. fortunately for humanity his mother, like a wise woman, determined to let her boy's genius have the scope which it required. he was accordingly sent back to grantham school, with the object of being trained in the knowledge which would fit him for entering the university of cambridge. [plate: trinity college, cambridge. showing newton's rooms; on the leads of the gateway he placed his telescope.] it was the th of june, , when isaac newton, a youth of eighteen, was enrolled as an undergraduate of trinity college, cambridge. little did those who sent him there dream that this boy was destined to be the most illustrious student who ever entered the portals of that great seat of learning. little could the youth himself have foreseen that the rooms near the gateway which he occupied would acquire a celebrity from the fact that he dwelt in them, or that the ante-chapel of his college was in good time to be adorned by that noble statue, which is regarded as one of the chief art treasures of cambridge university, both on account of its intrinsic beauty and the fact that it commemorates the fame of her most distinguished alumnus, isaac newton, the immortal astronomer. indeed, his advent at the university seemed to have been by no means auspicious or brilliant. his birth was, as we have seen, comparatively obscure, and though he had already given indication of his capacity for reflecting on philosophical matters, yet he seems to have been but ill-equipped with the routine knowledge which youths are generally expected to take with them to the universities. from the outset of his college career, newton's attention seems to have been mainly directed to mathematics. here he began to give evidence of that marvellous insight into the deep secrets of nature which more than a century later led so dispassionate a judge as laplace to pronounce newton's immortal work as pre-eminent above all the productions of the human intellect. but though newton was one of the very greatest mathematicians that ever lived, he was never a mathematician for the mere sake of mathematics. he employed his mathematics as an instrument for discovering the laws of nature. his industry and genius soon brought him under the notice of the university authorities. it is stated in the university records that he obtained a scholarship in . two years later we find that newton, as well as many residents in the university, had to leave cambridge temporarily on account of the breaking out of the plague. the philosopher retired for a season to his old home at woolsthorpe, and there he remained until he was appointed a fellow of trinity college, cambridge, in . from this time onwards, newton's reputation as a mathematician and as a natural philosopher steadily advanced, so that in , while still but twenty-seven years of age, he was appointed to the distinguished position of lucasian professor of mathematics at cambridge. here he found the opportunity to continue and develop that marvellous career of discovery which formed his life's work. the earliest of newton's great achievements in natural philosophy was his detection of the composite character of light. that a beam of ordinary sunlight is, in fact, a mixture of a very great number of different-coloured lights, is a doctrine now familiar to every one who has the slightest education in physical science. we must, however, remember that this discovery was really a tremendous advance in knowledge at the time when newton announced it. [plate: diagram of a sunbeam.] we here give the little diagram originally drawn by newton, to explain the experiment by which he first learned the composition of light. a sunbeam is admitted into a darkened room through an opening, h, in a shutter. this beam when not interfered with will travel in a straight line to the screen, and there reproduce a bright spot of the same shape as the hole in the shutter. if, however, a prism of glass, a b c, be introduced so that the beam traverse it, then it will be seen at once that the light is deflected from its original track. there is, however, a further and most important change which takes place. the spot of light is not alone removed to another part of the screen, but it becomes spread out into a long band beautifully coloured, and exhibiting the hues of the rainbow. at the top are the violet rays, and then in descending order we have the indigo, blue, green, yellow, orange, and red. the circumstance in this phenomenon which appears to have particularly arrested newton's attention, was the elongation which the luminous spot underwent in consequence of its passage through the prism. when the prism was absent the spot was nearly circular, but when the prism was introduced the spot was about five times as long as it was broad. to ascertain the explanation of this was the first problem to be solved. it seemed natural to suppose that it might be due to the thickness of the glass in the prism which the light traversed, or to the angle of incidence at which the light fell upon the prism. he found, however, upon careful trial, that the phenomenon could not be thus accounted for. it was not until after much patient labour that the true explanation dawned upon him. he discovered that though the beam of white light looks so pure and so simple, yet in reality it is composed of differently coloured lights blended together. these are, of course, indistinguishable in the compound beam, but they are separated or disentangled, so to speak, by the action of the prism. the rays at the blue end of the spectrum are more powerfully deflected by the action of the glass than are the rays at the red end. thus, the rays variously coloured red, orange, yellow, green, blue, indigo, violet, are each conducted to a different part of the screen. in this way the prism has the effect of exhibiting the constitution of the composite beam of light. to us this now seems quite obvious, but newton did not adopt it hastily. with characteristic caution he verified the explanation by many different experiments, all of which confirmed his discovery. one of these may be mentioned. he made a hole in the screen at that part on which the violet rays fell. thus a violet ray was allowed to pass through, all the rest of the light being intercepted, and on this beam so isolated he was able to try further experiments. for instance, when he interposed another prism in its path, he found, as he expected, that it was again deflected, and he measured the amount of the deflection. again he tried the same experiment with one of the red rays from the opposite end of the coloured band. he allowed it to pass through the same aperture in the screen, and he tested the amount by which the second prism was capable of producing deflection. he thus found, as he had expected to find, that the second prism was more efficacious in bending the violet rays than in bending the red rays. thus he confirmed the fact that the various hues of the rainbow were each bent by a prism to a different extent, violet being acted upon the most, and red the least. [plate: isaac newton.] not only did newton decompose a white beam into its constituent colours, but conversely by interposing a second prism with its angle turned upwards, he reunited the different colours, and thus reproduced the original beam of white light. in several other ways also he illustrated his famous proposition, which then seemed so startling, that white light was the result of a mixture of all hues of the rainbow. by combining painters' colours in the right proportion he did not indeed succeed in producing a mixture which would ordinarily be called white, but he obtained a grey pigment. some of this he put on the floor of his room for comparison with a piece of white paper. he allowed a beam of bright sunlight to fall upon the paper and the mixed colours side by side, and a friend he called in for his opinion pronounced that under these circumstances the mixed colours looked the whiter of the two. by repeated demonstrations newton thus established his great discovery of the composite character of light. he at once perceived that his researches had an important bearing upon the principles involved in the construction of a telescope. those who employed the telescope for looking at the stars, had been long aware of the imperfections which prevented all the various rays from being conducted to the same focus. but this imperfection had hitherto been erroneously accounted for. it had been supposed that the reason why success had not been attained in the construction of a refracting telescope was due to the fact that the object glass, made as it then was of a single piece, had not been properly shaped. mathematicians had abundantly demonstrated that a single lens, if properly figured, must conduct all rays of light to the same focus, provided all rays experienced equal refraction in passing through the glass. until newton's discovery of the composition of white light, it had been taken for granted that the several rays in a white beam were equally refrangible. no doubt if this had been the case, a perfect telescope could have been produced by properly shaping the object glass. but when newton had demonstrated that light was by no means so simple as had been supposed, it became obvious that a satisfactory refracting telescope was an impossibility when only a single object lens was employed, however carefully that lens might have been wrought. such an objective might, no doubt, be made to conduct any one group of rays of a particular shade to the same focus, but the rays of other colours in the beam of white light must necessarily travel somewhat astray. in this way newton accounted for a great part of the difficulties which had hitherto beset the attempts to construct a perfect refracting telescope. we now know how these difficulties can be, to a great extent, overcome, by employing for the objective a composite lens made of two pieces of glass possessing different qualities. to these achromatic object glasses, as they are called, the great development of astronomical knowledge, since newton's time, is due. but it must be remarked that, although the theoretical possibility of constructing an achromatic lens was investigated by newton, he certainly came to the conclusion that the difficulty could not be removed by employing a composite objective, with two different kinds of glass. in this his marvellous sagacity in the interpretation of nature seems for once to have deserted him. we can, however, hardly regret that newton failed to discover the achromatic objective, when we observe that it was in consequence of his deeming an achromatic objective to be impossible that he was led to the invention of the reflecting telescope. finding, as he believed, that the defects of the telescope could not be remedied by any application of the principle of refraction he was led to look in quite a different direction for the improvement of the tool on which the advancement of astronomy depended. the refraction of light depended as he had found, upon the colour of the light. the laws of reflection were, however, quite independent of the colour. whether rays be red or green, blue or yellow, they are all reflected in precisely the same manner from a mirror. accordingly, newton perceived that if he could construct a telescope the action of which depended upon reflection, instead of upon refraction, the difficulty which had hitherto proved an insuperable obstacle to the improvement of the instrument would be evaded. [plate: sir isaac newton's little reflector.] for this purpose newton fashioned a concave mirror from a mixture of copper and tin, a combination which gives a surface with almost the lustre of silver. when the light of a star fell upon the surface, an image of the star was produced in the focus of this mirror, and then this image was examined by a magnifying eye-piece. such is the principle of the famous reflecting telescope which bears the name of newton. the little reflector which he constructed, represented in the adjoining figure, is still preserved as one of the treasures of the royal society. the telescope tube had the very modest dimension of one inch in diameter. it was, however, the precursor of a whole series of magnificent instruments, each outstripping the other in magnitude, until at last the culminating point was attained in , by the construction of lord rosse's mammoth reflector of six feet in aperture. newton's discovery of the composition of light led to an embittered controversy, which caused no little worry to the great philosopher. some of those who attacked him enjoyed considerable and, it must be admitted, even well-merited repute in the ranks of science. they alleged, however, that the elongation of the coloured band which newton had noticed was due to this, to that, or to the other--to anything, in fact, rather than to the true cause which newton assigned. with characteristic patience and love of truth, newton steadily replied to each such attack. he showed most completely how utterly his adversaries had misunderstood the subject, and how slight indeed was their acquaintance with the natural phenomenon in question. in reply to each point raised, he was ever able to cite fresh experiments and adduce fresh illustrations, until at last his opponents retired worsted from the combat. it has been often a matter for surprise that newton, throughout his whole career, should have taken so much trouble to expose the errors of those who attacked his views. he used even to do this when it plainly appeared that his adversaries did not understand the subject they were discussing. a philosopher might have said, "i know i am right, and whether others think i am right or not may be a matter of concern to them, but it is certainly not a matter about which i need trouble. if after having been told the truth they elect to remain in error, so much the worse for them; my time can be better employed than in seeking to put such people right." this, however, was not newton's method. he spent much valuable time in overthrowing objections which were often of a very futile description. indeed, he suffered a great deal of annoyance from the persistency, and in some cases one might almost say from the rancour, of the attacks which were made upon him. unfortunately for himself, he did not possess that capacity for sublime indifference to what men may say, which is often the happy possession of intellects greatly inferior to his. the subject of optics still continuing to engross newton's attention, he followed up his researches into the structure of the sunbeam by many other valuable investigations in connection with light. every one has noticed the beautiful colours manifested in a soap-bubble. here was a subject which not unnaturally attracted the attention of one who had expounded the colours of the spectrum with such success. he perceived that similar hues were produced by other thin plates of transparent material besides soap-bubbles, and his ingenuity was sufficient to devise a method by which the thicknesses of the different films could be measured. we can hardly, indeed, say that a like success attended his interpretation of these phenomena to that which had been so conspicuous in his explanation of the spectrum. it implies no disparagement to the sublime genius of newton to admit that the doctrines he put forth as to the causes of the colours in the soap-bubbles can be no longer accepted. we must remember that newton was a pioneer in accounting for the physical properties of light. the facts that he established are indeed unquestionable, but the explanations which he was led to offer of some of them are seen to be untenable in the fuller light of our present knowledge. [plate: sir isaac newton's sun-dial.] had newton done nothing beyond making his wonderful discoveries in light, his fame would have gone down to posterity as one of the greatest of nature's interpreters. but it was reserved for him to accomplish other discoveries, which have pushed even his analysis of the sunbeam into the background; it is he who has expounded the system of the universe by the discovery of the law of universal gravitation. the age had indeed become ripe for the advent of the genius of newton. kepler had discovered with marvellous penetration the laws which govern the movements of the planets around the sun, and in various directions it had been more or less vaguely felt that the explanation of kepler's laws, as well as of many other phenomena, must be sought for in connection with the attractive power of matter. but the mathematical analysis which alone could deal with this subject was wanting; it had to be created by newton. at woolsthorpe, in the year , newton's attention appears to have been concentrated upon the subject of gravitation. whatever may be the extent to which we accept the more or less mythical story as to how the fall of an apple first directed the attention of the philosopher to the fact that gravitation must extend through space, it seems, at all events, certain that this is an excellent illustration of the line of reasoning which he followed. he argued in this way. the earth attracts the apple; it would do so, no matter how high might be the tree from which that apple fell. it would then seem to follow that this power which resides in the earth by which it can draw all external bodies towards it, extends far beyond the altitude of the loftiest tree. indeed, we seem to find no limit to it. at the greatest elevation that has ever been attained, the attractive power of the earth is still exerted, and though we cannot by any actual experiment reach an altitude more than a few miles above the earth, yet it is certain that gravitation would extend to elevations far greater. it is plain, thought newton, that an apple let fall from a point a hundred miles above this earth's surface, would be drawn down by the attraction, and would continually gather fresh velocity until it reached the ground. from a hundred miles it was natural to think of what would happen at a thousand miles, or at hundreds of thousands of miles. no doubt the intensity of the attraction becomes weaker with every increase in the altitude, but that action would still exist to some extent, however lofty might be the elevation which had been attained. it then occurred to newton, that though the moon is at a distance of two hundred and forty thousand miles from the earth, yet the attractive power of the earth must extend to the moon. he was particularly led to think of the moon in this connection, not only because the moon is so much closer to the earth than are any other celestial bodies, but also because the moon is an appendage to the earth, always revolving around it. the moon is certainly attracted to the earth, and yet the moon does not fall down; how is this to be accounted for? the explanation was to be found in the character of the moon's present motion. if the moon were left for a moment at rest, there can be no doubt that the attraction of the earth would begin to draw the lunar globe in towards our globe. in the course of a few days our satellite would come down on the earth with a most fearful crash. this catastrophe is averted by the circumstance that the moon has a movement of revolution around the earth. newton was able to calculate from the known laws of mechanics, which he had himself been mainly instrumental in discovering, what the attractive power of the earth must be, so that the moon shall move precisely as we find it to move. it then appeared that the very power which makes an apple fall at the earth's surface is the power which guides the moon in its orbit. [plate: sir isaac newton's telescope.] once this step had been taken, the whole scheme of the universe might almost be said to have become unrolled before the eye of the philosopher. it was natural to suppose that just as the moon was guided and controlled by the attraction of the earth, so the earth itself, in the course of its great annual progress, should be guided and controlled by the supreme attractive power of the sun. if this were so with regard to the earth, then it would be impossible to doubt that in the same way the movements of the planets could be explained to be consequences of solar attraction. it was at this point that the great laws of kepler became especially significant. kepler had shown how each of the planets revolves in an ellipse around the sun, which is situated on one of the foci. this discovery had been arrived at from the interpretation of observations. kepler had himself assigned no reason why the orbit of a planet should be an ellipse rather than any other of the infinite number of closed curves which might be traced around the sun. kepler had also shown, and here again he was merely deducing the results from observation, that when the movements of two planets were compared together, the squares of the periodic times in which each planet revolved were proportional to the cubes of their mean distances from the sun. this also kepler merely knew to be true as a fact, he gave no demonstration of the reason why nature should have adopted this particular relation between the distance and the periodic time rather than any other. then, too, there was the law by which kepler with unparalleled ingenuity, explained the way in which the velocity of a planet varies at the different points of its track, when he showed how the line drawn from the sun to the planet described equal areas around the sun in equal times. these were the materials with which newton set to work. he proposed to infer from these the actual laws regulating the force by which the sun guides the planets. here it was that his sublime mathematical genius came into play. step by step newton advanced until he had completely accounted for all the phenomena. in the first place, he showed that as the planet describes equal areas in equal times about the sun, the attractive force which the sun exerts upon it must necessarily be directed in a straight line towards the sun itself. he also demonstrated the converse truth, that whatever be the nature of the force which emanated from a sun, yet so long as that force was directed through the sun's centre, any body which revolved around it must describe equal areas in equal times, and this it must do, whatever be the actual character of the law according to which the intensity of the force varies at different parts of the planet's journey. thus the first advance was taken in the exposition of the scheme of the universe. the next step was to determine the law according to which the force thus proved to reside in the sun varied with the distance of the planet. newton presently showed by a most superb effort of mathematical reasoning, that if the orbit of a planet were an ellipse and if the sun were at one of the foci of that ellipse, the intensity of the attractive force must vary inversely as the square of the planet's distance. if the law had any other expression than the inverse square of the distance, then the orbit which the planet must follow would not be an ellipse; or if an ellipse, it would, at all events, not have the sun in the focus. hence he was able to show from kepler's laws alone that the force which guided the planets was an attractive power emanating from the sun, and that the intensity of this attractive power varied with the inverse square of the distance between the two bodies. these circumstances being known, it was then easy to show that the last of kepler's three laws must necessarily follow. if a number of planets were revolving around the sun, then supposing the materials of all these bodies were equally affected by gravitation, it can be demonstrated that the square of the periodic time in which each planet completes its orbit is proportional to the cube of the greatest diameter in that orbit. [plate: sir isaac newton's astrolabe.] these superb discoveries were, however, but the starting point from which newton entered on a series of researches, which disclosed many of the profoundest secrets in the scheme of celestial mechanics. his natural insight showed that not only large masses like the sun and the earth, and the moon, attract each other, but that every particle in the universe must attract every other particle with a force which varies inversely as the square of the distance between them. if, for example, the two particles were placed twice as far apart, then the intensity of the force which sought to bring them together would be reduced to one-fourth. if two particles, originally ten miles asunder, attracted each other with a certain force, then, when the distance was reduced to one mile, the intensity of the attraction between the two particles would be increased one-hundred-fold. this fertile principle extends throughout the whole of nature. in some cases, however, the calculation of its effect upon the actual problems of nature would be hardly possible, were it not for another discovery which newton's genius enabled him to accomplish. in the case of two globes like the earth and the moon, we must remember that we are dealing not with particles, but with two mighty masses of matter, each composed of innumerable myriads of particles. every particle in the earth does attract every particle in the moon with a force which varies inversely as the square of their distance. the calculation of such attractions is rendered feasible by the following principle. assuming that the earth consists of materials symmetrically arranged in shells of varying densities, we may then, in calculating its attraction, regard the whole mass of the globe as concentrated at its centre. similarly we may regard the moon as concentrated at the centre of its mass. in this way the earth and the moon can both be regarded as particles in point of size, each particle having, however, the entire mass of the corresponding globe. the attraction of one particle for another is a much more simple matter to investigate than the attraction of the myriad different points of the earth upon the myriad different points of the moon. many great discoveries now crowded in upon newton. he first of all gave the explanation of the tides that ebb and flow around our shores. even in the earliest times the tides had been shown to be related to the moon. it was noticed that the tides were specially high during full moon or during new moon, and this circumstance obviously pointed to the existence of some connection between the moon and these movements of the water, though as to what that connection was no one had any accurate conception until newton announced the law of gravitation. newton then made it plain that the rise and fall of the water was simply a consequence of the attractive power which the moon exerted upon the oceans lying upon our globe. he showed also that to a certain extent the sun produces tides, and he was able to explain how it was that when the sun and the moon both conspire, the joint result was to produce especially high tides, which we call "spring tides"; whereas if the solar tide was low, while the lunar tide was high, then we had the phenomenon of "neap" tides. but perhaps the most signal of newton's applications of the law of gravitation was connected with certain irregularities in the movements of the moon. in its orbit round the earth our satellite is, of course, mainly guided by the great attraction of our globe. if there were no other body in the universe, then the centre of the moon must necessarily perform an ellipse, and the centre of the earth would lie in the focus of that ellipse. nature, however, does not allow the movements to possess the simplicity which this arrangement would imply, for the sun is present as a source of disturbance. the sun attracts the moon, and the sun attracts the earth, but in different degrees, and the consequence is that the moon's movement with regard to the earth is seriously affected by the influence of the sun. it is not allowed to move exactly in an ellipse, nor is the earth exactly in the focus. how great was newton's achievement in the solution of this problem will be appreciated if we realise that he not only had to determine from the law of gravitation the nature of the disturbance of the moon, but he had actually to construct the mathematical tools by which alone such calculations could be effected. the resources of newton's genius seemed, however, to prove equal to almost any demand that could be made upon it. he saw that each planet must disturb the other, and in that way he was able to render a satisfactory account of certain phenomena which had perplexed all preceding investigators. that mysterious movement by which the pole of the earth sways about among the stars had been long an unsolved enigma, but newton showed that the moon grasped with its attraction the protuberant mass at the equatorial regions of the earth, and thus tilted the earth's axis in a way that accounted for the phenomenon which had been known but had never been explained for two thousand years. all these discoveries were brought together in that immortal work, newton's "principia." down to the year , when the "principia" was published, newton had lived the life of a recluse at cambridge, being entirely occupied with those transcendent researches to which we have referred. but in that year he issued from his seclusion under circumstances of considerable historical interest. king james the second attempted an invasion of the rights and privileges of the university of cambridge by issuing a command that father francis, a benedictine monk, should be received as a master of arts in the university, without having taken the oaths of allegiance and supremacy. with this arbitrary command the university sternly refused to comply. the vice-chancellor was accordingly summoned to answer for an act of contempt to the authority of the crown. newton was one of nine delegates who were chosen to defend the independence of the university before the high court. they were able to show that charles the second, who had issued a mandamus under somewhat similar circumstances, had been induced after due consideration to withdraw it. this argument appeared satisfactory, and the university gained their case. newton's next step in public life was his election, by a narrow majority, as member for the university, and during the years and , he seems to have attended to his parliamentary duties with considerable regularity. an incident which happened in was apparently the cause of considerable disturbance in newton's equanimity, if not in his health. he had gone to early morning chapel, leaving a lighted candle among his papers on his desk. tradition asserts that his little dog "diamond" upset the candle; at all events, when newton came back he found that many valuable papers had perished in a conflagration. the loss of these manuscripts seems to have had a serious effect. indeed, it has been asserted that the distress reduced newton to a state of mental aberration for a considerable time. this has, apparently, not been confirmed, but there is no doubt that he experienced considerable disquiet, for in writing on september th, , to mr. pepys, he says: "i am extremely troubled at the embroilment i am in, and have neither ate nor slept well this twelve-month, nor have my former consistency of mind." notwithstanding the fame which newton had achieved, by the publication of his, "principia," and by all his researches, the state had not as yet taken any notice whatever of the most illustrious man of science that this or any other country has ever produced. many of his friends had exerted themselves to procure him some permanent appointment, but without success. it happened, however, that mr. montagu, who had sat with newton in parliament, was appointed chancellor of the exchequer in . ambitious of distinction in his new office, mr. montagu addressed himself to the improvement of the current coin, which was then in a very debased condition. it fortunately happened that an opportunity occurred of appointing a new official in the mint; and mr. montagu on the th of march, , wrote to offer mr. newton the position of warden. the salary was to be five or six hundred a year, and the business would not require more attendance than newton could spare. the lucasian professor accepted this post, and forthwith entered upon his new duties. the knowledge of physics which newton had acquired by his experiments was of much use in connection with his duties at the mint. he carried out the re-coinage with great skill in the course of two years, and as a reward for his exertions, he was appointed, in , to the mastership of the mint, with a salary between , pounds and , pounds per annum. in , his duties at the mint being so engrossing, he resigned his lucasian professorship at cambridge, and at the same time he had to surrender his fellowship at trinity college. this closed his connection with the university of cambridge. it should, however, be remarked that at a somewhat earlier stage in his career he was very nearly being appointed to an office which might have enabled the university to retain the great philosopher within its precincts. some of his friends had almost succeeded in securing his nomination to the provostship of king's college, cambridge; the appointment, however, fell through, inasmuch as the statute could not be evaded, which required that the provost of king's college should be in holy orders. in those days it was often the custom for illustrious mathematicians, when they had discovered a solution for some new and striking problem, to publish that problem as a challenge to the world, while withholding their own solution. a famous instance of this is found in what is known as the brachistochrone problem, which was solved by john bernouilli. the nature of this problem may be mentioned. it was to find the shape of the curve along which a body would slide down from one point (a) to another point (b) in the shortest time. it might at first be thought that the straight line from a to b, as it is undoubtedly the shortest distance between the points, would also be the path of quickest descent; but this is not so. there is a curved line, down which a bead, let us say, would run on a smooth wire from a to b in a shorter time than the same bead would require to run down the straight wire. bernouilli's problem was to find out what that curve must be. newton solved it correctly; he showed that the curve was a part of what is termed a cycloid--that is to say, a curve like that which is described by a point on the rim of a carriage-wheel as the wheel runs along the ground. such was newton's geometrical insight that he was able to transmit a solution of the problem on the day after he had received it, to the president of the royal society. in newton, whose world wide fame was now established, was elected president of the royal society. year after year he was re-elected to this distinguished position, and his tenure, which lasted twenty-five years, only terminated with his life. it was in discharge of his duties as president of the royal society that newton was brought into contact with prince george of denmark. in april, , the queen paid a visit to cambridge as the guest of dr. bentley, the then master of trinity, and in a court held at trinity lodge on april th, , the honour of knighthood was conferred upon the discoverer of gravitation. urged by illustrious friends, who sought the promotion of knowledge, newton gave his attention to the publication of a new edition of the "principia." his duties at the mint, however, added to the supreme duty of carrying on his original investigations, left him but little time for the more ordinary task of the revision. he was accordingly induced to associate with himself for this purpose a distinguished young mathematician, roger coates, a fellow of trinity college, cambridge, who had recently been appointed plumian professor of astronomy. on july th, , newton, by this time a favourite at court, waited on the queen, and presented her with a copy of the new edition of the "principia." throughout his life newton appears to have been greatly interested in theological studies, and he specially devoted his attention to the subject of prophecy. he left behind him a manuscript on the prophecies of daniel and the apocalypse of st. john, and he also wrote various theological papers. many other subjects had from time to time engaged his attention. he studied the laws of heat; he experimented in pursuit of the dreams of the alchymist; while the philosopher who had revealed the mechanism of the heavens found occasional relaxation in trying to interpret hieroglyphics. in the last few years of his life he bore with fortitude a painful ailment, and on monday, march th, , he died in the eighty-fifth year of his age. on tuesday, march th, he was buried in westminster abbey. though newton lived long enough to receive the honour that his astonishing discoveries so justly merited, and though for many years of his life his renown was much greater than that of any of his contemporaries, yet it is not too much to say that, in the years which have since elapsed, newton's fame has been ever steadily advancing, so that it never stood higher than it does at this moment. we hardly know whether to admire more the sublime discoveries at which he arrived, or the extraordinary character of the intellectual processes by which those discoveries were reached. viewed from either standpoint, newton's "principia" is incomparably the greatest work on science that has ever yet been produced. [plate: sir isaac newton's sun-dial in the royal society.] flamsteed. among the manuscripts preserved at greenwich observatory are certain documents in which flamsteed gives an account of his own life. we may commence our sketch by quoting the following passage from this autobiography:--"to keep myself from idleness, and to recreate myself, i have intended here to give some account of my life, in my youth, before the actions thereof, and the providences of god therein, be too far passed out of my memory; and to observe the accidents of all my years, and inclinations of my mind, that whosoever may light upon these papers may see i was not so wholly taken up, either with my father's business or my mathematics, but that i both admitted and found time for other as weighty considerations." the chief interest which attaches to the name of flamsteed arises from the fact that he was the first of the illustrious series of astronomers royal who have presided over greenwich observatory. in that capacity flamsteed was able to render material assistance to newton by providing him with the observations which his lunar theory required. john flamsteed was born at denby, in derbyshire, on the th of august, . his mother died when he was three years old, and the second wife, whom his father took three years later, only lived until flamsteed was eight, there being also two younger sisters. in his boyhood the future astronomer tells us that he was very fond of those romances which affect boy's imagination, but as he writes, "at twelve years of age i left all the wild ones and betook myself to read the better sort of them, which, though they were not probable, yet carried no seeming impossibility in the picturing." by the time flamsteed was fifteen years old he had embarked in still more serious work, for he had read plutarch's "lives," tacitus' "roman history," and many other books of a similar description. in he became ill with some serious rheumatic affection, which obliged him to be withdrawn from school. it was then for the first time that he received the rudiments of a scientific education. he had, however, attained his sixteenth year before he made any progress in arithmetic. he tells us how his father taught him "the doctrine of fractions," and "the golden rule of three"--lessons which he seemed to have learned easily and quickly. one of the books which he read at this time directed his attention to astronomical instruments, and he was thus led to construct for himself a quadrant, by which he could take some simple astronomical observations. he further calculated a table to give the sun's altitudes at different hours, and thus displayed those tastes for practical astronomy which he lived to develop so greatly. it appears that these scientific studies were discountenanced by his father, who designed that his son should follow a business career. flamsteed's natural inclination, however, forced him to prosecute astronomical work, notwithstanding the impediments that lay in his path. unfortunately, his constitutional delicacy seems to have increased, and he had just completed his eighteenth year, "when," to use his own words, "the winter came on and thrust me again into the chimney, whence the heat and the dryness of the preceding summer had happily once before withdrawn me. but, it not being a fit season for physic, it was thought fit to let me alone this winter, and try the skill of another physician on me in the spring." it appears that at this time a quack named valentine greatrackes, was reputed to have effected most astonishing cures in ireland merely by the stroke of his hands, without the application of any medicine whatever. flamsteed's father, despairing of any remedy for his son from the legitimate branch of the profession, despatched him to ireland on august th, , he being then, as recorded with astronomical accuracy, "nineteen years, six days, and eleven hours old." the young astronomer, accompanied by a friend, arrived on a tuesday at liverpool but the wind not being favourable, they remained there till the following friday, when a shift of the wind to the east took place. they embarked accordingly on a vessel called the supply at noon, and on saturday night came in sight of dublin. ere they could land, however, they were nearly being wrecked on lambay island. this peril safely passed, there was a long delay for quarantine before they were at last allowed on shore. on thursday, september th, they set out from dublin, where they had been sojourning at the "ship" hotel, in dame street, towards assaune, where greatrackes received his patients. [plate: flamsteed's house.] flamsteed gives an interesting account of his travels in ireland. they dined at naas on the first day, and on september th they reached carlow, a town which is described as one of the fairest they saw on their journey. by sunday morning, september th, having lost their way several times, they reached castleton, called commonly four mile waters. flamsteed inquired of the host in the inn where they might find a church, but was told that the minister lived twelve miles away, and that they had no sermon except when he came to receive his tithes once a year, and a woman added that "they had plenty enough of everything necessary except the word of god." the travellers accordingly went on to cappoquin, which lies up the river blackwater, on the road to lismore, eight miles from youghal. thence they immediately started on foot to assaune. about a mile from cappoquin, and entering into the house of mr. greatrackes, they saw him touch several patients, "whereof some were nearly cured, others were on the mending hand, and some on whom his strokes had no effect." flamsteed was touched by the famous quack on the afternoon of september th, but we are hardly surprised to hear his remark that "he found not his disease to stir." next morning the astronomer came again to see mr. greatrackes, who had "a kind of majestical yet affable presence, and a composed carriage." even after the third touching had been submitted to, no benefit seems to have been derived. we must, however record, to the credit of mr. greatrackes, that he refused to accept any payment from flamsteed, because he was a stranger. finding it useless to protract his stay any longer, flamsteed and his friend set out on their return to dublin. in the course of his journey he seems to have been much impressed with clonmel, which he describes as an "exceedingly pleasantly seated town." but in those days a journey to ireland was so serious an enterprise that when flamsteed did arrive safely back at derby after an absence of a month, he adds, "for god's providence in this journey, his name be praised, amen." as to the expected benefits to his health from the expedition we may quote his own words: "in the winter following i was indifferent hearty, and my disease was not so violent as it used to be at that time formerly. but whether through god's mercy i received this through mr. greatrackes' touch, or my journey and vomiting at sea, i am uncertain; but, by some circumstances, i guess that i received a benefit from both." it is evident that by this time flamsteed's interest in all astronomical matters had greatly increased. he studied the construction of sun-dials, he formed a catalogue of seventy of the fixed stars, with their places on the heavens, and he computed the circumstances of the solar eclipse which was to happen on june nd, . it is interesting to note that even in those days the doctrines of the astrologers still found a considerable degree of credence, and flamsteed spent a good deal of his time in astrological studies and computations. he investigated the methods of casting a nativity, but a suspicion, or, indeed, rather more than a suspicion, seems to have crossed his mind as to the value of these astrological predictions, for he says in fine, "i found astrology to give generally strong conjectural hints, not perfect declarations." all this time, however, the future astronomer royal was steadily advancing in astronomical inquiries of a recondite nature. he had investigated the obliquity of the ecliptic with extreme care, so far as the circumstances of astronomical observation would at that time permit. he had also sought to discover the sun's distance from the earth in so far as it could be obtained by determining when the moon was exactly half illuminated, and he had measured, with much accuracy, the length of the tropical year. it will thus be seen that, even at the age of twenty, flamsteed had made marked progress, considering how much his time had been interfered with by ill-health. other branches of astronomy began also to claim his attention. we learn that in and he compared the planets jupiter and mars with certain fixed stars near which they passed. his instrumental means, though very imperfect, were still sufficient to enable him to measure the intervals on the celestial sphere between the planets and the stars. as the places of the stars were known, flamsteed was thus able to obtain the places of the planets. this is substantially the way in which astronomers of the present day still proceed when they desire to determine the places of the planets, inasmuch as, directly or indirectly those places are always obtained relatively to the fixed stars. by his observations at this early period, flamsteed was, it is true, not able to obtain any great degree of accuracy; he succeeded, however, in proving that the tables by which the places of the planets were ordinarily given were not to be relied upon. [plate: flamsteed.] flamsteed's labours in astronomy and in the allied branches of science were now becoming generally known, and he gradually came to correspond with many distinguished men of learning. one of the first occasions which brought the talents of the young astronomer into fame was the publication of some calculations concerning certain astronomical phenomena which were to happen in the year . in the monthly revolution of the moon its disc passes over those stars which lie along its track. the disappearance of a star by the interposition of the moon is called an "occultation." owing to the fact that our satellite is comparatively near us, the position which the moon appears to occupy on the heavens varies from different parts of the earth, it consequently happens that a star which would be occulted to an observer in one locality, would often not be occulted to an observer who was situated elsewhere. even when an occultation is visible from both places, the times at which the star disappears from view will, generally speaking, be different. much calculation is therefore necessary to decide the circumstances under which the occultations of stars may be visible from any particular station. having a taste for such computations, flamsteed calculated the occultations which were to happen in the year , it being the case that several remarkable stars would be passed over by the moon during this year. of course at the present time, we find such information duly set forth in the nautical almanac, but a couple of centuries ago there was no such source of astronomical knowledge as is now to be found in that invaluable publication, which astronomers and navigators know so well. flamsteed accordingly sent the results of his work to the president of the royal society. the paper which contained them was received very favourably, and at once brought flamsteed into notice among the most eminent members of that illustrious body, one of whom, mr. collins, became through life his faithful friend and constant correspondent. flamsteed's father was naturally gratified with the remarkable notice which his son was receiving from the great and learned; accordingly he desired him to go to london, that he might make the personal acquaintance of those scientific friends whom he had only known by correspondence previously. flamsteed was indeed glad to avail himself of this opportunity. thus he became acquainted with dr. barrow, and especially with newton, who was then lucasian professor of mathematics at cambridge. it seems to have been in consequence of this visit to london that flamsteed entered himself as a member of jesus college, cambridge. we have but little information as to his university career, but at all events he took his degree of m.a. on june th, . up to this time it would seem that flamsteed had been engaged, to a certain extent, in the business carried on by his father. it is true that he does not give any explicit details, yet there are frequent references to journeys which he had to take on business matters. but the time now approached when flamsteed was to start on an independent career, and it appears that he took his degree in cambridge with the object of entering into holy orders, so that he might settle in a small living near derby, which was in the gift of a friend of his father, and would be at the disposal of the young astronomer. this scheme was, however, not carried out, but flamsteed does not tell us why it failed, his only remark being, that "the good providence of god that had designed me for another station ordered it otherwise." sir jonas moore, one of the influential friends whom flamsteed's talents had attracted, seems to have procured for him the position of king's astronomer, with a salary of pounds per annum. a larger salary appears to have been designed at first for this office, which was now being newly created, but as flamsteed was resolved on taking holy orders, a lesser salary was in his case deemed sufficient. the building of the observatory, in which the first astronomer royal was to be installed, seems to have been brought about, or, at all events, its progress was accelerated, in a somewhat curious manner. a frenchman, named le sieur de s. pierre, came over to london to promulgate a scheme for discovering longitudes, then a question of much importance. he brought with him introductions to distinguished people, and his mission attracted a great deal of attention. the proposals which he made came under flamsteed's notice, who pointed out that the frenchman's projects were quite inapplicable in the present state of astronomical science, inasmuch as the places of the stars were not known with the degree of accuracy which would be necessary if such methods were to be rendered available. flamsteed then goes on to say:--"i heard no more of the frenchman after this; but was told that my letters had been shown king charles. he was startled at the assertion of the fixed stars' places being false in the catalogue, and said, with some vehemence, he must have them anew observed, examined, and corrected, for the use of his seamen." the first question to be settled was the site for the new observatory. hyde park and chelsea college were both mentioned as suitable localities, but, at sir christopher wren's suggestion, greenwich hill was finally resolved upon. the king made a grant of five hundred pounds of money. he gave bricks from tilbury fort, while materials, in the shape of wood, iron, and lead, were available from a gatehouse demolished in the tower. the king also promised whatever further material aid might be shown to be necessary. the first stone of the royal observatory was laid on august th, , and within a few years a building was erected in which the art of modern practical astronomy was to be created. flamsteed strove with extraordinary diligence, and in spite of many difficulties, to obtain a due provision of astronomical instruments, and to arrange for the carrying on of his observations. notwithstanding the king's promises, the astronomer was, however, but scantily provided with means, and he had no assistants to help him in his work. it follows that all the observations, as well as the reductions, and, indeed, all the incidental work of the observatory, had to be carried on by himself alone. flamsteed, as we have seen, had, however, many staunch friends. sir jonas moore in particular at all times rendered him most valuable assistance, and encouraged him by the warm sympathy and keen interest which he showed in astronomy. the work of the first astronomer royal was frequently interrupted by recurrent attacks of the complaints to which we have already referred. he says himself that "his distempers stick so close that that he cannot remove them," and he lost much time by prostration from headaches, as well as from more serious affections. the year found him in the full tide of work in his observatory. he was specially engaged on the problem of the earth's motion, which he sought to derive from observations of the sun and of venus. but this, as well as many other astronomical researches which he undertook, were only subsidiary to that which he made the main task of his life, namely, the formation of a catalogue of fixed stars. at the time when flamsteed commenced his career, the only available catalogue of fixed stars was that of tycho brahe. this work had been published at the commencement of the seventeenth century, and it contained about a thousand stars. the positions assigned to these stars, though obtained with wonderful skill, considering the many difficulties under which tycho laboured, were quite inaccurate when judged by our modern standards. tycho's instruments were necessarily most rudely divided, and he had, of course, no telescopes to aid him. consequently it was merely by a process of sighting that he could obtain the places of the stars. it must further be remembered that tycho had no clocks, and no micrometers. he had, indeed, but little correct knowledge of the motions of the heavenly bodies to guide him. to determine the longitudes of a few principal stars he conceived the ingenious idea of measuring by day the position of venus with respect to the sun, an observation which the exceptional brightness of this planet rendered possible without telescopic aid, and then by night he observed the position of venus with regard to the stars. it has been well remarked by mr. baily, in his introduction to the "british catalogue of stars," that "flamsteed's observations, by a fortunate combination of circumstances, commenced a new and a brilliant era. it happened that, at that period, the powerful mind of newton was directed to this subject; a friendly intercourse then existed between these two distinguished characters; and thus the first observations that could lay any claim to accuracy were at once brought in aid of those deep researches in which our illustrious geometer was then engaged. the first edition of the 'principia' bears testimony to the assistance afforded by flamsteed to newton in these inquiries; although the former considers that the acknowledgment is not so ample as it ought to have been." although flamsteed's observations can hardly be said to possess the accuracy of those made in more recent times, when instruments so much superior to his have been available, yet they possess an interest of a special kind from their very antiquity. this circumstance renders them of particular importance to the astronomer, inasmuch as they are calculated to throw light on the proper motions of the stars. flamsteed's work may, indeed, be regarded as the origin of all subsequent catalogues, and the nomenclature which he adopted, though in some respects it can hardly be said to be very defensible, is, nevertheless, that which has been adopted by all subsequent astronomers. there were also a great many errors, as might be expected in a work of such extent, composed almost entirely of numerical detail. many of these errors have been corrected by baily himself, the assiduous editor of "flamsteed's life and works," for flamsteed was so harassed from various causes in the latter part of his life, and was so subject to infirmities all through his career, that he was unable to revise his computations with the care that would have been necessary. indeed, he observed many additional stars which he never included in the british catalogue. it is, as baily well remarks, "rather a matter of astonishment that he accomplished so much, considering his slender means, his weak frame, and the vexations which he constantly experienced." flamsteed had the misfortune, in the latter part of his life, to become estranged from his most eminent scientific contemporaries. he had supplied newton with places of the moon, at the urgent solicitation of the author of the "principia," in order that the lunar theory should be carefully compared with observation. but flamsteed appears to have thought that in newton's further request for similar information, he appeared to be demanding as a right that which flamsteed considered he was only called upon to render as a favour. a considerable dispute grew out of this matter, and there are many letters and documents, bearing on the difficulties which subsequently arose, that are not, perhaps, very creditable to either party. notwithstanding his feeble constitution, flamsteed lived to the age of seventy-three, his death occurring on the last day of the year . halley. isaac newton was just fourteen years of age when the birth of edmund halley, who was destined in after years to become newton's warmly attached friend, and one of his most illustrious scientific contemporaries, took place. there can be little doubt that the fame as an astronomer which halley ultimately acquired, great as it certainly was, would have been even greater still had it not been somewhat impaired by the misfortune that he had to shine in the same sky as that which was illumined by the unparalleled genius of newton. edmund halley was born at haggerston, in the parish of st. leonard's, shoreditch, on october th, . his father, who bore the same name as his famous son, was a soap-boiler in winchester street, london, and he had conducted his business with such success that he accumulated an ample fortune. i have been unable to obtain more than a very few particulars with respect to the early life of the future astronomer. it would, however, appear that from boyhood he showed considerable aptitude for the acquisition of various kinds of learning, and he also had some capacity for mechanical invention. halley seems to have received a sound education at st. paul's school, then under the care of dr. thomas gale. here, the young philosopher rapidly distanced his competitors in the various branches of ordinary school instruction. his superiority was, however, most conspicuous in mathematical studies, and, as a natural development of such tastes, we learn that by the time he had left school he had already made good progress in astronomy. at the age of seventeen he was entered as a commoner at queen's college, oxford, and the reputation that he brought with him to the university may be inferred from the remark of the writer of "athenae oxonienses," that halley came to oxford "with skill in latin, greek, and hebrew, and such a knowledge of geometry as to make a complete dial." though his studies were thus of a somewhat multifarious nature, yet it is plain that from the first his most favourite pursuit was astronomy. his earliest efforts in practical observation were connected with an eclipse which he observed from his father's house in winchester street. it also appears that he had studied theoretical branches of astronomy so far as to be conversant with the application of mathematics to somewhat abstruse problems. up to the time of kepler, philosophers had assumed almost as an axiom that the heavenly bodies must revolve in circles and that the motion of the planet around the orbit which it described must be uniform. we have already seen how that great philosopher, after very persevering labour, succeeded in proving that the orbits of the planets were not circles, but that they were ellipses of small eccentricity. kepler was, however, unable to shake himself free from the prevailing notion that the angular motion of the planet ought to be of a uniform character around some point. he had indeed proved that the motion round the focus of the ellipse in which the sun lies is not of this description. one of his most important discoveries even related to the fact that at some parts of its orbit a planet swings around the sun with greater angular velocity than at others. but it so happens that in elliptic tracks which differ but little from circles, as is the case with all the more important planetary orbits, the motion round the empty focus of the ellipse is very nearly uniform. it seemed natural to assume, that this was exactly the case, in which event each of the two foci of the ellipse would have had a special significance in relation to the movement of the planet. the youthful halley, however, demonstrated that so far as the empty focus was concerned, the movement of the planet around it, though so nearly uniform, was still not exactly so, and at the age of nineteen, he published a treatise on the subject which at once placed him in the foremost rank amongst theoretical astronomers. but halley had no intention of being merely an astronomer with his pen. he longed to engage in the practical work of observing. he saw that the progress of exact astronomy must depend largely on the determination of the positions of the stars with all attainable accuracy. he accordingly determined to take up this branch of work, which had been so successfully initiated by tycho brahe. at the present day, astronomers of the great national observatories are assiduously engaged in the determination of the places of the stars. a knowledge of the exact positions of these bodies is indeed of the most fundamental importance, not alone for the purposes of scientific astronomy, but also for navigation and for extensive operations of surveying in which accuracy is desired. the fact that halley determined to concentrate himself on this work shows clearly the scientific acumen of the young astronomer. halley, however, found that hevelius, at dantzig, and flamsteed, the astronomer royal at greenwich, were both engaged on work of this character. he accordingly determined to direct his energies in a way that he thought would be more useful to science. he resigned to the two astronomers whom i have named the investigation of the stars in the northern hemisphere, and he sought for himself a field hitherto almost entirely unworked. he determined to go to the southern hemisphere, there to measure and survey those stars which were invisible in europe, so that his work should supplement the labours of the northern astronomers, and that the joint result of his labours and of theirs might be a complete survey of the most important stars on the surface of the heavens. in these days, after so many ardent students everywhere have devoted themselves to the study of nature, it seems difficult for a beginner to find a virgin territory in which to commence his explorations. halley may, however, be said to have enjoyed the privilege of commencing to work in a magnificent region, the contents of which were previously almost entirely unknown. indeed none of the stars which were so situated as to have been invisible from tycho brahe's observatory at uraniborg, in denmark, could be said to have been properly observed. there was, no doubt, a rumour that a dutchman had observed southern stars from the island of sumatra, and certain stars were indicated in the southern heavens on a celestial globe. on examination, however, halley found that no reliance could be placed on the results which had been obtained, so that practically the field before him may be said to have been unworked. at the age of twenty, without having even waited to take that degree at the university which the authorities would have been glad to confer on so promising an undergraduate, this ardent student of nature sought his father's permission to go to the southern hemisphere for the purpose of studying the stars which lie around the southern pole. his father possessed the necessary means, and he had likewise the sagacity to encourage the young astronomer. he was indeed most anxious to make everything as easy as possible for so hopeful a son. he provided him with an allowance of pounds a year, which was regarded as a very munificent provision in those days. halley was also furnished with letters of recommendation from king charles ii., as well as from the directors of the east india company. he accordingly set sail with his instruments in the year , in one of the east india company's ships, for the island of st. helena, which he had selected as the scene of his labours. [plate: halley.] after an uneventful voyage of three months, the astronomer landed on st. helena, with his sextant of five and a half feet radius, and a telescope feet long, and forthwith plunged with ardour into his investigation of the southern skies. he met, however, with one very considerable disappointment. the climate of this island had been represented to him as most favourable for astronomical observation; but instead of the pure blue skies he had been led to expect, he found that they were almost always more or less clouded, and that rain was frequent, so that his observations were very much interrupted. on this account he only remained at st. helena for a single year, having, during that time, and in spite of many difficulties, accomplished a piece of work which earned for him the title of "our southern tycho." thus did halley establish his fame as an astronomer on the same lonely rock in mid-atlantic, which nearly a century and a-half later became the scene of napoleon's imprisonment, when his star, in which he believed so firmly, had irretrievably set. on his return to england, halley prepared a map which showed the result of his labours, and he presented it to the king, in . like his great predecessor tycho, halley did not altogether disdain the arts of the courtier, for he endeavoured to squeeze a new constellation into the group around the southern pole which he styled "the royal oak," adding a description to the effect that the incidents of which "the royal oak" was a symbol were of sufficient importance to be inscribed on the surface of the heavens. there is reason to think that charles ii. duly appreciated the scientific renown which one of his subjects had achieved, and it was probably through the influence of the king that halley was made a master of arts at oxford on november th, . special reference was made on the occasion to his observations at st. helena, as evidence of unusual attainments in mathematics and astronomy. this degree was no small honour to such a young man, who, as we have seen, quitted his university before he had the opportunity of graduating in the ordinary manner. on november th, in the same year, the astronomer received a further distinction in being elected a fellow of the royal society. from this time forward he took a most active part in the affairs of the society, and the numerous papers which he read before it form a very valuable part of that notable series of volumes known as the "philosophical transactions." he was subsequently elected to the important office of secretary to the royal society, and he discharged the duties of his post until his appointment to greenwich necessitated his resignation. within a year of halley's election as a fellow of the royal society, he was chosen by the society to represent them in a discussion which had arisen with hevelius. the nature of this discussion, or rather the fact that any discussion should have been necessary, may seem strange to modern astronomers, for the point is one on which it would now seem impossible for there to be any difference of opinion. we must, however, remember that the days of halley were, comparatively speaking, the days of infancy as regards the art of astronomical observation, and issues that now seem obvious were often, in those early times, the occasions of grave and anxious consideration. the particular question on which halley had to represent the royal society may be simply stated. when tycho brahe made his memorable investigations into the places of the stars, he had no telescopes to help him. the famous instruments at uraniborg were merely provided with sights, by which the telescope was pointed to a star on the same principle as a rifle is sighted for a target. shortly after tycho's time, galileo invented the telescope. of course every one admitted at once the extraordinary advantages which the telescope had to offer, so far as the mere question of the visibility of objects was concerned. but the bearing of galileo's invention upon what we may describe as the measuring part of astronomy was not so immediately obvious. if a star be visible to the unaided eye, we can determine its place by such instruments as those which tycho used, in which no telescope is employed. we can, however, also avail ourselves of an instrument in which we view the star not directly but through the intervention of the telescope. can the place of the star be determined more accurately by the latter method than it can when the telescope is dispensed with? with our present knowledge, of course, there is no doubt about the answer; every one conversant with instruments knows that we can determine the place of a star far more accurately with the telescope than is possible by any mere sighting apparatus. in fact an observer would be as likely to make an error of a minute with the sighting apparatus in tycho's instrument, as he would be to make an error of a second with the modern telescope, or, to express the matter somewhat differently, we may say, speaking quite generally, that the telescopic method of determining the places of the stars does not lead to errors more than one-sixtieth part as great as which are unavoidable when we make use of tycho's method. but though this is so apparent to the modern astronomer, it was not at all apparent in the days of halley, and accordingly he was sent off to discuss the question with the continental astronomers. hevelius, as the representative of the older method, which tycho had employed with such success, maintained that an instrument could be pointed more accurately at a star by the use of sights than by the use of a telescope, and vigorously disputed the claims put forward by those who believed that the latter method was the more suitable. on may th, , halley started for dantzig, and the energetic character of the man may be judged from the fact that on the very night of his arrival he commenced to make the necessary observations. in those days astronomical telescopes had only obtained a fractional part of the perfection possessed by the instruments in our modern observatories, and therefore it may not be surprising that the results of the trial were not immediately conclusive. halley appears to have devoted much time to the investigation; indeed, he remained at dantzig for more than a twelve-month. on his return to england, he spoke highly of the skill which hevelius exhibited in the use of his antiquated methods, but halley was nevertheless too sagacious an observer to be shaken in his preference for the telescopic method of observation. the next year we find our young astronomer starting for a continental tour, and we, who complain if the channel passage lasts more than an hour or two, may note halley's remark in writing to hooke on june th, : "having fallen in with bad weather we took forty hours in the journey from dover to calais." the scientific distinction which he had already attained was such that he was received in paris with marked attention. a great deal of his time seems to have been passed in the paris observatory, where cassini, the presiding genius, himself an astronomer of well-deserved repute, had extended a hearty welcome to his english visitor. they made observations together of the place of the splendid comet which was then attracting universal attention, and halley found the work thus done of much use when he subsequently came to investigate the path pursued by this body. halley was wise enough to spare no pains to derive all possible advantages from his intercourse with the distinguished savants of the french capital. in the further progress of his tour he visited the principal cities of the continent, leaving behind him everywhere the memory of an amiable disposition and of a rare intelligence. after halley's return to england, in , he married a young lady named mary tooke, with whom he lived happily, till her death fifty-five years later. on his marriage, he took up his abode in islington, where he erected his instruments and recommenced his observations. it has often been the good fortune of astronomers to render practical services to humanity by their investigations, and halley's achievements in this respect deserve to be noted. a few years after he had settled in england, he published an important paper on the variation of the magnetic compass, for so the departure of the needle from the true north is termed. this subject had indeed early engaged his attention, and he continued to feel much interest in it up to the end of his life. with respect to his labours in this direction, sir john herschel says: "to halley we owe the first appreciation of the real complexity of the subject of magnetism. it is wonderful indeed, and a striking proof of the penetration and sagacity of this extraordinary man, that with his means of information he should have been able to draw such conclusions, and to take so large and comprehensive a view of the subject as he appears to have done." in , halley explained his theory of terrestrial magnetism, and begged captains of ships to take observations of the variations of the compass in all parts of the world, and to communicate them to the royal society, "in order that all the facts may be readily available to those who are hereafter to complete this difficult and complicated subject." the extent to which halley was in advance of his contemporaries, in the study of terrestrial magnetism, may be judged from the fact that the subject was scarcely touched after his time till the year . the interest which he felt in it was not of a merely theoretical kind, nor was it one which could be cultivated in an easy-chair. like all true investigators, he longed to submit his theory to the test of experiment, and for that purpose halley determined to observe the magnetic variation for himself. he procured from king william iii. the command of a vessel called the "paramour pink," with which he started for the south seas in . this particular enterprise was not, however, successful; for, on crossing the line, some of his men fell sick and one of his lieutenants mutinied, so that he was obliged to return the following year with his mission unaccomplished. the government cashiered the lieutenant, and halley having procured a second smaller vessel to accompany the "paramour pink," started once more in september, . he traversed the atlantic to the nd degree of southern latitude, beyond which his further advance was stopped. "in these latitudes," he writes to say, "we fell in with great islands of ice of so incredible height and magnitude, that i scarce dare write my thoughts of it." on his return in , halley published a general chart, showing the variation of the compass at the different places which he had visited. on these charts he set down lines connecting those localities at which the magnetic variation was identical. he thus set an example of the graphic representation of large masses of complex facts, in such a manner as to appeal at once to the eye, a method of which we make many applications in the present day. but probably the greatest service which halley ever rendered to human knowledge was the share in which he took in bringing newton's "principia" before the world. in fact, as dr. glaisher, writing in , has truly remarked, "but for halley the 'principia' would not have existed." it was a visit from halley in the year which seems to have first suggested to newton the idea of publishing the results of his investigations on gravitation. halley, and other scientific contemporaries, had no doubt some faint glimmering of the great truth which only newton's genius was able fully to reveal. halley had indeed shown how, on the assumptions that the planets move in circular orbits round the sun, and that the squares of their periodic times are proportional to the cubes of their mean distances, it may be proved that the force acting on each planet must vary inversely as the square of its distance from the sun. since, however, each of the planets actually moves in an ellipse, and therefore, at continually varying distances from the sun, it becomes a much more difficult matter to account mathematically for the body's motions on the supposition that the attractive force varies inversely as the square of the distance. this was the question with which halley found himself confronted, but which his mathematical abilities were not adequate to solve. it would seem that both hooke and sir christopher wren were interested in the same problem; in fact, the former claimed to have arrived at a solution, but declined to make known his results, giving as an excuse his desire that others having tried and failed might learn to value his achievements all the more. halley, however, confessed that his attempts at the solution were unsuccessful, and wren, in order to encourage the other two philosophers to pursue the inquiry, offered to present a book of forty shillings value to either of them who should in the space of two months bring him a convincing proof of it. such was the value which sir christopher set on the law of gravitation, upon which the whole fabric of modern astronomy may be said to stand. finding himself unequal to the task, halley went down to cambridge to see newton on the subject, and was delighted to learn that the great mathematician had already completed the investigation. he showed halley that the motions of all the planets could be completely accounted for on the hypothesis of a force of attraction directed towards the sun, which varies inversely as the square of the distance from that body. halley had the genius to perceive the tremendous importance of newton's researches, and he ceased not to urge upon the recluse man of science the necessity for giving his new discoveries publication. he paid another visit to cambridge with the object of learning more with regard to the mathematical methods which had already conducted newton to such sublime truths, and he again encouraged the latter both to pursue his investigations, and to give some account of them to the world. in december of the same year halley had the gratification of announcing to the royal society that newton had promised to send that body a paper containing his researches on gravitation. it seems that at this epoch the finances of the royal society were at a very low ebb. this impecuniosity was due to the fact that a book by willoughby, entitled "de historia piscium," had been recently printed by the society at great expense. in fact, the coffers were so low that they had some difficulty in paying the salaries of their permanent officials. it appears that the public did not care about the history of fishes, or at all events the volume did not meet with the ready demand which was expected for it. indeed, it has been recorded that when halley had undertaken to measure the length of a degree of the earth's surface, at the request of the royal society, it was ordered that his expenses be defrayed either in pounds sterling, or in fifty books of fishes. thus it happened that on june nd, the council, after due consideration of ways and means in connection with the issue of the principia, "ordered that halley should undertake the business of looking after the book and printing it at his own charge," which he engaged to do. it was, as we have elsewhere mentioned, characteristic of newton that he detested controversies, and he was, in fact, inclined to suppress the third book of the "principia" altogether rather than have any conflict with hooke with respect to the discoveries there enunciated. he also thought of changing the name of the work to de motu corporum libri duo, but upon second thoughts, he retained the original title, remarking, as he wrote to halley, "it will help the sale of the book, which i ought not to diminish, now it is yours," a sentence which shows conclusively, if further proof were necessary, that halley had assumed the responsibility of its publication. halley spared no pains in pushing forward the publication of his illustrious friend's great work, so that in the same year he was in a position to present a complete copy to king james ii., with a proper discourse of his own. halley also wrote a set of latin hexameters in praise of newton's genius, which he printed at the beginning of the work. the last line of this specimen of halley's poetic muse may be thus rendered: "nor mortals nearer may approach the gods." the intimate friendship between the two greatest astronomers of the time continued without interruption till the death of newton. it has, indeed, been alleged that some serious cause of estrangement arose between them. there is, however, no satisfactory ground for this statement; indeed, it may be regarded as effectually disposed of by the fact that, in the year , halley took up the defence of his friend, and wrote two learned papers in support of newton's "system of chronology," which had been seriously attacked by a certain ecclesiastic. it is quite evident to any one who has studied these papers that halley's friendship for newton was as ardent as ever. the generous zeal with which halley adopted and defended the doctrines of newton with regard to the movements of the celestial bodies was presently rewarded by a brilliant discovery, which has more than any of his other researches rendered his name a familiar one to astronomers. newton, having explained the movement of the planets, was naturally led to turn his attention to comets. he perceived that their journeyings could be completely accounted for as consequences of the attraction of the sun, and he laid down the principles by which the orbit of a comet could be determined, provided that observations of its positions were obtained at three different dates. the importance of these principles was by no one more quickly recognised than by halley, who saw at once that it provided the means of detecting something like order in the movements of these strange wanderers. the doctrine of gravitation seemed to show that just as the planets revolved around the sun in ellipses, so also must the comets. the orbit, however, in the case of the comet, is so extremely elongated that the very small part of the elliptic path within which the comet is both near enough and bright enough to be seen from the earth, is indistinguishable from a parabola. applying these principles, halley thought it would be instructive to study the movements of certain bright comets, concerning which reliable observations could be obtained. at the expense of much labour, he laid down the paths pursued by twenty-four of these bodies, which had appeared between the years and . amongst them he noticed three, which followed tracks so closely resembling each other, that he was led to conclude the so called three comets could only have been three different appearances of the same body. the first of these occurred in , the second was seen by kepler in , and the third by halley himself in . these dates suggested that the observed phenomena might be due to the successive returns of one and the same comet after intervals of seventy-five or seventy-six years. on the further examination of ancient records, halley found that a comet had been seen in the year , a date, it will be observed, seventy-five years before . another had been observed seventy-six years earlier than , viz., in , and another seventy-five years before that, in . as halley thus found that a comet had been recorded on several occasions at intervals of seventy-five or seventy-six years, he was led to the conclusion that these several apparitions related to one and the same object, which was an obedient vassal of the sun, performing an eccentric journey round that luminary in a period of seventy-five or seventy-six years. to realise the importance of this discovery, it should be remembered that before halley's time a comet, if not regarded merely as a sign of divine displeasure, or as an omen of intending disaster, had at least been regarded as a chance visitor to the solar system, arriving no one knew whence, and going no one knew whither. a supreme test remained to be applied to halley's theory. the question arose as to the date at which this comet would be seen again. we must observe that the question was complicated by the fact that the body, in the course of its voyage around the sun, was exposed to the incessant disturbing action produced by the attraction of the several planets. the comet therefore, does not describe a simple ellipse as it would do if the attraction of the sun were the only force by which its movement were controlled. each of the planets solicits the comet to depart from its track, and though the amount of these attractions may be insignificant in comparison with the supreme controlling force of the sun, yet the departure from the ellipse is quite sufficient to produce appreciable irregularities in the comet's movement. at the time when halley lived, no means existed of calculating with precision the effect of the disturbance a comet might experience from the action of the different planets. halley exhibited his usual astronomical sagacity in deciding that jupiter would retard the return of the comet to some extent. had it not been for this disturbance the comet would apparently have been due in or early in . but the attraction of the great planet would cause delay, so that halley assigned, for the date of its re-appearance, either the end of or the beginning of . halley knew that he could not himself live to witness the fulfilment of his prediction, but he says: "if it should return, according to our predictions, about the year , impartial posterity will not refuse to acknowledge that this was first discovered by an englishman." this was, indeed, a remarkable prediction of an event to occur fifty-three years after it had been uttered. the way in which it was fulfilled forms one of the most striking episodes in the history of astronomy. the comet was first seen on christmas day, , and passed through its nearest point to the sun on march th, . halley had then been lying in his grave for seventeen years, yet the verification of his prophecy reflects a glory on his name which will cause it to live for ever in the annals of astronomy. the comet paid a subsequent visit in , and its next appearance is due about . halley next entered upon a labour which, if less striking to the imagination than his discoveries with regard to comets, is still of inestimable value in astronomy. he undertook a series of investigations with the object of improving our knowledge of the movements of the planets. this task was practically finished in , though the results of it were not published until after his death in . in the course of it he was led to investigate closely the motion of venus, and thus he came to recognise for the first time the peculiar importance which attaches to the phenomenon of the transit of this planet across the sun. halley saw that the transit, which was to take place in the year , would afford a favourable opportunity for determining the distance of the sun, and thus learning the scale of the solar system. he predicted the circumstances of the phenomenon with an astonishing degree of accuracy, considering his means of information, and it is unquestionably to the exertions of halley in urging the importance of the matter upon astronomers that we owe the unexampled degree of interest taken in the event, and the energy which scientific men exhibited in observing it. the illustrious astronomer had no hope of being himself a witness of the event, for it could not happen till many years after his death. this did not, however, diminish his anxiety to impress upon those who would then be alive, the importance of the occurrence, nor did it lead him to neglect anything which might contribute to the success of the observations. as we now know, halley rather over-estimated the value of the transit of venus, as a means of determining the solar distance. the fact is that the circumstances are such that the observation of the time of contact between the edge of the planet and the edge of the sun cannot be made with the accuracy which he had expected. in , halley became a candidate for the savilian professorship of astronomy at oxford. he was not, however, successful, for his candidature was opposed by flamsteed, the astronomer royal of the time, and another was appointed. he received some consolation for this particular disappointment by the fact that, in , owing to newton's friendly influence, he was appointed deputy controller of the mint at chester, an office which he did not retain for long, as it was abolished two years later. at last, in , he received what he had before vainly sought, and he was appointed to the savilian chair. his observations of the eclipse of the sun, which occurred in , added greatly to halley's reputation. this phenomenon excited special attention, inasmuch as it was the first total eclipse of the sun which had been visible in london since the year . halley undertook the necessary calculations, and predicted the various circumstances with a far higher degree of precision than the official announcement. he himself observed the phenomenon from the royal society's rooms, and he minutely describes the outer atmosphere of the sun, now known as the corona; without, however, offering an opinion as to whether it was a solar or a lunar appendage. at last halley was called to the dignified office which he of all men was most competent to fill. on february th, , he was appointed astronomer royal in succession to flamsteed. he found things at the royal observatory in a most unsatisfactory state. indeed, there were no instruments, nor anything else that was movable; for such things, being the property of flamsteed, had been removed by his widow, and though halley attempted to purchase from that lady some of the instruments which his predecessor had employed, the unhappy personal differences which had existed between him and flamsteed, and which, as we have already seen, prevented his election as savilian professor of astronomy, proved a bar to the negotiation. greenwich observatory wore a very different appearance in those days, from that which the modern visitor, who is fortunate enough to gain admission, may now behold. not only did halley find it bereft of instruments, we learn besides that he had no assistants, and was obliged to transact the whole business of the establishment single-handed. in , however, he obtained a grant of pounds from the board of ordnance, and accordingly a transit instrument was erected in the same year. some time afterwards he procured an eight-foot quadrant, and with these instruments, at the age of sixty-four, he commenced a series of observations on the moon. he intended, if his life was spared, to continue his observations for a period of eighteen years, this being, as astronomers know, a very important cycle in connection with lunar movements. the special object of this vast undertaking was to improve the theory of the moon's motion, so that it might serve more accurately to determine longitudes at sea. this self-imposed task halley lived to carry to a successful termination, and the tables deduced from his observations, and published after his death, were adopted almost universally by astronomers, those of the french nation being the only exception. throughout his life halley had been singularly free from illness of every kind, but in he had a stroke of paralysis. notwithstanding this, however, he worked diligently at his telescope till , after which his health began rapidly to give way. he died on january th, , in the eighty-sixth year of his age, retaining his mental faculties to the end. he was buried in the cemetery of the church of lee in kent, in the same grave as his wife, who had died five years previously. we are informed by admiral smyth that pond, a later astronomer royal, was afterwards laid in the same tomb. halley's disposition seems to have been generous and candid, and wholly free from anything like jealousy or rancour. in person he was rather above the middle height, and slight in build; his complexion was fair, and he is said to have always spoken, as well as acted, with uncommon sprightliness. in the eloge pronounced upon him at the paris academie des sciences, of which halley had been made a member in it was said, "he possessed all the qualifications which were necessary to please princes who were desirous of instruction, with a great extent of knowledge and a constant presence of mind; his answers were ready, and at the same time pertinent, judicious, polite and sincere." [plate: greenwich observatory in halley's time.] thus we find that peter the great was one of his most ardent admirers. he consulted the astronomer on matters connected with shipbuilding, and invited him to his own table. but halley possessed nobler qualifications than the capacity of pleasing princes. he was able to excite and to retain the love and admiration of his equals. this was due to the warmth of his attachments, the unselfishness of his devotion to his friends, and to a vein of gaiety and good-humour which pervaded all his conversation. bradley. james bradley was descended from an ancient family in the county of durham. he was born in or , at sherbourne, in gloucestershire, and was educated in the grammar school at northleach. from thence he proceeded in due course to oxford, where he was admitted a commoner at balliol college, on march th, . much of his time, while an undergraduate, was passed in essex with his maternal uncle, the rev. james pound, who was a well-known man of science and a diligent observer of the stars. it was doubtless by intercourse with his uncle that young bradley became so expert in the use of astronomical instruments, but the immortal discoveries he subsequently made show him to have been a born astronomer. the first exhibition of bradley's practical skill seems to be contained in two observations which he made in and . they have been published by halley, whose acuteness had led him to perceive the extraordinary scientific talents of the young astronomer. another illustration of the sagacity which bradley manifested, even at the very commencement of his astronomical career, is contained in a remark of halley's, who says: "dr. pound and his nephew, mr. bradley, did, myself being present, in the last opposition of the sun and mars this way demonstrate the extreme minuteness of the sun's parallax, and that it was not more than twelve seconds nor less than nine seconds." to make the significance of this plain, it should be observed that the determination of the sun's parallax is equivalent to the determination of the distance from the earth to the sun. at the time of which we are now writing, this very important unit of celestial measurement was only very imperfectly known, and the observations of pound and bradley may be interpreted to mean that, from their observations, they had come to the conclusion that the distance from the earth to the sun must be more than millions of miles, and less than millions. we now, of course, know that they were not exactly right, for the true distance of the sun is about millions of miles. we cannot, however, but think that it was a very remarkable approach for the veteran astronomer and his brilliant nephew to make towards the determination of a magnitude which did not become accurately known till fifty years later. among the earliest parts of astronomical work to which bradley's attention was directed, were the eclipses of jupiter's satellites. these phenomena are specially attractive inasmuch as they can be so readily observed, and bradley found it extremely interesting to calculate the times at which the eclipses should take place, and then to compare his observations with the predicted times. from the success that he met with in this work, and from his other labours, bradley's reputation as an astronomer increased so greatly that on november the th, , he was elected a fellow of the royal society. up to this time the astronomical investigations of bradley had been more those of an amateur than of a professional astronomer, and as it did not at first seem likely that scientific work would lead to any permanent provision, it became necessary for the youthful astronomer to choose a profession. it had been all along intended that he should enter the church, though for some reason which is not told us, he did not take orders as soon as his age would have entitled him to do so. in , however, the bishop of hereford offered bradley the vicarage of bridstow, near ross, in monmouthshire, and on july th, , he having then taken priest's orders, was duly instituted in his vicarage. in the beginning of the next year, bradley had some addition to his income from the proceeds of a welsh living, which, being a sinecure, he was able to hold with his appointment at bridstow. it appears, however, that his clerical occupations were not very exacting in their demands upon his time, for he was still able to pay long and often-repeated visits to his uncle at wandsworth, who, being himself a clergyman, seems to have received occasional assistance in his ministerial duties from his astronomical nephew. the time, however, soon arrived when bradley was able to make a choice between continuing to exercise his profession as a divine, or devoting himself to a scientific career. the savilian professorship of astronomy in the university of oxford became vacant by the death of dr. john keill. the statutes forbade that the savilian professor should also hold a clerical appointment, and mr. pound would certainly have been elected to the professorship had he consented to surrender his preferments in the church. but pound was unwilling to sacrifice his clerical position, and though two or three other candidates appeared in the field, yet the talents of bradley were so conspicuous that he was duly elected, his willingness to resign the clerical profession having been first ascertained. there can be no doubt that, with such influential friends as bradley possessed, he would have made great advances had he adhered to his profession as a divine. bishop hoadly, indeed, with other marks of favour, had already made the astronomer his chaplain. the engrossing nature of bradley's interest in astronomy decided him, however, to sacrifice all other prospects in comparison with the opening afforded by the savilian professorship. it was not that bradley found himself devoid of interest in clerical matters, but he felt that the true scope for such abilities as he possessed would be better found in the discharge of the scientific duties of the oxford chair than in the spiritual charge of a parish. on april the th, , bradley read his inaugural lecture in that new position on which he was destined to confer such lustre. it must, of course, be remembered that in those early days the art of constructing the astronomical telescope was very imperfectly understood. the only known method for getting over the peculiar difficulties presented in the construction of the refracting telescope, was to have it of the most portentous length. in fact, bradley made several of his observations with an instrument of two hundred and twelve feet focus. in such a case, no tube could be used, and the object glass was merely fixed at the top of a high pole. notwithstanding the inconvenience and awkwardness of such an instrument, bradley by its means succeeded in making many careful measurements. he observed, for example, the transit of mercury over the sun's disc, on october th, ; he also observed the dimensions of the planet venus, while a comet which halley discovered on october the th, , was assiduously observed at wanstead up to the middle of the ensuing month. the first of bradley's remarkable contributions to the "philosophical transactions" relates to this comet, and the extraordinary amount of work that he went through in connection therewith may be seen from an examination of his book of calculations which is still extant. the time was now approaching when bradley was to make the first of those two great discoveries by which his name has acquired a lustre that has placed him in the very foremost rank of astronomical discoverers. as has been often the case in the history of science, the first of these great successes was attained while he was pursuing a research intended for a wholly different purpose. it had long been recognised that as the earth describes a vast orbit, nearly two hundred million miles in diameter, in its annual journey round the sun, the apparent places of the stars should alter, to some extent, in correspondence with the changes in the earth's position. the nearer the star the greater the shift in its apparent place on the heavens, which must arise from the fact that it was seen from different positions in the earth's orbit. it had been pointed out that these apparent changes in the places of the stars, due to the movement of the earth, would provide the means of measuring the distances of the stars. as, however, these distances are enormously great in comparison with the orbit which the earth describes around the sun, the attempt to determine the distances of the stars by the shift in their positions had hitherto proved ineffectual. bradley determined to enter on this research once again; he thought that by using instruments of greater power, and by making measurements of increased delicacy, he would be able to perceive and to measure displacements which had proved so small as to elude the skill of the other astronomers who had previously made efforts in the same direction. in order to simplify the investigation as much as possible, bradley devoted his attention to one particular star, beta draconis, which happened to pass near his zenith. the object of choosing a star in this position was to avoid the difficulties which would be introduced by refraction had the star occupied any other place in the heavens than that directly overhead. we are still able to identify the very spot on which the telescope stood which was used in this memorable research. it was erected at the house then occupied by molyneux, on the western extremity of kew green. the focal length was feet inches, and the eye-glass was and a half feet above the ground floor. the instrument was first set up on november th, . if there had been any appreciable disturbance in the place of beta draconis in consequence of the movement of the earth around the sun, the star must appear to have the smallest latitude when in conjunction with the sun, and the greatest when in opposition. the star passed the meridian at noon in december, and its position was particularly noticed by molyneux on the third of that month. any perceptible displacement by parallax--for so the apparent change in position, due to the earth's motion, is called--would would have made the star shift towards the north. bradley, however, when observing it on the th, was surprised to find that the apparent place of the star, so far from shifting towards the north, as they had perhaps hoped it would, was found to lie a little more to the south than when it was observed before. he took extreme care to be sure that there was no mistake in his observation, and, true astronomer as he was, he scrutinized with the utmost minuteness all the circumstances of the adjustment of his instruments. still the star went to the south, and it continued so advancing in the same direction until the following march, by which time it had moved no less than twenty seconds south from the place which it occupied when the first observation was made. after a brief pause, in which no apparent movement was perceptible, the star by the middle of april appeared to be returning to the north. early in june it reached the same distance from the zenith which it had in december. by september the star was as much as thirty-nine seconds more to the north than it had been in march, then it returned towards the south, regaining in december the same situation which it had occupied twelve months before. this movement of the star being directly opposite to the movements which would have been the consequence of parallax, seemed to show that even if the star had any parallax its effects upon the apparent place were entirely masked by a much larger motion of a totally different description. various attempts were made to account for the phenomenon, but they were not successful. bradley accordingly determined to investigate the whole subject in a more thorough manner. one of his objects was to try whether the same movements which he had observed in one star were in any similar degree possessed by other stars. for this purpose he set up a new instrument at wanstead, and there he commenced a most diligent scrutiny of the apparent places of several stars which passed at different distances from the zenith. he found in the course of this research that other stars exhibited movements of a similar description to those which had already proved so perplexing. for a long time the cause of these apparent movements seemed a mystery. at last, however, the explanation of these remarkable phenomena dawned upon him, and his great discovery was made. one day when bradley was out sailing he happened to remark that every time the boat was laid on a different tack the vane at the top of the boat's mast shifted a little, as if there had been a slight change in the direction of the wind. after he had noticed this three or four times he made a remark to the sailors to the effect that it was very strange the wind should always happen to change just at the moment when the boat was going about. the sailors, however, said there had been no change in the wind, but that the alteration in the vane was due to the fact that the boat's course had been altered. in fact, the position of the vane was determined both by the course of the boat and the direction of the wind, and if either of these were altered there would be a corresponding change in the direction of the vane. this meant, of course, that the observer in the boat which was moving along would feel the wind coming from a point different from that in which the wind appeared to be blowing when the boat was at rest, or when it was sailing in some different direction. bradley's sagacity saw in this observation the clue to the difficulty which had so long troubled him. it had been discovered before the time of bradley that the passage of light through space is not an instantaneous phenomenon. light requires time for its journey. galileo surmised that the sun may have reached the horizon before we see it there, and it was indeed sufficiently obvious that a physical action, like the transmission of light, could hardly take place without requiring some lapse of time. the speed with which light actually travelled was, however, so rapid that its determination eluded all the means of experimenting which were available in those days. the penetration of roemer had previously detected irregularities in the observed times of the eclipses of jupiter's satellites, which were undoubtedly due to the interval which light required for stretching across the interplanetary spaces. bradley argued that as light can only travel with a certain speed, it may in a measure be regarded like the wind, which he noticed in the boat. if the observer were at rest, that is to say, if the earth were a stationary object, the direction in which the light actually does come would be different from that in which it appears to come when the earth is in motion. it is true that the earth travels but eighteen miles a second, while the velocity with which light is borne along attains to as much as , miles a second. the velocity of light is thus ten thousand times greater than the speed of the earth. but even though the wind blew ten thousand times faster than the speed with which the boat was sailing there would still be some change, though no doubt a very small change, in the position of the vane when the boat was in progress from the position it would have if the boat were at rest. it therefore occurred to this most acute of astronomers that when the telescope was pointed towards a star so as to place it apparently in the centre of the field of view, yet it was not generally the true position of the star. it was not, in fact, the position in which the star would have been observed had the earth been at rest. provided with this suggestion, he explained the apparent movements of the stars by the principle known as the "aberration of light." every circumstance was accounted for as a consequence of the relative movements of the earth and of the light from the star. this beautiful discovery not only established in the most forcible manner the nature of the movement of light; not only did it illustrate the truth of the copernican theory which asserted that the earth revolved around the sun, but it was also of the utmost importance in the improvement of practical astronomy. every observer now knows that, generally speaking, the position which the star appears to have is not exactly the position in which the star does actually lie. the observer is, however, able, by the application of the principles which bradley so clearly laid down, to apply to an observation the correction which is necessary to obtain from it the true place in which the object is actually situated. this memorable achievement at once conferred on bradley the highest astronomical fame. he tested his discovery in every way, but only to confirm its truth in the most complete manner. halley, the astronomer royal, died on the th, january, , and bradley was immediately pointed out as his successor. he was accordingly appointed astronomer royal in february, . on first taking up his abode at greenwich he was unable to conduct his observations owing to the wretched condition in which he found the instruments. he devoted himself, however, assiduously to their repair, and his first transit observation is recorded on the th july, . he worked with such energy that on one day it appears that transit observations were taken by himself alone, and in september, , he had completed the series of observations which established his second great discovery, the nutation of the earth's axis. the way in which he was led to the detection of the nutation is strikingly illustrative of the extreme care with which bradley conducted his observations. he found that in the course of a twelve-month, when the star had completed the movement which was due to aberration, it did not return exactly to the same position which it had previously occupied. at first he thought this must be due to some instrumental error, but after closer examination and repeated study of the effect as manifested by many different stars, he came to the conclusion that its origin must be sought in some quite different source. the fact is that a certain change takes place in the apparent position of the stars which is not due to the movement of the star itself, but is rather to be attributed to changes in the points from which the star's positions are measured. we may explain the matter in this way. as the earth is not a sphere, but has protuberant parts at the equator, the attraction of the moon exercises on those protuberant parts a pulling effect which continually changes the direction of the earth's axis, and consequently the position of the pole must be in a state of incessant fluctuation. the pole to which the earth's axis points on the sky is, therefore, slowly changing. at present it happens to lie near the pole star, but it will not always remain there. it describes a circle around the pole of the ecliptic, requiring about , years for a complete circuit. in the course of its progress the pole will gradually pass now near one star and now near another, so that many stars will in the lapse of ages discharge the various functions which the present pole star does for us. in about , years, for instance, the pole will have come near the bright star, vega. this movement of the pole had been known for ages. but what bradley discovered was that the pole, instead of describing an uniform movement as had been previously supposed, followed a sinuous course now on one side and now on the other of its mean place. this he traced to the fluctuations of the moon's orbit, which undergoes a continuous change in a period of nineteen years. thus the efficiency with which the moon acts on the protuberant mass of the earth varies, and thus the pole is caused to oscillate. this subtle discovery, if perhaps in some ways less impressive than bradley's earlier achievements of the detection of the aberration of light, is regarded by astronomers as testifying even in a higher degree to his astonishing care and skill as an observer, and justly entitles him to a unique place among the astronomers whose discoveries have been effected by consummate practical skill in the use of astronomical instruments. of bradley's private or domestic life there is but little to tell. in , soon after he became astronomer royal, he married a daughter of samuel peach, of chalford, in gloucestershire. there was but one child, a daughter, who became the wife of her cousin, rev. samuel peach, rector of compton, beauchamp, in berkshire. bradley's last two years of life were clouded by a melancholy depression of spirits, due to an apprehension that he should survive his rational faculties. it seems, however, that the ill he dreaded never came upon him, for he retained his mental powers to the close. he died on th july, , aged seventy, and was buried at michinghamton. william herschel. william herschel, one of the greatest astronomers that has ever lived, was born at hanover, on the th november, . his father, isaac herschel, was a man evidently of considerable ability, whose life was devoted to the study and practice of music, by which he earned a somewhat precarious maintenance. he had but few worldly goods to leave to his children, but he more than compensated for this by bequeathing to them a splendid inheritance of genius. touches of genius were, indeed, liberally scattered among the members of isaac's large family, and in the case of his forth child, william, and of a sister several years younger, it was united with that determined perseverance and rigid adherence to principle which enabled genius to fulfil its perfect work. a faithful chronicler has given us an interesting account of the way in which isaac herschel educated his sons; the narrative is taken from the recollections of one who, at the time we are speaking of, was an unnoticed little girl five or six years old. she writes:-- "my brothers were often introduced as solo performers and assistants in the orchestra at the court, and i remember that i was frequently prevented from going to sleep by the lively criticisms on music on coming from a concert. often i would keep myself awake that i might listen to their animating remarks, for it made me so happy to see them so happy. but generally their conversation would branch out on philosophical subjects, when my brother william and my father often argued with such warmth that my mother's interference became necessary, when the names--euler, leibnitz, and newton--sounded rather too loud for the repose of her little ones, who had to be at school by seven in the morning." the child whose reminiscences are here given became afterwards the famous caroline herschel. the narrative of her life, by mrs. john herschel, is a most interesting book, not only for the account it contains of the remarkable woman herself, but also because it provides the best picture we have of the great astronomer to whom caroline devoted her life. this modest family circle was, in a measure, dispersed at the outbreak of the seven years' war in . the french proceeded to invade hanover, which, it will be remembered, belonged at this time to the british dominions. young william herschel had already obtained the position of a regular performer in the regimental band of the hanoverian guards, and it was his fortune to obtain some experience of actual warfare in the disastrous battle of hastenbeck. he was not wounded, but he had to spend the night after the battle in a ditch, and his meditations on the occasion convinced him that soldiering was not the profession exactly adapted to his tastes. we need not attempt to conceal the fact that he left his regiment by the very simple but somewhat risky process of desertion. he had, it would seem, to adopt disguises to effect his escape. at all events, by some means he succeeded in eluding detection and reached england in safety. it is interesting to have learned on good authority that many years after this offence was committed it was solemnly forgiven. when herschel had become the famous astronomer, and as such visited king george at windsor, the king at their first meeting handed to him his pardon for deserting from the army, written out in due form by his majesty himself. it seems that the young musician must have had some difficulty in providing for his maintenance during the first few years of his abode in england. it was not until he had reached the age of twenty-two that he succeeded in obtaining any regular appointment. he was then made instructor of music to the durham militia. shortly afterwards, his talents being more widely recognised, he was appointed as organist at the parish church at halifax, and his prospects in life now being fairly favourable, and the seven years' war being over, he ventured to pay a visit to hanover to see his father. we can imagine the delight with which old isaac herschel welcomed his promising son, as well as his parental pride when a concert was given at which some of william's compositions were performed. if the father was so intensely gratified on this occasion, what would his feelings have been could he have lived to witness his son's future career? but this pleasure was not to be his, for he died many years before william became an astronomer. in , about a couple of years after his return to england from this visit to his old home, we find that herschel had received a further promotion to be organist in the octagon chapel, at bath. bath was then, as now, a highly fashionable resort, and many notable personages patronised the rising musician. herschel had other points in his favour besides his professional skill; his appearance was good, his address was prepossessing, and even his nationality was a distinct advantage, inasmuch as he was a hanoverian in the reign of king george the third. on sundays he played the organ, to the great delight of the congregation, and on week-days he was occupied by giving lessons to private pupils, and in preparation for public performances. he thus came to be busily employed, and seems to have been in the enjoyment of comfortable means. [plate: , new king street, bath, where herschel lived.] from his earliest youth herschel had been endowed with that invaluable characteristic, an eager curiosity for knowledge. he was naturally desirous of perfecting himself in the theory of music, and thus he was led to study mathematics. when he had once tasted the charms of mathematics, he saw vast regions of knowledge unfolded before him, and in this way he was induced to direct his attention to astronomy. more and more this pursuit seems to have engrossed his attention, until at last it had become an absorbing passion. herschel was, however, still obliged, by the exigency of procuring a livelihood, to give up the best part of his time to his profession as a musician; but his heart was eagerly fixed on another science, and every spare moment was steadily devoted to astronomy. for many years, however, he continued to labour at his original calling, nor was it until he had attained middle age and become the most celebrated astronomer of the time, that he was enabled to concentrate his attention exclusively on his favourite pursuit. it was with quite a small telescope which had been lent him by a friend that herschel commenced his career as an observer. however, he speedily discovered that to see all he wanted to see, a telescope of far greater power would be necessary, and he determined to obtain this more powerful instrument by actually making it with his own hands. at first it may seem scarcely likely that one whose occupation had previously been the study and practice of music should meet with success in so technical an operation as the construction of a telescope. it may, however, be mentioned that the kind of instrument which herschel designed to construct was formed on a very different principle from the refracting telescopes with which we are ordinarily familiar. his telescope was to be what is termed a reflector. in this type of instrument the optical power is obtained by the use of a mirror at the bottom of the tube, and the astronomer looks down through the tube towards his mirror and views the reflection of the stars with its aid. its efficiency as a telescope depends entirely on the accuracy with which the requisite form has been imparted to the mirror. the surface has to be hollowed out a little, and this has to be done so truly that the slightest deviation from good workmanship in this essential particular would be fatal to efficient performance of the telescope. [plate: william herschel.] the mirror that herschel employed was composed of a mixture of two parts of copper to one of tin; the alloy thus obtained is an intensely hard material, very difficult to cast into the proper shape, and very difficult to work afterwards. it possesses, however, when polished, a lustre hardly inferior to that of silver itself. herschel has recorded hardly any particulars as to the actual process by which he cast and figured his reflectors. we are however, told that in later years, after his telescopes had become famous, he made a considerable sum of money by the manufacture and sale of great instruments. perhaps this may be the reason why he never found it expedient to publish any very explicit details as to the means by which his remarkable successes were obtained. [plate: caroline herschel.] since herschel's time many other astronomers, notably the late earl of rosse, have experimented in the same direction, and succeeded in making telescopes certainly far greater, and probably more perfect, than any which herschel appears to have constructed. the details of these later methods are now well known, and have been extensively practised. many amateurs have thus been able to make telescopes by following the instructions so clearly laid down by lord rosse and the other authorities. indeed, it would seem that any one who has a little mechanical skill and a good deal of patience ought now to experience no great difficulty in constructing a telescope quite as powerful as that which first brought herschel into fame. i should, however, mention that in these modern days the material generally used for the mirror is of a more tractable description than the metallic substance which was employed by herschel and by lord rosse. a reflecting telescope of the present day would not be fitted with a mirror composed of that alloy known as speculum metal, whose composition i have already mentioned. it has been found more advantageous to employ a glass mirror carefully figured and polished, just as a metallic mirror would have been, and then to impart to the polished glass surface a fine coating of silver laid down by a chemical process. the silver-on-glass mirrors are so much lighter and so much easier to construct that the more old-fashioned metallic mirrors may be said to have fallen into almost total disuse. in one respect however, the metallic mirror may still claim the advantage that, with reasonable care, its surface will last bright and untarnished for a much longer period than can the silver film on the glass. however, the operation of re-silvering a glass has now become such a simple one that the advantage this indicates is not relatively so great as might at first be supposed. [plate: street view, herschel house, slough.] some years elapsed after herschel's attention had been first directed to astronomy, before he reaped the reward of his exertions in the possession of a telescope which would adequately reveal some of the glories of the heavens. it was in , when the astronomer was thirty-six years old, that he obtained his first glimpse of the stars with an instrument of his own construction. night after night, as soon as his musical labours were ended, his telescopes were brought out, sometimes into the small back garden of his house at bath, and sometimes into the street in front of his hall-door. it was characteristic of him that he was always endeavouring to improve his apparatus. he was incessantly making fresh mirrors, or trying new lenses, or combinations of lenses to act as eye-pieces, or projecting alterations in the mounting by which the telescope was supported. such was his enthusiasm that his house, we are told, was incessantly littered with the usual indications of the workman's presence, greatly to the distress of his sister, who, at this time, had come to take up her abode with him and look after his housekeeping. indeed, she complained that in his astronomical ardour he sometimes omitted to take off, before going into his workshop, the beautiful lace ruffles which he wore while conducting a concert, and that consequently they became soiled with the pitch employed in the polishing of his mirrors. this sister, who occupies such a distinct place in scientific history is the same little girl to whom we have already referred. from her earliest days she seems to have cherished a passionate admiration for her brilliant brother william. it was the proudest delight of her childhood as well as of her mature years to render him whatever service she could; no man of science was ever provided with a more capable or energetic helper than william herschel found in this remarkable woman. whatever work had to be done she was willing to bear her share in it, or even to toil at it unassisted if she could be allowed to do so. she not only managed all his domestic affairs, but in the grinding of the lenses and in the polishing of the mirrors she rendered every assistance that was possible. at one stage of the very delicate operation of fashioning a reflector, it is necessary for the workman to remain with his hand on the mirror for many hours in succession. when such labours were in progress, caroline used to sit by her brother, and enliven the time by reading stories aloud, sometimes pausing to feed him with a spoon while his hands were engaged on the task from which he could not desist for a moment. when mathematical work had to be done caroline was ready for it; she had taught herself sufficient to enable her to perform the kind of calculations, not, perhaps, very difficult ones, that herschel's work required; indeed, it is not too much to say that the mighty life-work which this man was enabled to perform could never have been accomplished had it not been for the self-sacrifice of this ever-loving and faithful sister. when herschel was at the telescope at night, caroline sat by him at her desk, pen in hand, ready to write down the notes of the observations as they fell from her brother's lips. this was no insignificant toil. the telescope was, of course, in the open air, and as herschel not unfrequently continued his observations throughout the whole of a long winter's night, there were but few women who could have accomplished the task which caroline so cheerfully executed. from dusk till dawn, when the sky was clear, were herschel's observing hours, and what this sometimes implied we can realise from the fact that caroline assures us she had sometimes to desist because the ink had actually frozen in her pen. the night's work over, a brief rest was taken, and while william had his labours for the day to attend to, caroline carefully transcribed the observations made during the night before, reduced all the figures and prepared everything in readiness for the observations that were to follow on the ensuing evening. but we have here been anticipating a little of the future which lay before the great astronomer; we must now revert to the history of his early work, at bath, in , when herschel's scrutiny of the skies first commenced with an instrument of his own manufacture. for some few years he did not attain any result of importance; no doubt he made a few interesting observations, but the value of the work during those years is to be found, not in any actual discoveries which were accomplished, but in the practice which herschel obtained in the use of his instruments. it was not until that the great achievement took place by which he at once sprang into fame. [plate: garden view, herschel house, slough.] it is sometimes said that discoveries are made by accident, and, no doubt, to a certain extent, but only, i fancy to a very small extent, this statement may be true. it is, at all events, certain that such lucky accidents do not often fall to the lot of people unless those people have done much to deserve them. this was certainly the case with herschel. he appears to have formed a project for making a close examination of all the stars above a certain magnitude. perhaps he intended to confine this research to a limited region of the sky, but, at all events, he seems to have undertaken the work energetically and systematically. star after star was brought to the centre of the field of view of his telescope, and after being carefully examined was then displaced, while another star was brought forward to be submitted to the same process. in the great majority of cases such observations yield really nothing of importance; no doubt even the smallest star in the heavens would, if we could find out all about it, reveal far more than all the astronomers that were ever on the earth have even conjectured. what we actually learn about the great majority of stars is only information of the most meagre description. we see that the star is a little point of light, and we see nothing more. in the great review which herschel undertook he doubtless examined hundreds, or perhaps thousands of stars, allowing them to pass away without note or comment. but on an ever-memorable night in march, , it happened that he was pursuing his task among the stars in the constellation of gemini. doubtless, on that night, as on so many other nights, one star after another was looked at only to be dismissed, as not requiring further attention. on the evening in question, however, one star was noticed which, to herschel's acute vision seemed different from the stars which in so many thousands are strewn over the sky. a star properly so called appears merely as a little point of light, which no increase of magnifying power will ever exhibit with a true disc. but there was something in the star-like object which herschel saw that immediately arrested his attention and made him apply to it a higher magnifying power. this at once disclosed the fact that the object possessed a disc, that is, a definite, measurable size, and that it was thus totally different from any one of the hundreds and thousands of stars which exist elsewhere in space. indeed, we may say at once that this little object was not a star at all; it was a planet. that such was its true nature was confirmed, after a little further observation, by perceiving that the body was shifting its place on the heavens relatively to the stars. the organist at the octagon chapel at bath had, therefore, discovered a new planet with his home-made telescope. i can imagine some one will say, "oh, there was nothing so wonderful in that; are not planets always being discovered? has not m. palisa, for instance, discovered about eighty of such objects, and are there not hundreds of them known nowadays?" this is, to a certain extent, quite true. i have not the least desire to detract from the credit of those industrious and sharp-sighted astronomers who have in modern days brought so many of these little objects within our cognisance. i think, however, it must be admitted that such discoveries have a totally different importance in the history of science from that which belongs to the peerless achievement of herschel. in the first place, it must be observed that the minor planets now brought to light are so minute that if a score of them were rolled to together into one lump it would not be one-thousandth part of the size of the grand planet discovered by herschel. this is, nevertheless, not the most important point. what marks herschel's achievement as one of the great epochs in the history of astronomy is the fact that the detection of uranus was the very first recorded occasion of the discovery of any planet whatever. for uncounted ages those who watched the skies had been aware of the existence of the five old planets--jupiter, mercury, saturn, venus, and mars. it never seems to have occurred to any of the ancient philosophers that there could be other similar objects as yet undetected over and above the well-known five. great then was the astonishment of the scientific world when the bath organist announced his discovery that the five planets which had been known from all antiquity must now admit the company of a sixth. and this sixth planet was, indeed, worthy on every ground to be received into the ranks of the five glorious bodies of antiquity. it was, no doubt, not so large as saturn, it was certainly very much less than jupiter; on the other hand, the new body was very much larger than mercury, than venus, or than mars, and the earth itself seemed quite an insignificant object in comparison with this newly added member of the solar system. in one respect, too, herschel's new planet was a much more imposing object than any one of the older bodies; it swept around the sun in a majestic orbit, far outside that of saturn, which had previously been regarded as the boundary of the solar system, and its stately progress required a period of not less than eighty-one years. king george the third, hearing of the achievements of the hanoverian musician, felt much interest in his discovery, and accordingly herschel was bidden to come to windsor, and to bring with him the famous telescope, in order to exhibit the new planet to the king, and to tell his majesty all about it. the result of the interview was to give herschel the opportunity for which he had so long wished, of being able to devote himself exclusively to science for the rest of his life. [plate: view of the observatory, herschel house, slough.] the king took so great a fancy to the astronomer that he first, as i have already mentioned, duly pardoned his desertion from the army, some twenty-five years previously. as a further mark of his favour the king proposed to confer on herschel the title of his majesty's own astronomer, to assign to him a residence near windsor, to provide him with a salary, and to furnish such funds as might be required for the erection of great telescopes, and for the conduct of that mighty scheme of celestial observation on which herschel was so eager to enter. herschel's capacity for work would have been much impaired if he had been deprived of the aid of his admirable sister, and to her, therefore, the king also assigned a salary, and she was installed as herschel's assistant in his new post. with his usually impulsive determination, herschel immediately cut himself free from all his musical avocations at bath, and at once entered on the task of making and erecting the great telescopes at windsor. there, for more than thirty years, he and his faithful sister prosecuted with unremitting ardour their nightly scrutiny of the sky. paper after paper was sent to the royal society, describing the hundreds, indeed the thousands, of objects such as double stars; nebulae and clusters, which were first revealed to human gaze during those midnight vigils. to the end of his life he still continued at every possible opportunity to devote himself to that beloved pursuit in which he had such unparalleled success. no single discovery of herschel's later years was, however, of the same momentous description as that which first brought him to fame. [plate: the -foot telescope as it was in the year , herschel house, slough.] herschel married when considerably advanced in life and he lived to enjoy the indescribable pleasure of finding that his only son, afterwards sir john herschel, was treading worthily in his footsteps, and attaining renown as an astronomical observer, second only to that of his father. the elder herschel died in , and his illustrious sister caroline then returned to hanover, where she lived for many years to receive the respect and attention which were so justly hers. she died at a very advanced age in . laplace. the author of the "mecanique celeste" was born at beaumont-en-auge, near honfleur, in , just thirteen years later than his renowned friend lagrange. his father was a farmer, but appears to have been in a position to provide a good education for a son who seemed promising. considering the unorthodoxy in religious matters which is generally said to have characterized laplace in later years, it is interesting to note that when he was a boy the subject which first claimed his attention was theology. he was, however, soon introduced to the study of mathematics, in which he presently became so proficient, that while he was still no more than eighteen years old, he obtained employment as a mathematical teacher in his native town. desiring wider opportunities for study and for the acquisition of fame than could be obtained in the narrow associations of provincial life, young laplace started for paris, being provided with letters of introduction to d'alembert, who then occupied the most prominent position as a mathematician in france, if not in the whole of europe. d'alembert's fame was indeed so brilliant that catherine the great wrote to ask him to undertake the education of her son, and promised the splendid income of a hundred thousand francs. he preferred, however, a quiet life of research in paris, although there was but a modest salary attached to his office. the philosopher accordingly declined the alluring offer to go to russia, even though catherine wrote again to say: "i know that your refusal arises from your desire to cultivate your studies and your friendships in quiet. but this is of no consequence: bring all your friends with you, and i promise you that both you and they shall have every accommodation in my power." with equal firmness the illustrious mathematician resisted the manifold attractions with which frederick the great sought to induce him, to take up his residence at berlin. in reading of these invitations we cannot but be struck at the extraordinary respect which was then paid to scientific distinction. it must be remembered that the discoveries of such a man as d'alembert were utterly incapable of being appreciated except by those who possessed a high degree of mathematical culture. we nevertheless find the potentates of russia and prussia entreating and, as it happens, vainly entreating, the most distinguished mathematician in france to accept the positions that they were proud to offer him. it was to d'alembert, the profound mathematician, that young laplace, the son of the country farmer, presented his letters of introduction. but those letters seem to have elicited no reply, whereupon laplace wrote to d'alembert submitting a discussion on some point in dynamics. this letter instantly produced the desired effect. d'alembert thought that such mathematical talent as the young man displayed was in itself the best of introductions to his favour. it could not be overlooked, and accordingly he invited laplace to come and see him. laplace, of course, presented himself, and ere long d'alembert obtained for the rising philosopher a professorship of mathematics in the military school in paris. this gave the brilliant young mathematician the opening for which he sought, and he quickly availed himself of it. laplace was twenty-three years old when his first memoir on a profound mathematical subject appeared in the memoirs of the academy at turin. from this time onwards we find him publishing one memoir after another in which he attacks, and in many cases successfully vanquishes, profound difficulties in the application of the newtonian theory of gravitation to the explanation of the solar system. like his great contemporary lagrange, he loftily attempted problems which demanded consummate analytical skill for their solution. the attention of the scientific world thus became riveted on the splendid discoveries which emanated from these two men, each gifted with extraordinary genius. laplace's most famous work is, of course, the "mecanique celeste," in which he essayed a comprehensive attempt to carry out the principles which newton had laid down, into much greater detail than newton had found practicable. the fact was that newton had not only to construct the theory of gravitation, but he had to invent the mathematical tools, so to speak, by which his theory could be applied to the explanation of the movements of the heavenly bodies. in the course of the century which had elapsed between the time of newton and the time of laplace, mathematics had been extensively developed. in particular, that potent instrument called the infinitesimal calculus, which newton had invented for the investigation of nature, had become so far perfected that laplace, when he attempted to unravel the movements of the heavenly bodies, found himself provided with a calculus far more efficient than that which had been available to newton. the purely geometrical methods which newton employed, though they are admirably adapted for demonstrating in a general way the tendencies of forces and for explaining the more obvious phenomena by which the movements of the heavenly bodies are disturbed, are yet quite inadequate for dealing with the more subtle effects of the law of gravitation. the disturbances which one planet exercises upon the rest can only be fully ascertained by the aid of long calculation, and for these calculations analytical methods are required. with an armament of mathematical methods which had been perfected since the days of newton by the labours of two or three generations of consummate mathematical inventors, laplace essayed in the "mecanique celeste" to unravel the mysteries of the heavens. it will hardly be disputed that the book which he has produced is one of the most difficult books to understand that has ever been written. in great part, of course, this difficulty arises from the very nature of the subject, and is so far unavoidable. no one need attempt to read the "mecanique celeste" who has not been naturally endowed with considerable mathematical aptitude which he has cultivated by years of assiduous study. the critic will also note that there are grave defects in laplace's method of treatment. the style is often extremely obscure, and the author frequently leaves great gaps in his argument, to the sad discomfiture of his reader. nor does it mend matters to say, as laplace often does say, that it is "easy to see" how one step follows from another. such inferences often present great difficulties even to excellent mathematicians. tradition indeed tells us that when laplace had occasion to refer to his own book, it sometimes happened that an argument which he had dismissed with his usual formula, "il est facile a voir," cost the illustrious author himself an hour or two of hard thinking before he could recover the train of reasoning which had been omitted. but there are certain parts of this great work which have always received the enthusiastic admiration of mathematicians. laplace has, in fact, created whole tracts of science, some of which have been subsequently developed with much advantage in the prosecution of the study of nature. judged by a modern code the gravest defect of laplace's great work is rather of a moral than of a mathematical nature. lagrange and he advanced together in their study of the mechanics of the heavens, at one time perhaps along parallel lines, while at other times they pursued the same problem by almost identical methods. sometimes the important result was first reached by lagrange, sometimes it was laplace who had the good fortune to make the discovery. it would doubtless be a difficult matter to draw the line which should exactly separate the contributions to astronomy made by one of these illustrious mathematicians, and the contributions made by the other. but in his great work laplace in the loftiest manner disdained to accord more than the very barest recognition to lagrange, or to any of the other mathematicians, newton alone excepted, who had advanced our knowledge of the mechanism of the heavens. it would be quite impossible for a student who confined his reading to the "mecanique celeste" to gather from any indications that it contains whether the discoveries about which he was reading had been really made by laplace himself or whether they had not been made by lagrange, or by euler, or by clairaut. with our present standard of morality in such matters, any scientific man who now brought forth a work in which he presumed to ignore in this wholesale fashion the contributions of others to the subject on which he was writing, would be justly censured and bitter controversies would undoubtedly arise. perhaps we ought not to judge laplace by the standard of our own time, and in any case i do not doubt that laplace might have made a plausible defence. it is well known that when two investigators are working at the same subjects, and constantly publishing their results, it sometimes becomes difficult for each investigator himself to distinguish exactly between what he has accomplished and that which must be credited to his rival. laplace may probably have said to himself that he was going to devote his energies to a great work on the interpretation of nature, that it would take all his time and all his faculties, and all the resources of knowledge that he could command, to deal justly with the mighty problems before him. he would not allow himself to be distracted by any side issue. he could not tolerate that pages should be wasted in merely discussing to whom we owe each formula, and to whom each deduction from such formula is due. he would rather endeavour to produce as complete a picture as he possibly could of the celestial mechanics, and whether it were by means of his mathematics alone, or whether the discoveries of others may have contributed in any degree to the result, is a matter so infinitesimally insignificant in comparison with the grandeur of his subject that he would altogether neglect it. "if lagrange should think," laplace might say, "that his discoveries had been unduly appropriated, the proper course would be for him to do exactly what i have done. let him also write a "mecanique celeste," let him employ those consummate talents which he possesses in developing his noble subject to the utmost. let him utilise every result that i or any other mathematician have arrived at, but not trouble himself unduly with unimportant historical details as to who discovered this, and who discovered that; let him produce such a work as he could write, and i shall heartily welcome it as a splendid contribution to our science." certain it is that laplace and lagrange continued the best of friends, and on the death of the latter it was laplace who was summoned to deliver the funeral oration at the grave of his great rival. the investigations of laplace are, generally speaking, of too technical a character to make it possible to set forth any account of them in such a work as the present. he did publish, however, one treatise, called the "systeme du monde," in which, without introducing mathematical symbols, he was able to give a general account of the theories of the celestial movements, and of the discoveries to which he and others had been led. in this work the great french astronomer sketched for the first time that remarkable doctrine by which his name is probably most generally known to those readers of astronomical books who are not specially mathematicians. it is in the "systeme du monde" that laplace laid down the principles of the nebular theory which, in modern days, has been generally accepted by those philosophers who are competent to judge, as substantially a correct expression of a great historical fact. [plate: laplace.] the nebular theory gives a physical account of the origin of the solar system, consisting of the sun in the centre, with the planets and their attendant satellites. laplace perceived the significance of the fact that all the planets revolved in the same direction around the sun; he noticed also that the movements of rotation of the planets on their axes were performed in the same direction as that in which a planet revolves around the sun; he saw that the orbits of the satellites, so far at least as he knew them, revolved around their primaries also in the same direction. nor did it escape his attention that the sun itself rotated on its axis in the same sense. his philosophical mind was led to reflect that such a remarkable unanimity in the direction of the movements in the solar system demanded some special explanation. it would have been in the highest degree improbable that there should have been this unanimity unless there had been some physical reason to account for it. to appreciate the argument let us first concentrate our attention on three particular bodies, namely the earth, the sun, and the moon. first the earth revolves around the sun in a certain direction, and the earth also rotates on its axis. the direction in which the earth turns in accordance with this latter movement might have been that in which it revolves around the sun, or it might of course have been opposite thereto. as a matter of fact the two agree. the moon in its monthly revolution around the earth follows also the same direction, and our satellite rotates on its axis in the same period as its monthly revolution, but in doing so is again observing this same law. we have therefore in the earth and moon four movements, all taking place in the same direction, and this is also identical with that in which the sun rotates once every twenty-five days. such a coincidence would be very unlikely unless there were some physical reason for it. just as unlikely would it be that in tossing a coin five heads or five tails should follow each other consecutively. if we toss a coin five times the chances that it will turn up all heads or all tails is but a small one. the probability of such an event is only one-sixteenth. there are, however, in the solar system many other bodies besides the three just mentioned which are animated by this common movement. among them are, of course, the great planets, jupiter, saturn, mars, venus, and mercury, and the satellites which attend on these planets. all these planets rotate on their axes in the same direction as they revolve around the sun, and all their satellites revolve also in the same way. confining our attention merely to the earth, the sun, and the five great planets with which laplace was acquainted, we have no fewer than six motions of revolution and seven motions of rotation, for in the latter we include the rotation of the sun. we have also sixteen satellites of the planets mentioned whose revolutions round their primaries are in the same direction. the rotation of the moon on its axis may also be reckoned, but as to the rotations of the satellites of the other planets we cannot speak with any confidence, as they are too far off to be observed with the necessary accuracy. we have thus thirty circular movements in the solar system connected with the sun and moon and those great planets than which no others were known in the days of laplace. the significant fact is that all these thirty movements take place in the same direction. that this should be the case without some physical reason would be just as unlikely as that in tossing a coin thirty times it should turn up all heads or all tails every time without exception. we can express the argument numerically. calculation proves that such an event would not generally happen oftener than once out of five hundred millions of trials. to a philosopher of laplace's penetration, who had made a special study of the theory of probabilities, it seemed well-nigh inconceivable that there should have been such unanimity in the celestial movements, unless there had been some adequate reason to account for it. we might, indeed, add that if we were to include all the objects which are now known to belong to the solar system, the argument from probability might be enormously increased in strength. to laplace the argument appeared so conclusive that he sought for some physical cause of the remarkable phenomenon which the solar system presented. thus it was that the famous nebular hypothesis took its rise. laplace devised a scheme for the origin of the sun and the planetary system, in which it would be a necessary consequence that all the movements should take place in the same direction as they are actually observed to do. let us suppose that in the beginning there was a gigantic mass of nebulous material, so highly heated that the iron and other substances which now enter into the composition of the earth and planets were then suspended in a state of vapour. there is nothing unreasonable in such a supposition indeed, we know as a matter of fact that there are thousands of such nebulae to be discerned at present through our telescopes. it would be extremely unlikely that any object could exist without possessing some motion of rotation; we may in fact assert that for rotation to be entirety absent from the great primeval nebula would be almost infinitely improbable. as ages rolled on, the nebula gradually dispersed away by radiation its original stores of heat, and, in accordance with well-known physical principles, the materials of which it was formed would tend to coalesce. the greater part of those materials would become concentrated in a mighty mass surrounded by outlying uncondensed vapours. there would, however, also be regions throughout the extent of the nebula, in which subsidiary centres of condensation would be found. in its long course of cooling, the nebula would, therefore, tend ultimately to form a mighty central body with a number of smaller bodies disposed around it. as the nebula was initially endowed with a movement of rotation, the central mass into which it had chiefly condensed would also revolve, and the subsidiary bodies would be animated by movements of revolution around the central body. these movements would be all pursued in one common direction, and it follows, from well-known mechanical principles, that each of the subsidiary masses, besides participating in the general revolution around the central body, would also possess a rotation around its axis, which must likewise be performed in the same direction. around the subsidiary bodies other objects still smaller would be formed, just as they themselves were formed relatively to the great central mass. as the ages sped by, and the heat of these bodies became gradually dissipated, the various objects would coalesce, first into molten liquid masses, and thence, at a further stage of cooling, they would assume the appearance of solid masses, thus producing the planetary bodies such as we now know them. the great central mass, on account of its preponderating dimensions, would still retain, for further uncounted ages, a large quantity of its primeval heat, and would thus display the splendours of a glowing sun. in this way laplace was able to account for the remarkable phenomena presented in the movements of the bodies of the solar system. there are many other points also in which the nebular theory is known to tally with the facts of observation. in fact, each advance in science only seems to make it more certain that the nebular hypothesis substantially represents the way in which our solar system has grown to its present form. not satisfied with a career which should be merely scientific, laplace sought to connect himself with public affairs. napoleon appreciated his genius, and desired to enlist him in the service of the state. accordingly he appointed laplace to be minister of the interior. the experiment was not successful, for he was not by nature a statesman. napoleon was much disappointed at the ineptitude which the great mathematician showed for official life, and, in despair of laplace's capacity as an administrator, declared that he carried the spirit of his infinitesimal calculus into the management of business. indeed, laplace's political conduct hardly admits of much defence. while he accepted the honours which napoleon showered on him in the time of his prosperity, he seems to have forgotten all this when napoleon could no longer render him service. laplace was made a marquis by louis xviii., a rank which he transmitted to his son, who was born in . during the latter part of his life the philosopher lived in a retired country place at arcueile. here he pursued his studies, and by strict abstemiousness, preserved himself from many of the infirmities of old age. he died on march the th, , in his seventy-eighth year, his last words being, "what we know is but little, what we do not know is immense." brinkley. provost baldwin held absolute sway in the university of dublin for forty-one years. his memory is well preserved there. the bursar still dispenses the satisfactory revenues which baldwin left to the college. none of us ever can forget the marble angels round the figure of the dying provost on which we used to gaze during the pangs of the examination hall. baldwin died in , and was succeeded by francis andrews, a fellow of seventeen years' standing. as to the scholastic acquirements of andrews, all i can find is a statement that he was complimented by the polite professors of padua on the elegance and purity with which he discoursed to them in latin. andrews was also reputed to be a skilful lawyer. he was certainly a privy councillor and a prominent member of the irish house of commons, and his social qualities were excellent. perhaps it was baldwin's example that stimulated a desire in andrews to become a benefactor to his college. he accordingly bequeathed a sum of , pounds and an annual income of pounds wherewith to build and endow an astronomical observatory in the university. the figures just stated ought to be qualified by the words of cautious ussher (afterwards the first professor of astronomy), that "this money was to arise from an accumulation of a part of his property, to commence upon a particular contingency happening to his family." the astronomical endowment was soon in jeopardy by litigation. andrews thought he had provided for his relations by leaving to them certain leasehold interests connected with the provost's estate. the law courts, however, held that these interests were not at the disposal of the testator, and handed them over to hely hutchinson, the next provost. the disappointed relations then petitioned the irish parliament to redress this grievance by transferring to them the moneys designed by andrews for the observatory. it would not be right, they contended, that the kindly intentions of the late provost towards his kindred should be frustrated for the sake of maintaining what they described as "a purely ornamental institution." the authorities of the college protested against this claim. counsel were heard, and a committee of the house made a report declaring the situation of the relations to be a hard one. accordingly, a compromise was made, and the dispute terminated. the selection of a site for the new astronomical observatory was made by the board of trinity college. the beautiful neighbourhood of dublin offered a choice of excellent localities. on the north side of the liffey an observatory could have been admirably placed, either on the remarkable promontory of howth or on the elevation of which dunsink is the summit. on the south side of dublin there are several eminences that would have been suitable: the breezy heaths at foxrock combine all necessary conditions; the obelisk hill at killiney would have given one of the most picturesque sites for an observatory in the world; while near delgany two or three other good situations could be mentioned. but the board of those pre-railway days was naturally guided by the question of proximity. dunsink was accordingly chosen as the most suitable site within the distance of a reasonable walk from trinity college. the northern boundary of the phoenix park approaches the little river tolka, which winds through a succession of delightful bits of sylvan scenery, such as may be found in the wide demesne of abbotstown and the classic shades of glasnevin. from the banks of the tolka, on the opposite side of the park, the pastures ascend in a gentle slope to culminate at dunsink, where at a distance of half a mile from the stream, of four miles from dublin, and at a height of feet above the sea, now stands the observatory. from the commanding position of dunsink a magnificent view is obtained. to the east the sea is visible, while the southern prospect over the valley of the liffey is bounded by a range of hills and mountains extending from killiney to bray head, thence to the little sugar loaf, the two rock and the three rock mountains, over the flank of which the summit of the great sugar loaf is just perceptible. directly in front opens the fine valley of glenasmole, with kippure mountain, while the range can be followed to its western extremity at lyons. the climate of dunsink is well suited for astronomical observation. no doubt here, as elsewhere in ireland, clouds are abundant, but mists or haze are comparatively unusual, and fogs are almost unknown. the legal formalities to be observed in assuming occupation exacted a delay of many months; accordingly, it was not until the th december, , that a contract could be made with mr. graham moyers for the erection of a meridian-room and a dome for an equatorial, in conjunction with a becoming residence for the astronomer. before the work was commenced at dunsink, the board thought it expedient to appoint the first professor of astronomy. they met for this purpose on the nd january, , and chose the rev. henry ussher, a senior fellow of trinity college, dublin. the wisdom of the appointment was immediately shown by the assiduity with which ussher engaged in founding the observatory. in three years he had erected the buildings and equipped them with instruments, several of which were of his own invention. on the th of february, , a special grant of pounds was made by the board to dr. ussher as some recompense for his labours. it happened that the observatory was not the only scientific institution which came into being in ireland at this period; the newly-kindled ardour for the pursuit of knowledge led, at the same time, to the foundation of the royal irish academy. by a fitting coincidence, the first memoir published in the "transactions of the royal irish academy," was by the first andrews, professor of astronomy. it was read on the th of june, , and bore the title, "account of the observatory belonging to trinity college," by the rev. h. ussher, d.d., m.r.i.a., f.r.s. this communication shows the extensive design that had been originally intended for dunsink, only a part of which was, however, carried out. for instance, two long corridors, running north and south from the central edifice, which are figured in the paper, never developed into bricks and mortar. we are not told why the original scheme had to be contracted; but perhaps the reason may be not unconnected with a remark of ussher's, that the college had already advanced from its own funds a sum considerably exceeding the original bequest. the picture of the building shows also the dome for the south equatorial, which was erected many years later. ussher died in . during his brief career at the observatory, he observed eclipses, and is stated to have done other scientific work. the minutes of the board declare that the infant institution had already obtained celebrity by his labours, and they urge the claims of his widow to a pension, on the ground that the disease from which he died had been contracted by his nightly vigils. the board also promised a grant of fifty guineas as a help to bring out dr. ussher's sermons. they advanced twenty guineas to his widow towards the publication of his astronomical papers. they ordered his bust to be executed for the observatory, and offered "the death of ussher" as the subject of a prize essay; but, so far as i can find, neither the sermons nor the papers, neither the bust nor the prize essay, ever came into being. there was keen competition for the chair of astronomy which the death of ussher vacated. the two candidates were rev. john brinkley, of caius college, cambridge, a senior wrangler (born at woodbridge, suffolk, in ), and mr. stack, fellow of trinity college, dublin, and author of a book on optics. a majority of the board at first supported stack, while provost hely hutchinson and one or two others supported brinkley. in those days the provost had a veto at elections, so that ultimately stack was withdrawn and brinkley was elected. this took place on the th december, . the national press of the day commented on the preference shown to the young englishman, brinkley, over his irish rival. an animated controversy ensued. the provost himself condescended to enter the lists and to vindicate his policy by a long letter in the "public register" or "freeman's journal," of st december, . this letter was anonymous, but its authorship is obvious. it gives the correspondence with maskelyne and other eminent astronomers, whose advice and guidance had been sought by the provost. it also contends that "the transactions of the board ought not to be canvassed in the newspapers." for this reference, as well as for much other information, i am indebted to my friend, the rev. john stubbs, d.d. [plate: the observatory, dunsink. from a photograph by w. lawrence, upper sackville street, dublin.] the next event in the history of the observatory was the issue of letters patent ( geo. iii., a.d. ), in which it is recited that "we grant and ordain that there shall be forever hereafter a professor of astronomy, on the foundation of dr. andrews, to be called and known by the name of the royal astronomer of ireland." the letters prescribe the various duties of the astronomer and the mode of his election. they lay down regulations as to the conduct of the astronomical work, and as to the choice of an assistant. they direct that the provost and the senior fellows shall make a thorough inspection of the observatory once every year in june or july; and this duty was first undertaken on the th of july, . it may be noted that the date on which the celebration of the tercentenary of the university was held happens to coincide with the centenary of the first visitation of the observatory. the visitors on the first occasion were a. murray, matthew young, george hall, and john barrett. they record that they find the buildings, books and instruments in good condition; but the chief feature in this report, as well as in many which followed it, related to a circumstance to which we have not yet referred. in the original equipment of the observatory, ussher, with the natural ambition of a founder, desired to place in it a telescope of more magnificent proportions than could be found anywhere else. the board gave a spirited support to this enterprise, and negotiations were entered into with the most eminent instrument-maker of those days. this was jesse ramsden ( - ), famous as the improver of the sextant, as the constructor of the great theodolite used by general roy in the english survey, and as the inventor of the dividing engine for graduating astronomical instruments. ramsden had built for sir george schuckburgh the largest and most perfect equatorial ever attempted. he had constructed mural quadrants for padua and verona, which elicited the wonder of astronomers when dr. maskelyne declared he could detect no error in their graduation so large as two seconds and a half. but ramsden maintained that even better results would be obtained by superseding the entire quadrant by the circle. he obtained the means of testing this prediction when he completed a superb circle for palermo of five feet diameter. finding his anticipations were realised, he desired to apply the same principles on a still grander scale. ramsden was in this mood when he met with dr. ussher. the enthusiasm of the astronomer and the instrument-maker communicated itself to the board, and a tremendous circle, to be ten feet in diameter, was forthwith projected. projected, but never carried out. after ramsden had to some extent completed a -foot circle, he found such difficulties that he tried a -foot, and this again he discarded for an -foot, which was ultimately accomplished, though not entirely by himself. notwithstanding the contraction from the vast proportions originally designed, the completed instrument must still be regarded as a colossal piece of astronomical workmanship. even at this day i do not know that any other observatory can show a circle eight feet in diameter graduated all round. i think it is professor piazzi smith who tells us how grateful he was to find a large telescope he had ordered finished by the opticians on the very day they had promised it. the day was perfectly correct; it was only the year that was wrong. a somewhat remarkable experience in this direction is chronicled by the early reports of the visitors to dunsink observatory. i cannot find the date on which the great circle was ordered from ramsden, but it is fixed with sufficient precision by an allusion in ussher's paper to the royal irish academy, which shows that by the th june, , the order had been given, but that the abandonment of the -foot scale had not then been contemplated. it was reasonable that the board should allow ramsden ample time for the completion of a work at once so elaborate and so novel. it could not have been finished in a year, nor would there have been much reason for complaint if the maker had found he required two or even three years more. seven years gone, and still no telescope, was the condition in which the board found matters at their first visitation in . they had, however, assurances from ramsden that the instrument would be completed within the year; but, alas for such promises, another seven years rolled on, and in the place for the great circle was still vacant at dunsink. ramsden had fallen into bad health, and the board considerately directed that "inquiries should be made." next year there was still no progress, so the board were roused to threaten ramsden with a suit at law; but the menace was never executed, for the malady of the great optician grew worse, and he died that year. affairs had now assumed a critical aspect, for the college had advanced much money to ramsden during these fifteen years, and the instrument was still unfinished. an appeal was made by the provost to dr. maskelyne, the astronomer royal of england, for his advice and kindly offices in this emergency. maskelyne responds--in terms calculated to allay the anxiety of the bursar--"mr. ramsden has left property behind him, and the college can be in no danger of losing both their money and the instrument." the business of ramsden was then undertaken by berge, who proceeded to finish the circle quite as deliberately as his predecessor. after four years berge promised the instrument in the following august, but it did not come. two years later ( ) the professor complains that he can get no answer from berge. in , it is stated that berge will send the telescope in a month. he did not; but in the next year ( ), about twenty-three years after the great circle was ordered, it was erected at dunsink, where it is still to be seen. the following circumstances have been authenticated by the signatures of provosts, proctors, bursars, and other college dignitaries:--in the board ordered two of the clocks at the observatory to be sent to mr. crosthwaite for repairs. seven years later, in , mr. crosthwaite was asked if the clocks were ready. this impatience was clearly unreasonable, for even in four more years, , we find the two clocks were still in hand. two years later, in , the board determined to take vigorous action by asking the bursar to call upon crosthwaite. this evidently produced some effect, for in the following year, , the professor had no doubt that the clocks would be speedily returned. after eight years more, in , one of the clocks was still being repaired, and so it was in , which is the last record we have of these interesting time-pieces. astronomers are, however, accustomed to deal with such stupendous periods in their calculations, that even the time taken to repair a clock seems but small in comparison. the long tenure of the chair of astronomy by brinkley is divided into two nearly equal periods by the year in which the great circle was erected. brinkley was eighteen years waiting for his telescope, and he had eighteen years more in which to use it. during the first of these periods brinkley devoted himself to mathematical research; during the latter he became a celebrated astronomer. brinkley's mathematical labours procured for their author some reputation as a mathematician. they appear to be works of considerable mathematical elegance, but not indicating any great power of original thought. perhaps it has been prejudicial to brinkley's fame in this direction, that he was immediately followed in his chair by so mighty a genius as william rowan hamilton. after the great circle had been at last erected, brinkley was able to begin his astronomical work in earnest. nor was there much time to lose. he was already forty-five years old, a year older than was herschel when he commenced his immortal career at slough. stimulated by the consciousness of having the command of an instrument of unique perfection, brinkley loftily attempted the very highest class of astronomical research. he resolved to measure anew with his own eye and with his own hand the constants of aberration and of nutation. he also strove to solve that great problem of the universe, the discovery of the distance of a fixed star. these were noble problems, and they were nobly attacked. but to appraise with justice this work of brinkley, done seventy years ago, we must not apply to it the same criterion as we would think right to apply to similar work were it done now. we do not any longer use brinkley's constant of aberration, nor do we now think that brinkley's determinations of the star distances were reliable. but, nevertheless, his investigations exercised a marked influence on the progress of science; they stimulated the study of the principles on which exact measurements were to be conducted. brinkley had another profession in addition to that of an astronomer. he was a divine. when a man endeavours to pursue two distinct occupations concurrently, it will be equally easy to explain why his career should be successful, or why it should be the reverse. if he succeeds, he will, of course, exemplify the wisdom of having two strings to his bow. should he fail, it is, of course, because he has attempted to sit on two stools at once. in brinkley's case, his two professions must be likened to the two strings rather than to the two stools. it is true that his practical experience of his clerical life was very slender. he had made no attempt to combine the routine of a parish with his labours in the observatory. nor do we associate a special eminence in any department of religious work with his name. if, however, we are to measure brinkley's merits as a divine by the ecclesiastical preferment which he received, his services to theology must have rivalled his services to astronomy. having been raised step by step in the church, he was at last appointed to the see of cloyne, in , as the successor of bishop berkeley. now, though it was permissible for the archdeacon to be also the andrews professor, yet when the archdeacon became a bishop, it was understood that he should transfer his residence from the observatory to the palace. the chair of astronomy accordingly became vacant. brinkley's subsequent career seems to have been devoted entirely to ecclesiastical matters, and for the last ten years of his life he did not contribute a paper to any scientific society. arago, after a characteristic lament that brinkley should have forsaken the pursuit of science for the temporal and spiritual attractions of a bishopric, pays a tribute to the conscientiousness of the quondam astronomer, who would not even allow a telescope to be brought into the palace lest his mind should be distracted from his sacred duties. the good bishop died on the th september, . he was buried in the chapel of trinity college, and a fine monument to his memory is a familiar object at the foot of the noble old staircase of the library. the best memorial of brinkley is his admirable book on the "elements of plane astronomy." it passed through many editions in his lifetime, and even at the present day the same work, revised first by dr. luby, and more recently by the rev. dr. stubbs and dr. brunnow, has a large and well-merited circulation. john herschel. this illustrious son of an illustrious father was born at slough, near windsor, on the th march, . he was the only child of sir william herschel, who had married somewhat late in life, as we have already mentioned. [plate: astronometer made by sir j. herschel to compare the light of certain stars by the intervention of the moon.] the surroundings among which the young astronomer was reared afforded him an excellent training for that career on which he was to enter, and in which he was destined to attain a fame only less brilliant than that of his father. the circumstances of his youth permitted him to enjoy one great advantage which was denied to the elder herschel. he was able, from his childhood, to devote himself almost exclusively to intellectual pursuits. william herschel, in the early part of his career, had only been able to snatch occasional hours for study from his busy life as a professional musician. but the son, having been born with a taste for the student's life, was fortunate enough to have been endowed with the leisure and the means to enjoy it from the commencement. his early years have been so well described by the late professor pritchard in the "report of the council of the royal astronomical society for ," that i venture to make an extract here:-- "a few traits of john herschel's boyhood, mentioned by himself in his maturer life, have been treasured up by those who were dear to him, and the record of some of them may satisfy a curiosity as pardonable as inevitable, which craves to learn through what early steps great men or great nations become illustrious. his home was singular, and singularly calculated to nurture into greatness any child born as john herschel was with natural gifts, capable of wide development. at the head of the house there was the aged, observant, reticent philosopher, and rarely far away his devoted sister, caroline herschel, whose labours and whose fame are still cognisable as a beneficent satellite to the brighter light of her illustrious brother. it was in the companionship of these remarkable persons, and under the shadow of his father's wonderful telescope, that john herschel passed his boyish years. he saw them, in silent but ceaseless industry, busied about things which had no apparent concern with the world outside the walls of that well-known house, but which, at a later period of his life, he, with an unrivalled eloquence, taught his countrymen to appreciate as foremost among those living influences which but satisfy and elevate the noblest instincts of our nature. what sort of intercourse passed between the father and the boy may be gathered from an incident or two which he narrated as having impressed themselves permanently on the memory of his youth. he once asked his father what he thought was the oldest of all things. the father replied, after the socratic method, by putting another question: 'and what do you yourself suppose is the oldest of all things?' the boy was not successful in his answers, thereon the old astronomer took up a small stone from the garden walk: 'there, my child, there is the oldest of all the things that i certainly know.' on another occasion his father is said to have asked the boy, 'what sort of things, do you think, are most alike?' the delicate, blue-eyed boy, after a short pause, replied, 'the leaves of the same tree are most like each other.' 'gather, then, a handful of leaves of that tree,' rejoined the philosopher, 'and choose two that are alike.' the boy failed; but he hid the lesson in his heart, and his thoughts were revealed after many days. these incidents may be trifles; nor should we record them here had not john herschel himself, though singularly reticent about his personal emotions, recorded them as having made a strong impression on his mind. beyond all doubt we can trace therein, first, that grasp and grouping of many things in one, implied in the stone as the oldest of things; and, secondly, that fine and subtle discrimination of each thing out of many like things as forming the main features which characterized the habit of our venerated friend's philosophy." john herschel entered st. john's college, cambridge, when he was seventeen years of age. his university career abundantly fulfilled his father's eager desire, that his only son should develop a capacity for the pursuit of science. after obtaining many lesser distinctions, he finally came out as senior wrangler in . it was, indeed, a notable year in the mathematical annals of the university. second on that list, in which herschel's name was first, appeared that of the illustrious peacock, afterwards dean of ely, who remained throughout life one of herschel's most intimate friends. almost immediately after taking his degree, herschel gave evidence of possessing a special aptitude for original scientific investigation. he sent to the royal society a mathematical paper which was published in the philosophical transactions. doubtless the splendour that attached to the name he bore assisted him in procuring early recognition of his own great powers. certain it is that he was made a fellow of the royal society at the unprecedentedly early age of twenty-one. even after this remarkable encouragement to adopt a scientific career as the business of his life, it does not seem that john herschel at first contemplated devoting himself exclusively to science. he commenced to prepare for the profession of the law by entering as a student at the middle temple, and reading with a practising barrister. but a lawyer john herschel was not destined to become. circumstances brought him into association with some leading scientific men. he presently discovered that his inclinations tended more and more in the direction of purely scientific pursuits. thus it came to pass that the original intention as to the calling which he should follow was gradually abandoned. fortunately for science herschel found its pursuit so attractive that he was led, as his father had been before him, to give up his whole life to the advancement of knowledge. nor was it unnatural that a senior wrangler, who had once tasted the delights of mathematical research, should have been tempted to devote much time to this fascinating pursuit. by the time john herschel was twenty-nine he had published so much mathematical work, and his researches were considered to possess so much merit, that the royal society awarded him the copley medal, which was the highest distinction it was capable of conferring. at the death of his father in , john herschel, with his tastes already formed for a scientific career, found himself in the possession of ample means. to him also passed all his father's great telescopes and apparatus. these material aids, together with a dutiful sense of filial obligation, decided him to make practical astronomy the main work of his life. he decided to continue to its completion that great survey of the heavens which had already been inaugurated, and, indeed, to a large extent accomplished, by his father. the first systematic piece of practical astronomical work which john herschel undertook was connected with the measurement of what are known as "double stars." it should be observed, that there are in the heavens a number of instances in which two stars are seen in very close association. in the case of those objects to which the expression "double stars" is generally applied, the two luminous points are so close together that even though they might each be quite bright enough to be visible to the unaided eye, yet their proximity is such that they cannot be distinguished as two separate objects without optical aid. the two stars seem fused together into one. in the telescope, however, the bodies may be discerned separately, though they are frequently so close together that it taxes the utmost power of the instrument to indicate the division between them. the appearance presented by a double star might arise from the circumstance that the two stars, though really separated from each other by prodigious distances, happened to lie nearly in the same line of vision, as seen from our point of view. no doubt, many of the so-called double stars could be accounted for on this supposition. indeed, in the early days when but few double stars were known, and when telescopes were not powerful enough to exhibit the numerous close doubles which have since been brought to light, there seems to have been a tendency to regard all double stars as merely such perspective effects. it was not at first suggested that there could be any physical connection between the components of each pair. the appearance presented was regarded as merely due to the circumstance that the line joining the two bodies happened to pass near the earth. [plate: sir john herschel.] in the early part of his career, sir william herschel seems to have entertained the view then generally held by other astronomers with regard to the nature of these stellar pairs. the great observer thought that the double stars could therefore be made to afford a means of solving that problem in which so many of the observers of the skies had been engaged, namely, the determination of the distances of the stars from the earth. herschel saw that the displacement of the earth in its annual movement round the sun would produce an apparent shift in the place of the nearer of the two stars relatively to the other, supposed to be much more remote. if this shift could be measured, then the distance of the nearer of the stars could be estimated with some degree of precision. as has not unfrequently happened in the history of science, an effect was perceived of a very different nature from that which had been anticipated. if the relative places of the two stars had been apparently deranged merely in consequence of the motion of the earth, then the phenomenon would be an annual one. after the lapse of a year the two stars would have regained their original relative positions. this was the effect for which william herschel was looking. in certain of the so called double stars, he, no doubt, did find a movement. he detected the remarkable fact that both the apparent distance and the relative positions of the two bodies were changing. but what was his surprise to observe that these alterations were not of an annually periodic character. it became evident then that in some cases one of the component stars was actually revolving around the other, in an orbit which required many years for its completion. here was indeed a remarkable discovery. it was clearly impossible to suppose that movements of this kind could be mere apparent displacements, arising from the annual shift in our point of view, in consequence of the revolution of the earth. herschel's discovery established the interesting fact that, in certain of these double stars, or binary stars, as these particular objects are more expressively designated, there is an actual orbital revolution of a character similar to that which the earth performs around the sun. thus it was demonstrated that in these particular double stars the nearness of the two components was not merely apparent. the objects must actually lie close together at a distance which is small in comparison with the distance at which either of them is separated from the earth. the fact that the heavens contain pairs of twin suns in mutual revolution was thus brought to light. in consequence of this beautiful discovery, the attention of astronomers was directed to the subject of double stars with a degree of interest which these objects had never before excited. it was therefore not unnatural that john herschel should have been attracted to this branch of astronomical work. admiration for his father's discovery alone might have suggested that the son should strive to develop this territory newly opened up to research. but it also happened that the mathematical talents of the younger herschel inclined his inquiries in the same direction. he saw clearly that, when sufficient observations of any particular binary star had been accumulated, it would then be within the power of the mathematician to elicit from those observations the shape and the position in space of the path which each of the revolving stars described around the other. indeed, in some cases he would be able to perform the astonishing feat of determining from his calculations the weight of these distant suns, and thus be enabled to compare them with the mass of our own sun. [plate: nebula in southern hemisphere, drawn by sir john herschel.] but this work must follow the observations, it could not precede them. the first step was therefore to observe and to measure with the utmost care the positions and distances of those particular double stars which appear to offer the greatest promise in this particular research. in , herschel and a friend of his, mr. james south, agreed to work together with this object. south was a medical man with an ardent devotion to science, and possessed of considerable wealth. he procured the best astronomical instruments that money could obtain, and became a most enthusiastic astronomer and a practical observer of tremendous energy. south and john herschel worked together for two years in the observation and measurement of the double stars discovered by sir william herschel. in the course of this time their assiduity was rewarded by the accumulation of so great a mass of careful measurements that when published, they formed quite a volume in the "philosophical transactions." the value and accuracy of the work, when estimated by standards which form proper criteria for that period, is universally recognised. it greatly promoted the progress of sidereal astronomy, and the authors were in consequence awarded medals from the royal society, and the royal astronomical society, as well as similar testimonials from various foreign institutions. this work must, however, be regarded as merely introductory to the main labours of john herschel's life. his father devoted the greater part of his years as an observer to what he called his "sweeps" of the heavens. the great reflecting telescope, twenty feet long, was moved slowly up and down through an arc of about two degrees towards and from the pole, while the celestial panorama passed slowly in the course of the diurnal motion before the keenly watching eye of the astronomer. whenever a double star traversed the field herschel described it to his sister caroline, who, as we have already mentioned, was his invariable assistant in his midnight watches. when a nebula appeared, then he estimated its size and its brightness, he noticed whether it had a nucleus, or whether it had stars disposed in any significant manner with regard to it. he also dictated any other circumstance which he deemed worthy of record. these observations were duly committed to writing by the same faithful and indefatigable scribe, whose business it also was to take a memorandum of the exact position of the object as indicated by a dial placed in front of her desk, and connected with the telescope. john herschel undertook the important task of re-observing the various double stars and nebulae which had been discovered during these memorable vigils. the son, however, lacked one inestimable advantage which had been possessed by the father. john herschel had no assistant to discharge all those duties which caroline had so efficiently accomplished. he had, therefore, to modify the system of sweeping previously adopted in order to enable all the work both of observing and of recording to be done by himself. this, in many ways, was a great drawback to the work of the younger astronomer. the division of labour between the observer and the scribe enables a greatly increased quantity of work to be got through. it is also distinctly disadvantageous to an observer to have to use his eye at the telescope directly after he has been employing it for reading the graduations on a circle, by the light of a lamp, or for entering memoranda in a note book. nebulae, especially, are often so excessively faint that they can only be properly observed by an eye which is in that highly sensitive condition which is obtained by long continuance in darkness. the frequent withdrawal of the eye from the dark field of the telescope, and the application of it to reading by artificial light, is very prejudicial to its use for the more delicate purpose. john herschel, no doubt, availed himself of every precaution to mitigate the ill effects of this inconvenience as much as possible, but it must have told upon his labours as compared with those of his father. but nevertheless john herschel did great work during his "sweeps." he was specially particular to note all the double stars which presented themselves to his observation. of course some little discretion must be allowed in deciding as to what degree of proximity in adjacent stars does actually bring them within the category of "double stars." sir john set down all such objects as seemed to him likely to be of interest, and the results of his discoveries in this branch of astronomy amount to some thousands. six or seven great memoirs in the transactions of the royal astronomical society have been devoted to giving an account of his labours in this department of astronomy. [plate: the cluster in the centaur, drawn by sir john herschel.] one of the achievements by which sir john herschel is best known is his invention of a method by which the orbits of binary stars could be determined. it will be observed that when one star revolves around another in consequence of the law of gravitation, the orbit described must be an ellipse. this ellipse, however, generally speaking, appears to us more or less foreshortened, for it is easily seen that only under highly exceptional circumstances would the plane in which the stars move happen to be directly square to the line of view. it therefore follows that what we observe is not exactly the track of one star around the other; it is rather the projection of that track as seen on the surface of the sky. now it is remarkable that this apparent path is still an ellipse. herschel contrived a very ingenious and simple method by which he could discover from the observations the size and position of the ellipse in which the revolution actually takes place. he showed how, from the study of the apparent orbit of the star, and from certain measurements which could easily be effected upon it, the determination of the true ellipse in which the movement is performed could be arrived at. in other words, herschel solved in a beautiful manner the problem of finding the true orbits of double stars. the importance of this work may be inferred from the fact that it has served as the basis on which scores of other investigators have studied the fascinating subject of the movement of binary stars. the labours, both in the discovery and measurement of the double stars, and in the discussion of the observations with the object of finding the orbits of such stars as are in actual revolution, received due recognition in yet another gold medal awarded by the royal society. an address was delivered on the occasion by the duke of sussex ( th november, ), in the course of which, after stating that the medal had been conferred on sir john herschel, he remarks:-- "it has been said that distance of place confers the same privilege as distance of time, and i should gladly avail myself of the privilege which is thus afforded me by sir john herschel's separation from his country and friends, to express my admiration of his character in stronger terms than i should otherwise venture to use; for the language of panegyric, however sincerely it may flow from the heart, might be mistaken for that of flattery, if it could not thus claim somewhat of an historical character; but his great attainments in almost every department of human knowledge, his fine powers as a philosophical writer, his great services and his distinguished devotion to science, the high principles which have regulated his conduct in every relation of life, and, above all, his engaging modesty, which is the crown of all his other virtues, presenting such a model of an accomplished philosopher as can rarely be found beyond the regions of fiction, demand abler pens than mine to describe them in adequate terms, however much inclined i might feel to undertake the task." the first few lines of the eulogium just quoted allude to herschel's absence from england. this was not merely an episode of interest in the career of herschel, it was the occasion of one of the greatest scientific expeditions in the whole history of astronomy. herschel had, as we have seen, undertaken a revision of his father's "sweeps" for new objects, in those skies which are visible from our latitudes in the northern hemisphere. he had well-nigh completed this task. zone by zone the whole of the heavens which could be observed from windsor had passed under his review. he had added hundreds to the list of nebulae discovered by his father. he had announced thousands of double stars. at last, however, the great survey was accomplished. the contents of the northern hemisphere, so far at least as they could be disclosed by his telescope of twenty feet focal length, had been revealed. [plate: sir john herschel's observatory at feldhausen, cape of good hope.] but herschel felt that this mighty task had to be supplemented by another of almost equal proportions, before it could be said that the twenty-foot telescope had done its work. it was only the northern half of the celestial sphere which had been fully explored. the southern half was almost virgin territory, for no other astronomer was possessed of a telescope of such power as those which the herschels had used. it is true, of course, that as a certain margin of the southern hemisphere was visible from these latitudes, it had been more or less scrutinized by observers in northern skies. and the glimpses which had thus been obtained of the celestial objects in the southern sky, were such as to make an eager astronomer long for a closer acquaintance with the celestial wonders of the south. the most glorious object in the sidereal heavens, the great nebula in orion, lies indeed in that southern hemisphere to which the younger herschel's attention now became directed. it fortunately happens, however, for votaries of astronomy all the world over, that nature has kindly placed her most astounding object, the great nebula in orion, in such a favoured position, near the equator, that from a considerable range of latitudes, both north and south, the wonders of the nebula can be explored. there are grounds for thinking that the southern heavens contain noteworthy objects which, on the whole, are nearer to the solar system than are the noteworthy objects in the northern skies. the nearest star whose distance is known, alpha centauri, lies in the southern hemisphere, and so also does the most splendid cluster of stars. influenced by the desire to examine these objects, sir john herschel determined to take his great telescope to a station in the southern hemisphere, and thus complete his survey of the sidereal heavens. the latitude of the cape of good hope is such that a suitable site could be there found for his purpose. the purity of the skies in south africa promised to provide for the astronomer those clear nights which his delicate task of surveying the nebulae would require. on november , , sir john herschel, who had by this time received the honour of knighthood from william iv., sailed from portsmouth for the cape of good hope, taking with him his gigantic instruments. after a voyage of two months, which was considered to be a fair passage in those days, he landed in table bay, and having duly reconnoitred various localities, he decided to place his observatory at a place called feldhausen, about six miles from cape town, near the base of the table mountain. a commodious residence was there available, and in it he settled with his family. a temporary building was erected to contain the equatorial, but the great twenty-foot telescope was accommodated with no more shelter than is provided by the open canopy of heaven. as in his earlier researches at home, the attention of the great astronomer at the cape of good hope was chiefly directed to the measurement of the relative positions and distances apart of the double stars, and to the close examination of the nebulae. in the delineation of the form of these latter objects herschel found ample employment for his skilful pencil. many of the drawings he has made of the celestial wonders in the southern sky are admirable examples of celestial portraiture. the number of the nebulae and of those kindred objects, the star clusters, which herschel studied in the southern heavens, during four years of delightful labour, amount in all to one thousand seven hundred and seven. his notes on their appearance, and the determinations of their positions, as well as his measurements of double stars, and much other valuable astronomical research, were published in a splendid volume, brought out at the cost of the duke of northumberland. this is, indeed, a monumental work, full of interesting and instructive reading for any one who has a taste for astronomy. herschel had the good fortune to be at the cape on the occasion of the periodical return of halley's great comet in . to the study of this body he gave assiduous attention, and the records of his observations form one of the most interesting chapters in that remarkable volume to which we have just referred. [plate: column at feldhausen, cape town, to commemorate sir john herschel's survey of the southern heavens.] early in sir john herschel returned to england. he had made many friends at the cape, who deeply sympathised with his self- imposed labours while he was resident among them. they desired to preserve the recollection of this visit, which would always, they considered, be a source of gratification in the colony. accordingly, a number of scientific friends in that part of the world raised a monument with a suitable inscription, on the spot which had been occupied by the great twenty-foot reflector at feldhausen. his return to england after five years of absence was naturally an occasion for much rejoicing among the lovers of astronomy. he was entertained at a memorable banquet, and the queen, at her coronation, made him a baronet. his famous aunt caroline, at that time aged eighty, was still in the enjoyment of her faculties, and was able to estimate at its true value the further lustre which was added to the name she bore. but there is reason to believe that her satisfaction was not quite unmixed with other feelings. with whatever favour she might regard her nephew, he was still not the brother to whom her life had been devoted. so jealous was this vigorous old lady of the fame of the great brother william, that she could hardly hear with patience of the achievements of any other astronomer, and this failing existed in some degree even when that other astronomer happened to be her illustrious nephew. with sir john herschel's survey of the southern hemisphere it may be said that his career as an observing astronomer came to a close. he did not again engage in any systematic telescopic research. but it must not be inferred from this statement that he desisted from active astronomical work. it has been well observed that sir john herschel was perhaps the only astronomer who has studied with success, and advanced by original research, every department of the great science with which his name is associated. it was to some other branches of astronomy besides those concerned with looking through telescopes, that the rest of the astronomer's life was to be devoted. to the general student sir john herschel is best known by the volume which he published under the title of "outlines of astronomy." this is, indeed, a masterly work, in which the characteristic difficulties of the subject are resolutely faced and expounded with as much simplicity as their nature will admit. as a literary effort this work is admirable, both on account of its picturesque language and the ennobling conceptions of the universe which it unfolds. the student who desires to become acquainted with those recondite departments of astronomy, in which the effects of the disturbing action of one planet upon the motions of another planet are considered, will turn to the chapters in herschel's famous work on the subject. there he will find this complex matter elucidated, without resort to difficult mathematics. edition after edition of this valuable work has appeared, and though the advances of modern astronomy have left it somewhat out of date in certain departments, yet the expositions it contains of the fundamental parts of the science still remain unrivalled. another great work which sir john undertook after his return from the cape, was a natural climax to those labours on which his father and he had been occupied for so many years. we have already explained how the work of both these observers had been mainly devoted to the study of the nebulae and the star clusters. the results of their discoveries had been announced to the world in numerous isolated memoirs. the disjointed nature of these publications made their use very inconvenient. but still it was necessary for those who desired to study the marvellous objects discovered by the herschels, to have frequent recourse to the original works. to incorporate all the several observations of nebular into one great systematic catalogue, seemed, therefore, to be an indispensable condition of progress in this branch of knowledge. no one could have been so fitted for this task as sir john herschel. he, therefore, attacked and carried through the great undertaking. thus at last a grand catalogue of nebulae and clusters was produced. never before was there so majestic an inventory. if we remember that each of the nebulae is an object so vast, that the whole of the solar system would form an inconsiderable speck by comparison, what are we to think of a collection in which these objects are enumerated in thousands? in this great catalogue we find arranged in systematic order all the nebulae and all the clusters which had been revealed by the diligence of the herschels, father and son, in the northern hemisphere, and of the son alone in the southern hemisphere. nor should we omit to mention that the labours of other astronomers were likewise incorporated. it was unavoidable that the descriptions given to each of the objects should be very slight. abbreviations are used, which indicate that a nebula is bright, or very bright, or extremely bright, or faint, or very faint, or extremely faint. such phrases have certainly but a relative and technical meaning in such a catalogue. the nebulae entered as extremely bright by the experienced astronomer are only so described by way of contrast to the great majority of these delicate telescopic objects. most of the nebulae, indeed, are so difficult to see, that they admit of but very slight description. it should be observed that herschel's catalogue augmented the number of known nebulous objects to more than ten times that collected into any catalogue which had ever been compiled before the days of william herschel's observing began. but the study of these objects still advances, and the great telescopes now in use could probably show at least twice as many of these objects as are contained in the list of herschel, of which a new and enlarged edition has since been brought out by dr. dreyer. one of the best illustrations of sir john herschel's literary powers is to be found in the address which he delivered at the royal astronomical society, on the occasion of presenting a medal to mr. francis baily, in recognition of his catalogue of stars. the passage i shall here cite places in its proper aspect the true merit of the laborious duty involved in such a task as that which mr. baily had carried through with such success:-- "if we ask to what end magnificent establishments are maintained by states and sovereigns, furnished with masterpieces of art, and placed under the direction of men of first-rate talent and high-minded enthusiasm, sought out for those qualities among the foremost in the ranks of science, if we demand qui bono? for what good a bradley has toiled, or a maskelyne or a piazzi has worn out his venerable age in watching, the answer is--not to settle mere speculative points in the doctrine of the universe; not to cater for the pride of man by refined inquiries into the remoter mysteries of nature; not to trace the path of our system through space, or its history through past and future eternities. these, indeed, are noble ends and which i am far from any thought of depreciating; the mind swells in their contemplation, and attains in their pursuit an expansion and a hardihood which fit it for the boldest enterprise. but the direct practical utility of such labours is fully worthy of their speculative grandeur. the stars are the landmarks of the universe; and, amidst the endless and complicated fluctuations of our system, seem placed by its creator as guides and records, not merely to elevate our minds by the contemplation of what is vast, but to teach us to direct our actions by reference to what is immutable in his works. it is, indeed, hardly possible to over-appreciate their value in this point of view. every well-determined star, from the moment its place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can never deceive or fail him, the same for ever and in all places, of a delicacy so extreme as to be a test for every instrument yet invented by man, yet equally adapted for the most ordinary purposes; as available for regulating a town clock as for conducting a navy to the indies; as effective for mapping down the intricacies of a petty barony as for adjusting the boundaries of transatlantic empires. when once its place has been thoroughly ascertained and carefully recorded, the brazen circle with which that useful work was done may moulder, the marble pillar may totter on its base, and the astronomer himself survive only in the gratitude of posterity; but the record remains, and transfuses all its own exactness into every determination which takes it for a groundwork, giving to inferior instruments--nay, even to temporary contrivances, and to the observations of a few weeks or days--all the precision attained originally at the cost of so much time, labour, and expense." sir john herschel wrote many other works besides those we have mentioned. his "treatise on meteorology" is, indeed, a standard work on this subject, and numerous articles from the same pen on miscellaneous subjects, which have been collected and reprinted, seemed as a relaxation from his severe scientific studies. like certain other great mathematicians herschel was also a poet, and he published a translation of the iliad into blank verse. in his later years sir john herschel lived a retired life. for a brief period he had, indeed, been induced to accept the office of master of the mint. it was, however, evident that the routine of such an occupation was not in accordance with his tastes, and he gladly resigned it, to return to the seclusion of his study in his beautiful home at collingwood, in kent. his health having gradually failed, he died on the th may, , in the seventy-ninth year of his age. the earl of rosse. the subject of our present sketch occupies quite a distinct position in scientific history. unlike many others who have risen by their scientific discoveries from obscurity to fame, the great earl of rosse was himself born in the purple. his father, who, under the title of sir lawrence parsons, had occupied a distinguished position in the irish parliament, succeeded on the death of his father to the earldom which had been recently created. the subject of our present memoir was, therefore, the third of the earls of rosse, and he was born in york on june , . prior to his father's death in , he was known as lord oxmantown. the university education of the illustrious astronomer was begun in dublin and completed at oxford. we do not hear in his case of any very remarkable university career. lord rosse was, however, a diligent student, and obtained a first-class in mathematics. he always took a great deal of interest in social questions, and was a profound student of political economy. he had a seat in the house of commons, as member for king's county, from to , his ancestral estate being situated in this part of ireland. [plate: the earl of rosse.] lord rosse was endowed by nature with a special taste for mechanical pursuits. not only had he the qualifications of a scientific engineer, but he had the manual dexterity which qualified him personally to carry out many practical arts. lord rosse was, in fact, a skilful mechanic, an experienced founder, and an ingenious optician. his acquaintances were largely among those who were interested in mechanical pursuits, and it was his delight to visit the works or engineering establishments where refined processes in the arts were being carried on. it has often been stated--and as i have been told by members of his family, truly stated--that on one occasion, after he had been shown over some large works in the north of england, the proprietor bluntly said that he was greatly in want of a foreman, and would indeed be pleased if his visitor, who had evinced such extraordinary capacity for mechanical operations, would accept the post. lord rosse produced his card, and gently explained that he was not exactly the right man, but he appreciated the compliment, and this led to a pleasant dinner, and was the basis of a long friendship. i remember on one occasion hearing lord rosse explain how it was that he came to devote his attention to astronomy. it appears that when he found himself in the possession of leisure and of means, he deliberately cast around to think how that means and that leisure could be most usefully employed. nor was it surprising that he should search for a direction which would offer special scope for his mechanical tastes. he came to the conclusion that the building of great telescopes was an art which had received no substantial advance since the great days of william herschel. he saw that to construct mighty instruments for studying the heavens required at once the command of time and the command of wealth, while he also felt that this was a subject the inherent difficulties of which would tax to the uttermost whatever mechanical skill he might possess. thus it was he decided that the construction of great telescopes should become the business of his life. [plate: birr castle. plate: the mall, parsonstown.] in the centre of ireland, seventy miles from dublin, on the border between king's county and tipperary, is a little town whereof we must be cautious before writing the name. the inhabitants of that town frequently insist that its name is birr, * while the official designation is parsonstown, and to this day for every six people who apply one name to the town, there will be half a dozen who use the other. but whichever it may be, birr or parsonstown--and i shall generally call it by the latter name--it is a favourable specimen of an irish county town. the widest street is called the oxmantown mall. it is bordered by the dwelling-houses of the chief residents, and adorned with rows of stately trees. at one end of this distinctly good feature in the town is the parish church, while at the opposite end are the gates leading into birr castle, the ancestral home of the house of parsons. passing through the gates the visitor enters a spacious demesne, possessing much beauty of wood and water, one of the most pleasing features being the junction of the two rivers, which unite at a spot ornamented by beautiful timber. at various points illustrations of the engineering skill of the great earl will be observed. the beauty of the park has been greatly enhanced by the construction of an ample lake, designed with the consummate art by which art is concealed. even in mid-summer it is enlivened by troops of wild ducks preening themselves in that confidence which they enjoy in those happy localities where the sound of a gun is seldom heard. the water is led into the lake by a tube which passes under one of the two rivers just mentioned, while the overflow from the lake turns a water-wheel, which works a pair of elevators ingeniously constructed for draining the low-lying parts of the estate. * considering the fame acquired by parsonstown from lord rosse's mirrors, it may be interesting to note the following extract from "the natural history of ireland," by dr. gerard boate, thomas molyneux m.d., f.r.s., and others, which shows that years ago parsonstown was famous for its glass:-- "we shall conclude this chapter with the glass, there having been several glasshouses set up by the english in ireland, none in dublin or other cities, but all of them in the country; amongst which the principal was that of birre, a market town, otherwise called parsonstown, after one sir lawrence parsons, who, having purchased that lordship, built a goodly house upon it; his son william parsons having succeeded him in the possession of it; which town is situate in queen's county, about fifty miles (irish) to the southwest of dublin, upon the borders of the two provinces of leinster and munster; from this place dublin was furnished with all sorts of window and drinking glasses, and such other as commonly are in use. one part of the materials, viz., the sand, they had out of england; the other, to wit the ashes, they made in the place of ash-tree, and used no other. the chiefest difficulty was to get the clay for the pots to melt the materials in; this they had out of the north."--chap. xxi., sect. viii. "of the glass made in ireland." birr castle itself is a noble mansion with reminiscences from the time of cromwell. it is surrounded by a moat and a drawbridge of modern construction, and from its windows beautiful views can be had over the varied features of the park. but while the visitors to parsonstown will look with great interest on this residence of an irish landlord, whose delight it was to dwell in his own country, and among his own people, yet the feature which they have specially come to observe is not to be found in the castle itself. on an extensive lawn, sweeping down from the moat towards the lake, stand two noble masonry walls. they are turreted and clad with ivy, and considerably loftier than any ordinary house. as the visitor approaches, he will see between those walls what may at first sight appear to him to be the funnel of a steamer lying down horizontally. on closer approach he will find that it is an immense wooden tube, sixty feet long, and upwards of six feet in diameter. it is in fact large enough to admit of a tall man entering into it and walking erect right through from one end to the other. this is indeed the most gigantic instrument which has ever been constructed for the purpose of exploring the heavens. closely adjoining the walls between which the great tube swings, is a little building called "the observatory." in this the smaller instruments are contained, and there are kept the books which are necessary for reference. the observatory also offers shelter to the observers, and provides the bright fire and the cup of warm tea, which are so acceptable in the occasional intervals of a night's observation passed on the top of the walls with no canopy but the winter sky. almost the first point which would strike the visitor to lord rosse's telescope is that the instrument at which he is looking is not only enormously greater than anything of the kind that he has ever seen before, but also that it is something of a totally different nature. in an ordinary telescope he is accustomed to find a tube with lenses of glass at either end, while the large telescopes that we see in our observatories are also in general constructed on the same principle. at one end there is the object-glass, and at the other end the eye-piece, and of course it is obvious that with an instrument of this construction it is to the lower end of the tube that the eye of the observer must be placed when the telescope is pointed to the skies. but in lord rosse's telescope you would look in vain for these glasses, and it is not at the lower end of the instrument that you are to take your station when you are going to make your observations. the astronomer at parsonstown has rather to avail himself of the ingenious system of staircases and galleries, by which he is enabled to obtain access to the mouth of the great tube. the colossal telescope which swings between the great walls, like herschel's great telescope already mentioned, is a reflector, the original invention of which is due of course to newton. the optical work which is accomplished by the lenses in the ordinary telescope is effected in the type of instrument constructed by lord rosse by a reflecting mirror which is placed at the lower end of the vast tube. the mirror in this instrument is made of a metal consisting of two parts of copper to one of tin. as we have already seen, this mixture forms an alloy of a very peculiar nature. the copper and the tin both surrender their distinctive qualities, and unite to form a material of a very different physical character. the copper is tough and brown, the tin is no doubt silvery in hue, but soft and almost fibrous in texture. when the two metals are mixed together in the proportions i have stated, the alloy obtained is intensely hard and quite brittle being in both these respects utterly unlike either of the two ingredients of which it is composed. it does, however, resemble the tin in its whiteness, but it acquires a lustre far brighter than tin; in fact, this alloy hardly falls short of silver itself in its brilliance when polished. [plate: lord rosse's telescope. from a photograph by w. lawrence, upper sackville street, dublin.] the first duty that lord rosse had to undertake was the construction of this tremendous mirror, six feet across, and about four or five inches thick. the dimensions were far in excess of those which had been contemplated in any previous attempt of the same kind. herschel had no doubt fashioned one mirror of four feet in diameter, and many others of smaller dimensions, but the processes which he employed had never been fully published, and it was obvious that, with a large increase in dimensions, great additional difficulties had to be encountered. difficulties began at the very commencement of the process, and were experienced in one form or another at every subsequent stage. in the first place, the mere casting of a great disc of this mixture of tin and copper, weighing something like three or four tons, involved very troublesome problems. no doubt a casting of this size, if the material had been, for example, iron, would have offered no difficulties beyond those with which every practical founder is well acquainted, and which he has to encounter daily in the course of his ordinary work. but speculum metal is a material of a very intractable description. there is, of course, no practical difficulty in melting the copper, nor in adding the proper proportion of tin when the copper has been melted. there may be no great difficulty in arranging an organization by which several crucibles, filled with the molten material, shall be poured simultaneously so as to obtain the requisite mass of metal, but from this point the difficulties begin. for speculum metal when cold is excessively brittle, and were the casting permitted to cool like an ordinary copper or iron casting, the mirror would inevitably fly into pieces. lord rosse, therefore, found it necessary to anneal the casting with extreme care by allowing it to cool very slowly. this was accomplished by drawing the disc of metal as soon as it had entered into the solid state, though still glowing red, into an annealing oven. there the temperature was allowed to subside so gradually, that six weeks elapsed before the mirror had reached the temperature of the external air. the necessity for extreme precaution in the operation of annealing will be manifest if we reflect on one of the accidents which happened. on a certain occasion, after the cooling of a great casting had been completed, it was found, on withdrawing the speculum, that it was cracked into two pieces. this mishap was eventually traced to the fact that one of the walls of the oven had only a single brick in its thickness, and that therefore the heat had escaped more easily through that side than through the other sides which were built of double thickness. the speculum had, consequently, not cooled uniformly, and hence the fracture had resulted. undeterred, however, by this failure, as well as by not a few other difficulties, into a description of which we cannot now enter, lord rosse steadily adhered to his self-imposed task, and at last succeeded in casting two perfect discs on which to commence the tedious processes of grinding and polishing. the magnitude of the operations involved may perhaps be appreciated if i mention that the value of the mere copper and tin entering into the composition of each of the mirrors was about pounds. in no part of his undertaking was lord rosse's mechanical ingenuity more taxed than in the devising of the mechanism for carrying out the delicate operations of grinding and polishing the mirrors, whose casting we have just mentioned. in the ordinary operations of the telescope-maker, such processes had hitherto been generally effected by hand, but, of course, such methods became impossible when dealing with mirrors which were as large as a good-sized dinner table, and whose weight was measured by tons. the rough grinding was effected by means of a tool of cast iron about the same size as the mirror, which was moved by suitable machinery both backwards and forwards, and round and round, plenty of sand and water being supplied between the mirror and the tool to produce the necessary attrition. as the process proceeded and as the surface became smooth, emery was used instead of sand; and when this stage was complete, the grinding tool was removed and the polishing tool was substituted. the essential part of this was a surface of pitch, which, having been temporarily softened by heat, was then placed on the mirror, and accepted from the mirror the proper form. rouge was then introduced as the polishing powder, and the operation was continued about nine hours, by which time the great mirror had acquired the appearance of highly polished silver. when completed, the disc of speculum metal was about six feet across and four inches thick. the depression in the centre was about half an inch. mounted on a little truck, the great speculum was then conveyed to the instrument, to be placed in its receptacle at the bottom of the tube, the length of which was sixty feet, this being the focal distance of the mirror. another small reflector was inserted in the great tube sideways, so as to direct the gaze of the observer down upon the great reflector. thus was completed the most colossal instrument for the exploration of the heavens which the art of man has ever constructed. [plate: roman catholic church at parsonstown.] it was once my privilege to be one of those to whom the illustrious builder of the great telescope entrusted its use. for two seasons in and i had the honour of being lord rosse's astronomer. during that time i passed many a fine night in the observer's gallery, examining different objects in the heavens with the aid of this remarkable instrument. at the time i was there, the objects principally studied were the nebulae, those faint stains of light which lie on the background of the sky. lord rosse's telescope was specially suited for the scrutiny of these objects, inasmuch as their delicacy required all the light-grasping power which could be provided. one of the greatest discoveries made by lord rosse, when his huge instrument was first turned towards the heavens, consisted in the detection of the spiral character of some of the nebulous forms. when the extraordinary structure of these objects was first announced, the discovery was received with some degree of incredulity. other astronomers looked at the same objects, and when they failed to discern--and they frequently did fail to discern--the spiral structure which lord rosse had indicated, they drew the conclusion that this spiral structure did not exist. they thought it must be due possibly to some instrumental defect or to the imagination of the observer. it was, however, hardly possible for any one who was both willing and competent to examine into the evidence, to doubt the reality of lord rosse's discoveries. it happens, however, that they have been recently placed beyond all doubt by testimony which it is impossible to gainsay. a witness never influenced by imagination has now come forward, and the infallible photographic plate has justified lord rosse. among the remarkable discoveries which dr. isaac roberts has recently made in the application of his photographic apparatus to the heavens, there is none more striking than that which declares, not only that the nebulae which lord rosse described as spirals, actually do possess the character so indicated, but that there are many others of the same description. he has even brought to light the astonishingly interesting fact that there are invisible objects of this class which have never been seen by human eye, but whose spiral character is visible to the peculiar delicacy of the photographic telescope. in his earlier years, lord rosse himself used to be a diligent observer of the heavenly bodies with the great telescope which was completed in the year . but i think that those who knew lord rosse well, will agree that it was more the mechanical processes incidental to the making of the telescope which engaged his interest than the actual observations with the telescope when it was completed. indeed one who was well acquainted with him believed lord rosse's special interest in the great telescope ceased when the last nail had been driven into it. but the telescope was never allowed to lie idle, for lord rosse always had associated with him some ardent young astronomer, whose delight it was to employ to the uttermost the advantages of his position in exploring the wonders of the sky. among those who were in this capacity in the early days of the great telescope, i may mention my esteemed friend dr. johnston stoney. such was the renown of lord rosse himself, brought about by his consummate mechanical genius and his astronomical discoveries, and such the interest which gathered around the marvellous workshops at birr castle, wherein his monumental exhibitions of optical skill were constructed, that visitors thronged to see him from all parts of the world. his home at parsonstown became one of the most remarkable scientific centres in great britain; thither assembled from time to time all the leading men of science in the country, as well as many illustrious foreigners. for many years lord rosse filled with marked distinction the exalted position of president of the royal society, and his advice and experience in practical mechanical matters were always at the disposal of those who sought his assistance. personally and socially lord rosse endeared himself to all with whom he came in contact. i remember one of the attendants telling me that on one occasion he had the misfortune to let fall and break one of the small mirrors on which lord rosse had himself expended many hours of hard personal labour. the only remark of his lordship was that "accidents will happen." the latter years of his life lord rosse passed in comparative seclusion; he occasionally went to london for a brief sojourn during the season, and he occasionally went for a cruise in his yacht; but the greater part of the year he spent at birr castle, devoting himself largely to the study of political and social questions, and rarely going outside the walls of his demesne, except to church on sunday mornings. he died on october , . he was succeeded by his eldest son, the present earl of rosse, who has inherited his father's scientific abilities, and done much notable work with the great telescope. airy. in our sketch of the life of flamsteed, we have referred to the circumstances under which the famous observatory that crowns greenwich hill was founded. we have also had occasion to mention that among the illustrious successors of flamsteed both halley and bradley are to be included. but a remarkable development of greenwich observatory from the modest establishment of early days took place under the direction of the distinguished astronomer whose name is at the head of this chapter. by his labours this temple of science was organised to such a degree of perfection that it has served in many respects as a model for other astronomical establishments in various parts of the world. an excellent account of airy's career has been given by professor h. h. turner, in the obituary notice published by the royal astronomical society. to this i am indebted for many of the particulars here to be set down concerning the life of the illustrious astronomer royal. the family from which airy took his origin came from kentmere, in westmoreland. his father, william airy, belonged to a lincolnshire branch of the same stock. his mother's maiden name was ann biddell, and her family resided at playford, near ipswich. william airy held some small government post which necessitated an occasional change of residence to different parts of the country, and thus it was that his son, george biddell, came to be born at alnwick, on th july, . the boy's education, so far as his school life was concerned was partly conducted at hereford and partly at colchester. he does not, however, seem to have derived much benefit from the hours which he passed in the schoolroom. but it was delightful to him to spend his holidays on the farm at playford, where his uncle, arthur biddell, showed him much kindness. the scenes of his early youth remained dear to airy throughout his life, and in subsequent years he himself owned a house at playford, to which it was his special delight to resort for relaxation during the course of his arduous career. in spite of the defects of his school training he seems to have manifested such remarkable abilities that his uncle decided to enter him in cambridge university. he accordingly joined trinity college as a sizar in , and after a brilliant career in mathematical and physical science he graduated as senior wrangler in . it may be noted as an exceptional circumstance that, notwithstanding the demands on his time in studying for his tripos, he was able, after his second term of residence, to support himself entirely by taking private pupils. in the year after he had taken his degree he was elected to a fellowship at trinity college. having thus gained an independent position, airy immediately entered upon that career of scientific work which he prosecuted without intermission almost to the very close of his life. one of his most interesting researches in these early days is on the subject of astigmatism, which defect he had discovered in his own eyes. his investigations led him to suggest a means of correcting this defect by using a pair of spectacles with lenses so shaped as to counteract the derangement which the astigmatic eye impressed upon the rays of light. his researches on this subject were of a very complete character, and the principles he laid down are to the present day practically employed by oculists in the treatment of this malformation. on the th of december, , airy was elected to the lucasian professorship of mathematics in the university of cambridge, the chair which newton's occupancy had rendered so illustrious. his tenure of this office only lasted for two years, when he exchanged it for the plumian professorship. the attraction which led him to desire this change is doubtless to be found in the circumstance that the plumian professorship of astronomy carried with it at that time the appointment of director of the new astronomical observatory, the origin of which must now be described. those most interested in the scientific side of university life decided in that it would be proper to found an astronomical observatory at cambridge. donations were accordingly sought for this purpose, and upwards of , pounds were contributed by members of the university and the public. to this sum , pounds were added by a grant from the university chest, and in further sums amounting altogether to , pounds were given by the university for the same object. the regulations as to the administration of the new observatory placed it under the management of the plumian professor, who was to be provided with two assistants. their duties were to consist in making meridian observations of the sun, moon, and the stars, and the observations made each year were to be printed and published. the observatory was also to be used in the educational work of the university, for it was arranged that smaller instruments were to be provided by which students could be instructed in the practical art of making astronomical observations. the building of the cambridge astronomical observatory was completed in , but in , when airy entered on the discharge of his duties as director, the establishment was still far from completion, in so far as its organisation was concerned. airy commenced his work so energetically that in the next year after his appointment he was able to publish the first volume of "cambridge astronomical observations," notwithstanding that every part of the work, from the making of observations to the revising of the proof-sheets, had to be done by himself. it may here be remarked that these early volumes of the publications of the cambridge observatory contained the first exposition of those systematic methods of astronomical work which airy afterwards developed to such a great extent at greenwich, and which have been subsequently adopted in many other places. no more profitable instruction for the astronomical beginner can be found than that which can be had by the study of these volumes, in which the plumian professor has laid down with admirable clearness the true principles on which meridian work should be conducted. [plate: sir george airy. from a photograph by mr. e.p. adams, greenwich.] airy gradually added to the instruments with which the observatory was originally equipped. a mural circle was mounted in , and in the same year a small equatorial was erected by jones. this was made use of by airy in a well-known series of observations of jupiter's fourth satellite for the determination of the mass of the great planet. his memoir on this subject fully ex pounds the method of finding the weight of a planet from observations of the movements of a satellite by which the planet is attended. this is, indeed, a valuable investigation which no student of astronomy can afford to neglect. the ardour with which airy devoted himself to astronomical studies may be gathered from a remarkable report on the progress of astronomy during the present century, which he communicated to the british association at its second meeting in . in the early years of his life at cambridge his most famous achievement was connected with a research in theoretical astronomy for which consummate mathematical power was required. we can only give a brief account of the subject, for to enter into any full detail with regard to it would be quite out of the question. venus is a planet of about the same size and the same weight as the earth, revolving in an orbit which lies within that described by our globe. venus, consequently, takes less time than the earth to accomplish one revolution round the sun, and it happens that the relative movements of venus and the earth are so proportioned that in the time in which our earth accomplishes eight of her revolutions the other planet will have accomplished almost exactly thirteen. it, therefore, follows that if the earth and venus are in line with the sun at one date, then in eight years later both planets will again be found at the same points in their orbits. in those eight years the earth has gone round eight times, and has, therefore, regained its original position, while in the same period venus has accomplished thirteen complete revolutions, and, therefore, this planet also has reached the same spot where it was at first. venus and the earth, of course, attract each other, and in consequence of these mutual attractions the earth is swayed from the elliptic track which it would otherwise pursue. in like manner venus is also forced by the attraction of the earth to revolve in a track which deviates from that which it would otherwise follow. owing to the fact that the sun is of such preponderating magnitude (being, in fact, upwards of , times as heavy as either venus or the earth), the disturbances induced in the motion of either planet, in consequence of the attraction of the other, are relatively insignificant to the main controlling agency by which each of the movements is governed. it is, however, possible under certain circumstances that the disturbing effects produced upon one planet by the other can become so multiplied as to produce peculiar effects which attain measurable dimensions. suppose that the periodic times in which the earth and venus revolved had no simple relation to each other, then the points of their tracks in which the two planets came into line with the sun would be found at different parts of the orbits, and consequently the disturbances would to a great extent neutralise each other, and produce but little appreciable effect. as, however, venus and the earth come back every eight years to nearly the same positions at the same points of their track, an accumulative effect is produced. for the disturbance of one planet upon the other will, of course, be greatest when those two planets are nearest, that is, when they lie in line with the sun and on the same side of it. every eight years a certain part of the orbit of the earth is, therefore, disturbed by the attraction of venus with peculiar vigour. the consequence is that, owing to the numerical relation between the movements of the planets to which i have referred, disturbing effects become appreciable which would otherwise be too small to permit of recognition. airy proposed to himself to compute the effects which venus would have on the movement of the earth in consequence of the circumstance that eight revolutions of the one planet required almost the same time as thirteen revolutions of the other. this is a mathematical inquiry of the most arduous description, but the plumian professor succeeded in working it out, and he had, accordingly, the gratification of announcing to the royal society that he had detected the influence which venus was thus able to assert on the movement of our earth around the sun. this remarkable investigation gained for its author the gold medal of the royal astronomical society in the year . in consequence of his numerous discoveries, airy's scientific fame had become so well recognised that the government awarded him a special pension, and in , when pond, who was then astronomer royal, resigned, airy was offered the post at greenwich. there was in truth, no scientific inducement to the plumian professor to leave the comparatively easy post he held at cambridge, in which he had ample leisure to devote himself to those researches which specially interested him, and accept that of the much more arduous observatory at greenwich. there were not even pecuniary inducements to make the change; however, he felt it to be his duty to accede to the request which the government had made that he would take up the position which pond had vacated, and accordingly airy went to greenwich as astronomer royal on october st, . he immediately began with his usual energy to organise the systematic conduct of the business of the national observatory. to realise one of the main characteristics of airy's great work at greenwich, it is necessary to explain a point that might not perhaps be understood without a little explanation by those who have no practical experience in an observatory. in the work of an establishment such as greenwich, an observation almost always consists of a measurement of some kind. the observer may, for instance, be making a measurement of the time at which a star passes across a spider line stretched through the field of view; on another occasion his object may be the measurement of an angle which is read off by examining through a microscope the lines of division on a graduated circle when the telescope is so pointed that the star is placed on a certain mark in the field of view. in either case the immediate result of the astronomical observation is a purely numerical one, but it rarely happens, indeed we may say it never happens, that the immediate numerical result which the observation gives expresses directly the quantity which we are really seeking for. no doubt the observation has been so designed that the quantity we want to find can be obtained from the figures which the measurement gives, but the object sought is not those figures, for there are always a multitude of other influences by which those figures are affected. for example, if an observation were to be perfect, then the telescope with which the observation is made should be perfectly placed in the exact position which it ought to occupy; this is, however, never the case, for no mechanic can ever construct or adjust a telescope so perfectly as the wants of the astronomer demand. the clock also by which we determine the time of the observation should be correct, but this is rarely if ever the case. we have to correct our observations for such errors, that is to say, we have to determine the errors in the positions of our telescopes and the errors in the going of our clocks, and then we have to determine what the observations would have been had our telescopes been absolutely perfect, and had our clocks been absolutely correct. there are also many other matters which have to be attended to in order to reduce our observations so as to obtain from the figures as yielded to the observer at the telescope the actual quantities which it is his object to determine. the work of effecting these reductions is generally a very intricate and laborious matter, so that it has not unfrequently happened that while observations have accumulated in an observatory, yet the tedious duty of reducing these observations has been allowed to fall into arrear. when airy entered on his duties at greenwich he found there an enormous mass of observations which, though implicitly containing materials of the greatest value to astronomers, were, in their unreduced form, entirely unavailable for any useful purpose. he, therefore, devoted himself to coping with the reduction of the observations of his predecessors. he framed systematic methods by which the reductions were to be effected, and he so arranged the work that little more than careful attention to numerical accuracy would be required for the conduct of the operations. encouraged by the admiralty, for it is under this department that greenwich observatory is placed, the astronomer royal employed a large force of computers to deal with the work. by his energy and admirable organisation he managed to reduce an extremely valuable series of planetary observations, and to publish the results, which have been of the greatest importance to astronomical investigation. the astronomer royal was a capable, practical engineer as well as an optician, and he presently occupied himself by designing astronomical instruments of improved pattern, which should replace the antiquated instruments he found in the observatory. in the course of years the entire equipment underwent a total transformation. he ordered a great meridian circle, every part of which may be said to have been formed from his own designs. he also designed the mounting for a fine equatorial telescope worked by a driving clock, which he had himself invented. gradually the establishment at greenwich waxed great under his incessant care. it was the custom for the observatory to be inspected every year by a board of visitors, whose chairman was the president of the royal society. at each annual visitation, held on the first saturday in june, the visitors received a report from the astronomer royal, in which he set forth the business which had been accomplished during the past year. it was on these occasions that applications were made to the admiralty, either for new instruments or for developing the work of the observatory in some other way. after the more official business of the inspection was over, the observatory was thrown open to visitors, and hundreds of people enjoyed on that day the privilege of seeing the national observatory. these annual gatherings are happily still continued, and the first saturday in june is known to be the occasion of one of the most interesting reunions of scientific men which takes place in the course of the year. airy's scientific work was, however, by no means confined to the observatory. he interested himself largely in expeditions for the observation of eclipses and in projects for the measurement of arcs on the earth. he devoted much attention to the collection of magnetic observations from various parts of the world. especially will it be remembered that the circumstances of the transits of venus, which occurred in and in , were investigated by him, and under his guidance expeditions were sent forth to observe the transits from those localities in remote parts of the earth where observations most suitable for the determination of the sun's distance from the earth could be obtained. the astronomer royal also studied tidal phenomena, and he rendered great service to the country in the restoration of the standards of length and weight which had been destroyed in the great fire at the house of parliament in october, . in the most practical scientific matters his advice was often sought, and was as cheerfully rendered. now we find him engaged in an investigation of the irregularities of the compass in iron ships, with a view to remedying its defects; now we find him reporting on the best gauge for railways. among the most generally useful developments of the observatory must be mentioned the telegraphic method for the distribution of exact time. by arrangement with the post office, the astronomers at greenwich despatch each morning a signal from the observatory to london at ten o'clock precisely. by special apparatus, this signal is thence distributed automatically over the country, so as to enable the time to be known everywhere accurately to a single second. it was part of the same system that a time ball should be dropped daily at one o'clock at deal, as well as at other places, for the purpose of enabling ship's chronometers to be regulated. airy's writings were most voluminous, and no fewer than forty-eight memoirs by him are mentioned in the "catalogue of scientific memoirs," published by the royal society up to the year , and this only included ten years out of an entire life of most extraordinary activity. many other subjects besides those of a purely scientific character from time to time engaged his attention. he wrote, for instance, a very interesting treatise on the roman invasion of britain, especially with a view of determining the port from which caesar set forth from gaul, and the point at which he landed on the british coast. airy was doubtless led to this investigation by his study of the tidal phenomena in the straits of dover. perhaps the astronomer royal is best known to the general reading public by his excellent lectures on astronomy, delivered at the ipswich museum in . this book has passed through many editions, and it gives a most admirable account of the manner in which the fundamental problems in astronomy have to be attacked. as years rolled by almost every honour and distinction that could be conferred upon a scientific man was awarded to sir george airy. he was, indeed, the recipient of other honours not often awarded for scientific distinction. among these we may mention that in he received the freedom of the city of london, "as a recognition of his indefatigable labours in astronomy, and of his eminent services in the advancement of practical science, whereby he has so materially benefited the cause of commerce and civilisation." until his eightieth year airy continued to discharge his labours at greenwich with unflagging energy. at last, on august th, , he resigned the office which he had held so long with such distinction to himself and such benefit to his country. he had married in the daughter of the rev. richard smith, of edensor. lady airy died in , and three sons and three daughters survived him. one daughter is the wife of dr. routh, of cambridge, and his other daughters were the constant companions of their father during the declining years of his life. up to the age of ninety he enjoyed perfect physical health, but an accidental fall which then occurred was attended with serious results. he died on saturday, january nd, , and was buried in the churchyard at playford. hamilton. william rowan hamilton was born at midnight between the rd and th of august, , at dublin, in the house which was then , but subsequently , dominick street. his father, archibald hamilton, was a solicitor, and william was the fourth of a family of nine. with reference to his descent, it may be sufficient to notice that his ancestors appear to have been chiefly of gentle irish families, but that his maternal grandmother was of scottish birth. when he was about a year old, his father and mother decided to hand over the education of the child to his uncle, james hamilton, a clergyman of trim, in county meath. james hamilton's sister, sydney, resided with him, and it was in their home that the days of william's childhood were passed. in mr. graves' "life of sir william rowan hamilton" a series of letters will be found, in which aunt sydney details the progress of the boy to his mother in dublin. probably there is no record of an infant prodigy more extraordinary than that which these letters contain. at three years old his aunt assured the mother that william is "a hopeful blade," but at that time it was his physical vigour to which she apparently referred; for the proofs of his capacity, which she adduces, related to his prowess in making boys older than himself fly before him. in the second letter, a month later, we hear that william is brought in to read the bible for the purpose of putting to shame other boys double his age who could not read nearly so well. uncle james appears to have taken much pains with william's schooling, but his aunt said that "how he picks up everything is astonishing, for he never stops playing and jumping about." when he was four years and three months old, we hear that he went out to dine at the vicar's, and amused the company by reading for them equally well whether the book was turned upside down or held in any other fashion. his aunt assures the mother that "willie is a most sensible little creature, but at the same time has a great deal of roguery." at four years and five months old he came up to pay his mother a visit in town, and she writes to her sister a description of the boy;-- "his reciting is astonishing, and his clear and accurate knowledge of geography is beyond belief; he even draws the countries with a pencil on paper, and will cut them out, though not perfectly accurate, yet so well that a anybody knowing the countries could not mistake them; but, you will think this nothing when i tell you that he reads latin, greek, and hebrew." aunt sydney recorded that the moment willie got back to trim he was desirous of at once resuming his former pursuits. he would not eat his breakfast till his uncle had heard him his hebrew, and he comments on the importance of proper pronunciation. at five he was taken to see a friend, to whom he repeated long passages from dryden. a gentleman present, who was not unnaturally sceptical about willie's attainments, desired to test him in greek, and took down a copy of homer which happened to have the contracted type, and to his amazement willie went on with the greatest ease. at six years and nine months he was translating homer and virgil; a year later his uncle tells us that william finds so little difficulty in learning french and italian, that he wishes to read homer in french. he is enraptured with the iliad, and carries it about with him, repeating from it whatever particularly pleases him. at eight years and one month the boy was one of a party who visited the scalp in the dublin mountains, and he was so delighted with the scenery that he forthwith delivered an oration in latin. at nine years and six months he is not satisfied until he learns sanscrit; three months later his thirst for the oriental languages is unabated, and at ten years and four months he is studying arabic and persian. when nearly twelve he prepared a manuscript ready for publication. it was a "syriac grammar," in syriac letters and characters compiled from that of buxtorf, by william hamilton, esq., of dublin and trim. when he was fourteen, the persian ambassador, mirza abul hassan khan, paid a visit to dublin, and, as a practical exercise in his oriental languages, the young scholar addressed to his excellency a letter in persian; a translation of which production is given by mr. graves. when william was fourteen he had the misfortune to lose his father; and he had lost his mother two years previously. the boy and his three sisters were kindly provided for by different members of the family on both sides. it was when william was about fifteen that his attention began to be turned towards scientific subjects. these were at first regarded rather as a relaxation from the linguistic studies with which he had been so largely occupied. on november nd, , he notes in his journal that he had begun newton's "principia": he commenced also the study of astronomy by observing eclipses, occultations, and similar phenomena. when he was sixteen we learn that he had read conic sections, and that he was engaged in the study of pendulums. after an attack of illness, he was moved for change to dublin, and in may, , we find him reading the differential calculus and laplace's "mecanique celeste." he criticises an important part of laplace's work relative to the demonstration of the parallelogram of forces. in this same year appeared the first gushes of those poems which afterwards flowed in torrents. his somewhat discursive studies had, however, now to give place to a more definite course of reading in preparation for entrance to the university of dublin. the tutor under whom he entered, charles boyton, was himself a distinguished man, but he frankly told the young william that he could be of little use to him as a tutor, for his pupil was quite as fit to be his tutor. eliza hamilton, by whom this is recorded, adds, "but there is one thing which boyton would promise to be to him, and that was a friend; and that one proof he would give of this should be that, if ever he saw william beginning to be upset by the sensation he would excite, and the notice he would attract, he would tell him of it." at the beginning of his college career he distanced all his competitors in every intellectual pursuit. at his first term examination in the university he was first in classics and first in mathematics, while he received the chancellor's prize for a poem on the ionian islands, and another for his poem on eustace de st. pierre. there is abundant testimony that hamilton had "a heart for friendship formed." among the warmest of the friends whom he made in these early days was the gifted maria edgeworth, who writes to her sister about "young mr. hamilton, an admirable crichton of eighteen, a real prodigy of talents, who dr. brinkley says may be a second newton, quiet, gentle, and simple." his sister eliza, to whom he was affectionately attached, writes to him in :-- "i had been drawing pictures of you in my mind in your study at cumberland street with 'xenophon,' &c., on the table, and you, with your most awfully sublime face of thought, now sitting down, and now walking about, at times rubbing your hands with an air of satisfaction, and at times bursting forth into some very heroical strain of poetry in an unknown language, and in your own internal solemn ventriloquist-like voice, when you address yourself to the silence and solitude of your own room, and indeed, at times, even when your mysterious poetical addresses are not quite unheard." this letter is quoted because it refers to a circumstance which all who ever met with hamilton, even in his latest years, will remember. he was endowed with two distinct voices, one a high treble, the other a deep bass, and he alternately employed these voices not only in ordinary conversation, but when he was delivering an address on the profundities of quaternions to the royal irish academy, or on similar occasions. his friends had long grown so familiar with this peculiarity that they were sometimes rather surprised to find how ludicrous it appeared to strangers. hamilton was fortunate in finding, while still at a very early age, a career open before him which was worthy of his talents. he had not ceased to be an undergraduate before he was called to fill an illustrious chair in his university. the circumstances are briefly as follows. we have already mentioned that, in , brinkley was appointed bishop of cloyne, and the professorship of astronomy thereupon became vacant. such was hamilton's conspicuous eminence that, notwithstanding he was still an undergraduate, and had only just completed his twenty-first year, he was immediately thought of as a suitable successor to the chair. indeed, so remarkable were his talents in almost every direction that had the vacancy been in the professorship of classics or of mathematics, of english literature or of metaphysics, of modern or of oriental languages, it seems difficult to suppose that he would not have occurred to every one as a possible successor. the chief ground, however, on which the friends of hamilton urged his appointment was the earnest of original power which he had already shown in a research on the theory of systems of rays. this profound work created a new branch of optics, and led a few years later to a superb discovery, by which the fame of its author became world-wide. at first hamilton thought it would be presumption for him to apply for so exalted a position; he accordingly retired to the country, and resumed his studies for his degree. other eminent candidates came forward, among them some from cambridge, and a few of the fellows from trinity college, dublin, also sent in their claims. it was not until hamilton received an urgent letter from his tutor boyton, in which he was assured of the favourable disposition of the board towards his candidature, that he consented to come forward, and on june th, , he was unanimously chosen to succeed the bishop of cloyne as professor of astronomy in the university. the appointment met with almost universal approval. it should, however, be noted that brinkley, whom hamilton succeeded, did not concur in the general sentiment. no one could have formed a higher opinion than he had done of hamilton's transcendent powers; indeed, it was on that very ground that he seemed to view the appointment with disapprobation. he considered that it would have been wiser for hamilton to have obtained a fellowship, in which capacity he would have been able to exercise a greater freedom in his choice of intellectual pursuits. the bishop seems to have thought, and not without reason, that hamilton's genius would rather recoil from much of the routine work of an astronomical establishment. now that hamilton's whole life is before us, it is easy to see that the bishop was entirely wrong. it is quite true that hamilton never became a skilled astronomical observer; but the seclusion of the observatory was eminently favourable to those gigantic labours to which his life was devoted, and which have shed so much lustre, not only on hamilton himself, but also on his university and his country. in his early years at dunsink, hamilton did make some attempts at a practical use of the telescopes, but he possessed no natural aptitude for such work, while exposure which it involved seems to have acted injuriously on his health. he, therefore, gradually allowed his attention to be devoted to those mathematical researches in which he had already given such promise of distinction. although it was in pure mathematics that he ultimately won his greatest fame, yet he always maintained and maintained with justice, that he had ample claims to the title of an astronomer. in his later years he set forth this position himself in a rather striking manner. de morgan had written commending to hamilton's notice grant's "history of physical astronomy." after becoming acquainted with the book, hamilton writes to his friend as follows:-- "the book is very valuable, and very creditable to its composer. but your humble servant may be pardoned if he finds himself somewhat amused at the title, 'history of physical astronomy from the earliest ages to the middle of the nineteenth century,' when he fails to observe any notice of the discoveries of sir w. r. hamilton in the theory of the 'dynamics of the heavens.'" the intimacy between the two correspondents will account for the tone of this letter; and, indeed, hamilton supplies in the lines which follow ample grounds for his complaint. he tells how jacobi spoke of him in manchester in as "le lagrange de votre pays," and how donkin had said that, "the analytical theory of dynamics as it exists at present is due mainly to the labours of la grange poisson, sir w. r. hamilton, and jacobi, whose researches on this subject present a series of discoveries hardly paralleled for their elegance and importance in any other branch of mathematics." in the same letter hamilton also alludes to the success which had attended the applications of his methods in other hands than his own to the elucidation of the difficult subject of planetary perturbations. even had his contributions to science amounted to no more than these discoveries, his tenure of the chair would have been an illustrious one. it happens, however, that in the gigantic mass of his intellectual work these researches, though intrinsically of such importance, assume what might almost be described as a relative insignificance. the most famous achievement of hamilton's earlier years at the observatory was the discovery of conical refraction. this was one of those rare events in the history of science, in which a sagacious calculation has predicted a result of an almost startling character, subsequently confirmed by observation. at once this conferred on the young professor a world-wide renown. indeed, though he was still only twenty-seven, he had already lived through an amount of intellectual activity which would have been remarkable for a man of threescore and ten. simultaneously with his growth in fame came the growth of his several friendships. there were, in the first place, his scientific friendships with herschel, robinson, and many others with whom he had copious correspondence. in the excellent biography to which i have referred, hamilton's correspondence with coleridge may be read, as can also the letters to his lady correspondents, among them being maria edgeworth, lady dunraven, and lady campbell. many of these sheets relate to literary matters, but they are largely intermingled with genial pleasantry, and serve at all events to show the affection and esteem with which he was regarded by all who had the privilege of knowing him. there are also the letters to the sisters whom he adored, letters brimming over with such exalted sentiment, that most ordinary sisters would be tempted to receive them with a smile in the excessively improbable event of their still more ordinary brothers attempting to pen such effusions. there are also indications of letters to and from other young ladies who from time to time were the objects of hamilton's tender admiration. we use the plural advisedly, for, as mr. graves has set forth, hamilton's love affairs pursued a rather troubled course. the attention which he lavished on one or two fair ones was not reciprocated, and even the intense charms of mathematical discovery could not assuage the pangs which the disappointed lover experienced. at last he reached the haven of matrimony in , when he was married to miss bayly. of his married life hamilton said, many years later to de morgan, that it was as happy as he expected, and happier than he deserved. he had two sons, william and archibald, and one daughter, helen, who became the wife of archdeacon o'regan. [plate: sir w. rowan hamilton.] the most remarkable of hamilton's friendships in his early years was unquestionably that with wordsworth. it commenced with hamilton's visit to keswick; and on the first evening, when the poet met the young mathematician, an incident occurred which showed the mutual interest that was aroused. hamilton thus describes it in a letter to his sister eliza:-- "he (wordsworth) walked back with our party as far as their lodge, and then, on our bidding mrs. harrison good-night, i offered to walk back with him while my party proceeded to the hotel. this offer he accepted, and our conversation had become so interesting that when we had arrived at his home, a distance of about a mile, he proposed to walk back with me on my way to ambleside, a proposal which you may be sure i did not reject; so far from it that when he came to turn once more towards his home i also turned once more along with him. it was very late when i reached the hotel after all this walking." hamilton also submitted to wordsworth an original poem, entitled "it haunts me yet." the reply of wordsworth is worth repeating:-- "with a safe conscience i can assure you that, in my judgment, your verses are animated with the poetic spirit, as they are evidently the product of strong feeling. the sixth and seventh stanzas affected me much, even to the dimming of my eyes and faltering of my voice while i was reading them aloud. having said this, i have said enough. now for the per contra. you will not, i am sure, be hurt when i tell you that the workmanship (what else could be expected from so young a writer?) is not what it ought to be. . . "my household desire to be remembered to you in no formal way. seldom have i parted--never, i was going to say--with one whom after so short an acquaintance i lost sight of with more regret. i trust we shall meet again." the further affectionate intercourse between hamilton and wordsworth is fully set forth, and to hamilton's latest years a recollection of his "rydal hours" was carefully treasured and frequently referred to. wordsworth visited hamilton at the observatory, where a beautiful shady path in the garden is to the present day spoken of as "wordsworth's walk." it was the practice of hamilton to produce a sonnet on almost every occasion which admitted of poetical treatment, and it was his delight to communicate his verses to his friends all round. when whewell was producing his "bridgewater treatises," he writes to hamilton in :-- "your sonnet which you showed me expressed much better than i could express it the feeling with which i tried to write this book, and i once intended to ask your permission to prefix the sonnet to my book, but my friends persuaded me that i ought to tell my story in my own prose, however much better your verse might be." the first epoch-marking contribution to theoretical dynamics after the time of newton was undoubtedly made by lagrange, in his discovery of the general equations of motion. the next great step in the same direction was that taken by hamilton in his discovery of a still more comprehensive method. of this contribution hamilton writes to whewell, march st, :-- "as to my late paper, a day or two ago sent off to london, it is merely mathematical and deductive. i ventured, indeed, to call it the 'mecanique analytique' of lagrange, 'a scientific poem'; and spoke of dynamics, or the science of force, as treating of 'power acting by law in space and time.' in other respects it is as unpoetical and unmetaphysical as my gravest friends could desire." it may well be doubted whether there is a more beautiful chapter in the whole of mathematical philosophy than that which contains hamilton's dynamical theory. it is disfigured by no tedious complexity of symbols; it condescends not to any particular problems; it is an all embracing theory, which gives an intellectual grasp of the most appropriate method for discovering the result of the application of force to matter. it is the very generality of this doctrine which has somewhat impeded the applications of which it is susceptible. the exigencies of examinations are partly responsible for the fact that the method has not become more familiar to students of the higher mathematics. an eminent professor has complained that hamilton's essay on dynamics was of such an extremely abstract character, that he found himself unable to extract from it problems suitable for his examination papers. the following extract is from a letter of professor sylvester to hamilton, dated th of september, . it will show how his works were appreciated by so consummate a mathematician as the writer:-- "believe me, sir, it is not the least of my regrets in quitting this empire to feel that i forego the casual occasion of meeting those masters of my art, yourself chief amongst the number, whose acquaintance, whose conversation, or even notice, have in themselves the power to inspire, and almost to impart fresh vigour to the understanding, and the courage and faith without which the efforts of invention are in vain. the golden moments i enjoyed under your hospitable roof at dunsink, or moments such as they were, may probably never again fall to my lot. "at a vast distance, and in an humble eminence, i still promise myself the calm satisfaction of observing your blazing course in the elevated regions of discovery. such national honour as you are able to confer on your country is, perhaps, the only species of that luxury for the rich (i mean what is termed one's glory) which is not bought at the expense of the comforts of the million." the study of metaphysics was always a favourite recreation when hamilton sought for a change from the pursuit of mathematics. in the year we find him a diligent student of kant; and, to show the views of the author of quaternions and of algebra as the science of pure time on the "critique of the pure reason," we quote the following letter, dated th of july, , from hamilton to viscount adare:-- "i have read a large part of the 'critique of the pure reason,' and find it wonderfully clear, and generally quite convincing. notwithstanding some previous preparation from berkeley, and from my own thoughts, i seem to have learned much from kant's own statement of his views of 'space and time.' yet, on the whole, a large part of my pleasure consists in recognising through kant's works, opinions, or rather views, which have been long familiar to myself, although far more clearly and systematically expressed and combined by him. . . . kant is, i think, much more indebted than he owns, or, perhaps knows, to berkeley, whom he calls by a sneer, 'gutem berkeley'. . . as it were, 'good soul, well meaning man,' who was able for all that to shake to its centre the world of human thought, and to effect a revolution among the early consequences of which was the growth of kant himself." at several meetings of the british association hamilton was a very conspicuous figure. especially was this the case in , when the association met in dublin, and when hamilton, though then but thirty years old, had attained such celebrity that even among a very brilliant gathering his name was perhaps the most renowned. a banquet was given at trinity college in honour of the meeting. the distinguished visitors assembled in the library of the university. the earl of mulgrave, then lord lieutenant of ireland, made this the opportunity of conferring on hamilton the honour of knighthood, gracefully adding, as he did so: "i but set the royal, and therefore the national mark, on a distinction already acquired by your genius and labours." the banquet followed, writes mr. graves. "it was no little addition to the honour hamilton had already received that, when professor whewell returned thanks for the toast of the university of cambridge, he thought it appropriate to add the words, 'there was one point which strongly pressed upon him at that moment: it was now one hundred and thirty years since a great man in another trinity college knelt down before his sovereign, and rose up sir isaac newton.' the compliment was welcomed by immense applause." a more substantial recognition of the labours of hamilton took place subsequently. he thus describes it in a letter to mr. graves of th of november, :-- "the queen has been pleased--and you will not doubt that it was entirely unsolicited, and even unexpected, on my part--'to express her entire approbation of the grant of a pension of two hundred pounds per annum from the civil list' to me for scientific services. the letters from sir robert peel and from the lord lieutenant of ireland in which this grant has been communicated or referred to have been really more gratifying to my feelings than the addition to my income, however useful, and almost necessary, that may have been." the circumstances we have mentioned might lead to the supposition that hamilton was then at the zenith of his fame but this was not so. it might more truly be said, that his achievements up to this point were rather the preliminary exercises which fitted him for the gigantic task of his life. the name of hamilton is now chiefly associated with his memorable invention of the calculus of quaternions. it was to the creation of this branch of mathematics that the maturer powers of his life were devoted; in fact he gives us himself an illustration of how completely habituated he became to the new modes of thought which quaternions originated. in one of his later years he happened to take up a copy of his famous paper on dynamics, a paper which at the time created such a sensation among mathematicians, and which is at this moment regarded as one of the classics of dynamical literature. he read, he tells us, his paper with considerable interest, and expressed his feelings of gratification that he found himself still able to follow its reasoning without undue effort. but it seemed to him all the time as a work belonging to an age of analysis now entirely superseded. in order to realise the magnitude of the revolution which hamilton has wrought in the application of symbols to mathematical investigation, it is necessary to think of what hamilton did beside the mighty advance made by descartes. to describe the character of the quaternion calculus would be unsuited to the pages of this work, but we may quote an interesting letter, written by hamilton from his death-bed, twenty-two years later, to his son archibald, in which he has recorded the circumstances of the discovery:-- "indeed, i happen to be able to put the finger of memory upon the year and month--october, --when having recently returned from visits to cork and parsonstown, connected with a meeting of the british association, the desire to discover the laws of multiplication referred to, regained with me a certain strength and earnestness which had for years been dormant, but was then on the point of being gratified, and was occasionally talked of with you. every morning in the early part of the above-cited month, on my coming down to breakfast, your (then) little brother william edwin, and yourself, used to ask me, 'well papa, can you multiply triplets?' whereto i was always obliged to reply, with a sad shake of the head: 'no, i can only add and subtract them,' "but on the th day of the same month--which happened to be monday, and a council day of the royal irish academy--i was walking in to attend and preside, and your mother was walking with me along the royal canal, to which she had perhaps driven; and although she talked with me now and then, yet an undercurrent of thought was going on in my mind which gave at last a result, whereof it is not too much to say that i felt at once the importance. an electric circuit seemed to close; and a spark flashed forth the herald (as i foresaw immediately) of many long years to come of definitely directed thought and work by myself, if spared, and, at all events, on the part of others if i should even be allowed to live long enough distinctly to communicate the discovery. nor could i resist the impulse--unphilosophical as it may have been--to cut with a knife on a stone of brougham bridge as we passed it, the fundamental formula which contains the solution of the problem, but, of course, the inscription has long since mouldered away. a more durable notice remains, however, on the council books of the academy for that day (october , ), which records the fact that i then asked for and obtained leave to read a paper on 'quaternions,' at the first general meeting of the session; which reading took place accordingly, on monday, the th of november following." writing to professor tait, hamilton gives further particulars of the same event. and again in a letter to the rev. j. w. stubbs:-- "to-morrow will be the fifteenth birthday of the quaternions. they started into life full-grown on the th october, , as i was walking with lady hamilton to dublin, and came up to brougham bridge--which my boys have since called quaternion bridge. i pulled out a pocketbook which still exists, and made entry, on which at the very moment i felt that it might be worth my while to expend the labour of at least ten or fifteen years to come. but then it is fair to say that this was because i felt a problem to have been at that moment solved, an intellectual want relieved which had haunted me for at least fifteen years before. "but did the thought of establishing such a system, in which geometrically opposite facts--namely, two lines (or areas) which are opposite in space give always a positive product--ever come into anybody's head till i was led to it in october, , by trying to extend my old theory of algebraic couples, and of algebra as the science of pure time? as to my regarding geometrical addition of lines as equivalent to composition of motions (and as performed by the same rules), that is indeed essential in my theory but not peculiar to it; on the contrary, i am only one of many who have been led to this view of addition." pilgrims in future ages will doubtless visit the spot commemorated by the invention of quaternions. perhaps as they look at that by no means graceful structure quaternion bridge, they will regret that the hand of some old mortality had not been occasionally employed in cutting the memorable inscription afresh. it is now irrecoverably lost. it was ten years after the discovery that the great volume appeared under the title of "lectures on quaternions," dublin, . the reception of this work by the scientific world was such as might have been expected from the extraordinary reputation of its author, and the novelty and importance of the new calculus. his valued friend, sir john herschel, writes to him in that style of which he was a master:-- "now, most heartily let me congratulate you on getting out your book--on having found utterance, ore rotundo, for all that labouring and seething mass of thought which has been from time to time sending out sparks, and gleams, and smokes, and shaking the soil about you; but now breaks into a good honest eruption, with a lava stream and a shower of fertilizing ashes. "metaphor and simile apart, there is work for a twelve-month to any man to read such a book, and for half a lifetime to digest it, and i am glad to see it brought to a conclusion." we may also record hamilton's own opinion expressed to humphrey lloyd:-- "in general, although in one sense i hope that i am actually growing modest about the quaternions, from my seeing so many peeps and vistas into future expansions of their principles, i still must assert that this discovery appears to me to be as important for the middle of the nineteenth century as the discovery of fluxions was for the close of the seventeenth." bartholomew lloyd died in . he had been the provost of trinity college, and the president of the royal irish academy. three candidates were put forward by their respective friends for the vacant presidency. one was humphrey lloyd, the son of the late provost, and the two others were hamilton and archbishop whately. lloyd from the first urged strongly the claims of hamilton, and deprecated the putting forward of his own name. hamilton in like manner desired to withdraw in favour of lloyd. the wish was strongly felt by many of the fellows of the college that lloyd should be elected, in consequence of his having a more intimate association with collegiate life than hamilton; while his scientific eminence was world-wide. the election ultimately gave hamilton a considerable majority over lloyd, behind whom the archbishop followed at a considerable distance. all concluded happily, for both lloyd and the archbishop expressed, and no doubt felt, the pre-eminent claims of hamilton, and both of them cordially accepted the office of a vice-president, to which, according to the constitution of the academy, it is the privilege of the incoming president to nominate. in another chapter i have mentioned as a memorable episode in astronomical history, that sir j. herschel went for a prolonged sojourn to the cape of good hope, for the purpose of submitting the southern skies to the same scrutiny with the great telescope that his father had given to the northern skies. the occasion of herschel's return after the brilliant success of his enterprise, was celebrated by a banquet. on june th, , hamilton was assigned the high honour of proposing the health of herschel. this banquet is otherwise memorable in hamilton's career as being one of the two occasions in which he was in the company of his intimate friend de morgan. in the year a scheme was adopted by the royal irish academy for the award of medals to the authors of papers which appeared to possess exceptionally high merit. at the institution of the medal two papers were named in competition for the prize. one was hamilton's "memoir on algebra, as the science of pure time." the other was macullagh's paper on the "laws of crystalline reflection and refraction." hamilton expresses his gratification that, mainly in consequence of his own exertions, he succeeded in having the medal awarded to macullagh rather than to himself. indeed, it would almost appear as if hamilton had procured a letter from sir j. herschel, which indicated the importance of macullagh's memoir in such a way as to decide the issue. it then became hamilton's duty to award the medal from the chair, and to deliver an address in which he expressed his own sense of the excellence of macullagh's scientific work. it is the more necessary to allude to these points, because in the whole of his scientific career it would seem that macullagh was the only man with whom hamilton had ever even an approach to a dispute about priority. the incident referred to took place in connection with the discovery of conical refraction, the fame of which macullagh made a preposterous attempt to wrest from hamilton. this is evidently alluded to in hamilton's letter to the marquis of northampton, dated june th, , in which we read:-- "and though some former circumstances prevented me from applying to the person thus distinguished the sacred name of friend, i had the pleasure of doing justice...to his high intellectual merits... i believe he was not only gratified but touched, and may, perhaps, regard me in future with feelings more like those which i long to entertain towards him." hamilton was in the habit, from time to time, of commencing the keeping of a journal, but it does not appear to have been systematically conducted. whatever difficulties the biographer may have experienced from its imperfections and irregularities, seem to be amply compensated for by the practice which hamilton had of preserving copies of his letters, and even of comparatively insignificant memoranda. in fact, the minuteness with which apparently trivial matters were often noted down appears almost whimsical. he frequently made a memorandum of the name of the person who carried a letter to the post, and of the hour in which it was despatched. on the other hand, the letters which he received were also carefully preserved in a mighty mass of manuscripts, with which his study was encumbered, and with which many other parts of the house were not unfrequently invaded. if a letter was laid aside for a few hours, it would become lost to view amid the seething mass of papers, though occasionally, to use his own expression, it might be seen "eddying" to the surface in some later disturbance. the great volume of "lectures on quaternions" had been issued, and the author had received the honours which the completion of such a task would rightfully bring him. the publication of an immortal work does not, however, necessarily provide the means for paying the printer's bill. the printing of so robust a volume was necessarily costly; and even if all the copies could be sold, which at the time did not seem very likely, they would hardly have met the inevitable expenses. the provision of the necessary funds was, therefore, a matter for consideration. the board of trinity college had already contributed pounds to the printing, but yet another hundred was required. even the discoverer of quaternions found this a source of much anxiety. however, the board, urged by the representation of humphrey lloyd, now one of its members, and, as we have already seen, one of hamilton's staunchest friends, relieved him of all liability. we may here note that, notwithstanding the pension which hamilton enjoyed in addition to the salary of his chair, he seems always to have been in some what straitened circumstances, or, to use his own words in one of his letters to de morgan, "though not an embarrassed man, i am anything rather than a rich one." it appears that, notwithstanding the world-wide fame of hamilton's discoveries, the only profit in a pecuniary sense that he ever obtained from any of his works was by the sale of what he called his icosian game. some enterprising publisher, on the urgent representations of one of hamilton's friends in london, bought the copyright of the icosian game for pounds. even this little speculation proved unfortunate for the purchaser, as the public could not be induced to take the necessary interest in the matter. after the completion of his great book, hamilton appeared for awhile to permit himself a greater indulgence than usual in literary relaxations. he had copious correspondence with his intimate friend, aubrey de vere, and there were multitudes of letters from those troops of friends whom it was hamilton's privilege to possess. he had been greatly affected by the death of his beloved sister eliza, a poetess of much taste and feeling. she left to him her many papers to preserve or to destroy, but he said it was only after the expiration of four years of mourning that he took courage to open her pet box of letters. the religious side of hamilton's character is frequently illustrated in these letters; especially is this brought out in the correspondence with de vere, who had seceded to the church of rome. hamilton writes, august , :-- "if, then, it be painfully evident to both, that under such circumstances there cannot (whatever we may both desire) be now in the nature of things, or of minds, the same degree of intimacy between us as of old; since we could no longer talk with the same degree of unreserve on every subject which happened to present itself, but must, from the simplest instincts of courtesy, be each on his guard not to say what might be offensive, or, at least, painful to the other; yet we were once so intimate, and retain still, and, as i trust, shall always retain, so much of regard and esteem and appreciation for each other, made tender by so many associations of my early youth and your boyhood, which can never be forgotten by either of us, that (as times go) two or three very respectable friendships might easily be carved out from the fragments of our former and ever-to-be-remembered intimacy. it would be no exaggeration to quote the words: 'heu! quanto minus est cum reliquis versari, quam tui meminisse!'" in a correspondence on the subject of quaternions commenced between professor tait and sir william hamilton. it was particularly gratifying to the discoverer that so competent a mathematician as professor tait should have made himself acquainted with the new calculus. it is, of course, well known that professor tait subsequently brought out a most valuable elementary treatise on quaternions, to which those who are anxious to become acquainted with the subject will often turn in preference to the tremendous work of hamilton. in the year gratifying information came to hand of the progress which the study of quaternions was making abroad. especially did the subject attract the attention of that accomplished mathematician, moebius, who had already in his "barycentrische calculus" been led to conceptions which bore more affinity to quaternions than could be found in the writings of any other mathematician. such notices of his work were always pleasing to hamilton, and they served, perhaps, as incentives to that still closer and more engrossing labour by which he became more and more absorbed. during the last few years of his life he was observed to be even more of a recluse than he had hitherto been. his powers of long and continuous study seemed to grow with advancing years, and his intervals of relaxation, such as they were, became more brief and more infrequent. it was not unusual for him to work for twelve hours at a stretch. the dawn would frequently surprise him as he looked up to snuff his candles after a night of fascinating labour at original research. regularity in habits was impossible to a student who had prolonged fits of what he called his mathematical trances. hours for rest and hours for meals could only be snatched in the occasional the lucid intervals between one attack of quaternions and the next. when hungry, he would go to see whether anything could be found on the sideboard; when thirsty, he would visit the locker, and the one blemish in the man's personal character is that these latter visits were sometimes paid too often. as an example of one of hamilton's rare diversions from the all- absorbing pursuit of quaternions, we find that he was seized with curiosity to calculate back to the date of the hegira, which he found on the th july, . he speaks of the satisfaction with which he ascertained subsequently that herschel had assigned precisely the same date. metaphysics remained also, as it had ever been, a favourite subject of hamilton's readings and meditations and of correspondence with his friends. he wrote a very long letter to dr. ingleby on the subject of his "introduction to metaphysics." in it hamilton alludes, as he has done also in other places, to a peculiarity of his own vision. it was habitual to him, by some defect in the correlation of his eyes, to see always a distinct image with each; in fact, he speaks of the remarkable effect which the use of a good stereoscope had on his sensations of vision. it was then, for the first time, that he realised how the two images which he had always seen hitherto would, under normal circumstances, be blended into one. he cites this fact as bearing on the phenomena of binocular vision, and he draws from it the inference that the necessity of binocular vision for the correct appreciation of distance is unfounded. "i am quite sure," he says, "that i see distance with each eye separately." the commencement of , the last year of his life saw hamilton as diligent as ever, and corresponding with salmon and cayley. on april th he writes to a friend to say, that his health has not been good for years past, and that so much work has injured his constitution; and he adds, that it is not conducive to good spirits to find that he is accumulating another heavy bill with the printer for the publication of the "elements." this was, indeed, up to the day of his death, a cause for serious anxiety. it may, however, be mentioned that the whole cost, which amounted to nearly pounds, was, like that of the previous volume, ultimately borne by the college. contrary to anticipation, the enterprise, even in a pecuniary sense, cannot have been a very unprofitable one. the whole edition has long been out of print, and as much as pounds has since been paid for a single copy. it was on the th of may, , that hamilton was in dublin for the last time. a few days later he had a violent attack of gout, and on the th of june he became alarmingly ill, and on the next day had an attack of epileptic convulsions. however, he slightly rallied, so that before the end of the month he was again at work at the "elements." a gratifying incident brightened some of the last days of his life. the national academy of science in america had then been just formed. a list of foreign associates had to be chosen from the whole world, and a discussion took place as to what name should be placed first on the list. hamilton was informed by private communication that this great distinction was awarded to him by a majority of two-thirds. in august he was still at work on the table of contents of the "elements," and one of his very latest efforts was his letter to mr. gould, in america, communicating his acknowledgements of the honour which had been just conferred upon him by the national academy. on the nd of september mr. graves went to the observatory, in response to a summons, and the great mathematician at once admitted to his friend that he felt the end was approaching. he mentioned that he had found in the th psalm a wonderfully suitable expression of his thoughts and feelings, and he wished to testify his faith and thankfulness as a christian by partaking of the lord's supper. he died at half-past two on the afternoon of the nd of september, , aged sixty years and one month. he was buried in mount jerome cemetery on the th of september. many were the letters and other more public manifestations of the feelings awakened by hamilton's death. sir john herschel wrote to the widow:-- "permit me only to add that among the many scientific friends whom time has deprived me of, there has been none whom i more deeply lament, not only for his splendid talents, but for the excellence of his disposition and the perfect simplicity of his manners--so great, and yet devoid of pretensions." de morgan, his old mathematical crony, as hamilton affectionately styled him, also wrote to lady hamilton:-- "i have called him one of my dearest friends, and most truly; for i know not how much longer than twenty-five years we have been in intimate correspondence, of most friendly agreement or disagreement, of most cordial interest in each other. and yet we did not know each other's faces. i met him about at babbage's breakfast table, and there for the only time in our lives we conversed. i saw him, a long way off, at the dinner given to herschel (about ) on his return from the cape and there we were not near enough, nor on that crowded day could we get near enough, to exchange a word. and this is all i ever saw, and, so it has pleased god, all i shall see in this world of a man whose friendly communications were among my greatest social enjoyments, and greatest intellectual treats." there is a very interesting memoir of hamilton written by de morgan, in the "gentleman's magazine" for , in which he produces an excellent sketch of his friend, illustrated by personal reminiscences and anecdotes. he alludes, among other things, to the picturesque confusion of the papers in his study. there was some sort of order in the mass, discernible however, by hamilton alone, and any invasion of the domestics, with a view to tidying up, would throw the mathematician as we are informed, into "a good honest thundering passion." hardly any two men, who were both powerful mathematicians, could have been more dissimilar in every other respect than were hamilton and de morgan. the highly poetical temperament of hamilton was remarkably contrasted with the practical realism of de morgan. hamilton sends sonnets to his friend, who replies by giving the poet advice about making his will. the metaphysical subtleties, with which hamilton often filled his sheets, did not seem to have the same attraction for de morgan that he found in battles about the quantification of the predicate. de morgan was exquisitely witty, and though his jokes were always appreciated by his correspondent, yet hamilton seldom ventured on anything of the same kind in reply; indeed his rare attempts at humour only produced results of the most ponderous description. but never were two scientific correspondents more perfectly in sympathy with each other. hamilton's work on quaternions, his labours in dynamics, his literary tastes, his metaphysics, and his poetry, were all heartily welcomed by his friend, whose letters in reply invariably evince the kindliest interest in all hamilton's concerns. in a similar way de morgan's letters to hamilton always met with a heartfelt response. alike for the memory of hamilton, for the credit of his university, and for the benefit of science, let us hope that a collected edition of his works will ere long appear--a collection which shall show those early achievements in splendid optical theory, those achievements of his more mature powers which made him the lagrange of his country, and finally those creations of the quaternion calculus by which new capabilities have been bestowed on the human intellect. le verrier. the name of le verrier is one that goes down to fame on account of very different discoveries from those which have given renown to several of the other astronomers whom we have mentioned. we are sometimes apt to identify the idea of an astronomer with that of a man who looks through a telescope at the stars; but the word astronomer has really much wider significance. no man who ever lived has been more entitled to be designated an astronomer than le verrier, and yet it is certain that he never made a telescopic discovery of any kind. indeed, so far as his scientific achievements have been concerned, he might never have looked through a telescope at all. for the full interpretation of the movements of the heavenly bodies, mathematical knowledge of the most advanced character is demanded. the mathematician at the outset calls upon the astronomer who uses the instruments in the observatory, to ascertain for him at various times the exact positions occupied by the sun, the moon, and the planets. these observations, obtained with the greatest care, and purified as far as possible from the errors by which they may be affected form, as it were, the raw material on which the mathematician exercises his skill. it is for him to elicit from the observed places the true laws which govern the movements of the heavenly bodies. here is indeed a task in which the highest powers of the human intellect may be worthily employed. among those who have laboured with the greatest success in the interpretation of the observations made with instruments of precision, le verrier holds a highly honoured place. to him it has been given to provide a superb illustration of the success with which the mind of man can penetrate the deep things of nature. the illustrious frenchman, urban jean joseph le verrier, was born on the th march, , at st. lo, in the department of manche. he received his education in that famous school for education in the higher branches of science, the ecole polytechnique, and acquired there considerable fame as a mathematician. on leaving the school le verrier at first purposed to devote himself to the public service, in the department of civil engineering; and it is worthy of note that his earliest scientific work was not in those mathematical researches in which he was ultimately to become so famous. his duties in the engineering department involved practical chemical research in the laboratory. in this he seems to have become very expert, and probably fame as a chemist would have been thus attained, had not destiny led him into another direction. as it was, he did engage in some original chemical research. his first contributions to science were the fruits of his laboratory work; one of his papers was on the combination of phosphorus and hydrogen, and another on the combination of phosphorus and oxygen. his mathematical labours at the ecole polytechnique had, however, revealed to le verrier that he was endowed with the powers requisite for dealing with the subtlest instruments of mathematical analysis. when he was twenty-eight years old, his first great astronomical investigation was brought forth. it will be necessary to enter into some explanation as to the nature of this, inasmuch as it was the commencement of the life-work which he was to pursue. if but a single planet revolved around the sun, then the orbit of that planet would be an ellipse, and the shape and size, as well as the position of the ellipse, would never alter. one revolution after another would be traced out, exactly in the same manner, in compliance with the force continuously exerted by the sun. suppose, however, that a second planet be introduced into the system. the sun will exert its attraction on this second planet also, and it will likewise describe an orbit round the central globe. we can, however, no longer assert that the orbit in which either of the planets moves remains exactly an ellipse. we may, indeed, assume that the mass of the sun is enormously greater than that of either of the planets. in this case the attraction of the sun is a force of such preponderating magnitude, that the actual path of each planet remains nearly the same as if the other planet were absent. but it is impossible for the orbit of each planet not to be affected in some degree by the attraction of the other planet. the general law of nature asserts that every body in space attracts every other body. so long as there is only a single planet, it is the single attraction between the sun and that planet which is the sole controlling principle of the movement, and in consequence of it the ellipse is described. but when a second planet is introduced, each of the two bodies is not only subject to the attraction of the sun, but each one of the planets attracts the other. it is true that this mutual attraction is but small, but, nevertheless, it produces some effect. it "disturbs," as the astronomer says, the elliptic orbit which would otherwise have been pursued. hence it follows that in the actual planetary system where there are several planets disturbing each other, it is not true to say that the orbits are absolutely elliptic. at the same time in any single revolution a planet may for most practical purposes be said to be actually moving in an ellipse. as, however, time goes on, the ellipse gradually varies. it alters its shape, it alters its plane, and it alters its position in that plane. if, therefore, we want to study the movements of the planets, when great intervals of time are concerned, it is necessary to have the means of learning the nature of the movement of the orbit in consequence of the disturbances it has experienced. we may illustrate the matter by supposing the planet to be running like a railway engine on a track which has been laid in a long elliptic path. we may suppose that while the planet is coursing along, the shape of the track is gradually altering. but this alteration may be so slow, that it does not appreciably affect the movement of the engine in a single revolution. we can also suppose that the plane in which the rails have been laid has a slow oscillation in level, and that the whole orbit is with more or less uniformity moved slowly about in the plane. in short periods of time the changes in the shapes and positions of the planetary orbits, in consequence of their mutual attractions, are of no great consequence. when, however, we bring thousands of years into consideration, then the displacements of the planetary orbits attain considerable dimensions, and have, in fact, produced a profound effect on the system. it is of the utmost interest to investigate the extent to which one planet can affect another in virtue of their mutual attractions. such investigations demand the exercise of the highest mathematical gifts. but not alone is intellectual ability necessary for success in such inquiries. it must be united with a patient capacity for calculations of an arduous type, protracted, as they frequently have to be, through many years of labour. le verrier soon found in these profound inquiries adequate scope for the exercise of his peculiar gifts. his first important astronomical publication contained an investigation of the changes which the orbits of several of the planets, including the earth, have undergone in times past, and which they will undergo in times to come. as an illustration of these researches, we may take the case of the planet in which we are, of course, especially interested, namely, the earth, and we can investigate the changes which, in the lapse of time, the earth's orbit has undergone, in consequence of the disturbance to which it has been subjected by the other planets. in a century, or even in a thousand years, there is but little recognisable difference in the shape of the track pursued by the earth. vast periods of time are required for the development of the large consequences of planetary perturbation. le verrier has, however, given us the particulars of what the earth's journey through space has been at intervals of , years back from the present date. his furthest calculation throws our glance back to the state of the earth's track , years ago, while, with a bound forward, he shows us what the earth's orbit is to be in the future, at successive intervals of , years, till a date is reached which is , years in advance of a.d. . the talent which these researches displayed brought le verrier into notice. at that time the paris observatory was presided over by arago, a savant who occupies a distinguished position in french scientific annals. arago at once perceived that le verrier was just the man who possessed the qualifications suitable for undertaking a problem of great importance and difficulty that had begun to force itself on the attention of astronomers. what this great problem was, and how astonishing was the solution it received, must now be considered. ever since herschel brought himself into fame by his superb discovery of the great planet uranus, the movements of this new addition to the solar system were scrutinized with care and attention. the position of uranus was thus accurately determined from time to time. at length, when sufficient observations of this remote planet had been brought together, the route which the newly-discovered body pursued through the heavens was ascertained by those calculations with which astronomers are familiar. it happens, however, that uranus possesses a superficial resemblance to a star. indeed the resemblance is so often deceptive that long ere its detection as a planet by herschel, it had been observed time after time by skilful astronomers, who little thought that the star-like point at which they looked was anything but a star. from these early observations it was possible to determine the track of uranus, and it was found that the great planet takes a period of no less than eighty-four years to accomplish a circuit. calculations were made of the shape of the orbit in which it revolved before its discovery by herschel, and these were compared with the orbit which observations showed the same body to pursue in those later years when its planetary character was known. it could not, of course, be expected that the orbit should remain unaltered; the fact that the great planets jupiter and saturn revolve in the vicinity of uranus must necessarily imply that the orbit of the latter undergoes considerable changes. when, however, due allowance has been made for whatever influence the attraction of jupiter and saturn, and we may add of the earth and all the other planets, could possibly produce, the movements of uranus were still inexplicable. it was perfectly obvious that there must be some other influence at work besides that which could be attributed to the planets already known. astronomers could only recognise one solution of such a difficulty. it was impossible to doubt that there must be some other planet in addition to the bodies at that time known, and that the perturbations of uranus hitherto unaccounted for, were due to the disturbances caused by the action of this unknown planet. arago urged le verrier to undertake the great problem of searching for this body, whose theoretical existence seemed demonstrated. but the conditions of the search were such that it must needs be conducted on principles wholly different from any search which had ever before been undertaken for a celestial object. for this was not a case in which mere survey with a telescope might be expected to lead to the discovery. certain facts might be immediately presumed with reference to the unknown object. there could be no doubt that the unknown disturber of uranus must be a large body with a mass far exceeding that of the earth. it was certain, however, that it must be so distant that it could only appear from our point of view as a very small object. uranus itself lay beyond the range, or almost beyond the range, of unassisted vision. it could be shown that the planet by which the disturbance was produced revolved in an orbit which must lie outside that of uranus. it seemed thus certain that the planet could not be a body visible to the unaided eye. indeed, had it been at all conspicuous its planetary character would doubtless have been detected ages ago. the unknown body must therefore be a planet which would have to be sought for by telescopic aid. there is, of course, a profound physical difference between a planet and a star, for the star is a luminous sun, and the planet is merely a dark body, rendered visible by the sunlight which falls upon it. notwithstanding that a star is a sun thousands of times larger than the planet and millions of times more remote, yet it is a singular fact that telescopic planets possess an illusory resemblance to the stars among which their course happens to lie. so far as actual appearance goes, there is indeed only one criterion by which a planet of this kind can be discriminated from a star. if the planet be large enough the telescope will show that it possesses a disc, and has a visible and measurable circular outline. this feature a star does not exhibit. the stars are indeed so remote that no matter how large they may be intrinsically, they only exhibit radiant points of light, which the utmost powers of the telescope fail to magnify into objects with an appreciable diameter. the older and well-known planets, such as jupiter and mars, possess discs, which, though not visible to the unaided eye, were clearly enough discernible with the slightest telescopic power. but a very remote planet like uranus, though it possessed a disc large enough to be quickly appreciated by the consummate observing skill of herschel, was nevertheless so stellar in its appearance, that it had been observed no fewer than seventeen times by experienced astronomers prior to herschel. in each case the planetary nature of the object had been overlooked, and it had been taken for granted that it was a star. it presented no difference which was sufficient to arrest attention. as the unknown body by which uranus was disturbed was certainly much more remote than uranus, it seemed to be certain that though it might show a disc perceptible to very close inspection, yet that the disc must be so minute as not to be detected except with extreme care. in other words, it seemed probable that the body which was to be sought for could not readily be discriminated from a small star, to which class of object it bore a superficial resemblance, though, as a matter of fact, there was the profoundest difference between the two bodies. there are on the heavens many hundreds of thousands of stars, and the problem of identifying the planet, if indeed it should lie among these stars, seemed a very complex matter. of course it is the abundant presence of the stars which causes the difficulty. if the stars could have been got rid of, a sweep over the heavens would at once disclose all the planets which are bright enough to be visible with the telescopic power employed. it is the fortuitous resemblance of the planet to the stars which enables it to escape detection. to discriminate the planet among stars everywhere in the sky would be almost impossible. if, however, some method could be devised for localizing that precise region in which the planet's existence might be presumed, then the search could be undertaken with some prospect of success. to a certain extent the problem of localizing the region on the sky in which the planet might be expected admitted of an immediate limitation. it is known that all the planets, or perhaps i ought rather to say, all the great planets, confine their movements to a certain zone around the heavens. this zone extends some way on either side of that line called the ecliptic in which the earth pursues its journey around the sun. it was therefore to be inferred that the new planet need not be sought for outside this zone. it is obvious that this consideration at once reduces the area to be scrutinized to a small fraction of the entire heavens. but even within the zone thus defined there are many thousands of stars. it would seem a hopeless task to detect the new planet unless some further limitation to its position could be assigned. it was accordingly suggested to le verrier that he should endeavour to discover in what particular part of the strip of the celestial sphere which we have indicated the search for the unknown planet should be instituted. the materials available to the mathematician for the solution of this problem were to be derived solely from the discrepancies between the calculated places in which uranus should be found, taking into account the known causes of disturbance, and the actual places in which observation had shown the planet to exist. here was indeed an unprecedented problem, and one of extraordinary difficulty. le verrier, however, faced it, and, to the astonishment of the world, succeeded in carrying it through to a brilliant solution. we cannot here attempt to enter into any account of the mathematical investigations that were necessary. all that we can do is to give a general indication of the method which had to be adopted. let us suppose that a planet is revolving outside uranus, at a distance which is suggested by the several distances at which the other planets are dispersed around the sun. let us assume that this outer planet has started on its course, in a prescribed path, and that it has a certain mass. it will, of course, disturb the motion of uranus, and in consequence of that disturbance uranus will follow a path the nature of which can be determined by calculation. it will, however, generally be found that the path so ascertained does not tally with the actual path which observations have indicated for uranus. this demonstrates that the assumed circumstances of the unknown planet must be in some respects erroneous, and the astronomer commences afresh with an amended orbit. at last after many trials, le verrier ascertained that, by assuming a certain size, shape, and position for the unknown planet's orbit, and a certain value for the mass of the hypothetical body, it would be possible to account for the observed disturbances of uranus. gradually it became clear to the perception of this consummate mathematician, not only that the difficulties in the movements of uranus could be thus explained, but that no other explanation need be sought for. it accordingly appeared that a planet possessing the mass which he had assigned, and moving in the orbit which his calculations had indicated, must indeed exist, though no eye had ever beheld any such body. here was, indeed, an astonishing result. the mathematician sitting at his desk, by studying the observations which had been supplied to him of one planet, is able to discover the existence of another planet, and even to assign the very position which it must occupy, ere ever the telescope is invoked for its discovery. thus it was that the calculations of le verrier narrowed greatly the area to be scrutinised in the telescopic search which was presently to be instituted. it was already known, as we have just pointed out, that the planet must lie somewhere on the ecliptic. the french mathematician had now further indicated the spot on the ecliptic at which, according to his calculations, the planet must actually be found. and now for an episode in this history which will be celebrated so long as science shall endure. it is nothing less than the telescopic confirmation of the existence of this new planet, which had previously been indicated only by mathematical calculation. le verrier had not himself the instruments necessary for studying the heavens, nor did he possess the skill of the practical astronomer. he, therefore, wrote to dr. galle, of the observatory at berlin, requesting him to undertake a telescopic search for the new planet in the vicinity which the mathematical calculation had indicated for the whereabouts of the planet at that particular time. le verrier added that he thought the planet ought to admit of being recognised by the possession of a disc sufficiently definite to mark the distinction between it and the surrounding stars. it was the rd september, , when the request from le verrier reached the berlin observatory, and the night was clear, so that the memorable search was made on the same evening. the investigation was facilitated by the circumstance that a diligent observer had recently compiled elaborate star maps for certain tracts of the heavens lying in a sufficiently wide zone on both sides of the equator. these maps were as yet only partially complete, but it happened that hora. xxi., which included the very spot which le verrier's results referred to, had been just issued. dr. galle had thus before his, eyes a chart of all the stars which were visible in that part of the heavens at the time when the map was made. the advantage of such an assistance to the search could hardly be over-estimated. it at once gave the astronomer another method of recognising the planet besides that afforded by its possible possession of a disc. for as the planet was a moving body, it would not have been in the same place relatively to the stars at the time when the map was constructed, as it occupied some years later when the search was being made. if the body should be situated in the spot which le verrier's calculations indicated in the autumn of , then it might be regarded as certain that it would not be found in that same place on a map drawn some years previously. the search to be undertaken consisted in a comparison made point by point between the bodies shown on the map, and those stars in the sky which dr. galle's telescope revealed. in the course of this comparison it presently appeared that a star-like object of the eighth magnitude, which was quite a conspicuous body in the telescope, was not represented in the map. this at once attracted the earnest attention of the astronomer, and raised his hopes that here was indeed the planet. nor were these hopes destined to be disappointed. it could not be supposed that a star of the eighth magnitude would have been overlooked in the preparation of a chart whereon stars of many lower degrees of brightness were set down. one other supposition was of course conceivable. it might have been that this suspicious object belonged to the class of variables, for there are many such stars whose brightness fluctuates, and if it had happened that the map was constructed at a time when the star in question had but feeble brilliance, it might have escaped notice. it is also well known that sometimes new stars suddenly develop, so that the possibility that what dr. galle saw should have been a variable star or should have been a totally new star had to be provided against. fortunately a test was immediately available to decide whether the new object was indeed the long sought for planet, or whether it was a star of one of the two classes to which i have just referred. a star remains fixed, but a planet is in motion. no doubt when a planet lies at the distance at which this new planet was believed to be situated, its apparent motion would be so slow that it would not be easy to detect any change in the course of a single night's observation. dr. galle, however, addressed himself with much skill to the examination of the place of the new body. even in the course of the night he thought he detected slight movements, and he awaited with much anxiety the renewal of his observations on the subsequent evenings. his suspicions as to the movement of the body were then amply confirmed, and the planetary nature of the new object was thus unmistakably detected. great indeed was the admiration of the scientific world at this superb triumph. here was a mighty planet whose very existence was revealed by the indications afforded by refined mathematical calculation. at once the name of le verrier, already known to those conversant with the more profound branches of astronomy, became everywhere celebrated. it soon, however, appeared, that the fame belonging to this great achievement had to be shared between le verrier and another astronomer, j. c. adams, of cambridge. in our chapter on this great english mathematician we shall describe the manner in which he was independently led to the same discovery. directly the planetary nature of the newly-discovered body had been established, the great observatories naturally included this additional member of the solar system in their working lists, so that day after day its place was carefully determined. when sufficient time had elapsed the shape and position of the orbit of the body became known. of course, it need hardly be said that observations applied to the planet itself must necessarily provide a far more accurate method of determining the path which it follows, than would be possible to le verrier, when all he had to base his calculations upon was the influence of the planet reflected, so to speak, from uranus. it may be noted that the true elements of the planet, when revealed by direct observation, showed that there was a considerable discrepancy between the track of the planet which le verrier had announced, and that which the planet was actually found to pursue. the name of the newly-discovered body had next to be considered. as the older members of the system were already known by the same names as great heathen divinities, it was obvious that some similar source should be invoked for a suggestion as to a name for the most recent planet. the fact that this body was so remote in the depths of space, not unnaturally suggested the name "neptune." such is accordingly the accepted designation of that mighty globe which revolves in the track that at present seems to trace out the frontiers of our system. le verrier attained so much fame by this discovery, that when, in , arago's place had to be filled at the head of the great paris observatory, it was universally felt that the discoverer of neptune was the suitable man to assume the office which corresponds in france to that of the astronomer royal in england. it was true that the work of the astronomical mathematician had hitherto been of an abstract character. his discoveries had been made at his desk and not in the observatory, and he had no practical acquaintance with the use of astronomical instruments. however, he threw himself into the technical duties of the observatory with vigour and determination. he endeavoured to inspire the officers of the establishment with enthusiasm for that systematic work which is so necessary for the accomplishment of useful astronomical research. it must, however, be admitted that le verrier was not gifted with those natural qualities which would make him adapted for the successful administration of such an establishment. unfortunately disputes arose between the director and his staff. at last the difficulties of the situation became so great that the only possible solution was to supersede le verrier, and he was accordingly obliged to retire. he was succeeded in his high office by another eminent mathematician, m. delaunay, only less distinguished than le verrier himself. relieved of his official duties, le verrier returned to the mathematics he loved. in his non-official capacity he continued to work with the greatest ardour at his researches on the movements of the planets. after the death of m. delaunay, who was accidentally drowned in , le verrier was restored to the directorship of the observatory, and he continued to hold the office until his death. the nature of the researches to which the life of le verrier was subsequently devoted are not such as admit of description in a general sketch like this, where the language, and still less the symbols, of mathematics could not be suitably introduced. it may, however, be said in general that he was particularly engaged with the study of the effects produced on the movements of the planets by their mutual attractions. the importance of this work to astronomy consists, to a considerable extent, in the fact that by such calculations we are enabled to prepare tables by which the places of the different heavenly bodies can be predicted for our almanacs. to this task le verrier devoted himself, and the amount of work he has accomplished would perhaps have been deemed impossible had it not been actually done. the superb success which had attended le verrier's efforts to explain the cause of the perturbations of uranus, naturally led this wonderful computer to look for a similar explanation of certain other irregularities in planetary movements. to a large extent he succeeded in showing how the movements of each of the great planets could be satisfactorily accounted for by the influence of the attractions of the other bodies of the same class. one circumstance in connection with these investigations is sufficiently noteworthy to require a few words here. just as at the opening of his career, le verrier had discovered that uranus, the outermost planet of the then known system, exhibited the influence of an unknown external body, so now it appeared to him that mercury, the innermost body of our system, was also subjected to some disturbances, which could not be satisfactorily accounted for as consequences of any known agents of attraction. the ellipse in which mercury revolved was animated by a slow movement, which caused it to revolve in its plane. it appeared to le verrier that this displacement was incapable of explanation by the action of any of the known bodies of our system. he was, therefore, induced to try whether he could not determine from the disturbances of mercury the existence of some other planet, at present unknown, which revolved inside the orbit of the known planet. theory seemed to indicate that the observed alteration in the track of the planet could be thus accounted for. he naturally desired to obtain telescopic confirmation which might verify the existence of such a body in the same way as dr. galle verified the existence of neptune. if there were, indeed, an intramercurial planet, then it must occasionally cross between the earth and the sun, and might now and then be expected to be witnessed in the actual act of transit. so confident did le verrier feel in the existence of such a body that an observation of a dark object in transit, by lescarbault on th march, , was believed by the mathematician to be the object which his theory indicated. le verrier also thought it likely that another transit of the same object would be seen in march, . nothing of the kind was, however, witnessed, notwithstanding that an assiduous watch was kept, and the explanation of the change in mercury's orbit must, therefore, be regarded as still to be sought for. le verrier naturally received every honour that could be bestowed upon a man of science. the latter part of his life was passed during the most troubled period of modern french history. he was a supporter of the imperial dynasty, and during the commune he experienced much anxiety; indeed, at one time grave fears were entertained for his personal safety. early in his health, which had been gradually failing for some years, began to give way. he appeared to rally somewhat in the summer, but in september he sank rapidly, and died on sunday, the rd of that month. his remains were borne to the cemetery on mont parnasse in a public funeral. among his pallbearers were leading men of science, from other countries as well as france, and the memorial discourses pronounced at the grave expressed their admiration of his talents and of the greatness of the services he had rendered to science. adams. the illustrious mathematician who, among englishmen, at all events, was second only to newton by his discoveries in theoretical astronomy, was born on june the th, , at the farmhouse of lidcot, seven miles from launceston, in cornwall. his early education was imparted under the guidance of the rev. john couch grylls, a first cousin of his mother. he appears to have received an education of the ordinary school type in classics and mathematics, but his leisure hours were largely devoted to studying what astronomical books he could find in the library of the mechanics' institute at devonport. he was twenty years old when he entered st. john's college, cambridge. his career in the university was one of almost unparalleled distinction, and it is recorded that his answering at the wranglership examination, where he came out at the head of the list in , was so high that he received more than double the marks awarded to the second wrangler. among the papers found after his death was the following memorandum, dated july the rd, : "formed a design at the beginning of this week of investigating, as soon as possible after taking my degree, the irregularities in the motion of uranus, which are as yet unaccounted for, in order to find whether they may be attributed to the action of an undiscovered planet beyond it; and, if possible, thence to determine the elements of its orbit approximately, which would lead probably to its discovery." after he had taken his degree, and had thus obtained a little relaxation from the lines within which his studies had previously been necessarily confined, adams devoted himself to the study of the perturbations of uranus, in accordance with the resolve which we have just seen that he formed while he was still an undergraduate. as a first attempt he made the supposition that there might be a planet exterior to uranus, at a distance which was double that of uranus from the sun. having completed his calculation as to the effect which such a hypothetical planet might exercise upon the movement of uranus, he came to the conclusion that it would be quite possible to account completely for the unexplained difficulties by the action of an exterior planet, if only that planet were of adequate size and had its orbit properly placed. it was necessary, however, to follow up the problem more precisely, and accordingly an application was made through professor challis, the director of the cambridge observatory, to the astronomer royal, with the object of obtaining from the observations made at greenwich observatory more accurate values for the disturbances suffered by uranus. basing his work on the more precise materials thus available, adams undertook his calculations anew, and at last, with his completed results, he called at greenwich observatory on october the st, . he there left for the astronomer royal a paper which contained the results at which he had arrived for the mass and the mean distance of the hypothetical planet as well as the other elements necessary for calculating its exact position. [plate: john couch adams.] as we have seen in the preceding chapter, le verrier had been also investigating the same problem. the place which le verrier assigned to the hypothetical disturbing planet for the beginning of the year , was within a degree of that to which adams's computations pointed, and which he had communicated to the astronomer royal seven months before le verrier's work appeared. on july the th, , professor challis commenced to search for the unknown object with the northumberland telescope belonging to the cambridge observatory. he confined his attention to a limited region in the heavens, extending around that point to which mr. adams' calculations pointed. the relative places of all the stars, or rather star-like objects within this area, were to be carefully measured. when the same observations were repeated a week or two later, then the distances of the several pairs of stars from each other would be found unaltered, but any planet which happened to lie among the objects measured would disclose its existence by the alterations in distance due to its motion in the interval. this method of search, though no doubt it must ultimately have proved successful, was necessarily a very tedious one, but to professor challis, unfortunately, no other method was available. thus it happened that, though challis commenced his search at cambridge two months earlier than galle at berlin, yet, as we have already explained, the possession of accurate star-maps by dr. galle enabled him to discover the planet on the very first night that he looked for it. the rival claims of adams and le verrier to the discovery of neptune, or rather, we should say, the claims put forward by their respective champions, for neither of the illustrious investigators themselves condescended to enter into the personal aspect of the question, need not be further discussed here. the main points of the controversy have been long since settled, and we cannot do better than quote the words of sir john herschel when he addressed the royal astronomical society in :-- "as genius and destiny have joined the names of le verrier and adams, i shall by no means put them asunder; nor will they ever be pronounced apart so long as language shall celebrate the triumphs of science in her sublimest walks. on the great discovery of neptune, which may be said to have surpassed, by intelligible and legitimate means, the wildest pretensions of clairvoyance, it would now be quite superfluous for me to dilate. that glorious event and the steps which led to it, and the various lights in which it has been placed, are already familiar to every one having the least tincture of science. i will only add that as there is not, nor henceforth ever can be, the slightest rivalry on the subject between these two illustrious men--as they have met as brothers, and as such will, i trust, ever regard each other--we have made, we could make, no distinction between then, on this occasion. may they both long adorn and augment our science, and add to their own fame already so high and pure, by fresh achievements." adams was elected a fellow of st. john's college, cambridge, in ; but as he did not take holy orders, his fellowship, in accordance with the rules then existing came to an end in . in the following year he was, however, elected to a fellowship at pembroke college, which he retained until the end of his life. in he was appointed professor of mathematics in the university of st. andrews, but his residence in the north was only a brief one, for in the same year he was recalled to cambridge as lowndean professor of astronomy and geometry, in succession to peacock. in challis retired from the directorship of the cambridge observatory, and adams was appointed to succeed him. the discovery of neptune was a brilliant inauguration of the astronomical career of adams. he worked at, and wrote upon, the theory of the motions of biela's comet; he made important corrections to the theory of saturn; he investigated the mass of uranus, a subject in which he was naturally interested from its importance in the theory of neptune; he also improved the methods of computing the orbits of double stars. but all these must be regarded as his minor labours, for next to the discovery of neptune the fame of adams mainly rests on his researches upon certain movements of the moon, and upon the november meteors. the periodic time of the moon is the interval required for one circuit of its orbit. this interval is known with accuracy at the present day, and by means of the ancient eclipses the period of the moon's revolution two thousand years ago can be also ascertained. it had been discovered by halley that the period which the moon requires to accomplish each of its revolutions around the earth has been steadily, though no doubt slowly, diminishing. the change thus produced is not appreciable when only small intervals of time are considered, but it becomes appreciable when we have to deal with intervals of thousands of years. the actual effect which is produced by the lunar acceleration, for so this phenomenon is called, may be thus estimated. if we suppose that the moon had, throughout the ages, revolved around the earth in precisely the same periodic time which it has at present, and if from this assumption we calculate back to find where the moon must have been about two thousand years ago, we obtain a position which the ancient eclipses show to be different from that in which the moon was actually situated. the interval between the position in which the moon would have been found two thousand years ago if there had been no acceleration, and the position in which the moon was actually placed, amounts to about a degree, that is to say, to an arc on the heavens which is twice the moon's apparent diameter. if no other bodies save the earth and the moon were present in the universe, it seems certain that the motion of the moon would never have exhibited this acceleration. in such a simple case as that which i have supposed the orbit of the moon would have remained for ever absolutely unchanged. it is, however, well known that the presence of the sun exerts a disturbing influence upon the movements of the moon. in each revolution our satellite is continually drawn aside by the action of the sun from the place which it would otherwise have occupied. these irregularities are known as the perturbations of the lunar orbit, they have long been studied, and the majority of them have been satisfactorily accounted for. it seems, however, to those who first investigated the question that the phenomenon of the lunar acceleration could not be explained as a consequence of solar perturbation, and, as no other agent competent to produce such effects was recognised by astronomers, the lunar acceleration presented an unsolved enigma. at the end of the last century the illustrious french mathematician laplace undertook a new investigation of the famous problem, and was rewarded with a success which for a long time appeared to be quite complete. let us suppose that the moon lies directly between the earth and the sun, then both earth and moon are pulled towards the sun by the solar attraction; as, however, the moon is the nearer of the two bodies to the attracting centre it is pulled the more energetically, and consequently there is an increase in the distance between the earth and the moon. similarly when the moon happens to lie on the other side of the earth, so that the earth is interposed directly between the moon and the sun, the solar attraction exerted upon the earth is more powerful than the same influence upon the moon. consequently in this case, also, the distance of the moon from the earth is increased by the solar disturbance. these instances will illustrate the general truth, that, as one of the consequences of the disturbing influence exerted by the sun upon the earth-moon system, there is an increase in the dimensions of the average orbit which the moon describes around the earth. as the time required by the moon to accomplish a journey round the earth depends upon its distance from the earth, it follows that among the influences of the sun upon the moon there must be an enlargement of the periodic time, from what it would have been had there been no solar disturbing action. this was known long before the time of laplace, but it did not directly convey any explanation of the lunar acceleration. it no doubt amounted to the assertion that the moon's periodic time was slightly augmented by the disturbance, but it did not give any grounds for suspecting that there was a continuous change in progress. it was, however, apparent that the periodic time was connected with the solar disturbance, so that, if there were any alteration in the amount of the sun's disturbing effect, there must be a corresponding alteration in the moon's periodic time. laplace, therefore, perceived that, if he could discover any continuous change in the ability of the sun for disturbing the moon, he would then have accounted for a continuous change in the moon's periodic time, and that thus an explanation of the long-vexed question of the lunar acceleration might be forthcoming. the capability of the sun for disturbing the earth-moon system is obviously connected with the distance of the earth from the sun. if the earth moved in an orbit which underwent no change whatever, then the efficiency of the sun as a disturbing agent would not undergo any change of the kind which was sought for. but if there were any alteration in the shape or size of the earth's orbit, then that might involve such changes in the distance between the earth and the sun as would possibly afford the desired agent for producing the observed lunar effect. it is known that the earth revolves in an orbit which, though nearly circular, is strictly an ellipse. if the earth were the only planet revolving around the sun then that ellipse would remain unaltered from age to age. the earth is, however, only one of a large number of planets which circulate around the great luminary, and are guided and controlled by his supreme attracting power. these planets mutually attract each other, and in consequence of their mutual attractions the orbits of the planets are disturbed from the simple elliptic form which they would otherwise possess. the movement of the earth, for instance, is not, strictly speaking, performed in an elliptical orbit. we may, however, regard it as revolving in an ellipse provided we admit that the ellipse is itself in slow motion. it is a remarkable characteristic of the disturbing effects of the planets that the ellipse in which the earth is at any moment moving always retains the same length; that is to say, its longest diameter is invariable. in all other respects the ellipse is continually changing. it alters its position, it changes its plane, and, most important of all, it changes its eccentricity. thus, from age to age the shape of the track which the earth describes may at one time be growing more nearly a circle, or at another time may be departing more widely from a circle. these alterations are very small in amount, and they take place with extreme slowness, but they are in incessant progress, and their amount admits of being accurately calculated. at the present time, and for thousands of years past, as well as for thousands of years to come, the eccentricity of the earth's orbit is diminishing, and consequently the orbit described by the earth each year is becoming more nearly circular. we must, however, remember that under all circumstances the length of the longest axis of the ellipse is unaltered, and consequently the size of the track which the earth describes around the sun is gradually increasing. in other words, it may be said that during the present ages the average distance between the earth and the sun is waxing greater in consequence of the perturbations which the earth experiences from the attraction of the other planets. we have, however, already seen that the efficiency of the solar attraction for disturbing the moon's movement depends on the distance between the earth and the sun. as therefore the average distance between the earth and the sun is increasing, at all events during the thousands of years over which our observations extend, it follows that the ability of the sun for disturbing the moon must be gradually diminishing. [plate: cambridge observatory.] it has been pointed out that, in consequence of the solar disturbance, the orbit of the moon must be some what enlarged. as it now appears that the solar disturbance is on the whole declining, it follows that the orbit of the moon, which has to be adjusted relatively to the average value of the solar disturbance, must also be gradually declining. in other words, the moon must be approaching nearer to the earth in consequence of the alterations in the eccentricity of the earth's orbit produced by the attraction of the other planets. it is true that the change in the moon's position thus arising is an extremely small one, and the consequent effect in accelerating the moon's motion is but very slight. it is in fact almost imperceptible, except when great periods of time are involved. laplace undertook a calculation on this subject. he knew what the efficiency of the planets in altering the dimensions of the earth's orbit amounted to; from this he was able to determine the changes that would be propagated into the motion of the moon. thus he ascertained, or at all events thought he had ascertained, that the acceleration of the moon's motion, as it had been inferred from the observations of the ancient eclipses which have been handed down to us, could be completely accounted for as a consequence of planetary perturbation. this was regarded as a great scientific triumph. our belief in the universality of the law of gravitation would, in fact, have been seriously challenged unless some explanation of the lunar acceleration had been forthcoming. for about fifty years no one questioned the truth of laplace's investigation. when a mathematician of his eminence had rendered an explanation of the remarkable facts of observation which seemed so complete, it is not surprising that there should have been but little temptation to doubt it. on undertaking a new calculation of the same question, professor adams found that laplace had not pursued this approximation sufficiently far, and that consequently there was a considerable error in the result of his analysis. adams, it must be observed, did not impugn the value of the lunar acceleration which halley had deduced from the observations, but what he did show was, that the calculation by which laplace thought he had provided an explanation of this acceleration was erroneous. adams, in fact, proved that the planetary influence which laplace had detected only possessed about half the efficiency which the great french mathematician had attributed to it. there were not wanting illustrious mathematicians who came forward to defend the calculations of laplace. they computed the question anew and arrived at results practically coincident with those he had given. on the other hand certain distinguished mathematicians at home and abroad verified the results of adams. the issue was merely a mathematical one. it had only one correct solution. gradually it appeared that those who opposed adams presented a number of different solutions, all of them discordant with his, and, usually, discordant with each other. adams showed distinctly where each of these investigators had fallen into error, and at last it became universally admitted that the cambridge professor had corrected laplace in a very fundamental point of astronomical theory. though it was desirable to have learned the truth, yet the breach between observation and calculation which laplace was believed to have closed thus became reopened. laplace's investigation, had it been correct, would have exactly explained the observed facts. it was, however, now shown that his solution was not correct, and that the lunar acceleration, when strictly calculated as a consequence of solar perturbations, only produced about half the effect which was wanted to explain the ancient eclipses completely. it now seems certain that there is no means of accounting for the lunar acceleration as a direct consequence of the laws of gravitation, if we suppose, as we have been in the habit of supposing, that the members of the solar system concerned may be regarded as rigid particles. it has, however, been suggested that another explanation of a very interesting kind may be forthcoming, and this we must endeavour to set forth. it will be remembered that we have to explain why the period of revolution of the moon is now shorter than it used to be. if we imagine the length of the period to be expressed in terms of days and fractions of a day, that is to say, in terms of the rotations of the earth around its axis, then the difficulty encountered is, that the moon now requires for each of its revolutions around the earth rather a smaller number of rotations of the earth around its axis than used formerly to be the case. of course this may be explained by the fact that the moon is now moving more swiftly than of yore, but it is obvious that an explanation of quite a different kind might be conceivable. the moon may be moving just at the same pace as ever, but the length of the day may be increasing. if the length of the day is increasing, then, of course, a smaller number of days will be required for the moon to perform each revolution even though the moon's period was itself really unchanged. it would, therefore, seem as if the phenomenon known as the lunar acceleration is the result of the two causes. the first of these is that discovered by laplace, though its value was over-estimated by him, in which the perturbations of the earth by the planets indirectly affect the motion of the moon. the remaining part of the acceleration of our satellite is apparent rather than real, it is not that the moon is moving more quickly, but that our time-piece, the earth, is revolving more slowly, and is thus actually losing time. it is interesting to note that we can detect a physical explanation for the apparent checking of the earth's motion which is thus manifested. the tides which ebb and flow on the earth exert a brake-like action on the revolving globe, and there can be no doubt that they are gradually reducing its speed, and thus lengthening the day. it has accordingly been suggested that it is this action of the tides which produces the supplementary effect necessary to complete the physical explanation of the lunar acceleration, though it would perhaps be a little premature to assert that this has been fully demonstrated. the third of professor adams' most notable achievements was connected with the great shower of november meteors which astonished the world in . this splendid display concentrated the attention of astronomers on the theory of the movements of the little objects by which the display was produced. for the definite discovery of the track in which these bodies revolve, we are indebted to the labours of professor adams, who, by a brilliant piece of mathematical work, completed the edifice whose foundations had been laid by professor newton, of yale, and other astronomers. meteors revolve around the sun in a vast swarm, every individual member of which pursues an orbit in accordance with the well-known laws of kepler. in order to understand the movements of these objects, to account satisfactorily for their periodic recurrence, and to predict the times of their appearance, it became necessary to learn the size and the shape of the track which the swarm followed, as well as the position which it occupied. certain features of the track could no doubt be readily assigned. the fact that the shower recurs on one particular day of the year, viz., november th, defines one point through which the orbit must pass. the position on the heavens of the radiant point from which the meteors appear to diverge, gives another element in the track. the sun must of course be situated at the focus, so that only one further piece of information, namely, the periodic time, will be necessary to complete our knowledge of the movements of the system. professor h. newton, of yale, had shown that the choice of possible orbits for the meteoric swarm is limited to five. there is, first, the great ellipse in which we now know the meteors revolve once every thirty three and one quarter years. there is next an orbit of a nearly circular kind in which the periodic time would be a little more than a year. there is a similar track in which the periodic time would be a few days short of a year, while two other smaller orbits would also be conceivable. professor newton had pointed out a test by which it would be possible to select the true orbit, which we know must be one or other of these five. the mathematical difficulties which attended the application of this test were no doubt great, but they did not baffle professor adams. there is a continuous advance in the date of this meteoric shower. the meteors now cross our track at the point occupied by the earth on november th, but this point is gradually altering. the only influence known to us which could account for the continuous change in the plane of the meteor's orbit arises from the attraction of the various planets. the problem to be solved may therefore be attacked in this manner. a specified amount of change in the plane of the orbit of the meteors is known to arise, and the changes which ought to result from the attraction of the planets can be computed for each of the five possible orbits, in one of which it is certain that the meteors must revolve. professor adams undertook the work. its difficulty principally arises from the high eccentricity of the largest of the orbits, which renders the more ordinary methods of calculation inapplicable. after some months of arduous labour the work was completed, and in april, , adams announced his solution of the problem. he showed that if the meteors revolved in the largest of the five orbits, with the periodic time of thirty three and one quarter years, the perturbations of jupiter would account for a change to the extent of twenty minutes of arc in the point in which the orbit crosses the earth's track. the attraction of saturn would augment this by seven minutes, and uranus would add one minute more, while the influence of the earth and of the other planets would be inappreciable. the accumulated effect is thus twenty-eight minutes, which is practically coincident with the observed value as determined by professor newton from an examination of all the showers of which there is any historical record. having thus showed that the great orbit was a possible path for the meteors, adams next proved that no one of the other four orbits would be disturbed in the same manner. indeed, it appeared that not half the observed amount of change could arise in any orbit except in that one with the long period. thus was brought to completion the interesting research which demonstrated the true relation of the meteor swarm to the solar system. besides those memorable scientific labours with which his attention was so largely engaged, professor adams found time for much other study. he occasionally allowed himself to undertake as a relaxation some pieces of numerical calculation, so tremendously long that we can only look on them with astonishment. he has calculated certain important mathematical constants accurately to more than two hundred places of decimals. he was a diligent reader of works on history, geology, and botany, and his arduous labours were often beguiled by novels, of which, like many other great men, he was very fond. he had also the taste of a collector, and he brought together about eight hundred volumes of early printed works, many of considerable rarity and value. as to his personal character, i may quote the words of dr. glaisher when he says, "strangers who first met him were invariably struck by his simple and unaffected manner. he was a delightful companion, always cheerful and genial, showing in society but few traces of his really shy and retiring disposition. his nature was sympathetic and generous, and in few men have the moral and intellectual qualities been more perfectly balanced." in he married the daughter of haliday bruce, esq., of dublin and up to the close of his life he lived at the cambridge observatory, pursuing his mathematical work and enjoying the society of his friends. he died, after a long illness, on st january, , and was interred in st. giles's cemetery, on the huntingdon road, cambridge. astronomy for young folks [illustration: northern portion of the moon at last quarter taken with -inch hooker telescope of the mt. wilson observatory (see chapter xxi)] astronomy _for_ young folks by isabel martin lewis, a. m. (_connected with the nautical almanac office of the u. s. naval observatory_) new york duffield and company copyright, , by the century company copyright, , by duffield and company printed in u. s. a. contents chapter page preface xiii i. the constellations ii. january iii. february iv. march v. april vi. may vii. june viii. july ix. august x. september xi. october xii. november xiii. december xiv. stars of the southern hemisphere xv. the milky way or galaxy xvi. the surface of the sun xvii. the solar system xviii. the origin of the earth xix. jupiter and his nine moons xx. the rings and moons of saturn xxi. is the moon a dead world xxii. comets xxiii. meteorites xxiv. the earth as a magnet xxv. some effects of the earth's atmosphere upon sunlight xxvi. keeping track of the moon xxvii. the motions of the heavenly bodies xxviii. the evolution of the stars--from red giants to red dwarfs xxix. double and multiple stars xxx. astronomical distances xxxi. some astronomical facts worth remembering illustrations page northern portion of the moon at last quarter _frontispiece_ the great hercules cluster--a universe of suns _facing page_ a dark nebula: the dark bay or dark horse nebula in orion _facing page_ a. venus. b. mars. c. jupiter. d. saturn _facing page_ spiral nebula in canes venatici _facing page_ spiral nebula in andromeda viewed edgewise _facing page_ list of tables page i. the principal elements of the solar system ii. the satellites of the solar system iii. the twenty brightest stars in the heavens iv. a list of the principal constellations - v. pronunciations and meanings of names of stars and constellations - preface astronomy, it has been said, is the oldest and the noblest of the sciences. yet it is one of the few sciences for which most present-day educators seem to find little, if any, room in their curriculum of study for the young, in spite of its high cultural value. it is, we are told, too abstruse a subject for the youthful student. this is doubtless true of theoretical or mathematical astronomy and the practical astronomy of the navigator, surveyor and engineer, but it is not true of general, descriptive astronomy. there are many different aspects of this many-sided science, and some of the simplest and grandest truths of astronomy can be grasped by the intelligent child of twelve or fourteen years of age. merely as a branch of nature study the child should have some knowledge of the sun, moon, stars and planets, their motions and their physical features, for they are as truly a part of nature as are the birds, trees and flowers, and the man, woman or child who goes forth beneath the star-lit heavens at night absolutely blind to the wonders and beauties of the universe of which he is a part, loses as much as the one who walks through field or forest with no thought of the beauties of nature that surround him. the astronomer is the pioneer and explorer of today in realms unknown just as the pioneers and explorers of several centuries ago were to some extent astronomers as they sailed unknown seas and traversed unexplored regions. as the years pass by the astronomer extends more and more his explorations of the universe and brings back among the fruits of discovery measures of giant suns and estimates of the form and extent of the universe, views of whirling, seething nebulæ, mysterious dark clouds drifting through space, tremendous solar upheavals or glimpses of strangely marked surfaces of nearby planets. in the following pages the author has endeavored to tell in words not beyond the comprehension of the average fourteen-year-old child something of the nature of the heavenly bodies. in part i an effort is made to make the child familiar with the stars by indicating when and where they can be found in the early evening hours. in addition to identifying the principal constellations and their brightest stars by means of diagrams an attempt has been made to acquaint the child with the most interesting recent discoveries that have been made concerning the principal stars or objects in each group as well as with some of the stories and legends that have been associated with these groups of stars for centuries, and that have been handed down in the folk-lore of all nations. chapters - , inclusive, appeared originally with diagrams similar to those shown here, under the department of nature and science for young folk in _st. nicholas_ from may, , to april, , inclusive. the introductory chapter and chapters and , on the milky way and stars of the southern hemisphere, respectively, are published here for the first time, as is also the chapter in part ii on the evolution of the stars from red giants to red dwarfs, which gives the order of the evolution of the stars as now accepted as a result of the brilliant astronomical researches of dr. henry norris russell of the united states and prof. a. s. eddington, of england. the remaining chapters in part ii have been chosen from a series of articles that have appeared in _science and invention_, formerly _the electrical experimenter_, in the past four years, and have been considerably revised and in some parts rewritten to adapt them to the understanding of more youthful readers. these chapters deal with a variety of astronomical subjects of general popular interest and an effort has been made to select subjects that would cover as wide an astronomical field as possible in a limited space. the author's aim has not been to write a text-book of astronomy or to treat in detail of any one aspect of this extensive science, but simply to give the average child some general knowledge of the nature of the heavenly bodies, both those that form a part of our own solar system and those that lie in the depths of space beyond. it has been necessary to write very briefly, and we feel inadequately, of many topics of special interest such as the sun and moon. books have been written on these two subjects alone as well as upon such subjects as mars, eclipses, comets, meteors, etc., but the object has been to acquaint the child with the outstanding features of a variety of celestial objects rather than to treat of a few in detail. if the writer succeeds in arousing the child's interest in the stars so that he may look forth with intelligence at the heavens and greet the stars as friends and at the same time grasps some of the simplest and most fundamental of astronomical truths such as the distinction between stars and planets, the motions of the heavenly bodies and their relative distances from us and the place of our own planet-world in the universe, this book will have served its purpose. astronomy for young folks "the heavens declare the glory of god, and the firmament showeth his handiwork." psalm xix. i the constellations "canst thou bind the sweet influences of the pleiades or loose the bands of orion? canst thou bring forth mazzaroth in his season or canst thou guide arcturus with his sons?" --book of job. who would not like to know the stars and constellations by their names and in their seasons as we know the birds and the trees and the flowers, to recognize at their return, year by year, sirius and spica, arcturus and antares, vega and altair, to know when ursa major swings high overhead and orion sinks to rest beneath the western horizon, when leo comes into view in the east or the northern crown lies overhead? often we deprive ourselves of the pleasure of making friends with the stars and shut our eyes to the glories of the heavens above because we do not realize how simple a matter it is to become acquainted with the various groups of stars as they cross our meridian, one by one, day after day and month after month in the same orderly sequence. when the robin returns once more to nest in the same orchard in the spring time, leo and virgo may be seen rising above the eastern horizon in the early evening hours. when the first snow flies in the late fall and the birds have all gone southward the belt of orion appears in the east and cygnus dips low in the west. when we once come to know brilliant blue-white vega, ruddy arcturus, golden capella and sparkling sirius we watch for them to return each in its proper season and greet them as old friends. in the following pages we give for each month the constellations or star-groups that are nearest to our meridian, that is, that lie either due north or due south or exactly overhead in the early part of the month and the early part of the evening. we do not need to start our study of the constellations in january. we may start at any month in the year and we will find the constellations given for that month on or near the meridian at the time indicated. in using the charts or diagrams of the constellations, we should hold them in an inverted position with the top of the page toward the north or else remember that the left-hand side of the page is toward the _east_ and the right-hand side of the page toward the _west_, which is the opposite of the arrangement for charts and maps of the earth's surface. we should also bear in mind that the constellations are all continually shifting westward for the stars and the moon and the planets as well as the sun rise daily in the east and set in the west. this is due to the fact that the earth is turning in the opposite direction on its axis, that is from west to east. in twenty-four hours the earth turns completely around with respect to the heavens or through an angle of °, so in one hour it turns through an angle of ° ÷ or °. as a result the stars appear to shift westward ° every hour. this is a distance about equal in length to the handle of the big dipper, which i am sure we all know, even if we do not know another constellation in the heavens. if, then, we look at the heavens at a later hour than that for which the constellations are given we will find them farther westward and if our time of observation is earlier in the evening than the hour mentioned we will find them farther eastward. in the course of a year the earth makes one trip around the sun and faces in turn all parts of the heavens. that is, it turns through an angle of ° with respect to the heavens in a year or through an angle of ° ÷ or ° in one month. so as a result of our revolution around the sun, which is also in a west to east direction, we see that all the constellations are gradually shifting westward at the rate of ° a month. it is for this reason that we see different constellations in different months, and it is because of the turning of the earth on its axis that we see different constellations at different hours of the night. if we should sit up from sunset to sunrise and watch the stars rise in the east, pass the meridian and set in the west--as the sun does by day--we should see in turn the same constellations that are to pass across the heavens in the next six months. this is because in twelve hours' time we are carried through the same angle with respect to the heavens by the earth's rotation on its axis that we are in the next six months by the motion of the earth around the sun. let us suppose then that the time we choose for our observation of the heavens is the last of the month while our charts are given for the first of the month. we must look then farther westward for our constellations just as we must look farther westward if we chose a later hour in the evening for our observations. let us suppose that we choose for our time of observation half-past eight in the early part of december. on or close to the meridian we will find the constellations outlined in the charts for december. to the east of the meridian we will find the constellations that are given for january and february, and to the west of the meridian the constellations that are given for november and october. so if we are particularly ambitious or wish to become acquainted with the constellations more rapidly we may study at the same time the constellations for the preceding months now west of the meridian and the constellations for the following months now east of the meridian as well as the constellations for the month which will be due north or south or directly overhead as the case may be. if we were able to see the stars by day as well as by night we would observe that as the days go by the sun is apparently moving continuously eastward among certain constellations. this is a result of the earth's actual motion around the sun in the same direction. the apparent path of the sun among the stars is called the ecliptic and the belt of the heavens eight degrees wide on either side of the ecliptic is called the zodiac. the constellations that lie within this belt of the zodiac are called zodiacal constellations. the zodiac was divided by the astronomer hipparchus, who lived - b.c., into twelve signs ° wide, and the signs were named for the constellations lying at that time within each of these divisions. these zodiacal constellations are aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius and pisces. with the exception of libra, the scales, all of these constellations are named for people or animals and the word zodiac is derived from the greek word meaning "of animals." each month the sun moves eastward ° through one of these zodiacal constellations. in the days of hipparchus the sun was in aries at the beginning of spring, at the point where the ecliptic crosses the celestial equator--which lies directly above the earth's equator. this point where the ecliptic crosses the equator was then known as the first point in aries. the autumnal equinox was ° distant in aquarius and the two points were called the equinoxes because when the sun is at either equinox the day and night are equal in length all over the world. now for certain reasons which we will not explain here the equinoctial points are not fixed in position but shift gradually westward at the rate of ° in years. it is as if the equinoxes were advancing each year to meet the sun on its return and their westward motion is therefore called "the precession of the equinoxes." since the days of hipparchus this motion has amounted to about ° so that the constellations no longer occupy the signs of the zodiac that bear their names. the sun is now in pisces instead of aries at the beginning of spring and in virgo instead of aquarius at the beginning of fall. not only the sun but the moon and planets as well move through the zodiacal constellations. in fact a limit for the zodiac of ° on either side of the ecliptic was chosen because it marks the extent of the excursions of the moon and planets from the ecliptic. neither moon nor planets will be found at a greater distance than ° on either side of the ecliptic. for convenience in determining the positions of the heavenly bodies the astronomer assumes that they lie upon the surface of a celestial sphere that has its center at the center of the earth. the north pole of the celestial sphere lies directly above the north pole of the earth and the south pole of the celestial sphere directly above the south pole of the earth. the celestial equator is the great circle of the celestial sphere that lies midway between its north and south poles and directly above the earth's equator. the ecliptic is also a great circle of the celestial sphere and cuts the celestial equator at an angle of - / ° in the two points ° apart known as the equinoctial points, of which we have already spoken. the zodiacal constellations lie nearly overhead within the tropics and can be seen to advantage all over the world except in polar regions. for every position of the earth's surface except at the equator we have also our circumpolar constellations which are the ones that never pass below the horizon for the place of observation. in ° n. latitude the big dipper is a circumpolar constellation for it is above the horizon at all hours of the day and night and all times of the year. if our latitude is ° n., all stars within ° of the north pole of the heavens are circumpolar and never set, while stars within ° of the south pole of the heavens never rise. all other stars rise and set daily. if we were at the north pole all stars within ° of the north pole of the heavens would be circumpolar and would describe daily circuits of the pole parallel to the horizon remaining always above it. if we were at the equator all stars within _zero_ degrees of either pole would be circumpolar, that is _no_ stars would be circumpolar, all stars rising and setting daily. as a general rule, then, we may say that stars within an angular distance of the nearest pole of the heavens equal to the latitude never set and stars within an equal distance of the opposite pole never rise while all stars outside of these limits rise and set daily. the beginner who attempts to make the acquaintance of the principal stars and constellations occasionally may find a bright star in a constellation that is not noted in the diagrams. in this case he has probably happened upon one of the bright planets. it is not possible to include the planets in our diagrams for the reason that they are not fixed in position but apparently wander among the stars. the name planet is, in fact, derived from a greek word meaning "wanderer." the stars shine by their own light but the planets shine only by reflected light from the sun. of the seven planets in the solar system additional to our own planet earth, there are two, uranus and neptune that we need not consider for neptune is not visible without the aid of a telescope and uranus is fainter than any of the stars included in our diagrams. mercury will never appear except in the morning or evening twilight, when none but the very brightest stars are visible, since it never departs far from the sun. it will only be seen under certain favorable conditions, and usually it will escape our notice altogether unless we know exactly where to look for it although there are but two or three stars in the heavens that surpass it in brightness. venus, we will probably never mistake for any star in the heavens for it far surpasses all stars in brightness. it will always be seen in the west after sunset or in the east before sunrise and it is never seen more than three hours before or after the sun. this leaves us but three planets, jupiter, saturn and mars that we may mistake for bright stars. there is little chance that jupiter will be thus mistaken for it also is far brighter than all of the stars except sirius which differs greatly from jupiter in color. sirius is a brilliant white and jupiter is a golden yellow. the planets do not twinkle as the stars do and this is particularly true of jupiter which is remarkable for the quiet steadiness of its yellow light. this alone would serve to identify it. saturn is probably mistaken for a star oftener than any of the other planets. it moves so slowly among the stars that we would have to watch it for a number of successive evenings before we could discover that it is moving with respect to the stars. saturn is yellowish in color and we can probably best distinguish it by the steadiness of its light. if we find in one of the zodiacal groups of stars--for the planets appear among no other constellations--a bright yellowish star where no bright star is indicated on the diagram we may be reasonably certain that we have found the planet saturn. mars is the only planet that is reddish in color. once in fifteen or seventeen years, when it is particularly near to the earth, it surpasses even jupiter in brightness, but ordinarily it appears no more brilliant than one of the brighter stars. there are only two stars with which we are likely to confuse mars,--aldebaran and antares--which are very similar to it in color, and, at times, in brightness. moreover, both of these stars are zodiacal stars and mars frequently passes through the constellations to which they belong. there should be no trouble about identifying aldebaran and antares, however, from their distinctive positions in the diagrams so that any other reddish star appearing in any of the zodiacal groups we may feel certain is the planet mars. in the following diagrams of the constellations the brightest and most conspicuous stars, called first-magnitude stars, are represented by white stars. these are the stars we should all be able to recognize and call by name and in every instance the name of a first-magnitude star is given on the diagram. all other stars are represented by circles, and the size of the circle is an indication of the brightness of the star. stars visible without the aid of a telescope are referred to usually as "naked-eye stars." they are classed as first, second, third, four, fifth or sixth magnitude stars, according to their relative brightness. a star of the first magnitude is about two and one-half times brighter than a star of the second magnitude, which in turn is two and one-half times brighter than a star of the third magnitude and so on. a first-magnitude star is, then, one hundred times brighter than a sixth magnitude star which is the faintest star that can be seen without the aid of the telescope. this ratio between successive magnitudes continues among the telescopic stars. a star of the sixth magnitude is one hundred times brighter than a star of the eleventh magnitude which in turn is one hundred times brighter than a star of the sixteenth magnitude. the faintest stars that can be seen visually in the greatest telescopes are of the seventeenth or eighteenth magnitude, though stars two or three magnitudes fainter can be photographed. the faintest stars shown in the diagrams are fifth-magnitude stars and stars of this magnitude as well as stars of the fourth magnitude are only given when needed to fill out the distinctive outlines of the constellations which have been formed by connecting the principal stars in each group by dotted lines. all stars of first, second and third magnitude are given in the diagrams without exceptions as such stars are visible to everyone on clear nights. the constellations given in the following pages include practically all of the constellations that can be seen in ° n. latitude. a diagram is given for each constellation. in this latitude it is impossible to see the constellations of the southern hemisphere that lie within ° of the south pole of the heavens. a brief chapter with diagram treats of these constellations that are invisible in mid-latitudes of the northern hemisphere. ii january one of the most easily recognized constellations in the heavens is taurus, the bull, a zodiacal group which lies just south of the zenith in our latitudes in the early evening hours about the first of january. taurus is distinguished by the v-shaped group of the hyades, which contains the bright, red, first-magnitude star aldebaran, representing the fiery eye of the bull. it also contains the famous cluster of faint stars known as the pleiades, lying a short distance northwest of the hyades. no group of stars is more universally known than the pleiades. all tribes and nations of the world, from the remotest days of recorded history up to the present time, have sung the praises of the pleiades. they were "the many little ones" of the babylonians, "the seven sisters" of the greeks, "the seven brothers" of the american indians, "the hen and chickens" of many nations of europe, "the little eyes" of the south sea islanders. they were honored in the religious ceremonies of the aztecs, and the savage tribes of australia danced in their honor. many early tribes of men began their year with november, the pleiad month; and on november th, when the pleiades crossed the meridian at midnight, it was said that no petition was ever presented in vain to the kings of ancient persia. [illustration: january--taurus] poets of all ages have felt the charm of the pleiades. tennyson gives the following beautiful description of the pleiades in _locksley hall_: "many a night i saw the pleiades, rising through the mellow shade, glitter like a swarm of fireflies tangled in a silver braid." a well-known astronomer, not so many years ago, also felt the mysterious charm of the pleiades and seriously expressed the belief that alcyone, the brightest star of the pleiades, was a central sun about which all other suns were moving. but we know that there is no foundation whatever for such a belief. a fairly good eye, when the night is clear and dark, will make out six stars in this group arranged in the form of a small dipper. a seventh star lies close to the star at the end of the handle and is more difficult to find. it is called pleione, and is referred to in many legends as the lost pleiad. persons gifted with exceptionally fine eyesight have made out as many as eleven stars in the group; and with the aid of an ordinary opera-glass, anyone can see fully one hundred stars in this cluster. astronomers have found that the pleiades cluster contains at least two hundred and fifty stars, all drifting slowly in the same general direction through space, and that the entire group is enveloped in a fiery, nebulous mist which is most dense around the brightest stars. it is not known whether the stars are being formed from the nebula or whether the nebula is being puffed off from the stars. the brightest star, alcyone, is at least two hundred times more brilliant than our own sun, and all of the brighter stars in the group surpass the sun many times in brightness. it is believed that this cluster is so large that light takes many years to cross from one end of it to the other, and that it is so far from the earth that its light takes over three centuries to reach us, traveling at the rate of , miles a second. the hyades is a group of stars scarcely less famous than the pleiades, and the stars in the group also form a moving cluster of enormous extent at a distance of light-years from the earth. among the ancients, the hyades were called the rain-stars, and the word hyades is supposed to come from the greek word for rain. among the many superstitions of the past was the belief that the rising or setting of a group of stars with the sun had some special influence over human affairs. since the hyades set just after the sun in the showery springtime and just before sunrise in the stormy days of late fall, they were always associated with rain. in tennyson's _ulysses_ we read: "through scudding drifts the rainy hyades vex'd the dim sea." the hyades outline the forehead of taurus, while two bright stars some distance to the northeast of the v form the tips of the horns. only the head and forequarters of the bull are shown in the star-atlases that give the mythological groups, for, according to one legend, he is swimming through the sea and the rest of his body is submerged. according to another legend, taurus is charging down upon orion, the warrior, represented by the magnificent constellation just to the southeast of taurus, of which we shall have more to say next month. aldebaran is the arabic word for "the hindmost," and the star is so called because it follows the pleiades across the sky. it is one of the most beautiful of all the many brilliant stars visible at this time and we might profit by following the advice of mrs. sigourney in _the stars_: "go forth at night and talk with aldebaran, where he flames in the cold forehead of the wintry sky." next to aldebaran in the v is the interesting double star theta, which we can see as two distinct stars without a telescope. directly south of taurus is eridanus, sometimes called fluvius eridanus, or the river eridanus. starting a little to the southeast of taurus, close to the brilliant blue-white star rigel in orion, it runs to the westward for a considerable distance in a long curving line of rather faint stars, bends sharply southward for a short distance, then curves backward toward the east once more, and, after running for some distance, makes another sharp curve to the southwest and disappears below the southern horizon. its course is continued far into the southern hemisphere. its brightest star, achernar, is a star of the first magnitude, but it lies below the horizon in our latitudes. [illustration: january--eridanus] eridanus contains no star of particular interest to us. most of the numerous stars that mark its course are of the fourth and fifth magnitude. it contains but two stars of the third magnitude, one at the beginning of its course and one close to the southwestern horizon. the beautiful constellation of perseus lies just to the north of taurus and should rightfully be considered among the constellations lying nearest to the meridian in january, but we give this constellation among the star groups for december because of its close association with the nearby constellations andromeda and pegasus in legend and story. iii february across the meridian, due south, between eight and nine o'clock in the evening in the early part of february, lies orion, the warrior, generally considered to be the finest constellation in the heavens. orion is directly overhead at the equator, and so is seen to advantage from all parts of the world except the extreme northern and southern polar regions. a group of three faint stars outlines the head of orion. his right shoulder is marked by the deep-red, first-magnitude star betelgeuze (meaning armpit), and his left shoulder by the bright white star bellatrix, the amazon. orion stands facing taurus, the bull, and brandishes in his right hand a club, outlined by a number of faint stars extending from betelgeuze toward the northeast. the top of the club lies near the tips of the horns of taurus. in his left hand he holds up a lion's skin, which we can trace in another curving line of faint stars to the west and northwest of bellatrix. the brilliant, blue-white, first-magnitude star rigel lies in the left foot, and the second-magnitude star saiph, a little to the east of rigel, is in the right knee. three evenly spaced stars lying in a straight line that is exactly three degrees in length form the belt of orion, and from the belt hangs the sword of orion, outlined by three faint stars. the central star in the sword appears somewhat blurred and is the multiple star theta, in the midst of the great orion nebula, the finest object of its kind in the heavens. entangled in the meshes of this glowing nebula are a number of brilliant suns, appearing to us as faint stars because of their great distance. the star theta, in the heart of the nebula, is seen with a powerful telescope to consist of six stars; that is, it is a sextuple star. even with a small telescope, four of these stars can readily be seen, arranged in the form of a small trapezium. the lowest star in the sword is a triple star, and the entire constellation abounds in double, triple, and multiple stars. from the central portion of the nebula extend many branches and streamers of nebulous light, and it is known that the entire constellation of orion is enwrapped in the folds of this nebulosity, which forms a glowing, whirling mass of fiery gases in rapid rotation. this constellation is remarkable for the fact that all of its brighter stars, with the exception of the deep-red betelgeuze, form one enormous, connected group of stars. they are all more or less associated with the great nebula and its branches, and are all extremely hot, white or bluish-white stars, known as helium stars, because the gas helium is so conspicuous in their atmospheres. the orion stars are the hottest and brightest of all the stars. blazing rigel, bellatrix, and saiph, marking three corners of the great quadrilateral, of which betelgeuze marks the fourth corner, are all brilliant helium stars. so are the three stars in the belt and the fainter stars in the sword and the great nebula. [illustration: february--orion] it has been estimated that the great orion group of stars is over six hundred light-years from the earth, or about forty million times more distant than the sun. for more than six centuries the rays of light that now enter our eyes from these stars have been traveling through space with the speed of lightning. so we see orion not as it exists today, but as it was six centuries ago. the extent of the orion group of stars is also inconceivably great. even the central part of the great nebula, which appears to our unaided eyes only as a somewhat fuzzy star, would extend from here to the nearest star and beyond, while our entire solar system would be the merest speck in its midst. betelgeuze, the red star that marks the right shoulder of orion, is, as we have said, not a member of the orion group. it has been estimated that it is about two hundred light-years from the earth, or only about one-third as far away as the other stars of the constellation. betelgeuze very recently has attracted universal attention, and will probably be considered an object of historic interest in the future, because it is the first star to have its diameter measured with the new michelson interferometer, which is now being used so successfully to measure the diameters of the largest stars. the truly sensational discovery has been made that betelgeuze is a supergiant of the universe, with a diameter of about , , miles. our own sun, which is known as a "dwarf" star, has a diameter of , miles. that is, betelgeuze would make about thirty million suns the size of our own. if placed at the center of the solar system, it would fill all of the space within the orbit of mars; and the planets mercury, venus, and the earth would lie far beneath its surface. measurements of the diameters of other giant stars which are now being made with the interferometer give results quite as startling as have been obtained in the case of betelgeuze; and it has been found that several of these stars may even exceed betelgeuze in size. such a star is antares, the fiery-red star in the heart of scorpio, which is such a conspicuous object in the summer evening skies. all these huge stars are deep red in color, and some of them vary irregularly in brightness. betelgeuze is one of the stars that changes in brightness in a peculiar manner from time to time. when shining with its greatest brilliancy it is a brighter object than the nearby star aldebaran, in taurus; but a few months or a year later it may lose so much of its light as to be decidedly inferior to aldebaran. we may note for ourselves this remarkable change in the brightness of betelgeuze by comparing the two stars from time to time. directly south of orion lies the small constellation of lepus, the hare, which is made up of third-magnitude and fourth-magnitude stars. the four brighter stars are arranged in the form of a small, but distinct, quadrilateral, or four-sided figure, which may be easily seen in our latitudes. the small constellation of columba, the dove, which lies just south of lepus, is so close to the horizon that it can not be seen to advantage in the mid-latitudes of the northern hemisphere. neither lepus nor columba contain any object of unusual interest. [illustration: february--auriga] due north of orion, and lying in the zenith at this time, is auriga, the charioteer, represented, strange to say, with capella, a goat, in his arms. the beautiful first-magnitude star capella, golden-yellow in color, serves us in identifying the constellation. close at hand are the kids, represented by a group of three faint stars. capella is one of the most brilliant stars of the northern hemisphere. it is almost exactly equal in brightness to arcturus and vega, stars conspicuous in the summer months, and it is a shade brighter than magnificent blue-white rigel in orion. capella is about fifty light-years distant from the earth and is fully two hundred times more brilliant than our own sun. at the distance of capella, the sun would appear to be considerably fainter than any one of the three stars in the nearby group of the kids. capella is attended by a companion star so close to its brilliant ruler that it can not be seen as a separate star save with the aid of the most powerful telescopes. its distance from capella has been very accurately measured, however, by means of the interferometer, which is giving us the measurements of the diameters of the giant stars. it is known that this companion sun is closer to capella than our planet earth is to the sun. at no time of the year shall we find near the meridian so many brilliant and beautiful stars as appear in the month of february at this time in the evening. in addition to capella, which is one of the three most brilliant stars in the northern hemisphere of the heavens, we have, in orion alone, two stars of the first magnitude, betelgeuze and rigel, and five stars of the second magnitude, bellatrix and saiph and the three stars in the belt. in addition, we have not far distant in the western sky, fiery aldebaran in taurus, and close on the heel of orion in the east, sirius, the brightest star in the heavens, in the constellation of canis major, the greater dog, as well as the first-magnitude star procyon in canis minor, the lesser dog. of these two groups we shall have more to say under the constellations for march. iv march to the southeast of orion and almost due south at eight o'clock in the evening on the first of march lies the constellation of canis major, the greater dog, containing sirius, the dog-star, which far surpasses all other stars in the heavens in brilliancy. sirius lies almost in line with the three stars that form the belt of orion. we shall not have the slightest difficulty in recognizing it, owing to its surpassing brilliancy as well as to the fact that it follows so closely upon the heels of orion. sirius is the greek for "scorching" or "sparkling," and the ancients attributed the scorching heat of summer to the fact that sirius then rose with the sun. the torrid days of midsummer they called the "dog-days" for this reason, and we have retained the expression to the present time. since sirius was always associated with the discomforts of the torrid season, it did not have an enviable reputation among the greeks. we find in pope's translation of the _iliad_ this reference to sirius: "terrific glory! for his burning breath taints the red air with fever, plagues, and death." in egypt, however, many temples were dedicated to the worship of sirius, for the reason that some five thousand years ago it rose with the sun at the time of the summer solstice, which marks the beginning of summer, and heralded the approaching inundation of the nile, which was an occasion for great rejoicing among the egyptians. it was, therefore, called the nile star and regarded by them with the greatest reverence. sirius is an intensely white hydrogen star; but owing to its great brilliancy and to the fact that it does not attain a great height above the horizon in our latitudes, its rays are greatly refracted or broken up by the atmosphere, which is most dense near the horizon, and as a result, it twinkles or scintillates more noticeably than other stars and flashes the spectrum colors--chiefly red and green--like a true "diamond in the sky"--a magnificent object in the telescope. sirius is one of our nearest neighbors among the stars. only two stars are known to be nearer to the solar system. yet its light takes about eight and a half years to flash with lightning speed across the great intervening chasm. it is attended also by a very faint star that is so lost in the rays of its brilliant companion that it can only be found with the aid of a powerful telescope. the two stars are separated by a distance of , , , miles; that is they are about as far apart as neptune and the sun. they swing slowly and majestically about a common center, called their center of gravity, in a period of about forty-nine years. so faint is the companion of sirius that it is estimated that twenty thousand such stars would be needed to give forth as much light as sirius. the two stars together, sirius and its companion, give forth twenty-six times as much light as our own sun. they weigh only about three times as much, however. the companion of sirius, in spite of its extreme faintness, weighs fully half as much as the brilliant star. [illustration: march--canis major] there are a number of bright stars in the constellation of canis major. a fairly bright star a little to the west of sirius marks the uplifted paw of the dog, and to the southeast, in the tail and hind quarters, are several conspicuous stars of the second magnitude. a little to the east and much farther to the north, we find canis minor, the lesser dog, containing the beautiful first-magnitude star procyon, "precursor of the dog"--that is, of sirius. since procyon is so much farther north than sirius and very little to the east, we see its brilliant rays in the eastern sky some time before sirius appears above the southeastern horizon, hence its name. long after sirius has disappeared from view beneath the western horizon in the late spring, procyon may still be seen low in the western sky. procyon, also is one of our nearer neighbors among the stars, being only about ten light-years distant from the solar system. like sirius, it is a double star with a much fainter companion, that by its attraction sways the motion of procyon to such an extent that we should know of its existence, even if it were not visible, by the disturbances it produces in the motion of procyon. the period of revolution of procyon and its companion about a common center is about forty years, and the two stars combined weigh about a third more than our own sun and give forth six times as much light. canis minor contains only one other bright star, beta, a short distance to the northwest of procyon. originally, the name procyon was given to the entire constellation, but it was later used only with reference to the one star. procyon, sirius, and betelgeuze in orion form a huge equal-sided triangle that lies across the meridian at this time and is a most conspicuous configuration in the evening sky. [illustration: march--gemini and canis minor] directly south of the zenith we find gemini, the twins, one of the zodiacal constellations. it is in gemini that the sun is to be found at the beginning of summer. the two bright stars castor and pollux mark the heads of the twins, and the two stars in the opposite corners of the four-sided figure shown in the chart mark their feet. castor and pollux, according to the legend, were the twin brothers of helen of troy who went on the argonautic expedition. when a storm overtook the vessel on its return voyage, orpheus invoked the aid of apollo, who caused two stars to shine above the heads of the twins, and the storm immediately ceased. it was for this reason that castor and pollux became the special deities of seamen, and it was customary to place their effigies upon the prows of vessels. the "by jimini!" of today is but a corruption of the "by gemini!" heard so frequently among the sailors of the ancient world. the astronomical name for castor, the fainter star, is alpha geminorum, meaning alpha of gemini. as it was customary to call the brightest star in a constellation by the first letter in the greek alphabet, it is believed that castor has decreased considerably in brightness since the days of the ancients, for it is now decidedly inferior to pollux in brightness, which is called beta geminorum. of the two stars, castor is the more interesting because it is a double star that is readily separated into two stars with the aid of a small telescope. the two principal stars are known to be, in turn, extremely close double stars revolving almost in contact in periods of a few days. where we see but one star with the unaided eye, there is, then a system of four suns, the two close pairs revolving slowly about a common center of gravity in a period of several centuries and at a great distance apart. the star pollux, which we can easily distinguish by its superior brightness, is the more southerly of the twin stars and lies due north of procyon and about as far from procyon as procyon is from sirius. the appearance of gemini on the meridian in the early evening and of the huge triangle, with its corners marked by the brilliants, procyon, sirius, and betelgeuze, due south, with "great orion sloping slowly to the west," is as truly a sign of approaching spring as the gradual lengthening of the days, the appearance of crocuses and daffodils, and the first robin. it is only a few weeks later--as pictured by tennyson in _maud_-- "when the face of the night is fair on the dewy downs, and the shining daffodil dies, and the charioteer and starry gemini hang like glorious crowns over orion's grave low down in the west." v april in the early evening hours of april the western sky is still adorned with the brilliant jewels with which we became familiar on the clear frosty evenings of winter. orion is now sinking fast to his rest beneath the western horizon. beautiful, golden capella in auriga glows in the northwest. sirius sparkles and scintillates, a magnificent diamond of the sky, just above the southwestern horizon, while procyon in canis minor, the lesser dog, and castor and pollux, the twins, in the constellation of gemini, are still high in the western part of the heavens. in the northeast and east may be seen the constellations that will be close to the meridian at this time next month. ursa major, the greater bear, with its familiar big dipper, is now in a favorable position for observation. the sickle in leo is high in the eastern sky, and spica, the brilliant white diamond of the evening skies of spring, is low in the southeast in virgo. near the meridian this month we find between auriga and ursa major, and east of gemini, the inconspicuous constellation of lynx, which contains not a single bright star and is a modern constellation devised simply to fill the otherwise vacant space in circumpolar regions between ursa major and auriga. [illustration: april--cancer] just south of the zenith at this time, and lying between gemini and leo, is cancer, the crab, the most inconspicuous of all the zodiacal constellations. there are no bright stars in this group, and there is also nothing distinctive about the grouping of its faint stars, though we can readily find it, from its position between the two neighboring constellations of gemini and leo by reference to the chart. in the position indicated there we will see on clear evenings a faint, nebulous cloud of light, which is known as praesepe, the beehive, or as the manger, the two faint stars flanking it on either side being called aselli, the asses. this faint cloud can be easily resolved by an opera-glass into a coarse cluster of stars that lie just beyond the range of the unaided human vision. to the ancients, praesepe served as an indicator of weather conditions, and aratus, an ancient astronomer, wrote of this cluster: "a murky manger, with both stars shining unaltered, is a sign of rain. if while the northern ass is dimmed by vaporous shroud, he of the south gleam radiant, expect a south wind; the vaporous shroud and radiance exchanging stars, harbinger boreas." this was not entirely a matter of superstition, as we might possibly imagine, for the dimness of the cluster is simply an indication that vapor is gathering and condensing in the atmosphere, just as a ring around the moon is an indication of the same gathering and condensation of vapor that precedes a storm. some centuries ago the sun reached its greatest distance north of the equator--which occurs each year at the beginning of summer--at the time when it was passing through the constellation of cancer. our tropic of cancer, which marks the northern limit of the torrid zone, received its name from this fact. at the time when the sun reaches the point farthest north, its height above the horizon changes very little from day to day, and for a short time it appears to be slowly crawling sideways through the heavens, as a crab walks, and for this reason, possibly, the constellation was called cancer, the crab. at the present time the "precession of the equinoxes," or westward shifting of the vernal equinox--the point where the sun crosses the equator going north in the spring--brings the sun, when it is farthest north, in gemini instead of in cancer. at the present time, then it would be more accurate to speak of the tropic of gemini, though this in turn would be inaccurate after a lapse of centuries, as the sun passed into another constellation at the beginning of summer. the tropic of capricorn, which marks the farthest southern excursions of the sun in its yearly circuit of the heavens, should also more appropriately be called the tropic of sagittarius, as the sun is now in sagittarius instead of capricornus at the time when it is farthest south, though the point is slowly shifting westward into scorpio. mythology tells us that cancer was sent by juno to distract hercules by pinching his toes while he was contending with the many-headed serpent in the lernean swamp. hercules, the legend says, crushed the crab with a single blow, and juno by way of reward placed it in the heavens. in cancer, according to the belief of the chaldeans, was located the "gate of men," by which souls descended into human bodies, while in capricornus was the "gate of the gods," through which the freed souls of men returned to heaven. [illustration: april--hydra] hydra, the many-headed serpent with which hercules contended, is represented by a constellation of great length. it extends from a point just south of cancer, where a group of faint stars marks the heads, to the south and southeast in a long line of faint stars. it passes in its course just south of crater and corvus, the two small star-groups below leo (see constellations for may), which are sometimes called its riders, and it also stretches below the entire length of the long, straggling constellation of virgo. at this time we can trace it only to the point where it disappears below the horizon in the southeast. it contains but one bright star, alphard, or cor hydrae as it is also called, standing quite alone and almost due south at this time. hydra, as well as lynx and cancer, contains no noteworthy or remarkable object and consists chiefly of faint stars. alphard is, in fact, the only bright star that we have in the constellations for this month. it chances that these three inconspicuous star-groups, lynx, crater, and hydra, lie nearest to the meridian at this time, separating the brilliant groups of winter from those of the summer months. [illustration: april--lynx] vi may ursa major, the great bear, and ursa minor, the lesser bear, or, as they are more familiarly called, the big dipper and the little dipper, are the best known of all the constellations visible in northern latitudes. they are called circumpolar constellations, which means "around the pole." for those who live north of ° n. lat. they never set, but can be seen at all hours of the night and at all times of the year. in fall and winter evenings ursa major lies below the pole and near the horizon, and so is usually hidden more or less from view by trees or buildings. it is during the early evening hours of late spring and summer that this constellation is seen to the best advantage high in the sky above the pole. if one looks due north at the time mentioned, it will be impossible to miss either of these constellations. to complete the outline of the great bear, it is necessary to include faint stars to the east, which form the head of the bear, and other faint stars to the south, which form the feet, but these are all inconspicuous and of little general interest. the two stars in the bowl of the big dipper through which an arrow is drawn in the chart, are called the pointers, because an imaginary line drawn through these two stars and continued a distance about equal to the length of the big dipper, brings us to the star polaris, or the north star, at the end of the handle of the little dipper, which is very close to the north pole of the heavens, the direction in which the earth's axis points. the pole lies on the line connecting the star at the bend in the handle of the big dipper with polaris, and is only one degree distant from the pole-star. [illustration: may--ursa major and ursa minor] the distance between the pointers is five degrees of arc, and the distance from the more northerly of these two stars to polaris is nearly thirty degrees. we may find it useful to remember this in estimating distances between objects in the heavens, which are always given in angular measure. a small two and one-half inch telescope will separate polaris into two stars eighteen seconds of arc apart. the companion star is a faint white star of the ninth magnitude. twenty years or so ago it was discovered with the aid of the spectroscope that the brighter of the two stars was also a double star, but the two stars were so close together that they could not be seen as separate stars in any telescope. later it was found that the brighter star was in reality triple, that is, it consists of three suns close together. the faint white companion star formed with these three suns a system of four suns revolving about a common center of gravity. still more recently it has been discovered that the brightest of these four suns varies regularly in brightness in a period of a little less than four days. it belongs to the important class of stars known as cepheid variable stars, whose changes of light, it is believed, are produced by some periodic form of disturbance taking place within the stars themselves. with one exception, polaris is the nearest to the earth of all these cepheid variable stars, which are in most instances at very great distances from the solar system. the latest measurements of the distance of polaris show that its light takes about two centuries to travel to the earth, or, in other words, that it is distant two hundred light-years. like all cepheid variables, polaris is a giant star. it gives forth about five hundred and twenty-five times as much light as our own sun. if polaris and the sun were placed side by side at a distance of thirty-three light-years, the sun would appear as a star of the fifth magnitude, just well within the range of visibility of the human eye, while polaris would outshine sirius, the brightest star in the heavens. as a practical aid to navigators, polaris is unsurpassed in importance by any star of the northern hemisphere of the heavens. at the equator the pole-star lies in the horizon; at the north pole of the earth it is in the zenith or directly overhead. its altitude or height above the horizon is always equal to the latitude of the place of observation. as we travel northward from the equator toward the pole we see polaris rise higher and higher in the sky. in new york the elevation of polaris above the horizon is forty degrees, which is the latitude of the city. the pointers indicate the direction of polaris and the true north, while the height of polaris above the horizon tells us our latitude. these kindly stars direct us by night when we are uncertain of our bearings, whether we travel by land or sea or air. they are the friends and aids of explorers, navigators and aviators, who often turn to them for guidance. bryant writes thus beautifully of polaris in his _hymn to the north star_: constellations come and climb the heavens, and go. star of the pole! and thou dost see them set. alone in thy cold skies, thou keep'st thy old unmoving station yet, nor join'st the dances of that glittering train, nor dipp'st thy virgin orb in the blue western main. on thy unaltering blaze the half wrecked mariner, his compass lost, fixes his steady gaze, and steers, undoubting, to the friendly coast; and they who stray in perilous wastes by night, are glad when thou dost shine to guide their footsteps right. the star at the bend in the handle of the big dipper, called mizar, is of special interest. if one has good eyesight, he will see close to it a faint star. this is alcor, which is arabic for the test. the two stars are also called the horse and the rider. mizar forms with alcor what is known as a wide double star. it is, in fact, the widest of all double stars. many stars in the heavens that appear single to us are separated by the telescope into double or triple or multiple stars. they consist of two or more suns revolving about a common center, known as their center of gravity. sometimes the suns are so close together that even the most powerful telescope will not separate them. then a most wonderful little instrument, called the spectroscope, steps in and analyzes the light of the stars and shows which are double and which are single. a star shown to be double by the spectroscope, but not by the telescope, is called a spectroscopic binary star. mizar is of historic interest, as being the first double star to be detected with the aid of the telescope. a very small telescope will split mizar up into two stars. the brighter of the two is a spectroscopic binary star beside, so that it really consists of two suns instead of one, with the distance between the two so small that even the telescope cannot separate them. about this system of three suns which we know as the star mizar, the faint star alcor revolves at a distance equal to sixteen thousand times the distance of the earth from the sun. [illustration: may--leo] if we follow the imaginary line drawn through the pointers in a _southerly_ direction about forty-five degrees, we come to leo, the lion, one of the zodiacal constellations. there should be no difficulty in finding the constellation leo, as its peculiar sickle-shaped group of bright stars makes it distinctive from all other constellations. at the time we have mentioned, that is, the early evening hours, it will lie a little to the southwest of the zenith. leo is one of the finest constellations and is always associated with the spring months because it is then high in the sky in the evening. regulus is the beautiful white star which marks the handle of the sickle, and the heart of leo; and denebola is the second-magnitude star in the tail of leo. [illustration: may--corvus and crater] due south of denebola, about thirty degrees, we find the small star-group known as crater, the cup, which is composed of rather faint and inconspicuous stars. just east of crater is the group known as corvus, the crow, which forms a very characteristic little four-sided figure of stars differing very little from one another in brightness. these two star groups lie far to the south in our latitudes; but if we lived twenty degrees south of the equator, we would find them nearly overhead, at this time of the year. just south of corvus and crater we find hydra, one of the constellations for april which extends beneath these constellations and also beneath virgo, one of the june constellations. vii june the star-groups that occupy the center of the celestial stage in mid-latitudes of the northern hemisphere during the early evening hours of june are boötes, often called the hunter, (although the word means herdsman or shouter), which will be found overhead at this time; virgo, the maiden, largest of the zodiacal constellations, lying nearly due south; canes venatici, the hunting dogs; corona borealis, the northern crown, and coma berenices. the gorgeous orange-hued arcturus in boötes and the beautiful bluish-white spica in virgo, like a diamond in its sparkling radiance, form with denebola in leo, which we identified in may, a huge equal-sided triangle that is always associated with the spring and early summer months. to the west of boötes, below the handle of the big dipper, is a region where there are few conspicuous stars. here will be found canes venatici (the hunting dogs with which boötes is supposed to be pursuing the great bear around the north pole), and, further south, coma berenices (bernice's hair). the brighter of the two hunting dogs, which is also the brightest star in the entire region covered by these two constellations, appears as a beautiful blue-and-yellow double star in the telescope. it was named cor caroli (heart of charles) by the astronomer halley in honor of charles ii of england, at the suggestion of the court physician, who imagined it shone more brightly than usual the night before the return of charles to london. of more interest to astronomers is the magnificent spiral nebula in this constellation, known as the "whirlpool nebula," appearing as a faint, luminous patch in the sky, of which many photographs have been taken with the great telescopes. this entire region, from canes venatici to virgo, abounds in faint spiral nebulæ that for some reason not yet understood by astronomers are crowded together in this part of the heavens where stars are comparatively few. it is believed that there are between five hundred thousand and a million of these spiral nebulæ in the entire heavens, and the problem of their nature and origin and distance is one that the astronomers are very anxious to solve. many wonderful facts are now being learned concerning these faint nebulous wisps of light which, with a few exceptions, are observable only with great telescopes. they reveal their spiral structure more clearly to the photographic plate than to the human eye, and some magnificent photographs of them have been taken with powerful telescopes. coma berenices, south of canes venatici and southwest of boötes, is a constellation that consists of a great number of stars closely crowded together, and just barely visible to the unaided eye. as a result, it has the appearance of filmy threads of light, which doubtless suggested its name to the imaginative ancients, who loved to fill the heavens with fanciful creations associated with their myths and legends. these stars form a moving cluster of stars estimated to be at a distance of about light-years from the solar systems. [illustration: june--boÖtes, canes venatici and coma berenicis] this region, so lacking in interesting objects for the naked-eye observer, is a mine of riches to the fortunate possessors of telescopes; and the great telescopes of the world are frequently pointed in this direction, exploring the mysteries of space that abound here. just to the east of boötes is the exquisite little circlet of stars known as corona borealis, the northern crown. it consists of six stars arranged in a nearly perfect semicircle, and one will have no difficulty in recognizing it. its brightest star, alpha, known also by the name of alphacca, is a star of the second magnitude. boötes is one of the largest and finest of the northern constellations. it can be easily distinguished by its peculiar kite-shaped grouping of stars or by the conspicuous pentagon (five-sided figure) of stars which it contains. the most southerly star in this pentagon is known as epsilon boötes and is one of the finest double stars in the heavens. the two stars of which it consists are respectively orange and greenish-blue in color. by far the finest object in boötes, however, is the magnificent arcturus, which is the brightest star in the northern hemisphere of the heavens. this star will be conspicuous in the evening hours throughout the summer months, as will also the less brilliant spica in virgo. some recent measurements show that arcturus is one of our nearer neighbors among the stars. its distance is now estimated to be about twenty-one light-years. that is, a ray of light from this star takes twenty-one years to reach the earth, traveling at the rate of one hundred and eighty-six thousand miles per second. it would seem as if we should hardly speak of arcturus, twenty-one light-years away, as a near neighbor, yet there are millions of stars that are far more distant from the earth, and very few that are nearer to us than arcturus. the brightness of arcturus is estimated to be about forty times that of the sun. that is, if the two bodies were side by side, arcturus would give forth forty times as much light and heat as the sun. arcturus is also one of the most rapidly moving stars in the heavens. in the past sixteen centuries it has traveled so far as to have changed its position among the other stars by as much as the apparent width of the moon. most of the stars, in spite of their motions through the heavens in various directions, appear today in the same relative positions in which they were several thousand years ago. it is for this reason that the constellations of the egyptians and of the greeks and romans are the same constellations that we see in the heavens today. were all the stars as rapidly moving as arcturus, the distinctive forms of the constellations would be preserved for only a very few centuries. [illustration: june--virgo] virgo, which lies south and southwest of boötes, is a large, straggling constellation, consisting of a y-shaped configuration of rather inconspicuous stars. it lies in the path of our sun, moon and planets, and so is one of the zodiacal constellations. the cross in the diagram indicates the present position of the autumnal equinox, the point where the sun crosses the equator going south, and the position the sun now occupies at the beginning of fall. spica, the brightest star in virgo, is a bluish-white, first-magnitude star, standing very much alone in the sky. in fact, the arabs referred to this star as "the solitary one." its distance from the earth is not known, but must be very great as it cannot be found by the usual methods. the spectroscope shows that it consists of two suns very close together, revolving about a common center in a period of only four days. within the branches of the "y" in virgo, and just to the north of it, is the wonderful nebulous region of this constellation, but it takes a powerful telescope to show the faint spiral nebulæ that exist here in great profusion. all of these spirals are receding from the plane of the milky way with enormous velocities. the spiral nebulæ are, in fact, the most rapidly moving objects in the heavens. viii july due east of the little circlet of stars known as corona borealis, and almost directly overhead in our latitude ( ° n.) about nine o'clock in the evening in the early part of july, is the large constellation of hercules, named for the famous hero of grecian mythology. there are no stars of great brilliancy in this group, but it contains a large number of fairly bright stars arranged in the form outlined in the chart. the hero is standing with his head, marked by the star alpha herculis, toward the south, and his foot resting on the head of draco, the dragon, a far-northern constellation with which we become acquainted in august. [illustration: july--hercules] alpha herculis, the best known star in this constellation, is of unusual interest. not only is it a most beautiful double star, the brighter of the two stars of which it is composed being orange, and the fainter greenish-blue, but it is also a star that changes in brightness irregularly. both the orange and the blue star share in this change of brightness. there are a number of stars in the heavens that vary in brightness, some in very regular periods, and others, like alpha herculis, irregularly. these latter stars are nearly always deep orange or reddish in color. one may note this variation in the brightness of alpha herculis by comparing it from time to time with some nearby star that does not vary in brightness. [illustration: the great hercules cluster--a universe of suns taken with -inch reflector of the mt. wilson observatory] the constellation of hercules is a very rich field for the possessor of even a small telescope. here are to be found beautifully colored double stars in profusion, and, in addition, two remarkable clusters of stars. the brighter of the two is known as the great hercules cluster. its position is shown on the chart, and, under favorable conditions--that is, on a clear, dark night, when there is no moonlight--it may be seen without the aid of a telescope as a small, faint patch of light. one would never suspect from such a view what a wonderful object this cluster becomes when seen with the aid of a powerful telescope. photographs taken with the great telescopes show this faint wisp of light as a magnificent assemblage of thousands of stars, each a sun many times more brilliant than our own sun. the crowded appearance of the stars in the cluster is due partly to the fact that it is very distant from the earth, though neighboring stars in the cluster are indeed much nearer to one another than are the stars in the vicinity of our solar system. it has been found that this cluster is so far away that its light takes over thirty-six thousand years to reach the earth. at the distance of this cluster, a sun equal in brightness to our own sun would be so faint that the most powerful telescope in the world would not show it. so we know that the stars that are visible in the hercules cluster are far more brilliant than our sun. a fair-sized telescope will show about four thousand stars in this cluster, but the greatest telescopes show over one hundred thousand in it, and there are without doubt many more too faint to be seen at all. the hercules cluster is called a globular star-cluster, because the stars in it are arranged nearly in the form of a sphere. there are in the heavens about ninety such clusters whose distances have been found, and they are among the most distant of all objects. most of them are very faint, and a few are over two hundred thousand light-years distant from the earth. the hercules cluster is one of the nearest and is the most noted of all of these globular clusters. it is considered to be one of the finest objects in the heavens. the other cluster in hercules is also very fine, but not to be compared with this one. [illustration: july--ophiuchus and serpens] just to the south of hercules are two constellations, ophiuchus, the serpent-bearer, and serpens, the serpent, which are so intermingled that it is difficult to distinguish them. there are in these two constellations, as in hercules, no stars of unusual brilliancy, but a large number of fairly bright stars. the brightest star in ophiuchus is known as alpha ophiuchi and it marks the head of the serpent-bearer. the two stars, alpha ophiuchi and alpha herculis, are close together, being separated by a distance about equal to that between the pointers of the big dipper. alpha ophiuchi is the brighter of the two, and it is farther east. ophiuchus, according to one legend, was once a physician on earth, and was so successful as a healer that he could raise the dead. pluto, the god of the lower world, became alarmed for fear his kingdom would become depopulated, and persuaded jupiter to remove ophiuchus to a heavenly abode, where he would be less troublesome. the serpent is supposed to be a symbol of his healing powers. the head of serpens is marked by a group of faint stars just south of corona borealis and southwest of hercules. from here a line of fairly bright stars marks the course of serpens southward to the hand of ophiuchus. two stars close together and nearly equal in brightness mark the hand with which the hero grasps the body of the serpent. the other hand is marked by an equally bright single star some distance to the eastward where the two constellations again meet. ophiuchus is thus represented as holding the serpent with both hands. it is not an easy matter to make out the outlines of these straggling groups, but there are in them several pairs of stars nearly equal in brightness and about as evenly spaced as the two stars in the one hand of ophiuchus, and these, as well as the diagram, will be of aid in tracing the two groups. just south of serpens and ophiuchus lies one of the most beautiful and easily recognized constellations in the heavens. this is the constellation of scorpio, the scorpion, which will be found not far above the southern horizon at this time. the small constellation of libra, the scales, which lies just to the northwest of scorpio, was at one time a part of this constellation and represented the creature's claws, but some centuries ago its name was changed to libra. both scorpio and libra are numbered among the twelve zodiacal constellations--that is, they lie along the ecliptic, or apparent yearly path of the sun among the stars. scorpio is the most brilliant and interesting of all the zodiacal groups. the heart of the scorpion is marked by the magnificent first-magnitude star antares, which is of a deep reddish color. the name signifies rival of ares (mars). it is so called because it is the one star in the heavens that most closely resembles mars, and it might be mistaken for the ruddy planet if one were not familiar with the constellations. at times, when mars is at a considerable distance from the earth, it is almost equal in brightness and general appearance to this glowing red star in the heart of the scorpion. in its trips around the sun, mars passes occasionally very close to antares, and the two then present a very striking appearance. [illustration: july--libra and scorpio] with a telescope of medium size, one will find an exquisite little green companion-star close to antares. the little companion is so close to antares that it is difficult to find it in the glare of light from its more brilliant neighbor. antares is one of the giant stars of the universe. in fact it is, so far as we know, the greatest of all the giants. its diameter is more than five hundred times that of our own sun and nearly twice that of the giant star betelgeuze in orion. if placed at the center of the solar system its surface would lie far beyond the orbit of mars. both ophiuchus and scorpio are crossed by the milky way, that broad belt of numberless faint stars that encircles the heavens. some of the most wonderful and beautiful regions of the milky way are to be found in these two constellations. at various times in the past, there have suddenly flashed forth brilliant stars in the milky way which are known as "temporary stars," or "novæ." these outbursts signify that some celestial catastrophe has taken place, the nature or cause of which is not clearly understood. some of the most brilliant of these outbursts have occurred in these two constellations. the life of a nova is very short, a matter of a few months, and it rapidly sinks into oblivion, so nothing is to be seen of some of the most brilliant of all these stars that have appeared in this region in the past. a few are still faintly visible in large telescopes. ix august it was one of the twelve labors of hercules, the hero of grecian mythology, to vanquish the dragon that guarded the golden apples in the garden of the hesperides. among the constellations for july we found the large group of stars that represents the hero himself, and this month we find just to the north of hercules the head of draco, the dragon. the foot of the hero rests upon the dragon's head, which is outlined by a group of four fairly bright stars forming a quadrilateral or four-sided figure. the brightest star in this group passes in its daily circuit of the pole almost through the zenith of london. that is, as it crosses the meridian of london, it is almost exactly overhead. from the head of draco, the creature's body can be traced in a long line of stars curving first eastward, then northward, toward the pole-star to a point above hercules, where it bends sharply westward. the body of the monster lies chiefly between its head and the bowl of the little dipper. the tail extends in a long line of faint stars midway between the two dippers, or the constellations of ursa major and ursa minor, the tip of the tail lying on the line connecting the pointers of the big dipper with the pole-star polaris. draco, as well as ursa major and ursa minor, is a circumpolar constellation in our latitude; that is, it makes its circuit of the pole without at any time dipping below the horizon in latitudes north of °. it is, therefore, visible at all hours of the night in mid-latitudes of the northern hemisphere, but is seen to the best advantage during the early evening hours in the summer months. there are no remarkable stars in this constellation with the exception of alpha, which lies halfway between the bowl of the little dipper and mizar, the star at the bend in the handle of the big dipper. [illustration: august--draco and lyra] about four thousand seven hundred years ago, this star was the pole-star--lying even nearer to what was then the north pole of the heavens than polaris does to the present position of the pole. the sun and moon exert a pull on the bulging equatorial regions of the earth, which tends to draw the plane of the earth's equator down into the plane of the ecliptic. this causes the "precession of the equinoxes" and at the same time a slow revolution of the earth's axis of rotation about the pole of the ecliptic. the north pole of the heavens as a result describes a circle about the pole of the ecliptic of radius - / ° in a period of , years. each bright star that lies near the circumference of this circle becomes in turn the pole-star sometime within this period. the star alpha, in draco, had its turn at being pole-star some forty-seven centuries ago. polaris is now a little over a degree from the north pole of the heavens. during the next two centuries it will continue to approach the pole until it comes within a quarter of a degree of it, when its distance from the pole will begin to increase again. about twelve thousand years hence the magnificent vega, whose acquaintance we will now make, will be the most brilliant and beautiful of all pole-stars. vega (arabic for "falling eagle") is the resplendent, bluish-white, first-magnitude star that lies in the constellation of lyra, the lyre or harp, a small, but important, constellation just east of hercules and a little to the southeast of the head of draco. vega is almost exactly equal in brightness to arcturus, the orange-colored star in boötes, now lying west of the meridian in the early evening hours. it is also a near neighbor of the solar system, its light taking something like forty years to travel to the earth. vega is carried nearly through the zenith of washington and all places in the same latitude by the apparent daily rotation of the heavens. it is a star that we have no difficulty in recognizing, owing to the presence of two nearby stars that form, with it, a small equal-sided triangle with sides only two degrees in extent. if our own sun were at the distance of vega, it would not appear as bright as one of these faint stars, so much more brilliant is this magnificent sun than our own. the two faint stars that follow so closely after vega and form the little triangle with it are also of particular interest. epsilon lyræ, which is the northern one of these two stars, may be used as a test of keen eyesight. it is the finest example in the heavens of a quadruple star--that is, "a double-double star." a keen eye can just separate this star into two without a telescope, and with the aid of a telescope, each of the two splits up into two stars, making four stars in place of the one visible to the average eye. zeta, the other of the two stars that form the little triangle with vega, is also a fine double star. the star that lies almost in a straight line with epsilon and zeta and a short distance to the south of them is a very interesting variable star known as beta lyræ. its brightness changes very considerably in a period of twelve days and twenty-two hours. this change of brightness is due to the presence of a companion star. the two stars are in mutual revolution, and their motion is viewed at such an angle from the earth that, in each revolution, one star is eclipsed by the other, producing a variation in the amount of light that reaches our eyes. by comparing this star from day to day with the star just a short distance to the southeast of it, which does not vary in brightness, we can observe for ourselves this change in the light of beta lyræ. there are a number of stars in the heavens that vary in brightness in the same manner as beta lyræ, and they are called eclipsing-variable stars. on the line connecting beta lyræ with the star southeast of it and one-third of the distance from beta to this star, lies the noted ring nebula in lyra, which is a beautiful object even in a small telescope. it consists of a ring of luminous gas surrounding a central star. the star shines with a brilliant, bluish-white light and is visible only in powerful telescopes though it is easily photographed since it gives forth rays to which the photographic plate is particularly sensitive. in small telescopes the central part of this nebula appears dark but with a powerful telescope a faint light may be seen even in the central portion of the nebula. this is one of the most interesting and beautiful telescopic objects in the heavens. it is in the general direction of the constellation of lyra that our solar system is speeding at the rate of more than a million miles a day. this point toward which we are moving at such tremendous speed lies a little to the southwest of vega, on the border between the constellations of lyra and hercules, and is spoken of as the apex of the sun's way. [illustration: august--sagittarius] in the southern sky we have this month the constellation of sagittarius, the archer, which is just to the east of scorpio and a considerable distance south of lyra. it can be recognized by its peculiar form, which is that of a short-handled milk dipper, with the bowl turned toward the south and a trail of bright stars running from the end of the handle toward the southwest. this is one of the zodiacal groups which contain no first-magnitude stars, but a number of the second and third magnitude. it is crossed by the milky way, which is very wonderful in its structure at this point. some astronomers believe that here--among the star-clouds and mists of nebulous light which are intermingled with dark lanes and holes, in reality dark nebulæ--lies the center of the vast system of stars and nebulæ in which our entire solar system is but the merest speck. some of the grandest views through the telescope are also to be obtained in this beautiful constellation of sagittarius, which is so far south that it is seen to better advantage in the tropics than in the mid-latitudes of the northern hemispheres. x september one of the most beautiful constellations of the northern hemisphere is cygnus, the swan, which is in the zenith in mid-latitudes about nine o'clock in the evening the middle of september. it lies directly in the path of the milky way which stretches diagonally across the heavens from the northeast to the southwest at this time. in cygnus, the milky way divides into two branches, one passing through ophiuchus and serpens to scorpio, and the other through sagitta and aquila to sagittarius, to meet again in the southern constellation of ara, just south of scorpio and sagittarius. on clear, dark evenings, when there is no moonlight, this long, dark rift in the milky way can be seen very clearly. in cygnus, as in ophiuchus, scorpio, and sagittarius we find wonderful star-clouds, consisting of numberless stars so distant from us and, therefore, so faint that they do not appear as distinct points of light except in the greatest telescopes. it is the combined light from these numberless stars that cannot be seen separately that produces the impression of stars massed in clouds of nebulous light and gives to this girdle of the heavens its name of the milky way. in cygnus, as in a number of other constellations of both hemispheres, the milky way is crossed by dark rifts and bars and is very complicated in its structure. it is in cygnus, also, that one may see with the aid of powerful telescopes the vast, irregular, luminous nebulæ, that are like great clouds of fiery mist. these nebulæ are of enormous extent, for they cover space that could be occupied by hundreds of stars. [illustration: september--cygnus] cygnus is a constellation that is filled with the wonders and mysteries of space and that abounds in beautiful objects of varied kinds. it is a region one never tires of exploring with the telescope. the principal stars in cygnus form the well-known northern cross, with the beautiful, white, first-magnitude star deneb, or arided, as it is sometimes called, at the top of the cross, and albireo, the orange-and-blue double star at the foot. albireo, among all the pairs of contrasting hues, has the distinction of being considered the finest double star in the heavens for small telescopes. this star marks the head of the swan, as well as the foot of the northern cross, and deneb marks the tail of the swan. a short distance to the southeast of deneb, on the right wing of the swan, is a famous little star, cygni, barely visible to the naked eye and forming a little triangle with two brighter stars to the east. this star has the distinction of being the first one to have its distance from the solar system determined. the famous mathematician and astronomer bessel accomplished this difficult feat in the year . since that day, the distances of many stars have been found by various methods, but of all these stars only four or five are known to be nearer to us than cygni. its distance is about eight light-years, so its light takes about eight years to travel the distance that separates it from the solar system. as a result, we see it not as it is tonight, but as it was at the time when the light now entering our eyes first started on its journey eight years ago. cygni is also a double star, and the combined light of the two stars gives forth only one-tenth as much light as our own sun. most of the brilliant first-magnitude stars give forth many times as much light as the sun; but among the fainter stars, we find some that appear faint because they are very distant, and some that are faint because they are dwarf stars and have little light to give forth. to the class of nearby, feebly-shining dwarf stars cygni belongs. deneb, on the other hand, is one of the giant stars, and is at an immeasurably great distance from the solar system. just south of cygnus in the eastern branch of the milky way lie sagitta, the arrow, and aquila, the eagle. not far to the northeast of aquila is the odd little constellation of delphinus, the dolphin, popularly referred to as job's coffin. there will be no difficulty in finding this small star-group, owing to its peculiar diamond-shaped configuration. its five principal stars are of the fourth magnitude. it is in the constellation of delphinus that the most distant known object in the heavens is located. this is the globular star cluster known only by its catalogue number of n.g.c. . it is estimated to be at a distance of , light-years from the earth. sagitta, the arrow, lies midway between albireo and the brilliant altair in aquila. the point of the arrow is indicated by the star that is farthest east; and the feather, by the two faint stars to the west. like delphinus, this constellation is very small and contains no objects of particular interest. altair (flying eagle) is the brilliant white star of the first magnitude in aquila and is attended by two fainter stars, one on either side, at nearly equal distances from it. these two stars serve readily to distinguish this star, all three stars being nearly in a straight line. altair is one of the nearer stars, its distance from the earth being about sixteen light-years. it gives forth about ten times as much light as the sun. [illustration: september--delphinus, aquila and sagitta] we cannot leave the constellation of aquila without referring to the wonderful temporary star or nova, known as nova aquilæ no. (because it was the third nova to appear in this constellation), which appeared in the position indicated on the chart upon the eighth of june, . a few days previous to this date, there was in this position an extremely faint star, invisible to the naked eye and in small telescopes. this fact became known from later examinations of old photographs of this region that had been taken at the harvard college observatory, where the photographing of the heavens is carried on regularly for the purpose of having a record of celestial changes and happenings. clouds prevented the obtaining of any photographs of this part of the heavens on the four nights preceding the eighth of june, but on this evening there shone in the place of the faint telescopic star, a wonderful temporary star, or nova, which was destined on the next evening to outshine all stars in the heavens, with the exception of the brightest of all, sirius, which it closely rivaled in brilliancy at the height of its outburst. within less than a week's time, this faint star in the milky way for some mysterious reason increased in brightness many thousandfold. such outbursts have been recorded before, but on rare occasions, however. no star since the appearance of the nova known as kepler's star, in the year , which at its greatest brilliancy rivaled jupiter, shone with such splendor or attracted so much attention as nova aquilæ. in the year , there appeared in the constellation of perseus a star known as nova persei which at its brightest surpassed vega, but its splendor was not as great as that of the wonderful nova of . it speaks well for the zeal and interest of amateur astronomers, as well as for their acquaintance with the stars, that nova persei was discovered by an amateur astronomer, dr. anderson, and that among the deluge of telephone calls and telegrams received at the harvard college observatory on the night of june th, announcing independent discoveries of the "new star," were many from non-professional astronomers. like all stars of this class, nova aquilæ no. sank rapidly into oblivion. in a few weeks it was only a third-magnitude star; a few weeks more and it was invisible without a telescope. many wonderful and interesting changes have been recorded in the appearance of this star, however, even after it became visible only in the telescope. soon after its outburst it appeared to develop a nebulous envelope, as have other novas before it. it showed in addition many of the peculiarities of the nebulæ, though the central star remained visible as before the outburst. astronomers are still in doubt as to the cause of these outbursts, which certainly indicate celestial catastrophies of some form on a gigantic scale. all novas possess one characteristic in common--that of appearing exclusively in the milky way; and another characteristic is the development of a nebular envelope after the outburst of greatest brightness. in some cases temporary stars have been known to be variable in brightness for years before the great outburst. such a star was nova aquila, for the examination of photographs of this region taken some years previous showed variations in its brightness for a period of thirty years at least. up to the beginning of this century only about thirty novas had been discovered. since that date, thanks to the vigilance of the astronomers of today and to the aid of photography, more have been discovered than in all the preceding centuries. these outbursts of new stars appear to be not so rare as the earlier astronomers believed, though great outbursts as brilliant as that of nova aquila are very uncommon. xi october the constellations that will be found nearest the meridian in early october evenings are the circumpolar constellations cepheus and cassiopeia, and in the southern sky, capricornus and aquarius. cepheus, the king, and cassiopeia, his queen, of whom we shall have more to say later in connection with the constellations of andromeda and perseus, sit facing the north pole of the heavens opposite ursa major, the great bear, familiar to us under the name of the big dipper. the foot of cepheus rests upon the tail of the little bear, and the star farthest north in the diagram is in the left knee. the head is marked by a small triangle of faint stars, shown in the diagram. one of these three faint stars--the one farthest east--known as delta cephei, is a very remarkable variable star, changing periodically in brightness every five and one-third days. its name has been given to a large class of variable stars--the cepheid variables--that resemble delta cephei in being giant suns, faint only because they are at very great distances from the earth, and varying in brightness with the greatest regularity in periods that range from a few hours to several weeks. it has been found that the longer the period of light change the greater is the star in size and brightness. the cepheids of longest period are , times more brilliant than our own sun. cepheus contains no very bright or conspicuous stars. alpha cephei, the brightest star in the group, marks the king's right shoulder. it is the star farthest to the west in the diagram, and is only a third-magnitude star. [illustration: october--cassiopeia and cepheus] cassiopeia is a constellation with which every one in the northern hemisphere should be familiar, owing to its very distinctive w-shape and its far northern position, which brings it conspicuously into view throughout the clear fall and winter evenings. cassiopeia is pictured in all star atlases that show the mythological figures, with her face toward the north pole. the stars in the w outline the body. alpha, the star farthest south in the diagram marks the breast of cassiopeia. her head and uplifted hands are represented by faint stars south of alpha. this star is occasionally referred to by its arabic name of schedir. beta, the leader of all the stars in the w in their daily westward motion, is also known by an arabic name, caph. in the constellation of cassiopeia there appeared in the year a.d., a wonderful temporary star which suddenly, within a few days' time, became as brilliant as the planet venus and was clearly visible in broad daylight. this star is often referred to as tycho's star, because it was observed, and its position very accurately determined, by tycho brahe, one of the most famous of the old astronomers. this star remained visible to every one for about sixteen months, but it finally faded completely from view, and it is believed that a faint, nebulous red star, visible only in the telescope and close to the position recorded by tycho, represents the smoldering embers of the star that once struck terror to the hearts of the superstitious and ignorant among all the nations of europe, who took it to be a sign that the end of the world was at hand. both cassiopeia and cepheus lie in the path of the milky way, which reaches its farthest northern point in cassiopeia and passes from cepheus in a southerly direction into the constellation of cygnus. [illustration: october--aquarius and capricornus] turning now to southern skies, we find on and to the west of the meridian at this time the rather inconspicuous zodiacal constellation of capricornus, the goat. it contains no stars of great brightness and is chiefly remarkable for the fact that it contains one of the few double stars that can be seen without the aid of a telescope. the least distance in the heavens that the unaided human eye can separate is about four minutes of arc. the star alpha in capricornus is made up of two stars separated by a distance of six minutes of arc, so that any one can readily see that it consists of two stars very close together. this star, alpha, will be found in the extreme western part of the constellation, and can best be located in conjunction with the star beta, which is slightly brighter and lies but a short distance almost due south of alpha, the two stars standing somewhat alone in this part of the heavens. to the north and east of capricornus we find aquarius, which is also a zodiacal constellation. aquarius is the water-bearer, and the water jar which he carries is represented by a small, but distinct, y of stars from which flows a stream of faint stars toward the southeast and south. aquarius, like capricornus, is a rather uninteresting constellation, as it is made up of inconspicuous third- and fourth-magnitude stars. the entire region covered by these two groups of stars is remarkably barren, since it contains not a single first- or even second-magnitude star and little to attract the observer's eye. to relieve the barrenness of this region, there appears just to the south of aquarius and southeast of capricornus, sparkling low in the southern sky on crisp october evenings, the beautiful first-magnitude star fomalhaut in the small southern constellation of piscis australis, the southern fish. this star is the farthest south of all the brilliant first-magnitude stars that can be seen from the middle latitudes of the northern hemisphere. the constellation in which it lies is so close to the southern horizon in our latitudes that it cannot be seen to any advantage, and it is at best very inconspicuous, containing no other objects of interest. fomalhaut cannot be mistaken for any other star visible at this time of year in the evening, since it stands in such a solitary position far to the south. at the time of which we are writing it will be found a few degrees east of the meridian. xii november directly south of cassiopeia and cepheus, the circumpolar constellations with which we became acquainted last month, and almost overhead in our latitudes in the early evening hours of november, lie pegasus, the winged horse, and andromeda, the woman chained. according to the legend, cepheus was king of ethiopia, and cassiopeia was the beautiful, but vain, queen who dared to compare herself in beauty with the sea-nymphs. this so enraged the nymphs that, as a punishment for her presumption, they decided to send a terrible sea-monster to ravage the coast of the kingdom. the king and queen, upon consulting the oracle, found that the only way to avert this calamity would be to chain their daughter andromeda to the rocks and permit the monster to devour her. [illustration: november--andromeda and pegasus] as the story goes, the valiant hero, perseus, chanced to be riding through the air on his winged horse and saw, far beneath him the beautiful maiden chained to the rocks and the frightful monster approaching to devour her. he immediately went to the rescue, and, after a terrible struggle with the monster, succeeded in overpowering him and thus saved the maiden from a dreadful fate. perseus and the fair andromeda were married shortly afterward, and at the end of a happy life the pair were transferred to the heavens. cassiopeia, the vain queen, was ordered to be bound to a chair and, with the king cepheus at her side, to be swung continually around the north pole of the heavens that she might be taught a lesson in humility. the constellation cetus, representing the sea-monster, will be found to the southeast and south of pisces, the fishes, which lie south of andromeda and pegasus. the great square in pegasus is the most conspicuous configuration of stars to be seen in the heavens in autumn evenings. the star that marks the northeastern corner of the great square belongs to the constellation of andromeda and marks the head of the maiden, who is resting upon the shoulders of pegasus, the winged horse. the three bright stars nearly in a straight line outline the maiden's body, alpha, or alpheratz, as it is called, being the star in the head, beta or mirach in the waist, and gamma or almach in the left foot. the last-named star, which is farthest to the northeast in the diagram, was, in the opinion of the noted astronomer herschel, the finest double star in the heavens. the two stars into which the telescope splits it are of the beautifully contrasted shades of orange and sea green. a second most interesting object in andromeda and one of the finest in the entire heavens is the great andromeda nebula, which is faintly visible without the aid of a telescope as a hazy patch of light. it is believed that in reality this nebula is a great universe composed of many thousands of stars so distant that no telescope can show the individual members and that the light from it takes many thousands of years to span the abyss that separates it from the solar system. some magnificent photographs of the great andromeda nebula have been taken with powerful telescopes. it is through the use of photography that the nebulæ can best be studied, for a photographic plate after long exposure, reveals a wonderful detail in the structure of these objects that the human eye fails to see. on a clear, dark evening one may find the great andromeda nebula by the aid of two faint stars with which it makes a small triangle, as shown in the chart. this nebula is the only one of the spiral nebula that can be seen in these latitudes without the aid of a telescope, though there are several spiral nebulæ in the southern heavens that can be thus seen. lying to the northwest of the great square in pegasus are a number of faint stars that outline the shoulders and head of the winged steed, while the stars to the southwest of the square outline his forelegs. the creature is represented without hind quarters in all star atlases. the space within the great square contains no bright stars, and as a result, the outline of the square stands out with great distinctness. there are, in fact, no stars of the first magnitude in either pegasus or andromeda, though there are a number of the second and third magnitude which very clearly show the distinctive forms of these two star-groups. pisces, the fishes, the constellation just south of andromeda and pegasus, is the first of the twelve zodiacal constellations. it consists of the southern fish, lying in an east-to-west direction, and the northern fish, lying nearly north and south, the two touching at the southeastern extremity of the constellation. [illustration: november--pisces] there is in pisces not a single bright star, and its only point of interest is to be found in the fact that it contains the point, marked by the cross and letter v in the diagram, that is known variously as "the vernal equinox," "the equinoctial point" and "the first point in aries." this is a very important point of reference in the heavens, just as the meridian of greenwich is for the earth, and it marks the point where the sun crosses the equator going north in the spring. owing to the precession of the equinoxes, as it is called, this point is gradually shifting its position westward through the zodiacal constellations at a rate that will carry it completely around the heavens through the twelve zodiacal groups in a period of , years. since the beginning of the christian era, this point has backed from the constellation of aries, which lies just east of pisces, into pisces, though it still retains its name of "the first point in aries." xiii december the eastern half of the sky on early december evenings is adorned with some of the finest star-groups in the heavens; but as we are considering for each month only the constellations that lie on or near the meridian in the early evening hours, we must turn our eyes for the present from the sparkling brilliants in the east to the stars in the less conspicuous groups of aries, the ram, and cetus, the whale. we will also become acquainted this month with the beautiful and interesting constellation of perseus, the hero of mythical fame to whom we referred last month in connection with the legend of cepheus and cassiopeia. cetus, you will recall, represents the sea-monster sent to devour andromeda, the daughter of cepheus and cassiopeia. we have included the constellation of andromeda in our diagram for this month, since it is so closely associated in legend with the constellations of perseus and cetus, though we also showed it last month. the brightest star in perseus, known as alpha persei, is at the center of a curved line of stars that is concave or hollow toward the northeast. this line of stars is called the segment of perseus, and it lies along the path of the milky way, which passes from this point in a northwesterly direction into cassiopeia. according to the legend, perseus, in his great haste to rescue the maiden from cetus, the monster, stirred up a great dust, which is represented by the numberless faint stars in the milky way at this point. the star alpha is in the midst of one of the finest regions of the heavens for the possessor of a good field-glass or small telescope. a short distance to the southwest of alpha is one of the most interesting objects in the heavens. to the ancients, it represented the baleful, winking demon-eye in the head of the snaky-locked gorgon, medusa, whom perseus vanquished in one of his earliest exploits and whose head he carried in his hand at the time of the rescue of andromeda. to the astronomers, however, algol is known as beta persei, a star that has been found to consist of two stars revolving about each other and separated by a distance not much greater than their own diameters. one of the stars is so faint that we speak of it as a dark star, though it does emit a faint light. once in every revolution the faint star passes directly between us and the bright star and partly eclipses it, shutting off five-sixths of its light. this happens with great regularity once in a little less than three days. it is for this reason that algol varies in brightness in this period. there are a number of stars that vary in brightness in a similar manner. their periods of light-change are all very short, and the astronomers call them eclipsing variables. at its brightest, algol is slightly brighter than the star nearest to it in andromeda, which is an excellent star with which to compare it. [illustration: december--perseus, aries and cetus] perseus is another one of the constellations lying in the milky way in which temporary stars or novas have suddenly flashed forth. at the point indicated by a cross in the diagram, dr. anderson, an amateur astronomer of scotland, found on february , , a new star as brilliant as the pole-star. on the following day it became brighter than a star of the first magnitude. a day later it had lost a third of its light, and in a few weeks it was invisible without the aid of a telescope. in a year it was invisible in all except the most powerful telescopes. with such telescopes, it may still be seen as a very faint nebulous light. triangulum and aries are two rather inconspicuous constellations that lie on, or close to, the meridian at this time. there is nothing remarkable about either of these groups, except that aries is one of the twelve zodiacal constellations. some centuries ago, the sun was to be found in aries at the beginning of spring and the position it occupies in the sky at that time was called the first point in aries. as this point is slowly shifting westward, as we have explained elsewhere, the sun is now to be found in pisces, instead of aries at the beginning of spring and does not enter aries until a month later. pisces was one of the constellations for november and we showed in that constellation the present position of the sun at the beginning of spring. two stars in aries--alpha and beta--are fairly bright, alpha being fully as bright as the brightest star in andromeda. beta lies a short distance to the southwest of alpha, and a little to the southwest of beta is gamma, the three stars forming a short curved line of stars that distinguishes this constellation from other groups. the remaining stars in aries are all faint. just south of aries lies the head of cetus, the whale. this is an enormous constellation that extends far to the southwest, below a part of pisces, which runs in between andromeda and cetus. its brightest star, beta, diphda, or deneb kaitos, as it is severally called, stands quite alone not far above the southwestern horizon. it is almost due south of alpha andromedæ, the star in andromeda farthest to the west, which it exactly equals in brightness. the head of cetus is marked by a five-sided figure composed of stars that are all faint with the single exception of alpha, which is fairly bright, though inferior to beta or diphda. cetus, though made up chiefly of faint stars, and on the whole uninteresting, contains one of the most peculiar objects in the heavens, the star known as omicron ceti or mira (the wonderful). this star suddenly rises from invisibility nearly to the brightness of a first-magnitude star for a short period once every eleven months. mira was the first known variable star. its remarkable periodic change in brightness was discovered by fabricius in the year , so its peculiar behavior has been under observation for three hundred and twenty-five years. it is called a long-period variable star, because its variations of light take place in a period of months instead of a few hours or days, as is the case with stars such as algol. mira is not only a wonderful star, it is a mysterious star as well, for the cause of its light-changes are not known, as in the case of algol where the loss of light is produced by a dark star passing in front of a brighter star. mira is a deep-red star, as are all long-period variable stars that change irregularly in brightness. it is visible without a telescope for only one month or six weeks out of the eleven months. during the remainder of this time, it sometimes loses so much of its light that it cannot be found with telescopes of considerable size. its periods of light-change are quite variable as is also the amount of light it gains at different appearances. it is believed that the cause of the light-changes of mira is to be found within the star itself. it has been thought that dense clouds of vapors may surround these comparatively cool, red stars and that the imprisoned heat finally bursts through these vapors and we see for a short time the glowing gases below; then the vapors once more collect for a long period, to be followed by another sudden outburst of heat and light. it is interesting to remember in this connection that our own sun has been found to be slightly variable in the amount of light and heat that it gives forth at different times, and the cause of its changes in light and heat are believed to lie within the sun itself. xiv stars of the southern hemisphere as one travels southward from the mid-latitudes of the northern hemisphere into the tropics our familiar circumpolar constellations sink lower and lower in the northern heavens and strange and unfamiliar star-groups rise gradually above the southern horizon. if we make our southward journey in the winter months the first of the southern constellations to come fully into view is the small star-group just south of lepus known as columba (the dove), whose brightest star phact is of the second magnitude. a line drawn from procyon to sirius and extended as far again brings us to this star and a line from betelgeuze to sirius extended an equal distance brings us to zeta argus which is equal to phact in brightness. the two lines intersecting at sirius make the "egyptian x" as it is called. magnificent, blue-white canopus, the most brilliant star in the heavens next to sirius, a veritable diamond sparkling low in the southern sky, now commands our unqualified admiration. canopus lies about ° south of sirius and is invisible north of the th parallel of latitude. at nine o'clock in the evening of february th it can be seen just above the southern horizon in that latitude and is then a conspicuous object in georgia, florida and the gulf states. "the star of egypt whose proud light never hath beamed on those who rest in the white islands of the west." writes moore of canopus in "lalla rookh." along the nile canopus was an object of worship as the god of waters. at the time of their erection, b.c., a number of temples in upper egypt were oriented so as to show canopus at sunrise at the autumnal equinox, and other temples erected many centuries later were oriented in a similar manner. in china, as late as b.c., and in india also canopus was an object of worship. the astronomer tells us that canopus is immeasurably distant from the earth. it has been estimated to be forty thousand times more luminous than our sun. canopus is located in the constellation of argo navis which is the largest and most conspicuous constellation in the heavens. in addition to canopus it contains a number of second- and third-magnitude stars and is subdivided for convenience into puppis, the prow; carina, the keel; and vela, the sails. huge as it is, argo navis represents only half of a ship for the stern is lacking. according to the legend this ship was built by argos, aided by pallas athene, for jason, the leader of the expedition of the fifty argonauts who sailed from greece to colchis in search of the golden fleece. pallas athene placed in the bow of the ship a piece of timber from the speaking oak of dodona to guide the crew and warn them of dangers and after the voyage the ship was supposed to have been placed in the heavens. [illustration: southern constellations-- . in february] in argo navis is one of the finest telescopic objects in the heavens, the keyhole nebula, as it is usually called, from a peculiar-shaped dark patch in its brightest part. on the border of this nebula is the deep-red wonder star of the southern hemisphere, eta argus, which varies suddenly and unexpectedly in brightness between the seventh and first magnitudes. in it burst forth with a splendor rivaling sirius and maintained this brilliancy for nearly ten years and then slowly waned in brilliancy until it disappeared to the unaided eye in . the surrounding nebula also seems to share in its peculiar fluctuations of brightness. eta argus is now a star of the seventh magnitude and since it is still varying fitfully in brightness it is believed that the history of its light-changes is not complete. among the constellations of the southern heavens near the meridian in february we see in addition to argo navis the constellations of dorado, the goldfish; hydrus, the serpent, and tucana, the toucan. though insignificant in appearance dorado contains what was described by sir john herschel as one of the most extraordinary objects in the heavens, a worthy rival of the great orion nebula and in some respects very similar to it, the great looped nebula, "the center of a great spiral." in dorado also is located the greater magellanic cloud which looks like a detached portion of the milky way though it is far removed from it. to the naked eye it resembles a small white cloud about ° in extent. in the telescope it bears a close resemblance to a typical portion of the milky way. a similar formation known as the lesser magellanic cloud is located in hydrus. it has been estimated that the distance of the lesser cloud is , light-years and that it is receding from us. in tucana is located one of the finest globular star clusters in the heavens, known as tucanæ. this cluster and omega centauri, a globular star cluster in centaurus, are the two nearest of all the globular clusters. they are distant from the earth about , light-years and it is known that the combined light of the thousands of stars of which each cluster is composed is about one million times that of our own sun. in the western sky in the southern hemisphere in february may be seen the brilliant, white, first-magnitude star achernar in the river eridanus, the long, winding constellation that, we recall, starts near the brilliant rigel in orion and disappears from the view of northern observers below the southern horizon, extending its course far into the southern hemisphere. achernar means "the end of the river" and this is nearly its position in the constellation. though argo navis is the largest and most important constellation of the southern hemisphere, crux, the southern cross, far-famed in story and legend as well as for its historical associations, is beyond a doubt the most popular. the best time to view the southern cross is in june or july when it is near the meridian. it is not seen to advantage in the months of january or february. it then lies on its side and close to the horizon and therefore is dimmed by atmospheric haze so that it almost invariably is a disappointing object to the tourist from the north who usually views it for the first time in one of these months. the cross is viewed to advantage in the latitude of rio or valparaiso and it is best seen from the straits where it rides high overhead. it is not seen to advantage from the latitudes of cuba or jamaica. it is small, only ° in extent from north to south and less in width and it lies in the most brilliant portion of the milky way which is here a narrow stream only three or four degrees wide. directly below the cross is the noted coal sack, apparently a yawning chasm in the midst of its brilliant surroundings though probably in reality a dark nebula. viewed with the telescopes a number of stars are to be seen projected on this dark background. the southern cross is to the inhabitants of the southern hemisphere what the big dipper is to those who dwell in the northern hemisphere--an infallible timepiece. the upright of the cross points toward the south pole of the heavens which lies in a region where there is a singular dearth of bright stars, the nearest star to the south pole being a faint fifth-magnitude star called sigma octantis. when seen in the southeast or southwest the cross lies on its side, but when passing the meridian it stands nearly upright. humboldt, referring to this fact, says: "how often have we heard our guides exclaim in the savannahs of venezuela and in the desert extending from lima to truxillo, 'midnight is past, the cross begins to bend.'" by the explorers of the sixteenth century the cross was taken as a sign of heaven's approval of their attempt to establish the christian religion in the wilds of the new world. this thought is beautifully expressed in mrs. hemans' lines in "the cross of the south." "but to thee, as thy lode-stars resplendently burn in their clear depths of blue, with devotion i turn bright cross of the south! and beholding thee shine, scarce regret the loved land of the olive and vine. thou recallest the ages when first o'er the main my fathers unfolded the ensign of spain, and planted their faith in the regions that see its imperishing symbol ever blazoned in thee." alpha crucis, the brightest star in crux, is at the foot of the cross. it consists in reality of two second-magnitude stars forming a beautiful double while a third fifth-magnitude star one and one-half minutes of arc distant makes with this pair a combination similar to our mizar and alcor of the big dipper though the separation is not great enough to be visible to the naked eye. the second-magnitude star at the head of the cross is a deep orange in color and the two stars that mark the ends of the cross-arm are white third-magnitude stars. [illustration: southern constellations-- . in july] one of the finest constellations of the southern hemisphere is centaurus, the centaur, which surrounds crux on the north and is more than ° in length. its center lies about ° south of spica in virgo and below the tail of hydra. alpha centauri, its brightest star and the nearest star to the solar system, four and one-third light-years away, is a golden-yellow double star that forms with the star beta centauri on the west a configuration similar to that of castor and pollux in gemini, only one that is far more striking because of the superior brilliancy of the stars. alpha centauri lies in the milky way and transits the meridian at the same time with arcturus though it cannot be seen north of the th parallel. alpha centauri, like canopus, was an object of worship in egypt and a number of temples in northern egypt were oriented to its emergence from the sun's rays in the morning at the autumnal equinox, between and b.c. north of centaurus is the constellation lupus, the wolf, which is also crossed by the milky way. according to one myth lupus is held in the right hand of the centaur as an offering upon the altar which is represented by the constellation of ara next to centaurus on the east. ara also is crossed by the milky way. neither lupus nor ara contain any objects that are worthy of special attention. triangulum australe, the southern triangle, a little to the southeast of alpha centauri, is far more conspicuous than the triangulum of the northern hemisphere. the accompanying charts give two views of these principal southern constellations that lie within ° of the south pole of the heavens and that are below the horizon in ° north latitude. the first of these charts shows the constellations that are nearest the meridian in the early evening hours in february. canopus in argo navis and the greater magellanic cloud then lie close to the meridian. argo navis with its subdivisions puppis, vela and carina are found east of the meridian lying directly in the path of the milky way, which stretches diagonally across the sky from the northwest to the southeast. far over in the southeast appears crux, the southern cross, also directly in the path of the milky way. in the western heavens may be seen the lesser magellanic cloud in hydrus, brilliant achernar in eridanus and the inconspicuous star-group of tucana. in the early evening hours of july we find as shown on the second chart, alpha and beta centauri in centaurus close to the meridian, lupus due north of centaurus, ara and triangulum australe in the southeast and crux in fine position for observation just west of the meridian. carina of argo navis lies to the southwest of crux. the milky way now arches magnificently across the heavens from carina through crux, centaurus and lupus and ara to the zodiacal groups of scorpio and sagittarius in the northeast. in the northern part of the heavens, as seen from the southern hemisphere, appear the familiar zodiacal constellations that we of the northern hemisphere find south of the zenith, as well as the constellations of orion, lepus and canis major, hydra, corvus and crater, ophiuchus and serpens and aquila, all finely in view in their appropriate seasons. it is only our familiar circumpolar constellations of the north--the two bears, draco, cassiopeia, and cepheus, andromeda and perseus and auriga that are invisible in mid-latitudes of the southern hemisphere just as the constellations shown in the diagrams, and a few additional groups such as pavo, grus, phoenix, apus, mensa and volans which we have not shown, lie hidden from view beneath the southern horizon in mid-latitudes of the northern hemisphere. the northern visitor to the southern hemisphere familiar with the constellations of his own land is filled with a queer sensation of being in topsy-turvydom as he sees familiar orion standing on his head and all of the zodiacal constellations passing in their daily motions to the north instead of to the south of his zenith while by day the sun passes across the northern part of the heavens and culminates north instead of south of his zenith. he misses the familiar dippers of his own land and searches in vain for a pole-star in the unfamiliar circumpolar regions of the south. xv the milky way or galaxy "broad and ample road whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powder'd with stars." --milton, _paradise lost_. on clear, winter evenings one may see a portion of the zone of the milky way, which encircles the heavens, arching magnificently across the heavens as it passes from cassiopeia and cepheus in the northwest, through perseus and auriga and the eastern part of taurus, across the feet of gemini, between canis minor and orion and through the eastern part of canis major to the southern horizon. at this point it passes beyond our range of vision into the star-groups of puppis, vela and carina, subdivisions of the huge southern constellation of argo navis. it reaches its greatest distance south of the celestial equator and also attains its greatest brilliancy in crux, the far-famed southern cross. from here it turns northward once more, passing into centaurus, musca, circinus ara and lupus constellations of the southern hemisphere and comes within our range of vision again in sagittarius and scorpio. here the milky way divides into two branches, though some astronomers now believe that this apparent division into two branches is due to the presence of an enormous cloud of non-luminous matter lying along the course of the milky way at this point, similar in its nature to the dark "holes" and "caves" and streaks that appear in all portions of the milky way and most noticeably athwart its course in argo and centaurus. one of these branches of the milky way passes from sagittarius through aquila to cygnus and the other through scorpio, ophiuchus and serpens to cygnus, the two extending diagonally across the heavens in the late summer and early fall evenings from the northeast to the southwest. from cygnus, the milky way passes into cepheus and cassiopeia and thus completes its circuit of the heavens. it is not seen to advantage in spring or early summer evenings because it then rests nearly on the horizon. its plane is inclined about ° to the celestial equator and its poles lie in the constellations of coma berenicis and cetus. these are the two points that lie farthest from the milky way. the milky way has been called the groundwork of the universe. by far the greater number of all the stars are crowded towards its plane in the form of an enormous flattened disk or lens. our solar system, it has been estimated, lies close to the plane of the milky way and at a distance of some , or , light-years from its center. the diameter of the milky way as deduced from dr. harlow shapley's work on globular star clusters is about , light-years in extent, or ten times greater than the limit set some years ago. the apparent crowding together of the stars into dense clouds in the milky way is partly an effect due to our position in the milky way. when we look at the heavens in a direction at right angles to this plane we find comparatively few stars lying along our line of vision because the stars are actually fewer in number in this direction. if we look _along_ the plane of the milky way, however, we see to a greater distance through an enormous depth of stars. though individual stars may not be much closer together in the milky way than they are outside of it, there are on the whole more of them and the effect of greater density is produced. father hagen of the vatican observatory, who has for years made a study of the dark clouds of obscuring matter and dark nebulæ that abound in space, has found evidence of the existence of many vast clouds of dark obscuring matter over the entire heavens above and below the plane of the milky way as well as surrounding the milky way in its own plane. the existence of such clouds of non-luminous matter would account partly for the comparative fewness of stars in space outside of the plane of the milky way since many stars would be concealed from our eyes by these obscuring clouds. there is, however, in addition, an actual crowding of all the visible stars toward this plane. the peoples of all ages have honored the milky way in story and legend. it has been universally referred to as the sky river and the pathway of souls. to the little hiawatha, we remember, the "wrinkled old nakomis" "showed the broad white road in heaven pathway of the ghosts, the shadows, running straight across the heavens, crowded with the ghosts, the shadows. to the kingdom of ponemah to the land of the hereafter." in _the galaxy_, longfellow thus describes the milky way: "torrent of light and river of the air along whose bed the glimmering stars are seen like gold and silver sands in some ravine where mountain streams have left their channels bare!" in sweden, where the milky way arches high through the zenith in winter, it is called the winter street, and miss edith thomas writes thus beautifully of it in her poem entitled, "the winter street": "silent with star dust, yonder it lies-- the winter street, so fair and so white; winding along through the boundless skies, down heavenly vale, up heavenly height. faintly it gleams, like a summer road when the light in the west is sinking low, silent with star dust! by whose abode does the winter street in its windings go? and who are they, all unheard and unseen-- o who are they, whose blessèd feet pass over that highway smooth and sheen? what pilgrims travel the winter street? are they not those whom here we miss in the ways and the days that are vacant below? as the dust of that street their footfalls kiss does it not brighter and brighter grow?" beautiful indeed are these poetic fancies but none of them picture even remotely the awe-inspiring grandeur of the milky way as it actually exists. [illustration: a dark nebula: the dark bay or dark horse nebula in orion taken with -inch hooker telescope of the mt. wilson observatory] millions upon millions of far distant suns equal to or surpassing our own sun in brilliancy are gathered within this vast encircling zone of the heavens, their combined light giving to the naked eye the impression of a milky band of light. nine-tenths of all the stars, it has been estimated, lie close to the plane of the galaxy, as well as all the vast expanses of luminous gaseous nebulæ and clouds of dark obscuring matter all seemingly intermingled in chaotic confusion; yet law and order govern the motions of all. here also are the great moving star clusters such as the pleiades and the hyades and all of the brilliant "orion" stars. the structure of the milky way is not clearly understood but many astronomers believe there is evidence that it takes the form of a vast spiral nebula along whose arms the stars pass to and fro. beyond the milky way at enormous distances of many thousands of light-years, but apparently influenced by it, lie the globular star-clusters and the spiral nebulæ. the spirals appear to avoid the plane of the milky way for they are receding in the direction of its poles at high velocities; the globular clusters on the other hand are drawing in toward the milky way on either side, and in time will cross it. whether these objects external to the milky way form with it one enormous universe or whether the spiral nebulæ are in turn galaxies or "island universes," as the astronomer calls them, similar in form and structure to our own galaxy and at inconceivably great distances of millions of light-years from it, is still one of the riddles of the universe which the astronomers are attempting to solve. xvi the surface of the sun the visible surface of the sun is called the _photosphere_. even the smallest telescopes will show its peculiar "rice-grain" structure, consisting of intensely brilliant flecks or nodules about miles in diameter, which can be resolved by the more powerful telescopes into smaller particles about miles in diameter, against a darker background. it has been estimated that these bright nodules or rice-grains occupy only one-fifth of the total surface of the sun, yet radiate three-fourths of the total light. it is generally believed that the "rice grains" are the summits of highly heated columns of gas, arising from the sun's interior, and that the darker portions between are cooler descending currents. it is well known that the photosphere or visible surface of the sun appears to be much brighter in the center of the disk than near its circumference. this is due entirely to its gaseous nature and to the fact that it is surrounded by an atmosphere of dense enveloping cooler gases. rays from the center of the disk travel a shorter distance through this atmosphere than the rays from the rim and therefore are absorbed less by surrounding gases. we look further down into the sun's interior near the center of the disk than in the direction of its circumference and so the light appears more intense there. the photosphere is the region where sun-spots appear and they are found in zones extending from ° to ° on either side of the solar equator, never appearing exactly at the equator or near the poles. the disturbances that produce sun-spots and many allied phenomena occur cyclically in periods of eleven years on the average. the first outburst of the disturbance is manifested by the appearance of sun-spots in high solar latitudes. these break out and disappear and break out again with increased vigor, working gradually downward toward the solar equator, the maximum spottedness for a given period occurring in solar latitude about °. the disturbance finally dies out within ° or ° of the equator, but even before one cycle of disturbance has entirely passed away a new cycle has broken forth in high latitudes. so during the period of minimum spottedness there are four distinct belts, two in low latitudes, due to the dying disturbance, and two in high latitudes, due to the new disturbance. at sun-spot maximums there are two well-marked zones of great intensity, approximately ° north and south of the sun's equator. sun-spots are solar cyclones, occurring usually in groups, though large single spots appear less frequently. each spot is quite sharply divided into an umbra and a penumbra. the umbra is the darker central portion, the funnel of the whirling cyclone, and the penumbra is composed of the outspreading gases, and is less dark than the umbra. the peculiar "thatch-straw" structure of the penumbra is due, it is believed, to the fact that the columns of gases that usually rise vertically from the sun's interior and from the "rice grains" of the photosphere are drawn into a horizontal position by the whirling motion that exists in the penumbra regions of a sun-spot and therefore we get a longitudinal rather than a cross sectional view of them. the umbra of a sun-spot is anywhere from a few hundred miles to fifty thousand miles in diameter, frequently exceeding the earth in size, while the penumbra occasionally reaches a diameter of two hundred thousand miles. sun-spots of exceptional size can be seen even without the aid of a telescope. the darkness of sun-spots is only by comparison with their more brilliant background. owing to the rapid expansion and cooling of gases the temperature in sun-spot regions is far below the normal solar temperature of , ° centigrade, lying between , ° and , ° centigrade. at this temperature it is possible for the more refractory chemical compounds to form, the oxides and the hydrides, and the spectra of sun-spots reveal the presence of titanium oxide and magnesium and calcium hydride. at the higher solar temperatures that exist elsewhere in the photosphere and in its overlying gaseous envelopes all chemical elements occur in a free state, intermingling as incandescent vapors without the formation of any chemical compounds. strong magnetic fields exist in sun-spot regions and magnetic storms in our own atmosphere frequently accompany the appearance of exceptionally large sun-spots. directly above the photosphere of the sun lies the "reversing layer," which is about five hundred miles in depth and is composed of the incandescent vapors of all the chemical elements that exist on the sun, which are also the same familiar elements that exist on the earth, with the exception of coronium, the unknown element in the solar corona, there is no element in the sun that has not been found on our own planet. the "reversing layer" receives its name from the fact that it reverses the solar spectrum. it produces by its absorption of the rays of light from gases below the dark absorption lines found in the spectrum that serve to identify all the elements existing in the sun. during the time immediately preceding and following a total eclipse of the sun this reversing layer produces what is known as the flash spectrum. when the photosphere, which gives the bright continuous background of the solar spectrum, is concealed by the moon, the normally dark lines of the reversing layer--dark only by contrast with the bright background--become momentarily intensely bright lines against a dark background. the flash spectrum only lasts a second or so, as the reversing layer itself is soon covered by the moon. just above the reversing layer lies the _chromosphere_, which is between five thousand and ten thousand miles in depth. many of the gaseous vapors of the reversing layer are found in the chromosphere, thrown there continually by the vast upheavals of gases that are constantly disturbing the surface of the sun. the greater the solar activity the more is the chromosphere charged with the vapors of the lower strata of the sun's atmosphere. the gases that are most characteristic of the chromosphere, however, are the incandescent gases of hydrogen and calcium, which give it the pink or reddish tinge so noticeable during total solar eclipse. helium is also found in great abundance in the solar chromosphere. shooting upward from the photosphere with the tremendous velocity of one hundred or more miles per second, can be seen at all times, by properly screening off the light from the photosphere, the vast solar eruptions known as the _prominences_. these are composed chiefly of hydrogen and calcium gas, though other elements also appear, especially near the bases of the prominences. prominences are of two varieties, the quiescent, or cloud-like prominences, that float high above the solar surface for days at a time in some instances and resemble terrestrial clouds in form, and the eruptive, or metallic prominences, that dart up from the surface of the sun in an infinite variety of forms that may be entirely changed in the short interval of fifteen or twenty minutes. these eruptive prominences usually attain heights of thirty or forty thousand miles on the average, but _exceptional prominences reach heights of more than one hundred thousand miles and in a few rare cases have reached elevations of over five hundred thousand miles, or more than one-half of the solar diameter_. prominences are the most spectacular and beautiful of all solar phenomena, with the possible exception of the solar corona, which is the outermost of all the solar envelopes and also the most tenuous. the extent of the corona is enormous. its outer streamers extend usually to distances of several million miles from the center of the sun. measurements of the coronal light during total eclipses of the sun have shown that its intensity is only about one-half that of full moonlight, and it seems almost impossible to devise methods for detecting it, except during total eclipses, on account of the extreme faintness of its light. the sun, it is now known, is surrounded by a strong magnetic field in addition to the magnetic fields that exist in sun-spots. the cycle of sun-spot change is attended by marked changes in many forms of solar activity. the frequency of outbursts of eruptive prominences, the brightness and form of the corona, magnetic storms and weather changes on the earth are all closely associated with the sun-spot cycle. the cause of this sun-spot cycle, with all the attendant changes in the general solar activity, and the source of the apparently limitless supply of solar energy still remain the two chief unsolved secrets of the sun. xvii the solar system our sun is but a star traveling through the universe. it is accompanied in its journey to unknown parts of space, that lie in the general direction of the constellation hercules, by an extensive family of minor bodies consisting of the eight planets and their encircling moons, one thousand or more asteroids, numerous comets, and meteors without number, all moving in prescribed paths around their ruler. the most important members of the sun's family are the planets, mercury, venus, earth, mars, jupiter, saturn, uranus and neptune, named in the order of their position outward from the sun. we hear occasionally of the possibility of the existence of intra-mercurial and trans-neptunian planets and it is possible that some day an additional planet may be discovered within the orbit of mercury or beyond the orbit of neptune. the gravitational control of the sun extends far beyond the orbit of neptune and there are reasons for believing in the existence of at least one or two additional planets on the outskirts of the solar system. the existence of a planet within the orbit of mercury is now, after long continued and diligent search, believed to be very doubtful. were it possible to view the sun from the distance of the nearest star with the aid of the greatest telescope on earth all the members of his family would be hopelessly invisible. so, also, we cannot tell as we point our powerful telescopes at the stars whether these other suns are attended by planet families. we may only argue that it is very unlikely that there should be only one star among hundreds of millions that is attended by a group of comparatively small dark bodies that shine by the reflected light from the star they encircle. with the exception of the two planets, mercury and venus, which are known as the inferior planets, since their paths lie between the earth and the sun, all the planets have moons or satellites of their own that encircle the planet just as the planet encircles the sun. our planet earth has one satellite, the moon, that has the distinction of being the largest of all the moons in proportion to the size of the planet it encircles. jupiter and saturn have moons that surpass our moon in actual size; in fact, two of the moons of the outer planets are actually larger than the smallest planet mercury, but they are very small in proportion to the size of the planets around which they revolve. mars, the next planet beyond the earth, the nearest of the superior or outer planets, has two tiny moons that bear the names of deimos and phobos, respectively. they are both less than twenty miles in diameter and revolve very near to the surface of mars. they can only be seen with the aid of very powerful telescopes. the inner moon, phobos, is unique in the solar system for it makes three trips around mars while the planet is turning once on its axis. jupiter, the next planet outward from the sun, is almost a sun itself to its extensive family of nine moons. four of these moons were first seen about three hundred years ago when galileo pointed his first crude telescope at the heavens and any one can now see them with the aid of an opera glass. one of the four is equal in size to our own moon; the others surpass it in size. these moons are most interesting little bodies to observe. their eclipses in the shadow of jupiter, occultations or disappearances behind his disk, and the transits of the shadows as well as the moons themselves across the face of the planet can be easily seen even with the smallest telescope. the five remaining moons have all been discovered in modern times. they are extremely small bodies visible only in large telescopes. satellite v is the nearest of all the moons to jupiter. the other four are at great distances from the planet. the planet saturn has nine moons. titan, the largest, is nearly equal in size to jupiter's largest moon, and is larger than mercury; four of the other moons have diameters between one thousand and two thousand miles in length. since saturn is nearly twice as far from the sun as jupiter his moons are more difficult to observe, though the two largest are visible in small telescopes. saturn is unique in the solar system in possessing in addition to his nine satellites a most wonderful ring system, composed of swarms of minute moonlets, each pursuing its individual path around the planet. it is this unusual ring system that makes saturn the most interesting to observe telescopically of all the planets. the planet uranus has four satellites and neptune one. these planets and their satellites cannot be well observed on account of their great distances from the earth. the indistinctness of surface markings makes it impossible to determine the period of rotation of these two outer planets on their axes. it is believed that their rotation is very rapid, however, as is the case with the other planets jupiter and saturn. all the planets in the solar system fall naturally into two groups. jupiter, saturn, uranus, and neptune, the members of the outer group, have on the average, diameters ten times as great and, therefore, volumes one thousand times as great as mercury, venus, earth and mars, the members of the inner or terrestrial group. [illustration: a. venus. b. mars. c. jupiter. d. saturn. taken by prof. e. e. barnard with the -inch telescope of the yerkes observatory, with exception of saturn, which was taken by prof. barnard on mt. wilson.] note: the reader must bear in mind that the telescopic views of the four planets have not been reduced to the same scale and so are not to be compared in size. the terrestrial planets are the pigmies of the solar system, the outer planets are the giants. the densities of the planets mercury, venus, earth and mars are several times greater than the density of water. they are all extremely heavy bodies for their size, and probably have rigid interiors with surface crusts. the existence of life on mercury is made impossible by the absence of an atmosphere. venus and mars both have atmospheres and it is possible that both of these planets may support life. mars has probably been the most discussed of all the planets, though venus is the earth's twin planet in size, mass, density and surface gravity, just as uranus and neptune are the twins of the outer group. it is now believed that water and vegetation exist on mars. the reddish color of this planet is supposed to be due to its extensive desert tracts. the nature of certain peculiar markings on this planet, known as canals, still continues to be a matter of dispute. it is generally believed since air, water and vegetation exist on mars, that some form of animal life also exists there. the length of the day on mars is known very accurately, for the rareness of its atmosphere permits us to see readily many of its surface markings. the length of the day is about twenty-four and one-half hours, and the seasonal changes on mars strongly resemble our own, though the seasons on mars are twice as long as they are on our own planet since the martian year is twice as long as the terrestrial year. the question of life on venus depends largely upon the length of the planet's rotation period. this is still uncertain since no definite surface markings can be found on the planet by which the period of its rotation can be determined. so dense is the atmosphere of venus that its surface is, apparently, always hidden from view beneath a canopy of clouds. it is the more general belief that venus, as well as mercury, rotates on its axis in the same time that it takes to make a revolution around the sun. in this case the same side of the planet is always turned toward the sun and, as a result, the surface is divided into two hemispheres--one of perpetual day, the other of perpetual night. this peculiar form of rotation in which the period of rotation and revolution are equal is by no means unknown in the solar system. our own moon always keeps the same face turned toward the earth and there are reasons for believing that some of the satellites of jupiter and saturn rotate in the same manner. life on any one of the outer planets is impossible. the density of these planets averages about the same as the density of the sun, which is a little higher than the density of water. the density of saturn is even less than water. in other words, saturn would float in water and it is the lightest of all the planets. it is assumed from these facts that the four outer planets are largely in a gaseous condition. they all possess dense atmospheres and, in spite of their huge size, rotate on their axis with great rapidity. the two whose rotation periods are known, jupiter and saturn, turn on their axis in about ten hours. on account of this rapid rotation and their gaseous condition both jupiter and saturn are noticeably flattened at the poles. the terrestrial planets are separated from the outer group by a wide gap. within this space are to be found the asteroid or planetoid group. there are known to be over nine hundred and fifty of these minor bodies whose diameters range from five hundred miles for the largest to three or four miles for the smallest. there are only four asteroids whose diameters exceed one hundred miles and the majority have diameters of less than twenty miles. the total mass of the asteroids is much less than that of the smallest of the planets. it was believed at one time that these small bodies were fragments of a shattered planet, but this view is no longer held. the asteroids as well as the comets and meteors probably represent the material of the primitive solar nebula that was not swept up when the larger planets were formed. with few exceptions the asteroids are only to be seen in large telescopes and then only as star-like points of light. most of them are simply huge rocks and all are necessarily devoid of life since such small bodies have not sufficient gravitational force to hold an atmosphere. the revolution of the planets around the sun and of the satellites of the planets around the primary planets are performed according to known laws of motion that make it possible to foretell the positions of these bodies years in advance. asteroids and comets also obey these same laws, and after three observations of the positions of one of these bodies have been obtained its future movements can be predicted. all the planets and their satellites are nearly perfect spheres. they all, with few exceptions, rotate on their axes and revolve around the sun, or, in the case of moons, around their primaries, in the same direction, from west to east. only the two outermost satellites of jupiter, the outermost satellites of saturn and the satellites of uranus and neptune retrograde or travel in their orbits from east to west, which is opposite to the direction of motion of all the other planets and satellites. the paths of all the planets around the sun are ellipses that are nearly circular, and they all lie nearly in the same plane. the asteroids have orbits that are more flattened or elliptical and these orbits are in some instances highly inclined to the planetary orbits. the comets have orbits that are usually very elongated ellipses or parabolas. some of the comets may be only chance visitors to our solar system, though astronomers generally believe that they are all permanent members. paths of comets pass around the sun at all angles and some comets move in their orbits from west to east, while others move in the opposite direction or retrograde. the behavior of the asteroids and comets is not at all in accord with the theory that was, until recently, universally advanced to explain the origin of the solar system. some astronomers have made attempts to modify the nebular hypothesis that has held sway for so many years, in order to make it fit in with more recent discoveries, but others feel that a new theory is now required to explain the origin of the solar system. several theories have been advanced but no new theory has yet definitely replaced the famous nebular hypothesis of the noted french astronomer la place. xviii the origin of the earth it is not possible to consider the question of the origin of the earth apart from the question of the origin of the solar system. that all the planets, as well as the asteroids, originated from a common parent-mass has never been seriously questioned. all of these bodies revolve about the sun, and rotate upon their axes in the same direction--from west to east. moreover, all of the planetary orbits lie very nearly in the same plane and are nearly circular in form. the orbits of the asteroids are more elliptical and more highly inclined to one another than are the orbits of the planets, but on the average they are neither very elliptical nor very highly inclined to the planetary orbits. the sun rotates upon its axis in the same direction in which the planets rotate and perform their revolutions, and the orbits of the planets are inclined at small angles to the plane of the sun's equator. these facts are all significant and cannot be overlooked in formulating a theory to explain the origin of the planetary system in general and of the earth in particular. presumably the planets and asteroids formed at one time a part of a central body which rotated on its axis in the direction in which they now revolve about the sun. when and by the operation of what force, external or internal, they were separated from this central body is the question. in la place advanced his celebrated _nebular hypothesis_ to explain the origin of the solar system. it was received with favor both by scientists and laymen, and in a short time was almost universally accepted as closely approximating to the truth. according to the nebular hypothesis the solar nebula from which the planetary system was formed, originally extended at least as far as the orbit of neptune and rotated slowly in the direction in which the planets now revolve. as it lost heat by radiation and contracted under the gravitation of its parts its rate of rotation increased. when the centrifugal (center-fleeing) force at the equator equalled the gravitational force directed toward the center, a ring would be left behind by the contracting nebula. such a ring would not be absolutely uniform and would break at some point and gather into a planetary mass under the gravitation of its parts. this planetary mass would abandon rings in turn and these would break up to form satellites. successive rings were supposed to have been abandoned at intervals by the solar nebula at the present distances of the planets from the sun in the manner described above until the original solar nebula had contracted to its present size. the rings of saturn were supposed to be the single example remaining of this process of forming planets and satellites from a _contracting nebulous mass_. the la placian hypothesis attempted to explain why all the planets and their satellites revolve in the same direction in which the sun turns on its axis, in nearly circular orbits and nearly in the same plane. at the time it was advanced it appeared to be in accord with all the facts then known regarding the solar system. the planetoids with their interlacing and in some instances highly inclined and elliptical orbits were then undiscovered. it would have been impossible for them to have been formed by the abandonment of successive rings from a central, rotating mass. the constitution of saturn's rings was unknown at this time; also the fact that the moonlets of the inner ring revolve about saturn in _half_ the time required for the planet to turn on its axis--another impossibility under the nebular hypothesis, for, according to the assumptions of the nebular hypothesis it would be impossible for a satellite to revolve about a central body in a shorter time than that body turns on its axis. the satellites of mars were not discovered until many years later, as well as the retrograding satellites of jupiter and saturn, all presenting difficulties in the way of accepting the nebular hypothesis without radical changes. attempts, mostly unsuccessful, have been made from time to time to make these exceptional cases fit in with the requirements of the nebular hypothesis. the theory that the sun's heat was maintained by the contraction of the original solar nebula, which would cause its temperature to rise, appeared to give considerable support to the theory of la place, but the mathematicians got to work and showed that the amount of heat that would be furnished by the contraction of the sun from beyond the orbit of neptune to its present dimensions would be sufficient to supply heat to the earth at the present rate for only twenty-five million years, a period far too brief, the geologists and biologists said, to cover all the vast cyclical changes that are known to have taken place upon the surface of this planet since its surface crust was formed. evidently gravitational contraction is by no means the only or even the chief source of the sun's heat. it was also shown indisputably, that it would have been impossible for successive rings to have been abandoned at certain definite intervals by a contracting nebula, and granted a ring could have been formed it would have been impossible for it to condense into a planet, since forces residing in the sun would offset the gravitation of its parts. when la place advanced his famous theory it was, to use his own words, "with that distrust which everything ought to inspire that is not a result of observation or of calculation." were la place living today he would be, we believe, the first to abandon a theory that is now known to be in accord neither with observation nor calculation. deprived of a theory that has served to explain the outstanding features of the solar system more or less adequately for one hundred and twenty-five years, astronomers are seeking in the light of recent observations and discoveries to formulate a satisfactory theory of the origin of the solar system. in the planetesimal theory of chamberlin and moulton and the recently suggested theory of the well-known english mathematician, jeans, _a second sun passing close to our own sun is assumed to have been the cause of the origin of the planetary system_. the effect of the close approach of such a sun would be the ejection of a stream of matter from our sun, as we term it, in the direction of the passing body and also in a diametrically opposite direction. this ejection would be continuous as long as the stars remained near one another, the height attained by the ejected stream decreasing as the passing star receded. the result would be the formation of a _spiral nebula_ in which the motion of the ejected particles--planetesimals--would be across the spiral arms, toward and away from the passing star. after the sun had receded so far as to have no further effect upon these ejected particles they would revolve about the sun in more or less elliptical orbits which would gradually be reduced to nearly circular forms by repeated collisions between planetesimals. larger nuclei would be formed and these would gradually sweep up smaller fragments and become the planets of the present system. smaller nuclei in the vicinity of larger ones would become their satellites and in the course of many millions of years all of the larger fragments would be swept up by the planetary nuclei and their satellites--leaving only the asteroids, comets and meteors as survivors of the original spiral system. it must be borne in mind that a spiral nebula formed by the close approach of two suns would resemble in form only the great spiral nebulæ that are known to exist by hundreds of thousands in the heavens. these are far too extensive to form anything so small as a single solar system, but might condense into systems composed of many suns--either galaxies or star clusters. jean's suggested theory of the origin of the planetary system differs in its details from the above, though a passing sun is assumed to be the disturbing force that causes the ejection of a stream of matter which condenses to form the planets and their satellites. the origin of the inner planets is left greatly in doubt by this theory, however, and the system which interests us chiefly--the earth-moon system--is the one about which it is most difficult to arrive at any definite conclusion. our own sun, it is assumed, was dark and cold, of low density and with a diameter about equal to that of neptune's orbit at the time of the catastrophe which is placed at some , , years ago. in jean's words, "... the time for arriving at conclusions in cosmogony has not yet come--and it must be left to future investigators armed with more mathematical and observational knowledge than we at present possess to pronounce a final decision." however, since la place advanced his celebrated nebular hypothesis, great advances in astronomy have been made, and man is in a better position to theorize on this fascinating problem today than he was one hundred and twenty-five years ago. all such theories must necessarily be regarded as working hypotheses only, to be discarded or modified as our knowledge and understanding of the laws of the universe increase. no theory can ever be regarded as final or perfect. the discovery of radio-activity furnishes us with new material for new theories. the sun and the planets may be and probably are far older than we ever dreamed could be possible. it is no longer necessary or reasonable to assume that a greatly extended solar nebula once existed and supplied the planets with heat through gravitational contraction or to place a time limit upon the period required for the formation of the planets and their satellites that is not in accord with the requirements of other sciences. we know today that there exist within the sun powerful repulsive forces, which even under present conditions occasionally eject gaseous matter to heights of five hundred thousand miles or more with a velocity of over two hundred miles per second. small changes in the velocity of ejection produce great differences in the height of the ejected columns. with an initial velocity of three hundred and eighty miles per second, matter would be thrown from the solar surface to a height of fifty million miles. were the velocity of ejection three hundred and eighty-three miles per second the height of the column would be five hundred million miles, while a further increase in the initial velocity would send matter away from the sun, never to return. instead of suns and solar systems evolved from nebulæ we are now more familiar with the idea of nebulæ evolved from stars through some terrific cataclysm as in the case of novas or temporary stars. it is now known that there exist in certain parts of space a number of sharply defined stars surrounded by extensive nebulous envelopes. are these possibly suns that are going through the process of forming their planetary systems? it is now known that pressure of light and electrical repulsion are forces to be reckoned with in the evolution of stars and nebulæ as well as gravitational contraction. it has long been felt that the peculiar formations existing among the vast irregular gaseous nebulæ could not be explained as gravitational effects alone. _light-pressure and electrical repulsion_, as well as _gravitation_ are at work within the solar system and the sun is the seat of powerful disturbances which produce periodic outbursts of exceptional activity and which may have produced in the distant past more startling effects than any with which we are familiar at present. the earth and moon form a system that is in a way unique. no satellite in the solar system is so large in proportion to its primary as is our own moon. seen from the distance of venus or mars, the two bodies would apparently form a _double star_. the diameter of the moon is one-fourth that of the earth. satellite iii of jupiter far exceeds our own moon in actual size but its diameter is only about four-hundredths of the diameter of the planet about which it revolves. the diameter of titan, the largest satellite of the saturnian system, bears the same ratio to the diameter of saturn. moreover, all the nearer satellites of jupiter and saturn lie nearly in the equatorial planes of these planets, but the orbit of the moon is inclined at a high angle to the plane of the earth's equator. it is not difficult to believe that the satellites of jupiter or saturn were at some time thrown off from the equatorial belts of their primaries, just as the planets themselves may have been ejected from the equatorial belt of the sun, but we cannot so readily believe that our own satellite was formed from the earth in a similar manner. the moon's orbit lies nearly in the plane of the sun's equator, however, and it is conceivable that both earth and moon were simultaneously ejected from the equatorial zone of the sun, the two nuclei being so close together that the smaller one remained under the gravitational control of the larger. the difficulties in the way of believing that the moon once formed a part of the earth are very great. it can be shown mathematically that if the two bodies at one time formed a single mass it would have been impossible for the moon to break away from the earth, unless the force that caused the separation were sufficient to hurl the moon to a greater distance than two and a half times the earth's radius. the mathematician, roche, found out by computation that a satellite could not remain intact within this distance of the planet, but would be broken up into small fragments under the effects of the tides raised by the larger body. if, then, the moon had originally been ejected from the earth to a less distance than two and one-half radii of the earth ( . to be exact) it would have been disintegrated into small particles, or moonlets, under the tidal strains exerted upon it by the earth and would have been gradually distributed about the earth in the form of a meteoric ring which, in the course of ages, would be absorbed by the earth, just as saturn is now gradually absorbing its rings. the planets differ greatly in density. the more distant and larger planets--jupiter, saturn, uranus and neptune--have densities equal to or less than that of the sun. the densities of the inner planets--mercury, venus, earth and mars--are, relatively, extremely high, the density of the earth's core being about that of meteoric iron. the densities of mercury and venus are slightly less than that of the earth and the densities of mars and the moon about equal to that of the earth's crust. if a stream of matter were ejected from the sun under the influence of some external force, such as that exerted by a passing star, the outlying parts of the stream would consist of the lighter elements and the lower parts of the heavier elements, since the lighter solar elements lie at or near the surface of the sun and the heavier elements at greater depths. at the time of ejection the lighter elements would be thrown to great distances and would go to form the less dense outer planets; the heavier elements would go to form the inner planets of high density. it is conceivable that ejection of solar material might have taken place under the influence of certain forces at work within the sun itself, such as electrical repulsion or pressure of light which might become powerful enough under certain conditions to overcome the effect of gravitation. next to nothing is known about the physical state of matter at great solar depths, where abnormal conditions of temperature and pressure must exist, and where great physical changes and disturbances may have taken place in the past. even today solar activity goes through a cycle of change during the sun-spot period, and many millions of years ago the sun-spot cycle of solar activity may have been far different from what it is today and a far more powerful factor in producing changes in the solar system. outbursts of novas indicate that agencies making for peace and order are not the only ones at work among the stars. the cause of such outbursts has never been satisfactorily explained. the theory that they are caused by the close approach of two suns or by the encounter of a star with a dark nebula does not account for all of the circumstances of such outbursts. the nebulous matter seen about a nova after the outburst is now generally believed to have been expelled from the star itself at the time of the catastrophe and may conceivably be the stuff of which planetary systems are made. at some epoch in the past, probably at least one thousand million years ago, our own sun may have undergone some cataclysmic change and this may, conceivably, have been brought about by disturbances within the sun itself. elements may have been so formed and distributed within the interior of the sun that friction and internal instability resulted and in time produced an upheaval of solar elements with initial velocities so great that, possibly, through electrical repulsion and light-pressure, portions of the ejected streams were permanently detached from the sun and became the nuclei of future planets. in some such way, it is conceivable, our own planet earth and the other members of our solar system may have been brought into existence in the dim and distant past--many hundred million years ago. xix jupiter and his nine moons jupiter shines by reflected sunlight with a brilliancy that usually exceeds that of the brightest of the stars, sirius. when seen during the midnight hours the remarkable unflickering brightness of this largest and most distinguished member of the solar system at once serves to set it apart from the scintillating stars far beyond. there is but one planet, venus, that always surpasses jupiter in brilliancy, though mars on the occasions of its close approaches to the earth may equal or slightly surpass jupiter in brightness. as venus never departs more than forty-eight degrees from the sun, and so is never seen in the midnight hours, jupiter usually shines without a rival when visible at midnight. to one who has observed the two planets together the silvery radiance and surpassing brilliancy of venus, due not to its size, but to its comparative nearness to the earth, at once serves to distinguish it from the golden glow of jupiter. even the smallest telescopes of two- or three-inch aperture will show the four historic moons of jupiter which were the first celestial objects to be discovered when galileo turned his crude telescope to the heavens in the year . the fact that these tiny points of light were actually revolving around the great planet was soon detected by the famous astronomer and we can imagine with what breathless interest he observed these satellites of another world whose discovery dealt such a severe blow to the old ptolemaic theory that the earth was the center of the universe. it was not until the great telescopes of modern times were invented that the five additional moons of jupiter were discovered. the four satellites first observed by galileo were fancifully named io, europa, ganymede and callisto, in the order of their positions outward from the planet, but these names are rarely used now, the satellites being designated for convenience i, ii, iii and iv, respectively. the first of the new satellites to be discovered was satellite v, which is the nearest to jupiter of all the nine moons. it is an extremely small body, not more than one hundred miles in diameter, and to discover this tiny body as it skirted rapidly around the great planet within sixty-seven thousand miles of its surface, nearly lost in the glaring rays, was a difficult feat even for an experienced observer. it was accomplished, however, by prof. e. e. barnard with the great lick refractor in . satellite v is hopelessly beyond the reach of any but the greatest telescopes, as are also the four satellites discovered since that date. in fact, most of these tiny moons are observed photographically. satellites vi and vii were discovered photographically in . they are both about seven million miles from the planet and their paths loop through one another; they are, moreover, highly inclined to each other at an angle of nearly thirty degrees. when nearest together they are separated by a distance of two million miles. two more extremely small bodies, known as satellites viii and ix, have been discovered since then, one at greenwich, england, in , the other at the lick observatory in . these excessively faint bodies are the most remote satellites of jupiter and they are of particular interest because they travel around the planet in a retrograde direction, or from east to west, which is _opposite_ to the direction of revolution prevailing in the solar system. the ninth and most distant satellite of saturn also retrogrades, or revolves in a clockwise rather than a counter-clockwise direction around the planet. one explanation given for this peculiarity of the outermost satellites of jupiter and saturn is that this backward revolution around the planet is more stable when the satellites are at great distances from the primary, and the gravitational control that the planet exerts is therefore weak. the moons of the planets are, of course, subject to the attraction of the sun as well as to the attraction of the controlling planet, and the greater the distance of the satellite from the planet the stronger the pull exerted by the sun and the weaker the bonds that bind the moon to the planet. beyond a certain limit it would be impossible for the planet to hold the satellite against the sun's greater attraction and the satellite would leave the planet to revolve directly around the sun, thereby becoming a planet. it appears that as this danger limit is neared it is safer for the satellite to "back" around the planet than to follow the usual "west to east" direction of revolution. the eighth satellite of jupiter is more than fourteen million and the ninth more than fifteen million miles from the parent planet and they require about two years and three years, respectively, to complete one trip around jupiter. when we consider that satellite v darts around the planet in less than twelve hours at a distance of only sixty-seven thousand miles from its surface we realise what tremendous differences exist in the distances and periods of revolution of the nine moons. there is also great disparity in the sizes of the various moons. the five moons discovered in modern times are all excessively faint and extremely small. the diameter of the largest of these, satellite v, is less than one hundred miles. on the other hand, the four historic moons of jupiter are of planetary dimensions. the smallest, satellite ii, is slightly larger than our own moon, while the largest, satellite iii, has a diameter, according to measurements made with the -inch yerkes refractor in , of three thousand nine hundred and eight miles, which is only four hundred miles less than the diameter of mars. the periods of revolution of these four satellites range from one day and eighteen hours for the nearest, which is about two hundred and sixty-one thousand miles from the center of jupiter, to sixteen days and sixteen and one-half hours for the most distant, which is more than one million one hundred and sixty thousand miles from the planet. these four moons are so near to the great planet that they are continually dipping into his huge shadow and experiencing an eclipse of the sun which, owing to the nearness and great size of jupiter, lasts for two or three hours. at times of eclipse the moon suddenly disappears from the observer's view, though it may be considerably to one side of the planet. its reappearance later on is just as sudden, or it may pass out of the shadow while hidden from us behind the disk of the planet, in which case its reappearance is invisible from the earth. the occultations of the satellites, or, in other words, their disappearance behind the planet's disk, are also interesting phenomena to observe, as are their "transits" across the disk of the planet as the satellite passes in front of it. not only the satellite itself but its shadow as well can be seen, a small black dot passing over the surface of jupiter. the satellite is totally eclipsing the sun for this small dark portion of the planet's disk. two satellites and their shadows are frequently seen crossing the face of the planet at the same time. it is possible to observe all the phenomena of the satellite's transits and shadows, eclipses and occultations with very small telescopes. from observations of the eclipses of jupiter's satellites the important discovery of the finite velocity of light was first made as far back as the year . faint surface markings have been made out at certain times on the largest of the four satellites, satellite iii, and also on satellite i. observations of the markings on the former seem to indicate that it always keeps the same face turned toward jupiter as does our own moon toward the earth. there are also reasons for believing that the equatorial regions of satellite i are light colored and the polar regions dark. there is the possibility that forms of life may exist on these satellites of jupiter, though they are more likely barren, lifeless worlds, such as mercury and the moon. their great distance from the earth, never less than three hundred and sixty-eight million miles, makes observations of their surface markings very difficult. how beautiful beyond description must the heavens appear as viewed from the satellites of jupiter! viewed from the distance of io, or satellite i, the mighty planet jupiter presents a spectacle such as the eye of man has never been privileged to behold. the huge flattened globe, ninety thousand miles in equatorial diameter, equal in mass to _three hundred planets such as our own_ and in volume to nearly _fourteen hundred_, fills a space in the heavens nearly twenty degrees in extent as viewed from this satellite. fifteen hundred of our own full moons would hardly fill the same space. whirling on its axis with frightful speed in a period of less than ten hours, the huge ball glides rapidly but majestically onward through the sky. a far distant sun shrunk to but one-fifth the diameter of the full moon throws light and shade across the rapidly changing surface of the planet, rich in the reds, browns and yellows and all the gorgeous shades and tints of its dense, seething, gaseous envelope. the phases of the moon on a greatly enlarged scale rapidly succeed each other on jupiter as it is viewed from the satellite in all positions with reference to the sun. the cause of the belts of jupiter, that lie parallel to the planet's equator and are constantly changing in number, width and shade, as well as the nature of all the peculiar splashes of color and intensely white flecks that come and go in the dense atmosphere of the planet would not be such a mystery to us were it possible to view the great planet from the distance of satellite i, which is about as far from the surface of jupiter as the moon is from the earth. it is uncertain whether the planet is entirely gaseous throughout or has a central core of solid or liquid matter. its density is only one and one-quarter times that of water and slightly less than that of the sun, showing that it is composed largely, if not entirely, of matter in a gaseous state. jupiter is a world as different from our own as it is possible to imagine. there is no visible surface crust and there are no permanent markings. different spots on the planet's disk give different periods of rotation showing that it is atmospheric phenomena that we observe. all is constant flux and change on jupiter. dense vapors arise from a highly heated interior and spread out into belts parallel to the equator in the direction of the planet's rotation. from its nearest satellite all the interesting changes of color and form that constantly take place in the atmosphere of this great globe could be observed in great detail. the high percentage of light and heat that jupiter reflects from the sun to its nearer satellites makes it a secondary sun to them of tremendous size though feeble strength. as seen from satellite i the other three major moons of jupiter present all the phases of our own moon in rapid succession, due to their constantly changing positions with reference to the sun. the five small moons, discovered in modern times, are so minute that they are simply star-like points of light even when viewed from the other moons of jupiter. to keep track of the rapidly changing positions and various phases of the moons of jupiter as seen from any one of them, as well as the rapid _apparent_ motion of the planet through the sky due to the revolutions of the satellite around the planet, would be a troublesome task for an astronomer stationed on one of these far distant worlds. it would be a common sight to see in the sky at one time the huge planet, the far-distant, shrunken sun, and one, two or three moons. seen from the moons of jupiter the constellations would appear as they do to us on earth, for such a slight change in position as five hundred million miles, more or less, is trivial when one is looking at the stars. observations of the stars from the nearest moon of jupiter would be attended with great difficulties at times, since reflected sunlight from a body nearly twenty degrees in diameter would be extremely troublesome, especially were the phases of the planets near that of the full moon. we know how the presence of our own moon in the heavens at the full dims the brightness of the stars so that only the brightest stars are seen. even as viewed from the fourth or most distant of the major satellites the planet subtends an angle of nearly five degrees. occultations of the stars are many and frequent as the huge planet globe glides swiftly through the heavens. many a moonlight night appears almost as day owing to the presence of the enormous, brilliantly reflecting ball of light and at times two or three moons in addition. only the brightest stars could possibly be seen under such circumstances. when, however, the small worlds pass into the shadow of the great mother planet and not only the light of the sun but also the reflected light of jupiter disappears for many minutes, the stars shine forth in all their glory there as here. at such times some of the larger moons would usually be seen shining by the reflected light of the far distant sun. saturn also would be visible as a magnificent star, but beautiful venus and ruddy mars would fail to appear. tiny bodies, mere specks of light at this distance, they would be lost to view in the glare of the sun. xx the rings and moons of saturn nearly everyone has felt at some time or other a strong desire to gaze at some of the beauties and wonders of the heavens through a telescope and the one object that all of us wish to see, if, perchance, this desire is to be gratified is saturn, whose unusual ring system has, so far as we know, no counterpart in the sky. all the planets in the solar system with the exception of the two innermost, mercury and venus, are attended by satellites, but saturn, alone, has in addition to a family of nine moons, three distinct rings of great dimensions which are composed of swarms of minute particles revolving around the planet. why saturn should be the only planet to possess such a system of rings has never been explained in an entirely satisfactory manner. there is an interesting law known as "roche's law," however, named from its investigator, that states that no satellite of a planet can exist intact with . times the radius of the planet. this limit is spoken of as "roche's limit" and applying it to the planet saturn we find that the rings of saturn fall within this limit. it does not necessarily follow from this that the minute particles of which the rings are composed are the shattered remains of one small satellite, but rather that they are the material from which a satellite might have been formed were it not so close to the planet. within "roche's limit" the mutual attraction of the various particles for each other that would tend eventually to gather them into one body is overcome by tidal forces that arise from such close proximity to the huge planet. the stress and strain of such forces is so great that no grouping of particles can take place. this explains, possibly, why the rings continue to exist in their present condition. the total quantity of matter in the rings is known to be very small, for it does not disturb the motions of any of the nearer and smaller satellites, though tiny mimas, six hundred miles in diameter, is only thirty-one thousand miles beyond the outer edge of the outer ring. an interesting observation was made a few years ago of the passage of the rings of the planet between us and a star. though the light of the star was diminished to one-fourth of its normal brightness when the rings passed before it, at no time was its light entirely eclipsed by any of the particles. it was computed that if the diameters of any of the individual particles had amounted to as much as three or four miles the star would have been temporarily eclipsed. an upper limit for the size of the moonlets was thus obtained. the average diameter of the particles is probably much less than three miles. the thickness of the ring system is not over fifty or one hundred miles, but its total diameter is one hundred and seventy-two thousand miles. there are, in all, three concentric rings. the faint inner ring, known as the "crape" ring, is invisible in a telescope under four inches in aperture. the width of this inner ring is eleven thousand miles. just beyond the crape ring is the chief, bright ring, eighteen thousand miles in width. it shades gradually in brightness from its juncture with the crape ring to its most luminous portion at its outer edge, which is separated from the third or outer ring by a gap two thousand two hundred miles in width, known as cassini's division. the third or outer ring is eleven thousand miles wide and is less bright than the central ring. the inner edge of the inner ring is but six thousand miles above the surface of the planet. on account of the curvature of the planet the ring system is invisible from the north and south poles of saturn. as in the case of the satellites of a planet the inner particles of the rings revolve around the planet more rapidly than the outer particles. the innermost particles of the crape ring require but five hours for one journey around saturn while the outermost particles of the outer ring require one hundred and thirty-seven hours, or nearly six days to complete one revolution. in addition to the gap in the rings known as cassini's division several other fainter divisions exist. if a group of moonlets were to revolve around the planet in the positions marked by these gaps their periods of revolution would be commensurable with the periods of several of the satellites of saturn. as a result the attraction exerted on such particles by these satellites would gradually disturb their motion in such a way as to draw them away from these positions. it is owing, therefore, to the attraction of the satellites of saturn for the moonlets that these gaps in the rings exist. as a result of the disturbances produced in the motion of the moonlets by the satellites of saturn collisions are bound to occur occasionally among the various particles. when two particles collide the period of revolution of one or both of them is reduced and as a result collisions tend to bring the moonlets gradually closer and closer to the surface of the planet. the dusky inner ring, it is believed, may consist largely of particles whose periods have been continually shortened by collisions. saturn may, therefore, lose its ring system in the course of time through its gradually being drawn down upon the planet by collisions of the various particles until all of the material is finally swept up by the planet. such a change would probably require millions of years, however, as collisions are probably, on the whole, infrequent. it is possible that the ring system of saturn may have been much more extensive in the past than it is now and other members of our solar system may have had such appendages in the far distant past. the appearance of the rings of saturn as viewed from our planet changes periodically as a result of the revolution of the earth and saturn around the sun, which places them in constantly changing positions with reference to each other. the rings lie in the plane of saturn's equator, which is inclined twenty-seven degrees to its orbit and twenty-eight degrees to the earth's orbit. since the position of the equator remains parallel to itself while the planet is journeying around the sun it happens that half the time the earth is elevated above the plane of the rings and the remainder of the time it lies below the plane of the rings. twice in the period of saturn's revolution around the sun, which occupies nearly thirty years, the earth lies directly in the plane of the rings and at this time the rings entirely disappear from view for a short time. mid-way between the two dates of disappearance the rings are tilted at their widest angle with reference to the earth and they are then seen to the best advantage. as the date of their disappearance approaches they appear more and more like a line of light extending to either side of the planet's equator. even in the most powerful telescope the rings entirely disappear from view for a few hours at the time the earth lies exactly in the same plane. it is at this time that the ball of the planet is best seen. its flattening at the poles, which is nearly ten per cent. of its equatorial diameter then gives it a decidedly oval appearance. ordinarily one of the hemispheres of saturn is partly or entirely concealed by the rings so that the oblate form is not so noticeable. it was the change in the tilt and visibility of the rings that so perplexed galileo when he attempted to make out the nature of these appendages of saturn with his crude telescope of insufficient magnifying power. so great was his bewilderment when the rings finally disappeared that he cried out in despair that saturn must have swallowed his children, according to the legend. he finally became so exasperated with the results of his observations that he gave up observing the planet. the true nature of these appendages of saturn remained a mystery until huygens solved the problem in , some time after the death of galileo. in addition to the rings, saturn has nine satellites named, in the order of their distance outward from the planet, mimas, enceladus, tethys, dione, rhea, titan, hyperion, iapetus and phoebe. the last-mentioned satellite was discovered by w. h. pickering in . it aroused great interest at the time because it was the first satellite to be discovered with "retrograde" motion in its orbit. two satellites of jupiter since discovered revolve in the same direction around their primary. the satellites of saturn are approximate to those of jupiter in size and exactly equal them in number. the largest, titan, is three thousand miles in diameter and can be easily seen with the smallest telescopes. with a four-inch telescope five of the satellites can be readily found, though they are not as interesting to observe as the satellites of jupiter because they are far more distant from the earth. the time they require to make one journey around saturn varies from nearly twenty-three hours for mimas, the nearest, to approximately five hundred and twenty-four days for phoebe, the most distant. saturn as well as jupiter is marked by belts parallel to the equator though they appear more indistinct than those of jupiter on account of the greater distance of saturn. saturn also resembles jupiter in its physical composition which is largely, if not entirely, gaseous, and in the extremely short period of rotation on its axis which is approximately ten hours. in more ways than one saturn is a very unusual planet. in addition to possessing an enormous ring system it is the lightest of all the planets, its density being only sixty-three hundredths that of water, and it is the most oblate, its flattening at the poles amounting nearly to one-tenth of its diameter. its equator is more highly inclined to its orbit than is the case with any other planet, not even excepting the earth and mars. for this reason its seasonal changes are very great, in marked contrast to jupiter whose equator lies very nearly in the plane of its orbit. since saturn is so far away from the sun that it receives only one ninetieth as much light and heat per unit area as the earth, its outer gaseous surface must be extremely cold unless considerable heat is conveyed to the surface from within its hot interior. the late prof. lowell concluded from certain observations made at flagstaff, ariz., that saturn is composed of layers of different densities and that the inner layers are more flattened at the poles and rotate faster than the outer layers. marked variations in the color and brightness of the ball of the planet have been noted from time to time. in observers of saturn described the planet as pinkish-brown and conspicuously darker than the brighter portions of the rings. it is believed that these very noticeable changes in the color and brightness of saturn are due to slight, irregular changes in the intensity of the radiations of the sun which set up certain secondary effects in the atmosphere of the planets. similar changes in color and brightness have been observed also in the case of jupiter. xxi is the moon a dead world? it has been a generally accepted belief among astronomers for years that the moon is a dead world devoid of air and water and so, necessarily, lifeless. it is certain that the moon has no extensive atmosphere such as envelops our own planet. there is abundant proof of this fact. the edge of the lunar disk is clear-cut. whenever, as happens frequently, the moon passes between us and a star the disappearance of the star is instantaneous. there is no gradual dimming or refraction of the star's light by atmospheric vapors. moreover, lunar shadows are harsh and black. there is no evidence of diffusion of light on the moon by atmospheric gases. the absence of water or water vapor on the visible surface of the moon, at least in any appreciable quantity, is plainly evident to anyone who observes the moon through the telescope. even with small telescopes, objects five miles or so in diameter can be readily detected and clouds drifting over the surface could not possibly escape our observation if they existed. bodies of water, great or small, would be plainly visible and would besides give rise to water vapor and clouds, which we would not fail to detect. since the surface of the moon is unscreened by air and water vapor to absorb the incoming rays from the sun, and the outgoing radiations from the surface, the extremes of temperature between day and night are very great, and are augmented by the fact that the lunar day equals the lunar month in length, so that fourteen days of untempered heat are followed by fourteen days of frigid darkness. observations of the rate of radiation from the moon's surface during total eclipses of the moon indicate that the moon's radiation is very rapid, and that its temperature during the height of the lunar day probably approaches ° f., while at the lunar midnight it may have fallen to ° below zero f., or even lower. with air and water both lacking and such extremes of temperature existing why should we seriously consider the question of life on the moon? this is the point of view of the majority of astronomers and it seems well taken. yet many astronomers who have made a special study of the lunar surface for years under all conditions of illumination and phase, and have most carefully observed and mapped and photographed its characteristic markings, are agreed that there are evidences that changes are taking place on the moon, and recently prof. w. h. pickering has expressed the belief, substantiated by drawings, that there is a progressive change of color or darkening within certain lunar craters with the advance of the lunar day, indicating, in his opinion, a rapid vegetational growth that springs up in the height of the lunar day and dies out as the lunar night approaches. some years ago certain selenographers suggested that there might exist in the numberless crater-pits and craters, in the deep-lying _maria_ or "seas," and in the clefts and rills and cracks that form intricate systems all over the lunar surface, certain exhalations from the surface and heavy vapors including possibly carbon dioxide and water vapor to temper the extremes of the long lunar days and night and furnish the necessary medium for the support of certain forms of animal and vegetable life. many astronomers, including a number who are not in sympathy with the above view, believe that snow and ice exist on the moon, even though water in the form of liquid and vapor is not observable. all the extremely brilliant portions of the surface, according to some astronomers, are covered with snow and ice. certainly, some portions of the moon's surface reflect sunlight as brilliantly as if they were covered with freshly fallen snow, while other portions appear to be black by contrast. there also appears to be evidence that certain small markings, described as crater-cones and resembling our terrestrial volcanoes more than any other lunar feature, are at times temporarily obscured from view by a veil of vapors. many observers believe that these crater-cones are active volcanic vents, and that there is considerable volcanic activity still taking place upon the moon. these small crater-cones resemble, we are told, parasitic cones found on the sides of terrestrial volcanoes, and they are frequently seen on the floors of craters closely associated with light streaks. these crater-cones appear under a high sun as minute white spots and can be studied to advantage only with powerful instruments. the italian astronomer, maggini, observing the floor of the lunar crater, plato, in noted that one of the small crater-cones that exist there in great numbers, was temporarily obscured from view by a cloud of reddish vapors, and prof. w. h. pickering, at arequipa, peru, observing the same region some years ago, believed that he saw evidence of change in some of these small markings. the crater, plato, has probably been more carefully studied than any other portion of the lunar surface. it is sixty miles in diameter and may be seen even without a telescope as a dark "eye" not far from the northern edge of the moon. its floor is one of the darkest objects in the moon--a dark steel-grey in color--and there is no doubt that for some unknown reason its dark hue deepens from the time the sun has an altitude of twenty degrees until after full moon. it has a brilliant white wall rising from , to , feet above its floor, crowned with several lofty peaks and intersected by a number of valleys and passes. the spots and faint light markings on the floor have been the object of much study with small or moderate sized instruments, and at least six of them are known to be crater-cones. since they can only be studied to advantage with powerful instruments and as such instruments are rarely used for a systematic study of lunar markings, it is difficult to settle the controversy as to whether they have changed in appearance or have been at any time obscured by vapors. most lunar observing is done--necessarily--with smaller instruments because the majority of astronomers appear to have accepted the view that the moon is a dead world, and those who are engaged in astronomical work with our greatest telescopes seem to feel that other fields of research will prove more fruitful. possibly it is for this reason that we know so little about our nearest neighbor in space! there are at least as many unsolved problems confronting us on the moon as there are among the distant stars. geologists tell us that more oxygen is to be found in the first six feet of the earth's crust than in all of the atmosphere above. does oxygen not exist in the surface rocks of the moon as well? volcanic action, we are told, is primarily an escape of gases from the interior, chiefly hydrogen, nitrogen, hydrocarbons, sulphur, and various compounds, as well as vast quantities of steam. beneath the surface chemical change is continually taking place which results in the release of an enormous amount of heat. some of the gases mentioned above combine with the oxygen in the surface rocks and heat is evolved. it is a known fact that there is great inherent heat in the earth's surface crust. why not in the moon's surface crust as well? the water that would be expelled in the form of steam from volcanic vents on the moon would be transformed immediately into hoar-frost, snow and ice and would settle down upon the flanks of the crater-cones or vents. it should be borne in mind that only volcanic activity on an enormous scale would be plainly visible to us even with the powerful telescopes at our command. ordinary eruptions such as occur on our own planet would be very difficult to detect. since the escaping vapors would rapidly pass into the solid state and settle down upon the flanks of the crater-cones or vents, we would observe in general little if any change in an object unless we chanced to be looking at it at the time of the eruption, when it might appear to be temporarily obscured by a veil of vapors. what are the chances that we would be carefully observing at the precise time of an eruption, a minute marking, two or three miles in diameter, on a surface as large as all of north america, a surface that is covered with some , charted craters, numberless crater-pits, streaks, rays, spots, clefts and rills in intricate systems, mountain chains and valleys and a mass of intricate detail? if we were looking at the earth from the moon with the aid of a powerful telescope would we be apt to notice an eruption of vesuvius or katmai or mauna loa? objects four or five miles in diameter would appear as hazy spots with nothing distinctive or remarkable in their appearance. yet vapor and steam arising from terrestrial volcanoes would be carried by our atmosphere over an area of many square miles, while there is no atmosphere on the moon to spread the vapors that may arise from similar volcanic vents. it would have to be a cataclysmic change indeed to be accepted as indisputable evidence that change is taking place on the moon, and the days of gigantic upheavals are probably over on our satellite as well as on the earth. if volcanic activity is still taking place on the moon it is probably in a mild form such as a comparatively quiet emission of gases from volcanic vents and fumaroles. such forms of activity would not be plainly visible at this distance, even with the aid of powerful telescopes. the problem of detecting changes on the moon is complicated by the fact that a change of illumination greatly alters the appearance of all lunar markings. such a change is continually taking place in the course of the month. a marking that stands out in bold relief at lunar sunrise or sunset will change entirely in appearance a few days later under a high sun or even disappear from view entirely. these changes in phase or illumination have to be taken account of in the search for evidence of actual change. to decide whether or not change has actually taken place the object must be viewed under similar conditions, so far as they can be obtained. even when special care is taken in this respect the suspected evidence of change is usually "explained away" as due to differences in illumination or seeing, by those who have not observed the object themselves and are not in sympathy with the view that the moon is anything but a dead world. as regards the question of life on the moon, it is interesting to consider the facts brought out by investigations made by scientists connected with the geophysical laboratory of the carnegie institute in the valley of ten thousand smokes. the volcanic activity there takes the form of eruptions from numerous small vents or fumaroles and ninety-nine per cent. of the emanations are water vapor. it was observed that blue-green algae were living at the edge of active vents emitting ammonia compounds at a temperature of ° f. they were not found, however, near vents from which ammonia compounds were not being emitted. if life exists under such conditions it is conceivable that suitable conditions for the support of certain forms of life, animal as well as vegetable, may be found in low-lying valleys and crevices and upon the floors of craters, where certain gases essential to the support of life might be evolved from many small volcanic vents and fumaroles. many theories have been advanced to explain the origin of the lunar craters which have no counterpart on our own planet. they are saucer-like depressions in the surface of the moon, frequently of such great size that an observer standing in the center would not be able to see either side of the crater owing to the curvature of the moon's surface. craters fifty, sixty or one hundred miles in diameter are by no means uncommon, while there are thousands between five and fifty miles in diameter. a characteristic feature of many craters is a central peak, and the surrounding walls are often a mile or more high and in some instances are symmetrically terraced. new craters have been formed on the sides or floors of old craters, and these are always more clear-cut and sharper in outlines than the old formations, and generally much smaller. a number of craters are surrounded by a system of light streaks or rays of unknown origin that extend in some instances to enormous distances on all sides of the crater. the most conspicuous system is the one surrounding the lunar crater tycho near the south pole of the moon. the rays originating in this crater extend in all directions for hundreds of miles without turning aside for any obstructions, passing over mountains, craters and plains in their course in practically straight lines like spokes in a wheel. this ray system of tycho is the most noticeable marking on the moon's surface at the time of full moon. as these streaks cast no shadow they are apparently cracks in the surface that have filled up with some light-colored material from below. their origin has never been satisfactorily explained. as to the origin of the lunar craters, some believe that they were produced in past ages by a bombardment of the lunar surface by huge meteoric masses; but there are many objections to this theory that we will not take up here. it is more generally believed that the lunar craters are a result of volcanic activity on an enormous scale which took place on the moon many ages ago and which has now practically ceased, its only manifestations now taking the form of a quiet emission of gases from small volcanic vents or fumaroles which exist all over the lunar surface but which are to be found in greatest numbers on the floors and sides of craters. xxii comets the orbits of comets are inclined at all angles to each other and to the orbits of the planets which, on the other hand, lie very nearly in the same plane. the larger members of the sun's family, the planets and their satellites, revolve from west to east around the sun. comets on the contrary frequently retrograde or back around the sun in the opposite direction--from east to west. the paths that these erratic visitors follow in their journeys around the sun bear not the slightest resemblance to the paths of the planets, which are almost perfect circles. the orbits of comets are ellipses that are greatly elongated or parabolas. if the orbit is a parabola the comet makes one and only one visit to the sun, coming from interstellar space and returning thereto after a brief sojourn within our solar system. donati's comet of , one of the greatest comets of the nineteenth century, had a period of more than two thousand years and its aphelion (the point in its orbit farthest away from the sun) was five times more distant than the orbit of neptune. there is, however, a class of comets known as _periodic_ comets that have extremely short periods of revolution around the sun. to this class belongs halley's comet whose period of seventy-five years exceeds that of any other short period comet. encke's comet, on the other hand, has a period of three and a third years which is the shortest cometary period known. most of the periodic comets are inconspicuous and only visible telescopically even when comparatively near to the earth. halley's comet is the only one of this class that lays any pretensions to remarkable size or brilliancy and it also is showing the effects of disintegration resulting from too frequent visits to the sun. comets are bodies of great bulk or volume and small total mass. their tails, which only develop in the vicinity of the sun, are formed of the rarest gases, and the best vacuum that man can produce would not be in as tenuous a state as the material existing in the tails of comets. there are many proofs of the extreme tenuity of comets. the earth has on a number of occasions passed directly through the tails of comets without experiencing the slightest visible effects. stars shine undimmed in luster even through the heads of comets. if the earth should encounter a comet "head on" it is doubtful if it would experience anything more serious than a shower of meteors which would be consumed by friction with the earth's atmosphere, or a fall of meteorites over a small area of a few square miles. it is possible, however, that matter in the nucleus, the star-like condensation in the head of a comet, may consist of individual particles weighing in some instances a number of tons, surrounded by a gaseous envelope and held together by the loose bonds of their mutual attraction. if the earth should encounter the nucleus of a comet considerable damage might be done over a portion of the earth's surface, but the chances of such an occurrence are less than one in a million. since the total mass of a comet is so small, a close approach to one of the planets, especially jupiter, produces great changes in the form of the comet's orbit, though the motion of the planet is not disturbed in the slightest degree by the encounter. the majority of all the short-period comets have been "captured" by jupiter, that is, the original orbits have been so changed by the perturbations produced by close approaches to the giant planet that their aphelia, or the points in their orbits farthest from the sun, lie in the vicinity of jupiter's orbit. several of the other planets have also "captured" comets in this sense, and the fact that the aphelia of a number of comets are grouped at certain definite intervals beyond the orbit of neptune has been considered by some astronomers to be an indication that there are two or more additional planets in the solar system revolving around the sun at these distances. the most interesting feature of a comet is its characteristic tail which develops and increases in size and brilliancy as the comet approaches the sun. as the tail is always turned away from the sun it follows the comet as it draws near the sun and precedes it as it departs. its origin is due, it is believed, both to electrical repulsion and light-pressure acting upon minute particles of matter in the coma or head of the comet. the curvature of the tail depends upon the nature of the gases of which it is composed. long, straight tails consist chiefly of hydrogen, it has been found, curved tails of hydrocarbons and short, bushy tails of mixtures of iron, sodium and other metallic vapors. at times the same comet will have two or more tails of different types. since the material driven off from the nucleus or head of a comet by electrical repulsion and light-pressure is never recovered, it is evident that comets are continually disintegrating. also, comets that have passed close to the sun at perihelion have frequently been so disrupted by tidal forces that one nucleus has separated into several parts and the newly formed nuclei have pursued paths parallel to the original orbit, each nucleus developing a tail of its own. many periodic comets, it is now known, have gradually been broken up and dissipated into periodic swarms of meteors as a result of the disruptive effect produced by too frequent returns to the vicinity of the sun. these swarms of meteors continue to travel around the sun in the orbits of the former comets. the earth encounters a number of such swarms every year at certain definite times. the largest and best known of these swarms or showers are the leonids, which appear about november ; the andromedas (or bielids), which appear later in the same month and the perseids, which appear early in august. these swarms are named for the constellations in which their "radiant" lies, that is, the point in the heavens from which they appear to radiate. the position of the radiant depends upon the direction from which the swarm is coming. it is simply a matter of perspective that the individual particles appear to radiate from the one point, for they are actually travelling in parallel lines. the luminosity of these meteoric particles is caused by the friction produced by their passage through the atmosphere. they always appear noiselessly because they are mere particles of meteoric dust weighing at the most scarcely a grain. they differ greatly in this respect from their large and noisy relatives, the meteorites, bolides and fireballs. numberless small meteoric particles are entrapped by the earth's atmosphere every day. they are referred to as "shooting" stars or "falling" stars though, of course, they are not in any sense stars. it is only when these meteoric particles travel in well-defined cometary orbits and appear at certain definite times every year that they are referred to as swarms or showers of meteors. the luminosity of comets is due not only to reflected sunlight, but to certain unknown causes that produce sudden and erratic increases or decreases of brilliancy. the causes of these sudden changes in luminosity are unknown; possibly electrical discharges or chance collisions between fragments of considerable size may account for some of them. the peculiar behavior of the tails of comets at certain times has frequently been noted and suggests the existence of quantities of finely-divided meteoric or gaseous matter within the solar system that has no appreciable effect upon the huge planetary masses, but offers sensible resistance to the passage of the tenuous gases of which the tails of comets are composed. the fact that the earth daily encounters meteoric dust, meteorites and fireballs also indicates that meteoric matter exists in considerable quantities within our solar system. tails of comets appear at times to be twisted or brushed aside as if they had encountered some unknown force or some resisting medium. up to the present time several hundred comets have been discovered. nearly three-fourths of this number travel in orbits that appear to be parabolas. of the remaining number there are about forty that have been "captured" by the major planets, jupiter, saturn, uranus and neptune, though jupiter possesses the lion's share of these captured comets. scarcely a year passes by that several comets are not discovered. most of these are telescopic, however, even when they are near the sun and at their greatest brilliancy. naked-eye comets of great splendor and brilliancy are comparatively rare and there has been a particular dearth of such unusual comets during the past thirty years or so. the last spectacular comet, unless we make an exception of halley's periodic comet, which made its return according to prediction in , was the great comet of which was visible in broad daylight close to the sun and at its perihelion passage swept through the solar corona with a velocity that exceeded two hundred and fifty miles a second and carried it through one hundred and eighty degrees of its orbit in less than three hours. some comets approach much closer to the sun than others. the majority of all comets observed have come within the earth's orbit and no known comet has its perihelion beyond the orbit of jupiter. it is, of course, possible that there may be a number of comets that never come within the orbit of jupiter, but it is very unlikely that any such comet would ever be discovered. the majority of comets are simply small, fuzzy points of light that are only visible telescopically and the greater the perihelion distance of the comet the less likely is it to be seen with the naked eye. since comets as well as planets obey kepler's first law, known as the law of areas, and sweep over equal areas in equal times, it is evident that when a comet is at perihelion, or nearest to the sun, it is moving at maximum speed and when it is at aphelion, or farthest from the sun, it is moving at minimum speed. moreover, its speed at these two points in its orbit varies tremendously since the orbits of comets are ellipses of very high eccentricity. the speed with which the planets are traveling is, on the other hand, remarkably uniform since their orbits are nearly circular. the leisurely speed with which a comet travels through the frigid outer regions of the solar system is gradually accelerated as the comet draws nearer and nearer to the sun until it has acquired near the time of perihelion passage a velocity that occasionally exceeds two hundred miles a second. here, also, the great increase in light and heat and the strong magnetic field of the sun produce complex changes in the gaseous and meteoric substances of which the comet is composed until the characteristic tail and peculiar cometary features are fully developed. as the comet again recedes from the sun after perihelion passage its speed slackens once more. it soon parts with its tail and other spectacular features and fades rapidly from view even in the largest telescopes. xxiii meteorites meteorites, bolides or fireballs, as they are variously called, are stones that fall to the earth from the heavens. they furnish the one tangible evidence that we possess, aside from that furnished by the spectroscope, as to the composition of other bodies in space and it is a significant fact that no unknown elements have ever been found in meteorites, though the forms in which they appear are so characteristic that they make these stones readily distinguishable from stones of terrestrial origin. the origin of meteorites is not definitely known, but the evidence is very strong in favor of the theory that they are the larger fragments of disintegrated comets of which meteors and shooting stars are the smaller; the distinction between the two being simply that the latter class includes all bodies that are completely consumed by friction with the earth's atmosphere and, therefore, only reach the surface in the form of meteoric dust. according to other theories meteorites may be fragments of shattered worlds that have chanced to come too near to a larger body and have been disrupted, or they may possibly be the larger fragments of the disintegrated comets of which the meteoric swarms are the smaller. interplanetary space is not altogether a void. our own planet sweeps up in the course of a single day, it has been estimated, approximately twenty million shooting stars or meteors of sufficient size to be visible to the naked eye, while the estimate for the telescopic particles runs up to four hundred million. meteorites on the other hand are comparatively rare. on the average it has been estimated about one hundred meteorites strike the earth in the course of a year, of which number only two or three are actually seen. according to _bulletin , u. s. national museum_, approximately six hundred and fifty falls and finds of meteorites have been reported, representatives of which appear in museums and private collections. meteorites, as well as shooting stars and meteors, frequently appear in showers. in such instances the fall usually consists of several hundred or thousand individual stones and the area over which they fall is several square miles in extent and roughly ellipsoidal in shape. one of the most remarkable of such falls [...] hundred thousand stones, varying in weight from fifteen pounds to a small fraction of an ounce, fell near pultusk, poland. another remarkable fall of meteorites occurred at l'aigle, france, in . between two thousand and three thousand stones fell over an ellipsoidal area of six and two-tenths miles in greatest diameter, the aggregate weight of the stones being not less than seventy-five pounds. this fall of stones is of particular interest since it took place at a time when men were still very doubtful as to whether or not stones actually fell to earth from the heavens. after this fall had occurred in a most populous district of france in broad daylight and attended by violent explosions that lasted for five or six minutes and were heard for a distance of seventy-five miles, no reasonable doubt could longer be held as to the actuality of such phenomena. meteorites are without exception of an igneous nature, that is, they are rocks that have solidified from a molten condition. they can be classified into three groups, aerolites or stony meteorites, siderolites or stony-iron meteorites, and siderites or iron meteorites. more iron meteorites seem to have fallen in mexico and greenland than in any other part of the world--at least of its land surface. yet strange to say, of all the meteorites that have been seen to fall only nine belong to the group of siderites or iron meteorites, though the three largest meteorites known, peary's meteorite from cape york, greenland, weighing - / tons, the meteorite lying on the plain near bacubirito, mexico, weighing about tons, and the willamette, oregon, meteorite, weighing - / tons all belong to this group. moreover, all the canyon diablo meteorites, which are strewn concentrically around coon mountain crater in northern arizona to a distance of about five miles, are members of this same group. coon mountain or meteor crater itself is a perfectly round hole, about six hundred feet deep and over four thousand feet in diameter and was formed, it is believed, by the impact of a huge meteorite which has never been found. it is believed that the canyon diablo meteorites, of which there are nearly four hundred individuals in the u. s. national museum alone, were all members of this same fall. it is possible that these meteorites of the canyon diablo district, with the huge meteorite that produced the crater itself, formed the nucleus of a comet that struck the earth not more than five thousand years ago, according to the geological evidence. all iron meteorites or siderites (from the greek sideros, iron) are composed chiefly of alloys of nickel and iron. the percentage of nickel in these iron meteorites is very small, usually from five to ten per cent., while the iron forms about ninety or ninety-five per cent. of the whole. cobalt is also present in practically all iron meteorites in small quantities of per cent. or less. usually small quantities of iron sulphide and phosphide as well as graphite or some other form of carbon appear in the iron meteorites and in some instances black and white diamonds have been found, as in some of the canyon diablo irons. a very interesting and beautiful feature of many iron meteorites is the widmanstätten figures which appear when a section of such a stone is polished and treated by means of a weak acid. these figures are due to the unequal solubility of the three different alloys of nickel and iron of which the stones are composed. the irons giving the widmanstätten figures are known as octahedral irons. other irons known as hexahedral irons give figures of a different type known as neumann figures when the polished section is treated with weak acid, while other irons are so homogeneous in their composition that they show no figures at all. aerolites or stony meteorites occur more abundantly than iron or stony-iron types, and they are classified into many divisions and subdivisions according to their composition. in these stones appear certain compounds that are commonly met with in terrestrial igneous rocks. the mineral that is most abundant in the stony meteorites, composing sometimes nearly seventy-five per cent. of the stone, is a magnesium and iron silicate known as olivine, which is also usually present in terrestrial rocks of an igneous nature. certain compounds found in the stony meteorites are rarely if ever found in terrestrial rocks, however, and these serve to distinguish the stony meteorites readily from stones of terrestrial origin. the alloys of iron and nickel, for instance, that occur in minor quantities in the stony meteorites and make up usually about ninety-five per cent. of the mass of the iron meteorites, are never found in terrestrial rocks. although about thirty of the terrestrial elements are to be found in meteorites, the forms and compounds in which they appear are so characteristic and on the whole so different from those occurring in terrestrial rocks, that the analyst has no difficulty in distinguishing between the two. there are, for instance certain formations known as chondrules, peculiar spherical and oval shapes, varying in size from minute particles to objects the size of walnuts, appearing in many varieties of stony meteorites that are never found in terrestrial rocks, and that are one of the most puzzling features associated with the origin and nature of these stones. sometimes the chondrules are so loosely embedded in the stone that they fall away when it is broken. in some instances almost the entire stone is made up of these chondrules. according to one theory the chondrules were originally molten drops, like fiery rain, and their internal structure, which is greatly varied, depends upon their conditions of cooling. stony meteorites, in which these chondrules are to be found, are spoken of as chondrites. there are white and gray and black chondrites and crystalline and carbonaceous chondrites, according to the nature of the chondrules found in the stones. stony meteorites also contain minute quantities of iron and nickel alloys in the form of drops or stringers. upon entering the earth's atmosphere stony meteorites become coated with a thin black crust which is a glass formed by the fusion of its surface materials by the heat generated during its passage through the atmosphere. in many of the stony meteorites there also appear fine thread-like veins which are due to the fracturing of the stone prior to its entrance into the atmosphere. the material filling these veins is coal black in color, opaque and of an unknown composition. many meteorites show signs of collisions and encounters with other meteorites outside of the atmosphere as would be expected as they travel in swarms and groups. sometimes the entire meteorite is composed of fragments of two or more distinct stones cemented together. such a stone is spoken of as a _breccia_. in the third class of meteorites to which we now come, known as the stony-iron meteorites, there is a network or sponge of nickel-iron alloy, the interstices of which are filled with stony material. when this network or sponge is continuous the meteorite is spoken of as a stony-iron pallasite. when the network of metal is more or less disconnected the meteorite is a meso-siderite. if meteorites are heated in a vacuum, the conditions existing in interplanetary space being thus produced to a certain extent, they give forth their occluded gases and it has been found that these gases give spectra identical with the spectra of certain comets. meteoric irons give forth hydrogen as their characteristic gas while the gases occluded in the stony meteorites are chiefly the oxides of carbon, carbon monoxide and carbon dioxide. it has been found that the amount of gases contained in a large meteorite or shower of meteorites is sufficient to form the tail of a comet. these facts all tend to strengthen the belief that meteorites are indeed cometary fragments. in view of the fact that some geologists believe meteorites may be fragments of other worlds, it is of interest to know that so far no fossil-bearing meteorites have been found, and if meteorites are fragments of a shattered world, such worlds must have been reduced to a molten condition at the time of the catastrophe. the rapid passage of the meteorite through the air leaves a partial vacuum in its trail into which rush the molecules of air from all sides, producing the characteristic noises that accompany the passage of a meteorite, which have been variously compared to the rattle of artillery, the distant booming of cannons or the rumble of thunder. there may be, also, explosions of inflammable gases occluded in the crevices of the meteorite which will shatter it into fragments or the meteorite may be shattered by the resistance and pressure of the atmosphere or as a result of the extremes of temperature existing between the interior and its surface. many meteorites have actually been seen to burst into fragments in the air with a loud report. there is practically no foundation for the belief that germs of life have been brought to our planet on such igneous rocks. no microscopic examinations of meteorites have yielded any results that could be interpreted in favor of such a view. falls of meteorites are accompanied in nearly every instance by terrific explosions and sharp reports that can be heard for many miles around, often causing the ground to shake as in an earthquake. the meteorite itself has been described as resembling a ball of fire or the headlight of a locomotive, and is followed frequently by a trail of light or a cloud of smoke. at the time it enters our atmosphere a meteorite is moving with planetary velocity ranging from two to forty-five miles per second. its interior is intensely cold, approaching in temperature the absolute zero of interplanetary space, and it is, therefore, far more brittle than it would be at ordinary temperatures. as it ploughs its way into the earth's atmosphere its surface temperature is soon raised by friction to at least , ° or , ° c., which is sufficient to fuse all surface materials into the characteristic black crust, with which stony meteorites are coated. meteorites are usually first seen at an altitude of fifty or sixty miles. although they are moving with a velocity comparable to that of the planets, when they enter the earth's atmosphere, this velocity is so rapidly reduced by friction with the atmosphere that they usually drop to the surface of the earth with a velocity about equal to that of ordinary falling objects. the flight of a meteorite often extends over a path several hundred miles in length and the meteorite may be seen by many observers in several different states and yet finally fall in some unknown spot and never be found. the evidence gathered regarding the actual fall of meteorites is often contradictory. some stones are too hot to handle for hours after they fall, others are merely warm, while still others have been picked up cool or even intensely cold. meteorites have been seen to fall upon dried grass and upon straw without producing even charring effects. the evidence regarding the depths to which meteorites penetrate the ground is quite as conflicting. the largest of all the stony meteorites which fell at krnyahinya, hungary, weighed pounds and buried itself to a depth of eleven feet. yet peary's cape york iron meteorite, weighing - / tons, was only partially covered and showed no signs of abrasions of surface resulting from the fall. the willamette iron meteorite, weighing - / tons, lay in a forest when found and was not deeply buried. the bacubirito iron meteorite, weighing tons, lay in soft soil, barely beneath the level of the surface. on the other hand a fragment of a stony-iron meteorite, weighing pounds, that fell at estherville, iowa, buried itself eight feet in stiff clay. geologists in charge of the meteoric collections of various museums quite frequently have stones sent to them for analysis that are reputed to be of celestial origin. more often than not such stones are found to be purely terrestrial in their origin. the composition of a meteorite is so characteristic and unique that such a stone can never be mistaken. finds of bona-fide meteorites are on the whole extremely rare. it is also a peculiar fact that meteorites are usually observed in the months when ordinary meteors or periodic swarms of meteors are least prevalent, that is in the months of may, june and july. xxiv the earth as a magnet if a small, freely suspended compass needle is moved over a highly magnetized steel sphere, it will be seen that it constantly changes its position both horizontally and vertically so as to lie always along the "lines of force" of the sphere. there will be one point on the sphere which we will call the _north magnetic pole_, where the _north-seeking_ end of the needle will point vertically downward or make a "dip" of ° with the tangent plane. at the diametrically opposite point on the sphere, called the south magnetic pole, the opposite end of the compass, the south-seeking end, will point vertically downward; while at a point midway between the magnetic poles of the sphere the needle will lie parallel to the diameter connecting the two poles and there will be no dip. the total intensity of the magnetic field surrounding the sphere will be found to be greatest in the vicinity of the magnetic poles and least, midway between the poles. now, a freely suspended compass needle carried to all parts of the earth will behave very much in the same manner as the needle moved over the magnetized steel sphere. there are two points on the earth's surface, known as the north and south magnetic poles, where the needle points vertically downward and approximately midway between is the _magnetic equator_ where the compass needle places itself in a perfectly horizontal position and the "dip" of the needle is zero. in other words, the earth acts as a huge magnet and possesses a magnetic field with lines of force converging towards its poles similar to the lines of force of the steel sphere. there are, however, some very important differences between the sphere of steel and our earth. the matter of which the earth is composed is not homogeneous. it is believed to possess an iron core of considerable size, it is true, but its outer shell is composed of heterogeneous masses that in certain regions cause very appreciable local deflections of the needle. it is surrounded, moreover, by an atmosphere permeated by electrified particles of matter shot forth from the sun, which we now know is a still greater magnet surrounded by a magnetic field that is of the order of gausses at the poles and about eighty times more powerful than that of the earth. it is now a well-established fact that the sun's magnetic field exerts a powerful influence over the condition of the earth's magnetic field, and that vast solar disturbances affect very materially the direction and intensity of the lines of force. it is thus little wonder that this non-homogeneous and rapidly rotating terrestrial globe, surrounded by an electrified atmosphere and subject to the action of a still more powerful magnet, the sun, should not behave in a manner exactly analogous to a uniformly magnetized steel sphere. the earth's magnetic poles are neither symmetrically placed nor absolutely fixed in position. there is every reason to suspect that they shift about from year to year, and possibly fluctuate irregularly in position in the course of a few days or hours under the influence of disturbing forces. the position of the earth's north magnetic pole, last visited by amundsen in , now lies approximately in latitude ° n. longitude ° w. the position of the south magnetic pole, according to the latest determinations, is, in round numbers, in latitude ° s. and longitude ° e. of greenwich. it is evident, therefore, that the magnetic poles of the earth are not symmetrically placed and that they lie fully ° from the _geographical_ poles. the chord connecting the magnetic poles passes miles from the earth's center, and it is about , miles from the geographic pole to the nearest magnetic pole. there exist, moreover, in high latitudes local magnetic poles, due possibly to heavy local deposits of ore. one such pole was discovered at cape treadwell, near juneau, alaska, during dr. l. a. bauer's observations there in and . in the center of the observing tent at this point the needle pointed vertically downward and the compass _reversed_ its direction when carried from one side of the tent to the other. it is a well-known fact that there are very few points on the earth's surface where the compass needle points either to the true geographical pole or to the magnetic pole, and if it does chance to do so, it is a transient happening. the "variation of the compass" or the declination of the needle, as it is called, is the angle that the compass needle makes with the true north and south line or the meridian. it is an angle of greatest importance to navigators and explorers, for it gives them their bearings, yet it is unfortunately subject to ceaseless variations of a most complicated nature, since it depends on the constantly pulsating and never ceasing magnetic changes that sweep over the surface of the earth and through its crust. it is affected by long period or secular changes, as they are called, progressing more or less regularly in obscure cycles of unknown period. it is subject to a diurnal change that depends on the position of the sun relative to the meridian, and that varies with the seasons and with the hour of the day. it is affected by the sun spot cycle of . years which has a direct effect upon the intensity of the earth's magnetic field. the intensity of the magnetic field in sun spots is, according to abbot, sometimes as high as , gausses or , times the intensity of the earth's field. at times of maximum spottedness of the sun the intensity of the earth's magnetic field is reduced. moreover, when great and rapidly changing spots appear upon the sun, electrified particles are shot forth from the sun with great velocity and in great numbers, and are drawn in towards the magnetic poles of the earth. meeting the rarefied gases of the earth's upper atmosphere, they illuminate them as electric discharges illuminate a vacuum tube. some of these electrons are absorbed by gases at high elevations, other descend to lower levels. the most penetrating rays have been known to descend to an altitude of twenty-five miles which is about the lowest limit yet found for auroral displays. it is the passage of these rays through the atmosphere that cause the magnetic disturbances known as _magnetic storms_, that are associated with the appearance of great sun spots and auroral displays. at such times sudden changes take place in the intensity of the earth's magnetic field that cause the compass needle to shiver and tremble and temporarily lose its directive value. these _magnetic storms_ have been known to produce great temporal changes in the intensity of the earth's field. according to dr. l. a. bauer, director of the department of terrestrial magnetism of the carnegie institute of washington, the earth's intensity of magnetization was altered by about one-twentieth or one-thirtieth part by the magnetic storm of september , , which was one of the most remarkable on record, and the earth's magnetic condition was below par for fully three months afterwards as a result. in addition to these various regular and irregular changes in the variation of the compass, or declination of the needle, due to changes in the earth's magnetic field _as a whole_, there are local effects due to restricted regional disturbances of the earth's magnetic field or to local deposits of ore, or to volcanoes or other local causes. the effect of all these disturbances upon the declination of the needle must be determined by continual magnetic surveys of all portions of the earth's surface. as a whole the earth's magnetic field is more uniform over the oceans than over the land, with all its disturbing topographical features. yet this advantage is offset largely in navigation by the fact that every steel ship that sails the seas is a _magnet_, with its two magnetic poles and its neutral line where the two opposite magnetic forces are neutralized, as is the case with every magnet. the direction in which a steel ship lies with reference to the earth's magnetic field while it is being built determines the position of the magnetic poles in its hull and the position of its neutral line and this distribution of magnetism over a ship's hull must be taken account of in the installation of its standard compass. every piece of horizontal and vertical iron aboard ship has an effect upon the variation of the compass and compensation must be made for such disturbing forces. the direction of sailing, the position in which a ship lies at dock, storms encountered at sea, the firing of batteries (on warships) are some of the factors that are operative in producing changes in the variation of the magnetic compass aboard a ship. every ship must undergo at frequent intervals magnetic surveys for the purpose of determining its magnetic constants and its "table of deviations of the compass." the direction in which the compass needle points aboard ship is the _resultant_ of the effect of the earth's magnetic field and the magnetic field of the ship, and both fields are subject to continual and complicated variations from year to year, from day to day, _and even from hour to hour_! the elements of the earth's magnetic field are determined for any one epoch by long-continued magnetic surveys carried on to a greater or less extent by the various nations of the world, and the results are published in the form of _magnetic charts_ for land and sea, showing the values of the three magnetic elements, declination of the needle, dip or inclination, and horizontal intensity of the earth's field for a definite period. so rapid are even the long-period changes in the earth's magnetic field that a magnetic chart can be relied upon for only a very few years and fresh data for the construction of these charts that are so valuable to navigators and explorers must be gathered continually. the _department of terrestrial magnetism_ of the carnegie institute of washington is engaged in continual magnetic surveys of the earth by land and sea that are of the highest value not only to navigators but also to scientists interested in solving the great and mysterious problem of the underlying causes of the earth's magnetism. to give an idea of the extent and scope of the work of this department it may be mentioned that its non-magnetic ship _carnegie_ made in the period - a total run of , nautical miles, nearly nine times the earth's circumference, with an average day's run of nautical miles. magnetic observations were made practically every day at a distance of to miles apart. in this nine-year period five cruises were made. on her first cruise the _carnegie_ sailed from st. john's, newfoundland, to falmouth, england, over practically the same course followed by the famous astronomer, halley, in the _paramour pink_ two centuries earlier to determine the variation of the compass. in her fourth voyage the _carnegie_ circumnavigated the world in sub-antarctic regions in days--a record time. she has traversed all oceans from ° north to the parallel of ° south and has crossed and recrossed her own path and the path of her predecessor, the _galilee_, many times, thus making it possible to determine for the points of intersection the secular changes in the magnetic elements. after a thorough overhauling in and the installation of a four-cylinder gasoline engine, _made of bronze_ throughout, to take the place of the producer-gas engine used on earlier cruises, the _carnegie_ started on her sixth cruise with a crew of twenty-three officers and men on october , . a cruise of , miles was planned in the south atlantic, indian and pacific oceans to last approximately two years. unsurveyed regions in the south atlantic and indian ocean were to be covered and the route was planned so as to obtain a large number of observations of the progressive changes that have taken place in the magnetic elements. this is accomplished as stated above by intersecting former routes and obtaining new values of the element at the points of intersection. in addition to its ocean magnetic surveys the _department of terrestrial magnetism_ also carries on extensive land surveys in all parts of the globe. in special expeditions were sent out by the department to observe the total solar eclipse of may th at stations distributed over the entire zone of visibility of the eclipse and immediately outside. at dr. bauer's station in liberia the total phase was visible in a cloudless sky for more than six minutes, which is very close to the maximum length of phase that can possibly be observed. unmistakable evidence was gathered at all stations of an appreciable variation in the earth's magnetic field during a solar eclipse, which variation is the reverse of that causing the daylight portion of the solar diurnal variation of the needle. in addition to the magnetic survey work on land and sea which is the chief work of the _department of terrestrial magnetism_, atmospheric-electric observations are carried on continually on land and sea and experiments have been carried on at langley field, va., lately, in the development of methods and instruments for determining the geographical position of airplanes by astronomical observations. there has also been recently formed under this department a _section of terrestrial electricity_. the cause of the earth's magnetic field is still one of the greatest unsolved problems of astro-physics. the theory that has been advanced by schuster that all large rotating masses are magnets _as a result of their rotation_ has received considerable attention from astrophysicists, and attempts have been made to prove this experimentally. it has been found that iron globes spun at high velocities in the laboratory do _not_ exhibit magnetic properties. this may mean simply that the magnetic field is too weak to be detected in the case of a comparatively small iron sphere spun for a limited period under laboratory conditions. it must be remembered that the earth has been rotating rapidly on its axis for millions of years and is, compared to terrestrial objects, an extremely large mass. yet it has been shown that as a whole our earth is an extremely weak magnet, and that if it were made entirely of steel and magnetized as highly as an ordinary steel-bar magnet, the magnetic forces at its surface would be a thousand times greater than they actually are. if it is true that all rotating bodies are magnets, then all the heavenly bodies, planets, suns and nebulæ are surrounded by magnetic fields. we know nothing to the contrary. in fact, we know this to be true for the earth and sun, and strongly suspect that it is so in the case of the planets jupiter and saturn. when we understand more about the properties of matter, the nature of magnetism, as well as of gravity, may be revealed to us. xxv some effects of the earth's atmosphere upon sunlight it is impossible to exaggerate the importance of the atmosphere to all forms of life upon the surface of the earth. if there were no atmosphere there would be no life, because it is through the agency of the water-vapor, carbon-dioxide and oxygen in the atmosphere that all life-processes are maintained. if there were no atmosphere there would not only be no life upon the earth; there would be also none of the beautiful color effects produced by the passage of sunlight through the atmosphere. there would be no blue skies, no beautiful sunrise and sunset effects, no twilight, no rainbows, no halos, no auroral displays, no clouds, no rains, no rivers nor seas, no winds nor storms. the heavens would be perfectly black except in the direction of the heavenly bodies which would shine as brilliantly by day as by night. to understand how the atmosphere produces color effects such as blue skies, sunrise and sunset tints, rainbows and halos, as well as the twinkling of the stars, and numerous other phenomena, we must know something of the nature of light itself. light moves outward from any source, such as the sun, in all directions radially, or along straight lines (so long as it does not encounter a gravitational field) with the unimaginable velocity of , miles per second. as it advances it vibrates or oscillates back and forth across its path in all directions at right angles to this path, unless it is plane polarized light, in which case its vibrations are confined to one plane only. these vibrations or oscillations of light take the form of a wavelike motion, one wave-length being the distance passed over in the time of one vibration, measured from crest to crest or from trough to trough of adjacent waves. we may consider that a beam or ray of sunlight is made up of a great number of individual rays of different wave-lengths and different colors. the average wave-length of light, the wave-length of the green ray in sunlight, is about one-fifty-thousandth part of an inch, that is, it would take about fifty-thousand wave-lengths of green light to cover a space of one inch. now, since light makes one vibration in passing over a distance of one wave-length, it makes fifty thousand vibrations, while advancing one inch, and since it advances one hundred and eighty-six thousand miles in one second we can easily figure out that a ray of sunlight of average wave-length makes about six hundred trillion vibrations ( , , , , ) in a single second! the chief colors of which sunlight or white light is composed are red, orange, yellow, green, blue, indigo and violet, though there are an infinite number of gradations of color which blend into one another, gradually producing the intermediate tints and shades. the colors just mentioned are called the primary colors of the solar spectrum, which can be produced as a band of light of variegated colors, arranged in the order named by passing a ray of ordinary sunlight through a glass prism. the individual rays of different color and wave-length that make up a beam of sunlight, or white light, then separate out in the order of the wave-lengths. the red rays vibrate the most slowly and have the longest wave-length of all the rays of the visible spectrum. about four hundred trillion vibrations of red light reach the eye in one second. violet rays, on the other hand, vibrate the most rapidly of all the visible rays and have the shortest wave-length. about eight hundred trillion vibrations of violet light reach the eye every second. the wave-lengths of the intermediate colors decrease in length progressively from the red to the violet and, of course, the frequencies of their vibrations increase in the same order. all sunlight is made up of these rays of different colors and different vibration frequencies, and of other rays as well, to which the human eye is not sensitive, and which, therefore, do not appear in the visible spectrum. among these invisible rays are the infra-red rays which come just below the red of the visible spectrum, and which are of longer wave-length than the red rays, and the ultra-violet rays, which lie beyond the violet rays of the visible spectrum, and are of shorter wave-length than the violet rays. now a ray of ordinary sunlight is separated into the rays of various colors, which form the solar spectrum when it passes from a medium of one density obliquely to a medium of another density, as when it passes from air to glass, or from air to water, or from outer space into the earth's atmosphere. under such circumstances its velocity is slowed down when it passes from a rare to a denser medium, and the waves of different wave-lengths are bent from their former course, or refracted, by different amounts. the red rays, of longest wave-length, are bent from their former course the least, and the violet rays, of shortest wave-length, are bent the most upon passing from a rare to a denser medium. as a result the ray of sunlight is spread out or dispersed into its rays of different wave-length and color upon entering a medium of different density. it is this refraction and dispersion of sunlight that produces many color effects in the earth's atmosphere. the atmosphere is not of uniform density throughout. at high altitudes it is extremely rare. that is, there is little of it in a given volume. close to the earth's surface, however, it is comparatively dense. half of all the atmosphere is within three and one-half miles of the surface and half of the remainder lies within the next three and a half miles. we may consider it as made up, on the whole, of layers of different densities, strongly compressed near the surface. imagine a ray of sunlight entering the earth's atmosphere from without. if it comes from a point in the zenith its course is not changed upon entering the atmosphere, because light passing from a certain medium--as space--into a medium of different density, is not bent from its course, or refracted, provided it enters the new medium in a direction perpendicular to the surface. if it enters the atmosphere (which is the new medium of greater density) _obliquely_, refraction, or bending of the ray, takes place, and as the ray advances toward the earth, through layers of increasing densities, it is bent from its former course more and more. as the advancing rays of different colors and wave-lengths in the beam of sunlight are slowed down in the new medium, the red rays are turned from their course the least and the violet rays the most and the entire advancing wave-front of the beam of sunlight is bent down more and more toward the horizon, as it proceeds through the atmosphere. as we on the earth's surface see the ray not along its bent course through the atmosphere, but in the direction in which it finally enters our eyes, the effect of refraction upon a ray of light passing through the atmosphere is to displace the object in the direction of the zenith or increase its distance above the horizon. as a result of refraction we see the sun--or moon--above the western horizon after it has really set, and above the eastern horizon before it has really risen. the oval shape that the sun, or moon, often presents on rising or setting, is due to the fact that the light from the lower limb is passing through denser air than the light from the upper limb, and so is refracted more. as a result the lower limb is lifted proportionately more than the upper limb. this distorts the form of the solar or lunar disk, making it appear oval instead of circular. the familiar twinkling or scintillation of stars and, more rarely, of the planets, is a result of interference of light waves due to irregular and variable refraction in air that is not uniform in density, owing to the presence of constantly rising and descending atmospheric currents of different densities. this also produces the shimmering or unsteadiness of star images in the telescope, that interferes so greatly with accurate measurements of angles or observations of planetary markings. one may ask why it is, if light from an object, say a star, is bent from its course and separated into rays of various colors upon entering the earth's atmosphere, that we do not see the object drawn out into a band of spectral colors. it is because the angular separation of the various colors is so slight under ordinary circumstances that light from one point is blended with light from a neighboring point of complementary color to produce white light again. under certain circumstances, however, beautiful color effects may be seen in the earth's atmosphere as a result of the refraction of sunlight. the blue color of the sky and its brightness is caused by the scattering of the rays of shortest wave-length, the violet and blue rays, by the oxygen and nitrogen in the upper atmosphere. the molecules of these gases interfere with the passage of these rays, powerfully scattering and dispersing them, and thus increasing the length of their path through the air and diffusing their color and brightness in the upper atmosphere, while permitting rays of longer wave-length, the red and orange, to pass on practically undisturbed. when an object in the heavens lies close to the horizon, the rays of light from it have to travel a longer path through the atmosphere than when the object is overhead, and that too through the densest part of the atmosphere, which lies close to the earth's surface, and in which are floating many dust particles and impurities from the earth's surface. all these particles, as well as the increased density of the atmosphere, interfere with the free passage of the rays, especially of shorter wave-lengths. the violet and blue rays are sifted out and scattered in their long journey through the lower strata of air, far more than when they come to us from an object high in the sky. even the red and yellow rays are more or less scattered and bent aside--diffracted--by these comparatively large particles near the surface. the reddish color of the sun, moon and even of the stars and planets, when seen near the horizon, as well as the beautiful sunset tints, in which reds and pinks and yellows predominate, are due to the fact that the rays of longer wave-length are more successful in penetrating the dense, dust-laden layers of the lower atmosphere. it is to be free of the dust and impurities as well as the unsteadiness of the lower atmosphere, that observatories are built at high altitudes whenever possible. when there have been unusually violent volcanic eruptions, and great quantities of finely divided dust have been thrown into the upper atmosphere, the effect upon the blue and violet rays from the sun is very great. the volcanic dust particles are so large that instead of scattering these rays of shorter wave-length, as do the oxygen and nitrogen in our atmosphere, they reflect them back into space and so decrease the amount of light and heat received from the sun. for this reason the general temperature of the earth is lowered by violent volcanic eruptions. unusually cold periods, that lasted for months, followed the terrible eruption of krakatoa in and of katmai in . at times when much dust is present in the atmosphere, the sky is a milky white color by day as a result of the reflection of sunlight from the dust particles. sunrise and sunset colors are then particularly gorgeous, with reds predominating. at such times the blue and violet rays are almost completely shut out, and the red, orange and yellow rays are powerfully diffracted and scattered by the dust particles in the air. the twilight glow that is visible for some time before sunrise or after sunset is, of course, entirely an atmospheric effect caused by the reflection of sunlight to our eyes from the upper atmosphere, upon which the sun shines, while it is, itself, concealed from our view below the horizon. the atmosphere extends in quantities sufficient to produce twilight to an elevation of about sixty miles. when _all_ the rays of which sunlight is composed are reflected in equal proportions we get the impression of white light. dust and haze in the air reflect all rays strongly and give a whitish color to an otherwise blue sky. brilliant white clouds appear white, because they are reflecting all rays equally. clouds or portions of clouds appear black when they are in shade or, at times, by contrast with portions that are more strongly illuminated, or when they are moisture-laden and near the point of saturation, when they are absorbing more light than they reflect. at sunrise and sunset, when the light that falls upon the clouds is richest in red and orange and yellow, clouds reflect these colors to our eyes, and we see the brilliant sunset hues which are more intense the more the air is filled with dust and impurities. the familiar and beautiful phenomenon of the rainbow is produced by refraction, reflection and interference of sunlight by drops of falling water, such as rain or spray. as the ray of sunlight enters the drop of water, which acts as a tiny glass prism, it is refracted or bent from its course and spread out into its spectral colors. reflection of these rays next takes place (once or twice, as the case may be) from the inside of the drop and a second refraction of the reflected ray takes place as it leaves the drop. the smaller the drops the more brilliant is the rainbow and the richer in color. the most brilliant rainbows are produced by drops between . and . millimeters in diameter. in addition to the primary bow, which has a red outer border with a radius of °, there is the secondary bow with a radius of about ° and with colors reversed, the red being on the inner border; the supernumerary bows which are narrow bands of red, or green and red, appear parallel to the primary and secondary bows along the inner side of the primary bow and the outer side of the secondary bow. no rainbow arches ever appear between the primary and secondary bows, and it can be shown in fact, that the illumination between these two bows is at a minimum. the primary, secondary and supernumerary bows all lie opposite the sun in the direction of the observer's shadow and the observer must stand with his back to the sun in order to see them. the primary and secondary rainbow arches take the form of arcs of circles that have their common center on the line connecting the sun with the observer at a point as far below the horizon in angular distance as the sun is above the horizon. it is, therefore, never possible to see a rainbow arch of more than a semicircle in extent unless the observer is at an elevation above the surrounding country, under which circumstances it might be possible to see a complete circle formed by the rainbow. the highest and longest arch appears when the sun is on the horizon, and the greater the altitude of the sun the smaller and lower the visible arch. as the angular radius of the primary bow is ° and of the secondary bow ° and as the common center of the two circles is always as far below the horizon as the sun is above, it is never possible to see either primary or secondary rainbow when the altitude of the sun is over °, or the primary bow when the altitude is over °. for this reason rainbows are rarely seen at or near noon in mid-latitudes, since the sun is usually at an elevation of more than ° at noon, especially in the summer season, which is also the most favorable season for rainbows, owing to the great likelihood of rain and sunshine occurring at the same time. the light which comes to an observer from the primary bow is once reflected within the drop, and that which comes from the secondary bow is twice reflected within the drop. the sharper and brighter light therefore comes from the primary bow of ° radius. the space between the two bows is particularly dark, because it can be shown that the drops there do not reflect any light at all. the rainbow colors are rarely pure or arranged in spectral order, owing to interference of light waves. it is the interference of light waves from different parts of the same drop that produces the bands of alternate maximum and minimum brightness, that lie below the primary bow and beyond the secondary bow. the red or green and red bands of maximum brightness produced thus by interference, are called the supernumerary bows, and they are always found parallel to the primary and secondary bow within the former and above the latter. the distance of the rainbow from the observer is the distance of the drops that form it. a rainbow may be formed by clouds several miles distant or by the aid of the garden hose on our lawn. no two observers can see exactly the same rainbow because the rainbow arch encircles the surface of a cone whose vertex is at the observer's eye and no two such vertices can exactly coincide. two observers see rainbows formed by different drops. refraction of light by ice-crystals in clouds produces many beautiful color effects, such as halos of various types around sun or moon, vertical light pillars, circumzenithal arcs, and parhelia--"sun-dogs"--or paraselenæ--"moon-dogs"--which are luminous spots at equal altitudes with sun or moon--one to the left the other to the right, at an angular distance of °. the most usual form of halo is that of ° radius. this is a luminous ring of light surrounding sun or moon, with the inner edge red and sharply defined and the spectral colors proceeding outward in order; red is frequently the only color visible, the remainder of the ring appearing whitish. since the halo is produced by refraction of light by ice-crystals, which exist in clouds of a certain type gathering at high altitudes, it is always a very good indicator of an approaching storm. coronas are luminous rings showing the spectral colors in the reverse order, that is, with the inner edge blue instead of red. they are usually of very small radius, scarcely two degrees, closely surrounding sun or moon and are produced--not by refraction--but by _diffraction_ or a bending aside of the rays as they pass between--without entering--very small drops of water in clouds. as in the case of refraction, the red rays are turned from their course the least and the violet rays the most. many of these phenomena--halos, luminous spots, vertical pillars and arcs of light may, at times, be seen simultaneously, when clouds of ice-crystals are forming around the sun or moon. they then present a very complex and beautiful outline of luminous circles, arches and pillars that have a mysterious and almost startling appearance when the cause is not clearly understood. we have found then that sunlight is made up of rays of many different wave-lengths and colors and that the atmosphere acts upon these rays in various ways. it reflects them or turns them back on their course; it refracts them as they pass through the gases of which the atmosphere consists, or through the water-vapor and ice-crystals suspended in it, thus sifting out and dispersing the rays of different colors and wave-lengths and producing beautiful color effects; it _diffracts_ them or bends them aside as they pass between the fine dust particles and small drops of water in the air, again sifting out the rays of different colors and producing color effects similar to those produced by refraction; it also scatters and disperses, through the action of the molecules of oxygen and nitrogen in the upper strata, the blue and violet rays of shorter wave-length and thus produces the blue color and brightness of the sky; it produces beautifully colored auroral streamers and curtains and rays of light through the electrical discharges resulting when the rarefied gases in the upper air are bombarded by electrified particles shot forth from the sun. it is our atmosphere, then, that we have to thank for all these beautiful displays of color that delight our eyes and give pleasure to our existence, as well as for the very fact of our existence upon a planet that without its presence would be an uninhabitable waste, covered only with barren rocks. xxvi keeping track of the moon of all celestial objects the nearest and most familiar is our satellite, the moon. yet the mistakes and blunders that otherwise intelligent persons frequently make when they refer to the various aspects of the moon are quite unbelievable. who has not read in classics or in popular fiction of crescent moons riding high in midnight skies, of full moons rising above western cliffs or setting beyond eastern lakes? who has not seen the moon drawn in impossible positions with horns pointing toward the horizon, or a twinkling star shining through an apparently transparent moon? careful observation of the moon in all its various phases and at different seasons is the best method to be used in acquiring a knowledge of the elementary facts regarding the motion of the moon through the heavens from day to day, but that requires that one be up often after midnight and in the early hours preceding dawn and so it is that we feel so hazy in regard to what happens to the moon after it has passed the full. a few fundamental rules can be easily acquired, however, and these will enable us to locate the moon in the right quarter of the heavens at any time of the day or night when we know its phase and the approximate position of the sun at the same instant, and thus we may avoid some of the most obvious blunders that are made in dealing with the general aspect of the moon at any given time. as can be verified by direct observation, the moon is always moving continually eastward. since it makes a complete revolution around the earth from new moon back to new moon again in a little less than thirty days, it passes over about twelve degrees a day ( ° divided by ), on the average, or one-half a degree an hour, which is about the angular extent of its own diameter. therefore every hour the moon moves eastward a distance equal to its own diameter. this is of course only approximate as the moon moves more rapidly in some parts of its orbit than in others. in addition to its real eastward motion the moon shares the apparent daily westward motion of all celestial objects which is due to the daily rotation of the earth on its axis in the opposite direction. that is, the moon, as well as the sun, stars and planets, rises in the east and sets in the west daily. on account of its continuous eastward motion, however, the moon rises later every night, on the average about fifty minutes, though the amount of this daily retardation of moon-rise varies from less than half an hour to considerably over an hour at different seasons of the year and in different latitudes. in the course of a month then the moon has risen at all hours of the day and night and set at all hours of the day and night. it might seem unnecessary to emphasize the fact that the moon always rises in the east were it not that the astronomer occasionally meets the man who insists that he has at times seen the moon rise in the west. to be sure the new crescent moon first becomes visible above the western horizon shortly after sunset though it rises in the east the morning of the same day shortly after sunrise. as is also true of the sun the exact point on the horizon where the moon rises or sets varies from day to day and from season to season. in one month the moon passes over very nearly the same path through the heavens that the sun does in one year, for the moon's path is inclined only five degrees to the ecliptic or apparent path of the sun through the heavens. it can never pass more than - / ° ( - / ° + °) south of the celestial equator, nor more than - / ° north of it. it has a slightly greater range in altitude than the sun, therefore. north of - / ° north latitude it always crosses the meridian south of the zenith and below - / ° south latitude it crosses the meridian north of the zenith. in tropical regions the moon sometimes passes north of the zenith, sometimes south, or again directly through the zenith. since the full moon is always diametrically opposite to the sun it passes over nearly the same part of the heavens that the sun did six months before. in winter then when the sun is south of the equator the moon "rides high" at night north of the equator and, vice versa, in summer when the sun is north of the equator the full moon "rides low" south of the equator. in winter then we have more hours of moonlight than we have in summer. this may be of no great advantage in mid-latitudes but we may imagine what a boon it is to the inhabitants of the arctic and antarctic regions to have the friendly moon above the horizon during the long winter months when the sun is never seen for days at a time. at time of "new" moon the moon lies directly between us and the sun, but ordinarily passes just to the north or south of the sun since its orbit is inclined five degrees to the ecliptic or plane of the earth's orbit. if the moon's path lay exactly in the ecliptic we would have an eclipse of the sun every month at new moon and an eclipse of the moon two weeks later at full moon. now the moon crosses the ecliptic twice a month, the points of crossing being called the nodes of its orbit, but only twice a year is the moon nearly enough in line with the sun at the time it crosses to cause eclipses. every year, then, there are two "eclipse seasons," separated by intervals of six months, when the moon is in line with the sun at or close to the point where it crosses the ecliptic; then and only then can solar and lunar eclipses occur. the solar eclipses, of course, will occur when the moon is new, that is, when the moon passes directly between the earth and the sun and throws its shadow over the earth; and the lunar eclipses two weeks later when the earth passes between the sun and moon and throws its shadow over the face of the moon. probably there is no astronomical subject that has been more generally misunderstood than that of solar and lunar eclipses. it is well to remember that solar eclipses can only occur at time of new moon and lunar eclipses only at the time of full moon; and at the time of eclipses, whether lunar or solar, the moon is at or near its nodes, the points where its orbit crosses the ecliptic. there are always at least two solar eclipses every year and there may be as many as five. there are years when there are no lunar eclipses, though ordinarily both solar and lunar eclipses occur every year, some partial others total. the moon shines only by reflected sunlight. it is of itself a solid, dark body with its day surface intensely hot and its night surface intensely cold, a world of extreme temperatures. at new moon all of the night side of the moon is turned toward us, at full moon all of the day side. at other phases we see part of the day side and part of the night side and the illuminated side of the moon is always the side that is towards the sun. failure to observe this simple rule leads to many grievous blunders in depicting the moon. at the time of new moon the moon, moving continually eastward, passes north or south of the sun from west to east except when it passes directly in front of the sun, causing eclipses. a day or so later the waxing crescent moon or the "new moon," as it is popularly called, becomes visible low in the west immediately after sunset. the moon is now east of the sun and will remain east of the sun until the time of full moon. during the period from new moon to full moon it will, therefore, rise after the sun and set after the sun. the waxing crescent moon will not be visible in the morning hours because, inasmuch as it rises after the sun, it is lost to view in the sun's brilliant rays. nevertheless, it follows the sun across the sky and becomes visible in the west as soon as the sun has disappeared below the western horizon. the thin illuminated crescent has its horns or cusps turned _away_ from the point where the sun has set. the horns of the crescent can never point _toward_ the horizon since the illuminated side of the moon is always turned toward the sun whether the sun is above or below our horizon. as hour by hour and day by day the moon draws farther eastward and increases its angular distance from the sun, more and more of the illuminated side becomes visible; the crescent increases in width and area and the moon appears higher in the western sky each night at sunset. usually about seven and a fraction days after the date of new moon the moon completes the first quarter of its revolution around the earth. the period from one phase to the next is variable and irregular, being sometimes less than seven days and at other times more than eight days, since the moon does not move at a uniform rate in different parts of its orbit. when the moon has completed the first quarter of a revolution it is ninety degrees east of the sun and presents the phase known as "half-moon" since half of the surface that is turned toward the earth is illuminated and half is in darkness. it is said to be "at the first quarter." the illuminated half is of course the western half because the sun is to the west of the moon. the half moon is near the meridian at sunset and sets near midnight. up to the first quarter, then, the moon is a crescent in the western sky during the first part of the night and should never be represented as east of the meridian or near the meridian at midnight. after the moon has passed the first quarter and before it is full more than half of the side turned toward the earth is illuminated and it is in the "gibbous" phase. it is still the western limb that is fully illuminated. the moon is now east of the meridian at sunset and it crosses the meridian before midnight and sets before sunrise. all who are abroad during the first half of the night find this phase of the moon more favorable to them than the gibbous phase following full moon. the moon now being above the horizon at sunset is visible continuously from sunset to midnight but sets some time during the second half of the night, while the full moon shines throughout the night, rising in the east at sunset and setting in the west at sunrise. when the moon is full it is ° east, or west, of the sun and so both its eastern and western limbs are perfectly illuminated. after the full the moon goes through its phases in reverse order, being first gibbous, then a half-moon once more, and lastly a waning crescent. it is now west instead of east of the sun and so it is the eastern limb that is fully illuminated by the sun. being west of the sun it will now rise before the sun and set before the sun, the interval decreasing each day as the moon draws in toward the sun once more. the gibbous phase preceding full moon is favorable to all abroad before midnight but the gibbous phase following full moon is more favorable to those who are abroad after midnight, for from full moon to last quarter the moon is below the horizon at sunset, and of course, is rising later and later each night, while at sunrise it is still above the horizon, appearing each day higher and higher above the western horizon at sunrise as it approaches the third or last quarter. when it has reached this point it is once more a half-moon, though now it is the eastern half instead of the western half of the disk that is fully illuminated. the moon is ° west of the sun at third quarter and from this phase to the phase of new moon it is a crescent once more, but now a waning instead of a waxing crescent. it appears east of the meridian before sunrise and as the crescent grows thinner it draws nearer and nearer to the eastern horizon and the rising sun. as with the waxing crescent moon the horns are turned away from the horizon. the waning crescent moon is always to be looked for east of the meridian and to be associated with the rising sun, while the waxing crescent moon is to be looked for west of the meridian and associated with the setting sun. neither the waxing nor the waning crescent moon will be visible during the midnight hours. as the waning crescent moon grows thinner and draws in closer to the sun each successive night, its time of rising precedes that of the sun by an ever-decreasing interval until finally the crescent disappears from view in the eastern sky; the next day we see no crescent either in the eastern or western skies--the moon is once more in conjunction with the sun and "new." one revolution of the moon about the earth with respect to the sun has been completed and a day or so later we may look for a new crescent moon in the western sky after sunset. xxvii the motions of the heavenly bodies about three hundred and twenty years ago giordano bruno was burned at the stake for his audacity in believing in the existence of other worlds. a few decades later the famous astronomer galileo was forced to publicly recant his belief that the earth moved. yet the truth could not long be suppressed by such means, and since those dark days man's advance in knowledge has been so rapid that it seems to us today in this wonderful age of scientific discovery almost inconceivable that man ever believed that the earth, a tiny planet of a vast solar system, was "the hub of the universe," the fixed and immovable center about which revolved all the heavenly bodies. very reluctantly, however, and with bitter feeling, but in the light of overwhelming evidence man finally gave up his long-cherished idea of terrestrial importance, and when finally forced to move his fixed center of the universe, he moved it only so far as the comparatively nearby sun. this center he then regarded as fixed in space and also held to his belief that the stars, set in an imaginary celestial sphere, were immovable in space as well, and all at the same distance from the sun. so, scarcely two hundred years ago we find that the astronomer bradley was endeavoring to measure this common distance of the "fixed stars." though he failed in this attempt he made the important discovery that the observed positions of the stars are not their true positions, owing to the fact that the velocity of light is not infinite but takes a definite finite interval of time to travel a given distance. as a result the stars always appear displaced in the direction of the earth's motion around the sun, the amount of the displacement depending upon the velocity of the earth in its orbit and the velocity of light. this "aberration of light," as it is called, furnished additional proof that the earth revolves about the sun and was one more nail driven into the coffin of the old ptolemaic theory that the earth was the center of the universe. bradley also discovered that the positions of the stars were affected by the wabbling of the earth's axis, called its "nutation." although in the days of bradley neither the methods of observation nor the instruments were sufficiently accurate to show the minute shifts in the positions of the stars that reveal the individual motions of the stars and the distances of those nearest to us, yet the discovery of the two large displacements in the positions of all the stars, due to the aberration of light and the nodding of the earth's axis were of the greatest value, for they were a necessary step in the direction of the precise measurements of modern times. it is only through measurements of the greatest refinement and accuracy that it is possible to detect the motions and distances of the stars and to discover the wonderful truths about the nature and structure of the universe that they are revealing to us today. after unsuccessful attempts extending over several centuries the distance of one of the nearest stars, the faint cygni, as it is catalogued, was finally determined by the astronomer bessel in the year . this star is about ten light-years distant from the earth, which places it about six hundred and thirty thousand times farther away from us than the sun; that is, we would have to travel six hundred and thirty thousand times the distance from the earth to the sun to reach this very close stellar neighbor, cygni. the _nearest_ of all the stars, alpha centauri, is over two hundred and seventy thousand times the distance from the earth to the sun. it is, therefore, little wonder that the early astronomers believed that the stars were fixed in space since even the nearest is so far away that, viewed from opposite points in the earth's orbit, its apparent change in position due to our actual change in position of , , miles, amounts to only one and a half seconds of arc. two stars separated by _one hundred and sixty times_ this angular distance might possibly be glimpsed as two distinct stars by a person with good eyesight, though to most of us they would appear as one star. upon the measurement of such minute angles depended a knowledge of the distances of the nearest stars. it is to sir william herschel that we owe the discovery, more than a hundred years ago, of the motion of the sun through the universe. from the consideration of a long series of observations of the positions of the stars this famous astronomer discovered that the stars in the direction of the constellation hercules were separated by much greater angular distances than the stars diametrically opposite in the heavens. in other words, the stars were spreading apart in one portion of the heavens and crowding together in the opposite direction and he rightly interpreted this to mean that the sun was moving in the direction of the constellation of hercules. it was not until the spectroscope was applied to the study of the heavens in the latter part of the nineteenth century that the amount of this motion of the sun was found to be about twelve and a half miles per second, or four times the distance from the earth to the sun in a year. it is to sir william herschel that we owe also the discovery of binary systems of stars in which two stars swing around a point between them called their center of gravity. [illustration: spiral nebula in canes venatici taken with -inch reflector of the mt. wilson observatory] our first conception of the immensity and grandeur of the universe dates from the time of the older herschel only a century or so ago. the mysterious nebulæ and star clusters were then discovered, the wonders of the milky way were explored, and a new planet and satellites in our own solar system were discovered. it was found that the sun and the stars as well as the planets were in motion. neither sun nor earth could be regarded any longer as a fixed point in the universe. with the application of the spectroscope to the study of the heavens toward the end of the nineteenth century the key to a treasure-house of knowledge was placed in the hands of the astronomers of modern times and as a result we are now learning more, in a few decades, about the wonders and mysteries of the heavens than was granted to man to learn in centuries of earlier endeavor. yet it is the feeling of the astronomer of today that he is only standing on the threshold of knowledge and that the greatest of all discoveries, that of the nature of matter and of time and space is yet to be made. it is the spectroscope that tells us so many wonderful facts about the motions of the stars, nebulæ and star clusters. it tells us also practically all we know about the physical condition of our own sun and of the other suns of the universe, their temperature and age, and the peculiarities of their atmospheres. some of the most important astronomical discoveries that have been made in the past few years have to do with the distribution and velocities of the heavenly bodies as revealed by the spectroscope. it has been found, with the aid of the spectroscope, that the most slowly moving of all stars are the extremely hot bluish orion stars with an average velocity of eight miles per second, while the most rapidly moving stars are the deep-red stars with an average velocity of twenty-one miles per second, and there is in all cases a relationship existing between the color, or spectrum, of a star and its velocity. the reason for this connection between the two still remains undiscovered. the spectroscope has also told us some astonishing facts in recent years about the velocities of the spiral nebulæ. it is now known that these mysterious objects are moving with the tremendous average velocity of _four hundred and eighty miles per second_, which exceeds the average velocity of the stars fully twenty-five fold. they possess, moreover, internal motions of rotation that are almost as high as their velocities through space. it is now generally believed that spiral nebulæ are far distant objects of enormous size and mass, exterior to our own system of stars and similar to it in form. in place of the universe of the "fixed stars" and the immovable sun or earth of a few centuries ago we find that modern astronomical discovery is substituting a universe of inconceivable grandeur and immensity in a state of ceaseless flux and change. our earth--an atom spinning about on its axis and revolving rapidly around a huge sun that is equal in volume to more than a million earths--is carried onward with this sun through a vast universe of suns. only an average-sized star among several hundred million other stars is this huge sun of ours, moving with its planet family through the regions of the milky way, where are to be found not only moving clusters and groups of stars, speeding along their way in obedience to the laws of motion of the system to which they belong, but also strangely formed nebulæ covering vast stretches of space, whirling and seething internally and shining with mysterious light, and still other stretches of dark obscuring matter shutting off the rays of suns beyond. the extent and form of this enormous system of stars and nebulæ and the laws that govern the motions of its individual members are among the problems that the astronomers of today are attempting to solve. on both sides of these regions of the milky way, wherein lies our own solar system, lie other vast systems, such as the globular star clusters, composed of thousands, possibly hundreds of thousands, of suns; the magellanic clouds, which resemble detached portions of the milky way, and, probably, the much discussed spiral nebulæ, possible "island universes" similar to our own. we have come far in the past three hundred years from the conception of an immovable earth at the center of the universe to this awe-inspiring conception of the universe that we have today, which is based upon modern astronomical discoveries. whatever may be discovered in the future in regard to the form and extent of the universe the idea of a fixed and immovable center either within the solar system or among the stars beyond has gone from the minds of men at last. [illustration: spiral nebula in andromeda viewed edgewise taken with -inch reflector of the mt. wilson observatory] not more than a generation ago a survival of the old idea of a fixed center was seen in the belief that alcyone, in the pleiades was a "central sun" about which all the stars revolved. it is now well known that the pleiades form a moving star cluster. alcyone is therefore drifting slowly onward through the universe and the idea of a fixed and immovable center to which man may anchor his ideas is drifting away also. there are, it is true, local centers of systems, such, for instance, as the sun occupies in the solar system or some group of stars may occupy in the stellar system to which our sun belongs, yet _as a whole_ these systems move on and their centers with them. there is no evidence today that any absolutely immovable point exists in the heavens. no celestial object has been found to be without the attribute of _motion_, not only motion _onward_ through the universe, but also _rotational_ motion about an axis of the body. the planets rotate on their axes as well as revolve about the sun, and the sun also turns on its axis as it moves onward through space. this rotational motion is also found in the nebulæ and star clusters as well as in the stars and planets. no object in the heavens is known to be without it. even the slowly drifting orion nebula possesses a rapid internal velocity of rotation. there is no such thing as a body absolutely at rest in the universe. table showing the number and relative size, velocity and distribution of the various types of celestial objects. ================+====================+=====================+===========+===================== | | |velocities | object | number | diameter | miles | distribution | | | per sec. | ----------------+--------------------+---------------------+-----------+--------------------- . solar system | | | | | | | | a. planets |eight | , to , mi. | to |revolving in nearly | | | miles | circular orbits | | | per sec. | about the sun. b. sun | | , mi. | - / mi. |travelling through | | | | galactic systems of . stars | | | | stars (milky way). | | | | a. helium | | | mi. | (bluish) | | | | b. hydrogen |approx. | dwarfs | mi. |all types of stars are (white) | , , , (two | , to | | more or less crowded c. solar | thousand million) | , , mi. | - mi. | toward plane of milky (yellow) | | | | way in lens shaped d. type m |including all types | giants | mi. | formation. (milky (red) | | , , to | | way possibly a spiral | | , , mi. | | nebula.) . nebulæ | | | | | | | | a. diffuse or |numerous |very extensive, many |very low |in or close to milky gaseous | | light years. | | way. | | | | b. spiral |approx. , |size and distance |average |far external to milky | (seven hundred | doubtful but | mi. | way and numerous | thousand) | very great. | | near its poles. | | | | c. planetary |one hundred and |several times that |average |in or close to milky | fifty ( ) | of the solar system | mi. | way. | | on the average. | | | | | | . globular star|about one hundred | , - , |very high |external to milky way clusters | known | light-years. | | and spherically | | | | distributed about it. | | | | . magellanic |two (greater and |thousands of |very high |far beyond milky way. clouds | lesser) | light-years. | | ================+====================+=====================+===========+===================== xxviii the evolution of the stars--from red giants to red dwarfs the most casual of star-gazers is aware that the stars differ one from another in color and in brightness. there are red stars, yellow stars, white stars and bluish-white stars. there are the brilliant stars of first magnitude such as vega, capella and antares, and there are, on the other hand, stars so faint that they can barely be glimpsed with the most powerful telescopes. in general the most brilliant stars are the nearest and the faintest stars are the most distant, but there are many exceptions to the rule, since there are stars that appear faint even when comparatively near because they are small and shine with a feeble light. such a star is the faint, sixth-magnitude star, cygni, one of the nearest of all the stars. again, there are stars in far-distant clusters visible only in powerful telescopes that in actual brightness exceed our own sun several thousand times and in volume several million times. a star the size of the sun would be invisible in the most powerful telescope in existence if it were at the distance of many stars in the milky way or globular star clusters. stars differ in color because they differ in temperature. we are all aware of the fact that a piece of iron when heated first glows a deep red, then appears yellowish in color and finally attains to white heat. it is the same among the stars. the red stars are the coolest of all the stars and the bluish-white stars are the hottest of all the stars, while intermediate between them in temperature come the yellow and the white stars. now as the biologist and the geologist see in this world of ours signs of evolution, or gradual development and change from the simple to the more complex forms, and of growth and decay, so the astronomer sees among the stars signs of a continuous, progressive development from one type of star to another. stars share in the general evolution that is the law of the universe, and are born, reach the height of their development, decline to old age and die. within the past few years important astronomical discoveries have been made that show the true order of this evolution of the stars. it was believed not so long ago that the blue-white helium stars--the type b stars the astronomers called them, or the orion stars, since there are so many stars of this type in the constellation of orion--were not only the hottest but also the youngest of the stars and that they represented the first stage in the development of a star from a primitive gaseous nebula such as the great orion nebula. it is now known that these brilliant, hot helium stars represent the peak of development of the most massive of all the stars and not the first stages in the development. a star, it is now known, comes into existence as a giant, reddish star of enormous size and of a density only about one-thousandth that of the earth's atmosphere at sea-level. it is inconceivably tenuous or rare, and its temperature is comparatively low, about , ° centigrade or less. it is not evolved from the luminous, gaseous nebulæ because red stars are never found associated with the gaseous nebulæ, as are the blue-white stars. the origin of these red giant stars is uncertain, but it is possible that they may be gradually evolved in some manner from the dark clouds of obscuring matter or dark nebulæ that exists so abundantly in the heavens. in the next stage of its development the deep-red giant star increases in temperature as it contracts under the action of gravitation and its color gradually changes from red to yellow. its density increases slightly and its volume decreases. it is now a yellow giant star. as the evolution progresses in the course of ages the star continues to contract, its temperature increases greatly as does also its density and it continues to decrease in volume. it is now a brilliant white star, a hydrogen star, so called because its spectrum is chiefly characterized by the lines of hydrogen. as the star contracts under the gravitation of its parts and increases in temperature and density there comes more and more into play an important factor that has a great effect upon its future development. this is light-pressure or radiation pressure which acts in opposition to gravity and exerts a strong outward pressure upon matter within the depths of the star, tending to push it outward from the center where the temperature is greatest and the light is most intense. it is a most interesting fact that if the mass of a star, that is the quantity of matter that it contains, exceeds a certain value the pressure of light or radiation within it overbalances the gravitational attraction that draws matter towards its center and the star disintegrates or ceases to exist as a star. this accounts for the fact that the stars differ very little among themselves in the quantity of matter that they contain, that is, in their masses, though they may differ enormously in size. stars that exceed a certain mass will become unstable and this may account for the association of luminous nebulæ with the hottest of all stars, the nebulæ possibly being puffed off from the surfaces of these stars under the action of radiation pressure. after a star has reached the height of its development as a bluish-white helium star with a temperature of something like , ° centigrade and a density about one-tenth that of the sun, it begins to lose heat and to cool gradually though it continues to contract and increase in density. it is now on the descending scale of evolution and is to be counted among the dwarfs instead of the giants. a brilliant blue-white helium or orion star is about one hundred times more luminous than the sun, and its diameter is about ten times that of the sun. our own sun, we find, is on the descending scale of stellar evolution. it is a yellow dwarf star of temperature about , ° centigrade and density one and one-fourth that of water, which is probably about as great a density as is attained by any star since even the non-luminous planets jupiter and saturn have lower densities than the sun. the last stage in the development of a star is represented by the dwarf red star of high density and low temperature. the diameter of the dwarf red star probably averages about five hundred thousand miles and its temperature is , ° centigrade or less. after this we have the extinct stars, similar possibly to our planet jupiter, though considerably larger, with a dense gaseous atmosphere and a certain degree of internal heat. we have traced the evolution of a star from a red giant to a red dwarf through the intermediate stages from yellow giant to a giant helium star with increasing temperature and thence to yellow dwarf and red dwarf as the temperature decreases. only the most massive stars pass through this entire chain of evolution. stars of small mass never attain to the splendor of brilliant blue-white helium stars, but begin to decrease in temperature and brightness before this stage is reached. the time required for the evolution of a star from red giant to red dwarf is not known, but it must be very great. the age of the earth, which is probably equal to that of the solar system, is estimated as something like one thousand million years. it is probable that the average life of a star far exceeds this limit. xxix double and multiple stars the plan of the solar system which consists of a central sun encircled by satellites that are far inferior to their luminary in size, and that move about it in orbits that are almost perfect circles, is not the only, nor possibly, even the most general one in the universe. sweeping the heavens with powerful telescopes one is astonished to find that myriads of stars can be separated into two or more physically connected suns that are often, moreover, of exquisitely tinted and contrasting shades. green and red, orange and blue, white and golden or white and blue pairs exist in profusion, and strange to say there are well-authenticated instances of color changes taking place temporarily within the same system. a pair of white stars has been known to change within a few decades, first to golden yellow and bluish green and then to orange and green. the famous pair catalogued as " herculis" was noted to change from green and red to a palish yellow and back to the original strongly contrasting hues within the course of a single year, while at another time they appeared to be a perfectly white pair. at the present time both of these stars are decidedly yellowish in color. such changes in hue are probably due to temporary disturbances in the atmospheres of the stars, possibly of an electrical nature or to sudden or unusual outbursts of activity, concerning the origin of which we are as much in doubt as we are of the cause of the sun-spot cycle and periodic variation in the intensity of radiation of our own sun. temporary changes in the color of the components of double star systems sometimes take place when the two stars approach their "periastron" or point of nearest approach. owing to the great eccentricity of the orbits of double stars, such stars are anywhere from twice to nineteen times as near to each other at periastron as they are at "apastron," or point of greatest departure. such great changes in the relative distances of two physically connected suns would produce marked changes in the intensity of the tides raised upon each of them by their mutual gravitational attraction and unusual outbursts of gases or electrical excitement in the atmospheres of the stars might cause very noticeable changes in the color of these stars as they drew nearer to each other, which would subside as they receded toward apastron. in addition to "visual" double or multiple stars, there exists a very extensive class of stars known as "spectroscopic binaries," in which the two components are so close to each other that even the most powerful telescopes cannot divide them. it is only from the shifting of the lines of their overlapping spectra, caused by their alternate motion toward and from the earth as they revolve about their common center of gravity, that their duplex nature is revealed to us. in some instances one member of the system is so faint that its spectrum is not visible and its presence is disclosed only by the shifting of the lines of the bright star. according to doppler's law, when a star is approaching the earth the lines of its spectrum shift toward the blue end of the spectrum, and when the star is receding from the earth the lines are shifted toward the red end of the spectrum. the amount of this shift can be very accurately measured, and gives the relative velocities of the stars in their orbits directly in miles per second. knowing in addition, by observation, the period of mutual revolution of the stars, it is possible to find the dimensions of these spectroscopic binary systems compared to our own solar system, and also the masses of the stars compared to the mass of our own sun. if the spectrum of the fainter star is not visible, only the velocity of the brighter star with respect to the center of gravity of the system can be found and the mass found for the system comes out too small. in such cases we can obtain only a lower limit for the mass of the system. then, too, it must be remembered that these systems of stars lie at all angles with reference to our line of sight, and so we rarely see the orbits in their true form. the measured velocities are as a result smaller than the true velocities, and on the average amount to only sixty per cent. of the true orbital velocities. the calculated masses of spectroscopic binary stars are, therefore, in general only about sixty per cent. of the true masses. it has been found from calculating the masses of a number of binary systems, that the combined masses of the stars in these systems do not differ very greatly among themselves, nor as compared to our own sun, though in light-giving power these stars may differ hundreds, thousands, even millions of times. for instance, there are stars that give only one ten-thousandth part of the light of our own sun, and other stars that give ten thousand times as much light as the sun. moreover, there are many instances of physically connected stars differing thousands of times in luminosity, though in mass, or quantity of matter found in the stars, they differ only two or three times. why this is so remains one of the great mysteries of the heavens, and makes it extremely difficult to give any satisfactory theory of the origin of double-star systems. it has never been explained satisfactorily why of two suns physically connected and, therefore, presumably originating at the same time, one should be radiating with the greatest intensity, while the other is practically an extinct sun, in spite of the fact that the quantity of matter in the two bodies differs but slightly. in a few systems the plane in which the stars revolve passes so nearly through the earth that the two stars temporarily eclipse one another during each revolution. such systems are called _eclipsing binaries_. to such a system belongs the famous _algol_. its light waxes and wanes periodically with the greatest punctuality in a period of ^d ^h . ^m, owing to its temporary eclipse by a very large but extremely faint attendant sun. the diameter of the faint star is slightly greater than the diameter of the bright star which is about one million miles in extent. the distance between the _centers_ of the stars is only about three million miles, which brings their surfaces within two million miles of each other. the masses of the two stars are in the ratio of two to one, the brighter and more massive star being about half as heavy as our own sun, though its density is only about two-tenths that of the sun. the density of the fainter star is still less, being only about half that of the brighter star. very low density of both components and extreme faintness of one member compared to the other, appears to be a very general characteristic of closely associated eclipsing and spectroscopic binary stars. among the extremely hot and brilliant helium and hydrogen stars, spectroscopic binaries exist in great numbers. in fact, among these types there appear to be as many binary and multiple systems as there are systems of isolated suns. sometimes these close binary stars are egg-shaped or oval and revolve rapidly almost in contact about their common center of gravity. inhabitants of satellites of such a system would see in their heavens the, to us, strange and startling phenomenon of _two_ suns, each equal to our own or even greater in size, whirling rapidly about each other and separated by a space comparable in extent to their own diameters. eclipses in such a system would be of daily occurrence, and, if one star were dark, would produce for the satellite world the same effect of alternate day and night that results from axial rotation of a satellite. the two hemispheres of the faint companion sun would be very unequally illuminated owing to the fact that the side turned toward the brilliant sun would always reflect its neighbor's brightness in addition to shining with its own comparatively feeble inherent light, while the opposite hemisphere would shine only by its own dim light, and would, therefore, be in comparative darkness. the spectroscopic binaries generally revolve closely and rapidly about their common center of gravity; there are to be found, on the other hand, among the wider visual doubles, many systems wherein the components are separated by distances comparable to the distances of the outer planets, saturn, uranus and neptune, from the sun. it is evident that the individual stars of such binary systems could not possibly be encircled by any such extensive system of satellites as attends our own sun, though satellites such as our own planet earth, or the inferior planets mercury and venus, might conceivably encircle the individual components of such binary systems at distances not greater than that of the earth from the sun. no planet could safely exist at a much greater distance from one of these suns without being subject to most dangerous perturbations and disruptive tidal forces arising from the vicinity of the second sun. granted that planets might encircle one of these suns at a distance approximating that of venus or our own planet from the sun, the inhabitants of such worlds would behold the strange phenomenon of _two_ suns in the heavens, not almost in contact as in spectroscopic binary systems, but at one time comparatively near and again in opposite portions of the heavens as is the case with the sun and moon in our own heavens. as the planet advanced in its orbit about the ruling sun, the secondary sun would be visible at first by day and again by night. if the two suns were of contrasting hues, as, for instance, green and red, there might appear in the nearby heavens at a distance of one hundred million miles or so a magnificent sun of deep reddish hue, equal to or surpassing our own in splendor, while in a far distant part of the sky, at a distance as great as that which separates us from the planet saturn, might appear a rival sun of greenish hue, smaller and fainter, but nevertheless, hot and extremely brilliant and capable of exerting through its great gravitational attraction a most disturbing effect upon the motion of the planet of its neighbor. at times the rays of the two suns, red and green, would combine to produce a day characterized by terrific heat and intense illumination. again the green orb would rise in the east as the red sun set in the west and night would be turned into a weird, dimly-lighted day by the greenish rays of the secondary sun. compared to the wonders and beauties of the heavens in such a system, our own well-regulated and orderly planet family, undisturbed by the exciting proximity of a rival sun, seems to pale into insignificance. yet we have every good reason to be content with the ordering of affairs within our own solar system, and to feel relief rather than regret at the absence of a secondary sun. in a planet world revolving about one member of a double star system, we may imagine the dread rather than pleasure with which the periodic near-approach of a rival sun would be hailed, and even the possible hurried migration from exposed to sheltered portions of the planetary world to escape the rapidly increasing heat and intensity of light from the approaching sun. in such systems the coming and going of the seasons might indeed be a matter of life and death to the inhabitants of satellite worlds! within our solar system the masses of the planets are practically negligible compared to the mass of the sun, and it is for this reason that they appear to revolve about the _center_ of the sun. as a matter of fact, no body in the universe revolves about the _exact geometrical center_ of another body, but two mutually attracting bodies revolve in orbits about their common center of gravity, which always lies between the two bodies on the line connecting them and at a distance from each of them that is in inverse proportion to the mass of the body. the moon does not revolve about the _center of the earth_, but about the _center of gravity_ of the earth and moon, which lies on the line connecting the two bodies and at a distance from the earth's center that is one eighty-first of the distance from the center of the earth to the center of the moon, since this represents the ratio of the masses of the two bodies. this center of gravity of the earth and moon, lies, then, about two thousand miles from the earth's center, and about this point both earth and moon trace out orbits of revolution that are identical in form and differ only in size. in the same way each of the planets of the solar system revolves about the center of gravity of itself and the sun, but the mass of the sun is so far in excess of the combined masses of all the planets that we may consider, for all practical purposes, that the planets revolve about the sun's center, the center of gravity of the system being within the sun, just as the center of gravity of the earth and moon is within the earth. prof. t. j. j. see found from the investigation of forty binary star orbits that the average eccentricity of double star orbits is twelve times as great as the average eccentricity of a planetary orbit, and that the masses of the component suns never differ very greatly. the center of gravity of a binary system, therefore, lies at a great distance from the centers of the stars, and about this point, as a focus, the stars move in orbits that are exactly similar in form but differ in size in inverse proportion to the ratio of the masses. since the orbits of binaries are, moreover, very highly eccentric, the two suns are, as we have said, anywhere from two to nineteen times nearer to each other at periastron than they are at "apastron." we have spoken so far only of systems of two associated suns, but many systems exist in which three or more sun-like bodies are in revolution about a common center of gravity. frequently two fairly close suns are in revolution about a common center of gravity, in a period, say, of fifty or sixty years, while a third sun revolves at a comparatively great distance about the center of gravity of itself and the first pair in a period of several hundred years. or possibly the third sun also possesses a close attendant and the two pairs revolve in a period of great length about a common center of gravity. such, for instance, are the systems of _zeta cancri_ and _epsilon lyræ_. in the former system the closer components revolve rapidly about their center of gravity in a period of about sixty years, while the remote companion shows irregularities in its motion that indicate that it is revolving about a dark body in a period of seventeen and a half years, while the two together are revolving very slowly in a period of six or seven centuries, about a common center of gravity with the first pair in a retrograde direction. the wider pair of _epsilon lyræ_ is a naked-eye double for it can be seen as a double star by a keen eye, while even a three-inch telescope will separate each of the components into a double star. so extensive is this system that the periods of revolution of the closer components occupy several centuries, one pair appearing to revolve about twice as rapidly as the other, while the period of revolution of the two pairs about a common center is probably a matter of thousands of years. the gap that separates the two pairs may be so great that light requires months to cross it. these multiple systems are by no means exceptional. they are to be found in profusion among the brilliant _orion_ stars. they have been referred to as "knots" of stars and it has been suggested that they may have originated as local condensations in one vast nebulous tract. a system of only two components appears to be the exception rather than the rule, groups of several connected suns being more numerous than single pairs. in all of these double and multiple systems there exists the possibility of minute satellites, such as our own earth, in attendance upon some one component of the system. such tiny bodies shining only by reflected light from a nearby brilliant sun would be hopelessly invisible in the most powerful telescope. we can only assume that it is far more reasonable to believe in than to disprove the existence of such satellites. our own solar system, then, represents neither in its mechanical nor physical features, the only possibilities for the maintenance of life; it can neither be considered a unique form, nor even the most generally prevalent form in the universe. xxx astronomical distances the grandeur of the scale upon which the visible universe is fashioned lies almost beyond human comprehension. in measuring the vast extent of our own solar system, which is but a single unit in the system of the stars, we may have recourse to some earthly standard of measurement, such as the mile. but when we desire to express in terms of units that can be grasped by our imagination, the distances of the stars that lie far, far beyond, we find that all ordinary standards of measurement become utterly inadequate for our purpose. in the measurement of celestial distances within the solar system the unit employed is either the familiar mile or kilometer or the "astronomical unit," which is the mean distance from the earth to the sun (ninety-two million nine hundred thousand miles in round numbers). in the measurement of distances _beyond_ the solar system the unit employed is either the _light-year_ or more recently the _parsec_, which is rapidly replacing the light-year among astronomers. a "light-year" is the distance that light, with its finite but almost unimaginable velocity of one hundred and eighty-six thousand miles _per second, travels in a year_. it is equal in round numbers to sixty-three thousand times the distance from the earth to the sun or approximately six thousand billions of miles. the parsec is equal to three and twenty-six hundredths ( . ) light-years, and it is approximately two hundred thousand times the distance from the earth to the sun. it is "the distance of a star with the _parallax_ of a second," a fact which its name, parsec, conveys to us. in other words, at the distance of one parsec the distance from the earth to the sun, "the astronomical unit," would subtend an angle equal to one second of an arc. this angle is spoken of as the parallax of the star. the larger the parallax, that is, the larger the angle the astronomical unit or radius of the earth's orbit subtends, viewed from the star, the nearer the star is to us. the fact that there is no known star within one parsec, or three and twenty-six hundredths light-years, of the sun shows the immensity of the scale of the universe of stars. before considering the distances of the stars and the extent of the sidereal system of which our sun and his satellites form a part, let us undertake to express the distance of the sun, moon and planets from the earth and the extent of the solar system in terms with which we are familiar. the nearest to the earth of all celestial bodies is its satellite, the moon. so near is the moon that if we should make on some great plain a model of the solar system in which the astronomical unit, the distance from earth to sun, would be four hundred feet, the distance between the earth and moon would be only one foot. on the same scale the most distant planet neptune would be two and one-quarter miles away. granted that it were possible to escape the earth's gravitational bonds and to travel by our swiftest means of conveyance, the airplane, through interplanetary space, let us consider how long it would take us to reach the moon, sun and planets if our speed were maintained at a uniform rate of two hundred miles an hour. an airplane traveling at this rate would circumnavigate the earth in a little over five days and would reach the moon in seven weeks. a trip to the sun, however, would take fifty-three years. after traveling for fourteen and a fraction years we would pass the orbit of venus and eighteen years later the orbit of mercury. if we preferred to travel outward from the earth in the direction of mars and the outer planets instead of toward the sun, more than twenty-seven years would elapse before we would reach the orbit of mars. an airplane journey to jupiter would be a matter of more than two hundred years, to saturn four hundred and fifty years, to uranus nearly one thousand years, and to neptune, about one thousand five hundred years. to cross the solar system on the diameter of neptune's orbit in an airplane, traveling day and night without stopping at the rate of miles per hour would take more than three thousand years. the sun's attraction reaches far beyond neptune's orbit, however. there are comets belonging to the solar system compelled by the sun's attraction to accompany him on his travels through space that return periodically to the immediate vicinity of the sun from regions far beyond the orbit of neptune and there is also the possibility that one or more undiscovered planets may travel around the sun in orbits far exterior to neptune's orbit. measured in terms of familiar units, such as are employed for the measurement of distances on our own planet, the extent of the solar system is tremendously great. viewed from neptune, the sun is so far away that it presents no appreciable disk. it is in this sense star-like to the neptunians, but at the distance of neptune the stars appear no more brilliant and no nearer than they do to us. to neptune the sun, though star-like in form, supplies a very appreciable quantity of light and heat (one nine-hundredth of the amount the earth receives) while the amount of light and heat that neptune receives from the nearest stars is entirely inappreciable. when our airplane reaches neptune after a journey of one thousand five hundred years, it is, as it were, just clearing the ground for its flight to the stars. to cover the intervening space to the nearest star, traveled by light in four and a third years, an airplane would need _fourteen and a half million years_. in that time the solar system itself would be in some far distant part of the universe, since it is speeding onward through space at the rate of twelve miles a second or about four astronomical units a year. changing now our unit of measurement that we may express interstellar distances in comprehensible numbers, we prepare to travel from the earth to the stars with the velocity of light. with this velocity, one hundred and eighty-six thousand miles per second, we circumnavigate our globe in one-seventh of a second, reach the moon in one and a fourth seconds and the sun in eight minutes. in a little over four hours we pass the orbit of neptune and are started on our journey to the stars, penetrating further and further into interstellar space. for a year we travel and reach not a single star though we are speeding ever onward with the velocity of light. we have now covered the distance of one light-year, which means that the waves of light from the sun we have left behind must travel for a year before they reach us. we continue our journey and find ourselves next at a distance of one parsec from the sun. we have traveled a distance of approximately three and a quarter light-years, and were it possible to see the earth as well as the sun at this distance, the two would appear to be but one second of arc apart, a distance that requires the most careful adjustment and manipulation of the telescope to measure accurately. we are still one light-year distant from alpha centauri, the nearest of the bright stars. a few of the stars will now appear somewhat brighter than they appeared to us on earth, but the majority of the stars appear just as we see them here and the forms of the constellations remain practically unchanged in appearance, for we are only beginning our journey through the sidereal universe and our position in it has only shifted by a very slight amount. if we should continue our journey to the immediate vicinity of alpha centauri, we would find that it is not like our own sun, a single star, but is a binary star consisting of two suns in revolution around their common center of gravity. the distance of this binary system from the solar system has been measured with considerable accuracy and is known to be four and a third light-years. though there may be a few faint stars or non-luminous stars nearer to us than alpha centauri, this star has long held the distinction of being the nearest of the stars. as the sun continues his journey through the universe the two stars, alpha centauri and our sun, will finally draw away from each other after many ages have passed and some other sun of space will be our nearest star. the distances that separate the stars from each other probably average as great as the distance from the sun to alpha centauri. within a sphere whose center is at the earth and whose radius is five parsecs, or about sixteen light-years, there are only about twenty known stars. there is, therefore, small chance of collision among bodies that are so small in proportion to the tremendous intervals of space that separate them from each other. there is ample room for the individual stars to pursue their journey through space without interfering with each other's motion so long as they are as widely scattered as they appear to be in this portion of the universe. the fact that our own sun has continued its journey through the universe for some hundreds of millions of years without any catastrophe such as would result from closely approaching or colliding with another sun of space shows how enormous is the scale upon which our sidereal system is fashioned. stars that are ten, fifty or even one hundred light-years from the earth are our nearest neighbors in space. they are the stars that show a slight displacement in the heavens or measurable parallax, viewed from opposite sides of the earth's orbit. there are probably a thousand stars among the hundreds of millions of stars within reach of the greatest telescopes whose distances have been determined in light-years by direct measurement of their displacement in the heavens resulting from the change of position of the earth in its orbit. the most distant of the stars are apparently immovable in the heavens showing neither the effect of the sun's motion or their own motion through space. methods for finding the distances of many far remote stars and star-clusters have been devised, however, and some comparatively recent investigations have given results for the distances of these objects indicating that the diameter of the system of stars to which our sun belongs is approximately three hundred thousand light-years. it is difficult to grasp the full significance of this fact. it means that hundreds of millions of the suns of space throng the visible universe at distances from us and from each other running into hundreds, thousands and even hundreds of thousands of light-years. the light waves from some tiny object that we view today in one of our great reflectors may have started on their journey through space over one hundred thousand years ago when men of the old stone age inhabited our planet earth! astronomers have found as a result of their investigations that the sidereal system to which our solar system belongs is in the form of a flattened spheroid with its longest axis in the plane of the milky way. the extent of this star system composed of hundreds of millions of individual suns in addition to nebulæ and clusters is probably something like three hundred thousand light-years along its longest axis, while globular star clusters lying above and below its central plane are estimated to be at distances from it ranging from ten thousand to two hundred thousand light-years. this entire organized system is our sidereal universe. space beyond is unexplored. the globular star clusters are among the most distant celestial objects so far discovered. the spiral nebulæ may be entirely within the limits of this system or they may be even more distant than the globular clusters for their distances are not known as yet. there is a possibility that our sidereal universe, vast as it is known to be, may be but a unit in some still greater unit and that other similar systems lie beyond the reach of existing telescopes at unimaginable distances. the mind of man is overwhelmed by the thought of sidereal systems as vast as our own lying far beyond his ken. whether or not such external systems do exist and are with our own sidereal system units in some still vaster creation we cannot know. so vast, indeed, is this one visible universe of ours that the mind of man, accustomed to earthly standards, cannot comprehend its magnitude or the infinitesimal size of our whole solar system compared to it. xxxi some astronomical facts worth remembering kepler's three laws of planetary motion: i. the planets move in ellipses with the sun at one focus. ii. the radius vector of a planet (line adjoining sun and planet) sweeps over equal areas in equal times. iii. the square of the time of revolution (the year) of each planet is proportional to the cube of its mean distance from the sun. * * * * * sir isaac newton discovered that the law of gravitation extends to the stars. that is, every mass in the universe attracts every other mass with an attraction directly proportional to the product of the masses and inversely proportional to the square of the distances between them. * * * * * ocean tides are caused by the difference between the attraction of the sun and moon for the main body of the earth and their attraction for different particles of the earth's surface. the tide-raising force of the disturbing body is proportional to its mass and inversely proportional to the cube of its distance. the tides produced by the sun are, therefore, only two-fifths as great as the tides produced by the moon. * * * * * the celestial sphere is an imaginary sphere of infinite radius, with the earth at its center, upon which the celestial bodies are considered to be projected for convenience in determining their positions with respect to fixed points of reference in the heavens. the north and south poles of the heavens are the points on the celestial sphere directly above the north and south poles of the earth. the celestial equator is the great circle in which the plane of the earth's equator intersects the celestial sphere. it passes through the east and west points of the horizon and through the zenith--or point directly overhead--at the earth's equator. the ecliptic is the great circle in which the plane of the earth's orbit intersects the celestial sphere. the celestial equator and the ecliptic are inclined to each other at an angle of - / °, which is called the obliquity of the ecliptic. the two points in which the celestial equator and the ecliptic intersect are called respectively the vernal equinox and the autumnal equinox. the vernal equinox is an important point of reference on the celestial sphere. as the position of a point on the earth's surface is determined by its longitude and latitude so the position of an object on the celestial sphere--star, sun, planet--is determined by its right ascension and declination. the declination of a celestial object is its distance north or south of the celestial equator, measured in degrees, minutes and seconds of arc, on a great circle of the celestial sphere passing through the object and north and south poles of the heavens. these great circles are called hour circles and they correspond to the meridians or circles of longitude on the earth's surface. the declination of an object in the heavens corresponds to the latitude of a point on the earth's surface. the right ascension of a point on the celestial sphere corresponds to the longitude of a point on the earth's surface. it is measured--as longitude is measured--in degrees, minutes and seconds of arc or in hours, minutes and seconds of time--eastward along the celestial equator from the hour circle passing through the vernal equinox to the foot of the hour circle passing through the object. the hour circle passing through the vernal equinox is the zero meridian for the celestial sphere just as the meridian of greenwich is the zero meridian on the earth's surface. * * * * * the mean distance of the earth from the sun is , , miles and is called the astronomical unit. the sun with its satellites advances through the universe at the rate of astronomical units in a year or approximately one million miles a day. the parallax of a star is the angle at the star subtended by the radius of the earth's orbit, , , miles, or the astronomical unit. it is, in other words, the angular distance between the earth and sun as viewed from the star. the larger the parallax the nearer the star. the largest known stellar parallax is that of alpha centauri and its value is ". . the light-year is the distance that light travels in one year. it is equal to about , astronomical units or nearly six trillion ( , , , , ) miles. the velocity of light is , miles per second. the parsec is equal to . light-years. it is the distance of a star that has a parallax of one second of arc. the apparent magnitude of a star is its apparent brightness estimated on a scale in which a difference of one magnitude corresponds to a difference in brightness of . , or the fifth root of one hundred. a difference of five magnitudes corresponds to a difference one one hundredfold in brightness, of ten magnitudes to ten thousandfold in brightness. in exact measurements on this scale magnitudes are estimated to tenths. stars that are one magnitude brighter than stars of the standard first magnitude are of the zero magnitude and stars still brighter are of negative magnitudes. sirius is a star of the - . magnitude. jupiter at opposition is of - . magnitude and venus at greatest brilliancy of - . magnitude. the sun on this scale of comparative brightness is of the - . apparent magnitude. the faintest stars visible in the most powerful telescope in the world--the -inch mt. wilson hooker telescope--are of the twentieth magnitude. the _absolute_ magnitude of a star is its apparent magnitude at the standard distance of ten parsecs or . light years. the absolute magnitude of the sun is five. that is, the sun would be a fifth-magnitude star at the standard distance of . light-years. the absolute magnitudes of stars indicate how bright they would be relatively if they were all at the same standard distance. apparent magnitudes indicate how bright the stars appear to be at their true distances. * * * * * the mean distance of the moon from the earth is approximately , miles or sixty times the earth's radius. the sun is four hundred times farther away than the moon and its diameter is about four hundred times greater than the moon's diameter. the nearest star is about , times more distant than the sun, and the most distant known object, the globular star cluster, n.g.c. , is about fourteen billion times more distant than the sun. the earth is a spheroid flattened at the poles and its polar diameter is about twenty-seven miles shorter than its equatorial diameter. an object weighs less at the poles than at the equator. the earth's interior is as rigid as steel and probably consists of a core of magnetic iron surrounded by an outer stony shell. eclipses of the sun occur when the moon passes between the earth and sun. they can only occur at the time of new moon. there must be at least two solar eclipses every year separated by an interval of six months and there may be as many as five solar eclipses in a year. eclipses of the moon occur when the earth comes between the sun and moon, and the moon passes into the earth's shadow. eclipses of the moon can only occur at full moon. there may or may not be eclipses of the moon every year. the greatest number of eclipses than can occur in any one year, solar and lunar combined, is seven and the least number is two and in that case they are both solar eclipses. the sun is a yellow, dwarf star of a density of one and one-fourth that of water and with a surface temperature of about , ° f. except in sun-spot regions where the temperature is about , ° f. it is probably gaseous throughout. the sun, as well as the planets, rotates on its axis and different portions of the surface rotate at slightly different rates. the average period of the rotation of the sun on its axis is about twenty-six days. the sun is a variable star with a twofold variation. one is of long period during the eleven-year sun-spot cycle with a range of from three to five per cent. the other is a short irregular variation with a period of a few days, weeks or months and a range of from three to ten per cent. sun-spots are solar cyclones and appear black only by contrast with their hotter and brighter surroundings. they come in eleven-year cycles (approximately) with periods of maximum and minimum appearance. the brightness and blue color of the sky is due to the scattering of sunlight by the molecules of oxygen and nitrogen in the earth's upper atmosphere. if there were no atmosphere the skies would appear black except in the direction of the heavenly bodies, which would be visible by day as well as by night. the solar corona is the rare outer envelope of the sun and it is visible only during a total eclipse of the sun. it is partly of an electrical nature and it varies in form during the sun-spot cycle. it often extends to a distance of several solar diameters on either side of the sun. the warmth and the habitability of the earth's surface is due to the presence of water-vapor and carbon-dioxide in the atmosphere. without these substances in the atmosphere life on the earth's surface would be impossible. half of the earth's atmosphere and all clouds lie within seven miles of the earth's surface, and at high elevations above the earth the temperature is many degrees below zero. the temperature of space approaches the absolute zero of - ° f. the only planets in the solar system with the exception of the earth that might possibly support life are venus and mars. stars shine by their own light but planets shine only by reflected light from the sun. * * * * * if the earth were represented by a six-inch school globe the sun would be on the same scale a globe fifty-four feet in diameter. mercury would be a small ball two and a third inches in diameter. venus would be another six-inch globe. mars would be a ball about the size of a baseball, three and a fifth inches in diameter. the moon would be about the size of a golf ball, one and a half inches in diameter. the largest asteroids would be the size of marbles. average-sized asteroids would be the size of shot and the smallest would be merely grains of sand. jupiter would be a huge globe standing as tall as a man five feet six inches in height. saturn would be a smaller globe four and a half feet in diameter and its ring system would extend to a distance of five and a half feet on either side of the globe. uranus would be represented by a globe almost exactly two feet in diameter and neptune would be a slightly larger globe with a diameter of two feet two and a half inches. the satellites of the outer planets would range in size from tennis and golf balls for the largest, to marbles for the smaller and grains of sand for the smallest. on the same scale of measurement the distance of the six-inch globe of the earth from the fifty-four foot globe representing the sun would be one and one-tenth miles. the moon would be placed fifteen feet from the earth-globe and the diameter of the solar system on the same scale measured across the orbit of neptune would be sixty-six miles. the nearest star on this scale would be three hundred thousand miles away. * * * * * if the distance from the earth to the sun is taken as one inch so that the scale of the universe is reduced six trillion times, the diameter of the solar system across neptune's orbit is five feet and the distance of one light-year comes out almost exactly equal to one mile. the nearest star to the five-foot solar system would be four and a third miles away; the most distant known object would be two hundred and twenty thousand miles away, and the extent of the visible universe would be three hundred thousand miles. on the same scale the diameter of our sun would be about one hundredth of an inch and the diameters of the giant stars antares and betelgeuze would be four inches and two and three-fourth inches respectively. to see the earth we would need a microscope. tables table i the principal elements of the solar system =============+=============+========+============+========+============+=========== |mean distance from sun| |velocity| | +-------------+--------+ | in | |inclination | |relative| period | orbit |eccentricity| of planet | in miles | to | of |(miles | of | orbit to | |earth's | revolution | per | orbit | ecliptic | |distance| |second )| | ------------+-------------+--------+------------+--------+------------+----------- mercury | , , | . | . days | to | . | ° ' venus | , , | . | . days | . | . | earth | , , | . | . days | . | . | mars | , , | . | . years| . | . | asteroids[ ]| ...........| . - . | ......... | ...... | . to . | ° to ° jupiter | , , | . | . years| . | . | saturn | , , | . | . years| . | . | uranus | , , , | . | . years| . | . | neptune | , , , | . | . years| . | . | ============+=============+========+============+========+============+=========== footnote: [ ] about have been discovered up to the present time. the principal elements of the solar system (continued) =========+========+==========+==========+========+============+==========+==========+============+=========== | mean | | |density | surface |velocity |reflecting| period |inclination name |diameter| mass | volume |relative| gravity |of escape | power | of | of | in +----------+----------+to that | (relative |(miles per| in | axial | equator | miles | relative to earth's |of water| to earth's)| second) | per cent | rotation | to orbit ---------+--------+----------+----------+--------+------------+----------+----------+------------+----------- sun | , | , | , , | . | . | | ..... | d. h.| ° ' moon | , | . | . | . | . | . | | d. . h.| mercury | , | . | . | . ? | . ? | . | | d. ? | ? venus | , | . | . | . ? | . | . | | ? | ? earth | , | . | . | . | . | | | h. m. | mars | , | . | . | . | . | . | | | asteroids| - [ ]|very small|very small| . |. to . |. to . | | ..... | ..... jupiter | , | . | | . | . | | | ± | ° saturn | , | . | | . | . | | | ± | ° uranus | , | . | | . | . | | | ± | ? neptune | , | . | | . | . | | | ? | ? =========+========+==========+==========+========+============+==========+==========+============+=========== footnote: [ ] extreme values. table ii the satellites of the solar system =============+=========+=============+========+=====================+============+========= | | | | | | | |mean distance| | | | |apparent |from planet's|diameter| period of | | year of name |magnitude| center, |in miles| revolution | discoverer |discovery | | in miles | | | | -------------+---------+-------------+--------+---------------------+------------+--------- | | | | | | the earth | | | | | | moon | | , | | days, hours, | | | | | | minutes | | mars | | | | | | . phobos | | , | ? | days, hours, | asaph hall | | | | | minutes | | . deimos | | , | ? | day, hours, | asaph hall | | | | | minutes | | jupiter | | | | | | v. | | , | ? | day, hours, | barnard | | | | | minutes | | i. | . | , | | day, hours, | galileo | | | | | minutes | | ii. | . | , | | days, hours, | galileo | | | | | minutes | | iii. | | , | | days, hours, | galileo | | | | | minutes | | iv. | | , , | | days, hours, | galileo | | | | | minutes | | vi. | | , , | small | days, hours, | perrine | | | | | minutes | | vii. | | , , |very | days, hours, | perrine | | | | small | minutes | | viii. | | , , |very | days, hours, | melotte | | | | small | minutes | | ix. | | , , | ? | years | nicholson | | | | | | | saturn | | | | | | . meimas | | , | | days, hours, | herschel | | | | | minutes | | . enceladus | | , | | day, hours, | herschel | | | | | minutes | | . tethys | | , | | day, hours, | cassini | | | | | minutes | | . dione | | , | | days, hours, | cassini | | | | | minutes | | . rhea | | , | | days, hours, | cassini | | | | | minutes | | . titan | | , | | days, hours, | huygens | | | | | minutes | | . hyperion | | , | | days, hours, | bond | | | | | minutes | | . japetus | | , , | | days, hours, | cassini | | | | | minutes | | . phoebe | | , , | ? | days, hours, | w.h. | | | | | minutes | pickering | . themis | | , | ? | days, hours, | w.h. | | | | | minutes | pickering | uranus | | | | | | . ariel | | , | | days, hours, | lassell | | | | | minutes | | . umbriel | | , | | days, hours, | lassell | | | | | minutes | | . titania | | , | | days, hours, | herschel | | | | | minutes | | . oberon | | , | | days, hours, | herschel | | | | | minutes | | neptune | | | | | | . nameless | | , | | days, hours, | lassell | | | | | minutes | | =============+=========+=============+========+=====================+============+========= rings of saturn ===================+==========+===================+===================+==================== | | distance of | distance of |diameter of ring | | inner edge from | outer edge from | system from outer name | width, | surface of saturn,| surface of saturn,| edge to outer edge, | in miles | in miles | in miles | , miles. -------------------+----------+-------------------+-------------------|thickness of ring | | | | system, about one dark or crape ring | , | , | , | hundred miles. bright ring | , | , | , |size of individual cassini's division | , | , | , | moonlets, probably outer ring | , | , | , | less than three | | | | miles in diameter. ===================+==========+===================+===================+==================== table iii the twenty brightest stars in the heavens =============================+==========+============+============+===========+=========== | | | | passes | | | | on | through | distance name |magnitude | color | meridian |the zenith | in | | | p. m. |in latitude|light-years -----------------------------+----------+------------+------------+-----------+----------- sirius, alpha canis majoris | - . |white |february | s. | . canopus,[ ] alpha argus | - . |white |february | s. | ? alpha centauri[ ] | . |yellow |june | s. | . vega, alpha lyræ | . |white |august | n. | capella, alpha aurigæ | . |yellow |january | n. | arcturus, alpha boötis | . |orange |june | n. | rigel, beta orionis | . |bluish-white|january | s. | ? procyon, alpha canis minoris | . |white |february | n. | achernar,[ ] alpha eridani | . |bluish-white|december | s. | beta centauri | . |bluish-white|june | s. | betelgeuze, alpha orionis | var. |red |january | n. | - ? | . - . | | | | altair, alpha aquilæ | . |white |september | n. | alpha crucis[ ] (double star)| . - . |bluish-white|may | s. | aldebaran, alpha tauri | . |red |january | n. | pollux, beta geminorum | . |yellow |february | n. | spica, alpha virginis | . |bluish-white|may | s. | ? antares, alpha scorpii | . |red |july | s. | fomalhaut, alpha piscis | . |white |october | s. | australis | | | | | deneb, alpha cygni | . |white |september | n. | ? regulus, alpha leonis | . |white |april | n. | =============================+==========+============+============+===========+=========== footnote: [ ] invisible north of ° n. lat. (approximate). table iv a list of the principal constellations . visible in ° north latitude =============================================================== | | | passes name | chief star | on meridian | overhead | or | p. m. | in latitude[ ] | noted object | | (degrees) -----------------+---------------+-------------+--------------- andromeda | great nebula | november | n. aquarius | | october | s. aquila | altair | september | ° aries | | december | n. auriga | capella | february | n. boötes | arcturus | june | n. cancer | praesepe | march | n. canes venatici | cor caroli | june | n. canis major | sirius | march | s. canis minor | procyon | march | n. capricornus | | october | s. cassiopeia | | november | n. cepheus | | november | n. cetus | mira | december | s. columba | | february | s. coma berenices | | may | n. corona borealis | alphecca | july | n. corvus | | may | s. crater | | may | s. cygnus. | deneb | september | n. delphinus | most distant | september | n. | globular | | | cluster | | draco | alpha | august | n. eridanus | achernar | january | ° n. to ° s. gemini | pollux | march | n. hercules | great cluster | july | n. hydra | | april | s. leo | regulus | april | n. lepus | | february | s. libra | | june | s. lynx | | april | n. lyra | vega | august | n. ophiuchus | | july | s. orion | great nebula | february | ° piscis australis | fomalhaut | october | s. pegasus | | november | n. perseus | algol | january | n. pisces | | december | n. sagitta | | september | n. sagittarius | | august | s. scorpio | antares | july | s. serpens | | july | ° n. to ° s. taurus | pleiades | january | n. triangulum | | december | n. ursa major | mizar | may | n. ursa minor | polaris | | n. virgo | spica | june | ° =================+===============+=============+=============== footnote: [ ] the approximate position of the center of the constellation. . invisible in ° north latitude ===========+===============+===========+============== | | on | passes name | chief star | meridian | overhead | or | p. m. | in latitude[ ] | noted object | | (degrees) -----------+---------------+-----------+-------------- apus | | july | s. ara | | july | s. argo navis | canopus | march | s. . carina | | march | s. . puppis | | march | s. . vela | | march | s. centaurus | alpha centauri| june | s. crux | | | (southern | | | cross) | alpha crucis | june | s. dorado | gt. magellanic| | | cloud | february | s. grus | | october | s. hydrus | lesser mag. | | | cloud | | s. indus | | september | s. lupus | | june | s. musca | | june | s. octans | | | s. pavo | | october | s. phoenix | | november | s. telescopium| | july | s. triangulum | | | australe | | july | s. tucana | great cluster | november | s. volans | | march | s. ===========+===============+===========+============== table v pronunciations and meanings of names of stars and constellations . stars =============+====================+==================== name | pronunciation | meaning -------------+--------------------+-------------------- achernar | a-ke´r-när | end-of-the-river aldebaran | al-de´b-ar-an | the hindmost altair | al-ta´r | antares | an-ta´-rez | rival of ares (mars) arcturus | ärk-t´u-rus | bellatrix | bel-la´trix | the female warrior betelgeuze | be´t-el-gerz | | or be´t-el-gez | the arm-pit canopus | cän-o´-pus | capella | ca-pel-la | little she-goat deneb | de´n-eb | denebola | de-ne´b-o-la | the lion's tail fomalhaut | fo´-mal-o | the fish's mouth hyades | hi-a-dez | the rainy ones pleiades | ple´-ad-ez | pollux | po´l-lux | praesepe | pre-se´-pe | the beehive procyon | pro-si´-on | precursor of the dog regulus | reg´-u-lus | the ruler rigel | ri´-gel or ri-jel | sirius | sir´-i-us | the sparkling one spica | spi´-ka | the ear of wheat vega | ve´-ga | =============+====================+==================== . constellations ================+=======================+================== name | pronunciation | meaning ----------------+-----------------------+------------------ andromeda | an-d´rom-e-da | the woman chained aquarius | a-kwa´-ri-us | the water-bearer aquila | a´k-wi-la | the eagle ara | a´-ra | the altar argo navis | ä´r-go-n´a-vis | the ship argo aries | a´-res | the ram auriga | äw-ri´-ga | the charioteer boötes | bo-o´-tez | the herdsman cancer | ca´n-ser | the crab canes venatici | ca´-nez ven-a´t-i-si | the hunting dogs canis major | ca´-nis ma´jor | the greater dog canis minor | ca´-nis mi´nor | the lesser dog capricornus | ca´p-ri-kö´r-nus | the goat cassiopeia | ca´s-si-o-p´e-ya | centaurus | cen-tä´w-rus | the centaur cepheus | se-fe-us | cetus | s´e-tus | the whale columba | col-u´m-ba | the dove coma berenices | co´ma ber-e-ni-ses | berenice's hair corona borealis | co-ro´-na bo-re-a´-lis| the northern crown corvus | cô´r-vus | the crow crater | cr´a-ter | the cup crux | kru´x | the cross cygnus | si´g-nus | the swan delphinus | del-fi´-nus | the dolphin dorado | dôr-a´-do | the goldfish draco | dra´-co | the dragon eridanus | e-ri´d-a-nus | the river eridanus gemini | jem´-i-ni | the twins grus | gru´s | the crane hercules | her-ku-lez | hydra | hi´-dra | the water-snake hydrus | hi´-drus | the serpent indus | i´nd-us | the indian leo | le´-o | the lion lepus | le´-pus | the hare libra | li´-bra | the scales lupus | lu´-pus | the wolf lynx | | the fox lyra | li´-ra | the lyre musca | mus´-ca | the fly octans | o´ct-ans | the octant ophiuchus | o´-fi-u´-kus | the serpent-holder orion | o-ri´-on | the warrior pavo | pä´-vo | the peacock phoenix | fe´-nix | piscis australis| pi´s-sis aus-tra´-lis | the southern fish pegasus | peg´-a-sus | the winged horse perseus | pe´r-se-us or per-sus | pisces | pi´s-sez | the fishes sagitta | sa-ji´t-ta | the arrow sagittarius | sa-jit-ta´-ri-us | the archer scorpio | skô´r-pi-o | the scorpion serpens | ser-pens | the serpent taurus | täu-rus | the bull telescopium | tel-es-cop´-i-um | the telescope triangulum | tri-a´n-gu-lum | the triangle tucana | tu´c-an-a | the toucan ursa major | u´r-sa ma´-jor | the greater bear ursa minor | u´r-sa mi´-nor | the lesser bear virgo | ve´r-go | the maiden volans | vo´l-ans | the flying fish ================+=======================+================== * * * * * transcriber's note: obvious typographical errors have been repaired. _underscores_ surround italicized content. mid-paragraph illustrations were moved near to the text describing the illustrated material. redundant title--astronomy for young folks--on p. was deleted. p. : canst thou bring forth mazzaroth--"canst" is assumed in blank space. p. : brighter object than the nearby star aldebaran--"star" is assumed in blank space. p. : illustration originally stated "see note page ". that statement was removed, and the actual note from page was moved to its place with the illustration. p. : [...]--duplicate of later line "occurred at l'aigle, france, in . between two" appeared at this spot. possible missing text where the line occurred. p. : period of ^d ^h . ^m--carat (^) indicates that the letter immediately following appears as a superscript. data in tables retained as in original, but may be incorrect--for example, the escape velocity of mars, represented as . miles per second in table i, is closer to . . note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) a popular history of astronomy during the nineteenth century * * * * * by the same author problems in astrophysics. demy vo., cloth. containing over illustrations. price s. net. the system of the stars. second edition. thoroughly revised and largely rewritten. containing numerous and new illustrations. demy vo., cloth. price s. net. modern cosmogonies. crown vo., cloth. price s. d. net. a. and c. black, soho square, london, w. * * * * * [illustration: the great nebula in orion, _see p. _] a popular history of astronomy during the nineteenth century by agnes m. clerke [illustration: jupiter saturn ] london adam and charles black first edition, post vo., published second edition, post vo., published third edition, demy vo., published fourth edition, demy vo., published fourth edition, post vo., reprinted february, preface to the fourth edition since the third edition of the present work issued from the press, the nineteenth century has run its course and finished its record. a new era has dawned, not by chronological prescription alone, but to the vital sense of humanity. novel thoughts are rife; fresh impulses stir the nations; the soughing of the wind of progress strikes every ear. "the old order changeth" more and more swiftly as mental activity becomes intensified. already many of the scientific doctrines implicitly accepted fifteen years ago begin to wear a superannuated aspect. dalton's atoms are in process of disintegration; kirchhoff's theorem visibly needs to be modified; clerk maxwell's medium no longer figures as an indispensable factotum; "absolute zero" is known to be situated on an asymptote to the curve of cold. ideas, in short, have all at once become plastic, and none more completely so than those relating to astronomy. the physics of the heavenly bodies, indeed, finds its best opportunities in unlooked-for disclosures; for it deals with transcendental conditions, and what is strange to terrestrial experience may serve admirably to expound what is normal in the skies. in celestial science especially, facts that appear subversive are often the most illuminative, and the prospect of its advance widens and brightens with each divagation enforced or permitted from the strait paths of rigid theory. this readiness for innovation has undoubtedly its dangers and drawbacks. to the historian, above all, it presents frequent occasions of embarrassment. the writing of history is a strongly selective operation, the outcome being valuable just in so far as the choice what to reject and what to include has been judicious; and the task is no light one of discriminating between barren speculations and ideas pregnant with coming truth. to the possession of such prescience of the future as would be needed to do this effectually i can lay no claim; but diligence and sobriety of thought are ordinarily within reach, and these i shall have exercised to good purpose if i have succeeded in rendering the fourth edition of _a popular history of astronomy during the nineteenth century_ not wholly unworthy of a place in the scientific literature of the twentieth century. my thanks are due to sir david gill for the use of his photograph of the great comet of , which i have added to my list of illustrations, and to the council of the royal astronomical society for the loan of glass positives needed for the reproduction of those included in the third edition. london, _july_, . preface to the first edition the progress of astronomy during the last hundred years has been rapid and extraordinary. in its distinctive features, moreover, the nature of that progress has been such as to lend itself with facility to untechnical treatment. to this circumstance the present volume owes its origin. it embodies an attempt to enable the ordinary reader to follow, with intelligent interest, the course of modern astronomical inquiries, and to realize (so far as it can at present be realized) the full effect of the comprehensive change in the whole aspect, purposes, and methods of celestial science introduced by the momentous discovery of spectrum analysis. since professor grant's invaluable work on the _history of physical astronomy_ was published, a third of a century has elapsed. during the interval a so-called "new astronomy" has grown up by the side of the old. one effect of its advent has been to render the science of the heavenly bodies more popular, both in its needs and in its nature, than formerly. more popular in its needs, since its progress now primarily depends upon the interest in, and consequent efforts towards its advancement of the general public; more popular in its nature, because the kind of knowledge it now chiefly tends to accumulate is more easily intelligible--less remote from ordinary experience--than that evolved by the aid of the calculus from materials collected by the use of the transit-instrument and chronograph. it has thus become practicable to describe in simple language the most essential parts of recent astronomical discoveries, and, being practicable, it could not be otherwise than desirable to do so. the service to astronomy itself would be not inconsiderable of enlisting wider sympathies on its behalf, while to help one single mind towards a fuller understanding of the manifold works which have in all ages irresistibly spoken to man of the glory of god might well be an object of no ignoble ambition. the present volume does not profess to be a complete or exhaustive history of astronomy during the period covered by it. its design is to present a view of the progress of celestial science, on its most characteristic side, since the time of herschel. abstruse mathematical theories, unless in some of their more striking results, are excluded from consideration. these, during the eighteenth century, constituted the sum and substance of astronomy, and their fundamental importance can never be diminished, and should never be ignored. but as the outcome of the enormous development given to the powers of the telescope in recent times, together with the swift advance of physical science, and the inclusion, by means of the spectroscope, of the heavenly bodies within the domain of its inquiries, much knowledge has been acquired regarding the nature and condition of those bodies, forming, it might be said, a science apart, and disembarrassed from immediate dependence upon intricate, and, except to the initiated, unintelligible formulæ. this kind of knowledge forms the main subject of the book now offered to the public. there are many reasons for preferring a history to a formal treatise on astronomy. in a treatise, _what_ we know is set forth. a history tells us, in addition, _how_ we came to know it. it thus places facts before us in the natural order of their ascertainment, and narrates instead of enumerating. the story to be told leaves the marvels of imagination far behind, and requires no embellishment from literary art or high-flown phrases. its best ornament is unvarnished truthfulness, and this, at least, may confidently be claimed to be bestowed upon it in the ensuing pages. in them unity of treatment is sought to be combined with a due regard to chronological sequence by grouping in separate chapters the various events relating to the several departments of descriptive astronomy. the whole is divided into two parts, the line between which is roughly drawn at the middle of the present century. herschel's inquiries into the construction of the heavens strike the keynote of the first part; the discoveries of sun-spot and magnetic periodicity and of spectrum analysis determine the character of the second. where the nature of the subject required it, however, this arrangement has been disregarded. clearness and consistency should obviously take precedence of method. thus, in treating of the telescopic scrutiny of the various planets, the whole of the related facts have been collected into an uninterrupted narrative. a division elsewhere natural and helpful would here have been purely artificial, and therefore confusing. the interests of students have been consulted by a full and authentic system of references to the sources of information relied upon. materials have been derived, as a rule with very few exceptions, from the original authorities. the system adopted has been to take as little as possible at second-hand. much pains have been taken to trace the origin of ideas, often obscurely enunciated long before they came to resound through the scientific world, and to give to each individual discoverer, strictly and impartially, his due. prominence has also been assigned to the biographical element, as underlying and determining the whole course of human endeavour. the advance of knowledge may be called a vital process. the lives of men are absorbed into and assimilated by it. inquiries into the kind and mode of the surrender in each separate case must always possess a strong interest, whether for study or for example. the acknowledgments of the writer are due to professor edward s. holden, director of the washburn observatory, wisconsin, and to dr. copeland, chief astronomer of lord crawford's observatory at dunecht, for many valuable communications. london, _september_, . contents _introduction_ page three kinds of astronomy--progress of the science during the eighteenth century--popularity and rapid advance during the nineteenth century part i _progress of astronomy during the first half of the nineteenth century_ chapter i foundation of sidereal astronomy state of knowledge regarding the stars in the eighteenth century-- career of sir william herschel--constitution of the stellar system-- double stars--herschel's discovery of their revolutions-- his method of star-gauging--discoveries of nebulæ--theory of their condensation into stars--summary of results chapter ii progress of sidereal astronomy exact astronomy in germany--career of bessel--his _fundamenta astronomiæ_--career of fraunhofer--parallaxes of fixed stars--translation of the solar system--astronomy of the invisible--struve's researches in double stars--sir john herschel's exploration of the heavens--fifty years' progress chapter iii progress of knowledge regarding the sun early views as to the nature of sun-spots--wilson's observations and reasonings--sir william herschel's theory of the solar constitution--sir john herschel's trade-wind hypothesis--baily's beads--total solar eclipse of --corona and prominences--eclipse of chapter iv planetary discoveries bode's law--search for a missing planet--its discovery by piazzi-- further discoveries of minor planets--unexplained disturbance of uranus--discovery of neptune--its satellite--an eighth saturnian moon--saturn's dusky ring--the uranian system chapter v comets predicted return of halley's comet--career of olbers--acceleration of encke's comet--biela's comet--its duplication--faye's comet--comet of --electrical theory of cometary emanations--the earth in a comet's tail--second return of halley's comet--great comet of --results to knowledge chapter vi instrumental advances two principles of telescopic construction--early reflectors--three varieties--herschel's specula--high magnifying powers--invention of the achromatic lens--guinand's optical glass--the great rosse reflector--its disclosures--mounting of telescopes--astronomical circles--personal equation part ii _recent progress of astronomy_ chapter i foundation of astronomical physics schwabe's discovery of a decennial sun-spot period--coincidence with period of magnetic disturbance--sun-spots and weather--spectrum analysis--preliminary inquiries--fraunhofer lines--kirchhoff's principle--anticipations--elementary principles of spectrum analysis--unity of nature chapter ii solar observations and theories black openings in spots--carrington's observations--rotation of the sun--kirchhoff's theory of the solar constitution--faye's views--solar photography--kew observations--spectroscopic method--cyclonic theory of sun-spots--volcanic hypothesis--a solar outburst--sun-spot periodicity--planetary influence--structure of the photosphere chapter iii recent solar eclipses expeditions to spain--great indian eclipse--new method of viewing prominences--total eclipse visible in north america--spectrum of the corona--eclipse of --young's reversing layer--eclipse of --corona of --varying coronal types--egyptian eclipse--daylight coronal photography--observations at caroline island--photographs of corona in and --eclipses of , , , and --mechanical theory of corona--electro-magnetic theories--nature of corona chapter iv solar spectroscopy chemistry of prominences--study of their forms--two classes--photographs and spectrographs of prominences--their distribution--structure of the chromosphere--spectroscopic measurement of radial movements--spectroscopic determination of solar rotation--velocities of transport in the sun--lockyer's theory of dissociation--solar constituents--oxygen absorption in solar spectrum chapter v temperature of the sun thermal power of the sun--radiation and temperature--estimates of solar temperature--rosetti's and wilson's results--zöllner's method --langley's experiment at pittsburg--the sun's atmosphere--langley's bolometric researches--selective absorption by our air--the solar constant chapter vi the sun's distance difficulty of the problem--oppositions of mars--transits of venus--lunar disturbance--velocity of light--transit of --inconclusive result--opposition of mars in --measurements of minor planets--transit of --newcomb's determination of the velocity of light--combined result chapter vii planets and satellites schröter's life and work--luminous appearances during transits of mercury--mountains of mercury--intra-mercurian planets--schiaparelli's results for the rotation of mercury and venus--illusory satellite--mountains and atmosphere of venus--ashen light--solidity of the earth--variation of latitude--secular changes of climate--figure of the globe--study of the moon's surface--lunar atmosphere--new craters--thermal energy of moonlight--tidal friction chapter viii planets and satellites--(_continued_) analogy between mars and the earth--martian snowcaps, seas, and continents--climate and atmosphere--schiaparelli's canals--discovery of two martian satellites--photographic detection of minor planets--orbit of eros--distribution of the minor planets--their collective mass and estimated diameters--condition of jupiter--his spectrum--transits of his satellites--discovery of a fifth satellite--the great red spot--constitution of saturn's rings--period of rotation of the planet--variability of japetus--equatorial markings on uranus--his spectrum--rotation of neptune--trans-neptunian planets chapter ix theories of planetary evolution origin of the world according to kant--laplace's nebular hypothesis--maintenance of the sun's heat--meteoric hypothesis--radiation as an effect of contraction--regenerative theory--faye's scheme of planetary development--origin of the moon--effects of tidal friction chapter x recent comets donati's comet--the earth again involved in a comet's tail--comets of the august and november meteors--star showers--comets and meteors--biela's comet and the andromedes--holmes's comet--deflection of the leonids--orbits of meteorites--meteors with stationary radiants--spectroscopic analysis of cometary light--comet of --coggia's comet chapter xi recent comets--(_continued_) forms of comets' tails--electrical repulsion--brédikhine's three types--great southern comet--supposed previous appearances--tebbutt's comet and the comet of --successful photographs--schaeberle's comet--comet wells--sodium blaze in spectrum--great comet of --transit across the sun--relation to comets of and --cometary systems--spectral changes in comet of --brooks's comet of --swift's comet of --origin of comets chapter xii stars and nebulÆ stellar chemistry--four orders of stars--their relative ages--gaseous stars--spectroscopic star-catalogues--stellar chemistry--hydrogen spectrum in stars--the draper catalogue--velocities of stars in line of sight--spectroscopic binaries--eclipses of algol--catalogues of variables--new stars--outbursts in nebulæ--nova aurigæ--nova persei--gaseous nebulæ--variable nebulæ--movements of nebulæ--stellar and nebular photography--nebulæ in the pleiades--photographic star-charting--stellar parallax--double stars--stellar photometry--status of nebulæ--photographs and drawings of the milky way--star drift chapter xiii methods of research development of telescopic power--silvered glass reflectors--giant refractors--comparison with reflectors--the yerkes telescope--atmospheric disturbance--the lick observatory--mechanical difficulties--the equatoreal _coudé_--the photographic camera--retrospect and conclusion appendix chronology, - --chemical elements in the sun (rowland, )--epochs of sun-spot maximum and minimum from to --movements of sun and stars--list of great telescopes--list of observatories employed in the construction of the photographic chart and catalogue of the heavens index list of illustrations photograph of the great nebula in orion, _frontispiece_ photographs of jupiter, , and of saturn, _vignette_ plate i. photographs of the solar chromosphere and prominences _to face p. _ plate ii. photograph of the great comet of may, (taken at the royal observatory, cape of good hope) " plate iii. the great comet of september, (photographed at the cape of good hope) " plate iv. photographs of swift's comet, " plate v. photographic and visual spectrum of nova aurigæ " plate vi. photograph of the milky way in sagittarius " history of astronomy during the nineteenth century _introduction_ we can distinguish three kinds of astronomy, each with a different origin and history, but all mutually dependent, and composing, in their fundamental unity, one science. first in order of time came the art of observing the returns, and measuring the places, of the heavenly bodies. this was the sole astronomy of the chinese and chaldeans; but to it the vigorous greek mind added a highly complex geometrical plan of their movements, for which copernicus substituted a more harmonious system, without as yet any idea of a compelling cause. the planets revolved in circles because it was their nature to do so, just as laudanum sets to sleep because it possesses a _virtus dormitiva_. this first and oldest branch is known as "observational," or "practical astronomy." its business is to note facts as accurately as possible; and it is essentially unconcerned with schemes for connecting those facts in a manner satisfactory to the reason. the second kind of astronomy was founded by newton. its nature is best indicated by the term "gravitational"; but it is also called "theoretical astronomy."[ ] it is based on the idea of cause; and the whole of its elaborate structure is reared according to the dictates of a single law, simple in itself, but the tangled web of whose consequences can be unravelled only by the subtle agency of an elaborate calculus. the third and last division of celestial science may properly be termed "physical and descriptive astronomy." it seeks to know what the heavenly bodies are in themselves, leaving the how? and the wherefore? of their movements to be otherwise answered. now, such inquiries became possible only through the invention of the telescope, so that galileo was, in point of fact, their originator. but herschel first gave them a prominence which the whole progress of science during the nineteenth century served to confirm and render more exclusive. inquisitions begun with the telescope have been extended and made effective in unhoped-for directions by the aid of the spectroscope and photographic camera; and a large part of our attention in the present volume will be occupied with the brilliant results thus achieved. the unexpected development of this new physical-celestial science is the leading fact in recent astronomical history. it was out of the regular course of events. in the degree in which it has actually occurred it could certainly not have been foreseen. it was a seizing of the prize by a competitor who had hardly been thought qualified to enter the lists. orthodox astronomers of the old school looked with a certain contempt upon observers who spent their nights in scrutinising the faces of the moon and planets rather than in timing their transits, or devoted daylight energies, not to reductions and computations, but to counting and measuring spots on the sun. they were regarded as irregular practitioners, to be tolerated perhaps, but certainly not encouraged. the advance of astronomy in the eighteenth century ran in general an even and logical course. the age succeeding newton's had for its special task to demonstrate the universal validity, and trace the complex results, of the law of gravitation. the accomplishment of that task occupied just one hundred years. it was virtually brought to a close when laplace explained to the french academy, november , , the cause of the moon's accelerated motion. as a mere machine, the solar system, so far as it was then known, was found to be complete and intelligible in all its parts; and in the _mécanique céleste_ its mechanical perfections were displayed under a form of majestic unity which fitly commemorated the successive triumphs of analytical genius over problems amongst the most arduous ever dealt with by the mind of man. theory, however, demands a practical test. all its data are derived from observation; and their insecurity becomes less tolerable as it advances nearer to perfection. observation, on the other hand, is the pitiless critic of theory; it detects weak points, and provokes reforms which may be the beginnings of discovery. thus, theory and observation mutually act and react, each alternately taking the lead in the endless race of improvement. now, while in france lagrange and laplace were bringing the gravitational theory of the solar system to completion, work of a very different kind, yet not less indispensable to the future welfare of astronomy, was being done in england. the royal observatory at greenwich is one of the few useful institutions which date their origin from the reign of charles ii. the leading position which it still occupies in the science of celestial observation was, for near a century and a half after its foundation, an exclusive one. delambre remarked that, had all other materials of the kind been destroyed, the greenwich records alone would suffice for the restoration of astronomy. the establishment was indeed absolutely without a rival.[ ] systematic observations of sun, moon, stars, and planets were during the whole of the eighteenth century made only at greenwich. here materials were accumulated for the secure correction of theory, and here refinements were introduced by which the exquisite accuracy of modern practice in astronomy was eventually attained. the chief promoter of these improvements was james bradley. few men have possessed in an equal degree with him the power of seeing accurately, and reasoning on what they see. he let nothing pass. the slightest inconsistency between what appeared and what was to be expected roused his keenest attention; and he never relaxed his mental grip of a subject until it had yielded to his persistent inquisition. it was to these qualities that he owed his discoveries of the aberration of light and the nutation of the earth's axis. the first was announced in . what is meant by it is that, owing to the circumstance of light not being instantaneously transmitted, the heavenly bodies appear shifted from their true places by an amount depending upon the ratio which the velocity of light bears to the speed of the earth in its orbit. because light travels with enormous rapidity, the shifting is very slight; and each star returns to its original position at the end of a year. bradley's second great discovery was finally ascertained in . nutation is a real "nodding" of the terrestrial axis produced by the dragging of the moon at the terrestrial equatorial protuberance. from it results an _apparent_ displacement of the stars, each of them describing a little ellipse about its true or "mean" position, in a period of nearly nineteen years. now, an acquaintance with the fact and the laws of each of these minute irregularities is vital to the progress of observational astronomy; for without it the places of the heavenly bodies could never be accurately known or compared. so that bradley, by their detection, at once raised the science to a higher grade of precision. nor was this the whole of his work. appointed astronomer-royal in , he executed during the years - a series of observations which formed the real beginning of exact astronomy. part of their superiority must, indeed, be attributed to the co-operation of john bird, who provided bradley in with a measuring instrument of till then unequalled excellence. for not only was the art of observing in the eighteenth century a peculiarly english art, but the means of observing were furnished almost exclusively by british artists. john dollond, the son of a spitalfields weaver, invented the achromatic lens in , removing thereby the chief obstacle to the development of the powers of refracting telescopes; james short, of edinburgh, was without a rival in the construction of reflectors; the sectors, quadrants, and circles of graham, bird, ramsden, and cary were inimitable by continental workmanship. thus practical and theoretical astronomy advanced on parallel lines in england and france respectively, the improvement of their several tools--the telescope and the quadrant on the one side, and the calculus on the other--keeping pace. the whole future of the science seemed to be theirs. the cessation of interest through a too speedy attainment of the perfection towards which each spurred the other, appeared to be the only danger it held in store for them. when all at once, a rival stood by their side--not, indeed, menacing their progress, but threatening to absorb their popularity. the rise of herschel was the one conspicuous anomaly in the astronomical history of the eighteenth century. it proved decisive of the course of events in the nineteenth. it was unexplained by anything that had gone before; yet all that came after hinged upon it. it gave a new direction to effort; it lent a fresh impulse to thought. it opened a channel for the widespread public interest which was gathering towards astronomical subjects to flow in. much of this interest was due to the occurrence of events calculated to arrest the attention and excite the wonder of the uninitiated. the predicted return of halley's comet in verified, after an unprecedented fashion, the computations of astronomers. it deprived such bodies for ever of their portentous character; it ranked them as denizens of the solar system. again, the transits of venus in and were the first occurrences of the kind since the awakening of science to their consequence. imposing preparations, journeys to remote and hardly accessible regions, official expeditions, international communications, all for the purpose of observing them to the best advantage, brought their high significance vividly to the public consciousness; a result aided by the facile pen of lalande, in rendering intelligible the means by which these elaborate arrangements were to issue in an accurate knowledge of the sun's distance. lastly, herschel's discovery of uranus, march , , had the surprising effect of utter novelty. since the human race had become acquainted with the company of the planets, no addition had been made to their number. the event thus broke with immemorial traditions, and seemed to show astronomy as still young and full of unlooked-for possibilities. further popularity accrued to the science from the sequel of a career so strikingly opened. herschel's huge telescopes, his detection by their means of two saturnian and as many uranian moons, his piercing scrutiny of the sun, picturesque theory of its constitution, and sagacious indication of the route pursued by it through space; his discovery of stellar revolving systems, his bold soundings of the universe, his grandiose ideas, and the elevated yet simple language in which they were conveyed--formed a combination powerfully effective to those least susceptible of new impressions. nor was the evoked enthusiasm limited to the british isles. in germany, schröter followed--_longo intervallo_--in herschel's track. von zach set on foot from gotha that general communication of ideas which gives life to a forward movement. bode wrote much and well for unlearned readers. lalande, by his popular lectures and treatises, helped to form an audience which laplace himself did not disdain to address in the _exposition du système du monde_. this great accession of public interest gave the impulse to the extraordinarily rapid progress of astronomy in the nineteenth century. official patronage combined with individual zeal sufficed for the elder branches of the science. a few well-endowed institutions could accumulate the materials needed by a few isolated thinkers for the construction of theories of wonderful beauty and elaboration, yet precluded, by their abstract nature, from winning general applause. but the new physical astronomy depends for its prosperity upon the favour of the multitude whom its striking results are well fitted to attract. it is, in a special manner, the science of amateurs. it welcomes the most unpretending co-operation. there is no one "with a true eye and a faithful hand" but can do good work in watching the heavens. and not unfrequently, prizes of discovery which the most perfect appliances failed to grasp, have fallen to the share of ignorant or ill-provided assiduity. observers, accordingly, have multiplied; observatories have been founded in all parts of the world; associations have been constituted for mutual help and counsel. a formal astronomical congress met in at gotha--then, under duke ernest ii. and von zach, the focus of german astronomy--and instituted a combined search for the planet suspected to revolve undiscovered between the orbits of mars and jupiter. the astronomical society of london was established in , and the similar german institution in . both have been highly influential in promoting the interests, local and general, of the science they are devoted to forward; while functions corresponding to theirs have been discharged elsewhere by older or less specially constituted bodies, and new ones of a more popular character are springing up on all sides. modern facilities of communication have helped to impress more deeply upon modern astronomy its associative character. the electric telegraph gives a certain ubiquity which is invaluable to an observer of the skies. with the help of a wire, a battery, and a code of signals, he sees whatever is visible from any portion of our globe, depending, however, upon other eyes than his own, and so entering as a unit into a widespread organisation of intelligence. the press, again, has been a potent agent of co-operation. it has mainly contributed to unite astronomers all over the world into a body animated by the single aim of collecting "particulars" in their special branch for what bacon termed a history of nature, eventually to be interpreted according to the sagacious insight of some one among them gifted above his fellows. the first really effective astronomical periodical was the _monatliche correspondenz_, started by von zach in the year . it was followed in by the _astronomische nachrichten_, later by the _memoirs_ and _monthly notices_ of the astronomical society, and by the host of varied publications which now, in every civilised country, communicate the discoveries made in astronomy to divers classes of readers, and so incalculably quicken the current of its onward flow. public favour brings in its train material resources. it is represented by individual enterprise, and finds expression in an ample liberality. the first regular observatory in the southern hemisphere was founded at paramatta by sir thomas makdougall brisbane in . the royal observatory at the cape of good hope was completed in . similar establishments were set to work by the east india company at madras, bombay, and st. helena, during the first third of the nineteenth century. the organisation of astronomy in the united states of america was due to a strong wave of popular enthusiasm. in john quincy adams vainly urged upon congress the foundation of a national observatory; but in the lectures on celestial phenomena of ormsby macknight mitchel stirred an impressionable audience to the pitch of providing him with the means of erecting at cincinnati the first astronomical establishment worthy the name in that great country. on the st of january, , no less than one hundred and forty-four were active within its boundaries. the apparition of the great comet of gave an additional fillip to the movement. to the excitement caused by it the harvard college observatory--called the "american pulkowa"--directly owed its origin; and the example was not ineffective elsewhere. the united states naval observatory was built in , lieutenant maury being its first director. corporations, universities, municipalities, vied with each other in the creation of such institutions; private subscriptions poured in; emissaries were sent to europe to purchase instruments and to procure instruction in their use. in a few years the young republic was, in point of astronomical efficiency, at least on a level with countries where the science had been fostered since the dawn of civilisation. a vast widening of the scope of astronomy has accompanied, and in part occasioned, the great extension of its area of cultivation which our age has witnessed. in the last century its purview was a comparatively narrow one. problems lying beyond the range of the solar system were almost unheeded, because they seemed inscrutable. herschel first showed the sidereal universe as accessible to investigation, and thereby offered to science new worlds--majestic, manifold, "infinitely infinite" to our apprehension in number, variety, and extent--for future conquest. their gradual appropriation has absorbed, and will long continue to absorb, the powers which it has served to develop. but this is not the only direction in which astronomy has enlarged, or rather has levelled, its boundaries. the unification of the physical sciences is perhaps the greatest intellectual feat of recent times. the process has included astronomy; so that, like bacon, she may now be said to have "taken all knowledge" (of that kind) "for her province." in return, she proffers potent aid for its increase. every comet that approaches the sun is the scene of experiments in the electrical illumination of rarefied matter, performed on a huge scale for our benefit. the sun, stars, and nebulæ form so many celestial laboratories, where the nature and mutual relations of the chemical "elements" may be tried by more stringent tests than sublunary conditions afford. the laws of terrestrial magnetism can be completely investigated only with the aid of a concurrent study of the face of the sun. the solar spectrum will perhaps one day, by its recurrent modifications, tell us something of impending droughts, famines, and cyclones. astronomy generalises the results of the other sciences. she exhibits the laws of nature working over a wider area, and under more varied conditions, than ordinary experience presents. ordinary experience, on the other hand, has become indispensable to her progress. she takes in at one view the indefinitely great and the indefinitely little. the mutual revolutions of the stellar multitude during tracts of time which seem to lengthen out to eternity as the mind attempts to traverse them, she does not admit to be beyond her ken; nor is she indifferent to the constitution of the minutest atom of matter that thrills the ether into light. how she entered upon this vastly expanded inheritance, and how, so far, she has dealt with it, is attempted to be set forth in the ensuing chapters. footnotes: [footnote : the denomination "physical astronomy," first used by kepler, and long appropriated to this branch of the science, has of late been otherwise applied.] [footnote : _histoire de l'astronomie au xviii^e siècle_, p. .] part i progress of astronomy during the first half of the nineteenth century chapter i _foundation of sidereal astronomy_ until nearly a hundred years ago the stars were regarded by practical astronomers mainly as a number of convenient fixed points by which the motions of the various members of the solar system could be determined and compared. their recognised function, in fact, was that of milestones on the great celestial highway traversed by the planets, as well as on the byways of space occasionally pursued by comets. not that curiosity as to their nature, and even conjecture as to their origin, were at any period absent. both were from time to time powerfully stimulated by the appearance of startling novelties in a region described by philosophers as "incorruptible," or exempt from change. the catalogue of hipparchus probably, and certainly that of tycho brahe, some seventeen centuries later, owed each its origin to the temporary blaze of a new star. the general aspect of the skies was thus (however imperfectly) recorded from age to age, and with improved appliances the enumeration was rendered more and more accurate and complete; but the secrets of the stellar sphere remained inviolate. in a qualified though very real sense, sir william herschel may be called the founder of sidereal astronomy. before his time some curious facts had been noted, and some ingenious speculations hazarded, regarding the condition of the stars, but not even the rudiments of systematic knowledge had been acquired. the facts ascertained can be summed up in a very few sentences. giordano bruno was the first to set the suns of space in motion; but in imagination only. his daring surmise was, however, confirmed in , when halley announced[ ] that sirius, aldebaran, betelgeux, and arcturus had unmistakably shifted their quarters in the sky since ptolemy assigned their places in his catalogue. a similar conclusion was reached by j. cassini in , from a comparison of his own observations with those made at cayenne by richer in ; and tobias mayer drew up in a list showing the direction and amount of about fifty-seven proper motions,[ ] founded on star-places determined by olaus römer fifty years previously. thus the stars were no longer regarded as "fixed," but the question remained whether the movements perceived were real or only apparent; and this it was not yet found possible to answer. already, in the previous century, the ingenious robert hooke had suggested an "alteration of the very system of the sun,"[ ] to account for certain suspected changes in stellar positions; bradley in , and lambert in , pointed out that such apparent displacements (by that time well ascertained) were in all probability a combined effect of motions both of sun and stars; and mayer actually attempted the analysis, but without result. on the th of august, , david fabricius, an unprofessional astronomer in east friesland, saw in the neck of the whale a star of the third magnitude, which by october had disappeared. it was, nevertheless, visible in , when bayer marked it in his catalogue with the greek letter omicron, and was watched, in - , through its phases of brightening and apparent extinction by a dutch professor named holwarda.[ ] from hevelius this first-known periodical star received the name of "mira," or the wonderful, and boulliaud in fixed the length of its cycle of change at days. it was not a solitary instance. a star in the swan was perceived by janson in to show fluctuations of light, and montanari found in that algol in perseus shared the same peculiarity to a marked degree. altogether the class embraced in half-a-dozen members. when it is added that a few star-couples had been noted in singularly, but it was supposed accidentally, close juxtaposition, and that the failure of repeated attempts to measure stellar parallaxes pointed to distances _at least_ , times that of the earth from the sun,[ ] the picture of sidereal science, when the last quarter of the eighteenth century began, is practically complete. it included three items of information: that the stars have motions, real or apparent; that they are immeasurably remote; and that a few shine with a periodically variable light. nor were these scantily collected facts ordered into any promise of further development. they lay at once isolated and confused before the inquirer. they needed to be both multiplied and marshalled, and it seemed as if centuries of patient toil must elapse before any reliable conclusions could be derived from them. the sidereal world was thus the recognised domain of far-reaching speculations, which remained wholly uncramped by systematic research until herschel entered upon his career as an observer of the heavens. the greatest of modern astronomers was born at hanover, november , . he was the fourth child of isaac herschel, a hautboy-player in the band of the hanoverian guard, and was early trained to follow his father's profession. on the termination, however, of the disastrous campaign of , his parents removed him from the regiment, there is reason to believe, in a somewhat unceremonious manner. technically, indeed, he incurred the penalties of desertion, remitted--according to the duke of sussex's statement to sir george airy--by a formal pardon handed to him personally by george iii. on his presentation in .[ ] at the age of nineteen, then, his military service having lasted four years, he came to england to seek his fortune. of the life of struggle and privation which ensued little is known beyond the circumstances that in he was engaged in training the regimental band of the durham militia, and that in he was appointed organist at halifax. in the following year he removed to bath as oboist in linley's orchestra, and in october was promoted to the post of organist in the octagon chapel. the tide of prosperity now began to flow for him. the most brilliant and modish society in england was at that time to be met at bath, and the young hanoverian quickly found himself a favourite and the fashion in it. engagements multiplied upon him. he became director of the public concerts; he conducted oratorios, engaged singers, organised rehearsals, composed anthems, chants, choral services, besides undertaking private tuitions, at times amounting to thirty-five or even thirty-eight lessons a week. he in fact personified the musical activity of a place then eminently and energetically musical. but these multifarious avocations did not take up the whole of his thoughts. his education, notwithstanding the poverty of his family, had not been neglected, and he had always greedily assimilated every kind of knowledge that came in his way. now that he was a busy and a prosperous man, it might have been expected that he would run on in the deep professional groove laid down for him. on the contrary, his passion for learning seemed to increase with the diminution of the time available for its gratification. he studied italian, greek, mathematics; maclaurin's fluxions served to "unbend his mind"; smith's harmonics and optics and ferguson's astronomy were the nightly companions of his pillow. what he read stimulated without satisfying his intellect. he desired not only to know, but to discover. in he hired a small telescope, and through it caught a preliminary glimpse of the rich and varied fields in which for so many years he was to expatiate. henceforward the purpose of his life was fixed: it was to obtain "a knowledge of the construction of the heavens";[ ] and this sublime ambition he cherished to the end. a more powerful instrument was the first desideratum; and here his mechanical genius came to his aid. having purchased the apparatus of a quaker optician, he set about the manufacture of specula with a zeal which seemed to anticipate the wonders they were to disclose to him. it was not until fifteen years later that his grinding and polishing machines were invented, so the work had at that time to be entirely done by hand. during this tedious and laborious process (which could not be interrupted without injury, and lasted on one occasion sixteen hours), his strength was supported by morsels of food put into his mouth by his sister,[ ] and his mind amused by her reading aloud to him the arabian nights, don quixote, or other light works. at length, after repeated failures, he found himself provided with a reflecting telescope--a - / -foot gregorian--of his own construction. a copy of his first observation with it, on the great nebula in orion--an object of continual amazement and assiduous inquiry to him--is preserved by the royal society. it bears the date march , .[ ] in the following year he executed his first "review of the heavens," memorable chiefly as an evidence of the grand and novel conceptions which already inspired him, and of the enthusiasm with which he delivered himself up to their guidance. overwhelmed with professional engagements, he still contrived to snatch some moments for the stars; and between the acts at the theatre was often seen running from the harpsichord to his telescope, no doubt with that "uncommon precipitancy which accompanied all his actions."[ ] he now rapidly increased the power and perfection of his telescopes. mirrors of seven, ten, even twenty feet focal length, were successively completed, and unprecedented magnifying powers employed. his energy was unceasing, his perseverance indomitable. in the course of twenty-one years no less than parabolic specula left his hands. he had entered upon his forty-second year when he sent his first paper to the _philosophical transactions_; yet during the ensuing thirty-nine years his contributions--many of them elaborate treatises--numbered sixty-nine, forming a series of extraordinary importance to the history of astronomy. as a mere explorer of the heavens his labours were prodigious. he discovered , nebulæ, double stars, passed the whole firmament in review four several times, counted the stars in , "gauge-fields," and executed a photometric classification of the principal stars, founded on an elaborate (and the first systematically conducted) investigation of their relative brightness. he was as careful and patient as he was rapid; spared no time and omitted no precaution to secure accuracy in his observations; yet in one night he would examine, singly and attentively, up to separate objects. the discovery of uranus was a mere incident of the scheme he had marked out for himself--a fruit, gathered as it were by the way. it formed, nevertheless, the turning-point in his career. from a star-gazing musician he was at once transformed into an eminent astronomer; he was relieved from the drudgery of a toilsome profession, and installed as royal astronomer, with a modest salary of £ a year; funds were provided for the construction of the forty-foot reflector, from the great space-penetrating power of which he expected unheard-of revelations; in fine, his future work was not only rendered possible, but it was stamped as authoritative.[ ] on whit-sunday , william and caroline herschel played and sang in public for the last time in st. margaret's chapel, bath; in august of the same year the household was moved to datchet, near windsor, and on april , , to slough. here happiness and honours crowded on the fortunate discoverer. in he married mary, only child of james baldwin, a merchant of the city of london, and widow of mr. john pitt--a lady whose domestic virtues were enhanced by the possession of a large jointure. the fruit of their union was one son, of whose work--the worthy sequel of his father's--we shall have to speak further on. herschel was created a knight of the hanoverian guelphic order in , and in he became the first president of the royal astronomical society, his son being its first foreign secretary. but his health had now for some years been failing, and on august , , he died at slough, in the eighty-fourth year of his age, and was buried in upton churchyard. his epitaph claims for him the lofty praise of having "burst the barriers of heaven." let us see in what sense this is true. the first to form any definite idea as to the constitution of the stellar system was thomas wright, the son of a carpenter living at byer's green, near durham. with him originated what has been called the "grindstone theory" of the universe, which regarded the milky way as the projection on the sphere of a stratum or disc of stars (our sun occupying a position near the centre), similar in magnitude and distribution to the lucid orbs of the constellations.[ ] he was followed by kant,[ ] who transcended the views of his predecessor by assigning to nebulæ the position they long continued to occupy, rather on imaginative than scientific grounds, of "island universes," external to, and co-equal with, the galaxy. johann heinrich lambert,[ ] a tailor's apprentice from mühlhausen, followed, but independently. the conceptions of this remarkable man were grandiose, his intuitions bold, his views on some points a singular anticipation of subsequent discoveries. the sidereal world presented itself to him as a hierarchy of systems, starting from the planetary scheme, rising to throngs of suns within the circuit of the milky way--the "ecliptic of the stars," as he phrased it--expanding to include groups of many milky ways; these again combining to form the unit of a higher order of assemblage, and so onwards and upwards until the mind reels and sinks before the immensity of the contemplated creations. "thus everything revolves--the earth round the sun; the sun round the centre of his system; this system round a centre common to it with other systems; this group, this assemblage of systems, round a centre which is common to it with other groups of the same kind; and where shall we have done?"[ ] the stupendous problem thus speculatively attempted, herschel undertook to grapple with experimentally. the upshot of this memorable inquiry was the inclusion, for the first time, within the sphere of human knowledge, of a connected body of facts, and inferences from facts, regarding the sidereal universe; in other words, the foundation of what may properly be called a science of the stars. tobias mayer had illustrated the perspective effects which must ensue in the stellar sphere from a translation of the solar system, by comparing them to the separating in front and closing up behind of trees in a forest to the eye of an advancing spectator;[ ] but the appearances which he thus correctly described he was unable to detect. by a more searching analysis of a smaller collection of proper motions, herschel succeeded in rendering apparent the very consequences foreseen by mayer. he showed, for example, that arcturus and vega did, in fact, appear to recede from, and sirius and aldebaran to approach, each other by very minute amounts; and, with a striking effort of divinatory genius, placed the "apex," or point of direction of the sun's motion, close to the star lambda in the constellation hercules,[ ] within a few degrees of the spot indicated by later and indefinitely more refined methods of research. he resumed the subject in ,[ ] but though employing a more rigorous method, was scarcely so happy in his result. in ,[ ] he made a preliminary attempt to ascertain the speed of the sun's journey, fixing it, by doubtless much too low an estimate, at about three miles a second. yet the validity of his general conclusion as to the line of solar travel, though long doubted, has been triumphantly confirmed. the question as to the "secular parallax" of the fixed stars was in effect answered. with their _annual_ parallax, however, the case was very different. the search for it had already led bradley to the important discoveries of the aberration of light and the nutation of the earth's axis; it was now about to lead herschel to a discovery of a different, but even more elevated character. yet in neither case was the object primarily sought attained. from the very first promulgation of the copernician theory the seeming immobility of the stars had been urged as an argument against its truth; for if the earth really travelled in a vast orbit round the sun, objects in surrounding space should appear to change their positions, unless their distances were on a scale which, to the narrow ideas of the universe then prevailing, seemed altogether extravagant.[ ] the existence of such apparent or "parallactic" displacements was accordingly regarded as the touchstone of the new views, and their detection became an object of earnest desire to those interested in maintaining them. copernicus himself made the attempt; but with his "triquetrum," a jointed wooden rule with the divisions marked in ink, constructed by himself,[ ] he was hardly able to measure angles of ten minutes, far less fractions of a second. galileo, a more impassioned defender of the system, strained his ears, as it were, from arcetri, in his blind and sorrowful old age, for news of a discovery which two more centuries had still to wait for. hooke believed he had found a parallax for the bright star in the head of the dragon; but was deceived. bradley convinced himself that such effects were too minute for his instruments to measure. herschel made a fresh attempt by a practically untried method. it is a matter of daily experience that two objects situated at different distances seem to a beholder in motion to move relatively to each other. this principle galileo, in the third of his dialogues on the systems of the world,[ ] proposed to employ for the determination of stellar parallax; for two stars, lying apparently close together, but in reality separated by a great gulf of space, must shift their mutual positions when observed from opposite points of the earth's orbit; or rather, the remoter forms a virtually fixed point, to which the movements of the other can be conveniently referred. by this means complications were abolished more numerous and perplexing than galileo himself was aware of, and the problem was reduced to one of simple micrometrical measurement. the "double-star method" was also suggested by james gregory in , and again by wallis in ;[ ] huygens first, and afterwards dr. long of cambridge (about ), made futile experiments with it; and it eventually led, in the hands of bessel, to the successful determination of the parallax of cygni. its advantages were not lost upon herschel. his attempt to assign definite distances to the nearest stars was no isolated effort, but part of the settled plan upon which his observations were conducted. he proposed to sound the heavens, and the first requisite was a knowledge of the length of his sounding-line. thus it came about that his special attention was early directed to double stars. "i resolved," he writes,[ ] "to examine every star in the heavens with the utmost attention and a very high power, that i might collect such materials for this research as would enable me to fix my observations upon those that would best answer my end. the subject has already proved so extensive, and still promises so rich a harvest to those who are inclined to be diligent in the pursuit, that i cannot help inviting every lover of astronomy to join with me in observations that must inevitably lead to new discoveries." the first result of these inquiries was a classed catalogue of double stars presented to the royal society in , followed, after three years, by an additional list of . in both these collections the distances separating the individuals of each pair were carefully measured, and (with a few exceptions) the angles made with the hour-circle by the lines joining their centres (technically called "angles of position") were determined with the aid of a "revolving-wire micrometer," specially devised for the purpose. moreover, an important novelty was introduced by the observation of the various colours visible in the star-couples, the singular and vivid contrasts of which were now for the first time described. double stars were at that time supposed to be a purely optical phenomenon. their components, it was thought, while in reality indefinitely remote from each other, were brought into fortuitous contiguity by the chance of lying nearly in the same line of sight from the earth. yet bradley had noticed a change of °, between and , in the position-angle of the two stars forming castor, and was thus within a hair's breadth of the discovery of their physical connection.[ ] while the rev. john michell, arguing by the doctrine of probabilities, wrote as follows in :--"it is highly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together, do really consist of stars placed near together, and under the influence of some general law."[ ] and in :[ ] "it is not improbable that a few years may inform us that some of the great number of double, triple stars, etc., which have been observed by mr. herschel, are systems of bodies revolving about each other." this remarkable speculative anticipation had a practical counterpart in germany. father christian mayer, a jesuit astronomer at mannheim, set himself, in january , to collect examples of stellar pairs, and shortly after published the supposed discovery of "satellites" to many of the principal stars.[ ] but his observations were neither exact nor prolonged enough to lead to useful results in such an inquiry. his disclosures were derided; his planet-stars treated as results of hallucination. _on n'a point cru à des choses aussi extraordinaires_, wrote lalande[ ] within one year of a better-grounded announcement to the same effect. herschel at first shared the general opinion as to the merely optical connection of double stars. of this the purpose for which he made his collection is in itself sufficient evidence, since what may be called the _differential_ method of parallaxes depends, as we have seen, for its efficacy upon disparity of distance. it was "much too soon," he declared in ,[ ] "to form any theories of small stars revolving round large ones;" while in the year following,[ ] he remarked that the identical proper motions of the two stars forming, to the naked eye, the single bright orb of castor could only be explained as both equally due to the "systematic parallax" caused by the sun's movement in space. plainly showing that the notion of a physical tie, compelling the two bodies to travel together, had not as yet entered into his speculations. but he was eminently open to conviction, and had, moreover, by observations unparalleled in amount as well as in kind, prepared ample materials for convincing himself and others. in he was able to announce the fact of his discovery, and in the two ensuing years, to lay in detail before the royal society proofs, gathered from the labours of a quarter of a century, of orbital revolution in the case of as many as fifty double stars, henceforth, he declared, to be held as real binary combinations, "intimately held together by the bond of mutual attraction."[ ] the fortunate preservation in dr. maskelyne's note-book of a remark made by bradley about , to the effect that the line joining the components of castor was an exact prolongation of that joining castor with pollux, added eighteen years to the time during which the pair were under scrutiny, and confirmed the evidence of change afforded by more recent observations. approximate periods were fixed for many of the revolving suns--for castor years; for gamma leonis, , delta serpentis, , eta bootis, years; eta lyræ was noted as a "double-double-star," a change of relative situation having been detected in each of the two pairs composing the group; and the occultation was described of one star by another in the course of their mutual revolutions, as exemplified in by the rapidly circulating system of zeta herculis. thus, by the sagacity and perseverance of a single observer, a firm basis was at last provided upon which to raise the edifice of sidereal science. the analogy long presumed to exist between the mighty star of our system and the bright points of light spangling the firmament was shown to be no fiction of the imagination, but a physical reality; the fundamental quality of attractive power was proved to be common to matter so far as the telescope was capable of exploring, and law, subordination, and regularity to give testimony of supreme and intelligent design no less in those limitless regions of space than in our narrow terrestrial home. the discovery was emphatically (in arago's phrase) "one with a future," since it introduced the element of precise knowledge where more or less probable conjecture had previously held almost undivided sway; and precise knowledge tends to propagate itself and advance from point to point. we have now to speak of herschel's pioneering work in the skies. to explore with line and plummet the shining zone of the milky way, to delineate its form, measure its dimensions, and search out the intricacies of its construction, was the primary task of his life, which he never lost sight of, and to which all his other investigations were subordinate. he was absolutely alone in this bold endeavour. unaided, he had to devise methods, accumulate materials, and sift out results. yet it may safely be asserted that all the knowledge we possess on this sublime subject was prepared, and the greater part of it anticipated, by him. the ingenious method of "star-gauging," and its issue in the delineation of the sidereal system as an irregular stratum of evenly-scattered suns, is the best-known part of his work. but it was, in truth, only a first rude approximation, the principle of which maintained its credit in the literature of astronomy a full half-century after its abandonment by its author. this principle was the general equality of star distribution. if equal portions of space really held equal numbers of stars, it is obvious that the number of stars visible in any particular direction would be strictly proportional to the range of the system in that direction, apparent accumulation being produced by real extent. the process of "gauging the heavens," accordingly, consisted in counting the stars in successive telescopic fields, and calculating thence the depths of space necessary to contain them. the result of , such operations was the plan of the galaxy familiar to every reader of an astronomical text-book. widely-varying evidence was, as might have been expected, derived from an examination of different portions of the sky. some fields of view were almost blank, while others (in or near the milky way) blazed with the radiance of many hundred stars compressed into an area about one-fourth that of the full-moon. in the most crowded parts , were stated to have been passed in review within a quarter of an hour. here the "length of his sounding-line" was estimated by herschel at about times the distance of sirius--in other words, the bounding orb, or farthest sun of the system in that direction, so far as could be seen with the -foot reflector, was thus inconceivably remote. but since the distance of sirius, no less than of every other fixed star, was as yet an unknown quantity, the dimensions inferred for the galaxy were of course purely relative; a knowledge of its form and structure might (admitting the truth of the fundamental hypothesis) be obtained, but its real or absolute size remained altogether undetermined. even as early as , however, herschel perceived traces of a tendency which completely invalidated the supposition of any approach to an average uniformity of distribution. this was the action of what he called a "clustering power" in the milky way. "many gathering clusters"[ ] were already discernible to him even while he endeavoured to obtain a "true _mean_ result" on the assumption that each star in space was separated from its neighbours as widely as the sun from sirius. "it appears," he wrote in , "that the heavens consist of regions where suns are gathered into separate systems"; and in certain assemblages he was able to trace "a course or tide of stars setting towards a centre," denoting, not doubtfully, the presence of attractive forces.[ ] thirteen years later, he described our sun and his constellated companions as surrounded by "a magnificent collection of innumerable stars, called the milky way, which must occasion a very powerful balance of opposite attractions to hold the intermediate stars at rest. for though our sun, and all the stars we see, may truly be said to be in the plane of the milky way, yet i am now convinced, by a long inspection and continued examination of it, that the milky way itself consists of stars very differently scattered from those which are immediately about us." "this immense aggregation," he added, "is by no means uniform. its component stars show evident signs of clustering together into many separate allotments."[ ] the following sentences, written in , contain a definite retractation of the view frequently attributed to him:-- "i must freely confess," he says, "that by continuing my sweeps of the heavens my opinion of the arrangement of the stars and their magnitudes, and of some other particulars, has undergone a gradual change; and indeed, when the novelty of the subject is considered, we cannot be surprised that many things formerly taken for granted should on examination prove to be different from what they were generally but incautiously supposed to be. for instance, an equal scattering of the stars may be admitted in certain calculations; but when we examine the milky way, or the closely compressed clusters of stars of which my catalogues have recorded so many instances, this supposed equality of scattering must be given up."[ ] another assumption, the fallacy of which he had not the means of detecting since become available, was retained by him to the end of his life. it was that the brightness of a star afforded an approximate measure of its distance. upon this principle he founded in his method of "limiting apertures,"[ ] by which two stars, brought into view in two precisely similar telescopes, were "equalised" by covering a certain portion of the object-glass collecting the more brilliant rays. the distances of the orbs compared were then taken to be in the ratio of the reduced to the original apertures of the instruments with which they were examined. if indeed the absolute lustre of each were the same, the result might be accepted with confidence; but since we have no warrant for assuming a "standard star" to facilitate our computations, but much reason to suppose an indefinite range, not only of size but of intrinsic brilliancy, in the suns of our firmament, conclusions drawn from such a comparison are entirely worthless. in another branch of sidereal science besides that of stellar aggregation, herschel may justly be styled a pioneer. he was the first to bestow serious study on the enigmatical objects known as "nebulæ." the history of the acquaintance of our race with them is comparatively short. the only one recognised before the invention of the telescope was that in the girdle of andromeda, certainly familiar in the middle of the tenth century to the persian astronomer abdurrahman al-sûfi; and marked with dots on spanish and dutch constellation-charts of the fourteenth and fifteenth centuries.[ ] yet so little was it noticed that it might practically be said--as far as europe is concerned--to have been discovered in by simon marius (mayer of genzenhausen), who aptly described its appearance as that of a "candle shining through horn." the first mention of the great orion nebula is by a swiss jesuit named cysatus, who succeeded father scheiner in the chair of mathematics at ingolstadt. he used it, apparently without any suspicion of its novelty, as a term of comparison for the comet of december .[ ] a novelty, nevertheless, to astronomers it still remained in , when huygens discerned, "as it were, an hiatus in the sky, affording a glimpse of a more luminous region beyond."[ ] halley in knew of six nebulæ, which he believed to be composed of a "lucid medium" diffused through the ether of space.[ ] he appears, however, to have been unacquainted with some previously noticed by hevelius. lacaille brought back with him from the cape a list of forty-two--the first-fruits of observation in southern skies--arranged in three numerically equal classes;[ ] and messier (nicknamed by louis xv. the "ferret of comets"), finding such objects a source of extreme perplexity in the pursuit of his chosen game, attempted to eliminate by methodising them, and drew up a catalogue comprising, in , entries.[ ] these preliminary attempts shrank into insignificance when herschel began to "sweep the heavens" with his giant telescopes. in he presented to the royal society a descriptive catalogue of , nebulæ and clusters, followed, three years later, by a second of as many more; to which he added in a further gleaning of . on the subject of their nature his views underwent a remarkable change. finding that his potent instruments resolved into stars many nebulous patches in which no signs of such a structure had previously been discernible, he naturally concluded that "resolvability" was merely a question of distance and telescopic power. he was (as he said himself) led on by almost imperceptible degrees from evident clusters, such as the pleiades, to spots without a trace of stellar formation, the gradations being so well connected as to leave no doubt that all these phenomena were equally stellar. the singular variety of their appearance was thus described by him:-- "i have seen," he says, "double and treble nebulæ variously arranged; large ones with small, seeming attendants; narrow, but much extended lucid nebulæ or bright dashes; some of the shape of a fan, resembling an electric brush, issuing from a lucid point; others of the cometic shape, with a seeming nucleus in the centre, or like cloudy stars surrounded with a nebulous atmosphere; a different sort, again, contain a nebulosity of the milky kind, like that wonderful, inexplicable phenomenon about theta orionis; while others shine with a fainter, mottled kind of light, which denotes their being resolvable into stars."[ ] "these curious objects" he considered to be "no less than whole sidereal systems,"[ ] some of which might "well outvie our milky way in grandeur." he admitted, however, a wide diversity in condition as well as compass. the system to which our sun belongs he described as "a very extensive branching congeries of many millions of stars, which probably owes its origin to many remarkably large as well as pretty closely scattered small stars, that may have drawn together the rest."[ ] but the continued action of this same "clustering power" would, he supposed, eventually lead to the breaking-up of the original majestic galaxy into two or three hundred separate groups, already visibly gathering. such minor nebulæ, due to the "decay" of other "branching nebulæ" similar to our own, he recognised by the score, lying, as it were, stratified in certain quarters of the sky. "one of these nebulous beds," he informs us, "is so rich that in passing through a section of it, in the time of only thirty-six minutes, i detected no less than thirty-one nebulæ, all distinctly visible upon a fine blue sky." the stratum of coma berenices he judged to be the nearest to our system of such layers; nor did the marked aggregation of nebulæ towards both poles of the circle of the milky way escape his notice. by a continuation of the same process of reasoning, he was enabled (as he thought) to trace the life-history of nebulæ from a primitive loose and extended formation, through clusters of gradually increasing compression, down to the kind named by him "planetary" because of the defined and uniform discs which they present. these he regarded as "very aged, and drawing on towards a period of change or dissolution."[ ] "this method of viewing the heavens," he concluded, "seems to throw them into a new kind of light. they now are seen to resemble a luxuriant garden which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. for, to continue the simile which i have borrowed from the vegetable kingdom, is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?"[ ] but already this supposed continuity was broken. after mature deliberation on the phenomena presented by nebulous stars, herschel was induced, in , to modify essentially his original opinion. "when i pursued these researches," he says, "i was in the situation of a natural philosopher who follows the various species of animals and insects from the height of their perfection down to the lowest ebb of life; when, arriving at the vegetable kingdom, he can scarcely point out to us the precise boundary where the animal ceases and the plant begins; and may even go so far as to suspect them not to be essentially different. but, recollecting himself, he compares, for instance, one of the human species to a tree, and all doubt upon the subject vanishes before him. in the same manner we pass through gentle steps from a coarse cluster of stars, such as the pleiades ... till we find ourselves brought to an object such as the nebula in orion, where we are still inclined to remain in the once adopted idea of stars exceedingly remote and inconceivably crowded, as being the occasion of that remarkable appearance. it seems, therefore, to require a more dissimilar object to set us right again. a glance like that of the naturalist, who casts his eye from the perfect animal to the perfect vegetable, is wanting to remove the veil from the mind of the astronomer. the object i have mentioned above is the phenomenon that was wanting for this purpose. view, for instance, the th cluster of my th class, and afterwards cast your eye on this cloudy star, and the result will be no less decisive than that of the naturalist we have alluded to. our judgment, i may venture to say, will be, that _the nebulosity about the star is not of a starry nature_."[ ] the conviction thus arrived at of the existence in space of a widely diffused "shining fluid" (a conviction long afterwards fully justified by the spectroscope) led him into a field of endless speculation. what was its nature? should it "be compared to the coruscation of the electric fluid in the aurora borealis? or to the more magnificent cone of the zodiacal light?" above all, what was its function in the cosmos? and on this point he already gave a hint of the direction in which his mind was moving by the remark that this self-luminous matter seemed "more fit to produce a star by its condensation, than to depend on the star for its existence."[ ] this was not a novel idea. tycho brahe had tried to explain the blaze of the star of as due to a sudden concentration of nebulous material in the milky way, even pointing out the space left dark and void by the withdrawal of the luminous stuff; and kepler, theorising on a similar stellar apparition in , followed nearly in the same track. but under herschel's treatment the nebular origin of stars first acquired the consistency of a formal theory. he meditated upon it long and earnestly, and in two elaborate treatises, published respectively in and , he at length set forth the arguments in its favour. these rested entirely upon the "principle of continuity." between the successive classes of his assortment of developing objects there was, as he said, "perhaps not so much difference as would be in an annual description of the human figure, were it given from the birth of a child till he comes to be a man in his prime."[ ] from diffused nebulosity, barely visible in the most powerful light-gathering instruments, but which he estimated to cover nearly square degrees of the heavens,[ ] to planetary nebulæ, supposed to be already centrally solid, instances were alleged of every stage and phase of condensation. the validity of his reasoning, however, was evidently impaired by his confessed inability to distinguish between the dim rays of remote clusters and the milky light of true gaseous nebulæ. it may be said that such speculations are futile in themselves, and necessarily barren of results. but they gratify an inherent tendency of the human mind, and, if pursued in a becoming spirit, should be neither reproved nor disdained. herschel's theory still holds the field, the testimony of recent discoveries with regard to it having proved strongly confirmatory of its principle, although not of its details. strangely enough, it seems to have been propounded in complete independence of laplace's nebular hypothesis as to the origin of the solar system. indeed, it dated, as we have seen, in its first inception, from , while the french geometrician's view was not advanced until . we may now briefly sum up the chief results of herschel's long years of "watching the heavens." the apparent motions of the stars had been disentangled; one portion being clearly shown to be due to a translation towards a point in the constellation hercules of the sun and his attendant planets; while a large balance of displacement was left to be accounted for by real movements, various in extent and direction, of the stars themselves. by the action of a central force similar to, if not identical with, gravity, suns of every degree of size and splendour, and sometimes brilliantly contrasted in colour, were seen to be held together in systems, consisting of two, three, four, even six members, whose revolutions exhibited a wide range of variety both in period and in orbital form. a new department of physical astronomy was thus created,[ ] and rigid calculation for the first time made possible within the astral region. the vast problem of the arrangement and relations of the millions of stars forming the milky way was shown to be capable of experimental treatment, and of at least partial solution, notwithstanding the variety and complexity seen to prevail, to an extent previously undreamt of, in the arrangement of that majestic system. the existence of a luminous fluid, diffused through enormous tracts of space, and intimately associated with stellar bodies, was virtually demonstrated, and its place and use in creation attempted to be divined by a bold but plausible conjecture. change on a stupendous scale was inferred or observed to be everywhere in progress. periodical stars shone out and again decayed; progressive ebbings or flowings of light were indicated as probable in many stars under no formal suspicion of variability; forces were everywhere perceived to be at work, by which the very structure of the heavens themselves must be slowly but fundamentally modified. in all directions groups were seen to be formed or forming; tides and streams of suns to be setting towards powerful centres of attraction; new systems to be in process of formation, while effete ones hastened to decay or regeneration when the course appointed for them by infinite wisdom was run. and thus, to quote the words of the observer who "had looked farther into space than ever human being did before him,"[ ] the state into which the incessant action of the clustering power has brought the milky way at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that, since the breaking-up of the parts of the milky way affords a proof that it cannot last for ever, it equally bears witness that its past duration cannot be admitted to be infinite.[ ] footnotes: [footnote : _phil. trans._, vol. xxx., p. .] [footnote : out of eighty stars compared, fifty-seven were found to have changed their places by more than ". lesser discrepancies were at that time regarded as falling within the limits of observational error. _tobiæ mayeri op. inedita_, t. i., pp. , , and herschel in _phil. trans._, vol. lxxiii., pp. - .] [footnote : _posthumous works_, p. .] [footnote : arago in _annuaire du bureau des longitudes_, , p. .] [footnote : bradley to halley, _phil. trans._, vol. xxxv. ( ), p. . his observations were directly applicable to only two stars, gamma draconis and eta ursæ majoris, but some lesser ones were included in the same result.] [footnote : holden, _sir william herschel, his life and works_, p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : caroline lucretia herschel, born at hanover, march , , died in the same place, january , . she came to england in , and was her brother's devoted assistant, first in his musical undertakings, and afterwards, down to the end of his life, in his astronomical labours.] [footnote : holden, _op. cit._, p. .] [footnote : _memoir of caroline herschel_, p. .] [footnote : see holden's _sir william herschel_, p. .] [footnote : _an original theory or new hypothesis of the universe_, london, . see also de morgan's summary of his views in _philosophical magazine_, april, .] [footnote : _allgemeine naturgeschichte und theorie des himmels_, .] [footnote : _cosmologische briefe_, augsburg, .] [footnote : _the system of the world_, p. , london, (a translation of _cosmologische briefe_). lambert regarded nebulæ as composed of stars crowded together, but _not_ as external universes. in the case of the orion nebula, indeed, he throws out such a conjecture, but afterwards suggests that it may form a centre for that one of the subordinate systems composing the milky way to which our sun belongs.] [footnote : _opera inedita_, t. i., p. .] [footnote : _phil. trans._, vol. lxxiii. ( ), p. . pierre prévost's similar investigation, communicated to the berlin academy of sciences four months later, july , , was inserted in the _memoirs_ of that body for , and thus _seems_ to claim a priority not its due. georg simon klügel at halle gave about the same time an analytical demonstration of herschel's result. wolf, _gesch. der astronomie_, p. .] [footnote : _phil. trans._, vol. xcv., p. .] [footnote : _ibid._, vol. xcvi., p. .] [footnote : "ingens bolus devorandus est," kepler admitted to herwart in may, .] [footnote : described in "præfatio editoris" to _de revolutionibus_, p. xix. (ed. ).] [footnote : _opere_, t. i., p. .] [footnote : _phil. trans._, vol. xvii., p. .] [footnote : _ibid._, vol. lxxii., p. .] [footnote : doberck, _observatory_, vol. ii., p. .] [footnote : _phil. trans._, vol. lvii., p. .] [footnote : _ibid._, vol. lxxiv., p. .] [footnote : _beobachtungen von fixsterntrabanten_, ; and _de novis in coelo sidereo phænomenis_, .] [footnote : _bibliographie_, p. .] [footnote : _phil. trans._, vol. lxxii., p. .] [footnote : _ibid._, vol. lxxiii., p. .] [footnote : _ibid._, vol. xciii., p. .] [footnote : _phil. trans._, vol. lxxv., p. .] [footnote : _ibid._, vol. lxxix., pp. , .] [footnote : _ibid._, vol. xcii., pp. , .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _ibid._, vol. cvii., p. .] [footnote : bullialdus, _de nebulosâ stellâ in cingulo andromedæ_ ( ); see also g. p. bond, _mém. am. ac._, vol. iii., p. , holden's monograph on the orion nebula, _washington observations_, vol. xxv., (pub. ), and lady huggins's drawing, _atlas of spectra_, p. .] [footnote : _mathemata astronomica_, p. .] [footnote : _systema saturnium_, p. .] [footnote : _phil. trans._, vol. xxix., p. .] [footnote : _mém. ac. des sciences_, .] [footnote : _conn. des temps_, (pub. ), p. . a previous list of forty-five had appeared in _mém. ac. des sciences_, .] [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : _ibid._, vol. lxxix., p. .] [footnote : _ibid._, vol. lxxv., p. .] [footnote : _ibid._, vol. lxxix., p. .] [footnote : _phil. trans._, vol. lxxix., p. .] [footnote : _ibid._, vol. lxxxi., p. .] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _ibid._, p. .] [footnote : j. herschel, _phil. trans._, vol. cxvi., part iii., p. .] [footnote : his own words to the poet campbell cited by holden, _life and works_, p. .] [footnote : _phil. trans._, vol. civ., p. .] chapter ii _progress of sidereal astronomy_ we have now to consider labours of a totally different character from those of sir william herschel. exploration and discovery do not constitute the whole business of astronomy; the less adventurous, though not less arduous, task of gaining a more and more complete mastery over the problems immemorially presented to her, may, on the contrary, be said to form her primary duty. a knowledge of the movements of the heavenly bodies has, from the earliest times, been demanded by the urgent needs of mankind; and science finds its advantage, as in many cases it has taken its origin, in condescension to practical claims. indeed, to bring such knowledge as near as possible to absolute precision has been defined by no mean authority[ ] as the true end of astronomy. several causes concurred about the beginning of the last century to give a fresh and powerful impulse to investigations having this end in view. the rapid progress of theory almost compelled a corresponding advance in observation; instrumental improvements rendered such an advance possible; herschel's discoveries quickened public interest in celestial inquiries; royal, imperial, and grand-ducal patronage widened the scope of individual effort. the heart of the new movement was in germany. hitherto the observatory of flamsteed and bradley had been the acknowledged centre of practical astronomy; greenwich observations were the standard of reference all over europe; and the art of observing prospered in direct proportion to the fidelity with which greenwich methods were imitated. dr. maskelyne, who held the post of astronomer royal during forty-six years (from to ), was no unworthy successor to the eminent men who had gone before him. his foundation of the _nautical almanac_ (in ) alone constitutes a valid title to fame; he introduced at the observatory the important innovation of the systematic publication of results; and the careful and prolonged series of observations executed by him formed the basis of the improved theories, and corrected tables of the celestial movements, which were rapidly being brought to completion abroad. his catalogue of thirty-six "fundamental" stars was besides excellent in its way, and most serviceable. yet he was devoid of bradley's instinct for divining the needs of the future. he was fitted rather to continue a tradition than to found a school. the old ways were dear to him; and, indefatigable as he was, a definite purpose was wanting to compel him, by its exigencies, along the path of progress. thus, for almost fifty years after bradley's death, the acquisition of a small achromatic[ ] was the only notable change made in the instrumental equipment of the observatory. the transit, the zenith sector, and the mural quadrant, with which bradley had done his incomparable work, retained their places long after they had become deteriorated by time and obsolete by the progress of invention; and it was not until the very close of his career that maskelyne, compelled by pond's detection of serious errors, ordered a troughton's circle, which he did not live to employ. meanwhile, the heavy national disasters with which germany was overwhelmed in the early part of the nineteenth century seemed to stimulate rather than impede the intellectual revival already for some years in progress there. astronomy was amongst the first of the sciences to feel the new impulse. by the efforts of bode, olbers, schröter, and von zach, just and elevated ideas on the subject were propagated, intelligence was diffused, and a firm ground prepared for common action in mutual sympathy and disinterested zeal. they received powerful aid through the foundation, in , by a young artillery officer named von reichenbach, of an optical and mechanical institute at munich. here the work of english instrumental artists was for the first time rivalled, and that of english opticians--when fraunhofer entered the new establishment--far surpassed. the development given to the refracting telescope by this extraordinary man was indispensable to the progress of that fundamental part of astronomy which consists in the exact determination of the places of the heavenly bodies. reflectors are brilliant engines of discovery, but they lend themselves with difficulty to the prosaic work of measuring right ascensions and polar distances. a signal improvement in the art of making and working flint-glass thus most opportunely coincided with the rise of a german school of scientific mechanicians, to furnish the instrumental means needed for the reform which was at hand. of the leader of that reform it is now time to speak. friedrich wilhelm bessel was born at minden, in westphalia, july , . a certain taste for figures, coupled with a still stronger distaste for the latin accidence, directed his inclination and his father's choice towards a mercantile career. in his fifteenth year, accordingly, he entered the house of kuhlenkamp and sons, in bremen, as an apprenticed clerk. he was now thrown completely upon his own resources. from his father, a struggling government official, heavily weighted with a large family, he was well aware that he had nothing to expect; his dormant faculties were roused by the necessity for self-dependence, and he set himself to push manfully forward along the path that lay before him. the post of supercargo on one of the trading expeditions sent out from the hanseatic towns to china and the east indies was the aim of his boyish ambition, for the attainment of which he sought to qualify himself by the industrious acquisition of suitable and useful knowledge. he learned english in two or three months; picked up spanish with the casual aid of a gunsmith's apprentice; studied the geography of the distant lands which he hoped to visit; collected information as to their climates, inhabitants, products, and the courses of trade. he desired to add some acquaintance with the art (then much neglected) of taking observations at sea; and thus, led on from navigation to astronomy, and from astronomy to mathematics, he groped his way into a new world. it was characteristic of him that the practical problems of science should have attracted him before his mind was as yet sufficiently matured to feel the charm of its abstract beauties. his first attempt at observation was made with a sextant, rudely constructed under his own directions, and a common clock. its object was the determination of the longitude of bremen, and its success, he tells us himself,[ ] filled him with a rapture of delight, which, by confirming his tastes, decided his destiny. he now eagerly studied bode's _jahrbuch_ and von zach's _monatliche correspondenz_, overcoming each difficulty as it arose with the aid of lalande's _traité d'astronomie_, and supplying, with amazing rapidity, his early deficiency in mathematical training. in two years he was able to attack a problem which would have tasked the patience, if not the skill, of the most experienced astronomer. amongst the earl of egremont's papers von zach had discovered harriot's observations on halley's comet at its appearance in , and published them as a supplement to bode's annual. with an elaborate care inspired by his youthful ardour, though hardly merited by their loose nature, bessel deduced from them an orbit for that celebrated body, and presented the work to olbers, whose reputation in cometary researches gave a special fitness to the proffered homage. the benevolent physician-astronomer of bremen welcomed with surprised delight such a performance emanating from such a source. fifteen years previously, the french academy had crowned a similar work; now its equal was produced by a youth of twenty, busily engaged in commercial pursuits, self-taught, and obliged to snatch from sleep the hours devoted to study. the paper was immediately sent to von zach for publication, with a note from olbers explaining the circumstances of its author; and the name of bessel became the common property of learned europe. he had, however, as yet no intention of adopting astronomy as his profession. for two years he continued to work in the counting-house by day, and to pore over the _mécanique céleste_ and the differential calculus by night. but the post of assistant in schröter's observatory at lilienthal having become vacant by the removal of harding to göttingen in , olbers procured for him the offer of it. it was not without a struggle that he resolved to exchange the desk for the telescope. his reputation with his employers was of the highest; he had thoroughly mastered the details of the business, which his keen practical intelligence followed with lively interest; his years of apprenticeship were on the point of expiring, and an immediate, and not unwelcome prospect of comparative affluence lay before him. the love of science, however, prevailed; he chose poverty and the stars, and went to lilienthal with a salary of a hundred thalers yearly. looking back over his life's work, olbers long afterwards declared that the greatest service which he had rendered to astronomy was that of having discerned, directed, and promoted the genius of bessel.[ ] for four years he continued in schröter's employment. at the end of that time the prussian government chose him to superintend the erection of a new observatory at königsberg, which after many vexatious delays, caused by the prostrate condition of the country, was finished towards the end of . königsberg was the first really efficient german observatory. it became, moreover, a centre of improvement, not for germany alone, but for the whole astronomical world. during two-and-thirty years it was the scene of bessel's labours, and bessel's labours had for their aim the reconstruction, on an amended and uniform plan, of the entire science of observation. a knowledge of the places of the stars is the foundation of astronomy.[ ] their configuration lends to the skies their distinctive features, and marks out the shifting tracks of more mobile objects with relatively fixed, and generally unvarying points of light. a more detailed and accurate acquaintance with the stellar multitude, regarded from a purely uranographical point of view, has accordingly formed at all times a primary object of celestial science, and was, during the last century, cultivated with a zeal and success by which all previous efforts were dwarfed into insignificance. in lalande's _histoire céleste_, published in , the places of no less than , stars were given, but in the rough, as it were, and consequently needing laborious processes of calculation to render them available for exact purposes. piazzi set an example of improved methods of observation, resulting in the publication, in and , of two catalogues of about , stars--the second being a revision and enlargement of the first--which for their time were models of what such works should be.[ ] stephen groombridge at blackheath was similarly and most beneficially active. but something more was needed than the diligence of individual observers. a systematic reform was called for; and it was this which bessel undertook and carried through. direct observation furnishes only what has been called the "raw material" of the positions of the heavenly bodies.[ ] a number of highly complex corrections have to be applied before their _mean_ can be disengaged from their _apparent_ places on the sphere. of these, the most considerable and familiar is atmospheric refraction, by which objects seem to stand higher in the sky than they in reality do, the effect being evanescent at the zenith, and attaining, by gradations varying with conditions of pressure and temperature, a maximum at the horizon. moreover, the points to which measurements are referred are themselves in motion, either continually in one direction, or periodically to and fro. the _precession_ of the equinoxes is slowly progressive, or rather retrogressive; the _nutation_ of the pole oscillatory in a period of about eighteen years. added to which, the non-instantaneous transmission of light, combined with the movement of the earth in its orbit, causes a small annual displacement known as _aberration_. now it is easy to see that any uncertainty in the application of these corrections saps the very foundations of exact astronomy. extremely minute quantities, it is true, are concerned; but the life and progress of modern celestial science depends upon the sure recognition of extremely minute quantities. in the early years of the nineteenth century, however, no uniform system of "reduction" (so the complete correction of observational results is termed) had been established. much was left to the individual caprice of observers, who selected for the several "elements" of reduction such values as seemed best to themselves. hence arose much hurtful confusion, tending to hinder united action and mar the usefulness of laborious researches. for this state of things, bessel, by the exercise of consummate diligence, sagacity, and patience, provided an entirely satisfactory remedy. his first step was an elaborate investigation of the precious series of observations made by bradley at greenwich from until his death in . the catalogue of , stars which he extracted from them gave the earliest example of the systematic reduction on a uniform plan of such a body of work. it is difficult, without entering into details out of place in a volume like the present, to convey an idea of the arduous nature of this task. it involved the formation of a theory of the errors of each of bradley's instruments, and a difficult and delicate inquiry into the true value of each correction to be applied, before the entries in the greenwich journals could be developed into a finished and authentic catalogue. although completed in , it was not until five years later that the results appeared with the proud, but not inappropriate title of _fundamenta astronomiæ_. the eminent value of the work consisted in this, that by providing a mass of entirely reliable information as to the state of the heavens at the epoch , it threw back the beginning of _exact_ astronomy almost half a century. by comparison with piazzi's catalogues the amount of precession was more accurately determined, the proper motions of a considerable number of stars became known with certainty, and definite prediction--the certificate of initiation into the secrets of nature--at last became possible as regards the places of the stars. bessel's final improvements in the methods of reduction were published in in his _tabulæ regiomontanæ_. they not only constituted an advance in accuracy, but afforded a vast increase of facility in application, and were at once and everywhere adopted. thus astronomy became a truly universal science; uncertainties and disparities were banished, and observations made at all times and places rendered mutually comparable.[ ] more, however, yet remained to be done. in order to verify with greater strictness the results drawn from the bradley and piazzi catalogues, a third term of comparison was wanted, and this bessel undertook to supply. by a course of , observations, executed during the years - , with the utmost nicety of care, the number of accurately known stars was brought up to above , , and an ample store of trustworthy facts laid up for the use of future astronomers. in this department argelander, whom he attracted from finance to astronomy, and trained in his own methods, was his assistant and successor. the great "bonn durchmusterung,"[ ] in which , stars visible in the northern hemisphere are enumerated, and the corresponding "atlas" published in - , constituting a picture of our sidereal surroundings of heretofore unapproached completeness, may be justly said to owe their origin to bessel's initiative, and to form a sequel to what he commenced. but his activity was not solely occupied with the promotion of a comprehensive reform in astronomy; it embraced special problems as well. the long-baffled search for a parallax of the fixed stars was resumed with fresh zeal as each mechanical or optical improvement held out fresh hopes of a successful issue. illusory results abounded. piazza in perceived, as he supposed, considerable annual displacements in vega, aldebaran, sirius, and procyon; the truth being that his instruments were worn out with constant use, and could no longer be depended upon.[ ] his countryman, calandrelli, was similarly deluded. the celebrated controversy between the astronomer royal and dr. brinkley, director of the dublin college observatory, turned on the same subject. brinkley, who was in possession of a first-rate meridian-circle, believed himself to have discovered relatively large parallaxes for four of the brightest stars; pond, relying on the testimony of the greenwich instruments, asserted their nullity. the dispute, protracted for fourteen years, from until , was brought to no definite conclusion; but the strong presumption on the negative side was abundantly justified in the event. there was good reason for incredulity in the matter of parallaxes. announcements of their detection had become so frequent as to be discredited before they were disproved; and struve, who investigated the subject at dorpat in - , had clearly shown that the quantities concerned were too small to come within the reliable measuring powers of any instrument then in use. already, however, the means were being prepared of giving to those powers a large increase. on the st july, , two old houses in an alley of munich tumbled down, burying in their ruins the occupants, of whom one alone was extricated alive, though seriously injured. this was an orphan lad of fourteen named joseph fraunhofer. the elector maximilian joseph was witness of the scene, became interested in the survivor, and consoled his misfortune with a present of eighteen ducats. seldom was money better bestowed. part of it went to buy books and a glass-polishing machine, with the help of which young fraunhofer studied mathematics and optics, and secretly exercised himself in the shaping and finishing of lenses; the remainder purchased his release from the tyranny of one weichselberger, a looking-glass maker by trade, to whom he had been bound apprentice on the death of his parents. a period of struggle and privation followed, during which, however, he rapidly extended his acquirements; and was thus eminently fitted for the task awaiting him, when, in , he entered the optical department of the establishment founded two years previously by von reichenbach and utzschneider. he now zealously devoted himself to the improvement of the achromatic telescope; and, after a prolonged study of the theory of lenses, and many toilsome experiments in the manufacture of flint-glass, he succeeded in perfecting, december , , an object-glass of exquisite quality and finish, - / inches in diameter, and of feet focal length. this (as it was then considered) gigantic lens was secured by struve for the russian government, and the "great dorpat refractor"--the first of the large achromatics which have played such an important part in modern astronomy--was, late in , set up in the place which it still occupies. by ingenious improvements in mounting and fitting, it was adapted to the finest micrometrical work, and thus offered unprecedented facilities both for the examination of double stars (in which struve chiefly employed it), and for such subtle measurements as might serve to reveal or disprove the existence of a sensible stellar parallax. fraunhofer, moreover, constructed for the observatory at königsberg the first really available heliometer. the principle of this instrument (termed with more propriety a "divided object-glass micrometer") is the separation, by a strictly measurable amount, of two distinct images of the same object. if a double star, for instance, be under examination, the two half-lenses into which the object-glass is divided are shifted until the upper star (say) in one image is brought into coincidence with the lower star in the other, when their distance apart becomes known by the amount of motion employed.[ ] this virtually new engine of research was delivered and mounted in , three years after the termination of the life of its deviser. the dorpat lens had brought to fraunhofer a title of nobility and the sole management of the munich optical institute (completely separated since from the mechanical department). what he had achieved, however, was but a small part of what he meant to achieve. he saw before him the possibility of nearly quadrupling the light-gathering capacity of the great achromatic acquired by struve; he meditated improvements in reflectors as important as those he had already effected in refractors; and was besides eagerly occupied with investigations into the nature of light, the momentous character of which we shall by-and-by have an opportunity of estimating. but his health was impaired, it is said, from the weakening effects of his early accident, combined with excessive and unwholesome toil, and, still hoping for its restoration from a projected journey to italy, he died of consumption, june , , aged thirty-nine years. his tomb in munich bears the concise eulogy, _approximavit sidera_. bessel had no sooner made himself acquainted with the exquisite defining powers of the königsberg heliometer, than he resolved to employ them in an attack upon the now secular problem of star-distances. but it was not until that he found leisure to pursue the inquiry. in choosing his test-star he adopted a new principle. it had hitherto been assumed that our nearest neighbours in space must be found among the brightest ornaments of our skies. the knowledge of stellar proper motions afforded by the critical comparison of recent with earlier star-places, suggested a different criterion of distance. it is impossible to escape from the conclusion that the apparently swiftest-moving stars are, _on the whole_, also the nearest to us, however numerous the individual exceptions to the rule. now, as early as ,[ ] piazzi had noted as an indication of relative vicinity to the earth, the unusually large proper motion ( · " annually) of a double star of the fifth magnitude in the constellation of the swan. still more emphatically in [ ] bessel drew the attention of astronomers to the fact, and cygni became known as the "flying star." the _seeming_ rate of its flight, indeed, is of so leisurely a kind, that in a thousand years it will have shifted its place by less than - / lunar diameters, and that a quarter of a million would be required to carry it round the entire circuit of the visible heavens. nevertheless, it has few rivals in rapidity of movement, the apparent displacement of the vast majority of stars being, by comparison, almost insensible. this interesting, though inconspicuous object, then, was chosen by bessel to be put to the question with his heliometer, while struve made a similar and somewhat earlier trial with the bright gem of the lyre, whose arabic title of the "falling eagle" survives as a time-worn remnant in "vega." both astronomers agreed to use the "differential" method, for which their instruments and the vicinity to their selected stars of minute, physically detached companions offered special facilities. in the last month of bessel made known the result of one year's observations, showing for cygni a parallax of about a third of a second ( · ").[ ] he then had his heliometer taken down and repaired, after which he resumed the inquiry, and finally terminated a series of measures in march .[ ] the resulting parallax of · " (corresponding to a distance about , times that of the earth from the sun), seemed to be ascertained beyond the possibility of cavil, and is memorable as the first _published_ instance of the fathom-line, so industriously thrown into celestial space, having really and indubitably _touched bottom_. it was confirmed in - with curious exactness by c. a. f. peters at pulkowa; but later researches showed that it required increase to nearly half a second.[ ] struve's measurements inspired less confidence. they extended over three years ( - ), but were comparatively few, and were frequently interrupted. the parallax, accordingly, of about a quarter of a second ( · ") which he derived from them for alpha lyræ, and announced in ,[ ] has proved considerably too large.[ ] meanwhile a result of the same kind, but of a more striking character than either bessel's or struve's, had been obtained, one might almost say casually, by a different method and in a distant region. thomas henderson, originally an attorney's clerk in his native town of dundee, had become known for his astronomical attainments, and was appointed in to direct the recently completed observatory at the cape of good hope. he began observing in april, , and, the serious shortcomings of his instrument notwithstanding, executed during the thirteen months of his tenure of office a surprising amount of first-rate work. with a view to correcting the declination of the lustrous double star alpha centauri (which ranks after sirius and canopus as the third brightest orb in the heavens), he effected a number of successive determinations of its position, and on being informed of its very considerable proper motion ( · " annually), he resolved to examine the observations already made for possible traces of parallactic displacement. this was done on his return to scotland, where he filled the office of astronomer royal from until his premature death in . the result justified his expectations. from the declination measurements made at the cape and duly reduced, a parallax of about one second of arc clearly emerged (diminished by gill's and elkin's observations, - , to o· "); but, by perhaps an excess of caution, was withheld from publication until fuller certainty was afforded by the concurrent testimony of lieutenant meadows's determinations of the same star's right ascension.[ ] when at last, january , , henderson communicated his discovery to the astronomical society, he could no longer claim the priority which was his due. bessel had anticipated him with the parallax of cygni by just two months. thus from three different quarters, three successful and almost simultaneous assaults were delivered upon a long-beleaguered citadel of celestial secrets. the same work has since been steadily pursued, with the general result of showing that, as regards their overwhelming majority, the stars are far too remote to show even the slightest trace of optical shifting from the revolution of the earth in its orbit. in nearly a hundred cases, however, small parallaxes have been determined, some certainly (that is, within moderate limits of error), others more or less precariously. the list is an instructive one, in its omissions no less than in its contents. it includes stars of many degrees of brightness, from sirius down to a nameless telescopic star in the great bear;[ ] yet the vicinity to the earth of this minute object is so much greater than that of the brilliant vega, that the latter transported to its place would increase in lustre thirty-eight times. moreover, many of the brightest stars are found to have no sensible parallax, while the majority of those ascertained to be nearest to the earth are of fifth, sixth, even ninth magnitudes. the obvious conclusions follow that the range of variety in the sidereal system is enormously greater than had been supposed, and that estimates of distance based upon apparent magnitude must be wholly futile. thus, the splendid canopus, betelgeux, and rigel can be inferred, from their indefinite remoteness, to exceed our sun thousands of times in size and lustre; while many inconspicuous objects, which prove to be in our relative vicinity, must be notably his inferiors. the limits of real stellar magnitude are then set very widely apart. at the same time, the so-called "optical" and "geometrical" methods of relatively estimating star-distances are both seen to have a foundation of fact, although so disguised by complicated relations as to be of very doubtful individual application. on the whole, the chances are in favour of the superior vicinity of a bright star over a faint one; and, on the whole, the stars in swiftest _apparent_ motion are amongst those whose _actual_ remoteness is least. indeed, there is no escape from either conclusion, unless on the supposition of special arrangements in themselves highly improbable, and, we may confidently say, non-existent. the distances even of the few stars found to have measurable parallaxes are on a scale entirely beyond the powers of the human mind to conceive. in the attempt both to realize them distinctly, and to express them conveniently, a new unit of length, itself of bewildering magnitude, has originated. this is what we may call the _light-journey_ of one year. the subtle vibrations of the ether, propagated on all sides from the surface of luminous bodies, travel at the rate of , miles a second, or (in round numbers) six billions of miles a year. four and a third such measures are needed to span the abyss that separates us from the nearest fixed star. in other words, light takes four years and four months to reach the earth from alpha centauri; yet alpha centauri lies some ten billions of miles nearer to us (so far as is yet known) than any other member of the sidereal system! the determination of parallax leads, in the case of stars revolving in known orbits, to the determination of mass; for the distance from the earth of the two bodies forming a binary system being ascertained, the seconds of arc apparently separating them from each other can be translated into millions of miles; and we only need to add a knowledge of their period to enable us, by an easy sum in proportion, to find their combined mass in terms of that of the sun. thus, since--according to dr. doberck's elements--the components of alpha centauri revolve round their common centre of gravity at a mean distance nearly times the radius of the earth's orbit, in a period of years, the attractive force of the two together must be just twice the solar. we may gather some idea of their relations by placing in imagination a second luminary like our sun in circulation between the orbits of neptune and uranus. but systems of still more majestic proportions are reduced by extreme remoteness to apparent insignificance. a double star of the fourth magnitude in cassiopeia (eta), to which a small parallax is ascribed on the authority of o. struve, appears to be above eight times as massive as the central orb of our world; while a much less conspicuous pair-- pegasi--exerts, if the available data can be depended upon, no less than thirteen times the solar gravitating power. further, the actual rate of proper motions, so far as regards that part of them which is projected upon the sphere, can be ascertained for stars at known distance. the annual journey, for instance, of cygni _across the line of sight_ amounts to , , and that of alpha centauri to millions of miles. a small star, numbered , in groombridge's circumpolar catalogue, "devours the way" at the rate of at least miles a second--a speed, in newcomb's opinion, beyond the gravitating power of the entire sidereal system to control; and mu cassiopeiæ possesses above two-thirds of that surprising velocity; while for both objects, radial movements of just sixty miles a second were disclosed by professor campbell's spectroscopic measurements. herschel's conclusion as to the advance of the sun among the stars was not admitted as valid by the most eminent of his successors. bessel maintained that there was absolutely no preponderating evidence in favour of its supposed direction towards a point in the constellation hercules.[ ] biot, burckhardt, even herschel's own son, shared his incredulity. but the appearance of argelander's prize-essay in [ ] changed the aspect of the question. herschel's first memorable solution in was based upon the motions of thirteen stars, imperfectly known; his second, in , upon those of no more than six. argelander now obtained an entirely concordant result from the large number of , determined with the scrupulous accuracy characteristic of bessel's work and his own. the reality of the fact thus persistently disclosed could no longer be doubted; it was confirmed five years later by the younger struve, and still more strikingly in [ ] by galloway's investigations, founded exclusively on the apparent displacements of southern stars. in and , sir george airy and mr. dunkin ( - ),[ ] employing all the resources of modern science, and commanding the wealth of material furnished by , proper motions carefully determined by mr. main, reached conclusions closely similar to that indicated nearly eighty years previously by the first great sidereal astronomer; which mr. plummer's reinvestigation of the subject in [ ] served but slightly to modify. yet astronomers were not satisfied. dr. auwers of berlin completed in a splendid piece of work, for which he received in the gold medal of the royal astronomical society. it consisted in reducing afresh, with the aid of the most refined modern data, bradley's original stars, and comparing their places thus obtained for the year with those assigned to them from observations made at greenwich after the lapse of ninety years. in the interval, as was to be anticipated, most of them were found to have travelled over some small span of the heavens, and there resulted a stock of nearly three thousand highly authentic proper motions. these ample materials were turned to account by m. ludwig struve[ ] for a discussion of the sun's motion, of which the upshot was to shift its point of aim to the bordering region of the constellations hercules and lyra. and the more easterly position of the solar apex was fully confirmed by the experiments, with variously assorted lists of stars, of lewis boss of albany,[ ] and oscar stumpe of bonn.[ ] fresh precautions of refinement were introduced into the treatment of the subject by ristenpart of karlsruhe,[ ] by kapteyn of groningen,[ ] by newcomb[ ] and porter[ ] in america, who ably availed themselves of the copious materials accumulated before the close of the century. their results, although not more closely accordant than those of their predecessors, combined to show that the journey of our system is directed towards a point within a circle about ten degrees in radius, having the brilliant vega for its centre. to determine its rate was a still more arduous problem. it involved the assumption, very much at discretion, of an average parallax for the stars investigated; and otto struve's estimate of million miles as the span yearly traversed was hence wholly unreliable. fortunately, however, as will be seen further on, a method of determining the sun's velocity independently of any knowledge of star-distances, has now become available. as might have been expected, speculation has not been idle regarding the purpose and goal of the strange voyage of discovery through space upon which our system is embarked; but altogether fruitlessly. the variety of the conjectures hazarded in the matter is in itself a measure of their futility. long ago, before the construction of the heavens had as yet been made the subject of methodical inquiry, kant was disposed to regard sirius as the "central sun" of the milky way; while lambert surmised that the vast orion nebula might serve as the regulating power of a subordinate group including our sun. herschel threw out the hint that the great cluster in hercules might prove to be the supreme seat of attractive force;[ ] argelander placed his central body in the constellation perseus;[ ] fomalhaut, the brilliant of the southern fish, was set in the post of honour by boguslawski of breslau. mädler (who succeeded struve at dorpat in ) concluded from a more formal inquiry that the ruling power in the sidereal system resided, not in any single prepondering mass, but in the centre of gravity of the self-controlled revolving multitude.[ ] in the former case (as we know from the example of the planetary scheme), the stellar motions would be most rapid near the centre; in the latter, they would become accelerated with remoteness from it.[ ] mädler showed that no part of the heavens could be indicated as a region of exceptionally swift movements, such as would result from the presence of a gigantic (though possibly obscure) ruling body; but that a community of extremely sluggish movements undoubtedly existed in and near the group of the pleiades, where, accordingly, he placed the centre of gravity of the milky way.[ ] the bright star alcyone thus became the "central sun," but in a purely passive sense, its headship being determined by its situation at the point of neutralisation of opposing tendencies, and of consequent rest. by an avowedly conjectural method, the solar period of revolution round this point was fixed at , , years. the scheme of sidereal government framed by the dorpat astronomer was, it may be observed, of the most approved constitutional type; deprivation, rather than increase of influence accompanying the office of chief dignitary. but while we are still ignorant, and shall perhaps ever remain so, of the fundamental plan upon which the galaxy is organised, recent investigations tend more and more to exhibit it, not as monarchical (so to speak), but as federative. the community of proper motions detected by mädler in the vicinity of the pleiades may accordingly possess a significance altogether different from what he imagined. bessel's so-called "foundation of an astronomy of the invisible" now claims attention.[ ] his prediction regarding the planet neptune does not belong to the present division of our subject; a strictly analogous discovery in the sidereal system was, however, also very clearly foreshadowed by him. his earliest suspicions of non-uniformity in the proper motion of sirius dated from ; they extended to procyon in ; and after a series of refined measurements with the new repsold circle, he announced in his conclusion that these irregularities were due to the presence of obscure bodies round which the two bright dog-stars revolved as they pursued their way across the sphere.[ ] he even assigned to each an approximate period of half a century. "i adhere to the conviction," he wrote later to humboldt, "that procyon and sirius form real binary systems, consisting of a visible and an invisible star. there is no reason to suppose luminosity an essential quality of cosmical bodies. the visibility of countless stars is no argument against the invisibility of countless others."[ ] an inference so contradictory to received ideas obtained little credit, until peters found, in ,[ ] that the apparent anomalies in the movements of sirius could be completely explained by an orbital revolution in a period of fifty years. bessel's prevision was destined to be still more triumphantly vindicated. on the st of january, , while in the act of trying a new -inch refractor, mr. alvan g. clark (one of the celebrated firm of american opticians) actually discovered the hypothetical sirian companion in the precise position required by theory. it has now been watched through nearly an entire revolution (period · years), and proves to be very slightly luminous in proportion to its mass. its attractive power, in fact, is nearly half that of its primary, while it emits only / th of its light. sirius itself, on the other hand, possesses a far higher radiative intensity than our sun. it gravitates--admitting sir david gill's parallax of · " to be exact--like two suns, but shines like twenty. possibly it is much distended by heat, and undoubtedly its atmosphere intercepts a very much smaller proportion of its light than in stars of the solar class. as regards procyon, visual verification was awaited until november , , when professor schaeberle, with the great lick refractor, detected the long-sought object in the guise of a thirteenth-magnitude star. dr. see's calculations[ ] showed it to possess one-fifth the mass of its primary, or rather more than half that of our sun.[ ] yet it gives barely / th of the sun's light, so that it is still nearer to total obscurity than the dusky satellite of sirius. the period of forty years assigned to the system by auwers in [ ] appears to be singularly exact. but bessel was not destined to witness the recognition of "the invisible" as a legitimate and profitable field for astronomical research. he died march , , just six months before the discovery of neptune, of an obscure disease, eventually found to be occasioned by an extensive fungus-growth in the stomach. the place which he left vacant was not one easy to fill. his life's work might be truly described as "epoch-making." rarely indeed shall we find one who reconciled with the same success the claims of theoretical and practical astronomy, or surveyed the science which he had made his own with a glance equally comprehensive, practical, and profound. the career of friedrich georg wilhelm struve illustrates the maxim that science _differentiates_ as it develops. he was, while much besides, a specialist in double stars. his earliest recorded use of the telescope was to verify herschel's conclusion as to the revolving movement of castor, and he never varied from the predilection which this first observation at once indicated and determined. he was born at altona, of a respectable yeoman family, april , , and in took a degree in philology at the new russian university of dorpat. he then turned to science, was appointed in to a professorship of astronomy and mathematics, and began regular work in the dorpat observatory just erected by parrot for alexander i. it was not, however, until that the acquisition of a -foot refractor by troughton enabled him to take the position-angles of double stars with regularity and tolerable precision. the resulting catalogue of stellar systems gave the signal for a general resumption of the herschelian labours in this branch. his success, so far, and the extraordinary facilities for observation afforded by the fraunhofer achromatic encouraged him to undertake, february , , a review of the entire heavens down to ° south of the celestial equator, which occupied more than two years, and yielded, from an examination of above , stars, a harvest of about , previously unnoticed composite objects. the ensuing ten years were devoted to delicate and patient measurements, the results of which were embodied in _mensuræ micrometricæ_, published at st. petersburg in . this monumental work gives the places, angles of position, distances, colours, and relative brightness of , double and multiple stars, all determined with the utmost skill and care. the record is one which gains in value with the process of time, and will for ages serve as a standard of reference by which to detect change or confirm discovery. it appears from struve's researches that about one in forty of all stars down to the ninth magnitude is composite, but that the proportion is doubled in the brighter orders.[ ] this he attributed to the difficulty of detecting the faint companions of very remote orbs. it was also noticed, both by him and bessel, that double stars are in general remarkable for large proper motions. struve's catalogue included no star of which the components were more than " apart, because beyond that distance the chances of merely optical juxtaposition become considerable; but the immense preponderance of extremely close over (as it were) loosely yoked bodies is such as to demonstrate their physical connection, even if no other proof were forthcoming. many stars previously believed to be single divided under the scrutiny of the dorpat refractor; while in some cases, one member of a supposed binary system revealed itself as double, thus placing the surprised observer in the unexpected presence of a triple group of suns. five instances were noted of two pairs lying so close together as to induce a conviction of their mutual dependence;[ ] besides which, examples occurred of triple, quadruple, and multiple combinations, the reality of which was open to no reasonable doubt.[ ] it was first pointed out by bessel that the fact of stars exhibiting a common proper motion might serve as an unfailing test of their real association into systems. this was, accordingly, one of the chief criteria employed by struve to distinguish true binaries from merely optical couples. on this ground alone, cygni was admitted to be a genuine double star; and it was shown that, although its components appeared to follow almost strictly rectilinear paths, yet the probability of their forming a connected pair is actually greater than that of the sun rising to-morrow morning.[ ] moreover, this tie of an identical movement was discovered to unite bodies[ ] far beyond the range of distance ordinarily separating the members of binary systems, and to prevail so extensively as to lead to the conclusion that single do not outnumber conjoined stars more than twice or thrice.[ ] in struve was summoned by the emperor nicholas to superintend the erection of a new observatory at pulkowa, near st. petersburg, destined for the special cultivation of sidereal astronomy. boundless resources were placed at his disposal, and the institution created by him was acknowledged to surpass all others of its kind in splendour, efficiency, and completeness. its chief instrumental glory was a refractor of fifteen inches aperture by merz and mahler (fraunhofer's successors), which left the famous dorpat telescope far behind, and remained long without a rival. on the completion of this model establishment, august , , struve was installed as its director, and continued to fulfil the important duties of the post with his accustomed vigour until , when illness compelled his virtual resignation in favour of his son otto struve, born at dorpat in . he died november , . an inquiry into the laws of stellar distribution, undertaken during the early years of his residence at pulkowa, led struve to confirm in the main the inferences arrived at by herschel as to the construction of the heavens. according to his view, the appearance known as the milky way is produced by a collection of irregularly condensed star-clusters, within which the sun is somewhat eccentrically placed. the nebulous ring which thus integrates the light of countless worlds was supposed by him to be made up of stars scattered over a bent or "broken plane," or to lie in two planes slightly inclined to each other, our system occupying a position near their intersection.[ ] he further attempted to show that the limits of this vast assemblage must remain for ever shrouded from human discernment, owing to the gradual extinction of light in its passage through space,[ ] and sought to confer upon this celebrated hypothesis a definiteness and certainty far beyond the aspirations of its earlier advocates, chéseaux and olbers; but arbitrary assumptions vitiated his reasonings on this, as well as on some other points.[ ] in his special line as a celestial explorer of the most comprehensive type, sir william herschel had but one legitimate successor, and that successor was his son. john frederick william herschel was born at slough, march , , graduated with the highest honours from st. john's college, cambridge, in , and entered upon legal studies with a view to being called to the bar. but his share in an early compact with peacock and babbage, "to do their best to leave the world wiser than they found it," was not thus to be fulfilled. the acquaintance of dr. wollaston decided his scientific vocation. already, in , we find him reviewing some of his father's double stars; and he completed in the -inch speculum which was to be the chief instrument of his investigations. soon afterwards, he undertook, in conjunction with mr. (later sir james) south, a series of observations, issuing in the presentation to the royal society of a paper[ ] containing micrometrical measurements of binary stars, by which the elder herschel's inferences of orbital motion were, in many cases, strikingly confirmed. a star in the northern crown, for instance (eta coronæ), had completed more than one entire circuit since its first discovery; another, tau ophiuchi, had _closed up_ into apparent singleness; while the motion of a third, xi ursæ majoris, in an obviously eccentric orbit, was so rapid as to admit of being traced and measured from month to month. it was from the first confidently believed that the force retaining double stars in curvilinear paths was identical with that governing the planetary revolutions. but that identity was not ascertained until savary of paris showed, in ,[ ] that the movements of the above-named binary in the great bear could be represented with all attainable accuracy by an ellipse calculated on orthodox gravitational principles with a period of - / years. encke followed at berlin with a still more elegant method; and sir john herschel, pointing out the uselessness of analytical refinements where the data were necessarily so imperfect, described in a graphical process by which "the aid of the eye and hand" was brought in "to guide the judgment in a case where judgment only, and not calculation, could be of any avail."[ ] improved methods of the same kind were published by dr. see in ,[ ] and by mr. burnham in ;[ ] and our acquaintance with stellar orbits is steadily gaining precision, certainty, and extent. in herschel undertook, and executed with great assiduity during the ensuing eight years, a general survey of the northern heavens, directed chiefly towards the verification of his father's nebular discoveries. the outcome was a catalogue of , nebulæ and clusters, of which were observed for the first time, besides , double stars discovered almost incidentally.[ ] "strongly invited," as he tells us himself, "by the peculiar interest of the subject, and the wonderful nature of the objects which presented themselves," he resolved to attempt the completion of the survey in the southern hemisphere. with this noble object in view, he embarked his family and instruments on board the _mount stewart elphinstone_, and, after a prosperous voyage, landed at cape town on the th of january, . choosing as the scene of his observations a rural spot under the shelter of table mountain, he began regular "sweeping" on the th of march. the site of his great reflector is now marked with an obelisk, and the name of feldhausen has become memorable in the history of science; for the four years' work done there may truly be said to open the chapter of our knowledge as regards the southern skies. the full results of herschel's journey to the cape were not made public until , when a splendid volume[ ] embodying them was brought out at the expense of the duke of northumberland. they form a sequel to his father's labours such as the investigations of one man have rarely received from those of another. what the elder observer did for the northern heavens, the younger did for the southern, and with generally concordant results. reviving the paternal method of "star-gauging," he showed, from a count of , fields, that the milky way surrounds the solar system as a complete annulus of minute stars; not, however, quite symmetrically, since the sun was thought to lie somewhat nearer to those portions visible in the southern hemisphere, which display a brighter lustre and a more complicated structure than the northern branches. the singular cosmical agglomerations known as the "magellanic clouds" were now, for the first time, submitted to a detailed, though admittedly incomplete, examination, the almost inconceivable richness and variety of their contents being such that a lifetime might with great profit be devoted to their study. in the greater nubecula, within a compass of forty-two square degrees, herschel reckoned distinct nebulæ and clusters, besides fifty or sixty outliers, and a large number of stars intermixed with diffused nebulosity--in all, catalogued objects, and, for the lesser cloud, . yet this was only the most conspicuous part of what his twenty-foot revealed. such an extraordinary concentration of bodies so various led him to the inevitable conclusion that "the nubeculæ are to be regarded as systems _sui generis_, and which have no analogues in our hemisphere."[ ] he noted also the blankness of surrounding space, especially in the case of nubecula minor, "the access to which on all sides," he remarked, "is through a desert;" as if the cosmical material in the neighbourhood had been swept up and garnered in these mighty groups.[ ] of southern double stars, he discovered and gave careful measurements of , , and described , nebulæ, of which at least were new. the list was illustrated with a number of drawings, some of them extremely beautiful and elaborate. sir john herschel's views as to the nature of nebulæ were considerably modified by lord rosse's success in "resolving" with his great reflectors a crowd of these objects into stars. his former somewhat hesitating belief in the existence of phosphorescent matter, "disseminated through extensive regions of space in the manner of a cloud or fog,"[ ] was changed into a conviction that no valid distinction could be established between the faintest wisp of cosmical vapour just discernible in a powerful telescope, and the most brilliant and obvious cluster. he admitted, however, an immense range of possible variety in the size and mode of aggregation of the stellar constituents of various nebulæ. some might appear nebulous from the closeness of their parts; some from their smallness. others, he suggested, might be formed of "discrete luminous bodies floating in a non-luminous medium;"[ ] while the annular kind probably consisted of "hollow shells of stars."[ ] that a physical, and not merely an optical, connection unites nebulæ with the _embroidery_ (so to speak) of small stars with which they are in many instances profusely decorated, was evident to him, as it must be to all who look as closely and see as clearly as he did. his description of no. , in his northern catalogue as "a network or tracery of nebula following the lines of a similar network of stars,"[ ] would alone suffice to dispel the idea of accidental scattering; and many other examples of a like import might be quoted. the remarkably frequent occurrence of one or more minute stars in the close vicinity of "planetary" nebulæ led him to infer their dependent condition; and he advised the maintenance of a strict watch for evidences of circulatory movements, not only over these supposed stellar satellites, but also over the numerous "double nebulæ," in which, as he pointed out, "all the varieties of double stars as to distance, position, and relative brightness, have their counterparts." he, moreover, investigated the subject of nebular distribution by the simple and effectual method of graphic delineation or "charting," and succeeded in showing that while a much greater uniformity of scattering prevails in the southern than in the northern heavens, a condensation is nevertheless perceptible about the constellations pisces and cetus, roughly corresponding to the "nebular region" in virgo by its vicinity (within ° or °) to the opposite pole of the milky way. he concluded "that the nebulous system is distinct from the sidereal, though involving, and perhaps to a certain extent intermixed with, the latter."[ ] towards the close of his residence at feldhausen, herschel was fortunate enough to witness one of those singular changes in the aspect of the firmament which occasionally challenge the attention even of the incurious, and excite the deepest wonder of the philosophical observer. immersed apparently in the argo nebula is a star denominated eta carinæ. when halley visited st. helena in , it seemed of the fourth magnitude; but lacaille in the middle of the following century, and others after him, classed it as of the second. in the traveller burchell, being then at st. paul, near rio janeiro, remarked that it had unexpectedly assumed the first rank--a circumstance the more surprising to him because he had frequently, when in africa during the years to , noted it as of only fourth magnitude. this observation, however, did not become generally known until later. herschel, on his arrival at feldhausen, registered the star as a bright second, and had no suspicion of its unusual character until december , , when he suddenly perceived its light to be almost tripled. it then far outshone rigel in orion, and on the nd of january following it very nearly matched alpha centauri. from that date it declined; but a second and even brighter maximum occurred in april, , when maclear, then director of the cape observatory, saw it blaze out with a splendour approaching that of sirius. its waxings and wanings were marked by curious "trepidations" of brightness extremely perplexing to theory. in it had sunk below the fifth magnitude, and in was barely visible to the naked eye; yet it was not until eighteen years later that it touched a minimum of · magnitude. soon afterwards a recovery of brightness set in, but was not carried very far; and the star now shines steadily as of the seventh magnitude, its reddish light contrasting effectively with the silvery rays of the surrounding nebula. an attempt to include its fluctuations within a cycle of seventy years[ ] has signally failed; the extent and character of the vicissitudes to which it is subject stamping it rather as a species of connecting link between periodic and temporary stars.[ ] among the numerous topics which engaged herschel's attention at the cape was that of relative stellar brightness. having contrived an "astrometer" in which an "artificial star," formed by the total reflection of moonlight from the base of a prism, served as a standard of comparison, he was able to estimate the lustre of the _natural_ stars examined by the distances at which the artificial object appeared equal respectively to each. he thus constructed a table of of the principal stars,[ ] both in the northern and southern hemispheres, setting forth the numerical values of their apparent brightness relatively to that of alpha centauri, which he selected as a unit of measurement. further, the light of the full moon being found by him to exceed that of his standard star , times, and dr. wollaston having shown that the light of the full moon is to that of the sun as : , [ ] (zöllner made the ratio : , ), it became possible to compare stellar with solar radiance. hence was derived, in the case of the few stars at ascertained distances, a knowledge of real lustre. alpha centauri, for example, emits less than twice, capella one hundred times as much light as our sun; while arcturus, at its enormous distance, must display the splendour of , such luminaries. herschel returned to england in the spring of , bringing with him a wealth of observation and discovery such as had perhaps never before been amassed in so short a time. deserved honours awaited him. he was created a baronet on the occasion of the queen's coronation (he had been knighted in ); universities and learned societies vied with each other in showering distinctions upon him; and the success of an enterprise in which scientific zeal was tinctured with an attractive flavour of adventurous romance, was justly regarded as a matter of national pride. his career as an observing astronomer was now virtually closed, and he devoted his leisure to the collection and arrangement of the abundant trophies of his father's and his own activity. the resulting great catalogue of , nebulæ (including all then certainly known), published in the _philosophical transactions_ for , is, and will probably long remain, the fundamental source of information on the subject;[ ] but he unfortunately did not live to finish the companion work on double stars, for which he had accumulated a vast store of materials.[ ] he died at collingwood in kent, may , , in the eightieth year of his age, and was buried in westminster abbey, close beside the grave of sir isaac newton. the consideration of sir john herschel's cape observations brings us to the close of the period we are just now engaged in studying. they were given to the world, as already stated, three years before the middle of the century, and accurately represent the condition of sidereal science at that date. looking back over the fifty years traversed, we can see at a glance how great was the stride made in the interval. not alone was acquaintance with individual members of the cosmos vastly extended, but their mutual relations, the laws governing their movements, their distances from the earth, masses, and intrinsic lustre, had begun to be successfully investigated. _begun to be_; for only regarding a scarcely perceptible minority had even approximate conclusions been arrived at. nevertheless the whole progress of the future lay in that beginning; it was the thin end of the wedge of exact knowledge. the principle of measurement had been substituted for that of probability; a basis had been found large and strong enough to enable calculation to ascend from it to the sidereal heavens; and refinements had been introduced, fruitful in performance, but still more in promise. thus, rather the kind than the amount of information collected was significant for the time to come--rather the methods employed than the results actually secured rendered the first half of the nineteenth century of epochal importance in the history of our knowledge of the stars. footnotes: [footnote : bessel, _populäre vorlesungen_, pp. , .] [footnote : fitted to the old transit instrument, july , .] [footnote : _briefwechsel mit olbers_, p. xvi.] [footnote : r. wolf, _gesch. der astron._, p. .] [footnote : bessel, _pop. vorl._, p. .] [footnote : a new reduction of the observations upon which they were founded was undertaken in by herman s. davis, of the u.s. coast survey.] [footnote : bessel, _pop. vorl._, p. .] [footnote : durège, _bessel's leben und wirken_, p. .] [footnote : _bonner beobachtungen_, bd. iii.-v., - .] [footnote : bessel, _pop. vorl._, p. .] [footnote : the heads of the screws applied to move the halves of the object-glass in the königsberg heliometer are of so considerable a size that a thousandth part of a revolution, equivalent to / of a second of arc, can be measured with the utmost accuracy. main, _r. a. s. mem._, vol. xii., p. .] [footnote : _specola astronomica di palermo_, lib. vi., p. , _note_.] [footnote : _monatliche correspondenz_, vol. xxvi., p. .] [footnote : _astronomische nachrichten_, nos. - . it should be explained that what is called the "annual parallax" of a star is only half its apparent displacement. in other words, it is the angle subtended at the distance of that particular star by the _radius_ of the earth's orbit.] [footnote : _astr. nach._, nos. - .] [footnote : sir r. ball's measurements at dunsink gave to cygni a parallax of · "; professor pritchard obtained, by photographic determinations, one of · ".] [footnote : _additamentum in mensuras micrometricas_, p. .] [footnote : elkin's corrected result (in ) for the parallax of vega is · ".] [footnote : _mem. roy. astr. soc._, vol. xi., p. .] [footnote : that numbered , in lalande's _hist. cél._, found by argelander to have a proper motion of · ", and by winnecke a parallax of o· ". _month. not._, vol. xviii., p. .] [footnote : _fund. astr._, p. .] [footnote : _mém. prés. à l'ac. de st. pétersb._, t. iii.] [footnote : _phil. trans._, vol. cxxxvii., p. .] [footnote : _mem. roy. astr. soc._, vols. xxviii. and xxxii.] [footnote : _ibid._, vol. xlvii., p. .] [footnote : _mémoires de st. pétersbourg_, t. xxxv., no. , ; revised in _astr. nach._, nos. , - , .] [footnote : _astronomical journal_, nos. , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _veröffentlichungen der grossh. sternwarte zu karlsruhe_, bd. iv., .] [footnote : _proceedings amsterdam acad. of sciences_, jan. , .] [footnote : _astr. jour._, no. .] [footnote : _ibid._, nos. , .] [footnote : _phil. trans._, vol. xcvi., p. .] [footnote : _mém. prés. à l'ac. de st. pétersbourg_, t. iii., p. (read feb. , ).] [footnote : _die centralsonne, astr. nach._, nos. - , .] [footnote : sir j. herschel, note to _treatise on astronomy_, and _phil. trans._, vol. cxxiii., part ii., p. .] [footnote : the position is (as sir j. herschel pointed out, _outlines of astronomy_, p. , th ed.) placed beyond the range of reasonable probability by its remoteness (fully °) from the galactic plane.] [footnote : mädler in _westermann's jahrbuch_, , p. .] [footnote : letter from bessel to sir j. herschel, _month. not._, vol. vi., p. .] [footnote : wolf, _gesch. d. astr._, p. , _note_.] [footnote : _astr. nach._, nos. - .] [footnote : _astr. jour._, no. .] [footnote : adopting elkin's revised parallax for procyon of · ".] [footnote : _astr. nach._, nos. - .] [footnote : _ueber die doppelsterne_, bericht, , p. .] [footnote : _ueber die doppelsterne_, bericht, , p. .] [footnote : _mensuræ micr._, p. xcix.] [footnote : _stellarum fixarum imprimis duplicium et multiplicum positiones mediæ_, pp. cxc., cciii.] [footnote : for instance, the southern stars, a ophiuchi (itself double) and scorpii, which are ' " apart. _ibid._, p. cciii.] [footnote : _stellarum fixarum_, etc., p. ccliii.] [footnote : _Études d'astronomie stellaire_, , p. .] [footnote : _ibid._, p. .] [footnote : see encke's criticism in _astr. nach._, no. .] [footnote : _phil. trans._, vol. cxiv., part iii., .] [footnote : _conn. d. temps_, .] [footnote : _r. a. s. mem._, vol. v., p. , .] [footnote : _astr. and astrophysics_, vol. xii., p. .] [footnote : _popular astr._, vol. i., p. .] [footnote : _phil. trans._, vol. cxxiii., and _results_, etc., introd.] [footnote : _results of astronomical observations made during the years - at the cape of good hope._] [footnote : _results_, etc., p. .] [footnote : see proctor's _universe of stars_, p. .] [footnote : _a treatise on astronomy_, , p. .] [footnote : _results_, etc., p. .] [footnote : _ibid._, pp. , .] [footnote : _phil. trans._, vol. cxxiii., p. .] [footnote : _results_, etc., p. .] [footnote : loomis, _month. not._, vol. xxix., p. .] [footnote : see the author's _system of the stars_, pp. - .] [footnote : _outlines of astr._, app. i.] [footnote : _phil. trans._, vol. cxix., p. .] [footnote : dr. dreyer's new general catalogue, published in as vol. xlix. of the royal astronomical society's _memoirs_, is an enlargement of herschel's work. it includes , entries, and was supplemented, in , by an "index catalogue" of , nebulæ discovered to . _mem. r. a. s._, vol. li.] [footnote : a list of , composite stars was drawn out by him in order of right ascension, and has been published in vol. xl. of _mem. r. a. s._; but the data requisite for their formation into a catalogue were not forthcoming. see main's and pritchard's _preface_ to above, and dunkin's _obituary notices_, p. .] chapter iii _progress of knowledge regarding the sun_ the discovery of sun-spots in by fabricius and galileo first opened a way for inquiry into the solar constitution; but it was long before that way was followed with system or profit. the seeming irregularity of the phenomena discouraged continuous attention; casual observations were made the basis of arbitrary conjectures, and real knowledge received little or no increase. in we find jean tarde, canon of sarlat, arguing that because the sun is "the eye of the world," and the eye of the world _cannot suffer from ophthalmia_, therefore the appearances in question must be due, not to actual specks or stains on the bright solar disc, but to the transits of a number of small planets across it! to this new group of heavenly bodies he gave the name of "borbonia sidera," and they were claimed in for the house of hapsburg, under the title of "austriaca sidera" by father malapertius, a belgian jesuit.[ ] a similar view was temporarily maintained against galileo by the justly celebrated father scheiner of ingolstadt, and later by william gascoigne, the inventor of the micrometer; but most of those who were capable of thinking at all on such subjects (and they were but few) adhered either to the _cloud theory_ or to the _slag theory_ of sun-spots. the first was championed by galileo, the second by simon marius, "astronomer and physician" to the brother margraves of brandenburg. the latter opinion received a further notable development from the fact that in , a year remarkable for the appearance of three bright comets, the sun was almost free from spots; whence it was inferred that the cindery refuse from the great solar conflagration, which usually appeared as dark blotches on its surface, was occasionally thrown off in the form of comets, leaving the sun, like a snuffed taper, to blaze with renewed brilliancy.[ ] in the following century, derham gathered from observations carried on during the years - , "that the spots on the sun are caused by the eruption of some new volcano therein, which at first pouring out a prodigious quantity of smoke and other opacous matter, causeth the spots; and as that fuliginous matter decayeth and spendeth itself, and the volcano at last becomes more torrid and flaming, so the spots decay, and grow to umbræ, and at last to faculæ."[ ] the view, confidently upheld by lalande,[ ] that spots were rocky elevations uncovered by the casual ebbing of a luminous ocean, the surrounding penumbræ representing shoals or sandbanks, had even less to recommend it than derham's volcanic theory. both were, however, significant of a growing tendency to bring solar phenomena within the compass of terrestrial analogies. for years, then, after galileo first levelled his telescope at the setting sun, next to nothing was learned as to its nature; and the facts immediately ascertained, of its rotation on an axis nearly erect to the plane of the ecliptic, in a period of between twenty-five and twenty-six days, and of the virtual limitation of the spots to a so-called "royal" zone extending some thirty degrees north and south of the solar equator, gained little either in precision or development from five generations of astronomers. but in november, , a spot of extraordinary size engaged the attention of alexander wilson, professor of astronomy in the university of glasgow. he watched it day by day, and to good purpose. as the great globe slowly revolved, carrying the spot towards its western edge, he was struck with the gradual contraction and final disappearance of the penumbra _on the side next the centre of the disc_; and when on the th of december the same spot re-emerged on the eastern limb, he perceived, as he had anticipated, that the shady zone was now deficient _on the opposite side_, and resumed its original completeness as it returned to a central position. in other spots subsequently examined by him, similar perspective effects were visible, and he proved in ,[ ] by strict geometrical reasoning, that they could only arise in vast photospheric excavations. it was not, indeed, the first time that such a view had been suggested. father scheiner's later observations plainly foreshadowed it;[ ] a conjecture to the same effect was emitted by leonard rost of nuremburg early in the eighteenth century;[ ] both by lahire in and by j. cassini in spots had been seen as notches on the solar limb; while in pastor schülen of essingen, from the careful study of phenomena similar to those noted by wilson, concluded their depressed nature.[ ] modern observations, nevertheless, prove those phenomena to be by no means universally present. wilson's general theory of the sun was avowedly tentative. it took the modest form of an interrogatory. "is it not reasonable to think," he asks, "that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark, and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the sun would seem to derive the whole of his vivifying heat and energy?"[ ] he further suggests that the excavations or spots may be occasioned "by the working of some sort of elastic vapour which is generated within the dark globe," and that the luminous matter, being in some degree fluid, and being acted upon by gravity, tends to flow down and cover the nucleus. from these hints, supplemented by his own diligent observations and sagacious reasonings, herschel elaborated a scheme of solar constitution which held its ground until the physics of the sun were revolutionised by the spectroscope. a cool, dark, solid globe, its surface diversified with mountains and valleys, clothed in luxuriant vegetation, and "richly stored with inhabitants," protected by a heavy cloud-canopy from the intolerable glare of the upper luminous region, where the dazzling coruscations of a solar aurora some thousands of miles in depth evolved the stores of light and heat which vivify our world--such was the central luminary which herschel constructed with his wonted ingenuity, and described with his wonted eloquence. "this way of considering the sun and its atmosphere," he says,[ ] "removes the great dissimilarity we have hitherto been used to find between its condition and that of the rest of the great bodies of the solar system. the sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface, the rotation upon its axis, and the fall of heavy bodies, leads us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe." we smile at conclusions which our present knowledge condemns as extravagant and impossible, but such incidental flights of fancy in no way derogate from the high value of herschel's contributions to solar science. the cloud-like character which he attributed to the radiant shell of the sun (first named by schröter the "photosphere") is borne out by all recent investigations; he observed its mottled or corrugated aspect, resembling, as he described it, the roughness on the rind of an orange; showed that "faculæ" are elevations or heaped-up ridges of the disturbed photospheric matter; and threw out the idea that spots may ensue from an excess of the ordinary luminous emissions. a certain "empyreal" gas was, he supposed (very much as wilson had done), generated in the body of the sun, and rising everywhere by reason of its lightness, made for itself, when in moderate quantities, small openings or "pores,"[ ] abundantly visible as dark points on the solar disc. but should an uncommon quantity be formed, "it will," he maintained, "burst through the planetary[ ] regions of clouds, and thus will produce great openings; then, spreading itself above them, it will occasion large shallows (penumbræ), and mixing afterwards gradually with other superior gases, it will promote the increase, and assist in the maintenance, of the general luminous phenomena."[ ] this partial anticipation of the modern view that the solar radiations are maintained by some process of circulation within the solar mass, was reached by herschel through prolonged study of the phenomena in question. the novel and important idea contained in it, however, it was at that time premature to attempt to develop. but though many of the subtler suggestions of herschel's genius passed unnoticed by his contemporaries, the main result of his solar researches was an unmistakable one. it was nothing less than the definitive introduction into astronomy of the paradoxical conception of the central fire and hearth of our system as a cold, dark, terrestrial mass, wrapt in a mantle of innocuous radiance--an earth, so to speak, within--a sun without. let us pause for a moment to consider the value of this remarkable innovation. it certainly was not a step in the direction of truth. on the contrary, the crude notions of anaxagoras and xeno approached more nearly to what we now know of the sun, than the complicated structure devised for the happiness of a nobler race of beings than our own by the benevolence of eighteenth-century astronomers. and yet it undoubtedly constituted a very important advance in science. it was the first earnest attempt to bring solar phenomena within the compass of a rational system; to put together into a consistent whole the facts ascertained; to fabricate, in short, a solar machine that would in some fashion work. it is true that the materials were inadequate and the design faulty. the resulting construction has not proved strong enough to stand the wear and tear of time and discovery, but has had to be taken to pieces and remodelled on a totally different plan. but the work was not therefore done in vain. none of bacon's aphorisms show a clearer insight into the relations between the human mind and the external world than that which declares "truth to emerge sooner from error than from confusion."[ ] a definite theory (even if a false one) gives holding-ground to thought. facts acquire a meaning with reference to it. it affords a motive for accumulating them and a means of co-ordinating them; it provides a framework for their arrangement, and a receptacle for their preservation, until they become too strong and numerous to be any longer included within arbitrary limits, and shatter the vessel originally framed to contain them. such was the purpose subserved by herschel's theory of the sun. it helped to _clarify_ ideas on the subject. the turbid sense of groping and viewless ignorance gave place to the lucidity of a possible scheme. the persuasion of knowledge is a keen incentive to its increase. few men care to investigate what they are obliged to admit themselves entirely ignorant of; but once started on the road of knowledge, real or supposed, they are eager to pursue it. by the promulgation of a confident and consistent view regarding the nature of the sun, accordingly, research was encouraged, because it was rendered hopeful, and inquirers were shown a path leading indefinitely onwards where an impassable thicket had before seemed to bar the way. we have called the "terrestrial" theory of the sun's nature an innovation, and so, as far as its general acceptance is concerned, it may justly be termed; but, like all successful innovations, it was a long time brewing. it is extremely curious to find that herschel had a predecessor in its advocacy who never looked through a telescope (nor, indeed, imagined the possibility of such an instrument), who knew nothing of sun-spots, was still (mistaken assertions to the contrary notwithstanding) in the bondage of the geocentric system, and regarded nature from the lofty standpoint of an idealist philosophy. this was the learned and enlightened cardinal cusa, a fisherman's son from the banks of the moselle, whose distinguished career in the church and in literature extended over a considerable part of the fifteenth century ( - ). in his singular treatise _de doctâ ignorantiâ_, one of the most notable literary monuments of the early renaissance, the following passage occurs:--"to a spectator on the surface of the sun, the splendour which appears to us would be invisible, since it contains, as it were, an earth for its central mass, with a circumferential envelope of light and heat, and between the two an atmosphere of water and clouds and translucent air." the luminary of herschel's fancy could scarcely be more clearly portrayed; some added words, however, betray the origin of the cardinal's idea. "the earth also," he says, "would appear as a shining star to any one outside the fiery element." it was, in fact, an extension to the sun of the ancient elemental doctrine; but an extension remarkable at that period, as premonitory of the tendency, so powerfully developed by subsequent discoveries, to assimilate the orbs of heaven to the model of our insignificant planet, and to extend the brotherhood of our system and our species to the farthest limit of the visible or imaginable universe. in later times we find flamsteed communicating to newton, march , , his opinion "that the substance of the sun is terrestrial matter, his light but the liquid menstruum encompassing him."[ ] bode in arrived independently at the conclusion that "the sun is neither burning nor glowing, but in its essence a dark planetary body, composed like our earth of land and water, varied by mountains and valleys, and enveloped in a vaporous atmosphere";[ ] and the learned in general applauded and acquiesced. the view, however, was in still so far from popular, that the holding of it was alleged as a proof of insanity in dr. elliot when accused of a murderous assault on miss boydell. his friend dr. simmons stated on his behalf that he had received from him in the preceding january a letter giving evidence of a deranged mind, wherein he asserted "that the sun is not a body of fire, as hath been hitherto supposed, but that its light proceeds from a dense and universal aurora, which may afford ample light to the inhabitants of the surface beneath, and yet be at such a distance aloft as not to annoy them. no objection, he saith, ariseth to that great luminary's being inhabited; vegetation may obtain there as well as with us. there may be water and dry land, hills and dales, rain and fair weather; and as the light, so the season must be eternal, consequently it may easily be conceived to be by far the most blissful habitation of the whole system!" the recorder, nevertheless, objected that if an extravagant hypothesis were to be adduced as proof of insanity, the same might hold good with regard to some other speculators, and desired dr. simmons to tell the court what he thought of the theories of burnet and buffon.[ ] eight years later, this same "extravagant hypothesis," backed by the powerful recommendation of sir william herschel, obtained admittance to the venerable halls of science, there to abide undisturbed for nearly seven decades. individual objectors, it is true, made themselves heard, but their arguments had little effect on the general body of opinion. ruder blows were required to shatter an hypothesis flattering to human pride of invention in its completeness, in the plausible detail of observations by which it seemed to be supported, and in its condescension to the natural pleasure in discovering resemblance under all but total dissimilarity. sir john herschel included among the results of his multifarious labours at the cape of good hope a careful study of the sun-spots conspicuously visible towards the end of the year and in the early part of . they were remarkable, he tells us, for their forms and arrangement, as well as for their number and size; one group, measured on the th of march in the latter year, covering (apart from what may be called its outlying dependencies) the vast area of five square minutes or , million square miles.[ ] we have at present to consider, however, not so much these observations in themselves, as the chain of theoretical suggestions by which they were connected. the distribution of spots, it was pointed out, on two zones parallel to the equator, showed plainly their intimate connection with the solar rotation, and indicated as their cause fluid circulations analogous to those producing the terrestrial trade and anti-trade winds. "the spots, in this view of the subject," he went on to say,[ ] "would come to be assimilated to those regions on the earth's surface where, for the moment, hurricanes and tornadoes prevail; the upper stratum being temporarily carried downwards, displacing by its impetus the two strata of luminous matter beneath, the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the sun below. such processes cannot be unaccompanied by vorticose motions, which, left to themselves, die away by degrees and dissipate, with the peculiarity that their lower portions come to rest more speedily than their upper, by reason of the greater resistance below, as well as the remoteness from the point of action, which lies in a higher region, so that their centres (as seen in our waterspouts, which are nothing but small tornadoes) appear to retreat upwards. now this agrees perfectly with what is observed during the obliteration of the solar spots, which appear as if filled in by the collapse of their sides, the penumbra closing in upon the spot and disappearing after it." but when it comes to be asked whether a cause can be found by which a diversity of solar temperature might be produced corresponding with that which sets the currents of the terrestrial atmosphere in motion, we are forced to reply that we know of no such cause. for sir john herschel's hypothesis of an increased retention of heat at the sun's equator, due to the slightly spheroidal or bulging form of its outer atmospheric envelope, assuredly gives no sufficient account of such circulatory movements as he supposed to exist. nevertheless, the view that the sun's rotation is intimately connected with the formation of spots is so obviously correct, that we can only wonder it was not thought of sooner, while we are even now unable to explain with any certainty _how_ it is so connected. mere scrutiny of the solar surface, however, is not the only means of solar observation. we have a satellite, and that satellite from time to time acts most opportunely as a screen, cutting off a part or the whole of those dazzling rays in which the master-orb of our system veils himself from over-curious regards. the importance of eclipses to the study of the solar surroundings is of comparatively recent recognition; nevertheless, much of what we know concerning them has been snatched, as it were, by surprise under favour of the moon. in former times, the sole astronomical use of such incidents was the correction of the received theories of the solar and lunar movements; the precise time of their occurrence was the main fact to be noted, and subsidiary phenomena received but casual attention. now, their significance as a geometrical test of tabular accuracy is altogether overshadowed by the interest attaching to the physical observations for which they afford propitious occasions. this change may be said to date, in its pronounced form, from the great eclipse of . although a necessary consequence of the general direction taken by scientific progress, it remains associated in a special manner with the name of francis baily. the "philosopher of newbury" was by profession a london stockbroker, and a highly successful one. nevertheless, his services to science were numerous and invaluable, though not of the brilliant kind which attract popular notice. born at newbury in berkshire, april , , and placed in the city at the age of fourteen, he derived from the acquaintance of dr. priestley a love of science which never afterwards left him. it was, however, no passion such as flames up in the brain of the destined discoverer, but a regulated inclination, kept well within the bounds of an actively pursued commercial career. after travelling for a year or two in what were then the wilds of north america, he went on the stock exchange in , and earned during twenty-four years of assiduous application to affairs a high reputation for integrity and ability, to which corresponded an ample fortune. in the meantime the astronomical society (largely through his co-operation) had been founded; he had for three years acted as its secretary, and he now felt entitled to devote himself exclusively to a subject which had long occupied his leisure hours. he accordingly in retired from business, purchased a house in tavistock place, and fitted up there a small observatory. he was, however, by preference a computator rather than an observer. what sir john herschel calls the "archæology of practical astronomy" found in him an especially zealous student. he re-edited the star-catalogues of ptolemy, ulugh beigh, tycho brahe, hevelius, halley, flamsteed, lacaille, and mayer; calculated the eclipse of thales and the eclipse of agathocles, and vindicated the memory of the first astronomer royal. but he was no less active in meeting present needs than in revising past performances. the subject of the reduction of observations, then, as we have already explained,[ ] in a state of deplorable confusion, attracted his most earnest attention, and he was close on the track of bessel when made acquainted with the method of simplification devised at königsberg. anticipated as an inventor, he could still be of eminent use as a promoter of these valuable improvements; and, carrying them out on a large scale in the star-catalogue of the astronomical society (published in ), "he put" (in the words of herschel) "the astronomical world in possession of a power which may be said, without exaggeration, to have changed the face of sidereal astronomy."[ ] his reputation was still further enhanced by his renewal, with vastly improved apparatus, of the method, first used by henry cavendish in - , for determining the density of the earth. from a series of no less than , delicate and difficult experiments, conducted at tavistock place during the years - , he concluded our planet to weigh · as much as a globe of water of the same bulk; and this result slightly corrected is still accepted as a very close approximation of the truth. what we have thus glanced at is but a fragment of the truly surprising mass of work accomplished by baily in the course of a variously occupied life. a rare combination of qualities fitted him for his task. unvarying health, undisturbed equanimity, methodical habits, the power of directed and sustained thought, combined to form in him an intellectual toiler of the surest, though not perhaps of the highest quality. he was in harness almost to the end. he was destined scarcely to know the miseries of enforced idleness or of consciously failing powers. in he completed the laborious reduction of lalande's great catalogue, undertaken at the request of the british association, and was still engaged in seeing it through the press when he was attacked with what proved his last, as it was probably his first serious illness. he, however, recovered sufficiently to attend the oxford commemoration of july , , where an honorary degree of d.c.l. was conferred upon him in company with airy and struve; but sank rapidly after the effort, and died on the th of august following, at the age of seventy, lamented and esteemed by all who knew him. it is now time to consider his share in the promotion of solar research. eclipses of the sun, both ancient and modern, were a speciality with him, and he was fortunate in those which came under his observation. such phenomena are of three kinds--partial, annular, and total. in a partial eclipse, the moon, instead of passing directly between us and the sun, slips by, as it were, a little on one side, thus cutting off from our sight only a portion of his surface. an annular eclipse, on the other hand, takes place when the moon is indeed centrally interposed, but falls short of the apparent size required for the entire concealment of the solar disc, which consequently remains visible as a bright ring or annulus, even when the obscuration is at its height. in a total eclipse, on the contrary, the sun completely disappears behind the dark body of the moon. the difference of the two latter varieties is due to the fact that the apparent diameter of the sun and moon are so nearly equal as to gain alternate preponderance one over the other through the slight periodical changes in their respective distances from the earth. now, on the th of may, , an annular eclipse was visible in the northern parts of great britain, and was observed by baily at inch bonney, near jedburgh. it was here that he saw the phenomenon which obtained the name of "baily's beads," from the notoriety conferred upon it by his vivid description. "when the cusps of the sun," he writes, "were about ° asunder, a row of lucid points, like a string of bright beads, irregular in size and distance from each other, _suddenly_ formed round that part of the circumference of the moon that was about to enter on the sun's disc. its formation, indeed, was so rapid that it presented the appearance of having been caused by the ignition of a fine train of gunpowder. finally, as the moon pursued her course, the dark intervening spaces (which, at their origin, had the appearance of lunar mountains in high relief, and which still continued attached to the sun's border) were stretched out into long, black, thick, parallel lines, joining the limbs of the sun and moon; when all at once they _suddenly_ gave way, and left the circumference of the sun and moon in those points, as in the rest, comparatively smooth and circular, and the moon perceptibly advanced on the face of the sun."[ ] these curious appearances were not an absolute novelty. weber in , and von zach in , had seen the "beads"; van swinden had described the "belts" or "threads."[ ] these last were, moreover (as baily clearly perceived), completely analogous to the "black ligament" which formed so troublesome a feature in the transits of venus in and , and which, to the regret and confusion, though no longer to the surprise of observers, was renewed in that of . the phenomenon is largely an effect of what is called _irradiation_, by which a bright object seems to encroach upon a dark one; but under good atmospheric and instrumental conditions it becomes inconspicuous. the "beads" must always appear when the projected lunar edge is serrated with mountains. in baily's observation, they were exaggerated and distorted by an irradiative _clinging together_ of the limbs of sun and moon. the immediate result, however, was powerfully to stimulate attention to solar eclipses in their _physical_ aspect. never before had an occurrence of the kind been expected so eagerly or prepared for so actively as that which was total over central and southern europe on the th of july, . astronomers hastened from all quarters to the favoured region. the astronomer royal (airy) repaired to turin; baily to pavia; otto struve threw aside his work amidst the stars at pulkowa, and went south as far as lipeszk; schumacher travelled from altona to vienna; arago from paris to perpignan. nor did their trouble go unrewarded. the expectations of the most sanguine were outdone by the wonders disclosed. baily (to whose narrative we again have recourse) had set up his dollond's achromatic in an upper room of the university of pavia, and was eagerly engaged in noting a partial repetition of the singular appearances seen by him in , when he was "astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified at the sight of one of the most brilliant and splendid phenomena that can well be imagined. for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory similar in shape and relative magnitude to that which painters draw round the heads of saints, and which by the french is designated an _auréole_. pavia contains many thousand inhabitants, the major part of whom were, at this early hour, walking about the streets and squares or looking out of windows, in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was _instantaneous_, there was a universal shout from every observer, which 'made the welkin ring,' and, for the moment, withdrew my attention from the object with which i was immediately occupied. i had indeed anticipated the appearance of a luminous circle round the moon during the time of total obscurity; but i did not expect, from any of the accounts of preceding eclipses that i had read, to witness so magnificent an exhibition as that which took place.... the breadth of the corona, measured from the circumference of the moon, appeared to me to be nearly equal to half the moon's diameter. it had the appearance of brilliant rays. the light was most dense close to the border of the moon, and became gradually and uniformly more attenuate as its distance therefrom increased, assuming the form of diverging rays in a rectilinear line, which at the extremity were more divided, and of an unequal length; so that in no part of the corona could i discover the regular and well-defined shape of a ring at its _outer_ margin. it appeared to me to have the sun for its centre, but i had no means of taking any accurate measures for determining this point. its colour was quite white, not pearl-colour, nor yellow, nor red, and the rays had a vivid and flickering appearance, somewhat like that which a gaslight illumination might be supposed to assume if formed into a similar shape.... splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet i must confess that there was at the same time something in its singular and wonderful appearance that was appalling; and i can readily imagine that uncivilised nations may occasionally have become alarmed and terrified at such an object, more especially at times when the true cause of the occurrence may have been but faintly understood, and the phenomenon itself wholly unexpected. "but the most remarkable circumstance attending the phenomenon was the appearance of _three large protuberances_ apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. they had the appearance of mountains of a prodigious elevation; their colour was red, tinged with lilac or purple; perhaps the colour of the peach-blossom would more nearly represent it. they somewhat resembled the snowy tops of the alpine mountains when coloured by the rising or setting sun. they resembled the alpine mountains also in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona. all the three projections were of the same roseate cast of colour, and very different from the brilliant vivid white light that formed the corona; but they differed from each other in magnitude.... the whole of these three protuberances were visible even to the last moment of total obscuration; at least, i never lost sight of them when looking in that direction; and when the first ray of light was admitted from the sun, they vanished, with the corona, altogether, and daylight was instantaneously restored."[ ] notwithstanding unfavourable weather, the "red flames" were perceived with little less clearness and no less amazement from the superga than at pavia, and were even discerned by mr. airy with the naked eye. "their form" (the astronomer royal wrote) "was nearly that of saw-teeth in the position proper for a circular saw turned round in the same direction in which the hands of a watch turn.... their colour was a full lake-red, and their brilliancy greater than that of any other part of the ring."[ ] the height of these extraordinary objects was estimated by arago at two minutes of arc, representing, at the sun's distance, an actual elevation of , miles. when carefully watched, the rose-flush of their illumination was perceived to fade through violet to white as the light returned, the same changes in a reversed order having accompanied their first appearance. their forms, however, during about three minutes of visibility, showed no change, although of so apparently unstable a character as to suggest to arago "mountains on the point of crumbling into ruins" through topheaviness.[ ] the corona, both as to figure and extent, presented very different appearances at different stations. this was no doubt due to varieties in atmospheric conditions. at the superga, for instance, all details of structure seem to have been effaced by the murky air, only a comparatively feeble ring of light being seen to encircle the moon. elsewhere, a brilliant radiated formation was conspicuous, spreading at four opposite points into four vast luminous expansions, compared to feather-plumes or _aigrettes_.[ ] arago at perpignan noticed considerable irregularities in the divergent rays. some appeared curved and twisted, a few lay _across_ the others, in a direction almost tangential to the moon's limb, the general effect being described as that of a "hank of thread in disorder."[ ] at lipeszk, where the sun stood much higher above the horizon than in italy or france, the corona showed with surprising splendour. its apparent extent was judged by struve to be no less than twenty-five minutes (more than six times airy's estimate), while the great plumes spread their radiance to three or four degrees from the dark lunar edge. so dazzling was the light that many well-instructed persons denied the totality of the eclipse. nor was the error without precedent, although the appearances attending respectively a total and an annular eclipse are in reality wholly dissimilar. in the latter case, the surviving ring of sunlight becomes so much enlarged by irradiation, that the interposed dark lunar body is reduced to comparative insignificance, or even invisibility. maclaurin tells us[ ] that during an eclipse of this character which he observed at edinburgh in , "gentlemen by no means shortsighted declared themselves unable to discern the moon upon the sun without the aid of a smoked glass;" and baily (who, however, _was_ shortsighted) could distinguish, in , with the naked eye, no trace of "the globe of purple velvet" which the telescope revealed as projected upon the face of the sun.[ ] moreover, the diminution of light is described by him as "little more than might be caused by a temporary cloud passing over the sun"; the birds continued in full song, and "one cock in particular was crowing with all his might while the annulus was forming." very different were the effects of the eclipse of , as to which some interesting particulars were collected by arago.[ ] beasts of burthen, he tells us, paused in their labour, and could by no amount of punishment be induced to move until the sun reappeared. birds and beasts abandoned their food; linnets were found dead in their cages; even ants suspended their toil. diligence-horses, on the other hand, seemed as insensible to the phenomenon as locomotives. the convolvulus and some other plants closed their leaves, but those of the mimosa remained open. the little light that remained was of a livid hue. one observer described the general coloration as resembling the lees of wine, but human faces showed pale olive or greenish. we may, then, rest assured that none of the remarkable obscurations recorded in history were due to eclipses of the annular kind. the existence of the corona is no modern discovery. indeed, it is too conspicuous an apparition to escape notice from the least attentive or least practised observer of a total eclipse. nevertheless, explicit references to it are rare in early times. plutarch, however, speaks of a "certain splendour" compassing round the hidden edge of the sun, as a regular feature of total eclipses;[ ] and the corona is expressly mentioned in a description of an eclipse visible at corfu in a.d.[ ] the first to take the phenomenon into scientific consideration was kepler. he showed, from the orbital positions at the time of the sun and moon, that an eclipse observed by clavius at rome in could not have been annular,[ ] as the dazzling coronal radiance visible during the obscuration had caused it to be believed. although he himself never witnessed a total eclipse of the sun, he carefully collected and compared the remarks of those more fortunate, and concluded that the ring of "flame-like splendour" seen on such occasions was caused by the reflection of the solar rays from matter condensed in the neighbourhood either of the sun or moon.[ ] to the solar explanation he gave his own decided preference; but, with one of those curious flashes of half-prophetic insight characteristic of his genius, declared that "it should be laid by ready for use, not brought into immediate requisition."[ ] so literally was his advice acted upon, that the theory, which we now know to be (broadly speaking) the correct one, only emerged from the repository of anticipated truths after years of almost complete retirement, and even then timorously and with hesitation. the first eclipse of which the attendant phenomena were observed with tolerable exactness was that which was central in the south of france, may , . cassini then put forward the view that the "crown of pale light" seen round the lunar disc was caused by the illumination of the zodiacal light;[ ] but it failed to receive the attention which, as a step in the right direction, it undoubtedly merited. nine years later we meet with halley's comments on a similar event, the first which had occurred in london since march , . by nine in the morning of may , , the obscuration, he tells us, "was about ten digits,[ ] when the face and colour of the sky began to change from perfect serene azure blue to a more dusky livid colour, having an eye of purple intermixt.... a few seconds before the sun was all hid, there discovered itself round the moon a luminous ring, about a digit or perhaps a tenth part of the moon's diameter in breadth. it was of a pale whiteness, or rather pearl colour, seeming to be a little tinged with the colours of the iris, and to be concentric with the moon, whence i concluded it the moon's atmosphere. but the great height thereof, far exceeding our earth's atmosphere, and the observation of some, who found the breadth of the ring to increase on the west side of the moon as emersion approached, together with the contrary sentiments of those whose judgment i shall always revere" (newton is most probably referred to), "makes me less confident, especially in a matter whereto i confess i gave not all the attention requisite." he concludes by declining to decide whether the "enlightened atmosphere," which the appearance "in all respects resembled," "belonged to sun or moon."[ ] a french academician, who happened to be in london at the time, was less guarded in expressing an opinion. the chevalier de louville declared emphatically for the lunar atmospheric theory of the corona,[ ] and his authority carried great weight. it was, however, much discredited by an observation made by maraldi in , to the effect that the luminous ring, instead of travelling _with_ the moon, was traversed _by_ it.[ ] this was in reality decisive, though, as usual, belief lagged far behind demonstration. in a novel explanation had been offered by delisle and lahire,[ ] supported by experiments regarded at the time as perfectly satisfactory. the aureola round the eclipsed sun, they argued, is simply a result of the _diffraction_, or apparent bending of the sunbeams that graze the surface of the lunar globe--an effect of the same kind as the coloured fringes of shadows. and this view prevailed amongst men of science until (and even after) brewster showed, with clear and simple decisiveness, that such an effect could by no possibility be appreciable at our distance from the moon.[ ] don josé joaquim de ferrer, however, who observed a total eclipse of the sun at kinderhook, in the state of new york, on june , , ignoring this refined optical _rationale_, considered two alternative explanations of the phenomenon as alone possible. the bright ring round the moon must be due to the illumination either of a lunar or of a solar atmosphere. if the former, he calculated that it should have a height fifty times that of the earth's gaseous envelope. "such an atmosphere," he rightly concluded, "cannot belong to the moon, but must without any doubt belong to the sun."[ ] but he stood alone in this unhesitating assertion. the importance of the problem was first brought fully home to astronomers by the eclipse of . the brilliant and complex appearance which on that occasion challenged the attention of so many observers, demanded and received, no longer the casual attention hitherto bestowed upon it, but the most earnest study of those interested in the progress of science. nevertheless, it was only by degrees, and through a process of "exclusions" (to use a baconian phrase) that the corona was put in its right place as a solar appendage. as every other available explanation proved inadmissible and dropped out of sight, the broad presentation of fact remained, which, though of sufficiently obvious interpretation, was long and persistently misconstrued. nor was it until that absolutely decisive evidence on the subject was forthcoming, as we shall see further on. sir john herschel, writing to his venerable aunt, relates that when the brilliant red flames burst into view behind the dark moon on the morning of the th of july, , the populace of milan, with the usual inconsequence of a crowd, raised the shout, "_es leben die astronomen!_"[ ] in reality, none were less prepared for their apparition than the class to whom the applause due to the magnificent spectacle was thus adjudged. and in some measure through their own fault, for many partial hints and some distinct statements from earlier observers had given unheeded notice that some such phenomenon might be expected to attend a solar eclipse. what we now call the "chromosphere" is an envelope of glowing gases, by which the sun is completely covered, and from which the "prominences" are emanations, eruptive or flame-like. now, continual indications of the presence of this fire-ocean had been detected during eclipses in the eighteenth and nineteenth centuries. captain stannyan, describing in a letter to flamsteed an occurrence of the kind witnessed by him at berne on may (o.s.), , says that the sun's "getting out of the eclipse was preceded by a blood-red streak of light from its left limb."[ ] a precisely similar appearance was noted by both halley and de louville in ; during annular eclipses by lord aberdour in ,[ ] and by short in ,[ ] the tint of the ruby border being, however, subdued to "brown" or "dusky red" by the surviving sunlight; while observations identical in character were made at amsterdam in ,[ ] at edinburgh by henderson in , and at new york in .[ ] "flames" or "prominences," if more conspicuous, are less constant in their presence than the glowing stratum from which they spring. the first to describe them was a swedish professor named vassenius, who observed a total eclipse at gothenburg, may (o.s.), .[ ] his astonishment equalled his admiration when he perceived, just outside the edge of the lunar disc, and suspended, as it seemed, in the coronal atmosphere, three or four reddish spots or clouds, one of which was so large as to be detected with the naked eye. as to their nature, he did not even offer a speculation, further than by tacitly referring them to the moon. the observation was repeated in by a spanish admiral, but with no better success in directing efficacious attention to the phenomenon. don antonio ulloa was on board his ship the _espagne_ in passage from the azores to cape st. vincent on the th of june in that year, when a total eclipse of the sun occurred, of which he has left a valuable description. his notices of the corona are full of interest; but what just now concerns us is the appearance of "a red luminous point" "near the edge of the moon," which gradually increased in size as the moon moved away from it, and was visible during about a minute and a quarter.[ ] he was satisfied that it belonged to the sun because of its fiery colour and growth in magnitude, and supposed that it was occasioned by some crevice or inequality in the moon's limb, through which the solar light penetrated. allusions less precise, both prior and subsequent, which it is now easy to refer to similar objects (such as the "slender columns of smoke" seen by ferrer)[ ] might be detailed; but the evidence already adduced suffices to show that the prominences viewed with such amazement in were no unprecedented or even unusual phenomenon. it was more important, however, to decide what was their nature than whether their appearance might have been anticipated. they were generally, and not very incorrectly, set down as solar clouds. arago believed them to shine by reflected light,[ ] but the abbé peytal rightly considered them to be self-luminous. writing in a montpellier paper of july , , he declared that we had now become assured of the existence of a third or outer solar envelope, composed of a glowing substance of a bright rose tint, forming mountains of prodigious elevation, analogous in character to the clouds piled above our horizons.[ ] this first distinct recognition of a very important feature of our great luminary was probably founded on an observation made by bérard at toulon during the then recent eclipse, "of a very fine red band, irregularly dentelated, or, as it were, crevassed here and there,"[ ] encircling a large arc of the moon's circumference. it can hardly, however, be said to have attracted general notice until july , . on that day a total eclipse took place, which was observed with considerable success in various parts of sweden and norway by a number of english astronomers. mr. hind saw, on the south limb of the moon, "a long range of rose-coloured flames,"[ ] described by dawes as "a low ridge of red prominences, resembling in outline the tops of a very irregular range of hills."[ ] airy termed the portion of this "rugged lines of projections" visible to him the _sierra_, and was struck with its brilliant light and "nearly scarlet" colour.[ ] its true character of a continuous solar envelope was inferred from these data by grant, swan, and littrow, and was by father secchi, after the great eclipse of ,[ ] formally accepted as established. several prominences of remarkable forms, especially one variously compared to a turkish scimitar, a sickle, and a boomerang, were seen in . in connection with them two highly significant circumstances were pointed out. first, that of the approximate coincidence between their positions and those of sun-spots previously observed.[ ] next, that "the moon passed over them, leaving them behind, and revealing successive portions as she advanced."[ ] this latter perfectly well-attested fact was justly considered by the astronomer royal and others as affording absolute certainty of the solar dependence of these singular objects. nevertheless sceptics were still found. m. faye, of the french academy, inclined to a lunar origin for them;[ ] feilitsch of greifswald published in a treatise for the express purpose of proving all the luminous phenomena attendant on solar eclipses--corona, prominences and "sierra"--to be purely optical appearances.[ ] happily, however, the unanswerable arguments of the photographic camera were soon to be made available against such hardy incredulity. thus, the virtual discovery of the solar appendages, both coronal and chromospheric, may be said to have been begun in , and completed in . the current herschelian theory of the solar constitution remained, however, for the time, intact. difficulties, indeed, were thickening around it; but their discussion was perhaps felt to be premature, and they were permitted to accumulate without debate, until fortified by fresh testimony into unexpected and overwhelming preponderance. footnotes: [footnote : kosmos, bd. iii., p. ; lalande, _bibliographie astronomique_, pp. , .] [footnote : r. wolf, _die sonne und ihre flecken_, p. . marius himself, however, seems to have held the aristotelian terrestrial-exhalation theory of cometary origin. see his curious little tract, _astronomische und astrologische beschreibung der cometen_, nürnberg, .] [footnote : _phil. trans._, vol. xxvii., p. . _umbræ_ (now called _penumbræ_) are spaces of half-shadow which usually encircle spots. _faculæ_ ("little torches," so named by scheiner) are bright streaks or patches closely associated with spots.] [footnote : _mém. ac. sc._, (pub. ), p. . d. cassini, however, first put forward about the hypothesis alluded to in the text. see delambre, _hist. de l'astr. mod._, t. ii., p. ; and _kosmos_, bd. iii., p. .] [footnote : _phil. trans._, vol. lxiv., part i., pp. - .] [footnote : _rosa ursina_, lib. iv., p. .] [footnote : r. wolf, _die sonne und ihre flecken_, p. .] [footnote : schellen, _die spectralanalyse_, bd. ii., p. ( rd ed.).] [footnote : _phil. trans._, vol. lxiv., p. .] [footnote : _ibid._, vol. lxxxv., , p. .] [footnote : _phil. trans._, vol. xci., , p. .] [footnote : the supposed opaque or protective stratum beneath the photosphere was named by him "planetary," from the analogy of terrestrial clouds.] [footnote : _ibid._, p. .] [footnote : _novum organum_, lib. ii. aph. .] [footnote : brewster's _life of newton_, vol. ii., p. .] [footnote : _beschäftigungen d. berl. ges. naturforschender freunde_, bd. ii., p. .] [footnote : _gentleman's magazine_, , vol. ii., p. .] [footnote : _results_, etc., p. .] [footnote : _ibid._, p. .] [footnote : see _ante_, p. .] [footnote : _memoir of francis baily, mem. r. a. s._, vol. xv., p. .] [footnote : _mem. r. a. s._, vol. x., pp. - .] [footnote : _ibid._, pp. - .] [footnote : _mem. r. a. s._, vol. xv., pp. - .] [footnote : _ibid._, p. .] [footnote : _annuaire_, , p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. xl., p. .] [footnote : _mem. r. a. s._, vol. x., p. .] [footnote : _ann. du bureau des long._, , p. .] [footnote : _de facie in orbe lunæ_, xix., . cf. grant, _astr. nach._, no. . as to the phenomenon mentioned by philostratus in his _life of apollonius_ (viii. ), see w. t. lynn, _observatory_, vol. ix., p. .] [footnote : schmidt, _astr. nach._, no. .] [footnote : _astronomiæ pars optica, op. omnia_, t. ii., p. .] [footnote : _de stellâ novâ, op._, t. ii., pp. , .] [footnote : _astr. pars op._, p. .] [footnote : _mém. de l'ac. des sciences_, , p. .] [footnote : a digit = / of the solar diameter.] [footnote : _phil. trans._, vol. xxix., pp. - .] [footnote : _mém. de l'ac. des sciences_, ; _histoire_, p. ; _mémoires_, pp. - .] [footnote : _ibid._, , p. .] [footnote : _mém. de l'ac. des sciences_, , pp. , - .] [footnote : _ed. ency._, art. _astronomy_, p. .] [footnote : _trans. am. phil. soc._, vol. vi., p. .] [footnote : _memoir of caroline herschel_, p. .] [footnote : _phil. trans._, vol. xxv., p. .] [footnote : _ibid._, vol. xl., p. .] [footnote : _ibid._, vol. xlv., p. .] [footnote : _mem. r. a. s._, vol. i., pp. , .] [footnote : _american journal of science_, vol. xlii., p. .] [footnote : _phil. trans._, vol. xxxviii., p. . father secchi, however, adverted to a distinct mention of a prominence observed in a.d. a description of a total eclipse of that date includes the remark, "et quoddam foramen erat ignitum in circulo solis ex parte inferiore" (muratori, _rer. it. scriptores_, t. xiv., col. ). the "circulus solis" of course signifies the corona.] [footnote : _phil. trans._, vol. lxix., p. .] [footnote : _trans. am. phil. soc._, vol. vi., , p. .] [footnote : _annuaire_, , p. .] [footnote : _ibid._, p. , _note_.] [footnote : _ibid._, p. .] [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, pp. , .] [footnote : _le soleil_, t. i., p. .] [footnote : by williams and stanistreet, _mem. r. a. s._, vol. xxi., pp. , . santini had made a similar observation at padua in . grant, _hist. astr._, p. .] [footnote : lassell in _month. not._, vol. xii., p. .] [footnote : _comptes rendus_, t. xxxiv., p. .] [footnote : _optische untersuchungen_, and _zeitschrift für populäre mittheilungen_, bd. i., , p. .] chapter iv _planetary discoveries_ in the course of his early gropings towards a law of the planetary distances, kepler tried the experiment of setting a planet, invisible by reason of its smallness, to revolve in the vast region of seemingly desert space separating mars from jupiter.[ ] the disproportionate magnitude of the same interval was explained by kant as due to the overweening size of jupiter. the zone in which each planet moved was, according to the philosopher of königsberg, to be regarded as the empty storehouse from which its materials had been derived. a definite relation should thus exist between the planetary masses and the planetary intervals.[ ] lambert, on the other hand, sportively suggested that the body or bodies (for it is noticeable that he speaks of them in the plural) which once bridged this portentous gap in the solar system, might, in some remote age, have been swept away by a great comet, and forced to attend its wanderings through space.[ ] these speculations were destined before long to assume a more definite form. johann daniel titius, a professor at wittenberg (where he died in ), pointed out in , in a note to a translation of bonnet's _contemplation de la nature_,[ ] the existence of a remarkable symmetry in the disposition of the bodies constituting the solar system. by a certain series of numbers, increasing in regular progression,[ ] he showed that the distances of the six known planets from the sun might be represented with a close approach to accuracy. but with one striking interruption. the term of the series succeeding that which corresponded to the orbit of mars was without a celestial representative. the orderly flow of the sequence was thus singularly broken. the space where a planet should--in fulfilment of the "law"--have revolved, was, it appeared, untenanted. johann elert bode, then just about to begin his long career as leader of astronomical thought and work at berlin, marked at once the anomaly, and filled the vacant interval with a hypothetical planet. the discovery of uranus, at a distance falling but slightly short of perfect conformity with the law of titius, lent weight to a seemingly hazardous prediction, and von zach was actually at the pains, in , to calculate what he termed "analogical" elements[ ] for this unseen and (by any effect or influence) _unfelt_ body. the search for it, through confessedly scarcely less chimerical than that of alchemists for the philosopher's stone, he kept steadily in view for fifteen years, and at length (september , ) succeeded in organising, in combination with five other german astronomers assembled at lilienthal, a force of what he jocularly termed celestial police, for the express purpose of tracking and intercepting the fugitive subject of the sun. the zodiac was accordingly divided for purposes of scrutiny into twenty-four zones; their apportionment to separate observers was in part effected, and the association was rapidly getting into working order, when news arrived that the missing planet had been found, through no systematic plan of search, but by the diligent, though otherwise directed labours of a distant watcher of the skies. giuseppe piazzi was born at ponte in the valtelline, july , . he studied at various places and times under tiraboschi, beccaria, jacquier, and le sueur; and having entered the theatine order of monks at the age of eighteen, he taught philosophy, science, and theology in several of the italian cities, as well as in malta, until , when the chair of mathematics in the university of palermo was offered to and accepted by him. prince caramanico, then viceroy of sicily, had scientific leanings, and was easily won over to the project of building an observatory, a commodious foundation for which was afforded by one of the towers of the viceregal palace. this architecturally incongruous addition to an ancient saracenic edifice--once the abode of kelbite and zirite emirs--was completed in february, . piazzi, meanwhile, had devoted nearly three years to the assiduous study of his new profession, acquiring a practical knowledge of lalande's methods at the École militaire, and of maskelyne's at the royal observatory; and returned to palermo in , bringing with him, in the great five-foot circle which he had prevailed upon ramsden to construct, the most perfect measuring instrument hitherto employed by an astronomer. he had been above nine years at work on his star-catalogue, and was still profoundly unconscious that a place amongst the lilienthal band[ ] of astronomical detectives was being held in reserve for him, when, on the first evening of the nineteenth century, january , , he noticed the position of an eighth-magnitude star in a part of the constellation taurus to which an error of wollaston's had directed his special attention. reobserving, according to his custom, the same set of fifty stars on four consecutive nights, it seemed to him, on the nd, that the one in question had slightly shifted its position to the west; on the rd he assured himself of the fact, and believed that he had chanced upon a new kind of comet without tail or coma. the wandering body, whatever its nature, exchanged retrograde for direct motion on january ,[ ] and was carefully watched by piazzi until february , when a dangerous illness interrupted his observations. he had, however, not omitted to give notice of his discovery; but so precarious were communications in those unpeaceful times, that his letter to oriani of january did not reach milan until april , while a missive of one day later addressed to bode came to hand at berlin, march . the delay just afforded time for the publication, by a young philosopher of jena named hegel, of a "dissertation" showing, by the clearest light of reason, that the number of the planets could not exceed seven, and exposing the folly of certain devotees of induction who sought a new celestial body merely to fill a gap in a numerical series.[ ] unabashed by speculative scorn, bode had scarcely read piazzi's letter when he concluded that it referred to the precise body in question. the news spread rapidly, and created a profound sensation, not unmixed with alarm lest this latest addition to the solar family should have been found only to be again lost. for by that time piazzi's moving star was too near the sun to be any longer visible, and in order to rediscover it after conjunction a tolerably accurate knowledge of its path was indispensable. but a planetary orbit had never before been calculated from such scanty data as piazzi's observation afforded;[ ] and the attempts made by nearly every astronomer of note in germany to compass the problem were manifestly inadequate, failing even to account for the positions in which the body had been actually seen, and _à fortiori_ serving only to mislead as to the places where, from september, , it ought once more to have become discernible. it was in this extremity that the celebrated mathematician gauss came to the rescue. he was then in his twenty-fifth year, and was earning his bread by tuition at brunswick, with many possibilities, but no settled career before him. the news from palermo may be said to have converted him from an arithmetician into an astronomer. he was already in possession of a new and more general method of computing elliptical orbits; and the system of "least squares," which he had devised though not published, enabled him to extract the most probable result from a given set of observations. armed with these novel powers, he set to work; and the communication in november of his elements and ephemeris for the lost object revived the drooping hopes of the little band of eager searchers. their patience, however, was to be still further tried. clouds, mist, and sleet seemed to have conspired to cover the retreat of the fugitive; but on the last night of the year the sky cleared unexpectedly with the setting in of a hard frost, and there, in the north-western part of virgo, nearly in the position assigned by gauss to the runaway planet, a strange star was discerned by von zach[ ] at gotha, and on a subsequent evening--the anniversary of the original discovery--by olbers at bremen. the name of ceres (as the tutelary goddess of sicily) was, by piazzi's request, bestowed upon this first known of the numerous, and probably all but innumerable family of the minor planets. the recognition of the second followed as the immediate consequence of the detection of the first. olbers had made himself so familiar with the positions of the small stars along the track of the long-missing body, that he was at once struck (march , ) with the presence of an intruder near the spot where he had recently identified ceres. he at first believed the new-comer to be a variable star usually inconspicuous, but just then at its maximum of brightness; but within two hours he had convinced himself that it was no _fixed_ star, but a rapidly moving object. the aid of gauss was again invoked, and his prompt calculations showed that this fresh celestial acquaintance (named "pallas" by olbers), revolved round the sun at nearly the same mean distance as ceres, and was beyond question of a strictly analogous character. this result was perplexing in the extreme. the symmetry and simplicity of the planetary scheme appeared fatally compromised by the admission of many, where room could, according to old-fashioned rules, only be found for one. a daring hypothesis of olbers's invention provided an exit from the difficulty. he supposed that both ceres and pallas were fragments of a primitive trans-martian planet, blown to pieces in the remote past, either by the action of internal forces or by the impact of a comet; and predicted that many more such fragments would be found to circulate in the same region. he, moreover, pointed out that these numerous orbits, however much they might differ in other respects, must all have a common line of intersection,[ ] and that the bodies moving in them must consequently pass, at each revolution, through two opposite points of the heavens, one situated in the whale, the other in the constellation of the virgin, where already pallas had been found and ceres recaptured. the intimation that fresh discoveries might be expected in those particular regions was singularly justified by the detection of two bodies now known respectively as juno and vesta. the first was found near the predicted spot in cetus by harding, schröter's assistant at lilienthal, september , ; the second by olbers himself in virgo, after three years of persistent scrutiny, march , . the theory of an exploded planet now seemed to have everything in its favour. it required that the mean or average distances of the newly-discovered bodies should be nearly the same, but admitted a wide range of variety in the shapes and positions of their orbits, provided always that they preserved common points of intersection. these conditions were fulfilled with a striking approach to exactness. three of the four "asteroids" (a designation introduced by sir. w. herschel[ ]) conformed with very approximate precision to "bode's law" of distances; they all traversed, in their circuits round the sun, nearly the same parts of cetus and virgo; while the eccentricities and inclinations of their paths departed widely from the planetary type--that of pallas, to take an extreme instance, making with the ecliptic an angle of nearly °. the minuteness of these bodies appeared further to strengthen the imputation of a fragmentary character. herschel estimated the diameter of ceres at , that of pallas at miles.[ ] but these values are now known to be considerably too small. a suspected variability of brightness in some of the asteroids, somewhat hazardously explained as due to the irregularities of figure to be expected in cosmical _potsherds_ (so to speak), was added to the confirmatory evidence.[ ] the strong point of the theory, however, lay not in what it explained, but in what it had predicted. it had been twice confirmed by actual exploration of the skies, and had produced, in the recognition of vesta, the first recorded instance of the _premeditated_ discovery of a heavenly body. the view not only commended itself to the facile imagination of the unlearned, but received the sanction of the highest scientific authority. the great lagrange bestowed upon it his analytical _imprimatur_, showing that the explosive forces required to produce the supposed catastrophe came well within the bounds of possibility; since a velocity of less than twenty times that of a cannon-ball leaving the gun's mouth would have sufficed, according to his calculation, to launch the asteroidal fragments on their respective paths. indeed, he was disposed to regard the hypothesis of disruption as more generally available than its author had designed it to be, and proposed to supplement with it, as explanatory of the eccentric orbits of comets, the nebular theory of laplace, thereby obtaining, as he said, "a complete view of the origin of the planetary system more conformable to nature and mechanical laws than any yet proposed."[ ] nevertheless the hypothesis of olbers has not held its ground. it seemed as if all the evidence available for its support had been produced at once and spontaneously, while the unfavourable items were elicited slowly, and, as it were, by cross-examination. a more extended acquaintance with the group of bodies whose peculiarities it was framed to explain has shown them, after all, as recalcitrant to any such explanation. coincidences at the first view significant and striking have been swamped by contrary examples; and a hasty general conclusion has, by a not uncommon destiny, at last perished under the accumulation of particulars. moreover, as has been remarked by professor newcomb,[ ] mutual perturbations would rapidly efface all traces of a common disruptive origin, and the catastrophe, to be perceptible in its effects, should have been comparatively recent. a new generation of astronomers had arisen before any additions were made to the little family of the minor planets. piazzi died in , harding in , olbers in ; all those who had prepared or participated in the first discoveries passed away without witnessing their resumption. in , however, a certain hencke, ex-postmaster in the prussian town of driessen, set himself to watch for new planets, and after fifteen long years his patience was rewarded. the asteroid found by him, december , , received the name of astræa, and his further prosecution of the search resulted, july , , in the discovery of hebe. a few weeks later (august ), john russell hind ( - ), after many months' exploration from mr. bishop's observatory in the regent's park, picked up iris, and october , flora.[ ] the next on the list was metis, found by mr. graham, april , , at markree, in ireland.[ ] at the close of the period to which our attention is at present limited, the number of these small bodies known to astronomy was thirteen; and the course of discovery has since proceeded far more rapidly and with less interruption. both in itself and in its consequences the recognition of the minor planets was of the highest importance to science. the traditional ideas regarding the constitution of the solar system were enlarged by the admission of a new class of bodies, strongly contrasted, yet strictly co-ordinate with the old-established planetary order; the profusion of resource, so conspicuous in the living kingdoms of nature, was seen to prevail no less in the celestial spaces; and some faint preliminary notion was afforded of the indefinite complexity of relations underlying the apparent simplicity of the majestic scheme to which our world belongs. both theoretical and practical astronomy derived profit from the admission of these apparently insignificant strangers to the rights of citizenship of the solar system. the disturbance of their motions by their giant neighbours afforded a more accurate knowledge of the jovian mass, which laplace had taken about / too small; the anomalous character of their orbits presented geometers with highly stimulating problems in the theory of perturbation; while the exigencies of the first discovery had produced the _theoria motus_, and won gauss over to the ranks of calculating astronomy. moreover, the sure prospect of further detections powerfully incited to the exploration of the skies; observers became more numerous and more zealous in view of the prizes held out to them; star-maps were diligently constructed, and the sidereal multitude strewn along the great zodiacal belt acquired a fresh interest when it was perceived that its least conspicuous member might be a planetary shred or projectile in the dignified disguise of a distant sun. harding's "celestial atlas," designed for the special purpose of facilitating asteroidal research, was the first systematic attempt to represent to the eye the _telescopic_ aspect of the heavens. it was while engaged on its construction that the lilienthal observer successfully intercepted juno on her passage through the whale in ; whereupon promoted to göttingen, he there completed, in , the arduous task so opportunely entered upon a score of years previously. still more important were the great star-maps of the berlin academy, undertaken at bessel's suggestion, with the same object of distinguishing errant from fixed stars, and executed, under encke's supervision, during the years - . they have played a noteworthy part in the history of planetary discovery, nor of the minor kind alone. we have now to recount an event unique in scientific history. the discovery of neptune has been characterised as the result of a "movement of the age,"[ ] and with some justice. it had become necessary to the integrity of planetary theory. until it was accomplished, the phantom of an unexplained anomaly in the orderly movements of the solar system must have continued to haunt astronomical consciousness. moreover, it was prepared by many, suggested as possible by not a few, and actually achieved, simultaneously, independently, and completely, by two investigators. the position of the planet uranus was recorded as that of a fixed star no less than twenty times between and the epoch of its final detection by herschel. but these early observations, far from affording the expected facilities for the calculation of its orbit, proved a source of grievous perplexity. the utmost ingenuity of geometers failed to combine them satisfactorily with the later uranian places, and it became evident, either that they were widely erroneous, or that the revolving body was wandering from its ancient track. the simplest course was to reject them altogether, and this was done in the new tables published in by alexis bouvard, the indefatigable computating partner of laplace. but the trouble was not thus to be got rid of. after a few years fresh irregularities began to appear, and continued to increase until absolutely "intolerable." it may be stated as illustrative of the perfection to which astronomy had been brought, that divergencies regarded as menacing the very foundation of its theories never entered the range of unaided vision. in other words, if the theoretical and the real uranus had been placed side by side in the sky, they would have seemed, to the sharpest eye, to form a single body.[ ] the idea that these enigmatical disturbances were due to the attraction of an unknown exterior body was a tolerably obvious one; and we accordingly find it suggested in many different quarters. bouvard himself was perhaps the first to conceive it. he kept the possibility continually in view, and bequeathed to his nephew's diligence the inquiry into its reality when he felt that his own span was drawing to a close; but before any progress had been made with it, he had already (june , ) "ceased to breathe and to calculate." the rev. t. j. hussey actually entertained in the notion, but found his powers inadequate to the task, of assigning an approximate place to the disturbing body; and bessel, in , laid his plans for an assault in form upon the uranian difficulty, the triumphant exit from which fatal illness frustrated his hopes of effecting or even witnessing. the problem was practically untouched when, in , an undergraduate of st. john's college, cambridge, formed the resolution of grappling with it. the projected task was an arduous one. there were no guiding precedents for its conduct. analytical obstacles had to be encountered so formidable as to appear invincible even to such a mathematician as airy. john couch adams, however, had no sooner taken his degree, which he did as senior wrangler in january, , than he set resolutely to work, and on october , , was able to communicate to the astronomer royal numerical estimates of the elements and mass of the unknown planet, together with an indication of its actual place in the heavens. these results, it has been well said,[ ] gave "the final and inexorable proof" of the validity of newton's law. the date october , , "may therefore be regarded as marking a distinct epoch in the history of gravitational astronomy." sir george biddell airy had begun in his long and energetic administration of the royal observatory, and was already in possession of data vitally important to the momentous inquiry then on foot. at his suggestion, and under his superintendence, the reduction of all the planetary observations made at greenwich from onwards had been undertaken in . the results, published in , constituted a permanent and universal stock of materials for the correction of planetary theory. but in the meantime, investigators, both native and foreign, were freely supplied with the "places and errors," which, clearly exhibiting the discrepancies between observation and calculation--between what _was_ and what was _expected_--formed the very groundwork of future improvements. mr. adams had no reason to complain of official discourtesy. his labours received due and indispensable aid; but their purpose was regarded as chimerical. "i have always," sir george airy wrote,[ ] "considered the correctness of a distant mathematical result to be a subject rather of moral than of mathematical evidence." and that actually before him seemed, from its very novelty, to incur a suspicion of unlikelihood. no problem in planetary disturbance had heretofore been attacked, so to speak, from the rear. the inverse method was untried, and might well be deemed impracticable. for the difficulty of determining the perturbations produced by a given planet is small compared with the difficulty of finding a planet by its resulting perturbations. laplace might have quailed before it; yet it was now grappled with as a first essay in celestial dynamics. moreover, adams unaccountably neglected to answer until too late a question regarded by airy in the light of an _experimentum crucis_ as to the soundness of the new theory. nor did he himself take any steps to obtain a publicity which he was more anxious to merit than to secure. the investigation consequently remained buried in obscurity. it is now known that had a search been instituted in the autumn of for the remote body whose existence had been so marvellously foretold, it would have been found within _three and a half lunar diameters_ ( ° ') of the spot assigned to it by adams. a competitor, however, equally daring and more fortunate--_audax fortunâ adjutus_, as gauss said of him--was even then entering the field. urbain jean joseph leverrier, the son of a small government _employé_ in normandy, was born at saint-lô, march , . he studied with brilliant success at the École polytechnique, accepted the post of astronomical teacher there in , and, "docile to circumstance," immediately concentrated the whole of his vast, though as yet undeveloped powers upon the formidable problems, of celestial mechanics. he lost no time in proving to the mathematical world that the race of giants was not extinct. two papers on the stability of the solar system, presented to the academy of sciences, september and october , , showed him to be the worthy successor of lagrange and laplace, and encouraged hopes destined to be abundantly realised. his attention was directed by arago to the uranian difficulty in , when he cheerfully put aside certain intricate cometary researches upon which he happened to be engaged, in order to obey with dutiful promptitude the summons of the astronomical chief of france. in his first memoir on the subject (communicated to the academy, november , ), he proved the inadequacy of all known causes of disturbance to account for the vagaries of uranus; in a second (june , ), he demonstrated that only an exterior body, occupying at a certain date a determinate position in the zodiac, could produce the observed effects; in a third (august , ), he assigned the orbit of the disturbing body, and announced its visibility as an object with a sensible disc about as bright as a star of the eighth magnitude. the question was now visibly approaching an issue. on september , sir john herschel declared to the british association respecting the hypothetical new planet: "we see it as columbus saw america from the coast of spain. its movements have been felt, trembling along the far-reaching line of our analysis with a certainty hardly inferior to that of ocular demonstration." less than a fortnight later, september , professor galle, of the berlin observatory, received a letter from leverrier requesting his aid in the telescopic part of the inquiry already analytically completed. he directed his refractor to the heavens that same night, and perceived, within less than a degree of the spot indicated, an object with a measurable disc nearly three seconds in diameter. its absence from bremiker's recently-completed map of that region of the sky showed it to be no star, and its movement in the predicted direction confirmed without delay the strong persuasion of its planetary nature.[ ] in this remarkable manner the existence of the remote member of our system known as "neptune" was ascertained. but the discovery, which faithfully reflected the duplicate character of the investigation which led to it, had been already secured at cambridge before it was announced from berlin. sir george airy's incredulity vanished in the face of the striking coincidence between the position assigned by leverrier to the unknown planet in june, and that laid down by adams in the previous october; and on the th of july he wrote to professor challis, director of the cambridge observatory, recommending a search with the great northumberland equatoreal. had a good star-map been at hand, the process would have been a simple one; but of bremiker's "hora xxi." no news had yet reached england, and there was no other sufficiently comprehensive to be available for an inquiry which, in the absence of such aid, promised to be both long and laborious. as the event proved, it might have been neither. "after four days of observing," challis wrote, october , , to airy, "the planet was in my grasp if only i had examined or mapped the observations."[ ] had he done so, the first honours in the discovery, both theoretical and optical, would have fallen to the university of cambridge. but professor challis had other astronomical avocations to attend to, and, moreover, his faith in the precision of the indications furnished to him was, by his own confession, a very feeble one. for both reasons he postponed to a later stage of the proceedings the discussion and comparison of the data nightly furnished to him by his telescope, and thus allowed to lie, as it were, latent in his observations the momentous result which his diligence had insured, but which his delay suffered to be anticipated.[ ] nevertheless, it should not be forgotten that the berlin astronomer had two circumstances in his favour apart from which his swift success could hardly have been achieved. the first was the possession of a good star-map; the second was the clear and confident nature of leverrier's instructions. "look where i tell you," he seemed authoritatively to say, "and you will see an object such as i describe."[ ] and in fact, not only galle on the rd of september, but also challis on the th, immediately after reading the french geometer's lucid and impressive treatise, picked out from among the stellar points strewing the zodiac, a small planetary disc, which eventually proved to be that of the precise body he had been in search of during two months. the controversy that ensued had its ignominious side; but it was entered into by neither of the parties principally concerned. adams bore the disappointment, which the dilatory proceedings at greenwich and cambridge had inflicted upon him, with quiet heroism. his silence on the subject of what another man would have called his wrongs remained unbroken to the end of his life;[ ] and he took every opportunity of testifying his admiration for the genius of leverrier. personal questions, however, vanish in the magnitude of the event they relate to. by it the last lingering doubts as to the absolute exactness of the newtonian law were dissipated. recondite analytical methods received a confirmation brilliant and intelligible even to the minds of the vulgar, and emerged from the patient solitude of the study to enjoy an hour of clamorous triumph. for ever invisible to the unaided eye of man, a sister-globe to our earth was shown to circulate, in perpetual frozen exile, at thirty times its distance from the sun. nay, the possibility was made apparent that the limits of our system were not even thus reached, but that yet profounder abysses of space might shelter obedient, though little favoured, members of the solar family, by future astronomers to be recognised through the sympathetic thrillings of neptune, even as neptune himself was recognised through the tell-tale deviations of uranus. it is curious to find that the fruit of adams's and leverrier's laborious investigations had been accidentally all but snatched half a century before it was ripe to be gathered. on the th, and again on the th of may, , lalande noted the position of neptune as that of a fixed star, but perceiving that the two observations did not agree, he suppressed the first as erroneous, and pursued the inquiry no further. an immortality which he would have been the last to despise hung in the balance; the feather-weight of his carelessness, however, kicked the beam, and the discovery was reserved to be more hardly won by later comers. bode's law did good service in the quest for a trans-uranian planet by affording ground for a probable assumption as to its distance. a starting-point for approximation was provided by it; but it was soon found to be considerably at fault. even uranus is about millions of miles nearer to the sun than the order of progression requires; and neptune's vast distance of , million should be increased by no less than million miles, and its period of lengthened out to years,[ ] in order to bring it into conformity with the curious and unexplained rule which planetary discoveries have alternately tended to confirm and to invalidate. within seventeen days of its identification with the berlin achromatic, neptune was found to be attended by a satellite. this discovery was the first notable performance of the celebrated two-foot reflector[ ] erected by mr. lassell at his suggestively named residence of starfield, near liverpool. william lassell was a brewer by profession, but by inclination an astronomer. born at bolton in lancashire, june , , he closed a life of eminent usefulness to science, october , , thus spanning with his well-spent years four-fifths of the momentous period which we have undertaken to traverse. at the age of twenty-one, being without the means to purchase, he undertook to construct telescopes, and naturally turned his attention to the reflecting sort, as favouring amateur efforts by the comparative simplicity of its structure. his native ingenuity was remarkable, and was developed by the hourly exigencies of his successive enterprises. their uniform success encouraged him to enlarge his aims, and in he visited birr castle for the purpose of inspecting the machine used in polishing the giant speculum of parsonstown. in the construction of his new instrument, however, he eventually discarded the model there obtained, and worked on a method of his own, assisted by the supreme mechanical skill of james nasmyth. the result was a newtonian of exquisite definition, with an aperture of two, and a focal length of twenty feet, provided by a novel artifice with the equatoreal mounting, previously regarded as available only for refractors. this beautiful instrument afforded to its maker, october , , a cursory view of a neptunian attendant. but the planet was then approaching the sun, and it was not until the following july that the observation could be verified, which it was completely, first by lassell himself, and somewhat later by otto stuve and bond of cambridge (u.s.). when it is considered that this remote object shines by reflecting sunlight reduced by distance to / th of the intensity with which it illuminates our moon, the fact of its visibility, even in the most perfect telescopes, is a somewhat surprising one. it can only, indeed, be accounted for by attributing to it dimensions very considerable for a body of the secondary order. it shares with the moons of uranus the peculiarity of retrograde motion; that is to say, its revolutions, running counter to the grand current of movement in the solar system, are performed from east to west, in a plane inclined at an angle of ° to that of the ecliptic. their swiftness serves to measure the mass of the globe round which they are performed. for while our moon takes twenty-seven days and nearly eight hours to complete its circuit of the earth, the satellite of neptune, at a distance not greatly inferior, sweeps round its primary in five days and twenty-one hours, showing (according to a very simple principle of computation) that it is urged by a force seventeen times greater than the terrestrial pull upon the lunar orb. combining this result with those of professor barnard's[ ] and dr. see's[ ] recent measurements of the small telescopic disc of this farthest known planet, it is found that while in _mass_ neptune equals seventeen, in _bulk_ it is equivalent to forty-nine earths. this is as much as to say that it is composed of relatively very light materials, or more probably of materials distended by internal heat, as yet unwasted by radiation into space, to about five times the volume they would occupy in the interior of our globe. the fact, at any rate, is fairly well ascertained, that the average density of neptune is about twice that of water. we must now turn from this late-recognised member of our system to bestow some brief attention upon the still fruitful field of discovery offered by one of the immemorial five. the family of saturn, unlike that of its brilliant neighbour, has been gradually introduced to the notice of astronomers. titan, the sixth saturnian moon in order of distance, led the way, being detected by huygens, march , ; cassini made the acquaintance of four more between and ; while mimas and enceladus, the two innermost, were caught by herschel in , as they threaded their lucid way along the edge of the almost vanished ring. in the distances of these seven revolving bodies from their primary, an order of progression analogous to that pointed out by titius in the planetary intervals was found to prevail; but with one conspicuous interruption, similar to that which had first suggested the search for new members of the solar system. between titan and japetus--the sixth and seventh reckoning outwards--there was obviously room for another satellite. it was discovered on both sides of the atlantic simultaneously, on the th of september, . mr. w. c. bond, employing the splendid -inch refractor of the harvard observatory, noticed, september , a minute star situated in the plane of saturn's rings. the same object was discerned by mr. lassell on the th. on the following evening, both observers perceived that the problematical speck of light kept up with, instead of being left behind by the planet as it moved, and hence inferred its true character.[ ] hyperion, the seventh by distance and eighth by recognition of saturn's attendant train, is of so insignificant a size when compared with some of its fellow-moons (titan is but little inferior to the planet mars), as to have suggested to sir john herschel[ ] the idea that it might be only one of several bodies revolving very close together--in fact, an _asteroidal satellite_; but the conjecture has, so far, not been verified. the coincidence of its duplicate discovery was singularly paralleled two years later. galileo's amazement when his "optic glass" revealed to him the "triple" form of saturn--_planeta tergeminus_--has proved to be, like the laughter of the gods, "inextinguishable." it must revive in every one who contemplates anew the unique arrangements of that world apart known to us as the saturnian system. the resolution of the so-called _ansæ_, or "handles," into one encircling ring by huygens in , the discovery by cassini in of the division of that ring into two concentric ones, together with laplace's investigation of the conditions of stability of such a formation, constituted, with some minor observations, the sum of the knowledge obtained, up to the middle of the last century, on the subject of this remarkable formation. the first place in the discovery now about to be related belongs to an american astronomer. william cranch bond, born in at portland, in the state of maine, was a watchmaker, whom the solar eclipse of attracted to study the wonders of the heavens. when, in , the erection of an observatory in connection with harvard college, cambridge, was first contemplated, he undertook a mission to england for the purpose of studying the working of similar institutions there, and on his return erected a private observatory at dorchester, where he worked diligently for many years. then at last, in , the long-postponed design of the harvard authorities was resumed, and on the completion of the new establishment, bond, who had been from officially connected with the college and had carried on his scientific labours within its precincts, was offered and accepted the post of its director. placed in in possession of one of the finest instruments in the world--a masterpiece of merz and mahler--he headed the now long list of distinguished transatlantic observers. like the elder struve, he left an heir to his office and to his eminence, but george bond unfortunately died in , at the early age of thirty-nine, having survived his father but six years. on the night of november , --the air, remarkably enough, being so hazy that only the brightest stars could be perceived with the naked eye--william bond discerned a dusky ring, extending about halfway between the inner brighter one and the globe of saturn. a fortnight later, but before the observation had been announced in england, the same appearance was seen by the rev. w. r. dawes with the comparatively small refractor of his observatory at wateringbury, and on december was described by mr. lassell (then on a visit to him) as "something like a crape veil covering a part of the sky within the inner ring."[ ] next morning the _times_ containing the report of bond's discovery reached wateringbury. the most surprising circumstance in the matter was that the novel appendage had remained so long unrecognised. as the rings opened out to their full extent, it became obvious with very moderate optical assistance; yet some of the most acute observers who have ever lived, using instruments of vast power, had heretofore failed to detect its presence. it soon appeared, however, that galle of berlin[ ] had noticed, june , , a veil-like extension of the lucid ring across half the dark space separating it from the planet; but the observation, although communicated at the time to the berlin academy of sciences, had remained barren. traces of the dark ring, moreover, were found in drawings executed by campani in [ ] and by hooke in ;[ ] while picard (june , ),[ ] hadley (spring of ),[ ] and herschel,[ ] had all undoubtedly seen it under the aspect of a dark bar or belt crossing the saturnian globe. it was, then, of no recent origin; but there seemed reason to think that it had lately gained considerably in brightness. the full meaning of this suspected change it was reserved for later investigations to develop. what we may, in a certain sense, call the closing result of the race for discovery, in which several observers seemed at that time to be engaged, was the establishment, on a satisfactory footing, of our acquaintance with the dependent system of uranus. sir william herschel, whose researches formed, in so many distinct lines of astronomical inquiry, the starting-points of future knowledge, detected, january , ,[ ] two uranian moons, since called oberon and titania, and ascertained the curious circumstance of their motion in a plane almost at right angles to the ecliptic, in a direction contrary to that of all previously known denizens (other than cometary) of the solar kingdom. he believed that he caught occasional glimpses of four more, but never succeeded in assuring himself of their substantial existence. even the two first remained unseen save by himself until , when his son re-observed them with a -foot reflector, similar to that with which they had been originally discovered. thenceforward they were kept fairly within view, but their four questionable companions, in spite of some false alarms of detection, remained in the dubious condition in which herschel had left them. at last, on october , ,[ ] after some years of fruitless watching, lassell espied "ariel" and "umbriel," two uranian attendants, interior to oberon and titania, and of about half their brightness; so that their disclosure is still reckoned amongst the very highest proofs of instrumental power and perfection. in all probability they were then for the first time seen; for although professor holden[ ] made out a plausible case in favour of the fitful visibility to herschel of each of them in turn, lassell's argument[ ] that the glare of the planet in herschel's great specula must have rendered almost impossible the perception of objects so minute and so close to its disc, appears tolerably decisive to the contrary. uranus is thus attended by four moons, and, so far as present knowledge extends, by no more. among the most important of the "negative results"[ ] secured by lassell's observations at malta during the years - and - , were the convincing evidence afforded by them that, without great increase of optical power, no further neptunian or uranian satellites can be perceived, and the consequent relegation of herschel's baffling quartette, notwithstanding the unquestioned place long assigned to them in astronomical text-books, to the nirvana of non-existence. footnotes: [footnote : _op._, t. i., p. . he interposed, but tentatively only, another similar body between mercury and venus.] [footnote : _allgemeine naturgeschichte_ (ed. ), pp. , .] [footnote : _cosmologische briefe_, no. (quoted by von zach, _monat. corr._, vol. iii., p. ).] [footnote : second ed., p. . see bode, _von dem neuen hauptplaneten_, p. , _note_.] [footnote : the representative numbers are obtained by adding to the following series (irregular, it will be observed, in its first member, which should be / instead of ); , , , , , , etc. the formula is a purely empirical one, and is, moreover, completely at fault as regards the distance of neptune.] [footnote : _monat. corr._, vol. iii., p. .] [footnote : wolf, _geschichte der astronomie_, p. .] [footnote : such reversals of direction in the apparent movements of the planets are a consequence of the earth's revolution in its orbit.] [footnote : _dissertatio philosophica de orbitis planetarum_, . see wolf, _gesch. d. astr._, p. .] [footnote : observations on uranus, as a supposed fixed star, went back to .] [footnote : he had caught a glimpse of it on december , but was prevented by bad weather from verifying his suspicion. _monat. corr._, vol. v., p. .] [footnote : planetary fragments, hurled _in any direction_, and _with any velocity_ short of that which would for ever release them from the solar sway, would continue to describe elliptic orbits round the sun, all passing through the scene of the explosion, and thus possessing a common line of intersection.] [footnote : _phil. trans._, vol. xcii., part ii., p. .] [footnote : _ibid._, p. . in a letter to von zach of june , , he speaks of pallas as "almost incredibly small," and makes it only seventy english miles in diameter. _monat. corr._, vol. vi., pp. , .] [footnote : olbers, _monat. corr._, vol. vi., p. .] [footnote : _conn. d. tems_ for , p. .] [footnote : _popular astronomy_, p. .] [footnote : _month. not._, vol. vii., p. ; vol. viii., p. .] [footnote : _ibid._, p. .] [footnote : airy, _mem. r. a. s._, vol. xvi., p. .] [footnote : see newcomb's _pop. astr._, p. . the error of uranus amounted, in , to '; but even the tailor of breslau, whose extraordinary powers of vision humboldt commemorates (_kosmos_, bd. ii., p. ), could only see jupiter's first satellite at its greatest elongation, ' ". he might, however, possibly have distinguished two objects of _equal_ lustre at a lesser interval.] [footnote : j. w. l. glaisher, _observatory_, vol. xv., p. .] [footnote : _mem. r. a. s._, vol. xvi., p. .] [footnote : for an account of d'arrest's share in the detection see _copernicus_, vol. ii., pp. , .] [footnote : _mem. r. a. s._, vol. xvi., p. .] [footnote : he had recorded the places of , stars (three of which were different positions of the planet), and was preparing to map them, when, october , news of the discovery arrived from berlin. prof. challis's _report_, quoted in obituary notice, _month. not._, feb., , p. .] [footnote : see airy in _mem. r. a. s._, vol. xvi., p. .] [footnote : he died january , , in his st year.] [footnote : ledger, _the sun, its planets and their satellites_, p. .] [footnote : presented by the misses lassell, after their father's death, to the greenwich observatory.] [footnote : _astr. jour._, no. .] [footnote : _report of u.s. naval observatory for _, p. .] [footnote : grant, _hist. of astr._, p. .] [footnote : _month. not._, vol. ix., p. .] [footnote : _month. not._, vol. xi., p. .] [footnote : _astr. nach._, no. (may , ).] [footnote : _phil. trans._, vol. i., p. . see h. t. vivian, _engl. mech._, april , .] [footnote : secchi, _month. not._, vol. xiii., p. .] [footnote : hind, _ibid._, vol. xv., p. .] [footnote : lynn, _observatory_, oct. , ; hadley, _phil. trans._, vol. xxxii., p. .] [footnote : proctor, _saturn and its system_, p. .] [footnote : _phil. trans._, vol. lxxvii., p. .] [footnote : _month. not._, vol. xi., p. .] [footnote : _ibid._, vol. xxxv., pp. - .] [footnote : _ibid._, p. .] [footnote : _ibid._, vol. xli., p. .] chapter v _comets_ newton showed that the bodies known as "comets," or _hirsute_ stars, obey the law of gravitation; but it was by no means certain that the individual of the species observed by him in formed a permanent member of the solar system. the velocity, in fact, of its rush round the sun was quite possibly sufficient to carry it off for ever into the depths of space, there to wander, a celestial casual, from star to star. with another comet, however, which appeared two years later, the case was different. edmund halley, who afterwards succeeded flamsteed as astronomer royal, calculated the elements of its orbit on newton's principles, and found them to resemble so closely those similarly arrived at for comets observed by peter apian in , and by kepler in , as almost to compel the inference that all three were apparitions of a single body. this implied its revolution in a period of about seventy-six years, and halley accordingly fixed its return for - . so fully alive was he to the importance of the announcement that he appealed to a "candid posterity," in the event of its verification, to acknowledge that the discovery was due to an englishman. the prediction was one of the test-questions put by science to nature, on the replies to which largely depend both the development of knowledge and the conviction of its reality. in the present instance, the answer afforded may be said to have laid the foundation of this branch of astronomy. halley's comet punctually reappeared on christmas day, , and effected its perihelion passage on the th of march following, thus proving beyond dispute that some at least of these erratic bodies are domesticated within our system, and strictly conform, if not to its unwritten customs (so to speak), at any rate to its fundamental laws. their movements, in short, were demonstrated by the most unanswerable of all arguments--that of verified calculation--to be _calculable_, and their investigation was erected into a legitimate department of astronomical science. this notable advance was the chief _result_ obtained in the field of inquiry just now under consideration during the eighteenth century. but before it closed, its cultivation had received a powerful stimulus through the invention of an improved _method_. the name of olbers has already been brought prominently before our readers in connection with asteroidal discoveries; these, however, were but chance excursions from the path of cometary research which he steadily pursued through life. an early predilection for the heavens was fixed in this particular direction by one of the happy inspirations of genius. as he was watching, one night in the year , by the sick-bed of a fellow-student in medicine at göttingen, an important simplification in the mode of computing the paths of comets occurred to him. although not made public until , "olbers's method" was then universally adopted, and is still regarded as the most expeditious and convenient in cases where absolute rigour is not required. by its introduction, not only many a toilsome and thankless hour was spared, but workers were multiplied, and encouraged in the prosecution of labours more useful than attractive. the career of heinrich olbers is a brilliant example of what may be done by an amateur in astronomy. he at no time did regular work in an observatory; he was never the possessor of a transit or any other fixed instrument; moreover, all the best years of his life were absorbed in the assiduous exercise of a toilsome profession. born in at the village of arbergen, where his father was pastor, he settled in as a physician in the neighbouring town of bremen, and continued in active practice there for over forty years. it was thus only the hours which his robust constitution enabled him to spare from sleep that were available for his intellectual pleasures. yet his recreation was, as von zach remarked,[ ] no less prolific of useful results than the severest work of other men. the upper part of his house in the sandgasse was fitted up with such instruments and appliances as restrictions of space permitted, and there, night after night during half a century and upwards, he discovered, calculated, or observed the cometary visitants of northern skies. almost as effective in promoting the interests of science as the valuable work actually done by him, was the influence of his genial personality. he engaged confidence by his ready and discerning sympathy; he inspired affection by his benevolent disinterestedness; he quickened thought and awakened zeal by the suggestions of a lively and inventive spirit, animated with the warmest enthusiasm for the advancement of knowledge. nearly every astronomer in germany enjoyed the benefits of a frequently active correspondence with him, and his communications to the scientific periodicals of the time were numerous and striking. the motive power of his mind was thus widely felt and continually in action. nor did it wholly cease to be exerted even when the advance of age and the progress of infirmity rendered him incapable of active occupation. he was, in fact, _alive_ even to the last day of his long life of eighty-one years; and his death, which occurred march , , left vacant a position which a rare combination of moral and intellectual qualities had conspired to render unique. amongst the many younger men who were attracted and stimulated by intercourse with him was johann franz encke. but while olbers became a mathematician because he was an astronomer, encke became an astronomer because he was a mathematician. a born geometer, he was naturally sent to göttingen and placed under the tuition of gauss. but geometers are men; and the contagion of patriotic fervour which swept over germany after the battle of leipsic did not spare gauss's promising pupil. he took up arms in the hanseatic legion, and marched and fought until the oppressor of his country was safely ensconced behind the ocean-walls of st. helena. in the course of his campaigning he met lindenau, the militant director of the seeberg observatory, and by his influence was appointed his assistant, and eventually, in , became his successor. thence he was promoted in to berlin, where he superintended the building of the new observatory, so actively promoted by humboldt, and remained at its head until within some eighteen months of his death in august, . on the th of november, , pons of marseilles discovered a comet, whose inconspicuous appearance gave little promise of its becoming one of the most interesting objects in our system. encke at once took the calculation of its elements in hand, and brought out the unexpected result that it revolved round the sun in a period of about - / years.[ ] he, moreover, detected its identity with comets seen by méchain in , by caroline herschel in , by pons, huth, and bouvard in , and after six laborious weeks of research into the disturbances experienced by it from the planets during the entire interval since its first ascertained appearance, he fixed may , , as the date of its next return to perihelion. although on that occasion, owing to the position of the earth, invisible in the northern hemisphere, sir thomas brisbane's observatory at paramatta was fortunately ready equipped for its recapture, which rümker effected quite close to the spot indicated by encke's ephemeris. the importance of this event can be better understood when it is remembered that it was only the second instance of the recognised return of a comet (that of halley's, sixty-three years previously, having, as already stated, been the first); and that it, moreover, established the existence of a new class of celestial objects, somewhat loosely distinguished as "comets of short period." these bodies (of which about thirty have been found to circulate within the orbit of saturn) are remarkable as showing certain planetary affinities in the manners of their motions not at all perceptible in the wider travelling members of their order. they revolve, without exception, in the same direction as the planets--from west to east; they exhibit a marked tendency to conform to the zodiacal track which limits planetary excursions north and south; and their paths round the sun, although much more eccentric than the approximately circular planetary orbits, are far less so than the extravagantly long ellipses in which comets comparatively untrained (as it were) in the habits of the solar system ordinarily perform their revolutions. no _great_ comet is of the "planetary" kind. these are, indeed, only by exception visible to the naked eye; they possess extremely feeble tail-producing powers, and give small signs of central condensation. thin wisps of cosmical cloud, they flit across the telescopic field of view without sensibly obscuring the smallest star. their appearance, in short, suggests--what some notable facts in their history will presently be shown to confirm--that they are bodies already effete, and verging towards dissolution. if it be asked what possible connection can be shown to exist between the shortness of period by which they are essentially characterised, and what we may call their _superannuated_ condition, we are not altogether at a loss for an answer. kepler's remark,[ ] that comets are consumed by their own emissions, has undoubtedly a measure of truth in it. the substance ejected into the tail must, in overwhelmingly large proportion, be for ever lost to the central mass from which it issues. true, it is of a nature inconceivably tenuous; but unrepaired waste, however small in amount, cannot be persisted in with impunity. the incitement to such self-spoliation proceeds from the sun; it accordingly progresses more rapidly the more numerous are the returns to the solar vicinity. comets of short period may thus reasonably be expected to _wear out_ quickly. they are, moreover, subject to many adventures and vicissitudes. their aphelia--or the farthest points of their orbits from the sun--are usually, if not invariably, situated so near to the path either of jupiter or of saturn, as to permit these giant planets to act as secondary rulers of their destinies. by their influence they were, in all likelihood, originally fixed in their present tracks; and by their influence, exerted in an opposite sense, they may, in some cases, be eventually ejected from them. careers so varied, as can easily be imagined, are apt to prove instructive, and astronomers have not been backward in extracting from them the lessons they are fitted to convey. encke's comet, above all, has served as an index to much curious information, and it may be hoped that its function in that respect is by no means at an end. the great extent of the solar system traversed by its eccentric path makes it peculiarly useful for the determination of the planetary masses. at perihelion it penetrates within the orbit of mercury; it considerably transcends at aphelion the farthest excursion of pallas. its vicinity to the former planet in august, , offered the first convenient opportunity of placing that body in the astronomical balance. its weight or mass had previously been assumed, not ascertained; and the comparatively slight deviation from its regular course impressed upon the comet by its attractive power showed that it had been assumed nearly twice too great.[ ] that fundamental datum of planetary astronomy--the mass of jupiter--was corrected by similar means; and it was reassuring to find the correction in satisfactory accord with that already introduced from observations of the asteroidal movements. the fact that comets contract in approaching the sun had been noticed by hevelius; pingré admitted it with hesitating perplexity;[ ] the example of encke's comet rendered it conspicuous and undeniable. on the th of october, , the diameter of the nebulous matter composing this body was estimated at , miles. it was then about one and a half times further from the sun than the earth is at the time of the equinox. on the th of december following, its distance being reduced by nearly two-thirds, it was found to be only , miles across.[ ] that is to say, it had shrunk during those two months of approach to / th part of its original volume! yet it had still seventeen days' journey to make before reaching perihelion. the same curious circumstance was even more markedly apparent at its return in . its bulk, or the actual space occupied by it, appeared to be reduced, as it drew near the hearth of our system, in the enormous proportion of , to . a corresponding expansion accompanied on each occasion its retirement from the sphere of observation. similar changes of volume, though rarely to the same astounding extent, have been perceived in other comets. they still remain unexplained; but it can scarcely be doubted that they are due to the action of the same energetic internal forces which reveal themselves in so many splendid and surprising cometary phenomena. another question of singular interest was raised by encke's acute inquiries into the movements and disturbances of the first known "comet of short period." he found from the first that its revolutions were subject to some influence besides that of gravity. after every possible allowance had been made for the pulls, now backward, now forward, exerted upon it by the several planets, there was still a surplus of acceleration left unaccounted for. each return to perihelion took place about two and a half hours sooner than received theories warranted. here, then, was a "residual phenomenon" of the utmost promise for the disclosure of novel truths. encke (in accordance with the opinion of olbers) explained it as due to the presence in space of some such "subtle matter" as was long ago invoked by euler[ ] to be the agent of eventual destruction for the fair scheme of planetary creation. the apparent anomaly of accounting for an accelerative effect by a retarding cause disappears when it is considered that any check to the motion of bodies revolving round a centre of attraction causes them to draw closer to it, thus shortening their periods and quickening their circulation. if space were filled with a resisting medium capable of impeding, even in the most infinitesimal degree, the swift course of the planets, their orbits should necessarily be, not ellipses, but very close elliptical spirals along which they would slowly, but inevitably, descend into the burning lap of the sun. the circumstance that no such tendency can be traced in their revolutions by no means sets the question at rest. for it might well be that an effect totally imperceptible until after the lapse of countless ages, as regards the solid orbs of our system, might be obvious in the movements of bodies like comets of small mass and great bulk; just as a feather or a gauze veil at once yields its motion to the resistance of the air, while a cannon-ball cuts its way through with comparatively slight loss of velocity. it will thus be seen that issues of the most momentous character hang on the _time-keeping_ of comets; for plainly all must in some degree suffer the same kind of hindrance as encke's, if the cause of that hindrance be the one suggested. none of its congeners, however, show any trace of similar symptoms. true, the late professor oppolzer announced,[ ] in , that a comet, first seen by pons in , and rediscovered by winnecke in , having a period of , days ( · years), was accelerated at each revolution precisely in the manner required by encke's theory. but m. von haerdtl's subsequent investigation, the materials for which included numerous observations of the body in question at its return to the sun in , decisively negatived the presence of any such effect.[ ] moreover, the researches of von asten and backlund[ ] into the movements of encke's comet revealed a perplexing circumstance. they confirmed encke's results for the period covered by them, but exhibited the acceleration as having _suddenly diminished_ by nearly one-half in . the reality and permanence of this change were fully established by observations of the ensuing return in march, . some physical alteration of the retarded body seems indicated; but visual evidence countenances no such assumption. in aspect the comet is no less thin and diffuse than in or in . the character of the supposed resistance in inter-planetary space has, it may be remarked, been often misapprehended. what encke stipulated for was not a medium equally diffused throughout the visible universe, such as the ethereal vehicle of the vibrations of light, but a rare fluid, rapidly increasing in density towards the sun.[ ] this cannot be a solar atmosphere, since it is mathematically certain, as laplace has shown,[ ] that no envelope partaking of the sun's axial rotation can extend farther from his surface than nine-tenths of the mean distance of mercury; while physical evidence assures us that the _actual_ depth of the solar atmosphere bears a very minute proportion to the _possible_ depth theoretically assigned to it. that matter, however, not atmospheric in its nature--that is, neither forming one body with the sun nor altogether aëriform--exists in its neighbourhood, can admit of no reasonable doubt. the great lens-shaped mass of the zodiacal light, stretching out at times far beyond the earth's orbit, may indeed be regarded as an extension of the corona, the streamers of which themselves mark the wide diffusion, all round the solar globe, of granular or gaseous materials. yet comets have been known to penetrate the sphere occupied by them without perceptible loss of velocity. the hypothesis, then, of a resisting medium receives at present no countenance from the movements of comets, whether of short or of long periods. although encke's comet has made thirty-five complete rounds of its orbit since its first detection in , it shows no certain signs of decay. variations in its brightness are, it is true, conspicuous, but they do not proceed continuously.[ ] the history of the next known planet-like comet has proved of even more curious interest than that of the first. it was discovered by an austrian officer named wilhelm von biela at josephstadt in bohemia, february , , and ten days later by the french astronomer gambart at marseilles. both observers computed its orbit, showed its remarkable similarity to that traversed by comets visible in and , and connected them together as previous appearances of the body just detected by assigning to its revolutions a period of between six and seven years. the two brief letters conveying these strikingly similar inferences were printed side by side in the same number of the _astronomische nachrichten_ (no. ); but biela's priority in the discovery of the comet was justly recognised by the bestowal upon it of his name. the object in question was at no time, subsequently to , visible to the naked eye. its aspect in sir john herschel's great reflector on the rd of september, , was described by him as that of a "conspicuous nebula," nearly minutes in diameter. no trace of a tail was discernible. while he was engaged in watching it, a small knot of minute stars was directly traversed by it, "and when on the cluster," he tells us,[ ] it "presented the appearance of a nebula resolvable and partly resolved into stars, the stars of the cluster being visible through the comet." yet the depth of cometary matter through which such faint stellar rays penetrated undimmed, was, near the central parts of the globe, not less than , miles. it is curious to find that this seemingly harmless, and we may perhaps add effete body, gave occasion to the first (and not the last) cometary "scare" of an enlightened century. its orbit, at the descending node, may be said to have intersected that of the earth; since, according as it _bulged in or out_ under the disturbing influence of the planets, the passage of the comet was affected _inside_ or _outside_ the terrestrial track. now, certain calculations published by olbers in [ ] showed that, on october , , a considerable portion of its nebulous surroundings would actually sweep over the spot which, a month later, would be occupied by our planet. it needed no more to set the popular imagination in a ferment. astronomers, after all, could not, by an alarmed public, be held to be infallible. their computations, it was averred, which a trifling oversight would suffice to vitiate, exhibited clearly enough the danger, but afforded no guarantee of safety from a collision, with all the terrific consequences frigidly enumerated by laplace. nor did the panic subside until arago formally demonstrated that the earth and the comet could by no possibility approach within less than fifty millions of miles.[ ] the return of the same body in - was marked by an extraordinary circumstance. when first seen, november , it wore its usual aspect of a faint round patch of cosmical fog; but on december , mr. hind noticed that it had become distorted somewhat into the form of a pear; and ten days later, it had divided into two separate objects. this singular duplication was first perceived at new haven in america, december ,[ ] by messrs. herrick and bradley, and by lieutenant maury at washington, january , . the earliest british observer of the phenomenon (noticed by wichmann the same evening at königsberg) was professor challis. "i see _two_ comets!" he exclaimed, putting his eye to the great equatoreal of the cambridge observatory on the night of january ; then, distrustful of what his senses had told him, he called in his judgment to correct their improbable report by resolving one of the dubious objects into a hazy star.[ ] on the rd, however, both were again seen by him in unmistakable cometary shape, and until far on in march (otto struve caught a final glimpse of the pair on the th of april),[ ] continued to be watched with equal curiosity and amazement by astronomers in every part of the northern hemisphere. what seneca reproved ephorus for supposing to have taken place in b.c.--what pingré blamed kepler for conjecturing in --had then actually occurred under the attentive eyes of science in the middle of the nineteenth century! at a distance from each other of about two-thirds the distance of the moon from the earth, the twin comets meantime moved on tranquilly, so far, at least, as their course through the heaven was concerned. their extreme _lightness_, or the small amount of matter contained in each, could not have received a more signal illustration than by the fact that their revolutions round the sun were performed independently; that is to say, they travelled side by side without experiencing any appreciable mutual disturbance, thus plainly showing that at an interval of only , miles their attractive power was virtually inoperative. signs of internal agitation, however, were not wanting. each fragment threw out a short tail in a direction perpendicular to the line joining their centres, and each developed a bright nucleus, although the original comet had exhibited neither of these signs of cometary vitality. a singular interchange of brilliancy was, besides, observed to take place between the coupled objects, each of which alternately outshone and was outshone by the other, while an arc of light, apparently proceeding from the more lustrous, at times bridged the intervening space. obviously, the gravitational tie, rendered powerless by exiguity of matter, was here replaced by some other form of mutual action, the nature of which can as yet be dealt with only by conjecture. once more, in august, , the double comet returned to the neighbourhood of the sun, but under circumstances not the most advantageous for observation. indeed, the companion was not detected until september , when father secchi at rome perceived it to have increased its distance from the originating body to a million and a quarter of miles, or about eight times the average interval at the former appearance. both vanished shortly afterwards, and have never since been seen, notwithstanding the eager watch kept for objects of such singular interest, and the accurate knowledge of their track supplied by santini's investigations. a dangerously near approach to jupiter in is believed to have occasioned their disruption, and the disaggregating process thus started was likely to continue. we can scarcely doubt that the fate has overtaken them which newton assigned as the end of all cometary existence. _diffundi tandem et spargi per coelos universos._[ ] biela's is not the only vanished comet. brorsen's, discovered at kiel in , and observed at four subsequent returns, failed unaccountably to become visible in .[ ] yet numerous sentinels were on the alert to surprise its approach along a well-ascertained track, traversed in five and a half years. the object presented from the first a somewhat time-worn aspect. it was devoid of tail, or any other kind of appendage; and the rapid loss of the light acquired during perihelion passage was accompanied by inordinate expansion of an already tenuous globular mass. another lost or mislaid comet is one found by de vico at rome, august , . it was expected to return early in , but did not, and has never since been seen; unless its re-appearance as e. swift's comet of should be ratified by closer inquiry.[ ] a telescopic comet with a period of - / years, discovered november , , by m. faye of the paris observatory, formed the subject of a characteristically patient and profound inquiry on the part of leverrier, designed to test its suggested identity with lexell's comet of . the result was decisive against the hypothesis of valz, the divergences between the orbits of the two bodies being found to increase instead of to diminish, as the history of the new-comer was traced backward into the previous century.[ ] faye's comet pursues the most nearly circular path of any similar known object; even at its nearest approach to the sun it remains farther off than mars when he is most distant from it; and it was proved by the admirable researches of professor axel möller,[ ] director of the swedish observatory of lund, to exhibit no trace of the action of a resisting medium. periodical comets are evidently bodies which have each lived through a chapter of accidents, and a significant hint as to the nature of their adventures can be gathered from the fact that their aphelia are pretty closely grouped about the tracks of the major planets. halley's, and five other comets are thus related to neptune; three connect themselves with uranus, two with saturn, above a score with jupiter. some form of dependence is plainly indicated, and the researches of tisserand,[ ] callandreau,[ ] and newton[ ] of yale college, leave scarcely a doubt that the "capture-theory" represents the essential truth in the matter. the original parabolic paths of these comets were then changed into ellipses by the backward pull of a planet, whose sphere of attraction they chanced to enter when approaching the sun from outer space. moreover, since a body thus affected should necessarily return at each revolution to the scene of encounter, the same process of retardation may, in some cases, have been repeated many times, until the more restricted cometary orbits were reduced to their present dimensions. the prevalence, too, among periodical comets, of direct motion, is shown to be inevitable by m. callandreau's demonstration that those travelling in a retrograde direction would, by planetary action, be thrown outside the probable range of terrestrial observation. the scarcity of hyperbolic comets can be similarly explained. they would be created whenever the attractive influence of the disturbing planet was exerted in a forward or accelerative sense, but could come only by a rare exception to our notice. the inner planets, including the earth, have also unquestionably played their parts in modifying cometary orbits; and mr. plummer suggests, with some show of reason, that the capture of encke's comet may be a feat due to mercury.[ ] no _great_ comet appeared between the "star" which presided at the birth of napoleon and the "vintage" comet of . the latter was first described by flaugergues at viviers, march , ; wisniewski, at neu-tscherkask in southern russia, caught a final glimpse of it, august , . two disappearances in the solar rays as the earth moved round in its orbit, and two reappearances after conjunction, were included in this unprecedentedly long period of visibility of days. this relative permanence (so far as the inhabitants of europe were concerned) was due to the high northern latitude attained near perihelion, combined with a certain leisureliness of movement along a path everywhere external to that of the earth. the magnificent luminous train of this body, on october , the day of its nearest terrestrial approach, covered an arc of the heavens - / degrees in length, corresponding to a real extension of one hundred millions of miles. its form was described by sir william herschel as that of "an inverted hollow cone," and its colour as yellowish, strongly contrasted with the bluish-green tint of the "head," round which it was flung like a transparent veil. the planetary disc of the head, , miles across, appeared to be composed of strongly-condensed nebulous matter; but somewhat eccentrically situated within it was a star-like nucleus of a reddish tinge, which herschel presumed to be solid, and ascertained, with his usual care, to have a diameter of miles. from the total absence of phases, as well as from the vivacity of its radiance, he confidently inferred that its light was not borrowed, but inherent.[ ] this remarkable apparition formed the subject of a memoir by olbers,[ ] the striking yet steadily reasoned out suggestions contained in which there was at that time no means of following up with profit. only of late has the "electrical theory," of which zöllner[ ] regarded olbers as the founder, assumed a definite and measurable form, capable of being tested by the touchstone of fact, as knowledge makes its slow inroads on the fundamental mystery of the physical universe. the paraboloidal shape of the bright envelope separated by a dark interval from the head of the great comet of , and constituting, as it were, the _root_ of its tail, seemed to the astronomer of bremen to reveal the presence of a double repulsion; the expelled vapours accumulating where the two forces, solar and cometary, balanced each other, and being then swept backwards in a huge train. he accordingly distinguished three classes of these bodies:--first, comets which develop _no_ matter subject to solar repulsion. these have no tails, and are probably mere nebulosities, without solid nuclei. secondly, comets which are acted upon by solar repulsion _only_, and consequently throw out no emanations _towards_ the sun. of this kind was a bright comet visible in .[ ] thirdly, comets like that of , giving evidence of action of both kinds. these are distinguished by a dark _hoop_ encompassing the head and dividing it from the luminous envelope, as well as by an obscure caudal axis, resulting from the hollow, cone-like structure of the tail. again, the ingenious view subsequently propounded by m. bredikhin as to the connection between the _form_ of these appendages and the _kind_ of matter composing them, was very clearly anticipated by olbers. the amount of tail-curvature, he pointed out, depends in each case upon the proportion borne by the velocity of the ascending particles to that of the comet in its orbit; the swifter the outrush, the straighter the resulting tail. but the velocity of the ascending particles varies with the energy of their repulsion by the sun, and this again, it may be presumed, with their quality. thus multiple tails are developed when the same comet throws off, as it approaches perihelion, specifically distinct substances. the long, straight ray which proceeded from the comet of , for example, was doubtless made up of particles subject to a much more vigorous solar repulsion than those formed into the shorter curved emanation issuing from it nearly in the same direction. in the comet of he calculated that the particles expelled from the head travelled to the remote extremity of the tail in eleven minutes, indicating by this enormous rapidity of movement (comparable to that of the transmission of light) the action of a force much more powerful than the opposing one of gravity. the not uncommon phenomena of multiple envelopes, on the other hand, he explained as due to the varying amounts of repulsion exercised by the nucleus itself on the different kinds of matter developed from it. the movements and perturbations of the comet of were no less profoundly studied by argelander than its physical constitution by olbers. the orbit which he assigned to it is of such vast dimensions as to require no less that , years for the completion of its circuit; and to carry the body describing it at each revolution to fourteen times the distance from the sun of the frigid neptune. thus, when it last visited our neighbourhood, achilles may have gazed on its imposing train as he lay on the sands all night bewailing the loss of patroclus; and when it returns, it will perhaps be to shine upon the ruins of empires and civilizations still deep buried among the secrets of the coming time.[ ] on the th of june, , while the head of a comet passed across the face of the sun, the earth was in all probability involved in its tail. but of this remarkable double event nothing was known until more than a month later, when the fact of its past occurrence emerged from the calculations of olbers.[ ] nor had the comet itself been generally visible previous to the first days of july. several observers, however, on the publication of these results, brought forward accounts of singular spots perceived by them upon the sun at the time of the transit, and an original drawing of one of them, by pastorff of buchholtz, has been preserved. this undoubtedly authentic delineation[ ] represents a round nebulous object with a _bright_ spot in the centre, of decidedly cometary aspect, and not in the least like an ordinary solar "macula." mr. hind,[ ] nevertheless, showed its position on the sun to be irreconcilable with that which the comet must have occupied; and mr. ranyard's discovery of a similar smaller drawing by the same author, dated may , ,[ ] reduces to evanescence the probability of its connection with that body. indeed, recent experience renders very doubtful the possibility of such an observation. the return of halley's comet in was looked forward to as an opportunity for testing the truth of floating cometary theories, and did not altogether disappoint expectation. as early as , its movements and disturbances since were proposed by the turin academy of sciences as the subject of a prize ultimately awarded to baron damoiseau. pontécoulant was adjudged a similar distinction by the paris academy in ; while rosenberger's calculations were rewarded with the gold medal of the royal astronomical society.[ ] they were verified by the detection at rome, august , , of a nearly circular misty object not far from the predicted place of the comet. it was not, however, until the middle of september that it began to throw out a tail, which by the th of october had attained a length of about degrees (on the th, at madras, it extended to fully ),[ ] the head showing to the naked eye as a reddish star rather brighter than aldebaran or antares.[ ] some curious phenomena accompanied the process of tail-formation. an outrush of luminous matter, resembling in shape a partially opened fan, issued from the nucleus _towards_ the sun, and at a certain point, like smoke driven before a high wind, was vehemently swept backwards in a prolonged train. the appearance of the comet at this time was compared by bessel,[ ] who watched it with minute attention, to that of a blazing rocket. he made the singular observation that this fan of light, which seemed the source of supply for the tail, oscillated like a pendulum to and fro across a line joining the sun and nucleus, in a period of - / days; and he was unable to escape from the conclusion[ ] that a repulsive force, about twice as powerful as the attractive force of gravity, was concerned in the production of these remarkable effects. nor did he hesitate to recur to the analogy of magnetic polarity, or to declare, still more emphatically than olbers, "the emission of the tail to be a purely electrical phenomenon."[ ] the transformations undergone by this body were almost as strange and complete as those which affected the brigands in dante's _inferno_. when first seen, it wore the aspect of a nebula; later it put on the distinctive garb of a comet; it next appeared as a star; finally, it dilated, first in a spherical, then in a paraboloidal form, until may , , when it vanished from herschel's observation at feldhausen as if by melting into adjacent space from the excessive diffusion of its light. a very uncommon circumstance in its development was that it lost all trace of tail _previous_ to its arrival at perihelion on the th of november. nor did it begin to recover its elongated shape for more than two months afterwards. on the rd of january, boguslawski perceived it as a star of the sixth magnitude, _without measurable disc_.[ ] only two nights later, maclear, director of the cape observatory, found the head to be seconds across.[ ] and so rapidly did the augmentation of size progress, that sir john herschel estimated the actual bulk of this singular object to have increased forty-fold in the ensuing week. "i can hardly doubt," he remarks, "that the comet was fairly evaporated in perihelio by the heat, and resolved into transparent vapour, and is now in process of rapid condensation and re-precipitation on the nucleus."[ ] a plausible, but no longer admissible, interpretation of this still unexplained phenomenon. the next return of this body, which will be considerably accelerated by jupiter's influence, is expected to take place in .[ ] by means of an instrument devised to test the quality of light, arago obtained decisive evidence that some at least of the radiance proceeding from halley's comet was derived by reflection from the sun.[ ] indications of the same kind had been afforded[ ] by the comet which suddenly appeared above the north-western horizon of paris, july , , after having enveloped (as already stated) our terrestrial abode in its filmy appendages; but the "polariscope" had not then reached the perfection subsequently given to it, and its testimony was accordingly far less reliable than in . such experiments, however, are in reality more beautiful and ingenious than instructive, since ignited as well as obscure bodies possess the power of throwing back light incident upon them, and will consequently transmit to us from the neighbourhood of the sun rays partly direct, partly reflected, of which a certain proportion will exhibit the peculiarity known as polarisation. the most brilliant comets of the century were suddenly rivalled if not surpassed by the extraordinary object which blazed out beside the sun, february , . it was simultaneously perceived in mexico and the united states, in southern europe, and at sea off the cape of good hope, where the passengers on board the _owen glendower_ were amazed by the sight of a "short, dagger-like object," closely following the sun towards the western horizon.[ ] at florence, amici found its distance from the sun's centre at noon to be only ° '; and spectators at parma were able, when sheltered from the direct glare of mid-day, to trace the tail to a length of four or five degrees. the full dimensions of this astonishing appurtenance began to be disclosed a few days later. on the rd of march it measured °, and on the th, at calcutta, mr. clerihew observed a second streamer, nearly twice as long as the first, and making an angle with it of °, to have been emitted in a single day. this rapidity of projection, sir john herschel remarked, "conveys an astounding impression of the intensity of the forces at work." "it is clear," he continued, "that _if we have to deal here with matter, such as we conceive it_--viz., _possessing inertia--at all_, it must be under the dominion of forces incomparably more energetic than gravitation, and quite of a different nature."[ ] on the th of march a silvery ray, some ° long and slightly curved at its extremity, shone out above the sunset clouds in this country. no previous intimation had been received of the possibility of such an apparition, and even astronomers--no lightning messages across the seas being as yet possible--were perplexed. the nature of the phenomenon, indeed, soon became evident, but the wonder of it did not diminish with the study of its attendant circumstances. never before, within astronomical memory, had our system been traversed by a body pursuing such an adventurous career. the closest analogy was offered by the great comet of (newton's), which rushed past the sun at a distance of only , miles; but even this--on the cosmical scale--scarcely perceptible interval was reduced nearly one-half in the case we are now concerned with. the centre of the comet of approached the formidable luminary within , miles, leaving, it is estimated, a clear space of not more than , between the surfaces of the bodies brought into such perilous proximity. the escape of the wanderer was, however, secured by the extraordinary rapidity of its flight. it swept past perihelion at a rate-- miles a second--which, if continued, would have carried it right round the sun in _two hours_; and in only eleven minutes more than that short period it actually described half the _curvature_ of its orbit--an arc of °--although in travelling over the remaining half many hundreds of sluggish years will doubtless be consumed. the behaviour of this comet may be regarded as an _experimentum crucis_ as to the nature of tails. for clearly no fixed appendage many millions of miles in length could be whirled like a brandished sabre from one side of the sun to the other in minutes. cometary trains are then, as olbers rightly conceived them to be, emanations, not appendages--inconceivably rapid outflows of highly rarefied matter, the greater part, if not all, of which becomes permanently detached from the nucleus. that of the comet of reached, about the time that it became visible in this country, the extravagant length of millions of miles.[ ] it was narrow, and bounded by nearly parallel and nearly rectilinear lines, resembling--to borrow a comparison of aristotle's--a "road" through the constellations; and after the rd of march showed no trace of hollowness, the axis being, in fact, rather brighter than the edges. distinctly perceptible in it were those singular aurora-like coruscations which gave to the "tresses" of charles v.'s comet the appearance--as cardan described it--of "a torch agitated by the wind," and have not unfrequently been observed to characterise other similar objects. a consideration first adverted to by olbers proves these to originate in our own atmosphere. for owing to the great difference in the distances from the earth of the origin and extremity of such vast effluxes, the light proceeding from their various parts is transmitted to our eyes in notably different intervals of time. consequently a luminous undulation, even though propagated instantaneously from end to end of a comet's tail, would appear to us to occupy many minutes in its progress. but the coruscations in question pass as swiftly as a falling star. they are, then, of terrestrial production. periods of the utmost variety were by different computators assigned to the body, which arrived at perihelion, february , , at . p.m. professor hubbard of washington found that it required years to complete a revolution; mm. laugier and mauvais of paris considered the true term to be ;[ ] clausen looked for its return at the end of between six and seven. a recent discussion[ ] by professor kreutz of all the available data gives a probable period of years for this body, and precludes its hypothetical identity with the comet of , known as the "spina" of cassini. it may now be asked, what were the conclusions regarding the nature of comets drawn by astronomers from the considerable amount of novel experience accumulated during the first half of this century? the first and best assured was that the matter composing them is in a state of extreme tenuity. numerous and trustworthy observations showed that the feeblest rays of light might traverse some hundreds of thousands of miles of their substance, even where it was apparently most condensed, without being perceptibly weakened. nay, instances were recorded in which stars were said to have gained in brightness from the process![ ] on the th of june, , olbers[ ] saw the comet then visible all but obliterated by the central passage of a star too small to be distinguished with the naked eye, its own light remaining wholly unchanged. a similar effect was noted december , , when the great comet of that year approached so close to altair, the _lucida_ of the eagle, that the star seemed to be transformed into the nucleus of the comet.[ ] even the central blaze of halley's comet in was powerless to impede the passage of stellar rays. struve[ ] observed at dorpat, on september , an all but central occultation; glaisher[ ] one (so far as he could ascertain) absolutely so eight days later at cambridge. in neither case was there any appreciable diminution of the star's light. again, on the th of october, , mr. dawes,[ ] an exceptionally keen observer, distinctly saw a star of the tenth magnitude through the exact centre of a comet discovered on the first of that month by maria mitchell of nantucket. examples, on the other hand, are not wanting of the diminution of stellar light under similar circumstances;[ ] and we meet two alleged instances of the vanishing of a star behind a comet. wartmann of geneva observed the first, november , ;[ ] but his instrument was defective, and the eclipsing body, encke's comet, has shown itself otherwise perfectly translucent. the second case of occultation occurred september , , when an eleventh magnitude star was stated to have completely disappeared during the transit over it of denning's comet.[ ] from the failure to detect any effects of refraction in the light of stars occulted by comets, it was inferred (though, as we know now, erroneously) that their composition is rather that of dust than that of vapour; that they consist not of any continuous substance, but of discrete solid particles, very finely divided and widely scattered. in conformity with this view was the known smallness of their masses. laplace had shown that if the amount of matter forming lexell's comet had been as much as / of that contained in our globe, the effect of its attraction, on the occasion of its approach within , , miles of the earth, july , , must have been apparent in the lengthening of the year. and that some comets, at any rate, possess masses immeasurably below this maximum value was clearly proved by the undisturbed parallel march of the two fragments of biela's in . but the discovery in this branch most distinctive of the period under review is that of "short period" comets, of which four[ ] were known in . these, by the character of their movements, serve as a link between the planetary and cometary worlds, and by the nature of their construction, seem to mark a stage in cometary decay. for that comets are rather transitory agglomerations, than permanent products of cosmical manufacture, appeared to be demonstrated by the division and disappearance of one amongst their number, as well as by the singular and rapid changes in appearance undergone by many, and the seemingly irrevocable diffusion of their substance visible in nearly all. they might then be defined, according to the ideas respecting them prevalent fifty years ago, as bodies unconnected by origin with the solar system, but encountered, and to some extent appropriated, by it in its progress through space, owing their visibility in great part, if not altogether, to light reflected from the sun, and their singular and striking forms to the action of repulsive forces emanating from him, the penalty of their evanescent splendour being paid in gradual waste and final dissipation and extinction. footnotes: [footnote : _allgemeine geographische ephemeriden_, vol. iv., p. .] [footnote : _astr. jahrbuch_, , p. . the period ( , days) of this body is considerably shorter than that of any other known comet.] [footnote : "sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et denique mori."--_de cometis_, op., vol. vii., p. .] [footnote : considerable uncertainty, however, still prevails on the point. the inverse relation assumed by lagrange to exist between distance from the sun and density brought out the mercurian mass / that of the sun (laplace, _exposition du syst. du monde_, t. ii., p. , ed. ). von asten deduced from the movements of encke's comet, - , a value of / ; while backlund from its seven returns, - , derived / (_comptes rendus_, oct. , ).] [footnote : arago, _annuaire_ ( ), p. .] [footnote : hind, _the comets_, p. .] [footnote : _phil. trans._, vol. xlvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. cvii., p. .] [footnote : _mém. de st. pétersbourg_, t. xxxii., no. , ; _astr. nach._, no. , .] [footnote : _month. not._, vol. xix., p. .] [footnote : _mécanique céleste_, t. ii., p. .] [footnote : see berberich, _astr. nach._, nos. , - , , ; deichmüller, _ibid._, no. , .] [footnote : _month. not._, vol. ii., p. .] [footnote : _astr. nach._, no. .] [footnote : _annuaire_, , p. .] [footnote : _am. journ. of science_, vol. i. ( nd series), p. . prof. hubbard's calculations indicated a probability that the definitive separation of the two nuclei occurred as early as september , . _astronomical journal_ (gould's), vol. iv., p. . see also, on the subject of this comet, w. t. lynn, _intellectual observer_, vol. xi., p. ; e. ledger, _observatory_, august, , p. ; and h. a. newton, _am. journ. of science_, vol. xxxi., p. , february, .] [footnote : _month. not._, vol. vii., p. .] [footnote : _bulletin ac. imp. de st. pétersbourg_, t. vi., col. . the latest observation of the parent nucleus was that of argelander, april , at bonn.] [footnote : d'arrest, _astr. nach._, no. , .] [footnote : _der brorsen'sche comet._ von dr. e. lamp, kiel, ; plummer, _knowledge_, vol. xix., p. .] [footnote : schulhof, _astr. nach._, no. , ; _observatory_, vol. xviii., p. ; f. h. seares, _astr. nach._, nos. , - ; plummer, _knowledge_, vol. xix., p. .] [footnote : _comptes rendus_, t. xxv., p. .] [footnote : _month. not._, vol. xii., p. .] [footnote : _bull. astr._, t. vi., pp. , .] [footnote : _Étude sur la théorie des comètes périodiques. annales de l'observatoíre_, t. xx., paris, .] [footnote : _amer. journ. of science_, vol. xlii., pp. , , .] [footnote : _observatory_, vol. xiv., p. .] [footnote : _phil. trans._, vol. cii., pp. - .] [footnote : _ueber den schweif des grossen cometen von , monat. corr._, vol. xxv., pp. - . reprinted by zöllner. _ueber die natur der cometen_, pp. - .] [footnote : _natur der cometen_, p. .] [footnote : the subject of a classical memoir by bessel, published in .] [footnote : a fresh investigation of its orbit has been published by n. herz of vienna. see _bull. astr._, t. ix., p. .] [footnote : _astr. jahrbuch_ (bode's), , p. .] [footnote : reproduced in webb's _celestial objects_, th ed.] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _celestial objects_, p. , note.] [footnote : see airy's address, _mem. r. a. s._, vol. x., p. . rosenberger calculated no more, though he lived until . w. t. lynn, _observatory_, vol. xiii., p. .] [footnote : hind, _the comets_, p. .] [footnote : arago, _annuaire_, , p. .] [footnote : _astr. nach._, no. .] [footnote : it deserves to be recorded that robert hooke drew a very similar inference from his observations of the comets of and . _month. not._, vol. xiv., pp. - .] [footnote : _briefwechsel zwischen olbers und bessel_, bd. ii., p. .] [footnote : herschel, _results_, p. .] [footnote : _mem. r. a. s._, vol. x., p. ,] [footnote : _results_, p. .] [footnote : pontécoulant, _comptes rendus_, t. lviii., p. .] [footnote : _annuaire_, , p. .] [footnote : _cosmos_, vol. i., p. , _note_ (otté's trans.).] [footnote : herschel, _outlines of astronomy_, p. , th ed.] [footnote : _outlines_, p. .] [footnote : boguslawski calculated that it extended on the st of march to millions.--_report. brit. ass._, , p. .] [footnote : _comptes rendus_, t. xvi., p. .] [footnote : _observatory_, vol. xxiv., p. ; astr. nach., no. , .] [footnote : piazzi noticed a considerable increase of lustre in a very faint star of the twelfth magnitude viewed through a comet. mädler, _reden_, etc., p. , _note_.] [footnote : _astr. jahrbuch_, , p. .] [footnote : mädler, _gesch. d. astr._, bd. ii., p. .] [footnote : _recueil de l'ac. imp. de st. pétersbourg_, , p. .] [footnote : guillemin's _world of comets_, trans, by j. glaisher, p. , _note_.] [footnote : _month. not._, vol. viii., p. .] [footnote : a real, though only partial stoppage of light seems indicated by herschel's observations on the comet of . stars seen through the tail, october , lost much of their lustre. one near the head was only faintly visible by glimpses. _phil. trans._, vol. xcvii., p. .] [footnote : arago, _annuaire_, , p. .] [footnote : _ibid._, , p. .] [footnote : viz., encke's, biela's, faye's, and brorsen's.] chapter vi _instrumental advances_ it is impossible to follow with intelligent interest the course of astronomical discovery without feeling some curiosity as to the means by which such surpassing results have been secured. indeed, the bare acquaintance with _what_ has been achieved, without any corresponding knowledge of _how_ it has been achieved, supplies food for barren wonder rather than for fruitful and profitable thought. ideas advance most readily along the solid ground of practical reality, and often find true sublimity while laying aside empty marvels. progress is the result, not so much of sudden flights of genius, as of sustained, patient, often commonplace endeavour; and the true lesson of scientific history lies in the close connection which it discloses between the most brilliant developments of knowledge and the faithful accomplishment of his daily task by each individual thinker and worker. it would be easy to fill a volume with the detailed account of the long succession of optical and mechanical improvements by means of which the observation of the heavens has been brought to its present degree of perfection; but we must here content ourselves with a summary sketch of the chief amongst them. the first place in our consideration is naturally claimed by the telescope. this marvellous instrument, we need hardly remind our readers, is of two distinct kinds--that in which light is gathered together into a focus by _refraction_, and that in which the same end is attained by _reflection_. the image formed is in each case viewed through a magnifying lens, or combination of lenses, called the eye-piece. not for above a century after the "optic glasses" invented or stumbled upon by the spectacle-maker of middelburg ( ) had become diffused over europe, did the reflecting telescope come, even in england, the place of its birth, into general use. its principle (a sufficiently obvious one) had indeed been suggested by mersenne as early as ;[ ] james gregory in [ ] described in detail a mode of embodying that principle in a practical shape; and newton, adopting an original system of construction, actually produced in a tiny speculum, one inch across, by means of which the apparent distance of objects was reduced thirty-nine times. nevertheless, the exorbitantly long tubeless refractors, introduced by huygens, maintained their reputation until hadley exhibited to the royal society, january , ,[ ] a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpassed in manageability, one of the "aerial" kind of feet. the concave-mirror system now gained a decided ascendant, and was brought to unexampled perfection by james short of edinburgh during the years - . its resources were, however, first fully developed by william herschel. the energy and inventiveness of this extraordinary man marked an epoch wherever they were applied. his ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "space-penetrating power" by increasing their light-gathering surface. these, as he was the first to explain,[ ] are in a constant proportion one to the other. for a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. in other words, it will possess double the space-penetrating power of the smaller instrument. herschel's great mirrors--the first examples of the giant telescopes of modern times--were then primarily engines for extending the bounds of the visible universe; and from the sublimity of this "final cause" was derived the vivid enthusiasm which animated his efforts to success. it seems probable that the seven-foot telescope constructed by him in --that is within little more than a year after his experiments in shaping and polishing metal had begun--already exceeded in effective power any work by an earlier optician; and both his skill and his ambition rapidly developed. his efforts culminated, after mirrors of ten, twenty, and thirty feet focal length had successively left his hands, in the gigantic forty-foot, completed august , . it was the first reflector in which only a single mirror was employed. in the "gregorian" form, the focussed rays are, by a second reflection from a small concave[ ] mirror, thrown _straight back_ through a central aperture in the larger one, behind which the eye-piece is fixed. the object under examination is thus seen in the natural direction. the "newtonian," on the other hand, shows the object in a line of sight at right angles to the true one, the light collected by the speculum being diverted to one side of the tube by the interposition of a small plane mirror, situated at an angle of ° to the axis of the instrument. upon these two systems herschel worked until , when, becoming convinced of the supreme importance of economising light (necessarily wasted by the second reflection), he laid aside the small mirror of his forty-foot then in course of construction, and turned it into a "front-view" reflector. this was done--according to the plan proposed by lemaire in --by slightly inclining the speculum so as to enable the image formed by it to be viewed with an eye-glass fixed at the upper margin of the tube. the observer thus stood with his back turned to the object he was engaged in scrutinising. the advantages of the increased brilliancy afforded by this modification were strikingly illustrated by the discovery, august and september , , of the two saturnian satellites nearest the ring. nevertheless, the monster telescope of slough cannot be said to have realised the sanguine expectations of its constructor. the occasions on which it could be usefully employed were found to be extremely rare. it was injuriously affected by every change of temperature. the great weight ( cwt.) of a speculum four feet in diameter rendered it peculiarly liable to distortion. with all imaginable care, the delicate lustre of its surface could not be preserved longer than two years,[ ] when the difficult process of repolishing had to be undertaken. it was accordingly never used after , when, having _gone blind_ from damp, it lapsed by degrees into the condition of a museum inmate. the exceedingly high magnifying powers employed by herschel constituted a novelty in optical astronomy, to which he attached great importance. the work of ordinary observation would, however, be hindered rather than helped by them. the attempt to increase in this manner the efficacy of the telescope is speedily checked by atmospheric, to say nothing of other difficulties. precisely in the same proportion as an object is magnified, the disturbances of the medium through which it is seen are magnified also. even on the clearest and most tranquil nights, the air is never for a moment really still. the rays of light traversing it are continually broken by minute fluctuations of refractive power caused by changes of temperature and pressure, and the currents which these engender. with such luminous quiverings and waverings the astronomer has always more or less to reckon; their absence is simply a question of degree; if sufficiently magnified, they are at all times capable of rendering observation impossible. thus, such powers as , , , , , , even , ,[ ] which herschel now and again applied to his great telescopes, must, save on the rarest occasions, prove an impediment rather than an aid to vision. they were, however, used by him only for special purposes, experimentally, not systematically, and with the clearest discrimination of their advantages and drawbacks. it is obvious that perfectly different ends are subserved by increasing the _aperture_ and by increasing the _power_ of a telescope. in the one case, a larger quantity of light is captured and concentrated; in the other, the same amount is distributed over a wider area. a diminution of brilliancy in the image accordingly attends, _coeteris paribus_, upon each augmentation of its apparent size. for this reason, such faint objects as nebulæ are most successfully observed with moderate powers applied to instruments of a great capacity for light, the details of their structure actually disappearing when highly magnified. with stellar groups the reverse is the case. stars cannot be magnified, simply because they are too remote to have any sensible dimensions; but the space between them can. it was thus for the purpose of dividing very close double stars that herschel increased to such an unprecedented extent the magnifying capabilities of his instruments; and to this improvement incidentally the discovery of uranus, march , ,[ ] was due. for by the examination with strong lenses of an object which, even with a power of , presented a suspicious appearance, he was able at once to pronounce its disc to be real, not merely "spurious," and so to distinguish it unerringly from the crowd of stars amidst which it was moving. while the reflecting telescope was astonishing the world by its rapid development in the hands of herschel, its unpretending rival was slowly making its way towards the position which the future had in store for it. the great obstacle which long stood in the way of the improvement of refractors was the defect known as "chromatic aberration." this is due to no other cause than that which produces the rainbow and the spectrum--the separation, or "dispersion" in their passage through a refracting medium, of the variously coloured rays composing a beam of white light. in an ordinary lens there is no common point of concentration; each colour has its own separate focus; and the resulting image, formed by the superposition of as many images as there are hues in the spectrum, is indefinitely terminated with a tinted border, eminently baffling to exactness of observation. the extravagantly long telescopes of the seventeenth century were designed to _avoid_ this evil (as well as another source of indistinct vision in the spherical shape of lenses); but no attempt to _remedy_ it was made until an essex gentleman succeeded, in , in so combining lenses of flint and crown glass as to produce refraction without colour.[ ] mr. chester more hall was, however, equally indifferent to fame and profit, and took no pains to make his invention public. the _effective_ discovery of the achromatic telescope was, accordingly, reserved for john dollond, whose method of correcting at the same time chromatic and spherical aberration was laid before the royal society in . modern astronomy may be said to have been thereby rendered possible. refractors have always been found better suited than reflectors to the ordinary work of observatories. they are, so to speak, of a more robust, as well as of a more plastic nature. they suffer less from vicissitudes of temperature and climate. they retain their efficiency with fewer precautions and under more trying circumstances. above all, they co-operate more readily with mechanical appliances, and lend themselves with far greater facility to purposes of exact measurement. a practical difficulty, however, impeded the realisation of the brilliant prospects held out by dollond's invention. it was found impossible to procure flint-glass, such as was needed for optical use--that is, of perfectly homogeneous quality--except in fragments of insignificant size. discs of more than two or three inches in diameter were of extreme rarity; and the crushing excise duty imposed upon the article by the financial unwisdom of the government, both limited its production, and, by rendering experiments too costly for repetition, barred its improvement. up to this time, great britain had left foreign competitors far behind in the instrumental department of astronomy. the quadrants and circles of bird, cary and ramsden were unapproached abroad. the reflecting telescope came into existence and reached maturity on british soil. the refracting telescope was cured of its inherent vices by british ingenuity. but with the opening of the nineteenth century, the almost unbroken monopoly of skill and contrivance which our countrymen had succeeded in establishing was invaded, and british workmen had to be content to exchange a position of supremacy for one of at least partial temporary inferiority. somewhat about the time that herschel set about polishing his first speculum, pierre louis guinand, a swiss artisan, living near chaux-de-fonds, in the canton of neuchâtel, began to grind spectacles for his own use, and was thence led on to the rude construction of telescopes by fixing lenses in pasteboard tubes. the sight of an england achromatic stirred a higher ambition, and he took the first opportunity of procuring some flint glass from england (then the only source of supply), with the design of imitating an instrument the full capabilities of which he was destined to be the humble means of developing. the english glass proving of inferior quality, he conceived the possibility, unaided and ignorant of the art as he was, of himself making better, and spent seven years ( - ) in fruitless experiments directed to that end. failure only stimulated him to enlarge their scale. he bought some land near les brenets, constructed upon it a furnace capable of melting two quintals of glass, and reducing himself and his family to the barest necessaries of life, he poured his earnings (he at this time made bells for repeaters) unstintingly into his crucibles.[ ] his undaunted resolution triumphed. in he carried to paris and there showed to lalande several discs of flawless crystal four to six inches in diameter. lalande advised him to keep his secret, but in he was induced to remove to munich, where he became the instructor of the immortal fraunhofer. his return to les brenets in was signalised by the discovery of an ingenious mode of removing striated portions of glass by breaking and re-soldering the product of each melting, and he eventually attained to the manufacture of perfect discs up to inches in diameter. an object-glass for which he had furnished the material to cauchoix, procured him, in , a royal invitation to settle in paris; but he was no longer equal to the change, and died at the scene of his labours, february following. this same lens ( inches across) was afterwards purchased by sir james south, and the first observation made with it, february , , disclosed to sir john herschel the sixth minute star in the central group of the orion nebula, known as the "trapezium."[ ] bequeathed by south to trinity college, dublin, it was employed at the dunsink observatory by brünnow and ball in their investigations of stellar parallax. a still larger objective (of nearly inches) made of guinand's glass was secured in paris, about the same time, by mr. edward cooper of markree castle, ireland. the peculiarity of the method discovered at les brenets resided in the manipulation, not in the quality of the ingredients; the secret, that is to say, was not chemical, but mechanical.[ ] it was communicated by henry guinand (a son of the inventor) to bontemps, one of the directors of the glassworks at choisy-le-roi, and by him transmitted to messrs. chance of birmingham, with whom he entered into partnership when the revolutionary troubles of obliged him to quit his native country. the celebrated american opticians, alvan clark & sons, derived from the birmingham firm the materials for some of their early telescopes, notably the -inch chicago and -inch washington equatoreals; but the discs for the great lick refractor, and others shaped by them in recent years, have been supplied by feil of paris. two distinguished amateurs, meanwhile, were preparing to reassert on behalf of reflecting instruments their claim to the place of honour in the van of astronomical discovery. of mr. lassell's specula something has already been said.[ ] they were composed of an alloy of copper and tin, with a minute proportion of arsenic (after the example of newton[ ]), and were remarkable for perfection of figure and brilliancy of surface. the capabilities of the newtonian plan were developed still more fully--it might almost be said to the uttermost--by the enterprise of an irish nobleman. william parsons, known as lord oxmantown until , when, on his father's death, he succeeded to the title of earl of rosse, was born at york, june , . his public duties began before his education was completed. he was returned to parliament as member for king's county while still an undergraduate at oxford, and continued to represent the same constituency for thirteen years ( - ). from until his death, which took place, october , , he sat, silent but assiduous, in the house of lords as an irish representative peer; he held the not unlaborious post of president of the royal society from to ; presided over the meeting of the british association at cork in , and was elected vice-chancellor of dublin university in . in addition to these extensive demands upon his time and thoughts, were those derived from his position as practically the feudal chief of a large body of tenantry in times of great and anxious responsibility, to say nothing of the more genial claims of an unstinted hospitality. yet, while neglecting no public or private duty, this model nobleman found leisure to render to science services so conspicuous as to entitle his name to a lasting place in its annals. he early formed the design of reaching the limits of the attainable in enlarging the powers of the telescope, and the qualities of his mind conspired with the circumstances of his fortune to render the design a feasible one. from refractors it was obvious that no such vast and rapid advance could be expected. english glass-manufacture was still in a backward state. so late as , simms (successor to the distinguished instrumentalist edward troughton) reported a specimen of crystal scarcely - / inches in diameter, and perfect only over six, to be unique in the history of english glass-making.[ ] yet at that time the fifteen-inch achromatic of pulkowa had already left the workshop of fraunhofer's successors at munich. it was not indeed until , when the impost which had so long hampered their efforts was removed, that the optical artists of these islands were able to compete on equal terms with their rivals on the continent. in the case of reflectors, however, there seemed no insurmountable obstacle to an almost unlimited increase of light-gathering capacity; and it was here, after some unproductive experiments with fluid lenses, that lord oxmantown concentrated his energies. he had to rely entirely on his own invention, and to earn his own experience. james short had solved the problem of giving to metallic surfaces a perfect parabolic figure (the only one by which parallel incident rays can be brought to an exact focus); but so jealous was he of his secret, that he caused all his tools to be burnt before his death;[ ] nor was anything known of the processes by which herschel had achieved his astonishing results. moreover, lord oxmantown had no skilled workmen to assist him. his implements, both animate and inanimate, had to be formed by himself. peasants taken from the plough were educated by him into efficient mechanics and engineers. the delicate and complex machinery needed in operations of such hairbreadth nicety as his enterprise involved, the steam-engine which was to set it in motion, at times the very crucibles in which his specula were cast, issued from his own workshops. in experiments on the composition of speculum-metal were set on foot, and the first polishing-machine ever driven by steam-power was contrived in . but twelve arduous years of struggle with recurring difficulties passed before success began to dawn. a material less tractable than the alloy selected, of four chemical equivalents of copper to one of tin,[ ] can scarcely be conceived. it is harder than steel, yet brittle as glass, crumbling into fragments with the slightest inadvertence of handling or treatment;[ ] and the precision of figure requisite to secure good definition is almost beyond the power of language to convey. the quantities involved are so small as not alone to elude sight, but to confound imagination. sir john herschel tells us that "the _total_ thickness to be abraded from the edge of a spherical speculum inches in diameter and feet focus, to convert it into a paraboloid, is only / of an inch;"[ ] yet upon this minute difference of form depends the clearness of the image, and, as a consequence, the entire efficiency of the instrument. "almost infinite," indeed (in the phrase of the late dr. robinson), must be the exactitude of the operation adapted to bring about so delicate a result. at length, in , two specula, each three feet in diameter, were turned out in such perfection as to prompt a still bolder experiment. the various processes needed to insure success were now ascertained and under control; all that was necessary was to repeat them on a larger scale. a gigantic mirror, six feet across and fifty-four in focal length, was accordingly cast on the th of april, ; in two months it was ground down to figure by abrasion with emery and water, and daintily polished with rouge; and by the month of february, , the "leviathan of parsonstown" was available for the examination of the heavens. the suitable mounting of this vast machine was a problem scarcely less difficult than its construction. the shape of a speculum needs to be maintained with an elaborate care equal to that used in imparting it. in fact, one of the most formidable obstacles to increasing the size of such reflecting surfaces consists in their liability to bend under their own weight. that of the great rosse speculum was no less than four tons. yet, although six inches in thickness, and composed of a material only a degree inferior in rigidity to wrought iron, the strong pressure of a man's hand at its back produced sufficient flexure to distort perceptibly the image of a star reflected in it.[ ] thus the delicacy of its form was perishable equally by the stress of its own gravity, and by the slightest irregularity in the means taken to counteract that stress. the problem of affording a perfectly equable support in all possible positions was solved by resting the speculum upon twenty-seven platforms of cast iron, felt-covered, and carefully fitted to the shape of the areas they were to carry, which platforms were themselves borne by a complex system of triangles and levers, ingeniously adapted to distribute the weight with complete uniformity.[ ] a tube which resembled, when erect, one of the ancient round towers of ireland,[ ] served as the habitation of the great mirror. it was constructed of deal staves bound together with iron hoops, was fifty-eight feet long (including the speculum-box), and seven in diameter. a reasonably tall man may walk through it (as dean peacock once did) with umbrella uplifted. two piers of solid masonry, about fifty feet high, seventy long, and twenty-three apart, flanked the huge engine on either side. its lower extremity rested on a universal joint of cast iron; above, it was slung in chains, and even in a gale of wind remained perfectly steady. the weight of the entire, although amounting to fifteen tons, was so skilfully counterpoised, that the tube could with ease be raised or depressed by two men working a windlass. its horizontal range was limited by the lofty walls erected for its support to about ten degrees on each side of the meridian; but it moved vertically from near the horizon through the zenith as far as the pole. its construction was of the newtonian kind, the observer looking into the side of the tube near its upper end, which a series of galleries and sliding stages enabled him to reach in any position. it has also, though rarely, been used without a second mirror, as a "herschelian" reflector. the splendour of the celestial objects as viewed with this vast "light-grasper" surpassed all expectation. "never in my life," exclaimed sir james south, "did i see such glorious sidereal pictures."[ ] the orb of jupiter produced an effect compared to that of the introduction of a coach-lamp into the telescope;[ ] and certain star-clusters exhibited an appearance (we again quote sir james south) "such as man before had never seen, and which for its magnificence baffles all description." but it was in the examination of the nebulæ that the superiority of the new instrument was most strikingly displayed. a large number of these misty objects, which the utmost powers of herschel's specula had failed to resolve into stars, yielded at once to the parsonstown reflector; while many others showed under entirely changed forms through the disclosure of previously unseen details of structure. one extremely curious result of the increase of light was the abolition of any sharp distinction between the two classes of "annular" and "planetary" nebulæ. up to that time, only four ring-shaped systems--two in the northern and two in the southern hemisphere--were known to astronomers; they were now reinforced by five of the planetary kind, the discs of which were observed to be centrally perforated; while the definite margins visible in weaker instruments were replaced by ragged edges or filamentous fringes. still more striking was the discovery of an entirely new and most remarkable species of nebulæ. these were termed "spiral," from the more or less regular convolutions, resembling the whorls of a shell, in which the matter composing them appeared to be distributed. the first and most conspicuous specimen of this class was met with in april, ; it is situated in canes venatici, close to the tail of the great bear, and wore, in sir j. herschel's instruments, the aspect of a split ring encompassing a bright nucleus, thus presenting, as he supposed, a complete analogue to the system of the milky way. in the rosse mirror it shone out as a vast whirlpool of light--a stupendous witness to the presence of cosmical activities on the grandest scale, yet regulated by laws as to the nature of which we are profoundly ignorant. professor stephen alexander of new jersey, however, concluded, from an investigation (necessarily founded on highly precarious data) of the mechanical condition of these extraordinary agglomerations, that we see in them "the partially scattered fragments of enormous masses once rotating in a state of dynamical equilibrium." he further suggested "that the separation of these fragments may still be in progress,"[ ] and traced back their origin to the disruption, through its own continually accelerated rotation, of a "primitive spheroid" of inconceivably vast dimensions. such also, it was added (the curvilinear form of certain outliers of the milky way giving evidence of a spiral structure), is probably the history of our own cluster; the stars composing which, no longer held together in a delicately adjusted system like that of the sun and planets, are advancing through a period of seeming confusion towards an appointed goal of higher order and more perfect and harmonious adaptation.[ ] the class of spiral nebulæ included, in , fourteen members, besides several in which the characteristic arrangement seemed partial or dubious.[ ] a tendency in the exterior stars of other clusters to gather into curved branches (as in our galaxy) was likewise noted; and the existence of unsuspected analogies was proclaimed by the significant combination in the "owl" nebula (a large planetary in ursa major)[ ] of the twisted forms of a spiral with the perforated effect distinctive of an annular nebula. once more, by the achievements of the parsonstown reflector, the supposition of a "shining fluid" filling vast regions of space was brought into (as it has since proved) undeserved discredit. although lord rosse himself rejected the inference, that because many nebulæ had been resolved, all were resolvable, very few imitated his truly scientific caution; and the results of bond's investigations[ ] with the harvard college refractor quickened and strengthened the current of prevalent opinion. it is now certain that the evidence furnished on both sides of the atlantic as to the stellar composition of some conspicuous objects of this class (notably the orion and "dumb-bell" nebulæ) was delusive; but the spectroscope alone was capable of meeting it with a categorical denial. meanwhile there seemed good ground for the persuasion, which now, for the last time, gained the upper hand, that nebulæ are, without exception, true "island-universes," or assemblages of distant suns. lord rosse's telescope possesses a nominal power of , --that is, it shows the moon as if viewed with the naked eye at a distance of forty miles. but this seeming advantage is neutralised by the weakening of the available light through excessive diffusion, as well as by the troubles of the surging sea of air through which the observation must necessarily be made. professor newcomb, in fact, doubts whether with _any_ telescope our satellite has ever been seen to such advantage as it would be if brought within miles of the unarmed eye.[ ] the french opticians' rule of doubling the number of millimetres contained in the aperture of an instrument to find the highest magnifying power usually applicable to it, would give , as the maximum for the leviathan of birr castle; but in a climate like that of ireland the occasions must be rare when even that limit can be reached. indeed, the experience acquired by its use plainly shows that atmospheric rather than mechanical difficulties impede a still further increase of telescopic power. its construction may accordingly be said to mark the _ne plus ultra_ of effort in one direction, and the beginning of its conversion towards another. it became thenceforward more and more obvious that the conditions of observation must be ameliorated before any added efficacy could be given to it. the full effect of an uncertain climate in nullifying optical improvements was recognised, and the attention of astronomers began to be turned towards the advantages offered by more tranquil and more translucent skies. scarcely less important for the practical uses of astronomy than the optical qualities of the telescope is the manner of its mounting. the most admirable performance of the optician can render but unsatisfactory service if its mechanical accessories are ill-arranged or inconvenient. thus the astronomer is ultimately dependent upon the mechanician; and so excellently have his needs been served, that the history of the ingenious contrivances by which discoveries have been prepared would supply a subject (here barely glanced at) not far inferior in extent and instruction to the history of those discoveries themselves. there are two chief modes of using the telescope, to which all others may be considered subordinate.[ ] either it may be invariably directed towards the south, with no motion save in the plane of the meridian, so as to intercept the heavenly bodies at the moment of transit across that plain; or it may be arranged so as to follow the daily revolution of the sky, thus keeping the object viewed permanently in sight instead of simply noting the instant of its flitting across the telescopic field. the first plan is that of the "transit instrument," the second that of the "equatoreal." both were, by a remarkable coincidence, introduced about [ ] by olaus römer, the brilliant danish astronomer who first measured the velocity of light. the uses of each are entirely different. with the transit, the really fundamental task of astronomy--the determination of the movements of the heavenly bodies--is mainly accomplished; while the investigation of their nature and peculiarities is best conducted with the equatoreal. one is the instrument of mathematical, the other of descriptive astronomy. one furnishes the materials with which theories are constructed and the tests by which they are corrected; the other registers new facts, takes note of new appearances, sounds the depths and peers into every nook of the heavens. the great improvement of giving to a telescope equatoreally mounted an automatic movement by connecting it with clockwork, was proposed in by robert hooke. bradley in actually observed mars with a telescope "moved by a machine that made it keep pace with the stars;"[ ] and von zach relates[ ] that he had once followed sirius for twelve hours with a "heliostat" of ramsden's construction. but these eighteenth-century attempts were of no practical effect. movement by clockwork was virtually a complete novelty when it was adopted by fraunhofer in to the dorpat refractor. by simply giving to an axis unvaryingly directed towards the celestial pole an equable rotation with a period of twenty-four hours, a telescope attached to it, and pointed in _any_ direction, will trace out on the sky a parallel of declination, thus necessarily accompanying the movement of any star upon which it may be fixed. it accordingly forms part of the large sum of fraunhofer's merits to have secured this inestimable advantage to observers. sir john herschel considered that lassell's application of equatoreal mounting to a nine-inch newtonian in made an epoch in the history of "that eminently british instrument, the reflecting telescope."[ ] nearly a century earlier,[ ] it is true, short had fitted one of his gregorians to a complicated system of circles in such a manner that, by moving a handle, it could be made to follow the revolution of the sky; but the arrangement did not obtain, nor did it deserve, general adoption. lassell's plan was a totally different one; he employed the crossed axes of the true equatoreal, and his success removed, to a great extent, the fatal objection of inconvenience in use, until then unanswerably urged against reflectors. the very largest of these can now be mounted equatoreally; even the rosse, within its limited range, has been for some years provided with a movement by clockwork along declination-parallels. the art of accurately dividing circular arcs into the minute equal parts which serve as the units of astronomical measurement, remained, during the whole of the eighteenth century, almost exclusively in english hands. it was brought to a high degree of perfection by graham, bird and ramsden, all of whom, however, gave the preference to the old-fashioned mural quadrant and zenith-sector over the entire circle, which römer had already found the advantage of employing. the five-foot vertical circle, which piazzi with some difficulty induced ramsden to complete for him in , was the first divided instrument constructed in what may be called the modern style. it was provided with magnifiers for reading off the divisions (one of the neglected improvements of römer), and was set up above a smaller horizontal circle, forming an "altitude and azimuth" combination (again römer's invention), by which both the elevation of a celestial object above the horizon and its position as referred to the horizon could be measured. in the same year, borda invented the "repeating circle" (the principle of which had been suggested by tobias mayer in [ ]), a device for exterminating, so far as possible, errors of graduation by _repeating_ an observation with different parts of the limb. this was perhaps the earliest systematic effort to correct the imperfections of instruments by the manner of their use. the manufacture of astronomical circles was brought to a very refined state of excellence early in the nineteenth century by reichenbach at munich, and after by repsold at hamburg. bessel states[ ] that the "reading-off" on an instrument of the kind by the latter artist was accurate to about / th of a human hair. meanwhile the traditional reputation of the english school was fully sustained; and sir george airy did not hesitate to express his opinion that the new method of graduating circles, published by troughton in ,[ ] was the "greatest improvement ever made in the art of instrument-making."[ ] but a more secure road to improvement than that of mere mechanical exactness was pointed out by bessel. his introduction of a regular theory of instrumental errors might almost be said to have created a new art of observation. every instrument, he declared in memorable words,[ ] must be twice made--once by the artist, and again by the observer. knowledge is power. defects that are ascertained and can be allowed for are as good as non-existent. thus the truism that the best instrument is worthless in the hands of a careless or clumsy observer, became supplemented by the converse maxim, that defective appliances may, through skilful use, be made to yield valuable results. the königsberg observations--of which the first instalment was published in --set the example of regular "reduction" for instrumental errors. since then, it has become an elementary part of an astronomer's duty to study the _idiosyncrasy_ of each one of the mechanical contrivances at his disposal, in order that its inevitable, but now certified deviations from ideal accuracy may be included amongst the numerous corrections by which the pure essence of even approximate truth is distilled from the rude impressions of sense. nor is this enough; for the casual circumstances attending each observation have to be taken into account with no less care than the inherent or _constitutional_ peculiarities of the instrument with which it is made. there is no "once for all" in astronomy. vigilance can never sleep; patience can never tire. variable as well as constant sources of error must be anxiously heeded; one infinitesimal inaccuracy must be weighed against another; all the forces and vicissitudes of nature--frosts, dews, winds, the interchanges of heat, the disturbing effects of gravity, the shiverings of the air, the tremors of the earth, the weight and vital warmth of the observer's own body, nay, the rate at which his brain receives and transmits its impressions, must all enter into his calculations, and be sifted out from his results. it was in that bessel drew attention to discrepancies in the times of transits given by different astronomers.[ ] the quantities involved were far from insignificant. he was himself nearly a second in advance of all his contemporaries, argelander lagging behind him as much as a second and a quarter. each individual, in fact, was found to have a certain definite _rate of perception_, which, under the name of "personal equation," now forms so important an element in the correction of observations that a special instrument for accurately determining its amount in each case is in actual use at greenwich. such are the refinements upon which modern astronomy depends for its progress. it is a science of hairbreadths and fractions of a second. it exists only by the rigid enforcement of arduous accuracy and unwearying diligence. whatever secrets the universe still has in store for man will only be communicated on these terms. they are, it must be acknowledged, difficult to comply with. they involve an unceasing struggle against the infirmities of his nature and the instabilities of his position. but the end is not unworthy the sacrifices demanded. one additional ray of light thrown on the marvels of creation--a single, minutest encroachment upon the strongholds of ignorance--is recompense enough for a lifetime of toil. or rather, the toil is its own reward, if pursued in the lofty spirit which alone becomes it. for it leads through the abysses of space and the unending vistas of time to the very threshold of that infinity and eternity of which the disclosure is reserved for a life to come. footnotes: [footnote : grant, _hist. astr._, p. .] [footnote : _optica promota_, p. .] [footnote : _phil. trans._, vol. xxxii., p. .] [footnote : _ibid._, vol. xc., p. .] [footnote : cassegrain, a frenchman, substituted in a _convex_ for a _concave_ secondary speculum. the tube was thereby enabled to be shortened by twice the focal length of the mirror in question. the great melbourne reflector (four feet aperture, by grubb) is constructed upon this plan.] [footnote : _phil. trans._, vol. civ., p. , _note_.] [footnote : _phil. trans._, vol. xc., p. . with the forty-foot, however, only very moderate powers seemed to have been employed, whence dr. robinson argued a deficiency of defining power. _proc. roy. irish ac._, vol. ii., p. .] [footnote : _phil. trans._, vol. lxxi., p. .] [footnote : it is remarkable that, as early as , the possibility of an achromatic combination was inferred by david gregory from the structure of the human eye. see his _catoptricæ et dioptricæ sphericæ elementa_, p. .] [footnote : wolf, _biographien_, bd. ii., p. .] [footnote : _month. not._, vol. i., p. . _note_.] [footnote : henrivaux, _encyclopédie chimique_, t. v., fasc. , p. .] [footnote : see _ante_, p. .] [footnote : _phil. trans._, vol. vii., p. .] [footnote : j. herschel, _the telescope_, p. .] [footnote : _month. not._, vol. xxix., p. .] [footnote : a slight excess of copper renders the metal easier to work, but liable to tarnish. robinson, _proc. roy. irish ac._, vol. ii., p. .] [footnote : _brit. ass._, , dr. robinson's closing address. _athenæum_, sept. , p. .] [footnote : _the telescope_, p. .] [footnote : lord rosse in _phil. trans._, vol. cxl., p. .] [footnote : this method is the same in principle with that applied by grubb in to a -inch speculum for the observatory of armagh. _phil. trans._, vol. clix., p. .] [footnote : robinson, _proc. roy. ir. ac._, vol. iii., p. .] [footnote : _astr. nach._, no. .] [footnote : airy, _month. not._, vol. ix., p. .] [footnote : _astronomical journal_ (gould's), vol. ii., p. .] [footnote : _ibid._, p. .] [footnote : lord rosse in _phil. trans._, vol. cxl., p. .] [footnote : no. of herschel's ( ) catalogue. before a star was visible in each of the two larger openings by which it is pierced; since then, one only. webb, _celestial objects_ ( th ed.), p. .] [footnote : _mem. am. ac._, vol. iii., p. ; _astr. nach._, no. .] [footnote : _pop. astr._, p. .] [footnote : this statement must be taken in the most general sense. supplementary observations of great value are now made at greenwich with the altitude and azimuth instrument, which likewise served piazzi to determine the places of his stars; while a "prime vertical instrument" is prominent at pulkowa.] [footnote : as early as , according to r. wolf (_ges. der astr._, p. ), father scheiner made the experiment of connecting a telescope with an axis directed to the pole, while chinese "equatoreal armillæ," dating from the thirteenth century, existed at pekin until , when they were carried off as "loot" to berlin. j. l. e. dreyer, _copernicus_, vol. i., p. .] [footnote : _miscellaneous works_, p. .] [footnote : _astr. jahrbuch_, (published ), p. .] [footnote : _month. not._, vol. xli., p. .] [footnote : _phil. trans._, vol. xlvi., p. .] [footnote : grant, _hist. of astr._, p. .] [footnote : _pop. vorl._, p. .] [footnote : _phil. trans._, vol. xcix., p. .] [footnote : _report brit. ass._, , p. .] [footnote : _pop. vorl._, p. .] [footnote : c. t. anger, _grundzüge der neucren astronomischen beobachtungs-kunst_, p. .] part ii recent progress of astronomy chapter i _foundation of astronomical physics_ in the year , heinrich schwabe of dessau, elated with the hope of speedily delivering himself from the hereditary incubus of an apothecary's shop,[ ] obtained from munich a small telescope and began to observe the sun. his choice of an object for his researches was instigated by his friend harding of göttingen. it was a peculiarly happy one. the changes visible in the solar surface were then generally regarded as no less capricious than the changes in the skies of our temperate regions. consequently, the reckoning and registering of sun-spots was a task hardly more inviting to an astronomer than the reckoning and registering of summer clouds. cassini, keill, lemonnier, lalande, were unanimous in declaring that no trace of regularity could be detected in their appearances or effacements.[ ] delambre pronounced them "more curious than really useful."[ ] even herschel, profoundly as he studied them, and intimately as he was convinced of their importance as symptoms of solar activity, saw no reason to suspect that their abundance and scarcity were subject to orderly alternation. one man alone in the eighteenth century, christian horrebow of copenhagen, divined their periodical character, and foresaw the time when the effects of the sun's vicissitudes upon the globes revolving round him might be investigated with success; but this prophetic utterance was of the nature of a soliloquy rather than of a communication, and remained hidden away in an unpublished journal until , when it was brought to light in a general ransacking of archives.[ ] indeed, schwabe himself was far from anticipating the discovery which fell to his share. he compared his fortune to that of saul, who, seeking his father's asses, found a kingdom.[ ] for the hope which inspired his early resolution lay in quite another direction. his patient ambush was laid for a possible intramercurial planet, which, he thought, must sooner or later betray its existence in crossing the face of the sun. he took, however, the most effectual measures to secure whatever new knowledge might be accessible. during forty-three years his "imperturbable telescope"[ ] never failed, weather and health permitting, to bring in its daily report as to how many, or if any, spots were visible on the sun's disc, the information obtained being day by day recorded on a simple and unvarying system. in he made his first announcement of a probable decennial period,[ ] but it met with no general attention; although julius schmidt of bonn (afterwards director of the athens observatory) and gautier of geneva were impressed with his figures, and littrow had himself, in ,[ ] hinted at the likelihood of some kind of regular recurrence. schwabe, however, worked on, gathering each year fresh evidence of a law such as he had indicated; and when humboldt published in , in the third volume of his _kosmos_,[ ] a table of the sun-spot statistics collected by him from downwards, the strength of his case was perceived with, so to speak, a start of surprise; the reality and importance of the discovery were simultaneously recognised, and the persevering hofrath of dessau found himself famous among astronomers. his merit--recognised by the bestowal of the astronomical society's gold medal in --consisted in his choice of an original and appropriate line of work, and in the admirable tenacity of purpose with which he pursued it. his resources and acquirements were those of an ordinary amateur; he was distinguished solely by the unfortunately rare power of turning both to the best account. he died where he was born and had lived, april , , at the ripe age of eighty-six. meanwhile an investigation of a totally different character, and conducted by totally different means, had been prosecuted to a very similar conclusion. two years after schwabe began his solitary observations, humboldt gave the first impulse, at the scientific congress of berlin in , to a great international movement for attacking simultaneously, in various parts of the globe, the complex problem of terrestrial magnetism. through the genius and energy of gauss, göttingen became its centre. thence new apparatus, and a new system for its employment, issued; there, in , the first regular magnetic observatory was founded, whilst at göttingen was fixed the universal time-standard for magnetic observations. a letter addressed by humboldt in april, , to the duke of sussex as president of the royal society, enlisted the co-operation of england. a network of magnetic stations was spread all over the british dominions, from canada to van diemen's land; measures were concerted with foreign authorities, and an expedition was fitted out, under the able command of captain (afterwards sir james) clark ross, for the special purpose of bringing intelligence on the subject from the dismal neighbourhood of the south pole. in , the elaborate organisation created by the disinterested efforts of scientific "agitators" was complete; gauss's "magnetometers" were vibrating under the view of attentive observers in five continents, and simultaneous results began to be recorded. ten years later, in september, , dr. john lamont, the scotch director of the munich observatory, in reviewing the magnetic observations made at göttingen and munich from to , perceived with some surprise that they gave unmistakable indications of a period which he estimated at - / years.[ ] the manner in which this periodicity manifested itself requires a word of explanation. the observations in question referred to what is called the "declination" of the magnetic needle--that is, to the position assumed by it with reference to the points of the compass when moving freely in a horizontal plane. now this position--as was discovered by graham in --is subject to a small daily fluctuation, attaining its maximum towards the east about a.m., and its maximum towards the west shortly before p.m. in other words, the direction of the needle approaches (in these countries at the present time) nearest to the true north some four hours before noon, and departs farthest from it between one and two hours after noon. it was the _range_ of this daily variation that lamont found to increase and diminish once in every - / years. in the following winter, sir edward sabine, ignorant as yet of lamont's conclusion, undertook to examine a totally different set of observations. the materials in his hands had been collected at the british colonial stations of toronto and hobarton from to , and had reference, not to the regular diurnal swing of the needle, but to those curious spasmodic vibrations, the inquiry into the laws of which was the primary object of the vast organisation set on foot by humboldt and gauss. yet the upshot was practically the same. once in about ten years, magnetic disturbances (termed by humboldt "storms") were perceived to reach a maximum of violence and frequency. sabine was the first to note the coincidence between this unlooked-for result and schwabe's sun-spot period. he showed that, so far as observation had yet gone, the two cycles of change agreed perfectly both in duration and phase, maximum corresponding to maximum, minimum to minimum. what the nature of the connection could be that bound together by a common law effects so dissimilar as the rents in the luminous garment of the sun, and the swayings to and fro of the magnetic needle, was and still remains beyond the reach of well-founded theory; but the fact was from the first undeniable. the memoir containing this remarkable disclosure was presented to the royal society, march , and read may , .[ ] on the st of july following, rudolf wolf at berne,[ ] and on the th of august, alfred gautier at sion,[ ] announced, separately and independently, perfectly similar conclusions. this triple event is perhaps the most striking instance of the successful employment of the baconian method of co-operation in discovery, by which "particulars" are amassed by one set of investigators--corresponding to the "depredators" and "inoculators" of solomon's house--while inductions are drawn from them by another and a higher class--the "interpreters of nature." yet even here the convergence of two distinct lines of research was wholly fortuitous, and skilful combination owed the most brilliant part of its success to the unsought bounty of what we call fortune. the exactness of the coincidence thus brought to light was fully confirmed by further inquiries. a diligent search through the scattered records of sun-spot observations, from the time of galileo and scheiner onwards, put wolf[ ] in possession of materials by which he was enabled to correct schwabe's loosely-indicated decennial period to one of slightly over eleven ( . ) years; and he further showed that this fell in with the ebb and flow of magnetic change even better than lamont's - / year cycle. the analogy was also pointed out between the "light-curve," or zig-zagged line representing on paper the varying intensity in the lustre of certain stars, and the similar delineation of spot-frequency; the ascent from minimum to maximum being, in both cases, usually steeper than the descent from maximum to minimum; while an additional point of resemblance was furnished by the irregularities in height of the various maxima. in other words, both the number of spots on the sun and the brightness of variable stars increase, as a rule, more rapidly than they decrease; nor does the amount of that increase, in either instance, show any approach to uniformity. the endeavour, suggested by the very nature of the phenomenon, to connect sun-spots with weather was less successful. the first attempt of the kind was made by sir william herschel in , and a very notable one it was. meteorological statistics, save of the scantiest and most casual kind, did not then exist; but the price of corn from year to year was on record, and this, with full recognition of its inadequacy, he adopted as his criterion. nor was he much better off for information respecting the solar condition. what little he could obtain, however, served, as he believed, to confirm his surmise that a copious emission of light and heat accompanies an abundant formation of "openings" in the dazzling substance whence our supply of those indispensable commodities is derived.[ ] he gathered, in short, from his inquiries very much what he had expected to gather, namely, that the price of wheat was high when the sun showed an unsullied surface, and that food and spots became plentiful together.[ ] yet this plausible inference was scarcely borne out by a more exact collocation of facts. schwabe failed to detect any reflection of the sun-spot period in his meteorological register. gautier[ ] reached a provisional conclusion the reverse--though not markedly the reverse--of herschel's. wolf, in , derived from an examination of vogel's collection of zürich chronicles ( - a.d.) evidence showing (as he thought) that minimum years were usually wet and stormy, maximum years dry and genial;[ ] but a subsequent review of the subject in convinced him that no relation of any kind between the two kinds of effects was traceable.[ ] with the singular affection of our atmosphere known as the aurora borealis (more properly aurora polaris) the case was different. here the zürich chronicles set wolf on the right track in leading him to associate such luminous manifestations with a disturbed condition of the sun; since subsequent detailed observation has exhibited the curve of auroral frequency as following with such fidelity the jagged lines figuring to the eye the fluctuations of solar and magnetic activity, as to leave no reasonable doubt that all three rise and sink together under the influence of a common cause. as long ago as ,[ ] halley had conjectured that the northern lights were due to magnetic "effluvia," but there was no evidence on the subject forthcoming until hiorter observed at upsala in their agitating influence upon the magnetic needle. that the effect was no casual one was made superabundantly clear by arago's researches in and subsequent years. now both were perceived to be swayed by the same obscure power of cosmical disturbance. the sun is not the only one of the heavenly bodies by which the magnetism of the earth is affected. proofs of a similar kind of lunar action were laid by kreil in before the bohemian society of sciences, and with minor corrections were fully substantiated by sabine's more extended researches. it was thus ascertained that each lunar day, or the interval of twenty-four hours and about fifty-four minutes between two successive meridian passages of our satellite, is marked by a perceptible, though very small, double oscillation of the needle--two progressive movements from east to west, and two returns from west to east.[ ] moreover, the lunar, like the solar influence (as was proved in each case by sabine's analysis of the hobarton and toronto observations), extends to all three "magnetic elements," affecting not only the position of the horizontal or _declination_ needle, but also the dip and intensity. it seems not unreasonable to attribute some portion of the same subtle power to the planets and even to the stars, though with effects rendered imperceptible by distance. we have now to speak of the discovery and application to the heavenly bodies of a totally new method of investigation. spectrum analysis may be shortly described as a mode of distinguishing the various species of matter by the kind of light proceeding from each. this definition at once explains how it is that, unlike every other system of chemical analysis, it has proved available in astronomy. light, so far as _quality_ is concerned, ignores distance. no intrinsic change, that we yet know of, is produced in it by a journey from the farthest bounds of the visible universe; so that, provided only that in _quantity_ it remain sufficient for the purpose, its peculiarities can be equally well studied whether the source of its vibrations be one foot or a hundred billion miles distant. now the most obvious distinction between one kind of light and another resides in colour. but of this distinction the eye takes cognisance in an æsthetic, not in a scientific sense. it finds gladness in the "thousand tints" of nature, but can neither analyse nor define them. here the refracting prism--or the combination of prisms known as the "spectroscope"--comes to its aid, teaching it to measure as well as to perceive. it furnishes, in a word, an accurate scale of colour. the various rays which, entering the eye together in a confused crowd, produce a compound impression made up of undistinguishable elements, are, by the mere passage through a triangular piece of glass, separated one from the other, and ranged side by side in orderly succession, so that it becomes possible to tell at a glance what kinds of light are present, and what absent. thus, if we could only be assured that the various chemical substances when made to glow by heat, emit characteristic rays--rays, that is, occupying a place in the spectrum reserved for them, and for them _only_--we should at once be in possession of a mode of identifying such substances with the utmost readiness and certainty. this assurance, which forms the solid basis of spectrum analysis, was obtained slowly and with difficulty. the first to employ the prism in the examination of various flames (for it is only in a state of vapour that matter emits distinctive light) was a young scotchman named thomas melvill, who died in , at the age of twenty-seven. he studied the spectrum of burning spirits, into which were successively introduced sal ammoniac, potash, alum, nitre, and sea-salt, and observed the singular predominance, under almost all circumstances, of a particular shade of yellow light, perfectly definite in its degree of refrangibility[ ]--in other words, taking up a perfectly definite position in the spectrum. his experiments were repeated by morgan,[ ] wollaston, and--with far superior precision and diligence--by fraunhofer.[ ] the great munich optician, whose work was completely original, rediscovered melvill's deep yellow ray and measured its place in the colour-scale. it has since become well known as the "sodium line," and has played a very important part in the history of spectrum analysis. nevertheless, its ubiquity and conspicuousness long impeded progress. it was elicited by the combustion of a surprising variety of substances--sulphur, alcohol, ivory, wood, paper; its persistent visibility suggesting the accomplishment of some universal process of nature rather than the presence of one individual kind of matter. but if spectrum analysis were to exist as a science at all, it could only be by attaining certainty as to the unvarying association of one special substance with each special quality of light. thus perplexed, fox talbot[ ] hesitated in to enounce this fundamental principle. he was inclined to believe that the presence in the spectrum of any individual ray told unerringly of the volatilisation in the flame under scrutiny of some body as whose badge or distinctive symbol that ray might be regarded; but the continual prominence of the yellow beam staggered him. it appeared, indeed, without fail where sodium _was_; but it also appeared where it might be thought only reasonable to conclude that sodium _was not_. nor was it until thirty years later that william swan,[ ] by pointing out the extreme delicacy of the spectral test, and the singularly wide dispersion of sodium, made it appear probable (but even then only probable) that the questionable yellow line was really due invariably to that substance. common salt (chloride of sodium) is, in fact, the most diffusive of solids. it floats in the air; it flows with water; every grain of dust has its attendant particle; its absolute exclusion approaches the impossible. and withal, the light that it gives in burning is so intense and concentrated, that if a single grain be divided into million parts, and one alone of such inconceivably minute fragments be present in a source of light, the spectroscope will show unmistakably its characteristic beam. amongst the pioneers of knowledge in this direction were sir john herschel[ ]--who, however, applied himself to the subject in the interests of optics, not of chemistry--w. a. miller,[ ] and wheatstone. the last especially made a notable advance when, in the course of his studies on the "prismatic decomposition" of the electric light, he reached the significant conclusion that the rays visible in its spectrum were different for each kind of metal employed as "electrodes."[ ] thus indications of a wider principle were to be found in several quarters, but no positive certainty on any single point was obtained, until, in , gustav kirchhoff, professor of physics in the university of heidelberg, and his colleague, the eminent chemist robert bunsen, took the matter in hand. by them the general question as to the necessary and invariable connection of certain rays in the spectrum with certain kinds of matter, was first resolutely confronted, and first definitely answered. it was answered affirmatively--else there could have been no science of spectrum analysis--as the result of experiments more numerous, more stringent, and more precise than had previously been undertaken.[ ] and the assurance of their conclusion was rendered doubly sure by the discovery, through the peculiarities of their light alone, of two new metals, named from the blue and red rays by which they were respectively distinguished, "cæsium," and "rubidium."[ ] both were immediately afterwards actually obtained in small quantities by evaporation of the durckheim mineral waters. the link connecting this important result with astronomy may now be indicated. in the year it occurred to william hyde wollaston to substitute for the round hole used by newton and his successors for the admittance of light to be examined with the prism, an elongated "crevice" / th of an inch in width. he thereupon perceived that the spectrum, thus formed of light, as it were, _purified_ by the abolition of overlapping images, was traversed by seven dark lines. these he took to be natural boundaries of the various colours,[ ] and satisfied with this quasi-explanation, allowed the subject to drop. it was independently taken up after twelve years by a man of higher genius. in the course of experiments on light, directed towards the perfecting of his achromatic lenses, fraunhofer, by means of a slit and a telescope, made the surprising discovery that the solar spectrum is crossed, not by seven, but by thousands of obscure transverse streaks.[ ] of these he counted some , and carefully mapped , while a few of the most conspicuous he set up (if we may be permitted the expression) as landmarks, measuring their distances apart with a theodolite, and affixing to them the letters of the alphabet, by which they are still universally known. nor did he stop here. the same system of examination applied to the rest of the heavenly bodies showed the mild effulgence of the moon and planets to be deficient in precisely the same rays as sunlight; while in the stars it disclosed the differences in likeness which are always an earnest of increased knowledge. the spectra of sirius and castor, instead of being delicately ruled crosswise throughout, like that of the sun, were seen to be interrupted by three massive bars of darkness--two in the blue and one in the green;[ ] the light of pollux, on the other hand, seemed precisely similar to sunlight attenuated by distance or reflection, and that of capella, betelgeux, and procyon to share some of its peculiarities. one solar line especially--that marked in his map with the letter d--proved common to all the four last-mentioned stars; and it was remarkable that it exactly coincided in position with the conspicuous yellow beam (afterwards, as we have said, identified with the light of glowing sodium) which he had already found to accompany most kinds of combustion. moreover, both the _dark_ solar and the _bright_ terrestrial "d lines" were displayed by the refined munich appliances as double. in this striking correspondence, discovered by fraunhofer in , was contained the very essence of solar chemistry; but its true significance did not become apparent until long afterwards. fraunhofer was by profession, not a physicist, but a practical optician. time pressed; he could not and would not deviate from his appointed track; all that was possible to him was to indicate the road to discovery, and exhort others to follow it.[ ] partially and inconclusively at first this was done. the "fixed lines" (as they were called) of the solar spectrum took up the position of a standing problem, to the solution of which no approach seemed possible. conjectures as to their origin were indeed rife. an explanation put forward by zantedeschi[ ] and others, and dubiously favoured by sir david brewster and dr. j. h. gladstone,[ ] was that they resulted from "interference"--that is, a destruction of the motion producing in our eyes the sensation of light, by the superposition of two light-waves in such a manner that the crests of one exactly fill up the hollows of the other. this effect was supposed to be brought about by imperfections in the optical apparatus employed. a more plausible view was that the atmosphere of the earth was the agent by which sunlight was deprived of its missing beams. for a few of them this is actually the case. brewster found in that certain dark lines, which were invisible when the sun stood high in the heavens, became increasingly conspicuous as he approached the horizon.[ ] these are the well-known "atmospheric lines;" but the immense majority of their companions in the spectrum remain quite unaffected by the thickness of the stratum of air traversed by the sunlight containing them. they are then obviously due to another cause. there remained the true interpretation--absorption in the _sun's_ atmosphere; and this, too, was extensively canvassed. but a remarkable observation made by professor forbes of edinburgh[ ] on the occasion of the annular eclipse of may , , appeared to throw discredit upon it. if the problematical dark lines were really occasioned by the stoppage of certain rays through the action of a vaporous envelope surrounding the sun, they ought, it seemed, to be strongest in light proceeding from his edges, which, cutting that envelope obliquely, passed through a much greater depth of it. but the circle of light left by the interposing moon, and of course derived entirely from the rim of the solar disc, yielded to forbes's examination precisely the same spectrum as light coming from its central parts. this circumstance helped to baffle inquirers, already sufficiently perplexed. it still remains an anomaly, of which no satisfactory explanation has been offered. convincing evidence as to the true nature of the solar lines was however at length, in the autumn of , brought forward at heidelburg. kirchhoff's _experimentum crucis_ in the matter was a very simple one. he threw bright sunshine across a space occupied by vapour of sodium, and perceived with astonishment that the dark fraunhofer line d, instead of being effaced by flame giving a luminous ray of the same refrangibility, was deepened and thickened by the superposition. he tried the same experiment, substituting for sunbeams light from a drummond lamp, and with similar result. a dark furrow, corresponding in every respect to the solar d-line, was instantly seen to interrupt the otherwise unbroken radiance of its spectrum. the inference was irresistible, that the effect thus produced artificially was brought about naturally in the same way, and that sodium formed an ingredient in the glowing atmosphere of the sun.[ ] this first discovery was quickly followed up by the identification of numerous bright rays in the spectra of other metallic bodies with others of the hitherto mysterious fraunhofer lines. kirchhoff was thus led to the conclusion that (besides sodium) iron, magnesium, calcium, and chromium, are certainly solar constituents, and that copper, zinc, barium, and nickel are also present, though in smaller quantities.[ ] as to cobalt, he hesitated to pronounce, but its existence in the sun has since been established. these memorable results were founded upon a general principle first enunciated by kirchhoff in a communication to the berlin academy, december , , and afterwards more fully developed by him.[ ] it may be expressed as follows: substances of every kind are opaque to the precise rays which they emit at the same temperature; that is to say, they stop the kinds of light or heat which they are then actually in a condition to radiate. but it does not follow that _cool_ bodies absorb the rays which they would give out if sufficiently heated. hydrogen at ordinary temperatures, for instance, is almost perfectly transparent, but if raised to the glowing point--as by the passage of electricity--it _then_ becomes capable of arresting, and at the same time of displaying in its own spectrum light of four distinct colours. this principle is fundamental to solar chemistry. it gives the key to the hieroglyphics of the fraunhofer lines. the identical characters which are written _bright_ in terrestrial spectra are written _dark_ in the unrolled sheaf of sun-rays; the meaning remains unchanged. it must, however, be remembered that they are only _relatively_ dark. the substances stopping those particular tints in the neighbourhood of the sun are at the same time vividly glowing with the very same. remove the dazzling solar background, by contrast with which they show as obscure, and they will be seen, and, at critical moments, actually have been seen, in all their native splendour. it is because the atmosphere of the sun is cooler than the globe it envelops that the different kinds of vapour constituting that atmosphere take more than they give, absorb more light than they are capable of emitting; raise them to the same temperature as the sun itself, and their powers of emission and absorption being brought exactly to the same level, the thousands of dusky rays in the solar spectrum will be at once obliterated. the establishment of the terrestrial science of spectrum analysis was due, as we have seen, equally to kirchhoff and bunsen, but its celestial application to kirchhoff alone. he effected this object of the aspirations, more or less dim, of many other thinkers and workers, by the union of two separate, though closely related lines of research--the study of the different kinds of light _emitted_ by various bodies, and the study of the different kinds of light _absorbed_ by them. the latter branch appears to have been first entered upon by dr. thomas young in ;[ ] it was pursued by the younger herschel,[ ] by william allen miller, brewster, and gladstone. brewster indeed made, in ,[ ] a formal attempt to found what might be called an inverse system of analysis with the prism based upon absorption; and his efforts were repeated, just a quarter of a century later, by gladstone.[ ] but no general point of view was attained; nor, it may be added, was it by this path attainable. kirchhoff's map of the solar spectrum, drawn to scale with exquisite accuracy, and printed in three shades of ink to convey the graduated obscurity of the lines, was published in the transactions of the berlin academy for and .[ ] representations of the principal lines belonging to various elementary bodies formed, as it were, a series of marginal notes accompanying the great solar scroll, enabling the veriest tiro in the new science to decipher its meaning at a glance. where the dark solar and bright metallic rays agreed in position, it might safely be inferred that the metal emitting them was a solar constituent; and such coincidences were numerous. in the case of iron alone, no less than sixty occurred in one-half of the spectral area, rendering the chances[ ] absolutely overwhelming against mere casual conjunction. the preparation of this elaborate picture proved so trying to the eyes that kirchhoff was compelled by failing vision to resign the latter half of the task to his pupil hofmann. the complete map measured nearly eight feet in length. the conclusions reached by kirchhoff were no sooner announced than they took their place, with scarcely a dissenting voice, among the established truths of science. the broad result, that the dark lines in the spectrum of the sun afford an index to its chemical composition no less reliable than any of the tests used in the laboratory, was equally captivating to the imagination of the vulgar, and authentic in the judgment of the learned; and, like all genuine advances in the knowledge of nature, it stimulated curiosity far more than it gratified it. now the history of how discoveries were missed is often quite as instructive as the history of how they were made; it may then be worth while to expend a few words on the thoughts and trials by which, in the present case, the actual event was heralded. three times it seemed on the verge of being anticipated. the experiment, which in kirchhoff's hands proved decisive, of passing sunlight through glowing vapours and examining the superposed spectra, was performed by professor w. a. miller of king's college in .[ ] nay, more, it was performed with express reference to the question, then already (as has been noted) in debate, of the possible production of fraunhofer's lines by absorption in a solar atmosphere. yet it led to nothing. again, at paris in , with a view to testing the asserted coincidence between the solar d-line and the bright yellow beam in the spectrum of the electric arc (really due to the unsuspected presence of sodium), léon foucault threw a ray of sunshine across the arc and observed its spectrum.[ ] he was surprised to see that the d-line was rendered more intensely dark by the combination of lights. to assure himself still further, he substituted a reflected image of one of the white-hot carbon-points for the sunbeam, with an identical result. _the same ray was missing._ it needed but another step to have generalised this result, and thus laid hold of a natural truth of the highest importance; but that step was not taken. foucault, keen and brilliant though he was, rested satisfied with the information that the _voltaic arc_ had the power of stopping the kind of light emitted by it; he asked no further question, and was consequently the bearer of no further intelligence on the subject. the truth conveyed by this remarkable experiment was, however, divined by one eminent man. professor stokes of cambridge stated to sir william thomson (now lord kelvin), shortly after it had been made, his conviction that an absorbing atmosphere of sodium surrounded the sun. and so forcibly was his hearer impressed with the weight of the argument based upon the absolute agreement of the d-line in the solar spectrum with the yellow ray of burning sodium (then freshly certified by w. h. miller), combined with foucault's "reversal" of that ray, that he regularly inculcated, in his public lectures on natural philosophy at glasgow, five or six years before kirchhoff's discovery, not only the _fact_ of the presence of sodium in the solar neighbourhood, but also the _principle_ of the study of solar and stellar chemistry in the spectra of flames.[ ] yet it does not appear to have occurred to either of these two distinguished professors--themselves among the foremost of their time in the successful search for new truths--to verify practically a sagacious conjecture in which was contained the possibility of a scientific revolution. it is just to add, that kirchhoff was unacquainted, when he undertook his investigation, either with the experiment of foucault or the speculation of stokes. for c. j. Ångström, on the other hand, perhaps somewhat too much has been claimed in the way of anticipation. his _optical researches_ appeared at upsala in , and in their english garb two years later.[ ] they were undoubtedly pregnant with suggestion, yet made no epoch in discovery. the old perplexities continued to prevail after, as before their publication. to Ångström, indeed, belongs the great merit of having revived euler's principle of the equivalence of emission and absorption; but he revived it in its original crude form, and without the qualifying proviso which alone gave it value as a clue to new truths. according to his statement, a body absorbs all the series of vibrations it is, under any circumstances, capable of emitting, as well as those connected with them by simple harmonic relations. this is far too wide. to render it either true or useful, it had to be reduced to the cautious terms employed by kirchhoff. radiation strictly and necessarily corresponds with absorption only _when the temperature is the same_. in point of fact, Ångström was still, in , divided between adsorption and interference as the mode of origin of the fraunhofer dark rays. very important, however, was his demonstration of the compound nature of the spark-spectrum, which he showed to be made up of the spectrum of the metallic electrodes superposed upon that of the gas or gases across which the discharge passed. it may here be useful--since without some clear ideas on the subject no proper understanding of recent astronomical progress is possible--to take a cursory view of the elementary principles of spectrum analysis. to many of our readers they are doubtless already familiar; but it is better to appear trite to some than obscure even to a few. the spectrum, then, of a body is simply the light proceeding from it _spread out_ by refraction[ ] into a brilliant variegated band, passing from brownish-red through crimson, orange, yellow, green, and azure into dusky violet. the reason of this spreading-out or "dispersion" is that the various colours have different wave-lengths, and consequently meet with different degrees of retardation in traversing the denser medium of the prism. the shortest and quickest vibrations (producing the sensation we call "violet") are thrown farthest away from their original path--in other words, suffer the widest "deviation;" the longest and slowest (the red) travel much nearer to it. thus the sheaf of rays which would otherwise combine into a patch of white light are separated through the divergence of their tracks after refraction by a prism, so as to form a tinted riband. this _visible_ spectrum is prolonged _invisibly_ at both ends by a long range of vibrations, either too rapid or too sluggish to affect the eye as light, but recognisable through their chemical and heating effects. now all incandescent solid or liquid substances, and even gases ignited under great pressure, give what is called a "continuous spectrum;" that is to say, the light derived from them is of every conceivable hue. sorted out with the prism, its tints merge imperceptibly one into the other, uninterrupted by any dark spaces. no colours, in short, are missing. but gases and vapours rendered luminous by heat emit rays of only a few tints, which accordingly form an interrupted spectrum, usually designated as one of lines or bands. and since these rays are perfectly definite and characteristic--not being the same for any two substances--it is easy to tell what kind of matter is concerned in producing them. we may suppose that the inconceivably minute particles which by their rapid thrilling agitate the ethereal medium so as to produce light, are free to give out their peculiar tone of vibration only when floating apart from each other in gaseous form; but when crowded together into a condensed mass, the clear ring of the distinctive note is drowned, so to speak, in a universal molecular clang. thus prismatic analysis has no power to identify individual kinds of matter, except when they present themselves as glowing vapours. a spectrum is said to be "reversed" when lines previously seen bright on a dark background appear dark on a bright background. in this form it is equally characteristic of chemical composition with the "direct" spectrum, being due to _absorption_, as the latter is to _emission_. and absorption and emission are, by kirchhoff's law, strictly correlative. this is easily understood by the analogy of sound. for just as a tuning-fork responds to sound-waves of its own pitch, but remains indifferent to those of any other, so those particles of matter whose nature it is, when set swinging by heat, to vibrate a certain number of times in a second, thus giving rise to light of a particular shade of colour, appropriate those same vibrations, and those only, when transmitted past them,--or, phrasing it otherwise, are opaque to them, and transparent to all others. it should further be explained that the _shape_ of the bright or dark spaces in the spectrum has nothing whatever to do with the nature of the phenomena. the "lines" and "bands" so frequently spoken of are seen as such for no other reason than because the light forming them is admitted through a narrow, straight opening. change that opening into a fine crescent or a sinuous curve, and the "lines" will at once appear as crescents or curves. resuming in a sentence what has been already explained, we find that the prismatic analysis of the heavenly bodies was founded upon three classes of facts: first, the unmistakable character of the light given by each different kind of glowing vapour; secondly, the identity of the light absorbed with the light emitted by each; thirdly, the coincidence observed between rays missing from the solar spectrum and rays absorbed by various terrestrial substances. thus, a realm of knowledge, pronounced by morinus[ ] in the seventeenth century, and no less dogmatically by auguste comte[ ] in the nineteenth, hopelessly out of reach of the human intellect, was thrown freely open, and the chemistry of the sun and stars took at once a leading place among the experimental sciences. the immediate increase of knowledge was not the chief result of kirchhoff's labours; still more important was the change in the scope and methods of astronomy, which, set on foot in by the detection of a common period affecting at once the spots on the sun and the magnetism of the earth, was extended and accelerated by the discovery of spectrum analysis. the nature of that change is concisely indicated by the heading of the present chapter; we would now ask our readers to endeavour to realise somewhat distinctly what is implied by the "foundation of astronomical physics." just three centuries ago, kepler drew a forecast of what he called a "physical astronomy"--a science treating of the efficient causes of planetary motion, and holding the "key to the inner astronomy."[ ] what kepler dreamed of and groped after, newton realized. he showed the beautiful and symmetrical revolutions of the solar system to be governed by a uniformly acting cause, and that cause no other than the familiar force of gravity, which gives stability to all our terrestrial surroundings. the world under our feet was thus for the first time brought into physical connection with the worlds peopling space, and a very tangible relationship was demonstrated as existing between what used to be called the "corruptible" matter of the earth and the "incorruptible" matter of the heavens. this process of unification of the cosmos--this levelling of the celestial with the sublunary--was carried no farther until the fact unexpectedly emerged from a vast and complicated mass of observations, that the magnetism of the earth is subject to subtle influences, emanating, certainly from some, and presumably from all of the heavenly bodies; the inference being thus rendered at least plausible, that a force not less universal than gravity itself, but with whose modes of action we are as yet unacquainted, pervades the universe, and forms, it might be said, an intangible bond of sympathy between its parts. now for the investigation of this influence two roads are open. it may be pursued by observation either of the bodies from which it proceeds, or of the effects which it produces--that is to say, either by the astronomer or by the physicist, or, better still, by both concurrently. their acquisitions are mutually profitable; nor can either be considered as independent of the other. any important accession to knowledge respecting the sun, for example, may be expected to cast a reflected light on the still obscure subject of terrestrial magnetism; while discoveries in magnetism or its _alter ego_ electricity must profoundly affect solar inquiries. the establishment of the new method of spectrum analysis drew far closer this alliance between celestial and terrestrial science. indeed, they have come to merge so intimately one into the other, that it is no easier to trace their respective boundaries than it is to draw a clear dividing-line between the animal and vegetable kingdoms. yet up to the middle of the last century, astronomy, while maintaining her strict union with mathematics, looked with indifference on the rest of the sciences; it was enough that she possessed the telescope and the calculus. now the materials for her inductions are supplied by the chemist, the electrician, the inquirer into the most recondite mysteries of light and the molecular constitution of matter. she is concerned with what the geologist, the meteorologist, even the biologist, has to say; she can afford to close her ears to no new truth of the physical order. her position of lofty isolation has been exchanged for one of community and mutual aid. the astronomer has become, in the highest sense of the term, a physicist; while the physicist is bound to be something of an astronomer. this, then, is what is designed to be conveyed by the "foundation of astronomical or cosmical physics." it means the establishment of a science of nature whose conclusions are not only presumed by analogy, but are ascertained by observation, to be valid wherever light can travel and gravity is obeyed--a science by which the nature of the stars can be studied upon the earth, and the nature of the earth can be made better known by study of the stars--a science, in a word, which is, or aims at being, one and universal, even as nature--the visible reflection of the invisible highest unity--is one and universal. it is not too much to say that a new birth of knowledge has ensued. the astronomy so signally promoted by bessel[ ]--the astronomy placed by comte[ ] at the head of the hierarchy of the physical sciences--was the science of the _movements_ of the heavenly bodies. and there were those who began to regard it as a science which, from its very perfection, had ceased to be interesting--whose tale of discoveries was told, and whose farther advance must be in the line of minute technical improvements, not of novel and stirring disclosures. but the science of the _nature_ of the heavenly bodies is one only in the beginning of its career. it is full of the audacities, the inconsistencies, the imperfections, the possibilities of youth. it promises everything; it has already performed much; it will doubtless perform much more. the means at its disposal are vast and are being daily augmented. what has so far been secured by them it must now be our task to extricate from more doubtful surroundings and place in due order before our readers. footnotes: [footnote : wolf, _gesch. der astr._, p. .] [footnote : manuel johnson, _mem. r.a.s._, vol. xxvi., p. .] [footnote : _astronomie théorique et pratique_, t. iii., p. .] [footnote : wolf, _gesch. der astr._, p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : _mem. r.a.s._, vol. xxvi., p. .] [footnote : _astr. nach._, no. .] [footnote : gehler's _physikalisches wörterbuch_, art. _sonnenflecken_, p. .] [footnote : _zweite abth._, p. .] [footnote : _annalen der physik_ (poggendorff's), bd. lxxxiv., p. .] [footnote : _phil. trans._, vol. cxlii., p. .] [footnote : _mittheilungen der naturforschenden gesellschaft_, , p. .] [footnote : _archives des sciences_, t. xxi., p. .] [footnote : _neue untersuchungen, mitth. naturf. ges._, , p. .] [footnote : _phil. trans._, vol. xci., p. .] [footnote : evidence of an eleven-yearly fluctuation in the price of food-grains in india was collected some years ago by mr. frederick chambers. _nature_, vol. xxxiv., p. .] [footnote : _bibl. un. de genève_, t. li., p. .] [footnote : _neue untersuchungen_, p. .] [footnote : _die sonne und ihre flecken_, p. . arago was the first who attempted to decide the question by keeping, through a series of years, a parallel register of sun-spots and weather; but the data regarding the solar condition amassed at the paris observatory from to were not sufficiently precise to support any inference.] [footnote : _phil. trans._, vol. xxix., p. .] [footnote : _ibid._, vols. cxliii., p. , cxlvi., p. .] [footnote : _observations on light and colours_, p. .] [footnote : _phil. trans._, vol. lxxv., p. .] [footnote : _denkschriften_ (munich. ac. of sc.), , , bd. v., p. .] [footnote : _edinburgh journal of science_, vol. v., p. . see also _phil. mag._, feb., , vol. iv., p. .] [footnote : _ed. phil. trans.,_ vol. xxi., p. .] [footnote : _on the absorption of light by coloured media, ed. phil. trans._, vol. ix., p. ( ).] [footnote : _phil. mag._, vol. xxvii, (ser. iii.), p. .] [footnote : _report brit. ass._, , p. (pt. ii.). _electrodes_ are the terminals from one to the other of which the electric spark passes, volatilising and rendering incandescent in its transit some particles of their substance, the characteristic light of which accordingly flashes out in the spectrum.] [footnote : _phil. mag._, vol. xx., p. .] [footnote : _annalen der physik_, bd. cxiii., p. .] [footnote : _phil. trans._, vol. xcii., p. .] [footnote : _denkschriften_, bd. v., p. .] [footnote : _ibid._, p. ; _edin. jour. of science_, vol. viii., p. .] [footnote : _denkschriften_, bd. v., p. .] [footnote : _arch. des sciences_, , p. .] [footnote : _phil. trans._, vol. cl., p. , _note_.] [footnote : _ed. phil. trans._, vol. xii., p. .] [footnote : _phil. trans._, vol. cxxvi., p. . "i conceive," he says, "that this result proves decisively that the sun's atmosphere has nothing to do with the production of this singular phenomenon" (p. ). and brewster's well-founded opinion that it had much to do with it was thereby, in fact, overthrown.] [footnote : _monatsberichte_, berlin, , p. .] [footnote : _abhandlungen_, berlin, , pp. , .] [footnote : _ibid._, , p. ; _annalen der physik_, bd. cxix., p. . a similar conclusion, reached by balfour stewart in , for heat-rays (_ed. phil. trans._, vol. xxii., p. ), was, in , without previous knowledge of kirchhoff's work, extended to light (_phil. mag._, vol. xx., p. ); but his experiments wanted the precision of those executed at heidelburg.] [footnote : _miscellaneous works_, vol. i., p. .] [footnote : _ed. phil. trans._, vol. ix., p. .] [footnote : _ibid._, vol. xii., p. .] [footnote : _quart. jour. chem. soc._, vol. x. p. .] [footnote : a facsimile accompanied sir h. roscoe's translation of kirchhoff's "researches on the solar spectrum" (london, - ).] [footnote : estimated by kirchhoff's at a _trillion to one_. _abhandl._, , p. .] [footnote : _phil. mag._, vol. xxvii. ( rd series), p. .] [footnote : _l'institut_, feb. , , p. ; _phil. mag._, vol. xix. ( th series), p. .] [footnote : _ann. d. phys._, vol. cxviii., p. .] [footnote : _phil. mag._, vol. ix. ( th series), p. .] [footnote : spectra may be produced by _diffraction_ as well as by _refraction_; but we are here only concerned with the subject in its simplest aspect.] [footnote : _astrologia gallica_ ( ), p. .] [footnote : _pos. phil._, vol. i., pp. , (martineau's trans.).] [footnote : _proem astronomiæ pars optica_ ( ), _op._, t. ii.] [footnote : _pop. vorl._, pp. , , .] [footnote : _pos. phil._, p. .] chapter ii _solar observations and theories_ the zeal with which solar studies have been pursued during the last half century has already gone far to redeem the neglect of the two preceding ones. since schwabe's discovery was published in , observers have multiplied, new facts have been rapidly accumulated, and the previous comparative quiescence of thought on the great subject of the constitution of the sun, has been replaced by a bewildering variety of speculations, conjectures, and more or less justifiable inferences. it is satisfactory to find this novel impulse not only shared, but to a large extent guided, by our countrymen. william rutter dawes, one of many clergymen eminent in astronomy, observed, in , with the help of a solar eye-piece of his own devising, some curious details of spot-structure.[ ] the umbra--heretofore taken for the darkest part of the spot--was seen to be suffused with a mottled, nebulous illumination, in marked contrast with the striated appearance of the penumbra; while through this "cloudy stratum" a "black opening" permitted the eye to divine farther unfathomable depths beyond. the _hole_ thus disclosed--evidently the true nucleus--was found to be present in all considerable, as well as in many small maculæ. again, the whirling motions of some of these objects were noticed by him. the remarkable form of one sketched at wateringbury, in kent, january , , gave him the means of detecting and measuring a rotatory movement of the whole spot round the black nucleus at the rate of degrees in six days. "it appeared," he said, "as if some prodigious ascending force of a whirlwind character, in bursting through the cloudy stratum and the two higher and luminous strata, had given to the whole a movement resembling its own."[ ] an interpretation founded, as is easily seen, on the herschelian theory, then still in full credit. an instance of the same kind was observed by mr. w. r. birt in ,[ ] and cyclonic movements are now a recognised feature of sun-spots. they are, however, as father secchi[ ] concluded from his long experience, but temporary and casual. scarcely three per cent. of all spots visible exhibit the spiral structure which should invariably result if a conflict of opposing, or the friction of unequal, currents were essential, and not merely incidental to their origin. a whirlpool phase not unfrequently accompanies their formation, and may be renewed at periods of recrudescence or dissolution; but it is both partial and inconstant, sometimes affecting only one side of a spot, sometimes slackening gradually its movement in one direction, to resume it, after a brief pause, in the opposite. persistent and uniform notions, such as the analogy of terrestrial storms would absolutely require, are not to be found. so that the "cyclonic theory" of sun-spots, suggested by herschel in ,[ ] and urged, from a different point of view, by faye in , may be said to have completely broken down. the drift of spots over the sun's surface was first systematically investigated by carrington, a self-constituted astronomer, gifted with the courage and the instinct of thoughtful labour. born at chelsea in may, , richard christopher carrington entered trinity college, cambridge, in . he was intended for the church, but professor challis's lectures diverted him to astronomy, and he resolved, as soon as he had taken his degree, to prepare, with all possible diligence, to follow his new vocation. his father, who was a brewer on a large scale at brentford, offered no opposition; ample means were at his disposal; nevertheless, he chose to serve an apprenticeship of three years as observer in the university of durham, as though his sole object had been to earn a livelihood. he quitted the post only when he found that its restricted opportunities offered no farther prospect of self-improvement. he now built an observatory of his own at redhill in surrey, with the design of completing bessel's and argelander's survey of the northern heavens by adding to it the circumpolar stars omitted from their view. this project, successfully carried out between and , had another and still larger one superposed upon it before it had even begun to be executed. in , while the redhill observatory was in course of erection, the discovery of the coincidence between the sun-spot and magnetic periods was announced. carrington was profoundly interested, and devoted his enforced leisure to the examination of records, both written and depicted, of past solar observations. struck with their fragmentary and inconsistent character, he resolved to "appropriate," as he said, by "close and methodical research," the eleven-year period next ensuing.[ ] he calculated rightly that he should have the field pretty nearly to himself; for many reasons conspire to make public observatories slow in taking up new subjects, and amateurs with freedom to choose, and means to treat them effectually, were scarcer then than they are now. the execution of this laborious task was commenced november , . it was intended to be merely a _parergon_--a "second subject," upon which daylight energies might be spent, while the hours of night were reserved for cataloguing those stars that "are bereft of the baths of ocean." its results, however, proved of the highest interest, although the vicissitudes of life barred the completion, in its full integrity, of the original design. by the death, in , of the elder carrington, the charge of the brewery devolved upon his son; and eventually absorbed so much of his care that it was found advisable to bring the solar observations to a premature close, on march , . his scientific life may be said to have closed with them. attacked four years later with severe, and, in its results, permanent illness, he disposed of the brentford business, and withdrew to churt, near farnham, in surrey. there, in a lonely spot, on the top of a detached conical hill known as the "devil's jump," he built a second observatory, and erected an instrument which he was no longer able to use with pristine effectiveness; and there, november , , he died of the rupture of a blood vessel on the brain, before he had completed his fiftieth year.[ ] his observations of sun-spots were of a geometrical character. they concerned positions and movements, leaving out of sight physical peculiarities. indeed, the prudence with which he limited his task to what came strictly within the range of his powers to accomplish, was one of carrington's most valuable qualities. the method of his observations, moreover, was chosen with the same practical sagacity as their objects. as early as , sir john herschel had recommended the daily self-registration of sun-spots,[ ] and he enforced the suggestion, with more immediate prospect of success, in .[ ] the art of celestial photography, however, was even then in a purely tentative stage, and carrington wisely resolved to waste no time on dubious experiments, but employ the means of registration and measurement actually at his command. these were very simple, yet very effective. to the "helioscope" employed by father scheiner[ ] two centuries and a quarter earlier, a species of micrometer was added. the image of the sun was projected upon a screen by means of a firmly-clamped telescope, in the focus of which were placed two cross-wires forming angles of ° with the meridian. the six instants were then carefully noted at which these were met by the edges of the disc as it traversed the screen, and by the nucleus of the spot to be measured.[ ] a short process of calculation then gave the exact position of the spot as referred to the sun's centre. from a series of , observations made in this way, together with a great number of accurate drawings, carrington derived conclusions of great importance on each of the three points which he had proposed to himself to investigate. these were: the law of the sun's rotation, the existence and direction of systematic currents, and the distribution of spots on the solar surface. grave discrepancies were early perceived to exist between determinations of the sun's rotation by different observers. galileo, with "comfortable generality," estimated the period at "about a lunar month";[ ] scheiner, at twenty-seven days.[ ] cassini, in , made it · ; delambre, in , no more than twenty-five days. later inquiries brought these divergences within no more tolerable limits. laugier's result of · days--obtained in --enjoyed the highest credit, yet it differed widely in one direction from that of böhm ( ), giving · days, and in the other from that of kysæus ( ), giving · days. now the cause of these variations was really obvious from the first, although for a long time strangely overlooked. scheiner pointed out in that different spots gave different periods, adding the significant remark that one at a distance from the solar equator revolved more slowly than those nearer to it.[ ] but the hint was wasted. for upwards of two centuries ideas on the subject were either retrograde or stationary. what were called the "proper motions" of spots were, however, recognised by schröter,[ ] and utterly baffled laugier,[ ] who despaired of obtaining any concordant result as to the sun's rotation except by taking the mean of a number of discordant ones. at last, in , a valuable course of observations made at capo di monte, naples, in - , enabled c. h. f. peters[ ] to set in the clearest light the insecurity of determinations based on the assumption of fixity in objects plainly affected by movements uncertain both in amount and direction. such was the state of affairs when carrington entered upon his task. everything was in confusion; the most that could be said was that the confusion had come to be distinctly admitted and referred to its true source. what he discovered was this: that the sun, or at least the outer shell of the sun visible to us, has _no single period of rotation_, but drifts round, carrying the spots with it, at a rate continually accelerated from the poles to the equator. in other words, the time of axial revolution is shortest at the equator and lengthens with increase of latitude. carrington devised a mathematical formula by which the rate or "law" of this lengthening was conveniently expressed; but it was a purely empirical one. it was a concise statement, but implied no physical interpretation. it summarised, but did not explain the facts. an assumed "mean period" for the solar rotation of · days (twenty-five days nine hours, very nearly), was thus found to be _actually_ conformed to only in two parallels of solar latitude ( ° north and south), while the equatorial period was slightly less than twenty-five, and that of latitude ° rose to twenty-seven days and a half.[ ] these curious results gave quite a new direction to ideas on solar physics. the other two "elements" of the sun's rotation were also ascertained by carrington with hitherto unattained precision. he fixed the inclination of its axis to the ecliptic at ° '; the longitude of the ascending node at ° ' (for the epoch a.d.). these data--which have scarcely yet been improved upon--suffice to determine the position in space of the sun's equator. its north pole is directed towards a star in the coils of the dragon, midway between vega and the pole-star; its plane intersects that of the earth's orbit in such a way that our planet finds itself in the same level on or about the rd of june and the th of december, when any spots visible on the disc cross it in apparently straight lines. at other times, the paths pursued by them seem curved--downward (to an observer in the northern hemisphere) between june and december, upward between december and june. a singular peculiarity in the distribution of sun-spots emerged from carrington's studies at the time of the minimum of . two broad belts of the solar surface, as we have seen, are frequented by them, of which the limits may be put at ° and ° of north and south latitude. individual equatorial spots are not uncommon, but nearer to the poles than ° they are a rare exception. carrington observed--as an extreme instance--in july, , one in south latitude °; and peters, in june, , watched, during several days, a spot in ° ' north latitude. but beyond this no true macula has ever been seen; for lahire's reported observation of one in latitude ° is now believed to have had its place on the solar globe erroneously assigned; and the "veiled spots" described by trouvelot in [ ] as occurring within ° of the pole can only be regarded as, at the most, the same kind of disturbance in an undeveloped form. but the novelty of carrington's observations consisted in the detection of certain changes in distribution concurrent with the progress of the eleven-year period. as the minimum approached, the spot-zones contracted towards the equator, and there finally vanished; then, as if by a fresh impulse, spots suddenly reappeared in high latitude, and spread downwards with the development of the new phase of activity. scarcely had this remark been made public,[ ] when wolf[ ] found a confirmation of its general truth in böhm's observations during the years - ; and a perfectly similar behaviour was noted both by spörer and secchi at the minimum epoch of . the ensuing period gave corresponding indications; and it may now be looked upon as established that the spot-zones close in towards the equator with the advance of each cycle, their activity culminating, as a rule, in a mean latitude of about °, and expiring when it is reduced to °. before this happens, however, a completely new disturbance will have manifested itself some ° north and south of the equator, and will have begun to travel over the same course as its predecessor. each series of sun-spots is thus, to some extent, overlapped by the succeeding one; so that while the average interval from one maximum to the next is eleven years, the period of each distinct wave of agitation is twelve or fourteen.[ ] curious evidence of the retarded character of the maximum of - was to be found in the unusually low latitude of the spot-zones when it occurred. their movement downward having gone on regularly while the crisis was postponed, its final symptoms were hence displaced locally as well as in time. the "law of zones" was duly obeyed at the minima of [ ] and , and spörer found evidence of conformity to it so far back as .[ ] his researches, however, also showed that it was in abeyance during some seventy years previously to , during which period sun-spots remained persistently scarce, and auroral displays were feeble and infrequent even in high northern latitudes. an unaccountable suspension of solar activity is, in fact, indicated.[ ] gustav spörer, born at berlin in , began to observe sun-spots with the view of assigning the law of solar rotation in december, . his assiduity and success with limited means attracted attention, and a government endowment was procured for his little solar observatory at anclam, in pomerania, the crown prince (afterwards emperor frederick) adding a five-inch refractor to its modest equipment. unaware of carrington's discovery (not made known until january, ), he arrived at and published, in june, ,[ ] a similar conclusion as to the equatorial quickening of the sun's movement on its axis. appointed observer in the new astrophysical establishment at potsdam in , he continued his sun-spot determinations there for twenty years, and died july , . the time had now evidently come for a fundamental revision of current notions respecting the nature of the sun. herschel's theory of a cool, dark, habitable globe, surrounded by, and protected against, the radiations of a luminous and heat-giving envelope, was shattered by the first _dicta_ of spectrum analysis. traces of it may be found for a few years subsequent to ,[ ] but they are obviously survivals from an earlier order of ideas, doomed to speedy extinction. it needs only a moment's consideration of the meaning at last found for the fraunhofer lines to see the incompatibility of the new facts with the old conceptions. they implied not only the presence near the sun, as glowing vapours, of bodies highly refractory to heat, but that these glowing vapours formed the relatively cool envelope of a still hotter internal mass. kirchhoff, accordingly, included in his great memoir "on the solar spectrum," read before the berlin academy of sciences, july , , an exposition of the views on the subject to which his memorable investigations had led him. they may be briefly summarised as follows: since the body of the sun gives a continuous spectrum, it must be either solid or liquid,[ ] while the interruptions in its light prove it to be surrounded by a complex atmosphere of metallic vapours, somewhat cooler than itself. spots are simply clouds due to local depressions of temperature, differing in no respect from terrestrial clouds except as regards the kinds of matter composing them. these _sun-clouds_ take their origin in the zones of encounter between polar and equatorial currents in the solar atmosphere. this explanation was liable to all the objections urged against the "cumulus theory" on the one hand, and the "trade-wind theory" on the other. setting aside its propounder, it was consistently upheld perhaps by no man eminent in science except spörer; and his advocacy of it proved ineffective to secure its general adoption. m. faye, of the paris academy of sciences, was the first to propose a coherent scheme of the solar constitution covering the whole range of new discovery. the fundamental ideas on the subject now in vogue here made their first connected appearance. much, indeed, remained to be modified and corrected; but the transition was finally made from the old to the new order of thought. the essence of the change may be conveyed in a single sentence. the sun was thenceforth regarded, not as a mere heated body, or--still more remotely from the truth--as a cool body unaccountably spun round with a cocoon of fire, but as a vast _heat-radiating machine_. the terrestrial analogy was abandoned in one more particular besides that of temperature. the solar system of circulation, instead of being adapted, like that of the earth, to the distribution of heat received from without, was seen to be directed towards the transportation towards the surface of the heat contained within. polar and equatorial currents, tending to a purely superficial equalisation of temperature, were replaced by vertical currents bringing up successive portions of the intensely heated interior mass, to contribute their share in turn to the radiation into space which might be called the proper function of a sun. faye's views, which were communicated to the academy of sciences, january , ,[ ] were avowedly based on the anomalous mode of solar rotation discovered by carrington. this may be regarded either as an acceleration increasing from the poles to the equator, or as a retardation increasing from the equator to the poles, according to the rate of revolution we choose to assume for the unseen nucleus. faye preferred to consider it a retardation produced by ascending currents continually left behind as the sphere widened in which the matter composing them was forced to travel. he further supposed that the depth from which these vertical currents rose, and consequently the amount of retardation effected by their ascent to the surface, became progressively greater as the poles were approached, owing to the considerable flattening of the spheroidal surface from which they started;[ ] but the adoption of this expedient has been shown to involve inadmissible consequences. the extreme internal mobility betrayed by carrington's and spörer's observations led to the inference that the matter composing the sun was mainly or wholly gaseous. this had already been suggested by father secchi[ ] a year earlier, and by sir john herschel in april, ;[ ] but it first obtained general currency through faye's more elaborate presentation. a physical basis was afforded for the view by cagniard de la tour's experiments in ,[ ] proving that, under conditions of great heat and pressure, the vaporous state was compatible with a very considerable density. the position was strengthened when andrews showed, in ,[ ] that above a fixed limit of temperature, varying for different bodies, true liquefaction is impossible, even though the pressure be so tremendous as to retain the gas within the same space that enclosed the liquid. the opinion that the mass of the sun is gaseous now commands a very general assent; although the gaseity admitted is of such a nature as to afford the consistence rather of honey or pitch than of the aeriform fluids with which we are familiar. on another important point the course of subsequent thought was powerfully influenced by faye's conclusions in . arago somewhat hastily inferred from experiments with the polariscope the wholly gaseous nature of the visible disc of the sun. kirchhoff, on the contrary, believed (erroneously, as we now know) that the brilliant continuous spectrum derived from it proved it to be a white-hot solid or liquid. herschel and secchi[ ] indicated a cloud-like structure as that which would best harmonise the whole of the evidence at command. the novelty introduced by faye consisted in regarding the photosphere no longer "as a defined surface, in the mathematical sense, but as a limit to which, in the general fluid mass, ascending currents carry the physical or chemical phenomena of incandescence."[ ] uprushing floods of mixed vapours with strong affinities--say of calcium or sodium and oxygen--at last attain a region cool enough to permit their combination; a fine dust of solid or liquid compound particles (of lime or soda, for example) there collects into the photospheric clouds, and descending by its own weight in torrents of incandescent rain, is dissociated by the fierce heat below, and replaced by ascending and combining currents of similar constitution. this first attempt to assign the part played in cosmical physics by chemical affinities was marked by the importation into the theory of the sun of the now familiar phrase _dissociation_. it is indeed tolerably certain that no such combinations as those contemplated by faye occur at the photospheric level, since the temperature there must be enormously higher than would be needed to reduce all metallic earths and oxides; but molecular changes of some kind, dependent perhaps in part upon electrical conditions, in part upon the effects of radiation into space, most likely replace them. the conjecture was emitted by dr. johnstone stoney in [ ] that the photospheric clouds are composed of carbon-particles precipitated from their mounting vapour just where the temperature is lowered by expansion and radiation to the boiling-point of that substance. but this view, though countenanced by Ångström,[ ] and advocated by hastings of baltimore,[ ] and other authorities,[ ] is open to grave objections.[ ] in faye's theory, sun-spots were regarded as simply breaks in the photospheric clouds, where the rising currents had strength to tear them asunder. it followed that they were regions of increased heat--regions, in fact, where the temperature was too high to permit the occurrence of the precipitations to which the photosphere is due. their obscurity was attributed, as in dr. brester's more recent _théorie du soleil_, to deficiency of emissive power. yet here the verdict of the spectroscope is adverse and irreversible. after every deduction, however, has been made, we still find that several ideas of permanent value were embodied in this comprehensive sketch of the solar constitution. the principal of these were; first, that the sun is a mainly gaseous body; secondly, that its stores of heat are rendered available at the surface by means of vertical convection-currents--by the bodily transport, that is to say, of intensely hot matter upward, and of comparatively cool matter downward; thirdly, that the photosphere is a surface of condensation, forming the limit set by the cold of space to this circulating process, and that a similar formation must attend, at a certain stage, the cooling of every cosmical body. to warren de la rue belongs the honour of having obtained the earliest results of substantial value in celestial photography. what had been done previously was interesting in the way of promise, but much could not be claimed for it as actual performance. some "pioneering experiments" were made by dr. j. w. draper of new york in , resulting in the production of a few "moon-pictures" one inch in diameter;[ ] but slight encouragement was derived from them, either to himself or others. bond of cambridge (u.s.), however, secured in with the harvard -inch refractor that daguerreotype of the moon with which the career of extra-terrestrial photography may be said to have formally opened. it was shown in london at the great exhibition of , and determined the direction of de la rue's efforts. yet it did little more than prove the art to be a possible one. warren de la rue was born in guernsey in , and died in london april , . educated at the École sainte-barbe in paris, he made a large fortune as a paper manufacturer in england, and thus amply and early provided the material supplies for his scientific campaign. towards the end of he took some successful lunar photographs. they were remarkable as the first examples of the application to astronomical light-painting of the collodion process, invented by archer in ; and also of the use of reflectors (de la rue's was one of thirteen inches, constructed by himself) for that kind of work. the absence of a driving apparatus was, however, very sensibly felt; the difficulty of moving the instrument by hand so as accurately to follow the moon's apparent motion being such as to cause the discontinuance of the experiments until , when the want was supplied. de la rue's new observatory, built in that year at cranford, was expressly dedicated to celestial photography; and there he applied to the heavenly bodies the stereoscopic method of obtaining relief, and turned his attention to the delicate business of photographing the sun. a solar daguerreotype was taken at paris, april , ,[ ] by foucault and fizeau, acting on a suggestion from arago. but the attempt, though far from being unsuccessful, does not, at that time, seem to have been repeated. its great difficulty consisted in the enormous light-power of the object to be represented, rendering an inconceivably short period of exposure indispensable, under pain of getting completely "burnt-up" plates. in de la rue was commissioned by the royal society to construct an instrument specially adapted to the purpose for the kew observatory. the resulting "photoheliograph" may be described as a small telescope (of - / inches aperture and focus), with a plate-holder at the eye-end, guarded in front by a spring-slide, the rapid movement of which across the field of view secured for the sensitive plate a virtually instantaneous exposure. by its means the first solar light-pictures of real value were taken, and the autographic record of the solar condition recommended by sir john herschel was commenced and continued at kew during fourteen years-- - . the work of photographing the sun is now carried on in every quarter of the globe, from mauritius to massachusetts, and the days are few indeed on which the self-betrayal of the camera can be evaded by our chief luminary. in the year the incorporation of indian with greenwich pictures afforded a record of the state of the solar surface on days; and were similarly provided for in and . the conclusions arrived at by photographic means at kew were communicated to the royal society in a series of papers drawn up jointly by de la rue, balfour stewart, and benjamin loewy, in and subsequent years. they influenced materially the progress of thought on the subject they were concerned with. by its rotation the sun itself offers opportunities for bringing the stereoscope to bear upon it. two pictures, taken at an interval of twenty-six minutes, show just the amount of difference needed to give, by their combination, the maximum effect of solidity.[ ] de la rue thus obtained, in , a stereoscopic view of a sun-spot and surrounding faculæ, representing the various parts in their true mutual relations. "i have ascertained in this way," he wrote,[ ] "that the faculæ occupy the highest portions of the sun's photosphere, the spots appearing like holes in the penumbræ, which appeared lower than the regions surrounding them; in one case, parts of the faculæ were discovered to be sailing over a spot apparently at some considerable height above it." thus wilson's inference as to the depressed nature of spots received, after the lapse of not far from a century, proof of the most simple, direct, and convincing kind. a careful application of wilson's own geometrical test gave results only a trifle less decisive. of spots observed, per cent. showed, as they traversed the disc, the expected effects of perspective;[ ] and their absence in the remaining per cent. might be explained by internal commotions producing irregularities of structure. the absolute depth of spot-cavities--at least of their sloping sides--was determined by father secchi through measurement of the "parallax of profundity"[ ]--that is, of apparent displacements attendant on the sun's rotation, due to depression below the sun's surface. he found that in every case it fell short of , miles, and averaged not more than , , corresponding, on the terrestrial scale, to an excavation in the earth's crust of - / miles. of late, however, the reality of even this moderate amount of depression has been denied. mr. howlett's persevering observations, extending over a third of a century, the results of which were presented to the royal astronomical society in december, ,[ ] availed to shatter the consensus of opinion which had so long been maintained on the subject of spot-structure.[ ] it has become impossible any longer to hold that it is uniformly cavernous; and what seem like actually protruding umbræ are occasionally vouched for on unimpeachable authority.[ ] we can only infer that the forms of sun-spots are really more various than had been supposed; that they are peculiarly subject to disturbance; and that the level of the nuclei may rise and fall during the phases of commotion, like lavas within volcanic craters. the opinion of the kew observers as to the nature of such disturbances was strongly swayed by another curious result of the "statistical method" of inquiry. they found that of , instances of spots accompanied by faculæ, had those faculæ chiefly or entirely on the left, showed a nearly equal distribution, while only had faculous appendages mainly on the right side.[ ] now the rotation of the sun, as we see it, is performed from left to right; so that the marked tendency of the faculæ was a lagging one. this was easily accounted for by supposing the matter composing them to have been flung upwards from a considerable depth, whence it would reach the surface with the lesser _absolute_ velocity belonging to a smaller circle of revolution, and would consequently fall behind the cavities or "spots" formed by its abstraction. an attempt, it is true, made by m. wilsing at potsdam in [ ] to determine the solar rotation from photographs of faculæ had an outcome inconsistent with this view of their origin. they unexpectedly gave a uniform period. no trace of the retardation poleward from the equator, shown by the spots, could be detected in their movements. but the experiment was obviously inconclusive;[ ] and m. stratonoff's[ ] repetition of it with ampler materials gave a full assurance that faculæ rotate like spots in periods lengthening as latitude augments. the ideas of m. faye were, on two fundamental points, contradicated by the kew investigators. he held spots to be regions of _uprush_ and of heightened temperature; they believed their obscurity to be due to a _downrush_ of comparatively cool vapours. now m. chacornac, observing, at ville-urbanne, march , , saw floods of photospheric matter visibly precipitating themselves into the abyss opened by a great spot, and carrying with them small neighbouring maculæ.[ ] similar instances were repeatedly noted by father secchi, who considered the existence of a kind of _suction_ in spots to be quite beyond question.[ ] the tendency in their vicinity, to put it otherwise, is _centripetal_, not _centrifugal_; and this alone seems to negative the supposition of a central uprush. a fresh witness was by this time at hand. the application of the spectroscope to the direct examination of the sun's surface dates from march , , when sir norman lockyer (to give him his present title) undertook an inquiry into the cause of the darkening in spots.[ ] it was made possible by the simple device of throwing upon the slit of the spectroscope an _image_ of the sun, any part of which could be subjected to special scrutiny, instead of, as had hitherto been done, admitting rays from every portion of his surface indiscriminately. the answer to the inquiry was prompt and unmistakable, and was again, in this case, adverse to the french theorist's view. the obscurations in question were found to be produced by no deficiency of emissive power, but by an increase of absorptive action. the background of variegated light remains unchanged, but more of it is stopped by the interposition of a dense mass of relatively cool vapours. the spectrum of a sun-spot is crossed by the same set of multitudinous dark lines, with some minor differences, visible in the ordinary solar spectrum. we must then conclude that the same vapours (speaking generally) which are dispersed over the unbroken solar surface are accumulated in the umbral cavity, the compression incident to such accumulation being betrayed by the thickening of certain lines of absorption. but there is also a general absorption, extending almost continuously from one end of the spot-spectrum to the other. using, however, a spectroscope of exceptionally high dispersive power, professor young of princeton, new jersey, succeeded in in "resolving" the supposed continuous obscurity of spot-spectra into a countless multitude of fine dark lines set very close together.[ ] their structure was seen still more perfectly, about five years later, by m. dunér,[ ] director of the upsala observatory, who traced besides some shadowy vestiges of the crowded doublets and triplets forming the array, from the spots on to the general solar surface. they cease to be separable in the blue part of the spectrum; and the ultra-violet radiations of spots show nothing distinctive.[ ] as to the movements of the constipated vapours forming spots, the spectroscope is also competent to supply information. the principle of the method by which it is procured will be explained farther on. suffice it here to say that the transport, at any considerable velocity, to or from the eye of the gaseous material giving bright or dark lines, can be measured by the displacement of such lines from their previously known normal positions. in this way movements have been detected in or above spots of enormous rapidity, ranging up to _miles per second_. but the result, so far, has been to negative the ascription to them of any systematic direction. uprushes and downrushes are doubtless, as father cortie remarks,[ ] "correlated phenomena in the production of a sun-spot"; but neither seem to predominate as part of its regular internal economy. the same kind of spectroscopic evidence tells heavily against a theory of sun-spots started by faye in . he had been foremost in pointing out that the observations of carrington and spörer absolutely forbade the supposition that any phenomenon at all resembling our trade-winds exists in the sun. they showed, indeed, that beyond the parallels of ° there is a general tendency in spots to a slow poleward displacement, while within that zone they incline to approach the equator; but their "proper movements" gave no evidence of uniformly flowing currents in latitude. the systematic drift of the photosphere is strictly a drift in longitude; its direction is everywhere parallel to the equator. this fact being once clearly recognised, the "solar tornado" hypothesis at once fell to pieces; but m. faye[ ] perceived another source of vorticose motion in the unequal rotating velocities of contiguous portions of the photosphere. the "pores" with which the whole surface of the sun is studded he took to be the smaller eddies resulting from these inequalities; the spots to be such eddies developed into whirlpools. it only needs to thrust a stick into a stream to produce the kind of effect designated. and it happens that the differences of angular movement adverted to attain a maximum just about the latitudes where spots are most frequent and conspicuous. there are, however, grave difficulties in identifying the two kinds of phenomena. one (already mentioned) is the total absence of the regular swirling motion--in a direction contrary to that of the hands of a watch north of the solar equator, in the opposite sense south of it--which should impress itself upon every lineament of a sun-spot if the cause assigned were a primary producing, and not merely (as it possibly may be) a secondary determining one. the other, pointed out by young,[ ] is that the cause is inadequate to the effect. the difference of movement, or _relative drift_, supposed to occasion such prodigious disturbances, amounts, at the utmost, for two portions of the photosphere miles apart, to about five yards a minute. thus the friction of contiguous sections must be quite insignificant. a view better justified by observation was urged by secchi in and after the year , and was presented in an improved form by professor young in his excellent little book on _the sun_, published in .[ ] spots are manifestly associated with violent eruptive action, giving rise to the faculæ and prominences which usually garnish their borders. it is accordingly contended that upon the withdrawal of matter from below by the flinging up of a prominence must ensue a sinking-in of the surface, into which the partially cooled erupted vapours rush and settle, producing just the kind of darkening by increased absorption told of by the spectroscope. round the edges of the cavity the rupture of the photospheric shell will form lines of weakness provocative of further eruptions, which will, in their turn, deepen and enlarge the cavity. the phenomenon thus tends to perpetuate itself, until equilibrium is at last restored by internal processes. a sun-spot might then be described as an inverted terrestrial volcano, in which the outbursts of heated matter take place on the borders instead of at the centre of the crater, while the cooled products gather in the centre instead of at the borders. but on the earth, the solid crust forcibly represses the steam gathering beneath until it has accumulated strength for an explosion, while there is no such restraining power that we know of in the sun. zöllner, indeed, adapted his theory of the solar constitution to the special purpose of procuring it; yet with very partial success, since almost every new fact has proved adverse to his assumptions. volcanic action is essentially spasmodic. it implies habitual constraint varied by temporary outbreaks, inconceivable in a gaseous globe, such as we believe the sun to be. if the "volcanic hypothesis" represented the truth, no spot could possibly appear without a precedent eruption. the real order of the phenomenon, however, is exceedingly difficult to ascertain; nor is it perhaps invariable. although, in most cases, the "opening" shows first, that may be simply because it is more easily seen. according to father sidgreaves,[ ] the disturbance has then already passed the incipient stage. he considers it indeed "highly probable that the preparatory sign of a new spot is always a small, bright patch of facula." this sequence, if established, would be fatal to lockyer's theory of sun-spots, communicated to the royal society, may , ,[ ] and further developed some months later in his work on _the chemistry of the sun_. spots are represented in it as incidental to a vast system of solar atmospheric circulation, starting with the polar out- and up-flows indicated by observations during some total eclipses, and eventuating in the plunge downward from great heights upon the photosphere of prodigious masses of condensed materials. from these falls result, primarily, spots; secondarily, through the answering uprushes in which chemical and mechanical forces co-operate, their girdles of flame-prominences. the evidence is, however, slight that such a circulatory flow as would be needed to maintain this supposed cycle of occurrences really prevails in the sun's atmosphere; and a similar objection applies to an "anticyclonic theory" (so to designate it) elaborated by egon von oppolzer in .[ ] august schmidt's optical rationale of solar phenomena[ ] was, on the other hand, a complete novelty, both in principle and development. attractive to speculators from its recondite nature and far-reaching scope, it by no means commended itself to practical observers, intolerant of finding the all but palpable realities of their daily experience dealt with as illusory products of "circular refraction." a singular circumstance has now to be recounted. on the st of september, , while carrington was engaged in his daily work of measuring the positions of sun-spots, he was startled by the sudden appearance of two patches of peculiarly intense light within the area of the largest group visible. his first idea was that a ray of unmitigated sunshine had penetrated the screen employed to reduce the brilliancy of the image; but, having quickly convinced himself to the contrary, he ran to summon an additional witness of an unmistakably remarkable occurrence. on his return he was disappointed to find the strange luminous outburst already on the wane; shortly afterwards the last trace vanished. its entire duration was five minutes--from . to . a.m., greenwich time; and during those five minutes it had traversed a space estimated at , miles! no perceptible change took place in the details of the group of spots visited by this transitory conflagration, which, it was accordingly inferred, took place at a considerable height above it.[ ] carrington's account was precisely confirmed by an observation made at highgate. mr. r. hodgson described the appearance seen by him as that "of a very brilliant star of light, much brighter than the sun's surface, most dazzling to the protected eye, illuminating the upper edges of the adjacent spots and streaks, not unlike in effect the edging of the clouds at sunset."[ ] this unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. from the th of august to the th of september, , a magnetic storm of unparalleled intensity, extent, and duration, was in progress over the entire globe. telegraphic communication was everywhere interrupted--except, indeed, that it was, in some cases, found practicable to work the lines _without batteries_, by the agency of the earth-currents alone:[ ] sparks issued from the wires; gorgeous auroræ draped the skies in solemn crimson over both hemispheres, and even within the tropics; the magnetic needle lost all trace of continuity in its movements, and darted to and fro as if stricken with inexplicable panic. the coincidence was drawn even closer. _at the very instant_[ ] of the solar outburst witnessed by carrington and hodgson, the photographic apparatus at kew registered a marked disturbance of all the three magnetic elements; while, shortly after the ensuing midnight, the electric agitation culminated, thrilling the earth with subtle vibrations, and lighting up the atmosphere from pole to pole with the coruscating splendours which, perhaps, dimly recall the times when our ancient planet itself shone as a star. here then, at least, the sun was--in professor balfour stewart's phrase--"taken in the act"[ ] of stirring up terrestrial commotions. nor have instances since been wanting of an indubitable connection between outbreaks of individual spots and magnetic disturbances. four such were registered in ; and symptoms of the same kind, including the beautiful "rose aurora," marked the progress across the sun of the enormous spot-group of february, --the largest ever recorded at greenwich. this extraordinary formation, which covered about / of the sun's disc, survived through five complete rotations.[ ] it was remarkable for a persistent drift in latitude, its place altering progressively from ° to ° south of the solar equator. again, the central passage of an enormous spot on september , , synchronised with a sharp magnetic disturbance and brilliant aurora;[ ] and the coincidence was substantially repeated in march, ,[ ] when it was emphasised by the prevalent cosmic calm. the theory of the connection is indeed far from clear. lord kelvin, in ,[ ] pronounced against the possibility of any direct magnetic action by the sun upon the earth, on the ground of its involving an extravagant output of energy; but the fact is unquestionable that--in professor bigelow's words--"abnormal agitations affect the sun and the earth as a whole and at the same time."[ ] the nearer approach to the event of september , , was photographically observed by professor george e. hale at chicago, july , .[ ] an active spot in the southern hemisphere was the scene of this curiously sudden manifestation. during an interval of m. between two successive exposures, a bridge of dazzling light was found to have spanned the boundary-line dividing the twin-nuclei of the spot; and these, after another m., were themselves almost obliterated by an overflow of far-spreading brilliancy. yet two hours later, no trace of the outburst remained, the spot and its attendant faculæ remaining just as they had been previously to its occurrence. unlike that seen by carrington, it was accompanied by no exceptional magnetic phenomena, although a "storm" set in next day.[ ] possibly a terrestrial analogue to the former might be discovered in the "auroral beam" which traversed the heavens during a vivid display of polar lights, november , , and shared, there is every reason to believe, their electrical origin and character.[ ] meantime m. rudolf wolf, transferred to the direction of the zürich observatory, where he died, december , , had relaxed none of his zeal in the investigation of sun-spot periodicity. a laborious revision of the entire subject with the aid of fresh materials led him, in ,[ ] to the conclusion that while the _mean_ period differed little from that arrived at in of . years, very considerable fluctuations on either side of that mean were rather the rule than the exception. indeed, the phrase "sun-spot period" must be understood as fitting very loosely the great fact it is taken to represent; so loosely, that the interval between two maxima may rise to sixteen and a half or sink below seven and a half years.[ ] in [ ] wolf showed, and the remark was fully confirmed at kew, that the shortest periods brought the most acute crises, and _vice versâ_; as if for each wave of disturbance a strictly equal amount of energy were available, which might spend itself lavishly and rapidly, or slowly and parsimoniously, but could in no case be exceeded. the further inclusion of recurring solar commotions within a cycle of fifty-five and a half years was simultaneously pointed out; and hermann fritz showed soon afterwards that the aurora borealis is subject to an identical double periodicity.[ ] the same inquirer has more recently detected both for auroræ and sun-spots a "secular period" of years,[ ] and the kew observations indicate for the latter, oscillations accomplished within twenty-six and twenty-four days,[ ] depending, most likely, upon the rotation of the sun. this is certainly reflected in magnetic, and perhaps in auroral periodicity. the more closely, in fact, spot-fluctuations are looked into, the more complex they prove. maxima of one order are superposed upon, or in part neutralised by, maxima of another order;[ ] originating causes are masked by modifying causes; the larger waves of the commotion are indented with minor undulations, and these again crisped with tiny ripples, while the whole rises and falls with the swell of the great secular wave, scarcely perceptible in its progress because so vast in scale. the idea that solar maculation depends in some way upon the position of the planets occurred to galileo in .[ ] it has been industriously sifted by a whole bevy of modern solar physicists. wolf in [ ] found reason to believe that the eleven-year curve is determined by the action of jupiter, modified by that of saturn, and diversified by influences proceeding from the earth and venus. its tempting approach to agreement with jupiter's period of revolution round the sun, indeed, irresistibly suggested a causal connection; yet it does not seem that the most skilful "coaxing" of figures can bring about a fundamental harmony. carrington pointed out in , that while, during _eight successive periods_, from downwards, there were approximate coincidences between jupiter's aphelion passages and sun-spot maxima, the relation had been almost exactly reversed in the two periods preceding that date;[ ] and wolf himself finally concluded that the jovian origin must be abandoned.[ ] m. duponchel's[ ] prediction, nevertheless, of an abnormal retardation of the maximum due in through certain peculiarities in the positions of uranus and neptune about the time it fell due, was partially verified by the event, since, after an abortive phase of agitation in april, , the final outburst was postponed to january, . the interval was thus . instead of . years; and it is noticeable that the delay affected chiefly the southern hemisphere. alternations of activity in the solar hemispheres were indeed a marked feature of the maximum of , which, in m. faye's view,[ ] derived thence its indecisive character, while sharp, strong crises arise with the simultaneous advance of agitation north and south of the solar equator. the curve of magnetic disturbance followed with its usual strict fidelity the anomalous fluctuations of the sun-spot curve. the ensuing minimum occurred early in , and was succeeded in by a maximum slightly less feeble than its predecessor.[ ] it cannot be said that much progress has been made towards the disclosure of the cause, or causes, of the sun-spot cycle. no external influence adequate to the effect has, at any rate, yet been pointed out. most thinkers on this difficult subject provide a quasi-explanation of the periodicity in question through certain assumed vicissitudes affecting internal processes;[ ] sir norman lockyer and e. von oppolzer reach the same end by establishing self-compensatory fluctuations in the solar atmospheric circulation; dr. schuster resorts to changes in the electrical conductivity of space near the sun.[ ] in all these theories, however, the course of transition is arbitrarily arranged to suit a period, which imposes itself as a fact peremptorily claiming admittance, while obstinately defying explanation. the question so much discussed, as to the influence of sun-spots on weather, does not admit of a satisfactory answer. the facts of meteorology are too complex for easy or certain classification. effects owning dependence on one cause often wear the livery of another; the meaning of observed particulars may be inverted by situation; and yet it is only by the collection and collocation of particulars that we can hope to reach any general law. there is, however, a good deal of evidence to support the opinion--the grounds for which were primarily derived from the labours of dr. meldrum at mauritius--that increased rainfall and atmospheric agitation attend spot-maxima; while herschel's conjecture of a more copious emission of light and heat about the same epochs has recently obtained some countenance from savélieff's measures showing a gain in the strength of the sun's radiation _pari passu_ with increase in the number of spots visible on his surface.[ ] the examination of what we may call the _texture_ of the sun's surface derived new interest from a remarkable announcement made by mr. james nasmyth in .[ ] he had made (as he supposed) the discovery that the entire luminous stratum of the sun is composed of a multitude of elongated shining objects on a darker background, shaped much like willow-leaves, of vast size, crossing each other in all possible directions, and possessed of unceasing relative motions. a lively controversy ensued. in england and abroad the most powerful telescopes were directed to a scrutiny encompassed with varied difficulties. mr. dawes was especially emphatic in declaring that nasmyth's "willow-leaves" were nothing more than the "nodules" of sir william herschel seen under a misleading aspect of uniformity; and there is little doubt that he was right. it is, nevertheless, admitted that something of the kind may be seen in the penumbræ and "bridges" of spots, presenting an appearance compared by dawes himself in to that of a piece of coarse straw-thatching left untrimmed at the edges.[ ] the term "granulated," suggested by dawes in ,[ ] best describes the mottled aspect of the solar disc as shown by modern telescopes and cameras. the grains, or rather the "floccules," with which it is thickly strewn, have been resolved by langley, under exceptionally favourable conditions, into "granules" not above miles in diameter; and from these relatively minute elements, composing, jointly, about one-fifth of the visible photosphere,[ ] he estimates that three-quarters of the entire light of the sun are derived.[ ] janssen agrees, so far as to say that if the whole surface were as bright as its brightest parts, its luminous emission would be ten to twenty times greater than it actually is.[ ] the rapid changes in the forms of these solar cloud-summits are beautifully shown in the marvellous photographs taken by janssen at meudon, with exposures reduced at times to / of a second! by their means, also, the curious phenomenon known as the _réseau photosphérique_ has been made evident.[ ] this consists in the diffusion over the entire disc of fleeting blurred patches, separated by a reticulation of sharply-outlined and regularly-arranged granules. the imperfect definition in the smudged areas may be due to agitations in the solar or terrestrial atmosphere, unless it be--as dr. schemer thinks possible[ ]--merely a photographic effect. m. janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period;[ ] but this is as yet uncertain. the "grains," or more brilliant parts of the photosphere, are now generally held to represent the upper termination of ascending and condensing currents, while the darker interstices (herschel's "pores") mark the positions of descending cooler ones. in the penumbræ of spots, the glowing streams rushing up from the tremendous sub-solar furnace are bent sideways by the powerful indraught, so as to change their vertical for a nearly horizontal motion, and are thus taken, as it were, in flank by the eye, instead of being seen end-on in mamelon-form. this gives a plausible explanation of the channelled structure of penumbræ which suggested the comparison to a rude thatch. accepting this theory as in the main correct, we perceive that the very same circulatory process which, in its spasms of activity, gives rise to spots, produces in its regular course the singular "marbled" appearance, for the recording of which we are no longer at the mercy of the fugitive or delusive impressions of the human retina. and precisely this circulatory process it is which gives to our great luminary its permance as a _sun_, or warming and illuminating body. footnotes: [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxi., p. .] [footnote : _le soleil_, t. i., pp. - ( nd ed., ).] [footnote : see _ante_, p. .] [footnote : _observations at redhill ( )_, introduction.] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _cape observations_, p. , _note_.] [footnote : _month. not._, vol. x., p. .] [footnote : _rosa ursina_, lib. iii., p. .] [footnote : _observations at redhill_, p. .] [footnote : _op._, t. iii., p. .] [footnote : _rosa ursina_, lib. iv., p. . both galileo and scheiner spoke of the _apparent_ or "synodical" period, which is about one and a third days longer than the _true_ or "sidereal" one. the difference is caused by the revolution of the earth in its orbit in the same direction with the sun's rotation on its axis.] [footnote : _rosa ursina_, lib. iii., p. .] [footnote : faye, _comptes rendus_, t. lx., p. .] [footnote : _ibid._, t. xii., p. .] [footnote : _proc. am. ass. adv. of science_, , p. .] [footnote : _observations at redhill_, p. .] [footnote : _am. jour. of science_, vol. xi., p. .] [footnote : _month. not._, vol. xix., p. .] [footnote : _vierteljahrsschrift der naturfors. gesellschaft_ (zürich), , p. .] [footnote : lockyer, _chemistry of the sun_, p. .] [footnote : maunder, _knowledge_, vol. xv., p. .] [footnote : _month. mon._, vol. l., p. .] [footnote : maunder, _knowledge_, vol. xvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : as late as an elaborate treatise in its support was written by f. coyteux, entitled _qu'est-ce que le soleil? peut-il être habité?_ and answering the question in the affirmative.] [footnote : the subsequent researches of plücker, frankland, wüllner, and others, showed that gases strongly compressed give an absolutely unbroken spectrum.] [footnote : _comptes rendus_, t. lx., pp. , .] [footnote : _ibid._, t. c., p. .] [footnote : _bull. meteor. dell osservatorio dell coll. rom._, jan. , , p. .] [footnote : _quart. jour. of science_, vol. i., p. .] [footnote : _ann. de chim. et de phys._, t. xxii., p. .] [footnote : _phil. trans._, vol. clix., p. .] [footnote : _les mondes_, dec. , , p. .] [footnote : _comptes rendus_, t. lx., p. .] [footnote : _proc. roy. society_, vol. xvi., p. .] [footnote : _recherches sur le spectre solaire_, p. .] [footnote : _am. jour. of science_, , vol. xxi., p. . hastings stipulated only for some member of the triad, carbon, silicon, and boron.] [footnote : ranyard, _knowledge_, vol. xvi., p. .] [footnote : young, _the sun_, p. , ed. .] [footnote : h. draper, _quart. journ. of sc._, vol. i., p. ; also _phil. mag._, vol. xvii., , p. .] [footnote : reproduced in arago's _popular astronomy_, plate xii., vol. .] [footnote : _report brit. ass._, , p. .] [footnote : _phil. trans._, vol. clii., p. .] [footnote : _researches in solar physics_, part i., p. .] [footnote : both the phrase and the method were suggested by faye, who estimated the average depth of the luminous sheath of spots at , miles. _comptes rendus_, t. lxi., p. ; t. xcvi., p. .] [footnote : _month. not._, vol. lv., p. .] [footnote : sidgreaves, _ibid._, p. ; cortie, _ibid._, vol. lviii., p. .] [footnote : explained by east as refraction-effects. _jour. brit. astr. ass._, vol. viii., p. .] [footnote : _proc. roy. soc._, vol. xiv., p. .] [footnote : _potsdam publicationen_, no. ; _astr. nach._, nos. , , , .] [footnote : faye, _comptes rendus_, t. cxi., p. ; bélopolsky, _astr. nach._, no. , .] [footnote : _ibid._, nos. , , , .] [footnote : lockyer, _contributions to solar physics_, p. .] [footnote : _le soleil_, p. .] [footnote : _proc. roy. soc._, vol. xv., p. .] [footnote : _phil. mag._, vol. xvi., p. .] [footnote : _recherches sur la rotation du soleil_, p. .] [footnote : hale, _astr. and astrophysics_, vol. xi., p. .] [footnote : _jour. brit. astr. ass._, vol. i., p. .] [footnote : _comptes rendus_, t. lxxv., p. ; _revue scientifique_, t. v., p. ( ). mr. herbert spencer had already (in _the reader_, feb. , ) put forward an opinion that spots were of the nature of "cyclonic clouds."] [footnote : _the sun_, p. . for faye's answer to the objection, see _comptes rendus_, t. xcv., p. .] [footnote : a revised edition appeared in .] [footnote : _astr. and astrophysics_, vol. xii., p. .] [footnote : _proc. roy. soc._, no. .] [footnote : _astr. nach._, no. , ; _astr. and astrophysics_, vol. xii., pp. , .] [footnote : _sirius_, sept., ; _ibid._, vol. xxiii., p. ; _astrophy. jour._, vol. i., p. (wilczynski), p. (keeler); vol. ii., p. (hale).] [footnote : _month. not._, vol. xx., p. .] [footnote : _ibid._, p. .] [footnote : _am. jour._, vol. xxix. ( nd series), pp. , .] [footnote : the magnetic disturbance took place at . a.m., three minutes before the solar blaze compelled the attention of carrington.] [footnote : _phil. trans._, vol. cli., p. .] [footnote : maunder, _journal brit. astr. ass._, vol. ii., p. ; miss e. brown, _ibid._, p. ; month. not., vol. lii., p. .] [footnote : _observatory_, vol. xxi., p. ; maunder, _knowledge_, vol. xxi., p. ; fényi, _astroph. jour._, vol. x., p. .] [footnote : _ibid._, p. ; w. anderson, observatory, vol. xxii., p. .] [footnote : _proc. roy. society_, vol. lii., p. ; rev. w. sidgreaves, _mem. r. a. s._, vol. liv., p. .] [footnote : _report on solar and terrestrial magnetism_, washington, , p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, p. (sidgreaves).] [footnote : see j. rand capron, _phil. mag._, vol. xv., p. .] [footnote : _mittheilungen über die sonnenflecken_, no. ix., _vierteljahrsschrift der naturforschenden gesellschaft in zürich_, jahrgang .] [footnote : _mitth._, no. lii., p. ( ).] [footnote : _ibid._, no. xii., p. . baxendell, of manchester, reached independently a similar conclusion. see _month. not._, vol. xxi., p. .] [footnote : wolf, _mitth._, no. xv., p. , etc. olmsted, following hansteen, had already, in , sought to establish an auroral period of sixty-five years. _smithsonian contributions_, vol. viii., p. .] [footnote : hahn, _ueber die reziehungen der sonnenfleckenperiode zu meteorologischen erscheinungen_, p. ( ).] [footnote : _report brit. ass._, , p. ; , p. .] [footnote : the rev. a. cortie (_month. not._, vol. lx., p. ) detects the influence of a short subsidiary cycle, dr. w. j. s. lockyer that of a thirty-five year period (_nature_, june , ). professor newcomb (_astroph. jour._, vol. xiii., p. ) considers that solar activity oscillates uniformly in . years, with superposed periodic variations.] [footnote : _opere_, t. iii., p. .] [footnote : _mitth._, nos. vii. and xviii.] [footnote : _observations at redhill_, p. .] [footnote : _comptes rendus_, t. xcv., p. .] [footnote : _ibid._, t. xciii., p. ; t. xcvi., p. .] [footnote : _ibid._, t. c, p. .] [footnote : ellis, _proc. roy. society_, vol. lxiii., p. .] [footnote : schultz, _astr. nach._, nos. , - , , - ; wilsing, _ibid._, no. , ; bélopolsky, _ibid._, no. , .] [footnote : _report brit. ass._, , p. .] [footnote : a. w. augur, _astroph. jour._, vol. xiii., p. .] [footnote : _report brit. ass._, , p. (pt. ii.).] [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _am. jour. of science_, vol. vii., , p. .] [footnote : young, _the sun_, p. .] [footnote : _ann. bur. long._, , p. .] [footnote : _ibid._, , p. .] [footnote : _himmelsphotographie_, p. .] [footnote : ranyard, _knowledge_, vols. xiv., p. , xvi., p. ; see also the accompanying photographs.] chapter iii _recent solar eclipses_ by observations made during a series of five remarkable eclipses, comprised within a period of eleven years, knowledge of the solar surroundings was advanced nearly to its present stage. each of these events brought with it a fresh disclosure of a definite and unmistakable character. we will now briefly review this orderly sequence of discovery. photography was first systematically applied to solve the problems presented by the eclipsed sun, july , . it is true that a daguerreotype,[ ] taken by berkowski with the königsberg heliometer during the eclipse of , is still valuable as a record of the corona of that year; and some subsequent attempts were made to register partial phases of solar occultation, notably by professor bartlett at west point in ;[ ] but the ground remained practically unbroken until . in that year the track of totality crossed spain, and thither, accordingly, warren de la rue transported his photo-heliograph, and father secchi his six-inch cauchoix refractor. the question then primarily at issue was that relating to the nature of the red protuberances. although, as already stated, the evidence collected in gave a reasonable certainty of their connection with the sun, objectors were not silenced; and when the side of incredulity was supported by so considerable an authority as m. faye, it was impossible to treat it with contempt. two crucial tests were available. if it could be shown that the fantastic shapes suspended above the edge of the dark moon were seen under an identical aspect from two distant stations, that fact alone would annihilate the theory of optical illusion or "mirage"; while the certainty that they were progressively concealed by the advancing moon on one side, and uncovered on the other, would effectually detach them from dependence on our satellite, and establish them as solar appendages. now both these tests were eminently capable of being applied by photography. but the difficulty arose that nothing was known as to the chemical power of the rosy prominence-light, while everything depended on its right estimation. a shot had to be fired, as it were, in the dark. it was a matter of some surprise, and of no small congratulation, that, in both cases, the shot took effect. de la rue occupied a station at rivabellosa, in the upper ebro valley; secchi set up his instrument at desierto de las palmas, about miles to the south-east, overlooking the mediterranean. from the totally eclipsed sun, with its strange garland of flames, each observer derived several perfectly successful impressions, which were found, on comparison, to agree in the most minute details. this at once settled the fundamental question as to the substantial reality of these objects; while their solar character was demonstrated by the passage of the moon _in front_ of them, indisputably attested by pictures taken at successive stages of the eclipse. that forms seeming to defy all laws of equilibrium were, nevertheless, not wholly evanescent, appeared from their identity at an interval of seven minutes, during which the lunar shadow was in transit from one station to the other; and the singular energy of their actinic rays was shown by the record on the sensitive plates of some prominences invisible in the telescope. moreover, photographic evidence strongly confirmed the inference--previously drawn by grant and others, and now with fuller assurance by secchi--that an uninterrupted stratum of prominence-matter encompasses the sun on all sides, forming a reservoir from which gigantic jets issue, and into which they subside. thus, first-fruits of accurate knowledge regarding the solar surroundings were gathered, while the value of the brief moments of eclipse gained indefinite increase, by supplementing transient visual impressions with the faithful and lasting records of the camera. in the year the history of eclipse spectroscopy virtually began, as that of eclipse photography in ; that is to say, the respective methods then first gave definite results. on the th of august, , the indian and malayan peninsulas were traversed by a lunar shadow producing total obscuration during five minutes and thirty-eight seconds. two english and two french expeditions were despatched to the distant regions favoured by an event so propitious to the advance of knowledge, chiefly to obtain the verdict of the prism as to the composition of prominences. nor were they despatched in vain. an identical discovery was made by nearly all the observers. at jamkandi, in the western ghauts, where lieutenant (now colonel) herschel was posted, unremitting bad weather threatened to baffle his eager expectations; but during the lapse of the critical five and a half minutes the clouds broke, and across the driving wrack a "long, finger-like projection" jutted out over the margin of the dark lunar globe. in another moment the spectroscope was pointed towards it; three bright lines--red, orange, and blue--flashed out, and the problem was solved.[ ] the problem was solved in this general sense, that the composition out of glowing vapours of the objects infelicitously termed "protuberances" or "prominences" was no longer doubtful; although further inquiry was needed for the determination of the particular species to which those vapours belonged. similar, but more complete observations were made, with less atmospheric hindrance, by tennant and janssen at guntoor, by pogson at masulipatam, and by rayet at wha-tonne, on the coast of the malay peninsula, the last observer counting as many as nine bright lines.[ ] among them it was not difficult to recognise the characteristic light of hydrogen; and it was generally, though over-hastily, assumed that the orange ray matched the luminous emissions of sodium. but fuller opportunities were at hand. the eclipse of is chiefly memorable for having taught astronomers to do without eclipses, so far, at least, as one particular branch of solar inquiry is concerned. inspired by the beauty and brilliancy of the variously tinted prominence-lines revealed to him by the spectroscope, janssen exclaimed to those about him, "je verrai ces lignes-là en dehors des éclipses!" on the following morning he carried into execution the plan which formed itself in his brain while the phenomenon which suggested it was still before his eyes. it rests upon an easily intelligible principle. the glare of our own atmosphere alone hides the appendages of the sun from our daily view. to a spectator on an airless planet, the central globe would appear attended by all its splendid retinue of crimson prominences, silvery corona, and far-spreading zodiacal light projected on the star-spangled black background of an absolutely unilluminated sky. now the spectroscope offers the means of indefinitely weakening atmospheric glare by diffusing a constant amount of it over an area widened _ad libitum_. but monochromatic or "bright-line" light is, by its nature, incapable of being so diffused. it can, of course, be _deviated_ by refraction to any extent desired; but it always remains equally concentrated, in whatever direction it may be thrown. hence, when it is mixed up with continuous light--as in the case of the solar flames shining through our atmosphere--it derives a _relative_ gain in intensity from every addition to the dispersive power of the spectroscope with which the heterogeneous mass of beams is analysed. employ prisms enough, and eventually the undiminished rays of persistent colour will stand out from the continually fading rainbow-tinted band, by which they were at first effectually veiled. this janssen saw by a flash of intuition while the eclipse was in progress; and this he realised at a.m. next morning, august , --the date of the beginning of spectroscopic work at the margin of the unobscured sun. during the whole of that day and many subsequent ones, he enjoyed, as he said, the advantage of a prolonged eclipse. the intense interest with which he surveyed the region suddenly laid bare to his scrutiny was heightened by evidences of rapid and violent change. on the th of august, during the eclipse, a vast spiral structure, _at least_ , miles high, was perceived, planted in surprising splendour on the rim of the interposed moon. if was formed as general tennant judged from its appearance in his photographs, by the encounter of two mounting torrents of flame, and was distinguished as the "great horn." next day it was in ruins; hardly a trace remained to show where it had been.[ ] janssen's spectroscope furnished him besides with the strongest confirmation of what had already been reported by the telescope and the camera as to the continuous nature of the scarlet "sierra" lying at the base of the prominences. everywhere at the sun's edge the same bright lines appeared. it was not until the th of september that janssen thought fit to send news of his discovery to europe. it seemed little likely to be anticipated; yet a few minutes before his despatch was handed to the secretary of the paris academy of sciences, a communication similar in purport had been received from sir norman lockyer. there is no need to discuss the narrow and wearisome question of priority; each of the competitors deserves, and has obtained, full credit for his invention. with noteworthy and confident prescience, lockyer, in , before anything was yet known regarding the constitution of the "red flames," ordered a strongly dispersive spectroscope for the express purpose of viewing, apart from eclipses, the bright-line spectrum which he expected them to give. various delays, however, supervened, and the instrument was not in his hands until october , . on the th he picked up the vivid rays, of which the presence and (approximately) the positions had in the interim become known. but there is little doubt that, even without that previous knowledge, they would have been found; and that the eclipse of august only accelerated a discovery already assured. sir william huggins, meanwhile, had been tending towards the same goal during two and a half years in his observatory at tulse hill. the principle of the spectroscopic visibility of prominence-lines at the edge of an uneclipsed sun was quite explicitly stated by him in february, ,[ ] and he devised various apparatus for bringing them into actual view; but not until he knew where to look did he succeed in seeing them. astronomers, thus liberated, by the acquisition of power to survey them at any time, from the necessity of studying prominences during eclipses, were able to concentrate the whole of their attention on the corona. the first thing to be done was to ascertain the character of its spectrum. this was seen in only as a faintly continuous one; for rayet, who seems to have perceived its distinctive bright line far above the summits of the flames, connected it, nevertheless, with those objects. on the other hand, lieutenant campbell ascertained on the same occasion the polarisation of the coronal light in planes passing through the sun's centre,[ ] thereby showing that light to be, in whole or in part, reflected sunshine. but if reflected sunshine, it was objected, the chief at least of the dark fraunhofer lines should be visible in it, as they are visible in moonbeams, sky illumination, and all other sun-derived light. the objection was well founded, but was prematurely urged, as we shall see. on the th of august, , a track of total eclipse crossed the continent of north america diagonally, entering at behring's straits, and issuing on the coast of north carolina. it was beset with observers; but the most effective work was done in iowa. at des moines, professor harkness of the naval observatory, washington, obtained from the corona an "absolutely continuous spectrum," slightly less bright than that of the full moon, but traversed by a single green ray.[ ] the same green ray was seen at burlington and its position measured by professor young of dartmouth college.[ ] it appeared to coincide with that of a dark line of iron in the solar spectrum, numbered , on kirchhoff's scale. but in young was able, by the use of greatly increased dispersion, to resolve the fraunhofer line " " into a pair, the more refrangible member of which he considered to be the reversal of the green coronal ray.[ ] scarcely called in question for over twenty years, the identification nevertheless broke down through the testimony of the eclipse-photographs of . sir norman lockyer derived from them a position for the line in question notably higher up in the spectrum than that previously assigned to it. instead of , , its true wave-length proved to be , ten millionths of a millimetre;[ ] nor does it make any show by absorption in dispersed sunlight. the originating substance, designated "coronium," of which nothing is known to terrestrial chemistry, continues luminous[ ] at least , miles above the sun's surface, and is hence presumably much lighter even than hydrogen. a further trophy was carried off by american skill[ ] sixteen months after the determination due to it of the distinctive spectrum of the corona. the eclipse of december , , though lasting only two minutes and ten seconds, drew observers from the new, as well as from the old world to the shores of the mediterranean. janssen issued from beleaguered paris in a balloon, carrying with him the _vital parts_ of a reflector specially constructed to collect evidence about the corona. but he reached oran only to find himself shut behind a cloud-curtain more impervious than the prussian lines. everywhere the sky was more or less overcast. lockyer's journey from england to sicily, and shipwreck in the _psyche_, were recompensed with a glimpse of the solar aureola during _one second and a half_! three parties stationed at various heights on mount etna saw absolutely nothing. nevertheless important information was snatched in despite of the elements. the prominent event was young's discovery of the "reversing layer." as the surviving solar crescent narrowed before the encroaching moon, "the dark lines of the spectrum," he tells us, "and the spectrum itself, gradually faded away, until all at once, as suddenly as a bursting rocket shoots out its stars, the whole field of view was filled with bright lines more numerous than one could count. the phenomenon was so sudden, so unexpected, and so wonderfully beautiful, as to force an involuntary exclamation."[ ] its duration was about two seconds, and the impression produced was that of a complete reversal of the fraunhofer spectrum--that is, the substitution of a bright for every dark line. now something of the kind was theoretically necessary to account for the dusky rays in sunlight which have taught us so much, and have yet much more to teach us; so that, although surprising from its transitory splendour, the appearance could not strictly be called "unexpected." moreover, its premonitory symptom in the fading out of these rays had been actually described by secchi in ,[ ] and looked for by young as the moon covered the sun in august . but with the slit of his spectroscope placed _normally_ to the sun's limb, the bright lines gave a flash too thin to catch the eye. in the position of the slit was _tangential_--it ran along the shallow bed of incandescent vapours, instead of cutting across it: hence his success. the same observation was made at xerez de la frontera by mr. pye, a member of young's party; and, although an exceedingly delicate one, has since frequently been repeated. the whole fraunhofer series appeared bright (omitting other instances) to maclear, herschel, and fyers in , at the beginning or end of totality; to pogson, at the break-up of an annual eclipse, june , ; to stone at klipfontein, april , , when he saw "the field full of bright lines."[ ] but between the picture presented by the "véritable pluie de lignes brilliantes,"[ ] which descended into m. trépied's spectroscope for three seconds after the disappearance of the sun, may , , and the familiar one of the dark-line solar spectrum, certain differences were perceiving, showing their relation to be not simply that of a positive to a negative impression. a "reversing layer," or stratum of mixed vapours, glowing, but at a lower temperature than that of the actual solar surface, was an integral part of kirchhoff's theory of the production of the fraunhofer lines. here it was assumed that the missing rays were stopped, and here also it was assumed that the missing rays would be seen bright, could they be isolated from the overpowering splendour of their background. this isolation is effected by eclipses, with the result--beautifully confirmatory of theory--of _reversing_, or turning from dark to bright, the fraunhofer spectrum. the completeness and precision of the reversal, however, could not be visually attested; and a quarter of a century elapsed before a successful "snap-shot" provided photographic evidence on the subject. it was taken at novaya zemlya by mr. shackleton, a member of the late sir george baden-powell's expedition to observe the eclipse of august , ;[ ] and similar records in abundance were secured during the indian eclipse of january , ,[ ] and the spanish-american eclipse of may , .[ ] the result of their leisurely examination has been to verify the existence of a "reversing-layer," in the literal sense of the term. it is true that no single "flash" photograph is an inverted transcript of the fraunhofer spectrum. the lines are, indeed, in each case--speaking broadly--the same; but their relative intensities are widely different. yet this need occasion no surprise when we remember that the fraunhofer spectrum integrates the absorption of multitudinous strata, various in density and composition, while only the upper section of the formation comes within view of the sensitive plates exposed at totalities, the low-lying vaporous beds being necessarily covered by the moon. the total depth of this glowing envelope may be estimated at to miles, and its normal state seems to be one of profound tranquillity, judging from the imperturbable aspect of the array of dark lines due to its sifting action upon light. the last of the five eclipses which we have grouped together for separate consideration was visible in southern india and australia, december , . some splendid photographs were secured by the english parties on the malabar coast, showing, for the first time, the remarkable branching forms of the coronal emanations; but the most conspicuous result was janssen's detection of some of the dark fraunhofer lines, long vainly sought in the continuous spectrum of the corona. chief among these was the d-line of sodium, the original index, it might be said, to solar chemistry. no proof could be afforded more decisive that this faint _echoing back_ of the distinctive notes of the fraunhofer spectrum, that the polariscope had spoken the truth in asserting a large part of the coronal radiance to be reflected sunlight. but it is usually so drenched in original luminosity, that its special features are almost obliterated. janssen's success in seizing them was due in part to the extreme purity of the air at sholoor, in the neilgherries, where he was stationed; in part to the use of an instrument adapted by its large aperture and short focus to give an image of the utmost brilliancy. his observation, repeated during the caroline island eclipse of , was photographically verified ten years later by m. de la baume pluvinel in senegal.[ ] an instrument of great value for particular purposes was introduced into eclipse-work in . the "slitless spectroscope" consists simply of a prism placed outside the object-glass of a telescope or the lens of a camera, whereby the radiance encompassing the eclipsed sun is separated into as many differently tinted rings as it contains different kinds of light. these tinted rings were simultaneously viewed by respighi at poodacottah, and by lockyer at baikul. their photographic registration by the latter in initiated the transformation of the slitless spectroscope into the prismatic camera.[ ] meanwhile, the use of an ordinary spectroscope by herschel and tennant at dodabetta showed the green ray of coronium to be just as bright in a rift as in the adjacent streamer. the visible structure of the corona was thus seen to be independent of the distribution of the gases which enter into its composition. by means, then, of the five great eclipses of - it was ascertained: first, that the prominences, and at least the lower part of the corona, are genuine solar appurtenances; secondly, that the prominences are composed of hydrogen and other gases in a state of incandescence, and rise, as irregular outliers, from a continuous envelope of the same materials, some thousands of miles in thickness; thirdly, that the corona is of a highly complex constitution, being made up in part of glowing vapours, in part of matter capable of reflecting sunlight. we may now proceed to consider the results of subsequent eclipses. these have raised, and have helped to solve, some very curious questions. indeed, every carefully watched total eclipse of the sun stimulates as well as appeases curiosity, and leaves a legacy of outstanding doubt, continually, as time and inquiry go on, removed, but continually replaced. it cannot be denied that the corona is a perplexing phenomenon, and that it does not become less perplexing as we know more about it. it presented itself under quite a new and strange aspect on the occasion of the eclipse which visited the western states of north america, july , . the conditions of observation were peculiarly favourable. the weather was superb; above the rocky mountains the sky was of such purity as to permit the detection of jupiter's satellites with the naked eye on several successive nights. the opportunity for advancing knowledge was made the most of. nearly a hundred astronomers, including several englishmen, occupied twelve separate posts, and prepared for an attack in force. the question had often suggested itself, and was a natural one to ask, whether the corona sympathises with the general condition of the sun? whether, either in shape or brilliancy, it varies with the progress of the sun-spot period? a more propitious moment for getting this question answered could hardly have been chosen than that at which the eclipse occurred. solar disturbance was just then at its lowest ebb. the development of spots for the month of july, , was represented on wolf's system of "relative numbers" by the fraction · , as against · for december, , an epoch of maximum activity. the "chromosphere"[ ] was, for the most part, shallow and quiescent; its depth, above the spot zones, had sunk from about , to , miles;[ ] prominences were few and faint. obviously, if a type of corona corresponding to a minimum of sun-spots existed, it should be seen then or never. it _was_ seen; but while, in some respects, it agreed with anticipation, in others it completely set it at naught. the corona of , as compared with those of , , and , was generally admitted to be shrunken in its main outlines and much reduced in brilliancy. lockyer pronounced it ten times fainter than in ; harkness estimated its light at less than one-seventh that derived from the mist-blotted aureola of .[ ] in shape, too, it was markedly different. when sun-spots are numerous, the corona appears to be most fully developed above the spot-zones, thus offering to our eyes a rudely quadrilateral contour. the four great luminous sheaves forming the corners of the square are made up of rays curving together from each side into "synclinal" or ogival groups, each of which may be compared to the petal of a flower. to janssen, in , the eclipsing moon seemed like the dark heart of a gigantic dahlia, painted in light on the sky; and the similitude to the ornament on a compass-card, used by airy in , well conveys the decorative effect of the beamy, radiated kind of aureola, never, it would appear, absent when solar activity is at a tolerably high pitch. in his splendid volume on eclipses,[ ] with which the systematic study of coronal structure may be said to have begun, mr. ranyard first generalised the synclinal peculiarity by a comparison of records; but the symmetry of the arrangement, though frequently striking, is liable to be confused by secondary formations. he further pointed out, with the help of careful drawings from the photographs of made by mr. wesley, the curved and branching shapes assumed by the component filaments of massive bundles of rays. nothing of all this, however, was visible in . instead, there was seen, as the groundwork of the corona, a ring of pearly light, nebulous to the eye, but shown by telescopes and in photographs to have a fibrous texture, as if made up of tufts of fine hairs. north and south, a series of short, vivid, electrical-looking flame-brushes diverged with conspicuous regularity from each of the solar poles. their direction was not towards the centre of the sun, but towards each summit of his axis, so that the farther rays on either side started almost tangentially to the surface. but the leading, and a truly amazing, characteristic of the phenomenon was formed by two vast, faintly-luminous _wings_ of light, expanded on either side of the sun in the direction of the ecliptic. these were missed by very few careful onlookers; but the extent assigned to them varied with skill in, and facilities for seeing. by far the most striking observations were made by newcomb at separation (wyoming), by cleveland abbe from the shoulder of pike's peak, and by langley at its summit, an elevation of , feet above the sea. never before had an eclipse been viewed from anything approaching that altitude, or under so translucent a sky. a proof of the great reduction in atmospheric glare was afforded by the perceptibility of the corona four minutes after totality was over. for the seconds of its duration, the remarkable streamers above alluded to continued "persistently visible," stretching away right and left of the sun to a distance of at least ten million miles! one branch was traced over an apparent extent of fully twelve lunar diameters, without sign of a definite termination having been reached; and there were no grounds for supposing the other more restricted. the resemblance to the zodiacal light was striking; and a community of origin between that enigmatical member of our system and the corona was irresistibly suggested. we should, indeed, expect to see, under such exceptionally favourable atmospheric conditions as professor langley enjoyed on pike's peak, the _roots_ of the zodiacal light presenting near the sun just such an appearance as he witnessed; but we can imagine no reason why their visibility should be associated with a low state of solar activity. nevertheless this seems to be the case with the streamers which astonished astronomers in . for in august, , when similar equatorial emanations, accompanied by similar symptoms of polar excitement, were described and depicted by grosch[ ] of the santiago observatory, sun-spots were at a minimum; while the corona of , which appears from the record of it by roger cotes[ ] to have been of the same type, preceded by three years the ensuing maximum. the eclipsed sun was seen by him at cambridge, may , , encompassed with a ring of light about one-sixth of the moon's diameter in breadth, upon which was superposed a luminous cross formed of long bright branches lying very nearly in the plane of the ecliptic, and shorter polar arms so faint as to be only intermittently visible. the resemblance between his sketch and cleveland abbe's drawing of the corona of is extremely striking. it should, nevertheless, be noted that some conspicuous spots were visible on the sun's disc at the time of cotes's eclipse, and that the preceding minimum (according to wolf) occurred in . thus, the coincidence of epochs is imperfect. professor cleveland abbe was fully persuaded that the long rays carefully observed by him from pike's peak were nothing else than streams of meteorites rushing towards or from perihelion; and it is quite certain that the solar neighbourhood must be crowded with such bodies. but the peculiar structure at the base of the streamers displayed in the photographs, the curved rays meeting in pointed arches like gothic windows, the visible upspringing tendency, the filamentous texture,[ ] speak unmistakably of the action of forces proceeding _from_ the sun, not of extraneous matter circling round him. a further proof of sympathetic change in the corona is afforded by the analysis of its light. in the bright line so conspicuous in the coronal spectrum in and had faded to the very limit of visibility. several skilled observers failed to see it at all; but young and eastman succeeded in tracing the green "coronium" ray all round the sun, to a height estimated at , miles. the substance emitting it was thus present, though in a low state of incandescence. the continuous spectrum was relatively strong; faint traces of the fraunhofer lines attested for it an origin, in part by reflection; and polarisation was undoubted, increasing towards the limb, whereas in it reached a maximum at a considerable distance from it. experiments with edison's tasimeter seemed to show that the corona radiates a sensible amount of heat. the next promising eclipse occurred may , . the concourse of astronomers which has become usual on such occasions assembled this time at sohag, in upper egypt. rarely have seventy-four seconds been turned to such account. to each observer a special task was assigned, and the advantages of a strict division of labour were visible in the variety and amount of the information gained. the year was one of numerous sun-spots. on the eve of the eclipse twenty-three separate maculæ were counted. if there were any truth in the theory which connected coronal forms with fluctuations in solar activity, it might be anticipated that the vast equatorial expansions and polar "brushes" of would be found replaced by the star-like structure of . this expectation was literally fulfilled. no lateral streamers were to be seen. the universal failure to perceive them, after express search in a sky of the most transparent purity, justifies the emphatic assertion that _they were not there_. instead, the type of corona observed in india eleven years earlier, was reproduced with its shining aigrettes, complex texture and brilliant radiated aspect. concordant testimony was given by the spectroscope. the reflected light derived from the corona was weaker than in , while its original emissions were proportionately intensified. nevertheless, most of the bright lines recorded as coronal[ ] were really due, there can be no doubt, to diffused chromospheric light. on this occasion, the first successful attempt was made to photograph the coronal spectrum procured in the ordinary way with a slit and prisms, while the prismatic camera was also profitably employed. it served to bring out at least one important fact--that of the uncommon strength in chromospheric regions of the twin violet beams of calcium, designated "h" and "k"; and prominence-photography signalised its improvement by the registration, in the spectrum of one such object, of twenty-nine rays, including many of the ultra-violet hydrogen series discovered by sir william huggins in the emission of white stars.[ ] dr. schuster's photographs of the corona itself were the most extensive, as well as the most detailed, of any yet secured. one rift imprinted itself on the plates to a distance of nearly a diameter and a half from the limb; and the transparency of the streamers was shown by the delineation through them of the delicate tracery beyond. the singular and picturesque feature was added of a bright comet, self-depicted in all the exquisite grace of swift movement betrayed by the fine curve of its tail, hurrying away from one of its rare visits to our sun, and rendered momentarily visible by the withdrawal of the splendour in which it had been, and was again quickly veiled. from a careful study of these valuable records sir william huggins derived the idea of a possible mode of photographing the corona _without an eclipse_.[ ] as already stated, its ordinary invisibility is entirely due to the "glare" or reflected light diffused through our atmosphere. but huggins found, on examining schuster's negatives, that a large proportion of the light in the coronal spectrum, both continuous and interrupted, is collected in the violet region between the fraunhofer lines g and h. there, then, he hoped that, all other rays being excluded, it might prove strong enough to vanquish inimical glare, and stamp on prepared plates, through _local_ superiority in illuminative power, the forms of the appendage by which it is emitted. his experiments were begun towards the end of may, , and by september he had obtained a fair earnest of success. the exclusion of all other qualities of light save that with which he desired to operate, was accomplished by using chloride of silver as his sensitive material, that substance being chemically inert to all other but those precise rays in which the corona has the advantage.[ ] plates thus sensitised received impressions which it was hardly possible to regard as spurious. "not only the general features," captain abney affirmed,[ ] "are the same, but details, such as rifts and streamers, have the same position and form." it was found, moreover, that the corona photographed during the total eclipse of may , , was intermediate in shape between the coronas photographed by sir william huggins before and after that event, each picture taking its proper place in a series of progressive modifications highly interesting in themselves, and full of promise for the value of the method employed to record them.[ ] but experiments on the subject were singularly interrupted. the volcanic explosion in the straits of sunda in august, , brought to astronomers a peculiarly unwelcome addition to their difficulties. the magnificent sunglows due to the diffractive effects on light of the vapours and fine dust flung in vast volumes into the air, and rapidly diffused all round the globe, betokened an atmospheric condition of all others the most prejudicial to delicate researches in the solar vicinity. the filmy coronal forms, accordingly, which had been hopefully traced on the tulse hill plates ceased to appear there; nor were any substantially better results obtained by mr. c. ray woods, in the purer air either of the riffel or the cape of good hope, during the three ensuing years. moreover, attempts to obtain coronal photographs during the partial phases of the eclipse of august , , completely failed. no part of the lunar globe became visible in relief against circumfluous solar radiance on any of the plates exposed at grenada; and what vestiges of "structure" there were, came out almost better _upon_ the moon than _beside_ her, thus stamping themselves at once as of atmospheric origin. that the effect sought is a perfectly possible one is proved by the distinct appearance of the moon projected on the corona, in photographs of the partially eclipsed sun in , , and , and very notably in and .[ ] in the spring of , professor hale[ ] attacked the problem of coronal daylight photography, employing the "double-slit" method so eminently serviceable for the delineation of prominences.[ ] but neither at kenwood nor at the summit of pike's peak, whither, in the course of the summer, he removed his apparatus, was any action of the desired kind secured. similar ill success attended his and professor riccò's employment, on mount etna in july, , of a specially designed coronagraph. yet discouragement did not induce despair. the end in view is indeed too important to be readily abandoned; but it can be reached only when a more particular acquaintance with the nature of coronal light than we now possess indicates the appropriate device for giving it a preferential advantage in self-portraiture. moreover, the effectiveness of this device may not improbably be enhanced, through changes in the coronal spectrum at epochs of sun-spot maximum. the prosperous result of the sohag observations stimulated the desire to repeat them on the first favourable opportunity. this offered itself one year later, may , , yet not without the drawbacks incident to terrestrial conditions. the eclipse promised was of rare length, giving no less than five minutes and twenty-three seconds of total obscurity, but its path was almost exclusively a "water-track." it touched land only on the outskirts of the marquesas group in the southern pacific, and presented, as the one available foothold for observers, a coral reef named caroline island, seven and a half miles long by one and a half wide, unknown previously to , and visited only for the sake of its stores of guano. seldom has a more striking proof been given of the vividness of human curiosity as to the condition of the worlds outside our own, than in the assemblage of a group of distinguished men from the chief centres of civilisation, on a barren ridge, isolated in a vast and tempestuous ocean, at a distance, in many cases, of , miles and upwards from the ordinary scene of their labours. and all these sacrifices--the cost and care of preparation, the transport and readjustment of delicate instruments, the contrivance of new and more subtle means of investigating phenomena--on the precarious chance of a clear sky during one particular five minutes! the event, though fortunate, emphasised the hazard of the venture. the observation of the eclipse was made possible only by the happy accident of a serene interval between two storms. the american expedition was led by professor edward s. holden, and to it were courteously permitted to be attached messrs. lawrance and woods, photographers, sent out by the royal society of london. m. janssen was chief of the french academy mission; he was accompanied from meudon by trouvelot, and joined from vienna by palisa, and from rome by tacchini. a large share of the work done was directed to assuring or negativing previous results. the circumstances of an eclipse favour illusion. a single observation by a single observer, made under unfamiliar conditions, and at a moment of peculiar excitement, can scarcely be regarded as offering more than a suggestion for future inquiry. but incredulity may be carried too far. janssen, for instance, felt compelled, by the survival of unwise doubts, to devote some of the precious minutes of obscurity at caroline island to confirming what, in his own persuasion, needed no confirmation--that is, the presence of reflected fraunhofer lines in the spectrum of the corona. trouvelot and palisa, on the other hand, instituted an exhaustive, but fruitless search for the spurious "intramercurian" planets announced by swift and watson in . new information, however, was not deficient. the corona proved identical in type with that of ,[ ] agreeably to what was expected at an epoch of protracted solar activity. the characteristic aigrettes were of even greater brilliancy than in the preceding year, and the chemical effects of the coronal light proved unusually intense. janssen's photographs, owing to the considerable apertures (six and eight inches) of his object-glasses, and the long exposures permitted by the duration of totality, were singularly perfect; they gave a greater extension to the coronal than could be traced with the telescope,[ ] and showed its forms as absolutely fixed and of remarkable complexity. the english pictures, taken with exposures up to sixty seconds, were likewise of great value. they exhibited details of structure from the limb to the tips of the streamers, which terminated definitely, and as it seemed actually, where the impressions on the plates ceased. the coronal spectrum was also successfully photographed, and although the reversing layer in its entirety evaded record, a print was caught of some of its more prominent rays just before and after totality. the use of the prismatic camera was baffled by the anomalous scarcity of prominences. using an ingenious apparatus for viewing simultaneously the spectrum from both sides of the sun, professor hastings noticed at caroline island alternations, with the advance of the moon, in the respective heights above the right and left solar limbs of the coronal green line, which were thought to imply that the corona, with its rifts and sheaves and "tangled hanks" of rays, is, after all, merely an illusive appearance produced by the diffraction of sunlight at the moon's edge.[ ] but the observation was assuredly misleading or misinterpreted. atmospheric _diffusion_ may indeed, under favouring circumstances, be effective in deceptively enlarging solar appendages; but always to a very limited extent. the controversy is an old one as to the part played by our air in producing the radiance visible round the eclipsed sun. in its original form, it is true, it came to an end when professor harkness, in ,[ ] pointed out that the shadow of the moon falls equally over the air and on the earth, and that if the sun had no luminous appendages, a circular space of almost absolute darkness would consequently surround the apparent places of the superposed sun and moon. mr. proctor,[ ] with his usual ability, impressed this mathematically certain truth upon public attention; and sir john herschel calculated that the diameter of the "negative halo" thus produced would be, in general, no less than °. but about the same time a noteworthy circumstance relating to the state of things in the solar vicinity was brought into view. on february , , messrs. frankland and lockyer communicated to the royal society a series of experiments on gaseous spectra under varying conditions of heat and density, leading them to the conclusion that the higher solar prominences exist in a medium of excessive tenuity, and that even at the base of the chromosphere the pressure is far below that at the earth's surface.[ ] this inference was fully borne out by the researches of wüllner; and janssen expressed the opinion that the chromospheric gases are rarefied almost to the degree of an air-pump vacuum.[ ] hence was derived a general and fully justified conviction that there could be outside, and incumbent upon the chromosphere, no such vast atmosphere as the corona appeared to represent. upon the strength of which conviction the "glare" theory entered, chiefly under the auspices of sir norman lockyer, upon the second stage of its existence. the genuineness of the "inner corona" to the height of ' or ' from the limb was admitted; but it was supposed that by the detailed reflection of its light in our air the far more extensive "outer corona" was optically created, the irregularities of the moon's edge being called in to account for the rays and rifts by which its structure was varied. this view received some countenance from admiral maclear's observation, during the eclipse of , of bright lines "everywhere"--even at the centre of the lunar disc. here, indeed, was an undoubted case of atmospheric diffusion; but here, also, was a safe index to the extent of its occurrence. light scatters equally in all directions; so that when the moon's face at the time of an eclipse shows (as is the common case) a blank in the spectroscope, it is quite certain that the corona is not noticeably enlarged by atmospheric causes. a sky drifted over with thin cirrus clouds and air changed with aqueous vapour amply accounted for the abnormal amount of scattering in . but even in positive evidence was obtained of the substantial reality of the radiated outer corona, in the appearance on the photographic plates exposed by willard in spain and by brothers in sicily of identical dark rifts. the truth is, that far from being developed by misty air, it is peculiarly liable to be effaced by it. the purer the sky, the more extensive, brilliant, and intricate in the details of its structure the corona appears. take as an example general myer's description of the eclipse of , as seen from the summit of white top mountain, virginia, at an elevation above the sea of , feet, in an atmosphere of peculiar clearness. "to the unaided eye," he wrote,[ ] "the eclipse presented, during the total obscuration, a vision magnificent beyond description. as a centre stood the full and intensely black disc of the moon, surrounded by the aureola of a soft bright light, through which shot out, as if from the circumference of the moon, straight, massive, silvery rays, seeming distinct and separate from each other, to a distance of two or three diameters of the solar disc; the whole spectacle showing as on a background of diffused rose-coloured light." on the same day, at des moines, newcomb could perceive, through somewhat hazy air, no long rays, and the four-pointed outline of the corona reached at its farthest only a _single semidiameter_ of the moon from the limb. the plain fact, that our atmosphere acts rather as a veil to hide the coronal radiance than as the medium through which it is visually formed, emerges from further innumerable records. no observations of importance were made during the eclipse of september , . the path of total obscurity touched land only on the shores of new zealand, and two minutes was the outside limit of available time. hence local observers had the phenomenon to themselves; nor were they even favoured by the weather in their efforts to make the most of it. one striking appearance was, however, disclosed. it was that of two "white" prominences of unusual brilliancy, shining like a pair of electric lamps hung one at each end of a solar diameter, right above the places of two large spots.[ ] this coincidence of diametrically opposite disturbances is of too frequent occurrence to be accidental. m. trouvelot observed at meudon, june , , two active and evanescent prominences thus situated, each rising to the enormous height of , miles; and on august , one scarcely less remarkable, balanced by an antipodal spot-group.[ ] it towered upward, as if by a process of _unrolling_, to a quarter of a million of miles; after which, in two minutes, the light died out of it; it had become completely extinct. the development, again from the ends of a diameter, of a pair of similar objects was watched, september and , , by father fényi, director of the kalocsa observatory; and the phenomenon has been too often repeated to be accidental. the eclipse of august , , was total during about four minutes over tropical atlantic regions; and an english expedition, led by sir norman lockyer, was accordingly despatched to grenada in the west indies, for the purpose of using the opportunity it offered. but the rainy season was just then at its height: clouds and squalls were the order of the day; and the elaborately planned programme of observation could only in part be carried through. some good work, none the less, was done. professor tacchini, who had been invited to accompany the party, ascertained besides some significant facts about prominences. from a comparison of their forms and sizes during and after the eclipse, it appeared that only the growing vaporous cores of these objects are shown by the spectroscope under ordinary circumstances; their upper sections, giving a faint continuous spectrum, and composed of presumably cooler materials, can only be seen when the veil of scattered light usually drawn over them is removed by an eclipse. thus all modestly tall prominences have silvery summits; but all do not appear to possess the "red heart of flame," by which alone they can be rendered perceptible to daylight observation. some prove to be ordinarily invisible, because silvery throughout--"sheeted ghosts," as it were, met only in the dark. specimens of the class had been noted as far back as , but tacchini first drew particular attention to them. the one observed by him in rose in a branching form to a height of , miles, and gave a brilliantly continuous spectrum, with bright lines at h and k, but no hydrogen-lines.[ ] hence the total invisibility of the object before and after the eclipse. during the eclipse, it was seen framed, as it were, in a pointed arch of coronal light, the symmetrical arrangement of which with regard to it was obviously significant. both its unspringing shape, and the violet rays of calcium strongly emitted by it, contradicted the supposition that "white prominences" represent a downrush of refrigerated materials. the corona of , as photographed by dr. schuster and mr. maunder, showed neither the petals and plumes of , nor the streamers of . it might be called of a transition type.[ ] wide polar rifts were filled in with tufted radiations, and bounded on either side by irregularly disposed, compound luminous masses. in the south-western quadrant, a triangular ray, conspicuous to the naked eye, represented, mr. w. h. pickering thought, the projection of a huge, hollow cone.[ ] branched and recurving jets were curiously associated with it. the intrinsic photographic brightness of the corona proved, from pickering's measures, to be about / that of the average surface of the full moon. the russian eclipse of august , , can only be remembered as a disastrous failure. much was expected of it. the shadow-path ran overland from leipsic to the japanese sea, so that the solar appurtenances would, it was hoped, be disclosed to observers echeloned along a line of , miles. but the incalculable element of weather rendered all forecasts nugatory. the clouds never parted, during the critical three minutes, over central russia, where many parties were stationed, and professor d. p. todd was equally unfortunate in japan. some good photographs were, nevertheless, secured by professor arai, director of the tokio observatory, as well as by mm. bélopolsky and glasenapp at petrovsk and jurjevitch respectively. they showed a corona of simpler form than that of the year before, but not yet of the pronounced type first associated by mr. ranyard with the lowest stage of solar activity. the genuineness of the association was ratified by the duplicate spectacle of the next-ensuing minimum year. two total eclipses of the sun distinguished . the first took place on new year's day, when a narrow shadow-path crossed california, allowing less than two minutes for the numerous experiments prompted by the varied nature of modern methods of research. american astronomers availed themselves of the occasion to the full. the heavens were propitious. photographic records were obtained in unprecedented abundance, and of unusual excellence. their comparison and study placed it beyond reasonable doubt that the radiated corona belonging to periods of maximum sun-spots gives place, at periods of minimum, to the "winged" type of . professor holden perceived further that the equatorial extensions characterising the latter tend to assume a "trumpet-shape."[ ] their extremities diverge, as if mutually repellent, instead of flowing together along a medial plane. the maximum actinic brilliancy of the corona of january , , was determined at lick to be twenty-one times less than that of the full moon.[ ] its colour was described as "of an intense luminous silver, with a bluish tinge, similar to the light of an electric arc."[ ] its spectrum was comparatively simple. very few bright lines besides those of hydrogen and coronium, and apparently no dark ones, stood out from the prismatic background. "the marked structural features of the corona, as presented by the negatives" taken by professors nipher and charroppin, were the filaments and the streamers. the filaments issued from polar calottes of ° radius. "the impression conveyed to the eye," professor pritchett wrote,[ ] "is that the equatorial stream of denser coronal matter extends across and through the filaments, simply obscuring them by its greater brightness. the effect is just as if the equatorial belt were superposed upon, or passed through, the filamentary structure. there is nothing in the photographs to prove that the filaments do not exist all round the sun.[ ] the testimony from negatives of different lengths of exposure goes to show that the equatorial streamers are made up of numerous interlacing parts inclined at varying angles to the sun's equator." the coronal extensions, perceptible with the naked eye to a distance of more than ° from the sun, appeared barely one-third of that length on the best negatives. little more could be seen of them either in barnard's exquisite miniature pictures, or in the photographs obtained by w. h. pickering with a thirteen-inch refractor--the largest instrument so far used in eclipse-photography. the total eclipse of december , , held out a prospect, unfortunately not realized, of removing some of the doubts and difficulties that impeded the progress of coronal photography.[ ] messrs. burnham and schaeberle secured at cayenne some excellent impressions, showing enough of the corona to prove its identical character with that depicted in the beginning of the year, but not enough to convey additional information about its terminal forms or innermost structure. any better result was indeed impossible, the moisture-laden air having cut down the actinic power of the coronal light to one-fourth its previous value. two english expeditions organized by the royal astronomical society fared still worse. mr. taylor was stationed on the west coast of africa, one hundred miles south of loanda; father perry chose as the scene of his operations the salut islands, off french guiana. each was supplied with a reflector constructed by dr. common, endowed, by its extremely short focal length of forty-five, combined with an aperture of twenty inches, with a light-concentrating force capable, it was hoped, of compelling the very filmiest coronal branches to self-registration. had things gone well two sets of coronal pictures, absolutely comparable in every respect, and taken at an interval of two hours and a half, would have been at the disposal of astronomers. but things went very far from well. clouds altogether obscured the sun in africa; they only separated to allow of his shining through a saturated atmosphere in south america. father perry's observations were the last heroic effort of a dying man. stricken with malaria, he crawled to the hospital as soon as the eclipse was over, and expired five days later, at sea, on board the _comus_. he was buried at barbados. and the sacrifice of his life had, after all, purchased no decisive success. most of the plates exposed by him suffered deterioration from the climate, or from an inevitably delayed development. a drawing from the best of them by miss violet common[ ] represented a corona differing from its predecessor of january , chiefly through the oppositely unsymmetrical relations of its parts. then the western wing had been broader at its base than the eastern; now the inequality was conspicuously the other way.[ ] the next opportunity for retrieving the mischances of the past was offered april , . the line of totality charted for that day ran from chili to senegambia. american parties appropriated the andes; both shores of the atlantic were in english occupation; french expeditions, led by deslandres and bigourdan, took up posts south of cape verde. a long totality of more than four minutes was favoured by serene skies; hence an ample store of photographic data was obtained. professor schaeberle, of the lick observatory, took, almost without assistance, at mina bronces, a mining station , feet above the pacific, fifty-two negatives, eight of them with a forty-foot telescope, on a scale of four and a half inches to the solar diameter. not only the inner corona, but the array of prominences then conspicuous, appeared in them to be composed of fibrous jets and arches, held to be sections of elliptic orbits described by luminous particles about the sun's centre.[ ] one plate received the impression of a curious object,[ ] entangled amidst coronal streamers, and the belief in its cometary nature was ratified by the bestowal of a comet-medal in recognition of the discovery. similiar paraboloidal forms had, nevertheless, occasionally been seen to make an integral part of earlier coronas; and it remains extremely doubtful whether schaeberle's "eclipse-comet" was justly entitled to the character claimed for it. the eclipse of disclosed a radiated corona such as a year of spot-maximum was sure to bring. an unexpected fact about it was, however, ascertained. the coronal has been believed to have much in common with the chromospheric spectrum; it proved, on investigation with a large prismatic camera, employed under sir norman lockyer's directions by mr. fowler at fundium, to be absolutely distinct from it. the fundamental green ray had, on the west african plates, seven more refrangible associates;[ ] but all alike are of unknown origin. they may be due to many substances, or to one; future research will perhaps decide; we can at present only say that the gaseous emission of the corona include none from hydrogen, helium, calcium, or any other recognisable terrestrial element. deslandres' attempt to determine the rotation of the corona through opposite displacements, east and west of the interposed moon, of the violet calcium-lines supposed to make part of the coronal spectrum, was thus rendered nugatory. yet it gave an earnest of success, by definitely introducing the subject into the constantly lengthened programme of eclipse-work. there is, however, little prospect of its being treated effectively until the green line is vivified by a fresh access of solar activity. the flight of the moon's shadow was, on august , , dogged by atrocious weather. it traversed, besides, some of the most inhospitable regions on the earth's surface, and afforded, at the best, but a brief interval of obscurity. at novaya zemlya, however, of all places, the conditions were tolerably favourable, and, as we have seen, the trophy of a "flash-spectrograph" was carried off. some coronal photographs, moreover, taken by the late sir george baden-powell[ ] and by m. hansky, a member of a russian party, were marked by features of considerable interest. they made apparent a close connection between coronal outflows and chromospheric jets, cone-shaped beams serving as the sheaths, or envelopes, of prominences. m. hansky,[ ] indeed, thought that every streamer had a chromospheric eruption at its base. further, dark veinings of singular shapes unmistakably interrupted the coronal light, and bordered brilliant prominences,[ ] reminding us of certain "black lines" traced by swift across the "anvil protuberance" august , .[ ] in type the corona of reproduced that of , as befitted its intermediate position in the solar cycle. the eclipse-track on january , , crossed the indian peninsula from viziadrug, on the malabar coast, to mount everest in the himalayas. not a cloud obstructed the view anywhere, and an unprecedented harvest of photographic records was garnered. the flash-spectrum, in its successive phases, appeared on plates taken by sir norman lockyer, mr. evershed, professor campbell,[ ] and others; professor turner[ ] set on foot a novel mode of research by picturing the corona in the polarised ingredient of its light; mrs. maunder[ ] practically solved the problem of photographing the faint coronal extensions, one ray on her plates running out to nearly six diameters from the moon's limb. yet she used a dallmeyer lens of only one and a half inches aperture. her success accorded perfectly with professor wadsworth's conclusion that effectiveness in delineation by slight contrasts of luminosity varies inversely with aperture. triple-coated plates, and a comparatively long exposure of twenty seconds, contributed to a result unlikely, for some time, to be surpassed. the corona of presented a mixed aspect. the polar plumes due at minimum were combined in it with the quadrilateral ogives belonging to spot-maxima. a slow course of transformation, in fact, seemed in progress; and it was found to be completed in , when the eclipse of may revealed the typical halo of a quiescent sun. the obscurity on this occasion was short--less than seconds--but was well observed east and west of the atlantic. no striking gain in knowledge, however, resulted. important experiments were indeed made on the heat of the corona with langley's bolometer, but their upshot can scarcely be admitted as decisive. they indicated a marked deficiency of thermal radiations, implying for coronal light, in professor langley's opinion,[ ] an origin analogous to that of the electric glow-discharge, which, at low pressures, was found by k. Ångström in to have no invisible heat-spectrum.[ ] the corona was photographed by professor barnard, at wadesborough, north carolina, with a - / -foot horizontal "coelostat." in this instrument, of a type now much employed in eclipse operations and first recommended by professor turner, a six-inch photographic objective preserved an invariable position, while a silvered plane mirror, revolving by clockwork once in forty-eight hours (since the angle of movement is doubled by reflection), supplied the light it brought to a focus. a temporary wooden tube connected the lens with the photographic house where the plates were exposed. pictures thus obtained with exposures of from one to fourteen seconds, were described as "remarkably sharp and perfectly defined, showing the prominences and inner corona very beautifully. the polar fans came out magnificently."[ ] the great sumatra eclipse left behind it manifold memories of foiled expectations. a totality of above six minutes drew observers to the far east from several continents, each cherishing a plan of inquiry which few were destined to execute. all along the line of shadow, which, on may , , crossed réunion and mauritius, and again met land at sumatra and borneo, the meteorological forecast was dubious, and the meteorological actuality in the main deplorable. nevertheless, the corona was seen, and fairly well photographed through drifting clouds, and proved to resemble in essentials the appendage viewed a year previously. negatives taken by members of the lick observatory expedition led by mr. perrine[ ] disclosed the unique phenomenon of a violent coronal disturbance, with a small compact prominence as its apparent focus. tumbling masses and irregular streamers radiating from a point subsequently shown by the greenwich photographs to be the seat of a conspicuous spot, suggested the recent occurrence of an explosion, the far-reaching effects of which might be traced in the confused floccular luminosity of a vast surrounding region. again, photographs in polarised light attested the radiance of the outer corona to be in large measure reflected, while that of the inner ring was original; and the inference was confirmed by spectrographs, recording many fraunhofer lines when the slit lay far from the sun's limb, but none in its immediate vicinity. on plates exposed by mr. dyson and dr. humphrys with special apparatus, the coronal spectrum, continuous and linear, impressed itself more extensively in the ultra-violet than on any previous occasion; and dr. mitchell succeeded in photographing the reversing layer by means of a grating spectroscope. finally, mrs. maunder, at mauritius, despite mischievous atmospheric tremors, obtained with the newbegin telescope an excellent series of coronal pictures.[ ] the principles of explanation applied to the corona may be briefly described as eruptive and electrical. the first was adopted by professor schaeberle in his "mechanical theory," advanced in .[ ] according to this view, the eclipse-halo consists of streams of matter shot out with great velocity from the spot-zones by forces acting perpendicularly to the sun's surface. the component particles return to the sun after describing sections of extremely elongated ellipses, unless their initial speed happen to equal or exceed the critical rate of miles a second, in which case they are finally driven off into space. the perspective overlapping and interlacing of these incandescent outflows was supposed to occasion the intricacies of texture visible in the corona; and it should be recorded that a virtually identical conclusion was reached by mr. perrine in ,[ ] by a different train of reasoning, based upon a distinct set of facts. a theory on very much the same lines was, moreover, worked out by m. bélopolsky in .[ ] schaeberle, however, had the merit of making the first adequate effort to deduce the real shape of the corona, as it exists in three dimensions, from its projection upon the surface of the sphere. he failed, indeed, to account for the variation in coronal types by the changes in our situation with regard to the sun's equator. it is only necessary to remark that, if this were so, they should be subject to an annual periodicity, of which no trace can be discerned. electro-magnetic theories have the charm, and the drawback, of dealing largely with the unknown. but they are gradually losing the vague and intangible character which long clung to them; and the improved definition of their outlines has not, so far, brought them into disaccord with truth. the most promising hypothesis of the kind is due to professor bigelow of washington. his able discussion of the eclipse photographs of january , ,[ ] showed a striking agreement between the observed coronal forms and the calculated effects of a repulsive influence obeying the laws of electric potential, also postulated by huggins in .[ ] finely subdivided matter, expelled from the sun along lines of force emanating from the neighbourhood of his poles, thus tends to accumulate at "equipotential surfaces." in deference, however, to a doubt more strongly felt then than now, whether the presence of free electricity is compatible with the solar temperature, he avoided any express assertion that the coronal structure is an electrical phenomenon, merely pointing out that, if it were, its details would be just what they are. later, in , pupin in america,[ ] and ebert in germany,[ ] imitated the coronal streamers by means of electrical discharges in low vacua between small conducting bodies and strips of tinfoil placed on the outside of the containing glass receptacles. finally, a critical experiment made by ebert in served, as bigelow justly said, "to clear up the entire subject, and put the theory on a working basis." having obtained coronoidal effects in the manner described, he proceeded to subject them to the action of a strong magnetic field, with the result of marshalling the scattered rays into a methodical and highly suggestive array. they followed the direction of the magnetic lines of force, and, forsaking the polar collar of the magnetised sphere, surrounded it like a ruffle. the obvious analogy with the aurora polaris and the solar corona was insisted upon by ebert himself, and has been further developed by bigelow.[ ] according to a recent modification of his hypothesis, the latter appendage is controlled by two opposing systems of forces; the magnetic causing the rays to diverge from the poles towards the equator, and the electrostatic urging their spread, through the mutual repulsion of the particles accumulated in the "wings," from the equator towards either pole. the cyclical change in the corona, he adds, is probably due to a variation in the balance of power thus established, the magnetic polar influence dominating at minima, the electrostatic at maxima. and he may well feel encouraged by the fortunate combination of many experimental details into one explanatory whole, no less than by the hopeful prospect of further developments, both practical and theoretical, along the same lines. what we really know about the corona can be summed up in a few words. it is certainly _not_ a solar atmosphere. it does not gravitate upon the sun's surface and share his rotation, as our air gravitates upon and shares the rotation of the earth; and this for the simple reason that there is no visible growth of pressure downwards (of which the spectroscope would infallibly give notice) in its gaseous constituents; whereas under the sole influence of the sun's attractive power, their density should be multiplied many million times in the descent through a mere fraction of their actual depth.[ ] they are apparently in a perpetual state of efflux from, and influx to our great luminary, under the stress of opposing forces. it is not unlikely that some part, at least, of the coronal materials are provided by eruptions from the body of the sun;[ ] it is almost certain that they are organized and arranged round it through electro-magnetic action. this, however, would seem to be influential only upon their white-hot or reflective ingredients, out of which the streamers and aigrettes are composed; since the coronal gases appear, from observations during eclipses, to form a shapeless envelope, with condensations above the spot-zones, or at the bases of equatorial extensions. the corona is undoubtedly affected both in shape and constitution by the periodic ebb and flow of solar activity, its low-tide form being winged, its high-tide form stellate; while the rays emitted by the gases contained in it fade, and the continuous spectrum brightens, at times of minimum sun-spots. the appendage, as a whole, must be of inconceivable tenuity, since comets cut their way through it without experiencing sensible retardation. not even sir william crookes's vacua can give an idea of the rarefaction which this fact implies. yet the observed luminous effects may not in reality bear witness contradictory of it. one solitary molecule in each cubic inch of space might, in professor young's opinion, produce them; while in the same volume of ordinary air at the sea-level, the molecules number (according to dr. johnstone stoney) , trillions! the most important lesson, however, derived from eclipses is that of partial independence of them. some of its fruits in the daily study of prominences the next chapter will collect; and the harvest has been rendered more abundant, as well as more valuable, since it has been found possible to enlist, in this department too, the versatile aid of the camera. footnotes: [footnote : _vierteljahrsschrift astr. ges._, jahrg. xxvi., p. .] [footnote : _astr. jour._, vol. iv., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _month. not._, vol. xxvii., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _washington observations_, , app. ii., harkness's report, p. .] [footnote : _am. jour._, vol. xlviii. ( nd series), p. .] [footnote : _am. jour._, vol. xi. ( rd series), p. .] [footnote : campbell, _astroph. jour._, vol. x., p. .] [footnote : keeler, _reports on eclipse of january , _, p. .] [footnote : everything in such observations depends upon the proper manipulation of the slit of the spectroscope.] [footnote : _mem. r. a. s._, vol. xli., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _mem. r. a. s._, vol. xli., p. .] [footnote : _comptes rendus_, t. xciv., p. .] [footnote : young, _pop. astr._, oct., , p. .] [footnote : j. evershed, _indian eclipse_, , p. ; _month. not._, vol. lviii., p. ; _proc. roy. soc._, jan. , .] [footnote : frost, _astroph. jour._, vol. xii., p. ; lord, _ibid._, vol. xiii., p. .] [footnote : _comptes rendus_, t. cxvii., no. ; _jour. brit. astr. ass._, vol. iii., p. .] [footnote : lockyer, _phil. trans._, vol. clvii., p. .] [footnote : the rosy envelope of prominence-matter was so named by lockyer in (_phil. trans._, vol. clix., p. ).] [footnote : according to trouvelot (_wash. obs._, , app. iii., p. ), the subtracted matter was, at least to some extent, accumulated in the polar regions.] [footnote : _bull. phil. soc. washington_, vol. iii., p. .] [footnote : _mem. r. a. s._, vol. xli., .] [footnote : _astr. nach._, no. , .] [footnote : _correspondence with newton_, pp. - ; ranyard, _mem. astr. soc._, vol. xli., p. .] [footnote : s. p. langley, _wash. obs._, , app. iii., p. ; _nature_, vol. lxi., p. .] [footnote : schuster (_proc. roy. soc._, vol. xxxv., p. ) measured and photographed about thirty.] [footnote : abney, _phil. trans._, vol. clxxv., p. .] [footnote : _proc. roy. soc._, vol. xxxiv., p. . experiments directed to the same end had been made by dr. o. lohse at potsdam, - . _astr. nach._, no. , .] [footnote : the sensitiveness of chloride of silver extends from _h_ to h; that is, over the upper or more refrangible half of the space in which the main part of the coronal light is concentrated.] [footnote : _proc. roy. soc._, vol. xxxiv., p. .] [footnote : _report brit. assoc._, , p. .] [footnote : maunder, _indian eclipse_, p. ; _eclipse of _, p. .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] [footnote : see _infra_, p. .] [footnote : abney, _phil. trans._, vol. clxxx., p. .] [footnote : _comptes rendus_, t. xcvii., p. .] [footnote : _memoirs national ac. of sciences_, vol. ii., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : _the sun_, p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _comptes rendus_, t. lxxiii., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : stokes, anniversary address, _nature_, vol. xxxv., p. .] [footnote : _comptes rendus_, t. ci., p. .] [footnote : _harvard annals_, vol. xviii., p. .] [footnote : wesley, _phil. trans._, vol. clxxx., p. .] [footnote : _harvard annals_, vol. xviii, p. .] [footnote : _lick report_, p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _pub. astr. soc. of the pacific_, vol. iii., p. .] [footnote : professor holden concluded, with less qualification, "that so-called 'polar' rays exist at all latitudes on the sun's surface." _lick report_, p. .] [footnote : holden, _report on eclipse of december, _, p. ; charroppin, _pub. astr. soc. of the pacific_, vol. iii., p. .] [footnote : published as the frontispiece to the _observatory_, no. .] [footnote : wesley, _ibid._, p. .] [footnote : _lick observatory contributions_, no. , p. .] [footnote : _astr. and astrophysics_, vol. xiii. p. .] [footnote : lockyer, _phil. trans._, vol. clxxxvii., p. .] [footnote : he died in london, november , .] [footnote : _bull. acad. st. pétersbourg_, t. vi., p. .] [footnote : w. h. wesley, _phil. trans._, vol. cxc, p. .] [footnote : _lick reports on eclipse of january , _, p. .] [footnote : _astroph. jour._, vol. xi., p. .] [footnote : _observatory_, vol. xxi., p. .] [footnote : _the indian eclipse_, , p. .] [footnote : _science_, june , ; _astroph. jour._, vol. xii., p. .] [footnote : _ann. der physik_, bd. xlviii., p. . see also wood, _physical review_, vol. iv., p. , .] [footnote : _science_, august , .] [footnote : _lick observatory bulletin_, no. .] [footnote : _observatory_, vol. xxiv., pp. , .] [footnote : _lick report on eclipse of december , _, p. ; _month. not._, vol. l., p. .] [footnote : _lick obs. bull._, no. .] [footnote : _bull. de l'acad. st. pétersbourg_, t. iv., p. .] [footnote : _the solar corona discussed by spherical harmonics_, smithsonian institution, .] [footnote : bakerian lecture, _proc. roy. soc._, vol. xxxix.] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, vol. xii., p. .] [footnote : _am. journ. of science_, vol. xi., p. , .] [footnote : see huggins, _proc. roy. soc._, vol. xxxix., p. ; young, _north am. review_, february, , p. .] [footnote : professor w. a. norton, of yale college, appears to have been the earliest formal advocate of the expulsion theory of the solar surroundings, in the second ( ) and later editions of his _treatise on astronomy_.] chapter iv _solar spectroscopy_ the new way struck out by janssen and lockyer was at once and eagerly followed. in every part of europe, as well as in north america, observers devoted themselves to the daily study of the chromosphere and prominences. foremost among these were lockyer in england, zöllner at leipzig, spörer at anclam, young at hanover, new hampshire, secchi and respighi at rome. there were many others, but these names stood out conspicuously. the first point to be cleared up was that of chemical composition. leisurely measurements verified the presence above the sun's surface of hydrogen in prodigious volumes, but showed that sodium had nothing to do with the orange-yellow ray identified with it in the haste of the eclipse. from its vicinity to the d-pair (than which it is slightly more refrangible), the prominence-line was, however, designated d_ , and the unknown substance emitting it was named by lockyer "helium." its terrestrial discovery ensued after twenty-six years. in march, , professor ramsay obtained from the rare mineral clevite a volatile gas, the spectrum of which was found to include the yellow prominence-ray. helium was actually at hand, and available for examination. the identification cleared up many obscurities in chromospheric chemistry. several bright lines, persistently seen at the edge of the sun, and early suspected by young[ ] to emanate from the same source as d_ , were now derived from helium in the laboratory; and all the complex emissions of that exotic substance ranged themselves into six sets or series, the members of which are mutually connected by numerical relations of a definite and simple kind. helium is of rather more than twice the density of hydrogen, and has no chemical affinities. in almost evanescent quantities it lurks in the earth's crust, and is diffused through the earth's atmosphere. the importance of the part played in the prominence-spectrum by the violet line of calcium was noticed by professor young in , but since h and k lie near the limit of the visible spectrum, photography was needed for a thorough investigation of their appearances. aided by its resources, professor george e. hale, then at the beginning of his career, detected in their unfailing and conspicuous presence.[ ] the substance emitting them not only constitutes a fundamental ingredient of the chromosphere, but rises, in the fantastic jets thence issuing, to greater heights than hydrogen itself. the isolation of h and k in solar prominences from any other of the lines usually distinctive of calcium was experimentally proved by sir william and lady huggins in to be due to the extreme tenuity of the emitting vapour.[ ] hydrogen, helium, and calcium form, then, the chief and unvarying materials of the solar sierra and its peaks; but a number of metallic elements make their appearance spasmodically under the influence of disturbances in the layers beneath. in september, , young[ ] drew up at dartmouth college a list of lines significant of injections into the chromosphere of iron, titanium, chromium, magnesium, and many other substances. during two months' observation in the pure air of mount sherman ( , feet high) in the summer of , these tell-tale lines mounted up to ;[ ] and he believes their number might still be doubled by steady watching. indeed, both young and lockyer have more than once seen the whole field of the spectroscope momentarily inundated with bright rays, as if the "reversing layer" had been suddenly thrust upwards into the chromosphere, and as quickly allowed to drop back again. the opinion would thus appear to be well-grounded that the two form one continuous region, of which the lower parts are habitually occupied by the heaviest vapours, but where orderly arrangement is continually overturned by violent eruptive disturbances. the study of the _forms_ of prominences practically began with huggins's observation of one through an "open slit" february , .[ ] at first it had been thought possible to examine them only in sections--that is, by admitting mere narrow strips or "lines" of their various kinds of light; while the actual shape of the objects emitting those lines had been arrived at by such imperfect devices as that of giving to the slit of the spectroscope a vibratory moment rapid enough to enable the eye to retain the impression of one part while others were successively presented to it. it was an immense gain to find that their rays had strength to bear so much of dilution with ordinary light as was involved in opening the spectroscopic shutter wide enough to exhibit the tree-like, or horn-like, or flame-shaped bodies rising over the sun's rim in their undivided proportions. several diversely-coloured images of them are formed in the spectroscope; each may be seen under a crimson, a yellow, a green, and a deep blue aspect. the crimson, however (built up out of the c-line of hydrogen), is the most intense, and is commonly used for purposes of observation and illustration. friedrich zöllner was, by a few days, beforehand with huggins in describing the open-slit method, but was somewhat less prompt in applying it. his first survey of a complete prominence, pictured in, and not simply intersected by, the slit of his spectroscope, was obtained july , .[ ] shortly afterwards the plan was successfully adopted by the whole band of investigators. a difference in kind was very soon perceived to separate these objects into two well-marked classes. its natural and obvious character was shown by its having struck several observers independently. the distinction of "cloud-prominences" from "flame-prominences" was announced by lockyer, april ; by zöllner, june ; and by respighi, december , . the first description are tranquil and relatively permanent, sometimes enduring without striking change for many days. certain of the included species mimic terrestrial cloud-scenery--now appearing like fleecy cirrus transpenetrated with the red glow of sunset--now like prodigious masses of cumulo-stratus hanging heavily above the horizon. the solar clouds, however, have the peculiarity of possessing _stems_. slender columns can ordinarily be seen to connect the surface of the chromosphere with its outlying portions. hence the fantastic likeness to forest scenery presented by the long ranges of fiery trunks and foliage occasionally seeming to fringe the sun's limb. but while this mode of structure suggests an actual outpouring of incandescent material, certain facts require a different interpretation. at a distance, and quite apart from the chromosphere, prominences have been perceived, both by secchi and young, to _form_, just as clouds form in a clear sky, condensation being replaced by ignition. filaments were then thrown out downward towards the chromosphere, and finally the usual appearance of a "stemmed prominence" was assumed. still more remarkable was an observation made by trouvelot at harvard college observatory, june , .[ ] a gigantic comma-shaped prominence, , miles high, vanished from before his eyes by a withdrawal of light as sudden as the passage of a flash of lightning. the same observer has frequently witnessed a gradual illumination or gradual extinction of such objects, testifying to changes in the thermal or electrical condition of matter already _in situ_. the first photograph of a prominence, as shown by the spectroscope in daylight, was taken by professor young in .[ ] but neither his method, nor that described by dr. braun in ,[ ] had any practical success. this was reserved to reward the efforts towards the same end of professor hale. begun at harvard college in ,[ ] they were prosecuted soon afterwards at the kenwood observatory, chicago. the great difficulty was to extricate the coloured image of the gaseous structure, spectroscopically visible at the sun's limb, from the encompassing glare, a very little of which goes a long way in _fogging_ sensitive plates. to counteract its mischievous effects, a second slit,[ ] besides the usual narrow one in front of the collimator, was placed on guard, as it were, behind the dispersing apparatus, so as to shut out from the sensitised surface all light save that of the required quality. the sun's image being then allowed to drift across the outer slit, while the plate holder was kept moving at the same rate, the successive sectional impressions thus rapidly obtained finally "built up" a complete picture of the prominence. another expedient was soon afterwards contrived.[ ] the h and k rays of calcium are always, as we have seen, bright in the spectrum of prominences. they are besides fine and sharp, while the corresponding absorption-lines in the ordinary solar spectrum are wide and diffuse. hence, prominences formed by the spectroscope out of these particular qualities of violet light, can be photographed entire and at once, for the simple reason that they are projected upon a naturally darkened background. atmospheric glare is abolished by local absorption. this beautiful method was first realised by professor hale in june, . a "spectroheliograph," consisting of a spectroscopic and a photographic apparatus of special type, attached to the eye-end of an equatoreal twelve inches in aperture, was erected at kenwood in march, ; and with its aid, professor hale entered upon original researches of high promise for the advancement of solar physics. noteworthy above all is his achievement of photographing both prominences and faculæ on the very face of the sun. the latter had, until then, been very imperfectly observed. they were only visible, in fact, when relieved by their brilliancy against the dusky edge of the solar disc. their convenient emission of calcium light, however, makes it possible to photograph them in all positions, and emphasises their close relationship to prominences. the simultaneous picturing, moreover, of the entire chromospheric ring, with whatever trees or fountains of fire chance to be at the moment issuing from it, has been accomplished by a very simple device. the disc of the sun itself having been screened with a circular metallic diaphragm, it is only necessary to cause the slit to traverse the virtually eclipsed luminary, in order to get an impression of the whole round of its fringing appendages. and the record can be extended to the disc by removing the screen, and carrying the slit back at a quicker rate, when an "image of the sun's surface, with the faculæ and spots, is formed on the plate exactly within the image of the chromosphere formed during the first exposure. the whole operation," professor hale continues, "is completed in less than a minute, and the resulting photographs give the first true pictures of the sun, showing all of the various phenomena at its surface."[ ] most of these novel researches were, by a remarkable coincidence, pursued independently and contemporaneously by m. deslandres, of the paris observatory.[ ] the ultra-violet prominence spectrum was photographed for the first time from an uneclipsed sun, in june, , at chicago. besides h and k, four members of the huggins-series of hydrogen-lines imprinted themselves on the plate.[ ] meanwhile m. deslandres was enabled, by fitting quartz lenses to his spectroscope, and substituting a reflecting for a refracting telescope, to get rid of the obstructive action of glass upon the shorter light-waves, and thus to widen the scope of his inquiry into the peculiarities of those derived from prominences.[ ] as the result, not only all the nine white-star lines were photographed from a brilliant sun-flame, but five additional ones were found to continue the series upward. the wave-lengths of these last had, moreover, been calculated beforehand with singular exactness, from a simple formula known as "balmer's law."[ ] the new lines, accordingly, filled places in a manner already prepared for them, and were thus unmistakably associated with the hydrogen-spectrum. this is now known to be represented in prominences by twenty-seven lines,[ ] forming a kind of harmonic progression, only four of which are visibly darkened in the fraunhofer spectrum of the sun. plate i. [illustration: photographs of the solar chromosphere and prominences. taken with the spectroheliograph of the kenwood observatory, chicago, by professor george e. hale.] the chemistry of "cloud-prominences" is simple. hydrogen, helium, and calcium are their chief constituents. "flame-prominences," on the other hand, show, in addition, the characteristic rays of a number of metals, among which iron, titanium, barium, strontium, sodium, and magnesium are conspicuous. they are intensely brilliant; sharply defined in their varying forms of jets, spikes, fountains, waterspouts; of rapid formation and speedy dissolution, seldom attaining to the vast dimensions of the more tranquil kind. eruptive or explosive by origin, they occur in close connection with spots; whether causally, the materials ejected as "flames" cooling and settling down as dark, depressed patches of increased absorption;[ ] or consequentially, as a reactive effect of falls of solidified substances from great heights in the solar atmosphere.[ ] the two classes of phenomena, at any rate, stand in a most intimate relation; they obey the same law of periodicity, and are confined to the same portions of the sun's surface, while quiescent prominences may be found right up to the poles and close to the equator. the general distribution of prominences, including both genera, follows that of faculæ much more closely than that of spots. from father secchi's and professor respighi's observations, - , were derived the first clear ideas on the subject, which have been supplemented and modified by the later researches of professors tacchini and riccò at rome and palermo. the results are somewhat complicated, but may be stated broadly as follows. the district of greatest prominence-frequency covers and overlaps by several degrees that of the greatest spot-frequency. that is to say, it extends to about ° north and south of the equator.[ ] there is a visible tendency to a second pair of maxima nearer the poles. the poles themselves, as well as the equator, are regions of minimum occurrence. distribution in time is governed by the spot-cycle, but the maximum lasts longer for prominences than for spots. the structure of the chromosphere was investigated in and subsequent years by professor respighi, director of the capitoline observatory, as well as by spörer, and brédikhine of the moscow observatory. they found this supposed solar envelope to be of the same eruptive nature as the vast protrusions from it, and to be made up of a congeries of minute flames[ ] set close together like blades of grass. "the appearance," professor young writes,[ ] "which probably indicates a fact, is as if countless jets of heated gas were issuing through vents and spiracles over the whole surface, thus clothing it with flame which heaves and tosses like the blaze of a conflagration." the summits of these filaments of fire are commonly inclined, as if by a wind sweeping over them, when the sun's activity is near its height, but erect during his phase of tranquillity. spörer, in , inferred the influence of permanent polar currents,[ ] but tacchini showed in that the deflections upon which this inference was based ceased to be visible as the spot-minimum drew near.[ ] another peculiarity of the chromosphere, denoting the remoteness of its character from that of a true atmosphere,[ ] is the irregularity of its distribution over the sun's surface. there are no signs of its bulging out at the equator, as the laws of fluid equilibrium in a rotating mass would require; but there are some that the fluctuations in its depth are connected with the phases of solar agitation. at times of minimum it seems to accumulate and concentrate its activity at the poles; while maxima probably bring a more equable general distribution, with local depressions at the base of great prominences and above spots. a low-lying stratum of carbon-vapour was, in , detected in the chromosphere by professor hale with a grating-spectroscope attached to the -inch yerkes refractor.[ ] the eclipse-photographs of disclosed to hartley's examination the presence there of gallium;[ ] and those taken by evershed in were found by jewell[ ] to be crowded with ultra-violet lines of the equally rare metal scandium. the general rule had been laid down by sir norman lockyer that the metallic radiations from the chromosphere are those "enhanced" in the electric spark.[ ] hence, the comparative study of conditions prevalent in the arc and the spark has acquired great importance in solar physics. the reality of the appearance of violent disturbance presented by the "flaming" kind of prominence can be tested in a very remarkable manner. christian doppler,[ ] professor of mathematics at prague, enounced in the theorm that the colour of a luminous body, like the pitch of a sonorous body, must be changed by movements of approach or recession. the reason is this. both colour and pitch are physiological effects, depending, not upon absolute wave-length, but upon the number of waves entering the eye or ear in a given interval of time. and this number, it is easy to see, must be increased if the source of light or sound is diminishing its distance, and diminished if it is decreasing it. in the one case, the vibrating body _pursues_ and crowds together the waves emanating from it; in the other, it _retreats_ from them, and so lengthens out the space covered by an identical number. the principle may be thus illustrated. suppose shots to be fired at a target at fixed intervals of time. if the marksman advances, say twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals. the result will of course be the same whether the target or the marksman be in movement. so far doppler was altogether right. as regards sound, anyone can convince himself that the effect he predicted is a real one, by listening to the alternate shrilling and sinking of the steam-whistle when an express train rushes through a station. but in applying this principle to the colours of stars he went widely astray; for he omitted from consideration the double range of invisible vibrations which partake of, and to the eye exactly compensate, changes of refrangibility in the visible rays. there is, then, no possibility of finding a criterion of velocity in the hue of bodies shining, like the sun and stars, with continuous light. the entire spectrum is slightly shifted up or down in the scale of refrangibility; certain rays normally visible become exalted or degraded (as the case may be) into invisibility, and certain other rays at the opposite end undergo the converse process; but the sum total of impressions on the retina continues the same. we are not, however, without the means of measuring this sub-sensible transportation of the light-gamut. once more the wonderful fraunhofer lines came to the rescue. they were called by the earlier physicists "fixed lines;" but it is just because they are _not_ fixed that, in this instance, we find them useful. they share, and in sharing betray, the general shift of the spectrum. this aspect of doppler's principle was adverted to by fizeau in ,[ ] and the first tangible results in the estimation of movements of approach and recession between the earth and the stars, were communicated by sir william huggins to the royal society, april , . eighteen months later, zöllner devised his "reversion-spectroscope"[ ] for doubling the measurable effects of line-displacements; aided by which ingenious instrument, and following a suggestion of its inventor, professor h. c. vogel succeeded at bothkamp, june , ,[ ] in detecting effects of that nature due to the solar rotation. this application constitutes at once the test and the triumph of the method.[ ] the eastern edge of the sun is continually moving towards us with an equatorial speed of about a mile and a quarter per second, the western edge retreating at the same rate. the displacements--towards the violet on the east, towards the red on the west--corresponding to this velocity are very small; so small that it seems hardly credible that they should have been laid bare to perception. they amount to but / th part of the interval between the two constituents of the d-line of sodium; and the d-line of sodium itself can be separated into a pair only by a powerful spectroscope. nevertheless, professor young[ ] was able to show quite satisfactorily, in , not only deviations in the solar lines from their proper places indicating a velocity of rotation ( · miles per second) slightly in excess of that given by observations of spots, but the exemption of terrestrial lines (those produced by absorption in the earth's atmosphere) from the general push upwards or downwards. shortly afterwards, professor langley, then director of the allegheny observatory, having devised a means of comparing with great accuracy light from different portions of the sun's disc, found that while the obscure rays in two juxtaposed spectra derived from the solar poles were absolutely continuous, no sooner was the instrument rotated through °, so as to bring its luminous supplies from opposite extremities of the equator, than the same rays became perceptibly "notched." the telluric lines, meanwhile, remained unaffected, so as to be "virtually mapped" by the process.[ ] this rapid and unfailing mode of distinction was used by cornu with perfect ease during his investigation of atmospheric absorption near loiret in august and september, .[ ] a beautiful experiment of the same kind was performed by m. thollon, of m. bischoffsheim's observatory at nice, in the summer of .[ ] he confined his attention to one delicately defined group of four lines in the orange, of which the inner pair are solar (iron) and the outer terrestrial. at the centre of the sun the intervals separating them were sensibly equal; but when the light was taken alternately from the right and left limbs, a relative shift in alternate directions of the solar, towards and from the stationary telluric rays became apparent. a parallel observation was made at dunecht, december , , when it was noticed that a strong iron-line in the yellow part of the solar spectrum is permanently double on the sun's eastern, but single on his western limb;[ ] opposite motion-displacements bringing about this curious effect of coincidence with, and separation from, an adjacent stationary line of our own atmosphere's production, according as the spectrum is derived from the retreating or advancing margin of the solar globe. statements of fact so precise and authoritative amount to a demonstration that results of this kind are worthy of confidence; and they already occupy an important place among astronomical data. the subtle method of which they served to assure the validity was employed in - by m. dunér to test and extend carrington's and spörer's conclusions as to the anomalous nature of the sun's axial movement.[ ] his observations for the purpose, made with a fine diffraction-spectroscope, just then mounted at the observatory of upsala, were published in .[ ] their upshot was to confirm and widen the law of retardation with increasing latitude derived from the progressive motions of spots. determinations made within ° of the pole, consequently far beyond the region of spots, gave a rotation-period of - / , that of the equatorial belt being of - / days. spots near the equator indeed complete their rounds in a period shorter by at least half a day; and proportionate differences were found to exist elsewhere in corresponding latitudes; but dunér's observations, it must be remembered, apply to a distinct part of the complex solar machine from the disturbed photospheric surface. it is amply possible that the absorptive strata producing the fraunhofer lines, significant, by their varying displacements at either limb, of the inferred varying rates of rotation, may gyrate more slowly than the spot-generating level. moreover, faculæ appear to move at a quicker pace than either;[ ] so that we have, for three solar formations, three different periods of average rotation, the shortest of which belongs to the faculæ, one of intermediate length to the spots, and the most protracted to the reversing layer. all, however, agree in lengthening progressively from the equator towards the poles. professor holden aptly compared the sun to "a vast whirlpool where the velocities of rotation depend not only on the situation of the rotating masses as to latitude, but also as to depth beneath the exterior surface."[ ] sir norman lockyer[ ] promptly perceived the applicability of the surprising discovery of line-shiftings through end-on motion to the study of prominences, the discontinuous light of which affords precisely the same means of detecting movement without seeming change of place, as do lines of absorption in a continuous spectrum. indeed, his observations at the sun's edge almost compelled recourse to an explanation made available just when the need of it began to be felt. he saw bright lines, not merely pushed aside from their normal places by a barely perceptible amount, but bent, torn, broken, as if by the stress of some tremendous violence. these remarkable appearances were quite simply interpreted as the effects of movements varying in amount and direction in the different parts of the extensive mass of incandescent vapours falling within a single field of view. very commonly they are of a cyclonic character. the opposite distortions of the same coloured rays betray the fury of "counter-gales" rushing along at the rate of miles a second; while their undisturbed sections prove the persistence of a "heart of peace" in the midst of that unimaginable fiery whirlwind. velocities up to _miles a second_, or , times that of an express train at the top of its speed, were thus observed by young during his trip to mount sherman, august , ; and these were actually doubled in an extraordinary outburst observed by father jules fényi, on june , , at the haynald observatory in hungary, as well as by m. trouvelot at meudon.[ ] motions ascertainable in this way near the limb are, of course, horizontal as regards the sun's surface; the analogies they present might, accordingly, be styled _meteorological_ rather than _volcanic_. but vertical displacements on a scale no less stupendous can also be shown to exist. observations of the spectra of spots centrally situated (where motions in the line of sight are vertical) disclose the progress of violent uprushes and downrushes of ignited gases, for the most part in the penumbral or outlying districts. they appear to be occasioned by fitful and irregular disturbances, and have none of the systematic quality which would be required for the elucidation of sun-spot theories. indeed, they almost certainly take place at a great height above the actual openings in the photosphere. as to vertical motions above the limb, on the other hand, we have direct visual evidence of a truly amazing kind. the projected glowing matter has, by the aid of the spectroscope, been watched in its ascent. on september , , young examined at noon a vast hydrogen cloud , miles long, as it showed to the eye, and , high. it floated tranquilly above the chromosphere at an elevation of some , miles, and was connected with it by three or four upright columns, presenting the not uncommon aspect compared by lockyer to that of a grove of banyans. called away for a few minutes at . , on returning at . the observer found-- "that in the meantime the whole thing had been literally blown to shreds by some inconceivable uprush from beneath. in place of the quiet cloud i had left, the air, if i may use the expression, was filled with flying débris--a mass of detached, vertical, fusiform filaments, each from " to " long by " or " wide,[ ] brighter and closer together where the pillars had formerly stood, and rapidly ascending. they rose, with a velocity estimated at miles a second, to fully , miles above the sun's surface, then gradually faded away like a dissolving cloud, and at . only a few filmy wisps, with some brighter streamers low down near the photosphere, remained to mark the place."[ ] a velocity of projection of _at least_ miles per second was, by proctor's[ ] calculation, required to account for this extraordinary display, to which the earth immediately responded by a magnetic disturbance, and a fine aurora. it has proved by no means an isolated occurrence. young saw its main features repeated, october , ,[ ] on a still vaster scale; for the exploded prominence attained, this time, an altitude of , miles--the highest yet chronicled. lockyer, moreover, has seen a prominence , miles high shattered in ten minutes; while uprushes have been witnessed by respighi, of which the initial velocities were judged by him to be or miles a second. when it is remembered that a body starting from the sun's surface at the rate of miles a second would, if it encountered no resistance, escape for ever from his control, it is obvious that we have, in the enormous forces of eruption or repulsion manifested in the outbursts just described, the means of accounting for the vast diffusion of matter in the solar neighbourhood. nor is it possible to explain them away, as cornu,[ ] faye,[ ] and others have sought to do, by substituting for the rush of matter in motion, progressive illumination through electric discharges, chemical processes,[ ] or even through the mere reheating of gases cooled by expansion.[ ] all the appearances are against such evasions of the difficulty presented by velocities stigmatised as "fabulous" and "improbable," but which, there is the strongest reason to believe, really exist. on the th of december, , sir norman lockyer formally expounded before the royal society his hypothesis of the compound nature of the "chemical elements."[ ] an hypothesis, it is true, over and over again propounded from the simply terrestrial point of view. what was novel was the supra-terrestrial evidence adduced in its support; and even this had been, in a general and speculative way, anticipated by professor f. w. clarke of washington.[ ] lockyer had been led to his conclusion along several converging lines of research. in a letter to m. dumas, dated december , , he had sketched out the successive stages of "celestial dissociation" which he conceived to be represented in the sun and stars. the absence from the solar spectrum of metalloidal absorption he explained by the separation, in the fierce solar furnace, of such substances as oxygen, nitrogen, sulphur, and chlorine, into simpler constituents possessing unknown spectra; while metals were at that time still admitted to be capable of existing there in a state of integrity. three years later he shifted his position onward. he announced, as the result of a comparative study of the fraunhofer and electric-arc spectra of calcium, that the "molecular grouping" of that metal, which at low temperatures gives a spectrum with its chief line in the blue, is nearly broken up in the sun into another or others with lines in the violet.[ ] this came to be regarded by him as "a truly typical case."[ ] during four years ( - inclusive) this diligent observer was engaged in mapping a section of the more refrangible part of the solar spectrum (wave-lengths , - , ) on a scale of magnitude such that, if completed down to the infra-red, its length would have been about _half a furlong_. the attendant laborious investigation, by the aid of photography, of metallic spectra, seemed to indicate the existence of what he called "basic lines." these held their ground persistently in the spectra of two or more metals after all possible "impurities" had been eliminated, and were therefore held to attest the presence of a common substratum of matter in a simpler state of aggregation than any with which we are ordinarily acquainted. later inquiries have shown, however, that between the spectral lines of different substances there are probably no _absolute_ coincidences. "basic" lines are really formed of doublets or triplets merged together by insufficient dispersion. of thalèn's original list of seventy rays common to several spectra,[ ] very few resisted thollon's and young's powerful spectroscopes; and the process of resolution was completed by rowland. thus the argument from community of lines to community of substance has virtually collapsed. it was replaced by one founded on certain periodical changes on the spectra of sun-spots. they emerged from a series of observations begun at south kensington under sir norman lockyer's direction in , and continued for fifteen years.[ ] the principle of the method employed is this. the whole range of fraunhofer lines is visible when the light from a spot is examined with the spectroscope; but relatively few are widened. now these widened lines alone constitute (presumably) the true spot-spectrum; they, and they alone, tell what kinds of vapour are thrust down into the strange dusky pit of the nucleus, the unaffected lines taking their accustomed origin from the over-lying strata of the normal solar atmosphere. here then we have the criterion that was wanted--the means of distinguishing, spectroscopically and chemically, between the cavity and the absorbing layers piled up above it. by its persistent employment some marked peculiarities have been brought out, such as the unfamiliar character of numerous lines in spot-spectra, especially at epochs of disturbance; and the strange _individuality_ in the behaviour of every one of these darkened and distended rays. each seems to act on its own account; it comports itself as if it were the sole representative of the substance emitting it; its appearance is unconditioned by that of any of its terrestrial companions in the same spectrum. the most curious fact, however, elicited by these inquiries was that of the attendance of chemical vicissitudes upon the advance of the sun-spot period. as the maximum approached, unknown replaced known components of the spot-spectra in a most pronounced and unmistakable way.[ ] it seemed as if the vapours emitting lines of iron, titanium, nickel, etc., had ceased to exist as such, and their room been taken by others, total strangers in terrestrial laboratories. these were held by lockyer to be simply the finer constituents of their predecessors, dissociation having been effected by the higher temperature ensuing upon increased solar activity. but father cortie's supplementary investigations at stonyhurst[ ] modified, while they in the main substantiated, the south kensington results. they showed that the substitution of unknown for known lines characterizes disturbed spots, at all stages of the solar cycle, so that no systematic course of chemical change can be said to affect the sun as a whole. they showed further[ ]--from evidence independent of that obtained by young in [ ]--the remarkable conspicuousness in spot-spectra of vanadium lines excessively faint in the fraunhofer spectrum. lockyer's "unknown lines" may probably thus be accounted for. they represent absorption, not by new, but by scarce elements, especially, father cortie thinks, those with atomic weights of about . the circumstance of their development in solar commotions, largely to the exclusion of iron, is none the less curious; but it cannot be explained by any process of dissociation. the theory has, however, to be considered under still another aspect. it frequently happens that the contortions or displacements due to motion are seen to affect a single line belonging to a particular substance, while the other lines of _that same substance_ remain imperturbable. now, how is this most singular fact, which seems at first sight to imply that a body may be at rest and in motion at one and the same instant, to be accounted for? it is accounted for, on the present hypothesis, easily enough, by supposing that the rays thus discrepant in their testimony, do _not_ belong to one kind of matter, but to several, combined at ordinary temperatures to form a body in appearance "elementary." of these different vapours, one or more may of course be rushing rapidly towards or from the observer, while the others remain still; and since the line of sight across the average prominence-region penetrates, at the sun's edge, a depth of about , miles,[ ] all the incandescent materials separately occurring along which line are projected into a single "flame" or "cloud," it will be perceived that there is ample room for diversities of behaviour. the alternative mode of escape from the perplexity consists in assuming that the vapour in motion is rendered luminous under conditions which reduce its spectrum to a few rays, the unaffected lines being derived from a totally distinct mass of the same substance shining with its ordinary emissions.[ ] thus, calcium can be rendered virtually monochromatic by attenuation, and analogous cases are not rare. sir norman lockyer only asks us to believe that effects which follow certain causes on the earth are carried a stage further in the sun, where the same causes must be vastly intensified. we find that the bodies we call "compound" split asunder at fixed degrees of heat _within_ the range of our resources. why should we hesitate to admit that the bodies we call "simple" do likewise at degrees of heat _without_ the range of our resources? the term "element" simply expresses terrestrial incapability of reduction. that, in celestial laboratories, the means and their effect here absent should be present, would be an inference challenging, in itself, no expression of incredulity. there are indeed theoretical objections to it which, though probably not insuperable, are unquestionably grave. our seventy chemical "elements," for instance, are placed by the law of specific heats on a separate footing from their known compounds. we are not, it is true, compelled by it to believe their atoms to be really and absolutely such--to contain, that is, the "irreducible minimum" of material substance; but we do certainly gather from it that they are composed on a different principle from the salts and oxides made and unmade at pleasure by chemists. then the multiplication of the species of matter with which lockyer's results menace us, is at first sight startling. they may lead, we are told, to eventual unification, but the prospect appears remote. their only obvious outcome is the disruption into several constituents of each terrestrial "element." the components of iron alone should be counted by the dozen. and there are other metals, such as cerium, which, giving a still more complex spectrum, would doubtless be still more numerously resolved. sir norman lockyer interprets the observed phenomena as indicating the successive combinations, in varying proportions, of a very few original ingredients;[ ] but no definite sign of their existence is perceptible; "protyle" seems likely long to evade recognition; and the only intelligible underlying principle for the reasonings employed--that of "one line, one element"--implies a throng beyond counting of formative material units. thus, added complexity is substituted for that fundamental unity of matter which has long formed the dream of speculators. and it is extremely remarkable that sir william crookes, working along totally different lines, has been led to analogous conclusions. to take only one example. as the outcome of extremely delicate operations of sifting and testing carried on for years, he finds that the metal yttrium splits up into five, if not eight constituents.[ ] evidently, old notions are doomed, nor are any preconceived ones likely to take their place. it would seem, on the contrary, as if their complete reconstruction were at hand. subversive facts are steadily accumulating; the revolutionary ideas springing from them tend, if we interpret them aright, towards the substitution of electrical for chemical theories of matter. dissociation by the brute force of heat is already nearly superseded, in the thoughts of physicists, by the more delicate process of "ionisation." precisely what this implies and involves we do not know; but the symptoms of its occurrence are probably altogether different from those gathered by sir norman lockyer from the collation of celestial spectra. a. j. Ångström of upsala takes rank after kirchhoff as a subordinate founder, so to speak, of solar spectroscopy. his great map of the "normal" solar spectrum[ ] was published in , two years before he died. robert thalèn was his coadjutor in its execution, and the immense labour which it cost was amply repaid by its eminent and lasting usefulness. for more than a score of years it held its ground as the universal standard of reference in all spectroscopic inquiries within the range of the _visible_ emanations. those that are invisible by reason of the quickness of their vibrations were mapped by dr. henry draper, of new york, in , and with superior accuracy by m. cornu in . the infra-red part of the spectrum, investigated by langley, abney, and knut Ångström, reaches perhaps no definite end. the radiations oscillating too slowly to affect the eye as light may pass by insensible gradations into the long hertzian waves of electricity.[ ] professor rowland's photographic map of the solar spectrum, published in , and in a second enlarged edition in , opened fresh possibilities for its study, from far down in the red to high up in the ultra-violet, and the accompanying scale of absolute wave-lengths[ ] has been, with trifling modifications, universally adopted. his new table of standard solar lines was published in .[ ] through his work, indeed, knowledge of the solar spectrum so far outstripped knowledge of terrestrial spectra, that the recognition of their common constituents was hampered by intolerable uncertainties. thousands of the solar lines charted with minute precision remained unidentified for want of a corresponding precision in the registration of metallic lines. rowland himself, however, undertook to provide a remedy. aided by lewis e. jewell, he redetermined, at the johns hopkins university, the wave-lengths of about , solar lines,[ ] photographing for comparison with them the spectra of all the known chemical elements except gallium, of which he could procure no specimen. the labour of collation was well advanced when he died at the age of fifty-two, april , . investigations of metallic arc-spectra have also been carried out with signal success by hasselberg,[ ] kayser and runge, o. lohse,[ ] and others. another condition _sine quâ non_ of progress in this department is the separation of true solar lines from those produced by absorption in our own atmosphere. and here little remains to be done. thollon's great atlas[ ] was designed for this purpose of discrimination. each of its thirty-three maps exhibits in quadruplicate a subdivision of the solar spectrum under varied conditions of weather and zenith-distance. telluric effects are thus made easily legible, and they account wholly for , partly for , out of a total of , lines. but the death of the artist, april , , unfortunately interrupted the half-finished task of the last seven years of his life. a most satisfactory record, meanwhile, of selective atmospheric action has been supplied by the experiments and determinations of janssen, cornu and egoroff, by dr. becker's drawings,[ ] and mr. mcclean's photographs of the analysed light of the sun at high, low, and medium altitudes; and the autographic pictures obtained by mr. george higgs, of liverpool, of certain rhythmical groups in the red, emerging with surprising strength near sunset, excite general and well-deserved admiration.[ ] the main interest, however, of all these documents resides in the information afforded by them regarding the chemistry of the sun. the discovery that hydrogen exists in the atmosphere of the sun was made by Ångström in . his list of solar elements published in that year,[ ] the result of an investigation separate from, though conducted on the same principle as kirchhoff's, included the substance which we now know to be predominant among them. dr. plücker of bonn had identified in the fraunhofer line f with the green ray of hydrogen, but drew no inference from his observation. the agreement was verified by Ångström; two further coincidences were established; and in a fourth hydrogen line in the extreme violet (named _h_) was detected in the solar spectrum. with thalèn, he besides added manganese, titanium, and cobalt to the constituents of the sun enumerated by kirchhoff, and raised the number of identical rays in the solar and terrestrial spectra of iron to no less than .[ ] thus, when sir norman lockyer entered on that branch of inquiry in , fourteen substances were recognised as common to the earth and sun. early in he was able to increase the list provisionally to thirty-three,[ ] all except hydrogen metals. this rapid success was due to his adoption of the test of _length_ in lieu of that of _strength_ in the comparison of lines. he measured their relative significance, in other words, rather by their persistence through a wide range of temperature, than by their brilliancy at any one temperature. the distinction was easily drawn. photographs of the electric arc, in which any given metal had been volatilised, showed some of the rays emitted by it stretching across the axis of the light to a considerable distance on either side, while many others clung more or less closely to its central hottest core. the former "long lines," regarded as certainly representative, were those primarily sought in the solar spectrum; while the attendant "short lines," often, in point of fact, due to foreign admixtures, were set aside as likely to be misleading.[ ] the criterion is a valuable one, and its employment has greatly helped to quicken the progress of solar chemistry. carbon was the first non-metallic element discovered in the sun. messrs. trowbridge and hutchins of harvard college concluded in ,[ ] on the ground of certain spectral coincidences, that this protean substance is vaporised in the solar atmosphere at a temperature approximately that of the voltaic arc. partial evidence to the same effect had earlier been alleged by lockyer, as well as by liveing and dewar; and the case was rendered tolerably complete by photographs taken by kayser and runge in .[ ] it was by professor rowland shown to be irresistible. two hundred carbon-lines were, through his comparisons, sifted out from sunlight, and it contains others significant of the presence of silicon--a related substance, and one as important to rock-building on the earth, as carbon is to the maintenance of life. the general result of rowland's labours was the establishment among solar materials, not only of these two out of the fourteen metalloids, or non-metallic substances, but of thirty-three metals, including silver and tin. gold, mercury, bismuth, antimony, and arsenic were discarded from the catalogue; platinum and uranium, with six other metals, remained doubtful; while iron was recorded as crowding the spectrum with over two thousand obscure rays.[ ] gallium-absorption was detected in it by hartley and ramage in .[ ] dr. henry draper[ ] announced, in , his imagined discovery, in the solar spectrum, of eighteen especially brilliant spaces corresponding to oxygen-emissions. but the agreement proved, when put to the test of very high dispersion, to be wholly illusory.[ ] nor has it yet been found possible to identify, in analysed sunlight, any significant _bright_ beams.[ ] the book of solar chemistry must be read in characters exclusively of absorption. nevertheless, the whole truth is unlikely to be written there. that a substance displays none of its distinctive beams in the spectrum of the sun or of a star, affords scarcely a presumption against its presence. for it may be situated below the level where absorption occurs, or under a pressure such as to efface lines by widening and weakening them; it may be at a temperature so high that it gives out more light than it takes up, and yet its incandescence may be masked by the absorption of other bodies; finally, it may just balance absorption by emission, with the result of complete spectral neutrality. an instructive example is that of the chromospheric element helium. father secchi remarked in [ ] that there is no dark line in the solar spectrum matching its light; and his observation has been fully confirmed.[ ] helium-absorption is, however, occasionally noticed in the penumbræ of spots.[ ] our terrestrial vital element might then easily subsist unrecognisably in the sun. the inner organisation of the oxygen molecule is a considerably _plastic_ one. it is readily modified by heat, and these modifications are reflected in its varying modes of radiating light. dr. schuster enumerated in [ ] four distinct oxygen spectra, corresponding to various stages of temperature, or phases of electrical excitement; and a fifth has been added by m. egoroff's discovery in [ ] that certain well-known groups of dark lines in the red end of the solar spectrum (fraunhofer's a and b) are due to absorption by the cool oxygen of our air. these persist down to the lowest temperatures, and even survive a change of state. they are produced essentially the same by liquid, as by aërial oxygen.[ ] it seemed, however, possible to m. janssen that these bands owned a joint solar and terrestrial origin. oxygen in a fit condition to produce them might, he considered, exist in the outer atmosphere of the sun; and he resolved to decide the point. no one could bring more skill and experience to bear upon it than he.[ ] by observations on the summit of the faulhorn, as well as by direct experiment, he demonstrated, nearly thirty years ago, the leading part played by water-vapour in generating the atmospheric spectrum; and he had recourse to similar means for appraising the share in it assignable to oxygen. an electric beam, transmitted from the eiffel tower to meudon in the summer of , having passed through a weight of oxygen about equal to that piled above the surface of the earth, showed the groups a and b just as they appear in the high-sun spectrum.[ ] atmospheric action is then adequate to produce them. but m. janssen desired to prove, in addition, that they diminish proportionately to its amount. his ascent of mont blanc[ ] in was undertaken with this object. it was perfectly successful. in the solar spectrum, examined from that eminence, oxygen-absorption was so much enfeebled as to leave no possible doubt of its purely telluric origin. under another form, nevertheless, it has been detected as indubitably solar. a triplet of dark lines low down in the red, photographed from the sun by higgs and mcclean, was clearly identified by runge and paschen in [ ] with the fundamental group of an oxygen series, first seen by piazzi smyth in the spectrum of a vacuum-tube in .[ ] the _pabulum vitæ_ of our earth is then to some slight extent effective in arresting transmitted sunlight, and oxygen must be classed as a solar element. the rays of the sun, besides being stopped selectively in our atmosphere, suffer also a marked general absorption. this tells chiefly upon the shortest wave-lengths; the ultra-violet spectrum is in fact closed, as if by the interposition of an opaque screen. nor does the screen appear very sensibly less opaque from an elevation of , feet. dr. simony's spectral photographs, taken on the peak of teneriffe,[ ] extended but slightly further up than m. cornu's, taken in the valley of the loire. could the veil be withdrawn, some indications as to the originating temperature of the solar spectrum might be gathered from its range, since the proportion of quick vibrations given out by a glowing body grows with the intensity of its incandescence. and this brings us to the subject of our next chapter. footnotes: [footnote : _phil. mag._, vol. xlii., p. , .] [footnote : _astr. nach._, no. , , _amer. jour._, vol. xlii., p. ; deslandres, _comptes rendus_, t. cxiii., p. .] [footnote : _proc. roy. society_, vol. lxi., p. .] [footnote : _phil. mag._, vol. xlii., p. .] [footnote : frost-scheiner, _astr. spectroscopy_, pp. , .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _am. jour. of science_, vol. xv., p. .] [footnote : _journ. franklin institute_, vol. xl., p. _a_.] [footnote : _pogg. annalen_, bd. cxlvi., p. ; _astr. nach._, no. , .] [footnote : _astr. nach._, nos. , , , .] [footnote : this device was suggested by janssen in .] [footnote : _astr. and astrophysics_, vol. xi., pp. , .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _comptes rendus_, t. cxiii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, pp. , .] [footnote : wiedemann's _annalen der physik_, bd. xxv., p. .] [footnote : evershed, _knowledge_, vol. xxi., p. .] [footnote : secchi, _le soleil_, t. ii., p. .] [footnote : lockyer, _chemistry of the sun_, p. .] [footnote : _l'astronomie_, august, , p. (riccò); see also evershed, _jour. british astr. ass._, vol. ii., p. .] [footnote : averaging about miles across and high. _le soleil_, t. ii., p. .] [footnote : _the sun_, p. .] [footnote : _astr. nach._, no. , .] [footnote : _mem. degli spettroscopisti italiani_, t. v., p. ; secchi, _ibid._, t. vi., p. .] [footnote : its non-atmospheric character was early defined by proctor, _month. not._, vol. xxxi., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _ibid._, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _sun's place in nature_, pp. , .] [footnote : _abh. d. kön. böhm ges. d. wiss._, bd. ii., - , p. .] [footnote : in a paper read before the société philomathique de paris, december , , and first published _in extenso_ in _ann. de chim. et de phys._, t. xix., p. ( ). hippolyte fizeau died in september, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : a. cornu, _sur la méthode doppler-fizeau_, p. d. .] [footnote : _am. jour. of sc._, vol. xii., p. .] [footnote : _ibid._, vol. xiv., p. .] [footnote : _bull. astronom._, february, , p. .] [footnote : _comptes rendus_, t. xci., p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : see _ante_, p. .] [footnote : _recherches sur la rotation du soleil_, upsal, .] [footnote : harzer, _astr. nach._, no. , ; stratonoff, _ibid._, no. , .] [footnote : _publ. astr. pacific soc._, vol. ii., p. .] [footnote : _proc. roy. society_, vols. xvii., p. ; xviii., p. .] [footnote : _comptes rendus_, t. cxii., p. ; t. cxiii., p. .] [footnote : at the sun's distance, one second of arc represents about miles.] [footnote : _amer. jour. of sc._, vol. ii., p. , .] [footnote : _month. not._, vol. xxxii., p. .] [footnote : _nature_, vol. xxiii., p. .] [footnote : _comptes rendus_, t. lxxxvii., p. .] [footnote : _ibid._, t. xcvi., p. .] [footnote : a. brester, _théorie du soleil_, p. .] [footnote : such prominences as have been seen to grow by the spread of incandescence are of the quiescent kind, and present no deceptive appearance of violent motion.] [footnote : _proc. roy. soc._, vol. xxviii., p. .] [footnote : "evolution and the spectroscope," _pop. science monthly_, january, .] [footnote : _proc. roy. soc._, vol. xxiv., p. . these are the h and k of prominences. h. w. vogel discovered in a hydrogen-line nearly coincident with h (_monatsb. preuss. ak._, february, , p. ).] [footnote : _proc. roy. soc._, vol. xxviii., p. .] [footnote : many of these were referred by lockyer himself, who first sifted the matter, to traces of the metals concerned.] [footnote : _chemistry of the sun_, p. ; _proc. roy. society_, vol. lvii., p. .] [footnote : _lockyer's chemistry of the sun_, p. .] [footnote : _month. not._, vol. li., p. .] [footnote : _ibid._, vol. lviii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : thollon's estimate (_comptes rendus_, t. xcvii., p. ) of , _kilometres_, seems considerably too low. limiting the "average prominence region" to a shell , miles deep ( ' of arc as seen from the earth), the visual line will, at mid-height ( , miles from the sun's surface), travel through (in round numbers) , miles of that region.] [footnote : liveing and dewar, _phil. mag._, vol. xvi. ( th ser.), p. .] [footnote : _chemistry of the sun_, p. .] [footnote : _nature_, october , .] [footnote : the normal spectrum is that depending exclusively upon wave-length--the fundamental constant given by nature as regards light. it is obtained by the interference of rays, in the manner first exemplified by fraunhofer, and affords the only unvarying standard for measurement. in the refraction spectrum (upon which kirchhoff's map was founded), the relative positions of the lines vary with the material of the prisms.] [footnote : scheiner, _die spectralanalyse der gestirne_, p. .] [footnote : _phil. mag._, vol. xxvii., p. .] [footnote : _astr. and astrophysics_, vol. xii., p. ; frost-scheiner, _astr. spectr._, p. .] [footnote : published in _astroph. jour._, vols. i. to vi.] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _annales de l'observatoire de nice_, t. iii., .] [footnote : _trans. royal society of edinburgh_, vol. xxxvi., p. .] [footnote : rev. a. l. cortie, _astr. and astrophysics_, vol. xi., p. . specimens of his photographs were given by ranyard in _knowledge_, vol. xiii., p. .] [footnote : _ann. d. phys._, bd. cxvii., p. .] [footnote : _comptes rendus_, t. lxiii., p. .] [footnote : _ibid._, t. lxxxvi., p. . some half dozen of these identifications have proved fallacious.] [footnote : _chemistry of the sun_, p. .] [footnote : _amer. jour. of science_, vol. xxxiv., p. .] [footnote : _berlin abhandlungen_, .] [footnote : _amer. jour. of science_, vol. xli., p. . see appendix, table ii.] [footnote : _astrophy. jour._, vol. ix., p. ; fowler, _knowledge_, vol. xxiii., p. .] [footnote : _amer. jour, of science_, vol. xiv., p. ; _nature_, vol. xvi., p. ; _month. not._, vol. xxxix., p. .] [footnote : _month. not._, vol. xxxviii., p. ; trowbridge and hutchins, _amer. jour. of science_, vol. xxxiv., p. .] [footnote : scheiner, _die spectralanalyse_, p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : rev. a. l. cortie, _month. not._, vol. li., p. .] [footnote : young, _the sun_, p. ; hale, _astr. and astrophysics_, vol. xi., p. buss, _jour. brit. astr. ass._, vol. ix., p. .] [footnote : _phil. trans._, vol. clxx., p. .] [footnote : _comptes rendus_, t. xcvii., p. ; t. ci., p. .] [footnote : liveing and dewar, _astr. and astrophysics_, vol. xi., p. .] [footnote : _comptes rendus_, t. lx., p. ; t. lxiii., p. .] [footnote : _ibid._, t. cviii., p. .] [footnote : _ibid._, t. cxi., p. .] [footnote : _astroph. jour._, vols. iv., p. ; vi., p. .] [footnote : _trans. roy. soc. edin._, vol. xxxii., p. .] [footnote : _comptes rendus_, t. cxi., p. ; huggins, _proc. roy. soc._, vol. xlvi., p. .] chapter v _temperature of the sun_ newton was the first who attempted to measure the quantity of heat received by the earth from the sun. his object in making the experiment was to ascertain the temperature encountered by the comet of at its passage through perihelion. he found it, by multiplying the observed heating effects of direct sunshine according to the familiar rule of the "inverse squares of the distances," to be about , times that of red-hot iron.[ ] determinations of the sun's thermal power, made with some scientific exactness, date, however, from . a few days previous to the beginning of that year, herschel began observing at the cape of good hope with an "actinometer," and obtained results agreeing quite satisfactorily with those derived by pouillet from experiments made in france some months later with a "pyrheliometer."[ ] pouillet found that the vertical rays of the sun falling on each square centimetre of the earth's surface are competent (apart from atmospheric absorption) to raise the temperature of · grammes of water one degree centigrade per minute. this number ( · ) he called the "solar constant"; and the unit of heat chosen is known as the "calorie." hence it was computed that the total amount of solar heat received during a year would suffice to melt a layer of ice covering the entire earth to a depth of · metres, or feet; while the heat emitted would melt, at the sun's surface, a stratum · metres thick each minute. a careful series of observations showed that nearly half the heat incident upon our atmosphere is stopped in its passage through it. herschel got somewhat larger figures, though he assigned only a third as the spoil of the air. taking a mean between his own and pouillet's, he calculated that the ordinary expenditure of the sun per minute would have power to melt a cylinder of ice feet in diameter, reaching from his surface to that of alpha centauri; or, putting it otherwise, that an ice-rod · miles across, continually darted into the sun with the velocity of light, would scarcely consume, in dissolving, the thermal supplies now poured abroad into space.[ ] it is nearly certain that this estimate should be increased by about two-thirds in order to bring it up to the truth. nothing would, at first sight, appear simpler than to pass from a knowledge of solar emission--a strictly measurable quantity--to a knowledge of the solar temperature; this being defined as the temperature to which a surface thickly coated with lamp-black (that is, of standard radiating power) should be raised to enable it to send us, from the sun's distance, the amount of heat actually received from the sun. sir john herschel showed that heat-rays at the sun's surface must be , times as dense as when they reach the earth; but it by no means follows that either the surface emitting, or a body absorbing those heat-rays must be , times hotter than a body exposed here to the full power of the sun. the reason is, that the rate of emission--consequently the rate of absorption, which is its correlative--increases very much faster than the temperature. in other words, a body radiates or cools at a continually accelerated pace as it becomes more and more intensely heated above its surroundings. newton, however, took it for granted that radiation and temperature advance _pari passu_--that you have only to ascertain the quantity of heat received from, and the distance of a remote body in order to know how hot it is.[ ] and the validity of this principle, known as "newton's law" of cooling, was never questioned until de la roche pointed out, in ,[ ] that it was approximately true only over a low range of temperature; while five years later, dulong and petit generalised experimental results into the rule, that while temperature grows by arithmetical, radiation increases by geometrical progression.[ ] adopting this formula, pouillet derived from his observations on solar heat a solar temperature of somewhere between , ° and , ° c. now, the higher of these points--which is nearly that of melting platinum--is undoubtedly surpassed at the focus of certain burning-glasses which have been constructed of such power as virtually to bring objects placed there within a quarter of a million of miles of the photosphere. in the rays thus concentrated, platinum and diamond become rapidly vaporised, notwithstanding the great loss of heat by absorption, first in passing through the air, and again in traversing the lens. pouillet's maximum is then manifestly too low, since it involves the absurdity of supposing a radiating mass capable of heating a distant body more than it is itself heated. less demonstrably, but scarcely less surely, mr. j. j. waterston, who attacked the problem in , erred in the opposite direction. working up, on newton's principle, data collected by himself in india and at edinburgh, he got for the "potential temperature" of the sun , , ° fahr.,[ ] equivalent to , , ° c. the phrase _potential temperature_ (for which violle substituted, in , _effective temperature_) was designed to express the accumulation in a single surface, postulated for the sake of simplicity, of the radiations not improbably received from a multitude of separate solar layers reinforcing each other; and might thus (it was explained) be considerably higher than the _actual_ temperature of any one stratum. at rome, in , father secchi repeated waterston's experiments, and reaffirmed his conclusion;[ ] while soret's observations, made on the summit of mont blanc in ,[ ] furnished him with materials for a fresh and even higher estimate of ten million degrees centigrade.[ ] yet from the very same data, substituting dulong and petit's for newton's law, vicaire deduced in a _provisional_ solar temperature of , °.[ ] this is below that at which iron melts, and we know that iron-vapour exists high up in the sun's atmosphere. the matter was taken into consideration on the other side of the atlantic by ericsson in . he attempted to re-establish the shaken credit of newton's principle, and arrived, by its means, at a temperature of , , ° fahrenheit.[ ] subsequently, an "underrated computation," based upon observation of the quantity of heat received by his "sun motor," gave him , , °. and the result, as he insisted, followed inevitably from the principle that the temperature produced by radiant heat is proportional to its density, or inversely as its diffusion.[ ] the principle, however, is demonstrably unsound. in the sun's temperature was proposed as the subject of a prize by the paris academy of sciences; but although the essay of m. jules violle was crowned, the problem was declared to remain unsolved. violle (who adhered to dulong and petit's formula) arrived at an _effective_ temperature of , ° c., but considered that it might _actually_ reach , ° c., if the emissive power of the photospheric clouds fell far short (as seemed probable) of the lamp-black standard.[ ] experiments made in april and may, , giving a somewhat higher result, he raised this figure to , ° c.[ ] appraisements so outrageously discordant as those of waterston, secchi, and ericsson on the one hand, and those of the french _savants_ on the other, served only to show that all were based upon a vicious principle. professor f. rosetti,[ ] accordingly, of the paduan university, at last perceived the necessity for getting out of the groove of "laws" plainly in contradiction with facts. the temperature, for instance, of the oxy-hydrogen flame was fixed by bunsen at , ° c.--an estimate certainly not very far from the truth. but if the two systems of measurement applied to the sun be used to determine the heat of a solid body rendered incandescent in this flame, it comes out, by newton's mode of calculation, , ° c.; by dulong and petit's, ° c.[ ] both, then, are justly discarded, the first as convicted of exaggeration, the second of undervaluation. the formula substituted by rosetti in was tested successfully up to , ° c.; but since, like its predecessors, it was a purely empirical rule, guaranteed by no principle, and hence not to be trusted out of sight, it was, like them, liable to break down at still higher elevations. radiation by this new prescription increases as the _square_ of the _absolute_ temperature--that is, of the number of degrees counted from the "absolute zero" of - ° c. its employment gave for the sun's radiating surface an effective temperature of , ° c. (including a supposed loss of one-half in the solar atmosphere); and setting a probable deficiency in emission (as compared with lamp-black) against a probable mutual reinforcement of superposed strata, professor rosetti considered "effective" as nearly equivalent to "actual" temperature. a "law of cooling," proposed by m. stefan at vienna in ,[ ] was shown by boltzmann, many years later, to have a certain theoretical validity.[ ] it is that emission grows as the fourth power of absolute temperature. hence the temperature of the photosphere would be proportional to the square root of the square root of its heating effects at a distance, and appeared, by stefan's calculations from violle's measures of solar radiative intensity, to be just , ° c.; while m. h. le chatelier[ ] derived , ° from a formula, conveying an intricate and unaccountable relation between the temperature of an incandescent body and the intensity of its red radiations. from a series of experiments carefully conducted at daramona, ireland, with a delicate thermal balance, of the kind invented by boys and designated a "radio-micrometer," messrs. wilson and gray arrived in , with the aid of stefan's law, at a photospheric temperature of , ° c.,[ ] reduced by the first-named investigator in to , °.[ ] dr. paschen, of hanover, on the other hand, ascribed to the sun a temperature of , ° from comparisons between solar radiative intensity and that of glowing platinum;[ ] while f. w. very showed in [ ] that a minimum value of , ° c. for the same datum resulted from paschen's formula connecting temperature with the position of maximum spectral energy. a new line of inquiry was struck out by zöllner in . instead of tracking the solar radiations backward with the dubious guide of empirical formulæ, he investigated their intensity at their source. he showed[ ] that, taking prominences to be simple effects of the escape of powerfully compressed gases, it was possible, from the known mechanical laws of heat and gaseous constitution, to deduce minimum values for the temperatures prevailing in the area of their development. these came out , ° c. for the strata lying immediately above, and , ° c. for the strata lying immediately below the photosphere, the former being regarded as the region _into_ which, and the latter as the region _from_ which the eruptions took place. in this calculation, no prominences exceeding , miles ( · ') in height were included. but in , g. a. hirn of colmar, having regard to the enormous velocities of projection observed in the interim, fixed two million degrees centigrade as the lowest _internal_ temperature by which they could be accounted for; although admitting the photospheric condensations to be incompatible with a higher _external_ temperature than , ° to , ° c.[ ] this method of going straight to the sun itself, observing what goes on there, and inferring conditions, has much to recommend it; but its profitable use demands knowledge we are still very far from possessing. we are quite ignorant, for instance, of the actual circumstances attending the birth of the solar flames. the assumption that they are nothing but phenomena of elasticity is a purely gratuitous one. spectroscopic indications, again, give hope of eventually affording a fixed point of comparison with terrestrial heat sources; but their interpretation is still beset with uncertainties; nor can, indeed, the expression of transcendental temperatures in degrees of impossible thermometers be, at the best, other than a futile attempt to convey notions respecting a state of things altogether outside the range of our experience. a more tangible, as well as a less disputable proof of solar radiative intensity than any mere estimates of temperature, was provided in some experiments made by professor langley in .[ ] using means of unquestioned validity, he found the sun's disc to radiate times as much heat, and , times as much light as an equal area of metal in a bessemer converter after the air-blast had continued about twenty minutes. the brilliancy of the incandescent steel, nevertheless, was so blinding, that melted iron, flowing in a dazzling white-hot stream into the crucible, showed "deep brown by comparison, presenting a contrast like that of dark coffee poured into a white cup." its temperature was estimated (not quite securely)[ ] at about , ° c.; and no allowances were made, in computing relative intensities, for atmospheric ravages on sunlight, for the extra impediments to its passage presented by the smoke-laden air of pittsburgh, or for the obliquity of its incidence. thus, a very large balance of advantage lay on the side of the metal. a further element of uncertainty in estimating the intrinsic strength of the sun's rays has still to be considered. from the time that his disc first began to be studied with the telescope, it was perceived to be less brilliant near the edges. lucas valerius, of the lyncean academy, seems to have been the first to note this fact, which, strangely enough, was denied by galileo in a letter to prince cesi of january , .[ ] father scheiner, however, fully admitted it, and devoted some columns of his bulky tome to the attempt to find its appropriate explanation.[ ] in bouguer measured, with much accuracy, the amount of this darkening; and from his data, laplace, adopting a principle of emission now known to be erroneous, concluded that the sun loses eleven-twelfths of his light through absorption in his own atmosphere.[ ] the real existence of this atmosphere, which is totally distinct from the beds of ignited vapours producing the fraunhofer lines, is not open to doubt, although its nature is still a matter of conjecture. the separate effects of its action on luminous, thermal, and chemical rays were carefully studied by father secchi, who in [ ] inferred the total absorption to be / of all radiations taken together, and added the important observation that the light from the limb is no longer white, but reddish-brown. absorptive effects were thus seen to be unequally distributed; and they could evidently be studied to advantage only by taking the various rays of the spectrum separately, and finding out how much each had suffered in transmission. this was done by h. c. vogel in .[ ] using a polarising photometer, he found that only per cent. of the violet rays escape at the edge of the solar disc, of the blue and green, of the yellow, and per cent. of the red. midway between centre and limb, · of violet light and · of red penetrate the absorbing envelope, the abolition of which would increase the intensity of the sun's visible spectrum above two and a half times in the most, and once and a half times in the least refrangible parts. the nucleus of a small spot was ascertained to be of the same luminous intensity as a portion of the unbroken surface about two and a half minutes from the limb. these experiments having been made during a spot-minimum when there is reason to think that absorption is below its average strength, vogel suggested their repetition at a time of greater activity. they were extended to the heat-rays by edwin b. frost. detailed inquiries made at potsdam in [ ] went to show that, were the sun's atmosphere removed, his thermal power, as regards ourselves, would be increased · times. they established, too, the practical uniformity in radiation of all parts of his disc. a confirmatory result was obtained about the same time by wilson and rambaut, who found that the unveiled sun would be once and a half times hotter than the actual sun.[ ] professor langley, now of washington, gave to measures of the kind a refinement previously undreamt of. reliable determinations of the "energy" of the individual spectral rays were, for the first time, rendered possible by his invention of the "bolometer" in .[ ] this exquisitely sensitive instrument affords the means of measuring heat, not directly, like the thermopile, but in its effects upon the conduction of electricity. it represents, in the phrase of the inventor, the finger laid upon the throttle-valve of a steam-engine. a minute force becomes the modulator of a much greater force, and thus from imperceptible becomes conspicuous. by locally raising the temperature of an inconceivably fine strip of platinum serving as the conducting-wire in a circuit, the flow of electricity is impeded at that point, and the included galvanometer records a disturbance of the electrical flow. amounts of heat were thus detected in less than ten seconds, which, expended during a thousand years on the melting of a kilogramme of ice, would leave a part of the work still undone; and further improvements rendered this marvellous instrument capable of thrilling to changes of temperature falling short of one ten-millionth of a degree centigrade.[ ] the heat contained in the diffraction spectrum is, with equal dispersions, barely one-tenth of that in the prismatic spectrum. it had, accordingly, never previously been found possible to measure it in detail--that is, ray by ray. but it is only from the diffraction, or normal spectrum that any true idea can be gained as to the real distribution of energy among the various constituents, visible and invisible, of a sunbeam. the effect of passage through a prism is to crowd together the red rays very much more than the blue. to this prismatic distortion was owing the establishment of a pseudo-maximum of heat in the infra-red, which disappeared when the natural arrangement by wave-length was allowed free play. langley's bolometer has shown that the hottest part of the normal spectrum virtually coincides with its most luminous part, both lying in the orange, close to the d-line.[ ] thus the last shred of evidence in favour of the threefold division of solar radiations vanished, and it became obvious that the varying effects--thermal, luminous, or chemical--produced by them are due, not to any distinction of quality in themselves, but to the different properties of the substances they impinge upon. they are simply bearers of energy, conveyed in shorter or longer vibrations; the result in each separate case depending upon the capacity of the material particles meeting them for taking up those shorter or longer vibrations, and turning them variously to account in their inner economy. a long series of experiments at allegheny was completed in the summer of on the crest of mount whitney in the sierra nevada. here, at an elevation of , feet, in the driest and purest air, perhaps, in the world, atmospheric absorptive inroads become less sensible, and the indications of the bolometer, consequently, surer and stronger. an enormous expansion was at once given to the invisible region in the solar spectrum below the red. captain abney had got chemical effects from undulations twelve ten-thousandths of a millimetre in length. these were the longest recognised as, or indeed believed, on theoretical grounds, to be capable of existing. professor langley now got heating effects from rays of above twice that wave-length, his delicate thread of platinum groping its way down nearly to thirty ten-thousandths of a millimetre, or three "microns." the known extent of the solar spectrum was thus at once more than doubled. its visible portion covers a range of about one octave; bolometric indications already in comprised between three and four. the great importance of the newly explored region appears from the fact that three-fourths of the entire energy of sunlight reside in the infra-red, while scarcely more than one-hundredth part of that amount is found in the better known ultra-violet space.[ ] these curious facts were reinforced, in ,[ ] by further particulars learned with the help of rock-salt lenses and prisms, glass being impervious to very slow, as to very rapid vibrations. traces were thus detected of solar heat distributed into bands of transmission alternating with bands of atmospheric absorption, far beyond the measurable limit of · microns. in , langley described at the oxford meeting of the british association[ ] his new "bolographic" researches, in which the sensitive plate was substituted for the eye in recording deflections of the galvanometer responding to variations of invisible heat. finally, in ,[ ] he embodied in a splendid map of the infra-red spectrum absorption-lines of determinate wave-lengths, ranging from · to · microns. their chemical origin, indeed, remains almost entirely unknown, no extensive investigations having yet been undertaken of the slower vibrations distinctive of particular substances; but there is evidence that seven of the nine great bands crossing the "new spectrum" (as langley calls it)[ ] are telluric, and subject to seasonal change. here, then, he thought, might eventually be found a sure standing-ground for vitally important previsions of famines, droughts, and bonanza-crops. atmospheric absorption had never before been studied with such precision as it was by langley on mount whitney. aided by simultaneous observations from lone pine, at the foot of the sierra, he was able to calculate the intensity belonging to each ray before entering the earth's gaseous envelope--in other words, to construct an extra-atmospheric curve of energy in the spectrum. the result showed that the blue end suffered far more than the red, absorption varying inversely as wave-length. this property of stopping predominantly the quicker vibrations is shared, as both vogel and langley[ ] have conclusively shown, by the solar atmosphere. the effect of this double absorption is as if two plates of reddish glass were interposed between us and the sun, the withdrawal of which would leave his orb, not only three or four times more brilliant, but in colour distinctly greenish-blue.[ ] the fact of the uncovered sun being _blue_ has an important bearing upon the question of his temperature, to afford a somewhat more secure answer to which was the ultimate object of professor langley's persevering researches; for it is well known that as bodies grow hotter, the proportionate representation in their spectra of the more refrangible rays becomes greater. the lowest stage of incandescence is the familiar one of _red_ heat. as it gains intensity, the quicker vibrations come in, and an optical balance of sensation is established at _white_ heat. the final term of _blue_ heat, as we now know, is attained by the photosphere. on this ground alone, then, of the large original preponderance of blue light, we must raise our estimate of solar heat; and actual measurements show the same upward tendency. until quite lately, pouillet's figure of . calories per minute per square centimetre of terrestrial surface, was the received value for the "solar constant." forbes had, it is true, got . from observations on the faulhorn in ;[ ] but they failed to obtain the confidence they merited. pouillet's result was not definitely superseded until violle, from actinometrical measures at the summit and base of mont blanc in , computed the intensity of solar radiation at . ,[ ] and crova, about the same time, at montpellier, showed it to be above two calories.[ ] langley went higher still. working out the results of the mount whitney expedition, he was led to conclude atmospheric absorption to be fully twice as effective as had hitherto been supposed. scarcely per cent., in fact, of those solar radiations which strike perpendicularly through a seemingly translucent sky, were estimated to attain the sea-level. the rest are reflected, dispersed, or absorbed. this discovery involved a large addition to the original supply so mercilessly cut down in transmission, and the solar constant rose at once to three calories. nor did the rise stop there. m. savélieff deduced for it a value of . from actinometrical observations made at kieff in ;[ ] and knut Ångström, taking account of the arrestive power of carbonic acid, inferred enormous atmospheric absorption, and a solar constant of four calories.[ ] in other words, the sun's heat reaching the outskirts of our atmosphere is capable of doing without cessation the work of an engine of four-horse power for each square yard of the earth's surface. thus, modern inquiries tend to render more and more evident the vastness of the thermal stores contained in the great central reservoir of our system, while bringing into fair agreement the estimates of its probable temperature. this is in great measure due to the acquisition of a workable formula by which to connect temperature with radiation. stefan's rule of a fourth-power relation, if not actually a law of nature, is a colourable imitation of one; and its employment has afforded a practical certainty that the sun's temperature, so far as it is definable, neither exceeds , ° c., nor falls short of , ° c. footnotes: [footnote : _principia_, p. ( st ed.).] [footnote : _comptes rendus_, t. vii., p. .] [footnote : _results of astr. observations_, p. .] [footnote : "est enim calor solis ut radiorum densitas, hoc est, reciproce ut quadratum distantiæ locorum a sole."--_principia_, p. ( d ed., ).] [footnote : _jour. de physique_, t. lxxv., p. .] [footnote : _ann. de chimie_, t. vii., , p. .] [footnote : _phil. mag._, vol. xxiii. ( th ser.), p. .] [footnote : _nuovo cimento_, t. xvi., p. .] [footnote : _comptes rendus_, t. lxv., p. .] [footnote : the direct result of - / million degrees was doubled in allowance for absorption in the sun's own atmosphere. _comptes rendus_, t. lxxiv., p. .] [footnote : _ibid._, p. .] [footnote : _nature_, vols. iv., p. ; v., p. .] [footnote : _ibid._, vol. xxx., p. .] [footnote : _ann. de chim._, t. x. ( th ser.), p. .] [footnote : _comptes rendus_, t. xcvi., p. .] [footnote : _phil. mag._, vol. viii., p. , .] [footnote : _ibid._, p. .] [footnote : _sitzungsberichte_, wien, bd. lxxix., ii., p. .] [footnote : _wiedemann's annalen_, bd. xxii., p. ; _scheiner, strahlung und temperatur der sonne_, p. .] [footnote : _comptes rendus_, march , ; _astr. and astrophysics_, vol. xi., p. .] [footnote : _phil. trans._, vol. clxxxv., p. .] [footnote : _proc. roy. society_, december , .] [footnote : scheiner, _temp. der sonne_, p. .] [footnote : _astroph. jour._, vol. ii., p. .] [footnote : _astr. nach._, nos. , - .] [footnote : _l'astronomie_, september, , p. .] [footnote : _amer. jour. of science_, vol. i. ( rd ser.), p. .] [footnote : young, _the sun_, p. .] [footnote : _op._, t. vi., p. .] [footnote : _rosa ursina_, lib. iv., p. .] [footnote : _méc. cél._, liv. x., p. .] [footnote : _le soleil_ ( st ed.), p. .] [footnote : _monatsber._, berlin, , p. .] [footnote : _astr. nach._, nos. , - ; _astr. and astrophysics_, vol. xi., p. .] [footnote : _proc. roy. irish acad._, vol. ii., no. , .] [footnote : _am. jour. of sc._, vol. xxi., p. .] [footnote : _amer. jour. of science_, vol. v., p. , .] [footnote : for j. w. draper's partial anticipation of this result, see _ibid_. vol. iv., , p. .] [footnote : _phil. mag._, vol. xiv., p. , .] [footnote : "the solar and the lunar spectrum," _memoirs national acad. of science_, vol. xxxii.; "on hitherto unrecognised wave-lengths," _amer. jour. of science_, vol. xxxii., august, .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _annals of the smithsonian astroph. observatory_, vol. i.; _comptes rendus_, t. cxxxi., p. ; _astroph. jour._, vol. iii., p. .] [footnote : _phil. mag._, july, .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _nature_, vol. xxvi., p. .] [footnote : _phil. trans._, vol. cxxxii., p. .] [footnote : _ann. de chim._, t. x., p. .] [footnote : _ibid._, t. xi., p. .] [footnote : _comptes rendus_, t. cxii., p. .] [footnote : _wied. ann._, bd. xxxix., p. ; scheiner, _temperatur der sonne_, pp. , .] chapter vi _the sun's distance_ the question of the sun's distance arises naturally from the consideration of his temperature, since the intensity of the radiations emitted as compared with those received and measured, depends upon it. but the knowledge of that distance has a value quite apart from its connection with solar physics. the semi-diameter of the earth's orbit is our standard measure for the universe. it is the great fundamental datum of astronomy--the unit of space, any error in the estimation of which is multiplied and repeated in a thousand different ways, both in the planetary and sidereal systems. hence its determination was called by airy "the noblest problem in astronomy." it is also one of the most difficult. the quantities dealt with are so minute that their sure grasp tasks all the resources of modern science. an observational inaccuracy which would set the moon nearer to, or farther from us than she really is by one hundred miles, would vitiate an estimate of the sun's distance to the extent of sixteen million![ ] what is needed in order to attain knowledge of the desired exactness is no less than this: to measure an angle about equal to that subtended by a halfpenny , feet from the eye, within a little more than a thousandth part of its value. the angle thus represented is what is called the "horizontal parallax" of the sun. by this amount--the breadth of a halfpenny at , feet--he is, to a spectator on the rotating earth, removed at rising and setting from his meridian place in the heavens. such, in other terms, would be the magnitude of the terrestrial radius as viewed from the sun. if we knew this magnitude with certainty and precision, we should also know with certainty and precision--the dimensions of the earth being, as they are, well ascertained--the distance of the sun. in fact, the one quantity commonly stands for the other in works treating professedly of astronomy. but this angle of parallax or apparent displacement cannot be directly measured--cannot even be perceived with the finest instruments. not from its smallness. the parallactic shift of the nearest of the stars as seen from opposite sides of the earth's orbit, is many times smaller. but at the sun's limb, and close to the horizon, where the visual angle in question opens out to its full extent, atmospheric troubles become overwhelming, and altogether swamp the far more minute effects of parallax. there remain indirect methods. astronomers are well acquainted with the proportions which the various planetary orbits bear to each other. they are so connected, in the manner expressed by kepler's third law, that the periods being known, it only needs to find the interval between any two of them in order to infer at once the distances separating them all from one another and from the sun. the plan is given; what we want to discover is the scale upon which it is drawn; so that, if we can get a reliable measure of the distance of a single planet from the earth, our problem is solved. now some of our fellow-travellers in our unending journey round the sun, come at times well within the scope of celestial trigonometry. the orbit of mars lies at one point not more than thirty-five million miles outside that of the earth, and when the two bodies happen to arrive together in or near the favourable spot--a conjuncture which occurs every fifteen years--the desired opportunity is granted. mars is then "in opposition," or on the _opposite_ side of us from the sun, crossing the meridian consequently at midnight.[ ] it was from an opposition of mars, observed in by richer at cayenne in concert with cassini in paris, that the first scientific estimate of the sun's distance was derived. it appeared to be nearly eighty-seven millions of miles (parallax · "); while flamsteed deduced , , (parallax ") from his independent observations of the same occurrence--a difference quite insignificant at that stage of the inquiry. but picard's result was just half flamsteed's (parallax "; distance forty-one million miles); and lahire considered that we must be separated from the hearth of our system by an interval of _at least_ million miles.[ ] so that uncertainty continued to have an enormous range. venus, on the other hand, comes closest to the earth when she passes between it and the sun. at such times of "inferior conjunction" she is, however, still twenty-six million miles, or (in round numbers) times as distant as the moon. moreover, she is so immersed in the sun's rays that it is only when her path lies across his disc that the requisite facilities for measurement are afforded. these "partial eclipses of the sun by venus" (as encke termed them) are coupled together in pairs,[ ] of which the components are separated by eight years, recurring at intervals alternately of - / and - / years. thus, the first calculated transit took place in december, , and its companion (observed by horrocks) in the same month (n.s.), . then, after the lapse of - / years, came the june couple of and ; and again after - / , the two last observed, december , , and december , . throughout the twentieth century there will be no transit of venus; but the astronomers of the twenty-first will only have to wait four years for the first of a june pair. the rarity of these events is due to the fact that the orbits of the earth and venus do not lie in the same plane. if they did, there would be a transit each time that our twin-planet overtakes us in her more rapid circling--that is, on an average, every days. as things are actually arranged, she passes above or below the sun, except when she happens to be very near the line of intersection of the two tracks. such an occurrence as a transit of venus seems, at first sight, full of promise for solving the problem of the sun's distance. for nothing would appear easier than to determine exactly either the duration of the passage of a small, dark orb across a large brilliant disc, or the instant of its entry upon or exit from it. and the differences in these times (which, owing to the comparative nearness of venus, are quite considerable), as observed from remote parts of the earth, can be translated into differences of space--that is, into apparent or parallactic displacements, whereby the distance of venus becomes known, and thence, by a simple sum in proportion, the distance of the sun. but in that word "exactly" what snares and pitfalls lie hid! it is so easy to think and to say; so indefinitely hard to realise. the astronomers of the eighteenth century were full of hope and zeal. they confidently expected to attain, through the double opportunity offered them, to something like a permanent settlement of the statistics of our system. they were grievously disappointed. the uncertainty as to the sun's distance, which they had counted upon reducing to a few hundred thousand miles, remained at many millions. in , however, encke, then director of the seeberg observatory near gotha, undertook to bring order out of the confusion of discordant, and discordantly interpreted observations. his combined result for both transits ( and ) was published in ,[ ] and met universal acquiescence. the parallax of the sun thereby established was · ", corresponding to a mean distance[ ] of - / million miles. yet this abolition of doubt was far from being so satisfactory as it seemed. serenity on the point lasted exactly thirty years. it was disturbed in by hansen's announcement[ ] that the observed motions of the moon could be drawn into accord with theory only on the terms of bringing the sun considerably nearer to us than he was supposed to be. dr. matthew stewart, professor of mathematics in the university of edinburgh, had made a futile attempt in to deduce the sun's distance from his disturbing power over our satellite.[ ] tobias mayer of göttingen, however, whose short career was fruitful of suggestions, struck out the right way to the same end; and laplace, in the seventh book of the _mécanique céleste_,[ ] gave a solar parallax derived from the lunar "parallactic inequality" substantially identical with that issuing from encke's subsequent discussion of the eighteenth-century transits. thus, two wholly independent methods--the trigonometrical, or method by survey, and the gravitational, or method by perturbation--seemed to corroborate each the upshot of the use of the other until the nineteenth century was well past its meridian. it is singular how often errors conspire to lead conviction astray. hansen's note of alarm in was echoed by leverrier in .[ ] he found that an apparent monthly oscillation of the sun which reflects a real monthly movement of the earth round its common centre of gravity with the moon, and which depends for its amount solely on the mass of the moon and the distance of the sun, required a diminution in the admitted value of that distance by fully four million miles. three years later he pointed out that certain perplexing discrepancies between the observed and computed places both of venus and mars, would vanish on the adoption of a similar measure.[ ] moreover, a favourable opposition of mars gave the opportunity in for fresh observations, which, separately worked out by stone and winnecke, agreed with all the newer investigations in fixing the great unit at slightly over million miles. in newcomb's hands they gave - / million.[ ] the accumulating evidence in favour of a large reduction in the sun's distance was just then reinforced by an auxiliary result of a totally different and unexpected kind. the discovery that light does not travel instantaneously from point to point, but takes some short time in transmission, was made by olaus römer in , through observing that the eclipses of jupiter's satellites invariably occurred later, when the earth was on the far side, than when it was on the near side of its orbit. half the difference, or the time spent by a luminous vibration in crossing the "mean radius" of the earth's orbit, is called the "light-equation"; and the determination of its precise value has claimed the minute care distinctive of modern astronomy. delambre in made it seconds. glasenapp, a russian astronomer, raised the estimate in to , professor harkness adopts a safe medium value of seconds. hence, if we had any independent means of ascertaining how fast light travels, we could tell at once how far off the sun is. there is yet another way by which knowledge of the swiftness of light would lead us straight to the goal. the heavenly bodies are perceived, when carefully watched and measured, to be pushed forward out of their true places, in the direction of the earth's motion, by a very minute quantity. this effect (already adverted to) has been known since bradley's time as "aberration." it arises from a combination of the two movements of the earth round the sun and of the light-waves through the ether. if the earth stood still, or if light spent no time on the road from the stars, such an effect would not exist. its amount represents the proportion between the velocities with which the earth and the light-rays pursue their respective journeys. this proportion is, roughly, one to ten thousand. so that here again, if we knew the rate per second of luminous transmission, we should also know the rate per second of the earth's movement, consequently the size of its orbit and the distance of the sun. but, until lately, instead of finding the distance of the sun from the velocity of light, there has been no means of ascertaining the velocity of light except through the imperfect knowledge possessed as to the distance of the sun. the first successful terrestrial experiments on the point date from ; and it is certainly no slight triumph of human ingenuity to have taken rigorous account of the delay of a sunbeam in flashing from one mirror to another. fizeau led the way,[ ] and he was succeeded, after a few months, by léon foucault,[ ] who, in , had so far perfected wheatstone's method of revolving mirrors, as to be able to announce with authority that light travelled slower, and that the sun was in consequence nearer than had been supposed.[ ] thus a third line of separate research was found to converge to the same point with the two others. such a conspiracy of proof was not to be resisted, and at the anniversary meeting of the royal astronomical society in february, , the correction of the solar distance took the foremost place in the annals of the year. lest, however, a sudden bound of four million miles nearer to the centre of our system should shake public faith in astronomical accuracy, it was explained that the change in the solar parallax corresponding to that huge leap, amounted to no more than the breadth of a human hair feet from the eye![ ] the nautical almanac gave from the altered value of . ", for which newcomb's result of . ", adopted in in the berlin ephemeris, was substituted some ten years later. in astronomical literature the change was initiated by sir edmund beckett in the first edition ( ) of his _astronomy without mathematics_. if any doubt remained as to the misleading character of encke's deduction, so long implicitly trusted in, it was removed by powalky's and stone's rediscussions, in and respectively, of the transit observations of . using improved determinations of the longitude of the various stations, and a selective judgment in dealing with their materials, which, however indispensable, did not escape adverse criticism, they brought out results confirmatory of the no longer disputed necessity for largely increasing the solar parallax, and proportionately diminishing the solar distance. once more in , and this time with better success, the eighteenth-century transits were investigated by professor newcomb.[ ] turning to account the experience gained in the interim regarding the optical phenomena accompanying such events, he elicited from the mass of somewhat discordant observations at his command, a parallax ( · ") in close agreement with the value given by sundry modes of recent research. conclusions on the subject, however, were still regarded as purely provisional. a transit of venus was fast approaching, and to its arbitrament, as to that of a court of final appeal, the pending question was to be referred. it is true that the verdict in the same case by the same tribunal a century earlier had proved of so indecisive a character as to form only a starting-point for fresh litigation; but that century had not passed in vain, and it was confidently anticipated that observational difficulties, then equally unexpected and insuperable, would yield to the elaborate care and skill of forewarned modern preparation. the conditions of the transit of december , , were sketched out by sir george airy, then astronomer-royal, in ,[ ] and formed the subject of eager discussion in this and other countries down to the very eve of the occurrence. in these mr. proctor took a leading part; and it was due to his urgent representations that provision was made for the employment of the method identified with the name of halley,[ ] which had been too hastily assumed inapplicable to the first of each transit-pair. it depends upon the difference in the length of time taken by the planet to cross the sun's disc, as seen from various points of the terrestrial surface, and requires, accordingly, the visibility of both entrance and exit at the same station. since these were, in , separated by about three and a half hours, and the interval may be much longer, the choice of posts for the successful use of the "method of durations" is a matter of some difficulty. the system described by delisle in , on the other hand, involves merely noting the instant of ingress or egress (according to situation) from opposite extremities of a terrestrial diameter; the disparity in time giving a measure of the planet's apparent displacement, hence of its actual rate of travel in miles per minute, from which its distances severally from earth and sun are immediately deducible. its chief attendant difficulty is the necessity for accurately fixing the longitudes of the points of observation. but this was much more sensibly felt a century ago than it is now, the improved facility and certainty of modern determinations tending to give the delislean plan a decided superiority over its rival. these two traditional methods were supplemented in by the camera and the heliometer. from photography, above all, much was expected. observations made by its means would have the advantages of impartiality, multitude, and permanence. peculiarities of vision and bias of judgment would be eliminated; the slow progress of the phenomenon would permit an indefinite number of pictures to be taken, their epochs fixed to a fraction of a second; while subsequent leisurely comparison and measurement could hardly fail, it was thought, to educe approximate truth from the mass of accumulated evidence. the use of the heliometer (much relied on by german observers) was so far similar to that of the camera that the object aimed at by both was the determination of the relative positions of the _centres_ of the sun and venus viewed, at the same absolute instant, from opposite sides of the globe. so that the principle of the two older methods was to ascertain the exact times of meeting between the solar and planetary limbs; that of the two modern to determine the position of the dark body already thrown into complete relief by its shining background. the former are "methods by contact," the latter "methods by projection." every country which had a reputation to keep or to gain for scientific zeal was forward to co-operate in the great cosmopolitan enterprise of the transit. france and germany each sent out six expeditions; twenty-six stations were in russian, twelve in english, eight in american, three in italian, one in dutch occupation. in all, at a cost of nearly a quarter of a million, some fourscore distinct posts of observation were provided; among them such inhospitable, and all but inaccessible rocks in the bleak southern ocean, as st. paul's and campbell islands, swept by hurricanes, and fitted only for the habitation of seabirds, where the daring votaries of science, in the wise prevision of a long leaguer by the elements, were supplied with stores for many months, or even a whole year. siberia and the sandwich islands were thickly beset with observers; parties of three nationalities encamped within the mists of kerguelen island, expressively termed the "land of desolation," in the sanguine, though not wholly frustrated hope of a glimpse of the sun at the right moment. m. janssen narrowly escaped destruction from a typhoon in the china seas on his way to nagasaki; lord lindsay (now earl of crawford and balcarres) equipped, at his private expense, an expedition to mauritius, which was in itself an epitome of modern resource and ingenuity. during several years, the practical methods best suited to insure success for the impending enterprise formed a subject of european debate. official commissions were appointed to receive and decide upon evidence; and experiments were in progress for the purpose of defining the actual circumstances of contacts, the precise determination of which constituted the only tried, though by no means an assuredly safe road to the end in view. in england, america, france, and germany, artificial transits were mounted, and the members of the various expeditions were carefully trained to unanimity in estimating the phases of junction and separation between a moving dark circular body and a broad illuminated disc. in the previous century, a formidable and prevalent phenomenon, which acquired notoriety as the "black drop" or "black ligament," had swamped all pretensions to rigid accuracy. it may be described as substituting adhesion for contact, the limbs of the sun and planet, instead of meeting and parting with the desirable clean definiteness, _clinging_ together as if made of some glutinous material, and prolonging their connection by means of a dark band or dark threads stretched between them. some astronomers ascribed this baffling appearance entirely to instrumental imperfections; others to atmospheric agitation; others again to the optical encroachment of light upon darkness known as "irradiation." it is probable that all these causes conspired, in various measure, to produce it; and it is certain that its _conspicuous_ appearance may, by suitable precautions, be obviated. the organisation of the british forces reflected the utmost credit on the energy and ability of lieutenant-colonel tupman, who was responsible for the whole. no useful measure was neglected. each observer went out ticketed with his "personal equation," his senses drilled into a species of martial discipline, his powers absorbed, so far as possible, in the action of a cosmopolitan observing machine. instrumental uniformity and uniformity of method were obtainable, and were attained; but diversity of judgment unhappily survived the best-directed efforts for its extirpation. the eventful day had no sooner passed than telegrams began to pour in, announcing an outcome of considerable, though not unqualified success. the weather had proved generally favourable; the manifold arrangements had worked well; contacts had been plentifully observed; photographs in lavish abundance had been secured; a store of materials, in short, had been laid up, of which it would take years to work out the full results by calculation. gradually, nevertheless, it came to be known that the hope of a definitive issue must be abandoned. unanimity was found to be as remote as ever. the dreaded "black ligament" gave, indeed, less trouble than was expected; but another appearance supervened which took most observers by surprise. this was the illumination due to the atmosphere of venus. astronomers, it is true, were not ignorant that the planet had, on previous occasions, been seen girdled with a lucid ring; but its power to mar observations by the distorting effect of refraction had scarcely been reckoned with. it proved, however, to be very great. such was the difficulty of determining the critical instant of internal contact, that (in colonel tupman's words) "observers side by side, with adequate optical means, differed as much as twenty or thirty seconds in the times they recorded for phenomena which they have described in almost identical language."[ ] such uncertainties in the data admitted of a corresponding variety in the results. from the british observations of ingress and egress sir george airy[ ] derived, in , a solar parallax of · " (corrected to · "), indicating a mean distance of , , miles. mr. stone obtained a value of ninety-two millions (parallax · "), and held any parallax less than · " or more than · " to be "absolutely negatived" by the documents available.[ ] yet, from the same, colonel tupman deduced · ",[ ] implying a distance , miles greater than stone had obtained. the best french observations of contacts gave a parallax of about · "; french micrometric measures the obviously exaggerated one of · ".[ ] photography, as practised by most of the european parties, was a total failure. utterly discrepant values of the microscopic displacements designed to serve as sounding lines for the solar system, issued from attempts to measure even the most promising pictures. "you might as well try to measure the zodiacal light," it was remarked to sir george airy. those taken on the american plan of using telescopes of so great focal length as to afford, without further enlargement, an image of the requisite size, gave notably better results. from an elaborate comparison of those dating from vladivostock, nagasaki, and pekin, with others from kerguelen and chatham islands, professor d. p. todd, of amherst college, deduced a solar distance of about ninety-two million miles (parallax · " ± · "),[ ] and the value was much favoured by concurrent evidence. on the whole, estimates of the great spatial unit cannot be said to have gained any security from the combined effort of . a few months before the transit, mr. proctor considered that the uncertainty then amounted to , , miles;[ ] five years after the transit, professor harkness judged it to be still , , miles;[ ] yet it had been hoped that it would have been brought down to , . as regards the end for which it had been undertaken, the grand campaign had come to nothing. nevertheless, no sign of discouragement was apparent. there was a change of view, but no relaxation of purpose. the problem, it was seen, could be solved by no single heroic effort, but by the patient approximation of gradual improvements. astronomers, accordingly, looked round for fresh means or more refined expedients for applying those already known. a new phase of exertion was entered upon. on september , , mars came into opposition near the part of his orbit which lies nearest to that of the earth, and dr. gill (now sir david) took advantage of the circumstance to appeal once more to him for a decision on the _quæstio vexata_ of the sun's distance. he chose, as the scene of his labours, the island of ascension, and for their plan a method recommended by airy in ,[ ] but never before fairly tried. this is known as the "diurnal method of parallaxes." its principle consists in substituting successive morning and evening observations from the same spot, for simultaneous observations from remote spots, the rotation of the earth supplying the necessary difference in the points of view. its great advantage is that of unity in performance. a single mind, looking through the same pair of eyes, reinforced with the same optical appliances, is employed throughout, and the errors inseparable from the combination of data collected under different conditions are avoided. there are many cases in which one man can do the work of two better than two men can do the work of one. the result of gill's skilful determinations (made with lord lindsay's heliometer) was a solar parallax of · ", corresponding to a distance of , , miles.[ ] the bestowal of the royal astronomical society's gold medal stamped the merit of this distinguished service. but there are other subjects for this kind of inquiry besides mars and venus. professor galle of breslau suggested in [ ] that some of the minor planets might be got to repay astronomers for much disinterested toil spent in unravelling their motions, by lending aid to their efforts towards a correct celestial survey. ten or twelve come near enough, and are bright enough for the purpose; in fact, the absence of sensible magnitude is one of their chief recommendations, since a point of light offers far greater facilities for exact measurement than a disc. the first attempt to work this new vein was made at the opposition of phocæa in ; and from observations of flora in the following year at twelve observatories in the northern and southern hemispheres, galle deduced a solar parallax of · ".[ ] at mauritius in , lord lindsay and sir david gill applied the "diurnal method" to juno, then conveniently situated for the purpose; and the continued use of similar occasions affords an unexceptionable means for improving knowledge of the sun's distance. they frequently recur; they need no elaborate preparation; a single astronomer armed with a heliometer can do all the requisite work. dr. gill, however, organized a more complex plan of operations upon iris in , and upon victoria and sappho in . a novel method was adopted. its object was to secure simultaneous observations made from opposite sides of the globe just when the planet lay in the plane passing through the centre of the earth and the two observers, the same pair of reference-stars being used on each occasion. the displacements caused by parallax were thus in a sense doubled, since the star to which the planet seemed approximated in the northern hemisphere, showed as if slightly removed from it in the southern, and _vice versâ_. as the planet pursued its course, fresh star-couples came into play, during the weeks that the favourable period lasted. in these determinations, only heliometers were employed. dr. elkin, of yale college, co-operated throughout, and the heliometers of dresden, göttingen, bamberg, and leipzig, shared in the work, while dr. auwers of berlin was sir david gill's personal coadjutor at the cape. voluminous data were collected; meridian observations of the stars of reference for victoria occupied twenty-one establishments during four months; the direct work of triangulation kept four heliometers in almost exclusive use for the best part of a year; and the ensuing toilsome computations, carried out during three years at the cape observatory, filled two bulky tomes[ ] with their details. gill's final result, published in , was a parallax of · ", equivalent to a solar distance of , , ; and it was qualified by a probable error so small that the value might well have been accepted as definitive but for an unlooked-for discovery. the minor planet eros, detected august , , was found to pursue a course rendering it an almost ideal intermediary in solar parallax-determinations. once in thirty years, it comes within fifteen million miles of the earth; and although the next of these choice epochs must be awaited for some decades, an opposition too favourable to be neglected occurred in . at an international conference, accordingly, held at paris in july of that year, a plan of photographic operations was concerted between the representatives of no less than observatories.[ ] its primary object was to secure a large stock of negatives showing the planet with the comparison-stars along the route traversed by it from october, , to march, ,[ ] and this at least was successfully attained. their measurement will in due time educe the apparent displacements of the moving object as viewed simultaneously from remote parts of the earth; and the upshot should be a solar parallax adequate in accuracy to the exigent demands of the twentieth century. the second of the nineteenth-century pair of venus-transits was looked forward to with much abated enthusiasm. russia refused her active co-operation in observing it, on the ground that oppositions of the minor planets were trigonometrically more useful, and financially far less costly; and her example was followed by austria; while italian astronomers limited their sphere of action to their own peninsula. nevertheless, it was generally held that a phenomenon which the world could not again witness until it was four generations older should, at the price of any effort, not be allowed to pass in neglect. the persuasion of its importance justified the summoning of an international conference at paris in , from which, however, america, preferring independent action, held aloof. it was decided to give delisle's method another trial; and the ambiguities attending and marring its use were sought to be obviated by careful regulations for insuring agreement in the estimation of the critical moments of ingress and egress.[ ] but in fact (as m. puiseux had shown[ ]), contacts between the limbs of the sun and planet, so far from possessing the geometrical simplicity attributed to them, are really made up of a prolonged succession of various and varying phases, impossible either to predict or identify with anything like rigid exactitude. sir robert ball compared the task of determining the precise instant of their meeting or parting, to that of telling the hour with accuracy on a watch without a minute hand; and the comparison is admittedly inadequate. for not only is the apparent movement of venus across the sun extremely slow, being but the excess of her real motion over that of the earth; but three distinct atmospheres--the solar, terrestrial, and cytherean--combine to deform outlines and mask the geometrical relations which it is desired to connect with a strict count of time. the result was very much what had been expected. the arrangements were excellent, and were only in a few cases disconcerted by bad weather. the british parties, under the experienced guidance of mr. stone, the late radcliffe observer, took up positions scattered over the globe, from queensland to bermuda; the americans collected a whole library of photographs; the germans and belgians trusted to the heliometer; the french used the camera as an adjunct to the method of contacts. yet little or no approach was made to solving the problem. thus, from measures of venus on the sun, taken with a new kind of heliometer at santiago in chili, m. houzeau, of the brussels observatory, derived a solar parallax of . ", and a distance of , , miles.[ ] but the "probable errors" of this determination amounted to . " either way: it was subject to a "more or less" of , , or to a total uncertainty of , , miles. the "probable error" of the english result, published in , was less formidable,[ ] yet the details of the discussion showed that no great confidence could be placed in it. the sun's distance came out , , miles; while , , was given by professor harkness's investigation of , american photographs.[ ] finally, dr. auwers deduced from the german heliometric measures the unsatisfactorily small value of , , miles.[ ] the transit of had not, then, brought about the desired unanimity. the state and progress of knowledge on this important topic were summed up by faye and harkness in .[ ] the methods employed in its investigation fall (as we have seen) into three separate classes--the trigonometrical, the gravitational, and the "phototachymetrical"--an ungainly adjective used to describe the method by the velocity of light. each has its special difficulties and sources of error; each has counter-balancing advantages. the only trustworthy result from celestial surveys, was at that time furnished by gill's observations of mars in . but the method by lunar and planetary disturbances is unlike all the others in having time on its side. it is this which leverrier declared with emphasis must inevitably prevail, because its accuracy is continually growing.[ ] the scarcely perceptible errors which still impede its application are of such a nature as to accumulate year by year; eventually, then, they will challenge, and must receive, a more and more perfect correction. the light-velocity method, however, claimed, and for some years justified, m. faye's preference. by a beautiful series of experiments on foucault's principle, michelson fixed in the rate of luminous transmission at , (corrected later to , ) kilometres a second.[ ] this determination was held by professor todd to be entitled to four times as much confidence as any previous one; and if the solar parallax of · " deduced from it by professor harkness errs somewhat by defect, it is doubtless because glasenapp's "light-equation," with which it was combined, errs slightly by excess. but all earlier efforts of the kind were thrown into the shade by professor newcomb's arduous operations at washington in - .[ ] the scale upon which they were conducted was in itself impressive. foucault's entire apparatus in had been enclosed in a single room; newcomb's revolving and fixed mirrors, between which the rays of light were to run their timed course, were set up on opposite shores of the potomac, at a distance of nearly four kilometres. this advantage was turned to the utmost account by ingenuity and skill in contrivance and execution; and the deduced velocity of , kilometres = , miles a second, had an estimated error ( kilometres) only one-tenth that ascribed by cornu to his own result in . just as these experiments were concluded in , m. magnus nyrén, of st. petersburg, published an elaborate investigation of the small annular displacements of the stars due to the successive transmission of light, involving an increase of struve's "constant of aberration" from · " to · ". and from the new value, combined with newcomb's light-velocity, was derived a valuable approximation to the sun's distance, concluded at , , miles (parallax = · "). yet it is not quite certain that nyrén's correction was an improvement. a differential method of determining the amount of aberration, struck out by m. loewy of paris,[ ] avoids most of the objections to the absolute method previously in vogue; and the upshot of its application in was to show that struve's constant might better be retained than altered, loewy's of · " varying from it only to an insignificant extent. professor hall had, moreover, deduced nearly the same value ( · ") from the washington observations since , of alpha lyræ (vega); whence, in conjunction with newcomb's rate of light transmission, he arrived at a solar parallax of · ".[ ] inverting the process, sir david gill in derived the constant from the parallax. if the earth's orbit have a mean radius, as found by him, of , , miles, then, he calculated, the aberration of light--newcomb's measures of its velocity being supposed exact--amounts to . ". this figure can need very slight correction. professor harkness surveyed in ,[ ] from an eclectic point of view, the general situation as regarded the sun's parallax. convinced that no single method deserved an exclusive preference, he reached a plausible result through the combination, on the principle of least squares--that is, by the mathematical rules of probability--of all the various quantities upon which the great datum depends. it thus summed up and harmonised the whole of the multifarious evidence bearing upon the point, and, as modified in ,[ ] falls very satisfactorily into line with the cape determination. we may, then, at least provisionally, accept , , miles as the length of our measuring-rod for space. nor do we hazard much in fixing , miles as the outside limit of its future correction. footnotes: [footnote : airy, _month. not._, vol. xvii., p. .] [footnote : mars comes into opposition once in about days; but owing to the eccentricity of both orbits, his distance from the earth at those epochs varies from thirty-five to sixty-two million miles.] [footnote : j. d. cassini, _hist. abrégée de la parallaxe du soleil_, p. , .] [footnote : the present period of coupled eccentric transits will, in the course of ages, be succeeded by a period of single, nearly central transits. the alignments by which transits are produced, of the earth, venus, and the sun, close to the place of intersection of the two planetary orbits, now occur, the first a little in front of, the second, after eight years less two and a half days, a little behind the node. but when the first of these two meetings takes place very near the node, giving a nearly central transit, the second falls too far from it, and the planet escapes projection on the sun. the reason of the liability to an eight-yearly recurrence is that eight revolutions of the earth are accomplished in only a very little more time than thirteen revolutions of venus.] [footnote : _die entfernung der sonne: fortsetzung_, p. . encke slightly corrected his results of in _berlin abh._, , p. .] [footnote : owing to the ellipticity of its orbit, the earth is nearer to the sun in january than in june by , , miles. the quantity to be determined, or "mean distance," is that lying midway between these extremes--is, in other words, half the major axis of the ellipse in which the earth travels.] [footnote : _month. not._, vol. xv., p. .] [footnote : _the distance of the sun from the earth determined by the theory of gravity_, edinburgh, .] [footnote : _opera_, t. iii., p. .] [footnote : _comptes rendus_, t. xlvi., p. . the parallax · " derived by leverrier from the above-described inequality in the earth's motion, was corrected by stone to · ". _month. not._, vol. xxviii., p. .] [footnote : _month. not._, vol. xxxv., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : _comptes rendus_, t. xxix., p. .] [footnote : _ibid._, t. xxx., p. .] [footnote : _ibid._, t. lv., p. . the previously admitted velocity was million metres per second; foucault reduced it to million. combined with struve's "constant of aberration" this gave . " for the solar parallax, which exactly agreed with cornu's result from a repetition of fizeau's experiments in . _comptes rendus_, t. lxxvi., p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _astr. papers of the american ephemeris_, vol. ii., p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : because closely similar to that proposed by him in _phil. trans._ for .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _observatory_, vol. v., p. .] [footnote : _transits of venus_, p. ( st ed.).] [footnote : _am. jour. of sc._, vol. xx., p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : _mem. roy. astr. soc._, vol. xlvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : hilfiker, _bern mittheilungen_, , p. .] [footnote : _annals of the cape observatory_, vols. vi., vii.] [footnote : _rapport sur l'État de l'observatoire de paris pour l'année _, p. .] [footnote : _observatory_, vol. xxiii., p. ; newcomb, _astr. jour._, no. .] [footnote : _comptes rendus_, t. xciii., p. .] [footnote : _ibid._, t. xcii., p. .] [footnote : _bull. de l'acad._, t. vi., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _astr. jour._, no. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. xcii., p. ; _am. jour. of sc._, vol. xxii., p. .] [footnote : _month. not._, vol. xxxv., p. .] [footnote : _am. jour. of sc._, vol. xviii., p. .] [footnote : _nature_, vol. xxxiv., p. ; _astron. papers of the american ephemeris_, vol. ii., p. .] [footnote : _comptes rendus_, t. cxii., p. .] [footnote : _astr. journ._, nos. , ] [footnote : _the solar parallax and its related constants_, washington, .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] chapter vii _planets and satellites_ johann hieronymus schröter was the herschel of germany. he did not, it is true, possess the more brilliant gifts of his rival. herschel's piercing discernment, comprehensive intelligence, and inventive splendour were wanting to him. he was, nevertheless, the founder of descriptive astronomy in germany, as herschel was in england. born at erfurt in , he prosecuted legal studies at göttingen, and there imbibed from kästner a life-long devotion to science. from the law, however, he got the means of living, and, what was to the full as precious to him, the means of observing. entering the sphere of hanoverian officialism in , he settled a few years later at lilienthal, near bremen, as "oberamtmann," or chief magistrate. here he built a small observatory, enriched in with a seven-foot reflector by herschel, then one of the most powerful instruments to be found anywhere out of england. it was soon surpassed, through his exertions, by the first-fruits of native industry in that branch. schrader of kiel transferred his workshops to lilienthal in , and constructed there, under the superintendence and at the cost of the astronomical oberamtmann, a thirteen-foot reflector, declared by lalande to be the finest telescope in existence, and one twenty-seven feet in focal length, probably as inferior to its predecessor in real efficiency as it was superior in size. thus, with instruments of gradually increasing power, schröter studied during thirty-four years the topography of the moon and planets. the field was then almost untrodden; he had but few and casual predecessors, and has since had no equal in the sustained and concentrated patience of his hourly watchings. both their prolixity and their enthusiasm are faithfully reflected in his various treatises. yet the one may be pardoned for the sake of the other, especially when it is remembered that he struck out a substantially new line, and that one of the main lines of future advance. moreover, his infectious zeal communicated itself; he set the example of observing when there was scarcely an observer in germany; and under his roof harding and bessel received their training as practical astronomers. but he was reserved to see evil days. early in the french under vandamme occupied bremen. on the night of april , the vale of lilies was, by their wanton destructiveness, laid waste with fire; the government offices were destroyed, and with them the chief part of schröter's property, including the whole stock of his books and writings. there was worse behind. a few days later, his observatory, which had escaped the conflagration, was broken into, pillaged, and ruined. his life was wrecked with it. he survived the catastrophe three years without the means to repair, or the power to forget it, and gradually sank from disappointment into decay, terminated by death, august , . he had, indeed, done all the work he was capable of; and though not of the first quality, it was far from contemptible. he laid the foundation of the _comparative_ study of the moon's surface, and the descriptive particulars of the planets laboriously collected by him constituted a store of more or less reliable information hardly added to during the ensuing half century. they rested, it is true, under some shadow of doubt; but the most recent observations have tended on several points to rehabilitate the discredited authority of the lilienthal astronomer. we may now briefly resume, and pursue in its further progress, the course of his studies, taking the planets in the order of their distances from the sun. in april, , schröter saw reason to conclude, from the gradual degradation of light on its partially illuminated disc, that mercury possesses a tolerably dense atmosphere.[ ] during the transit of may , , he was, moreover, struck with the appearance of a ring of softened luminosity encircling the planet to an apparent height of three seconds, or about a quarter of its own diameter.[ ] although a "mere thought" in texture, it remained persistently visible both with the seven-foot and the thirteen-foot reflectors, armed with powers up to . it had a well-marked grayish boundary, and reminded him, though indefinitely fainter, of the penumbra of a sun-spot. a similar appendage had been noticed by de plantade at montpellier, november , , and again in and by prosperin and flaugergues; but herschel, on november , , saw the preceding limb of the planet projected on the sun cut the luminous solar clouds with the most perfect sharpness.[ ] the presence, however, of a "halo" was unmistakable in , when professor moll, of utrecht, described it as a "nebulous ring of a darker tinge approaching to the violet colour."[ ] again, to huggins and stone, november , , it showed as lucid and most distinct. no change in the colour of the glasses used, or the powers applied, could get rid of it, and it lasted throughout the transit.[ ] it was next seen by christie and dunkin at greenwich, may , ,[ ] and with much precision of detail by trouvelot at cambridge (u.s.).[ ] professor holden, on the other hand, noted at hastings-on-hudson the total absence of all anomalous appearances.[ ] nor could any vestige of them be perceived by barnard at lick on november , .[ ] various effects of irradiation and diffraction were, however, observed by lowell and w. h. pickering at flagstaff;[ ] and davidson was favoured at san francisco with glimpses of the historic aureola,[ ] as well as of a central whitish spot, which often accompanies it. that both are somehow of optical production can scarcely be doubted. nothing can be learned from them regarding the planet's physical condition. airy showed that refraction in a mercurian atmosphere could not possibly originate the noted aureola, which must accordingly be set down as "strictly an ocular nervous phenomenon."[ ] it is the less easy to escape from this conclusion that we find the virtually airless moon capable of exhibiting a like appendage. professor stephen alexander, of the united states survey, with two other observers, perceived, during the eclipse of the sun of july , , the advancing lunar limb to be bordered with a bright band;[ ] and photographic effects of the same kind appear in pictures of transits of venus and partial solar eclipses. the spectroscope affords little information as to the constitution of mercury. its light is of course that of the sun reflected, and its spectrum is consequently a faint echo of the fraunhofer spectrum. dr. h. c. vogel, who first examined it in april, , _suspected_ traces of the action of an atmosphere like ours,[ ] but, it would seem, on slight grounds. it is, however, certainly very poor in blue rays. more definite conclusions were, in ,[ ] derived by zöllner from photometric observations of mercurian phases. a similar study of the waxing and waning moon had afforded him the curious discovery that light-changes dependent upon phase vary with the nature of the reflecting surface, following a totally different law on a smooth homogeneous globe and on a rugged and mountainous one. now the phases of mercury--so far as could be determined from only two sets of observations--correspond with the latter kind of structure. strictly analogous to those of the moon, they seem to indicate an analogous mode of surface-formation. this conclusion was fully borne out by müller's more extended observations at potsdam during the years - .[ ] practical assurance was gained from them that the innermost planet has a rough rind of dusky rock, absorbing all but per cent. of the light poured upon it by the fierce adjacent sun. its "albedo," in other words, is · ,[ ] which is precisely that ascribed to the moon. the absence of any appreciable mercurian atmosphere followed almost necessarily from these results. on march , , schröter, observing with his -foot reflector in a peculiarly clear sky, perceived the southern horn of mercury's crescent to be quite distinctly blunted.[ ] interception of sunlight by a mercurian mountain rather more than eleven english miles high explained the effect to his satisfaction. by carefully timing its recurrence, he concluded rotation on an axis in a period of hours minutes. the first determination of the kind rewarded twenty years of unceasing vigilance. it received ostensible confirmation from the successive appearances of a dusky streak and blotch in may and june, .[ ] these, however, were inferred to be no permanent markings on the body of the planet, but atmospheric formations, the streak at times drifting forwards (it was thought) under the fluctuating influence of mercurian breezes. from a rediscussion of these somewhat doubtful observations bessel inferred that mercury rotates on an axis inclined ° to the plane of its orbit in hours seconds. the rounded appearance of the southern horn seen by schröter was more or less doubtfully caught by noble ( ), burton, and franks ( );[ ] but was obvious to mr. w. f. denning at bristol on the morning of november , .[ ] that the southern polar regions are usually less bright than the northern is well ascertained; but the cause of the deficiency remains dubious. if inequalities of surface are in question, they must be on a considerable scale; and a similar explanation might be given of the deformations of the "terminator"--or dividing-line between darkness and light in the planet's phases--first remarked by schröter, and again clearly seen by trouvelot in and .[ ] the displacement, during four days, of certain brilliant and dusky spaces on the disc indicated to mr. denning in rotation in about twenty-five hours; while the general aspect of the planet reminded him of that of mars.[ ] but the difficulties in the way of its observation are enormously enhanced by its constant close attendance on the sun. in his sustained study of the features of mercury, schröter had no imitator until schiaparelli took up the task at milan in . his observations were made in daylight. it was found that much more could be seen, and higher magnifying powers used, high up in the sky near the sun, than at low altitudes, through the agitated air of morning or evening twilight. a notable discovery ensued.[ ] following the planet hour by hour, instead of making necessarily brief inspections at intervals of about a day, as previous observers had done, it was found that the markings faintly visible remained sensibly fixed, hence, that there was no rotation in a period at all comparable with that of the earth. and after long and patient watching, the conclusion was at last reached that mercury turns on his axis in the same time needed to complete a revolution in his orbit. one of his hemispheres, then, is always averted from the sun, as one of the moon's hemispheres from the earth, while the other never shifts from beneath his torrid rays. the "librations," however, of mercury are on a larger scale than those of the moon, because he travels in a more eccentric path. the temporary inequalities arising between his "even pacing" on an axis and his alternately accelerated and retarded elliptical movement occasion, in fact, an oscillation to and fro of the boundaries of light and darkness on his globe over an arc of ° ', in the course of his year of days. thus the regions of perpetual day and perpetual night are separated by two segments, amounting to one-fourth of the entire surface, where the sun rises and sets once in days. else there is no variation from the intense glare on one side of the globe, and the nocturnal blackness on the other. to schiaparelli's scrutiny, mercury appeared as a "spotty globe," enveloped in a tolerably dense atmosphere. the brownish stripes and streaks, discerned on his rose-tinged disc, and judged to be permanent, were made the basis of a chart. they were not indeed always equally well seen. they disappeared regularly near the limb, and were at times veiled even when centrally situated. some of them had been clearly perceived by de ball at bothkamp in .[ ] mr. lowell followed schiaparelli's example by observing mercury in the full glare of noon. "the best time to study him," he remarked, "is when planetary almanacs state 'mercury invisible.'" a remarkable series of drawings executed, some at flagstaff in , the remainder at mexico in , supplied grounds for the following, among other, conclusions.[ ] mercury rotates synchronously with its revolution--that is, once in days--on an axis sensibly perpendicular to its orbital plane. no certain signs of a mercurian atmosphere are visible. the globe is seamed and furrowed with long narrow markings, explicable as cracks in cooling. it is, and always was, a dead world. from micrometrical measures, moreover, the inferences were drawn that the planet's mass has a probable value about / that of the earth, while its mean density falls considerably short of the terrestrial standard. the theory of mercury's movements has always given trouble. in lalande's,[ ] as in mästlin's time, the planet seemed to exist for no other purpose than to throw discredit on astronomers; and even to leverrier's powerful analysis it long proved recalcitrant. on the th of september, , however, he was able to announce before the academy of sciences[ ] the terms of a compromise between observation and calculation. they involved the addition of a new member to the solar system. the hitherto unrecognised presence of a body about the size of mercury itself revolving at somewhat less than half its mean distance from the sun (or, if farther, then of less mass, and _vice versâ_), would, it was pointed out, produce exactly the effect required, of displacing the perihelion of the former planet " a century more than could otherwise be accounted for. the planes of the two orbits, however, should not lie far apart, as otherwise a nodal disturbance would arise not perceived to exist. it was added that a ring of asteroids similarly placed would answer the purpose equally well, and was more likely to have escaped notice. upon the heels of this forecast followed promptly a seeming verification. dr. lescarbault, a physician residing at orgères, whose slender opportunities had not blunted his hopes of achievement, had, ever since , when he witnessed a transit of mercury, cherished the idea that an unknown planet might be caught thus projected on the solar background. unable to observe continuously until , he, on march , , saw what he had expected--a small perfectly round object slowly traversing the sun's disc. the fruitless expectation of reobserving the phenomenon, however, kept him silent, and it was not until december , after the news of leverrier's prediction had reached him, that he wrote to acquaint him with his supposed discovery.[ ] the imperial astronomer thereupon hurried down to orgères, and by personal inspection of the simple apparatus used, by searching cross-examination and local inquiry, convinced himself of the genuine character and substantial accuracy of the reported observation. he named the new planet "vulcan," and computed elements giving it a period of revolution slightly under twenty days.[ ] but it has never since been seen. m. liais, director of the brazilian coast survey, thought himself justified in asserting that it never had been seen. observing the sun for twelve minutes after the supposed ingress recorded at orgères, he noted those particular regions of its surface as "très uniformes d'intensité."[ ] he subsequently, however, admitted lescarbault's good faith, at first rashly questioned. the planet-seeking doctor was, in truth, only one among many victims of similar illusions. waning interest in the subject was revived by a fresh announcement of a transit witnessed, it was asserted, by weber at peckeloh, april , .[ ] the pseudo-planet, indeed, was detected shortly afterwards on the greenwich photographs, and was found to have been seen by m. ventosa at madrid in its true character of a sun-spot without penumbra; but leverrier had meantime undertaken the investigation of a list of twenty similar dubious appearances, collected by haase, and republished by wolf in .[ ] from these, five were picked out as referring in all likelihood to the same body, the reality of whose existence was now confidently asserted, and of which more or less probable transits were fixed for march , , and october , .[ ] but, widespread watchfulness notwithstanding, no suspicious object came into view at either epoch. the next announcement of the discovery of "vulcan" was on the occasion of the total solar eclipse of july , .[ ] this time it was stated to have been seen at some distance south-west of the obscured sun, as a ruddy star with a minute planetary disc; and its simultaneous detection by two observers--the late professor james c. watson, stationed at rawlins (wyoming territory), and professor lewis swift at denver (colorado)--was at first readily admitted. but their separate observations could, on a closer examination, by no possibility be brought into harmony, and, if valid, certainly referred to two distinct objects, if not to four; each astronomer eventually claiming a pair of planets. nor could any one of the four be identified with lescarbault's and leverrier's vulcan, which, if a substantial body revolving round the sun, must then have been found on the _east_ side of that luminary.[ ] the most feasible explanation of the puzzle seems to be that watson and swift merely saw each the same two stars in cancer: haste and excitement doing the rest.[ ] nevertheless, they strenuously maintained their opposite conviction.[ ] intra-mercurian planets have since been diligently searched for when the opportunity of a total eclipse offered, especially during the long obscuration at caroline island. not only did professor holden "sweep" in the solar vicinity, but palisa and trouvelot agreed to divide the field of exploration, and thus make sure of whatever planetary prey there might be within reach; yet with only negative results. photographic explorations during recent eclipses have been equally fruitless. belief in the presence of any considerable body or bodies within the orbit of mercury is, accordingly, at a low ebb. yet the existence of the anomaly in the mercurian movements indicated by leverrier has been made only surer by further research.[ ] its elucidation constitutes one of the "pending problems" of astronomy. * * * * * from the observation at bologna in - of some very faint spots, domenico cassini concluded a rotation or libration of venus--he was not sure which--in about twenty-three hours.[ ] by bianchini in the period was augmented to twenty-four _days_ eight hours. j. j. cassini, however, in , showed that the data collected by both observers were consistent with rotation in twenty-three hours twenty minutes.[ ] so the matter rested until schröter's time. after watching nine years in vain, he at last, february , , perceived the ordinarily uniform brightness of the planet's disc to be marbled with a filmy streak, which returned periodically to the same position in about twenty-three hours twenty-eight minutes. this approximate estimate was corrected by the application of a more definite criterion. on december , , the southern horn of the crescent venus was seen truncated, an outlying lucid point interrupting the darkness beyond. precisely the same appearance recurred two years later, giving for the planet's rotation a period of h. m.[ ] to this only twenty-two seconds were added by de vico, as the result of over , observations made with the cauchoix refractor of the collegio romano, - .[ ] the axis of rotation was found to be much more bowed towards the orbital plane than that of the earth, the equator making with it an angle of ° '. these conclusions inspired, it is true, much distrust, consequently there were no received ideas on the subject to be subverted. nevertheless, a shock of surprise was felt at schiaparelli's announcement, early in ,[ ] that venus most probably rotates after the fashion just previously ascribed to mercury. a continuous series of observations, from november, , to february, , with their records in above a hundred drawings, supplied the chief part of the data upon which he rested his conclusions. they certainly appeared exceptionally well-grounded; and the doubts at first qualifying them were removed by a fresh set of determinations in july, .[ ] most observers had depended, in their attempts to ascertain the rotation-period of venus, upon evanescent shadings, most likely of atmospheric origin, and scarcely recognisable from day to day. schiaparelli fixed his attention upon round, defined, lustrously white spots, the presence of which near the cusps of the illuminated crescent has been attested for close upon two centuries. his steady watch over them showed the invariability of their position with regard to the terminator; and this is as much as to say that the regions of day and night do not shift on the surface of the planet. in other words, she keeps the same face always turned towards the sun. moreover, since her orbit is nearly circular, libratory effects are very small. they amount in fact to only just one-thirtieth of those serving to modify the severe contrasts of climate in mercury. confirmatory evidence of schiaparelli's result for venus is not wanting. thus, observations irreconcilable with a swift rate of rotation were made at bothkamp in by vogel and lohse;[ ] and a drawing executed by professor holden with the great washington reflector, december , , showed the same markings in the positions recorded at milan to have been occupied by them eight hours previously. further, a series of observations, carried out by m. perrotin at nice, may to october , , and from mount mounier in - , with the special aim of testing the inference of synchronous rotation and revolution, proved strongly corroborative of it.[ ] a remarkable collection of drawings made by mr. lowell in appeared decisive in its favour;[ ] tacchini at rome,[ ] mascari at catania and etna,[ ] cerulli at terano,[ ] obtained in - evidence similar in purport. on the other hand, niesten of brussels found reason to revert to vico's discarded elements for the planet's rotation;[ ] and trouvelot,[ ] stanley williams,[ ] villiger,[ ] and leo brenner,[ ] so far agreed with him as to adopt a period of approximately twenty-four hours. finally, e. von oppolzer suggested an appeal to the spectroscope;[ ] and bélopolsky secured in [ ] spectrograms apparently marked by the minute displacements corresponding to a rapid rate of axial movement. but they were avowedly taken only as an experiment, with unsuitable apparatus; and the desirable verification of their supposed import is not yet forthcoming. until it is, schiaparelli's period of days must be allowed to hold the field. effects attributed to great differences of level in the surface of venus have struck many observers. francesco fontana at naples in noticed irregularities along the inner edge of the crescent.[ ] lahire in considered them--regard being had to difference of distance--to be much more strongly marked than those visible in the moon.[ ] schröter's assertions to the same effect, though scouted with some unnecessary vehemence by herschel,[ ] have since been repeatedly confirmed; amongst others by mädler, de vico, langdon, who in saw the broken line of the terminator with peculiar distinctness through a veil of auroral cloud;[ ] by denning,[ ] march , , despite preliminary impressions to the contrary, as well as by c. v. zenger at prague, january , . the great mountain mass, presumed to occasion the periodical blunting of the southern horn, was precariously estimated by the lilienthal observer to rise to the prodigious height of nearly twenty-seven miles, or just five times the elevation of mount everest! yet the phenomenon persists, whatever may be thought of the explanation. moreover, the speck of light beyond, interpreted as the visible sign of a detached peak rising high enough above the encircling shadow to catch the first and last rays of the sun, was frequently discerned by baron van ertborn in ;[ ] while an object near the northern horn of the crescent, strongly resembling a lunar ring-mountain, was delineated both by de vico in and by denning forty years later. we are almost equally sure that venus, as that the earth is encompassed with an atmosphere. yet, notwithstanding luminous appearances plainly due to refraction during the transits both of and , schröter, in , took the initiative in coming to a definite conclusion on the subject.[ ] it was founded, first, on the rapid diminution of brilliancy towards the terminator, attributed to atmospheric absorption; next, on the extension beyond a semicircle of the horns of the crescent; lastly, on the presence of a bluish gleam illuminating the early hours of the cytherean night with what was taken to be genuine twilight. even herschel admitted that sunlight, by the same effect through which the heavenly bodies show _visibly above_ our horizons while still _geometrically below_ them, appeared to be bent round the shoulder of the globe of venus. ample confirmation of the fact has since been afforded. at dorpat in may, , the planet being within ° ' of inferior conjunction, mädler found the arms of waning light upon the disc to embrace no less than ° of its extent;[ ] and in december, , mr. guthrie, of bervie, n.b., actually observed, under similar conditions, the whole circumference to be lit up with a faint nebulous glow.[ ] the same curious phenomenon was intermittently seen by mr. leeson prince at uckfield in september, ;[ ] but with more satisfactory distinctness by mr. c. s. lyman of yale college,[ ] before and after the conjunction of december , , and during nearly five hours previous to the transit of , when the yellowish ring of refracted light showed at one point an approach to interruption, possibly through the intervention of a bank of clouds. again, on december , , venus being ° ' from the sun's centre, mr. h. n. russell, of the halsted observatory, descried the coalescence of the cusps, and founded on the observation a valuable discussion of such effects.[ ] taking account of certain features in the case left unnoticed by neison[ ] and proctor,[ ] he inferred from them the presence of a cytherean atmosphere considerably less refractive than our own, although possibly, in its lower strata, encumbered with dust or haze. similar appearances are conspicuous during transits. but while the mercurian halo is characteristically seen on the sun, the "silver thread" round the limb of venus commonly shows on the part _off_ the sun. there are, however, instances of each description in both cases. mr. grant, in collecting the records of physical phenomena accompanying the transits of and , remarks that no one person saw both kinds of annulus, and argues a dissimilarity in their respective modes of production.[ ] such a dissimilarity probably exists, in the sense that the inner section of the ring is illusory, the outer, a genuine result of the bending of light in a gaseous envelope; but the distinction of separate visibility has not been borne out by recent experience. several of the australian observers during the transit of witnessed the complete phenomenon. mr. j. macdonnell, at eden, saw a "shadowy nebulous ring" surround the whole disc when ingress was two-thirds accomplished; mr. tornaghi, at goulburn, perceived a halo, entire and unmistakable, at half egress.[ ] similar observations were made at sydney,[ ] and were renewed in by lescarbault at orgères, by metzger in java, and by barnard at vanderbilt university.[ ] spectroscopic indications of aqueous vapour as present in the atmosphere of venus, were obtained in and , by tacchini and riccò in italy, and by young in new jersey.[ ] janssen, however, who made a special study of the point subsequently to the transit of , found them much less certain than he had anticipated;[ ] and vogel, by repeated examinations, - , could detect only the very slightest variations from the pattern of the solar spectrum. some additions there indeed seem to be in the thickening of a few water and oxygen-lines; but so nearly evanescent as to induce the persuasion that most of the light we receive from venus has traversed only the tenuous upper portion of its atmosphere.[ ] it is reflected, at any rate, with comparatively slight diminution. on the th and th of september, , a close conjunction gave mr. james nasmyth the rare opportunity of watching venus and mercury for several hours side by side in the field of his reflector; when the former appeared to him like clean silver, the latter as dull as lead or zinc.[ ] yet the light _incident_ upon mercury is, on an average, three and a half times as strong as the light reaching venus. thus, the reflective power of venus must be singularly strong. and we find, accordingly, from a combination of zöllner's with müller's results, that its albedo is but little inferior to that of new-fallen snow; in other words, it gives back per cent. of the luminous rays impinging upon it. this extraordinary brilliancy would be intelligible were it permissible to suppose that we see nothing of the planet but a dense canopy of clouds. but the hypothesis is discountenanced by the flagstaff observations, and is irreconcilable with the visibility of mountainous elevations, and permanent surface-markings. to mr. lowell these were so distinct and unchanging as to furnish data for a chart of the cytherean globe, and the peculiar arrangement of divergent shading exhibited in it cannot off-hand be set down as unreal, in view of perrotin's earlier discernment of analogous linear traces. gruithuisen's "snow-caps,"[ ] however--it is safe to say--do not exist as such; although shining regions near the poles form a well-attested trait of the strange cytherean landscape. the "secondary," or "ashen light," of venus was first noticed by riccioli in ; it was seen by derham about , by kirch in , by schröter and harding in ;[ ] and the reality of the appearance has since been authenticated by numerous and trustworthy observations. it is precisely similar to that of the "old moon in the new moon's arms"; and zenger, who witnessed it with unusual distinctness, january , ,[ ] supposes it due to the same cause--namely, to the faint gleam of reflected earth-light from the night-side of the planet. when we remember, however, that "full earth-light" on venus, at its nearest, has little more than / its intensity on the moon, we see at once that the explanation is inadequate. nor can professor safarik's,[ ] by phosphorescence of the warm and teeming oceans with which zöllner[ ] regarded the globe of venus as mainly covered, be seriously entertained. vogel's suggestion is more plausible. he and o. lohse, at bothkamp, november to , , saw the dark hemisphere _partially_ illuminated by secondary light, extending ° from the terminator, and thought the effect might be produced by a very extensive twilight.[ ] others have had recourse to the analogy of our auroræ, and j. lamp suggested that the grayish gleam, visible to him at bothkamp, october and , ,[ ] might be an accompaniment of electrical processes connected with the planet's meteorology. whatever the origin of the phenomenon, it may serve, on a night-enwrapt hemisphere, to dissipate some of the thick darkness otherwise encroached upon only by "the pale light of stars." venus was once supposed to possess a satellite. but belief in its existence has died out. no one, indeed, has caught even a deceptive glimpse of such an object during the last years. yet it was repeatedly and, one might have thought, well observed in the seventeenth and eighteenth centuries. fontana "discovered" it in ; cassini--an adept in the art of seeing--recognised it in , and again in ; short watched it for a full hour in with varied instrumental means; tobias mayer in , montaigne in ; several astronomers at copenhagen in march, , noted what they considered its unmistakable presence; as did horrebow in . but m. paul stroobant,[ ] who in submitted all the available data on the subject to a searching examination, identified horrebow's satellite with theta libræ, a fifth-magnitude star; and a few other apparitions were, by his industry, similarly explained away. nevertheless, several withstood all efforts to account for them, and together form a most curious case of illusion. for it is quite certain that venus has no such conspicuous attendant. * * * * * the third planet encountered in travelling outward from the sun is the abode of man. he has in consequence opportunities for studying its physical habitudes altogether different from the baffling glimpse afforded to him of the other members of the solar family. regarding the earth, then, a mass of knowledge so varied and comprehensive has been accumulated as to form a science--or rather several sciences--apart. but underneath all lie astronomical relations, the recognition and investigation of which constitute one of the most significant intellectual events of the present century. it is indeed far from easy to draw a line of logical distinction between items of knowledge which have their proper place here, and those which should be left to the historian of geology. there are some, however, of which the cosmical connections are so close that it is impossible to overlook them. among these is the ascertainment of the solidity of the globe. at first sight it seems difficult to conceive what the apparent positions of the stars can have to do with subterranean conditions; yet it was from star measurements alone that hopkins, in , concluded the earth to be solid to a depth of at least or , miles.[ ] his argument was, that if it were a mere shell filled with liquid, precession and nutation would be much larger than they are observed to be. for the shell alone would follow the pull of the sun and moon on its equatorial girdle, leaving the liquid behind; and being thus so much the lighter, would move the more readily. there is, it is true, grave reason to doubt whether this reasoning corresponds with the actual facts of the case;[ ] but the conclusion to which it led has been otherwise affirmed and extended. indications of an identical purport have been derived from another kind of external disturbance, affecting our globe through the same agencies. lord kelvin (then sir william thomson) pointed out in [ ] that tidal influences are brought to bear on land as well as on water, although obedience to them is perceptible only in the mobile element. some bodily distortion of the earth's figure _must_, however, take place, unless we suppose it of absolute or "preternatural" rigidity, and the amount of such distortion can be determined from its effect in diminishing oceanic tides below their calculated value. for if the earth were perfectly plastic to the stresses of solar and lunar gravity, tides--in the ordinary sense--would not exist. continents and oceans would swell and subside together. it is to the _difference_ in the behaviour of solid and liquid terrestrial constituents that the ebb and flow of the waters are due. six years later, the distinguished glasgow professor suggested that this criterion might, by the aid of a prolonged series of exact tidal observations, be practically applied to test the interior condition of our planet.[ ] in , accordingly, suitable data extending over thirty-three years having at length become available, mr. g. h. darwin performed the laborious task of their analysis, with the general result that the "effective rigidity" of the earth's mass must be _at least_ as great as that of steel.[ ] ratification from an unexpected quarter has lately been brought to this conclusion. the question of a possible mobility in the earth's axis of rotation has often been mooted. now at last it has received an affirmative reply. dr. küstner detected, in his observations of - , effects apparently springing from a minute variation in the latitude of berlin. the matter having been brought before the international geodetic association in , special observations were set on foot at berlin, potsdam, prague, and strasbourg, the upshot of which was to bring plainly to view synchronous, and seemingly periodic fluctuations of latitude to the extent of half a second of arc. the reality of these was verified by an expedition to honolulu in - , the variations there corresponding inversely to those simultaneously determined in europe.[ ] their character was completely defined by mr. s. c. chandler's discussion in october, .[ ] he showed that they could be explained by supposing the pole of the earth to describe a circle with a radius of thirty feet in a period of fourteen months. confirmation of this hypothesis was found by dr. b. a. gould in the cordoba observations,[ ] and it was provided with a physical basis through the able co-operation of professor newcomb.[ ] the earth, owing to its ellipsoidal shape, should, apart from disturbance, rotate upon its "axis of figure," or shortest diameter; since thus alone can the centrifugal forces generated by its spinning balance each other. temporary causes, however, such as heavy falls of snow or rain limited to one continental area, the shifting of ice-masses, even the movements of winds, may render the globe slightly lop-sided, and thus oblige it to forsake its normal axis, and rotate on one somewhat divergent from it. this "instantaneous axis" (for it is incessantly changing) must, by mathematical theory, revolve round the axis of figure in a period of days. provided, that is to say, the earth were a perfectly rigid body. but it is far from being so; it yields sensibly to every strain put upon it; and this yielding tends to protract the time of circulation of the displaced pole. the length of its period, then, serves as a kind of measure of the plasticity of the globe; which, according to newcomb's and s. s. hough's independent calculations,[ ] seems to be a little less than that of steel. in an earth compacted of steel, the instantaneous axis would revolve in days; in the actual earth, the process is accomplished in days. by this new path, accordingly, astronomers have been led to an identical estimate of the consistence of our globe with that derived from tidal investigations. variations of latitude are intrinsically complex. to produce them, an incalculable interplay of causes must be at work, each with its proper period and law of action.[ ] all the elements of the phenomenon are then in a perpetual state of flux,[ ] and absorb for their continual redetermination, the arduous and combined labours of many astronomers. nor is this trouble superfluous. minute in extent though they be, the shiftings of the pole menace the very foundations of exact celestial science; their neglect would leave the entire fabric insecure. just at the beginning of the present century they reached a predicted minimum, but are expected again to augment their range after the year . the interesting suggestion has been made by mr. j. halm that such fluctuations are, in some obscure way, affected by changes in solar activity, and conform like them to an eleven-year cycle.[ ] in a paper read before the geological society, december , ,[ ] sir john herschel threw out the idea that the perplexing changes of climate revealed by the geological record might be explained through certain slow fluctuations in the eccentricity of the earth's orbit, produced by the disturbing action of the other planets. shortly afterwards, however, he abandoned the position as untenable;[ ] and it was left to the late dr. james croll, in [ ] and subsequent years, to reoccupy and fortify it. within restricted limits (as lagrange and, more certainly and definitely, leverrier proved), the path pursued by our planet round the sun alternately contracts, in the course of ages, into a moderate ellipse, and expands almost to a circle, the major axis, and consequently the mean distance, remaining invariable. even at present, when the eccentricity approaches a minimum, the sun is nearer to us in january than in july by above three million miles, and some , years ago this difference was more than four times as great. dr. croll brought together[ ] a mass of evidence to support the view, that, at epochs of considerable eccentricity, the hemisphere of which the winter, occurring at aphelion, was both intensified and prolonged, must have undergone extensive glaciation; while the opposite hemisphere, with a short, mild winter, and long, cool summer, enjoyed an approach to perennial spring. these conditions were exactly reversed at the end of , years, through the shifting of the perihelion combined with the precession of the equinoxes, the frozen hemisphere blooming into a luxuriant garden as its seasons came round to occur at the opposite sites of the terrestrial orbit, and the vernal hemisphere subsiding simultaneously into ice-bound rigour.[ ] thus a plausible explanation was offered of the anomalous alternations of glacial and semi-tropical periods, attested, on incontrovertible geological evidence, as having succeeded each other in times past over what are now temperate regions. they succeeded each other, it is true, with much less frequency and regularity than the theory demanded; but the discrepancy was overlooked or smoothed away. the most recent glacial epoch was placed by dr. croll about , years ago, when the eccentricity of the earth's orbit was · times as great as it is now. at present a faint representation of such a state of things is afforded by the southern hemisphere. one condition of glaciation in the coincidence of winter with the maximum of remoteness from the sun, is present; the other--a high eccentricity--is deficient. yet the ring of ice-bound territory hemming in the southern pole is well known to be far more extensive than the corresponding region in the north. the verification of this ingenious hypothesis depends upon a variety of intricate meteorological conditions, some of which have been adversely interpreted by competent authorities.[ ] what is still more serious, its acceptance seems precluded by time-relations of a simple kind. dr. wright[ ] has established with some approach to certainty that glacial conditions ceased in canada and the united states about ten or twelve thousand years ago. the erosive action of the falls of niagara qualifies them to serve as a clepsydra, or water-clock on a grand scale; and their chronological indications have been amply corroborated elsewhere and otherwise on the same continent. the astronomical ice age, however, should have been enormously more antique. no reconciliation of the facts with the theory appears possible. the first attempt at an experimental estimate of the "mean density" of the earth was maskelyne's observation in of the deflection of a plumb-line through the attraction of schehallien. the conclusion thence derived, that our globe weighs - / times as much as an equal bulk of water,[ ] was not very exact. it was considerably improved upon by cavendish, who, in , brought into use the "torsion-balance" constructed for the same purpose by john michell. the resulting estimate of · was raised to · by francis baily's elaborate repetition of the process in - . from experiments on the subject made in - by cornu and baille the slightly inferior value of · was derived; and it was further shown that the data collected by baily, when corrected for a systematic error, gave practically the same result ( · ).[ ] m. wilsing's of · , obtained at potsdam in ,[ ] nearly agreed with it; while professor poynting, by means of a common balance, arrived at a terrestrial mean density of · .[ ] professor boys next entered the field with an exquisite apparatus, in which a quartz fibre performed the functions of a torsion-rod; and the figure · determined by him, and exactly confirmed by dr. braun's research at mariaschein, bohemia, in ,[ ] may be called the standard value of the required datum. newton's guess at the average weight of the earth as five or six times that of water has thus been curiously verified. operations for determining the figure of the earth were carried out during the last century on an unprecedented scale. the russo-scandinavian arc, of which the measurement was completed under the direction of the elder struve in , reached from hammerfest to ismailia on the danube, a length of ° '. but little inferior to it was the indian arc, begun by lambton in the first years of the century, continued by everest, revised and extended by walker. both were surpassed in compass by the anglo-french arc, which embraced °; and considerable segments of meridians near the atlantic and pacific shores of north america were measured under the auspices of the united states coast survey. but these operations shrink into insignificance by comparison with sir david gill's grandiose scheme for uniting two hemispheres by a continuous network of triangulation. the history of geodesy in south africa began with lacaille's measurements in . they were repeated and enlarged in scope by sir thomas maclear in - ; and his determinations prepared the way for a complete survey of cape colony and natal, executed during the ten years - by colonel morris, r.e., under the direction of sir david gill.[ ] bechuanaland and rhodesia were subsequently included in the work; and the royal astronomer obtained, in , the support of the international geodetic association for its extension to the mouth of the nile. nor was this the limit of his design. by carrying the survey along the levantine coast, connection can be established with struve's system, and the magnificent amplitude of ° will be given to the conjoined african and european arcs. meantime, the french have undertaken the remeasurement of bouguer's peruvian arc, and a corresponding russo-swedish[ ] enterprise is progressing in spitzbergen; so that abundant materials will ere long be provided for fresh investigations of the shape and size of our planet. the smallness of the outstanding uncertainty can be judged of by comparing j. b. listing's[ ] with general clarke's[ ] results, published in the same year ( ). listing stated the dimensions of the terrestrial spheroid as follows: equatorial radius = , miles; polar radius = , miles; ellipticity = / · . clarke's corresponding figures were: , and , miles, giving an ellipticity of / · . the value of the latter fraction at present generally adopted is / ; that is to say, the thickness of the protuberant equatorial ring is held to be / of the equatorial radius. from astronomical considerations, it is true, newcomb estimated the ratio at / ;[ ] but for obtaining this particular datum, geodetical methods are unquestionably to be preferred. * * * * * the moon possesses for us a unique interest. she in all probability shared the origin of the earth; she perhaps prefigures its decay. she is at present its minister and companion. her existence, so far as we can see, serves no other purpose than to illuminate the darkness of terrestrial nights, and to measure, by swiftly-recurring and conspicuous changes of aspect, the long span of terrestrial time. inquiries stimulated by visible dependence, and aided by relatively close vicinity, have resulted in a wonderfully minute acquaintance with the features of the single lunar hemisphere open to our inspection. selenography, in the modern sense, is little more than a hundred years old. it originated with the publication in of schröter's _selenotopographische fragmente_.[ ] not but that the lunar surface had already been diligently studied, chiefly by hevelius, cassini, riccioli, and tobias mayer; the idea, however, of investigating the moon's physical condition, and detecting symptoms of the activity there of natural forces through minute topographical inquiry, first obtained effect at lilienthal. schröter's delineations, accordingly, imperfect though they were, afforded a starting-point for a _comparative_ study of the superficial features of our satellite. the first of the curious objects which he named "rills" was noted by him in . before he had found eleven; lohrmann added ; mädler ; schmidt published in a catalogue of , of which had been detected by himself;[ ] and he eventually brought the number up to nearly , . they are, then, a very persistent lunar feature, though wholly without terrestrial analogue. there is no difference of opinion as to their nature. they are quite obviously clefts in a rocky surface, to yards deep, usually a couple of miles across, and pursuing straight, curved, or branching tracks up to miles in length. as regards their origin, the most probable view is that they are fissures produced in cooling; but neison inclines to consider them rather as dried watercourses.[ ] on february , , schröter perceived what he took to be distinct traces of a lunar twilight, and continued to observe them during nine consecutive years.[ ] they indicated, he thought, the presence of a shallow atmosphere, about times more tenuous than our own. bessel, on the other hand, considered that the only way of "saving" a lunar atmosphere was to deny it any refractive power, the sharpness and suddenness of star-occultations negativing the possibility of gaseous surroundings of greater density (admitting an extreme supposition) than / that of terrestrial air.[ ] newcomb places the maximum at / . sir john herschel concluded "the non-existence of any atmosphere at the moon's edge having / part of the density of the earth's atmosphere."[ ] this decision was fully borne out by sir william huggins's spectroscopic observation of the disappearance behind the moon's limb of the small star eta piscium, january , .[ ] not the slightest sign of selective absorption or unequal refraction was discernible. the entire spectrum went out at once, as if a slide had suddenly dropped over it. the spectroscope has uniformly told the same tale; for m. thollon's observation during the total solar eclipse at sohag of a supposed thickening at the moon's rim, of certain dark lines in the solar spectrum, is now acknowledged to have been illusory. moonlight, analysed with the prism, is found to be pure reflected sunlight, diminished in _quantity_, owing to the low reflective capability of the lunar surface, to less than one-fifth its incident intensity, but wholly unmodified in _quality_. nevertheless, the diameter of the moon appeared from the greenwich observations discussed by airy in [ ] to be " smaller than when directly measured; and the effect would be explicable by refraction in a lunar atmosphere , times thinner than our own at the sea-level. but the difference was probably illusory. it resulted in part, if not wholly, from the visual enlargement by irradiation of the bright disc of the moon. professor comstock, employing the -inch clark equatoreal of the washburn observatory, found in the refractive displacements of occulted stars so trifling as to preclude the existence of a permanent lunar atmosphere of much more than / the density of the terrestrial envelope.[ ] the possibility, however, was admitted that, on the illuminated side of the moon, temporary exhalations of aqueous vapour might arise from ice-strata evaporated by sun-heat. meantime, some renewed evidence of actual crepuscular gleams on the moon had been gathered by mm. paul and prosper henry of the paris observatory, as well as by mr. w. h. pickering, in the pure air of arequipa, at an altitude of , feet above the sea.[ ] an occultation of jupiter, too, observed by him august , ,[ ] was attended with a slight flattening of the planet's disc through the effect, it was supposed, of lunar refraction--but of refraction in an atmosphere possessing, at the most, / the density at the sea-level of terrestrial air, and capable of holding in equilibrium no more than / of an inch of mercury. yet this small barometric value corresponds, mr. pickering remarks, "to a pressure of hundreds of tons per square mile of the lunar surface." the compression downward of gaseous strata on the moon should, in any case, proceed very gradually, owing to the slight power of lunar gravity,[ ] and they might hence play an important part in the economy of our satellite while evading spectroscopic and other tests. thus--as mr. ranyard remarked[ ]--the cliffs and pinnacles of the moon bear witness, by their unworn condition, to the efficiency of atmospheric protection against meteoric bombardment; and mr. pickering shows that it could be afforded by such a tenuous envelope as that postulated by him. the first to emulate schröter's selenographical zeal was wilhelm gotthelf lohrmann, a land-surveyor of dresden, who, in , published four out of twenty-five sections of the first scientifically executed lunar chart, on a scale of - / inches to a lunar diameter. his sight, however, began to fail three years later, and he died in , leaving materials from which the work was completed and published in by dr. julius schmidt, late director of the athens observatory. much had been done in the interim. beer and mädler began at berlin in their great trigonometrical survey of the lunar surface, as yet neither revised nor superseded. a map, issued in four parts, - , on nearly the same scale as lohrmann's, but more detailed and authoritative, embodied the results. it was succeeded, in , by a descriptive volume bearing the imposing title, _der mond; oder allgemeine vergleichende selenographie_. this summation of knowledge in that branch, though in truth leaving many questions open, had an air of finality which tended to discourage further inquiry.[ ] it gave form to a reaction against the sanguine views entertained by hevelius, schröter, herschel and gruithuisen as to the possibilities of agreeable residence on the moon, and relegated the "selenites," one of whose cities schröter thought he had discovered, and of whose festal processions gruithuisen had not despaired of becoming a spectator, to the shadowy land of the ivory gate. all examples of change in lunar formations were, moreover, dismissed as illusory. the light contained in the work was, in short, a "dry light," not stimulating to the imagination. "a mixture of a lie," bacon shrewdly remarks, "doth ever add pleasure." for many years, accordingly, schmidt had the field of selenography almost to himself. reviving interest in the subject was at once excited and displayed by the appointment, in , of a lunar committee of the british association. the indirect were of greater value than the direct fruits of its labours. an english school of selenography rose into importance. popularity was gained for the subject by the diffusion of works conspicuous for ingenuity and research. nasmyth's and carpenter's beautifully illustrated volume ( ) was succeeded, after two years, by a still more weighty contribution to lunar science in mr. neison's well-known book, accompanied by a map, based on the survey of beer and mädler, but adding some measures of positions, besides the representation of several thousand new objects. with schmidt's _charte der gebirge der mondes_, germany once more took the lead. this splendid delineation, built upon lohrmann's foundation, embraced the detail contained in upwards of , original drawings, representing the labour of thirty-four years. no less than , craters are represented in it, on a scale of seventy-five inches to a diameter. an additional help to lunar inquiries was provided at the same time in this country by the establishment, through the initiative of the late mr. w. r. birt, of the selenographical society. but the strongest incentive to diligence in studying the rugged features of our celestial helpmate has been the idea of probable or actual variation in them. a change always seems to the inquisitive intellect of man like a breach in the defences of nature's secrets, through which it may hope to make its way to the citadel. what is desirable easily becomes credible; and thus statements and rumours of lunar convulsions have successively, during the last hundred years, obtained credence, and successively, on closer investigation, been rejected. the subject is one as to which illusion is peculiarly easy. our view of the moon's surface is a bird's-eye view. its conformation reveals itself indirectly through irregularities in the distribution of light and darkness. the forms of its elevations and depressions can be inferred only from the shapes of the black, unmitigated shadows cast by them. but these shapes are in a state of perpetual and bewildering fluctuation, partly through changes in the angle of illumination, partly through changes in our point of view, caused by what are called the moon's "librations."[ ] the result is, that no single observation can be _exactly_ repeated by the same observer, since identical conditions recur only after the lapse of a great number of years. local peculiarities of surface, besides, are liable to produce perplexing effects. the reflection of earth-light at a particular angle from certain bright summits completely, though temporarily, deceived herschel into the belief that he had witnessed, in and , volcanic outbursts on the dark side of the moon. the persistent recurrence, indeed, of similar appearances under circumstances less amenable to explanation inclined webb to the view that effusions of native light actually occur.[ ] more cogent proofs must, however, be adduced before a fact so intrinsically improbable can be admitted as true. but from the publication of beer and mädler's work until , the received opinion was that no genuine sign of activity had ever been seen, or was likely to be seen, on our satellite; that her face was a stereotyped page, a fixed and irrevisable record of the past. a profound sensation, accordingly, was produced by schmidt's announcement, in october, , that the crater "linné," in the mare serenitatis, had disappeared,[ ] effaced, as it was supposed, by an igneous outflow. the case seemed undeniable, and is still dubious. linné had been known to lohrmann and mädler, - , as a deep crater, five or six miles in diameter, the third largest in the dusky plain known as the "mare serenitatis"; and schmidt had observed and drawn it, - , under a practically identical aspect. now it appears under high light as a whitish spot, in the centre of which, as the rays begin to fall obliquely, a pit, scarcely two miles across, emerges into view.[ ] the crateral character of this comparatively minute depression was detected by father secchi, february , . this is not all. schröter's description of linné, as seen by him november , , tallies quite closely with modern observation;[ ] while its inconspicuousness in is shown by its omission from russell's lunar globe and maps.[ ] we are thus driven to adopt one of two suppositions: either lohrmann, mädler, and schmidt were entirely mistaken in the size and importance of linné, or a real change in its outward semblance supervened during the first half of the century, and has since passed away, perhaps again to recur. the latter hypothesis seems the more probable: and its probability is strengthened by much evidence of actual obscuration or variation of tint in other parts of the lunar surface, more especially on the floor of the great "walled plain" named "plato."[ ] from a re-examination with a -inch refractor at arequipa in - , of this region, and of the mare serenitatis, mr. w. h. pickering inclines to the belief that lunar volcanic action, once apparently so potent, is not yet wholly extinct.[ ] an instance of an opposite kind of change was alleged by dr. hermann j. klein of cologne in march, .[ ] in linné the obliteration of an old crater had been assumed; in "hyginus n.," the formation of a new crater was asserted. yet, quite possibly, the same cause may have produced the effects thought to be apparent in both. it is, however, far from certain that any real change has affected the neighbourhood of hyginus. the novelty of klein's observation of may , , may have consisted simply in the detection of a hitherto unrecognised feature. the region is one of complex formation, consequently of more than ordinary liability to deceptive variations in aspect under rapid and entangled fluctuations of light and shade.[ ] moreover, it seems to be certain, from messrs. pratt and capron's attentive study, that "hyginus n." is no true crater, but a shallow, saucer-like depression, difficult of clear discernment.[ ] under suitable illumination, nevertheless, it contains, and is marked by, an ample shadow.[ ] in both these controverted instances of change, lunar photography was invoked as a witness; but, notwithstanding the great advances made in the art by de la rue in this country, by draper, and, above all, by rutherford in america, without decisive results. investigations of the kind began to assume a new aspect in , when professor holden organised them at the lick observatory.[ ] autographic moon-pictures were no longer taken casually, but on system; and dr. weinek's elaborate study, and skilful reproductions of them at prague,[ ] gave them universal value. they were designed to provide materials for an atlas on the scale of beer and mädler's, of which some beautiful specimen-plates have been issued. at paris, in , with the aid of a large "equatoreal coudé," a work of similar character was set on foot by mm. loewy and puiseux. its progress has been marked by the successive publication of five instalments of a splendid atlas, on a scale of about eight feet to the lunar diameter, accompanied by theoretical dissertations, designed to establish a science of "selenology." the moon's formations are thus not only delineated under every variety of light-incidence, but their meaning is sought to be elicited, and their history and mutual relations interpreted.[ ] henceforth, at any rate, the lunar volcanoes can scarcely, without notice taken, breathe hard in their age-long sleep. melloni was the first to get undeniable heating effects from moonlight. his experiments, made on mount vesuvius early in ,[ ] were repeated with like result by zantedeschi at venice four years later. a rough measure of the intensity of those effects was arrived at by piazzi smyth at guajara, on the peak of teneriffe, in . at a distance of fifteen feet from the thermomultiplier, a price's candle was found to radiate just twice as much heat as the full moon.[ ] then, after thirteen years, in - , an exact and extensive series of observations on the subject were made by the present earl of rosse. the lunar radiations, from the first to the last quarter, displayed, when concentrated with the parsonstown three-foot mirror, appreciable thermal energy, increasing with the phase, and largely due to "dark heat," distinguished from the quicker-vibrating sort by inability to traverse a plate of glass. this was supposed to indicate an actual heating of the surface, during the long lunar day of hours, to about ° f.[ ] (corrected later to °),[ ] the moon thus acting as a direct radiator no less than as a reflector of heat. but the conclusion was very imperfectly borne out by dr. boeddicker's observations with the same instrument and apparatus during the total lunar eclipse of october , .[ ] this initial opportunity of measuring the heat phases of an eclipsed moon was used with the remarkable result of showing that the heat disappeared almost completely, though not quite simultaneously, with the light. confirmatory evidence of the extraordinary promptitude with which our satellite parts with heat already to some extent appropriated, was afforded by professor langley's bolometric observations at allegheny of the partial eclipse of september , .[ ] yet it is certain that the moon sends us a perceptible quantity of heat _on its own account_, besides simply throwing back solar radiations. for in february, , professor langley succeeded, after many fruitless attempts, in getting measures of a "lunar heat-spectrum." the incredible delicacy of the operation may be judged of from the statement that the sum-total of the thermal energy dispersed by his rock-salt prisms was insufficient to raise a thermometer fully exposed to it one-thousandth of a degree centigrade! the singular fact was, however, elicited that this almost evanescent spectrum is made up of two superposed spectra, one due to reflection, the other, with a maximum far down in the infra-red, to radiation.[ ] the corresponding temperature of the moon's sunlit surface professor langley considers to be about that of freezing water.[ ] repeated experiments having failed to get any thermal effects from the dark part of the moon, it was inferred that our satellite "has no internal heat sensible at the surface"; so that the radiations from the lunar soil giving the low maximum in the heat-spectrum, "must be due purely to solar heat which has been absorbed and almost immediately re-radiated." professor langley's explorations of the terra incognita of immensely long wave-lengths where lie the unseen heat-emissions from the earth into space, led him to the discovery that these, contrary to the received opinion, are in good part transmissible by our atmosphere, although they are completely intercepted by glass. another important result of the allegheny work was the abolition of the anomalous notion of the "temperature of space," fixed by pouillet at - ° c. for space in itself can have no temperature, and stellar radiation is a negligible quantity. thus, it is safe to assume "that a perfect thermometer suspended in space at the distance of the earth or moon from the sun, but shielded from its rays, would sensibly indicate the absolute zero,"[ ] ordinarily placed at - ° c. a "prize essay on the distribution of the moon's heat" (the hague), , by mr. frank w. very, who had taken an active part in professor langley's long-sustained inquiry, embodies the fruits of its continuation. they show the lunar disc to be tolerably uniform in thermal power. the brighter parts are also indeed hotter, but not much. the traces perceived of a slight retention of heat by the substances forming the lunar surface, agreed well with the parsonstown observations of the total eclipse of the moon, january , .[ ] for they brought out an unmistakable divergence between the heat and light phases. a curious decrease of heat previous to the first touch of the earth's shadow upon the lunar globe remains unexplained, unless it be admissible to suppose the terrestrial atmosphere capable of absorbing heat at an elevation of miles. the probable range of temperature on the moon was discussed by professor very in .[ ] he concluded it to be very wide. hotter than boiling water under the sun's vertical rays, the arid surface of our dependent globe must, he found, cool in the -day lunar night to about the temperature of liquid air. although that fundamental part of astronomy known as "celestial mechanics" lies outside the scope of this work, and we therefore pass over in silence the immense labours of plana, damoiseau, hansen, delaunay, g. w. hill, and airy in reconciling the observed and calculated motions of the moon, there is one slight but significant discrepancy which is of such importance to the physical history of the solar system, that some brief mention must be made of it. halley discovered in , by examining the records of ancient eclipses, that the moon was going faster then than , years previously--so much faster, as to have got ahead of the place in the sky she would otherwise have occupied, by about two of her own diameters. it was one of laplace's highest triumphs to have found an explanation of this puzzling fact. he showed, in , that it was due to a very slow change in the ovalness of the earth's orbit, tending, during the present age of the world, to render it more nearly circular. the pull of the sun upon the moon is thereby lessened; the counter-pull of the earth gets the upper hand; and our satellite, drawn nearer to us by something less than an inch each year,[ ] proportionately quickens her pace. many thousands of years hence the process will be reversed; the terrestrial orbit will close in at the sides, the lunar orbit will open out under the growing stress of solar gravity, and our celestial chronometer will lose instead of gaining time. this is all quite true as laplace put it; but it is not enough. adams, the virtual discoverer of neptune, found with surprise in that the received account of the matter was "essentially incomplete," and explained, when the requisite correction was introduced, only half the observed acceleration.[ ] what was to be done with the remaining half? here delaunay, the eminent french mathematical astronomer, unhappily drowned at cherbourg in by the capsizing of a pleasure-boat, came to the rescue.[ ] it is obvious to anyone who considers the subject a little attentively, that the tides must act to some extent as a friction-brake upon the rotating earth. in other words, they must bring about an almost infinitely slow lengthening of the day. for the two masses of water piled up by lunar influence on the hither and farther sides of our globe, strive, as it were, to detach themselves from the unity of the terrestrial spheroid, and to follow the movements of the moon. the moon, accordingly, holds them _against_ the whirling earth, which revolves like a shaft in a fixed collar, slowly losing motion and gaining heat, eventually dissipated through space.[ ] this must go on (so far as we can see) until the periods of the earth's rotation and of the moon's revolution coincide. nay, the process will be continued--should our oceans survive so long--by the feebler tide-raising power of the sun, ceasing only when day and night cease to alternate, when one side of our planet is plunged in perpetual darkness and the other seared by unchanging light. here, then, we have the secret of the moon's turning always the same face towards the earth. it is that in primeval times, when the moon was liquid or plastic, an earth-raised tidal wave rapidly and forcibly reduced her rotation to its present exact agreement with her period of revolution. this was divined by kant[ ] nearly a century before the necessity for such a mode of action presented itself to any other thinker. in a weekly paper published at königsberg in , the modern doctrine of "tidal friction" was clearly outlined by him, both as regards its effects actually in progress on the rotation of the earth, and as regards its effects already consummated on the rotation of the moon--the whole forming a preliminary attempt at what he called a "natural history" of the heavens. his sagacious suggestion, however, remained entirely unnoticed until revived--it would seem independently--by julius robert mayer in ;[ ] while similar, and probably original, conclusions were reached by william ferrel of allensville, kentucky, in .[ ] delaunay was not then the inventor or discoverer of tidal friction; he merely displayed it as an effective cause of change. he showed reason for believing that its action in checking the earth's rotation, far from being, as ferrel had supposed, completely neutralised by the contraction of the globe through cooling, was a fact to be reckoned with in computing the movements, as well as in speculating on the history, of the heavenly bodies. the outstanding acceleration of the moon was thus at once explained. it was explained as apparent only--the reflection of a real lengthening, by one second in , years, of the day. but on this point the last word has not yet been spoken. professor newcomb undertook in the onerous task of investigating the errors of hansen's lunar tables as compared with observations prior to . the results, published in ,[ ] proved somewhat perplexing. they tend, in general, to reduce the amount of acceleration left unaccounted for by laplace's gravitational theory, and proportionately to diminish the importance of the part played by tidal friction. but, in order to bring about this diminution, and at the same time conciliate alexandrian and arabian observations, it is necessary to reject _as total_ the ancient solar eclipses known as those of thales and larissa. this may be a necessary, but it must be admitted to be a hazardous expedient. its upshot was to indicate a possibility that the observed and calculated values of the moon's acceleration might after all prove to be identical; and the small outstanding discrepancy was still further diminished by tisserand's investigation, differently conducted, of the same arabian eclipses discussed by newcomb.[ ] the necessity of having recourse to a lengthening day is then less pressing than it seemed some time ago; and the effect, if perceptible in the moon's motion, should, m. tisserand remarked, be proportionately so in the motions of all the other heavenly bodies. the presence of the apparent general acceleration that should ensue can be tested with most promise of success, according to the same authority, by delicate comparisons of past and future transits of mercury. newcomb further showed that small residual irregularities are still found in the movements of our satellite, inexplicable either by any known gravitational influence, or by any _uniform_ value that could be assigned to secular acceleration.[ ] if set down to the account of imperfections in the "time-keeping" of the earth, it could only be on the arbitrary supposition of fluctuations in its rate of going themselves needing explanation. this, it is true, might be found in very slight changes of figure,[ ] not altogether unlikely to occur. but into this cloudy and speculative region astronomers for the present decline to penetrate. they prefer, if possible, to deal only with calculable causes, and thus to preserve for their "most perfect of sciences" its special prerogative of assured prediction. footnotes: [footnote : _neueste beyträge zur erweiterung der sternkunde_, bd. iii., p. ( ).] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. xciii., p. .] [footnote : _mem. roy. astr. soc._, vol. vi., p. .] [footnote : _month. not._, vol. xix., pp. , .] [footnote : _ibid._, vol. xxxviii., p. .] [footnote : _am. jour. of sc._, vol. xvi., p. .] [footnote : _wash. obs._ for , part ii., p. .] [footnote : _pop. astr._, vol. ii., p. ; _astr. jour._, no. .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _ibid._, vol. xxiii., p. (challis).] [footnote : _untersuchungen über die spectra der planeten_, p. .] [footnote : _sirius_, vol. vii., p. .] [footnote : _potsdam publ._, no. ; _astr. nach._, no. , ; frost, _astr. and astrophysics_, vol. xii., p. .] [footnote : zöllner and winnecke made it=o· , _astr. nach._, no. , .] [footnote : _neueste beyträge_, bd. iii., p. .] [footnote : _astr. jahrbuch_, , pp. - .] [footnote : webb, _celestial objects_, p. ( th ed.).] [footnote : _l'astronomie_, t. ii., p. .] [footnote : _observations sur les planètes vénus et mercure_, p. .] [footnote : _observatory_, vol. vi., p. .] [footnote : _atti dell' accad. dei lincei_, t. v. ii., p. , ; _astr. nach._, no. , .] [footnote : _astr. nach._ no. , .] [footnote : _memoirs amer. acad._, vol. xii., no. , p. .] [footnote : _hist. de l'astr._, p. .] [footnote : _comptes rendus_, t. xlix., p. .] [footnote : _comptes rendus_, t. l., p. .] [footnote : _ibid._, p. .] [footnote : _astr. nach._, nos. , and , .] [footnote : _comptes rendus_, t. lxxxiii., pp. , .] [footnote : _handbuch der mathematik_, bd. ii., p. .] [footnote : _comptes rendus_, t. lxxxiii., p. .] [footnote : _nature_, vol. xviii., pp. , , .] [footnote : oppolzer, _astr. nach._, no. , .] [footnote : _ibid._, nos. , - (c. h. f. peters).] [footnote : _ibid._, nos. , and , . see also tisserand in _ann. bur. des long._, , p. .] [footnote : see j. bauschinger's _untersuchungen_ ( ), summarised in _bull. astr._, t. i., p. , and _astr. nach._, no. , . newcomb finds the anomalous motion of the perihelion to be even larger ( " instead of ") than leverrier made it. _month. not._, february, , p. . harzer's attempt to account for it in _astr. nach._, no. , , is more ingenious than successful.] [footnote : _jour. des sçavans_, december, , p. .] [footnote : _Élémens d'astr._, p. . cf. chandler, _pop. astr._, february, , p. .] [footnote : _beobachtungen über die sehr beträchtlichen gebirge und rotation der venus_, , p. . schröter's final result in was h. m. · s. _monat. corr._, bd. xxv., p. .] [footnote : _astr. nach._, no. .] [footnote : _rendiconti del r. istituto lombardo_, t. xxiii., serie ii.] [footnote : _astr. nach._, no. , .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _comptes rendus_, t. cxi., p. ; t. cxxii., p. .] [footnote : _month. not._, vol. lvii., p. ; _astr. nach._, no. , .] [footnote : _mem. spettroscopisti italiani_, t. xxv., p. ; _nature_, vol. liii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _bull. de l'acad. de belgique_, t. xxi., p. , .] [footnote : _observations sur les planètes vénus et mercure_, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , . the velocity of a point on the equator of venus, if brenner's period of h. m. were exact, would be · miles per second; but the displacements due to this rate would be doubled by reflection.] [footnote : _novæ observationes_, p. .] [footnote : _mém. de l'ac._, , p. .] [footnote : _phil. trans._, vol. lxxxiii., p. .] [footnote : webb, _cel. objects_, p. .] [footnote : _month. not._, vol. xlii., p. .] [footnote : _bull. ac. de bruxelles_, t. xliii., p. .] [footnote : _phil. trans._, vol. lxxxii., p. ; _aphroditographische fragmente_, p. ( ).] [footnote : _astr. nach._, no. .] [footnote : _month. not._, vol. xiv., p. .] [footnote : _ibid._, vol. xxiv., p. .] [footnote : _am. jour. of sc._, vol. xliii., p. ( d ser.); vol. ix., p. ( d ser.).] [footnote : _astroph. jour._, vol. ix., p. .] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _old and new astronomy_, p. .] [footnote : _hist. phys. astr._, p. .] [footnote : _mem. roy. astr. soc._, vol. xlvii., pp. , .] [footnote : _astr. reg._, vol. xiii., p. .] [footnote : _l'astronomie_, t. ii., p. ; _astr. nach._, no. , ; _am. jour. of sc._, vol. xxv., p. .] [footnote : _mem. spettr. ital._, dicembre, ; _am. jour. of sc._, vol. xxv., p. .] [footnote : _comptes rendus_, t. cxvi., p. .] [footnote : vogel, _spectra der planeten_, p. .] [footnote : _nature_, vol. xix., p. .] [footnote : _nova acta acad. naturæ curiosorum_, bd. x., .] [footnote : _astr. jahrbuch_, , p. .] [footnote : _month not._, vol. xliii., p. .] [footnote : _report brit. ass._, , p. . the paper contains a valuable record of observations of the phenomenon.] [footnote : _photom. untersuchungen_, p. .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _mémoires de l'acad. de bruxelles_, t. xlix., no. , to; _astr. nach._, no. , ; _f._ schorr, _der venusmond_, .] [footnote : _phil. trans._, , , .] [footnote : delaunay objected (_comptes rendus_, t. lxvii., p. ) that the viscosity of the contained liquid (of which hopkins took no account) would, where the movements were so excessively slow as those of the earth's axis, almost certainly cause it to behave like a solid. lord kelvin, however (_report brit. ass._, , ii., p. ), considered hopkins's argument valid as regards the comparatively quick solar semi-annual and lunar fortnightly nutations.] [footnote : _phil. trans._, cliii., p. .] [footnote : _report brit. ass._, , p. .] [footnote : _ibid._, , p. .] [footnote : albrecht, _astr. nach._, no. , .] [footnote : _astr. jour._, nos. , .] [footnote : _ibid._, no. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , ; _phil. trans._, vol. clxxxvi., a., p. ; _proc. roy. soc._, vol. lix.] [footnote : see chandler's searching investigations, _astr. jour._, nos. , , , , , , , , , , , .] [footnote : rees, _pop. astr._, no. , .] [footnote : _nature_, vol. lxi., p. ; see also a. v. bäcklund, _astr. nach._, no. , .] [footnote : _trans. geol. soc._, vol. iii. ( d ser.), p. .] [footnote : see his _treatise on astronomy_, p. ( ).] [footnote : _phil. mag._, vol. xxviii. ( th ser.), p. .] [footnote : _climate and time_, ; _discussions on climate and cosmology_, .] [footnote : see for a popular account of the theory, sir r. ball's _the cause of an ice age_, .] [footnote : see a. woeikof, _phil. mag._, vol. xxi., p. .] [footnote : _the ice age in north america_, london, .] [footnote : _phil. trans._, vol. lxviii., p. .] [footnote : _comptes rendus_, t. lxxvi., p. .] [footnote : _potsdam publ._, nos. , .] [footnote : _phil. trans._, vol. clxxxii., p. ; _adams prize essay for ._] [footnote : _denkschriften akad. der wiss. wien_, bd. lxiv.; quoted by poynting. _nature_, vol. lxii., p. .] [footnote : _report on the geodetic survey of s. africa_, .] [footnote : _nature_, vol. lxii., p. ; hollis, _observatory_, vol. xxiii., p. ; poincaré, _comptes rendus_, july , .] [footnote : _astr. nach._, no. , .] [footnote : young's _gen. astr._, p. .] [footnote : _astr. constants_, p. .] [footnote : the second volume was published at göttingen in .] [footnote : _ueber rillen auf dem monde_, p. . _cf. the moon_, by t. gwyn elger, p. . w. h. pickering, _harvard annals_, vol. xxxii., p. .] [footnote : _the moon_, p. .] [footnote : _selen. fragm._, th. ii., p. .] [footnote : _astr. nach._, no. ( ); _pop. vorl._, pp. - ( ).] [footnote : _outlines of astr._, par. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _harvard annals_, vol. xxxii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : neison, _the moon_, p. .] [footnote : _knowledge_, vol. xvii., p. .] [footnote : neison, _the moon_, p. .] [footnote : the combination of a uniform rotational with an unequal orbital movement causes a slight swaying of the moon's globe, now east, now west, by which we are able to see round the edges of the averted hemisphere. there is also a "parallactic" libration, depending on the earth's rotation; and a species of nodding movement--the "libration in latitude"--is produced by the inclination of the moon's axis to her orbit, and by her changes of position with regard to the terrestrial equator. altogether, about / of the _invisible_ side come into view.] [footnote : _cel. objects_, p. ( th ed.).] [footnote : _astr. nach._, no. , .] [footnote : cf. leo brenner, _naturwiss. wochenschrift_, january , ; _jour. brit. astr. ass._, vol. v., pp. , .] [footnote : respighi, _les mondes_, t. xiv., p. ; huggins, _month. not._, vol. xxvii., p. .] [footnote : birt, _ibid._, p. .] [footnote : _report brit. ass._, , p. .] [footnote : _observatory_, vol. xv., p. .] [footnote : _astr. reg._, vol. xvi., p. ; _astr. nach._, no. , .] [footnote : lindsay and copeland, _month. not._, vol. xxxix., p. .] [footnote : _observatory_, vols. ii., p. ; iv., p. . n. e. green (_astr. reg._, vol. xvii., p. ) concluded the object a mere "spot of colour," dark under oblique light.] [footnote : webb, _cel. objects_, p. .] [footnote : _publ. lick observatory_, vol. iii., p. .] [footnote : _ibid._, p. ; mee, _knowledge_, vol. xviii., p. .] [footnote : _comptes rendus_, t. cxxii., p. ; _bull. astr._, august, ; _ann. bureau des long._, ; _nature_, vols. lii., p. ; lvi., p. ; lix., p. ; lx., p. ; _astroph. jour._ vol. vi., p. .] [footnote : _comptes rendus_, t. xxii., p. .] [footnote : _phil. trans._, vol. cxlviii., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _phil. trans._, vol. clxiii., p. .] [footnote : _trans. r. dublin soc._, vol. iii., p. .] [footnote : _science_, vol. vii., p. .] [footnote : _amer. jour. of science_, vol. xxxviii., p. .] [footnote : "the temperature of the moon," _memoirs national acad. of sciences_, vol. iv., p. , .] [footnote : _temperature of the moon_, p. iii.; see also app. ii., p. .] [footnote : _trans. r. dublin soc._, vol. iv., p. , ; rosse, _proc. roy. institution_, may , .] [footnote : _astroph. jour._, vol. viii., pp. , .] [footnote : airy, _observatory_, vol. iii., p. .] [footnote : _phil. trans._, vol. cxliii., p. ; _proc. roy. soc._, vol. vi., p. .] [footnote : _comptes rendus_, t. lxi., p. .] [footnote : professor darwin calculated that the heat generated by tidal friction in the course of lengthening the earth's period of rotation from to hours, equalled million times the amount of its present annual loss by cooling. _nature_, vol. xxxiv., p. .] [footnote : _sämmtl. werke_ (ed. ), th. vi., pp. - . see also c. j. monro's useful indications in _nature_, vol. vii., p. .] [footnote : _dynamik des himmels_, p. .] [footnote : gould's _astr. jour._, vol. iii., p. .] [footnote : _wash. obs._ for , vol. xxii., app. ii.] [footnote : _comptes rendus_, t. cxiii., p. ; _annuaire_, paris, .] [footnote : newcomb, _pop. astr._ ( th ed.), p. .] [footnote : sir w. thomson, _report brit. ass._, , p. .] chapter viii _planets and satellites_--(_continued_) "the analogy between mars and the earth is perhaps by far the greatest in the whole solar system." so herschel wrote in ,[ ] and so we may safely say to-day, after six score further years of scrutiny. the circumstance lends a particular interest to inquiries into the physical habitudes of our exterior planetary neighbour. fontana first caught glimpses, at naples in and ,[ ] of dusky stains on the ruddy disc of mars. they were next seen by hooke and cassini in , and this time with sufficient distinctness to serve as indexes to the planet's rotation, determined by the latter as taking place in a period of twenty-four hours forty minutes.[ ] increased confidence was given to this result through maraldi's precise verification of it in .[ ] among the spots observed by him, he distinguished two as stable in position, though variable in size. they were of a peculiar character, showing as bright patches round the poles, and had already been noticed during sixty years back. a current conjecture of their snowy nature obtained validity when herschel connected their fluctuations in extent with the progress of the martian seasons. the inference of frozen precipitations could scarcely be resisted when once it was clearly perceived that the shining polar zones did actually by turns diminish and grow with the alternations of summer and winter in the corresponding hemisphere. this, it may be said, was the opening of our acquaintance with the state of things prevailing on the surface of mars. it was accompanied by a steady assertion, on herschel's part, of permanence in the dark markings, notwithstanding partial obscurations by clouds and vapours floating in a "considerable but moderate atmosphere." hence the presumed inhabitants of the planet were inferred to "probably enjoy a situation in many respects similar to ours."[ ] schröter, on the other hand, went altogether wide of the truth as regards mars. he held that the surface visible to us is a mere shell of drifting cloud, deriving a certain amount of apparent stability from the influence on evaporation and condensation of subjacent but unseen areographical features;[ ] and his opinion prevailed with his contemporaries. it was, however, rejected by kunowsky in , and finally overthrown by beer and mädler's careful studies during five consecutive oppositions, - . they identified at each the same dark spots, frequently blurred with mists, especially when the local winter prevailed, but fundamentally unchanged.[ ] in lockyer established a "marvellous agreement" with beer and mädler's results of , leaving no doubt as to the complete fixity of the main features, amid "daily, nay, hourly," variations of detail through transits of clouds.[ ] on seventeen nights of the same opposition, f. kaiser of leyden obtained drawings in which nearly all the markings noted in at berlin reappeared, besides spots frequently seen respectively by arago in , by herschel in , and one sketched by huygens in with a writing-pen in his diary.[ ] from these data the leyden observer arrived at a period of rotation of h. m. · s., being just one second shorter than that deduced, exclusively from their own observations, by beer and mädler. the exactness of this result was practically confirmed by the inquiries of professor bakhuyzen of leyden.[ ] using for a middle term of comparison the disinterred observations of schröter, with those of huygens at one, and of schiaparelli at the other end of an interval of years, he was enabled to show, with something like certainty, that the time of rotation ( h. m. · s.) ascribed to mars by mr. proctor[ ] in reliance on a drawing executed by hooke in , was too long by _nearly one-tenth of a second_. the minuteness of the correction indicates the nicety of care employed. nor employed vainly; for, owing to the comparative antiquity of the records available in this case, an almost infinitesimal error becomes so multiplied by frequent repetition as to produce palpable discrepancies in the positions of the markings at distant dates. hence bakhuyzen's period of h. m. · s. is undoubtedly of a precision unapproached as regards any other heavenly body save the earth itself. two facts bearing on the state of things at the surface of mars were, then, fully acquired to science in or before the year . the first was that of the seasonal fluctuations of the polar spots; the second, that of the general permanence of certain dark gray or greenish patches, perceived with the telescope as standing out from the deep yellow ground of the disc. that these varieties of tint correspond to the real diversities of a terraqueous globe, the "ripe cornfield"[ ] sections representing land, the dusky spots and streaks, oceans and straits, has long been the prevalent opinion. sir j. herschel in led the way in ascribing the redness of the planet's light to an inherent peculiarity of soil.[ ] previously it had been assimilated to our sunset glows rather than to our red sandstone formations--set down, that is, to an atmospheric stoppage of blue rays. but the extensive martian atmosphere, implicitly believed in on the strength of some erroneous observations by cassini and römer in the seventeenth century, vanished before the sharp occultation of a small star in leo, witnessed by sir james south in ;[ ] and dawes's observation in ,[ ] that the ruddy tinge is deepest near the central parts of the disc, certified its non-atmospheric origin. the absolute whiteness of the polar snow-caps was alleged in support of the same inference by sir william huggins in .[ ] all recent operations tend to show that the atmosphere of mars is much thinner than our own. this was to have been expected _à priori_, since the same proportionate mass of air would on his smaller globe form a relatively sparse covering.[ ] besides, gravity there possesses less than four-tenths its force here, so that this sparser covering would weigh less, and be less condensed, than if it enveloped the earth. atmospheric pressure would accordingly be of about two and a quarter, instead of fifteen terrestrial pounds per square inch. this corresponds with what the telescope shows us. it is extremely doubtful whether any features of the earth's actual surface could be distinguished by a planetary spectator, however well provided with optical assistance. professor langley's inquiries[ ] led him to conclude that fully twice as much light is absorbed by our air as had previously been supposed--say per cent. of vertical rays in a clear sky. of the sixty reaching the earth, less than a quarter would be reflected even from white sandstone; and this quarter would again pay heavy toll in escaping back to space. thus not more than perhaps ten or twelve out of the original hundred sent by the sun would, under the most favourable circumstances, and from the very centre of the earth's disc, reach the eye of a martian or lunar observer. the light by which he views our world is, there is little doubt, light reflected from the various strata of our atmosphere, cloud or mist-laden or serene, as the case may be, with an occasional snow-mountain figuring as a permanent white spot. this consideration at once shows us how much more tenuous the martian air must be, since it admits of topographical delineations of the martian globe. the clouds, too, that form in it seem in general to be rather of the nature of ground-mists than of heavy cumulus.[ ] occasionally, indeed, durable and extensive strata become visible. during the latter half of october, , for instance, a region as large as europe remained apparently cloud-covered. yet most recent observers are unable to detect the traces of aqueous absorption in the martian spectrum noted by huggins in [ ] and by vogel in .[ ] campbell vainly looked for them,[ ] visually in , spectrographically in ; keeler was equally unsuccessful;[ ] jewell[ ] holds that they could, with present appliances, only be perceived if the atmosphere of mars were much richer in water-vapour than that of the earth. there can be little doubt, however, that its supply is about the minimum adequate to the needs of a _living_, and perhaps a life-nuturing planet. the climate of mars seems to be unexpectedly mild. its _theoretical_ mean temperature, taking into account both distance from the sun and albedo, is ° c. below freezing.[ ] yet its polar snows are both less extensive and less permanent than those on the earth. the southern white hood, noticed by schiaparelli in to have survived the summer only as a small lateral patch, melted completely in . moreover, mr. w. h. pickering observed with astonishment the disappearance, in the course of thirty-three days of june and july, , of , , square miles of southern snow.[ ] curiously enough, the initial stage of shrinkage in the white calotte was marked by its division into two unequal parts, as if in obedience to the mysterious principle of duplication governing so many martian phenomena.[ ] changes of the hues associated respectively with land and water accompanied in lower latitudes, and were thought to be occasioned by floods ensuing upon this rapid antarctic thaw. it is true that scarcity of moisture would account for the scantiness and transitoriness of snowy deposits easily liquefied because thinly spread. but we might expect to see the whole wintry hemisphere, at any rate, frost-bound, since the sun radiates less than half as much heat on mars as on the earth. water seems, nevertheless, to remain, as a rule, uncongealed everywhere outside the polar regions. we are at a loss to imagine by what beneficent arrangement the rigorous conditions naturally to be looked for can be modified into a climate which might be found tolerable by creatures constituted like ourselves. martian topography may be said to form nowadays a separate sub-department of descriptive astronomy. the amount of detail become legible by close scrutiny on a little disc which, once in fifteen years, attains a maximum of about / the area of the full moon, must excite surprise and might provoke incredulity. spurious discoveries, however, have little chance of holding their own where there are so many competitors quite as ready to dispute as to confirm. the first really good map of mars was constructed in by proctor from drawings by dawes. kaiser of leyden followed in with a representation founded upon data of his own providing in - ; and terby, in his valuable _aréographie_, presented to the brussels academy in [ ] a careful discussion of all important observations from the time of fontana downwards, thus virtually adding to knowledge by summarising and digesting it. the memorable opposition of september , , marked a fresh epoch in the study of mars. while executing a trigonometrical survey (the first attempted) of the disc, then of the unusual size of " across, g. v. schiaparelli, director of the milan observatory, detected a novel and curious feature. what had been taken for martian continents were found to be, in point of fact, agglomerations of islands, separated from each other by a network of so-called "canals" (more properly _channels_).[ ] these are obviously extensions of the "seas," originating and terminating in them, and sharing their gray-green hue, but running sometimes to a length of three or four thousand miles in a straight line, and preserving throughout a nearly uniform breadth of about sixty miles. further inquiries have fully substantiated the discovery made at the brera observatory. the "canals" of mars are an actually existent and permanent phenomenon. an examination of the drawings in his possession showed m. terby that they had been seen, though not distinctively recognised, by dawes, secchi, and holden; several were independently traced out by burton at the opposition of ; all were recovered by schiaparelli himself in and - ; and their indefinite multiplication resulted from lovell's observations in and . when the planet culminated at midnight, and was therefore in opposition, december , , its distance was greater, and its apparent diameter less than in , in the proportion of sixteen to twenty-five. its atmosphere was, however, more transparent, and ours of less impediment to northern observers, the object of scrutiny standing considerably higher in northern skies. never before, at any rate, had the true aspect of mars come out so clearly as at milan, with the - / -inch merz refractor of the observatory, between december, , and february, . the canals were all again there, but this time they were--in as many as twenty cases--_seen in duplicate_. that is to say, a twin-canal ran parallel to the original one at an interval of to miles.[ ] we are here brought face to face with an apparently insoluble enigma. schiaparelli regards the "germination" of his canals as a periodical phenomenon depending on the martian seasons. it is, assuredly, not an illusory one, since it was plainly apparent, during the opposition of , to mm. perrotin and thollon at nice,[ ] and to the former, using the new -inch refractor of that observatory, in ; mr. a. stanley williams, with the help of only a - / -inch reflector, distinctly perceived in seven of the duplicate objects noted at milan,[ ] and the lick observations, both of and of , together with the drawings made at flagstaff and mexico during the last favourable oppositions of the nineteenth century, brought unequivocal confirmation to the accuracy of schiaparelli's impressions.[ ] various conjectures have been hazarded in explanation of this bizarre appearance. the difficulty of conceiving a physical reality corresponding to it has suggested recourse to an optical rationale. proctor regarded it as an effect of diffraction;[ ] stanislas meunier, of oblique reflection from overlying mist-banks;[ ] flammarion considers it possible that companion-canals might, under special circumstances, be evoked by refraction as a kind of mirage.[ ] but none of these speculations are really admissible, when all the facts are taken into account. the view that the canals of mars are vast rifts due to the cooling of the globe, is recommended by the circumstance that they tend to follow great circles; nevertheless, it would break down if, as schiaparelli holds, the fluctuations in their visibility depend upon actual obliterations and re-emergencies. fantastic though the theory of their artificial origin appear, it is held by serious astronomers. its vogue is largely due to mr. lowell's ingenious advocacy. he considers the martian globe to be everywhere intersected by an elaborate system of irrigation-works, rendered necessary by a perennial water-famine, relieved periodically by the melting of the polar snows. nor does he admit the existence of oceans, or lakes. what have been taken for such are really tracts covered with vegetation, the bright areas intermixed with them representing sandy deserts. and it is noteworthy in this connection that professor barnard obtained in ,[ ] with the great lick refractor, "suggestive and impressive views" disclosing details of light and shade on the gray-green patches so intricate and minute as almost to preclude the supposition of their aqueous nature. the closeness of the terrestrial analogy has thus of late been much impaired. even if the surface of mars be composed of land and water, their distribution must be of a completely original type. the interlacing everywhere of continents with arms of the sea (if that be the correct interpretation of the visual effects) implies that their levels scarcely differ;[ ] and schiaparelli carries most observers with him in holding that their outlines are not absolutely constant, encroachments of dusky upon bright tints suggesting extensive inundations.[ ] the late n. e. green's observations at madeira in indicated, on the other hand, a rugged south polar region. the contour of the snow-cap not only appeared indented, as if by valleys and promontories, but brilliant points were discerned outside the white area, attributed to isolated snow-peaks.[ ] still more elevated, if similarly explained, must be the "ice island" first seen in a comparatively low latitude by dawes in january, . on august , , mars stood opposite to the sun at a distance of only , , miles from the earth. in point of vicinity, then, its situation was scarcely less favourable than in . the low altitude of the planet, however, practically neutralised this advantage for northern observers, and public expectation, which had been raised to the highest pitch by the announcements of sensation-mongers, was somewhat disappointed at the "meagreness" of the news authentically received from mars. valuable series of observations were, nevertheless, made at lick and arequipa; and they unite in testifying to the genuine prevalence of surface-variability, especially in certain regions of intermediate tint, and perhaps of the "crude consistence" of "boggy syrtes, neither sea, nor good dry land." professor holden insisted on the "enormous difficulties in the way of completely explaining the recorded phenomena by terrestrial analogies";[ ] mr. w. h. pickering spoke of "conspicuous and startling changes." they, however, merely overlaid, and partially disguised, a general stability. among the novelties detected by mr. pickering were a number of "lakes," or "oases" (in lowell's phraseology), under the aspect of black dots at the junctions of two or more canals;[ ] and he, no less than the lick astronomers and m. perrotin at nice,[ ] observed brilliant clouds projecting beyond the terminator, or above the limb, while carried round by the planet's rotation. they seemed to float at an altitude of at least twenty miles, or about four times the height of terrestrial cirrus; but this was not wonderful, considering the low power of gravity acting upon them. great capital was made in the journalistic interest out of these imaginary signals from intelligent martians, desirous of opening communications with (to them) problematical terrestrial beings. similar effects had, however, been seen before by mr. knobel in , by m. terby in , and at the lick observatory in ; and they were discerned again with particular distinctness by professor hussey at lick, august , .[ ] the first photograph of mars was taken by gould at cordoba in . little real service in planetary delineation has, it is true, been so far rendered by the art, yet one achievement must be recorded to its credit. a set of photographs obtained by mr. w. h. pickering on wilson's peak, california, april , , showed the southern polar cap of mars as of moderate dimensions, but with a large dim adjacent area. twenty-four hours later, on a corresponding set, the dim area was brilliantly white. the polar cap had become enlarged in the interim, apparently through a wide-spreading snow-fall, by the annexation of a territory equal to that of the united states. the season was towards the close of winter in mars. never until then had the process of glacial extension been actually (it might be said) superintended in that distant globe. mars was gratuitously supplied with a pair of satellites long before he was found actually to possess them. kepler interpreted galileo's anagram of the "triple" saturn in this sense; they were perceived by micromégas on his long voyage through space; and the laputan astronomers had even arrived at a knowledge, curiously accurate under the circumstances, of their distances and periods. but terrestrial observers could see nothing of them until the night of august , . the planet was then within one month of its second nearest approach to the earth during the last century; and in the washington -inch refractor was not in existence.[ ] professor asaph hall, accordingly, determined to turn the conjecture to account for an exhaustive inquiry into the surroundings of mars. keeping his glaring disc just outside the field of view, a minute attendant speck of light was "glimpsed" august . bad weather, however, intervened, and it was not until the th that it was ascertained to be what it appeared--a satellite. on the following evening a second, still nearer to the primary, was discovered, which, by the bewildering rapidity of its passages hither and thither, produced at first the effect of quite a crowd of little moons.[ ] both these delicate objects have since been repeatedly observed, both in europe and america, even with comparatively small instruments. at the opposition of , indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at washington. but the lick equatoreal showed them, july , , when their brightness was only · its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions. the names chosen for them were taken from the iliad, where "deimos" and "phobos" (fear and panic) are represented as the companions in battle of ares. in several respects, they are interesting and remarkable bodies. as to size, they may be said to stand midway between meteorites and satellites. from careful photometric measures executed at harvard in and , professor pickering concluded their diameters to be respectively six and seven miles.[ ] this is on the assumption that they reflect the same proportion of the light incident upon them that their primary does. but it may very well be that they are less reflective, in which case they would be more extensive. the albedo of mars is put by müller at · ; his surface, in other words, returns per cent. of the rays striking it. if we put the albedo of his satellites equal to that of our moon, · , their diameters will be increased from and to - / and miles, phobos, the inner one, being the larger. mr. lowell, however, formed a considerably larger estimate of their dimensions.[ ] it is interesting to note that deimos, according to professor pickering's very distinct perception, does not share the reddish tint of mars. deimos completes its nearly circular revolutions in thirty hours eighteen minutes, at a distance from the surface of its ruling body of , miles; phobos traverses an elliptical orbit[ ] in seven hours thirty-nine minutes twenty-two seconds, at a distance of only , miles. this is the only known instance of a satellite circulating faster than its primary rotates, and is a circumstance of some importance as regards theories of planetary development. to a martian spectator the curious effect would ensue of a celestial object, seemingly exempt from the general motion of the sphere, rising in the west, setting in the east, and culminating twice, or even thrice a day; which, moreover, in latitudes above ° north or south, would be permanently and altogether hidden by the intervening curvature of the globe. * * * * * the detection of new members of the solar system has come to be one of the most ordinary of astronomical events. since no single year has passed without bringing its tribute of asteroidal discovery. in the last of the seventies alone, a full score of miniature planets were distinguished from the thronging stars amid which they seem to move; brought seventeen such recognitions; their number touched a minimum of one in ; it rose in , and again in , to eleven; dropped to six in , and sprang up with the aid of photography to twenty-seven in . that high level has since, on an average, been maintained; and on january , , nearly asteroids were recognised as revolving between the orbits of mars and jupiter. of these, considerably more than one hundred are claimed by one investigator alone--dr. max wolf of heidelburg; m. charlois of nice comes second with ; while among the earlier observers palisa of vienna contributed , and c. h. f. peters of clinton (n. y.), whose varied and useful career terminated july , , to the grand total. the construction by chacornac and his successors at paris, and more recently by peters at clinton, of ecliptical charts showing all stars down to the thirteenth and fourteenth magnitudes respectively, rendered the picking out of moving objects above that brightness a mere question of time and diligence. both, however, are vastly economised by the photographic method. tedious comparisons of the sky with charts are no longer needed for the identification of unrecorded, because simulated stars. planetary bodies declare themselves by appearing upon the plate, not in circular, but in linear form. their motion converts their images into trails, long or short according to the time of exposure. the first asteroid (no. ) thus detected was by max wolf, december , .[ ] eighteen others were similarly discovered in , by the same skilful operator; and ten more through charlois's adoption at nice of the novel plan now in exclusive use for picking up errant light-specks. far more onerous than the task of their discovery is that of keeping them in view once discovered--of tracking out their paths, ixing their places, and calculating the disturbing effects upon them of the mighty jovian mass. these complex operations have come to be centralised at berlin under the superintendence of professor tietjen, and their results are given to the public through the medium of the _berliner astronomisches jahrbuch_. the _cui bono?_ however, began to be agitated. was it worth while to maintain a staff of astronomers for the sole purpose of keeping hold over the identity of the innumerable component particles of a cosmical ring? the prospect, indeed, of all but a select few of the asteroids being thrown back by their contemptuous captors into the sea of space seemed so imminent that professor watson provided by will against the dereliction of the twenty-two discovered by himself. but the fortunes of the whole family improved through the distinction obtained by one of them. on august , , the trail of a rapidly-moving, star-like object of the eleventh magnitude imprinted itself on a plate exposed by herr witt at the urania observatory, berlin. its originator proved to be unique among asteroids. "eros" is, in sober fact, 'one of those mysterious stars which hide themselves between the earth and mars,' divined or imagined by shelley.[ ] true, several of its congeners invade the martian sphere at intervals; but the proper habitat of eros is within that limit, although its excursions transcend it. in other words, its mean distance from the sun is about , as compared with the martian distance of million miles. further, its orbit being so fortunately circumstanced as to bring it once in sixty-seven years within some millions of miles of the earth, it is of extraordinary value to celestial surveyors. the calculation of its movements was much facilitated by detections, through a retrospective search,[ ] of many of its linear images among the star-dots on the harvard plates.[ ] the little body--which can scarcely be more than twenty miles in diameter--shows peculiarities of behaviour as well as of position. dr. von oppolzer, in february, ,[ ] announced it to be extensively and rapidly variable. once in hours minutes it lost about three-fourths of its light,[ ] but these fluctuations quickly diminished in range, and in the beginning of may ceased altogether.[ ] evidently, then, they depend upon the situation of the asteroid relatively to ourselves; and, so far, events lent countenance to m. andré's eclipse hypothesis, since mutual occultations of the supposed planetary twins could only take place when the plane of their revolutions passed through the earth, and this condition would be transitory. yet the recognition in eros of an "algol asteroid" seems on other grounds inadmissible;[ ] nor until the phenomenon is conspicuously renewed--as it probably will be at the opposition of --can there be much hope of finding its appropriate rationale. the crowd of orbits disclosed by asteroidal detections invites attentive study. d'arrest remarked in ,[ ] when only thirteen minor planets were known, that supposing their paths to be represented by solid hoops, not one of the thirteen could be lifted from its place without bringing the others with it. the complexity of interwoven tracks thus illustrated has grown almost in the numerical proportion of discovery. yet no two actually intersect, because no two lie exactly in the same plane, so that the chances of collision are at present _nil_. there is only one case, indeed, in which it seems to be eventually possible. m. lespiault has pointed out that the curves traversed by "fidés" and "maïa" approach so closely that a time may arrive when the bodies in question will either coalesce or unite to form a binary system.[ ] the maze threaded by the asteroids contrasts singularly with the harmoniously ordered and rhythmically separated orbits of the larger planets. yet the seeming confusion is not without a plan. the established rules of our system are far from being totally disregarded by its minor members. the orbit of pallas, with its inclination of ° ', touches the limit of departure from the ecliptic level; the average obliquity of the asteroidal paths is somewhat less than that of the sun's equator;[ ] their mean eccentricity is below that of the curve traced out by mercury, and all without exception are pursued in the planetary direction--from west to east. the zone in which these small bodies travel is about three times as wide as the interval separating the earth from the sun. it extends perilously near to jupiter, and dovetails into the sphere of mars. their distribution is very unequal. they are most densely congregated about the place where a single planet ought, by bode's law, to revolve; it may indeed be said that only stragglers from the main body are found more than fifty million miles within or without a mean distance from the sun · times that of the earth. significant gaps, too, occur where some force prohibitive of their presence would seem to be at work. the probable nature of that force was suggested by the late professor kirkwood, first in , when the number of known asteroids was only eighty-eight, and again with more confidence in , from the study of a list then run up to .[ ] it appears that these bare spaces are found just where a revolving body would have a period connected by a simple relation with that of jupiter. it would perform two or three circuits to his one, five to his two, nine to his five, and so on. kirkwood's inference was that the gaps in question were cleared of asteroids by the attractive influence of jupiter. for disturbances recurring time after time--owing to commensurability of periods--nearly at the same part of the orbit, would have accumulated until the shape of that orbit was notably changed. the body thus displaced would have come in contact with other cosmical particles of the same family with itself--then, it may be assumed, more evenly scattered than now--would have coalesced with them, and permanently left its original track. in this way the regions of maximum perturbation would gradually have become denuded of their occupants. we can scarcely doubt that this law of commensurability has largely influenced the present distribution of the asteroids. but its effects must have been produced while they were still in an unformed, perhaps a nebular condition. in a system giving room for considerable modification through disturbance, the recurrence of conjunctions with a dominating mass at the same orbital point need not involve instability.[ ] on the whole, the correspondence of facts with kirkwood's hypothesis has not been impaired by their more copious collection.[ ] some chasms of secondary importance have indeed been bridged; but the principal stand out more conspicuously through the denser scattering of orbits near their margins. nor is it doubtful that the influence of jupiter in some way produced them. m. de freycinet's study of the problem they present[ ] has, however, led him to the conclusion that they existed _ab origine_, thus testifying rather to the preventive than to the perturbing power of the giant planet. the existence, too, of numerous asteroidal pairs travelling in approximately coincident tracks, must date from a remote antiquity. they result, professor kirkwood[ ] believed, from the divellent action of jupiter upon embryo pigmy planets, just as comets moving in pursuit of one another are a consequence of the sundering influence of the sun. leverrier fixed, in ,[ ] one-fourth of the earth's mass as the outside limit for the combined masses of all the bodies circulating between mars and jupiter; but it is far from probable that this maximum is at all nearly approached. m. berberich[ ] held that the moon would more than outweigh the whole of them, a million of the lesser bodies shining like stars of the twelfth magnitude being needed, according to his judgment, to constitute her mass. and m. niesten estimated that the whole of the asteroids discovered up to august, , amounted in _volume_ to only / th of our globe,[ ] and we may safely add--since they are tolerably certain to be lighter, bulk for bulk, than the earth--that their proportionate _mass_ is smaller still. a fairly concordant result was published in by mr. b. m. roszel.[ ] he found that the lunar globe probably contains forty times, the terrestrial globe , times the quantity of matter parcelled out among the first minor planets. the actual size of a few of them may now be said to be known. professor pickering, from determinations of light-intensity, assigned to vesta a diameter of miles, to pallas , to juno , down to twelve and fourteen for the smaller members of the group.[ ] an albedo equal to that of mars was assumed as the basis of the calculation. moreover, professor g. müller[ ] of potsdam examined photometrically the phases of seven among them, of which four--namely, vesta, iris, massalia, and amphitrite--were found to conform precisely to the behaviour of mars as regards light-change from position, while ceres, pallas, and irene varied after the manner of the moon and mercury. the first group were hence inferred to resemble mars in physical constitution, nature of atmosphere, and reflective capacity; the second to be moon-like bodies. finally, professor barnard, directly measuring with the yerkes refractor the minute discs presented by the original quartette, obtained the following authentic data concerning them:[ ] diameter of ceres, miles, albedo = · ; diameter of pallas, miles, albedo = · ; diameter of vesta, miles, albedo = · ; diameter of juno, miles, albedo = · . thus, the rank of premier asteroid proves to belong to ceres, and to have been erroneously assigned to vesta in consequence of its deceptive brilliancy. what kind of surface this indicates, it is hard to say. the dazzling whiteness of snow can hardly be attributed to bare rock; yet the dynamical theory of gases--as dr. johnstone stoney pointed out in [ ]--prohibits the supposition that bodies of insignificant gravitative power can possess aerial envelopes. even our moon, it is calculated, could not permanently hold back the particles of oxygen, nitrogen, or water-gas from escaping into infinite space; still less, a globe one thousand times smaller. vogel's suspicion of an air-line in the spectrum of vesta[ ] has, accordingly, not been confirmed. * * * * * crossing the zone of asteroids on our journey outward from the sun, we meet with a group of bodies widely different from the "inferior" or terrestrial planets. their gigantic size, low specific gravity, and rapid rotation, obviously from the first threw the "superior" planets into a class apart; and modern research has added qualities still more significant of a dissimilar physical constitution. jupiter, a huge globe , miles in diameter, stands pre-eminent among them. he is, however, only _primus inter pares_; all the wider inferences regarding his condition may be extended, with little risk of error, to his fellows; and inferences in his case rest on surer grounds than in the case of the others, from the advantages offered for telescopic scrutiny by his comparative nearness. now the characteristic modern discovery concerning jupiter is that he is a body midway between the solar and terrestrial stages of cosmical existence--a decaying sun or a developing earth, as we choose to put it--whose vast unexpended stores of internal heat are mainly, if not solely, efficient in producing the interior agitations betrayed by the changing features of his visible disc. this view, impressed upon modern readers by mr. proctor's popular works, was anticipated in the last century. buffon wrote in his _Époques de la nature_ ( ):[ ]--"la surface de jupiter est, comme l'on sait, sujette à des changemens sensibles, qui semblent indiquer que cette grosse planète est encore dans un état d'inconstance et de bouillonnement." primitive incandescence, attendant, in his fantastic view, on planetary origin by cometary impacts with the sun, combined, he concluded, with vast bulk to bring about this result. jupiter has not yet had time to cool. kant thought similarly in ;[ ] but the idea did not commend itself to the astronomers of the time, and dropped out of sight until mr. nasmyth arrived at it afresh in .[ ] even still, however, terrestrial analogies held their ground. the dark belts running parallel to the equator, first seen at naples in , continued to be associated--as herschel had associated them in --with jovian trade-winds, in raising which the deficient power of the sun was supposed to be compensated by added swiftness of rotation. but opinion was not permitted to halt here. in g. p. bond of cambridge (u.s.) derived some remarkable indications from experiments on the light of jupiter.[ ] they showed that fourteen times more of the photographic rays striking it are reflected by the planet than by our moon, and that, unlike the moon, which sends its densest rays from the margin, jupiter is brightest near the centre. but the most perplexing part of his results was that jupiter actually seemed to give out more light than he received. bond, however, rightly considered his data too uncertain for the support of so bold an assumption as that of original luminosity, and, even if the presence of native light were proved, thought that it might emanate from auroral clouds of the terrestrial kind. the conception of a sun-like planet was still a remote, and seemed an extravagant one. only since it was adopted and enforced by zöllner in ,[ ] can it be regarded as permanently acquired to science. the rapid changes in the cloud-belts both of jupiter and saturn, he remarked, attest a high internal temperature. for we know that all atmospheric movements on the earth are sun-heat transformed into motion. but sun-heat at the distance of jupiter possesses but / , at that of saturn / of its force here. the large amount of energy, then, obviously exerted in those remote firmaments must have some other source, to be found nowhere else than in their own active and all-pervading fires, not yet banked in with a thick solid crust. the same acute investigator dwelt, in ,[ ] on the similarity between the modes of rotation of the great planets and of the sun, applying the same principles of explanation to each case. the fact of this similarity is undoubted. cassini[ ] and schröter both noticed that markings on jupiter travelled quicker the nearer they were to his equator; and cassini even hinted at their possible assimilation to sun-spots.[ ] it is now well ascertained that, as a rule (not without exceptions), equatorial spots give a period some - / minutes shorter than those in latitudes of about °. but, as mr. denning has pointed out,[ ] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. accelerations and retardations, depending upon processes of growth or change, take place in very much the same kind of way as in solar maculæ, inevitably suggesting similarity of origin. the interesting query as to jupiter's surface incandescence has been studied since bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. zöllner in , müller in , estimated his albedo at · and · respectively, that of fresh-fallen snow being · , and of white paper · .[ ] but the disc of jupiter is by no means purely white. the general ground is tinged with ochre; the polar zones are leaden or fawn coloured; large spaces are at times stained or suffused with chocolate-browns and rosy hues. it is occasionally seen ruled from pole to pole with dusky bars, and is never wholly free from obscure markings. the reflection, then, by it, as a whole, of about per cent. of the rays impinging upon it, might well suggest some original reinforcement. nevertheless, the spectroscope gives little countenance to the supposition of any considerable permanent light-emission. the spectrum of jupiter, as examined by huggins, - , and by vogel, - , shows the familiar fraunhofer rays belonging to reflected sunlight. but it also shows lines of native absorption. some of these are identical with those produced by the action of our own atmosphere, especially one or more groups due to aqueous vapours; others are of unknown origin; and it is remarkable that one among the latter--a strong band in the red--agrees in position with a dark line in the spectra of some ruddy stars.[ ] there is, besides, a general absorption of blue rays, intensified--as le sueur observed at melbourne in [ ]--in the dusky markings, evidently through an increase of depth in the atmospheric strata traversed by the light proceeding from them. all these observations, however (setting aside the stellar line as of doubtful significance), point to a cool planetary atmosphere. one spectrograph, it is true, taken by dr. henry draper, september , ,[ ] seemed to attest the action of intrinsic light; but the peculiarity was referred by dr. vogel, with convincing clearness, to a flaw in the film.[ ] so far, then, native emissions from any part of jupiter's diversified surface have not been detected; and, indeed, the blackness of the shadows cast by his satellites on his disc sufficiently proves that he sends out virtually none but reflected light.[ ] this conclusion, however, by no means invalidates that of his high internal temperature. the curious phenomena attending jovian satellite-transits may be explained, partly as effects of contrast, partly as due to temporary obscurations of the small discs projected on the large disc of jupiter. at their first entry upon its marginal parts, which are several times less luminous than those near the centre, they invariably show as bright spots, then usually vanish as the background gains lustre, to reappear, after crossing the disc, thrown into relief, as before, against the dusky limb. but instances are not rare, more especially of the third and fourth satellites standing out, during the entire middle part of their course, in such inky darkness as to be mistaken for their own shadows. the earliest witness of a "black transit" was cassini, september , ; römer in , and maraldi in and , made similar observations, which have been multiplied in recent years. in some cases the process of darkening has been visibly attended by the formation, or emergence into view, of spots on the transiting body, as noted by the two bonds at harvard, march , .[ ] the third satellite was seen by dawes, half dark, half bright, when crossing jupiter's disc, august , ;[ ] one-third dark by davidson of california, january , , under the same circumstances;[ ] and unmistakably spotted, both on and off the planet, by schröter, secchi, dawes, and lassell. the first satellite sometimes looks dusky, but never absolutely black, in travelling over the disc of jupiter. the second appears uniformly white--a circumstance attributed by dr. spitta[ ] to its high albedo. the singularly different aspects, even during successive transits, of the third and fourth satellites, are connected by professor holden[ ] with the varied luminosity of the segments of the planetary surface they are projected upon, and w. h. pickering inclines to the same opinion; but fluctuations in their own brightness[ ] may be a concurrent cause. herschel concluded in that, like our moon, they always turn the same face towards their primary, and as regards the outer satellite, engelmann's researches in , and c. e. burton's in , made this almost certain; while both for the third and fourth jovian moons it was completely assured by w. h. pickering's and a. e. douglass's observations at arequipa in ,[ ] and at flagstaff in - .[ ] strangely enough, however, the interior members of the system have preserved a relatively swift rotation, notwithstanding the enormous checking influence upon it of jove-raised tides. all the satellites are stated, on good authority, to be more or less egg-shaped. on september , , barnard saw the first elongated and bisected by a bright equatorial belt, during one of its dark transits;[ ] and his observation, repeated august , , was completely verified by schaeberle and campbell, who ascertained, moreover, that the longer axis of the prolate body was directed towards jupiter's centre.[ ] the ellipticity of its companions was determined by pickering and douglass; indeed, that of no. had long previously been noticed by secchi.[ ] no. also shows equatorial stripes, perceived in by schaeberle and campbell,[ ] and evident later to pickering and douglass;[ ] nor need we hesitate to admit as authentic their records of similar, though less conspicuous markings on the other satellites. a constitution analogous to that of jupiter himself was thus unexpectedly suggested; and vogel's detection of lines--or traces of lines--in their spectra, agreeing with absorption-rays derived from their primary, lends support to the conjecture that they possess gaseous envelopes similar to his. the system of jupiter, as it was discovered by galileo, and investigated by laplace, appeared in its outward aspect so symmetrical, and displayed in its inner mechanism such harmonious dynamical relations, that it might well have been deemed complete. nevertheless, a new member has been added to it. near midnight on september , , professor barnard discerned with the lick -inch "a tiny speck of light," closely following the planet.[ ] he instantly divined its nature, watched its hurried disappearance in the adjacent glare, and made sure of the reality of his discovery on the ensuing night. it was a delicate business throughout, the liliputian luminary subsiding into invisibility before the slightest glint of jovian light, and tarrying, only for brief intervals, far enough from the disc to admit of its exclusion by means of an occulting plate. the new satellite is estimated to be of the thirteenth stellar magnitude, and, if equally reflective of light with its next neighbour, io (satellite no. ), its diameter must be about one hundred miles. it revolves at a distance of , miles from jupiter's centre, and of , from his bulging equatorial surface. its period of h. m. s. is just two hours longer than jupiter's period of rotation, so that phobos still remains a unique example of a secondary body revolving faster than its primary rotates. jupiter's innermost moon conforms in its motions strictly, indeed inevitably, to the plane of his equatorial protuberance, following, however, a sensibly elliptical path the major axis of which is in rapid revolution.[ ] its very insignificance raises the suspicion that it may not prove solitary. possibly it belongs to a zone peopled by asteroidal satellites. more than fifteen thousand such small bodies could be furnished out of the materials of a single full-sized satellite spoiled in the making. but we must be content for the present to register the fact without seeking to penetrate the meaning of its existence. very high and very fine telescopic power is needed for its perception. outside the united states, it has been very little observed. the only instruments in this country successfully employed for its detection are, we believe, dr. common's -foot reflector and mr. newall's -inch refractor. in the course of his observations on jupiter at brussels in , m. niesten was struck with a rosy cloud attached to a whitish zone beneath the dark southern equatorial band.[ ] its size was enormous. at the distance of jupiter, its measured dimensions of " by " implied a real extension in longitude of , , in latitude of something short of , miles. the earliest record of its appearance seems to be by professor pritchett, director of the morrison observatory (u.s.), who figured and described it july , .[ ] it was again delineated august , by tempel at florence.[ ] in the following year it attracted the wonder and attention of almost every possessor of a telescope. its colour had by that time deepened into a full brick-red, and was set off by contrast with a white equatorial spot of unusual brilliancy. during three ensuing years these remarkable objects continued to offer a visible and striking illustration of the compound nature of the planet's rotation. the red spot completed a circuit in nine hours fifty-five minutes thirty-six seconds; the white spot in about five and a half minutes less. their _relative_ motion was thus no less than miles an hour, bringing them together in the same meridian at intervals of forty-four days ten hours forty-two minutes. neither, however, preserved continuously the same uniform rate of travel. the period of each had lengthened by some seconds in , while sudden displacements, associated with the recovery of lustre after recurrent fadings, were observed in the position of the white spot,[ ] recalling the leap forward of a reviving sun-spot. just the opposite effect attended the rekindling of the companion object. while semi-extinct, in - , it lost little motion; but a fresh access of retardation was observed by professor young[ ] in connection with its brightening in . this suggests very strongly that the red spot is _fed from below_. a shining aureola of "faculæ," described by bredichin at moscow, and by lohse at potsdam, as encircling it in september, ,[ ] was held to strengthen the solar analogy. the conspicuous visibility of this astonishing object lasted three years. when the planet returned to opposition in - , it had faded so considerably that riccò's uncertain glimpse of it at palermo, may , , was expected to be the last. it had, nevertheless, begun to recover in december, and presented to mr. denning in the beginning of much the same aspect as in october, .[ ] observed by him in an intermediate stage, february , , when "a mere skeleton of its former self," it bore a striking likeness to an "elliptical ring" descried in the same latitude by mr. gledhill in - . this, indeed, might be called the preliminary sketch for the famous object brought to perfection ten years later, but which mr. h. c. russell of sydney saw and drew still unfinished in june, ,[ ] before it had separated from its matrix, the dusky south tropical belt. in earlier times, too, a marking "at once fixed and transient" had been repeatedly perceived attached to the southernmost of the central belts. it gave cassini in a rotation-period of nine hours fifty-six minutes,[ ] reappeared and vanished eight times during the next forty-three years, and was last seen by maraldi in . it was, however, very much smaller than the recent object, and showed no unusual colour.[ ] the assiduous observations made on the "great red spot" by mr. denning at bristol and by professor hough at chicago afforded grounds only for negative conclusions as to its nature. it certainly did _not_ represent the outpourings of a jovian volcano; it was in no sense attached to the jovian soil--if the phrase have any application to that planet; it was _not_ a mere disclosure of a glowing mass elsewhere seethed over by rolling vapours. it was, indeed, certainly not self-luminous, a satellite projected upon it in transit having been seen to show as bright as upon the dusky equatorial bands. a fundamental objection to all three hypotheses is that the rotation of the spot was variable. it did not then ride at anchor, but floated free. some held that its surface was depressed below the average cloud-level, and that the cavity was filled with vapours. professor wilson, on the other hand, observing with the -inch equatorial of the goodsell observatory in minnesota, received a persistent impression of the object "being at a higher level than the other markings."[ ] a crucial experiment on this point was proposed by mr. stanley williams in .[ ] a dark spot moving faster along the same parallel was timed to overtake the red spot towards the end of july. a unique opportunity hence appeared to be at hand of determining the relative vertical depths of the two formations, one of which must inevitably, it was thought, pass above the other. no forecast included a third alternative, which was nevertheless adopted by the dark spot. it evaded the obstacle in its path by skirting round its southern edge.[ ] nothing, then, was gained by the conjunction, beyond an additional proof of the singular repellent influence exerted by the red spot over the markings in its vicinity. it has, for example, gradually carved out a deep bay for its accommodation in the gray belt just north of it. the effect was not at first steadily present. a premonitory excavation was drawn by schwabe at dessau, september , , and again by trouvelot, barnard, and elvins in ; yet there was no sign of it in the following year. its development can be traced in dr. boeddicker's beautiful delineations of jupiter, made with the parsonstown -foot reflector, from to .[ ] they record the belt as straight in , but as strongly indented from january, ; and the cavity now promises to outlast the spot. so long as it survives, however, the forces at work in the spot can have lost little of their activity. for it must be remembered that the belt has a shorter rotation-period than the red spot, which, accordingly (as mr. elvins of toronto has pointed out), breasts and diverts, by its interior energy, a current of flowing matter, ever ready to fill up its natural bed, and override the barrier of obstruction. the famous spot was described by keeler in , as "of a pale pink colour, slightly lighter in the middle. its outline was a fairly true ellipse, framed in by bright white clouds."[ ] the fading continuously in progress from was temporarily interrupted in . the revival, indeed, was brief. professor barnard wrote in august, : "the great red spot is still visible, but it has just passed through a crisis that seemingly threatened its very existence. for the past month it has been all but impossible to catch the feeblest trace of the spot, though the ever-persistent bay in the equatorial belt close north of it, and which has been so intimately connected with the history of the red spot, has been as conspicuous as ever. it is now, however, possible to detect traces of the entire spot. an obscuring medium seems to have been passing over it, and has now drifted somewhat preceding the spot."[ ] the object is now always inconspicuous, and often practically invisible, and may be said to float passively in the environing medium.[ ] yet there are sparks beneath the ashes. a rosy tinge faintly suffused it in april, ,[ ] and its absolute end may still be remote. the extreme complexity of the planet's surface-movements has been strikingly evinced by mr. stanley williams's detailed investigations. he enumerated in [ ] nine principal currents, all flowing parallel to the equator, but unsymmetrically placed north and south of it, and showing scant signs of conformity to the solar rule of retardation with increase of latitude. the linear rate of the planet's equatorial rotation was spectroscopically determined by bélopolsky and deslandres in . both found it to fall short of the calculated speed, whence an enlargement, by self-refraction, of the apparent disc was inferred.[ ] jupiter was systematically photographed with the lick -inch telescope during the oppositions of , , and , the image thrown on the plates (after eightfold direct enlargement) being one inch in diameter. mr. stanley williams's measurements and discussion of the set for showed the high value of the materials thus collected, although much more minute details can be seen than can at present be photographed. the red spot shows as "very distinctly annular" in several of these pictures.[ ] recently, the planet has been portrayed by deslandres with the -foot meudon refractor.[ ] the extreme actinic feebleness of the equatorial bands was strikingly apparent on his plates. in , mr. ranyard[ ]--whose death, december , , was a serious loss to astronomy--acting upon an earlier suggestion of sir william huggins, collected records of unusual appearances on the disc of jupiter, with a view to investigate the question of their recurrence at regular intervals. he concluded that the development of the deeper tinges of colour, and of the equatorial "port-hole" markings girdling the globe in regular alternations of bright and dusky, agreed, so far as could be ascertained, with epochs of sun-spot maximum. the further inquiries of dr. lohse at bothkamp in [ ] went to strengthen the coincidence, which had been anticipated _à priori_ by zöllner in .[ ] moreover, separate and distinct evidence was alleged by mr. denning in of decennial outbreaks of disturbance in north temperate regions.[ ] it may, indeed, be taken for granted that what hahn terms the universal pulse of the solar system[ ] affects the vicissitudes of jupiter; but the law of those vicissitudes is far from being so obviously subordinate to the rhythmical flow of central disturbance as are certain terrestrial phenomena. the great planet, being in fact himself a "semi-sun," may be regarded as an originator, no less than a recipient, of agitating influences, the combined effects of which may well appear insubordinate to any obvious law. it is likely that saturn is in a still earlier stage of planetary development than jupiter. he is the lightest for his size of all the planets. in fact, he would float in water. and since his density is shown, by the amount of his equatorial bulging, to increase centrally,[ ] it follows that his superficial materials must be of a specific gravity so low as to be inconsistent, on any probable supposition, with the solid or liquid states. moreover, the chief arguments in favour of the high temperature of jupiter, apply, with increased force, to saturn; so that it may be concluded, without much risk of error, that a large proportion of his bulky globe, , miles in diameter, is composed of heated vapours, kept in active and agitated circulation by the process of cooling. his unique set of appendages has, since the middle of the last century, formed the subject of searching and fruitful inquiries, both theoretical and telescopic. the mechanical problem of the stability of saturn's rings was left by laplace in a very unsatisfactory condition. considering them as rotating solid bodies, he pointed out that they could not maintain their position unless their weight were in some way unsymmetrically distributed; but made no attempt to determine the kind or amount of irregularity needed to secure this end. some observations by herschel gave astronomers an excuse for taking for granted the fulfilment of the condition thus vaguely postulated; and the question remained in abeyance until once more brought prominently forward by the discovery of the dusky ring in . the younger bond led the way, among modern observers, in denying the solidity of the structure. the fluctuations in its aspect were, he asserted in ,[ ] inconsistent with such a hypothesis. the fine dark lines of division, frequently detected in both bright rings, and as frequently relapsing into imperceptibility, were due, in his opinion, to the real nobility of their particles, and indicated a fluid formation. professor benjamin peirce of harvard university immediately followed with a demonstration, on abstract grounds, of their non-solidity.[ ] streams of some fluid denser than water were, he maintained, the physical reality giving rise to the anomalous appearance first disclosed by galileo's telescope. the mechanism of saturn's rings, proposed as the subject of the adams prize, was dealt with by james clerk maxwell in . his investigation forms the groundwork of all that is at present known in the matter. its upshot was to show that neither solid nor fluid rings could continue to exist, and that the only possible composition of the system was by an aggregated multitude of unconnected particles, each revolving independently in a period corresponding to its distance from the planet.[ ] this idea of a satellite-formation had been, remarkably enough, several times entertained and lost sight of. it was first put forward by roberval in the seventeenth century, again by jacques cassini in , and with perfect definiteness by wright of durham in .[ ] little heed, however, was taken of these casual anticipations of a truth which reappeared, a virtual novelty, as the legitimate outcome of the most refined modern methods. the details of telescopic observation accord, on the whole, admirably with this hypothesis. the displacements or disappearance of secondary dividing-lines--the singular striated appearance, first remarked by short in the eighteenth century, last by perrotin and lockyer at nice, march , [ ]--show the effects of waves of disturbance traversing a moving mass of gravitating particles;[ ] the broken and changing line of the planet's shadow on the ring gives evidence of variety in the planes of the orbits described by those particles. the whole ring-system, too, appears to be somewhat elliptical.[ ] the satellite-theory has derived unlooked-for support from photometric inquiries. professor seeliger pointed out in [ ] that the unvarying brilliancy of the outer rings under all angles of illumination, from ° to °, can be explained from no other point of view. nor does the constitution of the obscure inner ring offer any difficulty. for it is doubtless formed of similar small bodies to those aggregated in the lucid members of the system, only much more thinly strewn, and reflecting, consequently, much less light. it is not, indeed, at first easy to see why these sparser flights should show as a dense dark shading on the body of saturn. yet this is invariably the case. the objection has been urged by professor hastings of baltimore. the brightest parts of these appendages, he remarked,[ ] are more lustrous than the globe they encircle; but if the inner ring consists of identical materials, possessing presumably an equal reflective capacity, the mere fact of their scanty distribution would not cause them to show as dark against the same globe. professor seeliger, however, replied[ ] that the darkening is due to the never-ending swarms of their separate shadows transiting the planet's disc. sunlight is not, indeed, wholly excluded. many rays come and go between the open ranks of the meteorites. for the dusky ring is transparent. the planet it encloses shows through it, as if veiled with a strip of crape. a beautiful illustration of its quality in this respect was derived by professor barnard from an eclipse of japetus, november , .[ ] the eighth moon remained steadily visible during its passage through the shadow of the inner ring, but with a progressive loss of lustre in approaching its bright neighbour. there was no breach of continuity. the satellite met no gap, corresponding to that between the dusky ring and the body of saturn, through which it could shine with undiminished light, but was slowly lost sight of as it plunged into deeper and deeper gloom. the important facts were thus established, that the brilliant and obscure rings merge into each other, and that the latter thins out towards the saturnian globe. the meteoric constitution of these appendages was beautifully demonstrated in by professor keeler,[ ] then director of the alleghany observatory, pittsburgh. from spectrographs taken with the slit adjusted to coincidence with the equatorial plane of the system, he determined the comparative radial velocities of its different parts. and these supply a crucial test of clerk maxwell's theory. for if the rings were solid, the swiftest rates of rotation should be at their outer edges, corresponding to wider circles described in the same period; while, if they are pulverulent, the inverse relation must hold good. this proved to be actually the case. the motion slowed off outward, in agreement with the diminishing speed of particles travelling freely, each in its own orbit. keeler's result was promptly confirmed by campbell,[ ] as well as by deslandres and bélopolsky. a question of singular interest, and one which we cannot refrain from putting to ourselves, is--whether we see in the rings of saturn a finished structure, destined to play a permanent part in the economy of the system; or whether they represent merely a stage in the process of development out of the chaotic state in which it is impossible to doubt that the materials of all planets were originally merged. m. otto struve attempted to give a definite answer to this important query. a study of early and later records of observations disclosed to him, in , an apparent progressive approach of the inner edge of the bright ring to the planet. the rate of approach he estimated at about fifty-seven english miles a year, or , miles during the years elapsed since the time of huygens.[ ] were it to continue, a collapse of the system must be far advanced within three centuries. but was the change real or illusory--a plausible, but deceptive inference from insecure data? m. struve resolved to put it to the test. a set of elaborately careful micrometrical measures of the dimensions of saturn's rings, executed by himself at pulkowa in the autumn of , was provided as a standard of future comparison; and he was enabled to renew them, under closely similar circumstances, in .[ ] but the expected diminution of the space between saturn's globe and his rings had not taken place. a slight extension in the width of the system, both outward and inward, was indeed, hinted at; and it is worth notice that just such a separation of the rings was indicated by clerk maxwell's theory, so that there is an _à priori_ likelihood of its being in progress. yet hall's measures in - [ ] failed to supply evidence of alteration with time; and barnard's, executed at lick in - ,[ ] showed no sensible divergence from them. hence, much weight cannot be laid upon huygens's drawings and descriptions, which had been held to prove conclusively a partial filling up, since , of the interval between the ring and the planet.[ ] the rings of saturn replace, in professor g. h. darwin's view,[ ] an abortive satellite, scattered by tidal action into annular form. for they lie closer to the planet than is consistent with the integrity of a revolving body of reasonable bulk. the limit of possible existence for such a mass was fixed by roche of montpellier, in ,[ ] at · mean radii of its primary; while the outer edge of the ring-system is distant · radii of saturn from his centre. the virtual discovery of its pulverulent condition dates, then, according to professor darwin, from . he conjectures that the appendage will eventually disappear, partly through the dispersal of its constituent particles inward, and their subsidence upon the planet's surface, partly by their dispersal outward, to a region beyond "roche's limit," where coalescence might proceed unhindered by the strain of unequal attractions. one modest satellite, revolving inside mimas, would then be all that was left of the singular appurtenances we now contemplate with admiration. there seems reason to admit that kirkwood's law of commensurability has had some effect in bringing about the present distribution of the matter composing them. here the influential bodies are saturn's moons, while the divisions and boundaries of the rings represent the spaces where their disturbing action conspires to eliminate revolving particles. kirkwood, in fact, showed, in ,[ ] that a body circulating in the chasm between the bright rings known as "cassini's division," would have a period nearly commensurable with those of _four_ out of the eight moons; and meyer of geneva subsequently calculated all such combinations, with the result of bringing out coincidences between regions of maximum perturbation and the limiting and dividing lines of the system.[ ] this is in itself a strong confirmation of the view that the rings are made up of independently revolving small bodies. on december , , professor asaph hall discovered at washington a bright equatorial spot on saturn, which he followed and measured through above sixty rotations, each performed in ten hours fourteen minutes twenty-four seconds.[ ] this, he was careful to add, represented the period, not necessarily of the _planet_, but only of the individual spot. the only previous determination of saturn's axial movement (setting aside some insecure estimates by schröter) was herschel's in , giving a period of ten hours sixteen minutes. the substantial accuracy of hall's result was verified by mr. denning in .[ ] in may and june of that year, ten vague bright markings near the equator were watched by mr. stanley williams, who derived from them a rotation period only two seconds shorter than that determined at washington. nevertheless, similarly placed spots gave in and notably quicker rates;[ ] so that the task of timing the general drift of the saturnian surface by the displacements of such objects is hampered, to an indefinite extent, by their individual proper motions. saturn's outermost satellite, japetus, is markedly variable--so variable that it sends us, when brightest, just - / times as much light as when faintest. moreover, its fluctuations depend upon its orbital position in such a way as to make it a conspicuous telescopic object when west, a scarcely discernible one when east of the planet. herschel's inference[ ] of a partially obscured globe turning always the same face towards its primary seems the only admissible one, and is confirmed by pickering's measurements of the varying intensity of its light. he remarked further that the dusky and brilliant hemispheres must be so posited as to divide the disc, viewed from saturn, into nearly equal parts; so that this saturnian moon, even when "full," appears very imperfectly illuminated over one-half of its surface.[ ] zöllner estimated the albedo of saturn at · , müller at · , a value impossibly high, considering that the spectrum includes no vestige of original emissions. closely similar to that of jupiter, it shows the distinctive dark line in the red (wave-length ), which we may call the "red-star line"; and janssen, from the summit of etna in [ ] found traces in it of aqueous absorption. the light from the ring appears to be pure reflected sunshine unmodified by original atmospheric action.[ ] uranus, when favourably situated, can easily be seen with the naked eye as a star between the fifth and sixth magnitudes. there is indeed, some reason to suppose that he had been detected as a wandering orb by savage "watchers of the skies" in the pacific long before he swam into herschel's ken. nevertheless, inquiries into his physical habitudes are still in an early stage. they are exceedingly difficult of execution, even with the best and largest modern telescopes; and their results remain clouded with uncertainty. it will be remembered that uranus presents the unusual spectacle of a system of satellites travelling nearly at right angles to the plane of the ecliptic. the existence of this anomaly gives a special interest to investigations of his axial movement, which might be presumed, from the analogy of the other planets, to be executed in the same tilted plane. yet this is far from being certainly the case. mr. buffham in - caught traces of bright markings on the uranian disc, doubtfully suggesting a rotation in about twelve hours in a plane _not_ coincident with that in which his satellites circulate.[ ] dusky bands resembling those of jupiter, but very faint, were barely perceptible to professor young at princeton in . yet, though almost necessarily inferred to be equatorial, they made a considerable angle with the trend of the satellites' orbits.[ ] more distinctly by the brothers henry, with the aid of their fine refractor, two gray parallel rulings, separated by a brilliant zone, were discerned every clear night at paris from january to june, .[ ] what were taken to be the polar regions appeared comparatively dusky. the direction of the equatorial rulings (for so we may safely call them) made an angle of ° with the satellites' line of travel. similar observations were made at nice by mm. perrotin and thollon, march to june, , a lucid spot near the equator, in addition, indicating rotation in a period of about ten hours.[ ] the discrepancy was, however, considerably reduced by perrotin's study of the planet in with the new -inch equatoreal.[ ] the dark bands, thus viewed to better advantage than in , appeared to deviate no more than ° from the satellites' orbit-plane. no definitive results, on the other hand, were derived by professors holden, schaeberle, and keeler from their observations of uranus in - with the potent instrument on mount hamilton. shadings, it is true, were almost always, though faintly, seen; but they appeared under an anomalous, possibly an illusory aspect. they consisted, not of parallel, but of forked bands.[ ] measurements of the little sea-green disc which represents to us the massive bulk of uranus, by young, schiaparelli,[ ] safarik, h. c. wilson[ ] and perrotin, prove it to be quite distinctly _bulged_. the compression at once caught barnard's trained eye in ,[ ] when he undertook at lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about ° with the line of travel pursued by the satellites. nothing more can be learned on this curious subject for some years, since the pole of the planet is just now turned nearly towards the earth; but barnard's conclusion is unlikely to be seriously modified. he fixed the mean diameter of uranus at , miles. but this estimate was materially reduced through dr. see's elimination of irradiative effects by means of daylight measures, executed at washington in .[ ] the visual spectrum of this planet was first examined by father secchi in , and later, with more advantages for accuracy, by huggins, vogel,[ ] and keeler.[ ] it is a very remarkable one. in lieu of the reflected fraunhofer lines, imperceptible perhaps through feebleness of light, six broad bands of original absorption appear, one corresponding to the blue-green ray of hydrogen (f), another to the "red-star line" of jupiter and saturn, the rest as yet unidentified. the hydrogen band seems much too strong and diffuse to be the mere echo of a solar line, and might accordingly be held to imply the presence of free hydrogen in the uranian atmosphere. this, however, would be difficult of reconcilement with keeler's identification of an absorption-group in the yellow with a telluric waterband. notwithstanding its high albedo-- · , according to zöllner--proof is wanting that any of the light of uranus is inherent. mr. albert taylor announced, indeed, in , his detection, with common's giant reflector, of bright flutings in its spectrum;[ ] but professor keeler's examination proved them to be merely contrast effects.[ ] sir william and lady huggins, moreover, obtained about the same time a photograph purely solar in character. the spectrum it represented was crossed by numerous fraunhofer lines, and by no others. it was, then, presumably composed entirely of reflected light. * * * * * judging from the indications of an almost evanescent spectrum, neptune, as regards physical condition, is the twin of uranus, as saturn of jupiter. of the circumstances of his rotation we are as good as completely ignorant. mr. maxwell hall, indeed, noticed at jamaica, in november and december, , certain rhythmical fluctuations of brightness, suggesting revolution on an axis in slightly less than eight hours;[ ] but professor pickering reduces the supposed variability to an amount altogether too small for certain perception, and dr. g. müller denies its existence _in toto_. it is true their observations were not precisely contemporaneous with those of mr. hall[ ] who believes the partial obscurations recorded by himself to have been of a passing kind, and to have suddenly ceased after a fortnight of prevalence. their less conspicuous renewal was visible to him in november, , confirming a rotation period of · hours. it was ascertained at first by indirect means that the orbit of neptune's satellite is inclined about ° to his equator. mr. marth[ ] having drawn attention to the rapid shifting of its plane of motion, m. tisserand and professor newcomb[ ] independently published the conclusion that such shifting necessarily results from neptune's ellipsoidal shape. the movement is of the kind exemplified--although with inverted relations--in the precession of the equinoxes. the pole of the satellite, owing to the pull of neptune's equatorial protuberance, describes a circle round the pole of his equator in a retrograde direction, and in a period of over five hundred years. the amount of compression indicated for the primary body is, at the outside, / ; whence it can be inferred that neptune possesses a lower rotatory velocity than the other giant planets. direct verification of the trend theoretically inferred for the satellite's movement was obtained by dr. see in . the washington -inch refractor disclosed to him, under exceptionally favourable conditions, a set of equatorial belts on the disc of neptune, and they took just the direction prescribed by theory. their objective reality cannot be doubted, although barnard was unable, either with the lick or the yerkes telescope,[ ] to detect any definite markings on this planet. its diameter was found by him to be , miles. the possibility that neptune may not be the most remote body circling round the sun has been contemplated ever since he has been known to exist. within the last few years the position at a given epoch of a planet far beyond his orbital verge has been approximately fixed by two separate investigators. professor george forbes of edinburgh adopted in a novel plan of search for unknown members of the solar system, the first idea of which was thrown out by m. flammarion in november, .[ ] it depends upon the movements of comets. it is well known that those of moderately short periods are, for a reason already explained, connected with the larger planets in such a way that the cometary aphelia fall near some planetary orbit. jupiter claims a large retinue of such partial dependents, neptune owns six, and there are two considerable groups, the farthest distances of which from the sun lie respectively near and times that of the earth. at each of these vast intervals, one involving a period of , , the other of , years, professor forbes maintains that an unseen planet circulates. he even computed elements for the nearer of the two, and fixed its place on the celestial sphere;[ ] but the photographic searches made for it by dr. roberts at crowborough and by mr. wilson at daramona proved unavailing. undeterred by deichmüller's discouraging opinion that cometary orbits extending beyond the recognised bounds of the solar system are too imperfectly known to serve as the basis of trustworthy conclusions,[ ] the edinburgh professor returned to the attack in .[ ] he now sought to prove that the lost comet of actually returned in , but with elements so transformed by ultra-neptunian perturbations as to have escaped immediate identification. if so, the "wanted" planet has just entered the sign libra, and, being larger than jupiter, should be possible to find. almost simultaneously with forbes, professor todd set about groping for the same object by the help of a totally different set of indications. adams's approved method commended itself to him; but the hypothetical divagations of neptune having scarcely yet had time to develop, he was thrown back upon the "residual errors" of uranus. they gave him a virtually identical situation for the new planet with that derived from the clustering of cometary aphelia.[ ] yet its assigned distance was little more than half that of the nearer of professor forbes's remote pair, and it completed a revolution in instead of , years. the agreement in them between the positions determined, on separate grounds, for the ultra-neptunian traveller was merely an odd coincidence; nor can we be certain, until it is seen, that we have really got into touch with it. footnotes: [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : _novæ observationes_, p. .] [footnote : _phil. trans._, vol. i., p. .] [footnote : _mém. de l'ac._, , p. .] [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : a large work, entitled _areographische fragmente_, in which schröter embodied the results of his labours on mars, - , narrowly escaped the conflagration of , and was published at leyden in .] [footnote : _beiträge_, p. .] [footnote : _mem. r. a. soc._, vol. xxxii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. viii., p. .] [footnote : _month. not._, vols. xxviii., p. ; xxix., p. ; xxxiii., p. .] [footnote : flammarion, _l'astronomie_, t. i., p. .] [footnote : smyth, _cel. cycle_, vol. i., p. ( st ed.).] [footnote : _phil. trans._, vol. cxxi., p. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _phil. mag._, vol. xxxiv., p. .] [footnote : proctor, _quart. jour. of science_, vol. x., p. ; maunder, _sunday mag._, january, february, march, ; campbell, _publ. astr. pac. soc._, vol. vi., p. .] [footnote : _am. jour. of sc._, vol. xxviii., p. .] [footnote : burton, _trans. roy. dublin soc._, vol. i., , p. .] [footnote : _month. not._, vol. xxvii., p. ; _astroph. journ._, vol. i., p. .] [footnote : _untersuchungen über die spectra der planeten_, p. ; _astroph. journ._, vol. i., p. .] [footnote : _publ. astr. pac. soc._, vols. vi., p. ; ix., p. ; _astr. and astroph._, vol. xiii., p. ; _astroph. jour._, vol. ii., p. .] [footnote : _ibid._, vol. v., p. .] [footnote : _ibid._, vols. i., p. ; iii., p. .] [footnote : c. christiansen, _beiblätter_, , p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : flammarion, _la planète mars_, p. .] [footnote : _mémoires couronnés_, t. xxxix.] [footnote : lockyer, _nature_, vol. xlvi., p. .] [footnote : _mem. spettr. italiani_, t. xi., p. .] [footnote : _bull. astr._, t. iii., p. .] [footnote : _journ. brit. astr. ass._, vol. i., p. .] [footnote : _publ. pac. astr. soc._, vol. ii., p. ; percival lowell, _mars_, ; _annals of the lowell observatory_, vol. ii., .] [footnote : _old and new astr._, p. .] [footnote : _l'astronomie_, t. xi., p. .] [footnote : _la planète mars_, p. .] [footnote : _month. notices_, vol. lvi., p. .] [footnote : _l'astronomie_, t. viii.] [footnote : _astr. nach._, no. , ; _astr. and astrophysics_, vol. xiii., p. .] [footnote : _month. not._, vol. xxxviii., p. ; _mem. roy. astr. soc._, vol. xliv., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _comptes rendus_, t. cxv., p. .] [footnote : _astr. jour._, no. ; _publ. astr. pac. soc._, vol. vi., p. . _cf. observatory_ vol. xvii., pp. - .] [footnote : see mr. wentworth erck's remarks in _trans. roy. dublin soc._, vol. i., p. .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : _annals harvard coll. obs._, vol. xi., pt. ii., p. .] [footnote : young, _gen. astr._, p. .] [footnote : campbell, _publ. pac. astr. soc._, vol. vi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _witch of atlas_, stanza iii. i am indebted to dr. garnett for the reference.] [footnote : recommended by chandler, _astr. jour._, no. .] [footnote : _harvard circulars_, nos. , , .] [footnote : _astr. nach._, no. , .] [footnote : montangerand, _comptes rendus_, march , .] [footnote : pickering, _astroph. jour._, vol. xiii., p. .] [footnote : _harvard circular_, no. .] [footnote : _astr. nach._, no. .] [footnote : l. niesten, _annuaire_, bruxelles, , p. .] [footnote : according to svedstrup (_astr. nach._, nos. , - ), the inclination to the ecliptic of the "mean asteroid's" orbit is = °.] [footnote : _smiths._ report, , p. ; _the asteroids_ (kirkwood), p. , .] [footnote : tisserand, _annuaire_, paris, , p. b. ; newcomb, _astr. jour._, no. ; backlund, _bull. astr._, t. xvii., p. ; parmentier, _bull. soc. astr. de france_, march, ; observatory, vol. xviii., p. .] [footnote : berberich, _astr. nach._, no. , .] [footnote : _bull. astr._, t. xviii., p. .] [footnote : _the asteroids_, p. ; _publ. astr. pac. soc._, vols. ii., p. ; iii., p. .] [footnote : _comptes rendus_, t. xxxvii., p. .] [footnote : _bull. astr._, t. v., p. .] [footnote : _annuaire_, bruxelles, , p. .] [footnote : _johns hopkins un. circular_, january, ; _observatory_, vol. xviii., p. .] [footnote : _harvard annals_, vol. xi., part ii., p. .] [footnote : _astr. nach._, nos. , - .] [footnote : _month. not._, vol. lxi., p. .] [footnote : _astroph. jour._, vol. vii., p. .] [footnote : _spectra der planeten_, p. .] [footnote : tome i., p. .] [footnote : _berlinische monatsschrift_, , p. .] [footnote : _month. not._, vol. xiii., p. .] [footnote : _mem. am. ac._, vol. viii., p. .] [footnote : _photom. unters._, p. .] [footnote : _astr. nach._, no. , .] [footnote : _mém. de l'ac._, t. x., p. .] [footnote : _ibid._, , p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : _photom. unters._, pp. , ; _potsdam publ._, no. .] [footnote : vogel, _sp. der planeten_, p. , _note_.] [footnote : _proc. roy. soc._, vol. xviii., p. .] [footnote : _month. not._, vol. xl., p. .] [footnote : _sitzungsberichte_, berlin, , ii., p. .] [footnote : the anomalous shadow-effects recorded by webb (_cel. objects_, p. , th ed.) are obviously of atmospheric and optical origin.] [footnote : engelmann, _ueber die helligkeitsverhältnisse der jupiterstrabanten_, p. .] [footnote : _month. not._, vol. xxviii., p. .] [footnote : _observatory_, vol. vii., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : pickering failed to obtain any photometric evidence of their variability. _harvard annals_, vol. xi., p. .] [footnote : _astr. and astroph._, vol. xii., pp. , .] [footnote : _annals lowell obs._, vol. ii., pt. i.] [footnote : _astr. nach._, nos. , , , ; _month. not._, vols. li., p. ; liv., p. . barnard remains convinced that the oval forms attributed to jupiter's satellites are illusory effects of their markings. _astr. nach._, nos. , , , ; _astr. and astroph._, vol. xiii., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _astr. jour._, nos. , , , ; _observatory_, vol. xv., p. .] [footnote : tisserand, _comptes rendus_, october , ; cohn, _astr. nach._, no. , .] [footnote : _bull. ac. r. bruxelles_, t. xlviii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : denning, _month. not._, vol. xliv., pp. , ; _nature_, vol. xxv., p. .] [footnote : _sidereal mess._, december, , p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _proc. roy. soc. n. s. wales_, vol. xiv., p. .] [footnote : _phil. trans._, vol. i., p. .] [footnote : for indications relative to the early history of the red spot, see holden, _publ. astr. pac. soc._, vol. ii., p. ; noble, _month. not._, vol. xlvii., p. ; a. s. williams, _observatory_, vol. xiii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _month. not._, vol. l., p. .] [footnote : _observatory_, vol. xiii., pp. , .] [footnote : _trans. r. dublin soc._, vol. iv., p. , .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : denning, _knowledge_, vol. xxiii., p. ; _observatory_, vol. xxiv., p. ; _pop. astr._, vol. ix., p. ; _nature_, vol. lv., p. .] [footnote : williams, _observatory_, vol. xxiii., p. .] [footnote : _month. not._, vol. lvi., p. .] [footnote : bélopolsky, _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, vol. iv., p. .] [footnote : _bull. astr._, , p. .] [footnote : _month. not._, vol. xxxi., p. .] [footnote : _beobachtungen_, heft ii., p. .] [footnote : _ber. sächs. ges. der wiss._, , p. .] [footnote : _month. not._, vol. lix., p. .] [footnote : _beziehungen der sonnenfleckenperiode_, p. .] [footnote : a. hall, _astr. nach._, no. , .] [footnote : _astr. jour._ (gould's), vol. ii., p. .] [footnote : _ibid._, p. .] [footnote : _on the stability of the motion of saturn's rings_, p. .] [footnote : _mém. de l'ac._, , p. ; montucla, _hist. des math._, t. iv., p. ; _an original theory of the universe_, p. .] [footnote : _comptes rendus_, t. xcviii., p. .] [footnote : proctor, _saturn and its system_ ( ), p. .] [footnote : perrotin, _comptes rendus_, t. cvi., p. .] [footnote : _abhandl. akad. der wiss._, munich, bd. xvi., p. .] [footnote : _smiths. report_, (holden).] [footnote : quoted by dr. e. anding, _astr. nach._, no. , .] [footnote : _astr. and astrophysics_, vol. xi., p. ; _month. not._, vol. l., p. .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _ibid._, vol. ii., p. .] [footnote : _mém. de l'ac. imp._ (st. petersb.), t. vii., , p. .] [footnote : _astr. nach._, no. , .] [footnote : _washington observations_, app. ii., p. ] [footnote : _month. not._, vol. lvi., p. .] [footnote : t. lewis, _observatory_, vol. xviii., p. .] [footnote : _harper's magazine_, june, .] [footnote : _mém. de l'acad. de montpellier_, t. viii., p. , .] [footnote : _meteoric astronomy_, chap. xii. he carried the subject somewhat farther in . see _observatory_, vol. vi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _amer. jour. of sc._, vol. xiv., p. .] [footnote : _observatory_, vol. xiv., p. .] [footnote : _month. not._, vol. liv., p. .] [footnote : _phil. trans._, vol. lxxxii., p. .] [footnote : _smiths. report_, .] [footnote : _comptes rendus_, t. lxiv., p. .] [footnote : huggins, _proc. r. soc._, vol. xlvi., p. ; keeler, _astr. nach._, no. , ; vogel, _astroph. jour._, vol. i., p. .] [footnote : _month. not._, vol. xxxiii., p. .] [footnote : _astr. nach._, no , .] [footnote : _comptes rendus_, t. xcviii., p. .] [footnote : _comptes rendus_, t. xcviii., pp. , .] [footnote : _v. j. s. astr. ges._, jahrg. xxiv., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _astr. jour._, nos. , .] [footnote : _astr. nach._, no. , .] [footnote : _ann. der phys._, bd. clviii., p. ; _astroph. jour._, vol. i., p. .] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. xlix., p. .] [footnote : astr. nach., no. , ; scheiner's _spectralanalyse_, p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : _observatory_, vol. vii., pp. , , .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _comptes rendus_, t. cvii., p. ; _astr. and astroph._, vol. xiii., p. ; _astr. jour._, no. .] [footnote : _astr. jour._, nos. , , .] [footnote : _astr. pop._, p. ; _la nature_, january , .] [footnote : _proc. roy. soc. edinb._, vols. x., p. ; xi., p. .] [footnote : _vierteljahrsschrift. astr. ges._, jahrg. xxi., p. .] [footnote : _proc. roy. soc. edinb._, vol. xxiii., p. ; _nature_, vol. lxiv., p. .] [footnote : _amer. jour. of science_, vol. xx., p. .] chapter ix _theories of planetary evolution_ we cannot doubt that the solar system, as we see it, is the result of some process of growth--that, during innumerable ages, the forces of nature were at work upon its materials, blindly modelling them into the shape appointed for them from the beginning by omnipotent wisdom. to set ourselves to inquire what that process was may be an audacity, but it is a legitimate, nay, an inevitable one. for man's implanted instinct to "look before and after" does not apply to his own little life alone, but regards the whole history of creation, from the highest to the lowest--from the microscopic germ of an alga or a fungus to the visible frame and furniture of the heavens. kant considered that the inquiry into the mode of origin of the world was one of the easiest problems set by nature; but it cannot be said that his own solution of it was satisfactory. he, however, struck out in a track which thought still pursues. in his _allgemeine naturgeschichte_ the growth of sun and planets was traced from the cradle of a vast and formless mass of evenly diffused particles, and the uniformity of their movements was sought to be accounted for by the unvarying action of attractive and repulsive forces, under the dominion of which their development was carried forward. in its modern form, the "nebular hypothesis" made its appearance in .[ ] it was presented by laplace with diffidence, as a speculation unfortified by numerical buttresses of any kind, yet with visible exultation at having, as he thought, penetrated the birth-secret of our system. he demanded, indeed, more in the way of postulates than kant had done. he started with a sun ready made,[ ] and surrounded with a vast glowing atmosphere, extending into space out beyond the orbit of the farthest planet, and endowed with a slow rotatory motion. as this atmosphere or nebula cooled, it contracted; and as it contracted, its rotation, by a well-known mechanical law, became accelerated. at last a point arrived when tangential velocity at the equator increased beyond the power of gravity to control, and equilibrium was restored by the separation of a nebulous ring revolving in the same period as the generating mass. after a time, the ring broke up into fragments, all eventually reunited in a single revolving and rotating body. this was the first and farthest planet. meanwhile the parent nebula continued to shrink and whirl quicker and quicker, passing, as it did so, through successive crises of instability, each resulting in, and terminated by, the formation of a planet, at a smaller distance from the centre, and with a shorter period of revolution than its predecessor. in these secondary bodies the same process was repeated on a reduced scale, the birth of satellites ensuing upon their contraction, or not, according to circumstances. saturn's ring, it was added, afforded a striking confirmation of the theory of annular separation,[ ] and appeared to have survived in its original form in order to throw light on the genesis of the whole solar system; while the four first discovered asteroids offered an example in which the _débris_ of a shattered ring had failed to coalesce into a single globe. this scene of cosmical evolution was a characteristic bequest from the eighteenth century to the nineteenth. it possessed the self-sufficing symmetry and entireness appropriate to the ideas of a time of renovation, when the complexity of nature was little accounted of in comparison with the imperious orderliness of the thoughts of man. since its promulgation, however, knowledge has transgressed many boundaries, and set at naught much ingenious theorising. how has it fared with laplace's sketch of the origin of the world? it has at least not been discarded as effete. the groundwork of speculation on the subject is still furnished by it. it is, nevertheless, admittedly inadequate. of much that exists it gives no account, or an erroneous one. the march of events certainly did not everywhere--even if it did anywhere--follow the exact path prescribed for it. yet modern science attempts to supplement, but scarcely ventures to supersede it. thought has, in many directions, been profoundly modified by mayer's and joule's discovery, in , of the equivalence between heat and motion. its corollary was the grand idea of the "conservation of energy," now one of the cardinal principles of science. this means that, under the ordinary circumstances of observation, the old maxim _ex nihilo nihil fit_ applies to force as well as to matter. the supplies of heat, light, electricity, must be kept up, or the stream will cease to flow. the question of the maintenance of the sun's heat was thus inevitably raised; and with the question of maintenance that of origin is indissolubly connected. dr. julius robert mayer, a physician residing at heilbronn, was the first to apply the new light to the investigation of what sir john herschel had termed the "great secret." he showed that if the sun were a body either simply cooling or in a state of combustion, it must long since have "gone out." had an equal mass of coal been set alight four or five centuries after the building of the pyramid of cheops, and kept burning at such a rate as to supply solar light and heat during the interim, only a few cinders would now remain in lieu of our undiminished glorious orb. mayer looked round for an alternative. he found it in the "meteoric hypothesis" of solar conservation.[ ] the importance in the economy of our system of the bodies known as falling stars was then (in ) beginning to be recognised. it was known that they revolved in countless swarms round the sun; that the earth daily encountered millions of them; and it was surmised that the cone of the zodiacal light represented their visible condensation towards the attractive centre. from the zodiacal light, then, mayer derived the store needed for supporting the sun's radiations. he proved that, by the stoppage of their motion through falling into the sun, bodies would evolve from , to , times as much heat (according to their ultimate velocity) as would result from the burning of equal masses of coal, their precipitation upon the sun's surface being brought about by the resisting medium observed to affect the revolutions of encke's comet. there was, however, a difficulty. the quantity of matter needed to keep, by the sacrifice of its movement, the hearth of our system warm and bright would be very considerable. mayer's lowest estimate put it at , billion kilogrammes per second, or a mass equal to that of our moon bi-annually. but so large an addition to the gravitating power of the sun would quickly become sensible in the movement of the bodies dependent upon him. their revolutions would be notably accelerated. mayer admitted that each year would be shorter than the previous one by a not insignificant fraction of a second, and postulated an unceasing waste of substance, such as newton had supposed must accompany emission of the material corpuscles of light, to neutralise continual reinforcement. mayer's views obtained a very small share of publicity, and owned mr. waterston as their independent author in this country. the meteoric, or "dynamical," theory of solar sustentation was expounded by him before the british association in . it was developed with his usual ability by lord kelvin, in the following year. the inflow of meteorites, he remarked, "is the only one of all conceivable causes of solar heat which we know to exist from independent evidence."[ ] we know it to exist, but we now also know it to be entirely insufficient. the supplies presumed to be contained in the zodiacal light would be quickly exhausted; a constant inflow from space would be needed to meet the demand. but if moving bodies were drawn into the sun at anything like the required rate, the air, even out here at ninety-three millions of miles distance, would be thick with them; the earth would be red-hot from their impacts;[ ] geological deposits would be largely meteoric;[ ] to say nothing of the effects on the mechanism of the heavens. lord kelvin himself urged the inadmissibility of the "extra-planetary" theory of meteoric supply on the very tangible ground that, if it were true, the year would be shorter now, actually by six weeks, than at the opening of the christian era. the "intra-planetary" supply, however, is too scanty to be anything more than a temporary makeshift. the meteoric hypothesis was naturally extended from the maintenance of the sun's heat to the formation of the bodies circling round him. the earth--no less doubtless than the other planets--is still growing. cosmical matter in the shape of falling stars and aërolites, to the amount, adopting professor newton's estimate, of tons daily, is swept up by it as it pursues its orbital round. inevitably the idea suggested itself that this process of appropriation gives the key to the life-history of our globe, and that the momentary streak of fire in the summer sky represents a feeble survival of the glowing hailstorm by which in old times it was fashioned and warmed. mr. e. w. brayley supported this view of planetary production in ,[ ] and it has recommended itself to haidinger, helmholtz, proctor, and faye. but the negative evidence of geological deposits appears fatal to it. the theory of solar energy now generally regarded as the true one was enounced by helmholtz in a popular lecture in . it depends upon the same principle of the equivalence of heat and motion which had suggested the meteoric hypothesis. but here the movement surrendered and transformed belongs to the particles, not of any foreign bodies, but of the sun itself. drawn together from a wide ambit by the force of their own gravity, their fall towards the sun's centre must have engendered a vast thermal store, of which / are computed to be already spent. presumably, however, this stream of reinforcement is still flowing. in the very act of parting with heat, the sun develops a fresh stock. his radiations, in short, are the direct result of shrinkage through cooling. a diminution of the solar diameter by feet yearly would just suffice to cover the present rate of emission, and would for ages remain imperceptible with our means of observation, since, after the lapse of , years, the lessening of angular size would scarcely amount to one second.[ ] but the process, though not terminated, is strictly a terminable one. in less than five million years, the sun will have contracted to half its present bulk. in seven million more, it will be as dense as the earth. it is difficult to believe that it will then be a luminous body.[ ] nor can an unlimited past duration be admitted. helmholtz considered that radiation might have gone on with its actual intensity for twenty-two, langley allows only eighteen million years. the period can scarcely be stretched, by the most generous allowances, to double the latter figure. but this is far from meeting the demands of geologists and biologists. an attempt was made in to supply the sun with machinery analogous to that of a regenerative furnace, enabling it to consume the same fuel over and over again, and so to prolong indefinitely its beneficent existence. the inordinate "waste" of energy, which shocks our thrifty ideas, was simultaneously abolished. the earth stops and turns variously to account one , -millionth part of the solar radiations; each of the other planets and satellites takes a proportionate share; the rest, being all but an infinitesmal fraction of the whole, is dissipated through endless space, to serve what purpose we know not. now, on the late sir william siemens's plan, this reckless expenditure would cease; the solar incomings and outgoings would be regulated on approved economic principles, and the inevitable final bankruptcy would be staved off to remote ages. but there was a fatal flaw in its construction. he imagined a perpetual circulation of combustible materials, alternately surrendering and regaining chemical energy, the round being kept going by the motive force of the sun's rotation.[ ] this, however, was merely to perch the globe upon a tortoise, while leaving the tortoise in the air. the sun's rotation contains a certain definite amount of mechanical power--enough, according to lord kelvin, if directly converted into heat, to keep up the sun's emission during years and six days--a mere moment in cosmical time. more economically applied, it would no doubt go farther. its exhaustion would, nevertheless, under the most favourable circumstances, ensue in a comparatively short period.[ ] many other objections equally unanswerable have been urged to the "regenerative" hypothesis, but this one suffices. dr. croll's collision hypothesis[ ] is less demonstrably unsound, but scarcely less unsatisfactory. by the mutual impact of two dark masses rushing together with tremendous speed, he sought to provide the solar nebula with an immense _original_ stock of heat for the reinforcement of that subsequently evolved in the course of its progressive contraction. the sun, while still living on its capital, would thus have a larger capital to live on, and the time-demands of the less exacting geologists and biologists might be successfully met. but the primitive event, assumed for the purpose of dispensing them from the inconvenience of "hurrying up their phenomena," is not one that a sane judgment can readily admit to have ever, in point of actual fact, happened. there remains, then, as the only intelligible rationale of solar sustentation, helmholtz's shrinkage theory. and this has a very important bearing upon the nebular view of planetary formation; it may, in fact, be termed its complement. for it involves the idea that the sun's materials, once enormously diffused, gradually condensed to their present volume with development of heat and light, and, it may plausibly be added, with the separation of dependent globes. the data furnished by spectrum analysis, too, favour the supposition of a common origin for sun and planets by showing their community of substance; while gaseous nebulæ present examples of vast masses of tenuous vapour, such as our system may plausibly be conjectured to have primitively sprung from. but recent science raises many objections to the details, if it supplies some degree of confirmation to the fundamental idea of laplace's cosmogony. the detection of the retrograde movement of neptune's satellite made it plain that the anomalous conditions of the uranian world were due to no extraordinary disturbance, but to a systematic variety of arrangement at the outskirts of the solar domain. so that, were a trans-neptunian planet discovered, we should be fully prepared to find it rotating, and surrounded by satellites circulating from east to west. the uniformity of movement, upon the probabilities connected with which the french geometer mainly based his scheme, thus at once vanishes. the excessively rapid revolution of the inner martian moon is a further stumbling-block. on laplace's view, _no_ satellite can revolve in a shorter time than its primary rotates; for in its period of circulation survives the period of rotation of the parent mass which filled the sphere of its orbit at the time of giving it birth. and rotation quickens as contraction goes on; therefore, the older time of axial rotation should invariably be the longer. this obstacle can, however, as we shall presently see, be turned. more serious is one connected with the planetary periods, pointed out by babinet in .[ ] in order to make them fit in with the hypothesis of successive separation from a rotating and contracting body, certain arbitrary assumptions have to be made of fluctuations in the distribution of the matter forming that body at the various epochs of separation.[ ] such expedients usually merit the distrust which they inspire. primitive and permanent irregularities of density in the solar nebula, such as miss young's calculations suggest,[ ] do not, on the other hand, appear intrinsically improbable. again, it was objected by professor kirkwood in [ ] that there could be no sufficient cohesion in such an enormously diffused mass as the planets are supposed to have sprung from to account for the wide intervals between them. the matter separated through the growing excess of centrifugal speed would have been cast off, not by rarely recurring efforts, but continually, fragmentarily, _pari passu_ with condensation and acceleration. each wisp of nebula, as it found itself unduly hurried, would have declared its independence, and set about revolving and condensing on its own account. the result would have been a meteoric, not a planetary system. moreover, it is a question whether the relative ages of the planets do not follow an order just the reverse of that concluded by laplace. professor newcomb holds the opinion that the rings which eventually constituted the planets divided from the main body of the nebula almost simultaneously, priority, if there were any, being on the side of the inner and smaller ones;[ ] while in m. faye's cosmogony,[ ] the retrograde motion of the systems formed by the two outer planets is ascribed--on grounds, it is true, of dubious validity--to their comparatively late origin. this ingenious scheme was designed, not merely to complete, but to supersede that of laplace, which, undoubtedly, through the inclusion by our system of oppositely directed rotations, forfeits its claim simply and singly to account for the fundamental peculiarities of its structure. m. faye's leading contention is that, under the circumstances assumed by laplace, not the two outer planets alone, but the whole company must have been possessed of retrograde rotation. for they were formed--_ex hypothesi_--after the sun; central condensation had reached an advanced stage when the rings they were derived from separated; the principle of inverse squares consequently held good, and kepler's laws were in full operation. now, particles circulating in obedience to these laws can only--since their velocity decreases outward from the centre of attraction--coalesce into a globe with a _backward_ axial movement. nor was laplace blind to this flaw in his theory; but his effort to remove it, though it passed muster for the best part of a century,[ ] was scarcely successful. his planet-forming rings were made to rotate _all in one piece_, their outer parts thus necessarily travelling at a swifter linear rate than their inner parts, and eventually uniting, equally of necessity, into a _forward_-spinning body. the strength of cohesion involved may, however, safely be called impossible, especially when it is considered that nebulous materials were in question. the reform proposed by m. faye consists in admitting that all the planets inside uranus are of pre-solar origin--that they took globular form in the bosom of a nearly homogeneous nebula, revolving in a single period, with motion accelerated from centre to circumference, and hence agglomerating into masses with a direct rotation. uranus and neptune owe their exceptional characteristics to their later birth. when they came into existence, the development of the sun was already far advanced, central force had acquired virtually its present strength, unity of period had been abolished by its predominance, and motion was retarded outward. thus, what we may call the relative chronology of the solar system is thrown once more into confusion. the order of seniority of the planets is now no easier to determine than the "who first, who last?" among the victims of hector's spear. for m. faye's arrangements, notwithstanding the skill with which he has presented them, cannot be unreservedly accepted. the objections to them, thoughtfully urged by m. c. wolf[ ] and professor darwin,[ ] are grave. not the least so is his omission to take account of an agency of change presently to be noticed. a further valuable discussion of the matter was published by m. du ligondès in .[ ] his views are those of faye, modified to disarm the criticisms they had encountered; and special attention may be claimed for his weighty remark that each planet has a life-history of its own, essentially distinct from those of the others, and, despite original unity, not to be confounded with them. the drift of recent investigations seems, indeed, to be to find the embryonic solar system already potentially complete in the parent nebula, like the oak in an acorn, and to relegate detailed explanations of its peculiarities to the dim pre-nebular fore-time. we now come to a most remarkable investigation--one, indeed, unique in its profession to lead us back with mathematical certainty towards the origin of a heavenly body. we refer to professor darwin's inquiries into the former relations of the earth and moon.[ ] they deal exclusively with the effects of tidal friction, and primarily with those resulting, not from oceanic, but from "bodily" tides, such as the sun and moon must have raised in past ages on a liquid or viscous earth. the immediate effect of either is, as already explained, to destroy the rotation of the body on which the tide is raised, as regards the tide-raising body, bringing it to turn always the same face towards its disturber. this, we can see, has been completely brought about in the case of the moon. there is, however, a secondary or reactive effect. action is always mutual. precisely as much as the moon pulls the terrestrial tidal wave backward, the tidal wave pulls the moon forward. but pulling a body forward in its orbit implies the enlargement of that orbit; in other words, the moon is, as a consequence of tidal friction, very slowly receding from the earth. this will go on (other circumstances remaining unchanged) until the lengthening day overtakes the more tardily lengthening month, when each will be of about , hours.[ ] a position of what we may call tidal equilibrium between earth and moon will (apart from disturbance by other bodies) then be attained. if, however, it be true that, in the time to come, the moon will be much farther from us, it follows that in the time past she was much nearer to us than she now is. tracing back her history by the aid of professor darwin's clue, we at length find her revolving in a period of somewhere between two and four hours, almost in contact with an earth rotating just at the same rate. this was before tidal friction had begun its work of grinding down axial velocity and expanding orbital range. but the position was not one of stable equilibrium. the slightest inequality must have set on foot a series of uncompensated changes. if the moon had whirled the least iota faster than the earth spun she must have been precipitated upon it. her actual existence shows that the trembling balance inclined the other way. by a second or two to begin with, the month exceeded the day; the tidal wave crept ahead of the moon; tidal friction came into play, and our satellite started on its long spiral journey outward from the parent globe. this must have occurred, it is computed, _at least_ fifty-four million years ago. that this kind of tidal reactive effect played its part in bringing the moon into its present position, and is still, to some slight extent, at work in changing it, there can be no doubt whatever. an irresistible conjecture carried the explorer of its rigidly deducible consequences one step beyond them. the moon's time of revolution, when so near the earth as barely to escape contact with it, must have been, by kepler's law, more than two and less than two and a half hours. now it happens that the most rapid rate of rotation of a fluid mass of the earth's average density, consistent with spheroidal equilibrium, is two hours and twenty minutes. quicken the movement but by one second and the globe must fly asunder. hence the inference that the earth actually _did_ fly asunder through over-fast spinning, the ensuing disruption representing the birth-throes of the moon. it is likely that the event was hastened or helped by solar tidal disturbance. to recapitulate. analysis tracks backward the two bodies until it leaves them in very close contiguity, one rotating and the other revolving in approximately the same time, and that time certainly not far different from, and quite possibly identical with, the critical period of instability for the terrestrial spheroid. "is this," professor darwin asks, "a mere coincidence, or does it not rather point to the break-up of the primeval planet into two masses in consequence of a too rapid rotation?"[ ] we are tempted, but are not allowed to give an unqualified assent. mr. james nolan of victoria has made it clear that the moon could not have subsisted as a continuous mass under the powerful disruptive strain which would have acted upon it when revolving almost in contact with the present surface of the earth; and professor darwin, admitting the objection, concedes to our satellite, in its initial stage, the alternative form of a flock of meteorites.[ ] but such a congregation must have been quickly dispersed, by tidal action, into a meteoric ring. the same investigator subsequently fixed , miles from centre to centre as the minimum distance at which the moon could have revolved in its entirety; and he concluded it "necessary to suppose that, after the birth of a satellite, if it takes place at all in this way, a series of changes occur which are quite unknown."[ ] the evidence, however, for the efficiency of tidal friction in bringing about the actual configuration of the lunar-terrestrial system is not invalidated by this failure to penetrate its natal mystery. under its influence the principal elements of that system fall into interdependent mutual relations. it connects, casually and quantitatively, the periods of the moon's revolution and of the earth's rotation, the obliquity of the ecliptic, the inclination and eccentricity of the lunar orbit. all this can scarcely be accidental. professor darwin's first researches on this subject were communicated to the royal society, december , . they were followed, january , ,[ ] by an inquiry on the same principles into the earlier condition of the entire solar system. the results were a warning against hasty generalisation. they showed that the lunar-terrestrial system, far from being a pattern for their development, was a singular exception among the bodies swayed by the sun. its peculiarity resides in the fact that the moon is _proportionately_ by far the most massive attendant upon any known planet. its disturbing power over its primary is thus abnormally great, and tidal friction has, in consequence, played a predominant part in bringing their mutual relations into their present state. the comparatively late birth of the moon tends to ratify this inference. the dimensions of the earth did not differ (according to our present authority) very greatly from what they now are when her solitary offspring came, somehow, into existence. this is found not to have been the case with any other of the planets. it is unlikely that the satellites of jupiter, saturn, or mars (we may safely add, of uranus or neptune) ever revolved in much narrower orbits than those they now traverse; it is practically certain that they did not, like our moon, originate very near the _present_ surfaces of their primaries.[ ] what follows? the tide-raising power of a body grows with vicinity in a rapidly accelerated ratio. lunar tides must then have been on an enormous scale when the moon swung round at a fraction of its actual distance from the earth. but no other satellite with which we are acquainted occupied at any time a corresponding position. hence no other satellite ever possessed tide-raising capabilities in the least comparable to those of the moon. we conclude once more that tidal friction had an influence here very different from its influence elsewhere. quite possibly, however, that influence may be more nearly spent than in less advanced combinations of revolving globes. mr. nolan concluded in [ ] that it still retains appreciable efficacy in the several domains of the outer planets. the moons of jupiter and saturn are, by his calculations, in course of sensible retreat, under compulsion of the perennial ripples raised by them on the surfaces of their gigantic primaries. he thus connects the interior position of the fifth jovian satellite with its small mass. the feebleness of its tide-raising power obliged it to remain behind its companions; for there is no sign of its being more juvenile than the galilean quartette. the yielding of plastic bodies to the strain of unequal attractions is a phenomenon of far-reaching consequence. we know that the sun as well as the moon causes tides in our oceans. there must, then, be solar, no less than lunar, tidal friction. the question at once arises: what part has it played in the development of the solar system? has it ever been one of leading importance, or has its influence always been, as it now is, subordinate, almost negligible? to this, too, professor darwin supplies an answer. it can be stated without hesitation that the sun did _not_ give birth to the planets, as the earth has been supposed to have given birth to the moon, by the disruption of its already condensed, though viscous and glowing mass, pushing them then gradually backward from its surface into their present places. for the utmost possible increase in the length of the year through tidal friction is one hour; and five minutes is a more probable estimate.[ ] so far as the pull of tide-waves raised on the sun by the planets is concerned, then, the distances of the latter have never been notably different from what they now are; though that cause may have converted the paths traversed by them from circles into ellipses. over their _physical_ history, however, it was probably in a large measure influential. the first vital issue for each of them was--satellites or no satellites? were they to be governors as well as governed, or should they revolve in sterile isolation throughout the æons of their future existence? here there is strong reason to believe that solar tidal friction was the overruling power. it is remarkable that planetary fecundity increases--at least so far outward as saturn--with distance from the sun. can these two facts be in any way related? in other words, is there any conceivable way by which tidal influence could prevent or impede the throwingoff of secondary bodies? we have only to think for a moment in order to see that this is precisely one of its direct results.[ ] tidal friction, whether solar or lunar, tends to reduce the axial movement of the body it acts upon. but the separation of satellites depends--according to the received view--upon the attainment of a disruptive rate of rotation. hence, if solar tidal friction were strong enough to keep down the pace below this critical point, the contracting mass would remain intact--there would be no satellite-production. this, in all probability, actually occurred in the case both of mercury and venus. they cooled without dividing, because the solar friction-brake applied to them was too strong to permit acceleration to pass the limit of equilibrium. the complete destruction of their relative axial movement has been rendered probable by recent observations; and that the process went on rapidly is a reasonable further inference. the earth barely escaped the fate of loneliness incurred by her neighbours. her first and only epoch of instability was retarded until she had nearly reached maturity. the late appearance of the moon accounts for its large relative size--through the increased cohesion of an already strongly condensed parent mass--and for the distinctive peculiarities of its history and influence on the producing globe. solar tidal friction, although it did not hinder the formation of two minute dependents of mars, has been invoked to explain the anomalously rapid revolution of one of them. phobos, we have seen, completes more than three revolutions while mars rotates once. but this was probably not always so. the two periods were originally nearly equal. the difference, it is alleged, was brought about by tidal waves raised by the sun on the semi-fluid spheroid of mars. rotatory velocity was thereby destroyed, the martian day slowly lengthened, and, as a secondary consequence, the period of the inner satellite, become shorter than the augmented day, began progressively to diminish. so that phobos, unlike our moon, was in the beginning farther from its primary than now. but here again mr. nolan entered a _caveat_. applying the simple test of numerical evaluation, he showed that before solar tidal friction could lengthen the rotation-period of mars by so much as one minute, phobos should have been precipitated upon its surface.[ ] for the enormous disparity of mass between it and the sun is so far neutralised by the enormous disparity in their respective distances from mars that solar tidal force there is only fifty times that of the little satellite. but the tidal effects of a satellite circulating quicker than its primary rotates exactly reverse those of one moving, like our moon, comparatively slowly, so that the tides raised by phobos tend to _shorten_ both periods. its orbital momentum, however, is so extremely small in proportion to the rotational momentum of mars, that any perceptible inroad upon the latter is attended by a lavish and ruinous expenditure of the former. it is as if a man owning a single five-pound note were to play for equal stakes with a man possessing a million. the bankruptcy sure to ensue is typified by the coming fate of the martian inner satellite. the catastrophe of its fall needs to bring it about only a very feeble reactive pull compared with the friction which the sun should apply in order to protract the martian day by one minute. and from the proportionate strength of the forces at work, it is quite certain that one result cannot take place without the other. nor can things have been materially different in the past; hence the idea must be abandoned that the primitive time of rotation of mars survives in the period of its inner satellite. the anomalous shortness of the latter may, however, in m. wolf's opinion,[ ] be explained by the "traînées elliptiques" with which roche supplemented nebular annulation.[ ] these are traced back to the descent of separating strata from the _shoulders_ of the great nebulous spheroid towards its equatorial plane. their rotational velocity being thus relatively small, they formed "inner rings," very much nearer to the centre of condensation than would have been possible on the unmodified theory of laplace. phobos might, in this view, be called a polar offset of mars; and the rings of saturn are thought to own a similar origin. outside the orbit of mars, solar tidal friction can scarcely be said to possess at present any sensible power. but it is far from certain that this was always so. it seems not unlikely that its influence was the overruling one in determining the direction of planetary rotation. m. faye, as we have seen, objected to laplace's scheme that only retrograde secondary systems could be produced by it. in this he was anticipated by kirkwood, who, however, supplied an answer to his own objection.[ ] sun-raised tides must have acted with great power on the diffused masses of the embryo planets. by their means they doubtless very soon came to turn (in lunar fashion) the same hemisphere always towards their centre of motion. this amounts to saying that even if they started with retrograde rotation, it was, by solar tidal friction, quickly rendered direct.[ ] for it is scarcely necessary to point out that a planet turning an invariable face to the sun rotates in the same direction in which it revolves, and in the same period. as, with the progress of condensation, tides became feebler and rotation more rapid, the accelerated spinning necessarily proceeded in the sense thus prescribed for it. hence the backward axial movements of uranus and neptune may very well be a survival, due to the inefficiency of solar tides at their great distance, of a state of things originally prevailing universally throughout the system. the general outcome of mr. darwin's researches has been to leave laplace's cosmogony untouched. he concludes nothing against it, and, what perhaps tells with more weight in the long run, has nothing to substitute for it. in one form or the other, if we speculate at all on the development of the planetary system, our speculations are driven into conformity with the broad lines of the nebular hypothesis--to the extent, at least, of admitting an original material unity and motive uniformity. but we can see now, better than formerly, that these supply a bare and imperfect sketch of the truth. we should err gravely were we to suppose it possible to reconstruct, with the help of any knowledge our race is ever likely to possess, the real and complete history of our admirable system. "the subtlety of nature," bacon says, "transcends in many ways the subtlety of the intellect and senses of man." by no mere barren formula of evolution, indiscriminately applied all round, the results we marvel at, and by a fragment of which our life is conditioned, were brought forth; but by the manifold play of interacting forces, variously modified and variously prevailing, according to the local requirements of the design they were appointed to execute. footnotes: [footnote : _exposition du système du monde_, t. ii., p. .] [footnote : in later editions a retrospective clause was added admitting a prior condition of all but evanescent nebulosity.] [footnote : _méc. cél._, lib. xiv., ch. iii.] [footnote : _beiträge zur dynamik des himmels_, p. .] [footnote : _trans. roy. soc. of edinburgh_, vol. xxi., p. .] [footnote : newcomb, _pop. astr._, p. ( nd ed.).] [footnote : m. williams, _nature_, vol. iii., p. .] [footnote : _comp. brit. almanac_, p. .] [footnote : radau, _bull. astr._, t. ii., p. .] [footnote : newcomb, _pop. astr._, pp. - .] [footnote : _proc. roy. soc._, vol. xxxiii., p. .] [footnote : to this hostile argument, as urged by mr. e. douglas archibald, sir w. siemens opposed the increase of rotative velocity through contraction (_nature_, vol. xxv., p. ). but contraction cannot restore lost momentum.] [footnote : _stellar evolution, and its relations to geological time_, .] [footnote : _comptes rendus_, t. lii., p. . see also kirkwood, _observatory_, vol. iii., p. .] [footnote : fouché, _comptes rendus_, t. xcix., p. .] [footnote : _astroph. jour._, vol. xiii., p. .] [footnote : _month. not._, vol. xxix., p. .] [footnote : _pop. astr._, p. .] [footnote : _sur l'origine du monde_, .] [footnote : kirkwood adverted to it in , _am. jour._, vol. xxxviii., p. .] [footnote : _bull. astr._, t. ii.] [footnote : _nature_, vol. xxxi., p. .] [footnote : _formation mécanique du système du monde; bull. astr._, t. xiv., p. (o. callandreau). see also, _le problème solaire_, by l'abbé th. moreux, .] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : mr. j. nolan has pointed out (_nature_, vol. xxxiv., p. ) that the length of the equal day and month will be reduced to about , hours by the effects of _solar_ tidal friction.] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : _nature_, vol. xxxiii., p. ; see also nolan, _ibid._, vol. xxxiv., p. .] [footnote : _phil. trans._, vol. clxxviii., p. .] [footnote : _ibid._, vol. clxxii., p. .] [footnote : _ibid._, p. .] [footnote : _satellite evolution_, melbourne, ; _knowledge_, vol. xviii., p. .] [footnote : _phil. trans._, vol. clxxii., p. .] [footnote : this was perceived by m. ed. roche in . _mém. de l'acad. des sciences de montpellier_, t. viii., p. .] [footnote : _nature_, vol. xxxiv., p. .] [footnote : _bull. astr._, t. ii., p. .] [footnote : _montpellier méms._, t. viii., p. .] [footnote : _amer. jour._, vol. xxxviii. ( ), p. .] [footnote : wolf, _bull. astr._, t. ii., p. .] chapter x _recent comets_ on the nd of june, , giambattista donati discovered at florence a feeble round nebulosity in the constellation leo, about one-tenth the diameter of the full moon. it proved to be a comet approaching the sun. but it changed little in apparent place or brightness for some weeks. the gradual development of a central condensation of light was the first symptom of coming splendour. at harvard, in the middle of july, a strong stellar nucleus was seen; on august a tail began to be thrown out. as the comet wanted still over six weeks of the time of its perihelion-passage, it was obvious that great things might be expected of it. they did not fail of realisation. not before the early days of september was it generally recognised with the naked eye, though it had been detected without a glass at pulkowa, august . but its growth was thenceforward surprisingly rapid, as it swept with accelerated motion under the hindmost foot of the great bear, and past the starry locks of berenice. a sudden leap upward in lustre was noticed on september , when the nucleus shone with about the brightness of the pole-star, and the tail, notwithstanding large foreshortening, could be traced with the lowest telescopic power over six degrees of the sphere. the appendage, however, attained its full development only after perihelion, september , by which time, too, it lay nearly square to the line of sight from the earth. on october it stretched in a magnificent scimitar-like curve over a third and upwards of the visible hemisphere, representing a real extension in space of fifty-four million miles. but the most striking view was presented on october , when the brilliant star arcturus became involved in the brightest part of the tail, and during many hours contributed, its lustre undiminished by the interposed nebulous screen, to heighten the grandeur of the most majestic celestial object of which living memories retain the impress. donati's comet was, according to admiral smyth's testimony,[ ] outdone "as a mere _sight_-object" by the great comet of ; but what it lacked in splendour, it surely made up in grace, and variety of what we may call "scenic" effects. some of these were no less interesting to the student than impressive to the spectator. at pulkowa, on the th september, winnecke,[ ] the first director of the strasburg observatory, observed a faint outer envelope resembling a veil of almost evanescent texture flung somewhat widely over the head. next evening, the first of the "secondary" tails appeared, possibly as part of the same phenomenon. this was a narrow straight ray, forming a tangent to the strong curve of the primary tail, and reaching to a still greater distance from the nucleus. it continued faintly visible for about three weeks, during part of which time it was seen in duplicate. for from the chief train itself, at a point where its curvature abruptly changed, issued, as if through the rejection of some of its materials, a second beam nearly parallel to the first, the rigid line of which contrasted singularly with the softly diffused and waving aspect of the plume of light from which it sprang. olbers's theory of unequal repulsive forces was never more beautifully illustrated. the triple tail seemed a visible solar analysis of cometary matter. the processes of luminous emanation going on in this body forcibly recalled the observations made on the comets of and . from the middle of september, the nucleus, estimated by bond to be under five hundred miles in diameter, was the centre of action of the most energetic kind. seven distinct "envelopes" were detached in succession from the nebulosity surrounding the head, and after rising towards the sun during periods of from four to seven days, finally cast their material backward to form the right and left branches of the great train. the separation of these by an obscure axis--apparently as black, quite close up to the nucleus, as the sky--indicated for the tail a hollow, cone-like structure;[ ] while the repetition of certain spots and rays in the same corresponding situation on one envelope after another served to show that the nucleus--to some local peculiarity of which they were doubtless due--had no proper rotation, but merely shifted sufficiently on an axis to preserve the same aspect towards the sun as it moved round it.[ ] this observation of bond's was strongly confirmatory of bessel's hypothesis of opposite polarities in such bodies' opposite sides. the protrusion towards the sun, on september , of a brilliant luminous fan-shaped sector completed the resemblance to halley's comet. the appearance of the head was now somewhat that of a "bat's-wing" gaslight. there were, however, no oscillations to and fro, such as bessel had seen and speculated upon in . as the size of the nucleus contracted with approach to perihelion, its intensity augmented. on october , it outshone arcturus, and for a week or ten days was a conspicuous object half an hour after sunset. its lustre--setting aside the light derived from the tail--was, at that date, , times what it had been on june , though _theoretically_--taking into account, that is, only the differences of distance from sun and earth--it should have been only / of that amount. here, it might be thought, was convincing evidence of the comet itself becoming ignited under the growing intensity of the solar radiations. yet experiments with the polariscope were interpreted in an adverse sense, and bond's conclusion that the comet sent us virtually unmixed reflected sunshine was generally acquiesced in. it was, nevertheless, negatived by the first application of the spectroscope to these bodies. very few comets have been so well or so long observed as donati's. it was visible to the naked eye during days; it was telescopically discernible for , the last observation having been made by mr. william mann at the cape of good hope, march , . its course through the heavens combined singularly with the orbital place of the earth to favour curious inspection. the tail, when near its greatest development, lost next to nothing by the effects of perspective, and at the same time lay in a plane sufficiently inclined to the line of sight to enable it to display its exquisite curves to the greatest advantage. even the weather was, on both sides of the atlantic, propitious during the period of greatest interest, and the moon as little troublesome as possible. the volume compiled by the younger bond is a monument to the care and skill with which these advantages were turned to account. yet this stately apparition marked no turning-point in the history of cometary science. by its study knowledge was indeed materially advanced, but along the old lines. no quick and vivid illumination broke upon its path. quite insignificant objects--as we have already partly seen--have often proved more vitally instructive. donati's comet has been identified with no other. its path is an immensely elongated ellipse, lying in a plane far apart from that of the planetary movements, carrying it at perihelion considerably within the orbit of venus, and at aphelion out into space to - / times the distance from the sun of neptune. the entire circuit occupies over , years, and is performed in a retrograde direction, or against the order of the signs. before its next return, about the year a.d., the enigma of its presence and its purpose may have been to some extent--though we may be sure not completely--penetrated. on june , , the earth passed, for the second time in the century, through the tail of a great comet. some of our readers may remember the unexpected disclosure, on the withdrawal of the sun below the horizon on that evening, of an object so remarkable as to challenge universal attention. a golden-yellow planetary disc, wrapt in dense nebulosity, shone out while the june twilight of these latitudes was still in its first strength. the number and complexity of the envelopes surrounding the head produced, according to the late mr. webb,[ ] a magnificent effect. portions of six distinct emanations were traceable. "it was as though a number of light, hazy clouds were floating round a miniature full moon." as the sky darkened the tail emerged to view.[ ] although in brightness and sharpness of definition it could not compete with the display of , its dimensions proved to be extraordinary. it reached upwards beyond the zenith when the head had already set. by some authorities its extreme length was stated at °, and it showed no trace of curvature. most remarkable, however, was the appearance of two widely divergent rays, each pointing towards the head, though cut off from it by sky-illumination, of which one was seen by mr. webb, and both by mr. williams at liverpool, a quarter of an hour before midnight. there seems no doubt that webb's interpretation was the true one, and that these beams were, in fact, "the perspective representation of a conical or cylindrical tail, hanging closely above our heads, and probably just being lifted up out of our atmosphere."[ ] the cometary train was then rapidly receding from the earth, so that the sides of the "outspread fan" of light shown by it when we were right in the line of its axis must have appeared (as they did) to close up in departure. the swiftness with which the visually opened fan shut proved its vicinity; and, indeed, mr. hind's calculations showed that we were not so much near as actually within its folds at that very time. already m. liais, from his observations at rio de janeiro, june to , and mr. tebbutt, by whom the comet was discovered in new south wales on may , had anticipated such an encounter, while the former subsequently proved that it must have occurred in such a way as to cause an immersion of the earth in cometary matter to a depth of , miles.[ ] the comet then lay between the earth and the sun at a distance of about fourteen million miles from the former; its tail stretched outward just along the line of intersection of its own with the terrestrial orbit to an extent of fifteen million miles; so that our globe, happening to pass at the time, found itself during some hours involved in the flimsy appendage. no perceptible effects were produced by the meeting; it was known to have occurred by theory alone. a peculiar glare in the sky, thought by some to have distinguished the evening of june , was, at best, inconspicuous. nor were there any symptoms of unusual electric excitement. the greenwich instruments were, indeed, disturbed on the following night, but it would be rash to infer that the comet had art or part in their agitation. the perihelion-passage of this body occurred june , ; and its orbit has been shown by m. kreutz of bonn, from a very complete investigation founded on observations extending over nearly a year, to be an ellipse traversed in a period of years.[ ] towards the end of august, , a comet became visible to the naked eye high up in the northern hemisphere, with a nucleus equalling in brightness the lesser stars of the plough and a feeble tail ° in length. it thus occupied quite a secondary position among the members of its class. it was, nevertheless, a splendid object in comparison with a telescopic nebulosity discovered by tempel at marseilles, december , . this, the sole comet of , slipped past perihelion, january , without pomp of train or other appendages, and might have seemed hardly worth the trouble of pursuing. fortunately, this was not the view entertained by observers and computers; since upon the knowledge acquired of the movements of these two bodies has been founded one of the most significant discoveries of modern times. the first of them is now styled the comet ( iii.) of the august meteors, the second ( i.) that of the november meteors. the steps by which this curious connection came to be ascertained were many, and were taken in succession by a number of individuals. but the final result was reached by schiaparelli of milan, and remains deservedly associated with his name. the idea prevalent in the eighteenth century as to the nature of shooting stars was that they were mere aerial _ignes fatui_--inflammable vapours accidentally kindled in our atmosphere. but halley had already entertained the opinion of their cosmical origin; and chladni in formally broached the theory that space is filled with minute circulating atoms, which, drawn by the earth's attraction, and ignited by friction in its gaseous envelope, produce the luminous effects so frequently witnessed.[ ] acting on his suggestion, brandes and benzenberg, two students at the university of göttingen, began in to determine the heights of falling stars by simultaneous observations at a distance. they soon found that they move with planetary velocities in the most elevated regions of our atmosphere, and by the ascertainment of this fact laid a foundation of distinct knowledge regarding them. some of the data collected, however, served only to perplex opinion, and even caused chladni temporarily to renounce his. many high authorities, headed by laplace in , declared for the lunar-volcanic origin of meteorites; but thought on the subject was turbid, and inquiry seemed only to stir up the mud of ignorance. it needed one of those amazing spectacles, at which man assists, no longer in abject terror for his own frail fortunes, but with keen curiosity and the vivid expectation of new knowledge, to bring about a clarification. on the night of november - , , a tempest of falling stars broke over the earth. north america bore the brunt of its pelting. from the gulf of mexico to halifax, until daylight with some difficulty put an end to the display, the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. at boston the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. their numbers, while the first fury of their coming lasted, were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much diminished rate, that , must have been visible during the nine hours they continued to fall.[ ] now there was one very remarkable feature common to the innumerable small bodies which traversed, or were consumed in our atmosphere that night. _they all seemed to come from the same part of the sky._ traced backward, their paths were invariably found to converge to a point in the constellation leo. moreover, that point travelled with the stars in their nightly round. in other words, it was entirely independent of the earth and its rotation. it was a point in inter-planetary space. the _effective_ perception of this fact[ ] amounted to a discovery, as olmsted and twining, who had "simultaneous ideas" on the subject, were the first to realize. denison olmsted was then professor of mathematics in yale college. he showed early in [ ] that the emanation of the showering meteors from a fixed "radiant" proved their approach to the earth along nearly parallel lines, appearing to diverge by an effect of perspective; and that those parallel lines must be sections of orbits described by them round the sun and intersecting that of the earth. for the november phenomenon was now seen to be a periodical one. on the same night of the year , although with less dazzling and universal splendour than in america in , it had been witnessed over great part of europe and in arabia. olmsted accordingly assigned to the cloud of cosmical particles (or "comet," as he chose to call it), by terrestrial encounters with which he supposed the appearances in question to be produced, a period of about days; its path a narrow ellipse, meeting, near its farthest end from the sun, the place occupied by the earth on november . once for all, then, as the result of the star-fall of , the study of luminous meteors became an integral part of astronomy. their membership of the solar system was no longer a theory or a conjecture--it was an established fact. the discovery might be compared to, if it did not transcend in importance, that of the asteroidal group. "c'est un nouveau monde planétaire," arago wrote,[ ] "qui commence à se révéler à nous." evidences of periodicity continued to accumulate. it was remembered that humboldt and bonpland had been the spectators at cumana, after midnight on november , , of a fiery shower little inferior to that of , and reported to have been visible from the equator to greenland. moreover, in and some subsequent years, there were waning repetitions of the display, as if through the gradual thinning-out of the meteoric supply. the extreme irregularity of its distribution was noted by olbers in , who conjectured that we might have to wait until to see the phenomenon renewed on its former scale of magnificence.[ ] this was the first hint of a thirty-three or thirty-four year period. the falling stars of november did not alone attract the attention of the learned. similar appearances were traditionally associated with august by the popular phrase in which they figured as "the tears of st. lawrence." but the association could not be taken on trust from mediæval authority. it had to be proved scientifically, and this quetelet of brussels succeeded in doing in december, .[ ] a second meteoric revolving system was thus shown to exist. but its establishment was at once perceived to be fatal to the "cosmical cloud" hypothesis of olmsted. for if it be a violation of probability to attribute to one such agglomeration a period of an exact year, or sub-multiple of a year, it would be plainly absurd to suppose the movements of _two_ or more regulated by such highly artificial conditions. an alternative was proposed by adolf erman of berlin in .[ ] no longer in _clouds_, but in closed _rings_, he supposed meteoric matter to revolve round the sun. thus the mere circumstance of intersection by a meteoric of the terrestrial orbit, without any coincidence of period, would account for the earth meeting some members of the system at each annual passage through the "node" or point of intersection. this was an important step in advance, yet it decided nothing as to the forms of the orbits of such annular assemblages; nor was it followed up in any direction for a quarter of a century. hubert a. newton took up, in ,[ ] the dropped thread of inquiry. the son of a mathematical mother, he attained, at the age of twenty-five, to the dignity of professor of mathematics in yale university, and occupied the post until his death in . the diversion of his powers, however, from purely abstract studies stimulated their effective exercise, and constituted him one of the founders of meteoric astronomy. a search through old records carried the november phenomenon back to the year a.d., long distinguished as "the year of the stars." for in the same night in which taormina was captured by the saracens, and the cruel aghlabite tyrant ibrahim ibn ahmed died "by the judgment of god" before cosenza, stars fell from heaven in such abundance as to amaze and terrify beholders far and near. this was on october , and recurrences were traced down through the subsequent centuries, always with a day's delay in about seventy years. it was easy, too, to derive from the dates a cycle of - / years, so that professor newton did not hesitate to predict the exhibition of an unusually striking meteoric spectacle on november - , .[ ] for the astronomical explanation of the phenomena, recourse was had to a method introduced by erman of computing meteoric orbits. it was found, however, that conspicuous recurrences every thirty-three or thirty-four years could be explained on the supposition of five widely different periods, combined with varying degrees of extension in the revolving group. professor newton himself gave the preference to the shortest--of - / days, but indicated the means of deciding with certainty upon the true one. it was furnished by the advancing motion of the node, or that day's delay of the november shower every seventy years, which the old chronicles had supplied data for detecting. for this is a strictly measurable effect of gravitational disturbance by the various planets, the amount of which naturally depends upon the course pursued by the disturbed bodies. here the great mathematical resources of professor adams were brought to bear. by laborious processes of calculation, he ascertained that four out of newton's five possible periods were entirely incompatible with the observed nodal displacement, while for the fifth--that of - / years--a perfectly harmonious result was obtained.[ ] this was the last link in the chain of evidence proving that the november meteors--or "leonids," as they had by that time come to be called--revolve round the sun in a period of · years, in an ellipse spanning the vast gulf between the orbits of the earth and uranus, the group being so extended as to occupy nearly three years in defiling past the scene of terrestrial encounters. but before it was completed in march, , the subject had assumed a new aspect and importance. professor newton's prediction of a remarkable star-shower in november, , was punctually fulfilled. this time, europe served as the main target of the celestial projectiles, and observers were numerous and forewarned. the display, although, according to mr. baxendell's memory,[ ] inferior to that of , was of extraordinary impressiveness. dense crowds of meteors, equal in lustre to the brightest stars, and some rivalling venus at her best,[ ] darted from east to west across the sky with enormous apparent velocities, and with a certain determinateness of aim, as if let fly with a purpose, and at some definite object.[ ] nearly all left behind them trains of emerald green or clear blue light, which occasionally lasted many minutes, before they shrivelled and curled up out of sight. the maximum rush occurred a little after one o'clock on the morning of november , when attempts to count were overpowered by frequency. but during a previous interval of seven minutes five seconds, four observers at mr. bishop's observatory at twickenham reckoned , and during an hour , .[ ] before daylight the earth had fairly cut her way through the star-bearing stratum; the "ethereal rockets" had ceased to fly. this event brought the subject of shooting stars once more vividly to the notice of astronomers. schiaparelli had, indeed, been already attracted by it. the results of his studies were made known in four remarkable letters, addressed, before the close of the year , to father secchi, and published in the _bulletino_ of the roman observatory.[ ] their upshot was to show, in the first place, that meteors possess a real velocity considerably greater than that of the earth, and travel, accordingly, to enormously greater distances from the sun along tracks resembling those of comets in being very eccentric, in lying at all levels indifferently, and in being pursued in either direction. it was next inferred that comets and meteors equally have an origin foreign to the solar system, but are drawn into it temporarily by the sun's attraction, and occasionally fixed in it by the backward pull of some planet. but the crowning fact was reserved for the last. it was the astonishing one that the august meteors move in the same orbit with the bright comet of --that the comet, in fact, is but a larger member of the family named "perseids" because their radiant point is situated in the constellation perseus. this discovery was quickly capped by others of the same kind. leverrier published, january , ,[ ] elements for the november swarm, founded on the most recent and authentic observations; at once identified by dr. c. f. w. peters of altona with oppolzer's elements for tempel's comet of .[ ] a few days later, schiaparelli, having recalculated the orbit of the meteors from improved data, arrived at the same conclusion; while professor weiss of vienna pointed to the agreement between the orbits of a comet which had appeared in and of a star-shower found to recur on april (lyraïds), as well as between those of biela's comet and certain conspicuous meteors of november .[ ] these instances do not seem to be exceptional. the number of known or suspected accordances of cometary tracks with meteor streams contained in a list drawn up in [ ] by professor alexander s. herschel (who has made the subject peculiarly his own) amounts to seventy-six; although the four first detected still remain the most conspicuous, and perhaps the only absolutely sure examples of a relation as significant as it was, to most astronomers, unexpected. there had, indeed, been anticipatory ideas. not that kepler's comparison of shooting stars to "minute comets," or maskelyne's "forse risulterà che essi sono comete," in a letter to the abate cesaris, december , ,[ ] need count for much. but chladni, in ,[ ] considered both to be fragments or particles of the same primitive matter, irregularly scattered through space as nebulæ; and morstadt of prague suggested about [ ] that the meteors of november might be dispersed atoms from the tail of biela's comet, the path of which is cut across by the earth near that epoch. professor kirkwood, however, by a luminous intuition, penetrated the whole secret, so far as it has yet been made known. in an article published, or rather buried, in the _danville quarterly review_ for december, , he argued, from the observed division of biela, and other less noted instances of the same kind, that the sun exercises a "divellent influence" on the nuclei of comets, which may be presumed to continue its action until their corporate existence (so to speak) ends in complete pulverisation. "may not," he continued, "our periodic meteors be the débris of ancient but now disintegrated comets, whose matter has become distributed round their orbits?"[ ] the gist of schiaparelli's discovery could not be more clearly conveyed. for it must be borne in mind that with the ultimate destiny of comets' tails this had nothing to do. the tenuous matter composing them is, no doubt, permanently lost to the body from which it emanated; but science does not pretend to track its further wanderings through space. it can, however, state categorically that these will no longer be conducted along the paths forsaken under solar compulsion. from the central, and probably solid parts of comets, on the other hand, are derived the granules by the swift passage of which our skies are seamed with periodic fires. it is certain that a loosely agglomerated mass (such as cometary nuclei most likely are) must gradually separate through the unequal action of gravity on its various parts--through, in short, solar tidal influence. thenceforward its fragments will revolve independently in parallel orbits, at first as a swarm, finally--when time has been given for the full effects of the lagging of the slower moving particles to develop--as a closed ring. the first condition is still, more or less, that of the november meteors; those of august have already arrived at the second. for this reason, leverrier pronounced, in , the perseid to be of older formation than the leonid system. he even assigned a date at which the introduction of the last-named bodies into their present orbit was probably effected through the influence of uranus. in a.d. a close approach must have taken place between the planet and the parent comet of the november stars, after which its regular returns to perihelion, and the consequent process of its disintegration, set in. though not complete, it is already far advanced. the view that meteorites are the dust of decaying comets was now to be put to a definite test of prediction. biela's comet had not been seen since its duplicate return in . yet it had been carefully watched for with the best telescopes; its path was accurately known; every perturbation it could suffer was scrupulously taken into account. under these circumstances, its repeated failure to come up to time might fairly be thought to imply a cessation from visible existence. might it not, however, be possible that it would appear under another form--that a star-shower might have sprung from and would commemorate its dissolution? an unusually large number of falling stars were seen by brandes, december , . similar displays were noticed in the years , , and , and the point from which they emanated was shown by heis at aix-la-chapelle to be situated near the bright star gamma andromedæ.[ ] now this is precisely the direction in which the orbit of biela's comet would seem to lie, as it runs down to cut the terrestrial track very near the place of the earth at the above dates. the inference was, then, an easy one, that the meteors were pursuing the same path with the comet; and it was separately arrived at, early in , by weiss, d'arrest, and galle.[ ] but biela travels in the opposite direction to tempel's comet and its attendant "leonids"; its motion is direct, or from west to east, while theirs is retrograde. consequently, the motion of its node is in the opposite direction too. in other words, the meeting-place of its orbit with that of the earth retreats (and very rapidly) along the ecliptic instead of advancing. so that if the "andromedes" stood in the supposed intimate relation to biela's comet, they might be expected to anticipate the times of their recurrence by as much as a week in half a century. all doubt as to the fact may be said to have been removed by signor zezioli's observation of the annual shower in more than usual abundance at bergamo, november , . the missing comet was next due at perihelion in the year , and the probability was contemplated by both weiss and galle of its being replaced by a copious discharge of falling stars. the precise date of the occurrence was not easily determinable, but galle thought the chances in favour of november . the event anticipated the prediction by twenty-four hours. scarcely had the sun set in western europe on november , when it became evident that biela's comet was shedding over us the pulverised products of its disintegration. the meteors came in volleys from the foot of the chained lady, their numbers at times baffling the attempt to keep a reckoning. at moncalieri, about p.m., they constituted (as father denza said[ ]) a "real rain of fire." four observers counted, on an average, four hundred each minute and a half; and not a few fireballs, equalling the moon in diameter, traversed the sky. on the whole, however, the stars of , though about equally numerous, were less brilliant than those of ; the phosphorescent tracks marking their passage were comparatively evanescent and their movements sluggish. this is easily understood when we remember that the andromedes _overtake_ the earth, while the leonids rush to meet it; the velocity of encounter for the first class of bodies being under twelve, for the second above forty-four miles a second. the spectacle was, nevertheless, magnificent. it presented itself successively to various parts of the earth, from bombay and the mauritius to new brunswick and venezuela, and was most diligently and extensively observed. here it had well-nigh terminated by midnight.[ ] it was attended by a slight aurora, and although tacchini had telegraphed that the state of the sun rendered some show of polar lights probable, it has too often figured as an accompaniment of star-showers to permit the coincidence to rank as fortuitous. admiral wrangel was accustomed to describe how, during the prevalence of an aurora on the siberian coast, the passage of a meteor never failed to extend the luminosity to parts of the sky previously dark;[ ] and an enhancement of electrical disturbance may well be associated with the flittings of such cosmical atoms. a singular incident connected with the meteors of has now to be recounted. the late professor klinkerfues, who had observed them very completely at göttingen, was led to believe that not merely the débris strewn along its path, but the comet itself must have been in immediate proximity to the earth during their appearance.[ ] if so, it might be possible, he thought, to descry it as it retreated in the diametrically opposite direction from that in which it had approached. on november , accordingly, he telegraphed to mr. pogson, the madras astronomer, "biela touched earth november ; search near theta centauri"--the "anti-radiant," as it is called, being situated close to that star. bad weather prohibited observation during thirty-six hours, but when the rain clouds broke on the morning of december , there a comet was, just in the indicated position. in appearance it might have passed well enough for one of the biela twins. it had no tail, but a decided nucleus, and was about seconds across, being thus altogether below the range of naked-eye discernment. it was again observed december , when a short tail was perceptible; but overcast skies supervened, and it has never since been seen. its identity accordingly remains in doubt. it seems tolerably certain, however, that it was _not_ the lost comet, which ought to have passed that spot twelve weeks earlier, and was subject to no conceivable disturbance capable of delaying to that extent its revolution. on the other hand, there is the strongest likelihood that it belonged to the same system[ ]--that it was a third fragment, torn from the parent-body of the andromedes at a period anterior to our first observations of it. in thirteen years biela's comet (or its relics) travels nearly twice round its orbit, so that a renewal of the meteoric shower of was looked for on the same day of the year , the probability being emphasised by an admonitory circular from dunecht. astronomers were accordingly on the alert, and were not disappointed. in england, observation was partially impeded by clouds; but at malta, palermo, beyrout, and other southern stations, the scene was most striking. the meteors were both larger and more numerous than in . their numbers in the densest part of the drift were estimated by professor newton at , per hour, visible from one spot to so large a group of spectators that practically none could be missed. yet each of these multitudinous little bodies was found by him to travel in a clear cubical space of which the edge measured twenty miles![ ] thus the dazzling effect of a luminous throng was produced without jostling or overcrowding, by particles, it might almost be said, isolated in the void. their aspect was strongly characteristic of the andromede family of meteors. "they invariably," mr. denning wrote,[ ] "traversed short paths with very slow motions, and became extinct in evolved streams of yellowish sparks." the conclusion seemed obvious "that these meteors are formed of very soft materials, which expand while incalescent, and are immediately crumbled and dissipated into exiguous dust." the biela meteors of did not merely gratify astronomers with a fulfilled prediction, but were the means of communicating to them some valuable information. although their main body was cut through by the moving earth in six hours, and was not more than , miles across, skirmishers were thrown out to nearly a million miles on either side of the compact central battalions. members of the system were, on the th of november, recorded by mr. denning at the hourly rate of about ; and they did not wholly cease to be visible until december . they afforded besides a particularly well-marked example of that diffuseness of radiation previously observed in some less conspicuous displays. their paths seemed to diverge from an area rather than from a point in the sky. they came so ill to focus that divergences of several degrees were found between the most authentically determined radiants. these incongruities are attributed by professor newton to the irregular shape of the meteoroids producing unsymmetrical resistance from the air, and hence causing them to glance from their original direction on entering it. thus, their luminous tracks did not always represent (even apart from the effects of the earth's attraction) the true prolongation of their course through space. the andromedes of were laggards behind the comet from which they sprang; those of were its avant-couriers. that wasted and disrupted body was not due at the node until january , , sixty days, that is, after the earth's encounter with its meteoric fragments. these are now probably scattered over more than five hundred million miles of its orbits;[ ] yet professor newton considers that all must have formed one compact group with biela at the time of its close approach to jupiter about the middle of . for otherwise both comet and meteorites could not have experienced, as they seem to have done, the same kind and amount of disturbance. the rapidity of cometary disintegration is thus curiously illustrated. a short-lived persuasion that the missing heavenly body itself had been recovered, was created by mr. edwin holms's discovery, at london, november , , of a tolerably bright, tailless comet, just in a spot which biela's comet must have traversed in approaching the intersection of its orbit with that of the earth. a hasty calculation by berberich assigned elements to the newcomer seeming not only to ratify the identity, but to promise a quasi-encounter with the earth on november . the only effect of the prediction, however, was to raise a panic among the negroes of the southern states of america. the comet quietly ignored it, and moved away from instead of towards the appointed meeting-place. its projection, then, on the night of its discovery, upon a point of the biela-orbit was by a mere caprice of chance. north america, nevertheless, was visited on november by a genuine andromede shower. although the meteors were less numerous than in , professor young estimated that , , at the least, of their orange fire-streaks came, during five hours, within the range of view at princeton.[ ] brédikhine estimated the width of the space containing them at about , , miles.[ ] the anticipation of their due time by four days implied--if they were a prolongation of the main biela group, the nucleus of which passed the spot of encounter five months previously--a recession of the node since by no less than three degrees. unless, indeed, mr. denning were right in supposing the display to have proceeded from "an associated branch of the main swarm through which we passed in and ."[ ] the existence of separated detachments of biela meteors, due to disturbing planetary action, was contemplated as highly probable by schiaparelli.[ ] such may have been the belated flights met with in , , , and , and such the advance flight plunged through in . a shower looked for november , , did not fall, and no further display from this quarter is probable until november , , although one is possible a year earlier.[ ] the leonids, through the adverse influence of jupiter and saturn, inflicted upon multitudes of eager watchers a still more poignant disappointment. a dense part of the swarm, having nearly completed a revolution since , should, travelling normally, have met the earth november , ; in point of fact, it swerved sunward, and the millions of meteorites which would otherwise have been sacrificed for the illumination of our skies escaped a fiery doom. the contingency had been forecast in the able calculations of dr. johnstone stoney and dr. a. m. w. downing,[ ] superintendent of the nautical almanac office; but the verification scarcely compensated the failure. nor was the situation retrieved in the following years. only ragged fringes of the great tempest-cloud here and there touched our globe. as the same investigators warned us to expect, the course of the meteorites had been not only rendered sinuous by perturbation, but also broken and irregular. we can no longer count upon the leonids. their glory, for scenic purposes, is departed. the comet associated with them also evaded observation. although it doubtless kept its tryst with the sun in the spring of , the attendant circumstances were too unfavourable to allow it to be seen from the earth.[ ] by an almost fantastic coincidence, nevertheless, a faint comet was photographed, november , ,[ ] by dr. chase, of the yale college observatory, close to the leonid radiant, whither a "meteorograph" was directed with a view to recording trails left by precursors of the main leonid body. a promising start, too, was made on the same occasion with meteoric researches from sensitive plates.[ ] indeed, schaeberle and colton[ ] had already, in , determined the height of a leonid by means of photographs taken at stations on different ridges of mount hamilton; and professor pickering has prosecuted similar work at harvard, with encouraging results. everything in this branch of science depends upon how far they can be carried. without the meteorograph, rigid accuracy in the observation of shooting stars is unattainable, and rigid accuracy is the _sine quâ non_ for obtaining exact knowledge. biela does not offer the only example of cometary disruption. setting aside the unauthentic reports of early chroniclers, we meet the "double comet" discovered by liais at olinda (brazil), february , , of which the division appeared recent, and about to be carried farther.[ ] but a division once established, separation must continually progress. the periodic times of the fragments will never be identical; one must drop a little behind the other at each revolution, until at length they come to travel in remote parts of nearly the same orbit. thus the comet predicted by klinkerfues and discovered by pogson had already lagged to the extent of twelve weeks, and we shall meet instances farther on where the retardation is counted, not by weeks, but by years. here original identity emerges only from calculation and comparison of orbits. comets, then, die, as kepler wrote long ago, _sicut bombyces filo fundendo_. this certainty, anticipated by kirkwood in , we have at least acquired from the discovery of their generative connection with meteors. nay, their actual materials become, in smaller or larger proportions, incorporated with our globe. it is not, indeed, universally admitted that the ponderous masses of which, according to daubrée's estimate,[ ] at least fall annually from space upon the earth, ever formed part of the bodies known to us as comets. some follow tschermak in attributing to aerolites a totally different origin from that of periodical shooting-stars. that no clear line of demarcation can be drawn is no valid reason for asserting that no real distinction exists; and it is certainly remarkable that a meteoric fusillade may be kept up for hours without a single solid projectile reaching its destination. it would seem as if the celestial army had been supplied with blank cartridges. yet, since a few detonating meteors have been found to proceed from ascertained radiants of shooting-stars, it is difficult to suppose that any generic difference separates them. their assimilation is further urged--though not with any demonstrative force--by two instances, the only two on record, of the tangible descent of an aerolite during the progress of a star-shower. on april , , the saxon chronicle informs us that stars fell "so thickly that no man could count them," and adds that one of them having struck the ground in france, a bystander "cast water upon it, which was raised in steam with a great noise of boiling."[ ] and again, on november , , while the skirts of the andromede-tempest were trailing over mexico, "a ball of fire" was precipitated from the sky at mazapil, within view of a ranchman.[ ] scientific examination proved it to be a "siderite," or mass of "nickel-iron"; its weight exceeded eight pounds, and it contained many nodules of graphite. we are not, however, authorised by the circumstances of its arrival to regard the mazapil fragment of cosmic metal as a specimen torn from biela's comet. in this, as in the preceding case, the coincidence of the fall with the shower may have been purely casual, since no hint is given of any sort of agreement between the tracks followed by the sample provided for curious study, and the swarming meteors consumed in the upper air. professor newton's inquiries into the tracks pursued by meteorites previous to their collisions with the earth tend to distinguish them, at least specifically, from shooting-stars. he found that nearly all had been travelling with a direct movement in orbits the perihelia of which lay in the outer half of the space separating the earth from the sun.[ ] shooting-stars, on the contrary, are entirely exempt from such limitations. the yale professor concluded "that the larger meteorites moving in our solar system are allied much more closely with the group of comets of short period than with the comets whose orbits are nearly parabolic." they would thus seem to be more at home than might have been expected amid the planetary family. father carbonelle has, moreover, shown[ ] that meteorites, if explosion-products of the earth or moon, should, with rare exceptions, follow just the kind of paths assigned to them, from data of observation, by professor newton. yet it is altogether improbable that projectiles from terrestrial volcanoes should, at any geological epoch, have received impulses powerful enough to enable them, not only to surmount the earth's gravity, but to penetrate its atmosphere. a striking--indeed, an almost startling--peculiarity, on the other hand, divides from their congeners a class of meteors identified by mr. denning during ten years' patient watching of such phenomena at bristol.[ ] these are described as "meteors with stationary radiants," since for months together they seem to come from the same fixed points in the sky. now this implies quite a portentous velocity. the direction of meteor-radiants is affected by a kind of _aberration_, analogous to the aberration of light. it results from a composition of terrestrial with meteoric motion. hence, unless that of the earth in its orbit be by comparison insignificant, the visual line of encounter must shift, if not perceptibly from day to day, at any rate conspicuously from month to month. the fixity, then, of many systems observed by mr. denning seems to demand the admission that their members travel so fast as to throw the earth's movement completely out of the account. the required velocity would be, by mr. ranyard's calculation, at least miles a second.[ ] but the aspect of the meteors justifies no such extravagant assumption. their seeming swiftness is very various, and--what is highly significant--it is notably less when they pursue than when they meet the earth. yet the "incredible and unaccountable"[ ] fact of the existence of these "long radiants," although doubted by tisserand[ ] because of its theoretical refractoriness, must apparently be admitted. the first plausible explanation of them was offered by professor turner in .[ ] they represent, in his view, the cumulative effects of the earth's attraction. the validity of his reasoning is, however, denied by m. brédikhine,[ ] who prefers to regard them as a congeries of separate streams. the enigma they present has evidently not yet received its definitive solution. the perseids afford, on the contrary, a remarkable instance of a "shifting radiant." mr. denning's observations of these yellowish, leisurely meteors extend over nearly six weeks, from july to august ; the point of radiation meantime progressing no less than ° in right ascension. doubts as to their common origin were hence freely expressed, especially by mr. monck of dublin.[ ] but the late dr. kleiber[ ] showed, by strict geometrical reasoning, that the forty-nine radiants successively determined for the shower were all, in fact, comprised within one narrowly limited region of space. in other words, the application of the proper correction for the terrestrial movement, and the effects of attraction by which each individual shooting-star is compelled to describe a hyperbola round the earth's centre, reduces the extended line of radiants to a compact group, with the cometary radiant for its central point; the cometary radiant being the spot in the sky met by a tangent to the orbit of the perseid comet of at its intersection with the orbit of the earth. the reality of the connection between the comet and the meteors could scarcely be more clearly proved; while the vast dimensions of the stream into which the latter are found to be diffused cannot but excite astonishment not unmixed with perplexity. the first successful application of the spectroscope to comets was by donati in .[ ] a comet discovered by tempel, july , brightened until it appeared like a star somewhat below the second magnitude, with a feeble tail ° in length. it was remarkable as having, on august , almost totally eclipsed a small star--a very rare occurrence.[ ] on august donati admitted its light through his train of prisms, and found it, thus analysed, to consist of three bright bands--yellow, green, and blue--separated by wider dark intervals. this implied a good deal. comets had previously been considered, as we have seen, to shine mainly, if not wholly, by reflected sunlight. they were now perceived to be self-luminous, and to be formed, to a large extent, of glowing gas. the next step was to determine what _kind_ of gas it was that was thus glowing in them; and this was taken by sir william huggins in .[ ] a comet of subordinate brilliancy, known as comet ii., or sometimes as winnecke's, was the subject of his experiment. on comparing its spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by electricity, he found the agreement exact. it has since been abundantly confirmed. all the eighteen comets tested by light analysis, between and , showed the typical hydro-carbon spectrum[ ] common to the whole group of those compounds, but probably due immediately to the presence of acetylene. some minor deviations from the laboratory pattern, in the shifting of the maxima of light from the edge towards the middle of the yellow and blue bands, have been experimentally reproduced by vogel and hasselberg in tubes containing a mixture of carbonic oxide with olefiant gas.[ ] their illumination by disruptive electric discharges was, however, a condition _sine quâ non_ for the exhibition of the cometary type of spectrum. when a continuous current was employed, the carbonic oxide bands asserted themselves to the exclusion of the hydro-carbons. the distinction has great significance as regards the nature of comets. of particular interest in this connection is the circumstance that carbonic oxide is one of the gases evolved by meteoric stones and irons under stress of heat.[ ] for it must apparently have formed part of an aeriform mass in which they were immersed at an earlier stage of their history. plate ii. [illustration: great comet. photographed, may , , with the thirteen-inch astrographic refractor of the royal observatory, cape of good hope.] in a few exceptional comets the usual carbon-bands have been missed. two such were observed by sir william huggins in and respectively.[ ] in each a green ray, approximating in position to the fundamental nebular line, crossed an otherwise unbroken spectrum. and holmes's comet of displayed only a faint prismatic band devoid of any characteristic feature.[ ] now these three might well be set down as partially effete bodies; but a brilliant comet, visible in southern latitudes in april and may, , so far resembled them in the quality of its light as to give a spectrum mainly, if not purely, continuous. this, accordingly, is no symptom of decay. the earliest comet of first-class lustre to present itself for spectroscopic examination was that discovered by coggia at marseilles, april , . invisible to the naked eye till june, it blazed out in july a splendid ornament of our northern skies, with a just perceptibly curved tail, reaching more than half way from the horizon to the zenith, and a nucleus surpassing in brilliancy the brightest stars in the swan. brédikhine, vogel, and huggins[ ] were unanimous in pronouncing its spectrum to be that of marsh or olefiant gas. father secchi, in the clear sky of rome, was able to push the identification even closer than had heretofore been done. the _complete_ hydro-carbon spectrum consists of five zones of variously coloured light. three of these only--the three central ones--had till then been obtained from comets; owing, it was supposed, to their temperature not being high enough to develop the others. the light of coggia's comet, however, was found to contain all five, traces of the violet band emerging june , of the red, july .[ ] presumably, all five would show universally in cometary spectra, were the dispersed rays strong enough to enable them to be seen. the gaseous surroundings of comets are, then, largely made up of a compound of hydrogen with carbon. other materials are also present; but the hydro-carbon element is probably unfailing and predominant. its luminosity is, there is little doubt, an effect of electrical excitement. zöllner showed in [ ] that, owing to evaporation and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets; and that their original light is immediately connected with these, and depends upon solar radiation, rather through its direct or indirect electrifying effects, than through its more obvious thermal power, may be considered a truth permanently acquired to science.[ ] they are not, it thus seems, bodies incandescent through heat, but glowing by electricity; and this is compatible, under certain circumstances, with a relatively low temperature. the gaseous spectrum of comets is accompanied, in varying degrees, by a continuous spectrum. this is usually derived most strongly from the nucleus, but extends, more or less, to the nebulous appendages. in part, it is certainly due to reflected sunlight; in part, most likely, to the ignition of minute solid particles. footnotes: [footnote : _month. not._, vol. xix., p. .] [footnote : _mém. de l'ac. imp._, t. ii., , p. .] [footnote : _harvard annals_, vol. iii., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxii., p. .] [footnote : stothard in _ibid._, vol. xxi., p. .] [footnote : _intell. observer_, vol. i., p. .] [footnote : _comptes rendus_, t. lxi., p. .] [footnote : _smiths. report_, (holden); _nature_, vol. xxv., p. ; _observatory_, vol. xxi., p. (w. t. lynn).] [footnote : _ueber den ursprung der von pallas gefundenen eisenmassen_, p. .] [footnote : arago, _annuaire_, , p. .] [footnote : humboldt had noticed the emanation of the shooting stars of from a single point, or "radiant," as greg long afterwards termed it; but no reasoning was founded on the observation.] [footnote : _am. journ. of sc._, vol. xxvi., p. .] [footnote : _annuaire_, , p. .] [footnote : _ann. de l'observ._, bruxelles, , p. .] [footnote : _ibid._, , p. .] [footnote : _astr. nach._, nos. , .] [footnote : _am. jour. of sc._, vol. xxxviii. ( nd ser.), p. .] [footnote : _ibid._, vol. xxxviii., p. .] [footnote : _month. not._, vol. xxvii., p. .] [footnote : _am. jour. of sc._, vol. xliii. ( nd ser.), p. .] [footnote : grant, _month. not._, vol. xxvii., p. .] [footnote : p. smyth, _ibid._, p. .] [footnote : hind, _ibid._, p. .] [footnote : reproduced in _les mondes_, t. xiii.] [footnote : _comptes rendus_, t. lxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : schiaparelli, _le stelle cadenti_, p. .] [footnote : _ueber feuer-meteore_, p. .] [footnote : _astr. nach._, no. (mädler); see also boguslawski, _die kometen_, p. . .] [footnote : _nature_, vol. vi., p. .] [footnote : a. s. herschel, _month. not._, vol. xxxii., p. .] [footnote : _astr. nach._, nos. , , , , , .] [footnote : _nature_, vol. vii., p. .] [footnote : a. s. herschel, _report brit. ass._, , p. .] [footnote : humboldt, _cosmos_, vol. i., p. (otté's trans.).] [footnote : _month. not._, vol. xxxiii., p. .] [footnote : even this was denied by bruhns, _astr. nach._, no. , .] [footnote : _am. jour._, vol. xxxi., p. .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : in schiaparelli's opinion, centuries must have elapsed while the observed amount of scattering was being produced. _le stelle cadenti_, , p. .] [footnote : _astr. and astroph._, vol. xi., p. .] [footnote : _bull. de l'acad. st. petersbourg_, t. xxxv., p. . .] [footnote : _observatory_, vol. xvi., p. .] [footnote : _le stelle cadenti_, p. ; _rendiconti dell' istituto lombardo_, t. iii., ser. ii., p. .] [footnote : denning, _memoirs roy. astr. soc._, vol. liii., p. ; abelmann, _astr. nach._, no. , .] [footnote : _proc. roy. soc._, march , ; _nature_, november , .] [footnote : berberich, _astr. nach._, no. , .] [footnote : elkin, _astroph. jour._, vol. ix., p. .] [footnote : elkin, _astroph. jour._, vol. x., p. .] [footnote : _pop. astr._, september, , p. .] [footnote : _month. not._, vol. xx., p. .] [footnote : _revue des deux mondes_, december , , p. .] [footnote : palgrave, _phil. trans._, vol. cxxv., p. .] [footnote : w. e. hidden, _century mag._, vol. xxxiv., p. .] [footnote : _amer. jour. of science_, vol. xxxvi., p. i., .] [footnote : _revue des questions scientifiques_, january, , p. ; tisserand, _bull. astr._, t. viii., p. .] [footnote : _month. not._, vol. xlv., p. .] [footnote : _observatory_, vol. viii., p. .] [footnote : denning, _month. not._, vol. xxxviii., p. .] [footnote : _comptes rendus_, t. cix., p. .] [footnote : _month. not._, vol. lix., p. .] [footnote : _bull. de l'acad. st. petersb._, t. xii., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _annuaire_, paris, , p. .] [footnote : _phil. trans._, vol. clviii., p. .] [footnote : hasselberg, _mém. de l'ac. imp. de st. pétersbourg_, t. xxviii. ( th ser.), no. , p. .] [footnote : scheiner, _die spectralanalyse der gestirne_, p. . kayser (_astr. and astroph._, vol. xiii., p. ) refers the anomalies of the carbon-spectrum in comets wholly to instrumental sources.] [footnote : dewar, _proc. roy. inst._, vol. xi., p. .] [footnote : _proc. r. soc._, vol. xv., p. ; _month. not._, vol. xxvii., p. .] [footnote : keeler, _astr. and astrophysics_, vol. xi., p. ; vogel, _astr. nach._, no. , .] [footnote : _proc. roy. soc._, vol. xxiii., p. .] [footnote : hasselberg, _loc. cit._, p. .] [footnote : _ueber die natur der cometen_, p. .] [footnote : hasselberg, _loc. cit._, p. .] chapter xi _recent comets_ (_continued_) the mystery of comets' tails had been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. we have, then, the assurance that these extraordinary appendages are composed of no ethereal or supersensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity. olbers, as already stated, originated in the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. it was developed and enforced by bessel's discussion of the appearances presented by halley's comet in . he, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. professor w. a. norton, of yale college, considerably improved this by inquiries begun in , and resumed on the apparition of donati's comet; and dr. c. f. pape at altona[ ] gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object. the _physical_ theory of repulsion, however, was, it might be said, still in the air. nor did it even begin to assume consistency until zöllner took it in hand in .[ ] it is perfectly well ascertained that the energy of the push or pull produced by electricity depends (other things being the same) upon the _surface_ of the body acted on; that of gravity upon its _mass_. the efficacy of solar electrical repulsion relatively to solar gravitational attraction grows, consequently, as the size of the particle diminishes. make this small enough, and it will virtually cease to gravitate, and will unconditionally obey the impulse to recession. this principle zöllner was the first to realise in its application to comets. it gives the key to their constitution. admitting that the sun and they are similarly electrified, their more substantially aggregated parts will still follow the solicitations of his gravity, while the finely divided particles escaping from them will, simply by reason of their minuteness, fall under the sway of his repellent electric power. they will, in other words, form "tails." nor is any extravagant assumption called for as to the intensity of the electrical charge concerned in producing these effects. zöllner, in fact, showed[ ] that it need not be higher than that attributed by the best authorities to the terrestrial surface. forty years have elapsed since m. brédikhine, director successively of the moscow and of the pulkowa observatories, turned his attention to these curious phenomena. his persistent inquiries on the subject, however, date from the appearance of coggia's comet in . on computing the value of the repulsive force exerted in the formation of its tail, and comparing it with values of the same force arrived at by him in for some other conspicuous comets, it struck him that the numbers representing them fell into three well-defined classes. "i suspect," he wrote in , "that comets are divisible into groups, for each of which the repulsive force is perhaps the same."[ ] this idea was confirmed on fuller investigation. in the appendages of thirty-six well-observed comets had been reconstructed theoretically, without a single exception being met with to the rule of the three types. a further study of forty comets led, in , only to a modification of the numerical results previously arrived at. in the first of these, the repellent energy of the sun is fourteen times stronger than his attractive energy;[ ] the particles forming the enormously long straight rays projected outward from this kind of comet leave the nucleus with a mean velocity of just seven kilometres per second, which, becoming constantly accelerated, carries them in a few days to the limit of visibility. the great comets of , , and , that of (so far as its principal tail was concerned), and halley's comet at its various apparitions, belonged to this class. less narrow limits were assigned to the values of the repulsive force employed to produce the second type. for the axis of the tail, it exceeds by one-tenth (= · ) the power of solar gravity; for the anterior edge, it is more than twice ( · ), for the posterior only half as strong. the corresponding initial velocity (for the axis) is , metres a second, and the resulting appendage a scimitar-like or plumy tail, such as donati's and coggia's comets furnished splendid examples of. tails of the third type are constructed with forces of repulsion from the sun ranging from one-tenth to three-tenths that of his gravity, producing an accelerated movement of attenuated matter from the nucleus, beginning at the leisurely rate of to metres a second. they are short, strongly bent, brush-like emanations, and in bright comets seem to be only found in combination with tails of the higher classes. multiple tails, indeed--that is, tails of different types emitted simultaneously by one comet--are perceived, as experience advances and observation becomes closer, to be rather the rule than the exception.[ ] now what is the meaning of these three types? is any translation of them into physical fact possible? to this question brédikhine supplied, in , a plausible answer.[ ] it was already a current surmise that multiple tails are composed of different kinds of matter, differently acted on by the sun. both olbers and bessel had suggested this explanation of the straight and curved emanations from the comet of ; norton had applied it to the faint light tracks proceeding from that of donati;[ ] winnecke to the varying deviations of its more brilliant plumage. brédikhine defined and ratified the conjecture. he undertook to determine (provisionally as yet) the several kinds of matter appropriated severally to the three classes of tails. these he found to be hydrogen for the first, hydro-carbons for the second, and iron for the third. the ground of this apportionment is that the atomic weights of these substances bear to each other the same inverse proportion as the repulsive forces employed in producing the appendages they are supposed to form; and zöllner had pointed out in that the "heliofugal" power by which comets' tails are developed would, in fact, be effective just in that ratio.[ ] hydrogen, as the lightest known element--that is, the least under the influence of gravity--was naturally selected as that which yielded most readily to the counter-persuasions of electricity. hydro-carbons had been shown by the spectroscope to be present in comets, and were fitted by their specific weight, as compared with that of hydrogen, to form tails of the second type; while the atoms of iron were just heavy enough to compose those of the third, and, from the plentifulness of their presence in meteorites, might be presumed to enter, in no inconsiderable proportion, into the mass of comets. these three substances, however, were by no means supposed to be the sole constituents of the appendages in question. on the contrary, the great breadth of what, for the present, were taken to be characteristically "iron" tails was attributed to the presence of many kinds of matter of high and slightly different specific weights;[ ] while the expanded plume of donati was shown to be, in reality, a whole system of tails, made up of many substances, each spreading into a separate hollow cone, more or less deviating from, and partially superposed upon the others. yet these felicities of explanation must not make us forget that the chemical composition attributed to the first type of cometary trains has, so far, received no countenance from the spectroscope. the emission lines of free, incandescent hydrogen have never been derived from any part of these bodies. dissentient opinions, accordingly, were expressed as to the cause of their structural peculiarities. ranyard,[ ] zenker, and others advocated the agency of heat repulsion in producing them; kiaer somewhat obscurely explains them through the evolution of gases by colliding particles;[ ] herz of vienna concludes tails to be mere illusory appendages produced by electrical discharges through the rare medium assumed to fill space.[ ] but hirn[ ] conclusively showed that no such medium could possibly exist without promptly bringing ruin upon our "dædal earth" and its revolving companions. on the whole, modern researches tend to render superfluous the chemical diversities postulated by brédikhine. electricity alone seems competent to produce the varieties of cometary emanation they were designed to account for. the distinction of types rests on a solid basis of fact, but probably depends upon differences rather in the mode of action than in the kind of substance acted upon. suggestive sketches of electrical and "light-pressure" theories of comets have been published respectively by mr. fessenden of alleghany,[ ] and by m. arrhenius at stockholm.[ ] although evidently of a tentative character, they possess great interest. brédikhine's hypothesis was promptly and profusely illustrated. within three years of its promulgation, five bright comets made their appearance, each presenting some distinctive peculiarity by which knowledge of these curious objects was materially helped forward. the first of these is remembered as the "great southern comet." it was never visible in these latitudes, but made a short though stately progress through southern skies. its earliest detection was at cordoba on the last evening of january, ; and it was seen on february , as a luminous streak, extending just after sunset from the south-west horizon towards the pole, in new south wales, at monte video, and the cape of good hope. the head was lost in the solar rays until february , when dr. gould, then director of the national observatory of the argentine republic at cordoba, caught a glimpse of it very low in the west; and on the following evening, mr. eddie, at graham's town, discovered a faint nucleus, of a straw-coloured tinge, about the size of the annular nebula in lyra. its condensation, however, was very imperfect, and the whole apparition showed an exceedingly filmy texture. the tail was enormously long. on february it extended--large perspective retrenchment notwithstanding--over an arc of °; but its brightness nowhere exceeded that of the milky way in taurus. there was little curvature perceptible; the edges of the appendage ran parallel, forming a nebulous causeway from star to star; and the comparison to an auroral beam was appropriately used. the aspect of the famous comet of was forcibly recalled to the memory of mr. janisch, governor of st. helena; and the resemblance proved not merely superficial. but the comet of was less brilliant, and even more evanescent. after only eight days of visibility, it had faded so much as no longer to strike, though still discoverable by the unaided eye; and on february it was invisible with the great cordoba equatoreal pointed to its known place. but the most astonishing circumstance connected with this body is the identity of its path with that of its predecessor in . this is undeniable. dr. gould,[ ] mr. hind, and dr. copeland,[ ] each computed a separate set of elements from the first rough observations, and each was struck with an agreement between the two orbits so close as to render them virtually indistinguishable. "can it be possible," mr. hind wrote to sir george airy, "that there is such a comet in the system, almost grazing the sun's surface in perihelion, and revolving in less than thirty-seven years. i confess i feel a difficulty in admitting it, notwithstanding the above extraordinary resemblance of orbits."[ ] mr. hind's difficulty was shared by other astronomers. it would, indeed, be a violation of common-sense to suppose that a celestial visitant so striking in appearance had been for centuries back an unnoticed frequenter of our skies. various expedients, accordingly, were resorted to for getting rid of the anomaly. the most promising at first sight was that of the resisting medium. it was hard to believe that a body, largely vaporous, shooting past the sun at a distance of less than a hundred thousand miles from his surface, should have escaped powerful retardation. it must have passed through the very midst of the corona. it might easily have had an actual encounter with a prominence. escape from such proximity might, indeed, very well have been judged beforehand to be impossible. even admitting no other kind of opposition than that dubiously supposed to have affected encke's comet, the result in shortening the period ought to be of the most marked kind. it was proved by oppolzer[ ] that if the comet of had entered our system from stellar space with parabolic velocity it would, by the action of a medium such as encke postulated (varying in density inversely as the square of the distance from the sun), have been brought down, by its first perihelion passage, to elliptic movement in a period of twenty-four years, with such rapid diminution that its next return would be in about ten. but such restricted observations as were available on either occasion of its visibility gave no sign of such a rapid progress towards engulfment. another form of the theory was advocated by klinkerfues.[ ] he supposed that four returns of the same body had been witnessed within historical memory--the first in b.c., the next in , besides those of and ; an original period of , years being successively reduced by the withdrawal at each perihelion passage of / of the velocity acquired by falling from the far extremity of its orbit towards the sun, to and years. a continuance of the process would bring the comet of back in . unfortunately, the earliest of these apparitions cannot be identified with the recent ones unless by doing violence to the plain meaning of aristotle's words in describing it. he states that the comet was first seen "during the frosts and in the clear skies of winter," setting due west nearly at the same time as the sun.[ ] this implies some considerable north latitude. but the objects lately observed had practically _no_ north latitude. they accomplished their entire course _above_ the ecliptic in two hours and a quarter, during which space they were barely separated a hand's-breadth (one might say) from the sun's surface. for the purposes of the desired assimilation, aristotle's comet should have appeared in march. it is not credible, however, that even a native of thrace should have termed march "winter." with the comet of the case seemed more dubious. the circumstances of its appearance are barely reconcilable with the identity attributed to it, although too vaguely known to render certainty one way or the other attainable. it might however, be expected that recent observations would at least decide the questions whether the comet of could have returned in less than thirty-seven, and whether the comet of was to be looked for at the end of - / years. but the truth is that both these objects were observed over so small an arc-- ° and ° respectively--that their periods remained virtually undetermined. for while the shape and position of their orbits could be and were fixed with a very close approach to accuracy, the length of those orbits might vary enormously without any very sensible difference being produced in the small part of the curves traced out near the sun. dr. wilhelm meyer, however, arrived, by an elaborate discussion, at a period of thirty-seven years for the comet of ,[ ] while the observations of were admittedly best fitted by hubbard's ellipse of years; but these dr. meyer supposed to be affected by some constant source of error, such as would be produced by a mistaken estimate of the position of the comet's centre of gravity. he inferred finally that, in spite of previous non-appearances, the two comets represented a single regular denizen of our system, returning once in thirty-seven years along an orbit of such extreme eccentricity that its movement might be described as one of precipitation towards and rapid escape from the sun, rather than of sedate circulation round it. the _geometrical_ test of identity has hitherto been the only one which it was possible to apply to comets, and in the case before us it may fairly be said to have broken down. we may, then, tentatively, and with much hesitation, try a _physical_ test, though scarcely yet, properly speaking, available. we have seen that the comets of and were strikingly alike in general appearance, though the absence of a formed nucleus in the latter, and its inferior brilliancy, detracted from the convincing effect of the resemblance. nor was it maintained when tried by exact methods of inquiry. m. brédikhine found that the gigantic ray emitted in belonged to his type no. ; that of to type no. .[ ] the particles forming the one were actuated by a repulsive force ten times as powerful as those forming the other. it is true that a second noticeably curved tail was seen in chili, march , and at madras, march , ; and the conjecture was accordingly hazarded that the materials composing on that occasion the principal appendage having become exhausted, those of the secondary one remained predominant, and reappeared alone in the "hydro-carbon" train of . but the one known instance in point is against such a supposition. halley's comet, the only _great_ comet of which the returns have been securely authenticated and carefully observed, has preserved its "type" unchanged through many successive revolutions. the dilemma presented to astronomers by the great southern comet of was unexpectedly renewed in the following year. on the nd of may, , mr. john tebbutt of windsor, new south wales, scanning the western sky, discerned a hazy-looking object which he felt sure was a strange one. a marine telescope at once resolved it into two small stars and a comet, the latter of which quickly attracted the keen attention of astronomers; for dr. gould, computing its orbit from his first observations at cordoba, found it to agree so closely with that arrived at by bessel for the comet of that he telegraphed to europe, june , announcing the unexpected return of that body. so unexpected that theoretically it was not possible before the year ; and bessel's investigation was one which inspired and eminently deserved confidence. here, then, once more the perplexing choice had to be made between a premature and unaccountable reappearance and the admission of a plurality of comets moving nearly in the same path. but in this case facts proved decisive. tebbutt's comet passed the sun, june , at a distance of sixty-eight millions of miles, and became visible in europe six days later. it was, in the opinion of some, the finest object of the kind since . in traversing the constellation auriga on its _début_ in these latitudes, it outshone capella. on june and some subsequent nights, it was unmatched in brilliancy by any star in the heavens. in the telescope, the "two interlacing arcs of light" which had adorned the head of coggia's comet were reproduced; while a curious _dorsal spine_ of strong illumination formed the axis of the tail, which extended in clear skies over an arc of °. it belonged to the same "type" as donati's great plume; the particles composing it being driven _from_ the sun by a force twice as powerful as that urging them _towards_ it.[ ] but the appendage was, for a few nights, and by two observers perceived to be double. tempel, on june , and lewis boss, at albany (n.y.), june and , saw a long straight ray corresponding to a far higher rate of emission than the curved train, and shown by brédikhine to be a member of the (so-called) hydrogen class. it had vanished by july , but made a temporary reappearance july .[ ] the appendages of this comet were of remarkable transparency. small stars shone wholly undimmed across the tail, and a very nearly central transit of the head over one of the seventh magnitude on the night of june , produced--if any change--an increase of brilliancy in the object of this spontaneous experiment.[ ] dr. meyer, indeed, at the geneva observatory, detected apparent signs of refractive action upon rays thus transmitted;[ ] but his observations remain isolated, and were presumably illusory. the track pursued by this comet gave peculiar advantages for its observation. ascending from auriga through camelopardus, it stood, july , on a line between the pointers and the pole, within ° of the latter, and thus remained for a lengthened period constantly above the horizon of northern observers. its brightness, too, was no transient blaze, but had a lasting quality which enabled it to be kept steadily in view during nearly nine months. visible to the naked eye until the end of august, the last telescopic observation of it was made february , , when its distance from the earth considerably exceeded million miles. under these circumstances, the knowledge acquired of its orbit was of more than usual accuracy, and showed conclusively that the comet was not a simple return of bessel's; for this would involve a period of seventy-four years, whereas tebbutt's comet cannot revisit the sun until after the lapse of two and a half millenniums.[ ] nevertheless, the twin bodies move so nearly in the same path that an original connection of some kind is obvious; and the recent example of biela readily suggested a conjecture as to what the nature of that connection might have been. the comets of and are, then, regarded with much probability as fragments of a primitive disrupted body, one following in the wake of the other at an interval of seventy-four years. imperfect photographs were taken of donati's comet both in england and america;[ ] but tebbutt's comet was the first to which the process was satisfactorily applied. the difficulties to be overcome were very great. the chemical intensity of cometary light is, to begin with, extraordinarily small. janssen estimated it at / of moonlight.[ ] hence, if the ordinary process by which lunar photographs are taken had been applied to the comet of , an exposure of at least _three days_ would have been required in order to get an impression of the head with about a tenth part of the tail. but by that time a new method of vastly increased sensitiveness had been rendered available, by which dry gelatine-plates were substituted for the wet collodion-plates hitherto in use; and this improvement alone reduced the necessary time of exposure to two hours. it was brought down to half an hour by janssen's employment of a reflector specially adapted to give an image illuminated eight or ten times as strongly as that produced in the focus of an ordinary telescope.[ ] the photographic feebleness of cometary rays was not the only obstacle in the way of success. the proper motion of these bodies is so rapid as to render the usual devices for keeping a heavenly body steadily in view quite inapplicable. the machinery by which the diurnal movement of the sphere is followed, must be especially modified to suit each eccentric career. this, too, was done, and on june , , janssen secured a perfect photograph of the brilliant object then visible, showing the structure of the tail with beautiful distinctness to a distance of - / ° from the head. an impression to nearly ° was obtained about the same time by dr. henry draper at new york, with an exposure of minutes.[ ] tebbutt's (or comet iii.) was also the first comet of which the spectrum was so much as attempted to be chemically recorded. both huggins and draper were successful in this respect, but huggins was more completely so.[ ] the importance of the feat consisted in its throwing open to investigation a part of the spectrum invisible to the eye, and so affording an additional test of cometary constitution. the result was fully to confirm the origin from carbon-compounds assigned to the visible rays, by disclosing additional bands belonging to the same series in the ultra-violet; as well as to establish unmistakably the presence of a not inconsiderable proportion of reflected solar light by the clear impression of some of the principal fraunhofer lines. thus the polariscope was found to have told the truth, though not the whole truth. the photograph so satisfactorily communicative was taken by sir william huggins on the night of june ; and on the th, at greenwich, the tell-tale fraunhofer lines were perceived to interrupt the visible range of the spectrum. this was at first so vividly continuous, that the characteristic cometary bands could scarcely be detached from their bright background. but as the nucleus faded towards the end of june, they came out strongly, and were more and more clearly seen, both at greenwich and at princeton, to agree, not with the spectrum of hydro-carbons glowing in a vacuum tube, but with that of the same substances burning in a bunsen flame.[ ] it need not, however, be inferred that cometary materials are really in a state of combustion. this, from all that we know, may be called an impossibility. the additional clue furnished was rather to the manner of their electrical illumination.[ ] the spectrum of the tail was, in this comet, found to be not essentially different from that of the head. professor wright of yale college ascertained a large percentage of its light to be polarized in a plane passing through the sun, and hence to be reflected sunlight.[ ] a faint continuous spectrum corresponded to this portion of its radiance; but gaseous emissions were also present. at potsdam, on june , the hydro-carbon bands were indeed traced by vogel to the very end of the tail;[ ] and they were kept in sight by young at a greater distance from the nucleus than the more equably dispersed light. there seems little doubt that, as in the solar corona, the relative strength of the two orders of spectra is subject to fluctuations. the comet of iii. was thus of signal service to science. it afforded, when compared with the comet of , the first undeniable example of two such bodies travelling so nearly in the same orbit as to leave absolutely no doubt of the existence of a genetic tie between them. cometary photography came to its earliest fruition with it; and cometary spectroscopy made a notable advance by means of it. before it was yet out of sight, it was provided with a successor. at ann arbor observatory, michigan, on july , a comet was discovered by dr. schaeberle, which, as his claim to priority is undisputed, is often allowed to bear his name, although designated, in strict scientific parlance, comet iv. it was observed in europe after three days, became just discernible by the naked eye at the end of july, and brightened consistently up to its perihelion passage, august , when it was still about fifty million miles from the sun. during many days of that month, the uncommon spectacle was presented of two bright comets circling together, though at widely different distances, round the north pole of the heavens. the newcomer, however, never approached the pristine lustre of its predecessor. its nucleus, when brightest, was comparable to the star cor caroli, a narrow, perfectly straight ray proceeding from it to a distance of °. this was easily shown by brédikhine to belong to the hydrogen type of tails;[ ] while a "strange, faint second tail, or bifurcation of the first one," observed by captain noble, august ,[ ] fell into the hydro-carbon class of emanations. it was seen, august and , by dr. f. terby of louvain,[ ] as a short nebulous brush, like the abortive beginning of a congeries of curving trains; but appeared no more. its well-attested presence was significant of the complex constitution of such bodies, and the manifold kinds of action progressing in them. the only peculiarity in the spectrum of schaeberle's comet consisted in the almost total absence of continuous light. the carbon-bands were nearly isolated and very bright. barely from the nucleus proceeded a rainbow-tinted streak, indicative of solid or liquid matter, which, in this comet, must have been of very scanty amount. its visit to the sun in was, so far as is known, the first. the elements of its orbit showed no resemblance to those of any previous comet, nor any marked signs of periodicity. so that, although it may be considered probable, we do not _know_ that it is moving in a closed curve, or will ever again penetrate the precincts of the solar system. it was last seen from the southern hemisphere, october , . the third of a quartette of lucid comets visible within sixteen months, was discovered by mr. c. s. wells at the dudley observatory, albany, march , . two days later it was described by mr. lewis boss as "a great comet in miniature," so well defined and regularly developed were its various parts and appendages. discernible with optical aid early in may, it was on june observed on the meridian at albany just before noon--an astronomical event of extreme rarity. comet wells, however, never became an object so conspicuous as to attract general attention, owing to its immersion in the evening twilight of our northern june. but the study of its spectrum revealed new facts of the utmost interest. all the comets till then examined had been found (with the two transiently observed exceptions already mentioned) to conform to one invariable type of luminous emission. individual distinctions there had been, but no specific differences. now all these bodies had kept at a respectful distance from the sun; for of the great comet of no spectroscopic inquiries had been made. comet wells, on the other hand, approached its surface within little more than five million miles on june , ; and the vicinity had the effect of developing a novel feature in its incandescence. during the first half of april its spectrum was of the normal type, though the carbon bands were unusually weak; but with approach to the sun they died out, and the entire light seemed to become concentrated into a narrow, unbroken, brilliant streak, hardly to be distinguished from the spectrum of a star. this unusual behaviour excited attention, and a strict watch was kept. it was rewarded at the dunecht observatory, may , by the discernment of what had never before been seen in a comet--the yellow ray of sodium.[ ] by june , this had kindled into a blaze overpowering all other emissions. the light of the comet was practically monochromatic; and the image of the entire head, with the root of the tail, could be observed, like a solar prominence, depicted, in its new saffron vesture of vivid illumination, within the jaws of an open slit. at potsdam, the bright yellow line was perceived with astonishment by vogel on may , and was next evening identified with fraunhofer's "d." its character led him to infer a very considerable density in the glowing vapour emitting it.[ ] hasselberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet wells.[ ] for they were closely paralleled by some earlier experiments of wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. it seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. for the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons. sir william huggins succeeded in photographing the spectrum of comet wells by an exposure of one hour and a quarter.[ ] the result was to confirm the novelty of its character. none of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. otherwise the spectrum was strongly continuous, uninterrupted even by the fraunhofer lines detected in the spectrum of tebbutt's comet. hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival. all that is certainly known about the _extent_ of the orbit traversed by the first comet of is that it came from, and is now retreating towards, vastly remote depths of space. an american computer[ ] found a period indicated for it of no less than , years; a. thraen of dingelstädt arrived at one of .[ ] both are perhaps equally insecure. we have now to give some brief account of one of the most remarkable cometary apparitions on record, and--with the single exception of that identified with the name of halley--the most instructive to astronomers. the lessons learned from it were as varied and significant as its aspect was splendid; although from the circumstance of its being visible in general only before sunrise, the spectators of its splendour were comparatively few. the discovery of a great comet at rio janeiro, september , , became known in europe through a telegram from m. cruls, director of the observatory at that place. it had, however (as appeared subsequently), been already seen on the th by mr. finlay of the cape observatory, and at auckland as early as september . a later, but very singularly conditioned detection, quite unconnected with any of the preceding, was effected by dr. common at ealing. since the eclipse of may , when a comet--named "tewfik" in honour of the khedive of egypt--was caught on dr. schuster's photographs, entangled, one might almost say, in the outer rays of the corona, he had scrutinized the neighbourhood of the sun on the infinitesimal chance of intercepting another such body on its rapid journey thence or thither. we record with wonder that, after an interval of exactly four months, that infinitesimal chance turned up in his favour. on the forenoon of sunday, september , he saw a great comet close to, and rapidly approaching the sun. it was, in fact, then within a few hours of perihelion. some measures of position were promptly taken; but a cloud-veil covered the interesting spectacle before mid-day was long past. mr. finlay at the cape was more completely fortunate. divided from his fellow-observer by half the world, he unconsciously finished, under a clearer sky, his interrupted observation. the comet, of which the silvery radiance contrasted strikingly with the reddish-yellow glare of the sun's margin it drew near to, was followed "continuously right into the boiling of the limb"--a circumstance without precedent in cometary history.[ ] dr. elkin, who watched the progress of the event with another instrument, thought the intrinsic brilliancy of the nucleus scarcely surpassed by that of the sun's surface. nevertheless it had no sooner touched it than it vanished as if annihilated. so sudden was the disappearance (at h. m. s., cape mean time), that the comet was at first believed to have passed _behind_ the sun. but this proved not to have been the case. the observers at the cape had witnessed a genuine transit. nor could non-visibility be explained by equality of lustre. for the gradations of light on the sun's disc are amply sufficient to bring out against the dusky background of the limb any object matching the brilliancy of the centre; while an object just equally luminous with the limb must inevitably show dark at the centre. the only admissible view, then, is that the bulk of the comet was of too filmy a texture, and its presumably solid nucleus too small, to intercept any noticeable part of the solar rays--a piece of information worth remembering. plate iii. [illustration: the great comet of september, . photographed at the royal observatory, cape of good hope] on the following morning, the object of this unique observation showed (in sir david gill's words) "an astonishing brilliancy as it rose behind the mountains on the east of table bay, and seemed in no way diminished in brightness when the sun rose a few minutes afterward. it was only necessary to shade the eye from direct sunlight with the hand at arm's length, to see the comet, with its brilliant white nucleus and dense white, sharply bordered tail of quite half a degree in length."[ ] all over the world, wherever the sky was clear during that day, september , it was obvious to ordinary vision. since nothing had been seen like it. from spain, italy, algeria, southern france, despatches came in announcing the extraordinary appearance. at cordoba, in south america, the "blazing star near the sun" was the one topic of discourse.[ ] moreover--and this is altogether extraordinary--the records of its daylight visibility to the naked eye extend over three days. at reus, near tarragona, it showed bright enough to be seen through a passing cloud when only three of the sun's diameters from his limb, just before its final rush past perihelion on september ; while at carthagena in spain, on september , it was kept in view during two hours before and two hours after noon, and was similarly visible in algeria on the same day.[ ] but still more surprising than the appearance of the body itself were the nature and relations of the path it moved in. the first rough elements computed for it by mr. tebbutt, dr. chandler, and mr. white, assistant at the melbourne observatory, showed at once a striking resemblance to those of the twin comets of and . this suggestive fact became known in this country, september , through the medium of a dunecht circular. it was fully confirmed by subsequent inquiries, for which ample opportunities were luckily provided. the likeness was not, indeed, so absolutely perfect as in the previous case; it included some slight, though real differences; but it bore a strong and unmistakable stamp, broadly challenging explanation. two hypotheses only were really available. either the comet of was an accelerated return of those of and , or it was a fragment of an original mass to which they also had belonged. for the purposes of the first view the "resisting medium" was brought into full play; the opinion of its efficacy was for some time both prevalent and popular, and formed the basis, moreover, of something of a sensational panic. for a comet which, at a single passage through the sun's atmosphere, encountered sufficient resistance to shorten its period from thirty-seven to two years and eight months, must, in the immediate future, be brought to rest on his surface; and the solar conflagration thence ensuing was represented in some quarters, with more licence of imagination than countenance from science, as likely to be of catastrophic import to the inhabitants of our little planet. but there was a test available in which it had not been possible to apply either in or in . the two bodies visible in those years had been observed only after they had already passed perihelion;[ ] the third member of the group, on the other hand, was accurately followed for a week before that event, as well as during many months after it. finlay's and elkin's observation of its disappearance at the sun's edge formed, besides, a peculiarly delicate test of its motion. the opportunity was thus afforded, by directly comparing the comet's velocity before and after its critical plunge through the solar surroundings, of ascertaining with approximate certainty whether any considerable retardation had been experienced in the course of that plunge. the answer distinctly given was that there had not. the computed and observed places on both sides of the sun fitted harmoniously together. the effect, if any were produced, was too small to be perceptible. this result is, in itself, a memorable one. it seems to give the _coup de grâce_ to encke's theory--discredited, in addition, by backlund's investigation--of a resisting medium growing rapidly denser inwards. for the perihelion distance of the comet of , though somewhat greater than that of its predecessors, was nevertheless extremely small. it passed at less than , miles of the sun's surface. but the ethereal substance long supposed to obstruct the movement of encke's comet would there be nearly , times denser than at the perihelion of the smaller body, and must have exerted a conspicuous retarding influence. that none such could be detected seems to argue that no such medium exists. further evidence of a decisive kind was not wanting on the question of identity. the "great september comet" of was in no hurry to withdraw itself from curious terrestrial scrutiny. it was discerned with the naked eye at cordoba as late as march , , and still showed in the field of the great equatoreal on june as an "excessively faint whiteness."[ ] it was then about millions of miles from the earth--a distance to which no other comet--not even excepting the peculiar one of --had been pursued.[ ] moreover, an arc of out of the entire degrees of its circuit had been described under the eyes of astronomers; so that its course came to be very well known. that its movement is in a very eccentric ellipse, traversed in several hundred years, was ascertained.[ ] the later inquiries of dr. kreutz,[ ] completed in a volume published in ,[ ] demonstrated the period to be of about years, while that of its predecessor in might possibly agree with it, but is much more probably estimated at years. the hypothesis that they, or any of the comets associated with them, were returns of an individual body is peremptorily excluded. they may all, however, have been separated from one original mass by the divellent action of the sun at close quarters. each has doubtless its own period, since each has most likely suffered retardations or accelerations special to itself, which, though trifling in amount, would avail materially to alter the length of the major axis, while leaving the remaining elements of the common orbit virtually unchanged.[ ] a fifth member was added to the family in . on the th of january in that year, m. thome discovered at cordoba a comet reproducing with curious fidelity the lineaments of that observed in the same latitudes seven years previously. the narrow ribbon of light, contracting towards the sun, and running outward from it to a distance of thirty-five degrees; the unsubstantial head--a veiled nothingness, as it appeared, since no distinct nucleus could be made out; the quick fading into invisibility, were all accordant peculiarities, and they were confirmed by some rough calculations of its orbit, showing geometrical affinity to be no less unmistakable than physical likeness. the observations secured were indeed, from the nature of the apparition, neither numerous nor over-reliable; and the earliest of them dated from a week after perihelion, passed, almost by a touch-and-go escape, january . on january , this mysterious object could barely be discerned telescopically at cordoba.[ ] that it belonged to the series of "southern comets" can scarcely be doubted; but the inference that it was an actual return of the comet of , improbable in itself, was negatived by its non-appearance in . meyer's incorporation with this extraordinary group of the "eclipse-comet" of [ ] has been approved by kreutz, after searching examination. the idea of cometary systems was first suggested by thomas clausen in .[ ] it was developed by the late m. hoek, director of the utrecht observatory, in and some following years.[ ] he found that in quite a considerable number of cases, the paths of two or three comets had a common point of intersection far out in space, indicating with much likelihood a community of origin. this consisted, according to his surmise, in the disruption of a parent mass during its sweep round the star latest visited. be this as it may, the fact is undoubted that numerous comets fall into groups, in which similar conditions of motion betray a pre-existent physical connection. never before, however, had geometrical relationship been so notorious as between the comets now under consideration; and never before, in a comet still, it might be said, in the prime of life, had physical peculiarities tending to account for that affinity been so obvious as in the chief member of the group. observation of a granular structure in cometary nuclei dates far back into the seventeenth century, when cysatus and hevelius described the central parts of the comets of and respectively as made up of a congeries of minute stars. analogous symptoms of a loose state of aggregation have of late been not unfrequently detected in telescopic comets, besides the instances of actual division offered by those connected with the names of biela and liais. the forces concerned in producing these effects seem to have been peculiarly energetic in the great comet of . the segmentation of the nucleus was first noticed in the united states and at the cape of good hope, september . it proceeded rapidly. at kiel, on october and , professor krüger perceived two centres of condensation. a definite and progressive separation into _three_ masses was observed by professor holden, october and .[ ] a few days later, m. tempel found the head to consist of _four_ lucid aggregations, ranged nearly along the prolongation of the caudal axis;[ ] and dr. common, january , , saw _five_ nuclei in a line "like pearls on a string."[ ] this remarkable character was preserved to the last moment of the comet's distinct visibility. it was a consequence, according to dr. kreutz, of violent interior action in the comet itself while close to the sun. there were, however, other curious proofs of a disaggregative tendency in this body. on october , schmidt discovered at athens a nebulous object ° south-west of the great comet, and travelling in the same direction. it remained visible for a few days, and, from oppenheim's and hind's calculations, there can be little doubt that it was really the offspring by fission of the body it accompanied.[ ] this is rendered more probable by the unexampled spectacle offered, october , to professor barnard, then of nashville, tennessee, of _six or eight_ distinct cometary masses within ° south by west of the comet's head, none of which reappeared on the next opportunity for a search.[ ] a week later, however, one similar object was discerned by mr. w. r. brooks, in the opposite direction from the comet. thus space appeared to be strewn with the filmy débris of this beautiful but fragile structure all along the track of its retreat from the sun. its tail was only equalled (if it were equalled) in length by that of the comet of . it extended in space to the vast distance of millions of miles from the head; but, so imperfectly were its proportions displayed to terrestrial observers, that it at no time covered an arc of the sky of more than °. this apparent extent was attained, during a few days previous to september , by a faint, thin, rigid streak, noticed only by a few observers--by elkin at the cape observatory, eddie at grahamstown, and cruls at rio janeiro. it diverged at a low angle from the denser curved train, and was produced, according to brédikhine,[ ] by the action of a repulsive force twelve times as strong as the counter-pull of gravity. it belonged, that is, to type ; while the great bifurcate appendage, obvious to all eyes, corresponded to the lower rate of emission characteristic of type . this was remarkable for the perfect definiteness of its termination, for its strongly-forked shape, and for its unusual permanence. down to the end of january, , its length, according to schmidt's observations, was still million miles; and a week later it remained visible to the naked eye, without notable abridgment. most singular of all was an anomalous extension of the appendage _towards_ the sun. during the greater part of october and november, a luminous "tube" or "sheath," of prodigious dimensions, seemed to surround the head, and project in a direction nearly opposite to that of the usual outpourings of attentuated matter. (see plate iii.) its diameter was computed by schmidt to be, october , no less than four million miles, and it was described by cruls as a "truncated cone of nebulosity," stretching ° or ° sunwards.[ ] this, and the entire anterior part of the comet, were again surrounded by a thin, but enormously voluminous paraboloidal envelope, observed by schiaparelli for a full month from october .[ ] there can be little doubt that these abnormal effluxes were a consequence of the tremendous physical disturbance suffered at perihelion; and it is worth remembering that something analogous was observed in the comet of (newton's), also noted for its excessively close approach to the sun, and possibly moving in a related orbit. the only plausible hypothesis as to the mode of their production is that of an opposite state of electrification in the particles composing the ordinary and extraordinary appendages. the spectrum of the great comet of was, in part, a repetition of that of its immediate predecessor, thus confirming the inference that the previously unexampled sodium-blaze was in both a direct result of the intense solar action to which they were exposed. but the d line was, this time, not seen alone. at dunecht, on the morning of september , drs. copeland and j. g. lohse succeeded in identifying six brilliant rays in the green and yellow with as many prominent iron-lines;[ ] a very significant addition to our knowledge of cometary constitution, and one which lent countenance to brédikhine's assumption of various kinds of matter issuing from the nucleus with velocities inversely as their atomic weights. all the lines equally showed a slight displacement, indicating a recession from the earth of the radiating body at the rate of to miles a second. a similar observation, made by m. thollon at nice on the same day, gave emphatic sanction to the spectroscopic method of estimating movement in the line of sight. before anything was as yet known of the comet's path or velocity, he announced, from the position of the double sodium-line alone, that at p.m. on september it was increasing its distance from our planet by from to kilometres per second.[ ] m. bigourdan's subsequent calculations showed that its actual swiftness of recession was at that moment kilometres. changes in the inverse order to those seen in the spectrum of comet wells soon became apparent. in the earlier body, carbon bands had died out with _approach_ to perihelion, and had been replaced by sodium emissions; in its successor, sodium emissions became weakened and disappeared with _retreat_ from perihelion, and found their substitute in carbon bands. professor riccò was, in fact, able to infer, from the sequence of prismatic phenomena, that the comet had already passed the sun; thus establishing a novel criterion for determining the position of a comet in its orbit by the varying quality of its radiations. recapitulating what was learnt from the five conspicuous comets of - , we find that the leading facts acquired to science were these three. first, that comets may be met with pursuing each other, after intervals of many years, in the same, or nearly the same, track; so that identity of orbit can no longer be regarded as a sure test of individual identity. secondly, that at least the outer corona may be traversed by such bodies with perfect apparent impunity. finally, that their chemical constitution is highly complex, and that they possess, in some cases at least, a metallic core resembling the meteoric masses which occasionally reach the earth from planetary space. a group of five comets, including halley's, own a sort of cliental dependence upon the planet neptune. they travel out from the sun just to about his distance from it, as if to pay homage to a powerful protector, who gets the credit of their establishment as periodical visitors to the solar system. the second of these bodies to affect a looked-for return was a comet--the sixteenth within ten years--discovered by pons, july , , and found by encke to revolve in an elliptic orbit, with a period of nearly years. it was not, however, until september , , that mr. brooks caught its reappearance; it passed perihelion january , and was last seen june , . at its brightest, it had the appearance of a second magnitude star, furnished with a poorly developed double tail, and was fairly conspicuous to the naked eye in southern europe, from december to march. one exceptional feature distinguished it. its fluctuations in form and luminosity were unprecedented in rapidity and extent. on september , dr. chandler[ ] observed it at harvard as a very faint, diffused nebulosity, with slight central condensation. on the next night, there was found in its place a bright star of the eighth magnitude, scarcely marked out, by a bare trace of environing haze, from the genuine stars it counterfeited. the change was attended by an eight-fold augmentation of light, and was proved by schiaparelli's confirmatory observations[ ] to have been accomplished within a few hours. the stellar disguise was quickly cast aside. the comet appeared on september as a wide nebulous disc, and soon after faded down to its original dimness. its distance from the sun was then no less than million miles, and its spectrum showed nothing unusual. these strange variations recurred slightly on october , and with marked emphasis on january , when they were witnessed with amazement, and photometrically studied by müller of potsdam.[ ] the entire cycle this time was run through in less than four hours--the comet having, in that brief space, condensed, with a vivid outburst of light, into a seeming star, and the seeming star having expanded back again into a comet. scarcely less transient, though not altogether similar, changes of aspect were noted by m. perrotin,[ ] january and , . on the latter date, the continuous spectrum given by a reddish-yellow disc surrounding the true nucleus seemed intensified by bright knots corresponding to the rays of sodium. a comet discovered by mr. sawerthal at the royal observatory, cape of good hope, february , , distinguished itself by blazing up, on may , to four or five times its normal brilliancy, at the same time throwing out from the head two lustrous lateral branches.[ ] these had, on june , spread backward so as to join the tail, with an effect like the playing of a fountain; ten or eleven days later, they had completely disappeared, leaving the comet in its former shape and insignificance. its abrupt display of vitality occurred two full months after perihelion. on the morning of july , , mr. w. r. brooks, of geneva, new york, eminent as a successful comet-hunter, secured one of his customary trophies. the faint object in question was moving through the constellation cetus, and turned out to be a member of jupiter's numerous family of comets, revolving round the sun in a period of seven years. its past history came then, to a certain extent, within the scope of investigation, and proved to have been singularly eventful; nor had the body escaped scatheless from the vicissitudes to which it had been exposed. observing from mount hamilton, august and , professor barnard noticed this comet ( , v.) to be attended in its progress through space by four _outriders_, "the two brighter companions" (the fainter pair survived a very short time) "were perfect miniatures," professor barnard tells us,[ ] "of the larger comet, each having a small, fairly defined head and nucleus, with a faint, hazy tail, the more distant one being the larger and less developed. the three comets were in a straight line, nearly east and west, their tails lying along this line. there was no connecting nebulosity between these objects, the tails of the two smaller not reaching each other, or the large comet. to all appearance they were absolutely independent comets." nevertheless, spitaler, at vienna, in the early days of august, perceived, as it were, a thin cocoon of nebulosity woven round the entire trio.[ ] one of them faded from view september ; the other actually outshone the original comet on august , but was plainly of inferior vitality. it was last seen by barnard on november , with the thirty-six inch refractor, while its primary afforded an observation for position with the twelve-inch, march , .[ ] a cause for the disruption it had presumably undergone had, before then, been plausibly assigned. the adventures of lexell's comet have long served to exemplify the effects of jupiter's despotic sway over such bodies. although bright enough in to be seen with the naked eye, and ascertained to be circulating in five and a half years, it had never previously been seen, and failed subsequently to present itself. the explanation of this anomaly, suggested by lexell, and fully confirmed by the analytical inquiries both of laplace and leverrier,[ ] was that a very close approach to jupiter in had completely changed the character of its orbit, and brought it within the range of terrestrial observation; while in , after having only twice traversed its new path (at its second return it was so circumstanced as to be invisible from the earth), it was, by a fresh encounter, diverted into one entirely different. yet the possibility was not lost sight of that the great planet, by inverting its mode of action, might undo its own work, and fling the comet once more into the inner part of the solar system. this possibility seemed to be realized by chandler's identification of brooks's and lexell's comet.[ ] an exceedingly close approach to jupiter in had, he found reason to believe, produced such extensive alterations in the elements of its motion as to bring the errant body back to our neighbourhood in . but his inference, though ratified by mr. charles lane poor's preliminary calculations, proved dubious on closer inquiry, and was rendered wholly inadmissible by the circumstances attending the return of brooks's comet in .[ ] the companion-objects watched by barnard in had by that time, perhaps, become dissipated in space, for they were not redetected. they represented, in all likelihood, wreckage from a collision with jupiter, dating, perhaps, so far back as , when mr. lane poor found that one of the fateful meetings to which short-period comets are especially subject had taken place. the lexell-brooks case was almost duplicated by the resemblance to de vico's lost comet of [ ] of one detected november , , by edward, son of lewis swift. schulhof[ ] announced the identity, and chandler,[ ] under reserve, vouched for it. had the comet continued to pursue the track laboriously laid down for it at boston, and shown itself at the due epoch in , its individuality might have been considered assured; but the formidable vicegerent of the sun once more interposed, and, in , swept it out of the terrestrial range of view. hence the recognition remains ambiguous. on the morning of march , , professor lewis swift discovered the brightest comet that had been seen by northern observers since . about the time of perihelion, which occurred on april , it was conspicuous, as it crossed the celestial equator from aquarius towards pegasus, with a nucleus equal to a third magnitude star, and a tail twenty degrees long. this tail was multiple, and multiple in a most curiously variable manner. it divided up into many thin nebulous streaks, the number and relative lustre of which underwent rapid and marked changes. their permanent record on barnard's and w. h. pickering's plates marked a noteworthy advance in cometary photography. plate iv. reproduces two of the lick pictures, taken with a six-inch camera, on april and respectively, with, in each case, an exposure of about one hour. the tail is in the first composed of three main branches, the middle one having sprung out since the previous morning, and the branches are, in their turn, split up into finer rays, to the number of perhaps a dozen in all. in the second a very different state of things is exhibited. "the southern component," professor barnard remarked, "which was the brightest on the th, had become diffused and fainter, while the middle tail was very bright and broad. its southern side, which was the best defined, was wavy in numerous places, the tail appearing as if disturbing currents were flowing at right angles to it. at ° from the head the tail made an abrupt bend towards the south, as if its current was deflected by some obstacle. in the densest portion of the tail, at the point of deflection, are a couple of dark holes, similar to those seen in some of the nebulæ. the middle portion of the tail is brighter, and looks like crumpled silk in places."[ ] next morning the southern was the prominent branch, and it was loaded, at ° ' from the head, with a strange excrescence, suggesting the budding-out of a fresh comet in that incongruous situation.[ ] some of these changes, professor barnard thought, might possibly be explained by a rotation of the tail on an axis passing through the nucleus, and pickering, who formed a similar opinion on independent grounds, assigned about hours as the period of the gyrating movement.[ ] he, moreover, determined accelerative velocities outward from the sun of definite condensations in the tail, indicating for its materials, on brédikhine's theory, a density less than one half that of hydrogen.[ ] this conclusion applied also to rordame's comet, which exhibited a year later phenomena analogous to those remarked in swift's. their photographic study led professor hussey[ ] to significant inferences as to the structure and rapid changes of cometary appendages. plate iv. [illustration: photographs of swift's comet. by professor e. e. barnard. no. . taken april , ; exposure h. no. . taken april , ; exposure h. m ] seven comets were detected in , and all, strange to say, were visible together towards the close of the year.[ ] among them was a faint object, which unexpectedly left a trail on a plate exposed by professor barnard to the stars in aquila[ ] on october . this was the first comet actually discovered by photography, the sohag comet having been simultaneously seen and pictured. it has a period of about six years. holmes's comet is likewise periodical, in rather less than seven years. its path, which is wholly comprised between the orbits of mars and jupiter, is less eccentric than that of any other known comet. subsequently to its discovery, on november , it underwent some curious vicissitudes. at first bright and condensed, it expanded rapidly with increasing distance from the sun (to which it had made its nearest approach on june ), until, by the middle of december, it was barely discernible with powerful telescopes as "a feebly luminous mist on the face of the sky."[ ] but on january , , observers in europe and america were bewildered to find, as if substituted for it, a yellow star of the seventh magnitude, enveloped in a thin nebulous husk, which enclosed a faint miniature tail.[ ] this condensation and recovery of light lasted in its full intensity only a couple of days. the almost evanescent faintness of holmes's comet at its next return accounted for its invisibility previous to , when it was evidently in a state of peculiar excitement. mr. perrine was barely able, with the lick -inch, to find the vague nebulous patch which occupied its predicted place on june , . the origin of comets has been long and eagerly inquired into, not altogether apart from the cheering guidance of ascertained facts. sir william herschel regarded them as fragments of nebulæ[ ]--scattered débris of embryo worlds; and laplace approved of and adopted the idea.[ ] but there was a difficulty. no comet has yet been observed to travel in a decided hyperbola. the typical cometary orbit, apart from disturbance, is parabolic--that is to say, it is indistinguishable from an enormously long ellipse. but this circumstance could only be reconciled with the view that the bodies thus moving were casual visitors from outer space, by making, as laplace did, the tacit assumption that the solar system was at rest. his reasoning was, indeed, thereby completely vitiated, as gauss pointed out in ;[ ] and the objections then urged were reiterated by schiaparelli,[ ] who demonstrated in that a large preponderance of well-marked hyperbolic orbits should result if comets were picked up _en route_ by a swiftly-advancing sun. the fact that their native movement is practically parabolic shows it to have been wholly imparted from without. they passively obeyed the pull exerted upon them. in other words, their condition previous to being attracted by the sun was one very nearly of relative repose.[ ] they shared, accordingly, the movement of translation through space of the solar system. this significant conclusion had been indicated, on other grounds, as the upshot of researches undertaken independently by carrington[ ] and mohn[ ] in , with a view to ascertaining the anticipated existence of a relationship between the general _lie_ of the paths of comets and the direction of the sun's journey. it is tolerably obvious that if they wander at haphazard through interstellar regions their apparitions should markedly aggregate towards the vicinity of the constellation lyra; that is to say, we should meet considerably more comets than would overtake us, for the very same reason that falling stars are more numerous after than before midnight. moreover, the comets met by us should be, apparently, swifter-moving objects than those coming up with us from behind; because, in the one case, our own real movement would be added to, in the other subtracted from, theirs. but nothing of all this can be detected. comets approach the sun indifferently from all quarters, and with velocities quite independent of direction. we conclude, then, that the "cosmical current" which bears the solar system towards its unknown goal carries also with it nebulous masses of undefined extent, and at an undefined remoteness, fragments detached from which, continually entering the sphere of the sun's attraction, flit across our skies under the form of comets. these are, however, almost certainly so far strangers to our system that they had no part in the long processes of development by which its present condition was attained. they are, perhaps, survivals of an earlier, and by us scarcely and dimly conceivable state of things, when the swirling chaos from which sun and planets were, by a supreme edict, to emerge, had not as yet separately begun to be. footnotes: [footnote : _astr. nach._, nos. , - .] [footnote : _berichte sächs. ges._, , p. .] [footnote : _natur der cometen_, p. ; _astr. nach._, no. , .] [footnote : _annales de l'obs. de moscou_, t. iii., pt. i., p. .] [footnote : _bull. astr._, t. iii., p. . the value of the repellent force for the comet of (which offered peculiar facilities for its determination) was found = · .] [footnote : faye, _comptes rendus_, t. xciii., p. .] [footnote : _annales_, t. v., pt. ii., p. .] [footnote : _am. jour. of sc._, vol. xxxii. ( nd ser.), p. .] [footnote : _astr. nach._, no. , .] [footnote : _annales de l'obs. de moscou_, t. vi., pt. i., p. .] [footnote : _astr. register_, march, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _constitution de l'espace céleste_, p. .] [footnote : _astroph. jour._, vol. iii., p. .] [footnote : _physikalische zeitschrift_, november and , ; _astroph. jour._, vol. xiii., p. . _cf._ schwarzschild, _sitzungsb._, münchen, , heft iii.; j. hahn, _nature_, vols. lxv., p. ; lxvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _observatory_, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ueber die kometen von v. chr._, , , i. und i. göttingen, .] [footnote : _meteor._, lib. i., cap. .] [footnote : _mém. soc. phys. de genève_, t. xxviii., p. .] [footnote : _annales de l'obs. de moscou_, t. vii., pt. i., p. .] [footnote : brédikhine, _annales_, t. viii., p. .] [footnote : _am. jour. of sc._, vol. xxii., p. .] [footnote : messrs. burton and green observed a dilatation of the stellar image into a nebulous patch by the transmission of its rays through a nuclear jet of the comet. _am. jour. of sc._, vol. xxii., p. .] [footnote : _archives des sciences_, t. viii., p. . _cf._ perrine's negative results for swift's comet in , _astr. nach._, no. , .] [footnote : riem concluded in for a definitive period of , years; _observatory_, vol. xix., p. .] [footnote : holden, _publ. astr. pac. soc._, vol. ix., p. .] [footnote : _annuaire_, paris, , p. .] [footnote : _annuaire_, , p. .] [footnote : _am. jour. of sc._, vol. xxii., p. .] [footnote : _report brit. assoc._, , p. .] [footnote : _month. not._, vol. xlii., p. ; _am. jour. of sc._, vol. xxii., p. .] [footnote : piazzi smyth, _nature_, vol. xxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. xlii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , .] [footnote : _report brit. assoc._, , p. .] [footnote : j. j. parsons, _am. jour. of science_, vol. xxvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. v., p. . the transit had been foreseen by mr. tebbutt, but it occurred after sunset in new south wales.] [footnote : _observatory_, vol. v., p. .] [footnote : gould, _astr. nach._, no. , .] [footnote : flammarion, _comptes rendus_, t. xcv., p. .] [footnote : captain ray's sextant observation of the comet of , a few hours before perihelion, was too rough to be of use.] [footnote : _astr. nach._, no. , .] [footnote : _nature_, vol. xxix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _vierteljahrsschrift astr. ges._, jahrg. xxiv., p. ; _bull. astr._, t. vii., p. .] [footnote : _observatory_, vol. xxiv., p. .] [footnote : the attention of the author was kindly directed to this point by professor young of princeton (n. j.). _cf._ rebeur-paschwitz, _sirius_, bd. xvi., p. .] [footnote : oppenheim, _astr. nach._, no. , .] [footnote : _astr. nach._, no. , .] [footnote : gruithuisen's _analekten_, heft , p. .] [footnote : _month. not._, vols. xxv., xxvi., xxviii. _cf._ plummer, _observatory_, vol. xiii., p. .] [footnote : _nature_, vol. xxvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _athenæum_, february , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , .] [footnote : _annales_, moscow, t. ix., pt. ii., p. .] [footnote : _comptes rendus_, t. xcvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : _comptes rendus_, t. xcvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astr. nach._, no. , .] [footnote : _annales de l'observatoire de nice_, t. ii., c. .] [footnote : fényi, _astr. nach._, no. , ; kammermann, _ibid._, no. , .] [footnote : _publ. astr. pac. soc._, vol. i., p. .] [footnote : _annuaire_, paris, , p. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. xxv., p. .] [footnote : _astr. journ._, nos. , .] [footnote : _ibid._, nos. , , .] [footnote : _observatory_, vol. xviii., pp. , (denning and lynn).] [footnote : _astr. nach._, no. , ; plummer, _knowledge_, vol. xix., p. .] [footnote : _astr. jour._, nos. , .] [footnote : _astr. and astroph._, vol. xi., p. .] [footnote : _knowledge_, vol. xv., p. .] [footnote : _harvard annals_, vol. xxxii., pt. ii., p. .] [footnote : _ibid._, p. .] [footnote : _publ. astr. pac. soc._, vol. vii., p. .] [footnote : h. c. wilson, _astr. and astroph._, vol. xii., p. .] [footnote : _observatory_, vol. xvi., p. .] [footnote : barnard, _astr. and astroph._, vol. xii., p. ; _astroph. jour._, vol. iii., p. .] [footnote : palisa, _astr. nach._, no. , ; denning, _observatory_, vol. xvi., p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _conn. des temps_, , p. .] [footnote : _oeuvres_, t. vi., p. .] [footnote : _mem. dell' istit. lombardo_, t. xii., p. ; _rendiconti_, t. vii., p. , .] [footnote : w. förster, _pop. mitth._, , p. ; fabry, _Étude sur la probabilité des comètes hyperboliques_, marseille, , p. .] [footnote : _mem. r. a. soc._, vol. xxix., p. .] [footnote : _month. not._, vol. xxiii., p. .] chapter xii _stars and nebulÆ_ that a science of stellar chemistry should not only have become possible, but should already have made material advances, is assuredly one of the most amazing features in the swift progress of knowledge our age has witnessed. custom can never blunt the wonder with which we must regard the achievement of compelling rays emanating from a source devoid of sensible magnitude through immeasurable distance, to reveal, by its distinctive qualities, the composition of that source. the discovery of revolving double stars assured us that the great governing force of the planetary movements, and of our own material existence, sways equally the courses of the farthest suns in space; the application of prismatic analysis certified to the presence in the stars of the familiar materials, no less of the earth we tread, than of the human bodies built up out of its dust and circumambient vapours. we have seen that, as early as , fraunhofer ascertained the generic participation of stellar light in the peculiarity by which sunlight, spread out by transmission through a prism, shows numerous transverse rulings of interrupting darkness. no sooner had kirchhoff supplied the key to the hidden meaning of those ciphered characters than it was eagerly turned to the interpretation of the dim scrolls unfolded in the spectra of the stars. donati made at florence in the first efforts in this direction; but with little result, owing to the imperfections of the instrumental means at his command. his comparative failure, however, was a prelude to others' success. almost simultaneously, in , the novel line of investigation was entered upon by huggins near london, by father secchi at rome, and by lewis m. rutherfurd in new york. fraunhofer's device of using a cylindrical lens for the purpose of giving a second dimension to stellar spectra was adopted by all, and was, indeed, indispensable. for a luminous point, such as a star appears, becomes, when viewed through a prism, a variegated line, which, until broadened into a band by the intervention of a cylindrical lens, is all but useless for purposes of research. this process of _rolling out_ involves, it is true, much loss of light--a scanty and precious commodity, as coming from the stars; but the loss is an inevitable one. and so fully is it compensated by the great light-grasping power of modern telescopes that important information can now be gained from the spectroscopic examination of stars far below the range of the unarmed eye. the effective founders of stellar spectroscopy, then (since rutherfurd shortly turned his efforts elsewhither), were father secchi, the eminent jesuit astronomer of the collegio romano, where he died, february , , and sir william huggins, with whom the late professor w. a. miller was associated. the work of each was happily directed so as to supplement that of the other. with less perfect appliances, the roman astronomer sought to render his extensive rather than precise; at tulse hill searching accuracy over a narrow range was aimed at and attained. to father secchi is due the merit of having executed the first spectroscopic survey of the heavens. above , stars were passed in review by him, and classified according to the varying qualities of their light. his provisional establishment ( - ) of four types of stellar spectra[ ] has proved a genuine aid to knowledge through the facilities afforded by it for the arrangement and comparison of rapidly accumulating facts. moreover, it is scarcely doubtful that these spectral distinctions correspond to differences in physical condition of a marked kind. the first order comprises more than half the visible and probably an overwhelming proportion of the faintest stars. sirius, vega, regulus, altair, are amongst its leading members. their spectra are distinguished by the breadth and intensity of the four dark bars due to the absorption of hydrogen, and by the extreme faintness of the metallic lines, of which, nevertheless, hundreds are disclosed by careful examination. the light of these "sirian" orbs is white or bluish; and it is found to be rich in ultra-violet rays. capella and arcturus belong to the second, or solar type of stars, which is about one-sixth less numerously represented than the first. their spectra are quite closely similar to that of sunlight, in being ruled throughout by innumerable fine dark lines; and they share its yellowish tinge. the third class includes most red and variable stars (commonly synonymous), of which betelgeux in the shoulder of orion, and "mira" in the whale, are noted examples. their characteristic spectrum is of the "fluted" description. it shows like a strongly illuminated range of seven or eight variously tinted columns seen in perspective, the light falling from the red end towards the violet. this _kind_ of absorption is produced by the vapours of metalloids or of compound substances. to the fourth order of stars belongs also a colonnaded spectrum, but _reversed_; the light is thrown the other way. the three broad zones of absorption which interrupt it are sharp towards the red, insensibly gradated towards the violet end. the individuals composing class iv. are few and apparently insignificant, the brightest of them not exceeding the fifth magnitude. they are commonly distinguished by a deep red tint, and gleam like rubies in the field of the telescope. father secchi, who in detected the peculiarity of their analyzed light, ascribed it to the presence of carbon in some form in their atmospheres; and this was confirmed by the researches of h. c. vogel,[ ] director of the astro-physical observatory at potsdam. the hydro-carbon bands, in fact, seen bright in comets, are dark in these singular objects--the only ones in the heavens (save one bright-line star and a rare meteor)[ ] which display a cometary analogy of the fundamental sort revealed by the spectroscope. the members of all four orders are, however, emphatically suns. they possess, it would appear, photospheres radiating all kinds of light, and differ from each other mainly in the varying qualities of their absorptive atmospheres. the principle that the colours of stars depend, not on the intrinsic nature of their light, but on the kinds of vapours surrounding them, and stopping out certain portions of that light, was laid down by huggins in .[ ] the phenomena of double stars seem to indicate a connection between the state of the investing atmospheres, by the action of which their often brilliantly contrasted tints are produced, and their mutual physical relations. a tabular statement put forward by professor holden in june, ,[ ] made it, at any rate, clear that inequality of magnitude between the components of binary systems accompanies unlikeness in colour, and that stars more equally matched in one respect are pretty sure to be so in the other. besides, blue and green stars of a decided tinge are never solitary; they invariably form part of systems. so that association has undoubtedly a predominant influence upon colour. nevertheless, the crude notion thrown out by zöllner in ,[ ] that yellow and red stars are simply white stars in various stages of cooling, obtained for a time undeserved currency. d'arrest, indeed, protested against it, and Ångström, in ,[ ] substituted atmospheric quality for mere colour[ ] as a criterion of age and temperature. his lead was followed by lockyer in ,[ ] and by vogel in .[ ] the scheme of classification due to the potsdam astro-physicist differed from father secchi's only in presenting his third and fourth types as subdivisions of the same order, and in inserting three subordinate categories; but their variety was "rationalised" by the addition of the seductive idea of progressive development. thus, the white sirian stars were represented as the _youngest_ because the hottest of the sidereal family; those of the solar pattern as having already wasted much of their store by radiation, and being well advanced in middle life; while the red stars with banded spectra figured as effete suns, hastening rapidly down the road to final extinction. vogel's scheme is, however, incomplete. it traces the downward curve of decay, but gives no account of the slow ascent to maturity. the present splendour of vega, for instance, was prepared, according to all creative analogy, by almost endless processes of gradual change. what was its antecedent condition? the question has been variously answered. dr. johnstone stoney advocated, in , the comparative youth of red stars;[ ] a. ritter, of aix-la-chapelle, divided them, in ,[ ] into two squadrons, posted, the one on the ascending, the other on the descending branch of the temperature-curve, and corresponding, presumably, with secchi's third and fourth orders of stars with banded spectra. whether, in the interim, they should display spectra of the sirian or of the solar type was made to depend on their greater or less massiveness.[ ] but the relation actually existing perhaps inverts that contemplated by ritter. certainly, the evidence collected by mr. maunder in strongly supports the opinion that the average solar star is a weightier body than the average sirian star.[ ] on november , , sir norman lockyer communicated to the royal society the first of a series of papers embodying his "meteoritic hypothesis" of cosmical constitution, stated and supported more at large in a separate work bearing that name, published in . the fundamental proposition wrought out in it was that "all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat."[ ] on the basis of this supposed community of origin, sidereal objects were distributed in seven groups along a temperature-curve ascending from nebulæ and gaseous, or bright-line stars, through red stars of the third type, and a younger division of solar stars, to the high sirian level; then descending through the more strictly solar stars to red stars of the fourth type ("carbon-stars"), below which lay only the _caput mortuum_ entitled group vii. the ground-work of this classification was, however, insecure, and has given way. certain spectroscopic coincidences, avowedly only approximate, suggesting that stars and nebulæ of every species might be formed out of variously aggregated meteorites, failed of verification by exact inquiry. and spectroscopic coincidences admit of no compromise. those that are merely approximate are, as a rule, unmeaning. in his presidential address at the cardiff meeting of the british association in , dr. huggins adhered in the main to the line of advance traced by vogel. the inconspicuousness of metallic lines in the spectra of the white stars he attributed, not to the paucity, but to the high temperature of the vapours producing them, and the consequent deficiency of contrast between their absorption-rays and the continuous light of the photospheric background. "such a state of things would more probably," in his opinion, "be found in conditions anterior to the solar stage," while "a considerable cooling of the sun would probably give rise to banded spectra due to compounds." he adverted also to the influential effects upon stellar types of varying surface gravity, which being a function of both mass and bulk necessarily gains strength with wasting heat and consequent shrinkage. the same leading ideas were more fully worked out in "an atlas of representative stellar spectra," published by sir william and lady huggins in . they were, moreover, splendidly illustrated by a set of original spectrographic plates, while precision was added to the adopted classification by the separation of helium from hydrogen stars. the spectrum of the exotic substance terrestrially captured in is conspicuous by absorption, as vogel, lockyer, and deslandres promptly recognised in a considerable number of white stars, among them the pleiades and most of the brilliants in orion. mr. mcclean, whose valuable spectrographic survey of the heavens was completed at the cape in , found reason to conclude that they are in the first stage of development from gaseous nebulæ;[ ] and in this the tulse hill investigators unhesitatingly concur. the strongest evidence for the primitive state of white stars is found in their nebular relations. the components of groups, still involved and entangled with "silver braids" of cosmic mist, show, perhaps invariably, spectra of the helium type, occasionally crossed by bright rays. possibly all such stars have passed through a bright-line stage; but further evidence on the point is needed. relative density furnishes another important test of comparative age, and sirian stars are, on the whole, undoubtedly more bulky proportionately to their mass than solar stars. the rule, however, seems to admit of exceptions; hence the change from one kind of spectrum to the other is not inevitably connected with the attainment of a particular degree of condensation. there is reason to believe that it is anticipated in the more massive globes, despite their comparatively slow cooling, as a consequence of the greater power of gravity over their investing vaporous envelopes. this conclusion is enforced by the relations of double-star spectra. the fact that, in unequal pairs, the chief star most frequently shows a solar, its companion a sirian, spectrum can scarcely be otherwise explained than by admitting that, while the sequence of types is pursued in an invariable order, it is pursued much more rapidly in larger than in small orbs. it need not, indeed, be supposed that all stars are identical in constitution, and present identical life-histories.[ ] individualities in the one, and divergencies in the other, must be allowed for. yet the main track is plainly continuous, and leads by insensible gradations from nebulæ through helium stars to the sirian, and onward to the solar type, whence, by an inevitable transition, fluted, or "antarian,"[ ] spectra develop. the first-known examples of the class of gaseous stars--beta lyræ and gamma cassiopeiæ--were noticed by father secchi at the outset of his spectroscopic inquiries. both show _bright_ lines of hydrogen and helium, so that the peculiarity of their condition probably consists in the intense ignition of their chromospheric surroundings. their entire radiating surfaces might be described as _faculous_. that is to say, brilliant formations, such as have been photographed by professor hale on the sun's disc,[ ] cover, perhaps, the whole, instead of being limited to a small portion of the photospheric area. but this state of things is more or less inconstant. some at least of the bright rays indicative of it are subject to temporary extinctions. already in - , dr. vogel[ ] suspected the prevalence of such vicissitudes; and their reality was ascertained by m. eugen von gothard. after the completion of his new astrophysical observatory at herény in the autumn of , he repeatedly observed the spectra of both stars without perceiving a trace of bright lines; and was thus taken quite by surprise when he caught a twinkling of the crimson c in gamma cassiopeiæ, august , .[ ] a few days later, the whole range including d_ was lustrous. duly apprised of the recurrence of a phenomenon he had himself vainly looked for during some years, m. von konkoly took the opportunity of the great vienna refractor being placed at his disposal to examine with it the relighted spectrum on august .[ ] in its wealth of light c was dazzling; d_ and the green and blue hydrogen rays shone somewhat less vividly; d and the group _b_ showed faintly dark; while three broad absorption-bands, sharply terminated towards the red, diffuse towards the violet, shaded the spectrum near its opposite extremities. the previous absence of bright lines from the spectrum of this star was, however, by no means so protracted or complete as m. von gothard supposed. at dunecht, c was "superbly visible" december , [ ]; f was seen bright on october of the same year, and frequently at greenwich in - . the curious fact has, moreover, been adverted to by dr. copeland, that c _is much more variable than f_. to vogel, june , , the first was invisible, while the second was bright; at dunecht, january , , the conditions were so far inverted that c was resplendent, f comparatively dim. no spectral fluctuations were detected in gamma cassiopeiæ by keeler in ; but even with the giant telescope of mount hamilton, the helium-ray was completely invisible.[ ] it made, nevertheless, capricious appearances at south kensington during that autumn, and again october , ,[ ] while in september, , bélopolsky could obtain no trace of it on orthochromatic plates exposed with the -inch pulkowa refractor.[ ] still more noteworthy is the circumstance that the well-known green triplet of magnesium (_b_), recorded as dark by keeler in , came out bright on fifty-two spectrographs of the star taken by father sidgreaves during the years - .[ ] no fluctuations in the hydrogen-spectrum were betrayed by them; but subordinate lines of unknown origin showed alternate fading and vivification. the spectrum of beta lyræ undergoes transitions to some extent analogous, yet involving a different set of considerations. first noticed by von gothard in ,[ ] they were imperfectly made out, two years later, to be of a cyclical character.[ ] this, however, could only be effectively determined by photographic means. beta lyræ is a "short-period variable." its light changes with great regularity from · to · magnitude every twelve days and twenty-two hours, during which time it attains a twofold maximum, with an intervening secondary minimum. the question, then, is of singular interest, whether the changes of spectral quality visible in this object correspond to its changes in visual brightness. a distinct answer in the affirmative was supplied through mrs. fleming's examination of the harvard plates of the star's spectrum, upon which, in , she found recorded diverse complex changes of bright and dark lines obviously connected with the phases of luminous variation, and obeying, in the long-run, precisely the same period.[ ] something more will be said presently as to the import of this discovery. bright hydrogen lines have so far been detected--for the most part photographically at harvard college--in about sixty stars, including pleione, the surmised lost pleiad, p cygni, noted for instability of light in the seventeenth century, and the extraordinary southern variable, eta carinæ. in most of these objects other vivid rays are associated with those due to hydrogen. a blaze of hydrogen, moreover, accompanies the recurring outbursts of about one hundred and fifty "long-period variables," giving banded spectra of the third type. professor pickering discovered the first example of this class, towards the close of , in mira ceti; further detections were made visually by mr. espin; and the conjunction of bright hydrogen-lines with dusky bands has been proved by mrs. fleming's long experience in studying the harvard photographs, to indicate unerringly the subjection of the stars thus characterised to variations of lustre accomplished in some months. a third variety of gaseous star is named after mm. wolf and rayet, who discovered, at paris in ,[ ] its three typical representatives, close together in the constellation cygnus. six further specimens were discovered by dr. copeland, five of them in the course of a trip for the exploration of visual facilities in the andes in ;[ ] and a large number have been made known through spectral photographs taken in both hemispheres under professor pickering's direction. at the close of the nineteenth century, over a hundred such objects had been registered, none brighter than the sixth magnitude, with the single exception of gamma argûs, the resplendent continuous spectrum of which, first examined by respighi and lockyer in , is embellished with the yellow and blue rays distinctive of the type.[ ] here, then, we have a stellar globe apparently at the highest point of sunlike incandescence, sharing the peculiarities of bodies verging towards the nebulous state. examined with instruments of adequate power, their spectra are seen to be highly complex. they include a fairly strong continuous element, a numerous set of absorption-lines, and a range of emission-lines, more or less completely represented in different stars. especially conspicuous is a broad effluence of azure light, found by dr. vogel in ,[ ] and by sir william and lady huggins in ,[ ] to be of multiple structure, and hence to vary in its mode of display. its suggested identification with the blue carbon-fluting was disproved at tulse hill. metallic vapours give no certain sign of their presence in the atmospheres of these remarkable bodies; but nebulum is stated to shine in some.[ ] hydrogen and helium account for a large proportion of their spectral rays. thirty-two wolf-rayet stars were investigated, spectroscopically and spectrographically, by professor campbell with the great lick refractor in - ;[ ] and several disclosed the singularity, already noticed by him in gamma argûs, of giving out mixed series, the members of which change from vivid to obscure with increase of refrangibility. it is difficult to imagine by what chromospheric machinery this curious result can be produced. alcyone in the pleiades presents the same characteristic. alone among the hydrogen lines, crimson c glows in its spectrum, while all the others are dark. luminosity of the wolf-rayet kind is particularly constant, both in quantity and quality. it seems to be incapable of developing save under galactic conditions. all the stars marked by it lie near the central line of the milky way, or in the magellanic clouds. they tend also to gather into groups. circles of four degrees radius include respectively seven in argo, eight in cygnus. the first spectroscopic star catalogue was published by dr. vogel at potsdam in .[ ] it included , stars, distributed over a zone of the heavens extending from ° north to ° south of the celestial equator.[ ] more than half of these were white stars, while red stars with banded spectra occurred in the proportion of about one-thirteenth of the whole. to the latter genus, m. dunér, then of lund, now director of the upsala observatory, devoted a work of standard authority, issued at stockholm in . this was a catalogue with descriptive particulars of stars showing banded spectra, of which belong to secchi's third, to his fourth class (vogel's iii. _a_ and iii. _b_). since then discovery has progressed so rapidly, at first through the telescopic reviews of mr. espin, then in the course of the photographic survey carried on at harvard college, that considerably over one thousand stars are at present recognised as of the family of betelgeux and mira, while about have so far exhibited the spectral pattern of piscium. one fact well ascertained as regards both species is the invariability of the type. the prismatic flutings of the one, and the broader zones of the other, are as if stereotyped--they undergo, in their fundamental outlines, no modification, though varying in relative intensity from star to star. they are always accompanied by, or superposed upon, a spectrum of dark lines, in producing which sodium and iron have an obvious share; and certain bright rays, noticed by secchi with imperfect appliances as enhancing the chiaroscuro effects in carbon-stars, came out upon plates exposed by hale and ellerman in with the stellar spectrograph of the yerkes observatory.[ ] their genuineness was shortly afterwards visually attested by keeler, campbell, and dunér;[ ] but no chemical interpretation has been found for them. a fairly complete preliminary answer to the question, what are the stars made of? was given by sir william huggins in .[ ] by laborious processes of comparison between stellar dark lines and the bright rays emitted by terrestrial substances, he sought to assure his conclusions, regardless of cost in time and pains. he averred, indeed, that--taking into account restrictions by weather and position--the thorough investigation of a _single_ star-spectrum would be the work of some years. of two, however--those of betelgeux and aldebaran--he was able to furnish detailed and accurate drawings. the dusky flutings in the prismatic light of the first of these stars have not been identified with the absorption of any particular substance; but associated with them are metallic lines, of which were measured, and a good many identified by huggins, while the wave-lengths of were determined by vogel in .[ ] a photographic research, made by keeler at the alleghany observatory in , convinced him that the linear spectrum of third-type stars of the betelgeux pattern essentially repeats that of the sun, but with marked differences in the comparative strength of its components.[ ] hydrogen rays are inconspicuously present. that an exalted temperature reigns, at least in the lower strata of the atmosphere, is certified by the vaporisation there of matter so refractory to heat as iron.[ ] nine elements--among them iron, sodium, calcium, and magnesium--were recognised by huggins as having stamped their signature on the spectrum of aldebaran; while the existence in sirius, and nearly all the other stars inspected, of hydrogen, together with sundry metals, was rendered certain or highly probable. this was admitted to be a bare gleaning of results; nor is there reason to suppose any of his congeners inferior to our sun in complexity of constitution. definite knowledge on the subject, however, made little advance beyond the point to which it was brought by huggins's early experiments until spectroscopic photography became thoroughly effective as a means of research. in this, as in so many other directions, sir william huggins acted as pioneer. in march, , he obtained microscopic prints of the spectra of sirius and capella.[ ] but they told nothing. no lines were visible in them. they were mere characterless streaks of light. nine years later dr. henry draper of new york got an impression of four lines in the spectrum of vega. then huggins attacked the subject again in , when the -inch speculum of the royal society had come into his possession, using prisms of iceland spar and lenses of rock crystal; and this time with better success. a photograph of the spectrum of vega showed seven strong lines.[ ] still he was not satisfied. he waited and worked for three years longer. at length, on december , , he was able to communicate to the royal society[ ] results answering to his expectations. the delicacy of eye and hand needed to obtain them may be estimated from the single fact that the image of a star had to be kept, by continual minute adjustments, exactly projected upon a slit / of an inch in width during nearly an hour, in order to give it time to imprint the characters of its analyzed light upon a gelatine plate raised to the highest pitch of sensitiveness. but by this time he had secured in his wife a rarely qualified assistant. the ultra-violet spectrum of the white stars--of which vega was taken as the type--was thus shown to be a very remarkable one. a group of broad dark lines intersected it, arranged at intervals diminishing regularly upward, and falling into a rhythmical succession with the visible hydrogen lines. all belonged presumably to the same substance; and the presumption was rendered a certainty by direct photographs of the hydrogen spectrum taken by h. w. vogel at berlin a few months earlier.[ ] in them seven of the white-star series of grouped lines were visible; and the full complement of twelve appeared on cornu's plates in .[ ] in yellow stars, such as capella and arcturus, the same rhythmical series was _partially_ represented, but associated with a great number of other lines; their state, as regards ultra-violet absorption, approximating to that of the sun; while the redder stars betrayed so marked a deficiency in actinic rays that from betelgeux, with an exposure _forty times_ that required for sirius, only a faint spectral impression could be obtained, and from aldebaran, in the strictly invisible region, almost none at all. thus, by the means of stellar light-analysis, acquaintance was first made with the ultra-violet spectrum of hydrogen;[ ] and its harmonic character, as expressed by "balmer's law," supplies a sure test for discriminating, among newly discovered lines, those that appertain from those that are unrelated to it. deslandres' five additional prominence-rays, for instance, were at once seen to make part of the series, because conforming to its law;[ ] while a group of six dusky bands, photographed by sir william and lady huggins, april , ,[ ] near the extreme upper end of the spectrum of sirius, were pronounced without hesitation, for the opposite reason, to have nothing to do with hydrogen. their true affinities are still a matter for inquiry. as regards the hydrogen spectrum, however, the stars had further information in reserve. until recently, it was supposed to consist of a single harmonic series, although, by analogy, three should co-exist. in , accordingly, a second, bound to the first by unmistakable numerical relationships, was recognised by professor pickering in spectrographs of the · magnitude star zeta puppis,[ ] and the identification was shortly afterwards extended to prominent wolf-rayet emission lines. the discovery was capped by dr. rydberg's indication of the wolf-rayet blue band at lambda , as the fundamental member of the third, and principal, hydrogen series.[ ] none of the "pickering lines" (as they may be called to distinguish them from the "huggins series") can be induced to glimmer in vacuum-tubes. they seem to characterise bodies in a primitive state,[ ] and are in many cases associated with absorption rays of oxygen, the identification of which by mr. mcclean in [ ] was fully confirmed by sir david gill.[ ] the typical "oxygen star" is beta crucis, one of the brilliants of the southern cross; but the distinctive notes of its spectrum occur in not a few specimens of the helium class. thus, sir william and lady huggins photographed several ultra-violet oxygen lines in beta lyræ,[ ] and found in rigel signs of the presence of nitrogen,[ ] which, as well as silicium, proves to be a tolerably frequent constituent of such orbs.[ ] for some unknown reason, metalloids tend to become effaced, as metals, in the normal course of stellar development, exert a more and more conspicuous action. dr. scheiner's spectrographic researches at potsdam in and subsequently, exemplify the immense advantages of self-registration. in a restricted section of the spectrum of capella, he was enabled to determine nearly three hundred lines with more precision than had then been attained in the measurement of terrestrial spectra. this star appeared to be virtually identical with the sun in physical constitution, although it emits, according to the best available data, about times as much light, and is hence presumably , times more voluminous. an equally close examination of the spectrum of betelgeux showed the predominance in it of the linear absorption of iron;[ ] but its characteristic flutings do not extend to the photographic region. spectra of the second and third orders are for this reason not easily distinguished on the sensitive plate. a spectrographic investigation of all the brighter northern stars was set on foot in at the observatory of harvard college, under the form of a memorial to dr. h. draper, whose promising work in that line was brought to a close by his premature death in . no individual exertions could, however, have realized a tithe of what has been and is being accomplished under professor pickering's able direction, with the aid of the draper and other instruments, supplemented by mrs. draper's liberal provision of funds. a novel system was adopted, or, rather, an old one--originally used by fraunhofer--was revived.[ ] the use of a slit was discarded as unnecessary for objects like the stars, devoid of sensible dimensions, and giving hence a _naturally_ pure spectrum; and a large prism, placed in front of the object-glass, analysed at once, with slight loss of light, the rays of all the stars in the field. their spectra were taken, as it were, wholesale. as many as two hundred stars down to the eighth magnitude were occasionally printed on a single plate with a single exposure. no cylindrical lens was employed. the movement of the stars themselves was turned to account for giving the desirable width to their spectra. the star was allowed--by disconnecting or suitably regulating the clock--to travel slowly across the line of its own dispersed light, so broadening it gradually into a band. excellent results were secured in this way. about fifty lines appear in the photographed spectrum of aldebaran, and eight in that of vega. on january , , with an exposure of thirty-four minutes, a simultaneous impression was obtained of the spectra (among many others) of close upon forty pleiades. with few and doubtful exceptions, they all proved to belong to the same type. an additional argument for the common origin of the stars forming this beautiful group was thus provided.[ ] the "draper catalogue" of stellar spectra was published in .[ ] it gives the results of a rapid analytical survey of the heavens north of ° of southern declination, and includes , stars, down to about the eighth magnitude. the telescope used was of eight inches aperture and forty-five focus, its field of view--owing to the "portrait-lens" or "doublet" form given to it--embracing with fair definition no less than one hundred square degrees. an objective prism eight inches square was attached, and exposures of a few minutes were given to the most sensitive plates that could be procured. in this way the sky was twice covered in duplicate, each star appearing, as a rule, on four plates. the registration of their spectra was sought to be made more distinctive than had previously been attempted, secchi's first type being divided into four, his second into five subdivisions; but the differences regarded in them could be confidently established only for stars above the sixth magnitude. the work supplies none the less valuable materials for general inferences as to the distribution and relations of the spectral types. the labour of its actual preparation was borne by a staff of ladies under the direction of mrs. fleming. materials for its completion to the southern pole have been accumulated with the identical instrument used at cambridge, transferred for the purpose in to peru, and the forthcoming "second draper catalogue" will comprise , stars in both hemispheres. as supplements to this great enterprise, two important detailed discussions of stellar spectra were issued in and respectively.[ ] the first, by miss a. c. maury, dealt with bright stars visible in the northern hemisphere; the second, by miss a. j. cannon, with , southern stars. both authors traced, with care and ability, the minute gradations by which the long process of stellar evolution appears to be accomplished. the progress of the draper memorial researches was marked by discoveries of an unexampled kind. the principle upon which "motion in the line of sight" can be detected and measured with the spectroscope has already been explained.[ ] it depends, as our readers will remember, upon the removal of certain lines, dark or bright (it matters not which), from their normal places by almost infinitesimal amounts. the whole spectrum of the moving object, in fact, is very slightly _shoved_ hither or thither, according as it is travelling towards or from the eye; but, for convenience of measurement, one line is usually picked out from the rest, and attention concentrated upon it. the application of this method to the stars, however, is encompassed with difficulties. it needs a powerfully dispersive spectroscope to show line-displacements of the minute order in question; and powerful dispersion involves a strictly proportionate enfeeblement of light. this, where the supply is already to a deplorable extent niggardly, can ill be afforded; for which reason the operation of determining a star's approach or recession is, even apart from atmospheric obstacles, an excessively delicate one. it was first executed by sir william huggins early in .[ ] selecting the brightest star in the heavens as the most promising subject of experiment, he considered the f line in the spectrum of sirius to be just so much displaced towards the red as to indicate (the orbital motion of the earth being deducted) recession at the rate of twenty-nine miles a second; and the reality and direction of the movement were ratified by vogel and lohse's observation, march , , of a similar, but even more considerable displacement.[ ] the inquiry was resumed by huggins with improved apparatus in the following year, when the velocities of thirty stars were approximately determined.[ ] the retreat of sirius, which proved slower than had at first been supposed, was now announced to be shared, at rates varying from twelve to twenty-nine miles, by betelgeux, rigel, castor, regulus, and five of the principal stars in the plough. arcturus, on the contrary, gave signs of rapid approach, as well as pollux, vega, deneb in the swan, and the brightness of the pointers. numerically, indeed, these results were encompassed with uncertainty. thus, arcturus is now fully ascertained to be travelling towards the sun at the comparatively slow pace of less than five miles a second; and sirius moves twice as fast in the same direction. the great difficulty of measuring so distended a line as the sirian f might, indeed, well account for some apparent anomalies. the scope of sir william huggins's achievement was not, however, to provide definitive data, but to establish as practicable the method of procuring them. in this he was thoroughly successful, and his success was of incalculable value. spectroscopic investigations of stellar movements may confidently be expected to play a leading part in the unravelment of the vast and complex relations which we can dimly detect as prevailing among the innumerable orbs of the sidereal world; for it supplements the means which we possess of measuring by direct observation movements transverse to the line of sight, and thus completes our knowledge of the courses and velocities of stars at ascertained distances, while supplying for all a valuable index to the amount of perspective foreshortening of apparent movement. thus some, even if an imperfect, knowledge may at length be gained of the revolutions of the stars--of the systems they unite to form, of the paths they respectively pursue, and of the forces under the compulsion of which they travel. the applicability of the method to determining the orbital motions of double stars was pointed out by fox talbot in ;[ ] but its use for their discovery revealed itself spontaneously through the harvard college photographs. in "spectrograms" of zeta ursæ majoris (mizar), taken in , and again in , the k line was seen to be double; while on other plates it appeared single. a careful study of miss a. c. maury of a series of seventy impressions indicated for the doubling a period of fifty-two days, and showed it to affect all the lines in the spectrum.[ ] the only available, and no doubt the true, explanation of the phenomenon was that two similar and nearly equal stars are here merged into one telescopically indivisible; their combined light giving a single or double spectrum, according as their orbital velocities are directed across or along our line of sight. the movements of a revolving pair of stars must always be opposite in sense, and proportionately equal in amount. that is, they at all times travel with speeds in the inverse ratio of their masses. hence, unless the plane of their orbits be perpendicular to a plane passing through the eye, there must be two opposite points where their velocities in the line of sight reach a maximum, and two diametrically opposite points where they touch zero. the lines in their common spectrum would thus appear alternately double and single twice in the course of each revolution. to that of mizar, at first supposed to need days for its completion, a period of only twenty days fourteen hours was finally assigned by vogel.[ ] anomalous spectral effects, probably due to the very considerable eccentricity of the orbit, long impeded its satisfactory determination. the mean distance apart of the component stars, as now ascertained, is just twenty-two million miles, and their joint mass quadruples that of the sun. but these are minimum estimates. for if the orbital plane be inclined, much or little, to the line of sight, the dimensions and mass of the system should be proportionately increased. an analogous discovery was made by miss maury in . but in the spectrum of beta aurigæ, the lines open out and close up on alternate days, indicating a relative orbit[ ] with a radius of less than eight million miles, traversed in about four days. this implies a rate of travel for each star of sixty-five miles a second, and a combined mass · times that of the sun. the components are approximately equal, both in mass and light,[ ] and the system formed by them is transported towards us with a speed of some sixteen miles a second. the line-shiftings so singularly communicative proceed, in this star, with perfect regularity. this new class of "spectroscopic binaries" could never have been visually disclosed. the distance of beta aurigæ from the earth, as determined, somewhat doubtfully, by professor pritchard, is nearly three and a third million times that of the earth from the sun (parallax = · "); whence it has been calculated that the greatest angular separation of the revolving stars is only five-thousandths of a second of arc.[ ] to make this evanescent interval perceptible, a telescope eighty feet in aperture would be required. the zodiacal star, spica (alpha virginis), was announced by dr. vogel, april , ,[ ] to belong to the novel category, with the difference, however, of possessing a nearly dark, instead of a brilliantly lustrous companion. in this case, accordingly, the tell-tale spectroscopic variations consist merely in a slight swinging to and fro of single lines. no second spectrum leaves a legible trace on the plate. spica revolves in four days at the rate of fifty-seven miles a second,[ ] or quicker, in proportion as its orbit is more inclined to the line of sight, round a centre at a minimum distance of three millions of miles. but the position of the second star being unknown, the mass of the system remains indeterminate. the lesser component of the splendid, slowly revolving binary, castor, is also closely double. its spectral lines were found by bélopolsky in [ ] to oscillate once in nearly three days, the secondary globe being apparently quite obscure. further study of the movements thus betrayed elicited the fact that the major axis of the eclipse traversed revolves in a period of , days, as a consequence, most likely, of the flattened shape of the stars.[ ] still more unexpected was the simultaneous assignment, by campbell and newall, of a duplex character to capella.[ ] here both components shine, though with a different quality of light, one giving a pure solar spectrum, the other claiming prismatic affinity with procyon. their mutual circulation is performed in days, and the radius of their orbit cannot be less, and may be a great deal more, than , , miles. hence the possibility is not excluded that the star--which has an authentic parallax of · "--may be visually resolved. indeed, signs of "elongation" were thought to be perceptible with the greenwich -inch refractor,[ ] while only round images could be seen at lick.[ ] another noteworthy case is that of polaris, found by campbell to have certainly one, and probably two obscure attendants.[ ] through his systematic investigations of stellar radial velocities with the mills spectrograph, knowledge in this department has, since , progressed so rapidly that the spectroscopic binaries of our acquaintance already number half a hundred, and ten times as many more doubtless lie within easy range of detection. now it is evident that a spectroscopic binary, if the plane of its motion made a very small angle with the line of sight, would be a variable star. for, during a few hours of each revolution, some at least of its light should be cut off by a transit of its dusky companion. such "eclipse-stars" are actually found in the heavens. the best and longest-known member of the group is algol in the head of medusa, the "demon-star" of the arabs.[ ] this remarkable object, normally above the third magnitude, loses and regains three-fifths of its light once in · hours, the change being completed in about twelve hours. its definite and limited nature, and punctual recurrence, suggested to goodricke of york, by whom the periodicity of the star was discovered in ,[ ] the interposition of a large dark satellite. but the conditions involved by the explanation were first seriously investigated by pickering in .[ ] he found that the phenomena could be satisfactorily accounted for by supposing an obscure body · the bright star's diameter to revolve round it in a period identical with that of its observed variation. this theoretical forecast was verified with singular exactitude at potsdam in .[ ] a series of spectral photographs taken there showed each of algol's minima to be preceded by a rapid recession from the earth, and succeeded by a rapid movement of approach towards it. they take place, accordingly, when the star is at the furthest point from ourselves of an orbit described round an invisible companion, the transits of which across its disc betray themselves to notice by the luminous vicissitudes they occasion. the diameter of this orbit, traversed at the rate of twenty-six miles a second, is just , , miles; and it is an easy further inference from the duration and extent of the phases exhibited that algol itself must be (in round numbers) one million, its attendant , miles in diameter. assuming both to be of the same density, vogel found their respective masses to be four-ninths and two-ninths that of the sun, and their distance asunder to be , , miles. this singularly assorted pair of stars possibly form part of a larger system. their period of revolution is shorter now by six seconds than it was in goodricke's time; and dr. chandler has shown, by an exhaustive discussion, that its inequalities are comprised in a cycle of about years.[ ] they arise, in his view, from a common revolution, in that period, of the close couple about a third distant body, emitting little or no light, in an orbit inclined ° to our line of vision, and of approximately the size of that described by uranus round the sun. the time spent by light in crossing this orbit causes an apparent delay in the phases of the variable, when algol and its eclipsing satellite are on its further side from ourselves, balanced by acceleration while they traverse its hither side. dr. chandler derives confirmation for his plausible and ingenious theory from a supposed undulation in the line traced out by algol's small proper motion; but the reality of this disturbance has yet to be established.[ ] meanwhile, m. tisserand,[ ] late director of the paris observatory, preferred to account for algol's inequalities on the principle later applied by bélopolsky to those of castor. that is to say, he assumed a revolving line of apsides in an elliptical orbit traversed by a pretty strongly compressed pair of globes. the truth of this hypothesis can be tested by close observation of the phases of the star during the next few years. the variable in the head of medusa is the exemplar of a class including recognised members, all of which doubtless represent occulting combinations of stars. but their occultations result merely from the accident of their orbital planes passing through our line of sight; hence the heavens must contain numerous systems similarly constituted, though otherwise situated as regards ourselves, some of which, like spica virginis, will become known through their spectroscopic changes, while others, because revolving in planes nearly tangent to the sphere, or at right angles to the visual line, may never disclose to us their true nature. among eclipsing stars should probably be reckoned the peculiar variables, beta lyræ and v puppis, each believed to consist of a pair of bright stars revolving almost in contact.[ ] three stars, on the other hand, distinguished by rapid and regular fluctuations, have been proved by bélopolsky to be attended by non-occulting satellites, which circulate, nevertheless, in the identical periods of light-change. gore's "catalogue of known variables"[ ] included, in , entries, and the number was augmented to on its revision in .[ ] chandler's first list of such objects,[ ] published about the same time, received successive expansions in and ,[ ] and finally included entries. a new "catalogue of variable stars," still wider in scope, will shortly be issued by the german _astronomische gesellschaft_. mr. a. w. roberts's researches on southern variables[ ] have greatly helped to give precision, while adding to the extent of knowledge in this branch. dr. gould held the opinion that most stars fluctuate slightly in brightness through surface-alterations similar to, but on a larger scale than those of the sun; and the solar analogy might be pushed somewhat further. it perhaps affords a clue to much that is perplexing in stellar behaviour. wolf pointed out in the striking resemblance in character between curves representing sun-spot frequency and curves representing the changing luminous intensity of many variable stars. there were the same steep ascent to maximum and more gradual decline to minimum, the same irregularities in heights and hollows, and, it may be added, the same tendency to a double maximum, and complexity of superposed periods.[ ] it is impossible to compare the two sets of phenomena thus graphically portrayed without reaching the conclusion that they are of closely related origin. but the correspondence indicated is not, as has often been hastily assumed, between maxima of sun-spots and minima of stellar brightness, but just the reverse. the luminous outbursts, not the obscurations of variable stars, obey a law analogous to that governing the development of spots on the sun. objects of the kind do not, then, gain light through the closing-up of dusky chasms in their photospheres, but by an actual increase of surface-brilliancy, together with an immense growth of these brilliant formations--prominences and faculæ--which, in the sun, accompany, or are appended to spots. a comparison of light-curves with curves of spot-frequency leaves no doubt on this point, and the strongest corroborative evidence is derived from the emergence of bright lines in the spectra of long-period variables rising to their recurring maxima. every kind and degree of variability is exemplified in the heavens. at the bottom of the scales are stars like the sun, of which the lustre is--tried by our instrumental means--sensibly steady. at the other extreme are ranged the astounding apparitions of "new," or "temporary" stars. within the last thirty-six years eleven of these stellar guests (as the chinese call them) have presented themselves, and we meet with a twelfth no farther back than april , . but of the "new star" in ophiuchus found by mr. hind on that night, little more could be learnt than of the brilliant objects of the same kind observed by tycho and kepler. the spectroscope had not then been invented. let us hear what it had to tell of later arrivals. about thirty minutes before midnight of may , , mr. john birmingham of millbrook, near tuam, in ireland, saw with astonishment a bright star of the second magnitude unfamiliarly situated in the constellation of the northern crown. four hours earlier, schmidt of athens had been surveying the same part of the heavens, and was able to testify that it was not visible there. that is to say, a few hours, or possibly a few minutes, sufficed to bring about a conflagration, the news of which may have occupied hundreds of years in travelling to us across space. the rays which were its messengers, admitted within the slit of sir william huggins's spectroscope, may , proved to be of a composition highly significant as to the nature of the catastrophe. the star--which had already declined below the third magnitude--showed what was described as a double spectrum. to the dusky flutings of secchi's third type four brilliant rays were added.[ ] the chief of these agreed in position with lines of hydrogen; so that the immediate cause of the outburst was inferred to have been the eruption, or ignition, of vast masses of that subtle kind of matter, the universal importance of which throughout the cosmos is one of the most curious facts revealed by the spectroscope. t coronæ (as the new star was called) quickly lost its adventitious splendour. nine days after its discovery it was again invisible to the naked eye. it is now a pale yellow, slightly variable star near the tenth magnitude, and finds a place as such in argelander's charts.[ ] it was thus obscurely known before it made its sudden leap into notoriety. the next "temporary," discovered by dr. schmidt at athens, november , , could lay no claim to previous recognition even in that modest rank. it was strictly a parvenu. there was no record of its existence until it made its appearance as a star of nearly the third magnitude, in the constellation of the swan. its spectrum was examined, december , by cornu at paris,[ ] and a few days later by vogel and o. lohse at potsdam.[ ] it proved of a closely similar character to that of t coronæ. a range of bright lines, including those of hydrogen, and probably of helium, stood out from a continuous background impressed with strong absorption. it may be presumed that in reality the gaseous substances, which, by their sudden incandescence, had produced the apparent conflagration, lay comparatively near the surface of the star, while the screen of cooler materials intercepting large portions of its light was situated at a considerable elevation in its atmosphere. the object, meanwhile, steadily faded. by the end of the year it was of no more than seventh magnitude. after the second week of march, , strengthening twilight combined with the decline of its radiance to arrest further observation. it was resumed, september , at dunecht, with a strange result. practically the whole of its scanty light (it had then sunk below the tenth magnitude) was perceived to be gathered into a single bright line in the green, and that the most characteristic line of gaseous nebulæ.[ ] the star had, in fact, so far as outward appearance was concerned, become transformed into a planetary nebula, many of which are so minute as to be distinguishable from small stars only by the quality of their radiations. it is now, having sunk to about the fourteenth magnitude,[ ] entirely beyond the reach of spectroscopic scrutiny. perhaps none of the marvellous changes witnessed in the heavens has given a more significant hint as to their construction than the stellar blaze kindled in the heart of the great andromeda nebula some undetermined number of years or centuries before its rays reached the earth in the month of august, . the first published discovery was by dr. hartwig at dorpat on august ; but it was found to have been already seen, on the th, by mr. isaac w. ward of belfast, and on the th by m. ludovic gully of rouen. the _negative_ observations, on the th, of tempel[ ] and max wolf, limited very narrowly the epoch of the apparition. nevertheless, it did not, like most temporaries, attain its maximum brightness all at once. when first detected, it was of the ninth, by september it had risen to the seventh magnitude, from which it so rapidly fell off that in march it touched the limit of visibility (sixteenth magnitude) with the washington -inch. its light bleached very perceptibly as it faded.[ ] during the earlier stages of its decline, the contrast was striking between the sharply defined, ruddy disc of the star, and the hazy, greenish-white background upon which it was projected,[ ] and with which it was inevitably suggested to be in some sort of physical connection. let us consider what evidence was really available on this point. to begin with, the position of the star was not exactly central. it lay sixteen seconds of arc to the south-west of the true nebular nucleus. its appearance did not then signify a sudden advance of the nebula towards condensation, nor was it attended by any visible change in it save the transient effect of partial effacement through superior brightness. equally indecisive information was derived from the spectroscope. to vogel, hasselberg, and young, the light of the "nova" seemed perfectly continuous; but huggins caught traces of bright lines on september , confirmed on the th;[ ] and copeland succeeded, on september , in measuring three bright bands with an acute-angled prism specially constructed for the purpose.[ ] a shimmer of f was suspected, and had also been perceived by mr. o. t. sherman of yale college. still, the effect was widely different from that of the characteristic blazing spectrum of a temporary star, and prompted the surmise that here, too, a variable might be under scrutiny. the star, however, was certainly so far "new" that its rays, until their sudden accession of strength, were too feeble to affect even our reinforced senses. not one of the , small stars recorded in charts of the nebula could be identified with it; and a photograph taken by dr. common, august , , on which a multitude of stars down to the fifteenth magnitude had imprinted themselves, showed the uniform, soft gradation of nebulous light to be absolutely unbroken by a stellar indication in the spot reserved for the future occupation of the "nova."[ ] so far, then, the view that its relation to the nebula was a merely optical one might be justified; but it became altogether untenable when it was found that what was taken to be a chance coincidence had repeated itself within living memory. on the st of may, , m. auwers perceived at königsberg a seventh magnitude star shining close to the centre of a nebula in scorpio, numbered in messier's catalogue.[ ] three days earlier it certainly was not there, and three weeks later it had vanished. the effect to mr. pogson (who independently discovered the change, may )[ ] was as if the nebula had been _replaced_ by a star, so entirely were its dim rays overpowered by the concentrated blaze in their midst. now, it is simply incredible that two outbursts of so uncommon a character should have _accidentally_ occurred just on the line of sight between us and the central portions of two nebulæ; we must, then, conclude that they showed _on_ these objects because they took place _in_ them. the most favoured explanation is that they were what might be called effects of overcrowding--that some of the numerous small bodies, presumably composing the nebulæ, jostled together, in their intricate circlings, and obtained compensation in heat for their sacrifice of motion. but this is scarcely more than a plausible makeshift of perplexed thought. mr. w. h. s. monck, on the other hand, has suggested that new stars appear when dark bodies are rendered luminous by rushing through the gaseous fields of space,[ ] just as meteors kindle in our atmosphere. the idea, which has been revived and elaborated by dr. seeliger of munich,[ ] is ingenious, but was not designed to apply to our present case. neither of the objects distinguished by the striking variations just described is of gaseous constitution. that in scorpio appears under high magnifying powers as a "compressed cluster"; that in andromeda is perhaps, as sir j. herschel suggested, "optically nebulous through the smallness of its constituent stars"[ ]--if stars they deserve to be called. on the th of december, , dr. max wolf took a photograph of the region about chi aurigæ. no stranger so bright as the eighth magnitude was among the stars depicted upon it. on the th, nevertheless, a stellar object of the fifth magnitude, situated a couple of degrees to the north-east of beta tauri and previously unrecorded, where eleventh magnitude stars appeared, imprinted itself upon a harvard negative. subsequent photographs taken at the same place showed it to have gained about half a magnitude by the th; but the plates were not then examined, and the discovery was left to be modestly appropriated by an amateur, the rev. dr. anderson of edinburgh, by whom it was announced, february , , through the medium of an anonymous postcard, to dr. copeland, the astronomer royal for scotland.[ ] by him and others, the engines of modern research were promptly set to work. and to good purpose. nova aurigæ was the first star of its kind studied by the universal chemical method. it is the first, accordingly, of which authentic records can be handed down to posterity. they are of a most remarkable character. the spectrum of the new object was photographed at stonyhurst and south kensington on february ; a few days later, at harvard and lick in america, at potsdam and hérény on the continent of europe. but by far the most complete impression was secured, february , with an exposure of an hour and three-quarters, by sir william and lady huggins, through whose kindness it is reproduced in plate v., fig. . the range of bright lines displayed in it is of astonishing vividness and extent. it includes all the hydrogen rays dark in the spectrum of sirius (separately printed for comparison), besides many others still more refrangible, as yet unidentified. very significant, too, is the marked character of the great prominence lines h and k. the visual spectrum of the nova was splendidly effective. a quartette of brilliant green rays, two of them due to helium, caught the eye; and they had companions too numerous to be easily counted. the hydrogen lines were broad and bright; c blazed, as mr. espin said, "like a danger-signal on a dark night"; the sodium pair were identified at tulse hill, and the yellow helium ray was suspected to lurk close beside them. fig. in the same plate shows the spectrum as it was seen and mapped by lady huggins, february to , together with the spectra employed to test the nature of the emissions dispersed in it. one striking feature will be at once remarked. it is that of the pairing of bright with dark lines. both in the visible and the photographic regions this singular peculiarity was unmistakable; and since the two series plainly owned the same chemical origin, their separate visibility implied large displacement. otherwise they would have been superposed, not juxtaposed. measurements of the bright rays, accordingly, showed them to be considerably pushed down towards the red, while their dark companions were still more pushed up towards the blue end. thus the spectrum of nova aurigæ, like that of beta lyræ, with which it had many points in common, appeared to be really double. it was supposed to combine the light of two distinct bodies, one, of a gaseous nature, moving rapidly away from the earth, the other, giving a more sunlike spectrum, approaching it with even higher speed. the relative velocity determined at potsdam for these oppositely flying masses amounted to miles a second.[ ] and this prodigious rate of separation was fully maintained during six weeks! it did not then represent a mere periastral rush-past.[ ] to the bodies exhibiting its effects, and parting company for ever under its stress, it must have belonged, with slight diminution, in perpetuity. the luminous outburst by which they became visible was explained by sir william huggins, in a lecture delivered at the royal institution, may , , on the tidal theory of klinkerfues and wilsing. disturbances and deformations due to the mutual attraction of two bulky globes at a close approach would, he considered, "give rise to enormous eruptions of the hotter matters from within, immensely greater, but similar in kind, to solar eruptions; and accompanied, probably, by large electrical disturbances." the multiple aspect and somewhat variable character of both bright and dark lines were plausibly referred to processes of "reversal," such as are nearly always in progress above sun-spots; but the long duration of the star's suddenly acquired lustre did not easily fit in with the adopted rationale. a direct collision, on the other hand, was out of the question, since there had obviously been little, if any, sacrifice of motion; and the substitution of a nebula for one of the "stars"[ ] compelled recourse to scarcely conceivable modes of action for an explanation of the perplexing peculiarities of the compound spectrum. plate v. [illustration: photographic and visual spectrum of nova aurigæ. fig. .--from a photograph taken by sir william and lady huggins, feb. , . fig. .--from a drawing made by lady huggins, feb. to , .] an unexpected _dénouement_, however, threw all speculations off the track. the nova contained most of its brightness, fluctuations notwithstanding, until march ; after which date it ran swiftly and uniformly down towards what was apprehended to be total extinction. no marked change of spectrum attended its decline. when last examined at tulse hill, march , all the more essential features of its prismatic light were still faintly recognisable.[ ] the object was steadily sinking on april , when a (supposed) final glimpse of it was caught with the lick -inch.[ ] it was then of about the sixteenth magnitude. but on august it had sprung up to the tenth, as professors holden, schaeberle, and campbell perceived with amazement on turning the same instrument upon its place. and to professor barnard it appeared, two nights later, not only revived, but transformed into the nucleus of a planetary nebula, " across.[ ] the reality of this seeming distension, however, at once disputed, was eventually disproved. it unquestionably arose from the imperfect focussing power of the telescope for rays of unusual quality.[ ] the rekindled nova was detected in this country by mr. h. corder, on whose notification mr. espin, on august , examined its nearly monochromatic spectrum.[ ] the metamorphosis of nova cygni seemed repeated.[ ] the light of the new object, like that of its predecessor, was mainly concentrated in a vivid green band, identified with the chief nebular line by copeland,[ ] von gothard,[ ] and campbell.[ ] the second nebular line was also represented. indeed, the last-named observer recognised nearly all the eighteen lines measured by him in the nova as characteristic of planetary nebulæ.[ ] of particular interest is the emergence in the star-spectrum photographed by von gothard of an ultra-violet line originally discovered at tulse hill in the orion nebula, which is also very strong in the lyra annular nebula. obviously, then, the physical constitution of nova aurigæ became profoundly modified during the four months of its invisibility. the spectrum of february was or appeared compound; that of august was simple; it could be reasonably associated only with a single light-source. many of the former brilliant lines, too, had vanished, and been replaced by others, at first inconspicuous or absent. as a result, the solar-prominence type, to which the earlier spectrum had seemed to conform, was completely effaced in the later. the cause of these alterations remains mysterious, yet its effects continue. the chromatic behaviour of the semi-extinct nova, when scrutinised with great refractors, shows its waning light to be distinctly nebular.[ ] like nearly all its congeners, the star is situated in the full stream of the milky way, and we learn without surprise that micrometrical measures by burnham and barnard[ ] failed to elicit from it any sign of parallactic shifting. it is hence certain that the development of light, of which the news reached the earth in december, , must have been on a vast scale, and of ancient date. nova aurigæ at its maximum assuredly exceeded the sun many times in brightness; and its conflagration can scarcely have occurred less, and may have occurred much more, than a hundred years ago. by means of the photographic surveys of the skies, carried on in both hemispheres under professor pickering's superintendence, such amazing events have been proved to be of not infrequent occurrence. within six years five new stars were detected from draper memorial, or chart-plates by mrs fleming, besides the retrospective discovery of a sixth which had rapidly burnt itself out, eight years previously, in perseus.[ ] nova normæ was the immediate successor of nova aurigæ; nova carinæ and nova centauri lit up in , the latter in a pre-existent nebula; nova sagittarii and nova aquilæ attained brief maxima in and respectively. now, three out of these five stars reproduced with singular fidelity the spectrum of nova aurigæ; they displayed the same brilliant rays shadowed, invariably on their blue sides, by dark ones. palpably, then, the arrangement was systematic and significant; it could not result merely from the casually directed, opposite velocities of bodies meeting in space. the hypothesis of stellar encounters accordingly fell to the ground, and has been provided with no entire satisfactory substitute. most speculators now fully recognise that motion-displacements cannot be made to account for the doubled spectra of novæ, and seek recourse instead to some kind of physical agency for producing the observed effect.[ ] and since this is also visible in certain permanent, though peculiar objects--notably in p cygni, beta lyræ, and eta carinæ--the acting cause must also evidently be permanent and inherent. the "new star of the new century"[ ] was a visual discovery. dr. anderson duplicated, with added _éclat_, his performance of nine years back. in the early morning of february , , he perceived that algol had a neighbour of nearly its own brightness, which a photograph taken by mr. stanley williams, at brighton, proved to have risen from below the twelfth magnitude within the preceding hours. and it was still swiftly ascending. on the rd, it outshone capella; for a brief space it took rank as the premier star of the northern hemisphere. a decline set in promptly, but was pursued hesitatingly. the light fluctuated continually over a range of a couple of magnitudes, and with a close approach, during some weeks, to a three-day periodicity. a year after the original outburst, the star was still conspicuous with an opera-glass. the spectrum underwent amazing changes. at first continuous, save for fine dark lines of hydrogen and helium, it unfolded within forty-eight hours a composite range of brilliant and dusky bands disposed in the usual fashion of novæ. these lasted until far on in march, when hydrogen certainly, and probably other substances as well, ceased to exert any appreciable absorptive action. blue emissions of the wolf-rayet type then became occasionally prominent, in remarkable correspondence with the varying lustre of the star;[ ] finally, a band at lambda , found by wright at lick to characterise nebular spectra,[ ] assumed abnormal importance; and in july the nebular transformation might be said to be complete. striking alterations of colour attended these spectral vicissitudes. white to begin with, the star soon turned deep red, and its redness was visibly intensified at each of its recurring minima of light. blanching, however, ensued upon the development of its nebulous proclivities; and its surviving rays are of a steely hue. all the more important investigations of nova persei were conducted by photographic means. libraries of spectral plates were collected at the yerkes and lick observatories, at south kensington, stonyhurst, and potsdam, and await the more exhaustive interpretation of the future. meanwhile, extraordinary revelations have been supplied by immediate photographic delineation. on august and , , professor max wolf, by long exposures with the -inch bruce twin objectives of the königstuhl observatory (heidelberg), obtained indications of a large nebula finely ramified, extending south-east of the nova;[ ] and the entire formation came out in four hours with the yerkes -foot reflector, directed to it by mr. ritchey on september .[ ] it proved to be a great spiral encircling, and apparently emanating from, the star. but if so, tumultuously, and under stress of catastrophic impulsions. a picture obtained by mr. perrine with the crossley refractor, in h. m., on november and , disclosed the progress of a startling change.[ ] comparison with the yerkes photograph showed that during the intervening days four clearly identifiable condensations had become displaced, all to the same extent of about seconds of arc, and in fairly concordant directions, suggesting motion _round_ the nova as well as away from it. the velocity implied, however, is so prodigious as virtually to exclude the supposition of a bodily transport of matter. it should be at the rate of no less than twenty thousand miles a second, admitting the object to be at a distance from us corresponding to an annual parallax of one-tenth of a second, and actual measurements show it to be indefinitely more remote. the fact of rapid variations in the nebula was reaffirmed, though with less precision, from yerkes photographs of november and , mr. ritchey inferring a general expansion of its southern portions.[ ] much further evidence must be at hand before a sane judgment can be formed as to the nature of the strange events taking place in that secluded corner of the galaxy.[ ] and it is highly probable that the illumination of the nebulous wreaths round the star will prove no less evanescent than the blazing of the star itself. we have been compelled somewhat to anticipate our narrative as regards inquiries into the nature of nebulæ. the excursions of opinion on the point were abruptly restricted and defined by the application to them of the spectroscope. on august , , sir william huggins sifted through his prisms the rays of a bright planetary nebula in draco.[ ] to his infinite surprise, they proved to be mainly of one colour. in other words, they avowed their origin from a mass of glowing vapour. as to what _kind_ of vapour it might be by which herschel's conjecture of a "shining fluid" diffused at large throughout the cosmos was thus unexpectedly verified, an answer only partially satisfactory could be afforded. the conspicuous bright line of the draco nebula seemed to agree in position with one emitted by nitrogen, but has since proved to be distinct from it; of its two fainter companions, one was unmistakably the f line of hydrogen, while the other, in position intermediate between the two, still remains unidentified. by huggins had satisfactorily examined the spectra of about seventy nebulæ, of which one-third displayed a gaseous character.[ ] all of these gave the green ray fundamental to the nebular spectrum, and emanating from an unknown form of matter named by sir william huggins "nebulum." it is associated with seven or eight hydrogen lines, with three of "yellow" helium, and with a good many of undetermined origin. the absence of the crimson radiation of hydrogen--perceived with difficulty only in some highly condensed objects--is an anomaly very imperfectly explained as a physiological effect connected with the extreme faintness of nebular light.[ ] an approximate coincidence between the chief nebular line and a "fluting" of magnesium having been alleged by lockyer in support of his meteoritic hypothesis of nebular constitution, it became of interest to ascertain its reality. the task was accomplished by sir william and lady huggins in and ,[ ] and by professor keeler, with the advantages of the mount hamilton apparatus and atmosphere, in - .[ ] the upshot was to show a slight but sure discrepancy as to place, and a marked diversity as to character, between the two qualities of light. the nebular ray (wave-length , millionths of a millimetre) is slightly more refrangible than the magnesium fluting-edge, and it is sharp and fine, with no trace of the unilateral haze necessarily clinging even to the last "remnant" of a banded formation. planetary and annular nebulæ are, without exception, gaseous, as well as those termed "irregular," which frequent the region of the milky way. their constitution usually betrays itself to the eye by their blue or greenish colour; while those yielding a continuous spectrum are of a dull white. among the more remarkable of these are the well-known nebula in andromeda, and the great spiral in canes venatici; and, as a general rule, the emissions of all such nebulæ as present the appearance of star-clusters grown misty through excessive distance are of the same kind. it would, however, be eminently rash to conclude thence that they are really aggregations of sun-like bodies. the improbability of such an inference has been greatly enhanced by the occurrence, at an interval of a quarter of a century, of stellar outbursts in the midst of two of them. for it is practically certain that the temporary stars were equally remote with the hazy formations they illuminated; hence, if the constituent particles of the latter be suns, the incomparably vaster orbs by which their feeble light was well-nigh obliterated must, as was argued by mr. proctor, have been on a scale of magnitude such as the imagination recoils from contemplating. nevertheless, dr. scheiner, not without much difficulty, obtained, in january, , spectrographic prints of the andromeda nebula, indicative, he thought, of its being a cluster of solar stars.[ ] sir william and lady huggins, on the other hand, _saw_, in , bright intermixed with dark bands in the spectrum of the same object.[ ] and mr. maunder conjectures all "white" nebulæ to be made up of sunlets in which the coronal element predominates, while chromospheric materials assert their presence in nebulæ of the "green" variety.[ ] among the ascertained analogies between the stellar and nebular systems is that of variability of light. on october , , mr. hind discovered a small nebula in taurus. chacornac observed it at marseilles in , but was confounded four years later to find it vanished. d'arrest missed it october , and redetected it december , . it was easily seen in - , but invisible in the most powerful instruments from to .[ ] barnard, however, made out an almost evanescent trace of it, october , , with the great lick telescope,[ ] and saw it easily in the spring of , while six months later it evaded his most diligent search.[ ] then again, on september , , the yerkes -inch disclosed it to him as a mere shimmer at the last limit of visibility; and it came out in three diffuse patches on plates to which, on december and , , keeler gave prolonged exposures with the crossley reflector.[ ] moreover, a fairly bright adjacent nebula, perceived by o. struve in , and observed shortly afterwards by d'arrest, has totally vanished, and was most likely only a temporary apparition. these are the most authentic instances of nebular variability. many others have been more or less plausibly alleged;[ ] but professor holden's persuasion, acquired from an exhaustive study of the records since ,[ ] that the various parts of the orion nebula fluctuate continually in relative lustre, has not been ratified by photographic evidence. the case of the "trifid" nebula in sagittarius, investigated by holden in ,[ ] is less easily disposed of. what is certain is that a remarkable triple star, centrally situated, according to the observations of both the herschels, - , in a dark space between the three great _lobes_ of the nebula, is now, and has been since , densely involved in one of them; and since the hypothesis of relative motion is on many grounds inadmissible, the change that has apparently taken place must be in the distribution of light. one no less conspicuous was adduced by mr. h. c. russell, director of the sydney observatory.[ ] a particularly bright part of the great argo nebula, as drawn by sir john herschel, has, it would seem, almost totally disappeared. he noticed its absence in , using a -inch telescope, failed equally later on to find it with an - / -inch, and his long-exposure photographs show no vestige of it. the same structure is missing from, or scarcely traceable in, a splendid picture of the nebula taken by sir david gill in twelve hours distributed over four nights in march, .[ ] an immense gaseous expanse has, it would seem, sunk out of sight. materially it is no doubt there; but the radiance has left it. nebulæ have no ascertained proper motions. no genuine change of place in the heavens has yet been recorded for any one of them. all equally hold aloof, so far as telescopic observation shows, from the busy journeyings of the stars. this seeming immobility is partly an effect of vast distance. nebular parallax has, up to the present, proved evanescent, and nebular parallactic drift, in response to the sun's advance through space, remains likewise imperceptible.[ ] it may hence be presumed that no nebulæ occur within the sphere occupied by the nearer stars. but the difficulty of accurately measuring such objects must also be taken into account. displacements which would be conspicuous in stars might easily escape detection in ill-defined, hazy masses. thus the measures executed by d'arrest in [ ] have not yet proved effective for their designed purpose of contributing to the future detection of proper motions. some determinations made by mr. burnham with the lick refractor in ,[ ] will ultimately afford a more critical test. he found that nearly all planetary nebulæ include a sharp stellar nucleus, the position of which with reference to neighbouring stars could be fixed no less precisely than if it were devoid of nebulous surroundings. hence, the objects located by him cannot henceforward shift, were it only to the extent of a small fraction of a second, without the fact coming to the knowledge of astronomers. the spectroscope, however, here as elsewhere, can supplement the telescope; and what it has to tell, it tells at once, without the necessity of waiting on time to ripen results. sir william huggins made, in ,[ ] the earliest experiments on the radial movements of nebulæ. but with only a negative upshot. none of the six objects examined gave signs of spectral alteration, and it was estimated that they must have done so had they been in course of recession from or approach towards the earth by as much as twenty-five miles a second. with far more powerful appliances, professor keeler renewed the attempt at lick in - . his success was unequivocal. ten planetary nebulæ yielded perfectly satisfactory evidence of line-of-sight motion,[ ] the swiftest traveller being the well-known greenish globe in draco,[ ] found to be hurrying towards the earth at the rate of forty miles a second. for the orion nebula, a recession of about eleven miles was determined,[ ] the whole of which may, however, very well belong to the solar system itself, which, by its translation towards the constellation lyra, is certainly leaving the great nebula pretty rapidly behind. the anomaly of seeming nebular fixity has nevertheless been removed; and the problem of nebular motion has begun to be solved through the demonstrated possibility of its spectroscopic investigation. keeler's were the first trustworthy determinations of radial motion obtained visually. that the similar work on the stars begun at greenwich in , and carried on for thirteen years, remained comparatively unfruitful, was only what might have been expected, the instruments available there being altogether inadequate for the attainment of a high degree of accuracy. the various obstacles in the way of securing it were overcome by the substitution of the sensitive plate for the eye. air-tremors are thus rendered comparatively innocuous; and measurements of stellar lines displaced by motion with reference to fiducial lines from terrestrial sources, photographed on the same plates, can be depended upon within vastly reduced limits of error. studies for the realisation of the "spectrographic" method were begun by dr. vogel and his able assistant, dr. scheiner, at potsdam in . their preliminary results, communicated to the berlin academy of sciences, march , , already showed that the requirements for effective research in this important branch were at last about to be complied with. an improved instrument was erected in the autumn of the same year, and the fifty-one stars, bright enough for determination with a refractor of inches aperture, were promptly taken in hand. a list of their motions in the line of sight, published in ,[ ] was of high value, both in itself and for what it promised. one noteworthy inference from the data it collected was that the eye tends, under unfavourable circumstances, to exaggerate the line-displacements it attempts to estimate. the velocities photographically arrived at were of much smaller amounts than those visually assigned. the average speed of the potsdam stars came out only · miles a second, the quickest among them being aldebaran, with a recession of thirty miles a second. more lately, however, deslandres and campbell have determined for zeta herculis and eta cephei respectively approaching rates of forty-four and fifty-four miles a second. the installation, in , of a photographic refractor - / inches in aperture, coupled with a -inch guiding telescope, will enable dr. vogel to investigate spectrographically some hundreds of stars fainter than the second magnitude; and the materials thus accumulated should largely help to provide means for a definite and complete solution of the more than secular problem of the sun's advance through space. the solution should be complete, because including a genuine determination of the sun's velocity, apart from assumptions of any kind. m. homann's attempt, in ,[ ] to extract some provisional information on the subject from the radial movements of visually determined stars gave a fair earnest of what might be done with materials of a better quality. he arrived at a goal for the sun's way shifted eastward to the constellation cygnus--a result congruous with the marked tendency of recently determined apexes to collect in or near lyra; and the most probable corresponding velocity seemed to be about nineteen miles a second, or just that of the earth in its orbit. a more elaborate investigation of the same kind, based by professor campbell in [ ] upon the motions of stars, determined with extreme precision, suffered in completeness through lack of available data from the southern hemisphere. the outcome, accordingly, was an apex most likely correctly placed as regards right ascension, but displaced southward by some fifteen degrees. the speed of twelve miles a second, assigned to the solar translation, approximates doubtless very closely to the truth. a successful beginning was made in nebular spectrography by sir william huggins, march , .[ ] five lines in all stamped themselves upon the plate during forty-five minutes of exposure to the rays of the strange object in orion. of these, four were the known visible lines, and a fifth, high up in the ultra-violet, at wave-length , , has evidently peculiar relationships, as yet imperfectly apprehended. it is strong in the spectra of many planetaries; it helped to characterise the nebular metamorphosis of nova aurigæ, yet failed to appear in nova persei. two additional hydrogen lines, making six in all, were photographed at tulse hill, from the orion nebula, in ;[ ] and dr. copeland's detection in [ ] of the yellow ray d_ gave the first hint of the presence of helium in this prodigious formation. nor are there wanting spectroscopic indications of its physical connection with the stars visually involved in it. sir william and lady huggins found a plate exposed february , , impressed with four groups of fine bright lines, originating in the continuous light of two of the trapezium-stars, but extending some way into the surrounding nebula.[ ] and dr. scheiner[ ] argued a wider relationship from the common possession, by the nebula and the chief stars in the constellation orion, of a blue line, bright in the one case, dark in the others, since identified as a member of one of the helium series. the structural unity of the stellar and nebular orders in this extensive region of the sky has also, by direct photographic means, been unmistakably affirmed. the first promising autographic picture of the orion nebula was obtained by draper, september , .[ ] the marked approach towards a still more perfectly satisfactory result shown by his plates of march, and , was unhappily cut short by his death. meanwhile, m. janssen was at work in the same field from , with his accustomed success.[ ] but dr. a. ainslie common left all competitors far behind with a splendid picture, taken january , , by means of an exposure of thirty-seven minutes in the focus of his -foot silver-on-glass mirror.[ ] photography may thereby be said to have definitely assumed the office of historiographer to the nebulæ, since this one impression embodies a mass of facts hardly to be compassed by months of labour with the pencil, and affords a record of shape and relative brightness in the various parts of the stupendous object it delineates which must prove invaluable to the students of its future condition. its beauty and merit were officially recognised by the award of the astronomical society's gold medal in . a second picture of equal merit, obtained by the same means, february , , with an exposure of one hour, is reproduced in the frontispiece. the vignette includes two specimens of planetary photography. the jupiter, with the great red spot conspicuous in the southern hemisphere, is by dr. common. it dates from september , , and was accordingly one of the earliest results with his -inch, the direct image in which imprinted itself in a fraction of a second, and was subsequently enlarged on paper about twelve times. the exquisite little picture of saturn was taken at paris by mm. paul and prosper henry, december , , with their -inch photographic refractor. the telescopic image was in this case magnified eleven times previous to being photographed, an exposure of about five seconds being allowed; and the total enlargement, as it now appears, is nineteen times. a trace of the dusky ring perceptible on the original negative is lost in the print. a photograph of the orion nebula taken by dr. roberts in minutes, november , , made a striking disclosure of the extent of that prodigious object. more than six times the nebulous area depicted on dr. common's plates is covered by it, and it plainly shows an adjacent nebula, separately catalogued by messier, to belong to the same vast formation. this disposition to annex and appropriate has come out more strongly with every increase of photographic power. plates exposed at harvard college in march, , with an -inch portrait-lens (the same used in the preparation of the draper catalogue) showed the old-established "fish-mouth" nebula not only to involve the stars of the sword-handle, but to be in tolerably evident connection with the most easterly of the three belt-stars, from which a remarkable nebulous appendage was found to proceed.[ ] a still more curious discovery was made by w. h. pickering in .[ ] photographs taken in three hours from the summit of wilson's peak in california revealed the existence of an enormous, though faint spiral structure, enclosing in its span of nearly seventeen degrees the entire stellar and nebulous group of the belt and sword, from which it most likely, although not quite traceably, issues as if from a nucleus. a startling glimpse is thus afforded of the cosmical importance of that strange "hiatus" in the heavens which excited the wonder of huygens in . the inconceivable attenuation of the gaseous stuff composing it was virtually demonstrated by mr. ranyard.[ ] in march, , sir howard grubb mounted for dr. isaac roberts, at maghull, near liverpool (his observatory has since been transferred to crowborough in sussex), a silver-on-glass reflector of twenty inches aperture, constructed expressly for use in celestial photography. a series of nebula-pictures, obtained with this fine instrument, have proved highly instructive both as to the structure and extent of these wonderful objects; above all, one of the great andromeda nebula, to which an exposure of three hours was given on october , .[ ] in it a convoluted structure replaced and rendered intelligible the anomalously rifted mass seen by bond in .[ ] the effects of annular condensation appeared to have stamped themselves upon the plate, and two attendant nebulæ presented the aspect of satellites already separated from the parent body, and presumably revolving round it. the ring-nebula in lyra was photographed at paris in , and shortly afterwards by von gothard with a -inch reflector,[ ] and he similarly depicted in the two chief spiral and other nebulæ.[ ] photographs of the lyra nebula taken at algiers in ,[ ] and at the vatican observatory in ,[ ] were remarkable for the strong development of a central star, difficult of telescopic discernment, but evidently of primary importance to the annular structure around. the uses of photography in celestial investigations become every year more manifold and more apparent. the earliest chemical star-pictures were those of castor and vega, obtained with the cambridge refractor in by whipple of boston under the direction of w. c. bond. double-star photography was inaugurated under the auspices of g. p. bond, april , , with an impression, obtained in eight seconds, of mizar, the middle star in the handle of the plough. a series of measures from sixty-two similar images gave the distance and position-angle of its companion with about the same accuracy attainable by ordinary micrometrical operations; and the method and upshot of these novel experiments were described in three papers remarkably forecasting the purposes to be served by stellar photography.[ ] the matter next fell into the able hands of rutherfurd, who completed in a fine object glass (of - / inches) corrected for the ultra-violet rays, consequently useless for visual purposes. the sacrifice was recompensed by conspicuous success. a set of measurements from his photographs of nearly fifty stars in the pleiades, and their comparison with bessel's places, enabled dr. gould to announce, in , that during the intervening third of a century no changes of importance had occurred in their relative positions.[ ] and mr. harold jacoby[ ] similarly ascertained the fixity of seventy-five of rutherfurd's atlantids, between the epoch and that of dr. elkin's heliometric triangulation of the cluster in ,[ ] extending the interval to twenty-seven years by subsequent comparisons with plates taken at lick, september , .[ ] positive, however, as well as negative results have ensued from the application of modern methods to that antique group. on october , , wilhelm tempel, a saxon peasant by origin, later a skilled engraver, discovered with a small telescope, bought out of his scanty savings, an elliptical nebulosity, stretching far to the southward from the star merope. it attracted the attention of many observers, but was so often missed, owing to its extreme susceptibility to adverse atmospheric influences, as to rouse unfounded suspicions of its variability. the detection of this evasive object gave a hint, barely intelligible at the time, of further revelations of the same kind by more cogent means. a splendid photograph of , stars in the pleiades, taken by the mm. henry with three hours' exposure, november , , showed one of the brightest of them to have a small spiral nebula, somewhat resembling a strongly-curved comet's tail, attached to it. the reappearance of this strange appurtenance on three subsequent plates left no doubt of its real existence, visually attested at pulkowa, february , , by one of the first observations made with the -inch equatoreal.[ ] much smaller apertures, however, sufficed to disclose the "maia nebula," _once it was known to be there_. not only did it appear greatly extended in the vienna -inch,[ ] but mm. perrotin and thollon saw it with the nice -inch, and m. kammermann of geneva, employing special precautions, with a refractor of only ten inches aperture.[ ] the advantage derived by him for bringing it into view, from the insertion into the eye-piece of a uranium film, gives, with its photographic intensity, valid proof that a large proportion of the light of this remarkable object is of the ultra-violet kind. the beginning thus made was quickly followed up. a picture of the pleiades procured at maghull in eighty-nine minutes, october , , revealed nebulous surroundings to no less than four leading stars of the group, namely, alcyone, electra, merope, and maia; and a second impression, taken in three hours on the following night, showed further "that the nebulosity extends in streamers and fleecy masses till it seems almost to fill the spaces between the stars, and to extend far beyond them."[ ] the coherence of the entire mixed structure was, moreover, placed beyond doubt by the visibly close relationship of the stars to the nebulous formations surrounding them in dr. roberts's striking pictures. thus goldschmidt's notion that all the clustered pleiades constitute, as it were, a second orion trapezium in the midst of a huge formation of which tempel's nebula is but a fragment,[ ] has been to some extent verified. yet it seemed fantastic enough in . then in the mm. henry gave exposures of four hours each to several plates, which exhibited on development some new features of the entangled nebulæ. the most curious of these was the linking together of stars by nebulous chains. in one case seven aligned stars appeared strung on a silvery filament, "like beads on a rosary."[ ] the "rows of stars," so often noticed in the sky, may, then, be concluded to have more than an imaginary existence. of the , stars recorded in these pictures, a couple of hundred among the brightest can, at the outside, be reckoned as genuine pleiades. the great majority were relegated, by pickering's[ ] and stratonoff's[ ] counts of the stellar populace _in_ and _near_ the cluster, to the position of outsiders from it. they are undistinguished denizens of the abysmal background upon which it is projected. investigations of its condition were carried a stage further by barnard. on november , ,[ ] he discovered visually with the lick refractor a close nebulous satellite to merope, photographs of which were obtained by keeler in .[ ] it appears in them of a rudely pentagonal shape, a prominent angle being directed towards the adjacent star. finally, an exposure of ten hours made by barnard with the willard lens indicated the singular fact that the entire group is embedded in a nebulous matrix, streaky outliers of which blur a wide surface of the celestial vault.[ ] the artist's conviction of the reality of what his picture showed was confirmed by negatives obtained by bailey at arequipa in , and by h. c. wilson at northfield (minnesota) in .[ ] with the ealing -foot reflector, sold by dr. common to mr. crossley, and by him presented to the lick observatory, professor keeler took in a series of beautiful and instructive nebula[ ] photographs; one of the trifid may be singled out as of particular excellence. an astonishing multitude of new nebulæ were revealed by trial-exposures with this instrument. a "conservative estimate" gave , as the number coming within its scope. moreover, the majority of those actually recorded were of an unmistakable spiral character, and they included most of sir john herschel's "double nebulæ," previously supposed to exemplify the primitive history of binary stellar systems.[ ] dr. max wolf's explorations with a -inch voigtländer lens in emphatically reaffirmed the inexhaustible wealth of the nebular heavens. in one restricted region, midway between præsepe and the milky way, he located nebulæ, where only three had until then been catalogued; and he counted such objects clustering round the star comæ berenices,[ ] and so closely that all might be occulted together by the moon. the general photographic catalogue of nebulæ which dr. wolf has begun to prepare[ ] will thus be a most voluminous work. the history of celestial photography at the cape of good hope began with the appearance of the great comet of . no special apparatus was at hand; so sir david gill called in the services of a local artist, mr. allis of mowbray, with whose camera, strapped to the observatory equatoreal, pictures of conspicuous merit were obtained. but their particular distinction lay in the multitude of stars begemming the background. (see plate iii.) the sight of them at once opened to the royal astronomer a new prospect. he had already formed the project of extending argelander's "durchmusterung" from the point where it was left by schönfeld to the southern pole; and his ideas regarding the means of carrying it into execution crystallised at the needle touch of the cometary experiments. he resolved to employ photography for the purpose. the exposure of plates was accordingly begun, under the care of mr. ray woods, in ; and in less than six years, the sky, from ° of south latitude to the pole, had been covered in duplicate. their measurement, and the preparation of a catalogue of the stars imprinted upon them, were generously undertaken by professor kapteyn, and his laborious task has at length been successfully completed. the publication, in , of the third and concluding volume of the "cape photographic durchmusterung"[ ] placed at the disposal of astronomers a photographic census of the heavens fuller and surer than the corresponding visual enumeration executed at bonn. it includes , stars, nearly to the tenth magnitude, and their positions are reliable to about one second of arc. the production of this important work was thus a result of the cape comet-pictures; yet not the most momentous one. they turned the scale in favour of recourse to the camera when the mm. henry encountered, in their continuation of chacornac's half-finished enterprise of ecliptical charting, sections of the milky way defying the enumerating efforts of eye and hand. the perfect success of some preliminary experiments made with an instrument constructed by them expressly for the purpose was announced to the academy of sciences at paris, may , . by its means stars estimated as of the sixteenth magnitude clearly recorded their presence and their places; and the enormous increase of knowledge involved may be judged of from the fact that, in a space of the milky way in cygnus ° ' by °, where stars had been mapped by the old laborious method, about five thousand stamped their images on a single henry plate. these results suggested the grand undertaking of a general photographic survey of the heavens, and gill's proposal, june , , of an international congress for the purpose of setting it on foot was received with acclamation, and promptly acted upon. fifty-six delegates of seventeen different nationalities met in paris, april , , under the presidentship of admiral mouchez, to discuss measures and organise action. they resolved upon the construction of a photographic chart of the whole heavens, comprising stars of a fourteenth magnitude, to the surmised number of twenty millions; to be supplemented by a catalogue, framed from plates of comparatively short exposure, giving start to the eleventh magnitude. these will probably amount to about one million and a quarter. for procuring both sets of plates, instruments were constructed precisely similar to that of the mm. henry, which is a photographic refractor, thirteen inches in aperture, and eleven feet focus, attached to a guiding telescope of eleven inches aperture, corrected, of course, for the visual rays. each place covers an area of four square degrees, and since the series must be duplicated to prevent mistakes, about , plates will be needed for the chart alone. the task of preparing them was apportioned among eighteen observatories scattered over the globe, from mexico to melbourne; but three in south america having become disabled or inert, were replaced in by those at cordoba, montevideo, and perth, western australia. meanwhile, the publication of results has begun, and is likely to continue for at least a quarter of a century. the first volume of measures from the potsdam catalogue-plates was issued in , and its successors, if on the same scale, must number nearly . moreover, ninety-six heliogravure enlargements from the paris chart-plates, distributed in the same year, supplied a basis for the calculation that the entire atlas of the sky, composed of similar sheets, will form a pile thirty feet high and two tons in weight![ ] it will, however, possess an incalculable scientific value. for millions of stars can be determined by its means, from their imprinted images, with an accuracy comparable to that attainable by direct meridian observations. one of the most ardent promoters of the scheme it may be expected to realise was admiral mouchez, the successor of leverrier in the direction of the paris observatory. but it was not granted to him to see the fruition of his efforts. he died suddenly june , .[ ] although not an astronomer by profession, he had been singularly successful in pushing forward the cause of the science he loved, while his genial and open nature won for him wide personal regard. he was replaced by m. tisserand, whose mathematical eminence fitted him to continue the traditions of delaunay and leverrier. but his career, too, was unhappily cut short by an unforeseen death on october , ; and the more eminent among the many qualifications of his successor, m. maurice loewy, are of the practical kind. the sublime problem of the construction of the heavens has not been neglected amid the multiplicity of tasks imposed upon the cultivators of astronomy by its rapid development. but data of a far higher order of precision, and indefinitely greater in amount, than those at the disposal of herschel or struve must be accumulated before any definite conclusions on the subject are possible. the first organised effort towards realising this desideratum was made by the german astronomical society in , two years after its foundation at heidelberg. the original programme consisted in the _exact_ determination of the places of all argelander's stars to the ninth magnitude (exclusive of the polar zone), from the reobservation of which, say, in the year , astronomers of two generations hence may gather a vast store of knowledge--directly of the apparent motions, indirectly of the mutual relations binding together the suns and systems of space. thirteen observatories in europe and america joined in the work, now virtually terminated. its scope was, after its inception, widened to include southern zones as far as the tropic of capricorn; this having been rendered feasible by schönfeld's extension ( - ) of argelander's survey. thirty thousand additional stars thus taken in were allotted in zones to five observatories. another important undertaking of the same class is the reobservation of the , stars in lalande's _histoire céleste_. begun under arago in , its upshot has been the publication of the great paris catalogue, issued in eight volumes, between and . from a careful study of their secular changes in position, m. bossert has already derived the proper motions of a couple of thousand out of nearly fifty thousand stars enumerated in it. through dr. gould's unceasing labours during his fifteen years' residence at cordoba, a detailed acquaintance with southern stars was brought about. his _uranometria argentina_ ( ) enumerates the magnitudes of , out of , stars visible to the naked eye under those transparent skies; , down to - / magnitude are embraced in his "zones"; and the argentine general catalogue of , southern stars was published in . valuable work of the same kind has been done at the leander mccormick observatory, virginia, by professor o. stone; while the late redcliffe observer's "cape catalogue for " affords inestimable aid to the practical astronomer south of the line, which has been reinforced with several publications issued by the present astronomer royal at the cape. moreover, the gigantic task entered upon in by dr. c. h. f. peters, director of the litchfield observatory, clinton (n.y.), and of which a large instalment was finished in , deserves honourable mention. it was nothing less than to map all stars down to, and even below, the fourteenth magnitude, situated within ° on either side of the ecliptic, and so to afford "a sure basis for drawing conclusions with respect to the changes going on in the starry heavens."[ ] it is tolerably safe to predict that no work of its kind and for its purpose will ever again be undertaken. in a small part of one night stars can now be got to register themselves more numerously and more accurately than by the eye and hand of the most skilled observer in the course of a year. fundamental catalogues, constructed by the old, time-honoured method, will continue to furnish indispensable starting-points for measurement; and one of especial excellence was published by professor newcomb in ;[ ] but the relative places of the small crowded stars--the sidereal [greek: hoi pholloi]--will henceforth be derived from their autographic statements on the sensitive plate. even the secondary purpose--that of asteroidal discovery--served by detailed stellar enumeration, is more surely attained by photography than by laborious visual comparison. for planetary movement betrays itself in a comparatively short time by turning the imprinted image of the object affected by it from a dot into a trail. in the arduous matter of determining star distances progress has been steady, and bids fair to become rapidly accelerated. together, yet independently, gill and elkin carried out, at the cape observatory in - , an investigation of remarkable accuracy into the parallaxes of nine southern stars. one of these was the famous alpha centauri, the distance of which from the earth was ascertained to be just one-third greater than henderson had made it. the parallax of sirius, on the other hand, was doubled, or its distance halved; while canopus proved to be quite immeasurably remote--a circumstance which, considering that, among all the stellar multitude, it is outshone only by the radiant dog-star, gives a stupendous idea of its real splendour and dimensions. inquiries of this kind were, for some years, successfully pursued at the observatory of dunsink, near dublin. annual perspective displacements were by dr. brünnow detected in several stars, and in others remeasured with a care which inspired just confidence. his parallax for alpha lyræ ( · ") was authentic, though slightly too large (elkin's final results gave pi = · "); and the received value for the parallax of the swiftly travelling star "groombridge , " scarcely differs from that arrived at by him in (pi = · "). his successor as astronomer-royal for ireland, sir robert stawell ball (now lowndean professor of astronomy in the university of cambridge), has done good service in the same department. for besides verifying approximately struve's parallax of half a second of arc for cygni, he refuted, in , by a sweeping search for (so-called) "large" parallaxes, certain baseless conjectures of comparative nearness to the earth, in the case of red and temporary stars.[ ] of objects thus cursorily examined, only one star of the seventh magnitude, numbered , in groombridge's circumpolar catalogue, gave signs of measurable vicinity. similarly, a reconnaissance among rapidly moving stars lately made by dr. chase with the yale heliometer[ ] yielded no really large, and only eight appreciable parallaxes among the subjects of his experiments. a second campaign in stellar parallax was undertaken by gill and elkin in . but this time the two observers were in opposite hemispheres. both used heliometers. dr. elkin had charge of the fine instrument then recently erected in yale college observatory; sir david gill employed one of seven inches, just constructed under his directions, in first-rate style, by the repsolds of hamburg. dr. elkin completed in his share of the more immediate joint programme, which consisted in the determination, by direct measurement, of the average parallax of stars of the first magnitude. it came out, for the ten northern luminaries, after several revisions, · ", equivalent to a light-journey of thirty-three years. the deviations from this average were, indeed, exceedingly wide. two of the stars, betelgeux and alpha cygni, gave no certain sign of any perspective shifting; of the rest, procyon, with a parallax of · ", proved the nearest to our system. at the mean distance concluded for these ten brilliant stars, the sun would show as of only fifth magnitude; hence it claims a very subordinate rank among the suns of space. sir david gill's definitive results were published in .[ ] as the average parallax of the eleven brightest stars in the southern hemisphere, they gave · ", a value enhanced by the exceptional proximity of alpha centauri. yet four of these conspicuous objects--canopus, rigel, spica, and beta crucis--gave no sign of perspective response to the annual change in our point of view. the list included eleven fainter stars with notable proper motions, and most of these proved to have fairly large parallaxes. among other valuable contributions to this difficult branch may be instanced bruno peter's measurements of eleven stars with the leipzig heliometer, - ;[ ] kapteyn's application of the method by differences in right ascension to fifteen stars observed on the meridian - ;[ ] and flint's more recent similar determinations at madison, wisconsin.[ ] the great merit of having rendered photography available for the sounding of the celestial depths belongs to professor pritchard. the subject of his initial experiment was cygni. from measurements of negatives taken in , he derived for that classic star a parallax of · ", in satisfactory agreement with ball's of · ". a detailed examination convinced the astronomer-royal of its superior accuracy to bessel's result with the heliometer. the savilian professor carried out his project of determining all second magnitude stars to the number of about thirty,[ ] conveniently observable at oxford, obtaining as the general outcome of the research an average parallax of · ", for objects of that rank. but this value, though in itself probable, cannot be accepted as authoritative, in view of certain inaccuracies in the work adverted to by jacoby,[ ] hermann davis, and gill. the method has, nevertheless, very large capabilities. professor kapteyn showed, in ,[ ] the practicability of deriving parallaxes wholesale from plates exposed at due intervals, and applied his system, in , with encouraging success, to a group of stars.[ ] the apparent absence of spurious shiftings justified the proposal to follow up the completion of the astrographic chart with the initiation of a photographic "parallax durchmusterung." observers of double stars are among the most meritorious, and need to be among the most patient and painstaking workers in sidereal astronomy. they are scarcely as numerous as could be wished. dr. doberck, distinguished as a computer of stellar orbits, complained in [ ] that data sufficient for the purpose had not been collected for above or binaries out of between five and six hundred certainly or probably within reach. the progress since made is illustrated by mr. gore's useful catalogue of computed binaries, including fifty-nine entries, presented to the royal irish academy, june , .[ ] few have done more towards supplying the deficiency of materials than the late baron ercole dembowski of milan. he devoted the last thirty years of his life, which came to an end january , , to the revision of the dorpat catalogue, and left behind him a store of micrometrical measures as numerous as they are precise. of living observers in this branch, mr. s. w. burnham is beyond question the foremost. while pursuing legal avocations at chicago, he diverted his scanty leisure by exploring the skies with a -inch telescope mounted in his back-yard; and had discovered, in may, , one thousand close and mostly very difficult double stars.[ ] summoned as chief assistant to the new lick observatory in , he resumed the work of his predilection with the -inch and -inch refractors of that establishment. but although devoting most of his attention to much-needed remeasurements of known pairs, he incidentally divided no less than stars, the majority of which lay beyond the resolving power of less keen and effectually aided eyesight. one of his many interesting discoveries was that of a minute companion to alpha ursæ majoris (the first pointer), which already gives unmistakable signs of orbital movement round the shining orb it is attached to. another pair, kappa pegasi, detected in , was found in to have more than completed a circuit in the interim.[ ] its period of a little over eleven years is the shortest attributable to a _visible_ binary system, except that of delta equulei, provisionally determined by professor hussey in at · years,[ ] and indicated by spectroscopic evidence to be of uncommon brevity.[ ] burnham's catalogue of , double stars, discovered by him from to ,[ ] is a record of unprecedented interest. nearly all the pairs included in it, " or less than " apart, must be physically connected; and they offer a practically unlimited field for investigation; while the notes, diagrams, and orbits appended profusely to the various entries, are eminently helpful to students and computers. the author is continuing his researches at the yerkes observatory, having quitted the lick establishment in . the first complete enrolment of southern double stars was made by mr. r. t. a. innes in .[ ] the couples enumerated, twenty-one per cent. of which are separated by less than one second of arc, are , in number. they include discovered by himself. dr. see gathered a rich harvest of nearly new southern pairs with the lowell -inch refractor in .[ ] professor hough's discoveries in more northerly zones amount to ;[ ] hussey's at lick to ; and aitken's already to over . there is as yet no certainty that the stars of cygni form a true binary combination. mr. burnham, indeed, holds them to be in course of definitive separation; and professor hall's observations at washington, to , although favouring their physical connection, are far from decisive on the point.[ ] dr. wilsing, from certain anomalous displacements of their photographed images, concluded in [ ] the presence of an invisible third member of the system, revolving in a period of twenty-two months; but the effects noticed by him were probably illusory. important series of double-star observations were made by perrotin at nice in - ;[ ] by hall, with the -inch washington equatoreal, to ;[ ] by schiaparelli from onward; by glasenapp, o. stone, leavenworth, seabroke, and many besides. finally, professor hussey's revision of the pulkowa catalogue[ ] is a work of the _teres atque rotundus_ kind, which leaves little or nothing to be desired. the methods employed in double-star determinations remain, at the beginning of the twentieth century, essentially unchanged. the camera has scarcely encroached upon this part of the micrometer's domain.[ ] a research of striking merit into the origin of binary stars was published in by dr. t. j. j. see, in the form of an inaugural dissertation for his doctor's degree in the university of berlin. the main result was to show the powerful effects of tidal friction in prescribing the course of their development from double nebulæ, revolving almost in contact, to double suns, far apart, yet inseparable. the high eccentricities of their eventual orbits were shown to result necessarily from this mode of action, which must operate with enormous strength on closely conjoined, nearly equal masses, such as the rapidly revolving pairs disclosed by the spectroscope. that these are still in an early stage of their life-history is probable in itself, and is re-affirmed by the exceedingly small density indicated for eclipsing stars by the ratio of phase-duration to period. stellar photometry, initiated by the elder herschel, and provided with exact methods by his son at the cape, by steinheil and seidel at munich, has of late years assumed the importance of a separate department of astronomical research. two monumental works on the subject, compiled on opposite sides of the atlantic, were thus appropriately coupled in the bestowal of the royal astronomical society's gold medal in . harvard college observatory led the way under the able direction of professor e. c. pickering. his photometric catalogue of , stars,[ ] constructed from nearly , observations of light-intensity during the years - , constitutes a record of incalculable value for the detection and estimation of stellar variability. it was succeeded in by professor pritchard's "uranometria nova oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of , . the instrument employed was the "wedge photometer," which measures brightness by resistance to extinction. a wedge of neutral-tint glass, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. professor pickering's "meridian photometer," on the other hand, is based upon zöllner's principle of equalization effected by a polarising apparatus. after all, however, as professor pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range.[ ] absolute uniformity, then, in estimates made by various means, under varying conditions, and by different observers, is not to be looked for; and it is satisfactory to find substantial agreement attainable and attained. only in an insignificant fraction of the stars common to the harvard and oxford catalogues discordances are found exceeding one-third of a magnitude; a large proportion ( per cent.) agree within one-fourth, a considerable minority ( per cent.) within one-tenth of a magnitude.[ ] the harvard photometry was extended, on the same scale, to the opposite pole in a catalogue of the magnitudes of , southern stars,[ ] founded on professor bailey's observations in peru, - . measurements still more comprehensive were subsequently executed at the primary establishment. with a meridian photometer of augmented power, the surprising number of , settings were made during the years - , nearly all by the indefatigable director himself, and they afforded materials for a "photometric durchmusterung," published in , including all stars to · magnitude north of declination - °.[ ] a photometric zone, ° wide, has for some time been in course of observation at potsdam by mm. müller and kempf. the instrument employed by them is constructed on the polarising principle as adapted by zöllner. photographic photometry has meanwhile risen to an importance if anything exceeding that of visual photometry. for the usefulness of the great international star-chart now being prepared would be gravely compromised by systematic mistakes regarding the magnitudes of the stars registered upon it. no entirely trustworthy means of determining them have, however, yet been found. there is no certainty as to the relative times of exposure needed to get images of stars representative of successive photometric ranks. all that can be done is to measure the proportionate diameters of such images, and to infer, by the application of a law learned from experience, the varied intensities of light to which they correspond. the law is, indeed, neither simple nor constant. different investigators have arrived at different formulæ, which, being purely empirical, vary their nature with the conditions of experiment. probably the best expedient for overcoming the difficulty is that devised by pickering, of simultaneously photographing a star and its secondary image, reduced in brightness by a known amount.[ ] the results of its use will be exhibited in a catalogue of , stars to the tenth magnitude, one for each square degree of the heavens. a photographic photometry of all the lucid stars, modelled on the visual photometry of , is promised from the same copious source of novelties. the magnitudes of the stars in the draper catalogue were determined, so to speak, spectrographically. the quantity measured in all cases was the intensity of the hydrogen line near g. by the employment of this definite and uniform test, results were obtained, of special value indeed, but in strong disaccord with those given by less exclusive determinations. thought, meantime, cannot be held aloof from the great subject upon the future illustration of which so much patient industry is being expended. nor are partial glimpses denied to us of relations fully discoverable, perhaps, only through centuries of toil. some important points in cosmical economy have, indeed, become quite clear within the last fifty years, and scarcely any longer admit of a difference of opinion. one of these is that of the true status of nebulæ. this was virtually settled by sir j. herschel's description in of the structure of the magellanic clouds; but it was not until whewell, in , and herbert spencer, in ,[ ] enforced the conclusions necessarily to be derived therefrom that the conception of the nebulæ as remote galaxies, which lord rosse's resolution of many into stellar points had appeared to support, began to withdraw into the region of discarded and half-forgotten speculations. in the nubeculæ, as whewell insisted,[ ] "there coexist, in a limited compass and in indiscriminate position, stars, clusters of stars, nebulæ, regular and irregular, and nebulous streaks and patches. these, then, are different kinds of things in themselves, not merely different to us. there are such things as nebulæ side by side with stars and with clusters of stars. nebulous matter resolvable occurs close to nebulous matter irresolvable." this argument from coexistence in nearly the same region of space, reiterated and reinforced with others by mr. spencer, was urged with his accustomed force and freshness by mr. proctor. it is unanswerable. there is no maintaining nebulæ to be simply remote worlds of stars in the face of an agglomeration like the nubecula major, containing in its (certainly capacious) bosom _both_ stars and nebulæ. add the facts that a considerable proportion of these perplexing objects are gaseous, and that an intimate relation obviously subsists between the mode of their scattering and the lie of the milky way, and it becomes impossible to resist the conclusion that both nebular and stellar systems are parts of a single scheme.[ ] as to the stars themselves, the presumption of their approximate uniformity in size and brightness has been effectually dissipated. differences of distance can no longer be invoked to account for dissimilarity in lustre. minute orbs, altogether invisible without optical aid, are found to be indefinitely nearer to us than such radiant objects as canopus, betelgeux, or rigel. moreover, intensity of light is perceived to be a very imperfect index to real magnitude. brilliant suns are swayed from their course by the attractive power of massive yet faintly luminous companions, and suffer eclipse from obscure interpositions. besides, effective lustre is now known to depend no less upon the qualities of the investing atmosphere than upon the extent and radiative power of the stellar surface. red stars must be far larger in proportion to the light diffused by them than white or yellow stars.[ ] there can be no doubt that our sun would at least double its brightness were the absorption suffered by its rays to be reduced to the sirian standard; and, on the other hand, that it would lose half its present efficiency as a light-source if the atmosphere partially veiling its splendours were rendered as dense as that of aldebaran. thus, variety of all kinds is seen to abound in the heavens; and it must be admitted that the consequent insecurity of all hypotheses as to the relative distances of individual stars singularly complicates the question of their allocation in space. nevertheless, something has been learnt even on that point; and the tendency of modern research is, on the whole, strongly confirmatory of the views expressed by herschel in . he then no longer regarded the milky way as the mere visual effect of an enormously extended stratum of stars, but as an actual aggregation, highly irregular in structure, made up of stellar clouds and groups and nodosities. all the facts since ascertained fit in with this conception, to which proctor added arguments favouring the view, since adopted by barnard[ ] and easton,[ ] that the stars forming the galactic stream are not only situated more closely together, but are also really, as well as apparently, of smaller dimensions than the lucid orbs studding our skies. by the laborious process of isographically charting the whole of argelander's , stars, he brought out in [ ] signs of relationship between the distribution of the brighter stars and the complex branchings of the milky way, which has been stamped as authentic by newcomb's recent statistical inquiries.[ ] there is, besides, a marked condensation of stars, especially in the southern hemisphere, towards a great circle inclined some twenty degrees to the galactic plane; and these were supposed by gould to form with the sun a subordinate cluster, of which the components are seen projected upon the sky as a zone of stellar brilliants.[ ] the zone has, however, galactic rather than solar affinities, and represents, perhaps, not a group, but a stream. the idea is gaining ground that the milky way is designed, in its main outlines, on a spiral pattern, and that its various branches and sections are consequently situated at very different distances from ourselves. proctor gave a preliminarily interpretation of their complexities on this principle, and easton of rotterdam[ ] has renewed the attempt with better success. a most suggestive delineation of the milky way, completed in , after five years of labour, by dr. otto boedicker, lord rosse's astronomer at parsonstown, was published by lithography in . it showed a curiously intricate structure, composed of dimly luminous streams, and shreds, and patches, intermixed with dark gaps and channels. ramifications from the main trunk ran out towards the andromeda nebula and the "bee-hive" cluster in cancer, involved the pleiades and hyades, and, winding round the constellation of orion, just attained the sword-handle nebula. the last delicate touches had scarcely been put to the picture, when the laborious eye-and-hand method was, in this quarter, as already in so many others, superseded by a more expeditious process. professor barnard took the first photographs ever secured of the true milky way, july , august and , , at the lick observatory. special conditions were required for success; above all, a wide field and a strong light-grasp, both complied with through the use of a -inch portrait-lens. even thus, the sensitive plate needed some hours to pick out the exceedingly faint stars collected in the galactic clouds. these cannot be photographed under the nebulous aspect they wear to the eye; the camera takes note of their real nature, and registers their constituent stars rank by rank. hence the difficulty of disclosing them. "in the photographs made with the -inch portrait-lens," professor barnard wrote, "besides myriads of stars, there are shown, for the first time, the vast and wonderful cloud-forms, with all their remarkable structure of lanes, holes, and black gaps, and sprays of stars. they present to us these forms in all their delicacy and beauty, as no eye or telescope can ever hope to see them."[ ] in plate vi. one of these strange galactic landscapes is reproduced. it occurs in the bow of sagittarius, not far from the trifid nebula, where the aggregations of the milky way are more than usually varied and characteristic. one of their distinctive features comes out with particular prominence. it will be noticed that the bright mass near the centre of the plate is tunnelled with dark holes and furrowed by dusky lanes. such interruptions recur perpetually in the milky way. they are exemplified on the largest scale in the great rift dividing it into two branches all the way from cygnus to crux; and they are reproduced in miniature in many clusters. plate vi. [illustration: region of the milky way in sagittarius--showing a double black aperture. photographed by professor e. e. barnard.] mr. h. c. russell, at sydney in , successfully imitated professor barnard's example.[ ] his photographs of the southern milky way have many points of interest. they show the great rift, black to the eye, yet densely star-strewn to the perception of the chemical retina; while the "coal-sack" appears absolutely dark only in its northern portion. his most remarkable discovery, however, was that of the spiral character of the two nubeculæ. with an effective exposure of four and a half hours, the greater cloud came out as "a complex spiral, with two centres"; while the similar conformation of its minor companion developed only after eight hours of persistent actinic action. the revelation is full of significance. scarcely less so, although after a different fashion, is the disclosure on plates exposed by dr. max wolf, with a -inch lens, in june, , of a vastly extended nebula, bringing some of the leading stars in cygnus into apparently organic connection with the piles of galactic star-dust likewise involved by it.[ ] barnard has similarly found great tracts of the milky way to be photographically nebulous, and the conclusion seems inevitable that we see in it a prodigious mixed system, resembling that of the pleiades in point of composition, though differing widely from it in plan of structure. of corroborative testimony, moreover, is the discovery independently resulting from gill's and pickering's photographic reviews, that stars of the first type of spectrum largely prevail in the galactic zone of the heavens.[ ] with approach to that zone, kapteyn noticed a steady growth of actinic intensity relative to visual brightness in the stars depicted on the cape durchmusterung plates.[ ] in other words, stellar light is, in the milky way, _bluer_ than elsewhere. and the reality of the primitive character hence to be inferred for the entire structure was, in a manner, certified by mr. mcclean's observation that helium stars--the supposed immediate products of nebulous matter--crowd towards its medial plane. the first step towards the unravelment of the tangled web of stellar movements was taken when herschel established the reality, and indicated the direction of the sun's journey. but the gradual shifting backward of the whole of the celestial scenery amid which we advance accounts for only a part of the observed displacements. the stars have motions of their own besides those reflected upon them from ours. all attempts, however, to grasp the general scheme of these motions have hitherto failed. yet they have not remained wholly fruitless. the community of slow movement in taurus, upon which mädler based his famous theory, has proved to be a fact, and one of very extended significance. in mr. proctor undertook to chart down the directions and proportionate amounts of about , proper motions, as determined by messrs. stone and main, with the result of bringing to light the remarkable phenomenon termed by him "star-drift."[ ] quite unmistakably, large groups of stars, otherwise apparently disconnected, were seen to be in progress together, in the same direction and at the same rate, across the sky. an example of this kind of unanimity was alleged by him in the five intermediate stars of the plough; and that the agreement in thwartwise motion is no casual one is practically demonstrated by the concordant radial velocities determined at potsdam for four out of the five objects in question. all of these approach the earth at the rate of about eighteen miles a second; and the fifth and faintest, delta ursæ, though not yet measured, may be held to share their advance. one of them, moreover, zeta ursæ, alias mizar, carries with it three other stars--alcor, the arab "rider" of the horse, visible to the naked eye, besides a telescopic and a spectroscopic attendant. so that the group may be regarded as octuple. it is of vast compass. dr. höffler assigned to it in [ ]--although on grounds more or less hypothetical--a mean parallax corresponding to a light-journey of years, which would give to the marching squadron a total extent of at least fourteen times the distance from the sun to alpha centauri, while implying for its brightest member--eta ursæ majoris--the lustre of six hundred suns. the organising principle of this grand scheme must long remain mysterious. it is no solitary example. particular association, indeed--as was surmised by michell far back in the eighteenth century--appears to be the rule rather than an exception in the sidereal system. stars are bound together by twos, by threes, by dozens, by hundreds. our own sun is, perhaps, not exempt from this gregarious tendency. yet the search for its companions has, up to the present, been unavailing. gould's cluster[ ] seems remote and intangible; kapteyn's collection of solar stars proved to have been a creation of erroneous data, and was abolished by his unrelenting industry. rather, we appear to have secured a compartment to ourselves for our long journey through space. a practical certainty has, at any rate, been gained that whatever aggregation holds the sun as a constituent is of a far looser build than the pleiades or præsepe. of all such majestic communities the laws and revolutions remain, as yet, inaccessible to inquiry; centuries may elapse before even a rudimentary acquaintance with them begins to develop; while the economy of the higher order of association, which we must reasonably believe that they unite to compose, will possibly continue to stimulate and baffle human curiosity to the end of time. footnotes: [footnote : _report brit. assoc._, , p. . rutherfurd gave a rudimentary sketch of a classification of the kind in december, , but based on imperfect observation. see _am. jour. of sc._, vol. xxxv., p. .] [footnote : _publicationem_, potsdam, no. , , p. .] [footnote : von konkoly _once_ derived from a slow-moving meteor a hydro-carbon spectrum. a. s. herschel, _nature_, vol. xxiv., p. .] [footnote : _phil. trans._, vol. cliv., p. .] [footnote : _am. jour. of sc._, vol. xix., p. .] [footnote : _photom. unters._, p. .] [footnote : _spectre solaire_, p. .] [footnote : mr. j. birmingham, in the introduction to his catalogue of red stars, adduced sundry instances of colour-change in a direction the opposite to that assumed by zöllner to be the inevitable result of time. _trans. r. irish acad._, vol. xxvi., p. . a learned discussion by dr. t. j. j. see, moreover, enforces the belief that sirius was absolutely _red_ eighteen hundred years ago. _astr. and astroph._, vol. xi., p. .] [footnote : _phil. trans.,_ vol. clxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _proc. roy. soc._, vols. xvi., p. ; xvii., p. .] [footnote : _annalen der physik_, bd. xx., p. .] [footnote : _ibid._, p. .] [footnote : _knowledge_, vol. xiv., p. .] [footnote : _meteoritic hypothesis_, p. .] [footnote : _phil. trans._, vol. cxci. a., p. ; _spectra of southern stars_, p. .] [footnote : see the author's _system of the stars_, p. .] [footnote : a designation applied by sir norman lockyer to third-type stars.] [footnote : see _ante_, p. .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , ; _observatory_, vol. vi., p. .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _publ. astr. pac. soc._, vol. i., p. ; _observatory_, vol. xiii., p. .] [footnote : _lockyer, proc. roy. soc._, vol. lvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. lix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, nos. , - .] [footnote : _ibid._, no. , ; _astr. and astrophysics_, vol. xi., p. ; bélopolsky, _astr. nach._, no. , .] [footnote : _comptes rendus_, t. lxv., p. .] [footnote : _copernicus_, vol. iii., p. .] [footnote : _system of the stars_, p. ; _harvard annals_, vol. xxviii., pt. ii., p. (miss cannon).] [footnote : _potsdam publ._, no. , p. .] [footnote : _proc. roy. soc._, vol. xlix., p. .] [footnote : miss a. j. cannon, _harvard annals_, vol. xxviii., pt. ii., p. .] [footnote : _astr. and astroph._, vol. xiii., p. .] [footnote : _potsdam publ._, no. .] [footnote : the results of von konkoly's extension of vogel's work to ° of south declination were published in _o gyalla beobachtungen_, bd. viii., th. ii., .] [footnote : _astroph. jour._, vols. viii., p. ; ix., p. .] [footnote : _ibid._, vol. ix., p. .] [footnote : _phil. trans._, vol. cliv., p. . some preliminary results were embodied in a "note" communicated to the royal society, february , (_proc. roy. soc._, vol. xii., p. ).] [footnote : _bothkamp beob._, heft i., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _phil. trans._, vol. cliv., p. , _note_.] [footnote : _month. not._, vol. xxiii., p. .] [footnote : _proc. roy. soc._, vol. xxv., p. .] [footnote : _phil. trans._, vol. clxxi., p. ; _atlas of stellar spectra_, p. .] [footnote : _astr. nach._, no. , ; _monatsb._, berlin, , p. ; , p. .] [footnote : _jour. de physique_, t. v., p. .] [footnote : _system of the stars_, p. .] [footnote : see _ante_, p. .] [footnote : _proc. roy. soc._, vol. xlviii., p. .] [footnote : _harvard circulars_, nos. , ; _astroph. jour._, vol. v., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : mcclean, _phil. trans._, vol. cxci. a., p. .] [footnote : _proc. roy. soc._, vol. lxii., p. .] [footnote : _ibid._, april , ; _astroph. jour._, vol. x., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : lunt, _astroph. jour._, vol. xi., p. ; _proc. roy. soc._, vol. lxvi., p. ; lockyer, _ibid._, november , ; _nature_, vol. lxi., p. .] [footnote : _die spectralanalyse_, p. .] [footnote : _henry draper memorial, first ann. report_, .] [footnote : _mem. amer. acad._, vol. xi., p. .] [footnote : _harvard annals_, vol. xxvii.] [footnote : _harvard annals_, vol. xxviii., parts i. and ii.] [footnote : see _ante_, p. .] [footnote : _phil. trans._, vol. clviii., p. .] [footnote : schellen, _die spectralanalyse_, bd. ii., p. (ed. ).] [footnote : _proc. roy. soc._, vol. xx., p. .] [footnote : _system of the stars_, p. .] [footnote : pickering, _am. jour. of sc._, vol. xxxix., p. ; vogel, _astr. nach._ no. , .] [footnote : _sitzungsberichte_, berlin, may , ; _astroph. jour._, vol. xiii., p. .] [footnote : the "relative orbit" of a double star is that described by one round the other as a fixed point. micrometrical measures are always thus executed. but in reality both stars move in opposite directions, and at rates inversely as their masses round their common centre of gravity.] [footnote : vogel, _astr. nach._, nos. , , , .] [footnote : huggins, _pres. address_, ; cornu, _sur la méthode doppler-fizeau_ p. d. .] [footnote : _sitzungsb._, berlin, , p. ; _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astroph. jour._, vol. v., p. ; newall, _month. not._, vol. lvii., p. .] [footnote : _bull. de l'acad. de st. pétersb._, tt. vi., viii.] [footnote : _astroph. jour._, vol. x., p. ; _month. not._, vol. lx., p. ; vogel, _sitzungsb._, berlin, april , .] [footnote : _month. not._, vol. lx., p. .] [footnote : hussey, _astr. jour._, no. .] [footnote : _astroph. jour._, vols. x, p. ; xiv., p. ; _lick bulletin_, no. ; bélopolsky, _astr. nach._, no. , .] [footnote : the significance of the name "el ghoul" leaves little doubt that the arab astronomers took note of this star's variability. e. m. clerke, _observatory_, vol. xv., p. .] [footnote : _phil. trans._, vol. lxxiii., p. .] [footnote : _proc. amer. acad._, vol. xvi., p. ; _observatory_, vol. iv., p. . for a preliminary essay by t. s. aldis, see _phil. mag._, vol. xxxix., p. , .] [footnote : _astr. nach._, no. , .] [footnote : _astr. jour._, nos. - , - , . see also _knowledge_, vol. xv., p. .] [footnote : bauschinger, _v. j. s. astr. ges._, jahrg. xxix.; but _cf._ searle, _harvard annals_, vol. xxix., p. ; boss, _astr. jour._, no. .] [footnote : _comptes rendus_, t. cxx., p. .] [footnote : myers, _astroph. jour._, vol. vii., p. ; a. w. roberts, _ibid._, vol. xiii., p. .] [footnote : _proc. r. irish ac._, july, .] [footnote : _ibid._, vol. i., p. .] [footnote : _astr. jour._, nos. , .] [footnote : _ibid._, nos. , .] [footnote : _astr. jour._, nos. - .] [footnote : _system of the stars_, p. .] [footnote : _proc. roy. soc._, vol. xv., p. .] [footnote : weiss, _astr. nach._, no. , ; espin, _ibid._, no. , .] [footnote : _comptes rendus_, t. lxxxiii., p. .] [footnote : _monatsb._, berlin, , pp. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : burnham, _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , .] [footnote : a. hall, _am. jour. of sc._, vol. xxxi., p. .] [footnote : young, _sid. messenger_, vol. iv., p. ; hasselberg, _astr. nach._, no. , .] [footnote : _report brit. assoc._, , p. .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _nature_, vol. xxxii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _month. not._, vol. xxi., p. .] [footnote : _observatory_, vol. viii., p. .] [footnote : _astr. nach._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : _cape results_, p. .] [footnote : _trans. r. soc. of edinburgh_, vol. xxvii., p. ; _astr. and astroph._, august, , p. .] [footnote : vogel, _astr. nach._, no. , .] [footnote : _observatory_, vol. xv., p. ; seeliger, _astr. nach._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : ranyard, _knowledge_, vol. xv., p. .] [footnote : _proc. roy. soc._, vol. li., p. .] [footnote : burnham, _month. not._, vol. liii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : renz, _ibid._, nos. , , , ; huggins, _astr. and astroph._, vol. xiii., p. .] [footnote : _astr. nach._, no. , .] [footnote : bélopolsky, _astr. nach._, no. , .] [footnote : _nature_, september , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : _publ. astr. pac. soc._, vol. iv., p. .] [footnote : barnard, _astroph. jour._, vol. xiv., p. ; campbell, _observatory_, vol. xxiv., p. .] [footnote : _pop. astr._, march, , p. .] [footnote : _harvard circular_, no. , december , . the first nova persei was spectrographically recorded in .] [footnote : vogel, _sitzungsb._, berlin, april , , p. .] [footnote : sidgreaves, _observatory_, vol. xxiv., p. .] [footnote : _ibid._, _knowledge_, vol. xxv., p. .] [footnote : _lick bulletin_, no. .] [footnote : _astr. nach._, no. , .] [footnote : _astroph. jour._, vol. xiv., p. .] [footnote : _lick bulletin_, no. .] [footnote : _astroph. jour._, vols. xiv., p. ; xv., p. .] [footnote : _cf._ the theories on the subject of m. wolf, _astr. nach._, nos. , , , ; kapteyn, _ibid._, no. , ; f. w. very, _ibid._, no. , ; and w. e. wilson, _proc. roy. dublin soc._, no. , p. .] [footnote : _phil. trans._, vol. cliv., p. .] [footnote : _phil. trans._, vol. clviii., p. . the true proportion seems to be about one-tenth (_harvard annals_, vol. xxvi., pt. ii., p. ), the tulse hill working-list having been formed of specially selected objects.] [footnote : scheiner, _astr. nach._, no. , ; _astroph. jour._, vol. vii., p. ; campbell, _ibid._, vols. ix., p. ; x., p. .] [footnote : _proc. roy. soc._, vols. xlvi., p. ; xlviii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. ; _proc. roy. soc._, vol. xlix., p. .] [footnote : _astr. nach._, no , .] [footnote : _atlas of stellar spectra_, p. .] [footnote : _knowledge_, vol. xix., p. .] [footnote : _astr. nach._, nos. , , , , , ; chambers, _descriptive astr._ ( rd ed.), p. ; flammarion, _l'univers sidéral_, p. .] [footnote : _month. not._, vol. li., p. .] [footnote : _ibid._, vol. lix., p. .] [footnote : _ibid._, vol. lx., p. .] [footnote : dreyer, _ibid._, vol. lii., p. .] [footnote : _wash. obs._, vol. xxv., app. .] [footnote : _am. jour. of sc._, vol. xiv., p. ; c. dreyer, _month. not._, vol. xlvii., p. .] [footnote : _ibid._, vol. li., p. .] [footnote : reproduced in _knowledge_, april, .] [footnote : unless an exception be found in the pleiades nebulæ, which may be assumed to share the small apparent movement of the stars they adhere to.] [footnote : _abhandl. akad. der wiss._, leipzig, , bd. iii., p. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _proc. roy. soc._, , p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _system of the stars_, p. .] [footnote : _proc. roy. soc._, vol. xlix., p. .] [footnote : _potsdam publ._, bd. vii., th. i.] [footnote : _astr. nach._, no. , ; schönfeld, _v. j. s. astr. ges._, jahrg. xxi., p. .] [footnote : _astroph. journ._, vol. xiii., p. .] [footnote : _proc. roy. soc._, vol. xxxiii., p. ; _report brit. assoc._, , p. . an impression of the four lower lines in the same spectrum was almost simultaneously obtained by dr. draper. _comptes rendus_, t. xciv., p. .] [footnote : _proc. roy. soc._, vol. xlviii., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _proc. roy. soc._, vol. xlvi., p. ; _system of the stars_, p. .] [footnote : _sitzungsb._, berlin, february , .] [footnote : _wash. obs._, vol. xxv., app. i., p. .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _month. not._, vol. xliii., p. .] [footnote : _harvard annals_, vol. xviii., p. .] [footnote : _sid. mess._, vol. ix., p. .] [footnote : _knowledge_, vol. xv., p. .] [footnote : _month. not._, vol. xlix., p. .] [footnote : _system of the stars_, p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : vogel, _astr. nach._, , .] [footnote : _nature_, vol. xliii., p. .] [footnote : _l'astronomie_, t. xl., p. .] [footnote : _astr. nach._, bände xlvii., p. ; xlviii., p. ; xlix., p. . pickering, _mem. am. ac._, vol. xi., p. .] [footnote : gould on celestial photography, _observatory_, vol. ii., p. .] [footnote : _annals n. y. acad. of sciences_, vol. vi., p. , ; elkin, _publ. astr. pac. soc._, vol. iv., p. .] [footnote : _trans. yale observatory_, vol. i., pt. i.] [footnote : _astroph. jour._, vol. xiii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _les mondes_, t. iii., p. .] [footnote : mouchez, _comptes rendus_, t. cvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, nos. , , , .] [footnote : _journ. brit. astr. assoc._, vol. ix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. xxi., pp. , .] [footnote : reproduced in _astroph. journ._, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _astr. nach._, no. , .] [footnote : _sitzungsb. bayer. akad._, march , .] [footnote : _annals of the cape observatory_, vols. iii., iv., v.] [footnote : _month. not._, vol. lx., p. .] [footnote : d. klumpke, _observatory_, vol. xv., p. .] [footnote : gilbert, _sid. mess._, vol. i., p. .] [footnote : _astr. papers for the amer. ephemeris_, vol. viii., pt. ii.] [footnote : _nature_, vol. xxiv., p. ; _dunsink observations_, pt. v., .] [footnote : elkin, _report for - _, p. ; newcomb, _the stars_, p. .] [footnote : _annals of the cape observatory_, vol. viii., pt. ii. some of the measures were made by messrs. finlay and de sitter.] [footnote : _astr. nach._, no. , ; _observatory_, vol. xxi., p. .] [footnote : _annalen der sternwarte in leiden_, bd. vii.] [footnote : _report of harvard conference in _ (snyder).] [footnote : _researches in stellar parallax_, pt. ii., .] [footnote : _v. j. s. astr. ges._, jahrg., xxviii., p. .] [footnote : _bulletin de la carte du ciel_, no. , p. .] [footnote : _publ. of the astr. laboratory at groningen_, no. .] [footnote : _nature_, vol. xxvi., p. .] [footnote : _proc. r. irish acad._, vol. i., p. , ser. iii.] [footnote : _mem. r. a. s._, vol. xlvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, no. .] [footnote : campbell, _lick bulletin_, no. .] [footnote : _publ. yerkes observatory_, vol. i., .] [footnote : _annals cape observatory_, vol. ii., pt. ii.] [footnote : _astr. jour._, nos. - .] [footnote : w. j. hussey, _publ. astr. pac. soc._, no. .] [footnote : _astr. jour._, no. .] [footnote : _sitzungsberichte_, berlin, october , .] [footnote : _annales de l'obs. de nice_, t. ii.] [footnote : _washington observations_, , app. i.] [footnote : _publ. lick observatory_, vol. v., .] [footnote : t. lewis, _observatory_, vol. xvi., p. .] [footnote : _harvard annals_, vol. xiv., pt. i., .] [footnote : _observatory_, vol. viii., p. .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _harvard annals_, vol. xxxiv.] [footnote : _ibid._, vol. xlv.] [footnote : _carte phot. du ciel. réunion du comité permanent_, paris, , p. .] [footnote : _essays_ ( nd ser.), _the nebular hypothesis_.] [footnote : _on the plurality of worlds_, p. ( nd ed.).] [footnote : proctor, _month. not._, vol. xxix., p. .] [footnote : this remark was first made by j. michell, _phil. trans._, vol. lvii., p. ( ).] [footnote : _pop. astr._, no. .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _month. not._, vols. xxxi., p. ; xxxii., p. .] [footnote : _the stars_, p. .] [footnote : _system of the stars_, p. ; _old and new astronomy_, p. (ranyard).] [footnote : _astroph. jour._, vol. xii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _month. not._, vol. li., pp. , . for reproductions of some of the photographs in question, see _knowledge_, vol. xiv., p. .] [footnote : _astr. nach._, no. , ; _observatory_, vol. xiv., p. .] [footnote : _proc. roy. inst._, may , (gill).] [footnote : _annals cape obs._, iii., introduction, p. .] [footnote : _proc. roy. soc._, vol. xviii., p. .] [footnote : _astr. nach._, no. , ; _observatory_, vol. xxi., p. ; newcomb, _the stars_, p. .] [footnote : _month. not._, vol. xl., p. .] chapter xiii _methods of research_ comparing the methods now available for astronomical inquiries with those in use forty years ago, we are at once struck with the fact that they have multiplied. the telescope has been supplemented by the spectroscope and the photographic camera. now, this really involves a whole world of change. it means that astronomy has left the place where she dwelt apart in rapt union with mathematics, indifferent to all things on earth save only to those mechanical improvements which should aid her to penetrate further into the heavens, and has descended into the forum of human knowledge, at once a suppliant and a patron, alternately invoking help from and promising it to each of the sciences, and patiently waiting upon the advances of all. the science of the heavenly bodies has, in a word, become a branch of terrestrial physics, or rather a higher kind of integration of all their results. it has, however, this leading peculiarity, that the materials for the whole of its inquiries are telescopically furnished. they are such as come very imperfectly, or not at all, within the cognisance of the unarmed eye. spectroscopic and photographic apparatus are simply additions to the telescope. they do not supersede or render it of less importance. on the contrary, the efficacy of their action depends primarily upon the optical qualities of the instruments they are attached to. hence the development, to their fullest extent, of the powers of the telescope is of vital moment to the progress of modern physical astronomy, while the older mathematical astronomy could afford to remain comparatively indifferent to it. the colossal rosse reflector still marks, as to size, the _ne plus ultra_ of performance in that line. a mirror four feet in diameter was, however, sent out to melbourne by the late thomas grubb of dublin in . this is mounted in the cassegrainian manner, so that the observer looks straight through it towards the object viewed, of which he really sees a twice-reflected image. the dust-laden atmosphere of melbourne is said to impede very seriously the usefulness of this originally fine instrument. it may be doubted whether so large a spectrum will ever again be constructed. a new material for the mirrors of reflecting telescopes, proposed by steinheil in , and independently by foucault in ,[ ] has in a great measure superseded the use of a metallic alloy. this is glass upon which a thin film of silver has been deposited by a chemical process originally invented by liebig. it gives a peculiarly brilliant reflective surface, throwing back more light than a metallic mirror of the same area, in the proportion of about sixteen to nine. resilvering, too, involves much less risk and trouble than repolishing a speculum. the first use of this plan on a large scale was in an instrument of thirty-six inches aperture, finished by calver for dr. common in . to its excellent qualities turned to account with rare skill, his triumphs in celestial photography are mainly due. a more daring experiment was the construction and mounting, by dr. common himself, of a -foot reflector. but the first glass disc ordered from france for the purpose proved radically defective. when figured, polished, and silvered, towards the close of , it gave elliptical instead of circular star-images.[ ] a new one had to be procured, and was ready for astronomical use in . the satisfactory nature of its performance is vouched for by the observations made with it upon jupiter's new satellite in december, . this instrument, to which a newtonian form has been given, had no rival in respect of light-concentration at the time when it was built. it has now two--the paris -inch refractor and the yerkes -foot reflector. it is, however, in the construction of refracting telescopes that the most conspicuous advances have recently been made. the harvard college -inch achromatic was mounted and ready for work in june, . a similar instrument had already for some years been in its place at pulkowa. it was long before the possibility of surpassing these masterpieces of german skill presented itself to any optician. for fifteen years it seemed as if a line had been drawn just there. it was first transgressed in america. a portrait-painter of cambridgeport, massachusetts, named alvan clark, had for some time amused his leisure with grinding lenses, the singular excellence of which was discovered in england by mr. dawes in .[ ] seven years passed, and then an order came from the university of mississippi for an object-glass of the unexampled size of eighteen inches. an experimental glance through it to test its definition resulted, as we have seen, in the detection of the companion of sirius, january , . it never reached its destination in the south. war troubles supervened, and it was eventually sent to chicago, where it served professor hough in his investigations of jupiter, and mr. burnham in his scrutiny of double stars. the next step was an even longer one, and it was again taken by a self-taught optician, thomas cooke, the son of a shoemaker at allerthorpe, in the east riding of yorkshire. mr. newall of gateshead ordered from him in a -inch object-glass. it was finished early in , but at the cost of shortening the life of its maker, who died october , , before the giant refractor he had toiled at for five years was completely mounted. this instrument, the fine qualities of which had long been neutralized by an unfavourable situation, was presented by mr. newall to the university of cambridge, a few weeks before his death, april , . under the care of his son, mr. frank newall, it has proved highly efficient in the delicate work of measuring stellar radial motions. close upon its construction followed that of the washington -inch, for which twenty thousand dollars were paid to alvan clark. the most illustrious point in its career, entered upon in , has been the discovery of the satellites of mars. once known to be there, these were, indeed, found to be perceptible with very moderate optical means (mr. wentworth erck saw deimos with a -inch clark); but the first detection of such minute objects is a feat of a very different order from their subsequent observation. dr. see's perception with this instrument, in , of neptune's cloud-belts, and his refined series of micrometrical measures of the various planets, attest the unimpaired excellence of its optical qualities. it held the primacy for more than eight years. then, in december, , the place of honour had to be yielded to a -inch achromatic, built by howard grubb (son and successor of thomas grubb) for the vienna observatory. this, in its turn, was surpassed by two of respectively - / and inches, sent by gautier of paris to nice, and by alvan clark to pulkowa; and an object-glass, three feet in diameter, was in successfully turned out by the latter firm for the lick observatory in california. the difficulties, however, encountered in procuring discs of glass of the size and purity required for this last venture seemed to indicate that a term to progress in this direction was not far off. the flint was, indeed, cast with comparative ease in the workshops of m. feil at paris. the flawless mass weighed kilogrammes, was over inches across, and cost , dollars. but with the crown part of the designed achromatic combination things went less smoothly. the production of a perfect disc was only achieved after _nineteen_ failures, involving a delay of more than two years; and the glass for a third lens, designed to render the telescope available at pleasure for photographic purposes, proved to be strained, and consequently went to pieces in the process of grinding. it has been replaced by one of inches, with which a series of admirable lunar and other photographs have been taken. nor is the difficulty in obtaining suitable material the only obstacle to increasing the size of refractors. the "secondary spectrum," as it is called, also interposes a barrier troublesome to surmount. true achromatism cannot be obtained with ordinary flint and crown-glass; and although in lenses of "jena glass," outstanding colour is reduced to about one-sixth its usual amount, their term of service is fatally abridged by rapid deterioration. nevertheless, a -inch objective of the new variety was mounted at königsberg in ; and discs of jena crown and flint, inches across, were purchased by brashear at the chicago exhibition of . an achromatic combination of three kinds of glass, devised by mr. a. taylor[ ] for messrs. cooke of york, has less serious drawbacks, but has not yet come into extensive use. meanwhile, in giant telescopes affected to the full extent by chromatic aberration, such as the lick and yerkes refractors, the differences of focal length for the various colours are counted by inches,[ ] and this not through any lack of skill in the makers, but by the necessity of the case. embarrassing consequences follow. only a small part of the spectrum of a heavenly body, for instance, can be distinctly seen at one time; and a focal adjustment of half an inch is required in passing from the observation of a planetary nebula to that of its stellar nucleus. a refracting telescope loses, besides, one of its chief advantages over a reflector when its size is increased beyond a certain limit. that advantage is the greater luminosity of the images given by it. considerably more light is transmitted through a glass lens than is reflected from an equal metallic surface. but only so long as both are of moderate dimensions. for the glass necessarily grows in thickness as its area augments, and consequently stops a larger percentage of the rays it refracts. so that a point at length arrives--fixed by the late dr. robinson at a diameter a little short of feet[ ]--where the glass and the metal are, in this respect, on an equality; while above it the metal has the advantage. and since silvered glass gives back considerably more light than speculum metal, the stage of equalisation with lenses is reached proportionately sooner where this material is employed.[ ] the most distinctive faculty of reflectors, however, is that of bringing rays of all refrangibilities to a focus together. they are naturally achromatic. none of the beams they collect are thrown away in colour-fringes, obnoxious both in themselves and as a waste of the chief object of astrophysicists' greed--light. reflectors, then, are in this respect specially adapted to photographic and spectrographic use. but they have a countervailing drawback. the penalties imposed by bigness are for them peculiarly heavy. perfect definition becomes with increasing size, more and more difficult to attain; once attained, it becomes more and more difficult to keep. for the huge masses of material employed to form great object-glasses or mirrors tend with every movement to become deformed by their own weight. now, the slightest bending of a mirror is fatal to its performance, the effect being doubled by reflection; while in a lens alteration of figure is compensated by the equal and contrary flexures of the opposing surfaces, so that the emergent beams pursue much the same paths as if the curves of the refracting medium had remained theoretically perfect. for this reason work of precision must remain the province of refracting telescopes, although great reflectors retain the primacy in the portraiture of the heavenly bodies, as well as in certain branches of spectroscopy. professor hale, accordingly, summarised a valuable discussion on the subject by asserting[ ] "that the astrophysicist may properly consider the reflector to be an even more important part of his instrumental equipment than the refractor." a new era in its employment west of the atlantic opened with the transfer from halifax to mount hamilton of the crossley reflector. its prerogatives in nebular photography were splendidly indicated in by professor keeler's exquisite and searching portrayals of the cloud-worlds of space, and those obtained two years later, with a similar, though smaller, instrument, by professor ritchey of the yerkes observatory, were fully comparable with them. the performances of the yerkes -foot reflector still belong to the future. ambition as regards telescopic power is by no means yet satisfied. nor ought it to be. the advance of astrophysical researches of all kinds depends largely upon light-grasp. for the spectroscopic examination of stars, for the measurement of their motions in the line of sight, for the discovery and study of nebulæ, for stellar and nebular photography, the cry continually is "more light." there is no enterprising head of an observatory but must feel cramped in his designs if he can command no more than or inches of aperture, and he aspires to greater instrumental capacity, not merely with a view to the chances of discovery, but for the steady prosecution of some legitimate line of inquiry. thus projects of telescope-building on a large scale are rife, and some obtain realisation year by year. sir howard grubb finished in a -inch achromatic for the royal observatory, greenwich; the thompson equatoreal, mounted at the same establishment in , carries on a single axis a -inch photographic refractor and a -inch silvered-glass reflector; the victoria telescope, inaugurated at the cape in , comprises a powerful spectrographic apparatus, together with a chemically corrected -inch refractor, the whole being the munificent gift of mr. frank mcclean; at potsdam, at meudon, at paris, at alleghany, engines for light-concentration have been, or shortly will be, erected of dimensions which, two generations back, would have seemed extravagant and impossible. perhaps the finest, though not absolutely the greatest, among them, marked the summit and end of the performances of alvan g. clark, the last survivor of the cambridgeport firm. in october, , mr. yerkes of chicago offered an unlimited sum for the provision of the university of that city with a "superlative" telescope. and it happened, fortunately, that a pair of glass discs, nearly inches in diameter, and of perfect quality, were ready at hand. they had been cast by mantois for the university of southern california, when the erection of a great observatory on wilson's peak was under consideration. in the clark workshop they were combined into a superb objective, brought to perfection by trials and delicate touches extending over nearly five years. then the maker accompanied it to its destination, by the shore of a far western lake geneva, and died immediately after his return, june , . nor has the implement of celestial research he just lived to complete been allowed to "rust unburnished." manipulated by hale, burnham, and barnard, it has done work that would have been impracticable with less efficient optical aid. its construction thus marks a noticeable enlargement of astronomical possibilities, exemplified--to cite one among many performances--by barnard's success in keeping track of cluster-variables when below the common limit of visual perception. with the lick telescope results have also been achieved testifying to its unsurpassed excellence. holden's and schaeberle's views of planetary nebulæ, burnham's and hussey's hair's-breadth star-splitting operations, keeler's measurements of nebular radial motion, barnard's detections and prolonged pursuit of faint comets, his discovery of jupiter's tiny moon, campbell's spectroscopic determinations--all this could only have been accomplished, even by an exceptionally able and energetic staff, with the aid of an instrument of high power and quality. but there was another condition which should not be overlooked. the best telescope may be crippled by a bad situation. the larger it is, indeed, the more helpless is it to cope with atmospheric troubles. these are the worst plagues of all those that afflict the astronomer. no mechanical skill avails to neutralise or alleviate them. they augment with each increase of aperture; they grow with the magnifying powers applied. the rays from the heavenly bodies, when they can penetrate the cloud-veils that too often bar their path, reach us in an enfeebled, scattered, and disturbed condition. hence the twinkling of stars, the "boiling" effects at the edges of sun, moon, and planets; hence distortions of bright, effacements of feeble telescopic images; hence, too, the paucity of the results obtained with many powerful light-gathering machines. no sooner had the parsonstown telescope been built than it became obvious that the limit of profitable augmentation of size had, under climatic conditions at all nearly resembling those prevailing there, been reached, if not overpassed; and lord rosse himself was foremost to discern the need of pausing to look round the world for a clearer and stiller air than was to be found within the bounds of the united kingdom. with this express object mr. lassell transported his -foot newtonian to malta in , and mounted there, in , a similar instrument of fourfold capacity, with which, in the course of about two years, new nebulæ were discovered. professor piazzi smyth's experiences during a trip to the peak of teneriffe in in search of astronomical opportunities[ ] gave countenance to the most sanguine hopes of deliverance, at suitable elevated stations, from some of the oppressive conditions of low-level star-gazing; yet for a number of years nothing effectual was done for their realisation. now, at last, however, mountain observatories are not only an admitted necessity but an accomplished fact; and newton's long forecast of a time when astronomers would be compelled, by the developed powers of their telescopes, to mount high above the "grosser clouds" in order to use them,[ ] had been justified by the event. james lick, the millionaire of san francisco, had already chosen, when he died, october , , a site for the new observatory, to the building and endowment of which he had devoted a part of his large fortune. the situation of the establishment is exceptional and splendid. planted on one of the three peaks of mount hamilton, a crowning summit of the californian coast range, at an elevation of , feet above the sea, in a climate scarce rivalled throughout the world, it commands views both celestial and terrestrial which the lover of nature and astronomy may alike rejoice in. impediments to observation are there found to be most materially reduced. professor holden, who was appointed, in , president of the university of california and director of the new observatory affiliated to it, stated that during six or seven months of the year an unbroken serenity prevails, and that half the remaining nights are clear.[ ] the power of continuous work thus afforded is of itself an inestimable advantage; and the high visual excellences testified to by mr. burnham's discovery, during a two months' trip to mount hamilton in the autumn of , of forty-two new double stars with a -inch achromatic, gave hopes since fully realised of a brilliant future for the lick establishment. its advantages are shared, as professor holden desired them to be, by the whole astronomical world.[ ] a sort of appellate jurisdiction was at once accorded to the great equatoreal, and more than one disputed point has been satisfactorily settled by recourse to it. its performances, considered both as to quality and kind, are unlikely to be improved upon by merely outbidding it in size, unless the care expended upon the selection of its site be imitated. professor pickering thus showed his customary prudence in reserving his efforts to procure a great telescope until harvard college owned a dependent observatory where it could be employed to advantage. this was found by mr. w. h. pickering, after many experiments in colorado, california, and peru, at arequipa, on a slope of the andes, , feet above the sea-level. here the post provided for by the "boyden fund" was established in , under ideal meteorological conditions. temperature preserves a "golden mean"; the barometer is almost absolutely steady; the yearly rainfall amounts to no more than three or four inches. no wonder, then, that the "seeing" there is of the extraordinary excellence attested by mr. pickering's observations. in the absence of bright moonlight, he tells us,[ ] eleven pleiades can always be counted; the andromeda nebula appears to the naked eye conspicuously bright, and larger than the full moon; third magnitude stars have been followed to their disappearance at the true horizon; the zodiacal light spans the heavens as a complete arch, the "gegenschein" forming a regular part of the scenery of the heavens. corresponding telescopic facilities are enjoyed. the chief instrument at the station, a -inch equatoreal by clark, shows the fainter parts of the orion nebula, photographed at harvard college in , by which the dimensions given to it in bond's drawing are doubled; stars are at times seen encircled by half a dozen immovable diffraction rings, up to twelve of which have been counted round alpha centauri; while on many occasions no available increase of magnifying power availed to bring out any wavering in the limbs of the planets. moreover, the series of fine nights is nearly unbroken from march to november. the facilities thus offered for continuous photographic research rendered feasible professor bailey's amazing discovery of variable star-clusters. they belong exclusively to the "globular" class, and the peculiarity is most strikingly apparent in the groups known as omega centauri, and messier , , and . a large number of their minute components run through perfectly definite cycles of change in periods usually of a few hours.[ ] altogether, about "cluster-variables" have been recorded since . it should be mentioned that mr. david packer and dr. common discerned, about , some premonitory symptoms of light-fluctuation among the crowded stars of messier .[ ] with the bruce telescope, a photographic doublet inches in diameter, a store of , negatives was collected at arequipa between and . some were exposed directly, others with the intervention of a prism; and all are available for important purposes of detection or investigation. vapours and air-currents do not alone embarrass the use of giant telescopes. mechanical difficulties also oppose a formidable barrier to much further growth in size. but what seems a barrier often proves to be only a fresh starting-point; and signs are not wanting that it may be found so in this case. it is possible that the monumental domes and huge movable tubes of our present observatories will, in a few decades, be as much things of the past as huygens's "aerial" telescopes. it is certain that the thin edge of the wedge of innovation has been driven into the old plan of equatoreal mounting. m. loewy, the present director of the paris observatory, proposed to delaunay in the direction of a telescope on a novel system. the design seemed feasible, and was adopted; but the death of delaunay and the other untoward circumstances of the time interrupted its execution. its resumption, after some years, was rendered possible by m. bischoffsheim's gift of , francs for expenses, and the _coudé_ or "bent" equatoreal has been, since , one of the leading instruments at the paris establishment. its principle is briefly this: the telescope is, as it were, its own polar axis. the anterior part of the tube is supported at both ends, and is thus fixed in a direction pointing towards the pole, with only the power of twisting axially. the posterior section is joined on to it at right angles, and presents the object-glass, accordingly, to the celestial equator, in the plane of which it revolves. stars in any other part of the heavens have their beams reflected upon the object-glass by means of a plane rotating mirror placed in front of it. the observer, meanwhile, is looking steadfastly down the bent tube towards the invisible _southern_ pole. he would naturally see nothing whatever were it not that a second plane mirror is fixed at the "elbow" of the instrument, so as to send the rays which have traversed the object-glass to his eye. he never needs to move from his place. he watches the stars, seated in an arm-chair in a warm room, with as perfect convenience as if he were examining the seeds of a fungus with a microscope. nor is this a mere gain of personal ease. the abolition of hardship includes a vast accession of power.[ ] among other advantages of this method of construction are, first, that of added stability, the motion given to the ordinary equatoreal being transferred, in part, to an auxiliary mirror. next, that of increased focal length. the fixed part of the tube can be made almost indefinitely long without inconvenience, and with enormous advantage to the optical qualities of a large instrument. finally, the costly and unmanageable cupola is got rid of, a mere shed serving all purposes of protection required for the "coudé." the desirability of some such change as that which m. loewy has realised has been felt by others. professor pickering sketched, in , a plan for fixing large refractors in a permanently horizontal position, and reflecting into them, by means of a shifting mirror, the objects desired to be observed.[ ] the observations for his photometric catalogues are, in fact, made with a "broken transit," in which the line of sight remains permanently horizontal, whatever the altitude of the star examined. sir howard grubb, moreover, set up, in , a kind of siderostat at the crawford observatory, cork. in a paper read before the royal society, january , , he proposed to carry out the principle on a more extended scale;[ ] and shortly afterwards undertook its application to a telescope inches in aperture for the armagh observatory.[ ] the chief honours, however, remain to the paris inventor. none of the prognosticated causes of failure have proved effective. the loss of light from the double reflection is insignificant. the menaced deformation of images is, through the exquisite skill of the mm. henry in producing plane mirrors of all but absolute perfection, quite imperceptible. the definition was admitted to be singularly good. sir david gill stated in that he had never measured a double star so easily as he did gamma leonis by its means.[ ] sir norman lockyer pronounced it to be "one of the instruments of the future"; and the principle of its construction was immediately adopted by the directors of the besançon and algiers observatories, as well as for a -inch telescope destined for a new observatory at buenos ayres. at paris, it has since been carried out on a larger scale. a "coudé," of - / inches aperture and feet focal length was in installed at the national observatory, and has served m. loewy for his ingenious studies on refraction and aberration--above all, for taking the magnificent plates of his lunar atlas. the "bent" form is capable of being, but has not yet been, adapted to reflectors.[ ] the "coelostat," in the form given to it by professor turner, has proved an invaluable adjunct to eclipse-equipments. it consists essentially of a mirror rotating in forty-eight hours on an axis in its own plane, and parallel to the earth's axis. in the field of a telescope kept rigidly pointed towards such a mirror, stars appear immovably fixed. the employment of long-focus lenses for coronal photography is thus facilitated, and the size of the image is proportional to the length of the focus. professor barnard, accordingly, depicted the totality of with a horizontal telescope - / feet long, fed by a mirror inches across, the diameter of the moon on his plates being inches. the largest siderostat in the world is the paris -inch refractor, which formed the chief attraction of the palais d'optique at the exhibition of . it has a focal length of nearly feet, and can be used either for photographic or for visual purposes. celestial photography has not reached its grand climacteric; yet its earliest beginnings already seem centuries behind its present performances. the details of its gradual yet rapid improvement are of too technical a nature to find a place in these pages. suffice it to say that the "dry-plate" process, with which such wonderful results have been obtained, appears to have been first made available by sir william huggins in photographing the spectrum of vega in , and was then successively adopted by common, draper, and janssen. nor should captain abney's remarkable extension of the powers of the camera be left unnoticed. he began his experiments on the chemical action of red and infra-red rays in , and at length succeeded in obtaining a substance--the "blue" bromide of silver--highly sensitive to these slower vibrations of light. with its aid he explored a vast, unknown, and for ever invisible region of the solar spectrum, presenting to the royal society, december , ,[ ] a detailed map of its infra-red portion (wave-lengths , to , ), from which valuable inferences may yet be derived as to the condition of the various kinds of matter ignited in the solar atmosphere. upon plates rendered "orthochromatic" by staining with alizarine, or other dye-stuffs, the whole visible spectrum can now be photographed; but those with their maximum of sensitiveness near g are found preferable, except where the results of light-analysis are sought to be completely recorded. and since photographic refractors are corrected for the blue rays, exposures with them of orthochromatic surfaces would be entirely futile. the chemical plate has two advantages over the human retina:[ ] first, it is sensitive to rays which are utterly powerless to produce any visual effect; next, it can accumulate impression almost indefinitely, while from the retina they fade after one-tenth part of a second, leaving it a continually renewed _tabula rasa_. it is, accordingly, quite possible to photograph objects so faint as to be altogether beyond the power of any telescope to reveal--witness the chemical disclosure of the invisible nebula encircling nova persei--and we may thus eventually learn whether a blank space in the sky truly represents the end of the stellar universe in that direction, or whether farther and farther worlds roll and shine beyond, veiled in the obscurity of immeasurable distance. of many ingenious improvements in spectroscopic appliances the most fundamentally important relate to what are known as "gratings." these are very finely striated surfaces, by which light-waves are brought to interfere, and are thus sifted out, strictly according to their different lengths, into "normal" spectra. since no universally valid measures can be made in any others, their production is quite indispensable to spectroscopic science. fraunhofer, who initiated the study of the diffraction spectrum, used a real grating of very fine wires: but rulings on glass were adopted by his successors, and were by nobert executed with such consummate skill that a single square inch of surface was made to contain , hand-drawn lines. such rare and costly triumphs of art, however, found their way into very few hands, and practical availability was first given to this kind of instrument by the inventiveness and mechanical dexterity of two american investigators. both rutherfurd's and rowland's gratings are machine-ruled, and reflect instead of transmitting the rays they analyse; but rowland's present to them a very much larger diffractive surface, and consequently possess a higher resolving power. the first preliminary to his improvements was the production, in , of a faultless screw, those previously in use having been the inevitable source of periodical errors in striation, giving, in their turn, ghost-lines as subjects of spectroscopic study.[ ] their abolition was not one of rowland's least achievements. with his perfected machine a metallic area of - / by - / inches can be ruled with exquisite accuracy to almost any degree of fineness; he considered, however, , lines to the inch to be the limit of usefulness.[ ] the ruled surface is, moreover, concave, and hence brings the spectrum to a focus without a telescope. a slit and an eye-piece are alone needed to view it, and absorption of light by glass lenses is obviated--an advantage especially sensible in dealing with the ultra- or infra-visible rays. the high qualities of rowland's great photographic map of the solar spectrum were thus based upon his previous improvement of the instrumental means used in its execution. the amount of detail shown in it is illustrated by the appearance on the negatives of lines between h and k; and many lines depict themselves as double which, until examined with a concave grating, had passed for one and indivisible. a corresponding hand-drawing, for which m. thollon received in the lalande prize, exhibits, not the diffractive, but the prismatic spectrum as obtained with bisulphide of carbon prisms of large dispersive power. about one-third of the visible gamut of the solar radiations (a to _b_) is covered by it; it includes , lines, and is over ten metres long.[ ] the grating is an expensive tool in the way of light. where there is none to spare, its advantages must be foregone. they could not, accordingly, be turned to account in stellar spectroscopy until the lick telescope was at hand to supply more abundant material for research. by the use thus made possible of rowland's gratings, professor keeler was able to apply enormous dispersion to the rays of stars and nebulæ, and so to attain a previously unheard-of degree of accuracy in their measurement. his memorable detection of nebular movement in line of sight ensued as a consequence. professor campbell, his successor, has since obtained, by the same means, the first satisfactory photographs of stellar diffraction-spectra. the means at the disposal of astronomers have not multiplied faster than the tasks imposed upon them. looking back to the year , we cannot fail to be astonished at the change. the comparatively simple and serene science of the heavenly bodies known to our predecessors, almost perfect so far as it went, incurious of what lay beyond its grasp, has developed into a body of manifold powers and parts, each with its separate mode and means of growth, full of strong vitality, but animated by a restless and unsatisfied spirit, haunted by the sense of problems unsolved, and tormented by conscious impotence to sound the immensities it perpetually confronts. knowledge might be said, when the _mécanique céleste_ issued from the press, to be bounded by the solar system; but even the solar system presented itself under an aspect strangely different from what it now wears. it consisted of the sun, seven planets, and twice as many satellites, all circling harmoniously in obedience to a universal law, by the compensating action of which the indefinite stability of their mutual relations was secured. the occasional incursion of a comet, or the periodical presence of a single such wanderer chained down from escape to outer space by planetary attraction, availed nothing to impair the symmetry of the majestic spectacle. now, not alone the ascertained limits of the system have been widened by a thousand millions of miles, with the addition of one more giant planet and seven satellites to the ancient classes of its members, but a complexity has been given to its constitution baffling description or thought. five hundred circulating planetary bodies bridge the gap between jupiter and mars, the complete investigation of the movements of any one of which would overtask the energies of a lifetime. meteorites, strangers, apparently, to the fundamental ordering of the solar household, swarm, nevertheless, by millions in every cranny of its space, returning at regular intervals like the comets so singularly associated with them, or sweeping across it with hyperbolic velocities, brought, perhaps, from some distant star. and each of these cosmical grains of dust has a theory far more complex than that of jupiter; it bears within it the secret of its origin, and fulfils a function in the universe. the sun itself is no longer a semi-fabulous, fire-girt globe, but the vast scene of the play of forces as yet imperfectly known to us, offering a boundless field for the most arduous and inspiring researches. among the planets the widest variety in physical habitudes is seen to prevail, and each is recognised as a world apart, inviting inquiries which, to be effective, must necessarily be special and detailed. even our own moon threatens to break loose from the trammels of calculation, and commits "errors" which sap the very foundations of the lunar theory, and suggest the formidable necessity for its complete revision. nay, the steadfast earth has forfeited the implicit confidence placed in it as a time-keeper, and questions relating to the stability of the earth's axis and the constancy of the earth's rate of rotation are among those which it behoves the future to answer. everywhere there is multiformity and change, stimulating a curiosity which the rapid development of methods of research offers the possibility of at least partially gratifying. outside the solar system, the problems which demand a practical solution are virtually infinite in number and extent. and these have all arisen and crowded upon our thoughts within less than a hundred years. for sidereal science became a recognised branch of astronomy only through herschel's discovery of the revolutions of double stars in . yet already it may be, and has been called, "the astronomy of the future," so rapidly has the development of a keen and universal interest attended and stimulated the growth of power to investigate this sublime subject. what has been done is little--is scarcely a beginning; yet it is much in comparison with the total blank of a century past. and our knowledge will, we are easily persuaded, appear in turn the merest ignorance to those who come after us. yet it is not to be despised, since by it we reach up groping fingers to touch the hem of the garment of the most high. footnotes: [footnote : _comptes rendus_, t. xliv., p. .] [footnote : a. a. common, _memoirs r. astr. soc._, vol. i., p. .] [footnote : newcomb, _pop. astr._, p. .] [footnote : _month. not._, vol. liv., p. .] [footnote : keeler, _publ. astr. pac. soc._, vol. ii., p. .] [footnote : h. grubb, _trans. roy. dub. soc._, vol. i. (new ser.), p. .] [footnote : hale, nevertheless (_astroph. jour._, vol. v., p. ), considers that refractors preserve their superiority of visual light-grasp over newtonian reflectors up to an aperture of - / , while equalisation is reached for the photographic rays at inches.] [footnote : _astroph. jour._, vol. v., p. .] [footnote : _phil. trans._, vol. cxlviii., p. .] [footnote : _optics_, p. ( nd ed., ).] [footnote : _observatory_, vol. viii., p. .] [footnote : holden on celestial photography, _overland monthly_, nov., .] [footnote : _observatory_, vol. xv., p. .] [footnote : bailey, _astroph. jour._, vol. x., p. .] [footnote : _harvard circulars_, nos. , , , ;] [footnote : loewy, _bull. astr._, t. i., p. ; _nature_, vol. xxix., p. .] [footnote : _nature_, vol. xxiv., p. .] [footnote : _ibid._, vol. xxix., p. .] [footnote : _trans. r. dublin soc._, vol. iii., p. .] [footnote : _observatory_, vol. vii., p. .] [footnote : loewy, _bull. astr._, t. i., p. .] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : janssen, _l'astronomie_, t. ii., p. .] [footnote : rev. a. l. cortie, _astr. and astrophysics_, vol. xi., p. .] [footnote : _phil. mag._, vol. xiii., , p. .] [footnote : _bull. astr._, t. iii., p. .] appendix table i chronology, - , march herschel's first observation. subject, the orion nebula. sun-spots geometrically proved to be depressions by wilson. first experimental determination of the earth's mean density by maskelyne. , march discovery of uranus. herschel's first catalogue of double stars. herschel's first investigation of the sun's movement in space. goodricke's discovery of algol's law of variation. analogy between mars and the earth pointed out by herschel. construction of the heavens investigated by herschel's method of star-gauging. "cloven-disc" plan of the milky way. discovery of binary stars anticipated by michell. herschel's first catalogue of nebulæ. , jan. discovery by herschel of two uranian moons (oberon and titania). , nov. acceleration of the moon explained by laplace. herschel's second catalogue of nebulæ, and classification by age of these objects. completion of herschel's forty-foot reflector. , aug. his discovery with it of the two inner saturnian and sept. satellites. repeating-circle invented by borda. five-foot circle constructed by ramsden for piazzi. maskelyne's catalogue of thirty-six fundamental stars. herschel propounds the hypothesis of a fluid constitution for nebulæ. atmospheric refraction in venus announced by schröter. rotation-period of saturn fixed by herschel at h. m. herschel's theory of the solar constitution. herschel's first measures of comparative stellar brightness. laplace's nebular hypothesis published in _exposition du système du monde_. publication of olbers's method of computing cometary orbits. retrograde motions of uranian satellites detected by herschel. publication of first two volumes of _mécanique céleste_. , may transit of mercury observed by schröter. , nov. star-shower observed by humboldt at cumana. _monatliche correspondenz_ started by von zach. invisible heat-rays detected in the solar spectrum by herschel. , jan. discovery of ceres by piazzi. publication of lalande's _histoire céleste_. investigation by herschel of solar emissive variability in connection with spot-development. , march discovery of pallas by olbers. herschel's third catalogue of nebulæ. herschel's discovery of binary stars. marks of clustering in the milky way noted by herschel. wollaston records seven dark lines in the solar spectrum. , nov. transit of mercury observed by herschel. , sept. discovery of juno by harding. foundation of optical institute at munich. herschel's second determination of the solar apex. , march discovery of vesta by olbers. herschel's theory of the development of stars from nebulæ. , feb. death of maskelyne. pond appointed to succeed him as astronomer-royal. , sept. perihelion passage of great comet. theory of electrical repulsion in comets originated by olbers. , sept. perihelion passage of pons's comet. herschel demonstrates the irregular distribution of stars in space. fraunhofer maps dark lines in the solar spectrum. publication of bessel's _fundamenta astronomiæ_. recognition by encke of the first short-period comet. , june passage of the earth through the tail of a comet. foundation of the royal astronomical society. foundation of paramatta observatory. , september first number of _astronomische nachrichten_. , may first calculated return of encke's comet. , august death of herschel. bessel introduces the correction of observations for personal equation. fraunhofer examines the spectra of fixed stars. distance of the sun concluded by encke to be - / million miles. publication of lohrmann's lunar chart. dorpat refractor mounted equatoreally. commencement of schwabe's observations of sun-spots. , feb. biela's discovery of a comet. orbit of a binary star calculated by savary. completion of the royal observatory at the cape of good hope. the königsberg heliometer mounted. publication of bessel's _tabulæ regiomontanæ_. discovery by brewster of "atmospheric lines" in the solar spectrum. magnetic observatory established at göttingen. , nov. , star-shower visible in north america. completion of sir j. herschel's survey of the northern heavens. , jan. sir j. herschel's landing at the cape. , september airy appointed astronomer-royal in succession to pond. , nov. perihelion passage of halley's comet. solar movement determined by argelander. bessel's application of the heliometer to measurements of stellar parallax. publication of beer and mädler's _der mond_. publication of struve's _mensuræ micrometricæ_. , dec. outburst of eta carinæ observed by sir j. herschel. thermal power of the sun measured by herschel and pouillet. parallax of cygni determined by bessel. , jan. parallax of alpha centauri announced by henderson. completion of pulkowa observatory. solidity of the earth concluded by hopkins. , march death of olbers. first attempt to photograph the moon by j. w. draper. doppler enounces principle of colour-change by motion. conclusion of baily's experiments in weighing the earth. , july total solar eclipse. corona and prominences observed by airy, baily, arago, and struve. , feb. perihelion-passage of great comet. , february completion of parsonstown reflector. , april discovery with it of spiral nebulæ. , april daguerreotype of the sun taken by foucault and fizeau. , oct. place of neptune assigned by adams. , dec. discovery of astræa by hencke. , dec. duplication of biela's comet observed at yale college. melloni's detection of heating effects from moonlight. , march death of bessel. , sept. discovery of neptune by galle. , oct. neptune's satellite discovered by lassell. publication of sir j. herschel's _results of observations at the cape of good hope_. cyclonic theory of sun-spots stated by him. j. r. mayer's meteoric hypothesis of solar conservation. motion-displacements of fraunhofer lines adverted to by fizeau. , april new star in ophiuchus observed by hind. , sept. simultaneous discovery of hyperion by bond and lassell. first experimental determination of the velocity of light (fizeau). , july vega photographed at harvard college. , nov. discovery by bond of saturn's dusky ring. o. struve's first measurements of saturn's ring-system , july total solar eclipse observed in sweden. , oct. discovery by lassell of two inner uranian satellites. schwabe's discovery of sun-spot periodicity published by humboldt. , may coincidence of magnetic and sun-spot periods announced by sabine. , oct. variable nebula in taurus discovered by hind. lassell's two-foot reflector transported to malta. adams shows laplace's explanation of the moon's acceleration to be incomplete. hansen infers from lunar theory a reduced value for the distance of the sun. helmholtz's "gravitation theory" of solar energy. piazzi smyth's observations on the peak of teneriffe. saturn's rings shown by clerk maxwell to be of meteoric formation. , april double-star photography initiated at harvard college. solar photography begun at kew. , sept. perihelion of donati's comet. spectrum analysis established by kirchhoff and bunsen. carrington's discovery of the compound nature of the sun's rotation. , sept. luminous solar outburst and magnetic storm. , oct. merope nebula discovered by tempel. , dec. chemical constitution of the sun described by kirchhoff. , feb. discovery by liais of a "double comet." , may new star in scorpio detected by auwers. , july total solar eclipse observed in spain. prominences shown by photography to be solar appendages. , june the earth involved in the tail of a great comet. - kirchhoff's map of the solar spectrum. solar hydrogen-absorption recognised by Ångström. , jan. discovery by alvan g. clark of the companion of sirius. foucault determines the sun's distance by the velocity of light. opposition of mars. determination of solar parallax. completion of _bonner durchmusterung_. secchi's classification of stellar spectra. foundation of the german astronomical society. , march rotation period of mars determined by kaiser. huggins's first results in stellar spectrum analysis. , aug. spectroscopic examination of tempel's comet by donati shows it to be composed of glowing gas. , aug. discovery by huggins of gaseous nebulæ. value of , , miles adopted for the sun's distance. croll's explanation of glacial epochs. , nov. death of struve. , jan. spectroscopic observation by huggins of the occultation of eta piscium. , jan. faye's theory of the solar constitution. kew results published. zöllner argues for a high temperature in the great planets. identity of the orbits of the august meteors and of comet iii. demonstrated by schiaparelli. delaunay explains lunar acceleration by a lengthening of the day through tidal friction. , march spectroscopic study of the sun's surface by lockyer. , march new star in corona borealis detected by birmingham. , october schmidt announces the disappearance of the lunar crater linné. , nov. meteoric shower visible in europe. period of november meteors determined by adams. , aug. total solar eclipse. minimum sun-spot type of corona observed by grosch at santiago. discovery of gaseous stars in cygnus by wolf and rayet. , february principle of daylight spectroscopic visibility of prominences started by huggins. , aug. great indian eclipse. spectrum of prominences observed. , aug. janssen's first daylight view of a prominence. , oct. lockyer and janssen independently announce their discovery of the spectroscopic method. doppler's principle applied by huggins to measure stellar radial movements. publication of Ångström's map of the normal solar spectrum. spectrum of winnecke's comet found by huggins to agree with that of olefiant gas. , feb. tenuity of chromospheric gases inferred by lockyer and frankland. , feb. huggins observes a prominence with an "open slit." , aug. american eclipse. detection of bright-line coronal spectrum. mounting of newall's -inch achromatic at gateshead. proctor indicates the prevalence of drifting movements among the stars. a solar prominence photographed by young. , dec. sicilian eclipse. young discovers reversing layer. , may death of sir j. herschel. , june line-displacements due to solar rotation detected by vogel. , dec. total eclipse visible in india. janssen observes reflected fraunhofer lines in spectrum of corona. conclusion of a three years' series of observations on lunar heat by lord rosse. spectrum of vega photographed by h. draper. faye's cyclonic hypothesis of sun-spots. young's solar-spectroscopic observations at mount sherman. cornu's experiments on the velocity of light. , nov. meteoric shower connected with biela's comet. determination of mean density of the earth by cornu and baille. solar photographic work begun at greenwich. erection of -inch washington refractor. light-equation redetermined by glasenapp. vogel's classification of stellar spectra. , dec. transit of venus. publication of neison's _the moon_. , nov. new star in cygnus discovered by schmidt. spectrum of vega photographed by huggins. first use of dry gelatine plates in celestial photography. , may klein observes a supposed new lunar crater (hyginus n.). measurement by vogel of selective absorption in solar atmosphere. , aug. - discovery of two satellites of mars by hall at washington. , sept. death of leverrier. canals of mars discovered by schiaparelli. opposition of mars observed by gill at ascension. solar parallax deduced = . ". , january stationary meteor-radiants described by denning. publication of schmidt's _charte der gebirge des mondes_. first observations of great red spot on jupiter. conclusion of newcomb's researches on the lunar theory. , may transit of mercury. foundation of selenographical society. , july total eclipse visible in america. vast equatoreal extension of the corona. , october completion of potsdam astrophysical observatory. , dec. lockyer's theory of celestial dissociation communicated to the royal society. michelson's experiments on the velocity of light. publication of gould's _uranometria argentina_. , november observations of the spectra of sun-spots begun at south kensington. , dec. abney's map of the infra-red solar spectrum presented to the royal society. , dec. ultra-violet spectra of white stars described by huggins. , dec. communication of g. h. darwin's researches into the early history of the moon. , jan. discovery at cordoba of a great southern comet. conditions of algol's eclipses determined by pickering. pickering computes mass-brightness of binary stars. , sept. draper's photograph of the orion nebula. the bolometer invented by langley. , jan. communication of g. h. darwin's researches into the effects of tidal friction on the evolution of the solar system. langley's observations of atmospheric absorption on mount whitney. , june perihelion of tebbutt's comet. , june its spectrum photographed by huggins. , june photographs of tebbutt's comet by janssen and draper. , aug. retirement of sir george airy. succeeded by christie. , aug. perihelion of schaeberle's comet. publication of stone's cape catalogue for . struve's second measures of saturn's ring-system. newcomb's determination of the velocity of light. resulting solar parallax = · ". correction by nyrén of struve's constant of aberration. , march spectrum of orion nebula photographed by huggins. , may total solar eclipse observed at sohag in egypt. , may sodium-rays observed at dunecht in spectrum of comet wells. , june perihelion of comet wells. , sept. perihelion of great comet. daylight detection by common. transit observed at the cape. , sept. iron lines identified in spectrum by copeland and j. g. lohse. , september photographs of comet taken at the cape observatory, showing a background crowded with stars. , dec. transit of venus. duplication of martian canals observed by schiaparelli. completion by loewy at paris of first equatoreal coudé. rigidity of the earth concluded from tidal observations by g. h. darwin. experiments by huggins on photographing the corona without an eclipse. publication of holden's _monograph of the orion nebula_. , jan. orion nebula photographed by common. , may caroline island eclipse. , june great comet of observed from cordoba at a distance from the earth of million miles. parallaxes of nine southern stars measured by gill and elkin. catalogue of the spectra of , stars by vogel. , jan. return to perihelion of pons's comet. photometric catalogue by pickering of , stars. publication of gore's catalogue of variable stars. publication of faye's _origine du monde_. , oct. eclipse of the moon. heat-phases measured by boeddicker at parsonstown. dunér's catalogue of stars with banded spectra. backlund's researches into the movements of encke's comet. , february langley measures the lunar heat-spectrum. publication of _uranometria nova oxoniensis_. , aug. new star in andromeda nebula discerned by gully. , sept. thollon's drawing of the solar spectrum presented to the paris academy. , sept. solar eclipse visible in new zealand. , nov. photographic discovery by paul and prosper henry of a nebula in the pleiades. , nov. shower of biela meteors. thirty-inch achromatic mounted at pulkowa. publication of rowland's photographic map of the normal solar spectrum. bakhuyzen's determination of the rotation period of mars. stellar photographs by paul and prosper henry. , jan. spectra of forty pleiades simultaneously photographed at harvard college. , feb. first visual observation of the maia nebula with the pulkowa -inch refractor. , march photographs by the henrys of the pleiades, showing , stars with nebulæ intermixed. , may photographic investigations of stellar parallax undertaken by pritchard. , may periodical changes in spectra of sun-spots announced by lockyer. , june an international photographic congress proposed by gill. , aug. total eclipse of the sun observed at grenada. , oct. roberts's photograph showing annular structure of the andromeda nebula. , dec. roberts's photograph of the pleiades nebulosities. solar heat-spectrum extended by langley to below five microns. , dec. detection by copeland of helium-ray in spectrum of the orion nebula. thirty-inch refractor mounted at nice. publication of argentine general catalogue. completion of auwers's reduction of bradley's observations. draper memorial photographic work begun at harvard college. photographic detection at harvard college of bright hydrogen lines in spectra of variables (mira ceti and u orionis). , jan. discovery by thome at cordoba of a great comet belonging to the same group as the comet of . publication of lockyer's _chemistry of the sun_. , april meeting at paris of the international astrophotographic congress. heliometric triangulation of the pleiades by elkin. l. struve's investigation of the sun's motion, and redetermination of the constant of precession. von konkoly's extension to ° s. dec. of vogel's spectroscopic catalogue. auwers's investigation of the solar diameter. publication of schiaparelli's measures of double stars ( - ). , april death of thollon at nice. , aug. total eclipse of the sun. shadow-path crossed russia. observations marred by bad weather. , november langley's researches on the temperature of the moon. , nov. lockyer's _researches on meteorites_ communicated to the royal society. completion of -inch lick refractor. küstner's detection of variations in the latitude of berlin brought before the international geodetic association. chandler's first catalogue of variable stars. mean parallax of northern first magnitude stars determined by elkin. publication of dreyer's _new general catalogue_ of , nebulæ. vogel's first spectrographic determinations of stellar radial motion. carbon absorption recognised in solar spectrum by trowbridge and hutchins. , jan. total eclipse of the moon. heat-phases measured at parsonstown. , feb. remarkable photograph of the orion nebula spectrum taken at tulse hill. , june activity of the lick observatory begun. completion of dr. common's -foot reflector. heliometric measures of iris for solar parallax at the cape, newhaven (u.s.a.), and leipsic. loewy describes a comparative method of determining constant of aberration. presentation of the dunecht instrumental outfit to the nation by lord crawford. copeland succeeds piazzi smyth as astronomer-royal for scotland. , sept. death of r. a. proctor. photograph of the orion nebula taken by w. h. pickering, showing it to be the nucleus of a vast spiral. discovery at a harvard college of the first-known spectroscopic doubles, zeta ursæ majoris and beta aurigæ. eclipses of algol demonstrated spectrographically by vogel. completion of photographic work for the southern durchmusterung. boeddicker's drawing of the milky way. draper memorial photographs of southern star-spectra taken in peru. pernter's experiments on scintillation from the sonnblick. h. struve's researches on saturn's satellites. harkness's investigation of the masses of mercury, venus, and the earth. heliometric measures of victoria and sappho at the cape. , jan. total solar eclipse visible in california. , feb. foundation of the astronomical society of the pacific. , march investigation by sir william and lady huggins of the spectrum of the orion nebula. , july-aug. first photographs of the milky way taken by barnard. , august observation by barnard of four companions to brooks's comet. , nov. passage of japetus behind saturn's dusky ring observed by barnard. , december schiaparelli announces synchronous rotation and revolution of mercury. , dec. total eclipse of the sun visible in guiana. death of father perry, december . spectrum of uranus investigated visually by keeler, photographically by huggins. long-exposure photographs of ring-nebula in lyra. determinations of the solar translation by l. boss and o. stumpe. schiaparelli finds for venus an identical period of rotation and revolution. publication of thollon's map of the solar spectrum. bigelow's mathematical theory of coronal structures. foundation of the british astronomical association. measurements by keeler at lick of nebular radial movements. janssen's ascent of mont blanc, by which he ascertained the purely terrestrial origin of the oxygen-absorption in the solar spectrum. newcomb's discussion of the transits of venus of and . spiral structure of magellanic clouds displayed in photographs taken by h. c. russell of sydney. publication of the draper catalogue of stellar spectra. , april spica announced by vogel to be a spectroscopic binary. , june gore's catalogue of computed binaries. , november study by sir william and lady huggins of the spectra of wolf and rayet's stars in cygnus. , november discovery by barnard of a close nebulous companion to merope in the pleiades. , november mcclean spectrographs of the high and low sun. capture-theory of comets developed by callandreau, tisserand, and newton. dunér's spectroscopic researches on the sun's rotation. preponderance of sirian stars in the milky way concluded by pickering, gill, and kapteyn. detection by mrs. fleming of spectral variations corresponding to light-changes in beta lyræ. establishment of the harvard college station at arequipa in peru (height , feet). variations of latitude investigated by chandler. prominence-photography set on foot by hale at chicago and deslandres at paris. schmidt's theory of refraction in the sun. , april meeting at paris of the permanent committee for the photographic charting of the heavens. , may transit of mercury. , aug. presidential address by huggins at the cardiff meeting of the british association. , dec. nova aurigæ photographed at harvard college. , dec. photographic maximum of nova aurigæ. , dec. first photographic discovery of a minor planet by max wolf at heidelberg. commencement of international photographic charting work. photographic determination by scheiner of stars in the hercules cluster (m ). publication of vogel's spectrographic determinations for fifty-one stars. publication of pritchard's photographic parallaxes. , jan. death of sir george airy. , jan. death of professor adams. , feb. announcement by anderson of the outburst of a new star in auriga. , feb. appearance of the largest sun-spot ever photographed at greenwich. , march photograph of argo nebula taken by gill in twelve hours. , march discovery of a bright comet by swift. , june death of admiral mouchez. succeeded by tisserand as director of the national observatory, paris. , aug. favourable opposition of mars. , aug. rediscovery at lick of nova aurigæ. , sept. discovery by barnard of jupiter's inner satellite. , oct. first photographic discovery of a comet by barnard. , nov. discovery of holmes's comet. , nov. shower of andromede meteors visible in america. poynting's determination of the earth's mean density. dunér's investigation of the system of upsilon cygni. photographic investigation by deslandres of the spectra of prominences. photographs of the sun with faculæ and chromospheric surroundings taken by hale with a single exposure. investigation by t. j. j. see of the ancient colour of sirius. publication of t. j. j. see's thesis on the evolution of binary systems. chandler's theory of algol's inequalities. nebula in cygnus photographically discovered by max wolf. , jan. kapteyn's investigation of the structure of the stellar universe. , march gill announces his results from the opposition of victoria, among them a solar parallax = . ". , april total solar eclipse observed in south america and west africa. publication of kruger's _catalog der farbigen sterne_. conclusion of boys's series of experiments on the density of the earth. publication of _cordoba durchmusterung_, vol. i. fabry shows comets to be dependents of the solar system. publication of easton's _voie lactée_. campbell detects bright h alpha in gamma argûs and alcyone. nova normæ photographed july ; discovered on plates, october . , may death of professor pritchard. , july installation of -inch refractor at the royal observatory, greenwich. , december exterior nebulosities of pleiades photographed by barnard. , dec. death of rudolf wolf. , january sun-spot maximum. publication of potsdam _photometric durchmusterung_, part i. publication of roberts's _celestial photographs_, vol. i. wilson and gray's determination of the sun's temperature. barnard's micrometric measures of asteroids. mcclean's gift of an astrophysical outfit to the cape observatory. establishment of the lowell observatory at flagstaff, arizona. taylor's triple achromatic objective described. , april discovery of gale's comet. sampson's investigation of the sun's rotation. , oct. favourable opposition of mars. , nov. transit of mercury. , december howlett impugns the wilsonian theory of sun-spots. , dec. death of a. cowper ranyard. publication of newcomb's _astronomical constants_. bailey's photometric catalogue of , southern stars. bailey's photographic discovery of variable star clusters. publication of e. w. brown's _lunar theory_. tisserand's theory of the inequalities of algol. stratonoff's determination of the sun's rotation from photographs of faculæ. binary character of eta aquilæ spectroscopically recognised by bélopolsky. presentation of the crossley reflector to the lick observatory. , march great nebula in ophiuchus discovered photographically by barnard. , march ramsay's capture of helium. , april constitution of saturn's rings spectrographically demonstrated by keeler. binary character of delta cephei spectroscopically detected by bélopolsky. , june death of daniel kirkwood. , july death of f. w. g. spörer. , october nova carinæ spectrographically discovered by mrs. fleming. , dec. nova centauri spectrographically discovered by mrs. fleming. , dec. death of john russell hind. gill's report on the geodetic survey of south africa. appearance of loewy's photographic atlas of the moon, part i. , january fessenden's electrostatic theory of comets. chandler's third catalogue of variable stars. publication of lick observatory photographic atlas of the moon, part i. , february effects of pressure on wave-length described by humphreys and mohler. , april opening of new scottish royal observatory on blackford hill, edinburgh. , april pickering's photometric determinations of light curves of variable stars. one of the stars of castor spectroscopically resolved into two by bélopolsky. , may third astrographic chart conference at paris. , aug. total eclipse of the sun visible in novaya zemlya. reversing layer photographed by shackleton. , aug. death of hubert a. newton. , sept. death of hippolyte fizeau. , oct. death of f. tisserand. succeeded by maurice loewy. , nov. detection by schaeberle of procyon's missing satellite. , nov. death of benjamin apthorp gould. , november second series of hydrogen-lines discovered by pickering in stellar spectra. , december zeeman's discovery of spectral modifications through magnetic influence. , december oxygen-absorption identified in the sun by runge and paschen. study of lunar formations by loewy and puiseux. mounting of the mills spectrograph at the lick observatory. installation at greenwich of the thompson -inch photographic refractor. publication of miss maury's discussion of the photographed spectra of stars. callandreau's researches on cometary disaggregation. braun's determination of the earth's mean density. tenuity of calcium vapour in chromosphere demonstrated spectroscopically by sir william and lady huggins. completion at the cape observatory of mcclean's spectrographic survey of the heavens. twenty-one wolf-rayet stars found by mrs. fleming in magellanic cloud. percival lowell's _new observations on the planet mercury_ presented to the american academy. , april mcclean recognises oxygen-absorption in helium stars. , may death of e. j. stone, radcliffe observer. , june death of alvan g. clark. , june spectrum of a meteor photographed at arequipa. , oct. inauguration of the yerkes observatory. rabourdin's photographs of nebulæ with the meudon reflector. dr. see's discoveries of southern double stars with the lowell -inch refractor. , jan. total eclipse of the sun visible in india. , february binary character of zeta geminorum ascertained spectroscopically by bélopolsky. star with proper motion of nearly " discovered by innes and kapteyn from the cape durchmusterung plates. , march nova sagittarii photographed on draper memorial plates. , june opening of grand-ducal observatory at königsstuhl, heidelberg. keeler succeeds holden as director of the lick observatory. bruno peter's results in stellar parallax. lewis swift's discoveries of nebulæ at echo mountain, california. hale's photographic investigation of carbon stars. , aug. discovery of eros by witt. flint's investigations of stellar parallax by meridian differences. easton's spiral theory of the milky way. seeliger's research on star distribution. , october multiple hydrogen-bands observed by campbell in mira ceti. , november orbit of a leonid meteor photographically determined by elkin. publication of potsdam _photometric durchmusterung_, part ii. innes's _reference catalogue of southern double stars_. keeler's photographs of nebulæ with the crossley reflector and generalization of their spiral character. , january spectrum of andromeda nebula photographed by scheiner. , april photographic discovery of nova aquilæ by mrs. fleming. , aug. installation of -inch photographic refractor at potsdam. campbell's detection of polaris as spectroscopically triple. , october duplicate discovery by campbell and newall of capella as a spectroscopic binary. , nov. failure of the leonids. deflection of the stream predicted by johnstone stoney and downing. , december publication of sir william and lady huggins's _atlas of representative stellar spectra_. thirty-two-inch photographic refractor mounted at meudon. issue of first volume of potsdam measures of international catalogue plates. , jan. kapteyn's determination of the apex of solar motion. chase's measures for parallax of swiftly-moving stars. publication of gill's _researches on stellar parallax_. kapteyn proposes a method for a stellar parallax durchmusterung, and gives specimen results for stars. burnham's general catalogue of , double stars. publication of the concluding volume of the _cape photographic durchmusterung_. , may spanish-american total eclipse of the sun. , july international conference at paris. co-operation arranged of fifty-eight observatories in measures of eros for solar parallax. horizontal refractor, of inches aperture, feet focus, installed in paris exhibition. , aug. death of professor keeler. succeeded by campbell in direction of lick observatory. , november opposition of eros. publication of roberts's _celestial photographs_, vol. ii. complete publication of langley's researches on the infra-red spectrum. printing begun of paris section of international photographic catalogue. , feb. nova persei discovered by anderson. , february variability of eros announced by oppolzer. , april apparition of a great comet at the cape. publication of pickering's _photometric durchmusterung_. miss cannon's discussion of the spectra of , southern stars. kapteyn's investigation of mean stellar parallax. campbell's determination of the sun's velocity. porter's research on the solar motion in space. bigelow's magnetic theory of the solar corona. hussey's measurements of the pulkowa double stars. radial velocities of the components of delta equulei measured at lick. , april death of henry a. rowland. , june nebular spectrum derived from nova persei. , aug. nebula near nova persei photographed by max wolf. , sept. the same exhibited in spiral form on a plate taken by ritchey at the yerkes observatory. , nov. photograph taken by perrine with the crossley reflector showed nebula in course of rapid change. , sept. unveiling of the mcclean "victoria" telescope at the royal observatory, cape of good hope. sun-spot minimum. table ii. chemical elements in the sun (rowland, ). arranged according to the number of their representative lines in the solar spectrum. iron ( +). neodymium. cadmium. nickel. lanthanum. rhodium. titanium. yttrium. erbium. manganese. niobium. zinc. chromium. molybdenum. copper ( ). cobalt. palladium. silver ( ). carbon ( +). magnesium ( +). glucinum ( ). vanadium. sodium ( ). germanium. zirconium. silicon. tin. cerium. strontium. lead ( ). calcium ( +). barium. potassium ( ). scandium. aluminium ( ). _doubtful elements._--iridium, osmium, platinum, ruthenium, tantalum, thorium, tungsten, uranium. _not in solar spectrum._--antimony, arsenic, bismuth, boron, nitrogen (vacuum tube), cæsium, gold, iridium, mercury, phosphorus, rubidium, selenium, sulphur, thallium, praseodymium. oxygen was added to the solar ingredients by runge and paschen in , gallium by hartley and ramage in . lithium may be admitted provisionally, and the chromospheric constituent helium takes rank, since , as a chemical element. table iii. epochs of sun-spot maximum and minimum from to . +----------+----------++----------+----------+----------+----------+ | minima. | maxima. || minima. | maxima. | minima. | maxima. | +----------+----------++----------+----------+----------+----------+ | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | | +----------+----------++----------+----------+----------+----------+ table iv. movements of sun and stars. . translation of solar system. +--------------------+-------------------------+-------+ | apex of movement. | authority. | date. | +--------------------+-------------------------+-------+ | r. a. dec. | | | | | | | | ° ' + ° | newcomb | | | ° ' + ° ' | kapteyn | | | ° + ° | porter | | | ° + ° | boss | | | ° ' + ° | campbell (from stellar | | | | spectroscopic measures) | | +--------------------+-------------------------+-------+ | velocity = · miles per second (campbell). | +------------------------------------------------------+ . stellar velocities. +---------------------+------------+-------------+---------------------+ | name of star. | rate. | direction. | remarks. | | | miles per | | | | | sec. | | | +---------------------+------------+-------------+---------------------+ | delta leporis | | receding | campbell, | | eta cephei | | approaching | " | | theta canis majoris | | receding | " | | iota pegasi | | approaching | " " | | mu sagittarii | | approaching | " " | | eta andromedæ | | approaching | " " | | zeta herculis | | approaching | bélopolsky, | | cygni | | approaching | " " | | mu cassiopeiæ | | approaching | campbell, | | groombridge | | approaching | " " | | arcturus | . | approaching | keeler, | | arcturus | | tangential | accepting elkin's | | | | | parallax of · " | | groombridge | | tangential | parallax = · " | | mu cassiopeiæ | | tangential | parallax = · " | | | | | (peter) | | z. c. ^h | | tangential | parallax = · " | | | | | (gill) | | lacaille, , | | tangential | parallax = · " | | | | | (gill) | | lacaille, , | | tangential | parallax = · " | | | | | (gill) | | o_ , eridani | | tangential | parallax = · " | | | | | (gill) | | eta eridani | | tangential | parallax = · " | | | | | (gill) | +---------------------+------------+-------------+---------------------+ table v. list of great telescopes. . reflectors--a. metallic specula. +-------------+-----------+------------+-------------+------------------+ | locality. |aperture in|focal length| constructor.| remarks. | | | inches. | in feet. | | | +-------------+-----------+------------+-------------+------------------+ |birr castle, | | | third earl | | |parsonstown, | | | of rosse, |newtonian. | |ireland | | | | | +-------------+-----------+------------+-------------+------------------+ |melbourne | | | t. grubb, |cassegrain. | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | | | | third earl |newtonian. | |birr castle | | -- | of rosse, |remounted | | | | | |equatoreally .| +-------------+-----------+------------+-------------+------------------+ | | | | william |newtonian. | |royal | | | lassell, |presented | |observatory | | | |by the missess | |greenwich | | | |lassell to the | | | | | |royal observatory | +-------------+-----------+------------+-------------+------------------+ | b. silvered glass mirrors. | +-------------+-----------+------------+-------------+------------------+ |ealing, near | | |a. a. common,|newtonian. | |london | | | | | +-------------+-----------+------------+-------------+------------------+ | | | |g. w. richey,|can be employed | |yerkes | | | |at choice as a | |observatory | | | |coudé or a | | | | | |cassegrain. | +-------------+-----------+------------+-------------+------------------+ |national | | -- |martin, |newtonian. | |observatory, | | | |remodelled for | |paris | | | |spectrographic | | | | | |work by | | | | | |deslandres in | | | | | | . | +-------------+-----------+------------+-------------+------------------+ |meudon | | · | | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |lick | | · |calver, |mounted by | |observatory | | | |common at | | | | | |ealing in . | | | | | |sold by him to | | | | | |crossley, . | | | | | |presented by | | | | | |crossley to the | | | | | |lick | | | | | |observatory, .| +-------------+-----------+------------+-------------+------------------+ |toulouse | · | · | brothers | | |observatory | | | henry | | +-------------+-----------+------------+-------------+------------------+ |marseilles | · | -- |foucault | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |royal | | -- |common, |cassegrain. | |observatory, | | | |mounted as a | |greenwich | | | |counterpoise | | | | | |to the | | | | | |thompson | | | | | |equatoreal. | +-------------+-----------+------------+-------------+------------------+ |westgate- | | | common, |the property | |on-sea | | -- | |of sir norman | | | | | |lockyer. | +-------------+-----------+------------+-------------+------------------+ |harvard | | | h. draper, |mounted for | |college | | -- | |spectrographic | |observatory | | | |work, . | +-------------+-----------+------------+-------------+------------------+ |royal | | | t. grubb, | | |observatory, | | -- | | | |edinburgh | | | | | +-------------+-----------+------------+-------------+------------------+ |daramona, | | | sir h. |remounted . | |ireland | | · | grubb, |owned by mr. w. e.| | | | | |wilson. | +-------------+-----------+------------+-------------+------------------+ | | | | |can be used as a | |yerkes | · | · | ritchey, |cassegrain, with | |observatory | | | |an equivalent | | | | | |focal length of | | | | | | feet. | +-------------+-----------+------------+-------------+------------------+ |harvard | | | | | |college | | -- | common, | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |crowborough, | | · | sir h. |mounted with a | |sussex | | | grubb, | -inch | | | | | |refractor. | +-------------+-----------+------------+-------------+------------------+ | . refractors. | +-------------+-----------+------------+-------------+------------------+ |palais de | | | gautier, |mounted as a | |l'optique, | · | | |siderostat in | |paris | | | |connection with | | | | | |a plane mirror | | | | | |inches across. | +-------------+-----------+------------+-------------+------------------+ |yerkes | | | alvan g. | | |observatory | | | clark, | | +-------------+-----------+------------+-------------+------------------+ | | | | |for photographic | |lick | | | a. clark and|purposes a | |observatory | | · | sons, |correcting lens is| | | | | |available, of | | | | | |inches aperture, | | | | | | · feet focus. | +-------------+-----------+------------+-------------+------------------+ | | | | |mounted with a | |meudon | · | · | henrys and |photographic | |observatory | | |gautier, |refractor of · | | | | | |inches aperture. | +-------------+-----------+------------+-------------+------------------+ | | | | |photographic. | |astrophysical| | |steinheil and|mounted with a | |observatory, | · | · |repsold, |visual refractor | |potsdam | | | | inches in | | | | | |aperture. | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | |visual. mounted | |observatory, | · | · | henrys and |on mont gros, | |nice | | |gautier, | , feet above | | | | | |sea level. | +-------------+-----------+------------+-------------+------------------+ |imperial | | | a. clark and|visual. mounted | |observatory, | | | sons, |by repshold. | |pulkowa | | | | | +-------------+-----------+------------+-------------+------------------+ |national | | | | | |observatory, | · | -- | martin | | |paris | | | | | +-------------+-----------+------------+-------------+------------------+ |royal | | | sir h. |visual and | |observatory, | | | grubb, |photographic. | |greenwich | | | |mounted by | | | | | |ransome and simms.| +-------------+-----------+------------+-------------+------------------+ | university | | |sir h. grubb,| visual. | | observatory,| | | | | | vienna | | | | | +-------------+-----------+------------+-------------+------------------+ | royal | | |sir h. grubb,| the thompson | | observatory,| | | | photographic | | greenwich | | | | equatoreal. | +-------------+-----------+------------+-------------+------------------+ | naval | | | a. clark and| | | observatory,| | | sons, | | | washington | | | | | +-------------+-----------+------------+-------------+------------------+ | leander | | | a. clark and| | | mccormick | | · | sons, | | | observatory,| | | | | | virginia | | | | | +-------------+-----------+------------+-------------+------------------+ | cambridge | | | t. cooke and| presented to the | | university | | -- | sons, | university in | | observatory | | | | by | | | | | | mr. r. s. newall.| +-------------+-----------+------------+-------------+------------------+ | meudon | | | henrys and | photographic. | | observatory | · | · | gautier, | mounted with a | | | | | | visual . | | | | | | refractor. | +-------------+-----------+------------+-------------+------------------+ | harvard | | | a. clark and| photographic | | college | | · | sons, | doublet. the gift| | observatory | | | | of miss bruce. | | | | | | transfered in | | | | | | to arequipa,| | | | | | peru. | +-------------+-----------+------------+-------------+------------------+ | royal | | |sir h. grubb,| photographic. | | observatory,| | | | the gift or mr. | | cape of | | · | | mcclean. mounted | | good hope | | | | with an -inch | | | | | | visual refractor.| +-------------+-----------+------------+-------------+------------------+ | lowell | | | alvan g. | visual. first | | observatory,| | | clark, | mounted near the | | flagstaff, | | | | city of mexico. | | arizona | | | | installed at | | | | | | flagstaff, . | +-------------+-----------+------------+-------------+------------------+ | national | | | henrys and | visual and | | observatory,| · | | gautier, | photographic. | | paris | | | | mounted as an | | | | | | equatoreal coudé.| +-------------+-----------+------------+-------------+------------------+ | halsted | | | a. clark and| | | observatory,| | | sons, | | | princeton, | | | | | | n.j. | | | | | +-------------+-----------+------------+-------------+------------------+ | city | | | | mounted as a | | observatory,| | | | visual | | edinburgh | | | | equatoreal on | | | | | | the calton hill, | | | | | | . | +-------------+-----------+------------+-------------+------------------+ | etna | | | merz, | | | observatory | · | -- | | | +-------------+-----------+------------+-------------+------------------+ | buckingham | | | buckingham | | | observatory | · | -- | and wragge | | +-------------+-----------+------------+-------------+------------------+ | porro | | | porro | | | observatory,| · | -- | | | | turin | | | | | +-------------+-----------+------------+-------------+------------------+ | chamberlin | | | alvan g. | visual. | | observatory,| | | clark and | with a reversible| | colorado | | | saegmüller, | crown lens for | | | | | | photography. | +-------------+-----------+------------+-------------+------------------+ | manila | | | merz and | visual. | | observatory | | -- | saegmüller, | provided with a | | | | | | photographic | | | | | | correcting lens. | +-------------+-----------+------------+-------------+------------------+ | strasburg | · | |merz and | | | observatory | | |repsold, | | +-------------+-----------+------------+-------------+------------------+ | brera | | | merz and | | | observatory,| · | | repsold | | | milan | | | | | +-------------+-----------+------------+-------------+------------------+ | dearborn | | | a. clark and| mounted . | | observatory,| · | | sons, | | | illinois | | | | | +-------------+-----------+------------+-------------+------------------+ | national | | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | la plata | | | | | +-------------+-----------+------------+-------------+------------------+ | lowell | | | brashear, | mounted with a | | observatory,| | · | | -inch clark | | flagstaff, | | | | refractor as | | arizona | | | | counterpoise. | +-------------+-----------+------------+-------------+------------------+ | van der zee | | | fitz | dismounted. | | observatory,| | | | | | buffalo, | | -- | | | | n.y. | | | | | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | nice | | | | | +-------------+-----------+------------+-------------+------------------+ | university | | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | vienna | | | | | +-------------+-----------+------------+-------------+------------------+ | jesuit | | | henrys and | photographic. | | observatory,| · | · | gautier, | mounted with a | | zi-ka-wei | | | | visual refractor | | | | | |of equal aperture.| +-------------+-----------+------------+-------------+------------------+ | goodsell | | | brashear, | | | observatory,| · | -- | | | | northfield, | | | | | | minnesota. | | | | | +-------------+-----------+------------+-------------+------------------+ | warner | | | a. clark and| | | observatory,| | | sons, | | | rochester, | | | | | | n.y. | | | | | +-------------+-----------+------------+-------------+------------------+ | grand-ducal | | | brashear and| a twin | observatory,| | · | grubb, | photographic | | königsstuhl,| | | |doublet. the gift | | heidelberg | | | | of miss bruce. | | | | | | mounted with a | | | | | | visual -inch | | | | | | refractor by | | | | | | pauly. | +-------------+-----------+------------+-------------+------------------+ | meudon | | | | | | observatory | · | · | | | +-------------+-----------+------------+-------------+------------------+ | washburn | | | a. clark and| | | observatory,| · | · | sons, | | | wisconsin | | | | | +-------------+-----------+------------+-------------+------------------+ | teramo | | | t. cooke and| formerly the | | observatory,| · | -- | sons, | property of | | italy | | | | mr. wigglesworth.| +-------------+-----------+------------+-------------+------------------+ | royal | | | t. grubb, | presented by | | observatory,| · | -- | | lord crawford. | | edinburgh | | | | | +-------------+-----------+------------+-------------+------------------+ | madrid | | | merz | | | observatory | | -- | | | +-------------+-----------+------------+-------------+------------------+ | tulse hill | | |sir h. grubb,| lent by the | | observatory | | | | royal society to | | | | | | sir william | | | | | | huggins. mounted | | | | | | with an -inch | | | | | | cassegrain | | | | | | reflector. | +-------------+-----------+------------+-------------+------------------+ | national | | | lerebours | | | observatory,| | | | | | paris | | | | | +-------------+-----------+------------+-------------+------------------+ | harvard | | | merz, | | | college | | | | | | observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | national | | | | | | observatory,| | -- | | | | rio de | | | | | | janeiro | | | | | +-------------+-----------+------------+-------------+------------------+ | tacubaya | | |sir h. grubb,| | | observatory,| | | | | | mexico | | | | | +-------------+-----------+------------+-------------+------------------+ | stonyhurst | | |sir h. grubb,| | | college | | | | | | observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | brera | | | | | | observatory,| | -- | | | | milan | | | | | +-------------+-----------+------------+-------------+------------------+ | university | | |sir h. grubb,| visual. | | of | | | | mounted with a | | mississippi | | | | photographic | | | | | | -inch refractor.| +-------------+-----------+------------+-------------+------------------+ | imperial | | | merz and | | | observatory,| | · | mahler, | | | pulkowa | | | | | +-------------+-----------+------------+-------------+------------------+ | maidenhead | | |sir h. grubb,| the property of | | observatory | | -- | | mr. dunn. | | | | | | mounted with a | | | | | | twin photographic| | | | | | reflector. | +-------------+-----------+------------+-------------+------------------+ | odessa | | | merz, | | | observatory | · | -- | | | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | henrys and | | | observatory,| · | | gautier | | | nice | | | | | +-------------+-----------+------------+-------------+------------------+ | brussels | | | merz and | | | observatory | · | | cooke, | | +-------------+-----------+------------+-------------+------------------+ | observatory | | | merz and | | | of bordeaux | · | · |gautier, | | +-------------+-----------+------------+-------------+------------------+ | observatory | | | merz and | | | of lisbon | · | -- | mahler | | +-------------+-----------+------------+-------------+------------------+ table vi. list of observatories employed in the construction of the photographic chart and catalogue of the heavens. all are provided with -inch photographic, coupled with -inch visual refractors: ---------------------------------------------------------- | name of observatory. | constructors of instruments.| | |-------------------------------| | | optical part.|mechanical part.| |------------------------|--------------|----------------| |paris | henrys | gautier | |algiers | " | " | |bordeaux | " | " | |toulouse | " | " | |san fernando (spain) | " | " | |vatican | " | " | |cordoba | " | " | |montevideo | " | " | |perth, western australia| " | " | |helsingfors | " | repsold | |potsdam | steinheil | " | |catania | " | salmoiraghi | |greenwich | sir h. grubb | sir h. grubb | |oxford | " | " | |the cape | " | " | |melbourne | " | " | |sydney | " | " | |tacubaya (mexico) | " | " | ---------------------------------------------------------- index abbe, cleveland, corona of , , aberdour, lord, solar chromosphere, aberration, discovered by bradley, , ; cause of, , ; investigations of, , abney, daylight coronal photographs, ; infra-red photography, , , absorption, terrestrial atmospheric, , , - , ; solar, - , , , , , , ; correlative with emission, , , adams, discovery of neptune, - ; lunar acceleration, ; orbit of november meteors, aerolites, falls of, , airy, solar translation, ; observations during eclipses, , , ; astronomer-royal, ; search for neptune, , ; corona of , ; solar parallax, , ; transit of venus, ; mercurian halo, ; lunar atmosphere, aitken, double star discoveries, albedo, of mercury, ; of venus, ; of mars, ; of minor planets, ; of jupiter, ; of saturn, ; of uranus, alexander, spiral nebulæ, ; observation during eclipse, algol, variability of light, , ; eclipses, ; nature of system, altitude and azimuth instrument, _note_, amici, comet of , anderson, discovery of nova aurigæ, ; of nova persei, andrews, conditions of liquefaction, Ångström, c. j., _optical researches_, ; spark spectrum, ; nature of photosphere, ; solar spectroscopy, , ; hydrogen in sun, ; temperature of stars, Ångström, k., infra-red solar spectrum, ; solar constant, arago, eclipse of , , , ; prominences, ; polarization in comets, ; magnetic relations of auroræ, ; nature of photosphere, ; meteor-systems, arai, photographs of corona of , arcturus, spectrum, , ; radial movement, argelander, bonn durchmusterung, , ; solar motion, ; centre of milky way, ; comet of , aristotle, description of a comet, arrhenius, light-pressure theory of comets, asten, movements of encke's comet, asteroids, so designated by herschel, astronomical circles, , astronomical physics, , , astronomical society founded, ; herschel its first president, astronomy, classification, ; popularity and progress, ; in united states, ; in germany, ; practical reform, ; of the invisible, ; physical, atmosphere, solar, , , , , ; of venus, , , , ; of mercury, - ; of the moon, , ; of mars, ; of minor planets, auroræ, periodicity, , ; excited by meteors, auwers, reduction of bradley's observations, ; system of procyon, ; opposition of victoria, ; solar parallax, ; new star in scorpio, babinet, nebular hypothesis, backlund, movements of encke's comet, , baden-powell, sir george, eclipse expedition, bailey, nebulosity round pleiades, ; stellar photometric observations, ; discovery of variable clusters, baily, early life and career, - ; observations of eclipses, - ; density of the earth, , baily's beads, , bakhuyzen, rotation of mars, ball, sir robert, parallaxes of stars, _note_, ; contacts in transits, balmer's law, , barnard, micrometrical measures of neptune, ; of minor planets, ; of saturn's rings, ; photographs of solar corona, , ; transit of mercury, ; halo round venus, ; surface of mars, ; ellipticity of jupiter's first satellite, ; of uranus, ; discovery of inner jovian satellite, , ; red spot on jupiter, ; eclipse of japetus, ; attendants on comet of , ; on brooks's comet, , ; swift's comet, ; photographic discovery of a comet, ; observations of nova aurigæ, , ; hind's variable nebula, ; exterior pleiades nebulosities, ; galactic stars, ; photographs of milky way, , ; cluster variables, ; horizontal telescope, bartlett, photograph of a partial eclipse, basic lines, , baxendell, meteors of , becker, drawings of solar spectrum, beckett, sir e. (lord grimthorpe), value of solar parallax, beer and mädler, surveys of lunar surface, , ; studies of mars, bélopolsky, coronal photographs, ; theory of corona, ; rotation of venus, ; of jupiter, ; spectroscopic determinations of saturn's rings, ; spectrum of gamma cassiopeiæ, ; system of castor, , ; detection of variable stars as spectroscopic binaries, berberich, mass of asteroids, ; orbit of holmes's comet, berkowski, daguerrotype of eclipsed sun, bessel, biographical sketch, - ; reduction of bradley's observations, ; parallax of cygni, ; disturbed motion of sirius and procyon, ; trans-uranian planet, ; halley's comet, ; theory of instrumental errors, ; personal equation, ; rotation of mercury, ; lunar atmosphere, ; cometary emanations, , ; multiple tails, ; comet of , betelgeux, remoteness, , ; spectrum, , , , ; radial movement, bianchini, rotation of venus, biela, discovery of a comet, bigelow, magnetic and solar disturbances, ; theory of corona, bigourdan, eclipse of , ; velocity of comet of , bird's quadrants, , , birmingham, colours of stars, _note_; discovery of t coronæ, birt, rotation of a sun-spot, ; selenographical society, bischoffsheim, coudé telescope, black ligament, bode, popular writings, ; solar constitution, ; missing planet, , bode's law, , , boeddicker, heat-phases during lunar eclipses, , ; drawings of jupiter, ; of the milky way, boehm, solar observations, , boguslawski, centre of sidereal revolutions, ; observation of halley's comet, bolometer, principle of construction, bond, g. p., his father's successor, ; light of jupiter, ; saturn's rings, ; donati's comet, , ; andromeda nebula, ; double-star photography, bond, w. c., observation of neptune's satellite, ; discovery of hyperion, ; of saturn's dusky ring, ; resolution of nebulæ, ; celestial photography, , ; satellite-transit on jupiter, borda, repeating circle, boss, solar translation, ; observations on comets, , bossert, proper motions of stars, bouguer, solar atmospheric absorption, boulliaud, period of mira, bouvard, tables of uranus, ; encke's comet, boys, radio-micrometer, ; density of the earth, bradley, discoveries of aberration and nutation, ; solar translation, ; star-distances, , ; observation on castor, ; instruments, , ; observations reduced by bessel and auwers, , brahe, tycho, star of , brandes, observations of meteors, , ; braun, prominence photography, ; density of the earth, brayley, meteoric origin of planets, brédikhine, theory of cometary appendages, , ; repulsive forces, , ; chemical differences, , ; formative types, , , , , ; structure of chromosphere, ; red spot on jupiter, ; andromede meteors, ; stationary radiants, ; spectrum of coggia's comet, bremiker, star maps, brenner, rotation of venus, brester, _théorie du soleil_, brewster, diffraction theory of corona, ; telluric lines in solar spectrum, ; absorption spectra, brinkley, ostensible stellar parallaxes, brisbane, establishment of paramatta observatory, , brooks, fragment of comet, ; cometary discoveries, , brünnow, stellar parallaxes, , bruno, giordano, motion of stars, buffham, rotation of uranus, buffon, internal heat of jupiter, bunsen, discovery of spectrum analysis, burchell, magnitude of eta carinæ, burnham, stellar orbits, ; coronal photographs, ; measures of nova aurigæ, ; of planetary nebulæ, ; discoveries of double stars, , , , ; catalogue, ; system of cygni, burton, canals of mars, ; rotation of jupiter's satellites, calandrelli, stellar parallaxes, callandreau, capture theory of comets, campani, saturn's dusky ring, campbell, lieutenant, polarisation of corona, campbell, professor, stellar radial velocities, , , ; flash spectrum, ; spectroscopic observations of saturn's rings, ; wolf-rayet stars, ; spectroscopic binaries, ; nova aurigæ, ; translation of solar system, ; stellar diffraction-spectra, canals of mars, - cannon, miss a. j., spectrographic researches, canopus, remoteness, , ; spectrum, capella, spectrum, , , ; a spectroscopic binary, carbon, material of photosphere, ; absorption by, in sun, ; in stars, carbonelle, origin of meteorites, carinæ, eta, light variation, , ; spectrum, carrington, astronomical career, , ; sun-spot observations, ; solar rotation, ; spot-distribution, ; luminous outburst on sun, , ; jovian and sun-spot periods, ; origin of comets, cassini, domenico, discoveries of saturnian satellites, ; of division in ring, ; solar rotation period, ; solar parallax, ; rotation of venus, ; of mars, ; of jupiter, , ; satellite of venus, ; satellite-transit on jupiter, cassini, j. j., stellar proper motions, ; sun-spots on limb, ; theory of corona, ; rotation of venus, ; structure of saturn's rings, castor, system of, , cavendish experiment, , ceres, discovery, , ; diameter, , chacornac, observation of sun-spot, ; star-maps, , ; variable nebula, challis, search for neptune, , ; duplication of biela's comet, charlois, discoveries of minor planets, charroppin, coronal photographs, chase, photographic discovery of a comet, ; stellar parallaxes, chladni, origin of meteors, , christie, mercurian halo, chromosphere, early indications, ; distinct recognition, , , ; depth, , , ; metallic injections, ; eruptive character, ; spectrum, clark, alvan, large refractors, , , , clark, alvan g., discovery of sirian companion, , ; -inch refractor, clarke, colonel, figure of the earth, clarke, f. w., celestial dissociation, clausen, period of comet, ; cometary systems, clerihew, secondary tail of comet, clusters, variable stars in, coggia, discovery of a comet, comet, halley's, return in , , ; orbit computed by bessel, ; capture by neptune, , ; return in , - , ; type of tail, , ; of , , - ; type of tail, , ; relationships, - ; newton's, , ; encke's, ; changes of volume, ; of brightness, ; acceleration, , ; capture by mercury, ; winnecke's, , ; biela's, - , ; brorsen's, ; vico's, , ; faye's, ; of , - , ; of , , , ; of , , ; lexell's, , ; tewfik, , , , ; donati's, - , , ; of , , , ; perseid, , ; leonid, , , , ; klinkerfues's, ; holmes's, , , ; coggia's, , , ; of , ; of , , ; aristotle's, ; tebbutt's, - ; schaeberle's, , ; wells's, , ; of september, , - , - ; thome's, ; pons-brooks, , ; sawerthal's, ; brooks's, of , , ; swift's, cometary tails, repulsive action upon, , , , - ; coruscations in, ; three types, - , , ; multiple, , , , , , , comets, subject to gravitation, ; of short period, , ; translucency, , , , ; small masses, , ; capture by planets, , , ; changes of volume, , , ; polarisation of light, , , ; refractive inertness, , ; relations to meteor-systems, , - ; disintegration, , , , ; spectra, - , , , - ; luminous by electricity, , , ; systems, , , , , ; origin, - common, reflectors for eclipse photography, ; jupiter's inner satellite, ; detection of great comet near the sun, ; its five nuclei, ; photographs of andromeda nebula, ; of orion nebula and jupiter, , ; great reflectors, , ; cluster variables, common, miss, drawing of eclipsed sun, comstock, lunar atmosphere, comte, celestial chemistry, ; astronomy, cooke, -inch refractor, copeland, comets of and , ; spectrum of comet of , ; of gamma cassiopeiæ, ; of nova andromedæ, ; of orion nebula, ; discoveries of gaseous stars, ; nova aurigæ, , copernicus, stellar parallax, cornu, telluric lines in solar spectrum, ; velocities in prominences, ; ultra-violet solar spectrum, , ; velocity of light, _note_, ; spectrum of hydrogen, ; of nova cygni, cornu and bailie, density of the earth, corona of , - , ; early records and theories, - ; photographs, , , , , - ; spectrum, , , , , , ; varying types, - , ; of , - ; of , ; of , ; of , ; of , - ; of , ; of , ; of , ; of , ; daylight photography of, - ; glare theory, ; mechanical theory, ; electro-magnetic theories, , coronium, , , cortie, movements in sun-spots, ; their spectral changes, cotes, corona of , croll, secular changes of climate, , ; derivation of solar energy, crookes, chemical elements, crova, solar constant, cruls, comet of , , cusa, solar constitution, cysatus, orion nebula, ; comet of , damoiseau, theory of halley's comet, d'arrest, orbits of minor planets, ; andromede meteors, ; ages of stars, ; variable nebulæ, ; measures of nebulæ, darwin, g. h., rigidity of the earth, ; saturn's ring system, ; origin of the moon, - ; development of solar system, , , ; solar tidal friction, daubrée, falls of aerolites, davidson, satellite-transit on jupiter, davis, stellar parallaxes, dawes, prominences in , ; saturn's dusky ring, ; a star behind a comet, ; solar observations, , ; observations and drawings of mars, , , ; satellite-transits on jupiter, , de ball, markings on mercury, delambre, greenwich observations, ; solar rotation, ; light-equation, de la roche, newton's law of cooling, de la rue, celestial photography, , , ; solar investigations, ; expedition to spain, , de la tour, experiments on liquefaction, delaunay, tidal friction, , ; coudé telescope, delisle, diffraction theory of corona, ; transits of venus, , dembowski, double star measurements, denning, observations of mercury, , ; mountain on venus, ; rotation of jupiter, ; red spot, ; periodicity of markings, ; rotation of saturn, ; meteors of , ; of , ; stationary radiants, denza, meteors of , derham, theory of sun-spots, ; ashen light on venus, deslandres, eclipse expedition, ; rotation of corona, ; prominence photography, ; hydrogen spectrum in prominences, , ; photographs of jupiter, ; radial movements of saturn's rings, ; helium absorption in stars, ; stellar radial velocities, diffraction, corona explained by, , , ; spectrum, , , , dissociation in the sun, , - ; in space, doberck, orbits of double stars, , dollond, discovery of achromatic telescope, , donati, discovery of comet, ; spectra of comets, ; of stars, doppler, effect of motion on light, douglass, observations of jupiter's satellites, downing, perturbations of the leonids, draper, h., ultra-violet spectrum, ; oxygen in sun, ; photographs of the moon, ; of jupiter's spectrum, ; of tebbutt's comet, ; of spectrum of vega, ; of orion nebula draper, j. w., lunar photographs, ; distribution of energy in spectrum, _note_ draper memorial, - dreyer, catalogue of nebulæ, dulong and petit, law of radiation, , dunér, spectra of sun-spots, ; spectroscopic measurement of solar rotation, ; spectroscopic star catalogue, dunkin, solar translation, duponchel, sun-spot period, durchmusterung, bonn, , ; cape photographic, ; parallax, ; photometric, dyson, coronal photographs, earth, mean density, , ; knowledge regarding, ; rigidity, , ; variation of latitude, , ; figure, , ; effects of tidal friction, - ; bodily tides, ; primitive disruption, easton, structure of milky way, , ebert, coronoidal discharges, eclipse, solar, of , ; of , - , , ; of , , , ; of , , ; of , - ; of , ; of , ; of , ; of , - ; of , , ; of , , ; of , ; of , ; of , ; of , - ; of , , ; of , ; of , ; of , , ; of , eclipses, lunar, heat-phases during, , eclipses, solar, importance, ; ancient, , ; classification, ; results, , eddie, comet of , ; of , edison, tasimeter, egoroff, telluric lines in solar spectrum, , elements, chemical, dissociation in sun, , , elkin, star parallaxes, , , ; photography of meteors, ; transit of great comet, , ; secondary tail, ; triangulation of the pleiades, elliot, opinions regarding the sun, elvins, red spot on jupiter, encke, star maps, ; calculation of short-period comet, ; resisting medium, ; distance of the sun, , ; period of pons's comet, engelmann, rotation of jupiter's satellites, ericsson, solar temperature, erman, meteoric rings, eros, measures of, for solar parallax, ; discovery, ; variability, ertborn, mountain in venus, espin, spectra of variable stars, ; stars with banded spectra, ; nova aurigæ, , euler, resisting medium, evershed, eclipse photographs, , evolution, of solar system, , , - , ; of earth-moon system, - ; of stellar systems, fabricius, david, discovery of mira ceti, fabricius, john, detection of sun-spots, faculæ, relation to spots, , , ; solar rotation from, ; photographed, , , faye, nature of prominences, , ; discovery of a comet, ; cyclonic theory of sun-spots, , ; solar constitution, - ; maximum of , ; velocities in prominences, ; distance of the sun, ; planetary evolution, , , feilitsch, solar appendages, fényi, solar observations, , ferrel, tidal friction, ferrer, nature of corona, ; prominences, fessenden, electrical theory of comets, finlay, transit of great comet, , fizeau, daguerrotype of the sun, ; doppler's principle, ; velocity of light, flammarion, canals of mars, ; trans-neptunian planet, flamsteed, solar constitution, ; distance, flaugergues, detection of comet, ; transit of mercury, fleming, mrs., spectrum of beta lyræ, ; preparation of draper catalogue, ; discoveries of new stars, flint, star-parallaxes, fontana, mountains of venus, ; satellite, ; spots on mars, forbes, george, trans-neptunian planets, , forbes, james d., spectrum of annularly eclipsed sun, ; solar constant, foucault, spectrum of voltaic arc, ; photograph of the sun, ; velocity of light, , ; silvered glass reflectors, fraunhofer, early accident, ; improvement of refractors, ; clockwork motion, ; spectra of flames, ; of sun and stars, , , ; objective prism, ; diffraction gratings, fraunhofer lines, mapped, , ; origin, - , , ; reflected in coronal spectrum, , , ; in cometary spectra, , ; shifted by radial motion, freycinet, distribution of minor planets, fritz, auroral periodicity, frost, solar heat radiation, galileo, descriptive astronomy, ; double-star method of parallaxes, ; discovery of sun-spots, ; solar rotation, ; planets and sun-spots, ; darkening at sun's edge, galle, discovery of neptune, , ; saturn's dusky ring, ; distance of the sun, ; path of andromede meteors, galloway, solar translation, gambart, discovery of comet, gauss, orbits of minor planets, ; _theoria motus_, ; magnetic observations, , ; cometary orbits, gautier, sun-spot and magnetic periods, , ; sun-spots and weather, german astronomical society, , gill, star-parallaxes, , , , ; expedition to ascension, ; distance of the sun, , , ; constant of aberration, ; arc measurements, , ; comet of , , ; oxygen-absorption in stars, ; photograph of argo nebula, ; cape durchmusterung, ; photographic celestial survey, ; actinic intensity of galactic stars, ; coudé telescope, gladstone, j. h., spectrum analysis, , glaisher, occultation by halley's comet, glasenapp, coronal photographs, ; light equation, , ; double star measures, glass, optical, excise duty on, , ; guinand's, , ; jena, gledhill, spot on jupiter, goldschmidt, nebulæ in the pleiades, goodricke, periodicity of algol, gore, catalogue of variable stars, ; of computed binaries, gothard, bright-line stellar spectra, , ; spectrum of nova aurigæ, ; photographs of nebulæ, gould, variation of latitude, ; photograph of mars, ; comets of and , , ; luminous instability of stars, ; photographic measures of the pleiades, ; _uranometria argentina_, ; solar cluster, , graham, discovery of metis, grant, solar envelope, , ; transit phenomena, green, observation of mars, greenwich observations, , , gregory, david, achromatic lenses, _note_ gregory, james, double star method of parallaxes, ; reflecting telescopes, groombridge, star catalogue, grosch, corona of , grubb, sir howard, photographic reflector, ; great refractors, , ; siderostat, grubb, thomas, melbourne reflector, _note_, gruithuisen, snow-caps of venus, ; lunar inhabitants, gully, detection of nova andromedæ, guthrie, nebulous glow round venus, hadley, saturn's dusky ring, ; reflecting telescope, haerdtl, winnecke's comet, hale, luminous outburst on sun, ; daylight coronal photography, ; spectrum of prominences, , ; prominence photography, , ; photographs of faculæ, , ; carbon in chromosphere, ; bright lines in fourth-type stars, ; reflectors and refractors, hall, asaph, parallax of the sun, ; discovery of martian satellites, ; rotation of saturn, ; double star measurements, hall, chester more, invention of achromatic telescope, hall, maxwell, rotation of neptune, halley, stellar proper motions, ; composition of nebulæ, ; observation of eta carinæ, ; eclipse of , , ; predicted return of comet, ; magnetic theory of auroræ, ; transits of venus, ; lunar acceleration, ; origin of meteors, halm, magnetic relations of latitude variation, hansen, solar parallax from lunar theory, hansky, coronal photographs, , harding, discovery of juno, ; celestial atlas, harkness, spectrum of corona, ; corona of , ; shadow of the moon in solar eclipses, ; light equation, ; distance of the sun, , , , harriot, observations on halley's comet, hartley, gallium in the sun, , hartwig, nova andromedæ, hasselberg, metallic spectra, ; spectra of comets, , ; of nova andromedæ, hastings, composition of photosphere, ; observations at caroline island, ; saturn's dusky ring, hegel, number of the planets, heis, radiant of andromedes, heliometer, , , , , helium, a constituent of prominences, , , ; no absorption by, in solar spectrum, ; absorptive action in first-type stars, ; bright in gaseous stars, , , ; in orion nebula, helmholtz, gravitational theory of sun-heat, - hencke, discoveries of minor planets, henderson, parallax of alpha centauri, , ; observation of chromosphere, henry, paul and prosper, lunar twilight, ; markings on uranus, ; photograph of saturn, ; photographs of nebulæ in the pleiades, , ; stellar photography, ; plane mirrors, herrick and bradley, duplication of biela's comet, herschel, alexander s., cometary and meteoric orbits, herschel, caroline, her brother's assistant, ; observation of encke's comet, herschel, colonel, spectrum of prominences, ; of reversing layer, ; of corona, herschel, sir john, life and work, - ; magellanic clouds, , ; sun-spots, , , ; solar flames, ; anticipation of neptune's discovery, ; status of hyperion, ; biela's comet, ; halley's, ; comet of , ; sixth star in "trapezium," ; grinding of specula, ; spectrum analysis, ; solar photography, , ; solar constitution, ; shadow round eclipsed sun, ; actinometrical experiments, ; solar heat, ; climate and eccentricity, ; lunar atmosphere, ; surface of mars, ; andromeda nebula, ; observations of nebulæ, ; double nebulæ, herschel, sir william, discovery of uranus, ; founder of sidereal astronomy, ; biographical sketch, - ; sun's motion in space, , , ; revolutions of double stars, , ; structure of milky way, - , ; nature of nebulæ, - , ; results of his observations, ; centre of sidereal system, ; theory of the sun, - , ; asteroids, ; discoveries of saturnian and uranian satellites, , , ; comet of , ; reflecting telescopes, - ; sun-spots and weather, ; transit of mercury, ; refraction in venus, ; lunar volcanoes, ; terrestrial affinity of mars, ; jovian trade-winds, ; rotation of jupiter's satellites, ; ring of saturn, ; rotation of saturn, ; origin of comets, ; stellar photometry, herz, comets' tails, hevelius, "mira" ceti, ; contraction of comets, ; granular structure of comet, higgs, photographs of solar spectrum, , hind, solar flames, ; iris and flora discovered by, ; distortion of biela's comet, ; transit of a comet, ; earth in a comet's tail, ; comets of and , ; schmidt's comet, ; new star, ; variable nebula, hirn, solar temperature, ; resistance in space, hodgson, outburst on the sun, hoeffler, star-drift in ursa major, hoek, cometary systems, holden, uranian satellites, ; eclipse expedition, ; coronal extensions, ; solar rotation, ; transit of mercury, ; intra-mercurian planets, ; drawing of venus, ; lunar photographs, ; canals on mars, ; surface of mars, ; transits of jupiter's satellites, ; markings on uranus, ; disintegration of comet, ; colours of double stars, ; nova aurigæ, ; orion and trifid nebulæ, , ; director of lick observatory, holden and schaeberle, observations of nebulæ, holmes, discovery of a comet, homann, solar translation, hooke, solar translation, ; stellar parallax, ; repulsive action on comets, _note_; automatic movement of telescopes, ; spots on mars, , hopkins, solidity of the earth, horrebow, sun-spot periodicity, ; satellite of venus, hough, g. w., red spot on jupiter, , ; observations of double stars, houzeau, solar parallax, howlett, sun-spot observations, hubbard, period of comet of , , huggins, sir william, spectroscopic observations of prominences, , ; hydrogen spectrum in stars, , ; daylight coronal photography, , ; repulsive action in corona, ; stellar motions in line of sight, , , ; transit of mercury, ; occultation of a star, ; snowcaps on mars, ; spectrum of mars, ; of jupiter, ; jovian markings and sun-spots, ; spectrum of uranus, ; of comets, , ; photographs, , ; stellar spectroscopy, ; colours of stars, ; classification of star spectra, ; photographs, , , ; stellar chemistry, , ; spectra of new stars, , ; theory of nova aurigæ, ; spectra of nebulæ, , , ; nebular radial movement, huggins, sir william and lady, photograph of uranian spectrum, ; spectra of wolf-rayet stars, ; ultra-violet spectrum of sirius, ; nitrogen in stars, ; spectrum of nova aurigæ, - ; of andromeda nebula, ; of orion nebula, humboldt, sun-spot period, ; magnetic observations, ; meteoric shower, hussey, t. j., search for neptune, hussey, w. j., cloud effects on mars, ; cometary appendages, ; period of delta equulei, ; discoveries of double stars, , huygens, stellar parallax, ; orion nebula, ; discovery of titan, ; saturn's ring, , ; spot on mars, hydrogen, a constituent of prominences, , , ; spectrum, , , , ; absorption in stars, , , - ; in sun, ; theoretical material of comets' tails, ; emissions in stars, - , , , ; in nebulæ, , innes, southern double stars, jacoby, measurement of rutherfurd's plates, ; pritchard's parallax work, janssen, photographs of the sun, ; spectroscopic observations of prominences, , ; escape from paris in a balloon, ; coronal spectrum, , ; coronal photographs, ; rarefaction of chromospheric gases, ; oxygen absorption in solar spectrum, ; transit of , ; spectrum of venus, ; of saturn, ; photographs of tebbutt's comet, , ; of orion nebula, japetus, eclipse of, ; variability in light, jewell, solar spectroscopy, , joule, heat and motion, jupiter, mass corrected, , ; conjectured influence on sun-spot development, ; physical condition, , ; spectrum, , ; satellite-transits, , ; discovery of inner satellite, ; red spot, - ; photographs, , ; periodicity of markings, kaiser, rotation of mars, ; map of mars, kammermann, observation of maia nebula, kant, status of nebulæ, ; sirius the central sun, ; planetary intervals, ; tidal friction, ; condition of jupiter, ; cosmogony, kapteyn, solar translation, ; cape durchmusterung, ; stellar parallaxes, , ; actinic intensity of galactic stars, ; solar cluster, kayser and runge, spectroscopic investigations, , keeler, red spot on jupiter, ; spectroscopic determination of movements in saturn's rings, ; spectrum of uranus, ; of third type stars, ; of nebulæ, ; photographs of nebulæ, , , , ; nebular radial movements, , , ; grating spectroscope, kepler, star of , ; solar corona, ; missing planets, ; cometary decay, , ; comet of , ; physical astronomy, kiaer, comets' tails, kirchhoff, foundation of spectrum analysis, , - , ; map of solar spectrum, ; solar constitution, , , kirkwood, distribution of minor planets, ; grouped orbits, ; divisions in saturn's rings, , ; origin of planets, ; their mode of rotation, ; comets and meteors, , kleiber, perseid radiants, klein, hyginus n., , klinkerfues, comet predicted by, , ; apparitions of southern comet, ; tidal theory of new stars, knobel, cloud effects on mars, konkoly, spectrum of gamma cassiopeiæ, ; spectroscopic survey, _note_ kreil, lunar magnetic action, kreutz, period of comet, ; orbit of comet, ; period of great september comet, ; cause of disintegration, ; eclipse-comet of , krüger, segmentation of great comet, küstner, variation of latitude, kunowsky, spots on mars, lacaille, southern nebulæ, ; eta carinæ, lagrange, theory of solar system, ; planetary disruption, lahire, diffraction theory of corona, ; distance of the sun, ; mountains of venus, lalande, popularisation of astronomy, ; revolving stars, ; _histoire céleste_, , ; nature of sun-spots, ; observations of neptune, lambert, solar motion, ; construction of the universe, , ; missing planets, lamont, magnetic period, , lamp, ashen light on venus, langdon, mountains of venus, langley, solar granules, ; corona of , ; spectroscopic effects of solar rotation, ; infra-red spectrum, , , ; experiments at pittsburg, ; bolometer, ; distribution of energy in spectrum, , ; atmospheric absorption, , , ; solar constant, ; lunar heat-spectrum, ; temperature of lunar surface, ; age of the sun, laplace, lunar acceleration, , ; _système du monde_, ; nebular hypothesis, , , , , , ; stability of saturn's rings, , ; solar atmosphere, , ; lexell's comet, , ; solar distance by lunar theory, ; origin of meteors, ; of comets, lassell, discovery of neptune's satellite, ; of hyperion, ; saturn's dusky ring, ; observations at malta, , ; reflectors, ; equatoreal mounting, latitude, variation of, , laugier, period of comet, ; solar rotation, le chatelier, temperature of the sun, lescarbault, pseudo-discovery of vulcan, ; halo round venus, lespiault, orbits of minor planets, le sueur, spectrum of jupiter, leverrier, discovery of neptune, - ; lexell's comet, , ; distance of the sun, , ; revolutions of mercury, ; supposed transits of vulcan, ; mass of asteroids, ; orbit of november meteors, ; perseids and leonids, lexell, comet of , , , liais, supposed transit of vulcan, ; comet of , ; division of a comet, librations, of mercury, ; of venus, ; of the moon, lick, foundation of observatory, light, velocity, , , ; extinction in space, ; refrangibility changed by movement, light-equation, , ligondès, development of solar system, lindsay, lord, expedition to mauritius, line of sight, movements in, , ; of solar limbs, , ; in prominences, , ; of stars, , , ; binaries detected by, - listing, dimensions of the globe, littrow, chromosphere, ; sun-spot periodicity, liveing and dewar, carbon in the sun, lockyer, solar spectroscopy, , ; theory of sun-spots, , ; daylight observations of prominences, , , , ; eclipse of , ; slitless spectroscope, ; corona of , ; glare theory of corona, ; eclipse of , ; chromospheric spectrum, ; classification of prominences, ; their radial movements, ; celestial dissociation, - ; chemistry of sun-spots, ; spots on mars, ; meteoritic hypothesis, , ; equatoreal coudé, loewy, constant of aberration, , ; lunar photographs, ; director of paris observatory, ; equatoreal coudé, , lohrmann, lunar chart, ; linné, lohse, j. g., spectrum of great comet, lohse, o., daylight coronal photography, _note_; spectral investigations, ; twilight on venus, ; red spot on jupiter, ; periodicity of jupiter's markings, ; motion of sirius, ; spectrum of nova cygni, louville, nature of corona, ; chromosphere, lowell, rotation of mercury, ; of venus, ; markings on venus, ; observations of mars, , ; satellites, lyman, atmosphere of venus, mcclean, photographs of solar spectrum, , ; helium stars, ; oxygen stars, ; equipment of cape observatory, macdonnell, luminous ring round venus, maclaurin, eclipse of , maclear, admiral, observations during eclipses, , maclear, sir thomas, maximum of eta carinæ, ; observation of halley's comet, mädler, central sun, ; observations of venus, ; lunar rills, ; aspect of linné, ; common proper motions, magellanic clouds, , ; spiral character, magnetism, terrestrial, international observations, ; periodicity, , ; solar relations, , , , , ; lunar influence, mann, last observation of donati's comet, maraldi, solar corona, ; rotation of mars, ; satellite-transits on jupiter, ; spot on jupiter, marius, andromeda nebula, ; sun-spots, mars, oppositions, ; solar parallax from, , , ; polar spots, , , , , ; permanent markings, - ; rotation, , ; atmosphere, , ; climate, , ; canals, - ; photographs, ; satellites, , , , , marth, revolutions of neptune's satellite, maskelyne, components of castor, ; astronomer-royal, ; experiment at schehallien, ; comets and meteors, maunder, photographs of corona of , ; comparative massiveness of stars, ; constitution of nebulæ, maunder, mrs., coronal photographs, , maury, director of naval observatory, ; duplication of biela's comet, maury, miss a. c., spectrographic investigations, ; discoveries of spectroscopic binaries, , maxwell, j. clerk, structure of saturn's rings, , mayer, c., star satellites, mayer, julius r., tidal friction, ; meteoric sustentation of sun's heat, mayer, tobias, stellar motions, ; solar translation, ; repeating circle, ; solar distance, ; satellite of venus, ; lunar surface, mazapil meteorite, meldrum, sun-spots and cyclones, melloni, lunar heat, melvill, spectra of flames, mercury, mass, ; luminous phenomena during transits, , ; spectrum, ; mountainous conformation, , ; rotation, , ; theory of movements, , mersenne, reflecting telescope, messier, catalogue of nebulæ, meteoric hypothesis of solar sustentation, ; of planetary formation, meteoritic hypothesis of cosmical constitution, , meteors, origin, , ; relations to comets, , - , ; leonids, - , ; perseids, , , , ; andromedes, - ; stationary radiants, meunier, canals of mars, meyer, divisions of saturn's rings, ; comet of , ; cometary refraction, ; comet tewfik, michell, double stars, ; torsion balance, ; star systems, michelson, velocity of light, milky way, grindstone theory, ; clustering power, , ; structure, , , , , - ; centre of gravity, , ; frequented by wolf-rayet, temporary, and helium stars, , , ; by gaseous nebulæ, ; drawings and photographs, , miller, w. a., spectrum analysis, , , ; stellar chemistry, mira, light changes, ; spectrum, , mitchel, lectures at cincinnati, mitchell, photograph of reversing layer, möller, theory of faye's comet, mohn, origin of comets, moll, transit of mercury, monck, perseid meteors, ; new stars, moon, acceleration, , , ; magnetic influence, ; photographs, , , ; solar parallax from disturbed motion, , ; study of surface, ; atmosphere, - ; charts, - ; librations, ; superficial changes, , ; thermal radiations, , ; rotation, ; tables, , ; origin, - morinus, celestial chemistry, morstadt, andromede meteors, mouchez, photographic survey of the heavens, ; death, müller, phases of mercury, ; of minor planets, ; albedo of mars, ; of jupiter, ; of saturn, ; variability of neptune, ; of pons's comet, ; stellar photometry, munich, optical institute, , myer, solar eclipse, nasmyth, lassell's reflector, ; solar willow-leaves, ; comparative lustre of mercury and venus, ; condition of jupiter, nasmyth and carpenter, _the moon_, nebula, andromeda, early observations, ; new star in, , ; photographs, , ; structure, ; spectrum, , ; visibility at arequipa, nebula, orion, observed by herschel, ; mentioned by cysatus, ; apparent resolvability, ; suspected variability, ; radial movement, ; spectrum, ; photographs, , , nebulæ, first discoveries, ; catalogues, , , , ; distribution, , , ; composition, , , , ; resolution, , , ; double, , ; spiral, , , ; new stars in, - , , ; spectra, - , ; variability, , ; radial movements, ; photographs, - , nebular hypothesis, herschel's, , ; laplace's, , , , ; objections, - neison, atmosphere of venus, ; rills on the moon, ; _the moon_, neptune, discovery, - ; satellite, , ; density, ; comets captured by, , , ; mode of rotation, , , , newall, f., duplicity of capella, ; stellar radial motions, newall, r. s., -inch refractor, newcomb, runaway stars, ; solar translation, ; origin of minor planets, ; telescopic powers, ; corona of , ; of , ; distance of the sun, - ; velocity of light, ; variation of latitude, ; lunar atmosphere, ; lunar theory, , ; disturbance of neptune's satellite, ; formation of planets, ; star catalogue, ; structure of milky way, newton, h. a., capture of comets by planets, ; falls of aerolites, ; november meteors, , ; meteors of , , ; orbits of aerolites, newton, sir isaac, founder of theoretical astronomy, , ; comets subject to gravitation, ; first speculum, ; solar radiations, ; law of cooling, - ; telescopes and atmosphere, niesten, volume of asteroids, ; red spot on jupiter, nobert, diffraction gratings, noble, observations of mercury, ; secondary tail of comet, nolan, origin of the moon, ; period of phobos, norton, expulsion theory of solar appendages, _note_; comets' tails, , nova andromedæ, , nova aurigæ, - nova cygni, , , nova persei, , nutation, discovered by bradley, , ; a uranographical correction, nyrén, constant of aberration, observatory, greenwich, , , ; cape of good hope, , , ; paramatta, , ; harvard college, , ; königsberg, ; dorpat, ; pulkowa, ; palermo, ; berlin, ; anclam, ; potsdam, ; kew, ; arequipa, , , ; yerkes, ; lick, occultations of stars by comets, , , ; by the moon, ; by mars, ; of jupiter by the moon, olbers, bessel's first patron, , ; discoveries of minor planets, , ; origin by explosion, , ; career, , ; biela's comet, ; comet of , ; electrical theory of comets, , , , ; multiple tails, ; comet of , ; cometary coruscations, ; november meteors, olmsted, radiant of leonids, ; orbit, oppenheim, calculation of schmidt's comet, oppolzer, e. von, theory of sun-spots, ; variability of eros, oppolzer, th. von, winnecke's comet, ; comet of , oxygen in sun, - ; telluric absorption, ; in stars, packer, variable stars in cluster, palisa, search for vulcan, , ; discoveries of minor planets, pallas, discovery, ; inclination of orbit, , ; diameter, , , pape, donati's comet, parallax, annual, of stars, , , , , - ; horizontal, of sun, ; encke's result, , ; improved values from oppositions of mars, , ; from light velocity, , , ; from recent transits, , ; from observations of minor planets, , ; general result, paris catalogue of stars, paschen, oxygen in sun, ; solar temperature, pastorff, drawings of the sun, peirce, structure of saturn's rings, perrine, eclipse photographs, ; nature of corona, ; observation of holmes's comet, ; nebula round nova persei, perrotin, rotation of venus, ; markings on, ; canals of mars, ; clouds on mars, ; striation of saturn's rings, ; rotation and compression of uranus, , ; changes of pons's comet, ; maia nebula, ; measures of double stars, perry, eclipse of december, , personal equation, , peter, star-parallaxes, peters, c. a. f., parallax of cygni, ; disturbed motion of sirius, peters, c. f. w., orbit of leonid meteors and comet, peters, c. h. f., sun-spot observations, , ; discoveries of minor planets, ; star maps, , peytal, description of chromosphere, phobos, rapid revolution, , , ; tidal relations, , photography, solar, , , , ; of corona, , , , , , - ; without an eclipse, - ; of prominences, , , ; of coronal spectrum, , , ; of prominence-spectrum, , ; of arc-spectrum, , ; of solar spectrum, , , , , ; of uranian spectrum, ; of cometary spectra, , ; of stellar and nebular spectra, - , , , , ; lunar, , , ; detection of comets by, , , ; of asteroids, ; of new stars, ; use of, in transits of venus, , , ; mars depicted by, , ; jupiter, , ; comets, , , , ; nebulæ, , , - , , ; milky way, , ; star-charting by, , ; star-parallaxes by, ; rapid improvement, photometry, stellar, , , ; of planetary phases, , ; of saturn's rings, ; photographic, photosphere, named by schröter, ; structure, , , , piazzi, star catalogues, ; parallaxes, ; motion of cygni, ; birth and training, ; -foot circle, , ; discovery of ceres, , picard, saturn's dark ring, ; sun's distance, pickering, e. c., photometric measures of martian satellites, ; of minor planets, ; variability of japetus, ; of neptune, ; meteoric photography, ; gaseous stars, ; hydrogen spectrum in stars ; spectrographic results, ; eclipses of algol, ; photographic celestial surveys, ; star density in pleiades, ; photometric catalogues, , ; photographic photometry, ; white stars in milky way, ; climate of arequipa, ; horizontal telescope, pickering, w. h., corona of , ; coronal photographs, january , , ; lunar twilight, ; lunar volcanic action, ; melting of snow on mars, ; martian snowfall, ; jupiter's satellites, ; photographs of comets, ; of orion nebula, ; observatory at arequipa, pingré, phenomena of comets, , planets, influence on sun-spots, ; periods and distances, ; intra-mercurian, - ; inferior and superior, ; trans-neptunian, , ; origin, , ; relative ages, , planets, minor, existence inferred, , ; discoveries, - , , , ; solar parallax from, - ; distribution of orbits, , ; collective volume, ; atmospheres, plantade, halo round mercury, pleiades, community of movement near, ; photographed spectra, ; measurements, ; photographs, , ; nebulæ, , plücker, hydrogen in sun, plummer, solar translation, ; encke's comet, plutarch, solar corona, pogson, prominence spectrum, ; reversing layer, ; discovery of a comet, , ; new star in cluster, pond, errors of greenwich quadrant, ; controversy with brinkley, pons, discoveries of comets, , , pontécoulant, return of halley's comet, poor, c. lane, calculation of lexell's comet, porter, solar translation, pouillet, solar constant, , ; temperature of the sun, ; of space, poynting, mean density of the earth, prince, glow round venus, pritchard, parallax of beta aurigæ, ; photographic determinations of stellar parallax, ; photometric catalogue, pritchett, corona of january, , ; red spot on jupiter, proctor, glare theory of corona, ; speed of ejections from sun, ; transit of venus, ; distance of sun, ; atmosphere of venus, ; rotation of mars, ; map and canals of mars, , ; condition of great planets, ; nova andromedæ, ; status of nebulæ, , ; structure of milky way, ; star drift, procyon, satellite, ; parallax, prominences, observed in , , , ; described by vassenius, ; observed in , ; photographed during eclipse, , , ; without eclipse, , ; spectrum, , , , , , ; spectroscopic method of observing, - , - ; white, , ; chemistry, , ; classification, ; distribution, ; movements in, - ; heat of development, quetelet, periodicity of august meteors, ranyard, drawing of sun-spot, ; coronal types, , ; lunar atmosphere, ; jupiter's markings, ; meteors from fixed radiants, ; cometary trains, ; tenuity of nebulæ, rayet, spectrum of prominences, , red spot on jupiter, , reduction of observations, ; bessel's improvements, , ; baily's, refraction, atmospheric, ; effects looked for in comets, , ; cytherean, , , ; lunar , reichenbach, foundation of optical institute, , , repsold, astronomical circles, , ; cape heliometer, resisting medium, , , respighi, slitless spectroscope, ; prominences and chromosphere, , , ; solar uprushes, ; spectrum of gamma argûs, reversing layer, detected, , ; photographed, , ; depth, riccioli, secondary light of venus, riccò, trials with coronagraph, ; distribution of prominences, ; spectrum of venus, ; spot on jupiter, ; spectrum of great comet, richer, distance of the sun, ristenpart, solar translation, ritchey, nebula round nova persei, ; photographs of nebulæ, ritter, development of stars, roberts, a. w., southern variables, roberts, isaac, search for ultra-neptunian planet, ; photographs of orion nebula, ; of andromeda nebula, ; of the pleiades, roberval, structure of saturn's rings, robinson, reflectors and refractors, roche, inner limit of satellite-formation, ; modification of nebular hypothesis, römer, star places, ; invention of equatoreal and transit instrument, ; of altazimuth, ; velocity of light, ; satellite transit on jupiter, rosenberger, return of halley's comet, rosetti, temperature of the sun, rosse, third earl of, biographical sketch, ; great specula, - ; discovery of spiral nebulæ, ; resolution of nebulæ, ; climate and telescopes, rosse, fourth earl of, experiments on lunar heat, rost, nature of sun-spots, roszel, mass of asteroids, rowland, photographic maps of solar spectrum, , ; elements in run, ; concave gratings, , rümker, observation of encke's comet, russell, h. c., red spot on jupiter, ; change in argo nebula, ; photographs of nubeculæ, russell, h. n., atmosphere of venus, rutherfurd, lunar photography, ; star spectra, ; photographs of the pleiades, ; diffraction gratings, sabine, magnetic and sun-spot periods, , , safarik, secondary light of venus, ; compression of uranus, satellites, discoveries, , , ; transits, , ; variability, , ; origin, , saturn, low specific gravity, ; rotation, ; spectrum, saturn's rings, first disclosure, ; dusky ring, ; stability, , ; meteoric constitution, ; eventual dispersal, savary, orbits of double stars, savélieff, solar radiation, , sawerthal, discovery of a comet, schaeberle, discovery of procyon's satellite, ; coronal photographs, , ; theory of corona, ; meteoric photography, ; discovery of a comet, schaeberle and campbell, observations of jupiter's satellites, scheiner, father, nature of sun-spots, , ; equatoreal instrument, _note_; solar rotation, ; darkening of sun's limb, schiener, dr. j., photospheric structure, ; spectrographic researches, , ; spectrum of andromeda nebula, ; stars and nebulæ in orion, schiaparelli, rotation of mercury, ; of venus, , ; spots on mars, ; snow-cap, ; canals, - ; compression of uranus, ; comets and meteors, , , , ; anomalous tail of great comet, ; pons's comet, ; origin of comets, ; measures of double stars, schmidt, a., circular refraction in sun, schmidt, j., sun-spot period, ; lunar rills, ; lunar maps, ; disappearance of linné, ; cometary appendages, ; new stars, schönfeld, extension of bonn durchmusterung, , schrader, construction of reflectors, schröter, a follower of herschel, ; motions of sun-spots, ; biographical sketch, , ; observations on mercury, , , ; on venus, - , ; on the moon, ; a lunar city, ; linné, ; spots on mars, ; jovian markings, schülen, perspective effects in sun-spots, schuster, photographs of corona, , ; spectra of oxygen, schwabe, sun-spot periodicity, , secchi, chromosphere, ; biela's comet, ; cyclonic movements in sun-spots, ; distribution, ; profundity, ; nature, , ; constitution of photosphere, ; eclipse observations, , ; reversing layer, ; observations of prominences, , , ; absence of helium absorption, ; temperature of the sun, ; solar atmospheric absorption, ; martian canals, ; spectrum of uranus, ; of coggia's comet, ; stellar spectral researches, , ; carbon stars, , ; gaseous stars, see, stellar orbits, , ; measures of neptune, ; measures of uranus, ; belts of neptune, ; colour of sirius, _note_; southern double stars, ; evolution of stellar systems, seeliger, photometry of saturn's rings, ; rationale of new stars, seidel, stellar photometry, sherman, spectrum of nova andromedæ, short, reflectors, , , , ; chromosphere, ; satellite of venus, ; striation of saturn's rings, sidereal science, foundation, , ; condition in , ; progress, sidgreaves, spots and faculæ, siemens, regenerative theory of the sun, simony, photographs of ultra-violet spectrum, sirius, a binary star, ; mass, ; parallax, , ; spectrum, , , ; former redness, _note_; radial movement, , smyth, admiral, donati's comet, smyth, piazzi, oxygen spectrum, ; lunar radiations, ; expedition to teneriffe, solar constant, , solar spectrum, fixed lines in, - ; maps, , , , , , , ; distribution of energy, , solar system, translation through space, , , , ; development, , , - , ; complexity, soret, solar temperature, south, observations of double stars, ; -inch lens, ; rosse reflector, ; occultation by mars, spectroscopic binaries, - spectrum analysis, defined, ; first experiments, , ; applied to the sun, - , ; to the stars, , , ; kirchhoff's theorem, ; elementary principles, , ; effects on science, , ; radial motion determined by, , ; investigations of comets by, , ; of new stars, , ; of nebulæ, - spencer, position of nebulæ, spitaler, attendants on brooks's comet, spitta, transits of jupiter's satellites, spörer, solar rotation, , ; chromosphere, , stannyan, early notice of chromosphere, star catalogues, , , , , , ; spectroscopic, , , ; photographic, - ; photometric, , star-drift, star-gauging, , , star-maps, , , , , , ; photographic, , stars, movements, , , , , , ; radial, , , ; comparative brightness, , , , , ; distances, - , - ; chemistry, , , ; spectroscopic orders, ; colours, ; development, - ; actual magnitudes, ; gregarious, stars, double, physical connection surmised, ; proved, , ; masses, , ; catalogues, , , , , , ; orbits, , ; discoveries, , , , , , ; photographs, ; evolution, stars, gaseous, - stars, temporary, , - stars, variable, early discoveries, ; eta carinæ, , , ; sun-spot analogy, , ; spectra, ; algol class, , ; catalogues, , stefan, law of cooling, steinheil, stellar photometry, ; silvered glass reflectors, stewart, balfour, kirchhoff's principle, _note_; solar investigations, , stewart, matthew, solar distance by lunar theory, stokes, prevision of spectrum analysis, stone, e. j., reversal of fraunhofer spectrum, ; distance of the sun, , , ; transit of venus, ; cape catalogue, ; proper motions, stone, o., star catalogues, ; measures of double stars, stoney, carbon in photosphere, ; dynamical theory of planetary atmospheres, ; perturbations of leonids, ; status of red stars, stratonoff, star counts in pleiades, stroobant, satellite of venus, struve, f. g. w., stellar parallax, ; career and investigations, - ; occultation by halley's comet, ; russo-scandinavian arc, , struve, ludwig, solar translation, struve, otto, parallax of eta cassiopeiæ, ; solar velocity, ; his father's successor at pulkowa, ; eclipse of , , ; neptune's satellite, ; research on saturn's rings, , ; variable nebula, stumpe, solar translation, sun, herschel's theory, - , , ; atmospheric circulation, , ; chemical composition, , - ; mode of rotation, , ; kirchhoff's theory, ; faye's, - ; convection currents in, , , ; dissociation, , - ; luminous outbursts, - ; explosions, ; heat emission, , , , , , ; temperature, - , ; problem of distance, ; results from transits, , , , ; from oppositions of mars, , ; from light-velocity, , ; from measurements of minor planets, ; concluded value, ; maintenance of heat supply, - ; past and future duration, sun-spots, speculations regarding, , ; wilson's demonstration, , ; distribution, , , ; cyclonic aspect, , , , ; periodicity, , , , ; magnetic relations, , , ; meteorological, , ; auroral, , , , ; photographs, , ; level, ; spectra, , , ; volcanic hypothesis, ; lockyer's rationale, ; planetary influence, ; relation to jovian markings, swan, chromosphere, ; sodium line, swift, e., discovery of a comet, swift, l., fallacious glimpse of vulcan, , ; discovery of a comet, tacchini, eclipse of , ; white prominences, ; prominences and chromosphere, , ; spectrum of venus, talbot, fox, spectrum analysis, ; spectroscopic method of determining stellar orbits, tarde, nature of sun-spots, taylor, eclipse expedition, ; spectrum of uranus, ; achromatic lenses, tebbutt, comets discovered by, , ; comet of , telescopes, achromatic, , , telescopes, equatoreal, , , telescopes, reflecting, short's, , , , ; herschel's, , - ; lassell's, , , ; varieties of construction, , ; rosse's, - , ; common's, , , telescopes, refracting, fraunhofer's, , , ; clark's, , , , , ; grubb's, , ; with bent and horizontal mountings, - tempel, red spot on jupiter, ; comet discoveries, ; cometary observations, , ; andromeda nebula, ; discovery of merope nebula, temperature, of the sun, - , ; of the moon, , ; of space, ; on mars, tennant, eclipse observations, , , terby, surface of mars, , , ; secondary tail of comet, thalén, basic lines, ; map of solar spectrum, ; solar elements, thollon, line-displacements by motion, , ; atlas of solar spectrum, , ; lunar atmospheric absorption, thome, comet discovered by, thomson, sir william (lord kelvin), solar chemistry, ; magnetic influence of the sun, ; tidal strains, ; rotation of the earth, ; dynamical theory of solar heat, , thraen, period of wells's comet, tidal friction, effects on moon's rotation, , , ; month lengthened by, , ; influence on planets, - ; on development of binary systems, tietjen, asteroidal orbits, tisserand, capture of comets, ; lunar acceleration, ; revolutions of neptune's satellite, ; stationary radiants, ; perturbations of algol, ; director of paris observatory, titius, law of planetary intervals, , , todd, eclipse of , ; solar distance, , ; trans-neptunian planet, tornaghi, halo round venus, transit instrument, trépied, reversal of fraunhofer spectrum, troughton, method of graduation, trouvelot, veiled spots, ; chromosphere in , ; intra-mercurian planets, , ; observations of prominences, , , ; of mercury, , ; rotation of venus, ; red spot on jupiter, trowbridge and hutchins, carbon in sun, tschermak, origin of meteorites, tupman, transit expedition, ; results, turner, polariscopic coronal photography, ; employment of coelostat, , ; stationary radiants, ulloa, eclipse of , united states, observatories founded in, , uranus, discovery, , , ; unexplained disturbances, , , ; satellites, , ; equatoreal markings, , ; spectrum, , ; retrograde rotation, , , valerius, darkening of sun's limb, vassenius, description of prominences, venus, transits, , , ; of , - ; of , , ; atmosphere, , , ; mountains, , ; spectrum, ; albedo, ; ashen light, , ; pseudo-satellite, ; effects upon, of solar tidal friction, very, temperature of sun, ; lunar heat, vesta, discovery, , ; diameter, ; spectrum, vicaire, solar temperature, vico, comet discovered by, ; rotation of venus, ; cytherean mountain, violle, solar temperature, , ; solar constant, vogel, h. c., solar rotation, ; solar atmospheric absorption, , ; spectrum of mercury, ; of venus, ; of vesta, ; of jupiter, ; of jupiter's satellites, ; of uranus, ; rotation of venus, ; ashen light, ; intrinsic light of jupiter, ; cometary spectra, , , , ; carbon in stars, ; stellar development, , ; spectrum of gamma cassiopeiæ, ; of nova cygni, ; of nova andromedæ, ; spectroscopic star catalogue, ; radial motion of sirius, ; period of mizar, ; eclipses of algol, ; components of nova aurigæ, ; spectrographic determinations of radial motion, , vogel, h. w., spectrum of hydrogen, _note_, vulcan, existence predicted, ; pseudo-discoveries, , wadsworth, coronal photography, ward, nova andromedæ, waterston, solar temperature, ; meteoric infalls, watson, fallacious observations of vulcan, , ; asteroidal discoveries, webb, comet of , weber, baily's beads, ; illusory transit of vulcan, weinek, study of lunar photographs, weiss, comets and meteors, , wells, comet discovered by, wesley, drawings of corona, wheatstone, spectrum of electric arc, ; method of ascertaining light-velocity, whewell, stars and nebulæ, williams, a. stanley, canals of mars, ; markings on jupiter, , ; rotation, ; nova persei, wilsing, solar rotation from faculæ, ; density of the earth, ; system of, cygni, wilson, alexander, perspective effects in sun-spots, , wilson, h. c., red spot on jupiter, ; compression of uranus, ; exterior nebulosities of pleiades, wilson, w. e., solar temperature, , ; ultra-neptunian planets, winnecke, comet discovered by, ; distance of the sun, ; donati's comet, , wisniewski, last glimpse of comet, witt, discovery of eros, wolf, c., objections to faye's cosmogony, ; origin of phobos, wolf, max, photographic discoveries of minor planets, , ; nova andromedæ, ; nova aurigæ, ; nebula near nova persei, ; photographic nebular survey, ; galactic nebulosity, wolf, r., sun-spot and magnetic periodicity, , , ; analogy of variable stars, , ; auroræ, ; suspicious transits, wollaston, ratio of moonlight to sunlight, ; flame spectra, ; lines in solar spectrum, woods, coronal photography, , ; cape durchmusterung, wrangel, auroræ and meteors, wright, g. f., ice age in north america, wright, thomas, theory of milky way, ; structure of saturn's rings, wright, w. h., polarisation of cometary light, ; spectrum of nebulæ, yerkes, donation of a telescope, young, miss anne, nebular hypothesis, young, c. a., spectrum of sun-spots, ; origin, ; spectrum of corona, , ; detection of reversing layer, , ; prominences and chromosphere, - , ; photograph of a prominence, ; spectroscopic measurement of sun's rotation, ; solar cyclones and explosions, , ; basic lines, ; spectrum of venus, ; red spot on jupiter, ; observations of uranus, , ; andromedes of , ; spectrum of tebbutt's comet, ; of nova andromedæ, young, thomas, absorption spectra, zach, baron von, promotion of astronomy, , , ; baily's beads, ; search for missing planet, ; rediscovery of ceres, ; use of a heliostat, zantedeschi, lines in solar spectrum, ; lunar radiation, zenger, observations on venus, , zenker, cometary tails, zezioli, observation of andromedes, zodiacal light, relation to medium of space, ; to solar corona, ; meteoric constitution, zöllner, electrical theory of comets, , , , ; solar constitution, ; observations of prominences, , ; reversion spectroscope, ; solar temperature, ; mercurian phases, ; albedo of venus, ; of jupiter, ; of saturn, ; of uranus, ; condition of venus, ; of great planets, ; jovian markings, ; ages of stars, ; polarising photometer, , the end billing and sons, ltd., printers, guildford the system of the stars by agnes m. clerke hon. member of the royal astronomical society; author of "history of astronomy during the nineteenth century" and "problems in astrophysics" _second edition_ _demy vo. cloth. with many illustrations. *price s. net.*_ (post free, price * s. d.*) from the preface fifteen years have elapsed since the original publication of the present work; and fifteen years count as a long spell of time where sidereal research is in question. in preparing the second edition, accordingly, i have introduced extensive modifications. considerable sections of the book have been recast, and all have been thoroughly revised. new chapters have been inserted, old ones have been in large part suppressed. drastic measures of reform have, in short, been adopted, with results that certainly import progress and (it is hoped) constitute improvements. most of the illustrations are entirely new; and i am under great obligations for the use of valuable photographs and drawings, among others, to sir. david gill, f.r.s., to professor hale and the university press of chicago, to the rev. w. sidgreaves, s.j., to professors e. c. pickering, campbell, barnard, and frost, and to dr. max wolf of heidelberg. "the work was admirable from the first, imparting the best knowledge of a decade and a half ago; now it retains its high quality by incorporating the newer knowledge."--_the guardian_. "it has the remarkable feature of combining extraordinary profusion of precise information with an elegance of literary style quite unusual in scientific authors."--_the academy_. published by adam & charles black . soho square . london by the same author problems in astrophysics demy vo. pages. cloth. containing illustrations in the text and plates price s. net (post free, price s. d.) note "this is emphatically a "new century" book. it aims at stimulating progress along lines carefully marked out as immediately practicable. the same author's "history of astronomy" is a survey of the past; "problems in astrophysics" looks to the future. what we already know is regarded in it as means to the end of augmenting knowledge. astrophysics is a science still at the outset of a magnificent career. its ways are beset with claimants for its attention. there is often much difficulty in choosing between them, yet rapidity of progress depends upon prudence in selection. many hints for its guidance are accordingly offered in the present work, which deals, so far as possible, with answerable questions. it should, then, find its way into the hands of every astronomer who desires to keep up with the drift of thought, and to be informed of the prospects of work and discovery in the various departments of research connected with the physics of the heavenly bodies." *some press opinions* "the book shows every sign of profound and careful study, and the sense of scientific imagination, which is one of the greatest means of independent discovery."--_st. james's gazette._ "the book is written with all the charm that has characterised the authoress's previous volumes, and contains a wealth of information and suggestion for work yet to be accomplished which will appeal to all who are interested in the problems of the universe."--_daily telegraph._ "we feel that miss clerke has earned, and will surely receive, the admiration and gratitude of astronomers for this new proof of her devotion to their science."--_the times._ modern cosmogonies crown vo. bound in cloth. price s. d. net (post free, price s. d.) note this volume contains a series of brief and attractive studies on current theories regarding the origin and history of the visible universe. the difficulties besetting cosmical doctrines of evolution are pointed out in them, and the expedients are described by which those difficulties have been met, though not wholly overcome. the widened possibilities connected with the new science of radiology, the unification of the physical forces that may ensue upon further discoveries concerning electrical action, the function in the world of the impalpable ether, the nature of gravity, are in turn discussed or adverted to; while the final chapter takes into consideration the crowning problem of life. "these sixteen chapters are short studies on modern theories about the origin and mystery of the universe, by an astronomer whose writings have done much to help and popularise that science."--_the times._ published by adam & charles black . soho square . london * * * * * transcriber's notes: original page line original text left as is (sic) --- ---------- -------------------------------------------------- the search for it, through confessedly scarcely the first description are tranquil page line original text replaced with --- ---------- ----------------------- --------------------------- footnote xviiie xviii^e (the e is superscript) byeways byways concentation concentration is appears from it appears from appearances seem by him appearances seen by him ecole militaire École militaire forgotton forgotten footnote / / (confirmed by looking up reference quoted) phenenoma phenomena bredikhin brédikhine identifiying identifying terrestial terrestrial appearence appearance bloodvessel blood vessel angström Ångström undimished undiminished sympton symptom familar familiar photograpic photographic by which i structure by which its structure bredikhine brédikhine stata strata - of its orbit hours of its orbit in hours seconds. seconds garden at its seasons garden as its seasons throngh through oparator operator recognised. in a recognised in a footnote applie applied the gaseous fields o the gaseous fields of relatiouship relationship footnote optice optics ofter some years after some years footnote (two references given, (split into two footnotes, within a single footnote. and corrected references in the text footnote in the text) used twice) conclusion of a conclusion of a spectographically spectrographically spectographic spectrographic lyrae lyræ index wolf, r., sun-spot and wolf, r., sun-spot and magnetic periodicity, magnetic periodicity, , , ; , , ; generously made available by the internet archive.) university of nebraska studies in language, literature and criticism number astronomical lore in chaucer by florence m. grimm, a. m. _assistant in the university of nebraska library_ editorial committee louise pound, ph. d., department of english h. b. alexander, ph. d., department of philosophy f. w. sanford, a. b., department of latin. lincoln contents i. astronomy in the middle ages ii. chaucer's scientific knowledge iii. chaucer's cosmology iv. chaucer's astronomy v. astrological lore in chaucer appendix astronomical lore in chaucer i astronomy in the middle ages the conspicuousness of astronomical lore in the poetry of chaucer is due to its importance in the life of his century. in the mediaeval period, astronomy (or 'astrology,' for the two names were used indifferently to cover the same subject) was one of the vital interests of men. the ordinary man of the middle ages knew much more than do most men to-day about the phenomena of the heavens; conveniences such as clocks, almanacs, and charts representing celestial phenomena were rare, and direct observations of the apparent movements and the relative positions of the heavenly bodies were necessary for the regulation of man's daily occupations. furthermore, the belief in a geocentric system of the universe, which in chaucer's century was almost universally accepted, was of vast significance in man's way of thinking. accepting this view, all the heavenly bodies seemed to have been created for the sole benefit of man, inhabiting the central position in the universe; their movements, always with reference to the earth as a center, brought to man light, heat, changes of season--all the conditions that made human life possible on the earth. not only did the man of the middle ages see in the regular movements of the celestial spheres the instruments by which god granted him physical existence, but in the various aspects of heavenly phenomena he saw the governing principles of his moral life. the arrangement of the heavenly bodies with regard to one another at various times was supposed to exert undoubted power over the course of terrestrial events. each planet was thought to have special attributes and a special influence over men's lives. venus was the planet of love, mars, of war and hostility, the sun, of power and honor, and so forth. each was mysteriously connected with a certain color, with a metal, too, the alchemists said, and each had special power over some organ of the human body. the planet's influence was believed to vary greatly according to its position in the heavens, so that to determine a man's destiny accurately it was necessary to consider the aspect of the whole heavens, especially at the moment of his birth, but also at other times. this was called "casting the horoscope" and was regarded as of great importance in enabling a man to guard against threatening perils or bad tendencies, and to make the best use of favorable opportunities. it is not astonishing, then, that the great monuments of literature in the mediaeval period and even much later are filled with astronomical and astrological allusions; for these are but reflections of vital human interests of the times. the greatest poetical work of the middle ages, dante's _divina commedia_, is rich in astronomical lore, and its dramatic action is projected against a cosmographical background reflecting the view of dante's contemporaries as to the structure of the world. milton, writing in the seventeenth century, bases the cosmology of his _paradise lost_ in the main on the ptolemaic system, but makes adam and the archangel raphael discuss the relative merits of this system and the heliocentric view of the universe. the latter had been brought forth by copernicus a century earlier, but even in milton's day had not yet succeeded in supplanting the old geocentric cosmology. the view of the universe which we find reflected in chaucer's poetry is chiefly based on the ptolemaic system of astronomy, though it shows traces of very much more primitive cosmological ideas. the ptolemaic system owes its name to the famous alexandrian astronomer of the second century a. d., claudius ptolemy, but is based largely on the works and discoveries of the earlier greek philosophers and astronomers, especially eudoxus, hipparchus, and aristarchus, whose investigations ptolemy compiled and, along some lines, extended. ptolemaic astronomy was a purely geometrical or mathematical system which represented the observed movements and relative positions of the heavenly bodies so accurately that calculations as to their positions at any given time could be based upon it. ptolemy agreed with his contemporaries in the opinion that to assign causes for the celestial movements was outside the sphere of the astronomer. this was a proper field of philosophy; and the decisions of philosophers, especially those of aristotle, were regarded as final, and their teaching as the basis upon which observed phenomena should be described. according to the ptolemaic system the earth is a motionless sphere fixed at the center of the universe. it can have no motion, for there must be some fixed point in the universe to which all the motions of the heavenly bodies may be referred; if the earth had motion, it was argued, this would be proportionate to the great mass of the earth and would cause objects and animals to fly off into the air and be left behind. ptolemy believed this reason sufficient to make untenable the idea of a rotatory motion of the earth, although he was fully aware that to suppose such a motion of the earth would simplify exceedingly the representations of the celestial movements. it did not occur to him that to suppose the earth's atmosphere to participate in its motion would obviate this difficulty. the earth was but a point in comparison with the immense sphere to which the stars were attached and which revolved about the earth once in every twenty-four hours, imparting its motion to sun, moon, and planets, thus causing day and night and the rising and setting of the heavenly bodies. the irregular motions of the planets were accounted for by supposing them to move on circles of small spheres called 'epicycles', the centres of which moved around the 'deferents', or circles of large spheres which carried the planets in courses concentric to the star sphere. by giving each of the planets an epicycle and deferent of the proper relative size and velocity the varied oscillations of the planets, as far as they could be followed by means of the simple instruments then in use, were almost perfectly accounted for. though it was a purely mathematical system which only attempted to give a basis for computing celestial motions, ptolemaic astronomy is of great importance historically as it remained the foundation of theoretical astronomy for more than years. throughout the long dark centuries of the middle ages it survived in the studies of the retired students of the monasteries and of the few exceptionally enlightened men who still had some regard for pagan learning in the days when many of the church fathers denounced it as heretical. ptolemy was the last of the great original greek astronomers. the alexandrian school produced, after him, only copyists and commentators, and the theoretical astronomy of the greeks, so highly perfected in ptolemy's _almagest_, was for many centuries almost entirely neglected. the roman state gave no encouragement to the study of theoretical astronomy and produced no new school of astronomy. although it was the fashion for a roman to have a smattering of greek astronomy, and famous latin authors like cicero, seneca, strabo and pliny wrote on astronomy, yet the romans cared little for original investigations and contributed nothing new to the science. the romans, however, appreciated the value of astronomy in measuring time, and applied to the alexandrian school to satisfy their practical need for a calendar. what julius caesar obtained from the alexandrian sosigenes, he greatly improved and gave to the empire, as the calendar which, with the exception of the slight change made by gregory xiii, we still use. the pseudo-astronomical science of astrology, or the so-called 'judicial astronomy' was pursued during the roman empire and throughout the middle ages with much greater zeal than theoretical astronomy. the interest in astrology, to be sure, encouraged the study of observational astronomy to a certain extent; for the casting of horoscopes to foretell destinies required that the heavenly bodies be observed and methods of calculating their positions at any time or place be known. but there was no desire to inquire into the underlying laws of the celestial motions or to investigate the real nature of the heavenly phenomena. if the roman state did not encourage astronomy, the roman church positively discouraged it. the bible became and long remained the sole authority recognized by the church fathers as to the constitution of the universe. by many of the patristics ptolemaic astronomy was despised; not because it did not describe accurately the observed phenomena of the heavens, for it did this in a way that could scarcely have been improved upon with the facilities for observation then available; and not because it was founded upon the false assumption that the earth is the motionless center of the universe about which all heavenly bodies revolve; but because there was no authority in scripture for such a system, and it could not possibly be made consistent with the cosmology of genesis. allegorical descriptions of the universe based on the scriptures held almost complete sway over the mediaeval mind. the whole universe was represented allegorically by the tabernacle and its furniture. the earth was flat and rectangular like the table of shew bread, and surrounded on all four sides by the ocean. the walls of heaven beyond this supported the firmament shaped like a half-cylinder. angels moved the sun, moon, and stars across the firmament and let down rain through its windows from the expanse of water above. by no means all of the early church fathers were wholly without appreciation of the fruits of greek astronomical science. origen and clement of alexandria, while believing in the scriptural allegories, tried to reconcile them with the results of pagan learning. in the west, ambrose of milan and later augustine, were at least not opposed to the idea of the earth's sphericity, and of the existence of antipodes, although they could not get away from the queer notion of the waters above the firmament. a few enlightened students like philoponus of alexandria, isidore of seville, the venerable bede, and irish scholars like fergil and dicuil, studied the greek philosophers and accepted some of the pagan scientific teachings. fortunately the study of those ancient latin writers whose works had preserved some of the astronomy of the greeks had taken firm root among the patient scholars of the monasteries, and slowly but steadily the geocentric system of cosmology was making its way back into the realm of generally accepted fact, so that by the ninth century it was the system adopted by nearly all scholars. about the year began the impetus to learning which culminated in the great revival of the renaissance. one cause of this intellectual awakening was the contact of europe with arab culture through the crusades and through the saracens in sicily and the moors in spain. the arabian influence resulted in an increased sense of the importance of astronomy and astrology; for, while the scholars of the christian world had been devising allegorical representations of the world based on sacred literature, the arabian scholars had been delving into greek science, translating ptolemy and aristotle, and trying to make improvements upon ptolemaic astronomy. the spheres of the planets, which ptolemy had almost certainly regarded as purely symbolical, the arabs conceived as having concrete existence. this made it necessary to add a ninth sphere to the eight mentioned by ptolemy; for it was thought sufficient that the eighth sphere should carry the stars and give them their slow movement of precession from west to east. this ninth sphere was the outermost of all and it originated the "prime motion" by communicating to all the inner spheres its diurnal revolution from east to west. in mediaeval astronomy it came to be known as the _primum mobile_ or "first movable," while a tenth and motionless sphere was added as the abode of god and redeemed souls. the sun and moon were included among the planets, which revolved about the earth in the order moon, mercury, venus, sun, mars, jupiter, saturn. at first the astronomy taught in the universities was based on latin translations of arabic commentaries and paraphrases of aristotle, which had made their way into europe through the moors in spain. for several centuries aristotle represented in the eyes of most scholastics "the last possibility of wisdom and learning." but by the middle of the thirteenth century ptolemy began to be rediscovered. the ptolemaic system of planetary motions was briefly described in a handbook compiled by john halifax of holywood, better known as sacrobosco. roger bacon wrote on the spheres, the use of the astrolabe, and astrology, following ptolemy in his general ideas about the universe. the great mediaeval scholar and philosopher, thomas aquinas, was also familiar with the ptolemaic system; but to most of the men of the thirteenth century ptolemy's works remained quite unknown. the real revival of greek astronomy took place in the fourteenth century when scholars began to realize that new work in astronomy must be preceded by a thorough knowledge of the astronomy of the alexandrian school as exhibited in the _syntaxis_ of ptolemy. it was then that greek and latin manuscripts of works on astronomy began to be eagerly sought for and deciphered, and a firm foundation constructed for the revival of theoretical astronomy. ii chaucer's scientific knowledge it was in the fourteenth century that chaucer lived and wrote, and his interest in astronomical lore is, therefore, not surprising. although the theories of astronomy current in chaucer's century have been made untenable by the _de revolutionibus orbium_ of copernicus, and by kepler's discovery of the laws of planetary motion; although the inaccurate and unsatisfactory methods of astronomical investigation then in use have been supplanted by the better methods made possible through galileo's invention of the telescope and through the modern use of spectrum analysis; yet, of all scientific subjects, the astronomy of that period could most nearly lay claim to the name of science according to the present acceptation of the term. for, as we have seen, the interest in astrology during the middle ages had fostered the study of observational astronomy, and this in turn had furnished the science a basis of fact and observation far surpassing in detail and accuracy that of any other subject. practically all of chaucer's writings contain some reference to the movements and relative positions of the heavenly bodies, and to their influence on human and mundane affairs, and in some of his works, especially the treatise on _the astrolabe_, a very technical and detailed knowledge of astronomical and astrological lore is displayed. there is every reason to suppose that, so far as it satisfied his purposes, chaucer had made himself familiar with the whole literature of astronomical science. his familiarity with ptolemaic astronomy is shown in his writings both by specific mention[ ] of the name of ptolemy and his _syntaxis_, commonly known as the 'almagest,' and by many more general astronomical references. even more convincing evidence of chaucer's knowledge of the scientific literature of his time is given in his _treatise on the astrolabe_. according to skeat, part i and at least two-thirds of part ii are taken, with some expansion and alteration, from a work on the astrolabe by messahala[ ], called, in the latin translation which chaucer used, "compositio et operatio astrolabie." this work may have been ultimately derived from a sanskrit copy, but from chaucer's own words in the _prologue to the astrolabe_[ ] it is clear that he made use of the latin work. the rest of part ii may have been derived from some general compendium of astronomical and astrological knowledge, or from some other of the treatises on the astrolabe which chaucer says were common in his time.[ ] other sources mentioned by chaucer in _the astrolabe_ are the calendars of john some and nicholas lynne, carmelite friars who wrote calendars constructed for the meridian of oxford[ ]; and of the arabian astronomer abdilazi alkabucius.[ ] in _the frankeleyns tale_ chaucer mentions the tabulae toletanae,[ ] a set of tables composed by order of alphonso x, king of castile, and so called because they were adapted to the city of toledo. works which served chaucer not as sources of information on scientific subjects but as models for the treatment of astronomical lore in literature were the _de consolatione philosophiae_ of boethius, which chaucer translated and often made use of in his poetry; and the works of dante, whose influence on chaucer, probably considerable, has been pointed out by several writers, notably rambeau[ ] who discusses the parallels between _the hous of fame_ and the _divina commedia_. iii chaucer's cosmology chaucer wrote no poetical work having a cosmographical background as completely set forth as is that in dante's _divine comedy_ or that in milton's _paradise lost_. although his cosmological references are often incidental they are not introduced in a pedantic manner. whenever they are not parts of interpolations from other writers his use of them is due to their intimate relation to the life his poetry portrays or to his appreciation of their poetic value. when chaucer says, for example, that the sun has grown old and shines in capricorn with a paler light than is his wont, he is not using a merely conventional device for showing that winter has come, but is expressing this fact in truly poetic manner and in words quite comprehensible to the men of his day, who were accustomed to think of time relations in terms of heavenly phenomena. popular and scientific views of the universe in chaucer's century were by no means the same. the untaught man doubtless still thought of the earth as being flat, as it appears to be, as bounded by the waters of the ocean, and as covered by a dome-like material firmament through which the waters above sometimes came as rain; while, as we have seen, by the fourteenth century among scholars the geocentric system of astronomy was firmly established and the spheres and epicycles of ptolemy were becoming more widely known. it is the view held by the educated men of his century that chaucer's poetry chiefly reflects. . _the celestial spheres and their movements_ when we read chaucer we are transported into a world in which the heavenly bodies and their movements seem to bear a more intimate relation to human life than they do in the world in which we live. the thought of the revolving spheres carrying sun, moon, and planets, regulating light and heat on the earth, and exercising a mysterious influence over terrestrial events and human destiny was a sublime conception and one that naturally appealed to the imagination of a poet. chaucer was impressed alike by the vastness of the revolving spheres in comparison to the earth's smallness, by their orderly arrangement, and by the unceasing regularity of their appearance which seemed to show that they should eternally abide. in the _parlement of foules_ he interpolates a passage from cicero's _somnium scipionis_ in which africanus appears to the sleeping scipio, points out to him the insignificance of our little earth when compared with the vastness of the heavens and then admonishes him to regard the things of this world as of little importance when compared with the joys of the heavenly life to come.[ ] "than shewed he him the litel erthe, that heer is, at regard of the hevenes quantite; and after shewed he him the nyne speres." the regular arrangement of the planetary spheres clings often to the poet's fancy and he makes many allusions to their order in the heavens. he speaks of mars as "the thridde hevenes lord above"[ ] and of venus as presiding over the "fifte cercle."[ ] in _troilus and criseyde_ the poet invokes venus as the adorning light of the third heaven.[ ] "o blisful light, of which the bemes clere adorneth al the thridde hevene faire!"[ ] mediaeval astronomers as we have seen, imagined nine spheres, each of the seven innermost carrying with it one of the planets in the order mentioned below; the eighth sphere was that of the fixed stars, and to account for the precession of the equinoxes, men supposed it to have a slow motion from west to east, round the axis of the zodiac; the ninth or outermost sphere they called the _primum mobile_, or the sphere of first motion, and supposed it to revolve daily from east to west, carrying all the other spheres with it. the thought of the two outer spheres, the _primum mobile_, whirling along with it all the inner spheres, and the firmament, bearing hosts of bright stars, seems to have appealed strongly to the poet's imagination. in the _tale of the man of lawe_ the _primum mobile_ is described as crowding and hurling in diurnal revolution from east to west all the spheres that would naturally follow the slow course of the zodiac from west to east.[ ] elsewhere the _primum mobile_ is called the "whele that bereth the sterres" and is said to turn the heavens with a "ravisshing sweigh:" "o thou maker of the whele that bereth the sterres, which that art y-fastned to thy perdurable chayer, and tornest the hevene with a ravisshing sweigh, and constreinest the sterres to suffren thy lawe;"[ ] the firmament, which in chaucer is not restricted to the eighth sphere but generally refers to the whole expanse of the heavens, is many times mentioned by chaucer; and its appearance on clear or cloudy nights, its changing aspects before an impending storm or with the coming of dawn, beautifully described.[ ] . _the harmony of the spheres_ some of the cosmological ideas reflected in chaucer's writings can be traced back to systems older than the ptolemaic. the beautiful fancy that the universe is governed by harmony had its origin in the philosophy of the pythagoreans in the fourth century b. c., and continued to appeal to men's imagination until the end of the middle ages. it was thought that the distances of the planetary spheres from one another correspond to the intervals of a musical scale and that each sphere as it revolves sounds one note of the scale. when asked why men could not hear the celestial harmony, the pythagoreans said: a blacksmith is deaf to the continuous, regular beat of the hammers in his shop; so we are deaf to the music which the spheres have been sending forth from eternity. in ancient and mediaeval cosmology it was only the seven spheres of the planets that were generally supposed to participate in this celestial music; but the poets have taken liberties with this idea and have given it to us in forms suiting their own fancies. milton bids all the celestial spheres join in the heavenly melody: "ring out, ye crystal spheres, once bless our human ears, if ye have power to touch our senses so; and let your silver chime move in melodious time, and let the base of heaven's deep organ blow; and with your ninefold harmony, make up full consort to the angelic symphony."[ ] shakespeare lets every orb of the heavens send forth its note as it moves: "there's not the smallest orb which thou behold'st but in his motion like an angel sings, still quiring to the young-eyed cherubins;"[ ] chaucer, too, makes all nine spheres participate: "and after that the melodye herde he that cometh of thilke speres thryes three, that welle is of musyke and melodye in this world heer, and cause of armonye."[ ] only in unusual circumstances can the music of the spheres be heard by mortal ears. in the lines just quoted the celestial melody is heard during a dream or vision. in _troilus and criseyde_, after troilus' death his spirit is borne aloft to heaven whence he beholds the celestial orbs and hears the melody sent forth as they revolve: "and ther he saugh, with ful avysement, the erratik sterres, herkeninge armonye with sownes fulle of hevenish melodye."[ ] . _the cardinal points and the regions of the world_ more primitive in origin than the harmony of the spheres are references to the four elements, to the divisions of the world, and to the cardinal points or quarters of the earth. of these, probably the most primitive is the last. the idea of four cardinal points, the "before," the "behind," the "right," and the "left," later given the names north, south, east, and west, appears among peoples in their very earliest stages of civilization, and because of its great usefulness has remained and probably will remain throughout the history of the human race. only one of chaucer's many references to the cardinal points need be mentioned. in the _man of lawes tale_ (b. ff.) the cardinal points are first suggested by an allusion to the four 'spirits of tempest,' which were supposed to have their respective abodes in the four quarters of earth, and then specifically named in the lines following: "who bad the foure spirits of tempest, that power han tanoyen land and see, 'bothe north and south, and also west and est, anoyeth neither see, ne land, ne tree?'" of almost equal antiquity are ideas of the universe as a threefold world having heaven above, earth below, and a region of darkness and gloom beneath the earth. chaucer usually speaks of the threefold world, the "tryne compas," as comprising heaven, earth and sea. thus in the _knightes tale_:[ ] "'o chaste goddesse of the wodes grene, to whom bothe hevene and erthe and see is sene, quene of the regne of pluto derk and lowe,'" fame's palace is said to stand midway between heaven, earth and sea: "hir paleys stant, as i shal seye, right even in middes of the weye betwixen hevene, erthe, and see;"[ ] again in _the seconde nonnes tale_, the name 'tryne compas' is used of the threefold world and the three regions are mentioned: "that of the tryne compas lord and gyde is, whom erthe and see and heven, out of relees, ay herien;"[ ] . _heaven, hell and purgatory_ in mediaeval cosmology ideas of heaven, hell, and purgatory, as more or less definitely located regions where the spirits of the dead were either rewarded or punished eternally, or were purged of their earthly sins in hope of future blessedness, play an important part. according to dante's poetic conception hell was a conical shaped pit whose apex reached to the center of the earth, purgatory was a mountain on the earth's surface on the summit of which was located the garden of eden or the earthly paradise, and heaven was a motionless region beyond space and time, the motionless sphere outside of the _primum mobile_, called the empyrean. chaucer's allusions to heaven, hell and purgatory are frequent but chiefly incidental and give no such definite idea of their location as we find in the _divine comedy_. the nearest chaucer comes to indicating the place of heaven is in _the parlement of foules_, - , where africanus speaks of heaven and then points to the galaxy: "and rightful folk shal go, after they dye, to heven; and shewed him the galaxye." chaucer describes heaven as "swift and round and burning", thus to some extent departing from the conception of it usually held in his time: "and right so as thise philosophres wryte that heven is swift and round and eek brenninge, right so was fayre cecilie the whyte."[ ] in using the terms "swift and round" chaucer must have been thinking of the _primum mobile_ which, as we have seen, was thought to have a swift diurnal motion from east to west. his use of the epithet "burning" is in conformity with the mediaeval conception of the empyrean, or heaven of pure light as it is described by dante. chaucer does not describe the form and location of hell as definitely as does dante, but the idea which he presents of it by incidental allusions, whether or not this was the view of it he himself held, is practically the one commonly held in his day. that hell is located somewhere within the depths of the earth is suggested in the _knightes tale_;[ ]-- "his felawe wente and soghte him down in helle;" and in the _man of lawes tale_;[ ] "o serpent under femininitee, lyk to the serpent depe in helle y-bounde," in the _persones tale_ hell is described as a horrible pit to which no natural light penetrates, filled with smoking flames and presided over by devils who await an opportunity to draw sinful souls to their punishment.[ ] elsewhere in the same tale the parson describes hell as a region of disorder, the only place in the world not subject to the universal laws of nature, and attributes this idea of it to job: "and eek iob seith: that 'in helle is noon ordre of rule.' and al-be-it so that god hath creat alle thinges in right ordre, and no-thing with-outen ordre, but alle thinges been ordeyned and nombred; yet nathelees they that been dampned been no-thing in ordre, ne holden noon ordre."[ ] the word purgatory seldom occurs in a literal sense in chaucer's poetry, but the figurative use of it is frequent. when the wife of bath is relating her experiences in married life she tells us that she was her fourth husband's purgatory.[ ] the old man, ianuarie[ ], contemplating marriage, fears that he may lose hope of heaven hereafter, because he will have his heaven here on earth in the joys of wedded life. his friend iustinus sarcastically tells him that perhaps his wife will be his purgatory, god's instrument of punishment, so that when he dies his soul will skip to heaven quicker than an arrow from the bow. to arcite, released from prison on condition that he never again enter theseus' lands, banishment will be a worse fate than the purgatory of life imprisonment, for then even the sight of emelye will be denied him: "he seyde, 'allas that day that i was born! now is my prison worse than biforn; now is me shape eternally to dwelle noght in purgatorie, but in helle.'"[ ] the idea of purgatory, not as a place definitely located like dante's mount of purgatory, but rather as a period of punishment and probation, is expressed in these lines from _the parlement of foules_ ( - ): "'but brekers of the lawe, soth to seyne, and lecherous folk, after that they be dede, shul alwey whirle aboute therthe in peyne, til many a world be passed, out of drede, and than, for-yeven alle hir wikked dede, than shul they come unto that blisful place, to which to comen god thee sende his grace!'" chaucer uses the idea of paradise for poetical purposes quite as often as that of purgatory. he expresses the highest degree of earthly beauty or joy by comparing it with paradise. criseyde's face is said to be like the image of paradise.[ ] again, in extolling the married life, the poet says that its virtues are such "'that in this world it is a paradys.'"[ ] and later in the same tale, woman is spoken of as "mannes help and his confort, his paradys terrestre and his disport."[ ] when aeneas reaches carthage he "is come to paradys out of the swolow of helle, and thus in ioye remembreth him of his estat in troye."[ ] chaucer mentions paradise several times in its literal sense as the abode of adam and eve before their fall. in the _monkes tale_ we are told that adam held sway over all paradise excepting one tree.[ ] again, the pardoner speaks of the expulsion of adam and eve from paradise: "adam our fader, and his wyf also, fro paradys to labour and to wo were driven for that vyce, it is no drede; for whyl that adam fasted, as i rede, he was in paradys; and whan that he eet of the fruyt defended on the tree, anon he was out-cast to we and peyne."[ ] . _the four elements._ the idea of four elements[ ] has its origin in the attempts of the early greek cosmologists to discover the ultimate principle of reality in the universe. thales reached the conclusion that this principle was water, anaximines, that it was air, and heracleitus, fire, while parmenides supposed two elements, fire or light, subtle and rarefied, and earth or night, dense and heavy. empedocles of agrigentum (about b. c.) assumed as primary elements all four--fire, air, water, and earth--of which each of his predecessors had assumed only one or two. to explain the manifold phenomena of nature he supposed them to be produced by combinations of the elements in different proportions through the attractive and repulsive forces of 'love' and 'discord.' this arbitrary assumption of four elements, first made by empedocles, persisted in the popular imagination throughout the middle ages and is, like other cosmological ideas of antiquity, sometimes reflected in the poetry of the time. the elements in mediaeval cosmology were assigned to a definite region of the universe. being mortal and imperfect they occupied four spheres below the moon, the elemental region or region of imperfection, as distinguished from the ethereal region above the moon. immediately within the sphere of the moon came that of fire, below this the air, then water, and lowest of all the solid sphere of earth. fire being the most ethereal of the elements constantly tends to rise upward, while earth sinks towards the center of the universe. this contrast is a favorite idea with dante, who says in the _paradiso_ i. - : "'wherefore they move to diverse ports o'er the great sea of being, and each one with instinct given it to bear it on. this beareth the fire toward the moon; this is the mover in the hearts of things that die; this doth draw the earth together and unite it.'" elsewhere dante describes the lightning as fleeing its proper place when it strikes the earth: "'but lightning, fleeing its proper site, ne'er darted as dost thou who art returning thither.'"[ ] and again: "'so from this course sometimes departeth the creature that hath power, thus thrust, to swerve to-ward some other part, (even as fire may be seen to dart down from the cloud) if its first rush be wrenched aside to earth by false seeming pleasure.'"[ ] the same thought of the tendency of fire to rise and of earth to sink is found in chaucer's translation of boethius:[ ] "thou bindest the elements by noumbres proporcionables, ... that the fyr, that is purest, ne flee nat over hye, ne that the hevinesse ne drawe nat adoun over-lowe the erthes that ben plounged in the wateres." chaucer does not make specific mention of the spheres of the elements, but he tells us plainly that each element has been assigned its proper region from which it may not escape: "for with that faire cheyne of love he bond the fyr, the eyr, the water, and the lond in certeyn boundes, that they may nat flee;"[ ] the position of the elements in the universe is nevertheless made clear without specific reference to their respective spheres. the spirit of the slain troilus ascends through the spheres to the seventh heaven, leaving behind the elements: "and whan that he was slayn in this manere, his lighte goost ful blisfully is went up to the holownesse of the seventh spere, in converse letinge every element."[ ] "every element" here obviously means the sphere of each element; "holownesse" means concavity and "in convers" means 'on the reverse side.' the meaning of the passage is, then, that troilus' spirit ascends to the concave side of the seventh sphere from which he can look down upon the spheres of the elements, which have their convex surfaces towards him. this passage is of particular interest for the further reason that it shows that even in chaucer's century people still thought of the spheres as having material existence. the place and order of the elements is more definitely suggested in a passage from _boethius_ in which philosophical contemplation is figuratively described as an ascent of thought upward through the spheres: "'i have, forsothe, swifte fetheres that surmounten the heighte of hevene. when the swifte thought hath clothed it-self in the fetheres, it despyseth the hateful erthes, and surmounteth the roundnesse of the grete ayr; and it seeth the cloudes behinde his bak; and passeth the heighte of the region of the fyr, that eschaufeth by the swifte moevinge of the firmament, til that he areyseth him in-to the houses that beren the sterres, and ioyneth his weyes with the sonne phebus, and felawshipeth the wey of the olde colde saturnus.'"[ ] in this passage all the elemental regions except that of water are alluded to and in the order which, in the middle ages, they were supposed to follow. when in the _hous of fame_, chaucer is borne aloft into the heavens by jupiter's eagle, he is reminded of this passage in boethius and alludes to it: "and tho thoughte i upon boece, that writ, 'a thought may flee so hye, with fetheres of philosophye, to passen everich element; and whan he hath so fer y-went, than may be seen, behind his bak, cloud, and al that i of spak.'"[ ] empedocles, as we have seen, taught that the variety in the universe was caused by the binding together of the four elements in different proportions through the harmonizing principle of love, or by their separation through hate, the principle of discord. we find this idea also reflected in chaucer who obviously got it from boethius. love is the organizing principle of the universe; if the force of love should in any wise abate, all things would strive against each other and the universe be transformed into chaos.[ ] the elements were thought to be distinguished from one another by peculiar natures or attributes. thus the nature of fire was _hot_ and _dry_, that of water _cold_ and _moist_, that of air _cold_ and _dry_, and that of earth _hot_ and _moist_.[ ] chaucer alludes to these distinguishing attributes of the elements a number of times, as, for example, in _boethius_, iii.: metre . ff.: "thou bindest the elements by noumbres proporciounables, that the colde thinges mowen acorden with the hote thinges, and the drye thinges with the moiste thinges"; in conclusion it should be said that all creatures occupying the elemental region or realm of imperfection below the moon were thought to have been created not directly by god but by nature as his "vicaire" or deputy, or, in other words, by an inferior agency. chaucer alludes to this in _the parlement of foules_ briefly thus: "nature, the vicaire of thalmyghty lorde, that hoot, cold, hevy, light, (and) moist and dreye hath knit by even noumbre of acorde,"[ ] and more at length in _the phisiciens tale_. chaucer says of the daughter of virginius that nature had formed her of such excellence that she might have said of her creation: "'lo! i, nature, thus can i forme and peynte a creature, whan that me list; who can me countrefete? pigmalion noght, though he ay forge and bete, or grave, or peynte; for i dar wel seyn, apelles, zanzis, sholde werche in veyn, outher to grave or peynte or forge or bete, if they presumed me to countrefete. for he that is the former principal hath maked me his vicaire general, to forme and peynten erthely creaturis right as me list, and ech thing in my cure is under the mone, that may wane and waxe, and for my werk right no-thing wol i axe; my lord and i ben ful of oon accord; i made hir to the worship of my lord.'"[ ] what is of especial interest for our purposes is found in the five lines of this passage beginning "for he that is the former principal," etc. "former principal" means 'creator principal' or the chief creator. god is the chief creator; therefore there must be other or inferior creators. nature is a creator of inferior rank whom god has made his "vicaire" or deputy and whose work it is to create and preside over all things beneath the sphere of the moon. iv chaucer's astronomy chaucer's treatment of astronomical lore in his poetry differs much from his use of it in his prose writings. in poetical allusions to heavenly phenomena, much attention to detail and a pedantic regard for accuracy would be inappropriate. references to astronomy in chaucer's poetry are, as a rule rather brief, specific but not technical, often purely conventional but always truly poetic. there are, indeed, occasional passages in chaucer's poetry showing so detailed a knowledge of observational[ ] astronomy that they would seem astonishing and, to many people, out of place, in modern poetry. they were not so in chaucer's time, when the exigencies of practical life demanded of the ordinary man a knowledge of astronomy far surpassing that possessed by most of our contemporaries. harry bailly in the _introduction to the man of lawes tale_ determines the day of the month and hour of the day by making calculations from the observed position of the sun in the sky, and from the length of shadows, although, says chaucer, "he were not depe expert in lore."[ ] such references to technical details of astronomy as we find in this passage are, however, not common in chaucer's poetry; in his _treatise on the astrolabe_, on the other hand, a professedly scientific work designed to instruct his young son louis in those elements of astronomy and astrology that were necessary for learning the use of the astrolabe, we have sufficient evidence that he was thoroughly familiar with the technical details of the astronomical science of his day. in chaucer's poetry the astronomical references employed are almost wholly of two kinds: references showing the time of day or season of the year at which the events narrated are supposed to take place; and figurative allusions for purposes of illustration or comparison. figurative uses of astronomy in chaucer vary from simple similes as in the _prologue to the canterbury tales_, where the friar's eyes are compared to twinkling stars[ ] to extended allegories like the _compleynt of mars_ in which the myth of venus and mars is related by describing the motions of the planets venus and mars for a certain period during which venus overtakes mars, they are in conjunction[ ] for a short time, and then venus because of her greater apparent velocity leaves mars behind. one of the most magnificent astronomical figures employed by chaucer is in the _hous of fame_. chaucer looks up into the heavens and sees a great golden eagle near the sun, a sight so splendid that men could never have beheld its equal 'unless the heaven had won another sun:' "hit was of golde, and shone so bright, that never saw men such a sighte, but-if the heven hadde y-wonne al newe of golde another sonne; so shoon the egles fethres brighte, and somwhat dounward gan hit lighte."[ ] besides mentioning the heavenly bodies in time references and figurative allusions, chaucer also employs them often in descriptions of day and night, of dawn and twilight, and of the seasons. it is with a poet's joy in the warm spring sun that he writes: "bright was the day, and blew the firmament, phebus of gold his stremes doun hath sent, to gladen every flour with his warmnesse."[ ] and with a poet's delight in the new life and vigor that nature puts forth when spring comes that he writes the lines: "forgeten had the erthe his pore estat of winter, that him naked made and mat, and with his swerd of cold so sore greved; now hath the atempre sonne al that releved that naked was, and clad hit new agayn."[ ] chaucer's astronomical allusions, then, except in the _treatise on the astrolabe_ and in his translation of _boethius de consolatione philosophiae_, in which a philosophical interest in celestial phenomena is displayed, are almost invariably employed with poetic purpose. these poetical allusions to heavenly phenomena, however, together with the more technical and detailed references in chaucer's prose works give evidence of a rather extensive knowledge of astronomy. with all of the important observed movements of the heavenly bodies he was perfectly familiar and it is rather remarkable how many of these he uses in his poetry without giving one the feeling that he is airing his knowledge. . _the sun_ of all the heavenly bodies the one most often mentioned and employed for poetic purposes by chaucer is the sun. chaucer has many epithets for the sun, but speaks of him perhaps most often in the classical manner as phebus or apollo. he is called the "golden tressed phebus"[ ] or the "laurer-crowned phebus;"[ ] and when he makes mars flee from venus' palace he is called the "candel of ielosye."[ ] in the following passage chaucer uses three different epithets for the sun within two lines: "the dayes honour, and the hevenes ye, the nightes fo, al this clepe i the sonne, gan westren faste, and dounward for to wrye, as he that hadde his dayes cours y-ronne;"[ ] sometimes chaucer gives the sun the various accessories with which classical myth had endowed him--the four swift steeds, the rosy chariot and fiery torches: "and phebus with his rosy carte sone gan after that to dresse him up to fare."[ ] "'now am i war that pirous and tho swifte stedes three, which that drawen forth the sonnes char, hath goon some by-path in despyt of me;'"[ ] "phebus, that was comen hastely within the paleys-yates sturdely, with torche in honde, of which the stremes brighte on venus chambre knokkeden ful lighte."[ ] almost always when chaucer wishes to mention the time of day at which the events he is relating take place, he does so by describing the sun's position in the sky or the direction of his motion. we can imagine that chaucer often smiled as he did this, for he sometimes humorously apologizes for his poetical conceits and conventions by expressing his idea immediately afterwards in perfectly plain terms. such is the case in the passage already quoted where chaucer refers to the sun by the epithets "dayes honour," "hevenes ye," and "nightes fo" and then explains them by saying "al this clepe i the sonne;" and in the lines: "til that the brighte sonne loste his hewe; for thorisonte hath reft the sonne his light;" explained by the simple words: "this is as muche to seye as it was night."[ ] thus it is that chaucer's poetic references to the apparent daily motion of the sun about the earth are nearly always simply in the form of allusions to his rising and setting. canacee in the _squieres tale_, (f. ff.) is said to rise at dawn, looking as bright and fresh as the spring sun risen four degrees from the horizon. "up ryseth fresshe canacee hir-selve, as rody and bright as dooth the yonge sonne, that in the ram[ ] is four degrees up-ronne; noon hyer was he, whan she redy was;" many of these references to the rising and setting of the sun might be mentioned, if space permitted, simply for their beauty as poetry. one of the most beautiful is the following: "and fyry phebus ryseth up so brighte, that al the orient laugheth of the lighte, and with his stremes dryeth in the greves the silver dropes, hanging on the leves."[ ] when, in the _canterbury tales_, the manciple has finished his tale, chaucer determines the time by observing the position of the sun and by making calculations from the length of his own shadow: "by that the maunciple hadde his tale al ended, the sonne fro the south lyne was descended so lowe, that he nas nat, to my sighte, degrees nyne and twenty as in highte. foure of the clokke it was tho, as i gesse; for eleven foot, or litel more or lesse, my shadwe was at thilke tyme, as there, of swich feet as my lengthe parted were in six feet equal of porporcioun."[ ] we must not omit mention of the humorous touch with which chaucer, in the mock heroic tale of _chanticleer and the fox_ told by the nun's priest, makes even the rooster determine the time of day by observing the altitude of the sun in the sky: "chauntecleer, in al his pryde, his seven wyves walkyng by his syde, caste up his eyen to the brighte sonne, that in the signe of taurus hadde y-ronne twenty degrees and oon, and somewhat more; and knew by kynde, and by noon other lore, that it was pryme, and crew with blisful stevene. 'the sonne,' he sayde, 'is clomben up on hevene fourty degrees and oon, and more, y-wis.'"[ ] moreover, this remarkable rooster observed that the sun had passed the twenty-first degree in taurus, and we are told elsewhere that he knew each ascension of the equinoctial and crew at each; that is, he crew every hour, as ° of the equinoctial correspond to an hour: "wel sikerer was his crowing in his logge, than is a clokke, or an abbey orlogge. by nature knew he ech ascencioun[ ] of th' equinoxial in thilke toun; for whan degrees fiftene were ascended, thanne crew he, that it mighte nat ben amended."[ ] chaucer announces the approach of evening by describing the position and appearance of the sun more often than any other time of the day. in the _legend of good women_ he speaks of the sun's leaving the south point[ ] of his daily course and approaching the west: "whan that the sonne out of the south gan weste,"[ ] and again of his westward motion in the lines: "and whan that hit is eve, i rene blyve, as sone as ever the sonne ginneth weste,"[ ] elsewhere chaucer refers to the setting of the sun by saying that he has completed his "ark divine" and may no longer remain on the horizon,[ ] or by saying that the 'horizon has bereft the sun of his light.'[ ] chaucer's references to the daily motion of the sun about the earth are apt to sound to us like purely poetical figures, so accustomed are we to refer to the sun, what we know to be the earth's rotatory motion, by speaking of his apparent daily motion thus figuratively as if it were real. chaucer's manner of describing the revolution of the heavenly bodies about the earth and his application of poetic epithets to them are figurative, but the motion itself was meant literally and was believed in by the men of his century, because only the geocentric system of astronomy was then known. if chaucer had been in advance of his century in this respect there would certainly be some hint of the fact in his writings. references in chaucer to the sun's yearly motion are in the same sense literal. the apparent motion of the sun along the ecliptic,[ ] which we know to be caused by the earth's yearly motion in an elliptical orbit around the sun, was then believed to be an actual movement of the sun carried along by his revolving sphere. like the references to the sun's daily movements those that mention his yearly motion along the ecliptic are also usually time references. the season of the year is indicated by defining the sun's position among the signs of the zodiac. the canterbury pilgrims set out on their journey in april when "the yonge sonne hath in the ram his halfe course y-ronne."[ ] in describing the month of may, chaucer does not fail to mention the sun's position in the zodiac: "in may, that moder is of monthes glade, that fresshe floures, blewe, and whyte, and rede, ben quike agayn, that winter dede made, and ful of bawme is fletinge every mede; whan phebus doth his brighte bemes sprede right in the whyte bole, it so bitidde as i shal singe, on mayes day the thridde,"[ ] etc. the effect of the sun's declination in causing change of seasons[ ] is mentioned a number of times in chaucer's poetry. the poet makes a general reference to the fact in a passage of exquisite beauty from _troilus and criseyde_ where he says that the sun has thrice returned to his lofty position in the sky and melted away the snows of winter: "the golden-tressed phebus heighe on-lofte thryes hadde alle with his bemes shene the snowes molte, and zephirus as ofte y-brought ayein the tendre leves grene, sin that the sone of ecuba the quene bigan to love hir first, for whom his sorwe was al, that she departe sholde a-morwe."[ ] more interesting astronomically but of less interest as poetry is his reference to the sun's declination and its effect on the seasons in the _frankeleyns tale_, because here chaucer uses the word 'declination' and states that it is the cause of the seasons. the reference is the beginning of aurelius' prayer to apollo, or the sun: "'apollo, god and governour of every plaunte, herbe, tree and flour, that yevest, after thy declinacioun, to ech of hem his tyme and his sesoun, as thyn herberwe chaungeth lowe or hye;'"[ ] once again in the _frankeleyns tale_ chaucer refers to the sun's declination and the passage of the seasons: "phebus wex old, and hewed lyk latoun,[ ] that in his hote declinacioun shoon as the burned gold with stremes brighte; but now in capricorn adoun he lighte, wher-as he shoon ful pale, i dar wel seyn."[ ] chaucer is here contrasting the sun's appearance in summer and winter. in his hot declination (his greatest northward declination in cancer, about june ) he shines as burnished gold, but when he reaches capricornus, his greatest southward declination (about december ) he appears 'old' and has a dull coppery color, no longer that of brilliant gold. . _the moon_ from those references to the moon that occur in chaucer's poetry alone, it would be impossible to determine just how much he knew of the peculiarities of her apparent movements; for he alludes to the moon's motion and positions much less frequently and with much less detail than to those of the sun. but a passage in the prologue to the _astrolabe_ leaves it without doubt that chaucer was quite familiar with lunar phenomena. in stating what the treatise is to contain, he says of the fourth part: "the whiche ferthe partie in special shal shewen a table of the verray moeving of the mone from houre to houre, every day and in every signe, after thyn almenak; upon which table ther folwith a canon, suffisant to teche as wel the maner of the wyrking of that same conclusioun, as to knowe in oure orizonte with which degree of the zodiac that the mone ariseth in any latitude;"[ ] as a matter of fact the treatise as first contemplated by chaucer was never finished; only the first two parts were written. but chaucer would scarcely have written thus definitely of his plan for the fourth part of the work unless he had had fairly complete knowledge of the phenomena connected with the moon's movements. the moon, in chaucer's imagination, must have occupied rather an insignificant position among the heavenly bodies as far as appealing to his sense of beauty was concerned, for we find in his poetry no descriptions of her appearance that can compare with his descriptions of the sun or even of the stars. he speaks of moonrise in the most general way: "hit fil, upon a night, when that the mone up-reysed had her light, this noble quene un-to her reste wente;"[ ] he applies to her only a few epithets, the most eulogistic of which is "lucina the shene."[ ] in comparing the sun with the other heavenly bodies the poet mentions the moon among the rest without distinction, as inferior to the sun: "for i dar swere, withoute doute, that as the someres sonne bright is fairer, clerer, and hath more light than any planete, (is) in heven, the mone, or the sterres seven, for al the worlde, so had she surmounted hem alle of beaute," etc.[ ] on the other hand, the stars are elsewhere said to be like small candles in comparison with the moon: "and cleer as (is) the mone-light, ageyn whom alle the sterres semen but smale candels, as we demen."[ ] whenever chaucer mentions the moon's position in the heavens he does so by reference to the signs of the zodiac[ ] and, as in the case of the sun, usually with the purpose of showing time. in the _marchantes tale_ he expresses the passage of four days thus: "the mone that, at noon, was, thilke day that ianuarie hath wedded fresshe may, in two of taur, was in-to cancre gliden; so long hath maius in hir chambre biden,"[ ] and a few lines further on he states the fact explicitly: "the fourthe day compleet fro noon to noon, whan that the heighe masse was y-doon, in halle sit this ianuarie, and may as fresh as is the brighte someres day."[ ] when criseyde leaves troilus to go to the greek army she promises to return to troy within the time that it will take the moon to pass from aries through leo, that is, within ten days: "'and trusteth this, that certes, herte swete, er phebus suster, lucina the shene, the leoun passe out of this ariete, i wol ben here, with-outen any wene. i mene, as helpe me iuno, hevenes quene, the tenthe day, but-if that deeth me assayle, i wol yow seen, with-outen any fayle.'"[ ] but while the moon is quickly traversing the part of her course from aries to leo, criseyde, pressed by diomede, is changing her mind about returning to troy, and by the appointed tenth day has decided to remain with the greeks: "and cynthea[ ] hir char-hors over-raughte to whirle out of the lyon, if she mighte; and signifer[ ] his candeles shewed brighte, whan that criseyde un-to hir bedde wente in-with hir fadres faire brighte tente. . . . . . . . . . . . . and thus bigan to brede the cause why, the sothe for to telle, that she tok fully purpos for to dwelle."[ ] the passage of time is also indicated in chaucer's poetry by reference to the recurrence of the moon's phases. in the _legend of good women_, phillis writes to the false demophon saying that the moon has passed through its phases four times since he went away and thrice since the time he promised to return: "'your anker, which ye in our haven leyde, highte us, that ye wolde comen, out of doute, or that the mone ones wente aboute. but tymes foure the mone hath hid her face sin thilke day ye wente fro this place, and foure tymes light the world again.'"[ ] chaucer refers more often to the phases of the moon than to any other lunar phenomenon, but most of these references to her phases are used for the sake of comparison or illustration and give us little idea of the extent of chaucer's knowledge. mars in his 'compleynt' says that the lover "hath ofter wo then changed is the mone."[ ] the rumors in the house of fame are given times of waxing and waning like the moon: "thus out at holes gonne wringe every tyding streight to fame; and she gan yeven eche his name, after hir disposicioun, and yaf hem eek duracioun, some to wexe and wane sone, as dooth the faire whyte mone, and leet hem gon."[ ] chaucer briefly describes the crescent moon by calling her "the bente mone with hir hornes pale."[ ] in troilus' prayer to the moon, the line "'i saugh thyn hornes olde eek by the morwe,'"[ ] is practically the only one in which chaucer gives any hint of the times at which the moon in her various phases may be seen. the phase of the 'new moon,' when the moon is in conjunction with the sun (i. e., between the earth and the sun, so that we cannot see the illuminated hemisphere of the moon) is mentioned in the same poem: "right sone upon the chaunging of the mone, whan lightles is the world a night or tweyne."[ ] there is a very definite description of three of the moon's phases in the following passage from _boethius_:[ ] "so that the mone som-tyme shyning with hir ful hornes, meting with alle the bemes of the sonne hir brother, hydeth the sterres that ben lesse; and som-tyme, whan the mone, pale with hir derke hornes, approcheth the sonne, leseth hir lightes;" the moon 'shining with her full horns' means with her horns filled up as at full moon when she is in a position opposite both earth and sun so that she reflects upon the earth all the rays of the sun. the moon "with derke hornes" refers of course to the waning moon, a thin crescent near the sun and almost obscured in his light, which approaching nearer the sun is entirely lost to our view in his rays and becomes the new moon. chaucer's most interesting references to the moon are found in the prayer of aurelius to the sun in the _frankeleyns tale_. dorigen has jestingly promised to have pity on aurelius as soon as he shall remove all the rocks from along the coast of brittany, and aurelius prays to the sun, or apollo, to help him by enlisting the aid of the moon, in accomplishing this feat. the sun's sister, lucina, or the moon, is chief goddess of the sea; just as she desires to follow the sun and be quickened and illuminated by him, so the sea desires to follow her: "'your blisful suster, lucina the shene, that of the see is chief goddesse and quene, though neptunus have deitee in the see, yet emperesse aboven him is she: ye knowen wel, lord, that right as hir desyr is to be quiked and lightned of your fyr, for which she folweth yow ful bisily, right so the see desyreth naturelly to folwen hir, as she that is goddesse bothe in the see and riveres more and lesse.'"[ ] in calling lucina chief goddess of the sea and speaking of the sea's desire to follow her, chaucer is, of course alluding to the moon's effect upon the tides; and in the line: "'is to be quiked and lightned of your fyr,'" the reference is to the fact that the moon derives her light from the sun. instead of leaving it to the sun-god to find a way of removing the rocks for him, aurelius proceeds to give explicit instructions as to how this may be accomplished. as the highest tides occur when the moon is in opposition or in conjunction with the sun, if the moon could only be kept in either of these positions with regard to the sun for a long enough time, so great a flood would be produced, aurelius thinks, that the rocks would be washed away. so he prays phebus to induce the moon to slacken her speed at her next opposition in leo and for two years to traverse her sphere with the same (apparent) velocity as that of the sun, thus remaining in opposition with him: "'wherfore, lord phebus, this is my requeste-- do this miracle, or do myn herte breste-- that now, next at this opposicioun, which in the signe shal be of the leoun, as preyeth hir so greet a flood to bringe, that fyve fadme at the leeste it overspringe the hyeste rokke in armorik briteyne; and lat this flood endure yeres tweyne; . . . . . . . . . preye hir she go no faster cours than ye, i seye, preyeth your suster that she go no faster cours than ye thise yeres two. than shal she been evene atte fulle alway, and spring-flood laste bothe night and day.'"[ ] references to eclipses of the moon occur seldom in chaucer. in the second part of the _romaunt of the rose_, which is included in complete editions of chaucer's works but which he almost certainly did not write, there is a description of a lunar eclipse and of its causes. fickleness in love is compared to an eclipse: "for it shal chaungen wonder sone, and take eclips right as the mone, whan she is from us (y)-let thurgh erthe, that bitwixe is set the sonne and hir, as it may falle, be it in party, or in alle; the shadowe maketh her bemis merke, and hir hornes to shewe derke, that part where she hath lost hir lyght of phebus fully, and the sight; til, whan the shadowe is overpast, she is enlumined ageyn as faste, thurgh brightnesse of the sonne bemes that yeveth to hir ageyn hir lemes."[ ] this passage is so clear that it needs no explanation. an eclipse of the moon, since it is caused by the passing of the moon into the shadow of the earth, can only take place when the moon is full, that is, in _opposition_ to the sun. this fact is suggested in a reference in _boethius_ to a lunar eclipse: "the hornes of the fulle mone wexen pale and infect by the boundes of the derke night;"[ ] in the next lines chaucer mentions the fact that the stars which are lost to sight in the bright rays of the full moon become visible during an eclipse: "and ... the mone, derk and confuse, discovereth the sterres that she hadde y-covered by hir clere visage."[ ] . _the planets_ all the planets that are easily visible to the unaided eye were known in chaucer's time and are mentioned in his writings, some of them many times. these planets are mercury, venus, mars, jupiter, and saturn. according to the ptolemaic system, which as we have seen, held sway in the world of learning during chaucer's century, the sun and moon were also held to be planets, and all were supposed to revolve around the earth in concentric rings, the moon being nearest the earth, and the sun between venus and mars. the circular orbit of each planet was called its "deferent" and upon the deferent moved, not the planet itself, but an imaginary planet, represented by a point. the real planet moved upon a smaller circle called the "epicycle" whose center was the moving point representing the imaginary planet. the deferent of each planet was supposed to be traced as a great circle upon a transparent separate crystal sphere; and all of the crystal spheres revolved once a day around an axis passing through the poles of the heavens. as the sun and moon did not show the same irregularities[ ] of motion as the planets, ptolemy supposed these two bodies to have deferents but no epicycles. later investigators complicated the system by adding further secondary imaginary planets, revolving in ptolemy's epicycles and with the actual planets attached to additional corresponding epicycles. they even supposed the moon to have one, perhaps two epicycles and we shall find this notion reflected in chaucer. the eighth sphere had neither deferent nor epicycle but to it were attached the fixed stars. this sphere as we have seen earlier, revolved slowly from west to east to account for the precession of the equinoxes, while a ninth sphere, the _primum mobile_, imparted to all the inner spheres their diurnal motion from east to west. chaucer's poetical references to the planets, as we have found to be true in the case of the sun and moon, do not give us satisfactory evidence of the extent of his knowledge, but occasional passages from his prose works again throw light on these allusions. chaucer refers to the planets in general as 'the seven stars,' as, for instance, in the lines: "and with hir heed she touched hevene, ther as shynen sterres sevene."[ ] and "to have mo floures, swiche seven as in the welken sterres be."[ ] chaucer was undoubtedly familiar with the irregularities of the planetary movements, and with the theory of epicycles by which these irregular movements were in his day explained, although it is not from his poetry that we can learn the fact. he uses the word 'epicycle' only once in all his works. in the _astrolabe_ when comparing the moon's motion with that of the other planets, he says: "for sothly, the mone moeveth the contrarie from othere planetes as in hir episicle, but in non other manere."[ ] in the _astrolabe_[ ] chaucer explains a method of determining whether a planet's motion is retrograde or direct.[ ] the altitude of the planet and of any fixed star, is taken, and several nights later at the time when the fixed star has the same altitude as at the previous observation, the planet's altitude is again observed. if the planet is on the right or east side of the meridian, and its second altitude is less than its first, then the planet's motion is direct. if the planet is on the left or west side of the meridian, and has a smaller altitude at the second observation than at the first, then the planet's motion is retrograde. if the planet is on the east side of the meridional line when its altitude is taken and the second altitude is greater than the first, it is retrograde; and if it is on the west side and its second altitude is greater, it is direct. this method would be correct were it not that a change in the planet's declination or angular distance from the celestial equator might render the conclusions incorrect. chaucer mentions the irregularity of planetary movements in _boethius_ also when he says: "and whiche sterre in hevene useth wandering recourses, y-flit by dyverse speres."[ ] the expression "y-flit by dyverse speres" may have reference only to the one motion of the planets, that is, their motion concentric to the star-sphere; or it may be used to include also their epicyclic motion. skeat interprets the expression in the former way; but the context, it seems, would justify interpreting the words "dyverse speres" as meaning the various spheres of the planets to-gether with their epicycles; i. e., both deferents and epicycles. of all the planets, that most often mentioned by chaucer is venus, partly, no doubt, because of her greater brilliance, but probably in the main because of her greater astrological importance; for few of chaucer's references to venus, or to any other planet, indeed, are without astrological significance. chaucer refers to venus, in the classical manner, as hesperus when she appears as evening[ ] star and as lucifer when she is seen as the morning star: "and that the eve-sterre hesperus, which that in the firste tyme of the night bringeth forth hir colde arysinges, cometh eft ayein hir used cours, and is pale _by the morwe_ at the rysing of the sonne, and is thanne cleped lucifer."[ ] her appearance as morning star is again mentioned in the same work: "and after that lucifer the day-sterre hath chased awey the derke night, the day the fairere ledeth the rosene hors _of the sonne_,"[ ] and in _troilus and criseyde_ where it is said that "lucifer, the dayes messager, gan for to ryse, and out hir bemes throwe;"[ ] elsewhere in the same poem her appearance as evening star is mentioned but she is not this time called hesperus: "the brighte venus folwede and ay taughte the wey, ther brode phebus doun alighte;"[ ] occasionally venus is called cytherea, from the island near which greek myth represented her as having arisen from the sea. thus in the _knightes tale_: "he roos, to wenden on his pilgrimage un-to the blisful citherea benigne, i mene venus, honurable and digne."[ ] and in the _parlement of foules_; "citherea! thou blisful lady swete,"[ ] the relative positions of the different planets in the heavens is suggested by allusions to the different sizes of their spheres and to their different velocities. in the _compleynt of mars_ the comparative sizes and velocities of the spheres of mercury, venus and mars are made the basis for most of the action of the poem. the greater the sphere or orbit of a planet, the slower is its apparent motion. thus mars in his large sphere moves about half as fast as venus and in the poem it is planned that when mars reaches the next palace[ ] of venus, he shall by virtue of his slower motion, wait for her to overtake him: "that mars shal entre, as faste as he may glyde, into hir nexte paleys, to abyde, walking his cours til she had him a-take, and he preyde hir to haste hir for his sake."[ ] venus in compassion for his solitude hastens to overtake her knight: "she hath so gret compassion of hir knight, that dwelleth in solitude til she come; . . . . . . . . . wherefore she spedde hir as faste in her weye, almost in oon day, as he dide in tweye."[ ] when phebus comes into the palace with his fiery torch, mars will not flee and cannot hide, so he girds himself with sword and armour and bids venus flee. phebus, who in chaucer's time was regarded as the fourth planet, can overtake mars but not venus because his sphere is between theirs and his motion is consequently slower than that of venus but faster than that of mars: "flee wolde he not, ne mighte him-selven hyde. he throweth on his helm of huge wighte, and girt him with his swerde; and in his honde his mighty spere, as he was wont to fighte, he shaketh so that almost it to-wonde; ful hevy he was to walken over londe; he may not holde with venus companye, but bad hir fleen, lest phebus hir espye. "o woful mars! alas! what mayst thou seyn, that in the paleys of thy disturbaunce art left behinde, in peril to be sleyn? . . . . . . . . that thou nere swift, wel mayst thou wepe and cryen."[ ] in spite of his sorrow, mars patiently continues to follow venus, lamenting as he goes that his sphere is so large: "he passeth but oo steyre in dayes two, but ner the les, for al his hevy armure, he foloweth hir that is his lyves cure;[ ] . . . . . . . after he walketh softely a pas, compleyning, that hit pite was to here. he seyde, 'o lady bright, venus! alas! that ever so wyde a compass is my spere! alas! whan shal i mete yow, herte dere,'" etc.[ ] meanwhile venus has passed on to mercury's palace where he soon overtakes her and receives her as his friend:[ ] "hit happed for to be, that, whyl that venus weping made hir mone, cylenius, ryding in his chevauche, fro venus valance mighte his paleys see, and venus he salueth, and maketh chere, and hir receyveth as his frend ful dere."[ ] mercury's palace was the sign gemini and venus' valance, probably meaning her detrimentum or the sign opposite her palace, was aries. 'chevauche' means an equestrian journey or ride, and is here used in the sense of 'swift course.' the passage, then, simply refers to the swift motion by which in a very short time mercury passes from aries to a position near enough to that of venus in gemini so that he can see her and give her welcome. mercury's sphere being the smallest of the planets, his motion is also the swiftest. the size of jupiter's orbit is not mentioned in chaucer and that of saturn's only once. in the _knightes tale_ saturn, addressing venus, speaks of the great distance that he traverses with his revolving sphere but does not compare the size of his sphere with those of the other planets: "'my dere doghter venus,' quod saturne, 'my cours, that hath so wyde for to turne, hath more power than wot any man.'"[ ] besides the reference in the _compleynt of mars_ to the conjunction of venus and mars[ ], there are occasional references in chaucer to conjunctions of other planets. in the _astrolabe_[ ] chaucer explains a method of determining in what position in the heavens a conjunction of the sun and moon takes place, when the time of the conjunction is known. a conjunction of the moon with saturn and jupiter is mentioned in _troilus and criseyde_, in the lines: "the bente mone with hir hornes pale, saturne, and iove, in cancro ioyned were,"[ ] . _the galaxy_ the galaxy or milky way, which stretches across the heavens like a broad band whitish in color caused by closely crowded stars, has appealed to men's imagination since very early times. its resemblance to a road or street has been suggested in the names given to it by many peoples. ovid called it _via lactea_ and the roman peasants, _strada di roma_; pilgrims to spain referred to it as the _road to santiago_; dante refers to it as "the white circle commonly called st. janus's way"[ ]; and the english had two names for it, _walsingham way_ and _watling-street_. chaucer twice mentions the galaxy; once in the _parlement of foules_, where africanus shows scipio the location of heaven by pointing to the galaxy: "and rightful folk shal go, after they dye, to heven; and shewed him the galaxye."[ ] in the _hous of fame_, the golden eagle who bears chaucer through the heavens toward fame's palace, points out to him the galaxy and then relates the myth of phaeton driving the chariot of the sun, a story traditionally associated with the milky way: 'now,' quod he tho, 'cast up thyn ye; see yonder, lo, the galaxye, which men clepeth the milky wey, for hit is whyt: and somme, parfey, callen hit watlinge strete: that ones was y-brent with hete, whan the sonnes sone, the rede, that highte pheton, wolde lede algate his fader cart, and gye. the cart-hors gonne wel espye that he ne coude no governaunce, and gonne for to lepe and launce, and beren him now up, now doun, til that he saw the scorpioun, which that in heven a signe is yit. and he, for ferde, loste his wit, of that, and lest the reynes goon of his hors; and they anoon gonne up to mounte, and doun descende til bothe the eyr and erthe brende; til iupiter, lo, atte laste, him slow, and fro the carte caste.'[ ] in narrating this story here, chaucer may have been imitating dante who refers to the myth in the _divine comedy_: "what time abandoned phaeton the reins, whereby the heavens, as still appears, were scorched,"[ ] and states its source and the use made of it by some philosophers in the _convivio_: "for the pythagoreans affirmed that the sun at one time wandered in its course, and in passing through other regions not suited to sustain its heat, set on fire the place through which it passed; and so these traces of the conflagration remain there. and i believe that they were influenced by the fable of phaeton, which ovid tells at the beginning of the second book of the _metamorphoses_."[ ] v astrological lore in chaucer astrology, though resembling a science in that it makes use of observation and seeks to establish laws governing its data, is in reality a faith or creed. it had its beginning, so tradition tells us, in the faith of the ancient babylonians in certain astral deities who exerted an influence upon terrestrial events and human life. the basis of this faith was not altogether illogical but contained a germ of truth. of all the heavenly bodies, the sun exerted the most obvious effect upon the earth; the sun brought day and night, summer and winter; his rays lured growing things from mother earth and so gave sustenance to mankind. but to the ancient peoples of the orient the sun was also often a baneful power; he could destroy as well as give life. therefore, the ancients came to look upon the sun as a great and powerful god to be worshipped and propitiated by men. and if the sun was such a power, it was natural to believe that all the other bright orbs of the sky were lesser divinities who exercised more limited powers on the earth. from this beginning, based, as we have seen, on a germ of fact, by the power of his imagination and credulity, man extended more and more the powers of these sidereal divinities, attributing to their volition and influence all the most insignificant as well as the most important terrestrial events. and if the heavenly bodies, by revolving about the earth in ceaseless harmony, effected the recurrence of day and night and of the seasons, and if their configurations were responsible for the minutest events in nature, was it not natural to suppose that, besides affecting man thus indirectly, they also influenced him directly and were responsible for his conduct and for the very qualities of his mind and soul? perhaps the astonishing variety of the influences that the celestial bodies, from ancient until modern times, were supposed to exercise over the world and the life of mankind can be accounted for by imagining some such process of thought to have been involved in the beginnings of astrology. it was but a step from faith in stellar influence on our earth to the belief that, as the heavenly movements were governed by immutable laws, so their influence upon the world would follow certain laws and its effects in the future could be determined as certainly as could the coming revolutions and conjunctions of the stars. out of this two-fold belief was evolved a complex system of divination, the origin of which was forgotten as men, believing in it, invented reasons for believing, pretending that their faith was founded on a long series of observations. the chaldeans believed that in discovering the unceasing regularity of the celestial motions, they had found the very laws of life and they built upon this conviction a mass of absolutely rigid dogmas. but when experience belied these dogmas, unable to realize the falsity of their presuppositions and to give up their faith in the divine stars, the astrologers invented new dogmas to explain the old ones, thus piling up a body of complicated and often contradictory doctrines that will ever be to the student a source of perplexity and astonishment. on its philosophical side astrology was a system of astral theology developed, not by popular thought, but through the careful observations and speculations of learned priests and scholars. it was a purely eastern science which came into being on the chaldean plains and in the nile valley. as far as we know, it was entirely unknown to any of the primitive aryan races, from hindostan to scandinavia. astrology as a system of divination never gained a foothold in greece during the brightest period of her intellectual life. but the dogma of astral divinity was zealously maintained by the greatest of greek philosophers. plato, the great idealist, whose influence upon the theology of the ancient and even of the modern world was more profound than that of any other thinker, called the stars "visible gods" ranking them just below the supreme eternal being; and to plato these celestial gods were infinitely superior to the anthropomorphic gods of the popular religion, who resembled men in their passions and were superior to them only in beauty of form and in power. aristotle defended with no less zeal the doctrine of the divinity of the stars, seeing in them eternal substances, principles of movement, and therefore divine beings. in the hellenistic period, zeno, the stoic, and his followers proclaimed the supremacy of the sidereal divinities even more strongly than the schools of plato and aristotle had done. the stoics conceived the world as a great organism whose "sympathetic" forces constantly interacted upon one another, governed by reason which was of the essence of ethereal fire, the primordial substance of the universe. to the stars, the purest manifestation of the power of this ethereal substance, were attributed the greatest influence and the loftiest divine qualities. the stoics developed the doctrine of fatalism, which is the inevitable outcome of faith in stellar influence on human life, to its consequences; yet they proved by facts that fatalism is not incompatible with active and virtuous living. by the end of the roman imperial period astrology had transformed paganism, replacing the old society of immortals who were scarcely superior to mortals, except in being exempted from old age and death, by faith in the eternal beings who ran their course in perfect harmony throughout the ages, whose power, regulated by the unvarying celestial motions, extended over all the earth and determined the destiny of the whole human race. astrology, as a science and a system of divination, exerted a profound influence over the mediaeval mind. no court was without its practicing astrologer and the universities all had their professors of astrology. the practice of astrology was an essential part of the physician's profession, and before prescribing for a patient it was thought quite as important to determine the positions of the planets as the nature of the disease.[ ] interesting evidence of this fact is found in the _prologue to the canterbury tales_ where chaucer speaks of the doctour's knowledge and use of astrology as if it were his chief excellence as a physician: "in al this world ne was ther noon him lyk to speke of phisik and of surgerye; for he was grounded in astronomye. he kepte his pacient a ful greet del in houres, by his magik naturel. wel coude he fortunen the ascendent of his images for his pacient."[ ] yet in spite of the esteem in which astrological divination was held by most people in the middle ages, dante, the greatest exponent of the thought and learning of that period, shows practically no knowledge of the technical and practical side of astrology. when he refers to the specific effects of the planets it is only to those most familiarly known, and he nowhere uses such technical terms as "houses" or "aspects" of planets. but dante, like the great philosophers of the earlier periods, was undoubtedly influenced by the philosophical doctrines of astrology, and a general belief in the influence of the celestial spheres upon human life was deeply rooted in his mind. to him the ceaseless and harmonious movements of the celestial bodies were the manifestations and instruments of god's providence, and were ordained by the first mover to govern the destinies of the earth and human life. we can see this conviction of dante's with perfect certainty when we read the _divina commedia_. for dante's poetry is highly subjective; on every page his own personal thoughts and feelings are revealed quite openly. chaucer's poetry, on the other hand, is objective; he tells us almost nothing directly about himself and what we learn of him in his writings is almost entirely by inference. chaucer's frequent use of astrology in his poetry would make it hard to believe that he was not considerably influenced by its philosophical aspects, at least in the general way that dante was. part and parcel of the dramatic action in most of his poems is the idea of stellar influences. yet we cannot assert, with the same assurance that we can say it of dante, that chaucer believed, even in a general way, in the influence of the stars on human life. in dante's poetry, as we have said, the poet himself is always before us. chaucer, with socratic irony, always makes it plain to the reader that his attitude is purely objective, that he is only the narrator of what he has seen or dreamed, only the copyist of another's story. even when chaucer makes himself one of the protagonists, as in the _hous of fame_ and the _canterbury tales_, it is only that his narrative may be the more convincing. he tells a story and makes its protagonists actually live before us, as individual men and women. it is possible to imagine all of his use of astrology in his poetry not as the reflection of his own faith in its cosmic philosophy, but the expression of his genius for understanding people and truthfully describing life and character. considerable discussion as to chaucer's attitude towards astrology has been called forth by passages in which he speaks in words of scorn with regard to some of the practices and magic arts that were often used in connection with astrology. in the _astrolabe_ after describing somewhat at length the favorable and unfavorable positions of planets he says: "natheles, thise ben observauncez of iudicial matiere and rytes of payens, in which my spirit ne hath no feith, ne no knowing of hir horoscopum."[ ] again in the _franklin's tale_ he speaks in a similar disdainful tone of astrological magic: "he him remembred that, upon a day, at orliens in studie a book he say of magik naturel, which his felawe, that was that tyme a bacheler of lawe, al were he ther to lerne another craft, had prively upon his desk y-laft; which book spak muchel of the operaciouns, touchinge the eighte and twenty mansiouns that longen to the mone, and swich folye, as in our dayes is not worth a flye: for holy chirches feith in our bileve ne suffreth noon illusion us to greve."[ ] and elsewhere in the same tale he writes: "so atte laste he hath his tyme y-founde to maken his iapes and his wreccednesse of switch a supersticious cursednesse."[ ] here follows a long description of the clerk's instruments and astrological observances, ending in the lines "for swiche illusiouns and swiche meschaunces as hethen folk used in thilke dayes; for which no lenger maked he delayes, but thurgh his magik, for a wyke or tweye, it seemed that alle the rokkes were aweye."[ ] on the strength of these passages professor t. r. lounsbury[ ] holds that chaucer was far ahead of most of his contemporaries in his attitude toward the superstitious practices connected with the astrology of his day; that his attitude toward judicial astrology was one of total disbelief and scorn; and he even goes so far as to say that chaucer was guilty of a breach of artistic workmanship in expressing his disbelief so scornfully in a tale in which the very climax of the dramatic action depends upon a feat of astrological magic. a more satisfactory interpretation of the passages quoted above is advanced by professor j. s. p. tatlock,[ ] who shows that chaucer has taken great pains to place the setting of the _franklin's tale_ in ancient times and that he, in common with most of the educated men of his day, disapproved of the practices (except sometimes when employed for good purposes as, e. g. in the physician's profession) and the practicians of judicial astrology in his own day, but thought of the feats and observances of astrological magic as having been possible and efficacious in ancient times. according to this view chaucer's attitude was one of disapproval rather than disbelief, and his disapproval was not for the general theory of astrology, but for the shady observances and quackery connected with its application to the problems of life in his time. it is to be noted, further, that wherever chaucer speaks in the strongest terms against astrological observances he also uses religious language. this fact may point to a wise caution on his part lest his evident interest in astrology, (which was closely associated with magic, and hence, indirectly, with sorcery) might involve him in difficulties with mother church; and, as professor tatlock has pointed out, there is no reason to suppose that chaucer's religious expressions in these passages are insincere. the _franklin's tale_ falls in the group of tales called by professor kittredge the "marriage group,"[ ] that in which the wife of bath is the most conspicuous figure. the wife of bath's tale had aroused a rather heated controversy among a number of the canterbury pilgrims on the subject of the respective duties and relations of wives and husbands. if the critics have been right in placing the _franklin's tale_ where they do, it was chaucer's purpose to have the franklin soothe the ruffled feelings of certain members of the party by telling a tale in which a husband (and wife), a squire, and a clerk, all prove themselves capable of truly generous behavior. if the tale was to accomplish its purpose the clerk must accomplish his magic feat of removing the rocks from the coast of brittany, and must in the end generously refuse to accept pay from the squire when he learned that the latter had been too magnanimous to profit from his services. by setting the tale in pagan times, chaucer was able to express the scorn he felt for certain superstitious practices in his own time without debasing one of his chief characters, one of the three rivals in magnanimity, and so spoiling the noble temper of the story and entirely defeating its purpose. thus the astrological passages in the _franklin's tale_ do not suggest total disbelief in astrology on chaucer's part, and much less do they show him to have been lacking in true artistic sense. probably his attitude toward astrology was about this: he was very much interested in it, perhaps in much the same way that dante was, because of the philosophical ideas at the basis of astrology and out of curiosity as to the problems of free will, providence, and so on, that naturally arose from it. for the shady practices and quackery connected with its use in his own day he had nothing but scorn. but while chaucer was at one with the educated men of his century in his attitude toward astrology, and with them had a strong distaste for certain aspects of judicial astrology, nevertheless he made wide use of the greater faith of the majority of people of his time in portraying character in his poetry. for men's ideas and beliefs constitute a very important part of their character, and chaucer knew this very well. men believed that whatever happened to them, whether fortunate or unfortunate, could in some way be traced to the influence of the stars, the agents and instruments of destiny. the configuration of the heavens at the moment of one's birth was considered especially important, since the positions and interrelations of the different celestial bodies at this time could determine the most momentous events of one's life. now the nature of the influence exerted by the different stars, especially the planets and zodiacal constellations, varied greatly. mars and venus, for instance, bestowed vastly different qualities upon the soul that was coming into being. moreover, the power exerted by a planet or constellation fluctuated considerably according to its position. each planet had in the zodiac a position of greatest and a position of least power called its 'exaltation' and 'depression.' furthermore, the 'aspect' or angular distance of one planet from another altered its influence in various ways. if mars and jupiter, for instance, were in trine or sextile aspect the portent was favorable, if in opposition, it was unfavorable.[ ] these ideas are frequently expressed in chaucer, when the characters seek to understand their misfortunes or to justify their conduct by tracing them back to the determinations of the heavens at their birth. when palamon and arcite have been thrown into prison the latter pleads with his companion to have patience; this misfortune was fixed upon them at the time of their birth by the disposition of the planets and constellations, and complaining is of no avail: "'for goddes love, tak al in pacience our prisoun, for it may non other be; fortune hath yeven us this adversitee. som wikke aspect or disposicioun of saturne, by sum constellacioun hath yeven us this, al-though we hadde it sworn; so stood the heven whan that we were born; we moste endure it: this is the short and pleyn.'"[ ] in the _man of lawes tale_ the effect of the stars at the time of a man's nativity is discussed somewhat at length. the man of law predicts the fate of the sultan by saying that the destiny written in the stars had perhaps allotted to him death through love: "paraventure in thilke large book which that men clepe the heven, y-writen was with sterres, whan that he his birthe took, that he for love shulde han his deeth, allas! for in the sterres, clerer than is glas, is writen, god wot, who-so coude it rede, the deeth of every man, withouten drede."[ ] then he mentions the names of various ancient heroes whose death, he says was written in the stars "er they were born:" "in sterres, many a winter ther-biforn, was written the deeth of ector, achilles, of pompey, iulius, er they were born; the stryf of thebes; and of ercules, of sampson, turnus, and of socrates the deeth; but mennes wittes been so dulle, that no wight can wel rede it atte fulle."[ ] when criseyde learns that she is to be sent to the greeks in exchange for antenor she attributes her misfortune to the stars: "'alas!' quod she, 'out of this regioun i, woful wrecche and infortuned wight, and born in corsed constellacioun, mot goon, and thus departen fro my knight;'"[ ] in the _legend of good women_ we are told that hypermnestra was "born to all good things" or qualities, and then the various influences of the particular planets upon her destiny are mentioned: "the whiche child, of hir nativitee, to alle gode thewes born was she, as lyked to the goddes, or she was born, that of the shefe she sholde be the corn; the wirdes, that we clepen destinee, hath shapen her that she mot nedes be pitouse, sadde, wyse, and trewe as steel; and to this woman hit accordeth weel. for, though that venus yaf her great beautee, with jupiter compouned so was she that conscience, trouthe, and dreed of shame, and of hir wyfhood for to keep her name, this, thoughte her, was felicitee as here. and rede mars was, that tyme of the yere, so feble, that his malice is him raft, repressed hath venus his cruel craft; what with venus and other oppressioun of houses, mars his venim is adoun, that ypermistra dar nat handle a knyf in malice, thogh she sholde lese her lyf. but natheles, as heven gan tho turne, to badde aspectes hath she of saturne, that made her for to deyen in prisoun, as i shal after make mencioun."[ ] the purpose of this astrological passage is plainly to show why hypermnestra was doomed to die in prison. the qualities given her by the planets, as shown by her horoscope, were such that she was unable to violate a wife's duty and kill her husband in order to save her own life.[ ] venus gave her great beauty and was also influential in repressing the influence of mars who would have given her fighting qualities if his influence had been strong. the myth of the amour between venus and mars, which chaucer makes the basis of his poem the _compleynt of mars_, would explain why venus was able to influence mars in this way. the feeble influence of mars at hypermnestra's nativity is accounted for also in another way. his influence is feeble because of the time of year and through the "oppressioun of houses" both of which amount to the same thing, namely, a position in the zodiac in which his power is at a minimum.[ ] the influence of jupiter, we are told, was to give hypermnestra conscience, truth, and wifely loyalty. that of saturn was evil and the cause of her death in prison. the specific influences of saturn are mentioned in detail in the _knightes tale_. almost all the ills imaginable are attributable to his power: "'my dere doghter venus,' quod saturne, 'my cours, that hath so wyde for to turne, hath more power than wot any man. myn is the drenching in the see so wan; myn is the prison in the derke cote; myn is the strangling and hanging by the throte; the murmure, and the cherles rebelling, the groyning, and the pryvee empoysoning; i do vengeance and pleyn correccioun whyl i dwelle in the signe of the leoun. myn is the ruine of the hye halles, the falling of the toures and of the walles up-on the mynour or the carpenter. i slow sampsoun in shaking the piler; and myne be the maladyes colde, the derke tresons, and the castes olde; my loking is the fader of pestilence.'"[ ] in the line, "myn is the prison in the derke cote;" imprisonment is for the second time attributed to saturn's influence. in an earlier passage in the _knightes tale_[ ], (see p. ) it is suggested when palamon and arcite's imprisonment is said to be due to 'some wicked aspect or disposition of saturn' at the time of their birth. later in the story palamon specifically states that his imprisonment is through saturn: "but i mot been in prison thurgh saturne,"[ ] that mars and saturn were generally regarded as planets of evil influence is shown by a passage in the _astrolabe_. chaucer has just explained what the 'ascendant', means in astrology. it is that degree of the zodiac that at the given time is seen upon the eastern horizon. now, chaucer says, the ascendant may be 'fortunate or unfortunate,' thus: "a fortunat ascendent clepen they whan that no wykkid planete, as saturne or mars, or elles the tail of the dragoun, is in the house of the assendent, ne that no wikked planets have non aspects of enemite up-on the assendent;"[ ] the wife of bath attributes the two principal qualities of her disposition, amorousness and pugnaciousness, to the planets venus and mars: "for certes, i am al venerien in felinge, and myn herte is marcien. venus me yaf my lust, my likerousnesse, and mars yaf me my sturdy hardinesse. myn ascendent was taur, and mars ther-inne. allas! allas! that ever love was sinne! i folwed ay myn inclinacioun by vertu of my constellacioun."[ ] a little later in her _prologue_ the wife contrasts the influences of mercury and venus. as a jibe at the clerk who was in the company of canterbury pilgrims she has just said that clerks cannot possibly speak well of wives, and that women could tell tales of clerks if they would. she upholds her statement thus: wives are the children of venus, clerks, of mercury, two planets that are 'in their working full contrarious:' "the children of mercurie and of venus been in hir wirking ful contrarious; mercurie loveth wisdom and science, and venus loveth ryot and dispence. and, for hir diverse disposicioun, ech falleth in otheres exaltacioun; and thus, got woot! mercurie is desolat in pisces, wher venus is exaltat; and venus falleth ther mercurie is reysed; therefore no womman of no clerk is preysed."[ ] venus has her exaltation in the sign in which mercury has his depression. therefore the two signs have opposite virtues and influences, and the children of one can see little good in the children of the other. we have seen how the stars were supposed to control human destiny by bestowing certain qualities upon souls at birth. we shall next consider how they were thought to influence men more indirectly, through their effects on terrestrial events. certain positions of the heavenly bodies with regard to one another could cause heavy rains. the clerk in the _milleres tale_ predicts a great rain through observation of the moon's position: "'now john,' quod nicholas, 'i wol nat lye; i have y-founde in myn astrologye, as i have loked in the mone bright, that now, a monday next, at quarter-night, shal falle a reyn and that so wilde and wood, that half so greet was never noes flood.'"[ ] such predictions as this were, however, by no means always believed in even by uneducated people. in this case, for the purposes of the story, the flood does not take place. the carpenter, john, is taken in because the story requires it, but nicholas is a quack pure and simple, and of course the miller who tells the story has no delusions. in _troilus and criseyde_ we are told that the moon's conjunction with jupiter and saturn caused a heavy rain. pandarus had the day before suspected that there was to be rain from the condition of the moon: "right sone upon the chaunging of the mone, whan lightles is the world a night or tweyne, and that the welken shoop him for to reyne, he streight a-morwe un-to his nece wente;"[ ] and on the next night the rain came: "the bente mone with hir hornes pale, saturne, and iove, in cancro ioyned were, that swich a rayn from hevene gan avale, that every maner womman that was there hadde of that smoky reyn a verray fere;"[ ] perhaps the moon alone in cancer, which was her mansion, would have caused a rain, and it was the additional presence of saturn and jupiter that made it such a heavy downpour. chaucer humorously makes use of this astrological superstition that the planets cause rains in the _lenvoy a scogan_: "to-broken been the statuts hye in hevene that creat were eternally to dure, sith that i see the brighte goddes sevene mow wepe and wayle, and passioun endure, as may in erthe a mortal creature. allas, fro whennes may this thing procede? of whiche errour i deye almost for drede."[ ] here it is not the planets' positions that cause the rain, but the planets are weeping as mortals do and their tears are the rain. in the next stanza we learn that even venus, from whose sphere divine law once decreed no tear should ever fall, is weeping so that mortals are about to be drenched. and it is all scogan's fault! "by worde eterne whylom was hit shape that fro the fifte cercle, in no manere, ne mighte a drope of teres doun escape. but now so wepeth venus in hir spere, that with hir teres she wol drenche us here. allas! scogan! this is for thyn offence! thou causest this deluge of pestilence."[ ] so the ultimate cause of the rain was scogan's offense. and in the next stanza we learn what that offence was. instead of vowing to serve his lady forever, though his love is unrequited, scogan has rebelled against the law of love: "hast thou not seyd, in blaspheme of this goddes, through pryde, or through thy grete rakelnesse, swich thing as in the lawe of love forbode is? that, for thy lady saw nat thy distresse, therefor thou yave hir up at michelmesse!"[ ] i have said that chaucer makes wide use of the astrological beliefs of his century in portraying character and have shown how some of the strange astrological ideas of the people of his time are reflected in chaucer's poetry. it remains to consider somewhat more closely the relations between astrological faith and conduct, and chaucer's application of these relations to the dramatic action in his poems. the inevitable logical outcome of astrological faith is the doctrine of necessity. the invariability of the celestial motions suggested to early astrologers that there must be a higher power transcending and controlling them, and this power could be none other than necessity. but, since the stars by their movements and positions were the regulators of mundane events and human affairs, it followed that human destiny on the earth was also under the sway of this relentless power of necessity or fate. now it was the stoics alone who developed a thorough-going fatalism and at the same time made it consistent with practical life and virtue. they taught that man could best find himself in complete submission to the divine law of destiny. the early babylonian astrologers who originated the doctrine of necessity did not develop it to its logical consequences. reasoning from certain very unusual occurrences that sometimes took place in the heavens, such as the appearance of comets, meteors and falling stars, they reached the conclusion that divine will at times arbitrarily interfered in the destined course of nature. so priests foretold future events from the configuration of the heavens, but professed ability to ward off threatened evils by spells and incantations, or, by purifications and sacrifices, to make the promised blessings more secure. now the fatalism of chaucer's characters is something like this. the general belief in the determination of human destiny by fortune or necessity is present and is expressed usually at moments of deep despair, when the longings of the heart and the struggles of the will have been relentlessly thwarted. when the trojans decree that criseyde must go to the greeks in exchange for antenor, troilus pleads with fortune: "than seyde he thus, 'fortune! allas the whyle! what have i doon, what have i thus a-gilt? how mightestow for reuthe me bigyle? is ther no grace, and shall i thus be spilt? shal thus criseyde awey, for that thou wilt? allas! how maystow in thyn herte finde to been to me thus cruel and unkinde? . . . . . . . . . . . . . . . . . . . . . . . . allas! fortune! if that my lyf in ioye displesed hadde un-to thy foule envye, why ne haddestow my fader, king of troye, by-raft the lyf, or doon my bretheren dye, or slayn my-self, that thus compleyne and crye, i, combre-world, that may of no-thing serve, but ever dye, and never fulle sterve?'"[ ] but there is present, too, in spite of all obstacles and defeats, an undying hope that somehow--by prayers and sacrifices to the celestial powers, or by the choice of astrologically favorable times of doing things--that somehow the course of human lives, mapped out at birth by the stars under the control of relentless destiny, may be altered. so the characters in chaucer's poems pray to the orbs of the sky to help in their undertakings. the love-lorn troilus undertakes scarcely a single act without first beseeching some one of the celestial powers for help. when he has confessed his love to pandarus and the latter has promised to help him, troilus prays to venus: "'now blisful venus helpe, er that i sterve, of thee, pandare, i may som thank deserve.'"[ ] and when the first step has been taken and he knows that criseyde is not ill disposed to be his friend at least, he praises venus, looking up to her as a flower to the sun: "but right as floures, thorugh the colde of night y-closed, stoupen on hir stalkes lowe, redressen hem a-yein the sonne bright, and spreden on hir kinde cours by rowe; right so gan tho his eyen up to throwe this troilus, and seyde, 'o venus dere, thy might, thy grace, y-heried be it here!'"[ ] when troilus is about to undertake a step that will either win or lose criseyde he prays to all the planetary gods, but especially to venus, begging her to overcome by her aid whatever evil influences the planets exercised over him in his birth: "'yit blisful venus, this night thou me enspyre,' quod troilus, 'as wis as i thee serve, and ever bet and bet shal, til i sterve. and if i hadde, o venus ful of murthe, aspectes badde of mars or of saturne, or thou combust[ ] or let were in my birthe, thy fader prey al thilke harm disturne.'"[ ] troilus does not forget to praise venus when criseyde is won at last: "than seyde he thus, 'o, love, o, charitee, thy moder eek, citherea the swete, after thy-self next heried be she, venus mene i, the wel-willy planete;'"[ ] and after criseyde has gone away to the greeks, it is to venus still that the lover utters his lament and prayer, saying that without the guidance of her beams he is lost: "'o sterre, of which i lost have al the light, with herte soor wel oughte i to bewayle, that ever derk in torment, night by night, toward my deeth with wind in stere i sayle; for which the tenthe night if that i fayle the gyding of thy bemes brighte an houre, my ship and me caribdis wol devoure:'"[ ] another effect of astrological faith on conduct was the choice of times for doing things of importance with reference to astrological conditions. when a man wished to set out on any enterprise of importance he very often consulted the positions of the stars to see if the time was propitious. thus in the _squieres tale_ it is said that the maker of the horse of brass "wayted many a constellacioun, er he had doon this operacioun;"[ ] that is, he waited carefully for the moment when the stars would be in the most propitious position, so that his undertaking would have the greatest possible chance of success. pandarus goes to his niece criseyde to plead for troilus at a time when the moon is favorably situated in the heavens: "and gan to calle, and dresse him up to ryse, remembringe him his erand was to done from troilus, and eek his greet empryse; and caste and knew in good plyt was the mone-- to doon viage, and took his wey ful sone un-to his neces paleys ther bi-syde."[ ] the kind of fatalism that chaucer's characters, as a rule, represent is well illustrated in the story of palamon and arcite, told by the knight in the _canterbury tales_. these two young nobles of thebes, cousins by relationship, are captured by theseus, king of athens, and imprisoned in the tower of his palace. from the window of the tower palamon espies the king's beautiful sister emelye walking in the garden and instantly falls in love. arcite, seeing his cousin's sudden pallor and hearing his exclamation which, chaucer says, sounded "as though he stongen were un-to the herte."[ ] thinks that palamon is complaining because of his imprisonment and urges him to bear in patience the decree of the heavens: "'for goddes love, tak al in pacience our prisoun, for it may non other be; fortune hath yeven us this adversitee. som wikke aspect or disposicioun of saturne, by sum constellacioun, hath yeven us this, al-though we hadde it sworn; so stood the heven whan that we were born; we moste endure it; this is the short and pleyn.'"[ ] this is the doctrine of necessity, and it suggests the stoic virtue of submission to fate; yet arcite's attitude toword his misfortune is not truly stoic, for there is none of that joy in submission here that the stoic felt in surrendering himself to the will of the powers above. arcite would resist fate if he could. palamon explains the cause of his woe and when arcite looks out and sees emelye he too falls a victim to love. then palamon knits his brows in righteous indignation. did he not love the beautiful lady first and trust his secret to his cousin and sworn brother? and was it not arcite's duty and solemn pledge to help and not hinder him in his love? arcite's defence shows that the fatalism that dominates his thought is a fatalism that excuses him for doing as he pleases: love knows no law, but is a law unto itself. therefore he must needs love emelye. "wostow nat wel the olde clerkes sawe, that 'who shal yeve a lover any lawe?' love is a gretter lawe, by my pan, than may be yeve to any erthly man. and therefore positif lawe and swich decree is broke al-day for love, in ech degree. a man moot nedes love, maugree his heed."[ ] when arcite is released from prison but banished from athens with the threat of death should he return, both men are utterly unhappy, arcite, because he can no longer see emelye, and palamon because he fears that arcite will return to athens with a band of kinsmen to aid him, and carry off emelye by force. after arcite has gone palamon reproaches the gods for determining the destiny of men so irrevocably without consulting their wishes or their deserts: "'o cruel goddes, that governe this world with binding of your word eterne, and wryten in the table of athamaunt your parlement, and your eterne graunt, what is mankinde more un-to yow holde than is the sheep, that rouketh in the folde?'"[ ] many a man, palamon says, suffers sickness, imprisonment and other misfortunes unjustly because of the inexorable destiny imposed upon him by the gods. even the lot of the beasts is better, for they do as they will and have nothing to suffer for it after death; whereas man must suffer both in this life and the next. this, surely, is not willing submission to fate. after some years palamon escapes from prison and encounters arcite, who has returned in disguise and become theseus' chief squire. they arrange to settle their differences by a duel next day. but destiny was guiding theseus' conduct too, so the narrator of the story says, and was so powerful that it caused a coincidence that might not happen again in a thousand years: "the destinee, ministre general, that executeth in the world over-al the purveyaunce, that god hath seyn biforn, so strong it is, that, though the world had sworn the contrarie of a thing, by ye or nay, yet somtyme it shal fallen on a day that falleth nat eft with-inne a thousand yere. for certeinly, our appetytes here, be it of werre, or pees, or hate, or love, al is this reuled by the sighte above."[ ] theseus goes hunting and with him, the queen and emelye. they of course interrupt the duel between palamon and arcite. through the intercession of the two women the duelists are pardoned and it is arranged that they settle their dispute by a tournament set for about a year later. on the morning before the tournament palamon, arcite, and emelye all go, at different hours, to pray and sacrifice to their respective patron deities. the times of their prayers are chosen according to astrological considerations, each going to pray in the hour[ ] that was considered sacred to the planet with which his patron deity was identified. palamon prays to venus only that he may win his love, whether by victory or defeat in the tournament makes no difference to him. after his sacrifices are completed, the statute of venus shakes and palamon, regarding this as a favorable sign goes away with glad heart. arcite prays mars for victory and is answered by a portent even more favorable than that given to palamon. not only does the statue of mars tremble so that his coat of mail resounds, but the very doors of the temple shake, the fire on the altar burns more brightly and arcite hears the word "victory" uttered in a low dim murmur. emelye does not want to be given in marriage to any man and so she prays to diana[ ], as the protectress of maidenhood, to keep her a maid. diana, the goddess, appears in her characteristic form as a huntress and tells emelye that the gods have decreed her marriage either to palamon or to arcite, but that it cannot yet be revealed to which one she is to be given. but now there is trouble in heaven. venus has promised that palamon shall have his love, and mars has promised arcite the victory. how are both promises to be fulfilled? chaucer humorously expresses the dilemma thus: "and richt anon swich stryf ther is bigonne for thilke graunting, in the hevene above, bitwixe venus, the goddesse of love, and mars, the sterne god armipotente, that iupiter was bisy it to stente; til that the pale saturnus the colde, that knew so manye of aventures olde, fond in his old experience an art, that he ful sone hath plesed every part."[ ] we had almost forgotten that all the gods to whom prayers have been uttered and sacrifices offered were anything more than pagan gods. but now, by the reference to saturn, "the pale saturnus the colde" suggesting the dimness of his appearance in the sky, we are reminded that these gods are also planets. but, to resume the story, saturn finds the remedy for the embarrassing situation. he rehearses his powers and then tells venus that her knight shall have his lady, but that mars shall be able to help his knight also. "'my dere doghter venus,' quod saturne, 'my cours, that hath so wyde for to turne, hath more power that wot any man. . . . . . . . . now weep namore, i shal doon diligence that palamon, that is thyn owne knight, shal have his lady, as thou hast him hight. though mars shal helpe his knight, yet nathelees bitwixe yow ther moot be som tyme pees, al be ye noght of o complexioun, that causeth al day swich divisioun.'"[ ] when the appointed time for the tourney arrives, in order that no means of securing the god's favor and so assuring success may be left untried, arcite, with his knights, enters through the gate of mars, his patron deity, and palamon through that of venus. palamon is defeated in the fight but saturn fulfills his promise to venus by inducing pluto to send an omen which frightens arcite's horse causing an accident in which arcite is mortally injured. in the end palamon wins emelye. although the scene of this story is laid in ancient athens, the characters are plainly mediaeval knights and ladies. throughout the poem, as in many of chaucer's writings, there is a curious mingling of pagan and christian elements, a strange juxtaposition of astrological notions, greek anthropomorphism and mediaeval christian philosophy. but pervading the whole is the idea of determinism, of the inability of the human will to struggle successfully against the destiny imposed by the powers of heaven, or against the capricious wills of the gods. chaucer had too keen a sense of humor, too sympathetic an outlook on life not to see the irony in the ceaseless spectacle of mankind dashing itself against the relentless wall of circumstances, fate, or what you will, in undying hope of attaining the unattainable. he saw the humor in this maelstrom of human endeavor--and he saw the tragedy too. the _knightes tale_ presents largely, i think, the humorous side of it, _troilus and criseyde_, the tragic, although there is some tragedy in the _knightes tale_ and some comedy in _troilus_. it was fate that troilus should love criseyde, that he should win her love for a time, and that in the end he should be deserted by her. from the very first line of the poem we know that he is doomed to sorrow: "the double sorwe of troilus to tellen, that was the king priamus sone of troye, in lovinge, how his aventures fellen fro we to wele, and after out of ioye, my purpos is, er that i parte fro ye."[ ] the tragedy of troilus is also the tragedy of criseyde, for even at the moment of forsaking troilus for diomede she is deeply unhappy over her unfaithfulness; but circumstance is as much to blame as her own yielding nature, for troilus' fate is bound up with the inexorable doom of troy, and she could not return to him if she would. there is no doubt that chaucer feels the tragedy of the story as he writes. in his proem to the first book he invokes one of the furies to aid him in his task: "thesiphone, thou help me for tendyte thise woful vers, that wepen as i wryte!"[ ] throughout the poem he disclaims responsibility for what he narrates, saying that he is simply following his author and that, once begun, somehow he must keep on. in the proem to the second book he says: "wherefore i nil have neither thank ne blame of al this werk, but pray you mekely, disblameth me, if any word be lame, for as myn auctor seyde, so seye i."[ ] and concludes the proem with the words,-- "but sin i have begonne, myn auctor shal i folwen, if i conne."[ ] when fortune turns her face away from troilus, and chaucer must tell of the loss of criseyde his heart bleeds and his pen trembles with dread of what he must write: "but al to litel, weylawey the whyle, lasteth swich ioye, y-thonked be fortune! that semeth trewest, whan she wol bygyle, and can to foles so hir song entune, that she hem hent and blent, traytour comune; and whan a wight is from hir wheel y-throwe, than laugheth she, and maketh him the mowe. from troilus she gan hir brighte face awey to wrythe, and took of him non hede, but caste him clene oute of his lady grace, and on hir wheel she sette up diomede; for which right now myn herte ginneth blede, and now my penne, allas! with which i wryte, quaketh for drede of that i moot endyte."[ ] chaucer tells of criseyde's faithlessness reluctantly, reminding the reader often that so the story has it: "and after this the story telleth us, that she him yaf the faire baye stede, the which she ones wan of troilus; and eek a broche (and that was litel nede) that troilus was, she yaf this diomede. and eek, the bet from sorwe him to releve, she made him were a pencel of hir sleve. i finde eek in the stories elles-where, whan through the body hurt was diomede of troilus, tho weep she many a tere, whan that she saugh his wyde woundes blede; and that he took to kepen him good hede, and for to hele him of his sorwes smerte, men seyn, i not, that she yaf him hir herte."[ ] and in the end for very pity he tries to excuse her: "ne me ne list this sely womman chyde ferther than the story wol devyse, hir name, allas! is publisshed so wyde, that for hir gilt it oughte y-now suffyse. and if i mighte excuse hir any wyse, for she so sory was for hir untrouthe, y-wis, i wolde excuse hir yet for routhe."[ ] we have said that chaucer's attitude toward the philosophical aspects of astrology is hard to determine because in most of his poems he takes an impersonal ironic point of view towards the actions he describes or the ideas he presents. his attitude toward the idea of destiny is not so hard to determine. fortune, the executrix of the fates through the influence of the heavens rules men's lives; they are the herdsmen, we are their flocks: "but o, fortune, executrice of wierdes, o influences of thise hevenes hye! soth is, that, under god, ye ben our hierdes, though to us bestes been the causes wrye."[ ] perhaps chaucer did not mean this literally. but one is tempted to think that he, like dante, thought of the heavenly bodies in their spheres as the ministers and instruments of a providence that had foreseen and ordained all things. appendix i. most of the terms at present used to describe the movements of the heavenly bodies were used in chaucer's time and occur very frequently in his writings. the significance of chaucer's references will then be perfectly clear, if we keep in mind that the modern astronomer's description of the _apparent_ movements of the star-sphere and of the heavenly bodies individually would have been to chaucer a description of _real_ movements. when we look up into the sky on a clear night the stars and planets appear to be a host of bright dots on the concave surface, unimaginably distant, of a vast hollow sphere at the canter of which we seem to be. astronomers call this expanse of the heavens with its myriad bright stars the _celestial sphere_ or the _star sphere_, and have imagined upon its surface various systems of circles. in descriptions of the earth's relation to the celestial sphere it is customary to disregard altogether the earth's diameter which is comparatively infinitesimal. if we stand on a high spot in the open country and look about us in all directions the earth seems to meet the sky in a circle which we call the _terrestrial horizon_. now if we imagine a plane passing through the center of the earth and parallel to the plane in which the terrestrial horizon lies, and if we imagine this plane through the earth's center extended outward in all directions to an infinite distance, it would cut the celestial sphere in a great circle which astronomers call the _celestial horizon_. on the celestial horizon are the north, east, south and west points. the plane of the celestial horizon is, of course, different for different positions of the observer on the earth. if we watch the sky for some time, or make several observations on the same night, we notice, by observing the changing positions of the constellations, that the stars move very slowly across the blue dome above us. the stars that rise due east of us do not, in crossing the dome of the sky, pass directly over our heads but, from the moment that we first see them, curve some distance to the south, and, after passing their highest point in the heavens, turn toward the north and set due west. a star rising due east appears to move more rapidly than one rising some distance to the north or south of the east point, because it crosses a higher point in the heavens and has, therefore, a greater distance to traverse in the same length of time. when we observe the stars in the northern sky, we discover that many of them never set but seem to be moving around an apparently fixed point at somewhat more than an angle of °[ ] above the northern horizon and very near the north star. these are called _circum-polar stars_. the whole celestial sphere, in other words, appears to be revolving about an imaginary axis passing through this fixed point, which is called the _north pole_ of the heavens, through the center of the earth and through an invisible pole (the south pole of the heavens) exactly opposite the visible one. this apparent revolution of the whole star sphere, as we know, is caused by the earth's rotation on its axis once every twenty-four hours from west to east. chaucer and his contemporaries believed it to be the actual revolution of the nine spheres from east to west about the earth as a center. [illustration: fig. .] for determining accurately the position of stars on the celestial sphere astronomers make use of various circles which can be made clear by a few simple diagrams. in figure , the observer is imagined to be at o. then the circle nesw is the celestial horizon, which we have described above. z, the point immediately above the observer is called the _zenith_, and z', the point immediately underneath, as indicated by a plumb line at rest, is the _nadir_. the line pop' is the imaginary axis about which the star-sphere appears to revolve, and p and p' are the poles of the heavens. the north pole p is elevated, for our latitude, at an angle of approximately ° from the north point on the horizon. pp' is called the _polar axis_ and it is evident that the earth's axis extended infinitely would coincide with this axis of the heavens. in measuring positions of stars with reference to the horizon astronomers use the following circles: any great circle of the celestial sphere whose plane passes through the zenith and nadir is called a _vertical circle_. the verticle circle spnz', passing through the poles and meeting the horizon in the north and south points, n and s, is called the _meridian circle_, because the sun is on this circle at true mid-day. the _meridian_ is the plane in which this circle lies. the vertical circle, ez'wz, whose plane is at right angles to the meridian, is called the _prime vertical_ and it intersects the horizon at the east and west points, e and w. these circles, and the measurements of positions of heavenly bodies which involve their use, were all employed in chaucer's time and are referred to in his writings.[ ] the distance of a star from the horizon, measured on a vertical circle, toward the zenith is called the star's _altitude_. a star reaches its greatest altitude when on the part of the meridional circle between the south point of the horizon, s, and the north pole, p. a star seen between the north pole and the north point on the horizon, that is, on the arc pn, must obviously be a _circum-polar star_ and would have its highest altitude when between the pole and the zenith, or on the arc pz. when a star reaches the meridian in its course across the celestial sphere it is said to _culminate_ or reach its _culmination_. the highest altitude of any star would therefore be represented by the arc of the meridional circle between the star and the south point of the horizon. this is called the star's _meridian altitude_. the _azimuth_ of a star is its angular distance from the south point, measured westward on the horizon, to a vertical circle passing through the star. the _amplitude_ of a star is its distance from the prime vertical, measured on the horizon, north or south. for the other measurements used by astronomers in observations of the stars still other circles on the celestial sphere must be imagined. we know that the earth's surface is divided into halves, called the northern and southern hemispheres, by an imaginary circle called the _equator_, whose plane passes through the center of the earth and is perpendicular to the earth's axis. if the plane of the earth's equator were infinitely extended it would describe upon the celestial sphere a great circle which would divide that sphere into two hemispheres, just as the plane of the terrestrial equator divides the earth into two hemispheres. this great circle on the celestial sphere is called the _celestial equator_, or, by an older name, the _equatorial_, the significance of which we shall see presently. a star rising due east would traverse this great circle of the celestial sphere and set due west. the path of such a star is represented in figure by the great circle emwm', which also represents the celestial equator. all stars rise and set following circles whose planes are parallel to that of the celestial equator and these circles of the celestial sphere are smaller and smaller the nearer they are to the pole, so that stars very near the pole appear to be encircling it in very small concentric circles. stars in an area around the north celestial pole, whose limits vary with the position of the observer never set for an observer in the northern hemisphere. there is a similar group of stars around the south pole for an observer in the southern hemisphere. [illustration: fig. .] the angle of elevation of the celestial equator to the horizon varies according to the position of the observer. if, for example, the observer were at the north pole of the earth, the north celestial pole would be directly above him and would therefore coincide with the zenith; this would obviously make the celestial equator and the horizon also coincide. if the observer should pass slowly from the pole to the terrestrial equator it is clear that the two circles would no longer coincide and that the angle between them would gradually widen until it reached °. then the zenith would be on the celestial equator and the north and south poles of the heavens would be on the horizon. we have still to define a great circle of the celestial sphere that is of equal importance with the celestial equator and the celestial horizon. this is the sun's apparent yearly path, or the _ecliptic_. we know that the earth revolves about the sun once yearly in an orbit that is not entirely round but somewhat eliptical. now since the earth, the sun, and the earth's orbit around the sun are always in one plane, it follows that to an observer on the earth the sun would appear to be moving around the earth instead of the earth around the sun. the sun's apparent path, moreover, would be in the plane of the earth's orbit and when projected against the celestial sphere, which is infinite in extent, would appear as a great circle of that sphere. this great circle of the celestial sphere is the ecliptic. the sun must always appear to be on this circle, not only at all times of the year but at all hours of the day; for as the sun rises and sets, the ecliptic rises and sets also, since the earth's rotation causes an apparent daily revolution not only of the sun, moon, and planets but also of the fixed stars and so of the whole celestial sphere and of all the circles whose positions upon it do not vary. the ecliptic is inclined to the celestial equator approximately - / °, an angle which obviously measures the inclination of the plane of the earth's equator to the plane of its orbit, since the celestial equator and the ecliptic are great circles on the celestial sphere formed by extending the planes of the earth's equator and its orbit to an infinite distance. since both the celestial equator and the ecliptic are great circles of the celestial sphere each dividing it into equal parts, it is evident that these two circles must intersect at points exactly opposite each other on the celestial sphere. these points are called the vernal and the autumnal equinoxes. we shall next define the astronomical measurements that correspond to terrestrial latitude and longitude. for some reason astronomers have not, as we might expect, applied to these measurements the terms 'celestial longitude' and 'celestial latitude.' these two terms are now practically obsolete, having been used formerly to denote angular distance north or south of the ecliptic and angular distance measured east and west along circles parallel to the ecliptic. the measurements that correspond in astronomy to terrestrial latitude and longitude are called _declination_ and _right ascension_ and are obviously made with reference to the celestial equator, not the ecliptic. for taking these measurements astronomers employ circles on the celestial sphere perpendicular to the plane of the celestial equator and passing through the poles of the heavens. these are called _hour circles_. the hour circle of any star is the great circle passing through it and perpendicular to the plane of the equator. the angular distance of a star from the equator measured along its hour circle, is called the star's declination and is northern or southern according as the star is in the northern or southern of the two hemispheres into which the plane of the equator divides the celestial sphere. it is evident that declination corresponds exactly to terrestrial latitude. right ascension, corresponding to terrestrial longitude, is the angular distance of a heavenly body from the vernal equinox measured on the celestial equator eastward to the hour circle passing through the body. the _hour angle_ of a star is the angular distance measured on the celestial equator from the meridian to the foot of the hour circle passing through the star. [illustration: fig. .] it remains to describe in greater detail the apparent movements of the sun and the sun's effect upon the seasons. in figure , the great circle mwm'e represents the equinoctial and xvx'a the ecliptic. the point x represents the farthest point south that the sun reaches in its apparent journey around the earth, and this point is called the _winter solstice_, because, for the northern hemisphere the sun reaches this point in mid-winter. when the sun is south of the celestial equator its apparent daily path is the same as it would be for a star so situated. thus its daily path at the time of the winter solstice, about december , can be represented by the circle xmn'. the arc gxh represents the part of the sun's path that would be above the horizon, showing that night would last much longer than day and the rays of the sun would strike the northern hemisphere of the earth more indirectly than when the sun is north of the equator. as the sun passes along the ecliptic from x toward v, the part of its daily path that is above the horizon gradually increases until at v, the vernal equinox, the sun's path would, roughly speaking, coincide with the celestial equator so that half of it would be above the horizon and half below and day and night would be of equal length. this explains why the celestial equator was formerly called the equinoctial (chaucer's term for it). as the sun passes on toward x' its daily arc continues to increase and the days to grow longer until at x' it reaches its greatest declination north of the equator and we have the longest day, june , the summer solstice. when the sun reaches this point, its rays strike the northern hemisphere more directly than at any other time causing the hot or summer season in this hemisphere. next the sun's daily arc begins to decrease, day and night to become more nearly equal, at a the autumnal equinox[ ] is reached and the sun again shapes its course towards the point of maximum declination south of the equator. the two points of maximum declination are called _solstices_. the two small circles of the celestial sphere, parallel to the equator, which pass through the two points where the sun's declination is greatest, are called _tropics_; the one in the northern hemisphere is called the _tropic of cancer_, that in the southern hemisphere, the _tropic of capricorn_. they correspond to circles on the earth's surface having the same names. ii. by "artificial day" chaucer means the time during which the sun is above the horizon, the period from sunrise to sunset. the arc of the artificial day may mean the extent or duration of it, as measured on the rim of an astrolabe, or it may mean (as here), the arc extending from the point of sunrise to that of sunset. see _astrolabe_ ii. . there has been some controversy among editors as to the correctness of the date occurring in this passage, some giving it as the th instead of the th. in discussing the accuracy of the reading "eightetethe" skeat throws light also upon the accuracy of the rest of the passage considered from an astronomical point of view. he says (vol. , p. ): "the key to the whole matter is given by a passage in chaucer's 'astrolabe,' pt. ii, ch. , where it is clear that chaucer (who, however merely translates from messahala) actually confuses the hour-angle with the azimuthal arc (see appendix i); that is, he considered it correct to find the hour of the day by noting _the point of the horizon_ over which the sun appears to stand, and supposing this point to advance, with a _uniform_, not a _variable_, motion. the host's method of proceeding was this. wanting to know the hour, he observed how far the sun had moved southward along the horizon since it rose, and saw that it had gone more than half-way from the point of sunrise to the exact southern point. now the th of april in chaucer's time answers to the th of april at present. on april , , the sun rose at hr. m., and set at hr. m., giving a day of about hr. m., the fourth part of which is at hr. m., or, with sufficient exactness, at _half past eight_. this would leave a whole hour and a half to signify chaucer's 'half an houre and more', showing that further explanation is still necessary. the fact is, however, that the host reckoned, as has been said, in another way, viz. by observing the sun's position _with reference to the horizon_. on april the sun was in the th degree of taurus at that date, as we again learn from chaucer's treatise. set this th degree of taurus on the east horizon on a globe, and it is found to be degrees to the north of the east point, or degrees from the south. the half of this at degrees from the south; and the sun would seem to stand above this th degree, as may be seen even upon a globe, at about a quarter past nine; but mr. brae has made the calculation, and shows that it was at _twenty minutes past nine_. this makes chaucer's 'half an houre and more' to stand for _half an hour and ten minutes_; an extremely neat result. but this we can check again by help of the host's _other_ observation. he _also_ took note, that the lengths of a shadow and its object were equal, whence the sun's altitude must have been degrees. even a globe will shew that the sun's altitude, when in the th degree of taurus, and at o'clock in the morning, is somewhere about or degrees. but mr. brae has calculated it exactly, and his result is, that the sun attained its altitude of degrees at _two minutes to ten_ exactly. this is even a closer approximation than we might expect, and leaves no doubt about the right date being the _eighteenth_ of april." thus it appears that chaucer's method of determining the date was incorrect but his calculations in observing the sun's position were quite accurate. for fuller particulars see chaucer's _astrolabe_, ed. skeat (e. e. t. s.) preface, p. . iii. it was customary in ancient times and even as late as chaucer's century to determine the position of the sun, moon, or planets at any time by reference to the signs of the zodiac. the _zodiac_ is an imaginary belt of the celestial sphere, extending ° on each side of the ecliptic, within which the orbits of the sun, moon, and planets appear to lie. the zodiac is divided into twelve equal geometric divisions ° in extent called _signs_ to each of which a fanciful name is given. the signs were once identical with twelve constellations along the zodiac to which these fanciful names were first applied. since the signs are purely geometric divisions and are counted from the spring equinox in the direction of the sun's progress through them, and since through the precession of the equinoxes the whole series of signs shifts westward about one degree in seventy-two years, the signs and constellations no longer coincide. beginning with the sign in which the vernal equinox lies the names of the zodiacal signs are aries (ram), taurus (bull), gemini (twins), cancer (crab), leo (lion), virgo (virgin), libra (scales), scorpio (scorpion), sagittarius (archer), aquarius (water-carrier), and pisces (fishes). in this passage, the line "that in the ram is four degrees up-ronne" indicates the date march . this can be seen by reference to figure in skeat's edition of chaucer's _astrolabe_ (e. e. t. s.) the astrolabe was an instrument for making observations of the heavenly bodies and calculating time from these observations. the most important part of the kind of astrolabe described by chaucer was a rather heavy circular plate of metal from four to seven inches in diameter, which could be suspended from the thumb by a ring attached loosely enough so as to allow the instrument to assume a perpendicular position. one side of this plate was flat and was called the _back_, and it is this part that figure represents. the back of the astrolabe planisphere contained a series of concentric rings representing in order beginning with the outermost ring: the four quadrants of a circle each divided into ninety degrees; the signs of the zodiac divided into thirty degrees each; the days of the year, the circle being divided, for this purpose, into - / equal parts; the names of the months, the number of days in each, and the small divisions which represent each day, which coincide exactly with those representing the days of the year; and lastly the saints' days, with their sunday-letters. the purpose of the signs of the zodiac is to show the position of the sun in the ecliptic at different times. therefore, if we find on the figure the fourth degree of aries and the day of the month corresponding to it, we have the date march as nearly as we can determine it by observing the intricate divisions in the figure. the next passage "noon hyer was he, whan she redy was" means evidently, 'he was no higher than this (i. e. four degrees) above the horizon when she was ready'; that is, it was a little past six. the method used in determining the time of day by observation of the sun's position is explained in the astrolabe ii, and . first the sun's altitude is found by means of the revolving rule at the back of the astrolabe. the rule, a piece of metal fitted with sights, is moved up and down until the rays of the sun shine directly through the sights. then, by means of the degrees marked on the back of the astrolabe, the angle of elevation of the rule is determined, giving the altitude of the sun. the rest of the process involves the use of the _front_ of the astrolabe. this side of the circular plate, shown in fig. , had a thick rim with a wide depression in the middle. on the rim were three concentric circles, the first showing the letters a to z, representing the twenty-four hours of the day, and the two innermost circles giving the degrees of the four quadrants. the depressed central part of the front was marked with three circles, the 'tropicus cancri', the 'aequinoctialis,' and the 'tropicus capricorni'; and with the cross-lines from north to south, and from east to west. there were besides several thin plates or discs of metal of such a size as exactly to drop into the depression spoken of. the principal one of these was the 'rete' and is shown in fig. . "it consisted of a circular ring marked with the zodiacal signs, subdivided into degrees, with narrow branching limbs both within and without this ring, having smaller branches or tongues terminating in points, each of which denoted the exact position of some well-known star. * * * the 'rete' being thus, as it were, a skeleton plate, allows the 'tropicus cancri,' etc., marked upon the body of the instrument, to be partially seen below it. * * * but it was more usual to interpose between the 'rete' and the body of the instrument (called the 'mother') another thin plate or disc, such as that in fig. , so that portions of this latter plate could be seen beneath the skeleton-form of the 'rete' (i. ). these plates were called by chaucer 'tables', and sometimes an instrument was provided with several of them, differently marked, for use in places having different latitudes. the one in fig. is suitable for the latitude of oxford (nearly). the upper part, above the horizon obliquus, is marked with circles of altitude (i. ), crossed by incomplete arcs of azimuth tending to a common centre, the zenith (i. )." [skeat, _introduction to the astrolabe_, pp. lxxiv-lxxv.] now suppose we have taken the sun's altitude by § (pt. ii of the _astrolabe_) and found it to be - / °. "as the altitude was taken by the back of the astrolabe, turn it over, and then let the _rete_ revolve westward until the st point of aries is just within the altitude-circle marked , allowing for the / degree by guess. this will bring the denticle near the letter c, and the first point of aries near x, which means a.m." [skeat's note on the _astrolabe_ ii. , pp. - ]. iv. chaucer would know the altitude of the sun simply by inspection of an astrolabe, without calculation. skeat has explained this passage in his _preface to chaucer's astrolabe_ (e. e. t. s.), p. lxiii, as follows: "besides saying that the sun was ° high, chaucer says that his shadow was to his height in the proportion of to . changing this proportion, we can make it that of to - / ; that is, the point of the _umbra versa_ (which is reckoned by twelfth parts) is - / or - / nearly. (umbra recta and umbra versa were scales on the back of the astrolabe used for computing the altitudes of heavenly bodies from the height and shadows of objects. the _umbra recta_ was used where the angle of elevation of an object was greater than °; the _umbra versa_, where it was less.) this can be verified by fig. ; for a straight edge, laid across from the th degree above the word 'occidens,' and passing through the center, will cut the scale of umbra versa between the th and th points. the sun's altitude is thus established as ° above the western horizon, beyond all doubt." v. _herberwe_ means 'position.' chaucer says here, then, that the sun according to his declination causing his position to be low or high in the heavens, brings about the seasons for all living things. in the _astrolabe_, i. , there is a very interesting passage explaining in detail, declination, the solstices and equinoxes, and change of seasons. chaucer is describing the front of the astrolabe. he says: "the plate under thy rite is descryved with principal cercles; of whiche the leste is cleped the cercle of cancer, by-cause that the heved of cancer turneth evermor consentrik up-on the same cercle. (this corresponds to the tropic of cancer on the celestial sphere, which marks the greatest northern declination of the sun.) in this heved of cancer is the grettest declinacioun northward of the sonne. and ther-for is he cleped the solsticioun of somer; whiche declinacioun, aftur ptholome, is degrees and minutes, as wel in cancer as in capricorne. (the greatest declination of the sun measures the obliquity of the ecliptic, which is slightly variable. in chaucer's time it was about ° ', and in the time of ptolemy about ° '. ptolemy assigns it too high a value.) this signe of cancre is cleped the tropik of somer, of _tropos_, that is to seyn 'agaynward'; for thanne by-ginneth the sonne to passe fro us-ward. (see fig. in skeat's _preface to the astrolabe_, vol. iii, or e. e. t. s. vol. .) the middel cercle in wydnesse, of thise , is cleped the cercle equinoxial (the celestial equator of the celestial sphere); up-on whiche turneth evermo the hedes of aries and libra. (these are the two signs in which the ecliptic crosses the equinoctial.) and understond wel, that evermo this cercle equinoxial turneth iustly fro verrey est to verrey west; as i have shewed thee in the spere solide. (as the earth rotates daily from west to east, the celestial sphere appears to us to revolve about the earth once every twenty-four hours from east to west. chaucer, of course, means here that the equinoctial actually revolves with the _primum mobile_ instead of only appearing to revolve.) this same cercle is cleped also the weyere, _equator_, of the day; for whan the sonne is in the hevedes of aries and libra, than ben the dayes and the nightes ilyke of lengthe in al the world. and ther-fore ben thise two signes called equinoxies. the wydeste of thise three principal cercles is cleped the cercle of capricorne, by-cause that the heved of capricorne turneth evermo consentrix up-on the same cercle. (that is to say, the tropic of capricorn meets the ecliptic in the sign capricornus, or, in other words, the sun attains its greatest declination southward when in the sign capricornus.) in the heved of this for-seide capricorne is the grettest declinacioun southward of the sonne, and ther-for is it cleped the solsticioun of winter. this signe of capricorne is also cleped the tropik of winter, for thanne byginneth the sonne to come agayn to us-ward." vi. the moon's orbit around the earth is inclined at an angle of about ° to the earth's orbit around the sun. the moon, therefore, appears to an observer on the earth as if traversing a great circle of the celestial sphere just as the sun appears to do; and the moon's real orbit projected against the celestial sphere appears as a great circle similar to the ecliptic. this great circle in which the moon appears to travel will, therefore, be inclined to the ecliptic at an angle of ° and the moon will appear in its motion never far from the ecliptic; it will always be within the zodiac which extends eight or nine degrees on either side of the ecliptic. the angular velocity of the moon's motion in its projected great circle is much greater than that of the sun in the ecliptic. both bodies appear to move in the same direction, from west to east; but the solar apparent revolution takes about a year averaging ° daily, while the moon completes a revolution from any fixed star back to the same star in about - / days, making an average daily angular motion of about °. the actual daily angular motion of the moon varies considerably; hence in trying to test out chaucer's references to lunar angular velocity it would not be correct to make use only of the average angular velocity since his references apply to specific times and therefore the variation in the moon's angular velocity must be taken into account. vii. on the line "in two of taur," etc., skeat has the following note: "tyrwhitt unluckily altered _two_ to _ten_, on the plea that 'the time (_four days complete_, l. ) is not sufficient for the moon to pass from the second degree of taurus into cancer? and he then proceeds to shew this, taking the _mean_ daily motion of the moon as being degrees, minutes, and seconds. but, as mr. brae has shewn, in his edition of chaucer's astrolabe, p. , footnote, it is a mistake to reckon here the moon's _mean_ motion; we must rather consider her _actual_ motion. the question is simply, can the moon move from the nd degree of taurus to the st of cancer (through degrees) in four days? mr. brae says decidedly, that examples of such motion are to be seen 'in every almanac.' for example, in the nautical almanac, in june, , the moon's longitude at noon was ° ' on the th, and ° ' on the th; i. e., the moon was in the _first_ of taurus on the former day, and in the _first_ of cancer on the latter day, at the same hour; which gives (very nearly) a degree more of change of longitude than we here require. the mss all have _two_ or _tuo_, and they are quite right. the motion of the moon is so variable that the mean motion affords no safe guide." [skeat, _notes to the canterbury tales_, p. .] viii. the moon's "waxing and waning" is due to the fact that the moon is not self-luminous but receives its light from the sun and to the additional fact that it makes a complete revolution around the earth with reference to the sun in - / days. when the earth is on the side of the moon that faces the sun we see the full moon, that is, the whole illuminated hemisphere. but when we are on the side of the moon that is turned away from the sun we face its unilluminated hemisphere and we say that we have a 'new moon.' once in every - / days the earth is in each of these positions with reference to the moon and, of course, in the interval of time between these two phases we are so placed as to see larger or smaller parts of the illuminating hemisphere of the moon, giving rise to the other visible phases. when the moon is between the earth and the sun she is said to be in _conjunction_, and is invisible to us for a few nights. this is the phase called _new moon_. as she emerges from conjunction we see the moon as a delicate crescent in the west just after sunset and she soon sets below the horizon. half of the moon's surface is illuminated, but we can see only a slender edge with the horns turned away from the sun. the crescent appears a little wider each night, and, as the moon recedes ° further from the sun each night, she sets correspondingly later, until in her first quarter half of the illuminated hemisphere is turned toward us. as the moon continues her progress around the earth she gradually becomes gibbous and finally reaches a point in the heavens directly opposite the sun when she is said to be in _opposition_, her whole illumined hemisphere faces us and we have _full moon_. she then rises in the east as the sun sets in the west and is on the meridian at midnight. as the moon passes from opposition, the portion of her illuminated hemisphere visible to us gradually decreases, she rises nearly an hour later each evening and in the morning is seen high in the western sky after sunrise. at her _third quarter_ she again presents half of her illuminated surface to us and continues to decrease until we see her in crescent form again. but now her position with reference to the sun is exactly the reverse of her position as a waxing crescent, so that her horns are now turned toward the west away from the sun, and she appears in the eastern sky just before sunrise. the moon again comes into conjunction and is lost in the sun's rays and from this point the whole process is repeated. ix. that the apparent motions of the sun and moon are not so complicated as those of the planets will be clear at once if we remember that the sun's apparent motion is caused by our seeing the sun projected against the celestial sphere in the ecliptic, the path cut out by the plane of the earth's orbit, while in the case of the moon, what we see is the moon's actual motion around the earth projected against the celestial sphere in the great circle traced by the moon's own orbital plane produced to an indefinite extent. these motions are further complicated by the rotation of the earth on its own axis, causing the rising and setting of the sun and the moon. these two bodies, however, always appear to be moving directly on in their courses, each completing a revolution around the earth in a definite time, the sun in a year, the moon in - / days. what we see in the case of the planets, on the other hand, is a complex motion compounded of the effects of the earth's daily rotation, its yearly revolution around the sun, and the planets' own revolutions in different periods of time in elliptical orbits around the sun. these complex planetary motions are characterized by the peculiar oscillations known as 'direct' and 'retrograde' movements. [illustration: fig. .] the motion of a planet is said to be _direct_ when it moves in the direction of the succession of the zodiacal signs; _retrograde_ when in the contrary direction. all of the planets have periods of retrograde and direct motion, though their usual direction is direct, from west to east. retrograde motion can be explained by reference to the accompanying diagrams. in fig. , the outer circle represents the path of the zodiac on the celestial sphere. let the two inner circles represent the orbits of the earth and an inferior planet, venus, around the sun, at s. (an _inferior_ planet is one whose orbit around the sun is within that of the earth. a _superior_ planet is one whose orbit is outside that of the earth.) v, v' and v", and e, e', and e" are successive positions of the two planets in their orbits, the arc vv" being longer than the arc ee" because the nearer a planet is to the sun, the greater is its velocity. then when venus is at v and the earth at e, we shall see venus projected on the celestial sphere at v{ }. when venus has passed on to v' the earth will have passed to e' and we shall see venus on the celestial sphere at v{ }. the apparent motion of the planet thus far will have been direct, from west to east in the order of the signs. but when venus is at v" and the earth at e" venus will be seen at v{ } having apparently moved back about two signs in a direction the reverse of that taken at first. this is called the planet's retrograde motion. at some point beyond v", the planet will appear to stop moving for a very short period and then resume its direct motion. in fig. , the outer arc again represents the path of the zodiac on the celestial sphere. the smaller arcs represent the orbits of the superior planet, mars, and the earth around the sun, s. at the point of opposition of mars (when mars and the sun are at opposite points in the heavens to an observer on the earth) we should see mars projected on the zodiac at m{ }. after a month mars will be at m' and the earth at e', so that in its apparent motion mars will have retrograded to m{ }. after three months from opposition mars will be at m" and the earth at e", making mars appear at m{ } on the celestial sphere, its motion having changed from retrograde to direct. [illustration: fig. .] both figures and take no account of the fact that the earth's orbit and those of the planets are not in exactly the same planes. remembering this fact we see at once that the apparent oscillations of the planets are not back and forth in a straight line but in curves and spirals. it is easy to see why the apparent motions of the planets were accounted for by deferents and epicycles, before the copernican system revealed the true nature of the solar system as heliocentric and not geocentric. selected bibliography berry, arthur, _a short history of astronomy_. new york. . bryant, w. w., _a history of astronomy_. london. . cumont, franz, _astrology and religion among the greeks and romans_. new york. . cushman, h. e., _a beginner's history of philosophy_. boston. . dreyer, j. l. e., _history of the planetary systems from thales to kepler_. cambridge. . evershed, m. a., _dante and the early astronomers_. london. . gomperz, t., _greek thinkers, a history of ancient philosophy_. new york. . gore j. ellard, _astronomical essays, historical and descriptive_. london. . hinks, a. r., _astronomy_. london. . jacoby, harold, _astronomy_. new york. . jastrow, morris, "astrology," _encyclopaedia britannica_ ii, - . lea, h. c., _history of the inquisition of the middle ages_. new york. . iii. - . orchard, t. n., _milton's astronomy_. new york. . taylor, h. o., _the mediaeval mind_. vols. new york. . todd, mabel l., _steele's popular astronomy_. new york. . traill, h. d., _social england_. new york and london. . wallace, a. r., _man's place in the universe_. london. . white, a. d., _warfare of science with theology_. new york and london. . i. . * * * * * chaucer, _the complete works of geoffrey chaucer_. w. w. skeat, edit. clarendon press. . chaucer, _treatise on the astrolabe_, a. e. brae, edit. london. . _cambridge history of english literature, the_, ed. by a. w. ward and a. r. waller. vol. ii. . ten brink, bernard, _history of english literature_. vol. ii. new york. . courthope, w. j., _literary history of the english people_. vol. i. new york. . hadow, grace e., _chaucer and his times_. new york. . hammond, eleanor p., _chaucer: a bibliographical manual_. new york. . jusserand, j. j., _history of english poetry_. vol. ii. london. . kittredge, g. l., _chaucer and his poetry_. harvard university press. . legouis, emile, _geoffrey chaucer_. trans. by l. lailavoix. london. . lounsbury, t. r., _studies in chaucer_. new york. . morley, henry, _english writers_. vol. v. london. ff. root, robert k., _the poetry of chaucer_. boston and new york. . tatlock, john s. p., "astrology and magic in chaucer's _franklin's tale_." kittredge anniversary papers. . tatlock, john s. p., _the scene of the franklin's tale visited_. chaucer society publications. . footnotes: [ ] the name of ptolemy occurs once in _the somnours tale_ (d. ): "as wel as euclide or (as) ptholomee." and once in _the astrolabe_, i. . : "whiche declinacioun, aftur ptholome, is degrees and minutes, as wel in cancer as in capricorne." the _almagest_ is mentioned in _the milleres tale_ (a. ): "his almageste and bokes grete and smale," twice in _the wif of bathes prologue_ occur both the name of the _almagest_ and that of its author: "'who-so that nil be war by othere men, by him shul othere men corrected be. the same wordes wryteth ptholomee; rede in his almageste, and take it there.'" (d. - ) "of alle men y-blessed moot he be, the wyse astrologien dan ptholome, that seith this proverbe in his almageste, 'of alle men his wisdom is the hyeste, that rekketh never who hath the world in honde.'" (d. - ) professor lounsbury (_studies in chaucer_, ii p. and pp. - ) has difficulty in explaining why chaucer makes the wife of bath attribute these moral maxims to ptolemy. he is inclined to think that chaucer, so to speak, was napping when he put these utterances into the mouth of the wife of bath; yet elsewhere he acknowledges that the supposition of confused memory on chaucer's part in this case is hard to reconcile with the knowledge he elsewhere displays of ptolemy's work. i think it very probable that chaucer's seeming slip here is deliberate art. the wife of bath is one of chaucer's most humorous creations and the blunders he here attributes to her are quite in keeping with her character. from her fifth husband, who was a professional scholar and a wide reader, she has picked up a store of scattered and incomplete information about books and names, and she loses no opportunity for displaying it. at any rate, whether or not chaucer had read the _almagest_ in translation, his many cosmological and astronomical references show clearly his acquaintance with the ptolemaic system of astronomy. [ ] an arabian scholar of the eighth century. [ ] . ff. "this tretis, divided in fyve parties, wole i shewe thee under ful lighte rewles and naked wordes in english; for latin ne canstow yit but smal, my lyte sone." [ ] "and lowis, yif so be that i shewe thee in my lighte english as trewe conclusiouns touching this matere, and naught only as trewe but as many and as subtil conclusiouns as ben shewed in latin in any commune tretis of the astrolabie, con me the more thank;" _prologue to the astrolabe_, - . [ ] skeat, _notes on the astrolabe, prologue_, . "warton says that 'john some and nicholas lynne' were both carmelite friars, and wrote calendars constructed for the meridian of oxford. he adds that nicholas lynne is said to have made several voyages to the most northerly parts of the world, charts of which he presented to edward iii. these charts are, however, lost." [ ] _the astrolabe_, i. . . according to warton the work in question is an introduction to judicial astronomy. (lounsbury, ii. .) [ ] f. . "his tables toletanes forth he broght." [ ] _englische studien_ iii . see also j. s. p. tatlock, "chaucer and dante," in _modern philology_, iii, . . [ ] _parlement of foules_, - . [ ] _compleynt of mars_, . [ ] _lenvoy de chaucer a scogan_, - . "by worde eterne whylom was hit shape that fro the fifte cercle, in no manere, ne mighte a drope of teres doun escape. but now so wepeth venus in hir spere, that with hir teres she wol drenche us here." [ ] since chaucer calls mars the lord of the third heaven and elsewhere speaks of venus as presiding over that sphere it is evident that he sometimes reckons from the earth outwards, and sometimes from the outer sphere of saturn towards the earth. the regular order of the planets, counting from the earth, was supposed to be as follows: moon, mercury, venus, sun, mars, jupiter, saturn, making mars the third from the last. [ ] iii. - . [ ] "o firste moevyng cruel firmament, with thy diurnal sweigh that crowdest ay and hurlest al from est til occident, that naturelly wolde holde another way." (b. - ) chaucer does not use the term 'firmament' with sole reference to the star-sphere. here it clearly refers to the _primum mobile_; it often applies to the whole expanse of the heavens. [ ] _boethius_, book i: metre v, - . the conception of god as the creator and unmoved mover of the universe originated in the philosophy of aristotle, who was the one great authority, aside from scripture and the church fathers, recognized by the middle ages. god's abode was thought to be in the empyrean, the motionless sphere beyond the ninth, and the last heaven. this is the meaning in the reference to the eternal throne ("perdurable chayer") of god. [ ] many of these beautiful descriptions, however, are not strictly chaucer's own, since they occur in his translation of boethius. it will suffice to quote one of these descriptions: "and, right by ensaumple as the sonne is hid whan the sterres ben clustred (_that is to seyn, whan sterres ben covered with cloudes_) by a swifte winde that highte chorus, and that the firmament stant derked by wete ploungy cloudes, and that the sterres nat apperen up-on hevene, so that the night semeth sprad up-on erthe: yif thanne the wind that highte borias, y-sent out of the caves of the contres of trace, beteth this night (_that is to seyn, chaseth it a-wey_), and descovereth the closed day: than shyneth phebus y-shaken with sodein light, and smyteth with his bemes in mervelinge eyen." (_boethius_, book i.: metre iii. - .) [ ] _hymn on the nativity_, xiii. [ ] _the merchant of venice_, act. v. sc. i. [ ] _parlement of foules_, - . [ ] _troilus and criseyde_, v. - . [ ] a. - . [ ] _hous of fame_, ii. ff. [ ] _seconde nonnes tale_, g. - . [ ] _the seconde nonnes tale_, g. - . [ ] a. . [ ] b. ff. [ ] _the persones tale_, i. ff.: "ther shal the sterne and wrothe luge sitte above, and under him the horrible put of helle open to destroyen him that moot biknowen hise sinnes, whiche sinnes openly been shewed biforn god and biforn every creature. and on the left syde, mo develes than herte may bithinke, for to harie and drawe the sinful soules to the pyne of helle. and with-inne the hertes of folk shal be the bytinge conscience, and withoute-forth shal be the world al brenninge." [ ] _the persones tale_, i. - . [ ] _the wife of bath's prologue_, d. . [ ] _the marchantes tale_, e. ff. [ ] _the knightes tale_, a. - . [ ] _troilus and criseyde_, bk. iv. . [ ] _marchantes tale_, e. . [ ] _ibid._ e. - . [ ] _the legend of good women_, iii. ff. [ ] _the monkes tale_, b. . [ ] _the pardoneres tale_, c. - . [ ] in the time of hamurabi, , years before christ, the chaldeans worshipped as beneficent or formidable powers, the earth, that may give or refuse sustenance to man, the waters that fertilize or devastate, the winds that blow from the four quarters of the world, fire that warms or devours and all forces of nature which, in their sidereal religion, they confounded with the stars, giving them the generic name of 'elements.' but the system that recognizes only four elements as the original sources of all that exists in nature, was created by the greek philosophers. see f. cumont, _astrology and religion among the greeks and romans_ ( ), p. . [ ] _paradiso_ i. - . [ ] _paradiso_ i. - . [ ] book iii.: metre ix. ff. [ ] _the knightes tale_, a. - . [ ] _troilus and criseyde_, v. - . [ ] _boethius_, book iv.; metre i. l ff. [ ] _the hous of fame_, ii. - . [ ] _boethius_, book ii.: metre viii. l. ff. "that the world with stable feith varieth acordable chaunginges; that the contrarious qualitee of elements holden among hemself aliaunce perdurable; ... --al this acordaunce of things is bounden with love, that governeth erthe and see, and hath also commaundements to the hevenes. and yif this love slakede the brydeles, alle things that now loven hem to-gederes wolden maken a bataile continuely, and stryven to fordoon the fasoun of this worlde, the whiche they now leden in acordable feith by faire moevinges." the thought of love as the harmonizing bond between diverse elements is dealt with more poetically in _troilus and criseyde_, bk. iii. - . "'love, that of erthe and see hath governaunce, love, that his hestes hath in hevene hye, . . . . . . . . . . that that the world with feyth, which that is stable, dyverseth so his stoundes concordinge, that elements that been so discordable holden a bond perpetuely duringe. that phebus mote his rosy day forth bringe, and that the mone hath lordship over the nightes, al this doth love; ay heried be his mightes!'" [ ] skeat, _notes to boethius_, ii.: metre , . . [ ] . - . [ ] _the phisiciens tale_, c. - . [ ] see appendix, i. [ ] b. l ff. "our hoste sey wel that the brighte sonne the ark of his artificial day had ronne the fourthe part, and half an houre, and more; and though he were not depe expert in lore, he wiste it was the eightetethe day of april, that is messager to may; and sey wel that the shadwe of every tree was as in lengthe the same quantitee that was the body erect that caused it. and therefor by the shadwe he took his wit that phebus, which that shoon so clere and brighte, degrees was fyve and fourty clombe on highte; and for that day, as in that latitude, it was ten of the clokke, he gan conclude, and sodeynly he plighte his hors aboute." for chaucer's accuracy in this reference see appendix ii. [ ] _prologue_, - . [ ] planets are said to be in conjunction with one another when they appear as one object or very close together within a limited area of the sky. [ ] _the hous of fame_, book i. - . cf. dante, _paradiso_ i. - : "i not long endured him, nor yet so little but that i saw him sparkle all around, like iron issuing molten from the furnace. and, of a sudden, meseemed that day was added unto day, as though he who hath the power, had adorned heaven with a second sun." [ ] _the marchantes tale_, e. - . [ ] _prologue to the legend of good women_, - . [ ] _troilus and criseyde_, v. . [ ] _ibid._ v, . [ ] _compleynt of mars_, . the epithet "candel of ielosye" is an allusion to the classical myth according to which phoebus (the sun), having discovered the amour between mars and venus, revealed it to vulcan thus arousing him to jealousy. [ ] _troilus and criseyde_, ii, - . [ ] _ibid._ v. - . [ ] _troilus and criseyde_, iii. - . [ ] _compleynt of mars_, - . [ ] _frankeleyns tale_, f. - . [ ] see appendix iii. [ ] _knightes tale_, a. - . [ ] _parson's prologue_, i. - . see appendix iv. [ ] _nonne preestes tale_, b. - . chaucer has already indicated the date as may by saying that march is complete and thirty-two days have passed besides. (l. ). that the sun would on may have passed the st degree of aries can be verified by reference to fig. in skeat's _introduction to the astrolabe_. a straight edge ing may would cross the circle of the zodiacal signs at a point a little past the st degree of aries. [ ] ascension means 'ascending degree.' [ ] _nonne preestes tale_, b. - . [ ] the sun reaches his farthest point to the south at noon when on the meridian. see appendix i. [ ] _prologue_, . [ ] _ibid._ - . [ ] _marchantes tale_, e. - . [ ] _frankeleyns tale_, f. - . [ ] see appendix i. ff., ff. [ ] _prologue to the canterbury tales_, a. - . at the beginning of april the sun is a little past the middle of aries and at the beginning of may, roughly speaking, he is in the middle of taurus. thus the sun in april runs a half-course in aries and a half-course in taurus. chaucer means here that the former of these half-courses is completed, so that it is some time after the eleventh of april. [ ] _troilus and criseyde_, ii. - . on the third of may, in chaucer's time, the sun would be past the twentieth degree of taurus. [ ] the sun's declination means his angular distance north or south of the celestial equator. the solstices mark his maximum declination north or south. see appendix i. ff. [ ] v. - . [ ] _frankeleyns tale_, f. - . see appendix v. [ ] latoun was a compound metal containing chiefly copper and zinc. [ ] f. - . [ ] _astrolabe_, _prologue_, - . [ ] _legend of good women_, iii. - . [ ] _troilus and criseyde_, iv. . [ ] _book of the duchesse_, - . [ ] _romaunt of the rose_, - . [ ] see appendix vi. [ ] _marchantes tale_, e. - . to pass from the second degree of taurus into cancer the moon would have to traverse the remaining twenty-eight degrees of taurus, thirty of gemini and at least one of cancer, making ° of the zodiac in all. for the moon to do this is possible, as skeat has shown. see appendix vii. [ ] _marchantes tale_, e. - . [ ] _troilus and criseyde_, iv. - . chaucer's reference to the moon's motion is again correct. it would, in fact, take the moon about ten days to pass from aries through leo, traversing four signs, taurus, gemini, cancer, and leo, or about one-third of the whole zodiac. see skeat, _notes to troilus and criseyde_, p. . [ ] the moon. [ ] the 'sign-bearer'; that is, the zodiac. his candles are of course the stars and planets that appear in the zodiac. [ ] _troilus and criseyde_, v. - ; - . [ ] _legend of good women_, - . [ ] _compleynt of mars_, . [ ] _hous of fame_, - . [ ] _troilus and criseyde_, iii. . [ ] _ibid._ v. . "by the morwe" means 'early in the morning.' [ ] _troilus and criseyde_, iii. edt-ej. see appendix viii. p. . [ ] book i.: metre v. - . [ ] _frankeleyns tale_, f. - . [ ] _frankeleyns tale_, f. - . skeat explains the lines: "next at this opposicioun, which in the signe shal be of the leoun," thus: earlier in the poem (l. ) may is mentioned and it is on this date that the events narrated so far are supposed to have taken place. in may the sun is in taurus, so that the moon at her next opposition would have to be in the opposite sign, scorpio. the reference must mean therefore:--"at the next opposition that takes place with the sun in leo," not the very next one with the sun in taurus, nor the next with the sun in gemini or cancer. this reason for waiting until there should be an opposition with the sun in leo, was astrological. leo was the _mansion_ of the sun, so that the sun's power when in that sign would be greatest. [ ] b. - . [ ] book iv.: metre v. - . [ ] ibid. - . [ ] see appendix ix. p. ff. [ ] _hous of fame_, iii. - . [ ] _book of the duchesse_, iii. - . [ ] _astrolabe_, ii. . - . the attempt to explain the moon's motion by supposing her to move in an epicycle was hopelessly wrong. chaucer means here simply that the moon's motion in her deferent is direct like that of the other planets (their apparent motion is in the direction west to east except at short periods of retrogression) but that the moon's direction of motion in her epicycle is the reverse of that of the other planets. [ ] ii. . [ ] see appendix ix. p. ff. [ ] book i: metre ii. - . [ ] mercury and venus are always seen either just before sunrise or just after sunset because their distances from the sun are so comparatively small. [ ] _boethius_, bk. i.: metre v. - . [ ] _ibid._ bk. iii.: metre i. - . [ ] _troilus and criseyde_, bk. iii. - . [ ] _ibid._ v. - . [ ] a. - . [ ] . [ ] this is an astrological term. a _palace_, _mansion_ or _house_ was that zodiacal sign in which a planet was supposed to be peculiarly at home. [ ] _compleynt of mars_, - . mars is to hurry until he reaches venus' palace and then advance as slowly as possible, to wait for her. evidently chaucer was aware of the varying apparent velocities of planetary motions. [ ] _ibid._ - . when venus overtakes mars they are in conjunction. [ ] _ibid._ - . [ ] that is, the motions of both planets are direct, not retrograde. [ ] _ibid._ - . [ ] _ibid._ - . [ ] that is, the two planets appear very close together in the sky. [ ] _knightes tale_, a. - . [ ] - : "the grete ioye that was betwix hem two, whan they be met, ther may no tunge telle." [ ] ii. . [ ] iii. - . [ ] _convivio_, ii. xv. . [ ] - . [ ] _hous of fame_, ii. - . [ ] _inferno_, xvii. - . [ ] _convivio_, ii. xv. - . [ ] mrs. john evershed, _dante and the early astronomers_, p. . [ ] _prologue to the canterbury tales_, a. - . [ ] ii. . - . [ ] f. - . [ ] f. - . [ ] f. - . [ ] _studies in chaucer_, vol. ii. , ff. [ ] "the scene of _the franklin's tale_ visited," _chaucer society publications_, ( ); "astrology and magic in chaucer's _franklin's tale_;" _kittredge anniversary papers_ ( ). [ ] _chaucer and his poetry_, p. , ff. [ ] the principal aspects were conjunction, sextile, quartile, trine, and opposition, corresponding respectively to the angular distances °, °, °, ° and °. [ ] _knightes tale_, a. - . [ ] _tale of the man of lawe_, b. - . [ ] _ibid._ - . [ ] _troilus and criseyde_, iv. - . [ ] ix. - . [ ] her father, egistes, because he feared her husband, bade her kill him by cutting his throat, and threatened her with death if she refused. [ ] in astrology the signs of the zodiac were called 'houses' or 'mansions' and each was assigned to a particular planet. when a planet was in its house or mansion, its power was very great. each of the planets had also a sign called its 'exaltation' and in this sign its power was greatest of all. the sign opposite a planet's mansion was called its 'fall' and that opposite its exaltation was called its 'depression'; these were the positions of least influence. mars' mansions were aries and scorpio; his exaltation, capricornus; his fall, libra and taurus, and his depression, cancer. at the time of hypermnestra's birth, then, we may suppose that mars was in libra, taurus or in cancer. if he was in libra or taurus, his influence would be suppressed by venus, as these signs were in her mansions. [ ] _knightes tale_, a. - . [ ] _ibid._ - . [ ] _ibid._ . [ ] _astrolabe_, ii. . - . the term "hous" is here used in a different sense from that in the passage explained above, p. . the whole heavens were divided into twelve portions by great circles passing through the north and south points of the horizon. the one of these just rising was called the 'house of the ascendant.' [ ] _wife of bath's prologue_, d. - . the line "myn ascendent was taur, and mars ther-inne" means that at the time of her birth taurus was just rising in the east and mars was in this sign, and as taurus was the mansion of venus, the influences of the two planets would be mingled. [ ] d. - . [ ] a. - . [ ] iii. - . [ ] iii. - . [ ] - . [ ] - . [ ] - . [ ] _troilus and criseyde_, iv. - ; - . [ ] i. - . [ ] ii. - . [ ] a planet was said to be _combust_ when its light was extinguished by proximity to the sun. when venus and mercury were 'combust' their influence was lost. [ ] iii. - . it is sometimes hard to determine whether the beings prayed to are pagan gods and goddesses or heavenly bodies. this passage makes it clear that the planets were identified with the pagan divinities. in the rest of this prayer troilus addresses mars, mercury, jupiter, etc., as gods, referring in each case to some love affair, from ancient myth, that may win the god's sympathy and help. [ ] iii. - . the "wel-willy planete" means the propitious or favorable one. [ ] v. - . troilus needs the aid of venus especially on the tenth night after criseyde's departure, because she had promised to return on that night. [ ] f. - . [ ] ii. - . [ ] _knightes tale_, a. . [ ] _ibid._ - . [ ] a. - . [ ] a. - . [ ] a. - . this is the mediaeval christian idea of destiny or the fore-knowledge of god, and is appropriately uttered here by the knight. [ ] a. ff; ff; ff. [ ] diana was called _luna_ (or the moon) in heaven, on earth, _diana_ or _lucina_, and in hell, _proserpina_. [ ] a. - . [ ] a. - ; - . [ ] _troilus and criseyde_, i. - . [ ] _ibid._ i. - . [ ] ii. - . [ ] ii. - . [ ] iv. - . [ ] v. - . [ ] v. - . [ ] _troilus and criseyde_, iii. - . [ ] for chaucer's locality, °. [ ] see the _astrolabe_, i. , . vertical circles are called _azimuths_ by chaucer. [ ] strictly speaking, the equinoxes and solstices are each simply an instant of time. transcriber's notes: passages in italics are indicated by _italics_. subscripted characters are indicated by {subscript}. transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. there are several mathematical formulas within the text. they are represented as follows: superscripts: x^ subscripts: x_ square root: [square root] greek letters: [pi], [theta]. greek star names are represented as [alpha], [gamma], for example. pioneers of science [illustration] [illustration: newton _from the picture by kneller, , now at cambridge_] pioneers of science by oliver lodge, f.r.s. professor of physics in victoria university college, liverpool _with portraits and other illustrations_ london macmillan and co. and new york richard clay and sons, limited, london and bungay. preface this book takes its origin in a course of lectures on the history and progress of astronomy arranged for me in the year by three of my colleagues (a.c.b., j.m., g.h.r.), one of whom gave the course its name. the lectures having been found interesting, it was natural to write them out in full and publish. if i may claim for them any merit, i should say it consists in their simple statement and explanation of scientific facts and laws. the biographical details are compiled from all readily available sources, there is no novelty or originality about them; though it is hoped that there may be some vividness. i have simply tried to present a living figure of each pioneer in turn, and to trace his influence on the progress of thought. i am indebted to many biographers and writers, among others to mr. e.j.c. morton, whose excellent set of lives published by the s.p.c.k. saved me much trouble in the early part of the course. as we approach recent times the subject grows more complex, and the men more nearly contemporaries; hence the biographical aspect diminishes and the scientific treatment becomes fuller, but in no case has it been allowed to become technical and generally unreadable. to the friends (c.c.c., f.w.h.m., e.f.r.) who with great kindness have revised the proofs, and have indicated places where the facts could be made more readily intelligible by a clearer statement, i express my genuine gratitude. university college, liverpool, _november, _. contents _part i_ lecture i page copernicus and the motion of the earth lecture ii tycho brahÉ and the earliest observatory lecture iii kepler and the laws of planetary motion lecture iv galileo and the invention of the telescope lecture v galileo and the inquisition lecture vi descartes and his theory of vortices lecture vii sir isaac newton lecture viii newton and the law of gravitation lecture ix newton's "principia" _part ii_ lecture x roemer and bradley and the velocity of light lecture xi lagrange and laplace--the stability of the solar system, and the nebular hypothesis lecture xii herschel and the motion of the fixed stars lecture xiii the discovery of the asteroids lecture xiv bessel--the distances of the stars, and the discovery of stellar planets lecture xv the discovery of neptune lecture xvi comets and meteors lecture xvii the tides lecture xviii the tides, and planetary evolution illustrations fig. page . archimedes . leonardo da vinci . copernicus . homeric cosmogony . egyptian symbol of the universe . hindoo earth . order of ancient planets corresponding to the days of the week . ptolemaic system . specimens of apparent paths of venus and of mars among the stars . apparent epicyclic orbits of jupiter and saturn . egyptian system . true orbits of earth and jupiter . orbits of mercury and earth . copernican system as frequently represented . slow movement of the north pole in a circle among the stars . tychonic system, showing the sun with all the planets revolving round the earth . portrait of tycho . early out-door quadrant of tycho . map of denmark, showing the island of huen . uraniburg . astrolabe . tycho's large sextant . the quadrant in uraniburg . tycho's form of transit circle . a modern transit circle . orbits of some of the planets drawn to scale . many-sided polygon or approximate circle enveloped by straight lines . kepler's idea of the regular solids . diagram of equant . excentric circle supposed to be divided into equal areas . mode of drawing an ellipse . kepler's diagram proving equable description of areas for an ellipse . diagram of a planet's velocity in different parts of its orbit . portrait of kepler . curve described by a projectile . two forms of pulsilogy . tower of pisa . view of the half-moon in small telescope . portion of the lunar surface more highly magnified . another portion of the lunar surface . lunar landscape showing earth . galileo's method of estimating the height of lunar mountain . some clusters and nebulÆ . stages of the discovery of jupiter's satellites . eclipses of jupiter's satellites . old drawings of saturn by different observers, with the imperfect instruments of that day . phases of venus . sunspots as seen with low power . a portion of the sun's disk as seen in a powerful modern telescope . saturn and his rings . map of italy . portrait of galileo . portrait of descartes . descartes's eye diagram . descartes's diagram of vortices from his "principia" . manor-house of woolsthorpe . projectile diagram . } { . } diagrams illustrative of those near the beginning { . } of newton's "principia" { - . } { . prismatic dispersion . a single constituent of white light is capable of no more dispersion . parallel beam passing through a lens . newton's telescope . the sextant, as now made . newton when young . sir isaac newton . another "principia" diagram . well-known model exhibiting the oblate spheroidal form as a consequence of spinning about a central axis . jupiter . diagram of eye looking at a light reflected in a distant mirror through the teeth of a revolving wheel . fizeau's wheel, showing the appearance of distant image seen through its teeth . eclipses of one of jupiter's satellites . a transit instrument for the british astronomical expedition, . diagram of equatorially mounted telescope . aberration diagram . showing the three conjunction places in the orbits of jupiter and saturn . lord rosse's drawing of the spiral nebula in canes venatici . saturn . principle of newtonian reflector . herschel's -foot telescope . william herschel . caroline herschel . double stars . old drawing of the cluster in hercules . old drawing of the andromeda nebula . the great nebula in orion . planetary orbits to scale . diagram illustrating parallax . the kÖnigsberg heliometer . perturbations of uranus . uranus' and neptune's relative positions . meteorite . meteor stream crossing field of telescope . diagram of direction of earth's orbital motion . parabolic and elliptic orbits . orbit of halley's comet . various appearances of halley's comet when last seen . head of donati's comet of . comet . encke's comet . biela's comet as last seen in two portions . radiant point perspective . present orbit of november meteors . orbit of november meteors before and after encounter with uranus . the mersey . co-tidal lines, showing the way the tidal wave reaches the british isles from the atlantic . whirling earth model . earth and moon model . earth and moon (earth's rotation neglected) . maps showing how comparatively free from land obstruction the ocean in the southern hemisphere is . spring and neap tides . tidal clock . sir william thomson (lord kelvin) . tide-gauge for recording local tides . harmonic analyzer . tide-predicter . weekly sheet of curves pioneers of science part i _from dusk to daylight_ dates and summary of facts for lecture i _physical science of the ancients._ thales b.c., anaximander b.c., pythagoras b.c., anaxagoras b.c., eudoxus b.c., aristotle b.c., aristarchus b.c., archimedes b.c., eratosthenes b.c., hipparchus b.c., ptolemy a.d. _science of the middle ages._ cultivated only among the arabs; largely in the forms of astrology, alchemy, and algebra. _return of science to europe._ roger bacon , leonardo da vinci , (printing ), columbus , copernicus . _a sketch of copernik's life and work._ born at thorn in poland. studied mathematics at bologna. became an ecclesiastic. lived at frauenburg near mouth of vistula. substituted for the apparent motion of the heavens the real motion of the earth. published tables of planetary motions. motion still supposed to be in epicycles. worked out his ideas for years, and finally dedicated his work to the pope. died just as his book was printed, aged , a century before the birth of newton. a colossal statue by thorwaldsen erected at warsaw in . pioneers of science lecture i copernicus and the motion of the earth the ordinary run of men live among phenomena of which they know nothing and care less. they see bodies fall to the earth, they hear sounds, they kindle fires, they see the heavens roll above them, but of the causes and inner working of the whole they are ignorant, and with their ignorance they are content. "understand the structure of a soap-bubble?" said a cultivated literary man whom i know; "i wouldn't cross the street to know it!" and if this is a prevalent attitude now, what must have been the attitude in ancient times, when mankind was emerging from savagery, and when history seems composed of harassments by wars abroad and revolutions at home? in the most violently disturbed times indeed, those with which ordinary history is mainly occupied, science is quite impossible. it needs as its condition, in order to flourish, a fairly quiet, untroubled state, or else a cloister or university removed from the din and bustle of the political and commercial world. in such places it has taken its rise, and in such peaceful places and quiet times true science will continue to be cultivated. the great bulk of mankind must always remain, i suppose, more or less careless of scientific research and scientific result, except in so far as it affects their modes of locomotion, their health and pleasure, or their purse. but among a people hurried and busy and preoccupied, some in the pursuit of riches, some in the pursuit of pleasure, and some, the majority, in the struggle for existence, there arise in every generation, here and there, one or two great souls--men who seem of another age and country, who look upon the bustle and feverish activity and are not infected by it, who watch others achieving prizes of riches and pleasure and are not disturbed, who look on the world and the universe they are born in with quite other eyes. to them it appears not as a bazaar to buy and to sell in; not as a ladder to scramble up (or down) helter-skelter without knowing whither or why; but as a fact--a great and mysterious fact--to be pondered over, studied, and perchance in some small measure understood. by the multitude these men were sneered at as eccentric or feared as supernatural. their calm, clear, contemplative attitude seemed either insane or diabolic; and accordingly they have been pitied as enthusiasts or killed as blasphemers. one of these great souls may have been a prophet or preacher, and have called to his generation to bethink them of why and what they were, to struggle less and meditate more, to search for things of true value and not for dross. another has been a poet or musician, and has uttered in words or in song thoughts dimly possible to many men, but by them unutterable and left inarticulate. another has been influenced still more _directly_ by the universe around him, has felt at times overpowered by the mystery and solemnity of it all, and has been impelled by a force stronger than himself to study it, patiently, slowly, diligently; content if he could gather a few crumbs of the great harvest of knowledge, happy if he could grasp some great generalization or wide-embracing law, and so in some small measure enter into the mind and thought of the designer of all this wondrous frame of things. these last have been the men of science, the great and heaven-born men of science; and they are few. in our own day, amid the throng of inventions, there are a multitude of small men using the name of science but working for their own ends, jostling and scrambling just as they would jostle and scramble in any other trade or profession. these may be workers, they may and do advance knowledge, but they are never pioneers. not to them is it given to open out great tracts of unexplored territory, or to view the promised land as from a mountain-top. of them we shall not speak; we will concern ourselves only with the greatest, the epoch-making men, to whose life and work we and all who come after them owe so much. such a man was thales. such was archimedes, hipparchus, copernicus. such pre-eminently was newton. now i am not going to attempt a history of science. such a work in ten lectures would be absurd. i intend to pick out a few salient names here and there, and to study these in some detail, rather than by attempting to deal with too many to lose individuality and distinctness. we know so little of the great names of antiquity, that they are for this purpose scarcely suitable. in some departments the science of the greeks was remarkable, though it is completely overshadowed by their philosophy; yet it was largely based on what has proved to be a wrong method of procedure, viz the introspective and conjectural, rather than the inductive and experimental methods. they investigated nature by studying their own minds, by considering the meanings of words, rather than by studying things and recording phenomena. this wrong (though by no means, on the face of it, absurd) method was not pursued exclusively, else would their science have been valueless, but the influence it had was such as materially to detract from the value of their speculations and discoveries. for when truth and falsehood are inextricably woven into a statement, the truth is as hopelessly hidden as if it had never been stated, for we have no criterion to distinguish the false from the true. [illustration: fig. .--archimedes.] besides this, however, many of their discoveries were ultimately lost to the world, some, as at alexandria, by fire--the bigoted work of a mohammedan conqueror--some by irruption of barbarians; and all were buried so long and so completely by the night of the dark ages, that they had to be rediscovered almost as absolutely and completely as though they had never been. some of the names of antiquity we shall have occasion to refer to; so i have arranged some of them in chronological order on page , and as a representative one i may specially emphasize archimedes, one of the greatest men of science there has ever been, and the father of physics. the only effective link between the old and the new science is afforded by the arabs. the dark ages come as an utter gap in the scientific history of europe, and for more than a thousand years there was not a scientific man of note except in arabia; and with the arabs knowledge was so mixed up with magic and enchantment that one cannot contemplate it with any degree of satisfaction, and little real progress was made. in some of the _waverley novels_ you can realize the state of matters in these times; and you know how the only approach to science is through some arab sorcerer or astrologer, maintained usually by a monarch, and consulted upon all great occasions, as the oracles were of old. in the thirteenth century, however, a really great scientific man appeared, who may be said to herald the dawn of modern science in europe. this man was roger bacon. he cannot be said to do more than herald it, however, for we must wait two hundred years for the next name of great magnitude; moreover he was isolated, and so far in advance of his time that he left no followers. his own work suffered from the prevailing ignorance, for he was persecuted and imprisoned, not for the commonplace and natural reason that he frightened the church, but merely because he was eccentric in his habits and knew too much. the man i spoke of as coming two hundred years later is leonardo da vinci. true he is best known as an artist, but if you read his works you will come to the conclusion that he was the most scientific artist who ever lived. he teaches the laws of perspective (then new), of light and shade, of colour, of the equilibrium of bodies, and of a multitude of other matters where science touches on art--not always quite correctly according to modern ideas, but in beautiful and precise language. for clear and conscious power, for wide-embracing knowledge and skill, leonardo is one of the most remarkable men that ever lived. about this time the tremendous invention of printing was achieved, and columbus unwittingly discovered the new world. the middle of the next century must be taken as the real dawn of modern science; for the year marks the publication of the life-work of copernicus. [illustration: fig. .--leonardo da vinci.] nicolas copernik was his proper name. copernicus is merely the latinized form of it, according to the then prevailing fashion. he was born at thorn, in polish prussia, in . his father is believed to have been a german. he graduated at cracow as doctor in arts and medicine, and was destined for the ecclesiastical profession. the details of his life are few; it seems to have been quiet and uneventful, and we know very little about it. he was instructed in astronomy at cracow, and learnt mathematics at bologna. thence he went to rome, where he was made professor of mathematics; and soon afterwards he went into orders. on his return home, he took charge of the principal church in his native place, and became a canon. at frauenburg, near the mouth of the vistula, he lived the remainder of his life. we find him reporting on coinage for the government, but otherwise he does not appear as having entered into the life of the times. he was a quiet, scholarly monk of studious habits, and with a reputation which drew to him several earnest students, who received _vivâ voce_ instruction from him; so, in study and meditation, his life passed. he compiled tables of the planetary motions which were far more correct than any which had hitherto appeared, and which remained serviceable for long afterwards. the ptolemaic system of the heavens, which had been the orthodox system all through the christian era, he endeavoured to improve and simplify by the hypothesis that the sun was the centre of the system instead of the earth; and the first consequences of this change he worked out for many years, producing in the end a great book: his one life-work. this famous work, "de revolutionibus orbium coelestium," embodied all his painstaking calculations, applied his new system to each of the bodies in the solar system in succession, and treated besides of much other recondite matter. towards the close of his life it was put into type. he can scarcely be said to have lived to see it appear, for he was stricken with paralysis before its completion; but a printed copy was brought to his bedside and put into his hands, so that he might just feel it before he died. [illustration: fig. .--copernicus.] that copernicus was a giant in intellect or power--such as had lived in the past, and were destined to live in the near future--i see no reason whatever to believe. he was just a quiet, earnest, patient, and god-fearing man, a deep student, an unbiassed thinker, although with no specially brilliant or striking gifts; yet to him it was given to effect such a revolution in the whole course of man's thoughts as is difficult to parallel. you know what the outcome of his work was. it proved--he did not merely speculate, he proved--that the earth is a planet like the others, and that it revolves round the sun. yes, it can be summed up in a sentence, but what a revelation it contains. if you have never made an effort to grasp the full significance of this discovery you will not appreciate it. the doctrine is very familiar to us now, we have heard it, i suppose, since we were four years old, but can you realize it? i know it was a long time before i could. think of the solid earth, with trees and houses, cities and countries, mountains and seas--think of the vast tracts of land in asia, africa, and america--and then picture the whole mass spinning like a top, and rushing along its annual course round the sun at the rate of nineteen miles every second. were we not accustomed to it, the idea would be staggering. no wonder it was received with incredulity. but the difficulties of the conception are not only physical, they are still more felt from the speculative and theological points of view. with this last, indeed, the reconcilement cannot be considered complete even yet. theologians do not, indeed, now _deny_ the fact of the earth's subordination in the scheme of the universe, but many of them ignore it and pass it by. so soon as the church awoke to a perception of the tremendous and revolutionary import of the new doctrines, it was bound to resist them or be false to its traditions. for the whole tenor of men's thought must have been changed had they accepted it. if the earth were not the central and all-important body in the universe, if the sun and planets and stars were not attendant and subsidiary lights, but were other worlds larger and perhaps superior to ours, where was man's place in the universe? and where were the doctrines they had maintained as irrefragable? i by no means assert that the new doctrines were really utterly irreconcilable with the more essential parts of the old dogmas, if only theologians had had patience and genius enough to consider the matter calmly. i suppose that in that case they might have reached the amount of reconciliation at present attained, and not only have left scientific truth in peace to spread as it could, but might perhaps themselves have joined the band of earnest students and workers, as so many of the higher catholic clergy do at the present day. but this was too much to expect. such a revelation was not to be accepted in a day or in a century--the easiest plan was to treat it as a heresy, and try to crush it out. not in copernik's life, however, did they perceive the dangerous tendency of the doctrine--partly because it was buried in a ponderous and learned treatise not likely to be easily understood; partly, perhaps, because its propounder was himself an ecclesiastic; mainly because he was a patient and judicious man, not given to loud or intolerant assertion, but content to state his views in quiet conversation, and to let them gently spread for thirty years before he published them. and, when he did publish them, he used the happy device of dedicating his great book to the pope, and a cardinal bore the expense of printing it. thus did the roman church stand sponsor to a system of truth against which it was destined in the next century to hurl its anathemas, and to inflict on its conspicuous adherents torture, imprisonment, and death. to realize the change of thought, the utterly new view of the universe, which the copernican theory introduced, we must go back to preceding ages, and try to recall the views which had been held as probable concerning the form of the earth and the motion of the heavenly bodies. [illustration: fig. .--homeric cosmogony.] the earliest recorded notion of the earth is the very natural one that it is a flat area floating in an illimitable ocean. the sun was a god who drove his chariot across the heavens once a day; and anaxagoras was threatened with death and punished with banishment for teaching that the sun was only a ball of fire, and that it might perhaps be as big as the country of greece. the obvious difficulty as to how the sun got back to the east again every morning was got over--not by the conjecture that he went back in the dark, nor by the idea that there was a fresh sun every day; though, indeed, it was once believed that the moon was created once a month, and periodically cut up into stars--but by the doctrine that in the northern part of the earth was a high range of mountains, and that the sun travelled round on the surface of the sea behind these. sometimes, indeed, you find a representation of the sun being rowed round in a boat. later on it was perceived to be necessary that the sun should be able to travel beneath the earth, and so the earth was supposed to be supported on pillars or on roots, or to be a dome-shaped body floating in air--much like dean swift's island of laputa. the elephant and tortoise of the hindu earth are, no doubt, emblematic or typical, not literal. [illustration: fig. .--egyptian symbol of the universe. the earth a figure with leaves, the heaven a figure with stars, the principle of equilibrium and support, the boats of the rising and setting sun.] aristotle, however, taught that the earth must be a sphere, and used all the orthodox arguments of the present children's geography-books about the way you see ships at sea, and about lunar eclipses. to imagine a possible antipodes must, however, have been a tremendous difficulty in the way of this conception of a sphere, and i scarcely suppose that any one can at that time have contemplated the possibility of such upside-down regions being inhabited. i find that intelligent children invariably feel the greatest difficulty in realizing the existence of inhabitants on the opposite side of the earth. stupid children, like stupid persons in general, will of course believe anything they are told, and much good may the belief do them; but the kind of difficulties felt by intelligent and thoughtful children are most instructive, since it is quite certain that the early philosophers must have encountered and overcome those very same difficulties by their own genius. [illustration: fig. .--hindoo earth.] however, somehow or other the conception of a spherical earth was gradually grasped, and the heavenly bodies were perceived all to revolve round it: some moving regularly, as the stars, all fixed together into one spherical shell or firmament; some moving irregularly and apparently anomalously--these irregular bodies were therefore called planets [or wanderers]. seven of them were known, viz. moon, mercury, venus, sun, mars, jupiter, saturn, and there is little doubt that this number seven, so suggested, is the origin of the seven days of the week. the above order of the ancient planets is that of their supposed distance from the earth. not always, however, are they thus quoted by the ancients: sometimes the sun is supposed nearer than mercury or venus. it has always been known that the moon was the nearest of the heavenly bodies; and some rough notion of its distance was current. mars, jupiter, and saturn were placed in that order because that is the order of their apparent motions, and it was natural to suppose that the slowest moving bodies were the furthest off. the order of the days of the week shows what astrologers considered to be the order of the planets; on their system of each successive hour of the day being ruled over by the successive planets taken in order. the diagram (fig. ) shows that if the sun rule the first hour of a certain day (thereby giving its name to the day) venus will rule the second hour, mercury the third, and so on; the sun will thus be found to rule the eighth, fifteenth, and twenty-second hour of that day, venus the twenty-third, and mercury the twenty-fourth hour; so the moon will rule the first hour of the next day, which will therefore be monday. on the same principle (numbering round the hours successively, with the arrows) the first hour of the next day will be found to be ruled by mars, or by the saxon deity corresponding thereto; the first hour of the day after, by mercury (_mercredi_), and so on (following the straight lines of the pattern). the order of the planets round the circle counter-clockwise, _i.e._ the direction of their proper motions, is that quoted above in the text. to explain the motion of the planets and reduce them to any sort of law was a work of tremendous difficulty. the greatest astronomer of ancient times was hipparchus, and to him the system known as the ptolemaic system is no doubt largely due. but it was delivered to the world mainly by ptolemy, and goes by his name. this was a fine piece of work, and a great advance on anything that had gone before; for although it is of course saturated with error, still it is based on a large substratum of truth. its superiority to all the previously mentioned systems is obvious. and it really did in its more developed form describe the observed motions of the planets. each planet was, in the early stages of this system, as taught, say, by eudoxus, supposed to be set in a crystal sphere, which revolved so as to carry the planet with it. the sphere had to be of crystal to account for the visibility of other planets and the stars through it. outside the seven planetary spheres, arranged one inside the other, was a still larger one in which were set the stars. this was believed to turn all the others, and was called the _primum mobile_. the whole system was supposed to produce, in its revolution, for the few privileged to hear the music of the spheres, a sound as of some magnificent harmony. [illustration: fig. .--order of ancient planets corresponding to the days of the week.] the enthusiastic disciples of pythagoras believed that their master was privileged to hear this noble chant; and far be it from us to doubt that the rapt and absorbing pleasure of contemplating the harmony of nature, to a man so eminently great as pythagoras, must be truly and adequately represented by some such poetic conception. [illustration: fig. .--ptolemaic system.] the precise kind of motion supposed to be communicated from the _primum mobile_ to the other spheres so as to produce the observed motions of the planets was modified and improved by various philosophers until it developed into the epicyclic train of hipparchus and of ptolemy. it is very instructive to observe a planet (say mars or jupiter) night after night and plot down its place with reference to the fixed stars on a celestial globe or star-map. or, instead of direct observation by alignment with known stars, it is easier to look out its right ascension and declination in _whitaker's almanac_, and plot those down. if this be done for a year or two, it will be found that the motion of the planet is by no means regular, but that though on the whole it advances it sometimes is stationary and sometimes goes back.[ ] [illustration: fig. .--specimens of apparent paths of venus and of mars among the stars.] [illustration: fig. .--apparent epicyclic orbits of jupiter and saturn; the earth being supposed fixed at the centre, with the sun revolving in a small circle. a loop is made by each planet every year.] these "stations" and "retrogressions" of the planets were well known to the ancients. it was not to be supposed for a moment that the crystal spheres were subject to any irregularity, neither was uniform circular motion to be readily abandoned; so it was surmised that the main sphere carried, not the planet itself, but the centre or axis of a subordinate sphere, and that the planet was carried by this. the minor sphere could be allowed to revolve at a different uniform pace from the main sphere, and so a curve of some complexity could be obtained. a curve described in space by a point of a circle or sphere, which itself is carried along at the same time, is some kind of cycloid; if the centre of the tracing circle travels along a straight line, we get the ordinary cycloid, the curve traced in air by a nail on a coach-wheel; but if the centre of the tracing circle be carried round another circle the curve described is called an epicycloid. by such curves the planetary stations and retrogressions could be explained. a large sphere would have to revolve once for a "year" of the particular planet, carrying with it a subsidiary sphere in which the planet was fixed; this latter sphere revolving once for a "year" of the earth. the actual looped curve thus described is depicted for jupiter and saturn in the annexed diagram (fig. .) it was long ago perceived that real material spheres were unnecessary; such spheres indeed, though possibly transparent to light, would be impermeable to comets: any other epicyclic gearing would serve, and as a mere description of the motion it is simpler to think of a system of jointed bars, one long arm carrying a shorter arm, the two revolving at different rates, and the end of the short one carrying the planet. this does all that is needful for the first approximation to a planet's motion. in so far as the motion cannot be thus truly stated, the short arm may be supposed to carry another, and that another, and so on, so that the resultant motion of the planet is compounded of a large number of circular motions of different periods; by this device any required amount of complexity could be attained. we shall return to this at greater length in lecture iii. the main features of the motion, as shown in the diagram, required only two arms for their expression; one arm revolving with the average motion of the planet, and the other revolving with the apparent motion of the sun, and always pointing in the same direction as the single arm supposed to carry the sun. this last fact is of course because the motion to be represented does not really belong to the planet at all, but to the earth, and so all the main epicyclic motions for the superior planets were the same. as for the inferior planets (mercury and venus) they only appear to oscillate like the bob of a pendulum about the sun, and so it is very obvious that they must be really revolving round it. an ancient egyptian system perceived this truth; but the ptolemaic system imagined them to revolve round the earth like the rest, with an artificial system of epicycles to prevent their ever getting far away from the neighbourhood of the sun. it is easy now to see how the copernican system explains the main features of planetary motion, the stations and retrogressions, quite naturally and without any complexity. [illustration: fig. .--egyptian system.] let the outer circle represent the orbit of jupiter, and the inner circle the orbit of the earth, which is moving faster than jupiter (since jupiter takes days to make one revolution); then remember that the apparent position of jupiter is referred to the infinitely distant fixed stars and refer to fig. . let e_ , e_ , &c., be successive positions of the earth; j_ , j_ , &c., corresponding positions of jupiter. produce the lines e_ j_ , e_ j_ , &c., to an enormously greater circle outside, and it will be seen that the termination of these lines, representing apparent positions of jupiter among the stars, advances while the earth goes from e_ to e_ ; is almost stationary from somewhere about e_ to e_ ; and recedes from e_ to e_ ; so that evidently the recessions of jupiter are only apparent, and are due to the orbital motion of the earth. the apparent complications in the path of jupiter, shown in fig. , are seen to be caused simply by the motion of the earth, and to be thus completely and easily explained. [illustration: fig. .--true orbits of earth and jupiter.] the same thing for an inferior planet, say mercury, is even still more easily seen (_vide_ figure ). the motion of mercury is direct from m'' to m''', retrograde from m''' to m'', and stationary at m'' and m'''. it appears to oscillate, taking · days for its direct swing, and · for its return swing. [illustration: fig. .--orbit of mercury and earth.] on this system no artificiality is required to prevent mercury's ever getting far from the sun: the radius of its orbit limits its real and apparent excursions. even if the earth were stationary, the motions of mercury and venus would not be _essentially_ modified, but the stations and retrogressions of the superior planets, mars, jupiter, &c., would wholly cease. the complexity of the old mode of regarding apparent motion may be illustrated by the case of a traveller in a railway train unaware of his own motion. it is as though trees, hedges, distant objects, were all flying past him and contorting themselves as you may see the furrows of a ploughed field do when travelling, while you yourself seem stationary amidst it all. how great a simplicity would be introduced by the hypothesis that, after all, these things might be stationary and one's self moving. [illustration: fig. .--copernican system as frequently represented. but the cometary orbit is a much later addition, and no attempt is made to show the relative distances of the planets.] now you are not to suppose that the system of copernicus swept away the entire doctrine of epicycles; that doctrine can hardly be said to be swept away even now. as a description of a planet's motion it is not incorrect, though it is geometrically cumbrous. if you describe the motion of a railway train by stating that every point on the rim of each wheel describes a cycloid with reference to the earth, and a circle with reference to the train, and that the motion of the train is compounded of these cycloidal and circular motions, you will not be saying what is false, only what is cumbrous. the ptolemaic system demanded large epicycles, depending on the motion of the earth, these are what copernicus overthrew; but to express the minuter details of the motion smaller epicycles remained, and grew more and more complex as observations increased in accuracy, until a greater man than either copernicus or ptolemy, viz. kepler, replaced them all by a simple ellipse. one point i must not omit from this brief notice of the work of copernicus. hipparchus had, by most sagacious interpretation of certain observations of his, discovered a remarkable phenomenon called the precession of the equinoxes. it was a discovery of the first magnitude, and such as would raise to great fame the man who should have made it in any period of the world's history, even the present. it is scarcely expressible in popular language, and without some technical terms; but i can try. the plane of the earth's orbit produced into the sky gives the apparent path of the sun throughout a year. this path is known as the ecliptic, because eclipses only happen when the moon is in it. the sun keeps to it accurately, but the planets wander somewhat above and below it (fig. ), and the moon wanders a good deal. it is manifest, however, in order that there may be an eclipse of any kind, that a straight line must be able to be drawn through earth and moon and sun (not necessarily through their centres of course), and this is impossible unless some parts of the three bodies are in one plane, viz. the ecliptic, or something very near it. the ecliptic is a great circle of the sphere, and is usually drawn on both celestial and terrestrial globes. the earth's equator also produced into the sky, where it may still be called the equator (sometimes it is awkwardly called "the equinoctial"), gives another great circle inclined to the ecliptic and cutting it at two opposite points, labelled respectively [aries symbol] and [libra symbol], and together called "the equinoxes." the reason for the name is that when the sun is in that part of the ecliptic it is temporarily also on the equator, and hence is symmetrically situated with respect to the earth's axis of rotation, and consequently day and night are equal all over the earth. well, hipparchus found, by plotting the position of the sun for a long time,[ ] that these points of intersection, or equinoxes, were not stationary from century to century, but slowly moved among the stars, moving as it were to meet the sun, so that he gets back to one of these points again minutes - / seconds before it has really completed a revolution, _i.e._ before the true year is fairly over. this slow movement forward of the goal-post is called precession--the precession of the equinoxes. (one result of it is to shorten our years by about minutes each; for the shortened period has to be called a year, because it is on the position of the sun with respect to the earth's axis that our seasons depend.) copernicus perceived that, assuming the motion of the earth, a clearer account of this motion could be given. the ordinary approximate statement concerning the earth's axis is that it remains parallel to itself, _i.e._ has a fixed direction as the earth moves round the sun. but if, instead of being thus fixed, it be supposed to have a slow movement of revolution, so that it traces out a cone in the course of about , years, then, since the equator of course goes with it, the motion of its intersection with the fixed ecliptic is so far accounted for. that is to say, the precession of the equinoxes is seen to be dependent on, and caused by, a slow conical movement of the earth's axis. the prolongation of each end of the earth's axis into the sky, or the celestial north and south poles, will thus slowly trace out an approximate circle among the stars; and the course of the north pole during historic time is exhibited in the annexed diagram. it is now situated near one of the stars of the lesser bear, which we therefore call the pole star; but not always was it so, nor will it be so in the future. the position of the north pole years ago is shown in the figure; and a revolution will be completed in something like , years.[ ] [illustration: fig. .--slow movement of the north pole in a circle among the stars. (copied from sir r. ball.)] this perception of the conical motion of the earth's axis was a beautiful generalization of copernik's, whereby a multitude of facts were grouped into a single phenomenon. of course he did not explain the motion of the axis itself. he stated the fact that it so moved, and i do not suppose it ever struck him to seek for an explanation. an explanation was given later, and that a most complete one; but the idea even of seeking for it is a brilliant and striking one: the achievement of the explanation by a single individual in the way it actually was accomplished is one of the most astounding things in the history of science; and were it not that the same individual accomplished a dozen other things, equally and some still more extraordinary, we should rank that man as one of the greatest astronomers that ever lived. as it is, he is sir isaac newton. we are to remember, then, as the life-work of copernicus, that he placed the sun in its true place as the centre of the solar system, instead of the earth; that he greatly simplified the theory of planetary motion by this step, and also by the simpler epicyclic chain which now sufficed, and which he worked out mathematically; that he exhibited the precession of the equinoxes (discovered by hipparchus) as due to a conical motion of the earth's axis; and that, by means of his simpler theory and more exact planetary tables, he reduced to some sort of order the confused chaos of the ptolemaic system, whose accumulation of complexity and of outstanding errors threatened to render astronomy impossible by the mere burden of its detail. there are many imperfections in his system, it is true; but his great merit is that he dared to look at the facts of nature with his own eyes, unhampered by the prejudice of centuries. a system venerable with age, and supported by great names, was universally believed, and had been believed for centuries. to doubt this system, and to seek after another and better one, at a time when all men's minds were governed by tradition and authority, and when to doubt was sin--this required a great mind and a high character. such a mind and such a character had this monk of frauenburg. and it is interesting to notice that the so-called religious scruples of smaller and less truly religious men did not affect copernicus; it was no dread of consequences to one form of truth that led him to delay the publication of the other form of truth specially revealed to him. in his dedication he says:-- "if there be some babblers who, though ignorant of all mathematics, take upon them to judge of these things, and dare to blame and cavil at my work, because of some passage of scripture which they have wrested to their own purpose, i regard them not, and will not scruple to hold their judgment in contempt." i will conclude with the words of one of his biographers (mr. e.j.c. morton):-- "copernicus cannot be said to have flooded with light the dark places of nature--in the way that one stupendous mind subsequently did--but still, as we look back through the long vista of the history of science, the dim titanic figure of the old monk seems to rear itself out of the dull flats around it, pierces with its head the mists that overshadow them, and catches the first gleam of the rising sun, "'... like some iron peak, by the creator fired with the red glow of the rushing morn.'" dates and summary of facts for lecture ii copernicus lived from to , and was contemporary with paracelsus and raphael. tycho brahé from to . kepler from to . galileo from to . gilbert from to . francis bacon from to . descartes from to . _a sketch of tycho brahé's life and work._ tycho was a danish noble, born on his ancestral estate at knudstorp, near helsinborg, in . adopted by his uncle, and sent to the university of copenhagen to study law. attracted to astronomy by the occurrence of an eclipse on its predicted day, august st, . began to construct astronomical instruments, especially a quadrant and a sextant. observed at augsburg and wittenberg. studied alchemy, but was recalled to astronomy by the appearance of a new star. overcame his aristocratic prejudices, and delivered a course of lectures at copenhagen, at the request of the king. after this he married a peasant girl. again travelled and observed in germany. in was sent for to denmark by frederick ii., and established in the island of huen, with an endowment enabling him to devote his life to astronomy. built uraniburg, furnished it with splendid instruments, and became the founder of accurate instrumental astronomy. his theories were poor, but his observations were admirable. in frederick died, and five years later, tycho was impoverished and practically banished. after wandering till , he was invited to prague by the emperor rudolf, and there received john kepler among other pupils. but the sentence of exile was too severe, and he died in , aged years. a man of strong character, untiring energy, and devotion to accuracy, his influence on astronomy has been immense. lecture ii tycho brahÉ and the earliest observatory we have seen how copernicus placed the earth in its true position in the solar system, making it merely one of a number of other worlds revolving about a central luminary. and observe that there are two phenomena to be thus accounted for and explained: first, the diurnal revolution of the heavens; second, the annual motion of the sun among the stars. the effect of the diurnal motion is conspicuous to every one, and explains the rising, southing, and setting of the whole visible firmament. the effect of the annual motion, _i.e._ of the apparent annual motion, of the sun among the stars, is less obvious, but it may be followed easily enough by observing the stars visible at any given time of evening at different seasons of the year. at midnight, for instance, the position of the sun is definite, viz. due north always, but the constellation which at that time is due south or is rising or setting varies with the time of year; an interval of one month producing just the same effect on the appearance of the constellations as an interval of two hours does (because the day contains twice as many hours as the year contains months), _e.g._ the sky looks the same at midnight on the st of october as it does at p.m. on the st of november. all these simple consequences of the geocentric as opposed to the heliocentric point of view were pointed out by copernicus, in addition to his greater work of constructing improved planetary tables on the basis of his theory. but it must be admitted that he himself felt the hypothesis of the motion of the earth to be a difficulty. its acceptance is by no means such an easy and childish matter as we are apt now to regard it, and the hostility to it is not at all surprising. the human race, after having ridiculed and resisted the truth for a long time, is apt to end in accepting it so blindly and unimaginatively as to fail to recognize the real achievement of its first propounders, or the difficulties which they had to overcome. the majority of men at the present day have grown accustomed to hear the motion of the earth spoken of: their acceptance of it means nothing: the attitude of the paradoxer who denies it is more intelligent. it is not to be supposed that the idea of thus explaining some of the phenomena of the heavens, especially the daily motion of the entire firmament, by a diurnal rotation of the earth had not struck any one. it was often at this time referred to as the pythagorean theory, and it had been taught, i believe, by aristarchus. but it was new to the modern world, and it had the great weight of aristotle against it. consequently, for long after copernicus, only a few leading spirits could be found to support it, and the long-established venerable ptolemaic system continued to be taught in all universities. the main objections to the motion of the earth were such as the following:-- . the motion is unfelt and difficult to imagine. that it is unfelt is due to its uniformity, and can be explained mechanically. that it is difficult to imagine is and remains true, but a most important lesson we have to learn is that difficulty of conception is no valid argument against reality. . that the stars do not alter their relative positions according to the season of the year, but the constellations preserve always the same aspect precisely, even to careful measurement. this is indeed a difficulty, and a great one. in june the earth is million miles away from where it was in december: how can we see precisely the same fixed stars? it is not possible, unless they are at a practically infinite distance. that is the only answer that can be given. it was the tentative answer given by copernicus. it is the correct answer. not only from every position of the earth, but from every planet of the solar system, the same constellations are visible, and the stars have the same aspect. the whole immensity of the solar system shrinks to practically a point when confronted with the distance of the stars. not, however, so entirely a speck as to resist the terrific accuracy of the present century, and their microscopic relative displacement with the season of the year has now at length been detected, and the distance of many thereby measured. . that, if the earth revolved round the sun, mercury and venus ought to show phases like the moon. so they ought. any globe must show phases if it live nearer the sun than we do and if we go round it, for we shall see varying amounts of its illuminated half. the only answer that copernicus could give to this was that they might be difficult to see without extra powers of sight, but he ventured to predict that the phases would be seen if ever our powers of vision should be enhanced. . that if the earth moved, or even revolved on its own axis, a stone or other dropped body ought to be left far behind. this difficulty is not a real one, like the two last, and it is based on an ignorance of the laws of mechanics, which had not at that time been formulated. we know now that a ball dropped from a high tower, so far from lagging, drops a minute trifle _in front_ of the foot of a perpendicular, because the top of the tower is moving a trace faster than the bottom, by reason of the diurnal rotation. but, ignoring this, a stone dropped from the lamp of a railway carriage drops in the centre of the floor, whether the carriage be moving steadily or standing still; a slant direction of fall could only be detected if the carriage were being accelerated or if the brake were applied. a body dropped from a moving carriage shares the motion of the carriage, and starts with that as its initial velocity. a ball dropped from a moving balloon does not simply drop, but starts off in whatever direction the car was moving, its motion being immediately modified by gravity, precisely in the same way as that of a thrown ball is modified. this is, indeed, the whole philosophy of throwing--to drop a ball from a moving carriage. the carriage is the hand, and, to throw far, a run is taken and the body is jerked forward; the arm is also moved as rapidly as possible on the shoulder as pivot. the fore-arm can be moved still faster, and the wrist-joint gives yet another motion: the art of throwing is to bring all these to bear at the same instant, and then just as they have all attained their maximum velocity to let the ball go. it starts off with the initial velocity thus imparted, and is abandoned to gravity. if the vehicle were able to continue its motion steadily, as a balloon does, the ball when let go from it would appear to the occupant simply to drop; and it would strike the ground at a spot vertically under the moving vehicle, though by no means vertically below the place where it started. the resistance of the air makes observations of this kind inaccurate, except when performed inside a carriage so that the air shares in the motion. otherwise a person could toss and catch a ball out of a train window just as well as if he were stationary; though to a spectator outside he would seem to be using great skill to throw the ball in the parabola adapted to bring it back to his hand. the same circumstance enhances the apparent difficulty of the circus rider's jumping feats. all he has to do is to jump up and down on the horse; the forward motion which carries him through hoops belongs to him by virtue of the motion of the horse, without effort on his part. thus, then, it happens that a stone dropped sixteen feet on the earth appears to fall straight down, although its real path in space is a very flat trajectory of nineteen miles base and sixteen feet height; nineteen miles being the distance traversed by the earth every second in the course of its annual journey round the sun. no wonder that it was thought that bodies must be left behind if the earth was subject to such terrific speed as this. all that copernicus could suggest on this head was that perhaps the atmosphere might help to carry things forward, and enable them to keep pace with the earth. there were thus several outstanding physical difficulties in the way of the acceptance of the copernican theory, besides the biblical difficulty. it was quite natural that the idea of the earth's motion should be repugnant, and take a long time to sink into the minds of men; and as scientific progress was vastly slower then than it is now, we find not only all priests but even some astronomers one hundred years afterwards still imagining the earth to be at rest. and among them was a very eminent one, tycho brahé. it is interesting to note, moreover, that the argument about the motion of the earth being contrary to scripture appealed not only to ecclesiastics in those days, but to scientific men also; and tycho brahé, being a man of great piety, and highly superstitious also, was so much influenced by it, that he endeavoured to devise some scheme by which the chief practical advantages of the copernican system could be retained, and yet the earth be kept still at the centre of the whole. this was done by making all the celestial sphere, with stars and everything, rotate round the earth once a day, as in the ptolemaic scheme; and then besides this making all the planets revolve round the sun, and this to revolve round the earth. such is the tychonic system. so far as _relative_ motion is concerned it comes to the same thing; just as when you drop a book you may say either that the earth rises to meet the book, or that the book falls to meet the earth. or when a fly buzzes round your head, you may say that you are revolving round the fly. but the absurdity of making the whole gigantic system of sun and planets and stars revolve round our insignificant earth was too great to be swallowed by other astronomers after they had once had a taste of the copernican theory; and accordingly the tychonic system died a speedy and an easy death at the same time as its inventor. wherein then lay the magnitude of the man?--not in his theories, which were puerile, but in his observations, which were magnificent. he was the first observational astronomer, the founder of the splendid system of practical astronomy which has culminated in the present greenwich observatory. [illustration: fig. .--tychonic system showing the sun with all the planets revolving round the earth.] up to tycho the only astronomical measurements had been of the rudest kind. copernicus even improved upon what had gone before, with measuring rules made with his own hands. ptolemy's observations could never be trusted to half a degree. tycho introduced accuracy before undreamed of, and though his measurements, reckoned by modern ideas, are of course almost ludicrously rough (remember no such thing as a telescope or microscope was then dreamed of), yet, estimated by the era in which they were made, they are marvels of accuracy, and not a single mistake due to carelessness has ever been detected in them. in fact they may be depended on almost to minutes of arc, _i.e._ to sixtieths of a degree. for certain purposes connected with the proper motion of stars they are still appealed to, and they served as the certain and trustworthy data for succeeding generations of theorists to work upon. it was long, indeed, after tycho's death before observations approaching in accuracy to his were again made. in every sense, therefore, he was a pioneer: let us proceed to trace his history. born the eldest son of a noble family--"as noble and ignorant as sixteen undisputed quarterings could make them," as one of his biographers says--in a period when, even more than at present, killing and hunting were the only natural aristocratic pursuits, when all study was regarded as something only fit for monks, and when science was looked at askance as something unsavoury, useless, and semi-diabolic, there was little in his introduction to the world urging him in the direction where his genius lay. of course he was destined for a soldier; but fortunately his uncle, george brahé, a more educated man than his father, having no son of his own, was anxious to adopt him, and though not permitted to do so for a time, succeeded in getting his way on the birth of a second son, steno--who, by the way, ultimately became privy councillor to the king of denmark. tycho's uncle gave him what he would never have got at home--a good education; and ultimately put him to study law. at the age of thirteen he entered the university of copenhagen, and while there occurred the determining influence of his life. an eclipse of the sun in those days was not regarded with the cold-blooded inquisitiveness or matter-of-fact apathy, according as there is or is not anything to be learnt from it, with which such an event is now regarded. every occurrence in the heavens was then believed to carry with it the destiny of nations and the fate of individuals, and accordingly was of surpassing interest. ever since the time of hipparchus it had been possible for some capable man here and there to predict the occurrence of eclipses pretty closely. the thing is not difficult. the prediction was not, indeed, to the minute and second, as it is now; but the day could usually be hit upon pretty accurately some time ahead, much as we now manage to hit upon the return of a comet--barring accidents; and the hour could be predicted as the event approached. well, the boy tycho, among others, watched for this eclipse on august st, ; and when it appeared at its appointed time, every instinct for the marvellous, dormant in his strong nature, awoke to strenuous life, and he determined to understand for himself a science permitting such wonderful possibilities of prediction. he was sent to leipzig with a tutor to go on with his study of law, but he seems to have done as little law as possible: he spent all his money on books and instruments, and sat up half the night studying and watching the stars. in he observed a conjunction of jupiter and saturn, the precursor, and _cause_ as he thought it, of the great plague. he found that the old planetary tables were as much as a month in error in fixing this event, and even the copernican tables were several days out; so he formed the resolve to devote his life to improving astronomical tables. this resolve he executed with a vengeance. his first instrument was a jointed ruler with sights for fixing the position of planets with respect to the stars, and observing their stations and retrogressions. by thus measuring the angles between a planet and two fixed stars, its position can be plotted down on a celestial map or globe. [illustration: fig. .--portrait of tycho.] in his uncle george died, and made tycho his heir. he returned to denmark, but met with nothing but ridicule and contempt for his absurd drivelling away of time over useless pursuits. so he went back to germany--first to wittenberg, thence, driven by the plague, to rostock. here his fiery nature led him into an absurd though somewhat dangerous adventure. a quarrel at some feast, on a mathematical point, with a countryman, manderupius, led to the fixing of a duel, and it was fought with swords at p.m. at the end of december, when, if there was any light at all, it must have been of a flickering and unsatisfactory nature. the result of this insane performance was that tycho got his nose cut clean off. he managed however to construct an artificial one, some say of gold and silver, some say of putty and brass; but whatever it was made of there is no doubt that he wore it for the rest of his life, and it is a most famous feature. it excited generally far more interest than his astronomical researches. it is said, moreover, to have very fairly resembled the original, but whether this remark was made by a friend or by an enemy i cannot say. one account says that he used to carry about with him a box of cement to apply whenever his nose came off, which it periodically did. about this time he visited augsburg, met with some kindred and enlightened spirits in that town, and with much enthusiasm and spirit constructed a great quadrant. these early instruments were tremendous affairs. a great number of workmen were employed upon this quadrant, and it took twenty men to carry it to its place and erect it. it stood in the open air for five years, and then was destroyed by a storm. with it he made many observations. [illustration: fig. .--early out-door quadrant of tycho; for observing altitudes by help of the sights _d_, _l_ and the plumb line.] on his return to denmark in , his fame preceded him, and he was much better received; and in order to increase his power of constructing instruments he took up the study of alchemy, and like the rest of the persuasion tried to make gold. the precious metals were by many old philosophers considered to be related in some way to the heavenly bodies: silver to the moon, for instance--as we still see by the name lunar caustic applied to nitrate of silver; gold to the sun, copper to mars, lead to saturn. hence astronomy and alchemy often went together. tycho all his life combined a little alchemy with his astronomical labours, and he constructed a wonderful patent medicine to cure all disorders, which had as wide a circulation in europe in its time as holloway's pills; he gives a tremendous receipt for it, with liquid gold and all manner of ingredients in it; among them, however, occurs a little antimony--a well-known sudorific--and to this, no doubt, whatever efficacy the medicine possessed was due. so he might have gone on wasting his time, were it not that in november, , a new star made its appearance, as they have done occasionally before and since. on the average one may say that about every fifty years a new star of fair magnitude makes its temporary appearance. they are now known to be the result of some catastrophe or collision, whereby immense masses of incandescent gas are produced. this one seen by tycho became as bright as jupiter, and then died away in about a year and a half. tycho observed all its changes, and endeavoured to measure its distance from the earth, with the result that it was proved to belong to the region of the fixed stars, at an immeasurable distance, and was not some nearer and more trivial phenomenon. he was asked by the university of copenhagen to give a course of lectures on astronomy; but this was a step he felt some aristocratic aversion to, until a little friendly pressure was brought to bear upon him by a request from the king, and delivered they were. he now seems to have finally thrown off his aristocratic prejudices, and to have indulged himself in treading on the corns of nearly all the high and mighty people he came into contact with. in short, he became what we might now call a violent radical; but he was a good-hearted man, nevertheless, and many are the tales told of his visits to sick peasants, of his consulting the stars as to their fate--all in perfect good faith--and of the medicines which he concocted and prescribed for them. the daughter of one of these peasants he married, and very happy the marriage seems to have been. [illustration: fig. .--map of denmark, showing the island of huen. _walker & boutallse._] now comes the crowning episode in tycho's life. frederick ii., realizing how eminent a man they had among them, and how much he could do if only he had the means--for we must understand that tycho, though of good family and well off, was by no means what we would call a wealthy man--frederick ii. made him a splendid and enlightened offer. the offer was this: that if tycho would agree to settle down and make his astronomical observations in denmark, he should have an estate in norway settled upon him, a pension of £ a year for life, a site for a large observatory, and £ , to build it with. [illustration: fig. .--uraniburg.] [illustration: fig. .--astrolabe. an old instrument with sights for marking the positions of the celestial bodies roughly. a sort of skeleton celestial globe.] [illustration: sextans astronomicvs trigonicvs pro distantiis rimandis. fig. .--tycho's large sextant; for measuring the angular distance between two bodies by direct sighting.] well, if ever money was well spent, this was. by its means denmark before long headed the nations of europe in the matter of science--a thing it has not done before or since. the site granted was the island of huen, between copenhagen and elsinore; and here the most magnificent observatory ever built was raised, and called uraniburg--the castle of the heavens. it was built on a hill in the centre of the island, and included gardens, printing shops, laboratory, dwelling-houses, and four observatories--all furnished with the most splendid instruments that tycho could devise, and that could then be constructed. it was decorated with pictures and sculptures of eminent men, and altogether was a most gorgeous place. £ , no doubt went far in those days, but the original grant was supplemented by tycho himself, who is said to have spent another equal sum out of his own pocket on the place. [illustration: qvadrans maximvs chalibeus quadrato inclusus, et horizonti azimuthali chalybeo insistens. fig. .--the quadrant in uraniburg; or altitude and azimuth instrument.] for twenty years this great temple of science was continually worked in by him, and he soon became the foremost scientific man in europe. philosophers, statesmen, and occasionally kings, came to visit the great astronomer, and to inspect his curiosities. [illustration: qvadrans mvralis sive tichonicus. fig. .--tycho's form of transit circle. the method of utilising the extremely uniform rotation of the earth by watching the planets and stars as they cross the meridian, and recording their times of transit; observing also at the same time their meridian altitudes (see observer _f_), was the invention of tycho, and constitutes his greatest achievement. his method is followed to this day in all observatories.] [illustration: fig. .--a modern transit circle, showing essentially the same parts as in tycho's instrument, viz. the observer watching the transit, the clock, the recorder of the observation, and the graduated circle; the latter to be read by a second observer.] and very wholesome for some of these great personages must have been the treatment they met with. for tycho was no respecter of persons. his humbly-born wife sat at the head of the table, whoever was there; and he would snub and contradict a chancellor just as soon as he would a serf. whatever form his pride may have taken when a youth, in his maturity it impelled him to ignore differences of rank not substantially justified, and he seemed to take a delight in exposing the ignorance of shallow titled persons, to whom contradiction and exposure were most unusual experiences. for sick peasants he would take no end of trouble, and went about doctoring them for nothing, till he set all the professional doctors against him; so that when his day of misfortune came, as come it did, their influence was not wanting to help to ruin one who spoilt their practice, and whom they derided as a quack. but some of the great ignorant folk who came to visit his temple of science, and to inspect its curiosities, felt themselves insulted--not always without reason. he kept a tame maniac in the house, named lep, and he used to regard the sayings of this personage as oracular, presaging future events, and far better worth listening to than ordinary conversation. consequently he used to have him at his banquets and feed him himself; and whenever lep opened his mouth to speak, every one else was peremptorily ordered to hold his tongue, so that lep's words might be written down. in fact it was something like an exaggerated edition of betsy trotwood and mr. dick. "it must have been an odd dinner party" (says prof. stuart), "with this strange, wild, terribly clever man, with his red hair and brazen nose, sometimes flashing with wit and knowledge, sometimes making the whole company, princes and servants alike, hold their peace and listen humbly to the ravings of a poor imbecile." to people he despised he did not show his serious instruments. he had other attractions, in the shape of a lot of toy machinery, little windmills, and queer doors, and golden globes, and all manner of ingenious tricks and automata, many of which he had made himself, and these he used to show them instead; and no doubt they were well enough pleased with them. those of the visitors, however, who really cared to see and understand his instruments, went away enchanted with his genius and hospitality. i may, perhaps, be producing an unfair impression of imperiousness and insolence. tycho was fiery, no doubt, but i think we should wrong him if we considered him insolent. most of the nobles of his day were haughty persons, accustomed to deal with serfs, and very likely to sneer at and trample on any meek man of science whom they could easily despise. so tycho was not meek; he stood up for the honour of his science, and paid them back in their own coin, with perhaps a little interest. that this behaviour was not worldly-wise is true enough, but i know of no commandment enjoining us to be worldly-wise. if we knew more about his so-called imbecile _protégé_ we should probably find some reason for the interest which tycho took in him. whether he was what is now called a "clairvoyant" or not, tycho evidently regarded his utterances as oracular, and of course when one is receiving what may be a revelation from heaven it is natural to suppress ordinary conversation. among the noble visitors whom he received and entertained, it is interesting to notice james i. of england, who spent eight days at uraniburg on the occasion of his marriage with anne of denmark in , and seems to have been deeply impressed by his visit. among other gifts, james presented tycho with a dog (depicted in fig. ), and this same animal was subsequently the cause of trouble. for it seems that one day the chancellor of denmark, walchendorf, brutally kicked the poor beast; and tycho, who was very fond of animals, gave him a piece of his mind in no measured language. walchendorf went home determined to ruin him. king frederick, however, remained his true friend, doubtless partly influenced thereto by his queen sophia, an enlightened woman who paid many visits to uraniburg, and knew tycho well. but unfortunately frederick died; and his son, a mere boy, came to the throne. now was the time for the people whom tycho had offended, for those who were jealous of his great fame and importance, as well as for those who cast longing eyes on his estate and endowments. the boy-king, too, unfortunately paid a visit to tycho, and, venturing upon a decided opinion on some recondite subject, received a quiet setting down which he ill relished. letters written by tycho about this time are full of foreboding. he greatly dreads having to leave uraniburg, with which his whole life has for twenty years been bound up. he tries to comfort himself with the thought that, wherever he is sent, he will have the same heavens and the same stars over his head. gradually his norwegian estate and his pension were taken away, and in five years poverty compelled him to abandon his magnificent temple, and to take a small house in copenhagen. not content with this, walchendorf got a royal commission appointed to inquire into the value of his astronomical labours. this sapient body reported that his work was not only useless, but noxious; and soon after he was attacked by the populace in the public street. nothing was left for him now but to leave the country, and he went into germany, leaving his wife and instruments to follow him whenever he could find a home for them. his wanderings in this dark time--some two years--are not quite clear; but at last the enlightened emperor of bohemia, rudolph ii., invited him to settle in prague. thither he repaired, a castle was given him as an observatory, a house in the city, and crowns a year for life. so his instruments were set up once more, students flocked to hear him and to receive work at his hands--among them a poor youth, john kepler, to whom he was very kind, and who became, as you know, a still greater man than his master. but the spirit of tycho was broken, and though some good work was done at prague--more observations made, and the rudolphine tables begun--yet the hand of death was upon him. a painful disease seized him, attended with sleeplessness and temporary delirium, during the paroxysms of which he frequently exclaimed, _ne frustra vixisse videar_. ("oh that it may not appear that i have lived in vain!") quietly, however, at last, and surrounded by his friends and relatives, this fierce, passionate soul passed away, on the th of october, . his beloved instruments, which were almost a part of himself, were stored by rudolph in a museum with scrupulous care, until the taking of prague by the elector palatine's troops. in this disturbed time they got smashed, dispersed, and converted to other purposes. one thing only was saved--the great brass globe, which some thirty years after was recognized by a later king of denmark as having belonged to tycho, and deposited in the library of the academy of sciences at copenhagen, where i believe it is to this day. the island of huen was overrun by the danish nobility, and nothing now remains of uraniburg but a mound of earth and two pits. as to the real work of tycho, that has become immortal enough,--chiefly through the labours of his friend and scholar whose life we shall consider in the next lecture. summary of facts for lecture iii _life and work of kepler._ kepler was born in december, , at weil in würtemberg. father an officer in the duke's army, mother something of a virago, both very poor. kepler was utilized as a tavern pot-boy, but ultimately sent to a charity school, and thence to the university of tübingen. health extremely delicate; he was liable to violent attacks all his life. studied mathematics, and accepted an astronomical lectureship at graz as the first post which offered. endeavoured to discover some connection between the number of the planets, their times of revolution, and their distances from the sun. ultimately hit upon his fanciful regular-solid hypothesis, and published his first book in . in was invited by tycho to prague, and there appointed imperial mathematician, at a handsome but seldom paid salary. observed the new star of . endeavoured to find the law of refraction of light from vitellio's measurements, but failed. analyzed tycho's observations to find the true law of motion of mars. after incredible labour, through innumerable wrong guesses, and six years of almost incessant calculation, he at length emerged in his two "laws"--discoveries which swept away all epicycles, deferents, equants, and other remnants of the greek system, and ushered in the dawn of modern astronomy. law i. _planets move in ellipses, with the sun in one focus._ law ii. _the radius vector (or line joining sun and planet) sweeps out equal areas in equal times._ published his second book containing these laws in . death of rudolph in , and subsequent increased misery and misfortune of kepler. ultimately discovered the connection between the times and distances of the planets for which he had been groping all his mature life, and announced it in :-- law iii. _the square of the time of revolution (or year) of each planet is proportional to the cube of its mean distance from the sun._ the book in which this law was published ("on celestial harmonies") was dedicated to james of england. in had to intervene to protect his mother from being tortured for witchcraft. accepted a professorship at linz. published the rudolphine tables in , embodying tycho's observations and his own theory. made a last effort to overcome his poverty by getting the arrears of his salary paid at prague, but was unsuccessful, and, contracting brain fever on the journey, died in november, , aged . a man of keen imagination, indomitable perseverance, and uncompromising love of truth, kepler overcame ill-health, poverty, and misfortune, and placed himself in the very highest rank of scientific men. his laws, so extraordinarily discovered, introduced order and simplicity into what else would have been a chaos of detailed observations; and they served as a secure basis for the splendid erection made on them by newton. _seven planets of the ptolemaic system--_ moon, mercury, venus, sun, mars, jupiter, saturn. _six planets of the copernican system--_ mercury, venus, earth, mars, jupiter, saturn. _the five regular solids, in appropriate order--_ octahedron, icosahedron, dodecahedron, tetrahedron, cube. _table illustrating kepler's third law._ +---------+---------------+-----------+---------------+----------------+ | | mean distance | length | cube of the | square of the | | planet. | from sun. | of year. | distance. | time. | | | d | t | d^ | t^ | +---------+---------------+-----------+---------------+----------------+ | mercury | · | · | · | · | | venus | · | · | · | · | | earth | · | · | · | · | | mars | · | · | · | · | | jupiter | · | · | · | · | | saturn | · | · | · | · | +---------+---------------+-----------+---------------+----------------+ the length of the earth's year is · days; its mean distance from the sun, taken above as unity, is , , miles. lecture iii kepler and the laws of planetary motion it is difficult to imagine a stronger contrast between two men engaged in the same branch of science than exists between tycho brahé, the subject of last lecture, and kepler, our subject on the present occasion. the one, rich, noble, vigorous, passionate, strong in mechanical ingenuity and experimental skill, but not above the average in theoretical and mathematical power. the other, poor, sickly, devoid of experimental gifts, and unfitted by nature for accurate observation, but strong almost beyond competition in speculative subtlety and innate mathematical perception. the one is the complement of the other; and from the fact of their following each other so closely arose the most surprising benefits to science. the outward life of kepler is to a large extent a mere record of poverty and misfortune. i shall only sketch in its broad features, so that we may have more time to attend to his work. he was born (so his biographer assures us) in longitude ° ', latitude ° ', on the st of december, . his parents seem to have been of fair condition, but by reason, it is said, of his becoming surety for a friend, the father lost all his slender income, and was reduced to keeping a tavern. young john kepler was thereupon taken from school, and employed as pot-boy between the ages of nine and twelve. he was a sickly lad, subject to violent illnesses from the cradle, so that his life was frequently despaired of. ultimately he was sent to a monastic school and thence to the university of tübingen, where he graduated second on the list. meanwhile home affairs had gone to rack and ruin. his father abandoned the home, and later died abroad. the mother quarrelled with all her relations, including her son john; who was therefore glad to get away as soon as possible. all his connection with astronomy up to this time had been the hearing the copernican theory expounded in university lectures, and defending it in a college debating society. an astronomical lectureship at graz happening to offer itself, he was urged to take it, and agreed to do so, though stipulating that it should not debar him from some more brilliant profession when there was a chance. for astronomy in those days seems to have ranked as a minor science, like mineralogy or meteorology now. it had little of the special dignity with which the labours of kepler himself were destined so greatly to aid in endowing it. well, he speedily became a thorough copernican, and as he had a most singularly restless and inquisitive mind, full of appreciation of everything relating to number and magnitude--was a born speculator and thinker just as mozart was a born musician, or bidder a born calculator--he was agitated by questions such as these: why are there exactly six planets? is there any connection between their orbital distances, or between their orbits and the times of describing them? these things tormented him, and he thought about them day and night. it is characteristic of the spirit of the times--this questioning why there should be six planets. nowadays, we should simply record the fact and look out for a seventh. then, some occult property of the number six was groped for, such as that it was equal to + + and likewise equal to × × , and so on. many fine reasons had been given for the seven planets of the ptolemaic system (see, for instance, p. ), but for the six planets of the copernican system the reasons were not so cogent. again, with respect to their successive distances from the sun, some law would seem to regulate their distance, but it was not known. (parenthetically i may remark that it is not known even now: a crude empirical statement known as bode's law--see page --is all that has been discovered.) once more, the further the planet the slower it moved; there seemed to be some law connecting speed and distance. this also kepler made continual attempts to discover. [illustration: fig. .--orbits of some of the planets drawn to scale: showing the gap between mars and jupiter.] one of his ideas concerning the law of the successive distances was based on the inscription of a triangle in a circle. if you inscribe in a circle a large number of equilateral triangles, they envelop another circle bearing a definite ratio to the first: these might do for the orbits of two planets (see fig. ). then try inscribing and circumscribing squares, hexagons, and other figures, and see if the circles thus defined would correspond to the several planetary orbits. but they would not give any satisfactory result. brooding over this disappointment, the idea of trying solid figures suddenly strikes him. "what have plane figures to do with the celestial orbits?" he cries out; "inscribe the regular solids." and then--brilliant idea--he remembers that there are but five. euclid had shown that there could be only five regular solids.[ ] the number evidently corresponds to the gaps between the six planets. the reason of there being only six seems to be attained. this coincidence assures him he is on the right track, and with great enthusiasm and hope he "represents the earth's orbit by a sphere as the norm and measure of all"; round it he circumscribes a dodecahedron, and puts another sphere round that, which is approximately the orbit of mars; round that, again, a tetrahedron, the corners of which mark the sphere of the orbit of jupiter; round that sphere, again, he places a cube, which roughly gives the orbit of saturn. [illustration: fig. .--many-sided polygon or approximate circle enveloped by straight lines, as for instance by a number of equilateral triangles.] on the other hand, he inscribes in the sphere of the earth's orbit an icosahedron; and inside the sphere determined by that, an octahedron; which figures he takes to inclose the spheres of venus and of mercury respectively. the imagined discovery is purely fictitious and accidental. first of all, eight planets are now known; and secondly, their real distances agree only very approximately with kepler's hypothesis. [illustration: fig. .--frameworks with inscribed and circumscribed spheres, representing the five regular solids distributed as kepler supposed them to be among the planetary orbits. (see "summary" at beginning of this lecture, p. .)] nevertheless, the idea gave him great delight. he says:--"the intense pleasure i have received from this discovery can never be told in words. i regretted no more the time wasted; i tired of no labour; i shunned no toil of reckoning, days and nights spent in calculation, until i could see whether my hypothesis would agree with the orbits of copernicus, or whether my joy was to vanish into air." he then went on to speculate as to the cause of the planets' motion. the old idea was that they were carried round by angels or celestial intelligences. kepler tried to establish some propelling force emanating from the sun, like the spokes of a windmill. this first book of his brought him into notice, and served as an introduction to tycho and to galileo. tycho brahé was at this time at prague under the patronage of the emperor rudolph; and as he was known to have by far the best planetary observations of any man living, kepler wrote to him to know if he might come and examine them so as to perfect his theory. tycho immediately replied, "come, not as a stranger, but as a very welcome friend; come and share in my observations with such instruments as i have with me, and as a dearly beloved associate." after this visit, tycho wrote again, offering him the post of mathematical assistant, which after hesitation was accepted. part of the hesitation kepler expresses by saying that "for observations his sight was dull, and for mechanical operations his hand was awkward. he suffered much from weak eyes, and dare not expose himself to night air." in all this he was, of course, the antipodes of tycho, but in mathematical skill he was greatly his superior. on his way to prague he was seized with one of his periodical illnesses, and all his means were exhausted by the time he could set forward again, so that he had to apply for help to tycho. it is clear, indeed, that for some time now he subsisted entirely on the bounty of tycho, and he expresses himself most deeply grateful for all the kindness he received from that noble and distinguished man, the head of the scientific world at that date. to illustrate tycho's kindness and generosity, i must read you a letter written to him by kepler. it seems that kepler, on one of his absences from prague, driven half mad with poverty and trouble, fell foul of tycho, whom he thought to be behaving badly in money matters to him and his family, and wrote him a violent letter full of reproaches and insults. tycho's secretary replied quietly enough, pointing out the groundlessness and ingratitude of the accusation. kepler repents instantly, and replies:-- "most noble tycho," (these are the words of his letter), "how shall i enumerate or rightly estimate your benefits conferred on me? for two months you have liberally and gratuitously maintained me, and my whole family; you have provided for all my wishes; you have done me every possible kindness; you have communicated to me everything you hold most dear; no one, by word or deed, has intentionally injured me in anything; in short, not to your children, your wife, or yourself have you shown more indulgence than to me. this being so, as i am anxious to put on record, i cannot reflect without consternation that i should have been so given up by god to my own intemperance as to shut my eyes on all these benefits; that, instead of modest and respectful gratitude, i should indulge for three weeks in continual moroseness towards all your family, in headlong passion and the utmost insolence towards yourself, who possess so many claims on my veneration, from your noble family, your extraordinary learning, and distinguished reputation. whatever i have said or written against the person, the fame, the honour, and the learning of your excellency; or whatever, in any other way, i have injuriously spoken or written (if they admit no other more favourable interpretation), as, to my grief, i have spoken and written many things, and more than i can remember; all and everything i recant, and freely and honestly declare and profess to be groundless, false, and incapable of proof." tycho accepted the apology thus heartily rendered, and the temporary breach was permanently healed. in , kepler was appointed "imperial mathematician," to assist tycho in his calculations. the emperor rudolph did a good piece of work in thus maintaining these two eminent men, but it is quite clear that it was as astrologers that he valued them; and all he cared for in the planetary motions was limited to their supposed effect on his own and his kingdom's destiny. he seems to have been politically a weak and superstitious prince, who was letting his kingdom get into hopeless confusion, and entangling himself in all manner of political complications. while bohemia suffered, however, the world has benefited at his hands; and the tables upon which tycho was now engaged are well called the rudolphine tables. these tables of planetary motion tycho had always regarded as the main work of his life; but he died before they were finished, and on his death-bed he intrusted the completion of them to kepler, who loyally undertook their charge. the imperial funds were by this time, however, so taxed by wars and other difficulties that the tables could only be proceeded with very slowly, a staff of calculators being out of the question. in fact, kepler could not get even his own salary paid: he got orders, and promises, and drafts on estates for it; but when the time came for them to be honoured they were worthless, and he had no power to enforce his claims. so everything but brooding had to be abandoned as too expensive, and he proceeded to study optics. he gave a very accurate explanation of the action of the human eye, and made many hypotheses, some of them shrewd and close to the mark, concerning the law of refraction of light in dense media: but though several minor points of interest turned up, nothing of the first magnitude came out of this long research. the true law of refraction was discovered some years after by a dutch professor, willebrod snell. we must now devote a little time to the main work of kepler's life. all the time he had been at prague he had been making a severe study of the motion of the planet mars, analyzing minutely tycho's books of observations, in order to find out, if possible, the true theory of his motion. aristotle had taught that circular motion was the only perfect and natural motion, and that the heavenly bodies therefore necessarily moved in circles. so firmly had this idea become rooted in men's minds, that no one ever seems to have contemplated the possibility of its being false or meaningless. when hipparchus and others found that, as a matter of fact, the planets did _not_ revolve in simple circles, they did not try other curves, as we should at once do now, but they tried combinations of circles, as we saw in lecture i. the small circle carried by a bigger one was called an epicycle. the carrying circle was called the deferent. if for any reason the earth had to be placed out of the centre, the main planetary orbit was called an excentric, and so on. but although the planetary paths might be roughly represented by a combination of circles, their speeds could not, on the hypothesis of uniform motion in each circle round the earth as a fixed body. hence was introduced the idea of an equant, _i.e._ an arbitrary point, not the earth, about which the speed might be uniform. copernicus, by making the sun the centre, had been able to simplify a good deal of this, and to abolish the equant. but now that kepler had the accurate observations of tycho to refer to, he found immense difficulty in obtaining the true positions of the planets for long together on any such theory. he specially attacked the motion of the planet mars, because that was sufficiently rapid in its changes for a considerable collection of data to have accumulated with respect to it. he tried all manner of circular orbits for the earth and for mars, placing them in all sorts of aspects with respect to the sun. the problem to be solved was to choose such an orbit and such a law of speed, for both the earth and mars, that a line joining them, produced out to the stars, should always mark correctly the apparent position of mars as seen from the earth. he had to arrange the size of the orbits that suited best, then the positions of their centres, both being supposed excentric with respect to the sun; but he could not get any such arrangement to work with uniform motion about the sun. so he reintroduced the equant, and thus had another variable at his disposal--in fact, two, for he had an equant for the earth and another for mars, getting a pattern of the kind suggested in fig. . the equants might divide the line in any arbitrary ratio. all sorts of combinations had to be tried, the relative positions of the earth and mars to be worked out for each, and compared with tycho's recorded observations. it was easy to get them to agree for a short time, but sooner or later a discrepancy showed itself. [illustration: fig. .--_s_ represents the sun; _ec_, the centre of the earth's orbit, to be placed as best suited; _mc_, the same for mars; _ee_, the earth's equant, or point about which the earth uniformly revolved (_i.e._ the point determining the law of speed about the sun), likewise to be placed anywhere, but supposed to be in the line joining _s_ to _ec_; _me_, the same thing for mars; with _?me_ for an alternative hypothesis that perhaps mars' equant was on line joining _ec_ with _mc_.] i need not say that all these attempts and gropings, thus briefly summarized, entailed enormous labour, and required not only great pertinacity, but a most singularly constituted mind, that could thus continue groping in the dark without a possible ray of theory to illuminate its search. grope he did, however, with unexampled diligence. at length he hit upon a point that seemed nearly right. he thought he had found the truth; but no, before long the position of the planet, as calculated, and as recorded by tycho, differed by eight minutes of arc, or about one-eighth of a degree. could the observation be wrong by this small amount? no, he had known tycho, and knew that he was never wrong eight minutes in an observation. so he set out the whole weary way again, and said that with those eight minutes he would yet find out the law of the universe. he proceeded to see if by making the planet librate, or the plane of its orbit tilt up and down, anything could be done. he was rewarded by finding that at any rate the plane of the orbit did not tilt up and down: it was fixed, and this was a simplification on copernicus's theory. it is not an absolute fixture, but the changes are very small (see laplace, page ). [illustration: fig. .--excentric circle supposed to be divided into equal areas. the sun, _s_, being placed at a selected point, it was possible to represent the varying speed of a planet by saying that it moved from _a_ to _b_, from _b_ to _c_, and so on, in equal times.] at last he thought of giving up the idea of _uniform_ circular motion, and of trying _varying_ circular motion, say inversely as its distance from the sun. to simplify calculation, he divided the orbit into triangles, and tried if making the triangles equal would do. a great piece of luck, they did beautifully: the rate of description of areas (not arcs) is uniform. over this discovery he greatly rejoices. he feels as though he had been carrying on a war against the planet and had triumphed; but his gratulation was premature. before long fresh little errors appeared, and grew in importance. thus he announces it himself:-- "while thus triumphing over mars, and preparing for him, as for one already vanquished, tabular prisons and equated excentric fetters, it is buzzed here and there that the victory is vain, and that the war is raging anew as violently as before. for the enemy left at home a despised captive has burst all the chains of the equations, and broken forth from the prisons of the tables." still, a part of the truth had been gained, and was not to be abandoned any more. the law of speed was fixed: that which is now known as his second law. but what about the shape of the orbit--was it after all possible that aristotle, and every philosopher since aristotle, had been wrong? that circular motion was not the perfect and natural motion, but that planets might move in some other closed curve? suppose he tried an oval. well, there are a great variety of ovals, and several were tried: with the result that they could be made to answer better than a circle, but still were not right. now, however, the geometrical and mathematical difficulties of calculation, which before had been tedious and oppressive, threatened to become overwhelming; and it is with a rising sense of despondency that kepler sees his six years' unremitting labour leading deeper and deeper into complication. one most disheartening circumstance appeared, viz. that when he made the circuit oval his law of equable description of areas broke down. that seemed to require the circular orbit, and yet no circular orbit was quite accurate. while thinking and pondering for weeks and months over this new dilemma and complication of difficulties, till his brain reeled, an accidental ray of light broke upon him in a way not now intelligible, or barely intelligible. half the extreme breadth intercepted between the circle and oval was / , of the radius, and he remembered that the "optical inequality" of mars was also about / , . this coincidence, in his own words, woke him out of sleep; and for some reason or other impelled him instantly to try making the planet oscillate in the diameter of its epicycle instead of revolve round it--a singular idea, but copernicus had had a similar one to explain the motions of mercury. [illustration: fig. .--mode of drawing an ellipse. the two pins _f_ are the foci.] away he started through his calculations again. a long course of work night and day was rewarded by finding that he was now able to hit off the motions better than before; but what a singularly complicated motion it was. could it be expressed no more simply? yes, the curve so described by the planet is a comparatively simple one: it is a special kind of oval--the ellipse. strange that he had not thought of it before. it was a famous curve, for the greek geometers had studied it as one of the sections of a cone, but it was not so well known in kepler's time. the fact that the planets move in it has raised it to the first importance, and it is familiar enough to us now. but did it satisfy the law of speed? could the rate of description of areas be uniform with it? well, he tried the ellipse, and to his inexpressible delight he found that it did satisfy the condition of equable description of areas, if the sun was in one focus. so, moving the planet in a selected ellipse, with the sun in one focus, at a speed given by the equable area description, its position agreed with tycho's observations within the limits of the error of experiment. mars was finally conquered, and remains in his prison-house to this day. the orbit was found. [illustration: fig. .] in a paroxysm of delight kepler celebrates his victory by a triumphant figure, sketched actually on his geometrical diagram--the diagram which proves that the law of equable description of areas can hold good with an ellipse. the above is a tracing of it. such is a crude and bald sketch of the steps by which kepler rose to his great generalizations--the two laws which have immortalized his name. all the complications of epicycle, equant, deferent, excentric, and the like, were swept at once away, and an orbit of striking and beautiful properties substituted. well might he be called, as he was, "the legislator," or law interpreter, "of the heavens." [illustration: fig. .--if _s_ is the sun, a planet or comet moves from _p_ to _p_ _, from _p_ _ to _p_ _, and from _p_ _ to _p_ _ in the same time; if the shaded areas are equal.] he concludes his book on the motions of mars with a half comic appeal to the emperor to provide him with the sinews of war for an attack on mars's relations--father jupiter, brother mercury, and the rest--but the death of his unhappy patron in put an end to all these schemes, and reduced kepler to the utmost misery. while at prague his salary was in continual arrear, and it was with difficulty that he could provide sustenance for his family. he had been there eleven years, but they had been hard years of poverty, and he could leave without regret were it not that he should have to leave tycho's instruments and observations behind him. while he was hesitating what best to do, and reduced to the verge of despair, his wife, who had long been suffering from low spirits and despondency, and his three children, were taken ill; one of the sons died of small-pox, and the wife eleven days after of low fever and epilepsy. no money could be got at prague, so after a short time he accepted a professorship at linz, and withdrew with his two quite young remaining children. he provided for himself now partly by publishing a prophesying almanack, a sort of zadkiel arrangement--a thing which he despised, but the support of which he could not afford to do without. he is continually attacking and throwing sarcasm at astrology, but it was the only thing for which people would pay him, and on it after a fashion he lived. we do not find that his circumstances were ever prosperous, and though , crowns were due to him from bohemia he could not manage to get them paid. about this time occurred a singular interruption to his work. his old mother, of whose fierce temper something has already been indicated, had been engaged in a law-suit for some years near their old home in würtemberg. a change of judge having in process of time occurred, the defendant saw his way to turn the tables on the old lady by accusing her of sorcery. she was sent to prison, and condemned to the torture, with the usual intelligent idea of extracting a "voluntary" confession. kepler had to hurry from linz to interpose. he succeeded in saving her from the torture, but she remained in prison for a year or so. her spirit, however, was unbroken, for no sooner was she released than she commenced a fresh action against her accuser. but fresh trouble was averted by the death of the poor old dame at the age of nearly eighty. this narration renders the unflagging energy shown by her son in his mathematical wrestlings less surprising. interspersed with these domestic troubles, and with harassing and unsuccessful attempts to get his rights, he still brooded over his old problem of some possible connection between the distances of the planets from the sun and their times of revolution, _i.e._ the length of their years. it might well have been that there was no connection, that it was purely imaginary, like his old idea of the law of the successive distances of the planets, and like so many others of the guesses and fancies which he entertained and spent his energies in probing. but fortunately this time there was a connection, and he lived to have the joy of discovering it. the connection is this, that if one compares the distance of the different planets from the sun with the length of time they take to go round him, the cube of the respective distances is proportional to the square of the corresponding times. in other words, the ratio of r^ to t^ for every planet is the same. or, again, the length of a planet's year depends on the / th power of its distance from the sun. or, once more, the speed of each planet in its orbit is as the inverse square-root of its distance from the sun. the product of the distance into the square of the speed is the same for each planet. this (however stated) is called kepler's third law. it welds the planets together, and shows them to be one system. his rapture on detecting the law was unbounded, and he breaks out into an exulting rhapsody:-- "what i prophesied two-and-twenty years ago, as soon as i discovered the five solids among the heavenly orbits--what i firmly believed long before i had seen ptolemy's _harmonies_--what i had promised my friends in the title of this book, which i named before i was sure of my discovery--what sixteen years ago, i urged as a thing to be sought--that for which i joined tycho brahé, for which i settled in prague, for which i have devoted the best part of my life to astronomical contemplations, at length i have brought to light, and recognized its truth beyond my most sanguine expectations. it is not eighteen months since i got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze upon, burst upon me. nothing holds me; i will indulge my sacred fury; i will triumph over mankind by the honest confession that i have stolen the golden vases of the egyptians to build up a tabernacle for my god far away from the confines of egypt. if you forgive me, i rejoice; if you are angry, i can bear it; the die is cast, the book is written, to be read either now or by posterity, i care not which; it may well wait a century for a reader, as god has waited six thousand years for an observer." soon after this great work his third book appeared: it was an epitome of the copernican theory, a clear and fairly popular exposition of it, which had the honour of being at once suppressed and placed on the list of books prohibited by the church, side by side with the work of copernicus himself, _de revolutionibus orbium coelestium_. this honour, however, gave kepler no satisfaction--it rather occasioned him dismay, especially as it deprived him of all pecuniary benefit, and made it almost impossible for him to get a publisher to undertake another book. still he worked on at the rudolphine tables of tycho, and ultimately, with some small help from vienna, completed them; but he could not get the means to print them. he applied to the court till he was sick of applying: they lay idle four years. at last he determined to pay for the type himself. what he paid it with, god knows, but he did pay it, and he did bring out the tables, and so was faithful to the behest of his friend. this great publication marks an era in astronomy. they were the first really accurate tables which navigators ever possessed; they were the precursors of our present _nautical almanack_. after this, the grand duke of tuscany sent kepler a golden chain, which is interesting inasmuch as it must really have come from galileo, who was in high favour at the italian court at this time. once more kepler made a determined attempt to get his arrears of salary paid, and rescue himself and family from their bitter poverty. he travelled to prague on purpose, attended the imperial meeting, and pleaded his own cause, but it was all fruitless; and exhausted by the journey, weakened by over-study, and disheartened by the failure, he caught a fever, and died in his fifty-ninth year. his body was buried at ratisbon, and a century ago a proposal was made to erect a marble monument to his memory, but nothing was done. it matters little one way or the other whether germany, having almost refused him bread during his life, should, a century and a half after his death, offer him a stone. [illustration: fig. .--portrait of kepler, older.] the contiguity of the lives of kepler and tycho furnishes a moral too obvious to need pointing out. what kepler might have achieved had he been relieved of those ghastly struggles for subsistence one cannot tell, but this much is clear, that had tycho been subjected to the same misfortune, instead of being born rich and being assisted by generous and enlightened patrons, he could have accomplished very little. his instruments, his observatory--the tools by which he did his work--would have been impossible for him. frederick and sophia of denmark, and rudolph of bohemia, are therefore to be remembered as co-workers with him. kepler, with his ill-health and inferior physical energy, was unable to command the like advantages. much, nevertheless, he did; more one cannot but feel he might have done had he been properly helped. besides, the world would have been free from the reproach of accepting the fruits of his bright genius while condemning the worker to a life of misery, relieved only by the beauty of his own thoughts and the ecstasy awakened in him by the harmony and precision of nature. concerning the method of kepler, the mode by which he made his discoveries, we must remember that he gives us an account of all the steps, unsuccessful as well as successful, by which he travelled. he maps out his route like a traveller. in fact he compares himself to columbus or magellan, voyaging into unknown lands, and recording his wandering route. this being remembered, it will be found that his methods do not differ so utterly from those used by other philosophers in like case. his imagination was perhaps more luxuriant and was allowed freer play than most men's, but it was nevertheless always controlled by rigid examination and comparison of hypotheses with fact. brewster says of him:--"ardent, restless, burning to distinguish himself by discovery, he attempted everything; and once having obtained a glimpse of a clue, no labour was too hard in following or verifying it. a few of his attempts succeeded--a multitude failed. those which failed seem to us now fanciful, those which succeeded appear to us sublime. but his methods were the same. when in search of what really existed he sometimes found it; when in pursuit of a chimæra he could not but fail; but in either case he displayed the same great qualities, and that obstinate perseverance which must conquer all difficulties except those really insurmountable." to realize what he did for astronomy, it is necessary for us now to consider some science still in its infancy. astronomy is so clear and so thoroughly explored now, that it is difficult to put oneself into a contemporary attitude. but take some other science still barely developed: meteorology, for instance. the science of the weather, the succession of winds and rain, sunshine and frost, clouds and fog, is now very much in the condition of astronomy before kepler. we have passed through the stage of ascribing atmospheric disturbances--thunderstorms, cyclones, earthquakes, and the like--to supernatural agency; we have had our copernican era: not perhaps brought about by a single individual, but still achieved. something of the laws of cyclone and anticyclone are known, and rude weather predictions across the atlantic are roughly possible. barometers and thermometers and anemometers, and all their tribe, represent the astronomical instruments in the island of huen; and our numerous meteorological observatories, with their continual record of events, represent the work of tycho brahé. observation is heaped on observation; tables are compiled; volumes are filled with data; the hours of sunshine are recorded, the fall of rain, the moisture in the air, the kind of clouds, the temperature--millions of facts; but where is the kepler to study and brood over them? where is the man to spend his life in evolving the beginnings of law and order from the midst of all this chaos? perhaps as a man he may not come, but his era will come. through this stage the science must pass, ere it is ready for the commanding intellect of a newton. but what a work it will be for the man, whoever he be that undertakes it--a fearful monotonous grind of calculation, hypothesis, hypothesis, calculation, a desperate and groping endeavour to reconcile theories with facts. a life of such labour, crowned by three brilliant discoveries, the world owes (and too late recognizes its obligation) to the harshly treated german genius, kepler. summary of facts for lectures iv and v in , michael angelo died and galileo was born; in , galileo died and newton was born. milton lived from to . for teaching the plurality of worlds, with other heterodox doctrines, and refusing to recant, bruno, after six years' imprisonment in rome, was burnt at the stake on the th of february, a.d. a "natural" death in the dungeons of the inquisition saved antonio de dominis, the explainer of the rainbow, from the same fate, but his body and books were publicly burned at rome in . the persecution of galileo began in , became intense in , and so lasted till his death and after. * * * * * galileo galilei, eldest son of vincenzo de bonajuti de galilei, a noble florentine, was born at pisa, th of february, . at the age of was sent to the university of pisa to study medicine. observed the swing of a pendulum and applied it to count pulse-beats. read euclid and archimedes, and could be kept at medicine no more. at was appointed lecturer in mathematics at pisa. read bruno and became smitten with the copernican theory. controverted the aristotelians concerning falling bodies, at pisa. hence became unpopular and accepted a chair at padua, . invented a thermometer. wrote on astronomy, adopting the ptolemaic system provisionally, and so opened up a correspondence with kepler, with whom he formed a friendship. lectured on the new star of , and publicly renounced the old systems of astronomy. invented a calculating compass or "gunter's scale." in invented a telescope, after hearing of a dutch optician's discovery. invented the microscope soon after. rapidly completed a better telescope and began a survey of the heavens. on the th of january, , discovered jupiter's satellites. observed the mountains in the moon, and roughly measured their height. explained the visibility of the new moon by _earth-shine_. was invited to the grand ducal court of tuscany by cosmo de medici, and appointed philosopher to that personage. discovered innumerable new stars, and the nebulæ. observed a triple appearance of saturn. discovered the phases of venus predicted by copernicus, and spots on the sun. wrote on floating bodies. tried to get his satellites utilized for determining longitude at sea. went to rome to defend the copernican system, then under official discussion, and as a result was formally forbidden ever to teach it. on the accession of pope urban viii. in , galileo again visited rome to pay his respects, and was well received. in appeared his "dialogues" on the ptolemaic and copernican systems. summoned to rome, practically imprisoned, and "rigorously questioned." was made to recant nd of june, . forbidden evermore to publish anything, or to teach, or receive friends. retired to arcetri in broken down health. death of his favourite daughter, sister maria celeste. wrote and meditated on the laws of motion. discovered the moon's libration. in he became blind. the rigour was then slightly relaxed and many visited him: among them john milton. died th of january, , aged . as a prisoner of the inquisition his right to make a will or to be buried in consecrated ground was disputed. many of his manuscripts were destroyed. galileo, besides being a singularly clear-headed thinker and experimental genius, was also something of a musician, a poet, and an artist. he was full of humour as well as of solid common-sense, and his literary style is brilliant. of his scientific achievements those now reckoned most weighty, are the discovery of the laws of motion, and the laying of the foundations of mechanics. _particulars of jupiter's satellites, illustrating their obedience to kepler's third law._ -------------------------------------------------------------------------- | | | distance| | | t^ | | time of | from | | | ---- satellite.|diameter revolution | jupiter, | t^ | d^ | d^ | miles.| in hours. |in jovian | | | which is | miles | (t) | radii. | | |practically | | | (d) | | | constant. ----------|-------|------------|----------|---------|---------|----------- no. . | | · | · | · | · | · no. . | | · | · | · | · | · no. . | | · | · | · | · | · no. . | | · | · | · | · | · -------------------------------------------------------------------------- the diameter of jupiter is , miles. _falling bodies._ since all bodies fall at the same rate, except for the disturbing effect of the resistance of the air, a statement of their rates of fall is of interest. in one second a freely falling body near the earth is found to drop feet. in two seconds it drops feet altogether, viz. feet in the first, and feet in the next second; because at the beginning of every second after the first it has the accumulated velocity of preceding seconds. the height fallen by a dropped body is not proportional to the time simply, but to what is rather absurdly called the square of the time, _i.e._ the time multiplied by itself. for instance, in seconds it drops × = feet; in seconds × , or feet, and so on. the distances travelled in , , , , &c., seconds by a body dropped from rest and not appreciably resisted by the air, are , , , , , &c., respectively, each multiplied by the constant feet. another way of stating the law is to say that the heights travelled in successive seconds proceed in the proportion , , , , , &c.; again multiplied by feet in each case. [illustration: fig. .--curve described by a projectile, showing how it drops from the line of fire, _o d_, in successive seconds, the same distances _ap_, _bq_, _cr_, &c., as are stated above for a dropped body.] all this was experimentally established by galileo. a body takes half a second to drop feet; and a quarter of a second to drop foot. the easiest way of estimating a quarter of a second with some accuracy is to drop a bullet one foot. a bullet thrown or shot in any direction falls just as much as if merely dropped; but instead of falling from the starting-point it drops vertically from the line of fire. (see fig. ). the rate of fall depends on the intensity of gravity; if it could be doubled, a body would fall twice as far in the same time; but to make it fall a given distance in half the time the intensity of gravity would have to be quadrupled. at a place where the intensity of gravity is / of what it is here, a body would fall as far in a minute as it now falls in a second. such a place occurs at about the distance of the moon (_cf._ page ). the fact that the height fallen through is proportional to the square of the time proves that the attraction of the earth or the intensity of gravity is sensibly constant throughout ordinary small ranges. over great distances of fall, gravity cannot be considered constant; so for things falling through great spaces the galilean law of the square of the time does not hold. the fact that things near the earth fall feet in the first second proves that the intensity of ordinary terrestrial gravity is british units of force per pound of matter. the fact that all bodies fall at the same rate (when the resistance of the air is eliminated), proves that weight is proportional to mass; or more explicitly, that the gravitative attraction of the earth on matter near its surface depends on the amount of that matter, as estimated by its inertia, and on nothing else. lecture iv galileo and the invention of the telescope contemporary with the life of kepler, but overlapping it at both ends, comes the great and eventful life of galileo galilei,[ ] a man whose influence on the development of human thought has been greater than that of any man whom we have yet considered, and upon whom, therefore, it is necessary for us, in order to carry out the plan of these lectures, to bestow much time. a man of great and wide culture, a so-called universal genius, it is as an experimental philosopher that he takes the first rank. in this capacity he must be placed alongside of archimedes, and it is pretty certain that between the two there was no man of magnitude equal to either in experimental philosophy. it is perhaps too bold a speculation, but i venture to doubt whether in succeeding generations we find his equal in the domain of purely experimental science until we come to faraday. faraday was no doubt his superior, but i know of no other of whom the like can unhesitatingly be said. in mathematical and deductive science, of course, it is quite otherwise. kepler, for instance, and many men before and since, have far excelled galileo in mathematical skill and power, though at the same time his achievements in this department are by no means to be despised. born at pisa three centuries ago, on the very day that michael angelo lay dying in rome, he inherited from his father a noble name, cultivated tastes, a keen love of truth, and an impoverished patrimony. vincenzo de galilei, a descendant of the important bonajuti family, was himself a mathematician and a musician, and in a book of his still extant he declares himself in favour of free and open inquiry into scientific matters, unrestrained by the weight of authority and tradition. in all probability the son imbibed these precepts: certainly he acted on them. vincenzo, having himself experienced the unremunerative character of scientific work, had a horror of his son's taking to it, especially as in his boyhood he was always constructing ingenious mechanical toys, and exhibiting other marks of precocity. so the son was destined for business--to be, in fact, a cloth-dealer. but he was to receive a good education first, and was sent to an excellent convent school. here he made rapid progress, and soon excelled in all branches of classics and literature. he delighted in poetry, and in later years wrote several essays on dante, tasso, and ariosto, besides composing some tolerable poems himself. he played skilfully on several musical instruments, especially on the lute, of which indeed he became a master, and on which he solaced himself when quite an old man. besides this he seems to have had some skill as an artist, which was useful afterwards in illustrating his discoveries, and to have had a fine sensibility as an art critic, for we find several eminent painters of that day acknowledging the value of the opinion of the young galileo. perceiving all this display of ability, the father wisely came to the conclusion that the selling of woollen stuffs would hardly satisfy his aspirations for long, and that it was worth a sacrifice to send him to the university. so to the university of his native town he went, with the avowed object of studying medicine, that career seeming the most likely to be profitable. old vincenzo's horror of mathematics or science as a means of obtaining a livelihood is justified by the fact that while the university professor of medicine received , scudi a year, the professor of mathematics had only , that is £ a year, or - / _d._ a day. so the son had been kept properly ignorant of such poverty-stricken subjects, and to study medicine he went. but his natural bent showed itself even here. for praying one day in the cathedral, like a good catholic as he was all his life, his attention was arrested by the great lamp which, after lighting it, the verger had left swinging to and fro. galileo proceeded to time its swings by the only watch he possessed--viz., his own pulse. he noticed that the time of swing remained as near as he could tell the same, notwithstanding the fact that the swings were getting smaller and smaller. by subsequent experiment he verified the law, and the isochronism of the pendulum was discovered. an immensely important practical discovery this, for upon it all modern clocks are based; and huyghens soon applied it to the astronomical clock, which up to that time had been a crude and quite untrustworthy instrument. the best clock which tycho brahé could get for his observatory was inferior to one that may now be purchased for a few shillings; and this change is owing to the discovery of the pendulum by galileo. not that he applied it to clocks; he was not thinking of astronomy, he was thinking of medicine, and wanted to count people's pulses. the pendulum served; and "pulsilogies," as they were called, were thus introduced to and used by medical practitioners. the tuscan court came to pisa for the summer months, for it was then a seaside place, and among the suite was ostillio ricci, a distinguished mathematician and old friend of the galileo family. the youth visited him, and one day, it is said, heard a lesson in euclid being given by ricci to the pages while he stood outside the door entranced. anyhow he implored ricci to help him into some knowledge of mathematics, and the old man willingly consented. so he mastered euclid and passed on to archimedes, for whom he acquired a great veneration. his father soon heard of this obnoxious proclivity, and did what he could to divert him back to medicine again. but it was no use. underneath his galen and hippocrates were secreted copies of euclid and archimedes, to be studied at every available opportunity. old vincenzo perceived the bent of genius to be too strong for him, and at last gave way. [illustration: fig. .--two forms of pulsilogy. the string is wound up till the swinging weight keeps time with the pulse, and the position of a bead or of an index connected with the string is then read on a scale or dial.] with prodigious rapidity the released philosopher now assimilated the elements of mathematics and physics, and at twenty-six we find him appointed for three years to the university chair of mathematics, and enjoying the paternally dreaded stipend of - / _d._ a day. now it was that he pondered over the laws of falling bodies. he verified, by experiment, the fact that the velocity acquired by falling down any slope of given height was independent of the angle of slope. also, that the height fallen through was proportional to the square of the time. another thing he found experimentally was that all bodies, heavy and light, fell at the same rate, striking the ground at the same time.[ ] now this was clean contrary to what he had been taught. the physics of those days were a simple reproduction of statements in old books. aristotle had asserted certain things to be true, and these were universally believed. no one thought of trying the thing to see if it really were so. the idea of making an experiment would have savoured of impiety, because it seemed to tend towards scepticism, and cast a doubt on a reverend authority. young galileo, with all the energy and imprudence of youth (what a blessing that youth has a little imprudence and disregard of consequences in pursuing a high ideal!), as soon as he perceived that his instructors were wrong on the subject of falling bodies, instantly informed them of the fact. whether he expected them to be pleased or not is a question. anyhow, they were not pleased, but were much annoyed by his impertinent arrogance. it is, perhaps, difficult for us now to appreciate precisely their position. these doctrines of antiquity, which had come down hoary with age, and the discovery of which had reawakened learning and quickened intellectual life, were accepted less as a science or a philosophy, than as a religion. had they regarded aristotle as a verbally inspired writer, they could not have received his statements with more unhesitating conviction. in any dispute as to a question of fact, such as the one before us concerning the laws of falling bodies, their method was not to make an experiment, but to turn over the pages of aristotle; and he who could quote chapter and verse of this great writer was held to settle the question and raise it above the reach of controversy. it is very necessary for us to realize this state of things clearly, because otherwise the attitude of the learned of those days towards every new discovery seems stupid and almost insane. they had a crystallized system of truth, perfect, symmetrical--it wanted no novelty, no additions; every addition or growth was an imperfection, an excrescence, a deformity. progress was unnecessary and undesired. the church had a rigid system of dogma, which must be accepted in its entirety on pain of being treated as a heretic. philosophers had a cast-iron system of truth to match--a system founded upon aristotle--and so interwoven with the great theological dogmas that to question one was almost equivalent to casting doubt upon the other. in such an atmosphere true science was impossible. the life-blood of science is growth, expansion, freedom, development. before it could appear it must throw off these old shackles of centuries. it must burst its old skin, and emerge, worn with the struggle, weakly and unprotected, but free and able to grow and to expand. the conflict was inevitable, and it was severe. is it over yet? i fear not quite, though so nearly as to disturb science hardly at all. then it was different; it was terrible. honour to the men who bore the first shock of the battle! now aristotle had said that bodies fell at rates depending on their weight. a lb. weight would fall five times as quick as a lb. weight; a lb. weight fifty times as quick, and so on. why he said so nobody knows. he cannot have tried. he was not above trying experiments, like his smaller disciples; but probably it never occurred to him to doubt the fact. it seems so natural that a heavy body should fall quicker than a light one; and perhaps he thought of a stone and a feather, and was satisfied. galileo, however, asserted that the weight did not matter a bit, that everything fell at the same rate (even a stone and a feather, but for the resistance of the air), and would reach the ground in the same time. and he was not content to be pooh-poohed and snubbed. he knew he was right, and he was determined to make every one see the facts as he saw them. so one morning, before the assembled university, he ascended the famous leaning tower, taking with him a lb. shot and a lb. shot. he balanced them on the edge of the tower, and let them drop together. together they fell, and together they struck the ground. the simultaneous clang of those two weights sounded the death-knell of the old system of philosophy, and heralded the birth of the new. but was the change sudden? were his opponents convinced? not a jot. though they had seen with their eyes, and heard with their ears, the full light of heaven shining upon them, they went back muttering and discontented to their musty old volumes and their garrets, there to invent occult reasons for denying the validity of the observation, and for referring it to some unknown disturbing cause. they saw that if they gave way on this one point they would be letting go their anchorage, and henceforward would be liable to drift along with the tide, not knowing whither. they dared not do this. no; they _must_ cling to the old traditions; they could not cast away their rotting ropes and sail out on to the free ocean of god's truth in a spirit of fearless faith. [illustration: fig. .--tower of pisa.] yet they had received a shock: as by a breath of fresh salt breeze and a dash of spray in their faces, they had been awakened out of their comfortable lethargy. they felt the approach of a new era. yes, it was a shock; and they hated the young galileo for giving it them--hated him with the sullen hatred of men who fight for a lost and dying cause. we need scarcely blame these men; at least we need not blame them overmuch. to say that they acted as they did is to say that they were human, were narrow-minded, and were the apostles of a lost cause. but _they_ could not know this; _they_ had no experience of the past to guide them; the conditions under which they found themselves were novel, and had to be met for the first time. conduct which was excusable then would be unpardonable now, in the light of all this experience to guide us. are there any now who practically repeat their error, and resist new truth? who cling to any old anchorage of dogma, and refuse to rise with the tide of advancing knowledge? there may be some even now. well, the unpopularity of galileo smouldered for a time, until, by another noble imprudence, he managed to offend a semi-royal personage, giovanni de medici, by giving his real opinion, when consulted, about a machine which de medici had invented for cleaning out the harbour of leghorn. he said it was as useless as it in fact turned out to be. through the influence of the mortified inventor he lost favour at court; and his enemies took advantage of the fact to render his chair untenable. he resigned before his three years were up, and retired to florence. his father at this time died, and the family were left in narrow circumstances. he had a brother and three sisters to provide for. he was offered a professorship at padua for six years by the senate of venice, and willingly accepted it. now began a very successful career. his introductory address was marked by brilliant eloquence, and his lectures soon acquired fame. he wrote for his pupils on the laws of motion, on fortifications, on sundials, on mechanics, and on the celestial globe: some of these papers are now lost, others have been printed during the present century. kepler sent him a copy of his new book, _mysterium cosmographicum_, and galileo in thanking him for it writes him the following letter:--[ ] "i count myself happy, in the search after truth, to have so great an ally as yourself, and one who is so great a friend of the truth itself. it is really pitiful that there are so few who seek truth, and who do not pursue a perverse method of philosophising. but this is not the place to mourn over the miseries of our times, but to congratulate you on your splendid discoveries in confirmation of truth. i shall read your book to the end, sure of finding much that is excellent in it. i shall do so with the more pleasure, because _i have been for many years an adherent of the copernican system_, and it explains to me the causes of many of the appearances of nature which are quite unintelligible on the commonly accepted hypothesis. _i have collected many arguments for the purpose of refuting the latter_; but i do not venture to bring them to the light of publicity, for fear of sharing the fate of our master, copernicus, who, although he has earned immortal fame with some, yet with very many (so great is the number of fools) has become an object of ridicule and scorn. i should certainly venture to publish my speculations if there were more people like you. but this not being the case, i refrain from such an undertaking." kepler urged him to publish his arguments in favour of the copernican theory, but he hesitated for the present, knowing that his declaration would be received with ridicule and opposition, and thinking it wiser to get rather more firmly seated in his chair before encountering the storm of controversy. the six years passed away, and the venetian senate, anxious not to lose so bright an ornament, renewed his appointment for another six years at a largely increased salary. soon after this appeared a new star, the stella nova of , not the one tycho had seen--that was in --but the same that kepler was so much interested in. galileo gave a course of three lectures upon it to a great audience. at the first the theatre was over-crowded, so he had to adjourn to a hall holding persons. at the next he had to lecture in the open air. he took occasion to rebuke his hearers for thronging to hear about an ephemeral novelty, while for the much more wonderful and important truths about the permanent stars and facts of nature they had but deaf ears. but the main point he brought out concerning the new star was that it upset the received aristotelian doctrine of the immutability of the heavens. according to that doctrine the heavens were unchangeable, perfect, subject neither to growth nor to decay. here was a body, not a meteor but a real distant star, which had not been visible and which would shortly fade away again, but which meanwhile was brighter than jupiter. the staff of petrified professorial wisdom were annoyed at the appearance of the star, still more at galileo's calling public attention to it; and controversy began at padua. however, he accepted it; and now boldly threw down the gauntlet in favour of the copernican theory, utterly repudiating the old ptolemaic system which up to that time he had taught in the schools according to established custom. the earth no longer the only world to which all else in the firmament were obsequious attendants, but a mere insignificant speck among the host of heaven! man no longer the centre and cynosure of creation, but, as it were, an insect crawling on the surface of this little speck! all this not set down in crabbed latin in dry folios for a few learned monks, as in copernicus's time, but promulgated and argued in rich italian, illustrated by analogy, by experiment, and with cultured wit; taught not to a few scholars here and there in musty libraries, but proclaimed in the vernacular to the whole populace with all the energy and enthusiasm of a recent convert and a master of language! had a bombshell been exploded among the fossilized professors it had been less disturbing. but there was worse in store for them. a dutch optician, hans lippershey by name, of middleburg, had in his shop a curious toy, rigged up, it is said, by an apprentice, and made out of a couple of spectacle lenses, whereby, if one looked through it, the weather-cock of a neighbouring church spire was seen nearer and upside down. the tale goes that the marquis spinola, happening to call at the shop, was struck with the toy and bought it. he showed it to prince maurice of nassau, who thought of using it for military reconnoitring. all this is trivial. what is important is that some faint and inaccurate echo of this news found its way to padua, and into the ears of galileo. the seed fell on good soil. all that night he sat up and pondered. he knew about lenses and magnifying glasses. he had read kepler's theory of the eye, and had himself lectured on optics. could he not hit on the device and make an instrument capable of bringing the heavenly bodies nearer? who knew what marvels he might not so perceive! by morning he had some schemes ready to try, and one of them was successful. singularly enough it was not the same plan as the dutch optician's, it was another mode of achieving the same end. he took an old small organ pipe, jammed a suitably chosen spectacle glass into either end, one convex the other concave, and behold, he had the half of a wretchedly bad opera glass capable of magnifying three times. it was better than the dutchman's, however; it did not invert. it is easy to understand the general principle of a telescope. a general knowledge of the common magnifying glass may be assumed. roger bacon knew about lenses; and the ancients often refer to them, though usually as burning glasses. the magnifying power of globes of water must have been noticed soon after the discovery of glass and the art of working it. a magnifying glass is most simply thought of as an additional lens to the eye. the eye has a lens by which ordinary vision is accomplished, an extra glass lens strengthens it and enables objects to be seen nearer and therefore apparently bigger. but to apply a magnifying glass to distant objects is impossible. in order to magnify distant objects, another function of lenses has also to be employed, viz., their power of forming real images, the power on which their use as burning-glasses depends: for the best focus is an image of the sun. although the object itself is inaccessible, the image of it is by no means so, and to the image a magnifier can be applied. this is exactly what is done in the telescope; the object glass or large lens forms an image, which is then looked at through a magnifying glass or eye-piece. of course the image is nothing like so big as the object. for astronomical objects it is almost infinitely less; still it is an exact representation at an accessible place, and no one expects a telescope to show distant bodies as big as they really are. all it does is to show them bigger than they could be seen without it. but if the objects are not distant, the same principle may still be applied, and two lenses may be used, one to form an image, the other to magnify it; only if the object can be put where we please, we can easily place it so that its image is already much bigger than the object even before magnification by the eye lens. this is the compound microscope, the invention of which soon followed the telescope. in fact the two instruments shade off into one another, so that the reading telescope or reading microscope of a laboratory (for reading thermometers, and small divisions generally) goes by either name at random. the arrangement so far described depicts things on the retina the unaccustomed way up. by using a concave glass instead of a convex, and placing it so as to prevent any image being formed, except on the retina direct, this inconvenience is avoided. [illustration: fig. .--view of the half-moon in small telescope. the darker regions, or plains, used to be called "seas."] such a thing as galileo made may now be bought at a toy-shop for i suppose half a crown, and yet what a potentiality lay in that "glazed optic tube," as milton called it. away he went with it to venice and showed it to the signoria, to their great astonishment. "many noblemen and senators," says galileo, "though of advanced age, mounted to the top of one of the highest towers to watch the ships, which were visible through my glass two hours before they were seen entering the harbour, for it makes a thing fifty miles off as near and clear as if it were only five." among the people too the instrument excited the greatest astonishment and interest, so that he was nearly mobbed. the senate hinted to him that a present of the instrument would not be unacceptable, so galileo took the hint and made another for them. [illustration: fig. .--portion of the lunar surface more highly magnified, showing the shadows of a mountain range, deep pits, and other details.] they immediately doubled his salary at padua, making it florins, and confirmed him in the enjoyment of it for life. he now eagerly began the construction of a larger and better instrument. grinding the lenses with his own hands with consummate skill, he succeeded in making a telescope magnifying thirty times. thus equipped he was ready to begin a survey of the heavens. [illustration: fig. .--another portion of the lunar surface, showing a so-called crater or vast lava pool and other evidences of ancient heat unmodified by water.] the first object he carefully examined was naturally the moon. he found there everything at first sight very like the earth, mountains and valleys, craters and plains, rocks, and apparently seas. you may imagine the hostility excited among the aristotelian philosophers, especially no doubt those he had left behind at pisa, on the ground of his spoiling the pure, smooth, crystalline, celestial face of the moon as they had thought it, and making it harsh and rugged and like so vile and ignoble a body as the earth. [illustration: fig. .--lunar landscape showing earth. the earth would be a stationary object in the moon's sky: its only apparent motion being a slow oscillation as of a pendulum (the result of the moon's libration).] he went further, however, into heterodoxy than this--he not only made the moon like the earth, but he made the earth shine like the moon. the visibility of "the old moon in the new moon's arms" he explained by earth-shine. leonardo had given the same explanation a century before. now one of the many stock arguments against copernican theory of the earth being a planet like the rest was that the earth was dull and dark and did not shine. galileo argued that it shone just as much as the moon does, and in fact rather more--especially if it be covered with clouds. one reason of the peculiar brilliancy of venus is that she is a very cloudy planet.[ ] seen from the moon the earth would look exactly as the moon does to us, only a little brighter and sixteen times as big (four times the diameter). [illustration: fig. .--galileo's method of estimating the height of lunar mountain. _ab'bc_ is the illuminated half of the moon. _sa_ is a solar ray just catching the peak of the mountain _m_. then by geometry, as _mn_ is to _ma_, so is _ma_ to _mb'_; whence the height of the mountain, _mn_, can be determined. the earth and spectator are supposed to be somewhere in the direction _ba_ produced, _i.e._ towards the top of the page.] galileo made a very good estimate of the height of lunar mountains, of which many are five miles high and some as much as seven. he did this simply by measuring from the half-moon's straight edge the distance at which their peaks caught the rising or setting sun. the above simple diagram shows that as this distance is to the diameter of the moon, so is the height of the sun-tipped mountain to the aforesaid distance. wherever galileo turned his telescope new stars appeared. the milky way, which had so puzzled the ancients, was found to be composed of stars. stars that appeared single to the eye were some of them found to be double; and at intervals were found hazy nebulous wisps, some of which seemed to be star clusters, while others seemed only a fleecy cloud. [illustration: fig. .--some clusters and nebulæ.] [illustration: fig. .--jupiter's satellites, showing the stages of their discovery.] now we come to his most brilliant, at least his most sensational, discovery. examining jupiter minutely on january , , he noticed three little stars near it, which he noted down as fixing its then position. on the following night jupiter had moved to the other side of the three stars. this was natural enough, but was it moving the right way? on examination it appeared not. was it possible the tables were wrong? the next evening was cloudy, and he had to curb his feverish impatience. on the th there were only two, and those on the other side. on the th two again, but one bigger than the other. on the th the three re-appeared, and on the th there were four. no more appeared. jupiter then had moons like the earth, four of them in fact, and they revolved round him in periods which were soon determined. the reason why they were not all visible at first, and why their visibility so rapidly changes, is because they revolve round him almost in the plane of our vision, so that sometimes they are in front and sometimes behind him, while again at other times they plunge into his shadow and are thus eclipsed from the light of the sun which enables us to see them. a large modern telescope will show the moons when in front of jupiter, but small telescopes will only show them when clear of the disk and shadow. often all four can be thus seen, but three or two is a very common amount of visibility. quite a small telescope, such as a ship's telescope, if held steadily, suffices to show the satellites of jupiter, and very interesting objects they are. they are of habitable size, and may be important worlds for all we know to the contrary. the news of the discovery soon spread and excited the greatest interest and astonishment. many of course refused to believe it. some there were who having been shown them refused to believe their eyes, and asserted that although the telescope acted well enough for terrestrial objects, it was altogether false and illusory when applied to the heavens. others took the safer ground of refusing to look through the glass. one of these who would not look at the satellites happened to die soon afterwards. "i hope," says galileo, "that he saw them on his way to heaven." the way in which kepler received the news is characteristic, though by adding four to the supposed number of planets it might have seemed to upset his notions about the five regular solids. he says,[ ] "i was sitting idle at home thinking of you, most excellent galileo, and your letters, when the news was brought me of the discovery of four planets by the help of the double eye-glass. wachenfels stopped his carriage at my door to tell me, when such a fit of wonder seized me at a report which seemed so very absurd, and i was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of us both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or i of listening.... "on our separating, i immediately fell to thinking how there could be any addition to the number of planets without overturning my _mysterium cosmographicon_, published thirteen years ago, according to which euclid's five regular solids do not allow more than six planets round the sun. "but i am so far from disbelieving the existence of the four circumjovial planets that i long for a telescope to anticipate you if possible in discovering two round mars (as the proportion seems to me to require) six or eight round saturn, and one each round mercury and venus." [illustration: fig. .--eclipses of jupiter's satellites. the diagram shows the first (_i.e._ the nearest) moon in jupiter's shadow, the second as passing between earth and jupiter, and appearing to transit his disk, the third as on the verge of entering his shadow, and the fourth quite plainly and separately visible.] as an illustration of the opposite school, i will take the following extract from francesco sizzi, a florentine astronomer, who argues against the discovery thus:-- "there are seven windows in the head, two nostrils, two eyes, two ears, and a mouth; so in the heavens there are two favourable stars, two unpropitious, two luminaries, and mercury alone undecided and indifferent. from which and many other similar phenomena of nature, such as the seven metals, &c., which it were tedious to enumerate, we gather that the number of planets is necessarily seven. "moreover, the satellites are invisible to the naked eye, and therefore can have no influence on the earth, and therefore would be useless, and therefore do not exist. "besides, the jews and other ancient nations as well as modern europeans have adopted the division of the week into seven days, and have named them from the seven planets: now if we increase the number of the planets this whole system falls to the ground." to these arguments galileo replied that whatever their force might be as a reason for believing beforehand that no more than seven planets would be discovered, they hardly seemed of sufficient weight to destroy the new ones when actually seen. writing to kepler at this time, galileo ejaculates: "oh, my dear kepler, how i wish that we could have one hearty laugh together! here, at padua, is the principal professor of philosophy whom i have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do. why are you not here? what shouts of laughter we should have at this glorious folly! and to hear the professor of philosophy at pisa labouring before the grand duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky." a young german _protégé_ of kepler, martin horkey, was travelling in italy, and meeting galileo at bologna was favoured with a view through his telescope. but supposing that kepler must necessarily be jealous of such great discoveries, and thinking to please him, he writes, "i cannot tell what to think about these observations. they are stupendous, they are wonderful, but whether they are true or false i cannot tell." he concludes, "i will never concede his four new planets to that italian from padua though i die for it." so he published a pamphlet asserting that reflected rays and optical illusions were the sole cause of the appearance, and that the only use of the imaginary planets was to gratify galileo's thirst for gold and notoriety. when after this performance he paid a visit to his old instructor kepler, he got a reception which astonished him. however, he pleaded so hard to be forgiven that kepler restored him to partial favour, on this condition, that he was to look again at the satellites, and this time to see them and own that they were there. by degrees the enemies of galileo were compelled to confess to the truth of the discovery, and the next step was to outdo him. scheiner counted five, rheiter nine, and others went as high as twelve. some of these were imaginary, some were fixed stars, and four satellites only are known to this day.[ ] here, close to the summit of his greatness, we must leave him for a time. a few steps more and he will be on the brow of the hill; a short piece of table-land, and then the descent begins. lecture v galileo and the inquisition one sinister event occurred while galileo was at padua, some time before the era we have now arrived at, before the invention of the telescope--two years indeed after he had first gone to padua; an event not directly concerning galileo, but which i must mention because it must have shadowed his life both at the time and long afterwards. it was the execution of giordano bruno for heresy. this eminent philosopher had travelled largely, had lived some time in england, had acquired new and heterodox views on a variety of subjects, and did not hesitate to propound them even after he had returned to italy. the copernican doctrine of the motion of the earth was one of his obnoxious heresies. being persecuted to some extent by the church, bruno took refuge in venice--a free republic almost independent of the papacy--where he felt himself safe. galileo was at padua hard by: the university of padua was under the government of the senate of venice: the two men must in all probability have met. well, the inquisition at rome sent messengers to venice with a demand for the extradition of bruno--they wanted him at rome to try him for heresy. in a moment of miserable weakness the venetian republic gave him up, and bruno was taken to rome. there he was tried, and cast into the dungeons for six years, and because he entirely refused to recant, was at length delivered over to the secular arm and burned at the stake on th february, anno domini . this event could not but have cast a gloom over the mind of lovers and expounders of truth, and the lesson probably sank deep into galileo's soul. in dealing with these historic events will you allow me to repudiate once for all the slightest sectarian bias or meaning. i have nothing to do with catholic or protestant as such. i have nothing to do with the church of rome as such. i am dealing with the history of science. but historically at one period science and the church came into conflict. it was not specially one church rather than another--it was the church in general, the only one that then existed in those countries. historically, i say, they came into conflict, and historically the church was the conqueror. it got its way; and science, in the persons of bruno, galileo, and several others, was vanquished. such being the facts, there is no help but to mention them in dealing with the history of science. doubtless _now_ the church regards it as an unhappy victory, and gladly would ignore this painful struggle. this, however, is impossible. with their creed the churchmen of that day could act in no other way. they were bound to prosecute heresy, and they were bound to conquer in the struggle or be themselves shattered. but let me insist on the fact that no one accuses the ecclesiastical courts of crime or evil motives. they attacked heresy after their manner, as the civil courts attacked witchcraft after _their_ manner. both erred grievously, but both acted with the best intentions. we must remember, moreover, that his doctrines were scientifically heterodox, and the university professors of that day were probably quite as ready to condemn them as the church was. to realise the position we must think of some subjects which _to-day_ are scientifically heterodox, and of the customary attitude adopted towards them by persons of widely differing creeds. if it be contended now, as it is, that the ecclesiastics treated galileo well, i admit it freely: they treated him as well as they possibly could. they overcame him, and he recanted; but if he had not recanted, if he had persisted in his heresy, they would--well, they would still have treated his soul well, but they would have set fire to his body. their mistake consisted not in cruelty, but in supposing themselves the arbiters of eternal truth; and by no amount of slurring and glossing over facts can they evade the responsibility assumed by them on account of this mistaken attitude. i am not here attacking the dogma of papal infallibility: it is historically, i believe, quite unaffected by the controversy respecting the motion of the earth, no papal edict _ex cathedrâ_ having been promulgated on the subject. we left galileo standing at his telescope and beginning his survey of the heavens. we followed him indeed through a few of his first great discoveries--the discovery of the mountains and other variety of surface in the moon, of the nebulæ and a multitude of faint stars, and lastly of the four satellites of jupiter. this latter discovery made an immense sensation, and contributed its share to his removal from padua, which quickly followed it, as i shall shortly narrate; but first i think it will be best to continue our survey of his astronomical discoveries without regard to the place whence they were made. before the end of the year galileo had made another discovery--this time on saturn. but to guard against the host of plagiarists and impostors, he published it in the form of an anagram, which, at the request of the emperor rudolph (a request probably inspired by kepler), he interpreted; it ran thus: the furthest planet is triple. very soon after he found that venus was changing from a full moon to a half moon appearance. he announced this also by an anagram, and waited till it should become a crescent, which it did. this was a dreadful blow to the anti-copernicans, for it removed the last lingering difficulty to the reception of the copernican doctrine. [illustration: fig. .--old drawings of saturn by different observers, with the imperfect instruments of that day. the first is galileo's idea of what he saw.] copernicus had predicted, indeed, a hundred years before, that, if ever our powers of sight were sufficiently enhanced, venus and mercury would be seen to have phases like the moon. and now galileo with his telescope verifies the prediction to the letter. here was a triumph for the grand old monk, and a bitter morsel for his opponents. castelli writes: "this must now convince the most obstinate." but galileo, with more experience, replies:--"you almost make me laugh by saying that these clear observations are sufficient to convince the most obstinate; it seems you have yet to learn that long ago the observations were enough to convince those who are capable of reasoning, and those who wish to learn the truth; but that to convince the obstinate, and those who care for nothing beyond the vain applause of the senseless vulgar, not even the testimony of the stars would suffice, were they to descend on earth to speak for themselves. let us, then, endeavour to procure some knowledge for ourselves, and rest contented with this sole satisfaction; but of advancing in popular opinion, or of gaining the assent of the book-philosophers, let us abandon both the hope and the desire." [illustration: fig. .--phases of venus. showing also its apparent variations in size by reason of its varying distance from the earth. when fully illuminated it is necessarily most distant. it looks brightest to us when a broad crescent.] what a year's work it had been! in twelve months observational astronomy had made such a bound as it has never made before or since. why did not others make any of these observations? because no one could make telescopes like galileo. he gathered pupils round him however, and taught them how to work the lenses, so that gradually these instruments penetrated europe, and astronomers everywhere verified his splendid discoveries. but still he worked on, and by march in the very next year, he saw something still more hateful to the aristotelian philosophers, viz. spots on the sun. [illustration: fig. .] if anything was pure and perfect it was the sun, they said. was this impostor going to blacken its face too? well, there they were. they slowly formed and changed, and by moving all together showed him that the sun rotated about once a month. before taking leave of galileo's astronomical researches, i must mention an observation made at the end of , that the apparent triplicity of saturn (fig. ) had vanished. [illustration: fig. .--a portion of the sun's disk as seen in a powerful modern telescope.] "looking on saturn within these few days, i found it solitary, without the assistance of its accustomed stars, and in short perfectly round and defined, like jupiter, and such it still remains. now what can be said of so strange a metamorphosis? are perhaps the two smaller stars consumed like spots on the sun? have they suddenly vanished and fled? or has saturn devoured his own children? or was the appearance indeed fraud and illusion, with which the glasses have so long time mocked me and so many others who have so often observed with me? now perhaps the time is come to revive the withering hopes of those, who, guided by more profound contemplations, have fathomed all the fallacies of the new observations and recognized their impossibility! i cannot resolve what to say in a chance so strange, so new, so unexpected. the shortness of time, the unexampled occurrence, the weakness of my intellect, the terror of being mistaken, have greatly confounded me." however, he plucked up courage, and conjectured that the two attendants would reappear, by revolving round the planet. [illustration: fig. .--saturn and his rings, as seen under the most favourable circumstances.] the real reason of their disappearance is well known to us now. the plane of saturn's rings oscillates slowly about our line of sight, and so we sometimes see them edgeways and sometimes with a moderate amount of obliquity. the rings are so thin that, when turned precisely edgeways, they become invisible. the two imaginary attendants were the most conspicuous portions of the ring, subsequently called _ansæ_. i have thought it better not to interrupt this catalogue of brilliant discoveries by any biographical details; but we must now retrace our steps to the years and , the era of the invention of the telescope. by this time galileo had been eighteen years at padua, and like many another man in like case, was getting rather tired of continual lecturing. moreover, he felt so full of ideas that he longed to have a better opportunity of following them up, and more time for thinking them out. now in the holidays he had been accustomed to return to his family home at pisa, and there to come a good deal into contact with the grand-ducal house of tuscany. young cosmo di medici became in fact his pupil, and arrived at man's estate with the highest opinion of the philosopher. this young man had now come to the throne as cosmo ii., and to him galileo wrote saying how much he should like more time and leisure, how full he was of discoveries if he only had the chance of a reasonable income without the necessity of consuming so large a portion of his time in elementary teaching, and practically asking to be removed to some position in the court. nothing was done for a time, but negotiations proceeded, and soon after the discovery of jupiter's satellites cosmo wrote making a generous offer, which galileo gladly and enthusiastically accepted, and at once left padua for florence. all his subsequent discoveries date from florence. thus closed his brilliant and happy career as a professor at the university of padua. he had been treated well: his pay had become larger than that of any professor of mathematics up to that time; and, as you know, immediately after his invention of the telescope the venetian senate, in a fit of enthusiasm, had doubled it and secured it to him for life wherever he was. to throw up his chair and leave the place the very next year scarcely seems a strictly honourable procedure. it was legal enough no doubt, and it is easy for small men to criticize a great one, but nevertheless i think we must admit that it is a step such as a man with a keen sense of honour would hardly have taken. one quite feels and sympathizes with the temptation. not emolument, but leisure; freedom from harassing engagements and constant teaching, and liberty to prosecute his studies day and night without interference: this was the golden prospect before him. he yielded, but one cannot help wishing he had not. as it turned out it was a false step--the first false step of his public career. when made it was irretrievable, and it led to great misery. at first it seemed brilliant enough. the great philosopher of the tuscan court was courted and flattered by princes and nobles, he enjoyed a world-wide reputation, lived as luxuriously as he cared for, had his time all to himself, and lectured but very seldom, on great occasions or to a few crowned heads. his position was in fact analogous to that of tycho brahé in his island of huen. misfortune overtook both. in tycho's case it arose mainly from the death of his patron. in galileo's it was due to a more insidious cause, to understand which cause aright we must remember the political divisions of italy at that date. tuscany was a papal state, and thought there was by no means free. venice was a free republic, and was even hostile to the papacy. in the pope had placed it under an interdict. in reply it had ejected every jesuit. out of this atmosphere of comparative enlightenment and freedom into that hotbed of mediævalism and superstition went galileo with his eyes open. keen was the regret of his paduan and venetian friends; bitter were their remonstrances and exhortations. but he was determined to go, and, not without turning some of his old friends into enemies, he went. seldom has such a man made so great a mistake: never, i suppose, has one been so cruelly punished for it. [illustration: fig. .--map of italy.] we must remember, however, that galileo, though by no means a saint, was yet a really religious man, a devout catholic and thorough adherent of the church, so that he would have no dislike to place himself under her sway. moreover, he had been born a tuscan, his family had lived at florence or pisa, and it felt like going home. his theological attitude is worthy of notice, for he was not in the least a sceptic. he quite acquiesces in the authority of the bible, especially in all matters concerning faith and conduct; as to its statements in scientific matters, he argues that we are so liable to misinterpret their meaning that it is really easier to examine nature for truth in scientific matters, and that when direct observation and scripture seem to clash, it is because of our fallacious interpretation of one or both of them. he is, in fact, what one now calls a "reconciler." it is curious to find such a man prosecuted for heresy, when to-day his opinions are those of the orthodox among the orthodox. but so it ever is, and the heresy of one generation becomes the commonplace of the next. he accepts joshua's miracle, for instance, not as a striking poem, but as a literal fact; and he points out how much more simply it could be done on the copernican system by stopping the earth's rotation for a short time, than by stopping the sun and moon and all the host of heaven as on the old ptolemaic system, or again by stopping only the sun and not any of the other bodies, and so throwing astronomy all wrong. this reads to us like satire, but no doubt it was his genuine opinion. these scriptural reconciliations of his, however, angered the religious authorities still more. they said it was bad enough for this heretic to try and upset old _scientific_ beliefs, and to spoil the face of _nature_ with his infidel discoveries, but at least he might leave the bible alone; and they addressed an indignant remonstrance to rome, to protect it from the hands of ignorant laymen. thus, wherever he turned he encountered hostility. of course he had many friends--some of them powerful like cosmo, all of them faithful and sincere. but against the power of rome what could they do? cosmo dared no more than remonstrate, and ultimately his successor had to refrain from even this, so enchained and bound was the spirit of the rulers of those days; and so when his day of tribulation came he stood alone and helpless in the midst of his enemies. you may wonder, perhaps, why this man should excite so much more hostility than many another man who was suffered to believe and teach much the same doctrines unmolested. but no other man had made such brilliant and exciting discoveries. no man stood so prominently forward in the eyes of all christendom as the champion of the new doctrines. no other man stated them so clearly and forcibly, nor drove them home with such brilliant and telling illustrations. and again, there was the memory of his early conflict with the aristotelians at pisa, of his scornful and successful refutation of their absurdities. all this made him specially obnoxious to the aristotelian jesuits in their double capacity both of priests and of philosophers, and they singled him out for relentless official persecution. not yet, however, is he much troubled by them. the chief men at rome have not yet moved. messages, however, keep going up from tuscany to rome respecting the teachings of this man, and of the harm he is doing by his pertinacious preaching of the copernican doctrine that the earth moves. at length, in , pope paul v. wrote requesting him to come to rome to explain his views. he went, was well received, made a special friend of cardinal barberino--an accomplished man in high position, who became in fact the next pope. galileo showed cardinals and others his telescope, and to as many as would look through it he showed jupiter's satellites and his other discoveries. he had a most successful visit. he talked, he harangued, he held forth in the midst of fifteen or twenty disputants at once, confounding his opponents and putting them to shame. his method was to let the opposite arguments be stated as fully and completely as possible, himself aiding, and often adducing the most forcible and plausible arguments against his own views; and then, all having been well stated, he would proceed to utterly undermine and demolish the whole fabric, and bring out the truth in such a way as to convince all honest minds. it was this habit that made him such a formidable antagonist. he never shrank from meeting an opposing argument, never sought to ignore it, or cloak it in a cloud of words. every hostile argument he seemed to delight in, as a foe to be crushed, and the better and stronger they sounded the more he liked them. he knew many of them well, he invented a number more, and had he chosen could have out-argued the stoutest aristotelian on his own grounds. thus did he lead his adversaries on, almost like socrates, only to ultimately overwhelm them in a more hopeless rout. all this in rome too, in the heart of the catholic world. had he been worldly-wise, he would certainly have kept silent and unobtrusive till he had leave to go away again. but he felt like an apostle of the new doctrines, whose mission it was to proclaim them even in this centre of the world and of the church. well, he had an audience with the pope--a chat an hour long--and the two parted good friends, mutually pleased with each other. he writes that he is all right now, and might return home when he liked. but the question began to be agitated whether the whole system of copernicus ought not to be condemned as impious and heretical. this view was persistently urged upon the pope and college of cardinals, and it was soon to be decided upon. had galileo been unfaithful to the church he could have left them to stultify themselves in any way they thought proper, and himself have gone; but he felt supremely interested in the result, and he stayed. he writes:-- "so far as concerns the clearing of my own character, i might return home immediately; but although this new question regards me no more than all those who for the last eighty years have supported those opinions both in public and private, yet, as perhaps i may be of some assistance in that part of the discussion which depends on the knowledge of truths ascertained by means of the sciences which i profess, i, as a zealous and catholic christian, neither can nor ought to withhold that assistance which my knowledge affords, and this business keeps me sufficiently employed." it is possible that his stay was the worst thing for the cause he had at heart. anyhow, the result was that the system was condemned, and both the book of copernicus and the epitome of it by kepler were placed on the forbidden list,[ ] and galileo himself was formally ordered never to teach or to believe the motion of the earth. he quitted rome in disgust, which before long broke out in satire. the only way in which he could safely speak of these views now was as if they were hypothetical and uncertain, and so we find him writing to the archduke leopold, with a presentation copy of his book on the tides, the following:-- "this theory occurred to me when in rome whilst the theologians were debating on the prohibition of copernicus's book, and of the opinion maintained in it of the motion of the earth, which i at that time believed: until it pleased those gentlemen to suspend the book, and declare the opinion false and repugnant to the holy scriptures. now, as i know how well it becomes me to obey and believe the decisions of my superiors, which proceed out of more knowledge than the weakness of my intellect can attain to, this theory which i send you, which is founded on the motion of the earth, i now look upon as a fiction and a dream, and beg your highness to receive it as such. but as poets often learn to prize the creations of their fancy, so in like manner do i set some value on this absurdity of mine. it is true that when i sketched this little work i did hope that copernicus would not, after eighty years, be convicted of error; and i had intended to develop and amplify it further, but a voice from heaven suddenly awakened me, and at once annihilated all my confused and entangled fancies." this sarcasm, if it had been in print, would probably have been dangerous. it was safe in a private letter, but it shows us his real feelings. however, he was left comparatively quiet for a time. he was getting an old man now, and passed the time studiously enough, partly at his house in florence, partly at his villa in arcetri, a mile or so out of the town. here was a convent, and in it his two daughters were nuns. one of them, who passed under the name of sister maria celeste, seems to have been a woman of considerable capacity--certainly she was of a most affectionate disposition--and loved and honoured her father in the most dutiful way. this was a quiet period of his life, spoiled only by occasional fits of illness and severe rheumatic pains, to which the old man was always liable. many little circumstances are known of this peaceful time. for instance, the convent clock won't go, and galileo mends it for them. he is always doing little things for them, and sending presents to the lady superior and his two daughters. he was occupied now with problems in hydrostatics, and on other matters unconnected with astronomy: a large piece of work which i must pass over. most interesting and acute it is, however. in , when the old pope died, there was elected to the papal throne, as urban viii., cardinal barberino, a man of very considerable enlightenment, and a personal friend of galileo's, so that both he and his daughters rejoice greatly, and hope that things will come all right, and the forbidding edict be withdrawn. the year after this election he manages to make another journey to rome to compliment his friend on his elevation to the pontifical chair. he had many talks with urban, and made himself very agreeable. urban wrote to the grand duke ferdinand, son of cosmo:-- "for we find in him not only literary distinction but also love of piety, and he is strong in those qualities by which pontifical good will is easily obtainable. and now, when he has been brought to this city to congratulate us on our elevation, we have very lovingly embraced him; nor can we suffer him to return to the country whither your liberality recalls him without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honourable testimonial of virtue and piety. and we further signify that every benefit which you shall confer upon him, imitating or even surpassing your father's liberality, will conduce to our gratification." encouraged, doubtless, by these marks of approbation, and reposing too much confidence in the individual good will of the pope, without heeding the crowd of half-declared enemies who were seeking to undermine his reputation, he set about, after his return to florence, his greatest literary and most popular work, _dialogues on the ptolemaic and copernican systems_. this purports to be a series of four conversations between three characters: salviati, a copernican philosopher; sagredo, a wit and scholar, not specially learned, but keen and critical, and who lightens the talk with chaff; simplicio, an aristotelian philosopher, who propounds the stock absurdities which served instead of arguments to the majority of men. the conversations are something between plato's _dialogues_ and sir arthur helps's _friends in council_. the whole is conducted with great good temper and fairness; and, discreetly enough, no definite conclusion is arrived at, the whole being left in abeyance as if for a fifth and decisive dialogue, which, however, was never written, and perhaps was only intended in case the reception was favourable. the preface also sets forth that the object of the writer is to show that the roman edict forbidding the copernican doctrine was not issued in ignorance of the facts of the case, as had been maliciously reported, and that he wishes to show how well and clearly it was all known beforehand. so he says the dialogue on the copernican side takes up the question purely as a mathematical hypothesis or speculative figment, and gives it every artificial advantage of which the theory is capable. this piece of caution was insufficient to blind the eyes of the cardinals; for in it the arguments in favour of the earth's motion are so cogent and unanswerable, and are so popularly stated, as to do more in a few years to undermine the old system than all that he had written and spoken before. he could not get it printed for two years after he had written it, and then only got consent through a piece of carelessness or laziness on the part of the ecclesiastical censor through whose hands the manuscript passed--for which he was afterwards dismissed. however, it did appear, and was eagerly read; the more, perhaps, as the church at once sought to suppress it. the aristotelians were furious, and represented to the pope that he himself was the character intended by simplicio, the philosopher whose opinions get alternately refuted and ridiculed by the other two, till he is reduced to an abject state of impotence. the idea that galileo had thus cast ridicule upon his friend and patron is no doubt a gratuitous and insulting libel: there is no telling whether or not urban believed it, but certainly his countenance changed to galileo henceforward, and whether overruled by his cardinals, or actuated by some other motive, his favour was completely withdrawn. the infirm old man was instantly summoned to rome. his friends pleaded his age--he was now seventy--his ill-health, the time of year, the state of the roads, the quarantine existing on account of the plague. it was all of no avail, to rome he must go, and on the th of february he arrived. [illustration: fig. .--portrait of galileo.] his daughter at arcetri was in despair; and anxiety and fastings and penances self-inflicted on his account, dangerously reduced her health. at rome he was not imprisoned, but he was told to keep indoors, and show himself as little as possible. he was allowed, however, to stay at the house of the tuscan ambassador instead of in gaol. by april he was removed to the chambers of the inquisition, and examined several times. here, however, the anxiety was too much, and his health began to give way seriously; so, before long, he was allowed to return to the ambassador's house; and, after application had been made, was allowed to drive in the public garden in a half-closed carriage. thus in every way the inquisition dealt with him as leniently as they could. he was now their prisoner, and they might have cast him into their dungeons, as many another had been cast. by whatever they were influenced--perhaps the pope's old friendship, perhaps his advanced age and infirmities--he was not so cruelly used. still, they had their rules; he _must_ be made to recant and abjure his heresy; and, if necessary, torture must be applied. this he knew well enough, and his daughter knew it, and her distress may be imagined. moreover, it is not as if they had really been heretics, as if they hated or despised the church of rome. on the contrary, they loved and honoured the church. they were sincere and devout worshippers, and only on a few scientific matters did galileo presume to differ from his ecclesiastical superiors: his disagreement with them occasioned him real sorrow; and his dearest hope was that they could be brought to his way of thinking and embrace the truth. every time he was sent for by the inquisition he was in danger of torture unless he recanted. all his friends urged him repeatedly to submit. they said resistance was hopeless and fatal. within the memory of men still young, giordano bruno had been burnt alive for a similar heresy. this had happened while galileo was at padua. venice was full of it. and since that, only eight years ago indeed, antonio de dominis, archbishop of salpetria, had been sentenced to the same fate: "to be handed over to the secular arm to be dealt with as mercifully as possible without the shedding of blood." so ran the hideous formula condemning a man to the stake. after his sentence, this unfortunate man died in the dungeons in which he had been incarcerated six years--died what is called a "natural" death; but the sentence was carried out, notwithstanding, on his lifeless body and his writings. his writings for which he had been willing to die! these were the tender mercies of the inquisition; and this was the kind of meaning lurking behind many of their well-sounding and merciful phrases. for instance, what they call "rigorous examination," we call "torture." let us, however, remember in our horror at this mode of compelling a prisoner to say anything they wished, that they were a legally constituted tribunal; that they acted with well established rules, and not in passion; and that torture was a recognized mode of extracting evidence, not only in ecclesiastical but in civil courts, at that date. all this, however, was but poor solace to the pitiable old philosopher, thus ruthlessly haled up and down, questioned and threatened, threatened and questioned, receiving agonizing letters from his daughter week by week, and trying to keep up a little spirit to reply as happily and hopefully as he could. this condition of things could not go on. from february to june the suspense lasted. on the th of june he was summoned again, and told he would be wanted all next day for a rigorous examination. early in the morning of the st he repaired thither, and the doors were shut. out of those chambers of horror he did not reappear till the th. what went on all those three days no one knows. he himself was bound to secrecy. no outsider was present. the records of the inquisition are jealously guarded. that he was technically tortured is certain; that he actually underwent the torment of the rack is doubtful. much learning has been expended upon the question, especially in germany. several eminent scholars have held the fact of actual torture to be indisputable (geometrically certain, one says), and they confirm it by the hernia from which he afterwards suffered, this being a well-known and frequent consequence. other equally learned commentators, however, deny that the last stage was reached. for there are five stages all laid down in the rules of the inquisition, and steadily adhered to in a rigorous examination, at each stage an opportunity being given for recantation, every utterance, groan, or sigh being strictly recorded. the recantation so given has to be confirmed a day or two later, under pain of a precisely similar ordeal. the five stages are:-- st. the official threat in the court. nd. the taking to the door of the torture chamber and renewing the official threat. rd. the taking inside and showing the instruments. th. undressing and binding upon the rack. th. _territio realis._ through how many of these ghastly acts galileo passed i do not know. i hope and believe not the last. there are those who lament that he did not hold out, and accept the crown of martyrdom thus offered to him. had he done so we know his fate--a few years' languishing in the dungeons, and then the flames. whatever he ought to have done, he did not hold out--he gave way. at one stage or another of the dread ordeal he said: "i am in your hands. i will say whatever you wish." then was he removed to a cell while his special form of perjury was drawn up. the next day, clothed as a penitent, the venerable old man was taken to the convent of minerva, where the cardinals and prelates were assembled for the purpose of passing judgment upon him. the text of the judgment i have here, but it is too long to read. it sentences him-- st. to the abjuration. nd. to formal imprisonment for life. rd. to recite the seven penitential psalms every week. ten cardinals were present; but, to their honour be it said, three refused to sign; and this blasphemous record of intolerance and bigoted folly goes down the ages with the names of seven cardinals immortalized upon it. this having been read, he next had to read word for word the abjuration which had been drawn up for him, and then sign it. the abjuration of galileo. "i, galileo galilei, son of the late vincenzo galilei, of florence, aged seventy years, being brought personally to judgment, and kneeling before you most eminent and most reverend lords cardinals, general inquisitors of the universal christian republic against heretical depravity, having before my eyes the holy gospels, which i touch with my own hands, swear that i have always believed, and now believe, and with the help of god will in future believe, every article which the holy catholic and apostolic church of rome holds, teaches, and preaches. but because i have been enjoined by this holy office altogether to abandon the false opinion which maintains that the sun is the centre and immovable, and forbidden to hold, defend, or teach the said false doctrine in any manner, and after it hath been signified to me that the said doctrine is repugnant with the holy scripture, i have written and printed a book, in which i treat of the same doctrine now condemned, and adduce reasons with great force in support of the same, without giving any solution, and therefore have been judged grievously suspected of heresy; that is to say, that i held and believed that the sun is the centre of the universe and is immovable, and that the earth is not the centre and is movable; willing, therefore, to remove from the minds of your eminences, and of every catholic christian, this vehement suspicion rightfully entertained towards me, with a sincere heart and unfeigned faith, i abjure, curse, and detest the said errors and heresies, and generally every other error and sect contrary to holy church; and i swear that i will never more in future say or assert anything verbally, or in writing, which may give rise to a similar suspicion of me; but if i shall know any heretic, or any one suspected of heresy, that i will denounce him to this holy office, or to the inquisitor or ordinary of the place where i may be; i swear, moreover, and promise, that i will fulfil and observe fully, all the penances which have been or shall be laid on me by this holy office. but if it shall happen that i violate any of my said promises, oaths, and protestations (which god avert!), i subject myself to all the pains and punishments which have been decreed and promulgated by the sacred canons, and other general and particular constitutions, against delinquents of this description. so may god help me, and his holy gospels which i touch with my own hands. i, the above-named galileo galilei, have abjured, sworn, promised, and bound myself as above, and in witness thereof with my own hand have subscribed this present writing of my abjuration, which i have recited word for word. at rome, in the convent of minerva, nd june, . i, galileo galilei, have abjured as above with my own hand." those who believe the story about his muttering to a friend, as he rose from his knees, "e pur si muove," do not realize the scene. st. there was no friend in the place. nd. it would have been fatally dangerous to mutter anything before such an assemblage. rd. he was by this time an utterly broken and disgraced old man; wishful, of all things, to get away and hide himself and his miseries from the public gaze; probably with his senses deadened and stupefied by the mental sufferings he had undergone, and no longer able to think or care about anything--except perhaps his daughter,--certainly not about any motion of this wretched earth. far and wide the news of the recantation spread. copies of the abjuration were immediately sent to all universities, with instructions to the professors to read it publicly. at florence, his home, it was read out in the cathedral church, all his friends and adherents being specially summoned to hear it. for a short time more he was imprisoned in rome; but at length was permitted to depart, never more of his own will to return. he was allowed to go to siena. here his daughter wrote consolingly, rejoicing at his escape, and saying how joyfully she already recited the penitential psalms for him, and so relieved him of that part of his sentence. but the poor girl was herself, by this time, ill--thoroughly worn out with anxiety and terror; she lay, in fact, on what proved to be her death-bed. her one wish was to see her dearest lord and father, so she calls him, once more. the wish was granted. his prison was changed, by orders from rome, from siena to arcetri, and once more father and daughter embraced. six days after this she died. the broken-hearted old man now asks for permission to go to live in florence, but is met with the stern answer that he is to stay at arcetri, is not to go out of the house, is not to receive visitors, and that if he asks for more favours, or transgresses the commands laid upon him, he is liable to be haled back to rome and cast into a dungeon. these harsh measures were dictated, not by cruelty, but by the fear of his still spreading heresy by conversation, and so he was to be kept isolated. idle, however, he was not and could not be. he often complains that his head is too busy for his body. in the enforced solitude of arcetri he was composing those dialogues on motion which are now reckoned his greatest and most solid achievement. in these the true laws of motion are set forth for the first time (see page ). one more astronomical discovery also he was to make--that of the moon's libration. and then there came one more crushing blow. his eyes became inflamed and painful--the sight of one of them failed, the other soon went; he became totally blind. but this, being a heaven-sent infliction, he could bear with resignation, though it must have been keenly painful to a solitary man of his activity. "alas!" says he, in one of his letters, "your dear friend and servant is totally blind. henceforth this heaven, this universe, which by wonderful observations i had enlarged a hundred and a thousand times beyond the conception of former ages, is shrunk for me into the narrow space which i myself fill in it. so it pleases god; it shall therefore please me also." he was now allowed an amanuensis, and the help of his pupils torricelli, castelli, and viviani, all devotedly attached to him, and torricelli very famous after him. visitors also were permitted, after approval by a jesuit supervisor; and under these circumstances many visited him, among them a man as immortal as himself--john milton, then only twenty-nine, travelling in italy. surely a pathetic incident, this meeting of these two great men--the one already blind, the other destined to become so. no wonder that, as in his old age he dictated his masterpiece, the thoughts of the english poet should run on the blind sage of tuscany, and the reminiscence of their conversation should lend colour to the poem. well, it were tedious to follow the petty annoyances and troubles to which galileo was still subject--how his own son was set to see that no unauthorized procedure took place, and that no heretic visitors were admitted; how it was impossible to get his new book printed till long afterwards; and how one form of illness after another took possession of him. the merciful end came at last, and at the age of seventy-eight he was released from the inquisition. they wanted to deny him burial--they did deny him a monument; they threatened to cart his bones away from florence if his friends attempted one. and so they hoped that he and his work might be forgotten. poor schemers! before the year was out an infant was born in lincolnshire, whose destiny it was to round and complete and carry forward the work of their victim, so that, until man shall cease from the planet, neither the work nor its author shall have need of a monument. * * * * * here might i end, were it not that the same kind of struggle as went on fiercely in the seventeenth century is still smouldering even now. not in astronomy indeed, as then; nor yet in geology, as some fifty years ago; but in biology mainly--perhaps in other subjects. i myself have heard charles darwin spoken of as an atheist and an infidel, the theory of evolution assailed as unscriptural, and the doctrine of the ascent of man from a lower state of being, as opposed to the fall of man from some higher condition, denied as impious and un-christian. men will not learn by the past; still they brandish their feeble weapons against the truths of nature, as if assertions one way or another could alter fact, or make the thing other than it really is. as galileo said before his spirit was broken, "in these and other positions certainly no man doubts but his holiness the pope hath always an absolute power of admitting or condemning them; but it is not in the power of any creature to make them to be true or false, or otherwise than of their own nature and in fact they are." i know nothing of the views of any here present; but i have met educated persons who, while they might laugh at the men who refused to look through a telescope lest they should learn something they did not like, yet also themselves commit the very same folly. i have met persons who utterly refuse to listen to any view concerning the origin of man other than that of a perfect primæval pair in a garden, and i am constrained to say this much: take heed lest some prophet, after having excited your indignation at the follies and bigotry of a bygone generation, does not turn upon you with the sentence, "thou art the man." summary of facts for lecture vi _science before newton_ _dr. gilbert_, of colchester, physician to queen elizabeth, was an excellent experimenter, and made many discoveries in magnetism and electricity. he was contemporary with tycho brahé, and lived from to . _francis bacon_, lord verulam, - , though a brilliant writer, is not specially important as regards science. he was not a scientific man, and his rules for making discoveries, or methods of induction, have never been consciously, nor often indeed unconsciously, followed by discoverers. they are not in fact practical rules at all, though they were so intended. his really strong doctrines are that phenomena must be studied direct, and that variations in the ordinary course of nature must be induced by aid of experiment; but he lacked the scientific instinct for pursuing these great truths into detail and special cases. he sneered at the work and methods of both gilbert and galileo, and rejected the copernican theory as absurd. his literary gifts have conferred on him an artificially high scientific reputation, especially in england; at the same time his writings undoubtedly helped to make popular the idea of there being new methods for investigating nature, and, by insisting on the necessity for freedom from preconceived ideas and opinions, they did much to release men from the bondage of aristotelian authority and scholastic tradition. the greatest name between galileo and newton is that of descartes. _rené descartes_ was born at la haye in touraine, , and died at stockholm in . he did important work in mathematics, physics, anatomy, and philosophy. was greatest as a philosopher and mathematician. at the age of twenty-one he served as a volunteer under prince maurice of nassau, but spent most of his later life in holland. his famous _discourse on method_ appeared at leyden in , and his _principia_ at amsterdam in ; great pains being taken to avoid the condemnation of the church. descartes's main scientific achievement was the application of algebra to geometry; his most famous speculation was the "theory of vortices," invented to account for the motion of planets. he also made many discoveries in optics and physiology. his best known immediate pupils were the princess elizabeth of bohemia, and christina, queen of sweden. he founded a distinct school of thought (the cartesian), and was the precursor of the modern mathematical method of investigating science, just as galileo and gilbert were the originators of the modern experimental method. lecture vi descartes and his theory of vortices after the dramatic life we have been considering in the last two lectures, it is well to have a breathing space, to look round on what has been accomplished, and to review the state of scientific thought, before proceeding to the next great era. for we are still in the early morning of scientific discovery: the dawn of the modern period, faintly heralded by copernicus, brought nearer by the work of tycho and kepler, and introduced by the discoveries of galileo--the dawn has occurred, but the sun is not yet visible. it is hidden by the clouds and mists of the long night of ignorance and prejudice. the light is sufficient, indeed, to render these earth-born vapours more visible: it is not sufficient to dispel them. a generation of slow and doubtful progress must pass, before the first ray of sunlight can break through the eastern clouds and the full orb of day itself appear. it is this period of hesitating progress and slow leavening of men's ideas that we have to pass through in this week's lecture. it always happens thus: the assimilation of great and new ideas is always a slow and gradual process: there is no haste either here or in any other department of nature. _die zeit ist unendlich lang._ steadily the forces work, sometimes seeming to accomplish nothing; sometimes even the motion appears retrograde; but in the long run the destined end is reached, and the course, whether of a planet or of men's thoughts about the universe, is permanently altered. then, the controversy was about the _earth's_ place in the universe; now, if there be any controversy of the same kind, it is about _man's_ place in the universe; but the process is the same: a startling statement by a great genius or prophet, general disbelief, and, it may be, an attitude of hostility, gradual acceptance by a few, slow spreading among the many, ending in universal acceptance and faith often as unquestioning and unreasoning as the old state of unfaith had been. now the process is comparatively speedy: twenty years accomplishes a great deal: then it was tediously slow, and a century seemed to accomplish very little. periodical literature may be responsible for some waste of time, but it certainly assists the rapid spread of ideas. the rate with which ideas are assimilated by the general public cannot even now be considered excessive, but how much faster it is than it was a few centuries ago may be illustrated by the attitude of the public to darwinism now, twenty-five years after _the origin of species_, as compared with their attitude to the copernican system a century after _de revolutionibus_. by the way, it is, i know, presumptuous for me to have an opinion, but i cannot hear darwin compared to or mentioned along with newton without a shudder. the stage in which he found biology seems to me far more comparable with the ptolemaic era in astronomy, and he himself to be quite fairly comparable to copernicus. let us proceed to summarize the stage at which the human race had arrived at the epoch with which we are now dealing. the copernican view of the solar system had been stated, restated, fought, and insisted on; a chain of brilliant telescopic discoveries had made it popular and accessible to all men of any intelligence: henceforth it must be left to slowly percolate and sink into the minds of the people. for the nations were waking up now, and were accessible to new ideas. england especially was, in some sort, at the zenith of its glory; or, if not at the zenith, was in that full flush of youth and expectation and hope which is stronger and more prolific of great deeds and thoughts than a maturer period. a common cause against a common and detested enemy had roused in the hearts of englishmen a passion of enthusiasm and patriotism; so that the mean elements of trade, their cheating yard-wands, were forgotten for a time; the armada was defeated, and the nation's true and conscious adult life began. commerce was now no mere struggle for profit and hard bargains; it was full of the spirit of adventure and discovery; a new world had been opened up; who could tell what more remained unexplored? men awoke to the splendour of their inheritance, and away sailed drake and frobisher and raleigh into the lands of the west. for literature, you know what a time it was. the author of _hamlet_ and _othello_ was alive: it is needless to say more. and what about science? the atmosphere of science is a more quiet and less stirring one; it thrives best when the fever of excitement is allayed; it is necessarily a later growth than literature. already, however, our second great man of science was at work in a quiet country town--second in point of time, i mean, roger bacon being the first. dr. gilbert, of colchester, was the second in point of time, and the age was ripening for the time when england was to be honoured with such a galaxy of scientific luminaries--hooke and boyle and newton--as the world had not yet known. yes, the nations were awake. "in all directions," as draper says, "nature was investigated: in all directions new methods of examination were yielding unexpected and beautiful results. on the ruins of its ivy-grown cathedrals ecclesiasticism [or scholasticism], surprised and blinded by the breaking day, sat solemnly blinking at the light and life about it, absorbed in the recollection of the night that had passed, dreaming of new phantoms and delusions in its wished-for return, and vindictively striking its talons at any derisive assailant who incautiously approached too near." of the work of gilbert there is much to say; so there is also of roger bacon, whose life i am by no means sure i did right in omitting. but neither of them had much to do with astronomy, and since it is in astronomy that the most startling progress was during these centuries being made, i have judged it wiser to adhere mainly to the pioneers in this particular department. only for this reason do i pass gilbert with but slight mention. he knew of the copernican theory and thoroughly accepted it (it is convenient to speak of it as the copernican theory, though you know that it had been considerably improved in detail since the first crude statement by copernicus), but he made in it no changes. he was a cultivated scientific man, and an acute experimental philosopher; his main work lay in the domain of magnetism and electricity. the phenomena connected with the mariner's compass had been studied somewhat by roger bacon; and they were now examined still more thoroughly by gilbert, whose treatise _de magnete_, marks the beginning of the science of magnetism. as an appendix to that work he studied the phenomenon of amber, which had been mentioned by thales. he resuscitated this little fact after its burial of , years, and greatly extended it. he it was who invented the name electricity--i wish it had been a shorter one. mankind invents names much better than do philosophers. what can be better than "heat," "light," "sound"? how favourably they compare with electricity, magnetism, galvanism, electro-magnetism, and magneto-electricity! the only long-established monosyllabic name i know invented by a philosopher is "gas"--an excellent attempt, which ought to be imitated.[ ] of lord bacon, who flourished about the same time (a little later), it is necessary to say something, because many persons are under the impression that to him and his _novum organon_ the reawakening of the world, and the overthrow of aristotelian tradition, are mainly due. his influence, however, has been exaggerated. i am not going to enter into a discussion of the _novum organon_, and the mechanical methods which he propounded as certain to evolve truth if patiently pursued; for this is what he thought he was doing--giving to the world an infallible recipe for discovering truth, with which any ordinarily industrious man could make discoveries by means of collection and discrimination of instances. you will take my statement for what it is worth, but i assert this: that many of the methods which bacon lays down are not those which the experience of mankind has found to be serviceable; nor are they such as a scientific man would have thought of devising. true it is that a real love and faculty for science are born in a man, and that to the man of scientific capacity rules of procedure are unnecessary; his own intuition is sufficient, or he has mistaken his vocation,--but that is not my point. it is not that bacon's methods are useless because the best men do not need them; if they had been founded on a careful study of the methods actually employed, though it might be unconsciously employed, by scientific men--as the methods of induction, stated long after by john stuart mill, were founded--then, no doubt, their statement would have been a valuable service and a great thing to accomplish. but they were not this. they are the ideas of a brilliant man of letters, writing in an age when scientific research was almost unknown, about a subject in which he was an amateur. i confess i do not see how he, or john stuart mill, or any one else, writing in that age, could have formulated the true rules of philosophizing; because the materials and information were scarcely to hand. science and its methods were only beginning to grow. no doubt it was a brilliant attempt. no doubt also there are many good and true points in the statement, especially in his insistence on the attitude of free and open candour with which the investigation of nature should be approached. no doubt there was much beauty in his allegories of the errors into which men were apt to fall--the _idola_ of the market-place, of the tribe, of the theatre, and of the den; but all this is literature, and on the solid progress of science may be said to have had little or no effect. descartes's _discourse on method_ was a much more solid production. you will understand that i speak of bacon purely as a scientific man. as a man of letters, as a lawyer, a man of the world, and a statesman, he is beyond any criticism of mine. i speak only of the purely scientific aspect of the _novum organon_. _the essays_ and _the advancement of learning_ are masterly productions; and as a literary man he takes high rank. the over-praise which, in the british isles, has been lavished upon his scientific importance is being followed abroad by what may be an unnecessary amount of detraction. this is always the worst of setting up a man on too high a pinnacle; some one has to undertake the ungrateful task of pulling him down again. justus von liebig addressed himself to this task with some vigour in his _reden und abhandlung_ (leipzig, ), where he quotes from bacon a number of suggestions for absurd experimentation.[ ] the next paragraph i read, not because i endorse it, but because it is always well to hear both sides of a question. you have probably been long accustomed to read over-estimates of bacon's importance, and extravagant laudation of his writings as making an epoch in science; hear what draper says on the opposite side:--[ ] "the more closely we examine the writings of lord bacon, the more unworthy does he seem to have been of the great reputation which has been awarded to him. the popular delusion to which he owes so much originated at a time when the history of science was unknown. they who first brought him into notice knew nothing of the old school of alexandria. this boasted founder of a new philosophy could not comprehend, and would not accept, the greatest of all scientific doctrines when it was plainly set before his eyes. "it has been represented that the invention of the true method of physical science was an amusement of bacon's hours of relaxation from the more laborious studies of law, and duties of a court. "his chief admirers have been persons of a literary turn, who have an idea that scientific discoveries are accomplished by a mechanico-mental operation. bacon never produced any great practical result himself, no great physicist has ever made any use of his method. he has had the same to do with the development of modern science that the inventor of the orrery has had to do with the discovery of the mechanism of the world. of all the important physical discoveries, there is not one which shows that its author made it by the baconian instrument. "newton never seems to have been aware that he was under any obligation to bacon. archimedes, and the alexandrians, and the arabians, and leonardo da vinci did very well before he was born; the discovery of america by columbus and the circumnavigation by magellan can hardly be attributed to him, yet they were the consequences of a truly philosophical reasoning. but the investigation of nature is an affair of genius, not of rules. no man can invent an _organon_ for writing tragedies and epic poems. bacon's system is, in its own terms, an idol of the theatre. it would scarcely guide a man to a solution of the riddle of Ælia lælia crispis, or to that of the charade of sir hilary. "few scientific pretenders have made more mistakes than lord bacon. he rejected the copernican system, and spoke insolently of its great author; he undertook to criticize adversely gilbert's treatise _de magnete_; he was occupied in the condemnation of any investigation of final causes, while harvey was deducing the circulation of the blood from aquapendente's discovery of the valves in the veins; he was doubtful whether instruments were of any advantage, while galileo was investigating the heavens with the telescope. ignorant himself of every branch of mathematics, he presumed that they were useless in science but a few years before newton achieved by their aid his immortal discoveries. "it is time that the sacred name of philosophy should be severed from its long connection with that of one who was a pretender in science, a time-serving politician, an insidious lawyer, a corrupt judge, a treacherous friend, a bad man." this seems to me a depreciation as excessive as are the eulogies commonly current. the truth probably lies somewhere between the two extremes. it is unfair to judge bacon's methods by thinking of physical science in its present stage. to realise his position we must think of a subject still in its very early infancy, one in which the advisability of applying experimental methods is still doubted; one which has been studied by means of books and words and discussion of normal instances, instead of by collection and observation of the unusual and irregular, and by experimental production of variety. if we think of a subject still in this infantile and almost pre-scientific stage, bacon's words and formulæ are far from inapplicable; they are, within their limitations, quite necessary and wholesome. a subject in this stage, strange to say, exists,--psychology; now hesitatingly beginning to assume its experimental weapons amid a stifling atmosphere of distrust and suspicion. bacon's lack of the modern scientific instinct must be admitted, but he rendered humanity a powerful service in directing it from books to nature herself, and his genius is indubitable. a judicious account of his life and work is given by prof. adamson, in the _encyclopædia britannica_, and to this article i now refer you. * * * * * who, then, was the man of first magnitude filling up the gap in scientific history between the death of galileo and the maturity of newton? unknown and mysterious are the laws regulating the appearance of genius. we have passed in review a pole, a dane, a german, and an italian,--the great man is now a frenchman, rené descartes, born in touraine, on the st of march, . his mother died at his birth; the father was of no importance, save as the owner of some landed property. the boy was reared luxuriously, and inherited a fair fortune. nearly all the men of first rank, you notice, were born well off. genius born to poverty might, indeed, even then achieve name and fame--as we see in the case of kepler--but it was terribly handicapped. handicapped it is still, but far less than of old; and we may hope it will become gradually still less so as enlightenment proceeds, and the tremendous moment of great men to a nation is more clearly and actively perceived. it is possible for genius, when combined with strong character, to overcome all obstacles, and reach the highest eminence, but the struggle must be severe; and the absence of early training and refinement during the receptive years of youth must be a lifelong drawback. descartes had none of these drawbacks; life came easily to him, and, as a consequence perhaps, he never seems to have taken it quite seriously. great movements and stirring events were to him opportunities for the study of men and manners; he was not the man to court persecution, nor to show enthusiasm for a losing or struggling cause. in this, as in many other things, he was imbued with a very modern spirit, a cynical and sceptical spirit, which, to an outside and superficial observer like myself, seems rather rife just now. he was also imbued with a phase of scientific spirit which you sometimes still meet with, though i believe it is passing away, viz. an uncultured absorption in his own pursuits, and some feeling of contempt for classical and literary and æsthetic studies. in politics, art, and history he seems to have had no interest. he was a spectator rather than an actor on the stage of the world; and though he joined the army of that great military commander prince maurice of nassau, he did it not as a man with a cause at heart worth fighting for, but precisely in the spirit in which one of our own gilded youths would volunteer in a similar case, as a good opportunity for frolic and for seeing life. he soon tired of it and withdrew--at first to gay society in paris. here he might naturally have sunk into the gutter with his companions, but for a great mental shock which became the main epoch and turning-point of his life, the crisis which diverted him from frivolity to seriousness. it was a purely intellectual emotion, not excited by anything in the visible or tangible world; nor could it be called conversion in the common acceptation of that term. he tells us that on the th of november, , at the age of twenty-four, a brilliant idea flashed upon him--the first idea, namely, of his great and powerful mathematical method, of which i will speak directly; and in the flush of it he foresaw that just as geometers, starting with a few simple and evident propositions or axioms, ascend by a long and intricate ladder of reasoning to propositions more and more abstruse, so it might be possible to ascend from a few data, to all the secrets and facts of the universe, by a process of mathematical reasoning. "comparing the mysteries of nature with the laws of mathematics, he dared to hope that the secrets of both could be unlocked with the same key." that night he lapsed gradually into a state of enthusiasm, in which he saw three dreams or visions, which he interpreted at the time, even before waking, to be revelations from the spirit of truth to direct his future course, as well as to warn him from the sins he had already committed. his account of the dreams is on record, but is not very easy to follow; nor is it likely that a man should be able to convey to others any adequate idea of the deepest spiritual or mental agitation which has shaken him to his foundations. his associates in paris were now abandoned, and he withdrew, after some wanderings, to holland, where he abode the best part of his life and did his real work. even now, however, he took life easily. he recommends idleness as necessary to the production of good mental work. he worked and meditated but a few hours a day: and most of those in bed. he used to think best in bed, he said. the afternoon he devoted to society and recreation. after supper he wrote letters to various persons, all plainly intended for publication, and scrupulously preserved. he kept himself free from care, and was most cautious about his health, regarding himself, no doubt, as a subject of experiment, and wishful to see how long he could prolong his life. at one time he writes to a friend that he shall be seriously disappointed if he does not manage to see years. [illustration: fig. .--descartes.] this plan of not over-working himself, and limiting the hours devoted to serious thought, is one that might perhaps advantageously be followed by some over-laborious students of the present day. at any rate it conveys a lesson; for the amount of ground covered by descartes, in a life not very long, is extraordinary. he must, however, have had a singular aptitude for scientific work; and the judicious leaven of selfishness whereby he was able to keep himself free from care and embarrassments must have been a great help to him. and what did his versatile genius accomplish during his fifty-four years of life? in philosophy, using the term as meaning mental or moral philosophy and metaphysics, as opposed to natural philosophy or physics, he takes a very high rank, and it is on this that perhaps his greatest fame rests. (he is the author, you may remember, of the famous aphorism, "_cogito, ergo sum_.") in biology i believe he may be considered almost equally great: certainly he spent a great deal of time in dissecting, and he made out a good deal of what is now known of the structure of the body, and of the theory of vision. he eagerly accepted the doctrine of the circulation of the blood, then being taught by harvey, and was an excellent anatomist. you doubtless know professor huxley's article on descartes in the _lay sermons_, and you perceive in what high estimation he is there held. he originated the hypothesis that animals are automata, for which indeed there is much to be said from some points of view; but he unfortunately believed that they were unconscious and non-sentient automata, and this belief led his disciples into acts of abominable cruelty. professor huxley lectured on this hypothesis and partially upheld it not many years since. the article is included in his volume called _science and culture_. concerning his work in mathematics and physics i can speak with more confidence. he is the author of the cartesian system of algebraic or analytic geometry, which has been so powerful an engine of research, far easier to wield than the old synthetic geometry. without it newton could never have written the _principia_, or made his greatest discoveries. he might indeed have invented it for himself, but it would have consumed some of his life to have brought it to the necessary perfection. the principle of it is the specification of the position of a point in a plane by two numbers, indicating say its distance from two lines of reference in the plane; like the latitude and longitude of a place on the globe. for instance, the two lines of reference might be the bottom edge and the left-hand vertical edge of a wall; then a point on the wall, stated as being for instance feet along and feet up, is precisely determined. these two distances are called co-ordinates; horizontal ones are usually denoted by _x_, and vertical ones by _y_. if, instead of specifying two things, only one statement is made, such as _y_ = , it is satisfied by a whole row of points, all the points in a horizontal line feet above the ground. hence _y_ = may be said to represent that straight line, and is called the equation to that straight line. similarly _x_ = represents a vertical straight line feet (or inches or some other unit) from the left-hand edge. if it is asserted that _x_ = and _y_ = , only one point can be found to satisfy both conditions, viz. the crossing point of the above two straight lines. suppose an equation such as _x_ = _y_ to be given. this also is satisfied by a row of points, viz. by all those that are equidistant from bottom and left-hand edges. in other words, _x_ = _y_ represents a straight line slanting upwards at °. the equation _x_ = _y_ represents another straight line with a different angle of slope, and so on. the equation x^ + y^ = represents a circle of radius . the equation x^ + y^ = represents an ellipse; and in general every algebraic equation that can be written down, provided it involve only two variables, _x_ and _y_, represents some curve in a plane; a curve moreover that can be drawn, or its properties completely investigated without drawing, from the equation. thus algebra is wedded to geometry, and the investigation of geometric relations by means of algebraic equations is called analytical geometry, as opposed to the old euclidian or synthetic mode of treating the subject by reasoning consciously directed to the subject by help of figures. if there be three variables--_x_, _y_, and _z_,--instead of only two, an equation among them represents not a curve in a plane but a surface in space; the three variables corresponding to the three dimensions of space: length, breadth, and thickness. an equation with four variables usually requires space of four dimensions for its geometrical interpretation, and so on. thus geometry can not only be reasoned about in a more mechanical and therefore much easier, manner, but it can be extended into regions of which we have and can have no direct conception, because we are deficient in sense organs for accumulating any kind of experience in connexion with such ideas. [illustration: fig. .--the eye diagram. [from descartes' _principia_.] three external points are shown depicted on the retina: the image being appreciated by a representation of the brain.] in physics proper descartes' tract on optics is of considerable historical interest. he treats all the subjects he takes up in an able and original manner. in astronomy he is the author of that famous and long upheld theory, the doctrine of vortices. he regarded space as a plenum full of an all-pervading fluid. certain portions of this fluid were in a state of whirling motion, as in a whirlpool or eddy of water; and each planet had its own eddy, in which it was whirled round and round, as a straw is caught and whirled in a common whirlpool. this idea he works out and elaborates very fully, applying it to the system of the world, and to the explanation of all the motions of the planets. [illustration: fig. .--descartes's diagram of vortices, from his _principia_.] this system evidently supplied a void in men's minds, left vacant by the overthrow of the ptolemaic system, and it was rapidly accepted. in the english universities it held for a long time almost undisputed sway; it was in this faith that newton was brought up. something was felt to be necessary to keep the planets moving on their endless round; the _primum mobile_ of ptolemy had been stopped; an angel was sometimes assigned to each planet to carry it round, but though a widely diffused belief, this was a fantastic and not a serious scientific one. descartes's vortices seemed to do exactly what was wanted. it is true they had no connexion with the laws of kepler. i doubt whether he knew about the laws of kepler; he had not much opinion of other people's work; he read very little--found it easier to think. (he travelled through florence once when galileo was at the height of his renown without calling upon or seeing him.) in so far as the motion of a planet was not circular, it had to be accounted for by the jostling and crowding and distortion of the vortices. gravitation he explained by a settling down of bodies toward the centre of each vortex; and cohesion by an absence of relative motion tending to separate particles of matter. he "can imagine no stronger cement." the vortices, as descartes imagined them, are not now believed in. are we then to regard the system as absurd and wholly false? i do not see how we can do this, when to this day philosophers are agreed in believing space to be completely full of fluid, which fluid is certainly capable of vortex motion, and perhaps everywhere does possess that motion. true, the now imagined vortices are not the large whirls of planetary size, they are rather infinitesimal whirls of less than atomic dimensions; still a whirling fluid is believed in to this day, and many are seeking to deduce all the properties of matter (rigidity, elasticity, cohesion gravitation, and the rest) from it. further, although we talk glibly about gravitation and magnetism, and so on, we do not really know what they are. progress is being made, but we do not yet properly know. much, overwhelmingly much, remains to be discovered, and it ill-behoves us to reject any well-founded and long-held theory as utterly and intrinsically false and absurd. the more one gets to know, the more one perceives a kernel of truth even in the most singular statements; and scientific men have learned by experience to be very careful how they lop off any branch of the tree of knowledge, lest as they cut away the dead wood they lose also some green shoot, some healthy bud of unperceived truth. however, it may be admitted that the idea of a cartesian vortex in connexion with the solar system applies, if at all, rather to an earlier--its nebulous--stage, when the whole thing was one great whirl, ready to split or shrink off planetary rings at their appropriate distances. soon after he had written his great work, the _principia mathematica_, and before he printed it, news reached him of the persecution and recantation of galileo. "he seems to have been quite thunderstruck at the tidings," says mr. mahaffy, in his _life of descartes_.[ ] "he had started on his scientific journeys with the firm determination to enter into no conflict with the church, and to carry out his system of pure mathematics and physics without ever meddling with matters of faith. he was rudely disillusioned as to the possibility of this severance. he wrote at once--apparently, november th, --to mersenne to say he would on no account publish his work--nay, that he had at first resolved to burn all his papers, for that he would never prosecute philosophy at the risk of being censured by his church. 'i could hardly have believed,' he says, 'that an italian, and in favour with the pope as i hear, could be considered criminal for nothing else than for seeking to establish the earth's motion; though i know it has formerly been censured by some cardinals. but i thought i had heard that since then it was constantly being taught, even at rome; and i confess that if the opinion of the earth's movement is false, all the foundations of my philosophy are so also, because it is demonstrated clearly by them. it is so bound up with every part of my treatise that i could not sever it without making the remainder faulty; and although i consider all my conclusions based on very certain and clear demonstrations, i would not for all the world sustain them against the authority of the church.'" ten years later, however, he did publish the book, for he had by this time hit on an ingenious compromise. he formally denied that the earth moved, and only asserted that it was carried along with its water and air in one of those larger motions of the celestial ether which produce the diurnal and annual revolutions of the solar system. so, just as a passenger on the deck of a ship might be called stationary, so was the earth. he gives himself out therefore as a follower of tycho rather than of copernicus, and says if the church won't accept this compromise he must return to the ptolemaic system; but he hopes they won't compel him to do that, seeing that it is manifestly untrue. this elaborate deference to the powers that be did not indeed save the work from being ultimately placed upon the forbidden list by the church, but it saved himself, at any rate, from annoying persecution. he was not, indeed, at all willing to be persecuted, and would no doubt have at once withdrawn anything they wished. i should be sorry to call him a time-server, but he certainly had plenty of that worldly wisdom in which some of his predecessors had been so lamentably deficient. moreover, he was really a sceptic, and cared nothing at all about the church or its dogmas. he knew the church's power, however, and the advisability of standing well with it: he therefore professed himself a catholic, and studiously kept his science and his christianity distinct. in saying that he was a sceptic you must not understand that he was in the least an atheist. very few men are; certainly descartes never thought of being one. the term is indeed ludicrously inapplicable to him, for a great part of his philosophy is occupied with what he considers a rigorous proof of the existence of the deity. at the age of fifty-three he was sent for to stockholm by christina, queen of sweden, a young lady enthusiastically devoted to study of all kinds and determined to surround her court with all that was most famous in literature and science. thither, after hesitation, descartes went. he greatly liked royalty, but he dreaded the cold climate. born in touraine, a swedish winter was peculiarly trying to him, especially as the energetic queen would have lessons given her at five o'clock in the morning. she intended to treat him well, and was immensely taken with him; but this getting up at five o'clock on a november morning, to a man accustomed all his life to lie in bed till eleven, was a cruel hardship. he was too much of a courtier, however, to murmur, and the early morning audience continued. his health began to break down: he thought of retreating, but suddenly he gave way and became delirious. the queen's physician attended him, and of course wanted to bleed him. this, knowing all he knew of physiology, sent him furious, and they could do nothing with him. after some days he became quiet, was bled twice, and gradually sank, discoursing with great calmness on his approaching death, and duly fortified with all the rites of the catholic church. his general method of research was as nearly as possible a purely deductive one:--_i.e._, after the manner of euclid he starts with a few simple principles, and then, by a chain of reasoning, endeavours to deduce from them their consequences, and so to build up bit by bit an edifice of connected knowledge. in this he was the precursor of newton. this method, when rigorously pursued, is the most powerful and satisfactory of all, and results in an ordered province of science far superior to the fragmentary conquests of experiment. but few indeed are the men who can handle it safely and satisfactorily: and none without continual appeals to experiment for verification. it was through not perceiving the necessity for verification that he erred. his importance to science lies not so much in what he actually discovered as in his anticipation of the right conditions for the solution of problems in physical science. he in fact made the discovery that nature could after all be interrogated mathematically--a fact that was in great danger of remaining unknown. for, observe, that the mathematical study of nature, the discovery of truth with a piece of paper and a pen, has a perilous similarity at first sight to the straw-thrashing subtleties of the greeks, whose methods of investigating nature by discussing the meaning of words and the usage of language and the necessities of thought, had proved to be so futile and unproductive. a reaction had set in, led by galileo, gilbert, and the whole modern school of experimental philosophers, lasting down to the present day:--men who teach that the only right way of investigating nature is by experiment and observation. it is indeed a very right and an absolutely necessary way; but it is not the only way. a foundation of experimental fact there must be; but upon this a great structure of theoretical deduction can be based, all rigidly connected together by pure reasoning, and all necessarily as true as the premises, provided no mistake is made. to guard against the possibility of mistake and oversight, especially oversight, all conclusions must sooner or later be brought to the test of experiment; and if disagreeing therewith, the theory itself must be re-examined, and the flaw discovered, or else the theory must be abandoned. of this grand method, quite different from the gropings in the dark of kepler--this method, which, in combination with experiment, has made science what it now is--this which in the hands of newton was to lead to such stupendous results, we owe the beginning and early stages to rené descartes. summary of facts for lectures vii and viii otto guericke - hon. robert boyle - huyghens - christopher wren - robert hooke - newton - edmund halley - james bradley - _chronology of newton's life._ isaac newton was born at woolsthorpe, near grantham, lincolnshire, on christmas day, . his father, a small freehold farmer, also named isaac, died before his birth. his mother, _née_ hannah ayscough, in two years married a mr. smith, rector of north witham, but was again left a widow in . his uncle, w. ayscough, was rector of a near parish and a graduate of trinity college, cambridge. at the age of fifteen isaac was removed from school at grantham to be made a farmer of, but as it seemed he would not make a good one his uncle arranged for him to return to school and thence to cambridge, where he entered trinity college as a sub-sizar in . studied descartes's geometry. found out a method of infinite series in , and began the invention of fluxions. in the same year and the next he was driven from cambridge by the plague. in , at woolsthorpe, the apple fell. in he was elected a fellow of his college, and in was specially noted as possessing an unparalleled genius by dr. barrow, first lucasian professor of mathematics. the same year dr. barrow retired from his chair in favour of newton, who was thus elected at the age of twenty-six. he lectured first on optics with great success. early in he was elected a fellow of the royal society, and communicated his researches in optics, his reflecting telescope, and his discovery of the compound nature of white light. annoying controversies arose; but he nevertheless contributed a good many other most important papers in optics, including observations in diffraction, and colours of thin plates. he also invented the modern sextant. in a letter from paris was read at the royal society concerning a new and accurate determination of the size of the earth by picard. when newton heard of it he began the _principia_, working in silence. in arose a discussion between wren, hooke, and halley concerning the law of inverse square as applied to gravity and the path it would cause the planets to describe. hooke asserted that he had a solution, but he would not produce it. after waiting some time for it halley went to cambridge to consult newton on the subject, and thus discovered the existence of the first part of the _principia_, wherein all this and much more was thoroughly worked out. on his representations to the royal society the manuscript was asked for, and when complete was printed and published in at halley's expense. while it was being completed newton and seven others were sent to uphold the dignity of the university, before the court of high commission and judge jeffreys, against a high-handed action of james ii. in he was sent to parliament, and was present at the coronation of william and mary. made friends with locke. in montague, lord halifax, made him warden, and in master, of the mint. whiston succeeded him as lucasian professor. in the method of fluxions was published. in newton was made president of the royal society, and held the office to the end of his life. in he was knighted by anne. in cotes helped him to bring out a new edition of the _principia_, completed as we now have it. on the th of march , he died: having lived from charles i. to george ii. the laws of motion, discovered by galileo, stated by newton. _law ._--if no force acts on a body in motion, it continues to move uniformly in a straight line. _law ._--if force acts on a body, it produces a change of motion proportional to the force and in the same direction. _law ._--when one body exerts force on another, that other reacts with equal force upon the one. lecture vii sir isaac newton the little hamlet of woolsthorpe lies close to the village of colsterworth, about six miles south of grantham, in the county of lincoln. in the manor house of woolsthorpe, on christmas day, , was born to a widowed mother a sickly infant who seemed not long for this world. two women who were sent to north witham to get some medicine for him scarcely expected to find him alive on their return. however, the child lived, became fairly robust, and was named isaac, after his father. what sort of a man this father was we do not know. he was what we may call a yeoman, that most wholesome and natural of all classes. he owned the soil he tilled, and his little estate had already been in the family for some hundred years. he was thirty-six when he died, and had only been married a few months. of the mother, unfortunately, we know almost as little. we hear that she was recommended by a parishioner to the rev. barnabas smith, an old bachelor in search of a wife, as "the widow newton--an extraordinary good woman:" and so i expect she was, a thoroughly sensible, practical, homely, industrious, middle-class, mill-on-the-floss sort of woman. however, on her second marriage she went to live at north witham, and her mother, old mrs. ayscough, came to superintend the farm at woolsthorpe, and take care of young isaac. by her second marriage his mother acquired another piece of land, which she settled on her first son; so isaac found himself heir to two little properties, bringing in a rental of about £ a year. [illustration: fig. .--manor-house of woolsthorpe.] he had been sent to a couple of village schools to acquire the ordinary accomplishments taught at those places, and for three years to the grammar school at grantham, then conducted by an old gentleman named mr. stokes. he had not been very industrious at school, nor did he feel keenly the fascinations of the latin grammar, for he tells us that he was the last boy in the lowest class but one. he used to pay much more attention to the construction of kites and windmills and waterwheels, all of which he made to work very well. he also used to tie paper lanterns to the tail of his kite, so as to make the country folk fancy they saw a comet, and in general to disport himself as a boy should. it so happened, however, that he succeeded in thrashing, in fair fight, a bigger boy who was higher in the school, and who had given him a kick. his success awakened a spirit of emulation in other things than boxing, and young newton speedily rose to be top of the school. under these circumstances, at the age of fifteen, his mother, who had now returned to woolsthorpe, which had been rebuilt, thought it was time to train him for the management of his land, and to make a farmer and grazier of him. the boy was doubtless glad to get away from school, but he did not take kindly to the farm--especially not to the marketing at grantham. he and an old servant were sent to grantham every week to buy and sell produce, but young isaac used to leave his old mentor to do all the business, and himself retire to an attic in the house he had lodged in when at school, and there bury himself in books. after a time he didn't even go through the farce of visiting grantham at all; but stopped on the road and sat under a hedge, reading or making some model, until his companion returned. we hear of him now in the great storm of , the storm on the day cromwell died, measuring the force of the wind by seeing how far he could jump with it and against it. he also made a water-clock and set it up in the house at grantham, where it kept fairly good time so long as he was in the neighbourhood to look after it occasionally. at his own home he made a couple of sundials on the side of the wall (he began by marking the position of the sun by the shadow of a peg driven into the wall, but this gradually developed into a regular dial) one of which remained of use for some time; and was still to be seen in the same place during the first half of the present century, only with the gnomon gone. in the stone on which it was carved was carefully extracted and presented to the royal society, who preserve it in their library. the letters wton roughly carved on it are barely visible. all these pursuits must have been rather trying to his poor mother, and she probably complained to her brother, the rector of burton coggles: at any rate this gentleman found master newton one morning under a hedge when he ought to have been farming. but as he found him working away at mathematics, like a wise man he persuaded his sister to send the boy back to school for a short time, and then to cambridge. on the day of his finally leaving school old mr. stokes assembled the boys, made them a speech in praise of newton's character and ability, and then dismissed him to cambridge. at trinity college a new world opened out before the country-bred lad. he knew his classics passably, but of mathematics and science he was ignorant, except through the smatterings he had picked up for himself. he devoured a book on logic, and another on kepler's optics, so fast that his attendance at lectures on these subjects became unnecessary. he also got hold of a euclid and of descartes's geometry. the euclid seemed childishly easy, and was thrown aside, but the descartes baffled him for a time. however, he set to it again and again and before long mastered it. he threw himself heart and soul into mathematics, and very soon made some remarkable discoveries. first he discovered the binomial theorem: familiar now to all who have done any algebra, unintelligible to others, and therefore i say nothing about it. by the age of twenty-one or two he had begun his great mathematical discovery of infinite series and fluxions--now known by the name of the differential calculus. he wrote these things out and must have been quite absorbed in them, but it never seems to have occurred to him to publish them or tell any one about them. in he noticed some halos round the moon, and, as his manner was, he measured their angles--the small ones and degrees each, the larger one °· . later he gave their theory. small coloured halos round the moon are often seen, and are said to be a sign of rain. they are produced by the action of minute globules of water or cloud particles upon light, and are brightest when the particles are nearly equal in size. they are not like the rainbow, every part of which is due to light that has entered a raindrop, and been refracted and reflected with prismatic separation of colours; a halo is caused by particles so small as to be almost comparable with the size of waves of light, in a way which is explained in optics under the head "diffraction." it may be easily imitated by dusting an ordinary piece of window-glass over with lycopodium, placing a candle near it, and then looking at the candle-flame through the dusty glass from a fair distance. or you may look at the image of a candle in a dusted looking-glass. lycopodium dust is specially suitable, for its granules are remarkably equal in size. the large halo, more rarely seen, of angular radius °· , is due to another cause again, and is a prismatic effect, although it exhibits hardly any colour. the angle - / ° is characteristic of refraction in crystals with angles of ° and refractive index about the same as water; in other words this halo is caused by ice crystals in the higher regions of the atmosphere. he also the same year observed a comet, and sat up so late watching it that he made himself ill. by the end of the year he was elected to a scholarship and took his b.a. degree. the order of merit for that year never existed or has not been kept. it would have been interesting, not as a testimony to newton, but to the sense or non-sense of the examiners. the oldest professorship of mathematics at the university of cambridge, the lucasian, had not then been long founded, and its first occupant was dr. isaac barrow, an eminent mathematician, and a kind old man. with him newton made good friends, and was helpful in preparing a treatise on optics for the press. his help is acknowledged by dr. barrow in the preface, which states that he had corrected several errors and made some capital additions of his own. thus we see that, although the chief part of his time was devoted to mathematics, his attention was already directed to both optics and astronomy. (kepler, descartes, galileo, all combined some optics with astronomy. tycho and the old ones combined alchemy; newton dabbled in this also.) newton reached the age of twenty-three in , the year of the great plague. the plague broke out in cambridge as well as in london, and the whole college was sent down. newton went back to woolsthorpe, his mind teeming with ideas, and spent the rest of this year and part of the next in quiet pondering. somehow or other he had got hold of the notion of centrifugal force. it was six years before huyghens discovered and published the laws of centrifugal force, but in some quiet way of his own newton knew about it and applied the idea to the motion of the planets. we can almost follow the course of his thoughts as he brooded and meditated on the great problem which had taxed so many previous thinkers,--what makes the planets move round the sun? kepler had discovered how they moved, but why did they so move, what urged them? even the "how" took a long time--all the time of the greeks, through ptolemy, the arabs, copernicus, tycho: circular motion, epicycles, and excentrics had been the prevailing theory. kepler, with his marvellous industry, had wrested from tycho's observations the secret of their orbits. they moved in ellipses with the sun in one focus. their rate of description of area, not their speed, was uniform and proportional to time. yes, and a third law, a mysterious law of unintelligible import, had also yielded itself to his penetrating industry--a law the discovery of which had given him the keenest delight, and excited an outburst of rapture--viz. that there was a relation between the distances and the periodic times of the several planets. the cubes of the distances were proportional to the squares of the times for the whole system. this law, first found true for the six primary planets, he had also extended, after galileo's discovery, to the four secondary planets, or satellites of jupiter (p. ). but all this was working in the dark--it was only the first step--this empirical discovery of facts; the facts were so, but how came they so? what made the planets move in this particular way? descartes's vortices was an attempt, a poor and imperfect attempt, at an explanation. it had been hailed and adopted throughout europe for want of a better, but it did not satisfy newton. no, it proceeded on a wrong tack, and kepler had proceeded on a wrong tack in imagining spokes or rays sticking out from the sun and driving the planets round like a piece of mechanism or mill work. for, note that all these theories are based on a wrong idea--the idea, viz., that some force is necessary to maintain a body in motion. but this was contrary to the laws of motion as discovered by galileo. you know that during his last years of blind helplessness at arcetri, galileo had pondered and written much on the laws of motion, the foundation of mechanics. in his early youth, at pisa, he had been similarly occupied; he had discovered the pendulum, he had refuted the aristotelians by dropping weights from the leaning tower (which we must rejoice that no earthquake has yet injured), and he had returned to mechanics at intervals all his life; and now, when his eyes were useless for astronomy, when the outer world has become to him only a prison to be broken by death, he returns once more to the laws of motion, and produces the most solid and substantial work of his life. for this is galileo's main glory--not his brilliant exposition of the copernican system, not his flashes of wit at the expense of a moribund philosophy, not his experiments on floating bodies, not even his telescope and astronomical discoveries--though these are the most taking and dazzling at first sight. no; his main glory and title to immortality consists in this, that he first laid the foundation of mechanics on a firm and secure basis of experiment, reasoning, and observation. he first discovered the true laws of motion. i said little of this achievement in my lecture on him; for the work was written towards the end of his life, and i had no time then. but i knew i should have to return to it before we came to newton, and here we are. you may wonder how the work got published when so many of his manuscripts were destroyed. horrible to say, galileo's own son destroyed a great bundle of his father's manuscripts, thinking, no doubt, thereby to save his own soul. this book on mechanics was not burnt, however. the fact is it was rescued by one or other of his pupils, toricelli or viviani, who were allowed to visit him in his last two or three years; it was kept by them for some time, and then published surreptitiously in holland. not that there is anything in it bearing in any visible way on any theological controversy; but it is unlikely that the inquisition would have suffered it to pass notwithstanding. i have appended to the summary preceding this lecture (p. ) the three axioms or laws of motion discovered by galileo. they are stated by newton with unexampled clearness and accuracy, and are hence known as newton's laws, but they are based on galileo's work. the first is the simplest; though ignorance of it gave the ancients a deal of trouble. it is simply a statement that force is needed to change the motion of a body; _i.e._ that if no force act on a body it will continue to move uniformly both in speed and direction--in other words, steadily, in a straight line. the old idea had been that some force was needed to maintain motion. on the contrary, the first law asserts, some force is needed to destroy it. leave a body alone, free from all friction or other retarding forces, and it will go on for ever. the planetary motion through empty space therefore wants no keeping up; it is not the motion that demands a force to maintain it, it is the curvature of the path that needs a force to produce it continually. the motion of a planet is approximately uniform so far as speed is concerned, but it is not constant in direction; it is nearly a circle. the real force needed is not a propelling but a deflecting force. the second law asserts that when a force acts, the motion changes, either in speed or in direction, or both, at a pace proportional to the magnitude of the force, and in the same direction as that in which the force acts. now since it is almost solely in direction that planetary motion alters, a deflecting force only is needed; a force at right angles to the direction of motion, a force normal to the path. considering the motion as circular, a force along the radius, a radial or centripetal force, must be acting continually. whirl a weight round and round by a bit of elastic, the elastic is stretched; whirl it faster, it is stretched more. the moving mass pulls at the elastic--that is its centrifugal force; the hand at the centre pulls also--that is centripetal force. the third law asserts that these two forces are equal, and together constitute the tension in the elastic. it is impossible to have one force alone, there must be a pair. you can't push hard against a body that offers no resistance. whatever force you exert upon a body, with that same force the body must react upon you. action and reaction are always equal and opposite. sometimes an absurd difficulty is felt with respect to this, even by engineers. they say, "if the cart pulls against the horse with precisely the same force as the horse pulls the cart, why should the cart move?" why on earth not? the cart moves because the horse pulls it, and because nothing else is pulling it back. "yes," they say, "the cart is pulling back." but what is it pulling back? not itself, surely? "no, the horse." yes, certainly the cart is pulling at the horse; if the cart offered no resistance what would be the good of the horse? that is what he is for, to overcome the pull-back of the cart; but nothing is pulling the cart back (except, of course, a little friction), and the horse is pulling it forward, hence it goes forward. there is no puzzle at all when once you realise that there are two bodies and two forces acting, and that one force acts on each body.[ ] if, indeed, two balanced forces acted on one body that would be in equilibrium, but the two equal forces contemplated in the third law act on two different bodies, and neither is in equilibrium. so much for the third law, which is extremely simple, though it has extraordinarily far-reaching consequences, and when combined with a denial of "action at a distance," is precisely the principle of the conservation of energy. attempts at perpetual motion may all be regarded as attempts to get round this "third law." [illustration: fig. .] on the subject of the _second_ law a great deal more has to be said before it can be in any proper sense even partially appreciated, but a complete discussion of it would involve a treatise on mechanics. it is _the_ law of mechanics. one aspect of it we must attend to now in order to deal with the motion of the planets, and that is the fact that the change of motion of a body depends solely and simply on the force acting, and not at all upon what the body happens to be doing at the time it acts. it may be stationary, or it may be moving in any direction; that makes no difference. thus, referring back to the summary preceding lecture iv, it is there stated that a dropped body falls feet in the first second, that in two seconds it falls feet, and so on, in proportion to the square of the time. so also will it be the case with a thrown body, but the drop must be reckoned from its line of motion--the straight line which, but for gravity, it would describe. thus a stone thrown from _o_ with the velocity _oa_ would in one second find itself at _a_, in two seconds at _b_, in three seconds at _c_, and so on, in accordance with the first law of motion, if no force acted. but if gravity acts it will have fallen feet by the time it would have got to _a_, and so will find itself at _p_. in two seconds it will be at _q_, having fallen a vertical height of feet; in three seconds it will be at _r_, feet below _c_; and so on. its actual path will be a curve, which in this case is a parabola. (fig. .) if a cannon is pointed horizontally over a level plain, the cannon ball will be just as much affected by gravity as if it were dropped, and so will strike the plain at the same instant as another which was simply dropped where it started. one ball may have gone a mile and the other only dropped a hundred feet or so, but the time needed by both for the vertical drop will be the same. the horizontal motion of one is an extra, and is due to the powder. as a matter of fact the path of a projectile in vacuo is only approximately a parabola. it is instructive to remember that it is really an ellipse with one focus very distant, but not at infinity. one of its foci is the centre of the earth. a projectile is really a minute satellite of the earth's, and in vacuo it accurately obeys all kepler's laws. it happens not to be able to complete its orbit, because it was started inconveniently close to the earth, whose bulk gets in its way; but in that respect the earth is to be reckoned as a gratuitous obstruction, like a target, but a target that differs from most targets in being hard to miss. [illustration: fig. .] now consider circular motion in the same way, say a ball whirled round by a string. (fig. .) attending to the body at _o_, it is for an instant moving towards _a_, and if no force acted it would get to _a_ in a time which for brevity we may call a second. but a force, the pull of the string, is continually drawing it towards _s_, and so it really finds itself at _p_, having described the circular arc _op_, which may be considered to be compounded of, and analyzable into the rectilinear motion _oa_ and the drop _ap_. at _p_ it is for an instant moving towards _b_, and the same process therefore carries it to _q_; in the third second it gets to _r_; and so on: always falling, so to speak, from its natural rectilinear path, towards the centre, but never getting any nearer to the centre. the force with which it has thus to be constantly pulled in towards the centre, or, which is the same thing, the force with which it is tugging at whatever constraint it is that holds it in, is _mv^ /r_; where _m_ is the mass of the particle, _v_ its velocity, and _r_ the radius of its circle of movement. this is the formula first given by huyghens for centrifugal force. we shall find it convenient to express it in terms of the time of one revolution, say _t_. it is easily done, since plainly t = circumference/speed = _ [pi]r/v_; so the above expression for centrifugal force becomes _ [pi]^ mr/t^ _. as to the fall of the body towards the centre every microscopic unit of time, it is easily reckoned. for by euclid iii. , and fig. , _ap.aa' = ao^ _. take _a_ very near _o_, then _oa = vt_, and _aa' = r_; so _ap = v^ t^ / r = [pi]^ r t^ /t^ _; or the fall per second is _ [pi]^ r/t^ _, _r_ being its distance from the centre, and _t_ its time of going once round. in the case of the moon for instance, _r_ is earth radii; more exactly · ; and _t_ is a lunar month, or more precisely days, hours, minutes, and - / seconds. hence the moon's deflection from the tangential or rectilinear path every minute comes out as very closely feet (the true size of the earth being used). returning now to the case of a small body revolving round a big one, and assuming a force directly proportional to the mass of both bodies, and inversely proportional to the square of the distance between them: _i.e._ assuming the known force of gravity, it is _v mm/r^ _ where _v_ is a constant, called the gravitation constant, to be determined by experiment. if this is the centripetal force pulling a planet or satellite in, it must be equal to the centrifugal force of this latter, viz. (see above). _ [pi]^ mr/t^ equate the two together, and at once we get _r^ /t^ = v/ [pi]^ m;_ or, in words, the cube of the distance divided by the square of the periodic time for every planet or satellite of the system under consideration, will be constant and proportional to the mass of the central body. this is kepler's third law, with a notable addition. it is stated above for circular motion only, so as to avoid geometrical difficulties, but even so it is very instructive. the reason of the proportion between _r^ _ and _t^ _ is at once manifest; and as soon as the constant _v_ became known, _the mass of the central body_, the sun in the case of a planet, the earth in the case of the moon, jupiter in the case of his satellites, was at once determined. newton's reasoning at this time might, however, be better displayed perhaps by altering the order of the steps a little, as thus:-- the centrifugal force of a body is proportional to _r^ /t^ _, but by kepler's third law _r^ /t^ _ is constant for all the planets, reckoning _r_ from the sun. hence the centripetal force needed to hold in all the planets will be a single force emanating from the sun and varying inversely with the square of the distance from that body. such a force is at once necessary and sufficient. such a force would explain the motion of the planets. but then all this proceeds on a wrong assumption--that the planetary motion is circular. will it hold for elliptic orbits? will an inverse square law of force keep a body moving in an elliptic orbit about the sun in one focus? this is a far more difficult question. newton solved it, but i do not believe that even he could have solved it, except that he had at his disposal two mathematical engines of great power--the cartesian method of treating geometry, and his own method of fluxions. one can explain the elliptic motion now mathematically, but hardly otherwise; and i must be content to state that the double fact is true--viz., that an inverse square law will move the body in an ellipse or other conic section with the sun in one focus, and that if a body so moves it _must_ be acted on by an inverse square law. [illustration: fig. .] this then is the meaning of the first and third laws of kepler. what about the second? what is the meaning of the equable description of areas? well, that rigorously proves that a planet is acted on by a force directed to the centre about which the rate of description of areas is equable. it proves, in fact, that the sun is the attracting body, and that no other force acts. for first of all if the first law of motion is obeyed, _i.e._ if no force acts, and if the path be equally subdivided to represent equal times, and straight lines be drawn from the divisions to any point whatever, all these areas thus enclosed will be equal, because they are triangles on equal base and of the same height (euclid, i). see fig. ; _s_ being any point whatever, and _a_, _b_, _c_, successive positions of a body. now at each of the successive instants let the body receive a sudden blow in the direction of that same point _s_, sufficient to carry it from _a_ to _d_ in the same time as it would have got to _b_ if left alone. the result will be that there will be a compromise, and it will really arrive at _p_, travelling along the diagonal of the parallelogram _ap_. the area its radius vector sweeps out is therefore _sap_, instead of what it would have been, _sab_. but then these two areas are equal, because they are triangles on the same base _as_, and between the same parallels _bp_, _as_; for by the parallelogram law _bp_ is parallel to _ad_. hence the area that would have been described is described, and as all the areas were equal in the case of no force, they remain equal when the body receives a blow at the end of every equal interval of time, _provided_ that every blow is actually directed to _s_, the point to which radii vectores are drawn. [illustration: fig. .] [illustration: fig. .] it is instructive to see that it does not hold if the blow is any otherwise directed; for instance, as in fig. , when the blow is along _ae_, the body finds itself at _p_ at the end of the second interval, but the area _sap_ is by no means equal to _sab_, and therefore not equal to _soa_, the area swept out in the first interval. in order to modify fig. so as to represent continuous motion and steady forces, we have to take the sides of the polygon _oapq_, &c., very numerous and very small; in the limit, infinitely numerous and infinitely small. the path then becomes a curve, and the series of blows becomes a steady force directed towards _s_. about whatever point therefore the rate of description of areas is uniform, that point and no other must be the centre of all the force there is. if there be no force, as in fig. , well and good, but if there be any force however small not directed towards _s_, then the rate of description of areas about _s_ cannot be uniform. kepler, however, says that the rate of description of areas of each planet about the sun is, by tycho's observations, uniform; hence the sun is the centre of all the force that acts on them, and there is no other force, not even friction. that is the moral of kepler's second law. we may also see from it that gravity does not travel like light, so as to take time on its journey from sun to planet; for, if it did, there would be a sort of aberration, and the force on its arrival could no longer be accurately directed to the centre of the sun. (see _nature_, vol. xlvi., p. .) it is a matter for accuracy of observation, therefore, to decide whether the minutest trace of such deviation can be detected, _i.e._ within what limits of accuracy kepler's second law is now known to be obeyed. i will content myself by saying that the limits are extremely narrow. [reference may be made also to p. .] thus then it became clear to newton that the whole solar system depended on a central force emanating from the sun, and varying inversely with the square of the distance from him: for by that hypothesis all the laws of kepler concerning these motions were completely accounted for; and, in fact, the laws necessitated the hypothesis and established it as a theory. similarly the satellites of jupiter were controlled by a force emanating from jupiter and varying according to the same law. and again our moon must be controlled by a force from the earth, decreasing with the distance according to the same law. grant this hypothetical attracting force pulling the planets towards the sun, pulling the moon towards the earth, and the whole mechanism of the solar system is beautifully explained. if only one could be sure there was such a force! it was one thing to calculate out what the effects of such a force would be: it was another to be able to put one's finger upon it and say, this is the force that actually exists and is known to exist. we must picture him meditating in his garden on this want--an attractive force towards the earth. if only such an attractive force pulling down bodies to the earth existed. an apple falls from a tree. why, it does exist! there is gravitation, common gravity that makes bodies fall and gives them their weight. wanted, a force tending towards the centre of the earth. it is to hand! it is common old gravity that had been known so long, that was perfectly familiar to galileo, and probably to archimedes. gravity that regulates the motion of projectiles. why should it only pull stones and apples? why should it not reach as high as the moon? why should it not be the gravitation of the sun that is the central force acting on all the planets? surely the secret of the universe is discovered! but, wait a bit; is it discovered? is this force of gravity sufficient for the purpose? it must vary inversely with the square of the distance from the centre of the earth. how far is the moon away? sixty earth's radii. hence the force of gravity at the moon's distance can only be / of what it is on the earth's surface. so, instead of pulling it ft. per second, it should pull it / ft. per second, or ft. a minute.[ ] how can one decide whether such a force is able to pull the moon the actual amount required? to newton this would seem only like a sum in arithmetic. out with a pencil and paper and reckon how much the moon falls toward the earth in every second of its motion. is it / ? that is what it ought to be: but is it? the size of the earth comes into the calculation. sixty miles make a degree, degrees a circumference. this gives as the earth's diameter , miles; work it out. the answer is not feet a minute, it is · feet. surely a mistake of calculation? no, it is no mistake: there is something wrong in the theory, gravity is too strong. instead of falling toward the earth - / hundredths of an inch every second, as it would under gravity, the moon only falls - / hundredths of an inch per second. with such a discovery in his grasp at the age of twenty-three he is disappointed--the figures do not agree, and he cannot make them agree. either gravity is not the force in action, or else something interferes with it. possibly, gravity does part of the work, and the vortices of descartes interfere with it. he must abandon the fascinating idea for the time. in his own words, "he laid aside at that time any further thought of the matter." so far as is known, he never mentioned his disappointment to a soul. he might, perhaps, if he had been at cambridge, but he was a shy and solitary youth, and just as likely he might not. up in lincolnshire, in the seventeenth century, who was there for him to consult? true, he might have rushed into premature publication, after our nineteenth century fashion, but that was not his method. publication never seemed to have occurred to him. his reticence now is noteworthy, but later on it is perfectly astonishing. he is so absorbed in making discoveries that he actually has to be reminded to tell any one about them, and some one else always has to see to the printing and publishing for him. i have entered thus fully into what i conjecture to be the stages of this early discovery of the law of gravitation, as applicable to the heavenly bodies, because it is frequently and commonly misunderstood. it is sometimes thought that he discovered the force of gravity; i hope i have made it clear that he did no such thing. every educated man long before his time, if asked why bodies fell, would reply just as glibly as they do now, "because the earth attracts them," or "because of the force of gravity." his discovery was that the motions of the solar system were due to the action of a central force, directed to the body at the centre of the system, and varying inversely with the square of the distance from it. this discovery was based upon kepler's laws, and was clear and certain. it might have been published had he so chosen. but he did not like hypothetical and unknown forces; he tried to see whether the known force of gravity would serve. this discovery at that time he failed to make, owing to a wrong numerical datum. the size of the earth he only knew from the common doctrine of sailors that miles make a degree; and that threw him out. instead of falling feet a minute, as it ought under gravity, it only fell · feet, so he abandoned the idea. we do not find that he returned to it for sixteen years. lecture viii newton and the law of gravitation we left newton at the age of twenty-three on the verge of discovering the mechanism of the solar system, deterred therefrom only by an error in the then imagined size of the earth. he had proved from kepler's laws that a centripetal force directed to the sun, and varying as the inverse square of the distance from that body, would account for the observed planetary motions, and that a similar force directed to the earth would account for the lunar motion; and it had struck him that this force might be the very same as the familiar force of gravitation which gave to bodies their weight: but in attempting a numerical verification of this idea in the case of the moon he was led by the then received notion that sixty miles made a degree on the earth's surface into an erroneous estimate of the size of the moon's orbit. being thus baffled in obtaining such verification, he laid the matter aside for a time. the anecdote of the apple we learn from voltaire, who had it from newton's favourite niece, who with her husband lived and kept house for him all his later life. it is very like one of those anecdotes which are easily invented and believed in, and very often turn out on scrutiny to have no foundation. fortunately this anecdote is well authenticated, and moreover is intrinsically probable; i say fortunately, because it is always painful to have to give up these child-learnt anecdotes, like alfred and the cakes and so on. this anecdote of the apple we need not resign. the tree was blown down in and part of its wood is preserved. i have mentioned voltaire in connection with newton's philosophy. this acute critic at a later stage did a good deal to popularise it throughout europe and to overturn that of his own countryman descartes. cambridge rapidly became newtonian, but oxford remained cartesian for fifty years or more. it is curious what little hold science and mathematics have ever secured in the older and more ecclesiastical university. the pride of possessing newton has however no doubt been the main stimulus to the special pursuits of cambridge. he now began to turn his attention to optics, and, as was usual with him, his whole mind became absorbed in this subject as if nothing else had ever occupied him. his cash-book for this time has been discovered, and the entries show that he is buying prisms and lenses and polishing powder at the beginning of . he was anxious to improve telescopes by making more perfect lenses than had ever been used before. accordingly he calculated out their proper curves, just as descartes had also done, and then proceeded to grind them as near as he could to those figures. but the images did not please him; they were always blurred and rather indistinct. at length, it struck him that perhaps it was not the lenses but the light which was at fault. perhaps light was so composed that it _could_ not be focused accurately to a sharp and definite point. perhaps the law of refraction was not quite accurate, but only an approximation. so he bought a prism to try the law. he let in sunlight through a small round hole in a window shutter, inserted the prism in the light, and received the deflected beam on a white screen; turning the prism about till it was deviated as little as possible. the patch on the screen was not a round disk, as it would have been without the prism, but was an elongated oval and was coloured at its extremities. evidently refraction was not a simple geometrical deflection of a ray, there was a spreading out as well. [illustration: fig. .--a prism not only _deviates_ a beam of sunlight, but also spreads it out or _disperses_ it.] why did the image thus spread out? if it were due to irregularities in the glass a second prism should rather increase them, but a second prism when held in appropriate position was able to neutralise the dispersion and to reproduce the simple round white spot without deviation. evidently the spreading out of the beam was connected in some definite way with its refraction. could it be that the light particles after passing through the prism travelled in variously curved lines, as spinning racquet balls do? to examine this he measured the length of the oval patch when the screen was at different distances from the prism, and found that the two things were directly proportional to each other. doubling the distance of the screen doubled the length of the patch. hence the rays travelled in straight lines from the prism, and the spreading out was due to something that occurred within its substance. could it be that white light was compound, was a mixture of several constituents, and that its different constituents were differently bent? no sooner thought than tried. pierce the screen to let one of the constituents through and interpose a second prism in its path. if the spreading out depended on the prism only it should spread out just as much as before, but if it depended on the complex character of white light, this isolated simple constituent should be able to spread out no more. it did not spread out any more: a prism had no more dispersive power over it; it was deflected by the appropriate amount, but it was not analysed into constituents. it differed from sunlight in being simple. with many ingenious and beautifully simple experiments, which are quoted in full in several books on optics, he clinched the argument and established his discovery. white light was not simple but compound. it could be sorted out by a prism into an infinite number of constituent parts which were differently refracted, and the most striking of which newton named violet, indigo, blue, green, yellow, orange, and red. [illustration: fig. .--a single constituent of white light, obtained by the use of perforated screens is capable of no more dispersion.] at once the true nature of colour became manifest. colour resided not in the coloured object as had till now been thought, but in the light which illuminated it. red glass for instance adds nothing to sunlight. the light does not get dyed red by passing through the glass; all that the red glass does is to stop and absorb a large part of the sunlight; it is opaque to the larger portion, but it is transparent to that particular portion which affects our eyes with the sensation of red. the prism acts like a sieve sorting out the different kinds of light. coloured media act like filters, stopping certain kinds but allowing the rest to go through. leonardo's and all the ancient doctrines of colour had been singularly wrong; colour is not in the object but in the light. goethe, in his _farbenlehre_, endeavoured to controvert newton, and to reinstate something more like the old views; but his failure was complete. refraction analysed out the various constituents of white light and displayed them in the form of a series of overlapping images of the aperture, each of a different colour; this series of images we call a spectrum, and the operation we now call spectrum analysis. the reason of the defect of lenses was now plain: it was not so much a defect of the lens as a defect of light. a lens acts by refraction and brings rays to a focus. if light be simple it acts well, but if ordinary white light fall upon a lens, its different constituents have different foci; every bright object is fringed with colour, and nothing like a clear image can be obtained. [illustration: fig. .--showing the boundary rays of a parallel beam passing through a lens.] a parallel beam passing through a lens becomes conical; but instead of a single cone it is a sheaf or nest of cones, all having the edge of the lens as base, but each having a different vertex. the violet cone is innermost, near the lens, the red cone outermost, while the others lie between. beyond the crossing point or focus the order of cones is reversed, as the above figure shows. only the two marginal rays of the beam are depicted. if a screen be held anywhere nearer the lens than the place marked there will be a whitish centre to the patch of light and a red and orange fringe or border. held anywhere beyond the region , the border of the patch will be blue and violet. held about the colour will be less marked than elsewhere, but nowhere can it be got rid of. each point of an object will be represented in the image not by a point but by a coloured patch: a fact which amply explains the observed blurring and indistinctness. newton measured and calculated the distance between the violet and red foci--vr in the diagram--and showed that it was / th the diameter of the lens. to overcome this difficulty (called chromatic aberration) telescope glasses were made small and of very long focus: some of them so long that they had no tube, all of them egregiously cumbrous. yet it was with such instruments that all the early discoveries were made. with such an instrument, for instance, huyghens discovered the real shape of saturn's ring. the defects of refractors seemed irremediable, being founded in the nature of light itself. so he gave up his "glass works"; and proceeded to think of reflexion from metal specula. a concave mirror forms an image just as a lens does, but since it does so without refraction or transmission through any substance, there is no accompanying dispersion or chromatic aberration. the first reflecting telescope he made was in. diameter and in. long, and magnified forty times. it acted as well as a three or four feet refractor of that day, and showed jupiter's moons. so he made a larger one, now in the library of the royal society, london, with an inscription: "the first reflecting telescope, invented by sir isaac newton, and made with his own hands." this has been the parent of most of the gigantic telescopes of the present day. fifty years elapsed before it was much improved on, and then, first by hadley and afterwards by herschel and others, large and good reflectors were constructed. the largest telescope ever made, that of lord rosse, is a newtonian reflector, fifty feet long, six feet diameter, with a mirror weighing four tons. the sextant, as used by navigators, was also invented by newton. the year after the plague, in , newton returned to trinity college, and there continued his experiments on optics. it is specially to be noted that at this time, at the age of twenty-four, newton had laid the foundations of all his greatest discoveries:-- [illustration: fig. .--newton's telescope.] the theory of fluxions; or, the differential calculus. the law of gravitation; or, the complete theory of astronomy. the compound nature of white light; or, the beginning of spectrum analysis. [illustration: fig. .--the sextant, as now made.] his later life was to be occupied in working these incipient discoveries out. but the most remarkable thing is that no one knew about any one of them. however, he was known as an accomplished young mathematician, and was made a fellow of his college. you remember that he had a friend there in the person of dr. isaac barrow, first lucasian professor of mathematics in the university. it happened, about , that a mathematical discovery of some interest was being much discussed, and dr. barrow happened to mention it to newton, who said yes, he had worked out that and a few other similar things some time ago. he accordingly went and fetched some papers to dr. barrow, who forwarded them to other distinguished mathematicians, and it thus appeared that newton had discovered theorems much more general than this special case that was exciting so much interest. dr. barrow, being anxious to devote his time more particularly to theology, resigned his chair the same year in favour of newton, who was accordingly elected to the lucasian professorship, which he held for thirty years. this chair is now the most famous in the university, and it is commonly referred to as the chair of newton. still, however, his method of fluxions was unknown, and still he did not publish it. he lectured first on optics, giving an account of his experiments. his lectures were afterwards published both in latin and english, and are highly valued to this day. the fame of his mathematical genius came to the ears of the royal society, and a motion was made to get him elected a fellow of that body. the royal society, the oldest and most famous of all scientific societies with a continuous existence, took its origin in some private meetings, got up in london by the hon. robert boyle and a few scientific friends, during all the trouble of the commonwealth. after the restoration, charles ii. in incorporated it under royal charter; among the original members being boyle, hooke, christopher wren, and other less famous names. boyle was a great experimenter, a worthy follower of dr. gilbert. hooke began as his assistant, but being of a most extraordinary ingenuity he rapidly rose so as to exceed his master in importance. fate has been a little unkind to hooke in placing him so near to newton; had he lived in an ordinary age he would undoubtedly have shone as a star of the first magnitude. with great ingenuity, remarkable scientific insight, and consummate experimental skill, he stands in many respects almost on a level with galileo. but it is difficult to see stars even of the first magnitude when the sun is up, and thus it happens that the name and fame of this brilliant man are almost lost in the blaze of newton. of christopher wren i need not say much. he is well known as an architect, but he was a most accomplished all-round man, and had a considerable taste and faculty for science. these then were the luminaries of the royal society at the time we are speaking of, and to them newton's first scientific publication was submitted. he communicated to them an account of his reflecting telescope, and presented them with the instrument. their reception of it surprised him; they were greatly delighted with it, and wrote specially thanking him for the communication, and assuring him that all right should be done him in the matter of the invention. the bishop of salisbury (bishop burnet) proposed him for election as a fellow, and elected he was. in reply, he expressed his surprise at the value they set on the telescope, and offered, if they cared for it, to send them an account of a discovery which he doubts not will prove much more grateful than the communication of that instrument, "being in my judgment the oddest, if not the most considerable detection that has recently been made into the operations of nature." so he tells them about his optical researches and his discovery of the nature of white light, writing them a series of papers which were long afterwards incorporated and published as his _optics_. a magnificent work, which of itself suffices to place its author in the first rank of the world's men of science. the nature of white light, the true doctrine of colour, and the differential calculus! besides a good number of minor results--binomial theorem, reflecting telescope, sextant, and the like; one would think it enough for one man's life-work, but the masterpiece remains still to be mentioned. it is as when one is considering shakspeare: _king lear_, _macbeth_, _othello_,--surely a sufficient achievement,--but the masterpiece remains. comparisons in different departments are but little help perhaps, nevertheless it seems to me that in his own department, and considered simply as a man of science, newton towers head and shoulders over, not only his contemporaries--that is a small matter--but over every other scientific man who has ever lived, in a way that we can find no parallel for in other departments. other nations admit his scientific pre-eminence with as much alacrity as we do. well, we have arrived at the year and his election to the royal society. during the first year of his membership there was read at one of the meetings a paper giving an account of a very careful determination of the length of a degree (_i.e._ of the size of the earth), which had been made by picard near paris. the length of the degree turned out to be not sixty miles, but nearly seventy miles. how soon newton heard of this we do not learn--probably not for some years,--cambridge was not so near london then as it is now, but ultimately it was brought to his notice. armed with this new datum, his old speculation concerning gravity occurred to him. he had worked out the mechanics of the solar system on a certain hypothesis, but it had remained a hypothesis somewhat out of harmony with apparent fact. what if it should turn out to be true after all! he took out his old papers and began again the calculation. if gravity were the force keeping the moon in its orbit, it would fall toward the earth sixteen feet every minute. how far did it fall? the newly known size of the earth would modify the figures: with intense excitement he runs through the working, his mind leaps before his hand, and as he perceives the answer to be coming out right, all the infinite meaning and scope of his mighty discovery flashes upon him, and he can no longer see the paper. he throws down the pen; and the secret of the universe is, to one man, known. but of course it had to be worked out. the meaning might flash upon him, but its full detail required years of elaboration; and deeper and deeper consequences revealed themselves to him as he proceeded. for two years he devoted himself solely to this one object. during those years he lived but to calculate and think, and the most ludicrous stories are told concerning his entire absorption and inattention to ordinary affairs of life. thus, for instance, when getting up in a morning he would sit on the side of the bed half-dressed, and remain like that till dinner time. often he would stay at home for days together, eating what was taken to him, but without apparently noticing what he was doing. one day an intimate friend, dr. stukely, called on him and found on the table a cover laid for his solitary dinner. after waiting a long time, dr. stukely removed the cover and ate the chicken underneath it, replacing and covering up the bones again. at length newton appeared, and after greeting his friend, sat down to dinner, but on lifting the cover he said in surprise, "dear me, i thought i had not dined, but i see i have." it was by this continuous application that the _principia_ was accomplished. probably nothing of the first magnitude can be accomplished without something of the same absorbed unconsciousness and freedom from interruption. but though desirable and essential for the _work_, it was a severe tax upon the powers of the _man_. there is, in fact, no doubt that newton's brain suffered temporary aberration after this effort for a short time. the attack was slight, and it has been denied; but there are letters extant which are inexplicable otherwise, and moreover after a year or two he writes to his friends apologizing for strange and disjointed epistles, which he believed he had written without understanding clearly what he wrote. the derangement was, however, both slight and temporary: and it is only instructive to us as showing at what cost such a work as the _principia_ must be produced, even by so mighty a mind as that of newton. the first part of the work having been done, any ordinary mortal would have proceeded to publish it; but the fact is that after he had sent to the royal society his papers on optics, there had arisen controversies and objections; most of them rather paltry, to which he felt compelled to find answers. many men would have enjoyed this part of the work, and taken it as evidence of interest and success. but to newton's shy and retiring disposition these discussions were merely painful. he writes, indeed, his answers with great patience and ability, and ultimately converts the more reasonable of his opponents, but he relieves his mind in the following letter to the secretary of the royal society: "i see i have made myself a slave to philosophy, but if i get free of this present business i will resolutely bid adieu to it eternally, except what i do for my private satisfaction or leave to come out after me; for i see a man must either resolve to put out nothing new, or to become a slave to defend it." and again in a letter to leibnitz: "i have been so persecuted with discussions arising out of my theory of light that i blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow." this shows how much he cared for contemporary fame. so he locked up the first part of the _principia_ in his desk, doubtless intending it to be published after his death. but fortunately this was not so to be. in , among the leading lights of the royal society, the same sort of notions about gravity and the solar system began independently to be bruited. the theory of gravitation seemed to be in the air, and wren, hooke, and halley had many a talk about it. hooke showed an experiment with a pendulum, which he likened to a planet going round the sun. the analogy is more superficial than real. it does not obey kepler's laws; still it was a striking experiment. they had guessed at a law of inverse squares, and their difficulty was to prove what curve a body subject to it would describe. they knew it ought to be an ellipse if it was to serve to explain the planetary motion, and hooke said he could prove that an ellipse it was; but he was nothing of a mathematician, and the others scarcely believed him. undoubtedly he had shrewd inklings of the truth, though his guesses were based on little else than a most sagacious intuition. he surmised also that gravity was the force concerned, and asserted that the path of an ordinary projectile was an ellipse, like the path of a planet--which is quite right. in fact the beginnings of the discovery were beginning to dawn upon him in the well-known way in which things do dawn upon ordinary men of genius: and had newton not lived we should doubtless, by the labours of a long chain of distinguished men, beginning with hooke, wren, and halley, have been now in possession of all the truths revealed by the _principia_. we should never have had them stated in the same form, nor proved with the same marvellous lucidity and simplicity, but the facts themselves we should by this time have arrived at. their developments and completions, due to such men as clairaut, euler, d'alembert, lagrange, laplace, airy, leverrier, adams, we should of course not have had to the same extent; because the lives and energies of these great men would have been partially consumed in obtaining the main facts themselves. the youngest of the three questioners at the time we are speaking of was edmund halley, an able and remarkable man. he had been at cambridge, doubtless had heard newton lecture, and had acquired a great veneration for him. in january, , we find wren offering hooke and halley a prize, in the shape of a book worth forty shillings, if they would either of them bring him within two months a demonstration that the path of a planet subject to an inverse square law would be an ellipse. not in two months, nor yet in seven, was there any proof forthcoming. so at last, in august, halley went over to cambridge to speak to newton about the difficult problem and secure his aid. arriving at his rooms he went straight to the point. he said, "what path will a body describe if it be attracted by a centre with a force varying as the inverse square of the distance." to which newton at once replied, "an ellipse." "how on earth do you know?" said halley in amazement. "why, i have calculated it," and began hunting about for the paper. he actually couldn't find it just then, but sent it him shortly by post, and with it much more--in fact, what appeared to be a complete treatise on motion in general. with his valuable burden halley hastened to the royal society and told them what he had discovered. the society at his representation wrote to mr. newton asking leave that it might be printed. to this he consented; but the royal society wisely appointed mr. halley to see after him and jog his memory, in case he forgot about it. however, he set to work to polish it up and finish it, and added to it a great number of later developments and embellishments, especially the part concerning the lunar theory, which gave him a deal of trouble--and no wonder; for in the way he has put it there never was a man yet living who could have done the same thing. mathematicians regard the achievement now as men might stare at the work of some demigod of a bygone age, wondering what manner of man this was, able to wield such ponderous implements with such apparent ease. to halley the world owes a great debt of gratitude--first, for discovering the _principia_; second, for seeing it through the press; and third, for defraying the cost of its publication out of his own scanty purse. for though he ultimately suffered no pecuniary loss, rather the contrary, yet there was considerable risk in bringing out a book which not a dozen men living could at the time comprehend. it is no small part of the merit of halley that he recognized the transcendent value of the yet unfinished work, that he brought it to light, and assisted in its becoming understood to the best of his ability. though halley afterwards became astronomer-royal, lived to the ripe old age of eighty-six, and made many striking observations, yet he would be the first to admit that nothing he ever did was at all comparable in importance with his discovery of the _principia_; and he always used to regard his part in it with peculiar pride and pleasure. and how was the _principia_ received? considering the abstruse nature of its subject, it was received with great interest and enthusiasm. in less than twenty years the edition was sold out, and copies fetched large sums. we hear of poor students copying out the whole in manuscript in order to possess a copy--not by any means a bad thing to do, however many copies one may possess. the only useful way really to read a book like that is to pore over every sentence: it is no book to be skimmed. while the _principia_ was preparing for the press a curious incident of contact between english history and the university occurred. it seems that james ii., in his policy of catholicising the country, ordered both universities to elect certain priests to degrees without the ordinary oaths. oxford had given way, and the dean of christ church was a creature of james's choosing. cambridge rebelled, and sent eight of its members, among them mr. newton, to plead their cause before the court of high commission. judge jeffreys presided over the court, and threatened and bullied with his usual insolence. the vice-chancellor of cambridge was deprived of office, the other deputies were silenced and ordered away. from the precincts of this court of justice newton returned to trinity college to complete the _principia_. by this time newton was only forty-five years old, but his main work was done. his method of fluxions was still unpublished; his optics was published only imperfectly; a second edition of the _principia_, with additions and improvements, had yet to appear; but fame had now come upon him, and with fame worries of all kinds. by some fatality, principally no doubt because of the interest they excited, every discovery he published was the signal for an outburst of criticism and sometimes of attack. i shall not go into these matters: they are now trivial enough, but it is necessary to mention them, because to newton they evidently loomed large and terrible, and occasioned him acute torment. [illustration: fig. .--newton when young. (_from an engraving by b. reading after sir peter lely._)] no sooner was the _principia_ put than hooke put in his claims for priority. and indeed his claims were not altogether negligible; for vague ideas of the same sort had been floating in his comprehensive mind, and he doubtless felt indistinctly conscious of a great deal more than he could really state or prove. by indiscreet friends these two great men were set somewhat at loggerheads, and worse might have happened had they not managed to come to close quarters, and correspond privately in a quite friendly manner, instead of acting through the mischievous medium of third parties. in the next edition newton liberally recognizes the claims of both hooke and wren. however, he takes warning betimes of what he has to expect, and writes to halley that he will only publish the first two books, those containing general theorems on motion. the third book--concerning the system of the world, _i.e._ the application to the solar system--he says "i now design to suppress. philosophy is such an impertinently litigious lady that a man had as good be engaged in law-suits as have to do with her. i found it so formerly, and now i am no sooner come near her again but she gives me warning. the two books without the third will not so well bear the title 'mathematical principles of natural philosophy,' and therefore i had altered it to this, 'on the free motion of two bodies'; but on second thoughts i retain the former title: 'twill help the sale of the book--which i ought not to diminish now 'tis yours." however, fortunately, halley was able to prevail upon him to publish the third book also. it is, indeed, the most interesting and popular of the three, as it contains all the direct applications to astronomy of the truths established in the other two. some years later, when his method of fluxions was published, another and a worse controversy arose--this time with leibnitz, who had also independently invented the differential calculus. it was not so well recognized then how frequently it happens that two men independently and unknowingly work at the very same thing at the same time. the history of science is now full of such instances; but then the friends of each accused the other of plagiarism. i will not go into the controversy: it is painful and useless. it only served to embitter the later years of two great men, and it continued long after newton's death--long after both their deaths. it can hardly be called ancient history even now. but fame brought other and less unpleasant distractions than controversies. we are a curious, practical, and rather stupid people, and our one idea of honouring a man is to _vote_ for him in some way or other; so they sent newton to parliament. he went, i believe, as a whig, but it is not recorded that he spoke. it is, in fact, recorded that he was once expected to speak when on a royal commission about some question of chronometers, but that he would not. however, i dare say he made a good average member. then a little later it was realized that newton was poor, that he still had to teach for his livelihood, and that though the crown had continued his fellowship to him as lucasian professor without the necessity of taking orders, yet it was rather disgraceful that he should not be better off. so an appeal was made to the government on his behalf, and lord halifax, who exerted himself strongly in the matter, succeeding to office on the accession of william iii., was able to make him ultimately master of the mint, with a salary of some £ , a year. i believe he made rather a good master, and turned out excellent coins: certainly he devoted his attention to his work there in a most exemplary manner. but what a pitiful business it all is! here is a man sent by heaven to do certain things which no man else could do, and so long as he is comparatively unknown he does them; but so soon as he is found out, he is clapped into a routine office with a big salary: and there is, comparatively speaking, an end of him. it is not to be supposed that he had lost his power, for he frequently solved problems very quickly which had been given out by great continental mathematicians as a challenge to the world. we may ask why newton allowed himself to be thus bandied about instead of settling himself down to the work in which he was so pre-eminently great. well, i expect your truly great man never realizes how great he is, and seldom knows where his real strength lies. certainly newton did not know it. he several times talks of giving up philosophy altogether; and though he never really does it, and perhaps the feeling is one only born of some temporary overwork, yet he does not sacrifice everything else to it as he surely must had he been conscious of his own greatness. no; self-consciousness was the last thing that affected him. it is for a great man's contemporaries to discover him, to make much of him, and to put him in surroundings where he may flourish luxuriantly in his own heaven-intended way. however, it is difficult for us to judge of these things. perhaps if he had been maintained at the national expense to do that for which he was preternaturally fitted, he might have worn himself out prematurely; whereas by giving him routine work the scientific world got the benefit of his matured wisdom and experience. it was no small matter to the young royal society to be able to have him as their president for twenty-four years. his portrait has hung over the president's chair ever since, and there i suppose it will continue to hang until the royal society becomes extinct. the events of his later life i shall pass over lightly. he lived a calm, benevolent life, universally respected and beloved. his silver-white hair when he removed his peruke was a venerable spectacle. a lock of it is still preserved, with many other relics, in the library of trinity college. he died quietly, after a painful illness, at the ripe age of eighty-five. his body lay in state in the jerusalem chamber, and he was buried in westminster abbey, six peers bearing the pall. these things are to be mentioned to the credit of the time and the country; for after we have seen the calamitous spectacle of the way tycho and kepler and galileo were treated by their ungrateful and unworthy countries, it is pleasant to reflect that england, with all its mistakes, yet recognized _her_ great man when she received him, and honoured him with the best she knew how to give. [illustration: fig. .--sir isaac newton.] concerning his character, one need only say that it was what one would expect and wish. it was characterized by a modest, calm, dignified simplicity. he lived frugally with his niece and her husband, mr. conduit, who succeeded him as master of the mint. he never married, nor apparently did he ever think of so doing. the idea, perhaps, did not naturally occur to him, any more than the idea of publishing his work did. he was always a deeply religious man and a sincere christian, though somewhat of the arian or unitarian persuasion--so, at least, it is asserted by orthodox divines who understand these matters. he studied theology more or less all his life, and towards the end was greatly interested in questions of biblical criticism and chronology. by some ancient eclipse or other he altered the recognized system of dates a few hundred years; and his book on the prophecies of daniel and the revelation of st. john, wherein he identifies the beast with the church of rome in quite the orthodox way, is still by some admired. but in all these matters it is probable that he was a merely ordinary man, with natural acumen and ability doubtless, but nothing in the least superhuman. in science, the impression he makes upon me is only expressible by the words inspired, superhuman. and yet if one realizes his method of work, and the calm, uninterrupted flow of all his earlier life, perhaps his achievements become more intelligible. when asked how he made his discoveries, he replied: "by always thinking unto them. i keep the subject constantly before me, and wait till the first dawnings open slowly by little and little into a full and clear light." that is the way--quiet, steady, continuous thinking, uninterrupted and unharassed brooding. much may be done under those conditions. much ought to be sacrificed to obtain those conditions. all the best thinking work of the world has been thus done.[ ] buffon said: "genius is patience." so says newton: "if i have done the public any service this way, it is due to nothing but industry and patient thought." genius patience? no, it is not quite that, or, rather, it is much more than that; but genius without patience is like fire without fuel--it will soon burn itself out. notes for lecture ix the _principia_ published . newton died . the law of gravitation.--every particle of matter attracts every other particle of matter with a force proportional to the mass of each and to the inverse square of the distance between them. some of newton's deductions. . kepler's second law (equable description of areas) proves that each planet is acted on by a force directed towards the sun as a centre of force. . kepler's first law proves that this central force diminishes in the same proportion as the square of the distance increases. . kepler's third law proves that all the planets are acted on by the same kind of force; of an intensity depending on the mass of the sun.[ ] . so by knowing the length of year and distance of any planet from the sun, the sun's mass can be calculated, in terms of that of the earth. . for the satellites, the force acting depends on the mass of _their_ central body, a planet. hence the mass of any planet possessing a satellite becomes known. . the force constraining the moon in her orbit is the same gravity as gives terrestrial bodies their weight and regulates the motion of projectiles. [because, while a stone drops feet in a second, the moon, which is times as far from the centre of the earth, drops feet in a minute.] * * * * * . the moon is attracted not only by the earth, but by the sun also; hence its orbit is perturbed, and newton calculated out the chief of these perturbations, viz.:-- (the equation of the centre, discovered by hipparchus.) (_a_) the evection, discovered by hipparchus and ptolemy. (_b_) the variation, discovered by tycho brahé. (_c_) the annual equation, discovered by tycho brahé. (_d_) the retrogression of the nodes, then being observed at greenwich by flamsteed. (_e_) the variation of inclination, then being observed at greenwich by flamsteed. (_f_) the progression of the apses (with an error of one-half). (_g_) the inequality of apogee, previously unknown. (_h_) the inequality of nodes, previously unknown. . each planet is attracted not only by the sun but by the other planets, hence their orbits are slightly affected by each other. newton began the theory of planetary perturbations. . he recognized the comets as members of the solar system, obedient to the same law of gravity and moving in very elongated ellipses; so their return could be predicted (_e.g._ halley's comet). . applying the idea of centrifugal force to the earth considered as a rotating body, he perceived that it could not be a true sphere, and calculated its oblateness, obtaining miles greater equatorial than polar diameter. . conversely, from the observed shape of jupiter, or any planet, the length of its day could be estimated. . the so-calculated shape of the earth, in combination with centrifugal force, causes the weight of bodies to vary with latitude; and newton calculated the amount of this variation. lbs. at pole balance lbs. at equator. . a homogeneous sphere attracts as if its mass were concentrated at its centre. for any other figure, such as an oblate spheroid, this is not exactly true. a hollow concentric spherical shell exerts no force on small bodies inside it. . the earth's equatorial protuberance, being acted on by the attraction of the sun and moon, must disturb its axis of rotation in a calculated manner; and thus is produced the precession of the equinoxes. [the attraction of the planets on the same protuberance causes a smaller and rather different kind of precession.] . the waters of the ocean are attracted towards the sun and moon on one side, and whirled a little further away than the solid earth on the other side: hence newton explained all the main phenomena of the tides. . the sun's mass being known, he calculated the height of the solar tide. . from the observed heights of spring and neap tides he determined the lunar tide, and thence made an estimate of the mass of the moon. reference table of numerical data. +---------+---------------+----------------------+-----------------+ | |masses in solar| height dropped by a | length of day or| | | system. |stone in first second.|time of rotation.| +---------+---------------+----------------------+-----------------+ |mercury | · | · feet | hours | |venus | · | · " | - / " | |earth | · | · " | " | |mars | · | · " | - / " | |jupiter | · | · " | " | |saturn | · | · " | - / " | |the sun | · | · " | " | |the moon | about · | · " | " | +---------+---------------+----------------------+-----------------+ the mass of the earth, taken above as unity, is , trillion tons. _observatories._--uraniburg flourished from to ; the observatory of paris was founded in ; greenwich observatory in . _astronomers-royal._--flamsteed, halley, bradley, bliss, maskelyne, pond, airy, christie. lecture ix newton's "principia" the law of gravitation, above enunciated, in conjunction with the laws of motion rehearsed at the end of the preliminary notes of lecture vii., now supersedes the laws of kepler and includes them as special cases. the more comprehensive law enables us to criticize kepler's laws from a higher standpoint, to see how far they are exact and how far they are only approximations. they are, in fact, not precisely accurate, but the reason for every discrepancy now becomes abundantly clear, and can be worked out by the theory of gravitation. we may treat kepler's laws either as immediate consequences of the law of gravitation, or as the known facts upon which that law was founded. historically, the latter is the more natural plan, and it is thus that they are treated in the first three statements of the above notes; but each proposition may be worked inversely, and we might state them thus:-- . the fact that the force acting on each planet is directed to the sun, necessitates the equable description of areas. . the fact that the force varies as the inverse square of the distance, necessitates motion in an ellipse, or some other conic section, with the sun in one focus. . the fact that one attracting body acts on all the planets with an inverse square law, causes the cubes of their mean distances to be proportional to the squares of their periodic times. not only these but a multitude of other deductions follow rigorously from the simple datum that every particle of matter attracts every other particle with a force directly proportional to the mass of each and to the inverse square of their mutual distance. those dealt with in the _principia_ are summarized above, and it will be convenient to run over them in order, with the object of giving some idea of the general meaning of each, without attempting anything too intricate to be readily intelligible. [illustration: fig. .] no. . kepler's second law (equable description of areas) proves that each planet is acted on by a force directed towards the sun as a centre of force. the equable description of areas about a centre of force has already been fully, though briefly, established. (p. .) it is undoubtedly of fundamental importance, and is the earliest instance of the serious discussion of central forces, _i.e._ of forces directed always to a fixed centre. we may put it afresh thus:--oa has been the motion of a particle in a unit of time; at a it receives a knock towards c, whereby in the next unit it travels along ad instead of ab. now the area of the triangle cad, swept out by the radius vector in unit time, is / _bh_; _h_ being the perpendicular height of the triangle from the base ac. (fig. .) now the blow at a, being along the base, has no effect upon _h_; and consequently the area remains just what it would have been without the blow. a blow directed to any point other than c would at once alter the area of the triangle. one interesting deduction may at once be drawn. if gravity were a radiant force emitted from the sun with a velocity like that of light, the moving planet would encounter it at a certain apparent angle (aberration), and the force experienced would come from a point a little in advance of the sun. the rate of description of areas would thus tend to increase; whereas in reality it is constant. hence the force of gravity, if it travel at all, does so with a speed far greater than that of light. it appears to be practically instantaneous. (cf. "modern views of electricity," § , end of chap. xii.) again, anything like a retarding effect of the medium through which the planets move would constitute a tangential force, entirely un-directed towards the sun. hence no such frictional or retarding force can appreciably exist. it is, however, conceivable that both these effects might occur and just neutralize each other. the neutralization is unlikely to be exact for all the planets; and the fact is, that no trace of either effect has as yet been discovered. (see also p. .) the planets are, however, subject to forces not directed towards the sun, viz. their attractions for each other; and these perturbing forces do produce a slight discrepancy from kepler's second law, but a discrepancy which is completely subject to calculation. no. . kepler's first law proves that this central force diminishes in the same proportion as the square of the distance increases. to prove the connection between the inverse-square law of distance, and the travelling in a conic section with the centre of force in one focus (the other focus being empty), is not so simple. it obviously involves some geometry, and must therefore be left to properly armed students. but it may be useful to state that the inverse-square law of distance, although the simplest possible law for force emanating from a point or sphere, is not to be regarded as self-evident or as needing no demonstration. the force of a magnetic pole on a magnetized steel scrap, for instance, varies as the inverse cube of the distance; and the curve described by such a particle would be quite different from a conic section--it would be a definite class of spiral (called cotes's spiral). again, on an iron filing the force of a single pole might vary more nearly as the inverse fifth power; and so on. even when the thing concerned is radiant in straight lines, like light, the law of inverse squares is not universally true. its truth assumes, first, that the source is a point or sphere; next, that there is no reflection or refraction of any kind; and lastly, that the medium is perfectly transparent. the law of inverse squares by no means holds from a prairie fire for instance, or from a lighthouse, or from a street lamp in a fog. mutual perturbations, especially the pull of jupiter, prevent the path of a planet from being really and truly an ellipse, or indeed from being any simple re-entrant curve. moreover, when a planet possesses a satellite, it is not the centre of the planet which ever attempts to describe the keplerian ellipse, but it is the common centre of gravity of the two bodies. thus, in the case of the earth and moon, the point which really does describe a close attempt at an ellipse is a point displaced about miles from the centre of the earth towards the moon, and is therefore only miles beneath the surface. no. . kepler's third law proves that all the planets are acted on by the same kind of force; of an intensity depending on the mass of the sun. the third law of kepler, although it requires geometry to state and establish it for elliptic motion (for which it holds just as well as it does for circular motion), is very easy to establish for circular motion, by any one who knows about centrifugal force. if _m_ is the mass of a planet, _v_ its velocity, _r_ the radius of its orbit, and _t_ the time of describing it; [pi]_r_ = _vt_, and the centripetal force needed to hold it in its orbit is mv^ [pi]^ _mr_ -------- or ----------- _r_ t^ now the force of gravitative attraction between the planet and the sun is _vms_ -----, r^ where _v_ is a fixed quantity called the gravitation-constant, to be determined if possible by experiment once for all. now, expressing the fact that the force of gravitation _is_ the force holding the planet in, we write, [pi]^ _mr_ _vms_ ----------- = ---------, t^ r^ whence, by the simplest algebra, r^ _vs_ ------ = ---------. t^ [pi]^ the mass of the planet has been cancelled out; the mass of the sun remains, multiplied by the gravitation-constant, and is seen to be proportional to the cube of the distance divided by the square of the periodic time: a ratio, which is therefore the same for all planets controlled by the sun. hence, knowing _r_ and _t_ for any single planet, the value of _vs_ is known. no. . so by knowing the length of year and distance of any planet from the sun, the sun's mass can be calculated, in terms of that of the earth. no. . for the satellites, the force acting depends on the mass of _their_ central body, a planet. hence the mass of any planet possessing a satellite becomes known. the same argument holds for any other system controlled by a central body--for instance, for the satellites of jupiter; only instead of _s_ it will be natural to write _j_, as meaning the mass of jupiter. hence, knowing _r_ and _t_ for any one satellite of jupiter, the value of _vj_ is known. apply the argument also to the case of moon and earth. knowing the distance and time of revolution of our moon, the value of _ve_ is at once determined; _e_ being the mass of the earth. hence, _s_ and _j_, and in fact the mass of any central body possessing a visible satellite, are now known in terms of _e_, the mass of the earth (or, what is practically the same thing, in terms of _v_, the gravitation-constant). observe that so far none of these quantities are known absolutely. their relative values are known, and are tabulated at the end of the notes above, but the finding of their absolute values is another matter, which we must defer. but, it may be asked, if kepler's third law only gives us the mass of a _central_ body, how is the mass of a _satellite_ to be known? well, it is not easy; the mass of no satellite is known with much accuracy. their mutual perturbations give us some data in the case of the satellites of jupiter; but to our own moon this method is of course inapplicable. our moon perturbs at first sight nothing, and accordingly its mass is not even yet known with exactness. the mass of comets, again, is quite unknown. all that we can be sure of is that they are smaller than a certain limit, else they would perturb the planets they pass near. nothing of this sort has ever been detected. they are themselves perturbed plentifully, but they perturb nothing; hence we learn that their mass is small. the mass of a comet may, indeed, be a few million or even billion tons; but that is quite small in astronomy. but now it may be asked, surely the moon perturbs the earth, swinging it round their common centre of gravity, and really describing its own orbit about this point instead of about the earth's centre? yes, that is so; and a more precise consideration of kepler's third law enables us to make a fair approximation to the position of this common centre of gravity, and thus practically to "weigh the moon," i.e. to compare its mass with that of the earth; for their masses will be inversely as their respective distances from the common centre of gravity or balancing point--on the simple steel-yard principle. hitherto we have not troubled ourselves about the precise point about which the revolution occurs, but kepler's third law is not precisely accurate unless it is attended to. the bigger the revolving body the greater is the discrepancy: and we see in the table preceding lecture iii., on page , that jupiter exhibits an error which, though very slight, is greater than that of any of the other planets, when the sun is considered the fixed centre. let the common centre of gravity of earth and moon be displaced a distance _x_ from the centre of the earth, then the moon's distance from the real centre of revolution is not _r_, but _r-x_; and the equation of centrifugal force to gravitative-attraction is strictly [pi]^ _ve_ --------- (_r-x_) = ------, t^ r^ instead of what is in the text above; and this gives a slightly modified "third law." from this equation, if we have any distinct method of determining _ve_ (and the next section gives such a method), we can calculate _x_ and thus roughly weigh the moon, since _r-x_ e ----- = -----, _r_ e+m but to get anything like a reasonable result the data must be very precise. no. . the force constraining the moon in her orbit is the same gravity as gives terrestrial bodies their weight and regulates the motion of projectiles. here we come to the newtonian verification already several times mentioned; but because of its importance i will repeat it in other words. the hypothesis to be verified is that the force acting on the moon is the same kind of force as acts on bodies we can handle and weigh, and which gives them their weight. now the weight of a mass _m_ is commonly written _mg_, where _g_ is the intensity of terrestrial gravity, a thing easily measured; being, indeed, numerically equal to twice the distance a stone drops in the first second of free fall. [see table p. .] hence, expressing that the weight of a body is due to gravity, and remembering that the centre of the earth's attraction is distant from us by one earth's radius (r), we can write _vm_e _mg_ = ------, r^ or _v_e = gr^ = , cubic miles-per-second per second. but we already know _v_e, in terms of the moon's motion, as [pi]^ r^ ----------- t^ approximately, [more accurately, see preceding note, this quantity is _v_(e + m)]; hence we can easily see if the two determinations of this quantity agree.[ ] all these deductions are fundamental, and may be considered as the foundation of the _principia_. it was these that flashed upon newton during that moment of excitement when he learned the real size of the earth, and discovered his speculations to be true. the next are elaborations and amplifications of the theory, such as in ordinary times are left for subsequent generations of theorists to discover and work out. newton did not work out these remoter consequences of his theory completely by any means: the astronomical and mathematical world has been working them out ever since; but he carried the theory a great way, and here it is that his marvellous power is most conspicuous. it is his treatment of no. , the perturbations of the moon, that perhaps most especially has struck all future mathematicians with amazement. no. , no. , no. , these are the most inspired of the whole. no. . the moon is attracted not only by the earth, but by the sun also; hence its orbit is perturbed, and newton calculated out the chief of these perturbations. now running through the perturbations (p. ) in order:--the first is in parenthesis, because it is mere excentricity. it is not a true perturbation at all, and more properly belongs to kepler. (_a_) the first true perturbation is what ptolemy called "the evection," the principal part of which is a periodic change in the ellipticity or excentricity of the moon's orbit, owing to the pull of the sun. it is a complicated matter, and newton only partially solved it. i shall not attempt to give an account of it. (_b_) the next, "the variation," is a much simpler affair. it is caused by the fact that as the moon revolves round the earth it is half the time nearer to the sun than the earth is, and so gets pulled more than the average, while for the other fortnight it is further from the sun than the earth is, and so gets pulled less. for the week during which it is changing from a decreasing half to a new moon it is moving in the direction of the extra pull, and hence becomes new sooner than would have been expected. all next week it is moving against the same extra pull, and so arrives at quadrature (half moon) somewhat late. for the next fortnight it is in the region of too little pull, the earth gets pulled more than it does; the effect of this is to hurry it up for the third week, so that the full moon occurs a little early, and to retard it for the fourth week, so that the decreasing half moon like the increasing half occurs behind time again. thus each syzygy (as new and full are technically called) is too early; each quadrature is too late; the maximum hurrying and slackening force being felt at the octants, or intermediate ° points. (_c_) the "annual equation" is a fluctuation introduced into the other perturbations by reason of the varying distance of the disturbing body, the sun, at different seasons of the year. its magnitude plainly depends simply on the excentricity of the earth's orbit. both these perturbations, (_b_) and (_c_), newton worked out completely. (_d_) and (_e_) next come the retrogression of the nodes and the variation of the inclination, which at the time were being observed at greenwich by flamsteed, from whom newton frequently, but vainly, begged for data that he might complete their theory while he had his mind upon it. fortunately, halley succeeded flamsteed as astronomer-royal [see list at end of notes above], and then newton would have no difficulty in gaining such information as the national observatory could give. the "inclination" meant is the angle between the plane of the moon's orbit and that of the earth. the plane of the earth's orbit round the sun is called the ecliptic; the plane of the moon's orbit round the earth is inclined to it at a certain angle, which is slowly changing, though in a periodic manner. imagine a curtain ring bisected by a sheet of paper, and tilted to a certain angle; it may be likened to the moon's orbit, cutting the plane of the ecliptic. the two points at which the plane is cut by the ring are called "nodes"; and these nodes are not stationary, but are slowly regressing, _i.e._ travelling in a direction opposite to that of the moon itself. also the angle of tilt is varying slowly, oscillating up and down in the course of centuries. (_f_) the two points in the moon's elliptic orbit where it comes nearest to or farthest from the earth, _i.e._ the points at the extremity of the long axis of the ellipse, are called separately perigee and apogee, or together "the apses." now the pull of the sun causes the whole orbit to slowly revolve in its own plane, and consequently these apses "progress," so that the true path is not quite a closed curve, but a sort of spiral with elliptic loops. but here comes in a striking circumstance. newton states with reference to this perturbation that theory only accounts for - / ° per annum, whereas observation gives °, or just twice as much. this is published in the _principia_ as a fact, without comment. it was for long regarded as a very curious thing, and many great mathematicians afterwards tried to find an error in the working. d'alembert, clairaut, and others attacked the problem, but were led to just the same result. it constituted the great outstanding difficulty in the way of accepting the theory of gravitation. it was suggested that perhaps the inverse square law was only a first approximation; that perhaps a more complete expression, such as a b ---- + -----, r^ r^ must be given for it; and so on. ultimately, clairaut took into account a whole series of neglected terms, and it came out correct; thus verifying the theory. but the strangest part of this tale is to come. for only a few years ago, prof. adams, of cambridge (neptune adams, as he is called), was editing various old papers of newton's, now in the possession of the duke of portland, and he found manuscripts bearing on this very point, and discovered that newton had reworked out the calculations himself, had found the cause of the error, had taken into account the terms hitherto neglected, and so, fifty years before clairaut, had completely, though not publicly, solved this long outstanding problem of the progression of the apses. (_g_) and (_h_) two other inequalities he calculated out and predicted, viz. variation in the motions of the apses and the nodes. neither of these had then been observed, but they were afterwards detected and verified. a good many other minor irregularities are now known--some thirty, i believe; and altogether the lunar theory, or problem of the moon's exact motion, is one of the most complicated and difficult in astronomy; the perturbations being so numerous and large, because of the enormous mass of the perturbing body. the disturbances experienced by the planets are much smaller, because they are controlled by the sun and perturbed by each other. the moon is controlled only by the earth, and perturbed by the sun. planetary perturbations can be treated as a series of disturbances with some satisfaction: not so those of the moon. and yet it is the only way at present known of dealing with the lunar theory. to deal with it satisfactorily would demand the solution of such a problem as this:--given three rigid spherical masses thrown into empty space with any initial motions whatever, and abandoned to gravity: to determine their subsequent motions. with two masses the problem is simple enough, being pretty well summed up in kepler's laws; but with three masses, strange to say, it is so complicated as to be beyond the reach of even modern mathematics. it is a famous problem, known as that of "the three bodies," but it has not yet been solved. even when it is solved it will be only a close approximation to the case of earth, moon, and sun, for these bodies are not spherical, and are not rigid. one may imagine how absurdly and hopelessly complicated a complete treatment of the motions of the entire solar system would be. no. . each planet is attracted not only by the sun but by the other planets, hence their orbits are slightly affected by each other. the subject of planetary perturbation was only just begun by newton. gradually (by laplace and others) the theory became highly developed; and, as everybody knows, in neptune was discovered by means of it. no. . he recognized the comets as members of the solar system, obedient to the same law of gravity and moving in very elongated ellipses; so their return could be predicted. it was a long time before newton recognized the comets as real members of the solar system, and subject to gravity like the rest. he at first thought they moved in straight lines. it was only in the second edition of the _principia_ that the theory of comets was introduced. halley observed a fine comet in , and calculated its orbit on newtonian principles. he also calculated when it ought to have been seen in past times; and he found the year , when one was seen by kepler; also the year , when one was seen by appian; again, he reckoned , , . all these appearances were the same comet, in all probability, returning every seventy-five or seventy-six years. the period was easily allowed to be not exact, because of perturbing planets. he then predicted its return for , or perhaps , a date he could not himself hope to see. he lived to a great age, but he died sixteen years before this date. as the time drew nigh, three-quarters of a century afterwards, astronomers were greatly interested in this first cometary prediction, and kept an eager look-out for "halley's comet." clairaut, a most eminent mathematician and student of newton, proceeded to calculate out more exactly the perturbing influence of jupiter, near which it had passed. after immense labour (for the difficulty of the calculation was extreme, and the mass of mere figures something portentous), he predicted its return on the th of april, , but he considered that he might have made a possible error of a month. it returned on the th of march, , and established beyond all doubt the rule of the newtonian theory over comets. [illustration: fig. .--well-known model exhibiting the oblate spheroidal form as a consequence of spinning about a central axis. the brass strip _a_ looks like a transparent globe when whirled, and bulges out equatorially.] no. . applying the idea of centrifugal force to the earth considered as a rotating body, he perceived that it could not be a true sphere, and calculated its oblateness, obtaining miles greater equatorial than polar diameter. here we return to one of the more simple deductions. a spinning body of any kind tends to swell at its circumference (or equator), and shrink along its axis (or poles). if the body is of yielding material, its shape must alter under the influence of centrifugal force; and if a globe of yielding substance subject to known forces rotates at a definite pace, its shape can be calculated. thus a plastic sphere the size of the earth, held together by its own gravity, and rotating once a day, can be shown to have its equatorial diameter twenty-eight miles greater than its polar diameter: the two diameters being , and , respectively. now we have no guarantee that the earth is of yielding material: for all newton could tell it might be extremely rigid. as a matter of fact it is now very nearly rigid. but he argued thus. the water on it is certainly yielding, and although the solid earth might decline to bulge at the equator in deference to the diurnal rotation, that would not prevent the ocean from flowing from the poles to the equator and piling itself up as an equatorial ocean fourteen miles deep, leaving dry land everywhere near either pole. nothing of this sort is observed: the distribution of land and water is not thus regulated. hence, whatever the earth may be now, it must once have been plastic enough to accommodate itself perfectly to the centrifugal forces, and to take the shape appropriate to a perfectly plastic body. in all probability it was once molten, and for long afterwards pasty. thus, then, the shape of the earth can be calculated from the length of its day and the intensity of its gravity. the calculation is not difficult: it consists in imagining a couple of holes bored to the centre of the earth, one from a pole and one from the equator; filling these both with water, and calculating how much higher the water will stand in one leg of the gigantic v tube so formed than in the other. the answer comes out about fourteen miles. the shape of the earth can now be observed geodetically, and it accords with calculation, but the observations are extremely delicate; in newton's time the _size_ was only barely known, the _shape_ was not observed till long after; but on the principles of mechanics, combined with a little common-sense reasoning, it could be calculated with certainty and accuracy. no. . from the observed shape of jupiter or any planet the length of its day could be estimated. jupiter is much more oblate than the earth. its two diameters are to one another as is to ; the ellipticity of its disk is manifest to simple inspection. hence we perceive that its whirling action must be more violent--it must rotate quicker. as a matter of fact its day is ten [illustration: fig. .--jupiter.] hours long--five hours daylight and five hours night. the times of rotation of other bodies in the solar system are recorded in a table above. no. . the so-calculated shape of the earth, in combination with centrifugal force, causes the weight of bodies to vary with latitude; and newton calculated the amount of this variation. lbs. at pole balance lbs. at equator. but following from the calculated shape of the earth follow several interesting consequences. first of all, the intensity of gravity will not be the same everywhere; for at the equator a stone is further from the average bulk of the earth (say the centre) than it is at the poles, and owing to this fact a mass of pounds at the pole; would suffice to balance pounds at the equator, if the two could be placed in the pans of a gigantic balance whose beam straddled along an earth's quadrant. this is a _true_ variation of gravity due to the shape of the earth. but besides this there is a still larger _apparent_ variation due to centrifugal force, which affects all bodies at the equator but not those at the poles. from this cause, even if the earth were a true sphere, yet if it were spinning at its actual pace, pounds at the pole could balance pounds at the equator; because at the equator the true weight of the mass would not be fully appreciated, centrifugal force would virtually diminish it by / th of its amount. in actual fact both causes co-exist, and accordingly the total variation of gravity observed is compounded of the real and the apparent effects; the result is that pounds at a pole weighs as much as pounds at the equator. no. . a homogeneous sphere attracts as if its mass were concentrated at its centre. for any other figure, such as an oblate spheroid, this is not exactly true. a hollow concentric spherical shell exerts no force on small bodies inside it. a sphere composed of uniform material, or of materials arranged in concentric strata, can be shown to attract external bodies as if its mass were concentrated at its centre. a hollow sphere, similarly composed, does the same, but on internal bodies it exerts no force at all. hence, at all distances above the surface of the earth, gravity decreases in inverse proportion as the square of the distance from the centre of the earth increases; but, if you descend a mine, gravity decreases in this case also as you leave the surface, though not at the same rate as when you went up. for as you penetrate the crust you get inside a concentric shell, which is thus powerless to act upon you, and the earth you are now outside is a smaller one. at what rate the force decreases depends on the distribution of density; if the density were uniform all through, the law of variation would be the direct distance, otherwise it would be more complicated. anyhow, the intensity of gravity is a maximum at the surface of the earth, and decreases as you travel from the surface either up or down. no. . the earth's equatorial protuberance, being acted on by the attraction of the sun and moon, must disturb its axis of rotation in a calculated manner; and thus is produced the precession of the equinoxes. here we come to a truly awful piece of reasoning. a sphere attracts as if its mass were concentrated at its centre (no. ), but a spheroid does not. the earth is a spheroid, and hence it pulls and is pulled by the moon with a slightly uncentric attraction. in other words, the line of pull does not pass through its precise centre. now when we have a spinning body, say a top, overloaded on one side so that gravity acts on it unsymmetrically, what happens? the axis of rotation begins to rotate cone-wise, at a pace which depends on the rate of spin, and on the shape and mass of the top, as well as on the amount and leverage of the overloading. newton calculated out the rapidity of this conical motion of the axis of the earth, produced by the slightly unsymmetrical pull of the moon, and found that it would complete a revolution in , years--precisely what was wanted to explain the precession of the equinoxes. in fact he had discovered the physical cause of that precession. observe that there were three stages in this discovery of precession:-- first, the observation by hipparchus, that the nodes, or intersections of the earth's orbit (the sun's apparent orbit) with the plane of the equator, were not stationary, but slowly moved. second, the description of this motion by copernicus, by the statement that it was due to a conical motion of the earth's axis of rotation about its centre as a fixed point. third, the explanation of this motion by newton as due to the pull of the moon on the equatorial protuberance of the earth. the explanation _could_ not have been previously suspected, for the shape of the earth, on which the whole theory depends, was entirely unknown till newton calculated it. another and smaller motion of a somewhat similar kind has been worked out since: it is due to the unsymmetrical attraction of the other planets for this same equatorial protuberance. it shows itself as a periodic change in the obliquity of the ecliptic, or so-called recession of the apses, rather than as a motion of the nodes.[ ] no. . the waters of the ocean are attracted towards the sun and moon on one side, and whirled a little farther away than the solid earth on the other side: hence newton explained all the main phenomena of the tides. and now comes another tremendous generalization. the tides had long been an utter mystery. kepler likens the earth to an animal, and the tides to his breathings and inbreathings, and says they follow the moon. galileo chaffs him for this, and says that it is mere superstition to connect the moon with the tides. descartes said the moon pressed down upon the waters by the centrifugal force of its vortex, and so produced a low tide under it. everything was fog and darkness on the subject. the legend goes that an astronomer threw himself into the sea in despair of ever being able to explain the flux and reflux of its waters. newton now with consummate skill applied his theory to the effect of the moon upon the ocean, and all the main details of tidal action gradually revealed themselves to him. he treated the water, rotating with the earth once a day, somewhat as if it were a satellite acted on by perturbing forces. the moon as it revolves round the earth is perturbed by the sun. the ocean as it revolves round the earth (being held on by gravitation just as the moon is) is perturbed by both sun and moon. the perturbing effect of a body varies directly as its mass, and inversely as the cube of its distance. (the simple law of inverse square does not apply, because a perturbation is a differential effect: the satellite or ocean when nearer to the perturbing body than the rest of the earth, is attracted more, and when further off it is attracted less than is the main body of the earth; and it is these differences alone which constitute the perturbation.) the moon is the more powerful of the two perturbing bodies, hence the main tides are due to the moon; and its chief action is to cause a pair of low waves or oceanic humps, of gigantic area, to travel round the earth once in a lunar day, _i.e._ in about hours and minutes. the sun makes a similar but still lower pair of low elevations to travel round once in a solar day of hours. and the combination of the two pairs of humps, thus periodically overtaking each other, accounts for the well-known spring and neap tides,--spring tides when their maxima agree, neap tides when the maximum of one coincides with the minimum of the other: each of which events happens regularly once a fortnight. these are the main effects, but besides these there are the effects of varying distances and obliquity to be taken into account; and so we have a whole series of minor disturbances, very like those discussed in no. , under the lunar theory, but more complex still, because there are two perturbing bodies instead of only one. the subject of the tides is, therefore, very recondite; and though one may give some elementary account of its main features, it will be best to defer this to a separate lecture (lecture xvii). i had better, however, here say that newton did not limit himself to the consideration of the primary oceanic humps: he pursued the subject into geographical detail. he pointed out that, although the rise and fall of the tide at mid-ocean islands would be but small, yet on stretches of coast the wave would fling itself, and by its momentum would propel the waters, to a much greater height--for instance, or feet; especially in some funnel-shaped openings like the bristol channel and the bay of fundy, where the concentrated impetus of the water is enormous. he also showed how the tidal waves reached different stations in successive regular order each day; and how some places might be fed with tide by two distinct channels; and that if the time of these channels happened to differ by six hours, a high tide might be arriving by one channel and a low tide by the other, so that the place would only feel the difference, and so have a very small observed rise and fall; instancing a port in china (in the gulf of tonquin) where that approximately occurs. in fact, although his theory was not, as we now know, complete or final, yet it satisfactorily explained a mass of intricate detail as well as the main features of the tides. no. . the sun's mass being known, he calculated the height of the solar tide. no. . from the observed heights of spring and neap tides he determined the lunar tide, and thence made an estimate of the mass of the moon. knowing the sun's mass and distance, it was not difficult for newton to calculate the height of the protuberance caused by it in a pasty ocean covering the whole earth. i say pasty, because, if there was any tendency for impulses to accumulate, as timely pushes given to a pendulum accumulate, the amount of disturbance might become excessive, and its calculation would involve a multitude of data. the newtonian tide ignored this, thus practically treating the motion as either dead-beat, or else the impulses as very inadequately timed. with this reservation the mid-ocean tide due to the action of the sun alone comes out about one foot, or let us say one foot for simplicity. now the actual tide observed in mid-atlantic is at the springs about four feet, at the neaps about two. the spring tide is lunar plus solar; the neap tide is lunar minus solar. hence it appears that the tide caused by the moon alone must be about three feet, when unaffected by momentum. from this datum newton made the first attempt to approximately estimate the mass of the moon. i said that the masses of satellites must be estimated, if at all, by the perturbation they are able to cause. the lunar tide is a perturbation in the diurnal motion of the sea, and its amount is therefore a legitimate mode of calculating the moon's mass. the available data were not at all good, however; nor are they even now very perfect; and so the estimate was a good way out. it is now considered that the mass of the moon is about one-eightieth that of the earth. * * * * * such are some of the gems extracted from their setting in the _principia_, and presented as clearly as i am able before you. do you realize the tremendous stride in knowledge--not a stride, as whewell says, nor yet a leap, but a flight--which has occurred between the dim gropings of kepler, the elementary truths of galileo, the fascinating but wild speculations of descartes, and this magnificent and comprehensive system of ordered knowledge. to some his genius seemed almost divine. "does mr. newton eat, drink, sleep, like other men?" said the marquis de l'hôpital, a french mathematician of no mean eminence; "i picture him to myself as a celestial genius, entirely removed from the restrictions of ordinary matter." to many it seemed as if there was nothing more to be discovered, as if the universe were now explored, and only a few fragments of truth remained for the gleaner. this is the attitude of mind expressed in pope's famous epigram:-- "nature and nature's laws lay hid in night, god said, let newton be, and all was light." this feeling of hopelessness and impotence was very natural after the advent of so overpowering a genius, and it prevailed in england for fully a century. it was very natural, but it was very mischievous; for, as a consequence, nothing of great moment was done by england in science, and no englishman of the first magnitude appeared, till some who are either living now or who have lived within the present century. it appeared to his contemporaries as if he had almost exhausted the possibility of discovery; but did it so appear to newton? did it seem to him as if he had seen far and deep into the truths of this great and infinite universe? it did not. when quite an old man, full of honour and renown, venerated, almost worshipped, by his contemporaries, these were his words:-- "i know not what the world will think of my labours, but to myself it seems that i have been but as a child playing on the sea-shore; now finding some pebble rather more polished, and now some shell rather more agreeably variegated than another, while the immense ocean of truth extended itself unexplored before me." and so it must ever seem to the wisest and greatest of men when brought into contact with the great things of god--that which they know is as nothing, and less than nothing, to the infinitude of which they are ignorant. newton's words sound like a simple and pleasing echo of the words of that great unknown poet, the writer of the book of job:-- "lo, these are parts of his ways, but how little a portion is heard of him; the thunder of his power, who can understand?" end of part i. part ii _a couple of centuries' progress._ notes to lecture x _science during the century after newton_ the _principia_ published, roemer - james bradley - clairaut - euler - d'alembert - lagrange - laplace - william herschel - _olaus roemer_ was born in jutland, and studied at copenhagen. assisted picard in to determine the exact position of tycho's observatory on huen. accompanied picard to paris, and in read before the academy his paper "on successive propagation of light as revealed by a certain inequality in the motion of jupiter's first satellite." in he returned to copenhagen as professor of mathematics and astronomy, and died in . he invented the transit instrument, mural circle, equatorial mounting for telescopes, and most of the other principal instruments now in use in observatories. he made as many observations as tycho brahé, but the records of all but the work of three days were destroyed by a great fire in . _bradley_, professor of astronomy at oxford, discovered the aberration of light in , while examining stars for parallax, and the nutation of the earth's axis in . was appointed astronomer-royal in . lecture x roemer and bradley and the velocity of light at newton's death england stood pre-eminent among the nations of europe in the sphere of science. but the pre-eminence did not last long. two great discoveries were made very soon after his decease, both by professor bradley, of oxford, and then there came a gap. a moderately great man often leaves behind him a school of disciples able to work according to their master's methods, and with a healthy spirit of rivalry which stimulates and encourages them. newton left, indeed, a school of disciples, but his methods of work were largely unknown to them, and such as were known were too ponderous to be used by ordinary men. only one fresh result, and that a small one, has ever been attained by other men working according to the methods of the _principia_. the methods were studied and commented on in england to the exclusion of all others for nigh a century, and as a consequence no really important work was done. on the continent, however, no such system of slavish imitation prevailed. those methods of newton's which had been simultaneously discovered by leibnitz were more thoroughly grasped, modified, extended, and improved. there arose a great school of french and german mathematicians, and the laurels of scientific discovery passed to france and germany--more especially, perhaps, at this time to france. england has never wholly recovered them. during the present century this country has been favoured with some giants who, as they become distant enough for their true magnitude to be perceived, may possibly stand out as great as any who have ever lived; but for the mass and bulk of scientific work at the present day we have to look to germany, with its enlightened government and extensive intellectual development. england, however, is waking up, and what its government does not do, private enterprise is beginning to accomplish. the establishment of centres of scientific and literary activity in the great towns of england, though at present they are partially encumbered with the supply of education of an exceedingly rudimentary type, is a movement that in the course of another century or so will be seen to be one of the most important and fruitful steps ever taken by this country. on the continent such centres have long existed; almost every large town is the seat of a university, and they are now liberally endowed. the university of bologna (where, you may remember, copernicus learnt mathematics) has recently celebrated its th anniversary. the scientific history of the century after newton, summarized in the above table of dates, embraces the labours of the great mathematicians clairaut, euler, d'alembert, and especially of lagrange and laplace. but the main work of all these men was hardly pioneering work. it was rather the surveying, and mapping out, and bringing into cultivation, of lands already discovered. probably herschel may be justly regarded as the next true pioneer. we shall not, however, properly appreciate the stages through which astronomy has passed, nor shall we be prepared adequately to welcome the discoveries of modern times unless we pay some attention to the intervening age. moreover, during this era several facts of great moment gradually came into recognition; and the importance of the discovery we have now to speak of can hardly be over-estimated. our whole direct knowledge of the planetary and stellar universe, from the early observations of the ancients down to the magnificent discoveries of a herschel, depends entirely upon our happening to possess a sense of sight. to no other of our senses do any other worlds than our own in the slightest degree appeal. we touch them or hear them never. consequently, if the human race had happened to be blind, no other world but the one it groped its way upon could ever have been known or imagined by it. the outside universe would have existed, but man would have been entirely and hopelessly ignorant of it. the bare idea of an outside universe beyond the world would have been inconceivable, and might have been scouted as absurd. we do possess the sense of sight; but is it to be supposed that we possess every sense that can be possessed by finite beings? there is not the least ground for such an assumption. it is easy to imagine a deaf race or a blind race: it is not so easy to imagine a race more highly endowed with senses than our own; and yet the sense of smell in animals may give us some aid in thinking of powers of perception which transcend our own in particular directions. if there were a race with higher or other senses than our own, or if the human race should ever in the process of development acquire such extra sense-organs, a whole universe of existent fact might become for the first time perceived by us, and we should look back upon our past state as upon a blind chrysalid form of existence in which we had been unconscious of all this new wealth of perception. it cannot be too clearly and strongly insisted on and brought home to every mind, that the mode in which the universe strikes us, our view of the universe, our whole idea of matter, and force, and other worlds, and even of consciousness, depends upon the particular set of sense-organs with which we, as men, happen to be endowed. the senses of force, of motion, of sound, of light, of touch, of heat, of taste, and of smell--these we have, and these are the things we primarily know. all else is inference founded upon these sensations. so the world appears to us. but given other sense-organs, and it might appear quite otherwise. what it is actually and truly like, therefore, is quite and for ever beyond us--so long as we are finite beings. without eyes, astronomy would be non-existent. light it is which conveys all the information we possess, or, as it would seem, ever can possess, concerning the outer and greater universe in which this small world forms a speck. light is the channel, the messenger of information; our eyes, aided by telescopes, spectroscopes, and many other "scopes" that may yet be invented, are the means by which we read the information that light brings. light travels from the stars to our eyes: does it come instantaneously? or does it loiter by the way? for if it lingers it is not bringing us information properly up to date--it is only telling us what the state of affairs was when it started on its long journey. now, it is evidently a matter of interest to us whether we see the sun as he is now, or only as he was some three hundred years ago. if the information came by express train it would be three hundred years behind date, and the sun might have gone out in the reign of queen anne without our being as yet any the wiser. the question, therefore, "at what rate does our messenger travel?" is evidently one of great interest for astronomers, and many have been the attempts made to solve it. very likely the ancient greeks pondered over this question, but the earliest writer known to me who seriously discussed the question is galileo. he suggests a rough experimental means of attacking it. first of all, it plainly comes quicker than sound. this can be perceived by merely watching distant hammering, or by noticing that the flash of a pistol is seen before its report is heard, or by listening to the noise of a flash of lightning. sound takes five seconds to travel a mile--it has about the same speed as a rifle bullet; but light is much quicker than that. the rude experiment suggested by galileo was to send two men with lanterns and screens to two distant watch-towers or neighbouring mountain tops, and to arrange that each was to watch alternate displays and obscurations of the light made by the other, and to imitate them as promptly as possible. either man, therefore, on obscuring or showing his own light would see the distant glimmer do the same, and would be able to judge if there was any appreciable interval between his own action and the response of the distant light. the experiment was actually tried by the florentine academicians,[ ] with the result that, as practice improved, the interval became shorter and shorter, so that there was no reason to suppose that there was any real interval at all. light, in fact, seemed to travel instantaneously. well might they have arrived at this result. even if they had made far more perfect arrangements--for instance, by arranging a looking-glass at one of the stations in which a distant observer might see the reflection of his own lantern, and watch the obscurations and flashings made by himself, without having to depend on the response of human mechanism--even then no interval whatever could have been detected. if, by some impossibly perfect optical arrangement, a lighthouse here were made visible to us after reflection in a mirror erected at new york, so that the light would have to travel across the atlantic and back before it could be seen, even then the appearance of the light on removing a shutter, or the eclipse on interposing it, would seem to happen quite instantaneously. there would certainly be an interval: the interval would be the fiftieth part of a second (the time a stone takes to drop / th of an inch), but that is too short to be securely detected without mechanism. with mechanism the thing might be managed, for a series of shutters might be arranged like the teeth of a large wheel; so that, when the wheel rotates, eclipses follow one another very rapidly; if then an eye looked through the same opening as that by which the light goes on its way to the distant mirror, a tooth might have moved sufficiently to cover up this space by the time the light returned; in which case the whole would appear dark, for the light would be stopped by a tooth, either at starting or at returning, continually. at higher speeds of rotation some light would reappear, and at lower speeds it would also reappear; by noticing, therefore, the precise speed at which there was constant eclipse the velocity of light could be determined. [illustration: fig. .--diagram of eye looking at a light reflected in a distant mirror through the teeth of a revolving wheel.] this experiment has now been made in a highly refined form by fizeau, and repeated by m. cornu with prodigious care and accuracy. but with these recent matters we have no concern at present. it may be instructive to say, however, that if the light had to travel two miles altogether, the wheel would have to possess teeth and to spin times a second (at the risk of flying to pieces) in order that the ray starting through any one of the gaps might be stopped on returning by the adjacent tooth. well might the velocity of light be called instantaneous by the early observers. an ordinary experiment seemed (and was) hopeless, and light was supposed to travel at an infinite speed. but a phenomenon was noticed in the heavens by a quick-witted and ingenious danish astronomer, which was not susceptible of any ordinary explanation, and which he perceived could at once be explained if light had a certain rate of travel--great, indeed, but something short of infinite. this phenomenon was connected with the satellites of jupiter, and the astronomer's name was roemer. i will speak first of the observation and then of the man. [illustration: fig. .--fizeau's wheel, shewing the appearance of distant image seen through its teeth. st, when stationary, next when revolving at a moderate speed, last when revolving at the high speed just sufficient to cause eclipse.] jupiter's satellites are visible, precisely as our own moon is, by reason of the shimmer of sunlight which they reflect. but as they revolve round their great planet they plunge into his shadow at one part of their course, and so become eclipsed from sunshine and invisible to us. the moment of disappearance can be sharply observed. take the first satellite as an example. the interval between successive eclipses ought to be its period of revolution round jupiter. observe this period. it was not uniform. on the average it was hours minutes, but it seemed to depend on the time of year. when roemer observed in spring it was less, and in autumn it was more than usual. this was evidently a puzzling fact: what on earth can our year have to do with the motion of a moon of jupiter's? it was probably, therefore, only an apparent change, caused either by our greater or less distance from jupiter, or else by our greater or less speed of travelling to or from him. considering it thus, he was led to see that, when the time of revolution seemed longest, we were receding fastest from jupiter, and when shortest, approaching fastest. _if_, then, light took time on its journey, _if_ it travelled progressively, the whole anomaly would be explained. in a second the earth goes nineteen miles; therefore in - / hours (the time of revolution of jupiter's first satellite) it goes · million (say three million) miles. the eclipse happens punctually, but we do not see it till the light conveying the information has travelled the extra three million miles and caught up the earth. evidently, therefore, by observing how much the apparent time of revolution is lengthened in one part of the earth's orbit and shortened in another, getting all the data accurately, and assuming the truth of our hypothetical explanation, we can calculate the velocity of light. this is what roemer did. now the maximum amount of retardation is just about fifteen seconds. hence light takes this time to travel three million miles; therefore its velocity is three million divided by fifteen, say , , or, as we now know more exactly, , miles every second. note that the delay does not depend on our _distance_, but on our _speed_. one can tell this by common-sense as soon as we grasp the general idea of the explanation. a velocity cannot possibly depend on a distance only. [illustration: fig. .--eclipses of one of jupiter's satellites. a diagram intended to illustrate the dependence of its apparent time of revolution (from eclipse to eclipse) on the motion of the earth; but not illustrating the matter at all well. tt' t'' are successive positions of the earth, while jj' j'' are corresponding positions of jupiter.] roemer's explanation of the anomaly was not accepted by astronomers. it excited some attention, and was discussed, but it was found not obviously applicable to any of the satellites except the first, and not very simply and satisfactorily even to that. i have, of course, given you the theory in its most elementary and simple form. in actual fact a host of disturbing and complicated considerations come in--not so violently disturbing for the first satellite as for the others, because it moves so quickly, but still complicated enough. the fact is, the real motion of jupiter's satellites is a most difficult problem. the motion even of our own moon (the lunar theory) is difficult enough: perturbed as its motion is by the sun. you know that newton said it cost him more labour than all the rest of the _principia_. but the motion of jupiter's satellites is far worse. no one, in fact, has yet worked their theory completely out. they are perturbed by the sun, of course, but they also perturb each other, and jupiter is far from spherical. the shape of jupiter, and their mutual attractions, combine to make their motions most peculiar and distracting. hence an error in the time of revolution of a satellite was not _certainly_ due to the cause roemer suggested, unless one could be sure that the inequality was not a real one, unless it could be shown that the theory of gravitation was insufficient to account for it. this had not then been done; so the half-made discovery was shelved, and properly shelved, as a brilliant but unverified speculation. it remained on the shelf for half a century, and was no doubt almost forgotten. [illustration: fig. .--a transit-instrument for the british astronomical expedition, . shewing in its essential features the simplest form of such an instrument.] now a word or two about the man. he was a dane, educated at copenhagen, and learned in the mathematics. we first hear of him as appointed to assist picard, the eminent french geodetic surveyor (whose admirable work in determining the length of a degree you remember in connection with newton), who had come over to denmark with the object of fixing the exact site of the old and extinct tychonic observatory in the island of huen. for of course the knowledge of the exact latitude and longitude of every place whence numerous observations have been taken must be an essential to the full interpretation of those observations. the measurements being finished, young roemer accompanied picard to paris, and here it was, a few years after, that he read his famous paper concerning "an inequality in the motion of jupiter's first satellite," and its explanation by means of an hypothesis of "the successive propagation of light." the later years of his life he spent in copenhagen as a professor in the university and an enthusiastic observer of the heavens,--not a descriptive observer like herschel, but a measuring observer like sir george airy or tycho brahé. he was, in fact, a worthy follower of tycho, and the main work of his life is the development and devising of new and more accurate astronomical instruments. many of the large and accurate instruments with which a modern observatory is furnished are the invention of this dane. one of the finest observatories in the world is the russian one at pulkowa, and a list of the instruments there reads like an extended catalogue of roemer's inventions. he not only _invented_ the instruments, he had them made, being allowed money for the purpose; and he used them vigorously, so that at his death he left great piles of manuscript stored in the national observatory. unfortunately this observatory was in the heart of the city, and was thus exposed to a danger from which such places ought to be as far as possible exempt. some eighteen years after roemer's death a great conflagration broke out in copenhagen, and ruined large portions of the city. the successor to roemer, horrebow by name, fled from his house, with such valuables as he possessed, to the observatory, and there went on with his work. but before long the wind shifted, and to his horror he saw the flames coming his way. he packed up his own and his predecessor's manuscript observations in two cases, and prepared to escape with them, but the neighbours had resorted to the observatory as a place of safety, and so choked up the staircase with their property that he was barely able to escape himself, let alone the luggage, and everything was lost. [illustration: fig. .--diagram of equatorially mounted telescope; ce is the polar axis parallel to the axis of the earth; ab the declination axis. the diurnal motion is compensated by motion about the polar axis only, the other being clamped.] of all the observations, only three days' work remains, and these were carefully discussed by dr. galle, of berlin, in , and their nutriment extracted. these ancient observations are of great use for purposes of comparison with the present state of the heavens, and throw light upon possible changes that are going on. of course nowadays such a series of observations would be printed and distributed in many libraries, and so made practically indestructible. sad as the disaster was to the posthumous fame of the great observer, a considerable compensation was preparing. the very year that the fire occurred in denmark a quiet philosopher in england was speculating and brooding on a remarkable observation that he had made concerning the apparent motion of certain stars, and he was led thereby to a discovery of the first magnitude concerning the speed of light--a discovery which resuscitated the old theory of roemer about jupiter's satellites, and made both it and him immortal. james bradley lived a quiet, uneventful, studious life, mainly at oxford but afterwards at the national observatory at greenwich, of which he was third astronomer-royal, flamsteed and halley having preceded him in that office. he had taken orders, and lectured at oxford as savilian professor. it is said that he pondered his great discovery while pacing the long walk at magdalen college--and a beautiful place it is to meditate in. bradley was engaged in making observations to determine if possible the parallax of some of the fixed stars. parallax means the apparent relative shift of bodies due to a change in the observer's position. it is parallax which we observe when travelling by rail and looking out of window at the distant landscape. things at different distances are left behind at different apparent rates, and accordingly they seem to move relatively to each other. the most distant objects are least affected; and anything enormously distant, like the moon, is not subject to this effect, but would retain its position however far we travelled, unless we had some extraordinarily precise means of observation. so with the fixed stars: they were being observed from a moving carriage--viz. the earth--and one moving at the rate of nineteen miles a second. unless they were infinitely distant, or unless they were all at the same distance, they must show relative apparent motions among themselves. seen from one point of the earth's orbit, and then in six months from an opposite point, nearly million miles away, surely they must show some difference of aspect. remember that the old copernican difficulty had never been removed. if the earth revolved round the sun, how came it that the fixed stars showed no parallax? the fact still remained a surprise, and the question a challenge. picard, like other astronomers, supposed that it was only because the methods of observation had not been delicate enough; but now that, since the invention of the telescope and the founding of national observatories, accuracy hitherto undreamt of was possible, why not attack the problem anew? this, then, he did, watching the stars with great care to see if in six months they showed any change in absolute position with reference to the pole of the heavens; any known secular motion of the pole, such as precession, being allowed for. already he thought he detected a slight parallax for several stars near the pole, and the subject was exciting much interest. bradley determined to attempt the same investigation. he was not destined to succeed in it. not till the present century was success in that most difficult observation achieved; and even now it cannot be done by the absolute methods then attempted; but, as so often happens, bradley, in attempting one thing, hit upon another, and, as it happened, one of still greater brilliance and importance. let us trace the stages of his discovery. atmospheric refraction made horizon observations useless for the delicacy of his purpose, so he chose stars near the zenith, particularly one--[gamma] draconis. this he observed very carefully at different seasons of the year by means of an instrument specially adapted for zenith observations, viz. a zenith sector. the observations were made in conjunction with a friend of his, an amateur astronomer named molyneux, and they were made at kew. molyneux was shortly made first lord of the admiralty, or something important of that sort, and gave up frivolous pursuits. so bradley observed alone. they observed the star accurately early in the month of december, and then intended to wait six months. but from curiosity bradley observed it again only about a week later. to his surprise, he found that it had already changed its position. he recorded his observation on the back of an old envelope: it was his wont thus to use up odd scraps of paper--he was not, i regret to say, a tidy or methodical person--and this odd piece of paper turned up long afterwards among his manuscripts. it has been photographed and preserved as an historical relic. again and again he repeated the observation of the star, and continually found it moving still a little further and further south, an excessively small motion, but still an appreciable one--not to be set down to errors of observation. so it went on till march. it then waited, and after a bit longer began to return, until june. by september it was displaced as much to the north as it had been to the south, and by december it had got back to its original position. it had described, in fact, a small oscillation in the course of the year. the motion affected neighbouring stars in a similar way, and was called an "aberration," or wandering from their true place. for a long time bradley pondered over this observation, and over others like them which he also made. he found one group of stars describing small circles, while others at a distance from them were oscillating in straight lines, and all the others were describing ellipses. unless this state of things were cleared up, accurate astronomy was impossible. the fixed stars!--they were not fixed a bit. to refined and accurate observation, such as was now possible, they were all careering about in little orbits having a reference to the earth's year, besides any proper motion which they might really have of their own, though no such motion was at present known. not till herschel was that discovered; not till this extraordinary aberration was allowed for could it be discovered. the effect observed by bradley and molyneux must manifestly be only an apparent motion: it was absurd to suppose a real stellar motion regulating itself according to the position of the earth. parallax could not do it, for that would displace stars relatively among each other--it would not move similarly a set of neighbouring stars. at length, four years after the observation, the explanation struck him, while in a boat upon the thames. he noticed the apparent direction of the wind changed whenever the boat started. the wind veered when the boat's motion changed. of course the cause of this was obvious enough--the speed of the wind and the speed of the boat were compounded, and gave an apparent direction of the wind other than the true direction. but this immediately suggested a cause for what he had observed in the heavens. he had been observing an apparent direction of the stars other than the true direction, because he was observing from a moving vehicle. the real direction was doubtless fixed: the apparent direction veered about with the motion of the earth. it must be that light did not travel instantaneously, but gradually, as roemer had surmised fifty years ago; and that the motion of the light was compounded with the motion of the earth. think of a stream of light or anything else falling on a moving carriage. the carriage will run athwart the stream, the occupants of the carriage will mistake its true direction. a rifle fired through the windows of a railway carriage by a man at rest outside would make its perforations not in the true line of fire unless the train is stationary. if the train is moving, the line joining the holes will point to a place in advance of where the rifle is really located. so it is with the two glasses of a telescope, the object-glass and eye-piece, which are pierced by the light; an astronomer, applying his eye to the tube and looking for the origin of the disturbance, sees it apparently, but not in its real position--its apparent direction is displaced in the direction of the telescope's motion; by an amount depending on the ratio of the velocity of the earth to the velocity of light, and on the angle between those two directions. [illustration: fig. .--aberration diagram. the light-ray l penetrates the object-glass of the moving telescope at o, but does not reach the eye-piece until the telescope has travelled to the second position. consequently a moving telescope does not point out the true direction of the light, but aims at a point a little in advance.] but how minute is the displacement! the greatest effect is obtained when the two motions are at right angles to each other, _i.e._ when the star seen is at right angles to the direction of the earth's motion, but even then it is only ", or / th part of a degree; one-ninetieth of the moon's apparent diameter. it could not be detected without a cross-wire in the telescope, and would only appear as a slight displacement from the centre of the field, supposing the telescope accurately pointed to the true direction. but if this explanation be true, it at once gives a method of determining the velocity of light. the maximum angle of deviation, represented as a ratio of arc ÷ radius, amounts to ------------ - · = ------ × - / , (a gradient of foot in two miles). in other words, the velocity of light must be , times as great as the velocity of the earth in its orbit. this amounts to a speed of , miles a second--not so very different from what roemer had reckoned it in order to explain the anomalies of jupiter's first satellite. stars in the direction in which the earth was moving would not be thus affected; there would be nothing in mere approach or recession to alter direction or to make itself in any way visible. stars at right angles to the earth's line of motion would be most affected, and these would be all displaced by the full amount of seconds of arc. stars in intermediate directions would be displaced by intermediate amounts. but the line of the earth's motion is approximately a circle round the sun, hence the direction of its advance is constantly though slowly changing, and in one year it goes through all the points of the compass. the stars, being displaced always in the line of advance, must similarly appear to describe little closed curves, always a quadrant in advance of the earth, completing their orbits once a year. those near the pole of the ecliptic will describe circles, being always at right angles to the motion. those in the plane of the ecliptic (near the zodiac) will be sometimes at right angles to the motion, but at other times will be approached or receded from; hence these will oscillate like pendulums once a year; and intermediate stars will have intermediate motions--that is to say, will describe ellipses of varying excentricity, but all completed in a year, and all with the major axis ". this agreed very closely with what was observed. the main details were thus clearly and simply explained by the hypothesis of a finite velocity for light, "the successive propagation of light in time." this time there was no room for hesitation, and astronomers hailed the discovery with enthusiasm. not yet, however, did bradley rest. the finite velocity of light explained the major part of the irregularities he had observed, but not the whole. the more carefully he measured the amount of the deviation, the less completely accurate became its explanation. there clearly was a small outstanding error or discrepancy; the stars were still subject to an unexplained displacement--not, indeed, a displacement that repeated itself every year, but one that went through a cycle of changes in a longer period. the displacement was only about half that of aberration, and having a longer period was rather more difficult to detect securely. but the major difficulty was the fact that the two sorts of disturbances were co-existent, and the skill of disentangling them, and exhibiting the true and complete cause of each inequality, was very brilliant. for nineteen years did bradley observe this minor displacement, and in that time he saw it go through a complete cycle. its cause was now clear to him; the nineteen-year period suggested the explanation. it is the period in which the moon goes through all her changes--a period known to the ancients as the lunar cycle, or metonic cycle, and used by them to predict eclipses. it is still used for the first rough approximation to the prediction of eclipses, and to calculate easter. the "golden number" of the prayer-book is the number of the year in this cycle. the cause of the second inequality, or apparent periodic motion of the stars, bradley made out to be a nodding motion of the earth's axis. the axis of the earth describes its precessional orbit or conical motion every , years, as had long been known; but superposed upon this great movement have now been detected minute nods, each with a period of nineteen years. the cause of the nodding is completely accounted for by the theory of gravitation, just as the precession of the equinoxes was. both disturbances result from the attraction of the moon on the non-spherical earth--on its protuberant equator. "nutation" is, in fact, a small perturbation of precession. the motion may be observed in a non-sleeping top. the slow conical motion of the top's slanting axis represents the course of precession. sometimes this path is loopy, and its little nods correspond to nutation. the probable existence of some such perturbation had not escaped the sagacity of newton, and he mentions something about it in the _principia_, but thinks it too small to be detected by observation. he was thinking, however, of a solar disturbance rather than a lunar one, and this is certainly very small, though it, too, has now been observed. newton was dead before bradley made these great discoveries, else he would have been greatly pleased to hear of them. these discoveries of aberration and nutation, says delambre, the great french historian of science, secure to their author a distinguished place after hipparchus and kepler among the astronomers of all ages and all countries. notes to lecture xi _lagrange_ and _laplace_, both tremendous mathematicians, worked very much in alliance, and completed newton's work. the _mécanique céleste_ contains the higher intricacies of astronomy mathematically worked out according to the theory of gravitation. they proved the solar system to be stable; all its inequalities being periodic, not cumulative. and laplace suggested the "nebular hypothesis" concerning the origin of sun and planets: a hypothesis previously suggested, and to some extent, elaborated, by kant. a list of some of the principal astronomical researches of lagrange and laplace:--libration of the moon. long inequality of jupiter and saturn. perturbations of jupiter's satellites. perturbations of comets. acceleration of the moon's mean motion. improved lunar theory. improvements in the theory of the tides. periodic changes in the form and obliquity of the earth's orbit. stability of the solar system considered as an assemblage of rigid bodies subject to gravity. the two equations which establish the stability of the solar system are:-- _sum (me^ [square root]d) = constant,_ and _sum (m tan^ [theta][square root]d) = constant;_ where _m_ is the mass of each planet, _d_ its mean distance from the sun, _e_ the excentricity of its orbit, and [theta] the inclination of its plane. however the expressions above formulated may change for individual planets, the sum of them for all the planets remains invariable. the period of the variations in excentricity of the earth's orbit is , years; the period of conical revolution of the earth's axis is , years. about , years ago the excentricity was at a maximum. lecture xi lagrange and laplace--the stability of the solar system, and the nebular hypothesis laplace was the son of a small farmer or peasant of normandy. his extraordinary ability was noticed by some wealthy neighbours, and by them he was sent to a good school. from that time his career was one brilliant success, until in the later years of his life his prominence brought him tangibly into contact with the deteriorating influence of politics. perhaps one ought rather to say trying than deteriorating; for they seem trying to a strong character, deteriorating to a weak one--and unfortunately, laplace must be classed in this latter category. it has always been the custom in france for its high scientific men to be conspicuous also in politics. it seems to be now becoming the fashion in this country also, i regret to say. the _life_ of laplace is not specially interesting, and i shall not go into it. his brilliant mathematical genius is unquestionable, and almost unrivalled. he is, in fact, generally considered to come in this respect next after newton. his talents were of a more popular order than those of lagrange, and accordingly he acquired fame and rank, and rose to the highest dignities. nevertheless, as a man and a politician he hardly commands our respect, and in time-serving adjustability he is comparable to the redoubtable vicar of bray. his scientific insight and genius were however unquestionably of the very highest order, and his work has been invaluable to astronomy. i will give a short sketch of some of his investigations, so far as they can be made intelligible without overmuch labour. he worked very much in conjunction with lagrange, a more solid though a less brilliant man, and it is both impossible and unnecessary for us to attempt to apportion respective shares of credit between these two scientific giants, the greatest scientific men that france ever produced. first comes a research into the libration of the moon. this was discovered by galileo in his old age at arcetri, just before his blindness. the moon, as every one knows, keeps the same face to the earth as it revolves round it. in other words, it does not rotate with reference to the earth, though it does rotate with respect to outside bodies. its libration consists in a sort of oscillation, whereby it shows us now a little more on one side, now a little more on the other, so that altogether we are cognizant of more than one-half of its surface--in fact, altogether of about three-fifths. it is a simple and unimportant matter, easily explained. the motion of the moon may be analyzed into a rotation about its own axis combined with a revolution about the earth. the speed of the rotation is quite uniform, the speed of the revolution is not quite uniform, because the orbit is not circular but elliptical, and the moon has to travel faster in perigee than in apogee (in accordance with kepler's second law). the consequence of this is that we see a little too far round the body of the moon, first on one side, then on the other. hence it _appears_ to oscillate slightly, like a lop-sided fly-wheel whose revolutions have been allowed to die away so that they end in oscillations of small amplitude.[ ] its axis of rotation, too, is not precisely perpendicular to its plane of revolution, and therefore we sometimes see a few hundred miles beyond its north pole, sometimes a similar amount beyond its south. lastly, there is a sort of parallax effect, owing to the fact that we see the rising moon from one point of view, and the setting moon from a point , miles distant; and this base-line of the earth's diameter gives us again some extra glimpses. this diurnal or parallactic libration is really more effective than the other two in extending our vision into the space-facing hemisphere of the moon. these simple matters may as well be understood, but there is nothing in them to dwell upon. the far side of the moon is probably but little worth seeing. its features are likely to be more blurred with accumulations of meteoric dust than are those of our side, but otherwise they are likely to be of the same general character. the thing of real interest is the fact that the moon does turn the same face towards us; _i.e._ has ceased to rotate with respect to the earth (if ever it did so). the stability of this state of things was shown by lagrange to depend on the shape of the moon. it must be slightly egg-shape, or prolate--extended in the direction of the earth; its earth-pointing diameter being a few hundred feet longer than its visible diameter; a cause slight enough, but nevertheless sufficient to maintain stability, except under the action of a distinct disturbing cause. the prolate or lemon-like shape is caused by the gravitative pull of the earth, balanced by the centrifugal whirl. the two forces balance each other as regards motion, but between them they have strained the moon a trifle out of shape. the moon has yielded as if it were perfectly plastic; in all probability it once was so. it may be interesting to note for a moment the correlative effect of this aspect of the moon, if we transfer ourselves to its surface in imagination, and look at the earth (cf. fig. ). the earth would be like a gigantic moon of four times our moon's diameter, and would go through its phases in regular order. but it would not rise or set: it would be fixed in the sky, and subject only to a minute oscillation to and fro once a month, by reason of the "libration" we have been speaking of. its aspect, as seen by markings on its surface, would rapidly change, going through a cycle in twenty-four hours; but its permanent features would be usually masked by lawless accumulations of cloud, mainly aggregated in rude belts parallel to the equator. and these cloudy patches would be the most luminous, the whitest portions; for of course it would be their silver lining that we would then be looking on.[ ] next among the investigations of lagrange and laplace we will mention the long inequality of jupiter and saturn. halley had found that jupiter was continually lagging behind its true place as given by the theory of gravitation; and, on the other hand, that saturn was being accelerated. the lag on the part of jupiter amounted to about - / minutes in a century. overhauling ancient observations, however, halley found signs of the opposite state of things, for when he got far enough back jupiter was accelerated and saturn was being retarded. here was evidently a case of planetary perturbation, and laplace and lagrange undertook the working of it out. they attacked it as a case of the problem of three bodies, viz. the sun, jupiter, and saturn; which are so enormously the biggest of the known bodies in the system that insignificant masses like the earth, mars, and the rest, may be wholly neglected. they succeeded brilliantly, after a long and complex investigation: succeeded, not in solving the problem of the three bodies, but, by considering their mutual action as perturbations superposed on each other, in explaining the most conspicuous of the observed anomalies of their motion, and in laying the foundation of a general planetary theory. [illustration: fig. .--shewing the three conjunction places in the orbits of jupiter and saturn. the two planets are represented as leaving one of the conjunctions where jupiter was being pulled back and saturn being pulled forward by their mutual attraction.] one of the facts that plays a large part in the result was known to the old astrologers, viz. that jupiter and saturn come into conjunction with a certain triangular symmetry; the whole scheme being called a trigon, and being mentioned several times by kepler. it happens that five of jupiter's years very nearly equal two of saturn's,[ ] so that they get very nearly into conjunction three times in every five jupiter years, but not exactly. the result of this close approach is that periodically one pulls the other on and is itself pulled back; but since the three points progress, it is not always the same planet which gets pulled back. the complete theory shows that in the year there was no marked perturbation: before that it was in one direction, while afterwards it was in the other direction, and the period of the whole cycle of disturbances is of our years. the solution of this long outstanding puzzle by the theory of gravitation was hailed with the greatest enthusiasm by astronomers, and it established the fame of the two french mathematicians. next they attacked the complicated problem of the motions of jupiter's satellites. they succeeded in obtaining a theory of their motions which represented fact very nearly indeed, and they detected the following curious relationship between the satellites:--the speed of the first satellite + twice the speed of the second is equal to the speed of the third. they found this, not empirically, after the manner of kepler, but as a deduction from the law of gravitation; for they go on to show that even if the satellites had not started with this relation they would sooner or later, by mutual perturbation, get themselves into it. one singular consequence of this, and of another quite similar connection between their positions, is that all three satellites can never be eclipsed at once. the motion of the fourth satellite is less tractable; it does not so readily form an easy system with the others. after these great successes the two astronomers naturally proceeded to study the mutual perturbations of all other bodies in the solar system. and one very remarkable discovery they made concerning the earth and moon, an account of which will be interesting, though the details and processes of calculation are quite beyond us in a course like this. astronomical theory had become so nearly perfect by this time, and observations so accurate, that it was possible to calculate many astronomical events forwards or backwards, over even a thousand years or more, with admirable precision. now, halley had studied some records of ancient eclipses, and had calculated back by means of the lunar theory to see whether the calculation of the time they ought to occur would agree with the record of the time they did occur. to his surprise he found a discrepancy, not a large one, but still one quite noticeable. to state it as we know it now:--an eclipse a century ago happened twelve seconds later than it ought to have happened by theory; two centuries back the error amounted to forty-eight seconds, in three centuries it would be seconds, and so on; the lag depending on the square of the time. by research, and help from scholars, he succeeded in obtaining the records of some very ancient eclipses indeed. one in egypt towards the end of the tenth century a.d.; another in a.d.; another a little before christ; and one, the oldest of all of which any authentic record has been preserved, observed by the chaldæan astronomers in babylon in the reign of hezekiah. calculating back to this splendid old record of a solar eclipse, over the intervening , years, the calculated and the observed times were found to disagree by nearly two hours. pondering over an explanation of the discrepancy, halley guessed that it must be because the moon's motion was not uniform, it must be going quicker and quicker, gaining twelve seconds each century on its previous gain--a discovery announced by him as "the acceleration of the moon's mean motion." the month was constantly getting shorter. what was the physical cause of this acceleration according to the theory of gravitation? many attacked the question, but all failed. this was the problem laplace set himself to work out. a singular and beautiful result rewarded his efforts. you know that the earth describes an elliptic orbit round the sun: and that an ellipse is a circle with a certain amount of flattening or "excentricity."[ ] well, laplace found that the excentricity of the earth's orbit must be changing, getting slightly less; and that this change of excentricity would have an effect upon the length of the month. it would make the moon go quicker. one can almost see how it comes about. a decrease in excentricity means an increase in mean distance of the earth from the sun. this means to the moon a less solar perturbation. now one effect of the solar perturbation is to keep the moon's orbit extra large: if the size of its orbit diminishes, its velocity must increase, according to kepler's third law. laplace calculated the amount of acceleration so resulting, and found it ten seconds a century; very nearly what observation required; for, though i have quoted observation as demanding twelve seconds per century, the facts were not then so distinctly and definitely ascertained. this calculation for a long time seemed thoroughly satisfactory, but it is not the last word on the subject. quite lately an error has been found in the working, which diminishes the theoretical gravitation-acceleration to six seconds a century instead of ten, thus making it insufficient to agree exactly with fact. the theory of gravitation leaves an outstanding error. (the point is now almost thoroughly understood, and we shall return to it in lecture xviii). but another question arises out of this discussion. i have spoken of the excentricity of the earth's orbit as decreasing. was it always decreasing? and if so, how far back was it so excentric that at perihelion the earth passed quite near the sun? if it ever did thus pass near the sun, the inference is manifest--the earth must at one time have been thrown off, or been separated off, from the sun. if a projectile could be fired so fast that it described an orbit round the earth--and the speed of fire to attain this lies between five and seven miles a second (not less than the one, nor more than the other)--it would ever afterwards pass through its point of projection as one point of its elliptic orbit; and its periodic return through that point would be the sign of its origin. similarly, if a satellite does _not_ come near its central orb, and can be shown never to have been near it, the natural inference is that it has _not_ been born from it, but has originated in some other way. the question which presented itself in connexion with the variable ellipticity of the earth's orbit was the following:--had it always been decreasing, so that once it was excentric enough just to graze the sun at perihelion as a projected body would do? into the problem thus presented lagrange threw himself, and he succeeded in showing that no such explanation of the origin of the earth is possible. the excentricity of the orbit, though now decreasing, was not always decreasing; ages ago it was increasing: it passes through periodic changes. eighteen thousand years ago its excentricity was a maximum; since then it has been diminishing, and will continue to diminish for , years more, when it will be an almost perfect circle; it will then begin to increase again, and so on. the obliquity of the ecliptic is also changing periodically, but not greatly: the change is less than three degrees. this research has, or ought to have, the most transcendent interest for geologists and geographers. you know that geologists find traces of extraordinary variations of temperature on the surface of the earth. england was at one time tropical, at another time glacial. far away north, in spitzbergen, evidence of the luxuriant vegetation of past ages has been found; and the explanation of these great climatic changes has long been a puzzle. does not the secular variation in excentricity of the earth's orbit, combined with the precession of the equinoxes, afford a key? and if a key at all, it will be an accurate key, and enable us to calculate back with some precision to the date of the glacial epoch; and again to the time when a tropical flora flourished in what is now northern europe, _i.e._ to the date of the carboniferous era. this aspect of the subject has recently been taught with vigour and success by dr. croll in his book "climate and time." a brief and partial explanation of the matter may be given, because it is a point of some interest and is also one of fair simplicity. every one knows that the climatic conditions of winter and summer are inverted in the two hemispheres, and that at present the sun is nearest to us in our (northern) winter. in other words, the earth's axis is inclined so as to tilt its north pole away from the sun at perihelion, or when the earth is at the part of its elliptic orbit nearest the sun's focus; and to tilt it towards the sun at aphelion. the result of this present state of things is to diminish the intensity of the average northern winter and of the average northern summer, and on the other hand to aggravate the extremes of temperature in the southern hemisphere; all other things being equal. of course other things are not equal, and the distribution of land and sea is a still more powerful climatic agent than is the three million miles or so extra nearness of the sun. but it is supposed that the antarctic ice-cap is larger than the northern, and increased summer radiation with increased winter cold would account for this. but the present state of things did not always obtain. the conical movement of the earth's axis (now known by a curious perversion of phrase as "precession") will in the course of , years or so cause the tilt to be precisely opposite, and then we shall have the more extreme winters and summers instead of the southern hemisphere. if the change were to occur now, it might not be overpowering, because now the excentricity is moderate. but if it happened some time back, when the excentricity was much greater, a decidedly different arrangement of climate may have resulted. there is no need to say _if_ it happened some time back: it did happen, and accordingly an agent for affecting the distribution of mean temperature on the earth is to hand; though whether it is sufficient to achieve all that has been observed by geologists is a matter of opinion. once more, the whole diversity of the seasons depends on the tilt of the earth's axis, the ° by which it is inclined to a perpendicular to the orbital plane; and this obliquity or tilt is subject to slow fluctuations. hence there will come eras when all causes combine to produce a maximum extremity of seasons in the northern hemisphere, and other eras when it is the southern hemisphere which is subject to extremes. but a grander problem still awaited solution--nothing less than the fate of the whole solar system. here are a number of bodies of various sizes circulating at various rates round one central body, all attracted by it, and all attracting each other, the whole abandoned to the free play of the force of gravitation: what will be the end of it all? will they ultimately approach and fall into the sun, or will they recede further and further from him, into the cold of space? there is a third possible alternative: may they not alternately approach and recede from him, so as on the whole to maintain a fair approximation to their present distances, without great and violent extremes of temperature either way? if any one planet of the system were to fall into the sun, more especially if it were a big one like jupiter or saturn, the heat produced would be so terrific that life on this earth would be destroyed, even at its present distance; so that we are personally interested in the behaviour of the other planets as well as in the behaviour of our own. the result of the portentously difficult and profoundly interesting investigation, here sketched in barest outline, is that the solar system is stable: that is to say, that if disturbed a little it will oscillate and return to its old state; whereas if it were unstable the slightest disturbance would tend to accumulate, and would sooner or later bring about a catastrophe. a hanging pendulum is stable, and oscillates about a mean position; its motion is periodic. a top-heavy load balanced on a point is unstable. all the changes of the solar system are periodic, _i.e._ they repeat themselves at regular intervals, and they never exceed a certain moderate amount. the period is something enormous. they will not have gone through all their changes until a period of , , years has elapsed. this is the period of the planetary oscillation: "a great pendulum of eternity which beats ages as our pendulums beat seconds." enormous it seems; and yet we have reason to believe that the earth has existed through many such periods. the two laws of stability discovered and stated by lagrange and laplace i can state, though they may be difficult to understand:-- represent the masses of the several planets by m_ , m_ , &c.; their mean distances from the sun (or radii vectores) by r_ , r_ , &c.; the excentricities of their orbits by e_ , e_ , &c.; and the obliquity of the planes of these orbits, reckoned from a single plane of reference or "invariable plane," by [theta]_ , [theta]_ , &c.; then all these quantities (except m) are liable to fluctuate; but, however much they change, an increase for one planet will be accompanied by a decrease for some others; so that, taking all the planets into account, the sum of a set of terms like these, m_ e_ ^ [square root]r_ + m_ e_ ^ [square root]r_ + &c., will remain always the same. this is summed up briefly in the following statement: [sigma](me^ [square root]r) = constant. that is one law, and the other is like it, but with inclination of orbit instead of excentricity, viz.: [sigma](m[theta]^ [square root]r) = constant. the value of each of these two constants can at any time be calculated. at present their values are small. hence they always were and always will be small; being, in fact, invariable. hence neither _e_ nor _r_ nor [theta] can ever become infinite, nor can their average value for the system ever become zero. the planets may share the given amount of total excentricity and obliquity in various proportions between themselves; but even if it were all piled on to one planet it would not be very excessive, unless the planet were so small a one as mercury; and it would be most improbable that one planet should ever have all the excentricity of the solar system heaped upon itself. the earth, therefore, never has been, nor ever will be, enormously nearer the sun than it is at present: nor can it ever get very much further off. its changes are small and are periodic--an increase is followed by a decrease, like the swing of a pendulum. the above two laws have been called the magna charta of the solar system, and were long supposed to guarantee its absolute permanence. so far as the theory of gravitation carries us, they do guarantee its permanence; but something more remains to be said on the subject in a future lecture (xviii). and now, finally, we come to a sublime speculation, thrown out by laplace, not as the result of profound calculation, like the results hitherto mentioned, not following certainly from the theory of gravitation, or from any other known theory, and therefore not to be accepted as more than a brilliant hypothesis, to be confirmed or rejected as our knowledge extends. this speculation is the "nebular hypothesis." since the time of laplace the nebular hypothesis has had ups and downs of credence, sometimes being largely believed in, sometimes being almost ignored. at the present time it holds the field with perhaps greater probability of ultimate triumph than has ever before seemed to belong to it--far greater than belonged to it when first propounded. it had been previously stated clearly and well by the philosopher kant, who was intensely interested in "the starry heavens" as well as in the "mind of man," and who shewed in connexion with astronomy also a most surprising genius. the hypothesis ought by rights perhaps to be known rather by his name than by that of laplace. the data on which it was founded are these:--every motion in the solar system known at that time took place in one direction, and in one direction only. thus the planets revolve round the sun, all going the same way round; moons revolve round the planets, still maintaining the same direction of rotation, and all the bodies that were known to rotate on their own axis did so with still the same kind of spin. moreover, all these motions take place in or near a single plane. the ancients knew that sun moon and planets all keep near to the ecliptic, within a belt known as the zodiac: none strays away into other parts of the sky. satellites also, and rings, are arranged in or near the same plane; and the plane of diurnal spin, or equator of the different bodies, is but slightly tilted. now all this could not be the result of chance. what could have caused it? is there any connection or common ancestry possible, to account for this strange family likeness? there is no connection now, but there may have been once. must have been, we may almost say. it is as though they had once been parts of one great mass rotating as a whole; for if such a rotating mass broke up, its parts would retain its direction of rotation. but such a mass, filling all space as far as or beyond saturn, although containing the materials of the whole solar system in itself, must have been of very rare consistency. occupying so much bulk it could not have been solid, nor yet liquid, but it might have been gaseous. are there any such gigantic rotating masses of gas in the heaven now? certainly there are; there are the nebulæ. some of the nebulæ are now known to be gaseous, and some of them at least are in a state of rotation. laplace could not have known this for certain, but he suspected it. the first distinctly spiral nebula was discovered by the telescope of lord rosse; and quite recently a splendid photograph of the great andromeda nebula, by our townsman, mr. isaac roberts, reveals what was quite unsuspected--and makes it clear that this prodigious mass also is in a state of extensive and majestic whirl. very well, then, put this problem:--a vast mass of rotating gas is left to itself to cool for ages and to condense as it cools: how will it behave? a difficult mathematical problem, worthy of being attacked to-day; not yet at all adequately treated. there are those who believe that by the complete treatment of such a problem all the history of the solar system could be evolved. [illustration: fig. .--lord rosse's drawing of the spiral nebula in canes venatici, with the stub marks of the draughtsman unduly emphasised into features by the engraver.] laplace pictured to himself this mass shrinking and thereby whirling more and more rapidly. a spinning body shrinking in size and retaining its original amount of rotation, as it will unless a brake is applied, must spin more and more rapidly as it shrinks. it has what mathematicians call a constant moment of momentum; and what it loses in leverage, as it shrinks, it gains in speed. the mass is held together by gravitation, every particle attracting every other particle; but since all the particles are describing curved paths, they will tend to fly off tangentially, and only a small excess of the gravitation force over the centrifugal is left to pull the particles in, and slowly to concentrate the nebula. the mutual gravitation of the parts is opposed by the centrifugal force of the whirl. at length a point is reached where the two forces balance. a portion outside a certain line will be in equilibrium; it will be left behind, and the rest must contract without it. a ring is formed, and away goes the inner nucleus contracting further and further towards a centre. after a time another ring will be left behind in the same way, and so on. what happens to these rings? they rotate with the motion they possess when thrown or shrunk off; but will they remain rings? if perfectly regular they may; if there be any irregularity they are liable to break up. they will break into one or two or more large masses, which are ultimately very likely to collide and become one. the revolving body so formed is still a rotating gaseous mass; and it will go on shrinking and cooling and throwing off rings, like the larger nucleus by which it has been abandoned. as any nucleus gets smaller, its rate of rotation increases, and so the rings last thrown off will be spinning faster than those thrown off earliest. the final nucleus or residual central body will be rotating fastest of all. the nucleus of the whole original mass we now see shrunk up into what we call the sun, which is spinning on its axis once every twenty-five days. the rings successively thrown off by it are now the planets--some large, some small--those last thrown off rotating round him comparatively quickly, those outside much more slowly. the rings thrown off by the planetary gaseous masses as they contracted have now become satellites; except one ring which has remained without breaking up, and is to be seen rotating round saturn still. one other similar ring, an abortive attempt at a planet, is also left round the sun (the zone of asteroids). such, crudely and baldly, is the famous nebular hypothesis of laplace. it was first stated, as has been said above, by the philosopher kant, but it was elaborated into much fuller detail by the greatest of french mathematicians and astronomers. the contracting masses will condense and generate great quantities of heat by their own shrinkage; they will at a certain stage condense to liquid, and after a time will begin to cool and congeal with a superficial crust, which will get thicker and thicker; but for ages they will remain hot, even after they have become thoroughly solid. the small ones will cool fastest; the big ones will retain their heat for an immense time. bullets cool quickly, cannon-balls take hours or days to cool, planets take millions of years. our moon may be nearly cold, but the earth is still warm--indeed, very hot inside. jupiter is believed by some observers still to glow with a dull red heat; and the high temperature of the much larger and still liquid mass of the sun is apparent to everybody. not till it begins to scum over will it be perceptibly cooler. [illustration: fig. .--saturn.] many things are now known concerning heat which were not known to laplace (in the above paragraph they are only hinted at), and these confirm and strengthen the general features of his hypothesis in a striking way; so do the most recent telescopic discoveries. but fresh possibilities have now occurred to us, tidal phenomena are seen to have an influence then wholly unsuspected, and it will be in a modified and amplified form that the philosopher of next century will still hold to the main features of this famous old nebular hypothesis respecting the origin of the sun and planets--the evolution of the solar system. notes to lecture xii the subject of stellar astronomy was first opened up by sir william herschel, the greatest observing astronomer. _frederick william herschel_ was born in hanover in , and brought up as a musician. came to england in . first saw a telescope in . made a great many himself, and began a survey of the heavens. his sister caroline, born in , came to england in , and became his devoted assistant to the end of his life. uranus discovered in . music finally abandoned next year, and the -foot telescope begun. discovered two moons of saturn and two of uranus. reviewed, described, and gauged all the visible heavens. discovered and catalogued , nebulæ and double stars. speculated concerning the milky way, the nebulosity of stars, the origin and growth of solar systems. discovered that the stars were in motion, not fixed, and that the sun as one of them was journeying towards a point in the constellation hercules. died in , eighty-four years old. caroline herschel discovered eight comets, and lived on to the age of ninety-eight. lecture xii herschel and the motion of the fixed stars we may admit, i think, that, with a few notable exceptions, the work of the great men we have been recently considering was rather to complete and round off the work of newton, than to strike out new and original lines. this was the whole tendency of eighteenth century astronomy. it appeared to be getting into an adult and uninteresting stage, wherein everything could be calculated and predicted. labour and ingenuity, and a severe mathematical training, were necessary to work out the remote consequences of known laws, but nothing fresh seemed likely to turn up. consequently men's minds began turning in other directions, and we find chemistry and optics largely studied by some of the greatest minds, instead of astronomy. but before the century closed there was destined to arise one remarkable exception--a man who was comparatively ignorant of that which had been done before--a man unversed in mathematics and the intricacies of science, but who possessed such a real and genuine enthusiasm and love of nature that he overcame the force of adverse circumstances, and entering the territory of astronomy by a by-path, struck out a new line for himself, and infused into the science a healthy spirit of fresh life and activity. this man was william herschel. "the rise of herschel," says miss clerke, "is the one conspicuous anomaly in the otherwise somewhat quiet and prosy eighteenth century. it proved decisive of the course of events in the nineteenth. it was unexplained by anything that had gone before, yet all that came after hinged upon it. it gave a new direction to effort; it lent a fresh impulse to thought. it opened a channel for the widespread public interest which was gathering towards astronomical subjects to flow in." herschel was born at hanover in , the son of an oboe player in a military regiment. the father was a good musician, and a cultivated man. the mother was a german _frau_ of the period, a strong, active, business-like woman, of strong character and profound ignorance. herself unable to write, she set her face against learning and all new-fangled notions. the education of the sons she could not altogether control, though she lamented over it, but the education of her two daughters she strictly limited to cooking, sewing, and household management. these, however, she taught them well. it was a large family, and william was the fourth child. we need only remember the names of his younger brother alexander, and of his much younger sister caroline. they were all very musical--the youngest boy was once raised upon a table to play the violin at a public performance. the girls were forbidden to learn music by their mother, but their father sometimes taught them a little on the sly. alexander was besides an ingenious mechanician. at the age of seventeen, william became oboist to the hanoverian guards, shortly before the regiment was ordered to england. two years later he removed himself from the regiment, with the approval of his parents, though probably without the approbation or consent of the commanding officer, by whom such removal would be regarded as simple desertion, which indeed it was; and george iii. long afterwards handed him an official pardon for it. at the age of nineteen, he was thus launched in england with an outfit of some french, latin, and english, picked up by himself; some skill in playing the hautboy, the violin, and the organ, as taught by his father; and some good linen and clothing, and an immense stock of energy, provided by his mother. he lived as musical instructor to one or two militia bands in yorkshire, and for three years we hear no more than this of him. but, at the end of that time, a noted organist, dr. miller, of durham, who had heard his playing, proposed that he should come and live with him and play at concerts, which he was very glad to do. he next obtained the post of organist at halifax; and some four or five years later he was invited to become organist at the octagon chapel in bath, and soon led the musical life of that then very fashionable place. about this time he went on a short visit to his family at hanover, by all of whom he was very much beloved, especially by his young sister caroline, who always regarded him as specially her own brother. it is rather pitiful, however, to find that her domestic occupations still unfairly repressed and blighted her life. she says:-- "of the joys and pleasures which all felt at this long-wished-for meeting with my--let me say my dearest--brother, but a small portion could fall to my share; for with my constant attendance at church and school, besides the time i was employed in doing the drudgery of the scullery, it was but seldom i could make one in the group when the family were assembled together." while at bath he wrote many musical pieces--glees, anthems, chants, pieces for the harp, and an orchestral symphony. he taught a large number of pupils, and lived a hard and successful life. after fourteen hours or so spent in teaching and playing, he would retire at night to instruct his mind with a study of mathematics, optics, italian, or greek, in all of which he managed to make some progress. he also about this time fell in with some book on astronomy. in his father was struck with paralysis, and two years later he died. william then proposed that alexander should come over from hanover and join him at bath, which was done. next they wanted to rescue their sister caroline from her humdrum existence, but this was a more difficult matter. caroline's journal gives an account of her life at this time that is instructive. here are a few extracts from it:-- "my father wished to give me something like a polished education, but my mother was particularly determined that it should be a rough, but at the same time a useful one; and nothing further she thought was necessary but to send me two or three months to a sempstress to be taught to make household linen.... "my mother would not consent to my being taught french, ... so all my father could do for me was to indulge me (and please himself) sometimes with a short lesson on the violin, when my mother was either in good humour or out of the way.... she had cause for wishing me not to know more than was necessary for being useful in the family; for it was her certain belief that my brother william would have returned to his country, and my eldest brother not have looked so high, if they had had a little less learning." however, seven years after the death of their father, william went over to germany and returned to england in triumph, bringing caroline with him: she being then twenty-two. so now began a busy life in bath. for caroline the work must have been tremendous. for, besides having to learn singing, she had to learn english. she had, moreover, to keep accounts and do the marketing. when the season at bath was over, she hoped to get rather more of her brother william's society; but he was deep in optics and astronomy, used to sleep with the books under his pillow, read them during meals, and scarcely ever thought of anything else. he was determined to see for himself all the astronomical wonders; and there being a small gregorian reflector in one of the shops, he hired it. but he was not satisfied with this, and contemplated making a telescope feet long. he wrote to opticians inquiring the price of a mirror suitable, but found there were none so large, and that even the smaller ones were beyond his means. nothing daunted, he determined to make some for himself. alexander entered into his plans: tools, hones, polishers, and all sorts of rubbish were imported into the house, to the sister's dismay, who says:-- [illustration: fig. .--principle of newtonian reflector.] "and then, to my sorrow, i saw almost every room turned into a workshop. a cabinet-maker making a tube and stands of all descriptions in a handsomely furnished drawing-room; alex. putting up a huge turning-machine (which he had brought in the autumn from bristol, where he used to spend the summer) in a bed-room, for turning patterns, grinding glasses, and turning eye-pieces, &c. at the same time music durst not lie entirely dormant during the summer, and my brother had frequent rehearsals at home." finally, in , at the age of thirty-six, he had made himself a - / -foot telescope, and began to view the heavens. so attached was he to the instrument that he would run from the concert-room between the parts, and take a look at the stars. he soon began another telescope, and then another. he must have made some dozen different telescopes, always trying to get them bigger and bigger; at last he got a -foot and then a -foot instrument, and began a systematic survey of the heavens; he also began to communicate his results to the royal society. he now took a larger house, with more room for workshops, and a grass plot for a -foot telescope, and still he went on grinding mirrors--literally hundreds of them. i read another extract from the diary of his sister, who waited on him and obeyed him like a spaniel:-- "my time was taken up with copying music and practising, besides attendance on my brother when polishing, since by way of keeping him alive i was constantly obliged to feed him by putting the victuals by bits into his mouth. this was once the case when, in order to finish a -foot mirror, he had not taken his hands from it for sixteen hours together. in general he was never unemployed at meals, but was always at those times contriving or making drawings of whatever came in his mind. generally i was obliged to read to him whilst he was at the turning-lathe, or polishing mirrors--_don quixote_, _arabian nights' entertainments_, the novels of sterne, fielding, &c.; serving tea and supper without interrupting the work with which he was engaged, ... and sometimes lending a hand. i became, in time, as useful a member of the workshop as a boy might be to his master in the first year of his apprenticeship.... but as i was to take a part the next year in the oratorios, i had, for a whole twelvemonth, two lessons per week from miss fleming, the celebrated dancing-mistress, to drill me for a gentlewoman (god knows how she succeeded). so we lived on without interruption. my brother alex. was absent from bath for some months every summer, but when at home he took much pleasure in executing some turning or clockmaker's work for his brother." the music, and the astronomy, and the making of telescopes, all went on together, each at high pressure, and enough done in each to satisfy any ordinary activity. but the herschels knew no rest. grinding mirrors by day, concerts and oratorios in the evening, star-gazing at night. it is strange his health could stand it. the star-gazing, moreover, was no _dilettante_ work; it was based on a serious system--a well thought out plan of observation. it was nothing less than this--to pass the whole heavens steadily and in order through the telescope, noting and describing and recording every object that should be visible, whether previously known or unknown. the operation is called sweeping; but it is not a rapid passage from one object to another, as the term might suggest; it is a most tedious business, and consists in following with the telescope a certain field of view for some minutes, so as to be sure that nothing is missed, then shifting it to the next overlapping field, and watching again. and whatever object appears must be scrutinized anxiously to see what there is peculiar about it. if a star, it may be double, or it may be coloured, or it may be nebulous; or again it may be variable, and so its brightness must be estimated in order to compare with a subsequent observation. four distinct times in his life did herschel thus pass the whole visible heavens under review; and each survey occupied him several years. he discovered double stars, variable stars, nebulæ, and comets; and mr. william herschel, of bath, the amateur astronomer, was gradually emerging from his obscurity, and becoming a known man. tuesday, the th of march, , is a date memorable in the annals of astronomy. "on this night," he writes to the royal society, "in examining the small stars near _[eta]_ geminorum, i perceived one visibly larger than the rest. struck with its uncommon appearance, i compared it to _[eta]_ geminorum and another star, and finding it so much larger than either, i suspected it to be a comet." the "comet" was immediately observed by professional astronomers, and its orbit was computed by some of them. it was thus found to move in nearly a circle instead of an elongated ellipse, and to be nearly twice as far from the sun as saturn. it was no comet, it was a new planet; more than times as big as the earth, and nearly twice as far away as saturn. it was presently christened "uranus." this was a most striking discovery, and the news sped over europe. to understand the interest it excited we must remember that such a discovery was unique. since the most ancient times of which men had any knowledge, the planets mercury, venus, mars, jupiter, saturn, had been known, and there had been no addition to their number. galileo and others had discovered satellites indeed, but a new primary planet was an entire and utterly unsuspected novelty. one of the most immediate consequences of the event was the discovery of herschel himself. the royal society made him a fellow the same year. the university of oxford dubbed him a doctor; and the king sent for him to bring his telescope and show it at court. so to london and windsor he went, taking with him his best telescope. maskelyne, the then astronomer-royal, compared it with the national one at greenwich, and found herschel's home-made instrument far the better of the two. he had a stand made after herschel's pattern, but was so disgusted with his own instrument now that he scarcely thought it worthy of the stand when it was made. at windsor, george iii. was very civil, and mr. herschel was in great request to show the ladies of the court saturn and other objects of interest. mr. herschel exhibited a piece of worldly wisdom under these circumstances, that recalls faintly the behaviour of tycho brahé under similar circumstances. the evening when the exhibition was to take place threatened to become cloudy and wet, so herschel rigged up an artificial saturn, constructed of card and tissue paper, with a lamp behind it, in the distant wall of a garden; and, when the time came, his new titled friends were regaled with a view of this imitation saturn through the telescope--the real one not being visible. they went away much pleased. he stayed hovering between windsor and greenwich, and uncertain what was to be the outcome of all this regal patronizing. he writes to his sister that he would much rather be back grinding mirrors at bath. and she writes begging him to come, for his musical pupils were getting impatient. they had to get the better of their impatience, however, for the king ultimately appointed him astronomer or rather telescope-maker to himself, and so caroline and the whole household were sent for, and established in a small house at datchet. from being a star-gazing musician, herschel thus became a practical astronomer. henceforth he lived in his observatory; only on wet and moonlight nights could he be torn away from it. the day-time he devoted to making his long-contemplated -foot telescope. not yet, however, were all their difficulties removed. the house at datchet was a tumble-down barn of a place, chosen rather as a workshop and observatory than as a dwelling-house. and the salary allowed him by george iii. was scarcely a princely one. it was, as a matter of fact, £ a year. the idea was that he would earn his living by making telescopes, and so indeed he did. he made altogether some hundreds. among others, four for the king. but this eternal making of telescopes for other people to use or play with was a weariness to the flesh. what he wanted was to observe, observe, observe. sir william watson, an old friend of his, and of some influence at court, expressed his mind pretty plainly concerning herschel's position; and as soon as the king got to understand that there was anything the matter, he immediately offered £ , for a gigantic telescope to be made for herschel's own use. nothing better did he want in life. the whole army of carpenters and craftsmen resident in datchet were pressed into the service. furnaces for the speculum metal were built, stands erected, and the -foot telescope fairly begun. it cost £ , before it was finished, but the king paid the whole. [illustration: fig. .--herschel's -foot telescope.] with it he discovered two more satellites to saturn (five hitherto had been known), and two moons to his own planet uranus. these two are now known as oberon and titania. they were not seen again till some forty years after, when his son, sir john herschel, reobserved them. and in , mr. lassell, at his house, "starfield," near liverpool, discovered two more, called ariel and umbriel, making the number four, as now known. mr. lassell also discovered, with a telescope of his own making, an eighth satellite of saturn--hyperion--and a satellite to neptune. a letter from a foreign astronomer about this period describes herschel and his sister's method of work:-- "i spent the night of the th of january at herschel's, in datchet, near windsor, and had the good luck to hit on a fine evening. he has his -foot newtonian telescope in the open air, and mounted in his garden very simply and conveniently. it is moved by an assistant, who stands below it.... near the instrument is a clock regulated to sidereal time.... in the room near it sits herschel's sister, and she has flamsteed's atlas open before her. as he gives her the word, she writes down the declination and right ascension, and the other circumstances of the observation. in this way herschel examines the whole sky without omitting the least part. he commonly observes with a magnifying power of one hundred and fifty, and is sure that after four or five years he will have passed in review every object above our horizon. he showed me the book in which his observations up to this time are written, and i am astonished at the great number of them. each sweep covers ° ' in declination, and he lets each star pass at least three times through the field of his telescope, so that it is impossible that anything can escape him. he has already found about double stars, and almost as many nebulæ. i went to bed about one o'clock, and up to that time he had found that night four or five new nebulæ. the thermometer in the garden stood at ° fahrenheit; but, in spite of this, herschel observes the whole night through, except that he stops every three or four hours and goes into the room for a few moments. for some years herschel has observed the heavens every hour when the weather is clear, and this always in the open air, because he says that the telescope only performs well when it is at the same temperature as the air. he protects himself against the weather by putting on more clothing. he has an excellent constitution, and thinks about nothing else in the world but the celestial bodies. he has promised me in the most cordial way, entirely in the service of astronomy, and without thinking of his own interest, to see to the telescopes i have ordered for european observatories, and he will himself attend to the preparation of the mirrors." [illustration: _painted by abbott._ _engraved by ryder._ fig. .--william herschel. _from an original picture in the possession of_ wm. watson, m.d., f.r.s.] in , herschel married an estimable lady who sympathized with his pursuits. she was the only daughter of a city magnate, so his pecuniary difficulties, such as they were (they were never very troublesome to him), came to an end. they moved now into a more commodious house at slough. their one son, afterwards the famous sir john herschel, was born some nine years later. but the marriage was rather a blow to his devoted sister: henceforth she lived in lodgings, and went over at night-time to help him observe. for it must be remarked that this family literally turned night into day. whatever sleep they got was in the day-time. every fine night without exception was spent in observing: and the quite incredible fierceness of the pursuit is illustrated, as strongly as it can be, by the following sentence out of caroline's diary, at the time of the move from datchet to slough: "the last night at datchet was spent in sweeping till daylight, and by the next evening the telescope stood ready for observation at slough." caroline was now often allowed to sweep with a small telescope on her own account. in this way she picked up a good many nebulæ in the course of her life, and eight comets, four of which were quite new, and one of which, known since as encke's comet, has become very famous. the work they got through between them is something astonishing. he made with his own hands parabolic mirrors for reflecting telescopes, besides a great number of complete instruments. he was forty-two when he began contributing to the royal society; yet before he died he had sent them sixty-nine long and elaborate treatises. one of these memoirs is a catalogue of nebulæ. fifteen years after he sends in another ; and some years later another . he also discovered double stars, which he proved were really corrected from the fact that they revolved round each other (p. ). he lived to see some of them perform half a revolution. for him the stars were not fixed: they moved slowly among themselves. he detected their proper motions. he passed the whole northern firmament in review four distinct times; counted the stars in , gauge-fields, and estimated the brightness of hundreds of stars. he also measured as accurately as he could their proper motions, devising for this purpose the method which still to this day remains in use. and what is the outcome of it all? it is not uranus, nor the satellites, nor even the double stars and the nebulæ considered as mere objects: it is the beginning of a science of the stars. [illustration: fig. .--caroline herschel. _from a drawing from life, by_ george mÜller, .] hitherto the stars had only been observed for nautical and practical purposes. their times of rising and southing and setting had been noted; they had been treated as a clock or piece of dead mechanism, and as fixed points of reference. all the energies of astronomers had gone out towards the solar system. it was the planets that had been observed. tycho had observed and tabulated their positions. kepler had found out some laws of their motion. galileo had discovered their peculiarities and attendants. newton and laplace had perceived every detail of their laws. but for the stars--the old ptolemaic system might still have been true. they might still be mere dots in a vast crystalline sphere, all set at about one distance, and subservient to the uses of the earth. herschel changed all this. instead of sameness, he found variety; instead of uniformity of distance, limitless and utterly limitless fields and boundless distances; instead of rest and quiescence, motion and activity; instead of stagnation, life. [illustration: fig. .--the double-double star [epsilon] lyræ as seen under three different powers.] yes, that is what herschel discovered--the life and activity of the whole visible universe. no longer was our little solar system to be the one object of regard, no longer were its phenomena to be alone interesting to man. with herschel every star was a solar system. and more than that: he found suns revolving round suns, at distances such as the mind reels at, still obeying the same law of gravitation as pulls an apple from a tree. he tried hard to estimate the distance of the stars from the earth, but there he failed: it was too hopeless a problem. it was solved some time after his death by bessel, and the distances of many stars are now known but these distances are awful and unspeakable. our distance from the sun shrinks up into a mere speck--the whole solar system into a mere unit of measurement, to be repeated hundreds of thousands of times before we reach the stars. yet their motion is visible--yes, to very accurate measurement quite plain. one star, known as cygni, was then and is now rushing along at the rate of miles every second. not that you must imagine that this makes any obvious and apparent change in its position. no, for all ordinary and practical purposes they are still fixed stars; thousands of years will show us no obvious change; "adam" saw precisely the same constellations as we do: it is only by refined micrometric measurement with high magnifying power that their flight can be detected. but the sun is one of the stars--not by any means a specially large or bright one; sirius we now know to be twenty times as big as the sun. the sun is one of the stars: then is it at rest? herschel asked this question and endeavoured to answer it. he succeeded in the most astonishing manner. it is, perhaps, his most remarkable discovery, and savours of intuition. this is how it happened. with imperfect optical means and his own eyesight to guide him, he considered and pondered over the proper motion of the stars as he had observed it, till he discovered a kind of uniformity running through it all. mixed up with irregularities and individualities, he found that in a certain part of the heavens the stars were on the whole opening out--separating slowly from each other; on the opposite side of the heavens they were on the average closing up--getting slightly nearer to each other; while in directions at right angles to this they were fairly preserving their customary distances asunder. now, what is the moral to be drawn from such uniformity of behaviour among unconnected bodies? surely that this part of their motion is only apparent--that it is we who are moving. travelling over a prairie bounded by a belt of trees, we should see the trees in our line of advance opening out, and those behind closing up; we should see in fact the same kind of apparent motion as herschel was able to detect among the stars: the opening out being most marked near the constellation hercules. the conclusion is obvious: the sun, with all its planets, must be steadily moving towards a point in the constellation hercules. the most accurate modern research has been hardly able to improve upon this statement of herschel's. possibly the solar system may ultimately be found to revolve round some other body, but what that is no one knows. all one can tell is the present direction of the majestic motion: since it was discovered it has continued unchanged, and will probably so continue for thousands of years. [illustration: fig. .--old drawing of the cluster in hercules.] and, finally, concerning the nebulæ. these mysterious objects exercised a strong fascination for herschel, and many are the speculations he indulges in concerning them. at one time he regards them all as clusters of stars, and the milky way as our cluster; the others he regards as other universes almost infinitely distant; and he proceeds to gauge and estimate the shape of our own universe or galaxy of suns, the milky way. later on, however, he pictures to himself the nebulæ as nascent suns: solar systems before they are formed. some he thinks have begun to aggregate, while some are still glowing gas. [illustration: fig. .--old drawing of the andromeda nebula.] he likens the heavens to a garden in which there are plants growing in all manner of different stages: some shooting, some in leaf, some in flower, some bearing seed, some decaying; and thus at one inspection we have before us the whole life-history of the plant. just so he thinks the heavens contain worlds, some old, some dead, some young and vigorous, and some in the act of being formed. the nebulæ are these latter, and the nebulous stars are a further stage in the condensation towards a sun. and thus, by simple observation, he is led towards something very like the nebular hypothesis of laplace; and his position, whether it be true or false, is substantially the same as is held to-day. [illustration: fig. .--the great nebula in orion.] we _know_ now that many of the nebulæ consist of innumerable isolated particles and may be spoken of as gas. we know that some are in a state of whirling motion. we know also that such gas left to itself will slowly as it cools condense and shrink, so as to form a central solid nucleus; and also, if it were in whirling motion, that it would send off rings from itself, and that these rings could break up into planets. in two familiar cases the ring has not yet thus aggregated into planet or satellite--the zone of asteroids, and saturn's ring. the whole of this could not have been asserted in herschel's time: for further information the world had to wait. these are the problems of modern astronomy--these and many others, which are the growth of this century, aye, and the growth of the last thirty or forty, and indeed of the last ten years. even as i write, new and very confirmatory discoveries are being announced. the milky way _does_ seem to have some affinity with our sun. and the chief stars of the constellation of orion constitute another family, and are enveloped in the great nebula, now by photography perceived to be far greater than had ever been imagined. what is to be the outcome of it all i know not; but sure i am of this, that the largest views of the universe that we are able to frame, and the grandest manner of its construction that we can conceive, are certain to pale and shrink and become inadequate when confronted with the truth. notes to lecture xiii bode's law.--write down the series , , , , , , &c.; add to each, and divide by ; you get the series: · · · · · · · · · mercury venus earth mars ---- jupiter saturn uranus ---- numbers which very fairly represent the distances of the then known planets from the sun in the order specified. ceres was discovered on the st of january, , by piazzi; pallas in march, , by olbers; juno in , by harding; and vesta in , by olbers. no more asteroids were discovered till , but there are now several hundreds known. their diameters range from to miles. neptune was discovered from the perturbations of uranus by sheer calculation, carried on simultaneously and independently by leverrier in paris, and adams in cambridge. it was first knowingly seen by galle, of berlin, on the rd of september, . lecture xiii the discovery of the asteroids up to the time of herschel, astronomical interest centred on the solar system. since that time it has been divided, and a great part of our attention has been given to the more distant celestial bodies. the solar system has by no means lost its interest--it has indeed gained in interest continually, as we gain in knowledge concerning it; but in order to follow the course of science it will be necessary for us to oscillate to and fro, sometimes attending to the solar system--the planets and their satellites--sometimes extending our vision to the enormously more distant stellar spaces. those who have read the third lecture in part i. will remember the speculation in which kepler indulged respecting the arrangements of the planets, the order in which they succeeded one another in space, and the law of their respective distances from the sun; and his fanciful guess about the five regular solids inscribed and circumscribed about their orbits. the rude coincidences were, however, accidental, and he failed to discover any true law. no thoroughly satisfactory law is known at the present day. and yet, if the nebular hypothesis or anything like it be true, there must be some law to be discovered hereafter, though it may be a very complicated one. an empirical relation is, however, known: it was suggested by tatius, and published by bode, of berlin, in . it is always known as bode's law. bode's law asserts that the distance of each planet is approximately double the distance of the inner adjacent planet from the sun, but that the rate of increase is distinctly slower than this for the inner ones; consequently a better approximation will be obtained by adding a constant to each term of an appropriate geometrical progression. thus, form a doubling series like this, - / , , , , , &c. doubling each time; then add to each, and you get a series which expresses very fairly the relative distances of the successive planets from the sun, except that the number for mercury is rather erroneous, and we now know that at the other extreme the number for neptune is erroneous too. i have stated it in the notes above in a form calculated to give the law every chance, and a form that was probably fashionable after the discovery of uranus; but to call the first term of the doubling series is evidently not quite fair, though it puts mercury's distance right. neptune's distance, however, turns out to be more nearly times the earth's distance than · . the others are very nearly right: compare column d of the table preceding lecture iii. on p. , with the numbers in the notes on p. . the discovery of uranus a few years afterwards, in , at · times the earth's distance from the sun, lent great _éclât_ to the law, and seemed to establish its right to be regarded as at least a close approximation to the truth. the gap between mars and jupiter, which had often been noticed, and which kepler filled with a hypothetical planet too small to see, comes into great prominence by this law of bode. so much so, that towards the end of last century an enthusiastic german, von zach, after some search himself for the expected planet, arranged a committee of observing astronomers, or, as he termed it, a body of astronomical detective police, to begin a systematic search for this missing subject of the sun. [illustration: fig. .--planetary orbits to scale; showing the asteroidal region between jupiter and mars. (the orbits of satellites are exaggerated.)] in the preliminaries were settled: the heavens near the zodiac were divided into twenty-four regions, each of which was intrusted to one observer to be swept. meanwhile, however, quite independently of these arrangements in germany, and entirely unknown to this committee, a quiet astronomer in sicily, piazzi, was engaged in making a catalogue of the stars. his attention was directed to a certain region in taurus by an error in a previous catalogue, which contained a star really non-existent. in the course of his scrutiny, on the st of january, , he noticed a small star which next evening appeared to have shifted. he watched it anxiously for successive evenings, and by the th of january he was quite sure he had got hold of some moving body, not a star: probably, he thought, a comet. it was very small, only of the eighth magnitude; and he wrote to two astronomers (one of them bode himself) saying what he had observed. he continued to observe till the th of february, when he was attacked by illness and compelled to cease. his letters did not reach their destination till the end of march. directly bode opened his letter he jumped to the conclusion that this must be the missing planet. but unfortunately he was unable to verify the guess, for the object, whatever it was, had now got too near the sun to be seen. it would not be likely to be out again before september, and by that time it would be hopelessly lost again, and have just as much to be rediscovered as if it had never been seen. mathematical astronomers tried to calculate a possible orbit for the body from the observations of piazzi, but the observed places were so desperately few and close together. it was like having to determine a curve from three points close together. three observations ought to serve,[ ] but if they are taken with insufficient interval between them it is extremely difficult to construct the whole circumstances of the orbit from them. all the calculations gave different results, and none were of the slightest use. the difficulty as it turned out was most fortunate. it resulted in the discovery of one of the greatest mathematicians, perhaps the greatest, that germany has ever produced--gauss. he was then a young man of twenty-five, eking out a living by tuition. he had invented but not published several powerful mathematical methods (one of them now known as "the method of least squares"), and he applied them to piazzi's observations. he was thus able to calculate an orbit, and to predict a place where, by the end of the year, the planet should be visible. on the st of december of that same year, very near the place predicted by gauss, von zach rediscovered it, and olbers discovered it also the next evening. piazzi called it ceres, after the tutelary goddess of sicily. its distance from the sun as determined by gauss was · times the earth's distance. bode's law made it · . it was undoubtedly the missing planet. but it was only one hundred and fifty or two hundred miles in diameter--the smallest heavenly body known at the time of its discovery. it revolves the same way as other planets, but the plane of its orbit is tilted ° to the plane of the ecliptic, which was an exceptionally large amount. very soon, a more surprising discovery followed. olbers, while searching for ceres, had carefully mapped the part of the heavens where it was expected; and in march, , he saw in this place a star he had not previously noticed. in two hours he detected its motion, and in a month he sent his observations to gauss, who returned as answer the calculated orbit. it was distant · , like ceres, and was a little smaller, but it had a very excentric orbit: its plane being tilted - / °, an extraordinary inclination. this was called pallas. olbers at once surmised that these two planets were fragments of a larger one, and kept an eager look out for other fragments. in two years another was seen, in the course of charting the region of the heavens traversed by ceres and pallas. it was smaller than either, and was called juno. in the persevering search of olbers resulted in the discovery of another, with a very oblique orbit, which gauss named vesta. vesta is bigger than any of the others, being five hundred miles in diameter, and shines like a star of the sixth magnitude. gauss by this time had become so practised in the difficult computations that he worked out the complete orbit of vesta within ten hours of receiving the observational data from olbers. for many weary years olbers kept up a patient and unremitting search for more of these small bodies, or fragments of the large planet as he thought them; but his patience went unrewarded, and he died in without seeing or knowing of any more. in another was found, however, in germany, and a few weeks later two others by mr. hind in england. since then there seems no end to them; numbers have been discovered in america, where professors peters and watson have made a specialty of them, and have themselves found something like a hundred. vesta is the largest--its area being about the same as that of central europe, without russia or spain--and the smallest known is about twenty miles in diameter, or with a surface about the size of kent. the whole of them together do not nearly equal the earth in bulk. the main interest of these bodies to us lies in the question, what is their history? can they have been once a single planet broken up? or are they rather an abortive attempt at a planet never yet formed into one? the question is not _entirely_ settled, but i can tell you which way opinion strongly tends at the present time. imagine a shell travelling in an elliptic orbit round the earth to suddenly explode: the centre of gravity of all its fragments would continue moving along precisely the same path as had been traversed by the centre of the shell before explosion, and would complete its orbit quite undisturbed. each fragment would describe an orbit of its own, because it would be affected by a different initial velocity; but every orbit would be a simple ellipse, and consequently every piece would in time return through its starting-point--viz. the place at which the explosion occurred. if the zone of asteroids had a common point through which they all successively passed, they could be unhesitatingly asserted to be the remains of an exploded planet. but they have nothing of the kind; their orbits are scattered within a certain broad zone--a zone everywhere as broad as the earth's distance from the sun, , , miles--with no sort of law indicating an origin of this kind. it must be admitted, however, that the fragments of our supposed shell might in the course of ages, if left to themselves, mutually perturb each other into a different arrangement of orbits from that with which they began. but their perturbations would be very minute, and moreover, on laplace's theory, would only result in periodic changes, provided each mass were rigid. it is probable that the asteroids were at one time not rigid, and hence it is difficult to say what may have happened to them; but there is not the least reason to believe that their present arrangement is derivable in any way from an explosion, and it is certain that an enormous time must have elapsed since such an event if it ever occurred. it is far more probable that they never constituted one body at all, but are the remains of a cloudy ring thrown off by the solar system in shrinking past that point: a small ring after the immense effort which produced jupiter and his satellites: a ring which has aggregated into a multitude of little lumps instead of a few big ones. such an event is not unique in the solar system; there is a similar ring round saturn. at first sight, and to ordinary careful inspection, this differs from the zone of asteroids in being a solid lump of matter, like a quoit. but it is easy to show from the theory of gravitation, that a solid ring could not possibly be stable, but would before long get precipitated excentrically upon the body of the planet. devices have been invented, such as artfully distributed irregularities calculated to act as satellites and maintain stability; but none of these things really work. nor will it do to imagine the rings fluid; they too would destroy each other. the mechanical behaviour of a system of rings, on different hypotheses as to their constitution, has been worked out with consummate skill by clerk maxwell; who finds that the only possible constitution for saturn's assemblage of rings is a multitude of discrete particles each pursuing its independent orbit. saturn's ring is, in fact, a very concentrated zone of minor asteroids, and there is every reason to conclude that the origin of the solar asteroids cannot be very unlike the origin of the saturnian ones. the nebular hypothesis lends itself readily to both. the interlockings and motions of the particles in saturn's rings are most beautiful, and have been worked out and stated by maxwell with marvellous completeness. his paper constituted what is called "the adams prize essay" for . sir george airy, one of the adjudicators (recently astronomer-royal), characterized it as "one of the most remarkable applications of mathematics to physics that i have ever seen." there are several distinct constituent rings in the entire saturnian zone, and each perturbs the other, with the result that they ripple and pulse in concord. the waves thus formed absorb the effect of the mutual perturbations, and prevent an accumulation which would be dangerous to the persistence of the whole. the only effect of gravitational perturbation and of collisions is gradually to broaden out the whole ring, enlarging its outer and diminishing its inner diameter. but if there were any frictional resistance in the medium through which the rings spin, then other effects would slowly occur, which ought to be looked for with interest. so complete and intimate is the way maxwell works out and describes the whole circumstances of the motion of such an assemblage of particles, and so cogent his argument as to the necessity that they must move precisely so, and no otherwise, else the rings would not be stable, that it was a cambridge joke concerning him that he paid a visit to saturn one evening, and made his observations on the spot. notes to lecture xiv the total number of stars in the heavens visible to a good eye is about , . the total number at present seen by telescope is about , , . the number able to impress a photographic plate has not yet been estimated; but it is enormously greater still. of those which we can see in these latitudes, about are of the first magnitude, of the second, of the third, of the fourth, of the fifth, and , of the sixth; total, , . the quickest-moving stars known are a double star of the sixth magnitude, called cygni, and one of the seventh magnitude, called groombridge . the velocity of the latter is miles a second. the nearest known stars are cygni and [alpha] centauri. the distance of these from us is about , times the distance of the sun. their parallax is accordingly half a second of arc. sirius is more than a million times further from us than our sun is, and twenty times as big; many of the brightest stars are at more than double this distance. the distance of arcturus is too great to measure even now. stellar parallax was first securely detected in , by bessel, for cygni. bessel was born in , and died in , shortly before the discovery of neptune. the stars are suns, and are most likely surrounded by planets. one planet belonging to sirius has been discovered. it was predicted by bessel, its position calculated by peters, and seen by alvan clark in . another predicted one, belonging to procyon, has not yet been seen. a velocity of miles a second could carry a projectile right round the earth. a velocity of miles a second would carry it away from the earth, and round the sun. a velocity of miles a second would carry a projectile right out of the solar system never to return. lecture xiv bessel--the distances of the stars, and the discovery of stellar planets we will now leave the solar system for a time, and hastily sketch the history of stellar astronomy from the time of sir william herschel. you remember how greatly herschel had changed the aspect of the heavens for man,--how he had found that none of the stars were really fixed, but were moving in all manner of ways: some of this motion only apparent, much of it real. nevertheless, so enormously distant are they, that if we could be transported back to the days of the old chaldæan astronomers, or to the days of noah, we should still see the heavens with precisely the same aspect as they wear now. only by refined apparatus could any change be discoverable in all those centuries. for all practical purposes, therefore, the stars may still be well called fixed. another thing one may notice, as showing their enormous distances, is that from every planet of the solar system the aspect of the heavens will be precisely the same. inhabitants of mars, or jupiter, or saturn, or uranus, will see exactly the same constellations as we do. the whole dimensions of the solar system shrink up into a speck when so contemplated. and from the stars none of the planetary orbs of our system are visible at all; nothing but the sun is visible, and that merely as a twinkling star, brighter than some, but fainter than many others. the sun and the stars are one. try to realize this distinctly, and keep it in mind. i find it often difficult to drive this idea home. after some talk on the subject a friendly auditor will report, "the lecturer then described the stars, including that greatest and most magnificent of all stars, the sun." it would be difficult more completely to misapprehend the entire statement. when i say the sun is one of the stars, i mean one among the others; we are a long way from them, they are a long way from each other. they need be no more closely packed among each other than we are closely packed among them; except that some of them are double or multiple, and we are not double. it is highly desirable to acquire an intimate knowledge of the constellations and a nodding acquaintance with their principal stars. a description of their peculiarities is dull and uninteresting unless they are at least familiar by name. a little _vivâ voce_ help to begin with, supplemented by patient night scrutiny with a celestial globe or star maps under a tent or shed, is perhaps the easiest way: a very convenient instrument for the purpose of learning the constellations is the form of map called a "planisphere," because it can be made to show all the constellations visible at a given time at a given date, and no others. the greek alphabet also is a thing that should be learnt by everybody. the increased difficulty in teaching science owing to the modern ignorance of even a smattering of greek is becoming grotesque. the stars are named from their ancient grouping into constellations, and by the prefix of a greek letter to the larger ones, and of numerals to the smaller ones. the biggest of all have special arabic names as well. the brightest stars are called of "the first magnitude," the next are of "the second magnitude," and so on. but this arrangement into magnitudes has become technical and precise, and intermediate or fractional magnitudes are inserted. those brighter than the ordinary first magnitude are therefore now spoken of as of magnitude / , for instance, or · , which is rather confusing. small telescopic stars are often only named by their numbers in some specified catalogue--a dull but sufficient method. here is a list of the stars visible from these latitudes, which are popularly considered as of the first magnitude. all of them should be familiarly recognized in the heavens, whenever seen. star. constellation. sirius canis major procyon canis minor rigel orion betelgeux orion castor gemini pollux gemini aldebaran taurus arcturus boötes vega lyra capella auriga regulus leo altair aquila fomalhaut southern fish spica virgo [alpha] cygni is a little below the first magnitude. so, perhaps, is castor. in the southern heavens, canopus and [alpha] centauri rank next after sirius in brightness. [illustration: fig. .--diagram illustrating parallax.] the distances of the fixed stars had, we know, been a perennial problem, and many had been the attempts to solve it. all the methods of any precision have depended on the copernican fact that the earth in june was million miles away from its position in december, and that accordingly the grouping and aspect of the heavens should be somewhat different when seen from so different a point of view. an apparent change of this sort is called generally parallax; _the_ parallax of a star being technically defined as the angle subtended at the star by the radius of the earth's orbit: that is to say, the angle e[sigma]s; where e is the earth, s the sun, and [sigma] a star (fig. ). plainly, the further off [sigma] is, the more nearly parallel will the two lines to it become. and the difficulty of determining the parallax was just this, that the more accurately the observations were made, the more nearly parallel did those lines become. the angle was, in fact, just as likely to turn out negative as positive--an absurd result, of course, to be attributed to unavoidable very minute inaccuracies. for a long time absolute methods of determining parallax were attempted; for instance, by observing the position of the star with respect to the zenith at different seasons of the year. and many of these determinations appeared to result in success. hooke fancied he had measured a parallax for vega in this way, amounting to " of arc. flamsteed obtained " for [gamma] draconis. roemer made a serious attempt by comparing observations of vega and sirius, stars almost the antipodes of each other in the celestial vault; hoping to detect some effect due to the size of the earth's orbit, which should apparently displace them with the season of the year. all these fancied results however, were shown to be spurious, and their real cause assigned, by the great discovery of the aberration of light by bradley. after this discovery it was possible to watch for still outstanding very minute discrepancies; and so the problem of stellar parallax was attacked with fresh vigour by piazzi, by brinkley, and by struve. but when results were obtained, they were traced after long discussion to age and gradual wear of the instrument, or to some other minute inaccuracy. the more carefully the observation was made, the more nearly zero became the parallax--the more nearly infinite the distance of the stars. the brightest stars were the ones commonly chosen for the investigation, and vega was a favourite, because, going near the zenith, it was far removed from the fluctuating and tiresome disturbances of atmospheric refraction. the reason bright stars were chosen was because they were presumably nearer than the others; and indeed a rough guess at their probable distance was made by supposing them to be of the same size as the sun, and estimating their light in comparison with sunlight. by this confessedly unsatisfactory method it had been estimated that sirius must be , times further away than the sun is, if he be equally big. we now know that sirius is much further off than this; and accordingly that he is much brighter, perhaps sixty times as bright, though not necessarily sixty times as big, as our sun. but even supposing him of the same light-giving power as the sun, his parallax was estimated as "· , a quantity very difficult to be sure of in any absolute determination. relative methods were, however, also employed, and the advantages of one of these (which seems to have been suggested by galileo) so impressed themselves upon william herschel that he made a serious attempt to compass the problem by its means. the method was to take two stars in the same telescopic field and carefully to estimate their apparent angular distance from each other at different seasons of the year. all such disturbances as precession, aberration, nutation, refraction, and the like, would affect them both equally, and could thus be eliminated. if they were at the same distance from the solar system, relative parallax would, indeed, also be eliminated; but if, as was probable, they were at different distances, then they would apparently shift relatively to one another, and the amount of shift, if it could be observed, would measure, not indeed the distance of either from the earth, but their distance from each other. and this at any rate would be a step. it might be completed by similarly treating other stars in the same field, taking them in pairs together. a bright and a faint star would naturally be suitable, because their distances were likely to be unequal; and so herschel fixed upon a number of doublets which he knew of, containing one bright and one faint component. for up to that time it had been supposed that such grouping in occasional pairs or triplets was chance coincidence, the two being optically foreshortened together, but having no real connection or proximity. herschel failed in what he was looking for, but instead of that he discovered the real connection of a number of these doublets, for he found that they were slowly revolving round each other. there are a certain number of merely optical or accidental doublets, but the majority of them are real pairs of suns revolving round each other. this relative method of mapping micrometrically a field of neighbouring stars, and comparing their configuration now and six months hence, was, however, the method ultimately destined to succeed; and it is, i believe, the only method which has succeeded down to the present day. certainly it is the method regularly employed, at dunsink, at the cape of good hope, and everywhere else where stellar parallax is part of the work. between and the question was ripe for settlement, and, as frequently happens with a long-matured difficulty, it gave way in three places at once. bessel, henderson, and struve almost simultaneously announced a stellar parallax which could reasonably be accepted. bessel was a little the earliest, and by far the most accurate. his, indeed, was the result which commanded confidence, and to him the palm must be awarded. he was largely a self-taught student, having begun life in a counting-house, and having abandoned business for astronomy. but notwithstanding these disadvantages, he became a highly competent mathematician as well as a skilful practical astronomer. he was appointed to superintend the construction of germany's first great astronomical observatory, that of königsberg, which, by his system, zeal, and genius, he rapidly made a place of the first importance. struve at dorpat, bessel at königsberg, and henderson at the cape of good hope--all of them at newly-equipped observatories--were severally engaged at the same problem. but the russian and german observers had the advantage of the work of one of the most brilliant opticians--i suppose the most brilliant--that has yet appeared: fraunhofer, of munich. an orphan lad, apprenticed to a maker of looking-glasses, and subject to hard struggles and privations in early life, he struggled upwards, and ultimately became head of the optical department of a munich firm of telescope-makers. here he constructed the famous "dorpat refractor" for struve, which is still at work; and designed the "königsberg heliometer" for bessel. he also made a long and most skilful research into the solar spectrum, which has immortalized his name. but his health was broken by early trials, and he died at the age of thirty-nine, while planning new and still more important optical achievements. a heliometer is the most accurate astronomical instrument for relative measurements of position, as a transit circle is the most accurate for absolute determinations. it consists of an equatorial telescope with object-glass cut right across, and each half movable by a sliding movement one past the other, the amount by which the two halves are dislocated being read off by a refined method, and the whole instrument having a multitude of appendages conducive to convenience and accuracy. its use is to act as a micrometer or measurer of small distances.[ ] each half of the object-glass gives a distinct image, which may be allowed to coincide or may be separated as occasion requires. if it be the components of a double star that are being examined, each component will in general be seen double, so that four images will be seen altogether; but by careful adjustment it will be possible to arrange that one image of each pair shall be superposed on or coincide with each other, in which case only three images are visible; the amount of dislocation of the halves of the object-glass necessary to accomplish this is what is read off. the adjustment is one that can be performed with extreme accuracy, and by performing it again and again with all possible modifications, an extremely accurate determination of the angular distance between the two components is obtained. [illustration: fig. .--heliometer.] bessel determined to apply this beautiful instrument to the problem of stellar parallax; and he began by considering carefully the kind of star for which success was most likely. hitherto the brightest had been most attended to, but bessel thought that quickness of proper motion would be a still better test of nearness. not that either criterion is conclusive as to distance, but there was a presumption in favour of either a very bright or an obviously moving star being nearer than a faint or a stationary one; and as the "bright" criterion had already been often applied without result, he decided to try the other. he had already called attention to a record by piazzi in of a double star in cygnus whose proper motion was five seconds of arc every year--a motion which caused this telescopic object, cygni, to be known as "the flying star." its motion is not really very perceptible, for it will only have traversed one-third of a lunar diameter in the course of a century; still it was the quickest moving star then known. the position of this interesting double he compared with two other stars which were seen simultaneously in the field of the heliometer, by the method i have described, throughout the whole year ; and in the last month of that year he was able to announce with confidence a distinct though very small parallax; substantiating it with a mass of detailed evidence which commanded the assent of astronomers. the amount of it he gave as one-third of a second. we know now that he was very nearly right, though modern research makes it more like half a second.[ ] soon afterwards, struve announced a quarter of a second as the parallax of vega, but that is distinctly too great; and henderson announced for [alpha] centauri (then thought to be a double) a parallax of one second, which, if correct, would make it quite the nearest of all the stars, but the result is now believed to be about twice too big. knowing the distance of cygni, we can at once tell its real rate of travel--at least, its rate across our line of sight: it is rather over three million miles a day. now just consider the smallness of the half second of arc, thus triumphantly though only approximately measured. it is the angle subtended by twenty-six feet at a distance of , miles. if a telescope planted at new york could be directed to a house in england, and be then turned so as to set its cross-wire first on one end of an ordinary room and then on the other end of the same room, it would have turned through half a second, the angle of greatest stellar parallax. or, putting it another way. if the star were as near us as new york is, the sun, on the same scale, would be nine paces off. as twenty-six feet is to the distance of new york, so is ninety-two million miles to the distance of the nearest fixed star. suppose you could arrange some sort of telegraphic vehicle able to carry you from here to new york in the tenth part of a second--_i.e._ in the time required to drop two inches--such a vehicle would carry you to the moon in twelve seconds, to the sun in an hour and a quarter. travelling thus continually, in twenty-four hours you would leave the last member of the solar system behind you, and begin your plunge into the depths of space. how long would it be before you encountered another object? a month, should you guess? twenty years you must journey with that prodigious speed before you reach the nearest star, and then another twenty years before you reach another. at these awful distances from one another the stars are scattered in space, and were they not brilliantly self-luminous and glowing like our sun, they would be hopelessly invisible. i have spoken of cygni as a flying star, but there is another which goes still quicker, a faint star, in groombridge's catalogue. its distance is far greater than that of cygni, and yet it is seen to move almost as quickly. its actual speed is about miles a second--greater than the whole visible firmament of fifty million stars can control; and unless the universe is immensely larger than anything we can see with the most powerful telescopes, or unless there are crowds of invisible non-luminous stars mixed up with the others, it can only be a temporary visitor to this frame of things; it is rushing from an infinite distance to an infinite distance; it is passing through our visible universe for the first and only time--it will never return. but so gigantic is the extent of visible space, that even with its amazing speed of miles every second, this star will take two or three million years to get out of sight of our present telescopes, and several thousand years before it gets perceptibly fainter than it is now. have we any reason for supposing that the stars we see are all there are? in other words, have we any reason for supposing all celestial objects to be sufficiently luminous to be visible? we have every ground for believing the contrary. every body in the solar system is dull and dark except the sun, though probably jupiter is still red-hot. why may not some of the stars be dark too? the genius of bessel surmised this, and consistently upheld the doctrine that the astronomy of the future would have to concern itself with dark and invisible bodies; he preached "an astronomy of the invisible." moreover he predicted the presence of two such dark bodies--one a companion of sirius, the other of procyon. he noticed certain irregularities in the motions of these stars which he asserted must be caused by their revolving round other bodies in a period of half a century. he announced in that both sirius and procyon were double stars, but that their companions, though large, were dark, and therefore invisible. no one accepted this view, till peters, in america, found in that the hypothesis accurately explained the anomalous motion of sirius, and, in fact, indicated an exact place where the companion ought to be. the obscure companion of sirius became now a recognized celestial object, although it had never been seen, and it was held to revolve round sirius in fifty years, and to be about half as big. in , the firm of alvan clark and sons, of new york, were completing a magnificent -inch refractor, and the younger clark was trying it on sirius, when he said: "why, father, the star has a companion!" the elder clark also looked, and sure enough there was a faint companion due east of the bright star, and in just the position required by theory. not that the clarks knew anything about the theory. they were keen-sighted and most skilful instrument-makers, and they made the discovery by accident. after it had once been seen, it was found that several of the large telescopes of the world were able to show it. it is half as big, but it only gives / th part of the light that sirius gives. no doubt it shines partly with a borrowed light and partly with a dull heat of its own. it is a real planet, but as yet too hot to live on. it will cool down in time, as our earth has cooled and as jupiter is cooling, and no doubt become habitable enough. it does revolve round sirius in a period of · years--almost exactly what bessel assigned to it. but bessel also assigned a dark companion to procyon. it and its luminous neighbour are considered to revolve round each other in a period of forty years, and astronomers feel perfectly assured of its existence, though at present it has not been seen by man. lecture xv the discovery of neptune we approach to-night perhaps the greatest, certainly the most conspicuous, triumphs of the theory of gravitation. the explanation by newton of the observed facts of the motion of the moon, the way he accounted for precession and nutation and for the tides, the way in which laplace explained every detail of the planetary motions--these achievements may seem to the professional astronomer equally, if not more, striking and wonderful; but of the facts to be explained in these cases the general public are necessarily more or less ignorant, and so no beauty or thoroughness of treatment appeals to them, nor can excite their imaginations. but to predict in the solitude of the study, with no weapons other than pen, ink, and paper, an unknown and enormously distant world, to calculate its orbit when as yet it had never been seen, and to be able to say to a practical astronomer, "point your telescope in such a direction at such a time, and you will see a new planet hitherto unknown to man"--this must always appeal to the imagination with dramatic intensity, and must awaken some interest in almost the dullest. prediction is no novelty in science; and in astronomy least of all is it a novelty. thousands of years ago, thales, and others whose very names we have forgotten, could predict eclipses with some certainty, though with only rough accuracy. and many other phenomena were capable of prediction by accumulated experience. we have seen, for instance (coming to later times), how a gap between mars and jupiter caused a missing planet to be suspected and looked for, and to be found in a hundred pieces. we have seen, also, how the abnormal proper-motion of sirius suggested to bessel the existence of an unseen companion. and these last instances seem to approach very near the same class of prediction as that of the discovery of neptune. wherein, then, lies the difference? how comes it that some classes of prediction--such as that if you put your finger in fire it will get burnt--are childishly easy and commonplace, while others excite in the keenest intellects the highest feelings of admiration? mainly, the difference lies, first, in the grounds on which the prediction is based; second, on the difficulty of the investigation whereby it is accomplished; third, in the completeness and the accuracy with which it can be verified. in all these points, the discovery of neptune stands out pre-eminently among the verified predictions of science, and the circumstances surrounding it are of singular interest. * * * * * in , sir william herschel discovered the planet uranus. now you know that three distinct observations suffice to determine the orbit of a planet completely, and that it is well to have the three observations as far apart as possible so as to minimize the effects of minute but necessary errors of observation. (see p. .) directly uranus was found, therefore, old records of stellar observations were ransacked, with the object of discovering whether it had ever been unwittingly seen before. if seen, it had been thought of course to be a star (for it shines like a star of the sixth magnitude, and can therefore be just seen without a telescope if one knows precisely where to look for it, and if one has good sight), but if it had been seen and catalogued as a star it would have moved from its place, and the catalogue would by that entry be wrong. the thing to detect, therefore, was errors in the catalogues: to examine all entries, and see if the stars entered actually existed, or were any of them missing. if a wrong entry were discovered, it might of course have been due to some clerical error, though that is hardly probable considering the care taken over these things, or it might have been some tailless comet or other, or it might have been the newly found planet. so the next thing was to calculate backwards, and see if by any possibility the planet could have been in that place at that time. examined in this way the tabulated observations of flamsteed showed that he had unwittingly observed uranus five distinct times, the first time in , nearly a century before herschel discovered its true nature. but more remarkable still, le monnier, of paris, had observed it eight times in one month, cataloguing it each time as a different star. if only he had reduced and compared his observations, he would have anticipated herschel by twelve years. as it was, he missed it altogether. it was seen once by bradley also. altogether it had been seen twenty times. these old observations of flamsteed and those of le monnier, combined with those made after herschel's discovery, were very useful in determining an exact orbit for the new planet, and its motion was considered thoroughly known. it was not an _exact_ ellipse, of course: none of the planets describe _exact_ ellipses--each perturbs all the rest, and these small perturbations must be taken into account, those of jupiter and saturn being by far the most important. for a time uranus seemed to travel regularly and as expected, in the orbit which had been calculated for it; but early in the present century it began to be slightly refractory, and by its actual place showed quite a distinct discrepancy from its position as calculated with the aid of the old observations. it was at first thought that this discrepancy must be due to inaccuracies in the older observations, and they were accordingly rejected, and tables prepared for the planet based on the newer and more accurate observations only. but by it became apparent that it would not accurately obey even these. the error amounted to some ". by it was as much as ', or a minute and a half. this discrepancy is quite distinct, but still it is very small, and had two objects been in the heavens at once, the actual uranus and the theoretical uranus, no unaided eye could possibly have distinguished them or detected that they were other than a single star. [illustration: fig. .--perturbations of uranus. the chance observations by flamsteed, by le monnier, and others, are plotted in this diagram, as well as the modern determinations made after herschel had discovered the nature of the planet. the decades are laid off horizontally. vertical distance represents the difference between observed and subsequently calculated longitudes--in other words, the principal perturbations caused by neptune. to show the scale, a number of standard things are represented too by lengths measured upwards from the line of time, viz: the smallest quantity perceptible to the naked eye,--the maximum angle of aberration, of nutation, and of stellar parallax; though this last is too small to be properly indicated. the perturbations are much bigger than these; but compared with what can be seen without a telescope they are small--the distance between the component pairs of [epsilon] lyræ ( ") (see fig. , page ), which a few keen-eyed persons can see as a simple double star, being about twice the greatest perturbation.] the diagram shows all the irregularities plotted in the light of our present knowledge; and, to compare with their amounts, a few standard things are placed on the same scale, such as the smallest interval capable of being detected with the unaided eye, the distance of the component stars in [epsilon] lyræ, the constants of aberration, of nutation, and of stellar parallax. the errors of uranus therefore, though small, were enormously greater than things which had certainly been observed; there was an unmistakable discrepancy between theory and observation. some cause was evidently at work on this distant planet, causing it to disagree with its motion as calculated according to the law of gravitation. some thought that the exact law of gravitation did not apply to so distant a body. others surmised the presence of some foreign and unknown body, some comet, or some still more distant planet perhaps, whose gravitative attraction for uranus was the cause of the whole difficulty--some perturbations, in fact, which had not been taken into account because of our ignorance of the existence of the body which caused them. but though such an idea was mentioned among astronomers, it was not regarded with any special favour, and was considered merely as one among a number of hypotheses which could be suggested as fairly probable. it is perfectly right not to attach much importance to unelaborated guesses. not until the consequences of an hypothesis have been laboriously worked out--not until it can be shown capable of producing the effect quantitatively as well as qualitatively--does its statement rise above the level of a guess, and attain the dignity of a theory. a later stage still occurs when the theory has been actually and completely verified by agreement with observation. now the errors in the motion of uranus, _i.e._ the discrepancy between its observed and calculated longitudes--all known disturbing causes, such as jupiter and saturn, being allowed for--are as follows (as quoted by dr. haughton) in seconds of arc:-- ancient observations (casually made, as of a star). flamsteed + · " + · " + · le monnier - · bradley - · mayer - · le monnier - · " - · " - · modern observations. + · + · + · + · + · + · + · + · + · - · - · - · - · these are the numbers plotted in the above diagram (fig. ), where h marks the discovery of the planet and the beginning of its regular observation. something was evidently the matter with the planet. if the law of gravitation held exactly at so great a distance from the sun, there must be some perturbing force acting on it besides all those known ones which had been fully taken into account. could it be an outer planet? the question occurred to several, and one or two tried if they could solve the problem, but were soon stopped by the tremendous difficulties of calculation. the ordinary problem of perturbation is difficult enough: given a disturbing planet in such and such a position, to find the perturbations it produces. this problem it was that laplace worked out in the _mécanique céleste_. but the inverse problem: given the perturbations, to find the planet which causes them--such a problem had never yet been attacked, and by only a few had its possibility been conceived. bessel made preparations for trying what he could do at it in , but he was prevented by fatal illness. in the difficulties of the problem presented by these residual perturbations of uranus excited the imagination of a young student, an undergraduate of st. john's college, cambridge--john couch adams by name--and he determined to have a try at it as soon as he was through his tripos. in january, , he graduated as senior wrangler, and shortly afterwards he set to work. in less than two years he reached a definite conclusion; and in october, , he wrote to the astronomer-royal, at greenwich, professor airy, saying that the perturbations of uranus would be explained by assuming the existence of an outer planet, which he reckoned was now situated in a specified latitude and longitude. we know now that had the astronomer-royal put sufficient faith in this result to point his big telescope to the spot indicated and commence sweeping for a planet, he would have detected it within - / ° of the place assigned to it by mr. adams. but any one in the position of the astronomer-royal knows that almost every post brings an absurd letter from some ambitious correspondent or other, some of them having just discovered perpetual motion, or squared the circle, or proved the earth flat, or discovered the constitution of the moon, or of ether, or of electricity; and out of this mass of rubbish it requires great skill and patience to detect such gems of value as there may be. now this letter of mr. adams's was indeed a jewel of the first water, and no doubt bore on its face a very different appearance from the chaff of which i have spoken; but still mr. adams was an unknown man: he had graduated as senior wrangler it is true, but somebody must graduate as senior wrangler every year, and every year by no means produces a first-rate mathematician. those behind the scenes, as professor airy of course was, having been a senior wrangler himself, knew perfectly well that the labelling of a young man on taking his degree is much more worthless as a testimony to his genius and ability than the general public are apt to suppose. was it likely that a young and unknown man should have successfully solved so extremely difficult a problem? it was altogether unlikely. still, he would test him: he would ask for further explanations concerning some of the perturbations which he himself had specially noticed, and see if mr. adams could explain these also by his hypothesis. if he could, there might be something in his theory. if he failed--well, there was an end of it. the questions were not difficult. they concerned the error of the radius vector. mr. adams could have answered them with perfect ease; but sad to say, though a brilliant mathematician, he was not a man of business. he did not answer professor airy's letter. it may to many seem a pity that the greenwich equatoreal was not pointed to the place, just to see whether any foreign object did happen to be in that neighbourhood; but it is no light matter to derange the work of an observatory, and alter the work mapped out for the staff into a sudden sweep for a new planet, on the strength of a mathematical investigation just received by post. if observatories were conducted on these unsystematic and spasmodic principles, they would not be the calm, accurate, satisfactory places they are. of course, if any one could have known that a new planet was to be had for the looking, _any_ course would have been justified; but no one could know this. i do not suppose that mr. adams himself could feel all that confidence in his attempted prediction. so there the matter dropped. mr. adams's communication was pigeon-holed, and remained in seclusion for eight or nine months. meanwhile, and quite independently, something of the same sort was going on in france. a brilliant young mathematician, born in normandy in , had accepted the post of astronomical professor at the École polytechnique, then recently founded by napoleon. his first published papers directed attention to his wonderful powers; and the official head of astronomy in france, the famous arago, suggested to him the unexplained perturbations of uranus as a worthy object for his fresh and well-armed vigour. at once he set to work in a thorough and systematic way. he first considered whether the discrepancies could be due to errors in the tables or errors in the old observations. he discussed them with minute care, and came to the conclusion that they were not thus to be explained away. this part of the work he published in november, . he then set to work to consider the perturbations produced by jupiter and saturn, to see if they had been with perfect accuracy allowed for, or whether some minute improvements could be made sufficient to destroy the irregularities. he introduced several fresh terms into these perturbations, but none of them of sufficient magnitude to do more than slightly lessen the unexplained perturbations. he next examined the various hypotheses that had been suggested to account for them:--was it a failure in the law of gravitation? was it due to the presence of a resisting medium? was it due to some unseen but large satellite? or was it due to a collision with some comet? all these he examined and dismissed for various reasons one after the other. it was due to some steady continuous cause--for instance, some unknown planet. could this planet be inside the orbit of uranus? no, for then it would perturb saturn and jupiter also, and they were not perturbed by it. it must, therefore, be some planet outside the orbit of uranus, and in all probability, according to bode's empirical law, at nearly double the distance from the sun that uranus is. lastly he proceeded to examine where this planet was, and what its orbit must be to produce the observed disturbances. [illustration: fig. .--uranus's and neptune's relative positions. the above diagram, drawn to scale by dr. haughton, shows the paths of uranus and neptune, and their positions from to , and illustrates the _direction_ of their mutual perturbing force. in the planets were in conjunction, and the force would then perturb the radius vector (or distance from the sun), but not the longitude (or place in orbit). before that date uranus had been hurried along, and after that date it had been retarded, by the pull of neptune, and thus the observed discrepancies from its computed place were produced. the problem was first to disentangle the outstanding perturbations from those which would be caused by jupiter and saturn and all other known causes, and then to assign the place of an outer planet able to produce precisely those perturbations in uranus.] not without failures and disheartening complications was this part of the process completed. this was, after all, the real tug of war. so many unknown quantities: its mass, its distance, its excentricity, the obliquity of its orbit, its position at any time--nothing known, in fact, about the planet except the microscopic disturbance it caused in uranus, some thousand million miles away from it. without going into further detail, suffice it to say that in june, , he published his last paper, and in it announced to the world his theoretical position for the planet. professor airy received a copy of this paper before the end of the month, and was astonished to find that leverrier's theoretical place for the planet was within ° of the place mr. adams had assigned to it eight months before. so striking a coincidence seemed sufficient to justify a herschelian "sweep" for a week or two. but a sweep for so distant a planet would be no easy matter. when seen in a large telescope it would still only look like a star, and it would require considerable labour and watching to sift it out from the other stars surrounding it. we know that uranus had been seen twenty times, and thought to be a star, before its true nature was by herschel discovered; and uranus is only about half as far away as neptune is. neither in paris nor yet at greenwich was any optical search undertaken; but professor airy wrote to ask m. leverrier the same old question as he had fruitlessly put to mr. adams: did the new theory explain the errors of the radius vector or not? the reply of leverrier was both prompt and satisfactory--these errors were explained, as well as all the others. the existence of the object was then for the first time officially believed in. the british association met that year at southampton, and sir john herschel was one of its sectional presidents. in his inaugural address, on september th, , he called attention to the researches of leverrier and adams in these memorable words:-- "the past year has given to us the new [minor] planet astræa; it has done more--it has given us the probable prospect of another. we see it as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration." it was about time to begin to look for it. so the astronomer-royal thought on reading leverrier's paper. but as the national telescope at greenwich was otherwise occupied, he wrote to professor challis, at cambridge, to know if he would permit a search to be made for it with the northumberland equatoreal, the large telescope of cambridge university, presented to it by one of the dukes of northumberland. professor challis said he would conduct the search himself; and shortly commenced a leisurely and dignified series of sweeps round about the place assigned by theory, cataloguing all the stars which he observed, intending afterwards to sort out his observations, compare one with another, and find out whether any one star had changed its position; because if it had it must be the planet. he thus, without giving an excessive time to the business, accumulated a host of observations, which he intended afterwards to reduce and sift at his leisure. the wretched man thus actually saw the planet twice--on august th and august th, --without knowing it. if only he had had a map of the heavens containing telescopic stars down to the tenth magnitude, and if he had compared his observations with this map as they were made, the process would have been easy, and the discovery quick. but he had no such map. nevertheless one was in existence: it had just been completed in that country of enlightened method and industry--germany. dr. bremiker had not, indeed, completed his great work--a chart of the whole zodiac down to stars of the tenth magnitude--but portions of it were completed, and the special region where the new planet was expected happened to be among the portions already just done. but in england this was not known. meanwhile, mr. adams wrote to the astronomer-royal several additional communications, making improvements in his theory, and giving what he considered nearer and nearer approximations for the place of the planet. he also now answered quite satisfactorily, but too late, the question about the radius vector sent to him months before. let us return to leverrier. this great man was likewise engaged in improving his theory and in considering how best the optical search could be conducted. actuated, probably, by the knowledge that in such matters as cataloguing and mapping germany was then, as now, far ahead of all the other nations of the world, he wrote in september (the same september as sir john herschel delivered his eloquent address at southampton) to berlin. leverrier wrote, i say, to dr. galle, head of the observatory at berlin, saying to him, clearly and decidedly, that the new planet was now in or close to such and such a position, and that if he would point his telescope to that part of the heavens he would see it; and, moreover, that he would be able to tell it from a star by its having a sensible magnitude, or disk, instead of being a mere point. galle got the letter on the rd of september, . that same evening he did point his telescope to the place leverrier told him, and he saw the planet that very night. he recognized it first by its appearance. to his practised eye it did seem to have a small disk, and not quite the same aspect as an ordinary star. he then consulted bremiker's great star chart, the part just engraved and finished, and sure enough on that chart there was no such star there. undoubtedly it was the planet. the news flashed over europe at the maximum speed with which news could travel at that date (which was not very fast); and by the st of october professor challis and mr. adams heard it at cambridge, and had the pleasure of knowing that they were forestalled, and that england was out of the race. it was an unconscious race to all concerned, however. those in france knew nothing of the search going on in england. mr. adams's papers had never been published; and very annoyed the french were when a claim was set up on his behalf to a share in this magnificent discovery. controversies and recriminations, excuses and justifications, followed; but the discussion has now settled down. all the world honours the bright genius and mathematical skill of mr. adams, and recognizes that he first solved the problem by calculation. all the world, too, perceives clearly the no less eminent mathematical talents of m. leverrier, but it recognizes in him something more than the mere mathematician--the man of energy, decision, and character. lecture xvi comets and meteors we have now considered the solar system in several aspects, and we have passed in review something of what is known about the stars. we have seen how each star is itself, in all probability, the centre of another and distinct solar system, the constituents of which are too dark and far off to be visible to us; nothing visible here but the central sun alone, and that only as a twinkling speck. but between our solar system and these other suns--between each of these suns and all the rest--there exist vast empty spaces, apparently devoid of matter. we have now to ask, are these spaces really empty? is there really nothing in space but the nebulæ, the suns, their planets, and their satellites? are all the bodies in space of this gigantic size? may there not be an infinitude of small bodies as well? the answer to this question is in the affirmative. there appears to be no special size suited to the vastness of space; we find, as a matter of fact, bodies of all manner of sizes, ranging by gradations from the most tremendous suns, like sirius, down through ordinary suns to smaller ones, then to planets of all sizes, satellites still smaller, then the asteroids, till we come to the smallest satellite of mars, only about ten miles in diameter, and weighing only some billion tons--the smallest of the regular bodies belonging to the solar system known. but, besides all these, there are found to occur other masses, not much bigger and some probably smaller, and these we call comets when we see them. below these, again, we find masses varying from a few tons in weight down to only a few pounds or ounces, and these when we see them, which is not often, we call meteors or shooting-stars; and to the size of these meteorites there would appear to be no limit: some may be literal grains of dust. there seems to be a regular gradation of size, therefore, ranging from sirius to dust; and apparently we must regard all space as full of these cosmic particles--stray fragments, as it were, perhaps of some older world, perhaps going to help to form a new one some day. as kepler said, there are more "comets" in the sky than fish in the sea. not that they are at all crowded together, else they would make a cosmic haze. the transparency of space shows that there must be an enormous proportion of clear space between each, and they are probably much more concentrated near one of the big bodies than they are in interstellar space.[ ] even during the furious hail of meteors in november it was estimated that their average distance apart in the thickest of the shower was miles. consider the nature of a meteor or shooting-star. we ordinarily see them as a mere streak of light; sometimes they leave a luminous tail behind them; occasionally they appear as an actual fire-ball, accompanied by an explosion; sometimes, but very seldom, they are seen to drop, and may subsequently be dug up as a lump of iron or rock, showing signs of rough treatment by excoriation and heat. these last are the meteorites, or siderites, or aërolites, or bolides, of our museums. they are popularly spoken of as thunderbolts, though they have nothing whatever to do with atmospheric electricity. [illustration: fig. .--meteorite.] they appear to be travelling rocky or metallic fragments which in their journey through space are caught in the earth's atmosphere and instantaneously ignited by the friction. far away in the depths of space one of these bodies felt the attracting power of the sun, and began moving towards him. as it approached, its speed grew gradually quicker and quicker continually, until by the time it has approached to within the distance of the earth, it whizzes past with the velocity of twenty-six miles a second. the earth is moving on its own account nineteen miles every second. if the two bodies happened to be moving in opposite directions, the combined speed would be terrific; and the faintest trace of atmosphere, miles above the earth's surface, would exert a furious grinding action on the stone. a stream of particles would be torn off; if of iron, they would burn like a shower of filings from a firework, thus forming a trail; and the mass itself would be dissipated, shattered to fragments in an instant. [illustration: fig. .--meteor stream crossing field of telescope.] [illustration: fig. .--diagram of direction of earth's orbital motion, showing that after midnight, _i.e._ between midnight and noon, more asteroids are likely to be swept up by any locality than between noon and midnight. [from sir r.s. ball.]] even if the earth were moving laterally, the same thing would occur. but if earth and stone happened to be moving in the same direction, there would be only the differential velocity of seven miles a second; and though this is in all conscience great enough, yet there might be a chance for a residue of the nucleus to escape entire destruction, though it would be scraped, heated, and superficially molten by the friction; but so much of its speed would be rubbed out of it, that on striking the earth it might bury itself only a few feet or yards in the soil, so that it could be dug out. the number of those which thus reach the earth is comparatively infinitesimal. nearly all get ground up and dissipated by the atmosphere; and fortunate it is for us that they are so. this bombardment of the exposed face of the moon must be something terrible.[ ] thus, then, every shooting-star we see, and all the myriads that we do not and cannot see because they occur in the day-time, all these bright flashes or streaks, represent the death and burial of one of these flying stones. it had been careering on its own account through space for untold ages, till it meets a planet. it cannot strike the actual body of the planet--the atmosphere is a sufficient screen; the tremendous friction reduces it to dust in an instant, and this dust then quietly and leisurely settles down on to the surface. evidence of the settlement of meteoric dust is not easy to obtain in such a place as england, where the dust which accumulates is seldom of a celestial character; but on the snow-fields of greenland or the himalayas dust can be found; and by a committee of the british association distinct evidence of molten globules of iron and other materials appropriate to aërolites has been obtained, by the simple process of collecting, melting, and filtering long exposed snow. volcanic ash may be mingled with it, but under the microscope the volcanic and the meteoric constituents have each a distinctive character. the quantity of meteoric material which reaches the earth as dust must be immensely in excess of the minute quantity which arrives in the form of lumps. hundreds or thousands of tons per annum must be received; and the accretion must, one would think, in the course of ages be able to exert some influence on the period of the earth's rotation--the length of the day. it is too small, however, to have been yet certainly detected. possibly, it is altogether negligible. it has been suggested that those stones which actually fall are not the true cosmic wanderers, but are merely fragments of our own earth, cast up by powerful volcanoes long ago when the igneous power of the earth was more vigorous than now--cast up with a speed of close upon seven miles a second; and now in these quiet times gradually being swept up by the earth, and so returning whence they came. i confess i am unable to draw a clear distinction between one set and the other. some falling stars may have had an origin of this sort, but certainly others have not; and it would seem very unlikely that one set only should fall bodily upon the earth, while the others should always be rubbed to powder. still, it is a possibility to be borne in mind. we have spoken of these cosmic visitors as wandering masses of stone or iron; but we should be wrong if we associated with the term "wandering" any ideas of lawlessness and irregularity of path. these small lumps of matter are as obedient to the law of gravity as any large ones can be. they must all, therefore, have definite orbits, and these orbits will have reference to the main attracting power of our system--they will, in fact, be nearly all careering round the sun. each planet may, in truth, have a certain following of its own. within the limited sphere of the earth's predominant attraction, for instance, extending some way beyond the moon, we may have a number of satellites that we never see, all revolving regularly in elliptic orbits round the earth. but, comparatively speaking, these satellite meteorites are few. the great bulk of them will be of a planetary character--they will be attendant upon the sun. it may seem strange that such minute bodies should have regular orbits and obey kepler's laws, but they must. all three laws must be as rigorously obeyed by them as by the planets themselves. there is nothing in the smallness of a particle to excuse it from implicit obedience to law. the only consequence of their smallness is their inability to perturb others. they cannot appreciably perturb either the planets they approach or each other. the attracting power of a lump one million tons in weight is very minute. a pound, on the surface of such a body of the same density as the earth, would be only pulled to it with a force equal to that with which the earth pulls a grain. so the perturbing power of such a mass on distant bodies is imperceptible. it is a good thing it is so: accurate astronomy would be impossible if we had to take into account the perturbations caused by a crowd of invisible bodies. astronomy would then approach in complexity some of the problems of physics. but though we may be convinced from the facts of gravitation that these meteoric stones, and all other bodies flying through space near our solar system, must be constrained by the sun to obey kepler's laws, and fly round it in some regular elliptic or hyperbolic orbit, what chance have we of determining that orbit? at first sight, a very poor chance, for we never see them except for the instant when they splash into our atmosphere; and for them that instant is instant death. it is unlikely that any escape that ordeal, and even if they do, their career and orbit are effectually changed. henceforward they must become attendants on the earth. they may drop on to its surface, or they may duck out of our atmosphere again, and revolve round us unseen in the clear space between earth and moon. nevertheless, although the problem of determining the original orbit of any given set of shooting-stars before it struck us would seem nearly insoluble, it has been solved, and solved with some approach to accuracy; being done by the help of observations of certain other bodies. the bodies by whose help this difficult problem has been attacked and resolved are comets. what are comets? i must tell you that the scientific world is not entirely and completely decided on the structure of comets. there are many floating ideas on the subject, and some certain knowledge. but the subject is still, in many respects, an open one, and the ideas i propose to advocate you will accept for no more than they are worth, viz. as worthy to be compared with other and different views. up to the time of newton, the nature of comets was entirely unknown. they were regarded with superstitious awe as fiery portents, and were supposed to be connected with the death of some king, or with some national catastrophe. even so late as the first edition of the _principia_ the problem of comets was unsolved, and their theory is not given; but between the first and the second editions a large comet appeared, in , and newton speculated on its appearance and behaviour. it rushed down very close to the sun, spun half round him very quickly, and then receded from him again. if it were a material substance, to which the law of gravitation applied, it must be moving in a conic section with the sun in one focus, and its radius vector must sweep out equal areas in equal times. examining the record of its positions made at observatories, he found its observed path quite accordant with theory; and the motion of comets was from that time understood. up to that time no one had attempted to calculate an orbit for a comet. they had been thought irregular and lawless bodies. now they were recognized as perfectly obedient to the law of gravitation, and revolving round the sun like everything else--as members, in fact, of our solar system, though not necessarily permanent members. but the orbit of a comet is very different from a planetary one. the excentricity of its orbit is enormous--in other words, it is either a very elongated ellipse or a parabola. the comet of , newton found to move in an orbit so nearly a parabola that the time of describing it must be reckoned in hundreds of years at the least. it is now thought possible that it may not be quite a parabola, but an ellipse so elongated that it will not return till . until that date arrives, however, uncertainty will prevail as to whether it is a periodic comet, or one of those that only visit our system once. if it be periodic, as suspected, it is the same as appeared when julius cæsar was killed, and which likewise appeared in the years and a.d. should it appear in , our posterity will probably regard it as a memorial of newton. [illustration: fig. .--parabolic and elliptic orbits. the _a b_ (visible) portions are indistinguishable.] the next comet discussed in the light of the theory of gravitation was the famous one of halley. you know something of the history of this. its period is - / years. halley saw it in , and predicted its return in or --the first cometary prediction. clairaut calculated its return right within a month (p. ). it has been back once more, in ; and this time its date was correctly predicted within three days, because uranus was now known. it was away at its furthest point in . it will be back again in . [illustration: fig. .--orbit of halley's comet.] coming to recent times, we have the great comets of and of , the history of neither being known. quite possibly they arrived then for the first time. possibly the second will appear again in . but besides these great comets, there are a multitude of telescopic ones, which do not show these striking features, and have no gigantic tail. some have no tail at all, others have at best a few insignificant streamers, and others show a faint haze looking like a microscopic nebula. all these comets are of considerable extent--some millions of miles thick usually, and yet stars are clearly visible through them. hence they must be matter of very small density; their tails can be nothing more dense than a filmy mist, but their nucleus must be something more solid and substantial. [illustration: fig. .--various appearances of halley's comet when last seen.] i have said that comets arrive from the depths of space, rush towards and round the sun, whizzing past the earth with a speed of twenty-six miles a second, on round the sun with a far greater velocity than that, and then rush off again. now, all the time they are away from the sun they are invisible. it is only as they get near him that they begin to expand and throw off tails and other appendages. the sun's heat is evidently evaporating them, and driving away a cloud of mist and volatile matter. this is when they can be seen. the comet is most gorgeous when it is near the sun, and as soon as it gets a reasonable distance away from him it is perfectly invisible. the matter evaporated from the comet by the sun's heat does not return--it is lost to the comet; and hence, after a few such journeys, its volatile matter gets appreciably diminished, and so old-established periodic comets have no tails to speak of. but the new visitants, coming from the depths of space for the first time--these have great supplies of volatile matter, and these are they which show the most magnificent tails. [illustration: fig. .--head of donati's comet of .] the tail of a comet is always directed away from the sun as if it were repelled. to this rule there is no exception. it is suggested, and held as most probable, that the tail and sun are similarly electrified, and that the repulsion of the tail is electrical repulsion. some great force is obviously at work to account for the enormous distance to which the tail is shot in a few hours. the pressure of the sun's light can do something, and is a force that must not be ignored when small particles are being dealt with. (cf. _modern views of electricity_, nd edition, p. .) now just think what analogies there are between comets and meteors. both are bodies travelling in orbits round the sun, and both are mostly invisible, but both become visible to us under certain circumstances. meteors become visible when they plunge into the extreme limits of our atmosphere. comets become visible when they approach the sun. is it possible that comets are large meteors which dip into the solar atmosphere, and are thus rendered conspicuously luminous? certainly they do not dip into the actual main atmosphere of the sun, else they would be utterly destroyed; but it is possible that the sun has a faint trace of atmosphere extending far beyond this, and into this perhaps these meteors dip, and glow with the friction. the particles thrown off might be, also by friction, electrified; and the vaporous tail might be thus accounted for. [illustration: fig. .--halley's comet.] let us make this hypothesis provisionally--that comets are large meteors, or a compact swarm of meteors, which, coming near the sun, find a highly rarefied sort of atmosphere, in which they get heated and partly vaporized, just as ordinary meteorites do when they dip into the atmosphere of the earth. and let us see whether any facts bear out the analogy and justify the hypothesis. i must tell you now the history of three bodies, and you will see that some intimate connection between comets and meteors is proved. the three bodies are known as, first, encke's comet; second, biela's comet; third, the november swarm of meteors. encke's comet (one of those discovered by miss herschel) is an insignificant-looking telescopic comet of small period, the orbit of which was well known, and which was carefully observed at each reappearance after encke had calculated its orbit. it was the quickest of the comets, returning every - / years. [illustration: fig. .--encke's comet.] it was found, however, that its period was not quite constant; it kept on getting slightly shorter. the comet, in fact, returned to the sun slightly before its time. now this effect is exactly what friction against a solar atmosphere would bring about. every time it passed near the sun a little velocity would be rubbed out of it. but the velocity is that which carries it away, hence it would not go quite so far, and therefore would return a little sooner. any revolving body subject to friction must revolve quicker and quicker, and get nearer and nearer its central body, until, if the process goes on long enough, it must drop upon its surface. this seems the kind of thing happening to encke's comet. the effect is very small, and not thoroughly proved; but, so far as it goes, the evidence points to a greatly extended rare solar atmosphere, which rubs some energy out of it at every perihelion passage. [illustration: fig. .--biela's comet as last seen, in two portions.] next, biela's comet. this also was a well known and carefully observed telescopic comet, with a period of six years. in one of its distant excursions, it was calculated that it must pass very near jupiter, and much curiosity was excited as to what would happen to it in consequence of the perturbation it must experience. as i have said, comets are only visible as they approach the sun, and a watch was kept for it about its appointed time. it was late, but it did ultimately arrive. the singular thing about it, however, was that it was now double. it had apparently separated into two. this was in . it was looked for again in , and this time the components were further separated. sometimes one was brighter, sometimes the other. next time it ought to have come round no one could find either portion. the comet seemed to have wholly disappeared. it has never been seen since. it was then recorded and advertised as the missing comet. but now comes the interesting part of the story. the orbit of this biela comet was well known, and it was found that on a certain night in the earth would cross the orbit, and had some chance of encountering the comet. not a very likely chance, because it need not be in that part of its orbit at the time; but it was suspected not to be far off--if still existent. well, the night arrived, the earth did cross the orbit, and there was seen, not the comet, but a number of shooting-stars. not one body, nor yet two, but a multitude of bodies--in fact, a swarm of meteors. not a very great swarm, such as sometimes occurs, but still a quite noticeable one; and this shower of meteors is definitely recognized as flying along the track of biela's comet. they are known as the andromedes. this observation has been generalized. every cometary orbit is marked by a ring of meteoric stones travelling round it, and whenever a number of shooting-stars are seen quickly one after the other, it is an evidence that we are crossing the track of some comet. but suppose instead of only crossing the track of a comet we were to pass close to the comet itself, we should then expect to see an extraordinary swarm--a multitude of shooting-stars. such phenomena have occurred. the most famous are those known as the november meteors, or leonids. this is the third of those bodies whose history i had to tell you. professor h.a. newton, of america, by examining ancient records arrived at the conclusion that the earth passed through a certain definite meteor shoal every thirty-three years. he found, in fact, that every thirty-three years an unusual flight of shooting-stars was witnessed in november, the earliest record being a.d. their last appearance had been in , and he therefore predicted their return in or . sure enough, in november, , they appeared; and many must remember seeing that glorious display. although their hail was almost continuous, it is estimated that their average distance apart was thirty-five miles! their radiant point was and always is in the constellation leo, and hence their name leonids. [illustration: fig. .--radiant point perspective. the arrows represent a number of approximately parallel meteor-streaks foreshortened from a common vanishing-point.] a parallel stream fixed in space necessarily exhibits a definite aspect with reference to the fixed stars. its aspect with respect to the earth will be very changeable, because of the rotation and revolution of that body, but its position with respect to constellations will be steady. hence each meteor swarm, being a steady parallel stream of rushing masses, always strikes us from the same point in stellar space, and by this point (or radiant) it is identified and named. the paths do not appear to us to be parallel, because of perspective: they seem to radiate and spread in all directions from a fixed centre like spokes, but all these diverging streaks are really parallel lines optically foreshortened by different amounts so as to produce the radiant impression. the annexed diagram (fig. ) clearly illustrates the fact that the "radiant" is the vanishing point of a number of parallel lines. [illustration: fig. .--orbit of november meteors.] this swarm is specially interesting to us from the fact that we cross its orbit every year. its orbit and the earth's intersect. every november we go through it, and hence every november we see a few stragglers of this immense swarm. the swarm itself takes thirty-three years on its revolution round the sun, and hence we only encounter it every thirty-three years. the swarm is of immense size. in breadth it is such that the earth, flying nineteen miles a second, takes four or five hours to cross it, and this is therefore the time the display lasts. but in length it is far more enormous. the speed with which it travels is twenty-five miles a second, (for its orbit extends as far as uranus, although by no means parabolic), and yet it takes more than a year to pass. imagine a procession , miles broad, every individual rushing along at the rate of twenty-five miles every second, and the whole procession so long that it takes more than a year to pass. it is like a gigantic shoal of herrings swimming round and round the sun every thirty-three years, and travelling past the earth with that tremendous velocity of twenty-five miles a second. the earth dashes through the swarm and sweeps up myriads. think of the countless numbers swept up by the whole earth in crossing such a shoal as that! but heaps more remain, and probably the millions which are destroyed every thirty-three years have not yet made any very important difference to the numbers still remaining. the earth never misses this swarm. every thirty-three years it is bound to pass through some part of them, for the shoal is so long that if the head is just missed one november the tail will be encountered next november. this is a plain and obvious result of its enormous length. it may be likened to a two-foot length of sewing silk swimming round and round an oval sixty feet in circumference. but, you will say, although the numbers are so great that destroying a few millions or so every thirty-three years makes but little difference to them, yet, if this process has been going on from all eternity, they ought to be all swept up. granted; and no doubt the most ancient swarms have already all or nearly all been swept up. [illustration: fig. .--orbit of november meteors; showing their probable parabolic orbit previous to a.d., and its sudden conversion into an elliptic orbit by the violent perturbation caused by uranus, which at that date occupied the position shown.] the august meteors, or perseids, are an example. every august we cross their path, and we have a small meteoric display radiating from the sword-hand of perseus, but never specially more in one august than another. it would seem as if the main shoal has disappeared, and nothing is now left but the stragglers; or perhaps it is that the shoal has gradually become uniformly distributed all along the path. anyhow, these august meteors are reckoned much more ancient members of the solar system than are the november meteors. the november meteors are believed to have entered the solar system in the year a.d. this may seem an extraordinary statement. it is not final, but it is based on the calculations of leverrier--confirmed recently by mr. adams. a few moments will suffice to make the grounds of it clear. leverrier calculated the orbit of the november meteors, and found them to be an oval extending beyond uranus. it was perturbed by the outer planets near which it went, so that in past times it must have moved in a slightly different orbit. calculating back to their past positions, it was found that in a certain year it must have gone very near to uranus, and that by the perturbation of this planet its path had been completely changed. originally it had in all probability been a comet, flying in a parabolic orbit towards the sun like many others. this one, encountering uranus, was pulled to pieces as it were, and its orbit made elliptical as shown in fig. . it was no longer free to escape and go away into the depths of space: it was enchained and made a member of the solar system. it also ceased to be a comet; it was degraded into a shoal of meteors. this is believed to be the past history of this splendid swarm. since its introduction to the solar system it has made revolutions: its next return is due in november, , and i hope that it may occur in the english dusk, and (see fig. ) in a cloudless after-midnight sky, as it did in . notes for lecture xvii the tide-generating force of one body on another is directly as the mass of the one body and inversely as the cube of the distance between them. hence the moon is more effective in producing terrestrial tides than the sun. the tidal wave directly produced by the moon in the open ocean is about feet high, that produced by the sun is about feet. hence the average spring tide is to the average neap as about to . the lunar tide varies between apogee and perigee from · to · . the solar tide varies between aphelion and perihelion from · to · . hence the highest spring tide is to the lowest neap as · + · is to · - · , or as to · . the semi-synchronous oscillation of the southern ocean raises the magnitude of oceanic tides somewhat above these directly generated values. oceanic tides are true waves, not currents. coast tides are currents. the momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height--in some places up to or feet, or even more. early observed connections between moon and tides would be these:-- st. spring tides at new and full moon. nd. average interval between tide and tide is half a lunar, not a solar, day--a lunar day being the interval between two successive returns of the moon to the meridian: hours and minutes. rd. the tides of a given place at new and full moon occur always at the same time of day whatever the season of the year. lecture xvii the tides persons accustomed to make use of the mersey landing-stages can hardly fail to have been struck with two obvious phenomena. one is that the gangways thereto are sometimes almost level, and at other times very steep; another is that the water often rushes past the stage rather violently, sometimes south towards garston, sometimes north towards the sea. they observe, in fact, that the water has two periodic motions--one up and down, the other to and fro--a vertical and a horizontal motion. they may further observe, if they take the trouble, that a complete swing of the water, up and down, or to and fro, takes place about every twelve and a half hours; moreover, that soon after high and low water there is no current--the water is stationary, whereas about half-way between high and low it is rushing with maximum speed either up or down the river. to both these motions of the water the name _tide_ is given, and both are extremely important. sailors usually pay most attention to the horizontal motion, and on charts you find the tide-races marked; and the places where there is but a small horizontal rush of the water are labelled "very little tide here." landsmen, or, at any rate, such of the more philosophic sort as pay any attention to the matter at all, think most of the vertical motion of the water--its amount of rise and fall. dwellers in some low-lying districts in london are compelled to pay attention to the extra high tides of the thames, because it is, or was, very liable to overflow its banks and inundate their basements. sailors, however, on nearing a port are also greatly affected by the time and amount of high water there, especially when they are in a big ship; and we know well enough how frequently atlantic liners, after having accomplished their voyage with good speed, have to hang around for hours waiting till there is enough water to lift them over the bar--that standing obstruction, one feels inclined to say disgrace, to the liverpool harbour. [illustration: fig. .--the mersey] to us in liverpool the tides are of supreme importance--upon them the very existence of the city depends--for without them liverpool would not be a port. it may be familiar to many of you how this is, and yet it is a matter that cannot be passed over in silence. i will therefore call your attention to the ordnance survey of the estuaries of the mersey and the dee. you see first that there is a great tendency for sand-banks to accumulate all about this coast, from north wales right away round to southport. you see next that the port of chester has been practically silted up by the deposits of sand in the wide-mouthed dee, while the port of liverpool remains open owing to the scouring action of the tide in its peculiarly shaped channel. without the tides the mersey would be a wretched dribble not much bigger than it is at warrington. with them, this splendid basin is kept open, and a channel is cut of such depth that the _great eastern_ easily rode in it in all states of the water. the basin is filled with water every twelve hours through its narrow neck. the amount of water stored up in this basin at high tide i estimate as million tons. all this quantity flows through the neck in six hours, and flows out again in the next six, scouring and cleansing and carrying mud and sand far out to sea. just at present the currents set strongest on the birkenhead side of the river, and accordingly a "pluckington bank" unfortunately grows under the liverpool stage. should this tendency to silt up the gates of our docks increase, land can be reclaimed on the other side of the river between tranmere and rock ferry, and an embankment made so as to deflect the water over liverpool way, and give us a fairer proportion of the current. after passing new brighton the water spreads out again to the left; its velocity forward diminishes; and after a few miles it has no power to cut away that sandbank known as the bar. should it be thought desirable to make it accomplish this, and sweep the bar further out to sea into deeper water, it is probable that a rude training wall (say of old hulks, or other removable partial obstruction) on the west of queen's channel, arranged so as to check the spreading out over all this useless area, may be quite sufficient to retain the needed extra impetus in the water, perhaps even without choking up the useful old rock channel, through which smaller ships still find convenient exit. now, although the horizontal rush of the tide is necessary to our existence as a port, it does not follow that the accompanying rise and fall of the water is an unmixed blessing. to it is due the need for all the expensive arrangements of docks and gates wherewith to store up the high-level water. quebec and new york are cities on such magnificent rivers that the current required to keep open channel is supplied without any tidal action, although quebec is nearly , miles from the open ocean; and accordingly, atlantic liners do not hover in mid-river and discharge passengers by tender, but they proceed straight to the side of the quays lining the river, or, as at new york, they dive into one of the pockets belonging to the company running the ship, and there discharge passengers and cargo without further trouble, and with no need for docks or gates. however, rivers like the st. lawrence and the hudson are the natural property of a gigantic continent; and we in england may be well contented with the possession of such tidal estuaries as the mersey, the thames, and the humber. that by pertinacious dredging the citizens of glasgow manage to get large ships right up their small river, the clyde, to the quays of the town, is a remarkable fact, and redounds very highly to their credit. we will now proceed to consider the connection existing between the horizontal rush of water and its vertical elevation, and ask, which is cause and which is effect? does the elevation of the ocean cause the tidal flow, or does the tidal flow cause the elevation? the answer is twofold: both statements are in some sense true. the prime cause of the tide is undoubtedly a vertical elevation of the ocean, a tidal wave or hump produced by the attraction of the moon. this hump as it passes the various channels opening into the ocean raises their level, and causes water to flow up them. but this simple oceanic tide, although the cause of all tide, is itself but a small affair. it seldom rises above six or seven feet, and tides on islands in mid-ocean have about this value or less. but the tides on our coasts are far greater than this--they rise twenty or thirty feet, or even fifty feet occasionally, at some places, as at bristol. why is this? the horizontal motion of the water gives it such an impetus or momentum that its motion far transcends that of the original impulse given to it, just as a push given to a pendulum may cause it to swing over a much greater arc than that through which the force acts. the inrushing water flowing up the english channel or the bristol channel or st. george's channel has such an impetus that it propels itself some twenty or thirty feet high before it has exhausted its momentum and begins to descend. in the bristol channel the gradual narrowing of the opening so much assists this action that the tides often rise forty feet, occasionally fifty feet, and rush still further up the severn in a precipitous and extraordinary hill of water called "the bore." some places are subject to considerable rise and fall of water with very little horizontal flow; others possess strong tidal races, but very little elevation and depression. the effect observed at any given place entirely depends on whether the place has the general character of a terminus, or whether it lies _en route_ to some great basin. you must understand, then, that all tide takes its rise in the free and open ocean under the action of the moon. no ordinary-sized sea like the north sea, or even the mediterranean, is big enough for more than a just appreciable tide to be generated in it. the pacific, the atlantic, and the southern oceans are the great tidal reservoirs, and in them the tides of the earth are generated as low flat humps of gigantic area, though only a few feet high, oscillating up and down in the period of approximately twelve hours. the tides we, and other coast-possessing nations, experience are the overflow or back-wash of these oceanic humps, and i will now show you in what manner the great atlantic tide-wave reaches the british isles twice a day. [illustration: fig. .--co-tidal lines.] fig. shows the contour lines of the great wave as it rolls in east from the atlantic, getting split by the land's end and by ireland into three portions; one of which rushes up the english channel and through the straits of dover. another rolls up the irish sea, with a minor offshoot up the bristol channel, and, curling round anglesey, flows along the north wales coast and fills liverpool bay and the mersey. the third branch streams round the north coast of ireland, past the mull of cantyre and rathlin island; part fills up the firth of clyde, while the rest flows south, and, swirling round the west side of the isle of man, helps the southern current to fill the bay of liverpool. the rest of the great wave impinges on the coast of scotland, and, curling round it, fills up the north sea right away to the norway coast, and then flows down below denmark, joining the southern and earlier arriving stream. the diagram i show you is a rough chart of cotidal lines, which i made out of the information contained in _whitaker's almanac_. a place may thus be fed with tide by two distinct channels, and many curious phenomena occur in certain places from this cause. thus it may happen that one channel is six hours longer than the other, in which case a flow will arrive by one at the same time as an ebb arrives by the other; and the result will be that the place will have hardly any tide at all, one tide interfering with and neutralizing the other. this is more markedly observed at other parts of the world than in the british isles. whenever a place is reached by two channels of different length, its tides are sure to be peculiar, and probably small. another cause of small tide is the way the wave surges to and fro in a channel. the tidal wave surging up the english channel, for instance, gets largely reflected by the constriction at dover, and so a crest surges back again, as we may see waves reflected in a long trough or tilted bath. the result is that southampton has two high tides rapidly succeeding one another, and for three hours the high-water level varies but slightly--a fact of evident convenience to the port. places on a nodal line, so to speak, about the middle of the length of the channel, have a minimum of rise and fall, though the water rushes past them first violently up towards dover, where the rise is considerable, and then back again towards the ocean. at portland, for instance, the total rise and fall is very small: it is practically on a node. yarmouth, again, is near a less marked node in the north sea, where stationary waves likewise surge to and fro, and accordingly the tidal rise and fall at yarmouth is only about five feet (varying from four and a half to six), whereas at london it is twenty or thirty feet, and at flamborough head or leith it is from twelve to sixteen feet. it is generally supposed that water never flows up-hill, but in these cases of oscillation it flows up-hill for three hours together. the water is rushing up the english channel towards dover long after it is highest at the dover end; it goes on piling itself up, until its momentum is checked by the pressure, and then it surges back. it behaves, in fact, very like the bob of a pendulum, which rises against gravity at every quarter swing. to get a very large tide, the place ought to be directly accessible by a long sweep of a channel to the open ocean, and if it is situate on a gradually converging opening, the ebb and flow may be enormous. the severn is the best example of this on the british isles; but the largest tides in the world are found, i believe, in the bay of fundy, on the coast of north america, where they sometimes rise one hundred and twenty feet. excessive or extra tides may be produced occasionally in any place by the propelling force of a high wind driving the water towards the shore; also by a low barometer, _i.e._ by a local decrease in the pressure of the air. well, now, leaving these topographical details concerning tides, which we see to be due to great oceanic humps (great in area that is, though small in height), let us proceed to ask what causes these humps; and if it be the moon that does it, how does it do it? the statement that the moon causes the tides sounds at first rather an absurdity, and a mere popular superstition. galileo chaffed kepler for believing it. who it was that discovered the connection between moon and tides we know not--probably it is a thing which has been several times rediscovered by observant sailors or coast-dwellers--and it is certainly a very ancient piece of information. probably the first connection observed was that about full moon and about new moon the tides are extra high, being called spring tides, whereas about half-moon the tides are much less, and are called neap tides. the word spring in this connection has no reference to the season of the year; except that both words probably represent the same idea of energetic uprising or upspringing, while the word neap comes from nip, and means pinched, scanty, nipped tide. the next connection likely to be observed would be that the interval between two day tides was not exactly a solar day of twenty-four hours, but a lunar day of fifty minutes longer. for by reason of the moon's monthly motion it lags behind the sun about fifty minutes a day, and the tides do the same, and so perpetually occur later and later, about fifty minutes a day later, or hours and minutes on the average between tide and tide. a third and still more striking connection was also discovered by some of the ancient great navigators and philosophers--viz. that the time of high water at a given place at full moon is always the same, or very nearly so. in other words, the highest or spring tides always occur nearly at the same time of day at a given place. for instance, at liverpool this time is noon and midnight. london is about two hours and a half later. each port has its own time for receiving a given tide, and the time is called the "establishment" of the port. look out a day when the moon is full, and you will find the liverpool high tide occurs at half-past eleven, or close upon it. the same happens when the moon is new. a day after full or new moon the spring tides rise to their highest, and these extra high tides always occur in liverpool at noon and at midnight, whatever the season of the year. about the equinoxes they are liable to be extraordinarily high. the extra low tides here are therefore at a.m. and p.m., and the p.m. low tide is a nuisance to the river steamers. the spring tides at london are highest about half-past two. * * * * * it is, therefore, quite clear that the moon has to do with the tides. it and the sun together are, in fact, the whole cause of them; and the mode in which these bodies act by gravitative attraction was first made out and explained in remarkably full detail by sir isaac newton. you will find his account of the tides in the second and third books of the _principia_; and though the theory does not occupy more than a few pages of that immortal work, he succeeds not only in explaining the local tidal peculiarities, much as i have done to-night, but also in calculating the approximate height of mid-ocean solar tide; and from the observed lunar tide he shows how to determine the then quite unknown mass of the moon. this was a quite extraordinary achievement, the difficulty of which it is not easy for a person unused to similar discussions fully to appreciate. it is, indeed, but a small part of what newton accomplished, but by itself it is sufficient to confer immortality upon any ordinary philosopher, and to place him in a front rank. [illustration: fig. .--whirling earth model.] to make intelligible newton's theory of the tides, i must not attempt to go into too great detail. i will consider only the salient points. first, you know that every mass of matter attracts every other piece of matter; second, that the moon revolves round the earth, or rather that the earth and moon revolve round their common centre of gravity once a month; third, that the earth spins on its own axis once a day; fourth, that when a thing is whirled round, it tends to fly out from the centre and requires a force to hold it in. these are the principles involved. you can whirl a bucket full of water vertically round without spilling it. make an elastic globe rotate, and it bulges out into an oblate or orange shape; as illustrated by the model shown in fig. . this is exactly what the earth does, and newton calculated the bulging of it as fourteen miles all round the equator. make an elastic globe revolve round a fixed centre outside itself, and it gets pulled into a prolate or lemon shape; the simplest illustrative experiment is to attach a string to an elastic bag or football full of water, and whirl it round and round. its prolateness is readily visible. now consider the earth and moon revolving round each other like a man whirling a child round. the child travels furthest, but the man cannot merely rotate, he leans back and thus also describes a small circle: so does the earth; it revolves round the common centre of gravity of earth and moon (_cf._ p. ). this is a vital point in the comprehension of the tides: the earth's centre is not at rest, but is being whirled round by the moon, in a circle about / as big as the circle which the moon describes, because the earth weighs eighty times as much as the moon. the effect of the revolution is to make both bodies slightly protrude in the direction of the line joining them; they become slightly "prolate" as it is called--that is, lemon-shaped. illustrating still by the man and child, the child's legs fly outwards so that he is elongated in the direction of a radius; the man's coat-tails fly out too, so that he too is similarly though less elongated. these elongations or protuberances constitute the tides. [illustration: fig. .--earth and moon model, illustrating the production of statical or "equilibrium" tides when the whole is whirled about the point g.] fig. shows a model to illustrate the mechanism. a couple of cardboard disks (to represent globes of course), one four times the diameter of the other, and each loaded so as to have about the correct earth-moon ratio of weights, are fixed at either end of a long stick, and they balance about a certain point, which is their common centre of gravity. for convenience this point is taken a trifle too far out from the centre of the earth--that is, just beyond its surface. through the balancing point g a bradawl is stuck, and on that as pivot the whole readily revolves. now, behind the circular disks, you see, are four pieces of card of appropriate shape, which are able to slide out under proper forces. they are shown dotted in the figure, and are lettered a, b, c, d. the inner pair, b and c, are attached to each other by a bit of string, which has to typify the attraction of gravitation; the outer pair, a and d, are not attached to anything, but have a certain amount of play against friction in slots parallel to the length of the stick. the moon-disk is also slotted, so a small amount of motion is possible to it along the stick or bar. these things being so arranged, and the protuberant pieces of card being all pushed home, so that they are hidden behind their respective disks, the whole is spun rapidly round the centre of gravity, g. the result of a brief spin is to make a and d fly out by centrifugal force and show, as in the figure; while the moon, flying out too in its slot, tightens up the string, which causes b and c to be pulled out too. thus all four high tides are produced, two on the earth and two on the moon, a and d being caused by centrifugal force, b and c by the attraction of gravitation. each disk has become prolate in the same sort of fashion as yielding globes do. of course the fluid ocean takes this shape more easily and more completely than the solid earth can, and so here are the very oceanic humps we have been talking about, and about three feet high (fig. ). if there were a sea on the _moon_, its humps would be a good deal bigger; but there probably is no sea there, and if there were, the earth's tides are more interesting to us, at any rate to begin with. [illustration: fig. .--earth and moon (earth's rotation neglected).] the humps as so far treated are always protruding in the earth-moon line, and are stationary. but now we have to remember that the earth is spinning inside them. it is not easy to see what precise effect this spin will have upon the humps, even if the world were covered with a uniform ocean; but we can see at any rate that however much they may get displaced, and they do get displaced a good deal, they cannot possibly be carried round and round. the whole explanation we have given of their causes shows that they must maintain some steady aspect with respect to the moon--in other words, they must remain stationary as the earth spins round. not that the same identical water remains stationary, for in that case it would have to be dragged over the earth's equator at the rate of , miles an hour, but the hump or wave-crest remains stationary. it is a true wave, or form only, and consists of continuously changing individual particles. the same is true of all waves, except breaking ones. given, then, these stationary humps and the earth spinning on its axis, we see that a given place on the earth will be carried round and round, now past a hump, and six hours later past a depression: another six hours and it will be at the antipodal hump, and so on. thus every six hours we shall travel from the region in space where the water is high to the region where it is low; and ignoring our own motion we shall say that the sea first rises and then falls; and so, with respect to the place, it does. thus the succession of high and low water, and the two high tides every twenty-four hours, are easily understood in their easiest and most elementary aspect. a more complete account of the matter it will be wisest not to attempt: suffice it to say that the difficulties soon become formidable when the inertia of the water, its natural time of oscillation, the varying obliquity of the moon to the ecliptic, its varying distance, and the disturbing action of the sun are taken into consideration. when all these things are included, the problem becomes to ordinary minds overwhelming. a great many of these difficulties were successfully attacked by laplace. others remained for modern philosophers, among whom are sir george airy, sir william thomson, and professor george darwin. i may just mention that the main and simplest effect of including the inertia or momentum of the water is to dislocate the obvious and simple connexion between high water and high moon; inertia always tends to make an effect differ in phase by a quarter period from the cause producing it, as may be illustrated by a swinging pendulum. hence high water is not to be expected when the tide-raising force is a maximum, but six hours later; so that, considering inertia and neglecting friction, there would be low water under the moon. including friction, something nearer the equilibrium state of things occurs. with _sufficient_ friction the motion becomes dead-beat again, _i.e._ follows closely the force that causes it. returning to the elementary discussion, we see that the rotation of the earth with respect to the humps will not be performed in exactly twenty-four hours, because the humps are travelling slowly after the moon, and will complete a revolution in a month in the same direction as the earth is rotating. hence a place on the earth has to catch them up, and so each high tide arrives later and later each day--roughly speaking, an hour later for each day tide; not by any means a constant interval, because of superposed disturbances not here mentioned, but on the average about fifty minutes. we see, then, that as a result of all this we get a pair of humps travelling all over the surface of the earth, about once a day. if the earth were all ocean (and in the southern hemisphere it is nearly all ocean), then they would go travelling across the earth, tidal waves three feet high, and constituting the mid-ocean tides. but in the northern hemisphere they can only thus journey a little way without striking land. as the moon rises at a place on the east shores of the atlantic, for instance, the waters begin to flow in towards this place, or the tide begins to rise. this goes on till the moon is overhead and for some time afterwards, when the tide is at its highest. the hump then follows the moon in its apparent journey across to america, and there precipitates itself upon the coast, rushing up all the channels, and constituting the land tide. at the same time, the water is dragged away from the east shores, and so _our_ tide is at its lowest. the same thing repeats itself in a little more than twelve hours again, when the other hump passes over the atlantic, as the moon journeys beneath the earth, and so on every day. in the free southern ocean, where land obstruction is comparatively absent, the water gets up a considerable swing by reason of its accumulated momentum, and this modifies and increases the open ocean tides there. also for some reason, i suppose because of the natural time of swing of the water, one of the humps is there usually much larger than the other; and so places in the indian and other offshoots of the southern ocean get their really high tide only once every twenty-four hours. these southern tides are in fact much more complicated than those the british isles receive. ours are singularly simple. no doubt some trace of the influence of the southern ocean is felt in the north atlantic, but any ocean extending over ° of longitude is big enough to have its own tides generated; and i imagine that the main tides we feel are thus produced on the spot, and that they are simple because the damping-out being vigorous, and accumulated effects small, we feel the tide-producing forces more directly. but for authoritative statements on tides, other books must be read. i have thought, and still think, it best in an elementary exposition to begin by a consideration of the tide-generating forces as if they acted on a non-rotating earth. it is the tide generating forces, and not the tides themselves, that are really represented in figs. and . the rotation of the earth then comes in as a disturbing cause. a more complete exposition would begin with the rotating earth, and would superpose the attraction of the moon as a disturbing cause, treating it as a problem in planetary perturbation, the ocean being a sort of satellite of the earth. this treatment, introducing inertia but ignoring friction and land obstruction, gives low water in the line of pull, and high water at right angles, or where the pull is zero; in the same sort of way as a pendulum bob is highest where most force is pulling it down, and lowest where no force is acting on it. for a clear treatment of the tides as due to the perturbing forces of sun and moon, see a little book by mr. t.k. abbott of trinity college, dublin. (longman.) [illustration: fig. .--maps showing how comparatively free from land obstruction the ocean in the southern hemisphere is.] if the moon were the only body that swung the earth round, this is all that need be said in an elementary treatment; but it is not the only one. the moon swings the earth round once a month, the sun swings it round once a year. the circle of swing is bigger, but the speed is so much slower that the protuberance produced is only one-third of that caused by the monthly whirl; _i.e._ the simple solar tide in the open sea, without taking momentum into account, is but a little more than a foot high, while the simple lunar tide is about three feet. when the two agree, we get a spring tide of four feet; when they oppose each other, we get a neap tide of only two feet. they assist each other at full moon and at new moon. at half-moon they oppose each other. so we have spring tides regularly once a fortnight, with neap tides in between. [illustration: fig. .--spring and neap tides.] fig. gives the customary diagrams to illustrate these simple things. you see that when the moon and sun act at right angles (_i.e._ at every half-moon), the high tides of one coincide with the low tides of the other; and so, as a place is carried round by the earth's rotation, it always finds either solar or else lunar high water, and only experiences the difference of their two effects. whereas, when the sun and moon act in the same line (as they do at new and full moon), their high and low tides coincide, and a place feels their effects added together. the tide then rises extra high and falls extra low. [illustration: fig. .--tidal clock. the position of the disk b shows the height of the tide. the tide represented is a nearly high tide eight feet above mean level.] utilizing these principles, a very elementary form of tidal-clock, or tide-predicter, can be made, and for an open coast station it really would not give the tides so very badly. it consists of a sort of clock face with two hands, one nearly three times as long as the other. the short hand, ca, should revolve round c once in twelve hours, and the vertical height of its end a represents the height of the solar tide on the scale of horizontal lines ruled across the face of the clock. the long hand, ab, should revolve round a once in twelve hours and twenty-five minutes, and the height of its end b (if a were fixed on the zero line) would represent the lunar tide. the two revolutions are made to occur together, either by means of a link-work parallelogram, or, what is better in practice, by a string and pulleys, as shown; and the height of the end point, b, of the third side or resultant, cb, read off on a scale of horizontal parallel lines behind, represents the combination or actual tide at the place. every fortnight the two will agree, and you will get spring tides of maximum height ca + ab; every other fortnight the two will oppose, and you will get neap tides of maximum height ca-ab. such a clock, if set properly and driven in the ordinary way, would then roughly indicate the state of the tide whenever you chose to look at it and read the height of its indicating point. it would not indeed be very accurate, especially for such an inclosed station as liverpool is, and that is probably why they are not made. a great number of disturbances, some astronomical, some terrestrial, have to be taken into account in the complete theory. it is not an easy matter to do this, but it can be, and has been, done; and a tide-predicter has not only been constructed, but two of them are in regular work, predicting the tides for years hence--one, the property of the indian government, for coast stations of india; the other for various british and foreign stations, wherever the necessary preliminary observations have been made. these machines are the invention of sir william thomson. the tide-tables for indian ports are now always made by means of them. [illustration: fig. .--sir william thomson (lord kelvin).] [illustration: fig. .--tide-gauge for recording local tides, a pencil moved up and down by a float writes on a drum driven by clockwork.] the first thing to be done by any port which wishes its tides to be predicted is to set up a tide-gauge, or automatic recorder, and keep it working for a year or two. the tide-gauge is easy enough to understand: it marks the height of the tide at every instant by an irregular curved line like a barometer chart (fig. ). these observational curves so obtained have next to be fed into a fearfully complex machine, which it would take a whole lecture to make even partially intelligible, but fig. shows its aspect. it consists of ten integrating machines in a row, coupled up and working together. this is the "harmonic analyzer," and the result of passing the curve through this machine is to give you all the constituents of which it is built up, viz. the lunar tide, the solar tide, and eight of the sub-tides or disturbances. these ten values are then set off into a third machine, the tide-predicter proper. the general mode of action of this machine is not difficult to understand. it consists of a string wound over and under a set of pulleys, which are each set on an excentric, so as to have an up-and-down motion. these up-and-down motions are all different, and there are ten of these movable pulleys, which by their respective excursions represent the lunar tide, the solar tide, and the eight disturbances already analyzed out of the tide-gauge curve by the harmonic analyzer. one end of the string is fixed, the other carries a pencil which writes a trace on a revolving drum of paper--a trace which represents the combined motion of all the pulleys, and so predicts the exact height of the tide at the place, at any future time you like. the machine can be turned quite quickly, so that a year's tides can be run off with every detail in about half-an-hour. this is the easiest part of the operation. nothing has to be done but to keep it supplied with paper and pencil, and turn a handle as if it were a coffee-mill instead of a tide-mill. (figs. and .) [illustration: fig. .--harmonic analyzer; for analyzing out the constituents from a set of observational curves.] my subject is not half exhausted. i might go on to discuss the question of tidal energy--whether it can be ever utilized for industrial purposes; and also the very interesting question whence it comes. tidal energy is almost the only terrestrial form of energy that does not directly or indirectly come from the sun. the energy of tides is now known to be obtained at the expense of the earth's rotation; and accordingly our day must be slowly, very slowly, lengthening. the tides of past ages have destroyed the moon's rotation, and so it always turns the same face to us. there is every reason to believe that in geologic ages the moon was nearer to us than it is now, and that accordingly our tides were then far more violent, rising some hundreds of feet instead of twenty or thirty, and sweeping every six hours right over the face of a country, ploughing down hills, denuding rocks, and producing a copious sedimentary deposit. [illustration: fig. .--tide-predicter, for combining the ascertained constituents into a tidal curve for the future.] in thus discovering the probable violent tides of past ages, astronomy has, within the last few years, presented geology with the most powerful denuding agent known; and the study of the earth's past history cannot fail to be greatly affected by the modern study of the intricate and refined conditions attending prolonged tidal action on incompletely rigid bodies. [read on this point the last chapter of sir r. ball's _story of the heavens_.] [illustration: fig. .--weekly sheet of curves. tides for successive days are predicted on the same sheet of paper, to economise space.] i might also point out that the magnitude of our terrestrial tides enables us to answer the question as to the internal fluidity of the earth. it used to be thought that the earth's crust was comparatively thin, and that it contained a molten interior. we now know that this is not the case. the interior of the earth is hot indeed, but it is not fluid. or at least, if it be fluid, the amount of fluid is but very small compared with the thickness of the unyielding crust. all these, and a number of other most interesting questions, fringe the subject of the tides; the theoretical study of which, started by newton, has developed, and is destined in the future to further develop, into one of the most gigantic and absorbing investigations--having to do with the stability or instability of solar systems, and with the construction and decay of universes. these theories are the work of pioneers now living, whose biographies it is therefore unsuitable for us to discuss, nor shall i constantly mention their names. but helmholtz, and thomson, are household words, and you well know that in them and their disciples the race of pioneers maintains its ancient glory. notes for lecture xviii tides are due to incomplete rigidity of bodies revolving round each other under the action of gravitation, and at the same time spinning on their axes. two spheres revolving round each other can only remain spherical if rigid; if at all plastic they become prolate. if either rotate on its axis, in the same or nearly the same plane as it revolves, that one is necessarily subject to tides. the axial rotation tends to carry the humps with it, but the pull of the other body keeps them from moving much. hence the rotation takes place against a pull, and is therefore more or less checked and retarded. this is the theory of von helmholtz. the attracting force between two such bodies is no longer _exactly_ towards the centre of revolution, and therefore kepler's second law is no longer precisely obeyed: the rate of description of areas is subject to slight acceleration. the effect of this tangential force acting on the tide-compelling body is gradually to increase its distance from the other body. applying these statements to the earth and moon, we see that tidal energy is produced at the expense of the earth's rotation, and that the length of the day is thereby slowly increasing. also that the moon's rotation relative to the earth has been destroyed by past tidal action in it (the only residue of ancient lunar rotation now being a scarcely perceptible libration), so that it turns always the same face towards us. moreover, that its distance from the earth is steadily increasing. this last is the theory of professor g.h. darwin. long ago the moon must therefore have been much nearer the earth, and the day was much shorter. the tides were then far more violent. halving the distance would make them eight times as high; quartering it would increase them sixty-four-fold. a most powerful geological denuding agent. trade winds and storms were also more violent. if ever the moon were close to the earth, it would have to revolve round it in about three hours. if the earth rotated on its axis in three hours, when fluid or pasty, it would be unstable, and begin to separate a portion of itself as a kind of bud, which might then get detached and gradually pushed away by the violent tidal action. hence it is possible that this is the history of the moon. if so, it is probably an exceptional history. the planets were not formed from the sun in this way. mars' moons revolve round him more quickly than the planet rotates: hence with them the process is inverted, and they must be approaching him and may some day crash along his surface. the inner moon is now about , miles away, and revolves in - / hours. it appears to be about miles in diameter, and weighs therefore, if composed of rock, billion tons. mars rotates in - / hours. a similar fate may _possibly_ await our moon ages hence--by reason of the action of terrestrial tides produced by the sun. lecture xviii the tides, and planetary evolution in the last lecture we considered the local peculiarities of the tides, the way in which they were formed in open ocean under the action of the moon and the sun, and also the means by which their heights and times could be calculated and predicted years beforehand. towards the end i stated that the subject was very far from being exhausted, and enumerated some of the large and interesting questions which had been left untouched. it is with some of these questions that i propose now to deal. i must begin by reminding you of certain well-known facts, a knowledge of which i may safely assume. and first we must remind ourselves of the fact that almost all the rocks which form the accessible crust of the earth were deposited by the agency of water. nearly all are arranged in regular strata, and are composed of pulverized materials--materials ground down from pre-existing rocks by some denuding and grinding action. they nearly all contain vestiges of ancient life embedded in them, and these vestiges are mainly of marine origin. the strata which were once horizontal are now so no longer--they have been tilted and upheaved, bent and distorted, in many places. some of them again have been metamorphosed by fire, so that their organic remains have been destroyed, and the traces of their aqueous origin almost obliterated. but still, to the eye of the geologist, all are of aqueous or sedimentary origin: roughly speaking, one may say they were all deposited at the bottom of some ancient sea. the date of their formation no man yet can tell, but that it was vastly distant is certain. for the geological era is not over. aqueous action still goes on: still does frost chip the rocks into fragments; still do mountain torrents sweep stone and mud and _débris_ down the gulleys and watercourses; still do rivers erode their channels, and carry mud and silt far out to sea. and, more powerful than any of these agents of denudation, the waves and the tides are still at work along every coast-line, eating away into the cliffs, undermining gradually and submerging acre after acre, and making with the refuse a shingly, or a sandy, or a muddy beach--the nucleus of a new geological formation. of all denuding agents, there can be no doubt that, to the land exposed to them, the waves of the sea are by far the most powerful. think how they beat and tear, and drive and drag, until even the hardest rock, like basalt, becomes honeycombed into strange galleries and passages--fingal's cave, for instance--and the softer parts are crumbled away. but the area now exposed to the teeth of the waves is not great. the fury of a winter storm may dash them a little higher than usual, but they cannot reach cliffs feet high. they can undermine such cliffs indeed, and then grind the fragments to powder, but their direct action is limited. not so limited, however, as they would be without the tides. consider for a moment the denudation import of the tides: how does the existence of tidal rise and fall affect the geological problem? the scouring action of the tidal currents themselves is not to be despised. it is the tidal ebb and flow which keeps open channel in the mersey, for instance. but few places are so favourably situated as liverpool in this respect, and the direct scouring action of the tides in general is not very great. their geological import mainly consists in this--that they raise and lower the surface waves at regular intervals, so as to apply them to a considerable stretch of coast. the waves are a great planing machine attacking the land, and the tides raise and lower this planing machine, so that its denuding tooth is applied, now twenty feet vertically above mean level, now twenty feet below. making all allowance for the power of winds and waves, currents, tides, and watercourses, assisted by glacial ice and frost, it must be apparent how slowly the work of forming the rocks is being carried on. it goes on steadily, but so slowly that it is estimated to take years to wear away one foot of the american continent by all the denuding causes combined. to erode a stratum feet thick will require at this rate thirty million years. the age of the earth is not at all accurately known, but there are many grounds for believing it not to be much older than some thirty million years. that is to say, not greatly more than this period of time has elapsed since it was in a molten condition. it may be as old as a hundred million years, but its age is believed by those most competent to judge to be more likely within this limit than beyond it. but if we ask what is the thickness of the rocks which in past times have been formed, and denuded, and re-formed, over and over again, we get an answer, not in feet, but in miles. the laurentian and huronian rocks of canada constitute a stratum ten miles thick; and everywhere the rocks at the base of our stratified system are of the most stupendous volume and thickness. it has always been a puzzle how known agents could have formed these mighty masses, and the only solution offered by geologists was, unlimited time. given unlimited time, they could, of course, be formed, no matter how slowly the process went on. but inasmuch as the time allowable since the earth was cool enough for water to exist on it except as steam is not by any means unlimited, it becomes necessary to look for a far more powerful engine than any now existing; there must have been some denuding agent in those remote ages--ages far more distant from us than the carboniferous period, far older than any forms of life, fossil or otherwise, ages among the oldest known to geology--a denuding agent must have then existed, far more powerful than any we now know. such an agent it has been the privilege of astronomy and physics, within the last ten years, to discover. to this discovery i now proceed to lead up. our fundamental standard of time is the period of the earth's rotation--the length of the day. the earth is our one standard clock: all time is expressed in terms of it, and if it began to go wrong, or if it did not go with perfect uniformity, it would seem a most difficult thing to discover its error, and a most puzzling piece of knowledge to utilize when found. that it does not go much wrong is proved by the fact that we can calculate back to past astronomical events--ancient eclipses and the like--and we find that the record of their occurrence, as made by the old magi of chaldæa, is in very close accordance with the result of calculation. one of these famous old eclipses was observed in babylon about thirty-six centuries ago, and the chaldæan astronomers have put on record the time of its occurrence. modern astronomers have calculated back when it should have occurred, and the observed time agrees very closely with the actual, but not exactly. why not exactly? partly because of the acceleration of the moon's mean motion, as explained in the lecture on laplace (p. ). the orbit of the earth was at that time getting rounder, and so, as a secondary result, the speed of the moon was slightly increasing. it is of the nature of a perturbation, and is therefore a periodic not a progressive or continuous change, and in a sufficiently long time it will be reversed. still, for the last few thousand years the moon's motion has been, on the whole, accelerated (though there seems to be a very slight retarding force in action too). laplace thought that this fact accounted for the whole of the discrepancy; but recently, in , professor adams re-examined the matter, and made a correction in the details of the theory which diminishes its effect by about one-half, leaving the other half to be accounted for in some other way. his calculations have been confirmed by professor cayley. this residual discrepancy, when every known cause has been allowed for, amounts to about one hour. the eclipse occurred later than calculation warrants. now this would have happened from either of two causes, either an acceleration of the moon in her orbit, or a retardation of the earth in her diurnal rotation--a shortening of the month or a lengthening of the day, or both. the total discrepancy being, say, two hours, an acceleration of six seconds-per-century per century will in thirty-six centuries amount to one hour; and this, according to the corrected laplacian theory, is what has occurred. but to account for the other hour some other cause must be sought, and at present it is considered most probably due to a steady retardation of the earth's rotation--a slow, very slow, lengthening of the day. the statement that a solar eclipse thirty-six centuries ago was an hour late, means that a place on the earth's surface came into the shadow one hour behind time--that is, had lagged one twenty-fourth part of a revolution. the earth, therefore, had lost this amount in the course of × - / revolutions. the loss per revolution is exceedingly small, but it accumulates, and at any era the total loss is the sum of all the losses preceding it. it may be worth while just to explain this point further. suppose the earth loses a small piece of time, which i will call an instant, per day; a locality on the earth will come up to a given position one instant late on the first day after an event. on the next day it would come up two instants late by reason of the previous loss; but it also loses another instant during the course of the second day, and so the total lateness by the end of that day amounts to three instants. the day after, it will be going slower from the beginning at the rate of two instants a day, it will lose another instant on the fresh day's own account, and it started three instants late; hence the aggregate loss by the end of the third day is + + = . by the end of the fourth day the whole loss will be + + + , and so on. wherefore by merely losing one instant every day the total loss in _n_ days is ( + + + ... + _n_) instants, which amounts to / _n_ (_n_ + ) instants; or practically, when _n_ is big, to / n^ . now in thirty-six centuries there have been × - / days, and the total loss has amounted to an hour; hence the length of "an instant," the loss per diem, can be found from the equation / ( × )^ instants = hour; whence one "instant" equals the millionth part of a second. this minute quantity represents the retardation of the earth per day. in a year the aggregate loss mounts up to / th part of a second, in a century to about three seconds, and in thirty-six centuries to an hour. but even at the end of the thirty-six centuries the day is barely any longer; it is only × instants, that is / th of a second, longer than it was at the beginning. and even a million years ago, unless the rate of loss was different (as it probably was), the day would only be thirty-five minutes shorter, though by that time the aggregate loss, as measured by the apparent lateness of any perfectly punctual event reckoned now, would have amounted to nine years. (these numbers are to be taken as illustrative, not as precisely representing terrestrial fact.) what can have caused the slowing down? swelling of the earth by reason of accumulation of meteoric dust might do something, but probably very little. contraction of the earth as it goes on cooling would act in the opposite direction, and probably more than counterbalance the dust effect. the problem is thus not a simple one, for there are several disturbing causes, and for none of them are the data enough to base a quantitative estimate upon; but one certain agent in lengthening the day, and almost certainly the main agent, is to be found in the tides. remember that the tidal humps were produced as the prolateness of a sphere whirled round and round a fixed centre, like a football whirled by a string. these humps are pulled at by the moon, and the earth rotates on its axis against this pull. hence it tends to be constantly, though very slightly, dragged back. in so far as the tidal wave is allowed to oscillate freely, it will swing with barely any maintaining force, giving back at one quarter-swing what it has received at the previous quarter; but in so far as it encounters friction, which it does in all channels where there is an actual ebb and flow of the water, it has to receive more than it gives back, and the balance of energy has to be made up to it, or the tides would cease. the energy of the tides is, in fact, continually being dissipated by friction, and all the energy so dissipated is taken from the rotation of the earth. if tidal energy were utilized by engineers, the machines driven would be really driven at the expense of the earth's rotation: it would be a mode of harnessing the earth and using the moon as fixed point or fulcrum; the moon pulling at the tidal protuberance, and holding it still as the earth rotates, is the mechanism whereby the energy is extracted, the handle whereby the friction brake is applied. winds and ocean currents have no such effect (as mr. fronde in _oceania_ supposes they have), because they are all accompanied by a precisely equal counter-current somewhere else, and no internal rearrangement of fluid can affect the motion of a mass as a whole; but the tides are in different case, being produced, not by internal inequalities of temperature, but by a straightforward pull from an external body. the ultimate effect of tidal friction and dissipation of energy will, therefore, be to gradually retard the earth till it does not rotate with reference to the moon, _i.e._ till it rotates once while the moon revolves once; in other words, to make the day and the month equal. the same cause must have been in operation, but with eighty-fold greater intensity, on the moon. it has ceased now, because the rotation has stopped, but if ever the moon rotated on its axis with respect to the earth, and if it were either fluid itself or possessed any liquid ocean, then the tides caused by the pull of the earth must have been prodigious, and would tend to stop its rotation. have they not succeeded? is it not probable that this is _why_ the moon always now turns the same face towards us? it is believed to be almost certainly the cause. if so, there was a time when the moon behaved differently--when it rotated more quickly than it revolved, and exhibited to us its whole surface. and at this era, too, the earth itself must have rotated a little faster, for it has been losing speed ever since. we have thus arrived at this fact, that a thousand years ago the day was a trifle shorter than it is now. a million years ago it was, perhaps, an hour shorter. twenty million years ago it must have been much shorter. fifty million years ago it may have been only a few hours long. the earth may have spun round then quite quickly. but there is a limit. if it spun too fast it would fly to pieces. attach shot by means of wax to the whirling earth model, fig. , and at a certain speed the cohesion of the wax cannot hold them, so they fly off. the earth is held together not by cohesion but by gravitation; it is not difficult to reckon how fast the earth must spin for gravity at its surface to be annulled, and for portions to fly off. we find it about one revolution in three hours. this is a critical speed. if ever the day was three hours long, something must have happened. the day can never have been shorter than that; for if it were, the earth would have a tendency to fly in pieces, or, at least, to separate into two pieces. remember this, as a natural result of a three-hour day, which corresponds to an unstable state of things; remember also that in some past epoch a three-hour day is a probability. if we think of the state of things going on in the earth's atmosphere, if it had an atmosphere at that remote date, we shall recognize the existence of the most fearful tornadoes. the trade winds, which are now peaceful agents of commerce, would then be perpetual hurricanes, and all the denudation agents of the geologist would be in a state of feverish activity. so, too, would the tides: instead of waiting six hours between low and high tide, we should have to wait only three-quarters of an hour. every hour-and-a-half the water would execute a complete swing from high tide to high again. very well, now leave the earth, and think what has been happening to the moon all this while. we have seen that the moon pulls the tidal hump nearest to it back; but action and reaction are always equal and opposite--it cannot do that without itself getting pulled forward. the pull of the earth on the moon will therefore not be quite central, but will be a little in advance of its centre; hence, by kepler's second law, the rate of description of areas by its radius vector cannot be constant, but must increase (p. ). and the way it increases will be for the radius vector to lengthen, so as to sweep out a bigger area. or, to put it another way, the extra speed tending to be gained by the moon will fling it further away by extra centrifugal force. this last is not so good a way of regarding the matter; though it serves well enough for the case of a ball whirled at the end of an elastic string. after having got up the whirl, the hand holding the string may remain almost fixed at the centre of the circle, and the motion will continue steadily; but if the hand be moved so as always to pull the string a little in advance of the centre, the speed of whirl will increase, the elastic will be more and more stretched, until the whirling ball is describing a much larger circle. but in this case it will likewise be going faster--distance and speed increase together. this is because it obeys a different law from gravitation--the force is not inversely as the square, or any other single power, of the distance. it does not obey any of kepler's laws, and so it does not obey the one which now concerns us, viz. the third; which practically states that the further a planet is from the centre the slower it goes; its velocity varies inversely with the square root of its distance (p. ). if, instead of a ball held by elastic, it were a satellite held by gravity, an increase in distance must be accompanied by a diminution in speed. the time of revolution varies as the square of the cube root of the distance (kepler's third law). hence, the tidal reaction on the moon, having as its primary effect, as we have seen, the pulling the moon a little forward, has also the secondary or indirect effect of making it move slower and go further off. it may seem strange that an accelerating pull, directed in front of the centre, and therefore always pulling the moon the way it is going, should retard it; and that a retarding force like friction, if such a force acted, should hasten it, and make it complete its orbit sooner; but so it precisely is. gradually, but very slowly, the moon is receding from us, and the month is becoming longer. the tides of the earth are pushing it away. this is not a periodic disturbance, like the temporary acceleration of its motion discovered by laplace, which in a few centuries, more or less, will be reversed; it is a disturbance which always acts one way, and which is therefore cumulative. it is superposed upon all periodic changes, and, though it seems smaller than they, it is more inexorable. in a thousand years it makes scarcely an appreciable change, but in a million years its persistence tells very distinctly; and so, in the long run, the month is getting longer and the moon further off. working backwards also, we see that in past ages the moon must have been nearer to us than it is now, and the month shorter. now just note what the effect of the increased nearness of the moon was upon our tides. remember that the tide-generating force varies inversely as the cube of distance, wherefore a small change of distance will produce a great difference in the tide-force. the moon's present distance is thousand miles. at a time when it was only thousand miles, the earth's tides would have been twice as high as they are now. the pushing away action was then a good deal more violent, and so the process went on quicker. the moon must at some time have been just half its present distance, and the tides would then have risen, not or feet, but or feet. a little further back still, we have the moon at one-third of its present distance from the earth, and the tides feet high. now just contemplate the effect of a -foot tide. we are here only about feet above the level of the sea; hence, the tide would sweep right over us and rush far away inland. at high tide we should have some feet of blue water over our heads. there would be nothing to stop such a tide as that in this neighbourhood till it reached the high lands of derbyshire. manchester would be a seaport then with a vengeance! the day was shorter then, and so the interval between tide and tide was more like ten than twelve hours. accordingly, in about five hours, all that mass of water would have swept back again, and great tracts of sand between here and ireland would be left dry. another five hours, and the water would come tearing and driving over the country, applying its furious waves and currents to the work of denudation, which would proceed apace. these high tides of enormously distant past ages constitute the denuding agent which the geologist required. they are very ancient--more ancient than the carboniferous period, for instance, for no trees could stand the furious storms that must have been prevalent at this time. it is doubtful whether any but the very lowest forms of life then existed. it is the strata at the bottom of the geological scale that are of the most portentous thickness, and the only organism suspected in them is the doubtful _eozoon canadense_. sir robert ball believes, and several geologists agree with him, that the mighty tides we are contemplating may have been coæval with this ancient laurentian formation, and others of like nature with it. but let us leave geology now, and trace the inverted progress of events as we recede in imagination back through the geological era, beyond, into the dim vista of the past, when the moon was still closer and closer to the earth, and was revolving round it quicker and quicker, before life or water existed on it, and when the rocks were still molten. suppose the moon once touched the earth's surface, it is easy to calculate, according to the principles of gravitation, and with a reasonable estimate of its size as then expanded by heat, how fast it must then have revolved round the earth, so as just to save itself from falling in. it must have gone round once every three hours. the month was only three hours long at this initial epoch. remember, however, the initial length of the day. we found that it was just possible for the earth to rotate on its axis in three hours, and that when it did so, something was liable to separate from it. here we find the moon in contact with it, and going round it in this same three-hour period. surely the two are connected. surely the moon was a part of the earth, and was separating from it. that is the great discovery--the origin of the moon. once, long ages back, at date unknown, but believed to be certainly as much as fifty million years ago, and quite possibly one hundred million, there was no moon, only the earth as a molten globe, rapidly spinning on its axis--spinning in about three hours. gradually, by reason of some disturbing causes, a protuberance, a sort of bud, forms at one side, and the great inchoate mass separates into two--one about eighty times as big as the other. the bigger one we now call earth, the smaller we now call moon. round and round the two bodies went, pulling each other into tremendously elongated or prolate shapes, and so they might have gone on for a long time. but they are unstable, and cannot go on thus: they must either separate or collapse. some disturbing cause acts again, and the smaller mass begins to revolve less rapidly. tides at once begin--gigantic tides of molten lava hundreds of miles high; tides not in free ocean, for there was none then, but in the pasty mass of the entire earth. immediately the series of changes i have described begins, the speed of rotation gets slackened, the moon's mass gets pushed further and further away, and its time of revolution grows rapidly longer. the changes went on rapidly at first, because the tides were so gigantic; but gradually, and by slow degrees, the bodies get more distant, and the rate of change more moderate. until, after the lapse of ages, we find the day twenty-four hours long, the moon , miles distant, revolving in - / days, and the tides only existing in the water of the ocean, and only a few feet high. this is the era we call "to-day." the process does not stop here: still the stately march of events goes on; and the eye of science strives to penetrate into the events of the future with the same clearness as it has been able to descry the events of the past. and what does it see? it will take too long to go into full detail: but i will shortly summarize the results. it sees this first--the day and the month both again equal, but both now about , hours long. neither of these bodies rotating with respect to each other--the two as if joined by a bar--and total cessation of tide-generating action between them. the date of this period is one hundred and fifty millions of years hence, but unless some unforeseen catastrophe intervenes, it must assuredly come. yet neither will even this be the final stage; for the system is disturbed by the tide-generating force of the sun. it is a small effect, but it is cumulative; and gradually, by much slower degrees than anything we have yet contemplated, we are presented with a picture of the month getting gradually shorter than the day, the moon gradually approaching instead of receding, and so, incalculable myriads of ages hence, precipitating itself upon the surface of the earth whence it arose. such a catastrophe is already imminent in a neighbouring planet--mars. mars' principal moon circulates round him at an absurd pace, completing a revolution in - / hours, and it is now only , miles from his surface. the planet rotates in twenty-four hours as we do; but its tides are following its moon more quickly than it rotates after them; they are therefore tending to increase its rate of spin, and to retard the revolution of the moon. mars is therefore slowly but surely pulling its moon down on to itself, by a reverse action to that which separated our moon. the day shorter than the month forces a moon further away; the month shorter than the day tends to draw a satellite nearer. this moon of mars is not a large body: it is only twenty or thirty miles in diameter, but it weighs some forty billion tons, and will ultimately crash along the surface with a velocity of , miles an hour. such a blow must produce the most astounding effects when it occurs, but i am unable to tell you its probable date. so far we have dealt mainly with the earth and its moon; but is the existence of tides limited to these bodies? by no means. no body in the solar system is rigid, no body in the stellar universe is rigid. all must be susceptible of some tidal deformation, and hence, in all of them, agents like those we have traced in the history of the earth and moon must be at work: the motion of all must be complicated by the phenomena of tides. it is prof. george darwin who has worked out the astronomical influence of the tides, on the principles of sir william thomson: it is sir robert ball who has extended mr. darwin's results to the past history of our own and other worlds.[ ] tides are of course produced in the sun by the action of the planets, for the sun rotates in twenty-five days or thereabouts, while the planets revolve in much longer periods than that. the principal tide-generating bodies will be venus and jupiter; the greater nearness of one rather more than compensating for the greater mass of the other. it may be interesting to tabulate the relative tide-producing powers of the planets on the sun. they are as follows, calling that of the earth , :-- relative tide-producing powers of the planets on the sun. mercury , venus , earth , mars jupiter , saturn , uranus neptune the power of all of them is very feeble, and by acting on different sides they usually partly neutralize each other's action; but occasionally they get all on one side, and in that case some perceptible effect may be produced; the probable effect seems likely to be a gentle heaving tide in the solar surface, with breaking up of any incipient crust; and such an effect may be considered as evidenced periodically by the great increase in the number of solar spots which then break out. the solar tides are, however, much too small to appreciably push any planet away, hence we are not to suppose that the planets originated by budding from the sun, in contradiction of the nebular hypothesis. nor is it necessary to assume that the satellites, as a class, originated in the way ours did; though they may have done so. they were more probably secondary rings. our moon differs from other satellites in being exceptionally large compared with the size of its primary; it is as big as some of the moons of jupiter and saturn. the earth is the only one of the small planets that has an appreciable moon, and hence there is nothing forced or unnatural in supposing that it may have had an exceptional history. evidently, however, tidal phenomena must be taken into consideration in any treatment of the solar system through enormous length of time, and it will probably play a large part in determining its future. when laplace and lagrange investigated the question of the stability or instability of the solar system, they did so on the hypothesis that the bodies composing it were rigid. they reached a grand conclusion--that all the mutual perturbations of the solar system were periodic--that whatever changes were going on would reach a maximum and then begin to diminish; then increase again, then diminish, and so on. the system was stable, and its changes were merely like those of a swinging pendulum. but this conclusion is not final. the hypothesis that the bodies are rigid is not strictly true: and directly tidal deformation is taken into consideration it is perceived to be a potent factor, able in the long run to upset all their calculations. but it is so utterly and inconceivably minute--it only produces an appreciable effect after millions of years--whereas the ordinary perturbations go through their swings in some hundred thousand years or so at the most. granted it is small, but it is terribly persistent; and it always acts in one direction. never does it cease: never does it begin to act oppositely and undo what it has done. it is like the perpetual dropping of water. there may be only one drop in a twelvemonth, but leave it long enough, and the hardest stone must be worn away at last. * * * * * we have been speaking of millions of years somewhat familiarly; but what, after all, is a million years that we should not speak familiarly of it? it is longer than our lifetime, it is true. to the ephemeral insects whose lifetime is an hour, a year might seem an awful period, the mid-day sun might seem an almost stationary body, the changes of the seasons would be unknown, everything but the most fleeting and rapid changes would appear permanent and at rest. conversely, if our life-period embraced myriads of æons, things which now seem permanent would then appear as in a perpetual state of flux. a continent would be sometimes dry, sometimes covered with ocean; the stars we now call fixed would be moving visibly before our eyes; the earth would be humming on its axis like a top, and the whole of human history might seem as fleeting as a cloud of breath on a mirror. evolution is always a slow process. to evolve such an animal as a greyhound from its remote ancestors, according to mr. darwin, needs immense tracts of time; and if the evolution of some feeble animal crawling on the surface of this planet is slow, shall the stately evolution of the planetary orbs themselves be hurried? it may be that we are able to trace the history of the solar system for some thousand million years or so; but for how much longer time must it not have a history--a history, and also a future--entirely beyond our ken? those who study the stars have impressed upon them the existence of the most immeasurable distances, which yet are swallowed up as nothing in the infinitude of space. no less are we compelled to recognize the existence of incalculable æons of time, and yet to perceive that these are but as drops in the ocean of eternity. footnotes: [ ] the following account of mars's motion is from the excellent small manual of astronomy by dr. haughton of trinity college, dublin:--(p. ) "mars's motion is very unequal; when he first appears in the morning emerging from the rays of the sun, his motion is direct and rapid; it afterwards becomes slower, and he becomes stationary when at an elongation of ° from the sun; then his motion becomes retrograde, and its velocity increases until he is in opposition to the sun at °; at this time the retrograde motion is most rapid, and afterwards diminishes until he is ° distant from the sun on the other side, when mars again becomes stationary; his motion then becomes direct, and increases in velocity until it reaches a maximum, when the planet is again in conjunction with the sun. the retrograde motion of this planet lasts for days: and its arc of retrogradation is °." [ ] it is not so easy to plot the path of the sun among the stars by direct observation, as it is to plot the path of a planet; because sun and stars are not visible together. hipparchus used the moon as an intermediary; since sun and moon are visible together, and also moon and stars. [ ] this is, however, by no means the whole of the matter. the motion is not a simple circle nor has it a readily specifiable period. there are several disturbing causes. all that is given here is a first rough approximation. [ ] the proof is easy, and ought to occur in books on solid geometry. by a "regular" solid is meant one with all its faces, edges, angles, &c., absolutely alike: it is of these perfectly symmetrical bodies that there are only five. crystalline forms are practically infinite in number. [ ] best known to us by his christian name, as so many others of that time are known, _e.g._ raphael sanzio, dante alighieri, michael angelo buonarotti. the rule is not universal. tasso and ariosto are surnames. [ ] it would seem that the fact that all bodies of every material tend to fall at the same rate is still not clearly known. confusion is introduced by the resistance of the air. but a little thought should make it clear that the effect of the air is a mere disturbance, to be eliminated as far as possible, since the atmosphere has nothing to do with gravitation. the old fashioned "guinea and feather experiment" illustrates that in a vacuum things entirely different in specific gravity or surface drop at the same pace. [ ] karl von gebler (galileo), p. . [ ] it is of course the "silver lining" of clouds that outside observers see. [ ] l.u.k., _life of galileo_, p. . [ ] _note added september, ._ news from the lick observatory makes a very small fifth satellite not improbable. [ ] they remained there till this century. in they were quietly dropped. [ ] it was invented by van helmont, a belgian chemist, who died in . he suggested two names _gas_ and _blas_, and the first has survived. blas was, i suppose, from _blasen_, to blow, and gas seems to be an attempt to get at the sanskrit root underlying all such words as _geist_. [ ] such as this, among many others:--the duration of a flame under different conditions is well worth determining. a spoonful of warm spirits of wine burnt pulsations. the same spoonful of spirits of wine with addition of one-sixth saltpetre burnt pulsations. with one-sixth common salt, ; with one-sixth gunpowder, ; a piece of wax in the middle of the spirit, ; a piece of _kieselstein_, ; one-sixth water, ; and with equal parts water, only pulse-beats. this, says liebig, is given as an example of a "_licht-bringende versuch_." [ ] draper, _history of civilization in europe_, vol. ii. p. . [ ] professor knight's series of philosophical classics. [ ] to explain why the entire system, horse and cart together, move forward, the forces acting on the ground must be attended to. [ ] the distance being proportional to the _square_ of the time, see p. . [ ] the following letter, recently unearthed and published in _nature_, may , , seems to me well worth preserving. the feeling of a respiratory interval which it describes is familiar to students during the too few periods of really satisfactory occupation. the early guess concerning atmospheric electricity is typical of his extraordinary instinct for guessing right. "london, _dec. , _. "dear doctor,--he that in ye mine of knowledge deepest diggeth, hath, like every other miner, ye least breathing time, and must sometimes at least come to terr. alt. for air. "in one of these respiratory intervals i now sit down to write to you, my friend. "you ask me how, with so much study, i manage to retene my health. ah, my dear doctor, you have a better opinion of your lazy friend than he hath of himself. morpheous is my last companion; without or hours of him yr correspondent is not worth one scavenger's peruke. my practices did at ye first hurt my stomach, but now i eat heartily enou' as y' will see when i come down beside you. "i have been much amused at ye singular [greek: _phenomena_] resulting from bringing of a needle into contact with a piece of amber or resin fricated on silke clothe. ye flame putteth me in mind of sheet lightning on a small--how very small--scale. but i shall in my epistles abjure philosophy whereof when i come down to sakly i'll give you enou'. i began to scrawl at mins. from of ye clk. and have in writing consmd. mins. my ld. somerset is announced. "farewell, gd. bless you and help yr sincere friend. "isaac newton. "_to_ dr. law, suffolk." [ ] kepler's laws may be called respectively, the law of path, the law of speed, and the relationship law. by the "mass" of a body is meant the number of pounds or tons in it: the amount of matter it contains. the idea is involved in the popular word "massive." [ ] the equation we have to verify is [pi]^ r^ gr^ = -----------, t^ with the data that _r_, the moon's distance, is times r, the earth's radius, which is , miles; while t, the time taken to complete the moon's orbit, is days, hours, minutes, seconds. hence, suppose we calculate out _g_, the intensity of terrestrial gravity, from the above equation, we get [pi]^ · × × miles _g_ = ---------- × ( )^ r = ----------------------------- t ( days, hours, &c.)^ = · feet-per-second per second, which is not far wrong. [ ] the two motions may be roughly compounded into a single motion, which for a few centuries may without much error be regarded as a conical revolution about a different axis with a different period; and lieutenant-colonel drayson writes books emphasizing this simple fact, under the impression that it is a discovery. [ ] members of the accademia dei lyncei, the famous old scientific society established in the time of cosmo de medici--older than our own royal society. [ ] newton suspected that the moon really did so oscillate, and so it may have done once; but any real or physical libration, if existing at all, is now extremely minute. [ ] an interesting picture in the new gallery this year ( ), attempting to depict "earth-rise in moon-land," unfortunately errs in several particulars. first of all, the earth does not "rise," but is fixed relatively to each place on the moon; and two-fifths of the moon never sees it. next, the earth would not look like a map of the world with a haze on its edge. lastly, whatever animal remains the moon may contain would probably be rather in the form of fossils than of skeletons. the skeleton is of course intended as an image of death and desolation. it is a matter of taste: but a skeleton, it seems to me, speaks too recently of life to be as appallingly weird and desolate as a blank stone or ice landscape, unshaded by atmosphere or by any trace of animal or plant life, could be made. [ ] five of jupiter's revolutions occupy , days; two of saturn's revolutions occupy , days. [ ] _excircularity_ is what is meant by this term. it is called "excentricity" because the foci (not the centre) of an ellipse are regarded as the representatives of the centre of a circle. their distance from the centre, compared with the radius of the unflattened circle, is called the excentricity. [ ] a curve of the _n_th degree has / _n_(_n_+ ) arbitrary constants in its equation, hence this number of points specifically determine it. but special points, like focus or vertex, count as two ordinary ones. hence three points plus the focus act as five points, and determine a conic or curve of the second degree. three observations therefore fix an orbit round the sun. [ ] its name suggests a measure of the diameter of the sun's disk, and this is one of its functions; but it can likewise measure planetary and other disks; and in general behaves as the most elaborate and expensive form of micrometer. the königsberg instrument is shewn in fig. . [ ] it may be supposed that the terms "minute" and "second" have some necessary connection with time, but they are mere abbreviations for _partes minutæ_ and _partes minutæ secundæ_, and consequently may be applied to the subdivision of degrees just as properly as to the subdivision of hours. a "second" of arc means the th part of a degree, just as a second of time means the th part of an hour. [ ] a group of flying particles, each one invisible, obstructs light singularly little, even when they are close together, as one can tell by the transparency of showers and snowstorms. the opacity of haze may be due not merely to dust particles, but to little eddies set up by radiation above each particle, so that the air becomes turbulent and of varying density. (see a similar suggestion by mr. poynting in _nature_, vol. , p. .) [ ] the moon ought to be watched during the next great shower, if the line of fire happens to take effect on a visible part of the dark portion. [ ] address to birmingham midland institute, "a glimpse through the corridors of time." index index a abbott, t.k., on tides, adams, john couch, , , , , , , , , , , airy, sir george, , , , , , , anaxagoras, appian, arabs, the, form a link between the old and new science, archimedes, , , , , , aristarchus, aristotle, , , , , , . he taught that the earth was a sphere, ; his theories did not allow of the earth's motion, ; he was regarded as inspired, b bacon, francis, , , , . his _novum organum_, bacon, roger, , , . the herald of the dawn of science, brahé, george, uncle of tycho brahé, brahé, steno, brother of tycho brahé, brahé, tycho, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . he tried to adopt the main features of the copernican theory without admitting the motion of the earth, ; he was a poor theorist but a great observer, ; his medicine, ; his personal history, , _seq._; his observatory, uraniburg, ; his greatest invention, , note; his maniac lep, ; his kindness to kepler, ball, sir r., , ; his _story of the heavens_, barrow, dr., , bessel, , , , , , , , biela, , , bode's law, , , , , boyle, , bradley, prof. james, , , , , , , , bremiker, , brewster, on kepler, brinkley, bruno, giordano, , c castelli, , cayley, prof., challis, prof., , clairut, , , , , , clark, alvan and sons, columbus, , copernicus, , , _seq._, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his _de revolutionibus orbium coelestium_, , , ; he _proved_ that the earth went round the sun, ; the influence of his theory on the church, , _seq._; his life-work summarised, ; his life by mr. e.j.c. morton, copernican tables, ; copernican theory, , , , , copernik, nicolas; see copernicus cornu, croll, dr., his _climate and time_, d d'alembert, , darwin, charles, , , darwin, prof. george, , delambre, descartes, , , , , , , , , , , , , , ; his _discourse on method_, ; his dream, ; his system of algebraic geometry, , _seq._; his doctrine of vortices, , _seq._; his _principia mathematica_, ; his life by mr. mahaffy, e earth, the difficulties in the way of believing that it moved, , _seq._ "earth-rise in moon-land," , note encke, , epicyclic orbits explained, , _seq._ equinoxes, their precession discovered by hipparchus, eudoxus, euler, , f faraday, fizeau, , flamsteed, , , , , fraunhofer, froude, prof.; his _oceania_, g galen, galileo, galilei, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his youth, ; his discovery of the pendulum, ; his first observations about falling bodies, , _seq._; he invents a telescope, ; he adopts the copernican theory, ; he conceives "earth-shine," ; he discovers jupiter's moons, ; he studies saturn, , _seq._; his _dialogues on the ptolemaic and copernican systems_, ; his abjuration, ; he becomes blind, ; he discovered the laws of motion, , _seq._; he guessed that sight was not instantaneous, , galle, dr., , gauss, , gilbert, dr., , , , ; his _de magnete_, , greeks, their scientific methods, groombridge's catalogue, h hadley, halley, , , , , , , , , , , , , , ; he discovered the _principia_, harvey, , haughton, dr., ; his manual on astronomy, , note heliometer, described, helmholtz, helmont, van, invented the word "gas," henderson, , herschel, alexander, , , , herschel, caroline, , , , , ; her journal quoted, , _seq._; her work with william h. described, herschel, sir john, , , , herschel, william, , , , , , , , , , , , , , , , , , , , , , ; he "sweeps" the heavens, ; his discovery of uranus, , ; his artificial saturn, , ; his methods of work with his sister, described, ; he founded the science of astronomy, hind, hipparchus, , , , , , and note, , , , , ; an explanation of his discovery of the precession of the equinoxes, , seq. hippocrates, homeric cosmogony, , _seq._ hooke, , , , , , , hôpital, marquis de l', horkey, martin, horrebow, huxley, prof., huyghens, , , k kant, , kelvin, lord, see thomson, sir w. kepler, john, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; he replaced epicycles by an ellipse, ; he was a pupil of tycho brahé, ; he was a speculator more than an observer, ; his personal life, , _seq._; his theories about the numbers and distances of the planets, , ; he was helped by tycho, ; his main work, , _seq._; he gave up circular motion, ; his _mysterium cosmographicon_, ; his laws, , , , , , , , , _seq._ l lagrange, , , , , , , lagrange and laplace, , , ; they laid the foundations of the planetary theory, laplace, , , , , , , , , , , , , , , , , , ; his nebular hypothesis, , ; his _mécanique céleste_, lassell, mr., , leibnitz, , , le monnier, leonardo, see vinci, leonardo da leverrier, , , , , , lippershey, hans, m maskelyne, maxwell, clerk, , molyneux, , morton, mr. e.j. c, his life of copernicus, n newton, prof. h.a., newton, sir isaac, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his _principia_, , , , , , , , , , , , , , , ; his early life, , _seq._; his first experiments, ; his work at cambridge, ; his laws, ; his application of the laws of gravity to astronomy, , , , , ; his reticence, ; his discoveries in optics, , _seq._; his work summarised, ; his _optics_, ; anecdotes of him, ; his appearance in a court of justice, ; some of his manuscripts very recently discovered, ; his theories of the equinoxes and tides, , _seq._, , , _seq._ o olbers, , p peters, prof., , piazzi, , , , picard, , , , pioneers, genuine, planets and days of the week, poynting, printing, ptolemy, , , , , , , , ; his system of the heavens simplified by copernicus, , ; his system described, , _seq._; his system taught, ; his harmonies, pythagoras, , , q quadrant, an early, , r rheiter, ricci, ostillio, , roberts, isaac, roemer, , , , , , , rosse, lord, his telescope, , rudolphine tables, s scheiner, sizzi, francesca, an orthodox astronomer, snell, willebrod, and the law of refraction, solar system, its fate, stars, a list of, struve, , , , stuart, prof., quoted, t tatius, telescopes, early, thales, , , thomson, sir william, , , , , tide-gauge, described, , _seq._ tides, , _seq._ time, is not exactly uniform, torricelli, , tycho, see brahé, tycho v vinci, leonardo da, , , , viviani, , voltaire, w watson, prof., whewell, wren, sir christopher, , , , z zach, von, , zone of asteroids, , _seq._ the end. richard clay and sons, limited, london and bungay. the book of the damned a procession of the damned. by the damned, i mean the excluded. we shall have a procession of data that science has excluded. battalions of the accursed, captained by pallid data that i have exhumed, will march. you'll read them--or they'll march. some of them livid and some of them fiery and some of them rotten. some of them are corpses, skeletons, mummies, twitching, tottering, animated by companions that have been damned alive. there are giants that will walk by, though sound asleep. there are things that are theorems and things that are rags: they'll go by like euclid arm in arm with the spirit of anarchy. here and there will flit little harlots. many are clowns. but many are of the highest respectability. some are assassins. there are pale stenches and gaunt superstitions and mere shadows and lively malices: whims and amiabilities. the naïve and the pedantic and the bizarre and the grotesque and the sincere and the insincere, the profound and the puerile. a stab and a laugh and the patiently folded hands of hopeless propriety. the ultra-respectable, but the condemned, anyway. the aggregate appearance is of dignity and dissoluteness: the aggregate voice is a defiant prayer: but the spirit of the whole is processional. the power that has said to all these things that they are damned, is dogmatic science. but they'll march. the little harlots will caper, and freaks will distract attention, and the clowns will break the rhythm of the whole with their buffooneries--but the solidity of the procession as a whole: the impressiveness of things that pass and pass and pass, and keep on and keep on and keep on coming. the irresistibleness of things that neither threaten nor jeer nor defy, but arrange themselves in mass-formations that pass and pass and keep on passing. * * * * * so, by the damned, i mean the excluded. but by the excluded i mean that which will some day be the excluding. or everything that is, won't be. and everything that isn't, will be-- but, of course, will be that which won't be-- it is our expression that the flux between that which isn't and that which won't be, or the state that is commonly and absurdly called "existence," is a rhythm of heavens and hells: that the damned won't stay damned; that salvation only precedes perdition. the inference is that some day our accursed tatterdemalions will be sleek angels. then the sub-inference is that some later day, back they'll go whence they came. * * * * * it is our expression that nothing can attempt to be, except by attempting to exclude something else: that that which is commonly called "being" is a state that is wrought more or less definitely proportionately to the appearance of positive difference between that which is included and that which is excluded. but it is our expression that there are no positive differences: that all things are like a mouse and a bug in the heart of a cheese. mouse and a bug: no two things could seem more unlike. they're there a week, or they stay there a month: both are then only transmutations of cheese. i think we're all bugs and mice, and are only different expressions of an all-inclusive cheese. or that red is not positively different from yellow: is only another degree of whatever vibrancy yellow is a degree of: that red and yellow are continuous, or that they merge in orange. so then that, if, upon the basis of yellowness and redness, science should attempt to classify all phenomena, including all red things as veritable, and excluding all yellow things as false or illusory, the demarcation would have to be false and arbitrary, because things colored orange, constituting continuity, would belong on both sides of the attempted borderline. as we go along, we shall be impressed with this: that no basis for classification, or inclusion and exclusion, more reasonable than that of redness and yellowness has ever been conceived of. science has, by appeal to various bases, included a multitude of data. had it not done so, there would be nothing with which to seem to be. science has, by appeal to various bases, excluded a multitude of data. then, if redness is continuous with yellowness: if every basis of admission is continuous with every basis of exclusion, science must have excluded some things that are continuous with the accepted. in redness and yellowness, which merge in orangeness, we typify all tests, all standards, all means of forming an opinion-- or that any positive opinion upon any subject is illusion built upon the fallacy that there are positive differences to judge by-- that the quest of all intellection has been for something--a fact, a basis, a generalization, law, formula, a major premise that is positive: that the best that has ever been done has been to say that some things are self-evident--whereas, by evidence we mean the support of something else-- that this is the quest; but that it has never been attained; but that science has acted, ruled, pronounced, and condemned as if it had been attained. what is a house? it is not possible to say what anything is, as positively distinguished from anything else, if there are no positive differences. a barn is a house, if one lives in it. if residence constitutes houseness, because style of architecture does not, then a bird's nest is a house: and human occupancy is not the standard to judge by, because we speak of dogs' houses; nor material, because we speak of snow houses of eskimos--or a shell is a house to a hermit crab--or was to the mollusk that made it--or things seemingly so positively different as the white house at washington and a shell on the seashore are seen to be continuous. so no one has ever been able to say what electricity is, for instance. it isn't anything, as positively distinguished from heat or magnetism or life. metaphysicians and theologians and biologists have tried to define life. they have failed, because, in a positive sense, there is nothing to define: there is no phenomenon of life that is not, to some degree, manifest in chemism, magnetism, astronomic motions. white coral islands in a dark blue sea. their seeming of distinctness: the seeming of individuality, or of positive difference one from another--but all are only projections from the same sea bottom. the difference between sea and land is not positive. in all water there is some earth: in all earth there is some water. so then that all seeming things are not things at all, if all are inter-continuous, any more than is the leg of a table a thing in itself, if it is only a projection from something else: that not one of us is a real person, if, physically, we're continuous with environment; if, psychically, there is nothing to us but expression of relation to environment. our general expression has two aspects: conventional monism, or that all "things" that seem to have identity of their own are only islands that are projections from something underlying, and have no real outlines of their own. but that all "things," though only projections, are projections that are striving to break away from the underlying that denies them identity of their own. i conceive of one inter-continuous nexus, in which and of which all seeming things are only different expressions, but in which all things are localizations of one attempt to break away and become real things, or to establish entity or positive difference or final demarcation or unmodified independence--or personality or soul, as it is called in human phenomena-- that anything that tries to establish itself as a real, or positive, or absolute system, government, organization, self, soul, entity, individuality, can so attempt only by drawing a line about itself, or about the inclusions that constitute itself, and damning or excluding, or breaking away from, all other "things": that, if it does not so act, it cannot seem to be; that, if it does so act, it falsely and arbitrarily and futilely and disastrously acts, just as would one who draws a circle in the sea, including a few waves, saying that the other waves, with which the included are continuous, are positively different, and stakes his life upon maintaining that the admitted and the damned are positively different. our expression is that our whole existence is animation of the local by an ideal that is realizable only in the universal: that, if all exclusions are false, because always are included and excluded continuous: that if all seeming of existence perceptible to us is the product of exclusion, there is nothing that is perceptible to us that really is: that only the universal can really be. our especial interest is in modern science as a manifestation of this one ideal or purpose or process: that it has falsely excluded, because there are no positive standards to judge by: that it has excluded things that, by its own pseudo-standards, have as much right to come in as have the chosen. * * * * * our general expression: that the state that is commonly and absurdly called "existence," is a flow, or a current, or an attempt, from negativeness to positiveness, and is intermediate to both. by positiveness we mean: harmony, equilibrium, order, regularity, stability, consistency, unity, realness, system, government, organization, liberty, independence, soul, self, personality, entity, individuality, truth, beauty, justice, perfection, definiteness-- that all that is called development, progress, or evolution is movement toward, or attempt toward, this state for which, or for aspects of which, there are so many names, all of which are summed up in the one word "positiveness." at first this summing up may not be very readily acceptable. at first it may seem that all these words are not synonyms: that "harmony" may mean "order," but that by "independence," for instance, we do not mean "truth," or that by "stability" we do not mean "beauty," or "system," or "justice." i conceive of one inter-continuous nexus, which expresses itself in astronomic phenomena, and chemic, biologic, psychic, sociologic: that it is everywhere striving to localize positiveness: that to this attempt in various fields of phenomena--which are only quasi-different--we give different names. we speak of the "system" of the planets, and not of their "government": but in considering a store, for instance, and its management, we see that the words are interchangeable. it used to be customary to speak of chemic equilibrium, but not of social equilibrium: that false demarcation has been broken down. we shall see that by all these words we mean the same state. as every-day conveniences, or in terms of common illusions, of course, they are not synonyms. to a child an earth worm is not an animal. it is to the biologist. by "beauty," i mean that which seems complete. obversely, that the incomplete, or the mutilated, is the ugly. venus de milo. to a child she is ugly. when a mind adjusts to thinking of her as a completeness, even though, by physiologic standards, incomplete, she is beautiful. a hand thought of only as a hand, may seem beautiful. found on a battlefield--obviously a part--not beautiful. but everything in our experience is only a part of something else that in turn is only a part of still something else--or that there is nothing beautiful in our experience: only appearances that are intermediate to beauty and ugliness--that only universality is complete: that only the complete is the beautiful: that every attempt to achieve beauty is an attempt to give to the local the attribute of the universal. by stability, we mean the immovable and the unaffected. but all seeming things are only reactions to something else. stability, too, then, can be only the universal, or that besides which there is nothing else. though some things seem to have--or have--higher approximations to stability than have others, there are, in our experience, only various degrees of intermediateness to stability and instability. every man, then, who works for stability under its various names of "permanency," "survival," "duration," is striving to localize in something the state that is realizable only in the universal. by independence, entity, and individuality, i can mean only that besides which there is nothing else, if given only two things, they must be continuous and mutually affective, if everything is only a reaction to something else, and any two things would be destructive of each other's independence, entity, or individuality. all attempted organizations and systems and consistencies, some approximating far higher than others, but all only intermediate to order and disorder, fail eventually because of their relations with outside forces. all are attempted completenesses. if to all local phenomena there are always outside forces, these attempts, too, are realizable only in the state of completeness, or that to which there are no outside forces. or that all these words are synonyms, all meaning the state that we call the positive state-- that our whole "existence" is a striving for the positive state. the amazing paradox of it all: that all things are trying to become the universal by excluding other things. that there is only this one process, and that it does animate all expressions, in all fields of phenomena, of that which we think of as one inter-continuous nexus: the religious and their idea or ideal of the soul. they mean distinct, stable entity, or a state that is independent, and not a mere flux of vibrations or complex of reactions to environment, continuous with environment, merging away with an infinitude of other interdependent complexes. but the only thing that would not merge away into something else would be that besides which there is nothing else. that truth is only another name for the positive state, or that the quest for truth is the attempt to achieve positiveness: scientists who have thought that they were seeking truth, but who were trying to find out astronomic, or chemic, or biologic truths. but truth is that besides which there is nothing: nothing to modify it, nothing to question it, nothing to form an exception: the all-inclusive, the complete-- by truth i mean the universal. so chemists have sought the true, or the real, and have always failed in their endeavors, because of the outside relations of chemical phenomena: have failed in the sense that never has a chemical law, without exceptions, been discovered: because chemistry is continuous with astronomy, physics, biology--for instance, if the sun should greatly change its distance from this earth, and if human life could survive, the familiar chemic formulas would no longer work out: a new science of chemistry would have to be learned-- or that all attempts to find truth in the special are attempts to find the universal in the local. and artists and their striving for positiveness, under the name of "harmony"--but their pigments that are oxydizing, or are responding to a deranging environment--or the strings of musical instruments that are differently and disturbingly adjusting to outside chemic and thermal and gravitational forces--again and again this oneness of all ideals, and that it is the attempt to be, or to achieve, locally, that which is realizable only universally. in our experience there is only intermediateness to harmony and discord. harmony is that besides which there are no outside forces. and nations that have fought with only one motive: for individuality, or entity, or to be real, final nations, not subordinate to, or parts of, other nations. and that nothing but intermediateness has ever been attained, and that history is record of failures of this one attempt, because there always have been outside forces, or other nations contending for the same goal. as to physical things, chemic, mineralogic, astronomic, it is not customary to say that they act to achieve truth or entity, but it is understood that all motions are toward equilibrium: that there is no motion except toward equilibrium, of course always away from some other approximation to equilibrium. all biologic phenomena act to adjust: there are no biologic actions other than adjustments. adjustment is another name for equilibrium. equilibrium is the universal, or that which has nothing external to derange it. but that all that we call "being" is motion: and that all motion is the expression, not of equilibrium, but of equilibrating, or of equilibrium unattained: that life-motions are expressions of equilibrium unattained: that all thought relates to the unattained: that to have what is called being in our quasi-state, is not to be in the positive sense, or is to be intermediate to equilibrium and inequilibrium. so then: that all phenomena in our intermediate state, or quasi-state, represent this one attempt to organize, stabilize, harmonize, individualize--or to positivize, or to become real: that only to have seeming is to express failure or intermediateness to final failure and final success: that every attempt--that is observable--is defeated by continuity, or by outside forces--or by the excluded that are continuous with the included: that our whole "existence" is an attempt by the relative to be the absolute, or by the local to be the universal. in this book, my interest is in this attempt as manifested in modern science: that it has attempted to be real, true, final, complete, absolute: that, if the seeming of being, here, in our quasi-state, is the product of exclusion that is always false and arbitrary, if always are included and excluded continuous, the whole seeming system, or entity, of modern science is only quasi-system, or quasi-entity, wrought by the same false and arbitrary process as that by which the still less positive system that preceded it, or the theological system, wrought the illusion of its being. in this book, i assemble some of the data that i think are of the falsely and arbitrarily excluded. the data of the damned. i have gone into the outer darkness of scientific and philosophical transactions and proceedings, ultra-respectable, but covered with the dust of disregard. i have descended into journalism. i have come back with the quasi-souls of lost data. they will march. * * * * * as to the logic of our expressions to come-- that there is only quasi-logic in our mode of seeming: that nothing ever has been proved-- because there is nothing to prove. when i say that there is nothing to prove, i mean that to those who accept continuity, or the merging away of all phenomena into other phenomena, without positive demarcations one from another, there is, in a positive sense, no one thing. there is nothing to prove. for instance nothing can be proved to be an animal--because animalness and vegetableness are not positively different. there are some expressions of life that are as much vegetable as animal, or that represent the merging of animalness and vegetableness. there is then no positive test, standard, criterion, means of forming an opinion. as distinct from vegetables, animals do not exist. there is nothing to prove. nothing could be proved to be good, for instance. there is nothing in our "existence" that is good, in a positive sense, or as really outlined from evil. if to forgive be good in times of peace, it is evil in wartime. there is nothing to prove: good in our experience is continuous with, or is only another aspect of evil. as to what i'm trying to do now--i accept only. if i can't see universally, i only localize. so, of course then, that nothing ever has been proved: that theological pronouncements are as much open to doubt as ever they were, but that, by a hypnotizing process, they became dominant over the majority of minds in their era: that, in a succeeding era, the laws, dogmas, formulas, principles, of materialistic science never were proved, because they are only localizations simulating the universal; but that the leading minds of their era of dominance were hypnotized into more or less firmly believing them. newton's three laws, and that they are attempts to achieve positiveness, or to defy and break continuity, and are as unreal as are all other attempts to localize the universal: that, if every observable body is continuous, mediately or immediately, with all other bodies, it cannot be influenced only by its own inertia, so that there is no way of knowing what the phenomena of inertia may be; that, if all things are reacting to an infinitude of forces, there is no way of knowing what the effects of only one impressed force would be; that if every reaction is continuous with its action, it cannot be conceived of as a whole, and that there is no way of conceiving what it might be equal and opposite to-- or that newton's three laws are three articles of faith: or that demons and angels and inertias and reactions are all mythological characters: but that, in their eras of dominance, they were almost as firmly believed in as if they had been proved. * * * * * enormities and preposterousnesses will march. they will be "proved" as well as moses or darwin or lyell ever "proved" anything. * * * * * we substitute acceptance for belief. cells of an embryo take on different appearances in different eras. the more firmly established, the more difficult to change. that social organism is embryonic. that firmly to believe is to impede development. that only temporarily to accept is to facilitate. * * * * * but: except that we substitute acceptance for belief, our methods will be the conventional methods; the means by which every belief has been formulated and supported: or our methods will be the methods of theologians and savages and scientists and children. because, if all phenomena are continuous, there can be no positively different methods. by the inconclusive means and methods of cardinals and fortune tellers and evolutionists and peasants, methods which must be inconclusive, if they relate always to the local, and if there is nothing local to conclude, we shall write this book. if it function as an expression of its era, it will prevail. * * * * * all sciences begin with attempts to define. nothing ever has been defined. because there is nothing to define. darwin wrote _the origin of species_. he was never able to tell what he meant by a "species." it is not possible to define. nothing has ever been finally found out. because there is nothing final to find out. it's like looking for a needle that no one ever lost in a haystack that never was-- but that all scientific attempts really to find out something, whereas really there is nothing to find out, are attempts, themselves, really to be something. a seeker of truth. he will never find it. but the dimmest of possibilities--he may himself become truth. or that science is more than an inquiry: that it is a pseudo-construction, or a quasi-organization: that it is an attempt to break away and locally establish harmony, stability, equilibrium, consistency, entity-- dimmest of possibilities--that it may succeed. * * * * * that ours is a pseudo-existence, and that all appearances in it partake of its essential fictitiousness-- but that some appearances approximate far more highly to the positive state than do others. we conceive of all "things" as occupying gradations, or steps in series between positiveness and negativeness, or realness and unrealness: that some seeming things are more nearly consistent, just, beautiful, unified, individual, harmonious, stable--than others. we are not realists. we are not idealists. we are intermediatists--that nothing is real, but that nothing is unreal: that all phenomena are approximations one way or the other between realness and unrealness. so then: that our whole quasi-existence is an intermediate stage between positiveness and negativeness or realness and unrealness. like purgatory, i think. but in our summing up, which was very sketchily done, we omitted to make clear that realness is an aspect of the positive state. by realness, i mean that which does not merge away into something else, and that which is not partly something else: that which is not a reaction to, or an imitation of, something else. by a real hero, we mean one who is not partly a coward, or whose actions and motives do not merge away into cowardice. but, if in continuity, all things do merge, by realness, i mean the universal, besides which there is nothing with which to merge. that, though the local might be universalized, it is not conceivable that the universal can be localized: but that high approximations there may be, and that these approximate successes may be translated out of intermediateness into realness--quite as, in a relative sense, the industrial world recruits itself by translating out of unrealness, or out of the seemingly less real imaginings of inventors, machines which seem, when set up in factories, to have more of realness than they had when only imagined. that all progress, if all progress is toward stability, organization, harmony, consistency, or positiveness, is the attempt to become real. so, then, in general metaphysical terms, our expression is that, like a purgatory, all that is commonly called "existence," which we call intermediateness, is quasi-existence, neither real nor unreal, but expression of attempt to become real, or to generate for or recruit a real existence. our acceptance is that science, though usually thought of so specifically, or in its own local terms, usually supposed to be a prying into old bones, bugs, unsavory messes, is an expression of this one spirit animating all intermediateness: that, if science could absolutely exclude all data but its own present data, or that which is assimilable with the present quasi-organization, it would be a real system, with positively definite outlines--it would be real. its seeming approximation to consistency, stability, system--positiveness or realness--is sustained by damning the irreconcilable or the unassimilable-- all would be well. all would be heavenly-- if the damned would only stay damned. in the autumn of , and for years afterward, occurred brilliant-colored sunsets, such as had never been seen before within the memory of all observers. also there were blue moons. i think that one is likely to smile incredulously at the notion of blue moons. nevertheless they were as common as were green suns in . science had to account for these unconventionalities. such publications as _nature_ and _knowledge_ were besieged with inquiries. i suppose, in alaska and in the south sea islands, all the medicine men were similarly upon trial. something had to be thought of. upon the th of august, , the volcano of krakatoa, of the straits of sunda, had blown up. terrific. we're told that the sound was heard , miles, and that , persons were killed. seems just a little unscientific, or impositive, to me: marvel to me we're not told , miles and , persons. the volume of smoke that went up must have been visible to other planets--or, tormented with our crawlings and scurryings, the earth complained to mars; swore a vast black oath at us. in all text-books that mention this occurrence--no exception so far so i have read--it is said that the extraordinary atmospheric effects of were first noticed in the last of august or the first of september. that makes a difficulty for us. it is said that these phenomena were caused by particles of volcanic dust that were cast high in the air by krakatoa. this is the explanation that was agreed upon in -- but for seven years the atmospheric phenomena continued-- except that, in the seven, there was a lapse of several years--and where was the volcanic dust all that time? you'd think that such a question as that would make trouble? then you haven't studied hypnosis. you have never tried to demonstrate to a hypnotic that a table is not a hippopotamus. according to our general acceptance, it would be impossible to demonstrate such a thing. point out a hundred reasons for saying that a hippopotamus is not a table: you'll have to end up agreeing that neither is a table a table--it only seems to be a table. well, that's what the hippopotamus seems to be. so how can you prove that something is not something else, when neither is something else some other thing? there's nothing to prove. this is one of the profundities that we advertised in advance. you can oppose an absurdity only with some other absurdity. but science is established preposterousness. we divide all intellection: the obviously preposterousness and the established. but krakatoa: that's the explanation that the scientists gave. i don't know what whopper the medicine men told. we see, from the start, the very strong inclination of science to deny, as much as it can, external relations of this earth. this book is an assemblage of data of external relations of this earth. we take the position that our data have been damned, upon no consideration for individual merits or demerits, but in conformity with a general attempt to hold out for isolation of this earth. this is attempted positiveness. we take the position that science can no more succeed than, in a similar endeavor, could the chinese, or than could the united states. so then, with only pseudo-consideration of the phenomena of , or as an expression of positivism in its aspect of isolation, or unrelatedness, scientists have perpetrated such an enormity as suspension of volcanic dust seven years in the air--disregarding the lapse of several years--rather than to admit the arrival of dust from somewhere beyond this earth. not that scientists themselves have ever achieved positiveness, in its aspect of unitedness, among themselves--because nordenskiold, before , wrote a great deal upon his theory of cosmic dust, and prof. cleveland abbe contended against the krakatoan explanation--but that this is the orthodoxy of the main body of scientists. my own chief reason for indignation here: that this preposterous explanation interferes with some of my own enormities. it would cost me too much explaining, if i should have to admit that this earth's atmosphere has such sustaining power. later, we shall have data of things that have gone up in the air and that have stayed up--somewhere--weeks--months--but not by the sustaining power of this earth's atmosphere. for instance, the turtle of vicksburg. it seems to me that it would be ridiculous to think of a good-sized turtle hanging, for three or four months, upheld only by the air, over the town of vicksburg. when it comes to the horse and the barn--i think that they'll be classics some day, but i can never accept that a horse and a barn could float several months in this earth's atmosphere. the orthodox explanation: see the _report of the krakatoa committee of the royal society_. it comes out absolutely for the orthodox explanation--absolutely and beautifully, also expensively. there are pages in the "report," and plates, some of them marvelously colored. it was issued after an investigation that took five years. you couldn't think of anything done more efficiently, artistically, authoritatively. the mathematical parts are especially impressive: distribution of the dust of krakatoa; velocity of translation and rates of subsidence; altitudes and persistences-- _annual register_, - : that the atmospheric effects that have been attributed to krakatoa were seen in trinidad before the eruption occurred: _knowledge_, - : that they were seen in natal, south africa, six months before the eruption. * * * * * inertia and its inhospitality. or raw meat should not be fed to babies. we shall have a few data initiatorily. i fear me that the horse and the barn were a little extreme for our budding liberalities. the outrageous is the reasonable, if introduced politely. hailstones, for instance. one reads in the newspapers of hailstones the size of hens' eggs. one smiles. nevertheless i will engage to list one hundred instances, from the _monthly weather review_, of hailstones the size of hens' eggs. there is an account in _nature_, nov. , , of hailstones that weighed almost two pounds each. see chambers' encyclopedia for three-pounders. _report of the smithsonian institution_, - --two-pounders authenticated, and six-pounders reported. at seringapatam, india, about the year , fell a hailstone-- i fear me, i fear me: this is one of the profoundly damned. i blurt out something that should, perhaps, be withheld for several hundred pages--but that damned thing was the size of an elephant. we laugh. or snowflakes. size of saucers. said to have fallen at nashville, tenn., jan. , . one smiles. "in montana, in the winter of , fell snowflakes inches across, and inches thick." (_monthly weather review_, - .) in the topography of intellection, i should say that what we call knowledge is ignorance surrounded by laughter. * * * * * black rains--red rains--the fall of a thousand tons of butter. jet-black snow--pink snow--blue hailstones--hailstones flavored like oranges. punk and silk and charcoal. * * * * * about one hundred years ago, if anyone was so credulous as to think that stones had ever fallen from the sky, he was reasoned with: in the first place there are no stones in the sky: therefore no stones can fall from the sky. or nothing more reasonable or scientific or logical than that could be said upon any subject. the only trouble is the universal trouble: that the major premise is not real, or is intermediate somewhere between realness and unrealness. in , a committee, of whom lavoisier was a member, was appointed by the french academy, to investigate a report that a stone had fallen from the sky at luce, france. of all attempts at positiveness, in its aspect of isolation, i don't know of anything that has been fought harder for than the notion of this earth's unrelatedness. lavoisier analyzed the stone of luce. the exclusionists' explanation at that time was that stones do not fall from the sky: that luminous objects may seem to fall, and that hot stones may be picked up where a luminous object seemingly had landed--only lightning striking a stone, heating, even melting it. the stone of luce showed signs of fusion. lavoisier's analysis "absolutely proved" that this stone had not fallen: that it had been struck by lightning. so, authoritatively, falling stones were damned. the stock means of exclusion remained the explanation of lightning that was seen to strike something--that had been upon the ground in the first place. but positiveness and the fate of every positive statement. it is not customary to think of damned stones raising an outcry against a sentence of exclusion, but, subjectively, aerolites did--or data of them bombarded the walls raised against them-- _monthly review_, - "the phenomenon which is the subject of the remarks before us will seem to most persons as little worthy of credit as any that could be offered. the falling of large stones from the sky, without any assignable cause of their previous ascent, seems to partake so much of the marvelous as almost entirely to exclude the operation of known and natural agents. yet a body of evidence is here brought to prove that such events have actually taken place, and we ought not to withhold from it a proper degree of attention." the writer abandons the first, or absolute, exclusion, and modifies it with the explanation that the day before a reported fall of stones in tuscany, june , , there had been an eruption of vesuvius-- or that stones do fall from the sky, but that they are stones that have been raised to the sky from some other part of the earth's surface by whirlwinds or by volcanic action. it's more than one hundred and twenty years later. i know of no aerolite that has ever been acceptably traced to terrestrial origin. falling stones had to be undamned--though still with a reservation that held out for exclusion of outside forces. one may have the knowledge of a lavoisier, and still not be able to analyze, not be able even to see, except conformably with the hypnoses, or the conventional reactions against hypnoses, of one's era. we believe no more. we accept. little by little the whirlwind and volcano explanations had to be abandoned, but so powerful was this exclusion-hypnosis, sentence of damnation, or this attempt at positiveness, that far into our own times some scientists, notably prof. lawrence smith and sir robert ball, continued to hold out against all external origins, asserting that nothing could fall to this earth, unless it had been cast up or whirled up from some other part of this earth's surface. it's as commendable as anything ever has been--by which i mean it's intermediate to the commendable and the censurable. it's virginal. meteorites, data of which were once of the damned, have been admitted, but the common impression of them is only a retreat of attempted exclusion: that only two kinds of substance fall from the sky: metallic and stony: that the metallic objects are of iron and nickel-- butter and paper and wool and silk and resin. we see, to start with, that the virgins of science have fought and wept and screamed against external relations--upon two grounds: there in the first place; or up from one part of this earth's surface and down to another. as late as november, , in _nature notes_, - , a member of the selborne society still argued that meteorites do not fall from the sky; that they are masses of iron upon the ground "in the first place," that attract lightning; that the lightning is seen, and is mistaken for a falling, luminous object-- by progress we mean rape. butter and beef and blood and a stone with strange inscriptions upon it. so then, it is our expression that science relates to real knowledge no more than does the growth of a plant, or the organization of a department store, or the development of a nation: that all are assimilative, or organizing, or systematizing processes that represent different attempts to attain the positive state--the state commonly called heaven, i suppose i mean. there can be no real science where there are indeterminate variables, but every variable is, in finer terms, indeterminate, or irregular, if only to have the appearance of being in intermediateness is to express regularity unattained. the invariable, or the real and stable, would be nothing at all in intermediateness--rather as, but in relative terms, an undistorted interpretation of external sounds in the mind of a dreamer could not continue to exist in a dreaming mind, because that touch of relative realness would be of awakening and not of dreaming. science is the attempt to awaken to realness, wherein it is attempt to find regularity and uniformity. or the regular and uniform would be that which has nothing external to disturb it. by the universal we mean the real. or the notion is that the underlying super-attempt, as expressed in science, is indifferent to the subject-matter of science: that the attempt to regularize is the vital spirit. bugs and stars and chemical messes: that they are only quasi-real, and that of them there is nothing real to know; but that systematization of pseudo-data is approximation to realness or final awakening-- or a dreaming mind--and its centaurs and canary birds that turn into giraffes--there could be no real biology upon such subjects, but attempt, in a dreaming mind, to systematize such appearances would be movement toward awakening--if better mental co-ordination is all that we mean by the state of being awake--relatively awake. so it is, that having attempted to systematize, by ignoring externality to the greatest possible degree, the notion of things dropping in upon this earth, from externality, is as unsettling and as unwelcome to science as--tin horns blowing in upon a musician's relatively symmetric composition--flies alighting upon a painter's attempted harmony, and tracking colors one into another--suffragist getting up and making a political speech at a prayer meeting. if all things are of a oneness, which is a state intermediate to unrealness and realness, and if nothing has succeeded in breaking away and establishing entity for itself, and could not continue to "exist" in intermediateness, if it should succeed, any more than could the born still at the same time be the uterine, i of course know of no positive difference between science and christian science--and the attitude of both toward the unwelcome is the same--"it does not exist." a lord kelvin and a mrs. eddy, and something not to their liking--it does not exist. of course not, we intermediates say: but, also, that, in intermediateness, neither is there absolute non-existence. or a christian scientist and a toothache--neither exists in the final sense: also neither is absolutely non-existent, and, according to our therapeutics, the one that more highly approximates to realness will win. a secret of power-- i think it's another profundity. do you want power over something? be more nearly real than it. we'll begin with yellow substances that have fallen upon this earth: we'll see whether our data of them have a higher approximation to realness than have the dogmas of those who deny their existence--that is, as products from somewhere external to this earth. in mere impressionism we take our stand. we have no positive tests nor standards. realism in art: realism in science--they pass away. in , the thing to do was to accept darwinism; now many biologists are revolting and trying to conceive of something else. the thing to do was to accept it in its day, but darwinism of course was never proved: the fittest survive. what is meant by the fittest? not the strongest; not the cleverest-- weakness and stupidity everywhere survive. there is no way of determining fitness except in that a thing does survive. "fitness," then, is only another name for "survival." darwinism: that survivors survive. although darwinism, then, seems positively baseless, or absolutely irrational, its massing of supposed data, and its attempted coherence approximate more highly to organization and consistency than did the inchoate speculations that preceded it. or that columbus never proved that the earth is round. shadow of the earth on the moon? no one has ever seen it in its entirety. the earth's shadow is much larger than the moon. if the periphery of the shadow is curved--but the convex moon--a straight-edged object will cast a curved shadow upon a surface that is convex. all the other so-called proofs may be taken up in the same way. it was impossible for columbus to prove that the earth is round. it was not required: only that with a higher seeming of positiveness than that of his opponents, he should attempt. the thing to do, in , was nevertheless to accept that beyond europe, to the west, were other lands. i offer for acceptance, as something concordant with the spirit of this first quarter of the th century, the expression that beyond this earth are--other lands--from which come things as, from america, float things to europe. as to yellow substances that have fallen upon this earth, the endeavor to exclude extra-mundane origins is the dogma that all yellow rains and yellow snows are colored with pollen from this earth's pine trees. _symons' meteorological magazine_ is especially prudish in this respect and regards as highly improper all advances made by other explainers. nevertheless, the _monthly weather review_, may, , reports a golden-yellow fall, of feb. , , at peckloh, germany, in which four kinds of organisms, not pollen, were the coloring matter. there were minute things shaped like arrows, coffee beans, horns, and disks. they may have been symbols. they may have been objective hieroglyphics-- mere passing fancy--let it go-- in the _annales de chimie_, - , there is a list of rains said to have contained sulphur. i have thirty or forty other notes. i'll not use one of them. i'll admit that every one of them is upon a fall of pollen. i said, to begin with, that our methods would be the methods of theologians and scientists, and they always begin with an appearance of liberality. i grant thirty or forty points to start with. i'm as liberal as any of them--or that my liberality won't cost me anything--the enormousness of the data that we shall have. or just to look over a typical instance of this dogma, and the way it works out: in the _american journal of science_, - - , we are told of a yellow substance that fell by the bucketful upon a vessel, one "windless" night in june, in pictou harbor, nova scotia. the writer analyzed the substance, and it was found to "give off nitrogen and ammonia and an animal odor." now, one of our intermediatist principles, to start with, is that so far from positive, in the aspect of homogeneousness, are all substances, that, at least in what is called an elementary sense, anything can be found anywhere. mahogany logs on the coast of greenland; bugs of a valley on the top of mt. blanc; atheists at a prayer meeting; ice in india. for instance, chemical analysis can reveal that almost any dead man was poisoned with arsenic, we'll say, because there is no stomach without some iron, lead, tin, gold, arsenic in it and of it--which, of course, in a broader sense, doesn't matter much, because a certain number of persons must, as a restraining influence, be executed for murder every year; and, if detectives aren't able really to detect anything, illusion of their success is all that is necessary, and it is very honorable to give up one's life for society as a whole. the chemist who analyzed the substance of pictou sent a sample to the editor of the _journal_. the editor of course found pollen in it. my own acceptance is that there'd have to be some pollen in it: that nothing could very well fall through the air, in june, near the pine forests of nova scotia, and escape all floating spores of pollen. but the editor does not say that this substance "contained" pollen. he disregards "nitrogen, ammonia, and an animal odor," and says that the substance was pollen. for the sake of our thirty or forty tokens of liberality, or pseudo-liberality, if we can't be really liberal, we grant that the chemist of the first examination probably wouldn't know an animal odor if he were janitor of a menagerie. as we go along, however, there can be no such sweeping ignoring of this phenomenon: the fall of animal-matter from the sky. i'd suggest, to start with, that we'd put ourselves in the place of deep-sea fishes: how would they account for the fall of animal-matter from above? they wouldn't try-- or it's easy enough to think of most of us as deep-sea fishes of a kind. _jour. franklin inst._, - : that, upon the th of february, , there fell, at genoa, italy, according to director boccardo, of the technical institute of genoa, and prof. castellani, a yellow substance. but the microscope revealed numerous globules of cobalt blue, also corpuscles of a pearly color that resembled starch. see _nature_, - . _comptes rendus_, - : m. bouis says of a substance, reddish varying to yellowish, that fell enormously and successively, or upon april , may and may , in france and spain, that it carbonized and spread the odor of charred animal matter--that it was not pollen--that in alcohol it left a residue of resinous matter. hundreds of thousands of tons of this matter must have fallen. "odor of charred animal matter." or an aerial battle that occurred in inter-planetary space several hundred years ago--effect of time in making diverse remains uniform in appearance-- it's all very absurd because, even though we are told of a prodigious quantity of animal matter that fell from the sky--three days--france and spain--we're not ready yet: that's all. m. bouis says that this substance was not pollen; the vastness of the fall makes acceptable that it was not pollen; still, the resinous residue does suggest pollen of pine trees. we shall hear a great deal of a substance with a resinous residue that has fallen from the sky: finally we shall divorce it from all suggestion of pollen. _blackwood's magazine_, - : a yellow powder that fell at gerace, calabria, march , . some of this substance was collected by sig. simenini, professor of chemistry, at naples. it had an earthy, insipid taste, and is described as "unctuous." when heated, this matter turned brown, then black, then red. according to the _annals of philosophy_, - , one of the components was a greenish-yellow substance, which, when dried, was found to be resinous. but concomitants of this fall: loud noises were heard in the sky. stones fell from the sky. according to chladni, these concomitants occurred, and to me they seem--rather brutal?--or not associable with something so soft and gentle as a fall of pollen? * * * * * black rains and black snows--rains as black as a deluge of ink--jet-black snowflakes. such a rain as that which fell in ireland, may , , described in the _annals of scientific discovery_, , and the _annual register_, . it fell upon a district of square miles, and was the color of ink, and of a fetid odor and very disagreeable taste. the rain at castlecommon, ireland, april , --"thick, black rain." (_amer. met. jour._, - .) a black rain fell in ireland, oct. and , . (_symons' met. mag._ - .) "it left a most peculiar and disagreeable smell in the air." the orthodox explanation of this rain occurs in _nature_, march , --cloud of soot that had come from south wales, crossing the irish channel and all of ireland. so the black rain of ireland, of march, : ascribed in _symons' met. mag._ - , to clouds of soot from the manufacturing towns of north england and south scotland. our intermediatist principle of pseudo-logic, or our principle of continuity is, of course, that nothing is unique, or individual: that all phenomena merge away into all other phenomena: that, for instance--suppose there should be vast celestial super-oceanic, or inter-planetary vessels that come near this earth and discharge volumes of smoke at times. we're only supposing such a thing as that now, because, conventionally, we are beginning modestly and tentatively. but if it were so, there would necessarily be some phenomenon upon this earth, with which that phenomenon would merge. extra-mundane smoke and smoke from cities merge, or both would manifest in black precipitations in rain. in continuity, it is impossible to distinguish phenomena at their merging-points, so we look for them at their extremes. impossible to distinguish between animal and vegetable in some infusoria--but hippopotamus and violet. for all practical purposes they're distinguishable enough. no one but a barnum or a bailey would send one a bunch of hippopotami as a token of regard. so away from the great manufacturing centers: black rain in switzerland, jan. , . switzerland is so remote, and so ill at ease is the conventional explanation here, that _nature_, - , says of this rain that in certain conditions of weather, snow may take on an appearance of blackness that is quite deceptive. may be so. or at night, if dark enough, snow may look black. this is simply denying that a black rain fell in switzerland, jan. , . extreme remoteness from great manufacturing centers: _la nature_, , - : that aug. , , there fell at the cape of good hope, a rain so black as to be described as a "shower of ink." continuity dogs us. continuity rules us and pulls us back. we seemed to have a little hope that by the method of extremes we could get away from things that merge indistinguishably into other things. we find that every departure from one merger is entrance upon another. at the cape of good hope, vast volumes of smoke from great manufacturing centers, as an explanation, cannot very acceptably merge with the explanation of extra-mundane origin--but smoke from a terrestrial volcano can, and that is the suggestion that is made in _la nature_. there is, in human intellection, no real standard to judge by, but our acceptance, for the present, is that the more nearly positive will prevail. by the more nearly positive we mean the more nearly organized. everything merges away into everything else, but proportionately to its complexity, if unified, a thing seems strong, real, and distinct: so, in aesthetics, it is recognized that diversity in unity is higher beauty, or approximation to beauty, than is simpler unity; so the logicians feel that agreement of diverse data constitute greater convincingness, or strength, than that of mere parallel instances: so to herbert spencer the more highly differentiated and integrated is the more fully evolved. our opponents hold out for mundane origin of all black rains. our method will be the presenting of diverse phenomena in agreement with the notion of some other origin. we take up not only black rains but black rains and their accompanying phenomena. a correspondent to _knowledge_, - , writes of a black rain that fell in the clyde valley, march , : of another black rain that fell two days later. according to the correspondent, a black rain had fallen in the clyde valley, march , : then again march , . according to _nature_, - , a black rain fell at marlsford, england, sept. , ; more than twenty-four hours later another black rain fell in the same small town. the black rains of slains: according to rev. james rust (_scottish showers_): a black rain at slains, jan. , --another at carluke, miles from slains, may , --at slains, may , --slains, oct. , . but after two of these showers, vast quantities of a substance described sometimes as "pumice stone," but sometimes as "slag," were washed upon the sea coast near slains. a chemist's opinion is given that this substance was slag: that it was not a volcanic product: slag from smelting works. we now have, for black rains, a concomitant that is irreconcilable with origin from factory chimneys. whatever it may have been the quantity of this substance was so enormous that, in mr. rust's opinion, to have produced so much of it would have required the united output of all the smelting works in the world. if slag it were, we accept that an artificial product has, in enormous quantities, fallen from the sky. if you don't think that such occurrences are damned by science, read _scottish showers_ and see how impossible it was for the author to have this matter taken up by the scientific world. the first and second rains corresponded, in time, with ordinary ebullitions of vesuvius. the third and fourth, according to mr. rust, corresponded with no known volcanic activities upon this earth. _la science pour tous_, - : that, between october, , and january, , four more black rains fell at slains, scotland. the writer of this supplementary account tells us, with a better, or more unscrupulous, orthodoxy than mr. rust's, that of the eight black rains, five coincided with eruptions of vesuvius and three with eruptions of etna. the fate of all explanation is to close one door only to have another fly wide open. i should say that my own notions upon this subject will be considered irrational, but at least my gregariousness is satisfied in associating here with the preposterous--or this writer, and those who think in his rut, have to say that they can think of four discharges from one far-distant volcano, passing over a great part of europe, precipitating nowhere else, discharging precisely over one small northern parish-- but also of three other discharges, from another far-distant volcano, showing the same precise preference, if not marksmanship, for one small parish in scotland. nor would orthodoxy be any better off in thinking of exploding meteorites and their débris: preciseness and recurrence would be just as difficult to explain. my own notion is of an island near an oceanic trade-route: it might receive débris from passing vessels seven times in four years. other concomitants of black rains: in timb's _year book_, - , there is an account of "a sort of rumbling, as of wagons, heard for upward of an hour without ceasing," july , , bulwick rectory, northampton, england. on the th, a black rain fell. in _nature_, - , a correspondent writes of an intense darkness at preston, england, april , : page , another correspondent writes of black rain at crowle, near worcester, april : that a week later, or may , it had fallen again: another account of black rain, upon the th of april, near church shetton, so intense that the following day brooks were still dyed with it. according to four accounts by correspondents to _nature_ there were earthquakes in england at this time. or the black rain of canada, nov. , . this time it is orthodoxy to attribute the black precipitate to smoke of forest fires south of the ohio river-- zurcher, _meteors_, p. : that this black rain was accompanied by "shocks like those of an earthquake." _edinburgh philosophical journal_, - : that the earthquake had occurred at the climax of intense darkness and the fall of black rain. * * * * * red rains. orthodoxy: sand blown by the sirocco, from the sahara to europe. especially in the earthquake regions of europe, there have been many falls of red substance, usually, but not always, precipitated in rain. upon many occasions, these substances have been "absolutely identified" as sand from the sahara. when i first took this matter up, i came across assurance after assurance, so positive to this effect, that, had i not been an intermediatist, i'd have looked no further. samples collected from a rain at genoa--samples of sand forwarded from the sahara--"absolute agreement" some writers said: same color, same particles of quartz, even the same shells of diatoms mixed in. then the chemical analyses: not a disagreement worth mentioning. our intermediatist means of expression will be that, with proper exclusions, after the scientific or theological method, anything can be identified with anything else, if all things are only different expressions of an underlying oneness. to many minds there's rest and there's satisfaction in that expression "absolutely identified." absoluteness, or the illusion of it--the universal quest. if chemists have identified substances that have fallen in europe as sand from african deserts, swept up in african whirlwinds, that's assuasive to all the irritations that occur to those cloistered minds that must repose in the concept of a snug, isolated, little world, free from contact with cosmic wickednesses, safe from stellar guile, undisturbed by inter-planetary prowlings and invasions. the only trouble is that a chemist's analysis, which seems so final and authoritative to some minds, is no more nearly absolute than is identification by a child or description by an imbecile-- i take some of that back: i accept that the approximation is higher-- but that it's based upon delusion, because there is no definiteness, no homogeneity, no stability, only different stages somewhere between them and indefiniteness, heterogeneity, and instability. there are no chemical elements. it seems acceptable that ramsay and others have settled that. the chemical elements are only another disappointment in the quest for the positive, as the definite, the homogeneous, and the stable. if there were real elements, there could be a real science of chemistry. upon nov. and , , occurred the greatest fall of matter in the history of australia. upon the th of november, it "rained mud," in tasmania. it was of course attributed to australian whirlwinds, but, according to the _monthly weather review_, - , there was a haze all the way to the philippines, also as far as hong kong. it may be that this phenomenon had no especial relation with the even more tremendous fall of matter that occurred in europe, february, . for several days, the south of england was a dumping ground--from somewhere. if you'd like to have a chemist's opinion, even though it's only a chemist's opinion, see the report of the meeting of the royal chemical society, april , . mr. e.g. clayton read a paper upon some of the substance that had fallen from the sky, collected by him. the sahara explanation applies mostly to falls that occur in southern europe. farther away, the conventionalists are a little uneasy: for instance, the editor of the _monthly weather review_, - , says of a red rain that fell near the coast of newfoundland, early in : "it would be very remarkable if this was sahara dust." mr. clayton said that the matter examined by him was "merely wind-borne dust from the roads and lanes of wessex." this opinion is typical of all scientific opinion--or theological opinion--or feminine opinion--all very well except for what it disregards. the most charitable thing i can think of--because i think it gives us a broader tone to relieve our malices with occasional charities--is that mr. clayton had not heard of the astonishing extent of this fall--had covered the canary islands, on the th, for instance. i think, myself, that in , we passed through the remains of a powdered world--left over from an ancient inter-planetary dispute, brooding in space like a red resentment ever since. or, like every other opinion, the notion of dust from wessex turns into a provincial thing when we look it over. to think is to conceive incompletely, because all thought relates only to the local. we metaphysicians, of course, like to have the notion that we think of the unthinkable. as to opinions, or pronouncements, i should say, because they always have such an authoritative air, of other chemists, there is an analysis in _nature_, - , giving water and organic matter at . per cent. it's that carrying out of fractions that's so convincing. the substance is identified as sand from the sahara. the vastness of this fall. in _nature_, - , we are told that it had occurred in ireland, too. the sahara, of course--because, prior to february , there had been dust storms in the sahara--disregarding that in that great region there's always, in some part of it, a dust storm. however, just at present, it does look reasonable that dust had come from africa, via the canaries. the great difficulty that authoritativeness has to contend with is some other authoritativeness. when an infallibility clashes with a pontification-- they explain. _nature_, march , : another analysis-- per cent organic matter. such disagreements don't look very well, so, in _nature_, - , one of the differing chemists explains. he says that his analysis was of muddy rain, and the other was of sediment of rain-- we're quite ready to accept excuses from the most high, though i do wonder whether we're quite so damned as we were, if we find ourselves in a gracious and tolerant mood toward the powers that condemn--but the tax that now comes upon our good manners and unwillingness to be too severe-- _nature_, - : another chemist. he says it was . per cent water and organic matter. he "identifies" this matter as sand from an african desert--but after deducting organic matter-- but you and i could be "identified" as sand from an african desert, after deducting all there is to us except sand-- why we cannot accept that this fall was of sand from the sahara, omitting the obvious objection that in most parts the sahara is not red at all, but is usually described as "dazzling white"-- the enormousness of it: that a whirlwind might have carried it, but that, in that case it would be no supposititious, or doubtfully identified whirlwind, but the greatest atmospheric cataclysm in the history of this earth: _jour. roy. met. soc._, - : that, up to the th of february, this fall had continued in belgium, holland, germany and austria; that in some instances it was not sand, or that almost all the matter was organic: that a vessel had reported the fall as occurring in the atlantic ocean, midway between southampton and the barbados. the calculation is given that, in england alone, , , tons of matter had fallen. it had fallen in switzerland (_symons' met. mag._, march, ). it had fallen in russia (_bull. com. geolog._, - ). not only had a vast quantity of matter fallen several months before, in australia, but it was at this time falling in australia (_victorian naturalist_, june, )--enormously--red mud--fifty tons per square mile. the wessex explanation-- or that every explanation is a wessex explanation: by that i mean an attempt to interpret the enormous in terms of the minute--but that nothing can be finally explained, because by truth we mean the universal; and that even if we could think as wide as universality, that would not be requital to the cosmic quest--which is not for truth, but for the local that is true--not to universalize the local, but to localize the universal--or to give to a cosmic cloud absolute interpretation in terms of the little dusty roads and lanes of wessex. i cannot conceive that this can be done: i think of high approximation. our intermediatist concept is that, because of the continuity of all "things," which are not separate, positive, or real things, all pseudo-things partake of the underlying, or are only different expressions, degrees, or aspects of the underlying: so then that a sample from somewhere in anything must correspond with a sample from somewhere in anything else. that, by due care in selection, and disregard for everything else, or the scientific and theological method, the substance that fell, february, , could be identified with anything, or with some part or aspect of anything that could be conceived of-- with sand from the sahara, sand from a barrel of sugar, or dust of your great-great-grandfather. different samples are described and listed in the _journal of the royal meteorological society_, - --or we'll see whether my notion that a chemist could have identified some one of these samples as from anywhere conceivable, is extreme or not: "similar to brick dust," in one place; "buff or light brown," in another place; "chocolate-colored and silky to the touch and slightly iridescent"; "gray"; "red-rust color"; "reddish raindrops and gray sand"; "dirty gray"; "quite red"; "yellow-brown, with a tinge of pink"; "deep yellow-clay color." in _nature_, it is described as of a peculiar yellowish cast in one place, reddish somewhere else, and salmon-colored in another place. or there could be real science if there were really anything to be scientific about. or the science of chemistry is like a science of sociology, prejudiced in advance, because only to see is to see with a prejudice, setting out to "prove" that all inhabitants of new york came from africa. very easy matter. samples from one part of town. disregard for all the rest. there is no science but wessex-science. according to our acceptance, there should be no other, but that approximation should be higher: that metaphysics is super-evil: that the scientific spirit is of the cosmic quest. our notion is that, in a real existence, such a quasi-system of fables as the science of chemistry could not deceive for a moment: but that in an "existence" endeavoring to become real, it represents that endeavor, and will continue to impose its pseudo-positiveness until it be driven out by a higher approximation to realness: that the science of chemistry is as impositive as fortune-telling-- or no-- that, though it represents a higher approximation to realness than does alchemy, for instance, and so drove out alchemy, it is still only somewhere between myth and positiveness. the attempt at realness, or to state a real and unmodified fact here, is the statement: all red rains are colored by sands from the sahara desert. my own impositivist acceptances are: that some red rains are colored by sands from the sahara desert; some by sands from other terrestrial sources; some by sands from other worlds, or from their deserts--also from aerial regions too indefinite or amorphous to be thought of as "worlds" or planets-- that no supposititious whirlwind can account for the hundreds of millions of tons of matter that fell upon australia, pacific ocean and atlantic ocean and europe in and --that a whirlwind that could do that would not be supposititious. but now we shall cast off some of our own wessicality by accepting that there have been falls of red substance other than sand. we regard every science as an expression of the attempt to be real. but to be real is to localize the universal--or to make some one thing as wide as all things--successful accomplishment of which i cannot conceive of. the prime resistance to this endeavor is the refusal of the rest of the universe to be damned, excluded, disregarded, to receive christian science treatment, by something else so attempting. although all phenomena are striving for the absolute--or have surrendered to and have incorporated themselves in higher attempts, simply to be phenomenal, or to have seeming in intermediateness is to express relations. a river. it is water expressing the gravitational relation of different levels. the water of the river. expression of chemic relations of hydrogen and oxygen--which are not final. a city. manifestation of commercial and social relations. how could a mountain be without base in a greater body? storekeeper live without customers? the prime resistance to the positivist attempt by science is its relations with other phenomena, or that it only expresses those relations in the first place. or that a science can have seeming, or survive in intermediateness, as something pure, isolated, positively different, no more than could a river or a city or a mountain or a store. this intermediateness-wide attempt by parts to be wholes--which cannot be realized in our quasi-state, if we accept that in it the co-existence of two or more wholes or universals is impossible--high approximation to which, however, may be thinkable-- scientists and their dream of "pure science." artists and their dream of "art for art's sake." it is our notion that if they could almost realize, that would be almost realness: that they would instantly be translated into real existence. such thinkers are good positivists, but they are evil in an economic and sociologic sense, if, in that sense, nothing has justification for being, unless it serve, or function for, or express the relations of, some higher aggregate. so science functions for and serves society at large, and would, from society at large, receive no support, unless it did so divert itself or dissipate and prostitute itself. it seems that by prostitution i mean usefulness. there have been red rains that, in the middle ages, were called "rains of blood." such rains terrified many persons, and were so unsettling to large populations, that science, in its sociologic relations, has sought, by mrs. eddy's method, to remove an evil-- that "rains of blood" do not exist; that rains so called are only of water colored by sand from the sahara desert. my own acceptance is that such assurances, whether fictitious or not, whether the sahara is a "dazzling white" desert or not, have wrought such good effects, in a sociologic sense, even though prostitutional in the positivist sense, that, in the sociologic sense, they were well justified: but that we've gone on: that this is the twentieth century; that most of us have grown up so that such soporifics of the past are no longer necessary: that if gushes of blood should fall from the sky upon new york city, business would go on as usual. we began with rains that we accepted ourselves were, most likely, only of sand. in my own still immature hereticalness--and by heresy, or progress, i mean, very largely, a return, though with many modifications, to the superstitions of the past, i think i feel considerable aloofness to the idea of rains of blood. just at present, it is my conservative, or timid purpose, to express only that there have been red rains that very strongly suggest blood or finely divided animal matter-- débris from inter-planetary disasters. aerial battles. food-supplies from cargoes of super-vessels, wrecked in inter-planetary traffic. there was a red rain in the mediterranean region, march , . twelve days later, it fell again. whatever this substance may have been, when burned, the odor of animal matter from it was strong and persistent. (_l'astronomie_, - .) but--infinite heterogeneity--or débris from many different kinds of aerial cargoes--there have been red rains that have been colored by neither sand nor animal matter. _annals of philosophy_, - : that, nov. , --week before the black rain and earthquake of canada--there fell, at blankenberge, holland, a red rain. as to sand, two chemists of bruges concentrated ounces of the rain to ounces--"no precipitate fell." but the color was so marked that had there been sand, it would have been deposited, if the substance had been diluted instead of concentrated. experiments were made, and various reagents did cast precipitates, but other than sand. the chemists concluded that the rain-water contained muriate of cobalt--which is not very enlightening: that could be said of many substances carried in vessels upon the atlantic ocean. whatever it may have been, in the _annales de chimie_, - - , its color is said to have been red-violet. for various chemic reactions, see _quar. jour. roy. inst._, - , and _edin. phil. jour._, - . something that fell with dust said to have been meteoric, march , , , : described in the _chemical news_, - , as a "peculiar substance," consisted of red iron ocher, carbonate of lime, and organic matter. orange-red hail, march , , in tuscany. (notes and queries - - .) rain of lavender-colored substance, at oudon, france, dec. , . (_bull. soc. met. de france_, - .) _la nature_, - - : that, according to prof. schwedoff, there fell, in russia, june , , red hailstones, also blue hailstones, also gray hailstones. _nature_, - : a correspondent writes that he had been told by a resident of a small town in venezuela, that there, april , , had fallen hailstones, some red, some blue, some whitish: informant said to have been one unlikely ever to have heard of the russian phenomenon; described as an "honest, plain countryman." _nature_, july , , quotes a roman correspondent to the london _times_ who sent a translation from an italian newspaper: that a red rain had fallen in italy, june , , containing "microscopically small particles of sand." or, according to our acceptance, any other story would have been an evil thing, in the sociologic sense, in italy, in . but the english correspondent, from a land where terrifying red rains are uncommon, does not feel this necessity. he writes: "i am by no means satisfied that the rain was of sand and water." his observations are that drops of this rain left stains "such as sandy water could not leave." he notes that when the water evaporated, no sand was left behind. _l'année scientifique_, - : that, dec. , , there fell, in cochin china, a substance like blood, somewhat coagulated. _annales de chimie_, - : that a thick, viscous, red matter fell at ulm, in . we now have a datum with a factor that has been foreshadowed; which will recur and recur and recur throughout this book. it is a factor that makes for speculation so revolutionary that it will have to be reinforced many times before we can take it into full acceptance. _year book of facts_, - : quotation from a letter from prof. campini to prof. matteucci: that, upon dec. , , at about a.m., in the northwestern part of siena, a reddish rain fell copiously for two hours. a second red shower fell at o'clock. three days later, the red rain fell again. the next day another red rain fell. still more extraordinarily: each fall occurred in "exactly the same quarter of town." it is in the records of the french academy that, upon march , , in the town of châtillon-sur-seine, fell a reddish substance that was "thick, viscous, and putrid." _american journal of science_, - - : story of a highly unpleasant substance that had fallen from the sky, in wilson county, tennessee. we read that dr. troost visited the place and investigated. later we're going to investigate some investigations--but never mind that now. dr. troost reported that the substance was clear blood and portions of flesh scattered upon tobacco fields. he argued that a whirlwind might have taken an animal up from one place, mauled it around, and have precipitated its remains somewhere else. but, in volume , page , of the _journal_, there is an apology. the whole matter is, upon newspaper authority, said to have been a hoax by negroes, who had pretended to have seen the shower, for the sake of practicing upon the credulity of their masters: that they had scattered the decaying flesh of a dead hog over the tobacco fields. if we don't accept this datum, at least we see the sociologically necessary determination to have all falls accredited to earthly origins--even when they're falls that don't fall. _annual register_, - : that, upon the th of august, , something had fallen from the sky at amherst, mass. it had been examined and described by prof. graves, formerly lecturer at dartmouth college. it was an object that had upon it a nap, similar to that of milled cloth. upon removing this nap, a buff-colored, pulpy substance was found. it had an offensive odor, and, upon exposure to the air, turned to a vivid red. this thing was said to have fallen with a brilliant light. also see the _edinburgh philosophical journal_, - . in the _annales de chimie_, - , m. arago accepts the datum, and gives four instances of similar objects or substances said to have fallen from the sky, two of which we shall have with our data of gelatinous, or viscous matter, and two of which i omit, because it seems to me that the dates given are too far back. in the _american journal of science_, - - , is professor graves' account, communicated by professor dewey: that, upon the evening of august , , a light was seen in amherst--a falling object--sound as if of an explosion. in the home of prof. dewey, this light was reflected upon a wall of a room in which were several members of prof. dewey's family. the next morning, in prof. dewey's front yard, in what is said to have been the only position from which the light that had been seen in the room, the night before, could have been reflected, was found a substance "unlike anything before observed by anyone who saw it." it was a bowl-shaped object, about inches in diameter, and one inch thick. bright buff-colored, and having upon it a "fine nap." upon removing this covering, a buff-colored, pulpy substance of the consistency of soft-soap, was found--"of an offensive, suffocating smell." a few minutes of exposure to the air changed the buff color to "a livid color resembling venous blood." it absorbed moisture quickly from the air and liquefied. for some of the chemic reactions, see the _journal_. there's another lost quasi-soul of a datum that seems to me to belong here: london _times_, april , : fall of fish that had occurred in the neighborhood of allahabad, india. it is said that the fish were of the chalwa species, about a span in length and a seer in weight--you know. they were dead and dry. or they had been such a long time out of water that we can't accept that they had been scooped out of a pond, by a whirlwind--even though they were so definitely identified as of a known local species-- or they were not fish at all. i incline, myself, to the acceptance that they were not fish, but slender, fish-shaped objects of the same substance as that which fell at amherst--it is said that, whatever they were, they could not be eaten: that "in the pan, they turned to blood." for details of this story see the _journal of the asiatic society of bengal_, - . may or , , is the date given in the _journal_. in the _american journal of science_, - - , occurs the inevitable damnation of the amherst object: prof. edward hitchcock went to live in amherst. he says that years later, another object, like the one said to have fallen in , had been found at "nearly the same place." prof. hitchcock was invited by prof. graves to examine it. exactly like the first one. corresponded in size and color and consistency. the chemic reactions were the same. prof. hitchcock recognized it in a moment. it was a gelatinous fungus. he did not satisfy himself as to just the exact species it belonged to, but he predicted that similar fungi might spring up within twenty-four hours-- but, before evening, two others sprang up. or we've arrived at one of the oldest of the exclusionists' conventions--or nostoc. we shall have many data of gelatinous substance said to have fallen from the sky: almost always the exclusionists argue that it was only nostoc, an alga, or, in some respects, a fungous growth. the rival convention is "spawn of frogs or of fishes." these two conventions have made a strong combination. in instances where testimony was not convincing that gelatinous matter had been seen to fall, it was said that the gelatinous substance was nostoc, and had been upon the ground in the first place: when the testimony was too good that it had fallen, it was said to be spawn that had been carried from one place to another in a whirlwind. now, i can't say that nostoc is always greenish, any more than i can say that blackbirds are always black, having seen a white one: we shall quote a scientist who knew of flesh-colored nostoc, when so to know was convenient. when we come to reported falls of gelatinous substances, i'd like it to be noticed how often they are described as whitish or grayish. in looking up the subject, myself, i have read only of greenish nostoc. said to be greenish, in webster's dictionary--said to be "blue-green" in the new international encyclopedia--"from bright green to olive-green" (_science gossip_, - ); "green" (_science gossip_, - ); "greenish" (_notes and queries_, - - ). it would seem acceptable that, if many reports of white birds should occur, the birds are not blackbirds, even though there have been white blackbirds. or that, if often reported, grayish or whitish gelatinous substance is not nostoc, and is not spawn if occurring in times unseasonable for spawn. "the kentucky phenomenon." so it was called, in its day, and now we have an occurrence that attracted a great deal of attention in its own time. usually these things of the accursed have been hushed up or disregarded--suppressed like the seven black rains of slains--but, upon march , , something occurred, in bath county, kentucky, that brought many newspaper correspondents to the scene. the substance that looked like beef that fell from the sky. upon march , , at olympian springs, bath county, kentucky, flakes of a substance that looked like beef fell from the sky--"from a clear sky." we'd like to emphasize that it was said that nothing but this falling substance was visible in the sky. it fell in flakes of various sizes; some two inches square, one, three or four inches square. the flake-formation is interesting: later we shall think of it as signifying pressure--somewhere. it was a thick shower, on the ground, on trees, on fences, but it was narrowly localized: or upon a strip of land about yards long and about yards wide. for the first account, see the _scientific american_, - , and the _new york times_, march , . then the exclusionists. something that looked like beef: one flake of it the size of a square envelope. if we think of how hard the exclusionists have fought to reject the coming of ordinary-looking dust from this earth's externality, we can sympathize with them in this sensational instance, perhaps. newspaper correspondents wrote broadcast and witnesses were quoted, and this time there is no mention of a hoax, and, except by one scientist, there is no denial that the fall did take place. it seems to me that the exclusionists are still more emphatically conservators. it is not so much that they are inimical to all data of externally derived substances that fall upon this earth, as that they are inimical to all data discordant with a system that does not include such phenomena-- or the spirit or hope or ambition of the cosmos, which we call attempted positivism: not to find out the new; not to add to what is called knowledge, but to systematize. _scientific american supplement_, - : that the substance reported from kentucky had been examined by leopold brandeis. "at last we have a proper explanation of this much talked of phenomenon." "it has been comparatively easy to identify the substance and to fix its status. the kentucky 'wonder' is no more or less than nostoc." or that it had not fallen; that it had been upon the ground in the first place, and had swollen in rain, and, attracting attention by greatly increased volume, had been supposed by unscientific observers to have fallen in rain-- what rain, i don't know. also it is spoken of as "dried" several times. that's one of the most important of the details. but the relief of outraged propriety, expressed in the _supplement_, is amusing to some of us, who, i fear, may be a little improper at times. very spirit of the salvation army, when some third-rate scientist comes out with an explanation of the vermiform appendix or the os coccygis that would have been acceptable to moses. to give completeness to "the proper explanation," it is said that mr. brandeis had identified the substance as "flesh-colored" nostoc. prof. lawrence smith, of kentucky, one of the most resolute of the exclusionists: _new york times_, march , : that the substance had been examined and analyzed by prof. smith, according to whom it gave every indication of being the "dried" spawn of some reptile, "doubtless of the frog"--or up from one place and down in another. as to "dried," that may refer to condition when prof. smith received it. in the _scientific american supplement_, - , dr. a. mead edwards, president of the newark scientific association, writes that, when he saw mr. brandeis' communication, his feeling was of conviction that propriety had been re-established, or that the problem had been solved, as he expresses it: knowing mr. brandeis well, he had called upon that upholder of respectability, to see the substance that had been identified as nostoc. but he had also called upon dr. hamilton, who had a specimen, and dr. hamilton had declared it to be lung-tissue. dr. edwards writes of the substance that had so completely, or beautifully--if beauty is completeness--been identified as nostoc--"it turned out to be lung-tissue also." he wrote to other persons who had specimens, and identified other specimens as masses of cartilage or muscular fibers. "as to whence it came, i have no theory." nevertheless he endorses the local explanation--and a bizarre thing it is: a flock of gorged, heavy-weighted buzzards, but far up and invisible in the clear sky-- they had disgorged. prof. fassig lists the substance, in his "bibliography," as fish spawn. mcatee (_monthly weather review_, may, ) lists it as a jelly-like material, supposed to have been the "dried" spawn either of fishes or of some batrachian. or this is why, against the seemingly insuperable odds against all things new, there can be what is called progress-- that nothing is positive, in the aspects of homogeneity and unity: if the whole world should seem to combine against you, it is only unreal combination, or intermediateness to unity and disunity. every resistance is itself divided into parts resisting one another. the simplest strategy seems to be--never bother to fight a thing: set its own parts fighting one another. we are merging away from carnal to gelatinous substance, and here there is an abundance of instances or reports of instances. these data are so improper they're obscene to the science of today, but we shall see that science, before it became so rigorous, was not so prudish. chladni was not, and greg was not. i shall have to accept, myself, that gelatinous substance has often fallen from the sky-- or that, far up, or far away, the whole sky is gelatinous? that meteors tear through and detach fragments? that fragments are brought down by storms? that the twinkling of stars is penetration of light through something that quivers? i think, myself, that it would be absurd to say that the whole sky is gelatinous: it seems more acceptable that only certain areas are. humboldt (_cosmos_, - ) says that all our data in this respect must be "classed amongst the mythical fables of mythology." he is very sure, but just a little redundant. we shall be opposed by the standard resistances: there in the first place; up from one place, in a whirlwind, and down in another. we shall not bother to be very convincing one way or another, because of the over-shadowing of the datum with which we shall end up. it will mean that something had been in a stationary position for several days over a small part of a small town in england: this is the revolutionary thing that we have alluded to before; whether the substance were nostoc, or spawn, or some kind of a larval nexus, doesn't matter so much. if it stood in the sky for several days, we rank with moses as a chronicler of improprieties--or was that story, or datum, we mean, told by moses? then we shall have so many records of gelatinous substance said to have fallen with meteorites, that, between the two phenomena, some of us will have to accept connection--or that there are at least vast gelatinous areas aloft, and that meteorites tear through, carrying down some of the substance. _comptes rendus_, - : that, in , m. vallot, member of the french academy, placed before the academy some fragments of a gelatinous substance, said to have fallen from the sky, and asked that they be analyzed. there is no further allusion to this subject. _comptes rendus_, - : that, in wilna, lithuania, april , , in a rainstorm, fell nut-sized masses of a substance that is described as both resinous and gelatinous. it was odorless until burned: then it spread a very pronounced sweetish odor. it is described as like gelatine, but much firmer: but, having been in water hours, it swelled out, and looked altogether gelatinous-- it was grayish. we are told that, in and , a similar substance had fallen in asia minor. in _notes and queries_, - - , it is said that, early in august, , thousands of jellyfish, about the size of a shilling, had fallen at bath, england. i think it is not acceptable that they were jellyfish: but it does look as if this time frog spawn did fall from the sky, and may have been translated by a whirlwind--because, at the same time, small frogs fell at wigan, england. _nature_, - : that, june , , at eton, bucks, england, the ground was found covered with masses of jelly, the size of peas, after a heavy rainfall. we are not told of nostoc, this time: it is said that the object contained numerous eggs of "some species of chironomus, from which larvae soon emerged." i incline, then, to think that the objects that fell at bath were neither jellyfish nor masses of frog spawn, but something of a larval kind-- this is what had occurred at bath, england, years before. london _times_, april , : that, upon the nd of april, , a storm of glutinous drops neither jellyfish nor masses of frog spawn, but something of a [line missing here in original text. ed.] railroad station, at bath. "many soon developed into a worm-like chrysalis, about an inch in length." the account of this occurrence in the _zoologist_, - - , is more like the eton-datum: of minute forms, said to have been infusoria; not forms about an inch in length. _trans. ent. soc. of london_, -proc. xxii: that the phenomenon has been investigated by the rev. l. jenyns, of bath. his description is of minute worms in filmy envelopes. he tries to account for their segregation. the mystery of it is: what could have brought so many of them together? many other falls we shall have record of, and in most of them segregation is the great mystery. a whirlwind seems anything but a segregative force. segregation of things that have fallen from the sky has been avoided as most deep-dyed of the damned. mr. jenyns conceives of a large pool, in which were many of these spherical masses: of the pool drying up and concentrating all in a small area; of a whirlwind then scooping all up together-- but several days later, more of these objects fell in the same place. that such marksmanship is not attributable to whirlwinds seems to me to be what we think we mean by common sense: it may not look like common sense to say that these things had been stationary over the town of bath, several days-- the seven black rains of slains; the four red rains of siena. an interesting sidelight on the mechanics of orthodoxy is that mr. jenyns dutifully records the second fall, but ignores it in his explanation. r.p. greg, one of the most notable of cataloguers of meteoritic phenomena, records (_phil. mag._: - - ) falls of viscid substance in the years , , , , , , . he gives earlier dates, but i practice exclusions, myself. in the _report of the british association_, - , greg records a meteor that seemed to pass near the ground, between barsdorf and freiburg, germany: the next day a jelly-like mass was found in the snow-- unseasonableness for either spawn or nostoc. greg's comment in this instance is: "curious if true." but he records without modification the fall of a meteorite at gotha, germany, sept. , , "leaving a jelly-like mass on the ground." we are told that this substance fell only three feet away from an observer. in the _report of the british association_, - , according to a letter from greg to prof. baden-powell, at night, oct. , , near coblenz, a german, who was known to greg, and another person saw a luminous body fall close to them. they returned next morning and found a gelatinous mass of grayish color. according to chladni's account (_annals of philosophy_, n.s., - ) a viscous mass fell with a luminous meteorite between siena and rome, may, ; viscous matter found after the fall of a fire ball, in lusatia, march, ; fall of a gelatinous substance, after the explosion of a meteorite, near heidelberg, july, . in the _edinburgh philosophical journal_, - , the substance that fell at lusatia is said to have been of the "color and odor of dried, brown varnish." in the _amer. jour. sci._, - - , it is said that gelatinous matter fell with a globe of fire, upon the island of lethy, india, . in the _amer. jour. sci._, - - , in many observations upon the meteors of november, , are reports of falls of gelatinous substance: that, according to newspaper reports, "lumps of jelly" were found on the ground at rahway, n.j. the substance was whitish, or resembled the coagulated white of an egg: that mr. h.h. garland, of nelson county, virginia, had found a jelly-like substance of about the circumference of a twenty-five-cent piece: that, according to a communication from a.c. twining to prof. olmstead, a woman at west point, n.y., had seen a mass the size of a teacup. it looked like boiled starch: that, according to a newspaper, of newark, n.j., a mass of gelatinous substance, like soft soap, had been found. "it possessed little elasticity, and, on the application of heat, it evaporated as readily as water." it seems incredible that a scientist would have such hardihood, or infidelity, as to accept that these things had fallen from the sky: nevertheless, prof. olmstead, who collected these lost souls, says: "the fact that the supposed deposits were so uniformly described as gelatinous substance forms a presumption in favor of the supposition that they had the origin ascribed to them." in contemporaneous scientific publications considerable attention was given to prof. olmstead's series of papers upon this subject of the november meteors. you will not find one mention of the part that treats of gelatinous matter. i shall attempt not much of correlation of dates. a mathematic-minded positivist, with his delusion that in an intermediate state twice two are four, whereas, if we accept continuity, we cannot accept that there are anywhere two things to start with, would search our data for periodicities. it is so obvious to me that the mathematic, or the regular, is the attribute of the universal, that i have not much inclination to look for it in the local. still, in this solar system, "as a whole," there is considerable approximation to regularity; or the mathematic is so nearly localized that eclipses, for instance, can, with rather high approximation, be foretold, though i have notes that would deflate a little the astronomers' vainglory in this respect--or would if that were possible. an astronomer is poorly paid, uncheered by crowds, considerably isolated: he lives upon his own inflations: deflate a bear and it couldn't hibernate. this solar system is like every other phenomenon that can be regarded "as a whole"--or the affairs of a ward are interfered with by the affairs of the city of which it is a part; city by county; county by state; state by nation; nation by other nations; all nations by climatic conditions; climatic conditions by solar circumstances; sun by general planetary circumstances; solar system "as a whole" by other solar systems--so the hopelessness of finding the phenomena of entirety in the ward of a city. but positivists are those who try to find the unrelated in the ward of a city. in our acceptance this is the spirit of cosmic religion. objectively the state is not realizable in the ward of a city. but, if a positivist could bring himself to absolute belief that he had found it, that would be a subjective realization of that which is unrealizable objectively. of course we do not draw a positive line between the objective and the subjective--or that all phenomena called things or persons are subjective within one all-inclusive nexus, and that thoughts within those that are commonly called "persons" are sub-subjective. it is rather as if intermediateness strove for regularity in this solar system and failed: then generated the mentality of astronomers, and, in that secondary expression, strove for conviction that failure had been success. i have tabulated all the data of this book, and a great deal besides--card system--and several proximities, thus emphasized, have been revelations to me: nevertheless, it is only the method of theologians and scientists--worst of all, of statisticians. for instance, by the statistic method, i could "prove" that a black rain has fallen "regularly" every seven months, somewhere upon this earth. to do this, i'd have to include red rains and yellow rains, but, conventionally, i'd pick out the black particles in red substances and in yellow substances, and disregard the rest. then, too, if here and there a black rain should be a week early or a month late--that would be "acceleration" or "retardation." this is supposed to be legitimate in working out the periodicities of comets. if black rains, or red or yellow rains with black particles in them, should not appear at all near some dates--we have not read darwin in vain--"the records are not complete." as to other, interfering black rains, they'd be either gray or brown, or for them we'd find other periodicities. still, i have had to notice the year , for instance. i shall not note them all in this book, but i have records of extraordinary events in . someone should write a book upon the phenomena of this one year--that is, if books should be written. is notable for extraordinary falls, so far apart that a local explanation seems inadequate--not only the black rain of ireland, may, , but a red rain in sicily and a red rain in wales. also, it is said (timb's _year book_, - ) that, upon april or , , shepherds near mt. ararat, found a substance that was not indigenous, upon areas measuring to miles in circumference. presumably it had fallen there. we have already gone into the subject of science and its attempted positiveness, and its resistances in that it must have relations of service. it is very easy to see that most of the theoretic science of the th century was only a relation of reaction against theologic dogma, and has no more to do with truth than has a wave that bounds back from a shore. or, if a shop girl, or you or i, should pull out a piece of chewing gum about a yard long, that would be quite as scientific a performance as was the stretching of this earth's age several hundred millions of years. all "things" are not things, but only relations, or expressions of relations: but all relations are striving to be the unrelated, or have surrendered to, and subordinated to, higher attempts. so there is a positivist aspect to this reaction that is itself only a relation, and that is the attempt to assimilate all phenomena under the materialist explanation, or to formulate a final, all-inclusive system, upon the materialist basis. if this attempt could be realized, that would be the attaining of realness; but this attempt can be made only by disregarding psychic phenomena, for instance--or, if science shall eventually give in to the psychic, it would be no more legitimate to explain the immaterial in terms of the material than to explain the material in terms of the immaterial. our own acceptance is that material and immaterial are of a oneness, merging, for instance, in a thought that is continuous with a physical action: that oneness cannot be explained, because the process of explaining is the interpreting of something in terms of something else. all explanation is assimilation of something in terms of something else that has been taken as a basis: but, in continuity, there is nothing that is any more basic than anything else--unless we think that delusion built upon delusion is less real than its pseudo-foundation. in (timb's _year book_, - ) in persia fell a substance that the people said they had never seen before. as to what it was, they had not a notion, but they saw that the sheep ate it. they ground it into flour and made bread, said to have been passable enough, though insipid. that was a chance that science did not neglect. manna was placed upon a reasonable basis, or was assimilated and reconciled with the system that had ousted the older--and less nearly real--system. it was said that, likely enough, manna had fallen in ancient times--because it was still falling--but that there was no tutelary influence behind it--that it was a lichen from the steppes of asia minor--from one place in a whirlwind and down in another place. "in the _american almanac_, - , it is said that this substance--to the inhabitants of the region"--was "immediately recognized" by scientists who examined it: and that "the chemical analysis also identified it as a lichen." this was back in the days when chemical analysis was a god. since then his devotees have been shocked and disillusioned. just how a chemical analysis could so botanize, i don't know--but it was chemical analysis who spoke, and spoke dogmatically. it seems to me that the ignorance of inhabitants, contrasting with the local knowledge of foreign scientists, is overdone: if there's anything good to eat, within any distance conveniently covered by a whirlwind--inhabitants know it. i have data of other falls, in persia and asiatic turkey, of edible substances. they are all dogmatically said to be "manna"; and "manna" is dogmatically said to be a species of lichens from the steppes of asia minor. the position that i take is that this explanation was evolved in ignorance of the fall of vegetable substances, or edible substances, in other parts of the world: that it is the familiar attempt to explain the general in terms of the local; that, if we shall have data of falls of vegetable substance, in, say, canada or india, they were not of lichens from the steppes of asia minor; that, though all falls in asiatic turkey and persia are sweepingly and conveniently called showers of "manna," they have not been even all of the same substance. in one instance the particles are said to have been "seeds." though, in _comptes rendus_, the substance that fell in and is said to have been gelatinous, in the _bull. sci. nat. de neuchatel_, it is said to have been of something, in lumps the size of a filbert, that had been ground into flour; that of this flour had been made bread, very attractive-looking, but flavorless. the great difficulty is to explain segregation in these showers-- but deep-sea fishes and occasional falls, down to them, of edible substances; bags of grain, barrels of sugar; things that had not been whirled up from one part of the ocean-bottom, in storms or submarine disturbances, and dropped somewhere else-- i suppose one thinks--but grain in bags never has fallen-- object of amherst--its covering like "milled cloth"-- or barrels of corn lost from a vessel would not sink--but a host of them clashing together, after a wreck--they burst open; the corn sinks, or does when saturated; the barrel staves float longer-- if there be not an overhead traffic in commodities similar to our own commodities carried over this earth's oceans--i'm not the deep-sea fish i think i am. i have no data other than the mere suggestion of the amherst object of bags or barrels, but my notion is that bags and barrels from a wreck on one of this earth's oceans, would, by the time they reached the bottom, no longer be recognizable as bags or barrels; that, if we can have data of the fall of fibrous material that may have been cloth or paper or wood, we shall be satisfactory and grotesque enough. _proc. roy. irish acad._, - : "in the year , some workmen, who had been fetching water from a pond, seven german miles from memel, on returning to their work after dinner (during which there had been a snowstorm) found the flat ground around the pond covered with a coal-black, leafy mass; and a person who lived near said he had seen it fall like flakes with the snow." some of these flake-like formations were as large as a table-top. "the mass was damp and smelt disagreeably, like rotten seaweed, but, when dried, the smell went off." "it tore fibrously, like paper." classic explanation: "up from one place, and down in another." but what went up, from one place, in a whirlwind? of course, our intermediatist acceptance is that had this been the strangest substance conceivable, from the strangest other world that could be thought of; somewhere upon this earth there must be a substance similar to it, or from which it would, at least subjectively, or according to description, not be easily distinguishable. or that everything in new york city is only another degree or aspect of something, or combination of things, in a village of central africa. the novel is a challenge to vulgarization: write something that looks new to you: someone will point out that the thrice-accursed greeks said it long ago. existence is appetite: the gnaw of being; the one attempt of all things to assimilate all other things, if they have not surrendered and submitted to some higher attempt. it was cosmic that these scientists, who had surrendered to and submitted to the scientific system, should, consistently with the principles of that system, attempt to assimilate the substance that fell at memel with some known terrestrial product. at the meeting of the royal irish academy it was brought out that there is a substance, of rather rare occurrence, that has been known to form in thin sheets upon marsh land. it looks like greenish felt. the substance of memel: damp, coal-black, leafy mass. but, if broken up, the marsh-substance is flake-like, and it tears fibrously. an elephant can be identified as a sunflower--both have long stems. a camel is indistinguishable from a peanut--if only their humps be considered. trouble with this book is that we'll end up a lot of intellectual roués: we'll be incapable of being astonished with anything. we knew, to start with, that science and imbecility are continuous; nevertheless so many expressions of the merging-point are at first startling. we did think that prof. hitchcock's performance in identifying the amherst phenomenon as a fungus was rather notable as scientific vaudeville, if we acquit him of the charge of seriousness--or that, in a place where fungi were so common that, before a given evening two of them sprang up, only he, a stranger in this very fungiferous place, knew a fungus when he saw something like a fungus--if we disregard its quick liquefaction, for instance. it was only a monologue, however: now we have an all-star cast: and they're not only irish; they're royal irish. the royal irishmen excluded "coal-blackness" and included fibrousness: so then that this substance was "marsh paper," which "had been raised into the air by storms of wind, and had again fallen." second act: it was said that, according to m. ehrenberg, "the meteor-paper was found to consist partly of vegetable matter, chiefly of conifervæ." third act: meeting of the royal irishmen: chairs, tables, irishmen: some flakes of marsh-paper were exhibited. their composition was chiefly of conifervæ. this was a double inclusion: or it's the method of agreement that logicians make so much of. so no logician would be satisfied with identifying a peanut as a camel, because both have humps: he demands accessory agreement--that both can live a long time without water, for instance. now, it's not so very unreasonable, at least to the free and easy vaudeville standards that, throughout this book, we are considering, to think that a green substance could be snatched up from one place in a whirlwind, and fall as a black substance somewhere else: but the royal irishmen excluded something else, and it is a datum that was as accessible to them as it is to me: that, according to chladni, this was no little, local deposition that was seen to occur by some indefinite person living near a pond somewhere. it was a tremendous fall from a vast sky-area. likely enough all the marsh paper in the world could not have supplied it. at the same time, this substance was falling "in great quantities," in norway and pomerania. or see kirkwood, _meteoric astronomy_, p. : "substance like charred paper fell in norway and other parts of northern europe, jan. , ." or a whirlwind, with a distribution as wide as that, would not acceptably, i should say, have so specialized in the rare substance called "marsh paper." there'd have been falls of fence rails, roofs of houses, parts of trees. nothing is said of the occurrence of a tornado in northern europe, in january, . there is record only of this one substance having fallen in various places. time went on, but the conventional determination to exclude data of all falls to this earth, except of substances of this earth, and of ordinary meteoric matter, strengthened. _annals of philosophy_, - : the substance that fell in january, , is described as "a mass of black leaves, having the appearance of burnt paper, but harder, and cohering, and brittle." "marsh paper" is not mentioned, and there is nothing said of the "conifervæ," which seemed so convincing to the royal irishmen. vegetable composition is disregarded, quite as it might be by someone who might find it convenient to identify a crook-necked squash as a big fishhook. meteorites are usually covered with a black crust, more or less scale-like. the substance of is black and scale-like. if so be convenience, "leaf-likeness" is "scale-likeness." in this attempt to assimilate with the conventional, we are told that the substance is a mineral mass: that it is like the black scales that cover meteorites. the scientist who made this "identification" was von grotthus. he had appealed to the god chemical analysis. or the power and glory of mankind--with which we're not always so impressed--but the gods must tell us what we want them to tell us. we see again that, though nothing has identity of its own, anything can be "identified" as anything. or there's nothing that's not reasonable, if one snoopeth not into its exclusions. but here the conflict did not end. berzelius examined the substance. he could not find nickel in it. at that time, the presence of nickel was the "positive" test of meteoritic matter. whereupon, with a supposititious "positive" standard of judgment against him, von grotthus revoked his "identification." (_annals and mag. of nat. hist._, - - .) this equalization of eminences permits us to project with our own expression, which, otherwise, would be subdued into invisibility: that it's too bad that no one ever looked to see--hieroglyphics?--something written upon these sheets of paper? if we have no very great variety of substances that have fallen to this earth; if, upon this earth's surface there is infinite variety of substances detachable by whirlwinds, two falls of such a rare substance as marsh paper would be remarkable. a writer in the _edinburgh review_, - , says that, at the time of writing, he had before him a portion of a sheet of square feet, of a substance that had fallen at carolath, silesia, in --exactly similar to cotton-felt, of which clothing might have been made. the god microscopic examination had spoken. the substance consisted chiefly of conifervæ. _jour. asiatic soc. of bengal_, -pt. - : that march , --about the time of a fall of edible substance in asia minor--an olive-gray powder fell at shanghai. under the microscope, it was seen to be an aggregation of hairs of two kinds, black ones and rather thick white ones. they were supposed to be mineral fibers, but, when burned, they gave out "the common ammoniacal smell and smoke of burnt hair or feathers." the writer described the phenomenon as "a cloud of square miles of fibers, alkali, and sand." in a postscript, he says that other investigators, with more powerful microscopes, gave opinion that the fibers were not hairs; that the substance consisted chiefly of conifervæ. or the pathos of it, perhaps; or the dull and uninspired, but courageous persistence of the scientific: everything seemingly found out is doomed to be subverted--by more powerful microscopes and telescopes; by more refined, precise, searching means and methods--the new pronouncements irrepressibly bobbing up; their reception always as truth at last; always the illusion of the final; very little of the intermediatist spirit-- that the new that has displaced the old will itself some day be displaced; that it, too, will be recognized as myth-stuff-- but that if phantoms climb, spooks of ladders are good enough for them. _annual register_, - : that, according to a report by m. lainé, french consul at pernambuco, early in october, , there was a shower of a substance resembling silk. the quantity was as tremendous as might be a whole cargo, lost somewhere between jupiter and mars, having drifted around perhaps for centuries, the original fabrics slowly disintegrating. in _annales de chimie_, - - , it is said that samples of this substance were sent to france by m. lainé, and that they proved to have some resemblances to silky filaments which, at certain times of the year, are carried by the wind near paris. in the _annals of philosophy_, n.s., - , there is mention of a fibrous substance like blue silk that fell near naumberg, march , . according to chladni (_annales de chimie_, - - ), the quantity was great. he places a question mark before the date. one of the advantages of intermediatism is that, in the oneness of quasiness, there can be no mixed metaphors. whatever is acceptable of anything, is, in some degree or aspect, acceptable of everything. so it is quite proper to speak, for instance, of something that is as firm as a rock and that sails in a majestic march. the irish are good monists: they have of course been laughed at for their keener perceptions. so it's a book we're writing, or it's a procession, or it's a museum, with the chamber of horrors rather over-emphasized. a rather horrible correlation occurs in the _scientific american_, - . what interests us is that a correspondent saw a silky substance fall from the sky--there was an aurora borealis at the time--he attributes the substance to the aurora. since the time of darwin, the classic explanation has been that all silky substances that fall from the sky are spider webs. in , aboard the _beagle_, at the mouth of la plata river, miles from land, darwin saw an enormous number of spiders, of the kind usually known as "gossamer" spiders, little aeronauts that cast out filaments by which the wind carries them. it's difficult to express that silky substances that have fallen to this earth were not spider webs. my own acceptance is that spider webs are the merger; that there have been falls of an externally derived silky substance, and also of the webs, or strands, rather, of aeronautic spiders indigenous to this earth; that in some instances it is impossible to distinguish one from the other. of course, our expression upon silky substances will merge away into expressions upon other seeming textile substances, and i don't know how much better off we'll be-- except that, if fabricable materials have fallen from the sky-- simply to establish acceptance of that may be doing well enough in this book of first and tentative explorations. in _all the year round_, - , is described a fall that took place in england, sept. , , in the towns of bradly, selborne, and alresford, and in a triangular space included by these three towns. the substance is described as "cobwebs"--but it fell in flake-formation, or in "flakes or rags about one inch broad and five or six inches long." also these flakes were of a relatively heavy substance--"they fell with some velocity." the quantity was great--the shortest side of the triangular space is eight miles long. in the _wernerian nat. hist. soc. trans._, - , it is said that there were two falls--that they were some hours apart--a datum that is becoming familiar to us--a datum that cannot be taken into the fold, unless we find it repeated over and over and over again. it is said that the second fall lasted from nine o'clock in the morning until night. now the hypnosis of the classic--that what we call intelligence is only an expression of inequilibrium; that when mental adjustments are made, intelligence ceases--or, of course, that intelligence is the confession of ignorance. if you have intelligence upon any subject, that is something you're still learning--if we agree that that which is learned is always mechanically done--in quasi-terms, of course, because nothing is ever finally learned. it was decided that this substance was spiders' web. that was adjustment. but it's not adjustment to me; so i'm afraid i shall have some intelligence in this matter. if i ever arrive at adjustment upon this subject, then, upon this subject, i shall be able to have no thoughts, except routine-thoughts. i haven't yet quite decided absolutely everything, so i am able to point out: that this substance was of quantity so enormous that it attracted wide attention when it came down-- that it would have been equally noteworthy when it went up-- that there is no record of anyone, in england or elsewhere, having seen tons of "spider webs" going up, september, . further confession of intelligence upon my part: that, if it be contested, then, that the place of origin may have been far away, but still terrestrial-- then it's that other familiar matter of incredible "marksmanship" again--hitting a small, triangular space for hours--interval of hours--then from nine in the morning until night: same small triangular space. these are the disregards of the classic explanation. there is no mention of spiders having been seen to fall, but a good inclusion is that, though this substance fell in good-sized flakes of considerable weight, it was viscous. in this respect it was like cobwebs: dogs nosing it on grass, were blindfolded with it. this circumstance does strongly suggest cobwebs-- unless we can accept that, in regions aloft, there are vast viscous or gelatinous areas, and that things passing through become daubed. or perhaps we clear up the confusion in the descriptions of the substance that fell in and , in asia minor, described in one publication as gelatinous, and in another as a cereal--that it was a cereal that had passed through a gelatinous region. that the paper-like substance of memel may have had such an experience may be indicated in that ehrenberg found in it gelatinous matter, which he called "nostoc." (_annals and mag. of nat. hist._, - - .) _scientific american_, - : fall of a substance described as "cobwebs," latter part of october, , in milwaukee, wis., and other towns: other towns mentioned are green bay, vesburge, fort howard, sheboygan, and ozaukee. the aeronautic spiders are known as "gossamer" spiders, because of the extreme lightness of the filaments that they cast out to the wind. of the substance that fell in wisconsin, it is said: "in all instances the webs were strong in texture and very white." the editor says: "curiously enough, there is no mention in any of the reports that we have seen, of the presence of spiders." so our attempt to divorce a possible external product from its terrestrial merger: then our joy of the prospector who thinks he's found something: the _monthly weather review_, - , quotes the _montgomery_ (ala.) _advertiser_: that, upon nov. , , numerous batches of spider-web-like substance fell in montgomery, in strands and in occasional masses several inches long and several inches broad. according to the writer, it was not spiders' web, but something like asbestos; also that it was phosphorescent. the editor of the _review_ says that he sees no reason for doubting that these masses were cobwebs. _la nature_, - : a correspondent writes that he sends a sample of a substance said to have fallen at montussan (gironde), oct. , . according to a witness, quoted by the correspondent, a thick cloud, accompanied by rain and a violent wind, had appeared. this cloud was composed of a woolly substance in lumps the size of a fist, which fell to the ground. the editor (tissandier) says of this substance that it was white, but was something that had been burned. it was fibrous. m. tissandier astonishes us by saying that he cannot identify this substance. we thought that anything could be "identified" as anything. he can say only that the cloud in question must have been an extraordinary conglomeration. _annual register, - :_ that, march, , there fell, in the fields of kourianof, russia, a combustible yellowish substance, covering, at least two inches thick, an area of or square feet. it was resinous and yellowish: so one inclines to the conventional explanation that it was pollen from pine trees--but, when torn, it had the tenacity of cotton. when placed in water, it had the consistency of resin. "this resin had the color of amber, was elastic, like india rubber, and smelled like prepared oil mixed with wax." so in general our notion of cargoes--and our notion of cargoes of food supplies: in _philosophical transactions_, - , is an extract from a letter by mr. robert vans, of kilkenny, ireland, dated nov. , : that there had been "of late," in the counties of limerick and tipperary, showers of a sort of matter like butter or grease... having "a very stinking smell." there follows an extract from a letter by the bishop of cloyne, upon "a very odd phenomenon," which was observed in munster and leinster: that for a good part of the spring of there fell a substance which the country people called "butter"--"soft, clammy, and of a dark yellow"--that cattle fed "indifferently" in fields where this substance lay. "it fell in lumps as big as the end of one's finger." it had a "strong ill scent." his grace calls it a "stinking dew." in mr. vans' letter, it is said that the "butter" was supposed to have medicinal properties, and "was gathered in pots and other vessels by some of the inhabitants of this place." and: in all the following volumes of _philosophical transactions_ there is no speculation upon this extraordinary subject. ostracism. the fate of this datum is a good instance of damnation, not by denial, and not by explaining away, but by simple disregard. the fall is listed by chladni, and is mentioned in other catalogues, but, from the absence of all inquiry, and of all but formal mention, we see that it has been under excommunication as much as was ever anything by the preceding system. the datum has been buried alive. it is as irreconcilable with the modern system of dogmas as ever were geologic strata and vermiform appendix with the preceding system-- if, intermittently, or "for a good part of the spring," this substance fell in two irish provinces, and nowhere else, we have, stronger than before, a sense of a stationary region overhead, or a region that receives products like this earth's products, but from external sources, a region in which this earth's gravitational and meteorological forces are relatively inert--if for many weeks a good part of this substance did hover before finally falling. we suppose that, in , mr. vans and the bishop of cloyne could describe what they saw as well as could witnesses in : nevertheless, it is going far back; we shall have to have many modern instances before we can accept. as to other falls, or another fall, it is said in the _amer. jour. sci._, - - , that, april , --about a month after the fall of the substance of kourianof--fell a substance that was wine-yellow, transparent, soft, and smelling like rancid oil. m. herman, a chemist who examined it, named it "sky oil." for analysis and chemic reactions, see the _journal_. the _edinburgh new philosophical journal_, - , mentions an "unctuous" substance that fell near rotterdam, in . in _comptes rendus_, - , there is an account of an oily, reddish matter that fell at genoa, february, . whatever it may have been-- altogether, most of our difficulties are problems that we should leave to later developers of super-geography, i think. a discoverer of america should leave long island to someone else. if there be, plying back and forth from jupiter and mars and venus, super-constructions that are sometimes wrecked, we think of fuel as well as cargoes. of course the most convincing data would be of coal falling from the sky: nevertheless, one does suspect that oil-burning engines were discovered ages ago in more advanced worlds--but, as i say, we should leave something to our disciples--so we'll not especially wonder whether these butter-like or oily substances were food or fuel. so we merely note that in the _scientific american_, - , is an account of hail that fell, in the middle of april, , in mississippi, in which was a substance described as turpentine. something that tasted like orange water, in hailstones, about the first of june, , near nîmes, france; identified as nitric acid (_jour. de pharmacie_, - ). hail and ashes, in ireland, (_sci. amer._, - ). that, at elizabeth, n.j., june , , fell hail in which was a substance, said, by prof. leeds, of stevens institute, to be carbonate of soda (_sci. amer._, - ). we are getting a little away from the lines of our composition, but it will be an important point later that so many extraordinary falls have occurred with hail. or--if they were of substances that had had origin upon some other part of this earth's surface--had the hail, too, that origin? our acceptance here will depend upon the number of instances. reasonably enough, some of the things that fall to this earth should coincide with falls of hail. as to vegetable substances in quantities so great as to suggest lost cargoes, we have a note in the _intellectual observer_, - : that, upon the first of may, , a rain fell at perpignan, "bringing down with it a red substance, which proved on examination to be a red meal mixed with fine sand." at various points along the mediterranean, this substance fell. there is, in _philosophical transactions_, - , an account of a seeming cereal, said to have fallen in wiltshire, in --said that some of the "wheat" fell "enclosed in hailstones"--but the writer in _transactions_, says that he had examined the grains, and that they were nothing but seeds of ivy berries dislodged from holes and chinks where birds had hidden them. if birds still hide ivy seeds, and if winds still blow, i don't see why the phenomenon has not repeated in more than two hundred years since. or the red matter in rain, at siena, italy, may, ; said, by arago, to have been vegetable matter (arago, _oeuvres_, - ). somebody should collect data of falls at siena alone. in the _monthly weather review_, - , a correspondent writes that, upon feb. , , at pawpaw, michigan, upon a day that was so calm that his windmill did not run, fell a brown dust that looked like vegetable matter. the editor of the _review_ concludes that this was no widespread fall from a tornado, because it had been reported from nowhere else. rancidness--putridity--decomposition--a note that has been struck many times. in a positive sense, of course, nothing means anything, or every meaning is continuous with all other meanings: or that all evidences of guilt, for instance, are just as good evidences of innocence--but this condition seems to mean--things lying around among the stars a long time. horrible disaster in the time of julius caesar; remains from it not reaching this earth till the time of the bishop of cloyne: we leave to later research the discussion of bacterial action and decomposition, and whether bacteria could survive in what we call space, of which we know nothing-- _chemical news_, - : dr. a.t. machattie, f.c.s., writes that, at london, ontario, feb. , , in a violent storm, fell, with snow, a dark-colored substance, estimated at tons, over a belt miles by miles. it was examined under a microscope, by dr. machattie, who found it to consist mainly of vegetable matter "far advanced in decomposition." the substance was examined by dr. james adams, of glasgow, who gave his opinion that it was the remains of cereals. dr. machattie points out that for months before this fall the ground of canada had been frozen, so that in this case a more than ordinarily remote origin has to be thought of. dr. machattie thinks of origin to the south. "however," he says, "this is mere conjecture." _amer. jour. sci._, - : that, march , --during a thunderstorm--at rajkit, india, occurred a fall of grain. it was reported by col. sykes, of the british association. the natives were greatly excited--because it was grain of a kind unknown to them. usually comes forward a scientist who knows more of the things that natives know best than the natives know--but it so happens that the usual thing was not done definitely in this instance: "the grain was shown to some botanists, who did not immediately recognize it, but thought it to be either a spartium or a vicia." lead, silver, diamonds, glass. they sound like the accursed, but they're not: they're now of the chosen--that is, when they occur in metallic or stony masses that science has recognized as meteorites. we find that resistance is to substances not so mixed in or incorporated. of accursed data, it seems to me that punk is pretty damnable. in the _report of the british association_, - , there is mention of a light chocolate-brown substance that has fallen with meteorites. no particulars given; not another mention anywhere else that i can find. in this english publication, the word "punk" is not used; the substance is called "amadou." i suppose, if the datum has anywhere been admitted to french publications, the word "amadou" has been avoided, and "punk" used. or oneness of allness: scientific works and social registers: a goldstein who can't get in as goldstein, gets in as jackson. the fall of sulphur from the sky has been especially repulsive to the modern orthodoxy--largely because of its associations with the superstitions or principles of the preceding orthodoxy--stories of devils: sulphurous exhalations. several writers have said that they have had this feeling. so the scientific reactionists, who have rabidly fought the preceding, because it was the preceding: and the scientific prudes, who, in sheer exclusionism, have held lean hands over pale eyes, denying falls of sulphur. i have many notes upon the sulphurous odor of meteorites, and many notes upon phosphorescence of things that come from externality. some day i shall look over old stories of demons that have appeared sulphurously upon this earth, with the idea of expressing that we have often had undesirable visitors from other worlds; or that an indication of external derivation is sulphurousness. i expect some day to rationalize demonology, but just at present we are scarcely far enough advanced to go so far back. for a circumstantial account of a mass of burning sulphur, about the size of a man's fist, that fell at pultusk, poland, jan. , , upon a road, where it was stamped out by a crowd of villagers, see _rept. brit. assoc._, - . the power of the exclusionists lies in that in their stand are combined both modern and archaic systematists. falls of sandstone and limestone are repulsive to both theologians and scientists. sandstone and limestone suggest other worlds upon which occur processes like geological processes; but limestone, as a fossiliferous substance, is of course especially of the unchosen. in _science_, march , , we read of a block of limestone, said to have fallen near middleburg, florida. it was exhibited at the sub-tropical exposition, at jacksonville. the writer, in _science_, denies that it fell from the sky. his reasoning is: there is no limestone in the sky; therefore this limestone did not fall from the sky. better reasoning i cannot conceive of--because we see that a final major premise--universal--true--would include all things: that, then, would leave nothing to reason about--so then that all reasoning must be based upon "something" not universal, or only a phantom intermediate to the two finalities of nothingness and allness, or negativeness and positiveness. _la nature_, - - : fall, at pel-et-der (l'aube), france, june , , of limestone pebbles. identified with limestone at château-landon--or up and down in a whirlwind. but they fell with hail--which, in june, could not very well be identified with ice from château-landon. coincidence, perhaps. upon page , _science gossip_, , the editor says, of a stone that was reported to have fallen at little lever, england, that a sample had been sent to him. it was sandstone. therefore it had not fallen, but had been on the ground in the first place. but, upon page , _science gossip_, , is an account of "a large, smooth, water-worn, gritty sandstone pebble" that had been found in the wood of a full-grown beech tree. looks to me as if it had fallen red-hot, and had penetrated the tree with high velocity. but i have never heard of anything falling red-hot from a whirlwind-- the wood around this sandstone pebble was black, as if charred. dr. farrington, for instance, in his books, does not even mention sandstone. however, the british association, though reluctant, is less exclusive: _report_ of , p. : substance about the size of a duck's egg, that fell at raphoe, ireland, june , --date questioned. it is not definitely said that this substance was sandstone, but that it "resembled" friable sandstone. falls of salt have occurred often. they have been avoided by scientific writers, because of the dictum that only water and not substances held in solution, can be raised by evaporation. however, falls of salty water have received attention from dalton and others, and have been attributed to whirlwinds from the sea. this is so reasonably contested--quasi-reasonably--as to places not far from the sea-- but the fall of salt that occurred high in the mountains of switzerland-- we could have predicted that that datum could be found somewhere. let anything be explained in local terms of the coast of england--but also has it occurred high in the mountains of switzerland. large crystals of salt fell--in a hailstorm--aug. , , in switzerland. the orthodox explanation is a crime: whoever made it, should have had his finger-prints taken. we are told (_an. rec. sci._, ) that these objects of salt "came over the mediterranean from some part of africa." or the hypnosis of the conventional--provided it be glib. one reads such an assertion, and provided it be suave and brief and conventional, one seldom questions--or thinks "very strange" and then forgets. one has an impression from geography lessons: mediterranean not more than three inches wide, on the map; switzerland only a few more inches away. these sizable masses of salt are described in the _amer. jour. sci._, - - , as "essentially imperfect cubic crystals of common salt." as to occurrence with hail--that can in one, or ten, or twenty, instances be called a coincidence. another datum: extraordinary year : london _times_, dec. , : translation from a turkish newspaper; a substance that fell at scutari, dec. , ; described as an unknown substance, in particles--or flakes?--like snow. "it was found to be saltish to the taste, and to dissolve readily in water." miscellaneous: "black, capillary matter" that fell, nov. , , at charleston, s.c. (_amer. jour. sci._, - - ). fall of small, friable, vesicular masses, from size of a pea to size of a walnut, at lobau, jan. , (_rept. brit. assoc._, - ). objects that fell at peshawur, india, june, , during a storm: substance that looked like crystallized niter, and that tasted like sugar (_nature_, july , ). i suppose sometimes deep-sea fishes have their noses bumped by cinders. if their regions be subjacent to cunard or white star routes, they're especially likely to be bumped. i conceive of no inquiry: they're deep-sea fishes. or the slag of slains. that it was a furnace-product. the rev. james rust seemed to feel bumped. he tried in vain to arouse inquiry. as to a report, from chicago, april , , that slag had fallen from the sky, prof. e.s. bastian (_amer. jour. sci._, - - ) says that the slag "had been on the ground in the first place." it was furnace-slag. "a chemical examination of the specimens has shown that they possess none of the characteristics of true meteorites." over and over and over again, the universal delusion; hope and despair of attempted positivism; that there can be real criteria, or distinct characteristics of anything. if anybody can define--not merely suppose, like prof. bastian, that he can define--the true characteristics of anything, or so localize trueness anywhere, he makes the discovery for which the cosmos is laboring. he will be instantly translated, like elijah, into the positive absolute. my own notion is that, in a moment of super-concentration, elijah became so nearly a real prophet that he was translated to heaven, or to the positive absolute, with such velocity that he left an incandescent train behind him. as we go along, we shall find the "true test of meteoritic material," which in the past has been taken as an absolute, dissolving into almost utmost nebulosity. prof. bastian explains mechanically, or in terms of the usual reflexes to all reports of unwelcome substances: that near where the slag had been found, telegraph wires had been struck by lightning; that particles of melted wire had been seen to fall near the slag--which had been on the ground in the first place. but, according to the _new york times_, april , , about two bushels of this substance had fallen. something that was said to have fallen at darmstadt, june , ; listed by greg (_rept. brit. assoc._, - ) as "only slag." _philosophical magazine_, - - : that, in , a large stone was found far in the interior of a tree, in battersea fields. sometimes cannon balls are found embedded in trees. doesn't seem to be anything to discuss; doesn't seem discussable that any one would cut a hole in a tree and hide a cannon ball, which one could take to bed, and hide under one's pillow, just as easily. so with the stone of battersea fields. what is there to say, except that it fell with high velocity and embedded in the tree? nevertheless, there was a great deal of discussion-- because, at the foot of the tree, as if broken off the stone, fragments of slag were found. i have nine other instances. slag and cinders and ashes, and you won't believe, and neither will i, that they came from the furnaces of vast aerial super-constructions. we'll see what looks acceptable. as to ashes, the difficulties are great, because we'd expect many falls of terrestrially derived ashes--volcanoes and forest fires. in some of our acceptances, i have felt a little radical-- i suppose that one of our main motives is to show that there is, in quasi-existence, nothing but the preposterous--or something intermediate to absolute preposterousness and final reasonableness--that the new is the obviously preposterous; that it becomes the established and disguisedly preposterous; that it is displaced, after a while, and is again seen to be the preposterous. or that all progress is from the outrageous to the academic or sanctified, and back to the outrageous--modified, however, by a trend of higher and higher approximation to the impreposterous. sometimes i feel a little more uninspired than at other times, but i think we're pretty well accustomed now to the oneness of allness; or that the methods of science in maintaining its system are as outrageous as the attempts of the damned to break in. in the _annual record of science_, - , prof. daubrée is quoted: that ashes that had fallen in the azores had come from the chicago fire-- or the damned and the saved, and there's little to choose between them; and angels are beings that have not obviously barbed tails to them--or never have such bad manners as to stroke an angel below the waist-line. however this especial outrage was challenged: the editor of the _record_ returns to it, in the issue of : considers it "in the highest degree improper to say that the ashes of chicago were landed in the azores." _bull. soc. astro. de france_, - : account of a white substance, like ashes, that fell at annoy, france, march , : simply called a curious phenomenon; no attempt to trace to a terrestrial source. flake formations, which may signify passage through a region of pressure, are common; but spherical formations--as if of things that have rolled and rolled along planar regions somewhere--are commoner: _nature_, jan. , , quotes a kimberley newspaper: that, toward the close of november, , a thick shower of ashy matter fell at queenstown, south africa. the matter was in marble-sized balls, which were soft and pulpy, but which, upon drying, crumbled at touch. the shower was confined to one narrow streak of land. it would be only ordinarily preposterous to attribute this substance to krakatoa-- but, with the fall, loud noises were heard-- but i'll omit many notes upon ashes: if ashes should sift down upon deep-sea fishes, that is not to say that they came from steamships. data of falls of cinders have been especially damned by mr. symons, the meteorologist, some of whose investigations we'll investigate later--nevertheless-- notice of a fall, in victoria, australia, april , (_rept. brit. assoc._, - )--at least we are told, in the reluctant way, that someone "thought" he saw matter fall near him at night, and the next day found something that looked like cinders. in the _proc. of the london roy. soc._, - , there is an account of cinders that fell on the deck of a lightship, jan. , . in the _amer. jour. sci._, - - , there is a notice that the editor had received a specimen of cinders said to have fallen--in showery weather--upon a farm, near ottowa, ill., jan. , . but after all, ambiguous things they are, cinders or ashes or slag or clinkers, the high priest of the accursed that must speak aloud for us is--coal that has fallen from the sky. or coke: the person who thought he saw something like cinders, also thought he saw something like coke, we are told. _nature_, - : something that "looked exactly like coke" that fell--during a thunderstorm--in the orne, france, april , . or charcoal: dr. angus smith, in the _lit. and phil. soc. of manchester memoirs_, - - , says that, about --like a great deal in lyell's _principles_ and darwin's _origin_, this account is from hearsay--something fell from the sky, near allport, england. it fell luminously, with a loud report, and scattered in a field. a fragment that was seen by dr. smith, is described by him as having "the appearance of a piece of common wood charcoal." nevertheless, the reassured feeling of the faithful, upon reading this, is burdened with data of differences: the substance was so uncommonly heavy that it seemed as if it had iron in it; also there was "a sprinkling of sulphur." this material is said, by prof. baden-powell, to be "totally unlike that of any other meteorite." greg, in his catalogue (_rept. brit. assoc._, - ), calls it "a more than doubtful substance"--but again, against reassurance, that is not doubt of authenticity. greg says that it is like compact charcoal, with particles of sulphur and iron pyrites embedded. reassurance rises again: prof. baden-powell says: "it contains also charcoal, which might perhaps be acquired from matter among which it fell." this is a common reflex with the exclusionists: that substances not "truly meteoritic" did not fall from the sky, but were picked up by "truly meteoritic" things, of course only on their surfaces, by impact with this earth. rhythm of reassurances and their declines: according to dr. smith, this substance was not merely coated with charcoal; his analysis gives . per cent carbon. our acceptance that coal has fallen from the sky will be via data of resinous substances and bituminous substances, which merge so that they cannot be told apart. resinous substance said to have fallen at kaba, hungary, april , (_rept. brit. assoc._, - ). a resinous substance that fell after a fireball? at neuhaus, bohemia, dec. , (_rept. brit. assoc._, - ). fall, july , , at luchon, during a storm, of a brownish substance; very friable, carbonaceous matter; when burned it gave out a resinous odor (_comptes rendus_, - ). substance that fell, feb. , , , , at genoa, italy, said to have been resinous; said by arago (_oeuvres_, - ) to have been bituminous matter and sand. fall--during a thunderstorm--july, , near cape cod, upon the deck of an english vessel, the _albemarle_, of "burning, bituminous matter" (_edin. new phil. jour._, - ); a fall, at christiania, norway, june , , of bituminous matter, listed by greg as doubtful; fall of bituminous matter, in germany, march , , listed by greg. lockyer (_the meteoric hypothesis_, p. ) says that the substance that fell at the cape of good hope, oct. , --about five cubic feet of it: substance so soft that it was cuttable with a knife--"after being experimented upon, it left a residue, which gave out a very bituminous smell." and this inclusion of lockyer's--so far as findable in all books that i have read--is, in books, about as close as we can get to our desideratum--that coal has fallen from the sky. dr. farrington, except with a brief mention, ignores the whole subject of the fall of carbonaceous matter from the sky. proctor, in all of his books that i have read--is, in books, about as close as we can get to the admission that carbonaceous matter has been found in meteorites "in very minute quantities"--or my own suspicion is that it is possible to damn something else only by losing one's own soul--quasi-soul, of course. _sci. amer._, - : that the substance that fell at the cape of good hope "resembled a piece of anthracite coal more than anything else." it's a mistake, i think: the resemblance is to bituminous coal--but it is from the periodicals that we must get our data. to the writers of books upon meteorites, it would be as wicked--by which we mean departure from the characters of an established species--quasi-established, of course--to say that coal has fallen from the sky, as would be, to something in a barnyard, a temptation that it climb a tree and catch a bird. domestic things in a barnyard: and how wild things from forests outside seem to them. or the homeopathist--but we shall shovel data of coal. and, if over and over, we shall learn of masses of soft coal that have fallen upon this earth, if in no instance has it been asserted that the masses did not fall, but were upon the ground in the first place; if we have many instances, this time we turn down good and hard the mechanical reflex that these masses were carried from one place to another in whirlwinds, because we find it too difficult to accept that whirlwinds could so select, or so specialize in a peculiar substance. among writers of books, the only one i know of who makes more than brief mention is sir robert ball. he represents a still more antique orthodoxy, or is an exclusionist of the old type, still holding out against even meteorites. he cites several falls of carbonaceous matter, but with disregards that make for reasonableness that earthy matter may have been caught up by whirlwinds and flung down somewhere else. if he had given a full list, he would be called upon to explain the special affinity of whirlwinds for a special kind of coal. he does not give a full list. we shall have all that's findable, and we shall see that against this disease we're writing, the homeopathist's prescription availeth not. another exclusionist was prof. lawrence smith. his psycho-tropism was to respond to all reports of carbonaceous matter falling from the sky, by saying that this damned matter had been deposited upon things of the chosen by impact with this earth. most of our data antedate him, or were contemporaneous with him, or were as accessible to him as to us. in his attempted positivism it is simply--and beautifully--disregarded that, according to berthelot, berzelius, cloez, wohler and others these masses are not merely coated with carbonaceous matter, but are carbonaceous throughout, or are permeated throughout. how anyone could so resolutely and dogmatically and beautifully and blindly hold out would puzzle us were it not for our acceptance that only to think is to exclude and include; and to exclude some things that have as much right to come in as have the included--that to have an opinion upon any subject is to be a lawrence smith--because there is no definite subject. dr. walter flight (_eclectic magazine_, - ) says, of the substance that fell near alais, france, march , , that it "emits a faint bituminous substance" when heated, according to the observations of bergelius and a commission appointed by the french academy. this time we have not the reluctances expressed in such words as "like" and "resembling." we are told that this substance is "an earthy kind of coal." as to "minute quantities" we are told that the substance that fell at the cape of good hope has in it a little more than a quarter of organic matter, which, in alcohol, gives the familiar reaction of yellow, resinous matter. other instances given by dr. flight are: carbonaceous matter that fell in , in tennessee; cranbourne, australia, ; montauban, france, may , (twenty masses, some of them as large as a human head, of a substance that "resembled a dull-colored earthy lignite"); goalpara, india, about (about per cent of a hydrocarbon); at ornans, france, july , ; substance with "an organic, combustible ingredient," at hessle, sweden, jan. , . _knowledge_, - : that, according to m. daubrée, the substance that had fallen in the argentine republic, "resembled certain kinds of lignite and boghead coal." in _comptes rendus_, - , it is said that this mass fell, june , , in the province entre ríos, argentina: that it is "like" brown coal; that it resembles all the other carbonaceous masses that have fallen from the sky. something that fell at grazac, france, aug. , : when burned, it gave out a bituminous odor (_comptes rendus_, - ). carbonaceous substance that fell at rajpunta, india, jan. , : very friable: per cent of its soluble in water (_records geol. survey of india_, -pt. - ). a combustible carbonaceous substance that fell with sand at naples, march , (_amer. jour. sci._, - - ). _sci. amer. sup._, - : that, june , , a very friable substance, of a deep, greenish black, fell at mighei, russia. it contained per cent organic matter, which, when powdered and digested in alcohol, yielded, after evaporation, a bright yellow resin. in this mass was per cent of an unknown mineral. cinders and ashes and slag and coke and charcoal and coal. and the things that sometimes deep-sea fishes are bumped by. reluctances and the disguises or covered retreats of such words as "like" and "resemble"--or that conditions of intermediateness forbid abrupt transitions--but that the spirit animating all intermediateness is to achieve abrupt transitions--because, if anything could finally break away from its origin and environment, that would be a real thing--something not merging away indistinguishably with the surrounding. so all attempt to be original; all attempt to invent something that is more than mere extension or modification of the preceding, is positivism--or that if one could conceive of a device to catch flies, positively different from, or unrelated to, all other devices--up he'd shoot to heaven, or the positive absolute--leaving behind such an incandescent train that in one age it would be said that he had gone aloft in a fiery chariot, and in another age that he had been struck by lightning-- i'm collecting notes upon persons supposed to have been struck by lightning. i think that high approximation to positivism has often been achieved--instantaneous translation--residue of negativeness left behind, looking much like effects of a stroke of lightning. some day i shall tell the story of the _marie celeste_--"properly," as the _scientific american supplement_ would say--mysterious disappearance of a sea captain, his family, and the crew-- of positivists, by the route of abrupt transition, i think that manet was notable--but that his approximation was held down by his intense relativity to the public--or that it is quite as impositive to flout and insult and defy as it is to crawl and placate. of course, manet began with continuity with courbet and others, and then, between him and manet there were mutual influences--but the spirit of abrupt difference is the spirit of positivism, and manet's stand was against the dictum that all lights and shades must merge away suavely into one another and prepare for one another. so a biologist like de vries represents positivism, or the breaking of continuity, by trying to conceive of evolution by mutation--against the dogma of indistinguishable gradations by "minute variations." a copernicus conceives of helio-centricity. continuity is against him. he is not permitted to break abruptly with the past. he is permitted to publish his work, but only as "an interesting hypothesis." continuity--and that all that we call evolution or progress is attempt to break away from it-- that our whole solar system was at one time attempt by planets to break away from a parental nexus and set up as individualities, and, failing, move in quasi-regular orbits that are expressions of relations with the sun and with one another, all having surrendered, being now quasi-incorporated in a higher approximation to system: intermediateness in its mineralogic aspect of positivism--or iron that strove to break away from sulphur and oxygen, and be real, homogeneous iron--failing, inasmuch as elemental iron exists only in text-book chemistry: intermediateness in its biologic aspect of positivism--or the wild, fantastic, grotesque, monstrous things it conceived of, sometimes in a frenzy of effort to break away abruptly from all preceding types--but failing, in the giraffe-effort, for instance, or only caricaturing an antelope-- all things break one relation only by the establishing of some other relation-- all things cut an umbilical cord only to clutch a breast. so the fight of the exclusionists to maintain the traditional--or to prevent abrupt transition from the quasi-established--fighting so that here, more than a century after meteorites were included, no other notable inclusion has been made, except that of cosmic dust, data of which nordenskiold made more nearly real than data in opposition. so proctor, for instance, fought and expressed his feeling of the preposterous, against sir w.h. thomson's notions of arrival upon this earth of organisms on meteorites-- "i can only regard it as a jest" (_knowledge_, - ). or that there is nothing but jest--or something intermediate to jest and tragedy: that ours is not an existence but an utterance; that momus is imagining us for the amusement of the gods, often with such success that some of us seem almost alive--like characters in something a novelist is writing; which often to considerable degree take their affairs away from the novelist-- that momus is imagining us and our arts and sciences and religions, and is narrating or picturing us as a satire upon the gods' real existence. because--with many of our data of coal that has fallen from the sky as accessible then as they are now, and with the scientific pronouncement that coal is fossil, how, in a real existence, by which we mean a consistent existence, or a state in which there is real intelligence, or a form of thinking that does not indistinguishably merge away with imbecility, could there have been such a row as that which was raised about forty years ago over dr. hahn's announcement that he had found fossils in meteorites? accessible to anybody at that time: _philosophical magazine_, - - : that the substance that fell at kaba, hungary, april , , contained organic matter "analagous to fossil waxes." or limestone: of the block of limestone which was reported to have fallen at middleburg, florida, it is said (_science_, - ) that, though something had been seen to fall in "an old cultivated field," the witnesses who ran to it picked up something that "had been upon the ground in the first place." the writer who tells us this, with the usual exclusion-imagination known as stupidity, but unjustly, because there is no real stupidity, thinks he can think of a good-sized stone that had for many years been in a cultivated field, but that had never been seen before--had never interfered with plowing, for instance. he is earnest and unjarred when he writes that this stone weighs pounds. my own notion, founded upon my own experience in seeing, is that a block of stone weighing pounds might be in one's parlor twenty years, virtually unseen--but not in an old cultivated field, where it interfered with plowing--not anywhere--if it interfered. dr. hahn said that he had found fossils in meteorites. there is a description of the corals, sponges, shells, and crinoids, all of them microscopic, which he photographed, in _popular science_, - . dr. hahn was a well-known scientist. he was better known after that. anybody may theorize upon other worlds and conditions upon them that are similar to our own conditions: if his notions be presented undisguisedly as fiction, or only as an "interesting hypothesis," he'll stir up no prude rages. but dr. hahn said definitely that he had found fossils in specified meteorites: also he published photographs of them. his book is in the new york public library. in the reproductions every feature of some of the little shells is plainly marked. if they're not shells, neither are things under an oyster-counter. the striations are very plain: one sees even the hinges where bivalves are joined. prof. lawrence smith (_knowledge_, - ): "dr. hahn is a kind of half-insane man, whose imagination has run away with him." conservation of continuity. then dr. weinland examined dr. hahn's specimens. he gave his opinion that they are fossils and that they are not crystals of enstatite, as asserted by prof. smith, who had never seen them. the damnation of denial and the damnation of disregard: after the publication of dr. weinland's findings--silence. the living things that have come down to this earth: attempts to preserve the system: that small frogs and toads, for instance, never have fallen from the sky, but were--"on the ground, in the first place"; or that there have been such falls--"up from one place in a whirlwind, and down in another." were there some especially froggy place near europe, as there is an especially sandy place, the scientific explanation would of course be that all small frogs falling from the sky in europe come from that center of frogeity. to start with, i'd like to emphasize something that i am permitted to see because i am still primitive or intelligent or in a state of maladjustment: that there is not one report findable of a fall of tadpoles from the sky. as to "there in the first place": see _leisure hours_, - , for accounts of small frogs, or toads, said to have been seen to fall from the sky. the writer says that all observers were mistaken: that the frogs or toads must have fallen from trees or other places overhead. tremendous number of little toads, one or two months old, that were seen to fall from a great thick cloud that appeared suddenly in a sky that had been cloudless, august, , near toulouse, france, according to a letter from prof. pontus to m. arago. (_comptes rendus_, - .) many instances of frogs that were seen to fall from the sky. (_notes and queries_, - - ); accounts of such falls, signed by witnesses. (_notes and queries_, - - .) _scientific american_, july , : "a shower of frogs which darkened the air and covered the ground for a long distance is the reported result of a recent rainstorm at kansas city, mo." as to having been there "in the first place": little frogs found in london, after a heavy storm, july , . (_notes and queries_, - - ); little toads found in a desert, after a rainfall (_notes and queries_, - - ). to start with i do not deny--positively--the conventional explanation of "up and down." i think that there may have been such occurrences. i omit many notes that i have upon indistinguishables. in the london _times_, july , , there is an account of a shower of twigs and leaves and tiny toads in a storm upon the slopes of the apennines. these may have been the ejectamenta of a whirlwind. i add, however, that i have notes upon two other falls of tiny toads, in , one in france and one in tahiti; also of fish in scotland. but in the phenomenon of the apennines, the mixture seems to me to be typical of the products of a whirlwind. the other instances seem to me to be typical of--something like migration? their great numbers and their homogeneity. over and over in these annals of the damned occurs the datum of segregation. but a whirlwind is thought of as a condition of chaos--quasi-chaos: not final negativeness, of course-- _monthly weather review_, july, : "a small pond in the track of the cloud was sucked dry, the water being carried over the adjoining fields together with a large quantity of soft mud, which was scattered over the ground for half a mile around." it is so easy to say that small frogs that have fallen from the sky had been scooped up by a whirlwind; but here are the circumstances of a scoop; in the exclusionist-imagination there is no regard for mud, débris from the bottom of a pond, floating vegetation, loose things from the shores--but a precise picking out of frogs only. of all instances i have that attribute the fall of small frogs or toads to whirlwinds, only one definitely identifies or places the whirlwind. also, as has been said before, a pond going up would be quite as interesting as frogs coming down. whirlwinds we read of over and over--but where and what whirlwind? it seems to me that anybody who had lost a pond would be heard from. in _symons' meteorological magazine_, - , a fall of small frogs, near birmingham, england, june , , is attributed to a specific whirlwind--but not a word as to any special pond that had contributed. and something that strikes my attention here is that these frogs are described as almost white. i'm afraid there is no escape for us: we shall have to give to civilization upon this earth--some new worlds. places with white frogs in them. upon several occasions we have had data of unknown things that have fallen from--somewhere. but something not to be overlooked is that if living things have landed alive upon this earth--in spite of all we think we know of the accelerative velocity of falling bodies--and have propagated--why the exotic becomes the indigenous, or from the strangest of places we'd expect the familiar. or if hosts of living frogs have come here--from somewhere else--every living thing upon this earth may, ancestrally, have come from--somewhere else. i find that i have another note upon a specific hurricane: _annals and mag. of nat. hist._, - - : after one of the greatest hurricanes in the history of ireland, some fish were found "as far as yards from the edge of a lake." have another: this is a good one for the exclusionists: fall of fish in paris: said that a neighboring pond had been blown dry. (_living age_, - .) date not given, but i have seen it recorded somewhere else. the best-known fall of fishes from the sky is that which occurred at mountain ash, in the valley of abedare, glamorganshire, feb. , . the editor of the _zoologist_, - , having published a report of a fall of fishes, writes: "i am continually receiving similar accounts of frogs and fishes." but, in all the volumes of the _zoologist_, i can find only two reports of such falls. there is nothing to conclude other than that hosts of data have been lost because orthodoxy does not look favorably upon such reports. the _monthly weather review_ records several falls of fishes in the united states; but accounts of these reported occurrences are not findable in other american publications. nevertheless, the treatment by the _zoologist_ of the fall reported from mountain ash is fair. first appears, in the issue of - , a letter from the rev. john griffith, vicar of abedare, asserting that the fall had occurred, chiefly upon the property of mr. nixon, of mountain ash. upon page , dr. gray, of the british museum, bristling with exclusionism, writes that some of these fishes, which had been sent to him alive, were "very young minnows." he says: "on reading the evidence, it seems to me most probably only a practical joke: that one of mr. nixon's employees had thrown a pailful of water upon another, who had thought fish in it had fallen from the sky"--had dipped up a pailful from a brook. those fishes--still alive--were exhibited at the zoological gardens, regent's park. the editor says that one was a minnow and that the rest were sticklebacks. he says that dr. gray's explanation is no doubt right. but, upon page , he publishes a letter from another correspondent, who apologizes for opposing "so high an authority as dr. gray," but says that he had obtained some of these fishes from persons who lived at a considerable distance apart, or considerably out of range of the playful pail of water. according to the _annual register_, - , the fishes themselves had fallen by pailfuls. if these fishes were not upon the ground in the first place, we base our objections to the whirlwind explanation upon two data: that they fell in no such distribution as one could attribute to the discharge of a whirlwind, but upon a narrow strip of land: about yards long and yards wide-- the other datum is again the suggestion that at first seemed so incredible, but for which support is piling up, a suggestion of a stationary source overhead-- that ten minutes later another fall of fishes occurred upon this same narrow strip of land. even arguing that a whirlwind may stand still axially, it discharges tangentially. wherever the fishes came from it does not seem thinkable that some could have fallen and that others could have whirled even a tenth of a minute, then falling directly after the first to fall. because of these evil circumstances the best adaptation was to laugh the whole thing off and say that someone had soused someone else with a pailful of water in which a few "very young" minnows had been caught up. in the london _times_, march , , is a letter from mr. aaron roberts, curate of st. peter's, carmathon. in this letter the fishes are said to have been about four inches long, but there is some question of species. i think, myself, that they were minnows and sticklebacks. some persons, thinking them to be sea fishes, placed them in salt water, according to mr. roberts. "the effect is stated to have been almost instantaneous death." "some were placed in fresh water. these seemed to thrive well." as to narrow distribution, we are told that the fishes fell "in and about the premises of mr. nixon." "it was not observed at the time that any fish fell in any other part of the neighborhood, save in the particular spot mentioned." in the london _times_, march , , vicar griffith writes an account: "the roofs of some houses were covered with them." in this letter it is said that the largest fishes were five inches long, and that these did not survive the fall. _report of the british association_, - : "the evidence of the fall of fish on this occasion was very conclusive. a specimen of the fish was exhibited and was found to be the _gasterosteus leirus_." _gasterosteus_ is the stickleback. altogether i think we have not a sense of total perdition, when we're damned with the explanation that someone soused someone else with a pailful of water in which were thousands of fishes four or five inches long, some of which covered roofs of houses, and some of which remained ten minutes in the air. by way of contrast we offer our own acceptance: that the bottom of a super-geographical pond had dropped out. i have a great many notes upon the fall of fishes, despite the difficulty these records have in getting themselves published, but i pick out the instances that especially relate to our super-geographical acceptances, or to the principles of super-geography: or data of things that have been in the air longer than acceptably could a whirlwind carry them; that have fallen with a distribution narrower than is attributable to a whirlwind; that have fallen for a considerable length of time upon the same narrow area of land. these three factors indicate, somewhere not far aloft, a region of inertness to this earth's gravitation, of course, however, a region that, by the flux and variation of all things, must at times be susceptible--but, afterward, our heresy will bifurcate-- in amiable accommodation to the crucifixion it'll get, i think-- but so impressed are we with the datum that, though there have been many reports of small frogs that have fallen from the sky, not one report upon a fall of tadpoles is findable, that to these circumstances another adjustment must be made. apart from our three factors of indication, an extraordinary observation is the fall of living things without injury to them. the devotees of st. isaac explain that they fall upon thick grass and so survive: but sir james emerson tennant, in his _history of ceylon_, tells of a fall of fishes upon gravel, by which they were seemingly uninjured. something else apart from our three main interests is a phenomenon that looks like what one might call an alternating series of falls of fishes, whatever the significance may be: meerut, india, july, (_living age_, - ); fifeshire, scotland, summer of (_wernerian nat. hist. soc. trans._, - ); moradabad, india, july, (_living age_, - ); ross-shire, scotland, (_living age_, - ); moradabad, india, july , (_lin. soc. trans._, - ); perthshire, scotland (_living age_, - ); argyleshire, scotland, , march , (_recreative science_, - ); feridpoor, india, feb. , (_jour. asiatic soc. of bengal_, - ). a psycho-tropism that arises here--disregarding serial significance--or mechanical, unintelligent, repulsive reflex--is that the fishes of india did not fall from the sky; that they were found upon the ground after torrential rains, because streams had overflowed and had then receded. in the region of inertness that we think we can conceive of, or a zone that is to this earth's gravitation very much like the neutral zone of a magnet's attraction, we accept that there are bodies of water and also clear spaces--bottoms of ponds dropping out--very interesting ponds, having no earth at bottom--vast drops of water afloat in what is called space--fishes and deluges of water falling-- but also other areas, in which fishes--however they got there: a matter that we'll consider--remain and dry, or even putrefy, then sometimes falling by atmospheric dislodgment. after a "tremendous deluge of rain, one of the heaviest falls on record" (_all the year round_, - ) at rajkote, india, july , , "the ground was found literally covered with fishes." the word "found" is agreeable to the repulsions of the conventionalists and their concept of an overflowing stream--but, according to dr. buist, some of these fishes were "found" on the tops of haystacks. ferrel (_a popular treatise_, p. ) tells of a fall of living fishes--some of them having been placed in a tank, where they survived--that occurred in india, about miles south of calcutta, sept. , . a witness of this fall says: "the most strange thing which ever struck me was that the fish did not fall helter-skelter, or here and there, but they fell in a straight line, not more than a cubit in breadth." see _living age_, - . _amer. jour. sci._, - - : that, according to testimony taken before a magistrate, a fall occurred, feb. , , near feridpoor, india, of many fishes, of various sizes--some whole and fresh and others "mutilated and putrefying." our reflex to those who would say that, in the climate of india, it would not take long for fishes to putrefy, is--that high in the air, the climate of india is not torrid. another peculiarity of this fall is that some of the fishes were much larger than others. or to those who hold out for segregation in a whirlwind, or that objects, say, twice as heavy as others would be separated from the lighter, we point out that some of these fishes were twice as heavy as others. in the _journal of the asiatic society of bengal_, - , depositions of witnesses are given: "some of the fish were fresh, but others were rotten and without heads." "among the number which i got, five were fresh and the rest stinking and headless." they remind us of his grace's observation of some pages back. according to dr. buist, some of these fishes weighed one and a half pounds each and others three pounds. a fall of fishes at futtepoor, india, may , : "they were all dead and dry." (dr. buist, _living age_, - .) india is far away: about was long ago. _nature_, sept. , - : a correspondent writes, from the dove marine laboratory, cuttercoats, england, that, at hindon, a suburb of sunderland, aug. , , hundreds of small fishes, identified as sand eels, had fallen-- again the small area: about by yards. the fall occurred during a heavy rain that was accompanied by thunder--or indications of disturbance aloft--but by no visible lightning. the sea is close to hindon, but if you try to think of these fishes having described a trajectory in a whirlwind from the ocean, consider this remarkable datum: that, according to witnesses, the fall upon this small area occupied ten minutes. i cannot think of a clearer indication of a direct fall from a stationary source. and: "the fish were all dead, and indeed stiff and hard, when picked up, immediately after the occurrence." by all of which i mean that we have only begun to pile up our data of things that fall from a stationary source overhead: we'll have to take up the subject from many approaches before our acceptance, which seems quite as rigorously arrived at as ever has been a belief, can emerge from the accursed. i don't know how much the horse and the barn will help us to emerge: but, if ever anything did go up from this earth's surface and stay up--those damned things may have: _monthly weather review_, may, : in a tornado, in wisconsin, may , , "a barn and a horse were carried completely away, and neither horse nor barn, nor any portion of either have since been found." after that, which would be a little strong were it not for a steady improvement in our digestions that i note as we go along, there is little of the bizarre or the unassimilable in the turtle that hovered six months or so over a small town in mississippi: _monthly weather review_, may, : that, may , , at vicksburg, miss., fell a small piece of alabaster; that, at bovina, eight miles from vicksburg, fell a gopher turtle. they fell in a hailstorm. this item was widely copied at the time: for instance, _nature_, one of the volumes of , page , and _jour. roy. met. soc._, - . as to discussion--not a word. or science and its continuity with presbyterianism--data like this are damned at birth. the _weather review_ does sprinkle, or baptize, or attempt to save, this infant--but in all the meteorological literature that i have gone through, after that date--not a word, except mention once or twice. the editor of the _review_ says: "an examination of the weather map shows that these hailstorms occur on the south side of a region of cold northerly winds, and were but a small part of a series of similar storms; apparently some special local whirls or gusts carried heavy objects from this earth's surface up to the cloud regions." of all incredibilities that we have to choose from, i give first place to a notion of a whirlwind pouncing upon a region and scrupulously selecting a turtle and a piece of alabaster. this time, the other mechanical thing "there in the first place" cannot rise in response to its stimulus: it is resisted in that these objects were coated with ice--month of may in a southern state. if a whirlwind at all, there must have been very limited selection: there is no record of the fall of other objects. but there is no attempt in the _review_ to specify a whirlwind. these strangely associated things were remarkably separated. they fell eight miles apart. then--as if there were real reasoning--they must have been high to fall with such divergence, or one of them must have been carried partly horizontally eight miles farther than the other. but either supposition argues for power more than that of a local whirl or gust, or argues for a great, specific disturbance, of which there is no record--for the month of may, . nevertheless--as if i really were reasonable--i do feel that i have to accept that this turtle had been raised from this earth's surface, somewhere near vicksburg--because the gopher turtle is common in the southern states. then i think of a hurricane that occurred in the state of mississippi weeks or months before may , . no--i don't look for it--and inevitably find it. or that things can go up so high in hurricanes that they stay up indefinitely--but may, after a while, be shaken down by storms. over and over have we noted the occurrence of strange falls in storms. so then that the turtle and the piece of alabaster may have had far different origins--from different worlds, perhaps--have entered a region of suspension over this earth--wafting near each other--long duration--final precipitation by atmospheric disturbance--with hail--or that hailstones, too, when large, are phenomena of suspension of long duration: that it is highly unacceptable that the very large ones could become so great only in falling from the clouds. over and over has the note of disagreeableness, or of putrefaction, been struck--long duration. other indications of long duration. i think of a region somewhere above this earth's surface in which gravitation is inoperative and is not governed by the square of the distance--quite as magnetism is negligible at a very short distance from a magnet. theoretically the attraction of a magnet should decrease with the square of the distance, but the falling-off is found to be almost abrupt at a short distance. i think that things raised from this earth's surface to that region have been held there until shaken down by storms-- the super-sargasso sea. derelicts, rubbish, old cargoes from inter-planetary wrecks; things cast out into what is called space by convulsions of other planets, things from the times of the alexanders, caesars and napoleons of mars and jupiter and neptune; things raised by this earth's cyclones: horses and barns and elephants and flies and dodoes, moas, and pterodactyls; leaves from modern trees and leaves of the carboniferous era--all, however, tending to disintegrate into homogeneous-looking muds or dusts, red or black or yellow--treasure-troves for the palaeontologists and for the archaeologists--accumulations of centuries--cyclones of egypt, greece, and assyria--fishes dried and hard, there a short time: others there long enough to putrefy-- but the omnipresence of heterogeneity--or living fishes, also--ponds of fresh water: oceans of salt water. as to the law of gravitation, i prefer to take one simple stand: orthodoxy accepts the correlation and equivalence of forces: gravitation is one of these forces. all other forces have phenomena of repulsion and of inertness irrespective of distance, as well as of attraction. but newtonian gravitation admits attraction only: then newtonian gravitation can be only one-third acceptable even to the orthodox, or there is denial of the correlation and equivalence of forces. or still simpler: here are the data. make what you will, yourself, of them. in our intermediatist revolt against homogeneous, or positive, explanations, or our acceptance that the all-sufficing cannot be less than universality, besides which, however, there would be nothing to suffice, our expression upon the super-sargasso sea, though it harmonizes with data of fishes that fall as if from a stationary source--and, of course, with other data, too--is inadequate to account for two peculiarities of the falls of frogs: that never has a fall of tadpoles been reported; that never has a fall of full-grown frogs been reported-- always frogs a few months old. it sounds positive, but if there be such reports they are somewhere out of my range of reading. but tadpoles would be more likely to fall from the sky than would frogs, little or big, if such falls be attributed to whirlwinds; and more likely to fall from the super-sargasso sea if, though very tentatively and provisionally, we accept the super-sargasso sea. before we take up an especial expression upon the fall of immature and larval forms of life to this earth, and the necessity then of conceiving of some factor besides mere stationariness or suspension or stagnation, there are other data that are similar to data of falls of fishes. _science gossip_, - : that small snails, of a land species, had fallen near redruth, cornwall, july , , "during a heavy thunderstorm": roads and fields strewn with them, so that they were gathered up by the hatful: none seen to fall by the writer of this account: snails said to be "quite different to any previously known in this district." but, upon page , we have better orthodoxy. another correspondent writes that he had heard of the supposed fall of snails: that he had supposed that all such stories had gone the way of witch stories; that, to his astonishment, he had read an account of this absurd story in a local newspaper of "great and deserved repute." "i thought i should for once like to trace the origin of one of these fabulous tales." our own acceptance is that justice cannot be in an intermediate existence, in which there can be approximation only to justice or to injustice; that to be fair is to have no opinion at all; that to be honest is to be uninterested; that to investigate is to admit prejudice; that nobody has ever really investigated anything, but has always sought positively to prove or to disprove something that was conceived of, or suspected, in advance. "as i suspected," says this correspondent, "i found that the snails were of a familiar land-species"--that they had been upon the ground "in the first place." he found that the snails had appeared after the rain: that "astonished rustics had jumped to the conclusion that they had fallen." he met one person who said that he had seen the snails fall. "this was his error," says the investigator. in the _philosophical magazine_, - , there is an account of snails said to have fallen at bristol in a field of three acres, in such quantities that they were shoveled up. it is said that the snails "may be considered as a local species." upon page , another correspondent says that the numbers had been exaggerated, and that in his opinion they had been upon the ground in the first place. but that there had been some unusual condition aloft comes out in his observation upon "the curious azure-blue appearance of the sun, at the time." _nature_, - : that, according to _das wetter_, december, , upon aug. , , a yellow cloud appeared over paderborn, germany. from this cloud, fell a torrential rain, in which were hundreds of mussels. there is no mention of whatever may have been upon the ground in the first place, nor of a whirlwind. lizards--said to have fallen on the sidewalks of montreal, canada, dec. , . (_notes and queries_, - - .) in the _scientific american_, - , a correspondent writes, from south granville, n.y., that, during a heavy shower, july , , he heard a peculiar sound at his feet, and looking down, saw a snake lying as if stunned by a fall. it then came to life. gray snake, about a foot long. these data have any meaning or lack of meaning or degree of damnation you please: but, in the matter of the fall that occurred at memphis, tennessee, occur some strong significances. our quasi-reasoning upon this subject applies to all segregations so far considered. _monthly weather review_, jan. , : that, in memphis, tenn., jan. , , rather strictly localized, or "in a space of two blocks," and after a violent storm in which the rain "fell in torrents," snakes were found. they were crawling on sidewalks, in yards, and in streets, and in masses--but "none were found on roofs or any other elevation above ground" and "none were seen to fall." if you prefer to believe that the snakes had always been there, or had been upon the ground in the first place, and that it was only that something occurred to call special attention to them, in the streets of memphis, jan. , --why, that's sensible: that's the common sense that has been against us from the first. it is not said whether the snakes were of a known species or not, but that "when first seen, they were of a dark brown, almost black." blacksnakes, i suppose. if we accept that these snakes did fall, even though not seen to fall by all the persons who were out sight-seeing in a violent storm, and had not been in the streets crawling loose or in thick tangled masses, in the first place: if we try to accept that these snakes had been raised from some other part of this earth's surface in a whirlwind: if we try to accept that a whirlwind could segregate them-- we accept the segregation of other objects raised in that whirlwind. then, near the place of origin, there would have been a fall of heavier objects that had been snatched up with the snakes--stones, fence rails, limbs of trees. say that the snakes occupied the next gradation, and would be the next to fall. still farther would there have been separate falls of lightest objects: leaves, twigs, tufts of grass. in the _monthly weather review_ there is no mention of other falls said to have occurred anywhere in january, . again ours is the objection against such selectiveness by a whirlwind. conceivably a whirlwind could scoop out a den of hibernating snakes, with stones and earth and an infinitude of other débris, snatching up dozens of snakes--i don't know how many to a den--hundreds maybe--but, according to the account of this occurrence in the _new york times_, there were thousands of them; alive; from one foot to eighteen inches in length. the _scientific american_, - , records the fall, and says that there were thousands of them. the usual whirlwind-explanation is given--"but in what locality snakes exist in such abundance is yet a mystery." this matter of enormousness of numbers suggests to me something of a migratory nature--but that snakes in the united states do not migrate in the month of january, if ever. as to falls or flutterings of winged insects from the sky, prevailing notions of swarming would seem explanatory enough: nevertheless, in instances of ants, there are some peculiar circumstances. _l'astronomie_, - : fall of fishes, june , , in holland; ants, aug. , , strasbourg; little toads, aug. , , savoy. fall of ants, cambridge, england, summer of --"some were wingless." (_scientific american_, - .) enormous fall of ants, nancy, france, july , --"most of them were wingless." (_nature_, - .) fall of enormous, unknown ants--size of wasps--manitoba, june, . (_sci. amer._, - .) however, our expression will be: that wingless, larval forms of life, in numbers so enormous that migration from some place external to this earth is suggested, have fallen from the sky. that these "migrations"--if such can be our acceptance--have occurred at a time of hibernation and burial far in the ground of larvae in the northern latitudes of this earth; that there is significance in recurrence of these falls in the last of january--or that we have the square of an incredibility in such a notion as that of selection of larvae by whirlwinds, compounded with selection of the last of january. i accept that there are "snow worms" upon this earth--whatever their origin may have been. in the _proc. acad. nat. sci. of philadelphia_, - , there is a description of yellow worms and black worms that have been found together on glaciers in alaska. almost positively were there no other forms of insect-life upon these glaciers, and there was no vegetation to support insect-life, except microscopic organisms. nevertheless the description of this probably polymorphic species fits a description of larvae said to have fallen in switzerland, and less definitely fits another description. there is no opposition here, if our data of falls are clear. frogs of every-day ponds look like frogs said to have fallen from the sky--except the whitish frogs of birmingham. however, all falls of larvae have not positively occurred in the last of january: london _times_, april , : that, in the parish of bramford speke, devonshire, a large number of black worms, about three-quarters of an inch in length, had fallen in a snowstorm. in timb's _year book_, - , it is said that, in the winter of , at christiania, norway, worms were found crawling upon the ground. the occurrence is considered a great mystery, because the worms could not have come up from the ground, inasmuch as the ground was frozen at the time, and because they were reported from other places, also, in norway. immense number of black insects in a snowstorm, in , at pakroff, russia. (_scientific american_, - .) fall, with snow, at orenburg, russia, dec. , , of a multitude of small, black insects, said to have been gnats, but also said to have had flea-like motions. (_amer. jour. sci._, - - .) large number of worms found in a snowstorm, upon the surface of snow about four inches thick, near sangerfield, n.y., nov. , (_scientific american_, - ). the writer thinks that the worms had been brought to the surface of the ground by rain, which had fallen previously. _scientific american_, feb. , : "a puzzling phenomenon has been noted frequently in some parts of the valley bend district, randolph county, va., this winter. the crust of the snow has been covered two or three times with worms resembling the ordinary cut worms. where they come from, unless they fall with the snow is inexplicable." in the _scientific american_, march , , the editor says that similar worms had been seen upon the snow near utica, n.y., and in oneida and herkimer counties; that some of the worms had been sent to the department of agriculture at washington. again two species, or polymorphism. according to prof. riley, it was not polymorphism, "but two distinct species"--which, because of our data, we doubt. one kind was larger than the other: color-differences not distinctly stated. one is called the larvae of the common soldier beetle and the other "seems to be a variety of the bronze cut worm." no attempt to explain the occurrence in snow. fall of great numbers of larvae of beetles, near mortagne, france, may, . the larvae were inanimate as if with cold. (_annales société entomologique de france_, .) _trans. ent. soc. of london_, - , records "snowing of larvae," in silesia, ; "appearance of many larvae on the snow," in saxony, ; "larvae found alive on the snow," ; larvae and snow which "fell together," in the eifel, jan. , ; "fall of insects," jan. , , in lithuania; occurrence of larvae estimated at , on the snow in switzerland, in . the compiler says that most of these larvae live underground, or at the roots of trees; that whirlwinds uproot trees, and carry away the larvae--conceiving of them as not held in masses of frozen earth--all as neatly detachable as currants in something. in the _revue et magasin de zoologie_, - , there is an account of the fall in lithuania, jan. , --that black larvae had fallen in enormous numbers. larvae thought to have been of beetles, but described as "caterpillars," not seen to fall, but found crawling on the snow, after a snowstorm, at warsaw, jan. , . (_all the year round_, - .) flammarion (_the atmosphere_, p. ) tells of a fall of larvae that occurred jan. , , in a snowstorm, in upper savoy: "they could not have been hatched in the neighborhood, for, during the days preceding, the temperature had been very low"; said to have been of a species common in the south of france. in _la science pour tous_, - , it is said that with these larvae there were developed insects. _l'astronomie_, - : that, upon the last of january, , there fell, in a great tempest, in switzerland, incalculable numbers of larvae: some black and some yellow; numbers so great that hosts of birds were attracted. altogether we regard this as one of our neatest expressions for external origins and against the whirlwind explanation. if an exclusionist says that, in january, larvae were precisely and painstakingly picked out of frozen ground, in incalculable numbers, he thinks of a tremendous force--disregarding its refinements: then if origin and precipitation be not far apart, what becomes of an infinitude of other débris, conceiving of no time for segregation? if he thinks of a long translation--all the way from the south of france to upper savoy, he may think then of a very fine sorting over by differences of specific gravity--but in such a fine selection, larvae would be separated from developed insects. as to differences in specific gravity--the yellow larvae that fell in switzerland january, , were three times the size of the black larvae that fell with them. in accounts of this occurrence, there is no denial of the fall. or that a whirlwind never brought them together and held them together and precipitated them and only them together-- that they came from genesistrine. there's no escape from it. we'll be persecuted for it. take it or leave it-- genesistrine. the notion is that there is somewhere aloft a place of origin of life relatively to this earth. whether it's the planet genesistrine, or the moon, or a vast amorphous region super-jacent to this earth, or an island in the super-sargasso sea, should perhaps be left to the researches of other super--or extra--geographers. that the first unicellular organisms may have come here from genesistrine--or that men or anthropomorphic beings may have come here before amoebae: that, upon genesistrine, there may have been an evolution expressible in conventional biologic terms, but that evolution upon this earth has been--like evolution in modern japan--induced by external influences; that evolution, as a whole, upon this earth, has been a process of population by immigration or by bombardment. some notes i have upon remains of men and animals encysted, or covered with clay or stone, as if fired here as projectiles, i omit now, because it seems best to regard the whole phenomenon as a tropism--as a geotropism--probably atavistic, or vestigial, as it were, or something still continuing long after expiration of necessity; that, once upon a time, all kinds of things came here from genesistrine, but that now only a few kinds of bugs and things, at long intervals, feel the inspiration. not one instance have we of tadpoles that have fallen to this earth. it seems reasonable that a whirlwind could scoop up a pond, frogs and all, and cast down the frogs somewhere else: but, then, more reasonable that a whirlwind could scoop up a pond, tadpoles and all--because tadpoles are more numerous in their season than are the frogs in theirs: but the tadpole-season is earlier in the spring, or in a time that is more tempestuous. thinking in terms of causation--as if there were real causes--our notion is that, if x is likely to cause y, but is more likely to cause z, but does not cause z, x is not the cause of y. upon this quasi-sorites, we base our acceptance that the little frogs that have fallen to this earth are not products of whirlwinds: that they came from externality, or from genesistrine. i think of genesistrine in terms of biologic mechanics: not that somewhere there are persons who collect bugs in or about the last of january and frogs in july and august, and bombard this earth, any more than do persons go through northern regions, catching and collecting birds, every autumn, then casting them southward. but atavistic, or vestigial, geotropism in genesistrine--or a million larvae start crawling, and a million little frogs start hopping--knowing no more what it's all about than we do when we crawl to work in the morning and hop away at night. i should say, myself, that genesistrine is a region in the super-sargasso sea, and that parts of the super-sargasso sea have rhythms of susceptibility to this earth's attraction. i accept that, when there are storms, the damnedest of excluded, excommunicated things--things that are leprous to the faithful--are brought down--from the super-sargasso sea--or from what for convenience we call the super-sargasso sea--which by no means has been taken into full acceptance yet. that things are brought down by storms, just as, from the depths of the sea things are brought up by storms. to be sure it is orthodoxy that storms have little, if any, effect below the waves of the ocean--but--of course--only to have an opinion is to be ignorant of, or to disregard a contradiction, or something else that modifies an opinion out of distinguishability. _symons' meteorological magazine_, - : that, along the coast of new zealand, in regions not subject to submarine volcanic action, deep-sea fishes are often brought up by storms. iron and stones that fall from the sky; and atmospheric disturbances: "there is absolutely no connection between the two phenomena." (_symons._) the orthodox belief is that objects moving at planetary velocity would, upon entering this earth's atmosphere, be virtually unaffected by hurricanes; might as well think of a bullet swerved by someone fanning himself. the only trouble with the orthodox reasoning is the usual trouble--its phantom-dominant--its basing upon a myth--data we've had, and more we'll have, of things in the sky having no independent velocity. there are so many storms and so many meteors and meteorites that it would be extraordinary if there were no concurrences. nevertheless so many of these concurrences are listed by prof. baden-powell (_rept. brit. assoc._, - ) that one--notices. see _rept. brit. assoc._, --other instances. the famous fall of stones at siena, italy, --"in a violent storm." see _greg's catalogues_--many instances. one that stands out is--"bright ball of fire and light in a hurricane in england, sept. , ." the remarkable datum here is that this phenomenon was visible forty minutes. that's about times the duration that the orthodox give to meteors and meteorites. see the _annual register_--many instances. in _nature_, oct. , , and the london _times_, oct. , , something that fell in a gale of oct. , , is described as a "huge ball of green fire." this phenomenon is described by another correspondent, in _nature_, - , and an account of it by another correspondent was forwarded to _nature_ by w.f. denning. there are so many instances that some of us will revolt against the insistence of the faithful that it is only coincidence, and accept that there is connection of the kind called causal. if it is too difficult to think of stones and metallic masses swerved from their courses by storms, if they move at high velocity, we think of low velocity, or of things having no velocity at all, hovering a few miles above this earth, dislodged by storms, and falling luminously. but the resistance is so great here, and "coincidence" so insisted upon that we'd better have some more instances: aerolite in a storm at st. leonards-on-sea, england, sept. , --no trace of it found (_annual register_, ); meteorite in a gale, march , , described in the _monthly weather review_, march, ; meteorite in a thunderstorm, off coast of greece, nov. , (_nature_, - ); fall of a meteorite in a storm, july , , near lachine, quebec (_monthly weather review_, july, ); same phenomenon noted in _nature_, - ; meteorite in a whirlwind, sweden, sept. , (_nature_, - ). _london roy. soc. proc._, - : a triangular cloud that appeared in a storm, dec. , ; a red nucleus, about half the apparent diameter of the moon, and a long tail; visible minutes; explosion of the nucleus. nevertheless, in _science gossip_, n.s., - , it is said that, though meteorites have fallen in storms, no connection is supposed to exist between the two phenomena, except by the ignorant peasantry. but some of us peasants have gone through the _report of the british association_, . upon page , dr. buist, who had never heard of the super-sargasso sea, says that, though it is difficult to trace connection between the phenomena, three aerolites had fallen in five months, in india, during thunderstorms, in (may have been ). for accounts by witnesses, see page of the _report_. or--we are on our way to account for "thunderstones." it seems to me that, very strikingly here, is borne out the general acceptance that ours is only an intermediate existence, in which there is nothing fundamental, or nothing final to take as a positive standard to judge by. peasants believed in meteorites. scientists excluded meteorites. peasants believe in "thunderstones." scientists exclude "thunderstones." it is useless to argue that peasants are out in the fields, and that scientists are shut up in laboratories and lecture rooms. we cannot take for a real base that, as to phenomena with which they are more familiar, peasants are more likely to be right than are scientists: a host of biologic and meteorologic fallacies of peasants rises against us. i should say that our "existence" is like a bridge--except that that comparison is in static terms--but like the brooklyn bridge, upon which multitudes of bugs are seeking a fundamental--coming to a girder that seems firm and final--but the girder is built upon supports. a support then seems final. but it is built upon underlying structures. nothing final can be found in all the bridge, because the bridge itself is not a final thing in itself, but is a relationship between manhattan and brooklyn. if our "existence" is a relationship between the positive absolute and the negative absolute, the quest for finality in it is hopeless: everything in it must be relative, if the "whole" is not a whole, but is, itself, a relation. in the attitude of acceptance, our pseudo-base is: cells of an embryo are in the reptilian era of the embryo; some cells feel stimuli to take on new appearances. if it be of the design of the whole that the next era be mammalian, those cells that turn mammalian will be sustained against resistance, by inertia, of all the rest, and will be relatively right, though not finally right, because they, too, in time will have to give way to characters of other eras of higher development. if we are upon the verge of a new era, in which exclusionism must be overthrown, it will avail thee not to call us base-born and frowsy peasants. in our crude, bucolic way, we now offer an outrage upon common sense that we think will some day be an unquestioned commonplace: that manufactured objects of stone and iron have fallen from the sky: that they have been brought down from a state of suspension, in a region of inertness to this earth's attraction, by atmospheric disturbances. the "thunderstone" is usually "a beautifully polished, wedge-shaped piece of greenstone," says a writer in the _cornhill magazine_, - . it isn't: it's likely to be of almost any kind of stone, but we call attention to the skill with which some of them have been made. of course this writer says it's all superstition. otherwise he'd be one of us crude and simple sons of the soil. conventional damnation is that stone implements, already on the ground--"on the ground in the first place"--are found near where lightning was seen to strike: that are supposed by astonished rustics, or by intelligence of a low order, to have fallen in or with lightning. throughout this book, we class a great deal of science with bad fiction. when is fiction bad, cheap, low? if coincidence is overworked. that's one way of deciding. but with single writers coincidence seldom is overworked: we find the excess in the subject at large. such a writer as the one of the _cornhill magazine_ tells us vaguely of beliefs of peasants: there is no massing of instance after instance after instance. here ours will be the method of mass-formation. conceivably lightning may strike the ground near where there was a wedge-shaped object in the first place: again and again and again: lightning striking ground near wedge-shaped object in china; lightning striking ground near wedge-shaped object in scotland; lightning striking ground near wedge-shaped object in central africa: coincidence in france; coincidence in java; coincidence in south america-- we grant a great deal but note a tendency to restlessness. nevertheless this is the psycho-tropism of science to all "thunderstones" said to have fallen luminously. as to greenstone, it is in the island of jamaica, where the notion is general that axes of a hard greenstone fall from the sky--"during the rains." (_jour. inst. jamaica_, - .) some other time we shall inquire into this localization of objects of a specific material. "they are of a stone nowhere else to be found in jamaica." (_notes and queries_, - - .) in my own tendency to exclude, or in the attitude of one peasant or savage who thinks he is not to be classed with other peasants or savages, i am not very much impressed with what natives think. it would be hard to tell why. if the word of a lord kelvin carries no more weight, upon scientific subjects, than the word of a sitting bull, unless it be in agreement with conventional opinion--i think it must be because savages have bad table manners. however, my snobbishness, in this respect, loosens up somewhat before very widespread belief by savages and peasants. and the notion of "thunderstones" is as wide as geography itself. the natives of burma, china, japan, according to blinkenberg (_thunder weapons_, p. )--not, of course, that blinkenberg accepts one word of it--think that carved stone objects have fallen from the sky, because they think they have seen such objects fall from the sky. such objects are called "thunderbolts" in these countries. they are called "thunderstones" in moravia, holland, belgium, france, cambodia, sumatra, and siberia. they're called "storm stones" in lausitz; "sky arrows" in slavonia; "thunder axes" in england and scotland; "lightning stones" in spain and portugal; "sky axes" in greece; "lightning flashes" in brazil; "thunder teeth" in amboina. the belief is as widespread as is belief in ghosts and witches, which only the superstitious deny today. as to beliefs by north american indians, tyler gives a list of references (_primitive culture_, - ). as to south american indians--"certain stone hatchets are said to have fallen from the heavens." (_jour. amer. folk lore_, - .) if you, too, revolt against coincidence after coincidence after coincidence, but find our interpretation of "thunderstones" just a little too strong or rich for digestion, we recommend the explanation of one, tallius, written in : "the naturalists say they are generated in the sky by fulgurous exhalation conglobed in a cloud by the circumfused humor." of course the paper in the _cornhill magazine_ was written with no intention of trying really to investigate this subject, but to deride the notion that worked-stone objects have ever fallen from the sky. a writer in the _amer. jour. sci._, - - , read this paper and thinks it remarkable "that any man of ordinary reasoning powers should write a paper to prove that thunderbolts do not exist." i confess that we're a little flattered by that. over and over: "it is scarcely necessary to suggest to the intelligent reader that thunderstones are a myth." we contend that there is a misuse of a word here: we admit that only we are intelligent upon this subject, if by intelligence is meant the inquiry of inequilibrium, and that all other intellection is only mechanical reflex--of course that intelligence, too, is mechanical, but less orderly and confined: less obviously mechanical--that as an acceptance of ours becomes firmer and firmer-established, we pass from the state of intelligence to reflexes in ruts. an odd thing is that intelligence is usually supposed to be creditable. it may be in the sense that it is mental activity trying to find out, but it is confession of ignorance. the bees, the theologians, the dogmatic scientists are the intellectual aristocrats. the rest of us are plebeians, not yet graduated to nirvana, or to the instinctive and suave as differentiated from the intelligent and crude. blinkenberg gives many instances of the superstition of "thunderstones" which flourishes only where mentality is in a lamentable state--or universally. in malacca, sumatra, and java, natives say that stone axes have often been found under trees that have been struck by lightning. blinkenberg does not dispute this, but says it is coincidence: that the axes were of course upon the ground in the first place: that the natives jumped to the conclusion that these carved stones had fallen in or with lightning. in central africa, it is said that often have wedge-shaped, highly polished objects of stone, described as "axes," been found sticking in trees that have been struck by lightning--or by what seemed to be lightning. the natives, rather like the unscientific persons of memphis, tenn., when they saw snakes after a storm, jumped to the conclusion that the "axes" had not always been sticking in the trees. livingstone (_last journal_, pages , , , ) says that he had never heard of stone implements used by natives of africa. a writer in the _report of the smithsonian institution_, - , says that there are a few. that they are said, by the natives, to have fallen in thunderstorms. as to luminosity, it is my lamentable acceptance that bodies falling through this earth's atmosphere, if not warmed even, often fall with a brilliant light, looking like flashes of lightning. this matter seems important: we'll take it up later, with data. in prussia, two stone axes were found in the trunks of trees, one under the bark. (blinkenberg, _thunder weapons_, p. .) the finders jumped to the conclusion that the axes had fallen there. another stone ax--or wedge-shaped object of worked stone--said to have been found in a tree that had been struck by something that looked like lightning. (_thunder weapons_, p. .) the finder jumped to the conclusion. story told by blinkenberg, of a woman, who lived near kulsbjaergene, sweden, who found a flint near an old willow--"near her house." i emphasize "near her house" because that means familiar ground. the willow had been split by something. she jumped. cow killed by lightning, or by what looked like lightning (isle of sark, near guernsey). the peasant who owned the cow dug up the ground at the spot and found a small greenstone "ax." blinkenberg says that he jumped to the conclusion that it was this object that had fallen luminously, killing the cow. _reliquary_, - : a flint ax found by a farmer, after a severe storm--described as a "fearful storm"--by a signal staff, which had been split by something. i should say that nearness to a signal staff may be considered familiar ground. whether he jumped, or arrived at the conclusion by a more leisurely process, the farmer thought that the flint object had fallen in the storm. in this instance we have a lamentable scientist with us. it's impossible to have positive difference between orthodoxy and heresy: somewhere there must be a merging into each other, or an overlapping. nevertheless, upon such a subject as this, it does seem a little shocking. in most works upon meteorites, the peculiar, sulphurous odor of things that fall from the sky is mentioned. sir john evans (_stone implements_, p. ) says--with extraordinary reasoning powers, if he could never have thought such a thing with ordinary reasoning powers--that this flint object "proved to have been the bolt, by its peculiar smell when broken." if it did so prove to be, that settles the whole subject. if we prove that only one object of worked stone has fallen from the sky, all piling up of further reports is unnecessary. however, we have already taken the stand that nothing settles anything; that the disputes of ancient greece are no nearer solution now than they were several thousand years ago--all because, in a positive sense, there is nothing to prove or solve or settle. our object is to be more nearly real than our opponents. wideness is an aspect of the universal. we go on widely. according to us the fat man is nearer godliness than is the thin man. eat, drink, and approximate to the positive absolute. beware of negativeness, by which we mean indigestion. the vast majority of "thunderstones" are described as "axes," but meunier (_la nature_, - - ) tells of one that was in his possession; said to have fallen at ghardia, algeria, contrasting "profoundment" (pear-shaped) with the angular outlines of ordinary meteorites. the conventional explanation that it had been formed as a drop of molten matter from a larger body seems reasonable to me; but with less agreeableness i note its fall in a thunderstorm, the datum that turns the orthodox meteorologist pale with rage, or induces a slight elevation of his eyebrows, if you mention it to him. meunier tells of another "thunderstone" said to have fallen in north africa. meunier, too, is a little lamentable here: he quotes a soldier of experience that such objects fall most frequently in the deserts of africa. rather miscellaneous now: "thunderstone" said to have fallen in london, april, : weight about pounds: no particulars as to shape (timb's _year book_, - ). "thunderstone" said to have fallen at cardiff, sept. , (london _times_, sept. , ). according to _nature_, - , it was coincidence; only a lightning flash had been seen. stone that fell in a storm, near st. albans, england: accepted by the museum of st. albans; said, at the british museum, not to be of "true meteoritic material." (_nature_, - .) london _times_, april , : that, april , , near wolverhampton, fell a mass of meteoritic iron during a heavy fall of rain. an account of this phenomenon in _nature_, - , by h.s. maskelyne, who accepts it as authentic. also, see _nature_, - . for three other instances, see the _scientific american_, - ; - ; - . as to wedge-shape larger than could very well be called an "ax": _nature_, - : that, may , , at tysnas, norway, a meteorite had fallen: that the turf was torn up at the spot where the object had been supposed to have fallen; that two days later "a very peculiar stone" was found near by. the description is--"in shape and size very like the fourth part of a large stilton cheese." it is our acceptance that many objects and different substances have been brought down by atmospheric disturbance from what--only as a matter of convenience now, and until we have more data--we call the super-sargasso sea; however, our chief interest is in objects that have been shaped by means similar to human handicraft. description of the "thunderstones" of burma (_proc. asiatic soc. of bengal_, - ): said to be of a kind of stone unlike any other found in burma; called "thunderbolts" by the natives. i think there's a good deal of meaning in such expressions as "unlike any other found in burma"--but that if they had said anything more definite, there would have been unpleasant consequences to writers in the th century. more about the "thunderstones" of burma, in the _proc. soc. antiq. of london_, - - . one of them, described as an "adze," was exhibited by captain duff, who wrote that there was no stone like it in its neighborhood. of course it may not be very convincing to say that because a stone is unlike neighboring stones it had foreign origin--also we fear it is a kind of plagiarism: we got it from the geologists, who demonstrate by this reasoning the foreign origin of erratics. we fear we're a little gross and scientific at times. but it's my acceptance that a great deal of scientific literature must be read between the lines. it's not everyone who has the lamentableness of a sir john evans. just as a great deal of voltaire's meaning was inter-linear, we suspect that a captain duff merely hints rather than to risk having a prof. lawrence smith fly at him and call him "a half-insane man." whatever captain duff's meaning may have been, and whether he smiled like a voltaire when he wrote it, captain duff writes of "the extremely soft nature of the stone, rendering it equally useless as an offensive or defensive weapon." story, by a correspondent, in _nature_, - , of a malay, of "considerable social standing"--and one thing about our data is that, damned though they be, they do so often bring us into awful good company--who knew of a tree that had been struck, about a month before, by something in a thunderstorm. he searched among the roots of this tree and found a "thunderstone." not said whether he jumped or leaped to the conclusion that it had fallen: process likely to be more leisurely in tropical countries. also i'm afraid his way of reasoning was not very original: just so were fragments of the bath-furnace meteorite, accepted by orthodoxy, discovered. we shall now have an unusual experience. we shall read of some reports of extraordinary circumstances that were investigated by a man of science--not of course that they were really investigated by him, but that his phenomena occupied a position approximating higher to real investigation than to utter neglect. over and over we read of extraordinary occurrences--no discussion; not even a comment afterward findable; mere mention occasionally--burial and damnation. the extraordinary and how quickly it is hidden away. burial and damnation, or the obscurity of the conspicuous. we did read of a man who, in the matter of snails, did travel some distance to assure himself of something that he had suspected in advance; and we remember prof. hitchcock, who had only to smite amherst with the wand of his botanical knowledge, and lo! two fungi sprang up before night; and we did read of dr. gray and his thousands of fishes from one pailful of water--but these instances stand out; more frequently there was no "investigation." we now have a good many reported occurrences that were "investigated." of things said to have fallen from the sky, we make, in the usual scientific way, two divisions: miscellaneous objects and substances, and symmetric objects attributable to beings like human beings, sub-dividing into--wedges, spheres, and disks. _jour. roy. met. soc._, - : that, july , , a correspondent to a london newspaper wrote that something had fallen from the sky, during a thunderstorm of june , , at netting hill. mr. g.t. symons, of _symons' meteorological magazine_, investigated, about as fairly, and with about as unprejudiced a mind, as anything ever has been investigated. he says that the object was nothing but a lump of coal: that next door to the home of the correspondent coal had been unloaded the day before. with the uncanny wisdom of the stranger upon unfamiliar ground that we have noted before, mr. symons saw that the coal reported to have fallen from the sky, and the coal unloaded more prosaically the day before, were identical. persons in the neighborhood, unable to make this simple identification, had bought from the correspondent pieces of the object reported to have fallen from the sky. as to credulity, i know of no limits for it--but when it comes to paying out money for credulity--oh, no standards to judge by, of course--just the same-- the trouble with efficiency is that it will merge away into excess. with what seems to me to be super-abundance of convincingness, mr. symons then lugs another character into his little comedy: that it was all a hoax by a chemist's pupil, who had filled a capsule with an explosive, and "during the storm had thrown the burning mass into the gutter, so making an artificial thunderbolt." or even shakespeare, with all his inartistry, did not lug in king lear to make hamlet complete. whether i'm lugging in something that has no special meaning, myself, or not, i find that this storm of june , , was peculiar. it is described in the london _times_, july , : that "during the storm, the sky in many places remained partially clear while hail and rain were falling." that may have more meaning when we take up the possible extra-mundane origin of some hailstones, especially if they fall from a cloudless sky. mere suggestion, not worth much, that there may have been falls of extra-mundane substances, in london, june , . clinkers, said to have fallen, during a storm, at kilburn, july , : according to the _kilburn times_, july , , quoted by mr. symons, a street had been "literally strewn," during the storm, with a mass of clinkers, estimated at about two bushels: sizes from that of a walnut to that of a man's hand--"pieces of the clinkers can be seen at the _kilburn times_ office." if these clinkers, or cinders, were refuse from one of the super-mercantile constructions from which coke and coal and ashes occasionally fall to this earth, or, rather, to the super-sargasso sea, from which dislodgment by tempests occurs, it is intermediatistic to accept that they must merge away somewhere with local phenomena of the scene of precipitation. if a red-hot stove should drop from a cloud into broadway, someone would find that at about the time of the occurrence, a moving van had passed, and that the moving men had tired of the stove, or something--that it had not been really red-hot, but had been rouged instead of blacked, by some absent-minded housekeeper. compared with some of the scientific explanations that we have encountered, there's considerable restraint, i think, in that one. mr. symons learned that in the same street--he emphasizes that it was a short street--there was a fire-engine station. i had such an impression of him hustling and bustling around at notting hill, searching cellars until he found one with newly arrived coal in it; ringing door bells, exciting a whole neighborhood, calling up to second-story windows, stopping people in the streets, hotter and hotter on the trail of a wretched imposter of a chemist's pupil. after his efficiency at notting hill, we'd expect to hear that he went to the station, and--something like this: "it is said that clinkers fell, in your street, at about ten minutes past four o'clock, afternoon of july fifth. will you look over your records and tell me where your engine was at about ten minutes past four, july fifth?" mr. symons says: "i think that most probably they had been raked out of the steam fire-engine." june , , it was reported that a "thunderstone" had struck the house at oakley street, chelsea, falling down the chimney, into the kitchen grate. mr. symons investigated. he describes the "thunderstone" as an "agglomeration of brick, soot, unburned coal, and cinder." he says that, in his opinion, lightning had flashed down the chimney, and had fused some of the brick of it. he does think it remarkable that the lightning did not then scatter the contents of the grate, which were disturbed only as if a heavy body had fallen. if we admit that climbing up the chimney to find out is too rigorous a requirement for a man who may have been large, dignified and subject to expansions, the only unreasonableness we find in what he says--as judged by our more modern outlook, is: "i suppose that no one would suggest that bricks are manufactured in the atmosphere." sounds a little unreasonable to us, because it is so of the positivistic spirit of former times, when it was not so obvious that the highest incredibility and laughability must merge away with the "proper"--as the _sci. am. sup._ would say. the preposterous is always interpretable in terms of the "proper," with which it must be continuous--or--clay-like masses such as have fallen from the sky--tremendous heat generated by their velocity--they bake--bricks. we begin to suspect that mr. symons exhausted himself at notting hill. it's a warning to efficiency-fanatics. then the instance of three lumps of earthy matter, found upon a well-frequented path, after a thunderstorm, at reading, july , . there are so many records of the fall of earthy matter from the sky that it would seem almost uncanny to find resistance here, were we not so accustomed to the uncompromising stands of orthodoxy--which, in our metaphysics, represent good, as attempts, but evil in their insufficiency. if i thought it necessary, i'd list one hundred and fifty instances of earthy matter said to have fallen from the sky. it is his antagonism to atmospheric disturbance associated with the fall of things from the sky that blinds and hypnotizes a mr. symons here. this especial mr. symons rejects the reading substance because it was not "of true meteoritic material." it's uncanny--or it's not uncanny at all, but universal--if you don't take something for a standard of opinion, you can't have any opinion at all: but, if you do take a standard, in some of its applications it must be preposterous. the carbonaceous meteorites, which are unquestioned--though avoided, as we have seen--by orthodoxy, are more glaringly of untrue meteoritic material than was this substance of reading. mr. symons says that these three lumps were upon the ground "in the first place." whether these data are worth preserving or not, i think that the appeal that this especial mr. symons makes is worthy of a place in the museum we're writing. he argues against belief in all external origins "for our credit as englishmen." he is a patriot, but i think that these foreigners had a small chance "in the first place" for hospitality from him. then comes a "small lump of iron (two inches in diameter)" said to have fallen, during a thunderstorm, at brixton, aug. , . mr. symons says: "at present i cannot trace it." he was at his best at notting hill: there's been a marked falling off in his later manner: in the london _times_, feb. , , it is said that a roundish object of iron had been found, "after a violent thunderstorm," in a garden at brixton, aug. , . it was analyzed by a chemist, who could not identify it as true meteoritic material. whether a product of workmanship like human workmanship or not, this object is described as an oblate spheroid, about two inches across its major diameter. the chemist's name and address are given: mr. j. james morgan: ebbw vale. garden--familiar ground--i suppose that in mr. symons' opinion this symmetric object had been upon the ground "in the first place," though he neglects to say this. but we do note that he described this object as a "lump," which does not suggest the spheroidal or symmetric. it is our notion that the word "lump" was, because of its meaning of amorphousness, used purposely to have the next datum stand alone, remote, without similars. if mr. symons had said that there had been a report of another round object that had fallen from the sky, his readers would be attracted by an agreement. he distracts his readers by describing in terms of the unprecedented-- "iron cannon ball." it was found in a manure heap, in sussex, after a thunderstorm. however, mr. symons argues pretty reasonably, it seems to me, that, given a cannon ball in a manure heap, in the first place, lightning might be attracted by it, and, if seen to strike there, the untutored mind, or mentality below the average, would leap or jump, or proceed with less celerity, to the conclusion that the iron object had fallen. except that--if every farmer isn't upon very familiar ground--or if every farmer doesn't know his own manure heap as well as mr. symons knew his writing desk-- then comes the instance of a man, his wife, and his three daughters, at casterton, westmoreland, who were looking out at their lawn, during a thunderstorm, when they "considered," as mr. symons expresses it, that they saw a stone fall from the sky, kill a sheep, and bury itself in the ground. they dug. they found a stone ball. symons: coincidence. it had been there in the first place. this object was exhibited at a meeting of the royal meteorological society by mr. c. carus-wilson. it is described in the _journal's_ list of exhibits as a "sandstone" ball. it is described as "sandstone" by mr. symons. now a round piece of sandstone may be almost anywhere in the ground--in the first place--but, by our more or less discreditable habit of prying and snooping, we find that this object was rather more complex and of material less commonplace. in snooping through _knowledge_, oct. , , we read that this "thunderstone" was in the possession of mr. c. carus-wilson, who tells the story of the witness and his family--the sheep killed, the burial of something in the earth, the digging, and the finding. mr. c. carus-wilson describes the object as a ball of hard, ferruginous quartzite, about the size of a cocoanut, weight about twelve pounds. whether we're feeling around for significance or not, there is a suggestion not only of symmetry but of structure in this object: it had an external shell, separated from a loose nucleus. mr. carus-wilson attributes this cleavage to unequal cooling of the mass. my own notion is that there is very little deliberate misrepresentation in the writings of scientific men: that they are quite as guiltless in intent as are other hypnotic subjects. such a victim of induced belief reads of a stone ball said to have fallen from the sky. mechanically in his mind arise impressions of globular lumps, or nodules, of sandstone, which are common almost everywhere. he assimilates the reported fall with his impressions of objects in the ground, in the first place. to an intermediatist, the phenomena of intellection are only phenomena of universal process localized in human minds. the process called "explanation" is only a local aspect of universal assimilation. it looks like materialism: but the intermediatist holds that interpretation of the immaterial, as it is called, in terms of the material, as it is called, is no more rational than interpretation of the "material" in terms of the "immaterial": that there is in quasi-existence neither the material nor the immaterial, but approximations one way or the other. but so hypnotic quasi-reasons: that globular lumps of sandstone are common. whether he jumps or leaps, or whether only the frowsy and base-born are so athletic, his is the impression, by assimilation, that this especial object is a ball of sandstone. or human mentality: its inhabitants are conveniences. it may be that mr. symons' paper was written before this object was exhibited to the members of the society, and with the charity with which, for the sake of diversity, we intersperse our malices, we are willing to accept that he "investigated" something that he had never seen. but whoever listed this object was uncareful: it is listed as "sandstone." we're making excuses for them. really--as it were--you know, we're not quite so damned as we were. one does not apologize for the gods and at the same time feel quite utterly prostrate before them. if this were a real existence, and all of us real persons, with real standards to judge by, i'm afraid we'd have to be a little severe with some of these mr. symonses. as it is, of course, seriousness seems out of place. we note an amusing little touch in the indefinite allusion to "a man," who with his un-named family, had "considered" that he had seen a stone fall. the "man" was the rev. w. carus-wilson, who was well-known in his day. the next instance was reported by w.b. tripp, f.r.m.s.--that, during a thunderstorm, a farmer had seen the ground in front of him plowed up by something that was luminous. dug. bronze ax. my own notion is that an expedition to the north pole could not be so urgent as that representative scientists should have gone to that farmer and there spent a summer studying this one reported occurrence. as it is--un-named farmer--somewhere--no date. the thing must stay damned. another specimen for our museum is a comment in _nature_ upon these objects: that they are "of an amusing character, thus clearly showing that they were of terrestrial, and not a celestial, character." just why celestiality, or that of it which, too, is only of intermediateness should not be quite as amusing as terrestriality is beyond our reasoning powers, which we have agreed are not ordinary. of course there is nothing amusing about wedges and spheres at all--or archimedes and euclid are humorists. it is that they were described derisively. if you'd like a little specimen of the standardization of orthodox opinion-- _amer. met. jour._, - : "they are of an amusing character, thus clearly showing that they were of a terrestrial and not a celestial character." i'm sure--not positively, of course--that we've tried to be as easygoing and lenient with mr. symons as his obviously scientific performance would permit. of course it may be that sub-consciously we were prejudiced against him, instinctively classing him with st. augustine, darwin, st. jerome, and lyell. as to the "thunderstones," i think that he investigated them mostly "for the credit of englishmen," or in the spirit of the royal krakatoa committee, or about as the commission from the french academy investigated meteorites. according to a writer in _knowledge_, - , the krakatoa committee attempted not in the least to prove what had caused the atmospheric effects of , but to prove--that krakatoa did it. altogether i should think that the following quotation should be enlightening to anyone who still thinks that these occurrences were investigated not to support an opinion formed in advance: in opening his paper, mr. symons says that he undertook his investigation as to the existence of "thunderstones," or "thunderbolts" as he calls them--"feeling certain that there was a weak point somewhere, inasmuch as 'thunderbolts' have no existence." we have another instance of the reported fall of a "cannon ball." it occurred prior to mr. symons' investigations, but is not mentioned by him. it was investigated, however. in the _proc. roy. soc. edin._, - , is the report of a "thunderstone," "supposed to have fallen in hampshire, sept., ." it was an iron cannon ball, or it was a "large nodule of iron pyrites or bisulphuret of iron." no one had seen it fall. it had been noticed, upon a garden path, for the first time, after a thunderstorm. it was only a "supposed" thing, because--"it had not the character of any known meteorite." in the london _times_, sept. , , appears a letter from mr. george e. bailey, a chemist of andover, hants. he says that, in a very heavy thunderstorm, of the first week of september, , this iron object, had fallen in the garden of mr. robert dowling, of andover; that it had fallen upon a path "within six yards of the house." it had been picked up "immediately" after the storm by mrs. dowling. it was about the size of a cricket ball: weight four pounds. no one had seen it fall. in the _times_, sept. , , there is an account of this thunderstorm, which was of unusual violence. there are some other data relative to the ball of quartz of westmoreland. they're poor things. there's so little to them that they look like ghosts of the damned. however, ghosts, when multiplied, take on what is called substantiality--if the solidest thing conceivable, in quasi-existence, is only concentrated phantomosity. it is not only that there have been other reports of quartz that has fallen from the sky; there is another agreement. the round quartz object of westmoreland, if broken open and separated from its loose nucleus, would be a round, hollow, quartz object. my pseudo-position is that two reports of similar extraordinary occurrences, one from england and one from canada--are interesting. _proc. canadian institute_, - - : that, at the meeting of the institute, of dec. , , one of the members, mr. j.a. livingstone, exhibited a globular quartz body which he asserted had fallen from the sky. it had been split open. it was hollow. but the other members of the institute decided that the object was spurious, because it was not of "true meteoritic material." no date; no place mentioned; we note the suggestion that it was only a geode, which had been upon the ground in the first place. its crystalline lining was geode-like. quartz is upon the "index prohibitory" of science. a monk who would read darwin would sin no more than would a scientist who would admit that, except by the "up and down" process, quartz has ever fallen from the sky--but continuity: it is not excommunicated if part of or incorporated in a baptized meteorite--st. catherine's of mexico, i think. it's as epicurean a distinction as any ever made by theologians. fassig lists a quartz pebble, found in a hailstone (_bibliography_, part - ). "up and down," of course. another object of quartzite was reported to have fallen, in the autumn of , at schroon lake, n.y.--said in the _scientific american_, - to be a fraud--it was not--the usual. about the first of may, , the newspapers published a story of a "snow-white" meteorite that had fallen, at vincennes, indiana. the editor of the _monthly weather review_ (issue of april, ) requested the local observer, at vincennes, to investigate. the editor says that the thing was only a fragment of a quartz boulder. he says that anyone with at least a public school education should know better than to write that quartz has ever fallen from the sky. _notes and queries_, - - : that, in the leyden museum of antiquities, there is a disk of quartz: centimeters by millimeters by about centimeters; said to have fallen upon a plantation in the dutch west indies, after a meteoric explosion. bricks. i think this is a vice we're writing. i recommend it to those who have hankered for a new sin. at first some of our data were of so frightful or ridiculous mien as to be hated, or eyebrowed, was only to be seen. then some pity crept in? i think that we can now embrace bricks. the baked-clay-idea was all right in its place, but it rather lacks distinction, i think. with our minds upon the concrete boats that have been building terrestrially lately, and thinking of wrecks that may occur to some of them, and of a new material for the deep-sea fishes to disregard-- object that fell at richland, south carolina--yellow to gray--said to look like a piece of brick. (_amer. jour. sci._, - - .) pieces of "furnace-made brick" said to have fallen--in a hailstorm--at padua, august, . (_edin. new phil. jour._, - .) the writer offered an explanation that started another convention: that the fragments of brick had been knocked from buildings by the hailstones. but there is here a concomitant that will be disagreeable to anyone who may have been inclined to smile at the now digestible--enough notion that furnace-made bricks have fallen from the sky. it is that in some of the hailstones--two per cent of them--that were found with the pieces of brick, was a light grayish powder. _monthly notices of the royal astronomical society_, - : padre sechi explains that a stone said to have fallen, in a thunderstorm, at supino, italy, september, , had been knocked from a roof. _nature_, - : that it had been reported that a good-sized stone, of form clearly artificial, had fallen at naples, november, . the stone was described by two professors of naples, who had accepted it as inexplicable but veritable. they were visited by dr. h. johnstone-lavis, the correspondent to _nature_, whose investigations had convinced him that the object was a "shoemaker's lapstone." now to us of the initiated, or to us of the wider outlook, there is nothing incredible in the thought of shoemakers in other worlds--but i suspect that this characterization is tactical. this object of worked stone, or this shoemaker's lapstone, was made of vesuvian lava, dr. johnstone-lavis thinks: most probably of lava of the flow of , from the la scala quarries. we condemn "most probably" as bad positivism. as to the "men of position," who had accepted that this thing had fallen from the sky--"i have now obliged them to admit their mistake," says dr. johnstone-lavis--or it's always the stranger in naples who knows la scala lava better than the natives know it. explanation: that the thing had been knocked from, or thrown from, a roof. as to attempt to trace the occurrence to any special roof--nothing said upon that subject. or that dr. johnstone-lavis called a carved stone a "lapstone," quite as mr. symons called a spherical object a "cannon ball": bent upon a discrediting incongruity: shoemaking and celestiality. it is so easy to say that axes, or wedge-shaped stones found on the ground, were there in the first place, and that it is only coincidence that lightning should strike near one--but the credibility of coincidences decreases as the square root of their volume, i think. our massed instances speak too much of coincidences of coincidences. but the axes, or wedge-shaped objects that have been found in trees, are more difficult for orthodoxy. for instance, arago accepts that such finds have occurred, but he argues that, if wedge-shaped stones have been found in tree trunks, so have toads been found in tree trunks--did the toads fall there? not at all bad for a hypnotic. of course, in our acceptance, the irish are the chosen people. it's because they are characteristically best in accord with the underlying essence of quasi-existence. m. arago answers a question by asking another question. that's the only way a question can be answered in our hibernian kind of an existence. dr. bodding argued with the natives of the santal parganas, india, who said that cut and shaped stones had fallen from the sky, some of them lodging in tree trunks. dr. bodding, with orthodox notions of velocity of falling bodies, having missed, i suppose, some of the notes i have upon large hailstones, which, for size, have fallen with astonishingly low velocity, argued that anything falling from the sky would be "smashed to atoms." he accepts that objects of worked stone have been found in tree trunks, but he explains: that the santals often steal trees, but do not chop them down in the usual way, because that would be to make too much noise: they insert stone wedges, and hammer them instead: then, if they should be caught, wedges would not be the evidence against them that axes would be. or that a scientific man can't be desperate and reasonable too. or that a pickpocket, for instance, is safe, though caught with his hand in one's pocket, if he's gloved, say: because no court in the land would regard a gloved hand in the same way in which a bare hand would be regarded. that there's nothing but intermediateness to the rational and the preposterous: that this status of our own ratiocinations is perceptible wherein they are upon the unfamiliar. dr. bodding collected of these shaped stones, said to have fallen from the sky, in the course of many years. he says that the santals are a highly developed race, and for ages have not used stone implements--except in this one nefarious convenience to him. all explanations are localizations. they fade away before the universal. it is difficult to express that black rains in england do not originate in the smoke of factories--less difficult to express that black rains of south africa do not. we utter little stress upon the absurdity of dr. bedding's explanation, because, if anything's absurd everything's absurd, or, rather, has in it some degree or aspect of absurdity, and we've never had experience with any state except something somewhere between ultimate absurdity and final reasonableness. our acceptance is that dr. bedding's elaborate explanation does not apply to cut-stone objects found in tree trunks in other lands: we accept that for the general, a local explanation is inadequate. as to "thunderstones" not said to have fallen luminously, and not said to have been found sticking in trees, we are told by faithful hypnotics that astonished rustics come upon prehistoric axes that have been washed into sight by rains, and jump to the conclusion that the things have fallen from the sky. but simple rustics come upon many prehistoric things: scrapers, pottery, knives, hammers. we have no record of rusticity coming upon old pottery after a rain, reporting the fall of a bowl from the sky. just now, my own acceptance is that wedge-shaped stone objects, formed by means similar to human workmanship, have often fallen from the sky. maybe there are messages upon them. my acceptance is that they have been called "axes" to discredit them: or the more familiar a term, the higher the incongruity with vague concepts of the vast, remote, tremendous, unknown. in _notes and queries_, - - , a writer says that he had a "thunderstone," which he had brought from jamaica. the description is of a wedge-shaped object; not of an ax: "it shows no mark of having been attached to a handle." of ten "thunderstones," figured upon different pages in blinkenberg's book, nine show no sign of ever having been attached to a handle: one is perforated. but in a report by dr. c. leemans, director of the leyden museum of antiquities, objects, said by the japanese to have fallen from the sky, are alluded to throughout as "wedges." in the _archaeologic journal_, - , in a paper upon the "thunderstones" of java, the objects are called "wedges" and not "axes." our notion is that rustics and savages call wedge-shaped objects that fall from the sky, "axes": that scientific men, when it suits their purposes, can resist temptations to prolixity and pedantry, and adopt the simple: that they can be intelligible when derisive. all of which lands us in a confusion, worse, i think, than we were in before we so satisfactorily emerged from the distresses of--butter and blood and ink and paper and punk and silk. now it's cannon balls and axes and disks--if a "lapstone" be a disk--it's a flat stone, at any rate. a great many scientists are good impressionists: they snub the impertinences of details. had he been of a coarse, grubbing nature, i think dr. bodding could never have so simply and beautifully explained the occurrence of stone wedges in tree trunks. but to a realist, the story would be something like this: a man who needed a tree, in a land of jungles, where, for some unknown reason, everyone's very selfish with his trees, conceives that hammering stone wedges makes less noise than does the chopping of wood: he and his descendants, in a course of many years, cut down trees with wedges, and escape penalty, because it never occurs to a prosecutor that the head of an ax is a wedge. the story is like every other attempted positivism--beautiful and complete, until we see what it excludes or disregards; whereupon it becomes the ugly and incomplete--but not absolutely, because there is probably something of what is called foundation for it. perhaps a mentally incomplete santal did once do something of the kind. story told to dr. bodding: in the usual scientific way, he makes a dogma of an aberration. or we did have to utter a little stress upon this matter, after all. they're so hairy and attractive, these scientists of the th century. we feel the zeal of a sitting bull when we think of their scalps. we shall have to have an expression of our own upon this confusing subject. we have expressions: we don't call them explanations: we've discarded explanations with beliefs. though everyone who scalps is, in the oneness of allness, himself likely to be scalped, there is such a discourtesy to an enemy as the wearing of wigs. cannon balls and wedges, and what may they mean? bombardments of this earth-- attempts to communicate-- or visitors to this earth, long ago--explorers from the moon--taking back with them, as curiosities, perhaps, implements of this earth's prehistoric inhabitants--a wreck--a cargo of such things held for ages in suspension in the super-sargasso sea--falling, or shaken, down occasionally by storms-- but, by preponderance of description, we cannot accept that "thunderstones" ever were attached to handles, or are prehistoric axes-- as to attempts to communicate with this earth by means of wedge-shaped objects especially adapted to the penetration of vast, gelatinous areas spread around this earth-- in the _proc. roy. irish acad._, - , there is an account of a stone wedge that fell from the sky, near cashel, tipperary, aug. , . the phenomenon is not questioned, but the orthodox preference is to call it, not ax-like, nor wedge-shaped, but "pyramidal." for data of other pyramidal stones said to have fallen from the sky, see _rept. brit. assoc._, - . one fell at segowolee, india, march , . of the object that fell at cashel, dr. haughton says in the _proceedings_: "a singular feature is observable in this stone, that i have never seen in any other:--the rounded edges of the pyramid are sharply marked by lines on the black crust, as perfect as if made by a ruler." dr. haughton's idea is that the marks may have been made by "some peculiar tension in the cooling." it must have been very peculiar, if in all aerolites not wedge-shaped, no such phenomenon had ever been observed. it merges away with one or two instances known, after dr. haughton's time, of seeming stratification in meteorites. stratification in meteorites, however, is denied by the faithful. i begin to suspect something else. a whopper is coming. later it will be as reasonable, by familiarity, as anything else ever said. if someone should study the stone of cashel, as champollion studied the rosetta stone, he might--or, rather, would inevitably--find meaning in those lines, and translate them into english-- nevertheless i begin to suspect something else: something more subtle and esoteric than graven characters upon stones that have fallen from the sky, in attempts to communicate. the notion that other worlds are attempting to communicate with this world is widespread: my own notion is that it is not attempt at all--that it was achievement centuries ago. i should like to send out a report that a "thunderstone" had fallen, say, somewhere in new hampshire-- and keep track of every person who came to examine that stone--trace down his affiliations--keep track of him-- then send out a report that a "thunderstone" had fallen at stockholm, say-- would one of the persons who had gone to new hampshire, be met again in stockholm? but--what if he had no anthropological, lapidarian, or meteorological affiliations--but did belong to a secret society-- it is only a dawning credulity. of the three forms of symmetric objects that have, or haven't, fallen from the sky, it seems to me that the disk is the most striking. so far, in this respect, we have been at our worst--possibly that's pretty bad--but "lapstones" are likely to be of considerable variety of form, and something that is said to have fallen at sometime somewhere in the dutch west indies is profoundly of the unchosen. now we shall have something that is high up in the castes of the accursed: _comptes rendus_, - : that, upon june , , in a "violent storm"--two months before the reported fall of the symmetric iron object of brixton--a small stone had fallen from the sky at tarbes, france: millimeters in diameter; millimeters thick; weight grammes. reported to the french academy by m. sudre, professor of the normal school, tarbes. this time the old convenience "there in the first place" is too greatly resisted--the stone was covered with ice. this object had been cut and shaped by means similar to human hands and human mentality. it was a disk of worked stone--"tres regulier." "il a été assurement travaillé." there's not a word as to any known whirlwind anywhere: nothing of other objects or débris that fell at or near this date, in france. the thing had fallen alone. but as mechanically as any part of a machine responds to its stimulus, the explanation appears in _comptes rendus_ that this stone had been raised by a whirlwind and then flung down. it may be that in the whole nineteenth century no event more important than this occurred. in _la nature_, , and in _l'année scientifique_, , this occurrence is noted. it is mentioned in one of the summer numbers of _nature_, . fassig lists a paper upon it in the _annuaire de soc. met._, . not a word of discussion. not a subsequent mention can i find. our own expression: what matters it how we, the french academy, or the salvation army may explain? a disk of worked stone fell from the sky, at tarbes, france, june , . my own pseudo-conclusion: that we've been damned by giants sound asleep, or by great scientific principles and abstractions that cannot realize themselves: that little harlots have visited their caprices upon us; that clowns, with buckets of water from which they pretend to cast thousands of good-sized fishes have anathematized us for laughing disrespectfully, because, as with all clowns, underlying buffoonery is the desire to be taken seriously; that pale ignorances, presiding over microscopes by which they cannot distinguish flesh from nostoc or fishes' spawn or frogs' spawn, have visited upon us their wan solemnities. we've been damned by corpses and skeletons and mummies, which twitch and totter with pseudo-life derived from conveniences. or there is only hypnosis. the accursed are those who admit they're the accursed. if we be more nearly real we are reasons arraigned before a jury of dream-phantasms. of all meteorites in museums, very few were seen to fall. it is considered sufficient grounds for admission if specimens can't be accounted for in any way other than that they fell from the sky--as if in the haze of uncertainty that surrounds all things, or that is the essence of everything, or in the merging away of everything into something else, there could be anything that could be accounted for in only one way. the scientist and the theologian reason that if something can be accounted for in only one way, it is accounted for in that way--or logic would be logical, if the conditions that it imposes, but, of course, does not insist upon, could anywhere be found in quasi-existence. in our acceptance, logic, science, art, religion are, in our "existence," premonitions of a coming awakening, like dawning awarenesses of surroundings in the mind of a dreamer. any old chunk of metal that measures up to the standard of "true meteoritic material" is admitted by the museums. it may seem incredible that modern curators still have this delusion, but we suspect that the date on one's morning newspaper hasn't much to do with one's modernity all day long. in reading fletcher's catalogue, for instance, we learn that some of the best-known meteorites were "found in draining a field"--"found in making a road"--"turned up by the plow" occurs a dozen times. someone fishing in lake okeechobee, brought up an object in his fishing net. no meteorite had ever been seen to fall near it. the u.s. national museum accepts it. if we have accepted only one of the data of "untrue meteoritic material"--one instance of "carbonaceous" matter--if it be too difficult to utter the word "coal"--we see that in this inclusion-exclusion, as in every other means of forming an opinion, false inclusion and false exclusion have been practiced by curators of museums. there is something of ultra-pathos--of cosmic sadness--in this universal search for a standard, and in belief that one has been revealed by either inspiration or analysis, then the dogged clinging to a poor sham of a thing long after its insufficiency has been shown--or renewed hope and search for the special that can be true, or for something local that could also be universal. it's as if "true meteoritic material" were a "rock of ages" to some scientific men. they cling. but clingers cannot hold out welcoming arms. the only seemingly conclusive utterance, or seemingly substantial thing to cling to, is a product of dishonesty, ignorance, or fatigue. all sciences go back and back, until they're worn out with the process, or until mechanical reaction occurs: then they move forward--as it were. then they become dogmatic, and take for bases, positions that were only points of exhaustion. so chemistry divided and sub-divided down to atoms; then, in the essential insecurity of all quasi-constructions, it built up a system, which, to anyone so obsessed by his own hypnoses that he is exempt to the chemist's hypnoses, is perceptibly enough an intellectual anæmia built upon infinitesimal debilities. in _science_, n.s., - , e.d. hovey, of the american museum of natural history, asserts or confesses that often have objects of material such as fossiliferous limestone and slag been sent to him he says that these things have been accompanied by assurances that they have been seen to fall on lawns, on roads, in front of houses. they are all excluded. they are not of true meteoritic material. they were on the ground in the first place. it is only by coincidence that lightning has struck, or that a real meteorite, which was unfindable, has struck near objects of slag and limestone. mr. hovey says that the list might be extended indefinitely. that's a tantalizing suggestion of some very interesting stuff-- he says: "but it is not worth while." i'd like to know what strange, damned, excommunicated things have been sent to museums by persons who have felt convinced that they had seen what they may have seen, strongly enough to risk ridicule, to make up bundles, go to express offices, and write letters. i accept that over the door of every museum, into which such things enter, is written: "abandon hope." if a mr. symons mentions one instance of coal, or of slag or cinders, said to have fallen from the sky, we are not--except by association with the "carbonaceous" meteorites--strong in our impression that coal sometimes falls to this earth from coal-burning super-constructions up somewhere-- in _comptes rendus_, - , m. daubrée tells the same story. our acceptance, then, is that other curators could tell this same story. then the phantomosity of our impression substantiates proportionately to its multiplicity. m. daubrée says that often have strange damned things been sent to the french museums, accompanied by assurances that they had been seen to fall from the sky. especially to our interest, he mentions coal and slag. excluded. buried un-named and undated in science's potter's field. i do not say that the data of the damned should have the same rights as the data of the saved. that would be justice. that would be of the positive absolute, and, though the ideal of, a violation of, the very essence of quasi-existence, wherein only to have the appearance of being is to express a preponderance of force one way or another--or inequilibrium, or inconsistency, or injustice. our acceptance is that the passing away of exclusionism is a phenomenon of the twentieth century: that gods of the twentieth century will sustain our notions be they ever so unwashed and frowsy. but, in our own expressions, we are limited, by the oneness of quasiness, to the very same methods by which orthodoxy established and maintains its now sleek, suave preposterousnesses. at any rate, though we are inspired by an especial subtle essence--or imponderable, i think--that pervades the twentieth century, we have not the superstition that we are offering anything as a positive fact. rather often we have not the delusion that we're any less superstitious and credulous than any logician, savage, curator, or rustic. an orthodox demonstration, in terms of which we shall have some heresies, is that if things found in coal could have got there only by falling there--they fell there. so, in the _manchester lit. and phil. soc. mems._, - - , it is argued that certain roundish stones that have been found in coal are "fossil aerolites": that they had fallen from the sky, ages ago, when the coal was soft, because the coal had closed around them, showing no sign of entrance. _proc. soc. of antiq. of scotland_, - - : that, in a lump of coal, from a mine in scotland, an iron instrument had been found-- "the interest attaching to this singular relic arises from the fact of its having been found in the heart of a piece of coal, seven feet under the surface." if we accept that this object of iron was of workmanship beyond the means and skill of the primitive men who may have lived in scotland when coal was forming there-- "the instrument was considered to be modern." that our expression has more of realness, or higher approximation to realness, than has the attempt to explain that is made in the _proceedings_: that in modern times someone may have bored for coal, and that his drill may have broken off in the coal it had penetrated. why he should have abandoned such easily accessible coal, i don't know. the important point is that there was no sign of boring: that this instrument was in a lump of coal that had closed around it so that its presence was not suspected, until the lump of coal was broken. no mention can i find of this damned thing in any other publication. of course there is an alternative here: the thing may not have fallen from the sky: if in coal-forming times, in scotland, there were, indigenous to this earth, no men capable of making such an iron instrument, it may have been left behind by visitors from other worlds. in an extraordinary approximation to fairness and justice, which is permitted to us, because we are quite as desirous to make acceptable that nothing can be proved as we are to sustain our own expressions, we note: that in _notes and queries_, - - , there is an account of an ancient copper seal, about the size of a penny, found in chalk, at a depth of from five to six feet, near bredenstone, england. the design upon it is said to be of a monk kneeling before a virgin and child: a legend upon the margin is said to be: "st. jordanis monachi spaldingie." i don't know about that. it looks very desirable--undesirable to us. there's a wretch of an ultra-frowsy thing in the _scientific american_, - , which we condemn ourselves, if somewhere, because of the oneness of allness, the damned must also be the damning. it's a newspaper story: that about the first of june, , a powerful blast, near dorchester, mass., cast out from a bed of solid rock a bell-shaped vessel of an unknown metal: floral designs inlaid with silver; "art of some cunning workman." the opinion of the editor of the _scientific american_ is that the thing had been made by tubal cain, who was the first inhabitant of dorchester. though i fear that this is a little arbitrary, i am not disposed to fly rabidly at every scientific opinion. _nature_, - : a block of metal found in coal, in austria, . it is now in the salsburg museum. this time we have another expression. usually our intermediatist attack upon provincial positivism is: science, in its attempted positivism takes something such as "true meteoritic material" as a standard of judgment; but carbonaceous matter, except for its relative infrequency, is just as veritable a standard of judgment; carbonaceous matter merges away into such a variety of organic substances, that all standards are reduced to indistinguishability: if, then, there is no real standard against us, there is no real resistance to our own acceptances. now our intermediatism is: science takes "true meteoritic material" as a standard of admission; but now we have an instance that quite as truly makes "true meteoritic material" a standard of exclusion; or, then, a thing that denies itself is no real resistance to our own acceptances--this depending upon whether we have a datum of something of "true meteoritic material" that orthodoxy can never accept fell from the sky. we're a little involved here. our own acceptance is upon a carved, geometric thing that, if found in a very old deposit, antedates human life, except, perhaps, very primitive human life, as an indigenous product of this earth: but we're quite as much interested in the dilemma it made for the faithful. it is of "true meteoritic material." _l'astronomie_, - , it is said that, though so geometric, its phenomena so characteristic of meteorites exclude the idea that it was the work of man. as to the deposit--tertiary coal. composition--iron, carbon, and a small quantity of nickel. it has the pitted surface that is supposed by the faithful to be characteristic of meteorites. for a full account of this subject, see _comptes rendus_, - . the scientists who examined it could reach no agreement. they bifurcated: then a compromise was suggested; but the compromise is a product of disregard: that it was of true meteoritic material, and had not been shaped by man; that it was not of true meteoritic material, but telluric iron that had been shaped by man: that it was true meteoritic material that had fallen from the sky, but had been shaped by man, after its fall. the data, one or more of which must be disregarded by each of these three explanations, are: "true meteoritic material" and surface markings of meteorites; geometric form; presence in an ancient deposit; material as hard as steel; absence upon this earth, in tertiary times, of men who could work in material as hard as steel. it is said that, though of "true meteoritic material," this object is virtually a steel object. st. augustine, with his orthodoxy, was never in--well, very much worse--difficulties than are the faithful here. by due disregard of a datum or so, our own acceptance that it was a steel object that had fallen from the sky to this earth, in tertiary times, is not forced upon one. we offer ours as the only synthetic expression. for instance, in _science gossip_, - , it is described as a meteorite: in this account there is nothing alarming to the pious, because, though everything else is told, its geometric form is not mentioned. it's a cube. there is a deep incision all around it. of its faces, two that are opposite are rounded. though i accept that our own expression can only rather approximate to truth, by the wideness of its inclusions, and because it seems, of four attempts, to represent the only complete synthesis, and can be nullified or greatly modified by data that we, too, have somewhere disregarded, the only means of nullification that i can think of would be demonstration that this object is a mass of iron pyrites, which sometimes forms geometrically. but the analysis mentions not a trace of sulphur. of course our weakness, or impositiveness, lies in that, by anyone to whom it would be agreeable to find sulphur in this thing, sulphur would be found in it--by our own intermediatism there is some sulphur in everything, or sulphur is only a localization or emphasis of something that, unemphasized, is in all things. so there have, or haven't, been found upon this earth things that fell from the sky, or that were left behind by extra-mundane visitors to this earth-- a yarn in the london _times_, june , : that some workmen, quarrying rock, close to the tweed, about a quarter of a mile below rutherford mills, discovered a gold thread embedded in the stone at a depth of feet: that a piece of the gold thread had been sent to the office of the _kelso chronicle_. pretty little thing; not at all frowsy; rather damnable. london _times_, dec. , : that hiram de witt, of springfield, mass., returning from california, had brought with him a piece of auriferous quartz about the size of a man's fist. it was accidentally dropped--split open--nail in it. there was a cut-iron nail, size of a six-penny nail, slightly corroded. "it was entirely straight and had a perfect head." or--california--ages ago, when auriferous quartz was forming--super-carpenter, million of miles or so up in the air--drops a nail. to one not an intermediatist, it would seem incredible that this datum, not only of the damned, but of the lowest of the damned, or of the journalistic caste of the accursed, could merge away with something else damned only by disregard, and backed by what is called "highest scientific authority"-- communication by sir david brewster (_rept. brit. assoc._, - ): that a nail had been found in a block of stone from kingoodie quarry, north britain. the block in which the nail was found was nine inches thick, but as to what part of the quarry it had come from, there is no evidence--except that it could not have been from the surface. the quarry had been worked about twenty years. it consisted of alternate layers of hard stone and a substance called "till." the point of the nail, quite eaten with rust, projected into some "till," upon the surface of the block of stone. the rest of the nail lay upon the surface of the stone to within an inch of the head--that inch of it was embedded in the stone. although its caste is high, this is a thing profoundly of the damned--sort of a brahmin as regarded by a baptist. its case was stated fairly; brewster related all circumstances available to him--but there was no discussion at the meeting of the british association: no explanation was offered-- nevertheless the thing can be nullified-- but the nullification that we find is as much against orthodoxy in one respect as it is against our own expression that inclusion in quartz or sandstone indicates antiquity--or there would have to be a revision of prevailing dogmas upon quartz and sandstone and age indicated by them, if the opposing data should be accepted. of course it may be contended by both the orthodox and us heretics that the opposition is only a yarn from a newspaper. by an odd combination, we find our two lost souls that have tried to emerge, chucked back to perdition by one blow: _pop. sci. news_, - : that, according to the _carson appeal_, there had been found in a mine, quartz crystals that could have had only years in which to form: that, where a mill had been built, sandstone had been found, when the mill was torn down, that had hardened in years: that in this sandstone was a piece of wood "with a nail in it." _annals of scientific discovery_, - : that, at the meeting of the british association, , sir david brewster had announced that he had to bring before the meeting an object "of so incredible a nature that nothing short of the strongest evidence was necessary to render the statement at all probable." a crystal lens had been found in the treasure-house at nineveh. in many of the temples and treasure houses of old civilizations upon this earth have been preserved things that have fallen from the sky--or meteorites. again we have a brahmin. this thing is buried alive in the heart of propriety: it is in the british museum. carpenter, in _the microscope and its revelations_, gives two drawings of it. carpenter argues that it is impossible to accept that optical lenses had ever been made by the ancients. never occurred to him--someone a million miles or so up in the air--looking through his telescope--lens drops out. this does not appeal to carpenter: he says that this object must have been an ornament. according to brewster, it was not an ornament, but "a true optical lens." in that case, in ruins of an old civilization upon this earth, has been found an accursed thing that was, acceptably, not a product of any old civilization indigenous to this earth. early explorers have florida mixed up with newfoundland. but the confusion is worse than that still earlier. it arises from simplicity. very early explorers think that all land westward is one land, india: awareness of other lands as well as india comes as a slow process. i do not now think of things arriving upon this earth from some especial other world. that was my notion when i started to collect our data. or, as is a commonplace of observation, all intellection begins with the illusion of homogeneity. it's one of spencer's data: we see homogeneousness in all things distant, or with which we have small acquaintance. advance from the relatively homogeneous to the relatively heterogeneous is spencerian philosophy--like everything else, so-called: not that it was really spencer's discovery, but was taken from von baer, who, in turn, was continuous with preceding evolutionary speculation. our own expression is that all things are acting to advance to the homogeneous, or are trying to localize homogeneousness. homogeneousness is an aspect of the universal, wherein it is a state that does not merge away into something else. we regard homogeneousness as an aspect of positiveness, but it is our acceptance that infinite frustrations of attempts to positivize manifest themselves in infinite heterogeneity: so that though things try to localize homogeneousness they end up in heterogeneity so great that it amounts to infinite dispersion or indistinguishability. so all concepts are little attempted positivenesses, but soon have to give in to compromise, modification, nullification, merging away into indistinguishability--unless, here and there, in the world's history, there may have been a super-dogmatist, who, for only an infinitesimal of time, has been able to hold out against heterogeneity or modification or doubt or "listening to reason," or loss of identity--in which case--instant translation to heaven or the positive absolute. odd thing about spencer is that he never recognized that "homogeneity," "integration," and "definiteness" are all words for the same state, or the state that we call "positiveness." what we call his mistake is in that he regarded "homogeneousness" as negative. i began with a notion of some one other world, from which objects and substances have fallen to this earth; which had, or which, to less degree, has a tutelary interest in this earth; which is now attempting to communicate with this earth--modifying, because of data which will pile up later, into acceptance that some other world is not attempting but has been, for centuries, in communication with a sect, perhaps, or a secret society, or certain esoteric ones of this earth's inhabitants. i lose a great deal of hypnotic power in not being able to concentrate attention upon some one other world. as i have admitted before i'm intelligent, as contrasted with the orthodox. i haven't the aristocratic disregard of a new york curator or an eskimo medicine-man. i have to dissipate myself in acceptance of a host of other worlds: size of the moon, some of them: one of them, at least--tremendous thing: we'll take that up later. vast, amorphous aerial regions, to which such definite words as "worlds" and "planets" seem inapplicable. and artificial constructions that i have called "super-constructions": one of them about the size of brooklyn, i should say, offhand. and one or more of them wheel-shaped things a goodly number of square miles in area. i think that earlier in this book, before we liberalized into embracing everything that comes along, your indignation, or indigestion would have expressed in the notion that, if this were so, astronomers would have seen these other worlds and regions and vast geometric constructions. you'd have had that notion: you'd have stopped there. but the attempt to stop is saying "enough" to the insatiable. in cosmic punctuation there are no periods: illusion of periods is incomplete view of colons and semi-colons. we can't stop with the notion that if there were such phenomena, astronomers would have seen them. because of our experience with suppression and disregard, we suspect, before we go into the subject at all, that astronomers have seen them; that navigators and meteorologists have seen them; that individual scientists and other trained observers have seen them many times-- that it is the system that has excluded data of them. as to the law of gravitation, and astronomers' formulas, remember that these formulas worked out in the time of laplace as well as they do now. but there are hundreds of planetary bodies now known that were then not known. so a few hundred worlds more of ours won't make any difference. laplace knew of about only thirty bodies in this solar system: about six hundred are recognized now-- what are the discoveries of geology and biology to a theologian? his formulas still work out as well as they ever did. if the law of gravitation could be stated as a real utterance, it might be a real resistance to us. but we are told only that gravitation is gravitation. of course to an intermediatist, nothing can be defined except in terms of itself--but even the orthodox, in what seems to me to be the innate premonitions of realness, not founded upon experience, agree that to define a thing in terms of itself is not real definition. it is said that by gravitation is meant the attraction of all things proportionately to mass and inversely as the square of the distance. mass would mean inter-attraction holding together final particles, if there were final particles. then, until final particles be discovered, only one term of this expression survives, or mass is attraction. but distance is only extent of mass, unless one holds out for absolute vacuum among planets, a position against which we could bring a host of data. but there is no possible means of expressing that gravitation is anything other than attraction. so there is nothing to resist us but such a phantom as--that gravitation is the gravitation of all gravitations proportionately to gravitation and inversely as the square of gravitation. in a quasi-existence, nothing more sensible than this can be said upon any so-called subject--perhaps there are higher approximations to ultimate sensibleness. nevertheless we seem to have a feeling that with the system against us we have a kind of resistance here. we'd have felt so formerly, at any rate: i think the dr. grays and prof. hitchcocks have modified our trustfulness toward indistinguishability. as to the perfection of this system that quasi-opposes us and the infallibility of its mathematics--as if there could be real mathematics in a mode of seeming where twice two are not four--we've been told over and over of their vindication in the discovery of neptune. i'm afraid that the course we're taking will turn out like every other development. we began humbly, admitting that we're of the damned-- but our eyebrows-- just a faint flicker in them, or in one of them, every time we hear of the "triumphal discovery of neptune"--this "monumental achievement of theoretical astronomy," as the text-books call it. the whole trouble is that we've looked it up. the text-books omit this: that, instead of the orbit of neptune agreeing with the calculations of adams and leverrier, it was so different--that leverrier said that it was not the planet of his calculations. later it was thought best to say no more upon that subject. the text-books omit this: that, in , everyone who knew a sine from a cosine was out sining and cosining for a planet beyond uranus. two of them guessed right. to some minds, even after leverrier's own rejection of neptune, the word "guessed" may be objectionable--but, according to prof. peirce, of harvard, the calculations of adams and leverrier would have applied quite as well to positions many degrees from the position of neptune. or for prof. peirce's demonstration that the discovery of neptune was only a "happy accident," see _proc. amer. acad. sciences_, - . for references, see lowell's _evolution of worlds_. or comets: another nebulous resistance to our own notions. as to eclipses, i have notes upon several of them that did not occur upon scheduled time, though with differences only of seconds--and one delightful lost soul, deep-buried, but buried in the ultra-respectable records of the royal astronomical society, upon an eclipse that did not occur at all. that delightful, ultra-sponsored thing of perdition is too good and malicious to be dismissed with passing notice: we'll have him later. throughout the history of astronomy, every comet that has come back upon predicted time--not that, essentially, there was anything more abstruse about it than is a prediction that you can make of a postman's periodicities tomorrow--was advertised for all it was worth. it's the way reputations are worked up for fortune-tellers by the faithful. the comets that didn't come back--omitted or explained. or encke's comet. it came back slower and slower. but the astronomers explained. be almost absolutely sure of that: they explained. they had it all worked out and formulated and "proved" why that comet was coming back slower and slower--and there the damn thing began coming faster and faster. halley's comet. astronomy--"the perfect science, as we astronomers like to call it." (jacoby.) it's my own notion that if, in a real existence, an astronomer could not tell one longitude from another, he'd be sent back to this purgatory of ours until he could meet that simple requirement. halley was sent to the cape of good hope to determine its longitude. he got it degrees wrong. he gave to africa's noble roman promontory a retroussé twist that would take the pride out of any kaffir. we hear everlastingly of halley's comet. it came back--maybe. but, unless we look the matter up in contemporaneous records, we hear nothing of--the leonids, for instance. by the same methods as those by which halley's comet was predicted, the leonids were predicted. november, --no leonids. it was explained. they had been perturbed. they would appear in november, . november, --november, --no leonids. my notion of astronomic accuracy: who could not be a prize marksman, if only his hits be recorded? as to halley's comet, of --everybody now swears he saw it. he has to perjure himself: otherwise he'd be accused of having no interest in great, inspiring things that he's never given any attention to. regard this: that there never is a moment when there is not some comet in the sky. virtually there is no year in which several new comets are not discovered, so plentiful are they. luminous fleas on a vast black dog--in popular impressions, there is no realization of the extent to which this solar system is flea-bitten. if a comet have not the orbit that astronomers have predicted--perturbed. if--like halley's comet--it be late--even a year late--perturbed. when a train is an hour late, we have small opinion of the predictions of timetables. when a comet's a year late, all we ask is--that it be explained. we hear of the inflation and arrogance of astronomers. my own acceptance is not that they are imposing upon us: that they are requiting us. for many of us priests no longer function to give us seeming rapport with perfection, infallibility--the positive absolute. astronomers have stepped forward to fill a vacancy--with quasi-phantomosity--but, in our acceptance, with a higher approximation to substantiality than had the attenuations that preceded them. i should say, myself, that all that we call progress is not so much response to "urge" as it is response to a hiatus--or if you want something to grow somewhere, dig out everything else in its area. so i have to accept that the positive assurances of astronomers are necessary to us, or the blunderings, evasions and disguises of astronomers would never be tolerated: that, given such latitude as they are permitted to take, they could not be very disastrously mistaken. suppose the comet called halley's had not appeared-- early in , a far more important comet than the anæmic luminosity said to be halley's, appeared. it was so brilliant that it was visible in daylight. the astronomers would have been saved anyway. if this other comet did not have the predicted orbit--perturbation. if you're going to coney island, and predict there'll be a special kind of a pebble on the beach, i don't see how you can disgrace yourself, if some other pebble will do just as well--because the feeble thing said to have been seen in was no more in accord with the sensational descriptions given out by astronomers in advance than is a pale pebble with a brick-red boulder. i predict that next wednesday, a large chinaman, in evening clothes, will cross broadway, at nd street, at p.m. he doesn't, but a tubercular jap in a sailor's uniform does cross broadway, at th street, friday, at noon. well, a jap is a perturbed chinaman, and clothes are clothes. i remember the terrifying predictions made by the honest and credulous astronomers, who must have been themselves hypnotized, or they could not have hypnotized the rest of us, in . wills were made. human life might be swept from this planet. in quasi-existence, which is essentially hibernian, that would be no reason why wills should not be made. the less excitable of us did expect at least some pretty good fireworks. i have to admit that it is said that, in new york, a light was seen in the sky. it was about as terrifying as the scratch of a match on the seat of some breeches half a mile away. it was not on time. though i have heard that a faint nebulosity, which i did not see, myself, though i looked when i was told to look, was seen in the sky, it appeared several days after the time predicted. a hypnotized host of imbeciles of us: told to look up at the sky: we did--like a lot of pointers hypnotized by a partridge. the effect: almost everybody now swears that he saw halley's comet, and that it was a glorious spectacle. an interesting circumstance here is that seemingly we are trying to discredit astronomers because astronomers oppose us--that's not my impression. we shall be in the brahmin caste of the hell of the baptists. almost all our data, in some regiments of this procession, are observations by astronomers, few of them mere amateur astronomers. it is the system that opposes us. it is the system that is suppressing astronomers. i think we pity them in their captivity. ours is not malice--in a positive sense. it's chivalry--somewhat. unhappy astronomers looking out from high towers in which they are imprisoned--we appear upon the horizon. but, as i have said, our data do not relate to some especial other world. i mean very much what a savage upon an ocean island might vaguely think of in his speculations--not upon some other land, but complexes of continents and their phenomena: cities, factories in cities, means of communication-- now all the other savages would know of a few vessels sailing in their regular routes, passing this island in regularized periodicities. the tendency in these minds would be expression of the universal tendency toward positivism--or completeness--or conviction that these few regularized vessels constituted all. now i think of some especial savage who suspects otherwise--because he's very backward and unimaginative and insensible to the beautiful ideals of the others: not piously occupied, like the others, in bowing before impressive-looking sticks of wood; dishonestly taking time for his speculations, while the others are patriotically witch-finding. so the other higher and nobler savages know about the few regularized vessels: know when to expect them; have their periodicities all worked out; just about when vessels will pass, or eclipse each other--explaining that all vagaries were due to atmospheric conditions. they'd come out strong in explaining. you can't read a book upon savages without noting what resolute explainers they are. they'd say that all this mechanism was founded upon the mutual attraction of the vessels--deduced from the fall of a monkey from a palm tree--or, if not that, that devils were pushing the vessels--something of the kind. storms. débris, not from these vessels, cast up by the waves. disregarded. how can one think of something and something else, too? i'm in the state of mind of a savage who might find upon a shore, washed up by the same storm, buoyant parts of a piano and a paddle that was carved by cruder hands than his own: something light and summery from india, and a fur overcoat from russia--or all science, though approximating wider and wider, is attempt to conceive of india in terms of an ocean island, and of russia in terms of india so interpreted. though i am trying to think of russia and india in world-wide terms, i cannot think that that, or the universalizing of the local, is cosmic purpose. the higher idealist is the positivist who tries to localize the universal, and is in accord with cosmic purpose: the super-dogmatist of a local savage who can hold out, without a flurry of doubt, that a piano washed up on a beach is the trunk of a palm tree that a shark has bitten, leaving his teeth in it. so we fear for the soul of dr. gray, because he did not devote his whole life to that one stand that, whether possible or inconceivable, thousands of fishes had been cast from one bucket. so, unfortunately for myself, if salvation be desirable, i look out widely but amorphously, indefinitely and heterogeneously. if i say i conceive of another world that is now in secret communication with certain esoteric inhabitants of this earth, i say i conceive of still other worlds that are trying to establish communication with all the inhabitants of this earth. i fit my notions to the data i find. that is supposed to be the right and logical and scientific thing to do; but it is no way to approximate to form, system, organization. then i think i conceive of other worlds and vast structures that pass us by, within a few miles, without the slightest desire to communicate, quite as tramp vessels pass many islands without particularizing one from another. then i think i have data of a vast construction that has often come to this earth, dipped into an ocean, submerged there a while, then going away--why? i'm not absolutely sure. how would an eskimo explain a vessel, sending ashore for coal, which is plentiful upon some arctic beaches, though of unknown use to the natives, then sailing away, with no interest in the natives? a great difficulty in trying to understand vast constructions that show no interest in us: the notion that we must be interesting. i accept that, though we're usually avoided, probably for moral reasons, sometimes this earth has been visited by explorers. i think that the notion that there have been extra-mundane visitors to china, within what we call the historic period, will be only ordinarily absurd, when we come to that datum. i accept that some of the other worlds are of conditions very similar to our own. i think of others that are very different--so that visitors from them could not live here--without artificial adaptations. how some of them could breathe our attenuated air, if they came from a gelatinous atmosphere-- masks. the masks that have been found in ancient deposits. most of them are of stone, and are said to have been ceremonial regalia of savages-- but the mask that was found in sullivan county, missouri, in (_american antiquarian_, - ). it is made of iron and silver. one of the damnedest in our whole saturnalia of the accursed-- because it is hopeless to try to shake off an excommunication only by saying that we're damned by blacker things than ourselves; and that the damned are those who admit they're of the damned. inertia and hypnosis are too strong for us. we say that: then we go right on admitting we're of the damned. it is only by being more nearly real that we can sweep away the quasi-things that oppose us. of course, as a whole, we have considerable amorphousness, but we are thinking now of "individual" acceptances. wideness is an aspect of universalness or realness. if our syntheses disregard fewer data than do opposing syntheses--which are often not syntheses at all, but mere consideration of some one circumstance--less widely synthetic things fade away before us. harmony is an aspect of the universal, by which we mean realness. if we approximate more highly to harmony among the parts of an expression and to all available circumstances of an occurrence, the self-contradictors turn hazy. solidity is an aspect of realness. we pile them up, and we pile them up, or they pass and pass and pass: things that bulk large as they march by, supporting and solidifying one another-- and still, and for regiments to come, hypnosis and inertia rule us-- one of the damnedest of our data: in the _scientific american_, sept. , , charles f. holder writes: "many years ago, a strange stone resembling a meteorite, fell into the valley of the yaqui, mexico, and the sensational story went from one end to the other of the country that a stone bearing human inscriptions had descended to the earth." the bewildering observation here is mr. holder's assertion that this stone did fall. it seems to me that he must mean that it fell by dislodgment from a mountainside into a valley--but we shall see that it was such a marked stone that very unlikely would it have been unknown to dwellers in a valley, if it had been reposing upon a mountainside above them. it may have been carelessness: intent may have been to say that a sensational story of a strange stone said to have fallen, etc. this stone was reported by major frederick burnham, of the british army. later major burnham revisited it, and mr. holder accompanied him, their purpose to decipher the inscriptions upon it, if possible. "this stone was a brown, igneous rock, its longest axis about eight feet, and on the eastern face, which had an angle of about forty-five degrees, was the deep-cut inscription." mr. holder says that he recognized familiar mayan symbols in the inscription. his method was the usual method by which anything can be "identified" as anything else: that is to pick out whatever is agreeable and disregard the rest. he says that he has demonstrated that most of the symbols are mayan. one of our intermediatist pseudo-principles is that any way of demonstrating anything is just as good a way of demonstrating anything else. by mr. holder's method we could demonstrate that we're mayan--if that should be a source of pride to us. one of the characters upon this stone is a circle within a circle--similar character found by mr. holder is a mayan manuscript. there are two 's. 's can be found in mayan manuscripts. a double scroll. there are dots and there are dashes. well, then, we, in turn, disregard the circle within a circle and the double scroll and emphasize that 's occur in this book, and that dots are plentiful, and would be more plentiful if it were customary to use the small "i" for the first personal pronoun--that when it comes to dashes--that's demonstrated: we're mayan. i suppose the tendency is to feel that we're sneering at some valuable archaeologic work, and that mr. holder did make a veritable identification. he writes: "i submitted the photographs to the field museum and the smithsonian and one or two others, and, to my surprise, the reply was that they could make nothing out of it." our indefinite acceptance, by preponderance of three or four groups of museum-experts against one person, is that a stone bearing inscriptions unassimilable with any known language upon this earth, is said to have fallen from the sky. another poor wretch of an outcast belonging here is noted in the _scientific american_, - : that, of an object, or a meteorite, that fell feb. , , near brescia, italy, a false report was circulated that one of the fragments bore the impress of a hand. that's all that is findable by me upon this mere gasp of a thing. intermediatistically, my acceptance is that, though in the course of human history, there have been some notable approximations, there never has been a real liar: that he could not survive in intermediateness, where everything merges away or has its pseudo-base in something else--would be instantly translated to the negative absolute. so my acceptance is that, though curtly dismissed, there was something to base upon in this report; that there were unusual markings upon this object. of course that is not to jump to the conclusion that they were cuneiform characters that looked like finger-prints. altogether, i think that in some of our past expressions, we must have been very efficient, if the experience of mr. symons be typical, so indefinite are we becoming here. just here we are interested in many things that have been found, especially in the united states, which speak of a civilization, or of many civilizations not indigenous to this earth. one trouble is in trying to decide whether they fell here from the sky, or were left behind by visitors from other worlds. we have a notion that there have been disasters aloft, and that coins have dropped here: that inhabitants of this earth found them or saw them fall, and then made coins imitatively: it may be that coins were showered here by something of a tutelary nature that undertook to advance us from the stage of barter to the use of a medium. if coins should be identified as roman coins, we've had so much experience with "identifications" that we know a phantom when we see one--but, even so, how could roman coins have got to north america--far in the interior of north america--or buried under the accumulation of centuries of soil--unless they did drop from--wherever the first romans came from? ignatius donnelly, in _atlantis_, gives a list of objects that have been found in mounds that are supposed to antedate all european influence in america: lathe-made articles, such as traders--from somewhere--would supply to savages--marks of the lathe said to be unmistakable. said to be: of course we can't accept that anything is unmistakable. in the _rept. smithson. inst._, - , there is an account, by charles c. jones, of two silver crosses that were found in georgia. they are skillfully made, highly ornamented crosses, but are not conventional crucifixes: all arms of equal length. mr. jones is a good positivist--that de sota had halted at the "precise" spot where these crosses were found. but the spirit of negativeness that lurks in all things said to be "precise" shows itself in that upon one of these crosses is an inscription that has no meaning in spanish or any other known, terrestrial language: "iynkicidu," according to mr. jones. he thinks that this is a name, and that there is an aboriginal ring to it, though i should say, myself, that he was thinking of the far-distant incas: that the spanish donor cut on the cross the name of an indian to whom it was presented. but we look at the inscription ourselves and see that the letters said to be "c" and "d" are turned the wrong way, and that the letter said to be "k" is not only turned the wrong way, but is upside down. it is difficult to accept that the remarkable, the very extensive, copper mines in the region of lake superior were ever the works of american aborigines. despite the astonishing extent of these mines, nothing has ever been found to indicate that the region was ever inhabited by permanent dwellers-- "... not a vestige of a dwelling, a skeleton, or a bone has been found." the indians have no traditions relating to the mines. (_amer. antiquarian_, - .) i think that we've had visitors: that they have come here for copper, for instance. as to other relics of them--but we now come upon frequency of a merger that has not so often appeared before: fraudulency. hair called real hair--then there are wigs. teeth called real teeth--then there are false teeth. official money--counterfeit money. it's the bane of psychic research. if there be psychic phenomena, there must be fraudulent psychic phenomena. so desperate is the situation here that carrington argues that, even if palladino be caught cheating, that is not to say that all her phenomena are fraudulent. my own version is: that nothing indicates anything, in a positive sense, because, in a positive sense, there is nothing to be indicated. everything that is called true must merge away indistinguishably into something called false. both are expressions of the same underlying quasiness, and are continuous. fraudulent antiquarian relics are very common, but they are not more common than are fraudulent paintings. w.s. forest, _historical sketches of norfolk, virginia_: that, in september, , when some workmen, near norfolk, were boring for water, a coin was drawn up from a depth of about feet. it was about the size of an english shilling, but oval--an oval disk, if not a coin. the figures upon it were distinct, and represented "a warrior or hunter and other characters, apparently of roman origin." the means of exclusion would probably be--men digging a hole--no one else looking: one of them drops a coin into the hole--as to where he got a strange coin, remarkable in shape even--that's disregarded. up comes the coin--expressions of astonishment from the evil one who had dropped it. however, the antiquarians have missed this coin. i can find no other mention of it. another coin. also a little study in the genesis of a prophet. in the _american antiquarian_, - , is copied a story by a correspondent to the _detroit news_, of a copper coin about the size of a two-cent piece, said to have been found in a michigan mound. the editor says merely that he does not endorse the find. upon this slender basis, he buds out, in the next number of the _antiquarian_: "the coin turns out, as we predicted, to be a fraud." you can imagine the scorn of elijah, or any of the old more nearly real prophets. or all things are tried by the only kind of jurisprudence we have in quasi-existence: presumed to be innocent until convicted--but they're guilty. the editor's reasoning is as phantom-like as my own, or st. paul's, or darwin's. the coin is condemned because it came from the same region from which, a few years before, had come pottery that had been called fraudulent. the pottery had been condemned because it was condemnable. _scientific american_, june , : that a farmer, in cass co., ill., had picked up, on his farm, a bronze coin, which was sent to prof. f.f. hilder, of st. louis, who identified it as a coin of antiochus iv. inscription said to be in ancient greek characters: translated as "king antiochus epiphanes (illustrious) the victorius." sounds quite definite and convincing--but we have some more translations coming. in the _american pioneer_, - , are shown two faces of a copper coin, with characters very much like those upon the grave creek stone--which, with translations, we'll take up soon. this coin is said to have been found in connecticut, in . _records of the past_, - : that, early in , a coin, said to be a roman coin, was reported as discovered in an illinois mound. it was sent to dr. emerson, of the art institute, of chicago. his opinion was that the coin is "of the rare mintage of domitius domitianus, emperor in egypt." as to its discovery in an illinois mound, dr. emerson disclaims responsibility. but what strikes me here is that a joker should not have been satisfied with an ordinary roman coin. where did he get a rare coin, and why was it not missed from some collection? i have looked over numismatic journals enough to accept that the whereabouts of every rare coin in anyone's possession is known to coin-collectors. seems to me nothing left but to call this another "identification." _proc. amer. phil. soc._, - : that, in july, , a letter was received from mr. jacob w. moffit, of chillicothe, ill., enclosing a photograph of a coin, which he said had been brought up, by him, while boring, from a depth of feet. of course, by conventional scientific standards, such depth has some extraordinary meaning. palaeontologists, geologists, and archaeologists consider themselves reasonable in arguing ancient origin of the far-buried. we only accept: depth is a pseudo-standard with us; one earthquake could bury a coin of recent mintage feet below the surface. according to a writer in the _proceedings_, the coin is uniform in thickness, and had never been hammered out by savages--"there are other tokens of the machine shop." but, according to prof. leslie, it is an astrologic amulet. "there are upon it the signs of pisces and leo." or, with due disregard, you can find signs of your great-grand-mother, or of the crusades, or of the mayans, upon anything that ever came from chillicothe or from a five and ten cent store. anything that looks like a cat and a goldfish looks like leo and pisces: but, by due suppressions and distortions there's nothing that can't be made to look like a cat and a goldfish. i fear me we're turning a little irritable here. to be damned by slumbering giants and interesting little harlots and clowns who rank high in their profession is at least supportable to our vanity; but, we find that the anthropologists are of the slums of the divine, or of an archaic kindergarten of intellectuality, and it is very unflattering to find a mess of moldy infants sitting in judgment upon us. prof. leslie then finds, as arbitrarily as one might find that some joker put the brooklyn bridge where it is, that "the piece was placed there as a practical joke, though not by its present owner; and is a modern fabrication, perhaps of the sixteenth century, possibly hispano-american or french-american origin." it's sheer, brutal attempt to assimilate a thing that may or may not have fallen from the sky, with phenomena admitted by the anthropologic system: or with the early french or spanish explorers of illinois. though it is ridiculous in a positive sense to give reasons, it is more acceptable to attempt reasons more nearly real than opposing reasons. of course, in his favor, we note that prof. leslie qualifies his notions. but his disregards are that there is nothing either french or spanish about this coin. a legend upon it is said to be "somewhere between arabic and phoenician, without being either." prof. winchell (_sparks from a geologist's hammer_, p. ) says of the crude designs upon this coin, which was in his possession--scrawls of an animal and of a warrior, or of a cat and a goldfish, whichever be convenient--that they had been neither stamped nor engraved, but "looked as if etched with an acid." that is a method unknown in numismatics of this earth. as to the crudity of design upon this coin, and something else--that, though the "warrior" may be, by due disregards, either a cat or a goldfish, we have to note that his headdress is typical of the american indian--could be explained, of course, but for fear that we might be instantly translated to the positive absolute, which may not be absolutely desirable, we prefer to have some flaws or negativeness in our own expressions. data of more than the thrice-accursed: tablets of stone, with the ten commandments engraved upon them, in hebrew, said to have been found in mounds in the united states: masonic emblems said to have been found in mounds in the united states. we're upon the borderline of our acceptances, and we're amorphous in the uncertainties and mergings of our outline. conventionally, or, with no real reason for so doing, we exclude these things, and then, as grossly and arbitrarily and irrationally--though our attempt is always to approximate away from these negative states--as ever a kepler, newton, or darwin made his selections, without which he could not have seemed to be, at all, because every one of them is now seen to be an illusion, we accept that other lettered things have been found in mounds in the united states. of course we do what we can to make the selection seem not gross and arbitrary and irrational. then, if we accept that inscribed things of ancient origin have been found in the united states; that cannot be attributed to any race indigenous to the western hemisphere; that are not in any language ever heard of in the eastern hemisphere--there's nothing to it but to turn non-euclidian and try to conceive of a third "hemisphere," or to accept that there has been intercourse between the western hemisphere and some other world. but there is a peculiarity to these inscribed objects. they remind me of the records left, by sir john franklin, in the arctic; but, also, of attempts made by relief expeditions to communicate with the franklin expedition. the lost explorers cached their records--or concealed them conspicuously in mounds. the relief expeditions sent up balloons, from which messages were dropped broadcast. our data are of things that have been cached, and of things that seem to have been dropped-- or a lost expedition from--somewhere. explorers from somewhere, and their inability to return--then, a long, sentimental, persistent attempt, in the spirit of our own arctic relief-expeditions--at least to establish communication-- what if it may have succeeded? we think of india--the millions of natives who are ruled by a small band of esoterics--only because they receive support and direction from--somewhere else--or from england. in , mr. a.b. tomlinson, owner of the great mound at grave creek, west virginia, excavated the mound. he said that, in the presence of witnesses, he had found a small, flat, oval stone--or disk--upon which were engraved alphabetic characters. col. whittelsey, an expert in these matters, says that the stone is now "universally regarded by archaeologists as a fraud": that, in his opinion, mr. tomlinson had been imposed upon. avebury, _prehistoric times_, p. : "i mention it because it has been the subject of much discussion, but it is now generally admitted to be a fraud. it is inscribed with hebrew characters, but the forger has copied the modern instead of the ancient form of the letters." as i have said, we're as irritable here, under the oppressions of the anthropologists as ever were slaves in the south toward superiorities from "poor white trash." when we finally reverse our relative positions we shall give lowest place to the anthropologists. a dr. gray does at least look at a fish before he conceives of a miraculous origin for it. we shall have to submerge lord avebury far below him--if we accept that the stone from grave creek is generally regarded as a fraud by eminent authorities who did not know it from some other object--or, in general, that so decided an opinion must be the product of either deliberate disregard or ignorance or fatigue. the stone belongs to a class of phenomena that is repulsive to the system. it will not assimilate with the system. let such an object be heard of by such a systematist as avebury, and the mere mention of it is as nearly certainly the stimulus to a conventional reaction as is a charged body to an electroscope or a glass of beer to a prohibitionist. it is of the ideals of science to know one object from another before expressing an opinion upon a thing, but that is not the spirit of universal mechanics: a thing. it is attractive or repulsive. its conventional reaction follows. because it is not the stone from grave creek that is in hebrew characters, either ancient or modern: it is a stone from newark, ohio, of which the story is told that a forger made this mistake of using modern instead of ancient hebrew characters. we shall see that the inscription upon the grave creek stone is not in hebrew. or all things are presumed to be innocent, but are supposed to be guilty--unless they assimilate. col. whittelsey (_western reserve historical tracts, no. _) says that the grave creek stone was considered a fraud by wilson, squires, and davis. then he comes to the congress of archaeologists at nancy, france, . it is hard for col. whittelsey to admit that, at this meeting, which sounds important, the stone was endorsed. he reminds us of mr. symons, and "the man" who "considered" that he saw something. col. whittelsey's somewhat tortuous expression is that the finder of the stone "so imposed his views" upon the congress that it pronounced the stone genuine. also the stone was examined by schoolcraft. he gave his opinion for genuineness. or there's only one process, and "see-saw" is one of its aspects. three or four fat experts on the side against us. we find four or five plump ones on our side. or all that we call logic and reasoning ends up as sheer preponderance of avoirdupois. then several philologists came out in favor of genuineness. some of them translated the inscription. of course, as we have said, it is our method--or the method of orthodoxy--way in which all conclusions are reached--to have some awfully eminent, or preponderantly plump, authorities with us whenever we can--in this case, however, we feel just a little apprehensive in being caught in such excellently obese, but somewhat negativized, company: translation by m. jombard: "thy orders are laws: thou shinest in impetuous élan and rapid chamois." m. maurice schwab: "the chief of emigration who reached these places (or this island) has fixed these characters forever." m. oppert: "the grave of one who was assassinated here. may god, to revenge him, strike his murderer, cutting off the hand of his existence." i like the first one best. i have such a vivid impression from it of someone polishing up brass or something, and in an awful hurry. of course the third is more dramatic--still they're all very good. they are perturbations of one another, i suppose. in tract , col. whittelsey returns to the subject. he gives the conclusion of major de helward, at the congress of luxembourg, : "if prof. read and myself are right in the conclusion that the figures are neither of the runic, phoenician, canaanite, hebrew, lybian, celtic, or any other alphabet-language, its importance has been greatly over-rated." obvious to a child; obvious to any mentality not helplessly subjected to a system: that just therein lies the importance of this object. it is said that an ideal of science is to find out the new--but, unless a thing be of the old, it is "unimportant." "it is not worth while." (hovey.) then the inscribed ax, or wedge, which, according to dr. john c. evans, in a communication to the american ethnological society, was plowed up, near pemberton, n.j., . the characters upon this ax, or wedge, are strikingly similar to the characters on the grave creek stone. also, with a little disregard here and a little more there, they look like tracks in the snow by someone who's been out celebrating, or like your handwriting, or mine, when we think there's a certain distinction in illegibility. method of disregard: anything's anything. dr. abbott describes this object in the _report of the smithsonian institution_, - . he says he has no faith in it. all progress is from the outrageous to the commonplace. or quasi-existence proceeds from rape to the crooning of lullabies. it's been interesting to me to go over various long-established periodicals and note controversies between attempting positivists and then intermediatistic issues. bold, bad intruders of theories; ruffians with dishonorable intentions--the alarms of science; her attempts to preserve that which is dearer than life itself--submission--then a fidelity like mrs. micawber's. so many of these ruffians, or wandering comedians that were hated, or scorned, pitied, embraced, conventionalized. there's not a notion in this book that has a more frightful, or ridiculous, mien than had the notion of human footprints in rocks, when that now respectabilized ruffian, or clown, was first heard from. it seems bewildering to one whose interests are not scientific that such rows should be raised over such trifles: but the feeling of a systematist toward such an intruder is just about what anyone's would be if a tramp from the street should come in, sit at one's dinner table, and say he belonged there. we know what hypnosis can do: let him insist with all his might that he does belong there, and one begins to suspect that he may be right; that he may have higher perceptions of what's right. the prohibitionists had this worked out very skillfully. so the row that was raised over the stone from grave creek--but time and cumulativeness, and the very factor we make so much of--or the power of massed data. there were other reports of inscribed stones, and then, half a century later, some mounds--or caches, as we call them--were opened by the rev. mr. gass, near the city of davenport. (_american antiquarian_, - .) several stone tablets were found. upon one of them, the letters "tftowns" may easily be made out. in this instance we hear nothing of fraudulency--time, cumulativeness, the power of massed data. the attempt to assimilate this datum is: that the tablet was probably of mormon origin. why? because, at mendon, ill., was found a brass plate, upon which were similar characters. why that? because that was found "near a house once occupied by a mormon." in a real existence, a real meteorologist, suspecting that cinders had come from a fire engine--would have asked a fireman. tablets of davenport--there's not a record findable that it ever occurred to any antiquarian--to ask a mormon. other tablets were found. upon one of them are two "f's" and two " 's." also a large tablet, twelve inches by eight to ten inches "with roman numerals and arabic." it is said that the figure " " occurs three times, and the figure or letter "o" seven times. "with these familiar characters are others that resemble ancient alphabets, either phoenecian or hebrew." it may be that the discovery of australia, for instance, will turn out to be less important than the discovery and the meaning of these tablets-- but where will you read of them in anything subsequently published; what antiquarian has ever since tried to understand them, and their presence, and indications of antiquity, in a land that we're told was inhabited only by unlettered savages? these things that are exhumed only to be buried in some other way. another tablet was found, at davenport, by mr. charles harrison, president of the american antiquarian society. "... and other hieroglyphics are upon this tablet." this time, also, fraud is not mentioned. my own notion is that it is very unsportsmanlike ever to mention fraud. accept anything. then explain it your way. anything that assimilates with one explanation, must have assimilable relations, to some degree, with all other explanations, if all explanations are somewhere continuous. mormons are lugged in again, but the attempt is faint and helpless--"because general circumstances make it difficult to explain the presence of these tablets." altogether our phantom resistance is mere attribution to the mormons, without the slightest attempt to find base for the attribution. we think of messages that were showered upon this earth, and of messages that were cached in mounds upon this earth. the similarity to the franklin situation is striking. conceivably centuries from now, objects dropped from relief-expedition-balloons may be found in the arctic, and conceivably there are still undiscovered caches left by franklin, in the hope that relief expeditions would find them. it would be as incongruous to attribute these things to the eskimos as to attribute tablets and lettered stones to the aborigines of america. some time i shall take up an expression that the queer-shaped mounds upon this earth were built by explorers from somewhere, unable to get back, designed to attract attention from some other world, and that a vast sword-shaped mound has been discovered upon the moon--just now we think of lettered things and their two possible significances. a bizarre little lost soul, rescued from one of the morgues of the _american journal of science_: an account, sent by a correspondent, to prof. silliman, of something that was found in a block of marble, taken november, , from a quarry, near philadelphia (_am. j. sci._, - - ). the block was cut into slabs. by this process, it is said, was exposed an indentation in the stone, about one and a half inches by five-eighths of an inch. a geometric indentation: in it were two definite-looking raised letters, like "i u": only difference is that the corners of the "u" are not rounded, but are right angles. we are told that this block of stone came from a depth of seventy or eighty feet--or that, if acceptable, this lettering was done long, long ago. to some persons, not sated with the commonness of the incredible that has to be accepted, it may seem grotesque to think that an indentation in sand could have tons of other sand piled upon it and hardening into stone, without being pressed out--but the famous nicaraguan footprints were found in a quarry under eleven strata of solid rock. there was no discussion of this datum. we only take it out for an airing. as to lettered stones that may once upon a time have been showered upon europe, if we cannot accept that the stones were inscribed by indigenous inhabitants of europe, many have been found in caves--whence they were carried as curiosities by prehistoric men, or as ornaments, i suppose. about the size and shape of the grave creek stone, or disk: "flat and oval and about two inches wide." (sollas.) characters painted upon them: found first by m. piette, in the cave of mas d'azil, ariége. according to sollas, they are marked in various directions with red and black lines. "but on not a few of them, more complex characters occur, which in a few instances simulate some of the capital letters of the roman alphabet." in one instance the letters "f e i" accompanied by no other markings to modify them, are as plain as they could be. according to sollas (_ancient hunters_, p. ) m. cartailhac has confirmed the observations of piette, and m. boule has found additional examples. "they offer one of the darkest problems of prehistoric times." (sollas.) as to caches in general, i should say that they are made with two purposes: to proclaim and to conceal; or that caches documents are hidden, or covered over, in conspicuous structures; at least, so are designed the cairns in the arctic. _trans. n.y. acad. of sciences_, - : that mr. j.h. hooper, bradley co., tenn., having come upon a curious stone, in some woods upon his farm, investigated. he dug. he unearthed a long wall. upon this wall were inscribed many alphabetic characters. " characters have been examined, many of them duplicates, and a few imitations of animal forms, the moon, and other objects. accidental imitations of oriental alphabets are numerous." the part that seems significant: that these letters had been hidden under a layer of cement. and still, in our own heterogeneity, or unwillingness, or inability, to concentrate upon single concepts, we shall--or we sha'n't--accept that, though there may have been a lost colony or lost expedition from somewhere, upon this earth, and extra-mundane visitors who could never get back, there have been other extra-mundane visitors, who have gone away again--altogether quite in analogy with the franklin expedition and peary's flittings in the arctic-- and a wreck that occurred to one group of them-- and the loot that was lost overboard-- the chinese seals of ireland. not the things with the big, wistful eyes that lie on ice, and that are taught to balance objects on their noses--but inscribed stamps, with which to make impressions. _proc. roy. irish acad._, - : a paper was read by mr. j. huband smith, descriptive of about a dozen chinese seals that had been found in ireland. they are all alike: each a cube with an animal seated upon it. "it is said that the inscriptions upon them are of a very ancient class of chinese characters." the three points that have made a leper and an outcast of this datum--but only in the sense of disregard, because nowhere that i know of is it questioned: agreement among archaeologists that there were no relations, in the remote past, between china and ireland: that no other objects, from ancient china--virtually, i suppose--have ever been found in ireland: the great distances at which these seals have been found apart. after mr. smith's investigations--if he did investigate, or do more than record--many more chinese seals were found in ireland, and, with one exception, only in ireland. in , about had been found. of all archaeologic finds in ireland, "none is enveloped in greater mystery." (_chambers' journal_, - .) according to the writer in _chambers' journal_, one of these seals was found in a curiosity shop in london. when questioned, the shopkeeper said that it had come from ireland. in this instance, if you don't take instinctively to our expression, there is no orthodox explanation for your preference. it is the astonishing scattering of them, over field and forest, that has hushed the explainers. in the _proceedings of the royal irish academy_, - , dr. frazer says that they "appear to have been sown broadcast over the country in some strange way that i cannot offer solution of." the struggle for expression of a notion that did not belong to dr. frazer's era: "the invariable story of their find is what we might expect if they had been accidentally dropped...." three were found in tipperary; six in cork; three in down; four in waterford; all the rest--one or two to a county. but one of these chinese seals was found in the bed of the river boyne, near clonard, meath, when workmen were raising gravel. that one, at least, had been dropped there. astronomy. and a watchman looking at half a dozen lanterns, where a street's been torn up. there are gas lights and kerosene lamps and electric lights in the neighborhood: matches flaring, fires in stoves, bonfires, house afire somewhere; lights of automobiles, illuminated signs-- the watchman and his one little system. ethics. and some young ladies and the dear old professor of a very "select" seminary. drugs and divorce and rape: venereal diseases, drunkenness, murder-- excluded. the prim and the precise, or the exact, the homogeneous, the single, the puritanic, the mathematic, the pure, the perfect. we can have illusion of this state--but only by disregarding its infinite denials. it's a drop of milk afloat in acid that's eating it. the positive swamped by the negative. so it is in intermediateness, where only to "be" positive is to generate corresponding and, perhaps, equal negativeness. in our acceptance, it is, in quasi-existence, premonitory, or pre-natal, or pre-awakening consciousness of a real existence. but this consciousness of realness is the greatest resistance to efforts to realize or to become real--because it is feeling that realness has been attained. our antagonism is not to science, but to the attitude of the sciences that they have finally realized; or to belief, instead of acceptance; to the insufficiency, which, as we have seen over and over, amounts to paltriness and puerility of scientific dogmas and standards. or, if several persons start out to chicago, and get to buffalo, and one be under the delusion that buffalo is chicago, that one will be a resistance to the progress of the others. so astronomy and its seemingly exact, little system-- but data we shall have of round worlds and spindle-shaped worlds, and worlds shaped like a wheel; worlds like titanic pruning hooks; worlds linked together by streaming filaments; solitary worlds, and worlds in hordes: tremendous worlds and tiny worlds: some of them made of material like the material of this earth; and worlds that are geometric super-constructions made of iron and steel-- or not only fall from the sky of ashes and cinders and coke and charcoal and oily substances that suggest fuel--but the masses of iron that have fallen upon this earth. wrecks and flotsam and fragments of vast iron constructions-- or steel. sooner or later we shall have to take up an expression that fragments of steel have fallen from the sky. if fragments not of iron, but of steel have fallen upon this earth-- but what would a deep-sea fish learn even if a steel plate of a wrecked vessel above him should drop and bump him on the nose? our submergence in a sea of conventionality of almost impenetrable density. sometimes i'm a savage who has found something on the beach of his island. sometimes i'm a deep-sea fish with a sore nose. the greatest of mysteries: why don't they ever come here, or send here, openly? of course there's nothing to that mystery if we don't take so seriously the notion--that we must be interesting. it's probably for moral reasons that they stay away--but even so, there must be some degraded ones among them. or physical reasons: when we can specially take up that subject, one of our leading ideas, or credulities, will be that near approach by another world to this world would be catastrophic: that navigable worlds would avoid proximity; that others that have survived have organized into protective remotenesses, or orbits which approximate to regularity, though by no means to the degree of popular supposition. but the persistence of the notion that we must be interesting. bugs and germs and things like that: they're interesting to us: some of them are too interesting. dangers of near approach--nevertheless our own ships that dare not venture close to a rocky shore can send rowboats ashore-- why not diplomatic relations established between the united states and cyclorea--which, in our advanced astronomy, is the name of a remarkable wheel-shaped world or super-construction? why not missionaries sent here openly to convert us from our barbarous prohibitions and other taboos, and to prepare the way for a good trade in ultra-bibles and super-whiskeys; fortunes made in selling us cast-off super-fineries, which we'd take to like an african chief to someone's old silk hat from new york or london? the answer that occurs to me is so simple that it seems immediately acceptable, if we accept that the obvious is the solution of all problems, or if most of our perplexities consist in laboriously and painfully conceiving of the unanswerable, and then looking for answers--using such words as "obvious" and "solution" conventionally-- or: would we, if we could, educate and sophisticate pigs, geese, cattle? would it be wise to establish diplomatic relation with the hen that now functions, satisfied with mere sense of achievement by way of compensation? i think we're property. i should say we belong to something: that once upon a time, this earth was no-man's land, that other worlds explored and colonized here, and fought among themselves for possession, but that now it's owned by something: that something owns this earth--all others warned off. nothing in our own times--perhaps--because i am thinking of certain notes i have--has ever appeared upon this earth, from somewhere else, so openly as columbus landed upon san salvador, or as hudson sailed up his river. but as to surreptitious visits to this earth, in recent times, or as to emissaries, perhaps, from other worlds, or voyagers who have shown every indication of intent to evade and avoid, we shall have data as convincing as our data of oil or coal-burning aerial super-constructions. but, in this vast subject, i shall have to do considerable neglecting or disregarding, myself. i don't see how i can, in this book, take up at all the subject of possible use of humanity to some other mode of existence, or the flattering notion that we can possibly be worth something. pigs, geese, and cattle. first find out that they are owned. then find out the whyness of it. i suspect that, after all, we're useful--that among contesting claimants, adjustment has occurred, or that something now has a legal right to us, by force, or by having paid out analogues of beads for us to former, more primitive, owners of us--all others warned off--that all this has been known, perhaps for ages, to certain ones upon this earth, a cult or order, members of which function like bellwethers to the rest of us, or as superior slaves or overseers, directing us in accordance with instructions received--from somewhere else--in our mysterious usefulness. but i accept that, in the past, before proprietorship was established, inhabitants of a host of other worlds have--dropped here, hopped here, wafted, sailed, flown, motored--walked here, for all i know--been pulled here, been pushed; have come singly, have come in enormous numbers; have visited occasionally, have visited periodically for hunting, trading, replenishing harems, mining: have been unable to stay here, have established colonies here, have been lost here; far-advanced peoples, or things, and primitive peoples or whatever they were: white ones, black ones, yellow ones-- i have a very convincing datum that the ancient britons were blue ones. of course we are told by conventional anthropologists that they only painted themselves blue, but in our own advanced anthropology, they were veritable blue ones-- _annals of philosophy_, - : note of a blue child born in england. that's atavism. giants and fairies. we accept them, of course. or, if we pride ourselves upon being awfully far-advanced, i don't know how to sustain our conceit except by very largely going far back. science of today--the superstition of tomorrow. science of tomorrow--the superstition of today. notice of a stone ax, inches long: inches across broad end. (_proc. soc. of ants. of scotland_, - - .) _amer. antiquarian_, - : copper ax from an ohio mound: inches long; weight pounds. _amer. anthropologist_, n.s., - : stone ax found at birchwood, wisconsin--exhibited in the collection of the missouri historical society--found with "the pointed end embedded in the soil"--for all i know, may have dropped there-- inches long, wide, thick--weight pounds. or the footprints, in sandstone, near carson, nevada--each print to inches long. (_amer. jour. sci._, - - .) these footprints are very clear and well-defined: reproduction of them in the _journal_--but they assimilate with the system, like sour apples to other systems: so prof. marsh, a loyal and unscrupulous systematist, argues: "the size of these footprints and specially the width between the right and left series, are strong evidence that they were not made by men, as has been so generally supposed." so these excluders. stranglers of minerva. desperadoes of disregard. above all, or below all, the anthropologists. i'm inspired with a new insult--someone offends me: i wish to express almost absolute contempt for him--he's a systematistic anthropologist. simply to read something of this kind is not so impressive as to see for one's self: if anyone will take the trouble to look up these footprints, as pictured in the _journal_, he will either agree with prof. marsh or feel that to deny them is to indicate a mind as profoundly enslaved by a system as was ever the humble intellect of a medieval monk. the reasoning of this representative phantom of the chosen, or of the spectral appearances who sit in judgment, or condemnation, upon us of the more nearly real: that there never were giants upon this earth, because gigantic footprints are more gigantic than prints made by men who are not giants. we think of giants as occasional visitors to this earth. of course--stonehenge, for instance. it may be that, as time goes on, we shall have to admit that there are remains of many tremendous habitations of giants upon this earth, and that their appearances here were more than casual--but their bones--or the absence of their bones-- except--that, no matter how cheerful and unsuspicious my disposition may be, when i go to the american museum of natural history, dark cynicisms arise the moment i come to the fossils--or old bones that have been found upon this earth--gigantic things--that have been reconstructed into terrifying but "proper" dinosaurs--but my uncheerfulness-- the dodo did it. on one of the floors below the fossils, they have a reconstructed dodo. it's frankly a fiction: it's labeled as such--but it's been reconstructed so cleverly and so convincingly-- fairies. "fairy crosses." _harper's weekly_, - : that, near the point where the blue ridge and the allegheny mountains unite, north of patrick county, virginia, many little stone crosses have been found. a race of tiny beings. they crucified cockroaches. exquisite beings--but the cruelty of the exquisite. in their diminutive way they were human beings. they crucified. the "fairy crosses," we are told in _harper's weekly_, range in weight from one-quarter of an ounce to an ounce: but it is said, in the _scientific american_, - , that some of them are no larger than the head of a pin. they have been found in two other states, but all in virginia are strictly localized on and along bull mountain. we are reminded of the chinese seals in ireland. i suppose they fell there. some are roman crosses, some st. andrew's, some maltese. this time we are spared contact with the anthropologists and have geologists instead, but i am afraid that the relief to our finer, or more nearly real, sensibilities will not be very great. the geologists were called upon to explain the "fairy crosses." their response was the usual scientific tropism--"geologists say that they are crystals." the writer in _harper's weekly_ points out that this "hold up," or this anæsthetic, if theoretic science be little but attempt to assuage pangs of the unexplained, fails to account for the localized distributions of these objects--which make me think of both aggregation and separation at the bottom of the sea, if from a wrecked ship, similar objects should fall in large numbers but at different times. but some are roman crosses, some st. andrew's, some maltese. conceivably there might be a mineral that would have a diversity of geometric forms, at the same time restricted to some expression of the cross, because snowflakes, for instance, have diversity but restriction to the hexagon, but the guilty geologists, cold-blooded as astronomers and chemists and all the other deep-sea fishes--though less profoundly of the pseudo-saved than the wretched anthropologists--disregarded the very datum--that it was wise to disregard: that the "fairy crosses" are not all made of the same material. it's the same old disregard, or it's the same old psycho-tropism, or process of assimilation. crystals are geometric forms. crystals are included in the system. so then "fairy crosses" are crystals. but that different minerals should, in a few different regions, be inspired to turn into different forms of the cross--is the kind of resistance that we call less nearly real than our own acceptances. we now come to some "cursed" little things that are of the "lost," but for the "salvation" of which scientific missionaries have done their damnedest. "pigmy flints." they can't very well be denied. they're lost and well known. "pigmy flints" are tiny, prehistoric implements. some of them are a quarter of an inch in size. england, india, france, south africa--they've been found in many parts of the world--whether showered there or not. they belong high up in the froth of the accursed: they are not denied, and they have not been disregarded; there is an abundant literature upon this subject. one attempt to rationalize them, or assimilate them, or take them into the scientific fold, has been the notion that they were toys of prehistoric children. it sounds reasonable. but, of course, by the reasonable we mean that for which the equally reasonable, but opposing, has not been found out--except that we modify that by saying that, though nothing's finally reasonable, some phenomena have higher approximations to reasonableness than have others. against the notion of toys, the higher approximation is that where "pygmy flints" are found, all flints are pygmies--at least so in india, where, when larger implements have been found in the same place, there are separations by strata. (wilson.) the datum that, just at present, leads me to accept that these flints were made by beings about the size of pickles, is a point brought out by prof. wilson (_rept. national museum_, - ): not only that the flints are tiny but that the chipping upon them is "minute." struggle for expression, in the mind of a th-century-ite, of an idea that did not belong to his era: in _science gossip_, - , r.a. galty says: "so fine is the chipping that to see the workmanship a magnifying glass is necessary." i think that would be absolutely convincing, if there were anything--absolutely anything--either that tiny beings, from pickle to cucumber-stature, made these things, or that ordinary savages made them under magnifying glasses. the idea that we are now going to develop, or perpetrate, is rather intensely of the accursed, or the advanced. it's a lost soul, i admit--or boast--but it fits in. or, as conventional as ever, our own method is the scientific method of assimilating. it assimilates, if we think of the inhabitants of elvera-- by the way, i forgot to tell the name of the giant's world: monstrator. spindle-shaped world--about , miles along its major axis--more details to be published later. but our coming inspiration fits in, if we think of the inhabitants of elvera as having only visited here: having, in hordes as dense as clouds of bats, come here, upon hunting excursions--for mice, i should say: for bees, very likely--or most likely of all, or inevitably, to convert the heathen here--horrified with anyone who would gorge himself with more than a bean at a time; fearful for the souls of beings who would guzzle more than a dewdrop at a time--hordes of tiny missionaries, determined that right should prevail, determining right by their own minutenesses. they must have been missionaries. only to be is motion to convert or assimilate something else. the idea now is that tiny creatures coming here from their own little world, which may be eros, though i call it elvera, would flit from the exquisite to the enormous--gulp of a fair-sized terrestrial animal--half a dozen of them gone and soon digested. one falls into a brook--torn away in a mighty torrent-- or never anything but conventional, we adopt from darwin: "the geological records are incomplete." their flints would survive, but, as to their fragile bodies--one might as well search for prehistoric frost-traceries. a little whirlwind--elverean carried away a hundred yards--body never found by his companions. they'd mourn for the departed. conventional emotion to have: they'd mourn. there'd have to be a funeral: there's no getting away from funerals. so i adopt an explanation that i take from the anthropologists: burial in effigy. perhaps the elvereans would not come to this earth again until many years later--another distressing occurrence--one little mausoleum for all burials in effigy. london _times_, july , : that, early in july, , some boys were searching for rabbits' burrows in the rocky formation, near edinburgh, known as arthur's seat. in the side of a cliff, they came upon some thin sheets of slate, which they pulled out. little cave. seventeen tiny coffins. three or four inches long. in the coffins were miniature wooden figures. they were dressed differently both in style and material. there were two tiers of eight coffins each, and a third tier begun, with one coffin. the extraordinary datum, which has especially made mystery here: that the coffins had been deposited singly, in the little cave, and at intervals of many years. in the first tier, the coffins were quite decayed, and the wrappings had moldered away. in the second tier, the effects of age had not advanced so far. and the top coffin was quite recent-looking. in the _proceedings of the society of antiquarians of scotland_, - - , there is a full account of this find. three of the coffins and three of the figures are pictured. so elvera with its downy forests and its microscopic oyster shells--and if the elvereans be not very far-advanced, they take baths--with sponges the size of pin heads-- or that catastrophes have occurred: that fragments of elvera have fallen to this earth: in _popular science_, - , francis bingham, writing of the corals and sponges and shells and crinoids that dr. hahn had asserted that he had found in meteorites, says, judging by the photographs of them, that their "notable peculiarity" is their "extreme smallness." the corals, for instance, are about one-twentieth the size of terrestrial corals. "they represent a veritable pygmy animal world," says bingham. the inhabitants of monstrator and elvera were primitives, i think, at the time of their occasional visits to this earth--though, of course, in a quasi-existence, anything that we semi-phantoms call evidence of anything may be just as good evidence of anything else. logicians and detectives and jurymen and suspicious wives and members of the royal astronomic society recognize this indeterminateness, but have the delusion that in the method of agreement there is final, or real evidence. the method is good enough for an "existence" that is only semi-real, but also it is the method of reasoning by which witches were burned, and by which ghosts have been feared. i'd not like to be so unadvanced as to deny witches and ghosts, but i do think that there never have been witches and ghosts like those of popular supposition. but stories of them have been supported by astonishing fabrications of details and of different accounts in agreement. so, if a giant left impressions of his bare feet in the ground, that is not to say that he was a primitive--bulk of culture out taking the kneipp cure. so, if stonehenge is a large, but only roughly geometric construction, the inattention to details by its builders--signifies anything you please--ambitious dwarfs or giants--if giants, that they were little more than cave men, or that they were post-impressionist architects from a very far-advanced civilization. if there are other worlds, there are tutelary worlds--or that kepler, for instance, could not have been absolutely wrong: that his notion of an angel assigned to push along and guide each planet may not be very acceptable, but that, abstractedly, or in the notion of a tutelary relation, we may find acceptance. only to be is to be tutelary. our general expression: that "everything" in intermediateness is not a thing, but is an endeavor to become something--by breaking away from its continuity, or merging away, with all other phenomena--is an attempt to break away from the very essence of a relative existence and become absolute--if it have not surrendered to, or become part of, some higher attempt: that to this process there are two aspects: attraction, or the spirit of everything to assimilate all other things--if it have not given in and subordinated to--or have not been assimilated by--some higher attempted system, unity, organization, entity, harmony, equilibrium-- and repulsion, or the attempt of everything to exclude or disregard the unassimilable. universality of the process: anything conceivable: a tree. it is doing all it can to assimilate substances of the soil and substances of the air, and sunshine, too, into tree-substance: obversely it is rejecting or excluding or disregarding that which it cannot assimilate. cow grazing, pig rooting, tiger stalking: planets trying, or acting, to capture comets; rag pickers and the christian religion, and a cat down headfirst in a garbage can; nations fighting for more territory, sciences correlating the data they can, trust magnates organizing, chorus girl out for a little late supper--all of them stopped somewhere by the unassimilable. chorus girl and the broiled lobster. if she eats not shell and all she represents universal failure to positivize. also, if she does she represents universal failure to positivize: her ensuing disorders will translate her to the negative absolute. or science and some of our cursed hard-shelled data. one speaks of the tutelarian as if it were something distinct in itself. so one speaks of a tree, a saint, a barrel of pork, the rocky mountains. one speaks of missionaries, as if they were positively different, or had identity of their own, or were a species by themselves. to the intermediatist, everything that seems to have identity is only attempted identity, and every species is continuous with all other species, or that which is called the specific is only emphasis upon some aspect of the general. if there are cats, they're only emphasis upon universal felinity. there is nothing that does not partake of that of which the missionary, or the tutelary, is the special. every conversation is a conflict of missionaries, each trying to convert the other, to assimilate, or to make the other similar to himself. if no progress be made, mutual repulsion will follow. if other worlds have ever in the past had relations with this earth, they were attempted positivizations: to extend themselves, by colonies, upon this earth; to convert, or assimilate, indigenous inhabitants of this earth. or parent-worlds and their colonies here-- super-romanimus-- or where the first romans came from. it's as good as the romulus and remus story. super-israelimus-- or that, despite modern reasoning upon this subject, there was once something that was super-parental or tutelary to early orientals. azuria, which was tutelary to the early britons: azuria, whence came the blue britons, whose descendants gradually diluting, like blueing in a wash-tub, where a faucet's turned on, have been most emphasized of sub-tutelarians, or assimilators ever since. worlds that were once tutelarian worlds--before this earth became sole property of one of them--their attempts to convert or assimilate--but then the state that comes to all things in their missionary-frustrations--unacceptance by all stomachs of some things; rejection by all societies of some units; glaciers that sort over and cast out stones-- repulsion. wrath of the baffled missionary. there is no other wrath. all repulsion is reaction to the unassimilable. so then the wrath of azuria-- because surrounding peoples of this earth would not assimilate with her own colonists in the part of the earth that we now call england. i don't know that there has ever been more nearly just, reasonable, or logical wrath, in this earth's history--if there is no other wrath. the wrath of azuria, because the other peoples of this earth would not turn blue to suit her. history is a department of human delusion that interests us. we are able to give a little advancement to history. in the vitrified forts of a few parts of europe, we find data that the humes and gibbons have disregarded. the vitrified forts surrounding england, but not in england. the vitrified forts of scotland, ireland, brittany, and bohemia. or that, once upon a time, with electric blasts, azuria tried to swipe this earth clear of the peoples who resisted her. the vast blue bulk of azuria appeared in the sky. clouds turned green. the sun was formless and purple in the vibrations of wrath that were emanating from azuria. the whitish, or yellowish, or brownish peoples of scotland, ireland, brittany, and bohemia fled to hilltops and built forts. in a real existence, hilltops, or easiest accessibility to an aerial enemy, would be the last choice in refuges. but here, in quasi-existence, if we're accustomed to run to hilltops, in times of danger, we run to them just the same, even with danger closest to hilltops. very common in quasi-existence: attempt to escape by running closer to the pursuing. they built forts, or already had forts, on hilltops. something poured electricity upon them. the stones of these forts exist to this day, vitrified, or melted and turned to glass. the archaeologists have jumped from one conclusion to another, like the "rapid chamois" we read of a while ago, to account for vitrified forts, always restricted by the commandment that unless their conclusions conformed to such tenets as exclusionism, of the system, they would be excommunicated. so archaeologists, in their medieval dread of excommunication, have tried to explain vitrified forts in terms of terrestrial experience. we find in their insufficiencies the same old assimilating of all that could be assimilated, and disregard for the unassimilable, conventionalizing into the explanation that vitrified forts were made by prehistoric peoples who built vast fires--often remote from wood-supply--to melt externally, and to cement together, the stones of their constructions. but negativeness always: so within itself a science can never be homogeneous or unified or harmonious. so miss russel, in the _journal of the b.a.a._, has pointed out that it is seldom that single stones, to say nothing of long walls, of large houses that are burned to the ground, are vitrified. if we pay a little attention to this subject, ourselves, before starting to write upon it, which is one of the ways of being more nearly real than oppositions so far encountered by us, we find: that the stones of these forts are vitrified in no reference to cementing them: that they are cemented here and there, in streaks, as if special blasts had struck, or played, upon them. then one thinks of lightning? once upon a time something melted, in streaks, the stones of forts on the tops of hills in scotland, ireland, brittany, and bohemia. lightning selects the isolated and conspicuous. but some of the vitrified forts are not upon tops of hills: some are very inconspicuous: their walls too are vitrified in streaks. something once had effect, similar to lightning, upon forts, mostly on hills, in scotland, ireland, brittany, and bohemia. but upon hills, all over the rest of the world, are remains of forts that are not vitrified. there is only one crime, in the local sense, and that is not to turn blue, if the gods are blue: but, in the universal sense, the one crime is not to turn the gods themselves green, if you're green. one of the most extraordinary of phenomena, or alleged phenomena, of psychic research, or alleged research--if in quasi-existence there never has been real research, but only approximations to research that merge away, or that are continuous with, prejudice and convenience-- "stone-throwing." it's attributed to poltergeists. they're mischievous spirits. poltergeists do not assimilate with our own present quasi-system, which is an attempt to correlate denied or disregarded data as phenomena of extra-telluric forces, expressed in physical terms. therefore i regard poltergeists as evil or false or discordant or absurd--names that we give to various degrees or aspects of the unassimilable, or that which resists attempts to organize, harmonize, systematize, or, in short, to positivize--names that we give to our recognitions of the negative state. i don't care to deny poltergeists, because i suspect that later, when we're more enlightened, or when we widen the range of our credulities, or take on more of that increase of ignorance that is called knowledge, poltergeists may become assimilable. then they'll be as reasonable as trees. by reasonableness i mean that which assimilates with a dominant force, or system, or a major body of thought--which is, itself, of course, hypnosis and delusion--developing, however, in our acceptance, to higher and higher approximations to realness. the poltergeists are now evil or absurd to me, proportionately to their present unassimilableness, compounded, however, with the factor of their possible future assimilableness. we lug in the poltergeists, because some of our own data, or alleged data, merge away indistinguishably with data, or alleged data, of them: instances of stones that have been thrown, or that have fallen, upon a small area, from an unseen and undetectable source. london _times_, april , : "from o'clock, thursday afternoon, until half past eleven, thursday night, the houses, and reverdy road, bermondsey, were assailed with stones and other missiles coming from an unseen quarter. two children were injured, every window broken, and several articles of furniture were destroyed. although there was a strong body of policemen scattered in the neighborhood, they could not trace the direction whence the stones were thrown." "other missiles" make a complication here. but if the expression means tin cans and old shoes, and if we accept that the direction could not be traced because it never occurred to anyone to look upward--why, we've lost a good deal of our provincialism by this time. london _times_, sept. , : that, in the home of mrs. charton, at sutton courthouse, sutton lane, chiswick, windows had been broken "by some unseen agent." every attempt to detect the perpetrator failed. the mansion was detached and surrounded by high walls. no other building was near it. the police were called. two constables, assisted by members of the household, guarded the house, but the windows continued to be broken "both in front and behind the house." or the floating islands that are often stationary in the super-sargasso sea; and atmospheric disturbances that sometimes affect them, and bring things down within small areas, upon this earth, from temporarily stationary sources. super-sargasso sea and the beaches of its floating islands from which i think, or at least accept, pebbles have fallen: wolverhampton, england, june, --violent storm--fall of so many little black pebbles that they were cleared away by shoveling (_la sci. pour tous_, - ); great number of small black stones that fell at birmingham, england, august, --violent storm--said to be similar to some basalt a few leagues from birmingham (_rept. brit. assoc._, - ); pebbles described as "common water-worn pebbles" that fell at palestine, texas, july , --"of a formation not found near palestine" (w.h. perry, sergeant, signal corps, _monthly weather review_, july, ); round, smooth pebbles at kandahor, (_am. j. sci._, - - ); "a number of stones of peculiar formation and shapes, unknown in this neighborhood, fell in a tornado at hillsboro, ill., may , ." (_monthly weather review_, may, .) pebbles from aerial beaches and terrestrial pebbles as products of whirlwinds, so merge in these instances that, though it's interesting to hear of things of peculiar shape that have fallen from the sky, it seems best to pay little attention here, and to find phenomena of the super-sargasso sea remote from the merger: to this requirement we have three adaptations: pebbles that fell where no whirlwind to which to attribute them could be learned of: pebbles which fell in hail so large that incredibly could that hail have been formed in this earth's atmosphere: pebbles which fell and were, long afterward, followed by more pebbles, as if from some aerial, stationary source, in the same place. in september, , there was a story in a new york newspaper, of lightning--or an appearance of luminosity?--in jamaica--something had struck a tree: near the tree were found some small pebbles. it was said that the pebbles had fallen from the sky, with the lightning. but the insult to orthodoxy was that they were not angular fragments such as might have been broken from a stony meteorite: that they were "water-worn pebbles." in the geographical vagueness of a mainland, the explanation "up from one place and down in another" is always good, and is never overworked, until the instances are massed as they are in this book: but, upon this occasion, in the relatively small area of jamaica, there was no whirlwind findable--however "there in the first place" bobs up. _monthly weather review_, august, - : that the government meteorologist had investigated: had reported that a tree had been struck by lightning, and that small water-worn pebbles had been found near the tree: but that similar pebbles could be found all over jamaica. _monthly weather review_, september, - : prof. fassig gives an account of a fall of hail that occurred in maryland, june , : hailstones the size of baseballs "not at all uncommon." "an interesting, but unconfirmed, account stated that small pebbles were found at the center of some of the larger hail gathered at annapolis. the young man who related the story offered to produce the pebbles, but has not done so." a footnote: "since writing this, the author states that he has received some of the pebbles." when a young man "produces" pebbles, that's as convincing as anything else i've ever heard of, though no more convincing than, if having told of ham sandwiches falling from the sky, he should "produce" ham sandwiches. if this "reluctance" be admitted by us, we correlate it with a datum reported by a weather bureau observer, signifying that, whether the pebbles had been somewhere aloft a long time or not, some of the hailstones that fell with them, had been. the datum is that some of these hailstones were composed of from twenty to twenty-five layers alternately of clear ice and snow-ice. in orthodox terms i argue that a fair-sized hailstone falls from the clouds with velocity sufficient to warm it so that it would not take on even one layer of ice. to put on twenty layers of ice, i conceive of something that had not fallen at all, but had rolled somewhere, at a leisurely rate, for a long time. we now have a commonplace datum that is familiar in two respects: little, symmetric objects of metal that fell at orenburg, russia, september, (_phil. mag._, - - ). a second fall of these objects, at orenburg, russia, jan. , (_quar. jour. roy. inst._, - - ). i now think of the disk of tarbes, but when first i came upon these data i was impressed only with recurrence, because the objects of orenburg were described as crystals of pyrites, or sulphate of iron. i had no notion of metallic objects that might have been shaped or molded by means other than crystallization, until i came to arago's account of these occurrences (_oeuvres_, - ). here the analysis gives per cent. red oxide of iron, and sulphur and loss by ignition per cent. it seems to me acceptable that iron with considerably less than per cent. sulphur in it is not iron pyrites--then little, rusty iron objects, shaped by some other means, have fallen, four months apart, at the same place. m. arago expresses astonishment at this phenomenon of recurrence so familiar to us. altogether, i find opening before us, vistas of heresies to which i, for one, must shut my eyes. i have always been in sympathy with the dogmatists and exclusionists: that is plain in our opening lines: that to seem to be is falsely and arbitrarily and dogmatically to exclude. it is only that exclusionists who are good in the nineteenth century are evil in the twentieth century. constantly we feel a merging away into infinitude; but that this book shall approximate to form, or that our data shall approximate to organization, or that we shall approximate to intelligibility, we have to call ourselves back constantly from wandering off into infinitude. the thing that we do, however, is to make our own outline, or the difference between what we include and what we exclude, vague. the crux here, and the limit beyond which we may not go--very much--is: acceptance that there is a region that we call the super-sargasso sea--not yet fully accepted, but a provisional position that has received a great deal of support-- but is it a part of this earth, and does it revolve with and over this earth-- or does it flatly overlie this earth, not revolving with and over this earth-- that this earth does not revolve, and is not round, or roundish, at all, but is continuous with the rest of its system, so that, if one could break away from the traditions of the geographers, one might walk and walk, and come to mars, and then find mars continuous with jupiter? i suppose some day such queries will sound absurd--the thing will be so obvious-- because it is very difficult for me to conceive of little metallic objects hanging precisely over a small town in russia, for four months, if revolving, unattached, with a revolving earth-- it may be that something aimed at that town, and then later took another shot. these are speculations that seem to me to be evil relatively to these early years in the twentieth century-- just now, i accept that this earth is--not round, of course: that is very old-fashioned--but roundish, or, at least, that it has what is called form of its own, and does revolve upon its axis, and in an orbit around the sun. i only accept these old traditional notions-- and that above it are regions of suspension that revolve with it: from which objects fall, by disturbances of various kinds, and then, later, fall again, in the same place: _monthly weather review_, may, - : report from the signal service observer, at bismarck, dakota: that, at o'clock, in the evening of may , , sharp sounds were heard throughout the city, caused by a fall of flinty stones striking against windows. fifteen hours later another fall of flinty stones occurred at bismarck. there is no report of stones having fallen anywhere else. this is a thing of the ultra-damned. all editors of scientific publications read the _monthly weather review_ and frequently copy from it. the noise made by the stones of bismarck, rattling against those windows, may be in a language that aviators will some day interpret: but it was a noise entirely surrounded by silences. of this ultra-damned thing, there is no mention, findable by me, in any other publication. the size of some hailstones has worried many meteorologists--but not text-book meteorologists. i know of no more serene occupation than that of writing text-books--though writing for the _war cry_, of the salvation army, may be equally unadventurous. in the drowsy tranquillity of a text-book, we easily and unintelligently read of dust particles around which icy rain forms, hailstones, in their fall, then increasing by accretion--but in the meteorological journals, we read often of air-spaces nucleating hailstones-- but it's the size of the things. dip a marble in icy water. dip and dip and dip it. if you're a resolute dipper, you will, after a while, have an object the size of a baseball--but i think a thing could fall from the moon in that length of time. also the strata of them. the maryland hailstones are unusual, but a dozen strata have often been counted. ferrel gives an instance of thirteen strata. such considerations led prof. schwedoff to argue that some hailstones are not, and cannot, be generated in this earth's atmosphere--that they come from somewhere else. now, in a relative existence, nothing can of itself be either attractive or repulsive: its effects are functions of its associations or implications. many of our data have been taken from very conservative scientific sources: it was not until their discordant implications, or irreconcilabilities with the system, were perceived, that excommunication was pronounced against them. prof. schwedoff's paper was read before the british association (_rept. of _, p. ). the implication, and the repulsiveness of the implication to the snug and tight little exclusionists of --though we hold out that they were functioning well and ably relatively to -- that there is water--oceans or lakes and ponds, or rivers of it--that there is water away from, and yet not far-remote from, this earth's atmosphere and gravitation-- the pain of it: that the snug little system of would be ousted from its reposefulness-- a whole new science to learn: the science of super-geography-- and science is a turtle that says that its own shell encloses all things. so the members of the british association. to some of them prof. schwedoff's ideas were like slaps on the back of an environment-denying turtle: to some of them his heresy was like an offering of meat, raw and dripping, to milk-fed lambs. some of them bleated like lambs, and some of them turled like turtles. we used to crucify, but now we ridicule: or, in the loss of vigor of all progress, the spike has etherealized into the laugh. sir william thomson ridiculed the heresy, with the phantomosities of his era: that all bodies, such as hailstones, if away from this earth's atmosphere, would have to move at planetary velocity--which would be positively reasonable if the pronouncements of st. isaac were anything but articles of faith--that a hailstone falling through this earth's atmosphere, with planetary velocity, would perform , times as much work as would raise an equal weight of water one degree centigrade, and therefore never fall as a hailstone at all; be more than melted--super-volatalized-- these turls and these bleats of pedantry--though we insist that, relatively to , these turls and bleats should be regarded as respectfully as we regard rag dolls that keep infants occupied and noiseless--it is the survival of rag dolls into maturity that we object to--so these pious and naïve ones who believed that , times something could have--that is, in quasi-existence--an exact and calculable resultant, whereas there is--in quasi-existence--nothing that can, except by delusion and convenience, be called a unit, in the first place--whose devotions to st. isaac required blind belief in formulas of falling bodies-- against data that were piling up, in their own time, of slow-falling meteorites; "milk warm" ones admitted even by farrington and merrill; at least one icy meteorite nowhere denied by the present orthodoxy, a datum as accessible to thomson, in , as it is now to us, because it was an occurrence of . beans and needles and tacks and a magnet. needles and tacks adhere to and systematize relatively to a magnet, but, if some beans, too, be caught up, they are irreconcilables to this system and drop right out of it. a member of the salvation army may hear over and over data that seem so memorable to an evolutionist. it seems remarkable that they do not influence him--one finds that he cannot remember them. it is incredible that sir william thomson had never heard of slow-falling, cold meteorites. it is simply that he had no power to remember such irreconcilabilities. and then mr. symons again. mr. symons was a man who probably did more for the science of meteorology than did any other man of his time: therefore he probably did more to hold back the science of meteorology than did any other man of his time. in _nature_, - , mr. symons says that prof. schwedoff's ideas are "very droll." i think that even more amusing is our own acceptance that, not very far above this earth's surface, is a region that will be the subject of a whole new science--super-geography--with which we shall immortalize ourselves in the resentments of the schoolboys of the future-- pebbles and fragments of meteors and things from mars and jupiter and azuria: wedges, delayed messages, cannon balls, bricks, nails, coal and coke and charcoal and offensive old cargoes--things that coat in ice in some regions and things that get into areas so warm that they putrefy--or that there are all the climates of geography in super-geography. i shall have to accept that, floating in the sky of this earth, there often are fields of ice as extensive as those on the arctic ocean--volumes of water in which are many fishes and frogs--tracts of land covered with caterpillars-- aviators of the future. they fly up and up. then they get out and walk. the fishing's good: the bait's right there. they find messages from other worlds--and within three weeks there's a big trade worked up in forged messages. sometime i shall write a guide book to the super-sargasso sea, for aviators, but just at present there wouldn't be much call for it. we now have more of our expression upon hail as a concomitant, or more data of things that have fallen from the sky, with hail. in general, the expression is: these things may have been raised from some other part of the earth's surface, in whirlwinds, or may not have fallen, and may have been upon the ground, in the first place--but were the hailstones found with them, raised from some other part of the earth's surface, or were the hailstones upon the ground, in the first place? as i said before, this expression is meaningless as to a few instances; it is reasonable to think of some coincidence between the fall of hail and the fall of other things: but, inasmuch as there have been a good many instances,--we begin to suspect that this is not so much a book we're writing as a sanitarium for overworked coincidences. if not conceivably could very large hailstones and lumps of ice form in this earth's atmosphere, and so then had to come from external regions, then other things in or accompanying very large hailstones and lumps of ice came from external regions--which worries us a little: we may be instantly translated to the positive absolute. _cosmos_, - , quotes a virginia newspaper, that fishes said to have been catfishes, a foot long, some of them, had fallen, in , at norfolk, virginia, with hail. vegetable débris, not only nuclear, but frozen upon the surfaces of large hailstones, at toulouse, france, july , . (_la science pour tous_, - .) description of a storm, at pontiac, canada, july , , in which it is said that it was not hailstones that fell, but "pieces of ice, from half an inch to over two inches in diameter" (_canadian naturalist_, - - ): "but the most extraordinary thing is that a respectable farmer, of undoubted veracity, says he picked up a piece of hail, or ice, in the center of which was a small green frog." storm at dubuque, iowa, june , , in which fell hailstones and pieces of ice (_monthly weather review_, june, ): "the foreman of the novelty iron works, of this city, states that in two large hailstones melted by him were found small living frogs." but the pieces of ice that fell upon this occasion had a peculiarity that indicates--though by as bizarre an indication as any we've had yet--that they had been for a long time motionless or floating somewhere. we'll take that up soon. _living age_, - : that, june , , fishes, one of which was ten inches long, fell at boston; that, eight days later, fishes and ice fell at derby. in timb's _year book_, - , it is said that, at derby, the fishes had fallen in enormous numbers; from half an inch to two inches long, and some considerably larger. in the _athenæum_, - , copied from the sheffield _patriot_, it is said that one of the fishes weighed three ounces. in several accounts, it is said that, with the fishes, fell many small frogs and "pieces of half-melted ice." we are told that the frogs and the fishes had been raised from some other part of the earth's surface, in a whirlwind; no whirlwind specified; nothing said as to what part of the earth's surface comes ice, in the month of july--interests us that the ice is described as "half-melted." in the london _times_, july , , it is said that the fishes were sticklebacks; that they had fallen with ice and small frogs, many of which had survived the fall. we note that, at dunfermline, three months later (oct. , ) fell many fishes, several inches in length, in a thunderstorm. (london _times_, oct. , .) hailstones, we don't care so much about. the matter of stratification seems significant, but we think more of the fall of lumps of ice from the sky, as possible data of the super-sargasso sea: lumps of ice, a foot in circumference, derbyshire, england, may , (_annual register_, - ); cuboidal mass, six inches in diameter, that fell at birmingham, days later (thomson, _intro. to meteorology_, p. ); size of pumpkins, bangalore, india, may , (_rept. brit. assoc._, - ); masses of ice of a pound and a half each, new hampshire, aug. , (lummis, _meteorology_, p. ); masses of ice, size of a man's head, in the delphos tornado (ferrel, _popular treatise_, p. ); large as a man's hand, killing thousands of sheep, texas, may , (_monthly weather review_, may, ); "pieces of ice so large that they could not be grasped in one hand," in a tornado, in colorado, june , (_monthly weather review_, june, ); lumps of ice four and a half inches long, richmond, england, aug. , (_symons' met. mag._, - ); mass of ice, inches in circumference that fell with hail, iowa, june, (_monthly weather review_, june, ); "pieces of ice" eight inches long, and an inch and a half thick, davenport, iowa, aug. , (_monthly weather review_, aug., ); lump of ice size of a brick; weight two pounds, chicago, july , (_monthly weather review_, july, ); lumps of ice that weighed one pound and a half each, india, may (?), (_nature_, - ); lump of ice weighing four pounds, texas, dec. , (_sc. am._, - ); lumps of ice one pound in weight, nov. , , in a tornado, victoria (_meteorology of australia_, p. ). of course it is our acceptance that these masses not only accompanied tornadoes, but were brought down to this earth by tornadoes. flammarion, _the atmosphere_, p. : block of ice, weighing four and a half pounds that fell at cazorta, spain, june , ; block of ice, weighing eleven pounds, at cette, france, october, ; mass of ice three feet long, three feet wide, and more than two feet thick, that fell, in a storm, in hungary, may , . _scientific american_, - : that, according to the _salina journal_, a mass of ice weighing about pounds had fallen from the sky, near salina, kansas, august, . we are told that mr. w.j. hagler, the north santa fé merchant became possessor of it, and packed it in sawdust in his store. london _times_, april , : that, upon the th of march, , in a snowstorm, in upper wasdale, blocks of ice, so large that at a distance they looked like a flock of sheep, had fallen. _rept. brit. assoc._, - : that a mass of ice about a cubic yard in size had fallen at candeish, india, . against these data, though, so far as i know, so many of them have never been assembled together before, there is a silence upon the part of scientific men that is unusual. our super-sargasso sea may not be an unavoidable conclusion, but arrival upon this earth of ice from external regions does seem to be--except that there must be, be it ever so faint, a merger. it is in the notion that these masses of ice are only congealed hailstones. we have data against this notion, as applied to all our instances, but the explanation has been offered, and, it seems to me, may apply in some instances. in the _bull. soc. astro. de france_, - , it is said of blocks of ice the size of decanters that had fallen at tunis that they were only masses of congealed hailstones. london _times_, aug. , . that a block of ice, described as "pure" ice, weighing pounds, had been found in the meadow of mr. warner, of cricklewood. there had been a storm the day before. as in some of our other instances, no one had seen this object fall from the sky. it was found after the storm: that's all that can be said about it. letter from capt. blakiston, communicated by gen. sabine, to the royal society (_london roy. soc. proc._, - ): that, jan. , , in a thunderstorm, pieces of ice had fallen upon capt. blakiston's vessel--that it was not hail. "it was not hail, but irregular-shaped pieces of solid ice of different dimensions, up to the size of half a brick." according to the _advertiser-scotsman_, quoted by the edinburgh _new philosophical magazine_, - , an irregular-shaped mass of ice fell at ord, scotland, august, , after "an extraordinary peal of thunder." it is said that this was homogeneous ice, except in a small part, which looked like congealed hailstones. the mass was about feet in circumference. the story, as told in the london _times_, aug. , , is that, upon the evening of the th of august, , after a loud peal of thunder, a mass of ice said to have been feet in circumference, had fallen upon the estate of mr. moffat, of balvullich, ross-shire. it is said that this object fell alone, or without hailstones. altogether, though it is not so strong for the super-sargasso sea, i think this is one of our best expressions upon external origins. that large blocks of ice could form in the moisture of this earth's atmosphere is about as likely as that blocks of stone could form in a dust whirl. of course, if ice or water comes to this earth from external sources, we think of at least minute organisms in it, and on, with our data, to frogs, fishes; on to anything that's thinkable, coming from external sources. it's of great importance to us to accept that large lumps of ice have fallen from the sky, but what we desire most--perhaps because of our interest in its archaeologic and palaeontologic treasures--is now to be through with tentativeness and probation, and to take the super-sargasso sea into full acceptance in our more advanced fold of the chosen of this twentieth century. in the _report of the british association_, - , it is said that, at poorhundur, india, dec. , , flat pieces of ice, many of them weighing several pounds--each, i suppose--had fallen from the sky. they are described as "large ice-flakes." vast fields of ice in the super-arctic regions, or strata, of the super-sargasso sea. when they break up, their fragments are flake-like. in our acceptance, there are aerial ice-fields that are remote from this earth; that break up, fragments grinding against one another, rolling in vapor and water, of different constituency in different regions, forming slowly as stratified hailstones--but that there are ice-fields near this earth, that break up into just such flat pieces of ice as cover any pond or river when ice of a pond or river is broken, and are sometimes soon precipitated to the earth, in this familiar flat formation. _symons' met. mag._, - : a correspondent writes that, at braemar, july , , when the sky was clear overhead, and the sun shining, flat pieces of ice fell--from somewhere. the sun was shining, but something was going on somewhere: thunder was heard. until i saw the reproduction of a photograph in the _scientific american_, feb. , , i had supposed that these ice-fields must be, say, at least ten or twenty miles away from this earth, and invisible, to terrestrial observers, except as the blurs that have so often been reported by astronomers and meteorologists. the photograph published by the _scientific american_ is of an aggregation supposed to be clouds, presumably not very high, so clearly detailed are they. the writer says that they looked to him like "a field of broken ice." beneath is a picture of a conventional field of ice, floating ordinarily in water. the resemblance between the two pictures is striking--nevertheless, it seems to me incredible that the first of the photographs could be of an aerial ice-field, or that gravitation could cease to act at only a mile or so from this earth's surface-- unless: the exceptional: the flux and vagary of all things. or that normally this earth's gravitation extends, say, ten or fifteen miles outward--but that gravitation must be rhythmic. of course, in the pseudo-formulas of astronomers, gravitation as a fixed quantity is essential. accept that gravitation is a variable force, and astronomers deflate, with a perceptible hissing sound, into the punctured condition of economists, biologists, meteorologists, and all the others of the humbler divinities, who can admittedly offer only insecure approximations. we refer all who would not like to hear the hiss of escaping arrogance, to herbert spencer's chapters upon the rhythm of all phenomena. if everything else--light from the stars, heat from the sun, the winds and the tides; forms and colors and sizes of animals; demands and supplies and prices; political opinions and chemic reactions and religious doctrines and magnetic intensities and the ticking of clocks; and arrival and departure of the seasons--if everything else is variable, we accept that the notion of gravitation as fixed and formulable is only another attempted positivism, doomed, like all other illusions of realness in quasi-existence. so it is intermediatism to accept that, though gravitation may approximate higher to invariability than do the winds, for instance, it must be somewhere between the absolutes of stability and instability. here then we are not much impressed with the opposition of physicists and astronomers, fearing, a little mournfully, that their language is of expiring sibilations. so then the fields of ice in the sky, and that, though usually so far away as to be mere blurs, at times they come close enough to be seen in detail. for description of what i call a "blur," see _pop. sci. news_, february, --sky, in general, unusually clear, but, near the sun, "a white, slightly curdled haze, which was dazzlingly bright." we accept that sometimes fields of ice pass between the sun and the earth: that many strata of ice, or very thick fields of ice, or superimposed fields would obscure the sun--that there have been occasions when the sun was eclipsed by fields of ice: flammarion, _the atmosphere_, p. : that a profound darkness came upon the city of brussels, june , : there fell flat pieces of ice, an inch long. intense darkness at aitkin, minn., april , : sand and "solid chunks of ice" reported to have fallen (_science_, april , ). in _symons' meteorological magazine_, - , are outlined rough-edged but smooth-surfaced pieces of ice that fell at manassas, virginia, aug. , . they look as much like the roughly broken fragments of a smooth sheet of ice--as ever have roughly broken fragments of a smooth sheet of ice looked. about two inches across, and one inch thick. in _cosmos_, - , it is said that, at rouen, july , , fell irregular-shaped pieces of ice, about the size of a hand, described as looking as if all had been broken from one enormous block of ice. that, i think, was an aerial iceberg. in the awful density, or almost absolute stupidity of the th century, it never occurred to anybody to look for traces of polar bears or of seals upon these fragments. of course, seeing what we want to see, having been able to gather these data only because they are in agreement with notions formed in advance, we are not so respectful to our own notions as to a similar impression forced upon an observer who had no theory or acceptance to support. in general, our prejudices see and our prejudices investigate, but this should not be taken as an absolute. _monthly weather review_, july, : that, from the weather bureau, of portland, oregon, a tornado, of june , , was reported. fragments of ice fell from the sky. they averaged three to four inches square, and about an inch thick. in length and breadth they had the smooth surfaces required by our acceptance: and, according to the writer in the _review_, "gave the impression of a vast field of ice suspended in the atmosphere, and suddenly broken into fragments about the size of the palm of the hand." this datum, profoundly of what we used to call the "damned," or before we could no longer accept judgment, or cut and dried condemnation by infants, turtles, and lambs, was copied--but without comment--in the _scientific american_, - . our theology is something like this: of course we ought to be damned--but we revolt against adjudication by infants, turtles, and lambs. we now come to some remarkable data in a rather difficult department of super-geography. vast fields of aerial ice. there's a lesson to me in the treachery of the imaginable. most of our opposition is in the clearness with which the conventional, but impossible, becomes the imaginable, and then the resistant to modifications. after it had become the conventional with me, i conceived clearly of vast sheets of ice, a few miles above this earth--then the shining of the sun, and the ice partly melting--that note upon the ice that fell at derby--water trickling and forming icicles upon the lower surface of the ice sheet. i seemed to look up and so clearly visualized those icicles hanging like stalactites from a flat-roofed cave, in white calcite. or i looked up at the under side of an aerial ice-lump, and seemed to see a papillation similar to that observed by a calf at times. but then--but then--if icicles should form upon the under side of a sheet of aerial ice, that would be by the falling of water toward this earth; an icicle is of course an expression of gravitation--and, if water melting from ice should fall toward this earth, why not the ice itself fall before an icicle could have time to form? of course, in quasi-existence, where everything is a paradox, one might argue that the water falls, but the ice does not, because the ice is heavier--that is, in masses. that notion, i think, belongs in a more advanced course than we are taking at present. our expression upon icicles: a vast field of aerial ice--it is inert to this earth's gravitation--but by universal flux and variation, part of it sags closer to this earth, and is susceptible to gravitation--by cohesion with the main mass, this part does not fall, but water melting from it does fall, and forms icicles--then, by various disturbances, this part sometimes falls in fragments that are protrusive with icicles. of the ice that fell, some of it enclosing living frogs, at dubuque, iowa, june , , it is said (_monthly weather review_, june, ) that there were pieces from one to seventeen inches in circumference, the largest weighing one pound and three-quarters--that upon some of them were icicles half an inch in length. we emphasize that these objects were not hailstones. the only merger is that of knobby hailstones, or of large hailstones with protuberances wrought by crystallization: but that is no merger with terrestrial phenomena, and such formations are unaccountable to orthodoxy; or it is incredible that hail could so crystallize--not forming by accretion--in the fall of a few seconds. for an account of such hailstones, see _nature_, - . note the size--"some of them the size of turkeys' eggs." it is our expression that sometimes the icicles themselves have fallen, as if by concussion, or as if something had swept against the under side of an aerial ice floe, detaching its papillations. _monthly weather review_, june, : that, at oswego, n.y., june , , according to the turin (n.y.) _leader_, there fell, in a thunderstorm, pieces of ice that "resembled the fragments of icicles." _monthly weather review_, - : that on florence island, st. lawrence river, aug. , , with ordinary hail, fell pieces of ice "formed like icicles, the size and shape of lead pencils that had been cut into sections about three-eighths of an inch in length." so our data of the super-sargasso sea, and its arctic region: and, for weeks at a time, an ice field may hang motionless over a part of this earth's surface--the sun has some effect upon it, but not much until late in the afternoon, i should say--part of it has sagged, but is held up by cohesion with the main mass--whereupon we have such an occurrence as would have been a little uncanny to us once upon a time--or fall of water from a cloudless sky, day after day, in one small part of this earth's surface, late in the afternoon, when the sun's rays had had time for their effects: _monthly weather review_, october, : that, according to the charlotte _chronicle_, oct. , , for three weeks there had been a fall of water from the sky, in charlotte, n.c., localized in one particular spot, every afternoon, about three o'clock; that, whether the sky was cloudy or cloudless, the water or rain fell upon a small patch of land between two trees and nowhere else. this is the newspaper account, and, as such, it seems in the depths of the unchosen, either by me or any other expression of the salvation army. the account by the signal service observer, at charlotte, published in the _review_, follows: "an unusual phenomenon was witnessed on the st: having been informed that, for some weeks prior to date, rain had been falling daily, after p.m., on a particular spot, near two trees, corner of th and d streets, i visited the place, and saw precipitation in the form of rain drops at : and : p.m., while the sun was shining brightly. on the nd, i again visited the place, and from : to : p.m., a light shower of rain fell from a cloudless sky.... sometimes the precipitation falls over an area of half an acre, but always appears to center at these two trees, and when lightest occurs there only." we see conventionally. it is not only that we think and act and speak and dress alike, because of our surrender to social attempt at entity, in which we are only super-cellular. we see what it is "proper" that we should see. it is orthodox enough to say that a horse is not a horse, to an infant--any more than is an orange an orange to the unsophisticated. it's interesting to walk along a street sometimes and look at things and wonder what they'd look like, if we hadn't been taught to see horses and trees and houses as horses and trees and houses. i think that to super-sight they are local stresses merging indistinguishably into one another, in an all-inclusive nexus. i think that it would be credible enough to say that many times have monstrator and elvera and azuria crossed telescopic fields of vision, and were not even seen--because it wouldn't be proper to see them; it wouldn't be respectable, and it wouldn't be respectful: it would be insulting to old bones to see them: it would bring on evil influences from the relics of st. isaac to see them. but our data: of vast worlds that are orbitless, or that are navigable, or that are adrift in inter-planetary tides and currents: the data that we shall have of their approach, in modern times, within five or six miles of this earth-- but then their visits, or approaches, to other planets, or to other of the few regularized bodies that have surrendered to the attempted entity of this solar system as a whole-- the question that we can't very well evade: have these other worlds, or super-constructions, ever been seen by astronomers? i think there would not be much approximation to realness in taking refuge in the notion of astronomers who stare and squint and see only that which it is respectable and respectful to see. it is all very well to say that astronomers are hypnotics, and that an astronomer looking at the moon is hypnotized by the moon, but our acceptance is that the bodies of this present expression often visit the moon, or cross it, or are held in temporary suspension near it--then some of them must often have been within the diameter of an astronomer's hypnosis. our general expression: that, upon the oceans of this earth, there are regularized vessels, but also that there are tramp vessels: that, upon the super-ocean, there are regularized planets, but also that there are tramp worlds: that astronomers are like mercantile purists who would deny commercial vagabondage. our acceptance is that vast celestial vagabonds have been excluded by astronomers, primarily because their irresponsibilities are an affront to the pure and the precise, or to attempted positivism; and secondarily because they have not been seen so very often. the planets steadily reflect the light of the sun: upon this uniformity a system that we call primary astronomy has been built up; but now the subject-matter of advanced astronomy is data of celestial phenomena that are sometimes light and sometimes dark, varying like some of the satellites of jupiter, but with a wider range. however, light or dark, they have been seen and reported so often that the only important reason for their exclusion is--that they don't fit in. with dark bodies that are probably external to our own solar system, i have, in the provincialism that no one can escape, not much concern. dark bodies afloat in outer space would have been damned a few years ago, but now they're sanctioned by prof. barnard--and, if he says they're all right, you may think of them without the fear of doing something wrong or ridiculous--the close kinship we note so often between the evil and the absurd--i suppose by the ridiculous i mean the froth of evil. the dark companion of algol, for instance. though that's a clear case of celestial miscegenation, the purists, or positivists, admit that's so. in the _proceedings of the national academy of science_, - , prof. barnard writes of an object--he calls it an "object"--in cephus. his idea is that there are dark, opaque bodies outside this solar system. but in the _astrophysical journal_, - , he modifies into regarding them as "dark nebulæ." that's not so interesting. we accept that venus, for instance, has often been visited by other worlds, or by super-constructions, from which come ciders and coke and coal; that sometimes these things have reflected light and have been seen from this earth--by professional astronomers. it will be noted that throughout this chapter our data are accursed brahmins--as, by hypnosis and inertia, we keep on and keep on saying, just as a good many of the scientists of the th century kept on and kept on admitting the power of the system that preceded them--or continuity would be smashed. there's a big chance here for us to be instantaneously translated to the positive absolute--oh, well-- what i emphasize here is that our damned data are observations by astronomers of the highest standing, excommunicated by astronomers of similar standing--but backed up by the dominant spirit of their era--to which all minds had to equilibrate or be negligible, unheard, submerged. it would seem sometimes, in this book, as if our revolts were against the dogmatisms and pontifications of single scientists of eminence. this is only a convenience, because it seems necessary to personify. if we look over _philosophical transactions_, or the publications of the royal astronomical society, for instance, we see that herschel, for instance, was as powerless as any boy stargazer, to enforce acceptance of any observation of his that did not harmonize with the system that was growing up as independently of him and all other astronomers, as a phase in the development of an embryo compels all cells to take on appearances concordantly with the design and the predetermined progress and schedule of the whole. visitors to venus: evans, _ways of the planets_, p. : that, in , a body large enough to look like a satellite was seen near venus. four times in the first half of the th century, a similar observation was reported. the last report occurred in . a large body has been seen--seven times, according to _science gossip_, - --near venus. at least one astronomer, houzeau, accepted these observations and named the--world, planet, super-construction--"neith." his views are mentioned "in passing, but without endorsement," in the _trans. n.y. acad._, - . houzeau or someone writing for the magazine-section of a sunday newspaper--outer darkness for both alike. a new satellite in this solar system might be a little disturbing--though the formulas of laplace, which were considered final in his day, have survived the admittance of five or six hundred bodies not included in those formulas--a satellite to venus might be a little disturbing, but would be explained--but a large body approaching a planet--staying awhile--going away--coming back some other time--anchoring, as it were-- azuria is pretty bad, but azuria is no worse than neith. _astrophysical journal_, - : a light-reflecting body, or a bright spot near mars: seen nov. , , by prof. pickering and others, at the lowell observatory, above an unilluminated part of mars--self-luminous, it would seem--thought to have been a cloud--but estimated to have been about twenty miles away from the planet. luminous spot seen moving across the disk of mercury, in , by harding and schroeter. (_monthly notices of the r.a.s._, - .) in the first bulletin issued by the lowell observatory, in , prof. lowell describes a body that was seen on the terminator of mars, may , . on may , it was "suspected." if still there, it had moved, we are told, about miles--"probably a dust cloud." very conspicuous and brilliant spots seen on the disk of mars, october and november, . (_popular astronomy_, vol. , no. .) so one of them accepted six or seven observations that were in agreement, except that they could not be regularized, upon a world--planet--satellite--and he gave it a name. he named it "neith." monstrator and elvera and azuria and super-romanimus-- or heresy and orthodoxy and the oneness of all quasiness, and our ways and means and methods are the very same. or, if we name things that may not be, we are not of lonely guilt in the nomenclature of absences-- but now leverrier and "vulcan." leverrier again. or to demonstrate the collapsibility of a froth, stick a pin in the largest bubble of it. astronomy and inflation: and by inflation we mean expansion of the attenuated. or that the science of astronomy is a phantom-film distended with myth-stuff--but always our acceptance that it approximates higher to substantiality than did the system that preceded it. so leverrier and the "planet vulcan." and we repeat, and it will do us small good to repeat. if you be of the masses that the astronomers have hypnotized--being themselves hypnotized, or they could not hypnotize others--or that the hypnotist's control is not the masterful power that it is popularly supposed to be, but only transference of state from one hypnotic to another-- if you be of the masses that the astronomers have hypnotized, you will not be able even to remember. ten pages from here, and leverrier and the "planet vulcan" will have fallen from your mind, like beans from a magnet, or like data of cold meteorites from the mind of a thomson. leverrier and the "planet vulcan." and much the good it will do us to repeat. but at least temporarily we shall have an impression of a historic fiasco, such as, in our acceptance, could occur only in a quasi-existence. in , dr. lescarbault, an amateur astronomer, of orgères, france, announced that, upon march , of that year, he had seen a body of planetary size cross the sun. we are in a subject that is now as unholy to the present system as ever were its own subjects to the system that preceded it, or as ever were slanders against miracles to the preceding system. nevertheless few text-books go so far as quite to disregard this tragedy. the method of the systematists is slightingly to give a few instances of the unholy, and dispose of the few. if it were desirable to them to deny that there are mountains upon this earth, they would record a few observations upon some slight eminences near orange, n.j., but say that commuters, though estimable persons in several ways, are likely to have their observations mixed. the text-books casually mention a few of the "supposed" observations upon "vulcan," and then pass on. dr. lescarbault wrote to leverrier, who hastened to orgères-- because this announcement assimilated with his own calculations upon a planet between mercury and the sun-- because this solar system itself has never attained positiveness in the aspect of regularity: there are to mercury, as there are to neptune, phenomena irreconcilable with the formulas, or motions that betray influence by something else. we are told that leverrier "satisfied himself as to the substantial accuracy of the reported observation." the story of this investigation is told in _monthly notices_, - . it seems too bad to threaten the naïve little thing with our rude sophistications, but it is amusingly of the ingenuousness of the age from which present dogmas have survived. lescarbault wrote to leverrier. leverrier hastened to orgères. but he was careful not to tell lescarbault who he was. went right in and "subjected dr. lescarbault to a very severe cross-examination"--just the way you or i may feel at liberty to go into anybody's home and be severe with people--"pressing him hard step by step"--just as anyone might go into someone else's house and press him hard, though unknown to the hard-pressed one. not until he was satisfied, did leverrier reveal his identity. i suppose dr. lescarbault expressed astonishment. i think there's something utopian about this: it's so unlike the stand-offishness of new york life. leverrier gave the name "vulcan" to the object that dr. lescarbault had reported. by the same means by which he is, even to this day, supposed--by the faithful--to have discovered neptune, he had already announced the probable existence of an intra-mercurial body, or group of bodies. he had five observations besides lescarbault's upon something that had been seen to cross the sun. in accordance with the mathematical hypnoses of his era, he studied these six transits. out of them he computed elements giving "vulcan" a period of about days, or a formula for heliocentric longitude at any time. but he placed the time of best observation away up in . but even so, or considering that he still had probably a good many years to live, it may strike one that he was a little rash--that is if one has not gone very deep into the study of hypnoses--that, having "discovered" neptune by a method which, in our acceptance, had no more to recommend it than had once equally well-thought-of methods of witch-finding, he should not have taken such chances: that if he was right as to neptune, but should be wrong as to "vulcan," his average would be away below that of most fortune-tellers, who could scarcely hope to do business upon a fifty per cent. basis--all that the reasoning of a tyro in hypnoses. the date: march , . the scientific world was up on its hind legs nosing the sky. the thing had been done so authoritatively. never a pope had said a thing with more of the seeming of finality. if six observations correlated, what more could be asked? the editor of _nature_, a week before the predicted event, though cautious, said that it is difficult to explain how six observers, unknown to one another, could have data that could be formulated, if they were not related phenomena. in a way, at this point occurs the crisis of our whole book. formulas are against us. but can astronomic formulas, backed up by observations in agreement, taken many years apart, calculated by a leverrier, be as meaningless, in a positive sense, as all other quasi-things that we have encountered so far? the preparations they made, before march , . in england, the astronomer royal made it the expectation of his life: notified observers at madras, melbourne, sydney, and new zealand, and arranged with observers in chili and the united states. m. struve had prepared for observations in siberia and japan-- march , -- not absolutely, hypocritically, i think it's pathetic, myself. if anyone should doubt the sincerity of leverrier, in this matter, we note, whether it has meaning or not, that a few months later he died. i think we'll take up monstrator, though there's so much to this subject that we'll have to come back. according to the _annual register_, - , upon the th of august, , m. de rostan, of basle, france, was taking altitudes of the sun, at lausanne. he saw a vast, spindle-shaped body, about three of the sun's digits in breadth and nine in length, advancing slowly across the disk of the sun, or "at no more than half the velocity with which the ordinary solar spots move." it did not disappear until the th of september, when it reached the sun's limb. because of the spindle-like form, i incline to think of a super-zeppelin, but another observation, which seems to indicate that it was a world, is that, though it was opaque, and "eclipsed the sun," it had around it a kind of nebulosity--or atmosphere? a penumbra would ordinarily be a datum of a sun spot, but there are observations that indicate that this object was at a considerable distance from the sun: it is recorded that another observer, at paris, watching the sun, at this time, had not seen this object: but that m. croste, at sole, about forty-five german leagues northward from lausanne, had seen it, describing the same spindle-form, but disagreeing a little as to breadth. then comes the important point: that he and m. de rostan did not see it upon the same part of the sun. this, then, is parallax, and, compounded with invisibility at paris, is great parallax--or that, in the course of a month, in the summer of , a large, opaque, spindle-shaped body traversed the disk of the sun, but at a great distance from the sun. the writer in the _register_ says: "in a word, we know of nothing to have recourse to, in the heavens, by which to explain this phenomenon." i suppose he was not a hopeless addict to explaining. extraordinary--we fear he must have been a man of loose habits in some other respects. as to us-- monstrator. in the _monthly notices of the r.a.s._, february, , leverrier, who never lost faith, up to the last day, gives the six observations upon an unknown body of planetary size, that he had formulated: fritsche, oct. , ; stark, oct. , ; de cuppis, oct. , ; sidebotham, nov. , ; lescarbault, march , ; lummis, march , . if we weren't so accustomed to science in its essential aspect of disregard, we'd be mystified and impressed, like the editor of _nature_, with the formulation of these data: agreement of so many instances would seem incredible as a coincidence: but our acceptance is that, with just enough disregard, astronomers and fortune-tellers can formulate anything--or we'd engage, ourselves, to formulate periodicities in the crowds in broadway--say that every wednesday morning, a tall man, with one leg and a black eye, carrying a rubber plant, passes the singer building, at quarter past ten o'clock. of course it couldn't really be done, unless such a man did have such periodicity, but if some wednesday mornings it should be a small child lugging a barrel, or a fat negress with a week's wash, by ordinary disregard that would be prediction good enough for the kind of quasi-existence we're in. so whether we accuse, or whether we think that the word "accuse" over-dignifies an attitude toward a quasi-astronomer, or mere figment in a super-dream, our acceptance is that leverrier never did formulate observations-- that he picked out observations that could be formulated-- that of this type are all formulas-- that, if leverrier had not been himself helplessly hypnotized, or if he had had in him more than a tincture of realness, never could he have been beguiled by such a quasi-process: but that he was hypnotized, and so extended, or transferred, his condition to others, that upon march , , he had this earth bristling with telescopes, with the rigid and almost inanimate forms of astronomers behind them-- and not a blessed thing of any unusuality was seen upon that day or succeeding days. but that the science of astronomy suffered the slightest in prestige? it couldn't. the spirit of was behind it. if, in an embryo, some cells should not live up to the phenomena of their era, the others will sustain the scheduled appearances. not until an embryo enters the mammalian stage are cells of the reptilian stage false cells. it is our acceptance that there were many equally authentic reports upon large planetary bodies that had been seen near the sun; that, of many, leverrier picked out six; not then deciding that all the other observations related to still other large, planetary bodies, but arbitrarily, or hypnotically, disregarding--or heroically disregarding--every one of them--that to formulate at all he had to exclude falsely. the dénouement killed him, i think. i'm not at all inclined to place him with the grays and hitchcocks and symonses. i'm not, because, though it was rather unsportsmanlike to put the date so far ahead, he did give a date, and he did stick to it with such a high approximation-- i think leverrier was translated to the positive absolute. the disregarded: observation, of july , , by gruthinson--but that was of two bodies that crossed the sun together-- _nature_, - : that, according to the astronomer, j.r. hind, benjamin scott, city chamberlain of london, and mr. wray, had, in , seen a body similar to "vulcan" cross the sun. similar observation by hind and lowe, march , (_l'année scientifique_, - ). _nature_, - : body of apparent size of mercury, seen, jan. , , by f.a.r. russell and four other observers, crossing the sun. de vico's observation of july , (_observatory_, - ). _l'année scientifique_, - : that another amateur astronomer, m. coumbray, of constantinople, had written to leverrier, that, upon the th of march, , he had seen a black point, sharply outlined, traverse the disk of the sun. it detached itself from a group of sun spots near the limb of the sun, and took minutes to reach the other limb. figuring upon the diagram sent by m. coumbray, a central passage would have taken a little more than an hour. this observation was disregarded by leverrier, because his formula required about four times that velocity. the point here is that these other observations are as authentic as those that leverrier included; that, then, upon data as good as the data of "vulcan," there must be other "vulcans"--the heroic and defiant disregard, then, of trying to formulate one, omitting the others, which, by orthodox doctrine, must have influenced it greatly, if all were in the relatively narrow space between mercury and the sun. observation upon another such body, of april , , by m. weber, of berlin. as to this observation, leverrier was informed by wolf, in august, (_l'année scientifique_, - ). it made no difference, so far as can be known, to this notable positivist. two other observations noted by hind and denning--london _times_, nov. , , and march , . _monthly notices of the r.a.s._, - : standacher, february, ; lichtenberg, nov. , ; hoffman, may, ; dangos, jan. , ; stark, feb. , . an observation by schmidt, oct. , , is said to be doubtful: but, upon page , it is said that this doubt had arisen because of a mistaken translation, and two other observations by schmidt are given: oct. , , and feb. , --also an observation by lofft, jan. , . observation by steinheibel, at vienna, april , (_monthly notices_, ). haase had collected reports of twenty observations like lescarbault's. the list was published in , by wolf. also there are other instances like gruthinsen's: _amer. jour. sci._, - - : report by pastorff that he had seen twice in , and once in , two round spots of unequal size moving across the sun, changing position relatively to each other, and taking a different course, if not orbit, each time: that, in , he had seen similar bodies pass six times across the disk of the sun, looking very much like mercury in his transits. march , -- but to point out leverrier's poverty-stricken average--or discovering planets upon a fifty per cent. basis--would be to point out the low percentage of realness in the quasi-myth-stuff of which the whole system is composed. we do not accuse the text-books of omitting this fiasco, but we do note that theirs is the conventional adaptation here of all beguilers who are in difficulties-- the diverting of attention. it wouldn't be possible in a real existence, with real mentality, to deal with, but i suppose it's good enough for the quasi-intellects that stupefy themselves with text-books. the trick here is to gloss over leverrier's mistake, and blame lescarbault--he was only an amateur--had delusions. the reader's attention is led against lescarbault by a report from m. lias, director of the brazilian coast survey, who, at the time of lescarbault's "supposed" observation had been watching the sun in brazil, and, instead of seeing even ordinary sun spots, had noted that the region of the "supposed transit" was of "uniform intensity." but the meaninglessness of all utterances in quasi-existence-- "uniform intensity" turns our way as much as against us--or some day some brain will conceive a way of beating newton's third law--if every reaction, or resistance, is, or can be, interpretable as stimulus instead of resistance--if this could be done in mechanics, there's a way open here for someone to own the world--specifically in this matter, "uniform intensity" means that lescarbault saw no ordinary sun spot, just as much as it means that no spot at all was seen upon the sun. continuing the interpretation of a resistance as an assistance, which can always be done with mental forces--making us wonder what applications could be made with steam and electric forces--we point out that invisibility in brazil means parallax quite as truly as it means absence, and, inasmuch as "vulcan" was supposed to be distant from the sun, we interpret denial as corroboration--method of course of every scientist, politician, theologian, high-school debater. so the text-books, with no especial cleverness, because no especial cleverness is needed, lead the reader into contempt for the amateur of orgères, and forgetfulness of leverrier--and some other subject is taken up. but our own acceptance: that these data are as good as ever they were; that, if someone of eminence should predict an earthquake, and if there should be no earthquake at the predicted time, that would discredit the prophet, but data of past earthquakes would remain as good as ever they had been. it is easy enough to smile at the illusion of a single amateur-- the mass-formation: fritsche, stark, de cuppis, sidebotham, lescarbault, lummis, gruthinson, de vico, scott, wray, russell, hind, lowe, coumbray, weber, standacher, lichtenberg, dangos, hoffman, schmidt, lofft, steinheibel, pastorff-- these are only the observations conventionally listed relatively to an intra-mercurial planet. they are formidable enough to prevent our being diverted, as if it were all the dream of a lonely amateur--but they're a mere advance-guard. from now on other data of large celestial bodies, some dark and some reflecting light, will pass and pass and keep on passing-- so that some of us will remember a thing or two, after the procession's over--possibly. taking up only one of the listed observations-- or our impression that the discrediting of leverrier has nothing to do with the acceptability of these data: in the london _times_, jan. , , is benjamin scott's account of his observation: that, in the summer of , he had seen a body that had seemed to be the size of venus, crossing the sun. he says that, hardly believing the evidence of his sense of sight, he had looked for someone, whose hopes or ambitions would not make him so subject to illusion. he had told his little son, aged five years, to look through the telescope. the child had exclaimed that he had seen "a little balloon" crossing the sun. scott says that he had not had sufficient self-reliance to make public announcement of his remarkable observation at the time, but that, in the evening of the same day, he had told dr. dick, f.r.a.s., who had cited other instances. in the _times_, jan. , , is published a letter from richard abbott, f.r.a.s.: that he remembered mr. scott's letter to him upon this observation, at the time of the occurrence. i suppose that, at the beginning of this chapter, one had the notion that, by hard scratching through musty old records we might rake up vague, more than doubtful data, distortable into what's called evidence of unrecognized worlds or constructions of planetary size-- but the high authenticity and the support and the modernity of these of the accursed that we are now considering-- and our acceptance that ours is a quasi-existence, in which above all other things, hopes, ambitions, emotions, motivations, stands attempt to positivize: that we are here considering an attempt to systematize that is sheer fanaticism in its disregard of the unsystematizable--that it represented the highest good in the th century--that it is mono-mania, but heroic mono-mania that was quasi-divine in the th century-- but that this isn't the th century. as a doubly sponsored brahmin--in the regard of baptists--the objects of july , , stand out and proclaim themselves so that nothing but disregard of the intensity of mono-mania can account for their reception by the system: or the total eclipse of july , , and the reports by prof. watson, from rawlins, wyoming, and by prof. swift, from denver, colorado: that they had seen two shining objects at a considerable distance from the sun. it's quite in accord with our general expression: not that there is an intra-mercurial planet, but that there are different bodies, many vast things; near this earth sometimes, near the sun sometimes; orbitless worlds, which, because of scarcely any data of collisions, we think of as under navigable control--or dirigible super-constructions. prof. watson and prof. swift published their observations. then the disregard that we cannot think of in terms of ordinary, sane exclusions. the text-book systematists begin by telling us that the trouble with these observations is that they disagree widely: there is considerable respectfulness, especially for prof. swift, but we are told that by coincidence these two astronomers, hundreds of miles apart, were illuded: their observations were so different-- prof. swift (_nature_, sept. , ): that his own observation was "in close approximation to that given by prof. watson." in the _observatory_, - , swift says that his observations and watson's were "confirmatory of each other." the faithful try again: that watson and swift mistook stars for other bodies. in the _observatory_, - , prof. watson says that he had previously committed to memory all stars near the sun, down to the seventh magnitude-- and he's damned anyway. how such exclusions work out is shown by lockyer (_nature_, aug. , ). he says: "there is little doubt that an intra-mercurial planet has been discovered by prof. watson." that was before excommunication was pronounced. he says: "if it will fit one of leverrier's orbits"-- it didn't fit. in _nature_, - , prof. swift says: "i have never made a more valid observation, nor one more free from doubt." he's damned anyway. we shall have some data that will not live up to most rigorous requirements, but, if anyone would like to read how carefully and minutely these two sets of observations were made, see prof. swift's detailed description in the _am. jour. sci._, - ; and the technicalities of prof. watson's observations in _monthly notices_, - . our own acceptance upon dirigible worlds, which is assuredly enough, more nearly real than attempted concepts of large planets relatively near this earth, moving in orbits, but visible only occasionally; which more nearly approximates to reasonableness than does wholesale slaughter of swift and watson and fritsche and stark and de cuppis--but our own acceptance is so painful to so many minds that, in another of the charitable moments that we have now and then for the sake of contrast, we offer relief: the things seen high in the sky by swift and watson-- well, only two months before--the horse and the barn-- we go on with more observations by astronomers, recognizing that it is the very thing that has given them life, sustained them, held them together, that has crushed all but the quasi-gleam of independent life out of them. were they not systematized, they could not be at all, except sporadically and without sustenance. they are systematized: they must not vary from the conditions of the system: they must not break away for themselves. the two great commandments: thou shalt not break continuity; thou shalt try. we go on with these disregarded data, some of which, many of which, are of the highest degree of acceptability. it is the system that pulls back its variations, as this earth is pulling back the matterhorn. it is the system that nourishes and rewards, and also freezes out life with the chill of disregard. we do note that, before excommunication is pronounced, orthodox journals do liberally enough record unassimilable observations. all things merge away into everything else. that is continuity. so the system merges away and evades us when we try to focus against it. we have complained a great deal. at least we are not so dull as to have the delusion that we know just exactly what it is that we are complaining about. we speak seemingly definitely enough of "the system," but we're building upon observations by members of that very system. or what we are doing--gathering up the loose heresies of the orthodox. of course "the system" fringes and ravels away, having no real outline. a swift will antagonize "the system," and a lockyer will call him back; but, then, a lockyer will vary with a "meteoric hypothesis," and a swift will, in turn, represent "the system." this state is to us typical of all intermediatist phenomena; or that not conceivably is anything really anything, if its parts are likely to be their own opposites at any time. we speak of astronomers--as if there were real astronomers--but who have lost their identity in a system--as if it were a real system--but behind that system is plainly a rapport, or loss of identity in the spirit of an era. bodies that have looked like dark bodies, and lights that may have been sunlight reflected from inter-planetary--objects, masses, constructions-- lights that have been seen upon--or near?--the moon: in _philosophical transactions_, - , is herschel's report upon many luminous points, which he saw upon--or near?--the moon, during an eclipse. why they should be luminous, whereas the moon itself was dark, would get us into a lot of trouble--except that later we shall, or we sha'n't, accept that many times have luminous objects been seen close to this earth--at night. but numerousness is a new factor, or new disturbance, to our explorations-- a new aspect of inter-planetary inhabitancy or occupancy-- worlds in hordes--or beings--winged beings perhaps--wouldn't astonish me if we should end up by discovering angels--or beings in machines--argosies of celestial voyagers-- in and , herschel reported more lights on or near the moon, which he supposed were volcanic. the word of a herschel has had no more weight, in divergences from the orthodox, than has had the word of a lescarbault. these observations are of the disregarded. bright spots seen on the moon, november, (_proc. london roy. soc._, - ). for four other instances, see loomis (_treatise on astronomy_, p. ). a moving light is reported in _phil. trans._, - . to the writer, it looked like a star passing over the moon--"which, on the next moment's consideration i knew to be impossible." "it was a fixed, steady light upon the dark part of the moon." i suppose "fixed" applies to luster. in the _report of the brit. assoc._, - , there is an observation by rankin, upon luminous points seen on the shaded part of the moon, during an eclipse. they seemed to this observer like reflections of stars. that's not very reasonable: however, we have, in the _annual register_, - , a light not referable to a star--because it moved with the moon: was seen three nights in succession; reported by capt. kater. see _quart. jour. roy. inst._, - . _phil. trans._, - : report from the cape town observatory: a whitish spot on the dark part of the moon's limb. three smaller lights were seen. the call of positiveness, in its aspects of singleness, or homogeneity, or oneness, or completeness. in data now coming, i feel it myself. a leverrier studies more than twenty observations. the inclination is irresistible to think that they all relate to one phenomenon. it is an expression of cosmic inclination. most of the observations are so irreconcilable with any acceptance other than of orbitless, dirigible worlds that he shuts his eyes to more than two-thirds of them; he picks out six that can give him the illusion of completeness, or of all relating to one planet. or let it be that we have data of many dark bodies--still do we incline almost irresistibly to think of one of them as the dark-body-in-chief. dark bodies, floating, or navigating, in inter-planetary space--and i conceive of one that's the prince of dark bodies: melanicus. vast dark thing with the wings of a super-bat, or jet-black super-construction; most likely one of the spores of the evil one. the extraordinary year, : london _times_, dec. , : extract from a letter by hicks pashaw: that, in egypt, sept. , , he had seen, through glasses, "an immense black spot upon the lower part of the sun." sun spot, maybe. one night an astronomer was looking up at the sky, when something obscured a star, for three and a half seconds. a meteor had been seen nearby, but its train had been only momentarily visible. dr. wolf was the astronomer (_nature_, - ). the next datum is one of the most sensational we have, except that there is very little to it. a dark object that was seen by prof. heis, for eleven degrees of arc, moving slowly across the milky way. (greg's catalogue, _rept. brit. assoc._, - .) one of our quasi-reasons for accepting that orbitless worlds are dirigible is the almost complete absence of data of collisions: of course, though in defiance of gravitation, they may, without direction like human direction, adjust to one another in the way of vortex rings of smoke--a very human-like way, that is. but in _knowledge_, february, , are two photographs of brooks' comet that are shown as evidence of its seeming collision with a dark object, october, . our own wording is that it "struck against something": prof. barnard's is that it had "entered some dense medium, which shattered it." for all i know it had knocked against merely a field of ice. melanicus. that upon the wings of a super-bat, he broods over this earth and over other worlds, perhaps deriving something from them: hovers on wings, or wing-like appendages, or planes that are hundreds of miles from tip to tip--a super-evil thing that is exploiting us. by evil i mean that which makes us useful. he obscures a star. he shoves a comet. i think he's a vast, black, brooding vampire. _science_, july , : that, according to a newspaper account, mr. w.r. brooks, director of the smith observatory, had seen a dark round object pass rather slowly across the moon, in a horizontal direction. in mr. brooks' opinion it was a dark meteor. in _science_, sept. , , a correspondent writes that, in his opinion, it may have been a bird. we shall have no trouble with the meteor and bird mergers, if we have observations of long duration and estimates of size up to hundreds of miles. as to the body that was seen by brooks, there is a note from the dutch astronomer, muller, in the _scientific american_, - , that, upon april , , he had seen a similar phenomenon. in _science gossip_, n.s., - , are more details of the brooks object--apparent diameter about one-thirtieth of the moon's--moon's disk crossed in three or four seconds. the writer, in _science gossip_, says that, on june , , at one o'clock in the morning, he was looking at the moon with a -inch achromatic, power , when a long black object sailed past, from west to east, the transit occupying or seconds. he believed this object to be a bird--there was, however, no fluttering motion observable in it. in the _astronomische nachrichten_, no. , dr. brendel, of griefswald, pomerania, writes that postmaster ziegler and other observers had seen a body about feet in diameter crossing the sun's disk. the duration here indicates something far from the earth, and also far from the sun. this thing was seen a quarter of an hour before it reached the sun. time in crossing the sun was about an hour. after leaving the sun it was visible an hour. i think he's a vast, black vampire that sometimes broods over this earth and other bodies. communication from dr. f.b. harris (_popular astronomy_, - ): that, upon the evening of jan. , , dr. harris saw, upon the moon, "an intensely black object." he estimated it to be miles long and miles wide. "the object resembled a crow poised, as near as anything." clouds then cut off observation. dr. harris writes: "i cannot but think that a very interesting and curious phenomenon happened." short chapter coming now, and it's the worst of them all. i think it's speculative. it's a lapse from our usual pseudo-standards. i think it must mean that the preceding chapter was very efficiently done, and that now by the rhythm of all quasi-things--which can't be real things, if they're rhythms, because a rhythm is an appearance that turns into its own opposite and then back again--but now, to pay up, we're what we weren't. short chapter, and i think we'll fill in with several points in intermediatism. a puzzle: if it is our acceptance that, out of the negative absolute, the positive absolute is generating itself, recruiting, or maintaining, itself, via a third state, or our own quasi-state, it would seem that we're trying to conceive of universalness manufacturing more universalness from nothingness. take that up yourself, if you're willing to run the risk of disappearing with such velocity that you'll leave an incandescent train behind, and risk being infinitely happy forever, whereas you probably don't want to be happy--i'll sidestep that myself, and try to be intelligible by regarding the positive absolute from the aspect of realness instead of universalness, recalling that by both realness and universalness we mean the same state, or that which does not merge away into something else, because there is nothing else. so the idea is that out of unrealness, instead of nothingness, realness, instead of universalness, is, via our own quasi-state, manufacturing more realness. just so, but in relative terms, of course, all imaginings that materialize into machines or statues, buildings, dollars, paintings or books in paper and ink are graduations from unrealness to realness--in relative terms. it would seem then that intermediateness is a relation between the positive absolute and the negative absolute. but the absolute cannot be the related--of course a confession that we can't really think of it at all, if here we think of a limit to the unlimited. doing the best we can, and encouraged by the reflection that we can't do worse than has been done by metaphysicians in the past, we accept that the absolute can't be the related. so then that our quasi-state is not a real relation, if nothing in it is real. on the other hand, it is not an unreal relation, if nothing in it is unreal. it seems thinkable that the positive absolute can, by means of intermediateness, have a quasi-relation, or be only quasi-related, or be the unrelated, in final terms, or, at least, not be the related, in final terms. as to free will and intermediatism--same answer as to everything else. by free will we mean independence--or that which does not merge away into something else--so, in intermediateness, neither free-will nor slave-will--but a different approximation for every so-called person toward one or the other of the extremes. the hackneyed way of expressing this seems to me to be the acceptable way, if in intermediateness, there is only the paradoxical: that we're free to do what we have to do. i am not convinced that we make a fetish of the preposterous. i think our feeling is that in first gropings there's no knowing what will afterward be the acceptable. i think that if an early biologist heard of birds that grow on trees, he should record that he had heard of birds that grow on trees: then let sorting over of data occur afterward. the one thing that we try to tone down but that is to a great degree unavoidable is having our data all mixed up like long island and florida in the minds of early american explorers. my own notion is that this whole book is very much like a map of north america in which the hudson river is set down as a passage leading to siberia. we think of monstrator and melanicus and of a world that is now in communication with this earth: if so, secretly, with certain esoteric ones upon this earth. whether that world's monstrator and monstrator's melanicus--must be the subject of later inquiry. it would be a gross thing to do: solve up everything now and leave nothing to our disciples. i have been very much struck with phenomena of "cup marks." they look to me like symbols of communication. but they do not look to me like means of communication between some of the inhabitants of this earth and other inhabitants of this earth. my own impression is that some external force has marked, with symbols, rocks of this earth, from far away. i do not think that cup marks are inscribed communications among different inhabitants of this earth, because it seems too unacceptable that inhabitants of china, scotland, and america should all have conceived of the same system. cup marks are strings of cup-like impressions in rocks. sometimes there are rings around them, and sometimes they have only semi-circles. great britain, america, france, algeria, circassia, palestine: they're virtually everywhere--except in the far north, i think. in china, cliffs are dotted with them. upon a cliff near lake como, there is a maze of these markings. in italy and spain and india they occur in enormous numbers. given that a force, say, like electric force, could, from a distance, mark such a substance as rocks, as, from a distance of hundreds of miles, selenium can be marked by telephotographers--but i am of two minds-- the lost explorers from somewhere, and an attempt, from somewhere, to communicate with them: so a frenzy of showering of messages toward this earth, in the hope that some of them would mark rocks near the lost explorers-- or that somewhere upon this earth, there is an especial rocky surface, or receptor, or polar construction, or a steep, conical hill, upon which for ages have been received messages from some other world; but that at times messages go astray and mark substances perhaps thousands of miles from the receptor: that perhaps forces behind the history of this earth have left upon the rocks of palestine and england and india and china records that may some day be deciphered, of their misdirected instructions to certain esoteric ones--order of the freemasons--the jesuits-- i emphasize the row-formation of cup marks: prof. douglas (_saturday review_, nov. , ): "whatever may have been their motive, the cup-markers showed a decided liking for arranging their sculpturings in regularly spaced rows." that cup marks are an archaic form of inscription was first suggested by canon greenwell many years ago. but more specifically adumbratory to our own expression are the observations of rivett-carnac (_jour. roy. asiatic soc._, - ): that the braille system of raised dots is an inverted arrangement of cup marks: also that there are strong resemblances to the morse code. but no tame and systematized archaeologist can do more than casually point out resemblances, and merely suggest that strings of cup marks look like messages, because--china, switzerland, algeria, america--if messages they be, there seems to be no escape from attributing one origin to them--then, if messages they be, i accept one external origin, to which the whole surface of this earth was accessible, for them. something else that we emphasize: that rows of cup marks have often been likened to footprints. but, in this similitude, their unilinear arrangement must be disregarded--of course often they're mixed up in every way, but arrangement in single lines is very common. it is odd that they should so often be likened to footprints: i suppose there are exceptional cases, but unless it's something that hops on one foot, or a cat going along a narrow fence-top, i don't think of anything that makes footprints one directly ahead of another--cop, in a station house, walking a chalk line, perhaps. upon the witch's stone, near ratho, scotland, there are twenty-four cups, varying in size from one and a half to three inches in diameter, arranged in approximately straight lines. locally it is explained that these are tracks of dogs' feet (_proc. soc. antiq. scotland_, - - ). similar marks are scattered bewilderingly all around the witch's stone--like a frenzy of telegraphing, or like messages repeating and repeating, trying to localize differently. in inverness-shire, cup marks are called "fairies' footmarks." at valna's church, norway, and st. peter's, ambleteuse, there are such marks, said to be horses' hoofprints. the rocks of clare, ireland, are marked with prints supposed to have been made by a mythical cow (_folklore_, - ). we now have such a ghost of a thing that i'd not like to be interpreted as offering it as a datum: it simply illustrates what i mean by the notion of symbols, like cups, or like footprints, which, if like those of horses or cows, are the reverse of, or the negatives of, cups--of symbols that are regularly received somewhere upon this earth--steep, conical hill, somewhere, i think--but that have often alighted in wrong places--considerably to the mystification of persons waking up some morning to find them upon formerly blank spaces. an ancient record--still worse, an ancient chinese record--of a courtyard of a palace--dwellers of the palace waking up one morning, finding the courtyard marked with tracks like the footprints of an ox--supposed that the devil did it. (_notes and queries_, - - .) . angels. hordes upon hordes of them. beings massed like the clouds of souls, or the commingling whiffs of spirituality, or the exhalations of souls that doré pictured so often. it may be that the milky way is a composition of stiff, frozen, finally-static, absolute angels. we shall have data of little milky ways, moving swiftly; or data of hosts of angels, not absolute, or still dynamic. i suspect, myself, that the fixed stars are really fixed, and that the minute motions said to have been detected in them are illusions. i think that the fixed stars are absolutes. their twinkling is only the interpretation by an intermediatist state of them. i think that soon after leverrier died, a new fixed star was discovered--that, if dr. gray had stuck to his story of the thousands of fishes from one pail of water, had written upon it, lectured upon it, taken to street corners, to convince the world that, whether conceivable or not, his explanation was the only true explanation: had thought of nothing but this last thing at night and first thing in the morning--his obituary--another "nova" reported in _monthly notices_. i think that milky ways, of an inferior, or dynamic, order, have often been seen by astronomers. of course it may be that the phenomena that we shall now consider are not angels at all. we are simply feeling around, trying to find out what we can accept. some of our data indicate hosts of rotund and complacent tourists in inter-planetary space--but then data of long, lean, hungry ones. i think that there are, out in inter-planetary space, super tamerlanes at the head of hosts of celestial ravagers--which have come here and pounced upon civilizations of the past, cleaning them up all but their bones, or temples and monuments--for which later historians have invented exclusionist histories. but if something now has a legal right to us, and can enforce its proprietorship, they've been warned off. it's the way of all exploitation. i should say that we're now under cultivation: that we're conscious of it, but have the impertinence to attribute it all to our own nobler and higher instincts. against these notions is the same sense of finality that opposes all advance. it's why we rate acceptance as a better adaptation than belief. opposing us is the strong belief that, as to inter-planetary phenomena, virtually everything has been found out. sense of finality and illusion of homogeneity. but that what is called advancing knowledge is violation of the sense of blankness. a drop of water. once upon a time water was considered so homogeneous that it was thought of as an element. the microscope--and not only that the supposititiously elementary was seen to be of infinite diversity, but that in its protoplasmic life there were new orders of beings. or the year --and a european looking westward over the ocean--his feeling that that suave western droop was unbreakable; that gods of regularity would not permit that smooth horizon to be disturbed by coasts or spotted with islands. the unpleasantness of even contemplating such a state--wide, smooth west, so clean against the sky--spotted with islands--geographic leprosy. but coasts and islands and indians and bison, in the seemingly vacant west: lakes, mountains, rivers-- one looks up at the sky: the relative homogeneity of the relatively unexplored: one thinks of only a few kinds of phenomena. but the acceptance is forced upon me that there are modes and modes and modes of inter-planetary existence: things as different from planets and comets and meteors as indians are from bison and prairie dogs: a super-geography--or celestiography--of vast stagnant regions, but also of super-niagaras and ultra-mississippis: and a super-sociology--voyagers and tourists and ravagers: the hunted and the hunting: the super-mercantile, the super-piratic, the super-evangelical. sense of homogeneity, or our positivist illusion of the unknown--and the fate of all positivism. astronomy and the academic. ethics and the abstract. the universal attempt to formulate or to regularize--an attempt that can be made only by disregarding or denying. or all things disregard or deny that which will eventually invade and destroy them-- until comes the day when some one thing shall say, and enforce upon infinitude: "thus far shalt thou go: here is absolute demarcation." the final utterance: "there is only i." in the _monthly notices of the r.a.s._, - , there is a letter from the rev. w. read: that, upon the th of september, , at : a.m., he had seen a host of self-luminous bodies, passing the field of his telescope, some slowly and some rapidly. they appeared to occupy a zone several degrees in breadth. the direction of most of them was due east to west, but some moved from north to south. the numbers were tremendous. they were observed for six hours. editor's note: "may not these appearances be attributed to an abnormal state of the optic nerves of the observer?" in _monthly notices_, - , mr. read answers that he had been a diligent observer, with instruments of a superior order, for about years--"but i have never witnessed such an appearance before." as to illusion he says that two other members of his family had seen the objects. the editor withdraws his suggestion. we know what to expect. almost absolutely--in an existence that is essentially hibernian--we can predict the past--that is, look over something of this kind, written in , and know what to expect from the exclusionists later. if mr. read saw a migration of dissatisfied angels, numbering millions, they must merge away, at least subjectively, with commonplace terrestrial phenomena--of course disregarding mr. read's probable familiarity, of years' duration, with the commonplaces of terrestrial phenomena. _monthly notices_, - : letter from rev. w.r. dawes: that he had seen similar objects--and in the month of september--that they were nothing but seeds floating in the air. in the _report of the british association_, - , there is a communication from mr. read to prof. baden-powell: that the objects that had been seen by him and by mr. dawes were not similar. he denies that he had seen seeds floating in the air. there had been little wind, and that had come from the sea, where seeds would not be likely to have origin. the objects that he had seen were round and sharply defined, and with none of the feathery appearance of thistledown. he then quotes from a letter from c.b. chalmers, f.r.a.s., who had seen a similar stream, a procession, or migration, except that some of the bodies were more elongated--or lean and hungry--than globular. he might have argued for sixty-five years. he'd have impressed nobody--of importance. the super-motif, or dominant, of his era, was exclusionism, and the notion of seeds in the air assimilates--with due disregards--with that dominant. or pageantries here upon our earth, and things looking down upon us--and the crusades were only dust clouds, and glints of the sun on shining armor were only particles of mica in dust clouds. i think it was a crusade that read saw--but that it was right, relatively to the year , to say that it was only seeds in the wind, whether the wind blew from the sea or not. i think of things that were luminous with religious zeal, mixed up, like everything else in intermediateness, with black marauders and from gray to brown beings of little personal ambitions. there may have been a richard coeur de lion, on his way to right wrongs in jupiter. it was right, relatively to , to say that he was a seed of a cabbage. prof. coffin, u.s.n. (_jour. frank. inst._, - ): that, during the eclipse of august, , he had noted the passage, across his telescope, of several bright flakes resembling thistleblows, floating in the sunlight. but the telescope was so focused that, if these things were distinct, they must have been so far away from this earth that the difficulties of orthodoxy remain as great, one way or another, no matter what we think they were-- they were "well-defined," says prof. coffin. henry waldner (_nature_, - ): that, april , , he had seen great numbers of small, shining bodies passing from west to east. he had notified dr. wolf, of the observatory of zurich, who "had convinced himself of this strange phenomenon." dr. wolf had told him that similar bodies had been seen by sig. capocci, of the capodimonte observatory, at naples, may , . the shapes were of great diversity--or different aspects of similar shapes? appendages were seen upon some of them. we are told that some were star-shaped, with transparent appendages. i think, myself, it was a mohammed and his hegira. may have been only his harem. astonishing sensation: afloat in space with ten million wives around one. anyway, it would seem that we have considerable advantage here, inasmuch as seeds are not in season in april--but the pulling back to earth, the bedraggling by those sincere but dull ones of some time ago. we have the same stupidity--necessary, functioning stupidity--of attribution of something that was so rare that an astronomer notes only one instance between and , to an every-day occurrence-- or mr. waldner's assimilative opinion that he had seen only ice crystals. whether they were not very exclusive veils of a super-harem, or planes of a very light material, we have an impression of star-shaped things with transparent appendages that have been seen in the sky. hosts of small bodies--black, this time--that were seen by the astronomers herrick, buys-ballot, and de cuppis (_l'année scientifique_, - ); vast numbers of bodies that were seen by m. lamey, to cross the moon (_l'année scientifique_, - ); another instance of dark ones; prodigious number of dark, spherical bodies reported by messier, june , (arago, _oeuvres_, - ); considerable number of luminous bodies which appeared to move out from the sun, in diverse directions; seen at havana, during eclipse of the sun, may , , by prof. auber (poey); m. poey cites a similar instance, of aug. , ; m. lotard's opinion that they were birds (_l'astronomie_, - ); large number of small bodies crossing disk of the sun, some swiftly, some slowly; most of them globular, but some seemingly triangular, and some of more complicated structure; seen by m. trouvelet, who, whether seeds, insects, birds, or other commonplace things, had never seen anything resembling these forms (_l'année scientifique_, - ); report from the rio de janeiro observatory, of vast numbers of bodies crossing the sun, some of them luminous and some of them dark, from some time in december, , until jan. , (_la nature_, - ). of course, at a distance, any form is likely to look round or roundish: but we point out that we have notes upon the seeming of more complex forms. in _l'astronomie_, - , is recorded m. briguiere's observation, at marseilles, april and april , , upon the crossing of the sun by bodies that were irregular in form. some of them moved as if in alignment. letter from sir robert inglis to col. sabine (_rept. brit. assoc._, - ): that, at p.m., aug. , , at gais, switzerland, inglis had seen thousands and thousands of brilliant white objects, like snowflakes in a cloudless sky. though this display lasted about twenty-five minutes, not one of these seeming snowflakes was seen to fall. inglis says that his servant "fancied" that he had seen something like wings on these--whatever they were. upon page , of the _report_, sir john herschel says that, in or , his attention had been attracted by objects of considerable size, in the air, seemingly not far away. he had looked at them through a telescope. he says that they were masses of hay, not less than a yard or two in diameter. still there are some circumstances that interest me. he says that, though no less than a whirlwind could have sustained these masses, the air about him was calm. "no doubt wind prevailed at the spot, but there was no roaring noise." none of these masses fell within his observation or knowledge. to walk a few fields away and find out more would seem not much to expect from a man of science, but it is one of our superstitions, that such a seeming trifle is just what--by the spirit of an era, we'll call it--one is not permitted to do. if those things were not masses of hay, and if herschel had walked a little and found out, and had reported that he had seen strange objects in the air--that report, in , would have been as misplaced as the appearance of a tail upon an embryo still in its gastrula era. i have noticed this inhibition in my own case many times. looking back--why didn't i do this or that little thing that would have cost so little and have meant so much? didn't belong to that era of my own development. _nature_, - : that, at kattenau, germany, about half an hour before sunrise, march , , "an enormous number of luminous bodies rose from the horizon, and passed in a horizontal direction from east to west." they are described as having appeared in a zone or belt. "they shone with a remarkably brilliant light." so they've thrown lassos over our data to bring them back to earth. but they're lassos that cannot tighten. we can't pull out of them: we may step out of them, or lift them off. some of us used to have an impression of science sitting in calm, just judgment: some of us now feel that a good many of our data have been lynched. if a crusade, perhaps from mars to jupiter, occur in the autumn--"seeds." if a crusade or outpouring of celestial vandals is seen from this earth in the spring--"ice crystals." if we have record of a race of aerial beings, perhaps with no substantial habitat, seen by someone in india--"locusts." this will be disregarded: if locusts fly high, they freeze and fall in thousands. _nature_, - : locusts that were seen in the mountains of india, at a height of , feet--"in swarms and dying by thousands." but no matter whether they fly high or fly low, no one ever wonders what's in the air when locusts are passing overhead, because of the falling of stragglers. i have especially looked this matter up--no mystery when locusts are flying overhead--constant falling of stragglers. _monthly notices_, - : "an unusual phenomenon noticed by lieut. herschel, oct. and , , while observing the sun, at bangalore, india." lieut. herschel had noticed dark shadows crossing the sun--but away from the sun there were luminous, moving images. for two days bodies passed in a continuous stream, varying in size and velocity. the lieutenant tries to explain, as we shall see, but he says: "as it was, the continuous flight, for two whole days, in such numbers, in the upper regions of the air, of beasts that left no stragglers, is a wonder of natural history, if not of astronomy." he tried different focusing--he saw wings--perhaps he saw planes. he says that he saw upon the objects either wings or phantom-like appendages. then he saw something that was so bizarre that, in the fullness of his nineteenth-centuriness, he writes: "there was no longer doubt: they were locusts or flies of some sort." one of them had paused. it had hovered. then it had whisked off. the editor says that at that time "countless locusts had descended upon certain parts of india." we now have an instance that is extraordinary in several respects--super-voyagers or super-ravagers; angels, ragamuffins, crusaders, emigrants, aeronauts, or aerial elephants, or bison or dinosaurs--except that i think the thing had planes or wings--one of them has been photographed. it may be that in the history of photography no more extraordinary picture than this has ever been taken. _l'astronomie_, - : that, at the observatory of zacatecas, mexico, aug. , , about , meters above sea level, were seen a large number of small luminous bodies, entering upon the disk of the sun. m. bonilla telegraphed to the observatories of the city of mexico and of puebla. word came back that the bodies were not visible there. because of this parallax, m. bonilla placed the bodies "relatively near the earth." but when we find out what he called "relatively near the earth"--birds or bugs or hosts of a super-tamerlane or army of a celestial richard coeur de lion--our heresies rejoice anyway. his estimate is "less distance than the moon." one of them was photographed. see _l'astronomie_, - . the photograph shows a long body surrounded by indefinite structures, or by the haze of wings or planes in motion. _l'astronomie_, - ; signer ricco, of the observatory of palermo, writes that, nov. , , at : o'clock in the morning, he was watching the sun, when he saw, slowly traversing its disk, bodies in two long, parallel lines, and a shorter, parallel line. the bodies looked winged to him. but so large were they that he had to think of large birds. he thought of cranes. he consulted ornithologists, and learned that the configuration of parallel lines agrees with the flight-formation of cranes. this was in : anybody now living in new york city, for instance, would tell him that also it is a familiar formation of aeroplanes. but, because of data of focus and subtended angles, these beings or objects must have been high. sig. ricco argues that condors have been known to fly three or four miles high, and that heights reached by other birds have been estimated at two or three miles. he says that cranes have been known to fly so high that they have been lost to view. our own acceptance, in conventional terms, is that there is not a bird of this earth that would not freeze to death at a height of more than four miles: that if condors fly three or four miles high, they are birds that are especially adapted to such altitudes. sig. ricco's estimate is that these objects or beings or cranes must have been at least five and a half miles high. the vast dark thing that looked like a poised crow of unholy dimensions. assuming that i shall ever have any readers, let him, or both of them, if i shall ever have such popularity as that, note how dim that bold black datum is at the distance of only two chapters. the question: was it a thing or the shadow of a thing? acceptance either way calls not for mere revision but revolution in the science of astronomy. but the dimness of the datum of only two chapters ago. the carved stone disk of tarbes, and the rain that fell every afternoon for twenty--if i haven't forgotten, myself, whether it was twenty-three or twenty-five days!--upon one small area. we are all thomsons, with brains that have smooth and slippery, though corrugated, surfaces--or that all intellection is associative--or that we remember that which correlates with a dominant--and a few chapters go by, and there's scarcely an impression that hasn't slid off our smooth and slippery brains, of leverrier and the "planet vulcan." there are two ways by which irreconcilables can be remembered--if they can be correlated in a system more nearly real than the system that rejects them--and by repetition and repetition and repetition. vast black thing like a crow poised over the moon. the datum is so important to us, because it enforces, in another field, our acceptance that dark bodies of planetary size traverse this solar system. our position: that the things have been seen: also that their shadows have been seen. vast black thing poised like a crow over the moon. so far it is a single instance. by a single instance, we mean the negligible. in _popular science_, - , serviss tells of a shadow that schroeter saw, in , in the lunar alps. first he saw a light. but then, when this region was illuminated, he saw a round shadow where the light had been. our own expression: that he saw a luminous object near the moon: that that part of the moon became illuminated, and the object was lost to view; but that then its shadow underneath was seen. serviss explains, of course. otherwise he'd not be prof. serviss. it's a little contest in relative approximations to realness. prof. serviss thinks that what schroeter saw was the "round" shadow of a mountain--in the region that had become lighted. he assumes that schroeter never looked again to see whether the shadow could be attributed to a mountain. that's the crux: conceivably a mountain could cast a round--and that means detached--shadow, in the lighted part of the moon. prof. serviss could, of course, explain why he disregards the light in the first place--maybe it had always been there "in the first place." if he couldn't explain, he'd still be an amateur. we have another datum. i think it is more extraordinary than-- vast thing, black and poised, like a crow, over the moon. but only because it's more circumstantial, and because it has corroboration, do i think it more extraordinary than-- vast poised thing, black as a crow, over the moon. mr. h.c. russell, who was usually as orthodox as anybody, i suppose--at least, he wrote "f.r.a.s." after his name--tells in the _observatory_, - , one of the wickedest, or most preposterous, stories that we have so far exhumed: that he and another astronomer, g.d. hirst, were in the blue fountains, near sydney, n.s.w., and mr. hirst was looking at the moon-- he saw on the moon what russell calls "one of those remarkable facts, which being seen should be recorded, although no explanation can at present be offered." that may be so. it is very rarely done. our own expression upon evolution by successive dominants and their correlates is against it. on the other hand, we express that every era records a few observations out of harmony with it, but adumbratory or preparatory to the spirit of eras still to come. it's very rarely done. lashed by the phantom-scourge of a now passing era, the world of astronomers is in a state of terrorism, though of a highly attenuated, modernized, devitalized kind. let an astronomer see something that is not of the conventional, celestial sights, or something that it is "improper" to see--his very dignity is in danger. some one of the corralled and scourged may stick a smile into his back. he'll be thought of unkindly. with a hardihood that is unusual in his world of ethereal sensitivenesses, russell says, of hirst's observation: "he found a large part of it covered with a dark shade, quite as dark as the shadow of the earth during an eclipse of the moon." but the climax of hardihood or impropriety or wickedness, preposterousness or enlightenment: "one could hardly resist the conviction that it was a shadow, yet it could not be the shadow of any known body." richard proctor was a man of some liberality. after a while we shall have a letter, which once upon a time we'd have called delirious--don't know that we could read such a thing now, for the first time, without incredulous laughter--which mr. proctor permitted to be published in _knowledge_. but a dark, unknown world that could cast a shadow upon a large part of the moon, perhaps extending far beyond the limb of the moon; a shadow as deep as the shadow of this earth-- too much for mr. proctor's politeness. i haven't read what he said, but it seems to have been a little coarse. russell says that proctor "freely used" his name in the _echo_, of march , , ridiculing this observation which had been made by russell as well as hirst. if it hadn't been proctor, it would have been someone else--but one notes that the attack came out in a newspaper. there is no discussion of this remarkable subject, no mention in any other astronomic journal. the disregard was almost complete--but we do note that the columns of the _observatory_ were open to russell to answer proctor. in the answer, i note considerable intermediateness. far back in , it would have been a beautiful positivism, if russell had said-- "there was a shadow on the moon. absolutely it was cast by an unknown body." according to our religion, if he had then given all his time to the maintaining of this one stand, of course breaking all friendships, all ties with his fellow astronomers, his apotheosis would have occurred, greatly assisted by means well known to quasi-existence when its compromises and evasions, and phenomena that are partly this and partly that, are flouted by the definite and uncompromising. it would be impossible in a real existence, but mr. russell, of quasi-existence, says that he did resist the conviction; that he had said that one could "hardly resist"; and most of his resentment is against mr. proctor's thinking that he had not resisted. it seems too bad--if apotheosis be desirable. the point in intermediatism here is: not that to adapt to the conditions of quasi-existence is to have what is called success in quasi-existence, but is to lose one's soul-- but is to lose "one's" chance of attaining soul, self, or entity. one indignation quoted from proctor interests us: "what happens on the moon may at any time happen to this earth." or: that is just the teaching of this department of advanced astronomy: that russell and hirst saw the sun eclipsed relatively to the moon by a vast dark body: that many times have eclipses occurred relatively to this earth, by vast, dark bodies: that there have been many eclipses that have not been recognized as eclipses by scientific kindergartens. there is a merger, of course. we'll take a look at it first--that, after all, it may have been a shadow that hirst and russell saw, but the only significance is that the sun was eclipsed relatively to the moon by a cosmic haze of some kind, or a swarm of meteors close together, or a gaseous discharge left behind by a comet. my own acceptance is that vagueness of shadow is a function of vagueness of intervention; that a shadow as dense as the shadow of this earth is cast by a body denser than hazes and swarms. the information seems definite enough in this respect--"quite as dark as the shadow of this earth during the eclipse of the moon." though we may not always be as patient toward them as we should be, it is our acceptance that the astronomic primitives have done a great deal of good work: for instance, in the allaying of fears upon this earth. sometimes it may seem as if all science were to us very much like what a red flag is to bulls and anti-socialists. it's not that: it's more like what unsquare meals are to bulls and anti-socialists--not the scientific, but the insufficient. our acceptance is that evil is the negative state, by which we mean the state of maladjustment, discord, ugliness, disorganization, inconsistency, injustice, and so on--as determined in intermediateness, not by real standards, but only by higher approximations to adjustment, harmony, beauty, organization, consistency, justice, and so on. evil is outlived virtue, or incipient virtue that has not yet established itself, or any other phenomenon that is not in seeming adjustment, harmony, consistency with a dominant. the astronomers have functioned bravely in the past. they've been good for business: the big interests think kindly, if at all, of them. it's bad for trade to have an intense darkness come upon an unaware community and frighten people out of their purchasing values. but if an obscuration be foretold, and if it then occur--may seem a little uncanny--only a shadow--and no one who was about to buy a pair of shoes runs home panic-stricken and saves the money. upon general principles we accept that astronomers have quasi-systematized data of eclipses--or have included some and disregarded others. they have done well. they have functioned. but now they're negatives, or they're out of harmony-- if we are in harmony with a new dominant, or the spirit of a new era, in which exclusionism must be overthrown; if we have data of many obscurations that have occurred, not only upon the moon, but upon our own earth, as convincing of vast intervening bodies, usually invisible, as is any regularized, predicted eclipse. one looks up at the sky. it seems incredible that, say, at the distance of the moon, there could be, but be invisible, a solid body, say, the size of the moon. one looks up at the moon, at a time when only a crescent of it is visible. the tendency is to build up the rest of it in one's mind; but the unillumined part looks as vacant as the rest of the sky, and it's of the same blueness as the rest of the sky. there's a vast area of solid substance before one's eyes. it's indistinguishable from the sky. in some of our little lessons upon the beauties of modesty and humility, we have picked out basic arrogances--tail of a peacock, horns of a stag, dollars of a capitalist--eclipses of astronomers. though i have no desire for the job, i'd engage to list hundreds of instances in which the report upon an expected eclipse has been "sky overcast" or "weather unfavorable." in our super-hibernia, the unfavorable has been construed as the favorable. some time ago, when we were lost, because we had not recognized our own dominant, when we were still of the unchosen and likely to be more malicious than we now are--because we have noted a steady tolerance creeping into our attitude--if astronomers are not to blame, but are only correlates to a dominant--we advertised a predicted eclipse that did not occur at all. now, without any especial feeling, except that of recognition of the fate of all attempted absolutism, we give the instance, noting that, though such an evil thing to orthodoxy, it was orthodoxy that recorded the non-event. _monthly notices of the r.a.s._, - : "remarkable appearances during the total eclipse of the moon on march , ": in an extract from a letter from mr. forster, of bruges, it is said that, according to the writer's observations at the time of the predicted total eclipse, the moon shone with about three times the intensity of the mean illumination of an eclipsed lunar disk: that the british consul, at ghent, who did not know of the predicted eclipse, had written enquiring as to the "blood-red" color of the moon. this is not very satisfactory to what used to be our malices. but there follows another letter, from another astronomer, walkey, who had made observations at clyst st. lawrence: that, instead of an eclipse, the moon became--as is printed in italics--"most beautifully illuminated" ... "rather tinged with a deep red"... "the moon being as perfect with light as if there had been no eclipse whatever." i note that chambers, in his work upon eclipses, gives forster's letter in full--and not a mention of walkey's letter. there is no attempt in _monthly notices_ to explain upon the notion of greater distance of the moon, and the earth's shadow falling short, which would make as much trouble for astronomers, if that were not foreseen, as no eclipse at all. also there is no refuge in saying that virtually never, even in total eclipses, is the moon totally dark--"as perfect with light as if there had been no eclipse whatever." it is said that at the time there had been an aurora borealis, which might have caused the luminosity, without a datum that such an effect, by an aurora, had ever been observed upon the moon. but single instances--so an observation by scott, in the antarctic. the force of this datum lies in my own acceptance, based upon especially looking up this point, that an eclipse nine-tenths of totality has great effect, even though the sky be clouded. scott (_voyage of the discovery_, vol. ii, p. ): "there may have been an eclipse of the sun, sept. , , as the almanac said, but we should, none of us, have liked to swear to the fact." this eclipse had been set down at nine-tenths of totality. the sky was overcast at the time. so it is not only that many eclipses unrecognized by astronomers as eclipses have occurred, but that intermediatism, or impositivism, breaks into their own seemingly regularized eclipses. our data of unregularized eclipses, as profound as those that are conventionally--or officially?--recognized, that have occurred relatively to this earth: in _notes and queries_ there are several allusions to intense darknesses that have occurred upon this earth, quite as eclipses occur, but that are not referable to any known eclipsing body. of course there is no suggestion here that these darknesses may have been eclipses. my own acceptance is that if in the nineteenth century anyone had uttered such a thought as that, he'd have felt the blight of a dominant; that materialistic science was a jealous god, excluding, as works of the devil, all utterances against the seemingly uniform, regular, periodic; that to defy him would have brought on--withering by ridicule--shrinking away by publishers--contempt of friends and family--justifiable grounds for divorce--that one who would so defy would feel what unbelievers in relics of saints felt in an earlier age; what befell virgins who forgot to keep fires burning, in a still earlier age--but that, if he'd almost absolutely hold out, just the same--new fixed star reported in _monthly notices_. altogether, the point in positivism here is that by dominants and their correlates, quasi-existence strives for the positive state, aggregating, around a nucleus, or dominant, systematized members of a religion, a science, a society--but that "individuals" who do not surrender and submerge may of themselves highly approximate to positiveness--the fixed, the real, the absolute. in _notes and queries_, - - , there is an account of a darkness in holland, in the midst of a bright day, so intense and terrifying that many panic-stricken persons lost their lives stumbling into the canals. _gentleman's magazine_, - : a darkness that came upon london, aug. , , "greater than at the great eclipse of ." however, our preference is not to go so far back for data. for a list of historic "dark days," see humboldt, _cosmos_, - . _monthly weather review_, march, - : that, according to the _la crosse daily republican_, of march , , darkness suddenly settled upon the city of oshkosh, wis., at p.m., march . in five minutes the darkness equaled that of midnight. consternation. i think that some of us are likely to overdo our own superiority and the absurd fears of the middle ages-- oshkosh. people in the streets rushing in all directions--horses running away--women and children running into cellars--little modern touch after all: gas meters instead of images and relics of saints. this darkness, which lasted from eight to ten minutes, occurred in a day that had been "light but cloudy." it passed from west to east, and brightness followed: then came reports from towns to the west of oshkosh: that the same phenomenon had already occurred there. a "wave of total darkness" had passed from west to east. other instances are recorded in the _monthly weather review_, but, as to all of them, we have a sense of being pretty well-eclipsed, ourselves, by the conventional explanation that the obscuring body was only a very dense mass of clouds. but some of the instances are interesting--intense darkness at memphis, tenn., for about fifteen minutes, at a.m., dec. , --"we are told that in some quarters a panic prevailed, and that some were shouting and praying and imagining that the end of the world had come." (_m.w.r._, - .) at louisville, ky., march , , at about a.m.: duration about half an hour; had been raining moderately, and then hail had fallen. "the intense blackness and general ominous appearance of the storm spread terror throughout the city." (_m.w.r._, - .) however, this merger between possible eclipses by unknown dark bodies and commonplace terrestrial phenomena is formidable. as to darknesses that have fallen upon vast areas, conventionality is--smoke from forest fires. in the _u.s. forest service bulletin_, no. , f.g. plummer gives a list of eighteen darknesses that have occurred in the united states and canada. he is one of the primitives, but i should say that his dogmatism is shaken by vibrations from the new dominant. his difficulty, which he acknowledges, but which he would have disregarded had he written a decade or so earlier, is the profundity of some of these obscurations. he says that mere smokiness cannot account for such "awe-inspiring dark days." so he conceives of eddies in the air, concentrating the smoke from forest fires. then, in the inconsistency or discord of all quasi-intellection that is striving for consistency or harmony, he tells of the vastness of some of these darknesses. of course mr. plummer did not really think upon this subject, but one does feel that he might have approximated higher to real thinking than by speaking of concentration and then listing data of enormous area, or the opposite of circumstances of concentration--because, of his nineteen instances, nine are set down as covering all new england. in quasi-existence, everything generates or is part of its own opposite. every attempt at peace prepares the way for war; all attempts at justice result in injustice in some other respect: so mr. plummer's attempt to bring order into his data, with the explanation of darkness caused by smoke from forest fires, results in such confusion that he ends up by saying that these daytime darknesses have occurred "often with little or no turbidity of the air near the earth's surface"--or with no evidence at all of smoke--except that there is almost always a forest fire somewhere. however, of the eighteen instances, the only one that i'd bother to contest is the profound darkness in canada and northern parts of the united states, nov. , --which we have already considered. its concomitants: lights in the sky; fall of a black substance; shocks like those of an earthquake. in this instance, the only available forest fire was one to the south of the ohio river. for all i know, soot from a very great fire south of the ohio might fall in montreal, canada, and conceivably, by some freak of reflection, light from it might be seen in montreal, but the earthquake is not assimilable with a forest fire. on the other hand, it will soon be our expression that profound darkness, fall of matter from the sky, lights in the sky, and earthquakes are phenomena of the near approach of other worlds to this world. it is such comprehensiveness, as contrasted with inclusion of a few factors and disregard for the rest, that we call higher approximation to realness--or universalness. a darkness, of april , , at wimbledon, england (_symons' met. mag._, - ). it came from a smokeless region: no rain, no thunder; lasted minutes; too dark to go "even out in the open." as to darknesses in great britain, one thinks of fogs--but in _nature_, - , there are some observations by major j. herschel, upon an obscuration in london, jan. , , at : a.m., so great that he could hear persons upon the opposite side of the street, but could not see them--"it was obvious that there was no fog to speak of." _annual register_, - : an account by charles a. murray, british envoy to persia, of a darkness of may , , that came upon bagdad--"a darkness more intense than ordinary midnight, when neither stars nor moon are visible...." "after a short time the black darkness was succeeded by a red, lurid gloom, such as i never saw in any part of the world." "panic seized the whole city." "a dense volume of red sand fell." this matter of sand falling seems to suggest conventional explanation enough, or that a simoon, heavily charged with terrestrial sand, had obscured the sun, but mr. murray, who says that he had had experience with simoons, gives his opinion that "it cannot have been a simoon." it is our comprehensiveness now, or this matter of concomitants of darknesses that we are going to capitalize. it is all very complicated and tremendous, and our own treatment can be but impressionistic, but a few of the rudiments of advanced seismology we shall now take up--or the four principal phenomena of another world's close approach to this world. if a large substantial mass, or super-construction, should enter this earth's atmosphere, it is our acceptance that it would sometimes--depending upon velocity--appear luminous or look like a cloud, or like a cloud with a luminous nucleus. later we shall have an expression upon luminosity--different from the luminosity of incandescence--that comes upon objects falling from the sky, or entering this earth's atmosphere. now our expression is that worlds have often come close to this earth, and that smaller objects--size of a haystack or size of several dozen skyscrapers lumped, have often hurtled through this earth's atmosphere, and have been mistaken for clouds, because they were enveloped in clouds-- or that around something coming from the intense cold of inter-planetary space--that is of some regions: our own suspicion is that other regions are tropical--the moisture of this earth's atmosphere would condense into a cloud-like appearance around it. in _nature_, - , there is an account by mr. s.w. clifton, collector of customs, at freemantle, western australia, sent to the melbourne observatory--a clear day--appearance of a small black cloud, moving not very swiftly--bursting into a ball of fire, of the apparent size of the moon-- or that something with the velocity of an ordinary meteorite could not collect vapor around it, but that slower-moving objects--speed of a railway train, say--may. the clouds of tornadoes have so often been described as if they were solid objects that i now accept that sometimes they are: that some so-called tornadoes are objects hurtling through this earth's atmosphere, not only generating disturbances by their suctions, but crushing, with their bulk, all things in their way, rising and falling and finally disappearing, demonstrating that gravitation is not the power that the primitives think it is, if an object moving at relatively low velocity be not pulled to this earth, or being so momentarily affected, bounds away. in finley's _reports on the character of tornadoes_ very suggestive bits of description occur: "cloud bounded along the earth like a ball"-- or that it was no meteorological phenomenon, but something very much like a huge solid ball that was bounding along, crushing and carrying with it everything within its field-- "cloud bounded along, coming to the earth every eight hundred or one thousand yards." here's an interesting bit that i got somewhere else. i offer it as a datum in super-biology, which, however, is a branch of advanced science that i'll not take up, restricting to things indefinitely called "objects"-- "the tornado came wriggling, jumping, whirling like a great green snake, darting out a score of glistening fangs." though it's interesting, i think that's sensational, myself. it may be that vast green snakes sometimes rush past this earth, taking a swift bite wherever they can, but, as i say, that's a super-biologic phenomenon. finley gives dozens of instances of tornado clouds that seem to me more like solid things swathed in clouds, than clouds. he notes that, in the tornado at americus, georgia, july , , "a strange sulphurous vapor was emitted from the cloud." in many instances, objects, or meteoritic stones, that have come from this earth's externality, have had a sulphurous odor. why a wind effect should be sulphurous is not clear. that a vast object from external regions should be sulphurous is in line with many data. this phenomenon is described in the _monthly weather review_, july, , as "a strange sulphurous vapor ... burning and sickening all who approached close enough to breathe it." the conventional explanation of tornadoes as wind-effects--which we do not deny in some instances--is so strong in the united states that it is better to look elsewhere for an account of an object that has hurtled through this earth's atmosphere, rising and falling and defying this earth's gravitation. _nature_, - : that, according to a correspondent to the _birmingham morning news_, the people living near king's sutton, banbury, saw, about one o'clock, dec. , , something like a haycock hurtling through the air. like a meteor it was accompanied by fire and a dense smoke and made a noise like that of a railway train. "it was sometimes high in the air and sometimes near the ground." the effect was tornado-like: trees and walls were knocked down. it's a late day now to try to verify this story, but a list is given of persons whose property was injured. we are told that this thing then disappeared "all at once." these are the smaller objects, which may be derailed railway trains or big green snakes, for all i know--but our expression upon approach to this earth by vast dark bodies-- that likely they'd be made luminous: would envelop in clouds, perhaps, or would have their own clouds-- but that they'd quake, and that they'd affect this earth with quakes-- and that then would occur a fall of matter from such a world, or rise of matter from this earth to a nearby world, or both fall and rise, or exchange of matter--process known to advanced seismology as celestio-metathesis-- except that--if matter from some other world--and it would be like someone to get it into his head that we absolutely deny gravitation, just because we cannot accept orthodox dogmas--except that, if matter from another world, filling the sky of this earth, generally, as to a hemisphere, or locally, should be attracted to this earth, it would seem thinkable that the whole thing should drop here, and not merely its surface-materials. objects upon a ship's bottom. from time to time they drop to the bottom of the ocean. the ship does not. or, like our acceptance upon dripping from aerial ice-fields, we think of only a part of a nearby world succumbing, except in being caught in suspension, to this earth's gravitation, and surface-materials falling from that part-- explain or express or accept, and what does it matter? our attitude is: here are the data. see for yourself. what does it matter what my notions may be? here are the data. but think for yourself, or think for myself, all mixed up we must be. a long time must go by before we can know florida from long island. so we've had data of fishes that have fallen from our now established and respectabilized super-sargasso sea--which we've almost forgotten, it's now so respectable--but we shall have data of fishes that have fallen during earthquakes. these we accept were dragged down from ponds or other worlds that have been quaked, when only a few miles away, by this earth, some other world also quaking this earth. in a way, or in its principle, our subject is orthodox enough. only grant proximity of other worlds--which, however, will not be a matter of granting, but will be a matter of data--and one conventionally conceives of their surfaces quaked--even of a whole lake full of fishes being quaked and dragged down from one of them. the lake full of fishes may cause a little pain to some minds, but the fall of sand and stones is pleasantly enough thought of. more scientific persons, or more faithful hypnotics than we, have taken up this subject, unpainfully, relatively to the moon. for instance, perrey has gone over , records of earthquakes, and he has correlated many with proximities of the moon, or has attributed many to the pull of the moon when nearest this earth. also there is a paper upon this subject in the _proc. roy. soc. of cornwall_, . or, theoretically, when at its closest to this earth, the moon quakes the face of this earth, and is itself quaked--but does not itself fall to this earth. as to showers of matter that may have come from the moon at such times--one can go over old records and find what one pleases. that is what we now shall do. our expressions are for acceptance only. our data: we take them from four classes of phenomena that have preceded or accompanied earthquakes: unusual clouds, darkness profound, luminous appearances in the sky, and falls of substances and objects whether commonly called meteoritic or not. not one of these occurrences fits in with principles of primitive, or primary, seismology, and every one of them is a datum of a quaked body passing close to this earth or suspended over it. to the primitives there is not a reason in the world why a convulsion of this earth's surface should be accompanied by unusual sights in the sky, by darkness, or by the fall of substances or objects from the sky. as to phenomena like these, or storms, preceding earthquakes, the irreconcilability is still greater. it was before that perrey made his great compilation. we take most of our data from lists compiled long ago. only the safe and unpainful have been published in recent years--at least in ambitious, voluminous form. the restraining hand of the "system"--as we call it, whether it has any real existence or not--is tight upon the sciences of today. the uncanniest aspect of our quasi-existence that i know of is that everything that seems to have one identity has also as high a seeming of everything else. in this oneness of allness, or continuity, the protecting hand strangles; the parental stifles; love is inseparable from phenomena of hate. there is only continuity--that is in quasi-existence. _nature_, at least in its correspondents' columns, still evades this protective strangulation, and the _monthly weather review_ is still a rich field of unfaithful observation: but, in looking over other long-established periodicals, i have noted their glimmers of quasi-individuality fade gradually, after about , and the surrender of their attempted identities to a higher attempted organization. some of them, expressing intermediateness-wide endeavor to localize the universal, or to localize self, soul, identity, entity--or positiveness or realness--held out until as far as ; traces findable up to --and then, expressing the universal process--except that here and there in the world's history there may have been successful approximations to positiveness by "individuals"--who only then became individuals and attained to selves or souls of their own--surrendered, submitted, became parts of a higher organization's attempt to individualize or systematize into a complete thing, or to localize the universal or the attributes of the universal. after the death of richard proctor, whose occasional illiberalities i'd not like to emphasize too much, all succeeding volumes of _knowledge_ have yielded scarcely an unconventionality. note the great number of times that the _american journal of science_ and the _report of the british association_ are quoted: note that, after, say, , they're scarcely mentioned in these inspired but illicit pages--as by hypnosis and inertia, we keep on saying. about . throttle and disregard. but the coercion could not be positive, and many of the excommunicated continued to creep in; or, even to this day, some of the strangled are faintly breathing. some of our data have been hard to find. we could tell stories of great labor and fruitless quests that would, though perhaps imperceptibly, stir the sympathy of a mr. symons. but, in this matter of concurrence of earthquakes with aerial phenomena, which are as unassociable with earthquakes, if internally caused, as falls of sand on convulsed small boys full of sour apples, the abundance of so-called evidence is so great that we can only sketchily go over the data, beginning with robert mallet's catalogue (_rept. brit. assoc._, ), omitting some extraordinary instances, because they occurred before the eighteenth century: earthquake "preceded" by a violent tempest, england, jan. , --"preceded" by a brilliant meteor, switzerland, nov. , --"luminous cloud, moving at high velocity, disappearing behind the horizon," florence, dec. , --"thick mists in the air, through which a dim light was seen: several weeks before the shock, globes of light had been seen in the air," swabia, may , --rain of earth, carpentras, france, oct. , --a black cloud, london, march , --violent storm and a strange star of octagonal shape, slavange, norway, april , --balls of fire from a streak in the sky, augermannland, --numerous meteorites, lisbon, oct. , --"terrible tempests" over and over--"falls of hail" and "brilliant meteors," instance after instance--"an immense globe," switzerland, nov. , --oblong, sulphurous cloud, germany, april, --extraordinary mass of vapor, boulogne, april, --heavens obscured by a dark mist, grenada, aug. , --"strange, howling noises in the air, and large spots obscuring the sun," palermo, italy, april , --"luminous meteor moving in the same direction as the shock," naples, nov. , --fire ball appearing in the sky: apparent size of the moon, thuringerwald, nov. , . and, unless you be polarized by the new dominant, which is calling for recognition of multiplicities of external things, as a dominant, dawning new over europe in , called for recognition of terrestrial externality to europe--unless you have this contact with the new, you have no affinity for these data--beans that drop from a magnet--irreconcilables that glide from the mind of a thomson-- or my own acceptance that we do not really think at all; that we correlate around super-magnets that i call dominants--a spiritual dominant in one age, and responsively to it up spring monasteries, and the stake and the cross are its symbols: a materialist dominant, and up spring laboratories, and microscopes and telescopes and crucibles are its ikons--that we're nothing but iron filings relatively to a succession of magnets that displace preceding magnets. with no soul of your own, and with no soul of my own--except that some day some of us may no longer be intermediatisms, but may hold out against the cosmos that once upon a time thousands of fishes were cast from one pail of water--we have psycho-valency for these data, if we're obedient slaves to the new dominant, and repulsion to them, if we're mere correlates to the old dominant. i'm a soulless and selfless correlate to the new dominant, myself: i see what i have to see. the only inducement i can hold out, in my attempt to rake up disciples, is that some day the new will be fashionable: the new correlates will sneer at the old correlates. after all, there is some inducement to that--and i'm not altogether sure it's desirable to end up as a fixed star. as a correlate to the new dominant, i am very much impressed with some of these data--the luminous object that moved in the same direction as an earthquake--it seems very acceptable that a quake followed this thing as it passed near this earth's surface. the streak that was seen in the sky--or only a streak that was visible of another world--and objects, or meteorites, that were shaken down from it. the quake at carpentras, france: and that, above carpentras, was a smaller world, more violently quaked, so that earth was shaken down from it. but i like best the super-wolves that were seen to cross the sun during the earthquake at palermo. they howled. or the loves of the worlds. the call they feel for one another. they try to move closer and howl when they get there. the howls of the planets. i have discovered a new unintelligibility. in the _edinburgh new philosophical journal_--have to go away back to --days of less efficient strangulation--sir david milne lists phenomena of quakes in great britain. i pick out a few that indicate to me that other worlds were near this earth's surface: violent storm before a shock of --ball of fire "preceding," --a large ball of fire seen upon day following a quake, --"uncommon phenomenon in the air: a large luminous body, bent like a crescent, which stretched itself over the heavens, --vast ball of fire, --black rains and black snows, --numerous instances of upward projection--or upward attraction?--during quakes--preceded by a cloud, very black and lowering," --fall of black powder, preceding a quake, by six hours, . some of these instances seem to me to be very striking--a smaller world: it is greatly racked by the attraction of this earth--black substance is torn down from it--not until six hours later, after an approach still closer, does this earth suffer perturbation. as to the extraordinary spectacle of a thing, world, super-construction, that was seen in the sky, in , i have not yet been able to find out more. i think that here our acceptance is relatively sound: that this occurrence was tremendously of more importance than such occurrence as, say, transits of venus, upon which hundreds of papers have been written--that not another mention have i found, though i have not looked so especially as i shall look for more data--that all but undetailed record of this occurrence was suppressed. altogether we have considerable agreement here between data of vast masses that do not fall to this earth, but from which substances fall, and data of fields of ice from which ice may not fall, but from which water may drip. i'm beginning to modify: that, at a distance from this earth, gravitation has more effect than we have supposed, though less effect than the dogmatists suppose and "prove." i'm coming out stronger for the acceptance of a neutral zone--that this earth, like other magnets, has a neutral zone, in which is the super-sargasso sea, and in which other worlds may be buoyed up, though projecting parts may be subject to this earth's attraction-- but my preference: here are the data. i now have one of the most interesting of the new correlates. i think i should have brought it in before, but, whether out of place here, because not accompanied by earthquake, or not, we'll have it. i offer it as an instance of an eclipse, by a vast, dark body, that has been seen and reported by an astronomer. the astronomer is m. lias: the phenomenon was seen by him, at pernambuco, april , . _comptes rendus_, - : it was about noon--sky cloudless--suddenly the light of the sun was diminished. the darkness increased, and, to illustrate its intensity, we are told that the planet venus shone brilliant. but venus was of low visibility at this time. the observation that burns incense to the new dominant is: that around the sun appeared a corona. there are many other instances that indicate proximity of other world's during earthquakes. i note a few--quake and an object in the sky, called "a large, luminous meteor" (_quar. jour. roy. inst._, - ); luminous body in the sky, earthquake, and fall of sand, italy, feb. and , (_la science pour tous_, - ); many reports upon luminous object in the sky and earthquake, connecticut, feb. , (_monthly weather review_, february, ); luminous object, or meteor, in the sky, fall of stones from the sky, and earthquake, italy, jan. , (_l'astronomie_, - ); earthquake and prodigious number of luminous bodies, or globes, in the air, boulogne, france, june , (sestier, "_la foudre_," - ); earthquake at manila, , and "curious luminous appearance in the sky" (ponton, _earthquakes_, p. ). the most notable appearance of fishes during an earthquake is that of riobamba. humboldt sketched one of them, and it's an uncanny-looking thing. thousands of them appeared upon the ground during this tremendous earthquake. humboldt says that they were cast up from subterranean sources. i think not myself, and have data for thinking not, but there'd be such a row arguing back and forth that it's simpler to consider a clearer instance of the fall of living fishes from the sky, during an earthquake. i can't quite accept, myself, whether a large lake, and all the fishes in it, was torn down from some other world, or a lake in the super-sargasso sea, distracted between two pulling worlds, was dragged down to this earth-- here are the data: _la science pour tous_, - : feb. , . an earthquake at singapore. then came an extraordinary downpour of rain--or as much water as any good-sized lake would consist of. for three days this rain or this fall of water came down in torrents. in pools on the ground, formed by this deluge, great numbers of fishes were found. the writer says that he had, himself, seen nothing but water fall from the sky. whether i'm emphasizing what a deluge it was or not, he says that so terrific had been the downpour that he had not been able to see three steps away from him. the natives said that the fishes had fallen from the sky. three days later the pools dried up and many dead fishes were found, but, in the first place--though that's an expression for which we have an instinctive dislike--the fishes had been active and uninjured. then follows material for another of our little studies in the phenomena of disregard. a psycho-tropism here is mechanically to take pen in hand and mechanically write that fishes found on the ground after a heavy rainfall came from overflowing streams. the writer of the account says that some of the fishes had been found in his courtyard, which was surrounded by high walls--paying no attention to this, a correspondent (_la science pour tous_, - ) explains that in the heavy rain a body of water had probably overflowed, carrying fishes with it. we are told by the first writer that these fishes of singapore were of a species that was very abundant near singapore. so i think, myself, that a whole lakeful of them had been shaken down from the super-sargasso sea, under the circumstances we have thought of. however, if appearance of strange fishes after an earthquake be more pleasing in the sight, or to the nostrils, of the new dominant, we faithfully and piously supply that incense--an account of the occurrence at singapore was read by m. de castelnau, before the french academy. m. de castelnau recalled that, upon a former occasion, he had submitted to the academy the circumstance that fishes of a new species had appeared at the cape of good hope, after an earthquake. it seems proper, and it will give luster to the new orthodoxy, now to have an instance in which, not merely quake and fall of rocks or meteorites, or quake and either eclipse or luminous appearances in the sky have occurred, but in which are combined all the phenomena, one or more of which, when accompanying earthquake, indicate, in our acceptance, the proximity of another world. this time a longer duration is indicated than in other instances. in the _canadian institute proceedings_, - - , there is an account, by the deputy commissioner at dhurmsalla, of the extraordinary dhurmsalla meteorite--coated with ice. but the combination of events related by him is still more extraordinary: that within a few months of the fall of this meteorite there had been a fall of live fishes at benares, a shower of red substance at furruckabad, a dark spot observed on the disk of the sun, an earthquake, "an unnatural darkness of some duration," and a luminous appearance in the sky that looked like an aurora borealis-- but there's more to this climax: we are introduced to a new order of phenomena: visitors. the deputy commissioner writes that, in the evening, after the fall of the dhurmsalla meteorite, or mass of stone covered with ice, he saw lights. some of them were not very high. they appeared and went out and reappeared. i have read many accounts of the dhurmsalla meteorite--july , --but never in any other of them a mention of this new correlate--something as out of place in the nineteenth century as would have been an aeroplane--the invention of which would not, in our acceptance, have been permitted, in the nineteenth century, though adumbrations to it were permitted. this writer says that the lights moved like fire balloons, but: "i am sure that they were neither fire balloons, lanterns, nor bonfires, or any other thing of that sort, but bona fide lights in the heavens." it's a subject for which we shall have to have a separate expression--trespassers upon territory to which something else has a legal right--perhaps someone lost a rock, and he and his friends came down looking for it, in the evening--or secret agents, or emissaries, who had an appointment with certain esoteric ones near dhurmsalla--things or beings coming down to explore, and unable to stay down long-- in a way, another strange occurrence during an earthquake is suggested. the ancient chinese tradition--the marks like hoof marks in the ground. we have thought--with a low degree of acceptance--of another world that may be in secret communication with certain esoteric ones of this earth's inhabitants--and of messages in symbols like hoof marks that are sent to some receptor, or special hill, upon this earth--and of messages that at times miscarry. this other world comes close to this world--there are quakes--but advantage of proximity is taken to send a message--the message, designed for a receptor in india, perhaps, or in central europe, miscarries all the way to england--marks like the marks of the chinese tradition are found upon a beach, in cornwall, after an earthquake-- _phil. trans._, - : after the quake of july , , upon the sands of penzance, cornwall, in an area of more than square yards, were found marks like hoof prints, except that they were not crescentic. we feel a similarity, but note an arbitrary disregard of our own, this time. it seems to us that marks described as "little cones surrounded by basins of equal diameter" would be like hoof prints, if hoofs printed complete circles. other disregards are that there were black specks on the tops of cones, as if something, perhaps gaseous, had issued from them; that from one of these formations came a gush of water as thick as a man's wrist. of course the opening of springs is common in earthquakes--but we suspect, myself, that the negative absolute is compelling us to put in this datum and its disorders. there's another matter in which the negative absolute seems to work against us. though to super-chemistry, we have introduced the principle of celestio-metathesis, we have no good data of exchange of substances during proximities. the data are all of falls and not of upward translations. of course upward impulses are common during earthquakes, but i haven't a datum upon a tree or a fish or a brick or a man that ever did go up and stay up and that never did come down again. our classic of the horse and barn occurred in what was called a whirlwind. it is said that, in an earthquake in calabria, paving stones shot up far in the air. the writer doesn't specifically say that they came down again, but something seems to tell me they did. the corpses of riobamba. humboldt reported that, in the quake of riobamba, "bodies were torn upward from graves"; that "the vertical motion was so strong that bodies were tossed several hundred feet in the air." i explain. i explain that, if in the center of greatest violence of an earthquake, anything ever has gone up, and has kept on going up, the thoughts of the nearest observers were very likely upon other subjects. the quay of lisbon. we are told that it went down. a vast throng of persons ran to the quay for refuge. the city of lisbon was in profound darkness. the quay and all the people on it disappeared. if it and they went down--not a single corpse, not a shred of clothing, not a plank of the quay, nor so much as a splinter of it ever floated to the surface. the new dominant. i mean "primarily" all that opposes exclusionism-- that development or progress or evolution is attempt to positivize, and is a mechanism by which a positive existence is recruited--that what we call existence is a womb of infinitude, and is itself only incubatory--that eventually all attempts are broken down by the falsely excluded. subjectively, the breaking down is aided by our own sense of false and narrow limitations. so the classic and academic artists wrought positivist paintings, and expressed the only ideal that i am conscious of, though we so often hear of "ideals" instead of different manifestations, artistically, scientifically, theologically, politically, of the one ideal. they sought to satisfy, in its artistic aspect, cosmic craving for unity or completeness, sometimes called harmony, called beauty in some aspects. by disregard they sought completeness. but the light-effects that they disregarded, and their narrow confinement to standardized subjects brought on the revolt of the impressionists. so the puritans tried to systematize, and they disregarded physical needs, or vices, or relaxations: they were invaded and overthrown when their narrowness became obvious and intolerable. all things strive for positiveness, for themselves, or for quasi-systems of which they are parts. formality and the mathematic, the regular and the uniform are aspects of the positive state--but the positive is the universal--so all attempted positiveness that seems to satisfy in the aspects of formality and regularity, sooner or later disqualifies in the aspect of wideness or universalness. so there is revolt against the science of today, because the formulated utterances that were regarded as final truths in a past generation, are now seen to be insufficiencies. every pronouncement that has opposed our own acceptances has been found to be a composition like any academic painting: something that is arbitrarily cut off from relations with environment, or framed off from interfering and disturbing data, or outlined with disregards. our own attempt has been to take in the included, but also to take in the excluded into wider expressions. we accept, however, that for every one of our expressions there are irreconcilables somewhere--that final utterance would include all things. however, of such is the gossip of angels. the final is unutterable in quasi-existence, where to think is to include but also to exclude, or be not final. if we admit that for every opinion we have expressed, there must somewhere be an irreconcilable, we are intermediatists and not positivists; not even higher positivists. of course it may be that some day we shall systematize and dogmatize and refuse to think of anything that we may be accused of disregarding, and believe instead of merely accepting: then, if we could have a wider system, which would acknowledge no irreconcilables we'd be higher positivists. so long as we only accept, we are not higher positivists, but our feeling is that the new dominant, even though we have thought of it only as another enslavement, will be the nucleus for higher positivism--and that it will be the means of elevating into infinitude a new batch of fixed stars--until, as a recruiting instrument, it, too, will play out, and will give way to some new medium for generating absoluteness. it is our acceptance that all astronomers of today have lost their souls, or, rather, all chance of attaining entity, but that copernicus and kepler and galileo and newton, and, conceivably, leverrier are now fixed stars. some day i shall attempt to identify them. in all this, i think we're quite a moses. we point out the promised land, but, unless we be cured of our intermediatism, will never be reported in _monthly notices_, ourself. in our acceptance, dominants, in their succession, displace preceding dominants not only because they are more nearly positive, but because the old dominants, as recruiting mediums, play out. our expression is that the new dominant, of wider inclusions, is now manifesting throughout the world, and that the old exclusionism is everywhere breaking down. in physics exclusionism is breaking down by its own researches in radium, for instance, and in its speculations upon electrons, or its merging away into metaphysics, and by the desertion that has been going on for many years, by such men as gurney, crookes, wallace, flammarion, lodge, to formerly disregarded phenomena--no longer called "spiritualism" but now "psychic research." biology is in chaos: conventional darwinites mixed up with mutationists and orthogenesists and followers of wisemann, who take from darwinism one of its pseudo-bases, and nevertheless try to reconcile their heresies with orthodoxy. the painters are metaphysicians and psychologists. the breaking down of exclusionism in china and japan and in the united states has astonished history. the science of astronomy is going downward so that, though pickering, for instance, did speculate upon a trans-neptunian planet, and lowell did try to have accepted heretical ideas as to marks on mars, attention is now minutely focused upon such technicalities as variations in shades of jupiter's fourth satellite. i think that, in general acceptance, over-refinement indicates decadence. i think that the stronghold of inclusionism is in aeronautics. i think that the stronghold of the old dominant, when it was new, was in the invention of the telescope. or that coincidentally with the breakdown of exclusionism appears the means of finding out--whether there are vast aerial fields of ice and floating lakes full of frogs and fishes or not--where carved stones and black substances and great quantities of vegetable matter and flesh, which may be dragons' flesh, come from--whether there are inter-planetary trade routes and vast areas devastated by super-tamerlanes--whether sometimes there are visitors to this earth--who might be pursued and captured and questioned. i have industriously sought data for an expression upon birds, but the prospecting has not been very quasi-satisfactory. i think i rather emphasize our industriousness, because a charge likely to be brought against the attitude of acceptance is that one who only accepts must be one of languid interest and little application of energy. it doesn't seem to work out: we are very industrious. i suggest to some of our disciples that they look into the matter of messages upon pigeons, of course attributed to earthly owners, but said to be undecipherable. i'd do it, ourselves, only that would be selfish. that's more of the intermediatism that will keep us out of the firmament: positivism is absolute egoism. but look back in the time of andrée's polar expedition. pigeons that would have no publicity ordinarily, were often reported at that time. in the _zoologist_, - - , is recorded an instance of a bird (puffin) that had fallen to the ground with a fractured head. interesting, but mere speculation--but what solid object, high in the air, had that bird struck against? tremendous red rain in france, oct. and , ; great storm at the time, and red rain supposed to have been colored by matter swept up from this earth's surface, and then precipitated (_comptes rendus_, - ). but in _comptes rendus_, - , the description of this red rain differs from one's impression of red, sandy or muddy water. it is said that this rain was so vividly red and so blood-like that many persons in france were terrified. two analyses are given (_comptes rendus_, - ). one chemist notes a great quantity of corpuscles--whether blood-like corpuscles or not--in the matter. the other chemist sets down organic matter at per cent. it may be that an inter-planetary dragon had been slain somewhere, or that this red fluid, in which were many corpuscles, came from something not altogether pleasant to contemplate, about the size of the catskill mountains, perhaps--but the present datum is that with this substance, larks, quail, ducks, and water hens, some of them alive, fell at lyons and grenoble and other places. i have notes upon other birds that have fallen from the sky, but unaccompanied by the red rain that makes the fall of birds in france peculiar, and very peculiar, if it be accepted that the red substance was extra-mundane. the other notes are upon birds that have fallen from the sky, in the midst of storms, or of exhausted, but living, birds, falling not far from a storm-area. but now we shall have an instance for which i can find no parallel: fall of dead birds, from a clear sky, far-distant from any storm to which they could be attributed--so remote from any discoverable storm that-- my own notion is that, in the summer of , something, or some beings, came as near to this earth as they could, upon a hunting expedition; that, in the summer of , an expedition of super-scientists passed over this earth, and let down a dragnet--and what would it catch, sweeping through the air, supposing it to have reached not quite to this earth? in the _monthly weather review_, may, , w.l. mcatee quotes from the baton rouge correspondence to the _philadelphia times_: that, in the summer of , into the streets of baton rouge, la., and from a "clear sky," fell hundreds of dead birds. there were wild ducks and cat birds, woodpeckers, and "many birds of strange plumage," some of them resembling canaries. usually one does not have to look very far from any place to learn of a storm. but the best that could be done in this instance was to say: "there had been a storm on the coast of florida." and, unless he have psycho-chemic repulsion for the explanation, the reader feels only momentary astonishment that dead birds from a storm in florida should fall from an unstormy sky in louisiana, and with his intellect greased like the plumage of a wild duck, the datum then drops off. our greasy, shiny brains. that they may be of some use after all: that other modes of existence place a high value upon them as lubricants; that we're hunted for them; a hunting expedition to this earth--the newspapers report a tornado. if from a clear sky, or a sky in which there were no driven clouds, or other evidences of still-continuing wind-power--or, if from a storm in florida, it could be accepted that hundreds of birds had fallen far away, in louisiana, i conceive, conventionally, of heavier objects having fallen in alabama, say, and of the fall of still heavier objects still nearer the origin in florida. the sources of information of the weather bureau are widespread. it has no records of such falls. so a dragnet that was let down from above somewhere-- or something that i learned from the more scientific of the investigators of psychic phenomena: the reader begins their works with prejudice against telepathy and everything else of psychic phenomena. the writers deny spirit-communication, and say that the seeming data are data of "only telepathy." astonishing instances of seeming clairvoyance--"only telepathy." after a while the reader finds himself agreeing that it's only telepathy--which, at first, had been intolerable to him. so maybe, in , a super-dragnet did not sweep through this earth's atmosphere, gathering up all the birds within its field, the meshes then suddenly breaking-- or that the birds of baton rouge were only from the super-sargasso sea-- upon which we shall have another expression. we thought we'd settled that, and we thought we'd establish that, but nothing's ever settled, and nothing's ever established, in a real sense, if, in a real sense, there is nothing in quasiness. i suppose there had been a storm somewhere, the storm in florida, perhaps, and many birds had been swept upward into the super-sargasso sea. it has frigid regions and it has tropical regions--that birds of diverse species had been swept upward, into an icy region, where, huddling together for warmth, they had died. then, later, they had been dislodged--meteor coming along--boat--bicycle--dragon--don't know what did come along--something dislodged them. so leaves of trees, carried up there in whirlwinds, staying there years, ages, perhaps only a few months, but then falling to this earth at an unseasonable time for dead leaves--fishes carried up there, some of them dying and drying, some of them living in volumes of water that are in abundance up there, or that fall sometimes in the deluges that we call "cloudbursts." the astronomers won't think kindly of us, and we haven't done anything to endear ourselves to the meteorologists--but we're weak and mawkish intermediatists--several times we've tried to get the aeronauts with us--extraordinary things up there: things that curators of museums would give up all hope of ever being fixed stars, to obtain: things left over from whirlwinds of the time of the pharaohs, perhaps: or that elijah did go up in the sky in something like a chariot, and may not be vega, after all, and that there may be a wheel or so left of whatever he went up in. we basely suggest that it would bring a high price--but sell soon, because after a while there'd be thousands of them hawked around-- we weakly drop a hint to the aeronauts. in the _scientific american_, - , there is an account of some hay that fell from the sky. from the circumstances we incline to accept that this hay went up, in a whirlwind, from this earth, in the first place, reached the super-sargasso sea, and remained there a long time before falling. an interesting point in this expression is the usual attribution to a local and coinciding whirlwind, and identification of it--and then data that make that local whirlwind unacceptable-- that, upon july , , small masses of damp hay had fallen at monkstown, ireland. in the _dublin daily express_, dr. j.w. moore had explained: he had found a nearby whirlwind, to the south of monkstown, that coincided. but, according to the _scientific american_, a similar fall had occurred near wrexham, england, two days before. in november, , i made some studies upon light objects thrown into the air. armistice-day. i suppose i should have been more emotionally occupied, but i made notes upon torn-up papers thrown high in the air from windows of office buildings. scraps of paper did stay together for a while. several minutes, sometimes. _cosmos_, - - : that, upon the th of april, , at autriche (indre-et-loire) a great number of oak leaves--enormous segregation of them--fell from the sky. very calm day. so little wind that the leaves fell almost vertically. fall lasted about ten minutes. flammarion, in _the atmosphere_, p. , tells this story. he has to find a storm. he does find a squall--but it had occurred upon april rd. flammarion's two incredibilities are--that leaves could remain a week in the air: that they could stay together a week in the air. think of some of your own observations upon papers thrown from an aeroplane. our one incredibility: that these leaves had been whirled up six months before, when they were common on the ground, and had been sustained, of course not in the air, but in a region gravitationally inert; and had been precipitated by the disturbances of april rains. i have no records of leaves that have so fallen from the sky in october or november, the season when one might expect dead leaves to be raised from one place and precipitated somewhere else. i emphasize that this occurred in april. _la nature_, - - : that, upon april , , dried leaves, of different species, oak, elm, etc., fell from the sky. this day, too, was a calm day. the fall was tremendous. the leaves were seen to fall fifteen minutes, but, judging from the quantity on the ground, it is the writer's opinion that they had already been falling half an hour. i think that the geyser of corpses that sprang from riobamba toward the sky must have been an interesting sight. if i were a painter, i'd like that subject. but this cataract of dried leaves, too, is a study in the rhythms of the dead. in this datum, the point most agreeable to us is the very point that the writer in _la nature_ emphasizes. windlessness. he says that the surface of the loire was "absolutely smooth." the river was strewn with leaves as far as he could see. _l'astronomie_, - : that, upon the th of april, , dried leaves fell at clairvaux and outre-aube, france. the fall is described as prodigious. half an hour. then, upon the th, a fall of dried leaves occurred at pontcarré. it is in this recurrence that we found some of our opposition to the conventional explanation. the editor (flammarion) explains. he says that the leaves had been caught up in a cyclone which had expended its force; that the heavier leaves had fallen first. we think that that was all right for , and that it was quite good enough for . but, in these more exacting days, we want to know how wind-power insufficient to hold some leaves in the air could sustain others four days. the factors in this expression are unseasonableness, not for dried leaves, but for prodigious numbers of dried leaves; direct fall, windlessness, month of april, and localization in france. the factor of localization is interesting. not a note have i upon fall of leaves from the sky, except these notes. were the conventional explanation, or "old correlate" acceptable, it would seem that similar occurrences in other regions should be as frequent as in france. the indication is that there may be quasi-permanent undulations in the super-sargasso sea, or a pronounced inclination toward france-- inspiration: that there may be a nearby world complementary to this world, where autumn occurs at the time that is springtime here. let some disciple have that. but there may be a dip toward france, so that leaves that are borne high there, are more likely to be held in suspension than highflying leaves elsewhere. some other time i shall take up super-geography, and be guilty of charts. i think, now, that the super-sargasso sea is an oblique belt, with changing ramifications, over great britain, france, italy, and on to india. relatively to the united states i am not very clear, but think especially of the southern states. the preponderance of our data indicates frigid regions aloft. nevertheless such phenomena as putrefaction have occurred often enough to make super-tropical regions, also, acceptable. we shall have one more datum upon the super-sargasso sea. it seems to me that, by this time, our requirements of support and reinforcement and agreement have been quite as rigorous for acceptance as ever for belief: at least for full acceptance. by virtue of mere acceptance, we may, in some later book, deny the super-sargasso sea, and find that our data relate to some other complementary world instead--or the moon--and have abundant data for accepting that the moon is not more than twenty or thirty miles away. however, the super-sargasso sea functions very well as a nucleus around which to gather data that oppose exclusionism. that is our main motive: to oppose exclusionism. or our agreement with cosmic processes. the climax of our general expression upon the super-sargasso sea. coincidentally appears something else that may overthrow it later. _notes and queries_, - - : that in the province of macerata, italy (summer of ?) an immense number of small, blood-colored clouds covered the sky. about an hour later a storm broke, and myriad seeds fell to the ground. it is said that they were identified as products of a tree found only in central africa and the antilles. if--in terms of conventional reasoning--these seeds had been high in the air, they had been in a cold region. but it is our acceptance that these seeds had, for a considerable time, been in a warm region, and for a time longer than is attributable to suspension by wind-power: "it is said that a great number of the seeds were in the first stage of germination." the new dominant. inclusionism. in it we have a pseudo-standard. we have a datum, and we give it an interpretation, in accordance with our pseudo-standard. at present we have not the delusions of absolutism that may have translated some of the positivists of the nineteenth century to heaven. we are intermediatists--but feel a lurking suspicion that we may some day solidify and dogmatize and illiberalize into higher positivists. at present we do not ask whether something be reasonable or preposterous, because we recognize that by reasonableness and preposterousness are meant agreement and disagreement with a standard--which must be a delusion--though not absolutely, of course--and must some day be displaced by a more advanced quasi-delusion. scientists in the past have taken the positivist attitude--is this or that reasonable or unreasonable? analyze them and we find that they meant relatively to a standard, such as newtonism, daltonism, darwinism, or lyellism. but they have written and spoken and thought as if they could mean real reasonableness and real unreasonableness. so our pseudo-standard is inclusionism, and, if a datum be a correlate to a more widely inclusive outlook as to this earth and its externality and relations with externality, its harmony with inclusionism admits it. such was the process, and such was the requirement for admission in the days of the old dominant: our difference is in underlying intermediatism, or consciousness that though we're more nearly real, we and our standards are only quasi-- or that all things--in our intermediate state--are phantoms in a super-mind in a dreaming state--but striving to awaken to realness. though in some respects our own intermediatism is unsatisfactory, our underlying feeling is-- that in a dreaming mind awakening is accelerated--if phantoms in that mind know that they're only phantoms in a dream. of course, they too are quasi, or--but in a relative sense--they have an essence of what is called realness. they are derived from experience or from senes-relations, even though grotesque distortions. it seems acceptable that a table that is seen when one is awake is more nearly real than a dreamed table, which, with fifteen or twenty legs, chases one. so now, in the twentieth century, with a change of terms, and a change in underlying consciousness, our attitude toward the new dominant is the attitude of the scientists of the nineteenth century to the old dominant. we do not insist that our data and interpretations shall be as shocking, grotesque, evil, ridiculous, childish, insincere, laughable, ignorant to nineteenth-centuryites as were their data and interpretations to the medieval-minded. we ask only whether data and interpretations correlate. if they do, they are acceptable, perhaps only for a short time, or as nuclei, or scaffolding, or preliminary sketches, or as gropings and tentativenesses. later, of course, when we cool off and harden and radiate into space most of our present mobility, which expresses in modesty and plasticity, we shall acknowledge no scaffoldings, gropings or tentativenesses, but think we utter absolute facts. a point in intermediatism here is opposed to most current speculations upon development. usually one thinks of the spiritual as higher than the material, but, in our acceptance, quasi-existence is a means by which the absolutely immaterial materializes absolutely, and, being intermediate, is a state in which nothing is finally either immaterial or material, all objects, substances, thoughts, occupying some grade of approximation one way or the other. final solidification of the ethereal is, to us, the goal of cosmic ambition. positivism is puritanism. heat is evil. final good is absolute frigidity. an arctic winter is very beautiful, but i think that an interest in monkeys chattering in palm trees accounts for our own intermediatism. visitors. our confusion here, out of which we are attempting to make quasi-order, is as great as it has been throughout this book, because we have not the positivist's delusion of homogeneity. a positivist would gather all data that seem to relate to one kind of visitors and coldly disregard all other data. i think of as many different kinds of visitors to this earth as there are visitors to new york, to a jail, to a church--some persons go to church to pick pockets, for instance. my own acceptance is that either a world or a vast super-construction--or a world, if red substances and fishes fell from it--hovered over india in the summer of . something then fell from somewhere, july , , at dhurmsalla. whatever "it" was, "it" is so persistently alluded to as "a meteorite" that i look back and see that i adopted this convention myself. but in the london _times_, dec. , , syed abdoolah, professor of hindustani, university college, london, writes that he had sent to a friend in dhurmsalla, for an account of the stones that had fallen at that place. the answer: "... divers forms and sizes, many of which bore great resemblance to ordinary cannon balls just discharged from engines of war." it's an addition to our data of spherical objects that have arrived upon this earth. note that they are spherical stone objects. and, in the evening of this same day that something--took a shot at dhurmsalla--or sent objects upon which there may be decipherable markings--lights were seen in the air-- i think, myself, of a number of things, beings, whatever they were, trying to get down, but resisted, like balloonists, at a certain altitude, trying to get farther up, but resisted. not in the least except to good positivists, or the homogeneous-minded, does this speculation interfere with the concept of some other world that is in successful communication with certain esoteric ones upon this earth, by a code of symbols that print in rock, like symbols of telephotographers in selenium. i think that sometimes, in favorable circumstances, emissaries have come to this earth--secret meetings-- of course it sounds-- but: secret meetings--emissaries--esoteric ones in europe, before the war broke out-- and those who suggested that such phenomena could be. however, as to most of our data, i think of super-things that have passed close to this earth with no more interest in this earth than have passengers upon a steamship in the bottom of the sea--or passengers may have a keen interest, but circumstances of schedules and commercial requirements forbid investigation of the bottom of the sea. then, on the other hand, we may have data of super-scientific attempts to investigate phenomena of this earth from above--perhaps by beings from so far away that they had never even heard that something, somewhere, asserts a legal right to this earth. altogether, we're good intermediatists, but we can't be very good hypnotists. still another source of the merging away of our data: that, upon general principles of continuity, if super-vessels, or super-vehicles, have traversed this earth's atmosphere, there must be mergers between them and terrestrial phenomena: observations upon them must merge away into observations upon clouds and balloons and meteors. we shall begin with data that we cannot distinguish ourselves and work our way out of mergers into extremes. in the _observatory_, - , it is said that, according to a newspaper, march , , residents of warmley, england, were greatly excited by something that was supposed to be "a splendidly illuminated aeroplane, passing over the village." "the machine was apparently traveling at a tremendous rate, and came from the direction of bath, and went on toward gloucester." the editor says that it was a large, triple-headed fireball. "tremendous indeed!" he says. "but we are prepared for anything nowadays." that is satisfactory. we'd not like to creep up stealthily and then jump out of a corner with our data. this editor, at least, is prepared to read-- _nature_, oct. , : a correspondent writes that, in the county wicklow, ireland, at about o'clock in the evening, he had seen, in the sky, an object that looked like the moon in its three-quarter aspect. we note the shape which approximates to triangularity, and we note that in color it is said to have been golden yellow. it moved slowly, and in about five minutes disappeared behind a mountain. the editor gives his opinion that the object may have been an escaped balloon. in _nature_, aug. , , there is a story, taken from the july number of the _canadian weather review_, by the meteorologist, f.f. payne: that he had seen, in the canadian sky, a large, pear-shaped object, sailing rapidly. at first he supposed that the object was a balloon, "its outline being sharply defined." "but, as no cage was seen, it was concluded that it must be a mass of cloud." in about six minutes this object became less definite--whether because of increasing distance or not--"the mass became less dense, and finally it disappeared." as to cyclonic formation--"no whirling motion could be seen." _nature_, - : that, upon july , , a correspondent had seen, at kiel, an object in the sky, colored red by the sun, which had set. it was about as broad as a rainbow, and about twelve degrees high. "it remained in its original brightness about five minutes, and then faded rapidly, and then remained almost stationary again, finally disappearing about eight minutes after i first saw it." in an intermediate existence, we quasi-persons have nothing to judge by because everything is its own opposite. if a hundred dollars a week be a standard of luxurious living to some persons, it is poverty to others. we have instances of three objects that were seen in the sky in a space of three months, and this concurrence seems to me to be something to judge by. science has been built upon concurrence: so have been most of the fallacies and fanaticisms. i feel the positivism of a leverrier, or instinctively take to the notion that all three of these observations relate to the same object. however, i don't formulate them and predict the next transit. here's another chance for me to become a fixed star--but as usual--oh, well-- a point in intermediatism: that the intermediatist is likely to be a flaccid compromiser. our own attitude: ours is a partly positive and partly negative state, or a state in which nothing is finally positive or finally negative-- but, if positivism attract you, go ahead and try: you will be in harmony with cosmic endeavor--but continuity will resist you. only to have appearance in quasiness is to be proportionately positive, but beyond a degree of attempted positivism, continuity will rise to pull you back. success, as it is called--though there is only success-failure in intermediateness--will, in intermediateness, be yours proportionately as you are in adjustment with its own state, or some positivism mixed with compromise and retreat. to be very positive is to be a napoleon bonaparte, against whom the rest of civilization will sooner or later combine. for interesting data, see newspaper accounts of fate of one dowie, of chicago. intermediatism, then, is recognition that our state is only a quasi-state: it is no bar to one who desires to be positive: it is recognition that he cannot be positive and remain in a state that is positive-negative. or that a great positivist--isolated--with no system to support him--will be crucified, or will starve to death, or will be put in jail and beaten to death--that these are the birth-pangs of translation to the positive absolute. so, though positive-negative, myself, i feel the attraction of the positive pole of our intermediate state, and attempt to correlate these three data: to see them homogeneously; to think that they relate to one object. in the aeronautic journals and in the london _times_ there is no mention of escaped balloons, in the summer or fall of . in the _new york times_ there is no mention of ballooning in canada or the united states, in the summer of . london _times_, sept. , : a clipping from the _royal gazette_, of bermuda, of sept. , , sent to the _times_ by general lefroy: that, upon aug. , , at about : a.m., there was observed by mrs. adelina d. bassett, "a strange object in the clouds, coming from the north." she called the attention of mrs. l. lowell to it, and they were both somewhat alarmed. however, they continued to watch the object steadily for some time. it drew nearer. it was of triangular shape, and seemed to be about the size of a pilot-boat mainsail, with chains attached to the bottom of it. while crossing the land it had appeared to descend, but, as it went out to sea, it ascended, and continued to ascend, until it was lost to sight high in the clouds. or with such power to ascend, i don't think much myself of the notion that it was an escaped balloon, partly deflated. nevertheless, general lefroy, correlating with exclusionism, attempts to give a terrestrial interpretation to this occurrence. he argues that the thing may have been a balloon that had escaped from france or england--or the only aerial thing of terrestrial origin that, even to this date of about thirty-five years later, has been thought to have crossed the atlantic ocean. he accounts for the triangular form by deflation--"a shapeless bag, barely able to float." my own acceptance is that great deflation does not accord with observations upon its power to ascend. in the _times_, oct. , , charles harding, of the r.m.s., argues that if it had been a balloon from europe, surely it would have been seen and reported by many vessels. whether he was as good a briton as the general or not, he shows awareness of the united states--or that the thing may have been a partly collapsed balloon that had escaped from the united states. general lefroy wrote to _nature_ about it (_nature_, - ), saying--whatever his sensitivenesses may have been--that the columns of the _times_ were "hardly suitable" for such a discussion. if, in the past, there had been more persons like general lefroy, we'd have better than the mere fragments of data that in most cases are too broken up very well to piece together. he took the trouble to write to a friend of his, w.h. gosling, of bermuda--who also was an extraordinary person. he went to the trouble of interviewing mrs. bassett and mrs. lowell. their description to him was somewhat different: an object from which nets were suspended-- deflated balloon, with its network hanging from it-- a super-dragnet? that something was trawling overhead? the birds of baton rouge. mr. gosling wrote that the item of chains, or suggestion of a basket that had been attached, had originated with mr. bassett, who had not seen the object. mr. gosling mentioned a balloon that had escaped from paris in july. he tells of a balloon that fell in chicago, september , or three weeks later than the bermuda object. it's one incredibility against another, with disregards and convictions governed by whichever of the two dominants looms stronger in each reader's mind. that he can't think for himself any more than i can is understood. my own correlates: i think that we're fished for. it may be that we're highly esteemed by super-epicures somewhere. it makes me more cheerful when i think that we may be of some use after all. i think that dragnets have often come down and have been mistaken for whirlwinds and waterspouts. some accounts of seeming structure in whirlwinds and waterspouts are astonishing. and i have data that, in this book, i can't take up at all--mysterious disappearances. i think we're fished for. but this is a little expression on the side: relates to trespassers; has nothing to do with the subject that i shall take up at some other time--or our use to some other mode of seeming that has a legal right to us. _nature_, - : "our paris correspondent writes that in relation to the balloon which is said to have been seen over bermuda, in september, no ascent took place in france which can account for it." last of august: not september. in the london _times_ there is no mention of balloon ascents in great britain, in the summer of , but mention of two ascents in france. both balloons had escaped. in _l'aéronaute_, august, , it is said that these balloons had been sent up from fêtes of the fourteenth of july-- days before the observation at bermuda. the aeronauts were gower and eloy. gower's balloon was found floating on the ocean, but eloy's balloon was not found. upon the th of july it was reported by a sea captain: still in the air; still inflated. but this balloon of eloy's was a small exhibition balloon, made for short ascents from fêtes and fair grounds. in _la nature_, - - , it is said that it was a very small balloon, incapable of remaining long in the air. as to contemporaneous ballooning in the united states, i find only one account: an ascent in connecticut, july , . upon leaving this balloon, the aeronauts had pulled the "rip cord," "turning it inside out." (_new york times_, aug. , .) to the intermediatist, the accusation of "anthropomorphism" is meaningless. there is nothing in anything that is unique or positively different. we'd be materialists were it not quite as rational to express the material in terms of the immaterial as to express the immaterial in terms of the material. oneness of allness in quasiness. i will engage to write the formula of any novel in psycho-chemic terms, or draw its graph in psycho-mechanic terms: or write, in romantic terms, the circumstances and sequences of any chemic or electric or magnetic reaction: or express any historic event in algebraic terms--or see boole and jevons for economic situations expressed algebraically. i think of the dominants as i think of persons--not meaning that they are real persons--not meaning that we are real persons-- or the old dominant and its jealousy, and its suppression of all things and thoughts that endangered its supremacy. in reading discussions of papers, by scientific societies, i have often noted how, when they approached forbidden--or irreconcilable--subjects, the discussions were thrown into confusion and ramification. it's as if scientific discussions have often been led astray--as if purposefully--as if by something directive, hovering over them. of course i mean only the spirit of all development. just so, in any embryo, cells that would tend to vary from the appearances of their era are compelled to correlate. in _nature_, - , charles tilden smith writes that, at chisbury, wiltshire, england, april , , he saw something in the sky-- "--unlike anything that i had ever seen before." "although i have studied the skies for many years, i have never seen anything like it." he saw two stationary dark patches upon clouds. the extraordinary part: they were stationary upon clouds that were rapidly moving. they were fan-shaped--or triangular--and varied in size, but kept the same position upon different clouds as cloud after cloud came along. for more than half an hour mr. smith watched these dark patches-- his impression as to the one that appeared first: that it was "really a heavy shadow cast upon a thin veil of clouds by some unseen object away in the west, which was intercepting the sun's rays." upon page , of this volume of _nature_, is a letter from another correspondent, to the effect that similar shadows are cast by mountains upon clouds, and that no doubt mr. smith was right in attributing the appearance to "some unseen object, which was intercepting the sun's rays." but the old dominant that was a jealous dominant, and the wrath of the old dominant against such an irreconcilability as large, opaque objects in the sky, casting down shadows upon clouds. still the dominants are suave very often, or are not absolute gods, and the way attention was led away from this subject is an interesting study in quasi-divine bamboozlement. upon page , charles j.p. cave, the meteorologist, writes that, upon april and , at ditcham park, petersfield, he had observed a similar appearance, while watching some pilot balloons--but he describes something not in the least like a shadow on clouds, but a stationary cloud--the inference seems to be that the shadows at chisbury may have been shadows of pilot balloons. upon page , another correspondent writes upon shadows cast by mountains; upon page someone else carries on the divergence by discussing this third letter: then someone takes up the third letter mathematically; and then there is a correction of error in this mathematic demonstration--i think it looks very much like what i think it looks like. but the mystery here: that the dark patches at chisbury could not have been cast by stationary pilot balloons that were to the west, or that were between clouds and the setting sun. if, to the west of chisbury, a stationary object were high in the air, intercepting the sun's rays, the shadow of the stationary object would not have been stationary, but would have moved higher and higher with the setting of the sun. i have to think of something that is in accord with no other data whatsoever: a luminous body--not the sun--in the sky--but, because of some unknown principle or atmospheric condition, its light extended down only about to the clouds; that from it were suspended two triangular objects, like the object that was seen in bermuda; that it was this light that fell short of the earth that these objects intercepted; that the objects were drawn up and lowered from something overhead, so that, in its light, their shadows changed size. if my grope seem to have no grasp in it, and, if a stationary balloon will, in half an hour, not cast a stationary shadow from the setting sun, we have to think of two triangular objects that accurately maintained positions in a line between sun and clouds, and at the same time approached and receded from clouds. whatever it may have been, it's enough to make the devout make the sign of the crucible, or whatever the devotees of the old dominant do in the presence of a new correlate. vast, black thing poised like a crow over the moon. it is our acceptance that these two shadows of chisbury looked, from the moon, like vast things, black as crows, poised over the earth. it is our acceptance that two triangular luminosities and then two triangular patches, like vast black things, poised like crows over the moon, and, like the triangularities at chisbury, have been seen upon, or over, the moon: _scientific american_, - : two triangular, luminous appearances reported by several observers in lebanon, conn., evening of july , , on the moon's upper limb. they disappeared, and two dark triangular appearances that looked like notches were seen three minutes later upon the lower limb. they approached each other, met and instantly disappeared. the merger here is notches that have at times been seen upon the moon's limb: thought to be cross sections of craters (_monthly notices, r.a.s._, - ). but these appearances of july , , were vast upon the moon--"seemed to be cutting off or obliterating nearly a quarter of its surface." something else that may have looked like a vast black crow poised over this earth from the moon: _monthly weather review_, - : description of a shadow in the sky, of some unseen body, april , , fort worth, texas--supposed to have been cast by an unseen cloud--this patch of shade moved with the declining sun. _rept. brit. assoc._, - : account by two observers of a faint but distinctly triangular object, visible for six nights in the sky. it was observed from two stations that were not far apart. but the parallax was considerable. whatever it was, it was, acceptably, relatively close to this earth. i should say that relatively to phenomena of light we are in confusion as great as some of the discords that orthodoxy is in relatively to light. broadly and intermediatistically, our position is: that light is not really and necessarily light--any more than is anything else really and necessarily anything--but an interpretation of a mode of force, as i suppose we have to call it, as light. at sea level, the earth's atmosphere interprets sunlight as red or orange or yellow. high up on mountains the sun is blue. very high up on mountains the zenith is black. or it is orthodoxy to say that in inter-planetary space, where there is no air, there is no light. so then the sun and comets are black, but this earth's atmosphere, or, rather, dust particles in it, interpret radiations from these black objects as light. we look up at the moon. the jet-black moon is so silvery white. i have about fifty notes indicating that the moon has atmosphere: nevertheless most astronomers hold out that the moon has no atmosphere. they have to: the theory of eclipses would not work out otherwise. so, arguing in conventional terms, the moon is black. rather astonishing--explorers upon the moon--stumbling and groping in intense darkness--with telescopes powerful enough, we could see them stumbling and groping in brilliant light. or, just because of familiarity, it is not now obvious to us how the preposterousnesses of the old system must have seemed to the correlates of the system preceding it. ye jet-black silvery moon. altogether, then, it may be conceivable that there are phenomena of force that are interpretable as light as far down as the clouds, but not in denser strata of air, or just the opposite of familiar interpretations. i now have some notes upon an occurrence that suggests a force not interpreted by air as light, but interpreted, or reflected by the ground as light. i think of something that, for a week, was suspended over london: of an emanation that was not interpreted as light until it reached the ground. _lancet_, june , : that every night for a week, a light had appeared in woburn square, london, upon the grass of a small park, enclosed by railings. crowds gathering--police called out "for the special service of maintaining order and making the populace move on." the editor of the _lancet_ went to the square. he says that he saw nothing but a patch of light falling upon an arbor at the northeast corner of the enclosure. seems to me that that was interesting enough. in this editor we have a companion for mr. symons and dr. gray. he suggests that the light came from a street lamp--does not say that he could trace it to any such origin himself--but recommends that the police investigate neighboring street lamps. i'd not say that such a commonplace as light from a street lamp would not attract and excite and deceive great crowds for a week--but i do accept that any cop who was called upon for extra work would have needed nobody's suggestion to settle that point the very first thing. or that something in the sky hung suspended over a london square for a week. _knowledge_, dec. , : "seeing so many meteorological phenomena in your excellent paper, _knowledge_, i am tempted to ask for an explanation of the following, which i saw when on board the british india company's steamer _patna_, while on a voyage up the persian gulf. in may, , on a dark night, about : p.m., there suddenly appeared on each side of the ship an enormous luminous wheel, whirling around, the spokes of which seemed to brush the ship along. the spokes would be or yards long, and resembled the birch rods of the dames' schools. each wheel contained about sixteen spokes, and, although the wheels must have been some or yards in diameter, the spokes could be distinctly seen all the way round. the phosphorescent gleam seemed to glide along flat on the surface of the sea, no light being visible in the air above the water. the appearance of the spokes could be almost exactly represented by standing in a boat and flashing a bull's eye lantern horizontally along the surface of the water, round and round. i may mention that the phenomenon was also seen by captain avern, of the _patna_, and mr. manning, third officer. "lee fore brace. "p.s.--the wheels advanced along with the ship for about twenty minutes.--l.f.b." _knowledge_, jan. , : letter from "a. mc. d.": that "lee fore brace," "who sees 'so many meteorological phenomena in your excellent paper,' should have signed himself 'the modern ezekiel,' for his vision of wheels is quite as wonderful as the prophet's." the writer then takes up the measurements that were given, and calculates a velocity at the circumference of a wheel, of about yards per second, apparently considering that especially incredible. he then says: "from the nom de plume he assumes, it might be inferred that your correspondent is in the habit of 'sailing close to the wind.'" he asks permission to suggest an explanation of his own. it is that before : p.m. there had been numerous accidents to the "main brace," and that it had required splicing so often that almost any ray of light would have taken on a rotary motion. in _knowledge_, jan. , , mr. "brace" answers and signs himself "j.w. robertson": "i don't suppose a. mc. d. means any harm, but i do think it's rather unjust to say a man is drunk because he sees something out of the common. if there's one thing i pride myself upon, it's being able to say that never in my life have i indulged in anything stronger than water." from this curiosity of pride, he goes on to say that he had not intended to be exact, but to give his impressions of dimensions and velocity. he ends amiably: "however, 'no offense taken, where i suppose none is meant.'" to this letter mr. proctor adds a note, apologizing for the publication of "a. mc. d's." letter, which had come about by a misunderstood instruction. then mr. proctor wrote disagreeable letters, himself, about other persons--what else would you expect in a quasi-existence? the obvious explanation of this phenomenon is that, under the surface of the sea, in the persian gulf, was a vast luminous wheel: that it was the light from its submerged spokes that mr. robertson saw, shining upward. it seems clear that this light did shine upward from origin below the surface of the sea. but at first it is not so clear how vast luminous wheels, each the size of a village, ever got under the surface of the persian gulf: also there may be some misunderstanding as to what they were doing there. a deep-sea fish, and its adaptation to a dense medium-- that, at least in some regions aloft, there is a medium dense even to gelatinousness-- a deep-sea fish, brought to the surface of the ocean: in a relatively attenuated medium, it disintegrates-- super-constructions adapted to a dense medium in inter-planetary space--sometimes, by stresses of various kinds, they are driven into this earth's thin atmosphere-- later we shall have data to support just this: that things entering this earth's atmosphere disintegrate and shine with a light that is not the light of incandescence: shine brilliantly, even if cold-- vast wheel-like super-constructions--they enter this earth's atmosphere, and, threatened with disintegration, plunge for relief into an ocean, or into a denser medium. of course the requirements now facing us are: not only data of vast wheel-like super-constructions that have relieved their distresses in the ocean, but data of enormous wheels that have been seen in the air, or entering the ocean, or rising from the ocean and continuing their voyages. very largely we shall concern ourselves with enormous fiery objects that have either plunged into the ocean or risen from the ocean. our acceptance is that, though disruption may intensify into incandescence, apart from disruption and its probable fieriness, things that enter this earth's atmosphere have a cold light which would not, like light from molten matter, be instantly quenched by water. also it seems acceptable that a revolving wheel would, from a distance, look like a globe; that a revolving wheel, seen relatively close by, looks like a wheel in few aspects. the mergers of ball-lightning and meteorites are not resistances to us: our data are of enormous bodies. so we shall interpret--and what does it matter? our attitude throughout this book: that here are extraordinary data--that they never would be exhumed, and never would be massed together, unless-- here are the data: our first datum is of something that was once seen to enter an ocean. it's from the puritanic publication, _science_, which has yielded us little material, or which, like most puritans, does not go upon a spree very often. whatever the thing could have been, my impression is of tremendousness, or of bulk many times that of all meteorites in all museums combined: also of relative slowness, or of long warning of approach. the story, in _science_, - , is from an account sent to the hydrographic office, at washington, from the branch office, at san francisco: that, at midnight, feb. , , lat. ° n., and long. ° e., or somewhere between yokohama and victoria, the captain of the bark _innerwich_ was aroused by his mate, who had seen something unusual in the sky. this must have taken appreciable time. the captain went on deck and saw the sky turning fiery red. "all at once, a large mass of fire appeared over the vessel, completely blinding the spectators." the fiery mass fell into the sea. its size may be judged by the volume of water cast up by it, said to have rushed toward the vessel with a noise that was "deafening." the bark was struck flat aback, and "a roaring, white sea passed ahead." "the master, an old, experienced mariner, declared that the awfulness of the sight was beyond description." in _nature_, - , and _l'astronomie_; - , we are told that an object, described as "a large ball of fire," was seen to rise from the sea, near cape race. we are told that it rose to a height of fifty feet, and then advanced close to the ship, then moving away, remaining visible about five minutes. the supposition in _nature_ is that it was "ball lightning," but flammarion, _thunder and lightning_, p. , says that it was enormous. details in the american _meteorological journal_, - --nov. , --british steamer _siberian_--that the object had moved "against the wind" before retreating--that captain moore said that at about the same place he had seen such appearances before. _report of the british association_, - : that, upon june , , according to the _malta times_, from the brig _victoria_, about miles east of adalia, asia minor ( ° ' ", n. lat.: ° ' " e. long.), three luminous bodies were seen to issue from the sea, at about half a mile from the vessel. they were visible about ten minutes. the story was never investigated, but other accounts that seem acceptably to be other observations upon this same sensational spectacle came in, as if of their own accord, and were published by prof. baden-powell. one is a letter from a correspondent at mt. lebanon. he describes only two luminous bodies. apparently they were five times the size of the moon: each had appendages, or they were connected by parts that are described as "sail-like or streamer-like," looking like "large flags blown out by a gentle breeze." the important point here is not only suggestion of structure, but duration. the duration of meteors is a few seconds: duration of fifteen seconds is remarkable, but i think there are records up to half a minute. this object, if it were all one object, was visible at mt. lebanon about one hour. an interesting circumstance is that the appendages did not look like trains of meteors, which shine by their own light, but "seemed to shine by light from the main bodies." about miles west of the position of the _victoria_ is the town of adalia, asia minor. at about the time of the observation reported by the captain of the _victoria_, the rev. f. hawlett, f.r.a.s., was in adalia. he, too, saw this spectacle, and sent an account to prof. baden-powell. in his view it was a body that appeared and then broke up. he places duration at twenty minutes to half an hour. in the _report of the british association_, - , the phenomenon was reported from syria and malta, as two very large bodies "nearly joined." _rept. brit. assoc._, - : that, at cherbourg, france, jan. , , was seen a luminous body, seemingly two-thirds the size of the moon. it seemed to rotate on an axis. central to it there seemed to be a dark cavity. for other accounts, all indefinite, but distortable into data of wheel-like objects in the sky, see _nature_, - ; london _times_, oct. , ; _nature_, - ; _monthly weather review_, - . _l'astronomie_, - : that, upon the morning of dec. , , an appearance in the sky was seen by many persons in virginia, north carolina, and south carolina. a luminous body passed overhead, from west to east, until at about degrees in the eastern horizon, it appeared to stand still for fifteen or twenty minutes. according to some descriptions it was the size of a table. to some observers it looked like an enormous wheel. the light was a brilliant white. acceptably it was not an optical illusion--the noise of its passage through the air was heard. having been stationary, or having seemed to stand still fifteen or twenty minutes, it disappeared, or exploded. no sound of explosion was heard. vast wheel-like constructions. they're especially adapted to roll through a gelatinous medium from planet to planet. sometimes, because of miscalculations, or because of stresses of various kinds, they enter this earth's atmosphere. they're likely to explode. they have to submerge in the sea. they stay in the sea awhile, revolving with relative leisureliness, until relieved, and then emerge, sometimes close to vessels. seamen tell of what they see: their reports are interred in scientific morgues. i should say that the general route of these constructions is along latitudes not far from the latitudes of the persian gulf. _journal of the royal meteorological society_, - : that, upon april , , about : , in the persian gulf, captain hoseason, of the steamship _kilwa_, according to a paper read before the society by captain hoseason, was sailing in a sea in which there was no phosphorescence--"there being no phosphorescence in the water." i suppose i'll have to repeat that: "... there being no phosphorescence in the water." vast shafts of light--though the captain uses the word "ripples"--suddenly appeared. shaft followed shaft, upon the surface of the sea. but it was only a faint light, and, in about fifteen minutes, died out: having appeared suddenly, having died out gradually. the shafts revolved at a velocity of about miles an hour. phosphorescent jellyfish correlate with the old dominant: in one of the most heroic compositions of disregards in our experience, it was agreed, in the discussion of capt. hoseason's paper, that the phenomenon was probably pulsations of long strings of jellyfish. _nature_, - : reprint of a letter from r.e. harris, commander of the a.h.n. co.'s steamship _shahjehan_, to the calcutta _englishman_, jan. , : that upon the th of june, , off the coast of malabar, at p.m., water calm, sky cloudless, he had seen something that was so foreign to anything that he had ever seen before, that he had stopped his ship. he saw what he describes as waves of brilliant light, with spaces between. upon the water were floating patches of a substance that was not identified. thinking in terms of the conventional explanation of all phosphorescence at sea, the captain at first suspected this substance. however, he gives his opinion that it did no illuminating but was, with the rest of the sea, illuminated by tremendous shafts of light. whether it was a thick and oily discharge from the engine of a submerged construction or not, i think that i shall have to accept this substance as a concomitant, because of another note. "as wave succeeded wave, one of the most grand and brilliant, yet solemn, spectacles that one could think of, was here witnessed." _jour. roy. met. soc._, - : extract from a letter from mr. douglas carnegie, blackheath, england. date some time in -- "this last voyage we witnessed a weird and most extraordinary electric display." in the gulf of oman, he saw a bank of apparently quiescent phosphorescence: but, when within twenty yards of it, "shafts of brilliant light came sweeping across the ship's bows at a prodigious speed, which might be put down as anything between and miles an hour." "these light bars were about feet apart and most regular." as to phosphorescence--"i collected a bucketful of water, and examined it under the microscope, but could not detect anything abnormal." that the shafts of light came up from something beneath the surface--"they first struck us on our broadside, and i noticed that an intervening ship had no effect on the light beams: they started away from the lee side of the ship, just as if they had traveled right through it." the gulf of oman is at the entrance to the persian gulf. _jour. roy. met. soc._, - : extract from a letter by mr. s.c. patterson, second officer of the p. and o. steamship _delta_: a spectacle which the _journal_ continues to call phosphorescent: malacca strait, a.m., march , : "... shafts which seemed to move round a center--like the spokes of a wheel--and appeared to be about yards long. the phenomenon lasted about half an hour, during which time the ship had traveled six or seven miles. it stopped suddenly." _l'astronomie_, - : a correspondent writes that, in october, , in the china sea, he had seen shafts or lances of light that had had the appearance of rays of a searchlight, and that had moved like such rays. _nature_, - : report to the admiralty by capt. evans, the hydrographer of the british navy: that commander j.e. pringle, of h.m.s. _vulture_, had reported that, at lat. ° ' n., and long. ° ' e.--in the persian gulf--may , , he had noticed luminous waves or pulsations in the water, moving at great speed. this time we have a definite datum upon origin somewhere below the surface. it is said that these waves of light passed under the _vulture_. "on looking toward the east, the appearance was that of a revolving wheel with a center on that bearing, and whose spokes were illuminated, and, looking toward the west, a similar wheel appeared to be revolving, but in the opposite direction." or finally as to submergence--"these waves of light extended from the surface well under the water." it is commander pringle's opinion that the shafts constituted one wheel, and that doubling was an illusion. he judges the shafts to have been about feet broad, and the spaces about . velocity about miles an hour. duration about minutes. time : p.m. before and after this display the ship had passed through patches of floating substance described as "oily-looking fish spawn." upon page of this number of _nature_, e.l. moss says that, in april, , when upon h.m.s. _bulldog_, a few miles north of vera cruz, he had seen a series of swift lines of light. he had dipped up some of the water, finding in it animalcule, which would, however, not account for phenomena of geometric formation and high velocity. if he means vera cruz, mexico, this is the only instance we have out of oriental waters. _scientific american_, - : that, in the _nautical meteorological annual_, published by the danish meteorological institute, appears a report upon a "singular phenomenon" that was seen by capt. gabe, of the danish east asiatic co.'s steamship _bintang_. at a.m., june , , while sailing through the straits of malacca, captain gabe saw a vast revolving wheel of light, flat upon the water--"long arms issuing from a center around which the whole system appeared to rotate." so vast was the appearance that only half of it could be seen at a time, the center lying near the horizon. this display lasted about fifteen minutes. heretofore we have not been clear upon the important point that forward motions of these wheels do not synchronize with a vessel's motions, and freaks of disregard, or, rather, commonplaces of disregard, might attempt to assimilate with lights of a vessel. this time we are told that the vast wheel moved forward, decreasing in brilliancy, and also in speed of rotation, disappearing when the center was right ahead of the vessel--or my own interpretation would be that the source of light was submerging deeper and deeper and slowing down because meeting more and more resistance. the danish meteorological institute reports another instance: that, when capt. breyer, of the dutch steamer _valentijn_, was in the south china sea, midnight, aug. , , he saw a rotation of flashes. "it looked like a horizontal wheel, turning rapidly." this time it is said that the appearance was above water. "the phenomenon was observed by the captain, the first and second mates, and the first engineer, and upon all of them it made a somewhat uncomfortable impression." in general, if our expression be not immediately acceptable, we recommend to rival interpreters that they consider the localization--with one exception--of this phenomenon, to the indian ocean and adjacent waters, or persian gulf on one side and china sea on the other side. though we're intermediatists, the call of attempted positivism, in the aspect of completeness, is irresistible. we have expressed that from few aspects would wheels of fire in the air look like wheels of fire, but, if we can get it, we must have observation upon vast luminous wheels, not interpretable as optical illusions, but enormous, substantial things that have smashed down material resistances, and have been seen to plunge into the ocean: _athenæum_, - : that at the meeting of the british association, , sir w.s. harris said that he had recorded an account sent to him of a vessel toward which had whirled "two wheels of fire, which the men described as rolling millstones of fire." "when they came near, an awful crash took place: the topmasts were shivered to pieces." it is said that there was a strong sulphurous odor. _journal of the royal meteorological society_, - : extract from the log of the bark _lady of the lake_, by capt. f.w. banner: communicated by r.h. scott, f.r.s.: that, upon the nd of march, , at lat. ° ' n., long. ° ' w., the sailors of the _lady of the lake_ saw a remarkable object, or "cloud," in the sky. they reported to the captain. according to capt. banner, it was a cloud of circular form, with an included semi-circle divided into four parts, the central dividing shaft beginning at the center of the circle and extending far outward, and then curving backward. geometricity and complexity and stability of form: and the small likelihood of a cloud maintaining such diversity of features, to say nothing of appearance of organic form. the thing traveled from a point at about degrees above the horizon to a point about degrees above. then it settled down to the northeast, having appeared from the south, southeast. light gray in color, or it was cloud-color. "it was much lower than the other clouds." and this datum stands out: that, whatever it may have been, it traveled against the wind. "it came up obliquely against the wind, and finally settled down right in the wind's eye." for half an hour this form was visible. when it did finally disappear that was not because it disintegrated like a cloud, but because it was lost to sight in the evening darkness. capt. banner draws the following diagram: [illustration] text-books tell us that the dhurmsalla meteorites were picked up "soon," or "within half an hour." given a little time the conventionalists may argue that these stones were hot when they fell, but that their great interior coldness had overcome the molten state of their surfaces. according to the deputy commissioner of dhurmsalla, these stones had been picked up "immediately" by passing coolies. these stones were so cold that they benumbed the fingers. but they had fallen with a great light. it is described as "a flame of fire about two feet in depth and nine feet in length." acceptably this light was not the light of molten matter. in this chapter we are very intermediatistic--and unsatisfactory. to the intermediatist there is but one answer to all questions: sometimes and sometimes not. another form of this intermediatist "solution" of all problems is: yes and no. everything that is, also isn't. a positivist attempts to formulate: so does the intermediatist, but with less rigorousness: he accepts but also denies: he may seem to accept in one respect and deny in some other respect, but no real line can be drawn between any two aspects of anything. the intermediatist accepts that which seems to correlate with something that he has accepted as a dominant. the positivist correlates with a belief. in the dhurmsalla meteorites we have support for our expression that things entering this earth's atmosphere sometimes shine with a light that is not the light of incandescence--or so we account, or offer an expression upon, "thunderstones," or carved stones that have fallen luminously to this earth, in streaks that have looked like strokes of lightning--but we accept, also, that some things that have entered this earth's atmosphere, disintegrate with the intensity of flame and molten matter--but some things, we accept, enter this earth's atmosphere and collapse non-luminously, quite like deep-sea fishes brought to the surface of the ocean. whatever agreement we have is an indication that somewhere aloft there is a medium denser than this earth's atmosphere. i suppose our stronghold is in that such is not popular belief-- or the rhythm of all phenomena: air dense at sea level upon this earth--less and less dense as one ascends--then denser and denser. a good many bothersome questions arise-- our attitude: here are the data: luminous rains sometimes fall (_nature_, march , ; _nature_, - ). this is light that is not the light of incandescence, but no one can say that these occasional, or rare, rains come from this earth's externality. we simply note cold light of falling bodies. for luminous rain, snow, and dust, see hartwig, _aerial world_, p. . as to luminous clouds, we have more nearly definite observations and opinions: they mark transition between the old dominant and the new dominant. we have already noted the transition in prof. schwedoffs theory of external origin of some hailstones--and the implications that, to a former generation, seemed so preposterous--"droll" was the word--that there are in inter-planetary regions volumes of water--whether they have fishes and frogs in them or not. now our acceptance is that clouds sometimes come from external regions, having had origin from super-geographical lakes and oceans that we shall not attempt to chart, just at present--only suggesting to enterprising aviators--and we note that we put it all up to them, and show no inclination to go columbusing on our own account--that they take bathing suits, or, rather, deep-sea diving-suits along. so then that some clouds come from inter-planetary oceans--of the super-sargasso sea--if we still accept the super-sargasso sea--and shine, upon entering this earth's atmosphere. in _himmel und erde_, february, --a phenomenon of transition of thirty years ago--herr o. jesse, in his observations upon luminous night-clouds, notes the great height of them, and drolly or sensibly suggests that some of them may have come from regions external to this earth. i suppose he means only from other planets. but it's a very droll and sensible idea either way. in general i am accounting for a great deal of this earth's isolation: that it is relatively isolated by circumstances that are similar to the circumstances that make for relative isolation of the bottom of the ocean--except that there is a clumsiness of analogy now. to call ourselves deep-sea fishes has been convenient, but, in a quasi-existence, there is no convenience that will not sooner or later turn awkward--so, if there be denser regions aloft, these regions should now be regarded as analogues of far-submerged oceanic regions, and things coming to this earth would be like things rising to an attenuated medium--and exploding--sometimes incandescently, sometimes with cold light--sometimes non-luminously, like deep-sea fishes brought to the surface--altogether conditions of inhospitality. i have a suspicion that, in their own depths, deep-sea fishes are not luminous. if they are, darwinism is mere jesuitism, in attempting to correlate them. such advertising would so attract attention that all advantages would be more than offset. darwinism is largely a doctrine of concealment: here we have brazen proclamation--if accepted. fishes in the mammoth cave need no light to see by. we might have an expression that deep-sea fishes turn luminous upon entering a less dense medium--but models in the american museum of natural history: specialized organs of luminosity upon these models. of course we do remember that awfully convincing "dodo," and some of our sophistications we trace to him--at any rate disruption is regarded as a phenomenon of coming from a dense to a less dense medium. an account by m. acharius, in the _transactions of the swedish academy of sciences_, - , translated for the _north american review_, - : that m. acharius, having heard of "an extraordinary and probably hitherto unseen phenomenon," reported from near the town of skeninge, sweden, investigated: that, upon the th of may, , at about p.m., the sun suddenly turned dull brick-red. at the same time there appeared, upon the western horizon, a great number of round bodies, dark brown, and seemingly the size of a hat crown. they passed overhead and disappeared in the eastern horizon. tremendous procession. it lasted two hours. occasionally one fell to the ground. when the place of a fall was examined, there was found a film, which soon dried and vanished. often, when approaching the sun, these bodies seemed to link together, or were then seen to be linked together, in groups not exceeding eight, and, under the sun, they were seen to have tails three or four fathoms long. away from the sun the tails were invisible. whatever their substance may have been, it is described as gelatinous--"soapy and jellied." i place this datum here for several reasons. it would have been a good climax to our expression upon hordes of small bodies that, in our acceptance, were not seeds, nor birds, nor ice-crystals: but the tendency would have been to jump to the homogeneous conclusion that all our data in that expression related to this one kind of phenomena, whereas we conceive of infinite heterogeneity of the external: of crusaders and rabbles and emigrants and tourists and dragons and things like gelatinous hat crowns. or that all things, here, upon this earth, that flock together, are not necessarily sheep, presbyterians, gangsters, or porpoises. the datum is important to us, here, as indication of disruption in this earth's atmosphere--dangers in entering this earth's atmosphere. i think, myself, that thousands of objects have been seen to fall from aloft, and have exploded luminously, and have been called "ball lightning." "as to what ball lightning is, we have not yet begun to make intelligent guesses." (_monthly weather review_, - .) in general, it seems to me that when we encounter the opposition "ball lightning" we should pay little attention, but confine ourselves to guesses that are at least intelligent, that stand phantom-like in our way. we note here that in some of our acceptances upon intelligence we should more clearly have pointed out that they were upon the intelligent as opposed to the instinctive. in the _monthly weather review_, - , there is an account of "ball lightning" that struck a tree. it made a dent such as a falling object would make. some other time i shall collect instances of "ball lightning," to express that they are instances of objects that have fallen from the sky, luminously, exploding terrifically. so bewildered is the old orthodoxy by these phenomena that many scientists have either denied "ball lightning" or have considered it very doubtful. i refer to dr. sestier's list of one hundred and fifty instances, which he considered authentic. in accord with our disaccord is an instance related in the _monthly weather review_, march, --something that fell luminously from the sky, accompanied by something that was not so affected, or that was dark: that, according to capt. c.d. sweet, of the dutch bark, _j.p.a._, upon march , , n. ° ', w. ° ', he encountered a severe storm. he saw two objects in the air above the ship. one was luminous, and might be explained in several ways, but the other was dark. one or both fell into the sea, with a roar and the casting up of billows. it is our acceptance that these things had entered this earth's atmosphere, having first crashed through a field of ice--"immediately afterward lumps of ice fell." one of the most astonishing of the phenomena of "ball lightning" is a phenomenon of many meteorites: violence of explosion out of all proportion to size and velocity. we accept that the icy meteorites of dhurmsalla could have fallen with no great velocity, but the sound from them was tremendous. the soft substance that fell at the cape of good hope was carbonaceous, but was unburned, or had fallen with velocity insufficient to ignite it. the tremendous report that it made was heard over an area more than seventy miles in diameter. that some hailstones have been formed in a dense medium, and violently disintegrate in this earth's relatively thin atmosphere: _nature_, - : large hailstones noted at the university of missouri, nov. , : they exploded with sounds like pistol shots. the writer says that he had noticed a similar phenomenon, eighteen years before, at lexington, kentucky. hailstones that seemed to have been formed in a denser medium: when melted under water they gave out bubbles larger than their central air spaces. (_monthly weather review_, - .) our acceptance is that many objects have fallen from the sky, but that many of them have disintegrated violently. this acceptance will co-ordinate with data still to come, but, also, we make it easy for ourselves in our expressions upon super-constructions, if we're asked why, from thinkable wrecks of them, girders, plates, or parts recognizably of manufactured metal have not fallen from the sky. however, as to composition, we have not this refuge, so it is our expression that there have been reported instances of the fall of manufactured metal from the sky. the meteorite of rutherford, north carolina, is of artificial material: mass of pig iron. it is said to be fraudulent. (_amer. jour. sci._, - - .) the object that was said to have fallen at marblehead, mass., in , is described in the _amer. jour. sci._, - - , as "a furnace product, formed in smelting copper ores, or iron ores containing copper." it is said to be fraudulent. according to ehrenberg, the substance reported by capt. callam to have fallen upon his vessel, near java, "offered complete resemblance to the residue resulting from combustion of a steel wire in a flask of oxygen." (zurcher, _meteors_, p. .) _nature_, nov. , , publishes a notice that, according to the _yuma sentinel_, a meteorite that "resembles steel" had been found in the mohave desert. in _nature_, feb. , , we read that one of the meteorites brought to the united states by peary, from greenland, is of tempered steel. the opinion is that meteoric iron had fallen in water or snow, quickly cooling and hardening. this does not apply to composition. nov. , , _nature_ publishes a notice of a paper by prof. berwerth, of vienna, upon "the close connection between meteoric iron and steel-works' steel." at the meeting of nov. , , of the essex field club, was exhibited a piece of metal said to have fallen from the sky, oct. , , at braintree. according to the _essex naturalist_, dr. fletcher, of the british museum, had declared this metal to be smelted iron--"so that the mystery of its reported 'fall' remained unexplained." we shall have an outcry of silences. if a single instance of anything be disregarded by a system--our own attitude is that a single instance is a powerless thing. of course our own method of agreement of many instances is not a real method. in continuity, all things must have resemblances with all other things. anything has any quasi-identity you please. some time ago conscription was assimilated with either autocracy or democracy with equal facility. note the need for a dominant to correlate to. scarcely anybody said simply that we must have conscription: but that we must have conscription, which correlates with democracy, which was taken as a base, or something basically desirable. of course between autocracy and democracy nothing but false demarcation can be drawn. so i can conceive of no subject upon which there should be such poverty as a single instance, if anything one pleases can be whipped into line. however, we shall try to be more nearly real than the darwinites who advance concealing coloration as darwinism, and then drag in proclaiming luminosity, too, as darwinism. i think the darwinites had better come in with us as to the deep-sea fishes--and be sorry later, i suppose. it will be amazing or negligible to read all the instances now to come of things that have been seen in the sky, and to think that all have been disregarded. my own opinion is that it is not possible, or very easy, to disregard them, now that they have been brought together--but that, if prior to about this time we had attempted such an assemblage, the old dominant would have withered our typewriter--as it is the letter "e" has gone back on us, and the "s" is temperamental. "most extraordinary and singular phenomenon," north wales, aug. , ; a disk from which projected an orange-colored body that looked like "an elongated flatfish," reported by admiral ommanney (_nature_, - ); disk from which projected a hook-like form, india, about ; diagram of it given; disk about size of the moon, but brighter than the moon; visible about twenty minutes; by g. pettit, in prof. baden-powell's catalogue (_rept. brit. assoc._, ); very brilliant hook-like form, seen in the sky at poland, trumbull co., ohio, during the stream of meteors, of ; visible more than an hour: large luminous body, almost stationary "for a time"; shaped like a square table; niagara falls, nov. , (_amer. jour. sci._, - - ); something described as a bright white cloud, at night, nov. , , at hamar, norway; from it were emitted brilliant rays of light; drifted across the sky; "retained throughout its original form" (_nature_, dec. , - ); thing with an oval nucleus, and streamers with dark bands and lines very suggestive of structure; new zealand, may , (_nature_, - ); luminous object, size of full moon, visible an hour and a half, chili, nov. , (_comptes rendus_, - ); bright object near sun, dec. , (_knowledge_, - ); light that looked like a great flame, far out at sea, off ryook phyoo, dec. , (_london roy. soc. proc._, - ); something like a gigantic trumpet, suspended, vertical, oscillating gently, visible five or six minutes, length estimated at feet, at oaxaca, mexico, july , (_sci. am. sup._, - ); two luminous bodies, seemingly united, visible five or six minutes, june , (_la nature_, - - ); thing with a tail, crossing moon, transit half a minute, sept. , (london _times_, sept. , ); object four or five times size of moon, moving slowly across sky, nov. , , near adrianople (_l'astronomie_, - ); large body, colored red, moving slowly, visible minutes, reported by coggia, marseilles, aug. , (_chem. news_, - ); details of this observation, and similar observation by guillemin, and other instances by de fonville (_comptes rendus_, - , ); thing that was large and that was stationary twice in seven minutes, oxford, nov. , ; listed by lowe (_rec. sci._, - ); grayish object that looked to be about three and a half feet long, rapidly approaching the earth at saarbruck, april , ; sound like thunder; object expanding like a sheet (_am. jour. sci._, - - ; _quar. jour. roy. inst._, - ); report by an astronomer, n.s. drayton, upon an object duration of which seemed to him extraordinary; duration three-quarters of a minute, jersey city, july , (_sci. amer._, - ); object like a comet, but with proper motion of degrees an hour; visible one hour; reported by purine and glancy from the cordoba observatory, argentina, march , (_sci. amer._, - ); something like a signal light, reported by glaisher, oct. , ; bright as jupiter, "sending out quick flickering waves of light" (_year book of facts_, - ). i think that with the object known as eddie's "comet" passes away the last of our susceptibility to the common fallacy of personifying. it is one of the most deep-rooted of positivist illusions--that people are persons. we have been guilty too often of spleens and spites and ridicules against astronomers, as if they were persons, or final unities, individuals, completenesses, or selves--instead of indeterminate parts. but, so long as we remain in quasi-existence, we can cast out illusion only with some other illusion, though the other illusion may approximate higher to reality. so we personify no more--but we super-personify. we now take into full acceptance our expression that development is an autocracy of successive dominants--which are not final--but which approximate higher to individuality or self-ness, than do the human tropisms that irresponsibly correlate to them. eddie reported a celestial object, from the observatory at grahamstown, south africa. it was in . the new dominant was only heir presumptive then, or heir apparent but not obvious. the thing that eddie reported might as well have been reported by a night watchman, who had looked up through an unplaced sewer pipe. it did not correlate. the thing was not admitted to _monthly notices_. i think myself that if the editor had attempted to let it in--earthquake--or a mysterious fire in his publishing house. the dominants are jealous gods. in _nature_, presumably a vassal of the new god, though of course also plausibly rendering homage to the old, is reported a comet-like body, of oct. , , observed at grahamstown, by eddie. it may have looked comet-like, but it moved degrees while visible, or one hundred degrees in three-quarters of an hour. see _nature_, - , . in _nature_, - , prof. copeland describes a similar appearance that he had seen, sept. , . dreyer says (_nature_, - ) that he had seen this object at the armagh observatory. he likens it to the object that was reported by eddie. it was seen by dr. alexander graham bell, sept. , , in nova scotia. but the old dominant was a jealous god. so there were different observations upon something that was seen in november, . these observations were philistines in . in the _amer. met. jour._, - , a correspondent reports having seen an object like a comet, with two tails, one up and one down, nov. or , . very likely this phenomenon should be placed in our expression upon torpedo-shaped bodies that have been seen in the sky--our data upon dirigibles, or super-zeppelins--but our attempted classifications are far from rigorous--or are mere gropes. in the _scientific american_, - , a correspondent writes from humacao, porto rico, that, nov. , , he and several other--persons--or persons, as it were--had seen a majestic appearance, like a comet. visible three successive nights: disappeared then. the editor says that he can offer no explanation. if accepted, this thing must have been close to the earth. if it had been a comet, it would have been seen widely, and the news would have been telegraphed over the world, says the editor. upon page of this volume of the _scientific american_, a correspondent writes that, at sulphur springs, ohio, he had seen "a wonder in the sky," at about the same date. it was torpedo-shaped, or something with a nucleus, at each end of which was a tail. again the editor says that he can offer no explanation: that the object was not a comet. he associates it with the atmospheric effects general in . but it will be our expression that, in england and holland, a similar object was seen in november, . in the _scientific american_, - , is published a letter from henry harrison, of jersey city, copied from the _new york tribune_: that upon the evening of april , , mr. harrison was searching for brorsen's comet, when he saw an object that was moving so rapidly that it could not have been a comet. he called a friend to look, and his observation was confirmed. at two o'clock in the morning this object was still visible. in the _scientific american supplement_, - , mr. harrison disclaims sensationalism, which he seems to think unworthy, and gives technical details: he says that the object was seen by mr. j. spencer devoe, of manhattanville. "a formation having the shape of a dirigible." it was reported from huntington, west virginia (_sci. amer._, - ). luminous object that was seen july , , at about p.m. observed through "rather powerful field glasses," it looked to be about two degrees long and half a degree wide. it gradually dimmed, disappeared, reappeared, and then faded out of sight. another person--as we say: it would be too inconvenient to hold to our intermediatist recognitions--another person who observed this phenomenon suggested to the writer of the account that the object was a dirigible, but the writer says that faint stars could be seen behind it. this would seem really to oppose our notion of a dirigible visitor to this earth--except for the inconclusiveness of all things in a mode of seeming that is not final--or we suggest that behind some parts of the object, thing, construction, faint stars were seen. we find a slight discussion here. prof. h.m. russell thinks that the phenomenon was a detached cloud of aurora borealis. upon page of this volume of the _scientific american_, another correlator suggests that it was a light from a blast furnace--disregarding that, if there be blast furnaces in or near huntington, their reflections would be commonplaces there. we now have several observations upon cylindrical-shaped bodies that have appeared in this earth's atmosphere: cylindrical, but pointed at both ends, or torpedo-shaped. some of the accounts are not very detailed, but out of the bits of description my own acceptance is that super-geographical routes are traversed by torpedo-shaped super-constructions that have occasionally visited, or that have occasionally been driven into this earth's atmosphere. from data, the acceptance is that upon entering this earth's atmosphere, these vessels have been so racked that had they not sailed away, disintegration would have occurred: that, before leaving this earth, they have, whether in attempted communication or not, or in mere wantonness or not, dropped objects, which did almost immediately violently disintegrate or explode. upon general principles we think that explosives have not been purposely dropped, but that parts have been racked off, and have fallen, exploding like the things called "ball lightning." many have been objects of stone or metal with inscriptions upon them, for all we know, at present. in all instances, estimates of dimensions are valueless, but ratios of dimensions are more acceptable. a thing said to have been six feet long may have been six hundred feet long; but shape is not so subject to the illusions of distance. _nature_, - : that, aug. , , during a violent storm, an object that looked to be about inches long and inches wide, fell, rather slowly, at east twickenham, england. it exploded. no substance from it was found. _l'année scientifique_, - : that, oct. , , m. leverrier had sent to the academy three letters from witnesses of a long luminous body, tapering at both ends, that had been seen in the sky. in _thunder and lightning_, p. , flammarion says that on aug. , , during a rather violent storm, m.a. trécul, of the french academy, saw a very brilliant yellowish-white body, apparently to centimeters long, and about centimeters wide. torpedo-shaped. or a cylindrical body, "with slightly conical ends." it dropped something, and disappeared in the clouds. whatever it may have been that was dropped, it fell vertically, like a heavy object, and left a luminous train. the scene of this occurrence may have been far from the observer. no sound was heard. for m. trécul's account, see _comptes rendus_, - . _monthly weather review_, - : that, july , , in the town of burlington, vermont, a terrific explosion had been heard throughout the city. a ball of light, or a luminous object, had been seen to fall from the sky--or from a torpedo-shaped thing, or construction, in the sky. no one had seen this thing that had exploded fall from a larger body that was in the sky--but if we accept that at the same time there was a larger body in the sky-- my own acceptance is that a dirigible in the sky, or a construction that showed every sign of disrupting, had barely time to drop--whatever it did drop--and to speed away to safety above. the following story is told, in the _review_, by bishop john s. michaud: "i was standing on the corner of church and college streets, just in front of the howard bank, and facing east, engaged in conversation with ex-governor woodbury and mr. a.a. buell, when, without the slightest indication, or warning, we were startled by what sounded like a most unusual and terrific explosion, evidently very nearby. raising my eyes, and looking eastward along college street, i observed a torpedo-shaped body, some feet away, stationary in appearance, and suspended in the air, about feet above the tops of the buildings. in size it was about feet long by inches in diameter, the shell, or covering, having a dark appearance, with here and there tongues of fire issuing from spots on the surface, resembling red-hot, unburnished copper. although stationary when first noticed, this object soon began to move, rather slowly, and disappeared over dolan brothers' store, southward. as it moved, the covering seemed rupturing in places, and through these the intensely red flames issued." bishop michaud attempts to correlate it with meteorological observations. because of the nearby view this is perhaps the most remarkable of the new correlates, but the correlate now coming is extraordinary because of the great number of recorded observations upon it. my own acceptance is that, upon nov. , , a vast dirigible crossed england, but by the definiteness-indefiniteness of all things quasi-real, some observations upon it can be correlated with anything one pleases. e.w. maunder, invited by the editors of the _observatory_ to write some reminiscences for the th number of their magazine, gives one that he says stands out (_observatory_, - ). it is upon something that he terms "a strange celestial visitor." maunder was at the royal observatory, greenwich, nov. , , at night. there was an aurora, without features of special interest. in the midst of the aurora, a great circular disk of greenish light appeared and moved smoothly across the sky. but the circularity was evidently the effect of foreshortening. the thing passed above the moon, and was, by other observers, described as "cigar-shaped," "like a torpedo," "a spindle," "a shuttle." the idea of foreshortening is not mine: maunder says this. he says: "had the incident occurred a third of a century later, beyond doubt everyone would have selected the same simile--it would have been 'just like a zeppelin.'" the duration was about two minutes. color said to have been the same as that of the auroral glow in the north. nevertheless, maunder says that this thing had no relation to auroral phenomena. "it appeared to be a definite body." motion too fast for a cloud, but "nothing could be more unlike the rush of a meteor." in the _philosophical magazine_, - - , j. rand capron, in a lengthy paper, alludes throughout to this phenomenon as an "auroral beam," but he lists many observations upon its "torpedo-shape," and one observation upon a "dark nucleus" in it--host of most confusing observations--estimates of height between and miles--observations in holland and belgium. we are told that according to capron's spectroscopic observations the phenomenon was nothing but a beam of auroral light. in the _observatory_, - , is maunder's contemporaneous account. he gives apparent approximate length and breadth at twenty-seven degrees and three degrees and a half. he gives other observations seeming to indicate structure--"remarkable dark marking down the center." in _nature_, - , capron says that because of the moonlight he had been able to do little with the spectroscope. color white, but aurora rosy (_nature_, - ). bright stars seen through it, but not at the zenith, where it looked opaque. this is the only assertion of transparency (_nature_, - ). too slow for a meteor, but too fast for a cloud (_nature_, - ). "surface had a mottled appearance" (_nature_, - ). "very definite in form, like a torpedo" (_nature_, - ). "probably a meteoric object" (dr. groneman, _nature_, - ). technical demonstration by dr. groneman, that it was a cloud of meteoric matter (_nature_, - ). see _nature_, - , , , , , . "very little doubt it was an electric phenomenon" (proctor, _knowledge_, - ). in the london _times_, nov. , , the editor says that he had received a great number of letters upon this phenomenon. he publishes two. one correspondent describes it as "well-defined and shaped like a fish... extraordinary and alarming." the other correspondent writes of it as "a most magnificent luminous mass, shaped somewhat like a torpedo." _notes and queries_, - - : about lights that were seen in wales, over an area of about miles, all keeping their own ground, whether moving together perpendicularly, horizontally, or over a zigzag course. they looked like electric lights--disappearing, reappearing dimly, then shining as bright as ever. "we have seen them three or four at a time afterward, on four or five occasions." london _times_, oct. , : "from time to time the west coast of wales seems to have been the scene of mysterious lights.... and now we have a statement from towyn that within the last few weeks lights of various colors have been seen moving over the estuary of the dysynni river, and out to sea. they are generally in a northerly direction, but sometimes they hug the shore, and move at high velocity for miles toward aberdovey, and suddenly disappear." _l'année scientifique_, - : lights that appeared in the sky, above vence, france, march , ; described as balls of fire of dazzling brightness; appeared from a cloud about a degree in diameter; moved relatively slowly. they were visible more than an hour, moving northward. it is said that eight or ten years before similar lights or objects had been seen in the sky, at vence. london _times_, sept. , : that, at inverness, scotland, two large, bright lights that looked like stars had been seen in the sky: sometimes stationary, but occasionally moving at high velocity. _l'année scientifique_, - : observed near st. petersburg, july , , in the evening: a large spherical light and two smaller ones, moving along a ravine: visible three minutes; disappearing without noise. _nature_, - : that, at yloilo, sept. , , was seen a luminous object the size of the full moon. it "floated" slowly "northward," followed by smaller ones close to it. "the false lights of durham." every now and then in the english newspapers, in the middle of the nineteenth century, there is something about lights that were seen against the sky, but as if not far above land, oftenest upon the coast of durham. they were mistaken for beacons by sailors. wreck after wreck occurred. the fishermen were accused of displaying false lights and profiting by wreckage. the fishermen answered that mostly only old vessels, worthless except for insurance, were so wrecked. in (london _times_, jan. , ) popular excitement became intense. there was an investigation. before a commission, headed by admiral collinson, testimony was taken. one witness described the light that had deceived him as "considerably elevated above ground." no conclusion was reached: the lights were called "the mysterious lights." but whatever the "false lights of durham" may have been, they were unaffected by the investigation. in , the tyne pilotage board took the matter up. opinion of the mayor of tyne--"a mysterious affair." in the _report of the british association_, - , there is a description of a group of "meteors" that traveled with "remarkable slowness." they were in sight about three minutes. "remarkable," it seems, is scarcely strong enough: one reads of "remarkable" as applied to a duration of three seconds. these "meteors" had another peculiarity; they left no train. they are described as "seemingly huddled together like a flock of wild geese, and moving with the same velocity and grace of regularity." _jour. roy. astro. soc. of canada_, november and december, : that, according to many observations collected by prof. chant, of toronto, there appeared, upon the night of feb. , , a spectacle that was seen in canada, the united states, and at sea, and in bermuda. a luminous body was seen. to it there was a long tail. the body grew rapidly larger. "observers differ as to whether the body was single, or was composed of three or four parts, with a tail to each part." the group, or complex structure, moved with "a peculiar, majestic deliberation." "it disappeared in the distance, and another group emerged from its place of origin. onward they moved, at the same deliberate pace, in twos or threes or fours." they disappeared. a third group, or a third structure, followed. some observers compared the spectacle to a fleet of airships: others to battleships attended by cruisers and destroyers. according to one writer: "there were probably or bodies, and the peculiar thing about them was their moving in fours and threes and twos, abreast of one another; and so perfect was the lining up that you would have thought it was an aerial fleet maneuvering after rigid drilling." _nature_, may , : a letter from capt. charles j. norcock, of h.m.s. _caroline_: that, upon the th of february, , at p.m., between shanghai and japan, the officer of the watch had reported "some unusual lights." they were between the ship and a mountain. the mountain was about , feet high. the lights seemed to be globular. they moved sometimes massed, but sometimes strung out in an irregular line. they bore "northward," until lost to sight. duration two hours. the next night the lights were seen again. they were, for a time, eclipsed by a small island. they bore north at about the same speed and in about the same direction as speed and direction of the _caroline_. but they were lights that cast a reflection: there was a glare upon the horizon under them. a telescope brought out but few details: that they were reddish, and seemed to emit a faint smoke. this time the duration was seven and a half hours. then capt. norcock says that, in the same general locality, and at about the same time, capt. castle, of h.m.s. _leander_, had seen lights. he had altered his course and had made toward them. the lights had fled from him. at least, they had moved higher in the sky. _monthly weather review_, march, - : report from the observations of three members of his crew by lieut. frank h. schofield, u.s.n, of the u.s.s. _supply_: feb. , . three luminous objects, of different sizes, the largest having an apparent area of about six suns. when first sighted, they were not very high. they were below clouds of an estimated height of about one mile. they fled, or they evaded, or they turned. they went up into the clouds below which they had, at first, been sighted. their unison of movement. but they were of different sizes, and of different susceptibilities to all forces of this earth and of the air. _monthly weather review_, august, - : two letters from c.n. crotsenburg, crow agency, montana: that, in the summer of , when this writer was a railroad postal clerk--or one who was experienced in train-phenomena--while his train was going "northward," from trenton, mo., he and another clerk saw, in the darkness of a heavy rain, a light that appeared to be round, and of a dull-rose color, and seemed to be about a foot in diameter. it seemed to float within a hundred feet of the earth, but soon rose high, or "midway between horizon and zenith." the wind was quite strong from the east, but the light held a course almost due north. its speed varied. sometimes it seemed to outrun the train "considerably." at other times it seemed to fall behind. the mail-clerks watched until the town of linville, iowa, was reached. behind the depot of this town, the light disappeared, and was not seen again. all this time there had been rain, but very little lightning, but mr. crotsenburg offers the explanation that it was "ball lightning." the editor of the _review_ disagrees. he thinks that the light may have been a reflection from the rain, or fog, or from leaves of trees, glistening with rain, or the train's light--not lights. in the december number of the _review_ is a letter from edward m. boggs--that the light was a reflection, perhaps, from the glare--one light, this time--from the locomotive's fire-box, upon wet telegraph wires--an appearance that might not be striated by the wires, but consolidated into one rotundity--that it had seemed to oscillate with the undulations of the wires, and had seemed to change horizontal distance with the varying angles of reflection, and had seemed to advance or fall behind, when the train had rounded curves. all of which is typical of the best of quasi-reasoning. it includes and assimilates diverse data: but it excludes that which will destroy it: that, acceptably, the telegraph wires were alongside the track beyond, as well as leading to linville. mr. crotsenburg thinks of "ball lightning," which, though a sore bewilderment to most speculation, is usually supposed to be a correlate with the old system of thought: but his awareness of "something else" is expressed in other parts of his letters, when he says that he has something to tell that is "so strange that i should never have mentioned it, even to my friends, had it not been corroborated... so unreal that i hesitated to speak of it, fearing that it was some freak of the imagination." vast and black. the thing that was poised, like a crow over the moon. round and smooth. cannon balls. things that have fallen from the sky to this earth. our slippery brains. things like cannon balls have fallen, in storms, upon this earth. like cannon balls are things that, in storms, have fallen to this earth. showers of blood. showers of blood. showers of blood. whatever it may have been, something like red-brick dust, or a red substance in a dried state, fell at piedmont, italy, oct. , (_electric magazine_, - ). a red powder fell, in switzerland, winter of (_pop. sci. rev._, - )-- that something, far from this earth, had bled--super-dragon that had rammed a comet-- or that there are oceans of blood somewhere in the sky--substance that dries, and falls in a powder--wafts for ages in powdered form--that there is a vast area that will some day be known to aviators as the desert of blood. we attempt little of super-topography, at present, but ocean of blood, or desert of blood--or both--italy is nearest to it--or to them. i suspect that there were corpuscles in the substance that fell in switzerland, but all that could be published in was that in this substance there was a high proportion of "variously shaped organic matter." at giessen, germany, in , according to the _report of the british association_, - , fell a rain of a peach-red color. in this rain were flakes of a hyacinthine tint. it is said that this substance was organic: we are told that it was pyrrhine. but distinctly enough, we are told of one red rain that it was of corpuscular composition--red snow, rather. it fell, march , , near the crystal palace, london (_year book of facts_, - ; _nature_, - ). as to the "red snow" of polar and mountainous regions, we have no opposition, because that "snow" has never been seen to fall from the sky: it is a growth of micro-organisms, or of a "protococcus," that spreads over snow that is on the ground. this time nothing is said of "sand from the sahara." it is said of the red matter that fell in london, march , , that it was composed of corpuscles-- of course: that they looked like "vegetable cells." a note: that nine days before had fallen the red substance--flesh--whatever it may have been--of bath county, kentucky. i think that a super-egotist, vast, but not so vast as it had supposed, had refused to move to one side for a comet. we summarize our general super-geographical expressions: gelatinous regions, sulphurous regions, frigid and tropical regions: a region that has been source of life relatively to this earth: regions wherein there is density so great that things from them, entering this earth's thin atmosphere, explode. we have had a datum of explosive hailstones. we now have support to the acceptance that they had been formed in a medium far denser than air of this earth at sea-level. in the _popular science news_, - , is an account of ice that had been formed, under great pressure, in the laboratory of the university of virginia. when released and brought into contact with ordinary air, this ice exploded. and again the flesh-like substance that fell in kentucky: its flake-like formation. here is a phenomenon that is familiar to us: it suggests flattening, under pressure. but the extraordinary inference is--pressure not equal on all sides. in the _annual record of science_, - , it is said that, in , after a heavy thunderstorm in louisiana, a tremendous number of fish scales were found, for a distance of forty miles, along the banks of the mississippi river: bushels of them picked up in single places: large scales that were said to be of the gar fish, a fish that weighs from five to fifty pounds. it seems impossible to accept this identification: one thinks of a substance that had been pressed into flakes or scales. and round hailstones with wide thin margins of ice irregularly around them--still, such hailstones seem to me more like things that had been stationary: had been held in a field of thin ice. in the _illustrated london news_, - , are drawings of hailstones so margined, as if they had been held in a sheet of ice. some day we shall have an expression which will be, to our advanced primitiveness, a great joy: that devils have visited this earth: foreign devils: human-like beings, with pointed beards: good singers; one shoe ill-fitting--but with sulphurous exhalations, at any rate. i have been impressed with the frequent occurrence of sulphurousness with things that come from the sky. a fall of jagged pieces of ice, orkney, july , (_trans. roy. soc. edin._, - ). they had a strong sulphurous odor. and the coke--or the substance that looked like coke--that fell at mortrée, france, april , : with it fell a sulphurous substance. the enormous round things that rose from the ocean, near the _victoria_. whether we still accept that they were super-constructions that had come from a denser atmosphere and, in danger of disruption, had plunged into the ocean for relief, then rising and continuing on their way to jupiter or uranus--it was reported that they spread a "stench of sulphur." at any rate, this datum of proximity is against the conventional explanation that these things did not rise from the ocean, but rose far away above the horizon, with illusion of nearness. and the things that were seen in the sky july, : i have another note. in _nature_, - , a correspondent writes that, upon july , , at sedberg, he had seen in the sky--a red object--or, in his own wording, something that looked like the red part of a rainbow, about degrees long. but the sky was dark at the time. the sun had set. a heavy rain was falling. throughout this book, the datum that we are most impressed with: successive falls. or that, if upon one small area, things fall from the sky, and then, later, fall again upon the same small area, they are not products of a whirlwind, which though sometimes axially stationary, discharges tangentially-- so the frogs that fell at wigan. i have looked that matter up again. later more frogs fell. as to our data of gelatinous substance said to have fallen to this earth with meteorites, it is our expression that meteorites, tearing through the shaky, protoplasmic seas of genesistrine--against which we warn aviators, or they may find themselves suffocating in a reservoir of life, or stuck like currants in a blanc mange--that meteorites detach gelatinous, or protoplasmic, lumps that fall with them. now the element of positiveness in our composition yearns for the appearance of completeness. super-geographical lakes with fishes in them. meteorites that plunge through these lakes, on their way to this earth. the positiveness in our make-up must have expression in at least one record of a meteorite that has brought down a lot of fishes with it-- _nature_, - : that, near the bank of a river, in peru, feb. , , a meteorite fell. "on the spot, it is reported, several dead fishes were found, of different species." the attempt to correlate is--that the fishes "are supposed to have been lifted out of the river and dashed against the stones." whether this be imaginable or not depends upon each one's own hypnoses. _nature_, - : that the fishes had fallen among the fragments of the meteorite. _popular science review_, - : that one day, mr. le gould, an australian scientist, was traveling in queensland. he saw a tree that had been broken off close to the ground. where the tree had been broken was a great bruise. near by was an object that "resembled a ten-inch shot." a good many pages back there was an instance of over-shadowing, i think. the little carved stone that fell at tarbes is my own choice as the most impressive of our new correlates. it was coated with ice, remember. suppose we should sift and sift and discard half the data in this book--suppose only that one datum should survive. to call attention to the stone of tarbes would, in my opinion, be doing well enough, for whatever the spirit of this book is trying to do. nevertheless, it seems to me that a datum that preceded it was slightingly treated. the disk of quartz, said to have fallen from the sky, after a meteoric explosion: said to have fallen at the plantation bleijendal, dutch guiana: sent to the museum of leyden by m. van sypesteyn, adjutant to the governor of dutch guiana (_notes and queries_, - - ). and the fragments that fall from super-geographic ice fields: flat pieces of ice with icicles on them. i think that we did not emphasize enough that, if these structures were not icicles, but crystalline protuberances, such crystalline formations indicate long suspension quite as notably as would icicles. in the _popular science news_, - , it is said that in , near tiflis, fell large hailstones with long protuberances. "the most remarkable point in connection with the hailstones is the fact that, judging from our present knowledge, a very long time must have been occupied in their formation." according to the _geological magazine_, - , this fall occurred may , . the writer in the _geological magazine_ says that of all theories that he had ever heard of, not one could give him light as to this occurrence--"these growing crystalline forms must have been suspended a long time"-- again and again this phenomenon: fourteen days later, at about the same place, more of these hailstones fell. rivers of blood that vein albuminous seas, or an egg-like composition in the incubation of which this earth is a local center of development--that there are super-arteries of blood in genesistrine: that sunsets are consciousness of them: that they flush the skies with northern lights sometimes: super-embryonic reservoirs from which life-forms emanate-- or that our whole solar system is a living thing: that showers of blood upon this earth are its internal hemorrhages-- or vast living things in the sky, as there are vast living things in the oceans-- or some one especial thing: an especial time: an especial place. a thing the size of the brooklyn bridge. it's alive in outer space--something the size of central park kills it-- it drips. we think of the ice fields above this earth: which do not, themselves, fall to this earth, but from which water does fall-- _popular science news_, - : that, according to prof. luigi palazzo, head of the italian meteorological bureau, upon may , , at messignadi, calabria, something the color of fresh blood fell from the sky. this substance was examined in the public-health laboratories of rome. it was found to be blood. "the most probable explanation of this terrifying phenomenon is that migratory birds (quails or swallows) were caught and torn in a violent wind." so the substance was identified as birds' blood-- what matters it what the microscopists of rome said--or had to say--and what matters it that we point out that there is no assertion that there was a violent wind at the time--and that such a substance would be almost infinitely dispersed in a violent wind--that no bird was said to have fallen from the sky--or said to have been seen in the sky--that not a feather of a bird is said to have been seen-- this one datum: the fall of blood from the sky-- but later, in the same place, blood again fell from the sky. _notes and queries_, - - : a correspondent who had been to devonshire writes for information as to a story that he had heard there: of an occurrence of about thirty-five years before the date of writing: of snow upon the ground--of all south devonshire waking up one morning to find such tracks in the snow as had never before been heard of--"clawed footmarks" of "an unclassifiable form"--alternating at huge but regular intervals with what seemed to be the impression of the point of a stick--but the scattering of the prints--amazing expanse of territory covered--obstacles, such as hedges, walls, houses, seemingly surmounted-- intense excitement--that the track had been followed by huntsmen and hounds, until they had come to a forest--from which the hounds had retreated, baying and terrified, so that no one had dared to enter the forest. _notes and queries_, - - : whole occurrence well-remembered by a correspondent: a badger had left marks in the snow: this was determined, and the excitement had "dropped to a dead calm in a single day." _notes and queries_, - - : that for years a correspondent had had a tracing of the prints, which his mother had taken from those in the snow in her garden, in exmouth: that they were hoof-like marks--but had been made by a biped. _notes and queries_, - - : well remembered by another correspondent, who writes of the excitement and consternation of "some classes." he says that a kangaroo had escaped from a menagerie--"the footprints being so peculiar and far apart gave rise to a scare that the devil was loose." we have had a story, and now we shall tell it over from contemporaneous sources. we have had the later accounts first very largely for an impression of the correlating effect that time brings about, by addition, disregard and distortion. for instance, the "dead calm in a single day." if i had found that the excitement did die out rather soon, i'd incline to accept that nothing extraordinary had occurred. i found that the excitement had continued for weeks. i recognize this as a well-adapted thing to say, to divert attention from a discorrelate. all phenomena are "explained" in the terms of the dominant of their era. this is why we give up trying really to explain, and content ourselves with expressing. devils that might print marks in snow are correlates to the third dominant back from this era. so it was an adjustment by nineteenth-century correlates, or human tropisms, to say that the marks in the snow were clawed. hoof-like marks are not only horsey but devilish. it had to be said in the nineteenth century that those prints showed claw-marks. we shall see that this was stated by prof. owen, one of the greatest biologists of his day--except that darwin didn't think so. but i shall give reference to two representations of them that can be seen in the new york public library. in neither representation is there the faintest suggestion of a claw-mark. there never has been a prof. owen who has explained: he has correlated. another adaptation, in the later accounts, is that of leading this discorrelate to the old dominant into the familiar scenery of a fairy story, and discredit it by assimilation to the conventionally fictitious--so the idea of the baying, terrified hounds, and forest like enchanted forests, which no one dared to enter. hunting parties were organized, but the baying, terrified hounds do not appear in contemporaneous accounts. the story of the kangaroo looks like adaptation to needs for an animal that could spring far, because marks were found in the snow on roofs of houses. but so astonishing is the extent of snow that was marked that after a while another kangaroo was added. but the marks were in single lines. my own acceptance is that not less than a thousand one-legged kangaroos, each shod with a very small horseshoe, could have marked that snow of devonshire. london _times_, feb , : "considerable sensation has been caused in the towns of topsham, lymphstone, exmouth, teignmouth, and dawlish, in devonshire, in consequence of the discovery of a vast number of foot tracks of a most strange and mysterious description." the story is of an incredible multiplicity of marks discovered in the morning of feb. , , in the snow, by the inhabitants of many towns and regions between towns. this great area must of course be disregarded by prof. owen and the other correlators. the tracks were in all kinds of unaccountable places: in gardens enclosed by high walls, and up on the tops of houses, as well as in the open fields. there was in lymphstone scarcely one unmarked garden. we've had heroic disregards but i think that here disregard was titanic. and, because they occurred in single lines, the marks are said to have been "more like those of a biped than of a quadruped"--as if a biped would place one foot precisely ahead of another--unless it hopped--but then we have to think of a thousand, or of thousands. it is said that the marks were "generally inches in advance of each other." "the impression of the foot closely resembles that of a donkey's shoe, and measured from an inch and a half, in some instances, to two and a half inches across." or the impressions were cones in incomplete, or crescentic basins. the diameters equaled diameters of very young colts' hoofs: too small to be compared with marks of donkey's hoofs. "on sunday last the rev. mr. musgrave alluded to the subject in his sermon and suggested the possibility of the footprints being those of a kangaroo, but this could scarcely have been the case, as they were found on both sides of the este. at present it remains a mystery, and many superstitious people in the above-named towns are actually afraid to go outside their doors after night." the este is a body of water two miles wide. london _times_, march , : "the interest in this matter has scarcely yet subsided, many inquiries still being made into the origin of the footprints, which caused so much consternation upon the morning of the th ult. in addition to the circumstances mentioned in the _times_ a little while ago, it may be stated that at dawlish a number of persons sallied out, armed with guns and other weapons, for the purpose, if possible, of discovering and destroying the animal which was supposed to have been so busy in multiplying its footprints. as might have been expected, the party returned as they went. various speculations have been made as to the cause of the footprints. some have asserted that they are those of a kangaroo, while others affirm that they are the impressions of claws of large birds driven ashore by stress of weather. on more than one occasion reports have been circulated that an animal from a menagerie had been caught, but the matter at present is as much involved in mystery as ever it was." in the _illustrated london news_, the occurrence is given a great deal of space. in the issue of feb. , , a sketch is given of the prints. i call them cones in incomplete basins. except that they're a little longish, they look like prints of hoofs of horses--or, rather, of colts. but they're in a single line. it is said that the marks from which the sketch was made were inches apart, and that this spacing was regular and invariable "in every parish." also other towns besides those named in the _times_ are mentioned. the writer, who had spent a winter in canada, and was familiar with tracks in snow, says that he had never seen "a more clearly defined track." also he brings out the point that was so persistently disregarded by prof. owen and the other correlators--that "no known animal walks in a line of single footsteps, not even man." with these wider inclusions, this writer concludes with us that the marks were not footprints. it may be that his following observation hits upon the crux of the whole occurrence: that whatever it may have been that had made the marks, it had removed, rather than pressed, the snow. according to his observations the snow looked "as if branded with a hot iron." _illustrated london news_, march , - : prof. owen, to whom a friend had sent drawings of the prints, writes that there were claw-marks. he says that the "track" was made by "a" badger. six other witnesses sent letters to this number of the _news_. one mentioned, but not published, is a notion of a strayed swan. always this homogeneous-seeing--"a" badger--"a" swan--"a" track. i should have listed the other towns as well as those mentioned in the _times_. a letter from mr. musgrave is published. he, too, sends a sketch of the prints. it, too, shows a single line. there are four prints, of which the third is a little out of line. there is no sign of a claw-mark. the prints look like prints of longish hoofs of a very young colt, but they are not so definitely outlined as in the sketch of february th, as if drawn after disturbance by wind, or after thawing had set in. measurements at places a mile and a half apart, gave the same inter-spacing--"exactly eight inches and a half apart." we now have a little study in the psychology and genesis of an attempted correlation. mr. musgrave says: "i found a very apt opportunity to mention the name 'kangaroo' in allusion to the report then current." he says that he had no faith in the kangaroo-story himself, but was glad "that a kangaroo was in the wind," because it opposed "a dangerous, degrading, and false impression that it was the devil." "mine was a word in season and did good." whether it's jesuitical or not, and no matter what it is or isn't, that is our own acceptance: that, though we've often been carried away from this attitude controversially, that is our acceptance as to every correlate of the past that has been considered in this book--relatively to the dominant of its era. another correspondent writes that, though the prints in all cases resembled hoof marks, there were indistinct traces of claws--that "an" otter had made the marks. after that many other witnesses wrote to the _news_. the correspondence was so great that, in the issue of march th, only a selection could be given. there's "a" jumping-rat solution and "a" hopping-toad inspiration, and then someone came out strong with an idea of "a" hare that had galloped with pairs of feet held close together, so as to make impressions in a single line. london _times_, march , : "among the high mountains of that elevated district where glenorchy, glenlyon and glenochay are contiguous, there have been met with several times, during this and also the former winter, upon the snow, the tracks of an animal seemingly unknown at present in scotland. the print, in every respect, is an exact resemblance to that of a foal of considerable size, with this small difference, perhaps, that the sole seems a little longer, or not so round; but as no one has had the good fortune as yet to have obtained a glimpse of this creature, nothing more can be said of its shape or dimensions; only it has been remarked, from the depth to which the feet sank in the snow, that it must be a beast of considerable size. it has been observed also that its walk is not like that of the generality of quadrupeds, but that it is more like the bounding or leaping of a horse when scared or pursued. it is not in one locality that its tracks have been met with, but through a range of at least twelve miles." in the _illustrated london news_, march , , a correspondent from heidelberg writes, "upon the authority of a polish doctor of medicine," that on the piashowa-gora (sand hill) a small elevation on the border of galicia, but in russian poland, such marks are to be seen in the snow every year, and sometimes in the sand of this hill, and "are attributed by the inhabitants to supernatural influences." astronomical myths. [illustration] [illustration: the cliffs of flamanville.] astronomical myths, based on flammarion's "history of the heavens." by john f. blake. [illustration] london: macmillan and co. . london: r. clay, sons, and taylor, printers, bread street hill, queen victoria street. [illustration] preface. the book which is here presented to the public is founded upon a french work by m. flammarion which has enjoyed considerable popularity. it contained a number of interesting accounts of the various ideas, sometimes mythical, sometimes intended to be serious, that had been entertained concerning the heavenly bodies and our own earth; with a popular history of the earliest commencement of astronomy among several ancient peoples. it was originally written in the form of conversations between the members of an imaginary party at the seaside. it was thought that this style would hardly be so much appreciated by english as by french readers, and therefore in presenting the materials of the french author in an english dress the conversational form has been abandoned. several facts of extreme interest in relation to the early astronomical myths and the development of the science among the ancients having been brought to light, especially by the researches of mr. haliburton, a considerable amount of new matter, including the whole chapter on the pleiades, has been introduced, which makes the present issue not exactly a translation, but rather a book founded on the french author's work. it is hoped that it may be found of interest to those who care to know about the early days of the oldest of our sciences, which is now attracting general attention again by the magnitude of its recent advances. astronomy also, in early days, as will be seen by a perusal of this book, was so mixed up with all the affairs of life, and contributed so much even to religion, that a history of its beginnings is found to reveal the origin of several of our ideas and habits, now apparently quite unconnected with the science. there is matter of interest here, therefore, for those who wish to know only the history of the general ideas of mankind. [illustration: the annual revolution of the earth round the sun, with the signs of the zodiac and the constellations.] list of illustrations. the cliffs of flamanville _frontispiece._ the annual revolution of the earth round the sun, with the signs of the zodiac and the constellations page ix the earth's year, and the months " xiv an astronomer at work to face page the northern constellations " the constellations from the sea-shore " the zodiac of denderah " i. babylonian astronomers ii. druidical worship iii. chaldean astronomers iv. the zodiac and the dead in egypt v. the legends of the druids vi. the nemÆan lion vii. heavens of the fathers viii. death of copernicus ix. the solar system x. the discovery of the telescope xi. the foundation of the paris observatory xii. the legend of owen xiii. christopher columbus and the eclipse of the moon xiv. prodigies in the middle ages xv. an astrologer at work xvi. the end of the world . the earliest (aryan) representation of the earth . ancient gaulish medals, bearing astronomical signs . ancient celestial sphere . positions of the great bear on september . constellation of the bear . constellation of orion . chart of constellations in sixteenth and seventeenth centuries . flamsteed's chart . arabian sphere of the eleventh century . ancient chinese pieces of money, bearing representations of the zodiac . the zodiac . diagram illustrating the position of certain stars, b.c. . curious fifteenth century figure, representing eleven different heavens . ptolemy's astronomical system . the epicycles of ptolemy . heavens of the middle ages . emblematic drawing from ancient astronomical work . egyptian system . capella's system . the copernican system . tycho brahe's system . descartes' theory of vortices . vortices of the stars . variation of descartes' theory . the earth floating . the earth with roots . the earth of the vedic priests . hindoo earth . the earth of anaximander . plato's cubical earth . egyptian representation of the earth . homeric cosmography . the earth of the later greeks . pomponius mela's cosmography . the earth's shadow . ditto . ditto . ditto . the cosmography of cosmas . the square earth . explanation of sunrise . the earth as an egg . the earth as a floating egg . eighth-century map of the world . tenth-century maps . the map of andrea bianco . from the map in hereford cathedral . ditto . cosmography of st. denis . the map of marco polo . map on a medal of charles v . dante's infernal regions . paradise of fra mauro . the paradise of the fifteenth century . representation of a comet, sixteenth century . an egg marked with a comet . the roman calendar . diagram illustrating the order of the days of the week [illustration: the earth's year, and the months.] contents. page chapter i. the first beginnings of astronomy chapter ii. astronomy of the celts chapter iii. origin of the constellations chapter iv. the zodiac chapter v. the pleiades chapter vi. the nature and structure of the heavens according to the ancients chapter vii. the celestial harmony chapter viii. astronomical systems chapter ix. the terrestrial world of the ancients.--cosmography and geography chapter x. cosmography and geography of the church chapter xi. legendary worlds of the middle ages chapter xii. eclipses and comets chapter xiii. the greatness and the fall of astrology chapter xiv. time and the calendar chapter xv. the end of the world [illustration: an astronomer at work.] [illustration] history of the heavens. chapter i. the first beginnings of astronomy. astronomy is an ancient science; and though of late it has made a fresh start in new regions, and we are opening on the era of fresh and unlooked-for discoveries which will soon reveal our present ignorance, our advance upon primitive ideas has been so great that it is difficult for us to realize what they were without an attentive and not uninstructive study of them. no other science, not even geology, can compare with astronomy for the complete revolution which it has effected in popular notions, or for the change it has brought about in men's estimate of their place in creation. it is probable that there will always be men who believe that the whole universe was made for their benefit; but, however this may be, we have already learned from astronomy that our habitation is not that central spot men once deemed it, but only an ordinary planet circulating round an ordinary star, just as we are likely also to learn from biology, that we occupy the position, as animals, of an ordinary family in an ordinary class. that we may more perfectly realize this strange revolution of ideas, we must throw ourselves as far as possible into the feeling and spirit of our ancestors, when, without the knowledge we now possess, they contemplated, as they could not fail to do, the marvellous and awe-inspiring phenomena of the heavens by night. to them, for many an age, the sun and moon and stars, with all the planets, seemed absolutely to rise, to shine, and to set; the constellations to burst out by night in the east, and travel slowly and in silence to the west; the ocean waves to rise and fall and beat against the rock-bound shore as if endowed with life; and even in the infancy of the intellect they must have longed to pierce the secrets of this mysterious heavenly vault, and to know the nature of the starry firmament as it seemed to them, and the condition of the earth which appeared in the centre of these universal movements. the simplest hypothesis was for them the truth, and they believed that the sky was in reality a lofty and extended canopy bestudded with stars, and the earth a vast plain, the solid basis of the universe, on which dwelt man, sole creature that lifted his eyes and thoughts above. two distinct regions thus appeared to compose the whole system--the upper one, or the air, in which were the moving stars, the lights of heaven, and the firmament over all; and the lower one, or earth and sea, adorned on the surface with the products of life, and below with the minerals, metals, and stones. for a long time the various theories of the universe, grotesque and changing as they might be, were but modifications of this one central idea, the earth below, the heavens above, and on this was based every religious system that was promulgated--the very phrases founded upon it remaining to this day for a testimony to the intimate relation thus manifested between the infant ideas in astronomy and theology. no wonder that early revolutions in the conceptions in one science were thought to militate against the other. it is only when the thoughts on both are enlarged that it is seen that their connection is not necessary, but accidental, or, at least, inevitable only in the infancy of both. it is scarcely possible to estimate fully the enormous change from these ideas representing the appearances to those which now represent the reality; or to picture to ourselves the total revolution in men's minds before they could transform the picture of a vast terrestrial surface, to which the sun and all the heavenly bodies were but accessories for various purposes, to one in which the earth is but a planet like mars, moving in appearance among the stars, as it does, and rotating with a rapidity that brings a whole hemisphere of the heavens into view through the course of a single day and night. at first sight, what a loss of dignity! but, on closer thought, what a gain of grandeur! no longer some little neighbouring lights shine down upon us from a solid vault; but we find ourselves launched into the sea of infinity; with power to gaze into its almost immeasurable depths. to appreciate rightly our position, we have to plant ourselves, in imagination, in some spot removed from the surface of the earth, where we may be uninfluenced by her motion, and picture to ourselves what we should see. were we placed in some spot far enough removed from the earth, we should find ourselves in eternal day; the sun would ever shine, for no great globe would interpose itself between it and our eyes; there would be no night there. were we in the neighbourhood of the earth's orbit, and within it, most wonderful phenomena would present themselves. at one time the earth would appear but an ordinary planet, smaller than venus, but, as time wore on, unmeasured by recurring days or changing seasons, it would gradually be seen to increase in size--now appearing like the moon at the full, and shining like her with a silver light. as it came nearer, and its magnitude increased, the features of the surface would be distinguished; the brighter sea and the darker shining continents, with the brilliant ice-caps at the poles; but, unlike what we see in the moon, these features would appear to move, and, one after another, every part of the earth would be visible. the actual time required for all to pass before us would be what we here call a day and night. and still, as it rotates, the earth passes nearer to us, assumes its largest apparent size, and so gradually decreasing again, becomes once more, after the interval we here call a year, an ordinary-looking star-like planet. to us, in these days, this description is easy of imagination; we find no difficulty in picturing it to ourselves; but, if we will think for a moment what such an idea would have been to the earliest observers of astronomy, we shall better appreciate the vast change that has taken place--how we are removed from them, as we may say, _toto coelo_. but not only as to the importance of the earth in the universe, but on other matters connected with astronomy, we perceive the immensity of the change in our ideas--in that of distance, for instance. this celestial vault of the ancients was near enough for things to pass from it to us; it was in close connection with the earth, supported by it, and therefore of less diameter; but now, when our distance from the sun is expressed by numbers that we may write, indeed, but must totally fail to adequately appreciate, and the distance from the _next_ nearest star is such, that with the velocity of light--a velocity we are accustomed to regard as instantaneous--we should only reach it after a three years' journey, we are reminded of the pathetic lines of thomas hood: "i remember, i remember, the fir trees straight and high, and how i thought their slender tops were close against the sky; it was a childish fantasy, but now 'tis little joy, to know i'm further off from heaven than when i was a boy." the astronomer's answer to the last line would be that as far as the material heaven goes, we are just as much in it as the stars or as any other member of the universe; we cannot, therefore, be far off or near to it. it is probable that we are even yet but little awake to true cosmical ideas in other respects;--as to velocity, for instance. we know indeed, of light and electricity and the motions of the earth, but revelations are now being made to us of motions of material substances in the sun with such velocities that in comparison with them any motions on the earth appear infinitesimally small. our progress to our present notions, and appreciations of the truth of nature in the heavens, will thus occupy much of our thoughts; but we must also recount the history of the acquirement of those facts which have ultimately become the basis for our changes of idea. our rustic forefathers, whatever their nation, were not so enamoured of the "wonders of science"--that their astronomy was greatly a collection of theories, though theories, and wild ones, they had; it was a more practical matter, and was believed too by them to be more practical than we now find reason to believe to be the case. they noticed the various seasons, and they marked the changes in the appearances of the heavens that accompanied them; they connected the two together, and conceived the latter to be the cause of the former, and so, with other apparently uncertain events. the celestial phenomena thus acquired a fictitious importance which rendered their study of primary necessity, but gave no occasion for a theory. that we may better appreciate the earliest observations on astronomy, it may be well to mention briefly what are the varying phenomena which may most easily be noticed. if we except the phases of the moon, which almost without observation would force their recognition on people who had no other than lunar light by night, and which must therefore, from the earliest periods of human history have divided time into lunar months; there are three different sets of phenomena which depend on the arrangement of our planetary system, and which were early observed. the first of these depends upon the earth's rotation on its axis, the result of which is that the stars appear to revolve with a uniform motion from east to west; the velocity increasing with the distance from the pole star, which remains nearly fixed. this circumstance is almost as easy of observation as the phases of the moon, and was used from the earliest ages to mark the passage of time during the night. the next arises from the motion of the earth in her orbit about the sun, by which it happens that the earth is in a different position with respect to the sun every night, and, therefore, a different set of stars are seen in his neighbourhood; these are setting with him, and therefore also a different set are just rising at sunset every evening. these changes, which would go through the cycle in a year, are, of course, less obvious, but of great importance as marking the approach of the various seasons during ages in which the hour of the sun's rising could not be noted by a clock. the last depends on the proper motions of the moon and planets about the earth and sun respectively, by reason of which those heavenly bodies occupy varying positions among the stars. only a careful and continuous scrutiny of the heavens would detect these changes, except, perhaps, in the case of the moon, and but little of importance really depends on them; nevertheless, they were very early the subject of observation, as imagination lent them a false value, and in some cases because their connection with eclipses was perceived. the practical cultivation of astronomy amongst the earliest people had always reference to one or other of these three sets of appearances, and the various terms and signs that were invented were intended for the clearer exposition of the results of their observations on these points. in looking therefore into extreme antiquity we shall find in many instances our only guide to what their knowledge was is the way in which they expressed these results. we do not find, and perhaps we should scarcely expect to find, any one man or even one nation who laid the foundation of astronomy--for it was an equal necessity for all, and was probably antecedent to the practice of remembering men by their names. we cannot, either, conjecture the antiquity of ideas and observations met with among races who are themselves the only record of their past; and if we are to find any origins of the science, it is only amongst those nations which have been cultivators of arts by which their ancient doings are recorded. amongst the earliest cultivators of astronomy we may refer to the primitive greeks, the chinese, the egyptians, the babylonians, and the aryans, and also to certain traditions met with amongst many savage as well as less barbarous races, the very universality of which proclaims as loudly as possible their extreme antiquity. each of the four above-mentioned races have names with which are associated the beginnings of astronomy--uranus and atlas amongst the greeks; folic amongst the chinese; thaut or mercury in egypt; zoroaster and bel in persia and babylonia. names such as these, if those of individuals, are not necessarily those of the earliest astronomers--but only the earliest that have come down to us. indeed it is very far from certain whether these ancient celebrities have any real historical existence. the acts and labours of the earliest investigators are so wrapped in obscurity, there is such a mixture of fable with tradition, that we can have no reliance that any of them, or that others mentioned in ancient mythology, are not far more emblematical than personal. some, such as uranus, are certainly symbolical; but the very existence of the name handed down to us, if it prove nothing else, proves that the science was early cultivated amongst those who have preserved or invented them. if we attempt to name in years the date of the commencement--not of astronomy itself--for that probably in some form was coeval with the race of man itself, but of recorded observations, we are met with a new difficulty arising from the various ways in which they reckoned time. this was in every case by the occurrence of the phases of one or other of the above-mentioned phenomena; sometimes however they selected the apparent rotation of the sun in twenty-four hours, sometimes that of the moon in a month, sometimes the interval from one solstice to the next, and yet they apparently gave to each and all of these the same title--such as _annus_--obviously representing a cycle only, but without reference to its length. by these different methods of counting, hopeless confusion has often been introduced into chronology; and the moderns have in many instances unjustly accused the ancients of vanity and falsehood. bailly attempted to reconcile all these various methods and consequent dates with each other, and to prove that practical astronomy commenced "about , years before the deluge, or that it is about , years old;" but we shall see reason in the sequel for suspecting any such attempt, and shall endeavour to arrive at more reliable dates from independent evidence. perhaps the remotest antiquity to which we can possibly mount is that of the aryans, amongst whom the hymns of the _rig veda_ were composed. the short history of hebrew and greco-roman civilization seems to be lost in comparison with this the earliest work of human imagination. when seeking for words to express their thoughts, these primitive men by the banks of the oxus personified the phenomena of the heavens and earth, the storm, the wind, the rain, the stars and meteors. here, of course, it is not practical but theoretical astronomy we find. we trace the first figuring of that primitive idea alluded to before--the heaven above, the earth below. here, as we see, is the earth represented as an indefinite plane surface and passive being forming the foundation of the world; and above it the sky, a luminous and variable vault beneath which shines out the fertile and life-giving light. thus to the earth they gave the name p'rthovi, "the wide expanse;" the blue and star-bespangled heavens they called varuna, "the vault;" and beneath it in the region of the clouds they enthroned the light dyaus, _i.e._ "the luminous air." [illustration: fig. .] from hence, it would appear, or on this model, the early ideas of all peoples have been formed. among the greeks the name for heaven expresses the same idea of a hollow vault ([greek: koilos], hollow, concave) and the earth is called [greek: gê], or mother. among the latins the name _coelum_ has the same signification, while the earth _terra_ comes from the participle _tersa_ (the dry element) in contradistinction to _mare_ the wet. in this original aryan notion, however, as represented by the figure, we have more than this, the origin of the names _jupiter_ and _deus_ comes out. for it is easy to trace the connection between _dyaus_ (the luminiferous air) and the greek word _zeus_ from whence _dios_, [greek: _theos_], _deus_, and the french word _dieu_, and then by adding _pater_ or father we get _deuspater_, _zeuspater_, jupiter. these etymologies are not however matters beyond dispute, and there are at least two other modes of deriving the same words. thus we are told the earliest name for the deity was jehovah, the word _jehov_ meaning father of life; and that the greeks translated this into _dis_ or _zeus_, a word having, according to this theory, the same sense, being derived from [greek: zaô] to live. of course there can be no question of the later word _deus_ being the direct translation of _dios_. a third theory is that there exists in one of the dialects which formed the basis of the old languages of asia, a word _yahouh_, a participle of the verb _nîh_, to exist, to be; which therefore signifies the self-existent, the principle of life, the origin of all motion, and this is supposed to be the allusion of diodorus, who explaining the theology of the greeks, says that the egyptians according to manetho, priest of memphis, in giving names to the five elements have called the spirit or ether youpiter in the _proper sense_ of the word, for the spirit is the source of life, the author of the vital principle in animals, and is hence regarded as the father or generator of all beings. the people of the homeric ages thought the lightning-bearing jupiter was the commencement, origin, end, and middle of all things, a single and universal power, governing the heavens, the earth, fire, water, day and night, and all things. porphyry says that when the philosophers discoursed on the nature and parts of the deity, they could not imagine any single figure that should represent all his attributes, though they presented him under the appearance of a man, who was _seated_ to represent his immovable essence; uncovered in his upper part, because the upper parts of the universe or region of the stars manifest most of his nature; but clothed below the loins, because he is more hidden in terrestrial things; and holding a sceptre in his left hand, because his heart is the ruler of all things. there are, besides, the etymologies which assert that jupiter is derived from _juvare_ to help, meaning the assisting father; or again that he is _dies pater_--the god of the day--in which case no doubt the sun would be alluded to. it appears then that the ancient aryan scheme, though _possibly_ supplying us with the origin of one of the widest spread of our words, is not universally allowed to do so. this origin, however, appears to derive support from the apparent occurrence of the original of another well-known ancient classical word in the same scheme, that is varuna, obviously the same word as [greek: ouranos], and uranus, signifying the heavens. less clearly too perhaps we may trace other such words to the same source. thus the sun, which according to these primitive conceptions is the husband of the earth, which it nourishes and makes fruitful, was called _savitr_ and _surya_, from which the passage to the gothic _sauil_ is within the limits of known etymological changes, and so comes the lithuanian _saull_, the cymric _haul_, the greek _heilos_, the latin _sol_, and the english _solar_. so from their _nakt_, the destructive, we get _nux_, _nacht_, _night_. from _glu_, the shining, whence the participle _glucina_, and so to _lucina_, _luena_, _luna_, _lune_. turning from the ancient aryans, whose astronomy we know only from poems and fables, and so learn but little of their actual advance in the science of observation, we come to the babylonians, concerning whose astronomical acquirements we have lately been put in possession of valuable evidence by the tablets obtained by mr. smith from kouyunjik, an account the contents of which has been given by mr. sayce (_nature_, vol. xii. p. ). as the knowledge thus obtained is more certain, being derived from their actual records, than any that we previously possessed, it will be well to give as full an account of it as we are able. the originators of babylonian astronomy were not the chaldæans, but another race from the mountains of elam, who are generally called acadians. of the astronomy of this race we have no complete records, but can only judge of their progress by the words and names left by them to the science, as afterwards cultivated by the semitic babylonians. these last were a subsequent race, who entering the country from the east, conquered the original inhabitants about b.c., and borrowed their civilization, and with it their language in the arts and sciences. but even this latter race is one of considerable antiquity, and when we see, as we shortly shall, the great advances they had made in observations of the sun and moon, and consider the probable slowness of development in those early ages, we have some idea of the remoteness of the date at which astronomical science was there commenced. our chief source of information is an extremely ancient work called the _observations of bell_, supposed to have been written before b.c., which was compiled for a certain king saigou, of agave in babylonia. this work is in seventy books or parts, and is composed of numerous small earthen tablets having impressed upon them the cuneiform character in which they printed, and which we are now able to read. we generally date the art of printing from caxton, in , because it took the place of manuscript that had been previously in use in the west; but that method of writing, if in some respects an improvement on previous methods of recording ideas as more easily executed, was in others a retrogression as being less durable: while the manuscripts have perished the impressions on stone have remained to this day, and will no doubt last longer than even our printed books. these little tablets represented so many leaves, and in large libraries, such as that from which those known have been derived, they were numbered as our own are now, so that any particular one could be asked for by those who might wish to consult it. the great difficulty of interpreting these records, which are written in two different dialects, and deal often with very technical matters, may well be imagined. these difficulties however have been overcome, and a good approach to the knowledge of their contents has been made. the chaldæans, as is well known, were much given to astronomy and many of their writings deal with this subject; but they did practical work as well, and did not indulge so much in theory as the aryans. we shall have future occasion in this book to refer to their observations on various points, as they did not by any means confine themselves to the simplest matters; much, in fact, of that with which modern astronomy deals, the dates and duration of eclipses of the sun and moon, the accurate measurement of time, the existence of cycles in lunar and solar phenomena, was studied and recorded by them. we can make some approach to the probable dates of the invention of some part of their system, by means of the signs of the zodiac, which were invented by them and which we will discuss more at length hereafter. we need only say at present that what is now the sign of spring, was not reckoned so with them, and that we can calculate how long ago it is that the sign they reckoned the spring sign was so. semiramis also raised in the centre of babylon a temple consecrated to jupiter, whom the babylonians called bel. it was of an extraordinary height and served for an observatory. the whole edifice was constructed with great art in asphalte and brick. on its summit were placed the statues of jupiter, juno, and rhea, covered with gold. the egyptians have always been named as the earliest cultivators of astronomy by the grecian writers, by whom the science has been handed down to us, and the chaldæans have even been said to have borrowed from them. the testimony of such writers however is not to be received implicitly, but to be weighed with the knowledge we may now obtain, as we have noticed above with respect to the babylonians, from the actual records they have left us, whether by actual records, or by words and customs remaining to the present day. [illustration: plate i.--babylonian astronomers.] herodotus declares that the egyptians had made observations for , years and had seen the course of the sun change four times, and the ecliptic placed perpendicular to the equator. this is the style of statement on which opinions of the antiquity of egyptian astronomy have been founded, and it is obviously unworthy of credit. diodorus says that there is no country in which the positions and motions of the stars have been so accurately observed as in egypt (_i.e._ to his knowledge). they have preserved, he says, for a great number of years registers in which their observations are recorded. expositions are found in these registers of the motions of the planets, their revolutions and their stations, and, moreover, the relation which each bears to the birthdays of animals, and its good or evil influence. they often predicted the future with success. the earthquakes, inundations, the appearance of comets, and many other phenomena which it is impossible for the vulgar to know beforehand, were foreseen by them by means of the observations they had made over a long series of years. on the occasion of the french expedition to egypt, a long passage was discovered leading from karnak to lucksor. this passage was adorned on each side of the way with a range of sphinxes with the body of a lion and the head of a ram. now in egyptian architecture, the ornaments are never the result of caprice or chance; on the contrary, all is done with intention, and what often appears at first sight strange, appears, after having been carefully examined and studied, to present allegories full of sense and reason, founded on a profound knowledge of natural phenomena, that the ornaments are intended to record. these sphinxes and rams of the passage were probably the emblems of the different signs of the zodiac along the route of the sun. the date of the avenue is not known; but it would doubtless lead us to a high antiquity for the egyptian observations. the like may be said of the great pyramid, which according to piazzi smyth was built about b.c. certainly there are no carvings about it exhibiting any astronomical designs; but the exact way in which it is executed would seem to indicate that the builders had a very clear conception of the importance of the meridian line. it should, however, be stated that piazzi smyth does not consider it to have been built by the egyptians for themselves; but under the command of some older race. there seem, however, to be indications in various festivals and observances, which are met with widely over the earth's surface, as will be indicated more in detail in the chapter on the pleiades, that some astronomical observations, though of the rudest, were made by races anterior even to those whose history we partially possess; and that not merely because of its naturalness, but because of positive evidence, we must trace back astronomy to a source from whence egyptians, indians, and perhaps babylonians themselves derived it. the chinese astronomy is totally removed from these and stands on its own basis. with them it was a matter concerning the government, and stringent laws were enforced on the state astronomers. the advance, however, that they made would appear to be small; but if we are to believe their writers, they made observations nearly three thousand years before our era. under the reign of hoangti, yuchi recorded that there was a large star near the poles of the heavens. by a method which we shall enlarge upon further on, it can be astronomically ascertained that about the epoch this observation was said to be made there was a star ([greek: a] draconis) so near the pole as to appear immovable, which is so far a confirmation of his statement. in the first of a series of eclipses was recorded by them; but the value of their astronomy seems to be doubtful when we learn that calculation proves that not one of them previous to the age of ptolemy can be identified with the dates given. amongst all nations except the chinese, where it was political, and the greeks, where it was purely speculative, astronomy has been intimately mixed with religious ideas, and we consequently find it to have taken considerable hold on the mind. just as we have seen among the indians that the basis of their astronomical ideas was the two-fold division into heaven and earth, so among other nations this duality has formed the basis of their religion. two aspects of things have been noticed by men in the constitution of things--that which remains always, and that which is merely transitory, causes and effects. the heaven and the earth have presented the image of this to their minds--one being the eternal existence, the other the passing form. in heaven nothing seems to be born, increase, decrease, or die above the sphere of the moon. that alone showed the traces of alteration in its phases; while on the other hand there was an image of perpetuity in its proper substance, in its motion, and the invariable succession of the same phases. from another point of view, the heavens were regarded as the father, and the earth as the mother of all things. for the principle of fertility in the rains, the dew and the warmth, came from above; while the earth brought forth abundantly of the products of nature. such is the idea of plutarch, of hesiod, and of virgil. from hence have arisen the fictions which have formed the basis of theogony. uranus is said to have espoused ghe, or the heavens took the earth to wife, and from their marriage was born the god of time or saturn. another partly religious, and partly astronomical antagonism has been drawn between light and darkness, associated respectively with good and evil. in the days when artificial lights, beyond those of the flickering fire, were unknown, and with the setting of the sun all the world was enveloped in darkness and seemed for a time to be without life, or at least cut off entirely from man, it would seem that the sun and its light was the entire origin of life. hence it naturally became the earliest divinity whose brilliant light leaping out of the bosom of chaos, had brought with it man and all the universe, as we see it represented in the theologies of orpheus and of moses; whence the god bel of the chaldeans, the oromaza of the persians, whom they invoke as the source of all that is good in nature, while they place the origin of all evil in darkness and its god ahrinam. we find the glories of the sun celebrated by all the poets, and painted and represented by numerous emblems and different names by the artists and sculptors who have adorned the temples raised to nature or the great first cause. among the jews there are traditions of a very high antiquity for their astronomy. josephus assures us that it was cultivated before the mosaic deluge. according to him it is to the public spirit and the labour of the antediluvians that we owe the science of astrology: "and since they had learnt from adam that the world should perish by water and by fire, the fear that their science should be lost, made them erect two columns, one of brick the other of stone, on which they engraved the knowledge they had acquired, so that if a deluge should wash away the column of brick, the stone one might remain to preserve for posterity the memory of what they had written. the prescience was rewarded, and the column of stone is still to be seen in syria." whatever we may think of this statement it would certainly be interesting if we could find in syria or anywhere else a monument that recorded the ancient astronomical observations of the jews. ricard and others believe that they were very far advanced in the science, and that we owe a great part of our present astronomy to them; but such a conjecture must remain without proof unless we could prove them anterior to the other nations, whom, we have seen, cultivated astronomy in very remote times. one observation seems peculiar to them, if indeed it be a veritable observation. josephus says, "god prolonged the life of the patriarchs that preceded the deluge, both on account of their virtues, and to give them the opportunity of perfecting the sciences of geometry and astronomy which they had discovered; which they could not have done if they had not lived for years, because it is only after the lapse of years that the _great year_ is accomplished." now what is this great year or cycle of years? m. cassini, the director of the observatory of paris, has discussed it astronomically. he considers it as a testimony of the high antiquity of their astronomy. "this period," he says, "is one of the most remarkable that have been discovered; for, if we take the lunar month to be days h. m. s. we find that , - / days make , lunar months, and that this number of days gives solar years of days h. m. s. if this year was in use before the deluge, it appears very probable it must be acknowledged that the patriarchs were already acquainted to a considerable degree of accuracy with the motions of the stars, for this lunar month agrees to a second almost with that which has been determined by modern astronomers." a very similar argument has been used by prof. piazzi smyth to prove that the great pyramids were built by the descendants of abraham near the time of noah; namely, that measures of two different elements in the measurement of time or space when multiplied or divided produce a number which may be found to represent some proportion of the edifice, and hence to assume that the two numbers were known to the builders. we need scarcely point out that numbers have always been capable of great manipulation, and the mere fact of one number being so much greater than another, is no proof that _both_ were known, unless we knew that _one_ of them was known independently, or that they are intimately connected. in the case of josephus' number the cycle during which the lunar months and solar years are commensurable has been long discussed and if the number had been instead of , we should have had little doubt of its reference; yet is a very simple number and might refer to many other cycles than the complicated one pointed out by m. cassini. a similar case may be quoted with regard to the indians, which, according to our temperament, may be either considered a proof that these reasonings are correct, or that they are easy to make. they say that there are two stars diametrically opposite which pass through the zodiac in years; nothing can be made of this period, nor yet of another equally problematical one of years; but if we multiply the two together we obtain , , which is very nearly the length of the cycle for the precession of the equinoxes. in this review of the ancient ideas of different peoples, we have followed the most probable order in considering that the observation of nature came first, and the different parts of it were afterwards individualized and named. it is proper to add that according to some ancient authors--such as diodorus siculus--the process was considered to have been the other way. that uranus was an actual individual, that atlas and saturn were his sons or descendants or followers, and that because atlas was a great astronomer he was said to support the heavens, and that his seven daughters were real, and being very spiritual they were regarded as goddesses after death and placed in heaven under the name of the pleiades. however, the universality of the ideas seems to forbid this interpretation, which is also in itself much less natural. these various opinions lead us to remark, in conclusion, that the fables of ancient mythological astronomy must be interpreted by means of various keys. allegory is the first--the allegory employed by philosophers and poets who have spoken in figurative language. their words taken in the letter are quite unnatural, but many of the fables are simply the description or explanation of physical facts. hieroglyphics are another key. having become obscure by the lapse of time they sometimes, however, present ideas different from those which they originally expressed. it is pretty certain that hieroglyphics have been the source of the men with dogs' heads, or feet of goats, &c. fables also arise from the adoption of strange words whose sound is something like another word in the borrowing language connected with other ideas, and the connection between the two has to be made by fable. chapter ii. astronomy of the celts. the numerous stone monuments that are to be found scattered over this country, and over the neighbouring parts of normandy, have given rise to many controversies as to their origin and use. by some they have been supposed to be mere sepulchral monuments erected in late times since the roman occupation of great britain. such an idea has little to rest upon, and we prefer to regard them, as they have always been regarded, as relics of the druidical worship of the celtic or gaulish races that preceded us in this part of europe. if we were to believe the accounts of ordinary historians, we might believe that the druids were nothing more than a kind of savage race, hidden, like the fallow-deer in the recesses of their woods. thought to be sanguinary, brutal, superstitious, we have learned nothing of them beyond their human sacrifices, their worship of the oak, their raised stones; without inquiring whether these characteristics which scandalize our tastes, are not simply the legacy of a primitive era, to which, by the side of the tattered religions of the old paganism, druidism remained faithful. nevertheless the druids were not without merit in the order of thought. for the celts, as for all primitive people, astronomy and religion were intimately associated. they considered that the soul was eternal, and the stars were worlds successively inhabited by the spiritual emigrants. they considered that the stars were as much the abodes of human life as our own earth, and this image of the future life constituted their power and their grandeur. they repelled entirely the idea of the destruction of life, and preferred to see in the phenomena of death, a voyage to a region already peopled by friends. under what form did druidical science represent the universe? their scientific contemplation of the heavens was at the same time a religious contemplation. it is therefore impossible to separate in our history their astronomical and theological heavens. in their theological astronomy, or astronomical theology, the druids considered the totality of all living beings as divided into three circles. the first of these circles, the circle of immensity, _ceugant_, corresponding to incommunicable, infinite attributes, belonged to god alone; it was properly the absolute, and none, save the ineffable being, had a right there. the second circle, that of blessedness, _gwyn-fyd_, united in it the beings that have arrived at the superior degrees of existence; this was heaven. the third, the circle of voyages, _abred_, comprised all the noviciate; it was there, at the bottom of the abysses, in the great oceans, as taliesin says, that the first breath of man commenced. the object proposed to men's perseverance and courage was to attain to what the bards called the point of liberty, very probably the point at which, being suitably fortified against the assaults of the lower passions, they were not exposed to be troubled, against their wills, in their celestial aspirations; and when they arrived at such a point--so worthy of the ambition of every soul that would be its own master--they quitted the circle of abred and entered that of gwyn-fyd; the hour of their recompense had come. demetrius, cited by plutarch, relates that the druids believed that these souls of the elect were so intimately connected with our circle that they could not emerge from it without disturbing its equilibrium. this writer states, that being in the suite of the emperor claudius, in some part of the british isles, he heard suddenly a terrible hurricane, and the priests, who alone inhabited these sacred islands, immediately explained the phenomenon, by telling him that a vacuum had been produced on the earth, by the departure of an important soul. "the great men," he said, "while they live are like torches whose light is always beneficent and never harms any one, but when they are extinguished their death generally occasions, as you have just seen, winds, storm, and derangements of the atmosphere." the palingenetic system of the druids is complete in itself, and takes the being at his origin, and conducts him to the ultimate heaven. at the moment of his creation, as henry martyn says in his commentary, the being has no conscience of the gifts that are latent in him. he is created in the lowest stage of life, in _annwfn_, the shadowy abyss at the base of _abred_. there, surrounded by nature, submitted to necessity, he rises obscurely through the successive degrees of inorganic matter, and then through the organic. his conscience at last awakes. he is man. "three things are primarily contemporaneous--man, liberty, and light." before man there was nothing in creation but fatal obedience to physical laws; with man commences the great battle between liberty and necessity, good and evil. the good and the evil present themselves to man in equilibrium, "and he can at his pleasure attach himself to one or the other of them." it might appear at first sight that it was carrying things too far to attribute to the druids the knowledge, not indeed of the true system of the world, but the general idea on which it was constructed. but, on closer examination, this opinion seems to have some consistency. if it was from the druids that pythagoras derived the basis of his theology, why should it not be from them that he derived also that of his astronomy? why, if there is no difficulty in seeing that the principle of the subordination of the earth might arise from the meditations of an isolated spirit, should there be any more difficulty in thinking that the principles of astronomy should take birth in the midst of a corporation of theologians embued with the same ideas as the philosophers on the circulation of life, and applied with continued diligence to the study of celestial phenomena. the druid, not having to receive mythological errors, might be led by that circumstance to imagine in space other worlds similar to our own. independently of its intrinsic value, this supposition rests also upon the testimony of historians. a singular statement made by hecatæus with regard to the religious rites of great britain exhibits this in a striking manner. this historian relates that the moon, seen in this island, appears much larger than it does anywhere else, and that it is possible to distinguish mountains on its surface, such as there are on the earth. now, how had the druids made an observation of this kind? it is of not much consequence whether they had actually seen the lunar mountains or had only imagined them, the curious thing is that they were persuaded that that body was like the earth, and had mountains and other features similar to our own. plutarch, in his treatise _de facie in orbe lunæ_, tells us that, according to the druids, and conformably to an idea which had long been held in science, the surface of the moon is furrowed with several mediterraneans, which the grecian philosophers compare to the red and caspian seas. it was also thought that immense abysses were seen, which were supposed to be in communication with the hemisphere that is turned away from the earth. lastly, the dimensions of this sky-borne country were estimated; (ideas very different to those that were current in greece): its size and its breadth, says the traveller depicted by the writer, are not at all such as the geometers say, but much larger. it is through the same author, who is in accordance in this respect with all the bards, that we know that this celestial earth was considered by the theologians of the west as the residence of happy souls. they rose and approached it in proportion as their preparation had been complete, but, in the agitation of the whirlwind, many reached the moon that it would not receive. "the moon repelled a great number, and rejected them by its fluctuations, at the moment they reached it; but those that had better success fixed themselves there for good; their soul is like the flame, which, raising itself in the ether of the moon, as fire raises itself on that of the earth receives force and solidity in the same way that red-hot iron does when plunged into the water." they thus traced an analogy between the moon and the earth, which they doubtless carried out to its full development, and made the moon an image of what they knew here, picturing there the lunar fields and brooks and breezes and perfumes. what a charm such a belief must have given to the heavens at night. the moon was the place and visible pledge of immortality. on this account it was placed in high position in their religion; the order of all the festivals was arranged after that which was dedicated to it; its presence was sought in all their ceremonies, and its rays were invoked. the druids are always therefore represented as having the crescent in their hands. astronomy and theology being so intimately connected in the spirit of the druids, we can easily understand that the two studies were brought to the front together in their colleges. from certain points of view we may say that the druids were nothing more than astronomers. this quality was not less striking to the ancients in them than in the chaldæans. the observation of the stars was one of their official functions. cæsar tells us, without entering more into particulars, that they taught many things about _the form and dimensions of the earth, the size and arrangements of the different parts of heaven, and the motions of the stars_, which includes the greater part of the essential problems of celestial geometry, which we see they had already proposed to themselves. we can see the same fact in the magnificent passage of taliesin. "i will ask the bards," he says in his _hymn of the world_, "and why will not the bards answer me? i will ask of them what sustains the earth, since having no support it does not fall? or if it falls which way does it go? but what can serve for its support? is the world a great traveller? although it moves without ceasing, it remains tranquil in its route; and how admirable is that route, seeing that the world moves not in any direction." this suffices to show that the ideas of the druids on material phenomena were not at all inferior to their conceptions of the destiny of the soul, and that they had scientific views of quite another origin from the alexandrian greeks, the latins, their disciples, or the middle ages. an anecdote of the eighth century furnishes another proof in favour of druidical science. every one knows that virgilius, bishop of salzburg, was accused of heresy by boniface before the pope zacharias, because he had asserted that there were antipodes. now virgilius was educated in one of the learned monasteries of ireland, which were fed by the christian bards, who had preserved the scientific traditions of druidism. [illustration: plate ii.--druidical worship.] the fundamental alliance between the doctrine of the plurality of worlds and of the eternity of the soul is perhaps the most memorable character in the thoughts of this ancient race. the death upon earth was for them only a psychological and astronomical fact, not more grave than that which happened to the moon when it was eclipsed, nor the fall of the verdant clothing of the oak under the breath of the autumnal breeze. we see these conceptions and manners, at first sight so extraordinary, clothe themselves with a simple and natural aspect. the druids were so convinced of the future life in the stars, that they used _to lend money to be repaid in the other world_. such a custom must have made a profound impression on the minds of those who daily practised it. pomponius mela and valerius maximus both tell us of this custom. the latter says, "after having left marseilles i found that ancient custom of the gauls still in force, namely, of lending one another money to be paid back in the infernal regions, for they are persuaded that the souls of men are immortal." in passing to the other world they lost neither their personality, their memory, nor their friends; they there re-encountered the business, the laws, the magistrates of this world. they had capitals and everything the same as here. they gave one another rendezvous as emigrants might who were going to america. this superstition, so laudable as far as it had the effect of pressing on the minds of men the firm sentiment of immortality, led them to burn, along with the dead, all the objects which had been dear to them, or of which they thought they might still wish to make use. "the gauls," says pomponius mela, "burn and bury with the dead that which had belonged to the living." they had another custom prompted by the same spirit, but far more touching. when any one bade farewell to the earth, each one charged him to take letters to his absent friends, who should receive him on his arrival and doubtless load him with questions as to things below. it is to diodorus that we owe the preservation of the remembrance of this custom. "at their funerals," he says, "they place letters with the dead which are written to those already dead by their parents, so that they may be read by them." they followed the soul in thought in its passage to the other planets, and the survivors often regretted that they could not accomplish the voyage in their company; sometimes, indeed, they could not resist the temptation. "there are some," says mela, "who burn themselves with their friends in order that they may continue to live together." they entertained another idea also, which led even to worse practices than this, namely, that death was a sort of recruiting that was commanded by the laws of the universe for the sustenance of the army of existences. in certain cases they would replace one death by another. posidonius, who visited gaul at an epoch when it had not been broken up, and who knew it far better than cæsar, has left us some very curious information on this subject. if a man felt himself seriously warned by his disease that he must hold himself in readiness for departure, but who, nevertheless, had, for the moment, some important business on hand, or the needs of his family chained him to this life, or even that death was disagreeable to him; if no member of his family or his clients were willing to offer himself instead, he looked out for a substitute; such a one would soon arrive accompanied by a troop of friends, and stipulating for his price a certain sum of money, he distributed it himself as remembrances among his companions,--often even he would only ask for a barrel of wine. then they would erect a stage, improvise a sort of festival, and finally, after the banquet was over, our hero would lie down on the shield, and driving a sword into his bosom, would take his departure for the other world. such a custom, indeed, shows anything but what we should rightly call civilization, however admirable may have been their opinions; but it receives its only palliation from the fact that their indifference to death did not arise from their undervaluing life here, but that they had so firm a belief in the existence and the happiness of a life hereafter. that these beliefs were not separated from their astronomical ideas is seen from the fact that they peopled the firmament with the departed. the milky way was called the town of gwyon (coër or ker gwydion, ker in breton, caer in gaulish, kohair in gaelic); certain bardic legends gave to gwyon as father a genius called don, who resides in the constellation of cassiopeia, and who figures as "the king of the fairies" in the popular myths of ireland. the empyrean is thus divided between various heavenly spirits. arthur had for residence the great bear, called by the druids "arthur's chariot." we are not, however, entirely limited to tradition and the reports of former travellers for our information as to the astronomy of the druids, but we have also at our service numerous coins belonging to the old gauls, who were of one family with those who cultivated druidism in our island, which have been discovered buried in the soil of france. the importance which was given to astronomy in that race becomes immediately evident upon the discovery of the fact that these coins are marked with figures having reference to the heavenly bodies, in other words are astronomical coins. if we examine, from a general point of view, a large collection of gaulish medals such as that preserved in the national museum of paris, we observe that among the essential symbols that occupy the fields are types of the horse, the bull, the boar, the eagle, the lion, the horseman, and the bear. we remark next a great number of signs, most often astronomical, ordinarily accessory, but occasionally the chief, such as the sign [symbol: rotated mirrored s], globules surrounded by concentric circles, stars of five, six, or eight points, radiated and flaming bodies, crescents, triangles, wheels with four spokes, the sign [symbol: infinity], the lunar crescent, the zigzag, &c. lastly, we remark other accessory types represented by images of real objects or imaginary figures, such as the lyre, the diota, the serpent, the hatchet, the human eye, the sword, the bough, the lamp, the jewel, the bird, the arrow, the ear of corn, the fishes, &c. on a great number of medals, on the stateres of vercingetorix, on the reverses of the coins of several epochs, we recognize principally the sign of the waterer, which appears to symbolize for one part of antiquity the knowledge of the heavenly sphere. on the gaulish types this sign (an amphora with two handles) bears the name of diota, and represents amongst the druids as amongst the magi the sciences of astronomy and astrology. some of these coins are represented in the woodcut below. [illustration: fig. .] the first of these represents the course of the sun-horse reaching the tropic of cancer (summer solstice), and brought back to the tropic of capricorn (winter solstice). on the second is seen the symbol of the year between the south (represented by the sun [symbol: sun]) and the north (represented by the northern bear). in the third the calendar (or course of the year) between the sun [symbol: sun] and the moon [symbol: moon]. time the sun, and the bear are visible on the fourth. the diurnal motion of the heavens is represented on the fifth; and lastly, on the sixth, appears the watering-pot, the sun-horse, and the sign of the course of the heavenly bodies. on other groups of money the presence of the zodiac may be made out. these medals would seem to show that some part of the astronomical knowledge of the druids was not invented by themselves, but borrowed from the chaldeans or others who in other lands invented them in previous ages, and from whom they may have possibly derived them from the phenicians. we may certainly expect, however, from these pieces of money, if found in sufficient number and carefully studied, to discover a good many positive facts now wanting to us, of the religion, sciences, manners, language, commercial relation, &c. which belonged to the celtic civilization. it was far from being so barbarous as is ordinarily supposed, and we shall do more justice to it when we know it better. m. fillioux, the curator of the museum of guéret, who has studied these coins with care, after having sought for a long time for a clear and concise method of determining exactly the symbolic and religious character of the gaulish money, has been able to give the following general statements. the coins have for their ordinary field the heavens. on the right side they present almost universally the ideal heads of gods or goddesses, or in default of these, the symbols that are representative of them. on the reverse for the most part, they reproduce, either by direct types or by emblems artfully combined, the principal celestial bodies, the divers aspects of the constellations, and probably the laws, which, according to their ancient science, presided over their course; in a smaller proportion they denote the religious myths which form the base of the national belief of the gauls. as we have seen above, for them the present life was but a transitory state of the soul, only a prodrome of the future life, which should develop itself in heaven and the astronomical worlds with which it is filled. borrowed from an elevated spiritualism, incessantly tending towards the celestial worlds, these ideas were singularly appropriate to a nation at once warlike and commercial. these circumstances explain the existence of these strange types, founded at the same time on those of other nations, and on the symbolism which was the soul of the druidical religion. to this religious caste, indeed, we must give the merit of this ingenious and original conception, of turning the reverses of the coins into regular charts of the heavens. nothing indeed could be better calculated to inspire the people with respect and confidence than these mysterious and learned symbols, representing the phenomena of the heavens. not making use of writing to teach their dogmas, which they wished to maintain as part of the mysteries of their caste, the druids availed themselves of this method of placing on the money that celestial symbolism of which they alone possessed the key. the religious ideas founded on astronomical observations were not peculiar to, or originated by, the druids, any more than their zodiac. there seems reason to believe that they had come down from a remote antiquity, and been widely spread over many nations, as we shall see in the chapter on the pleiades; but we can certainly trace them to the east, where they first prevailed in persia and egypt, and were afterwards brought to greece, where they disappeared before the new creations of anthropomorphism, though they were not forgotten in the days of the poet anacreon, who says, "do not represent for me, around this vase" (a vase he had ordered of the worker in silver), "either the heavenly bodies, or the chariot, or the melancholy orion; i have nothing to do with the pleiades or the herdsman." he only wanted mythological subjects which were more to his taste. the characters which are made use of in these astronomical moneys of the druids would appear to have a more ancient origin than we are able to trace directly, since they are most of them found on the arms and implements of the bronze age. some of them, such as the concentric pointed circles, the crescent with a globule or a star, the line in zigzag, were used in egypt; where they served to mark the sun, the month, the year, the fluid element; and they appear to have had among the druids the same signification. the other signs, such as the [symbol: wave], and its multiple combinations, the centred circles, grouped in one or two, the little rings, the alphabetical characters recalling the form of a constellation, the wheel with rays, the radiating discs, &c. are all represented on the bronze arms found in the celtic, germanic, breton, and scandinavian lands. from this remote period, which was strongly impressed with the oriental genius, we must date the origin of the celtic symbolism. it has been supposed, and not without reason, that this epoch, besides being contemporaneous with the phenician establishments on the borders of the ocean, was an age of civilization and progress in gaul, and that the ideas of the druids became modified at the same time that they acquired just notions in astronomy and in the art of casting metals. at a far later period, the druidic theocracy having, with religious care, preserved the symbols of its ancient traditions, had them stamped on the coins which they caused to be struck. this remarkable fact is shown in an incontestable manner in the rougher attempts in gaulish money, and this same state of things was perpetuated even into the epoch of the high arts, since we find on the imitation statues of macedonia the old celtic symbols associated with emblems of a grecian origin. in italy a different result was arrived at, because the warlike element of the nobles soon predominated over the religious. nevertheless the most ancient roman coins, those which are known to us under the name of consular, have not escaped the common law which seems to have presided, among all nations, over the origin of money. the two commonest types, one in bronze of _janus bifrons_ with the _palus_; the other in silver, the _dioscures_ with their stars, have an eminently astronomical aspect. the comparison between the gaulish and roman coins may be followed in a series of analogies which are very remarkable from an astronomical point of view. to cite only a few examples, we may observe on a large number of pennies of different families, the impression of auriga "the coachman" conducting a quadriga; or the sun under another form (with his head radiated and drawn in profile); or diana with her lunar attributes; or the five planets well characterised; for example, venus by a double star, as that of the morning or of the evening; or the constellations of the dog, hercules, the kid, the lyre, and almost all those of the zodiac and of the circumpolar region and the seven-kine (septemtriones). in later times, under the cæsars, in the villa of borghèse, is found a calendar whose arrangements very much recall the ancient gaulish coin. the head of the twelve great gods and the twelve signs of the zodiac are represented, and the drawing of the constellations establishes a correspondence between their rising and the position of the sun in the zodiac. it may therefore be affirmed that in the coinage and works of art in italy and greece, the characteristic influence of astronomical worship is found as strongly as among the druids. nor have the western nations alone had the curious habit of impressing their astronomical ideas upon their coinage, for in china and japan coins of a similar description have been met with, containing on their reverse all the signs of the zodiac admitted by them. in conclusion, we may say, that it was cosmography, that constructed the dogmas of the druidical religion, which was, in its essential elements, the same as that of the old oriental theocracies. the outward ceremonies were addressed to the sun, the moon, the stars, and other visible phenomena; but, above nature, there was the great generating and moving principle, which the celts placed, at a later period perhaps, among the attributes of their supreme deities. [illustration: the northern constellations. the lyre--cassiopeia--the little bear--the dragon--andromeda--the great bear--capella--algol, or medusa's head.] chapter iii. origin of the constellations. when we look upon the multitude of heavenly bodies with which the celestial vault is strewed, our attention is naturally arrested by certain groupings of brilliant stars, apparently associated together on account of their great proximity; and also by certain remarkable single stars which have excessive brilliancy or are completely isolated from the rest. these natural groups seem to have some obscure connection with or dependence on each other. they have always been noticed, even by the most savage races. the languages of several such races contain different names for the same identical groups, and these names, mostly borrowed from terrestrial beings, give an imaginary life to the solitude and silence of the skies. a celestial globe, as we know, presents us with a singular menagerie, rich in curious monsters placed in inconceivable positions. how these constellations, as they are called, were first invented, and by whom, is an interesting question which by the aid of comparative philology we must endeavour now to answer. among these constellations there are twelve which have a more than ordinary importance, and to which more attention has always been paid. they are those through which the sun appears to pass in his annual journey round the ecliptic, entering one region each month. at least, this is what they were when first invented. they were called the zodiacal constellations or signs of the zodiac--the name being derived from their being mostly named after living beasts. in our own days the zodiacal constellations are no longer the signs of the zodiac. when they were arranged the sun entered each one on a certain date. he now is no longer at the same point in the heavens at that date, nevertheless he is still said to enter the same sign of the zodiac--which therefore no longer coincides with the zodiacal constellation it was named from--but merely stands for a certain twelfth part of the ecliptic, which varies from time to time. it will be of course of great interest to discover the origin of these particular constellations, the date of their invention, &c.; and we shall hope to do so after having discussed the origin of those seen in the northern hemisphere which may be more familiar even than those. we have represented in the frontispiece the two halves of the grecian celestial sphere--the northern and the southern, with the various constellations they contain. this sphere was not invented by the greeks, but was received by them from more ancient peoples, and corrected and augmented. it was used by hipparchus two thousand years ago; and ptolemy has given us a description of it. it contained constellations, of which belonged to the northern, to the southern hemisphere, and the remaining twelve were those of the zodiac, situated along the ecliptic. the constellations reckoned by ptolemy contained altogether , stars, whose relative positions were determined by hipparchus; with reference to which accomplishment pliny says, "hipparchus, with a height of audacity too great even for a god, has ventured to transmit to posterity the number of the stars!" ptolemy's catalogue contains:-- for the northern constellations stars for the zodiacal " for the southern " or ----- for all the constellations , " or, since of these are named twice , " of course this number is not to be supposed to represent the whole of the stars visible even to the naked eye; there are twice as many in the northern hemisphere alone, while there are about , in the whole sky. the number visible in a telescope completely dwarfs this, so that more than , are now catalogued; while the number visible in a large telescope may be reckoned at not less than millions. the principal northern constellations named by ptolemy are contained in the following list, with the stars of the first magnitude that occur in each:-- the great bear, or david's chariot, near the centre. the little bear, with the pole star at the end of the tail. the dragon. cepheus, situated to the right of the pole. the herdsman, or the keeper of the bear, with the star arcturus. the northern crown to the right. hercules, or the man who kneels. the lyre, or falling vulture, with the beautiful star vega. the swan, or bird, or cross. cassiopeia, or the chair, or the throne. perseus. the carter, or the charioteer, with capella ophiuchus, or serpentarius, or esculapius. the serpent. the bow and arrow, or the dart. the eagle, or the flying vulture, with altaïr. the dolphin. the little horse, or the bust of the horse. pegasus, or the winged horse, or the great cross. andromeda, or the woman with the girdle. the northern triangle, or the delta. the fifteen constellations on the south of the ecliptic were:-- the whale. orion, with the beautiful stars rigel and betelgeuse. the river endanus, or the river orion, with the brilliant achernar. the hare. the great dog, with the magnificent sirius. the little dog, or the dog which runs before, with procyon. the ship argo, with its fine alpha (canopus) and eta. the female hydra, or the water snake. the cup, or the urn, or the vase. the raven. the altar, or the perfuming pot. the centaur, whose star alpha is the nearest to the earth. the wolf, or the centaur's lance, or the panther, or the beast. the southern crown, or the wand of mercury, or uraniscus. the southern fish, with fomalhaut. the twelve zodiacal constellations, which are of more importance than the rest, are generally named in the order in which the sun passes through them in its passage along the ecliptic, and both latins and english have endeavoured to impress their names on the vulgar by embodying them in verses. the poet ausonius thus catalogues them:-- "sunt: aries, taurus, gemini, cancer, leo, virgo, libraque, scorpius, arcitenens, caper, amphora, pisces." and the english effusion is as follows:-- "the ram, the bull, the heavenly twins, and next the crab the lion shines, the virgin and the scales. the scorpion, archer, and he goat, the man that holds the watering-pot, and fish with glittering scales." these twelve have hieroglyphics assigned to them, by which they are referred to in calendars and astronomical works, some of the marks being easily traced to their origin. thus [symbol: aries] refers to the horns of the ram; [symbol: taurus] to the head of the bull; [symbol: scorpion] to the joints and tail-sting of the scorpion; [symbol: saggitarius] is very clearly connected with an archer; [symbol: capricorn] is formed by the junction of the first two letters [greek: t] and [greek: r] in [greek: tragos], the sea-goat, or capricorn; [symbol: libra] for the balance, is suggestive of its shape; [symbol: aquarius] refers to the water in the watering-pot; and perhaps [symbol: pisces] to the two fishes; [symbol: gemini] for twins may denote two sides alike; [symbol: cancer] for the crab, has something of its side-walking appearance; while [symbol: leo] for the lion, and [symbol: virgo] for the virgin, seem to have no reference that is traceable. these constellations contain the following stars of the first magnitude--aldebaran, antares, and spica. to these constellations admitted by the greeks should be added the locks of berenice, although it is not named by ptolemy. it was invented indeed by the astronomer conon. the story is that berenice was the spouse and the sister of ptolemy euergetes, and that she made a vow to cut off her locks and devote them to venus if her husband returned victorious; to console the king the astronomer placed her locks among the stars. if this is a true account arago must be mistaken in asserting that the constellation was created by tycho brahe in . the one he did add to the former ones was that of antinöus, by collecting into one figure some unappropriated stars near the eagle. at about the same time j. bayer, from the information of vespuccius and the sailors, added twelve to the southern constellations of ptolemy; among which may be mentioned the peacock, the toucan, the phoenix, the crane, the fly, the chameleon, the bird of paradise, the southern triangle, and the indian. augustus royer, in , formed five new groups, among which we may name the great cloud, the fleur-de-lis, and the southern cross. hevelius, in , added ; the most important being the giraffe, the unicorn, the little lion, the lynx, the little triangle. among these newer-named constellations none is more interesting than the southern cross, which is by some considered as the most brilliant of all that are known. some account of it, possibly from the arabs, seems to have reached dante, who evidently refers to it, before it had been named by royer, in a celebrated passage in his "purgatory." some have thought that his reference to such stars was only accidental, and that he really referred only to the four cardinal virtues of theology, chiefly on account of the difficulty of knowing how he could have heard of them; but as the arabs had establishments along the entire coast of africa, there is no difficulty in understanding how the information might reach italy. americus vespuccius, who in his third voyage refers to these verses of dante, does not mention the name of the southern cross. he simply says that the four stars form a rhomboidal figure. as voyages round the cape multiplied, however, the constellation became rapidly more celebrated, and it is mentioned as forming a brilliant cross by the florentine andrea corsali, in , and a little later by pigafetta, in . all these constellations have not been considered sufficient, and many subsequent additions have been made. thus lacaille, in , created fourteen new ones, mostly characterized by modern names--as the sculptor's studio, the chemical furnace, the clock, the compass, the telescope, the microscope, and others. lemonnier, in , added the reindeer, the solitaire, and the indian bird, and lalande the harvestman. poczobut, in , added one more, and p. hell another. finally, in the charts drawn by bode, eight more appear, among which the aerostat, and the electrical and printing machines. we thus arrive at a total of constellations. to which we may add that the following groups are generally recognized. the head of medusa, near perseus; the pleiades, on the back, and the hyades on the forehead of the bull; the club of hercules; the shield of orion, sometimes called the rake; the three kings; the staff of s. james; the sword of orion; the two asses in the crab, having between them the star cluster, called the stall, or the manger; and the kids, near capella, in the constellation of the coachman. this brings the list of the constellations to , which is the total number now admitted. a curious episode with respect to these star arrangements may here be mentioned. about the eighth century bede and certain other theologians and astronomers wished to depose the olympian gods. they proposed, therefore, to change the names and arrangements of the constellations; they put s. peter in the place of the ram; s. andrew instead of the bull; and so on. in more recent calendars david, solomon, the magi, and other new and old testament characters were placed in the heavens instead of the former constellations; but these changes of name were not generally adopted. as an example of these celestial spheres we figure a portion of one named _coeli stellati christiani hemisphericum prius_. we here see the great bear replaced by the barque of s. peter, the little bear by s. michael, the dragon by the innocents, the coachman by s. jerome, perseus by s. paul, cassiopeia by the magdalene, andromache by s. sepulchre, and the triangle by s. peter's mitre; while for the zodiac were substituted the twelve apostles. [illustration: fig. .] in the seventeenth century a proposal was made by weigel, a professor in the university of jena, to form a series of heraldic constellations, and to use for the zodiac the arms of the twelve most illustrious families in europe; but these attempts at change have been in vain, the old names are still kept. having now explained the origin in modern times of out of the constellations, there remain the which were acknowledged by the greeks, whose origin is involved in more obscurity. one of the first to be noticed and named, as it is now the most easily recognized and most widely known, is the _great bear_, which attracts all the more attention that it is one of those that never sets, being at a less distance from the pole than the latter is from the horizon. every one knows the seven brilliant stars that form this constellation. the four in the rectangle and the three in a curved line at once call to mind the form of a chariot, especially one of antique build. it is this resemblance, no doubt, that has obtained for the constellation the name of "the chariot" that it bears among many people. among the ancient gauls it was "arthur's chariot." in france it is "david's chariot," and in england it goes by the name of "king charles' wain," and by that of the "plough." the latter name was in vogue, too, among the latins (_plaustrum_), and the three stars were three oxen, from whence it would appear that they extended the idea to all the seven stars, and at last called them the _seven_ oxen, _septem-triones_, from whence the name sometimes used for the north--septentrional. the greeks also called it the chariot ([greek: hamaxa]), and the same word seems to have stood sometimes for a plough. it certainly has some resemblance to this instrument. if we take the seven stars as representing the characteristic points of a chariot, the four stars of the quadrilateral will represent the four wheels, and the three others will represent the three horses. above the centre of the three horses any one with clear sight may perceive a small star of the fifth or sixth magnitude, called the cavalier. each of these several stars is indicated, as is usual with all the constellations, by a greek letter, the largest being denoted by the first letter. thus the stars in the quadrilateral are [greek: a], [greek: b], [greek: g], [greek: d], and the tail stars [greek: e], [greek: x], [greek: ê]. the arabs give to each star its special name, which in this case are as follows:--dubhé and mérak are the stars at the back; phegda and megrez those of the front; alioth, mizat, and ackïar the other three, while the little one over mizat is alcor. another name for it is saidak, or the tester, the being able to see it being a mark of clear vision. there is some little interest in the great bear on account of the possibility of its being used as a kind of celestial time-keeper, and its easy recognition makes it all the more available. the line through [greek: a] and [greek: b] passes almost exactly through the pole. now this line revolves of course with the constellation round the pole in hours; in every such interval being once, vertical above the pole, and once vertical below, taking the intermediate positions to right and left between these times. the instant at which this line is vertical over the pole is not the same on any two consecutive nights, since the stars advance each day minutes on the sun. on the st of march the superior passage takes place at minutes to at night; on the following night four minutes earlier, or at minutes to . in three months the culmination takes place hours earlier, or at minutes to . in six months, _i.e._ on sept. , it culminates at . in the morning, being vertically below the pole at the same hour in the evening. the following woodcut exhibits the positions of the great bear at the various hours of september th. it is plain from this that, knowing the day of the month, the hour of the night may be told by observing what angle the line joining [greek: a] and [greek: b] of this constellation makes with the vertical. [illustration: fig. .] we have used the name _great bear_, by which the constellation is best known. it is one of the oldest names also, being derived from the greeks, who called it arctos megale ([greek: arktos megalê]), whence the name arctic; and singularly enough the iroquois, when america was discovered, called it okouari, their name for a bear. the explanation of this name is certainly not to be found in the resemblance of the constellation to the animal. the three stars are indeed in the tail, but the four are in the middle of the back; and even if we take in the smaller stars that stand in the feet and head, no ingenuity can make it in this or any other way resemble a bear. it would appear, as aristotle observes, that the name is derived from the fact, that of all known animals the bear was thought to be the only one that dared to venture into the frozen regions of the north and tempt the solitude and cold. [illustration: fig. .] other origins of the name, and other names, have been suggested, of which we may mention a few. for example, "ursa" is said to be derived from _versus_, because the constellation is seen to _turn_ about the pole. it has been called the screw ([greek: elikê]), or helix, which has plainly reference to its turning. another name is callisto, in reference to its beauty; and lastly, among the arabs the great and little bears were known as the great and little coffins in reference to their slow and solemn motion. these names referred to the four stars of each constellation, the other three being the mourners following the bearers. the christian arabs made it into the grave of lazarus and the three weepers, mary, martha, and their maid. next as to the little bear. this constellation has evidently received its name from the similarity of its form to that of the great bear. in fact, it is composed of seven stars arranged in the same way, only in an inverse order. if we follow the line from [greek: b] to [greek: a] of the great bear to a distance of five times as great as that between these stars we reach the brightest star of the little bear, called the pole star. all the names of the one constellation have been applied to the other, only at a later date. the new constellations were added one by one to the celestial sphere by the greeks before they arranged certain of them as parts of the zodiac. the successive introduction of the constellations is proved completely by a long passage of strabo, which has been often misunderstood. "it is wrong," he says, "to accuse homer of ignorance because he speaks only of one of the two celestial bears. the second was probably not formed at that time. the phenicians were the first to form them and to use them for navigation. they came later to the greeks." [illustration: the constellations from the sea-shore. the swan--the lyre--hercules--the crown--the herdsman--the eagle--the serpent--the balance--the scorpion--sagittarius.] all the commentators on homer, hygin and diogenes laertes, attribute to thales the introduction of this constellation. pseudo-eratosthenes called the little bear [greek: phoinikê], to indicate that it was a guide to the phenicians. a century later, about the seventeenth olympiad, cleostrates of tenedos enriched the sphere with the archer ([greek: toxotês], sagittarius) and the ram ([greek: krios], aries), and about the same time the zodiac was introduced into the grecian sphere. with regard to the little bear there is another passage of strabo which it will be interesting to quote. he says--"the position of the people under the parallel of cinnamomophore, _i.e._ , stadia south of meroe and , stadia north of the equator, represents about the middle of the interval between the equator and the tropic, which passes by syene, which is , stadia north of meroe. these same people are the first for whom the little bear is comprised entirely in the arctic circle and remains always visible; the most southern star of the constellation, the brilliant one that ends the tail being placed on the circumference of the arctic circle, so as just to touch the horizon." the remarkable thing in this passage is that it refers to an epoch anterior to strabo, when the star [greek: a] of the little bear, which now appears almost immovable, owing to its extreme proximity to the pole, was then more to the south than the other stars of the constellation, and moved in the arctic circle so as to touch the horizon of places of certain latitudes, and to set for latitudes nearer the equator. in those days it was not the _pole_ star--if that word has any relation to [greek: poleô], i turn--for the heavens did not turn about it then as they do now. the grecian geographer speaks in this passage of a period when the most brilliant star in the neighbourhood of the pole was [greek: a] of the dragon. this was more than three thousand years ago. at that time the little bear was nearer to the pole than what we now call the polar star, for this latter was "the most southern star in the constellation." if we could alight upon documents dating back fourteen thousand years, we should find the star vega ([greek: a] lyra) referred to as occupying the pole of the world, although it now is at a distance of degrees from it, the whole cycle of changes occupying a period of about twenty-six thousand years. before leaving these two constellations we may notice the origin of the names according to plutarch. he would have it that the names are derived from the use that they were put to in navigation. he says that the phenicians called that constellation that guided them in their route the _dobebe_, or _doube_, that is, the speaking constellation, and that this same word happens to mean also in that language a bear; and so the name was confounded. certainly there is still a word _dubbeh_ in arabic having this signification. next as to the herdsman. the name of its characteristic star and of itself, arcturus ([greek: arktos], bear; [greek: ouros], guardian), is explained without difficulty by its position near the bears. there are six small stars of the third magnitude in the constellation round its chief one--three of its stars forming an equilateral triangle. arcturus is in the continuation of the curved line through the three tail stars of the great bear. the constellation has also been called atlas, from its nearness to the pole--as if it held up the heavens, as the fable goes. beyond this triangle, in the direction of the line continued straight from the great bear, is the northern crown, whose form immediately suggests its name. among the stars that compose it one, of the second magnitude, is called the pearl of the crown. it was in this point of the heavens that a temporary star appeared in may, , and disappeared again in the course of a few weeks. among the circumpolar constellations we must now speak of cassiopeia, or the chair--or throne--which is situated on the opposite side of the pole from the great bear; and which is easily found by joining its star [greek: d] to the pole and continuing it. the chair is composed principally of five stars, of the third magnitude, arranged in the form of an m. a smaller star of the fourth magnitude completes the square formed by the three [greek: b], [greek: a], and [greek: g]. the figure thus formed has a fair resemblance to a chair or throne, [greek: d] and [greek: e] forming the back; and hence the justification for its popular name. the other name cassiopeia has its connection and meaning unknown. we may suitably remark in this place, with arago, that no precise drawing of the ancient constellations has come down to us. we only know their forms by written descriptions, and these often very short and meagre. a verbal description can never take the place of a drawing, especially if it is a complex figure, so that there is a certain amount of doubt as to the true form, position, and arrangement of the figures of men, beasts, and inanimate objects which composed the star-groups of the grecian astronomers--so that unexpected difficulties attend the attempt to reproduce them on our modern spheres. add to this that alterations have been avowedly introduced by the ancient astronomers themselves, among others by ptolemy, especially in those given by hipparchus. ptolemy says he determined to make these changes because it was necessary to give a better proportion to the figures, and to adapt them better to the real positions of the stars. thus in the constellation of the virgin, as drawn by hipparchus, certain stars corresponded to the shoulders; but ptolemy placed them in the sides, so as to make the figure a more beautiful one. the result is that modern designers give scope to their imagination rather than consult the descriptions of the greeks. _cassiopeia_, _cepheus_, _andromeda_, and _perseus_ holding in his hand the _head of medusa_, appear to have been established at the same epoch, no doubt subsequently to the great bear. they form one family, placed together in one part of the heavens, and associated in one drama; the ardent perseus delivering the unfortunate andromeda, daughter of cepheus and cassiopeia. we can never be sure, however, whether the constellations suggested the fable, or the fable the constellations: the former may only mean that perseus, rising before andromeda, seems to deliver it from the night and from the constellation of the whale. the head of medusa, a celebrated woman, that perseus cut off and holds in his hand, is said by volney to be only the head of the constellation virgo, which passes beneath the horizon precisely as the perseus rises, and the serpents which surround it are ophiucus and the polar dragon, which then occupies the zenith. either way, we have no account of the origin of the _names_, and it is possible that we may have to seek it, if ever we find it, from other sources--for it would appear that similar names were used for the same constellations by the indians. this seems inevitably proved by what is related by wilford (_asiatic researches_, iii.) of his conversation with his pundit, an astronomer, on the names of the indian constellations. "asking him," he says, "to show me in the heavens the constellation of antarmada, he immediately pointed to andromeda, though i had not given him any information about it beforehand. he afterwards brought me a very rare and curious work in sanscrit, which contained a chapter devoted to _upanacchatras_, or extra-zodiacal constellations, with drawings of _capuja_ (cepheus), and of _casyapi_ (cassiopeia) seated and holding a lotus flower in her hand, of _antarmada_ charmed with the fish beside her, and last of _parasiea_ (perseus) who, according to the explanation of the book, held the head of a monster which he had slain in combat; blood was dropping from it, and for hair it had snakes." as the stars composing a constellation have often very little connection with the figure they are supposed to form, when we find the same set of stars called by the same name by two different nations, as was the case, for instance, in some of the indian names of constellations among the americans, it is a proof that one of the nations copied it from the other, or that both have copied from a common source. so in the case before us, we cannot think these similar names have arisen independently, but must conclude that the grecian was borrowed from the indian. another well-known constellation in this neighbourhood, forming an isosceles triangle with arcturus and the pole star, is the lyre. lucian of samosatus says that the greeks gave this name to the constellation to do honour to the lyre of orpheus. another possible explanation is this. the word for lyre in greek [greek: chelys] and in latin (_testudo_) means also a tortoise. now at the time when this name was imposed the chief star in the lyre may have been very near to the pole of the heavens and therefore have had a very slow motion, and hence it might have been named the tortoise, and this in greek would easily be interpreted into lyre instead. indeed this double meaning of the word seems certainly to have given rise to the fable of mercury having constructed a lyre out of the back of a tortoise. circling round the pole of the ecliptic, and formed by a sinuous line of stars passing round from the great bear to the lyre, is the dragon, which owes its name to its form. its importance is derived from its relation to the ecliptic, the pole of which is determined by reference to the stars of the first coil of the body. the centre of the zodiacal circle is a very important point, that circle being traced on the most ancient spheres, and probably being noticed even before the pole of the heavens. closely associated with the dragon both in mythology and in the celestial sphere is hercules. he is always drawn kneeling; in fact, the constellation is rather a man in a kneeling posture than any particular man. the poets called it engonasis with reference to this, which is too melancholy or lowly a position than would agree well with the valiant hero of mythology. there is a story related by Æschylus about the stones in the champ des cailloux, between marseilles and the embouchure of the rhône, to the effect that hercules, being amongst the ligurians, found it necessary to fight with them; but he had no more missiles to throw; when jupiter, touched by the danger of his son, sent a rain of round stones, with which hercules repulsed his enemies. the engonasis is thus considered by some to represent him bending down to pick up the stones. posidonius remarks that it was a pity jupiter did not rain the stones on the ligurians at once, without giving hercules the trouble to pick them up. ophiucus, which comes close by, simply means the man that holds the serpent [greek: ophi-ouchos]. it is obviously impossible to know the origins of all the names, as those we now use are only the surviving ones of several that from time to time have been applied to the various constellations according to their temporary association with the local legends. the prominent ones are favoured with quite a crowd of names. we need only cite a few. hercules, for instance, has been called [greek: okalzôn korynêtês], engonasis, ingeniculus, nessus, thamyris, desanes, maceris, almannus, al-chete, &c. the swan has the names of [greek: kyknos], [greek: iktin], [greek: ornis], olar, helenæ genitor, ales jovis, ledæus, milvus, gallina, the cross, while the coachman has been [greek: ippilatês], [greek: elastippos], [greek: airôêlatês], [greek: Êniochos], auriga, acator, hemochus, erichthonus, mamsek, alánat, athaiot, alatod, &c. with respect to the coachman, in some old maps he is drawn with a whip in his left hand turned towards the chariot, and is called the charioteer. no doubt its proximity to the former constellation has acquired for it its name. the last we need mention, as of any celebrity, is that of orion, which is situated on the equator, which runs exactly through its midst. regel forms its left foot, and the hare serves for a footstool to the right foot of the hero. three magnificent stars in the centre of the quadrilateral, which lie in one straight line are called the rake, or the three kings, or the staff of jacob, or the belt. these names have an obvious origin; but the meaning of orion itself is more doubtful. in the grecian sphere it is written [greek: Ôriôn], which also means a kind of bird. the allied word [greek: ôros] has very numerous meanings, the only one of which that could be conjectured to be connected with the constellations is a "guardian." the word [greek: horion], on the contrary, the diminutive of [greek: hôros], means a limit, and has been assigned to jupiter; and in this case may have reference to the constellation being situated on the confines of the two hemispheres. in mythology orion was an intrepid hunter of enormous size. he was the same personage as orus, arion, the minotaur, and nimrod, and afterwards became saturn. orion is called _tsan_ in chinese, which signifies three, and corresponds to the three kings. [illustration: fig. .] the asiatics used not to trace the images of their constellations, but simply joined the component stars by straight lines, and placed at the side the hieroglyphic characters that represented the object they wished to name. thus joining by five lines the principal stars in orion, they placed at the side the hieroglyphics representing a man and a sword, from whence the greeks derived the figure they afterwards drew of a giant armed with a sword. we must include in this series that brightest of all stars, sirius. it forms part of the constellation of the great dog, and lies to the south of orion near the extreme limit of our vision into the southern hemisphere in our latitudes. this star seems to have been intimately connected with egypt, and to have derived its name--as well as the name of the otherwise unimportant constellation it forms part of--from that country, and in this way:-- the overflowing of the nile was always preceded by an etesian wind, which, blowing from north to south about the time of the passage of the sun beneath the stars of the crab, drove the mists to the south, and accumulated them over the country whence the nile takes its source, causing abundant rains, and hence the flood. the greatest importance attached to the foretelling the time of this event, so that people might be ready with their provisions and their places of security. the moon was no use for this purpose, but the stars were, for the inundation commenced when the sun was in the stars of the lion. at this time the stars of the crab just appeared in the morning, but with them, at some distance from the ecliptic, the bright star sirius also rose. the morning rising of this star was a sure precursor of the inundation. it seemed to them to be the warning star, by whose first appearance they were to be ready to move to safer spots, and thus acted for each family the part of a faithful dog. whence they gave it the name of the dog, or monitor, in egyptian _anubis_, in phenician _hannobeach_, and it is still the dog-star--_caniculus_, and its rising commences our _dog-days_. the intimate connection between the rising of this star and the rising of the nile led people to call it also the nile star, or simply the nile; in egyptian and hebrew, _sihor_; in greek, [greek: sothis]; in latin, _sirius_. in the same way the egyptians and others characterised the different days of the year by the stars which first appeared in the evening--as we shall see more particularly with reference to the pleiades--and in this way certain stars came to be associated in their calendar with variations of temperature and operations of agriculture. they soon took for the cause what was originally but the sign, and thus they came to talk of moist stars, whose rising brought rain, and arid stars, which brought drought. some made certain plants to grow, and others had influence over animals. in the case of egypt, no other so great event could occur as that which the dog-star foretold, and its appearance was consequently made the commencement of the year. instead, therefore, of painting it as a simple star, in which case it would be indistinguishable from others, they gave it shape according to its function and name. when they wished to signify that it opened the year, it was represented as a porter bearing keys, or else they gave it two heads, one of an old man, to represent the passing year, the other of a younger, to denote the succeeding year. when they would represent it as giving warning of the inundation they painted it as a dog. to illustrate what they were to do when it appeared, anubis had in his arms a stew-pot, wings to his feet, a large feather under his arm, and two reptiles behind him, a tortoise and a duck. there is also in the celestial sphere a constellation called the little dog and procyon; the latter name has an obvious meaning, as appearing _before_ the dog-star. we cannot follow any farther the various constellations of the northern sphere, nor of the southern. the zodiacal constellations we must reserve for the present, while we conclude by referring to some of the changes in form and position that some of the above-mentioned have undergone in the course of their various representations. these changes are sometimes very curious, as, for example, in a coloured chart, printed at paris in , we have the charioteer drawn in the costume of adam, with his knees on the milky way, and turning his back to the public; the she-goat appears to be climbing over his neck, and two little she-goats seem to be running towards their mother. cassiopeia is more like king solomon than a woman. compare this with the _phenomena of aratus_, published , where cassiopeia is represented sitting on an oak chair with a ducal back, holding the holy palm in her left hand, while the coachman, "erichthon," is in the costume of a minion of henry the third of france. now compare the cassiopeia of the greeks with that drawn in the sixteenth and seventeenth centuries, or the coachman of the same periods, and we can easily see the fancies of the painters have been one of the most fertile sources of change. they seem, too, to have had the fancy in the middle ages to draw them all hideous and turning their backs. compare, for instance, the two pictures of andromeda and hercules, as given below, where those on one side are as heavy and gross as the others are artistic and pretty. unfortunately for the truth of andromeda's beauty, as depicted in these designs, she was supposed to be a negress, being the daughter of the ethiopians, cepheus and cassiopeia. not one of the drawings indicates this; indeed they all take after their local beauties. [illustration: fig. .] in flamsteed's chart, as drawn above, the coachman is a female; and instead of the she-goat being on the back, she holds it in her arms. no one, indeed, from any of the figures of this constellation would ever dream it was intended to represent a coachman. [illustration: fig. .] one more fundamental cause of changes has been the confusion of names derived by one nation from another, these having sometimes followed their signification, but at others being translated phonetically. thus the latins, in deriving names from the greek [greek: arktos], have partly translated it by ursa, and partly have copied it in the form arcticus. so also with reference to the three stars in the head of the bull, called by the greeks hyades. the romans thought it was derived from [greek: hyes], sows, so they called them _suculæ_, or little sows; whereas the original name was derived from [greek: hyein], to rain, and signified stars whose appearance indicated the approach of the rainy season. more curious still is the transformation of the pearl of the northern crown (margarita coronæ) in a saint--s. marguerite. the names may have had many origins whose signification is lost, owing to their being misunderstood. thus figurative language may have been interpreted as real, as when a conjunction is called a marriage; a disappearance, death; and a reappearance, a resurrection; and then stories must be invented to fit these words; or the stars that have in one country given notice of certain events lose the meaning of their names when these are used elsewhere; as when a boat painted near the stars that accompany an inundation, becomes the ship argo; or when, to represent the wind, the bird's wing is drawn; or those stars that mark a season are associated with the bird of passage, the insect or the animal that appears at that time: such as these would soon lose their original signification. the celestial sphere, therefore, as we now possess it, is not simply a collection of unmeaning names, associated with a group of stars in no way connected with them, which have been imposed at various epochs by capricious imagination, but in most instances, if not in all, they embody a history, which, if we could trace it, would probably lead us to astronomical facts, indicating the where and the when of their first introduction; and the story of their changes, so far as we can trace it, gives us some clue to the mental characteristics or astronomical progress of the people who introduced the alterations. we shall find, indeed, in a subsequent chapter, that many of our conclusions as to the birth and growth of astronomy are derived from considerations connected with the various constellations, more especially those of the zodiac. with regard to the date when and the country where the constellations of the sphere were invented, we will here give what evidence we possess, independent of the origin of the zodiac. in the first place it seems capable of certain proof that they were not invented by the greeks, from whom we have received them, but adopted from an older source, and it is possible to give limits to the date of introduction among them. newton, who attributes its introduction to musæus, a contemporary of chiron, remarks, that it must have been settled _after_ the expedition of the argonauts, and _before_ the destruction of troy; because the greeks gave to the constellation names that were derived from their history and fables, and devoted several to celebrate the memory of the famous adventurers known as the argonauts, and they would certainly have dedicated some to the heroes of troy, if the siege of that place had happened at the time. we remark that at this time astronomy was in too infant a state in greece for them to have fixed with so much accuracy the position of the stars, and that we have in this a proof they must have borrowed their knowledge from older cultivators of the science. the various statements we meet with about the invention of the sphere may be equally well interpreted of its introduction only into greece. such, for instance, as that eudoxus first constructed it in the thirteenth or fourteenth century b.c., or that by clement of alexandria, that chiron was the originator. the oldest direct account of the names of the constellations and their component stars is that of hesiod, who cites by name in his _works and days_ the pleiades, arcturus, orion, and sirius. he lived, according to herodotus, about years before christ. the knowledge of all the constellations did not reach the greeks at the same time, as we have seen from the omission by homer of any mention of the little bear, when if he had known it, he could hardly have failed to speak of it. for in his description of the shield of achilles, he mentions the pleiades, the hyades, orion and the bear, "which alone does not bathe in the ocean." he could never have said this last if he had known of the dragon and little bear. we may then safely conclude that the greeks received the idea of the constellations from some older source, probably the chaldeans. they received it doubtless as a sphere, with figured, but nameless constellations; and the greeks by slight changes adapted them to represent the various real or imaginary heroes of their history. it would be a gracious task, for their countrymen would glory in having their great men established in the heavens. when they saw a ship represented, what more suitable than to name it the ship argo? the swan must be jupiter transformed, the lyre is that of orpheus, the eagle is that which carried away ganymede, and so on. this would be no more than what other nations have done, as, for example, the chinese, who made greater changes still, unless we consider theirs to have had an entirely independent origin. [illustration: fig. .] that the celestial sphere was a conception known to others than the greeks is easily proved. the arabians, for instance, certainly did not borrow it from them; yet they have the same things represented. above is a figure of a portion of an arabian sphere drawn in the eleventh century, where we get represented plainly enough the great and little bears, the dragon, cassiopeia, andromeda, perseus, with the triple head of medusa; the triangle, one of the fishes, auriga, the ram, the bull obscurely, and the twins. there is also the famous so-called zodiac of denderah, brought from egypt to paris. this in reality contains more constellations than those of the zodiac. most of the northern ones can be traced, with certain modifications. its construction is supposed to belong to the eighth century b.c. most conspicuous on it is the lion, in a kind of barque, recalling the shape of the hydra. below it is the calf isis, with sirius, or the dog-star, on the forehead; above it is the crab, to the right the twins, over these along instrument, the plough, and above that a small animal, the little bear, and so we may go on:--all the zodiacal constellations, especially the balance, the scorpion, and the fishes being very clear. this sphere is indeed of later date than that supposed for the grecian, but it certainly appears to be independent. the remains we possess of older spheres are more particularly connected with the zodiac, and will be discussed hereafter. from what people the greeks received the celestial sphere, is a question on which more than one opinion has been formed. one is that it was originated in the tropical latitudes of egypt. the other, that it came from the chaldeans, and a third that it came from more temperate latitudes further to the east. the arguments for the last of these are as follow: there is an empty space of about °, formed by the last constellations of the sphere, towards the south pole, that is by the centaur, the altar, the archer, the southern fish, the whale, and the ship. now in a systematic plan, if the author were situated near the equator there would be no vacant space left in this way, for in this case the southern stars, attracting as much attention as the northern, would be inevitably inserted in the system of constellations which would be extended to the horizon on all sides. but a country of sufficiently high latitude to be unable to see at any time the stars about the southern pole must be north both of egypt and chaldea. this empty space remained unfilled until the discovery of the cape of good hope, except that the star canopus was included in the constellation argo, and the river eridan had an arbitrary extension given to it, instead of terminating in latitude °. another less cogent argument is derived from the interpretation of the fable of the phoenix. this is supposed to represent the course of the sun, which commences its growth at the time of its death. a similar fable is found among the swedes. now a tropical nation would find the difference of days too little to lead it to invent such a fable to represent it. it must needs have arisen where the days of winter were very much shorter than those of summer. the book of zoroaster, in which some of the earliest notices of astronomy are recorded, states that the length of a summer day is twice as long as that of winter. this fixes the latitude in which that book must have been composed, and makes it °. whence it follows, that to such a place must we look for the origin of these spheres, and not to egypt or chaldea. [illustration: plate iii.--chaldean astronomers.] diodorus siculus speaks of a nation in that part of the world, whom he calls hyperboreans, who had a tradition that their country is the nearest to the moon, on which they discovered mountains like those on the earth, and that apollo comes there once every nineteen years. this period being that of the metonic cycle of the moon, shows that if this could have really been discovered by them, they must have had a long acquaintance with astronomy. the babylonian tablets lead us to the belief that astronomy, and with it the sphere, and the zodiac were introduced by a nation coming from the east, from the mountains of elam, called the accadians, before b.c., and these may have been the nation to whom the whole is due. on the other hand, the arguments for the egyptians, or chaldeans being the originators depend solely on the tradition handed down by many, that one or other are the oldest people in the world, with the oldest civilization, and they have long cultivated astronomy. more precise information, however, seems to render these traditions, to say the least, doubtful, and certainly incapable of overthrowing the arguments adduced above. chapter iv. the zodiac. the zodiac, as already stated, is the course in the heavens apparently pursued by the sun in his annual journey through the stars. let us consider for a moment, however, the series of observations and reflections that must have been necessary to trace this zone as representing such a course. first, the diurnal motion of the whole heavens from east to west must have been noticed during the night, and the fact that certain stars never set, but turn in a circle round a fixed point. what becomes then, the next question would be, of those stars that do descend beneath the horizon, since they rise in the same relative positions as those in which they set. they could not be thought to be destroyed, but must complete the part of the circle that is invisible _beneath the earth_. the possibility of any stars finding a path beneath the earth must have led inevitably to the conception of the earth as a body suspended in the centre with nothing to support it. but leaving this alone, it would also be concluded that the sun went with the stars, and was in a certain position among them, even when both they and it were invisible. the next observations necessary would be that the zodiacal constellations visible during the nights of winter were not the same as those seen in summer, that such and such a group of stars passed the meridian at midnight at a certain time, and that six months afterwards the group exactly opposite in the heavens passed at the same hour. now since at midnight the sun will be exactly opposite the meridian, if it continues uniformly on its course, it will be among that group of stars that is opposite the group that culminates at midnight, and so the sign of the zodiac the sun occupies would be determined. this method would be checked by comparisons made in the morning and evening with the constellations visible nearest to the sun at its rising and setting. the difficulty and indirectness of these observations would make it probable that originally the zodiac would be determined rather by the path of the moon, which follows nearly the same path as the sun, and which could be observed at the same time as, and actually associated with, the constellations. now the moon is found each night so far to the east of its position on the previous night that it accomplishes the whole circumference in twenty-seven days eight hours. the two nearest whole number of days have generally been reckoned, some taking twenty-eight, and others twenty-seven. the zodiac, or, as the chinese called it, the yellow way, was thus divided into twenty-eight parts, which were called _nakshatras_ (mansions, or hotels), because the moon remains in each of them for a period of twenty-four hours. these mansions were named after the brightest stars in each, though sometimes they went a long way off to fix upon a characteristic star, as in the sixteenth indian constellation, _vichaca_, which was named after the northern crown, in latitude °. this arose from the brightness of the moon extinguishing the light of those that lie nearest to it. this method of dividing the zodiac was very widely spread, and was common to almost all ancient nations. the chinese have twenty-eight constellations, but the word _siou_ does not mean a group of stars, but simply a mansion or hotel. in the coptic and ancient egyptian the word for constellation has the same meaning. they also had twenty-eight, and the same number is found among the arabians, persians, and indians. among the chaldeans, or accadians, we find no sign of the number twenty-eight. the ecliptic or "yoke of the sky," with them, as we see in the newly-discovered tablets, was divided into twelve divisions as now, and the only connection that can be imagined between this and the twenty-eight is the opinion of m. biot, who thinks that the chinese had originally only twenty-four mansions, four more being added by chenkung (b.c. ), and that they corresponded with the twenty-four stars, twelve to the north and twelve to the south, that marked the twelve signs of the zodiac among the chaldeans. but under this supposition the twenty-eight has no reference to the moon, whereas we have every reason to believe that it has. the siamese only reckoned twenty-seven, and occasionally inserted an extra one, called _abigitten_, or intercalary moon. they made use, moreover, of the constellations to tell the hour of the night by their position in the heavens, and their method of doing this appears to have involved their having twenty-eight constellations. the names of the twenty-eight divisions among the arabs were derived from parts of the larger constellations that made the twelve signs, the first being the horns, and the second the belly, of the ram. the twenty-eight divisions among the persians, of which we may notice that the second was formed by the pleiades, and called _pervis_, soon gave way to the twelve, the names of which, recorded in the works of zoroaster, and therefore not less ancient than he, were not quite the same as those now used. they were the lamb, the bull, the twins, the crab, the lion, the ear of corn, the balance, the scorpion, the bow, the sea-goat, the watering-pot, and the fishes. nor were the chinese continually bound to the number twenty-eight. they, too, had a zodiac for the sun as well as the moon, as may be seen on some very curious pieces of money, of which those figured below are specimens. [illustration: fig. .] on some of these the various constellations of the northern hemisphere are engraved, especially the great bear--under innumerable disguises--and on others the twelve signs of the zodiac. these are very different, however, from the grecian set--they are the mouse, the bull, the tiger, the hare, the dragon, the serpent, the horse, the ram, the monkey, the cock, the dog, and the pig. the japanese series were the same. the mongolians had a series of zodiacal coins struck in the reign of jehanjir shah ( ). he had pieces of gold stamped, representing the sun in the constellation of the lion; and some years afterwards other coins were made, with one side having the impress of the particular sign in which the sun happened to be when the coin was struck. in this way a series is preserved having all the twelve signs. tavernier tells the story that one of the wives of the sultan, wishing to immortalise herself, asked jehanjir to be allowed to reign for four-and-twenty hours, and took the opportunity to have a large quantity of new gold and silver zodiacal coins struck and distributed among the people. the twenty-eight divisions are less known now, simply from the fact that the greeks did not adopt them; but they were much used by the early asian peoples, who distinguished them, like the twelve, by a series of animals, and they are still used by the arabs. so far for the nature of the zodiac, as used in various countries, and as adopted from more ancient sources by the greeks and handed on to us. it is very remarkable that the arrangement of it, and its relation to the pole of the equator, carries with it some indication of the age in which it must have been invented, as we now proceed to show. we may remark, in the first place, that from very early times the centre of the zodiacal circle has been marked in the celestial sphere, though there is no remarkable star near the spot; and the centre of the equatorial circle, or pole, has been even less noticed, though much more obvious. we cannot perhaps conclude that the instability of the pole was known, but that the necessity for drawing the zodiac led to attention being paid to its centre. both the persians and the chinese noted in addition four bright stars, which they said watched over the rest, _taschter_ over the east, _sateris_ over the west, _venaud_ over the south, and _hastorang_ over the north. now we must understand these points to refer to the sun, the east being the spring equinox, the west the autumnal, and the north and south the summer and winter solstices. there are no stars of any brilliancy that we could now suppose referred to in these positions; but if we turn the zodiac through ° we shall find aldebaran, the antares, regulus, and fomalhaut, four stars of the first magnitude, pretty nearly in the right places. does the zodiac then turn in this way? the answer is, it does. the effect of the attraction of the sun and moon upon the equatorial protuberance of the earth is to draw it round from west to east by a very slow motion, and make the ecliptic cross the equator each year about one minute of arc to the east of where it crossed it the year before. so, then, the sidereal year, or interval between the times at which the sun is in a certain position amongst the stars, is longer than the solar year, or interval between the times at which the sun crosses the equator at the vernal equinox. now the sun's position in the zodiac refers to the former, his appearance at the equinox to the latter kind of year. each solar year then--and these are the years we usually reckon by--the equinox is at a point fifty seconds of arc to the east on the zodiac, an effect which is known by the name of the precession of the equinoxes. now it is plain that if it keeps moving continuously to the east it will at last come round to the same point again, and the whole period of its revolution can easily be calculated from the distance it moves each year. the result of such a calculation shows that the whole revolution is completed in , years, after which time all will be again as it is now in this respect. [illustration: fig. .] if we draw a figure of the zodiac, as below, and know that at this time the vernal equinox takes place when the sun is in the fishes, then, the constellation of the ram being to the west of this, the date at which the equinox was there must be before our present date, while at some time in the future it will be in aquarius. now if in any old description we find that the equinox is referred to as being in the ram or in the bull, it tells us at once how long ago such a description was a true one, and, therefore, when it was written. this is the way in which the zodiac carries with it an intimation of its date. thus in the example lately referred to of the persians and their four stars, it must have been about , years ago, according to the above calculation, that these were in the positions assigned, which is therefore the date of this part of persian astronomy, if we have rightly conjectured the stars referred to. we have already said that the signs of the zodiac are not now the same as the zodiacal constellations, and this is now easily understood. it is not worth while to say that the sun enters such and such a part of the fishes at the equinox, and changes every year. so the part of the heavens it _does_ then enter--be it fishes, or aquarius, or the ram--is called by the same name--and is called a _sign_; the name chosen is the ram or aries, which coincided with the constellation of that name when the matter was arranged. there is another equally important and instructive result of this precession of the equinox. for the earth's axis is always perpendicular to the plane of the equator, and if the latter moves, the former must too, and change its position with respect to the axis of the ecliptic, which remains immovable. and the ends of these axes, or the points they occupy among the stars, called their poles, will change in the same way; the pole of the equator, round which the heavens appear to move, describing a curve about the pole of the ecliptic; and since the ecliptic and equator are always _nearly_ at the same angle, this curve will be very nearly a circle, as represented on preceding page. [illustration: fig. .] now the pole of the equator is a very marked point in the heavens, because the star nearest to it appears to have no motion. if then we draw such a figure as above, so as to see where this pole would be at any given date, and then read in any old record that such and such a star had no motion, we know at once at what date such a statement must have been made. this means of estimating dates is less certain than the other, because any star that is nearer to the pole than any other will appear to have no motion _relatively_ to the rest, unless accurate measurements were made. nevertheless, when we have any reason to believe that observations were carefully made, and there is any evidence that some particular star was considered the pole star, we have some confidence in concluding the date, examples of which will appear in the sequel; and we may give one illustration now, though not a very satisfactory one. hipparchus cites a passage from the sphere of eudoxus, in which he says, _est vero stella quædam in eodem consistens loco, quæ quidem polus est mundi._ (there is a certain immovable star, which is the pole of the world.) now referring to our figure, we find that about b.c. the two stars, [greek: b] ursæ minoris and [greek: k] draconis were fairly near the pole, and this fact leads us to date the invention of this sphere at about this epoch, rather than a little before or a little after, although, of course, there is nothing in _this_ argument (though there may be in others), to prevent us dating it when [greek: a] draconis was near the pole, b.c. this star was indeed said by the chinese astronomers in the reign of hoangti to mark the pole, which gives a date to their observations. the chief use of this latter method is to _confirm_ our conclusions from the former, rather than to originate any. let us now apply our knowledge to the facts. in the first place we may notice that in the time of hipparchus the vernal equinox was in the first degree of the ram, from which our own arrangement has originated. hipparchus lived years b.c., or nearly , years ago, at which time the equinox was exactly at [greek: b] aries. secondly, there are many reasons for believing that at the time of the invention of the zodiac, indeed in the first dawning of astronomy, the bull was the first sign into which the sun entered at the vernal equinox. now it takes , years to retrograde through a sign, and therefore the bull might occupy this position any time between and b.c., and any nearer approximation must depend on our ability to fix on any particular _part_ of the constellation as the original equinoctial point. we may say that whoever invented the zodiac would no doubt make this point the _beginning_ of a sign, and therefore date its invention b.c.; or on the other hand, if it can be proved that the constellations were known and observed before this, we may have to put back the date to near the end of the sign, and make its last remarkable stars the equinoctial ones, say those in the horns of taurus. compare the line of virgil, "candidus auratis aperit cum cornibus annum taurus." the date in this case would be about b.c.--or once more some remarkable part of the constellation may give proof that its appearance with the sun commenced the year--and our date would be intermediate between these two. in fact, the remarkable group of stars known as the _pleiades_ actually does play this part. so much interest clusters, however, round this group, so much light is thrown by it on the past history of astronomical ideas--and so much new information has recently been obtained about it--that it requires a chapter to itself, and we shall therefore pass over its discussion here. let us now review some of the indications that some part of the constellation of the bull was originally the first sign of the zodiac. we need perhaps only mention the astrological books of the jews--the cabal--in which the bull is dealt with as the first zodiacal sign. among the persians, who designate the successive signs by the letters of the alphabet, _a_ stands for taurus, _b_ for the twins, and so on. the chinese attribute the commencement of the sun's apparent motion to the stars of taurus. in thebes is a sepulchral chamber with zodiacal signs, and taurus at the head of them. the zodiac of the pagoda of elephanta (salsette) commences with the same constellation. however, reasons have been given for assigning to the zodiac a still earlier date than this would involve. thus laplace writes:--"the names of the constellations of the zodiac have not been given to them by chance--they embody the results of a large number of researches and of astronomical systems. some of the names appear to have reference to the motion of the sun. the crab, for instance, and the he-goat, indicate its retrogression at the solstices. the balance marks the equality of the days and nights at the equinoxes, and the other names seem to refer to agriculture and to the climate of the country in which the zodiac was invented. the he-goat appears better placed at the highest point of the sun's course than the lowest. in this position, which it occupied fifteen thousand years ago, the balance was at the vernal equinox, and the zodiacal constellations match well with the climate and agriculture of egypt." if we examine this, however, we see that all that is probable in it is satisfied by the ram being at the vernal, and the balance at the autumnal equinox, which corresponds much better with other evidence. [illustration: the zodiac of denderah.] in the first instance, no doubt, the names of the zodiacal constellations would depend on the principal star or stars in each, and these stars and the portion of the ecliptic assigned to each may have been noticed before the stars round them were grouped into constellations with different names. in any case, the introduction of the zodiac into greece seems to have been subsequent to that of the celestial sphere, and not to have taken place more than five or six centuries before our era. eudemus, of rhodes, one of the most distinguished of the pupils of aristotle, and author of a history of astronomy, attributes the introduction of the zodiac to oenopides of chio, a contemporary of anaxagoras. they did not receive it complete, as at first it had only eleven constellations, one of them, the scorpion, being afterwards divided, to complete the necessary number. their zodiacal divisions too would have been more regular had they derived them directly from the east, and would not have stretched in some instances over ° to °, like the lion, the bull, the fishes, or the virgin--while the crab, the ram, and the he-goat, have only ° to °. nor would their constellations be disposed so irregularly, some to the north and some to the south of the ecliptic, nor some spreading out widely and others crammed close together, so that we see that they only borrowed the idea from the easterns, and filled it out with their ancient constellations. such is the opinion of humboldt. with regard to the origin of the names of the signs of the zodiac, we must remember that a certain portion of the zodiacal circle, and not any definite group of stars, forms each sign, and that the constellations may have been formed separately, and have received independent names, though afterwards receiving those of the sign in which they were. the only rational suggestion for the origin of the names is that they were connected with some events which took place, or some character of the sun's motion observed, when it was in each sign. thus we have seen that the balance may refer to equal nights and days (though only introduced among the _greeks_ in the time of hipparchus), and the crab to the retrogression or stopping of the sun at the solstice. the various pursuits of husbandry, having all their necessary times, which in the primeval days were determined by the positions of the stars, would give rise to more important names. thus the ethiopian, at thebes, would call the stars that by their rising at a particular time indicated the inundation, aquarius, or the waterer; those beneath which it was necessary to put the plough to the earth, the bull stars. the lion stars would be those at whose appearance this formidable animal, driven from the deserts by thirst, showed himself on the borders of the river. those of the ear of corn, or the virgin of harvest, those beneath which the harvest was to be gathered in; and the sign of the goat, that in which the sun was when these animals were born. there can be but little doubt but that such was the origin of the names imposed, and for a time they would be understood in that sense. but afterwards, when time was more accurately kept, and calendars regulated, without each man studying the stars for himself, when the precession of the equinoxes made the periods not exactly coincide, the original meaning would be lost, the stars would be associated with the animals, as though there was a real bull, a real lion, &c., in the heavens; and then the step would be easy to represent these by living animals, whom they would endow with the heavenly attributes of what they represented; and so the people came at last to pray to and worship the several creatures for the sake of their supposed influence. they asked of the ram from their flocks the influences they thought depended on the constellation. they prayed the scorpion not to spread his evil venom on the world; they revered the crab, the scarabæus, and the fish, without perceiving the absurdity of it. it is certain at least that the gods of many nations are connected or are identical with the signs of the zodiac, and it seems at least more reasonable to suppose the former derived from the latter than _vice versâ_. among the greeks indeed, who had, so to speak, their gods ready made before they borrowed the idea of the zodiac, the process appears to have been the reverse, they made the signs to represent as far as they could their gods. in the more pastoral peoples, however, of the east, and in egypt, this process can be very clearly traced. among the jews there seems to be some remarkable connection between their patriarchs and these signs, though the history of that connection may not well be made out. the twelve signs are mentioned as being worshipped, along with the sun and moon, in the book of kings. but what is more remarkable is the dream of joseph, in which the sun and moon and the other eleven stars worshipped him, coupled with the various designations or descriptions given to each son in the blessing of jacob. in reuben we have the man who is said to be "unstable as water," in which we may recognise aquarius. in simeon and levi "the brethren," we trace the twins. judah is the "lion." zebulun, "that dwells at the haven of the sea," represents fishes. issachar is the bull, or "strong ass couching down between two burdens." dan, "the serpent by the way, the adder in the path," represents the scorpion. gad is the ram, the leader to a flock or troop of sheep. asher the balance, as the weigher of bread. naphtali, "the hind let loose," is the capricorn, joseph the archer, whose bow abode in strength. brujanin the crab, changing from morning to evening, and dinah, the only daughter, represents the virgin. there is doubtless something far-fetched in some of these comparisons, but when we consider the care with which the number twelve was retained, and that the four chief tribes carried on their sacred standards these very signs--namely, judah a lion, reuben a man, ephraim a bull, and dan a scorpion--and notice the numerous traces of astronomical culture in the jewish ceremonies, the seven lights of the candlestick, the twelve stones of the high priest, the feasts at the two equinoxes, the ceremonies connected with a ram and a bull, we cannot doubt that there is something more than chance in the matter, but rather conclude that we have an example of the process by which, in the hands of the egyptians themselves, astronomical representations became at last actually deified. it has been thought possible indeed to assign definitely each god of the egyptians to one of the twelve zodiacal signs. the ram was consecrated to jupiter ammon, who was represented with a ram's head and horns. the bull became the god apis, who was worshipped under that similitude. the twins correspond to horus and harpocrates, two sons of osiris. the crab was consecrated to anubis or mercury. the lion belonged to the summer sun, osiris; the virgin to isis. the balance and the scorpion were included together under the name of scorpion, which animal belongs to typhon, as did all dangerous animals. the archer was the image of hercules, for whom the egyptians had great veneration. the capricorn was consecrated to pan or mendes. the waterer--or man carrying a water-pot--is found on many egyptian monuments. this process of deification was rendered easier by the custom they had of celebrating a festival each month, under the name _neomenia_. they characterised the neomenias of the various months by making the animal whose sign the sun was entering accompany the isis which announced the _fête_. they were not content with a representation only, but had the animal itself. the dog, being the symbol of cannulus, with which the year commenced, a living dog was made to head the ceremonial of the first neomenia. diodorus testifies to this as an eye-witness. these neomenias thus came to be called the festival of the bull, of the ram, the dog, or the lion. that of the ram would be the most solemn and important in places where they dealt much in sheep. that of the bull in the fat pasture-lands of memphis and lower egypt. that of capella would be brilliant at mendes, where they bred goats more than elsewhere. we may fortify these opinions by a quotation from lucian, who gives expression to them very clearly. "it is from the divisions of the zodiac," he says, "that the crowd of animals worshipped in egypt have had their origin. some employed one constellation, and some another. those who used to consult that of the ram came to adore a ram. those who took their presages from the fishes would not eat fish. the goat was not killed in places were they observed capricornus, and so on, according to the stars whose influence they cared most for. if they adored a bull it was certainly to do honour to the celestial bull. the apis, which was a sacred object with them, and wandered at liberty through the country, and for which they founded an oracle, was the astrological symbol of the bull that shone in the heavens." [illustration: plate iv.--the zodiac and the dead in egypt.] their use of the zodiac is illustrated in an interesting manner by a mummy found some years ago in egypt. at the bottom of the coffin was found painted a zodiac, something like that of denderah; underneath the lid, along the body of a great goddess, were drawn eleven signs, but with that of _capricornus_ left out. the inscription showed that the mummy was that of a young man, aged years, months, and days, who died the th year of trajan, on the th of the month pazni, which corresponds to the nd of june, a.d. . the embalmed was therefore born on the th of january, a.d. , at which time the sun was in the constellation of capricornus. this shows that the zodiac was the representation of the astrological theories about the person embalmed, who was doubtless a person of some importance. (see plate iv.) any such use as this, however, must have been long subsequent to the invention of the signs themselves, as it involves a much more complicated idea. chapter v. the pleiades. among the most remarkable of the constellations is a group of seven stars arranged in a kind of triangular cluster, and known as the pleiades. it is not, strictly speaking, one of the constellations, as it forms only part of one. we have seen that one of the ancient signs of the zodiac is the bull, or taurus; the group of stars we are now speaking of forms part of this, lying towards the eastern part in the shoulders of the bull. the pleiades scarcely escape anybody's observation now, and we shall not be, therefore, surprised that they have always attracted great attention. so great indeed has been the attention paid to them that festivals and seasons, calendars and years, have by many nations been regulated by their rising or culmination, and they have been thus more mixed up with the early history of astronomy, and have left more marks on the records of past nations, than any other celestial object, except the sun and moon. the interesting details of the history of the pleiades have been very carefully worked out by r. g. haliburton, f.s.a., to whom we owe the greater part of the information we possess on the subject.[ ] let us first explain what may be observed with respect to the pleiades. it is a group possessing peculiar advantages for observation; it is a compact group, the whole will appear at once; and it is an unmistakable group and it is near the equator, and is therefore visible to observers in either hemisphere. now suppose the sun to be in the same latitude as the pleiades on some particular day; owing to the proximity of the group to the ecliptic, it will be then very near the sun, and it will set with it and be invisible during the night. if the sun were to the east of the pleiades they would have already set, and the first view of the heavens at sunset would not contain this constellation; and so it would be so long as the sun was to the east, or for nearly half a year; though during some portion of this time it would rise later on in the night. during the other half year, while the sun was to the west, the pleiades would be visible at sunset, and we immediately see how they are thus led to divide the whole year into two portions, one of which might be called _the pleiades below_, and the other _the pleiades above_. it is plain that the pleiades first become visible at sunset, when they are then just rising, in which case they will culminate a little after midnight (not at midnight, on account of the twilight) and be visible all night. this will occur when the sun is about half a circle removed from them--that is, at this time, about the beginning of november; which would thus be the commencement of one half of the year, the other half commencing in may. the culmination of the pleiades at midnight takes place a few days later, when they rise at the time that the sun is really on the horizon, in which case they are exactly opposite to it; and this will happen on the same day all over the earth. the opposite effect to this would be when the sun was close to the pleiades--a few days before which the latter would be just setting after sunset, and a few days after would be just rising before sunrise. [ ] mr. haliburton's observations are contained in an interesting pamphlet, entitled _new materials for the history of man_, which is quoted by prof. piazzi smyth, but which is not easy to obtain. it may be seen, however, in the british museum. we have thus the following observations, that might be made with respect to this, or any other well-marked constellation. first, the period during which it was visible at sunset; secondly, the date of its culmination at midnight; thirdly, its setting in the evening; and fourthly, its rising in the morning: the last two dates being nearly six months removed from the second. there are also the dates of its culmination at sunrise and sunset, which would divide these intervals into two equal halves. on account of the precession of the equinoxes, as explained in the last chapter, the time at which the sun has any particular position with respect to the stars, grows later year by year in relation to the equinoctial points. and as we regulate our year by the date of the sun's entrance on the northern hemisphere, the sidereal dates, as we may call them, keep advancing on the months. as, however, the change is slow, it has not prevented years being commenced and husbandry being regulated by the dates above mentioned. any date that is regulated by the stars we might expect to be nearly the same all over the world, and the customs observed to be universal, though the date itself might alter, and in this way. so long as the date was directly obtained from the position of the star, all would agree; but as soon as a solar calendar was arranged, and it was found that at that time this position coincided with a certain day, say the pleiades culminating at midnight on november , then some would keep on the date november as the important day, even when the pleiades no longer culminated at midnight then, and others would keep reckoning by the stars, and so have a different date. with these explanations we shall be able to recognise how much the configurations of the pleiades have had to do with the festivals and calendars of nations, and have even left their traces on customs and names in use among ourselves to the present day. we have evidences from two very different quarters of the universality of the division of the year into two parts by means of the pleiades. on the one hand we learn from hesiod that the greeks commenced their winter seasons in his days by the setting of the pleiades in the morning, and the summer season by their rising at that time. and mr. ellis, in his _polynesian researches_, tells us that "the society islanders divided the year into two seasons of the pleiades, or _matarii_. the first they called _matarii i nia_, or the _pleiades above_. it commenced when, in the evening, these stars appeared at or near the horizon, and the half year during which, immediately after sunset, they were seen above the horizon was called _matarii i nia_. the other season commenced when at sunset these stars are invisible, and continued until at that time they appeared again above the horizon. this season was called _matarii i raro_, i.e. _the pleiades below_." besides these direct evidences we shall find that many semi-annual festivals connected with these stars indicate the commencement of the two seasons among other nations. one of these festivals was of course always taken for the commencement of the year, and much was made of it as new-year's day. a new-year's festival connected with and determined by the pleiades appears to be one of the most universal of all customs; and though some little difficulty arises, as we have already pointed out, in fixing the date with reference to solar calendars, and differences and coincidences in this respect among different nations may be to a certain extent accidental, yet the fact of the wide-spread observance of such a festival is certain and most interesting. the actual observance at the present day of this festival is to be found among the australian savages. at their midnight culmination in november, they still hold a new-year's _corroboree_, in honour of the _mormodellick_, as they call the pleiades, which they say are "very good to the black fellows." with them november is somewhat after the beginning of spring, but in former days it would mark the actual commencement, and the new year would be regulated by the seasons. in the northern hemisphere this culmination of the pleiades has the same relation to the autumnal equinox, which would never be taken as the commencement of the year; and we must therefore look to the southern hemisphere for the origin of the custom; especially as we find the very pleiades themselves called _vergiliæ_, or stars of spring. of course we might suppose that the rising of the constellation in the _morning_ had been observed in the northern hemisphere, which would certainly have taken place in the beginning of spring some , years ago; but this seems improbable, first, because it is unlikely that different phenomena of the pleiades should have been most noticed, and secondly, because neither april nor may are among any nations connected with this constellation by name. whereas in india the year commenced in the month they called _cartiguey_, which means the pleiades. among the ancient egyptians we find the same connection between _athar-aye_, the name of the pleiades, with the chaldeans and hebrews, and _athor_ in the egyptian name of november. the arabs also call the constellation _atauria_. we shall have more to say on this etymology presently, but in the meantime we learn that it was the phenomenon connected with the pleiades at or about november that was noticed by all ancient nations, from which we must conclude that the origin of the new-year's spring festival came from the southern hemisphere. there is some corroboration of this in the ancient traditions as to the stars having changed their courses. in the southern hemisphere a man standing facing the position of the sun at noon would see the stars rise on his right hand and move towards his left. in the northern hemisphere, if he also looked in the direction of the sun at noon, he would see them rise on his left hand. now one of a race migrating from one side to the other of the equator would take his position from the sun, and fancy he was facing the same way when he looked at it at noon, and so would think the motion of the stars to have altered, instead of his having turned round. such a tradition, then, seems to have arisen from such a migration, the fact of which seems to be confirmed by the calling the pleiades stars of spring, and commencing the year with their culmination at midnight. in order to trace this new-year's festival into other countries, and by this means to show its connection with the pleiades, we must remark that every festival has its peculiar features and rites, and it is by these that we must recognise it, where the actual date of its occurrence has slightly changed; bearing, of course, in mind that the actual change of date must not be too great to be accounted for by the precession of the equinoxes, or about seventy-one years for each day of change, since the institution of the festival, and that the change is in the right direction. now we find that everywhere this festival of the pleiades' culmination at midnight (or it may be of the slightly earlier one of their first appearance at the horizon at apparent sunset) was always connected with the memory of the dead. it was a "feast of ancestors." among the australians themselves, the _corroborees_ of the natives are connected with a worship of the dead. they paint a white stripe over their arms, legs, and ribs, and, dancing by the light of their fires by night, appear like so many skeletons rejoicing. what is also to be remarked, the festival lasts three days, and commences in the evening; the latter a natural result of the date depending on the appearance of the pleiades on the horizon at that time. the society islanders, who, as we have seen, divided their year by the appearance of the pleiades at sunset, commenced their year on the first day of the appearance, about november, and also celebrated the closing of one and the opening of a new year by a "usage resembling much the popish custom of mass for souls in purgatory," each man returning to his home to offer special prayers for the spirits of departed relatives. in the tonga islands, which belong to the fiji group, the festival of _inachi_, a vernal first-fruits' celebration, and also a commemoration of the dead takes place towards the end of october, and commences at sunset. in peru the new-year's festival occurs in the beginning of november, and is "called _ayamarca_ from _aya_, a corpse, and _marca_, carrying in arms, because they celebrated the solemn festival of the dead, with tears, lugubrious songs, and plaintive music; and it was customary to visit the tombs of relations, and to leave in them food and drink." the fact that this took place at the time of the discovery of peru on the very same day as a similar ceremony takes place in europe, was only an accidental coincidence, which is all the more remarkable because the two appear, as will be seen in the sequel, to have had the same origin, and therefore at first the same date, and to have altered from it by exactly the same amount. these instances from races south of the equator prove clearly that there exists a very general connection with new-year's day, as determined by the rising of the pleiades at sunset, and a festival of the dead; and in some instances with an offering of first-fruits. what the origin of this connection may be is a more difficult matter. at first sight one might conjecture that with the year that was passed it was natural to connect the men that had passed away; and this may indeed be the true interpretation: but there are traditions and observances which may be thought by some to point to some ancient wide-spread catastrophe which happened at this particular season, which they yearly commemorated, and reckoned a new year from each commemoration. such traditions and observances we shall notice as we trace the spread of this new-year's festival of the dead among various nations, and its connection, with the pleiades. we have seen that in india november is called the month of the pleiades. now on the th day of that month is celebrated the hindoo durga, a festival of the dead, and said by greswell to have been a new-year's commemoration at the earliest time to which indian calendars can be carried back. among the ancient egyptians the same day was very noticeable, and they took care to regulate their solar calendars that it might remain unchanged. numerous altered calendars have been discovered, but they are all regulated by this one day. this was determined by the culmination of the pleiades at midnight. on this day commenced the solemn festival of the isia, which, like the _corroborees_ of the australians, lasted three days, and was celebrated in honour of the dead, and of osiris, the lord of tombs. now the month athyr was undoubtedly connected with the pleiades, being that "in which the pleiades are most distinct"--that is, in which they rise near and before sunset. among the egyptians, however, more attention was paid to astronomy than amongst the savage races with which the year of the pleiades would appear to have originated, and they studied very carefully the connection between the positions of the stars and the entrance of the sun into the northern hemisphere, and regulated their calendar accordingly; as we shall see shortly in speaking of the pyramid builders. the persians formerly called the month of november _mordâd_, the angel of death, and the feast of the dead took place at the same time as in peru, and was considered a new-year's festival. it commenced also in the evening. in ceylon a combined festival of agriculture and of the dead takes place at the beginning of november. among the better known of the ancient nations of the northern hemispheres, such as the greeks and romans, the anomaly of having the beginning of the year at the autumnal equinox seems to have induced them to make a change to that of spring, and with this change has followed the festival of the dead, although some traces of it were left in november. the commemoration of the dead was connected among the egyptians with a deluge, which was typified by the priest placing the image of osiris in a sacred coffer or ark, and launching it out into the sea till it was borne out of sight. now when we connect this fact, and the celebration taking place on the th day of athyr, with the date on which the mosaic account of the deluge of noah states it to have commenced, "in the second month (of the jewish year, which corresponds to november), the th day of the month," it must be acknowledged that this is no chance coincidence, and that the precise date here stated must have been regulated by the pleiades, as was the egyptian date. this coincidence is rendered even stronger by the similiarity of traditions among the two nations concerning the dove and the tree as connected with the deluge. we find, however, no festival of the dead among the hebrews; their better form of faith having prevented it. we have not as yet learnt anything of the importance of the pleiades among the ancient babylonian astronomers, but as through their tablets we have lately become acquainted with their version of the story of the deluge, we may be led in this way to further information about their astronomical appreciation of this constellation. from whatever source derived, it is certain that the celtic races were partakers in this general culture, we might almost call it, of the pleiades, as shown by the time and character of their festival of the dead. this is especially interesting to ourselves, as it points to the origin of the superstitions of the druids, and accounts for customs remaining even to this day amongst us. [illustration: plate v.--the legends of the druids.] the first of november was with the druids a night full of mystery, in which they annually celebrated the reconstruction of the world. a terrible rite was connected with this; for the druidess nuns were obliged at this time to pull down and rebuild each year the roof of their temple, as a symbol of the destruction and renovation of the world. if one of them, in bringing the materials for the new roof, let fall her sacred burden, she was lost. her companions, seized with a fanatic transport, rushed upon her and tore her to pieces, and scarcely a year is said to have passed without there being one or more victims. on this same night the druids extinguished the sacred fire, which was kept continually burning in the sacred precincts, and at that signal all the fires in the island were one by one put out, and a primitive night reigned throughout the land. then passed along to the west the phantoms of those who had died during the preceding year, and were carried away by boats to the judgment-seat of the god of the dead. (plate v.) although druidism is now extinct, the relics of it remain to this day, for in our calendar we still find november marked as all saints' day, and in the pre-reformation calendars the last day of october was marked all hallow eve, and the nd of november as all souls'; indicating clearly a three days' festival of the dead, commencing in the evening, and originally regulated by the pleiades--an emphatic testimony how much astronomy has been mixed up with the rites and customs even of the english of to-day. in former days the relics were more numerous, in the hallowe'en torches of the irish, the bonfires of the scotch, the _coel-coeth_ fires of the welsh, and the _tindle_ fires of cornwall, all lighted on hallowe'en. in france it still lingers more than here, for to this very day the parisians at this festival repair to the cemeteries, and lunch at the graves of their ancestors. if the extreme antiquity of a rite can be gathered from the remoteness of the races that still perform it, the fact related to us by prescott in his _history of the conquest of mexico_ cannot fail to have great interest. there we find that the great festival of the mexican cycle was held in november, at the time of the midnight culmination of the pleiades. it began at sunset, and at midnight as that constellation approached the zenith, a human victim, was offered up, to avert the dread calamity which they believed impended over the human race. they had a tradition that the world had been previously destroyed at this time, and they were filled with gloom and dismay, and were not at rest until the pleiades were seen to culminate, and a new cycle had begun; this great cycle, however, was only accomplished in fifty-two years. it is possible that the festival of lanthorns among the japanese, which is celebrated about november, may be also connected with this same day, as it is certain that that nation does reckon days by the pleiades. these instances of a similar festival at approximately the same period of the year, and regulated (until fixed to a particular day in a solar calendar) by the midnight culmination of the pleiades, show conclusively how great an influence that constellation has had on the manners and customs of the world, and throw some light on the history of man. even where we find no festival connected with the particular position of the pleiades which is the basis of the above, they still are used for the regulation of the seasons--as amongst the dyaks of borneo. this race of men are guided in their farming operations by this constellation. "when it is low in the east at early morning, before sunrise, the elders know it is time to cut down the jungle; when it approaches mid-heaven, then it is time to burn what they have cut down; when it is declining towards the west, then they plant; and when in the early evening it is seen thus declining, then they may reap in safety and in peace;" the latter period is also that of their feast of _nycapian_, or first-fruits. we find the same regulations amongst the ancient greeks in the days of hesiod, who tells us that the corn is to be cut when the pleiades rise, and ploughing is to be done when they set. also that they are invisible for forty days, and reappear again at harvest. when the pleiades rise, the care of the vine must cease; and when, fleeing from orion, they are lost in the waves, sailing commences to be dangerous. the name, indeed, by which we now know these stars is supposed to be derived from the word [greek: plein], to sail--because sailing was safe after they had risen; though others derive it from [greek: peleiai], a flight of doves. any year that is regulated by the pleiades, or by any other group of stars, must, as we have seen before, be what is called a sidereal, and not a solar year. now a year in uncivilised countries can only mean a succession of seasons, as is illustrated by the use of the expression "a person of so many summers." it is difficult of course to say when any particular season begins by noticing its characteristics as to weather; even the most regular phenomena are not certain enough for that; we cannot say that when the days and nights become exactly equal any marked change takes place in the temperature or humidity of the atmosphere, or in any other easily-noticed phenomena. the day therefore on which spring commences is arbitrary, except that, inasmuch as spring depends on the position of the sun, its commencement, ought to be regulated by that luminary, and not by some star-group which has no influence in the matter. nevertheless the position of such a group is much more easily observed, and in early ages could almost alone be observed; and so long as the midnight culmination of the pleiades--judged of, it must be noticed, by their appearance _on the horizon_ at sunset--fairly coincided with that state of weather which might be reckoned the commencement of spring conditions, no error would be detected, because the change in their position is so slow. the solar spring is probably a later discovery, which now, from its greater reasonableness and constancy, has superseded the old one. but since the time of the sun's crossing the equator is the natural commencement of spring, whether discovered or not, it is plain that no group of stars could be taken as a guide instead, if their indication did not approximately coincide with this. if then we can determine the exact date at which the pleiades indicated by their midnight culmination the sun's passage across the equator, we can be sure that the spring could only have been regulated by this during, say, a thousand years at most, on either side of this date. it is very certain that if the method of reckoning spring by the stars had been invented at a more remote date, some other set of stars would have been chosen instead. now when was this date? it is a matter admitting of certain calculation, depending only on numbers derived from observation in our own days and records of the past few centuries, and the answer is that this date is about b.c. we have seen that, though it was probably brought from the southern hemisphere, the egyptians adopted the year of the pleiades, and celebrated the new-year's festival of the dead; but they were also advanced astronomers, and would soon find out the change that took place in the seasons when regulated by the stars. and to such persons the date at which the two periods coincided, or at least were exactly half a year apart, would be one of great importance and interest, and there seems to be evidence that they did commemorate it in a very remarkable manner. the evidence, however, is all circumstantial, and the conclusion therefore can only claim probability. the evidence is as follows:--the most remarkable buildings of egypt are the pyramids. these are of various sizes and importance, but are built very much after the same plan. they seem, however, to be all copies from one, the largest, namely, the pyramid of gizeh, and to be of subsequent date to this. their object has long been a puzzle, and the best conclusion has been supposed to be that they were for sepulchral purposes, as in some of them coffins have been found. the large one, however, shows far more than the rest of the structure, and cannot have been meant for a funeral pile alone. its peculiarities come out on a careful examination and measurement such as it has been subjected to at the devoted hands of piazzi smyth, the astronomer royal for scotland. he has shown that it is not built at random, as a tomb might be, but it is adjusted with exquisite design, and with surprising accuracy. in the first place it lies due north, south, east, and west, and the careful ascertainment of the meridian of the place, by modern astronomical instruments, could not suggest any improvement in its position in this respect. the outside of it is now, so to speak, pealed, that is to say, there was originally, covering the whole, another layer of stones which have been taken away. these stones, which were of a different material, were beautifully polished, as some of the remaining ones, now covered and concealed, can testify. the angle at which they are cut, and which of course gives the angle and elevation of the whole pyramid, is such that the height of it is in the same proportion to its circumference or perimeter, as the radius of a circle is to its circumference approximately. the height, in fact, is proved by measurement and observation to be ft., and the four sides together to be , ft., or about - / times the height. it does not seem improbable that, considering their advancement, the egyptians might have calculated approximately how much larger the circumference of the circle is than its diameter, and it is a curious coincidence that the pyramid expresses it. professor piazzi smyth goes much further and believes that they knew, or were divinely taught, the shape and size of the earth, and by a little manipulation of the length of their unit, or as he expresses it the "pyramid inch," he makes the base of the pyramid express the number of miles in the diameter of the earth. now in the interior of the apparently solid structure, besides the usual slanting passage down to a kind of cellar or vault beneath the middle of the base, which may have been used for a sepulchral resting-place, there are two slanting passages, one running north and the other running south, and slanting up at different angles. part of that which leads south is much enlarged, and is known as the grand gallery. it is of a very remarkable shape, being perfectly smooth and polished along its ascending base, as indeed it is in every part, and having a number of steps or projections, pointing also upwards at certain angles, very carefully maintained. whether we understand its use or not, it is very plain that it has been made with a very particular design, and one not easily comprehended. this leads into a chamber known as the king's chamber, whose walls are exquisitely polished and which contains a coffer known as _cheops' coffin_. this coffer has been villainously treated by travellers, who have chipped and damaged it, but originally it was very carefully made and polished. it is too large to have been brought in by the only entrance into the chamber after it was finished, and therefore is obviously no coffin at all, as is proved also by the elaborateness of the means of approach. professor piazzi smyth has made the happy suggestion that it represents their standard of length and capacity, and points out the remarkable fact that it contains exactly as much as four quarters of our dry measure. as no one has ever suggested what our "quarters" are quarters of, professor smyth very naturally supplies the answer--"of the contents of the pyramid coffer." there are various other measurements that have been made by the same worker, and their meaning suggested in his interesting book, _our inheritance in the great pyramid_, which we may follow or agree to as we can; but from all that has been said above, it will appear probable that this pyramid was built with a definite design to mark various natural phenomena or artificial measures, which is all we require for our present purpose. now we come to the question, what is the meaning of the particular angles at which the north-looking and south-looking passages rise, if, as we now believe, they must have _some_ meaning. the exits of these passages were closed, and they could not therefore have been for observation, but they may have been so arranged as to be a memorial of any remarkable phenomena to be seen in those directions. to ascertain if there be any such to which they point, we must throw back the heavens to their position in the days of the egyptians, because, as we have seen, the precession of the equinoxes alters the meridian altitude of every star. as the passages point north and south, if they refer to any star at all, it must be to their passing the meridian. now let us take the heavens as they were b.c., the date at which the pleiades _really_ commenced the spring, by their midnight culmination, and ask how high they would be then. the answer of astronomy is remarkable--"_exactly at that height that they could be seen in the direction of the southward-pointing passage of the pyramid._" and would any star then be in a position to be seen in the direction of the other or northward-looking passage? yes, the largest star in the constellation of the dragon, which would be so near the pole ( ° ´) as to be taken as the pole star in those days. these are such remarkable coincidences in a structure admittedly made with mathematical accuracy and design, and truly executed, that we cannot take them to be accidental, but must endeavour to account for them. the simplest explanation seems to be, that everything in the pyramid is intended to represent some standard or measure, and that these passages have to do with their year. they had received the year of the pleiades from a remoter antiquity than their own, they had discovered the true commencement of solar spring, as determined from the solar autumnal equinox, and they commemorated by the building of the pyramid the coincidence of the two dates, making passages in it which would have no meaning except at that particular time. whether the pyramid was built _at that time_, or whether their astronomical knowledge was sufficient to enable them to predict it and build accordingly, just as we calculate back to it, we have no means of knowing. it is very possible that the pyramid may have been built by some immigrating race more learned in astronomy, like the accadians among the babylonians. either the whole of the conclusions respecting the pyramid is founded on pure imagination and the whole work upon it thrown away, or we have here another very remarkable proof of the influence of the pleiades on the reckoning of the year, and a very interesting chapter in the history of the heavens. following the guidance of mr. haliburton, we shall find still more customs, and names depending in all probability on the influence the pleiades once exerted, and the observances connected with the feasts in their honour. the name by which the pleiades are known among the polynesians is the "tau," which means a season, and they speak of the years of the tau, that is of the pleiades. now we have seen that the egyptians had similar feasts at similar times, in relation to this constellation, and argued that they did not arise independently. this seems still further proved by their name for these stars--the atauria. now the egyptians do not appear to have derived their signs of the zodiac from the same source; these had a babylonian origin, and the constellation in which the pleiades were placed by the latter people was the bull, by whatever name he went. the egyptians, we may make the fair surmise, adopted from both sources; they took the pleiades to indicate the bull, and they called this animal after the atauria. from thence we got the latin taurus, and the german thier. it is possible that this somehow got connected with the letter "tau" in greek, which seems itself connected with the sacred scarabæus or tau-beetle of egypt; but the nature of the connection is by no means obvious. mr. haliburton even suggests that the "tors" and "arthur's seat," which are names given to british hill-tops, may be connected with the "high places," of the worship of the pleiades, but of this we have no proof. among the customs possibly derived from the ancients, through the phoenicians, though now adopted as conveying a different meaning in a christian sense, is that of the "hot cross bun," or "bull cake." it is found on egyptian monuments, signifying the four quarters of the year, and sometimes stamped with the head and horns of the bull. it is found among ourselves too, essentially connected with the dead, and something similar to it appears in the "soul cake" connected originally with all souls' day. among the scotch it was traditionally thought that on new year's eve the candlemas bull can be seen, rising at twilight and sailing over the heavens--a very near approach to a matter-of-fact statement. we have seen that among the ancient indians there was some notice taken of the pleiades, and that they in all probability guided their year by them or by some other stars: it would therefore behove them to know something of the precession of the equinoxes. it seems very well proved that their days of brahma and other periods were meant to represent some astronomical cycles, and among these we find one that is applicable to the above. they said that in every thousand divine ages, or in every day of brahma, fourteen menus are successively invested with the sovereignty of the earth. each menu transmits his empire to his sons during seventy-one divine ages. we may find a meaning for this by putting it that the equinox goes forward fourteen days in each thousand years, and each day takes up seventy-one years. these may not be the only ones among the various customs, sayings, and names that are due in one way or other to this primitive method of arranging the seasons by the positions of the stars, especially of those most remarkable and conspicuous ones the pleiades, but they are those that are best authenticated. if the connection between the pleiades and the festival of the dead, the new year and a deluge, can be clearly made out; if the tradition of the latter be found as universal as that of the former, and be connected with it in the mosaic narrative; if we can trace all these traditions to the south of the equator, and find numerous further traditions connected with islands, we may find some reason for believing in their theory who suggest that the early progenitors of the human race (? all of them) were inhabitants of some fortunate islands of even temperature in the southern hemisphere, where they made some progress in civilisation, but that their island was swallowed up by the sea, and that they only escaped by making huge vessels, and, being carried by the waves, they landed on continental shores, where they commemorated yearly the great catastrophe that had happened to them, notifying its time by the position of the pleiades, making it a feast of the dead whom they had left behind, and opening the year with the day, whether it were spring or not, and handing down to their descendants and to those among whom they came, the traditions and customs which such events had impressed upon them. whether such an account be probable, mythical, or unnatural, there are certainly some strange things to account for in connection with the pleiades. chapter vi. the nature and structure of the heavens according to the ancients. many and various have been the ideas entertained by reflecting men in former times on the nature and construction of the heavenly vault, wherein appeared those stars and constellations whose history we have already traced. is it solid? or liquid? or gaseous? each of these and many other suppositions have been duly formulated by the ancient philosophers and sages, although, as we are told by modern astronomy, it does not exist at all. in our study of the ancient ideas about the structure of the universe, we will commence with that early and curious system which considered the heavenly vault to be material and solid. the theory of a solid sky received the assent of all the most ancient philosophers. in his commentary on aristotle's work on the heavens, simplicius reveals the repugnance the ancient philosophers felt in admitting that a star could stand alone in space, or have a free motion of its own. it must have a support, and they therefore conceived that the sky must be solid. however strange this idea may now appear, it formed for many centuries the basis of all astronomical theories. thus anaximenas (in the sixth century b.c.) is related by plutarch to have said that "the outer sky is solid and crystalline," and that the stars are "fixed to its surface like studs," but he does not say on what this opinion was founded, though it is probable that, like his master anaximander, he could not understand how the stars could move without being supported. pythagoras, who lived about the same epoch, is also supposed by some to have held the same views, and it is possible that they all borrowed these ideas from the persians, whose earliest astronomers are said in the _zend avesta_ to have believed in concentric solid skies. eudoxus of cnidus, in the fifth century b.c., is said by his commentator aratus to have also believed in the solidity of the heavens, but his reasons are not assigned. notwithstanding these previously expressed opinions, aristotle (fourth century, b.c.) has for a long time been generally supposed to be the inventor of solid skies, but in fact he only gave the idea his valuable and entire support. the sphere of the stars was his eighth heaven. the less elevated heavens, in which he also believed, were invented to explain as well as they might, the proper motions of the sun, moon, and planets. the philosopher of stagira said that the motion of his eighth or outermost solid sky was uniform, nor ever troubled by any perturbation. "within the universe there is," he says, "a fixed and immovable centre, the earth; and without there is a bounding surface enclosing it on all sides. the outermost part of the universe is the sky. it is filled with heavenly bodies which we know as stars, and it has a perpetual motion, carrying round with it these immortal bodies in its unaltering and unending revolution." euclid, to whom we may assign a date of about before our present era, also considered the stars to be set in a solid sphere, having the eye of the observer as centre; though for him this conception was simply a deduction from exact and fundamental observations, namely, that their revolution took place as a whole, the shape and size of the constellation being never altered. cicero, in the last century before christ, declared himself a believer in the solidity of the sky. according to him the ether was too rarefied to enable it to move the stars, which must therefore require to be fixed to a sphere of their own, independent of the ether. in the time of seneca there seem to have been difficulties already raised about the solidity of the heavens, for he only mentions it in the form of a question--"is the sky solid and of a firm and compact substance?" (_questions_, book ii.) in the fifth century the idea of the star sphere still lingered, and in the eyes of simplicius, the commentator of aristotle, it was not merely an artifice suitable for the representation of the apparent motions, but a firm and solid reality; while mahomet and most of the fathers of the christian church had the same conception of these concentric spheres. it appears then from this review that the phrases "starry vault," and especially "fixed stars," have been used in two very distinct senses. when we meet with them in aristotle or ptolemy, it is obvious that they have reference to the crystal sphere of anaximenas, to which they were supposed to be affixed, and to move with it; but that later the word "fixed" carried with it the sense of immovable, and the stars were conceived as fixed in this sense, independently of the sphere to which they were originally thought to be attached. thus seneca speaks of them as the _fixum et immobilem populum_. if we would inquire a little further into the supposed nature of this solid sphere, we find that empedocles considered it to be a solid mass, formed of a portion of the ether which the elementary fire has converted into crystal, and his ideas of the connection between cold and solidification being not very precise, he described it by names that give the best idea of transparence, and, like lactantius, called it _vitreum cælum_, or said _cælum ærem glaciatum esse_, though we cannot suppose that he made any allusion to what we now call glass, but simply meant some body eminently transparent into which the fire had transformed the air; while so far from having any idea of cold, as we might imagine possible from observations of the snowy tops of mountains, they actually believed in a warm region above the lower atmosphere. thus aristotle considers that the spheres heat by their motion the air below them, without being heated themselves, and that there is thus a production of heat. "the motion of the sphere of fixed stars," he says, "is the most rapid, as it moves in a circle with all the bodies attached to it, and the spaces immediately below are strongly heated by the motion, and the heat, thus engendered, is propagated downwards to the earth." this however, strangely enough, does not appear to have prevented their supposing an eternal cold to reign in the regions next below, for macrobius, in his commentary on cicero, speaks of the decrease of temperature with the height, and concludes that the extreme zones of the heavens where saturn moves must be eternally cold; but this they reckoned as part of the atmosphere, beyond whose limits alone was to be found the fiery ether. it is to the fathers of the church that we owe the transmission during the middle ages of the idea of a crystal vault. they conceived a heaven of glass composed of eight or ten superposed layers, something like so many skins in an onion. this idea seems to have lingered on in certain cloisters of southern europe even into the nineteenth century, for a venerable prince of the church told humboldt in , that a large aërolite lately fallen, which was covered with a vitrified crust, must be a fragment of the crystalline sky. on these various spheres, one enveloping without touching another, they supposed the several planets to be fixed, as we shall see in a subsequent chapter. whether the greater minds of antiquity, such as plato, plutarch, eudoxus, aristotle, apollonius, believed in the reality of these concentric spheres to carry the planets, or whether this conception was not rather with them an imaginary one, serving only to simplify calculation and assist the mind in the solution of the difficult problem of their motion, is a point on which even humboldt cannot decide. it is certain, however, that in the middle of the sixteenth century, when the theory involved no less than seventy-seven concentric spheres, and later, when the adversaries of copernicus brought them all into prominence to defend the system of ptolemy, the belief in the existence of these solid spheres, circles and epicycles, which was under the especial patronage of the church, was very widespread. tycho brahe expressly boasts of having been the first, by considerations concerning the orbits of the comets, to have demonstrated the impossibility of solid spheres, and to have upset this ingenious scaffolding. he supposed the spaces of our system to be filled with air, and that this medium, disturbed by the motion of the heavenly bodies, opposed a resistance which gave rise to the harmonic sounds. it should be added also that the grecian philosophers, though little fond of observation, but rejoicing rather in framing systems for the explanation of phenomena of which they possessed but the faintest glimpse, have left us some ideas about the nature of shooting stars and aërolites that come very close to those that are now accepted. "some philosophers think," says plutarch in his life of lysander, "that shooting stars are not detached particles of ether which are extinguished by the atmosphere soon after being ignited, nor do they arise from the combustion of the rarefied air in the upper regions, but that they are rather heavenly bodies which fall, that is to say, which escaping in some way from the general force of rotation are precipitated in an irregular manner, sometimes on inhabited portions of the earth, but sometimes also in the ocean, where of course they cannot be found." diogenes of apollonius expresses himself still more clearly: "amongst the stars that are visible move others that are invisible, to which in consequence we are unable to give any name. these latter often fall to the earth and take fire like that star-stone which fell all on fire near Ægos potamos." these ideas were no doubt borrowed from some more ancient source, as he believed that all the stars were made of something like pumice-stone. anaxagoras, in fact, thought that all the heavenly bodies were fragments of rocks which the ether, by the force of its circular motion, had detached from the earth, set fire to, and turned into stars. thus the ionic school, with diogenes of apollonius, placed the aërolites and the stars in one class, and assigned to all of them a terrestrial origin, though in this sense only, that the earth, being the central body, had furnished the matter for all those that surround it. plutarch speaks thus of this curious combination:--"anaxagoras teaches that the ambient ether is of an igneous nature, and by the force of its gyratory motion it tears off blocks of stone, renders them incandescent, and transforms them into stars." it appears that he explained also by an analogous effect of the circular motion the descent of the nemæan lion, which, according to an old tradition, fell out of the moon upon the peloponnesus. according to boeckh, this ancient myth of the nemæan lion had an astronomical origin, and was symbolically connected in chronology with the cycle of intercalation of the lunar year, with the worship of the moon in nemaea, and the games by which it was accompanied. [illustration: plate vi.--the nemÆan lion.] anaxagoras explains the apparent motion of the celestial sphere from east to west by the hypothesis of a general revolution, the interruption of which, as we have just seen, caused the fall of meteoric stones. this hypothesis is the point of departure of the theory of vortices, which more than two thousand years later, by the labours of descartes, huyghens, and hooke, took so prominent a place among the theories of the world. it may be worth adding with regard to the famous aërolite of Ægos potamus, alluded to above, that when the heavens were no longer believed to be solid, the faith in the celestial origin of this, as of other aërolites, was for a long time destroyed. thus bailly the astronomer, alluding to it, says, "if the fact be true, this stone must have been thrown out by a volcano." indeed it is only within the last century that it has been finally accepted for fact that stones do fall from the sky. laplace thought it probable that they came from the moon; but it has now been demonstrated that aërolites, meteors, and shooting stars belong all to one class of heavenly bodies, that they are fragments scattered through space, and circulate like the planets round the sun. when the earth in its motion crosses this heavenly host, those which come near enough to touch its atmosphere leave a luminous train behind them by their heating by friction with the air: these are the _shooting stars_. sometimes they come so close as to appear larger than the moon, then they are _meteors;_ and sometimes too the attraction of the earth makes them fall to it, and these become _aërolites_. but to return to our ancient astronomers:-- they believed the heavens to be in motion, not only because they saw the motion with their eyes, but because they believed them to be animated, and regarded motion as the essence of life. they judged of the rapidity of the stars' motion by a very ingenious means. they perceived that it was greater than that of a horse, a bird, an arrow, or even of the voice, and cleomenas endeavoured to estimate it in the following way. he remarks that when the king of persia made war upon greece he placed men at certain intervals, so as to lie in hearing of each other, and thus passed on the news from athens to susa. now this news took two days and nights to pass over this distance. the voice therefore only accomplished a fraction of the distance that the stars had accomplished twice in the same time. the heavens, as we have seen, were not supposed to consist of a single sphere, but of several concentric ones, the arrangement and names of which we must now inquire into. the early chaldeans established three. the first was the empyreal heaven, which was the most remote. this, which they called also the solid firmament, was made of fire, but of fire of so rare and penetrating a nature, that it easily passed through the other heavens, and became universally diffused, and in this way reached the earth. the second was the ethereal heaven, containing the stars, which were simply formed of the more compact and denser parts of this substance; and the third heaven was that of the planets. the persians, however, gave a separate heaven to the sun, and another to the moon. the system which has enjoyed the longest and most widely-spread reign is that which places above, or rather round, the solid firmament a heaven of water--(the nature of which is not accurately defined), and round this a _primum mobile_, prime mover, or originator of all the motions, and round all this the empyreal heaven, or abode of the blessed. in the most anciently printed scientific encyclopædia known, the _magarita philosophica_, edited in the fifteenth century, that is, two centuries before the adoption of the true system of the world, we have the curious figure represented on the next page, in which we find no less than eleven different heavens. we here see on the exterior the solid empyreal heaven, which is stated in the body of the work to be the abode of the blessed and to be immovable, while the next heaven gives motion to all within, and is followed by the aqueous heaven, then the crystal firmament, and lastly by the several heavens of the planets, sun, and moon. the revolution of these spheres was not supposed to take place, like the motion of the earth in modern astronomy, round an imaginary axis, but round one which had a material existence, which was provided with pivots moving in fixed sockets. thus vitruvius, architect to augustus, teaches it expressly in these words:-- "the heaven turns continually round the earth and sea upon an axis, where two extremities are like two pivots that sustain it: for there are two places in which the governor of nature has fashioned and set these pivots as two centres; one is above the earth among the northern stars; the other is at the opposite end beneath the earth to the south; and around these pivots, as round two centres, he has placed little naves, like those of a wheel upon which the heaven turns continually." [illustration: fig. .] similarly curious ideas we shall find to have prevailed with respect to the meaning of everything that they observed in the heavens: thus what a number of opinions have been hazarded on the nature of the "milky way" alone! some of which we may learn from plutarch. the milky way, he says, is a nebulous circle, which constantly appears in the sky, and which owes its name to its white appearance. certain pythagoreans assert that when phaeton lit up the universe, one star, which escaped from its proper place, set light to the whole space it passed over in its circular course, and so formed the milky way. others thought that this circle was where the sun had been moving at the beginning of the world. according to others it is but an optical phenomenon produced by the reflection of the sun's rays from the vault of the sky as from a mirror, and comparable with the effects seen in the rainbow and illuminated clouds. metrodorus says it is the mark of the sun's passage which moves along this circle. parmenidas pretends that the milky colour arises from a mixture of dense and rare air. anaxagoras thinks it an effect of the earth's shadow projected on this part of the heavens, when the sun is below. democritus says that it is the lustre of several little stars which are very near together, and which reciprocally illuminate each other. aristotle believes it to be a vast mass of arid vapours, which takes fire from a glowing tress, above the region of the ether, and far below that of the planets. posidonius says that the circle is a compound of fire less dense than that of the stars, but more luminous. all such opinions, except that of democritus, are of little value, because founded on nothing; perhaps the worst is that of theophrastus, who said it was the junction between the two hemispheres, which together formed the vault of heaven: and that it was so badly made that it let through some of the light that he supposed to exist everywhere behind the solid sky. we now know that the milky way, like many of the nebulæ, is an immense agglomeration of suns. the milky way is itself a nebula, a mass of sidereal systems, with our own among them, since our sun is a single star in this vast archipelago of eighteen million orbs. the greeks called it the galaxy. the chinese and arabians call it the river of heaven. it is the path of souls among the north american indians, and the road of s. jacques de compostelle among french peasants. in tracing the history of ideas concerning the structure of the heavens among the greek philosophers, we meet with other modifications which it will be interesting to recount. thus eudoxus, who paid greater attention than others to the variations of the motions of the planets, gave more than one sphere to each of them to represent these observed changes. each planet, according to him, has a separate part of the heaven to itself, which is composed of several concentric spheres, whose movements, modifying each other, produce that of the planet. he gave three spheres to the sun: one which turned from east to west in twenty-four hours, to represent the diurnal rotation; a second, which turned about the pole of the ecliptic in - / days, and produced its annual movement; and a third was added to account for a certain supposed motion, by which the sun was drawn out of the ecliptic, and turned about an axis, making such an angle with that of the ecliptic, as represented the supposed aberration. the moon also had three spheres to produce its motions in longitude and latitude, and its diurnal motion. each of the other planets had four, the extra one being added to account for their stations and retrogressions. it should be added that these concentric spheres were supposed to fit each other, so that the different planets were only separated by the thicknesses of these crystal zones. polemarch, the disciple of eudoxus, who went to athens with his pupil calippus for the express purpose of consulting aristotle on these subjects, was not satisfied with the exactness with which these spheres represented the planetary motions, and made changes in the direction of still greater complication. instead of the twenty-six spheres which represented eudoxus' system, calippus established thirty-three, and by adding also intermediary spheres to prevent the motion of one planet interfering with that of the adjacent ones, the number was increased to fifty-six. there is extant a small work, ascribed to aristotle, entitled "letter of aristotle to alexander on the system of the world," which gives so clear an account of the ideas entertained in his epoch that we shall venture to give a somewhat long extract from it. the work, it should be said, is not by all considered genuine, but is ascribed by some to nicolas of damas, by others to anaximenas of lampsacus, a contemporary of alexander's, and by others to the stoic posidonius. it is certain, however, that aristotle paid some attention to astronomy, for he records the rare phenomena of an eclipse of mars by the moon, and the occultation of one of the gemini by the planet jupiter, and the work may well be genuine. it contains the following:-- "there is a fixed and immovable centre to the universe. this is occupied by the earth, the fruitful mother, the common focus of every kind of living thing. immediately surrounding it on all sides is the air. above this in the highest region is the dwelling-place of the gods, which is called the heavens. the heavens and the universe being spherical and in continual motion, there must be two points on opposite sides, as in a globe which turns about an axis, and these points must be immovable, and have the sphere between them, since the universe turns about them. they are called the poles. if a line be drawn from one of these points to the other it will be the diameter of the universe, having the earth in the centre and the two poles at the extremities; of these two poles the northern one is always visible above our horizon, and is called the arctic pole; the other, to the south, is always invisible to us--it is called the antarctic pole. "the substance of the heavens and of the stars is called ether; not that it is composed of flame, as pretended by some who have not considered its nature, which is very different from that of fire, but it is so called because it has an eternal circular motion, being a divine and incorruptible element, altogether different from the other four. "of the stars contained in the heavens some are fixed, and turn with the heavens, constantly maintaining their relative positions. in their middle portion is the circle called the _zoophore_, which stretches obliquely from one tropic to the other, and is divided into twelve parts, which are the twelve signs (of the zodiac). the others are wandering stars, and move neither with the same velocity as the fixed stars, nor with a uniform velocity among themselves, but all in different circles, and with velocities depending on the distances of these circles from the earth. "although all the fixed stars move on the same surface of the heavens, their number cannot be determined. of the movable stars there are seven, which circulate in as many concentric circles, so arranged that the lower circle is smaller than the higher, and that the seven so placed one within the other are all within the spheres of the fixed stars. "on the nearer, that is inner, side of this ethereal, immovable, unalterable, impassible nature is placed our movable, corruptible, and mortal nature. of this there are several kinds, the first of which is fire, a subtle inflammable essence, which is kindled by the great pressure and rapid motion of the ether. it is in this region of air, when any disturbance takes place in it, that we see kindled shooting-stars, streaks of light, and shining motes, and it is there that comets are lighted and extinguished. "below the fire comes the air, by nature cold and dark, but which is warmed and enflamed, and becomes luminous by its motion. it is in the region of the air, which is passive and changeable in any manner, that the clouds condense, and rain, snow, frost, and hail are formed and fall to the earth. it is the abode of stormy winds, of whirlwinds, thunder, lightning, and many other phenomena. "the cause of the heaven's motion is god. he is not in the centre, where the earth is a region of agitation and trouble, but he is above the outermost circumference, which is the purest of all regions, a place which we call rightly _ouranos_, because it is the highest part of the universe, and _olympos_, that is, perfectly bright, because it is altogether separated from everything like the shadow and disordered movements which occur in the lower regions." we notice in this extract a curious etymology of the word ether, namely, as signifying perpetual motion ([greek: aei teein]), though it is more probable that its true, as its more generally accepted derivation is from [greek: aithein], to burn or shine, a meaning doubtless alluded to in a remarkable passage of hippocrates, [greek: peri sarkôn]. "it appears to me," he says, "that what we call the principle of heat is immortal, that it knows all, sees all, hears all, perceives all, both in the past and in the future. at the time when all was in confusion, the greater part of this principle rose to the circumference of the universe; it is this that the ancients have called _ether_." the first greek that can be called an astronomer was thales, born at miletus b.c., who introduced into greece the elements of astronomy. his opinions were these: that the stars were of the same substance as the earth, but that they were on fire; that the moon borrowed its light from the sun, and caused the eclipses of the latter, while it was itself eclipsed when it entered the earth's shadow; that the earth was round, and divisible into five zones, by means of five circles, _i.e._ the arctic and antarctic, the two tropics, and the equator; that the latter circle is cut obliquely by the ecliptic, and perpendicularly by the meridian. up to his time no division of the sphere had been made beyond the description of the constellations. these opinions do not appear to have been rapidly spread, since herodotus, one of the finest intellects of greece, who lived two centuries later, was still so ill-instructed as to say, in speaking of an eclipse, "the sun abandoned its place, and night took the place of day." anaxagoras, of whom we have spoken before, asserted that the sun was a mass of fire larger than the peloponnesus. plutarch says he regarded it as a burning stone, and diogenes laertius looked upon it as hot iron. for this bold idea he was persecuted. they considered it a crime that he taught the causes of the eclipses of the moon, and pretended that the sun is larger than it looks. he first taught the existence of one god, and he was taxed with impiety and treason against his country. when he was condemned to death, "nature," he said, "has long ago condemned me to the same; and as to my children, when i gave them birth i had no doubt but they would have to die some day." his disciple pericles, however, defended him so eloquently that his life was spared, and he was sent into exile. pythagoras, who belonged to the school of thales, and who travelled in phoenicia, chaldea, judæa, and egypt, to learn their ideas, ventured, in spite of the warnings of the priests, to submit to the rites of initiation at heliopolis, and thence returned to samos, but meeting with poor reception there, he went to italy to teach. from him arose the _italian school_, and his disciples took the name of philosophers (lovers of wisdom) instead of that of sages. we shall learn more about him in the chapter on the harmony of the spheres. his first disciple, empedocles, famous for the curiosity which led him to his death in the crater of Ætna, as the story goes, thought that the true sun, the fire that is in the centre of the universe, illuminated the other hemisphere, and that what we see is only the reflected image of that, which is invisible to us, and all of whose movements it follows. his disciple, philolaus, also taught that the sun was a mass of glass, which sent us by reflection all the light that it scattered through the universe. we must not, however, forget that these opinions are recorded by historians who probably did not understand them, and who took in the letter what was only intended for a comparison or figure. if we are to believe plutarch, xenophanes, who flourished about b.c., was very wild in his opinions. he thought the stars were lighted every night and extinguished every morning; that the sun is a fiery cloud; that eclipses take place by the sun being extinguished and afterwards rekindled; that the moon is inhabited, but is eighteen times larger than the earth; that there are several suns and several moons for giving light to different countries. this can only be matched by those who said the sun went every night through a hole in the earth round again to the east; or that it went above ground, and if we did not see it going back it was because it accomplished the journey in the night. parmenidas was the disciple of xenophanes. he divided the earth, like thales, into zones; and he added that it was suspended in the centre of the universe, and that it did not fall because there was no reason why it should move in one direction rather than another. this argument is perfectly philosophical, and illustrates a principle employed since the time of archimedes, and of which leibnitz made so much use. such are some of the general ideas which were held by the greeks and others on the nature of the heavens, omitting that of ptolemy, of which we shall give a fuller account hereafter. we see that they were all affected by the dominant idea of the superiority of the earth over the rest of the universe, and were spoiled for want of the grand conception of the immensity of space. the universe was for them a closed space, outside of which there was _nothing_; and they busied themselves with metaphysical questions as to the possibility of space being infinite. in the meantime their conceptions of the distances separating us from other visible parts of the universe were excessively cramped. hesiod, for instance, thinks to give a grand idea of the size of the universe by saying that vulcan's anvil took seven days to fall from heaven to earth, when in reality, as now calculated, it would take no less than seventy-two years for the light, even travelling at a far greater rate, to reach us from one of the nearest of the fixed stars. chapter vii. the celestial harmony. nature presents herself to us under various aspects. at times, it may be, she presents to us the appearance of discord, and we fail to perceive the unity that pervades the whole of her actions. at others, however, and most often to an instructed mind, there is a concord between her various powers, a harmony even in her sounds, that will not escape us. even the wild notes of the tempest and the bass roll of the thunder form themselves into part of the grand chorus which in the great opera are succeeded by the solos of the evening breeze, the songs of birds, or the ripple of the waves. these are ideas that would most naturally present themselves to contemplative minds, and such must have been the students of the silent, but to them harmonious and tuneful, star-lit sky, under the clear atmosphere of greece. the various motions they observed became indissolubly connected in their minds with music, and they did not doubt that the heavenly spheres made harmony, if imperceptible to human ears. but their ideas were more precise than this. they discovered that harmony depended on number, and they attempted to prove that whether the music they might make were audible or not, the celestial spheres had motions which were connected together in the same way as the numbers belonging to a harmony. the study of their opinions on this point reveals some very curious as well as very interesting ideas. we may commence by referring to an ancient treatise by timæus of locris on the soul of the universe. to him we owe the first serious exposition of the complete harmonic cosmography of pythagoras. we must premise that, according to this school, god employed all existing matter in the formation of the universe--so that it comprehends all things, and all is in it. "it is a unique, perfect, and spherical production, since the sphere is the most perfect of figures; animated and endowed with reason, since that which is animated and endowed with reason is better than that which is not." so begins timæus, and then follows, as a quotation from plato, a comparison of the earth to what would appear to us nowadays to be a very singular animal. not only, says plato, is the earth a sphere, but this sphere is perfect, and its maker took care that its surface should be perfectly uniform for many reasons. the universe in fact has no need of eyes, since there is nothing outside of it to see; nor yet of ears, since there is nothing but what is part of itself to make a sound; nor of breathing organs, as it is not surrounded by air: any organ that should serve to take in nourishment, or to reject the grosser parts, would be absolutely useless, for there being nothing outside it, it could not receive or reject anything. for the same reason it needs no hands with which to defend itself, nor yet of feet with which to walk. of the seven kinds of motion, its author has given it that which is most suitable for its figure in making it turn about its axis, and since for the execution of this rotatory motion no arms or legs are wanted, its maker gave it none. with regard to the soul of the universe, plato, according to timæus, says that god composed it "of a mixture of the divisible and indivisible essences, so that the two together might be united into one, uniting two forces, the principles of two kinds of motion, one that which is _always the same_, and the other that which is _always changing_. the mixture of these two essences was difficult, and was not accomplished without considerable skill and pains. the proportions of the mixture were according to harmonic numbers, so chosen that it is possible to know of what, and by what rule, the soul of the universe is compounded." by harmonic numbers timæus means those that are proportional to those representing the consonances of the musical scale. the consonances known to the ancients were three in number: the diapason, or octave, in the proportion of to , the diapent, or fifth, in that of to , and the diatessaron, or fourth, in that of to ; when to these are joined the tones which fill the intervals of the consonances, and are in the proportion of to , and the semitones in that of to , all the degrees of the musical scale is complete. the discovery of these harmonic numbers is due to pythagoras. it is stated that when passing one day near a forge, he noticed that the hammers gave out very accurate musical concords. he had them weighed, and found that of those which sounded the octave, one weighed twice as much as the other; that of those which made a perfect fifth, one weighed one third more than the other, and in the case of a fourth, one quarter more. after having tried the hammers, he took a musical string stretched with weights, and found that when he had applied a given weight in the first instance to make any particular note, he had to double the weight to obtain the octave, to add one third extra only to obtain a fifth, a quarter for the fourth, and eight for one tone, and about an eighteenth for a half-tone; or more simply still, he stretched a cord once for all, and then when the whole length sounded any note, when stopped in the middle it gave the octave, at the third it gave the fifth, at the quarter the fourth, at the eighth the tone, and at the eighteenth the semi-tone. since the ancients conceived of the soul by means of motion, the quantity of motion developed in anything was their measure of the quantity of its soul. now the motion of the heavenly bodies seemed to them to depend on their distance from the centre of the universe, the fastest being those at the circumference of the whole. to determine the relative degrees of velocity, they imagined a straight line drawn outwards from the centre of the earth, as far as the empyreal heaven, and divided it according to the proportions of the musical scale, and these divisions they called the harmonic degrees of the soul of the universe. taking the earth's radius for the first number, and calling it unity, or, in order to avoid fractions, denoting it by , the second degree, which is at the distance of an harmonic third, will be represented by plus its eighth part, or . the third degree will be , plus its eighth part, or . the fourth, being a semitone, will be as to , which will give ; and so on. the eighth degree will in this way be the double of or , and represents the first octave. they continued this series to degrees, as in the following table:-- the earth. mi + / = re + / = ut : : : : si + / = la + / = sol + / = fa : : : : mi + / = re + / = ut : : : : si + / = la + / = sol + / = fa : : : : mi + / = re + / = ut : : : si + = si : : : : la + / = sol + / = fa : : : : mi + / = re + / = ut + / = si : : : : la + / = sol + / = fa : : : : mi + = mi : : : : re + / = ut + / = si : : : : la + / = sol = + the empyreal heaven. sum of all the terms, , . this series they considered a complete one, because by taking the terms in their proper intervals, the last becomes times the original number, and in the school of pythagoras this had a mystic signification, and was considered as the perfect number. the reason for considering a perfect number was curious. it is the sum of the first linear, square, and cubic numbers added to unity. first there is , which represents the point, then and , the first linear numbers, even and uneven, then and , the first square or surface numbers, even and uneven, and the last and , the first solid or cubic numbers, even and uneven, and is the sum of all the former. whence, taking the number as the symbol of the universe, and the numbers which compose it as the elements, it appeared right that the soul of the universe should be composed of the same elements. on this scale of distances, with corresponding velocities, they arranged the various planets, and the universe comprehended all these spheres, from that of the fixed stars (which was excluded) to the centre of the earth. the sphere of the fixed stars was the common envelope, or circumference of the universe, and saturn, immediately below it, corresponded to the thirty-sixth tone, and the earth to the first, and the other planets with the sun and moon at the various harmonic distances. they reckoned one tone from the earth to the moon, half a tone from the moon to mercury, another half-tone to venus, one tone and a half from venus to the sun, one from the sun to mars, a semitone from mars to jupiter, half a tone from jupiter to saturn, and a tone and a half from saturn to the fixed stars; but these distances were not, as we shall see, universally agreed upon. according to timæus, the sphere of the fixed stars, which contains within it no principle of contrariety, being entirely divine and pure, always moves with an equal motion in the same direction from east to west. but the stars which are within it, being animated by the mixed principle, whose composition has been just explained, and thus containing two contrary forces, yield on account of one of these forces to the motion of the sphere of fixed stars from east to west, and by the other they resist it, and move in a contrary direction, in proportion to the degree with which they are endowed with each; that is to say, that the greater the proportion of the material to the divine force that they possess, the greater is their motion from west to east, and the sooner they accomplish their periodic course. now the amount of this force depends on the matter they contain. thus, according to this system, the planets turn each day by the common motion with all the heavens about the earth from east to west, but they also retrograde towards the east, and accomplish their periods according to their component parts. the additions which plato made to this theory have always been a proverb of obscurity, and none of his commentators have been able to make anything of them, and very possibly they were never intended to. so far the harmony of the heavenly bodies has been explained with reference to numbers only, and we may add to this that they reckoned , stadia, or , miles, to represent a tone, which was thus the distance of the earth to the moon, and the same measurement made it , from the earth to the sun, and the same distance from the sun to the fixed stars. but plato teaches in his _republic_ that there is actual musical, harmony between the planets. each of the spheres, he said, carried with it a siren, and each of these sounding a different note, they formed by their union a perfect concert, and being themselves delighted with their own harmony, they sang divine songs, and accompanied them by a sacred dance. the ancients said there were nine muses, eight of whom, according to plato, presided over celestial, and the ninth over terrestrial things, to protect them from disorder and irregularity. cicero and macrobius also express opinions on this harmonious concert. such great motions, says cicero, cannot take place in silence, and it is natural that the two extremes should have related sounds as in the octave. the fixed stars must execute the upper note, and the moon the base. kepler has improved on this, and says jupiter and saturn sing bass, mars takes the tenor, the earth and venus are contralto, and mercury is soprano! true, no one has ever heard these sounds, but pythagoras himself may answer this objection. we are always surrounded, he says, by this melody, and our ears are accustomed to it from our birth, so that, having nothing different to compare it with, we cannot perceive it. we may here recall the further development of the idea of the soul of the universe, which was the source of this harmony, and endeavour to find a rational interpretation of their meaning. they said that nature had made the animals mortal and ephemeral, and had infused their souls into them, as they had been extracts from the sun or moon, or even from one of the planets. a portion of the unchangeable essence was added to the reasoning part of man, to form a germ of wisdom in privileged individuals. for the human soul there is one part which possesses intelligence and reason, and another part which has neither the one nor the other. the various portions of the general soul of the universe resided, according to timæus, in the different planets, and depended on their various characters. some portions were in the moon, others in mercury, venus, or mars, and so on, and thus they give rise to the various characters and dispositions that are seen among men. but to these parts of the human soul that are taken from the planets is joined a spark of the supreme divinity, which is above them all, and this makes man a more holy animal than all the rest, and enables him to have immediate converse with the deity himself. all the different substances in nature were supposed to be endowed with more or less of this soul, according to their material nature or subtilty, and were placed in the same order along the line, from the centre to the circumference, on which the planets were situated, as we have seen above. in the centre was the earth, the heaviest and grossest of all, which had but little if any soul at all. between the earth and the moon, timæus placed first water, then the air, and lastly elementary fire, which he considered to be principles, which were less material in proportion as they were more remote and partook of a larger quantity of the soul of the universe. beyond the moon came all the planets, and thus were filled up the greater number of the harmonic degrees, the motions of the various bodies being guided by the principle enunciated above. when we carefully consider this theory we find that by a slight change of name we may bring it more into harmony with modern ideas. it would appear indeed that the ancients called that "soul" which we now call "force," and while we say that this force of attraction is in proportion to the masses and the inverse square of the distance, they put it that it was proportional to the matter, and to the divine substance on which the distance depended. so that we may interpret timæus as stating this proposition: _the distances of the stars and their forces are proportional among themselves to their periodic times._ "some people," says plutarch, "seek the proportions of the soul of the universe in the velocities (or periodic times), others in the distances from the centre; some in the masses of the heavenly bodies, and others more acute in the ratios of the diameters of their orbits. it is probable that the mass of each planet, the intervals between the spheres and the velocities of their motions, are like well-tuned musical instruments, all proportional harmonically with each other and with all other parts of the universe, and by necessary consequence that there are the same relative proportions in the soul of the universe by which they were formed by the deity." it is marvellous how deeply occupied were all the best minds in greece and italy on this subject, both poets and philosophers; ocellus, democritus, timæus, aristotle, and lucretius have all left treatises on the same subject, and almost with the same title, "the nature of the universe." though somewhat similar to that of timæus, it will be interesting to give an account of the ideas of one of these, ocellus of lucania. ocellus represents the universe as having a spherical form. this sphere is divided into concentric layers; above that of the moon they were called celestial spheres, while below it and inwards as far as the centre of the earth they were called the elementary spheres, and the earth was the centre of them all. in the celestial spheres all the stars were situated, which were so many gods, and among them the sun, the largest and most powerful of all. in these spheres is never any disturbance, storm, or destruction, and consequently no reparation, no reproduction, no action of any kind was required on the part of the gods. below the moon all is at war, all is destroyed and reconstructed, and here therefore it is that generations are possible. but these take place under the influence of the stars, and particularly that of the sun, which in its course acts in different ways on the elementary spheres, and produces continual variations in them, from whence arises the replenishing and diversifying of nature. it is the sun that lights up the region of fire, that dilates the air, melts the water, and renders fertile the earth, in its daily course from east to west, as well as in this annual journey into the two tropics. but to what does the earth owe its germs and its species? according to some philosophers these germs were celestial ideas which both gods and demons scattered from above over every part of nature, but according to ocellus they arise continually under the influence of the heavenly bodies. the divisions of the heavens were supposed to separate the portion that is unalterable from that which is in ceaseless change. the line dividing the mortal from the immortal is that described by the moon: all that lies above that, inclusive, is the habitation of the gods; all that lies below is the abode of nature and discord; the latter tending constantly to destruction, the former to the reconstruction of all created things. ideas such as these, of which we could give other examples more remotely connected with harmony, whatever amount of truth we may discover in them, prove themselves to have been made before the sciences of observation had enabled men to make anything better than empty theories, and to support them with false logic. no better example of the latter can perhaps be mentioned here than the way in which ocellus pretends to prove that the world is eternal. "the universe," he says, "_having_ always existed, it follows that everything in it and every arrangement of it must always have been as it is now. the several parts of the universe _having_ always existed with it, we may say the same of the parts of these parts; thus the sun, the moon, the fixed stars, and the planets have always existed with the heavens; animals, vegetables, gold, and silver with the earth; the currents of air, winds, and changes from hot to cold, from cold to hot, with the air. _therefore_ the heaven, with all that it now contains; the earth, with all that it produces and supports; and lastly, the whole aërial region, with all its phenomena, have always existed." when this system of argument passed away, and exact observation took its place, it was soon found that so far from what the ancients had argued _must be_ really being the case, no such relation as they indicated between the distances or velocities of the planets could be traced, and therefore no harmony in the heavens in this sense. it is not indeed that we can say no sounds exist because we hear none; but considering harmony really to consist of the relations of numbers, no such relations exist between the planets' distances, as measured now of course from the sun, instead of being, as then, imagined from the earth. the gamut is nothing else than the series of numbers:-- do re mi fa sol la si do / / / / / / and is independent of our perception of the corresponding notes. a concert played before a deaf assembly would be a concert still. if one note is made by , vibrations per second, and another by , , we should hear them as an octave, but if one had only and the other , they would still be an octave, though inaudible as notes to us; so too we may speak even of the harmony of luminous vibrations of ether, though they do not affect our ears. the velocities of the planets do not coincide with the terms of this series. the nearer they are to the sun the faster is their motion, mercury travelling at the mean rate of , metres a second, venus, , , the earth , , mars , , jupiter , , saturn , , uranus , , and neptune , , numbers which are in the proportion roundly of , , , , , , , , which have no sufficient relation to the terms of an harmonic series, to make any harmony obvious. returning, however, to the ancient philosophers, we are led by their ideas about the soul of the universe to discover the origin of their gods and natural religion. they were persuaded that only living things could move, and consequently that the moving stars must be endowed with superior intelligence. it may very well be that from the number seven of the planets, including the sun and moon, which were their earliest gods, arose the respect and superstition with which all nations, and especially the orientals, regarded that number. from these arose the seven superior angels that are found in the theologies of the chaldeans, persians, and arabians; the seven gates of mithra, through which all souls must pass to reach the abode of bliss; the seven worlds of purification of the indians, and all the other applications of the number seven which so largely figure in judaism, and have descended from it to our own time. on the other hand, as we have seen, this number seven may have been derived from the number of the stars in the pleiades. we have noticed in our chapter on the history of the zodiac how the various signs as they came round and were thought to influence the weather and other natural phenomena, came at last to be worshipped. not less, of course, were the sun and moon deified, and that by nations who had no zodiac. among the egyptians the sun was painted in different forms according to the time of year, very much as he is represented in our own days in pictures of the old and new years. at the winter solstice with them he was an infant, at the spring equinox he was a young man, in summer a man in full age with flowing beard, and in the autumn an old man. their fable of osiris was founded on the same idea. they represented the sun by the hawk, and the moon by the ibis, and to these two, worshipped under the names of osiris and isis they attributed the government of the world, and built a city, heliopolis, to the former, in the temple of which they placed his statue. the phenicians in the same way, who were much influenced by ideas of religion, attributed divinity to the sun, moon, and stars, and regarded them as the sole causes of the production and destruction of all things. the sun, under the name of hercules, was their great divinity. the ethiopians worshipped the same, and erected the famous table of the sun. those who lived above meroë, admitted the existence of eternal and incorruptible gods, among which they included the sun, moon, and the universe. like the incas of peru, they called themselves the children of the sun, whom they regarded as their common father. the moon was the great divinity of the arabs. the saracens called it cabar, or the great, and its crescent still adorns the religious monuments of the turks. each of their tribes was under the protection of some particular star. sabeism was the principal religion of the east. the heavens and the stars were its first object. in reading the sacred books of the ancient persians contained in the _zendavesta_, we find on every page invocations addressed to mithra, to the moon, the stars, the elements, the mountains, the trees, and every part of nature. the ethereal fire circulating through all the universe, and of which the sun is the principal focus, was represented among the fire-worshippers by the sacred and perpetual fire of their priests. each planet had its own particular temple, where incense was burnt in its honour. these ancient peoples embodied in their religious systems the ideas which, as we have seen, led among the greeks to the representation of the harmony of heaven. all the world seemed to them animated by a principle of life which circulated through all parts, and which preserved it in an eternal activity. they thought that the universe lived like man and the other animals, or rather that these latter only lived because the universe was essentially alive, and communicated to them for an instant an infinitely small portion of its own immortality. they were not wise, it may be, in this, but they appear to have caught some of the ideas that lie at the basis of religious thought, and to have traced harmony where we have almost lost the perception of it. chapter viii. astronomical systems. in our former chapters we have gained some idea of the general structure of the heavens as represented by ancient philosophers, and we no longer require to know what was thought in the infancy of astronomy, when any ideas promulgated were more or less random ones; but in this chapter we hope to discuss those arrangements of the heavenly bodies which have been promulgated by men as complete systems, and were supposed to represent the totality of the facts. the earliest thoroughly-established system is that of ptolemy. it was not indeed invented by him. the main ideas had been entertained long before his time, but he gave it consistence and a name. we obtain an excellent view of the general nature of this system from cicero. he writes:-- "the universe is composed of nine circles, or rather of nine moving globes. the outermost sphere is that of the heavens which surrounds all the others, and on which are fixed the stars. beneath this revolve seven other globes, carried round by a motion in a direction contrary to that of the heavens. on the first circle revolves the star which men call saturn; on the second jupiter shines, that beneficent and propitious star to human eyes; then follows mars, ruddy and awful. below, and occupying the middle region, revolves the sun, the chief, prince, and moderator of the other stars, the soul of the world, whose immense globe spreads its light through space. after him come, like two companions, venus and mercury. lastly, the lowest globe is occupied by the moon, which borrows its light from the star of day. below this last celestial circle, there is nothing but what is mortal and corruptible, except the souls given by a beneficent divinity to the race of men. above the moon all is eternal. the earth, situated in the centre of the world, and separated from heaven on all sides, forms the ninth sphere; it remains immovable, and all heavy bodies are drawn to it by their own weight." the earth, we should add, is surrounded by the sphere of air, and then by that of fire, and by that of ether and the meteors. with respect to the motions of these spheres. the first circle described about the terrestrial system, namely, that of the moon, was accomplished in days, hours, and minutes. next to the moon, mercury in the second, and venus in the third, and the sun in the fourth circle, all turned about the earth in the same time, days, hours, and minutes. but these planets, in addition to the general movement, which carried them in hours round from east to west and west to east, and the annual revolution, which made them run through the zodiacal circle, had a third motion by which they described a circle about each point of their orbit taken as a centre. [illustration: fig. .--ptolemy's astronomical system.] the fifth sphere, carrying mars, accomplished its revolution in two years. jupiter took years, days, and hours to complete his orbit, and saturn in the seventh sphere took years and days. above all the planets came the sphere of the fixed stars, or firmament, turning from east to west in hours with inconceivable rapidity, and endued also with a proper motion from west to east, which was measured by hipparchus, and which we now call the precession of the equinoxes, and know that it has a period of , years. above all these spheres, a _primum mobile_ gave motion to the whole machine, making it turn from east to west, but each planet and each fixed star made an effort against this motion, by means of which each of them accomplished their revolution about the earth in greater or less time, according to its distance, or the magnitude of the orbit it had to accomplish. one immense difficulty attended this system. the apparent motions of the planets is not uniform, for sometimes they are seen to advance from west to east, when their motion is called _direct_, sometimes they are seen for several nights in succession at the same point in the heavens, when they are called _stationary_, and sometimes they return from east to west, and then their motion is called _retrograde_. we know now that this apparent variation in the motion of the planets is simply due to the annual motion of the earth in its orbit round the sun. for example, saturn describes its vast orbit in about thirty years, and the earth describes in one year a much smaller one inside. now if the earth goes faster in the same direction as saturn, it is plain that saturn will be left behind and appear to go backwards, while if the earth is going in the same direction the velocity of saturn will appear to be decreased, but his direction of motion will appear unaltered. to explain these variations, however, according to his system, ptolemy supposed that the planets did not move exactly in the circumference of their respective orbits, but about an _ideal centre_, which itself moved along this circumference. instead therefore of describing a circle, they described parts of a series of small circles, which would combine, as is easy to see, into a series of uninterrupted waves, and these he called _epicycles_. another objection, which even this arrangement did not overcome, was the variation of the size of the planets. to overcome this hipparchus gave to the sphere of each planet a considerable thickness, and saw that the planet did not turn centrally round the earth, but round a centre of motion placed outside the earth. its revolution took place in such a manner, that at one time it reached the inner boundary, at another time the outer boundary of its spherical heaven. but this reply was not satisfactory, for the differences in the apparent sizes proved by the laws of optics such a prodigious difference between their distances from the earth at the times of conjunction and opposition, that it would be extremely difficult to imagine spheres thick enough to allow of it. it was a gigantic and formidable piece of machinery to which it was necessary to be continually adding fresh pieces to make observation accord with theory. in the thirteenth century, in the times of the king-astronomer, alphonso x. of castile, there were already seventy-five circles, one within the other. it is said that one day he exclaimed, in a full assemblage of bishops, that if the deity had done him the honour to ask his advice before creating the world, he could have told him how to make it a little better, or at all events more simply. he meant to express how unworthy this complication was of the dignity of nature. [illustration: fig. .--the epicycles of ptolemy.] fracastor, in his _homocentrics_, says that nothing is more monstrous or absurd than all the excentrics and epicycles of ptolemy, and proposes to explain the difference of velocity in the planets at different parts of their orbits by the medium offering greater or less resistance, and their alteration in apparent size by the effect of refraction. the essential element of this system was that it took appearances for realities, and was founded on the assumption that the earth is fixed in the centre of the universe, and of course therefore neglected all the appearances produced by its motion, or had to explain them by some peculiarity in the other planets. although it was corrected from time to time to make it accord better with observation, it was the same essentially that was taught officially everywhere. it reigned supreme in egypt, greece, italy, and arabia, and in the great school of alexandria, which consolidated it and enriched it by its own observations. but though the same in essence, the details, and especially the means of overcoming the difficulties raised by increased observations, have much varied, and it will be interesting and instructive to record some of the chief of them. one of the most important influences in modifying the astronomical systems taught to the world has been that of the fathers of the christian church. when, after five centuries of patient toil, of hopes, ambitions, and discussions, the christian church took possession of the thrones and consciences of men, they founded their physical edifice on the ancient system, which they adapted to their special wants. with them aristotle and ptolemy reigned supreme. they decreed that the earth constituted the universe, that the heavens were made for it, that god, the angels, and the saints inhabited an eternal abode of joy situated above the azure sphere of the fixed stars, and they embodied this gratifying illusion in all their illuminated manuscripts, their calendars, and their church windows. the doctors of the church all acknowledged a plurality of heavens, but they differed as to the number. st. hilary of poitiers would not fix it, and the same doubt held st. basil back; but the rest, for the most part borrowing their ideas from paganism, said there were six or seven, or up to ten. they considered these heavens to be so many hemispheres supported on the earth, and gave to each a different name. in the system of bede, which had many adherents, they were the air, ether, fiery space, firmament, heaven of the angels, and heaven of the trinity. the two chief varieties in the systems of the middle ages may be represented as follows:-- those who wished to have everything as complete as possible combined the system of ptolemy with that of the fathers of the church, and placed in the centre of the earth the infernal regions which they surrounded by a circle. another circle marked the earth itself, and after that the surrounding ocean, marked as water, then the circle of air, and lastly that of fire. enveloping these, and following one after the other, were the seven circles of the seven planets; the eighth represented the sphere of the fixed stars on the firmament, then came the ninth heaven, then a tenth, the _coelum cristallinum_, and lastly an eleventh and outermost, which was the empyreal heaven, where dwelt the cherubim and seraphim, and above all the spheres was a throne on which sat the father, as jupiter olympus. the others who wished for more simplicity, represented the earth in the centre of the universe, with a circle to indicate the ocean, the second sphere was that of the moon; the third was that of the sun; on the fourth were placed the four planets, jupiter, mars, venus and mercury; there was a fifth for the space outside the planets, and the last outside one was the firmament; altogether seven spheres instead of eleven. as a specimen of the style of representation of the astronomical systems of the middle ages, we may take the figure on the following page:-- here we see the earth placed immovable in the centre of the universe, and represented by a disc traversed by the mediterranean, and surrounded by the ocean. round this are circumscribed the celestial spheres. that of the moon first, then that of mercury, in which several constellations, as the lyre, cassiopeia, the crown, and others, are roughly indicated, then comes the sphere of venus with sagittarius and the swan. after this comes the _celestis_ _paradisus_, and the legend that, "the paradise to which paul was raised is in this third locality; some of these must reach to us, since in them repose the souls of the prophets." in the other circles are yet other constellations: for example pegasus, andromeda, the dog, argo, the he-goat, aquarius, the fishes, and canopus, figured by a star of the first magnitude. to the north is seen near the constellation of the swan a large star with seven rays, meant to represent the brightest of those which compose the great bear. the stars of cassiopeia are not only misplaced, but roughly represented. the lyre is curiously drawn. the positions of the constellations just named are all wrong in this figure, just as we find those of towns in maps of the earth. the cartographers of the middle ages, with incredible ignorance, misplaced in general every locality. they did the same for the constellations in the celestial hemispheres. in the heaven of jupiter, and in that of saturn we read the words--seraphim, dominationes, potestates, archangeli, virtutes coelorum, principatus, throni, cherubim, all derived from their theology. a veritable muddle! the angels placed with the heroes of mythology, the immortal virgins with venus and andromeda, and the saints with the great bear, the hydra, and the scorpion! [illustration: fig. .--heavens of the middle ages.] another such richly illuminated manuscript in the library at ghent, entitled liber floridus, contains a drawing similar to this under the title _astrologia secundum bedum_. only, instead of the earth, there is a serpent in the centre with the name great bear, and the twins are represented by a man and woman, andromeda in a chasuble, and venus as a nun! several similar ones might be quoted, varying more or less from this; one, executed in a geographical manuscript of the fifteenth century, has the tenth sphere, being that of the fixed stars, then the crystalline heaven, and then the immovable heaven, "which," it says, "according to sacred and certain theology, is the dwelling-place of the blessed, where may we live for ever and ever, amen;" "this is also called the empyreal heaven." near each planet the author marks the time of its revolution, but not at all correctly. [illustration: plate vii.--heavens of the fathers.] the constructors of these systems were not in the least doubt as to their reality, for they actually measured the distance between one sphere and another, though in every case their numbers were far from the truth as we now know it. we may cite as an example an italian system whose spheres were as follows:--terra, aqua, aria, fuoco, luna, mercurio, venus, sol, marte, giove, saturno, stelle fixe, sfera nona, cielo empyreo. attached to the design is the following table of dimensions which we may copy:-- miles. from the centre of the earth to the surface , " " " " inner side of the heaven of the moon , diameter of moon , from the centre of the earth to mercury , diameter of mercury from the centre of the earth to venus , diameter of venus , from the centre of the earth to the sun , , diameter of the sun , from the centre of the earth to mars , , diameter of mars , from the centre of the earth to jupiter , , diameter of jupiter , from the centre of the earth to outside of saturn's heaven , , diameter of saturn , from the centre of the earth to the fixed stars , , the author states that he cannot pursue his calculations further, and condescends to acknowledge that it is very difficult to know accurately what is the thickness of the ninth and of the crystalline heavens! perhaps, however, these reckonings are better than those of the egyptians, who came to the conclusion that saturn was only distant miles, the sun only , and the moon . these numerous variations and adaptations of the ptolemaic system, prove what a firm hold it had taken, and how it reigned supreme over all minds. nor are we merely left to gather this. they consciously looked to ptolemy as their great light, if we may judge from an emblematic drawing taken from an authoritative astronomical work, the _margarita philosophica_, which we give on the opposite page. [illustration: fig. .] in all the systems derived from ptolemy, the order of the planets remained the same, and mercury and venus were placed nearer to the earth than the sun is. according to many authors, however, plato made a variation in this respect, by putting them outside the sun, on the ground that they never were seen to pass across its surface. he had obviously never heard of the "transit of venus." this arrangement was adopted by theon, in his commentary on the _almagesta_ of ptolemy, and afterwards by geber, who alone among the arabians departed from the strict ptolemaic system. [illustration: fig. .--egyptian system.] the egyptians improved upon this idea, and made the first step towards the true system, by representing these two planets, mercury and venus, as revolving round the sun instead of the earth. all the rest of their system was the same as that of ptolemy, for the sun itself, and the other planets and the fixed stars all revolved round the earth in the centre. this system of course accounted accurately for the motions of the two inferior planets, whose nearness to the sun may have suggested their connection with it. this system was in vogue at the same time as ptolemy's, and numbers vitruvius amongst its supporters. [illustration: fig. .--capella's system.] in the fifth century of our era martian capella taught a variation on the egyptian system, in which he made mercury and venus revolve in the same orbit round the sun. in the treatise entitled _quod tellus non sit centrum omnibus planetis_, he explains that when mercury is on this side of the orbit it is nearer to us than venus, and farther off from us than that planet when it is on the other side. this hypothesis was also adopted in the middle ages. we have here indicated the time of the revolution of the various planets, and notice that the firmament is said to move round from west to east in , years; the second heaven in , , while the _primum mobile_ outside moved in the contrary direction in twenty-four hours. these egyptian systems survived in some places the true one, as they were thought to overcome the chief difficulties of the ptolemaic without interfering with the stability of the earth, and they were known as the _common system_, _i.e._ containing the elements of both. such were the astronomical systems in vogue before the time of copernicus--all of them based upon the principle of the earth being the immovable centre of the universe. we must now turn to trace the history of the introduction of that system which has completely thrown over all these former ones, and which every one knows now to be the true one--the copernican. no revolution is accomplished, whether in science or politics, without having been long in preparation. the theory of the motion of the earth had been conceived, discussed, and even taught many ages before the birth of copernicus. and the best proof of this is the acknowledgment of copernicus himself in his great work _de revolutionibus orbium cælestium_, in which he laid down the principles of his system. we will quote the passage in which it is contained. "i have been at the trouble," he writes, "to read over all the works of philosophers that i could procure, to see if i could find in them any different opinion to that which is now taught in the schools respecting the motions of the celestial spheres. and i saw first in cicero that mætas had put forth the opinion that the earth moves. (mætam sensisse terram moveri.) afterwards i found in plutarch that others had entertained the same idea." here copernicus quotes the original as far as it relates to the system of philolaus, to the effect "that the earth turns round the region of fire (ethereal region), and runs through the zodiac like the sun and the moon." the principal pythagoreans, such as archytas of tarentum, heraclides of pontium, taught also the same doctrine, saying that "the earth is not immovable in the centre of the universe, but revolves in a circle, and is far from occupying the chief place among the celestial bodies." pythagoras learnt this doctrine, it is said, from the egyptians, who in their hieroglyphics represented the symbol of the sun by the stercoral beetle, because this insect forms a ball with the excrement of the oxen, and lying down on its back, turns it round and round with its legs. timæus of locris was more precise than the other pythagoreans in calling "the five planets the organs of time, on account of their revolutions," adding that we must conclude that the earth is not immovable in one place, but that it turns, on the contrary, about itself, and travels also through space. plutarch records that plato, who had always taught that the sun turned round the earth, had changed his opinion towards the end of his life, regretting that he had not placed the sun in the centre of the universe, which was the only place, he then thought, that was suitable for that star. three centuries before jesus christ, aristarchus of samos is said by aristotle to have composed a special work to defend the motion of the earth against the contrary opinions of philosophers. in this work, which is now lost, he laid down in the most positive manner that "the sun remains immovable, and that the earth moves round it in a circular curve, of which that star is the centre." it would be impossible to state this in clearer terms; and what makes his meaning more clear, if possible, is that he was persecuted for it, being accused of irreligion and of troubling the repose of vesta--"because," says plutarch, "in order to explain the phenomena, he taught that the heavens were immovable, and that the earth accomplished a motion of translation in an oblique line, at the same time that it turned round its own axis." this is exactly the opinion that copernicus took up, after an interval of eighteen centuries--and he too was accused of irreligion. in passing from the greeks to the romans, and from them to the middle ages, the doctrine of aristarchus underwent a curious modification, assimilating it to the system of tycho brahe, which we shall hereafter consider, rather than to that of copernicus. this consisted in making the planets move round the sun, while the sun itself revolved round the earth, and carried them with him, and the heavens revolved round all. vitruvius and macrobius both taught this doctrine. although cicero and seneca, with aristotle and the stoics, taught the immobility of the earth in the centre of the universe, the question seemed undecided, to seneca at least, who writes:--"it would be well to examine whether it is the universe that turns about the immovable earth, or the earth that moves, while the universe remains at rest. indeed some men have taught that the earth is carried along, unknown to ourselves, that it is not the motion of the heavens that produces the rising and setting of the stars, but that it is we who rise and set relatively to them. it is a matter worthy of contemplation, to know in what state we are--whether we are assigned an immovable or rapidly-moving home--whether god makes all things revolve round us, or we round them." the double motion of the earth, then, is an idea revived from the grecian philosophers. the theory was known indeed to ptolemy, who devotes a whole chapter in his celebrated _almagesta_ to combat it. from his point of view it seemed very absurd, and he did not hesitate to call it so; and it was in reality only when fresh discoveries had altered the method of examining the question that the absurdities disappeared, and were transferred to the other side. not until it was discovered that the earth was no larger and no heavier than the other planets could the idea of its revolution and translation have appeared anything else than absurd. we are apt to laugh at the errors of former great men, while we forget the scantiness of the knowledge they then possessed. so it will be instructive to draw attention to ptolemy's arguments, that we may see where it is that new knowledge and ideas have led us, as they would doubtless have led him, had he possessed them, to a different conclusion. his argument depends essentially on the observed effects of weight. "light bodies," he says, "are carried towards the circumference, they appear to us to go _up_; because we so speak of the space that is over our heads, as far as the surface which appears to surround us. heavy bodies tend, on the contrary, towards the middle, as towards a centre, and they appear to us to fall _down_, because we so speak of whatever is under our feet, in the direction of the centre of the earth. these bodies are piled up round the centre by the opposed forces of their impetus and friction. we can easily see that the whole mass of the earth, being so large compared with the bodies that fall upon it, can receive them without their weight or their velocity communicating to it any perceptible oscillation. now if the earth had a motion in common with all the other heavy bodies, it would not be long, on account of its weight, in leaving the animals and other bodies behind it, and without support, and it would soon itself fall out of heaven. such would be the consequences of its motion, which are most ridiculous even to imagine." against the idea of the earth's diurnal rotation he argued as follows:--"there are some who pretend that nothing prevents us from supposing that the heaven remains immovable, and the earth turns round upon its axis from west to east, accomplishing the rotation each day. it is true that, as far as the stars are concerned, there is nothing against our supposing this, if guided only by appearances, and for greater simplicity; but those who do so forget how thoroughly ridiculous it is when we consider what happens near us and in the air. for even if we admit, which is not the case, that the lighter bodies have no motion, or only move as bodies of a contrary nature, although we see that aërial bodies move with greater velocity than terrestrial--if we admit that very dense and heavy bodies have a rapid and constant motion of their own, whereas in reality they obey but with difficulty the impulses communicated to them--we should then be obliged to assert that the earth, by its rotation, has a more rapid motion than any of the bodies that are round it, as it makes so large a circuit in so short a time. in this case the bodies which are not supported by it would appear to have a motion contrary to it, and no cloud or any flying bird could ever appear to go to the east, since the earth would always move faster than it in that direction." the _almagesta_ was for a long time the gospel of astronomers; to believe in the motion of the earth was to them more than an innovation, it was simply folly. copernicus himself well expresses the state of opinion in which he found the question, and the process of his own change, in the following words:--"and i too, taking occasion by these testimonies, commenced to cogitate on the motion of the earth, and although that opinion appeared absurd, i thought that as others before me had invented an assemblage of circles to explain the motion of the stars, i might also try if, by supposing the earth to move, i could not find a better account of the motions of the heavenly bodies than that with which we are at present contented. after long researches, i am at last convinced that if we assign to the circulation of the earth the motions of the other planets, calculation and observation will agree better together. and i have no doubt that mathematicians will be of my opinion, if they will take the trouble to consider carefully and not superficially the demonstrations i shall give in this work." although the opinions of copernicus had been held before, it is very just that his should be the name by which they are known; for during the time that elapsed before he wrote, the adherents of such views became fewer and fewer, until at last the very remembrance of them was almost forgotten, and it required research to know who had held them and taught them. it took him thirty years' work to establish them on a firm basis. we shall make no excuse for quoting further from his book, that we may know exactly the circumstances, as far as he tells us, of his giving this system to the world. "i hesitated for a long time whether i should publish my commentaries on the motions of the heavenly bodies, or whether it would not be better to follow the example of certain pythagoreans, who left no writings, but communicated the mysteries of their philosophy orally from man to man among their adepts and friends, as is proved by the letter of lysidas to hipparchus. they did not do this, as some suppose, from a spirit of jealousy, but in order that weighty questions, studied with great care by illustrious men, might not be disparaged by the idle, who do not care to undertake serious study, unless it be lucrative, or by shallow-minded men, who, though devoting themselves to science, are of so indolent a spirit that they only intrude among philosophers, like drones among bees. "when i hesitated and held back, my friends pressed me on. the first was nicolas schonberg, cardinal of capua, a man of great learning. the other was my best friend, tideman gysius, bishop of culm, who was as well versed in the holy scriptures as in the sciences. the latter pressed me so much that he decided me at last to give to the public the work i had kept for more than twenty-seven years. many illustrious men urged me, in the interest of mathematics, to overcome my repugnance and to let the fruit of my labours see the light. they assured me that the more my theory of the motion of the earth appeared absurd, the more it would be admired when the publication of my work had dissipated doubts by the clearest demonstrations. yielding to these entreaties, and buoying myself with the same hope, i consented to the printing of my work." he tried to guard himself against the attacks of dogmatists by saying, "if any evil-advised person should quote against me any texts of scripture, i deprecate such a rash attempt. mathematical truths can only be judged by mathematicians." notwithstanding this, however, his work, after his death, was condemned by the index in , under paul v. on examining the ancient systems, copernicus was struck by the want of harmony in the arrangements proposed, and by the arbitrary manner in which new principles were introduced and old ones neglected, comparing the system to a collection of legs and arms not united to any trunk, and it was the simplicity and harmony which the one idea of the motion of the earth introduced into the whole system that convinced him most thoroughly of its truth. he knew well that new views and truths would appear as paradoxes, and be rejected by men who were wedded to old doctrines, and on this account he took such pains to show that these views had been held before, and thus to disarm them of their apparent novelty. [illustration: fig. .--the copernican system.] copernicus dealt only with the six planets then known and the sun and moon. as to the stars, he had no idea that they were suns like our own, at immense and various distances from us. the knowledge of the magnitude of the sidereal universe was reserved for our own century, when it was discovered by the method of parallaxes. we will give copernicus's own sketch of the planetary system:-- "in the highest place is the sphere of the fixed stars, an immovable sphere, which surrounds the whole of the universe. among the movable planets the first is saturn, which requires thirty years to make its revolution. after it jupiter accomplishes its journey in twelve years; mars follows, requiring two years. in the fourth line come the earth and the moon which in the course of one year return to their original position. the fifth place is occupied by venus, which requires nine months for its journey. mercury occupies the sixth place, whose orbit is accomplished in eighty days. in the midst of all is the sun. what man is there, who in this majestic temple could choose another and better place for that brilliant lamp which illuminates all the planets with their satellites? it is not without reason that the sun is called the lantern of the world, the soul and thought of the universe. in placing it in the centre of the planets, as on a regal throne, we give it the government of the great family of celestial bodies." the hypothesis of the motion of the earth in its orbit appeared simply to copernicus as a good basis for the exact determination of the ratios of the distances of the several planets about the sun. but he did not give up the excentrics and epicycles for the explanation of the irregular motions of the planets, and certain imaginary variations in the precession of the equinoxes and the obliquity of the ecliptic. according to him the earth was endowed with three different motions, the first about its axis, the second along the ecliptic, and a third, which he called the declination, moving it backwards along the signs of the zodiac from east to west. this last motion was invented to explain the phenomena of the seasons. he thought, like many other ancient philosophers, that a body could not turn about another without being fixed in some way to it--by a crystal sphere, or something--and in this case that the same surface would each day be presented to the sun, and so it requires a third rotation, by which its axis may remain constantly parallel to itself. galileo, however, afterwards demonstrated the independence of the two motions in question, and proved that the third was unnecessary. copernicus was born in the polish village of thorn, in , and died in , at warmia, of which he was canon, and where he built an observatory. the voyages of his youth, his labours, adversities, and old age at last broke him down, and in the winter of he took to his bed, and was incapable of further work. his work, which was just finished printing at nuremberg, was brought to him by his friends before he died. he soon after completely failed in strength, and passed away tranquilly on the rd of may, . [illustration: plate viii.--death of copernicus.] the copernican system required, however, establishing in the minds of astronomers generally before it took the place it now holds, and this work was done by galileo--a name as celebrated as that of copernicus himself, if not more so. this perhaps is due not only to his demonstration of the motion of the earth, but to his introduction of experimental philosophy, and his observational method in astronomy. the next advance was made by kepler, who overthrew at one blow all the excentrics and epicycles of the ancients, when by his laborious calculations he proved the ellipticity of the orbit of mars. the grecian hypotheses were the logical consequences of two propositions which were universally admitted as axioms in the early and middle ages. first, that the motions of the heavenly bodies were uniform; second, that their orbits were perfect circles. nothing appeared more natural than this belief, though false. so then when kepler, in , recognised the fact, by incontestable geometrical measurements, that mars described an oval orbit round the sun, in which its velocity varied periodically, he could not believe either his observation or his calculation, and he puzzled his brain to discover what secret principle it was that forced the planet to approach and depart from the sun by turns. fortunately for him, in this inquietude he came across a treatise by gilbert, _de magnate_, which had been published in london nine years before. in this remarkable work gilbert proved by experiment that the earth acts on magnetized needles and on bars of iron placed near its surface just as a magnet does--and by a conjectural extension of this fact, which was a vague presentiment of the truth, he supposed that the earth itself might be retained in its constant orbit round the sun by a magnetic attraction. this idea was a ray of light to kepler. it led him to see the secret cause of the alternating motions that had troubled him so much, and in the joy of that discovery he said, "if we find it impossible to attribute the vibration to a magnetic power residing in the sun, acting on the planet without any material medium between, we must conclude that the planet is itself endowed with a kind of intelligent perception which gives it power to know at each instant the proper angles and distances for its motion." in the result kepler was led to enunciate to the world his three celebrated laws:-- st. that the planets move in ellipses, of which the sun is in one of the foci. nd. the spaces described by the ideal radius which joins each planet to the sun are proportional to the times of their description. in other words, the nearer a planet is to the sun, the faster it moves. rd. the squares of the times of revolution are as the cubes of the major axes of the orbits. such were the laws of kepler, the basis of modern astronomy, which led in the hands of newton to the simple explanation by universal gravitation, which itself is now asking to be explained. we are not to suppose that the system of copernicus was universally accepted even by astronomers of note. by some an attempt was made to invent a system which should have all the advantages of this, and yet if possible save the immobility of the earth. such was that of tycho brahe, who was born three years after the death of copernicus, and died in . he was one of the most laborious and painstaking observers of his time, although by the peculiarity of fate he is known generally only by his false system. [illustration: fig. .--tycho brahe's system.] in , tycho brahe wrote a little treatise, _tychonis brahe, dani, de mundi Ætherei recentioribus phenomenis, à propos_ of a comet that had lately appeared. he speaks at length of his system as follows:--"i have remarked that the ancient system of ptolemy is not at all natural, and too complicated. but neither can i approve of the new one introduced by the great copernicus after the example of aristarchus of samos. this heavy mass of earth, so little fit for motion, could not be displaced in this manner, and moved in three ways, like the celestial bodies, without a shock to the principles of physics. besides, it is opposed to scripture! i think then," he adds, "that we must decidedly and without doubt place the earth immovable in the centre of world, according to the belief of the ancients and the testimony of scripture. in my opinion the celestial motions are arranged in such a way that the sun, the moon, and the sphere of the fixed stars, which incloses all, have the earth for their centre. the five planets turn about the sun as about their chief and king, the sun being constantly in the centre of their orbits, and accompany it in its annual motion round the earth." this system perfectly accounts for the apparent motions of the planets as seen from the earth, and is essentially a variation on the copernican, rather than on the ptolemaic system, but it lent itself less readily to future discoveries. it simply amounts, as far as the solar system is concerned, to impressing upon all the rest of it the motions of the earth, so as to leave the latter at rest; and were the sun only as large with respect to the earth as it seems, were the planets really smaller than the moon, and the stars only at a short distance, and smaller than the planets, it might seem more natural that they should move than the earth; but when all these suppositions were disproved, the very argument of tycho brahe for the stability of the earth turned the other way, and proved as incontestably that it moved. in the copernican system, however, these questions are of no consequence; if the sun be at rest, this mass makes no difference; if the earth moves like the planets, their relative size does not alter anything; and if stars are immovable they may be at any distance and of any magnitude. the objections of tycho brahe to the earth's motion were: first, that it was too heavy--we know now, however, that some other planets are heavier--and that the sun, which he would make move instead, is , times as heavy. secondly, that if the earth moved, all loose things would be carried from east to west; but we have experience of many loose things being kept by friction on moving bodies, and can conceive how, all things may be kept by the attraction of the earth under the influence of its own motion. thirdly, that he could not imagine that the earth was turned upside down every day, and that for twelve hours our heads are downwards. but the existence of the antipodes overcomes this objection, and shows that there is no up and down in the universe, but each man calls that _down_ which is nearer to the centre of the earth than himself. a variation on tycho brahe's system was attempted by one longomontanus, who had lived with him for ten years. it consisted in admitting the diurnal rotation, but not the annual revolution, of the earth; but it made no progress, and was soon forgotten. more remarkable than this was the attempt by descartes in the same direction, namely, to hold the principles of copernicus, and yet to teach the immobility of the earth. his idea of immobility was however very different from that of tycho brahe, or of any one else, and would only be called so by those who were bound to believe it at all costs. his theory of vortices, as it is called, will be best given in his own words as contained in his _les principes de la philosophie_, third part, chap. xxvi., entitled, "that the earth is at rest in its heaven, which does not prevent its being carried along with it, and that it is the same with all the planets." "i adhere," he says, "to the hypothesis of copernicus, because it seems to me the simplest and clearest. there is no vacuum anywhere in space.... the heavens are full of a universal liquid substance. this is an opinion now commonly received among astronomers, because they cannot see how the phenomena can be explained without it. the substance of the heavens has the common property of all liquids, that its minutest particles are easily moved in any direction, and when it happens that they all move in one way, they necessarily carry with them all the bodies they surround, and which are not prevented from moving by any external cause. the matter of the heaven in which the planets are turns round continually like a vortex, which has the sun for its centre. the parts that are nearest the sun move faster than those that are at a greater distance; and all the planets, including the earth, remain always suspended in the same place in the matter of the heaven. and just as in the turns of rivers, when the water turns back on itself and twists round in circles, if any twig or light body floats on it, we see it carry them round, and make them move with it, and even among these twigs we may see some turning on their own centre, and those that are nearest to the middle of the vortex moving quicker than those on the outside; so we may easily imagine it to be with the planets, and this is all that is necessary to explain the phenomena. the matter that is round saturn takes about thirty years to run its circle; that which surrounds jupiter carries it and its satellites round in twelve years, and so on.... the satellites are carried round their primaries by smaller vortices.... the earth is not sustained by columns, nor suspended in the air by ropes, but it is environed on all sides by a very liquid heaven. it is at rest, and has no propulsion or motion, since we do not perceive any in it. this does not prevent it being carried round by its heaven, and following its motion without moving itself, just as a vessel which is not moved by winds or oars, and is not retained by anchors, remains in repose in the middle of the sea, although the flood of the great mass of water carries it insensibly with it. like the earth, the planets remain at rest in the region of heaven where each one is found. copernicus made no difficulty in allowing that the earth moves. tycho, to whom this opinion seemed absurd and unworthy of common sense, wished to correct him, but the earth has far more motion in his hypothesis than in that of copernicus." [illustration: fig. .--descartes' theory of vortices.] such is the celebrated theory of vortices. the comparison of the rotation of the earth and planets and their revolution round the sun to the turning of small portions of a rapid stream, may contain an idea yet destined to be developed to account for these motions; but as used by descartes it is a mere playing upon words admirably adapted to secure the concurrence of all parties; those who believed in the motion of the earth seeing that it did not interfere with their ideas in the least, and those who believed in its stability being gratified to find some way by which they might still cling to that belief and yet adopt the new ideas. this was its purpose, and that purpose it well served; but as a philosophical speculation it was worthless. when former astronomers declared that any planet moved, whether it were the earth or any other, they had no idea of attraction, but supposed the planet fixed to a sphere; this sphere moving and carrying the planet with it was what they meant by the planet moving: the theory of vortices merely substituted a liquid for a solid sphere, with this disadvantage, that if the planet were fixed to a solid moving sphere, it _must_ move; if only placed in a liquid one, that liquid might pass it if it did not have motion of its own. [illustration: fig. .--vortices of the stars.] a variation on descartes' system of vortices was proposed in the eighteenth century, which supposed that the sun, instead of being fixed in the centre of the system, itself circulated round another centre, carrying mercury with it. this motion of the sun was intented to explain the changes of magnitude of its disc as seen from the earth, and the diurnal and annual variations in its motion, without discarding its circular path. [illustration: fig. .--variation of descartes' theory.] we have thus noticed all the chief astronomical systems that have at any time been entertained by astronomers. they one and all have given way before the universally acknowledged truth about which there is no longer any dispute. systems are not now matters of opinion or theory. we speak of facts as certain as any that can be ascertained in any branch of knowledge. we have much to learn, but what we have settled as the basis of our knowledge will never more be altered as far as we can see. of course there have been always fantastic fancies put forth about the solar system, but they are more amusing than instructive. some have said that there is no sun, moon, or stars, but that they are reflections from an immense light under the earth. some savage races say that the moon when decreasing breaks up into stars, and is renewed each month by a creative act. the indians used to say that it was full of nectar which the gods ate up when it waned, and which grew again when it waxed. the brahmins placed the earth in the centre, and said that the stars moved like fishes in a sea of liquid. they counted nine planets, of which two are invisible dragons which cause eclipses; which, since they happen in various parts of the zodiac, show that these dragons revolve like the rest. they said the sun was nearer than the moon, perhaps because it is hotter and brighter. berosus the chaldean gave a very original explanation of the phases and eclipses of the moon. he said it had one side bright, and the other side just the colour of the sky, and in turning it represented the different colours to us. before concluding this chapter we may notice what information we possess as to the origin of the names by which the planets are known. these names have not always been given to them, and date only from the time when the poets began to associate the grecian mythology with astronomy. the earlier names had reference rather to their several characters, although there appear to have been among every people two sets of names applied to them. the earliest greek names referred to their various degrees of brilliancy: thus saturn, which is not easily distinguished, was called phenon, or _that which appears_; jupiter was named phaëton, _the brilliant_; mars was pysoïs, or _flame-coloured_; mercury, stilbon, _the sparkling_; venus, phosphorus; and lucifer, _the light-bearer_. they called the latter also calliste, _the most beautiful_. it was also known then as now under the appellations of the morning star and evening star, indicating its special position. with the ancient accadians, the planets had similar names, among others. thus, "mars was sometimes called _the vanishing star_, in allusion to its recession from the earth, and jupiter the _planet of the ecliptic_, from its neighbourhood to the latter" (sayce). the name of mars raises the interesting question as to whether they had noticed its phases as well as its movements--especially when, with reference to venus, it is recorded in the "observations of bel," that "it rises, and in its orbit duly grows in size." they had also a rather confusing system of nomenclature by naming each planet after the star that it happened to be the nearest to at any point of its course round the ecliptic. among less cultivated nations also the same practice held, as with the natives of south america, whose name for the sun is a word meaning _it brings the day_; for the moon, _it brings the night_; and for venus, _it announces the day_. but even among the eastern nations, from whom the greeks and romans borrowed their astronomical systems, it soon became a practice to associate these planets with the names of the several divinities they worshipped. this was perhaps natural from the adoration they paid to the celestial luminaries themselves on account of their real or supposed influence on terrestrial affairs; and, moreover, as time went on, and heroes had appeared, and they had to find them dwelling-places in the heavens, they would naturally associate them with one or other of the most brilliant and remarkable luminaries, to which they might suppose them translated. beyond these general remarks, only conjectures can be made why any particular divinity should among the greeks be connected with the several planets as we now know them. such conjectures as the following we may make. thus jupiter, the largest, would take first rank, and be called after the name of the chief divinity. the soft and sympathising venus--appearing at the twilight--would well denote the evening star. mars would receive its name from its red appearance, naturally suggesting carnage and the god of war. saturn, or kronos, the god of time, is personified by the slow and almost imperceptible motion of that remote planet. while mercury, the fiery and quick god of thieves and commerce, is well matched with the hide-and-seek planet which so seldom can be seen, and moves so rapidly. these were the only planets known to the ancients, and were indeed all that could be discovered without a telescope. if the ancient babylonians possessed telescopes, as has been conjectured from their speaking, as we have noticed above, of the increase of the size of venus, and from the finding a crystal lens among the ruins of nineveh, they did not use them for this purpose. the other planets now known have a far shorter history. uranus was discovered by sir william herschel on the th of march, , and was at first taken for a comet. herschel proposed to call it georgium sidus, after king george iii. lalande suggested it should be named herschel, after its discoverer, and it bore this name for some time. afterwards the names, neptune, astroea, cybele, and uranus were successively proposed, and the latter, the suggestion of bode, was ultimately adopted. it is the name of the most ancient of the gods, connected with the then most modern of planets in point of discovery, though also most ancient in formation, if recent theories be correct. neptune, as everybody knows, was calculated into existence, if one may so speak, by adams and leverrier independently, and was first seen, in the quarter indicated, by dr. galle at berlin, in september, , and by universal consent it received the name it now bears. there are now also known a long series of what are called minor planets, all circulating between mars and jupiter, with their irregular orbits inextricably mingled together. their discovery was led to in a remarkable manner. it was observed that the distances of the several planets might approximately be expressed by the terms of a certain mathematical series, if one term was supplied between mars and jupiter--a fact known by the name of bode's law. when the new planet, uranus, was found to obey this law, the feeling was so strong that there must be something to represent this missing term, that strong efforts were made to discover it, which led to success, and several, whose names are derived from the minor gods and goddesses, are now well known. all these planets, like the signs of the zodiac, are indicated by astronomers by certain symbols, which, as they derive their form from the names or nature of the planets, may properly here be explained. the sign of neptune is [symbol: neptune], representing the trident of the sea; for uranus [symbol: uranus], which is the first letter of herschel with a little globe below; [symbol: saturn] is the sickle of time, or saturn; [symbol: jupiter] is the representation of the first letter of zeus or jupiter; [symbol: mars] is the lance and buckler of mars; [symbol: venus] the mirror of venus; [symbol: mercury] the wand of mercury; [symbol: sun] the sun's disc; and [symbol: moon] the crescent of the moon. [illustration: plate ix.--the solar system.] the more modern discoveries have, of course, been all made by means of the telescope, and a few words on the history of its discovery may fitly close this chapter. according to olbers, a concave and convex lens were first used in combination, to render objects less distant in appearance, in the year . in that year the children of one jean lippershey, an optician of middelburg, in zealand, were playing with his lenses, and happened to hold one before the other to look at a distant clock. their great surprise in seeing how near it seemed attracted their father's attention, and he made several experiments with them, at last fixing them as in the modern telescope--in draw tubes. on the nd of october, , he made a petition to the states-general of holland for a patent. the aldermen, however, saw no advantage in it, as you could only look with one eye instead of two. they refused the patent, and though the discovery was soon found of value, lippershey reaped no benefit. galileo was the first to apply the telescope to astronomical observations. he did not have it made in holland, but constructed it himself on lippershey's principle. this was in . its magnifying power was at first , and he afterwards increased it to , and then to . with this he discovered the phases of venus, the spots on the sun, the four satellites of jupiter, and the mountains of the moon. [illustration: plate x.--the discovery of the telescope.] kepler, in , made the first astronomical telescope with two concave glasses. huyghens increased the magnifying power successively to , , and , and discovered saturn's ring and his satellite no. . cassini, the first director of the paris observatory, brought it to , aided by auzout campani of rome, and rives of london. he observed the rotation of jupiter ( ), that of venus and mars ( ), the fifth and third satellites of saturn ( ), and afterwards the two nearer ones ( ); the other satellites of this planet were discovered, the sixth and seventh, by sir william herschel ( ), and the eighth by bond and lasel ( ). we may add here that the satellites of uranus were discovered, six by herschel from to , and two by lassel in , the latter also discovering neptune's satellite in . the rotation of saturn was discovered by herschel in , and that of mercury by schroeter in . the earliest telescopes which were reflectors were made by gregory in and newton in . the greatest instruments of our century are that of herschel, which magnifies , times, and lord rosse's, magnifying , times, the foucault telescope at marseilles, of , , the reflector at melbourne, of , , and the newall refractor. [illustration: plate xi.--the foundation of paris observatory.] the exact knowledge of the heavens, which makes so grand a feature in modern science, is due, however, not only to the existence of instruments, but also to the establishment of observatories especially devoted to their use. the first astronomical observatory that was constructed was that at paris. in colbert submitted the designs of it to louis xiv., and four years afterwards it was completed. the greenwich observatory was established in , that of berlin in , and that of st. petersburg in . since then numerous others have been erected, private as well as public, in all parts of the world, and no night passes without numerous observations being taken as part of the ordinary duty of the astronomers attached to them. chapter ix. the terrestrial world of the ancients.--cosmography and geography. with respect to the shape and position of the earth itself in the material universe, and the question of its motion or immobility, we cannot go so far back as in the case of the heavens, since it obviously requires more observation, and is not so pressing for an answer. amongst the greeks several authors appear to have undertaken the subject, but only one complete work has come down to us which undertakes it directly. this is a work attributed to aristotle, _de mundo_. it is addressed to alexander, and by some is considered to be spurious, because it lacks the majestic obscurity that in his acknowledged works repels the reader. although, however, it is not as obscure as it might be, for the writer, it is quite bad enough, and its dryness and vagueness, its mixture of metaphysical and physical reasoning, logic and observation, and the change that has naturally passed over the meanings of many common words since they were written, render it very tedious and unpleasant reading. nevertheless, as presenting us with the first recorded ideas on these questions of the nature and properties of the earth, it deserves attentive study. it is not a system of observations like those of ptolemy and the alexandrian school, but an entirely theoretical work. it is founded entirely on logic; but unfortunately, if the premisses are bad, the better the syllogism the more erroneous will be the conclusion; and it is just this which we find here. thus if he be asked whether the earth turns or the heavens, he will reply that the earth is _evidently_ in repose, and that this is the case not only because we observe it to be so, but because it is a necessity that it should be; because repose is _natural_ to the earth, and it is _naturally_ in equilibrium. this idea of "natural" leads very often astray. he is guided to his idea of what is natural by seeing what is, and then argues that what is, or appears to be, must be, because it is natural--thus arguing in a circle. another example may be given in his answer to the question, why must the stars move round the earth? he says it is natural, because a circle is a more perfect line, and must therefore be described by the perfect stars, and a circle is perfect because it has no ends! unfortunately there are other curves that have no ends; but the circle was considered, without more reason, the most perfect curve, and therefore the planets must move in circles--an idea which had to wait till kepler's time to be exploded. one more specimen of this style may be quoted, namely, his proof that every part of heaven must be eternally moving, while the earth must be in the centre and at rest. the proof is this. everything which performs any act has been made for the purpose of that act. now the work of god is immortality, from which it follows that all that is divine must have an eternal motion. but the heavens have a divine quality, and for this reason they have a spherical shape and move eternally in a circle. now when a body has a circular motion, one part of it must remain at rest in its place, namely, that which is in the centre; the earth is in the centre--therefore it is at rest. aristotle says in this work that there are two kinds of simple motion, that in a circle and that in a straight line. the latter belongs to the elements, which either go up or down, and the former to the celestial bodies, whose nature is more divine, and which have never been known to change; and the earth and world must be the only bodies in existence, for if there were another, it must be the contrary to this, and there is no contrary to a circle; and again, if there were any other body, the earth would be attracted towards it, and move, which it does not. such is the style of argument which was in those days thought conclusive, and which with a little development and inflation of language appeared intensely profound. but what brings these speculations to the subject we have now in hand is this: that when aristotle thus proves the earth to be immovable in the centre of the universe, he is led on to inquire how it is possible for it to remain in one fixed place. he observed that even a small fragment of earth, when it is raised into the air and then let go, immediately falls without ever stopping in one place--falling, as he supposed, all the quicker according to its weight; and he was therefore puzzled to know why the whole mass of the earth, notwithstanding its weight, could be kept from falling. aristotle examines one by one the answers that have been given to this question. thus xenophanes gave to the earth infinitely extended roots, against which empedocles uses such arguments as we should use now. thales of miletus makes the earth rest upon water, without finding anything on which the water itself can rest, or answering the question how it is that the heavier earth can be supported on the lighter water. anaxemenes, anaxagoras, and democritus, who make the earth flat, consider it to be sustained by the air, which is accumulated below it, and also presses down upon it like a great coverlet. aristotle himself says that he agrees with those philosophers who think that the earth is brought to the centre by the primitive rotation of things, and that we may compare it, as empedocles does, to the water in glasses which are made to turn rapidly, and which does not fall out or move, even though upside down. he also quotes with approval another opinion somewhat similar to this, namely, that of anaximander, which states that the earth is in repose, on account of its own equilibrium. placed in the centre and at an equal distance from its extremities, there is no reason why it should move in one direction rather than the other, and rests immovable in the centre without being able to leave it. the result of all is that aristotle concludes that the earth is immovable, in the centre of the universe, and that it is not a star circulating in space like other stars, and that it does not rotate upon its axis; and he completes the system by stating that the earth is spherical, which is proved by the different aspects of the heavens to a voyager to the north or to the south. such was the aristotelian system, containing far more error than truth, which was the first of any completeness. scattered ideas, however, on the shape and method of support of the earth and the cause of various phenomena, such as the circulation of the stars, are met with besides in abundance. the original ideas of the earth were naturally tinged by the prepossessions of each race, every one thinking his own country to be situated in the centre. thus among the hindoos, who lived near the equator, and among the scandinavians, inhabiting regions nearer the pole, the same meaning attaches to the words by which they express their own country, _medpiama_ and _medgard_, both meaning the central habitation. olympus among the greeks was made the centre of the earth, and afterwards the temple of delphi. for the egyptians the central point was thebes; for the assyrians it was babylon; for the indians it was the mountain mero; for the hebrews jerusalem. the chinese always called their country the central empire. it was then the custom to denote the world by a large disc, surrounded on all sides by a marvellous and inaccessible ocean. at the extremities of the earth were placed imaginary regions and fortunate isles, inhabited by giants or pigmies. the vault of the sky was supposed to be supported by enormous mountains and mysterious columns. numerous variations have been suggested on the earliest supposed form of the earth, which was, as we have seen in a former chapter, originally supposed to be an immense flat of infinite depth, and giving support to the heavens. as travels extended and geography began to be a science, it was remarked that an immense area of water circumscribed the solid earth by irregular boundaries--whence the idea of a universal ocean. when, however, it was perceived that the horizon at sea was always circular, it was supposed that the ocean was bounded, and the whole earth came to be represented as contained in a circle, beneath which were roots reaching downwards without end, but with no imagined support. [illustration: fig. .--the earth floating.] [illustration: fig. .--the earth with roots.] the vedic priests asserted that the earth was supported on twelve columns, which they very ingeniously turned to their own account by asserting that these columns were supported by virtue of the sacrifices that were made to the gods, so that if these were not made the earth would collapse. [illustration: fig. .--the earth of the vedic priests.] these pillars were invented in order to account for the passing of the sun beneath the earth after his setting, for which at first they were obliged to imagine a system of tunnels, which gradually became enlarged to the intervals between the pillars. the hindoos made the hemispherical earth to be supported upon four elephants, and the four elephants to stand on the back of an immense tortoise, which itself floated on the surface of a universal ocean. we are not however to laugh at this as intended to be literal; the elephants symbolised, it may be, the four elements, or the four directions of the compass, and the tortoise was the symbol for strength and for eternity, which was also sometimes represented by a serpent. [illustration: fig. .--hindoo earth.] the floating of the earth on water or some other liquid long held ground. it was adopted by thales, and six centuries later seneca adopts the same opinion, saying that the humid element that supports the earth's disc like a vessel may be either the ocean or some liquid more simple than water. diodorus tells us that the chaldeans considered the earth hollow and boat-shaped--perhaps turned upside down--and this doctrine was introduced into greece by heraclitus of ephesus. [illustration: fig. .--the earth of anaximander.] anaximander represents the earth as a cylinder, the upper face of which alone is inhabited. this cylinder, he states, is one-third as high as its diameter, and it floats freely in the centre of the celestial vault, because there is no reason why it should move to one side rather than the other. leucippus, democritus, heraclitus, and anaxagoras all adopted this purely imaginary form. europe made the northern half, and lybia (africa) and asia the southern, while delphi was in the centre. anaximenes, without giving a precise opinion as to the form of the earth, made it out to be supported on compressed air, though he gave no idea as to how the air was to be compressed. plato thought to improve upon these ideas by making the earth cubical. the cube, which is bound by six equal faces, appeared to him the most perfect of solids, and therefore most suitable for the earth, which was to stand in the centre of the universe. [illustration: fig. .--plato's cubical earth.] eudoxus, who in his long voyages throughout greece and egypt had seen new constellations appear as he went south, while others to the north disappeared, deduced the sphericity of the earth, in which opinion he was followed by archimedes, and, as we have seen, by aristotle. according to achilles tatius, xenophanes gave to the earth the shape of an immense inclined plane, which stretched out to infinity. he drew it in the form of a vast mountain. the summit only was inhabited by men, and round it circulated the stars, and the base was at an infinite depth. hesiod had before this obscurely said: "the abyss is surrounded by a brazen barrier; above it rest the roots of the earth." epicurus and his school were well pleased with this representation. if such were the foundations of the earth, then it was impossible that the sun, and moon, and stars should complete their revolutions beneath it. a solid and indefinite support being once admitted, the epicurean ideas about the stars were a necessary consequence; the stars must inevitably be put out each day in the west, since they are not seen to return to the place whence they started, and they must be rekindled some hours afterwards in the east. in the days of augustus, cleomedes still finds himself obliged to combat these epicurean ideas about the setting and rising of the sun and stars. "these stupid ideas," he says, "have no other foundation than an old woman's story--that the iberians hear each night the hissing noise made by the burning sun as it is extinguished, like a hot iron in the waters of the ocean." modern travellers have shown us that similar ideas about the support of the earth have been entertained by more remote people. thus, in the opinion of the greenlanders, handed down from antiquity to our own days, the earth is supported on pillars, which are so consumed by time that they often crack, and were it not that they are supported by the incantations of the magicians, they would long since have broken down. this idea of the breaking of the pillars may possibly have originated in the known sinking of the land beneath the sea, which is still going on even at the present day. [illustration: fig. .--egyptian representation of the earth.] an ancient egyptian papyrus in the library of paris gives a very curious hieroglyphical representation of the universe. the earth is here figured under the form of a reclining figure, and is covered with leaves. the heavens are personified by a goddess, which forms the vault by her star-bespangled body, which is elongated in a very peculiar manner. two boats, carrying, one the rising sun, and the other the setting sun, are represented as moving along the heavens over the body of the goddess. in the centre of the picture is the god, maon, a divine intelligence, which presides over the equilibrium of the universe. we will now pass on from the early ideas of the general shape and situation of the world to inquire into the first outlines of geographical knowledge of details. of all the ancient writings which deal with such questions, the hebrew scriptures have the greatest antiquity, and in them are laid down many details of known countries, from which a fair map of the world as known to them might be made out. the prophet esdras believed that six-sevenths of the earth was dry land--an idea which could not well be exploded till the great oceans had been traversed and america discovered. more interesting, as being more complete, and written to a certain extent for the very purpose of relating what was known of the geography of the earth, are the writings of the oldest grecian poets. the first elements of grecian geography are contained in the two national and almost sacred poems, the _iliad_ and _odyssey_. so important have these writings been considered in regard to ancient geography, that for many centuries discussions have been carried on with regard to the details, though evidently fictitious, of the voyage of ulysses, and twenty lines of the _iliad_ have furnished matter for a book of thirty volumes. the shield of achilles, forged by vulcan and described in the eighteenth book of the _iliad_, gives us an authentic representation of the primitive cosmographical ideas of the age. the earth is there figured as a disc, surrounded on all sides by the _river ocean_. however strange it may appear to us, to apply the term _river_ to the ocean, it occurs too often in homer and the other ancient poets to admit of a doubt of its being literally understood by them. hesiod even describes the sources of the ocean at the western extremity of the world, and the representation of these sources was preserved from age to age amongst authors posterior to homer by nearly a thousand years. herodotus says plainly that the geographers of his time drew their maps of the world according to the same ideas; the earth was figured with them as a round disc, and the ocean as a river, which washed it on all sides. the earth's disc, the _orbis terrarum_, was covered according to homer by a solid vault or firmament, beneath which the stars of the day and night were carried by chariots supported by the clouds. in the morning the sun rose from the eastern ocean, and in the evening it declined into the western; and a vessel of gold, the mysterious work of vulcan, carried it quickly back by the north, to the east again. beneath the earth homer places, not the habitation of the dead, the caverns of hades, but a vault called tartarus, corresponding to the firmament. here lived the titans, the enemies of the gods, and no breath of wind, no ray of light, ever penetrated to this subterranean world. writers subsequent to homer by a century determined even the height of the firmament and the depth of tartarus. an anvil, they said, would take nine days to fall from heaven to earth, and as many more to fall from earth to the bottom of tartarus. this estimate of the height of heaven was of course far too small. if a body were to fall for nine days and nights, or , seconds under the attraction of the earth, it would only pass over , miles, that is not much more than half as far again as the moon. a ray of light would only take two seconds to pass over that distance, whereas it takes eight minutes to reach us from the sun, and four hours to come from neptune--to say nothing of the distance of the stars. the limits of the world in the homeric cosmography were surrounded by obscurity. the columns of which atlas was the guardian were supported on unknown foundations, and disappeared in the systems subsequent to homer. beyond the mysterious boundary where the earth ended and the heavens began an indefinite chaos spread out--a confused medley of life and inanity, a gulf where all the elements of heaven, tartarus, and earth and sea are mixed together, a gulf of which the gods themselves are afraid. ideas such as these prevailed long after geometers and astronomers had proved the spherical form of the globe, and they were revived by the early christian geographers and have left their trace even on the common language of to-day. [illustration: fig. .--homeric cosmography.] the centre of the terrestrial disc was occupied by the continent and isles of greece, which in the time of homer possessed no general name. the centre of greece passed therefore for the centre of the whole world; and in homer's system it was reckoned to be olympus in thessaly, but the priests of the celebrated temple of apollo at delphi (known then under the name of python) gave out a tradition that that sacred place was the real centre of the habitable world. the straits which separate italy from sicily were so to speak the vestibule of the fabulous world of homer. the threefold ebb and flow, the howling of the monster scylla, the whirlpools of charybdis, the floating rocks--all tell us that we are quitting here the region of truth. sicily itself, although already known under the name of _trinacria_, was filled with marvels; here the flocks of the sun wandered in a charming solitude under the guardianship of nymphs; here the cyclops, with one eye only, and the anthropophagous lestrigons scared away the traveller from a land that was otherwise fertile in corn and wine. two historical races were placed by homer in sicily, namely the _sicani_, and the _siceli_, or _siculi_. to the west of sicily we find ourselves in the midst of a region of fables. the enchanted islands of circe and calypso, and the floating island of eolus can no longer be found, unless we imagine them to have originated, like graham's island in this century, from volcanic eruptions or elevations, and to have disappeared again by the action of the sea. the homeric map of the world terminated towards the west by two fabulous countries which have given rise to many traditions among the ancients, and to many discussions among moderns. near to the entrance of the ocean, and not far from the sombre caverns where the dead are congregated, ulysses found the _cimmerians_, "an unhappy people, who, constantly surrounded by thick shadows, never enjoyed the rays of the sun, neither when it mounted the skies, nor when it descended below the earth." still farther away, and in the ocean itself, and therefore beyond the limits of the earth, beyond the region of winds and seasons, the poet paints for us a fortunate land, which he calls _elysium_, a country where tempests and winter are unknown, where a soft zephyr always blows, and where the elect of jupiter, snatched from the common lot of mortals, enjoy a perpetual felicity. whether these fictions had an allegory for their basis, or were founded on the mistaken notions of voyagers--whether they arose in greece, or, as the hebrew etymology of the name cimmerian might seem to indicate, in the east, or in phenicia, it is certain that the images they present, transferred to the world of reality, and applied successively to various lands, and confused by contradictory explanations, have singularly embarrassed the progress of geography through many centuries. the roman travellers thought they recognised the fortunate isles in a group to the west of africa, now known as the canaries. the philosophical fictions of plato and theopompus about atlantes and meropis have been long perpetuated in historical theories; though of course it is possible that in the numerous changes that have taken place in the surface of the earth, some ancient vast and populous island may have descended beneath the level of the sea. on the other side, the poetic imagination created the _hyperboreans_, beyond the regions where the northern winds were generated, and according to a singular kind of meteorology, they believed them for that reason to be protected from the cold winds. herodotus regrets that he has not been able to discover the least trace of them; he took the trouble to ask for information about them from their neighbours, the _arimaspes_, a very clear-sighted race, though having but a single eye; but they could not inform him where the hyperboreans dwelt. the enchanted isles, where the hesperides used to guard the golden fruit, and which the whole of antiquity placed in the west, not far from the fortunate isles, are sometimes called hyperborean by authors well versed in the ancient traditions. it is also in this sense that sophocles speaks of the garden of phoebus, near the vault of heaven, and not far from the _sources of the night_, _i.e._ of the setting of the sun. avienus explains the mild temperature of the hyperborean country by the temporary proximity of the sun, since, according to the homeric ideas, it passes during the night by the northern ocean to return to its palace in the east. this ancient tradition was not entirely exploded in the time of tacitus, who states that on the confines of germany might be seen the veritable setting of apollo beyond the water, and he believes that as in the east the sun gives rise to incense and balm by its great proximity to the earth, so in the regions where it sets it makes the most precious of juices to transude from the earth and form amber. it is this idea that is embedded in the fables of amber being the tears of gold that apollo shed when he went to the hyperborean land to mourn the loss of his son Æsculapius, or by the sisters of phaëton, changed into poplars; and it is denoted by the greek name for amber, _electron_--a sun-stone. the grecian sages, long before the time of tacitus, said that this very precious material was an exhalation from the earth that was produced and hardened by the rays of the sun, which they thought came nearer to the earth in the west and in the north. florus, in relating the expedition of decimus brutus along the coast of spain, gives great effect to the epicurean views about the sun, by declaring that brutus only stopped his conquests after having witnessed the actual descent of the sun into the ocean, and having heard with horror the terrible noise occasioned by its extinction. the ancients also believed that the sun and the other heavenly bodies were nourished by the waters--partly the fresh water of the rivers, and partly the salt water of the sea. cleanthes gave the reason for the sun returning towards the equator on reaching the solstices, that it could not go too far away from the source of its nourishment. pytheas relates that in the island of thule, six days' journey north of great britain, and in all that neighbourhood, there was no land nor sea nor air, but a compound of all three, on which the earth and the sea were suspended, and which served to unite together all the parts of the universe, though it was not possible to go into these places, neither on foot nor in ships. perhaps the ice floating in the frozen seas and the hazy northern atmosphere had been seen by some navigator, and thus gave rise to this idea. as it stands, the history may be perhaps matched by that of the amusing monk who said he had been to the end of the world and had to stoop down, as there was not room to stand between heaven and earth at their junction. homer lived in the tenth century before our era. herodotus, who lived in the fifth, developed the homeric chart to three times its size. he remarks at the commencement of his book that for several centuries the world has been divided into three parts--europe, asia, and libya; the names given to them being female. the exterior limits of these countries remained in obscurity notwithstanding that those boundaries of them that lay nearest to greece were clearly defined. one of the greatest writers on ancient geography was strabo, whose ideas we will now give an account of. he seems to have been a disciple of hipparchus in astronomy, though he criticises and contradicts him several times in his geography. he had a just idea of the sphericity of the earth; but considered it as the centre of the universe, and immovable. he takes pains to prove that there is only one inhabited earth--not in this refuting the notion that the moon and stars might have inhabitants, for these he considered to be insignificant meteors nourished by the exhalations of the ocean; but he fought against the fact of there being on this globe any other inhabited part than that known to the ancients. it is remarkable to notice that the proofs then used by geographers of the sphericity of the earth are just those which we should use now. thus strabo says, "the indirect proof is drawn from the centripetal force in general, and the tendency that all bodies have in particular towards a centre of gravity. the direct proof results from the phenomena observed on the sea and in the sky. it is evident, for example, that it is the curvature of the earth that alone prevents the sailor from seeing at a distance the lights that are placed at the ordinary height of the eye, and which must be placed a little higher to become visible even at a greater distance; in the same way, if the eye is a little raised it will see things which previously were hidden." homer had already made the same remark. on this globe, representing the world, strabo and the cosmographers of his time placed the habitable world in a surface which he describes in the following way: "suppose a great circle, perpendicular to the equator, and passing through the poles to be described about the sphere. it is plain that the surface will be divided by this circle, and by the equator into four equal parts. the northern and southern hemispheres contain, each of them, two of these parts. now on any one of these quarters of the sphere let us trace a quadrilateral which shall have for its southern boundary the half of the equator, for northern boundary a circle marking the commencement of polar cold, and for the other sides two equal and opposite segments of the circle that passes through the poles. it is on one such quadrilateral that the habitable world is placed." he figures it as an island, because it is surrounded on all sides by the sea. it is plain that strabo had a good idea of the nature of gravity, because he does not distinguish in any way an upper or a lower hemisphere, and declares that the quadrilateral on which the habitable world is situated may be any one of the four formed in this way. the form of the habitable world is that of a "chlamys," or cloak. this follows, he says, both from geometry and the great spread of the sea, which, enveloping the land, covers it both to the east and to the west and reduces it to a shortened and truncated form of such a figure that its greatest breadth preserved has only a third of its length. as to the actual length and breadth, he says, "it measures seventy thousand stadia in length, and is bounded by a sea whose immensity and solitude renders it impassable; while the breadth is less than thirty thousand stadia, and has for boundaries the double region where the excess of heat on one side and the excess of cold on the other render it uninhabitable." the habitable world was thus much longer from east to west than it was broad from north to south; from whence come our terms _longitude_, whose degrees are counted in the former direction, and _latitude_, reckoned in the latter direction. eratosthenes, and after him hipparchus, while he gives larger numbers than the preceding for the dimensions of the inhabited part of the earth, namely, thirty-eight thousand stadia of breadth and eighty thousand of length, declares that physical laws accord with calculations to prove that the length of the habitable earth must be taken from the rising to the setting of the sun. this length extends from the extremity of india to that of iberia, and the breadth from the parallel of ethiopia to that of ierne. that the earth is an island, strabo considers to be proved by the testimony of our senses. for wherever men have reached to the extremities of the earth they have found the sea, and for regions where this has not been verified it is established by reasoning. those who have retraced their steps have not done so because their passage was barred by any continent, but because their supplies have run short, and they were afraid of the solitude; the water always ran freely in front of them. it is extraordinary that strabo and the astronomers of that age, who recognised so clearly the sphericity of the earth and the real insignificance of mountains, should yet have supposed the stars to have played so humble a part, but so it was; and we find strabo arguing in what we may call quite the contrary direction. he says, "the larger the mass of water that is spread round the earth, so much more easy is it to conceive how the vapours arising from it are sufficient to nourish the heavenly bodies." [illustration: fig. .--the earth of the later greeks.] among the latin cosmographers we may here cite one who flourished in the first century after christ, pomponius mela, who wrote a treatise, called _de situ orbis_. from whatever source, whether traditional or otherwise, he arrived at the conclusion, he divided the earth into two continents, our own and that of the antichthones, which reached to our antipodes. this map was in use till the time of christopher columbus, who modified it in the matter of the position of this second continent, which till then remained a matter of mystery. [illustration: fig. .--pomponius mela's cosmography.] of those who in ancient times added to the knowledge then possessed of cosmography, we should not omit to mention the name of pytheas, of marseilles, who flourished in the fourth century before our era. his chief observations, however, were not so closely related to geography as to the relation of the earth with the heavenly bodies. by the observation of the gnomon at mid-day on the day of the solstice he determined the obliquity of the ecliptic in his epoch. by the observation of the height of the pole, he discovered that in his time it was not marked by any star, but formed a quadrilateral with three neighbouring stars, [greek: b] of the little bear and [greek: k] and [greek: a] of the dragon. chapter x. cosmography and geography of the church. after the writers mentioned in the last chapter a long interval elapsed without any progress being made in the knowledge of the shape or configuration of the earth. from the fall of the roman empire, whose colonies themselves gave a certain knowledge of geography, down to the fifteenth century, when the great impetus was given to discovery by the adventurous voyagers of spain and portugal, there was nothing but servile copying from ancient authors, who were even misrepresented when they were not understood. even the peninsula of india was only known by the accounts of orientals and the writings of the ancients until the beginning of the fifteenth century. vague notions, too, were held as to the limits of africa, and even of europe and asia--while of course they knew nothing of america, in spite of their marking on their maps an antichthonal continent to the south. denys, the traveller, a greek writer of the first century, and priscian, his latin commentator of the fourth, still maintained the old errors with regard to the earth. according to them the earth is not round, but leaf-shaped; its boundaries are not so arranged as to form everywhere a regular circle. macrobius, in his system of the world, proves clearly that he had no notion that africa was continued to the south of ethiopia, that is of the tenth degree of n. latitude. he thought, like cleanthus and crates and other ancient authors, that the regions that lay nearest the tropics, and were burnt by the sun, could not be inhabited; and that the equatorial regions were occupied by the ocean. he divided the hemisphere into five zones, of which only two were habitable. "one of them," he said, "is occupied by us, and the other by men of whose nature we are ignorant." orosus, writing in the same century (fourth), and whose work exercised so great an influence on the cosmographers of the middle ages and on those who made the maps of the world during that long period, was ignorant of the form or boundaries of africa, and of the contours of the peninsulas of southern asia. he made the heavens rest upon the earth. s. basil, also of the fourth century, placed the firmament on the earth, and on this heaven a second, whose upper surface was flat, notwithstanding that the inner surface which is turned towards us is in the form of a vault; and he explains in this way how the waters can be held there. s. cyril shows how useful this reservoir of water is to the life of men and of plants. diodorus, bishop of tarsus, in the same century, also divided the world into two stages, and compared it to a tent. severianus, bishop of gabala, about the same time, compared the world to a house of which the earth is the ground floor, the lower heavens the ceiling, and the upper, or heaven of heavens, the roof. this double heaven was also admitted by eusebius of cæsaræa. in the fifth, sixth, and seventh centuries science made no progress whatever. it was still taught that there were limits to the ocean. thus lactantius asserted that there could not be inhabitants beyond the line of the tropics. this father of the church considered it a monstrous opinion that the earth is round, that the heavens turn about it, and that all parts of the earth are inhabited. "there are some people," he says, "so extravagant as to persuade themselves that there are men who have their heads downwards and their feet upwards; that all that lies down here is hung up there; that the trees and herbs grow downwards; and that the snow and hail fall upwards.... those people who maintain such opinions do so for no other purpose than to amuse themselves by disputation, and to show their spirit; otherwise it would be easy to prove by invincible argument that it is impossible for the heavens to be underneath the earth." (divine institution). saint augustin also, in his _city of god_, says: "there is no reason to believe in that fabulous hypothesis of the antipodes, that is to say, of men who inhabit the other side of the earth--where the sun rises when it sets with us, and who have their feet opposed to ours." ... "but even if it were demonstrated by any argument that the earth and world have a spherical form, it would be too absurd to pretend that any hardy voyagers, after having traversed the immensity of the ocean, had been able to reach that part of the world and there implant a detached branch of the primæval human family." in the same strain wrote s. basil, s. ambrose, s. justin martyr, s. chrysostom, procopius of gaza, severianus, diodorus bishop of tarsus, and the greater number of the thinkers of that epoch. eusebius of cæsaræa was bold enough on one occasion to write in his commentaries on the psalms, that, "according to the opinion of some the earth is round;" but he draws back in another work from so rash an assertion. even in the fifteenth century the monks of salamanca and alcala opposed the old arguments against the antipodes to all the theories of columbus. [illustration: fig. .--the earth's shadow.] in the middle of the sixteenth century gregory of tours adopted also the opinion that the intertropical zone was uninhabitable, and, like other historians, he taught that the nile came from the unknown land in the east, descended to the south, crossed the ocean which separated the antichthone from africa, and then alone became: visible. the geographical and cosmographical ideas that were then prevalent may also be judged of by what s. avitus, a latin poet of the sixth century and nephew of the emperor flavius avitus, says in his poem on the creation, where he describes the terrestrial paradise. "beyond india," he writes, "_where the world commences_, where the confines of heaven and earth are joined, is an exalted asylum, inaccessible to mortals, and closed by eternal barriers, since the first sin was committed." in a treatise on astronomy, published a little after this in , by apian and gemma frison, they very distinctly state their belief in a round earth, though they do not go into details of its surface. the argument is the old one from eclipses, but the figures they give in illustration are very amusing, with three or four men of the size of the moon disporting themselves on the earth's surface. as, however, they all have their feet to the globe representing the earth, and consequently have their feet in opposite directions at the antipodes, the idea is very clearly shown. [illustration: fig. .] "if," they say, "the earth were square, its shadow on the moon would be square also. "if the earth were triangular, its shadow, during an eclipse of the moon, would also be triangular. [illustration: fig. .] "if the earth had six sides, its shadow would have the same figure. [illustration: fig. .] "since, then, the shadow of the earth is round, it is a proof that the earth is round also." this of course is one of the proofs that would be employed in the present day for the same purpose. the most remarkable of all the fantastical systems, however, the _chef d'oeuvre_ of the cosmography of that age, was the famous system of the square earth, with solid walls for supporting the heavens. its author was _cosmas_, surnamed _indicopleustes_ after his voyage to india and ethiopia. he was at first a merchant, and afterwards a monk. he died in . his manuscript was entitled "christian topography," and was written in . it was with the object of refuting the opinions of those who gave a spherical form to the earth that cosmas composed his work after the systems of the church fathers, and in opposition to the cosmography of the gentiles. he reduced to a systematic form the opinions of the fathers, and undertook to explain all the phenomena of the heavens in accordance with the scriptures. in his first book he refutes the opinion of the sphericity of the earth, which he regarded as a heresy. in the second he expounds his own system, and the fifth to the ninth he devotes to the courses of the stars. this mongrel composition is a singular mixture of the doctrines of the indians, chaldeans, greeks, and christian fathers. with respect to his opponents he says, "there are on all sides vigorous attacks against the church," and accuses them of misunderstanding scripture, being misled by the eclipses of the sun and moon. he makes great fun of the idea of rain falling upwards, and yet accuses his opponents of making the earth at the same time the centre and the base of the universe. the zeal with which these pretended refutations are used proves, no doubt, that in the sixth century there were some men, more sensible and better instructed than others, who preserved the deposit of progress accomplished by the grecian genius in the alexandrian school, and defended the labours of hipparchus and ptolemy; while it is manifest that the greater number of their contemporaries kept the old indian and homeric traditions, which were easier to understand, and more accessible to the false witness of the senses, and not improved by combination with texts of scripture misinterpreted. in fact, cosmographical science in the general opinion retrograded instead of advancing. according to cosmas and his map of the world, the habitable earth is a plane surface. but instead of being supposed, as in the time of thales, to be a disc, he represented it in the form of a parallelogram, whose long sides are twice the shorter ones, so that man is on the earth like a bird in a cage. this parallelogram is surrounded by the ocean, which breaks in in four great gulfs, namely, the mediterranean and caspian seas, and the persian and arabian gulfs. beyond the ocean in every direction there exists another continent which cannot be reached by man, but of which one part was once inhabited by him before the deluge. to the east, just as in other maps of the world, and in later systems, he placed the _terrestrial paradise_, and the four rivers that watered eden, which come by subterranean channels to water the post-diluvian earth. after the fall, adam was driven from paradise; but he and his descendants remained on its coasts until the deluge carried the ark of noah to our present earth. on the four outsides of the earth rise four perpendicular walls, which surround it, and join together at the top in a vault, the heavens forming the cupola of this singular edifice. the world, according to cosmas, was therefore a large oblong box, and it was divided into two parts; the first, the abode of men, reaches from the earth to the firmament, above which the stars accomplish their revolutions; there dwell the angels, who cannot go any higher. the second reaches upwards from the firmament to the upper vault, which crowns and terminates the world. on this firmament rest the waters of the heavens. cosmas justifies this system by declaring that, according to the doctrine of the fathers and the commentators on the bible, the earth has the form of the tabernacle that moses erected in the desert; which was like an oblong box, twice as long as broad. but we may find other similarities,--for this land beyond the ocean recalls the atlantic of the ancients, and the mahomedans, and orientals in general, say that the earth is surrounded by a high mountain, which is a similar idea to the walls of cosmas. [illustration: fig. .--the cosmography of cosmas.] "god," he says, "in creating the earth, rested it on nothing. the earth is therefore sustained by the power of god, the creator of all things, supporting all things by the word of his power. if below the earth, or outside of it, anything existed, it would fall of its own accord. so god made the earth the base of the universe, and ordained that it should sustain itself by its own proper gravity." after having made a great square box of the universe, it remained for him to explain the celestial phenomena, such as the succession of days and nights and the vicissitudes of the seasons. [illustration: fig. .--the square earth.] this is the remarkable explanation he gives. he says that the earth, that is, the oblong table circumscribed on all sides by high walls, is divided into three parts; first the habitable earth, which occupies the middle; secondly, the ocean which surrounds this on all sides; and thirdly, another dry land which surrounds the ocean, terminated itself by these high walls on which the firmament rests. according to him the habitable earth is always higher as we go north, so that southern countries are always much lower than northern. for this reason, he says, the tigris and euphrates, which run towards the south, are much more rapid than the nile, which runs northwards. at the extreme north there is a large conical mountain, behind which the sun, moon, planets, and comets all set. these stars never pass below the earth, they only pass behind this great mountain, which hides them for a longer or shorter time from our observation. according as the sun departs from or approaches the north, and consequently is lower or higher in the heavens, he disappears at a point nearer to or further from the base of the mountain, and so is behind it a longer or shorter time, whence the inequality of the days and nights, the vicissitudes of the seasons, eclipses, and other phenomena. this idea is not peculiar to cosmas, for according to the indians, the mountain of someirat is in the centre of the earth, and when the sun appears to set, he is really only hiding behind this mountain. his idea, too, of the manner in which the motions are performed is strange, but may be matched elsewhere. "all the stars are created," he says, "to regulate the days and nights, the months and the years, and they move, not at all by the motion of the heaven itself, but by the action of certain divine beings, or _lampadophores_. god made the angels for his service, and he has charged some of them with the motion of the air, others with that of the sun, or the moon, or the other stars, and others again with the collecting of clouds and preparing the rain." similar to this were the ideas of other doctors of the [illustration: fig. .--explanation of sunrise.] church, such as s. hilary and theodorus, some of whom supposed that the angels carried the stars on their shoulders like the _omophores_ of the manichees; others that they rolled them in front of them or drew them behind; while the jesuit riccioli, who made astronomical observations, remarks that each angel that pushes a star takes great care to observe what the others are doing, so that the relative distances between the stars may always remain what they ought to be. the abbot trithemus gives the exact succession of the seven angels or spirits of the planets, who take it in turns during a cycle of three hundred and fifty-four years to govern the celestial motions from the creation to the year . the system thus introduced seems to have been spread abroad, and to have lingered even into the nineteenth century among the arabs. a guide of that nationality hired at cairo in , remarked to two travellers how the earth had been made square and covered with stones, but the stones had been thrown into the four corners, now called france, italy, england, and russia, while the centre, forming a circle round mount sinai, had been given to the arabians. alongside of this system of the square was another equally curious--that of the egg. its author was the famous venerable bede, one of the most enlightened men of his time, who was educated at the university of armagh, which produced alfred and alcuin. he says: "the earth is an element placed in the middle of the world, as the yolk is in the middle of an egg; around it is the water, like the white surrounding the yolk; outside that is the air, like the membrane of the egg; and round all is the fire which closes it in as the shell does. the earth being thus in the centre receives every weight upon itself, and though by its nature it is cold and dry in its different parts, it acquires accidentally different qualities; for the portion which is exposed to the torrid action of the air is burnt by the sun, and is uninhabitable; its two extremities are too cold to be inhabited, but the portion that lies in the temperate region of the atmosphere is habitable. the ocean, which surrounds it by its waves as far as the horizon, divides it into two parts, the upper of which is inhabited by us, while the lower is inhabited by our antipodes; although not one of them can come to us, nor one of us to them." [illustration: fig. .--the earth as an egg.] this last sentence shows that however far he may have been from the truth, he did not, like so many of his contemporaries, stumble over the idea of up and down in the universe, and so consider the notion of antipodes absurd. [illustration: fig. .--the earth as a floating egg.] a great number of the maps of the world of the period followed this idea, and drew the world in the shape of an egg at rest. it was broached, however, in another form by edrisi, an arabian geographer of the eleventh century, who, with many others, considered the earth to be like an egg with one half plunged into the water. the regularity of the surface is only interrupted by valleys and mountains. he adopted the system of the ancients, who supposed that the torrid zone was uninhabited. according to him the known world only forms a single half of the egg, the greater part of the water belonging to the surrounding ocean, in the midst of which earth floats like an egg in a basin. several artists and map-makers adopted this theory in the geographical representations, and so, whether in this way or the last, the egg has had the privilege of representing the form of the earth for nearly a thousand years. the celebrated raban maur, of mayence, composed in the ninth century a treatise, entitled _de universo_, divided into twenty-two books. it is a kind of encyclopædia, in which he gives an abridged view of all the sciences. according to his cosmographic system the earth is in the form of a wheel, and is placed in the middle of the universe, being surrounded by the ocean; on the north it is bounded by the caucasus, which he supposes to be mountains of gold, which no one can reach because of dragons, and griffins, and men of monstrous shape that dwell there. he also places jerusalem in the centre of the earth. the treatise of honorus, entitled _imago mundi_, and many other authors of the same kind, represent, st, the terrestrial paradise in the most easterly portion of the world, in a locality inaccessible to man; nd, the four rivers which had their sources in paradise; rd, the torrid zone, uninhabited; th, fantastic islands, transformed from the atlantis into _antillia_. [illustration: fig .--eighth century map of the world.] in a manuscript commentary on the apocalypse, which is in the library of turin, is a very curious chart, referred to the tenth, but belonging possibly to the eighth century. it represents the earth as a circular planisphere. the four sides of the earth are each accompanied by a figure of a wind, as a horse on a bellows, from which air is poured out, as well as from a shell in his mouth. above, or to the east, are adam, and eve with the serpent. to their right is asia with two very elevated mountains--cappadocia and caucasus. from thence comes the river _eusis_, and the sea into which it falls forms an arm of the ocean which surrounds the earth. this arm joins the mediterranean, and separates europe from asia. towards the middle is jerusalem, with two curious arms of the sea running past it; while to the south there is a long and straight sea in an east and west direction. the various islands of the mediterranean are put in a square patch, and rome, france, and germany are indicated, while thula, britannia, and scotia are marked as islands in the north-west of the ocean that surrounds the whole world. [illustration: fig. .--tenth century maps.] we figure below two very curious maps of the world of the tenth century--one of which is round, the other square. the first is divided into three triangles; that of the east, or asia, is marked with the name of _shem_; that of the north, or europe, with that of _japhet_; that of the south, or africa, with that of _cham_. the second is also divided between the three sons of noah; the ocean surrounds it, the mediterranean forms the upright portion of a cross of water which divides the adamic world. omons, the author of a geographical poem entitled _the image of the world_, composed in , who was called the lucretius of the thirteenth century, was not more advanced than the cosmographers of the former centuries of which we have hitherto spoken. the cosmographical part of his poem is borrowed from the system of pythagoras and the venerable bede. he maintains that the earth is enveloped in the heavens, as the yoke in the white of an egg, and that it is in the middle as the centre is within the circle, and he speaks like pythagoras of the harmony of the celestial spheres. omons supposed also that in his time the terrestrial paradise was still existing in the east, with its tree of life, its four rivers, and its angel with a flaming sword. he appears to have confounded hecla with the purgatory of st. patrick, and he places the latter in iceland, saying that it never ceases to burn. the volcanoes were only, according to him, the breathing places or mouths of the infernal regions. the latter he placed with other cosmographers in the centre of the earth. another author, nicephorus blemmyde, a monk who lived during the same century, composed three cosmographical works, among them the following: _on the heavens and the earth, on the sun and moon, the stars, and times and days_. according to his system the earth is flat, and he adopts the homeric theory of the ocean surrounding the world, and that of the seven climates. nicolas of oresmus, a celebrated cosmographer of the fourteenth century, although his celebrity as a mathematician attracted the attention of king john of france, who made him tutor to his son charles v., was not wiser than those we have enumerated above. he composed among other works a _treatise on the sphere_. he rejected the theory of an antichthonal continent as contrary to the faith. a map of the world, prepared by him about the year , represents the earth as round, with one hemisphere only inhabited, the other, or lower one, being plunged in the water. he seems to have been led by various borrowed ideas, as, for instance, theological ones, such as the statement in the psalms that god had founded the earth upon the waters, and grecian ones borrowed from the school of thales, and the theories of the arabian geographers. in fact we have seen that edrisi thought that half of the earth was in the water, and aboulfeda thought the same. the earth was placed by nicolas in the centre of the universe, which he represented by painting the sky blue, and dotting it over with stars in gold. leonardo dati, who composed a geographical poem entitled _della spera_, during this century, advanced no further. a coloured planisphere showed the earth in the centre of the universe surrounded by the ocean, then the air, then the circles of the planets after the ptolemaic system, and in another representation of the same kind he figures the infernal regions in the centre of the earth, and gives its diameter as seven thousand miles. he proves himself not to have known one half of the globe by his statement of the shape of the earth--that it is like a t inside an o. this is a comparison given in many maps of the world in the middle ages, the mean parallel being about the th degree of north latitude, that is to say at the straits of gibraltar; the mediterranean is thus placed so as to divide the earth into two equal parts. john beauvau, bishop of angiers under louis xl, expresses his ideas as follows:-- "the earth is situated and rests in the middle of the firmament, as the centre or point is in the middle of a circle. of the whole earth mentioned above only one quarter is inhabited. the earth is divided into four parts, as an apple is divided through the centre by cutting it lengthways and across. if one part of such an apple is taken and peeled, and the peel is spread out over anything flat, such as the palm of the hand, then it resembles the habitable earth, one side of which is called the east, and the other the west." the arabians adopted not only the ideas of the ancients, but also the fundamental notions of the cosmographical system of the greeks. some of them, as _bakouy_, regarded the earth as a flat surface, like a table, others as a ball, of which one half is cut off, others as a complete revolving ball, and others that it was hollow within. others again went as far as to say that there were several suns and moons for the several parts of the earth. in a map, preserved in the library at cambridge, by henry, canon of st. marie of mayence, the form of the world is given after herodotus. the four cardinal points are indicated, and the orientation is that of nearly all the cartographic monuments of the middle ages, namely, the east at the top of the map. the four cardinal points are four angels, one foot placed on the disc of the earth; the colours of their vestments are symbolical. the angel placed at the boreal extremity of the earth, or to the north of the scythians, points with his finger to people enclosed in the ramparts of gog and magog, _gens immunda_ as the legend says. in his left hand he holds a die to indicate, no doubt, that there are shut up the jews who cast lots for the clothes of christ. his vestments are green, his mantle and his wings are red. the angel placed to the left of paradise has a green mantle and wings, and red vestments. in his left hand he holds a kind of palm, and by the right he seems to mark the way to paradise. the position of the other angels placed at the west of the world is different. they seem occupied in stopping the passage beyond the _columns_ (that is, the entrance to the atlantic ocean). all of them have golden aureolas. the surrounding ocean is painted of a clear green. another remarkable map of the world is that of andrea bianco. in it we see eden at the top, which represents the east, and the four rivers are running out of it. much of europe is indicated, including spain, paris, sweden, norway, ireland, which are named, england, iceland, spitzbergen, &c., which are not named. the portion round the north pole to the left is indicated as "cold beneath the pole star." in these maps the systematic theories of the ancient geographers seem mixed with the doctrines of the fathers of the church. they place generally in the red sea some mark denoting the passage of the hebrews, the terrestrial paradise at the extreme east, and jerusalem in the centre. the towns are figured often by edifices, as in the list of theodosius, but without any regard to their respective positions. each town is ordinarily represented by two towers, but the principal ones are distinguished by a little wall that appears between these two towers, on which are painted several windows, or else they may be known by the size of the edifices. st. james of compostella in gallicia and rome are represented by edifices of considerable size, as are nazareth, troy, antioch, damascus, babylon, and nineveh. [illustration: fig. .--the map of andrea bianco.] one of the most remarkable monuments of the geography of the last centuries of the middle ages is the map in hereford cathedral, by richard of haldingham, not only on account of its numerous legends, but because of its large dimensions, being several square yards in area. on the upper part of this map is represented the last judgment; jesus christ, with raised arms, holding in his hands a scroll with these words, _ecce testimonium meum_. at his side two angels carry in their hands the instruments of his passion. on the right hand stands an angel with a trumpet to his mouth, out of which come these words, _levez si vendres vous par_. an angel brings forward a bishop by the hand, behind whom is a king, followed by other personages; the angel introduces them by a door formed of two columns, which seems to serve as an entrance to an edifice. the virgin is kneeling at the feet, of her son. behind her is another woman kneeling, who holds a crown, which she seems ready to place on the head of the mother of christ, and by the side of the woman is a kneeling angel, who appears to be supporting the maternal intercessor. the virgin uncovers her breast and pronounces the words of a scroll which is held by an angel kneeling in front of her, _vei i b' fiz mon piz de deuiz lauele chare preistes--eles mame lettes dont leit de virgin qui estes--syes merci de tous si com nos mesmes deistes.--r ... em ... ont servi kaut sauveresse me feistes_. to the left another angel, also with a trumpet to his mouth, gives out the following words, which are written on a scroll, _leves si alles all fu de enfer estable_. a gate, drawn like that of the entrance, represents probably the passage by which those must go out who are condemned to eternal pains. in fact the devil is seen dragging after him a crowd of men, who are tied by a cord which he holds in his hand. [illustration: fig. .--from the map in hereford cathedral.] the map itself commences at its upper part, that is, the east, by the terrestrial paradise. it is a circle, in the centre of which is represented the tree of the knowledge of good and evil. adam and eve are there in company with the serpent that beguiled them. the four legendary rivers come out of the base of the tree, and they are seen below crossing the map. outside eden the flight of the first couple, and the angel that drove them away, are represented. at this extreme eastern portion is the region of giants with the heads of beasts. there, too, is seen the first human habitation, or town, built by enoch. below appears the tower of babel. near this are two men seated on a hill close to the river jaxartes; one of them is eating a human leg and the other an arm, which the legend explains thus:--"here live the essedons, whose custom it is to sing at the funerals of their parents; they tear the corpses with their teeth, and prepare their food with these fragments of flesh, mixed with that of animals. in their opinion it is more honourable to the dead to be enclosed in the bodies of their relations than in those of worms." [illustration: fig. .--from the map in hereford cathedral. tower of babel. essedons. dragons. pigmies. the monoceros. the mantichore. a sphinx. the king of the cyclops. blemmye. parasol lip. monocle.] below are seen dragons and pigmies, always to the east of asia, and a little further away in the midst of a strange country, _the king of the cyclops_. this extraordinary geography shows us in india the "mantichore, who has a triple range of teeth, the face of a man, blue eyes, the red colour of blood, the body of a lion, and the tail of a scorpion; its voice is a whistle." on the north of the ganges is represented a man with one leg, shading his head with his foot, which is explained by the following legend:--"in india dwell the monocles, who have only one leg, but who nevertheless move with surprising velocity; when they wish to protect themselves from the heat of the sun they make a shadow with the sole of their foot, which is very large." the blemmys have their mouth and eyes in their chest; others have their mouths and eyes on their shoulders. the parvini are ethiopians that have four eyes. to the east of syene is a man seated who is covering his head with his lip, "people who with their prominent lip shade their faces from the sun." above is drawn a little sun, with the word _sol_. then comes an animal of human form, having the feet of a horse and the head and beak of a bird; he rests on a stick, and the legend tells us it is a satyr; the fauns, half men and half horses; the cynocephales--men with the head of a dog; and the cyanthropes--dogs with the heads of men. the sphinx has the wings of a bird, the tail of a serpent, the head of a woman. it is placed in the midst of the cordilleras, which are joined to a great chain of mountains. here lastly is seen the _monoceros_, a terrible animal; but here is the marvel: "when one shows to this _monoceros_ a young girl, who, when the animal approaches, uncovers her breast, the monster, forgetting his ferocity, lays his head there, and when he is asleep may be taken defenceless." near to the lake meotides is a man clothed in oriental style, with a hat that terminates in a point, and holding by the bridle a horse whose harness is a human skin, and which is explained thus by the latin legend: "here live the griffins, very wicked men, for among other crimes they proceed so far as to make clothes for themselves and their horses out of the skins of their enemies." more to the south is a large bird, the ostrich; according to the legend, "the ostrich has the head of a goose, the body of a crane, the feet of a calf; it eats iron." not far from the riphean mountains two men with long tunics and round bonnets are represented in the attitude of fighting; one brandishes a sword, the other a kind of club, and the legend tells us, "the customs of the people of the interior of scythia are somewhat wild; they inhabit caves; they drink the blood of the slain by sucking their wounds; they pride themselves on the number of people they have slain--not to have slain any one in combat is reckoned disgraceful." near the river that empties itself into the caspian sea it is written: "this river comes from the infernal regions; it enters the sea after having descended from mountains covered with wood, and it is there, they say, that the mouth of hell opens." to the south of this river, and to the north of hyrcania, is represented a monster having the body of a man, the head, tail, and feet of a bull: this is the minotaur. further on are the mountains of armenia, and the ark of noah on one of its plateaux. here, too, is seen a large tiger, and we read: "the tiger, when he sees that he has been deprived of his young, pursues the ravisher precipitately; but the latter, hastening away on his swift horse, throws a mirror to him and is safe." elsewhere appears lot's wife changed into a pillar of salt; the lynx who can see through a stone wall; the river lethe; so called because all who drink of it forget everything. numerous other details might be mentioned, but enough has been said to show the curious nature and exceeding interest of this map, in which matters of observation and imagination are strangely mixed. another very curious geographical document of that epoch is the map of the world of the _grandes chroniques de saint-denis_. this belongs to the fourteenth century. the capitals here too are represented by edifices. the mediterranean is a vertical canal, which goes from the columns of hercules to jerusalem. the caspian sea communicates with it to the north, and the red sea to the south-east, by the nile. it preserves the same position for paradise and for the land of gog and magog that we have seen before. the geography of europe is very defective. britannia and anglia figure as two separate islands, being represented off the west coast of spain, with allemania and germania, also two distinct countries, to the north. the ocean is represented as round the whole, and the various points of the compass are represented by different kinds of winds on the outside. [illustration: fig. .--cosmography of st. denis.] this was the general style of the maps of the world at that period, as we may perceive from the various illustrations we have been able to give, and it curiously initiates us into the mediæval ideas. sometimes they are surrounded by laughable figures of the winds with inflated cheeks, sometimes there are drawn light children of eolus seated on leathern bottles, rotating the liquid within; at other times, saints, angels, adam and eve, or other people, adorn the circumference of the map. within are shown a profusion of animals, trees, populations, monuments, tents, draperies, and monarchs seated on their thrones--an idea which was useful, no doubt, and which gave the reader some knowledge of the local riches, the ethnography, the local forms of government and of architecture in the various countries represented; but the drawings were for the most part childish, and more fantastic than real. the language, too, in which they were written was as mixed as the drawings; no regularity was preserved in the orthography of a name, which on the same map may be written in ten different ways, being expressed in barbarous latin, roman, or old french, catalan, italian, castilian, or portuguese! during the same epoch other forms of maps in less detail and of smaller size show the characters that we have seen in the maps of earlier centuries. marco polo, the traveller, at the end of the fourteenth century, has preserved in his writings all the ancient traditions, and united them in a singular manner with the results of his own observations. he had not seen paradise, but he had seen the ark of noah resting on the top of ararat. his map of the world, preserved in the library at stockholm, is oval, and represents two continents. in that which we inhabit, the only seas indicated are the mediterranean and the black sea. asia appears at the east, europe to the north, and africa to the south. the other continent to the south of the equator, which is not marked, is antichthonia. in a map of the world engraved on a medal of the fifteenth century during the reign of charles v. there is still a reminiscence of the ideas of the concealed earth and meropides, as described by theopompus. we see the winds as cherubim; europe more accurately represented than usual; but africa still unknown, and a second continent, called brumæ, instead of antichthonia, with imaginary details upon it. [illustration: fig. --the map of marco polo.] if such were the ideas entertained amongst the most enlightened nations, what may we expect among those who were less advanced? it would take us too long to describe all that more eastern nations have done upon this point since the commencement of our present era, but we may give an example or two from the arabians. [illustration: fig. .--map on a medal of charles v.] in the ancient arabian chronicle of tabari is a system founded on the earth being the solid foundation of all things; we read: "the prophet says, the all-powerful and inimitable deity has created the mountain of kaf round about the earth; it has been called the foundation pile of the earth, as it is said in the koran, 'the mountains are the piles.' this world is in the midst of the mountain of kaf, just as the finger is in the midst of the ring. this mountain is emerald, and blue in colour; no man can go to it, because he would have to pass four months in darkness to do so. there is in that mountain neither sun, nor moon, nor stars; it is so blue that the azure colour you see in the heavens comes from the brilliancy of the mountain of kaf, which is reflected in the sky. if this were not so the sky would not be blue. all the mountains that you see are supported by kaf; if it did not exist, all the earth would be in a continual tremble, and not a creature could live upon its surface. the heavens rest upon it like a tent." another arabian author, benakaty, writing in , says: "know that the earth has the form of a globe suspended in the centre of the heavens. it is divided by the two great circles of the meridian and equator, which cut each other at light angles, into four equal parts, namely, those of the north-west, north-east, south-west, and south-east. the inhabited portion of the earth is situated in the southern hemisphere, of which one half is inhabited." ibn-wardy, who lived in the same century, adopted the idea of the ocean surrounding all the earth, and said we knew neither its depth nor its extent. this ocean was also acknowledged by the author of the kaf mountain; he says it lies between the earth and that mountain, and calls it bahr-al-mohith. the end of the fifteenth century saw the dawn of a new era in knowledge and science. the discoveries of columbus changed entirely the aspect of matters, the imagination was excited to fresh enterprises, and the hardihood of the adventurers through good or bad success was such as want of liberty could not destroy. nevertheless, as we have seen, columbus imagined the earth to have the shape of a pear. not that he obtained this idea from his own observations, but rather retained it as a relic of past traditions. it is probable that it really dates from the seventh century. we may read in several cosmographical manuscripts of that epoch, that the earth has the form of a cone or a top, its surface rising from south to north. these ideas were considerably spread by the compilations of john of beauvais in , from whom probably columbus derived his notion. although columbus is generally and rightly known as the discoverer of the new world, a very curious suit was brought by pinzon against his heirs in . pinzon pretended that the discovery was due to him alone, as columbus had only followed his advice in making it. pinzon told the admiral himself that the required route was intimated by an inspiration, or revelation. the truth was that this "revelation" was due to a flock of parrots, flying in the evening towards the south-west, which pinzon concluded must be going in the direction of an invisible coast to pass the night in the bushes. certainly the consequences of columbus resisting the advice of pinzon would have been most remarkable; for had he continued to sail due west he would have been caught by the gulf stream and carried to florida, or possibly to virginia, and in this case the united states would have received a spanish and catholic population, instead of an english and protestant one. the discoveries of those days were often commemorated by the formation of heraldic devices for the authors of them, and we have in this way some curious coats of arms on record. that, for instance, of sebastian cano was a globe, with the legend, _primus circumdedisti me_. the arms given to columbus in consisted of the first map of america, with a range of islands in a gulf. charles v. gave to diego of ordaz the figure of the peak of orizaba as his arms, to commemorate his having ascended it; and to the historian oviedo, who passed thirty-four years without interruption ( - ) in tropical america, the four beautiful stars of the southern cross. we have arrived at the close of our history of the attempts that preceded the actual discovery of the form and constitution of the globe; since these were established our further progress has been in matters of detail. there now remains briefly to notice the attempts at discovering the size of the earth on the supposition, and afterwards certainty, of its being a globe. the earliest attempt at this was made by eratosthenes, years before our era, and it was founded on the following reasoning. the sun illuminates the bottom of pits at syene at the summer solstice; on the same day, instead of being vertical over the heads of the inhabitants of alexandria, it is - / degrees from the zenith. seven-and-a-quarter degrees is the fiftieth part of an entire circumference; and the distance between the two towns is five thousand stadia; hence the circumference of the earth is fifty times this distance, or thousand stadia. a century before our era posidonius arrived at an analogous result by remarking that the star canopus touched the horizon at rhodes when it was degrees minutes above that of alexandria. these measurements, which, though rough, were ingenious, were, followed in the eighth century by similar ones by the arabian caliph, almamoun, who did not greatly modify them. the first men who actually went round the world were the crew of the ship under magellan, who started to the west in ; he was slain by the philippine islanders in , but his ship, under his lieutenant, sebastian cano, returned by the east in . the first attempt at the actual measurement of a part of the earth's surface along the meridian was made by fernel in . his process was a singular, but simple one, namely, by counting the number of the turns made by the wheels of his carriage between paris and amiens. he made the number , , and accurate measurements of the distance many years after showed he had not made an error of more than four turns. the astronomer picard attempted it again under louis xiv. by triangulation. the french astronomers have always been forward in this inquiry, and to them we owe the systematic attempts to arrive at a truer knowledge of the length of an arc of the meridian which were made in - in lapland and in peru; and later under mechain and delambre, by order of the national assembly, for the basis of the metrical system. observations of this kind have also been made by the english, as at lough foyle in ireland, and in india. the review which has here been made of the various ideas on what now seems so simple a matter cannot but impress us with the vast contrast there is between the wild attempts of the earlier philosophers and our modern affirmations. what progress has been made in the last two thousand years! and all of this is due to hard work. the true revelation of nature is that which we form ourselves, by our persevering efforts. we now know that the earth is approximately spherical, but flattened by about / at the poles, is three-quarters covered with water, and enveloped everywhere by a light atmospheric mantle. the distance from the centre of the earth to its surface is , miles, its area is million square miles, its volume is , millions of cubic miles, its weight is six thousand trillion tons. so, thanks to the bold measurements of its inhabitants, we know as much about it as we are likely to know for a long time to come. chapter xi. legendary worlds of the middle ages. the legends that were for so many ages prevalent in europe had their foundation in the attempt to make the accounts of scripture and the ideas and dogmas of the fathers of the church fit into the few and insignificant facts that were known with respect to the earth, and the system of which it forms a part, and the far more numerous imaginations that were entertained about it. we are therefore led on to examine some of these legends, that we may appreciate how far a knowledge of astronomy will effect the eradication of errors and fantasies which, under the aspect of truth, have so long enslaved the people. no doubt the authors of the legendary stories knew well enough their allegorical nature; but those who received them supposed that they gave true indications of the nature of the earth and world, and therefore accepted them as facts. some indeed considered that the whole physical constitution of the world was a scaffold or a model, and that there was a real theological universe hidden beneath this semblance. no one omitted from his system the spiritual heaven in which the angels and just men might spend their existence; but in addition to this there were places whose reality was believed in, but whose locality is more difficult to settle, and which therefore were moved from one place to another by various writers, viz., the infernal regions, purgatory, and the terrestrial paradise. we will here recount some of those legends, which wielded sufficient sway over men's minds as to gain their belief in the veritable existence of the places described, and in this way to influence their astronomical and cosmographical ideas. and for the first we will descend to the infernal regions with plutarch and thespesius. this thespesius relates his adventures in the other world. having fallen head-first from an elevated place, he found himself unwounded, but was contused in such a way as to be insensible. he was supposed to be dead, but, after three days, as they were about to bury him, he came to life again. in a few days he recovered his former powers of mind and body; but made a marvellous change for the better in his life. he said that at the moment that he lost consciousness he found himself like a sailor at the bottom of the sea; but afterwards, having recovered himself a little, he was able to breathe perfectly, and seeing only with the eyes of his soul, he looked round on all that was about him. he saw no longer the accustomed sights, but stars of prodigious magnitude, separated from each other by immense distances. they were of dazzling brightness and splendid colour. his soul, carried like a vessel on the luminous ocean, sailed along freely and smoothly, and moved everywhere with rapidity. passing over in silence a large number of the sights that met his eye, he stated that the souls of the dead, taking the form of bubbles of fire, rise through the air, which opens a passage above them; at last the bubbles, breaking without noise, let out the souls in a human form and of a smaller size, and moving in different ways. some, rising with astonishing lightness, mounted in a straight line; others, running round like a whipping-top, went up and down by turns with a confused and irregular motion, making small advance by long and painful efforts. among this number he saw one of his parents, whom he recognised with difficulty, as she had died in his infancy; but she approached him, and said, "good day, thespesius." surprised to hear himself called by this name, he told her that he was called arideus, and not thespesius. "that was once your name," she replied, "but in future you will bear that of thespesius, for you are not dead, only the intelligent part of your soul has come here by the particular will of the gods; your other faculties are still united to your body, which keeps them like an anchor. the proof i will give you is that the souls of the dead do not cast any shadow, and they cannot move their eyes." further on, in traversing a luminous region, he heard, as he was passing, the shrill voice of a female speaking in verse, who presided over the time thespesius should die. his genius told him that it was the voice of the sibyl, who, turning on the orbit of the moon, foretold the future. thespesius would willingly have heard more, but, driven off by a rapid whirlwind, he could make out but little of her predictions. in another place he remarked several parallel lakes, one filled with melted and boiling gold, another with lead colder than ice, and a third with very rough iron. they were kept by genii, who, armed with tongs like those used in forges, plunged into these lakes, and then withdrew by turns, the souls of those whom avarice or an insatiable cupidity had led into crime; after they had been plunged into the lake of gold, where the fire made them red and transparent, they were thrown into the lake of lead. then, frozen by the cold, and made as hard as hail, they were put into the lake of iron, where they became horribly black. broken and bruised on account of their hardness, they changed their form, and passed once more into the lake of gold, and suffered in these changes inexpressible pain. in another place he saw the souls of those who had to return to life and be violently forced to take the form of all sorts of animals. among the number he saw the soul of nero, which had already suffered many torments, and was bound with red-hot chains of iron. the workmen were seizing him to give him the form of a viper, under which he was destined to live, after having devoured the womb that bore him. the locality of these infernal regions was never exactly determined. the ancients were divided upon the point. in the poems of homer the infernal regions appear under two different forms: thus, in the _iliad_, it is a vast subterranean cavity; while in the _odyssey_, it is a distant and mysterious country at the extremity of the earth, beyond the ocean, in the neighbourhood of the cimmerians. the description which homer gives of the infernal region proves that in his time the greeks imagined it to be a copy of the terrestrial world, but one which had a special character. according to the philosophers it was equally remote from all parts of the earth. thus cicero, in order to show that it was of no consequence where one died, said, wherever we die there is just as long a journey to be made to reach the "infernal regions." the poets fixed upon certain localities as the entrance to this dismal empire: such was the river lethe, on the borders of the scythians; the cavern acherusia in epirus, the mouth of pluto, in laodicoea, the cave of zenarus near lacedæmon. in the map of the world in the _polychronicon_ of ranulphus uygden, now in the british museum, it is stated: "the island of sicily was once a part of italy. there is mount etna, containing the infernal regions and purgatory, and it has scylla and charybdis, two whirlpools." ulysses was said to reach the place of the dead by crossing the ocean to the cimmerian land, Æneas to have entered it by the lake of avernus. xenophon says that hercules went there by the peninsula of arechusiade. much of this, no doubt, depends on the exaggeration and misinterpretation of the accounts of voyagers; as when the phoenicians related that, after passing the columns of hercules, to seek tin in thule and amber in the baltic, they came, at the extremity of the world, to the fortunate isles, the abode of eternal spring, and further on to the hyperborean regions, where a perpetual night enveloped the country--the imagination of the people developed from this the elysian fields, as the places of delight in the lower regions, having their own sun, moon, and stars, and tartarus, a place of shades and desolation. in every case, however, both among pagans and christians, the locality was somewhere in the centre of the earth. the poets and philosophers of greece and rome made very detailed and circumstantial maps of the subterranean regions. they enumerated its rivers, its lakes, and woods, and mountains, and the places where the furies perpetually tormented the wicked souls who were condemned to eternal punishment. these ideas passed naturally into the creeds of christians through the sect of the essenes, of whom josephus writes as follows:--"they thought that the souls of the just go beyond the ocean to a place of repose and delight, where they were troubled by no inconvenience, no change of seasons. those of the wicked, on the contrary, were relegated to places exposed to all the inclemencies of the weather, and suffered eternal torments. the essenes," adds the same author, "have similar ideas about these torments to those of the greeks about tartarus and the kingdom of pluto. the greater part of the gnostic sects, on the contrary, considered the lower regions as simply a place of purgatory, where the soul is purified by fire." amongst all the writings of christian ages in which matters such as we are now passing in review are described, there is one that stands out beyond all others as a masterpiece, and that is the magnificent poem of dante, his _divine comedy_, wherein he described the infernal regions as they presented themselves to his lively and fertile imagination. we have in it a picture of mediæval ideas, painted for us in indelible lines, before the remembrance of them was lost in the past. the poem is at once a tomb and a cradle--the tomb of a world that was passing, the cradle of the world that was to come: a portico between two temples, that of the past and that of the future. in it are deposited the traditions, the ideas, the sciences of the past, as the egyptians deposited their kings and symbolic gods in the sepulchres of thebes and memphis. the future brings into it its aspirations and its germs enveloped in the swaddling clothes of a rising language and a splendid poetry--a mysterious infant that is nourished by the two teats of sacred tradition and profane fiction, moses and st. paul, homer and virgil. the theology of dante, strictly orthodox, was that of st. thomas and the other doctors of the church. natural philosophy, properly so called, was not yet in existence. in astronomy, ptolemy reigned supreme, and in the explanation of celestial phenomena no one dreamt or dared to dream of departing in any way from the traditionally sacred system. in those days astronomy was indissolubly linked with a complete series of philosophical and theological ideas, and included the physics of the world, the science of life in every being, of their organisation, and the causes on which depended the aptitudes, inclinations, and even in part the actions, of men, the destinies of individuals, and the events of history. in this theological, astronomical, and terrestrial universe everything emanated from god; he had created everything, and the creation embraced two orders of beings, the immaterial and the corporeal. the pure spirits composed the nine choirs of the celestial hierarchy. like so many circles, they were ranged round a fixed point, the eternal being, in an order determined by their relative perfection. first the seraphim, then the cherubim, and afterwards the simple angels. those of the first circle received immediately from the central point the light and the virtue which they communicated to those of the second; and so on from circle to circle, like mirrors which reflect, with an ever-lessening light, the brilliancy of a single luminous point. the nine choirs, supported by love, turned without ceasing round their centre in larger and larger circles according to their distance; and it was by their means that the motion and the divine inflatus was communicated to the material creation. this latter had in the upper part of it the empyreal, or heaven of pure light. below that, was the _primum mobile_, the greatest body in the heavens, as dante calls it, because it surrounds all the rest of the circle, and bounds the material world. then came the heaven of the fixed stars; then, continuing to descend, the heavens of saturn, jupiter, mars, the sun, venus, mercury, the moon, and lastly, the earth, whose solid and compact nucleus is surrounded by the spheres of water, air, and fire. as the choirs of angels turn about a fixed point, so the nine material circles turn also about another fixed point, and are moved by the pure spirits. let us now descend to the geography of the interior of the earth. within the earth is a large cone, whose layers are the frightful abodes of the condemned, and which ends in the centre, where the divine justice keeps bound up to his chest in ice the prince of the rebellious angels, the emperor of the kingdom of woe. such are the infernal regions which dante describes according to ideas generally admitted in the middle ages. the form of the infernal regions was that of a funnel or reversed cone. all its circles were concentric, and continually diminished; the principal ones were nine in number. virgil also admitted nine divisions--three times three, a number sacred _par excellence_. the seventh, eighth, and ninth circles were divided into several regions; and the space between the entrance to the infernal regions and the river acheron, where the resting-place of the damned really commenced, was divided into two parts. dante, guided by virgil, traversed all these circles. it was in that the poet, "in the midst of the course of life," at the age of thirty-five, passed in spirit through the three regions of the dead. lost in a lonely, wild, and dismal forest, he reached the base of a hill, which he attempted to climb. but three animals, a panther, a lion, and a thin and famished wolf, prevented his passage; so, returning again where the sun was powerless, into the shades of the depths of the valley, there met him a shadow of the dead. this human form, whom a long silence had deprived of speech, was virgil, who was sent to guide and succour him by a celestial dame, beatrice, the object of his love, who was at the same time a real and a mystically ideal being. virgil and dante arrived at the gate of the infernal regions; they read the terrible inscription placed over the gate; they entered and found first those unhappy souls who had lived without virtue and without vice. they reached the banks of acheron and saw charon, who carried over the souls in his bark to the other side; and dante was surprised by a profound sleep. he woke beyond the river, and he descended into the limbo which is the first circle of the infernal regions. he found there the souls of those who had died without baptism, or who had been indifferent to religion. they descended next to the second circle, where minos, the judge of those below, is enthroned. here the luxurious are punished. the poet here met with francesca of rimini and paul, her friend. he completely recovered the use of his senses, and passed through the third circle, where the gourmands are punished. in the fourth he found plutus, who guards it. here are tormented the prodigal and the avaricious. in the fifth are punished those who yield to anger. dante and virgil there saw a bark approaching, conducted by phlegias; they entered it, crossed a river, and arrived thus at the base of the red-hot iron walls of the infernal town of dite. the demons that guarded the gates refused them admittance, but an angel opened them, and the two travellers there saw the heretics that were enclosed in tombs surrounded by flames. the travellers then visited the circles of violence, fraud, and usury, when they came to a river of blood guarded by a troop of centaurs; suddenly they saw coming to them geryon, who represents fraud, and this beast took them behind him to carry them across the rest of the infernal space. [illustration: fig. .--dante's infernal regions.] the eighth circle was divided into ten valleys, comprising: the flatterers; the simoniacal; the astrologers; the sorcerers; the false judges; the hypocrites who walked about clothed with heavy leaden garments; the thieves, eternally stung by venomous serpents; the heresiarchs; the charlatans, and the forgers. at last the poets descended into the ninth circle, divided into four regions, where are punished four kinds of traitors. here is recounted the admirable episode of count ugolin. in the last region, called the region of judas, lucifer is enchained. there is the centre of the earth, and dante, hearing the noise of a little brook, reascended to the other hemisphere, on the surface of which he found, surrounded by the southern ocean, the mountain of purgatory. such was the famous _inferno_ of dante. not only was the geography of the infernal regions attempted in the middle ages, but even their size. dexelius calculated that the number of the damned was a hundred millions, and that their abode need not measure more than one german mile in every direction. cyrano of bergerac amusingly said that it was the damned that kept turning the earth, by hanging on the ceiling like bats, and trying to get away. in an english clergyman, dr. swinden, published a book entitled, _researches on the nature of the fire of hell and the place where it is situated_. he places it in the sun. according to him the christians of the first century had placed it beneath the earth on account of a false interpretation of the descent of jesus into hell after his crucifixion, and by false ideas of cosmography. he attempted to show, st, that the terrestrial globe is too small to contain even the angels that fell from heaven after their battle; nd, that the fire of hell is real, and that the closed globe of earth could not support it a sufficiently long period; rd, that the sun alone presents itself as the necessary place, being a well-sustained fire, and directly opposite in situation to heaven, since the empyreal is round the outside of the universe, and the sun in the centre. what a change to the present ideas, even of doctors of divinity, in a hundred years! so far, then, for mediæval ideas on the position and character of hell. next as to purgatory. the voyage to purgatory that has met with most success is certainly the celebrated irish legend of st. patrick, which for several centuries was admitted as authentic, and the account of which was composed certainly a century before the poem of dante. this purgatory, the entrance to which is drawn in more than one illuminated manuscript, is situated in ireland, on one of the islands of lough derg, county donegal, where there are still two chapels and a shrine, at which annual ceremonies are performed. a knight, called owen, resolved to visit it for penance; and the chronicle gives us an account of his adventures. first he had his obsequial rites performed, as if he had been dead, and then he advanced boldly into the deep ravine; he marched on courageously, and entered into the semi-shadows; he marched on, and even this funereal twilight abandoned him, and "when he had gone for a long time in this obscurity, there appeared to him a little light as it were from a glimmer of day." he arrived at a house, built with much care, an imposing mansion of grief and hope, a marvellous edifice, but similar nevertheless to a monkish cloister, where there was no more light than there is in this world in winter at vesper-time. the knight was in dreadful suspense. suddenly he heard a terrible noise, as if the universe was in a riot; for it seemed certainly to him as if every kind of beast and every man in the world were together, and each gave utterance to their own cry, at one time and with one voice, so that they could not make a more frightful noise. then commenced his trials, and discourse with the infernal beings; the demons yelled with delight or with fury round him. "miserable wretch," said some, "you are come here to suffer." "fly," said others, "for you have not behaved well in the time that is passed: if you will take our advice, and will go back again to the world, we will take it as a great favour and courtesy." [illustration: plate xii.--the legend of owen.] owen was thrown on the dark shadowy earth, where the demons creep like hideous serpents. a mysterious wind, which he scarcely heard, passed over the mud, and it seemed to the knight as if he had been pierced by a spear-head. after a while the demons lifted him up; they took him straight off to the east, where the sun rises, as if they were going to the place where the universe ends. "now, after they had journeyed for a long time here and there over divers countries, they brought him to an open field, very long and very full of griefs and chastisements; he could not see the end of the field, it was so long; there were men and women of various ages, who lay down all naked on the ground with their bellies downwards, who had hot nails driven into their hands and feet; and there was a fiery dragon, who sat upon them and drove his teeth into their flesh, and seemed as if he would eat them; hence they suffered great agony, and bit the earth in spite of its hardness, and from time to time they cried most piteously 'mercy, mercy;' but there was no one there who had pity or mercy, for the devils ran among them and over them, and beat them most cruelly." the devils brought the knight towards a house of punishment, so broad and long that one could not see the end. this house is the house of baths, like those of the infernal regions, and the souls that are bathed in ignominy are there heaped in large vats. "now so it was, that each of these vats was filled with some kind of metal, hot and boiling, and there they plunged and bathed many people of various ages, some of whom were plunged in over their heads, others up to the eyebrows, others up to the eyes, and others up to the mouth. now all in truth of these people cried out with a loud voice and wept most piteously." scarcely had the knight passed this terrible place, and left behind in his mysterious voyage that column of fire which rose like a lighthouse in the shades, and which shone so sadly betwixt hope and eternal despair, than a vast and magnificent spectacle displayed itself in the subterranean space. this luminous and odorescent region, where one might see so many archbishops, bishops, and monks of every order, was the terrestrial paradise; man does not stay there always; they told the knight that he could not taste too long its rapid delights; it is a place of transition between purgatory and the abodes of heaven, just as the dark places which he had traversed were made by the creator between the world and the infernal regions. "in spite of our joys," said the souls, "we shall pass away from here." then they took him to a mountain, and told him to look, and asked of him what colour the heavens seemed to be there where he was standing, and he replied it was the colour of burning gold, such as is in the furnace; and then they said to him, "that which you see is the entrance to heaven and the gate of paradise." the attempts at identification of hell and purgatory have not been so numerous, perhaps because the subjects were not very attractive, except as the spite of men might think of them in reference to other people; but when we come to the terrestrial paradise, quite a crowd of attempts by every kind of writer to fix its position in any and every part of the globe is met with on every side. in the seventeenth century, under louis xiv., daniel huet, bishop of avranches, gave great attention to the question, and collected every opinion that had been expressed upon it, with a view to arriving at some definite conclusion for himself. he was astonished at the number of writings and the diversity of the opinions they expressed. "nothing," he says, "could show me better how little is really known about the situation of the terrestrial paradise than the differences in the opinions of those who have occupied themselves about the question. some have placed it in the third heaven, some in the fourth, in the heaven of the moon, in the moon itself, on a mountain near the lunar heaven, in the middle region of the air, out of the earth, upon the earth, beneath the earth, in a place that is hidden and separated from man. it has been placed under the north pole, in tartary, or in the place now occupied by the caspian sea. others placed it in the extreme south, in the land of fire. others in the levant, or on the borders of the ganges, or in the island of ceylon, making the name india to be derived from eden, the land where the paradise was situated. it has been placed in china, or in an inaccessible place beyond the black sea; by others in america, in africa, beneath the equator, in the east, &c. &c." notwithstanding this formidable array, the good bishop was bold enough to make his choice between them all. his opinion was that the dwelling-place of the first man was situated between the tigris and euphrates, above the place where they separate before falling into the persian gulf; and, founding this opinion on very extensive reading, he declared that of all his predecessors, calvin had come nearest to the truth. among the other authors of greater or less celebrity that have occupied themselves in this question, we may instance the following:-- raban maur (ninth century) believed that the terrestrial paradise was at the eastern extremity of the earth. he described the tree of life, and added that there was neither heat nor cold in that garden; that immense rivers of water nourished all the forest; and that the paradise was surrounded by a wall of fire, and its four rivers watered the earth. james of vitry supposed pison to come out of the terrestrial paradise. he describes also the garden of eden; and, like all the cosmographers of the middle ages, he placed it in the most easterly portion of the world in an inaccessible place, and surrounded by a wall of fire, which rose up to heaven. dati placed also the terrestrial paradise in asia, like the cosmographers that preceded him, and made the nile come from the east. stenchus, the librarian of st. siége, who lived in the sixteenth century, devoted several years to the problem, but discovered nothing. the celebrated orientalist and missionary bochart wrote a treatise on this subject in . thévenot published also in the seventeenth century a map representing the country of the lybians, and adds that "several great doctors place the terrestrial paradise there." an armenian writer who translated and borrowed from st. epiphanius (eighth century) produced a _memorial on the four rivers of the terrestrial paradise_. he supposes they rise in the unknown land of the amazons, whence also arise the danube and the hellespont, and they deliver their waters into that great sea that is the source of all seas, and which surrounds the four quarters of the globe. he afterwards says, following up the same theory, that the rivers of paradise surround the world and enter again into the sea, which is the universal ocean." gervais and robert of st. marien d'auxerre taught that the terrestrial paradise was on the eastern border of the _square_ which formed the world. alain de lille, who lived in the thirteenth century, maintained in his _anticlaudianus_ that the earth is circular, and the garden of eden is in the east of asia. joinville, the friend of st. louis, gives us a curious notion of his geographical ideas, since, with regard to paradise, he assures us that the four great rivers of the south come out of it, as do the spices. "here," he says, referring to the nile, "it is advisable to speak of the river which passes by the countries of egypt, and comes from the terrestrial paradise. where this river enters egypt there are people very expert and experienced, as thieves are here, at stealing from the river, who in the evening throw their nets on the streams and rivers, and in the morning they often find and carry off the spices which are sold here in europe as coming from egypt at a good rate, and by weight, such as cinnamon, ginger, rhubarb, cloves, lignum, aloes, and several other good things, and they say that these good things come _from the terrestrial paradise_, and that the wind blows them off the trees that are growing there." and he says that near the end of the world are the peoples of gog and magog, who will come at the end of the world with antichrist. we find, however, more than descriptions--we have representations of the terrestrial paradise by cartographers of the middle ages, some of which we have seen in speaking of their general ideas of geography, and we will now introduce others. [illustration: fig. .--paradise of fra mauro.] fra mauro, a religious cosmographer of the fifteenth century, gives on the east side of a map of the world a representation which shows us that at that epoch the "garden of delights" had become very barren. it is a vast plain, on which we see jehovah and the first human couple, with a circular rampart surrounding it. the four rivers flow out of it by bifurcating. an angel protects the principal gate, which cannot be reached but by crossing barren mountains. the cosmographical map of gervais, dedicated to the emperor otho iv., shows the terrestrial paradise in the centre of the earth, which is square, and is situated in the midst of the seas. adam and eve appear in consultation. the map of the world prepared by andreas bianco, in the fifteenth century, represents eden, adam and eve, and the tree of life. on the left, on a peninsula, are seen the reprobated people of gog and magog, who are to accompany antichrist. alexander is also represented there, but without apparent reason. the paradisaical peninsula has a building on it with this inscription, "ospitius macarii." formalconi says, on this subject, that a certain macarius lives near paradise, who is a witness to all that the author states, and as bianco has indicated, his cell was close to the gates of paradise. this legend has reference to the pilgrims of st. macarius, a tradition that was spread on the return of the crusaders, of three monks who undertook a voyage to discover the point where the earth and heaven meet, that is to say, the place of the terrestrial paradise. the map of rudimentum, a vast compilation published at lübeck in by the dominican brocard, represents the terrestrial paradise surrounded by walls, but it is less sterile that in the last picture, as may be seen on the next page. in the year , when varthema, the adventurous bolognian, went to the indies by the route of palestine and syria, he was shown the evil-reputed house which cain dwelt in, which was not far from the terrestrial paradise. master gilius, the learned naturalist who travelled at the expense of francis i., had the same satisfaction. the simple faith of our ancestors had no hesitation in accepting such archæology. [illustration: fig. .--the paradise of the fifteenth century.] the most curious and interesting of all attempts to discover the situation of paradise was that made half unconsciously by columbus when he first found the american shore. in his third voyage, when for the first time he reached the main land, he was persuaded not only that he had arrived at the extremity of asia, but that he could not be far from the position of paradise. the orinoco seemed to be one of those four great rivers which, according to tradition, came out of the garden inhabited by our first parents, and his hopes were supported by the fragrant breezes that blew from the beautiful forests on its banks. this, he thought, was but the entrance to the celestial dwelling-place, and if he had dared--if a religious fear had not held back him who had risked everything amidst the elements and amongst men, he would have liked to push forward to where he might hope to find the celestial boundaries of the world, and, a little further, to have bathed his eyes, with profound humility, in the light of the flaming swords which were wielded by two seraphim before the gate of eden. he thus expresses himself on this subject in his letter to one of the monarchs of spain, dated hayti, october, . "the holy scriptures attest that the lord created paradise, and placed in it the tree of life, and made the four great rivers of the earth to pass out of it, the ganges of india, the tigris, the euphrates (passing from the mountains to form mesopotamia, and ending in persia), and the nile, which rises in ethiopia and goes to the sea of alexander. i cannot, nor have been ever able to find in the books of the latins or greeks anything authentic on the site of this terrestrial paradise, nor do i see anything more certain in the maps of the world. some place it at the source of the nile, in ethiopia; but the travellers who have passed through those countries have not found either in the mildness of the climate or in the elevation of the site towards heaven anything that could lead to the presumption that paradise was there, and that the waters of the deluge were unable to reach it or cover it. several pagans have written for the purpose of proving it was in the fortunate isles, which are the canaries. st. isidore, bede, and strabo, st. ambrosius, scotus, and all judicious theologians affirm with one accord that paradise was in the east. it is from thence only that the enormous quantity of water can come, seeing that the course of the rivers is extremely long; and these waters (of paradise) arrive here, where i am, and form a lake. there are great signs here of the neighbourhood of the terrestrial paradise, for the site is entirely conformable to the opinion of the saints and judicious theologians. the climate is of admirable mildness. i believe that if i passed beneath the equinoctial line, and arrived at the highest point of which i have spoken, i should find a milder temperature, and a change in the stars and the waters; not that i believe that the point where the greatest height is situated is navigable, or even that there is water there, or that one could reach it, but i am convinced that _there_ is the terrestrial paradise, where no one can come except by the will of god." in the opinion of this illustrious navigator the earth had the form of a pear, and its surface kept rising towards the east, indicated by the point of the fruit. it was there that he supposed might be found the garden where ancient tradition imagined the creation of the first human couple was accomplished. we can scarcely think without astonishment of the great amount of darkness that obscured scientific knowledge, when this great man appeared on the scene of the world, nor of the rapidity with which the obscurity and vagueness of ideas were dissipated almost immediately after his marvellous discoveries. scarcely had a half century elapsed after his death, than all the geographical fables of the middle ages did no more than excite smiles of incredulity, although during his life the universal opinion was not much advanced upon the times of the famous knight john of mandeville, who wrote gravely as follows:-- "no mortal man can go to or approach this paradise. by land no one can go there on account of savage beasts which are in the deserts, and because of mountains and rocks that cannot be passed over, and dark places without number; nor can one go there any better by sea; the water rushes so wildly, it comes in so great waves, that no vessel dare sail against them. the water is so rapid, and makes so great a noise and tempest, that no one can hear however loud he is spoken to, and so when some great men with good courage have attempted several times to go by this river to paradise, in large companies, they have never been able to accomplish their journey. on the contrary, many have died with fatigue in swimming against the watery waves. many others have become blind, others have become deaf by the noise of the water, and others have been suffocated and lost in the waves, so that no mortal man can approach it except by the special grace of god." with one notable exception, no attempts have been made of late years to solve such a question. that exception is by the noble and indefatigable livingstone, who declared his conviction to sir roderick murchison, in a letter published in the _athenæum_, that paradise was situated somewhere near the sources of the nile. those generally who now seek an answer to the question of the birthplace of the human race do not call it paradise. since man is here, and there was a time quite recent, geologically speaking, when he was not, there must have been some actual locality on the earth's surface where he was first a man. whether we have, or even can hope to have, enough information to indicate where that locality was situated, is a matter of doubt. we have not at present. those who have attended most to the subject appear to think some island the most probable locality, but it is quite conjectural. the name "paradise" appears to have been derived from the persian, in which it means a garden; similarly derived words express the same idea in other languages; as in the hebrew _pardês_, in the arabian _firdaus_, in the syriac _pardiso_, and in the armenian _partes_. it has been thought that the persian word itself is derived from the sanscrit _pradesa_, or _paradesa_, which means a circle, a country, or strange region; which, though near enough as to sound, does not quite agree as to meaning. "eden" is from a hebrew root meaning delights. chapter xii. eclipses and comets. we have seen in the earlier chapters on the systems of the ancients and their ideas of the world how everything was once supposed to have exclusive reference to man, and how he considered himself not only chief of animate objects, but that his own city was the centre of the material world, and his own world the centre of the material universe; that the sun was made to shine, as well as the moon and stars, for his benefit; and that, were it not for him they would have no reason for existence. and we have seen how, step by step, these illusions have been dispelled, and he has learnt to appreciate his own littleness in proportion as he has realised the immensity of the universe of which he forms part. if such has been his history, and such his former ideas on the regular parts, as we may call them, of nature, much more have similar ideas been developed in relation to those other phenomena which, coming at such long intervals, have not been recognised by him as periodic, but have seemed to have some relation to mundane affairs, often of the smallest consequence. such are eclipses of the sun and moon, comets, shooting-stars, and meteors. among the less instructed of men, even when astronomers of the same age and nation knew their real nature, eclipses have always been looked upon as something ominous of evil. among the ancient nations people used to come to the assistance of the moon, by making a confused noise with all kinds of instruments, when it was eclipsed. it is even done now in persia and some parts of china, where they fancy that the moon is fighting with a great dragon, and they think the noise will make him loose his hold and take to flight. among the east indians they have the same belief that when the sun and the moon are eclipsed, a dragon is seizing them, and astronomers who go there to observe eclipses are troubled by the fears of their native attendants, and by their endeavours to get into the water as the best place under the circumstances. in america the idea is that the sun and moon are tired when they are eclipsed. but the more refined greeks believed for a long time that the moon was bewitched, and that the magicians made it descend from heaven, to put into the herbs a certain maleficent froth. perhaps the idea of the dragon arose from the ancient custom of calling the places in the heavens at which the eclipses of the moon took place the head and tail of the dragon. in ancient history we have many curious instances of the very critical influence that eclipses have had, especially in the case of events in a campaign, where it was thought unfavourable to some projected attempt. thus an eclipse of the moon was the original cause of the death of the athenian general nicias. just at a critical juncture, when he was about to depart from the harbour of syracuse, the eclipse filled him and his whole army with dismay. the result of his terror was that he delayed the departure of his fleet, and the athenian army was cut in pieces and destroyed, and nicias lost his liberty and life. plutarch says they could understand well enough the cause of the eclipse of the sun by the interposition of the moon, but they could not imagine by the opposition of what body the moon itself could be eclipsed. one of the most famous eclipses of antiquity was that of thales, recorded by herodotus, who says:--"the lydians and the medes were at war for five consecutive years. now while the war was sustained on both sides with equal chance, in the sixth year, one day when the armies were in battle array, it happened that in the midst of the combat the day suddenly changed into night. thales of miletus had predicted this phenomenon to the ionians, and had pointed out precisely that very year as the one in which it would take place. the lydians and medes, seeing the night succeeding suddenly to the day, put an end to the combat, and only cared to establish peace." another notable eclipse is that related by diodorus siculus. it was a total eclipse of the sun, which took place while agathocles, fleeing from the port of syracuse, where he was blockaded by the carthaginians, was hastening to gain the coast of africa. "when agathocles was already surrounded by the enemy, night came on, and he escaped contrary to all hope. on the day following so complete an eclipse of the sun took place that it seemed altogether night, for the stars shone out in all places. the soldiers therefore of agathocles, persuaded that the gods were intending them some misfortune, were in the greatest perturbation about the future. agathocles was equal to the occasion. when disembarked in africa, where, in spite of all his fine words, he was unable to reassure his soldiers, whom the eclipse of the sun had frightened, he changed his tactics, and pretending to understand the prodigy, "i grant, comrades," he said, "that had we perceived this, eclipse before our embarkation we should indeed have been in a critical situation, but now that we have seen it after our departure, and as it always signifies a change in the present state of affairs, it follows that our circumstances, which were very bad in sicily, are about to amend, while we shall indubitably ruin those of the carthaginians, which have been hitherto so flourishing." we are reminded by this of the story of pericles, who, when ready to set sail with his fleet on a great expedition, saw himself stopped by a similar phenomenon. he spread his mantle over the eyes of the pilot, whom fear had prevented acting, and asked him if that was any sign of misfortune, when the pilot answered in the negative. "what misfortune then do you suppose," said he, "is presaged by the body that hides the sun, which differs from this in nothing but being larger?" with reference to these eclipses, when their locality and approximate date is known, astronomy comes to the assistance of history, and can supply the exact day, and even hour, of the occurrence. for the eclipses depend on the motions of the moon, and just as astronomers can calculate both the time and the path of a solar eclipse in the future, so they can for the past. if then the eclipses are calculated back to the epoch when the particular one is recorded, it can be easily ascertained which one it was that about that time passed over the spot at which it was observed, and as soon as the particular eclipse is fixed upon, it may be told at what hour it would be seen. thus the eclipse of thales has been assigned by different authors to various dates, between the st of october, b.c., and the rd of february, b.c. the only eclipse of the sun that is suitable between those dates has been found by the astronomer-royal to be that which would happen in lydia on the th of may, b.c., which must therefore be the date of the event. so of the eclipse of agathocles, m. delaunay has fixed its date to the th august, b.c. in later days, when christopher columbus had to deal with the ignorant people of america, the same kind of story was repeated. he found himself reduced to famine by the inhabitants of the country, who kept him and his companions prisoners; and being aware of the approach of the eclipse, he menaced them with bringing upon them great misfortunes, and depriving them of the light of the moon, if they did not instantly bring him provisions. they cared little for his menaces at first; but as soon as they saw the moon disappear, they ran to him with abundance of victuals, and implored pardon of the conqueror. this was on the st of march, , a date which may be tested by the modern tables of the moon, and columbus's account proved to be correct. the eclipse was indeed recorded in other places by various observers. eclipses in their natural aspect have thus had considerable influence on the vulgar, who knew nothing of their cause. this of course was the state with all in the early ages, and it is interesting to trace the gradual progress from their being quite unexpected to their being predicted. it is very probable, if not certain, that their recurrence in the case of the moon at least was recognised long before their nature was understood. [illustration: plate xiii.--christopher columbus and the eclipse of the moon.] among the chinese they were long calculated, and, in fact, it is thought by some that they have pretended to a greater antiquity by calculating backwards, and recording as observed eclipses those which happened before they understood or noticed them. it seems, however, authenticated that they did in the year b.c. observe an eclipse of the sun, and that at that date they were in the habit of predicting them. for this particular eclipse is said to have cost several of the astronomers their lives, as they had not calculated it rightly. as the lives of princes were supposed to be dependent on these eclipses, it became high treason to expose them to such a danger without forewarning them. they paid more attention to the eclipses of the sun than of the moon. among the babylonians the eclipses of the moon were observed from a very early date, and numerous records of them are contained in the observations of bel in sargon's library, the tablets of which have lately been discovered. in the older portion they only record that on the th day of such and such a (lunar) month an eclipse takes place, and state in what watch it begins, and when it ends. in a later portion the observations were more precise, and the descriptions of the eclipse more accurate. long before b.c. the discovery of the lunar cycle of lunar months had been made, and by means of it they were able to state of each lunar eclipse, that it was either "according to calculation" or "contrary to calculation." they dealt also with solar eclipses, and tried to trace on a sphere the path they would take on the earth. accordingly, like the eclipses of the moon, these too were spoken of as happening either "according to calculation" or "contrary to calculation." "in a report sent in to one of the later kings of assyria by the state astronomer, abil islar states that a watch had been kept on the th, th, and th of sivan, or may, for an eclipse of the sun, which did not, however, take place after all. the shadow, it is clear, must have fallen outside the field of observation." besides the more ordinary kind of solar eclipses, mention is made in the observations of bel of annular eclipses which, strangely enough, are seldom alluded to by classical writers. a record of a later eclipse has been found by sir henry rawlinson on one of the nineveh tablets. this occurred near that city in b.c. , and from the character of the inscription it may be inferred that it was a rare occurrence with them, indeed that it was nearly, if not quite, a total eclipse. this has an especial interest as being the earliest that we have any approximate date for. it is possible that the remarkable phenomenon, alluded to by the prophet isaiah, of the shadow going backwards ten degrees on the dial of ahaz, may be really a record of an eclipse of the sun, such as astronomy proves to have occurred at jerusalem in the year b.c. we have very little notice of the calculation of eclipses by the egyptians; all that is told us is more or less fabulous. thus diogenes laertius says that they reckoned that during a period of , years, eclipses of the sun and eclipses of the moon had occurred, which is far fewer than the right number for so long a time, and which, of course, has no basis in fact. among the greeks, anaxagoras was the first who entertained clear ideas about the nature of eclipses; and it was from him that pericles learnt their harmlessness. plutarch relates that helicon of cyzicus predicted an eclipse of the sun to dionysius of syracuse, and received as a reward a talent of silver. livy records an eclipse of the sun as having taken place on the th of quintilis, which corresponds to the th of july. it happened during the appollinarian games, b.c. the same author tells us of an eclipse of the moon that was predicted by one gallus, a tribune of the second legion, on the eve of the battle of pydna--a prediction which was duly fulfilled on the following night. the fact of its having been foretold quieted the superstitious fears of the soldiers, and gave them a very high opinion of gallus. other authors, among them cicero, do not give so flattering a story, but state that gallus's part consisted only in explaining the cause of the eclipse after it had happened. the date of this eclipse was the rd of september, b.c. ennius, writing towards the end of the second century b.c., describes an eclipse which was said to have happened nearly two hundred years before ( , b.c.), in the following remarkable words:--"on the nones of july the moon passed over the sun, and there was night." aristarchus, three centuries before christ, understood and explained the nature of eclipses; but the chief of the ancient authors upon this subject was hipparchus. he and his disciples were able to predict eclipses with considerable accuracy, both as to their time and duration. geminus and cleomedes were two other writers, somewhat later, who explained and predicted eclipses. in later times regular tables were drawn up, showing when the eclipses would happen. one that ptolemy was the author of was founded on data derived from ancient observers--callipus, democritus, eudoxus, hipparchus--aided by his own calculations. after the days of ptolemy the knowledge of the eclipses advanced _pari passu_ with the advance of astronomy generally. so long as astronomy itself was empirical, the time of the return of an eclipse was only reckoned by the intervals that had elapsed during the same portion of previous cycles; but after the discovery of elliptic orbits and the force of gravitation the whole motion of the moon could be calculated with as great accuracy as any other astronomical phenomenon. in point of fact, if the new moon is in the plane of the ecliptic there must be an eclipse of the sun; if the full moon is there, there must be an eclipse of the moon; and if it should in these cases be only partially in that plane, the eclipses also will be partial. the cycle of changes that the position of the moon can undergo when new and full occupies a period of eighteen years and eleven days, in which period there are forty-one eclipses of the sun and twenty-nine of the moon. each year there are at most seven and at least two eclipses; if only two, they are eclipses of the sun. although more numerous in reality for the whole earth, eclipses of the sun are more rarely observed in any particular place, because they are not seen everywhere, but only where the shadow of the moon passes; while all that part of the earth that sees the moon at all at the time sees it eclipsed. we now come to comets. the ancients divided comets into different classes, the chief points of distinction being derived from the shape, length, and brilliancy of the tails. pliny distinguished twelve kinds, which he thus characterised:--"some frighten us by their blood-coloured mane; their bristling hair rises towards the heaven. the bearded ones let their long hair fall down like a majestic beard. the javelin-shaped ones seem to be projected forwards like a dart, as they rapidly attain their shape after their first appearance; if the tail is shorter, and terminates in a point, it is called a sword; this is the palest of all the comets; it has the appearance of a bright sword without any diverging rays. the plate or disc derives its name from its shape, its colour is that of amber, it gives out some diverging rays from its sides, but not in large quantity. the cask has really the form of a cask, which one might suppose to be staved in smoke enveloped in light. the retort imitates the figure of a horn, and the lamp that of a burning flame. the horse-comet represents the mane of a horse which is violently agitated, as by a circular, or rather cylindrical, motion. such a comet appears also of singular whiteness, with hair of a silver hue; it is so bright that one can scarcely look at it. there are bristling comets, they are like the skins of beasts with their hair on, and are surrounded by a nebulosity. lastly, the hair of the comet sometimes takes the form of a lance." pingré, a celebrated historian of comets, tells us that one of the first comets noticed in history is that which appeared over rome forty years before christ, and in which the roman people imagined they saw the soul of cæsar endowed with divine honours. next comes that which threw its light on jerusalem when it was being besieged and remained for a whole year above the city, according to the account of josephus. it was of this kind that pliny said it "is of so great a whiteness that one can scarcely look at it, and _one may see in it the image of god in human form_." diodorus tells us that, a little after the subversion of the towns of helix and bura, there were seen, for several nights in succession, a brilliant light, which was called a beam of fire, but which aristotle says was a true comet. plutarch, in his life of timoleon, says a burning flame preceded the fleet of this general until his arrival at sicily, and that during the consulate of caius servilius a bright shield was seen suspended in the heavens. the historians sazoncenas and socrates relate that in the year a.d. a comet in the form of a sword shone over constantinople, and appeared to touch the town just at the time when great misfortunes were impending through the treachery of gainas. the same phenomenon appeared over rome previous to the arrival of alaric. in fact the ancient chroniclers always associated the appearance of a comet with some terrestrial event, which it was not difficult to do, seeing that critical situations were at all times existing in some one country or other where the comet would be visible, and probably those which could not be connected with any were not thought worthy of being recorded. it is well known that the year a.d. was for a long time predicted to be the end of the world. in this year the astronomers and chroniclers registered the fall of an enormous burning meteor and the appearance of a comet. pingré says: "on the th of the calends of january"--that is the th of december--"the heavens being dark, a kind of burning sword fell to the earth, leaving behind it a long train of light. its brilliancy was such that it frightened not only those who were in the fields, but even those who were shut up in their houses. this great opening in the heavens was gradually closed, and then was seen the figure of a dragon, whose feet were blue, and whose head kept continually increasing. a comet having appeared at the same time as this chasm, or meteor, they were confounded." this relation is given in the chronicles of seigbert in hermann corner, in the chronique de tours, in albert casin, and other historians of the time. bodin, resuscitating an idea of democritus, wrote that the comets were the souls of illustrious personages, who, after having lived on the earth a long series of centuries, and being ready at last to pass away, were carried in a kind of triumph to heaven. for this reason, famine, epidemics, and civil wars followed on the apparition of comets, the towns and their inhabitants finding themselves then deprived of the help of the illustrious souls who had laboured to appease their intestinal feuds. one of the comets of the middle ages which made the greatest impression on the minds of the people was that which appeared during holy week of the year , and frightened louis the debonnaire. the first morning of its appearance he sent for his astrologer. "go," he said, "on to the terrace of the palace, and come back again immediately and tell me what you have seen, for i have not seen that star before, and you have not shown it to me; but i know that this sign is a comet: it announces a change of reign and the death of a prince." the son of charlemagne having taken counsel with his bench of bishops, was convinced that the comet was a notice sent from heaven expressly for him. he passed the nights in prayer, and gave large donations to the monasteries, and finally had a number of masses performed out of fear for himself and forethought for the church committed to his care. the comet, however, was a very inoffensive one, being none other than that known as halley's comet, which returned in . while they were being thus frightened in france, the chinese were observing it astronomically. the historian of merlin the enchanter relates that a few days after the _fêtes_ which were held on the occasion of the erection of the funeral monument of salisbury, a sign appeared in heaven. it was a comet of large size and excessive splendour. it resembled a dragon, out of whose mouth came a long two-forked tongue, one part of which turned towards the north and the other to the east. the people were in a state of fear, each one asking what this sign presaged. uter, in the absence of the king, ambrosius, his brother, who was engaged in pursuing one of the sons of vortigern, consulted all the wise men of britain, but no one could give him any answer. then he thought of merlin the enchanter, and sent for him to the court. "what does this apparition presage?" demanded the king's brother. merlin began to weep. "o son of britain, you have just had a great loss--the king is dead." after a moment of silence he added, "but the britons have still a king. haste thee, uter, attack the enemy. all the island will submit to you, for the figure of the fiery dragon is thyself. the ray that goes towards gaul represents a son who shall be born to thee, who will be great by his achievements, and not less so by his power. the ray that goes towards ireland represents a daughter of whom thou shalt be the father, and her sons and grandsons shall reign over all the britons." these predictions were realised; but it is more than probable that they were made up after the event. the comet of was regarded as a presage of the conquest under william of normandy. in the bayeaux tapestry, on which matilda of flanders had drawn all the most memorable episodes in the transmarine expedition of her husband, the comet appears in one of the corners with the inscription, _isti mirantur stellam_, which proves that the comet was considered a veritable marvel. it is said even to be traditionally reported that one of the jewels of the british crown was taken from the tail of this comet. nevertheless it was no more than halley's comet again in its periodical visit every seventy-six years. in july, , a brilliant comet appeared which was lost to view on the same day as the pope, urban iv., died, _i.e._ the third of october. in june, , a similar body of enormous size, with a very long and extraordinarily bright tail, put all christendom in a fright. the pope, calixtus iii., was engaged in a war at that time with the saracens. he showed the christians that the comet "had the form of a cross," and announced some great event. at the same time mahomet announced to his followers that the comet, "having the form of a yataghan," was a blessing of the prophet's. it is said that the pope afterwards recognised that it had this form, and excommunicated it. nevertheless, the christians obtained the victory under the walls of belgrade. this was another appearance of halley's comet. in the early months of appeared a large comet, which historians agree in saying was very horrible and alarming. belleforest said it was a hideous and frightening comet, which threw its rays from east to west, giving great cause for fear to great people, who were not ignorant that comets are the menacing rods of god, which admonish those who are in authority, that they may be converted. pingré, who has told us of so many of the comets that were seen before his time, wrote of this epoch: "comets became the most efficacious signs of the most important and doubtful events. they were charged to announce wars, seditions, and the internal movements of republics; they presaged famines, pestilence, and epidemics; princes, or even persons of dignity, could not pay the tribute of nature without the previous appearance of that universal oracle, a comet; men could no longer be surprised by any unexpected event; the future might be as easily read in the heavens as the past in history. their effect depended on the place in the heavens where they shone, the countries over which they directly lay, the signs of the zodiac that they measured by their longitude, the constellations they traversed, the form and length of their tails, the place where they went out, and a thousand other circumstances more easily indicated than distinguished; they also announced in general wars, and the death of princes, or some grand personage, but there were few years that passed without something of this kind occurring. the devout astrologers--for there were many of that sort--risked less than the others. according to them, the comet threatened some misfortune; if it did not happen, it was because the prayers of penitence had turned aside the wrath of god; he had returned his sword to the scabbard. but a rule was invented which gave the astrologers free scope, for they said that events announced by a comet might be postponed for one or more periods of forty years, or even as many years as the comet had appeared days; so that one which had appeared for six months need not produce its effect for years." [illustration: fig. .--representation of a comet, th century.] the most frightful of the comets of this period, according to simon goulart, was that of . "it put some into so great a fright that they died; others fell sick. it was seen by several thousand people, and appeared very long, and of the colour of blood. at the summit was seen the representation of a curved arm, holding a large sword in its hand, as if it would strike; at the top of the point of the sword were three stars, but that which touched the point was more brilliant than the others. on the two sides of the rays of this comet were seen large hatchets, poignards, bloody swords, among which were seen a great number of men decapitated, having their heads and beards horribly bristling." a view of this comet is given in the _history of prodigies_. there was another comet remarked in , and another in , like the head of an owl, followed by a mantle of scattered light, with pointed ends. of this comet we read in the same book that recorded the last described: "the comet is an infallible sign of a very evil event. whenever eclipses of the sun or moon, or comets, or earthquakes, conversions of water into blood, and such like prodigies happen, it has always been known that very soon after these miserable portents afflictions, effusion of human blood, massacres, deaths of great monarchs, kings, princes, and rulers, seditions, treacheries, raids, overthrowings of empires, kingdoms, or villages; hunger and scarcity of provisions, burning and overthrowing of towns; pestilences, widespread mortality, both of beasts and men; in fact all sorts of evils and misfortunes take place. nor can it be doubted that all these signs and prodigies give warning that the end of the world is come, and with it the terrible last judgment of god." but even now comets were being observed astronomically, and began to lose their sepulchral aspect. a remarkable comet, however, which appeared in , was not without its fears for the vulgar. we are told that it was recognised as the same which appeared the year of cæsar's death, then in , and afterwards in , having a period of about years. the terror it produced in the towns was great; timid spirits saw in it the sign of a new deluge, as they said water was always announced by fire. while the fearful were making their wills, and, in anticipation of the end of the world, were leaving their money to the monks, who in accepting them showed themselves better physicists than the testators, people in high station were asking what great person it heralded the death of, and it is reported of the brother of louis xiv., who apparently was afraid of becoming too suddenly like cæsar, that he said sharply to the courtiers who were discussing it, "ah, gentlemen, you may talk at your ease, if you please; you are not princes." this same comet gave rise to a curious story of an "extraordinary prodigy, how at rome a hen laid an egg on which was drawn a picture of the comet. "the fact was attested by his holiness, by the queen of sweden, and all the persons of first quality in rome. on the th december, , a hen laid an egg on which was seen the figure of the comet, accompanied by other marks such as are here represented. the cleverest naturalists in rome have seen and examined it, and have never seen such a prodigy before." of this same comet bernouilli wrote, "_that if the body of the comet is not a visible sign of the anger of god, the tail may be_." it was this too that suggested to whiston the idea that he put forward, not as a superstitious, but as a physical speculation, that a comet approaching the earth was the cause of the deluge. [illustration: fig. .--an egg marked with a comet.] the last blow to the superstitious fear of the comets was given by halley, when he proved that they circulated like planets round the sun, and that the comets noticed in , , , , , , were all one, whose period was about years, and which would return in , which prediction was verified, and the comet went afterwards by the name of this astronomer. it returned again in , and will revisit us in . even after the fear arising from the relics of astrology had died away, another totally different alarm was connected with comets--an alarm which has not entirely subsided even in our own times. this is that a comet may come in contact with the earth and destroy it by the collision. the most remarkable panic in this respect was that which arose in paris in . at the previous meeting of the academy of sciences, m. lalande was to have read an interesting paper, but the time failed. it was on the subject of comets that could, by approaching the earth, cause its destruction, with special reference to the one that was soon to come. from the title only of the paper the most dreadful fears were spread abroad, and, increasing day by day, were with great difficulty allayed. the house of m. lalande was filled with those who came to question him on the memoir in question. the fermentation was so great that some devout people, as ignorant as weak, asked the archbishop to make a forty hours' prayer to turn away the enormous deluge that they feared, and the prelate was nearly going to order these prayers, if the members of the academy had not persuaded him how ridiculous it would be. finally, m. lalande, finding it impossible to answer all the questions put to him about his fatal memoir, and wishing to prevent the real evils that might arise from the frightened imaginations of the weak, caused it to be printed, and made it as clear as was possible. when it appeared, it was found that he stated that of the sixty comets known there were eight which could, by coming too near the earth, say within , miles, occasion such a pressure that the sea would leave its bed and cover part of the globe, but that in any case this could not happen till after twenty years. this was too long to make it worth while to make provision for it, and the effervescence subsided. a similar case to this occurred with respect to biela's comet, which was to return in . in calculating its reappearance in this year, damoiseau found that it would pass through the plane of the earth's orbit on the th of october. rushing away with this, the papers made out that a collision was inevitable, and the end of the world was come. but no one thought to inquire where the earth would be when the comet passed through the plane in which it revolved. arago, however, set people's minds at rest by pointing out that at that time the earth would be a month's journey from the spot, which with the rate at which the earth is moving would correspond to a distance of sixty millions of miles. this, like other frights, passed away, but was repeated again in and with like results, and even in a similar end to the world was announced to the public for the th of august, on the supposed authority of a professor at geneva, but who had never said what was supposed. but in reality all cause of fear has now passed away, since it has been proved that the comet is made of gaseous matter in a state of extreme tenuity, so that, though it may make great show in the heavens, the whole mass may not weigh more than a few pounds; and we have in addition the testimony of experience, which might have been relied on on the occasions above referred to, for in lexele's comet was seen to pass through the satellites of jupiter without deranging them in the least, but was itself thrown entirely out of its path, while there is reason to believe that on the th of june, , the earth remained several hours in the tail of a comet without having experienced the slightest inconvenience. as to the nature of comets, the opinions that have been held have been mostly very vague. metrodorus thought they were reflections from the sun; democritus, a concourse of several stars; aristotle, a collection of exhalations which had become dry and inflamed; strabo, that they are the splendour of a star enveloped in a cloud; heracletes of pontus, an elevated cloud which gave out much light; epigenes, some terrestrial matter that had caught fire, and was agitated by the wind; boecius, part of the air, coloured; anaxagoras, sparks fallen from the elementary fire; xenophanes, a motion and spreading out of clouds which caught fire; and descartes, the débris of vortices that had been destroyed, the fragments of which were coming towards us. it is said that the chaldæans held the opinion that they were analogous to planets by their regular course, and that when we ceased to see them, it was because they had gone too far from us; and seneca followed this explanation, since he regarded them as globes turning in the heavens, and which appear and disappear in certain times, and whose periodical motions might be known by regular observation. we have thus traced the particular ideas that have attached themselves to eclipses and comets, as the two most remarkable of the extraordinary phenomena of the heavens, and have seen how the fears and superstitions of mankind have been inevitably linked with them in the earlier days of ignorance and darkness, but they are only part of a system of phenomena, and have been no more connected with superstition than others less remarkable, except in proportion to their remarkableness. other minor appearances that are at all unusual have, on the same belief in the inextricable union of celestial and terrestrial matters, been made the signs of calamities or extra-prosperity; the doleful side of human nature being usually the strongest, the former have been chosen more often than the latter. according to seneca, the tradition of the chaldees announced that a universal deluge would be caused by the conjunction of all the planets in the sign of capricorn, and that a general breaking up of the earth would take place at the moment of their conjunction in cancer. "the general break-up of the world," they said, "will happen when the stars which govern the heaven, penetrated with a quality of heat and dryness, meet one another in a fiery triplicity." [illustration: plate xiv.--prodigies in the middle ages.] everywhere, and in all ages of the past, men have thought that a protecting providence, always watching over them, has taken care to warn them of the destinies which await them; thence the good and evil _presages_ taken from the appearance of certain heavenly bodies, of divers meteors, or even the accidental meeting of certain animate or inanimate objects. the indian of north america dying of famine in his miserable cabin, will not go out to the chase if he sees certain presages in the atmosphere. nor need we be astonished at such ideas in an uncultivated man, when even among europeans, a salt-cellar upset, a glass broken, a knife and fork crossed, the number thirteen at dinner, and such things are regarded as unlucky accidents. the employment of sorcery and divination is closely connected with these superstitions. besides eclipses and comets, meteors were taken as the signs of divine wrath. we learn from s. maximus of turin, that the christians of his time admitted the necessity of making a noise during eclipses, so as to prevent the magicians from hurting the sun or moon, a superstition entirely pagan. they used to fancy they could see celestial armies in the air, coming to bring miraculous assistance to man. they thought the hurricanes and tempests the work of evil spirits, whose rage kept them set against the earth. s. thomas aquinas, the great theologian of the thirteenth century, accepted this opinion, just as he admitted the reality of sorceries. but the full development, as well as the nourishment of these superstitious ideas, was derived from the storehouse of astrology, which dealt with matters of ordinary occurrence, both in the heavens and on the earth--and to the history of which our next chapter is devoted. chapter xiii. the greatness and the fall of astrology. our study of the opinions of the ancients on the various phenomena of astronomy, leads us inevitably to the discussion of their astrology, which has in every age and among every people accompanied it--and though astrology be now no more as a science, or lingers only with those who are ignorant and desirous of taking advantage of the still greater ignorance of others--yet it is not lacking in interest as showing the effect of the phenomena of the heavens on the human mind, when that effect is brought to its most technical and complete development. we must distinguish in the first place two kinds of astrology, viz., natural and judicial. the first proposed to foresee and announce the changes of the seasons, the rains, wind, heat, cold, abundance, or sterility of the ground, diseases, &c., by means of a knowledge of the causes which act on the air and on the atmosphere. the other is occupied with objects which would be still more interesting to men. it traced at the moment of his birth, or at any other period of his life, the line that each must travel according to his destiny. it pretended to determine our characters, our passions, fortune, misfortunes, and perils in reserve for each mortal. we have not here to consider the natural astrology, which is a veritable science of observation and does not deserve the name of astrology. it is rather worthy to be called the meteorological calendar of its cultivators. more rural than their descendants of the nineteenth century, the ancients had recognised the connection between the celestial phenomena and the vicissitudes of the seasons; they observed these phenomena carefully to discover the return of the same inclemencies; and they were able (or thought they were) to state the date of the return of particular kinds of weather with the same positions of the stars. but the very connection with the stars soon led the way to a degeneracy. the autumnal constellations, for example, orion and hercules, were regarded as rainy, because the rains came at the time when these stars rose. the egyptians who observed in the morning, called sirius "the burning," because his appearance in the morning was followed by the great heat of the summer: and it was the same with the other stars. soon they regarded them as the cause of the rain and the heat--although they were but remote witnesses. the star sirius is still connected with heat--since we call it the dog-star--and the hottest days of the year, july nd to august rd, we call dog-days. at the commencement of our era, the morning rising of sirius took place on the earlier of those days--though it does not now rise in the morning till the middle of august--and , years ago it rose about the th of june, and preceded the annual rise of the nile. the belief in the meteorological influence of the stars is one of the causes of judicial astrology. this latter has simply subjected man, like the atmosphere, to the influence of the stars; it has made dependent on them the risings of his passions, the good and ill fortune of his life, as well as the variations of the seasons. indeed, it was very easy to explain. it is the stars, or heavenly bodies in general, that bring the winds, the rains, and the storms; their influences mixed with the action of the rays of the sun modify the cold or heat; the fertility of the fields, health or sickness, depend on these beneficial or injurious influences; not a blade of grass can grow without all the stars having contributed to its increase; man breathes the emanations which escaping from the heavenly bodies fill the air; man is therefore in his entire nature subjected to them; these stars must therefore influence his will and his passions; the good and evil passages in his career, in a word, must direct his life. as soon as it was established that the rising of a certain star or planet, and its aspect with regard to other planets, announced a certain destiny to man, it was natural to believe that the rarer configurations signified extraordinary events, which concerned great empires, nations, and towns. and lastly, since errors grow faster than truth, it was natural to think that the configurations which were still more rare, such as the reunion of all the planets in conjunction with the same star, which can occur only after thousands of centuries, while nations have been renewed an infinity of times, and empires have been ruined, had reference to the earth itself, which had served as the theatre for all these events. joined to these superstitious ideas was the tradition of a deluge, and the belief that the world must one day perish by fire, and so it was announced that the former event took place when all the planets were in conjunction in the sign of the fishes, and the latter would occur when they all met in the sign of the lion. the origin of astrology, like that of the celestial sphere, was in all probability in upper asia. there, the starry heavens, always pure and splendid, invited observation and struck the imagination. we have already seen this with respect to the more matter-of-fact portions of astronomy. the assyrians looked upon the stars as divinities endued with beneficent or maleficent power. the adoration of the heavenly bodies was the earliest form of religion among the pastoral population that came down from the mountains of kurdestan to the plains of babylon. the chaldæans at last set apart a sacerdotal and learned caste devoted to the observation of the heavens; and the temples became regular observatories. such doubtless was the tower of babel--a monument consecrated to the seven planets, and of which the account has come down to us in the ancient book of genesis. a long series of observations put the chaldæans in possession of a theological astronomy, resting on a more or less chimerical theory of the influence of the celestial bodies on the events of nations and private individuals. diodorus siculus, writing towards the commencement of our era, has put us in possession of the most circumstantial details that have reached us with regard to the chaldæan priests. at the head of the gods, the assyrians placed the sun and moon, whose courses and daily positions they had noted in the constellation of the zodiac, in which the sun remained, one month in each. the twelve signs were governed by as many gods, who had the corresponding months under their influence. each of these months were divided into three parts, which made altogether thirty-six subdivisions, over which as many stars presided, called gods of consultation. half of these gods had under their control the things which happen above the earth, and the other half those below. the sun and moon and the five planets occupied the most elevated rank in the divine hierarchy and bore the name of gods of interpretation. among these planets saturn or old bel, which was regarded as the highest star and the most distant from us, was surrounded by the greatest veneration; he was the interpreter _par excellence_--the revealer. each of the other planets had his own particular name. some of them, such as _bel_ (jupiter), _merodaez_ (mars), _nebo_ (mercury), were regarded as male, and the others, as _sin_ (the moon), and _mylitta_ or _baulthis_ (venus), as females; and from their position relative to the zodiacal constellations, which were also called _lords_ or masters of the _gods_, the chaldæans derived the knowledge of the destiny of the men who were born under such and such a conjunction--predictions which the greeks afterwards called horoscopes. the chaldæans invented also relations between each of the planets and meteorological phenomena, an opinion partly founded on fortuitous coincidences which they had more or less frequently observed. in the time of alexander their credit was considerable, and the king of macedonia, either from superstition or policy, was in the habit of consulting them. it is probable that the babylonian priests, who referred every natural property to sidereal influences, imagined there were some mysterious relations between the planets and the metals whose colours were respectively somewhat analogous to theirs. gold corresponded to the sun, silver to the moon, lead to saturn, iron to mars, tin to jupiter, and mercury still retains the name of the planet with which it was associated. it is less than two centuries ago, since the metals have ceased to be designated by the signs of their respective planets. alchemy, the mother of chemistry, was an intimately connected sister of astrology, the mother of astronomy. egyptian civilisation dates back to a no less remote period than that of babylon. not less careful observers than the babylonish astrologers of the meteors and the atmospheric revolutions, they could predict certain phenomena, and they gave it out that they had themselves been the cause of them. diodorus siculus tells us that the egyptian priests pretty generally predicted the years of barrenness or abundance, the contagions, the earthquakes, inundations, and comets. the knowledge of celestial phenomena made an essential part of the theology of the egyptians as it did of the chaldæans. they had colleges of priests specially attached to the study of the stars, at which pythagoras, plato, and eudoxus were instructed. religion was besides completely filled with the symbols relating to the sun or moon. each month, each decade, each day was consecrated to a particular god. these gods, to the number of thirty, were called in the alexandrine astronomy _decans_ ([greek: dekavoi]). the festivals were marked by the periodical return of certain astronomical phenomena, and those heliacal risings to which any mythological ideas were attached, were noted with great care. we find even now proof of this old sacerdotal science in the zodiac sculptured on the ceilings of certain temples, and in the hieroglyphic inscriptions relating to celestial phenomena. according to the egyptians, who were no less aware than the greeks, of the influence of atmospheric changes on our organs, the different stars had a special action on each part of the body. in the funeral rituals which were placed at the bottom of the coffins, constant allusion is made to this theory. each limb of the dead body was placed under the protection of a particular god. the divinities divided between them, so to speak, the spoils of the dead. the head belonged to ra, or the sun, the nose and lips to anubis, and so on. to establish the horoscope of anyone, this theory of specific influences was combined with the state of the heavens at the time of his birth. it seems even to have been the doctrine of the egyptians, that a particular star indicated the coming of each man into the world, and this opinion was held also by the medes, and is alluded to in the gospels. in egypt, as in persia and chaldæa, the science of nature was a sacred doctrine, of which magic and astrology constituted the two branches, and in which the phenomena of the universe were attached very firmly to the divinities or genii with which they believed it filled. it was the same in the primitive religions of greece. the thessalian women had an especially great reputation in the art of enchantments. all the poets rival one another in declaring how they are able, by their magical hymns, to bring down the moon. menander, in his comedy entitled _the thessalian_, represents the mysterious ceremonies by the aid of which these sorcerers force the moon to leave the heavens, a prodigy which so completely became the type of enchantments that nonnus tells us it is done by the brahmins. there was, in addition, another _cultus_ in greece, namely, that of hecate with mysterious rays, the patron of sorcerers. lucian of samosate--if the work on astrology which is ascribed to him be really his--justifies his belief in the influence of the stars in the following terms:--"the stars follow their orbit in the heaven; but independently of their motion, they act upon what passes here below. if you admit that a horse in a gallop, that birds in flying, and men in walking, make the stones jump or drive the little floating particles of dust by the wind of their course, why should you deny that the stars have any effect? the smallest fire sends us its emanations, and although it is not for us that the stars burn, and they care very little about warming us, why should we not receive any emanations from them? astrology, it is true, cannot make that good which is evil. it can effect no change in the course of events; but it renders a service to those who cultivate it by announcing to them good things to come; it procures joy by anticipation at the same time that it fortifies them against the evil. misfortune, in fact, does not take them by surprise, the foreknowledge of it renders it easier and lighter. that is my way of looking at astrology." very different is the opinion of the satirist juvenal, who says that women are the chief cultivators of it. "all that an astrologer predicts to them," he says, "they think to come from the temple of jupiter. avoid meeting with a lady who is always casting up her _ephemerides_, who is so good an astrologer that she has ceased to consult, and is already beginning to be consulted; such a one on the inspection of the stars will refuse to accompany her husband to the army or to his native land. if she only wishes to drive a mile, the hour of departure is taken from her book of astrology. if her eye itches and wants rubbing, she will do nothing till she has run through her conjuring book. if she is ill in bed, she will take her food only at the times fixed in her _petosiris_. women of second-rate condition," he adds, "go round the circus before consulting their destiny, after which they show their hands and face to the diviner." when octavius came into the world a senator versed in astrology, nigidius figulus, predicted the glorious destiny of the future emperor. livia, the wife of tiberius, asked another astrologer, scribius, what would be the destiny of her infant; his reply was, they say, like the other's. the house of poppea, the wife of nero, was always full of astrologers. it was one of the soothsayers attached to her house, ptolemy, who predicted to otho his elevation to the empire, at the time of the expedition into spain, where he accompanied him. the history of astrology under the roman empire supplies some very curious stories, of which we may select an illustrative few. octavius, in company with agrippa, consulted one day the astrologer theagenes. the future husband of julia, more credulous or more curious than the nephew of cæsar, was the first to take the horoscope. theagenes foretold astonishing prosperity for him. octavius, jealous of so happy a destiny, and fearing that the reply would be less favourable to him, instead of following the example of his companion, refused at first to state the day of his birth. but, curiosity getting the better of him, he decided to reply. no sooner had he told the day of his birth than the astrologer threw himself at his feet, and worshipped him as the future master of the empire. octavius was transported with joy, and from that moment was a firm believer in astrology. to commemorate the happy influence of the zodiacal sign under which he was born, he had the picture of it struck on some of the medals that were issued in his reign. the masters of the empire believed in astrological divination, but wished to keep the advantages to themselves. they wanted to know the future without allowing their subjects to do the same. nero would not permit anyone to study philosophy, saying it was a vain and frivolous thing, from which one might take a pretext to divine future events. he feared lest some one should push his curiosity so far as to wish to find out when and how the emperor should die--a sort of indiscreet question, replies to which lead to conspiracies and attempts. this was what the heads of the state were most afraid of. tiberius had been to rhodes, to a soothsayer of renown, to instruct himself in the rules of astrology. he had attached to his person the celebrated astrologer thrasyllus, whose fate-revealing science he proved by one of those pleasantries which are only possible with tyrants. whenever tiberius consulted an astrologer he placed him in the highest part of his palace, and employed for his purpose an ignorant and powerful freedman, who brought by difficult paths, bounded by precipices, the astrologer whose science his majesty wished to prove. on the return journey, if the astrologer was suspected of indiscretion or treachery, the freedman threw him into the sea, to bury the secret. thrasyllus having been brought by the same route across these precipices, struck tiberius with awe while he questioned him, by showing him his sovereign power, and easily disclosing the things of the future. cæsar asked him if he had taken his own horoscope, and with what signs were marked that day and hour for himself. thrasyllus then examined the position and the distance of the stars; he hesitated at first, then he grew pale; then he looked again, and finally, trembling with astonishment and fear, he cried out that the moment was perilous, and he was very near his last hour. tiberius then embraced him and congratulated him on having escaped a danger by foreseeing it; and accepting henceforth all his predictions as oracles, he admitted him to the number of his intimate friends. tiberius had a great number of people put to death who were accused of having taken their horoscope to know what honours were in store for them, although in secret he took the horoscopes of great people, that he might ascertain that he had no rivalry to fear from them. septimus severus was very nearly paying with his head for one of those superstitious curiosities that brought the ambitious of the time to the astrologer. in prosperous times he had gained faith in their predictions, and consulted them about important acts. having lost his wife, and wishing to contract a second marriage, he took the horoscopes of the well-connected ladies who were at the time open to marriage. none of their fortunes, taken by the rules of astrology, were encouraging. he learnt at last that there was living in syria a young woman to whom the chaldæans had predicted that she should be the wife of a king. severus was as yet but a legate. he hastened to demand her in marriage, and he obtained her; julia was the name of the woman who was born under so happy a star; but was he the crowned husband which the stars had promised to the young syrian? this reflection soon began to perplex severus, and to get out of his perplexity he went to sicily to consult an astrologer of renown. the matter came to the ears of the emperor commodus; and judge of his anger! the anger of commodus was rage and frenzy; but the event soon gave the response that severus was seeking in sicily,--commodus was strangled. divination which had the emperor for its object at last came to be a crime of high treason. the rigorous measures resorted to against the indiscreet curiosity of ambition took more terrible proportions under the christian emperors. under constantine, a number of persons who had applied to the oracles were punished with cruel tortures. under valens, a certain palladius was the agent of a terrible persecution. everyone found himself exposed to being denounced for having relations with soothsayers. traitors slipped secretly into houses magic formulæ and charms, which then became so many proofs against the inhabitant. the fear was so great in the east, says ammienus marcellinus, that a great number burned their books, lest matter should be found in them for an accusation of magic or sorcery. one day in anger, vitellius commanded all the astrologers to leave italy by a certain day. they responded by a poster, which impudently commanded the prince to leave the earth before that date, and at the end of the year vitellius was put to death; on the other hand, the confidence accorded to astrologers led sometimes to the greatest extremes. for instance, after having consulted babylus, nero put to death all those whose prophecies promised the elevation of heliogabalus. another instance was that of marcus aurelius and his wife faustina. the latter was struck with the beauty of a gladiator. for a long time she vainly strove in secret with the passion that consumed her, but the passion did nothing but increase. at last faustina revealed the matter to her husband, and asked him for some remedy that should restore peace to her troubled soul. the philosophy of marcus aurelius could not suggest anything. so he decided to consult the chaldæans, who were adepts at the art of mixing philters and composing draughts. the means prescribed were more simple than might have been expected from their complicated science; it was that the gladiator should be cut in pieces. they added that faustina should afterwards be anointed with the blood of the victim. the remedy was applied, the innocent athlete was immolated, and the empress afterwards only dreamed of him with great pleasure. the first christians were as much addicted to astrology as the other sects. the councils of laodicea ( , a.d.), of arles ( ), of agdus ( ), orleans ( ), auxerre ( ), and narbonne ( ), condemned the practice. according to a tradition of the commencement of our era, which appears to have been borrowed from mazdeism, it was the rebel angels who taught men astrology and the use of charms. under constantius the crime of high treason served as a pretext for persecution. a number of people were accused of it, who simply continued to practise the ancient religion. it was pretended that they had recourse to sorceries against the life of the emperor, in order to bring about his fall. those who consulted the oracles were menaced with severe penalties and put to death by torture, under the pretence that by dealing with questions of fate they had criminal intentions. plots without number multiplied the accusations; and the cruelty of the judges aggravated the punishments. the pagans, in their turn had to suffer the martyrdom which they had previously inflicted on the early disciples of christ--or rather, to be truer, it was authority, always intolerable, whether pagan or christian, that showed itself inexorable against those who dared to differ from the accepted faith. libanius and jamblicus were accused of having attempted to discover the name of the successor to the empire. jamblicus, being frightened at the prosecution brought against him, poisoned himself. the name only of philosopher was sufficient to found an accusation upon. the philosopher maximus diogenes alypius, and his son hierocles, were condemned to lose their lives on the most frivolous pretence. an old man was put to death because he was in the habit of driving off the approach of fever by incantations, and a young man who was surprised in the act of putting his hands alternately to a marble and his breast, because he thought that by counting in this way seven times seven, he might cure the stomach-ache, met with the same fate. theodosius prohibited every kind of manifestation or usage connected with pagan belief. whoever should dare to immolate a victim, said his law, or consult the entrails of the animals he had killed, should be regarded as guilty of the crime of high treason. the fact of having recourse to a process of divination was sufficient for an accusation against a man. theodosius ii. thought that the continuation of idolatrous practices had drawn down the wrath of heaven, and brought upon them the recent calamities that had afflicted his empire--the derangement of the seasons and the sterility of the soil--and he thundered out terrible threats when his faith and his anger united themselves into fanaticism. he wrote as follows to florentius, prefect of the prætorium in , the year that preceded his death:-- "are we to suffer any longer from the seasons being upset by the effect of the divine wrath, on account of the atrocious perfidy of the pagans, which disturbs the equilibrium of nature? for what is the cause that now the spring has no longer its ordinary beauty, that the autumn no longer furnishes a harvest to the laborious workman and that the winter, by its rigour, freezes the soil and renders it sterile?" perhaps we are unduly amused with these ideas of theodosius so long as we retain the custom of asking the special intervention of providence for the presence or absence of rain! in the middle ages, when astrology took such a hold on the world, several philosophers went so far as to consider the celestial vault as a book, in which each star, having the value of one of the letters of the alphabet, told in ineffaceable characters the destiny of every empire. the book of _unheard-of curiosities_, by gaffarel, gives us the configuration of these celestial characters, and we find them also in the writings of cornelius agrippa. the middle ages took their astrological ideas from the arabians and jews. the jews themselves at this epoch borrowed their principles from such contaminated sources that we are not able to trace in them the transmission of the ancient ideas. to give an example, simeon ben-jochai, to whom is attributed the famous book called _zohar_, had attained in their opinion such a prodigious acquaintance with celestial mysteries as indicated by the stars, that he could have read the divine law in the heavens before it had been promulgated on the earth. during the whole of the middle ages, whenever they wanted to clear up doubts about geography or astronomy, they always had recourse to this oriental science, as cultivated by the jews and arabians. in the thirteenth century alphonse x. was very importunate with the jews to make them assist him with their advice in his vast astronomical and historical works. nicholas oresmus, when the most enlightened monarch in europe was supplying du guesclin with an astrologer to guide him in his strategical operations, was physician to charles v. of france, who was himself devoted to astrology, and gave him the bishopric of lisieux. he composed the _treatise of the sphere_, of which we have already spoken. a few years later, a learned man, the bishop peter d'ailly, actually dared to take the horoscope of jesus christ, and proved by most certain rules that the great event which inaugurated the new era was marked with very notable signs in astrology. mathias corvin, king of hungary, never undertook anything without first consulting the astrologers. the duke of milan and pope paul also governed themselves by their advice. king louis xi., who so heartily despised the rest of mankind, and had as much malice in him as he had weakness, had a curious adventure with an astrologer. it was told him that an astrologer had had the hardihood to predict the death of a woman of whom the king was very fond. he sent for the wretched prophet, gave him a severe reprimand, and then asked him the question, "you, who know everything, when will _you_ die?" the astrologer, suspecting a trick, replied immediately, "sire, three days before your majesty." fear and superstition overcame the monarch's resentment, and the king took particular care of the adroit impostor. it is well known how much catherine de medicis was under the influence of the astrologers. she had one in her hôtel de soissons in paris, who watched constantly at the top of a tower. this tower is still in existence, by the wool-market, which was built in on the site of the hotel. it is surmounted by a sphere and a solar dial, placed there by the astronomer pingré. one of the most celebrated of the astrologers who was under her patronage was nostradamus. he was a physician of provence, and was born at st. reny in . to medicine he joined astrology, and undertook to predict future events. he was called to paris by catherine in , and attempted to write his oracles in poetry. his little book was much sought after during the whole of the remainder of the sixteenth century, and even in the beginning of the next. according to contemporary writers many imitations were made of it. it was written in verses of four lines, and was called _quatrains astronomiques_. as usual, the prophecies were obscure enough to suit anything, and many believers have thought they could trace in the various verses prophecies of known events, by duly twisting and manipulating the sense. a very amusing prophecy, which happened to be too clear to leave room for mistakes as to its meaning, and which turned out to be most ludicrously wrong, was one contained in a little book published in with this title:--_prognostication touching the marriage of the very honourable and beloved henry, by the grace of god king of navarre, and the very illustrious princess marguerite of france, calculated by master bernard abbatio, doctor in medicine, and astrologer to the very christian king of france._ first he asked if the marriage would be happy, and says:--"having in my library made the figure of the heavens, i found that the lord of the ascendant is joined to the lord of the seventh house, which is for the woman of a trine aspect, from whence i have immediately concluded, according to the opinion of ptolemy, haly, zael, messahala, and many other sovereign astrologers, that they will love one another intensely all their lives." in point of fact they always detested each other. again, "as to length of life, i have prepared another figure, and have found that jupiter and venus are joined to the sun with fortification, and that they will approach a hundred years;" after all henri iv. died before he was sixty. "our good king of navarre will have by his most noble and virtuous queen many children; since, after i had prepared another figure of heaven, i found the ascendant and its lord, together with the moon, all joined to the lord of the fifth house, called that of children, which will be pretty numerous, on account of jupiter and also of venus;" and yet they had no children! "jupiter and venus are found domiciled on the aquatic signs, and since these two planets are found concordant with the lord of the ascendant, all this proves that the children will be upright and good, and that they will love their father and mother, without doing them any injury, nor being the cause of their destruction, as is seen in the fruit of the nut, which breaks, opens, and destroys the stock from which it took its birth. the children will live long, they will be good christians, and with their father will make themselves so benign and favourable towards those of our religion, that at last they will be as beloved as any man of our period, and there will be no more wars among the french, as there would have been but for the present marriage. god grant us grace that so long as we are in this transitory life we may see no other king but charles ix., the present king of france." and yet these words were written in the year of the massacre of st. bartholomew's day! and the marriage was broken off, and henri iv. married to marie de medici. so much for the astrological predictions! the aspect in which astrology was looked upon by the better minds even when it was flourishing may be illustrated by two quotations we may make, from shakespeare and voltaire. our immortal poet puts into the mouth of edmund in _king lear_:--"this is the excellent foppery of the world, that when we are sick in fortune (often the surfeit of our own behaviour) we make guilty of our disasters the sun, the moon, and the stars, as if we were villains by necessity; fools by heavenly compulsion; knaves, thieves, and treacherous, by spherical predominance; drunkards, liars, and adulterers by an enforced obedience of planetary influence; and all that we are evil in, by a divine thrusting on. an admirable evasion of a libertine to lay his goatish disposition to the charge of a star! my father married my mother under the dragon's tail; and my nativity was under _ursa major_; so that it follows i am rough lecherous. tut, i should have been that i am, had the maidenliest star in the firmament twinkled on my birth." voltaire writes thus:--"this error is ancient, and that is enough. the egyptians, the chaldæans, the jews could predict, and therefore we can predict now. if no more predictions are made it is not the fault of the art. so said the alchemists of the philosopher's stone. if you do not find to-day it is because you are not clever enough; but it is certain that it is in the clavicle of solomon, and on that certainty more than two hundred families in germany and france have been ruined. do you wonder either that so many men, otherwise much exalted above the vulgar, such as princes or popes, who knew their interests so well, should be so ridiculously seduced by this impertinence of astrology. they were very proud and very ignorant. there were no stars but for them; the rest of the universe was _canaille_, for whom the stars did not trouble themselves. i have not the honour of being a prince. nevertheless, the celebrated count of boulainvilliers and an italian, called colonne, who had great reputation in paris, both predicted to me that i should infallibly die at the age of thirty-two. i have had the malice already to deceive them by thirty years, for which i humbly beg their pardon." the method by which these predictions were arrived at consisted in making the different stars and planets responsible for different parts of the body, different properties, and different events, and making up stories from the association of ideas thus obtained, which of course admitted of the greatest degree of latitude. the principles are explained by manilius in his great poem entitled _the astronomicals_, written two thousand years ago. according to him the sun presided over the head, the moon over the right arm, venus over the left, jupiter over the stomach, mars the parts below, mercury over the right leg, and saturn over the left. among the constellations, the ram governed the head; the bull the neck; the twins the arms and shoulders; the crab the chest and the heart; the lion the stomach; the abdomen corresponded to the sign of the virgin; the reins to the balance; then came the scorpion; the archer, governing the thighs; the he-goat the knees; the waterer the legs; and the fishes the feet. albert the great assigned to the stars the following influences:--saturn was thought to rule over life, changes, sciences, and buildings; jupiter over honour, wishes, riches, and cleanness; mars over war, prisons, marriages, and hatred; the sun over hope, happiness, gain, and heritages; venus over friendships and amours; mercury over illness, debts, commerce, and fear; the moon over wounds, dreams, and larcenies. each of these stars also presides over particular days of the week, particular colours, and particular metals. the sun governed the sunday; the moon, monday; mars, tuesday; mercury, wednesday; jupiter, thursday; venus, friday; and saturn, saturday; which is partially indicated by our own names of the week, but more particularly in the french names, which are each and all derived from these stars. the sun represented yellow; the moon, white; venus, green; mars, red; jupiter, blue; saturn, black; mercury, shaded colours. we have already indicated the metals that corresponded to each. the sun was reckoned to be beneficent and favourable; saturn to be sad, morose, and cold; jupiter, temperate and benign; mars, vehement; venus, benevolent and fertile; mercury, inconstant; and the moon, melancholy. among the constellations, the ram, the lion, and the archer were hot, dry and vehement. the bull, the virgin, and the he-goat were heavy, cold, and dry; the twins, the balance, and the waterer were light, hot, and moist; the crab, scorpion, and the fishes were moist, soft, and cold. [illustration: plate xv.--an astrologer at work.] in this way the heavens were made to be intimately connected with the affairs of earth; and astrology was in equally intimate connection with astronomy, of which it may in some sense be considered the mother. the drawers of horoscopes were at one time as much in request as lawyers or doctors. one thurneisen, a famous astrologer and an extraordinary man, who lived last century at the electoral court of berlin, was at the same time physician, chemist, drawer of horoscopes, almanack maker, printer, and librarian. his astrological reputation was so widespread that scarcely a birth took place in families of any rank in germany, poland, hungary, or even england without there being sent an immediate envoy to him to announce the precise moment of birth. he received often three and sometimes as many as ten messages a day, and he was at last so pressed with business that he was obliged to take associates and agents. in the days of kepler we know that astrology was more thought of than astronomy, for though on behalf of the world he worked at the latter, for his own daily bread he was in the employ of the former, making almanacks and drawing horoscopes that he might live. chapter xiv. time and the calendar. the opinions of thinkers on the nature of time have been very varied. some have considered time as an absolute reality, which is exactly measured by hours, days, and years, and is as known and real as any other object whose existence is known to us. others maintain that time is only a matter of sensation, or that it is an illusion, or a hallucination of a lively brain. the definitions given of it by different great writers is as various. thus kant calls it "one of the forms of sensibility." schelling declares it is "pure activity with the negation of all being." leibnitz defines it "the order of successions" as he defined space to be the order of co-existences. newton and clarke make space and time two attributes of the deity. a study of the astronomical phenomena of the universe, and a consideration of their teaching, give us authority for saying, that neither space nor time are realities, but that the only things absolute are eternity and infinity. in fact, we give the name of time to the succession of the terrestrial events measured by the motion of the earth. if the earth were not to move, we should have no means of measuring, and consequently no idea of time as we have it now. so long as it was believed that the earth was at rest, and that the sun and all the stars turned round us, this apparent motion was then, as the real motion of the earth is now, the method of generating time. in fact, the fathers said that at the end of the world the diurnal motion would cease, and there would be no more time. but let us examine the fact a little further. suppose for an instant that the earth was, as it was formerly believed to be, an immense flat surface, which was illuminated by a sun which remained always immovable at the zenith, or by an invariable diffused light--such an earth being supposed to be alone by itself in the universe and immovable. now if there were a man created on that earth, would there be such a thing as "time" for him? the light which illumines him is immovable. no moving shadow, no gnomon, no sun-dial would be possible. no day nor night, no morning nor evening, no year. nothing that could be divided into days, hours, minutes, and seconds. in such a case one would have to fall back upon some other terminating events, which would indicate a lapse of time; such for instance as the life of a man. this, however, would be no universal measure, for on one planet the life might be a thousand years, and on another only a hundred. or we may look at it in another way. suppose the earth were to turn twice as fast about itself and about the sun, the persons who lived sixty of such years would only have lived thirty of our present years, but they would have seen sixty revolutions of the earth, and, rigorously speaking, would have lived sixty years. if the earth turned ten times as fast, sixty years would be reduced to ten, but they would still be sixty of those years. we should live just as long; there would be four seasons, days, &c., only everything would be more rapid: but it would be exactly the same thing for us, and the other apparently celestial motions having a similar diminution, there would be no change perceived by us. again, consider the minute animals that are observable under the microscope, which live but for five minutes. during that period, they have time to be born and to grow. from embryos they become adult, marry, so to speak, and have a numerous progeny, which they develop and send into the world. afterwards they die, and all this in a few minutes. the impressions which, in spite of their minuteness, we are justified in presuming them to possess, though rapid and fleeting, may be as profound for them in proportion as ours are to us, and their measure of time would be very different from ours. all is relative. in absolute value, a life completed in a hundred years is not longer than one that is finished in five minutes. it is the same for space. the earth has a diameter of , miles, and we are five or six feet high. now if, by any process, the earth should diminish till it became as small as a marble, and if the different elements of the world underwent a corresponding diminution, our mountains might become as small as grains of sand, the ocean might be but a drop, and we ourselves might be smaller than the microscopic animals adverted to above. but for all that nothing would have changed for us. we should still be our five or six feet high, and the earth would remain exactly the same number of our miles. a value then that can be decreased and diminished at pleasure without change is not a mathematical absolute value. in this sense then it may be said that neither time nor space have any real existence. or once again. suppose that instead of our being on the globe, we were placed in pure space. what time should we find there? no time. we might remain ten years, twenty, a hundred, or a thousand years, but we should never arrive at the next year! in fact each planet makes its own time for its inhabitants, and where there is no planet or anything answering to it there is no time. jupiter makes for its inhabitants a year which is equal to twelve years of ours, and a day of ten of our hours. saturn has a year equal to thirty of ours, and days of ten hours and a quarter. in other solar systems there are two or three suns, so that it is difficult to imagine what sort of time they can have. all this infinite diversity of time takes place in eternity, the only thing that is real. the whole history of the earth and its inhabitants takes place, not in time, but in eternity. before the existence of the earth and our solar system, there was another time, measured by other motions, and having relation to other beings. when the earth shall exist no longer, there may be in the place we now occupy, another time again, for other beings. but they are not realities. a hundred millions of centuries, and a second, have the same real length in eternity. in the middle of space, we could not tell the difference. our finite minds are not capable of grasping the infinite, and it is well to know that our only idea of time is relative, having relation to the regular events that befall this planet in its course, and not a thing which we can in any way compare with that, which is so alarming to the ideas of some--eternity. we have then to deal with the particular form of time that our planet makes for us, for our personal use. it turns about the sun. an entire circuit forms a period, which we can use for a measure in our terrestrial affairs. we call it a year, or in latin _annus_, signifying a circle, whence our word _annual_. a second, shorter revolution, turns the earth upon itself, and brings each meridian directly facing the sun, and then round again to the opposite side. this period we call a _day_, from the latin _dies_, which in italian becomes _giorne_, whence the french _jour_. in sanscrit we have the same word in _dyaus_. the length of time that it takes for the earth to arrive at the same position with respect to the stars, which is called a sidereal year, amounts to · days. but during this time, as we have seen, the equinox is displaced among the stars. this secular retrogression brings it each year a little to the east of its former position, so that the sun arrives there about eleven minutes too soon. by taking this amount from the sidereal we obtain the tropical year, which has reference to the seasons and the calendar. its length is · days, or days, hours, minutes, · seconds. in what way was the primitive year regulated? was it a solar or a sidereal year? there can be no doubt that when there was an absence of all civilisation and a calendar of any sort was unknown, the year meant simply the succession of seasons, and that no attempt would be made to reckon any day as its commencement. and as soon as this was attempted a difficulty would arise from there not being an exact number of days in the year. so that when reckoned as the interval between certain positions of the sun they would be of different lengths, which would introduce some difficulty as to the commencement of the year. be this the case, however, or not, mr. haliburton's researches seem to show that the earliest form of year was the sidereal one, and that it was regulated by the pleiades. in speaking of that constellation we have noticed that among the islanders of the southern hemisphere and others there are two years in one of ours, the first being called the pleiades above and the second the pleiades below; and we have seen how the same new year's day has been recognised in very many parts of the world and among the ancient egyptians and hindoos. this year would begin in november, and from the intimate relation of all the primitive calendars that have been discovered to a particular day, taken as november by the egyptians, it would appear probable that for a long time corrections were made both by the egyptians and others in order to keep the phenomenon of the pleiades just rising at sunset to one particular named day of their year--showing that the year they used was a sidereal one. this can be traced back as far as b.c. among the egyptians, and to b.c. among the hindoos. there seem to have been in use also shorter periods of three months, which, like the two-season year, appear to have been, as they are now among the japanese, regulated by the different positions of the pleiades. among the siamese of the present day, there are both forms of the year existing, one sidereal, beginning in november, and regulated by the fore-named constellation; and the other tropical, beginning in april. whether, however, the year be reckoned by the stars or by the sun, there will always be a difficulty in arranging the length of the year, because in each case there will be about a quarter of a day over. it seems, too, to have been found more convenient in early times to take days as the length of the year, and to add an intercalary month now and then, rather than and add a day. thus among the earliest egyptians the year was of days, which were reckoned in the months, and five days were added each year, between the commencement of one and the end of the other, and called unlucky days. it was the belief of the egyptians that these five days were the birthdays of their principal gods; osiris being born on the first, anieris (or apollo) on the second, typhon on the third, isis on the fourth, nephys (or aphrodite) on the fifth. these appear to have some relation with similar unlucky days among the greeks and romans, and other nations. the days of the egyptian year were represented at acantho, near memphis, in a symbolical way, there being placed a perforated vessel, which each day was filled with water by one of a company of priests, each priest having charge over one day in the year. a similar symbolism was used at the tomb of osiris, around which were placed pitchers, one of which each day was filled with milk. on the other hand, the days were represented by the tomb of osymandyas, at thebes, being surrounded by a circle of gold which was one cubit broad and cubits in circumference. on the side were written the risings and settings of the stars, with the prognostications derived from them by the egyptian astrologers. it was destroyed, however, by cambyses when the persians conquered egypt. they divided their year according to herodotus into twelve months, the names of which have come down to us. even with the days, which their method of reckoning would practically come to, they would still be a quarter of a day each year short; so that in four years it would amount to a whole day, an error which would amount to something perceptible even during the life of a single man, by its bringing the commencement of the civil year out of harmony with the seasons. in fact the first day of the year would gradually go through all the seasons, and at the end of solar years there would have been completed civil years, which would bring back the day to its original position. this period represents a cycle of years in which approximately the sun and the earth come to the same relative position again, as regards the earth's rotation on its axis and revolution round the sun. this cycle was noticed by firmicius. another more accurate cycle of the same kind, noticed by syncellus, is obtained by multiplying by , making , years, which takes into account the defect which the extra hours over have from six. the egyptians, however, did not allow their year to get into so large an error, though it was in error by their using sidereal time, regulating their year, and intercalating days, first according to the risings of the pleiades, and after according to that of sirius, the dog-star, which announced to them the approaching overflowing of the nile, a phenomenon of such great value to egypt that they celebrated it with annual fêtes of the greatest magnificence. among the babylonians, as we are informed by mr. sayce, the year was divided into twelve lunar months and days, an intercalary month being added whenever a certain star, called the "star of stars," or icu, also called dilgan, by the ancient accadians, meaning the "messenger of light," and what is now called aldebaran, which was just in advance of the sun when it crossed the vernal equinox, was not parallel with the moon until the third of nisan, that is, two days after the equinox. they also added shorter months of a few days each when this system became insufficient to keep their calendar correct. they divided their year into four quarters of three months each; the spring quarter not commencing with the beginning of the year when the sun entered the spring equinox, proving that the arrangement of seasons was subsequent to the settling of the calendar. the names of their months were given them from the corresponding signs of the zodiac; which was the same as our own, though the zodiac began with aries and the year with nisan. they too had cycles, but they arose from a very different cause; not from errors of reckoning in the civil year or the revolution of the earth, but from the variations of the weather. every twelve solar years they expected to have the same weather repeated. when we connect this with their observations on the varying brightness of the sun, especially at the commencement of the year on the first of nisan, which they record at one time as "bright yellow" and at another as "spotted," and remember that modern researches have shown that weather is certainly in some way dependent on the solar spots, which have a period _now_ of about eleven years, we cannot help fancying that they were very near to making these discoveries. the year of the ancient persians consisted of days. the extra quarter of a day was not noticed for years, at the end of which they intercalated a month--in the first instance, at the end of the first month, which was thus doubled. at the end of another years they inserted an intercalary month after the second month, and so on through all their twelve months. so that after years the series began again. this period they called the intercalary cycle. the calendar among the greeks was more involved, but more useful. it was _luni-solar_, that is to say, they regulated it at the same time by the revolutions of the moon and the motion about the sun, in the following manner:-- the year commenced with the new moon nearest to the th or st of june, the time of the summer solstice; it was composed in general of twelve months, each of which commenced on the day of the new moon, and which had alternately twenty-nine and thirty days. this arrangement, conformable to the lunar year, only gave days to the civil year, and as this was too short by ten days, twenty-one hours, this difference, by accumulation, produced nearly eighty-seven days at the end of eight years, or three months of twenty-nine days each. to bring the lunar years into accordance with the solstices, it was necessary to add three intercalary months every eight years. the phases of the moon being thus brought into comparison with the rotation of the earth, a cycle was discovered by meton, now known as the metonic cycle, useful also in predicting eclipses, which comprised nineteen years, during which time lunations will have very nearly occurred, and the full moons will return to the same dates. in fact, the year and the lunation are to one another very nearly in the proportion of to . by observing for nineteen years the positions and phases of the moon, they will be found to return again in the same order at the same times, and they can therefore be predicted. this lunar cycle was adopted in the year b.c. to regulate the luni-solar calendar, and it was engraved in letters of gold on the walls of the temple of minerva, from whence comes the name _golden number_, which is given to the number that marks the place of the given year in this period of nineteen. caliphus made a larger and more exact cycle by multiplying by four and taking away one day. thus he made of , days julian years, during which there were lunations. the roman calendar was even more complicated than the greek, and not so good. romulus is said to have given to his subjects a strange arrangement that we can no longer understand. more of a warrior than a philosopher, this founder of rome made the year to consist of ten months, some being of twenty days and others of fifty-five. these unequal lengths were probably regulated by the agricultural works to be done, and by the prevailing religious ideas. after the conclusion of these days they began counting again in the same order; so that the year had only days. the first of these ten months was called _mars_ after the name of the god from whom romulus pretended to have descended. the name of the second, aprilis, was derived from the word _aperire_, to open, because it was at the time that the earth opened; or it may be, from aphrodite, one of the names of venus, the supposed grandmother of Æneas. the third month was consecrated to _maïa_, the mother of mercury. the names of the six others expressed simply their order--quintilis, the fifth; sextilis, the sixth; september, the seventh; and so on. numa added two months to the ten of romulus; one took the name of _januarius_, from _janus_: the name of the other was derived either from the sacrifices (_februalia_), by which the faults committed during the course of the past year were expiated, or from _februo___, the god of the dead, to which the last month was consecrated. the year then had days. these roman months have become our own, and hence a special interest attaches to the consideration of their origin, and the explanation of the manner in which they have been modified and supplemented. each of them was divided into unequal parts, by the days which were known as the calends, nones, and ides. the calends were invariably fixed to the first day of each month; the nones came on the th or th, and the ides the th or th. the romans, looking forward, as children do to festive days, to the fête which came on these particular days, named each day by its distance from the next that was following. immediately after the calends of a month, the dates were referred to the nones, each day being called seven, six, five, and so on days before the nones; on the morrow of the nones they counted to the ides; and so the days at the end of the month always bore the name of the calends of the month following. to complete the confusion the nd day before the fête was called the rd, by counting the fête itself as the st, and so they added one throughout to the number that _we_ should now say expressed our distance from a certain date. since there were thus ten days short in each year, it was soon found necessary to add them on, so a supplementary month was created, which was called mercedonius. this month, by another anomaly, was placed between the rd and th of february. thus, after february rd, came st, nd, rd of mercedonius; and then after the dates of this supplementary month were gone through, the original month was taken up again, and they went on with the th of february. and finally, to complete the medley, the priests who had the charge of regulating this complex calendar, acquitted themselves as badly as they could; by negligence or an arbitrary use of their power they lengthened or shortened the year without any uniform rule. often, indeed, they consulted in this nothing but their own convenience, or the interests of their friends. the disorder which this license had introduced into the calendar proceeded so far that the months had changed from the seasons, those of winter being advanced to the autumn, those of the autumn to the summer. the fêtes were celebrated in seasons different from those in which they were instituted, so that of ceres happened when the wheat was in the blade, and that of bacchus when the raisins were green. julius cæsar, therefore, determined to establish a solar year according to the known period of revolution of the sun, that is days and a quarter. he ordained that each fourth year a day should be intercalated in the place where the month mercedonius used to be inserted, _i.e._ between the rd and th of february. the th of the calends of march in ordinary years was the th of february; it was called _sexto-kalendas_. when an extra day was put in every fourth year before the th, this was a second th day, and was therefore called _bissexto-kalendas_, whence we get the name bissextile, applied to leap year. but it was necessary also to bring back the public fêtes to the seasons they ought to be held in: for this purpose cæsar was obliged to insert in the current year, b.c. (or a.u.c.), two intercalary months beside the month mercedonius. there was, therefore, a year of fifteen months divided into days, and this was called the year of confusion. cæsar gave the strictest injunctions to sosigenes, a celebrated alexandrian astronomer whom he brought to rome for this purpose; and on the same principles flavius was ordered to compose a new calendar, in which all the roman fêtes were entered--following, however, the old method of reckoning the days from the calends, nones, and ides. antonius, after the death of cæsar, changed the name of quintilis, in which julius cæsar was born, into the name _julius_, whence we derive our name july. the name of _augustus_ was given to the month _sextilis_, because the emperor augustus obtained his greatest victories during that month. tiberius, nero, and other imperial monsters attempted to give their names to the other months. but the people had too much independence and sense of justice to accord them such a flattery. the remaining months we have as they were named in the days of numa pompilius. [illustration: fig. .--the roman calendar.] a cubical block of white marble has been found at pompeii which illustrates this very well. each of the four sides is divided into three columns, and on each column is the information about the month. each month is surmounted by the sign of the zodiac through which the sun is passing. beneath the name of the month is inscribed the number of days it contains; the date of the nones, the number of the hours of the day, and of the night; the place of the sun, the divinity under whose protection the month is placed, the agricultural works that are to be done in it, the civil and ecclesiastical ceremonies that are to be performed. these inscriptions are to be seen under the month january to the left of the woodcut. the reform thus introduced by julius cæsar is commonly known as the _julian reform_. the first year in which this calendar was followed was b.c. the julian calendar was in use, without any modification, for a great number of years; nevertheless, the mean value which had been assigned to the civil year being a little different to that of the tropical, a noticeable change at length resulted in the dates in which, each year, the seasons commenced; so that if no remedy had been introduced, the same season would be displaced little by little each year, so as to commence successively in different months. the council of nice, which was held in the year of the christian era, adopted a fixed rule to determine the time at which easter falls. this rule was based on the supposed fact that the spring equinox happened every year on the st of march, as it did at the time of the meeting of the council. this would indeed be the case if the mean value of the civil year of the julian calendar was exactly equal to the tropical year. but while the first is · days, the second is · days; so that the tropical year is too small by minutes and seconds. it follows hence that after the lapse of four julian years the vernal equinox, instead of happening exactly at the same time as it did four years before, will happen minutes seconds too soon; and will gain as much in each succeeding four years. so that at the end of a certain number of years, after the year , the equinox will happen on the th of march, afterwards on the th, and so on. this continual advance notified by the astronomers, determined pope gregory xiii. to introduce a new reform into the calendar. it was in the year that the _gregorian reform_ was put into operation. at that epoch the vernal equinox happened on the th instead of the st of march. to get rid of this advance of ten days that the equinox had made and to bring it back to the original date, pope gregory decided that the day after the th of october, , should be called the th instead of the th. this change only did away with the inconvenience at the time attaching to the julian calendar; it was necessary to make also some modification in the rule which served to determine the lengths of the civil years, in order to avoid the same error for the future. so the pope determined that in each years there should be only bissextile years, instead of , as there used to be in the julian calendar. this made three days taken off the years, and in consequence the mean value of the civil year is reduced to · days, which is not far from the true tropical year. the gregorian year thus obtained is still too great by · of a day; the date of the vernal equinox will still then advance in virtue of this excess, but it is easy to see that the gregorian reform will suffice for a great number of centuries. the method in which it is carried out is as follows:--in the julian calendar each year that divided by four when expressed in its usual way, by a.d., was a leap year, and therefore each year that completed a century was such, as , and so on--but in the gregorian reform, all these century numbers are to be reckoned common years, unless the number without the two cyphers divides by four; thus , will be a common year and , a leap year. it is easy to see that this will leave out three leap years in every years. the gregorian calendar was immediately adopted in france and germany, and a little later in england. now it is in operation in all the christian countries of europe, except russia, where the julian calendar is still followed. it follows that russian dates do not agree with ours. in , the difference was ten days, and this difference remained the same till the end of the seventeenth century, when the year was bissextile in the julian, but not in the gregorian calendar, so the difference increased to eleven days, and now in the same way is twelve days. next to the year, comes the day as the most natural division of time in connection with the earth, though it admits of less difference in its arrangements, as we cannot be mistaken as to its length. it is the natural standard too of our division of time into shorter intervals such as hours, minutes, and seconds. by the word _day_ we mean of course the interval during which the earth makes a complete revolution round itself, while _daytime_ may be used to express the portion of it during which our portion of the earth is towards the sun. the greeks to avoid ambiguity used the word _nyctemere_, meaning night and day. no ancient nation is known that did not divide the day into twenty-four hours, when they divided it at all into such small parts, which seems to show that such a division was comparatively a late institution, and was derived from the invention of a single nation. it would necessarily depend on the possibility of reckoning shorter periods of time than the natural one of the day. in the earliest ages, and even afterwards, the position of the sun in the heavens by day, and the position of the constellations by night, gave approximately the time. instead of asking what "o'clock" is it? the greeks would say, "what star is passing?" the next method of determining time depended on the uniform motion of water from a cistern. it was invented by the egyptians, and was called a clepsydra, and was in use among the babylonians, the greeks, and the romans. the more accurate measurement of time by means of clocks was not introduced till about b.c., when trimalcion had one in his dining chamber. the use of them, however, had been so lost that in a.d. they were considered quite novelties. the clocks, of course, have to be regulated by the sun, an operation which has been the employment of astronomers, among other things, for centuries. each locality had its own time according to the moment when the sun passed the meridian of the place, a moment which was determined by observation. before the introduction of the hour, the day and night appear to have been divided into watches. among the babylonians the night was reckoned from what we call a.m. to p.m., and divided into three watches of four hours each--called the "evening," "middle," and "morning" watch. these were later superseded by the more accurate hour, or rather "double hour" or _casbri_, each of which was divided into sixty minutes and sixty seconds, and the change taking place not earlier than , b.c. whether the babylonians (or accadians) were the inventors of the hour it is difficult to say, though they almost certainly were of other divisions of time. it is remarkable that in the ancient jewish scriptures we find no mention of any such division until the date at which the prophecy of daniel was written, that is, until the jews had come in contact with the babylonians. some nations have counted the twenty-four hours consecutively from one to twenty-four as astronomers do now, but others and the majority have divided the whole period into two of twelve hours each. the time of the commencement of the day has varied much with the different nations. the jews, the ancient athenians, the chinese, and several other peoples, more or less of the past, have commenced the day with the setting of the sun, a custom which perhaps originated with the determination of the commencement of the year, and therefore of the day, by the observation of some stars that were seen at sunset, a custom continued in our memory by the well-known words, "the evening and the morning were the first day." the italians, till recently, counted the hours in a single series, between two settings of the sun. the only gain in such a method would be to sailors, that they might know how many hours they had before night overtook them; the sun always setting at twenty-four o'clock; if the watch marked nineteen or twenty, it would mean they had five or four hours to see by--but such a gain would be very small against the necessity of setting their watches differently every morning, and the inconvenience of never having fixed hours for meals. among the babylonians, syrians, persians, the modern greeks, and inhabitants of the balearic isles, &c., the day commenced with the rising of the sun. nevertheless, among all the astronomical phenomena that may be submitted to observation, none is so liable to uncertainty as the rising and setting of the heavenly bodies, owing among other things to the effects of refraction. among the ancient arabians, followed in this by the author of the _almagesta_, and by ptolemy, the day commenced at noon. modern astronomers adopt this usage. the moment of changing the date is then always marked by a phenomenon easy to observe. lastly, that we may see how every variety possible is sure to be chosen when anything is left to the free choice of men, we know that with the egyptians, hipparchus, the ancient romans, and all the european nations at present, the day begins at midnight. copernicus among the astronomers of our era followed this usage. we may remark that the commencement of the astronomical day commences twelve hours _after_ the civil day. of the various periods composed of several days, the week of seven days is the most widely spread--and of considerable antiquity. yet it is not the universal method of dividing months. among the egyptians the month was divided into periods of ten days each; and we find no sign of the seven days--the several days of the whole month having a god assigned to each. among the hindoos no trace has been found by max müller in their ancient vedic literature of any such division, but the month is divided into two according to the moon; the _clear_ half from the new to the full moon, the _obscure_ half from the full to the new, and a similar division has been found among the aztecs. the chinese divide the month like the egyptians. among the babylonians two methods of dividing the month existed, and both of them from the earliest times. the first method was to separate it into two halves of fifteen days each, and each of these periods into three shorter ones of five days, making six per month. the other method is the week of seven days. the days of the week with them, as they are with many nations now, were named after the sun and moon and the five planets, and the th, th, th, st, and th days of each month--days separated by seven days each omitting the th--were termed "days of rest," on which certain works were forbidden to be done. from this it is plain that we have here all the elements of our modern week. we find it, as is well known, in the earliest of hebrew writings, but without the mark which gives reason for the number seven, that is the names of the seven heavenly bodies. it would seem most probable, then, that we must look to the accadians as the originators of our modern week, from whom the hebrews may have--and, if so, at a very early period--borrowed the idea. it is known that the week was not employed in the ancient calendars of the romans, into which it was afterwards introduced through the medium of the biblical traditions, and became a legal usage under the first christian emperors. from thence it has been propagated together with the julian calendar amongst all the populations that have been subjected to the roman power. we find the period of seven days employed in the astronomical treatises of hindoo writers, but not before the fifth century. dion cassius, in the third century, represents the week as universally spread in his times, and considers it a recent invention which he attributes to the egyptians; meaning thereby, doubtless, the astrologers of the alexandrian school, at that time very eager to spread the abstract speculations of plato and pythagoras. if the names of the days of the week were derived from the planets, the sun and moon, as is easy to see, it is not so clear how they came to have their present order. the original order in which they were supposed to be placed in the various heavens that supported them according to their distance from the earth was thus:--saturn, jupiter, mars, the sun, venus, mercury, the moon. one supposition is that each hour of the day was sacred to one of these, and that each day was named from the god that presided over the first hours. now, as seven goes three times into twenty-four, and leaves three over, it is plain that if saturn began the first hour of saturday, the next day would begin with the planet three further on in the series, which would bring us to the sun for sunday, three more would bring us next day to the moon for monday, and so to mars for tuesday, to mercury for wednesday, to jupiter for thursday, to venus for friday, and so round again to saturn for saturday. the same method is illustrated by putting the symbols in order round the circumference of a circle, and joining them by lines to the one most opposite, following always in the same order as in the following figure. we arrive in this way at the order of the days of the week. [illustration: fig. .] all the nations who have adopted the week have not kept to the same names for them, but have varied them according to taste. thus sunday was changed by the christian church to the "lord's day," a name it still partially retains among ourselves, but which is the regular name among several continental nations, including the corrupted _dimanche_ of the french. the four middle days have also been very largely changed, as they have been among ourselves and most northern nations to commemorate the names of the great scandinavian gods tuesco, woden, thor, and friga. this change was no doubt due to the old mythology of the druids being amalgamated with the new method of collecting the days into weeks. we give below a general table of the names of the days of the week in several different languages. +------------+-----------+------------+------------+------------------+ | english. | french. | italian. | spanish. | portuguese. | +------------+-----------+------------+------------+------------------+ | sunday. | dimanche. | domenica. | domingo. | domingo. | | monday. | lundi. | lunedi. | luneo. | secunda feira. | | tuesday. | mardi. | marteti. | martes. | terça feira. | | wednesday. | mercredi | mercoledi. | miercoles. | quarta feira. | | thursday. | jeudi. | giovedi. | jueves. | quinta feira. | | friday. | vendredi. | venerdi. | viernes. | sexta feira. | | saturday. | samedi. | sabbato. | sabado. | sabbado. | +------------+-----------+------------+------------+------------------+ +------------+--------------+-------------+---------------+-----------+ | german. | anglo-saxon. | ancient | ancient | dutch. | | | | frisian. | northmen. | | +------------+--------------+-------------+---------------+-----------+ | sonntag. | sonnan däg. | sonna dei. | sunnu dagr. | zondag. | | montag. | monan däg. | mona dei. | mâna dagr. | maandag. | | dienstag. | tives däg. | tys dei. | tyrs dagr. | dingsdag. | | mitwoch. | vôdenes däg. | werns dei. | odins dagr. | woensdag. | | donnerstag.| thunores däg.| thunres dei.| thors dagr. | donderdag.| | freitag. | frige däg. | frigen dei. | fria dagr. | vrijdag. | | samstag. | soeternes | sater dei. | laugar dagr | zaturdag. | | | däg. | | (washing day)| | +------------+--------------+-------------+---------------+-----------+ the cycle which must be completed with the present calendar to bring the same day of the year to the same day of the week, is twenty-eight years, since there is one day over every ordinary year, and two every leap year; which will make an overlapping of days which, except at the centuries, will go through all the changes in twenty-eight times, which forms what is called the solar cycle. there is but one more point that will be interesting about the calendar, namely, the date from which we reckon our years. among the jews it was from the creation of the world, as recorded in their sacred books--but no one can determine when that was with sufficient accuracy to make it represent anything but an agreement of the present day. different interpreters do not come within a thousand years of one another for its supposed date; although some of them have determined it very accurately to their own satisfaction--one going so far as to say that creation finished at nine o'clock one sunday morning! in other cases the date has been reckoned from national events--as in the olympiads, the foundation of rome, &c. the word we now use, Æra, points to a particular date from which to reckon, since it is composed of the initials of the words ab exordio regni augusti "from the commencement of the reign of augustus." at the present day the point of departure, both forwards and backwards, is the year of the birth of jesus christ--a date which is itself controverted, and the use of which did not exist among the first christians. they exhibited great indifference, for many centuries, as to the year in which jesus christ entered the world. it was a monk who lived in obscurity at rome, about the year , who was a native of so unknown a country that he has been called a scythian, and whose name was denys, surnamed _exiguus_, or the little, who first attempted to discover by chronological calculations the year of the birth of jesus christ. the era of denys the little was not adopted by his contemporaries. two centuries afterwards, the venerable bede exhorted christians to make use of it--and it only came into general use about the year . among those who adopted the christian era, some made the year commence with march, which was the first month of the year of romulus; others in january, which commences the year of numa; others commenced on christmas day; and others on lady day, march . another form of nominal year was that which commenced with easter day, in which case, the festival being a movable one, some years were shorter than others, and in some years there might be two nd, rd, &c., of april, if easter fell in one year on the nd, and next year a few days later. the st of january was made to begin the year in germany in . an edict of charles ix. prescribes the same in france in . but it was not till that the change was made in england by lord chesterfield's act. the year , as the year that had preceded it, began on march th, and it should have lasted till the next lady day; but according to the act, the months of january, february, and part of march were to be reckoned as part of the year . by this means the unthinking seemed to have grown old suddenly by three months, and popular clamour was raised against the promoter of the bill, and cries raised of "give us our three months." such have been the various changes that our calendar has undergone to bring it to its present state. chapter xv. the end of the world. perhaps the most anxious question that has been asked of the astronomer is when the world is to come to an end. it is a question which, of course, he has no power to answer with truth; but it is also one that has often been answered in good faith. it has perhaps been somewhat natural to ask such a question of an astronomer, partly because his science naturally deals with the structure of the universe, which might give some light as to its future, and partly because of his connection with astrology, whose province it was supposed to be to open the destiny of all things. yet the question has been answered by others than by astronomers, on grounds connected with their faith. in the early ages of the church, the belief in the rapid approach of the end of the world was universally spread amongst christians. the apocalypse of st. john and the acts of the apostles seemed to announce its coming before that generation passed away. afterwards, it was expected at the year ; and though these beliefs did not rest in any way on astronomical grounds, yet to that science was recourse had for encouragement or discouragement of the idea. the middle ages, fall of simple faith and superstitious credulity, were filled with fear of this terrible catastrophe. as the year approached, the warnings became frequent and very pressing. thus, for example, bernard of thuringia, about , began to announce publicly that the world was about to end, declaring that he had had a particular revelation of the fact. he took for his text the enigmatical words of the apocalypse: "at the end of one thousand years, satan shall be loosed from his prison, and shall seduce the people that are in the four quarters of the earth. the book of life shall be open, and the sea shall give up her dead." he fixed the day when the annunciation of the virgin should coincide with good friday as the end of all things. this happened in , but nothing extraordinary happened. during the tenth century the royal proclamations opened by this characteristic phrase: _whereas the end of the world is approaching_.... in the astrologers frightened europe by announcing a conjunction of all the planets. rigord, a writer of that period, says in his _life of philip augustus_: "the astrologers of the east, jews, saracens, and even christians, sent letters all over the world, in which they predicted, with perfect assurance, that in the month of september there would be great tempests, earthquakes, mortality among men, seditions and discords, revolutions in kingdoms, and the destruction of all things. but," he adds, "the event very soon belied their predictions." some years after, in , another alarm of the end of the world was raised, but this time it was not dependent on celestial phenomena. it was said that antichrist was born in babylon, and therefore all the human race would be destroyed. it would be a curious list to make of all the years in which it was said that antichrist was born; they might be counted by hundreds, to say nothing of the future. at the commencement of the fourteenth century, the alchemist arnault of villeneuve announced the end of the world for . in his treatise _de sigillis_ he applies the influence of the stars to alchemy, and expounds the mystical formula by which demons are to be conjured. st. vincent ferrier, a famous spanish preacher, gave to the world as many years' duration as there were verses in the psalms--about . the sixteenth century produced a very plentiful crop of predictions of the final catastrophe. simon goulart, for example, gave the world an appalling account of terrible sights seen in assyria--where a mountain opened and showed a scroll with letters of greek--"the end of the world is coming." this was in ; but after that year had passed in safety, leovitius, a famous astrologer, predicted it again for . louis gayon reports that the fright at this time was great. the churches could not hold those who sought a refuge in them, and a great number made their wills, without reflecting that there was no use in it if the whole world was to finish. one of the most famous mathematicians of europe, named stoffler, who flourished in the th century, and who worked for a long time at the reform of the calendar proposed by the council of constance, predicted a universal deluge for . this deluge was to happen in the month of february, because saturn, jupiter, and mars were then together in the sign of the fishes. everyone in europe, asia, and africa, to whom these tidings came, was in a state of consternation. they expected a deluge, in spite of the rainbow. many contemporary authors report that the inhabitants of the maritime provinces of germany sold their lands for a mere trifle to those who had more money and less credulity. each built himself a boat like an ark. a doctor of toulouse, named auriol, made a very large ark for himself, his family, and his friends, and the same precautions were taken by a great many people in italy. at last the month of february came, and not a drop of rain fell. never was a drier month or a more puzzled set of astrologers. nevertheless they were not discouraged nor neglected for all that, and stoffler himself, associated with the celebrated regiomontanus, predicted once more that the end of the world would come in , or at least that there would be frightful events which would overturn the earth. this new prediction was a new deception; nothing extraordinary occurred in . the year , however, witnessed a strange phenomenon, capable of justifying all their fears. an unknown star came suddenly into view in the constellation of cassiopeia, so brilliant that it was visible even in full daylight, and the astrologers calculated that it was the star of the magi which had returned, and that it announced the second coming of jesus christ. the seventeenth and eighteenth centuries were filled with new predictions of great variety. even our own century has not been without such. a religious work, published in , by the count sallmard montfort, demonstrated perfectly that the world had no more than ten years to exist. "the world," he said, "is old, and its time of ending is near, and i believe that the epoch of that terrible event is not far off. jacob, the chief of the twelve tribes of israel, and consequently of the ancient church, was born in of the world, _i.e._, b.c. the ancient church, which was the figure of the new, lasted years. hence the new one will only last till a.d." similar prophecies by persons of various nations have in like manner been made, without being fulfilled. indeed, we have had our own prophets; but they have proved themselves incredulous of their own predictions, by taking leases that should _commence_ in the year of the world's destruction. but we have one in store for us yet. in , pierre louis of paris calculated that the end would be in , and he calculated in this way:--the apocalypse says the gentiles shall occupy the holy city for forty-two months. the holy city was taken by omar in . forty-two months of years is , which brings the return of the jews to , which will precede by a few years the final catastrophe. daniel also announces the arrival of antichrist , days after the establishment of artaxerxes on the throne of persia, b.c., which again brings us to . some again have put it at a.d., which will make , years, as they think, from the creation; these are the days of work; then comes the , years of millennial sabbath. we are led far away by these vain speculations from the wholesome study of astronomy; they are useful only in showing how by a little latitude that science may wind itself into all the questions that in any way affect the earth. indeed, since the world began, the world will doubtless end, and astronomers are still asked how could it be brought about? certainly it is not an impossible event, and there are only too many ways in which it has been imagined it might occur. the question is one that stands on a very different footing from that it occupied before the days of galileo and copernicus. _then_ the earth was believed to be the centre of the universe, and all the heavens and stars created for it. _then_ the commencement of the world was the commencement of the universe, its destruction would be the destruction of all. _now_, thanks to the revolution in feeling that has been accomplished by the progress of astronomy, we have learned our own insignificance, and that amongst the infinite number of stars, each supporting their own system of inhabited planets, our earth occupies an infinitesimally small portion, and the destruction of it would make no difference whatever--still less its becoming uninhabitable. it is an event which must have happened and be happening to other worlds, without affecting the infinite life of the universe in any marked degree. nevertheless, for ourselves, the question remains as interesting as if we were the all in all, but must be approached in a different manner. numerous hypotheses have been put forth on the question but they may mostly be dismissed as vain. buffon calculated that it had taken , years for the earth to cool down to its present temperature, and that it will take , years more before it would be too cold for men to live upon it. but sir william thomson has shown that the internal heat of the earth, supposed to be due to its cooling from fusion, cannot have seriously modified climate for a long series of years, and that life depends essentially on the heat of the sun. another hypothesis, the most ancient of all, is that which supposes the earth will be destroyed by fire. it comes down from zoroaster and the jews; and on the improbable supposition of the thin crust of the earth over a molten mass, this is thought possible. however, as the tendency in the past has been all the other way, namely, to make the effect of the inner heat of the earth less marked on the surface, we have no reason to expect a reversal. a third theory would make the earth die more gradually and more surely. it is known that by the wearing down of the surface by the rains and rivers, there is a tendency to reduce mountains and all high parts of the earth to a uniform level, a tendency which is only counteracted by some elevating force within the earth. if these elevating forces be supposed to be due to the internal heat--a hypothesis which cannot be proved--then with the cooling of the earth the elevating forces would cease, and, finally, the whole of the continent would be brought beneath the sea and terrestrial life perish. another interesting but groundless hypothesis is that of adhémar on the periodicity of deluges. this theory depends on the fact of the unequal length of the seasons in the two hemispheres. our autumn and our winter last days. in the southern hemisphere they last days. these seven days, or hours, of difference, increase each year the coldness of the pole. during , years the ice accumulates at one pole and melts at the other, thereby displacing the earth's centre of gravity. now a time will arrive when, after the maximum of elevation of temperature on one side, a catastrophe will happen, which will bring back the centre of gravity to the centre of figure, and cause an immense deluge. the deluge of the north pole was , years ago, therefore the next will be , hence. it is very obvious to ask on this--_why_ should there be a _catastrophe_? and why should not the centre of gravity return _gradually_ as it was gradually displaced? another theory has been that it would perish by a comet. that it will not be by the shock we have already seen from the light weight of the comet and from experience; but it has been suggested that the gas may combine with the air, and an explosion take place that would destroy us all; but is not that also contradicted by experience? another idea is that we shall finally fall into the sun by the resistance of the ether to our motion. encke's comet loses in thirty-three years a thousandth part of its velocity. it appears then that we should have to wait millions of centuries before we came too near the sun. in reality, however, we are simply dependent on our sun, and our destiny depends upon that. in the first place, in its voyage through space it might encounter or come within the range of some dark body we at present know nothing of, and the attraction might put out of harmony all our solar system with calamitous results. or since we are aware that the sun is a radiating body giving out its heat on all sides, and therefore growing colder, it may one day happen that it will be too cold to sustain life on the earth. it is, we know, a variable star, and stars have been seen to disappear, or even to have a catastrophe happen to them, as the kindling of enormous quantities of gas. a catastrophe in the sun will be our own end. fontenelle has amusingly described in verse the result of the sun growing cold, which may be thus englished:-- "of this, though, i haven't a doubt, one day when there isn't much light, the poor little sun will go out and bid us politely--good-night. look out from the stars up on high, some other to help you to see; i can't shine any longer, not i, since shining don't benefit me. "then down on our poor habitation what numberless evils will fall, when the heavens demand liquidation, why all will go smash, and then all society come to an end. soon out of the sleepy affair his way will each traveller wend, no testament leaving, nor heir." the cooling of the sun must, however, take place very gradually, as no cooling has been perceived during the existence of man; and the growth of plants in the earliest geological ages, and the life of animals, prove that for so long a time it has been within the limits within which life has been possible--and we may look forward to as long in the future. it is not of course the time when the sun will become a dark ball, surrounded by illuminated planets, that we must reckon as the end of the earth. life would have ceased long before that stage--no man will witness the death of the sun. [illustration: plate xvi.--the end of the world.] the diminution of the sun's heat would have for its natural effect the enlargement of the glacial zones! the sea and the land in those parts of the earth would cease to support life, which would gradually be drawn closer to the equatorial belt. man, who by his nature and his intelligence is best fitted to withstand cold climates, would remain among the last of the inhabitants, reduced to the most miserable nourishment. drawn together round the equator, the last of the sons of earth would wage a last combat with death, and exactly as the shades approached, would the human genius, fortified by all the acquirements of ages past--give out its brightest light, and attempt in vain to throw off the fatal cover that was destined to engulf him. at last, the earth, fading, dry, and sterile, would become an immense cemetery. and it would be the same with the other planets. the sun, already become red, would at last become black, and the planetary system would be an assemblage of black balls revolving round a larger black ball. of course this is all imaginary, and cannot affect ourselves, but the very idea of it is melancholy, and enough to justify the words of campbell:-- "for this hath science searched on weary wing by shore and sea--each mute and living thing, or round the cope her living chariot driven and wheeled in triumph through the signs of heaven. oh, star-eyed science, hast thou wandered there to waft us home the message of despair?" in reality, as we know nothing of the origin, so we know nothing of the end of the world; and where so much has been accomplished, there are obviously infinite possibilities enough to satisfy the hopes of every one. while some stars may be fading, others may be rising into their place, and man need not be identified with one earth alone, but may rest content in the idea that the life universal is eternal. the end. london: p. clay, sons, and taylor, printers. transcriber's notes: . passages in italics are surrounded by _underscores_. . images have been moved from the middle of a paragraph to the closest paragraph break. . the original text includes greek characters. for this text version these letters have been replaced with transliterations, for example, [greek: a] represents first greek letter alpha. . the original text includes certain symbols for planets and zodiac signs. for this text version these symbols are replaced by text name of the corresponding symbol. for example, [symbol: sun] replaces the symbolic representation of sun. . in this text version, fractions are represented using hyphen and forward slash. for example, - / stands for three and a half. . certain words use oe ligature in the original. . obvious errors in punctuation and a few misprints have been silently corrected. . other than the corrections listed above, printer's inconsistencies in hyphenation and ligature usage have been retained. available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/royalobservatory maun transcriber's note: text enclosed by underscores is in italics (_italics_). small capital text has been replaced with all capitals. the carat character (^) indicates that the following letter is superscripted (example: ii^s). if two or more letters are superscripted they are enclosed in curly brackets (example: d^{ni}). [illustration: flamsteed, the first astronomer royal. (_from the portrait in the 'historia coelestis.'_)] the royal observatory greenwich a glance at its history and work by e. walter maunder, f.r.a.s. with many portraits and illustrations from old prints and original photographs london the religious tract society paternoster row, and st. paul's churchyard london: printed by william clowes and sons, limited, stamford street and charing cross. preface i was present on one occasion at a popular lecture delivered in greenwich, when the lecturer referred to the way in which so many english people travel to the ends of the earth in order to see interesting or wonderful places, and yet entirely neglect places of at least equal importance in their own land. 'ten minutes' walk from this hall,' he said, 'is greenwich observatory, the most famous observatory in the world. most of you see it every day of your lives, and yet i dare say that not one in a hundred of you has ever been inside.' whether the lecturer was justified in the general scope of his stricture or not, the particular instance he selected was certainly unfortunate. it was not the fault of the majority of his audience that they had not entered greenwich observatory, since the regulations by which it is governed forbade them doing so. these rules are none too stringent, for the efficiency of the institution would certainly suffer if it were made a 'show' place, like a picture gallery or museum. the work carried on therein is too continuous and important to allow of interruption by daily streams of sightseers. to those who may at some time or other visit the observatory it may be of interest to have at hand a short account of its history, principal instruments, and work. to the far greater number who will never be able to enter it, but who yet feel an interest in it, i would trust that this little book may prove some sort of a substitute for a personal visit. i would wish to take this opportunity of thanking the astronomer royal for his kind permission to reproduce some of the astronomical photographs taken at the observatory and to photograph the domes and instruments. i would also express my thanks to miss airy, for permission to reproduce the photograph of sir g. b. airy; to mr. j. nevil maskelyne, f.r.a.s., for the portrait of dr. maskelyne; to mr. bowyer, for procuring the portraits of bliss and pond; to messrs. edney and lacey, for many photographs of the royal observatory; and to the editor of _engineering_, for permission to copy two engravings of the astrographic telescope. e. w. m. royal observatory, greenwich, _august, _. [illustration: the new building. (_from a photograph by mr. lacey._)] contents chapter page i. introduction ii. flamsteed iii. halley and his successors iv. airy v. the observatory buildings vi. the time department vii. the transit and circle departments viii. the altazimuth department ix. the magnetic and meteorological departments x. the heliographic department xi. the spectroscopic department xii. the astrographic department xiii. the double-star department index list of illustrations page flamsteed, the first astronomer royal _frontispiece_ the new building general view of the observatory buildings from the new dome flamsteed's sextant the royal observatory in flamsteed's time the 'camera stellata' in flamsteed's time edmund halley halley's quadrant james bradley graham's zenith sector nathaniel bliss nevil maskelyne hadley's quadrant john pond george biddell airy, astronomer royal the astronomer royal's room the south-east tower w. h. m. christie, astronomer royal the astronomer royal's house the courtyard plan of observatory at present time the great clock and porter's lodge the chronograph the time-desk harrison's chronometer the chronometer room the chronometer oven the transit pavilion 'lost in the birkenhead' the transit circle the mural circle airy's altazimuth new altazimuth building the new altazimuth the new observatory as seen from flamsteed's observatory the self-registering thermometers the anemometer room, north-west turret the anemometer trace magnetic pavilion--exterior magnetic pavilion--interior the dallmeyer photo-heliograph photograph of a group of sun-spots the great nebula in orion the half-prism spectroscope on the south-east equatorial the workshop the -inch reflector with the new spectroscope attached 'chart plate' of the pleiades the control pendulum and the base of the thompson telescope the astrographic telescope the driving clock of the astrographic telescope the thompson telescope in the new dome the nebulÆ of the pleiades double-star observation with the south-east equatorial the south-east dome with the shutter open [illustration: general view of the observatory buildings from the new dome. (_from a photograph by mr. lacey._)] the royal observatory greenwich chapter i introduction i had parted from a friend one day just as he met an acquaintance of his to whom i was unknown. 'who is that?' said the newcomer, referring to me. my friend replied that i was an astronomer from greenwich observatory. 'indeed; and what does he do there?' this question completely exhausted my friend's information, for as his tastes did not lead him in the direction of astronomy, he had at no time ever concerned himself to inquire as to the nature of my official duties. 'oh--er--why--he _observes_, don't you know?' and the answer, vague as it was, completely slaked the inquirer's thirst for knowledge. it is not every one who has such exceedingly nebulous ideas of an astronomer's duties. more frequently we find that the inquirer has already formed a vivid and highly-coloured picture of the astronomer at his 'soul-entrancing work.' resting on a comfortable couch, fixed at a luxurious angle, the eye-piece of some great and perfect instrument brought most conveniently to his eye, there passes before him, in grand procession, a sight such as the winter nights, when clear and frosty, give to the ordinary gazer, but increased ten thousand times in beauty, brilliance, and wonder by the power of his telescope. for him jupiter reveals his wind-drifted clouds and sunset colours; for him saturn spreads his rings; for him the snows of mars fall and melt, and a thousand lunar plains are ramparted with titanic crags; his are the star-clusters, where suns in their first warm youth swarm thicker than hiving bees; his the faint veils of nebulous smoke, the first hint of shape in worlds about to be, or, perchance, the last relics of worlds for ever dead. and beside the enjoyment of all this entrancing spectacle of celestial beauty, the fortunate astronomer sits at his telescope and _discovers_--always he _discovers_. this, or something like it, is a very popular conception of an astronomer's experiences and duty; and consequently many, when they are told that 'discoveries' are not made at greenwich, are inclined to consider that the observatory has failed in its purpose. an astronomer without 'discoveries' to his record is like an angler who casts all day and comes home without fish--obviously an idle or incompetent person. again, it is considered that astronomy is a most transcendental science. it deals with infinite distances, with numbers beyond all power of human intellect to appreciate, and therefore it is supposed, on the one hand, that it is a most elevating study, keeping the mind continually on the stretch of ecstasy, and, on the other hand, that it is utterly removed from all connection with practical, everyday, ordinary life. these ideas as to the royal observatory, or ideas like them, are very widely current, and they are, in every respect, exactly and wholly wrong. first of all, greenwich observatory was originally founded, and has been maintained to the present day, for a strictly practical purpose. next, instead of leading a life of dreamy ecstasy or transcendental speculation, the astronomer has, perhaps, more than any man, to give the keenest attention to minute practical details. his life, on the one side, approximates to that of the engineer; on the other, to that of the accountant. thirdly, the professional astronomer has hardly anything to do with the show places of the sky. it is quite possible that there are many people whose sole opportunity of looking through a telescope is the penny peep through the instrument of some itinerant showman, who may have seen more of these than an active astronomer in a lifetime; while as to 'discoveries,' these lie no more within the scope of our national observatory than do geographical discoveries within that of the captain and officers of an ocean liner. if it is not to afford the astronomer beautiful spectacles, nor to enable him to make thrilling discoveries, what is the purpose of greenwich observatory? first and foremost, it is to assist navigation. the ease and certainty with which to-day thousands of miles of ocean are navigated have ceased to excite any wonder. we do not even think about it. we go down to the docks and see, it may be, one steamer bound for halifax, another for new york, a third for charleston, a fourth for the west indies, a fifth for rio de janeiro; and we unhesitatingly go on board the one bound for our chosen destination, without the faintest misgiving as to its direction. we have no more doubt about the matter than we have in choosing our train at a railway station. yet, whilst the train is obliged to follow a narrow track already laid for it, from which it cannot swerve an inch, the steamer goes forth to traverse for many days an ocean without a single fixed mark or indication of direction; and it is exposed, moreover, to the full force of winds and currents, which may turn it from its desired path. but for this facility of navigation, great britain could never have obtained her present commercial position and world-wide empire. 'for the lord our god most high, he hath made the deep as dry; he has smote for us a pathway, to the ends of all the earth.' part of this facility is, of course, due to the invention of the steam engine, but much less than is generally supposed. even yet the clippers, with their roods of white canvas, are not entirely superseded; and if we could conceive of all steamships being suddenly annihilated, ere long the sailing vessels would again, as of yore, prove the 'swift shuttles of an empire's loom, that weave us main to main.' but with the art of navigation thrust back into its condition of a hundred and fifty years ago, it is doubtful whether a sufficient tide of commerce could be carried on to keep our home population supplied, or to maintain a sufficiently close political connection between these islands and our colonies. navigation was in a most primitive condition even as late as the middle of last century. then the method of finding a ship's longitude at sea was the insufficient one of dead reckoning. in other words, the direction and speed of the ship were estimated as closely as possible, and so the position was carried on from day to day. the uncertainty of the method was very great, and many terrible stories might be told of the disastrous consequences which might, and often did, follow in the train of this method by guess-work. it will be sufficient, however, to cite the instance of commodore anson. he wanted to make the island of juan fernandez, where he hoped to obtain fresh water and provisions, and to recruit his crew, many of whom were suffering from that scourge of old-time navigators--scurvy. he got into its latitude easily enough, and ran eastward, believing himself to be west of the island. he was, however, really east of it, and therefore made the mainland of america. he had therefore to turn round and sail westwards, losing many days, during which the scurvy increased upon his crew, many of whom died from the terrible disease before he reached the desired island. the necessity for finding out a ship's place when at sea had not been very keenly felt until the end of the fifteenth century. it was always possible for the sailor to ascertain his latitude pretty closely, either by observing the height of the pole-star at night or the height of the sun at noonday; and so long as voyages were chiefly confined to the mediterranean sea, and the navigators were content for the most part to coast from point to point, rarely losing sight of land, the urgency of solving the second problem--the longitude of the ship--was not so keenly felt. but immediately the discoveries of the great portuguese and spanish navigators brought a wider, bolder navigation into vogue, it became a matter of the first necessity. to take, for example, the immortal voyage of christopher columbus. his purpose in setting out into the west was to discover a new way to india. the venetians and genoese practically possessed the overland route across the isthmus of suez and down the red sea. vasco da gama had opened out the route eastward round the cape. firmly convinced that the world was a globe, columbus saw that a third route was possible, namely, one nearly due west; and when, therefore, he reached the bahamas, after traversing some ° of longitude, he believed that he was in the islands of the china sea, some ° from spain. those who followed him still laboured under the same impression, and when they reached the mainland of america, believed that they were close to the shores of india, which was still distant from them by half the circumference of the globe. little by little the intrepid sailors of the sixteenth century forced their way to a true knowledge of the size of the globe, and of the relative position of the great continents. but this knowledge was only attained after many disasters and terrible miseries; and though a new kind of navigation was established--the navigation of the open ocean, far away from any possible landmark, a navigation as different as could be conceived from the old method of coasting--yet it remained terribly risky and uncertain throughout the sixteenth century. therefore many mathematicians endeavoured to solve the problem of determining the position of a ship when at sea. their suggestions, however, remained entirely fruitless at the time, though in several instances they struck upon principles which are being employed at the present day. the first country to profit by the discovery of america was spain, and hence spain was the first to feel keenly the pinch of the problem. in , therefore, philip iii. offered a prize of , crowns to any one who would devise a method by which a captain of a vessel could determine his position when out of sight of land. holland, which had recently started on its national existence, and which was challenging the colonial empire of spain, followed very shortly after with the offer of a reward of , florins. not very long after the offer of these rewards, a master mind did work out a simple method for determining the longitude, a method theoretically complete, though practically it proved inapplicable. this was galileo, who, with his newly invented telescope, had discovered that jupiter was attended by four satellites. at first sight such a discovery, however interesting, would seem to have not the slightest bearing upon the sailor's craft, or upon the commercial progress of one nation or another. but galileo quickly saw in it the promise of great practical usefulness. the question of the determination of the place of a ship when in the open ocean really resolved itself into this: how could the navigator ascertain at any time what was the true time, say at the port from which he sailed? as already pointed out, it was possible, by observing the height of the sun at noon, or of the pole-star at night, to infer the latitude of the ship. the longitude was the point of difficulty. now, the longitude may be expressed as the difference between the local time of the place of observation and the local time at the place chosen as the standard meridian. the sailor could, indeed, obtain his own local time by observations of the height of the sun. the sun reached its greatest height at local noon, and a number of observations before and after noon would enable him to determine this with sufficient nicety. but how was he to determine when he, perhaps, was half-way across the atlantic, what was the local time at genoa, cadiz, lisbon, bristol, or amsterdam, or whatever was the port from which he sailed? galileo thought out a way by which the satellites of jupiter could give him this information. for as they circle round their primary, they pass in turn into its shadow, and are eclipsed by it. it needed, then, only that the satellites should be so carefully watched, that their motions, and, consequently, the times of their eclipses could be foretold. it would follow, then, that if the mariner had in his almanac the local time of the standard city at which a given satellite would enter into eclipse, and he were able to note from the deck of his vessel the disappearance of the tiny point, he would ascertain the difference between the local times of the two places, or, in other words, the difference of their longitudes. the plan was simplicity itself, but there were difficulties in carrying it out, the greatest being the impossibility of satisfactorily making telescopic observations from the moving deck of a ship at sea. nor were the observations sufficiently sharp to be of much help. the entry of a satellite into the shadow of jupiter is in most cases a somewhat slow process, and the moment of complete disappearance would vary according to the size of the telescope, the keenness of the observer's sight, and the transparency of the air. as the power and commerce of spain declined, two other nations entered into the contest for the sovereignty of the seas, and with that sovereignty predominance in the new world of america--france and england. the problem of the longitude at sea, or, as already pointed out, what amounts to the same thing, the problem how to determine when at sea the local time at some standard place, became, in consequence, of greater necessity to them. the standard time would be easily known, if a thoroughly good chronometer which did not change its rate, and which was set to the standard time before starting, was carried on board the ship. this plan had been proposed by gemma frisius as early as , but at the time was a mere suggestion, as there were no chronometers or watches sufficiently good for the purpose. there was, however, another method of ascertaining the standard time. the moon moves pretty quickly amongst the stars, and at the present time, when its motions are well known, it is possible to draw up a table of its distances from a number of given stars at definite times for long periods in advance. this is actually done to-day in the _nautical almanac_, the moon's distance from certain stars being given for every three hours of greenwich time. it is possible, then, by measuring these distances, and making, as in the case of the latitude, certain corrections, to find out the time at greenwich. in short, the whole sky may be considered as a vast clock set to greenwich time, the stars being the numbers on the dial face, and the moon the hand (for this clock has only one hand) moving amongst them. the local apparent time--that is, the time at the place at which the ship itself was--is a simpler matter. it is noon at any place when the sun is due south--or, as we may put it a little differently, when it culminates--that is, when it reaches its highest point. to find the longitude at sea, therefore, it was necessary to be able to predict precisely the apparent position of the moon in the sky for any time throughout the entire year, and it was also necessary that the places of the stars themselves should be very accurately known. it was therefore to gather the materials for a better knowledge of the motions of the moon and the position of the stars that greenwich observatory was founded, whilst the _nautical almanac_ was instituted to convey this information to mariners in a convenient form. this proposal was actually made in the reign of charles ii. by a frenchman, le sieur de saint-pierre, who, having secured an introduction to the duchess of portsmouth, endeavoured to obtain a reward for his scheme. it would appear that he had simply borrowed the idea from a book which an eminent french mathematician brought out forty years before, without having himself any real knowledge of the subject. but when the matter was brought before the king's notice, he desired some of the leading scientific men of the day to report upon its practicability, and the rev. john flamsteed was the man selected for the task. he reported that the scheme in itself was a good one, but impracticable in the then state of science. the king, who, in spite of the evil reputation which he has earned for himself, took a real interest in science, was startled when this was reported to him, and commanded the man who had drawn his attention to these deficiencies 'to apply himself,' as the king's astronomer, 'with the most exact care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, in order to find out the so much desired longitude at sea, for the perfecting the art of navigation.' this man, the rev. john flamsteed, was accordingly appointed first astronomer royal at the meagre salary of £ a year, with full permission to provide himself with the instruments he might require, at his own expense. he followed out the task assigned to him with extreme devotion, amidst many difficulties and annoyances, until his death in . he has been succeeded by seven astronomers royal, each of whom has made it his first object to carry out the original scheme of the institution; and the chief purpose of greenwich observatory to-day, as when it was founded in , is to observe the motions of the sun, moon, and planets, and to issue accurate star catalogues. it will be seen, therefore, that the establishment of greenwich observatory arose from the actual necessity of the nation. it was an essential step in its progress towards its present position as the first commercial nation. no thoughts of abstract science were in the minds of its founders; there was no desire to watch the cloud-changes on jupiter, or to find out what sirius was made of. the observatory was founded for the benefit of the royal navy and of the general commerce of the realm; and, in essence, that which was the sole object of its foundation at the beginning has continued to be its first object down to the present time. it was impossible that the work of the observatory should be always confined within the above limits, and it will be my purpose, in the pages which follow, to describe when and how the chief expansions of its programme have taken place. but assistance to navigation is now, and has always been, the dominant note in its management. chapter ii flamsteed for the first century of its existence, the lives of its astronomers royal formed practically the history of the royal observatory. during this period, the observatory was itself so small that the astronomer royal, with a single assistant, sufficed for the entire work. everything, therefore, depended upon the ability, energy, and character of the actual director. there was no large organized staff, established routine, or official tradition, to keep the institution moving on certain lines, irrespective of the personal qualities of the chief. it was specially fortunate, therefore, that the first four astronomers royal, flamsteed, halley, bradley, and maskelyne (for bliss, the immediate successor of bradley, reigned for so short a time that he may be practically left out of the count), were all men of the most conspicuous ability. it will be convenient to divide the history of the first seven astronomers royal into three sections. in the first, we have the founder, john flamsteed, a pathetic and interesting figure, whom we seem to know with especial clearness, from the fulness of the memorials which he has left to us. he was succeeded by the man who was, indeed, best fitted to succeed him, but whom he most hated. the second to the sixth astronomers royal formed what we might almost speak of as a dynasty, each in turn nominating his successor, who had entered into more or less close connection with the observatory during the lifetime of the previous director; and the lives of these five may well form the second section. the line was interrupted after the resignation of the sixth astronomer royal, and the third section will be devoted to the seventh director, airy, under whom the observatory entered upon its modern period of expansion. 'god suffers not man to be idle, although he swim in the midst of delights; for when he had placed his own image (adam) in a paradise so replenished (of his goodness) with varieties of all things, conducing as well to his pleasure as sustenance, that the earth produced of itself things convenient for both,--he yet (to keep him out of idleness) commands him to till, prune, and dress his pleasant, verdant habitation; and to add (if it might be) some lustre, grace, or conveniency to that place, which, as well as he, derived its original from his creator.' in these words john flamsteed begins the first of several autobiographies which he has handed down to us; this particular one being written before he attained his majority, 'to keep myself from idleness and to recreate myself.' 'i was born,' he goes on, 'at denby, in derbyshire, in the year , on the th day of august, at hours minutes after noon. my father, named stephen, was the third son of mr. william flamsteed, of little hallam; my mother, mary, was the daughter of mr. john spateman, of derby, ironmonger. from these two i derived my beginning, whose parents were of known integrity, honesty, and fortune, as they [were] of equal extraction and ingenuity; betwixt whom i [was] tenderly educated (by reason of my natural weakness, which required more than ordinary care) till i was aged three years and a fortnight; when my mother departed, leaving my father a daughter, then not a month old, with me, then weak, to his fatherly care and provision.' the weakly, motherless boy became at an early age a voracious reader. at first, he says-- 'i began to affect the volubility and ranting stories of romances; and at twelve years of age i first left off the wild ones, and betook myself to read the better sort of them, which, though they were not probable, yet carried no seeming impossibility in the fiction. afterwards, as my reason increased, i gathered other real histories; and by the time i was fifteen years old i had read, of the ancients, plutarch's _lives_, appian's and tacitus's _roman histories_, holingshed's _history of the kings of england_, davies's _life of queen elizabeth_, saunderson's of _king charles the first_, heyling's _geography_, and many others of the moderns; besides a company of romances and other stories, of which i scarce remember a tenth at present.' flamsteed received his education at the free school at derby, where he continued until the whitsuntide of , when he was nearly sixteen years of age. two years earlier than this, however, a great misfortune fell upon him. 'at fourteen years of age,' he writes, 'when i was nearly arrived to be the head of the free-school, [i was] visited with a fit of sickness, that was followed with a consumption and other distempers, which yet did not so much hinder me in my learning, but that i still kept my station till the form broke up, and some of my fellows went to the universities; for which, though i was designed, my father thought it not advisable to send me, by reason of my distemper.' this was a keen disappointment to him, but seems to have really been the means of determining his career. the sickly, suffering boy could not be idle, though 'a day's short reading caused so violent a headache;' and a month or two after he had left school, he had a book lent to him--sacrobosco's _de sphæra_, in latin--which was the beginning of his mathematical studies. a partial eclipse of the sun in september of the same year seems to have first drawn his attention to astronomical observation, and during the winter his father, who had himself a strong passion for arithmetic, instructed him in that science. it was astonishing how quickly his appetite for his new subjects grew. the _art of dialling_, the calculation of tables of the sun's altitudes for all hours of the day, and for different latitudes, and the construction of a quadrant--'of which i was not meanly joyful'--were the occupations of this winter of illness. in he made the acquaintanceship of two friends, mr. george linacre and mr. william litchford; the former of whom taught him to recognize many of the fixed stars, whilst the latter was the means of his introduction to a knowledge of the motions of the planets. 'i had now completed eighteen years, when the winter came on, and thrust me again into the chimney; whence the heat and dryness of the preceding summer had happily once before withdrawn me.' the following year, , was memorable to him 'for the appearance of the comet,' and for a journey which he made to ireland to be 'stroked' for his rheumatic disorder by valentine greatrackes, a kind of mesmerist, who had the repute of effecting wonderful cures. the journey, of which he gives a full and vivid account, occupied a month; but though he was a little better, the following winter brought him no permanent benefit. but, ill or well, he pressed on his astronomical studies. a large partial eclipse of the sun was due the following june; he computed the particulars of it for derby, and observed the eclipse itself to the best of his ability. he argued out for himself 'the equation of time'; the difference, that is, between time as given by the actual sun, or 'apparent time,' and that given by a perfect clock, or 'mean time.' he drew up a catalogue of seventy stars, computing their right ascensions, declinations, longitudes, and latitudes for the year ; he attempted to determine the inclination of the ecliptic, the mean length of the tropical year, and the actual distance of the earth from the sun. and these were the recreations of a sickly, suffering young man, not yet twenty-one years of age, and who had only begun the study of arithmetic, such as fractions and the rule of three, four years previously! his next attempt was almanac-making, in the which he improved considerably upon those current at the time. his almanac for was rejected, however, and returned to him, and, not to lose his whole labour, he sent his calculations of an eclipse of the sun, and of five occultations of stars by the moon, which he had undertaken for the almanac, to the royal society. he sent the paper anonymously, or, rather, signed it with an anagram, 'in mathesi a sole fundes,' for 'johannes flamsteedius.' his covering letter ends thus:-- 'excuse, i pray you, this juvenile heat for the concerns of science and want of better language, from one who, from the sixteenth year of his age to this instant, hath only served one bare apprenticeship in these arts, under the discouragement of friends, the want of health, and all other instructors except his better genius.' this letter was dated november , , and on january , mr. oldenburg, the secretary of the society, replied to him in a letter which the young man cannot but have felt encouraging and flattering to the highest degree. 'though you did what you could to hide your name from us,' he writes, 'yet your ingenious and useful labours for the advancement of astronomy addressed to the noble president of the royal society, and some others of that illustrious body, did soon discover you to us, upon our solicitous inquiries after their worthy author.' and after congratulating him upon his skill, and encouraging him to furnish further similar papers, he signs himself, 'your very affectionate friend and real servant'--no unmeaning phrase, for the friendship then commenced ceased only with oldenburg's life. the following june, his father, pleased with the notice that some of the leading scientific men of the day were taking of his son, sent him up to london, that he might be personally acquainted with them; and he then was introduced to sir jonas moore, the surveyor of the ordnance, who made him a present of townley's micrometer, and promised to furnish him with object-glasses for telescopes at moderate rates. on his return journey he called at cambridge, where he visited dr. barrow and newton, and entered his name in jesus college. it was not until the following year, , that he was enabled to complete his own observatory, as he had had to wait long for the lenses which sir jonas moore and collins had promised to procure for him. he still laboured under several difficulties, in that he had no good means for measuring time, pendulum clocks not then being common. he, therefore, with a practical good sense which was characteristic, refrained from attempting anything which lay out of his power to do well, and he devoted himself to such observations as did not require any very accurate knowledge of the time. at the same time, he was careful to ascertain the time of his observations as closely as possible, by taking the altitudes of the stars. the next four years seem to have passed exceedingly pleasantly to him. the notes of ill-health are few. he was making rapid progress in his acquaintanceship with the work of other astronomers, particularly with those of the three marvellously gifted young men--horrox, crabtree, and gascoigne--who had passed away shortly before his own birth. he was making new friends in scientific circles, and, in particular, sir jonas moore was evidently esteeming him more and more highly. in he became more intimate with newton, the occasion which led to this acquaintanceship being the amusing one, that his assistance was asked by newton, who had found himself unable to adjust a microscope, having forgotten its object-glass--not the only instance of the great mathematician's absent-mindedness. the same year he took his degree of a.m. at cambridge, designing to enter the church; but sir jonas moore was extremely anxious to give him official charge of an observatory, and was urging the royal society to build an astronomical observatory at chelsea college, which then belonged to that body. he therefore came up to london, and resided some months with sir jonas moore at the tower. but shortly after his coming up to london, 'an accident happened,' to use his own expression, that hastened, if it did not occasion, the building of greenwich observatory. 'a frenchman that called himself le sieur de st. pierre, having some small skill in astronomy, and made an interest with a french lady, then in favour at court, proposed no less than the discovery of the longitude, and had procured a kind of commission from the king to the lord brouncker, dr. ward (bishop of salisbury), sir christopher wren, sir charles scarborough, sir jonas moore, colonel titus, dr. pell, sir robert murray, mr. hook, and some other ingenious gentlemen about the town and court, to receive his proposals, with power to elect, and to receive into their number, any other skilful persons; and having heard them, to give the king an account of them, with their opinion whether or no they were practicable, and would show what he pretended. sir jonas moore carried me with him to one of their meetings, where i was chosen into their number; and, after, the frenchman's proposals were read, which were: '( ) to have the year and day of the observations. '( ) the height of two stars, and on which side of the meridian they appeared. '( ) the height of the moon's two limbs. '( ) the height of the pole--all to degrees and minutes. 'it was easy to perceive, from these demands, that the sieur understood not that the best lunar tables differed from the heavens; and that, therefore, his demands were not sufficient for determining the longitude of the place where such observations were, or should be, made, from that to which the lunar tables were fitted, which i represented immediately to the company. but they, considering the interests of his patroness at court, desired to have him furnished according to his demands. i undertook it; and having gained the moon's true place by observations made at derby, february , , and november , , gave him observations such as he demanded. the half-skilled man did not think they could have been given him, and cunningly answered "_they were feigned_." i delivered them to dr. pell, february , - , who, returning me his answer some time after, i wrote a letter in english to the commissioners, and another in latin to the sieur, to assure him they were not feigned, and to show them that, if they had been, yet if we had astronomical tables that would give us the two places of the fixed stars and the moon's true places, both in longitude and latitude, nearer than to half a minute, we might hope to find the longitude of places by lunar observations, but not by such as he demanded. but that we were so far from having the places of the fixed stars true, that the tychonic catalogues often erred ten minutes or more; that they were uncertain to three or four minutes, by reason that tycho assumed a faulty obliquity of the ecliptic, and had employed only plain sights in his observations: and that the best lunar tables differ one-quarter, if not one-third, of a degree from the heavens; and lastly, that he might have learnt better methods than he proposed, from his countryman morin, whom he had best consult before he made any more demands of this nature.' this was in effect to tell st. pierre that his proposal was neither original nor practicable. if st. pierre had but consulted morin's writings (morin himself had died more than eighteen years before), he would have known that practically the same proposal had been laid before cardinal richelieu in , and had been rejected, as quite impracticable in the then state of astronomical knowledge. possibly flamsteed meant further to intimate that st. pierre had simply stolen his method from morin, hoping to trade it off upon the government of another country; in which case he would no doubt regard flamsteed's letter as a warning that he had been found out. flamsteed continues:-- 'i heard no more of the frenchman after this; but was told that, my letters being shown king charles, he startled at the assertion of the fixed stars' places being false in the catalogue; said, with some vehemence, "he must have them anew observed, examined, and corrected, for the use of his seamen;" and further (when it was urged to him how necessary it was to have a good stock of observations taken for correcting the motions of the moon and planets), with the same earnestness, "he must have it done." and when he was asked who could, or who should do it? "the person (says he) that informs you of them." whereupon i was appointed to it, with the incompetent allowance aforementioned; but with assurances, at the same time, of such further additions as thereafter should be found requisite for carrying on the work.' [illustration: flamsteed's sextant. (_from an engraving in the 'historia coelestis.'_)] thus, in his twenty-ninth year, john flamsteed became the first astronomer royal. in many ways he was an ideal man for the post. in the twelve years which had passed since he left school he had accomplished an amazing amount of work. despite his constant ill-health and severe sufferings, and the circumstance--which may be inferred from many expressions in his autobiographies--that he assisted his father in his business, he had made himself master, perhaps more thoroughly than any of his contemporaries, of the entire work of a practical astronomer as it was then understood. he was an indefatigable computer; the calculation of tables of the motions of the moon and planets, which should as faithfully as possible represent their observed positions, had had an especial attraction for him, and, as has been already mentioned, some years before his appointment he had drawn up a catalogue of stars, based upon the observations of tycho brahe. more than that, he had not been a merely theoretical worker, he had been a practical observer of very considerable skill, and, in the dearth of suitable instruments, had already made one or two for himself, and had contemplated the making of others. in his first letter to sir jonas moore he asks for instruction as to the making of object-glasses for telescopes, for he was quite prepared to set about the task of making his own. in addition to his tireless industry, which neither illness nor suffering could abate, he was a man of singularly exact and business-like habits. the precision with which he preserves and records the dates of all letters received or sent is an illustration of this. on the other hand, he had the defects of his circumstances and character. his numerous autobiographical sketches betray him, not indeed as a conceited man, in the ordinary sense of the word, but as an exceedingly self-conscious one. devout and high-principled he most assuredly was, but, on the other hand, he shows in almost every line he wrote that he was one who could not brook anything like criticism or opposition. such a man, however efficient, was little likely to be happy as the first incumbent of a new and important government post; but there was another circumstance which was destined to cause him greater unhappiness still. if we believe, as surely we must, that not only the moral and the physical progress of mankind is watched over and controlled by god's good providence, but its intellectual progress as well, then there can be no doubt that john flamsteed was raised up at this particular time, not merely to found greenwich observatory, and to assist the solution of the problem of the longitude at sea, but also, and chiefly, to become the auxiliary to a far greater mind, the journeyman to a true master-builder. but for the founding of greenwich observatory, and for john flamsteed's observations made therein, the working out of newton's grand theory of gravitation must have been hindered, and its acceptance by the men of science of his time immensely delayed. we cannot regard as accidental the combination, so fortunate for us, of newton, the great world-genius, to work out the problem, of flamsteed, the painstaking observer, to supply him with the materials for his work, and of the newly-founded institution, greenwich observatory, where flamsteed was able to gather those materials together. this is the true debt that we owe to flamsteed, that, little as he understood the position in which he had been placed from the standpoint from which we see it to-day, yet, to the extent of his ability, and as far as he conceived it in accordance with his duty, he gave newton such assistance as he could. this is how we see the matter to-day. it wore a very different aspect in flamsteed's eyes; and the two following documents, the one, the warrant founding the observatory and making him astronomer royal; the other, the warrant granting him a salary, will go far to explain his position in the matter. he had a high-sounding, official position, which could not fail to impress him with a sense of importance; whilst his salary was so insufficient that he naturally regarded himself as absolute owner of his own work. _'warrant for the payment of mr. flamsteed's salary._ 'charles rex. 'whereas, we have appointed our trusty and well-beloved john flamsteed, master of arts, our astronomical observator, forthwith to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so-much-desired longitude of places for the perfecting the art of navigation, our will and pleasure is, and we do hereby require and authorize you, for the support and maintenance of the said john flamsteed, of whose abilities in astronomy we have very good testimony, and are well satisfied, that from time to time you pay, or cause to be paid, unto him, the said john flamsteed, or his assigns, the yearly salary or allowance of one hundred pounds per annum; the same to be charged and borne upon the quarter-books of the office of the ordnance, and paid to him quarterly, by even and equal portions, by the treasurer of our said office, the first quarter to begin and be accompted from the feast of st. michael the archangel last past, and so to continue during our pleasure. and for so doing, this shall be as well unto you, as to the auditors of the exchequer, for allowing the same, and all other our officers and ministers whom it may concern, a full and sufficient warrant. 'given at our court at whitehall, the th day of march, - . 'by his majesty's command, 'j. williamson. 'to our right-trusty and well-beloved counsellor, sir thomas chichely, knt., master of our ordnance, and to the lieutenant-general of our ordnance, and to the rest of the officers of our ordnance, now and for the time being, and to all and every of them.' _'warrant for building the observatory._ 'charles rex. 'whereas, in order to the finding out of the longitude of places for perfecting navigation and astronomy, we have resolved to build a small observatory within our park at greenwich, upon the highest ground, at or near the place where the castle stood, with lodging-rooms for our astronomical observator and assistant, our will and pleasure is, that according to such plot and design as shall be given you by our trusty and well-beloved sir christopher wren, knight, our surveyor-general of the place and scite of the said observatory, you cause the same to be fenced in, built and finished with all convenient speed, by such artificers and workmen as you shall appoint thereto, and that you give order unto our treasurer of the ordnance for the paying of such materials and workmen as shall be used and employed therein, out of such monies as shall come to your hands for old and decayed powder, which hath or shall be sold by our order of the st of january last, provided that the whole sum, so to be expended or paid, shall not exceed five hundred pounds; and our pleasure is, that all our officers and servants belonging to our said park be assisting to those that you shall appoint, for the doing thereof, and for so doing, this shall be to you, and to all others whom it may concern, a sufficient warrant. 'given at our court at whitehall, the nd day of june, , in the th year of our reign. 'by his majesty's command, 'j. williamson. 'to our right-trusty and well-beloved counsellor, sir thomas chichely, knt., master-general of our ordnance.' the first question that arose, when it had been determined to found the new observatory, was where it was to be placed. hyde park was suggested, and sir jonas moore recommended chelsea college, where he had already thought of establishing flamsteed in a private observatory. fortunately, both these localities were set aside in favour of one recommended by sir christopher wren. there was a small building on the top of the hill in the royal park of greenwich belonging to the crown, and which was now of little or no use. visible from the city, and easily accessible by that which was then the best and most convenient roadway, the river thames, it was yet so completely out of town as to be entirely safe from the smoke of london. in greenwich park, too, but on the more easterly hill, charles i. had contemplated setting up an observatory, but the pressure of events had prevented him carrying out his intention. a further practical advantage was that materials could be easily transported thither. the management of public affairs under charles ii. left much to be desired in the matter of efficiency and economy, and it was not very easy to procure what was wanted for the erection of a purely scientific building. however, the matter was arranged. a gate-house demolished in the tower supplied wood; iron, and lead, and bricks were supplied from tilbury fort, and these could be easily brought by water to the selected site. the sum of £ , actually £ , was further allotted from the results of a sale of spoilt gunpowder; and with these limited resources greenwich observatory was built. the foundation-stone was laid august , , and flamsteed amused himself by drawing the horoscope of the observatory, a fact which--in spite of his having written across the face of the horoscope _risum teneatis amici?_ (can you keep from laughter, my friends?), and his having two or three years before written very severely against the imposture of astrology--has led some modern astrologers to claim him as a believer in their cult. he actually entered into residence july , . [illustration: the royal observatory in flamsteed's day. (_from an engraving in the 'historia coelestis.'_)] his position was not a bright one. the government had, indeed, provided him with a building for his observatory, and a small house for his own residence, but he had no instrument and no assistant. the first difficulty was partly overcome for the moment by gifts or loans from sir jonas moore, and by one or two small loans from the royal society. the death of this great friend and patron, four years after the founding of the observatory, and only three years after his entering into residence, deprived him of several of these; it was with difficulty that he maintained against sir jonas' heirs his claim to the instruments which sir jonas had given him. there was nothing for him to do but to make his instruments himself, and in he built a mural quadrant of fifty inches radius. his circumstances improved the following year, when lord north gave him the living of burstow, near horley, surrey, flamsteed having received ordination almost at the time of his appointment to the astronomer royalship. we have little or no account of the way in which he fulfilled his duties as a clergyman. evidently he considered that his position as astronomer royal had the first claim upon him. at the same time, comparatively early in life he had expressed his desire to fill the clerical office, and he was a man too conscientious to neglect any duty that lay upon him. that in spite of his feeble health he often journeyed to and fro between burstow and greenwich we know; and we may take it as certain that at a time when the standard of clerical efficiency was extremely low, he was not one of those who 'for their bellies' sake, creep and intrude and climb into the fold.' his chief source of income, however, seems to have been the private pupils whom he took in mathematics and astronomy. these numbered in the years to no fewer than ; and as many of them were of the very first and wealthiest families in the kingdom, the gain to flamsteed in money and influence must have been considerable. but it was most distasteful work. it was in no sense that which he felt to be his duty, and which he had at heart. it was undertaken from sheer, hard necessity, and he grudged bitterly the time and strength which it diverted from his proper calling. how faithfully he followed that, one single circumstance will show. in the thirteen years ending , he made , observations, and had revised single-handed the whole of the theories and tables of the heavenly bodies then in use. in the death of his father brought him a considerable accession of means, and, far more important, the assistance of abraham sharp,[ ] the first and most distinguished of the long list of greenwich assistants, men who, though far less well known than the astronomers royal, have contributed scarcely less in their own field to the high reputation of the observatory. [ ] abraham sharp had been with flamsteed earlier than this--in and . sharp was not only a most careful and indefatigable calculator, he was what was even more essential for flamsteed--a most skilful instrument-maker; and he divided for him a new mural arc of ° and seven feet radius, with which he commenced operations on december , . above all, sharp became his faithful and devoted friend and adherent, and no doubt his sympathy strengthened flamsteed to endure the trouble which was at hand. that trouble began in , when newton visited the royal observatory. at that time flamsteed, though he had done so much, had published nothing, and newton, who had made his discovery of the laws of gravitation some few years before, was then employed in deducing from them a complete theory of the moon's motion. this work was one of absolutely first importance. in the first place and chiefly, upon the success with which it could be carried out, depended undoubtedly the acceptance of the greatest discovery which has yet been made in physical science. secondarily--and this should, and no doubt did, appeal to flamsteed--the perfecting of our knowledge of the movements of the moon was a primary part of the very work which he was commissioned to do as astronomer royal. newton was, therefore, anxious beyond everything to receive the best possible observations of the moon's places, and he came to flamsteed, as to the man from whom he had a right to expect to receive a supply of them. at first flamsteed seems to have given these as fully as he was able; but it is evident that newton chafed at the necessity for these frequent applications to flamsteed, and to the constant need of putting pressure upon him. flamsteed, on the other hand, as clearly evidently resented this continual demand. feeling, as he keenly did, that, though he had been named astronomer royal, he had been left practically entirely without support; his instruments were entirely his own, either made or purchased by himself; his nominal salary of £ was difficult to get, and did not nearly cover the actual current expenses of his position, he not unnaturally regarded his observations as his own exclusive property. he had a most natural dislike for his observations to be published, except after such reduction as he himself had carried through, and in the manner which he himself had chosen. the idea which was ever before him was that of carrying out a single great work that should not only be a monument to his own industry and skill, but should also raise the name of england amongst scientific nations. he complained of it, therefore, both as a personal wrong and an injury to the country when some observations of cassini's were combined with some observations of his own in order to deduce a better orbit for a comet. unknown to himself, therefore, he was called upon to decide a question that has proved fundamental to the policy of greenwich observatory, and he decided it wrongly--the question of publication. newton had urged upon him as early as that he should not wait until he had formed an exhaustive catalogue of all the brighter stars, but that he should publish at once a catalogue of a few, which might serve as standards; but flamsteed would not hear of it. he failed to see that his office had been created for a definite practical purpose, not for the execution of some great scheme, however important to science. all his work of thirty years had done nothing to forward navigation so long as he published nothing. but if, year by year, he had published the places of the moon and of a few standard stars, he would have advanced the art immensely and yet have not hindered himself from eventually bringing out a great catalogue. no doubt the little incident of newton's difficulty with the microscope, of which he had forgotten the object-glass, had given flamsteed a low opinion of newton's qualifications as a practical astronomer. if so, he was wrong, for newton's insight into practical matters was greater than flamsteed's own, and his practical skill was no less, though his absent-mindedness might occasionally lead him into an absurd mistake. the following extract from flamsteed's own 'brief history of the observatory' gives an account of his view of newton's action towards him in desiring the publication of his star catalogue, and at the same time it illustrates flamsteed's touchy and suspicious nature. 'whilst mr. flamsteed was busied in the laborious work of the catalogue of the fixed stars, and forced often to watch and labour by night, to fetch the materials for it from the heavens, that were to be employed by day, he often, on sir isaac newton's instances, furnished him with observations of the moon's places, in order to carry on his correction of the lunar theory. a civil correspondence was carried on between them; only mr. flamsteed could not but take notice that as sir isaac was advanced in place, so he raised himself in his conversation and became more magisterial. at last, finding that mr. flamsteed had advanced far in his designed catalogue by the help of his country calculators, that he had made new lunar tables, and was daily advancing on the other planets, sir isaac newton came to see him (tuesday, april , ); and desiring, after dinner, to be shown in what forwardness his work was, had so much of the catalogue of the fixed stars laid before him as was then finished; together with the maps of the constellations, both those drawn by t. weston and p. van somer, as also his collation of the observed places of saturn and jupiter, with the rudolphine numbers. having viewed them well, he told mr. flamsteed he would (_i.e._ he was desirous to) recommend them to the prince _privately_. mr. flamsteed (who had long been sensible of his partiality, and heard how his two flatterers cried sir isaac's performances up, was sensible of the snare in the word _privately_) answered that would not do; and (upon sir isaac's demanding "why not?") that then the prince's attendants would tell him these were but curiosities of no great use, and persuade him to save that expense, that there might be the more for them to beg of him: and that the recommendation must be made _publicly_, to prevent any such suggestions. sir isaac apprehended right, that he was understood, and his designs defeated: and so took his leave not well satisfied with the refusal. 'it was november following ere mr. flamsteed heard from him any more: when, considering with himself that what he had done was not well understood, he set himself to examine how many folio pages his work when printed would fill; and found upon an easy computation that they would at least take up . being amazed at this, he set himself to consider them more seriously; drew up an estimate of them; and, to obviate the misrepresentations of dr. s[loane] and some others, who had given out that what he had was inconsiderable, he delivered a copy of the estimate to mr. hodgson, then lately chosen a member of the royal society, with directions to deliver it to a friend, who he knew would do him justice; and, on this fair account, obviate those unjust reports which had been studiously spread to his prejudice. it happened soon after, mr. hodgson being at a meeting, spied this person there, at the other side of the room; and therefore gave the paper to one that stood in some company betwixt them, to be handed to him. but the gentleman, mistaking his request, handed to the secretary [dr. sloane], who, being a physician, and not acquainted with astronomical terms, did not read it readily. whereupon another in the company took it out of his hands; and, having read it distinctly, desired that the works therein mentioned might be recommended to the prince; the charge of printing them being too great either for the author or the royal society. sir isaac closed in with this.' [illustration: the 'camera stellata' in flamsteed's time. (_from an engraving in the 'historia coelestis.'_)] the work was in consequence recommended to prince george of denmark, the queen's consort; but it was not till november , , that the contract for the printing was signed. two years later, the observations which he had made with his sextant in his first thirteen years of office were printed. then came the difficulty of the catalogue. it was not complete to flamsteed's satisfaction, and he was most unwilling to let it pass out of his hands. however, two manuscripts, comprising some three-quarters of the whole, were deposited with referees, the first of these being sealed up. the seal was broken with flamsteed's concurrence; but the fact that it had been so broken was made by him the subject of bitter complaint later. at this critical juncture prince george died, and a stop was put to the progress of the printing. two years more elapsed without any advance being made, and then, in order to check any further obstruction, a committee of the royal society was appointed as a board of visitors to visit and inspect the observatory, and so maintain a control over the astronomer royal. this was naturally felt by so sensitive a man as flamsteed as a most intolerable wrong, and when he found that the printing of his catalogue had been placed in the hands of halley as editor, a man for whom he had conceived the most violent distrust, he absolutely refused to furnish the visitors with any further material. this led to, perhaps, the most painful scene in the lives either of newton or flamsteed. flamsteed was summoned to meet the council of the royal society at their rooms in crane court. a quorum was not present, and so the interview was not official, and no record of it is preserved in the archives. flamsteed has himself described it with great particularity in more than one document, and it is only too easy to understand the scene that took place. newton was a man who had an absolutely morbid dread of anything like controversy, and over and over again would have preferred to have buried his choicest researches, rather than to have encountered the smallest conflict of the kind. he was perhaps, therefore, the worst man to deal with a high-principled, sensitive, and obstinate man who was in the wrong, and yet who had been so hardly dealt with that it was most natural for him to think himself wholly in the right. flamsteed adhered absolutely to his position, from which it is clear it would have been extremely difficult for the greatest tact and consideration to have dislodged him. newton, on his part, simply exerted his authority, and, that failing, was reduced to the miserable extremity of calling names. the scene is described by flamsteed himself, in a letter to abraham sharp, as follows:-- 'i have had another contest with the president[ ] of the royal society, who had formed a plot to make my instruments theirs; and sent for me to a committee, where only himself and two physicians (dr. sloane, and another as little skilful as himself) were present. the president ran himself into a great heat, and very indecent passion. i had resolved aforehand his kn--sh talk should not move me; showed him that all the instruments in the observatory were my own; the mural arch and voluble quadrant having been made at my own charge, the rest purchased with my own money, except the sextant and two clocks, which were given me by sir jonas moore, with mr. towneley's micrometer, his gift, some years before i came to greenwich. this nettled him; for he has got a letter from the secretary of state for the royal society to be visitors of the observatory, and he said, "_as good have no observatory as no instruments_." i complained then of my catalogue being printed by raymer, without my knowledge, and that i was _robbed of the fruit of my labours_. at this he fired, and called me all the ill names, puppy, etc., that he could think of. all i returned was, i put him in mind of his passion, desired him to govern it, and keep his temper: this made him rage worse, and he told me how much i had received from the government in thirty-six years i had served. i asked what he had done for the £ per annum that he had received ever since he had settled in london. this made him calmer; but finding him going to burst out again, i only told him my catalogue, half finished, was delivered into his hands, on his own request, sealed up. he could not deny it, but said dr. arbuthnott had procured the queen's order for opening it. this, i am persuaded, was false; or it was got after it had been opened. i said nothing to him in return; but, with a little more spirit than i had hitherto showed, told them that god (who was seldom spoken of with due reverence in that meeting) had hitherto prospered all my labours, and i doubted not would do so to a happy conclusion; took my leave and left them. dr. sloane had said nothing all this while; the other doctor told me i was proud, and insulted the president, and ran into the same passion with the president. at my going out, i called to dr. sloane, told him he had behaved himself civilly, and thanked him for it. i saw raymer after, drank a dish of coffee with him, and told him, still calmly, of the villany of his conduct, and called it _blockish_. since then they let me be quiet; but how long they will do so i know not, nor am i solicitous.' [ ] sir isaac newton. the visitors continued the printing, halley being the editor, and the work appeared in under the title of _historia coelestis_. this seemed to flamsteed the greatest wrong of all. the work as it appeared seemed to him so full of errors, wilfully or accidentally inserted, as to be the greatest blot upon his fair fame, and he set himself, though now an old man, to work it out _de novo_ and at his own expense. to that purpose he devoted the remaining seven years of his life. few things can be more pathetic than the letters which he wrote in that period referring to it. he was subject to the attacks of one of the cruelest of all diseases--the stone; he was at all times liable to distracting headaches. he had been, from his boyhood, a great sufferer from rheumatism, and yet, in spite of all, he resolutely pushed on his self-appointed task. the following extract from one of his letters will give a more vivid idea of the brave old man than much description:-- 'i can still, i praise god for it, walk from my door to the blackheath gate and back, with a little resting at some benches i have caused to be set up betwixt them. but i found myself so tired with getting up the hill when i return from church, that at last i have bought a sedan, and am carried thither in state on sunday mornings and back; i hope i may employ it in the afternoons, though i have not hitherto, by reason of the weather is too cold for me.' after the death of queen anne, a change in the ministry enabled him to secure that three hundred copies of the total impression of four hundred of the _historia coelestis_ were handed over to him. these, except the first volume, containing his sextant observations (which had received his own approval), he burned, 'as a sacrifice to heavenly truth.' his own great work had advanced so far that the first volume was printed, and much of the second, when he himself died, on the last day of . he was buried in the chancel of burstow church. the completion of his work took ten years more; a work of piety and regard on the part of his assistant, joseph crosthwait. when compared with the catalogues that have gone before, it was a work of wonderful accuracy. nevertheless, as caroline herschel showed, nearly a century later, not a few errors had crept into it. some of the stars are non-existent, others have been catalogued in more than one constellation; important stars have been altogether omitted. perhaps the most serious fault arises from the neglect of flamsteed to accept from newton a practical hint, namely, to read the barometer and thermometer at the time of his observations. nevertheless, the work accomplished was not only wonderful under the untoward conditions in which flamsteed was placed; it was wonderful in itself, winning from airy the following high encomium:-- 'in regard not only to accuracy of observation, and to detail in publication of the methods of observing, but also to steadiness of system followed through many years, and to completeness of calculation of the useful results deduced from the observations, this work may shame any other collection of observations in this or any other country.' this catalogue was not flamsteed's only achievement. he had determined the latitude of the observatory, the obliquity of the ecliptic, and the position of the equinoctial points. he thought out an original method of obtaining the absolute right ascensions of stars by differential observations of the places of the stars and the sun near to both equinoxes. he had revised and improved horrox's theory of the lunar motions, which was by far the best existing in flamsteed's day. he showed the existence of the long inequality of jupiter and saturn; that is to say, the periodic influence which they exercise upon each other. he determined the time in which the sun rotates on its axis, and the position of that axis. he observed an apparent movement of the stars in the course of a year, which he ascribed, though erroneously, to the stellar parallax, and which was explained by the third astronomer royal, bradley. flamsteed not only met with harsh treatment during his lifetime; he has not yet received, except from a few, anything like the meed of appreciation which is his just due; but, at least, his successors in the office have not forgotten him. they have been proud that their official residence should be known as flamsteed house, and his name is inscribed over the main entrance of the latest and finest of the observatory buildings, and his bust looks forth from its front towards the home where he laboured so devotedly for nearly fifty years. but he has received little honour, save at greenwich, and--in spite of the proverb--in his other home, the village of burstow, in surrey, of which he was for many years the rector. here a stained glass window representing, appropriately, the adoration of the magi, has been recently set up to his memory, largely through the interest taken in his history by an amateur astronomer of the neighbourhood, mr. w. tebb, f.r.a.s. no instrument of flamsteed's remains in the observatory, his wife removing them after his death. but we may consider his principal instrument, the mural quadrant made for him by abraham sharp, as represented by the remains of a quadrant by the same artist, which was presented to the observatory by the rev. n. s. heineken, in , and now hangs over the door of the transit room. chapter iii halley and his successors there is no need to give the lives of the succeeding astronomers royal so fully as that of flamsteed. not that they were inferior men to him; on the contrary, there can be little doubt that we ought to reckon some of them as his superiors, but, in the case of several, their best work was done apart from greenwich observatory, and before they came to it. this was particularly the case with edmund halley. born on october , , he was ten years the junior of flamsteed. like flamsteed, he came of a derbyshire family, though he was born at haggerston, in the parish of st. leonard's, shoreditch. he was educated at st. paul's school, where he made very rapid progress, and already showed the bent of his mind. he learnt to make dials; he made himself so thoroughly acquainted with the heavens that it is said, 'if a star were displaced in the globe he would presently find it out,' and he observed the changes in the direction of the mariner's compass. in he went to queen's college, oxford, where he observed a sunspot in july and august, , and an occultation of mars. this was not his first astronomical observation, as, in june, , he had observed an eclipse of the moon from his father's house in winchester street. [illustration: edmund halley. (_from an old print._)] a much wider scheme of work than such merely casual observations now entered his mind, possibly suggested to him by flamsteed's appointment to the direction of the new royal observatory. this was to make a catalogue of the southern stars. tycho's places for the northern stars were defective enough, but there was no catalogue at all of stars below the horizon of tycho's observatory. here, then, was a field entirely unworked, and young halley was so eager to enter upon it that he would not wait at oxford to obtain his degree, but was anxious to start at once for the southern hemisphere. his father, who was wealthy and proud of his gifted son, strongly supported him in his project. the station he selected was st. helena, an unfortunate choice, as the skies there were almost always more or less clouded, and rain was frequent during his stay. however, he remained there a year and a half, and succeeded in making a catalogue of stars. this catalogue was finally reduced by sharp, and included in the third volume of flamsteed's _historia coelestis_. in he was elected fellow of the royal society, and the following year he was chosen to represent that society in a discussion with hevelius. the question at issue was as to whether more accurate observations of the place of a star could be obtained by the use of sights without optical assistance, or by the use of a telescope. the next year he visited the paris observatory, and, later in the same tour, the principal cities of the continent. not long after his return from this tour, halley was led to that undertaking for which we owe him the greatest debt of gratitude, and which must be regarded as his greatest achievement. some fifty years before, the great kepler had brought out the third of his well-known laws of planetary motion. these laws stated that the planets move round the sun in ellipses, of which the sun occupies one of the foci; that the straight line joining any planet with the sun moves over equal areas of space in equal periods of time; and, lastly, that the squares of the times in which the several planets complete a revolution round the sun are proportional to the cubes of their mean distances from it. these three laws were deduced from actual examination of the movements of the planets. kepler did not work out any underlying cause of which these three laws were the consequence. but the desire to find such an underlying cause was keen amongst astronomers, and had given rise to many researches. amongst those at work on the subject was halley himself. he had seen, and been able to prove, that if the planets moved in circles round the sun, with the sun in the centre, then the law of the relation between the times of revolution and the distances of the planets would show that the attractive force of the sun varied inversely as the square of the distance. the actual case, however, of motion in an ellipse was too hard for him, and he could not deal with it. halley therefore went up to cambridge to consult newton, and, to his wonder and delight, found that the latter had already completely solved the problem, and had proved that kepler's three laws of planetary motion were summed up in one, namely, that the sun attracted the planets to it with a force inversely proportional to the square of the distance. halley was most enthusiastic over this great discovery, and he at once strongly urged newton to publish it. newton's unwillingness to do so was great, but at length halley overcame his reluctance; and the royal society not being able at the time to afford the expense, halley took the charges upon himself, although his own resources had been recently seriously damaged by the death of his father. the publication of newton's _principia_, which, but for him, might never have seen the light, and most certainly would have been long delayed, is halley's highest claim to our gratitude. but, apart from this, his record of scientific achievement is indeed a noble one. always, from boyhood, he had taken a great interest in the behaviour of the magnetic compass, and he now followed up the study of its variations with the greatest energy. for this purpose it was necessary that he should travel, in view of the great importance of the subject to navigation. king william iii. gave him a captain's commission in the royal navy--a curious and interesting illustration of the close connection between astronomy and the welfare of our navy--and placed him in command of a 'pink,' that is to say, a small vessel with pointed stern, named the paramour, in which he proceeded to the southern ocean. his first voyage was unfortunate, but the paramour was recommissioned in , and he sailed in it as far as south latitude °. in and the succeeding year he made further voyages in the paramour, surveying the tides and coasts of the british channel and of the adriatic, and helping in the fortification of trieste. he became savilian professor of geometry at oxford in , having failed twelve years previously to secure the savilian professorship of astronomy, mainly through the opposition of flamsteed, who had already formed a strong prejudice against him, which some writers have traced to halley's detection of several errors in one of flamsteed's tide-tables, others to halley's supposed materialistic views. probably the difference was innate in the two men. there was likely to be but little sympathy between the strong, masterful man of action and society and the secluded, self-conscious, suffering invalid. at any rate, in the contest between newton and flamsteed, which has been already described, halley took warmly the side of the former, and was appointed to edit the publication of flamsteed's results, and, on the death of the latter, to succeed him at the royal observatory. the condition of things at greenwich when halley succeeded to the post of astronomer royal in was most discouraging. the instruments there had all belonged to flamsteed, and therefore, most naturally, had been removed by his widow. the observatory had practically to be begun _de novo_, and halley had now almost attained the age at which in the present day an astronomer royal would have to retire. more fortunate, however, than his predecessor, he was able to get a grant for instruments, and he equipped the observatory as well as the resources of the time permitted, and his transit instrument and great eight-foot quadrant still hang upon the observatory walls. as astronomer royal his great work was the systematic observation of the positions of the moon through an entire _saros_. as is well known, a period of eighteen years and ten or eleven days brings the sun and moon very nearly into the same positions relatively to the earth which they occupied at the commencement of the period. this period was well known to the ancient chaldeans, who gave it its name, since they had noticed that eclipses of the sun or eclipses of the moon recurred at intervals of the above length. it was halley's desire to obtain such a set of observations of the moon through an entire _saros_ period as to be able to deduce therefrom an improved set of tables of the moon's motion. it was an ambitious scheme for a man so much over sixty to undertake, nevertheless he carried it through successfully. his desire to complete this scheme, and to found upon it improved lunar tables, hindered him from publishing his observations, for he feared that others might make use of them before he was in a position to complete his work himself. this omission to publish troubled newton, who, as president of the royal society--the greenwich board of visitors having lapsed at queen anne's death--drew attention at a meeting of the royal society, march , , to halley's disobedience of the order issued under queen anne, for the prompt communication of the observatory results. that newton should thus have put public pressure upon halley, the man to whom he was so much indebted, and with whom there was so close an affection, is sufficient proof that his similar attitude towards flamsteed was one of principle and not of arbitrariness. halley, on his side, stood firm, as flamsteed had done, urging the danger that, by publishing before he had completed his task, he might give an opportunity to others to forestall his results. it is said--probably without sufficient ground--that this refusal broke newton's heart and caused his death. certainly halley's writings in that very year show his reverence and affection for newton to have been as keen and lively as ever. halley's work at the observatory went on smoothly, on the lines he had laid down for himself, for ten years after newton's death; but in he had a stroke of paralysis, and his health, which had been remarkably robust up to that time, began to give way. he died january , , and was buried in the cemetery of lee church. as an astronomer, his services to the science rank higher than those of his predecessor; but as astronomer royal, as director, that is to say, of greenwich observatory, he by no means accomplished as much as flamsteed had done. professor grant, in his _history of physical astronomy_, says that he seems to have undervalued those habits of minute attention which are indispensable to the attainment of a high degree of excellence in the practice of astronomical observation. he was far from being sufficiently careful as to the adjustment of his instruments, the going of his clocks, or the recording of his own observations. the important feature of his administration was that under him the observatory was first supplied with instruments which belonged to it. [illustration: halley's quadrant. (_from an old print._)] his astronomical work apart from the observatory was of the first importance. he practically inaugurated the study of terrestrial magnetism, and his map giving the results of his observations during his voyage in the paramour introduced a new and most useful style of recording observations. he joined together by smooth curves places of equal variation, the result being that the chart shows at a glance, not merely the general course of the variation over the earth's surface, but its value at any spot within the limits of the chart. another work which has justly made his name immortal was the prediction of the return of the comet which is called by his name, to which reference will be made later. another great scheme, and one destined to bear much fruit, was the working out of a plan to determine the distance of the sun by observations of the transit of venus. of attractive appearance, pleasing manners, and ready wit, loyal, generous, and free from self-seeking, he probably was one of the most personally engaging men who ever held the office. the salary of the astronomer royal remained under halley at the same inadequate rate which it had done under flamsteed--£ , without provision for an assistant. but in queen caroline, learning that halley had actually had a captain's commission in the royal navy, secured for him a post-captain's pay. [illustration: james bradley. (_from the painting by hudson._)] halley's work is represented at the observatory by two of his instruments which are still preserved there, and which hang on the west wall of the present transit room: the iron quadrant afterwards made famous by the observations of bradley, and 'halley's transit,' the first of the great series of instruments upon which the fame of greenwich chiefly rests. this transit instrument seems to have been set up in a small room at the west end of what is now known as the north terrace. his quadrant was mounted on the pier which is now the base of the pier of the astrographic telescope. this pier was the first extension which the observatory received from the original building. on the breakdown of his health halley nominated as his successor, james bradley; indeed, it is stated that he offered to resign in his favour. he had known him then for over twenty years, and that keen and generous appreciation of merit in others which was characteristic of halley had led him very early to recognize bradley's singular ability. * * * * * james bradley was born in or , of an old north of england family. his birthplace was sherbourne, in gloucestershire, and he was educated at north leach grammar school and at baliol college, oxford. during the years of his undergraduateship he resided much with his uncle, the rev. james pound, rector of wanstead, essex, an ardent amateur astronomer, a frequent visitor at the observatory in flamsteed's time, and one of the most accurate observers in the country. from him, no doubt, he derived his love of the science, and possibly some of his skill in observation. bradley's earliest observations seem to have been devoted to the phenomena of jupiter's satellites and to the measures of double stars. the accuracy with which he followed up the first drew the attention of halley, and so began a friendship which lasted through life. his observations of double stars, particularly of castor, only just failed to show him the orbital movement of the pair, because his attention was drawn to other subjects before it had become sufficiently obvious. in bradley and his uncle made an attempt to determine the distance of the sun through observations of mars when in opposition, observations which were so accurate that they sufficed to show that the distance of the sun could not be greater than millions of miles, nor less than about millions. the lower limit which they thus found has proved to be almost exactly correct, our best modern determinations giving it as millions. the instrument with which the observations were made was a novel one, being 'moved by a machine that made it to keep pace with the stars;' in other words, it was the first, or nearly the first, example of what we should now call a clock-driven equatorial. that same year he was offered the vicarage of bridstow, near ross, in monmouthshire, where, having by that time taken priest's orders, he was duly installed, july, . to this was added the sinecure rectory of llandewi-velgry; but he held both livings only a very short time. in the death of dr. john keill rendered vacant the savilian professorship of astronomy at oxford, for which bradley became a candidate, and was duly elected, and resigned his livings in consequence. it was whilst he was savilian professor that bradley made that great discovery which will always be associated with his name. though professor at oxford, he had continued to assist his uncle, mr. pound, at his observations at wanstead, and after the death of the latter he still lived there as much as possible, and continued his astronomical work. but in he was invited by mr. samuel molyneux, who had set up a twenty-four-foot telescope made by graham as a zenith tube at his house on kew green, to verify some observations which he was making. these were of the star gamma draconis, a star which passes through the zenith of london, and which, therefore, had been much observed both by flamsteed and hooke, inasmuch as by fixing a telescope in an absolutely vertical position--a position which could be easily verified--it was easy to ascertain if there was any minute change in the apparent position of the star. dr. hooke had declared that there was such a change, a change due to the motion of the earth in its orbit, which would prove that the star was not an infinite distance from the earth, the seeming change of its place in the sky corresponding to the change in the place of the earth from which the observer was viewing it. bradley found at once that there was such a change--a marked one. it amounted to as much as ´´ of arc in three days; but it was not in the direction in which the parallax of the star would have moved it, but in the opposite. whether, therefore, the star was near enough to show any parallax or not, some other cause was giving rise to an apparent displacement of the star, which entirely masked and overcame the effect of parallax. so far, bradley had but come to the same point which flamsteed had reached. flamsteed had detected precisely the same apparent displacement of stars, and, like hooke, had ascribed it to parallax. cassini had shown that this could not be the case, as the displacement was in the wrong direction; and there the matter had rested. bradley now set to follow the question up. other stars beside gamma draconis were found to show a displacement of the same general character, but the amount varied with their distance from the plane of the ecliptic, the earth's orbit. the first explanation suggested was that the axis of the earth, which moves very nearly parallel to itself as the earth moves round the sun, underwent a slight regular 'wobble' in the course of a year. to check this, a star was observed on the opposite side of the pole from gamma draconis; then bradley investigated as to whether refraction might explain the difficulty, but again without success. he now was most keenly interested in the problem, and he purchased a zenith telescope of his own, made, like that of molyneux, by graham, and mounted it in his aunt's house at wanstead, and observed continuously with it. the solution of the problem came at last to him as he was boating on the thames. watching a vane at the top of the mast, he saw with surprise that it shifted its direction every time that the boat was put about. remarking to the boatmen that it was very odd that the wind should change just at the same moment that there was a shift in the boat's course, they replied that there was no change in the wind at all, and that the apparent change of the vane was simply due to the change of direction of the motion of the boat. [illustration: graham's zenith sector. (_from an old print._)] this supplied bradley with a key to the solution of the mystery that had troubled him so long. it had been discovered long before this that light does not travel instantaneously from place to place, but takes an appreciable time to pass from one member of the solar system to another. this had been discovered by römer from observations of the satellites of jupiter. he had noted that the eclipses of the satellites always fell late of the computed time, when jupiter was at his greatest distance from the earth; and bradley's own work in the observation of those satellites had brought the fact most intimately under his own acquaintance. the result of the boating incident taught him, then, that he might look upon light as analogous to the wind blowing on the boat. as the wind, so long as it was steady, would seem to blow from one fixed quarter so long as the boat was also in rest, but as it seemed to shift its direction when the boat was moving and changed its direction, so he saw that the light coming from a particular star must seem to slightly change the direction in which it came, or, in other words, the apparent position of the star, to correspond with the movement of the earth in its orbit round the sun. this was the celebrated discovery of the aberration of light, a triumph of exact observation and of clear insight. as to the exactness of bradley's observations, it is sufficient to say that his determination of the value of the 'constant of aberration' gave it as · ´´; the value adopted to-day is · ´´. on the death of halley, in , bradley was appointed to succeed him. he found the observatory in as utterly disheartening a condition as his predecessors had done. as already mentioned, halley had not the same qualifications as an observer that flamsteed had. he was, further, an old man when appointed to the post, he had no assistant provided for him, and the last five years of his life his health and strength had entirely given way. under these circumstances, it was no wonder that bradley found the instruments of the observatory in a deplorable state. nevertheless, he set to work most energetically, and in the year of his appointment he made observations in the last five months of the year. he was particularly earnest in examining the condition and the errors of his instruments; and as their defects became known to him, he was more and more anxious for a better equipment. he moved the royal society, therefore, to apply on his behalf for the instruments he required; and a petition from that body, in , obtained what in those days must be considered the generous grant of £ , the proceeds of the sale of old admiralty stores. the principal instruments purchased therewith were a mural quadrant and a transit instrument, both eight feet in focal length, still preserved on the walls of the transit-room. it is interesting also to note that, following in the steps of halley, and forecasting, as it were, the magnetic observatory which airy would found, he devoted £ of the grant to purchasing magnetic instruments. meantime he had continued his observations on aberration, and had discovered that the aberration theory was not sufficient entirely to account for the apparent changes in places of stars which he had discovered. a second cause was at work, a movement of the earth's axis, a 'wobble' in its inclination, technically known as nutation, which is due to the action of the moon, and goes through its course in a period of nineteen years. beside these two great discoveries of aberration and nutation, bradley's reputation rests upon his magnificent observations of the places of more than three thousand stars. this part of his work was done with such thoroughness, that the star-places deduced from them form the basis of most of our knowledge as to the actual movements of individual stars. in particular, he was careful to investigate and to correct for the errors of his instrument, and to determine the laws of refraction, introducing corrections for changes in the readings of thermometer and barometer. his tables of refraction were used, indeed, for seventy years after his death. of his other labours it may be sufficient to refer to his determination of the longitudes of lisbon and of new york, and to his effort to ascertain the parallax of the sun and moon, in combination with la caille, who was observing at the cape of good hope. as astronomer royal, bradley's great achievement was the high standard to which he raised the practical work of observation. from his day onwards, also, there was always at least one assistant. his first assistant was his own nephew, john bradley, who received the munificent salary of ten shillings a week. still, this was not out of proportion to the then salary of the astronomer royal, which practically amounted only to £ . however, in , bradley was awarded a crown pension of £ a year. he refused the living of greenwich, which was offered him in order to increase his emoluments, on the ground that he could not suitably fulfil the double office. bradley's later assistants were charles mason and charles green. bradley's last work was the preparation for the observations of the transit of venus of , according to the lines laid down by his predecessor, halley. his health gave way, and he became subject to melancholia, so that the actual observations were taken by the rev. nathaniel bliss, who succeeded him in his office after his death, in . he was buried at minchinhampton. so far as we know bradley's character, he seems to have been a gentle, modest, unassuming man, entirely free from self-seeking, and indifferent to personal gain. he was in many ways an ideal astronomer, exact, methodical, and conscientious to the last degree. his skill as an observer was his chief characteristic; and though his abilities were not equal as a mathematician or a mechanician, yet, on the one hand, he had a very clear insight into the meaning of his observations, and, on the other, he was skilful enough to himself adjust, repair, and improve his instruments. of bradley's instruments, there are still preserved his famous twelve-and-a-half-foot zenith sector, with which he made his two great discoveries; his brass quadrant, which in he substituted for halley's iron quadrant; his transit instrument, and equatorial sector. bradley added to the buildings of the observatory that portion which is now represented by the upper and lower computing rooms, and the chronometer room, which adjoins the latter. this room--the chronometer room--was his transit room, and the position of the shutters is still marked by the window in the roof. * * * * * the rev. nathaniel bliss, who succeeded bradley, only held the office for a couple of years, and during that time was much at oxford. he, therefore, has left no special mark behind him as astronomer royal. he was born november , . his father, like himself, nathaniel bliss, was a gentleman, of bisley, gloucestershire. [illustration: nathaniel bliss. (_from an engraving on an old pewter flagon._)] bliss graduated at pembroke college, oxford, as b.a. in , and m.a. in . he became the rector of st. ebb's, oxford, in , and on halley's death succeeded him as savilian professor of geometry. he supplied bradley with his observations of jupiter's satellites, and from time to time, at his request, rendered him some assistance at the royal observatory. this was particularly the case, as has been already mentioned, with respect to the transit of venus of , the observations of which were carried out by bliss, owing to bradley's ill-health. it was natural, therefore, that on bradley's death he should succeed to the vacant post; but he held it too short a time to do any distinctive work. such observations as he made seem to have been entirely in continuation of bradley's. he took a great interest, however, in the improvement of clocks, a department in which so much was being done at this time by graham, ellicott, and others. * * * * * nevil maskelyne, the fifth astronomer royal, was, like bliss, a close friend of bradley's. he was the third son of a wealthy country gentleman, edmund maskelyne, of purton, in wiltshire. maskelyne was born in london, october , , and was educated at westminster school. thence he proceeded to cambridge, where he graduated seventh wrangler in . he was ordained to the curacy of barnet in , and, twenty years later, was presented by his nephew, lord clive, to the living of shrawardine, in shropshire. in he was presented by his college to the rectory of north runcton, norfolk. the event which turned his thoughts in the direction of astronomy was the solar eclipse of july , ; and about the time that he was appointed to the curacy of barnet he became acquainted with bradley, then the astronomer royal, to whom he gave great assistance in the preparation of his table of refractions. like halley before him, he made an astronomical expedition to the island of st. helena. this was for the special purpose of observing the transit of venus of june , , bradley having induced the royal society to send him out for that purpose. here he stayed ten months, and made many observations. but though the transit of venus was his special object, it was not the chief result of the expedition: not because clouds hindered his observations, but because the voyage gave him the especial bent of his life. halley had actually held a captain's commission in the royal navy, and commanded a ship; maskelyne, more than any of the astronomers royal before or since, made the improvement of the practical business of navigation his chief aim. none of all the incumbents of the office kept its original charter--'to find the so much desired longitude at sea, for the perfecting the art of navigation,' so closely before him. the solution of the problem was at hand at this time--its solution in two different ways. on the one hand, the offer by the government of a reward of £ , for a clock or watch which should go so perfectly at sea, notwithstanding the tossing of the ship and the wide changes of temperature to which it might be exposed, that the navigator might at any moment learn the true greenwich time from it, had brought out the invention of harrison's time-keeper; on the other hand, the great improvement that had now taken place in the computation of tables of the moon's motion, and the more accurate star-catalogues now procurable, had made the method of 'lunars,' suggested a hundred and thirty years before by the frenchman, morin, and others, a practicable one. [illustration: nevil maskelyne.] in principle, the method of finding the longitude from 'lunars,' that is to say, from measurements of the distances between the moon and certain stars, is an exceedingly simple one. in actual practice, it involves a very toilsome calculation, beside exact and careful observation. the principle, as already mentioned, is simply this: the moon travels round the sky, making a complete circuit of the heavens in between twenty-seven and twenty-eight days. it thus moves amongst the stars, roughly speaking, its own diameter, in about an hour. when once its movements were sufficiently well known to be exactly predicted, almanacs could be drawn up in which the greenwich time of its reaching any definite point of the sky could be predicted long beforehand; or, what comes to the same thing, its distances from a number of suitable stars could be given for definite intervals of greenwich time. it is only necessary, then, to measure the distances between the moon and some of these stars, and by comparing them with the distances given in the almanac, the exact time at greenwich can be inferred. as has been already pointed out, the determination of the latitude of the ship and of the local time at any place where the ship is, is not by any means so difficult a matter; but the local time being known and the greenwich time, the difference between these gives the longitude; and the latitude having been also ascertained, the exact position of the ship is known. there are, of course, difficulties in the way of working out this method. one is, that whilst it takes the sun but twenty-four hours to move round the sky from one noon to the next, and consequently its movements, from which the local time is inferred, are fairly rapid, the moon takes nearly twenty-eight days to move amongst the stars from the neighbourhood of one particular star round to that particular star again. consequently, it is much easier to determine the local time with a given degree of exactness than the greenwich time; it is something like the difference of reading a clock from both hands and from the hour hand alone. there are other difficulties in the case which make the computation a long and laborious one, and difficult in that sense; but they do not otherwise affect its practicability. during this voyage to st. helena, both when outward bound and when returning, maskelyne gave the method of 'lunars' a very thorough testing, and convinced himself that it was capable of giving the information required. for by this time the improvement of the sextant, or quadrant as it then was, by the introduction of a second mirror, by hadley, had rendered the actual observation at sea of lunar distances, and of altitudes generally, a much more exact operation. this conclusion he put at once to practical effect, and, in , he published the _british mariner's guide_, a handbook for the determination of the longitude at sea by the method of lunars. at the same time, the other method, that by the time-keeper or chronometer, was practically tested by him. the time-keeper constructed by john harrison had been tested by a voyage to jamaica in , and now, in , another time-keeper was tested in a voyage to barbadoes. charles green, the assistant at greenwich observatory, was sent in charge of the chronometer, and maskelyne went with him to test its performance, in the capacity of chaplain to his majesty's ship louisa. [illustration: hadley's quadrant. (_from an old print._)] the position which maskelyne had already won for himself as a practical astronomer, and the intimate relations into which he had entered with bradley and bliss, made his appointment to the astronomer royalship, on the death of the latter, most suitable. at once he bent his mind to the completion of the revolution in nautical astronomy which his _british mariner's guide_ had inaugurated, and in the year after his appointment he published the first number of the _nautical almanac_, together with a volume entitled, _tables requisite to be used with the nautical ephemeris_, the value of which was so instantly appreciated, that , copies were sold at once. the _nautical almanac_ was maskelyne's greatest work, and it must be remembered that he carried it on from this time up to the day of his death--truly a formidable addition to the routine labours of an astronomer royal who had but a single assistant on his staff. the _nautical almanac_ was, however, in the main not computed at the observatory; the calculations were effected by computers living in different parts of the country, the work being done in duplicate, on the principle which flamsteed had inaugurated in the preparation of his _historia coelestis_. maskelyne's next service to science was almost as important. he arranged that the regular and systematic publication of the observations made at greenwich should be a distinct part of the duties of an astronomer royal, and he procured an arrangement by which a special fund was set apart by the royal society for printing them. his observations covering the years to fill four large folio volumes, and though, as already stated, he had but one assistant, they are , in number. thus it was maskelyne who first rendered effective the design which charles ii. had in the establishment of the observatory. flamsteed and halley had been too jealous of their own observations to publish; bradley's observations--though he himself was entirely free from this jealousy--were made, after his death, the subject of litigation by his heirs and representatives, who claimed an absolute property in them, a claim which the government finally allowed. none of the three, however much their work ultimately tended to the improvement of the art of navigation, made that their first object. whereas maskelyne set this most eminently practical object in the forefront, and so gave to the royal observatory, which under his predecessors somewhat resembled a private observatory, its distinctive characteristics of a public institution. it fell to maskelyne to have to advise the government as to the assignment of their great reward of £ , for the discovery of the longitude at sea. maskelyne, while reporting favourably of the behaviour of harrison's time-keeper, considered that the method of 'lunars' was far too important to be ignored, and he therefore recommended that half the sum should be given to harrison for his watch, whilst the other half was awarded for the lunar tables which mayer, before his death, had sent to the board of longitude. this decision, though there can be no doubt it was the right one, led to much dissatisfaction on the part of harrison, who urged his claim for the whole grant very vigorously; and eventually the whole £ , was paid him. the whole question of rewards to chronometer-makers must have been one which caused maskelyne much vexation. he was made the subject of a bitter and most voluminous attack by thomas mudge, for having preferred the work of arnold and earnshaw to his own. otherwise his reign at the observatory seems to have been a singularly peaceful one, and there is little to record about it beyond the patient prosecution, year by year, of an immense amount of sober, practical work. to maskelyne, however, we owe the practice of taking a transit of a star over five wires instead of over one, and he provided the transit instrument with a sliding eye-piece, to get over the difficulty of the displacement which might ensue if the star were observed askew when out of the centre of the field. to maskelyne, too, we owe in a pre-eminent degree the orderly form of recording, reducing, and printing the observations. much of the work in this direction which is generally ascribed to airy was really due to maskelyne. indeed, without a wonderful gift of organization, it would have been impossible to plan and to carry the _nautical almanac_. beside the editing of various works intended for use in nautical astronomy or in general computation, the chief events of his long reign at greenwich were the transit of venus in , which he himself observed, and for which he issued instructions in the _nautical almanac_; and his expedition in to scotland, where he measured the deviation of the plumb-line from the vertical caused by the attraction of the mountain schiehallion, deducing therefrom the mean density of the earth to be four and a half times that of water. [illustration: john pond. (_from an old engraving._)] he died at the observatory, february , , aged , leaving but one child, a daughter, who married mr. anthony mervin story, to whom she brought the family estates in wiltshire, inherited by maskelyne on the deaths of his elder brothers, and, in consequence, mr. story added the name of maskelyne to his own. maskelyne's character and policy as astronomer royal have been sufficiently dwelt upon. his private character was mild, amiable, and generous. 'every astronomer, every man of learning, found in him a brother;' and, in particular, when the french revolution drove some french astronomers to this country to find a refuge, they received from the astronomer royal the kindest reception and most delicate assistance. maskelyne added no instrument to the observatory during his reign, though he improved bradley's transit materially. he designed the mural circle, but it was not completed until after his death. his additions to the observatory buildings consisted of three new rooms in the astronomer royal's house, and the present transit circle room. * * * * * john pond was recommended by maskelyne as his successor at greenwich. at the time of his succession he was forty-four years of age, having been born in . he was educated at trinity college, cambridge, and then spent some considerable time travelling in the south of europe and egypt. on his return home he settled at westbury, where he erected an altazimuth by troughton, with a two-and-a-half-foot circle. a born observer, his observations of the declinations of some of the principal fixed stars showed that the instrument which maskelyne was using at greenwich--the quadrant by bird--could no longer be trusted. maskelyne, in consequence, ordered a six-foot mural circle from troughton, but did not live to see it installed, and in this was supplemented by troughton's transit instrument of five inches aperture and ten feet focal length. the introduction of these two important instruments, and of other new instruments, together with new methods of observation, form one of the chief characteristics of pond's administration. under this head must be specially mentioned the introduction of the mercury trough, both for determining the position of the vertical, and for obtaining a check upon the flexure of the mural circle in different positions; and the use in combination of a pair of mural circles for determining the declinations of stars. another characteristic of his reign was that under him there was the first attempt to give the astronomer royal a salary somewhat higher than that of a mechanic, and to support him with an adequate staff of assistants. his salary was fixed at £ a year, and the single assistant of maskelyne was increased to six. this multiplication of assistants was for the purpose of multiplying observations, for pond was the first astronomer to recognize the importance of greatly increasing the number of all observations upon which the fundamental data of astronomy were to be based. in he finished his standard catalogue of stars, at that time the fullest of any catalogue prepared on the same scale of accuracy. 'it is not too much to say,' was the verdict of the royal astronomical society, 'that meridian sidereal observation owes more to him than to all his countrymen put together since the time of bradley.' a yet higher testimony to the exactness of his work is given by his successor, airy. 'the points upon which, in my opinion, mr. pond's claims to the gratitude of astronomers are founded, are principally the following. _first_ and chief, the accuracy which he introduced into all the principal observations. this is a thing which, from its nature, it is extremely difficult to estimate now, so long after the change has been made; and i can only say that, so far as i can ascertain from books, the change is one of very great extent; for certainty and accuracy, astronomy is quite a different thing from what it was, and this is mainly due to mr. pond.' the same authority eulogizes him further for his laborious working out of every conceivable cause or indication of error in his declination instruments, for the system which he introduced in the observation of transits, for the thoroughness with which he determined all his fundamental data, and for the regularity which he infused into the greenwich observations. one result of this great increase of accuracy was that pond was able at once authoritatively to discard the erroneous stellar parallaxes that had been announced by brinkley, royal astronomer for ireland. but pond's administration was open, in several particulars, to serious censure, and the board of visitors, which had been for many years but a committee of the royal society, but which had recently been reconstituted, proved its value and efficiency by the remonstrances which it addressed to him, and which eventually brought about his resignation. his personal skill and insight as an observer were of the highest order; but either from lack of interest or failing health, he absented himself almost entirely from the observatory in later years, visiting it only every ninth or tenth day. he had caused the staff of assistants to be increased from one to six, but had stipulated that the men supplied to him should be 'drudges.' his minute on the subject ran-- 'i want indefatigable, hard-working, and, above all, obedient drudges (for so i must call them, although they are drudges of a superior order), men who will be contented to pass half their day in using their hands and eyes in the mechanical act of observing, and the remainder of it in the dull process of calculation.' this was a fatal mistake, and one which it is very hard to understand how any one with a real interest in the science could have made. men who had the spirit of 'drudges,' to whom observation was a mere 'mechanical act,' and calculation a 'dull process,' were not likely to maintain the honour of the observatory, particularly under an absentee astronomer royal. pond tried to overcome the difficulty by devising rules for their guidance of iron rigidity. the result was that after his resignation, in , the first lord and the secretary of the admiralty expressed their feeling to airy, pond's successor, 'that the observatory had fallen into such a state of disrepute that the whole establishment should be cleared out.' a further evil was the excessive development of chronometer business, so as practically to swamp the real work of the observatory, whilst the prices paid for the chronometers at this time were often much larger than would have been the case under a more business-like administration. with all his merits, therefore, as an observer, the administration of pond was, in some respects, the least satisfactory of all that the observatory has known, and he alone of all the astronomers royal retired under pressure. he did not long survive his resignation, dying in september, . he was buried by the side of halley, in the churchyard at lee. of pond's instruments, the observatory retains the fine transit instrument which was constructed by troughton at his direction, and the mural circle, designed by maskelyne, but which pond was the first to use. both of these have, of course, long been obsolete, and now hang on the walls of the transit room. the small equatorial, called, after its donor, the shuckburgh equatorial, was also added in pond's day, and though practically never used, still remains mounted in its special dome. chapter iv airy one hundred and sixty years from the day when flamsteed laid the foundation stone of the observatory, the royal warrant under the sign manual was issued, appointing the seventh and strongest of the astronomers royal, august , . he actually entered on his office in the following october, but did not remove to the observatory until the end of the year. george biddell airy was born at alnwick, in northumberland, on july , . his father was william airy, of luddington, in lincolnshire, a collector of excise; his mother was the daughter of george biddell, a well-to-do farmer, of playford, near ipswich. he was educated at the grammar school, colchester, and so distinguished himself there that although his father was at this time very straitened in his circumstances, it was resolved that young airy should go to cambridge. here he was entered as sizar at trinity college, and his robust, self-reliant character was seen in the promptness with which he rendered himself independent of all pecuniary help from his relatives. in he graduated as bachelor of arts, being senior wrangler and smith's prizeman, entirely distancing all other men of his year. he had already begun to pay attention to astronomy, at first from the side of optics, to the study of which he had been very early attracted; a paper of his on the achromatism of eye-pieces and microscopes, written in , being one of especial value. in he attempted to determine 'the diminution of gravity in a deep mine'--that of dolcoath, in cornwall. in the winter of - he was invited to london by mr. (afterwards sir) james south, who took him, amongst other places, to greenwich observatory, and gave him his first introduction to practical astronomy. in he was appointed lucasian professor at cambridge, and in , plumian professor, with the charge of the new university observatory. prior to his election he had definitely told the electors that the salary proposed was not sufficient for him to undertake the responsibility of the observatory. he followed this up by a formal application for an increase, which created not a little commotion at the time, the action being so unprecedented; and after a delay of a little over a year he obtained what he had asked for. the delay gave rise, however, to the remark of a local wit, that the university had given 'to airy, nothing, a local habitation and a name.' [illustration: george biddell airy.] the seven years which he spent in the cambridge observatory were the best possible preparation for that greater charge which he was to assume later. when he entered on his duties the observatory had been completed four years, but no observations had been published; there was no assistant, and the only instruments were a couple of good clocks and a transit instrument. but airy set to work at once with so much energy that the observations for were published early in the following year, and he had very quickly worked out the best methods for correcting and reducing his observations. in an assistant was granted to him, in a second, and in the latter year mr. baldrey, the senior assistant, observed about transits, and mr. glaisher, the junior, about the same number of zenith distances. a syndicate had been appointed at cambridge for the purpose of visiting the observatory once in each term, and making an annual report to the senate. a smaller-minded and less acute man than airy might have resented such an arrangement. he, on the contrary, threw himself heartily into it, and made such formal written reports to the syndicate as best helped them in the performance of their duty, and at the same time secured for the observatory the support and assistance which from time to time it required. on his appointment to greenwich, he at once entered into the same relations to the board of visitors of that observatory, and from that time forth the friction that had occasionally existed between the board and the astronomer royal in the past entirely ceased. the board was henceforth no longer a body whose chief function was to reprove, to check, or to quicken the astronomer royal, but rather a company of experts, before whom he might lay the necessities of the observatory, that they in turn might present them to the government. such representations were not likely to be in vain. for, as mr. sheepshanks has left on record-- 'when mr. airy wants to carry anything into effect by government assistance, he states, clearly and briefly, why he wants it; what advantages he expects from it; and what is the probable expense. he also engages to direct and superintend the execution, making himself personally responsible, and giving his labour gratis. when he has obtained permission (which is very seldom refused), he arranges everything with extraordinary promptitude and foresight, conquers his difficulties by storm, and presents his results and his accounts in perfect order, before men like ... or myself would have made up our minds about the preliminaries. now, men in office naturally like persons of this stamp. there is no trouble, no responsibility, no delay, no inquiries in the house; the matter is done, paid for, and published, before the seekers of a grievance can find an opportunity to be heard. this mode of proceeding is better relished by busy statesmen than recommendations from influential noblemen or fashionable ladies.' his first action towards the board was, however, a very bold and independent one. he made strong representations on the subject of the growth of the chronometer business, which proved displeasing to the hydrographer, captain beaufort, who was one of the official visitors, and by his influence the report was not printed. airy 'kept it, and succeeding reports, safe for three years, and then the board of visitors agreed to print them, and four reports were printed together, and bound with the greenwich observations of .' with the completion of arrangements which put the chronometer business in proper subordination to the scientific charge of the observatory, airy was free to push forward its development on the lines which he had already marked out for himself. to go through these in detail is simply to describe the observatory as he left it. little by little he entirely renovated the equipment. greatly as pond had improved the instruments of the observatory, airy carried that work much further still. though he did not observe much himself, and was not pond's equal in the actual handling of a telescope, he had a great mechanical gift, and the detail in its minutest degree of every telescope set up during his long reign was his own design. in the work of reduction he introduced the use of printed skeleton forms, to which pond had been a stranger. the publication of the greenwich results was carried on with the utmost regularity; and, in striking contrast to the reluctance of flamsteed and halley, he was always most prompt in communicating any observations to every applicant who could show cause for his request for them. it is most difficult to give any adequate impression of his far-reaching ability and measureless activity. perhaps the best idea of these qualities may be obtained from a study of his autobiography, edited and published some four years after his death by his son. the book, to any one who was not personally acquainted with airy, is heavy and monotonous, chiefly for the reason that its pages are little but a mere catalogue of the works which he undertook and carried through; and catalogues, except to the specialist, are the dullest of reading. to enter into the details of his work might fill a library. [illustration: the astronomer royal's room.] as astronomer royal he seems to have inherited and summed up all the great qualities of his predecessors: flamsteed's methodical habits and unflagging industry; halley's interest in the lunar theory; bradley's devotion to star observation and catalogue making; maskelyne's promptitude in publishing, and keen interest in practical navigation; pond's refinement of observation. nor did he allow this inheritance to be merely metaphorical; he made it an actual reality. he discussed, reduced, and published, in forms suitable for use and comparison to-day, the whole vast mass of planetary and lunar observations made at the royal observatory from the year to his own accession, a work of prodigious labour, but of proportionate importance. airy has been accused--and with some reason--of being a strong, selfish, aggressive man; yet nothing can show more clearly than this great work how thoroughly he placed the fame and usefulness of the observatory before all personal considerations. with far less labour he could have carried on a dozen investigations that would have brought him more fame than this great enterprise, the purpose of which was to render the work of his predecessors of the highest possible use. the light in which he regarded his office may best be expressed in his own words:-- 'the observatory was expressly built for the aid of astronomy and navigation, for promoting methods of determining longitude at sea, and (as the circumstances that led to its foundation show) more especially for determination of the moon's motions. all these imply, as their first step, the formation of accurate catalogues of stars, and the determination of the fundamental elements of the solar system. these objects have been steadily pursued from the foundation of the observatory; in one way by flamsteed; in another way by halley, and by bradley in the earlier part of his career; in a third form by bradley in his later years; by maskelyne (who contributed most powerfully both to lunar and to chronometric nautical astronomy), and for a time by pond; then with improved instruments by pond, and by myself for some years; and subsequently, with the instruments now in use. it has been invariably my own intention to maintain the principles of the long-established system in perfect integrity; varying the instruments, the modes of employing them, and the modes of utilizing the observations of calculation and publication, as the progress of science might seem to require.' the result of this keen appreciation of the essential continuity of the astronomer royalship has been that it is to airy, more than to any of his predecessors, or than to all of them put together, that the high reputation of greenwich observatory is due. professor newcomb, the greatest living authority on the subject outside our own land--and other great foreign astronomers have independently pronounced the same verdict--has said:-- 'the most useful branch of astronomy has hitherto been that which, treating of the positions and motions of the heavenly bodies, is practically applied to the determination of geographical positions on land and at sea. the greenwich observatory has, during the past century, been so far the largest contributor in this direction as to give rise to the remark that, if this branch of astronomy were entirely lost, it could be reconstructed from the greenwich observations alone.' early in airy proposed to the board of visitors the creation of the magnetic and meteorological department of the observatory, and in a system of regular two-hourly observations was set on foot. this was the first great enlargement of programme for the observatory beyond the original one expressed in flamsteed's warrant. it was followed in with the formation of the solar photographic department, to which the spectroscope was added a little later. though he had objected strongly on his first coming to the observatory to the excessive time devoted to the merely commercial side of the care of chronometers, yet the perfecting of these instruments was one that he had much at heart, and many recent appliances are either of his own invention or are due to suggestions which he threw out. much work lying outside the observatory, and yet intimately connected with it, was carried out either by him or in accordance with his directions. the transit of venus expeditions of , the delimitation of the boundary line between canada and the united states, and, later, that of the oregon boundary; the determination of the longitudes of valencia, cambridge, edinburgh, brussels, and paris; assistance in the determination of the longitude of altona--all came under airy's direction. nor did he neglect expeditions in connection with what we would now call the physical side of astronomy. on three occasions, , , and , he himself personally took part in successful eclipse expeditions. the determination of the increase of gravity observable in the descent of a deep mine was also the subject of another expedition, to the harton colliery, near south shields. but with all these, and many other inquiries--for he was the confidential adviser of the government in a vast number of subjects: lighthouses, railways, standard weights and measures, drainage, bridges--he yet always kept the original objects of the observatory in the very first place. it was in order to get more frequent observations of the moon that he had the altazimuth erected, which was completed in may, . this was followed, in , by the transit circle, as he had long felt the need for more powerful light grasp in the fundamental instrument of the observatory. the transit circle took the place both of the old transit instrument and of the mural circle. above all, he arranged for the observations of moon and stars to be carried out with practical continuity. the observations were made and reduced at once, and published in such a way that any one wishing to discuss them afresh could for himself go over every step of the reduction from the commencement, and could see precisely what had been done. the greatest addition made to the equipment of the observatory in airy's day was the erection of the - / -inch merz equatorial, which proved of great service when spectroscopy became a department of the observatory. [illustration: the south-east tower. (_from a photograph by mr. lacey._)] so strong and gifted a man as airy was bound to make enemies, and at different times of his life bitter attacks were made on him from one quarter or another. one of these, curiously enough, was from sir james south, the man who, as he said, first introduced him to practical astronomy. later came the discovery of neptune, and airy was subjected to much bitter criticism, since, as it appeared on the surface, it was owing to his supineness that adams missed being held the sole discoverer of the new planet, and narrowly missed all credit for it altogether. last of all was the vehement attack made upon him by richard anthony proctor, in connection with his preparations for the transit of venus. all such attacks, however, simply realized the old fable of the viper and the file. attacks which would have agonized flamsteed's every nerve, and have called forth full and dignified rejoinders from maskelyne, were absolutely and entirely disregarded by airy. he had done his duty, and in his own estimation--and, it should be added, in the estimation of those best qualified to judge--had done it well. he was perfectly satisfied with himself, and what other people thought or said about him influenced him no more than the opinions of the inhabitants of saturn. but great as airy was, he had the defects of his qualities, and some of these were serious. his love of method and order was often carried to an absurd extreme, and much of the time of one of the greatest intellects of the century was often devoted to doing what a boy at fifteen shillings a week could have done as well, or better. the story has often been told, and it is exactly typical of him, that on one occasion he devoted an entire afternoon to himself labelling a number of wooden cases 'empty,' it so happening that the routine of the establishment kept every one else engaged at the time. his friend dr. morgan jocularly said that if airy wiped his pen on a piece of blotting-paper he would duly endorse the blotting-paper with the date and particulars of its use, and file it away amongst his papers. his mind had that consummate grasp of detail which is characteristic of great organizers, but the details acquired for him an importance almost equal to the great principles, and the statement that he had put a new pane of glass into a window would figure as prominently in his annual report to the board of visitors as the construction of the new transit circle. his son remarks of him that 'in his last days he seemed to be more anxious to put letters which he received into their proper place for reference than even to master their contents,' his system having grown with him from being a means to an end, to becoming the end itself. so, too, his regulation of his subordinates was, especially in his earlier days, despotic in the extreme--despotic to an extent which would scarcely be tolerated in the present day, and which was the cause of not a little serious suffering to some of his staff, whom, at that time, he looked upon in the true spirit of pond, as mere mechanical 'drudges.' for thirty-five years of his administration the salaries of his assistants remained discreditably low, and his treatment of the supernumerary members of his staff would now probably be characterized as 'remorseless sweating.' the unfortunate boys who carried out the computations of the great lunar reductions were kept at their desks from eight in the morning till eight at night, without the slightest intermission, except an hour at midday. as an example of the extreme detail of the oversight which he exercised over his assistants, it may be mentioned that he drew up for each one of those who took part in the harton colliery experiment, instructions, telling them by what trains to travel, where to change, and so forth, with the same minuteness that one might for a child who was taking his first journey alone; and he himself packed up soap and towels with the instruments, lest his astronomers should find themselves, in co. durham, out of reach of these necessaries of civilization. a regime so essentially personal may indeed have been necessary after pond's administration, and to give the observatory a fresh start. but it would not have been to the advantage of the observatory, had it become a permanent feature of its administration, as it militated--was almost avowedly intended to militate--against the growth of real zeal and intelligence in the staff, and necessarily occasioned labour and discomfort out of proportion to the results obtained. fortunately, in airy's later years, the extension of the work of the observatory, a slight failing in his own powers, and the efforts he was devoting to the working out of the lunar theory, compelled him to relax something of that microscopic imperiousness which had been the chief characteristic of his rule for so long. airy had, in the fullest degree, the true spirit of the public servant; his sense of duty to the state was very high. he was always ready to undertake any duty which he felt to be of public usefulness, and many of these he discharged without fee or reward. so great an astronomer was necessarily most highly esteemed by astronomers. he was president of the royal society for two years; he was five times president of the royal astronomical society, and twice received its gold medal, beside a special testimonial for his reduction of the greenwich lunar observations. from the royal society he received the copley medal and the royal medal, beside honorary titles from the universities of oxford, cambridge, and edinburgh. so invaluable a public servant, he received the distinction of a knight commandership of the bath in . he had been repeatedly offered knighthood before, but had not thought it well to receive it. he was in the receipt of decorations also from a great number of foreign countries; for, for many years, he was looked up to, not only by english astronomers, but by scientific men in all countries, as the very head and representative of his science. and he also received a more popular appreciation--and most justly so. for whilst no one could have less of the arts of the ordinary popularizer about him, no one has ever given popular lectures on astronomy which more fully corresponded to the ideal of what such should be than airy's six lectures to working men, delivered at ipswich. and we may count the bestowal upon him of the honorary freedom of the city of london, in , as one of the tokens that his services in this direction had not been unappreciated. during the last seven years of his official career he undertook the working out of a lunar theory, and, to allow himself more leisure for its completion, he resigned his position august , , after forty-six years of office. he was now eighty years of age, and he took up his residence at the white house, just outside greenwich park. he resided there till his death, more than ten years later--january , . * * * * * airy was succeeded in the astronomer royalship by the present and eighth holder of the office, w. h. m. christie. he was born at woolwich, in , his father having been professor samuel hunter christie, f.r.s. he was educated at king's college, london, and trinity college, cambridge, graduating as fourth wrangler in . in he was appointed chief assistant at greenwich, in succession to mr. stone, who had become her majesty's astronomer at the cape, and in he succeeded airy as astronomer royal. [illustration: w. h. m. christie, astronomer royal. (_from a photograph by elliott and fry._)] during mr. christie's office, the two new departments of the astrographic chart and double-star observations have come into being. the following buildings have been erected under his administration: the great new observatory in the south ground, the new altazimuth, the new library, nearly opposite to it, the transit pavilion, the porter's lodge, and the magnetic pavilion out in the park. whilst in the old buildings the astrographic dome has been added, and the upper and lower computing rooms have been rebuilt and enlarged. as to the instruments, the -inch refractor, the astrographic twin telescope, the new altazimuth, the -inch and -inch thompson photographic refractors, and the -inch reflector are all additions during the present reign. roughly speaking, therefore, we may say that three-fourths of the present observatory has been added during the nineteen years of the present astronomer royal. one exceedingly important improvement should not be overlooked. airy observed little himself whilst at greenwich, and had an inadequate idea of the necessity for room in a dome and breadth in a shutter-opening. with the sole exception, perhaps, of the transit circle, every instrument set up by airy was crammed into too small a dome or looked out through too narrow an opening. the increase of shutter-opening of the newer domes may be well seen by contrasting, say, the old altazimuth or the sheepshanks dome with that of the astrographic. this reform has had much to do with the success of later work. chapter v the observatory buildings like a living organism, greenwich observatory bears the record of its life-history in its structure. it was not one of those favoured institutions that have sprung complete and fully equipped from the liberality of some great king or private millionaire. as we have seen, it was originally established on the most modest--not to say meagre--scale, and has been enlarged just as it has been absolutely necessary. to quote again from professor newcomb-- 'whenever any part of it was found insufficient for its purpose, new rooms were built for the special object in view, and thus it has been growing from the beginning by a process as natural and simple as that of the growth of a tree. even now the very value of its structure is less than that of several other public observatories, though it eclipses them all in the results of its work.' entering the courtyard--an enclosure some eighty feet deep by ninety feet in extreme breadth--by the great gate, we see before us flamsteed house, the original building of the observatory. flamsteed's little domain was only some twenty-seven yards wide by fifty deep, and for buildings comprised little beyond a small dwelling-house on the ground floor, and one fine room above it. this room--the original greenwich observatory--still remains, and is used as a council room by the official board of visitors, who come down to the observatory on the first saturday in june, to examine into its condition and to receive the astronomer royal's report. the room is called, from its shape, the octagon room, and is well known to londoners from the great north window which looks out straight over the river between the twin domes of the hospital. in bradley's time, about , the first extension of the domains of the observatory took place to the south and east of the original building, the direction in which, on the whole, all subsequent extensions have taken place, owing to the fact that the original building was constructed at the extremity of what sir george airy was accustomed to call a 'peninsula'--a projecting spur of the blackheath plateau, from which the ground falls away very sharply on three sides and on part of the fourth. the observatory domain at present is fully two hundred yards in greatest length, with an average breadth of about sixty. nearly the whole of this accession took place under the directorates of pond and airy. the present instruments are, therefore, as a rule, the more modern in direct proportion to their distance from the octagon room--the old original observatory. there is one notable exception. the very first extension of the observatory buildings, made in the time of halley, the second astronomer royal, consisted in the setting up of a strong pier, to carry two quadrant telescopes. the pier still remains, but now forms the base of the support of the twin telescopes devoted to the photographic survey of the heavens for the international chart. standing just within the gate of the courtyard, and looking westward, that is toward flamsteed house, we have immediately on our right hand the porter's lodge; a little farther forward, also on the right, the transit pavilion, a small building sheltering a portable transit instrument; and farther forward, still on the right, the entrance to the chronograph room. above the chronograph room is a little, inconveniently-placed dome, containing a small equatorially-mounted telescope, known as the shuckburgh. beyond the chronograph room a door opens on to the north terrace, over which is seen the great north window of the octagon room. close by the door of the chronograph room a great wooden staircase rises to the roof of the main building. it is not an attractive-looking ascent, as the steps overlap inconveniently. still, there is no record of an accident upon them, and those who venture on the climb to the roof, where are placed the anemometers and the turret carrying the time-ball, which is dropped daily at p.m., will be well repaid by the splendid view of the river which is there afforded to them. passing under this staircase, on the wall by its side is seen the following inscription:-- carolus ii^s rex optimus astronomiÆ et nauticÆ artis patronus maximus speculam hanc in utriusque commodum fecit anno d^{ni} mdclxxvi. regni sui xxviii. curante iona moore milite r. t. s. g. [illustration: the astronomer royal's house. (_from a photograph by mr. lacey._)] in the extreme angle of the courtyard is the entrance to the mean solar clock cupboard, and to the staircase leading up to the octagon room. at the head of this staircase in a small closet is the winch for winding up the time-ball. coming back into the courtyard, and crossing the face of the astronomer royal's private house, the range of buildings is reached which form the left hand or south side of the enclosure. entering the first of these, we find ourselves in the lower computing room, which is devoted to the 'time department.' the next room which opens out of it, as we turn eastwards, was bradley's transit room, but is now used for the storage of chronometers. passing through bradley's transit room, we come to the present transit room, which brings us close to the great gate. the range of buildings is, however, continued somewhat farther, containing on the ground floor some small sitting-rooms and a fire-proof room for records. [illustration: the courtyard. (_from a photograph by mr. lacey._)] turning back to the lower computing room, we notice in it the stone pier, already alluded to, which was set up by halley, and formed the first addition to the original observatory of flamsteed. the lower computing room itself and bradley's transit room were due to the astronomer after which the latter is named. an iron spiral staircase in the middle of the lower computing room leads up to the upper computing room, and above that to the astrographic dome, so called because the twin telescope housed therein is devoted to the work of the astrographic chart--a chart of the entire sky to be made by eighteen co-operating observatories by means of photography. in this way it is intended to secure a record of the places of far more stars than could be done by the ordinary methods, and in this project greenwich has necessarily taken a premier place. this is a work which, whilst it is the legitimate and natural outcome of the original purpose of the observatory, is yet pushed beyond what is necessary for any mere utilitarian assistance to navigation. for the sailor it will always be sufficient to know the places of a mere handful of the brightest stars, and the vast majority of those in the great photographic map will never be visible in the little portable telescope of the sailor's sextant. but it will be freely admitted that in the case of an enterprise of this nature, in which the observatories of so many different nations were uniting, and which was so precisely on the lines of its original charter, though an extension of it, it was impossible for greenwich to hold back on the plea that the work was not entirely utilitarian. descending again to the lower computing room, and passing through it, not to the east, into bradley's transit room, but through a little lobby to the south, we come upon an inconvenient wooden staircase winding round a great stone pillar with three rays. this pillar is the support of airy's altazimuth, and very nearly marks the place where flamsteed set up his original sextant. returning again to the lower computing room, and passing out to the east, just in front of the time superintendent's desk, we enter a small passage running along the back of bradley's transit room, and from this passage enter the present transit room near its south end. just before reaching the transit room, however, we pass the reflex zenith tube, a telescope of a very special kind. immediately outside the transit room is a staircase leading on the first floor to two rooms long used as libraries, and to the leads above them, on which is a small dome containing the sheepshanks equatorial. these libraries are over the small sitting-rooms already referred to. the fire-proof record rooms, two stories in height, terminate this range of buildings. beyond the record rooms the boundary turns sharply south, where stands a large octagonal building surmounted by a dome of oriental appearance, a 'circular versatile roof,' as the visitors would have called it a hundred years ago. this dome--which has been likened, according to the school of æsthetics in which its critics have been severally trained, to the taj at agra, a collapsed balloon, or a mammoth spanish onion--houses the largest refractor in england, the 'south-east equatorial' of twenty-eight inches aperture. but, though the largest that england possesses, it would appear but as a pigmy beside some of the great telescopes for which america is famous. beyond this dome the hollow devoted to the astronomer royal's private garden reduces the observatory ground to a mere 'wasp's waist,' a narrow, inconvenient passage from the old and north observatory to the younger southern one. the first building, as the grounds begin to widen out to the south, contains the new altazimuth, a transit instrument which can be turned into any meridian. a library of white brick and a low wooden cruciform building--the magnetic observatory--follow it closely. this latter building houses the magnetic department, a department which, though it lies aside from the original purposes of the observatory, as defined in the warrant given to flamsteed, is yet intimately connected with navigation, and was founded by airy very early in his period of office. this deals with the observation of the changes in the force and direction of the earth's magnetism, an inquiry which the greater delicacy of modern compasses, and, in more recent times, the use of iron instead of wood in the construction of ships, has rendered imperative. closely associated with the magnetic department is the meteorological. weather forecasts, so necessary for the safety of shipping round our coasts, are not issued from greenwich observatory, any more than the _nautical almanac_ is now issued from it. but just as the observatory furnishes the astronomical data upon which the almanac is based, so also a considerable department is set apart for furnishing observations to be used by the meteorological office at westminster for their daily predictions. so far, the development of the observatory had been along the central line of assistance to navigation. but the 'magnetic department' led on to a new one, which had but a secondary connection with it. it had been discovered that the extent of the daily range of the magnetic needle, and the amount of the disturbances to which it was subjected, were in close connection with the numbers and size of the spots on the sun's surface. this led to the institution of a daily photographic record of the state of the sun's surface, a record of which greenwich has now the complete monopoly. [illustration: plan of observatory at present time. (for key to plan, see p. .)] key to the plan of the observatory on page . . chronograph room. . old altazimuth dome. . safe room. . computing room. . bradley's transit room. . transit circle room. . assistants' room. . chief assistant's room. . computers' room. . record rooms. . chronometer rooms and south-east dome. . greenhouse and outbuildings. . new library. . magnetic observatory. . offices. . sheds. . winch room for time-ball. . porter's lodge. . new transit pavilion. . new altazimuth pavilion. . museum: new building. . south wing " . north wing " . west wing " . east wing " f. rooms built for flamsteed. h. added by halley. b. " bradley. m. " maskelyne. a. " airy. f'f'. flamsteed's boundaries. m'm'. maskelyne's " . p'p'. pond's " . a'a'. airy's " . a"a". airy's " . beyond the magnetic observatory the ground widens out into an area about equal to that of the northern part, and the new building just completed, and which is now emphatically 'the observatory,' stands clear before us. the transfer to this stately building of the computing rooms, libraries, and store rooms has been aptly described as a shift in the latitude of greenwich observatory, which still preserves its longitude. it may be noted that the only two buildings of any architectural pretensions in the whole range are--flamsteed's original observatory, built by sir christopher wren, and containing little beyond the octagon room, in the extreme north; and this newest building in the extreme south. this 'new observatory,' like the old, and like the great south-eastern tower, is an octagon in its central portion. but whilst the two other great buildings are simply octagonal, here the octagon serves only as the centre from which radiate four great wings to the four points of the compass. the building is by far the largest on the ground, but in little accord with the popular idea of an astronomer as perpetually looking through a telescope, carries but a single dome; its best rooms being set apart as 'computing rooms,' for the use of those members of the staff who are employed in the calculations and other clerical work, which form, after all, much the greater portion of the observatory routine. an observer with the transit instrument, for instance, will take only three or four minutes to make a complete determination of the place of a single star. but that observation will furnish work to the computers for many hours afterwards. or, to take a photograph of the sun will occupy about five minutes in setting the instrument, whilst the actual exposure will take but the one-thousandth part of a second. but the plate, once exposed, will have to be developed, fixed, and washed; then measured, and the measures reduced, and, _on the average_, will provide one person with work for four days before the final results have been printed and published. it is easy to see, then, that observing, though the first duty of the observatory, makes the smallest demand on its time. the visitor who comes to the observatory by day (and none are permitted to do so by night) finds the official rooms not unlike those of somerset house or whitehall, and its occupants for the most part similarly engaged in what is, apparently, merely clerical work. an examination of the big folios would of course show that instead of being ledgers of sales of stamps, or income-tax schedules, they referred to stars, planets, and sun-spots; but for one person actively engaged at a telescope, the visitor would see a dozen writing or computing at a desk. the staff, like the building, is the result of a gradual development, and bears traces of its life history in its composition. first comes the astronomer royal, the representative and successor of the original 'king's astronomer,' the rev. john flamsteed. but the 'single surly and clumsy labourer,' which was all that the 'merry monarch' could grant for his assistance, is now represented by a large and complex body of workers; each varied class and rank of which is a relic of some stage in the progress of the observatory to its present condition. the following extract from the annual report of the astronomer royal to the board of visitors, june, , describes the present _personnel_ of the establishment:-- 'the staff at the present time is thus constituted, the names in each class being arranged in alphabetical order:-- 'chief assistants--mr. cowell, mr. dyson. 'assistants--mr. hollis, mr. lewis, mr. maunder, mr. nash, mr. thackeray. 'second-class assistants--mr. bryant, mr. crommelin. 'clerical assistant--mr. outhwaite. 'established computers--mr. bowyer, mr. davidson, mr. edney, mr. furner, mr. rendell, and one vacancy. 'the two second-class assistants will be replaced by higher grade established computers as vacancies occur. 'mr. dyson and mr. cowell have the general superintendence of all the work of the observatory. mr. maunder is charged with the heliographic photography and reductions, and with the preparation of the library catalogue. mr. lewis has charge of the time-signals and chronometers, and of the -inch equatorial. mr. thackeray superintends the miscellaneous astronomical computations, including the preparation of the new ten-year catalogue. mr. hollis has charge of the photographic mapping of the heavens, the measurement of the plates, and the computations for the astrographic catalogue. mr. crommelin undertakes the altazimuth and sheepshanks equatorial reductions, and mr. bryant the transit and meridian zenith distance reductions and time-determinations. in the magnetic and meteorological branch, mr. nash has charge of the whole of the work. mr. outhwaite acts as responsible accountant officer; has charge of the library, records, manuscripts, and stores, and conducts the official correspondence. as regards the established computers, mr. bowyer, mr. furner, mr. davidson, and mr. rendell assist mr. lewis, mr. thackeray, mr. hollis, and mr. bryant respectively, and mr. edney assists mr. nash. 'there are at the present time twenty-four supernumerary computers employed at the observatory, ten being attached to the astronomical branch, two the chronometer branch, six to the astrographic, one to the heliographic, four to the magnetic and meteorological, and one to the clerical. 'a foreman of works, with two carpenters, and two labourers; a skilled mechanic with an assistant; a gate porter, two messengers, a watchman, a gardener, and a charwoman, are also attached to the observatory. 'the whole number of persons regularly employed at the observatory is fifty-three.' the day work, as said before, is by far the greatest in amount, the 'office hours' being from nine till half-past four, with an hour's interval. the arrangements for the night watches present some complications. for many years the instruments in regular use were two only, the transit circle and the altazimuth. the arrangements for observing were simple. four assistants divided the work between them thus: an assistant was on duty with the transit circle one day, his watch beginning about six a.m. or a little later, and ending about three the following morning; a watch of twenty-one hours in maximum length. the second day his duties were entirely computational, and were only two or three hours in length. the third day he had a full day's work on the calculations, followed by a night duty with the altazimuth. the latter instrument might give him a very easy watch or a terribly severe one. if the moon were a young one it was easy, especially if the night was clear, as in that case an hour was enough to secure the observations required. very different was the case with a full moon, especially in the long, often cloudy, nights of winter. then a vigilant watch had to be kept from sunset to sunrise, so that in case of a short break in the clouds the moon might yet be observed. such a watch was the severest (with one exception) that an assistant had to undergo. his fourth day would then resemble his second, and with the fifth day a second cycle of his quartan fever would commence, the symptoms following each other in the same sequence as before. such a routine carried on with iron inflexibility was exceedingly trying, as it was absolutely impossible for an observer to keep any regularity in his hours of rest or times for meals. this routine has been considerably modified by the present astronomer royal, partly because the instruments now in regular daily use are five instead of two, and partly because a less stringent system has proved not merely far less wearing to the observers, but also much more prolific of results. it was impossible for a man to be at his best for long under the old _régime_, and from forty-six to forty-seven has been an ordinary age for an assistant to break down under the strain. one point in which the observing work has been lightened has been in the discontinuance of the altazimuth observations at the full of the moon, another in the shortening of the hours of the transit circle watch; and a further and most important one in the arrangement that the observers with the larger instruments should have help at their work. the net result of these changes has been a most striking increase in the amount of work achieved. thus, whilst in the year ending may , , transits were taken with the transit circle, and determinations of north polar distance; in that ending may , , the numbers had risen to , and , respectively, the telescope remaining precisely the same. one principle of airy's rule still remains. so far as possible no observer is on duty for two consecutive days, but a long day of desk work and observing is followed by a short day of desk work without observing. it will be readily understood that with five principal telescopes in constant work and one or two minor ones, some demanding two observers, others only one, each telescope having its special programme and its special hours of work, whilst by no means every member of the staff is authorized to observe with all instruments indifferently, it becomes a somewhat intricate matter to arrange the weekly _rota_ in strict accordance with the foregoing principle, and with the further one, that whilst a considerable amount of sunday observing is inevitable, the average duty of an observer should be three days a week, not seven days a fortnight. there is a story, received with much reserve at cambridge, that there was once a man at that university who had mastered all the colours and combinations of shades and colours of the various colleges and clubs. if so gifted a being ever existed, he may be paralleled by the greenwich assistant who can predict for any future epoch the sequence of duties throughout the entire establishment. at any rate, one of the first items in the week's programme is the preparation of the _rota_ for the week, or rather, to use an ecclesiastical term, for the 'octave,' _i.e._ from the monday to the monday following. the special work to be carried out on any telescope is likewise a matter of programme. for the transit circle a list of the most important objects to be observed is supplied for the observer's use, and the general lines upon which the other stars are to be selected from a huge 'working catalogue' are well understood. with some of the other telescopes the principles upon which the objects are to be selected are laid down, but the actual choice is left to the discretion of the observer at the time. there is no time for the watcher to spend in what the outsider would regard as 'discovery'; such as sweeping for comets or asteroids, hunting for variable stars, sketching planets, and so forth. indeed, there is a story current in the observatory that some fifty years ago, when the tide of asteroid discovery first set in, airy found an assistant, since famous, working with a telescope on his 'off-duty' night. that stern disciplinarian asked what business the assistant had to be there on his free night, and on being told he was 'searching for new planets,' he was severely reprimanded and ordered to discontinue at once. a similar energy would not meet so gruff a discouragement to-day; but the routine work so fully occupies both staff and telescopes that an assistant may be most thoroughly devoted to his science, and yet pass a decade at the observatory without ever seeing those 'show places' of the sky which an amateur would have run over in the first week after receiving his telescope. for example, there is no refractor in the british isles so competent to bring out the vivid green light of the great orion nebula--that marvellous mass of glowing, curdling, emerald cloud--or the indescribable magnificence of the myriad suns that cluster like swarming bees or the grapes of eshcol in the constellation of hercules; yet probably most of the staff have never seen either spectacle through it. the professional astronomer who is worth his salt will find abundance of charm and interest in his work, but he will not, 'like a girl, valuing the giddy pleasures of the eyes,' consider the charm to lie mainly in the occasional sight of wonderful beauty which his work may bring to him, nor the interest in some chance phenomenon which may make his name known. it is not every field of astronomy that is cultivated at greenwich. the search for comets and for 'pocket planets' forms no part of its programme; and the occupation so fascinating to those who take it up, of drawing the details on the surfaces of the moon, mars, jupiter, or saturn, has been but little followed. such work is here incidental, not fundamental, and the same may be said of certain spectroscopic observations of new or variable stars, and of many similar subjects. work such as this is most interesting to the general public, and is followed with much devotion by many amateur astronomers. for that very reason it does not form an integral part of the programme of our state observatory. but work which is necessary for the general good, or for the advancement of the science, and which demands observations carried on continuously for many years, and strict unity of instruments and methods, cannot possibly be left to chance individual zeal, and is therefore rightly made the first object at greenwich. those striking discoveries which from time to time appeal strongly to the popular imagination, and which have rendered so justly famous some of the great observatories of the sister continent, have not often been made here. its work has, none the less, been not only useful but essential. a century ago, when we were engaged in the hand-to-hand struggle with napoleon, by far the most brilliant part of that naval war which we waged against the french, and the most productive of prize-money, was carried on by our cruisers, who captured valuable prizes in every sea. but a much greater service, indeed an absolutely vital one, was rendered to the state by those line-of-battle ships which were told off to watch the harbours wherein the french fleet was taking refuge. this was a work void of the excitement, interest, and profit of cruising. it was monotonous, wearing, and almost inglorious, but absolutely necessary to the very existence of england. so the continuance for more than two centuries of daily observations of places of moon, stars, and planets is likewise 'monotonous, wearing, and almost inglorious;' the one compensation is that it is essential to the life of astronomy. the eight astronomers royal have, as already said, kept the observatory strictly on the lines originally laid down for it, subject, of course, to that enlargement which the growth of the science has inevitably brought. but had they been inclined to change its course, the board of visitors has been specially appointed to bring them back to the right way. as already mentioned in the account of flamsteed, the board dates from , when it practically consisted of the president and council of the royal society. its royal warrant lapsed on the death of queen anne, and was not renewed at the accession of the two following sovereigns; but in the reign of george iii. a new warrant was issued under date february , ; and this was renewed at the accession of george iv. when william iv. came to the throne, the constitution of the board was extended, so as to give a representation to the new royal astronomical society, founded in . the president of the royal society is still chairman of the board, but the admiralty, of which the observatory is a department, the two universities of oxford and cambridge, and the royal astronomical society are all represented on it by _ex officio_ members, and twelve other members are contributed by the royal and royal astronomical societies respectively, six by each. the first saturday in june is the appointed day for the annual inspection by the board, and for the presentation to it of the astronomer royal's report. to this all-important business meeting has been added something of a social function, by the invitation of many well-known astronomers and the leading men of the allied sciences to inspect the results of the year, and to partake of the chocolate and cracknels, which have been the traditional refreshments offered on these occasions for a period 'whereof the memory of man runneth not to the contrary.' chapter vi the time department one day two scotchmen stood just outside the main entrance of greenwich observatory, looking intently at the great twenty-four-hour clock, which is such an object of attention to the passers through the park. 'jock,' said one of them to the other, 'd'ye ken whaur ye are?' jock admitted his ignorance. 'ye are at the vara ceentre of the airth.' geographers tell us that there is a sense in which this statement as it stands may be accepted as true. for if the surface of the globe be divided into two hemispheres, so related to each other that the one contains as much land as possible, and the other as little, then london will occupy the centre or thereabouts of the hemisphere with most land. this was not, however, what the scotchman meant. he meant to tell his companion that he was standing on the prime meridian of the world, the imaginary base line from which all distances, east or west, are reckoned; in short, that he was on 'longitude nought.' he was not absolutely correct, however, for the great twenty-four-hour clock does not mark the exact meridian of greenwich. to find the instrument which marks it out and defines it we must step inside the observatory precincts, and just within the gate we see before us on the left hand a door which leads through a little lobby straight into the most important room of the whole observatory--the transit room. [illustration: the great clock and porter's lodge. (_from a photograph by mr. lacey._)] this room is not well adapted for representation by artist or photographer. four broad stone pillars occupy the greater part of the space, and leave little more than mere passage room beside. two of these pillars are tall, as well as broad and massive, and stand east and west of the centre of the room, carrying between them the fundamental instrument of the observatory, the transit circle. the optical axis of this telescope marks 'longitude nought,' which is further continued by a pair of telescopes, one to the north of it, the other to the south, mounted on the third and fourth of the pillars alluded to above. this room has not always marked the meridian of greenwich, for it stands outside the original boundary of the observatory. but it is only a few feet to the east of the first transit instrument which was set up by halley, the second astronomer royal, in the extreme n.-w. corner of the observatory domain, a distance equivalent to very much less than one-tenth of a second of time, an utterly insensible quantity with the instruments of two hundred years ago. it would be a long story to tell in detail how the greenwich transit room has come to define one of the two fundamental lines that encircle the earth. the other, the equator, is fixed for us by the earth itself, and is independent of any political considerations, or of any effort or enterprise of man. but of all the infinite number of great circles which could be drawn at right angles to the equator, and passing through the north and south poles, it was not easy to select one with such an overwhelming amount of argument in its favour as to obtain a practically universal acceptance. the meridians of jerusalem and of rome have both been urged, upon what we may call religious or sentimental grounds; that of the great pyramid at ghizeh has been pressed in accordance with the fantastic delusion that the pyramid was erected under divine inspiration and direction; that of ferro, in the canaries, as being an oceanic station, well to the west of the old world, and as giving a base line without preference or distinction for one nation rather than another. the actual decision has been made upon no such grounds as these. it has been one of pure practical convenience, and has resulted from the amazing growth of great britain as a naval and commercial power. like tyre of old, she is 'situate at the entry of the sea, a merchant of the people for many isles,' and 'her merchants are the great men of the earth.' to tell in full, therefore, the steps by which the greenwich meridian has overcome all others is practically to tell again, from a different standpoint, the story of the 'expansion of england.' the need for a supreme navy, the development of our empire beyond the seven seas, the vast increase of our carrying trade--these have made it necessary that englishmen should be well supplied with maps and charts. the hydrographic and geographic surveys carried on, either officially by this country, or by englishmen in their own private capacity, have been so numerous, complete, and far-reaching as not only to outweigh those of all other countries put together, but to induce the surveyors and explorers of not a few other countries to adopt in their work the same prime meridian as that which they found in the british charts of regions bordering on those which they were themselves studying. naturally, the meridian of greenwich has not only been adopted for great britain, but also for the british possessions over-sea, and, from these, for a large number of foreign countries; whilst our american cousins retain it, an historic relic of their former political connection with us. the victories of clive at arcot and plassy, of nelson at the nile and trafalgar, the voyages and surveys of cook and flinders, and many more; the explorations of bruce, park, livingstone, speke, cameron, and stanley; these are some of the agencies which have tended to fix 'longitude nought' in the greenwich transit room. there are two somewhat different senses in which the meridian of greenwich is the standard meridian for nearly the entire world. the first is the sense about which we have already been speaking; it constitutes the fundamental line whence distances east and west are measured, just as distances north and south are measured from the equator. but there is another, though related sense, in which it has become the standard. _it gives the time to the world._ there are few questions more frequently put than, 'what time is it?' 'can you tell me the true time?' a stickler for exactitude might reply, 'what kind of time do you mean?' 'do you mean solar or sidereal time?' 'apparent time or mean time?' 'local time or standard time?' there are all these six kinds of time, but it is only within the last two generations, within, indeed, the reign of our sovereign, queen victoria, that the subject of the differences of most of these kinds of time has become of pressing importance to any but theorists. in one of the public gardens of paris a little cannon is set up with a burning-glass attached to it in such a manner that the sun itself fires the cannon as it reaches the meridian. this, of course, is the time of paris noon--apparent noon--but it would be exceedingly imprudent of any traveller through paris who wished, say, to catch the one o'clock express, to set his watch by the gun. for if it happened to be in february, he would find when he reached the railway station that the station clock was faster than the sun by nearly a full quarter of an hour, and that his train had gone; whilst towards the end of october or the beginning of november, he would find himself as much too soon. until machines for accurately measuring time were invented, apparent time--time, that is to say, given by the sun itself, as by a sun-dial--was the only time about which men knew or cared. but when reasonably good clocks and watches were made, it was very soon seen that at different times in the year there was a marked difference between sun-dial time and that shown by the clock, the reason being simply that the apparent rate of motion of the sun across the sky was not always quite the same, whilst the movement of the clock was, of course, as regular as it could be made. this difference between time as shown by the actual sun and by a perfect clock is known as the 'equation of time.' it is least about april , june , august , and december . it is greatest, the sun being after the clock, about february ; and the sun being before the clock, about november . flamsteed, before he became astronomer royal, investigated the question, and so clearly demonstrated the existence, cause, and amount of the equation of time as entirely to put an end to controversy on the subject. we had thus, early in the century, the two kinds of time in common use, apparent time and mean time, or clock time. but as the sun can only be on one particular meridian at any given instant, the time as shown by the clocks in one particular town will differ from that of another town several miles to the east or west of it. it is thus noon at moscow hr. min. before it is noon at berlin, and noon at berlin min. before it is noon in london. this was all well enough known, but occasioned no inconvenience until the introduction of railway travelling; then a curious difficulty arose. suppose an express train was running at the rate of sixty miles an hour from london to bristol. the guard of the train sets his watch to london time before he leaves paddington, but if the various towns through which the train passes, reading, swindon, etc., each keep their own local time, he will find his watch apparently fast at each place he reaches; but on his return journey, if he sets to bristol time before starting, he will in a similar way find it apparently slow by the swindon, reading, and paddington clocks as he reaches them in succession. it became at once necessary to settle upon one uniform system of time for use in the railway guides. apart from this, a passenger taking train, say, at swindon, might have been very troubled to know whether the advertised time of his train was that of exeter, the place whence it started, or swindon, the station where he was getting in, or london, its destination. 'railway time,' therefore, was very early fixed for the whole of great britain to be the same as london time, which is, of course, time as determined at greenwich observatory. at first it was the custom to keep at the various stations two clocks, one showing local time, the other 'railway,' or greenwich time, or else the clocks would be provided with a double minute hand, one branch of which pointed to the time of the place, the other to the time of greenwich. it was soon found, however, that there was no sufficient reason for keeping up local time. even in the extreme west of england the difference between the two only amounted to twenty-three minutes, and it was found that no practical inconvenience resulted from saying that the sun rose at twenty-three minutes past six on march , rather than at six o'clock. the hours of work and business were practically put twenty-three minutes earlier in the day, a change of which very few people took any notice. other countries besides england felt the same difficulty, and solved it in the same way, each country as a rule taking as its standard time the time of its own chief city. there were two countries for which this expedient was not sufficient--the united states and canada. the question was of no importance until the iron road had linked the atlantic to the pacific in both countries. then it became pressing. no fewer than seventy different standards prevailed in the united states only some twenty years ago. the case was a very different one here from that of england, where east and west differed in local time by only a little over twenty minutes. in north america, in the extreme case, the difference amounted to four hours, and it seemed asking too much of men to call eight o'clock in their morning, or it might be four o'clock in their afternoon, their noonday. the device was therefore adopted of keeping the minutes and seconds the same for all places right across the continent, but of changing the hour at every ° of longitude. the question then arose what longitude should be adopted as the standard. the americans might very naturally have taken their standard time from their great national observatory at washington, or from that of their chief city, new york, or of their principal central city, chicago. but, guided partly no doubt by a desire to have their standard times correspond directly to the longitudes of their maps, and partly from a desire to fall in, if possible, with some universal time scheme, if such could be brought forward, they fixed upon the meridian of greenwich as their ultimate reference line, and defined their various hour standards as being exactly so many hours slow of greenwich mean time. the decision of the united states and of canada brought with it later a similar decision on the part of all the principal states of europe; and greenwich is not only 'longitude nought' for the bulk of the civilized world, but greenwich mean time, increased or decreased by an exact number of hours or half-hours, is the standard time all over the planet. no; the statement requires correction. two countries hold out, both close to our own doors. france, instead of adopting greenwich time as such, adopts _paris time less_ m. s. (that being the precise difference in longitude between the two national observatories). ireland disdains even such a veiled surrender, and dublin time is the only one recognized from the hill of howth to far valentia. so the distressful country preserves her old grievance, that she does not even get her time until after england has been served. the alteration in national habits following on the adoption of this european system has had a very perceptible effect in some cases. thus, switzerland has adopted mid-european time, one hour fast of greenwich; the true local time for berne being just half an hour later. the result of putting the working hours this thirty minutes earlier in the day has had such a noticeable effect on the consumption of gas, as to lead the gas company to contemplate agitating for a return to the old system. thus, greenwich time, as well as the greenwich meridian, has practically been adopted the world over. it follows, then, that the determination of time is the most important duty of the royal observatory; and the time department, the one to which is entrusted the duty of determining, keeping, and distributing the time, calls for the first attention. entering the transit room, the first thing that strikes the visitor is the extreme solidity with which the great telescope is mounted. it turns but in one plane, that of 'longitude nought,' and its pivots are supported by the pair of great stone pillars which we have already spoken of as occupying the principal part of the transit-room area, and the foundations of which go deep down under the surface of the hill. on the west side of the telescope, and rigidly connected with it, is a large wheel some six feet in diameter, and with a number of wooden handles attached to it, resembling the steering-wheel of a large steamer. this wheel carries the setting circle, which is engraved upon a band of silver let into its face near its circumference, a similar circle being at the back of the wheel nearer the pillar. eleven microscopes, of which only seven are ordinarily used, penetrate through the pier, and are directed on to this second circle. the present transit is the fourth which the observatory has possessed, and its three predecessors, known as halley's, bradley's, and troughton's, respectively, are still preserved and hang on the walls of the transit room, affording by their comparison an interesting object-lesson in the evolution of a modern astronomical instrument. the watcher who wishes to observe the passing of a star must note two things: he must know in what direction to point his telescope, and at what time to look for the star. then, about two minutes before the appointed time, he takes his place at the eyepiece. as he looks in he sees a number of vertical lines across his field of view. these are spider-threads placed in the focus of the eye-piece. presently, as he looks, a bright point of silver light, often surrounded by little flashing, vibrating rays of colour, comes moving quickly, steadily onward--'swims into his ken,' as the poet has it. the watcher's hand seeks the side of the telescope till his finger finds a little button, over which it poises itself to strike. on comes the star, 'without haste, without rest,' till it reaches one of the gleaming threads. tap! the watcher's finger falls sharply on the button. some three or four seconds later and the star has reached another 'wire,' as the spider-threads are commonly called. tap! again the button is struck. another brief interval and the third wire is reached, and so on, until ten wires have been passed, and the transit is over. the intervals are not, however, all the same, the ten wires being grouped into three sets, two of three apiece, and the third of four. [illustration: the chronograph.] each tap of the observer's finger completed for an instant an electric circuit, and recorded a mark on the 'chronograph.' this is a large metal cylinder covered with paper, and turned by a carefully-regulated clock once in every two minutes. once in every two seconds a similar mark was made by a current sent by means of the standard sidereal clock of the observatory. the paper cover of the chronograph after an hour's work shows a spiral trace of little dots encircling it some thirty times. these dots are at regular intervals, about an inch apart, and are the marks made by the clock. interspersed between them are certain other dots, in sets of ten; and these are the signals sent from the telescope by the transit observer. if, then, one of the clock dots and one of the observer's dots come exactly side by side, we know that the star was on one of the wires at a given precise second. if the observer's dot comes between two clock dots, it is easy, by measuring its distance from them with a divided scale, to tell the instant the star was on the wire to the tenth of a second, or even to a smaller fraction. whilst, since the transit was taken over ten wires, and the distance of each wire from the centre of the field of view is known, we have practically ten separate observations, and the average of these will give a much better determination of the time of transit than a single one would. but let the watcher be ever so little too slow in setting his telescope, or ever so little late in placing himself at his eye-piece, and the star will have passed the wire, and as it smoothly, resistlessly moves on its inexorable way, will tell the tardy watcher in a language there is no mistaking, 'lost moments can never be recalled.' the opportunity let slip, not until twenty-four hours have gone by will another chance come of observing that same star. it is the stars that are chiefly used in this determination, partly because the stars are so many, whilst there is but one sun. if, therefore, clouds cover the sun at the important moment of transit, the astronomer may well exclaim, so far as this observation is concerned, 'i have lost a day!' the chance will not be offered him again until the following noon. but if one star is lost by cloud, there are many others, and the chance is by no means utterly gone. beside, the sun enables us to tell the time only at noon; the stars enable us to find it at various times throughout the entire night; indeed, throughout both day and night, since the brighter stars can be observed in a large telescope even during the day. there are two great standard clocks at the observatory: the mean solar clock and the sidereal clock. the latter registers twenty-four hours in the precise time that the earth rotates on its axis. a 'day' in our ordinary use of the term is somewhat longer than this; it is the average time from one noon to the next, and as the earth whilst turning round on its axis is also travelling round the sun, it has to rather more than complete a rotation in order to bring the sun again on to the same meridian. a solar day is therefore some four minutes longer than an actual rotation of the earth, _i.e._ a sidereal day, as it is called, since such rotation brings a star back again to the same meridian. the sidereal clock can therefore be readily checked by the observation of star transits, for the time when the star ought to be on the meridian is known. if, therefore, the comparison of the transit taps on the chronograph with the taps of the sidereal clock show that the clock was not indicating this time at the instant of the transit, we know the clock must be so much fast or slow. similarly, the difference which should be shown between the sidereal and solar clocks at any moment is known; and hence when the error of the sidereal clock is known, that of the solar can be readily found. it is often quite sufficient to know how much a clock is wrong without actually setting its hands right; but it is not possible to treat the greenwich clock so, for it controls a number of other clocks continually, and sends hourly signals out over the whole country, by which the clocks and watches all over the kingdom are set right. in the lower computing room, below the south window, we find the time-desk, the head-quarters of the time department. this is a very convenient place for the department, since one of the chronometer rooms, formerly bradley's transit room, opens out of the lower computing room; the transit instrument is just beyond; it is close to the main gate of the observatory, and so convenient for chronometer makers or naval officers bringing chronometers or coming for them, whilst just across the courtyard is the chronograph room, with the battery basement, in which the batteries for the electric currents are kept, and the mean solar clock lobby, with the winch for the winding of the time-ball at the head of the stairs above it. these rooms do not exhaust the territory of the department, since it owns two other chronometer rooms on the ground floor and first floor respectively of the s.-e. tower. at the time-desk means are provided for setting the clock right very easily and exactly. just above the desk are a range of little dials and bright brass knobs, that almost suggest the stops of a great organ. two of these little dials are clock faces, electrically connected with the solar and sidereal standard clocks, so that, though these clocks are themselves a good way off, in entirely different parts of the observatory, the time superintendent, seated here at the time-desk, can see at once what they are indicating. between the two is a dial labelled 'commutator.' from this dial a little handle usually hangs vertically downwards, but it can be turned either to the right or to the left, and when thus switched hard over, an electric current is sent through to the mean solar clock. if now we leave the computing room and cross the courtyard to the extreme north-west corner, we find the mean solar clock in a little lobby, carefully guarded by double doors and double windows against rapid changes of temperature. opening the door of the clock case, we see that the pendulum carries on its side a long steel bar, and that this bar as the pendulum swings passes just over the upper end of an electro-magnet. when the current is switched on at the commutator, this electro-magnet attracts or repels the steel bar according to the direction of the current, and the action of the clock is accordingly quickened or retarded. to put the commutator in action for one minute will alter the clock by the tenth of a second. as the error of the clock is determined twice a day, shortly before ten o'clock in the morning, and shortly before one o'clock in the afternoon, its error is always small, usually only one or two tenths. these two times are chosen because, though time-signals are sent over the metropolitan area every hour from the greenwich clock through the medium of the post office, at ten and at one o'clock signals are also sent to all the great provincial centres. further, at one o'clock the time balls at greenwich and at deal are dropped, so that the captains of ships in the docks, on the river, or in the downs may check their chronometers. the time-ball is dropped directly by the mean solar clock itself. it is raised by means of a windlass turned by hand-power to the top of its mast just before one o'clock. connected with it is a piston working in a stout cylinder. when the ball has reached the top of the mast, the piston is lightly supported by a pair of catches. these catches are pulled back by the hourly signal current, and the piston at once falls sharply, bringing the ball with it. but after a fall of a few feet, the air compressed by the piston acts as a cushion and checks the fall, the ball then gently and slowly finishing its descent. the instant of the beginning of the fall is, of course, the true moment to be noted. the other dials on the time-desk are for various purposes connected with the signals. one little needle in a continual state of agitation shows that the electric current connecting the various sympathetic clocks of the observatory is in full action. another receives a return signal from various places after the despatch of the time-signal from greenwich, and shows that the signal has been properly received at the distant station, whilst all the many electric wires within the observatory or radiating from it are made to pass through the great key-board, where they can be at once tested, disconnected, or joined up, as may be required. [illustration: the time-desk.] the distribution of greenwich time over the island in this way is thus a simple matter. the far more important one of the distribution of greenwich time to ships at sea is more difficult. the difficulty lay in the construction of a clock or watch, the rate of which would not be altered by the uneasy motion of a ship, or by the changes of temperature which are inevitable on a voyage. two hundred years ago it was not deemed possible to construct a watch of anything like sufficient accuracy. they would not even keep going whilst they were being wound, and would lose or gain as much as a minute in the day for a fall or rise of ° in temperature. this was owing to the extreme sensitiveness of the balance spring--which takes the place in a watch of a pendulum in a clock--to the effects of temperature. the british government, therefore, in offered a prize of the amount of £ , for a means of finding the longitude at sea within half a degree, or, in other words, for a watch that would keep greenwich time correct to two minutes in a voyage across the atlantic. in , james harrison, the son of a yorkshire carpenter, succeeded in solving the problem. his method was to attach a sort of automatic regulator to the spring which should push the regulator over in one direction as the temperature rose, and bring it back as it fell. this he effected by fastening together two strips of brass and steel. the brass expanded with heat more rapidly than the steel, and hence with a rise of temperature the strip bent over on the steel side. this was the first germ of the idea of making watches 'compensated for temperature;' watches, that is, which maintain practically the same rate whether they are in heat or cold, an idea now brought to great perfection in the modern chronometer. [illustration: harrison's chronometer.] the great reward the government had offered stimulated many men to endeavour to solve the problem. of these, dr. halley, the second astronomer royal, and graham, the inventor of the astronomical clock, were the most celebrated. but when harrison, then poor and unknown, came to london in , and laid his invention before them, with an utter absence of self-seeking, and in the true scientific spirit, they gave him every assistance. harrison's first four time-keepers are still preserved at the royal observatory. he did not, however, receive his reward until a facsimile of the fourth had been made by his apprentice, larcum kendall. the latter is preserved at the royal observatory. there is a larcum kendall at the royal institution which is said to have been used by captain cook. harrison's chronometer was sent on a trial voyage to jamaica in , and on its return to portsmouth in the following year it was found that its complete variation was under the two minutes for which the government had stipulated. since harrison's day the improvement of the chronometer has been carried on almost to perfection, and now the care and rating of chronometers for the royal navy is one of the most important duties of the observatory. [illustration: the chronometer room.] a visitor who should make the attempt to compare a single chronometer with a standard clock would probably feel very disheartened when, after many minutes of comparison, he had got out its error to the nearest second, were he told that it was his duty to compare the entire army here collected, some five hundred or more, and to do it not to the second, but to the nearest tenth of a second. practice and system make, however, the impossible easy, and one assistant will quietly walk round the room calling out the error of each chronometer as he passes it, as fast as a second assistant seated at the table can enter it at his dictation in the chronometer ledgers. the seconds beat of a clock sympathetic with the solar standard, rings out loud and clear above the insect-like chatter of the ticking of the hundreds of chronometers, and wherever the assistant stands, he has but to lift his eyes to see straight before him, if not a complete clock-face, at least a seconds dial moving in exact accordance with the solar standard. the test to which chronometers are subjected is not merely one of rate, but one of rate under carefully altered conditions. thus they may be tried with the xii pointing in succession to the four points of the compass, or, in the case of chronometer watches, they may be laid flat down on the table or hung from the ring or pendant, or with the ring right or left, as it would be likely to be when carried in the waistcoat pocket. but the chief test is the performance of a chronometer when subjected to considerable heat for a long period. this is a matter of great consequence, since a chronometer travelling from england to india, australia, or the cape, would necessarily be subjected to very different conditions of temperature from those to which it would be exposed in england. they are therefore kept for eight weeks in a closed stove at a temperature of about ° or °. at one time a cold test was also applied, and sir george airy, the late astronomer royal, in one of his popular lectures, drew a humorous comparison between the unhappy chronometers thus doomed to trial, now in heat and now in frost, and the lost spirits whom dante describes as alternately plunged in flame and ice. the cold test has, however, been done away with. it is perfectly easy on the modern ship to keep the chronometer comfortably warm even on an arctic expedition. the elaborate cold testing applied to sir george nares' chronometers before he started on his polar journey was found to have been practically quite superfluous; the chronometers were, if anything, kept rather too warm. the exposure of the chronometer in the cooling box, moreover, was found to be attended with a risk of rusting its springs. [illustration: the chronometer oven.] once the determination of the longitude at sea became possible, it was clearly the next duty to fix with precision the position of the principal places, cities, ports, capes, islands, the world over. of all the work done in this department none has ever been done better, in proportion to the means at command, than that accomplished by captain cook in his celebrated three voyages. as has already been pointed out, it is the extent and thoroughness of the hydrographic surveys of the british admiralty which have largely contributed to the honour done to england by the international selection of the english meridian, and of english standard time, as in principle those for the whole civilized world. the generosity and public spirit therefore which led the second astronomer royal to help forward and support his rival, has almost directly led to this great distinction accruing to the observatory of which he was the head. three different methods have successively been used in the determination of longitudes of distant places. in each case the problem required was to ascertain the time at the standard place, say greenwich, at the same time that it was being determined in the ordinary way at the given station. one method of ascertaining greenwich time when at a distance from it was, as stated in chapter i., to use the moon, as it were, as the hand of a vast clock, of which the sky was the face and the stars the dial figures. this is the method of 'lunar distances,' the distances of the moon from a certain number of bright stars being given in the _nautical almanac_ for every three hours of greenwich time. as chronometers were brought to a greater point of perfection, it was found easier and better in many cases to use 'chronometer runs,' that is, to carry backwards and forwards between the two stations a number of good chronometers, and by constant comparison and re-comparison to get over the errors which might attach to any one of them. [illustration: the transit pavilion. (_from a photograph by mr. lacey._)] but of late years another method has proved available. distant nations are now woven together across thousands of miles of ocean by the submarine telegraph. the american reads in his morning paper a summary of the debates of the previous night in the house of commons at westminster. the londoner watches with interest the scores of the english cricket team in australia. it is now therefore possible for an astronomer in england to record, should he so desire, the time of the transit of a star across the wires of his instrument, not only on his own chronograph, but upon that of another observatory, it may be miles away. or, much more conveniently, each observer may independently determine the error of his own clock, and then bring his clock into the current, so that it may send a signal to the chronograph of the other station. in one way or another this work of the determination of geographical longitudes has been an important part of the extra-routine work at greenwich, part of the work which has built up and sustained its claim to define 'longitude nought'; and many distinguished astronomers, especially from the leading observatories of the continent, have come here from time to time to obtain more accurately the longitude of their own cities. the traces of their visits may be seen here and there about the observatory grounds in flat stones which lie level with the surface, and bear a name and date like the gravestones in some old country churchyard. these are not, as one might suppose, to mark the burial-places of deceased astronomers, but record the sites where, on their visits for longitude purposes, different foreign astronomers have set up their transit instruments. now, however, a permanent pier has been erected in the courtyard, and a neat house--the transit pavilion--built over it, so that in all probability no fresh additions will be made to these sepulchral-looking little monuments. it might be asked, what reason is there for a foreign observer to come over to england for such a purpose? would it not be sufficient for the clock signals to be exchanged? but a curious little fact has come out with the increase of accuracy of transit observation, and that is, that each observer has his own particular habit or method of observation. a hundred years ago, maskelyne, the fifth astronomer royal, was greatly disturbed to find that his assistant, david kinnebrook, constantly and regularly observed a star-transit a little later than he did himself. the offender was scolded, warned, exhorted, and finally, when all proved useless to bring his observations into exact agreement with the astronomer royal's, dismissed as an incompetent observer. as a matter of fact, poor kinnebrook has a right to be regarded as one of the martyrs of science, and maskelyne, by this most natural but mistaken judgment, missed the chance of making an important discovery, which was not made until some thirty years later. astronomers now would be more cautious of concluding that observations were bad simply because they differed from what had been expected. they have learnt by experience that these unexpected differences are the most likely hunting-ground in which to look for new discoveries. in a modern transit observation with the use of the chronograph it will be seen at once that before the observer can register a star-transit on the chronograph, he has to perceive with his eye that the star has reached the wire, he has to mentally recognize the fact, and consciously or unconsciously to exert the effort of will necessary to bring his finger down on the button. a very slight knowledge of character will show that this will require different periods of time for different people. it will be but a fraction of a second in any case, but there will be a distinct difference, a constant difference, between the eager, quick, impulsive man who habitually anticipates, as it were, the instant when he sees star and wire together, and the phlegmatic, slow-and-sure man who carefully waits till he is quite sure that the contact has taken place, and then deliberately and firmly records it. these differences are so truly personal to the observer that it is quite possible to correct for them, and after a given observer's habit has become known, to reduce his transit times to those of some standard observer. it must, of course, be remembered that this 'personal equation' is an exceedingly minute quantity, and in most cases is rather a question of hundredths of seconds than of tenths. it will be seen from the foregoing description how little of what may be termed the picturesque or sensational side of astronomy enters into the routine of the time department, the most important of all the departments of the observatory. the daily observation of sun and of many stars--selected from a carefully chosen list of some hundreds, and known as 'clock stars'--the determination of the error of the standard clock to the hundredth of a second if possible, and its correction twice a day, the sending out of time signals to the general post office and other places, whence they are distributed all over the country; the care, winding, and rating of hundreds of chronometers and chronometer watches, and from time to time the determination of the longitude of foreign or colonial cities, make up a heavy, ceaseless routine in which there is little opportunity for the realization of an astronomer's life as it is apt to be popularly conceived. yet there is interest enough in the work. there is the charm which always attaches to work of precision, the delight of using delicate and exact instruments, and of obtaining results of steadily increasing perfection. it may be akin to the sporting passion for record-breaking, but surely it is a noble form of it which has led the assistants, in recent years, to steadily increase the number of observations in a normal night's work up to the very limit, taking care the while that their accuracy has in no degree suffered. in longitude work also 'the better is the enemy of the good,' and there is the ambition that each fresh determination shall be markedly more precise than all that have preceded it. the constant care of chronometers soon reveals a kind of individuality in them which forms a fresh source of interest, whilst if a man has but a spark of imagination, how easily he will wrap them round with a halo of romance! glance through the ledgers, and you will see how some of them have heard the guns at the siege of alexandria, others have been carried far into the frozen north, others have wandered with livingstone or cameron in the trackless forests of equatorial africa. more striking still are those pages across which the closing line has been drawn; never again will the time-keeper there scheduled return to the kindly inquisition of flamsteed hill. this sailed away in the wasp, and was swallowed up in the eastern typhoon; that went down in the sudden squall that smote the eurydice off the isle of wight; these foundered with the captain. the last fatal journey of sir john franklin to find the north-west passage leaves its record here; the chronometers of the erebus and terror will never again appear on the greenwich muster roll. land exploration claims its victims too. sturt's ill-fated expedition across australia, and livingstone's last wandering, are represented. [illustration: 'lost in the birkenhead.'] sometimes an amusing entry interrupts the silent pathos of these closed pages. 'lost by mr. smith on the coast of africa,' reads at first sight like a rather thin attempt of some one to shift the responsibility of his own carelessness on to the broad shoulders of mr. nobody. in reality it probably gives a hint of the necessary, dangerous, and exciting work of slave-dhow chasing which gives employment to our ships on the african coast. 'mr. smith' was no doubt a petty officer who was told off to carry the chronometer for a boat's crew sent to search for a slave-dhow up some equatorial estuary. probably the dhow was found, and the arabs who manned it gave so stout a resistance that 'mr. smith' and his men had other things to do than take care of chronometers before they could overcome them. we may take it that the real story outlined here was one of courage and hard fighting, not of carelessness and shirking. stories of higher valour and nobler courage yet are also hinted: the calm discipline of the crew of the victoria as she sank from the ram of the camperdown, the yet nobler devotion of the men of the birkenhead, as they formed up in line on deck and cheered the boats that bore away the women and children to safety, whilst they themselves went down with the ship into the shark-crowded sea. 'there rose no murmur from the ranks, no thought by shameful strength, unhonoured life to seek; our post to quit we were not trained, nor taught to trample down the weak. 'what followed, why recall? the brave who died died without flinching in that bloody surf. they sleep as well beneath that purple tide as others under turf.' chapter vii the transit and circle departments the determination of time is a duty the importance of which readily commends itself to the general public. it is easy to see that in any civilized country it is very necessary to have an accurate standard of time. our railways and telegraphs make it quite impossible for us to be content with the rough-and-ready sun-dial which satisfied our forefathers. but it should be remembered that it was neither to establish a 'longitude nought,' nor to create a system of standard time, that greenwich observatory was founded in . it was for 'the rectifying the tables of the motions of the heavens and the places of the fixed stars, in order to find out the so-much-desired longitude at sea for the perfecting the art of navigation.' the two related departments, therefore, those of the transit and the circle, which are concerned in the work of making star-catalogues, come next in order to the time department. though both departments deal with the same instrument, the transit circle, they are at present placed at opposite ends of the observatory domain; the circle department being lodged in the upper computing room of the old building; the transit department in the south wing of the new observatory in the south ground. it may be asked why, if this were the purpose of the observatory at its foundation, two and a quarter centuries ago; if, as was the case, the work was set on foot from the beginning and was carried out with every possible care, how comes it that it is still the fundamental work of the observatory, and, instead of being completed, has assumed greater proportions at the present day than ever before? the answer to this inquiry may be found in a special application of the old proverb, originally directed against the discontent of man: 'the more he has, the more he wants.' for, however paradoxical it may seem, it is true that the fuller a star-catalogue is, and the more accurate the places of the stars that it contains, the greater is the need for a yet fuller catalogue, with places more accurate still. it is worth while following up this paradox in some detail, as it affords a very instructive example of the way in which a work started on purely utilitarian grounds extends itself till it crosses the undefined boundary and enters the region of pure science. we have no idea who made the earliest census of the sky. it is written for us in no book; it is not even engraved on any monument. and yet no small portion of it is in our hands to-day, and, strangest of all, we are able to fix fairly closely the time at which it was made, and the latitude in which its compiler lived. the catalogue is very unlike our star-catalogues of to-day. the places of the stars are very roughly indicated; and yet this catalogue has left a more enduring mark than all those that have succeeded it. the catalogue simply consists of the star names. an old lady who had attended a university extension lecture on astronomy was heard to exclaim: 'what wonderful men these astronomers are! i can understand how they can find out how far off the stars are, how big they are, and what they weigh--that is all easy enough; and i think i can see how they find out what they are made of. but there is one thing that i can't understand--i don't know how they can find out what are their names!' this same difficulty, though with a much deeper meaning than the old lady in her simplicity was able to grasp, has occurred to many students of astronomy. many have wished to know what was the meaning of, and whence were derived, the sonorous names which are found attached to all the brighter stars on our celestial globes: adhara, alderamin, betelgeuse, denebola, schedar, zubeneschamal, and many more. the explanation lies here. some years ago, a man, or college of men, living in latitude ° north, in order that they might better remember the stars, associated certain groups of them with certain fancied figures, and the individual star names are simply arabic words designed to indicate whereabouts in its peculiar figure or constellation that special star was situated. thus adhara means 'back,' and is the name of the bright star in the back of the great dog. alderamin means 'right arm,' and is the brightest star in the right arm of cepheus, the king. betelgeuse is 'giant's shoulder,' the giant being orion; denebola is 'lion's tail.' schedar is the star on the 'breast' of cassiopeia, and zubeneschamal is 'northern claw,' that is, of the scorpion. so far is clear enough. the names of the stars for the most part explain themselves; but whence the constellations derived their names, how it was that so many snakes and fishes and centaurs were pictured out in the sky, is a much more difficult problem, and one which does not concern us here. one point, however, these old constellations do tell us, and tell us plainly. they show that the axis of the earth, which, as the earth travels round the sun, moves parallel with itself, yet, in the course of ages, itself rotates so as in a period of some , years to trace out a circle amongst the stars. this is the cause of what is called 'precession,' and explains how it is that the star we call the pole-star to-day was not always the pole-star, nor will always remain so. we learn this fact from the circumstance that the old constellations do not cover the entire celestial sphere. they leave a great circular space of ° radius unmapped in the southern heavens. this simply means that the originators of the constellations lived in ° north latitude, and stars within ° of their south pole never rose above their horizon, and consequently were never seen, and could not be mapped, by them. in like manner, the star census taken at greenwich observatory does not include the whole sky, but leaves a space some ° in radius round our south pole. since the latitude of greenwich is nearly ° north, stars within that distance of the south pole do not rise above our horizon, and are never seen here. but if we compare the vacant space left by the old original constellations with the vacant space left by a greenwich catalogue of to-day, we see that the centre of the first space, which must have been the south pole of that time, is a long way from the centre of the second space--our south pole of to-day. the difference tells us how far the pole has moved since those old forgotten astronomers did their work. we know the rate at which the pole appears to move, by comparing our more modern catalogues one with another; and so we are able to fix pretty nearly the time when lived those old first census-takers of the stars, whose names have perished so completely, but whose work has proved so immortal. these old workers gave us the constellation groupings and names which still remain to us, and are still in common, every-day use. their work affords us the most striking illustration of the result of precession, but precession itself was not recognized till nearly years after their day, when a marvellous genius, hipparchus, established the fact, and 'built himself an everlasting name' by the creation of a catalogue of over stars prepared on modern principles. that catalogue formed the basis of one which survives to us at the present time, and was made some years ago by claudius ptolemy, the great astronomer of alexandria, whose work, which still bears the proud name of _almagest_, 'the greatest,' remained for fourteen centuries the one universal astronomical text-book. a modern catalogue contains, like that of ptolemy, four columns of entry. the first gives the star's designation; the second an indication of its brightness; the third and fourth the determinations of its place. these are expressed in two directions, which, in modern catalogues, not in ptolemy's, correspond on the celestial sphere to longitude and latitude on the terrestrial. distance north or south of the celestial equator is termed 'declination,' corresponding to terrestrial latitude. distance in a direction parallel to the equator is termed 'right ascension,' corresponding to terrestrial longitude. for geographical purposes we conceive the earth to be encircled by two imaginary lines at right angles to each other--the one, the equator, marked out for us by the earth itself; the other, 'longitude nought,' the meridian of greenwich, fixed for us by general consent, after the lapse of centuries, by a kind of historical evolution. on the celestial globe in like manner we have two fundamental lines--one, the celestial equator, marked out for us by nature; the other at right angles to it, and passing through the poles of the sky, adopted as a matter of convenience. but a difficulty at once confronts us--where can we fix our 'right ascension nought'? what star has the right to be considered the greenwich of the sky? the difficulty is met in the following manner: for six months of the year, the summer months, the sun is north of the celestial equator; for the other six months of the year, the months of winter, it is south of it. it crosses the equator, therefore, twice in the year--once when moving northward at the spring equinox; once when moving southward at the equinox of autumn. the point where it crosses the equator at the first of these times is taken as the fundamental point of the heavens, and the first sign of the zodiac, aries the ram, is said to begin here, and it is called, therefore, 'the first point of aries.' one of the very first facts noticed in the very early days of astronomy was that, as the stars seemed to move across the sky night by night, they seemed to move in one solid piece, as if they were lamps rigidly fixed in one and the same solid vault. of course it has long been perfectly understood that this apparent movement was not in the least due to any motion of the stars, but simply to the rotation of the earth on its axis. this rotation is the smoothest, most constant, and regular movement of which we know. it follows, therefore, that the interval of time between the passage of one star across the meridian of greenwich and that of any other given star is always the same. this interval of time is simply the difference of their right ascension. if we are able, then, to turn our transit instrument to the sun, and to a number of stars, each in its proper turn, and by pressing the tapping-piece on the instrument as the sun or star comes up to each of the ten wires in succession, to record the times of its transit on the chronograph, we shall have practically determined their right ascensions--one of the two elements of their places. the other element, that of declination, is found in a different manner. the celestial equator, like the terrestrial, is ° from the pole. the bright star polaris is not exactly at the north pole, but describes a small circle round it. twice in the twenty-four hours it transits across the meridian--once when going from east to west it passes above the pole, once when going from west to east below the pole. the mean between these two altitudes of polaris above the horizon gives the position of the true pole. [illustration: the transit circle.] a complete transit observation of a star consists therefore of two operations. the observer, as we have already described, sees a star entering the field of the telescope, and as it swims forward, he presses the galvanic button, which sends a signal to the chronograph as the star comes up to each of the ten vertical wires in succession. but, beside the ten wires, there are others. two vertical wires lie outside the ten of which we have already spoken, and there is also a horizontal wire. the latter can be moved by a graduated screw-head just above the eye-piece, and as the star comes in succession to these two vertical wires, this horizontal wire is moved by the screw-head, so as to meet the star at the moment it is crossing the vertical wire, and the observer presses a second little button, which records the position of the horizontal wire on a small paper-covered drum. then, the transit over, the observer leaves the telescope and comes round to the outside of the west pier. here he finds seven large microscopes, which pierce the whole thickness of the pier, and are directed towards the circumference of a large wheel which is rigidly attached to the telescope and revolves with it. this wheel is six feet in diameter, and has a silver circle upon both faces. each circle is divided extremely carefully into divisions--these divisions, therefore, being about the one-twentieth of an inch apart. there are, therefore, twelve divisions to every degree ( × = ), and each division equals five minutes of arc. the lowest microscope is the least powerful, and shows a large part of the circle, enabling the observer to see at once to what degree and division of a degree the microscope is pointing. the other six microscopes are very carefully placed ° apart--as equally placed as they possibly can be. these microscopes are all fitted with movable wires--wires moved by a very fine and delicate screw; the screw-head having divisions upon it so that the exact amount of its movement can be told. each of the six screw-heads will read to the one five-thousandth part of a division of the circle; in other words, to the one hundred thousandth part of an inch. using all six microscopes, and taking their mean, we are able to _read_ to the one-hundredth of a second of arc. if, therefore, the observations could be made with perfect certainty down to the extremest nicety of reading which the instrument supplies, we should be able to read the declination of a star to this degree of refinement. it may be added that a halfpenny, at a distance of three miles, appears to be one second of arc in diameter; at three hundred miles it would be one-hundredth of a second. it need scarcely be said that we cannot observe with quite such refinement of exactness as this would indicate. nevertheless, this exactness is one after which the observer is constantly striving, and tenths, even hundredths, of seconds of arc are quantities which the astronomer cannot now neglect. the observer has then to read the heads of all these seven microscopes on the pier side, and also two positions of the horizontal wire on the screw-head at the eye-piece. the following morning he will also read off from the chronograph-sheet the times when he made the ten taps as the star passed each of the ten vertical wires. there are, therefore, nine entries to make for one position of a star in declination, and ten for one position of a star in right ascension. the observer will also have to read the barometer to get the pressure of the air at the time of observation, and one thermometer inside the transit room, and another outside, to get the temperature of the air. in some cases thermometers at different heights in the room are also read. a complete observation of a single star means, therefore, the entry of two-and-twenty different numbers. it may be asked, what is the use of reading the barometer and thermometer? the answer to the question can only be given by contradicting a statement made above, that the true pole lay midway between the position of the telescope when pointing to the pole-star at its upper transit, and its position when pointing to it at its lower transit. the pole being very high in the heavens in this country, there are a great number of stars that, like the pole-star, cross the meridian twice in the twenty-four hours--once when they pass above the pole, moving from east to west, once when they pass below it, moving from west to east as the real distance of a star from the true pole does not alter, it follows that we ought to get the position of the pole from the mean of the two transits of any of these stars, and they ought all to exactly agree with each other. but they do not. so, too, i said that the stars all appeared to move as in a single piece. if, then, we constructed an instrument with its axis parallel to the axis of the earth, and fixed a telescope to it, pointing to any particular star, if we turn the telescope round as fast from east to west as the earth itself is turning from west to east--if we built an equatorial, that is to say--we ought to find that the star once in the centre of the field would remain there. as a matter of fact, when the star got near the horizon it would soon be a long way from the centre of the field. sir george airy, the seventh astronomer royal, makes, with reference to this very point, the following remarks: 'perhaps you may be surprised to hear me say the rule is established as true, and yet there is a departure from it. this is the way we go on in science, as in everything else; we have to make out that something is true, then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained.' in this particular case, the disturbing cause is found in the action of our own atmosphere. the rays of light from the star are bent out of a perfectly straight course as they pass through the various layers of that atmosphere, layers which necessarily become denser the closer we get to the actual surface of the earth. every celestial body therefore appears to be a little higher in the sky than it really is. this action is most noticeable at the horizon, where it amounts to about half a degree. as both sun and moon are about half a degree in diameter, it follows that when they have really just entirely sunk below the horizon they appear to be just entirely above it. it happens, in consequence, on rare occasions, that an eclipse of the moon will take place when both sun and moon are together seen above the horizon. it was a great matter to discover this effect of refraction. it was soon seen that it was not constant, that it varied with both temperature and pressure. it is, indeed, the most troublesome of all the hindrances to exact observation with which the astronomer has to contend; partly because of its large amount--half a degree, as has been already said, in the extreme case--and partly because it is difficult in many cases to determine its exact effect. the double observation with the transit circle gives us, then, the place in the sky where the star _appeared_ to be at the moment of observation, not its true place; to find that true place we have to calculate how much refraction had displaced the star at the particular height in the sky, and at the particular temperature and atmospheric pressure at which the observation was made. [illustration: the mural circle.] the transit circle is a comparatively recent instrument. in earlier times the two observations of right ascension and declination were entrusted to perfectly separate instruments. the transit instrument was mounted as the transit circle is, between two solid stone piers, and moved in precisely the same way. but the great six-foot wheel, which was made as stiff as it possibly could be, was mounted on the face of a great stone pier or wall, from which circumstance it was called the 'mural circle,' and a light telescope was attached to it which turned about its centre. this arrangement had a double disadvantage--that the two observations had to be made separately, and the mural circle, not being a symmetrical instrument, was liable to small errors which it was difficult to detect. thus, being supported on one side only, a flexure or bending outwards of either telescope or circle, or both, might be feared. it was for this reason that pond set up a pair of mural circles, one on the east side of its supporting pier and the other on the west.[ ] his plan was not only to have each star observed simultaneously in the two instruments, a plan by which, at the cost of some additional labour, he would have got rid, to a large extent, of the individual errors of the two separate instruments, inasmuch as, on the whole, it might have been expected that the errors of the two instruments would have been very nearly equal in amount, but of opposite character. the differences, too, between the two instruments would have afforded the means for tracing these small errors to their respective causes, and so ascertaining the laws to which they were subject. [ ] the second circle was intended for the cape observatory, but pond obtained leave to retain it. in it was transferred to the observatory of queen's college, belfast. pond went further still. he added to the mural circle a simple instrument, the extreme value of which every astronomer recognizes to-day--the mercury trough. not only was the star to be observed by both circles when the two telescopes were pointing directly to it, it was also to be observed by reflection; the telescopes were to be pointed down towards a basin of mercury, in which the image of the star would be seen reflected. the mercury being a liquid, its surface is perfectly horizontal; and, since the law of reflection is that the angle of incidence is equal to the angle of reflection, it follows that the telescope, when pointed down toward the mercury trough, points just at as great an angle below the horizon as, when it is set directly on the star, it points above it. if the circle, therefore, be carefully read at both settings, half the difference between the two readings will give the angular elevation of the star above the horizon. a combination, therefore, of all four observations, that is to say, one reflection and one direct with each of the telescopes, would give an exceedingly exact value for the star's altitude. the conception of this method gives a striking idea of pond's thoroughness and skill as a practical observer, and it is a distinct blot upon airy's justly high reputation in the same line that he discontinued the system upon his accession to office. however, in , as already mentioned, airy substituted for the two separate instruments--the transit and mural circle--the transit circle, which, unlike the mural circle, is equally supported on both sides. this, however, does not free it from the liability to some minute flexure in the direction of its length, from the weight of its two ends, and the mercury trough is used for the detection of such bending, should it exist. the present practice is to observe a star both by reflection and directly in the course of the same transit. the observer sets the telescope carefully before ever the star comes into the field of view, and reads his seven microscopes. then he climbs up a narrow wooden staircase and watches the star transit nearly half across the field. then comes a rush, the observer swings himself down the ladder, unclamps the telescope, turns it rapidly up to the star itself, clamps it again, flings himself on his back on a bench below the telescope, and does it so quickly that he is able to observe the star across the second half of the field. there is no time for dawdling, no room for making any mistakes; the stars never forgive; 'they haste not, they rest not;' and if the unfortunate observer is too slow, or makes some slip in his second setting, the star, cold and inexorable, takes no pity, and moves regardless on. it will be seen that a considerable amount of work is involved in taking a single observation of a star-place. but in making a star-catalogue it is always deemed necessary to obtain at least three observations of each star; and many are observed much more frequently. a modern star-catalogue contains, like ptolemy's, four columns. it contains also several more. of these the principal are devoted to the effect of precession. as precession is caused by a movement of the earth's axis making the pole of the sky seem to describe a circle in the heavens, it follows that the celestial poles, and the celestial equator with them are slowly, but continually, changing their place with respect to the stars, and therefore that the declinations of the stars are always undergoing change, and as the equator changes, the point where the sun crosses it in spring--the first point of aries--changes also, and with it the stars' right ascensions. to make one determination of a star's place comparable with another made at another time, it is clear that we must correct for the effects of precession in the interval of time between the two observations, and for the effects of refraction. but observations made with the transit circle must also be corrected for errors in the instrument itself. the astronomer will see to it that his instrument is made and is set up as perfectly as possible. the pivots on which it turns must be exactly on the same level; they must point exactly east and west, and the axis of the telescope must be exactly at right angles to the line joining the pivots in all positions of the instrument. these conditions are very nearly fulfilled, but never absolutely. day by day, therefore, the astronomer has to ascertain just how much his instrument is in error in each of these three matters. were his instrument absolutely without error to-day, he could not assume that it would remain so, nor, if he had measured the amount of its errors yesterday, would it be safe to assume that those errors would not change to-day. in the examination of these sources of error the mercury trough comes again into use. the transit circle is turned directly downwards, and the mercury trough brought below it. a light is so arranged as to illuminate the field of the telescope, and the observer, looking in, sees the ten transit wires and the one declination wire, and also sees their images reflected from the surface of the mercury. if the telescope be pointing _exactly_ down towards the surface of the mercury, then the image of the declination wire will fall exactly on the declination wire itself, and by reading the circle we can tell where the zenith point of the circle is. similarly, if the pivots of the telescope are precisely on the same level, the centre wire of the right ascension series would coincide with its reflected image. a third point is determined by looking through the eye-piece of the north collimator telescope--that is to say, the telescope mounted in a horizontal position at the north end of the room--at the spider lines in the focus of the south collimator. in order to get this view, the transit telescope has either to be lifted up out of its usual position, or else the middle of the tube has to be opened. the spider lines in the north collimator are then made to coincide with the image of the wires of the south collimator. the transit telescope is then turned first to one collimator, then to the other, and the central wire of the right ascension series is turned till it coincides with the wire of the collimator; the amount by which it has to be moved giving an index of the error of collimation; that is to say, of the deviation of the optical axis of the telescope from perpendicularity to the line joining the pivots. i have said enough to show that the making of an observation is a small matter as compared with those corrections which have to be made to it afterwards, before it is available for use. but i have only mentioned some of the reductions and corrections which have to be made. there are several more, and it is a just pride of greenwich that her third ruler, bradley, as has been already told in the notice of his life, discovered two of the most important. the one, aberration, is due to the fact that light, though it moves so swiftly-- , miles per second--yet does not move with an infinitely greater velocity than that of the earth. the other, nutation, might be called a correction to precession, inasmuch as, moved by the moon's attraction, the earth's axis does not swing round smoothly, but with a slight nodding or staggering motion. but when these observations of the places of a star have been made, and have been properly 'reduced,' even then we do not find an exact correspondence between two different determinations. little differences still remain. some of these are to be accounted for by changes in the actual crust of the earth, which, solid and stable as we think it, is yet always in motion. professor milne, our greatest authority on earth movements, says, 'the earth is so elastic that a comparatively small impetus will set it vibrating; why, even two hills tip together when there is a heavy load of moisture in the valley between them. and then, when the moisture evaporates in a hot sun, they tip away from each other.' so there is a perceptible rocking to and fro even of the huge stone piers of a transit circle, as seasons of rain and drought, heat and cold, follow each other. more than that, the earth is so sensitive to pressure that it was found, at the oxford university observatory, that there was a distinct swaying shown by a horizontal pendulum when the whole of a party of seventy-six undergraduates stood on one side of the instrument and close up to it, from the position it had when the party stood ninety feet away. more wonderful still, a comparison of the star-places, obtained at a number of observatories, including greenwich, has shown that the earth is continually changing her axis of rotation. and so the star-places determined at greenwich have shown that the north pole of the earth, miles away, moves about in an irregular curve about thirty feet in radius. nothing is stable, nothing is immovable, nothing is constant. the astronomer even finds that his own presence near the instrument is sufficient to disturb it. the great interest attaching to transit-circle work is this striving after ever greater and greater precision, with the result of bringing out fresh little discordances, which, at first sight, appear purely accidental, but which, under further scrutiny, show themselves to be subject to some law. then comes the hunt for this new unknown law. its discovery follows. it explains much, but when it is allowed for, though the observations now come much closer together, little deviations still remain, to form the subject of a fresh inquiry. astronomy has well been called the exact science, and yet exactitude ever eludes its pursuer. if it be asked, 'what is the use of this ever-increasing refinement of observation?' no better answer can be given than the words of sir john herschel in one of his presidential addresses to the royal astronomical society:-- 'if we ask to what end magnificent establishments are maintained by states and sovereigns, furnished with masterpieces of art, and placed under the direction of men of first-rate talent and high-minded enthusiasm, sought out for those qualities among the foremost in the ranks of science, if we demand, _cui bono?_ for what good a bradley has toiled, or a maskelyne or a piazzi has worn out his venerable age in watching?--the answer is, not to settle mere speculative points in the doctrine of the universe; not to cater for the pride of man by refined inquiries into the remoter mysteries of nature; not to trace the path of our system through space, or its history through past and future eternities. these, indeed, are noble ends, and which i am far from any thought of depreciating; the mind swells in their contemplation, and attains in their pursuit an expansion and a hardihood which fit it for the boldest enterprise. but the direct practical utility of such labours is fully worthy of their speculative grandeur. the stars are the landmarks of the universe; and, amidst the endless and complicated fluctuations of our system, seem placed by its creator as guides and records, not merely to elevate our minds by the contemplation of what is vast, but to teach us to direct our actions by reference to what is immutable in his works. it is, indeed, hardly possible to over-appreciate their value in this point of view. every well-determined star, from the moment its place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can never deceive or fail him, the same for ever and in all places; of a delicacy so extreme as to be a test for every instrument yet invented by man, yet equally adapted for the most ordinary purposes; as available for regulating a town clock as for conducting a navy to the indies; as effective for mapping down the intricacies of a petty barony as for adjusting the boundaries of transatlantic empires. when once its place has been thoroughly ascertained and carefully recorded, the brazen circle with which that useful work was done may moulder, the marble pillar may totter on its base, and the astronomer himself survive only in the gratitude of posterity; but the record remains, and transfuses all its own exactness into every determination which takes it for a groundwork, giving to inferior instruments--nay, even to temporary contrivances, and to the observations of a few weeks or days--all the precision attained originally at the cost of so much time, labour, and expense.' but for these strictly utilitarian purposes a comparatively small number of stars would meet our every requisite. in the narrow sense of which sir john herschel is here speaking, we have no use for anything beyond the smallest of catalogues; and if the question before us is, 'why are we continually extending our catalogues?' the following words of a more recent writer[ ] on the subject will set forth the real explanation:-- 'a word in conclusion, suggested by the history of star-catalogues. we have no difficulty in understanding that it is necessary to study the planets, and a reasonable number of the brighter stars, for the purpose of determining the figure of the earth, and the positions of points upon its surface; but the use for a catalogue of ten thousand stars, such as la caille compiled, is not just so apparent. nay, what did ptolemy want with a thousand stars, or tamerlane's grandson, born, reared, and destined to die amidst a horde of savages, however splendid in their trappings? there is not, and there never was, any real, practical use for the great volumes of star-catalogues that weigh down the shelves of our libraries. the navigator and explorer need never see them at all. why, then, were these pages compiled? why have astronomers, from hipparchus's time to the present, spent their lives in the weary routine-work of observing the places of tiny points in the stellar depths? does it not seem that there is something in the mind of man that impels him to seek after knowledge--truly--for its own sake? something heaven-born, heaven-nurtured, god-given ... that there is something in man common to him and his creator, and therefore eternal ... in beautiful accord with the plain statement that "god made man in his own image?"' [ ] mr. thomas lindsay, _transactions of the astronomical and physical society of toronto_, , p. . chapter viii the altazimuth department the determining of the places of the fixed stars which flamsteed carried out so efficiently in his _british catalogue of stars_--the first 'census of the sky' made with the aid of a telescope--was but half of the work imposed upon him. the other half, equally necessary for the solution of the problem of the longitude at sea, was the 'rectifying the tables of the motions of the heavens.' this second duty was not less necessary than the other, for, if we may again use the old simile of the clock-face, the fixed stars may be taken to represent the figures on the vast dial of the sky, whilst the moon, as it moves amongst them, corresponds to the moving hand of the timepiece. to know the places of the stars, then, without being able to predict the place of the moon, would be much like having a clock without its hands. but if not less necessary, it was certainly more difficult. the secret of the movements of the moon and planets had not then been grasped, and the only tables which had been calculated were based upon observations made before the days of telescopes. it is one of the most fortunate and remarkable coincidences in the whole history of science, that at the very time that greenwich observatory was being called into existence, the greatest of all astronomers was working out his demonstration of the great fundamental law of the material universe--the law that every particle of matter attracts every other particle with a force which varies directly with the mass and inversely with the square of the distance. several other of the great minds of that time, in particular dr. hooke, the gresham professor of astronomy, had seen that it was possible that some such law might supply the secret of planetary motion; but it is one thing to make a suggestion, and a very different matter indeed to be able to demonstrate it; and the latter was in newton's power alone. he did much more than demonstrate it--he brought out a whole series of most important and far-reaching consequences. he showed that the ebb and flow of the tides was due to the attraction of both sun and moon, especially the latter, upon the waters of our oceans. he pointed out certain irregularities which must take place in the motion of our moon, due to the influence of the sun upon it. he showed, too, what was the cause of that swinging of the axis of the earth which gives rise to precession. he deduced the relative weights of the earth, the sun, and of jupiter and saturn, the planets with satellites. he proved also that comets, which had seemed hitherto to men as perfectly lawless wanderers, obeyed in their orbits the self-same law which governed the moon and planets. the whole vast system of celestial movements, which had long seemed to men irregular and uncontrolled, now fell, every one of them, into its place, as but the necessary manifestations of one grand, simple order. this great discovery gave a new and additional importance to the regular observation of the moon and planets. they were needed now, not only to assist in the practical work of navigation, but for the development of questions of pure science. halley, the second astronomer royal, and maskelyne, the fifth, devoted themselves chiefly to this department of work, to the partial neglect of the observation of the places of stars. airy, the seventh, whilst making catalogue-work a part of the regular routine of the observatory, developed the observation of the members of the solar system, and especially of the moon, in a most marked degree, and collected and completely reduced the vast mass of material which the industry of his predecessors had gathered. it is pre-eminently of the work of airy that the memorable words quoted before of professor newcomb, the great american mathematician and astronomer, are applicable: 'that if this branch of astronomy were entirely lost, it could be reconstructed from the greenwich observations alone.' a most important step taken by airy was the construction of an altazimuth. an altazimuth is practically a theodolite on a large scale. its purpose is to determine, not the declination and right ascension of some celestial body, as is the case with the transit circle, but its altitude, _i.e._ its height above the horizon, and its azimuth, _i.e._ the angle measured on the horizontal plane from the north point. the altazimuth, then, like the transit circle, consists of a telescope revolving on a horizontal axis, but, unlike the transit circle, both the telescope and the piers which carry its pivots can be rotated so as to point not merely due north and south, but in any direction whatsoever. [illustration: airy's altazimuth.] the observations with the altazimuth are rather more complicated than those with the transit circle. looking in the telescope, the observer sees a double set of spider threads or 'wires'; and when a star or other heavenly body enters the field, it will generally be observed to move obliquely across both sets of wires. the observer usually determines to make an observation either in altitude or azimuth. in the former case he presses the little contact button, which, as in the transit circle, is provided close to the eyepiece, as the star reaches each of the horizontal wires in succession. if in azimuth, it is the times of crossing the vertical wires that are in like manner telegraphed to the chronograph. the transit over, the appropriate circle is read; for the telescope itself is rigidly attached to a vertical wheel having a carefully engraved circle on its face and read by four microscopes, whilst the entire instrument carries another set of microscopes, pointing to a fixed horizontal circle, and upon which the azimuth can be read. a complete observation involves four such transits and sets of circle readings, two of altitude, and two of azimuth; for after one of altitude and one of azimuth the telescope is turned round, and a second observation is taken in each element. the observation gives us the altitude and azimuth of the star. these particulars are of no direct value to us. but it is a mere matter of computation, though a long and laborious one, to convert these elements into right ascension and declination. the usefulness of the altazimuth will be seen at once. it will be remembered that with the transit circle any particular object can only be observed as it crosses the meridian. if the weather should be cloudy, or the observer late, the chance of observation is lost for four and twenty hours, and in the case of the moon, for which the altazimuth is specially used, it is on the meridian only in broad daylight during that part of the month which immediately precedes and follows new moon. at such times it is practically impossible to observe it with the transit circle; with the altazimuth it may be caught in the twilight before sunrise or after sunset; and at other times in the month, if lost on the meridian in the transit circle, the altazimuth still gives the observer a chance of catching it any time before it sets. but for this instrument, our observations of the moon would have been practically impossible over at least one-fourth of its orbit. airy's altazimuth was but a small instrument of three and three-quarter inches aperture, mounted in a high tower built on the site of flamsteed's mural arc; and, after a life history of about half a century, has been succeeded by a far more powerful instrument. the 'new altazimuth' has an aperture of eight inches, and is housed in a very solidly constructed building of striking appearance, the connection of the observatory with navigation being suggested by a row of circular lights which strongly recall a ship's portholes. this building is at the southern end of the narrow passage, 'the wasp's waist,' which connects the older observatory domain with the newer. it is the first building we come to in the south ground. the computations of the department are carried on in the south wing of the new observatory. it will be seen from the photograph that the instrument is much larger, heavier, and less easy to move in azimuth than the old altazimuth. it is, therefore, not often moved in azimuth, but is set in some particular direction, not necessarily north and south, in which it is used practically as a transit circle. [illustration: new altazimuth building.] there is quite another way of determining the place of the moon, which is sometimes available, and which offers one of the prettiest of observations to the astronomer. as the moon travels across the sky, moving amongst the stars from west to east, it necessarily passes in front of some of them, and hides them from us for a time. such a passage, or 'occultation,' offers two observations: the 'disappearance,' as the moon comes up to the star and covers it; the 'reappearance,' as it leaves it again, and so discloses it. [illustration: the new altazimuth. (_from a photograph by mr. lacey._)] except at the exact time of full moon, we do not see the entire face of our satellite; one edge or 'limb' is in darkness. as the moon therefore passes over the star, either the limb at which the star disappears, or that at which it reappears, is invisible to us. to watch an occultation at the bright limb is pretty; the moon, with its shining craters and black hollows, its mountain ranges in bright relief, like a model in frosted silver, slowly, surely, inevitably comes nearer and nearer to the little brilliant which it is going to eclipse. the movement is most regular, most smooth, yet not rapid. the observer glances at his clock, and marks the minute as the two heavenly bodies come closer and closer to each other. then he counts the clock beats: 'five, six, seven,' it may be, as the star has been all but reached by the advancing moon. 'eight,' it is still clear; ere the beat of the clock rings to the 'nine,' perhaps the little diamond point has been touched by the wide arch of the moon's limb, and has gone! less easy to exactly time is a reappearance at the bright limb. in this case the observer must ascertain from the _nautical almanac_ precisely where the star will reappear; then a little before the predicted time he takes his place at the telescope, watches intently the moon's circumference at the point indicated, and, listening for the clock-beats, counts the seconds as they fly. suddenly, without warning, a pin-point of light flashes out at the edge of the moon, and at once draws away from it. the star has 'reappeared.' far more striking is a disappearance or reappearance at the 'dark limb.' in this case the limb of the moon is absolutely invisible, and it may be that no part of the moon is visible in the field of the telescope. in this case the observer sees a star shining brightly and alone in the middle of the field of his telescope. he takes the time from his faithful clock, counting beat after beat, when suddenly the star is gone! so sudden is the disappearance that the novice feels almost as astonished as if he had received a slap in the face, and not unfrequently he loses all count or recollection of the clock beats. the reappearance at the dark limb is quite as startling; with a bright star it is almost as if a shell had burst in his very face, and it would require no very great imagination to make him think that he had heard the explosion. one moment nothing was visible; now a great star is shining down serenely on the watcher. a little practice soon enables the observer to accustom himself to these effects, and an old hand finds no more difficulty in observing an occultation of any kind than in taking a transit. such an observation is useful for more purposes than one. if the position of the star occulted is known--and it can be determined at leisure afterwards--we necessarily know where the limb of the moon was at the time of the observation. then the time which the moon took to pass over the star enables us to calculate the diameter of our satellite; the different positions of the moon relative to the star, as seen from different observatories, enable us to calculate its distance. but if the disappearance takes place at the bright limb, the reappearance usually takes place at the dark, and _vice versâ_; and the two observations are not quite comparable. there is one occasion, however, when both observations are made under similar circumstances, namely, at the full. and if the moon happens also to be totally eclipsed, the occultations of quite faint stars can be successfully observed, much fainter than can ordinarily be seen close up to the moon. total eclipses of the moon, therefore, have recently come to be looked upon as important events for the astronomer, and observatories the world over usually co-operate in watching them. october , , was the first occasion when such an organised observation was made; there have been several since, and on these nights every available telescope and observer at greenwich is called into action. it may be asked why these different modes of observing the moon are still kept up, year in and year out. 'do we not know the moon's orbit sufficiently well, especially since the discovery of gravitation?' no; we do not. this simple and beautiful law--simple enough in itself, gives rise to the most amazing complexity of calculation. if the earth and moon were the only two bodies in the universe, the problem would be a simple one. but the earth, sun, and moon are members of a triple system, each of which is always acting on both of the others. more, the planets, too, have an appreciable influence, and the net result is a problem so intricate that our very greatest mathematicians have not thoroughly worked it out. our calculations of the moon's motions need, therefore, to be continually compared with observation, need even to be continually corrected by it. there is a further reason for this continual observation, not only in the case of the sun, which is our great standard star, since from it we derive the right ascensions of the stars, and it is also our great timekeeper; not only in that of the moon, but also in the case of the planets. their places as computed need continually to be compared with their places as observed, and the discordances, if any, inquired into. the great triumph which resulted to science from following this course--to pure science, since uranus is too faint a planet to be any help to the sailor in navigation--is well known. the observed movements of uranus proved not to be in accord with computation, and from the discordances between calculation and observation adams and leverrier were able to predicate the existence of a hitherto unseen planet beyond-- 'to see it, as columbus saw america from spain. its movements were felt by them trembling along the far-reaching line of their analysis, with a certainty hardly inferior to that of ocular demonstration.'[ ] [ ] from sir john herschel's address to the british association, september , , thirteen days before galle's first observation of the planet. the discovery of neptune was not made at greenwich, and airy has been often and bitterly attacked because he did not start on the search for the predicted planet the moment adams addressed his first communication to him, and so allowed the french astronomer to engross so much of the honour of the exploit. the controversy has been argued over and over again, and we may be content to leave it alone here. there is one point, however, which is hardly ever mentioned, which must have had much effect in determining airy's conduct. in , the year in which adams sent his provisional elements of the unseen disturbing planet to airy, the largest telescope available for the search at greenwich was an equatorial of only six and three-quarter inches aperture, provided with small and insufficient circles for determining positions, and housed in a very small and inconvenient dome; whilst at cambridge, within a mile or so of adams' own college, was the 'northumberland' equatorial, of nearly twelve inches aperture, under the charge of the university professor of astronomy, professor challis, and which was then much the largest, best mounted and housed equatorial in the entire country. the 'northumberland' had been begun from airy's designs and under his own superintendence, when he was professor of astronomy at cambridge. naturally, then, knowing how much superior the cambridge telescope was to any which he had under his care, he thought the search should be made with it. he had no reason to believe that his own instrument was competent for the work. [illustration: the new observatory as seen from flamsteed's observatory.] on the other hand, it is hard for the ordinary man to understand how it was that adams not only left unnoticed and unanswered for three-quarters of a year, an inquiry of airy's with respect to his calculations, but also never took the trouble to visit challis, whom he knew well, and who was so near at hand, to stir him up to the search. but, in truth, the whole interest of the matter for adams rested in the mathematical problem. the irregularities in the motion of uranus were interesting to him simply for the splendid opportunity which they gave him for their analysis. a purely imaginary case would have served his purpose nearly as well. the actuality of the planet which he predicted was of very little moment; the _éclat_ and popular reputation of the discovery was less than nothing; the problem itself and the mental exercise in its solution, were what he prized. but it was not creditable to the nation that the royal observatory should have been so ill-provided with powerful telescopes; and a few years later airy obtained the sanction of the government for the erection of an equatorial larger than the 'northumberland,' but on the same general plan and in a much more ample dome. this was for thirty-four years the 'great' or 'south-east' equatorial, and the mounting still remains and bears the old name, though the original telescope has been removed elsewhere. the object-glass had an aperture of twelve and three-quarter inches and a focal length of eighteen feet, and was made by merz of munich, the engineering work by ransomes and sims of ipswich, and the graduations and general optical work by simms, now of charlton, kent. the mounting was so massive and stable that the present astronomer royal has found it quite practicable and safe to place upon it a telescope (with its counterpoises) of many times the weight, one made by sir howard grubb, of dublin, of twenty-eight inches aperture and twenty-eight feet focal length, the largest refractor in the british empire, though surpassed by several american and continental instruments. the stability of the mounting was intended to render the telescope suitable for a special work. this was the observation of 'minor planets.' on the first day of the present century the first of these little bodies was discovered by piazzi at palermo. three more were discovered at no great interval afterwards, and then there was an interval of thirty-eight years without any addition to their number. but from december , , up to the present time, the work of picking up fresh individuals of these 'pocket planets' has gone on without interruption, until now more than are known. most of these are of no interest to us, but a few come sufficiently near to the earth for their distance to be very accurately determined; and when the distance of one member of the solar system is determined, those of all the others can be calculated from the relations which the law of gravitation reveals to us. it is a matter of importance, therefore, to continue the work of discovery, since we may at any time come across an interesting or useful member of the family; and that we may be able to distinguish between minor planets already discovered and new ones, their orbits must be determined as they are discovered, and some sort of watch kept on their movements. a striking example of the scientific prizes which we may light upon in the process of the rather dreary and most laborious work which the minor planets cause, has been recently supplied by the discovery of eros. on august , , herr witt, of the urania observatory, berlin, discovered a very small planet that was moving much faster in the sky than is common with these small bodies. the great majority are very much farther from the sun than the planet mars, many of them twice as far, and hence, since the time of a planet's revolution round the sun increases, in accordance with kepler's law, more rapidly than does its distance, it follows that they move much more slowly than mars. but this new object was moving at almost the same speed as mars; it must, therefore, be most unusually near to us. further observations soon proved that this was the case, and eros, as the little stranger has been called, comes nearer to us than any other body of which we are aware except the moon. venus when in transit is - / millions of miles from us, mars at its nearest is - / millions, eros at its nearest approach is but little over millions. the use of such a body to us is, of course, quite apart from any purpose of navigation, except very indirectly. but it promises to be of the greatest value in the solution of a question in which astronomers must always feel an interest, the determination of the distance of the earth from the sun. we know the _relative_ distances of the different planets, and, consequently if we could determine the absolute distance of any one, we should know the distances of all. as it is practically impossible to measure our distance from the sun directly, several attempts have been made to determine the distances of venus, mars, or such of the minor planets as come the nearest to us. three of these in particular, the little planets iris, victoria, and sappho, have given us the most accurate determinations of the sun's distance ( , , miles) which we have yet obtained; but eros at its nearest approach will be six times as near to us as either of the three mentioned above, and therefore should give us a value with only one-sixth of the uncertainty attaching to that just mentioned. the discovery of minor planets has lain outside the scope of greenwich work, but their observation has formed an integral part of it. the general public is apt to lay stress rather on the first than on the second, and to think it rather a reproach to greenwich that it has taken no part in such explorations. experience has, however, shown that they may be safely left to amateur activity, whilst the monotonous drudgery of the observation of minor planets can only be properly carried out in a permanent institution. the observation of these minute bodies with the transit circle and altazimuth is attended with some difficulties; but precise observations of various objects may be made with an equatorial; indeed, comets are usually observed by its means. the most ordinary way of observing a comet with an equatorial is as follows: two bars are placed in the eye-piece of the telescope, at right angles to each other, and at an angle of forty-five degrees to the direction of the apparent daily motion of the stars. the telescope is turned to the neighbourhood of the comet, and moved about until it is detected. the telescope is then put a little in front of the comet, and very firmly fixed. the observer soon sees the comet entering his field, and by pressing the contact button he telegraphs to the chronograph the time when the comet is exactly bisected by each of the bars successively. he then waits until a bright star, or it may be two or three, have entered the telescope and been observed in like manner. the telescope is then unclamped, and moved forward until it is again ahead of the comet, and the observations are repeated; and this is done as often as is thought desirable. the places of the stars have, of course, to be found out from catalogues, or have to be observed with the transit circle, but when they are known the position of the comet or minor planet can easily be inferred. next to the glory of having been the means of bringing about the publication of newton's _principia_, the greatest achievement of halley, the second astronomer royal, was that he was the first to predict the return of a comet. newton had shown that comets were no lawless wanderers, but were as obedient to gravitation as were the planets themselves, and he also showed how the orbit of a comet could be determined from observations on three different dates. following these principles, halley computed the orbits of no fewer than twenty-four comets, and found that three of them, visible at intervals of about seventy-five years, pursued practically the same path. he concluded, therefore, that these were really different appearances of the same object, and, searching old records, he found reason to believe that it had been observed frequently earlier still. it seems, in fact, to have been the comet which is recorded to have been seen in in england at the time of the norman invasion; in a.d. , shortly before the commencement of that war which ended in the destruction of jerusalem by titus; and earlier still, so far back as b.c. . halley, however, experienced a difficulty in his investigation. the period of the comet's revolution was not always the same. this, he concluded, must be due to the attraction of the planets near which the comet might chance to travel. in the summer of it had passed very close to jupiter, for instance, and in consequence he expected that instead of returning in august , seventy-five years after its last appearance, it would not return until the end of or the beginning of . it has returned twice since halley's day, a triumphant verification of the law of gravitation; and we are looking for it now for a third return some ten years hence, in . halley's comet, therefore, is an integral member of our solar system, as much so as the earth or neptune, though it is utterly unlike them in appearance and constitution, and though its path is so utterly unlike theirs that it approaches the sun nearer than our earth, and recedes farther than neptune. but there are other comets, which are not permanent members of our system, but only passing visitors. from the unfathomed depths of space they come, to those depths they go. they obey the law of gravitation so far as our sight can follow them, but what happens to them beyond? do they come under some other law, or, perchance, in outermost space is there still a region reserved to primeval chaos, the 'anarch old,' where no law at all prevails? gravitation is the bond of the solar system; is it also the bond of the universe? chapter ix the magnetic and meteorological departments passing out of the south door of the new altazimuth building, we come to a white cruciform erection, constructed entirely of wood. this is the magnet house or magnetic observatory, the home of a double department, the magnetic and meteorological. this department does not, indeed, lie within the original purpose of the observatory as that was defined in the warrant given to flamsteed, and yet is so intimately connected with it, through its bearing on navigation, that there can be no question as to its suitability at greenwich. indeed, its creation is a striking example of the thorough grasp which airy had upon the essential principles which should govern the great national observatory of an essentially naval race, and of the keen insight with which he watched the new development of science. the magnetic observatory, therefore, the purpose of which was to deal with the observation of the changes in the force and direction of the earth's magnetism--an inquiry which the greater delicacy of modern compasses, and, in more recent times, the use of iron instead of wood in the construction of ships has rendered imperative--was suggested by airy on the first possible occasion after he entered on his office, and was sanctioned in . the meteorological department has a double bearing on the purpose of the observatory. on the one side, a knowledge of the temperature and pressure of the atmosphere is, as we have already seen, necessary in order to correct astronomical observations for the effect of refraction. on the other hand, meteorology proper, the study of the movements of the atmosphere, the elucidation of the laws which regulate those movements, leading to accurate forecasts of storms, are of the very first necessity for the safety of our shipping. it is true that weather forecasts are not issued from greenwich observatory, any more than the _nautical almanac_ is now issued from it; but just as the observatory furnishes the astronomical data upon which the almanac is based, so also it takes its part in furnishing observations to be used by the meteorological office at westminster for its daily predictions. those predictions are often made the subject of much cheap ridicule; but, however far short they may fall of the exact and accurate predictions which we would like to have, yet they mark an enormous advance upon the weather-lore of our immediate forefathers. 'he that is weather wise is seldom other wise,' says the proverb, and the saying is not without a shrewd amount of truth. for perhaps nowhere can we find a more striking combination of imperfect observation and inconsequent deduction than in the saws which form the stock-in-trade of the ordinary would-be weather prophet. how common it is to find men full of the conviction that the weather must change at the co-called 'changes of the moon,' forgetful that 'if we'd no moon at all-- and that may seem strange-- we still should have weather that's subject to change.' they will say, truly enough, no doubt, that they have known the weather to change at 'new' or 'full,' as the case may be, and they argue that it, therefore, must always do so. but, in fact, they have only noted a few chance coincidences, and have let the great number of discordances pass by unnoticed. but observations of this kind seem scientific and respectable compared with those numerous weather proverbs which are based upon the mere jingle of a rhyme, as 'if the ash is out before the oak, you may expect a thorough soak'-- a proverb which is deftly inverted in some districts by making 'oak' rhyme to 'choke.' others, again, are based upon a mere childish fancy, as, for example, when the young moon 'lying on her back' is supposed to bode a spell of dry weather, because it looks like a cup, and so might be thought of as able to hold the water. during the present reign, however, a very different method of weather study has come into action, and the foundations of a true weather wisdom have been laid. these have been based, not on fancied analogies or old wives' rhymes, or a few forechosen coincidences, but upon observations carried on for long periods of time and over wide areas of country, and discussed in their entirety without selection and bias. above all, mathematical analysis has been applied to the motions of the air, and ideas, ever gaining in precision and exactness, have been formulated of the general circulation of the atmosphere. as compared with its sister science, astronomy, meteorology appears to be still in a very undeveloped state. there is such a difference between the power of the astronomer to foretell the precise position of sun, moon, and planets for years, even for centuries, beforehand, and the failure of the meteorologist to predict the weather for a single season ahead, that the impression has been widely spread that there is yet no true meteorological science at all. it is forgotten that astronomy offered us, in the movements of the heavenly bodies, the very simplest and easiest problem of related motion. yet for how many thousands of years did men watch the planets, and speculate concerning their motions, before the labours of tycho, kepler, and newton culminated in the revelation of their meaning? for countless generations it was supposed that their movements regulated the lives, characters, and private fortunes of individual men; just as quite recently it was fancied that a new moon falling on a saturday, or two full moons coming within the same calendar month, brought bad weather! it is still impossible to foresee the course of weather change for long ahead; but the difference between the modern navigator, surely and confidently making a 'bee-line' across thousands of miles of ocean to his destination, and the timid sailor of old, creeping from point to point of land, is hardly greater than the contrast between the same two men, the one watching his barometer, the other trusting in the old wives' rhymes which afforded him his only indication as to coming storms. it is still impossible to foresee the weather change for long ahead, but in our own and in many other countries, especially the united states, it has been found possible to predict the weather of the coming four-and-twenty hours with very considerable exactness, and often to forecast the coming of a great storm several days ahead. this is the chief purpose of the two great observatories of the storm-swept indian and chinese seas, hong kong and mauritius; and the value of the work which they have done in preventing the loss of ships, and the consequent loss of lives and property, has been beyond all estimate. the royal observatory, greenwich, is a meteorological as well as an astronomical observatory, but, as remarked above, it does not itself issue any weather forecasts. just as the greenwich observations of the places of the moon and stars are sent to the _nautical almanac_ office, for use in the preparation of that ephemeris; just as the greenwich determinations of time are used for the issue of signals to the post office, whence they are distributed over the kingdom, so the greenwich observations of weather are sent to the meteorological office, there to be combined with similar records from every part of the british isles, to form the basis of the daily forecasts which the latter office publishes. to each of these three offices, therefore, the royal observatory, greenwich, stands in the relation of purveyor. it supplies them with the original observations more or less in reduced and corrected form, without which they could not carry on most important portions of their work. let it be noted how closely these three several departments, the _nautical almanac_ office, the time department, and the meteorological office, are related to practical navigation. whatever questions of pure science--of knowledge, that is, apart from its useful applications--may arise out of the following up of these several inquiries, yet the first thought, the first principle of each, is to render navigation more sure and more safe. the first of all meteorological instruments is the barometer, which, under its two chief forms of mercurial and aneroid, is simply a means of measuring the pressure exerted by the atmosphere. there are two important corrections to which its readings are subject. the first is for the height of the station above the level of the sea; the second is for the effect of temperature upon the mercury in the barometer itself, lengthening the column. to overcome these, the height of the standard barometer at greenwich above sea-level has been most carefully ascertained, and the heights relative to it of the other barometers of the observatory, particularly those in rooms occupied by fundamental telescopes, have also been determined, whilst the self-recording barometer is mounted in a basement, where it is almost completely protected from changes of temperature. next in importance to the barometer as a meteorological instrument comes the thermometer. the great difficulty in the observatory use of the thermometer is to secure a perfectly unexceptionable exposure, so that the thermometer may be in free and perfect contact with the air, and yet completely sheltered from any direct ray from the sun. this is secured in the great thermometer shed at greenwich by a double series of 'louvre' boards, on the east, south, and west sides of the shed, the north side being open. the shed itself is made a very roomy one, in order to give access to a greater body of air. a most important use of the thermometer is in the measurement of the amount of moisture in the air. to obtain this, a pair of thermometers are mounted close together, the bulb of one being covered by damp muslin, and the other being freely exposed. if the air is completely saturated with moisture, no evaporation can take place from the damp muslin, and consequently the two thermometers will read the same. but if the air be comparatively dry, more or less evaporation will take place from the wet bulb, and its temperature will sink to that at which the air would be fully saturated with the moisture which it already contained. for the higher the temperature, the greater is its power of containing moisture. the difference of the reading of the two thermometers is, therefore, an index of humidity. the greater the difference, the greater the power of absorbing moisture, or, in other words, the dryness of the air. the great shed already alluded to is devoted to these companion thermometers. [illustration: the self-registering thermometers.] very closely connected with atmospheric pressure, as shown us by the barometer, is the study of the direction of winds. if we take a map of the british isles and the neighbouring countries, and put down upon them the barometer readings from a great number of observing stations, and then join together the different places which show the same barometric pressure, we shall find that these lines of equal pressure--technically called 'isobars'--are apt to run much nearer together in some places than in others. clearly, where the isobars are close together it means that in a very short distance of country we have a great difference of atmospheric pressure. in this case we are likely to get a very strong wind blowing from the region of high pressure to the region of low pressure, in order to restore the balance. if, further, we had information from these various observing stations of the direction in which the wind was blowing, we should soon perceive other relationships. for instance, if we found that the barometer read about the same in a line across the country from east to west, but that it was higher in the north of the islands than in the south, we should then have a general set of winds from the east, and a similar relation would hold good if the barometer were highest in some other quarter; that is, the prevailing wind will come from a quarter at right angles to the region of highest barometer, or, as it is expressed in what is known as 'buys ballot's law,' 'stand with your back to the wind, and the barometer will be lower on your left hand than on your right.' this law holds good for the northern hemisphere generally, except near to the equator; in the southern hemisphere the right hand is the side of low barometer. the instruments for wind observation are of two classes: vanes to show its direction, and anemometers to show its speed and its pressure. these may be regarded as two different modes in which the strength of the wind manifests itself. pressure anemometers are usually of two forms: one in which a heavy plate is allowed to swing by its upper edge in a position fronting the wind, the amount of its deviation from the vertical being measured; and the other in which the plate is supported by springs, the degree of compression of the springs being the quantity registered in that case. of the speed anemometers, the best known form is the 'robinson,' in which four hemispherical cups are carried at the extremities of a couple of cross bars. for the mounting of these wind instruments the old original observatory, known as the octagon room, has proved an excellent site, with its flat roof surmounted by two turrets in the north-east and north-west corners, and raised some two hundred feet above high-water mark. [illustration: the anemometer room, north-west turret.] the two chief remaining instruments are those for measuring the amount of rainfall and of full sunshine. the rain gauge consists essentially of a funnel to collect the rain, and a graduated glass to measure it. the sunshine recorder usually consists of a large glass globe arranged to throw an image of the sun on a piece of specially prepared paper. this image, as the sun moves in the sky, moves along the paper, charring it as it moves, and at the end of the day it is easy to see, from the broken, burnt trace, at what hours the sun was shining clear, and when it was hidden by cloud. an amusing difficulty was encountered in an attempt to set on foot another inquiry. the superintendent of the meteorological department at the time wished to have a measure of the rate at which evaporation took place, and therefore exposed carefully measured quantities of water in the open air in a shallow vessel. for a few days the record seemed quite satisfactory. then the evaporation showed a sudden increase, and developed in the most erratic and inexplicable manner, until it was found that some sparrows had come to the conclusion that the saucer full of water was a kindly provision for their morning 'tub,' and had made use of it accordingly. a large proportion of the meteorological instruments at greenwich and other first-class observatories are arranged to be self-recording. it was early felt that it was necessary that the records of the barometer and thermometer should be as nearly as possible continuous; and at one time, within the memory of members of the staff still living, it was the duty of the observer to read a certain set of instruments at regular two-hour intervals during the whole of the day and night--a work probably the most monotonous, trying, and distasteful of any that the observatory had to show. the two-hour record was no doubt practically equivalent to a continuous one, but it entailed a heavy amount of labour. automatic registers were, therefore, introduced whenever they were available. the earliest of these were mechanical, and several still make their records in this manner. on the roof of the octagon room we find, beside the two turrets already referred to, a small wooden cabin built on a platform several feet above the roof level. this cabin and the north-western turret contain the wind-registering instruments. opening the turret door, we find ourselves in a tiny room which is nearly filled by a small table. upon this table lies a graduated sheet of paper in a metal frame, and as we look at it, we see that a clock set up close to the table is slowly drawing the paper across it. three little pencils rest lightly on the face of the paper at different points. one of these, and usually the most restless, is connected with a spindle which comes down into the turret from the roof, and which is, in fact, the spindle of the wind vane. the gearing is so contrived that the motion on a pivot of the vane is turned into motion in a straight line at right angles to the direction in which the paper is drawn by the clock. a second pencil is connected with the wind-pressure anemometer. the third pencil indicates the amount of rain that has fallen since the last setting, the pencil being moved by a float in the receiver of the rain gauge. [illustration: the anemometer trace.] an objection to all the mechanical methods of continuous registration is that, however carefully the gearing between the instrument itself and the pencil is contrived, however lightly the pencil moves over the paper, yet some friction enters in and affects the record: this is of no great moment in wind registration, when we are dealing with so powerful an agent as the wind, but it becomes a serious matter when the barometer is considered, since its variations require to be registered with the greatest minuteness. when photography, therefore, was invented, meteorologists were very prompt to take advantage of this new ally. a beam of light passing over the head of the column of mercury in a thermometer or barometer could easily be made to fall upon a drum revolving once in the twenty-four hours, and covered with a sheet of photographic paper. in this case, when the sensitive paper is developed, we find its upper half blackened, the lower edge of the blackened part showing an irregular curve according as the mercury in the thermometer or barometer rose or fell, and admitted less or more light through the space above it. here we have a very perfect means of registration: the passage of the light exercises no friction or check on the free motion of the mercury in the tube, or on the turning of the cylinder covered by the sensitive paper, whilst it is easy to obtain a time scale on the register by cutting off the light for an instant--say at each hour. in this way the wet and dry bulb thermometers in the great shed make their registers. the supply of material to the meteorological office is not the only use of the greenwich meteorological observations. two elements of meteorology, the temperature and the pressure of the atmosphere, have the very directest bearing upon astronomical work. and this in two ways. an instrument is sensible to heat and cold, and undergoes changes of form, size, or scale, which, however absolutely minute, yet become, with the increased delicacy of modern work, not merely appreciable, but important. so too with the density of the atmosphere: the light from a distant star, entering our atmosphere, suffers refraction; and being thus bent out of its path, the star appears higher in the heavens than it really is. the amount of this bending varies with the density of the layers of air through which the light has to pass. the two great meteorological instruments, the thermometer and barometer, are therefore astronomical instruments as well. in the arrangements at greenwich the magnetic department is closely connected with the meteorological, and it is because the two departments have been associated together that the building devoted to both is constructed of wood, not brick, since ordinary bricks are made of clay which is apt to be more or less ferruginous. copper nails have alone been employed in the construction of the buildings. the fire-grates, coal-scuttles, and fire-irons are all of the same metal. the growth of the observatory has, however, made it necessary to set up some of the new telescopes, into the mounting of which much iron enters, very close to the magnetic building. the present astronomer-royal has therefore erected a magnetic pavilion right out in the park at an ample distance from these disturbing causes. the double department is, therefore, the most widely scattered in the whole observatory. it is located for computing purposes in the west wing of the new observatory; many of its magnetic instruments are in the old magnet house, others are across the park in the new magnetic pavilion; the anemometers are on the roof of the octagon room, flamsteed's original observatory, and the self-registering thermometers are in the south ground between the old magnet house and the new observatory. [illustration: magnetic pavilion--exterior. (_from a photograph by mr. lacey._)] the object of the magnetic observatory is to study the movements of the magnetic needle. the quaintest answer that i ever received in an examination was in reply to the question, 'what is meant by magnetic inclination and declination?' the examinee replied: 'to make a magnet, you take a needle, and rub it on a lodestone. if it refuses or _declines_ to become a magnet, that is magnetic declination; if it is easily made a magnet, or is _inclined_ to become one, that is magnetic inclination.' one greatly regretted that it was necessary to mark the reply according to its ignorance, and not, as one would have wished, in proportion to its ingenuity. magnetic declination, however, as everybody knows, measures the deviation of the 'needle' from the true geographical north and south direction; the inclination or dip is the angle which a 'needle' makes with the horizon. at one time the only method of watching the movements of the magnetic needles was by direct observation, just precisely as it was wont to be in the case of the barometer and thermometer. but the same agent that has been called in to help in their case has enabled the magnets also to give us a direct and continuous record of their movements. in principle the arrangement is as follows: a small light mirror is attached to the magnetic needle, and a beam of light is arranged to fall upon the mirror, and is reflected away from it to a drum covered with sensitive paper. if, then, the needle is perfectly at rest, a spot of light falls on the drum and blackens the paper at one particular point. the drum is made to revolve by clockwork once in twenty-four hours, and the black dot is therefore lengthened out into a straight line encircling the drum. if, however, the needle moves, then the spot of light travels up or down, as the case may be. now, if we look at one of these sheets of photographic paper after it has been taken from the drum, we shall see that the north pole of the magnet has moved a little, a very little, towards the west in the early part of the day, say from sunrise to p.m., and has swung backwards from that hour till about p.m., remaining fairly quiet during the night. the extent of this daily swing is but small, but it is greater in summer than in winter, and it varies also from year to year. [illustration: magnetic pavilion--interior. (_from a photograph by mr. lacey._)] besides this daily swing, there occasionally happen what are called 'magnetic storms;' great convulsive twitchings of the needle, as if some unseen operator were endeavouring, whilst in a state of intense excitement, to telegraph a message of vast importance, so rapid and so sharp are the movements of the needle to and fro. these great storms are felt, so far as we know, simultaneously over the whole earth, and the more characteristic begin with a single sharp twitch of the needle towards the east. besides the movements of the magnetic needle, the intensity of the currents of electricity which are always passing through the crust of the earth are also determined at greenwich; but this work has been rendered practically useless for the last few years by the construction of the electric railway from stockwell to the city. since it was opened, the photographic register of earth currents has shown a broad blurring from the moment of the starting of the first train in the morning to the stopping of the last train at night. as an indication of the delicacy of modern instruments, it may be mentioned that distinct indications of the current from this railway have been detected as far off as north walsham, in norfolk, a distance of more than a hundred miles. a further illustration of the delicacy of the magnetic needles was afforded shortly after the opening of the railway referred to. on one occasion the then superintendent of the magnetic department visited the generating station at stockwell, and on his return it was noticed day after day that the traces from the magnets showed a curious deflection from a.m. to p.m., the hours of his attendance. this gave rise to some speculation, as it did not seem possible that the gentleman could himself have become magnetized. eventually, the happy accident of a fine day solved the mystery. that morning the superintendent left his umbrella at home, and the magnets were undisturbed. the secret was out. the umbrella had become a permanent magnet, and its presence in the lobby of the magnetic house had been sufficient to influence the needles. chapter x the heliographic department so far the development of the observatory had been along the central line of assistance to navigation. but the magnetic department led on to one which had but a very secondary connection with it. a greatly enhanced interest was given to the observations of earth magnetism, when it was found that the intensity and frequency of its disturbances were in close accord with changes that were in progress many millions of miles away. that the surface of the sun was occasionally diversified by the presence of dark spots, had been known almost from the first invention of the telescope; but it was not until the middle of the present century that any connection was established between these solar changes and the changes which took place in the magnetism of the earth. then two observers, the one interesting himself entirely with the spots on the sun, the other as wholly devoted to the study of the movements of the magnetic needle, independently found that the particular phenomenon which each was watching was one which varied in a more or less regular cycle. and further, when the cycles were compared, they proved to be the same. whatever the secret of the connection, it is now beyond dispute that as the spots on the sun become more and more numerous, so the daily swing of the magnetic needle becomes stronger; and, on the other hand, as the spots diminish, so the magnetic needle moves more and more feebly. this discovery has given a greatly increased significance to the study of the earth's magnetism. the daily swing, the occasional 'storms,' are seen to be something more than matters of merely local interest; they have the closest connection with changes going on in the vast universe beyond; they have an astronomical importance. and it was soon felt to be necessary to supplement the magnetic observatory at greenwich by one devoted to the direct study of the solar surface; and here again that invaluable servant of modern science, photography, was ready to lend its help. just as, by the means of photography, the magnets recorded their own movements, so even more directly the sun himself makes register of his changes by the same agency, and gives us at once his portrait and his autograph. this new department was again due to airy, and in the 'kew' photo-heliograph, which had been designed by de la rue for this work, was installed at greenwich. [illustration: the dallmeyer photo-heliograph.] in order to photograph so bright a body as the sun, it is not in the least necessary to have a very large telescope. the one in common use at greenwich from to , is only four inches in aperture and even that is usually diminished by a cap to three inches, and its focal length is but five feet. this is not very much larger than what is commonly called a 'student's telescope,' but it is amply sufficient for its work. this 'dallmeyer' telescope, so called from the name of its maker, is one of five identical instruments which were made for use in the observation of the transit of venus of , and which, since they are designed for photographing the sun, are called 'photo-heliographs.' the image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. this is managed by arranging a very narrow slit in a strip of brass. the strip is made to run in a groove across the principal focus. before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. when all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second--in midsummer for less than a thousandth of a second. two of these photographs are taken every fine day at greenwich; occasionally more, if anything specially interesting appears to be going on. but in our cloudy climate at least one day in three gives no good opportunity for taking photographs of the sun, and in the winter time long weeks may pass without a chance. the present astronomer-royal, mr. christie, has therefore arranged that photographs with precisely similar instruments should be taken in india and in the mauritius, and these are sent over to greenwich as they are required, to fill up the gaps in the greenwich series. we have therefore at greenwich, from one source or another, practically a daily record of the state of the sun's surface. more recently the 'dallmeyer' photo-heliograph, though still retained for occasional use, has been superseded generally by the 'thompson'; a photographic refractor of nine inches aperture, and nearly nine feet focal length, presented to the observatory by sir henry thompson. the image of the sun obtained after enlargement in the telescope, with this instrument, is seven and a half inches in diameter. the 'thompson' is mounted below the great twenty-six-inch photographic refractor,--also presented to the observatory by sir henry thompson,--in the dome which crowns the centre of the new observatory. a photograph of the sun taken, it has next to be measured, the four following particulars being determined for each spot: first, its distance from the centre of the image of the sun; next, the angle between it and the north point; thirdly, the size of the spot; and fourthly, the size of the umbra of the spot, that is to say, of its dark central portion. the size or area of the spot is measured by placing a thin piece of glass, on which a number of cross-lines have been ruled one-hundredth of an inch apart, in contact with the photograph. these cross-lines make up a number of small squares, each the ten-thousandth ( / in.) part of a square inch in area. when the photograph and the little engraved glass plate are nearly in contact, the photograph is examined with a magnifying glass, and the number of little squares covered by a given spot are counted. it will give some idea of the vast scale of the sun when it is stated that a tiny spot, so small that it only just covers one of these little squares, and which is only one-millionth of the visible hemisphere of the sun in area, yet covers in actual extent considerably more than one million of square miles. the dark spots are not the only objects on the sun's surface. here and there, and especially near the edge of the sun, are bright marks, generally in long branching lines, so bright as to appear bright even against the dazzling background of the sun itself. these are called 'faculæ,' and they, like the spots, have their times of great abundance and of scarcity, changing on the whole at the same time as the spots. after the solar photographs have been measured, the measures must be 'reduced,' and the positions of the spots as expressed in longitude and latitude on the sun computed. there is no difficulty in doing this, for the position of the sun's equator and poles have long been known approximately, the sun revolving on its axis in a little more than twenty-five days, and carrying of course the spots and faculæ round with him. there are few studies in astronomy more engrossing than the watch on the growth and changes of the solar spots. their strange shapes, their rapid movements, and striking alterations afford an unfailing interest. for example, the amazing spectacle is continually being afforded of a spot, some two, three, or four hundred millions of square miles in area, moving over the solar surface at a speed of three hundred miles an hour, whilst other spots in the same group are remaining stationary. but a higher interest attaches to the behaviour of the sun as a whole than to the changes of any particular single spot; and the curious fact has been brought to light, that not only do the spots increase and diminish in a regular cycle of about eleven years in length, but they also affect different regions of the sun at different points of the cycle. at the time when spots are most numerous and largest, they are found occupying two broad belts, the one with its centre about ° north of the equator, the other about as far south, the equator itself being very nearly free from them. but as the spots begin to diminish, so they appear continually in lower and lower latitudes, until instead of having two zones of spots there is only one, and this one lies along the equator. by this time the spots have become both few and small. the next stage is that a very few small spots are seen from time to time in one hemisphere or the other at a great distance from the equator, much farther than any were seen at the time of greatest activity. there are then for a little time three sun-spot belts, but the equatorial one soon dies out. the two belts in high latitude, on the other hand, continually increase; but as they increase, so do they move downwards in latitude, until at length they are again found in about latitude ° north or south, when the spots have attained their greatest development. [illustration: photograph of a group of sun-spots. (_from a photograph taken at the royal observatory, greenwich, april, , d. h. m._)] the clearest connection between the magnetic movements and the sun-spot changes is seen when we take the mean values of either for considerable periods of time, as, for instance, year by year. but occasionally we have much more special instances of this connection. some three or four times within the last twenty years an enormous spot has broken out on the sun, a spot so vast that worlds as great as our own could lie in it like peas in a breakfast saucer, and in each case there has been an immediate and a threefold answer from the earth. one of the most remarkable of these occurred in november, . a great spot was then seen covering an area of more than three thousand millions of square miles. the weather in london happened to be somewhat foggy, and the sun loomed, a dull red ball, through the haze, a ball it was perfectly easy to look at without specially shading the eyes. so large a spot under such circumstances was quite visible to the naked eye, and it caught the attention of a great number of people, many of whom knew nothing about the existence of spots on the sun. this great disturbance, evidently something of the nature of a storm in the solar atmosphere, stretched over one hundred thousand miles on the surface of the sun. the disturbance extended farther still, even to nearly one hundred millions of miles. for simultaneously with the appearance of the spot the magnetic needles at greenwich began to suffer from a strange excitement, an excitement which grew from day to day until it had passed half-way across the sun's disc. as the twitchings of the magnetic needle increased in frequency and violence, other symptoms were noticed throughout the length of the british isles. telegraphic communication was greatly interfered with. the telegraph lines had other messages to carry more urgent than those of men. the needles in the telegraph instruments twitched to and fro. the signal bells on many of the railway lines were rung, and some of the operators received shocks from their instruments. lastly, on november , a superb aurora was witnessed, the culminating feature of which was the appearance, at about six o'clock in the evening, of a mysterious beam of greenish light, in shape something like a cigar, and many degrees in length, which rose in the east and crossed the sky at a pace much quicker than but nearly as even as that of sun, moon, or stars, till it set in the west two minutes after its rising. so far we have been dealing only with effects. their causes still rest hidden from us. there is clearly a connection between the solar activity as shown by the spots and the agitation of the magnetic needles. but many great spots find no answer in any magnetic vibration, and not a few considerable magnetic storms occur when we can detect no great solar changes to correspond. thus even in the simplest case before us we have still very much to explain. two far more difficult problems are still offered us for solution. what is the cause of these mysterious solar spots? and have they any traceable connection with the fitful vagaries of earthly weather? it was early suggested that probably the first problem might find an answer in the ever-varying combinations and configurations of the various planets, and that the sun-spots in their turn might hold the key of our meteorology. both ideas were eagerly followed up--not that there was much to support either, but because they seemed to offer the only possible hope of our being able to foretell the general current of weather change for any long period in advance. so far, however, the first idea may be considered as completely discredited. as to the second, there would appear to be, in the case of certain great tropical and continental countries like india, some slight but by no means conclusive evidence of a connection between the changes in the annual rainfall and the changes in the spotted surface of the sun. dr. meldrum, the late veteran director of the great meteorological observatory in mauritius, has expressed himself as confident that the years of most spots are the years of most violent cyclones in the indian ocean. but this is about as far as real progress has been made, and it may be taken as certain that many years more of observation will be required, and the labours of many skilful investigators, before we can hope to carry much farther our knowledge as to any connection between storm and sun. a further relation of great interest has come to light within the last few years. the year opened a new epoch in the study of eclipses of the sun. these, perhaps, scarcely lie within the scope of a book on the royal observatory, since greenwich has seen but one in all its history. that fell in the year ; for the next it must wait many centuries. yet, as the late astronomer royal conducted three expeditions to see total eclipses, and as the present astronomer royal has undertaken a like number, and members of the staff have been sent on other occasions, it may not be deemed quite a digression to refer to one feature which they have brought to light. when the dark body of the moon has entirely hidden the sun, we have revealed to us, there and then only, that strange and beautiful surrounding of the sun which we call the corona. the earlier observations of the corona seem to reveal it as a body of the most weird and intricate form, a form which seemed to change quite lawlessly from one eclipse to another. but latterly it has been abundantly clear that the forms which it assumes may be grouped under a few well-defined types. in the corona was of a particularly simple and striking character. two great wings shot out east and west in the direction of the sun's equator; round either pole was a cluster of beautiful radiating 'plumes.' it was then recollected that the corona of had been of precisely the same character, both years being years when sun-spots were at their fewest. the coronæ, on the other hand, seen at times when sun-spots are more abundant, were of an altogether different character, the streamers being irregularly distributed all round the sun. other types also have been recognized, and it is perfectly apparent that the corona changes its shape in close accordance with the eleven-year period. the eclipses of and , for example, showed coronæ that bore the very closest resemblance to those of and , the interval of eleven years bringing a return to the same form. the further problem, therefore, now confronts us: does the corona produce the sun-spots, or do the sun-spots produce the corona, or are both the result of some mysterious magnetic action of the sun, an action powerful enough on occasion to thrill through and through this world of ours, ninety-three millions of miles away? chapter xi the spectroscopic department another department was set on foot by airy at the same time as the heliographic department, and in connection with it; and it is the department which has the greatest of interest for the general public. this deals with astronomical physics, or astrophysics, as it is sometimes more shortly called; the astronomy, that is, which treats of the constitution and condition of the heavenly bodies, not with their movements. the older astronomy, on the other hand, confined itself to the movements of the heavens so entirely that bessel, the man whose practical genius revolutionized the science of observation, and whose influence may be traced throughout in airy's great reconstitution of greenwich observatory, denied that anything but the study of the celestial movements had a right to the title of astronomy at all. hardly more than sixty years ago he wrote: 'what astronomy is expected to accomplish is evidently at all times the same. it may lay down rules by which the movements of the celestial bodies, as they appear to us upon the earth, can be computed. all else which we may learn respecting these bodies, as, for example, their appearance, and the character of their surfaces, is, indeed, not undeserving of attention, but possesses no proper astronomical interest. whether the mountains of the moon are arranged in this way or in that is no further an object of interest to astronomers than is a knowledge of the mountains of the earth to others. whether jupiter appears with dark stripes upon its surface, or is uniformly illuminated, pertains as little to the inquiries of the astronomer; and its four moons are interesting to him only for the motions they have. to learn so perfectly the motions of the celestial bodies that for any specified time an accurate computation of these can be given--that was, and is, the problem which astronomy has to solve.' there is a curious irony of progress which seems to delight in falsifying the predictions of even master minds as to the limits beyond which it cannot advance. bessel laid down his dictum as to the true subjects of astronomical inquiry, comte declared that we could never learn what were the elements of which the stars were composed, at the very time that the first steps were being taken towards the creation of a research which should begin by demonstrating the existence in the heavenly bodies of the elements with which we are familiar on the earth, and should go on to prove itself a true astronomy, even in bessel's restricted sense, by supplying the means for determining motion in a direction which he would have thought impossible--that is to say, directly to or from us. the years that followed kirchhoff's application of the spectroscope to the study of the sun, and his demonstration that sodium and iron existed in the solar atmosphere, were crowded with a succession of brilliant discoveries in the same field. kirchhoff, bunsen, angström, thalèn, added element after element to the list of those recognized in the sun. huggins and miller carried the same research into a far more difficult field, and showed us the same elements in the stars. rutherfurd and secchi grouped the stars according to the types of their spectra, and so laid the foundations of what may be termed stellar comparative anatomy. huggins discovered true gaseous nebulæ, and so revived the nebular theory, which had been supposed crushed when the great telescope of lord rosse appeared to have resolved several portions of the orion nebula into separate stars. the great riddle of 'new stars'--which still remains a riddle--was at least attacked, and glowing hydrogen was seen to be a feature in their constitution. glowing hydrogen, again, was, in the observation of total eclipses, seen to be a principal constituent of those surroundings of our own sun which we now call prominences and chromosphere. then the method was discovered of observing the prominences without an eclipse, and they were found to wax and wane in more or less sympathy with the solar spots. sun-spots, planets, comets, meteors, variable stars, all were studied with the new instrument, and all yielded to it fresh and valuable, and often unexpected, information. [illustration: the great nebula in orion. (_from a photograph taken at the royal observatory greenwich, december , , with an exposure of - / hours._)] in this activity greenwich observatory practically took no part. airy, ever mindful of the original purpose of the observatory, and deeply imbued with views similar to those which we have quoted from bessel, considered that the new science lay outside the scope of his duties, until in mr., now sir william, huggins's skilful hands the spectroscope showed itself not only as a means for determining the condition and constitution of the stars, but also their movements--until, in short, it had shown itself as an astronomical instrument even within bessel's narrow definition. the principle of this inquiry is as follows: if a source of light is approaching us very rapidly, then the waves of light coming from it necessarily appear a little shorter than they really are, or, in other words, that light appears to be slightly more blue--the blue waves being shorter than the red--than it really is. a similar thing with regard to the waves of sound is often noticed in connection with a railway train. if an express train, the whistle of which is blowing the whole time, dashes past us at full speed, there is a perceptible drop in the note of the whistle after it has gone by. the sound waves as it was coming were a little shortened, and the whistle therefore appeared to have a sharper note than it had in reality. and in the same way, when it had gone by, the sound waves were a little lengthened, making the note of the whistle appear a very little flatter. such a change of colour in a star could never have been detected without the spectroscope; but since when light passes through a prism the shorter waves are refracted more strongly, that is to say, are more turned out of their course than the longer, the spectroscope affords us the means of detecting and measuring this change. let us suppose that the lines of hydrogen are recognized in a given star. if we compare the spectrum of this star with the spectrum of a tube containing hydrogen and through which the electric spark is passing, we shall be able to see whether any particular hydrogen line occupies the same place as shown by the two spectra. if the line from the star is a little to the red of the line from the tube, the star must be receding from us; if to the blue, approaching us. the amount of displacement may be measured by a delicate micrometer, and the rate of motion concluded from it. [illustration: the half-prism spectroscope on the south-east equatorial.] the principle is clear enough. the actual working out of the observation was one of very great difficulty. the movements of the stars towards us, or away from us, are, in general, extremely slow as compared with the speed of light itself; and hence the apparent shift in the position of a line is only perceptible when a very powerful spectroscope is used. this means that the feeble light of a star has to be spread out into a great length of spectrum, and a very powerful telescope is necessary. the work of observing the motions of stars in the line of sight was started at greenwich in , the 'great equatorial' being devoted to it. this telescope, of - / inches aperture, was not powerful enough to do much more than afford a general indication of the direction in which the principal stars were moving, and to confirm in a general way the inference which various astronomers had found, from discussing the proper motions of stars, that the sun and the solar system were moving towards that part of the heavens where the constellations hercules and lyra are placed. in , therefore, the work was discontinued, and as already mentioned, the - / telescope by merz was removed to make room for the present much larger instrument by sir howard grubb, upon the same mounting. the new telescope being much larger than the one for which mounting and observing room were originally built, it was not possible to put the spectroscope in the usual position, in the same straight line as the great telescope. it was therefore mounted under it, and parallel to it, and the light of the star was brought into it after two reflections. the observer therefore stood with his back to the object and looked down into the spectroscope. it had, however, become apparent by this time that this most delicate field of work was one for which photography possessed several advantages, and as sir henry thompson had made the munificent gift to the observatory of a great photographic equatorial, it was resolved to devote the -inch telescope chiefly to double-star work, and to transfer the spectroscope to the 'new building.' the 'new observatory' in the south ground is crowned indeed with the dome devoted to the great thompson photographic refractor, but this is not its chief purpose. its principal floor contains four fine rooms which are used as 'computing rooms'--for the office work, that is to say, of the observatory. of these the principal is in the north wing, where the main entrance is placed, and is occupied by the astronomer royal and the two chief assistants. the basement contains the libraries and the workshops of the mechanics and carpenters. the upper floor will eventually be used for the storage of photographs and manuscripts, and the terrace roofs of the four wings will be exceedingly convenient for occasional observations, as, for example, of meteor showers. the central dome, which rises high above the level of the terraces, is the only room in the building devoted to telescopic work. as in the new altazimuth building, a ring of circular lights just below the coping of the wall recalls the portholes of a ship, and again reminds us of the connection of the observatory with navigation. [illustration: the workshop.] here the spectroscope is now placed, but not, as it happens, on the thompson refractor. the equatorial mounting in this new dome is a modification of what is usually called the 'german' form of mounting--that is to say, there is but one pier to support the telescope, and the telescope rides on one side of the pier and a counterpoise balances it on the other the 'great equatorial,' on the other hand, is an example of the english mounting, and has two piers, one north and the other south, whilst the telescope swings in a frame between them. in the new dome three telescopes are found rigidly connected with each other on one side of the pier, the telescopes being ( ) the great thompson photographic telescope, double the aperture and double the focal length of the standard astrographic telescope used for the international photographic survey; ( ) the - / telescope by merz, that used to be in the great south-east dome, but which is now rigidly connected with the thompson refractor as a guide telescope; and ( ) a photographic telescope of inches aperture, already described as the 'thompson' photo-heliograph, and used for photographing the sun or in eclipse expeditions. the counterpoise to this collection of instruments is not a mere mass of lead, but a powerful reflector of inches' aperture, and it is to this telescope that the spectroscope is now attached. at the present time, however (august, ), regular work has not been commenced with it. [illustration: the -inch reflector with the new spectroscope attached.] beside this attempt to determine the motions of the stars as they approach us or retreat from us, on rare occasions the spectroscope has been turned on the planets. as these shine by reflected light, their spectra are normally the same as that of the sun. mars appeared to the writer, as to huggins and others, to show some slight indication of the presence of water vapour in its atmosphere. jupiter and saturn show that their atmospheres contain some absorbing vapour unknown to ours. and uranus and neptune, faint and distant as they are, not only show the same dark band given by the two nearer planets, but several others. more attractive has been the examination of the spectra of the brighter comets that have visited us. the years and were especially rich in these. the two principal comets of were called after their respective discoverers, tebbutt's and schaeberle's. they were not bright enough to attract popular attention, though they could be seen with the naked eye, and both gave clear indications of the presence of carbon, their spectra closely resembling that of the blue part of a gas or candle flame. there was nothing particularly novel in these observations, since comets usually show this carbon spectrum, though why they should is still a matter for inquiry; but the two comets of the following year were much more interesting. both comets came very near indeed to the sun. the earlier one, called from its discoverer comet wells, as it drew near to the sun, began to grow more and more yellow, until in the first week of june it looked as full an orange as even the so-called red planet, mars. the spectroscope showed the reason of this at a glance. the comet had been rich in sodium. so long as it was far from the sun the sodium made no sign, but as it came close to it the sodium was turned into glowing vapour under the fierce solar heat. and as the writer saw it in the early dawn of june , the comet itself was a disc of much the same colour as mars, whilst its spectrum resembled that of a spirit lamp that has been plentifully fed with carbonate of soda or common salt. the 'great comet' of the autumn of the same year, and which was so brilliant an object in the early morning, came yet nearer to the sun, and the heating process went on further. the sodium lines blazed up as they had done with comet wells, but under the fiercer stress of heat to which the great comet was subjected, the lines of iron also flashed out, a significant indication of the tremendous temperature to which it was exposed. there are two other departments of spectroscopic work which it was attempted for a time to carry on as part of the greenwich routine. these were the daily mapping of the prominences round the sun, and the detailed examination of the spectra of sun-spots. both are almost necessary complements of the work done in the heliographic department--that is to say, the work of photographing the appearance of the sun day by day, and of measuring the positions and areas of the spots. for the spots afford but one index out of several, of the changes in the sun's activity. the prominences afford another, nor can we at the present moment say authoritatively which is the more significant. then again, with regard to the spots themselves, it is not certain that either their extent or their changes of appearance are the features which it is most important for us to study. we want, if possible, to get down to the soul of the spot, to find out what makes one spot differ from another; and here the spectroscope can help us. great sun-spots are often connected with violent agitation of the magnetic needles, and with displays of auroræ. but they are not always so, and the inquiry, 'what makes them to differ?' has been made again and again, without as yet receiving any unmistakable answer. the great spot of november, , which was connected with so remarkable an aurora and so violent a magnetic storm, was as singular in its spectrum as in its earthly effects. the sun was only seen through much fog, and the spectrum was therefore very faint, but shooting up from almost every part of its area, except the very darkest, were great masses of intensely brilliant hydrogen, evidently under great pressure. the sodium lines were extremely broadened, and on november a broad bright flame of hydrogen was seen shooting up at an immense speed from one edge of the nucleus. a similar effect--an outburst of intensely luminous hydrogen--has often been observed in spots which have been accompanied by great magnetic storms; and it may even be that it is this violent eruption of intensely heated gas which has the directest connection with the magnetic and auroral disturbances here upon earth. this sun-spot work was not carried on for very long, as only one assistant could be spared for the entire solar work of whatever character. yet in that time an interesting discovery was made by the writer--namely, that in the green part of the spectrum of certain spots a number of broad diffused lines or narrow bands made their appearance from time to time, and especially when sun-spots were increasing in number, or were at their greatest development. the prominence work had also to be dropped, partly for the same reason, but chiefly because the atmospheric conditions at greenwich are not suitable for these delicate astrophysical researches. when the observatory was founded 'in the golden days' of charles ii., greenwich was a little country town far enough removed from the great capital, and no interference from its smoke and dust had to be feared or was dreamt of. now the 'great wen,' as cobbett called it, has spread far around and beyond it, and the days when the sky is sufficiently pure round the sun for successful spectrum work on the spots or prominences are few indeed. whether in the future it will be thought advisable for the royal observatory to enter into serious competition in inquiries of this description with the great 'astrophysical' observatories of the continent and of america--potsdam, meudon, the lick, and the yerkes--we cannot say. that would involve a very considerable departure from its original programme, and probably also a departure from its original site. for the conditions at greenwich tend to become steadily less favourable for such work, and it would most probably be found that full efficiency could only be secured by setting up a branch or branches far from the monster town. with the older work it is otherwise. so long as greenwich park and blackheath are kept--as it is to be hoped they always will be--sacred from the invasion of the builder; so long as no new railways burrow their tunnels in the neighbourhood of the observatory, so long the fundamental duties laid upon flamsteed, 'of rectifying the tables of the motions of the heavens and the places of the fixed stars,' will be carried out by his successors on flamsteed hill. chapter xii the astrographic department the two last departments mentioned, the heliographic and spectroscopic, lie clearly and unmistakably outside the terms of the original warrant of the observatory, though the progress of science has led naturally and inevitably to their being included in the greenwich programme. but the astrographic department, though it could no more have been conceived in the days of charles ii. than the spectroscopic, does come within the terms of the warrant, and is but an expansion of that work of 'rectifying the places of the fixed stars,' which formed part of the programme enjoined upon flamsteed, the first astronomer royal, at the first foundation of the observatory, and which was so diligently carried out by him, the first greenwich catalogue, containing about stars, being due to his labours. [illustration: 'chart plate' of the pleiades. (_from a photograph taken at the royal observatory, greenwich, with an exposure of forty minutes._)] his immediate successors did much less in this field, though bradley's observations were published, long after his death, as a catalogue of stars, in some aspects the most important ever issued. pond, the sixth astronomer royal, restored catalogue-making to a prominent place in the greenwich routine, and his precedent is sedulously followed to-day. but each of these was confined to about stars. the necessity has long been felt for a much ampler census, and argelander, at the bonn observatory, brought out a catalogue of , stars north of south declination °, a work which has been completed by schönfeld, who carried the census down to south declination °, and by the two great astronomers of cordoba, south america, dr. gould and dr. thome, by whom it was extended to the south pole. these last three catalogues embrace stars of all magnitudes down to the th or th; but certain astronomers had endeavoured to go much lower, and to make charts of limited portions of the sky down to even the th magnitude. from the very earliest days that men observed the stars, they could not help noticing that 'one star differeth from another star in glory,' and consequently they divided them into six classes, according to their brightness--classes which are commonly spoken of now as magnitudes. the ordinary th magnitude star is one which can be clearly seen by average sight on a good night, and it gives us about one-hundredth the light of an average st magnitude star. sirius, the brightest of all the fixed stars, is called a st magnitude star, but is really some six or seven times as bright as the average. it would take, therefore, more than two and a half million stars of the th magnitude to give as much light as sirius. it is evident that so searching a census as to embrace stars of the th magnitude would involve a most gigantic chart. but the work went on in more than one observatory for a considerable time, until at last the observers entered on to the region of the milky way. here the numbers of the stars presented to them were so great as to baffle all ordinary means of observation. what could be done? just at this time immense interest was caused in the astronomical world by the appearance of the great comet of . it was watched and observed and sketched by countless admirers, but more important still, it was photographed, and some of its photographs, taken at the royal observatory, cape of good hope, showed not only the comet with marvellous beauty of detail, but also thousands of stars, and the success of these photographs suggested to her majesty's astronomer at the cape, dr. gill, that in photography we possessed the means for making a complete sky census even to the th magnitude. the project was thought over in all its bearings, and in a great conference of astronomers at paris resolved upon an international scheme for photographing the entire heavens. the work was to be divided between eighteen observatories of different nationalities. it was to result in a photographic chart extending to the th magnitude, and probably embracing some forty million stars, and a catalogue made from measures of the photographs down to the th magnitude, which would probably include between two and three million stars. [illustration: the control pendulum and the base of the thompson telescope.] the eighteen observatories all undertook to use instruments of the same capacity. this was to be a photographic refractor, with an object-glass of inches aperture and feet focus. at greenwich this telescope is mounted equatorially--that is, so as to follow the stars in their courses--and is mounted on the top of the pier that once supported halley's quadrant. the telescope is driven by a most efficient clock, whose motive power is a heavy weight. the rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. a seconds pendulum is mounted in a glass case on the wall of the observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. on one of the wheels of the clock are arranged a number of little brass points, at such intervals apart that the wheel, when going at the proper rate, takes exactly one second to move through the distance between any pair. a little spring is arranged above the wheel, so that these points touch it as they pass. if this occurs exactly as the pendulum point passes through the mercury nothing happens, but if the clock is ever so little late or early, the electric current from the pendulum brings into action a second wheel, which accelerates or retards the driving of the clock, as the case may be. the total motion, therefore, is most beautifully even. [illustration: the astrographic telescope. (_reproduced from 'engineering' by permission._)] but even this is not quite sufficient, especially as the plates for the great chart have to be exposed for at least forty minutes. rigidly united with the -inch refractor, so that the two look like the two barrels of a huge double-barrelled gun, is a second telescope for the use of the observer. in its eyepiece are fixed two pairs of cross spider lines, commonly called wires, and a bright star, as near as possible to the centre of the field to be photographed, is brought to the junction of two wires. should the star appear to move away from the wire, the observer has but to press one of two buttons on a little plate which he carries in his hand, and which is connected by an electric wire with the driving clock, to bring it back to its position. the photographs taken with this instrument are of two kinds. those for the great chart have but a single exposure, but this lasts for forty minutes. those for the great catalogue have three exposures on them, the three images of a star being some seconds of arc apart. these exposures are of six minutes', three minutes', and twenty seconds' duration, and the last exposure is given as a test, since, if stars of the th magnitude are visible with an exposure of twenty seconds, stars of the th magnitude should be visible with three minutes' exposure. thus it will be seen that in three minutes an impression is got of many scores of stars, whose places it would require many hours to determine at the transit instrument. but the positions of these stars on the plate still remain to be measured. for this purpose a net-work of lines, at right angles to each other, is printed on the photograph before its development, and, after it has been developed, washed and dried, the distances of the stars from their nearest cross-lines are measured in the measuring machine. [illustration: the driving clock of the astrographic telescope. (_reproduced from 'engineering' by permission._)] the measuring machine is constructed to hold two plates, one half its breadth higher than the other. in fact, in each of the two series of photographs the whole sky is taken twice, but the two photographs of any region are not simply duplicates of each other. the centre of each plate is at a corner of four other plates, and in the micrometer the stars on the quarter common to two plates are measured simultaneously. in this way will be carried out a great census of the sky that will exceed flamsteed's ten thousand fold. and just as flamsteed's was but the first of many similar catalogues, so, no doubt, will this be followed by others--not superseded, for its value will increase with its age and the number of those that follow it, by comparison with which it will prove an inexhaustible mine of information concerning the motions of the stars and the structure of the universe. there is a great difference between the work of the observer with the 'astrographic telescope,' as this great twin photographic instrument is called, and the work of the transit observer. the latter sees the star gliding past him, and telegraphs the instant that the star threads itself on each of the ten vertical wires in succession. the astrographic observer, on the other hand, sees his star shining almost immovably in the centre of his field, threaded on the two cross wires placed there, for the driving-clock moves the telescope so as to almost exactly compensate for the rotation movement of the earth. the observer's duty in this case is to telegraph to his driving-clock, when it has in the least come short of or exceeded its duty, and so to bring back the 'guiding star' to its exact proper place on the cross wires. so far, the work of the astrographic department has been, as mentioned above, a development on an extraordinary scale, but a development still, of the original programme of the observatory. but the munificent gift of sir henry thompson has put it within the power of the astronomer royal to push this work of sidereal photography a stage further. sir henry thompson gave to the observatory, not merely the photographic refractor of inches' aperture, now used for solar photography, and known as the 'thompson photo-heliograph,' but also one of inches' aperture and - / feet focal length. this instrument was specially designed of exactly double the dimensions of the standard astrographic telescope used for the international photographic survey, the idea being that, in the case of a field of special interest and importance, a photograph could be obtained with the larger instrument on exactly double the scale given by the smaller. it has rather, however, found its usefulness in a slightly different field. the observation of the satellites of jupiter was suggested by galileo as a means of determining the longitude at sea. as already pointed out, the suggestion did not prove to be a practical one for that purpose, but observations of the satellites have been made none the less with a view simply to improving our knowledge of their movements, and of the mass of jupiter. the utilitarian motive for the work having fallen through, it has been carried on as a matter of pure science. and the work has not stopped with the satellites of jupiter; eight satellites were in due time discovered to saturn, four to uranus, and two to mars; and though these could give not the remotest assistance to navigation, they too have been made the subjects of observation for precisely the same reason as those of jupiter have been. [illustration: the thompson telescope in the new dome.] in just the same way, when the discovery of neptune was followed by that of a solitary companion to it, this also had to be followed. the difficulties in the way of observing the fainter of all these satellites were considerable, and the work has been mostly confined to two or three observatories possessing very large telescopes. as the largest telescope at greenwich was only inches in aperture up to , and only - / inches up to , it is only very recently that it has been able to take any very substantial part in satellite measures. but since the thompson photographic telescope was set up, it has been found that a photograph of neptune and its satellite can be taken in considerably less time than a complete set of direct measures can be made, whilst the photograph, which can be measured at leisure during the day, gives distinctly the more accurate results. so, too, the places of the minor planets can be got more accurately and quickly by means of photographs with this great telescope than by direct observation, and photographs of the most interesting of them all, the little planet eros, have been very successfully obtained. so that, though doing nothing directly to improve the art of navigation, or to find the longitude at sea, the great photographic refractor takes its share in the work of 'rectifying the tables of the planets.' [illustration: the nebulÆ of the pleiades. (_from a photograph taken at the royal observatory, greenwich, december , , with an exposure of three hours._)] the reflector of inches' aperture, which acts as a counterpoise to the sheaf of telescopes of the thompson, is intended for use with the spectroscope, the quality which mirrors possess of bringing all rays, whatever their colour, to the same focus being of great importance for spectroscopic work. but the experiments which have been made with it in celestial photography have proved so extremely successful as to cause the postponement of the recommencement of the spectroscopic researches. chief amongst these photographs are some good ones of the moon, and more recently some exceedingly fine photographs of the principal nebulæ. in no department of astronomy has photography brought us such striking results as in regard to the nebulæ. dr. roberts' photograph of the great nebula in andromeda converted the two or three meaningless rifts--which some of the best drawings had shown--into the divisions between concentric rings; and what had appeared a mere shapeless cloud was seen to be a vast symmetrical structure, a great sidereal system in the making. the great nebula in orion has grown in successive photographs in detail and extent, until we have a large part of the constellation bound together in the convolutions of a single nebula of the most exquisite detail and most amazing complexity. the group of the pleiades has had a more wonderful record still. manifestly a single system even to the naked eye, and showing some faint indications of nebulosity in the telescope, the photographs have revealed its principal stars shining out from nebulous masses, in appearance like carded wool, and have shown smaller stars threaded on nebulous lines like pearls upon a string. such photographs are, of course, of no utilitarian value, and at present they lead us to no definite scientific conclusions. they lie, therefore, doubly outside the limits of the purely practical, but they attract us by their extreme beauty, and by the amazing difficulty of the problems they suggest. how are these weird masses of gas retained in such complex form over distances which must be reckoned by millions of millions of miles? by what agency are they made to glow so as to be visible to us here? what conceivable condition threads together suns on a line of nebula? what universes are here in the making, or perhaps it may be falling into ruin and decay? chapter xiii the double-star department the foregoing chapters will have shown that though the original purpose of the observatory has always been kept in view, yet the progress of science has caused many researches to be undertaken which overstep its boundaries. thus in the present transit room, beside the successive transit instruments we find upon the wall two long thin tubes, labelled respectively alpha aquilæ and alpha cygni. these were two telescopes set up by pond for a special purpose. dr. brinkley, royal astronomer for ireland, had announced that he had found that several stars shifted their apparent place in the sky in the course of a year, due to the change in the position of the earth from which we view them, by an amount which would show that they were only about six to nine billions of miles distant from us; or, in other words, they showed a parallax of from two to three seconds of arc. pond was not able to confirm these parallaxes from his observations, and to decide the point he set up these two telescopes, the alpha aquilæ telescope being rigidly fixed on the west side of the pier of troughton's mural circles; the alpha cygni telescope on another pier, the one which now forms the base of the pier of the astrographic telescope. pond's method was to compare the position of these two stars with that of a star almost exactly the same distance from the pole, but at a great distance from it in time of crossing the meridian; in other words, of almost the same declination, but widely different right ascension. the result proved that brinkley was wrong, and vindicated the delicacy and accuracy of pond's observations. these two telescopes, therefore, had their day and ceased to be. others have followed them. an ingenious telescope was set up by sir george airy in order to ascertain if the speed of light were different when passing through water than when passing through air. or, in other words, if the aberration of light would give the same value as at present if we observed through water. the water telescope, as it was called, is kept on the ground floor of the central octagon of the new observatory. the observations obtained with it were hardly quite satisfactory, but gave on the whole a negative result. turning back to the transit room, and leaving it by the south-west door, we come into the little passage which leads at the back of bradley's transit room into the lower computing room. just inside this passage, on the left-hand side, there is a little room of a most curious shape, the 'reflex zenith room.' here is fixed a telescope pointing straight upwards, the eye-piece being fixed by the side of the object-glass. the light from a star--the star gamma draconis--which passes exactly over the zenith of greenwich, enters the object-glass, passes downwards to a basin of mercury, and is reflected upwards from the surface of the mercury to a little prism placed over the centre of the object-glass, from which it is reflected again into the eye-piece. by means of this telescope the distance of the star gamma draconis from the zenith could be measured very exactly, and, consequently, the changes in the apparent position of the star due to aberration, parallax, and other causes could be very exactly followed, and the corrections to be applied on account of these causes precisely determined. this particular telescope was devised by airy, and the observations with it were continued to the end of his reign. the germ of the idea may be traced back, however, to the time of flamsteed, who would seem to have occasionally observed gamma draconis from the bottom of a deep well; the precise position of the well is not, however, now known. later, bradley set up his celebrated - / -foot zenith sector, still preserved in the transit room, first at wanstead and then at greenwich, for the determination of the amount of aberration. later, a zenith tube by troughton, of feet focus, was used by pond in conjunction with the mural circle for observations of gamma draconis in order to determine the zenith point of the latter instrument. these telescopes for special purposes have passed out of use. observations with the spectroscope have been suspended for some years. the work of the astrographic department will come to an end, in the ordinary course of events, when the programme assigned to greenwich in the international scheme is completed. within the last few years a new department has come into being at greenwich--a department which has been steadily worked at many foreign public observatories, but only recently here. this is the department of double-star observation. the first double star, zeta ursæ majoris, was discovered years ago. bradley discovered two exceedingly famous double stars whilst still a young man observing with his uncle at wanstead--gamma virginis and castor. bradley made also other discoveries of double stars after his appointment to greenwich, and maskelyne succeeded him in the same line, but the great foundation of double-star astronomy was laid by sir william herschel. at first it was supposed that double stars were double only in appearance; one star comparatively near us 'happened' to lie in almost exactly the same direction as another star much further off. it was, indeed, in the very expectation that this would prove to be the case, that the elder herschel first took up their study. but he was soon convinced that many of the objects were true double stars--members of the same system of which the smaller revolved round the larger--not merely apparently double, one star appearing by chance to be close to another with which it had no connection--but real double stars. the discovery of these has led to the establishment of a new department of astronomy, again scientific rather than utilitarian. [illustration: double-star observation with the south-east equatorial. (_from a photograph by mr. edney._)] as mentioned above, it is only recently that greenwich has taken any appreciable part in this work. under airy, the largest equatorial of the time had been furnished with a good micrometer, and observations of one or two double stars been made now and again; but airy's programme of work was far too rigid, and kept the staff too closely engaged for such observations to be anything but extremely rare. and, indeed, when the micrometers of the equatorials were brought into use, they were far more generally devoted to the satellites of saturn than to the companions of stars. in the main, double-star astronomy has been in the hands of amateurs, at least in england. but the discovery in recent years of many pairs so close that a telescope of the largest size is required for their successful observation, has put an important section of double stars beyond the reach of most private observers, and therefore the great telescope at greenwich is now mainly devoted to their study. the astronomer royal, therefore, soon after the completion of the great equatorial of -inches aperture placed in the south-east dome, added this work to the observatory programme. the -inch equatorial is a remarkable-looking instrument, its mounting being of an entirely different kind to that of the other equatorials in the observatory, with the solitary exception of the shuckburgh, which is set up in a little dome over the chronograph room. the shuckburgh was presented to the observatory in the year , by sir g. shuckburgh. it was first intended to be mounted as an altazimuth, but proved to be unsteady in that position, and was then converted into an equatorial without clockwork, and mounted in its present position. the position is about as hopelessly bad a one as a telescope could well have, completely overshadowed as it is by the trees and buildings close at hand. the dome is a small one, and the arrangements for the shutters and for turning the dome are as bad as they could possibly be. it has practically been useless for the last forty years. its only interest is that the method of mounting employed is a small scale model of that of the great telescope in the s.-e. dome. in the german or fraunhofer form of mounting for an equatorial there is but a single pillar, which carries a comparatively short polar axis. at the upper end of the polar axis we find the declination axis, and at one end of the declination axis is the telescope, whilst at the other end is a heavy weight to counterpoise it. the german mounting has the advantage that the telescope can easily point to the pole of the heavens; its drawbacks are that, except in certain special forms, the telescope cannot travel very far when it is on the same side of the meridian as the star to which it is pointed, the end of the telescope coming into contact under such circumstances with the central pier, whilst the introduction of mere deadweight as the necessary counterpoise, is not economical. it has been already pointed out that the present astronomer royal has not only considerably modified the german mounting in the great collection of telescopes in the thompson dome, but has used a powerful reflector as a counterpoise to the sheaf of refractors at the other end of the declination axis. the english equatorial requires two piers. between these two piers is a long polar axis. both in the little shuckburgh and in the great -inch equatorial the frame of the polar axis consists of six parallel rods disposed in two equilateral triangles, with their bases parallel to each other, the telescope swinging in the space between the two bases. the construction of this form of equatorial, therefore, is expensive, as it requires two piers. it takes much more room than the german form, and the telescope cannot be directed precisely to the pole. but the instrument is symmetrical, there is no deadweight, and the telescope can follow a star from rising to setting without having to be reversed on crossing the meridian. the great stability of the english form of mounting, therefore, commended it very highly to airy, and he designed the great northumberland equatorial of the cambridge observatory on that plan, as well as one for the liverpool observatory at bidston, and in the s.-e. equatorial at greenwich. the telescope at first mounted upon it had an object-glass of - / inches' aperture, and feet focal length. that was dismounted in , and is now used as the guiding telescope of the thompson -inch photographic refractor. its place was taken by an immensely heavier instrument, the present refractor of inches' aperture, and feet focal length; and that this change was effected safely was an eloquent testimony to the solidity of the original mounting. the clock that drives this great instrument, so that it can follow a star or other celestial object in its apparent daily motion across the sky, is in the basement of the s.-e. tower. it is a very simple looking instrument, a conical pendulum in a glass case. the pendulum makes a complete revolution once in two seconds. below it in a closed case is a water turbine. a cistern on the roof of the staircase supplies this turbine with water, having a fall of about thirty feet. the water rushing out of the arms of the turbine forces it backward, and the turbine spins rapidly round, driving a spindle which runs up into the dome, and gears through one or two intermediate wheels with the great circle of the telescope; the extremely rapid rotation of the spindle, four times in a second, being converted by these intermediate wheels into the exceedingly slow one of once in twenty-four hours. just above the centre of motion of the turbine is a set of three small wheels, all of exactly the same size, and of the same number of teeth. of these the bottom wheel is horizontal, and is turned by the turbine. the top wheel is also horizontal, and is turned by the pendulum. the third wheel gears into both these, and is vertical. if the top and bottom wheels are moving exactly at the same rate, the intermediate wheel simply turns on its axis, but does not travel; but if the turbine and pendulum are moving at different rates, then the vertical wheel is forced to run in one direction or the other, and, doing so, it opens or closes a throttle valve, which controls the supply of water to the turbine, and so speedily brings the turbine into accord with the pendulum. the control of the motion of the great telescope is therefore almost as perfect as that of the astrographic and thompson equatorials, though the principle employed is very different. and the control needs to be perfect, for, as said above, the great telescope is mostly devoted to the observation of double stars, and there can be no greater hindrance to this work than a telescope which does not move accurately with the star. there is a striking contrast between the great telescope and all the massive machinery for its direction and movement, and the objects on which it is directed--two little points of light separated by a delicate hair of darkness. the observation is very unlike those of which we have hitherto spoken. the object is not to ascertain the actual position in the sky of the two stars, but their relative position to each other. a spider's thread of the finest strands is moved from one star to the other by turning an exquisitely fine screw; this enables us to measure their distance apart. another spider thread at right angles to the first is laid through the centres of both stars, and a divided circle enables us to read the angle which this line makes to the true east and west direction. such observations repeated year after year on many stars have enabled the orbits of not a few to be laid down with remarkable precision; and we find that their movements are completely consistent with the law of gravitation. further, just as neptune was pre-recognized and discovered from noting the irregularities in the motion of uranus, so the discordances in the place of sirius led to the belief that it was attracted by a then unseen companion, whose position with respect to the brighter star was predicted and afterwards seen. [illustration: the south-east dome with the shutter open.] gravitation thus appears, indeed, to be the bond of the universe, yet it leaves us with several weighty problems. the observation of the positions of stars shows that though we call them fixed they really have motions of their own. of these motions, a great part consists of a drift away from one portion of the heavens towards a point diametrically opposite to it, a drift such as must be due, not to a true motion of the individual stars, but to a motion through space of our sun and its attendant system. the elder herschel was the first to discover this mysterious solar motion. sir george airy and mr. edwin dunkin, for forty-six years a member of the greenwich staff, and from - the chief assistant, contributed important determinations of its direction. what is the cause of this motion, what is the law of this motion, is at present beyond our power to find out. many years ago a german astronomer made the random suggestion that possibly we were revolving in an orbit round the pleiades as a centre. the suggestion was entirely baseless, but unfortunately has found its way into many popular works, and still sometimes is brought forward as if it were one of the established truths of astronomy. we can at present only say that this solar motion is a mystery. there is a greater mystery still. the stars have their own individual motions, and in the case of a few these are of the most amazing swiftness. the earth in its motion round the sun travels nearly nineteen miles in a second, say one thousand times faster than the quickest rush of an express train. the sun's rate of motion is probably not quite so swift, but arcturus, a sun far larger than our own, has a pace some twenty times as swift as the orbital motion of the earth. this is not a motion that we can conceive of as being brought about by gravitation, for if there were some unseen body so vast as to draw arcturus with this swiftness, other stars too would be hurtling across the sky as quickly. such 'runaway stars' afford a problem to which we have as yet no key, and, like job of old, we are speechless when the question comes to us from heaven, 'canst thou guide arcturus and his sons?' it will be seen then that, fundamentally, greenwich observatory was founded and has been maintained for distinctly practical purposes, chiefly for the improvement of the eminently practical science of navigation. other inquiries relating to navigation, as, for instance, terrestrial magnetism and meteorology, have been added since. the pursuit of these objects has of necessity meant that the observatory was equipped with powerful and accurate instruments, and the possession of these again has led to their use in fields which lay outside the domain of the purely utilitarian, fields from which the only harvest that could be reaped was that of the increase of our knowledge. so we have been led step by step from the mere desire to help the mariner to find his way across the trackless ocean, to the establishment of the secret law which rules the movements of every body of the universe, till at length we stand face to face with the mysteries of vast systems in the making, with the intimate structure of the stellar universe, with the apparently aimless, causeless wanderings of vast suns in lightning flight; with problems that we cannot solve, nor hope to solve, yet cannot cease from attempting, problems to which the only answer we can give is the confession of the magicians of egypt--'this is the finger of god.' index aberration of light, adams, john c., his discovery of neptune, adhara, airy, george biddell, seventh astronomer royal, his early life, ; his work at cambridge, ; comes to greenwich, ; his relations with the visitors, ; his autobiography, ; his character, ; his labours, ; attacks on, ; his distinctions, ; his resignation, ; his death, ; anecdote of, ; his conduct _re_ adams, ; his water telescope, alderamin, _almagest_, almanac making, alpha aquilæ, telescope for, ---- cygni, telescope for, altazimuth the, ; description and work of, , _et seq._ altazimuth department, , _et seq._ american time, andromeda nebula, anemometer, use of, ; trace of, angström, anson, commodore, apparent time, arcturus, motion of, argelander, star catalogue of, _art of dialling_, the, assistants, position of the, , , , astrographic chart, ---- department, , _et seq._ ---- dome, ---- telescope, , _et seq._ astronomers royal, the, astrophysical researches, auroræ, automatic register, axis of the earth, precession of, ball, time, barometer, use of the, , battery basement, beaufort, captain, bessel quoted, betelgeuse, birkenhead, wreck of the, bliss, nathaniel, fourth astronomer royal, history of, bradley, james, third astronomer royal, his life, ; his ordination, ; vicar of bridstow, ; savilian professor of astronomy, ; discovers aberration of light, , _et seq._; becomes astronomer royal, ; labours of, ; character of, bradley's transit room, brinkley, dr., _british mariner's guide_, the, bunsen, buys ballot's law, canadian time, castor, , catalogues, star, , , _et seq._, , cepheus, charles ii., warrants of, , christie, w. h. m., eighth astronomer royal, work of, chromosphere of the sun, chronograph, the, ---- room, chronometer business, , chronometers, harrison's improvements in, , _et seq._; tests of, ; 'runs' of, ; romance of, circle department, , _et seq._ clock, astrographic driving, ; driving -inch telescope, clocks, standard, columbus, aim of voyage of, comet, appearance of a, ---- wells, comets, observation of, ; spectra of, commutator, the, comte, assertion of, constant of aberration, cook, captain, work of, copper, use of in observatory, corona of the sun, crabtree, james, crosthwait, joseph, dallmeyer telescope, declination, , _et seq._ denebola, distances of planets, ; of sun, double-star department, , _et seq._ double stars, dublin time, dunkin, edwin, earth, the, movements of, eclipses of the moon, ; of the sun, july , ... ; other eclipses of the sun, , _et seq._ electric railway, influence of, equation of time, the, , equatorial, shuckburgh's, ----, the great -inch, ----, the merz, - / -inch, ----, -inch, driving clock of, ; use of, ----, clock-driven, eros, discovery of, ; photographs of, errors in observations, noting of, , _et seq._ evaporation, faculæ of the sun, flamsteed, john, his report on saint-pierre's proposal, , ; appointed first astronomer royal, , ; his autobiography, ; his studies, ; his almanac, ; sent to london, ; enters jesus college, cambridge, ; completes his observatory, ; acquaintance with newton, ; takes his degree, ; his work, ; warrant for his salary, ; position of, ; his ordination, ; his pupils, ; his trouble with newton, , _et seq._; his catalogue, ; his letter to sharp, ; his death, ; his labours, flamsteed house, fraunhofer mounting, french time, galileo, his discovery of jupiter's satellites, gamma draconis, , ---- virginis, gascoigne, william, gemma frisius, plan of, george of denmark, prince, german mounting, , gould, dr., graham, gravitation, the bond of the universe, great comet of , the, , greatrackes, valentine, green, charles, greenwich time, ; distribution of, halley, edmund, his life, ; his early work, ; his catalogue of stars, ; elected f.r.s., ; his work on kepler's laws, ; becomes captain, ; savilian professor of geometry, ; astronomer royal, ; observations on saros of the moon, ; pressed by newton, ; his death, ; his services to science, ; his pay, ; nominates his successor, ; his transit instrument, halley's comet, harrison, james, timekeepers of, , , , heineken, rev. n. s., heineken quadrant, heliographic department, , _et seq._ herschel, caroline, hipparchus, catalogue of, hodgson, mr., hooke, robert, , horrox, jeremiah, huggins, sir w., his use of spectroscope, inscription, an, international photographic survey, ireis, iron quadrant, isobars, jupiter, satellites of, , ; atmosphere of, keill, john, kendall, larcum, kepler, laws of, kew, photo-heliograph, the, kinnebrook, david, kirchhoff's use of spectroscope, latitude, finding the, ledgers, chronometer, romance of, leverrier, his discovery of neptune, libraries, linacre, g., lindsay, thomas, quoted, litchford, w., local apparent time, longitude, finding the, ; at sea, problem of, ; determination of, longitude nought, lower computing room, lunars, method of, magnetic department, work of, ; description of, , _et seq._ magnetic inclination and declination, ---- needles, movements of, , ---- observatory, ---- pavilion, ---- storms, , mars, distance of, ; atmosphere of, ; satellites of, maskelyne, nevil, fifth astronomer royal, ; practical work of, ; astronomer royal, ; his work, ; his publications, ; his observations and work, , _et seq._; his death, ; his character, ; recommends his successor, ; his mural circle, mean solar clock, mean time, meldrum, dr., on sun spots, meridian, the, merz telescope, meteorological department, work of, ; description of, , _et seq._ micrometers, use of, microscopes, use of, milky way, miller, professor, milne, professor, on earth movements, minor planets, molyneux, samuel, moon, observation of the, , _et seq._; eclipses of, moore, sir jonas, ; death of, morin, mounting telescopes, modes of, mudge, thomas, mural arc, -feet, mural circles, , names of stars, origin of, nares, sir george, _nautical almanac_, the, , , navigation, state of primitive, neptune, discovery of, ; atmosphere of, ; satellite of, new altazimuth, the, , new observatory, the, , new stars, newcomb, professor, on growth of observatory, ; on greenwich observations, newton, sir i., his absent-mindedness, ; his trouble with flamsteed, , _et seq._; on kepler's laws, ; his _principia_, ; his pressure on halley, ; his discovery of gravitation, north terrace, the, northumberland equatorial, nutation of the earth, observation, modes of, , , ; by reflection, ; of comets, observatory, greenwich, work of, ; foundation of, ; warrant for building, ; position of, ; foundation stone laid, ; condition of, ; enlargement of, ; recent extensions of, ; description of, , _et seq._; staff of, ; work of, , _et seq._; visitors to, ; new altazimuth building, ; magnet house, ; magnetic pavilion, ; new observatory, ; future of, ; reflex zenith room, ; objects of, occultations by the moon, , _et seq._ octagon room, , , oldenburg, mr., orion nebula, , parallax of stars, paramour, the, paris, conference at, ----, noon at, philip iii., offer of, photographic registration, , , , ; refractors, photographs, star, photo-heliographs, , _et seq._, piazzi, discovery of, pleiades, the, polar plumes of the corona, polaris, pole-star, variation of, pond, john, sixth astronomer royal, his life, ; his reign, ; his salary, ; his assistants, ; his observations, ; censured by visitors, ; his observations of stars, pound, james, precession of earth's axis, _principia_, publication of, proctor, r. a., attack of, ptolemy, claudius, catalogue of, publication, the problem of, , quadrant, heineken, ----, the iron, railway time, rain gauge, record rooms, reflection, observation by, reflex zenith room, ---- ---- tube, refraction, effects of, right ascension, , _et seq._ roberts, dr. isaac, römer, discovery of, rosse, lord, royal society and flamsteed, , _et seq._ saint-pierre, le sieur de, proposal of, , sappho, saros of the moon, satellites, discovery of, saturn, atmosphere of, ; satellites of, schaeberle's comet, schedar, schiehallion, attraction of, schönfeld, scotchmen, anecdote of, sharp, abraham, sheepshanks, rev. james, on airy, shuckburgh equatorial, sidereal clock, sirius, sloane, dr., 'smith, mr.,' his chronometer, solar photographs, ---- storms, , sound waves, south, sir james, , south-east equatorial, the, , spectroscope, use of, spectroscopic department, , _et seq._ spots, sun, , _et seq._, staff of observatory, ; work of, , _et seq._ standard time, stars, observations of, , , ; origin of names of, ; movements of, ; catalogues of, , , _et seq._; composition of, , _et seq._; colour of, ; classes of, ; census of, ; photographs of, , _et seq._; motions of, , story, mr. a. m., sun, distance of the, , ; spots on, , _et seq._, ; eclipses of, , _et seq._; chromosphere of, ; motions of, sunshine recorder, swiss time, tebb, mr. w., tebbutt's comet, telescope, the great transit, ----, -inch, ----, astrographic, ----, shuckburgh, ----, thompson, , , thalèn, thermometer, use of, , thome, dr., thompson photo-heliograph, , , time ball, ---- department, the, , _et seq._ ---- desk, ----, foreign, ---- signals, ---- standard, transit, halley's, transit circle, the, ; mode of observation with, , _et seq._ transit circle, troughton's, ---- department, , _et seq._ ---- observations, number of, ---- pavilion, , ---- room, , troughton's transit circle, uranus, discovery of, ; atmosphere of, ; satellites of, vanes, use of, venus, distance of, victoria, visitors, the board of, ; censures pond, ; work of, ; constitution of, visitors to observatory, warrant for flamsteed's salary, water telescope, weather predictions, , _et seq._ winds, study of, witt, herr, discovery of, working catalogue, the, zenith sector, , ---- tube, , zeta ursæ majoris, zubeneschamal, the end london: printed by william clowes and sons, limited, stamford street and charing cross. * * * * * transcriber's note: minor typographical errors have been corrected without note. irregularities and inconsistencies in the text have been retained as printed. the illustrations have been moved so that they do not break up paragraphs. mismatched quotation marks were not corrected if it was not clear where the missing quotation mark should be placed. generously made available by the internet archive/american libraries.) the beauties of nature [illustration: _frontispiece._ group of beeches, burnham. _page ._] the beauties of nature and the wonders of the world we live in by the right hon. sir john lubbock, bart., m.p. f.r.s., d.c.l., ll.d. new york macmillan and co. and london _all rights reserved_ copyright, , by macmillan and co. typography by j. s. cushing & co., boston, u.s.a. presswork by berwick & smith, boston, u.s.a. contents chapter i page introduction beauty and happiness the love of nature enjoyment of scenery scenery of england foreign scenery the aurora the seasons chapter ii on animal life love of animals growth and metamorphoses rudimentary organs modifications colour communities of animals ants chapter iii on animal life--_continued_ freedom of animals sleep senses sense of direction number of species importance of the smaller animals size of animals complexity of animal structure length of life on individuality animal immortality chapter iv on plant life structure of flowers insects and flowers past history of flowers fruits and seeds leaves aquatic plants on hairs influence of soil on seedlings sleep of plants behaviour of leaves in rain mimicry ants and plants insectivorous plants movements of plants imperfection of our knowledge chapter v woods and fields fairy land tropical forests structure of trees ages of trees meadows downs chapter vi mountains alpine flowers mountain scenery the afterglow the origin of mountains glaciers swiss mountains volcanoes origin of volcanoes chapter vii water rivers and witchcraft water plants water animals origin of rivers the course of rivers deltas chapter viii rivers and lakes on the directions of rivers the conflicts and adventures of rivers on lakes on the configuration of valleys chapter ix the sea the sea coast sea life the ocean depths coral islands the southern skies the poles chapter x the starry heavens the moon the sun the planets mercury venus the earth mars the minor planets jupiter saturn uranus neptune origin of the planetary system comets shooting stars the stars nebulæ illustrations fig. page . larva of choerocampa porcellus . bougainvillea fruticosa; natural size. (after allman) . do. do. magnified . do. do. medusa-form . medusa aurita, and progressive stages of development. (after steenstrup) . white dead-nettle . do. . do. . salvia . do. . do. . primrose . do. . arum . twig of beech . arrangement of leaves in acer platanoides . diagram to illustrate the formation of mountain chains . section across the jura from brenets to neuchâtel. (after jaccard) . section from the spitzen across the brunnialp, and the maderanerthal. (after heim) . glacier of the blümlis alp. (after reclus) . cotopaxi. (after judd) . lava stream. (after judd) . stromboli, viewed from the north-west, april . (after judd) . upper valley of st. gotthard . section of a river valley. the dotted line shows a slope or talus of debris . valley of the rhone, with the waterfall of sallenches, showing a talus of debris . section across a valley. _a_, present river valley; _b_, old river terrace . diagram of an alpine valley, showing a river cone. front view . diagram of an alpine valley, showing a river cone. lateral view . map of the valais near sion . view in the rhone valley, showing a lateral cone . do. showing the slope of a river cone . shore of the lake of geneva, near vevey . view in the district of the broads, norfolk . delta of the po . do. mississippi . map of the lake district . section of the weald of kent, _a, a_, upper cretaceous strata, chiefly chalk, forming the north and south downs; _b, b_, escarpment of lower greensand, with a valley between it and the chalk; _c, c_, weald clay, forming plains; _d_, hills formed of hastings sand and clay. the chalk, etc., once spread across the country, as shown in the dotted lines . map of the weald of kent . sketch map of the swiss rivers . diagram in illustration of mountain structure . sketch map of the aar and its tributaries . river system round chur, as it used to be . river system round chur, as it is . river system of the maloya . final slope of a river . do. do. with a lake . diagrammatic section of a valley (exaggerated). _r r_, rocky basis of a valley; _a a_, sedimentary strata; _b_, ordinary level of river; _c_, flood level . whitsunday island. (after darwin) . a group of lunar volcanoes; maurolycus, barocius, etc. (after judd) . orbits of the inner planets. (after ball) . relative distances of the planets from the sun. (after ball) . saturn, with the surrounding series of rings. (after lockyer) . the parallactic ellipse. (after ball) . displacement of the hydrogen line in the spectrum of rigel. (after clarke) plates burnham beeches _frontispiece_ windsor castle. (from a drawing by j. finnemore) _to face page_ aquatic vegetation, rio. (published by spooner and co.) tropical forest, west indies. (after kingsley) summit of mont blanc the mer de glace, mont blanc rydal water. (from a photograph by frith and co., published by spooner and co.) windermere view in the valais below st. maurice view up the valais from the lake of geneva the land's end. (from a photograph by frith and co., published by spooner and co.) view of the moon near the third quarter. (from a photograph by prof. draper) chapter i introduction if any one gave you a few acres, you would say that you had received a benefit; can you deny that the boundless extent of the earth is a benefit? if any one gave you money, you would call that a benefit. god has buried countless masses of gold and silver in the earth. if a house were given you, bright with marble, its roof beautifully painted with colours and gilding, you would call it no small benefit. god has built for you a mansion that fears no fire or ruin ... covered with a roof which glitters in one fashion by day, and in another by night.... whence comes the breath you draw; the light by which you perform the actions of your life? the blood by which your life is maintained? the meat by which your hunger is appeased?... the true god has planted, not a few oxen, but all the herds on their pastures throughout the world, and furnished food to all the flocks; he has ordained the alternation of summer and winter ... has invented so many arts and varieties of voice, so many notes to make music.... we have implanted in us the seed of all ages, of all arts; and god our master brings forth our intellects from obscurity.--seneca. chapter i introduction the world we live in is a fairyland of exquisite beauty, our very existence is a miracle in itself, and yet few of us enjoy as we might, and none as yet appreciate fully, the beauties and wonders which surround us. the greatest traveller cannot hope even in a long life to visit more than a very small part of our earth, and even of that which is under our very eyes how little we see! what we do see depends mainly on what we look for. when we turn our eyes to the sky, it is in most cases merely to see whether it is likely to rain. in the same field the farmer will notice the crop, geologists the fossils, botanists the flowers, artists the colouring, sportsmen the cover for game. though we may all look at the same things, it does not at all follow that we should see them. it is good, as keble says, "to have our thoughts lift up to that world where all is beautiful and glorious,"--but it is well to realise also how much of this world is beautiful. it has, i know, been maintained, as for instance by victor hugo, that the general effect of beauty is to sadden. "comme la vie de l'homme, même la plus prospère, est toujours au fond plus triste que gaie, le ciel sombre nous est harmonieux. le ciel éclatant et joyeux nous est ironique. la nature triste nous ressemble et nous console; la nature rayonnante, magnifique, superbe ... a quelque chose d'accablant."[ ] this seems to me, i confess, a morbid view. there are many no doubt on whom the effect of natural beauty is to intensify feeling, to deepen melancholy, as well as to raise the spirits. as mrs. w. r. greg in her memoir of her husband tells us: "his passionate love for nature, so amply fed by the beauty of the scenes around him, intensified the emotions, as all keen perception of beauty does, but it did not add to their joyousness. we speak of the pleasure which nature and art and music give us; what we really mean is that our whole being is quickened by the uplifting of the veil. something passes into us which makes our sorrows more sorrowful, our joys more joyful,--our whole life more vivid. so it was with him. the long solitary wanderings over the hills, and the beautiful moonlight nights on the lake served to make the shadows seem darker that were brooding over his home." but surely to most of us nature when sombre, or even gloomy, is soothing and consoling; when bright and beautiful, not only raises the spirits, but inspires and elevates our whole being-- nature never did betray the heart that loved her; 'tis her privilege, through all the years of this our life, to lead from joy to joy: for she can so inform the mind that is within us, so impress with quietness and beauty, and so feed with lofty thoughts, that neither evil tongues, rash judgments, nor the sneers of selfish men, nor greetings where no kindness is, nor all the dreary intercourse of daily life, shall e'er prevail against us, or disturb our cheerful faith, that all which we behold is full of blessings.[ ] kingsley speaks with enthusiasm of the heaths and moors round his home, "where i have so long enjoyed the wonders of nature; never, i can honestly say, alone; because when man was not with me, i had companions in every bee, and flower and pebble; and never idle, because i could not pass a swamp, or a tuft of heather, without finding in it a fairy tale of which i could but decipher here and there a line or two, and yet found them more interesting than all the books, save one, which were ever written upon earth." those who love nature can never be dull. they may have other temptations; but at least they will run no risk of being beguiled, by ennui, idleness, or want of occupation, "to buy the merry madness of an hour with the long penitence of after time." the love of nature, again, helps us greatly to keep ourselves free from those mean and petty cares which interfere so much with calm and peace of mind. it turns "every ordinary walk into a morning or evening sacrifice," and brightens life until it becomes almost like a fairy tale. in the romances of the middle ages we read of knights who loved, and were loved by, nature spirits,--of sir launfal and the fairy tryamour, who furnished him with many good things, including a magic purse, in which as oft as thou puttest thy hand therein a mark of gold thou shalt iwinne, as well as protection from the main dangers of life. such times have passed away, but better ones have come. it is not now merely the few, who are so favoured. all those who love nature she loves in return, and will richly reward, not perhaps with the good things, as they are commonly called, but with the best things, of this world; not with money and titles, horses and carriages, but with bright and happy thoughts, contentment and peace of mind. happy indeed is the naturalist: to him the seasons come round like old friends; to him the birds sing: as he walks along, the flowers stretch out from the hedges, or look up from the ground, and as each year fades away, he looks back on a fresh store of happy memories. though we can never "remount the river of our years," he who loves nature is always young. but what is the love of nature? some seem to think they show a love of flowers by gathering them. how often one finds a bunch of withered blossoms on the roadside, plucked only to be thrown away! is this love of nature? it is, on the contrary, a wicked waste, for a waste of beauty is almost the worst waste of all. if we could imagine a day prolonged for a lifetime, or nearly so, and that sunrise and sunset were rare events which happened but a few times to each of us, we should certainly be entranced by the beauty of the morning and evening tints. the golden rays of the morning are a fortune in themselves, but we too often overlook the loveliness of nature, because it is constantly before us. for "the senseless folk," says king alfred, is far more struck at things it seldom sees. "well," says cicero, "did aristotle observe, 'if there were men whose habitations had been always underground, in great and commodious houses, adorned with statues and pictures, furnished with everything which they who are reputed happy abound with; and if, without stirring from thence, they should be informed of a certain divine power and majesty, and, after some time, the earth should open, and they should quit their dark abode to come to us; where they should immediately behold the earth, the seas, the heavens; should consider the vast extent of the clouds and force of the winds; should see the sun, and observe his grandeur and beauty, and also his creative power, inasmuch as day is occasioned by the diffusion of his light through the sky; and when night has obscured the earth, they should contemplate the heavens bespangled and adorned with stars; the surprising variety of the moon, in her increase and wane; the rising and setting of all the stars, and the inviolable regularity of their courses; when,' says he, 'they should see these things, they would undoubtedly conclude that there are gods, and that these are their mighty works.'"[ ] is my life vulgar, my fate mean, which on such golden memories can lean?[ ] at the same time the change which has taken place in the character of our religion has in one respect weakened the hold which nature has upon our feelings. to the greeks--to our own ancestors,--every river or mountain or forest had not only its own special deity, but in some sense was itself instinct with life. they were not only peopled by nymphs and fauns, elves and kelpies, were not only the favourite abodes of water, forest, or mountain spirits, but they had a conscious existence of their own. in the middle ages indeed, these spirits were regarded as often mischievous, and apt to take offence; sometimes as essentially malevolent--even the most beautiful, like the venus of tannhäuser, being often on that very account all the more dangerous; while the mountains and forests, the lakes and seas, were the abodes of hideous ghosts and horrible monsters, of giants and ogres, sorcerers and demons. these fears, though vague, were none the less extreme, and the judicial records of the middle ages furnish only too conclusive evidence that they were a terrible reality. the light of science has now happily dispelled these fearful nightmares. unfortunately, however, as men have multiplied, their energies have hitherto tended, not to beautify, but to mar. forests have been cut down, and replaced by flat fields in geometrical squares, or on the continent by narrow strips. here and there indeed we meet with oases, in which beauty has not been sacrificed to profit, and it is then happily found that not only is there no loss, but the earth seems to reward even more richly those who treat her with love and respect. scarcely any part of the world affords so great a variety in so small an area as our own island. commencing in the south, we have first the blue sea itself, the pebbly beaches, the white chalk cliffs of kent, the tinted sands of alum bay, the red sandstone of devonshire, granite and gneiss in cornwall: inland we have the chalk downs and clear streams, the well-wooded weald and the rich hop gardens; farther westwards the undulating gravelly hills, and still farther the granite tors: in the centre of england we have to the east the norfolk broads and the fens; then the fertile midlands, the cornfields, rich meadows, and large oxen; and to the west the welsh mountains; farther north the yorkshire wolds, the lancashire hills, the lakes of westmoreland; lastly, the swelling hills, bleak moors, and picturesque castles of northumberland and cumberland. there are of course far larger rivers, but perhaps none lovelier than the crystal thamis wont to glide in silver channel, down along the lee,[ ] [illustration: windsor castle. _to face page ._] by lawns and parks, meadows and wooded banks, dotted with country houses and crowned by windsor castle itself (see frontispiece). by many scotland is considered even more beautiful. and yet too many of us see nothing in the fields but sacks of wheat, in the meadows but trusses of hay, and in woods but planks for houses, or cover for game. even from this more prosaic point of view, how much there is to wonder at and admire, in the wonderful chemistry which changes grass and leaves, flowers and seeds, into bread and milk, eggs and cream, butter and honey! almost everything, says hamerton, "that the peasant does, is lifted above vulgarity by ancient, and often sacred, associations." there is, indeed, hardly any business or occupation with reference to which the same might not be said. the triviality or vulgarity does not depend on what we do, but on the spirit in which it is done. not only the regular professions, but every useful occupation in life, however humble, is honourable in itself, and may be pursued with dignity and peace. working in this spirit we have also the satisfaction of feeling that, as in some mountain track every one who takes the right path, seems to make the way clearer for those who follow; so may we also raise the profession we adopt, and smooth the way for those who come after us. but, even for those who are not agriculturists, it must be admitted that the country has special charms. one perhaps is the continual change. every week brings some fresh leaf or flower, bird or insect. every month again has its own charms and beauty. we sit quietly at home and nature decks herself for us. in truth we all love change. some think they do not care for it, but i doubt if they know themselves. "not," said jefferies, "for many years was i able to see why i went the same round and did not care for change. i do not want change: i want the same old and loved things, the same wild flowers, the same trees and soft ash-green; the turtle-doves, the blackbirds, the coloured yellow-hammer sing, sing, singing so long as there is light to cast a shadow on the dial, for such is the measure of his song, and i want them in the same place. let me find them morning after morning, the starry-white petals radiating, striving upwards up to their ideal. let me see the idle shadows resting on the white dust; let me hear the humble-bees, and stay to look down on the yellow dandelion disk. let me see the very thistles opening their great crowns--i should miss the thistles; the reed grasses hiding the moor-hen; the bryony bine, at first crudely ambitious and lifted by force of youthful sap straight above the hedgerow to sink of its weight presently and progress with crafty tendrils; swifts shot through the air with outstretched wings like crescent-headed shaftless arrows darted from the clouds; the chaffinch with a feather in her bill; all the living staircase of the spring, step by step, upwards to the great gallery of the summer, let me watch the same succession year by year." after all then he did enjoy the change and the succession. kingsley again in his charming prose idyll "my winter garden" tries to persuade himself that he was glad he had never travelled, "having never yet actually got to paris." monotony, he says, "is pleasant in itself; morally pleasant, and morally useful. marriage is monotonous; but there is much, i trust, to be said in favour of holy wedlock. living in the same house is monotonous; but three removes, say the wise, are as bad as a fire. locomotion is regarded as an evil by our litany. the litany, as usual, is right. 'those who travel by land or sea' are to be objects of our pity and our prayers; and i do pity them. i delight in that same monotony. it saves curiosity, anxiety, excitement, disappointment, and a host of bad passions." but even as he writes one can see that he does not convince himself. possibly, he admits, "after all, the grapes are sour"; and when some years after he did travel, how happy he was! at last, he says, triumphantly, "at last we too are crossing the atlantic. at last the dream of forty years, please god, would be fulfilled, and i should see (and happily not alone), the west indies and the spanish main. from childhood i had studied their natural history, their charts, their romances; and now, at last, i was about to compare books with facts, and judge for myself of the reported wonders of the earthly paradise." no doubt there is much to see everywhere. the poet and the naturalist find "tropical forests in every square foot of turf." it may even be better, and especially for the more sensitive natures, to live mostly in quiet scenery, among fields and hedgerows, woods and downs; but it is surely good for every one, from time to time, to refresh and strengthen both mind and body by a spell of sea air or mountain beauty. on the other hand we are told, and told of course with truth, that though mountains may be the cathedrals of nature, they are generally remote from centres of population; that our great cities are grimy, dark, and ugly; that factories are creeping over several of our counties, blighting them into building ground, replacing trees by chimneys, and destroying almost every vestige of natural beauty. but if this be true, is it not all the more desirable that our people should have access to pictures and books, which may in some small degree, at any rate, replace what they have thus unfortunately lost? we cannot all travel; and even those who can, are able to see but a small part of the world. moreover, though no one who has once seen, can ever forget, the alps, the swiss lakes, or the riviera, still the recollection becomes less vivid as years roll on, and it is pleasant, from time to time, to be reminded of their beauties. there is one other advantage not less important. we sometimes speak as if to visit a country, and to see it, were the same thing. but this is not so. it is not every one who can see switzerland like a ruskin or a tyndall. their beautiful descriptions of mountain scenery depend less on their mastery of the english language, great as that is, than on their power of seeing what is before them. it has been to me therefore a matter of much interest to know which aspects of nature have given the greatest pleasure to, or have most impressed, those who, either from wide experience or from their love of nature, may be considered best able to judge. i will begin with an english scene from kingsley. he is describing his return from a day's trout-fishing:-- "what shall we see," he says, "as we look across the broad, still, clear river, where the great dark trout sail to and fro lazily in the sun? white chalk fields above, quivering hazy in the heat. a park full of merry hay-makers; gay red and blue waggons; stalwart horses switching off the flies; dark avenues of tall elms; groups of abele, 'tossing their whispering silver to the sun'; and amid them the house,--a great square red-brick mass, made light and cheerful though by quoins and windows of white sarsden stone, with high peaked french roofs, broken by louvres and dormers, haunted by a thousand swallows and starlings. old walled gardens, gay with flowers, shall stretch right and left. clipt yew alleys shall wander away into mysterious glooms, and out of their black arches shall come tripping children, like white fairies, to laugh and talk with the girl who lies dreaming and reading in the hammock there, beneath the black velvet canopy of the great cedar tree, like some fair tropic flower hanging from its boughs; and we will sit down, and eat and drink among the burdock leaves, and then watch the quiet house, and lawn, and flowers, and fair human creatures, and shining water, all sleeping breathless in the glorious light beneath the glorious blue, till we doze off, lulled by the murmur of a thousand insects, and the rich minstrelsy of nightingale and blackcap, thrush and dove. "peaceful, graceful, complete english country life and country houses; everywhere finish and polish; nature perfected by the wealth and art of peaceful centuries! why should i exchange you, even for the sight of all the alps?" though jefferies was unfortunately never able to travel, few men have loved nature more devotedly, and speaking of his own home he expresses his opinion that: "of all sweet things there is none so sweet as fresh air--one great flower it is, drawn round about; over, and enclosing us, like aphrodite's arms; as if the dome of the sky were a bell-flower drooping down over us, and the magical essence of it filling all the room of the earth. sweetest of all things is wild-flower air. full of their ideal the starry flowers strained upwards on the bank, striving to keep above the rude grasses that push by them; genius has ever had such a struggle. the plain road was made beautiful by the many thoughts it gave. i came every morning to stay by the star-lit bank." passing to countries across the ocean, humboldt tells us that: "if i might be allowed to abandon myself to the recollection of my own distant travels, i would instance, amongst the most striking scenes of nature, the calm sublimity of a tropical night, when the stars, not sparkling, as in our northern skies, shed their soft and planetary light over the gently heaving ocean; or i would recall the deep valleys of the cordilleras, where the tall and slender palms pierce the leafy veil around them, and waving on high their feathery and arrow-like branches, form, as it were, 'a forest above a forest'; or i would describe the summit of the peak of teneriffe, when a horizon layer of clouds, dazzling in whiteness, has separated the cone of cinders from the plain below, and suddenly the ascending current pierces the cloudy veil, so that the eye of the traveller may range from the brink of the crater, along the vine-clad slopes of orotava, to the orange gardens and banana groves that skirt the shore. in scenes like these, it is not the peaceful charm uniformly spread over the face of nature that moves the heart, but rather the peculiar physiognomy and conformation of the land, the features of the landscape, the ever-varying outline of the clouds, and their blending with the horizon of the sea, whether it lies spread before us like a smooth and shining mirror, or is dimly seen through the morning mist. all that the senses can but imperfectly comprehend, all that is most awful in such romantic scenes of nature, may become a source of enjoyment to man, by opening a wide field to the creative power of his imagination. impressions change with the varying movements of the mind, and we are led by a happy illusion to believe that we receive from the external world that with which we have ourselves invested it." humboldt also singles out for especial praise the following description given of tahiti by darwin[ ]:-- "the land capable of cultivation is scarcely in any part more than a fringe of low alluvial soil, accumulated round the base of mountains, and protected from the waves of the sea by a coral reef, which encircles at a distance the entire line of coast. the reef is broken in several parts so that ships can pass through, and the lake of smooth water within, thus affords a safe harbour, as well as a channel for the native canoes. the low land which comes down to the beach of coral sand is covered by the most beautiful productions of the inter-tropical regions. in the midst of bananas, orange, cocoa-nut, and breadfruit trees, spots are cleared where yams, sweet potatoes, sugar-cane, and pine-apples are cultivated. even the brushwood is a fruit tree, namely, the guava, which from its abundance is as noxious as a weed. in brazil i have often admired the contrast of varied beauty in the banana, palm, and orange tree; here we have in addition the breadfruit tree, conspicuous from its large, glossy, and deeply digitated leaf. it is admirable to behold groves of a tree, sending forth its branches with the force of an english oak, loaded with large and most nutritious fruit. however little on most occasions utility explains the delight received from any fine prospect, in this case it cannot fail to enter as an element in the feeling. the little winding paths, cool from the surrounding shade, led to the scattered houses; and the owners of these everywhere gave us a cheerful and most hospitable reception." darwin himself has told us, after going round the world that "in calling up images of the past, i find the plains of patagonia frequently cross before my eyes; yet these plains are pronounced by all to be most wretched and useless. they are characterised only by negative possessions; without habitations, without water, without trees, without mountains, they support only a few dwarf plants. why then--and the case is not peculiar to myself--have these arid wastes taken so firm possession of my mind? why have not the still more level, the greener and more fertile pampas, which are serviceable to mankind, produced an equal impression? i can scarcely analyse these feelings, but it must be partly owing to the free scope given to the imagination. the plains of patagonia are boundless, for they are scarcely practicable, and hence unknown; they bear the stamp of having thus lasted for ages, and there appears no limit to their duration through future time. if, as the ancients supposed, the flat earth was surrounded by an impassable breadth of water, or by deserts heated to an intolerable excess, who would not look at these last boundaries to man's knowledge with deep but ill-defined sensations?" hamerton, whose wide experience and artistic power make his opinion especially important, says:-- "i know nothing in the visible world that combines splendour and purity so perfectly as a great mountain entirely covered with frozen snow and reflected in the vast mirror of a lake. as the sun declines, its thousand shadows lengthen, pure as the cold green azure in the depth of a glacier's crevasse, and the illuminated snow takes first the tender colour of a white rose, and then the flush of a red one, and the sky turns to a pale malachite green, till the rare strange vision fades into ghastly gray, but leaves with you a permanent recollection of its too transient beauty."[ ] wallace especially, and very justly, praises the description of tropical forest scenery given by belt in his charming _naturalist in nicaragua_:-- "on each side of the road great trees towered up, carrying their crowns out of sight amongst a canopy of foliage, and with lianas hanging from nearly every bough, and passing from tree to tree, entangling the giants in a great network of coiling cables. sometimes a tree appears covered with beautiful flowers which do not belong to it, but to one of the lianas that twines through its branches and sends down great rope-like stems to the ground. climbing ferns and vanilla cling to the trunks, and a thousand epiphytes perch themselves on the branches. amongst these are large arums that send down long aerial roots, tough and strong, and universally used instead of cordage by the natives. amongst the undergrowth several small species of palms, varying in height from two to fifteen feet, are common; and now and then magnificent tree ferns send off their feathery crowns twenty feet from the ground to delight the sight by their graceful elegance. great broad-leaved heliconias, leathery melastomæ, and succulent-stemmed, lop-sided leaved and flesh-coloured begonias are abundant, and typical of tropical american forests; but not less so are the cecropia trees, with their white stems and large palmated leaves standing up like great candelabra. sometimes the ground is carpeted with large flowers, yellow, pink, or white, that have fallen from some invisible tree-top above; or the air is filled with a delicious perfume, the source of which one seeks around in vain, for the flowers that cause it are far overhead out of sight, lost in the great over-shadowing crown of verdure." "but," he adds, "the uniformity of climate which has led to this rich luxuriance and endless variety of vegetation is also the cause of a monotony that in time becomes oppressive." to quote the words of mr. belt: "unknown are the autumn tints, the bright browns and yellows of english woods; much less the crimsons, purples, and yellows of canada, where the dying foliage rivals, nay, excels, the expiring dolphin in splendour. unknown the cold sleep of winter; unknown the lovely awakening of vegetation at the first gentle touch of spring. a ceaseless round of ever-active life weaves the fairest scenery of the tropics into one monotonous whole, of which the component parts exhibit in detail untold variety of beauty." siberia is no doubt as a rule somewhat severe and inhospitable, but m. patrin mentions with enthusiasm how one day descending from the frozen summits of the altai, he came suddenly on a view of the plain of the obi--the most beautiful spectacle, he says, which he had ever witnessed. behind him were barren rocks and the snows of winter, in front a great plain, not indeed entirely green, or green only in places, and for the rest covered by three flowers, the purple siberian iris, the golden hemerocallis, and the silvery narcissus--green, purple, gold, and white, as far as the eye could reach. wallace tells us that he himself has derived the keenest enjoyment from his sense of colour:-- "the heavenly blue of the firmament, the glowing tints of sunset, the exquisite purity of the snowy mountains, and the endless shades of green presented by the verdure-clad surface of the earth, are a never-failing source of pleasure to all who enjoy the inestimable gift of sight. yet these constitute, as it were, but the frame and background of a marvellous and ever-changing picture. in contrast with these broad and soothing tints, we have presented to us in the vegetable and animal worlds an infinite variety of objects adorned with the most beautiful and most varied hues. flowers, insects, and birds are the organisms most generally ornamented in this way; and their symmetry of form, their variety of structure, and the lavish abundance with which they clothe and enliven the earth, cause them to be objects of universal admiration. the relation of this wealth of colour to our mental and moral nature is indisputable. the child and the savage alike admire the gay tints of flowers, birds, and insects; while to many of us their contemplation brings a solace and enjoyment which is both intellectually and morally beneficial. it can then hardly excite surprise that this relation was long thought to afford a sufficient explanation of the phenomena of colour in nature; and although the fact that-- full many a flower is born to blush unseen, and waste its sweetness on the desert air, might seem to throw some doubt on the sufficiency of the explanation, the answer was easy,--that in the progress of discovery man would, sooner or later, find out and enjoy every beauty that the hidden recesses of the earth have in store for him." professor colvin speaks with special admiration of greek scenery:-- "in other climates, it is only in particular states of the weather that the remote ever seems so close, and then with an effect which is sharp and hard as well as clear; here the clearness is soft; nothing cuts or glitters, seen through that magic distance; the air has not only a new transparency so that you can see farther into it than elsewhere, but a new quality, like some crystal of an unknown water, so that to see into it is greater glory." speaking of the ranges and promontories of sterile limestone, the same writer observes that their colours are as austere and delicate as the forms. "if here the scar of some old quarry throws a stain, or there the clinging of some thin leafage spreads a bloom, the stain is of precious gold, and the bloom of silver. between the blue of the sky and the tenfold blue of the sea these bare ranges seem, beneath that daylight, to present a whole system of noble colour flung abroad over perfect forms. and wherever, in the general sterility, you find a little moderate verdure--a little moist grass, a cluster of cypresses--or whenever your eye lights upon the one wood of the district, the long olive grove of the cephissus, you are struck with a sudden sense of richness, and feel as if the splendours of the tropics would be nothing to this." most travellers have been fascinated by the beauty of night in the tropics. our evenings no doubt are often delicious also, though the mild climate we enjoy is partly due to the sky being so often overcast. in parts of the tropics, however, the air is calm and cloudless throughout nearly the whole of the year. there is no dew, and the inhabitants sleep on the house-tops, in full view of the brightness of the stars and the beauty of the sky, which is almost indescribable. "il faisait," says bernardin de st. pierre of such a scene, "une de ces nuits délicieuses, si communes entre les tropiques, et dont le plus abile pinceau ne rendrait pas le beauté. la lune paraissait au milieu du firmament, entourée d'un rideau de nuages, que ses rayons dissipaient par degrés. sa lumière se répandait insensiblement sur les montagnes de l'île et sur leurs pitons, qui brillaient d'un vert argenté. les vents retenaient leurs haleines. on entendait dans les bois, au fond des vallées, au haut des rochers, de petits cris, de doux murmures d'oiseaux, qui se caressaient dans leurs nids, réjouis par la clarté de la nuit et la tranquillité de l'air. tous, jusqu'aux insectes, bruissaient sous l'herbe. les étoiles étincelaient au ciel, et se réfléchissaient au sein de la mer, qui répétait leurs images tremblantes." in the arctic and antarctic regions the nights are often made quite gorgeous by the northern lights or aurora borealis, and the corresponding appearance in the southern hemisphere. the aurora borealis generally begins towards evening, and first appears as a faint glimmer in the north, like the approach of dawn. gradually a curve of light spreads like an immense arch of yellowish-white hue, which gains rapidly in brilliancy, flashes and vibrates like a flame in the wind. often two or even three arches appear one over the other. after a while coloured rays dart upwards in divergent pencils, often green below, yellow in the centre, and crimson above, while it is said that sometimes almost black, or at least very dark violet, rays are interspersed among the rings of light, and heighten their effect by contrast. sometimes the two ends of the arch seem to rise off the horizon, and the whole sheet of light throbs and undulates like a fringed curtain of light; sometimes the sheaves of rays unite into an immense cupola; while at others the separate rays seem alternately lit and extinguished. gradually the light flickers and fades away, and has generally disappeared before the first glimpse of dawn. we seldom see the aurora in the south of england, but we must not complain; our winters are mild, and every month has its own charm and beauty. in january we have the lengthening days. " february " the first butterfly. " march " the opening buds. " april " the young leaves and spring flowers. " may " the song of birds. " june " the sweet new-mown hay. " july " the summer flowers. " august " the golden grain. " september " the fruit. " october " the autumn tints. " november " the hoar frost on trees and the pure snow. " december " last not least, the holidays of christmas, and the bright fireside. it is well to begin the year in january, for we have then before us all the hope of spring. oh wind, if winter comes, can spring be long behind?[ ] spring seems to revive us all. in the song of solomon-- my beloved spake, and said unto me, rise up, my love, my fair one, and come away. for, lo, the winter is past, the rain is over and gone; the flowers appear on the earth; the time of the singing of birds is come, the voice of the turtle is heard in our land, the fig tree putteth forth her green figs, and the vines with the tender grape give a good smell. "but indeed there are days," says emerson, "which occur in this climate, at almost any season of the year, wherein the world reaches its perfection, when the air, the heavenly bodies, and the earth make a harmony, as if nature would indulge her offspring.... these halcyon days may be looked for with a little more assurance in that pure october weather, which we distinguish by the name of the indian summer. the day, immeasurably long, sleeps over the broad hills and warm wide fields. to have lived through all its sunny hours, seems longevity enough." yet does not the very name of indian summer imply the superiority of the summer itself,--the real, the true summer, "when the young corn is bursting into ear; the awned heads of rye, wheat, and barley, and the nodding panicles of oats, shoot from their green and glaucous stems, in broad, level, and waving expanses of present beauty and future promise. the very waters are strewn with flowers: the buck-bean, the water-violet, the elegant flowering rush, and the queen of the waters, the pure and splendid white lily, invest every stream and lonely mere with grace."[ ] for our greater power of perceiving, and therefore of enjoying nature, we are greatly indebted to science. over and above what is visible to the unaided eye, the two magic tubes, the telescope and microscope, have revealed to us, at least partially, the infinitely great and the infinitely little. science, our fairy godmother, will, unless we perversely reject her help, and refuse her gifts, so richly endow us, that fewer hours of labour will serve to supply us with the material necessaries of life, leaving us more time to ourselves, more leisure to enjoy all that makes life best worth living. even now we all have some leisure, and for it we cannot be too grateful. "if any one," says seneca, "gave you a few acres, you would say that you had received a benefit; can you deny that the boundless extent of the earth is a benefit? if a house were given you, bright with marble, its roof beautifully painted with colours and gilding, you would call it no small benefit. god has built for you a mansion that fears no fire or ruin ... covered with a roof which glitters in one fashion by day, and in another by night. whence comes the breath which you draw; the light by which you perform the actions of your life? the blood by which your life is maintained? the meat by which your hunger is appeased?... the true god has planted, not a few oxen, but all the herds on their pastures throughout the world, and furnished food to all the flocks; he has ordained the alternation of summer and winter ... he has invented so many arts and varieties of voice, so many notes to make music.... we have implanted in us the seeds of all ages, of all arts; and god our master brings forth our intellects from obscurity."[ ] footnotes: [ ] _choses vues._ [ ] wordsworth. [ ] cicero, _de natura deorum_. [ ] thoreau. [ ] spenser. [ ] darwin's _voyage of the beagle_. [ ] hamerton's _landscape_. [ ] shelley. [ ] howitt's _book of the seasons_. [ ] seneca, _de beneficiis_. chapter ii on animal life if thy heart be right, then will every creature be to thee a mirror of life, and a book of holy doctrine. thomas À kempis. chapter ii on animal life there is no species of animal or plant which would not well repay, i will not say merely the study of a day, but even the devotion of a lifetime. their form and structure, development and habits, geographical distribution, relation to other living beings, and past history, constitute an inexhaustible study. when we consider how much we owe to the dog, man's faithful friend, to the noble horse, the patient ox, the cow, the sheep, and our other domestic animals, we cannot be too grateful to them; and if we cannot, like some ancient nations, actually worship them, we have perhaps fallen into the other extreme, underrate the sacredness of animal life, and treat them too much like mere machines. some species, however, are no doubt more interesting than others, especially perhaps those which live together in true communities, and which offer so many traits--some sad, some comical, and all interesting,--which reproduce more or less closely the circumstances of our own life. the modes of animal life are almost infinitely diversified; some live on land, some in water; of those which are aquatic some dwell in rivers, some in lakes or pools, some on the sea-shore, others in the depths of the ocean. some burrow in the ground, some find their home in the air. some live in the arctic regions, some in the burning deserts; one little beetle (hydrobius) in the thermal waters of hammam-meskoutin, at a temperature of °. as to food, some are carnivorous and wage open war; some, more insidious, attack their victims from within; others feed on vegetable food, on leaves or wood, on seeds or fruits; in fact, there is scarcely an animal or vegetable substance which is not the special and favourite food of one or more species. hence to adapt them to these various requirements we find the utmost differences of form and size and structure. even the same individual often goes through great changes. growth and metamorphoses the development, indeed, of an animal from birth to maturity is no mere question of growth. the metamorphoses of insects have long excited the wonder and admiration of all lovers of nature. they depend to a great extent on the fact that the little creatures quit the egg at an early stage of development, and lead a different life, so that the external forces acting on them, are very different from those by which they are affected when they arrive at maturity. a remarkable case is that of certain beetles which are parasitic on solitary bees. the young larva is very active, with six strong legs. it conceals itself in some flower, and when the bee comes in search of honey, leaps upon her, but is so minute as not to be perceived. the bee constructs her cell, stores it with honey, and lays her egg. at that moment the little larva quits the bee and jumps on to the egg, which she proceeds gradually to devour. having finished the egg, she attacks the honey; but under these circumstances the activity which was at first so necessary has become useless; the legs which did such good service are no longer required; and the active slim larva changes into a white fleshy grub, which floats comfortably in the honey with its mouth just below the surface. even in the same group we may find great differences. for instance, in the family of insects to which bees and wasps belong, some have grub larvæ, such as the bee and ant; some have larvæ like caterpillars, such as the sawflies; and there is a group of minute forms the larvæ of which live inside the eggs of other insects, and present very remarkable and abnormal forms. these differences depend mainly on the mode of life and the character of the food. rudimentary organs such modifications may be called adaptive, but there are others of a different origin that have reference to the changes which the race has passed through in bygone ages. in fact the great majority of animals do go through metamorphoses (many of them as remarkable, though not so familiar as those of insects), but in many cases they are passed through within the egg and thus escape popular observation. naturalists who accept the theory of evolution, consider that the development of each individual represents to a certain extent that which the species has itself gone through in the lapse of ages; that every individual contains within itself, so to say, a history of the race. thus the rudimentary teeth of cows, sheep, whales, etc. (which never emerge from their sockets), the rudimentary toes of many mammals, the hind legs of whales and of the boa-constrictor, which are imbedded in the flesh, the rudimentary collar-bone of the dog, etc., are indications of descent from ancestors in which these organs were fully developed. again, though used for such different purposes, the paddle of a whale, the leg of a horse and of a mole, the wing of a bird or a bat, and the arm of a man, are all constructed on the same model, include corresponding bones, and are similarly arranged. the long neck of the giraffe, and the short one of the whale (if neck it can be called), contain the same number of vertebræ. even after birth the young of allied species resemble one another much more than the mature forms. the stripes on the young lion, the spots on the young blackbird, are well-known cases; and we find the same law prevalent among the lower animals, as, for instance, among insects and crustacea. the lobster, crab, shrimp, and barnacle are very unlike when full grown, but in their young stages go through essentially similar metamorphoses. no animal is perhaps in this respect more interesting than the horse. the skull of a horse and that of a man, though differing so much, are, says flower,[ ] "composed of exactly the same number of bones, having the same general arrangement and relation to each other. not only the individual bones, but every ridge and surface for the attachment of muscles, and every hole for the passage of artery or nerve, seen in the one can be traced in the other." it is often said that the horse presents a remarkable peculiarity in that the canine teeth grow but once. there are, however, in most horses certain spicules or minute points which are shed before the appearance of the permanent canines, and which are probably the last remnants of the true milk canines. the foot is reduced to a single toe, representing the third digit, but the second and fourth, though rudimentary, are represented by the splint bones; while the foot also contains traces of several muscles, originally belonging to the toes which have now disappeared, and which "linger as it were behind, with new relations and uses, sometimes in a reduced, and almost, if not quite, functionless condition." even man himself presents traces of gill-openings, and indications of other organs which are fully developed in lower animals. modifications there is in new zealand a form of crow (hura), in which the female has undergone a very curious modification. it is the only case i know, in which the bill is differently shaped in the two sexes. the bird has taken on the habits of a woodpecker, and the stout crow-like bill of the cock-bird is admirably adapted to tap trees, and if they sound hollow, to dig down to the burrow of the insect; but it lacks the horny-pointed tip of the tongue, which in the true woodpecker is provided with recurved hairs, thus enabling that bird to pierce the grub and draw it out. in the hura, however, the bill of the hen-bird has become much elongated and slightly curved, and when the cock has dug down to the burrow, the hen inserts her long bill and draws out the grub, which they then divide between them: a very pretty illustration of the wife as helpmate to the husband. it was indeed until lately the general opinion that animals and plants came into existence just as we now see them. we took pleasure in their beauty; their adaptation to their habits and mode of life in many cases could not be overlooked or misunderstood. nevertheless the book of nature was like some missal richly illuminated, but written in an unknown tongue. the graceful forms of the letters, the beauty of the colouring, excited our wonder and admiration; but of the true meaning little was known to us; indeed we scarcely realised that there was any meaning to decipher. now glimpses of the truth are gradually revealing themselves, we perceive that there is a reason, and in many cases we know what the reason is, for every difference in form, in size, and in colour; for every bone and every feather, almost for every hair.[ ] colour the colours of animals, generally, i believe, serve as a protection. in some, however, they probably render them more attractive to their mates, of which the peacock is one of the most remarkable illustrations. in richness of colour birds and insects vie even with flowers. "one fine red admiral butterfly," says jefferies,[ ] "whose broad wings, stretched out like fans, looked simply splendid floating round and round the willows which marked the margin of a dry pool. his blue markings were really blue--blue velvet--his red and the white stroke shone as if sunbeams were in his wings. i wish there were more of these butterflies; in summer, dry summer, when the flowers seem gone and the grass is not so dear to us, and the leaves are dull with heat, a little colour is so pleasant. to me colour is a sort of food; every spot of colour is a drop of wine to the spirit." the varied colours which add so much to the beauty of animals and plants are not only thus a delight to the eye, but afford us also some of the most interesting problems in natural history. some probably are not in themselves of any direct advantage. the brilliant mother-of-pearl of certain shells, which during life is completely hidden, the rich colours of some internal organs of animals, are not perhaps of any direct benefit, but are incidental, like the rich and brilliant hues of many minerals and precious stones. but although this may be true, i believe that most of these colours are now of some advantage. "the black back and silvery belly of fishes" have been recently referred to by a distinguished naturalist as being obviously of no direct benefit. i should on the contrary have quoted this case as one where the advantage was obvious. the dark back renders the fish less conspicuous to an eye looking down into the water; while the white under-surface makes them less visible from below. the animals of the desert are sand-coloured; those of the arctic regions are white like snow, especially in winter; and pelagic animals are blue. let us take certain special cases. the lion, like other desert animals, is sand-coloured; the tiger which lives in the jungle has vertical stripes, making him difficult to see among the upright grass; leopards and the tree-cats are spotted, like rays of light seen through leaves. an interesting case is that of the animals living in the sargasso or gulf-weed of the atlantic. these creatures--fish, crustacea, and mollusks alike--are characterised by a peculiar colouring, not continuously olive like the seaweed itself, but blotched with rounded more or less irregular patches of bright, opaque white, so as closely to resemble fronds covered with patches of flustra or barnacles. take the case of caterpillars, which are especially defenceless, and which as a rule feed on leaves. the smallest and youngest are green, like the leaves on which they live. when they become larger, they are characterised by longitudinal lines, which break up the surface and thus render them less conspicuous. on older and larger ones the lines are diagonal, like the nerves of leaves. conspicuous caterpillars are generally either nauseous in taste, or protected by hairs. [illustration: fig. .--_choerocampa porcellus._] i say "generally," because there are some interesting exceptions. the large caterpillars of some of the elephant hawkmoths are very conspicuous, and rendered all the more so by the presence of a pair of large eyelike spots. every one who sees one of these caterpillars is struck by its likeness to a snake, and the so-called "eyes" do much to increase the deception. moreover, the ring on which they are placed is swollen, and the insect, when in danger, has the habit of retracting its head and front segments, which gives it an additional resemblance to some small reptile. that small birds are, as a matter of fact, afraid of these caterpillars (which, however, i need not say, are in reality altogether harmless) weismann has proved by actual experiment. he put one of these caterpillars in a tray, in which he was accustomed to place seed for birds. soon a little flock of sparrows and other small birds assembled to feed as usual. one of them lit on the edge of this tray, and was just going to hop in, when she spied the caterpillar. immediately she began bobbing her head up and down in the odd way which some small birds have, but was afraid to go nearer. another joined her and then another, until at last there was a little company of ten or twelve birds all looking on in astonishment, but not one ventured into the tray; while one bird, which lit in it unsuspectingly, beat a hasty retreat in evident alarm as soon as she perceived the caterpillar. after waiting for some time, weismann removed it, when the birds soon attacked the seeds. other caterpillars also are probably protected by their curious resemblance to spotted snakes. one of the large indian caterpillars has even acquired the power of hissing. among perfect insects many resemble closely the substances near which they live. some moths are mottled so as to mimic the bark of trees, or moss, or the surface of stones. one beautiful tropical butterfly has a dark wing on which are painted a series of green leaf tips, so that it closely resembles the edge of a pinnate leaf projecting out of shade into sunshine. the argument is strengthened by those cases in which the protection, or other advantage, is due not merely to colour, but partly also to form. such are the insects which resemble sticks or leaves. again, there are cases in which insects mimic others, which, for some reason or other, are less liable to danger. so also many harmless animals mimic others which are poisonous or otherwise well protected. some butterflies, as mr. bates has pointed out, mimic others which are nauseous in taste, and therefore not attacked by birds. in these cases it is generally only the females that are mimetic, and in some cases only a part of them, so that there are two, or even three, kinds of females, the one retaining the normal colouring of the group, the other mimicking another species. some spiders closely resemble ants, and several other insects mimic wasps or hornets. some reptiles and fish have actually the power of changing the colour of their skin so as to adapt themselves to their surroundings. many cases in which the colouring does not at first sight appear to be protective, will on consideration be found to be so. it has, for instance, been objected that sheep are not coloured green; but every mountaineer knows that sheep could not have had a colour more adapted to render them inconspicuous, and that it is almost impossible to distinguish them from the rocks which so constantly crop up on hill sides. even the brilliant blue of the kingfisher, which in a museum renders it so conspicuous, in its native haunts, on the contrary, makes it difficult to distinguish from a flash of light upon the water; and the richly-coloured woodpecker wears the genuine dress of a forester--the green coat and crimson cap. it has been found that some brilliantly coloured and conspicuous animals are either nauseous or poisonous. in these cases the brilliant colour is doubtless a protection by rendering them more unmistakable. communities some animals may delight us especially by their beauty, such as birds or butterflies; others may surprise us by their size, as elephants and whales, or the still more marvellous monsters of ancient times; may fascinate us by their exquisite forms, such as many microscopic shells; or compel our reluctant attention by their similarity to us in structure; but none offer more points of interest than those which live in communities. i do not allude to the temporary assemblages of starlings, swallows, and other birds at certain times of year, nor even to the permanent associations of animals brought together by common wants in suitable localities, but to regular and more or less organised associations. such colonies as those of rooks and beavers have no doubt interesting revelations and surprises in store for us, but they have not been as yet so much studied as those of some insects. among these the hive bees, from the beauty and regularity of their cells, from their utility to man, and from the debt we owe them for their unconscious agency in the improvement of flowers, hold a very high place; but they are probably less intelligent, and their relations with other animals and with one another are less complex than in the case of ants, which have been so well studied by gould, huber, forel, m'cook, and other naturalists. the subject is a wide one, for there are at least a thousand species of ants, no two of which have the same habits. in this country we have rather more than thirty, most of which i have kept in confinement. their life is comparatively long: i have had working ants which were seven years old, and a queen ant lived in one of my nests for fifteen years. the community consists, in addition to the young, of males, which do no work, of wingless workers, and one or more queen mothers, who have at first wings, which, however, after one marriage flight, they throw off, as they never leave the nest again, and in it wings would of course be useless. the workers do not, except occasionally, lay eggs, but carry on all the affairs of the community. some of them, and especially the younger ones, remain in the nest, excavate chambers and tunnels, and tend the young, which are sorted up according to age, so that my nests often had the appearance of a school, with the children arranged in classes. in our english ants the workers in each species are all similar except in size, but among foreign species there are some in which there are two or even more classes of workers, differing greatly not only in size, but also in form. the differences are not the result of age, nor of race, but are adaptations to different functions, the nature of which, however, is not yet well understood. among the termites those of one class certainly seem to act as soldiers, and among the true ants also some have comparatively immense heads and powerful jaws. it is doubtful, however, whether they form a real army. bates observed that on a foraging expedition the large-headed individuals did not walk in the regular ranks, nor on the return did they carry any of the booty, but marched along at the side, and at tolerably regular intervals, "like subaltern officers in a marching regiment." he is disposed, however, to ascribe to them a much humbler function, namely, to serve merely "as indigestible morsels to the ant thrushes." this, i confess, seems to me improbable. solomon was, so far as we yet know, quite correct in describing ants as having "neither guide, overseer, nor ruler." the so-called queens are really mothers. nevertheless it is true, and it is curious, that the working ants and bees always turn their heads towards the queen. it seems as if the sight of her gave them pleasure. on one occasion, while moving some ants from one nest into another for exhibition at the royal institution, i unfortunately crushed the queen and killed her. the others, however, did not desert her, or draw her out as they do dead workers, but on the contrary carried her into the new nest, and subsequently into a larger one with which i supplied them, congregating round her for weeks just as if she had been alive. one could hardly help fancying that they were mourning her loss, or hoping anxiously for her recovery. the communities of ants are sometimes very large, numbering even up to , individuals; and it is a lesson to us, that no one has ever yet seen a quarrel between any two ants belonging to the same community. on the other hand it must be admitted that they are in hostility, not only with most other insects, including ants of different species, but even with those of the same species if belonging to different communities. i have over and over again introduced ants from one of my nests into another nest of the same species, and they were invariably attacked, seized by a leg or an antenna, and dragged out. it is evident therefore that the ants of each community all recognise one another, which is very remarkable. but more than this, i several times divided a nest into two halves, and found that even after a separation of a year and nine months they recognised one another, and were perfectly friendly; while they at once attacked ants from a different nest, although of the same species. it has been suggested that the ants of each nest have some sign or password by which they recognise one another. to test this i made some insensible. first i tried chloroform, but this was fatal to them; and as therefore they were practically dead, i did not consider the test satisfactory. i decided therefore to intoxicate them. this was less easy than i had expected. none of my ants would voluntarily degrade themselves by getting drunk. however, i got over the difficulty by putting them into whisky for a few moments. i took fifty specimens, twenty-five from one nest and twenty-five from another, made them dead drunk, marked each with a spot of paint, and put them on a table close to where other ants from one of the nests were feeding. the table was surrounded as usual with a moat of water to prevent them from straying. the ants which were feeding soon noticed those which i had made drunk. they seemed quite astonished to find their comrades in such a disgraceful condition, and as much at a loss to know what to do with their drunkards as we are. after a while, however, to cut my story short, they carried them all away: the strangers they took to the edge of the moat and dropped into the water, while they bore their friends home into the nest, where by degrees they slept off the effects of the spirit. thus it is evident that they know their friends even when incapable of giving any sign or password. this little experiment also shows that they help comrades in distress. if a wolf or a rook be ill or injured, we are told that it is driven away or even killed by its comrades. not so with ants. for instance, in one of my nests an unfortunate ant, in emerging from the chrysalis skin, injured her legs so much that she lay on her back quite helpless. for three months, however, she was carefully fed and tended by the other ants. in another case an ant in the same manner had injured her antennæ. i watched her also carefully to see what would happen. for some days she did not leave the nest. at last one day she ventured outside, and after a while met a stranger ant of the same species, but belonging to another nest, by whom she was at once attacked. i tried to separate them, but whether by her enemy, or perhaps by my well-meant but clumsy kindness, she was evidently much hurt and lay helplessly on her side. several other ants passed her without taking any notice, but soon one came up, examined her carefully with her antennæ, and carried her off tenderly to the nest. no one, i think, who saw it could have denied to that ant one attribute of humanity, the quality of kindness. the existence of such communities as those of ants or bees implies, no doubt, some power of communication, but the amount is still a matter of doubt. it is well known that if one bee or ant discovers a store of food, others soon find their way to it. this, however, does not prove much. it makes all the difference whether they are brought or sent. if they merely accompany on her return a companion who has brought a store of food, it does not imply much. to test this, therefore, i made several experiments. for instance, one cold day my ants were almost all in their nests. one only was out hunting and about six feet from home. i took a dead bluebottle fly, pinned it on to a piece of cork, and put it down just in front of her. she at once tried to carry off the fly, but to her surprise found it immovable. she tugged and tugged, first one way and then another for about twenty minutes, and then went straight off to the nest. during that time not a single ant had come out; in fact she was the only ant of that nest out at the time. she went straight in, but in a few seconds--less than half a minute,--came out again with no less than twelve friends, who trooped off with her, and eventually tore up the dead fly, carrying it off in triumph. now the first ant took nothing home with her; she must therefore somehow have made her friends understand that she had found some food, and wanted them to come and help her to secure it. in all such cases, however, so far as my experience goes, the ants brought their friends, and some of my experiments indicated that they are unable to send them. certain species of ants, again, make slaves of others, as huber first observed. if a colony of the slave-making ants is changing the nest, a matter which is left to the discretion of the slaves, the latter carry their mistresses to their new home. again, if i uncovered one of my nests of the fuscous ant (formica fusca), they all began running about in search of some place of refuge. if now i covered over one small part of the nest, after a while some ant discovered it. in such a case, however, the brave little insect never remained there, she came out in search of her friends, and the first one she met she took up in her jaws, threw over her shoulder (their way of carrying friends), and took into the covered part; then both came out again, found two more friends and brought them in, the same manoeuvre being repeated until the whole community was in a place of safety. this i think says much for their public spirit, but seems to prove that, in f. fusca at least, the powers of communication are but limited. one kind of slave-making ant has become so completely dependent on their slaves, that even if provided with food they will die of hunger, unless there is a slave to put it into their mouth. i found, however, that they would thrive very well if supplied with a slave for an hour or so once a week to clean and feed them. but in many cases the community does not consist of ants only. they have domestic animals, and indeed it is not going too far to say that they have domesticated more animals than we have. of these the most important are aphides. some species keep aphides on trees and bushes, others collect root-feeding aphides into their nests. they serve as cows to the ants, which feed on the honey-dew secreted by the aphides. not only, moreover, do the ants protect the aphides themselves, but collect their eggs in autumn, and tend them carefully through the winter, ready for the next spring. many other insects are also domesticated by ants, and some of them, from living constantly underground, have completely lost their eyes and become quite blind. but i must not let myself be carried away by this fascinating subject, which i have treated more at length in another work.[ ] i will only say that though their intelligence is no doubt limited, still i do not think that any one who has studied the life-history of ants can draw any fundamental line of separation between instinct and reason. when we see a community of ants working together in perfect harmony, it is impossible not to ask ourselves how far they are mere exquisite automatons; how far they are conscious beings? when we watch an ant-hill tenanted by thousands of industrious inhabitants, excavating chambers, forming tunnels, making roads, guarding their home, gathering food, feeding the young, tending their domestic animals--each one fulfilling its duties industriously, and without confusion,--it is difficult altogether to deny to them the gift of reason; and all our recent observations tend to confirm the opinion that their mental powers differ from those of men, not so much in kind as in degree. footnotes: [ ] _the horse._ [ ] lubbock, _fifty years of science_. [ ] _the open air._ [ ] _ants, bees, and wasps._ chapter iii on animal life--_continued_ an organic being is a microcosm--a little universe, formed of a host of self-propagating organisms, inconceivably minute and numerous as the stars of heaven. darwin. chapter iii on animal life--_continued._ we constantly speak of animals as free. a fish, says ruskin, "is much freer than a man; and as to a fly, it is a black incarnation of freedom." it is pleasant to think of anything as free, but in this case the idea is, i fear, to a great extent erroneous. young animals may frolic and play, but older ones take life very seriously. about the habits of fish and flies, indeed, as yet we know very little. any one, however, who will watch animals will soon satisfy himself how diligently they work. even when they seem to be idling over flowers, or wandering aimlessly about, they are in truth diligently seeking for food, or collecting materials for nests. the industry of bees is proverbial. when collecting honey or pollen they often visit over twenty flowers in a minute, keeping constantly to one species, without yielding a moment's dalliance to any more sweet or lovely tempter. ants fully deserve the commendation of solomon. wasps have not the same reputation for industry; but i have watched them from before four in the morning till dark at night working like animated machines without a moment's rest or intermission. sundays and bank holidays are all the same to them. again, birds have their own gardens and farms from which they do not wander, and within which they will tolerate no interference. their ideas of the rights of property are far stricter than those of some statesmen. as to freedom, they have their daily duties as much as a mechanic in a mill or a clerk in an office. they suffer under alarms, moreover, from which we are happily free. mr. galton believes that the life of wild animals is very anxious. "from my own recollection," he says, "i believe that every antelope in south africa has to run for its life every one or two days upon an average, and that he starts or gallops under the influence of a false alarm many times in a day. those who have crouched at night by the side of pools in the desert, in order to have a shot at the beasts that frequent it, see strange scenes of animal life; how the creatures gambol at one moment and fight at another; how a herd suddenly halts in strained attention, and then breaks into a maddened rush as one of them becomes conscious of the stealthy movements or rank scent of a beast of prey. now this hourly life-and-death excitement is a keen delight to most wild creatures, but must be peculiarly distracting to the comfort-loving temperament of others. the latter are alone suited to endure the crass habits and dull routine of domesticated life. suppose that an animal which has been captured and half-tamed, received ill-usage from his captors, either as punishment or through mere brutality, and that he rushed indignantly into the forest with his ribs aching from blows and stones. if a comfort-loving animal, he will probably be no gainer by the change, more serious alarms and no less ill-usage awaits him: he hears the roar of the wild beasts, and the headlong gallop of the frightened herds, and he finds the buttings and the kicks of other animals harder to endure than the blows from which he fled: he has peculiar disadvantages from being a stranger; the herds of his own species which he seeks for companionship constitute so many cliques, into which he can only find admission by more fighting with their strongest members than he has spirit to undergo. as a set-off against these miseries, the freedom of savage life has no charms for his temperament; so the end of it is, that with a heavy heart he turns back to the habitation he had quitted." but though animals may not be free, i hope and believe that they are happy. dr. hudson, an admirable observer, assures us with confidence that the struggle for existence leaves them much leisure and famous spirits. "in the animal world," he exclaims,[ ] "what happiness reigns! what ease, grace, beauty, leisure, and content! watch these living specks as they glide through their forests of algæ, all 'without hurry and care,' as if their 'span-long lives' really could endure for the thousand years that the old catch pines for. here is no greedy jostling at the banquet that nature has spread for them; no dread of each other; but a leisurely inspection of the field, that shows neither the pressure of hunger nor the dread of an enemy. "'to labour and to be content' (that 'sweet life' of the son of sirach)--to be equally ready for an enemy or a friend--to trust in themselves alone, to show a brave unconcern for the morrow, all these are the admirable points of a character almost universal among animals, and one that would lighten many a heart were it more common among men. that character is the direct result of the golden law 'if one will not work, neither let him eat'; a law whose stern kindness, unflinchingly applied, has produced whole nations of living creatures, without a pauper in their ranks, flushed with health, alert, resolute, self-reliant, and singularly happy." it has often been said that man is the only animal gifted with the power of enjoying a joke, but if animals do not laugh, at any rate they sometimes play. we are, indeed, apt perhaps to credit them with too much of our own attributes and emotions, but we can hardly be mistaken in supposing that they enjoy certain scents and sounds. it is difficult to separate the games of kittens and lambs from those of children. our countryman gould long ago described the "amusements or sportive exercises" which he had observed among ants. forel was at first incredulous, but finally confirmed these statements; and, speaking of certain tropical ants, bates says "the conclusion that they were engaged in play was irresistible." sleep we share with other animals the great blessing of sleep, nature's soft nurse, "the mantle that covers thought, the food that appeases hunger, the drink that quenches thirst, the fire that warms cold, the cold that moderates heat, the coin that purchases all things, the balance and weight that equals the shepherd with the king, and the simple with the wise." some animals dream as we do; dogs, for instance, evidently dream of the chase. with the lower animals which cannot shut their eyes it is, however, more difficult to make sure whether they are awake or asleep. i have often noticed insects at night, even when it was warm and light, behave just as if they were asleep, and take no notice of objects which would certainly have startled them in the day. the same thing has also been observed in the case of fish. but why should we sleep? what a remarkable thing it is that one-third of our life should be passed in unconsciousness. "half of our days," says sir t. browne, "we pass in the shadow of the earth, and the brother of death extracteth a third part of our lives." the obvious suggestion is that we require rest. but this does not fully meet the case. in sleep the mind is still awake, and lives a life of its own: our thoughts wander, uncontrolled, by the will. the mind, therefore, is not necessarily itself at rest; and yet we all know how it is refreshed by sleep. but though animals sleep, many of them are nocturnal in their habits. humboldt gives a vivid description of night in a brazilian forest. "everything passed tranquilly till eleven at night, and then a noise so terrible arose in the neighbouring forest that it was almost impossible to close our eyes. amid the cries of so many wild beasts howling at once the indians discriminated such only as were (at intervals) heard separately. these were the little soft cries of the sapajous, the moans of the alouate apes, the howlings of the jaguar and couguar, the peccary and the sloth, and the cries of (many) birds. when the jaguars approached the skirt of the forest our dog, which till then had never ceased barking, began to howl and seek for shelter beneath our hammocks. sometimes, after a long silence, the cry of the tiger came from the tops of the trees; and then it was followed by the sharp and long whistling of the monkeys, which appeared to flee from the danger which threatened them. we heard the same noises repeated during the course of whole months whenever the forest approached the bed of the river. "when the natives are interrogated on the causes of the tremendous noise made by the beasts of the forest at certain hours of the night, the answer is, they are keeping the feast of the full moon. i believe this agitation is most frequently the effect of some conflict that has arisen in the depths of the forest. the jaguars, for instance, pursue the peccaries and the tapirs, which, having no defence, flee in close troops, and break down the bushes they find in their way. terrified at this struggle, the timid and distrustful monkeys answer, from the tops of the trees, the cries of the large animals. they awaken the birds that live in society, and by degrees the whole assembly is in commotion. it is not always in a fine moonlight, but more particularly at the time of a storm of violent showers, that this tumult takes place among the wild beasts. 'may heaven grant them a quiet night and repose, and us also!' said the monk who accompanied us to the rio negro, when, sinking with fatigue, he assisted in arranging our accommodation for the night." life is indeed among animals a struggle for existence, and in addition to the more usual weapons--teeth and claws--we find in some animals special and peculiar means of offence and defence. if we had not been so familiarised with the fact, the possession of poison might well seem a wonderful gift. that a fluid, harmless in one animal itself, should yet prove so deadly when transferred to others, is certainly very remarkable; and though the venom of the cobra or the rattlesnake appeal perhaps more effectively to our imagination, we have conclusive evidence of concentrated poison even in the bite of a midge, which may remain for days perceptible. the sting of a bee or wasp, though somewhat similar in its effect, is a totally different organ, being a modified ovipositor. some species of ants do not sting in the ordinary sense, but eject their acrid poison to a distance of several inches. another very remarkable weapon is the electric battery of certain eels, of the electric cat fish, and the torpedoes, one of which is said to be able to discharge an amount of electricity sufficient to kill a man. some of the medusæ and other zoophytes are armed by millions of minute organs known as "thread cells." each consists of a cell, within which a firm, elastic thread is tightly coiled. the moment the medusa touches its prey the cells burst and the threads spring out. entering the flesh as they do by myriads, they prove very effective weapons. the ink of the sepia has passed into a proverb. the animal possesses a store of dark fluid, which, if attacked, it at once ejects, and thus escapes under cover of the cloud thus created. the so-called bombardier beetles, when attacked, discharge at the enemy, from the hinder part of their body, an acrid fluid which, as soon as it comes in contact with air, explodes with a sound resembling a miniature gun. westwood mentions, on the authority of burchell, that on one occasion, "whilst resting for the night on the banks of one of the large south american rivers, he went out with a lantern to make an astronomical observation, accompanied by one of his black servant boys; and as they were proceeding, their attention was directed to numerous beetles running about upon the shore, which, when captured, proved to be specimens of a large species of brachinus. on being seized they immediately began to play off their artillery, burning and staining the flesh to such a degree that only a few specimens could be captured with the naked hand, and leaving a mark which remained a considerable time. upon observing the whitish vapour with which the explosions were accompanied, the negro exclaimed in his broken english, with evident surprise, 'ah, massa, they make smoke!'" many other remarkable illustrations might be quoted; as for instance the web of the spider, the pit of the ant lion, the mephitic odour of the skunk. senses we generally attribute to animals five senses more or less resembling our own. but even as regards our own senses we really know or understand very little. take the question of colour. the rainbow is commonly said to consist of seven colours--red, orange, yellow, green, blue, indigo, and violet. but it is now known that all our colour sensations are mixtures of three simple colours, red, green, and violet. we are, however, absolutely ignorant how we perceive these colours. thomas young suggested that we have three different systems of nerve fibres, and helmholtz regards this as "a not improbable supposition"; but so far as microscopical examination is concerned, there is no evidence whatever for it. or take again the sense of hearing. the vibrations of the air no doubt play upon the drum of the ear, and the waves thus produced are conducted through a complex chain of small bones to the fenestra ovalis and so to the inner ear or labyrinth. but beyond this all is uncertainty. the labyrinth consists mainly of two parts ( ) the cochlea, and ( ) the semicircular canals, which are three in number, standing at right angles to one another. it has been supposed that they enable us to maintain the equilibrium of the body, but no satisfactory explanation of their function has yet been given. in the cochlea, corti discovered a remarkable organ consisting of some four thousand complex arches, which increase regularly in length and diminish in height. they are connected at one end with the fibres of the auditory nerve, and helmholtz has suggested that the waves of sound play on them, like the fingers of a performer on the keys of a piano, each separate arch corresponding to a different sound. we thus obtain a glimpse, though but a glimpse, of the manner in which perhaps we hear; but when we pass on to the senses of smell and taste, all we know is that the extreme nerve fibres terminate in certain cells which differ in form from those of the general surface; but in what manner the innumerable differences of taste or smell are communicated to the brain, we are absolutely ignorant. if then we know so little about ourselves, no wonder that with reference to other animals our ignorance is extreme. we are too apt to suppose that the senses of animals must closely resemble, and be confined to ours. no one can doubt that the sensations of other animals differ in many ways from ours. their organs are sometimes constructed on different principles, and situated in very unexpected places. there are animals which have eyes on their backs, ears in their legs, and sing through their sides. we all know that the senses of animals are in many cases much more acute than ours, as for instance the power of scent in the dog, of sight in the eagle. moreover, our eye is much more sensitive to some colours than to others; least so to crimson, then successively to red, orange, yellow, blue, and green; the sensitiveness for green being as much as times as great as for red. this alone may make objects appear of very different colours to different animals. nor is the difference one of degree merely. the rainbow, as we see it, consists of seven colours--red, orange, yellow, green, blue, indigo, and violet. but though the red and violet are the limits of the visible spectrum, they are not the limits of the spectrum itself, there are rays, though invisible to us, beyond the red at the one end, and beyond the violet at the other: the existence of the ultra red can be demonstrated by the thermometer; while the ultra violet are capable of taking a photograph. but though the red and violet are respectively the limits of our vision, i have shown[ ] by experiments which have been repeated and confirmed by other naturalists, that some of the lower animals are capable of perceiving the ultra-violet rays, which to us are invisible. it is an interesting question whether these rays may not produce on them the impression of a new colour, or colours, differing from any of those known to us. so again with hearing, not only may animals in some cases hear better than we do, but sounds which are beyond the reach of our ears, may be audible to theirs. even among ourselves the power of hearing shrill sounds is greater in some persons than in others. sound, as we know, is produced by vibration of the air striking on the drum of the ear, and the fewer are the vibrations in a second, the deeper is the sound, which becomes shriller and shriller as the waves of sound become more rapid. in human ears the limits of hearing are reached when about , vibrations strike the drum of the ear in a second. whatever the explanation of the gift of hearing in ourselves may be, different plans seem to be adopted in the case of other animals. in many crustacea and insects there are flattened hairs each connected with a nerve fibre, and so constituted as to vibrate in response to particular notes. in others the ear cavity contains certain minute solid bodies, known as otoliths, which in the same way play upon the nerve fibres. sometimes these are secreted by the walls of the cavity itself, but certain crustacea have acquired the remarkable habit of selecting after each moult suitable particles of sand, which they pick up with their pincers and insert into their ears. many insects, besides the two large "compound" eyes one on each side of the head, have between them three small ones, known as the "ocelli," arranged in a triangle. the structure of these two sets of eyes is quite different. the ocelli appear to see as our eyes do. the lens throws an inverted image on the back of the eye, so that with these eyes they must see everything reversed, as we ourselves really do, though long practice enables us to correct the impression. on the other hand, the compound eyes consist of a number of facets, in some species as many as , in each eye, and the prevailing impression among entomologists now is that each facet receives the impression of one pencil of rays, that in fact the image formed in a compound eye is a sort of mosaic. in that case, vision by means of these eyes must be direct; and it is indeed difficult to understand how an insect can obtain a correct impression when it looks at the world with five eyes, three of which see everything reversed, while the other two see things the right way up! on the other hand, some regard each facet as an independent eye, in which case many insects realise the epigram of plato-- thou lookest on the stars, my love, ah, would that i could be yon starry skies with thousand eyes, that i might look on thee! even so, therefore, we only substitute one difficulty for another. but this is not all. we have not only no proof that animals are confined to our five senses, but there are strong reasons for believing that this is not the case. in the first place, many animals have organs which from their position, structure, and rich supply of nerves, are evidently organs of sense; and yet which do not appear to be adapted to any one of our five senses. as already mentioned, the limits of hearing are reached when about , vibrations of the air strike on the drums of our ears. light, as was first conclusively demonstrated by our great countryman young, is the impression produced by vibration of the ether on the retina of the eye. when millions of millions of vibrations strike the eye in a second, we see violet; and the colour changes as the number diminishes, millions of millions giving us the impression of red. between thousand and millions of millions the interval is immense, and it is obvious that there might be any number of sensations. when we consider how greatly animals differ from us, alike in habits and structure, is it not possible, nay, more, is it not likely that some of these problematical organs are the seats of senses unknown to us, and give rise to sensations of which we have no conception? in addition to the capacity for receiving and perceiving, some animals have the faculty of emitting light. in our country the glow-worm is the most familiar case, though some other insects and worms have, at any rate under certain conditions, the same power, and it is possible that many others are really luminous, though with light which is invisible to us. in warmer climates the fire-fly, lanthorn-fly, and many other insects, shine with much greater brilliance, and in these cases the glow seems to be a real love-light, like the lamp of hero. many small marine animals, medusæ, crustacea, worms, etc., are also brilliantly luminous at night. deep-sea animals are endowed also in many cases with special luminous organs, to which i shall refer again. sense of direction it has been supposed that animals possess also what has been called a sense of direction. many interesting cases are on record of animals finding their way home after being taken a considerable distance. to account for this fact it has been suggested that animals possess a sense with which we are not endowed, or of which, at any rate, we possess only a trace. the homing instinct of the pigeon has also been ascribed to the same faculty. my brother alfred, however, who has paid much attention to pigeons, informs me that they are never taken any great distance at once; but if they are intended to take a long flight, they are trained to do so by stages. darwin suggested that it would be interesting to test the case by taking animals in a close box, and then whirling them round rapidly before letting them out. this is in fact done with cats in some parts of france, when the family migrates, and is considered the only way of preventing the cat from returning to the old home. fabre has tried the same thing with some wild bees (chalicodoma). he took some, marked them on the back with a spot of white, and put them into a bag. he then carried them a quarter of a mile, stopping at a point where an old cross stands by the wayside, and whirled the bag rapidly round his head. while he was doing so a good woman came by, who seemed not a little surprised to find the professor solemnly whirling a black bag round his head in front of the cross; and, he fears, suspected him of satanic practices. he then carried his bees a mile and a half in the opposite direction and let them go. three out of ten found their way home. he tried the same experiment several times, in one case taking them a little over two miles. on an average about a third of the bees found their way home. "la démonstration," says fabre, "est suffisante. ni les mouvements enchevêtrés d'une rotation comme je l'ai décrite; ni l'obstacle de collines à franchir et de bois à traverser; ni les embûches d'une voie qui s'avance, rétrograde, et revient par un ample circuit, ne peuvent troubler les chalicodomes dépaysés et les empêcher de revenir au nid." i must say, however, that i am not convinced. in the first place, the distances were i think too short; and in the second, though it is true that some of the bees found their way home, nearly two-thirds failed to do so. it would be interesting to try the experiment again, taking the bees say five miles. if they really possess any such sense, that distance would be no bar to their return. i have myself experimented with ants, taking them about fifty yards from the nest, and i always found that they wandered aimlessly about, having evidently not the slightest idea of their way home. they certainly did not appear to possess any "sense of direction." number of species the total number of species may probably be safely estimated as at least , , , of which but a fraction have yet been described or named. of extinct species the number was probably at least as great. in the geological history of the earth there have been at least twelve periods, in each of which by far the greatest number were distinct. the ancient poets described certain gifted mortals as having been privileged to descend into the interior of the earth, and exercised their imagination in recounting the wonders thus revealed. as in other cases, however, the realities of science have proved far more varied and surprising than the dreams of fiction. of these extinct species our knowledge is even more incomplete than that of the existing species. but even of our contemporaries it is not too much to say that, as in the case of plants, there is not one the structure, habits, and life-history of which are yet fully known to us. the male of the cynips, which produces the common king charles oak apple, has only recently been discovered, those of the root-feeding aphides, which live in hundreds in every nest of the yellow meadow ant (lasius flavus) are still unknown; the habits and mode of reproduction of the common eel have only just been discovered; and we may even say generally that many of the most interesting recent discoveries have relation to the commonest and most familiar animals. importance of the smaller animals whatever pre-eminence man may claim for himself, other animals have done far more to affect the face of nature. the principal agents have not been the larger or more intelligent, but rather the smaller, and individually less important, species. beavers may have dammed up many of the rivers of british columbia, and turned them into a succession of pools or marshes, but this is a slight matter compared with the action of earthworms and insects[ ] in the creation of vegetable soil; of the accumulation of animalcules in filling up harbours and lakes; or of zoophytes in the construction of coral islands. microscopic animals make up in number what they lack in size. paris is built of infusoria. the peninsula of florida, , square miles in extent, is entirely composed of coral debris and fragments of shells. chalk consists mainly of foraminifera and fragments of shells deposited in a deep sea. the number of shells required to make up a cubic inch is almost incredible. ehrenberg has estimated that of the bilin polishing slate which caps the mountain, and has a thickness of forty feet, a cubic inch contains many hundred million shells of infusoria. in another respect these microscopic organisms are of vital importance. many diseases are now known, and others suspected, to be entirely due to bacteria and other minute forms of life (microbes), which multiply incredibly, and either destroy their victims, or after a while diminish again in numbers. we live indeed in a cloud of bacteria. at the observatory of montsouris at paris it has been calculated that there are about in each cubic meter of air. elsewhere, however, they are much more numerous. pasteur's researches on the silkworm disease led him to the discovery of bacterium anthracis, the cause of splenic fever. microbes are present in persons suffering from cholera, typhus, whooping-cough, measles, hydrophobia, etc., but as to their history and connection with disease we have yet much to learn. it is fortunate, indeed, that they do not all attack us. in surgical cases, again, the danger of compound fractures and mortification of wounds has been found to be mainly due to the presence of microscopic organisms; and lister, by his antiseptic treatment which destroys these germs or prevents their access, has greatly diminished the danger of operations, and the sufferings of recovery. size of animals in the size of animals we find every gradation from these atoms which even in the most powerful microscopes appear as mere points, up to the gigantic reptiles of past ages and the whales of our present ocean. the horned ray or skate is feet in length, by in width. the cuttle-fishes of our seas, though so hideous as to resemble a bad dream, are too small to be formidable; but off the newfoundland coast is a species with arms sometimes feet long, so as to be feet from tip to tip. the body, however, is small in proportion. the giraffe attains a height of over feet; the elephant, though not so tall, is more bulky; the crocodile reaches a length of over feet, the python of feet, the extinct titanosaurus of the american jurassic beds, the largest land animal yet known to us, feet in length and in height; the whalebone whale over feet, sibbald's whale is said to have reached - , which is perhaps the limit. captain scoresby indeed mentions a rorqual no less than feet in length, but this is probably too great an estimate. complexity of animal structure the complexity of animal structure is even more marvellous than their mere magnitude. a caterpillar contains more than muscles. in our own body are some , , perspiration glands, communicating with the surface by ducts having a total length of some miles; while that of the arteries, veins, and capillaries must be very great; the blood contains millions of millions of corpuscles, each no doubt a complex structure in itself; the rods in the retina, which are supposed to be the ultimate recipient of light, are estimated at , , ; and meinert has calculated that the gray matter of the brain is built up of at least , , cells. no verbal description, however, can do justice to the marvellous complexity of animal structure, which the microscope alone, and even that but faintly, can enable us to realise. length of life how little we yet know of the life-history of animals is illustrated by the vagueness of our information as to the age to which they live. professor lankester[ ] tells us that "the paucity and uncertainty of observations on this class of facts is extreme." the rabbit is said to reach years, the dog and sheep - , the pig , the horse , the camel , the elephant , the greenland whale (?): among birds, the parrot to attain years, the raven even more. the atur parrot mentioned by humboldt, talked, but could not be understood, because it spoke in the language of an extinct indian tribe. it is supposed from their rate of growth that among fish the carp is said to reach years; and a pike, feet long, and weighing lbs., is said to have been taken in suabia in carrying a ring, on which was inscribed, "i am the fish which was first of all put into the lake by the hands of the governor of the universe, frederick the second, the th oct. ." this would imply an age of over years. many reptiles are no doubt very long-lived. a tortoise is said to have reached years. as regards the lower animals, the greatest age on record is that of sir j. dalzell's sea anemone, which lived for over years. insects are generally short-lived; the queen bee, however, is said by aristotle, whose statement has not been confirmed by recent writers, to live years. i myself had a queen ant which attained the age of years. the may fly (ephemera) is celebrated as living only for a day, and has given its name to all things short-lived. the statement usually made is, indeed, very misleading, for in its larval condition the ephemera lives for weeks. many writers have expressed surprise that in the perfect state its life should be so short. it is, however, so defenceless, and, moreover, so much appreciated by birds and fish, that unless they laid their eggs very rapidly none would perhaps survive to continue the species. many of these estimates are, as will be seen, very vague and doubtful, so that we must still admit with bacon that, "touching the length and shortness of life in living creatures, the information which may be had is but slender, observation is negligent, and tradition fabulous. in tame creatures their degenerate life corrupteth them, in wild creatures their exposing to all weathers often intercepteth them." on individuality when we descend still lower in the animal scale, the consideration of this question opens out a very curious and interesting subject connected with animal individuality. as regards the animals with which we are most familiar no such question intrudes. among quadrupeds and birds, fishes and reptiles, there is no difficulty in deciding whether a given organism is an individual, or a part of an individual. nor does the difficulty arise in the case of most insects. the bee or butterfly lays an egg which develops successively into a larva and pupa, finally producing bee or butterfly. in these cases, therefore, the egg, larva, pupa, and perfect insect, are regarded as stages in the life of a single individual. in certain gnats, however, the larva itself produces young larvæ, each of which develops into a gnat, so that the egg produces not one gnat but many gnats. the difficulty of determining what constitutes an individual becomes still greater among the zoophytes. these beautiful creatures in many cases so closely resemble plants, that until our countryman ellis proved them to be animals, crabbe was justified in saying-- involved in seawrack here we find a race, which science, doubting, knows not where to place; on shell or stone is dropped the embryo seed, and quickly vegetates a vital breed. we cannot wonder that such organisms were long regarded as belonging to the vegetable kingdom. the cups which terminate the branches contain, however, an animal structure, resembling a small sea anemone, and possessing arms which capture the food by which the whole colony is nourished. some of these cups, moreover, differ from the rest, and produce eggs. these then we might be disposed to term ovaries. but in many species they detach themselves from the group and lead an independent existence. thus we find a complete gradation from structures which, regarded by themselves, we should unquestionably regard as mere organs, to others which are certainly separate and independent beings. [illustration: fig. .--bougainvillea fruticosa; natural size. (after allman.)] fig. represents, after allman, a colony of bougainvillea fruticosa of the natural size. it is a british species, which is found growing on buoys, floating timber, etc., and, says allman, "when in health and vigour, offers a spectacle unsurpassed in interest by any other species--every branchlet crowned by its graceful hydranth, and budding with medusæ in all stages of development (fig. ), some still in the condition of minute buds, in which no trace of the definite medusa-form can yet be detected; others, in which the outlines of the medusa can be distinctly traced within the transparent ectotheque (external layer); others, again, just casting off this thin outer pellicle, and others completely freed from it, struggling with convulsive efforts to break loose from the colony, and finally launched forth in the full enjoyment of their freedom into the surrounding water. i know of no form in which so many of the characteristic features of a typical hydroid are more finely expressed than in this beautiful species." [illustration: fig. .--bougainvillea fruticosa; magnified to show development.] fig. represents the medusa or free form of this beautiful species. [illustration: fig. .--bougainvillea fruticosa, medusa-form.] if we pass to another great group of zoophytes, that of the jelly-fishes, we have a very similar case. for our first knowledge of the life-history of these zoophytes we are indebted to the norwegian naturalist sars. take, for instance, the common jelly-fish (medusa aurita) (fig. ) of our shores. the egg is a pear-shaped body (_ _), covered with fine hairs, by the aid of which it swims about, the broader end in front. after a while it attaches itself, not as might have been expected by the posterior but by the anterior extremity (_ _). the cilia then disappear, a mouth is formed at the free end, tentacles, first four (_ _), then eight, and at length as many as thirty (_ _), are formed, and the little creature resembles in essentials the freshwater polyp (hydra) of our ponds. [illustration: fig. .--medusa aurita, and progressive stages of development.] at the same time transverse wrinkles (_ _) are formed round the body, first near the free extremity and then gradually descending. they become deeper and deeper, and develop lobes or divisions one under the other, as at _ _. after a while the top ring (and subsequently the others one by one) detaches itself, swims away, and gradually develops into a medusa (_ _). thus, then, the life-history is very similar to that of the hydroids, only that while in the hydroids the fixed condition is the more permanent, and the free swimming more transitory, in the medusæ, on the contrary, the fixed condition is apparently only a phase in the production of the free swimming animal. in both the one and the other, however, the egg gives rise not to one but to many mature animals. steenstrup has given to these curious phenomena, many other cases of which occur among the lower animals, and to which he first called attention, the name of alternations of generations. in the life-history of infusoria (so called because they swarm in most animal or vegetable infusions) similar difficulties encounter us. the little creatures, many of which are round or oval in form, from time to time become constricted in the middle; the constriction becomes deeper and deeper, and at length the two halves twist themselves apart and swim away. in this case, therefore, there was one, and there are now two exactly similar; but are these two individuals? they are not parent and offspring--that is clear, for they are of the same age; nor are they twins, for there is no parent. as already mentioned, we regard the caterpillar, chrysalis, and butterfly as stages in the life-history of a single individual. but among zoophytes, and even among some insects, one larva often produces several mature forms. in some species these mature forms remain attached to the larval stock, and we might be disposed to regard the whole as one complex organism. but in others they detach themselves and lead an independent existence. these considerations then introduce much difficulty into our conception of the idea of an individual. animal immortality but, further than this, we are confronted by by another problem. if we regard a mass of coral as an individual because it arises by continuous growth from a single egg, then it follows that some corals must be thousands of years old. some of the lower animals may be cut into pieces, and each piece will develop into an entire organism. in fact the realisation of the idea of an individual gradually becomes more and more difficult, and the continuity of existence, even among the highest animals, gradually forces itself upon us. i believe that as we become more rational, as we realise more fully the conditions of existence, this consideration is likely to have important moral results. it is generally considered that death is the common lot of all living beings. but is this necessarily so? infusoria and other unicellular animals multiply by division. that is to say, if we watch one for a certain time, we shall observe, as already mentioned, that a constriction takes place, which grows gradually deeper and deeper, until at last the two halves become quite detached, and each swims away independently. the process is repeated over and over again, and in this manner the species is propagated. here obviously there is no birth and no death. such creatures may be killed, but they have no natural term of life. they are, in fact, theoretically immortal. those which lived millions of years ago may have gone on dividing and subdividing, and in this sense multitudes of the lower animals are millions of years old. footnotes: [ ] address to microscopical society, . [ ] _ants, bees, and wasps_, and _the senses of animals_. [ ] prof. drummond (_tropical africa_) dwells with great force on the manner in which the soil of central africa is worked up by the white ants. [ ] lankester, _comparative longevity_. see also weismann, _duration of life_. chapter iv on plant life flower in the crannied wall, i pluck you out of the crannies, i hold you here, root and all, in my hand, little flower--but _if_ i could understand what you are, root and all, and all in all, i should know what god and man is. tennyson. chapter iv on plant life we are told that in old days the fairies used to give presents of flowers and leaves to those whom they wished to reward, or whom they loved best; and though these gifts were, it appears, often received with disappointment, still it will probably be admitted that flowers have contributed more to the happiness of our lives than either gold or silver or precious stones; and that our happiest days have been spent out-of-doors in the woods and fields, when we have ... found in every woodland way the sunlight tint of fairy gold.[ ] to many minds flowers acquired an additional interest when it was shown that there was a reason for their colour, size, and form--in fact, for every detail of their organisation. if we did but know all that the smallest flower could tell us, we should have solved some of the greatest mysteries of nature. but we cannot hope to succeed--even if we had the genius of plato or aristotle--without careful, patient, and reverent study. from such an inquiry we may hope much; already we have glimpses, enough to convince us that the whole history will open out to us conceptions of the universe wider and grander than any which the imagination alone would ever have suggested. attempts to explain the forms, colours, and other characteristics of animals and plants are by no means new. our teutonic forefathers had a pretty story which explained certain points about several common plants. balder, the god of mirth and merriment, was, characteristically enough, regarded as deficient in the possession of immortality. the other divinities, fearing to lose him, petitioned thor to make him immortal, and the prayer was granted on condition that every animal and plant would swear not to injure him. to secure this object, nanna, balder's wife, descended upon the earth. loki, the god of envy, followed her, disguised as a crow (which at that time were white), and settled on a little blue flower, hoping to cover it up, so that nanna might overlook it. the flower, however, cried out "forget-me-not, forget-me-not," and has ever since been known under that name. loki then flew up into an oak and sat on a mistletoe. here he was more successful. nanna carried off the oath of the oak, but overlooked the mistletoe. she thought, however, and the divinities thought, that she had successfully accomplished her mission, and that balder had received the gift of immortality. one day, supposing balder proof, they amused themselves by shooting at him, posting him against a holly. loki tipped an arrow with a piece of mistletoe, against which balder was not proof, and gave it to balder's brother. this, unfortunately, pierced him to the heart, and he fell dead. some drops of his blood spurted on to the holly, which accounts for the redness of the berries; the mistletoe was so grieved that she has ever since borne fruit like tears; and the crow, whose form loki had taken, and which till then had been white, was turned black. this pretty myth accounts for several things, but is open to fatal objections. recent attempts to explain the facts of nature are not less fascinating, and, i think, more successful. why then this marvellous variety? this inexhaustible treasury of beautiful forms? does it result from some innate tendency in each species? is it intentionally designed to delight the eye of man? or has the form and size and texture some reference to the structure and organisation, the habits and requirements of the whole plant? i shall never forget hearing darwin's paper on the structure of the cowslip and primrose, after which even sir joseph hooker compared himself to peter bell, to whom a primrose by a river's brim a yellow primrose was to him, and it was nothing more. we all, i think, shared the same feeling, and found that the explanation of the flower then given, and to which i shall refer again, invested it with fresh interest and even with new beauty. a regular flower, such, for instance, as a geranium or a pink, consists of four or more whorls of leaves, more or less modified: the lowest whorl is the calyx, and the separate leaves of which it is composed, which however are sometimes united into a tube, are called sepals; ( ) a second whorl, the corolla, consisting of coloured leaves called petals, which, however, like those of the calyx, are often united into a tube; ( ) of one or more stamens, consisting of a stalk or filament, and a head or anther, in which the pollen is produced; and ( ) a pistil, which is situated in the centre of the flower, and at the base of which is the ovary, containing one or more seeds. almost all large flowers are brightly coloured, many produce honey, and many are sweet-scented. what, then, is the use and purpose of this complex organisation? it is, i think, well established that the main object of the colour, scent, and honey of flowers is to attract insects, which are of use to the plant in carrying the pollen from flower to flower. in many species the pollen is, and no doubt it originally was in all, carried by the air. in these cases the chance against any given grain of pollen reaching the pistil of another flower of the same species is of course very great, and the quantity of pollen required is therefore immense. in species where the pollen is wind-borne as in most of our trees--firs, oaks, beech, ash, elm, etc., and many herbaceous plants, the flowers are as a rule small and inconspicuous, greenish, and without either scent or honey. moreover, they generally flower early, so that the pollen may not be intercepted by the leaves, but may have a better chance of reaching another flower. and they produce an immense quantity of pollen, as otherwise there would be little chance that any would reach the female flower. every one must have noticed the clouds of pollen produced by the scotch fir. when, on the contrary, the pollen is carried by insects, the quantity necessary is greatly reduced. still it has been calculated that a peony flower produces between , , and , , pollen grains; in the dandelion, which is more specialised, the number is reduced to about , ; while in such a flower as the dead-nettle it is still smaller. the honey attracts the insects; while the scent and colour help them to find the flowers, the scent being especially useful at night, which is perhaps the reason why evening flowers are so sweet. it is to insects, then, that flowers owe their beauty, scent, and sweetness. just as gardeners, by continual selection, have added so much to the beauty of our gardens, so to the unconscious action of insects is due the beauty, scent, and sweetness of the flowers of our woods and fields. let us now apply these views to a few common flowers. take, for instance, the white dead-nettle. the corolla of this beautiful and familiar flower (fig. ) consists of a narrow tube, somewhat expanded at the upper end (fig. ), where the lower lobe forms a platform, on each side of which is a small projecting tooth (fig. , _m_). the upper portion of the corolla is an arched hood (_co_), under which lie four anthers (_a a_), in pairs, while between them, and projecting somewhat downwards, is the pointed pistil (_st_); the tube at the lower part contains honey, and above the honey is a row of hairs running round the tube. [illustration: fig. --white dead-nettle.] now, why has the flower this peculiar form? what regulates the length of the tube? what is the use of the arch? what lesson do the little teeth teach us? what advantage is the honey to the flower? of what use is the fringe of hairs? why does the stigma project beyond the anthers? why is the corolla white, while the rest of the plant is green? [illustration: fig. .] [illustration: fig. .] the honey of course serves to attract the humble bees by which the flower is fertilised, and to which it is especially adapted; the white colour makes the flower more conspicuous; the lower lip forms the stage on which the bees may alight; the length of the tube is adapted to that of their proboscis; its narrowness and the fringe of fine hairs exclude small insects which might rob the flower of its honey without performing any service in return; the arched upper lip protects the stamens and pistil, and prevents rain-drops from choking up the tube and washing away the honey; the little teeth are, i believe, of no use to the flower in its present condition, they are the last relics of lobes once much larger, and still remaining so in some allied species, but which in the dead-nettle, being no longer of any use, are gradually disappearing; the height of the arch has reference to the size of the bee, being just so much above the alighting stage that the bee, while sucking the honey, rubs its back against the hood and thus comes in contact first with the stigma and then with the anthers, the pollen-grains from which adhere to the hairs on the bee's back, and are thus carried off to the next flower which the bee visits, when some of them are then licked off by the viscid tip of the stigma.[ ] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] in the salvias, the common blue salvia of our gardens, for instance,--a plant allied to the dead-nettle,--the flower (fig. ) is constructed on the same plan, but the arch is much larger, so that the back of the bee does not nearly reach it. the stamens, however, have undergone a remarkable modification. two of them have become small and functionless. in the other two the anthers or cells producing the pollen, which in most flowers form together a round knob or head at the top of the stamen, are separated by a long arm, which plays on the top of the stamen as on a hinge. of these two arms one hangs down into the tube, closing the passage, while the other lies under the arched upper lip. when the bee pushes its proboscis down the tube (fig. ) it presses the lower arm to one side, and the upper arm consequently descends, tapping the bee on the back, and dusting it with pollen. when the flower is a little older the pistil (fig. , _p_) has elongated so that the stigma (fig. , _st_) touches the back of the bee and carries off some of the pollen. this sounds a little complicated, but is clear enough if we take a twig or stalk of grass and push it down the tube, when one arm of each of the two larger stamens will at once make its appearance. it is one of the most beautiful pieces of plant mechanism which i know, and was first described by sprengel, a poor german schoolmaster. snapdragon at first sight it may seem an objection to the view here advocated that the flowers in some species--as, for instance, the common snapdragon (antirrhinum), which, according to the above given tests, ought to be fertilised by insects--are entirely closed. a little consideration, however, will suggest the reply. the snapdragon is especially adapted for fertilisation by humble bees. the stamens and pistil are so arranged that smaller species would not effect the object. it is therefore an advantage that they should be excluded, and in fact they are not strong enough to move the spring. the antirrhinum is, so to speak, a closed box, of which the humble bees alone possess the key. furze, broom, and laburnum other flowers such as the furze, broom, laburnum, etc., are also opened by bees. the petals lock more or less into one another, and the flower remains at first closed. when, however, the insect alighting on it presses down the keel, the flower bursts open, and dusts it with pollen. sweet pea in the above cases the flower once opened does not close again. in others, such as the sweet pea and the bird's-foot lotus, nature has been more careful. when the bee alights it clasps the "wings" of the flower with its legs, thus pressing them down; they are, however, locked into the "keel," or lower petal, which accordingly is also forced down, thus exposing the pollen which rubs against, and part of which sticks to, the breast of the bee. when she leaves the flower the keel and wings rise again, thus protecting the rest of the pollen and keeping it ready until another visitor comes. it is easy to carry out the same process with the fingers. [illustration: fig. . fig. . flower and pollen of primrose] primula in the primrose and cowslip, again, we find quite a different plan. it had long been known that if a number of cowslips or primroses are examined, about half would be found to have the stigma at the top of the tube and the stamens half way down, while in the other half the stamens are at the top and the stigma half way down. these two forms are about equally numerous, but never occur on the same stock. they have been long known to children and gardeners, who call them thrum-eyed and pin-eyed. mr. darwin was the first to explain the significance of this curious difference. it cost him several years of patient labour, but when once pointed out it is sufficiently obvious. an insect thrusting its proboscis down a primrose of the long-styled form (fig. ) would dust its proboscis at a part (_a_) which, when it visited a short-styled flower (fig. ), would come just opposite the head of the pistil (_st_), and could not fail to deposit some of the pollen on the stigma. conversely, an insect visiting a short-styled plant would dust its proboscis at a part farther from the tip; which, when the insect subsequently visited a long-styled flower, would again come just opposite to the head of the pistil. hence we see that by this beautiful arrangement insects must carry the pollen of the long-styled form to the short-styled, and _vice versâ_. the economy of pollen is not the only advantage which plants derive from these visits of insects. a second and scarcely less important is that they tend to secure "cross fertilisation"; that is to say, that the seed shall be fertilised by pollen from another plant. the fact that "cross fertilisation" is of advantage to the plant doubtless also explains the curious arrangement that in many plants the stamen and pistil do not mature at the same time--the former having shed their pollen before the pistil is mature; or, which happens less often, the pistil having withered before the pollen is ripe. in most geraniums, pinks, etc., for instance, and many allied species, the stamens ripen first, and are followed after an interval by the pistil. the nottingham catchfly the nottingham catchfly (silene nutans) is a very interesting case. the flower is adapted to be fertilised by moths. accordingly it opens towards evening, and as is generally the case with such flowers, is pale in colour, and sweet-scented. there are two sets of stamens, five in each set. the first evening that the flower opens one set of stamens ripen and expose their pollen. towards morning these wither away, the flower shrivels up, ceases to emit scent, and looks as if it were faded. so it remains all next day. towards evening it reopens, the second set of stamens have their turn, and the flower again becomes fragrant. by morning, however, the second set of stamens have shrivelled, and the flower is again asleep. finally on the third evening it reopens for the last time, the long spiral stigmas expand, and can hardly fail to be fertilised with the pollen brought by moths from other flowers. the heath in the hanging flowers of heaths the stamens form a ring, and each one bears two horns. when the bee inserts its proboscis into the flower to reach the honey, it is sure to press against one of these horns, the ring is dislocated, and the pollen falls on to the head of the insect. in fact, any number of other interesting cases might be mentioned. bees and flies bees are intelligent insects, and would soon cease to visit flowers which did not supply them with food. flies, however, are more stupid, and are often deceived. thus in our lovely little parnassia, five of the ten stamens have ceased to produce pollen, but are prolonged into fingers, each terminating in a shining yellow knob, which looks exactly like a drop of honey, and by which flies are continually deceived. paris quadrifolia also takes them in with a deceptive promise of the same kind. some foreign plants have livid yellow and reddish flowers, with a most offensive smell, and are constantly visited by flies, which apparently take them for pieces of decaying meat. [illustration: fig. .--arum.] the flower of the common lords and ladies (arum) of our hedges is a very interesting case. the narrow neck bears a number of hairs pointing downwards. the stamens are situated above the stigma, which comes to maturity first. small flies enter the flower apparently for shelter, but the hairs prevent them from returning, and they are kept captive until the anthers have shed their pollen. then, when the flies have been well dusted, the hairs shrivel up, leaving a clear road, and the prisoners are permitted to escape. the tubular flowers of aristolochia offer a very similar case. past history of flowers if the views here advocated are correct, it follows that the original flowers were small and green, as wind-fertilised flowers are even now. but such flowers are inconspicuous. those which are coloured, say yellow or white, are of course much more visible and more likely to be visited by insects. i have elsewhere given my reasons for thinking that under these circumstances some flowers became yellow, that some of them became white, others subsequently red, and some finally blue. it will be observed that red and blue flowers are as a rule highly specialised, such as aconites and larkspurs as compared with buttercups; blue gentians as compared with yellow, etc. i have found by experiment that bees are especially partial to blue and pink. tubular flowers almost always, if not always, contain honey, and are specially suited to butterflies and moths, bees and flies. those which are fertilised by moths generally come out in the evening, are often very sweetly scented, and are generally white or pale yellow, these colours being most visible in the twilight. aristotle long ago noticed the curious fact that in each journey bees confine themselves to some particular flower. this is an economy of labour to the bee, because she has not to vary her course of proceeding. it is also an advantage to the plants, because the pollen is carried from each flower to another of the same species, and is therefore less likely to be wasted. fruits and seeds after the flower comes the seed, often contained in a fruit, and which itself encloses the future plant. fruits and seeds are adapted for dispersion, beautifully and in various ways: some by the wind, being either provided with a wing, as in the fruits of many trees--sycamores, ash, elms, etc.; or with a hairy crown or covering, as with thistles, dandelions, willows, cotton plant, etc. some seeds are carried by animals; either as food--such as most edible fruits and seeds, acorns, nuts, apples, strawberries, raspberries, blackberries, plums, grasses, etc.--or involuntarily, the seeds having hooked hairs or processes, such as burrs, cleavers, etc. some seeds are scattered by the plants themselves, as, for instance, those of many geraniums, violets, balsams, shamrocks, etc. our little herb robert throws its seeds some feet. some seeds force themselves into the ground, as those of certain grasses, cranes'-bills (erodiums), etc. some are buried by the parent plants, as those of certain clovers, vetches, violets, etc. some attach themselves to the soil, as those of the flax; or to trees, as in the case of the mistletoe. leaves again, as regards the leaves there can, i think, be no doubt that similar considerations of utility are applicable. their forms are almost infinitely varied. to quote ruskin's vivid words, they "take all kinds of strange shapes, as if to invite us to examine them. star-shaped, heart-shaped, spear-shaped, arrow-shaped, fretted, fringed, cleft, furrowed, serrated, sinuated, in whorls, in tufts, in spires, in wreaths, endlessly expressive, deceptive, fantastic, never the same from foot-stalk to blossom, they seem perpetually to tempt our watchfulness and take delight in outstepping our wonder." but besides these differences of mere form, there are many others: of structure, texture, and surface; some are scented or have a strong taste, or acrid juice, some are smooth, others hairy; and the hairs again are of various kinds. i have elsewhere[ ] endeavoured to explain some of the causes which have determined these endless varieties. in the beech, for instance (fig. ), the leaf has an area of about square inches. the distance between the buds is about - / inch, and the leaves lie in the general plane of the branch, which bends slightly at each internode. the basal half of the leaf fits the swell of the twig, while the upper half follows the edge of the leaf above; and the form of the inner edge being thus determined, decides that of the outer one also. [illustration: fig. .--beech.] the weight, and consequently the size of the leaf, is limited by the strength of the twig; and, again, in a climate such as ours it is important to plants to have their leaves so arranged as to secure the maximum of light. hence in leaves which lie parallel to the plane of the boughs, as in the beech, the width depends partly on the distance between the buds; if the leaves were broader, they would overlap, if they were narrower, space would be wasted. consequently the width being determined by the distance between the buds, and the size depending on the weight which the twig can safely support, the length also is determined. this argument is well illustrated by comparing the leaves of the beech with those of the spanish chestnut. the arrangement is similar, and the distance between the buds being about the same, so is the width of the leaves. but the terminal branches of the spanish chestnut being much stronger, the leaves can safely be heavier; hence the width being fixed, they grow in length and assume the well-known and peculiar sword-blade shape. in the sycamores, maples (fig. ), and horse-chestnuts the arrangement is altogether different. the shoots are stiff and upright with leaves placed at right angles to the branches instead of being parallel to them. the leaves are in pairs and decussate with one another; while the lower ones have long petioles which bring them almost to the level of the upper pairs, the whole thus forming a beautiful dome. for leaves arranged as in the beech the gentle swell at the base is admirably suited; but in a crown of leaves such as those of the sycamore, space would be wasted, and it is better that they should expand at once, so soon as their stalks have carried them free from the upper and inner leaves. [illustration: fig. .--acer platanoides.] in the black poplar the arrangement of the leaves is again quite different. the leaf stalk is flattened, so that the leaves hang vertically. in connection with this it will be observed that while in most leaves the upper and under surfaces are quite unlike, in the black poplar on the contrary they are very similar. the stomata or breathing holes, moreover, which in the leaves of most trees are confined to the under surface, are in this species nearly equally numerous on both. the "compass" plant of the american prairies, a plant not unlike a small sunflower, is another species with upright leaves, which growing in the wide open prairies tend to point north and south, thus exposing both surfaces equally to the light and heat. such a position also affects the internal structure of the leaf, the two sides becoming similar in structure, while in other cases the upper and under surfaces are very different. in the yew the leaves are inserted close to one another, and are linear; while in the box they are further apart and broader. in other cases the width of the leaves is determined by what botanists call the "phyllotaxy." some plants have the leaves opposite, each pair being at right angles with the pairs above and below. in others they are alternate, and arranged round the stem in a spiral. in one very common arrangement the sixth leaf stands directly over the first, the intermediate ones forming a spiral which has passed twice round the stem. this, therefore, is known as the / arrangement. common cases are / , / , / , / , and / . in the first the leaves are generally broad, in the / arrangement they are elliptic, in the / and more complicated arrangements nearly linear. the willows afford a very interesting series. salix herbacea has the / arrangement and rounded leaves, salix caprea elliptic leaves and / , salix pentandra lancet-shaped leaves and / , and s. incana linear leaves and a / arrangement. the result is that whether the series consists of , , , , or leaves, in every case, if we look perpendicularly at a twig the leaves occupy the whole circle. in herbaceous plants upright leaves as a rule are narrow, which is obviously an advantage, while prostrate ones are broad. [illustration: aquatic vegetation, brazil. _to face page ._] aquatic plants many aquatic plants have two kinds of leaves; some more or less rounded, which float on the surface; and others cut up into narrow segments, which remain below. the latter thus present a greater extent of surface. in air such leaves would be unable even to support their own weight, much less to resist the force of the wind. in still air, however, for the same reason, finely-divided leaves may be an advantage, while in exposed positions compact and entire leaves are more suitable. hence herbaceous plants tend to have divided, bushes and trees entire, leaves. there are many cases when even in the same family low and herb-like species have finely-cut leaves, while in shrubby or ligneous ones they more or less resemble those of the laurel or beech. these considerations affect trees more than herbs, because trees stand more alone, while herbaceous plants are more affected by surrounding plants. upright leaves tend to be narrow, as in the case of grasses; horizontal leaves, on the contrary, wider. large leaves are more or less broken up into leaflets, as in the ash, mountain-ash, horse-chestnut, etc. the forms of leaves depend also much on the manner in which they are packed into the buds. the leaves of our english trees, as i have already said, are so arranged as to secure the maximum of light; in very hot countries the reverse is the case. hence, in australia, for instance, the leaves are arranged not horizontally, but vertically, so as to present, not their surfaces, but their edges, to the sun. one english plant, a species of lettuce, has the same habit. this consideration has led also to other changes. in many species the leaves are arranged directly under, so as to shelter, one another. the australian species of acacia have lost their true leaves, and the parts which in them we generally call leaves are in reality vertically-flattened leaf stalks. in other cases the stem itself is green, and to some extent replaces the leaves. in our common broom we see an approach to this, and the same feature is more marked in cactus. or the leaves become fleshy, thus offering, in proportion to their volume, a smaller surface for evaporation. of this the stonecrops, mesembryanthemum, etc., are familiar instances. other modes of checking transpiration and thus adapting plants to dry situations are by the development of hairs, by the formation of chalky excretions, by the sap becoming saline or viscid, by the leaf becoming more or less rolled up, or protected by a covering of varnish. our english trees are for the most part deciduous. leaves would be comparatively useless in winter when growth is stopped by the cold; moreover, they would hold the snow, and thus cause the boughs to be broken down. hence perhaps the glossiness of evergreen leaves, as, for instance, of the holly, from which the snow slips off. in warmer climates trees tend to retain their leaves, and some species which are deciduous in the north become evergreen, or nearly so, in the south of europe. evergreen leaves are as a rule tougher and thicker than those which drop off in autumn; they require more protection from the weather. but some evergreen leaves are much longer lived than others; those of the evergreen oak do not survive a second year, those of the scotch pine live for three, of the spruce fir, yew, etc., for eight or ten, of the pinsapo even eighteen. as a general rule the conifers with short leaves keep them on for several years, those with long ones for fewer, the length of the leaf being somewhat in the inverse ratio to the length of its life; but this is not an invariable criterion, as other circumstances also have to be taken into consideration. leaves with strong scent, aromatic taste, or acrid juice, are characteristic of dry regions, where they run especial danger of being eaten, and where they are thus more or less effectively protected. on hairs the hairs of plants are useful in various ways. in some cases ( ) they keep off superfluous moisture; in others ( ) they prevent too rapid evaporation; in some ( ) they serve as a protection against too glaring light; in some ( ) they protect the plant from browsing quadrupeds; in others ( ) from being eaten by insects; or, ( ) serve as a quickset hedge to prevent access to the flowers. in illustration of the first case i may refer to many alpine plants, the well-known edelweiss, for instance, where the woolly covering of hairs prevents the "stomata," or minute pores leading into the interior of the leaf, from being clogged up by rain, dew, or fog, and thus enable them to fulfil their functions as soon as the sun comes out. as regards the second case many desert and steppe-plants are covered with felty hairs, which serve to prevent too rapid evaporation and consequent loss of moisture. the woolly hairy leaves of the mulleins (verbascum) doubtless tend to protect them from being eaten, as also do the spines of thistles, and those of hollies, which, be it remarked, gradually disappear on the upper leaves which browsing quadrupeds cannot reach. i have already alluded to the various ways in which flowers are adapted to fertilisation by insects. but ants and other small creeping insects cannot effectually secure this object. hence it is important that they should be excluded, and not allowed to carry off the honey, for which they would perform no service in return. in many cases, therefore, the opening of the flower is either contracted to a narrow passage, or is itself protected by a fringe of hairs. in others the peduncle, or the stalk of the plant, is protected by a hedge, or chevaux de frise, of hairs. in this connection i might allude to the many plants which are more or less viscid. this also is in most cases a provision to preclude creeping insects from access to the flowers. there are various other kinds of hairs to which i might refer--glandular hairs, secretive hairs, absorbing hairs, etc. it is marvellous how beautifully the form and structure of leaves is adapted to the habits and requirements of the plants, but i must not enlarge further on this interesting subject. the time indeed will no doubt come when we shall be able to explain every difference of form and structure, almost infinite as these differences are. influence of soil the character of the vegetation is of course greatly influenced by that of the soil. in this respect granitic and calcareous regions offer perhaps the best marked contrast. there are in switzerland two kinds of rhododendrons, very similar in their flowers, but contrasted in their leaves: rhododendron hirsutum having them hairy at the edges as the name indicates; while in r. ferrugineum they are rolled, but not hairy, at the edges, and become ferrugineous on the lower side. this species occurs in the granitic regions, where r. hirsutum does not grow. the yarrows (achillea) afford us a similar case. achillea atrata and a. moschata will live either on calcareous or granitic soil, but in a district where both occur, a. atrata grows so much the more vigorously of the two if the soil is calcareous that it soon exterminates a. moschata; while in granite districts, on the contrary, a. moschata is victorious and a. atrata disappears. every keen sportsman will admit that a varied "bag" has a special charm, and the botanist in a summer's walk may see at least a hundred plants in flower, all with either the interest of novelty, or the charm of an old friend. on seedlings in many cases the seedlings afford us an interesting insight into the former condition of the plant. thus the leaves of the furze are reduced to thorns; but those of the seedling are herbaceous and trifoliate like those of the herb genet and other allied species, subsequent ones gradually passing into spines. this is evidence that the ancestors of the furze bore leaves. plants may be said to have their habits as well as animals. sleep of plants many flowers close their petals during rain; the advantage of which is that it prevents the honey and pollen from being spoilt or washed away. everybody, however, has observed that even in fine weather certain flowers close at particular hours. this habit of going to sleep is surely very curious. why should flowers do so? in animals we can better understand it; they are tired and require rest. but why should flowers sleep? why should some flowers do so, and not others? moreover, different flowers keep different hours. the daisy opens at sunrise and closes at sunset, whence its name "day's-eye." the dandelion (leontodon) is said to open about seven and to close about five; arenaria rubra to be open from nine to three; the white water lily (nymphæa), from about seven to four; the common mouse-ear hawk-weed (hieracium) from eight to three; the scarlet pimpernel (anagallis) to waken at seven and close soon after two; tragopogon pratensis to open at four in the morning, and close just before twelve, whence its english name, "john go to bed at noon." farmers' boys in some parts are said to regulate their dinner time by it. other flowers, on the contrary, open in the evening. now it is obvious that flowers which are fertilised by night-flying insects would derive no advantage from being open by day; and on the other hand, that those which are fertilised by bees would gain nothing by being open at night. nay it would be a distinct disadvantage, because it would render them liable to be robbed of their honey and pollen, by insects which are not capable of fertilising them. i have ventured to suggest then that the closing of flowers may have reference to the habits of insects, and it may be observed also in support of this, that wind-fertilised flowers do not sleep; and that many of those flowers which attract insects by smell, open and emit their scent at particular hours; thus hesperis matronalis and lychnis vespertina smell in the evening, and orchis bifolia is particularly sweet at night. but it is not the flowers only which "sleep" at night; in many species the leaves also change their position, and darwin has given strong reasons for considering that the object is to check transpiration and thus tend to a protection against cold. behaviour of leaves in rain the behaviour of plants with reference to rain affords many points of much interest. the germander speedwell (veronica) has two strong rows of hairs, the chickweed (stellaria) one, running down the stem and thus conducting the rain to the roots. plants with a main tap-root, like the radish or the beet, have leaves sloping inwards so as to conduct the rain towards the axis of the plant, and consequently to the roots; while, on the contrary, where the roots are spreading the leaves slope outwards. in other cases the leaves hold the rain or dew drops. every one who has been in the alps must have noticed how the leaves of the lady's mantle (alchemilla) form little cups containing each a sparkling drop of icy water. kerner has suggested that owing to these cold drops, the cattle and sheep avoid the leaves. mimicry in many cases plants mimic others which are better protected than themselves. thus matricaria chamomilla mimics the true chamomile, which from its bitterness is not eaten by quadrupeds. ajuga chamæpitys mimics euphorbia cyparissias, with which it often grows, and which is protected by its acrid juice. the most familiar case, however, is that of the stinging and the dead nettles. they very generally grow together, and though belonging to quite different families are so similar that they are constantly mistaken for one another. some orchids have a curious resemblance to insects, after which they have accordingly been named the bee orchis, fly orchis, butterfly orchis, etc., but it has not yet been satisfactorily shown what advantage the resemblance is to the plant. ants and plants the transference of pollen from plant to plant is by no means the only service which insects render. ants, for instance, are in many cases very useful to plants. they destroy immense numbers of caterpillars and other insects. forel observing a large ants' nest counted more than insects brought in as food per minute. in some cases ants attach themselves to particular trees, constituting a sort of bodyguard. a species of acacia, described by belt, bears hollow thorns, while each leaflet produces honey in a crater-formed gland at the base, as well as a small, sweet, pear-shaped body at the tip. in consequence it is inhabited by myriads of a small ant, which nests in the hollow thorns, and thus finds meat, drink, and lodging all provided for it. these ants are continually roaming over the plant, and constitute a most efficient bodyguard, not only driving off the leaf-eating ants, but, in belt's opinion, rendering the leaves less liable to be eaten by herbivorous mammalia. delpino mentions that on one occasion he was gathering a flower of clerodendrum, when he was himself suddenly attacked by a whole army of small ants. insectivorous plants in the cases above mentioned the relation between flowers and insects is one of mutual advantage. but this is by no means an invariable rule. many insects, as we all know, live on plants, but it came upon botanists as a surprise when our countryman ellis first discovered that some plants catch and devour insects. this he observed in a north american plant, dionsea, the leaves of which are formed something like a rat-trap, with a hinge in the middle, and a formidable row of spines round the edge. on the surface are a few very sensitive hairs, and the moment any small insect alights on the leaf and touches one of these hairs the two halves of the leaf close up quickly and catch it. the surface then throws out a glutinous secretion, by means of which the leaf sucks up the nourishment contained in the insect. our common sun-dews (drosera) are also insectivorous, the prey being in their case captured by glutinous hairs. again, the bladderwort (utricularia), a plant with pretty yellow flowers, growing in pools and slow streams, is so called because it bears a great number of bladders or utricles, each of which is a real miniature eel-trap, having an orifice guarded by a flap opening inwards which allows small water animals to enter, but prevents them from coming out again. the butterwort (pinguicula) is another of these carnivorous plants. movements of plants while considering plant life we must by no means confine our attention to the higher orders, but must remember also those lower groups which converge towards the lower forms of animals, so that in the present state of our knowledge the two cannot always be distinguished with certainty. many of them differ indeed greatly from the ordinary conception of a plant. even the comparatively highly organised sea-weeds multiply by means of bodies called spores, which an untrained observer would certainly suppose to be animals. they are covered by vibratile hairs or "cilia," by means of which they swim about freely in the water, and even possess a red spot which, as being especially sensitive to light, may be regarded as an elementary eye, and with the aid of which they select some suitable spot, to which they ultimately attach themselves. it was long considered as almost a characteristic of plants that they possessed no power of movement. this is now known to be an error. in fact, as darwin has shown, every growing part of a plant is in continual and even constant rotation. the stems of climbing plants make great sweeps, and in other cases, when the motion is not so apparent, it nevertheless really exists. i have already mentioned that many plants change the position of their leaves or flowers, or, as it is called, sleep at night. the common dandelion raises its head when the florets open, opens and shuts morning and evening, then lies down again while the seeds are ripening, and raises itself a second time when they are ready to be carried away by the wind. valisneria spiralis is a very interesting case. it is a native of european rivers, and the female flower has a long spiral stalk which enables it to float on the surface of the water. the male flowers have no stalks, and grow low down on the plant. they soon, however, detach themselves altogether, rise to the surface, and thus are enabled to fertilise the female flowers among which they float. the spiral stalk of the female flower then contracts and draws it down to the bottom of the water so that the seeds may ripen in safety. many plants throw or bury their seeds. the sensitive plants close their leaves when touched, and the leaflets of desmodium gyrans are continually revolving. i have already mentioned that the spores of sea-weeds swim freely in the water by means of cilia. some microscopic plants do so throughout a great part of their lives. a still lower group, the myxomycetes, which resemble small, more or less branched, masses of jelly, and live in damp soil, among decaying leaves, under bark and in similar moist situations, are still more remarkably animal like. they are never fixed, but in almost continual movement, due to differences of moisture, warmth, light, or chemical action. if, for instance, a moist body is brought into contact with one of their projections, or "pseudopods," the protoplasm seems to roll itself in that direction, and so the whole organism gradually changes its place. so again, while a solution of salt, carbonate of potash, or saltpetre causes them to withdraw from the danger, an infusion of sugar, or tan, produces a flow of protoplasm towards the source of nourishment. in fact, in the same way it rolls over and round its food, absorbing what is nutritious as it passes along. in cold weather they descend into the soil, and one of them (oethalium), which lives in tan pits, descends in winter to a depth of several feet. when about to fructify it changes its habits, seeks the light instead of avoiding it, climbs upwards, and produces its fruit above ground. imperfection of our knowledge the total number of living species of plants may be roughly estimated at , , and there is not one, of which we can say that the structure, uses, and life-history are yet fully known to us. our museums contain large numbers which botanists have not yet had time to describe and name. even in our own country not a year passes without some additional plant being discovered; as regards the less known regions of the earth not half the species have yet been collected. among the lichens and fungi especially many problems of their life-history, some, indeed, of especial importance to man, still await solution. our knowledge of the fossil forms, moreover, falls far short even of that of existing species, which, on the other hand, they must have greatly exceeded in number. every difference of form, structure, and colour has doubtless some cause and explanation, so that the field for research is really inexhaustible. footnotes: [ ] thomson. [ ] lubbock, _flowers and insects_. [ ] _flowers, fruits, and leaves._ chapter v woods and fields "by day or by night, summer or winter, beneath trees the heart feels nearer to that depth of life which the far sky means. the rest of spirit, found only in beauty, ideal and pure, comes there because the distance seems within touch of thought." jefferies. chapter v woods and fields rural life, says cicero, "is not delightful by reason of cornfields only and meadows, and vineyards and groves, but also for its gardens and orchards, for the feeding of cattle, the swarms of bees, and the variety of all kinds of flowers." bacon considered that a garden is "the greatest refreshment to the spirits of man, without which buildings and palaces are but gross handyworks, and a man shall ever see, that when ages grow to civility and elegancy men come to build stately sooner than to garden finely, as if gardening were the greater perfection." no doubt "the pleasure which we take in a garden is one of the most innocent delights in human life."[ ] elsewhere there may be scattered flowers, or sheets of colour due to one or two species, but in gardens one glory follows another. here are brought together all the quaint enamelled eyes, that on the green turf sucked the honeyed showers, and purple all the ground with vernal flowers. bring the rathe primrose that forsaken dies, the tufted crow-toe, and pale jessamine, the white pink and the pansy freaked with jet, the glowing violet, the musk rose, and the well attired woodbine, with cowslips wan that hang the pensive head, and every flower that sad embroidery wears.[ ] we cannot, happily we need not try to, contrast or compare the beauty of gardens with that of woods and fields. and yet to the true lover of nature wild flowers have a charm which no garden can equal. cultivated plants are but a living herbarium. they surpass, no doubt, the dried specimens of a museum, but, lovely as they are, they can be no more compared with the natural vegetation of our woods and fields than the captives in the zoological gardens with the same wild species in their native forests and mountains. often indeed, our woods and fields rival gardens even in the richness of colour. we have all seen meadows white with narcissus, glowing with buttercups, cowslips, early purple orchis, or cuckoo flowers; cornfields blazing with poppies; woods carpeted with bluebells, anemones, primroses, and forget-me-nots; commons with the yellow lady's bedstraw, harebells, and the sweet thyme; marshy places with the yellow stars of the bog asphodel, the sun-dew sparkling with diamonds, ragged robin, the beautifully fringed petals of the buckbean, the lovely little bog pimpernel, or the feathery tufts of cotton grass; hedgerows with hawthorn and traveller's joy, wild rose and honeysuckle, while underneath are the curious leaves and orange fruit of the lords and ladies, the snowy stars of the stitchwort, succory, yarrow, and several kinds of violets; while all along the banks of streams are the tall red spikes of the loosestrife, the hemp agrimony, water groundsel, sedges, bulrushes, flowering rush, sweet flag, etc. many other sweet names will also at once occur to us--snowdrops, daffodils and hearts-ease, lady's mantles and lady's tresses, eyebright, milkwort, foxgloves, herb roberts, geraniums, and among rarer species, at least in england, columbines and lilies. but nature does not provide delights for the eye only. the other senses are not forgotten. a thousand sounds--many delightful in themselves, and all by association--songs of birds, hum of insects, rustle of leaves, ripple of water, seem to fill the air. flowers again are sweet, as well as lovely. the scent of pine woods, which is said to be very healthy, is certainly delicious, and the effect of woodland scenery is good for the mind as well as for the body. "resting quietly under an ash tree, with the scent of flowers, and the odour of green buds and leaves, a ray of sunlight yonder lighting up the lichen and the moss on the oak trunk, a gentle air stirring in the branches above, giving glimpses of fleecy clouds sailing in the ether, there comes into the mind a feeling of intense joy in the simple fact of living."[ ] the wonderful phenomenon of phosphorescence is not a special gift to the animal kingdom. henry o. forbes describes a forest in sumatra: "the stem of every tree blinked with a pale greenish-white light which undulated also across the surface of the ground like moonlight coming and going behind the clouds, from a minute thread-like fungus invisible in the day-time to the unassisted eye; and here and there thick dumpy mushrooms displayed a sharp, clear dome of light, whose intensity never varied or changed till the break of day; long phosphorescent caterpillars and centipedes crawled out of every corner, leaving a trail of light behind them, while fire-flies darted about above like a lower firmament."[ ] woods and forests were to our ancestors the special scenes of enchantment. the great ash tree yggdrasil bound together heaven, earth, and hell. its top reached to heaven, its branches covered the earth, and the roots penetrated into hell. the three normas or fates sat under it, spinning the thread of life. of all the gods and goddesses of classical mythology or our own folk-lore, none were more fascinating than the nature spirits--elves and fairies, neckans and kelpies, pixies and ouphes, mermaids, undines, water spirits, and all the elfin world which have their haunts in dale and piny mountain, or forests, by slow stream or tingling brook. they come out, as we are told, especially on moonlight nights. but while evening thus clothes many a scene with poetry, forests are fairy land all day long. almost any wood contains many and many a spot well suited for fairy feasts; where one might most expect to find titania, resting, as once we are told, she lay upon a bank, the favourite haunt of the spring wind in its first sunshine hour, for the luxuriant strawberry blossoms spread like a snow shower then, and violets bowed down their purple vases of perfume about her pillow,--linked in a gay band floated fantastic shapes; these were her guards, her lithe and rainbow elves. the fairies have disappeared, and, so far as england is concerned, the larger forest animals have vanished almost as completely. the elk and bear, the boar and wolf have gone, the stag has nearly disappeared, and but a scanty remnant of the original wild cattle linger on at chillingham. still the woods teem with life; the fox and badger, stoat and weasel, hare and rabbit, and hedgehog, the tawny squirrel vaulting through the boughs, hawk, buzzard, jay, the mavis and the merle,[ ] the owls and nightjar, the woodpecker, nuthatch, magpie, doves, and a hundred more. in early spring the woods are bright with the feathery catkins of the willow, followed by the soft green of the beech, the white or pink flowers of the thorn, the pyramids of the horse-chestnut, festoons of the laburnum and acacia, and the oak slowly wakes from its winter sleep, while the ash leaves long linger in their black buds. under foot is a carpet of flowers--anemones, cowslips, primroses, bluebells, and the golden blossoms of the broom, which, however, while gorse and heather continue in bloom for months, "blazes for a week or two, and is then completely extinguished, like a fire that has burnt itself out."[ ] in summer the tints grow darker, the birds are more numerous and full of life; the air teems with insects, with the busy murmur of bees and the idle hum of flies, while the cool of morning and evening, and the heat of the day, are all alike delicious. as the year advances and the flowers wane, we have many beautiful fruits and berries, the red hips and haws of the wild roses, scarlet holly berries, crimson yew cups, the translucent berries of the guelder rose, hanging coral beads of the black bryony, feathery festoons of the traveller's joy, and others less conspicuous, but still exquisite in themselves--acorns, beech nuts, ash keys, and many more. it is really difficult to say which are most beautiful, the tender greens of spring or the rich tints of autumn, which glow so brightly in the sunshine. tropical fruits are even more striking. no one who has seen it can ever forget a grove of orange trees in full fruit; while the more we examine the more we find to admire; all perfectly and exquisitely finished "usque ad ungues," perfect inside and outside, for nature does in the pomegranate close jewels more rare than ormus shows.[ ] in winter the woods are comparatively bare and lifeless, even the brambles and woodbine, which straggle over the tangle of underwood being almost leafless. still even then they have a beauty and interest of their own; the mossy boles of the trees; the delicate tracery of the branches which can hardly be appreciated when they are covered with leaves; and under foot the beds of fallen leaves; while the evergreens seem brighter than in summer; the ruddy stems and rich green foliage of the scotch pines, and the dark spires of the firs, seeming to acquire fresh beauty. again in winter, though no doubt the living tenants of the woods are much less numerous, many of our birds being then far away in the dense african forests, on the other hand those which remain are much more easily visible. we can follow the birds from tree to tree, and the squirrel from bough to bough. it requires little imagination to regard trees as conscious beings, indeed it is almost an effort not to do so. "the various action of trees rooting themselves in inhospitable rocks, stooping to look into ravines, hiding from the search of glacier winds, reaching forth to the rays of rare sunshine, crowding down together to drink at sweetest streams, climbing hand in hand among the difficult slopes, opening in sudden dances among the mossy knolls, gathering into companies at rest among the fragrant fields, gliding in grave procession over the heavenward ridges--nothing of this can be conceived among the unvexed and unvaried felicities of the lowland forest; while to all these direct sources of greater beauty are added, first the power of redundance, the mere quantity of foliage visible in the folds and on the promontories of a single alp being greater than that of an entire lowland landscape (unless a view from some cathedral tower); and to this charm of redundance, that of clearer visibility--tree after tree being constantly shown in successive height, one behind another, instead of the mere tops and flanks of masses as in the plains; and the forms of multitudes of them continually defined against the clear sky, near and above, or against white clouds entangled among their branches, instead of being confused in dimness of distance."[ ] there is much that is interesting in the relations of one species to another. many plants are parasitic upon others. the foliage of the beech is so thick that scarcely anything will grow under it, except those spring plants, such as the anemone and the wood buttercup or goldilocks, which flower early before the beech is in leaf. there are other cases in which the reason for the association of species is less evident. the larch and the arolla (pinus cembra) are close companions. they grow together in siberia; they do not occur in scandinavia or russia, but both reappear in certain swiss valleys, especially in the cantons of lucerne and valais and the engadine. another very remarkable case which has recently been observed is the relation existing between some of our forest trees and certain fungi, the species of which have not yet been clearly ascertained. the root tips of the trees are as it were enclosed in a thin sheet of closely woven mycelium. it was at first supposed that the fungus was attacking the roots of the tree, but it is now considered that the tree and the fungus mutually benefit one another. the fungus collects nutriment from the soil, which passes into the tree and up to the leaves, where it is elaborated into sap, the greater part being utilized by the tree, but a portion reabsorbed by the fungus. there is reason to think that, in some cases at any rate, the mycelium is that of the truffle. [illustration: tropical forest. _to face page ._] the great tropical forests have a totally different character from ours. i reproduce here the plate from kingsley's _at last_. the trees strike all travellers by their magnificence, the luxuriance of their vegetation, and their great variety. our forests contain comparatively few species, whereas in the tropics we are assured that it is far from common to see two of the same species near one another. but while in our forests the species are few, each tree has an independence and individuality of its own. in the tropics, on the contrary, they are interlaced and interwoven, so as to form one mass of vegetation; many of the trunks are almost concealed by an undergrowth of verdure, and intertwined by spiral stems of parasitic plants; from tree to tree hang an inextricable network of lianas, and it is often difficult to tell to which tree the fruits, flowers, and leaves really belong. the trunks run straight up to a great height without a branch, and then form a thick leafy canopy far overhead; a canopy so dense that even the blaze of the cloudless blue sky is subdued, one might almost say into a weird gloom, the effect of which is enhanced by the solemn silence. at first such a forest gives the impression of being more open than an english wood, but a few steps are sufficient to correct this error. there is a thick undergrowth matted together by wiry creepers, and the intermediate space is traversed in all directions by lines and cords. the english traveller misses sadly the sweet songs of our birds, which are replaced by the hoarse chatter of parrots. now and then a succession of cries even harsher and more discordant tell of a troop of monkeys passing across from tree to tree among the higher branches, or lower sounds indicate to a practised ear the neighbourhood of an ape, a sloth, or some other of the few mammals which inhabit the great forests. occasionally a large blue bee hums past, a brilliant butterfly flashes across the path, or a humming-bird hangs in the air over a flower like, as st. pierre says, an emerald set in coral, but "how weak it is to say that that exquisite little being, whirring and fluttering in the air, has a head of ruby, a throat of emerald, and wings of sapphire, as if any triumph of the jeweller's art could ever vie with that sparkling epitome of life and light."[ ] sir wyville thomson graphically describes a morning in a brazilian forest:-- "the night was almost absolutely silent, only now and then a peculiarly shrill cry of some night bird reached us from the woods. as we got into the skirt of the forest the morning broke, but the _réveil_ in a brazilian forest is wonderfully different from the slow creeping on of the dawn of a summer morning at home, to the music of the thrushes answering one another's full rich notes from neighbouring thorn-trees. suddenly a yellow light spreads upwards in the east, the stars quickly fade, and the dark fringes of the forest and the tall palms show out black against the yellow sky, and almost before one has time to observe the change the sun has risen straight and fierce, and the whole landscape is bathed in the full light of day. but the morning is yet for another hour cool and fresh, and the scene is indescribably beautiful. the woods, so absolutely silent and still before, break at once into noise and movement. flocks of toucans flutter and scream on the tops of the highest forest trees hopelessly out of shot, the ear is pierced by the strange wild screeches of a little band of macaws which fly past you like the wrapped-up ghosts of the birds on some gaudy old brocade."[ ] mr. darwin tells us that nothing can be better than the description of tropical forests given by bates. "the leafy crowns of the trees, scarcely two of which could be seen together of the same kind, were now far away above us, in another world as it were. we could only see at times, where there was a break above, the tracery of the foliage against the clear blue sky. sometimes the leaves were palmate, or of the shape of large outstretched hands; at others finely cut or feathery like the leaves of mimosæ. below, the tree trunks were everywhere linked together by sipos; the woody flexible stems of climbing and creeping trees, whose foliage is far away above, mingled with that of the taller independent trees. some were twisted in strands like cables, others had thick stems contorted in every variety of shape, entwining snake-like round the tree trunks or forming gigantic loops and coils among the larger branches; others, again, were of zigzag shape, or indented like the steps of a staircase, sweeping from the ground to a giddy height." the reckless and wanton destruction of forests has ruined some of the richest countries on earth. syria and asia minor, palestine and the north of africa were once far more populous than they are at present. they were once lands "flowing with milk and honey," according to the picturesque language of the bible, but are now in many places reduced to dust and ashes. why is there this melancholy change? why have deserts replaced cities? it is mainly owing to the ruthless destruction of the trees, which has involved that of nations. even nearer home a similar process may be witnessed. two french departments--the hautes- and basses-alpes--are being gradually reduced to ruin by the destruction of the forests. cultivation is diminishing, vineyards are being washed away, the towns are threatened, the population is dwindling, and unless something is done the country will be reduced to a desert; until, when it has been released from the destructive presence of man, nature reproduces a covering of vegetable soil, restores the vegetation, creates the forests anew, and once again fits these regions for the habitation of man. in another part of france we have an illustration of the opposite process. the region of the landes, which fifty years ago was one of the poorest and most miserable in france, has now been made one of the most prosperous owing to the planting of pines. the increased value is estimated at no less than , , , francs. where there were fifty years ago only a few thousand poor and unhealthy shepherds whose flocks pastured on the scanty herbage, there are now sawmills, charcoal kilns, and turpentine works, interspersed with thriving villages and fertile agricultural lands. in our own country, though woodlands are perhaps on the increase, true forest scenery is gradually disappearing. this is, i suppose, unavoidable, but it is a matter of regret. forests have so many charms of their own. they give a delightful impression of space and of abundance. the extravagance is sublime. trees, as jefferies says, "throw away handfuls of flower; and in the meadows the careless, spendthrift ways of grass and flower and all things are not to be expressed. seeds by the hundred million float with absolute indifference on the air. the oak has a hundred thousand more leaves than necessary, and never hides a single acorn. nothing utilitarian--everything on a scale of splendid waste. such noble, broadcast, open-armed waste is delicious to behold. never was there such a lying proverb as 'enough is as good as a feast.' give me the feast; give me squandered millions of seeds, luxurious carpets of petals, green mountains of oak-leaves. the greater the waste the greater the enjoyment--the nearer the approach to real life." it is of course impossible here to give any idea of the complexity of structure of our forest trees. a slice across the stem of a tree shows many different tissues with more or less technical names, bark and cambium, medullary rays, pith, and more or less specialised tissue; air-vessels, punctate vessels, woody fibres, liber fibres, scalariform vessels, and other more or less specialised tissues. let us take a single leaf. the name is synonymous with anything very thin, so that we might well fancy that a leaf would consist of only one or two layers of cells. far from it, the leaf is a highly complex structure. on the upper surface are a certain number of scattered hairs, while in the bud these are often numerous, long, silky, and serve to protect the young leaf, but the greater number fall off soon after the leaf expands. the hairs are seated on a layer of flattened cells--the skin or epidermis. below this are one or more layers of "palisade cells," the function of which seems to be to regulate the quantity of light entering the leaf. under these again is the "parenchyme," several layers of more or less rounded cells, leaving air spaces and passages between them. from place to place in the parenchyme run "fibro-vascular bundles," forming a sort of skeleton to the leaf, and comprising air-vessels on the upper side, rayed or dotted vessels with woody fibre below, and vessels of various kinds. the under surface of the leaf is formed by another layer of flattened cells, supporting generally more or less hairs, and some of them specially modified so as to leave minute openings or "stomata" leading into the air passages. these stomata are so small that there are millions on a single leaf, and on plants growing in dry countries, such as the evergreen oak, oleander, etc., they are sunk in pits, and further protected by tufts of hair. the cells of the leaf again are themselves complex. they consist of a cell wall perforated by extremely minute orifices, of protoplasm, cell fluid, and numerous granules of "chlorophyll," which give the leaf its green colour. while these are, stated very briefly, the essential parts of a leaf, the details differ in every species, while in the same species and even in the same plant, the leaves present minor differences according to the situation in which they grow. since, then, there is so much complex structure in a single leaf, what must it be in a whole plant? there is a giant sea-weed (macrocystis), which has been known to reach a length of feet, as also do some of the lianas of tropical forests. these, however, attain no great bulk, and the most gigantic specimens of the vegetable kingdom yet known are the wellingtonia (sequoia) gigantea, which grows to a height of feet, and the blue gum (eucalyptus) even to . one is apt to look on animal structure as more delicate, and of a higher order, than that of plants. and so no doubt it is. yet an animal, even man himself, will recover from a wound or an operation more rapidly and more perfectly than a tree.[ ] trees again derive a special interest from the venerable age they attain. in some cases, no doubt, the age is more or less mythical, as, for instance, the olive of minerva at athens, the oaks mentioned by pliny, "which were thought coeval with the world itself," the fig tree, "under which the wolf suckled the founder of rome and his brother, lasting (as tacitus calculated) years, putting out new shoots, and presaging the translation of that empire from the cæsarian line, happening in nero's reign."[ ] but in other cases the estimates rest on a surer foundation, and it cannot be doubted that there are trees still living which were already of considerable size at the time of the conquest. the soma cypress of lombardy, which is feet high and in circumference, is calculated to go back to forty years before the birth of christ. francis the first is said to have driven his sword into it in despair after the battle of padua, and napoleon altered his road over the simplon so as to spare it. ferdinand and isabella in swore to maintain the privileges of the biscayans under the old oak of guernica. in the ardennes an oak cut down in contained a funeral urn and some samnite coins. a writer at the time drew the conclusion that it must have been already a large tree when rome was founded, and though the facts do not warrant this conclusion, the tree did, no doubt, go back to pagan times. the great yew of fountains abbey is said to have sheltered the monks when the abbey was rebuilt in , and is estimated at an age of years; that at brabourne in kent at . de candolle gives the following as the ages attainable:-- the ivy years larch " plane " cedar of lebanon " lime " oak " taxodium distichum to baobab years nowhere is woodland scenery more beautiful than where it passes gradually into the open country. the separate trees, having more room both for their roots and branches, are finer, and can be better seen, while, when they are close together, "one cannot see the wood for the trees." the vistas which open out are full of mystery and of promise, and tempt us gradually out into the green fields. what pleasant memories these very words recall, games in the hay as children, and sunny summer days throughout life. "consider," says ruskin,[ ] "what we owe to the meadow grass, to the covering of the dark ground by that glorious enamel, by the companies of those soft countless and peaceful spears. the fields! follow but forth for a little time the thought of all that we ought to recognise in those words. all spring and summer is in them--the walks by silent scented paths, the rests in noonday heat, the joy of herds and flocks, the power of all shepherd life and meditation, the life of sunlight upon the world, falling in emerald streaks, and soft blue shadows, where else it would have struck on the dark mould or scorching dust, pastures beside the pacing brooks, soft banks and knolls of lowly hills, thymy slopes of down overlooked by the blue line of lifted sea, crisp lawns all dim with early dew, or smooth in evening warmth of barred sunshine, dinted by happy feet, and softening in their fall the sound of loving voices. * * * * * "go out, in the spring time, among the meadows that slope from the shores of the swiss lakes to the roots of their lower mountains. there, mingled with the taller gentians and the white narcissus, the grass grows deep and free, and as you follow the winding mountain paths, beneath arching boughs all veiled and dim with blossom,--paths, that for ever droop and rise over the green banks and mounds sweeping down in scented undulation, steep to the blue water, studded here and there with new mown heaps, filling all the air with fainter sweetness,--look up towards the higher hills, where the waves of everlasting green roll silently into their long inlets among the shadows of the pines; and we may, perhaps, at last know the meaning of those quiet words of the th psalm, 'he maketh the grass to grow upon the mountains.'" "on fine days," he tells us again in his _autobiography_, "when the grass was dry, i used to lie down on it, and draw the blades as they grew, with the ground herbage of buttercup or hawkweed mixed among them, until every square foot of meadow, or mossy bank, became an infinite picture and possession to me, and the grace and adjustment to each other of growing leaves, a subject of more curious interest to me than the composition of any painter's masterpieces." in the passage above quoted, ruskin alludes especially to swiss meadows. they are especially remarkable in the beauty and variety of flowers. in our fields the herbage is mainly grass, and if it often happens that they glow with buttercups or are white with ox-eye-daisies, these are but unwelcome intruders and add nothing to the value of the hay. swiss meadows, on the contrary, are sweet and lovely with wild geraniums, harebells, bluebells, pink restharrow, yellow lady's bedstraw, chervil, eyebright, red and white silenes, geraniums, gentians, and many other flowers which have no familiar english names; all adding not only to the beauty and sweetness of the meadows, but forming a valuable part of the crop itself.[ ] on the other hand "turf" is peculiarly english, and no turf is more delightful than that of our downs--delightful to ride on, to sit on, or to walk on. the turf indeed feels so springy under our feet that walking on it seems scarcely an exertion: one could almost fancy that the downs themselves were still rising, even higher, into the air. the herbage of the downs is close rather than short, hillocks of sweet thyme, tufts of golden potentilla, of milkwort--blue, pink, and white--of sweet grass and harebells: here and there pink with heather, or golden with furze or broom, while over all are the fresh air and sunshine, sweet scents, and the hum of bees. and if the downs seem full of life and sunshine, their broad shoulders are types of kindly strength, they give also an impression of power and antiquity, while every now and then we come across a tumulus, or a group of great grey stones, the burial place of some ancient hero, or a sacred temple of our pagan forefathers. on the downs indeed things change slowly, and in parts of sussex the strong slow oxen still draw the waggons laden with warm hay or golden wheat sheaves, or drag the wooden plough along the slopes of the downs, just as they did a thousand years ago. i love the open down most, but without hedges england would not be england. hedges are everywhere full of beauty and interest, and nowhere more so than at the foot of the downs, when they are in great part composed of wild guelder roses and rich dark yews, decked with festoons of traveller's joy, the wild bryonies, and garlands of wild roses covered with thousands of white or delicate pink flowers, each with a centre of gold. at the foot of the downs spring clear sparkling streams; rain from heaven purified still further by being filtered through a thousand feet of chalk; fringed with purple loosestrife and willowherb, starred with white water ranunculuses, or rich watercress, while every now and then a brown water rat rustles in the grasses at the edge, and splashes into the water, or a pink speckled trout glides out of sight. in many of our midland and northern counties most of the meadows lie in parallel undulations or "rigs." these are generally about a furlong ( yards) in length, and either one or two poles ( - / or yards) in breadth. they seldom run straight, but tend to curve towards the left. at each end of the field a high bank, locally called a balk, often or feet high, runs at right angles to the rigs. in small fields there are generally eight, but sometimes ten, of these rigs, which make in the one case , in the other acres. these curious characters carry us back to the old tenures, and archaic cultivation of land, and to a period when the fields were not in pasture, but were arable. they also explain our curious system of land measurement. the "acre" is the amount which a team of oxen were supposed to plough in a day. it corresponds to the german "morgen" and the french "journée." the furlong or long "furrow" is the distance which a team of oxen can plough conveniently without stopping to rest. oxen, as we know, were driven not with a whip, but with a goad or pole, the most convenient length for which was - / feet, and the ancient ploughman used his "pole" or "perch" by placing it at right angles to his first furrow, thus measuring the amount he had to plough. hence our "pole" or "perch" of - / feet, which at first sight seems a very singular unit to have selected. this width is also convenient both for turning the plough, and also for sowing. hence the most convenient unit of land for arable purposes was a furlong in length and a perch or pole in width. the team generally consisted of eight oxen. few peasants, however, possessed a whole team, several generally joining together, and dividing the produce. hence the number of "rigs," one for each ox. we often, however, find ten instead of eight; one being for the parson's tithe, the other tenth going to the ploughman. when eight oxen were employed the goad would not of course reach the leaders, which were guided by a man who walked on the near side. on arriving at the end of each furrow he turned them round, and as it was easier to pull than to push them, this gradually gave the furrow a turn towards the left, thus accounting for the slight curvature. lastly, while the oxen rested on arriving at the end of the furrow, the ploughmen scraped off the earth which had accumulated on the coulter and ploughshare, and the accumulation of these scrapings gradually formed the balk. it is fascinating thus to trace indications of old customs and modes of life, but it would carry us away from the present subject. even though the swiss meadows may offer a greater variety, our english fields are yet rich in flowers: yellow with cowslips and primroses, pink with cuckoo flowers and purple with orchis, while, however, unwelcome to the eye of the farmer, the rich buttercup its tiny polished urn holds up, filled with ripe summer to the edge,[ ] turning many a meadow into a veritable field of the cloth of gold, and there are few prettier sights in nature than an english hay field on a summer evening, with a copse perhaps at one side and a brook on the other; men with forks tossing the hay in the air to dry; women with wooden rakes arranging it in swathes ready for the great four-horse waggon, or collecting it in cocks for the night; while some way off the mowers are still at work, and we hear from time to time the pleasant sound of the whetting of the scythe. all are working with a will lest rain should come and their labour be thrown away. this too often happens. but though we often complain of our english climate, it is yet, take it all in all, one of the best in the world, being comparatively free from extremes either of heat or cold, drought or deluge. to the happy mixture of sunshine and of rain we owe the greenness of our fields, sparkling with dewdrops indwelt with little angels of the sun,[ ] lit and warmed by golden sunshine and fed by silver rain, which now and again sprinkles the whole earth with diamonds. footnotes: [ ] _the spectator._ [ ] milton. [ ] jefferies. [ ] forbes, _a naturalist's wanderings in the eastern archipelago_. [ ] tennyson. [ ] hamerton. [ ] marvell. [ ] ruskin. [ ] thomson, _voyage of the challenger_. [ ] thomson, _voyage of the challenger_. [ ] sir j. paget, _on the pathology of plants_. [ ] evelyn's _sylva_. [ ] _modern painters._ [ ] m. correvon informs me that the gruyère cheese is supposed to owe its peculiar flavour to the alpine alchemilla, which is now on that account often purposely sown elsewhere. [ ] j. r. lowell. [ ] hamerton. chapter vi mountains mountains "seem to have been built for the human race, as at once their schools and cathedrals; full of treasures of illuminated manuscript for the scholar, kindly in simple lessons for the worker, quiet in pale cloisters for the thinker, glorious in holiness for the worshipper. they are great cathedrals of the earth, with their gates of rock, pavements of cloud, choirs of stream and stone, altars of snow, and vaults of purple traversed by the continual stars."--ruskin. [illustration: summit of mont blanc. _to face page ._] chapter vi mountains the alps are to many of us an inexhaustible source of joy and peace, of health, and even of life. we have gone to them jaded and worn, feeling, perhaps without any external cause, anxious and out of spirits, and have returned full of health, strength, and energy. among the mountains nature herself seems freer and happier, brighter and purer, than elsewhere. the rush of the rivers, and the repose of the lakes, the pure snowfields and majestic glaciers, the fresh air, the mysterious summits of the mountains, the blue haze of the distance, the morning tints and the evening glow, the beauty of the sky and the grandeur of the storm, have all refreshed and delighted us time after time, and their memories can never fade away. even now as i write comes back to me the bright vision of an alpine valley--blue sky above, glittering snow, bare grey or rich red rock, dark pines here and there, mixed with bright green larches, then patches of smooth alp, with clumps of birch and beech, and dotted with brown châlets; then below them rock again, and wood, but this time with more deciduous trees; and then the valley itself, with emerald meadows, interspersed with alder copses, threaded together by a silver stream; and i almost fancy i can hear the tinkling of distant cowbells coming down from the alp, and the delicious murmur of the rushing water. the endless variety, the sense of repose and yet of power, the dignity of age, the energy of youth, the play of colour, the beauty of form, the mystery of their origin, all combine to invest mountains with a solemn beauty. i feel with ruskin that "mountains are the beginning and the end of all natural scenery; in them, and in the forms of inferior landscape that lead to them, my affections are wholly bound up; and though i can look with happy admiration at the lowland flowers, and woods, and open skies, the happiness is tranquil and cold, like that of examining detached flowers in a conservatory, or reading a pleasant book." and of all mountain views which he has seen, the finest he considers is that from the montanvert: "i have climbed much and wandered much in the heart of the high alps, but i have never yet seen anything which equalled the view from the cabin of the montanvert." it is no mere fancy that among mountains the flowers are peculiarly large and brilliant in colour. not only are there many beautiful species which are peculiar to mountains,--alpine gentians, yellow, blue, and purple; alpine rhododendrons, alpine primroses and cowslips, alpine lychnis, columbine, monkshood, anemones, narcissus, campanulas, soldanellas, and a thousand others less familiar to us,--but it is well established that even within the limits of the same species those living up in the mountains have larger and brighter flowers than their sisters elsewhere. various alpine species belonging to quite distinct families form close moss-like cushions, gemmed with star-like flowers, or covered completely with a carpet of blossom. on the lower mountain slopes and in alpine valleys trees seem to flourish with peculiar luxuriance. pines and firs and larches above; then, as we descend, beeches and magnificent chestnuts, which seem to rejoice in the sweet, fresh air and the pure mountain streams. to any one accustomed to the rich bird life of english woods and hedgerows, it must be admitted that swiss woods and alps seem rather lonely and deserted. still the hawk, or even eagle, soaring high up in the air, the weird cry of the marmot, and the knowledge that, even if one cannot see chamois, they may all the time be looking down on us, give the alps, from this point of view also, a special interest of their own. another great charm of mountain districts is the richness of colour. "consider,[ ] first, the difference produced in the whole tone of landscape colour by the introductions of purple, violet, and deep ultra-marine blue which we owe to mountains. in an ordinary lowland landscape we have the blue of the sky; the green of the grass, which i will suppose (and this is an unnecessary concession to the lowlands) entirely fresh and bright; the green of trees; and certain elements of purple, far more rich and beautiful than we generally should think, in their bark and shadows (bare hedges and thickets, or tops of trees, in subdued afternoon sunshine, are nearly perfect purple and of an exquisite tone), as well as in ploughed fields, and dark ground in general. but among mountains, in addition to all this, large unbroken spaces of pure violet and purple are introduced in their distances; and even near, by films of cloud passing over the darkness of ravines or forests, blues are produced of the most subtle tenderness; these azures and purples passing into rose colour of otherwise wholly unattainable delicacy among the upper summits, the blue of the sky being at the same time purer and deeper than in the plains. nay, in some sense, a person who has never seen the rose colour of the rays of dawn crossing a blue mountain twelve or fifteen miles away can hardly be said to know what tenderness in colour means at all; bright tenderness he may, indeed, see in the sky or in a flower, but this grave tenderness of the far-away hill-purples he cannot conceive." "i do not know," he says elsewhere, "any district possessing a more pure or uninterrupted fulness of mountain character (and that of the highest order), or which appears to have been less disturbed by foreign agencies, than that which borders the course of the trient between valorsine and martigny. the paths which lead to it, out of the valley of the rhone, rising at first in steep circles among the walnut trees, like winding stairs among the pillars of a gothic tower, retire over the shoulders of the hills into a valley almost unknown, but thickly inhabited by an industrious and patient population. along the ridges of the rocks, smoothed by old glaciers, into long, dark, billowy swellings, like the backs of plunging dolphins, the peasant watches the slow colouring of the tufts of moss and roots of herb, which, little by little, gather a feeble soil over the iron substance; then, supporting the narrow strip of clinging ground with a few stones, he subdues it to the spade, and in a year or two a little crest of corn is seen waving upon the rocky casque." tyndall, speaking of the scene from the summit of the little scheideck,[ ] says: "the upper air exhibited a commotion which we did not experience; clouds were wildly driven against the flanks of the eiger, the jungfrau thundered behind, while in front of us a magnificent rainbow, fixing one of its arms in the valley of grindelwald, and, throwing the other right over the crown of the wetterhorn, clasped the mountain in its embrace. through jagged apertures in the clouds floods of golden light were poured down the sides of the mountain. on the slopes were innumerable châlets, glistening in the sunbeams, herds browsing peacefully and shaking their mellow bells; while the blackness of the pine trees, crowded into woods, or scattered in pleasant clusters over alp and valley, contrasted forcibly with the lively green of the fields." few men had more experience of mountains than mr. whymper, and from him, i will quote one remarkable passage describing the view from the summit of the matterhorn just before the terrible catastrophe which overshadows the memory of his first ascent. "the day was one of those superlatively calm and clear ones which usually precede bad weather. the atmosphere was perfectly still and free from all clouds or vapours. mountains fifty, nay, a hundred miles off looked sharp and near. all their details--ridge and crag, snow and glacier--stood out with faultless definition. pleasant thoughts of happy days in bygone years came up unbidden as we recognised the old familiar forms. all were revealed, not one of the principal peaks of the alps was hidden. i see them clearly now, the great inner circle of giants, backed by the ranges, chains, and _massifs_.... ten thousand feet beneath us were the green fields of zermatt, dotted with châlets, from which blue smoke rose lazily. eight thousand feet below, on the other side, were the pastures of breuil. there were black and gloomy forests; bright and cheerful meadows, bounding waterfalls and tranquil lakes, fertile lands and savage wastes, sunny plains and frigid plateaux. there were the most rugged forms and the most graceful outlines, bold perpendicular cliffs and gentle undulating slopes; rocky mountains and snowy mountains, sombre and solemn, or glittering and white, with walls, turrets, pinnacles, pyramids, domes, cones, and spires! there was every combination that the world can give, and every contrast that the heart could desire." these were summer scenes, but the autumn and winter again have a grandeur and beauty of their own. "autumn is dark on the mountains; grey mist rests on the hills. the whirlwind is heard on the heath. dark rolls the river through the narrow plain. the leaves twirl round with the wind, and strew the grave of the dead."[ ] even bad weather often but enhances the beauty and grandeur of mountains. when the lower parts are hidden, and the peaks stand out above the clouds, they look much loftier than if the whole mountain side is visible. the gloom lends a weirdness and mystery to the scene, while the flying clouds give it additional variety. rain, moreover, adds vividness to the colouring. the leaves and grass become a brighter green, "every sunburnt rock glows into an agate," and when fine weather returns the new snow gives intense brilliance, and invests the woods especially with the beauty of fairyland. how often in alpine districts does one long "for the wings of a dove," more thoroughly to enjoy and more completely to explore, the mysteries and recesses of the mountains. the mind, however, can go, even if the body must remain behind. each hour of the day has a beauty of its own. the mornings and evenings again glow with different and even richer tints. in mountain districts the cloud effects are brighter and more varied than in flatter regions. the morning and evening tints are seen to the greatest advantage, and clouds floating high in the heavens sometimes glitter with the most exquisite iridescent hues that blush and glow like angels' wings.[ ] on low ground one may be in the clouds, but not above them. but as we look down from mountains and see the clouds floating far below us, we almost seem as if we were looking down on earth from one of the heavenly bodies. not even in the alps is there anything more beautiful than the "after glow" which lights up the snow and ice with a rosy tint for some time after the sun has set. long after the lower slopes are already in the shade, the summit of mont blanc for instance is transfigured by the light of the setting sun glowing on the snow. it seems almost like a light from another world, and vanishes as suddenly and mysteriously as it came. as we look up from the valleys the mountain peaks seem like separate pinnacles projecting far above the general level. this, however, is a very erroneous impression, and when we examine the view from the top of any of the higher mountains, or even from one of very moderate elevation, if well placed, such say as the well-known piz languard, we see that in many cases they must have once formed a dome, or even a table land, out of which the valleys have been carved. many mountain chains were originally at least twice as high as they are now, and the highest peaks are those which have suffered least from the wear and tear of time. we used to speak of the everlasting hills, and are only beginning to realise the vast and many changes which our earth has undergone. there rolls the deep where grew the tree. o earth, what changes hast thou seen! there where the long street roars, hath been the stillness of the central sea. the hills are shadows, and they flow from form to form, and nothing stands; they melt like mist, the solid lands, like clouds they shape themselves and go.[ ] the origin of mountains geography moreover acquires a new interest when we once realise that mountains are no mere accidents, but that for every mountain chain, for every peak and valley, there is a cause and an explanation. the origin of mountains is a question of much interest. the building up of volcanoes is even now going on before our eyes. some others, the dolomites for instance, have been regarded by richthofen and other geologists as ancient coral islands. the long lines of escarpment which often stretch for miles across country, are now ascertained, mainly through the researches of whitaker, to be due to the differential action of aerial causes. the general origin of mountain chains, however, was at first naturally enough attributed to direct upward pressure from below. to attribute them in any way to subsidence seems almost a paradox, and yet it appears to be now well established that the general cause is lateral compression, due to contraction of the underlying mass. the earth, we know, has been gradually cooling, and as it contracted in doing so, the strata of the crust would necessarily be thrown into folds. when an apple dries and shrivels in winter, the surface becomes covered with ridges. or again, if we place some sheets of paper between two weights on a table, and then bring the weights nearer together, the paper will be crumpled up. [illustration: fig. .--adapted from ball's paper "on the formation of alpine valleys and lakes," _lond. and ed. phil. mag._ , p. .] in the same way let us take a section of the earth's surface ab (fig. ), and suppose that, by the gradual cooling and consequent contraction of the mass, ab sinks to a'b', then to a''b'', and finally to a'''b'''. of course if the cooling of the surface and of the deeper portion were the same, then the strata between a and b would themselves contract, and might consequently still form a regular curve between a''' and b'''. as a matter of fact, however, the strata at the surface of our globe have long since approached a constant temperature. under these circumstances there would be no contraction of the strata between a and b corresponding to that of those in the interior, and consequently they could not lie flat between a''' and b''', but must be thrown into folds, commencing along any line of least resistance. sometimes indeed the strata are completely inverted, as in fig. , and in other cases they have been squeezed for miles out of their original position. this explanation was first, i believe, suggested by steno. it has been recently developed by ball and suess, and especially by heim. in this manner it is probable that most mountain chains originated.[ ] the structure of mountain districts confirms this theoretical explanation. it is obvious of course that when strata are thrown into folds, they will, if strained too much, give way at the summit of the fold. before doing so, however, they are stretched and consequently loosened, while on the other hand the strata at the bottom of the fold are compressed: the former, therefore, are rendered more susceptible of disintegration, the latter on the contrary acquire greater powers of resistance. hence denudation will act with more effect on the upper than on the lower portion of the folds, and if continued long enough, so that, as shown in the above diagram, the dotted portion is removed, we find the original hill tops replaced by valleys, and the original valleys forming the hill tops. every visitor to switzerland must have noticed hills where the strata lie as shown in parts of fig. , and where it is obvious that strata corresponding to those in dots must have been originally present. in the jura, for instance, a glance at any good map of the district will show a succession of ridges running parallel to one another in a slightly curved line from s.w. to n.e. that these ridges are due to folds of the earth's surface is clear from the following figure in jaccard's work on the geology of the jura, showing a section from brenets due south to neuchâtel by le locle. these folds are comparatively slight and the hills of no great height. further south, however, the strata are much more violently dislocated and compressed together. the mont salève is the remnant of one of these ridges. [illustration: fig. .--section across the jura from brenets to neuchâtel.] in the alps the contortions are much greater than in the jura. fig. shows a section after heim, from the spitzen across the brunnialp, and the maderanerthal. it is obvious that the valleys are due mainly to erosion, that the maderaner valley has been cut out of the crystalline rocks _s_, and was once covered by the jurassic strata _j_, which must have formerly passed in a great arch over what is now the valley. however improbable it may seem that so great an amount of rock should have disappeared, evidence is conclusive. ramsay has shown that in some parts of wales not less than , feet have been removed, while there is strong reason for the belief that in switzerland an amount has been carried away equal to the present height of the mountains; though of course it does not follow that the alps were once twice as high as they are at present, because elevation and erosion must have gone on contemporaneously. [illustration: fig. .--_e_, eocene strata; _j_, jurassic; _s_, crystalline rocks.] it has been calculated that the strata between bâle and the st. gotthard have been compressed from miles to miles, the ardennes from to miles, and the appalachians from miles to ! prof. gumbel has recently expressed the opinion that the main force to which the elevation of the alps was due acted along the main axis of elevation. exactly the opposite inference would seem really to follow from the facts. if the centre of force were along the axis of elevation, the result would, as suess and heim have pointed out, be to extend, not to compress, the strata; and the folds would remain quite unaccounted for. the suggestion of compression is on the contrary consistent with the main features of swiss geography. the principal axis follows a curved line from the maritime alps towards the north-east by mont blanc and monte rosa and st. gotthard to the mountains overlooking the engadine. the geological strata follow the same direction. north of a line running through chambery, yverdun, neuchâtel, solothurn, and olten to waldshut on the rhine are jurassic strata; between that line and a second nearly parallel and running through annecy, vevey, lucerne, wesen, appenzell, and bregenz on the lake of constance, is the lowland occupied by later tertiary strata; between this second line and another passing through albertville, st. maurice, lenk, meiringen, and altdorf lies a more or less broken band of older tertiary strata; south of which are a cretaceous zone, one of jurassic age, then a band of crystalline rocks, while the central core, so to say, of the alps, as for instance at st. gotthard, consists mainly of gneiss or granite. the sedimentary deposits reappear south of the alps, and in the opinion of some high authorities, as, for instance, of bonney and heim, passed continuously over the intervening regions. the last great upheaval commenced after the miocene period, and continued through the pliocene. miocene strata attain in the righi a height of feet. for neither the hills nor the mountains are everlasting, or of the same age. the welsh mountains are older than the vosges, the vosges than the pyrenees, the pyrenees than the alps, and the alps than the andes, which indeed are still rising; so that if our english mountains are less imposing so far as mere height is concerned, they are most venerable from their great antiquity. but though the existing alps are in one sense, and speaking geologically, very recent, there is strong reason for believing that there was a chain of lofty mountains there long previously. "the first indication," says judd, "of the existence of a line of weakness in this portion of the earth's crust is found towards the close of the permian period, when a series of volcanic outbursts on the very grandest scale took place" along a line nearly following that of the present alps, and led to the formation of a range of mountains, which, in his opinion, must have been at least to feet high. ramsay and bonney have also given strong reasons for believing that the present line of the alps was, at a still earlier period, occupied by a range of mountains no less lofty than those of to-day. thus then, though the present alps are comparatively speaking so recent, there are good grounds for the belief that they were preceded by one or more earlier ranges, once as lofty as they are now, but which were more or less completely levelled by the action of air and water, just as is happening now to the present mountain ranges. movements of elevation and subsidence are still going on in various parts of the world. scandinavia is rising in the north, and sinking at the south. south america is rising on the west and sinking in the east, rotating in fact on its axis, like some stupendous pendulum. the crushing and folding of the strata to which mountain chains are due, and of which the alps afford such marvellous illustrations, necessarily give rise to earthquakes, and the slight shocks so frequent in parts of switzerland[ ] appear to indicate that the forces which have raised the alps are not yet entirely spent, and that slow subterranean movements are still in progress along the flanks of the mountains. but if the mountain chains are due to compression, the present valleys are mainly the result of denudation. as soon as a mountain range is once raised, all nature seems to conspire against it. sun and frost, heat and cold, air and water, ice and snow, every plant, from the lichen to the oak, and every animal, from the worm to man himself, combine to attack it. water, however, is the most powerful agent of all. the autumn rains saturate every pore and cranny; the water as it freezes cracks and splits the hardest rocks; while the spring sun melts the snow and swells the rivers, which in their turn carry off the debris to the plains. perhaps, however, it would after all be more correct to say that nature, like some great artist, carves the shapeless block into form, and endows the rude mass with life and beauty. "what more," said hutton long ago, "is required to explain the configuration of our mountains and valleys? nothing but time. it is not any part of the process that will be disputed; but, after allowing all the parts, the whole will be denied; and for what? only because we are not disposed to allow that quantity of time which the absolution of so much wasted mountain might require." the tops of the swiss mountains stand, and since their elevation have probably always stood, above the range of ice, and hence their bold peaks. in scotland, on the contrary, and still more in norway, the sheet of ice which once, as is the case with greenland now, spread over the whole country, has shorn off the summits and reduced them almost to gigantic bosses; while in wales the same causes, together with the resistless action of time--for, as already mentioned, the welsh hills are far older than the mountains of switzerland--has ground down the once lofty summits and reduced them to mere stumps, such as, if the present forces are left to work out their results, the swiss mountains will be thousands, or rather tens of thousands, of years hence. the "snow line" in switzerland is generally given as being between and feet. above this level the snow or _névé_ gradually accumulates until it forms "glaciers," solid rivers of ice which descend more or less far down the valleys. no one who has not seen a glacier can possibly realise what they are like. fig. represents the glacier of the blümlis alp, and the plate the mer de glace. [illustration: fig. .--glacier of the blümlis alp.] [illustration: the mer de glace. _to face page ._] they are often very beautiful. "mount beerenberg," says lord dufferin, "in size, colour, and effect far surpassed anything i had anticipated. the glaciers were quite an unexpected element of beauty. imagine a mighty river, of as great a volume as the thames, started down the side of a mountain, bursting over every impediment, whirled into a thousand eddies, tumbling and raging on from ledge to ledge in quivering cataracts of foam, then suddenly struck rigid by a power so instantaneous in its action that even the froth and fleeting wreaths of spray have stiffened to the immutability of sculpture. unless you had seen it, it would be almost impossible to conceive the strangeness of the contrast between the actual tranquillity of these silent crystal rivers and the violent descending energy impressed upon their exterior. you must remember too all this is upon a scale of such prodigious magnitude, that when we succeeded subsequently in approaching the spot--where with a leap like that of niagara one of these glaciers plunges down into the sea--the eye, no longer able to take in its fluvial character, was content to rest in simple astonishment at what then appeared a lucent precipice of grey-green ice, rising to the height of several hundred feet above the masts of the vessel."[ ] the cliffs above glaciers shower down fragments of rock which gradually accumulate at the sides and at the end of the glaciers, forming mounds known as "moraines." many ancient moraines occur far beyond the present region of glaciers. in considering the condition of alpine valleys we must remember that the glaciers formerly descended much further than they do at present. the glaciers of the rhone for instance occupied the whole of the valais, filled the lake of geneva--or rather the site now occupied by that lake--and rose feet up the slopes of the jura; the upper ticino, and contributory valleys, were occupied by another which filled the basin of the lago maggiore; a third occupied the valley of the dora baltea, and has left a moraine at ivrea some twenty miles long, and which rises no less than feet above the present level of the river. the scotch and scandinavian valleys were similarly filled by rivers of ice, which indeed at one time covered the whole country with an immense sheet, as greenland is at present. enormous blocks of stone, the pierre à niton at geneva and the pierre à bot above neuchâtel, for instance, were carried by these glaciers for miles and miles; and many of the stones in the norfolk cliffs were brought by ice from norway (perhaps, however, by icebergs), across what is now the german ocean. again wherever the rocks are hard enough to have withstood the weather, we find them polished and ground, just as, and even more so than, those at the ends and sides of existing glaciers. the most magnificent glacier tracks in the alps are, in ruskin's opinion, those on the rocks of the great angle opposite martigny; the most interesting those above the channel of the trient between valorsine and the valley of the rhone. in great britain i know no better illustration of ice action than is to be seen on the road leading down from glen quoich to loch hourn, one of the most striking examples of desolate and savage scenery in scotland. its name in celtic is said to mean the lake of hell. all along the roadside are smoothed and polished hummocks of rock, most of them deeply furrowed with approximately parallel striæ, presenting a gentle slope on the upper end, and a steep side below, clearly showing the direction of the great ice flow. many of the upper swiss valleys contain lakes, as, for instance, that of the upper rhone, the lake of geneva, of the reuss, the lake of lucerne, of the rhine, that of constance. these lakes are generally very deep. the colour of the upper rivers, which are white with the diluvium from the glaciers, is itself evidence of the erosive powers which they exercise. this finely-divided matter is, however, precipitated in the lakes, which, as well as the rivers issuing from them, are a beautiful rich blue. "is it not probable that this action of finely-divided matter may have some influence on the colour of some of the swiss lakes--as that of geneva for example? this lake is simply an expansion of the river rhone, which rushes from the end of the rhone glacier, as the arveiron does from the end of the mer de glace. numerous other streams join the rhone right and left during its downward course; and these feeders, being almost wholly derived from glaciers, join the rhone charged with the finer matter which these in their motion have ground from the rocks over which they have passed. but the glaciers must grind the mass beneath them to particles of all sizes, and i cannot help thinking that the finest of them must remain suspended in the lake throughout its entire length. faraday has shown that a precipitate of gold may require months to sink to the bottom of a bottle not more than five inches high, and in all probability it would require ages of calm subsidence to bring all the particles which the lake of geneva contains to its bottom. it seems certainly worthy of examination whether such particles suspended in the water contribute to the production of that magnificent blue which has excited the admiration of all who have seen it under favourable circumstances."[ ] among the swiss mountains themselves each has its special character. tyndall thus describes a view in the alps, certainly one of the most beautiful--that, namely, from the summit of the Ægischhorn. "skies and summits are to-day without a cloud, and no mist or turbidity interferes with the sharpness of the outlines. jungfrau, monk, eiger, trugberg, cliffy strahlgrat, stately lady-like aletschhorn, all grandly pierce the empyrean. like a saul of mountains, the finsteraarhorn overtops all his neighbours; then we have the oberaarhorn, with the riven glacier of viesch rolling from his shoulders. below is the mârjelin see, with its crystal precipices and its floating icebergs, snowy white, sailing on a blue green sea. beyond is the range which divides the valais from italy. sweeping round, the vision meets an aggregate of peaks which look as fledglings to their mother towards the mighty dom. then come the repellent crags of mont cervin; the ideal of moral savagery, of wild untameable ferocity, mingling involuntarily with our contemplation of the gloomy pile. next comes an object, scarcely less grand, conveying, it may be, even a deeper impression of majesty and might than the matterhorn itself--the weisshorn, perhaps the most splendid object in the alps. but beauty is associated with its force, and we think of it, not as cruel, but as grand and strong. further to the right the great combin lifts up his bare head; other peaks crowd around him; while at the extremity of the curve round which our gaze has swept rises the sovran crown of mont blanc. and now, as day sinks, scrolls of pearly clouds draw themselves around the mountain crests, being wafted from them into the distant air. they are without colour of any kind; still, by grace of form, and as the embodiment of lustrous light and most tender shade, their beauty is not to be described."[ ] volcanoes volcanoes belong to a totally different series of mountains. it is practically impossible to number the volcanoes on our earth. humboldt enumerated , which keith johnston raised to nearly . some, no doubt, are always active, but in the majority the eruptions are occasional, and though some are undoubtedly now extinct, it is impossible in all cases to distinguish those which are only in repose from those whose day of activity is over. then, again, the question would arise, which should be regarded as mere subsidiary cones and which are separate volcanoes. the slopes of etna present more than small cones, and on hawaii there are several thousands. in fact, most of the very lofty volcanoes present more or less lateral cones. the molten matter, welling up through some fissure, gradually builds itself up into a cone, often of the most beautiful regularity, such as the gigantic peaks of chimporazo, cotopaxi (fig. ), and fusiyama, and hence it is that the crater is so often at, or very near, the summit. [illustration: fig. .--cotopaxi.] perhaps no spectacle in nature is more magnificent than a volcano in activity. it has been my good fortune to have stood more than once at the edge of the crater of vesuvius during an eruption, to have watched the lava seething below, while enormous stones were shot up high into the air. such a spectacle can never be forgotten. the most imposing crater in the world is probably that of kilauea, at a height of about feet on the side of mouna loa, in the island of hawaii. it has a diameter of miles, and is elliptic in outline, with a longer axis of about , and a circumference of about miles. the interior is a great lake of lava, the level of which is constantly changing. generally, it stands about feet below the edge, and the depth is about feet. the heat is intense, and, especially at night, when the clouds are coloured scarlet by the reflection from the molten lava, the effect is said to be magnificent. gradually the lava mounts in the crater until it either bursts through the side or runs over the edge, after which the crater remains empty, sometimes for years. a lava stream flows down the slope of the mountain like a burning river, at first rapidly, but as it cools, scoriæ gradually form, and at length the molten matter covers itself completely (fig. ), both above and at the sides, with a solid crust, within which, as in a tunnel, it continues to flow slowly as long as it is supplied from the source, here and there breaking through the crust which, as continually, re-forms in front. thus the terrible, inexorable river of fire slowly descends, destroying everything in its course. [illustration: fig. .--lava stream.] the stream of lava which burst from mouna loa in had a length of miles; that of skaptar-jokul in iceland in had a length of miles, and a maximum depth of nearly feet. it has been calculated that the mass of lava equalled that of mont blanc. the stones, ashes, and mud ejected during eruptions are even more destructive than the rivers of lava. in tomboro, a volcano on the island of sumbava, cost more lives than fell in the battle of waterloo. the earthquake of lisbon in destroyed , persons. during the earthquake of riobamba and the mud eruption of tunguragua, and again in that of krakatoa, it is estimated that the number who perished was between , and , . at the earthquake of antioch in no less than , persons are said to have lost their lives. perhaps the most destructive eruption of modern times has been that on cosequina. for miles it covered the ground with muddy water feet in depth. the dust and ashes formed a dense cloud, extending over many miles, some of it being carried degrees to the west. the total mass ejected has been estimated at milliards of square yards. stromboli, in the mediterranean (fig. ), though only feet in height, is very imposing from its superb regularity, and its roots plunge below the surface to a depth of feet. it is, moreover, very interesting from the regularity of its action, which has a period of minutes or a little less. on looking down into the crater one sees at a depth of say feet a seething mass of red-hot lava; this gradually rises, and then explodes, throwing up a cloud of vapour and stones, after which it sinks again. so regular is it that the volcano has been compared to a "flashing" lighthouse, and this wonderful process has been going on for ages. [illustration: fig. .--stromboli, viewed from the north-west, april .] though long extinct, volcanoes once existed in the british isles; arthur's seat, near edinburgh, for instance, appears to be the funnel of a small volcano, belonging to the carboniferous period. the summit of a volcanic mountain is sometimes entirely blown away. between my first two visits to vesuvius feet of the mountain had thus disappeared. vesuvius itself stands in a more ancient crater, part of which still remains, and is now known as somma, the greater portion having disappeared in the great eruption of , when the mountain, waking from its long sleep, destroyed herculaneum and pompeii. as regards the origin of volcanoes there have been two main theories. impressed by the magnitude and grandeur of the phenomena, enhanced as they are by their destructive character, many have been disposed to regard the craters of volcanoes as gigantic chimneys, passing right through the solid crust of the globe, and communicating with a central fire. recent researches, however, have indicated that, grand and imposing as they are, volcanoes must yet be regarded as due mainly to local and superficial causes. a glance at the map shows that volcanoes are almost always situated on, or near, the sea coast. from the interior of continents they are entirely wanting. the number of active volcanoes in the andes, contrasted with their absence in the alps and ourals, the himalayas, and central asian chains, is very striking. indeed, the pacific ocean is encircled, as ritter has pointed out, by a ring of fire. beginning with new zealand, we have the volcanoes of tongariro, whakaii, etc.; thence the circle passes through the fiji islands, solomon islands, new guinea, timor, flores, sumbava, lombock, java, sumatra, the philippines, japan, the aleutian islands, along the rocky mountains, mexico, peru, and chili, to tierra del fuego, and, in the far south, to the two great volcanoes of erebus and terror on victoria land. we know that the contraction of the earth's surface with the strains and fractures, the compression and folds, which must inevitably result, is still in operation, and must give rise to areas of high temperature, and consequently to volcanoes. we must also remember that the real mountain chains of our earth are the continents, compared to which even the alps and andes are mere wrinkles. it is along the lines of the great mountain chains, that is to say, along the main coast lines, rather than in the centres of the continents, which may be regarded as comparatively quiescent, that we should naturally expect to find the districts of greatest heat, and this is perhaps why volcanoes are generally distributed along the coast lines. another reason for regarding volcanoes as local phenomena is that many even of those comparatively near one another act quite independently. this is so with kilauea and mouna loa, both on the small island of hawaii. again, if volcanoes were in connection with a great central sea of fire, the eruptions must follow the same laws as regulate the tides. this, however, is not the case. there are indeed indications of the existence of slight tides in the molten lake which underlies vesuvius, and during the eruption of there was increased activity twice a day, as we should expect to find in any great fluid reservoir, but very different indeed from what must have been the case if the mountain was in connection with a central ocean of molten matter. indeed, unless the "crust" of our earth was of great thickness we should be subject to perpetual earthquakes. no doubt these are far more frequent than is generally supposed; indeed, with our improved instruments it can be shown that instead of occasional vibrations, with long intermediate periods of rest, we have in reality short intervals of rest with long periods of vibration, or rather perhaps that the crust of the earth is in constant tremor, with more violent oscillation from time to time. it appears, moreover, that earthquakes are not generally deep-seated. the point at which the shock is vertical can be ascertained, and it is also possible in some cases to determine the angle at which it emerges elsewhere. when this has been done it has always been found that the seat of disturbance must have been within geographical miles of the surface. yet, though we cannot connect volcanic action with the central heat of the earth, but must regard it as a minor and local manifestation of force, volcanoes still remain among the grandest, most awful, and at the same time most magnificent spectacles which the earth can afford. footnotes: [ ] ruskin. [ ] _the glaciers of the alps._ [ ] ossian. [ ] bullar, _azores_. [ ] tennyson. [ ] see especially heim's great work, _unt. ü. d. mechanismus der gebirgsbildung_. [ ] in the last years more than are recorded. [ ] _letters from high latitudes._ [ ] _glaciers of the alps._ [ ] _mountaineering in ._ chapter vii water of all inorganic substances, acting in their own proper nature, and without assistance or combination, water is the most wonderful. if we think of it as the source of all the changefulness and beauty which we have seen in the clouds; then as the instrument by which the earth we have contemplated was modelled into symmetry, and its crags chiselled into grace; then as, in the form of snow, it robes the mountains it has made, with that transcendent light which we could not have conceived if we had not seen; then as it exists in the foam of the torrent, in the iris which spans it, in the morning mist which rises from it, in the deep crystalline pools which mirror its hanging shore, in the broad lake and glancing river, finally, in that which is to all human minds the best emblem of unwearied, unconquerable power, the wild, various, fantastic, tameless unity of the sea; what shall we compare to this mighty, this universal element, for glory and for beauty? or how shall we follow its eternal cheerfulness of feeling? it is like trying to paint a soul.--ruskin. [illustration: rydal water. _to face page ._] chapter vii water in the legends of ancient times running water was proof against all sorcery and witchcraft: no spell could stay the living tide or charm the rushing stream.[ ] there was much truth as well as beauty in this idea. flowing waters, moreover, have not only power to wash out material stains, but they also clear away the cobwebs of the brain--the results of over incessant work--and restore us to health and strength. snowfields and glaciers, mountain torrents, sparkling brooks, and stately rivers, meres and lakes, and last, not least, the great ocean itself, all alike possess this magic power. "when i would beget content," says izaak walton, "and increase confidence in the power and wisdom and providence of almighty god, i will walk the meadows by some gliding stream, and there contemplate the lilies that take no care, and those very many other little living creatures that are not only created, but fed (man knows not how) by the goodness of the god of nature, and therefore trust in him;" and in his quaint old language he craves a special blessing on all those "that are true lovers of virtue, and dare trust in his providence, and be quiet, and go a angling." at the water's edge flowers are especially varied and luxuriant, so that the banks of a river are a long natural garden of tall and graceful grasses and sedges, the meadow sweet, the flowering rush, the sweet flag, the bull rush, purple loosestrife, hemp agrimony, dewberry, forget-me-not, and a hundred more, backed by willows, alders, poplars, and other trees. the animal world, if less conspicuous to the eye, is quite as fascinating to the imagination. here and there a speckled trout may be detected (rather by the shadow than the substance) suspended in the clear water, or darting across a shallow; if we are quiet we may see water hens or wild ducks swimming among the lilies, a kingfisher sitting on a branch or flashing away like a gleam of light; a solemn heron stands maybe at the water's edge, or slowly rises flapping his great wings; water rats, neat and clean little creatures, very different from their coarse brown namesakes of the land, are abundant everywhere; nor need we even yet quite despair of seeing the otter himself. insects of course are gay, lively, and innumerable; but after all the richest fauna is that visible only with a microscope. "to gaze," says dr. hudson, "into that wonderful world which lies in a drop of water, crossed by some stems of green weed, to see transparent living mechanism at work, and to gain some idea of its modes of action, to watch a tiny speck that can sail through the prick of a needle's point; to see its crystal armour flashing with ever varying tint, its head glorious with the halo of its quivering cilia; to see it gliding through the emerald stems, hunting for its food, snatching at its prey, fleeing from its enemy, chasing its mate (the fiercest of our passions blazing in an invisible speck); to see it whirling in a mad dance, to the sound of its own music, the music of its happiness, the exquisite happiness of living--can any one, who has once enjoyed this sight, ever turn from it to mere books and drawings, without the sense that he has left all fairyland behind him?"[ ] the study of natural history has indeed the special advantage of carrying us into the country and the open air. lakes are even more restful than rivers or the sea. rivers are always flowing, though it may be but slowly; the sea may rest awhile, now and then, but is generally full of action and energy; while lakes seem to sleep and dream. lakes in a beautiful country are like silver ornaments on a lovely dress, like liquid gems in a beautiful setting, or bright eyes in a lovely face. indeed as we gaze down on a lake from some hill or cliff it almost looks solid, like some great blue crystal. [illustration: windermere. _to face page ._] it is not merely for purposes of commerce or convenience that men love to live near rivers. let me live harmlessly, and near the brink of trent or avon have my dwelling-place; where i may see my quill, or cork, down sink, with eager bite of pike, or bleak, or dace; and on the world and my creator think: while some men strive ill-gotten goods t' embrace: and others spend their time in base excess of wine; or worse, in war, or wantonness. let them that will, these pastimes still pursue, and on such pleasing fancies feed their fill: so i the fields and meadows green may view and daily by fresh rivers walk at will, among the daisies and the violets blue, red hyacinth and yellow daffodil.[ ] it is interesting and delightful to trace a river from its source to the sea. "beginning at the hill-tops," says geikie, "we first meet with the spring or 'well-eye,' from which the river takes its rise. a patch of bright green, mottling the brown heathy slope, shows where the water comes to the surface, a treacherous covering of verdure often concealing a deep pool beneath. from this source the rivulet trickles along the grass and heath, which it soon cuts through, reaching the black, peaty layer below, and running in it for a short way as in a gutter. excavating its channel in the peat, it comes down to the soil, often a stony earth bleached white by the peat. deepening and widening the channel as it gathers force with the increasing slope, the water digs into the coating of drift or loose decomposed rock that covers the hillside. in favourable localities a narrow precipitous gully, twenty or thirty feet deep, may thus be scooped out in the course of a few years." if, however, we trace one of the swiss rivers to its source we shall generally find that it begins in a snow field or _névé_ nestled in a shoulder of some great mountain. below the _névé_ lies a glacier, on, in, and under which the water runs in a thousand little streams, eventually emerging at the end, in some cases forming a beautiful blue cavern, though in others the end of the glacier is encumbered and concealed by earth and stones. [illustration: fig. .--upper valley of st. gotthard.] the uppermost alpine valleys are perhaps generally, though by no means always, a little desolate and severe, as, for instance, that of st. gotthard (fig. ). the sides are clothed with rough pasture, which is flowery indeed, though of course the flowers are not visible at a distance, interspersed with live rock and fallen masses, while along the bottom rushes a white torrent. the snowy peaks are generally more or less hidden by the shoulders of the hills. the valleys further down widen and become more varied and picturesque. the snowy peaks and slopes are more often visible, the "alps" or pastures to which the cows are taken in summer, are greener and dotted with the huts or châlets of the cow-herds, while the tinkling of the cowbells comes to one from time to time, softened by distance, and suggestive of mountain rambles. below the alps there is generally a steeper part clothed with firs or with larches and pines, some of which seem as if they were scaling the mountains in regiments, preceded by a certain number of skirmishers. below the fir woods again are beeches, chestnuts, and other deciduous trees, while the central cultivated portion of the valley is partly arable, partly pasture, the latter differing from our meadows in containing a greater variety of flowers--campanulas, wild geraniums, chervil, ragged robin, narcissus, etc. here and there is a brown village, while more or less in the centre hurries along, with a delightful rushing sound, the mountain torrent, to which the depth, if not the very existence of the valley, is mainly due. the meadows are often carefully irrigated, and the water power is also used for mills, the streams seeming to rush on, as ruskin says, "eager for their work at the mill, or their ministry to the meadows." apart from the action of running water, snow and frost are continually disintegrating the rocks, and at the base of almost any steep cliff may be seen a slope of debris (as in figs. , ). this stands at a regular angle--the angle of repose--and unless it is continually removed by a stream at the base, gradually creeps up higher and higher, until at last the cliff entirely disappears. [illustration: fig. .--section of a river valley. the dotted line shows a slope or talus of debris.] sometimes the two sides of the valley approach so near that there is not even room for the river and the road: in that case nature claims the supremacy, and the road has to be carried in a cutting, or perhaps in a tunnel through the rock. in other cases nature is not at one with herself. in many places the debris from the rocks above would reach right across the valley and dam up the stream. then arises a struggle between rock and river, but the river is always victorious in the end; even if dammed back for a while, it concentrates its forces, rises up the rampart of rock, rushes over triumphantly, resumes its original course, and gradually carries the enemy away. [illustration: fig. .--valley of the rhone, with the waterfall of sallenches, showing talus of debris.] another prominent feature in many valleys is afforded by the old river, or lake, terraces, which were formed at a time when the river ran at a level far above its present bed. thus many a mountain valley gives some such section as the following. [illustration: fig. .--_a_, present river valley; _b_, old river terrace.] first, a face of rock, very steep, and in some places almost perpendicular; secondly, a regular talus of fallen rocks, stones, etc., as shown in the view of the rhone valley (fig. ), which takes what is known as the slope of repose, at an angle which depends on the character of the material. as a rule for loose rock fragments it may be taken roughly to be an angle of about °. then an irregular slope followed in many places by one or more terraces, and lastly the present bed of the river. [illustration: fig. .--diagram of an alpine valley showing a river cone. front view.] the width or narrowness of the valley in relation to its depth depends greatly on the condition of the rocks, the harder and tougher they are the narrower as a rule being the valley. from time to time a side stream enters the main valley. this is itself composed of many smaller rivulets. if the lateral valleys are steep, the streams bring with them, especially after rains, large quantities of earth and stones. when, however, they reach the main valley, the rapidity of the current being less, their power of transport also diminishes, and they spread out the material which they carry down in a depressed cone (figs. , , , ). a side stream with its terminal cone, when seen from the opposite side of the valley, presents the appearance shown in figs. , , or, if we are looking down the valley, as in figs. , , the river being often driven to one side of the main valley, as, for instance, is the case in the valais, near sion, where the rhone (fig. ) is driven out of its course by, and forms a curve round, the cone brought down by the torrent of the borgne. [illustration: fig. .--diagram of an alpine valley, showing a river cone. lateral view.] sometimes two lateral valleys (see plate) come down nearly opposite one another, so that the cones meet, as, for instance, some little way below vernayaz, and, indeed, in several other places in the valais (fig. ). or more permanent lakes may be due to a ridge of rock running across the valley, as, for instance, just below st. maurice in the valais. [illustration: fig. .] [illustration: view in the valais below st. maurice. _to face page ._] [illustration: fig. .--view in the rhone valley, showing a lateral cone.] almost all river valleys contain, or have contained, in their course one or more lakes, and where a river falls into a lake a cone like those just described is formed, and projects into the lake. thus on the lake of geneva, between vevey and villeneuve (see fig. ), there are several such promontories, each marking the place where a stream falls into the lake. [illustration: fig. .--view in the rhone valley, showing the slope of a river cone.] the rhone itself has not only filled up what was once the upper end of the lake, but has built out a strip of land into the water. [illustration: fig. .--shore of the lake of geneva, near vevey.] that the lake formerly extended some distance up the valais no one can doubt who looks at the flat ground about villeneuve. the plate opposite, from a photograph taken above vevey, shows this clearly. it is quite evident that the lake must formerly have extended further up the valley, and that it has been filled up by material brought down by the rhone, a process which is still continuing. at the other end of the lake the river rushes out feet deep of "not flowing, but flying water; not water neither--melted glacier matter, one should call it; the force of the ice is in it, and the wreathing of the clouds, the gladness of the sky, and the countenance of time."[ ] [illustration: view up the valais from the lake of geneva. _to face page ._] in flat countries the habits of rivers are very different. for instance, in parts of norfolk there are many small lakes or "broads" in a network of rivers--the bure, the yare, the ant, the waveney, etc.--which do not rush on with the haste of some rivers, or the stately flow of others which are steadily set to reach the sea, but rather seem like rivers wandering in the meadows on a holiday. they have often no natural banks, but are bounded by dense growths of tall grasses, bulrushes, reeds, and sedges, interspersed with the spires of the purple loosestrife, willow herb, hemp agrimony, and other flowers, while the fields are very low and protected by dykes, so that the red cattle appear to be browsing below the level of the water; and as the rivers take most unexpected turns, the sailing boats often seem (fig. ) as if they were in the middle of the fields. [illustration: fig. .--view in the district of the broads, norfolk.] at present these rivers are restrained in their courses by banks; when left free they are continually changing their beds. their courses at first sight seem to follow no rule, but, as it is termed, from a celebrated river of asia minor, to "meander" along without aim or object, though in fact they follow very definite laws. finally, when the river at length reaches the sea, it in many cases spreads out in the form of a fan, forming a very flat cone or "delta," as it is called, from the greek capital [greek: delta], a name first applied to that of the nile, and afterwards extended to other rivers. this is due to the same cause, and resembles, except in size, the comparatively minute cones of mountain streams. [illustration: fig. .] fig. represents the delta of the po, and it will be observed that adria, once a great port, and from which the adriatic was named, is now more than miles from the sea. perhaps the most remarkable case is that of the mississippi (fig. ), the mouths of which project into the sea like a hand, or like the petals of a flower. for miles the mud is too soft to support trees, but is covered by sedges (miegea); the banks of mud gradually become too soft and mobile even for them. the pilots who navigate ships up the river live in frail houses resting on planks, and kept in place by anchors. still further, and the banks of the mississippi, if banks they can be called, are mere strips of reddish mud, intersected from time to time by transverse streams of water, which gradually separate them into patches. these become more and more liquid, until the land, river, and sea merge imperceptibly into one another. the river is so muddy that it might almost be called land, and the mud so saturated by water that it might well be called sea, so that one can hardly say whether a given spot is on the continent, in the river, or on the open ocean. [illustration: fig. .] footnotes: [ ] leyden. [ ] dr. hudson, address to the microscopical society, . [ ] f. davors. [ ] ruskin. chapter viii rivers and lakes on the directions of rivers in the last chapter i have alluded to the wanderings of rivers within the limits of their own valleys; we have now to consider the causes which have determined the directions of the valleys themselves. if a tract of country were raised up in the form of a boss or dome, the rain which fell on it would partly sink in, partly run away to the lower ground. the least inequality in the surface would determine the first directions of the streams, which would carry down any loose material, and thus form little channels, which would be gradually deepened and enlarged. it is as difficult for a river as for a man to get out of a groove. in such a case the rivers would tend to radiate with more or less regularity from the centre or axis of the dome, as, for instance, in our english lake district (fig. ). derwent water, thirlmere, coniston water, and windermere, run approximately n. and s.; crummock water, loweswater, and buttermere n.w. by s.e.; waste water, ullswater, and hawes water n.e. by s.w.; while ennerdale water lies nearly e. by w. can we account in any way, and if so how, for these varied directions? the mountains of cumberland and westmoreland form a more or less oval boss, the axis of which, though not straight, runs practically from e.n.e. to w.s.w., say from scaw fell to shap fell; and a sketch map shows us almost at a glance that derwent water, thirlmere, ullswater, coniston water, and windermere run at right angles to this axis; ennerdale water is just where the boss ends and the mountains disappear; while crummock water and waste water lie at the intermediate angles. [illustration: fig. .--map of the lake district.] so much then for the direction. we have still to consider the situation and origin, and it appears that ullswater, coniston water, the river dudden, waste water, and crummock water lie along the lines of old faults, which no doubt in the first instance determined the flow of the water. take another case. in the jura the valleys are obviously (see fig. ) in many cases due to the folding of the strata. it seldom happens, however, that the case is so simple. if the elevation is considerable the strata are often fractured, and fissures are produced. again if the part elevated contains layers of more than one character, this at once establishes differences. take, for instance, the weald of kent (figs. , ). here we have (omitting minor layers) four principal strata concerned, namely, the chalk, greensand, weald clay, and hastings sands. [illustration: fig. .--_a_, _a_, upper cretaceous strata, chiefly chalk, forming the north and south downs; _b_, _b_, escarpment of lower greensand, with a valley between it and the chalk; _c_, _c_, weald clay, forming plains; _d_, hills formed of hastings sand and clay. the chalk, etc., once spread across the country, as shown in the dotted lines.] the axis of elevation runs (fig. ) from winchester by petersfield, horsham, and winchelsea to boulogne, and as shown in the following section, taken from professor ramsay, we have on each side of the axis two ridges or "escarpments," one that of the chalk, the other that of the greensand, while between the chalk and the greensand is a valley, and between the greensand and the ridge of hastings sand an undulating plain, in each case with a gentle slope from about where the london and brighton railway crosses the weald towards the east. under these circumstances we might have expected that the streams draining the weald would have run in the direction of the axis of elevation, and at the bases of the escarpments, as in fact the rother does for part of its course, into the sea between the north and south downs, instead of which as a rule they run north and south, cutting in some cases directly through the escarpments; on the north, for instance, the wye, the mole, the darenth, the medway, and the stour; and on the south the arun, the addur, the ouse, and the cuckmere. [illustration: fig. .--map of the weald of kent.] they do not run in faults or cracks, and it is clear that they could not have excavated their present valleys under circumstances such as now exist. they carry us back indeed to a time when the greensand and chalk were continued across the weald in a great dome, as shown by the dotted lines in fig. . they then ran down the slope of the dome, and as the chalk and greensand gradually weathered back, a process still in operation, the rivers deepened and deepened their valleys, and thus were enabled to keep their original course. other evidence in support of this view is afforded by the presence of gravel beds in some places at the very top of the chalk escarpment--beds which were doubtless deposited when, what is now the summit of a hill, was part of a continuous slope. the course of the thames offers us a somewhat similar instance. it rises on the oolites near cirencester, and cuts through the escarpment of the chalk between wallingford and reading. the cutting through the chalk has evidently been effected by the river itself. but this could not have happened under existing conditions. we must remember, however, that the chalk escarpment is gradually moving eastwards. the chalk escarpments indeed are everywhere, though of course only slowly, crumbling away. between farnham and guildford the chalk is reduced to a narrow ridge known as the hog's back. in the same way no doubt the area of the chalk formerly extended much further west than it does at present, and, indeed, there can be little doubt, somewhat further west than the source of the thames, almost to the valley of the severn. at that time the thames took its origin in a chalk spring. gradually, however, the chalk was worn away by the action of weather, and especially of rain. the river maintained its course while gradually excavating, and sinking deeper and deeper into, the chalk. at present the river meets the chalk escarpment near wallingford, but the escarpment itself is still gradually retreating eastward. so, again, the elbe cuts right across the erz-gebirge, the rhine through the mountains between bingen and coblenz, the potomac, the susquehannah, and the delaware through the alleghanies. the case of the dranse will be alluded to further on (p. ). in these cases the rivers preceded the mountains. indeed as soon as the land rose above the waters, rivers would begin their work, and having done so, unless the rate of elevation of the mountain exceeded the power of erosion of the river, the two would proceed simultaneously, so that the river would not alter its course, but would cut deeper and deeper as the mountain range gradually rose. rivers then are in many cases older than mountains. moreover, the mountains are passive, the rivers active. since it seems to be well established that in switzerland a mass, more than equal to what remains, has been removed; and that many of the present mountains are not sites which were originally raised highest, but those which have suffered least, it follows that if in some cases the course of the river is due to the direction of the mountain ridges, on the other hand the direction of some of the present ridges is due to that of the rivers. at any rate it is certain that of the original surface not a trace or a fragment remains _in situ_. many of our own english mountains were once valleys, and many of our present valleys occupy the sites of former mountain ridges. heim and rütimeyer point out that of the two factors which have produced the relief of mountain regions, the one, elevation, is temporary and transitory; the other, denudation, is constant, and gains therefore finally the upper hand. we must not, however, expect too great regularity. the degree of hardness, the texture, and the composition of the rocks cause great differences. on the other hand, if the alteration of level was too rapid, the result might be greatly to alter the river courses. mr. darwin mentions such a case, which, moreover, is perhaps the more interesting as being evidently very recent. "mr. gill," he says, "mentioned to me a most interesting, and as far as i am aware, quite unparalleled case, of a subterranean disturbance having changed the drainage of a country. travelling from casma to huaraz (not very far distant from lima) he found a plain covered with ruins and marks of ancient cultivation, but now quite barren. near it was the dry course of a considerable river, whence the water for irrigation had formerly been conducted. there was nothing in the appearance of the water-course to indicate that the river had not flowed there a few years previously; in some parts beds of sand and gravel were spread out; in others, the solid rock had been worn into a broad channel, which in one spot was about yards in breadth and feet deep. it is self-evident that a person following up the course of a stream will always ascend at a greater or less inclination. mr. gill therefore, was much astonished when walking up the bed of this ancient river, to find himself suddenly going downhill. he imagined that the downward slope had a fall of about or feet perpendicular. we here have unequivocal evidence that a ridge had been uplifted right across the old bed of a stream. from the moment the river course was thus arched, the water must necessarily have been thrown back, and a new channel formed. from that moment also the neighbouring plain must have lost its fertilising stream, and become a desert."[ ] the strata, moreover, often--indeed generally, as we have seen, for instance, in the case of switzerland--bear evidence of most violent contortions, and even where the convulsions were less extreme, the valleys thus resulting are sometimes complicated by the existence of older valleys formed under previous conditions. in the alps then the present configuration of the surface is mainly the result of denudation. if we look at a map of switzerland we can trace but little relation between the river courses and the mountain chains. [illustration: fig. .--sketch map of the swiss rivers.] the rivers, as a rule (fig. ), run either s.e. by n.w., or, at right angles to this, n.e. and s.w. the alps themselves follow a somewhat curved line from the maritime alps, commencing with the islands of hyères, by briancon, martigny, the valais, urseren thal, vorder rhein, innsbruck, radstadt, and rottenmann to the danube, a little below vienna,--at first nearly north and south, but gradually curving round until it becomes s.w. by n.e. the central mountains are mainly composed of gneiss, granite, and crystalline schists: the line of junction between these rocks and the secondary and tertiary strata on the north, runs, speaking roughly, from hyères to grenoble, and then by albertville, sion, chur, inns, bruck, radstadt, and hieflau, towards vienna. it is followed (in some part of their course) by the isère, the rhone, the rhine, the inn, and the enns. one of the great folds shortly described in the preceding chapter runs up the isère, along the chamouni valley, up the rhone, through the urseren thal, down the rhine valley to chur, along the inn nearly to kufstein, and for some distance along the enns. thus, then, five great rivers have taken advantage of this main fold, each of them eventually breaking through into a transverse valley. the pusterthal in the tyrol offers us an interesting case of what is obviously a single valley, which has, however, been slightly raised in the centre, near toblach, so that from this point the water flows in opposite directions--the drau eastward, and the rienz westward. in this case the elevation is single and slight: in the main valley there are several, and they are much loftier, still we may, i think, regard that of the isère from chambery to albertville, of the rhone from martigny to its source, of the urseren thal, of the vorder rhine from its source to chur, of the inn from landeck to below innsbruck, even perhaps of the enns from radstadt to hieflau as in one sense a single valley, due to one of these longitudinal folds, but interrupted by bosses of gneiss and granite,--one culminating in mont blanc, and another in the st. gotthard,--which have separated the waters of the isère, the rhone, the vorder rhine, the inn, and the enns. that the valley of chamouni, the valais, the urseren thal, and that of the vorder rhine really form part of one great fold is further shown by the presence of a belt of jurassic strata nipped in, as it were, between the crystalline rocks. this seems to throw light on the remarkable turns taken by the rhone at martigny and the vorder rhine at chur, where they respectively quit the great longitudinal fold, and fall into secondary transverse valleys. the rhone for the upper part of its course, as far as martigny, runs in the great longitudinal fold of the valais; at martigny it falls into and adopts the transverse valley, which properly belongs to the dranse; for the dranse is probably an older river and ran in the present course even before the great fold of the valais. this would seem to indicate that the oberland range is not so old as the pennine, and that its elevation was so gradual that the dranse was able to wear away a passage as the ridge gradually rose. after leaving the lake of geneva the rhone follows a course curving gradually to the south, until it reaches st. genix, where it falls into and adopts a transverse valley which properly belongs to the little river guiers; it subsequently joins the ain and finally falls into the saône. if these valleys were attributed to their older occupiers we should therefore confine the name of the rhone to the portion of its course from the rhone glacier to martigny. from martigny it occupies successively the valleys of the dranse, guiers, ain, and saône. in fact, the saône receives the ain, the ain the guiers, the guiers the dranse, and the dranse the rhone. this is not a mere question of names, but also one of antiquity. the saône, for instance, flowed past lyons to the mediterranean for ages before it was joined by the rhone. in our nomenclature, however, the rhone has swallowed up the others. this is the more curious because of the three great rivers which unite to form the lower rhone, namely, the saône, the doubs, and the rhone itself, the saône brings for a large part of the year the greatest volume of water, and the doubs has the longest course. other similar cases might be mentioned. the aar, for instance, is a somewhat larger river than the rhine. [illustration: fig. .--diagram in illustration of mountain structure.] but why should the rivers, after running for a certain distance in the direction of the main axis, so often break away into lateral valleys? if the elevation of a chain of mountains be due to the causes suggested in p. , it is evident, though, so far as i am aware, stress has not hitherto been laid upon this, that the compression and consequent folding of the strata (fig. ) would not be in the direction _a b_ only, but also at right angles to it, in the direction _a c_, though the amount of folding might be much greater in one direction than in the other. thus in the case of switzerland, while the main folds run south-west by north-east, there would be others at right angles to the main axis. the complex structure of the swiss mountains may be partly due to the coexistence of these two directions of pressure at right angles to one another. the presence of a fold so originating would often divert the river to a course more or less nearly at right angles to its original direction. switzerland, moreover, slopes northwards from the alps, so that the lowest part of the great swiss plain is that along the foot of the jura. hence the main drainage runs along the line from yverdun to neuchâtel, down the zihl to soleure, and then along the aar to waldshut: the upper aar, the emmen, the wiggern, the suhr, the wynen, the lower reuss, the sihl, and the limmat, besides several smaller streams, running approximately parallel to one another north-north-east, and at angles to the main axis of elevation, and all joining the aar from the south, while on the north it does not receive a single contributary of any importance. on the south side of the alps again we have the dora baltea, the sesia, the ticino, the olonna, the adda, the adige, etc., all running south-south-east from the axis of elevation to the po. [illustration: fig. .] indeed, the general slope of switzerland, being from the ridge of the alps towards the north, it will be observed (fig. ) that almost all the large affluents of these rivers running in longitudinal valleys fall in on the south, as, for instance, those of the isère from albertville to grenoble, of the rhone from its source to martigny, of the vorder rhine from its source to chur, of the inn from landeck to kufstein, of the enns from its source to near admont, of the danube from its source to vienna, and as just mentioned, of the aar from bern to waldshut. hence also, whenever the swiss rivers running east and west break into a transverse valley, as the larger ones all do, and some more than once, they invariably, whether originally running east or westwards, turn towards the north. but although we thus get a clue to the general structure of switzerland, the whole question is extremely complex, and the strata have been crumpled and folded in the most complicated manner, sometimes completely reversed, so that older rocks have been folded back on younger strata, and even in some cases these folds again refolded. moreover, the denudation by aerial action, by glaciers, frosts, and rivers has removed hundreds, or rather thousands, of feet of strata. in fact, the mountain tops are not by any means the spots which have been most elevated, but those which have been least denuded; and hence it is that so many of the peaks stand at about the same altitude. the conflicts and adventures of rivers our ancestors looked upon rivers as being in some sense alive, and in fact in their "struggle for existence" they not only labour to adapt their channel to their own requirements, but in many cases enter into conflict with one another. in the plain of bengal, for instance, there are three great rivers, the brahmapootra coming from the north, the ganges from the west, and the megna from the east, each of them with a number of tributary streams. mr. fergusson[ ] has given us a most interesting and entertaining account of the struggles between these great rivers to occupy the fertile plain of bengal. the megna, though much inferior in size to the brahmapootra, has one great advantage. it depends mainly on the monsoon rains for its supply, while the brahmapootra not only has a longer course to run, but relies for its floods, to a great extent, on the melting of the snow, so that, arriving later at the scene of the struggle, it finds the country already occupied by the megna to such an extent that it has been driven nearly miles northwards, and forced to find a new channel. under these circumstances it has attacked the territory of the ganges, and being in flood earlier than that river, though later than the megna, it has in its turn a great advantage. whatever the ultimate result may be the struggle continues vigorously. at sooksaghur, says fergusson, "there was a noble country house, built by warren hastings, about a mile from the banks of the hoogly. when i first knew it in half the avenue of noble trees, which led from the river to the house, was gone; when i last saw it, some eight years afterwards, the river was close at hand. since then house, stables, garden, and village are all gone, and the river was on the point of breaking through the narrow neck of high land that remained, and pouring itself into some weak-banded nullahs in the lowlands beyond: and if it had succeeded, the hoogly would have deserted calcutta. at this juncture the eastern bengal railway company intervened. they were carrying their works along the ridge, and they have, for the moment at least, stopped the oscillation in this direction." this has affected many of the other tributaries of the ganges, so that the survey made by rennell in - is no longer any evidence as to the present course of the rivers. they may now be anywhere else; in some cases all we can say is that they are certainly not now where they were then. the association of the three great european rivers, the rhine, the rhone, and the danube, with the past history of our race, invests them with a singular fascination, and their past history is one of much interest. they all three rise in the group of mountains between the galenstock and the bernardino, within a space of a few miles; on the east the waters run into the black sea, on the north into the german ocean, and on the west into the mediterranean. but it has not always been so. their head-waters have been at one time interwoven together. at present the waters of the valais escape from the lake of geneva at the western end, and through the remarkable defile of fort de l'ecluse and malpertius, which has a depth of feet, and is at one place not more than feet across. moreover, at various points round the lake of geneva, remains of lake terraces show that the water once stood at a level much higher than the present. one of these is rather more than feet[ ] above the lake. a glance at the map will show that between lausanne and yverdun there is a low tract of land, and the venoge, which falls into the lake of geneva between lausanne and morges, runs within about half a mile of the nozon, which falls into the lake of neuchâtel at yverdun, the two being connected by the canal d'entreroches, and the height of the watershed being only metres ( feet), corresponding with the above mentioned lake terrace. it is evident, therefore, that when the lake of geneva stood at the level of the feet terrace the waters ran out, not as now at geneva and by lyons to the mediterranean, but near lausanne by cissonay and entreroches to yverdun, and through the lake of neuchâtel into the aar and the rhine. but this is not the whole of the curious history. at present the aar makes a sharp turn to the west at waldshut, where it falls into the rhine, but there is reason to believe that at a former period, before the rhine had excavated its present bed, the aar continued its course eastward to the lake of constance, by the valley of the klettgau, as is indicated by the presence of gravel beds containing pebbles which have been brought, not by the rhine from the grisons, but by the aar from the bernese oberland, showing that the river which occupied the valley was not the rhine but the aar. it would seem also that at an early period the lake of constance stood at a considerably higher level, and that the outlet was, perhaps, from frederichshaven to ulm, along what are now the valleys of the schussen and the ried, into the danube. thus the head-waters of the rhone appear to have originally run by lausanne and the lake of constance into the danube, and so to the black sea. then, after the present valley was opened between waldshut and basle, they flowed by basle and the present rhine, and after joining the thames, over the plain which now forms the german sea into the arctic ocean between scotland and norway. finally, after the opening of the passage at fort de l'ecluse, by geneva, lyons, and the valley of the saône, to the mediterranean. it must not, however, be supposed that these changes in river courses are confined to the lower districts. mountain streams have also their adventures and vicissitudes, their wars and invasions. take for instance the upper rhine, of which we have a very interesting account by heim. it is formed of three main branches, the vorder rhine, hinter rhine, and the albula. the two latter, after meeting near thusis, unite with the vorder rhine at reichenau, and run by chur, mayenfeld, and sargans into the lake of constance at rheineck. at some former period, however, the drainage of this district was very different, as is shown in fig. . the vorder and hinter rhine united then (fig. ) as they do now at reichenau, but at a much higher level, and ran to mayenfeld, not by chur, but by the kunckel pass to sargans, and so on, not to the lake of constance, but to that of zurich. the landwasser at that time rose in the schlappina joch, and after receiving as tributaries the vereina and the sardasca, joined the albula, as it does now at tiefenkasten; but instead of going round to meet the hinter rhine near thusis, the two together travelled parallel with, but at some distance from, the hinter rhine, by heide to chur, and so to mayenfeld. in the meanwhile, however, the landquart was stealthily creeping up the valley, attacked the ridge which then united the casanna and the madrishorn, and gradually forcing the passage, invaded (fig. ) the valleys of the schlappina, vereina, and sardasca, absorbed them as tributaries, and, detaching them from their allegiance to the landwasser, annexed the whole of the upper province which had formerly belonged to that river. [illustration: fig. .--river system round chur, as it used to be.] the schyn also gradually worked its way upwards from thusis till it succeeded in sapping the albula, and carried it down the valley to join the vorder rhine near thusis. in what is now the main valley of the rhine above chur another stream ate its way back, and eventually tapped the main river at reichenau, thus diverting it from the kunckel, and carrying it round by chur. [illustration: fig. .--river system round chur, as it is.] at sargans a somewhat similar process was repeated, with the addition that the material brought down by the weisstannen, or perhaps a rockfall, deflected the rhine, just as we see in fig. that the rhone was pushed on one side by the borgne. the rhone, however, had no choice, it was obliged to force, and has forced its way over the cone deposited by the borgne. the rhine, on the contrary, had the option of running down by vaduz to rheinach, and has adopted this course. the watershed between it and the weisstannen is, however, only about feet in height, and the people of zurich watch it carefully, lest any slight change should enable the river to return to its old bed. the result of all these changes is that the rivers have changed their courses from those shown in fig. to their present beds as shown in fig. . another interesting case is that of the upper engadine (fig. ), to which attention has been called by bonney and heim. the fall of the val bregaglia is much steeper than that of the inn, and the maira has carried off the head-waters of that river away into italy. the col was formerly perhaps as far south as stampa: the albegna, the upper maira, and the stream from the forgno glacier, originally belonged to the inn, but have been captured by the lower maira. their direction still indicates this; they seem as if they regretted the unwelcome change, and yearned to rejoin their old companions. [illustration: fig. .--river system of the maloya.] moreover, as rivers are continually cutting back their valleys they must of course sometimes meet. in these cases when the valleys are at different levels the lower rivers have drained the upper ones, and left dry, deserted valleys. in other cases, especially in flatter districts, we have bifurcations, as, for instance, at sargans, and several of the italian lakes. every one must have been struck by the peculiar bifurcation of the lakes of como and lugano, while a very slight depression would connect the lake varese with the maggiore, and give it also a double southern end. on lakes the problem of the origin of lakes is by no means identical with that of valleys. the latter are due, primarily as a rule to geological causes, but so far as their present condition is concerned, mainly to the action of rain and rivers. flowing water, however, cannot give rise to lakes. it is of course possible to have valleys without lakes, and in fact the latter are, now at least, exceptional. there can be no lakes if the slope of the valley is uniform. to what then are lakes due? professor ramsay divides lakes into three classes:-- . those due to irregular accumulations of drift, and which are generally quite shallow. . those formed by moraines. . those which occupy true basins scooped by glacier ice out of the solid rock. to these must, however, i think be added at least one other great class and several minor ones, namely,-- . those due to inequalities of elevation or depression. . lakes in craters of extinct volcanoes, for instance, lake avernus. . those caused by subsidence due to the removal of underlying soluble rocks, such as some of the cheshire meres. . loop lakes in deserted river courses, of which there are many along the course of the rhine. . those due to rockfalls, landslips, or lava currents, damming up the course of a river. . those caused by the advance of a glacier across a lateral valley, such as the mergelen see, or the ancient lake whose margins form the celebrated "parallel roads of glen roy." as regards the first class we find here and there on the earth's surface districts sprinkled with innumerable shallow lakes of all sizes, down to mere pools. such, for instance, occur in the district of le doubs between the rhone and the saône, that of la sologne near orleans, in parts of north america, and in finland. such lakes are, as a rule, quite shallow. some geologists, geikie, for instance, ascribe them to the fact of these regions having been covered by sheets of ice which strewed the land with irregular masses of clay, gravel, and sand, lying on a stratum impervious to water, either of hard rock such as granite or gneiss, or of clay, so that the rain cannot percolate through it, and without sufficient inclination to throw it off. . to ramsay's second class of lakes belong those formed by moraines. the materials forming moraines being, however, comparatively loose, are easily cut through by streams. there are in switzerland many cases of valleys crossed by old moraines, but they have generally been long ago worn through by the rivers. . ramsay and tyndall attribute most of the great swiss and italian lakes to the action of glaciers, and regard them as rock basins. it is of course obvious that rivers cannot make basin-shaped hollows surrounded by rock on all sides. the lake of geneva, feet above the sea, is over feet deep; the lake of brienz is feet above the sea, and feet deep, so that its bottom is really below the sea level. the italian lakes are even more remarkable. the lake of como, feet above the sea, is feet deep. lago maggiore, feet above the sea, is no less than feet deep. if the mind is at first staggered at the magnitude of the scale, we must remember that the ice which is supposed to have scooped out the valley in which the lake of geneva now reposes, was once at least feet thick; while the moraines were also of gigantic magnitude, that of ivrea, for instance, being no less than feet above the river, and several miles long. indeed it is obvious that a glacier many hundred, or in some cases several thousand, feet in thickness, must exercise great pressure on the bed over which it travels. we see this from the striæ and grooves on the solid rocks, and the fine mud which is carried down by glacial streams. the deposit of glacial rivers, the "loess" of the rhine itself, is mainly the result of this ice-waste, and that is why it is so fine, so impalpable. that glaciers do deepen their beds seems therefore unquestionable. moreover, though the depth of some of these lakes is great, the true slope is very slight. tyndall and ramsay do not deny that the original direction of valleys, and consequently of lakes, is due to cosmical causes and geological structure, while even those who have most strenuously opposed the theory which attributes lakes to glacial erosion do not altogether deny the action of glaciers. favre himself admits that "it is impossible to deny that valleys, after their formation, have been swept out and perhaps enlarged by rivers and glaciers." even ruskin admits "that a glacier may be considered as a vast instrument of friction, a white sand-paper applied slowly but irresistibly to all the roughness of the hill which it covers." it is obvious that sand-paper applied "irresistibly" and long enough, must gradually wear away and lower the surface. i cannot therefore resist the conclusion that glaciers have taken an important part in the formation of lakes. the question has sometimes been discussed as if the point at issue were whether rivers or glaciers were the most effective as excavators. but this is not so. those who believe that lakes are in many cases due to glaciers might yet admit that rivers have greater power of erosion. there is, however, an essential difference in the mode of action. rivers tend to regularise their beds; they drain, rather than form lakes. their tendency is to cut through any projections so that finally their course assumes some such curve as that below, from the source (_a_) to its entrance into the sea (_b_). [illustration: fig. .--final slope of a river.] glaciers, however, have in addition a scooping power, so that if similarly _a d b_ in fig. represent the course of a glacier, starting at _a_ and gradually thinning out to _e_, it may scoop out the rock to a certain extent at _d_; in that case if it subsequently retires say to _c_, there would be a lake lying in the basin thus formed between _c_ and _e_. [illustration: fig. .] on the other hand i am not disposed to attribute the swiss lakes altogether to the action of glaciers. in the first place it does not seem clear that they occupy true rock basins. on this point more evidence is required. that some lakes are due to unequal changes of level will hardly be denied. no one, for instance, as bonney justly observes,[ ] would attribute the dead sea to glacial erosion. the alps, as we have seen, are a succession of great folds, and there is reason to regard the central one as the oldest. if then the same process continued, and the outer fold was still further raised, or a new one formed, more quickly than the rivers could cut it back, they would be dammed up, and lakes would result. moreover, if the formation of a mountain region be due to subsidence, and consequent crumpling, as indicated on p. , so that the strata which originally occupied the area a b c d are compressed into a' b' c' d', it is evident, as already mentioned, that while the line of least resistance, and, consequently, the principal folds might be in the direction a' b', there must also be a tendency to the formation of similar folds at right angles, or in the direction a' c'. thus, in the case of switzerland, while the main folds run south-west by north-east there would also be others at right angles, though the amount of folding might be much greater in the one direction than in the other. to this cause the bosses, for instance--at martigny, the furca, and the ober alp,--which intersect the great longitudinal valley of switzerland, are perhaps due. the great american lakes also are probably due to differences of elevation. round lake ontario, for instance, there is a raised beach which at the western end of the lake is feet above the sea level, but rises towards the east and north until near fine it reaches an elevation of feet. as this terrace must have been originally horizontal we have here a lake barrier, due to a difference of elevation, amounting to over feet. in the same way we get a clue to the curious cruciform shape of the lake of lucerne as contrasted with the simple outline of such lakes as those of neuchâtel or zurich. that of lucerne is a complex lake. soundings have shown that the bottom of the urner see is quite flat. it is in fact the old bed of the reuss, which originally ran, not as now by lucerne, but by schwytz and through the lake of zug. in the same way the alpnach see is the old bed of the aa, which likewise ran through the lake of zug. the old river terraces of the reuss can be traced in places between brunnen and goldau. now these terraces must have originally sloped from the upper part downwards, from brunnen towards goldau. but at present the slope is the other way, _i.e._ from goldau towards brunnen. from this and other evidence we conclude that in the direction from lucerne towards rapperschwyl there has been an elevation of the land, which has dammed up the valleys and thus turned parts of the aa and the reuss into lakes--the two branches of the lake of lucerne known as the alpnach see and urner see. during the earthquakes of while part of the runn of cutch, square miles in area, sunk several feet, a ridge of land, called by the natives the ulla-bund or "the wall of god," thirty miles long, and in parts sixteen miles wide, was raised across an ancient arm of the indus, and turned it temporarily into a lake. in considering the great italian lakes, which descend far below the sea level, we must remember that the valley of the po is a continuation of the adriatic, now filled up and converted into land, by the materials brought down from the alps. hence we are tempted to ask whether the lakes may not be remains of the ancient sea which once occupied the whole plain. moreover just as the seals of lake baikal in siberia carry us back to the time when that great sheet of fresh water was in connection with the arctic ocean, so there is in the character of the fauna of the italian lakes, and especially the presence of a crab in the lake of garda, some confirmation of such an idea. further evidence, however, is necessary before these interesting questions can be definitely answered. lastly, some lakes and inland seas seem to be due to even greater cosmical causes. thus a line inclined ten degrees to the pole beginning at gibraltar would pass through a great chain of inland waters--the mediterranean, black sea, caspian, aral, baikal, and back again through the great american lakes. but though many causes have contributed to the original formation and direction of valleys, their present condition is mainly due to the action of water. when we contemplate such a valley, for example, as that which is called _par excellence_ the "valais," we can at first hardly bring ourselves to realise this; but we can trace up valleys, from the little water-course made by last night's rains up to the greatest valleys of all. these considerations, however, do not of course apply to such depressions as those of the great oceans. these were probably formed when the surface of the globe began to solidify, and, though with many modifications, have maintained their main features ever since. on the configuration of valleys the conditions thus briefly described repeat themselves in river after river, valley after valley, and it adds, i think, very much to the interest with which we regard them if, by studying the general causes to which they are due, we can explain their origin, and thus to some extent understand the story they have to tell us, and the history they record. what, then, has that history been? the same valley may be of a very different character, and due to very different causes, in different parts of its course. some valleys are due to folds (see fig. ) caused by subterranean changes, but by far the greater number are, in their present features, mainly the result of erosion. as soon as any tract of land rose out of the sea, the rain which fell on the surface would trickle downwards in a thousand rills, forming pools here and there (see fig. ), and gradually collecting into larger and larger streams. wherever the slope was sufficient the water would begin cutting into the soil and carrying it off to the sea. this action would be the same in any case, but, of course, would differ in rapidity according to the hardness of the ground. on the other hand, the character of the valley would depend greatly on the character of the strata, being narrow where they were hard and tough; broader, on the contrary, where they were soft, so that they crumbled readily into the stream, or where they were easily split by the weather. gradually the stream would eat into its bed until it reached a certain slope, the steepness of which would depend on the volume of water. the erosive action would then cease, but the weathering of the sides and consequent widening would continue, and the river would wander from one part of its valley to another, spreading the materials and forming a river plain. at length, as the rapidity still further diminished, it would no longer have sufficient power even to carry off the materials brought down. it would form, therefore, a cone or delta, and instead of meandering, would tend to divide into different branches. these three stages, we may call those of-- . deepening and widening; . widening and levelling; . filling up; and every place in the second stage has passed through the first; every one in the third has passed through the second. a velocity of inches per second will lift fine sand, inches will move sand as coarse as linseed, inches will sweep along fine gravel, inches will roll along rounded pebbles an inch diameter, and it requires feet per second at the bottom to sweep along angular stones of the size of an egg. when a river has so adjusted its slope that it neither deepens its bed in the upper portion of its course, nor deposits materials, it is said to have acquired its "regimen," and in such a case if the character of the soil remains the same, the velocity must also be uniform. the enlargement of the bed of a river is not, however, in proportion to the increase of its waters as it approaches the sea. if, therefore, the slope did not diminish, the regimen would be destroyed, and the river would again commence to eat out its bed. hence as rivers enlarge, the slope diminishes, and consequently every river tends to assume some such "regimen" as that shown in fig. . now, suppose that the fall of the river is again increased, either by a fresh elevation, or locally by the removal of a barrier. then once more the river regains its energy. again it cuts into its old bed, deepening the valley, and leaving the old plain as a terrace high above its new course. in many valleys several such terraces may be seen, one above the other. in the case of a river running in a transverse valley, that is to say of a valley lying at right angles to the "strike" or direction of the strata (such, for instance, as the reuss), the water acts more effectively than in longitudinal valleys running along the strike. hence the lateral valleys have been less deeply excavated than that of the reuss itself, and the streams from them enter the main valley by rapids or cascades. again, rivers running in transverse valleys cross rocks which in many cases differ in hardness, and of course they cut down the softer strata more rapidly than the harder ones; each ridge of harder rock will therefore form a dam and give rise to a rapid, or cataract. we often as we ascend a river, after a comparatively flat plain, find ourselves in a narrow defile, down which the water rushes in an impetuous torrent, but at the summit of which, to our surprise, we find another broad flat valley. another lesson which we learn from the study of river valleys, is that, just as geological structure was shown by sir c. lyell to be no evidence of cataclysms, but the result of slow action; so also the excavation of valleys is due mainly to the regular flow of rivers; and floods, though their effects are more sudden and striking, have had, after all, comparatively little part in the result. the mouths of rivers fall into two principal classes. if we look at any map we cannot but be struck by the fact that some rivers terminate in a delta, some in an estuary. the thames, for instance, ends in a noble estuary, to which london owes much of its wealth and power. it is obvious that the thames could not have excavated this estuary while the coast was at its present level. but we know that formerly the land stood higher, that the german ocean was once dry land, and the thames, after joining the rhine, ran northwards, and fell eventually into the arctic ocean. the estuary of the thames, then, dates back to a period when the south-east of england stood at a higher level than the present, and even now the ancient course of the river can be traced by soundings under what is now sea. the sites of present deltas, say of the nile, were also once under water, and have been gradually reclaimed by the deposits of the river. it would indeed be a great mistake to suppose that rivers always tend to deepen their valleys. this is only the case when the slope exceeds a certain angle. when the fall is but slight they tend on the contrary to raise their beds by depositing sand and mud brought down from higher levels. hence in the lower part of their course many of the most celebrated rivers--the nile, the po, the mississippi, the thames, etc.--run upon embankments, partly of their own creation. [illustration: fig. .--diagrammatic section of a valley (exaggerated) _r r_, rocky basis of valley; _a a_, sedimentary strata; _b_, ordinary level of river; _c_, flood level.] the reno, the most dangerous of all the apennine rivers, is in some places as much as feet above the adjoining country. rivers under such conditions, when not interfered with by man, sooner or later break through their banks, and leaving their former bed, take a new course along the lowest part of their valley, which again they gradually raise above the rest. hence, unless they are kept in their own channels by human agency, such rivers are continually changing their course. if we imagine a river running down a regularly inclined plane in a more or less straight line; any inequality or obstruction would produce an oscillation, which when once started would go on increasing until the force of gravity drawing the water in a straight line downwards equals that of the force tending to divert its course. hence the radius of the curves will follow a regular law depending on the volume of water and the angle of inclination of the bed. if the fall is feet per mile and the soil homogeneous, the curves would be so much extended that the course would appear almost straight. with a fall of foot per mile the length of the curve is, according to fergusson, about six times the width of the river, so that a river feet wide would oscillate once in feet. this is an important consideration, and much labour has been lost in trying to prevent rivers from following their natural law of oscillation. but rivers are very true to their own laws, and a change at any part is continued both upwards and downwards, so that a new oscillation in any place cuts its way through the whole plain of the river both above and below. the curves of the mississippi are, for instance, for a considerable part of its course so regular that they are said to have been used by the indians as a measure of distance. if the country is flat a river gradually raises the level on each side, the water which overflows during floods being retarded by reeds, bushes, trees, and a thousand other obstacles, gradually deposits the solid matter which it contains, and thus raising the surface, becomes at length suspended, as it were, above the general level. when this elevation has reached a certain point, the river during some flood bursts its banks, and deserting its old bed takes a new course along the lowest accessible level. this then it gradually fills up, and so on; coming back from time to time if permitted, after a long cycle of years, to its first course. in evidence of the vast quantity of sediment which rivers deposit, i may mention that the river-deposits at calcutta are more than feet in thickness. in addition to temporary "spates," due to heavy rain, most rivers are fuller at one time of year than another, our rivers, for instance, in winter, those of switzerland, from the melting of the snow, in summer. the nile commences to rise towards the beginning of july; from august to october it floods all the low lands, and early in november it sinks again. at its greatest height the volume of water sometimes reaches twenty times that when it is lowest, and yet perhaps not a drop of rain may have fallen. though we now know that this annual variation is due to the melting of the snow and the fall of rain on the high lands of central africa, still when we consider that the phenomenon has been repeated annually for thousands of years it is impossible not to regard it with wonder. in fact egypt itself may be said to be the bed of the nile in flood time. some rivers, on the other hand, offer no such periodical differences. the lower rhone, for instance, below the junction with the saône, is nearly equal all through the year, and yet we know that the upper portion is greatly derived from the melting of the swiss snows. in this case, however, while the rhone itself is on this account highest in summer and lowest in winter, the saône, on the contrary, is swollen by the winter's rain, and falls during the fine weather of summer. hence the two tend to counterbalance one another. periodical differences are of course comparatively easy to deal with. it is very different with floods due to irregular rainfall. here also, however, the mere quantity of rain is by no means the only matter to be considered. for instance a heavy rain in the watershed of the seine, unless very prolonged, causes less difference in the flow of the river, say at paris, than might at first have been expected, because the height of the flood in the nearer affluents has passed down the river before that from the more distant streams has arrived. the highest level is reached when the rain in the districts drained by the various affluents happens to be so timed that the different floods coincide in their arrival at paris. footnotes: [ ] darwin's _voyage of a naturalist_. [ ] _geol. jour._, . [ ] favre, _rech. geol. de la savoie._ [ ] _growth and structure of the alps._ chapter ix the sea there is a pleasure in the pathless woods, there is a rapture on the lonely shore, there is society, where none intrudes, by the deep sea, and music in its roar: i love not man the less, but nature more, from these our interviews, in which i steal from all i may be, or have been before, to mingle with the universe, and feel what i can ne'er express, yet cannot all conceal. roll on, thou deep and dark-blue ocean--roll! byron. [illustration: the land's end. _to face page ._] chapter ix the sea when the glorious summer weather comes, when we feel that by a year's honest work we have fairly won the prize of a good holiday, how we turn instinctively to the sea. we pine for the delicious smell of the sea air, the murmur of the waves, the rushing sound of the pebbles on the sloping shore, the cries of the sea-birds; and long to linger, where the pebble-paven shore, under the quick, faint kisses of the sea, trembles and sparkles as with ecstasy.[ ] how beautiful the sea-coast is! at the foot of a cliff, perhaps of pure white chalk, or rich red sandstone, or stern grey granite, lies the shore of gravel or sand, with a few scattered plants of blue sea holly, or yellow-flowered horned poppies, sea-kale, sea convolvulus, saltwort, artemisia, and sea-grasses; the waves roll leisurely in one by one, and as they reach the beach, each in turn rises up in an arch of clear, cool, transparent, green water, tipped with white or faintly pinkish foam, and breaks lovingly on the sands; while beyond lies the open sea sparkling in the sunshine. ... o pleasant sea earth hath not a plain so boundless or so beautiful as thine.[ ] the sea is indeed at times overpoweringly beautiful. at morning and evening a sheet of living silver or gold, at mid-day deep blue; even too deeply blue; too beautiful; too bright; oh, that the shadow of a cloud might rest somewhere upon the splendour of thy breast in momentary gloom.[ ] there are few prettier sights than the beach at a seaside town on a fine summer's day; the waves sparkling in the sunshine, the water and sky each bluer than the other, while the sea seems as if it had nothing to do but to laugh and play with the children on the sands; the children perseveringly making castles with spades and pails, which the waves then run up to and wash away, over and over and over again, until evening comes and the children go home, when the sea makes everything smooth and ready for the next day's play. many are satisfied to admire the sea from shore, others more ambitious or more free prefer a cruise. they feel with tennyson's voyager: we left behind the painted buoy that tosses at the harbour-mouth; and madly danced our hearts with joy, as fast we fleeted to the south: how fresh was every sight and sound on open main or winding shore! we knew the merry world was round, and we might sail for evermore. many appreciate both. the long roll of the mediterranean on a fine day (and i suppose even more of the atlantic, which i have never enjoyed), far from land in a good ship, and with kind friends, is a joy never to be forgotten. to the gulf stream and the atlantic ocean northern europe owes its mild climate. the same latitudes on the other side of the atlantic are much colder. to find the same average temperature in the united states we must go far to the south. immediately opposite us lies labrador, with an average temperature the same as that of greenland; a coast almost destitute of vegetation, a country of snow and ice, whose principal wealth consists in its furs, and a scattered population, mainly composed of indians and esquimaux. but the atlantic would not alone produce so great an effect. we owe our mild and genial climate mainly to the gulf stream--a river in the ocean, twenty million times as great as the rhone--the greatest, and for us the most important, river in the world, which brings to our shores the sunshine of the west indies. the sea is outside time. a thousand, ten thousand, or a million years ago it must have looked just as it does now, and as it will ages hence. with the land this is not so. the mountains and hills, rivers and valleys, animals and plants are continually changing: but the sea is always the same, steadfast, serene, immovable, the same year after year. directly we see the coast, or even a ship, the case is altered. boats may remain the same for centuries, but ships are continually being changed. the wooden walls of old england are things of the past, and the ironclads of to-day will soon be themselves improved off the face of the ocean. the great characteristic of lakes is peace, that of the sea is energy, somewhat restless, perhaps, but still movement without fatigue. the earth lies quiet like a child asleep, the deep heart of the heaven is calm and still, must thou alone a restless vigil keep, and with thy sobbing all the silence fill.[ ] a lake in a storm rather gives us the impression of a beautiful water spirit tormented by some evil demon; but a storm at sea is one of the grandest manifestations of nature. yet more; the billows and the depths have more; high hearts and brave are gathered to thy breast; they hear not now the booming waters roar, the battle thunders will not break their rest. keep thy red gold and gems, thou stormy grave; give back the true and brave.[ ] the most vivid description of a storm at sea is, i think, the following passage from ruskin's _modern painters_: "few people, comparatively, have ever seen the effect on the sea of a powerful gale continued without intermission for three or four days and nights; and to those who have not, i believe it must be unimaginable, not from the mere force or size of the surge, but from the complete annihilation of the limit between sea and air. the water from its prolonged agitation is beaten, not into mere creaming foam, but into masses of accumulated yeast, which hangs in ropes and wreaths from wave to wave, and, where one curls over to break, form a festoon like a drapery from its edge; these are taken up by the wind, not in dissipating dust, but bodily, in writhing, hanging, coiling masses, which make the air white and thick as with snow, only the flakes are a foot or two long each: the surges themselves are full of foam in their very bodies underneath, making them white all through, as the water is under a great cataract; and their masses, being thus half water and half air, are torn to pieces by the wind whenever they rise, and carried away in roaring smoke, which chokes and strangles like actual water. add to this, that when the air has been exhausted of its moisture by long rain, the spray of the sea is caught by it as described above, and covers its surface not merely with the smoke of finely divided water, but with boiling mist; imagine also the low rain-clouds brought down to the very level of the sea, as i have often seen them, whirling and flying in rags and fragments from wave to wave; and finally, conceive the surges themselves in their utmost pitch of power, velocity, vastness, and madness, lifting themselves in precipices and peaks, furrowed with their whirl of ascent, through all this chaos, and you will understand that there is indeed no distinction left between the sea and air; that no object, nor horizon, nor any landmark or natural evidence of position is left; and the heaven is all spray, and the ocean all cloud, and that you can see no further in any direction than you see through a cataract." sea life the sea teems with life. the great sea serpent is, indeed, as much a myth as the kraken of pontoppidan, but other monsters, scarcely less marvellous, are actual realities. the giant cuttle fish of newfoundland, though the body is comparatively small, may measure feet from the tip of one arm to that of another. the whalebone whale reaches a length of over feet, but is timid and inoffensive. the cachalot or sperm whale, which almost alone among animals roams over the whole ocean, is as large, and much more formidable. it is armed with powerful teeth, and is said to feed mainly on cuttle fish, but sometimes on true fishes, or even seals. when wounded it often attacks boats, and its companions do not hesitate to come to the rescue. in one case, indeed, an american ship was actually attacked, stove in, and sunk by a gigantic male cachalot. the great roqual is still more formidable, and has been said to attain a length of feet, but this is probably an exaggeration. so far as we know, the largest species of all is simmond's whale, which reaches a maximum of to feet. in former times whales were frequent on our coasts, so that, as bishop pontoppidan said, the sea sometimes appeared as if covered with smoking chimneys, but they have been gradually driven further and further north, and are still becoming rarer. as they retreated man followed, and to them we owe much of our progress in geography. is it not, however, worth considering whether they might not also be allowed a "truce of god," whether some part of the ocean might not be allotted to them where they might be allowed to breed in peace? as a mere mercantile arrangement the maritime nations would probably find this very remunerative. the reckless slaughter of whales, sea elephants, seals, and other marine animals is a sad blot, not only on the character, but on the common sense, of man. the monsters of the ocean require large quantities of food, but they are supplied abundantly. scoresby mentions cases in which the sea was for miles tinged of an olive green by a species of medusa. he calculates that in a cubic mile there must have been , , , , , , and though no doubt the living mass did not reach to any great depth, still, as he sailed through water thus discoloured for many miles, the number must have been almost incalculable. this is, moreover, no rare or exceptional case. navigators often sail for leagues through shoals of creatures, which alter the whole colour of the sea, and actually change it, as reclus says, into "une masse animée." still, though the whole ocean teems with life, both animals and plants are most abundant near the coast. air-breathing animals, whether mammals or insects, are naturally not well adapted to live far from dry land. even seals, though some of them make remarkable migrations, remain habitually near the shore. whales alone are specially modified so as to make the wide ocean their home. of birds the greatest wanderer is the albatross, which has such powers of flight that it is said even to sleep on the wing. many pelagic animals--jelly-fishes, molluscs, cuttle-fishes, worms, crustacea, and some true fishes--are remarkable for having become perfectly transparent; their shells, muscles, and even their blood have lost all colour, or even undergone the further modification of having become blue, often with beautiful opalescent reflections. this obviously renders them less visible, and less liable to danger. the sea-shore, wherever a firm hold can be obtained, is covered with sea-weeds, which fall roughly into two main divisions, olive-green and red, the latter colour having a special relation to light. these sea-weeds afford food and shelter to innumerable animals. the clear rocky pools left by the retiring tide are richly clothed with green sea-weeds, while against the sides are tufts of beautiful filmy red algæ, interspersed with sea-anemones,--white, creamy, pink, yellow, purple, with a coronet of blue beads, and of many mixed colours; sponges, corallines, starfish, limpets, barnacles, and other shell-fish; feathery zoophytes and annelides expand their pink or white disks, while here and there a crab scuttles across; little fish or shrimps timidly come out from crevices in the rocks, or from among the fronds of the sea-weeds, or hastily dart from shelter to shelter; each little pool is, in fact, a miniature ocean in itself, and the longer one looks the more and more one will see. the dark green and brown sea-weeds do not live beyond a few--say about --fathoms in depth. below them occur delicate scarlet species, with corallines and a different set of shells, sea-urchins, etc. down to about fathoms the animals and plants are still numerous and varied. but they gradually diminish in numbers, and are replaced by new forms. to appreciate fully the extreme loveliness of marine animals they must be seen alive. "a tuft of sertularia, laden with white, or brilliantly tinted polypites," says hincks, "like blossoms on some tropical tree, is a perfect marvel of beauty. the unfolding of a mass of plumularia, taken from amongst the miscellaneous contents of the dredge, and thrown into a bottle of clear sea-water, is a sight which, once seen, no dredger will forget. a tree of campanularia, when each one of its thousand transparent calycles--itself a study of form--is crowned by a circlet of beaded arms, drooping over its margin like the petals of a flower, offers a rare combination of the elements of beauty. "the rocky wall of some deep tidal pool, thickly studded with the long and slender stems of tubularia, surmounted by the bright rose-coloured heads, is like the gay parterre of a garden. equally beautiful is the dense growth of campanularia, covering (as i have seen it in plymouth sound) large tracts of the rock, its delicate shoots swaying to and fro with each movement of the water, like trees in a storm, or the colony of obelia on the waving frond of the tangle looking almost ethereal in its grace, transparency, and delicacy, as seen against the coarse dark surface that supports it." few things are more beautiful than to look down from a boat into transparent water. at the bottom wave graceful sea-weeds, brown, green, or rose-coloured, and of most varied forms; on them and on the sands or rocks rest starfishes, mollusca, crustaceans, sea-anemones, and innumerable other animals of strange forms and varied colours; in the clear water float or dart about endless creatures; true fishes, many of them brilliantly coloured; cuttle-fishes like bad dreams; lobsters and crabs with graceful, transparent shrimps; worms swimming about like living ribbons, some with thousands of coloured eyes, and medusæ like living glass of the richest and softest hues, or glittering in the sunshine with all the colours of the rainbow. and on calm, cool nights how often have i stood on the deck of a ship watching with wonder and awe the stars overhead, and the sea-fire below, especially in the foaming, silvery wake of the vessel, where often suddenly appear globes of soft and lambent light, given out perhaps from the surface of some large medusa. "a beautiful white cloud of foam," says coleridge, "at momently intervals coursed by the side of the vessel with a roar, and little stars of flame danced and sparkled and went out in it; and every now and then light detachments of this white cloud-like foam darted off from the vessel's side, each with its own small constellation, over the sea, and scoured out of sight like a tartar troop over a wilderness." fish also are sometimes luminous. the sun-fish has been seen to glow like a white-hot cannon-ball, and in one species of shark (squalus fulgens) the whole surface sometimes gives out a greenish lurid light which makes it a most ghastly object, like some great ravenous spectre. the ocean depths the land bears a rich harvest of life, but only at the surface. the ocean, on the contrary, though more richly peopled in its upper layers, which swarm with such innumerable multitudes of living creatures that they are, so to say, almost themselves alive--teems throughout with living beings. the deepest abysses have a fauna of their own, which makes up for the comparative scantiness of its numbers, by the peculiarity and interest of their forms and organisation. the middle waters are the home of various fishes, medusæ, and animalcules, while the upper layers swarm with an inexhaustible variety of living creatures. it used to be supposed that the depths of the ocean were destitute of animal life, but recent researches, and especially those made during our great national expedition in the "challenger," have shown that this is not the case, but that the ocean depths have a wonderful and peculiar life of their own. fish have been dredged up even from a depth of fathoms. the conditions of life in the ocean depths are very peculiar. the light of the sun cannot penetrate beyond about two hundred fathoms; deeper than this complete darkness prevails. hence in many species the eyes have more or less completely disappeared. sir wyville thomson mentions a kind of crab (ethusa granulata), which when living near the surface has well developed eyes; in deeper water, to fathoms, eyestalks are present, but the animal is apparently blind, the eyes themselves being absent; while in specimens from a depth of - fathoms the eyestalks themselves have lost their special character, and have become fixed, their terminations being combined into a strong, pointed beak. in other deep sea creatures, on the contrary, the eyes gradually become more and more developed, so that while in some species the eyes gradually dwindle, in others they become unusually large. many of the latter species may be said to be a light to themselves, being provided with a larger or smaller number of curious luminous organs. the deep sea fish are either silvery, pink, or in many cases black, sometimes relieved with scarlet, and when the luminous organs flash out must present a very remarkable appearance. we have still much to learn as to the structure and functions of these organs, but there are cases in which their use can be surmised with some probability. the light is evidently under the will of the fish.[ ] it is easy to imagine a photichthys (light fish) swimming in the black depths of the ocean, suddenly flashing out light from its luminous organs, and thus bringing into view any prey which may be near; while, if danger is disclosed, the light is again at once extinguished. it may be observed that the largest of these organs is in this species situated just under the eye, so that the fish is actually provided with a bull's eye lantern. in other cases the light may rather serve as a defence, some having, as, for instance, in the genus scopelus, a pair of large ones in the tail, so that "a strong ray of light shot forth from the stern-chaser may dazzle and frighten an enemy." in other cases they appear to serve as lures. the "sea-devil" or "angler" of our coasts has on its head three long, very flexible, reddish filaments, while all round its head are fringed appendages, closely resembling fronds of sea-weed. the fish conceals itself at the bottom, in the sand or among sea-weed, and dangles the long filaments in front of its mouth. little fishes, taking these filaments for worms, unsuspectingly approach, and thus fall victims. several species of the same family live at great depths, and have very similar habits. a mere red filament would be invisible in the dark and therefore useless. they have, however, developed a luminous organ, a living "glow-lamp," at the end of the filament, which doubtless proves a very effective lure. in the great depths, however, fish are comparatively rare. nor are molluscs much more abundant. sea-urchins, sea slugs, and starfish are more numerous, and on one occasion , specimens of an echinus were brought up at a single haul. true corals are rare, nor are hydrozoa frequent, though a giant species, allied to the little hydra of our ponds but upwards of feet in height, has more than once been met with. sponges are numerous, and often very beautiful. the now well known euplectella, "venus's flower-basket," resembles an exquisitely delicate fabric woven in spun silk; it is in the form of a gracefully curved tube, expanding slightly upwards and ending in an elegant frill. the wall is formed of parallel bands of glassy siliceous fibres, crossed by others at right angles, so as to form a square meshed net. these sponges are anchored on the fine ooze by wisps of glassy filaments, which often attain a considerable length. many of these beautiful organisms, moreover, glow when alive with a soft diffused light, flickering and sparkling at every touch. what would one not give to be able to wander a while in these wonderful regions! it is curious that no plants, so far as we know, grow in the depths of the ocean, or, indeed, as far as our present information goes, at a greater depth than about fathoms. as regards the nature of the bottom itself, it is in the neighbourhood of land mainly composed of materials, brought down by rivers or washed from the shore, coarser near the coast, and tending to become finer and finer as the distance increases and the water deepens. the bed of the atlantic from to fathoms is covered with an ooze, or very fine chalky deposit, consisting to a great extent of minute and more or less broken shells, especially those of globigerina. at still greater depths the carbonate of lime gradually disappears, and the bottom consists of fine red clay, with numerous minute particles, some of volcanic, some of meteoric, origin, fragments of shooting stars, over , , of which are said to strike the surface of our earth every year. how slow the process of deposition must be, may be inferred from the fact that the trawl sometimes brings up many teeth of sharks and ear-bones of whales (in one case no less than teeth and ear-bones), often semi-fossil, and which from their great density had remained intact for ages, long after all the softer parts had perished and disappeared. the greatest depth of the ocean appears to coincide roughly with the greatest height of the mountains. there are indeed cases recorded in which it is said that "no bottom" was found even at , feet. it is, however, by no means easy to sound at such great depths, and it is now generally considered that these earlier observations are untrustworthy. the greatest depth known in the atlantic is fathoms--a little to the north of the virgin islands, but the soundings as yet made in the deeper parts of the ocean are few in number, and it is not to be supposed that the greatest depth has yet been ascertained. coral islands in many parts of the world the geography itself has been modified by the enormous development of animal life. most islands fall into one of three principal categories: firstly, those which are in reality a part of the continent near which they lie, being connected by comparatively shallow water, and standing to the continent somewhat in the relation of planets to the sun; as, for instance, the cape de verde islands to africa, ceylon to india, or tasmania to australia. secondly, volcanic islands; and thirdly, those which owe their origin to the growth of coral reefs. [illustration: fig. .--whitsunday island.] coral islands are especially numerous in the indian and pacific oceans, where there are innumerable islets, in the form of rings, or which together form rings, the rings themselves being sometimes made up of ringlets. these "atolls" contain a circular basin of yellowish green, clear, shallow water, while outside is the dark blue deep water of the ocean. the islands themselves are quite low, with a beach of white sand rising but a few feet above the level of the water, and bear generally groups of tufted cocoa palms. it used to be supposed that these were the summits of submarine volcanoes on which the coral had grown. but as the reef-making coral does not live at greater depths than about twenty-five fathoms, the immense number of these reefs formed an almost insuperable objection to this theory. the laccadives and maldives for instance--meaning literally the "lac of or , islands," and the "thousand islands"--are a series of such atolls, and it was impossible to imagine so great a number of craters, all so nearly of the same altitude. in shallow tracts of sea, coral reefs no doubt tend to assume the well-known circular form, but the difficulty was to account for the numerous atolls which rise to the surface from the abysses of the ocean, while the coral-forming zoophytes can only live near the surface. darwin showed that so far from the ring of corals resting on a corresponding ridge of rocks, the lagoons, on the contrary, now occupy the place which was once the highest land. he pointed out that some lagoons, as for instance that of vanikoro, contain an island in the middle; while other islands, such as tahiti, are surrounded by a margin of smooth water separated from the ocean by a coral reef. now if we suppose that tahiti were to sink slowly it would gradually approximate to the condition of vanikoro; and if vanikoro gradually sank, the central island would disappear, while on the contrary the growth of the coral might neutralise the subsidence of the reef, so that we should have simply an atoll with its lagoon. the same considerations explain the origin of the "barrier reefs," such as that which runs for nearly a thousand miles, along the north-east coast of australia. thus darwin's theory explains the form and the approximate identity of altitude of these coral islands. but it does more than this, because it shows that there are great areas in process of subsidence, which though slow, is of great importance in physical geography. the lagoon islands have received much attention; which "is not surprising, for every one must be struck with astonishment, when he first beholds one of these vast rings of coral-rock, often many leagues in diameter, here and there surmounted by a low verdant island with dazzling white shores, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection is generally of a bright but pale green colour. the naturalist will feel this astonishment more deeply after having examined the soft and almost gelatinous bodies of these apparently insignificant coral-polypifers, and when he knows that the solid reef increases only on the outer edge, which day and night is lashed by the breakers of an ocean never at rest. well did françois pyrard de laval, in the year exclaim, 'c'est une merveille de voir chacun de ces atollons, environné d'un grand banc de pierre tout autour, n'y ayant point d'artifice humain.'"[ ] of the enchanting beauty of the coral beds themselves we are assured that language conveys no adequate idea. "there were corals," says prof. ball, "which, in their living state, are of many shades of fawn, buff, pink, and blue, while some were tipped with a magenta-like bloom. sponges which looked as hard as stone spread over wide areas, while sprays of coralline added their graceful forms to the picture. through the vistas so formed, golden-banded and metallic-blue fish meandered, while on the patches of sand here and there holothurias and various mollusca and crustaceans might be seen slowly crawling." abercromby also gives a very graphic description of a coral reef. "as we approached," he says, "the roaring surf on the outside, fingery lumps of beautiful live coral began to appear of the palest lavender-blue colour; and when at last we were almost within the spray, the whole floor was one mass of living branches of coral. "but it is only when venturing as far as is prudent into the water, over the outward edge of the great sea wall, that the true character of the reef and all the beauties of the ocean can be really seen. after walking over a flat uninteresting tract of nearly bare rock, you look down and see a steep irregular wall, expanding deeper into the ocean than the eye can follow, and broken into lovely grottoes and holes and canals, through which small resplendent fish of the brightest blue or gold flit fitfully between the lumps of coral. the sides of these natural grottoes are entirely covered with endless forms of tender-coloured coral, but all beautiful, and all more or less of the fingery or branching species, known as madrepores. it is really impossible to draw or describe the sight, which must be taken with all its surroundings as adjuncts."[ ] the vegetation of these fairy lands is also very lovely; the coral tree (erythrina) with light green leaves and bunches of scarlet blossoms, the cocoa-nut always beautiful, the breadfruit, the graceful tree ferns, the barringtonia, with large pink and white flowers, several species of convolvulus, and many others unknown to us even by name. the southern skies in considering these exquisite scenes, the beauty of the southern skies must not be omitted. "from the time we entered the torrid zone," says humboldt, "we were never wearied with admiring, every night, the beauty of the southern sky, which, as we advanced towards the south, opened new constellations to our view. we feel an indescribable sensation, when, on approaching the equator, and particularly on passing from one hemisphere to the other, we see those stars which we have contemplated from our infancy, progressively sink, and finally disappear. nothing awakens in the traveller a livelier remembrance of the immense distance by which he is separated from his country, than the aspect of an unknown firmament. the grouping of the stars of the first magnitude, some scattered nebulæ rivalling in splendour the milky way, and tracts of space remarkable for their extreme blackness, give a particular physiognomy to the southern sky. this sight fills with admiration even those, who, uninstructed in the branches of accurate science, feel the same emotions of delight in the contemplation of the heavenly vault, as in the view of a beautiful landscape, or a majestic river. a traveller has no need of being a botanist to recognise the torrid zone on the mere aspect of its vegetation; and, without having acquired any notion of astronomy, he feels he is not in europe, when he sees the immense constellation of the ship, or the phosphorescent clouds of magellan, arise on the horizon. the heaven and the earth, in the equinoctial regions, assume an exotic character." "the sunsets in the eastern archipelago," says h. o. forbes,[ ] "were scenes to be remembered for a lifetime. the tall cones of sibissie and krakatoa rose dark purple out of an unruffled golden sea, which stretched away to the south-west, where the sun went down; over the horizon gray fleecy clouds lay in banks and streaks, above them pale blue lanes of sky, alternating with orange bands, which higher up gave place to an expanse of red stretching round the whole heavens. gradually as the sun retreated deeper and deeper, the sky became a marvellous golden curtain, in front of which the gray clouds coiled themselves into weird forms before dissolving into space...." the poles the arctic and antarctic regions have always exercised a peculiar fascination over the human mind. until now every attempt to reach the north pole has failed, and the south has proved even more inaccessible. in the north, parry all but reached lat. ; in the south no one has penetrated beyond lat. . . and yet, while no one can say what there may be round the north pole, and some still imagine that open water might be found there, we can picture to ourselves the extreme south with somewhat more confidence. whenever ships have sailed southwards, except at a few places where land has been met with, they have come at last to a wall of ice, from fifty to four hundred feet high. in those regions it snows, if not incessantly, at least very frequently, and the snow melts but little. as far as the eye can reach nothing is to be seen but snow. now this snow must gradually accumulate, and solidify into ice, until it attains such a slope that it will move forward as a glacier. the enormous icebergs of the southern ocean, moreover, show that it does so, and that the snow of the extreme south, after condensing into ice, moves slowly outward and at length forms a wall of ice, from which icebergs, from time to time, break away. we do not exactly know what, under such circumstances, the slope would be; but mr. croll points out that if we take it at only half a degree, and this seems quite a minimum, the ice cap at the south pole must be no less than twelve miles in thickness. it is indeed probably even more, for some of the southern tabular icebergs attain a height of eight hundred, or even a thousand feet above water, indicating a total thickness of the ice sheet even at the edge, of over a mile. sir james ross mentions that--"whilst measuring some angles for the survey near mount lubbock an island suddenly appeared, which he was quite sure was not to be seen two or three hours previously. he was much astonished, but it eventually turned out to be a large iceberg, which had turned over, and so exposed a new surface covered with earth and stones." the condition of the arctic regions is quite different. there is much more land, and no such enormous solid cap of ice. spitzbergen, the land of "pointed mountains," is said to be very beautiful. lord dufferin describes his first view of it as "a forest of thin lilac peaks, so faint, so pale, that had it not been for the gem-like distinctness of their outline one could have deemed them as unsubstantial as the spires of fairyland." it is, however, very desolate; scarcely any vegetation excepting a dark moss, and even this goes but a little way up the mountain side. scoresby ascended one of the hills near horn sound, and describes the view as "most extensive and grand. a fine sheltered bay was seen to the east of us, an arm of the same on the north-east, and the sea, whose glassy surface was unruffled by a breeze, formed an immense expanse on the west; the glaciers, rearing their proud crests almost to the tops of mountains between which they were lodged, and defying the power of the solar beams, were scattered in various directions about the sea-coast and in the adjoining bays. beds of snow and ice filling extensive hollows, and giving an enamelled coat to adjoining valleys, one of which, commencing at the foot of the mountain where we stood, extended in a continual line towards the north, as far as the eye could reach--mountain rising above mountain, until by distance they dwindled into insignificance, the whole contrasted by a cloudless canopy of deepest azure, and enlightened by the rays of a blazing sun, and the effect, aided by a feeling of danger, seated as we were on the pinnacle of a rock almost surrounded by tremendous precipices--all united to constitute a picture singularly sublime." one of the glaciers of spitzbergen is miles in breadth when it reaches the sea-coast, the highest part of the precipitous front adjoining the sea being over feet, and it extends far upwards towards the summit of the mountain. the surface forms an inclined plane of smooth unsullied snow, the beauty and brightness of which render it a conspicuous landmark on that inhospitable shore. from the perpendicular face great masses of ice from time to time break away, whose blocks of sapphire seem to mortal eye hewn from cærulean quarries of the sky.[ ] field ice is comparatively flat, though it may be piled up perhaps as much as feet. it is from glaciers that true icebergs, the beauty and brilliance of which arctic travellers are never tired of describing, take their origin. the attempts to reach the north pole have cost many valuable lives; willoughby and hudson, behring and franklin, and many other brave mariners; but yet there are few expeditions more popular than those to "the arctic," and we cannot but hope that it is still reserved for the british navy after so many gallant attempts at length to reach the north pole. footnotes: [ ] shelley. [ ] campbell. [ ] holmes. [ ] bell. [ ] hemans. [ ] gunther, _history of fishes_. [ ] darwin, _coral reefs_. [ ] abercromby, _seas and skies in many latitudes_. [ ] _a naturalist's wanderings in the eastern archipelago._ [ ] montgomery. chapter x the starry heavens a man can hardly lift up his eyes towards the heavens without wonder and veneration, to see so many millions of radiant lights, and to observe their courses and revolutions, even without any respect to the common good of the universe.--seneca. chapter x the starry heavens many years ago i paid a visit to naples, and ascended vesuvius to see the sun rise from the top of the mountain. we went up to the observatory in the evening and spent the night outside. the sky was clear; at our feet was the sea, and round the bay the lights of naples formed a lovely semicircle. far more beautiful, however, were the moon and the stars overhead; the moon throwing a silver path over the water, and the stars shining in that clear atmosphere with a brilliance which i shall never forget. for ages and ages past men have admired the same glorious spectacle, and yet neither the imagination of man nor the genius of poetry had risen to the truer and grander conceptions of the heavens for which we are indebted to astronomical science. the mechanical contrivances by which it was attempted to explain the movements of the heavenly bodies were clumsy and prosaic when compared with the great discovery of newton. ruskin is unjust i think when he says "science teaches us that the clouds are a sleety mist; art, that they are a golden throne." i should be the last to disparage the debt we owe to art, but for our knowledge, and even more, for our appreciation, feeble as even yet it is, of the overwhelming grandeur of the heavens, we are mainly indebted to science. there is scarcely a form which the fancy of man has not sometimes detected in the clouds,--chains of mountains, splendid cities, storms at sea, flights of birds, groups of animals, monsters of all kinds,--and our superstitious ancestors often terrified themselves by fantastic visions of arms and warriors and battles which they regarded as portents of coming calamities. there is hardly a day on which clouds do not delight and surprise us by their forms and colours. they belong, however, to our earth, and i must now pass on to the heavenly bodies. [illustration: the moon. _to face page ._] the moon the moon is the nearest, and being the nearest, appears to us, with the single exception of the sun, the largest, although it is in reality one of the smallest, of the heavenly bodies. just as the earth goes round the sun, and the period of revolution constitutes a year, so the moon goes round the earth approximately in a period of one month. but while we turn on our axis every twenty-four hours, thus causing the alternation of light and darkness--day and night--the moon takes a month to revolve on hers, so that she always presents the same, or very nearly the same, surface to us. seeing her as we do, not like the sun and stars, by light of her own, but by the reflected light of the sun, her form appears to change, because the side upon which the sun shines is not always that which we see. hence the "phases" of the moon, which add so much to her beauty and interest. who is there who has not watched them with admiration? "we first see her as an exquisite crescent of pale light in the western sky after sunset. night after night she moves further and further to the east, until she becomes full, and rises about the same time that the sun sets. from the time of full moon the disc of light begins to diminish, until the last quarter is reached. then it is that the moon is seen high in the heavens in the morning. as the days pass by, the crescent shape is again assumed. the crescent wanes thinner and thinner as the moon draws closer to the sun. finally, she becomes lost in the overpowering light of the sun, again to emerge as the new moon, and again to go through the same cycle of changes."[ ] but although she is so small the moon is not only, next to the sun, by far the most beautiful, but also for us the most important, of the heavenly bodies. her attraction, aided by that of the sun, causes the tides, which are of such essential service to navigation. they carry our vessels in and out of port, and, indeed, but for them many of our ports would themselves cease to exist, being silted up by the rivers running into them. the moon is also of invaluable service to sailors by enabling them to determine where they are, and guiding them over the pathless waters. the geography of the moon, so far as concerns the side turned towards us, has been carefully mapped and studied, and may almost be said to be as well known as that of our own earth. the scenery is in a high degree weird and rugged; it is a great wilderness of extinct volcanoes, and, seen with even a very moderate telescope, is a most beautiful object. the mountains are of great size. our loftiest mountain, mount everest, is generally stated as about , feet in height. the mountains of the moon reach an altitude of over , , but this reckons to the lowest depression, and it must be remembered that we reckon the height of mountains to the sea level only. several of the craters on the moon have a diameter of or --one of them even as much as --miles. many also have central cones, closely resembling those in our own volcanic regions. in some cases the craters are filled nearly to the brim with lava. the volcanoes seem, however, to be all extinct; and there is not a single case in which we have conclusive evidence of any change in a lunar mountain. [illustration: fig. .--a group of lunar volcanoes.] the moon, being so much smaller than the earth, cooled, of course, much more rapidly, and it is probable that these mountains are millions of years old--much older than many of our mountain chains. yet no one can look at a map of the moon without being struck with the very rugged character of its mountain scenery. this is mainly due to the absence of air and water. to these two mighty agencies, not merely "the cloud-capped towers, the gorgeous palaces, the solemn temples," but the very mountains themselves, are inevitable victims. not merely storms and hurricanes, but every gentle shower, every fall of snow, tends to soften our scenery and lower the mountain peaks. these agencies are absent from the moon, and the mountains stand to-day just as they were formed millions of years ago. but though we find on our own globe (see, for instance, fig. ) volcanic regions closely resembling those of the moon, there are other phenomena on the moon's surface for which our earth presents as yet no explanation. from tycho, for instance, a crater , feet high and miles across, a number of rays or streaks diverge, which for hundreds, or in some cases two or three thousand, miles pass straight across plains, craters, and mountains. the true nature of these streaks is not yet understood. the sun the sun is more than times as distant as the moon; a mighty glowing globe, infinitely hotter than any earthly fiery furnace, , times as heavy, and , , times as large as the earth. its diameter is , miles, and it revolves on its axis in between and days. its distance is , , miles. and yet it is only a star, and by no means one of the first magnitude. the surface of the sun is the seat of violent storms and tempests. from it gigantic flames, consisting mainly of hydrogen, flicker and leap. professor young describes one as being, when first observed, , miles high. suddenly it became very brilliant, and in half an hour sprang up , more. for another hour it soared higher and higher, reaching finally an elevation of no less than , miles, after which it slowly faded away, and in a couple of hours had entirely disappeared. this was no doubt an exceptional case, but a height of , miles is not unusual, and the velocity frequently reaches miles in a second. the proverbial spots on the sun in many respects resemble the appearances which would be presented if a comparatively dark central mass was here and there exposed by apertures through the more brilliant outer gases, but their true nature is still a matter of discussion. during total eclipses it is seen that the sun is surrounded by a "corona," or aureola of light, consisting of radiant filaments, beams, and sheets of light, which radiate in all directions, and the true nature of which is still doubtful. another stupendous problem connected with the sun is the fact that, as geology teaches us, it has given off nearly the same quantity of light and heat for millions of years. how has this come to pass? certainly not by any process of burning such as we are familiar with. indeed, if the heat of the sun were due to combustion it would be burnt up in years. it has been suggested that the meteors, which fall in showers on to the sun, replace the heat which is emitted. to some slight extent perhaps they do so, but the main cause seems to be the slow condensation of the sun itself. mathematicians tell us that a contraction of about feet a year would account for the whole heat emitted, and as the present diameter of the sun is about , miles, the potential store of heat is still enormous. to the sun we owe our light and heat; it is not only the centre of our planetary system, it is the source and ruler of our lives. it draws up water from the ocean, and pours it down in rain to fill the rivers and refresh the plants; it raises the winds, which purify the air and waft our ships over the seas; it draws our carriages and drives our steam-engines, for coal is but the heat of former ages stored up for our use; animals live and move by the sun's warmth; it inspires the song of birds, paints the flowers, and ripens the fruit. through it the trees grow. for the beauties of nature, for our food and drink, for our clothing, for our light and life, for the very possibility of our existence, we are indebted to the sun. what is the sun made of? comte mentioned as a problem, which it was impossible that man could ever solve, any attempt to determine the chemical composition of the heavenly bodies. "nous concevons," he said, "la possibilité de déterminer leurs formes, leurs distances, leurs grandeurs, et leurs mouvements, tandis que nous ne saurions jamais étudier par aucun moyen leur composition chimique ou leur structure minéralogique." to do so might well have seemed hopeless, and yet the possibility has been proved, and a beginning has been made. in the early part of this century wollaston observed that the bright band of colours thrown by a prism, and known as the spectrum, was traversed by dark lines, which were also discovered, and described more in detail, by fraunhofer, after whom they are generally called "fraunhofer's lines." the next step was made by wheatstone, who showed that the spectrum formed by incandescent vapours was formed of bright lines, which differed for each substance, and might, therefore, be used as a convenient mode of analysis. in fact, by this process several new substances have actually been discovered. these bright lines were found on comparison to coincide with the dark lines in the spectrum, and to kirchhoff and bunsen is due the credit of applying this method of research to astronomical science. they arranged their apparatus so that one-half was lighted by the sun, the other by the incandescent gas they were examining. when the vapour of sodium was treated in this way they found that the bright line in the flame of soda exactly coincided with a line in the sun's spectrum. the conclusion was obvious; there is sodium in the sun. it must, indeed, have been a glorious moment when the thought flashed upon them; and the discovery, with its results, is one of the greatest triumphs of human genius. the sun has thus been proved to contain hydrogen, sodium, barium, magnesium, calcium, aluminium, chromium, iron, nickle, manganese, titanium, cobalt, lead, zinc, copper, cadmium, strontium, cerium, uranium, potassium, etc., in all of our terrestrial elements, while as regards some others the evidence is not conclusive. we cannot as yet say that any of our elements are absent, nor though there are various lines which cannot as yet be certainly referred to any known substance, have we clear proof that the sun contains any element which does not exist on our earth. on the whole, then, the chemical composition of the sun appears closely to resemble that of our earth. the planets the syrian shepherds watching their flocks by night long ago noticed--and they were probably not the first--that there were five stars which did not follow the regular course of the rest, but, apparently at least, moved about irregularly. these they appropriately named planets, or wanderers. further observations have shown that this irregularity of their path is only apparent, and that, like our own earth, they really revolve round the sun. to the five first observed--mercury, venus, mars, jupiter, and saturn--two large ones, uranus and neptune, and a group of minor bodies, have since been added. the following two diagrams give the relative orbits of the planets. [illustration: fig. .--orbits of the inner planets.] mercury it is possible, perhaps probable, that there may be an inner planet, but, so far as we know for certain, mercury is the one nearest to the sun, its average distance being , , miles. it is much smaller than the earth, its weight being only about / th of ours. mercury is a shy though beautiful object, for being so near the sun it is not easily visible; it may, however, generally be seen at some time or other during the year as a morning or evening star. [illustration: fig. .--relative distances of the planets from the sun.] venus the true morning or evening star, however, is venus--the peerless and capricious venus. venus, perhaps, "has not been noticed, not been thought of, for many months. it is a beautifully clear evening; the sun has just set. the lover of nature turns to admire the sunset, as every lover of nature will. in the golden glory of the west a beauteous gem is seen to glisten; it is the evening star, the planet venus. a week or two later another beautiful sunset is seen, and now the planet is no longer a glistening point low down; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. again a little longer and venus has gained its full brilliancy and splendour. all the heavenly host--even sirius and jupiter--must pale before the splendid lustre of venus, the unrivalled queen of the firmament."[ ] venus is about as large as our earth, and when at her brightest outshines about fifty times the most brilliant star. yet, like all the other planets, she glows only with the reflected light of the sun, and consequently passes through phases like those of the moon, though we cannot see them with the naked eye. to venus also owe we mainly the power of determining the distance, and consequently the magnitude, of the sun. the earth our own earth has formed the subject of previous chapters. i will now, therefore, only call attention to her movements, in which, of course, though unconsciously, we participate. in the first place, the earth revolves on her axis in hours. her circumference at the tropics is , miles. hence a person at the tropics is moving in this respect at the rate of miles an hour, or over miles a minute. but more than this, astronomers have ascertained that the whole solar system is engaged in a great voyage through space, moving towards a point on the constellation of hercules at the rate of at least , miles an hour, or over miles a minute.[ ] but even more again, we revolve annually round the sun in a mighty orbit , , miles in circumference. in this respect we are moving at the rate of no less than , miles an hour, or miles a minute--a rate far exceeding of course, in fact by some times, that of a cannon ball. how few of us know, how little we any of us realise, that we are rushing through space with such enormous velocity. mars to the naked eye mars appears like a ruddy star of the first magnitude. it has two satellites, which have been happily named phobos and deimos--fear and dismay. it is little more than half as large as the earth, and, though generally far more distant, it sometimes approaches us within , , miles. this has enabled us to study its physical structure. it seems very probable that there is water in mars, and the two poles are tipped with white, as if capped by ice and snow. it presents also a series of remarkable parallel lines, the true nature of which is not yet understood. the minor planets a glance at figs. and will show that the distances of the planets from the sun follow a certain rule. if we take the numbers , , , , , , , each one (after the second) the double of that preceding, and add four, we have the series. now the distances of the planets from the sun are as follow:-- mercury. venus. earth. mars. jupiter. saturn. . . . . . for this sequence, which was first noticed by bode, and is known as bode's law, no explanation can yet be given. it was of course at once observed that between mars and jupiter one place is vacant, and it has now been ascertained that this is occupied by a zone of minor planets, the first of which was discovered by piazzi on january , , a worthy prelude to the succession of scientific discoveries which form the glory of our century. at present over are known, but certainly these are merely the larger among an immense number, some of them doubtless mere dust. jupiter beyond the minor planets we come to the stupendous jupiter, containing times the mass, and being times the size of our earth--larger indeed than all the other planets put together. it is probably not solid, and from its great size still retains a large portion of the original heat, if we may use such an expression. jupiter usually shows a number of belts, supposed to be due to clouds floating over the surface, which have a tendency to arrange themselves in belts or bands, owing to the rotation of the planet. jupiter has four moons or satellites. saturn [illustration: fig. .--saturn.] next to jupiter in size, as in position, comes saturn, which, though far inferior in dimensions, is much superior in beauty. to the naked eye saturn appears as a brilliant star, but when galileo first saw it through a telescope it appeared to him to be composed of three bodies in a line, a central globe with a small one on each side. huyghens in first showed that in reality saturn was surrounded by a series of rings (see fig. ). of these there are three, the inner one very faint, and the outer one divided into two by a dark line. these rings are really enormous shoals of minute bodies revolving round the planet, and rendering it perhaps the most marvellous and beautiful of all the heavenly bodies. while we have one moon, mars two, and jupiter four, saturn has no less than eight satellites. uranus saturn was long supposed to be the outermost body belonging to the solar system. in , however, on the th march, william herschel was examining the stars in the constellation of the twins. one struck him because it presented a distinct disc, while the true fixed stars, however brilliant, are, even with the most powerful telescope, mere points of light. at first he thought it might be a comet, but careful observations showed that it was really a new planet. though thus discovered by herschel it had often been seen before, but its true nature was unsuspected. it has a diameter of about , miles. four satellites of uranus have been discovered, and they present the remarkable peculiarity that while all the other planets and their satellites revolve nearly in one plane, the satellites of uranus are nearly at right angles, indicating the presence of some local and exceptional influence. neptune the study of uranus soon showed that it followed a path which could not be accounted for by the influence of the sun and the other then known planets. it was suspected, therefore, that this was due to some other body not yet discovered. to calculate where such a body must be so as to account for these irregularities was a most complex and difficult, and might have seemed almost a hopeless, task. it was, however, solved almost simultaneously and independently by adams in this country, and le verrier in france. neptune, so far as we yet know the out-most of our companions, is , miles in diameter, and its mean distance from the sun is , , , miles. origin of the planetary system the theory of the origin of the planetary system known as the "nebular hypothesis," which was first suggested by kant, and developed by herschel and laplace, may be fairly said to have attained a high degree of probability. the space now occupied by the solar system is supposed to have been filled by a rotating spheroid of extreme tenuity and enormous heat, due perhaps to the collision of two originally separate bodies. the heat, however, having by degrees radiated into space, the gas cooled and contracted towards a centre, destined to become the sun. through the action of centrifugal force the gaseous matter also flattened itself at the two poles, taking somewhat the form of a disc. for a certain time the tendency to contract, and the centrifugal force, counterbalanced one another, but at length a time came when the latter prevailed and the outer zone detached itself from the rest of the sphere. one after another similar rings were thrown off, and then breaking up, formed the planets and their satellites. that each planet and satellite did form originally a ring we still have evidence in the wonderful and beautiful rings of saturn, which, however, in all probability will eventually form spherical satellites like the rest. thus then our earth was originally a part of the sun, to which again it is destined one day to return. m. plateau has shown experimentally that by rotating a globe of oil in a mixture of water and spirit having the same density this process may be actually repeated in miniature. this brilliant, and yet simple, hypothesis is consistent with, and explains many other circumstances connected with the position, magnitude, and movements of the planets and their satellites. the planets, for instance, lie more or less in the same plane, they revolve round the sun and rotate on their own axis in the same direction--a series of coincidences which cannot be accidental, and for which the theory would account. again the rate of cooling would of course follow the size; a small body cools more rapidly than a large one. the moon is cold and rigid; the earth is solid at the surface, but intensely hot within; jupiter and saturn, which are immensely larger, still retain much of their original heat, and have a much lower density than the earth; and astronomers tell us on other grounds that the sun itself is still contracting, and that to this the maintenance of its temperature is due. although, therefore, the nebular theory cannot be said to have been absolutely proved, it has certainly been brought to a high state of probability, and is, in its main features, generally accepted by astronomers. the question has often been asked whether any of the heavenly bodies are inhabited, and as yet it is impossible to give any certain answer. it seems _à priori_ probable that the millions of suns which we see as stars must have satellites, and that some at least of them may be inhabited. so far as our own system is concerned the sun is of course too hot to serve as a dwelling-place for any beings with bodies such as ours. the same may be said of mercury, which is at times probably ten times as hot as our tropics. the outer planets appear to be still in a state of vapour. the moon has no air or water. mars is in a condition which most nearly resembles ours. all, however, that can be said is that, so far as we can see, the existence of living beings on mars is not impossible. comets the sun, moon, and stars, glorious and wonderful as they are, though regarded with great interest, and in some cases worshipped as deities, excited the imagination of our ancestors less than might have been expected, and even now attract comparatively little attention, from the fact that they are always with us. comets, on the other hand, both as rare and occasional visitors, from their large size and rapid changes, were regarded in ancient times with dread and with amazement. some comets revolve round the sun in ellipses, but many, if not the majority, are visitors indeed, for having once passed round the sun they pass away again into space, never to return. the appearance which is generally regarded as characteristic of a comet is that of a head with a central nucleus and a long tail. many, however, of the smaller ones possess no tail, and in fact comets present almost innumerable differences. moreover the same comet changes rapidly, so that when they return, they are identified not in any way by their appearance, but by the path they pursue. comets may almost be regarded as the ghosts of heavenly bodies. the heads, in some cases, may consist of separate solid fragments, though on this astronomers are by no means agreed, but the tails at any rate are in fact of almost inconceivable tenuity. we know that a cloud a few hundred feet thick is sufficient to hide, not only the stars, but even the sun himself. a comet is thousands of miles in thickness, and yet even extremely minute stars can be seen through it with no appreciable diminution of brightness. this extreme tenuity of comets is moreover shown by their small weight. enormous as they are i remember sir g. airy saying that there was probably more matter in a cricket ball than there is in a comet. no one, however, now doubts that the weight must be measured in tons; but it is so small, in relation to the size, as to be practically inappreciable. if indeed they were comparable in mass even to the planets, we should long ago have perished. the security of our system is due to the fact that the planets revolve round the sun in one direction, almost in circles, and very nearly in the same plane. comets, however, enter our system in all directions, and at all angles; they are so numerous that, as kepler said, there are probably more comets in the sky than there are fishes in the sea, and but for their extreme tenuity they would long ago have driven us into the sun. when they first come in sight comets have generally no tail; it grows as they approach the sun, from which it always points away. it is no mere optical illusion; but while the comet as a whole is attracted by the sun, the tail, how or why we know not, is repelled. when once driven off, moreover, the attraction of the comet is not sufficient to recall it, and hence perhaps so many comets have now no tails. donati's comet, the great comet of , was first noticed on the d june as a faint nebulous spot. for three months it remained quite inconspicuous, and even at the end of august was scarcely visible to the naked eye. in september it grew rapidly, and by the middle of october the tail extended no less than degrees, after which it gradually disappeared. faint as is the light emitted by comets, it is yet their own, and spectrum analysis has detected the presence in them of carbon, hydrogen, nitrogen, sodium, and probably of iron. comets then remain as wonderful, and almost as mysterious, as ever, but we need no longer regard "a comet as a sign of impending calamity; we may rather look upon it as an interesting and a beautiful visitor, which comes to please us and to instruct us, but never to threaten or to destroy."[ ] we are free, therefore, to admire them in peace, and beautiful, indeed, they are. "the most wonderful sight i remember," says hamerton, "as an effect of calm, was the inversion of donati's comet, in the year , during the nights when it was sufficiently near the horizon to approach the rugged outline of graiganunie, and be reflected beneath it in loch awe. in the sky was an enormous aigrette of diamond fire, in the water a second aigrette, scarcely less splendid, with its brilliant point directed upwards, and its broad, shadowy extremity ending indefinitely in the deep. to be out on the lake alone, in a tiny boat, and let it rest motionless on the glassy water, with that incomparable spectacle before one, was an experience to be remembered through a lifetime. i have seen many a glorious sight since that now distant year, but nothing to equal it in the association of solemnity with splendour."[ ] shooting stars on almost any bright night, if we watch a short time some star will suddenly seem to drop from its place, and, after a short plunge, to disappear. this appearance is, however, partly illusory. while true stars are immense bodies at an enormous distance, shooting stars are very small, perhaps not larger than a paving stone, and are not visible until they come within the limits of our atmosphere, by the friction with which they are set on fire and dissipated. they are much more numerous on some nights than others. from the th to the th august we pass through one cluster which is known as the perseids; and on the th and th november a still greater group called by astronomers the leonids. the leonids revolve round the sun in a period of years, and in an elliptic orbit, one focus of which is about at the same distance from the sun as we are, the other at about that of uranus. the shoal of stars is enormous; its diameter cannot be less than , miles, and its length many hundreds of thousands. there are, indeed, stragglers scattered over the whole orbit, with some of which we come in contact every year, but we pass through the main body three times in a century--last in --capturing millions on each occasion. one of these has been graphically described by humboldt: "from half after two in the morning the most extraordinary luminary meteors were seen in the direction of the east. m. bonpland, who had risen to enjoy the freshness of the air, perceived them first. thousands of bodies and falling stars succeeded each other during the space of four hours. their direction was very regular from north to south. they filled a space in the sky extending from due east ° to north and south. in an amplitude of ° the meteors were seen to rise above the horizon at east-north-east, and at east, to describe arcs more or less extended, and to fall towards the south, after having followed the direction of the meridian. some of them attained a height of °, and all exceeded ° or °. no trace of clouds was to be seen. m. bonpland states that, from the first appearance of the phenomenon, there was not in the firmament a space equal in extent to three diameters of the moon which was not filled every instant with bolides and falling stars. the first were fewer in number, but as they were of different sizes it was impossible to fix the limit between these two classes of phenomena. all these meteors left luminous traces from five to ten degrees in length, as often happens in the equinoctial regions. the phosphorescence of these traces, or luminous bands, lasted seven or eight seconds. many of the falling stars had a very distinct nucleus, as large as the disc of jupiter, from which darted sparks of vivid light. the bodies seemed to burst as by explosion; but the largest, those from ° to ° ' in diameter, disappeared without scintillation, leaving behind them phosphorescent bands (trabes), exceeding in breadth fifteen or twenty minutes. the light of these meteors was white, and not reddish, which must doubtless be attributed to the absence of vapour and the extreme transparency of the air."[ ] the past history of the leonids, which le verrier has traced out with great probability, if not proved, is very interesting. they did not, he considers, approach the sun until a.d., when, in their career through the heavens, they chanced to come near to uranus. but for the influence of that planet they would have passed round the sun, and then departed again for ever. by his attraction, however, their course was altered, and they will now continue to revolve round the sun. there is a remarkable connection between star showers and comets, which, however, is not yet thoroughly understood. several star showers follow paths which are also those of comets, and the conclusion appears almost irresistible that these comets are made up of shooting stars. we are told, indeed, that , , of meteors, including only those visible with a moderate telescope, fall on the earth annually. at any rate, there can be no doubt that every year millions of them are captured by the earth, thus constituting an appreciable, and in the course of ages a constantly increasing, part of the solid substance of the globe. the stars we have been dealing in the earlier part of this chapter with figures and distances so enormous that it is quite impossible for us to realise them; and yet we have still others to consider compared with which even the solar system is insignificant. in the first place, the number of the stars is enormous. when we look at the sky at night they seem, indeed, almost innumerable; so that, like the sands of the sea, the stars of heaven have ever been used as effective symbols of number. the total number visible to the naked eye is, however, in reality only about , while that shown by the telescope is about , , . photography, however, has revealed to us the existence of others which no telescope can show. we cannot by looking long at the heavens see more than at first; in fact, the first glance is the keenest. in photography, on the contrary, no light which falls on the plate, however faint, is lost; it is taken in and stored up. in an hour the effect is times as great as in a second. by exposing the photographic plate, therefore, for some hours, and even on successive nights, the effect of the light is as it were accumulated, and stars are rendered visible, the light of which is too feeble to be shown by any telescope. the distances and magnitudes of the stars are as astonishing as their numbers, sirius, for instance, being about twenty times as heavy as the sun itself, times as bright, and no less than , , times as far away; while, though like other stars it seems to us stationary, it is in reality sweeping through the heavens at the rate of miles a minute; maia, electra, and alcyone, three of the pleiades, are considered to be respectively , , and times as brilliant as the sun, canopus times, and arcturus, incredible as it may seem, even times, so that, in fact, the sun is by no means one of the largest stars. even the minute stars not separately visible to the naked eye, and the millions which make up the milky way, are considered to be on an average fully equal to the sun in lustre. arcturus is, so far as we know at present, the swiftest, brightest, and largest of all. its speed is over miles a second, it is said to be times as bright as the sun, and times as large, while its distance is so great that its light takes years in reaching us. the distances of the heavenly bodies are ascertained by what is known as "parallax." suppose the ellipse (fig. ), marked jan., apr., july, oct., represents the course of the earth round the sun, and that a b are two stars. if in january we look at the star a, we see it projected against the front of the sky marked . three months later it would appear to be at , and thus as we move round our orbit the star itself appears to move in the ellipse , , , . the more distant star b also appears to move in a similar, but smaller, ellipse; the difference arising from the greater distance. the size of the ellipse is inversely proportional to the distance, and hence as we know the magnitude of the earth's orbit we can calculate the distance of the star. the difficulty is that the apparent ellipses are so minute that it is in very few cases possible to measure them. [illustration: fig. .--the parallactic ellipse.] the distances of the fixed stars thus tested are found to be enormous, and indeed generally incalculable; so great that in most cases, whether we look at them from one end of our orbit or the other--though the difference of our position, corresponding to the points marked january and july in fig. , is , , miles--no apparent change of position can be observed. in some, however, the parallax, though very minute, is yet approximately measurable. the first star to which this test was applied with success was that known as cygni, which is thus shown to be no less than billions of miles away from us--many thousand times as far as we are from the sun. the nearest of the stars, so far as we yet know, is [greek: alpha] centauri, the distance of which is about billions of miles. the pleiades are considered to be at a distance of nearly billions of miles. as regards the chemical composition of the stars, it is, moreover, obvious that the powerful engine of investigation afforded us by the spectroscope is by no means confined to the substances which form part of our system. the incandescent body can thus be examined, no matter how great its distance, so long only as the light is strong enough. that this method was theoretically applicable to the light of the stars is indeed obvious, but the practical difficulties are very great. sirius, the brightest of all, is, in round numbers, a hundred millions of millions of miles from us; and, though as bright as fifty of our suns, his light when it reaches us, after a journey of sixteen years, is at most one two-thousand-millionth part as bright. nevertheless, as long ago as fraunhofer recognised the fixed lines in the light of four of the stars; in miller and huggins in our own country, and rutherford in america, succeeded in determining the dark lines in the spectrum of some of the brighter stars, thus showing that these beautiful and mysterious lights contain many of the material substances with which we are familiar. in aldebaran, for instance, we may infer the presence of hydrogen, sodium, magnesium, iron, calcium, tellurium, antimony, bismuth, and mercury. as might have been expected, the composition of the stars is not uniform, and it would appear that they may be arranged in a few well-marked classes, indicating differences of temperature, or perhaps of age. thus we can make the stars teach us their own composition with light, which started from its source years ago, in many cases long before we were born. spectrum analysis has also thrown an unexpected light on the movements of the stars. ordinary observation, of course, is powerless to inform us whether they are moving towards or away from us. spectrum analysis, however, enables us to solve the problem, and we know that some are approaching, some receding. [illustration: fig. .--displacement of the hydrogen line in the spectrum of rigel.] if a star, say for instance sirius, were motionless, or rather if it retained a constant distance from the earth, fraunhofer's lines would occupy exactly the same position in the spectrum as they do in that of the sun. on the contrary, if sirius were approaching, the lines would be slightly shifted towards the blue, or if it were receding towards the red. fig. shows the displacement of the hydrogen line in the spectrum of rigel, due to the fact that it is receding from us at the rate of miles a second. the sun affords us an excellent test of this theory. as it revolves on its axis one edge is always approaching and the other receding from us at a known rate, and observation shows that the lines given by the light of the two edges differ accordingly. so again as regards the stars, we obtain a similar test derived from the earth's movement. as we revolve in our orbit we approach or recede any given star, and our rate of motion being known we thus obtain a second test. the results thus examined have stood their ground satisfactorily, and in huggins' opinion may be relied on within about an english mile a second. the effect of this movement is, moreover, independent of the distance. a lateral motion, say of miles a second, which in a nearer object would appear to be a stupendous velocity, becomes in the stars quite imperceptible. a motion of the same rapidity, on the other hand, towards or away from us, displaces the dark lines equally, whatever the distance of the object may be. we may then affirm that sirius, for instance, is receding from us at the rate of about miles a second. betelgeux, rigel, castor, regulus, and others are also moving away; while some--vega, arcturus, and pollux, for example--are approaching us. by the same process it is shown that some groups of stars are only apparently in relation to one another. thus in charles' wain some of the stars are approaching, others receding. i have already mentioned that sirius, though it seems, like other stars, so stationary that we speak of them as "fixed," is really sweeping along at the rate of miles a minute. even this enormous velocity is exceeded in other cases. one, which is numbered as in groombridge's _catalogue of the stars_, and is therefore known as "groombridge's ," moves no less than , miles a minute, and arcturus , miles a minute, or , , of miles a day; and yet the distances of the stars are so great that years would make hardly any difference in the appearance of the heavens. changes, however, there certainly would be. even in the short time during which we have any observations, some are already on record. one of the most interesting is the fading of the th pleiad, due, according to ovid, to grief at the taking of troy. again, the "fiery dogstar," as it used to be, is now, and has been for centuries, a clear white. the star known as nova cygni--the "new star in the constellation of the swan"--was first observed on the th november by dr. schmidt of athens, who had examined that part of the heavens only four days before, and is sure that no such star was visible then. at its brightest it was a brilliant star of the third magnitude, but this only lasted for a few days; in a week it had ceased to be a conspicuous object, and in a fortnight became invisible without a telescope. its sudden splendour was probably due to a collision between two bodies, and was probably little, if at all, less than that of the sun itself. it is still a mystery how so great a conflagration can have diminished so rapidly. but though we speak of some stars as specially variable, they are no doubt all undergoing slow change. there was a time when they were not, and one will come when they will cease to shine. each, indeed, has a life-history of its own. some, doubtless, represent now what others once were, and what many will some day become. for, in addition to the luminous heavenly bodies, we cannot doubt that there are countless others invisible to us, some from their greater distance or smaller size, but others, doubtless, from their feebler light; indeed, we know that there are many dark bodies which now emit no light, or comparatively little. thus in the case of procyon the existence of an invisible body is proved by the movement of the visible star. again, i may refer to the curious phenomena presented by algol, a bright star in the head of medusa. the star shines without change for two days and thirteen hours; then in three hours and a half dwindles from a star of the second to one of the fourth magnitude; and then, in another three and a half hours, reassumes its original brilliancy. these changes led astronomers to infer the presence of an opaque body, which intercepts at regular intervals a part of the light emitted by algol; and vogel has now shown by the aid of the spectroscope that algol does in fact revolve round a dark, and therefore invisible, companion. the spectroscope, in fact, makes known to us the presence of many stars which no telescope could reveal. thus the floor of heaven is not only "thick inlaid with patines of bright gold," but studded also with extinct stars, once probably as brilliant as our own sun, but now dead and cold, as helmholtz tells us that our sun itself will be some seventeen millions of years hence. such dark bodies cannot of course be seen, and their existence, though we cannot doubt it, is a matter of calculation. in one case, however, the conclusion has received a most interesting confirmation. the movements of sirius led mathematicians to conclude that it had also a mighty and massive neighbour, the relative position of which they calculated, though no such body had ever been seen. in february , however, the messrs. alvan clark of cambridgeport were completing their -inch glass for the chicago observatory. "'why, father,'" exclaimed the younger clark, "'the star has a companion.' the father looked, and there was a faint star due east from the bright one, and distant about ten seconds. this was exactly the predicted direction for that time, though the discoverers knew nothing of it. as the news went round the world many observers turned their attention to sirius; and it was then found that, though it had never before been noticed, the companion was really shown under favourable circumstances by any powerful telescope. it is, in fact, one-half of the size of sirius, though only / th of the brightness."[ ] stars are, we know, of different magnitudes and different degrees of glory. they are also of different colours. most, indeed, are white, but some reddish, some ruddy, some intensely red; others, but fewer, green, blue, or violet. it is possible that the comparative rarity of these colours is due to the fact that our atmosphere especially absorbs green and blue, and it is remarkable that almost all of the green, blue, or violet stars are one of the pairs of a double star, and in every case the smaller one of the two, the larger being red, orange, or yellow. one of the most exquisite of these is [greek: beta] cygni, a double star, the larger one being golden yellow, the smaller light blue. with a telescope the effect is very beautiful, but it must be magnificent if one could only see it from a lesser distance. double stars occur in considerable numbers. in some cases indeed the relation may only be apparent, one being really far in front of the other. in very many cases, however, the association is real, and they revolve round one another. in some cases the period may extend to thousands of years; for the distance which separates them is enormous, and, even when with a powerful telescope it is indicated only by a narrow dark line, amounts to hundreds of millions of miles. the pole star itself is double. andromeda is triple, with perhaps a fourth dark and therefore invisible companion. these dark bodies have a special interest, since it is impossible not to ask ourselves whether some at any rate of them may not be inhabited. in [greek: epsilon] lyræ there are two, each again being itself double. [greek: xi] cancri, and probably also [greek: theta] orionis, consist of six stars, and from such a group we pass on to star clusters in which the number is very considerable. the cluster in hercules consists of from to . a stellar swarm in the southern cross contains several hundred stars of various colours, red, green, greenish blue, and blue closely thronged together, so that they have been compared to a "superb piece of fancy jewellery."[ ] the cluster in the sword handle of perseus contains innumerable stars, many doubtless as brilliant as our sun. we ourselves probably form a part of such a cluster. the milky way itself, as we know, entirely surrounds us; it is evident, therefore, that the sun, and of course we ourselves, actually lie in it. it is, therefore, a star cluster, one of countless numbers, and containing our sun as a single unit. it has as yet been found impossible to determine even approximately the distance of these star clusters. nebulÆ from stars we pass insensibly to nebulæ, which are so far away that their distance is at present quite immeasurable. all that we can do is to fix a minimum, and this is so great that it is useless to express it in miles. astronomers, therefore, take the velocity of light as a unit. it travels at the rate of , miles a second, and even at this enormous velocity it must have taken hundreds of years to reach us, so that we see them not as they now are but as they were hundreds of years ago. it is no wonder, therefore, that in many of these clusters it is impossible to distinguish the separate stars of which they are composed. as, however, our telescopes are improved, more and more clusters are being resolved. photography also comes to our aid, and, as already mentioned, by long exposure stars can be made visible which are quite imperceptible to the eye, even with aid of the most powerful telescope. spectrum analysis also seems to show that such a nebula as that in andromeda, which with our most powerful instruments appears only as a mere cloud, is really a vast cluster of stellar points. this, however, by no means applies to all the nebulæ. the spectrum of a star is a bright band of colour crossed by dark lines; that of a gaseous nebula consists of bright lines. this test has been made use of, and indicates that some of the nebulæ are really immense masses of incandescent and very attenuated gas; very possibly, however, in a condition of which we have no experience, and arranged in discs, bands, rings, chains, wisps, knots, rays, curves, ovals, spirals, loops, wreaths, fans, brushes, sprays, lace, waves, and clouds. huggins has shown that many of them are really stupendous masses of glowing gas, especially of hydrogen, and perhaps of nitrogen, while the spectrum also shows other lines which perhaps may indicate some of the elements which, so far as our earth is concerned, appear to be missing between hydrogen and lithium. many of the nebulæ are exquisitely beautiful, and their colour very varied. in some cases, moreover, nebulæ seem to be gradually condensing into groups of stars, and in many cases it is difficult to say whether we should consider a given group as a cluster of stars surrounded by nebulous matter or a gaseous nebula condensed here and there into stars. "besides the single sun," says proctor, "the universe contains groups and systems and streams of primary suns; there are galaxies of minor orbs; there are clustering stellar aggregations showing every variety of richness, of figure, and of distribution; there are all the various forms of star cloudlets, resolvable and irresolvable, circular, elliptical, and spiral; and lastly, there are irregular masses of luminous gas clinging in fantastic convolutions around stars and star systems. nor is it unsafe to assert that other forms and varieties of structure will yet be discovered, or that hundreds more exist which we may never hope to recognise." nor is it only as regards the magnitude and distances of the heavenly bodies that we are lost in amazement and admiration. the lapse of time is a grander element in astronomy even than in geology, and dates back long before geology begins. we must figure to ourselves a time when the solid matter which now composes our earth was part of a continuous and intensely heated gaseous body, which extended from the centre of the sun to beyond the orbit of neptune, and had, therefore, a diameter of more than , , , miles. as this slowly contracted, neptune was detached, first perhaps as a ring, and then as a spherical body. ages after this uranus broke away. then after another incalculable period saturn followed suit, and here the tendencies to coherence and disruption were so evenly balanced that to this day a portion circulates as rings round the main body instead of being broken up into satellites. again after successive intervals jupiter, mars, the asteroids, the earth, venus, and mercury all passed through the same marvellous phases. the time which these changes would have required must have been incalculable, and they all of course preceded, and preceded again by another incalculable period, the very commencement of that geological history which itself indicates a lapse of time greater than human imagination can realise. thus, then, however far we penetrate in time or in space, we find ourselves surrounded by mystery. just as in time we can form no idea of a commencement, no anticipation of an end, so space also extends around us, boundless in all directions. our little earth revolves round the mighty sun; the sun itself and the whole solar system are moving with inconceivable velocity towards a point in the constellation of hercules; together with all the nearer stars it forms a cluster in the heavens, which appears to our eyes as the milky way; while outside our star cluster again are innumerable others, which far transcend, alike in magnitude, in grandeur, and in distance, the feeble powers of our finite imagination. footnotes: [ ] ball, _story of the heavens_. [ ] ball, _story of the heavens_. [ ] some authorities estimate it even higher. [ ] ball. [ ] hamerton, _landscape_. [ ] humboldt, _travels_. [ ] clarke, _system of the stars_. [ ] kosmos. this material taken from pages i-ii, iv and v, and - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p i cosmos volume i [p ii is blank] [p iii - not copied; pertains to reprint series] p iv [portrait] p v cosmos a sketch or a physical description of the universe by alexander von humboldt translated from the german by e. c. otte naturae vero rerum vis atque majestas in omnibus momentis fides caret, si quis modo partes ejus ac non totam complectatur animo. -- plin., 'hist. nat.', lib. vii, c. . volume i with an introduction by nicolaas a. rupke the johns hopkins university press baltimore and london [page vi and introduction to the edition not copied] p cosmos volume i [p is blank] p translator's preface. ----------------------- i can not more appropriately introduce the cosmos than by presenting a brief sketch of the life of its illustrious author.* while the name of alexander von humboldt is familiar to every one, few, perhaps, are aware of the peculiar circumstances of his scientific career and of the extent of his labors in almost every department of physical knowledge. he was born on the th of september, , and is, therefore, now in his th year. after going through the ordinary course of education at gottingen, and having made a rapid tour through holland, england, and france, he became a pupil of werner at the mining school of freyburg, and in his st year published an "essay on the basalts of the rhine." though he soon became officially connected with the mining corps, he was enabled to continue his excursions in foreign countries, for, during the six or seven years succeeding the publication of his first essay, he seems to have visited austria, switzerland, italy, and france. his attention to mining did not, however, prevent him from devoting his attention to other scientific pursuits, among which botany and the then recent discovery of galvanism may be especially noticed. botany, indeed, we know from his own authority, occupied him almost exclusively for some years; but even at this time he was practicing the use of those astronomical and physical instruments which he afterward turned to so singularly excellent an account. [footnote] *for the following remarks i am mainly indebted to the articles on the cosmos in the two leading quarterly reviews. the political disturbances of the civilized world at the close p of the last century prevented our author from carrying out various plans of foreign travel which he had contemplated, and detained him an unwilling prisoner in europe. in the year he went to spain, with the hope of entering africa from cadiz, but the unexpected patronage which he received at the court of madrid led to a great alteration in his plans, and decided him to proceed directly to the spanish possessions in america, "and there gratify the longings for foreign adventure, and the scenery of the tropics, which had haunted him from boyhood, but had all along been turned in the diametrically opposite direction of asia." after encountering various risks of capture, he succeeded in reaching america, and from to prosecuted there extensive researches in the physical geography of the new world, which has indelibly stamped his name in the undying records of science. excepting an excursion to naples with gay-lussac and von buch in (the year after his return from america), the succeeding twenty years of his life were spent in paris, and were almost exclusively employed in editing the results of his american journey. in order to bring these results before the world in a manner worthy of their importance, he commenced a series of gigantic publications in almost every branch of science on which he had instituted observations. in , after twelve years of incessant toil, four fifths were completed, and an ordinary copy of the part then in print cost considerably more than one hundred pounds sterling. since that time the publication has gone on more slowly, and even now after the lapse of nearly half a century, it remains, and probably ever will remain, incomplete. in the year , when the greatest portion of his literary labor had been accomplished, he undertook a scientific journey to siberia, under the special protection of the russian government. in this journey -- a journey for which he had prepared himself by a course of study unparalleled in the history of travel -- he was accompanied by two companions hardly less distinguished than himself, ehrenberg and gustav rose, and p the results obtained during their expedition are recorded by our author in his 'fragments asiatiques', and in his 'asie centrale', and by rose in his 'reise nach dem oural'. if the 'asie centrale' had been his only work, constituting, as it does, an epitome of all the knowledge acquired by himself and by former travelers on the physical geography of northern and central asia, that work alone would have sufficed to form a reputation of the highest order. i proceed to offer a few remarks on the work of which i now present a new translation to the english public, a work intended by its author "to embrace a summary of physical knowledge, as connected with a delineation of the material universe." the idea of such a physical description of the universe had, it appears, been present to his mind from a very early epoch. it was a work which he felt he must accomplish, and he devoted almost a lifetime to the accumulation of materials for it. for almost half a century it had occupied his thoughts; and at length, in the evening of life, he felt himself rich enough in the accumulation of thought, travel, reading, and experimental research, to reduce into form and reality the undefined vision that has so long floated before him. the work, when completed, will form three volumes. the 'first' volume comprises a sketch of all that is at present known of the physical phenomena of the universe; the 'second' comprehends two distinct parts, the first of which treats of the incitements to the study of nature, afforded in descriptive poetry, landscape painting, and the cultivation of exotic plants; while the second and larger part enters into the consideration of the different epochs in the progress of discovery and of the corresponding stages of advance in human civilization. the 'third' volume, the publication of which, as m. humboldt himself informs me in a letter addressed to my learned friend and publisher, mr. h. g. bohn, "has been somewhat delayed, owing to the present state of public affairs, will comprise the special and scientific development of the great picture of nature p each of the three parts of the 'cosmos' is therefore, to a certain extent, distinct in its object, and may be considered complete in itself. we can not better terminate this brief notice than in the words of one of the most eminent philosophers of our own country, that, "should the conclusion correspond (as we doubt not) with these beginnings, a work will have been accomplished every way worthy of the author's fame, and a crowning laurel added to that wreath with which europe will always delight to surround the name of alexander von humboldt." in venturing to appear before the english public as the interpreter of "the great work of our age,"* i have been encouraged by the assistance of many kind literary and scientific friends, and i gladly avail myself of this opportunity of expressing my deep obligations to mr. brooke, dr. day, professor edward forbes, mr. hind, mr. glaisher, dr. percy, and mr. ronalds, for the valuable aid they have afforded me. [footnote] *the expression applied to the cosmos by the learned bunsen, in his late report on ethnology, in the 'report of the british association for' , p. . it would be scarcely right to conclude these remarks without a reference to the translations that have preceded mine. the translation executed by mrs. sabine is singularly accurate and elegant. the other translation is remarkable for the opposite qualities, and may therefore be passed over in silence. the present volumes differ from those of mrs. sabine in having all the foreign measures converted into corresponding english terms, in being published at considerably less than one third of the price, and in being a translation of the entire work, for i have not conceived myself justified in omitting passages, sometimes amounting to pages, simply because they might be deemed slightly obnoxious to our national prejudices. p author's preface. ------------------- in the late evening of an active life i offer to the german public a work, whose undefined image has floated before my mind for almost half a century. i have frequently looked upon its completion as impracticable, but as often as i have been disposed to relinquish the undertaking, i have again -- although perhaps imprudently -- resumed the task. this work i now present to my contemporaries with a diffidence inspired by a just mistrust of my own powers, while i would willingly forget that writings long expected are usually received with less indulgence. although the outward relations of life, and an irresistible impulse toward knowledge of various kinds, have led me to occupy myself for many years -- and apparently exclusively -- with separate branches of science, as, for instance, with descriptive botany, geognosy, chemistry, astronomical determinations of position, and terrestrial magnetism, in order that i might the better prepare myself for the extensive travels in which i was desirous of engaging, the actual object of my studies has nevertheless been of a higher character. the principal impulse by which i was directed was the earnest endeavor to comprehend the phenomena of physical objects in their general connection, and to represent nature as one great whole, moved and animated by internal forces. my intercourse with highly-gifted men early led me to discover that, without an earnest striving to attain to a knowledge of special branches of study, all attempts to give a grand and general view of the universe would be nothing more than a vain illusion. these special departments in the great domain of natural p science are, moreover, capable of being reciprocally fructified by means of the appropriative forces by which they are endowed. descriptive botany, no longer confined to the narrow circle of the determination of genera and species, leads the observer who traverses distant lands and lofty mountains to the study of the geographical distribution of plants of the earth's surface, according to distance from the equator and vertical elevation above the sea. it is further necessary to investigate the laws which regulate the differences of temperature and climate, and the meteorological processes of the atmosphere, before we can hope to explain the involved causes of vegetable distribution; and it is thus that the observer who earnestly pursues the path of knowledge is led from one class of phenomena to another, by means of the mutual dependence and connection existing between them. i have enjoyed an advantage which few scientific travelers have shared to an equal extent, viz., that of having seen not only littoral districts, such as are alone visited by the majority of those who take part in voyages of circumnavigation, but also those portions of the interior of two vast continents which present the most striking contrasts manifested in the alpine tropical landscapes of south america, and the dreary wastes of the steppes in northern asia. travels, undertaken in districts such as these, could not fail to encourage the natural tendency of my mind toward a generalization of views, and to encourage me to attempt, in a special work, to treat of the knowledge which we at present possess, regarding the sidereal and terrestrial phenomena of the cosmos in their empirical relations. the hitherto undefined idea of a physical geography has thus, by an extended and perhaps too boldly imagined a plan, been comprehended under the idea of a physical description of the universe, embracing all created things in the regions of space and in the earth. the very abundance of the materials which are presented to the mind for arrangement and definition, necessarily impart no inconsiderable difficulties in the choice of the form under p which such a work must be presented, if it would aspire to the honor of being regarded as a literary composition. descriptions of nature ought not to be deficient in a tone of life-like truthfulness, while the mere enumeration of a series of general results is productive of a no less wearying impression than the elaborate accumulation of the individual data of observation. i scarcely venture to hope that i have succeeded in satisfying these various requirements of composition, or that i have myself avoided the shoals and breakers which i have known how to indicate to others. my faint hope of success rests upon the special indulgence which the german public have bestowed upon a small work bearing the title of 'ansichten der natur', which i published soon after my return from mexico. this work treats, under general points of view, of separate branches of physical geography (such as the forms of vegetation, grassy plains, and deserts). the effect produced by this small volume has doubtlessly been more powerfully manifested in the influence it has exercised on the sensitive minds of the young, whose imaginative faculties are so strongly manifested, than by means of any thing which it could itself impart. in the work on the cosmos on which i am now engaged, i have endeavored to show, as in that entitled 'ansichten der natur', that a certain degree of scientific completeness in the treatment of individual facts is not wholly incompatible with a picturesque animation of style. since public lectures seemed to me to present an easy and efficient means of testing the more or less successful manner of connecting together the detached branches of any one science, i undertook, for many months consecutively, first in the french language, at paris, and afterward in my own native german, at berlin (almost simultaneously at two different places of assembly), to deliver a course of lectures on the physical description of the universe, according to my conception of the science. my lectures were given extemporaneously, both in french and german, and without the aid of written notes, nor have i, in any way, made use, in the present work, p of those portions of my discourses which have been preserved by the industry of certain attentive auditors. with the exception of the first forty pages, the whole of the present work was written, for the first time, in the years and . a character of unity, freshness, and animation must, i think, be derived from an association with some definite epoch, where the object of the writer is to delineate the present condition of knowledge and opinions. since the additions constantly made to the latter give rise to fundamental changes in pre-existing views, my lectures and the cosmos have nothing in common beyond the succession in which the various facts are treated. the first portion of my work contains introductory considerations regarding the diversity in the degrees of enjoyment to be derived from nature, and the knowledge of the laws by which the universe is governed; it also considers the limitation and scientific mode of treating a physical description of the universe, and gives a general picture of nature which contains a view of all the phenomena comprised in the cosmos. this general picture of nature, which embraces within its wide scope the remotest nebulous spots, and the revolving double stars in the regions of space, no less than the telluric phenomena included under the department of the geography of organic forms (such as plants, animals, and races of men), comprises all that i deem most specially important with regard to the connection existing between generalities and specialities, while it moreover exemplifies, by the form and style of the composition, the mode of treatment pursued in the selection of the results obtained from experimental knowledge. the two succeeding volumes will contain a consideration of the particular means of incitement toward the study of nature (consisting in animated delineations, landscape painting, and the arrangement and cultivation of exotic vegetable forms), of the history of the contemplation of the universe, or the gradual development of the reciprocal action of natural forces constituting one natural whole; and lastly, of the special p branches of the several departments of science, whose mutual connection is indicated in the beginning of the work. wherever it has been possible to do so, i have adduced the authorities from whence i derived my facts, with a view of affording testimony both to the accuracy of my statements and to the value of the observations to which reference was made. in those instances where i have quoted from my own writings (the facts contained in which being, from their very nature, scattered through different portions of my works), i have always referred to the original editions, owing to the importance of accuracy with regard to numerical relations, and to my own distrust of the care and correctness of translators. in the few cases where i have extracted short passages from the works of my friends, i have indicated them by marks of quotation; and, in imitation of the practice of the ancients, i have invariably preferred the repetition of the same words to any arbitrary substitution of my own paraphrases. the much-contested question of priority of claim to a first discovery, which it is so dangerous to treat of in a work of this uncontroversial kind, has rarely been touched upon. where i have occasionally referred to classical antiquity, and to that happy period of transition which has rendered the sixteenth and seventeenth centuries so celebrated, owing to the great geographical discoveries by which the age was characterized, i have been simply led to adopt this mode of treatment, from the desire we experience from time to time, when considering the general views of nature, to escape from the circle of more strictly dogmatical modern opinions, and enter the free and fanciful domain of earlier presentiments. it has frequently been regarded as a subject of discouraging consideration, that while purely literary products of intellectual activity are rooted in the depths of feeling, and interwoven with the creative force of imagination, all works treating of empirical knowledge, and of the connection of natural phenomena and physical laws, are subject to the most marked modifications of form in the lapse of short periods of time, both p by the improvement in the instruments used, and by the consequent expansion of the field of view opened to rational observation, and that those scientific works which have, to use a common expression, become 'antiquated' by the acquisition of new funds of knowledge, are thus continually being consigned to oblivion as unreadable. however discouraging such a prospect must be, no one who is animated by a genuine love of nature, and by a sense of the dignity attached to its study, can view with regret any thing which promises future additions and a greater degree of perfection to general knowledge. many important branches of knowledge have been based upon a solid foundation which will not easily be shaken, both as regards the phenomena in the regions of space and on the earth; while there are other portions of science in which general views will undoubtedly take the place of merely special; where new forces will be discovered and new substances will be made known, and where those which are now considered as simple will be decomposed. i would, therefore, venture to hope that an attempt to delineate nature in all its vivid animation and exalted grandeur, and to trace the 'stable' amid the vacillating, ever-recurring alternation of physical metamorphoses, will not be wholly disregarded even at a future age. 'potsdam, nov.', . this material taken from pages - nb - the page numbers will be properly aligned in courier font. cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p contents of vol. i. ---------------------- page the translator's preface . . . . . . . . . . . . . . . . . . . . . . the author's preface . . . . . . . . . . . . . . . . . . . . . . . . summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . introduction. the results of the study of physical phenomena . . . . . . . . . . the different epochs of the contemplation of the external world . . the different degrees of enjoyment presented by the contemplation of nature . . . . . . . . . . . . . . . . . . . . . . . . . . instances of this species of enjoyment . . . . . . . . . . . . . . means by which it is induced . . . . . . . . . . . . . . . . . . . the elevations and climatic relations of many of the most celebrated mountains in the world, considered with reference to the effect produced on the mind of the observer . . . . . . . . . . . . . . . . . . . . . . . . . . - the impressions awakened by the aspect of tropical regions . . . . the more accurate knowledge of the physical forces of the universe, acquired by the inhabitants of a small section of the temperate zone . . . . . . . . . . . . . . . . . . . . . the earliest dawn of the science of the cosmos . . . . . . . . . . the difficulties that opposed the progress of inquiry . . . . . . . consideration of the effect produced on the mind by the observation of nature, and the fear entertained by some of its injurious influence . . . . . . . . . . . . . . . . . . . illustrations of the manner in which many recent discoveries have tended to remove the groundless fears entertained regarding the agency of certain natural phenomena . . . . . . the amount of scientific knowledge required to enter on the consideration of physical phenomena . . . . . . . . . . . . . the object held in view by the present work . . . . . . . . . . . . the nature of the study of the cosmos . . . . . . . . . . . . . . . the special requirements of the present age . . . . . . . . . . . . limits and method of exposition of the physical description of the universe . . . . . . . . . . . . . . . . . . . . . . . . . . . considerations on the terms physiology and physics . . . . . . . . . physical geography . . . . . . . . . . . . . . . . . . . . . . . . celestial phenomena . . . . . . . . . . . . . . . . . . . . . . . . the natural philosophy of the ancients directed more to celestial than to terrestrial phenomena . . . . . . . . . . . . . . . . . the able treatises of varenius and carl ritter . . . . . . . . . , signification of the word cosmos . . . . . . . . . . . . . . . . - the domain embraced by cosmography . . . . . . . . . . . . . . . . empiricism and experiments . . . . . . . . . . . . . . . . . . . . the process of reason and induction . . . . . . . . . . . . . . . . p general review of natural phenomena. connection between the material and the ideal world . . . . . . . . delineation of nature . . . . . . . . . . . . . . . . . . . . . . . celestial phenomena . . . . . . . . . . . . . . . . . . . . . . . . sidereal systems . . . . . . . . . . . . . . . . . . . . . . . . . planetary systems . . . . . . . . . . . . . . . . . . . . . . . . . comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . aerolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . zodiacal light . . . . . . . . . . . . . . . . . . . . . . . . . . translatory motion of the solar system . . . . . . . . . . . . . . the milky way . . . . . . . . . . . . . . . . . . . . . . . . . . . starless openings . . . . . . . . . . . . . . . . . . . . . . . terrestrial phenomena . . . . . . . . . . . . . . . . . . . . . . . geographical distribution . . . . . . . . . . . . . . . . . . . . . figure of the earth . . . . . . . . . . . . . . . . . . . . . . . . density of the earth . . . . . . . . . . . . . . . . . . . . . . . internal heat of the earth . . . . . . . . . . . . . . . . . . . . mean temperature of the earth . . . . . . . . . . . . . . . . . . . terrestrial magnetism . . . . . . . . . . . . . . . . . . . . . . magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . aurora borealis . . . . . . . . . . . . . . . . . . . .. . . . . . geognostic phenomena . . . . . . . . . . . . . . . . . . . . . . . earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . gaseous emanations . . . . . . . . . . . . . . . . . . . . . . . . hot springs . . . . . . . . . . . . . . . . . . . . . . . . . . . . salses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . volcanoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . palaeontology . . . . . . . . . . . . . . . . . . . . . . . . . . . geognostic periods . . . . . . . . . . . . . . . . . . . . . . . . physical geography . . . . . . . . . . . . . . . . . . . . . . . . meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . atmospheric pressure . . . . . . . . . . . . . . . . . . . . . . . climatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . the snow-line . . . . . . . . . . . . . . . . . . . . . . . . . . . hygrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . atmospheric electricity . . . . . . . . . . . . . . . . . . . . . . organic life . . . . . . . . . . . . . . . . . . . . . . . . . . . motion in plants . . . . . . . . . . . . . . . . . . . . . . . . . universality of animal life . . . . . . . . . . . . . . . . . . . . geography of plants and animals . . . . . . . . . . . . . . . . . . floras of different countries . . . . . . . . . . . . . . . . . . . man . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . races . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . conclusion of the subject . . . . . . . . . . . . . . . . . . . . . p summary. ----------- translator's preface. author's preface. vol i. general summary of the contents. introduction. -- reflections on the different degrees of enjoyment presented to us by the aspect of nature and the scientific exposition of the laws of the universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page - insight into the connection of phenomena as the aim of all natural investigation. nature presents itself to meditative contemplation as a unity in diversity. differences in the grades of enjoyment yielded by nature. effect of contact with free nature; enjoyment derived from nature independently of a knowledge of the action of natural forces, or of the physiognomy and configuration of the surface, or of the character of vegetation. reminiscences of the woody valleys of the cordilleras and of the peak of teneriffe. advantages of the mountainous region near the equator, where the multiplicity of natural impressions attains its maximum within the most circumscribed limits, and where it is permitted to man simultaneously to behold all the stars of the firmament and all the forms of vegetation -- p. - . tendency toward the investigation of the causes of physical phenomena. erroneous views of the character of natural forces arising from an imperfect mode of observation or of induction. the crude accumulation of physical dogmas transmitted from one country to another. their diffusion among the higher classes. scientific physics are associated with another and a deep-rooted system of untried and misunderstood experimental positions. investigation of natural laws. apprehension that nature may lose a portion of its secret charm by an inquiry into the internal character of its forces, and that the enjoyment of nature must necessarily be weakened by a study of its domain. advantages of general views which impart an exalted and solemn character to natural science. the possibility of separating generalities from specialties. examples drawn from astronomy, recent optical discoveries, physical geognosy, and the geography of plants. practicability of the study of physical cosmography -- p. - . misunderstood popular knowledge, confounding cosmography with a mere encyclopedic enumeration of natural sciences. necessity for a simultaneous regard for all branches of natural science. influence of this study on national prosperity and the welfare of nations; its more earnest and characteristic aim is an inner one, arising from exalted mental activity. mode of treatment with regard to the object and presentation; reciprocal connection existing between thought and speech -- p. - . the notes to p. - . comparative hypsometrical data of the elevations of the dhawalagiri, jawahir, chimborazo, aetna (according to the measurement of sir john herschel), the swiss alps, etc. -- p. . rarity p of palms and ferns in the himalaya mountains -- p. . european vegetable forms in the indian mountains -- p. . northern and southern limits of perpetual snow on the himalaya; influence of the elevated plateau of thibet -- p. - . fishes of an earlier world -- p. . limits and method of exposition of the physical description of the universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . page - subjects embraced by the study of the cosmos or of physical cosmography. separation of other kindred studies -- p. - . the uranological portion of the cosmos is more simple than the telluric; the impossibility of ascertaining the diversity of matter simplifies the study of the mechanism of the heavens. origin of the word 'cosmos', its signification of adornment and order of the universe. the 'existing' can not be absolutely separated in our contemplation of nature from the 'future'. history of the world and description of the world -- p. - . attempts to embrace the multiplicity of the phenomena of the cosmos in the unity of thought and under the form of a purely rational combination. natural philosophy, which preceded all exact observation in antiquity, is a natural, but not unfrequently ill-directed, effort of reason. two forms of abstraction rule in the whole mass of knowledge, viz.: the 'quantitative', relative determinations according to number and magnitude, and 'qualitative', material characters. means of submitting phenomena to calculation. atoms, mechanical methods of construction. figurative representations; mythical conception of imponderable matters, and the peculiar vital forces in every organism. that which is attained by observation and experiment (calling forth phenomena) leads, by analogy and induction, to a knowledge of 'empirical laws'; their gradual simplification and generalization. arrangement of the facts discovered in accordance with leading ideas. the treasure of empirical contemplation, collected through ages, is in no danger of experiencing any hostile agency from philosophy -- p. - . [in the notes appended to p. - are considerations of the general and comparative geography of varenius. philological investigation into the meaning of the words [greek word] and 'mundus'.] delineation of nature. general review of natural phenomena. . . . . p. - introduction -- p. - . a descriptive delineation of the world embraces the whole universe ([greek words]) in the celestial and terrestrial spheres. form and course of the representation. it begins with the laws of gravitation, and with the region of the remotest nebulous spots and double stars, and then, gradually descending through the starry stratum to which our solar system belongs, it contemplates this terrestrial spheroid, surrounded by air and water, and finally, proceeds to the consideration of the form of our planet, its temperature and magnetic tension, and the fullness of organic vitality which is unfolded on its surface under the action of light. partial insight into the relative dependence existing among all phenomena. amid all the mobile and unstable elements in space, 'mean numerical values' are the ultimate aim of investigation, being the expression of the physical laws, or forces of the cosmos. the delineation of the universe does not begin with the earth, from which a merely subjective point of view might have led us to start, but rather with the objects comprised in the regions of space. distribution of matter, which is partially conglomerated into rotating p and circling heavenly bodies of very different density and magnitude, and partly scattered as self-luminous vapor. review of the separate portions of the picture of nature, for the purpose of explaining the reciprocal connection of all phenomena. i. celestial portion of the cosmos . . . . . . . . . . . . . . . . .page - ii. terrestrial portion of the cosmos . . . . . . . . . . . . . . . .p. - a. form of the earth, its mean density, quantity of heat, electro-magnetic activity, process of light -- p. - . b. vital activity of the earth toward its external surface. reaction of the interior of a planet on its crust and surface. subterranean noise without waves of concussion. earthquakes dynamic phenomena -- p. - . c. material products which frequently accompany earthquakes. gaseous and aqueous springs. salses and mud volcanoes. upheavals of the soil by elastic forces -- p. - . d. fire-emitting mountains. craters of elevation. distribution of volcanoes on the earth -- p. - . e. volcanic forces form new kinds of rock, and metamorphose those already existing. geognostical classification of rocks into four groups. phenomena of contact. fossiliferous strata; their vertical arrangement. the faunas and floras of an earlier world. distribution of masses of rock -- p. - . f. geognostical epochs, which are indicated by the mineralogical difference of rocks, have determined the distribution of solids and fluids into continents and seas. individual configuration of solids into horizontal expansion and vertical elevation. relations of area. articulation. probability of the continued elevation of the earth's crust in ridges -- p. - . g. liquid and aeriform envelopes of the solid surface of our planet. distribution of heat in both. the sea. the tides. currents and their effects -- p. - . h. the atmosphere. its chemical composition. fluctuations in its density. law of the direction of the winds. mean temperature. enumeration of the causes which tend to raise and lower the temperature. continental and insular climates. east and west coasts. cause of the curvature of the isothermal lines. limits of perpetual snow. quantity of vapor. electricity in the atmosphere. forms of the clouds -- p. - . i. separation of inorganic terrestrial life from the geography of vital organisms; the geography of vegetables and animals. physical gradations of the human race -- p. - . special analysis of the delineation of nature, including references to the subjects treated of in the notes. i. celestial portion of the cosmos . . . . . . . . . . . . . . . . . p. - the universe and all that it comprises -- multiform nebulous spots, planetary vapor, and nebulous stars. the picturesque charm of a southern sky -- note, p. . conjectures on the position in space of the world. our stellar masses. a cosmical island. gauging stars. double stars revolving round a common center. distance of the star cygni -- p. and note. our solar system more complicated than was conjectured at the close of the last century. primary planets with neptune, astrea, hebe, iris, and flora, now constitute ; secondary planets ; myriad of comets of which many of the inner ones are inclosed p in the orbits of the planets; a rotating ring (the zodiacal light) and meteoric stones, probably to be regarded as small cosmical bodies. the telescopic planets, vesta, juno, ceres, pallas, astrea, hebe, iris and flora, with their frequently intersecting, strongly inclined, and more eccentric orbits, constitute a central group of separation between the inner planetary group (mercury, venus, the earth, and mars) and the outer group (jupiter, saturn, uranus, and neptune). contrasts of these planetary groups. relations of distance from one central body. differences of absolute magnitude, density, period of revolution, eccentricity, and inclination of the orbits. the so-called law of the distances of the planets from their central sun. the planets which have the largest number of moons -- p. and note. relations in space, both absolute and relative, of the secondary planets. largest and smallest of the moons. greatest approximation to a primary planet. retrogressive movement of the moons of uranus. libration of the earth's satellite -- p. and note. comets; the nucleus and tail; various forms and directions of the emanations in conoidal envelopes, with more or less dense walls. several tails inclined toward the sun; change of form of fixed stars by the nuclei of comets. eccentricity of their orbits and periods of revolution. greatest distance and greatest approximation of comets. passage through the system of jupiter's satellites. comets of short periods of revolution, more correctly termed inner comets (encke, biela, faye) -- p. and note. revolving aerolites (meteoric stones, fire-balls, falling stars). their planetary velocity, magnitude, form, observed height. periodic return in streams; the november stream and the stream of st. lawrence. chemical composition of meteoric asteroids -- p. and notes. ring of zodiacal light. limitation of the present solar atmosphere -- p. and note. translatory motion of the whole solar system -- p. - and note. the existence of the law of gravitation beyond our solar system. the milky way of stars and its conjectured breaking up. milky way of nebulous spots, at right angles with that of the stars. periods of revolutions of bi-colored double stars. canopy of stars; openings in the stellar stratum. events in the universe; the apparition of new stars. propagation of light, the aspect of the starry vault of the heavens conveys to the mind an idea of inequality of time -- p. - and notes. ii. terrestrial portion of the cosmos . . . . . . . . . . . . . . page - a. figure of the earth. density, quantity of heat, electro-magnetic tension, and terrestrial light -- p. - and note. knowledge of the compression and curvature of the earth's surface acquired by measurements of degrees, pendulum oscillations, and certain inequalities in the moon's orbit. mean density of the earth. the earth's crust, and the depth to which we are able to penetrate -- p. , , note. threefold movement of the heat of the earth; its thermic condition. law of the increase of heat with the increase of depth -- p. , and note. magnetism electricity in motion. periodical variation of terrestrial magnetism. disturbance of the regular course of the magnetic needle. magnetic storms; extension of their action. manifestations of magnetic force on the earth's surface presented under three classes of phenomena, namely, lines of equal force (isodynamic), equal inclination (isoclinic), and equal deviation (isogonic). position of the magnetic pole. its probable connection with the poles of cold. change of all the magnetic phenomena of the earth. erection of magnetic observatories p since ; a far-extending net-work of magnetic stations -- p. and note. development of light at the magnetic poles; terrestrial light as a consequence of the electro-magnetic activity of our planet. elevation of polar light. whether magnetic storms are accompanied by noise. connection of polar light (an electro-magnetic development of light) with the formation of cirrus clouds. other examples of the generation of terrestrial light -- p. and note. b. the vital activity of a planet manifested from within outward, the principal source of geognostic phenomena. connection between merely dynamic concussions or the upheaval of whole portions of the earth's crust, accompanied by the effusion of matter, and the generation of gaseous and liquid fluids, of hot mud and fused earths, which solidify into rocks. volcanic action, in the most general conception of the idea, is the reaction of the interior of a planet on its outer surface. earthquakes. extent of the circles of commotion and their gradual increase. whether there exists any connection between the changes in terrestrial magnetism and the processes of the atmosphere. noises, subterranean thunder without any perceptible concussion. the rocks which modify the propagation of the waves of concussion. upheavals; eruption of water, hot steam, mud mofettes, smoke, and flame during an earthquake -- p. - and notes. c. closer consideration of material products as a consequence of internal planetary activity. there rise from the depths of the earth, through fissures and cones of eruption, various gases, liquid fluids (pure or acidulated), mud, and molten earths. volcanoes are a species of intermittent spring. temperature of thermal springs; their constancy and change. depth of the foci -- p. - and notes. salses, mud volcanoes. while fire-emitting mountains, being sources of molten earths, produce volcanic rocks, spring water forms, by precipitation, strata of limestone. continued generation of sedimentary rocks -- p. and note. d. diversity of volcanic elevations. dome-like closed trachytic mountains. actual volcanoes which are formed from craters of elevations or among the detritus of their original structure. permanent connection of the interior of our earth with the atmosphere. relation to certain rocks. influence of the relations of height on the frequency of the eruptions. heights of the cone of cinders. characteristics of those volcanoes which rise above the snow-line. columns of ashes and fire. volcanic storm during the eruption. mineral composition of lavas -- p. and notes. distribution of volcanoes on the earth's surface; central and linear volcanoes; insular and littoral volcanoes. distance of volcanoes from the sea-coast. extinction of volcanic forces -- p. and notes. e. relation of volcanoes to the character of rocks. volcanic forces form new rocks, and metamorphose the more ancient ones. the study of these relations leads, by a double course, to the mineral portion of geognosy (the study of the textures and of the position of the earth's strata), and to the configuration of continents and insular groups elevated above the level of the sea (the study of the geographical form and outlines of the different parts of the earth. classification of rocks according to the scale of the phenomena of structure and metamorphosis, which are still passing before our eyes. rocks of eruption, sedimentary rocks, changed (metamorphosed) rocks, conglomerates -- compound rocks are definite associations of cryctognostically simple fossils. there are four phases in the formative condition; rocks of eruption, p endogenous (granite, sienite, porphyry, greenstone, hyperathene, rock, euphotide, melaphyre, basalt, and phonolithe); sedimentary rocks (silurian schist, coal measures, limestone, travertino, infusorial deposit); metamorphosed rock, which contains also, together with the detritus mica schist, and more ancient metamorphic masses. aggregate and sandstone formations. the phenomenon of contact explained by the artificial imitation of minerals. effects of pressure and the various rapidity of cooling. origin of granular or saccharoidal marble, silicification of schist into ribbon jasper. metamorphosis of calcareous marl into micaceous schist through granite. conversion of dolomite and granite into argillaceous schist, by contact with basaltic and doleritic rocks. filling up of the veins from below. processes of cementation in agglomerate structures. friction conglomerates -- p. and note. relative age of rocks, chronometry of the earth's crust. fossiliferous strata. relative age of organisms. simplicity of the first vital forms. dependence of physiological gradations on the age of the formations. geognostic horizon, whose careful investigation may yield certain data regarding the identity or the relative age of formations, the periodic recurrence of certain strata, their parallelism, or their total suppression. types of the sedimentary structures considered in their most simple and general characters; silurian and devonian formations (formerly known as rocks of transition); the lower trias (mountain limestone, coal measures, together with 'todilegende' and zechstein); the upper trias (butter sandstone, muschelkalk, and keuper); jura limestone (lias and oolite); freestone, lower and upper chalk, as the last of the flotz strata, which begin with mountain limestone; tertiary formations in three divisions, which are designated by granular limestone, lignite, and south apennine gravel -- p. - . the faunas and floras of an earlier world, and their relations to existing organisms. colossal bones of antediluvian mammalia in the upper alluvium. vegetation of an earlier world; monuments of the history of its vegetation. the points at which certain vegetable groups attain their maximum; cycadeae in the keuper and lias, and coniferae in the butter sandstone. lignite and coal measures (amber-tree). deposition of large masses of rock; doubts regarding their origin -- p. and note. f. the knowledge of geognostic epochs -- of the upheaval of mountain chains and elevated plateaux, by which lands are both formed and destroyed, leads, by an internal causal connection, to the distribution into solids and fluids, and to the peculiarities in the natural configuration of the earth's surface. existing areal relations of the solid to the fluid differ considerably from those presented by the maps of the physical portion of a more ancient geography. importance of the eruption of quartzose, porphyry with reference to the then existing configuration of continental masses. individual conformation in horizontal extension (relations of articulation) and in vertical elevation (hypsometrical views). influence of the relations of the area of land and sea on the temperature, direction of the winds, abundance or scarcity of organic products, and on all meteorological processes collectively. direction of the major axes of continental masses. articulation and pyramidal termination toward the south. series of peninsulas. valley-like formation of the atlantic ocean. forms which frequently recur -- p. - and notes. ramifications and systems of mountain chains, and the means of determining their relative ages. attempts to determine the centre of gravity of the volume of the lands upheaved above the level p of the sea. the elevation of continents is still progressing slowly, and is being compensated for at some definite points by a perceptible sinking. all geognostic phenomena indicate a periodical alteration of activity in the interior of our planet. probability of new elevations of ridges -- p. - and notes. g. the solid surface of the earth has two envelopes, one liquid, and the other aeriform. contrasts and analogies which these envelopes -- the sea and the atmosphere -- present in their conditions of aggregation and electricity, and in their relations of currents and temperature. depths of the ocean and of the atmosphere, the shoals of which constitute our highlands and mountain chains. the degree of heat at the surface of the sea in different latitudes and in the lower strata. tendency of the sea to maintain the temperature of the surface in the strata nearest to the atmosphere, in consequence of the mobility of its particles and the alteration in its density. maximum of the density of salt water. position of the zones of the hottest water, and of those having the greatest saline contents. thermic influence of the lower polar current and the counter currents in the straits of the sea -- p. - and notes. general level of the sea, and permanent local disturbances of equilibrium; the periodic disturbances manifested as tides. oceanic currents; the equatorial or rotation current, the atlantic warm gulf stream, and the further impulse which it receives; the cold peruvian stream in the eastern portion of the pacific ocean of the southern zone. temperature of shoals. the universal diffusion of life in the ocean. influence of the small submarine sylvan region at the bottom of beds of rooted algae, or on far-extending floating layers of fucus -- p. - and notes. h. the gaseous envelope of our planet, the atmosphere. chemical composition of the atmosphere, its transparency, its polarization, pressure, temperature, humidity, and electric tension. relation of oxygen to nitrogen; amount of carbonic acid; carbureted hydrogen; ammoniacal vapors. miamata. regular (horary) changes in the pressure of the atmosphere. mean barometrical height at the level of the sea in different zones of the earth. isobarometrical curves. barometrical windroses. law of rotation of the winds, and its importance with reference to the knowledge of many meteorological processes. land and sea winds, trade winds and monsoons -- p. - . climatic distribution of heat in the atmosphere, as the effect of the relative position of transparent and opaque masses (fluid and solid superficial area), and of the hypsometrical configuration of continents. curvature of the isothermal lines in a horizontal and vertical direction, on the earth's surface and in the superimposed strata of air. convexity and concavity of the isothermal lines. mean heat of the year, seasons, months, and days. enumeration of the causes which produce disturbances in the form of isothermal lines, i.e., their deviation from the position of the geographical parallels. isochimenal and isotheral lines are the lines of equal winter and summer heat. causes which raise or lower the temperature. radiation of the earth's surface, according to its inclination, color, density, dryness, and chemical composition. the form of the cloud which announces what is passing in the upper strata of the atmosphere is the image of the strongly radiating ground projected on a hot summer sky. contrast between an insular or littoral climate, such as is experienced by all deeply-articulated continents, and the climate of the interior of large tracts of land. east and west coasts. difference between the southern and northern hemispheres. thermal scales of p cultivated plants, going down from the vanilla, cacoa, and musaceae, by citrous and olives, and to vines yielding potable wines. the influence which these scales exercise on the geographical distribution of cultivated plants. the favorable ripening and the immaturity of fruits are essentially influenced by the difference in the action of direct or scattered light in a clear sky or in one overcast with mist. general summary of the causes which yield a more genial climate to the greater portion of europe considered as the western peninsula of asia -- p. . determination of the changes in the mean annual and summer temperature, which correspond to one degree of geographical latitude. equality of the mean temperature of a mountain station, and of the polar distance of any point lying at the level of the sea. decrease of temperature with the decrease in elevation. limits of perpetual snow, and the fluctuations in these limits. causes of disturbance in the regularity of the phenomenon. northern and southern chains of the himalaya; habitability of the elevated plateaux of thibet -- p. . quantity of moisture in the atmosphere, according to the hours of the day, the seasons of the year, degrees of latitude, and elevation. greatest dryness of the atmosphere observed in northern asia, between the river districts of the irtysch and the obi. dew, a consequence of radiation. quantity of rain -- p. . electricity of the atmosphere, and disturbance of the electric tension. geographical distribution of storms. predettermination of atmospheric changes. the most important climatic disturbances can not be traced, at the place of observation, to any local cause, but are rather the consequence of some occurrence by which the equilibrium in the atmospheric currents has been destroyed at some considerable distance -- p. - . i. physical geography is not limited to elementary inorganic terrestrial life, but, elevated to a higher point of view, it embraces the sphere of organic life, and the numerous gradations of its typical development. animal and vegetable life. general diffusion of life in the sea and on the land; microscopic vital forms discovered in the polar ice no less than in the depths of the ocean within the tropics. extension imparted to the horizon of life by ehrenberg's discoveries. estimation of the mass (volume) of animal and vegetable organisms -- p. - . geography of plants and animals. migrations of organisms in the ovum, or by means of organs capable of spontaneous motion. spheres of distribution depending on climatic relations. regions of vegetation, and classification of the genera of animals. isolated and social living plants and animals. the character of flora and fauna is not determined so much by the predominance of separate families, in certain parallels of latitude, as by the highly complicated relations of the association of many families, and the relative numerical value of their species. the forms of natural families which increase or decrease from the equator to the poles. investigations into the numerical relation existing in different districts of the earth between each one of the large families to the whole mass of phanerogamia -- p. - . the human race considered according to its physical gradations, and the geographical distribution of its simultaneously occurring types. races and varieties. all races of men are forms of one single species. unity of the human race. languages considered as the intellectual creations of mankind, or as portions of the history of mental activity, manifest a character of nationality, although certain historical occurrences have been the means of diffusing idioms of the same family of languages among nations of wholly different descent -- p. - . in this material taken from pages to cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p introduction. ---------------- reflections on the different degrees of enjoyment presented to us by the aspect of nature and the study of her laws. in attempting, after a long absence from my native country, to develop the physical phenomena of the globe, and the simultaneous action of the forces that pervade the regions of space, i experience a two-fold cause of anxiety. the subject before me is so inexhaustible and so varied, that i fear either to fall into the superficiality of the encyclopedist, or to weary the mind of my reader by aphorisms consisting of mere generalities clothed in dry and dogmatical forms. undue conciseness often checks the flow of expression, while diffuseness is alike detrimental to a clear and precise exposition of our ideas. nature is a free domain, and the profound conceptions and enjoyments she awakens within us can only be vividly delineated by thought clothed in exalted forms of speech, worthy of bearing witness to the majesty and greatness of the creation. in considering the study of physical phenomena, not merely in its bearings on the material wants of life, but in its general influence on the intellectual advancement of mankind, we find its noblest and most important result to be a knowledge of the chain of connection, by which all natural forces are linked together, and made mutually dependent upon each other; and it is the perception of these relations that exalts our views and ennobles our enjoyments. such a result can, however, only be reaped as the fruit of observation and intellect, combined with the spirit of the age, in which are reflected all the varied phases of thought. he who can trace, through by-gone times, the stream of our knowledge to its primitive source, will learn from history how, for thousands of years, man has labored, amid the ever-recurring changes of form, to recognize the invariability of natural laws, and has thus, by the force of mind, gradually subdued a great portion of the physical world to his dominion. in interrogating the history of the past, we trace the mysterious course of ideas yielding the first glimmering perception of the same image of p a cosmos, or harmoniously ordered whole, which, dimly shadowed forth to the human mind in the primitive ages of the world, is now fully revealed to the maturer intellect of mankind as the result of long and laborious observation. each of these epochs of the contemplation of the external world -- the earliest dawn of thought and the advanced stage of civilization -- has its own source of enjoyment. in the former, this enjoyment, in accordance with the simplicity of the primitive ages, flowed from an intuitive feeling of the order that was proclaimed by the invariable and successive reappearance of the heavenly bodies, and by the progressive development of organized beings; while in the latter, this sense of enjoyment springs from a definite knowledge of the phenomena of nature. when man began to interrogate nature, and, not content with observing, learned to evoke phenomena under definite conditions; when once he sought to collect and record facts, in order that the fruit of his labors might aid investigation after his own brief existence had passed away, the 'philosophy of nature' cast aside the vague and poetic garb in which she had been enveloped from her origin, and, having assumed a severer aspect, she now weighs the value of observations, and substitutes induction and reasoning for conjecture and assumption. the dogmas of former ages survive now only in the superstitions of the people and the prejudices of the ignorant, or are perpetuated in a few systems, which, conscious of their weakness, shroud themselves in a vail of mystery. we may also trace the same primitive intuitions in languages exuberant in figurative expressions; and a few of the best chosen symbols engendered by the happy inspiration of the earliest ages, having by degrees lost their vagueness through a better mode of interpretation, are still preserved among our scientific terms. nature considered 'rationally', that is to say, submitted to the process of thought, is a unity in diversity of phenomena; a harmony blending together all created things, however dissimilar in form and attributes; one great whole ([greek words]) animated by the breath of life. the most important result of a rational inquiry into nature is, therefore, to establish the unity and harmony of this stupendous mass of force and matter, to determine with impartial justice what is due to the discoveries of the past and to those of the present, and to analyze the individual parts of natural phenomena without succumbing beneath the weight of the whole. thus, and thus alone, is it permitted to man, while mindful of the high destiny p of his race, to comprehend nature, to lift the vail that shrouds her phenomena, and as it were, submit the results of observation to the test of reason and of intellect. in reflecting upon the different degrees of enjoyment presented to us in the contemplation of nature, we find that the first place must be assigned to a sensation, which is wholly independent of an intimate acquaintance with the physical phenomena presented to our view, or of the peculiar character of the region surrounding us. in the uniform plain bounded only by a distant horizon, where the lowly heather, the cistus, or waving grasses, deck the soil; on the ocean shore, where the waves, softly rippling over the beach, leave a track, green with the weeds of the sea; every where, the mind is penetrated by the same sense of the grandeur and vast expanse of nature, revealing to the soul, by a mysterious inspiration, the existence of laws that regulate the forces of the universe. mere communion with nature, mere contact with the free air, exercise a soothing yet strengthening influence on the wearied spirit, calm the storm of passion, and soften the heart when shaken by sorrow to its inmost depths. every where, in every region of the globe, in every stage of intellectual culture, the same sources of enjoyment are alike vouchsafed to man. the earnest and solemn thoughts awakened by a communion with nature intuitively arise from a presentiment of the order and harmony pervading the whole universe, and from the contrast we draw between the narrow limits of our own existence and the image of infinity revealed on every side, whether we look upward to the starry vault of heaven, scan the far-stretching plain before us, or seek to trace the dim horizon across the vast expanse of ocean. the contemplation of the individual characteristics of the landscape, and of the conformation of the land in any definite region of the earth, gives rise to a different source of enjoyment, awakening impressions that are more vivid, better defined, and more congenial to certain phases of the mind, than those of which we have already spoken. at one time the heart is stirred by a sense of the grandeur of the face of nature, by the strife of the elements, or, as in northern asia by the aspect of the dreary barrenness of the far-stretching steppes; at another time, softer emotions are excited by the contemplation of rich harvests wrested by the hand of man from the wild fertility of nature, or by the sight of human habitations raised beside some wild and foaming torrent. here i regard less the degree of intensity than the difference existing in the p various sensations that derive their charm and permanence from the peculiar character of the scene. if i might be allowed to abandon myself to the recollections of my own distant travels, i would instance, among the most striking scenes of nature, the calm sublimity of a tropical night, when the stars, not sparkling, as in our northern skies, shed their soft and planetary light over the gently-heaving ocean; or i would recall the deep valleys of the cordilleras, where the tall and slender palms pierce the leafy vail around them, and waving on high their feathery and arrow-like branches for, as it were, "a forest above a forest;"* or i would describe the summit of the peak of teneriffe, when a horizontal layer of clouds, dazzling in whiteness, has separated the cone of cinders from the plain below, and suddenly the ascending current pierces the cloudy vail, so that the eye of the traveler may range from the brink of the crater, along the vine-clad slopes of orotava, to the orange gardens and banana groves that skirt the shore. in scenes like these, it is not the peaceful charm uniformly spread over the face of nature that moves the heart, but rather the peculiar physiognomy and conformation of the land, the features of the landscape, the ever varying outline of the clouds, and their blending with the horizon of the sea, whether it lies spread before us like a smooth and shining mirror, or is dimly seen through the morning mist. all that the senses can but imperfectly comprehend, all that is most awful in such romantic scenes of nature, may become a source of enjoyment to man, by opening a wide field to the creative powers of his imagination. impressions change with the varying movements of the mind, and we are led by a happy illusion to believe that we receive from the external world that with which we have ourselves invested it. [footnote] *this expression is taken from a beautiful description of tropical forest scenery in 'paul and virginia', by bernardia de saint pierre. when far from our native country, after a long voyage, we tread for the first time the soil of a tropical land, we experience a certain feeling of surprise and gratification in recognizing, in the rocks that surround us, the same inclined schistose strata, and the same columnar basalt covered with cellular amygdaloids, that we had left in europe, and whose identity of character, in latitudes so widely different, reminds us that the solidification of the earth's crust is altogether independent of climatic influences. but these rocky masses of schist and of basalt are covered with vegetation of a character with which we are unacquainted, and of a physiognomy wholly p unknown to us; and it is then, amid the colossal and majestic forms of an exotic flora, that we feel how wonderfully the flexibility of our nature fits us to receive new impressions, linked together by a certain secret analogy. we so readily perceive the affinity existing among all the forms of organic life, that although the sight of a vegetation similar to that of our native country might at first be most welcome to the eye, as the sweet familiar sounds of our mother tongue are to the ear, we nevertheless, by degrees, and almost imperceptibly, become familiarized with a new home and a new climate. as a true citizen of the world, man every where habituates himself to that which surrounds him; yet fearful, as it were, of breaking the links of association that bind him to the home of his childhood, the colonist applies to some few plants in a far-distant clime the names he had been familiar with in his native land; and by the mysterious relations existing among all types of organization, the forms of exotic vegetation present themselves to his mind as nobler and more perfect developments of those he had loved in earlier days. thus do the spontaneous impressions of the untutored mind lead, like the laborious deductions of cultivated intellect, to the same intimate persuasion, that one sole and indissoluble chain binds together all nature. it may seem a rash attempt to endeavor to separate, into its different elements, the magic power exercised upon our minds by the physical world, since the character of the landscape, and of every imposing scene in nature, depends so materially upon the mutual relation of the ideas and sentiments simultaneously excited in the mind of the observer. the powerful effect exercised by nature springs, as it were, from the connection and unity of the impressions and emotions produced; and we can only trace their different sources by analyzing the individuality of objects and the diversity of forces. the richest and most varied elements for pursuing an analysis of this nature present themselves to the eyes of the traveler in the scenery of southern asia, in the great indian archipelago, and more especially, too, in the new continent, where the summits of the lofty cordilleras penetrate the confines of the aerial ocean surrounding our globe, and where the same subterranean forces that once raised these mountain chains still shake them to their foundation and threaten their downfall. graphic delineations of nature, arranged according to systematic views, are not only suited to please the imagination, p but may also, when properly considered, indicate the grades of the impressions of which i have spoken, from the uniformity of the sea-shore, or the barren steppes of siberia, to the inexhaustible fertility of the torrid zone. if we were even to picture to ourselves mount pilatus placed on the schreckhorn,* or the schneekoppe of silesia on mont blanc, we should p not have attained to the height of that great colossus of the andes, the chimborazo, whose height is twice that of mont aetna; and we must pile the righi, or mount athos, on the summit of the chimborazo, in order to form a just estimate of the elevation of the dhawalagiri, the highest point of the himalaya. [footnote] *these comparisons are only approximative. the several elevations above the level of the sea are, in accurate numbers, as follows: the schneekoppe or riesenkoppe, in silesia about feet, according to hallaschka. the righi, feet, taking the height of the lake of lucerne at feet, according to eschman. (see 'compte rendu des mesures trigonometriques en suisse', , p. .) mount athos, feet, according to captain gaultier; mount pilatus, feet; mount aetna, , feet, according to captain smyth; or , feet, according to the barometrical measurement made by sir john herschel, and communicated to me in writing in , and , feet, according to angles of altitude taken by cacciatore at palermo (calculated by assuming the terrestrial refraction to be . ); the schreckhorn, , feet; the jungfrau, , feet, according to tralles; mount blanc, , feet, according to the different measurements considered by roger ('bibl. univ.', may, , . - ), , feet, according to the measurements taken from mount columbier by carlini in , and , feet, as measured by the austrian engineers from trelod and the glacier d'ambin. [footnote continued] the actual height of the swiss mountains fluctuates, according to eschman's observations, as much as english feet, owing to the varying thickness of the stratum of snow that covers the summits. chimborazo is, according to my trigonometrical measurements, , feet (see humboldt, 'recueil d'obs. astr.', tome i., p. ), and dhawalagiri, , feet. as there is a difference of feet between the determinations of blake and webb, the elevation assigned to the dhawalagiri (or white mountain, from the sanscrit 'dhawala', white, and 'giri', mountain) can not be received with the same confidence as that of the jawahir, , feet, since the latter rests on a complete trigonomietrical measurement (see herbert and hodgson in the 'asiat. res.', vol. xiv., p. , and suppl. to 'encycl. brit.', vol. iv., p. ). i have shown elsewhere ('ann. des sciences naturelles', mars, ) that the height of the dhawalagiri ( , feet) depends on several elements that have not been ascertained with certainty, as azimuths and latitudes (humboldt, 'asie centrale', t. iii., p. ). it has been believed, but without foundation, that in the tartaric chain, north of thibet, opposite to the chain of kuen-lun, there are several snowy summits, whose elevation is about , english feet (almost twice that of mont blanc), or, at any rate, , feet (see captain alexander gerard's and john gerard's 'journey to the boorendo pass', , vol. i., p. and ). chimborazo is spoken of in the text only as 'one' of the highest summits of the chain of the andes; for in the year , the learned and highly-gifted traveler, pentland, in his memorable expedition to upper peru (bolivia), measured the elevation of two mountains situated to the east of lake titicaca, viz., the sorata, , feet, and the illimani, , feet, both greatly exceeding the height of chimborazo, which is only , feet, and being nearly equal in elevation to the jawahir, which is the highest mountain in the himalaya that has as yet been accurately measured. thus mont blanc is feet below chimborazo; chimborazo, feet below the sorata; the sorata, feet below the jawahir, and probably about feet below the dhawalagiri. according to a new measurement of the illimani, by pentland, in , the elevation of this mountain is given at , feet, varying only feet from the measurement taken in . the elevations have been given in this note with minute exactness, as erroneous numbers have been introduced into many maps and tables recently published, owing to incorrect reductions of the measurements. [in the preceding note, taken from those appended to the introduction in the french translation, rewritten by humboldt himself, the measurements are given in meters, but these have been converted into english feet, for the greater convenience of the general reader.] -- 'tr.' but although the mountains of india greatly surpass the cordilleras of south america by their astonishing elevation (which, after being long contested, has at last been confirmed by accurate measurements), they can not, from their geographical position, present the same inexhaustible variety of phenomena by which the latter are characterized. the impression produced by the grander aspects of nature dies not depend exclusively on height. the chain of the himalaya is placed far beyond the limits of the torrid zone, and scarcely is a solitary palm-tree to be found in the beautiful valleys of kumaoun and garhwal.* [footnote] *the absence of palms and tree-ferns on the temperate slopes of the himalaya is shown in don's 'flora nepalensis', , and in the remarkable series of lithographs of wallich's 'flora indica', whose catalogue contains the enormous number of himalaya species, almost all phanerogamic plants, which have as yet been but imperfectly classified. in nepaul (lat. / degrees to / degrees) there has hitherto been observed only one species of palm, chamaerops martiana, wall. ('plantae asiat.', lib. iii., p. , ), which is found at the height of english feet above the level of the sea, in the shady valley of bunipa. the magnificent tree-fern, alsophila brunoniana, wall. (of which a stem feet long has been in the possession of the british museum since ), does not grow in nepaul, but is found on the mountains of silhet, to the northwest of calcutta, in lat. degrees minutes. the nepaul fern, paranema cyathoides, don, formerly known as sphaeroptera barbata, wall. ('plantae asiat.', lib. i., p. , ), is indeed, nearly related to cyathea, a species of which i have seen in the south american missions of caripe, measuring feet in height; this is not, however, properly speaking a tree. on the southern slope of the ancient paropamisus, in the latitudes of degrees and degrees, nature no longer displays the same abundance of tree-ferns and arborescent grasses, heliconias and orchideous plants, which in tropical p regions are to be found even on the highest plateaux of the mountains. on the slope of the himalaya, under the shade of the deodora and the broad-leaved oak, peculiar to these indian alps, the rocks of granite and of mica schist are covered with vegetable forms almost similar to those which characterize europe and northern asia. the species are not identical, but closely analogous in aspect and physiognomy, as, marsh parnassia, and the prickly species of ribes.* the chain of the himalaya is also wanting in the imposing phenomena of volcanoes, which in the andes and in the indian archipelago often reveal to the inhabitants, under the most terrific forms, the existence of the forces pervading the interior of our planet. [footnote] *ribes nubicola, r. glaciale, r. grossularia. the species which compose the vegetation of the himalaya are four pines, notwithstanding the assertion of the ancients regarding eastern asia (strabo, lib. , p. , cas.), twenty-five oaks, four birches, two chestnuts, seven maples, twelve willows, fourteen roses, three species of strawberry, seven species of alpine roses ('rhododendra'), one of which attains a height of feet, and many other northern genera. large white apes, having black faces, inhabit the wild chestnut-tree of kashmir, which grows to a height of feet, in lat. degrees (see carl von hugel's 'kaschmir', , d pt. ). among the coniferae, we find the pinus deodwara, or deodara (in sanscrit, 'dewa-daru', the timber of the gods), which is nearly allied to pinus cedrus. near the limit of perpetual snow flourish the large and showy flowers of the gentiana venusta, g. moorcroftiana, swertia purpurescens, s. speciosa, parnassia armata, p. nubicola, poenia emode, tulipa stellata; and besides varieties of european genera peculiar to these indian mountains, true european species as leontodon taraxacum, prunella vulgaris, galium aparine, and thlaspi arvense. the heath mentioned by saunders, in turner's 'travels', and which had been confounded with calluna vulgaris, is an andromeda, a fact of the greatest importance in the geography of asiatic plants. if i have made use, in this work, of the unphilosophical expressions of european genera, 'european' special, 'growing wild in asia', etc., it has been in consequence of the old botanical language, which, instead of the idea of a large dissemination, or, rather, of the coexistence of organic productions, has dogmatically substituted the false hypothesis of a migration, which, from predilection for europe, is further assumed to have been from west to east. moreover, on the southern declivity of the himalaya, where the ascending current deposits the exhalations rising from a vigorous indian vegetation, the region of perpetual snow begins at an elevation of , or , feet above the level of the sea,* thus setting a limit to the development of organic p life in a zone that is nearly feet lower than that to which it attains in the equinoctial region of the cordilleras. [footnote] *on the southern declivity of the himalaya, the limit of perpetual snow is , feet above the level of the sea; on the northern declivity, or, rather, on the peaks which rise above the thibet, or tartarian plateau, this limit is at , feet from / degrees to degrees of latitude, while at the equator, in the andes of quito, it is , feet. such is the result i have deduced from the combination of numerous data furnished by webb, gerard, herbert, and moorcroft. (see my two memoirs on the mountains of india, in and , in the 'ann. de chimie et de physique', t. iii., p. ; t. xiv., p. , , .) the greater elevation to which the limit of perpetual snow recedes on the tartarian declivity is owing to the radiation of heat from the neighboring elevated plains, to the purity of the atmosphere, and to the infrequent formation of snow in an air which is both very cold and very dry. (humboldt, 'asie centrale', t. iii., p. - .) my opinion on the difference of height of the snow-line on the two sides of the himalaya has the high authority of colebrooke in its favor. he wrote to me in june, , as follows: "i also find, from the data in my possession, that the elevation of the line of perpetual snow is , feet. on the southern declivity, and at latitude degrees, webb's measurements give me , feet, consequently feet more than the height deduced from captain hodgson's observations. gerard's measurements fully confirm your opinion that the line of snow is higher on the northern than on the southern side." it was not until the present year ( ) that we obtained the complete and collected journal of the brothers gerard, published under the supervision of mr. lloyd. ('narrative of a journey from cawnpoor to the boorendo pass, in the himalaya, by captain alexander gerard and john gerard, edited by george lloyd', vol. i., p. , , , and .) many interesting details regarding some localities may be found in the narrative of 'a visit to the shatool, for the purpose of determining the line of perpetual snow on the southern face of the himalaya, in august', . unfortunately, however, these travelers always confound the elevation at which sporadic snow falls with the maximum of the height that the snow-line attains on the thibetian plateau. captain gerard distinguishes between the summits that rise in the middle of the plateau, where he states the elevation of the snow-line to be between , and , feet, and the northern slopes of the chain of the himalaya, which border on the defile of the sutledge, and can radiate but little heat, owing to the deep ravines with which they are intersected. the elevation of the village of tangno is given at only feet, while that of the plateau surrounding the sacred lake of maqasa is , feet. captain gerard finds the snow-line feet lower on the northern slopes, where the chain of the himalaya is broken through, than toward the southern declivities facing hindostan, and he there estimates the line of perpetual snow at , feet. the most striking differences are presented between the vegetation on the thibetian plateau and that characteristic of the southern slopes of the himalaya. on the latter the cultivation of grain is arrested at feet and even there the corn has often to be cut when the blades are still green. the extreme limit of forests of tall oaks and deodars is , feet; that of dwarf birches, , feet. on the plains, captain gerard found pastures up to the height of , feet; the cereals will grow at , feet, or even at , feet; birches with tall stems at , feet, and copse or brush wood applicable for fuel is found at an elevation of upward of , feet, that is to say, feet and above the lower limits of the snow-line at the equator, in the province of quito. it is very desirable that the 'mean' elevation of the thibetian plateau, which i have estimated at only about feet between the himalaya and the kuen-lun, and the difference in the height of the line of perpetual snow on the southern and on the northern slopes of the himalaya, should be again investigated by travelers who are accustomed to judge of the general conformation of the land. hitherto simple calculations have too often been confounded with actual measurements, and the elevations of isolated summits with that of the surrounding plateau. (compare carl zimmerman's excellent hypsometrical remarks in his 'geographischen analyse der karte von inner asien', , s. .) lord draws attention to the difference presented by the two faces of the himalaya and those of the alpine chain of hindoo-coosh, with respect to the limits of the snow-line. "the latter chain," he says, "has the table-land to the south, in consequence of which the snow-line is higher on the southern side, contrary to what we find to be the case with respect to the himalaya, which is bounded on the south by sheltered plains, as hindoo-coosh is on the north." it must, however, be admitted that the hypsometrical data on which these statements are based require a critical revision with regard to several of their details; but still they suffice to establish the main fact, that the remarkable configuration of the land in central asia affords man all that is essential to the maintenance of life, as habitation, food, and fuel, at an elevation above the level of the sea which in almost all other parts of the globe is covered with perpetual ice. we must except the very dry districts of bolivia, where snow is so rarely met with, and where pentland (in ) fixed the snow-line at , feet, between degrees and / degrees south latitude. the opinion that i had advanced regarding the difference in the snow-line on the two faces of the himalaya has been most fully confirmed by the barometrical observations of victor jacquemont, who fell an early sacrifice to his noble and unwearied ardor. (see his 'correspondance pendant son voyage dans l'inde', 'a' , liv. , p. , , .) "perpetual snow," says jacquemont, "descends lower on the southern than on the northern slopes of the himalaya, and the limit constantly rises as we advance to the north of the chain bordering on india. on the kionbrong, about , feet in elevation, according to captain gerard, i was still considerably below the limit of perpetual snow which i believe to be , feet in this part of hindostan." (this estimate i consider much too high.) [footnote continues] the same traveler says, "to whatever height we rise on the southern declivity of the himalaya, the climate retains the same character, and the same division of the seasons as in the plains of india; the summer solstice being every year marked by the same prevalence of rain which continues to fall without intermission until the autumnal equinox. but a new, a totally different climate begins at kashmir, whose elevation i estimate to be feet, nearly equal to that of the cities of mexico and popayan" ('correspond. de jacquemont', t. ii., p. et ). the warm and humid air of the sea, as leopold von buch well observes, is carried by the monsoons across the plains of india to the skirts of the himalaya which arrest its course, and hinder it from diverging to the thibetian districts of ladak and lassa. carl von hugel estimates the elevation of the valley of kashmir above the level of the sea at feet, and bases his observation on the determination of the boiling point of water (see theil , s. , and 'journal of geog. soc.', vol. vi., p. ). in this valley, where the atmosphere is scarcely ever agitated by storms, and in degrees minutes lat., snow is found, several feet in thickness, from december to march. p but the countries bordering on the equator possess another advantage, to which sufficient attention has not hitherto been p directed. this portion of the surface of the globe affords in the smallest space the greatest possible variety of impressions from the contemplation of nature. among the colossal mountains of cundinamarea, of quito, and of peru, furrowed by deep ravines, man is enabled to contemplate alike all the families of plants, and all the stars of the firmament. there, at a single glance, the eye surveys majestic palms, humid forests of bambusa, and the varied species of musaceae, while above these forms of tropical vegetation appear oaks, medlars, the sweet-brier, and umbelliferous plants, as in our european homes. there as the traveler turns his eyes to the vault of heaven, a single glance embraces the constellation of the southern cross, the magellanic clouds, and the guiding stars of the constellation of the bear, as they circle round the arctic pole. there the depths of the earth and the vaults of heaven display all the richness of their forms and the variety of their phenomena. there the different climates are ranged the one above the other, stage by stage, like the vegetable zones, whose succession they limit; and there the observer may readily trace the laws that regulate the diminution of heat, as they stand indelibly inscribed on the rocky walls and abrupt declivities of the cordilleras. not to weary the reader with the details of the phenomena which i long since endeavored graphically to represent,* i will here limit myself to the consideration of a few of the general results whose combination constitutes the 'physical delineation of the torrid zone.' that which, in the vagueness of our p impressions, loses all distinctness of form, like some distant mountain shrouded from view by a vail of mist, is clearly revealed by the light of mind, which, by its scrutiny into the causes of phenomena, learns to resolve and analyze their different elements, assigning to each its individual character. thus, in the sphere of natural investigation, as in poetry and painting, the delineation of that which appeals most strongly to the imagination, derives its collective interest from the vivid truthfulness with which the individual features are portrayed. [footnote] *see, generally my 'essai sur la geographie des plantes, et le tableau physique des regions equinoxiales', , p. - . on the diurnal and nocturnal variations of temperature, see plate of my 'atlas geogr. et phys. du nouveau continent'; and the tables in my work, entitled 'de distributione geographica plantarum, secundum coeli tempriem, et altitudinem montium', , p. - ; the meteorological portion of my 'asie centrale', t. iii., p. , ; and, finally, the more recent and far more exact exposition of the variations of temperature experienced in correspondence with the increase of altitude on the chain of the andes, given in boussingault's memoir, 'sur la profondeur a laquelle on trouve, sous les tropiques, la couche de temperature invariable.' (ann. de chimie et de physique, , t. liii., p. - .) this treatise contains the elevations of points, included between the level of the sea and the declivity of the antisana ( , feet), as well as the mean temperature of the atmosphere, which varies with the height between degrees and degrees f. the regions of the torrid zone not only give rise to the most powerful impressions by their organic richness and their abundant fertility, but they likewise afford the inestimable advantage of revealing to man, by the uniformity of the variations of the atmosphere and the development of vital forces, and by the contrasts of climate and vegetation exhibited at the different elevations, the invariability of the laws that regulate the course of the heavenly bodies, reflected, as it were, in terrestrial phenomena. let us dwell, then, for a few moments, on the proofs of this regularity, which is such that it may be submitted to numerical calculation and computation. in the burning plains that rise but little above the level of the sea, reign the families of the banana, the cycas, and the palm, of which the number of species comprised in the flora of tropical regions has been so wonderfully increased in the present day by the zeal of botanical travelers. to these groups succeed, in the alpine valleys, and the humid and shaded clefts on the slopes of the cordilleras, the tree-ferns, whose thick cylindrical trunks and delicate lace-like foliage stand out in bold relief against the azure of the sky, and the cinchona, from which we derive the febrifuge bark. the medicinal strength of this bark is said to increase in proportion to the degree of moisture imparted to the foliage of the tree by the light mists which form the upper surface of the clouds resting over the plains. every where around, the confines of the forest are encircled by broad bands of social plants, as the delicate aralia, the thibaudia, and the myrtle-leaved andromeda, while the alpine rose, the magnificent befaria, weaves a purple girdle round the spiry peaks. in the cold regions of the paramos, which is continually exposed to the fury of storms and winds, we find that flowering shrubs and herbaceous plants, bearing large and variegated blossoms, have given place to monocotyledons, whose slender spikes constitute the sole covering of the soil. this is the zone of the p grasses, one vast savannah extending over the immense mountain plateaux, and reflecting a yellow, almost golden tinge, to the slopes of the cordilleras, on which graze the lama and the cattle domesticated by the european colonist. where the naked trachyte rock pierces the grassy turf, and penetrates into those higher strata of air which are supposed to be less charged with carbonic acid, we meet only with plants of an inferior organization, as lichens, lecideas, and the brightly-colored, dust-like lepraria, scattered around in circular patches. islets of fresh-fallen snow, varying in form and extent, arrest the last feeble traces of vegetable development, and to these succeeds the region of perpetual snow, whose elevation undergoes but little change, and may be easily determined. it is but rarely that the elastic forces at work within the interior of our globe have succeeded in breaking through the spiral domes, which, resplendent in the brightness of eternal snow, crown the summits of the cordilleras; and even where these subterranean forces have opened a permanent communication with the atmosphere, through circular craters or long fissures, they rarely send forth currents of lava, but merely eject ignited scoriae, steam, sulphureted hydrogen gas, and jets of carbonic acid. in the earliest stages of civilization, the grand and imposing spectacle presented to the minds of the inhabitants of the tropics could only awaken feelings of astonishment and awe. it might, perhaps, be supposed, as we have already said, that the periodical return of the same phenomena, and the uniform manner in which they arrange themselves in successive groups, would have enabled man more readily to attain to a knowledge of the laws of nature; but, as far as tradition and history guide us, we do not find that any application was made of the advantages presented by these favored regions. recent researches have rendered it very doubtful whether the primitive seat of hindoo civilization -- one of the most remarkable phases in the progress of mankind -- was actually within the tropics. airyana vaedjo, the ancient cradle of the zend, was situated to the northwest of the upper indus, and after the great religious schism, that is to say, after the separation of the iranians from the brahminical institution, the language that had previously been common to them and to the hindoos assumed among the latter people (together with the literature, habits, and conditions of society) an individual form in the magodha of madhya desa,* a district that is bounded by the great chain p of himalaya and the smaller range of the vindhya. [footnote] *see, on the madhjadeca, properly so called, lassen's excellent work, entitled 'indische alterthumskunde', bd. i., s. . the chinese give the name of mo-kie-thi to the southern bahar, situated to the south of the ganges (see 'foe-koue-ki' by, 'chy-fa-hian', , p. ). djambu-dwipa is the name given to the whole of india; but the words also indicate one of the four buddhist continents. in less ancient times the sanscrit language and civilization advanced toward the southeast, penetrating further within the torrid zone, as my brother wilhelm von humboldt has shown in his great work on the kavi and other languages of analogous structure.* [footnote] *'ueber die kawi sprache auf der insel java, nebst einer einleitung uber die verschiedenheit des menschlichen sprachbaues und ihren ein fluss auf die geistige entwickelung des menschengrshlecht's' von wilhelm v. humboldt, , bd. i., s. . notwithstanding the obstacles opposed in northern latitudes to the discovery of the laws of nature, owing to the excessive complication of phenomena, and the perpetual local variations and the distribution of organic forms, it is to the inhabitants of a small section of the temperate zone that the rest of mankind owe the earliest revelation of an intimate and rational acquaintance with the forces governing the physical world. moreover, it is from the same zone (which is apparently more favorable to the progress of reason, the softening of manners, and the security of public liberty) that the germs of civilization have been carried to the regions of the tropics, as much by the migratory movement of races as by the establishment of colonies, differing widely in their institution from those of the phoenicians or greeks. in speaking of the influence exercised by the succession of phenomena on the greater or lesser facility of recognizing the causes producing them, i have touched upon that important stage of our communion with the external world, when the enjoyment arising from a knowledge of the laws, and the mutual connection of phenomena, associates itself with the charm of a simple contemplation of nature. that which for a long time remains merely an object of vague intuition, by degrees acquires the certainty of positive truth; and man, as an immortal poet has said, in our own tongue -- amid ceaseless change seeks the unchanging pole.* [footnote] *this verse occurs in a poem of schiller, entitled 'der spaziergang' which first appeared in , in the 'horen.' in order to trace to its primitive source the enjoyment derived from the exercise of thought, it is sufficient to cast a rapid glance on the earliest dawnings of the philosophy of nature, or of the ancient doctrine of the 'cosmos.' we find even p among the most savage nations (as my own travels enable me to attest) a certain vague, terror-stricken sense of the all-powerful unity of natural forces, and of the existence of an invisible, spiritual essence manifested in these forces, whether in unfolding the flower and maturing the fruit of the nutrient tree, in upheaving the soil of the forest, or in rending the clouds with the might of the storm. we may here trace the revelation of a bond of union, linking together the visible world and that higher spiritual world which escapes the grasp of the senses. the two become unconsciously blended together, developing in the mind of man, as a simple product of ideal conception and independently of the aid of observation, the first germ of a 'philosophy of nature.' among nations least advanced in civilization, the imagination revels in strange and fantastic creations, and, by its predilection for symbols, alike influences ideas and language. instead of examining, men are led to conjecture, dogmatize, and interpret supposed facts that have never been observed. the inner world of thought and of feeling does not reflect the image of the external world in its primitive purity. that which in some regions of the earth manifested itself as the rudiments of natural philosophy, only to a small number of persons endowed with superior intelligence, appears in other regions, and among entire races of men, to be the result of mystic tendencies and instinctive intuitions. an intimate communion with nature, and the vivid and deep emotions thus awakened, are likewise the source from which have sprung the first impulses toward the worship and deification of the destroying and preserving forces of the universe. but by degrees, as man, after having passed through the different gradations of intellectual development, arrives at the free enjoyment of the regulating power of reflection, and learns by gradual progress, as it were, to separate the world of ideas from that of sensations, he no longer rests satisfied merely with a vague presentiment of the harmonious unity of natural forces; thought begins to fulfill its noble mission; and observation, aided by reason, endeavors to trace phenomena to the causes from which they spring. the history of science teaches us the difficulties that have opposed the progress of this active spirit of inquiry. inaccurate and imperfect observations have led, by false inductions, to the great number of physical views that have been perpetuated as popular prejudices among all classes of society. thus by the side of a solid and scientific knowledge of natural phenomena there has been preserved a system of the pretended p results of observation, which is so much the more difficult to shake, as it denies the validity of the facts by which it may be refuted. this empiricism, the melancholy heritage transmitted to us from former times, invariably contends for the truth of its axioms with the arrogance of a narrow-minded spirit. physical philosophy, on the other hand, when based upon science, doubts because it seeks to investigate, distinguishes between that which is certain and that which is merely probable, and strives incessantly to perfect theory by extending the circle of observation. this assemblage of imperfect dogmas, bequeathed by one age to another -- this physical philosophy, which is composed of popular prejudices -- is not only injurious because it perpetuates error with the obstinacy engendered by the evidence of ill-observed facts, but also because it hinders the mind from attaining to higher views of nature. instead of seeking to discover the 'mean' or 'medium' point, around which oscillate, in apparent independence of forces, all the phenomena of the external world, this system delights in multiplying exceptions to the law, and seeks, amid phenomena and in organic forms for something beyond the marvel of a regular succession, and an internal and progressive development. ever inclined to believe that the order of nature is disturbed, it refuses to recognize in the present any analogy with the past, and guided by its own varying hypotheses, seeks at hazard, either in the interior of the globe or in the regions of space, for the cause of these pretended perturbations. it is the special object of the present work to combat those errors which derive their source from a vicious empiricism and from imperfect inductions. the higher enjoyments yielded by the study of nature depend upon the correctness and the depth of our views, and upon the extent of the subjects that may be comprehended in a single glance. increased mental cultivation has given rise, in all classes of society, to an increased desire of embellishing life by augmenting the mass of ideas, and by multiplying means for their generalization; and this sentiment fully refutes the vague accusations advanced against the age in which we live, showing that other interests, besides the material wants of life, occupy the minds of men. it is almost with reluctance that i am about to speak of a sentiment, which appears to arise from narrow-minded views, or from a certain weak and morbid sentimentality -- i allude to the 'fear' entertained by some persons, that nature may by degrees lose a portion of the charm and magic of her power, p as we learn more and more how to unvail her secrets, comprehend the mechanism of the movements of the heavenly bodies, and estimate numerically the intensity of natural forces. it is true that, properly speaking, the forces of nature can only exercise a magical power over us as long as their action is shrouded in mystery and darkness, and does not admit of being classed among the conditions with which experience has made us acquainted. the effect of such a power is, therefore, to excite the imagination, but that, assuredly, is not the faculty of mind we would evoke to preside over the laborious and elaborate observations by which we strive to attain to a knowledge of the greatness and excellence of the laws of the universe. the astronomer who, by the aid of the heliometer or a double-refracting prism,* determines the diameter of planetary bodies; who measures patiently year after year, the meridian altitude and the relative distances of stars, or who seeks a telescopic comet in a group of nebulae, does not feel his imagination more excited -- and this is the very guarantee of the precision of his labors -- than the botanist who counts the divisions of the calyx, or the number of stamens in a flower, or examines the connected or the separate teeth of the peristoma surrounding the capsule of a moss. yet the multiplied angular measurements on the one hand, and the detail of organic relations on the other, alike aid in preparing the way for the attainment of higher views of the laws of the universe. [footnote] *arago's ocular micrometer, a happy improvement upon rochon's prismatic or double-refraction micrometer. see m. mathieu's note in delambre's 'histoire de l'astronomie au dix-huitieme siecle', . we must not confound the disposition of mind in the observer at the time he is pursuing his labors, with the ulterior greatness of the views resulting from investigation and the exercise of thought. the physical philosopher measures with admirable sagacity the waves of light of unequal length which by interference mutually strengthen or destroy each other, even with respect to their chemical actions; the astronomer, armed with powerful telescopes, penetrates the regions of space, contemplates, on the extremest confines of our solar system, the satellites of uranus, or decomposes faintly sparkling points into double stars differing in color. the botanist discovers the constancy of the gyratory motion of the chara in the greater number of vegetable cells, and recognizes in the genera and natural families of plants the intimate relations or organic forms. the vault of heaven, studded with nebulae p and stars, and the rich vegetable mantle that covers the soil in the climate of palms, can not surely fail to produce on the minds of these laborious observers of nature an impression more imposing and more worthy of the majesty of creation than on those who are unaccustomed to investigate the great mutual relations of phenomena. i can not, therefore, agree with burke when he says, "it is our ignorance of natural things that causes all our admiration and chiefly excites our passions." while the illusion of the senses would make the stars stationary in the vault of heaven, astronomy, by her aspiring labors, has assigned indefinite bounds to space; and if she have set limits to the great nebula to which our solar system belongs, it has only been to show us in those remote regions of our optic powers, islet on islet of scattered nebulae. the feeling of the sublime, so far as it arises from a contemplation of the distance of the stars, of their greatness and physical extent, reflects itself in the feeling of the infinite, which belongs to another sphere of ideas included in the domain of mind. the solemn and imposing impressions excited by this sentiment are owing to the combination of which we have spoken, and to the analogous character of the enjoyment and emotions awakened in us, whether we float on the surface of the great deep, stand on some lonely mountain summit enveloped in the half-transparent vapory vail of the atmosphere, or by the aid of powerful optical instruments scan the regions of space, and see the remote nebulous mass resolve itself into worlds of stars. the mere accumulation of unconnected observations of details, devoid of generalization of ideas, may doubtlessly have tended to create and foster the deeply-rooted prejudice, that the study of the exact sciences must necessarily chill the feelings, and diminish the nobler enjoyments attendant upon a contemplation of nature. those who still cherish such erroneous views in the present age, and amid the progress of public opinion, and the advancement of all branches of knowledge, fail in duly appreciating the value of every enlargement of the sphere of intellect, and the importance of the detail of isolated facts in leading us on to general results. the fear of sacrificing the free enjoyment of nature, under the influence of scientific reasoning, is often associated with an apprehension that every mind may not be capable of grasping the truths of the philosophy of nature. it is certainly true that in the midst of the universal fluctuation of phenomena and vital p forces -- in that inextricable net-work of organisms by turns developed and destroyed -- each step that we make in the more intimate knowledge of nature leads us to the entrance of new labyrinths; but the excitement produced by a presentiment of discovery, the vague intuition of the mysteries to be unfolded, and the multiplicity of the paths before us, all tend to stimulate the exercise of thought in every stage of knowledge. the discovery of each separate law of nature leads to the establishment of some other more general law, or at least indicates to the intelligent observer its existence. nature, as a celebrated physiologist* has defined it, and as the word was interpreted by the greeks and romans, is "that which is ever growing and ever unfolding itself in new forms." [footnote] *carus, 'von den urtheilen des knochen und schalen gerustes', . the series of organic types becomes extended or perfected in proportion as hitherto unknown regions are laid open to our view by the labors and researches of travelers and observers; as living organisms are compared with those which have disappeared in the great revolutions of our planet; and as microscopes are made more perfect, and are more extensively and efficiently employed. in the midst of this immense variety, and this periodic transformation of animal and vegetable productions, we see incessantly revealed the primordial mystery of all organic development, that same great problem of 'metamorphosis' which gÃ�Â�the has treated with more than common sagacity, and to the solution of which man is urged by his desire of reducing vital forms to the smallest number of fundamental types. as men contemplate the riches of nature, and see the mass of observations incessantly increasing before them, they become impressed with the intimate conviction that the surface and the interior of the earth, the depths of the ocean, and the regions of air will still, when thousands and thousands of years have passed away, open to the scientific observer untrodden paths of discovery. the regret of alexander can not be applied to the progress of observation and intelligence.* [footnote] * plut., in 'vita alex. magni', cap. general considerations, whether they treat of the agglomeration of matter in the heavenly bodies, or of the geographical distribution of terrestrial organisms, are not only in themselves more attractive than special studies, but they also afford superior advantages to those who are unable to devote much time to occupations of this nature. the different branches of the study of natural history are only accessible in certain positions of social life, and do not, at every season p and in every climate, present like enjoyments. thus, in the dreary regions of the north, man is deprived for a long period of the year of the spectacle presented by the activity of the productive forces of organic nature; and if the mind be directed to one sole class of objects, the most animated narratives of voyages in distant lands will fail to interest and attract us, if they do not touch upon the subjects to which we are most partial. as the history of nations -- if it were always able to trace events to their true causes -- might solve the ever-recurring enigma of the oscillations experienced by the alternately progressive and retrograde movement of human society, so might also the physical description of the world, the science of the 'cosmos', if it were grasped by a powerful intellect, and based upon a knowledge of all the results of discovery up to a given period, succeed in dispelling a portion of the contradictions which, at first sight, appear to arise from the complication or phenomena and the multitude of the perturbations simultaneously manifested. the knowledge of the laws of nature, whether we can trace them in the alternate ebb and flow of the ocean, in the measured path of comets, or in the mutual attractions of multiple stars, alike increases our sense of the calm of nature, while the chimera so long cherished by the human mind in its early and intuitive contemplations, the belief in a "discord of the elements," seems gradually to vanish in proportion as science extends her empire. general views lead us habitually to consider each organism as a part of the entire creation, and to recognize in the plant or the animal not merely an isolated species, but a form linked in the chain of being to other forms either living or extinct. they aid us in comprehending the relations that exist between the most recent discoveries and those which have prepared the way for them. although fixed to one point of space, we eagerly grasp at a knowledge of that which has been observed in different and far-distant regions. we delight in tracking the course of the bold mariner through seas of polar ice, or in following him to the summit of that volcano of the antarctic pole, whose fires may be seen from afar, even at mid-day. it is by an acquaintance with the results of distant voyages that we may learn to comprehend some of the marvels of terrestrial magnetism, and be thus led to appreciate the importance of the establishments of the numerous observatories which in the present day cover both hemispheres, and are designed to note p the simultaneous occurrence of perturbations, and the frequency and duration of 'magnetic storms.' let me be permitted here to touch upon a few points connected with discoveries, whose importance can only be estimated by those who have devoted themselves to the study of the physical sciences generally. examples chosen from among the phenomena to which special attention has been directed in recent times, will throw additional light upon the preceding considerations. without a preliminary knowledge of the orbits of comets, we should be unable duly to appreciate the importance attached to the discovery of one of these bodies, whose elliptical orbit is included in the narrow limits of our solar system, and which has revealed the existence of an ethereal fluid, tending to diminish its centrifugal force and the period of its revolution. the superficial half-knowledge, so characteristic of the present day, which leads to the introduction of vaguely comprehended scientific views into general conversation, also gives rise, under various forms, to the expression of alarm at the supposed danger of a collision between the celestial bodies, or of disturbance in the climatic relations of our globe. these phantoms of the imagination are so much the more injurious as they derive their source from dogmatic pretensions to true science. the history of the atmosphere, and of the annual variations of its temperature, extends already sufficiently far back to show the recurrence of slight disturbances in the mean temperature of any given place, and thus affords sufficient guarantee against the exaggerated apprehension of a general and progressive deterioration of the climates of europe. encke's comet, which is one of the three 'interior comets', completes its course in days, but from the form and position of its orbit it is as little dangerous to the earth as halley's great comet, whose revolution is not completed in less than seventy-six years (and which appeared less brilliant in than it had done in ): the interior comet of biela intersects the earth's orbit, it is true, but it can only approach our globe when its proximity to the sun coincides with our winter solstice. the quantity of heat received by a planet, and whose unequal distribution determines the meteorological variations of its atmosphere, depends alike upon the light-engendering force of the sun; that is to say, upon the condition of its gaseous coverings, and upon the relative position of the planet and the central body. p there are variations, it is true, which, in obedience to the laws of universal gravitation, affect the form of the earth's orbit and the inclination of the ecliptic, that is, the angle which the axis of the earth makes with the plane of its orbit; but these periodical variations are so slow, and are restricted within such narrow limits, that their thermic effects would hardly be appreciable by our instruments in many thousands of years. the astronomical causes of a refrigeration of our globe, and of the diminution of moisture at its surface, and the nature and frequency of certain epidemics -- phenomena which are often discussed in the present day according to the benighted views of the middle ages -- ought to be considered as beyond the range of our experience in physics and chemistry. physical astronomy presents us with other phenomena, which can not be fully comprehended in all their vastness without a previous acquirement of general views regarding the forces that govern the universe. such, for instance, are the innumerable double stars, or rather suns, which revolve round one common center of gravity, and thus reveal in distant worlds the existence of the newtonian law; the larger or smaller number of spots upon the sun, that is to say, the openings formed through the luminous and opaque atmosphere surrounding the solid nucleus; and the regular appearance about the th of november and the th of august, of shooting stars, which probably form part of a belt of asteroids, intersecting the earth's orbit, and moving with planetary velocity. descending from the celestial regions to the earth, we would fain inquire into the relations that exist between the oscillations of the pendulum in air (the theory of which has been perfected by bessel) and the density of our planet; and how the pendulum, acting the part of a plummet, can, to a certain extent, throw light upon the geological constitution of strata at great depths? by means of this instrument we are enabled to trace the striking analogy which exists between the formation of the granular rocks composing the lava currents ejected from active volcanoes, and those endogenous masses of granite, porphyry, and serpentine, which, issuing from the interior of the earth, have broken, as eruptive rocks, through the secondary strata, and modified them by contact, either in rendering them harder by the introduction of silex, or reducing them into dolomite, or, finally, by inducing within them the formation of crystals of the most varied composition. the elevation of sporadic islands, of p domes of trachyte, and cones of basalt, by the elastic forces emanating from the fluid interior of our globe, has led one of the first geologists of the age, leopold von buch, to the theory of the elevation of continents, and of mountain chains generally. this action of subterranean forces in breaking through and elevating strata of sedimentary rocks, of which the coast of chili, in consequence of a great earthquake, furnished a recent example, leads to the assumption that the pelagic shells found by m. bonpland and myself on the ridge of the andes, at an elevation of more than , english feet, may have been conveyed to so extraordinary a position, not by a rising of the ocean, but by the agency of volcanic forces capable of elevating into ridges the softened crust of the earth. i apply the term 'volcanic', in the widest sense of the word, to every action exercised by the interior of a planet on its external crust. the surface of our globe, and that of the moon, manifest traces of this action, which in the former, at least, has varied during the course of ages. those who are ignorant of the fact that the internal heat of the earth increases so rapidly with the increase of depth that granite is in a state of fusion about twenty or thirty geographical miles below the surface,* can not have a clear conception of the causes, and the simultaneous occurrence of volcanic eruptions at places widely removed from one another, or of the extent and intersection of 'circles of commotion' in earthquakes, or of the uniformity of temperature, and equality of chemical composition observed in thermal springs during a long course of years. [footnote] * the determinations usually given of the point of fusion are in general much too high for refracting substances. according to the very accurate researches of mitscherlich, the melting point of granite can hardly exceed degrees f. [dr. mantell states in 'the wonders of geology', , vol. i., p. , that this increase of temperature amounts to degree of fahrenheit for every fifty-four feet of vertical depth.] -- tr. the quantity of heat peculiar to a planet is, however, a matter of such importance -- being the result of its primitive condensation, and varying according to the nature and duration of the radiation -- that the study of this subject may throw some degree of light on the history of the atmosphere, and the distribution of the organic bodies imbedded in the solid crust of the earth. this study enables us to understand how a tropical temperature, independent of latitude (that is, of the distance from the poles), may have been produced by deep fissures remaining open, and exhaling heat from the interior p of the globe, at a period when the earth's crust was still furrowed and rent, and only in a state of semi-solidification; and a primordial condition is thus revealed to us, in which the temperature of the atmosphere, and climates generally, were owing rather to a liberation of caloric and of different gaseous emanations (that is to say, rather to the energetic reaction of the interior on the exterior) than to the position of the earth with respect to the central body, the sun. the cold regions of the earth contain, deposited in sedimentary strata, the products of tropical climates; thus, in the coal formations, we find the trunks of palms standing upright amid coniferae, tree ferns, goniatites, and fishes having rhomboidal osseous scales;* in the jura limestone, colossal skeletons of crocodiles, plesiosauri, planulites, and stems of the cycadeae; in the chalk formations, small polythalmia and bryozoa, whose species still exist in our seas; in tripoli, or polishing slate, in the semi-opal and the farina-like opal or mountain meal, agglomerations of siliceous infusoria, which have been brought to light by the powerful microscope of ehrenberg;** and, lastly, in transported soils, and in certain caves, the bones of elephants, hyenas, and lions. [footnote] *see the classical work on the fishes of the old world by agassiz, 'rech. sur les poissons fossiles', , vol. i., p. ; vol. ii., p. , , , app., p. . the whole genus of amblypterus, ag., nearly allied to palaeoniscus (called also palaeothrissum), lies buried beneath the jura formations in the old carboniferous strata. scales which, in some fishes, as in the family of lepidoides (order of ganoides), are formed like teeth, and covered in certain parts with enamel, belong, after the placoides, to the oldest forms of fossil fishes; their living representatives are still found in two genera, the 'bichir' of the nile and senegal, and the 'lepidosteus' of the ohio. [footnote] **[the 'polishing slate' of bilin is stated by m. ehrenberg to form a 'series' of strata fourteen feet in thickness, entirely made up of the siliceous shells of 'gaillonellae', of such extreme minuteness that a cubic inch of the stone contains forty-one thousand millions! the 'bergmehl' ('mountain meal' or 'fossil farina') of san fiora, in tuscany, is one mass of animalculites. see the interesting work of g. a. mantell, 'on the medals of creation', vol. i., p. .] -- tr. an intimate acquaintance with the physical phenomena of the universe leads us to regard the products of warm latitudes that are thus found in a fossil condition in northern regions not merely as incentives to barren curiosity, but as subjects awakening deep reflection, and opening new sources of study. the number and the variety of the objects i have alluded to give rise to the question whether general considerations of physical phenomena can be made sufficiently clear to persons who have not acquired a detailed and special knowledge of p descriptive natural history, geology, or mathematical astronomy? i think we ought to distinguish here between him whose task it is to collect the individual details of various observations, and study the mutual relations existing among them, and him to whom these relations are to be revealed, under the form of general results. the former should be acquainted with the specialities of phenomena, that he may arrive at a generalization of ideas as the result, at least in part, of his own observations, experiments, and calculations. it can not be denied, that where there is an absence of positive knowledge of physical phenomena, the general results which impart so great a charm to the study of nature can not all be made equally clear and intelligible to the reader, but still i venture to hope, that in the work which i am now preparing on the physical laws of the universe, the greater part of the facts advanced can be made manifest without the necessity of appealing to fundamental views and principles. the picture of nature thus drawn, notwithstanding the want of distinctness of some of its outlines, will not be the less able to enrich the intellect, enlarge the sphere of ideas, and nourish and vivify the imagination. there is, perhaps, some truth in the accusation advanced against many german scientific works, that they lessen the value of general views by an accumulation of detail, and do not sufficiently distinguish between those great results which form, as it were, the beacon lights of science, and the long series of means by which they have been attained. this method of treating scientific subjects led the most illustrious of our poets* to exclaim with impatience, "the germans have the art of making science inaccessible." an edifice can not produce a striking effect until the scaffolding is removed, that had of necessity been used during its erection. [footnote] *gothe, in 'die aphorismen uber naturwissenschaft', bd. i., s. ('werke kleine ausgabe','von' .) thus the uniformity of figure observed in the distribution of continental masses, which all terminate toward the south in a pyramidal form, and expand toward the north (a law that determines the nature of climates, the direction of currents in the ocean and the atmosphere, and the transition of certain types of tropical vegetation toward the southern temperate zone), may be clearly apprehended without any knowledge of the geodesical and astronomical operations by means of which these pyramidal forms of continents have been determined. in like manner, physical geography teaches us by how many leagues p the equatorial axis exceeds the polar axis of the globe, and shows us the mean equality of the flattening of the two hemispheres, without entailing on us the necessity of giving the detail of the measurement of the degrees in the meridian, or the observations on the pendulum, which have led us to know that the true figure of our globe is not exactly that of a regular ellipsoid of revolution, and that this irregularity is reflected in the corresponding irregularity of the movements of the moon. the views of comparative geography have been specially enlarged by that admirable work, 'erdkunde im verhÃ�Â�ltniss zur natur und sur geschichte', in which carl ritter so ably delineates the physiognomy of our globe, and shows the influence of its external configuration on the physical phenomena on its surface, on the migrations, laws, and manners of nations, and on all the principal historical events enacted upon the face of the earth. france possesses an immortal work, 'l'exposition du systÃ�Â�me du monde', in which the author has combined the results of the highest astronomical and mathematical labors, and presented them to his readers free from all processes of demonstration. the structure of the heavens is here reduced to the simple solution of a great problem in mechanics; yet laplace's work has never yet been accused of incompleteness and want of profundity. the distinction between dissimilar subjects, and the separation of the general from the special, are not only conducive to the attainment of perspicuity in the composition of a physical history of the universe, but are also the means by which a character of greater elevation may be imparted to the study of nature. by the suppression of all unnecessary detail, the great masses are better seen, and the reasoning faculty is enabled to grasp all that might otherwise escape the limited range of the senses. the exposition of general results has, it must be owned, been singularly facilitated by the happy revolution experienced since the close of the last century, in the condition of all the special sciences, more particularly of geology, chemistry, and descriptive natural history. in proportion as laws admit of more general application, and as sciences mutually enrich each other, and by their extension become connected together in more numerous and more intimate relations, the development of general truths may be given with conciseness devoid of superficiality. on being first examined, all phenomena appear to be p isolated, and it is only by the result of a multiplicity of observations, combined by reason, that we are able to trace the mutual relations existing between them. if, however, in the present age, which is so strongly characterized by a brilliant course of scientific discoveries, we perceive a want of connection in the phenomena of certain sciences, we may anticipate the revelation of new facts, whose importance will probably be commensurate with the attention directed to these branches of study. expectations of this nature may be entertained with regard to meteorology, several parts of optics, and to radiating heat, and electro-magnetism, since the admirable discoveries of melloni and faraday. a fertile field is here opened to discovery, although the voltaic pile has already taught us the intimate connection existing between electric, magnetic, and chemical phenomena. who will venture to affirm that we have any precise knowledge, in the present day, of that part of the atmosphere which is not oxygen, or that thousands of gaseous substances affecting our organs may not be mixed with the nitrogen, or, finally, that we have even discovered the whole number of the forces which pervade the universe? it is not the purpose of this essay on the physical history of the world to reduce all sensible phenomena to a small number of abstract principles, based on reason only. the physical history of the universe, whose exposition i attempt to develop, does not pretend to rise to the perilous abstractions of a purely rational science of nature, and is simply a 'physical geography, combined with a description of the regions of space and the bodies occupying them.' devoid of the profoundness of a purely speculative philosophy, my essay on the 'cosmos' treats of the contemplation of the universe, and is based upon a rational empiricism, that is to say, upon the results of the facts registered by science, and tested by the operations of the intellect. it is within these limits alone that the work, which i now venture to undertake, appertains to the sphere of labor to which i have devoted myself throughout the course of my long scientific career. the path of inquiry is not unknown to me, although it may be pursued by others with greater success. the unity which i seek to attain in the development of the great phenomena of the universe, is analogous to that which historical composition is capable of acquiring. all points relating to the accidental individualities, and the essential variations of the actual, whether in the form and arrangement of natural objects in the struggle of man against the elements, or of nations against nations, do not admit of being p based only on a 'rational foundation' -- that is to say, of being deduced from ideas alone. it seems to me that a like degree of empiricism attaches to the description of the universe and to civil history; but in reflecting upon physical phenomena and events, and tracing their causes by the process of reason, we become more and more convinced of the truth of the ancient doctrine, that the forces inherent in matter, and those which govern the moral necessity, and in accordance with movements occurring periodically after longer or shorter intervals. it is this necessity, this occult but permanent connection, this periodical recurrence in the progressive development of forms, phenomena, and events, which constitute 'nature', obedient to the first impulse imparted to it. physics, as the term signifies, is limited to the explanation of the phenomena of the material world by the properties of matter. the ultimate object of the experimental sciences is, therefore, to discover laws, and to trace their progressive generalization. all that exceeds this goes beyond the province of the physical description of the universe, and appertains to a range of higher speculative views. emmanuel kant, one of the few philosophers who have escaped the imputation of impiety, has defined with rare sagacity the limits of physical explanations, in his celebrated essay 'on the theory and structure of the heavens', published at konigsberg in . the study of a science that promises to lead us through the vast range of creation may be compared to a journey in a far-distant land. before we set forth, we consider, and often with distrust, our own strength, and that of the guide we have chosen. but the apprehensions which have originated in the abundance and the difficulties attached to the subjects we would embrace, recede from view as we remember that with the increase of observations in the present day there has also arisen a more intimate knowledge of the connection existing among all phenomena. it has not unfrequently happened, that the researches made at remote distances have often and unexpectedly thrown light upon subjects which had long resisted the attempts made to explain them within the narrow limits of our own sphere of observation. organic forms that had long remained isolated, both in the animal and vegetable kingdom, have been connected by the discovery of intermediate links or stages of transition. the geography of beings endowed p with life attains completeness as we see the species, genera, and entire families belonging to one hemisphere, reflected as it were, in analogous animal and vegetable forms in the opposite hemisphere. there are, so to speak, the 'equivalents' which mutually personate and replace one another in the great series of organisms. these connecting links and stages of transition may be traced, alternately, in a deficiency or an excess of development of certain parts, in the mode of junction of distinct organs, in the differences in the balance of forces, or in a resemblance to intermediate forms which are not permanent, but merely characteristic of certain phases of normal development. passing from the consideration of beings endowed with life to that of inorganic bodies, we find many striking illustrations of the high state of advancement to which modern geology has attained. we thus see, according to the grand views of elie de beaumont, how chains of mountains dividing different climates and floras and different races of men, reveal to us their 'relative age', both by the character of the sedimentary strata they have uplifted, and by the directions which they follow over the long fissures and which the earth's crust is furrowed. relations of superposition of trachyte and of syenitic porphyry, of diorite and of serpentine, which remain in the rich platinum districts of the oural, and on the south-western declivity of the siberian alti, are elucidated by the observations that have been made on the plateaux of mexico and antioquia, and in the unhealthy ravines of choco. the most important facts on which the physical history of the world has been based in modern times, have not been accumulated by chance. it has at length been fully acknowledged, and the conviction is characteristic of the age, that the narratives of distant travels, too long occupied in the mere recital of hazardous adventures, can only be made a source of instruction where the traveler is acquainted with the condition of the science he would enlarge, and is guided by reason in his researches. it is by this tendency to generalization, which is only dangerous in its abuse, that a great portion of the physical knowledge already acquired may be made the common property of all classes of society; but, in order to render the instruction impaired by these means commensurate with the importance of the subject, it is desirable to deviate as widely as possible from the imperfect compilations designated, till the close of the eighteenth century, by the inappropriate term of 'popular p knowledge.' i take pleasure in persuading myself that scientific subjects may be treated of in language at once dignified, grave, and animated, and that those who are restricted within the circumscribed limits of ordinary life, and have long remained strangers to an intimate communion with nature, may thus have opened to them one of the richest sources of enjoyment, by which the mind is invigorated by the acquisition of new ideas. communion with nature awakens within us perceptive faculties that had long lain dormant; and we thus comprehend at a single glance the influence exercised by physical discoveries on the enlargement of the sphere of intellect, and perceive how a judicious application of mechanics, chemistry, and other sciences may be made conducive to national prosperity. a more accurate knowledge of the connection of physical phenomena will also tend to remove the prevalent error that all branches of natural science are not equally important in relation to general cultivation and industrial progress. an arbitrary distinction is frequently made between the various degrees of importance appertaining to mathematical sciences, to the study of organized beings, the knowledge of electro-magnetism, and investigations of the general properties of matter in its different conditions of molecular aggregation; and it is not uncommon presumptuously to affix a supposed stigma upon researches of this nature, by terming them "purely theoretical," forgetting , although the fact has been long attested, that in the observation of a phenomenon, which at first sight appears to be wholly isolated, may be concealed the germ of a great discovery. when aloysio galvani first stimulated the nervous fiber by the accidental contact of two heterogeneous metals, his contemporaries could never have anticipated that the action of the voltaic pile would discover to us, in the alkalies, metals of a silvery luster, so light as to swim on water, and eminently inflammable; or that it would become a powerful instrument of chemical analysis, and at the same time a thermoscope and a magnet. when hygens first observed, in , the phenomenon of the polarization of light, exhibited in the difference between the two rays into which a pencil of light divides itself in passing through a doubly refracting crystal, it could not have been foreseen that, a century and a half later, the great philosopher arago would, by his discovery of 'chromatic polarization', be led to discern, by means of a small fragment of iceland spar, whether solar light emanates from a solid body or a gaseous covering, or p whether comets transmit light directly or merely by reflection.* [footnote] *arago's discoveries in the year . -- delambro's 'histoire de l'ast.', p. . (passage already quoted.) an equal appreciation of all branches of the mathematical, physical, and natural sciences is a special requirement of the present age, in which the material wealth and the growing prosperity of nations are principally based upon a more enlightened employment of the products and forces of nature. the most superficial glance at the present condition of europe shows that a diminution, or even a total annihilation of national prosperity, must be the award of those states who shrink with slothful indifference from the great struggle of rival nations in the career of the industrial arts. it is with nations as with nature, which, according to a happy expression of gÃ�Â�the,* "knows no pause in progress and development, and attaches her curse on all inaction." [footnote] *gothe, in 'die aphorismen uber naturwissenschaft.' -- 'werke', bd. ., s. the propagation of an earnest and sound knowledge of science can therefore alone avert the dangers of which i have spoken. man can not act upon nature, or appropriate her forces to his own use, without comprehending their full extent, and having an intimate acquaintance with the laws of the physical world. bacon has said that, in human societies, knowledge is power. both must rise and sink together. but the knowledge that results from the free action of thought is at once the delight and the indestructible prerogative of man; and in forming part of the wealth of mankind, it not unfrequently serves as a substitute for the natural riches, which are but sparingly scattered over the earth. those states which take no active part in the general industrial movement, in the choice and preparation of natural substances, or in the application of mechanics and chemistry, and among whom this activity is not appreciated by all classes of society, will infallibly see their prosperity diminish in proportion as neighboring countries become strengthened and invigorated under the genial influence of arts and sciences. as in nobler spheres of thought and sentiment, in philosophy, poetry, and the fine arts, the object at which we aim ought to be an inward one -- an ennoblement of the intellect -- so ought we likewise in our pursuit of science, to strive after a knowledge of the laws and the principles of unity that pervade the vital forces of the universe; and it is by such a course that p physical studies may be made subservient to the progress of industry, which is a conquest of mind over matter. by a happy connection of causes and effects, we often see the useful linked to the beautiful and the exalted. the improvement of agriculture in the hands of freemen, and on properties of a moderate extent -- the flourishing state of the mechanical arts freed from the trammels of municipal restrictions -- the increased impetus imparted to commerce by the multiplied means of the intellectual progress of mankind, and of the amelioration of political institutions, in which this progress is reflected. the picture presented by modern history ought to convince those who are tardy in awakening to the truth of the lesson it teaches. nor let it be feared that the marked predilection for the study of nature, and for industrial progress, which is so characteristic of the present age, should necessarily have a tendency to retard the noble exertions of the intellect in the domains of philosophy, classical history, and antiquity, or to deprive the arts by which life is embellished of the vivifying breath of imagination. where all the germs of civilization are developed beneath the aegis of free institutions and wise legislation, there is no cause for apprehending that any one branch of knowledge should be cultivated to the prejudice of others. all afford the state precious fruits, whether they yield nourishment to man and constitute his physical wealth, or whether, more permanent in their nature, they transmit in the works of mind the glory of nations to remotest posterity. the spartans, notwithstanding their doric austerity, prayed the gods to grant them "the beautiful with the good."* [footnote] *pseudo-plato, -- 'alcib.', xi., p. , ed. steph.; plut., 'instituta laconica', p. , ed. hatten. i will no longer dwell upon the considerations of the influence exercised by the mathematical and physical sciences on all that appertains to the material wants of social life, for the vast extent of the course on which i am entering forbids me to insist further upon the utility of these applications. accustomed to distant excursions, i may, perhaps, have erred in describing the path before us as more smooth and pleasant than it really is, for such is wont to be the practice of those who delight in guiding others to the summits of lofty mountains: they praise the view even when great part of the distant plains lie hidden by clouds, knowing that this half-transparent vapory vail imparts to the scene a certain charm from p the power exercised by the imagination over the domain of the senses. in like manner, from the height occupied by the physical history of the world, all parts of the horizon will not appear equally clear and well defined. this indistinctness will not, however, be wholly owing to the present imperfect state of some of the sciences, but in part, likewise, to the unskillfulness of the guide who has imprudently ventured to ascend these lofty summits. the object of this introductory notice is not, however, solely to draw attention to the importance and greatness of the physical history of the universe, for in the present day these are too well understood to be contested, but likewise to prove how, without detriment to the stability of special studies, we may be enabled to generalize our ideas by concentrating them in one common focus, and thus arrive at a point of view from which all the organisms and forces of nature may be seen as one living active whole, animated by one sole impulse. "nature," as schelling remarks in his poetic discourse on art, "is not an inert mass; and to him who can comprehend her vast sublimity, she reveals herself as the creative force of the universe -- before all time, eternal, ever active, she calls to life all things, whether perishable or imperishable." by uniting, under one point of view, both the phenomena of our own globe and those presented in the regions of space, we embrace the limits of the science of the 'cosmos', and convert the physical history of the globe into the physical history of the universe, the one term being modeled upon that of the other. this science of the cosmos is not, however, to be regarded as a mere encyclopedic aggregation of the most important and general results that have been collected together from special branches of knowledge. these results are nothing more than the materials for a vast edifice, and their combination can not constitute the physical history of the world, whose exalted part it is to show the simultaneous action and the connecting links of the forces which pervade the universe. the distribution of organic types in different climates and at different elevations -- that is to say, the geography of plants and animals -- differs as widely from botany and descriptive zoology as geology does from mineralogy, properly so called. the physical history of the universe must not, therefore, be confounded with the 'encyclopedias of the natural sciences', as they have hitherto been compiled, and whose title is as vague as their limits are ill defined. in the work before us, partial facts will be considered only in relation to the whole. p the higher the point of view, the greater is the necessity for a systematic mode of treating the subject in language at once animated and picturesque. but thought and language have ever been most intimately allied. if language, by its originality of structure and its native richness, can, in its delineations, interpret thought with grace and clearness, and if, by its happy flexibility, it can paint with vivid truthfulness the objects of the external world, it reacts at the same time upon thought, and animates it, as it were, with the breath of life. it is this mutual reaction which makes words more than mere signs and forms of thought; and the beneficent influence of a language is most strikingly manifested on its native soil, where it has sprung spontaneously from the minds of the people, whose character it embodies. proud of a country that seeks to concentrate her strength in intellectual unity, the writer recalls with delight the advantages he has enjoyed in being permitted to express his thoughts in his native language; and truly happy is he who, in attempting to give a lucid exposition of the great phenomena of the universe, is able to draw from the depths of a language, which, through the free exercise of thought, and by the effusions of creative fancy, has for centuries past exercised so powerful an influence over the destinies of man. this material taken from pages to cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p limits and method of exposition of the physical description of the universe. i have endeavored, in the preceding part of my work, to explain and illustrate, by various examples, how the enjoyments presented by the aspect of nature, varying as they do in the sources from when they flow, may be multiplied and ennobled by an acquaintance with the connection of phenomena and the laws by which they are regulated. it remains, then, for me to examine the spirit of the method in which the exposition of the 'physical description of the universe' should be conducted, and to indicate the limits of this science in accordance with the views i have acquired in the course of my studies and travels in various parts of the earth. i trust i may flatter myself with a hope that a treatise of this nature will justify the title i have ventured to adopt for my work, and exonerate me from the reproach of a presumption that would be doubly reprehensible in a scientific discussion. before entering upon the delineation of the partial phenomena p which are found to be distributed in various groups, i would consider a few general questions intimately connected together, and bearing upon the nature of our knowledge of the external world and its different relations, in all epochs of history and in all phases of intellectual advancement. under this head will be comprised the following considerations: . the precise limits of the physical description of the universe, considered as a distinct science. . a brief enumeration of the totality of natural phenomena, presented under the form of a 'general delineation of nature.' . the influence of the external world on the imagination and feelings, which has acted in modern times as a powerful impulse toward the study of natural science, by giving animation to the description of distant regions and to the delineation of natural scenery, as far as it is characterized by vegetable physiognomy and by the cultivation of exotic plants, and their arrangement in well-contrasted groups. . the history of the contemplation of nature, or the progressive development of the idea of the cosmos, considered with reference to the historical and geographical facts that have led to the discovery of the connection of phenomena. the higher the point of view from which natural phenomena may be considered, the more necessary it is to circumscribe the science within its just limits, and to distinguish it from all other analogous or auxiliary studies. physical cosmography is founded on the contemplation of all created things -- all that exists in space, whether as substances or forces -- that is, all the material beings that constitute the universe. the science which i would attempt to define presents itself, therefore, to man, as the inhabitant of the earth, under a two-fold form -- as the earth itself and the regions of space. it is with a view of showing the actual character and the independence of the study of physical cosmography, and at the same time indicating the nature of its relations to 'general physics, descriptive natural history, geology, and comparative geography', that i will pause for a few moments to consider that portion of the science of the cosmos which concerns the earth. as the history of philosophy does not consist of a mere material enumeration of the philosophical views entertained in different ages, neither should the physical description of the universe be a simple encyclopedic compilation of the sciences we have enumerated. the difficulty of defining the limits of intimately-connected studies has been increased, because for centuries it has been customary to designate various branches p of empirical knowledge by terms which admit either of too wide or too limited a definition of the ideas which they were intended to convey, and are, besides, objectionable from having had a different signification in those classical languages of antiquity from thish chey have been borrowed. the terms physiology, physics, natural history, geology and geography arose, and were commonly used, long before clear ideas were entertained of the diversity of objects embraced by these sciences, and consequently of their reciprocal limitation. such is the influence of long habit upon language, that by one of the nations of europe most advanced in civilization the word "physic" is applied to medicine, while in a society of justly deserved universal reputation, technical chemistry, geology and astronomy (purely experimental sciences) are comprised under the head of "philosophical transactions." an attempt has often been made, and almost always in vain, to substitute new and more appropriate terms for these ancient designations, which, notwithstanding their undoubted vagueness, are now generally understood. these changes have been proposed, for the most part, by those who have occupied themselves with the general classification of the various branches of knowledge, from the first appearance of the great encyclopedia ('margarita philosophica') of gregory reisch,* prior of the chartreuse at freiburg, toward the close of the fifteenth century, to lord bacon, and from bacon to d'alembert; and in recent times to an eminent physicist, andre marie ampere.** [footnote] *the 'margarita philosophica' of gregory reisch, prior of the chartreuse at freiburg, first appeared under the following title: aepitome omnis philosophiÃ�¾, alias margarita philosophica, tractans de omni generi scibili. the heidelberg edition ( ), and that of strasburg ( ), both bear this title, but the first part was suppressed in the freiburg edition of the same year, as well as in the twelve subsequent editions, which succeeded one another, at short intervals, till . this work exercised a great influence on the diffusion of mathematical and physical sciences toward the beginning of the sixteenth century, and crasles, the learned author of 'l'aperÃ�Â�u historique des methodes en gÃ�Â�ometrica' ( ) has shown the great importance of reisch's 'encyclopedia' in the history of mathematics in the middle ages. i have had recourse to a passage in the 'margarita philosophica', found only in the edition of , to elucidate the important question of the relations between the statements of the geographer of saint-die, hylacomilus (martin waldseemuller), the first who gave the name of america to the new continent, and those of amerigo vespucci, rene, king of jerusalem and duke of lorraine, as also those contained in the celebrated editions of ptolemy of and . see my 'examen critique de la gegraphie du nouveau continent, et des progres de l'astronomie nautique aux e et e siecles', t. iv., p. - . [footnote] ii ampÃ�Â�re, 'essai sur la phil. des sciences', , p. . whewell, 'philosophy of the inductive sciences', vol. ii., p. . park, 'pantology', p. . p the selection of an inappropriate greek nomenclature has perhaps been even more prejudicial to the last of these attempts than the injudicious use of binary divisions and the excessive multiplication of groups. the physical description of the world, considering the universe as an object of the external senses, does undoubtedly require the aid of general physics and of descriptive natural history, but thecontemplation of all created things, which are linked together, and form one 'whole', animated by internal forces, given to the science we are considering a peculiar character. phyical science considers only the general properties of bodies; it is the product of abstraction -- a generalization of perceptible phenomena; and even in the work in which were laid the first foundations of general physics, in the eight books on physics of aristotle,* all the phenomena of nature are considered as depending upon the primitive and vital action of one sole force, from which emaate all the movements of the universe. [footnote] * all changes in the physical world may be reduced to motion. aristot., 'phys. ausc.', iii., and , p. , . bekker, viii., , , and , p. , , . 'de genere et corr.', ii., , p. . pseudo-aristot., 'de mundo.' cap. vi., p. . the terrestrial portion of physical cosmography, for which i would willingly retain the expressive designation of 'physical geography', treats of the distribution of magnetism in our planet with relation to its intensity and direction, but does not enter into a consideration of the laws of attraction or repulsion of the poles, or the means of eliciting either permanent or transitory electro-magnetic currents. physical geography depicts in broad outlines the even or irregular configuration of continents, the relations of superficial area, and the distribution of continental masses in the two hemispheres, a distribution which exercises a powerful influence on the diversity of climate and the meteorological modifications of the atmosphere; this science defines the character of mountain chains, which, having been elevated at different epochs, constitute distinct systems, whether they run in parallel lines or intersect one another; determines the mean height of continents above the level of the sea, the position of the center of gravity of their volume, and the relation of the highest summits of mountain chains to the mean elevation of their crests, or to their proximity with the sea-shore. it depicts the eruptive rocks as principles of movement, acting upon the sedimentary rocks by traversing, uplifting, and inclining them at various angles; it p considers volcanoes either as isolated, or ranged in single or in double series, and extending their sphere of action to various distances, either by raising long and narrow lines of rocks, or by means of circles of commotion, which expand or diminish in diameter in the course of ages. this terrestrial portion of the science of the cosmos describes the strife of the liquid element with the solid land; it indicates the features possessed in common by all great rivers in the upper and lower portion of their course, and in their mode of bifurcation when their basins are unclosed; and shows us rivers breaking through the highest mountain chains, or following for a long time a course parallel to them, either at their base, or at a considerable distance, where the elevation of the strata of the mountain system and the direction of their inclination correspond to the configuration of the table-land. it is only the general results of comparative orography and hydrography that belong to the science whose true limits i am desirous of determining, and not the special enumeration of the greatest elevations of our globe, of active volcanoes, of rivers, and the number of their tributaries, these details falliing rather within the domain of geography, properly so called. we would here only consider phenomena in their mutual connection, and in their relations to different zones of our planet, and to its physical constitution generally. the specialties both of inorganic and organized matter, classed according to analogy of form and composition, undoubtedly constitute a most interesting branch of study, but they appertain to a sphere of ideas having no affinity with the subject of this work. the description of different countries certainly furnishes us with the most important materials for the composition of a physical geography; but the combination of these different descriptions, ranged in series, would as little give us a true image of the general conformation of the irregular surface of our globe, as a succession of all the floras of different regions would constitute that which i designate as a 'geography of plants.' it is by subjecting isolated observations to the process of thought, and by combining and comparing them, that we are enabled to discover the relations existing in common between the climatic distribution of beings and the individuality of organic forms (in the morphology or descriptive natural history of plants and animals); and it is by induction that we are led to comprehend numerical laws, the proportion of natural families to the whole number of species, and to designate the latitude or geographical position of the zones in whose p plains each organic form attains the maximum of its development. considerations of this nature, by their tendency to generalization, impress a nobler character on the physical description of the globe, and enable us to understand how the aspect of the scenery, that is to say, the impression produced upon the mind by the physiognomy of the vegetation, depends upon the local distribution, the number, and the luxuriance of growth of the vegetable forms predominating in the general mass. the catalogues of organized beings to which was formerly given the pompous title of 'systems of nature', present us with an admirably connected arrangement by analogies of structure, either in the perfected development of these beings, or in the different phases which, in accordance with the views of a spiral evolution, affect in vegetables the leaves, bracts, calyx, corolla and fructifying organs; and in animals, with more or less symmetrical regularity, the cellular and fibrous tissues, and their perfect or but obscurely developed articulations. but these pretended systems of nature, however ingenious their mode of classification may be, do not show us organic beings as they are distributed in groups throughout our planet, according to their different relations of latitude and elevation above the level of the sea, and to climatic influences, which are owing to general and often very remote causes. the ultimate aim of physical geography is, however, as we have already said, to recognise unity in the vast diversity of phenomena, and by the exercise of thought and the combination of observations, to discern the constancy of phenomena in the midst of apparent changes. in the exposition of the terrestrial portion of the cosmos, it will occasionally be necessary to descend to very special facts; but this will only be in order to recall the connection existing between the actual distribution of organic beings over the globe, and the laws of the ideal classification by natural families, analogy of internal organization and progressive evolution. it follows from these discussions on the limits of the various sciences, and more particularly from the distinction which must necessarily be made between descriptive botany (morphology of vegetables) and the geography of plants, that in the physical history of the globe, the innumerable multitude of organized bodies which embellish creation are considered rather according to 'zones of habitation' or 'stations', and to differently inflected 'isothermal bands', than with reference to the principles of gradation in the development of internal organism. notwithstanding this, botany and zoology, which constitute p the descriptive natural history of all organized beings, are the fruitful sources whence we draw the materials necessary to give a solid basis to the study of the mutual relations and connection of phenomena. we will here subjoin one important observation by way of elucidating the connection of which we have spoken. the first general glance over the vegetation of a vast extent of a continent shows us forms the most dissimilar -- graminae and orchideae, coniferae and oaks, in local approximation to one another; while natural families and genera, instead of being locally associated, are dispersed as if by chance. this dispersion is, however, only apparent. the physical description of the globe teaches us that vegetation every where presents numerically constant relations in the development of its forms and types; that in the same climates, the species which are wanting in one country are replaced in a neighboring one by other species of the same family; and that this 'law of substitution', which seems to depend upon some inherent mysteries of the organism, considered with reference to its origin, maintains in contiguous regions a numerical relation between the species of various great families and the general mass of the phanerogamic plants constituting the two floras. we thus revealed in the multiplicity of the distinct organizations by which these regions are occupied; and we also discover in each zone, and diversified according to the families of plants, a slow but continuous action on the aerial ocean, depending upon the influence of light -- the primary condition of all organic vitality -- on the solid and liquid surface of our planet. it might be said, in accordance with a beautiful expression of lavoisier, that the ancient marvel of the myth of prometheus was incessantly renewed before our eyes. if we extend the course which we have proposed, following in the exposition of the physical description of the earth to the sidereal part of the science of the cosmos, the delineation of the regions of space and the bodies by which they are occupied, we shall find our task simplified in no common degree. if, according to ancient but unphilosophical forms of nomenclature, we would distinguish between 'physics', that is to say, general considerations on the essence of matter, and the forces by which it is actuated, and 'chemistry', which treats of the nature of substances, their elementary composition, and those attractions that are not determined solely by the relations of mass, we must admit that the description of the earth comprises at p once 'physical' and 'chemical' actions. in addition to gravitation, which must be considered as a primitive force in nature, we observe that attractions of another kind are at work around us, both in the interior of our planet and on its surface. these forces, to which we apply the term 'chemical affinity', act upon molecules in contact, or at infinitely minute distances from one another,* and which, being differently modified by electricity, heat, condensation in porous bodies, or by the contact of an intermediate substance, animate equally the inorganic world and animal and vegetable tissues. [footnote] * on the question already discussed by newton, regarding the difference existing between the attraction of masses and molecular attraction, see laplace, 'exposition du systeme du monde', p. , and supplement to book x. of the 'mecanique celeste', p. , ; kant, 'metaph. anfangegrunde der naturwissenschaft, sÃ�Â�m. werke', , bd. v., s. (metaphysical principles of the natural sciences); pectet, 'physique', , vol. i., p. - . if we except the small asteroids, which appear to us under the forms of aerolites and shooting stars, the regions of space have hitherto presented to our direct observation physical phenomena alone; and in the case of these, we know only with certainty the effects depending upon the quantitative relations of matter of the distribution of masses. the phenomena of the regions of space may consequently be considered as influenced by simple dynamical laws -- the laws of motion. the effects that may arise from the specific difference and the hererogeneous nature of matter have not hitherto entered into our calculations of the mechanism of the heavens. the only means by which the inhabitants of our planet can enter into relation with the matter contained within the regions of space, whether existing in scattered forms or united into large spheroids, is by the phenomena of light, the propagation of the force of gravitation or the attraction of masses. the existence of a periodical action of the sun and moon on the variations of terrestrial magnetism is even at the present day extremely problematical. we have no direct experimental knowledge regarding the properties and specific qualities of the masses circulating in space, or of the matter of which they are probably composed, if we except what may be derived from the fall of aerolites or meteoric stones, which, as we have already observed, enter within the limits of our terrestrial sphere. it will be sufficient here to remark, that the direction and the excessive velocity of projection (a velocity wholly planetary) manifested by these masses, render it more than probable that p they are small celestial bodies, which, being attracted by our planet, are made to deviate from their original course, and thus reach the earth enveloped in vapors, and in a high state of actual incandescence. the familiar aspect of these asteroids, and the analogies which they present with the minerals composing the earth's crust, undoubtedly afford ample grounds for surprise,* but, in my opinion, the only conclusion to be drawn from these facts is that, in general, planets and other sidereal masses, which by the influence of a central body, have been agglomerated into rings of vapor, and subsequently into spheroids, being integrant parts of the same system, and having one common origin, may likewise be composed of substances chemically identical. [footnote] i[the analysis of an aerolite which fell a few years since in maryland, united states, and was examined by professor silliman, of new haven, connecticut, gave the following results: oxyd of iron, ; oxyd of nickel, . ; silica, with earthy matter, . ; sulphur, a trace - . . dr. mantell's 'wonders of geology', , vol. i., p. .] -- 'tr.' again, experiments with the pendulum, particularly those prosecuted with such rare precision by bessel, confirm the newtonian axiom, that bodies the most heterogeneous in their nature (as water, gold, quartz, granular limestone, and different masses of aerolites) experience a perfectly similar degree of acceleration from the attraction of the earth. to the experiments of the pendulum may be added the proofs furnished by purely astronomical observations. the almost perfect identity of the mass of jupiter, deduced from the influence exercised by this stupendous planet on its own satellites, on enck's comet of short period, and on the small planets vesta, juno, ceres, and pallas, indicates with equal certainty that within the limits of actual observation attraction is determined solely by the quantity of matter.* [footnote] *poisson, 'connaissances des temps pour l'anne' , p. - . bessel, poggendorf's 'annalen', bd. xxv., s. . encke, 'abhandlungen der berliner academie' (trans. of the berlin academy), , s. . mitscherlich, 'lehrbuch der chemie' (manual of chemistry), bd. i. s. . this absence of any perceptible difference in the nature of matter, alike proved by direct observation and theoretical deductions, imparts a high degree of simplicity to the mechanism of the heavens. the immeasurable extent of the regions of space being subjected to laws of motion alone, the sidereal portion of the science of the cosmos is based on the pure and abundant source of mathematical astronomy, as is the terrestrial portion on physics, chemistry, and organic morphology; but the domain of these three last-named sciences embraces p the consideration of phenomena which are so complicated and have, up to the present time, been found so little susceptible of the application of rigorous method, that the physical science of the earth can not boast of the same certainty and simplicity in the exposition of facts and their mutual connection which characterize the celestial portion of the cosmos. it is not improbable that the difference to which we allude may furnish an explanation of the cause which, in the earliest ages of intellectual culture among the greeks, directed the natural philosophy of the pythagoreans with more ardor to the heavenly bodies and the regions of space than to the earth and its productions, and how through philolaus, and subsequently through the analogous views of aristarchus of samos, and of seleucus of erythrea, this science has been made more conducive to the attainment of a knowledge of the true system of the world than the natural philosophy of the ionian school could ever be to the physical history of the earth. giving but little attention to the properties and specific differences of matter filling space, the great italian school, in its doric gravity, turned by preference toward all that relates to measure, to the form of bodies, and to the number and distances of the planets,* while the ionian physicists directed their attention to the qualities of matter, its true or supposed metamorphoses, and to relations of origin. [footnote] *compare otfried muller's 'dorien', bd. i., s. . it was reserved for the powerful genius of aristotle, alike profoundly speculative and practical to sound with equal success the depths of abstraction and the inexhaustible resources of vital activity pervading the material world. several highly distinguished treatises on physical geography are prefaced by an introduction, whose purely astronomical sections are directed to the consideration of the earth in its planetary dependence, and as constituting a part of that great system which is animated by one central body, the sun. this course is diametrically opposed to the one which i propose following. in order adequately to estimate the dignity of the cosmos, it is requisite that the sidereal portion, termed by kant the 'natural history of the heavens', should not be made subordinate to the terrestrial. in the science of the cosmos, according to the expression of aristarchus of samos, the pioneer of the copernican system, the sun, with its satellites, was nothing more than one of the innumerable stars by which space is occupied. the physical history of the world must, therefore, begin with the description of the heavenly bodies, p and with a geographical sketch of the universe, or, i would rather say, a true 'map of th world', such as was traced by the bold hand of the elder herschel. if, notwithstanding the smallness of our planet, the most considerable space and the most attentive consideration be here afforded to that which exclusively concerns it, this arises solely from the disproportion in the extent of our knowledge of that which is accessible and of that which is closed to our observation. this subordination of the celestial to the terrestrial portion is met with in the great work of bernard varenius,* which appeared in the middle of the seventeenth century. [footnote] *'geographia generalis in qua affectiones generales telluris explicantur.' the oldest elzevir edition bears date , the second , and the third ; these were published at cambridge, under newton's supervision. this excellent work by varenius is, in the true sense of the words, a physical description of the earth. since the work 'historia natural de las indias', , in which the jesuit joseph de acosta sketched in so masterly a manner the delineation of the new continent, questions relating to the physical history of the earth have never been considered with such admirable generality. acosta is richer in original observations, while varenius embraces a wider circle of ideas, since his sojourn in holland, which was at that period the center of vast commercial relations, had brought him in contact with a great number of well-iinformed travelers. 'generalis sive universalis geographia dictur quae tellurem in genere considerat atque affectiones explicat, non habita particularium regionum ratione.' the general description of the earth by varenius ('pars absoluta', cap. i.-xxii.) may be considered as a treatise of comparative geography, if we adopt the term used by the author himself ('geographia comparativa', cap. xxxiii.-xl.), although this must be understood in a limited acceptation. we may cite the following among the most remarkable passages of this book: the enumeration of the systems of mountains; the examination of the relations existing between their directions and the general form of continents (p. , , ed. cantab., ); a list of extinct volcanoes, and such as were still in a state of activity; the discussion of facts relative to the general distribution of islands and archipelagoes (p. ); the depth of the ocean relatively to the height of neighboring coasts (p. ); the uniformity of level observed in all open seas (p. ); the dependence of currents on the prevailing winds; the unequal saltness of the sea; the configuration of shores (p. ); the direction of the winds as the result of differences of temperature, etc. we may further instance the remarkable considerations of varenius regarding the equinoctial current from east to west, to which he attributes the origin of the gulf stream, beginning at cape st. augustin, and issuing forth between cuba and florida (p. ). nothing can be more accurate than his description of the current which skirts the western coast of africa, between cape verde and the island of fernando po in the gulf of guinea. varenius explains the formation of sporadic islands by supposing them to be "the raised bottom of the sea:" 'magna spirituum inclusorum vi, sicut aliquando montes e terra protusos esse quidam scribunt' (p. ). the edition published by newton in ('auctior et emendatior' unfortunately contains no additions from this great authority; and there is not even mention made of the polar compression of the globe, although the experiments on the pendulum by richer had been made nine years prior to the appearance of the cambridge edition. newton's 'principia mathematica philosophie naturalis' were not communicated in manuscript to the royal society until april, . much uncertainty seems to prevail regarding the birth-place of varenius. jaecher says it was england, while, according to 'la biographie universelle' (b.xlvii., p. ), he is stated to have been born at amsterdam; but it would appear, from the dedicatory address to the burgomaster of that city (see his 'geographia comparativa', that both suppositions are false. varenius expressly says that he had sought refuge in amsterdam, "because his native city had been burned and completely destroyed during a long war," words which appear to apply to the north of germany, and to the devastations of the thirty years' war. in his dedication of another work, 'descriptio regni japoniae' (amst., ), to the senate of hamburgh, varenius says that he prosecuted his elementary mathematical studies in the gymnasium of that city. there is, therefore, every reason to believe that this admirable geographer was a native of germany, and was probably born at luneburg ('witten. mem. theol.', , p. ; zedler, 'universal lexicon', vol. xlvi., , p. ). p he was the first to distinguish between 'general and special geography', the former of which he subdivides into an 'absolute', or, properly speaking, 'terrestrial' part, and a 'relative or planetary' portion, according to the mode of considering our planet either with reference to its surface in its different zones, or to its relations to the sun and moon. it redounds to the glory of varenius that his work on 'general and comparative geography' should in so high a degree have arrested the attention of newton. the imperfect state of many of the auxiliary sciences from which this writer was obliged to draw his materials prevented his work from corresponding to the greatness of the design, and it was reserved for the present age, and for my own country, to see the delineation of comparative geography, drawn in its full extent, and in all its relations with the history of man, by the skillful hand of carl ritter.* [footnote] *carl ritter's 'erdkunde im verhÃ�Â�ltniss zur natur und zur geschichte des menschen, oder allgemeine vergleichende geographie' (geography in relation to nature and the history of man, or general comparative geography). the enumeration of the most important results of the astronomical and physical sciences which in the history of the cosmos radiate toward one common focus, may perhaps, to a certain degree, justify the designation i have given to my work, and, considered within the circumscribed limits i have proposed to myself, the undertaking may be esteemed less adventurous than the title. the introduction of new terms, especially with reference to the general results of a science which p ought to be accessible to all, has always been greatly in opposition to my own practice; and whenever i have enlarged upon the established nomenclature, it has only been in the specialities of descriptive botany and zoology, where the introduction of hitherto unknown objects rendered new names necessary. the denominations of physical descriptions of the universe, or physical cosmography, which i use indiscriminantely, have been modeled upon those of 'physical descriptions of the earth', that is to say, 'physical geography', terms that have long been in common use. descartes, whose genius was one of the most powerful manifested in any age, has left us a few fragments of a great work, which he intended publishing under the title of 'monde', and for which he had prepared hiimself by special studies, including even that of human anatomy. the uncommon, but definite expression of the 'science of the cosmos' recalls to the mind of the inhabitant of the earth that we are treating of a more widely-extended horizon -- of the assemblage of all things with which space is filled, from the remotest nebulae to the climatic distribution of those delicate tissues of vegetable matter which spread a variegated covering over the surface of our rocks. the influence of narrow-minded views peculiar to the earlier ages of civilization led in all languages to a confusion of ideas in the synonymic use of the words 'earth' and 'world', while the common expressions 'voyages round the world', 'map of the world', and 'new world', afford further illustrations of the same confusion. the more noble and precisely-defined expressions of 'system of the world', 'the planetary world', and 'creation and age of the world', relate either to the totality of the substances by which space is filled, or to the origin of the whole universe. it was natural that, in the midst of the extreme variability of phenomena presented by the surface of our globe, and the aerial ocean by which it is surrounded, man should have been impressed by the aspect of the vault of heaven, and the uniform and regular movements of the sun and planets. thus the word cosmos, which primitively, in the homeric ages, indicated an idea of order and harmony, was subsequently adopted in scientific language, where it was gradually applied to the order observed in the movements of the heavenly bodies, to the whole universe, and then finally to the world in which this harmony was reflected to us. according to the assertion of philolaus, whose fragmentary works have been so ably commented upon by bÃ�Â�ckh, and conformably to the general testimony p of antiquity, pythagoras was the first who used the word cosmos to designate the order that reigns in the universe, or entire world.* [footnote] *[greek word], in the most ancient, and at the same time most precise, definition of the word, signified 'ornament' (as an adornment for a man, a woman, or a horse); taken figuratively for [greek word], it implied the order or adornment of a discourse. according to the testimony of all the ancients, it was pythagoras who first used the word to designate the order in the universe, and the universe itself. pythagoras left no writings; but ancient attestation to the truth of this assertion is to be found in several passages of the fragmentary works of philolaus (stob., 'eclog.', p. and , heeren), p. , , in bockh's german edition. i do not, according to the example of nake, cite timof locris, since his authenticity is doubtful. plutarch ('de plac. phil.', ii., i) says, in the most express manner, that pythatoras gave the name of cosmos to the universe on account of the order which reigned throughout it; so likewise does galen ('hist. phil.', p. ). this word, together with its novel signification, passed from the schools of philosophy into the language of poets and prose writers. plato designates the heavenly bodies by the name of 'uranos', but the order pervading the regions of space he too terms the cosmos, and in his 'timus' (p. a.) he says 'that the world is an animal endowed with a soul' [greek words]. compare anaxag. claz., ed. schaubach, p. iii, and plut. ('de plac. phil.', in aristotle ('de caelo', i, ), 'cosmos' signifies "the universe and the order pervading it," but it is likewise considered as divided in space into two parts -- the sublunary world, and the world above the moon. ('meteor.', i., w, , and i., , , p. , 'a', and , 'b', bekk.) the definition of cosmos, which i have already cited is taken from pseudo-aristoteles 'de mundo', cap. ii. (p. ); the passage referred to is as follows: [greek words]. most of the passages occurring in greek writers on the word 'cosmos' may be found collected together in the controversy between richard bentley and charles boyle ('opuscula philologica', , p. , ; 'dissertation upon the epistles of phalaris', , p. ); on the historical existence of zaleucus, legislator of leucris, in nake's excellent work, 'sched. crit.', , p. , ; and, finally in theophilus schmidt, 'ad cleom. cycl. theor.', met. i., , p. ix., and . taken in a more limited sense, the word cosmos is also used in the plural (plut., , ), either to designate the stars (stob., , p. ; plut., , ) or the innumerable systems scattered like islands through the immensity of space, and each composed of a sun and a moon. (anax. claz., 'fragm.', p. , , ; brandis, 'gesch. der griechisch-rÃ�Â�mischen philosophie', b. i., s. (history of the greco-roman philosophy). each of these groups forming thus a 'cosmos', the universe, [greek words], the word must be understood in a wider sense (plut., ii., ). it was not until long after the time of the ptolemies that the word was applied to the earth. bockh has made known inscriptions in praise of trajan and adrian ('corpus inscr. graec.', i, n. and ), in which [greek word] occurs for [greek word] in the same manner as we still use the term 'world' to signify the earth alone. we have already mentioned the singular division of the regions of space p [footnote continues] into three parts, the 'olympus, cosmos' and 'ouranos' (stob., i., p. ; philolaus, p. , ); this division applies to the different regions surrounding that mysterious focus of the universe, the [greek words] of the pythagoreans. in the fragmentary passage in which this division is found, the term [greek word] designates the innermost region, situated between the moon and earth; this is the domain of changing things. the middle region, where the planets circulate in an invariable and harmonious order, is, in accordance with the special conceptions entertained of the universe, exclusively termed 'cosmos', while the word 'olympus' is used to express the exterior or igneous region. bopp, the profound philologist, has remarked that we may deduce, as pott has done, 'etymol. forschungen', th.i., s. and ('etymol. researches'), the word [greek word] from the sanscrit root 'sud', 'purificari', by assuming two conditions; first that the greek letter 'kappa' in [greek word] comes from the palatial 'epsilon', which bopp represents by 's' and pott by 'Ã�Â�' (in the same manner as [greek word], 'decem, taihun' in gothic, comes from the indian word 'dasan'), and, next, that the indian 'd'' corresponds, as a general rule, with the greek 'theta' ('vergleichende grammatik' -- comparative grammar), which shows the relation of [greek word] (for [greek word]) with the sanscrit root 'sud', whence is also derived [greek word]. another indian term for the world is 'gagat' (pronounced 'dschagat'), which is, properly speaking the present participle of the verb 'gagami' (i go), the root of which is 'ga.' in restricting ourselves to the circle of hellenic etymologies, we find ('etymol. m.', p. , ) that [greek word] is intimately associated with [greek word] or rather with [greek word], whence we have [greek word] or [greek word] welcker ('eine kretische col in theben', s. -- a cretan colony in thebes) combines with this the name [greek word] , as in hesychius [greek word] signifies a cretan suit of arms. when the scientific language of greece was introduced among the romans, the word 'mundus', which at first had only the primary meaning of [greek word] (female ornament), was applied to designate the entire universe. ennius seems to have been the first who ventured upon this innovation. in one of the fragments of this poet, preserved by macrobius, on the occasion of his quarrel with virgil, we find the word used in its novel mode of acceptation: "mundus caeli vastus constitit silentio" (sat., vi., ). cicero also says, "quem nos lucentem mundum vocamus" (timÃ�¾us, 's.de univer.', cap. x.) the sanscrit root 'mand' from which pott derives the latin 'mundus' ('etym. forsch.', th. i., s. ), combines the double signification of shining and adorning. 'loka' designates in sanscrit the world and people in general, in the same manner as the french word 'monde', and is derived according to bopp, from 'lok' (to see and shine); it is the same with the slavonic root 'swjet', which means both 'light' and 'world.' (grimm, 'deutsche gramm.', b. iii., s. -- german grammar.) the word 'welt', which the germans make use of at the present day, and which was 'weralt' in old german, 'worold' in old saxon, and 'weruld' in anglo-saxon, was, according to james grimm's interpretation, a period of time, an age ('saeculum') rather than a term used for the world in space. the etruscans figured to themselves 'mundus' as an inverted dome, symmetrically opposed to the celestial vault (otfried muller's 'etrusken', th. ii., s. , etc.). taken in a still more limited sense, the word appears to have signified among the goths the terrestrial surface girded by seas ('marei, meri',) the 'merigard', literally, 'garden of seas.' from the italian school of philosophy, the expression passed, in this signification, into the language of those early poets p of nature, parmenides and empedocles, and from thence into the works of prose writers. we will not here enter into a discussion of the manner in which, according to the pythagorean views, philolaus distinguishes between olympus, uranus, or the heavens, and cosmos, or how the same word, used in a plural sense, could be applied to certain heavenly bodies (the planets) revolving round one central focus of the world, or to groups of stars. in this work i use the word cosmos in conformity with the hellenic usage of the term subsequently to the time of pythagorus, and in accordance with the precise definition given of it in the treatise entitled 'de mundo', which was long erroneously attributed to aristotle. it is the assemblage of all things in heaven and earth, the universality of created things constituting the perceptible world. if scientific terms had not long been diverted from their true verbal signification, the present work ought rather to have borne the title of 'cosmography', divided into 'uranography' and 'geography.' the romans, in their feeble essays on philosophy, imitated the greeks by applying to the universe the term 'mundus', which, in its primary meaning, indicated nothing more than ornament, and did not even imply order or regularity in the disposition of parts. it is probable that the introduction into the language of latium of this technical term as an equivalent for cosmos, in its double signification, is due to ennius,* who was a follower of the italian school, and the translator of the writings of epicharmus and some of his pupils on the pythagorean philosophy. [footnote] *see, on ennius, the ingenious researches of leopold krahner, in his 'grundlinien zur geschichte des verfalls der romischen staats-reigion', , s. - (outlines of the history of the decay of the established religion among the romans). in all probability, ennius did not quote from writings of epicharmus himself, but from poems composed in the name of that philosopher, and in accordance with his views. we would first distinguish between the physical 'history' and the physical 'description' of the world. the former, conceived in the most general sense of the word, ought, if materials for writing it existed, to trace the variations experienced by the universe in the course of ages from the new stars which have suddenly appeared and disappeared in the vault of heaven, from nebulÃ�¾ dissolving or condensing -- to the first stratum of cryptogamic vegetation on the still imperfectly cooled surface of the earth, or on a reef of coral uplifted from the depths of ocean. 'the physical description of the world' presents a picture of all that exists in space -- of the siimultaneous action of p natural forces, together with the phenomena which they produce. but if we would correctly comprehend nature, we must not entirely or absolutely separate the consideration of the present state of things from that of the successive phases through which they have passed. we can not form a just conception of their nature without looking back on the mode of their formation. it is not organic matter alone that is continually undergoing change, and being dissolved to form new combinations. the globe itself reveals at every phase of its existence the mystery of its former conditions. we can not survey the crust of our planet without recognizing the traces of the prior existence and destruction of an organic world. the sedimentary rocks present a succession of organic forms, associated in groups, which have successively displaced and succeeded each other. the different super-imposed strata thus display to us the faunas and floras of different epochs. in this sense the description of nature is intimately connected with its history; and the geologist, who is guided by the connection existing among the facts observed, can not form a conception of the present without pursuing, through countless ages, the history of the past. in tracing the physical delineation of the globe, we behold the present and the past reciprocally incorporated, as it were, with one another; for the domain of nature is like that of languages, in which etymological research reveals a successive development, by showing us the primary condition of an idiom reflected in the forms of speech in use at the present day. the study of the material world renders this reflection of the past peculiarly manifest, by displaying in the process of formation rocks of eruption and sedimentary strata similar to those of former ages. if i may be allowed to borrow a striking illustration from the geological relations by which the physiognomy of a country is determined, i would say that domes of trachyte, cones of basalt, lava streams ('coules')of amygdaloid with elongated and parallel pores, and white deposits of pumice, intermixed with black scoriae, animate the scenery by the associations of the past which they awaken, acting upon the imagination of the enlightened observer like traditional records of an earlier world. their form is their history. the sense in which the greeks and romans originally employed the word 'history' proves that they too were intimately convinced that, to form a complete idea of the present state of the universe, it was necessary to consider it in its successive p phases. it is not, however, in the definition given by valerius flaccus,* but in the zoological writings of aristotle, that the word 'history' presents itself as an exposition of the results of experience and observation. [footnote] *aul. gell., 'nect. att.', v., . the physical description of the word by pliny the elder bears the title of 'natural history', while in the letters of his nephew it is designated by the nobler term of 'history of nature.' the earlier greek historians did not separate the description of countries from the narrative of events of which they had been the theater. with these writers, physical geography and history were long intimately associated, and remained simply but elegantly blended until the period of the development of political interests, when the agitation in which the lives of men were passed caused the geographical portion to be banished from the history of nations, and raised into an independent science. it remains to be considered whether by the operation of thought, we may hope to reduce the immense diversity of phenomena comprised by the cosmos to the unity of a principle, and the evidence afforded by rational truths. in the present state of empirical knowledge, we can scarcely flatter ourselves with such a hope. experimental sciences, based on the observation of the external world, can not aspire to completeness; the nature of things, and the imperfection of our organs, are alike opposed to it. we shall never succeed in exhausting the immeasurable riches of nature; and no generation of men will ever have cause to boast of having comprehended the total aggregation of phenomena. it is only by distributing them into groups that we have been able, in the case of a few, to discover the empire of certain natural laws, grand and simple as nature itself. the extent of this empire will no doubt increase in proportion as physical sciences are more perfectly developed. striking proofs of this advancement have been made manifest in our own day, in the phenomena of electro-magnetism, the propagation of luminous waves and radiating heat. in the same manner, the fruitful doctrine of evolution shows us how, in organic development, all that is formed is sketched out beforehand, and how the tissues of vegetable and animal matter uniformly arise from the multiplication and transformation of cells. the generalization of laws, which, being at first bounded by narrow limits, had been applied solely to isolated groups of phenomena, acquires in time more marked gradations, and gains in extent and certainty as long as the process of reasoning p is applied strictly to analogous phenomena; but as soon as dynamical views prove insufficient where the specific properties and heterogeneous nature of matter come into play; it is to be feared that, by persisting in the pursuit of laws, we may find our course suddenly arrested by an impassible chasm. the principle of unity is lost sight of, and the guiding clew is rent asunder whenever any specific and peculiar kind of action manifests itself amid the active forces of nature. the law of equivalents and the numerical proportions of composition, so happily recognized by modern chemists, and proclaimed under the ancient form of atomic symbols, still remains isolated and independent of mathematicl laws of motion and gravitation. those productions of nature which are objects of direct observation may be logically distributed in classes, orders, and families. this form of distribution undoubtedly sheds some light on descriptive natural history, but the study of organized bodies, considered in their linear connection, although it may impart a greater degree of unity and simplicity to the distribution of groups, can not rise to the height of a classification based on one sole principle of composition and internal organization. as different gradations are presented by the laws of nature according to the extent of the horizon, or the limits of the phenomena to be considered, so there are likewise differently graduated phases in the investigation of the external world. empiricism originates in isolated views, which are subsequently grouped according to their analogy or dissimilarity. to direct observation succeeds, although long afterward, the wish to prosecute experiments; that is to say, to evoke phenomena under different determined conditions. the rational experimentalist does not proceed at hazard, but acts under the guidance of hypotheses, founded on a half indistinct and more or less just intuition of the connection existing among natural objects or forces. that which has been conquered by observation or by means of experiments, leads, by analysis and induction, to the discovery of empirical laws. these are the phases in human intellect that have marked the different epochs in the life of nations, and by means of which that great mass of facts has been accumulated which constitutes at the present day the solid basis of the natural sciences. two forms of abstraction conjointly regulate our knowledge, namely, relations of 'quantity', comprising ideas of number and size, and relations of 'quality', embracing the consideration of the specific properties and the heterogeneous nature p of matter. the former, as being more accessible to the exercise of thought, appertains to mathematics; the latter, from the apparent mysteries and greater difficulties, falls under the domain of the chemical sciences. in order to submit phenomena to calculation, recourse is had to a hypothetical construction of matter by a combination of molecules and atoms, whose number, form, position, and polarity determine, modify, or vary phenomena. the mythical ideas long entertained of the imponderable substances and vital forces peculiar to each mode of organization, have complicated our views generally, and shed an uncertain light on the path we ought to pursue. the most various forms of intuition have thus, age after age, aided in augmenting the prodigious mass of empirical knowledge, which, in our own day has been enlarged with ever-increasing rapidity. the investigating spirit of man strives from time to time, with varying success, to break through those ancient forms and symbols invented, to subject rebellious matter to rules of mechanical construction. we are still very far from the time when it will be possible for us to reduce, by the operation of thought, all that we perceive by the senses, to the unity of a rational principle. it may even be doubted if such a victory could ever be achieved in the field of natural philosophy. the complication of phenomena, and of the vast extent of the cosmos, would seem to oppose such a result; but even a partial solution of the problem -- the tendency toward a comprehension of the phenomena of the universe -- will not the less remain the eternal and sublime aim of every investigation of nature. in conformity with the character of my former writings, as well as with the labors in which i have been engaged during my scientific career, in measurements, experiments, and the investigation of facts, i limit myself to the domain of empirical ideas. the exposition of mutually connected facts does not exclude the classification of phenomena according to their rational connection, the generalization of many specialities in the great mass of observations, or the attempt to discover laws. conceptions of the universe solely based upon reason, and the principles of speculative philosophy, would no doubt assign a still more exalted aim to the science of the cosmos. i am far from blaming the efforts of others solely because their success has hitherto remained very doubtful. contrary to the wishes and counsel of of those profound and powerful thinkers who p have given new life to speculations which were already familiar to the ancients, systems of natural philosophy have in our own country for some time past turned aside the minds of men from the graver study of mathematical and physical sciences. the abuse of better powers, which has led many of our noble but ill-judging youth into the saturnalia of a purely ideal science of nature, has been signalized by the intoxication of pretended conquests, by a novel and fantastically symbolical phraseology, and by a predilection for the formulae of a scholastic rationalism, more contracted in its views than any known to the middle ages. i use the expression "abuse of better powers," because superior intellects devoted to philosophical pursuits and experimental sciences have remained strangers to these saturnalia. the results yielded by an earnest investigation in the path of experiment can not be at variance with a true philosophy of nature. if there be any contradiction, the fault must lie either in the unsoundness of speculation, or in the exaggerated pretensions of empiricism, which thinks that more is proved by experiment than is actually derivable from it. external nature may be opposed to the intellectual world, as if the latter were not comprised within the limits of the former, or nature may be opposed to art when the latter is defined as a manifestation of the intellectual power of man; but these contrasts, which we find reflected in the most cultivated languages, must not lead us to separate the sphere of nature from that of mind, since such a separation would reduce the physical science of the world to a mere aggregation of empirical specialities. science does not present itself to man until mind conquers matter in striving to subject the result of experimental investigation to rational combinations. science is the labor of mind applied to nature, but the external world has no real existence for us beyond the image reflected within ourselves through the medium of the senses. as intelligence and forms of speech, thought and its verbal symbols, are united by secret and indissoluble links, so does the external world blend almost unconsciously to ourselves with our ideas and feelings. "external phenomena," says hegel, in his 'philosophy of history', "are in some degree translated in our inner representations." the objective world, conceived and reflected within us by thought, is subjected to the eternal and necessary conditions of our intellectual being. the activity of the mind exercises itself on the elements furnished to it by the perceptions of the senses. thus, in the p early ages of mankind, there manifests itself in the simple intuition of natural facts, and in the efforts made to comprehend them, the germ of the philosophy of nature. these ideal tendencies vary, and are more or less powerful, according to the individual characteristics and moral dispositions of nations, and to the degrees of their mental culture, whether attained amid scenes of nature that excite or chill the imagination. history has preserved the record of the numerous attempts that have been made to form a rational conception of the whole world of phenomena, and to recognize in the universe the action of one sole active force by which matter is penetrated, transformed, and animated. these attempts are traced in classical antiquity in those treatises on the principles of things which emanated from the ionian school, and in which all the phenomena of nature were subjected to hazardous speculations, based upon a small number of observations. by degrees, as the influence of great historical events has favored the development of every branch of science supported by observation, that ardor has cooled which formerly led men to seek the essential nature and connection of things by ideal construction and in purely rational principles. in recent times, the mathematical portion of natural philosophy has been most remarkably and admirably enlarged. the method and the instrument (analysis) have been simultaneously perfected. that which has been acquired by means so different -- by the ingenious application of atomic suppositions, by the more general and intimate study of phenomena, and by the improved construction of new apparatus -- is the common property of mankind, and shouldnot, in our opinion, now, more than in ancient times, be withdrawn from the free exercise of speculative thought. it can not be denied that in this process of thought, the results of experience have had to contend with many disadvantages; we must not, therefore, be surprised if, in the perpetual vicissitude of theoretical views, as is ingeniously expressed by the author of 'giordano bruno', "most men see nothing in philosophy but a succession of passing meteors, while even the grander forms in which she has revealed herself share the fate of comets, bodies that do not rank in popular opinion among the eternal and permanent works of nature, p but are regarded as mere fugitive apparitions of igncor vapor." [footnote] *schelling's bruno, 'eber das gottliche und naturaliche princip. der dinge', (bruno, on the 'divine and natural principle of things') we would here remark that the abuse of thought, and the false track it too often pursues, ought not to sanction an opinion derogatory to the intellect, which would imply that the domain of mind is essentially a world of vague fantastic illusions, and that the treasures accumulated by laborious observations in philosophy are powers hostile to its own empire. it does not become the spirit which characterizes the present age distrustfully to reject every generalization of views and every attempt to examine into the nature of things by the process of reason and induction. it would be a denial of the dignity of human nature and the relative importance of the faculties with which we are endowed, were we to condemn at one time austere reason engaged in investigating causes and their natural connections, and at another that exercise of the imagination which prompts and excites discoveries by its creative powers. this material taken from pages to cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p cosmos. ------------------------- delineation of nature. general review of natural phenomena. when the human mind first attempts to subject to its control the world of physical phenomena, and strives by meditative contemplation to penetrate the rich luxuriance of living nature, and the mingled web of free and restricted natural forces, man feels himself raised to a height from whence, as he embraces the vast horizon, individual things blend together in varied groups, and appear as if shrouded in a vapory vail. these figurative expressions are used in order to illustrate the point of view from whence we would consider the universe both in its celestial and terrestrial sphere. i am not insensible of the boldness of such an undertaking. among all the forms of exposition to which these pages are devoted, there is none more difficult than the general delineation of nature, which we purpose sketching, since we must not allow ourselves to be overpowered by a sense of the stupendous richness and variety of the forms presented to us, but must dwell only on the consideration of masses either possessing actual magnitude, or borrowing its semblance from the associations awakened within the subjective sphere of ideas. it is by a separation and classification of phenomena by an intuitive insight into the play of obscure forces, and by animated expressions, in which the perceptible spectacle is reflected with vivid truthfulness, that we may hope to comprehend and describe the 'universal all' [greek words] in a manner worthy of the dignity of the word 'cosmos' in its signification of 'universe, order of the world', and 'adornment' of this universal order. may the immeasurable diversity of phenomena which crowd into the picture of nature in no way detract from that harmonious impression of rest and unity which is the ultimate object of every literary or purely artistical composition. beginning with the depths of space and the regions of remotest nebulae, we will gradually descend through the starry zone to which our solar system belongs, to our own terrestrial spheroid, circled by air and ocean, there to direct our attention p to its form, temperature, and magnetic tension, and to consider the fullness of organic life unfolding itself upon its surface beneath the vivifying influence of light. in this manner a picture of the world may, with a few strokes, be made to include the realms of infinity no less than the minute microscopic animal and vegetable organisms which exist in standing waters and on the weather-beaten surface of our rocks. all that can be perceived by the senses, and all that has been accumulated up to the present day by an attentive and variously directed study of nature, constitute the materials from which this representation is to be drawn, whose character is an evidence of its fidelity and truth. but the descriptive picture of nature which we purpose drawing must not enter too fully into detail, since a minute enumeration of all vital forms, natural objects, and processes is not requisite to the completeness of the undertaking. the delineator of nature must resist the tendency toward endless division, in order to avoid the dangers presented by the very abundance of our empirical knowledge. a considerable portion of the qualitative properties of matter -- or, to speak more in accordance with the language of natural philosophy, of the qualitative expression of forces -- is doubtlessly still unknown to us, and the attempt perfectly to represent unity in diversity must therefore necessarily prove unsuccessful. thus, besides the pleasure derived and tinged with a shade of sadness, an unsatisfied longing for something beyond the present -- a striving toward regions yet unknown and unopened. such a sense of longing binds still faster the links which, in accordance with the supreme laws of our being, connect the material with the ideal world, and animates the mysterious relation existing between that which the mind receives from without, and that which it reflects from its own depths to the external world. if, then, nature (understanding by the term all natural objects and phenomena) be illimitable in extent and contents, it likewise presents itself to the human intellect as a problem which can not be grasped, and whose solution is impossible, since it requires a knowledge of the combined action of all natural forces. such an acknowledgement is due where the actual state and prospective development of phenomena constitute the sole objects of direct investigation, which does not venture to depart from the strict rules of induction. but, although the incessant effort to embrace nature in its universality may remain unsatisfied, the history of the contemplation of the universe (which p will be considered in another part of this work) will teach us how, in the course of ages, mankind has gradually attained to a partial insight into the relative dependence of phenomena. my duty is to depict the results of our knowledge in all their bearings with reference to the present. in all that is subject to motion and change in space, the ultimate aim, the very expression of physical laws, depend upon 'mean numerical values', which show us the constant amid change, and the stable amid apparent fluctuations of phenomena. thus the progress of modern physical science is especially characterized by the attainment and the rectification of the mean values of certain quantities by means of the processes of weighing and measuring; and it may be said, that the only remaining and widely-diffused hieroglyphic characters still in our writing -- 'numbers' -- appear to us again, as powers of the cosmos, although in a wider sense than that applied to them by the italian school. the earnest investigator delights in the simplicity of numerical relations, indicating the dimensions of the celestial regions, the magnitudes and periodical disturbances of the heavenly bodies, the triple elements of terrestrial magnetism, the mean pressure of the atmosphere, and the quantity of heat which the sun imparts in each year, and in every season of the year, to all points of the solid and liquid surface of our planet. these sources of enjoyment do not, however, satisfy the poet of nature, or the mind of the inquiring many. to both of these the present state of science appears as a blank, now that she answers doubtingly, or wholly rejects as unanswerable, questions to which former ages deemed they could furnish satisfactory replies. in her severer aspect, and clothed with less luxuriance, she shows herself deprived of that seductive charm with which a dogmatizing and symbolizing physical philosophy knew how to deceive the understanding and give the rein to imagination. long before the discovery of the new world, it was believed that new lands in the far west might be seen from the shores of the canaries and the azores. these illusive images were owing, not to any extraordinary refraction of the rays of light, but produced by an eager longing for the distant and the unattained. the philosophy of the greeks, the physical views of the middle ages, and even those of a more recent period, have been eminently imbued with the charm springing from similar illusive phantoms of the imagination. at the limits of circumscribed knowledge, as from some lofty island shore, the eye delights to penetrate p to distant regions. the belief in the uncommon and the wonderful lends a definite outline to every manifestation of ideal creation; and the realm of fancy -- a fairy-land of cosmological, geognostical, and magnetic visions -- becomes thus involuntarily blended with the domain of reality. nature, in the manifold signification of the word -- whether considered as the universality of all that is and ever will be -- as the inner moving force of all phenomena, or as their mysterious prototype -- reveals itself to the simple mind and feelings of man as something earthly, and closely allied to himself. it is only within the animated circles of organic structure that we feel ourselves peculiarly at home. thus, wherever the earth unfolds her fruits and flowers, and gives food to countless tribes of animals, there the image of nature impresses itself most vividly upon our senses. the impression thus produced upon our minds limits itself almost exclusively to the reflection of the earthly. the starry vault and the wide expanse of the heavens belong to a picture of the universe, in which the magnitude of masses, the number of congregated suns and faintly glimmering nebulae, although they excite our wonder and astonishment, manifest themselves to us in apparent isolation, and as utterly devoid of all evidence of their being the scenes of organic life. thus, even in the earliest physical views of mankind, heaven and earth have been separated and opposed to one another as an upper and lower portion of space. if, then, a picture of nature were to correspond to the requirements of contemplation by the senses, it ought to begin with a delineation of our native earth. it should depict, first, the terrestrial planet as to its size and form; its increasing density and heat at increasing depths in its superimposed solid and liquid strate; the separation of sea and land, and the vital forms animating both, developed in the cellular tissues of plants and animals; the atmospheric ocean, with its waves and currents, through which pierce the forest-crowned summits of our mountain chains. after this delineation of purely telluric relations, the eye would rise to the celestial regions, and the earth would then, as the well-known seat of organic development, be considered as a planet, occupying a place in the series of those heavenly bodies which circle round one of the innumerable host of self-luminous stars. this succession of ideas indicates the course pursued in the earliest stages of perceptive contemplation, and reminds us of the ancient conception of the "sea-girt disk of earth," supporting the vault of heaven. it begins to exercise in action p at the spot where it originated, and passes from the consideration of the known to the unknown, of the near to the distant. it corresponds with the method pursued in our elementary works on astronomy (and which is so admirable in a mathematical point of view), of proceeding from the apparent to the real movements of the heavenly bodies. another course of ideas must, however, be pursued in a work which proposes merely to give an exposition of what is known -- of what may in the present state of our knowledge be regarded as certain, or as merely probable in a greater or lesser degree -- and does not enter into a consideration of the proofs on which such results have been based. here, therefore, we do not proceed from the subjective point of view of human interests. the terrestrial must be treated only as grand and free, uninfluenced by motives of proximity, social sympathy, or relative utility. a physical cosmography -- a picture of the universe -- does not begin, therefore, with the picture of the universe -- does not begin, therefore, with the terrestrial, but with that which fills the regions of space. but as the sphere of contemplation contracts in dimension our perception of the richness of individual parts, the fullness of physical phenomena, and of the heterogeneous properties of matter becomes enlarged. from the regions in which we recognize ony the dominion of the laws of attraction, we descend to our own planet, and to the intricate play of terrestrial forces. the method here described for the delineation of nature is opposed to that which mst be pursued in establishing conclusive results. the one enumerates what the other demonstrates. man learns to know the external world through the organs of the senses. phenomena of light proclaim the existence of matter in remotest space, and the eye is thus made the medium through which we may contemplate the universe. the discovery of telescopic vision more than two centuries ago, has transmitted to latest generations a power whose limits are as yet unattained. the first and most general consideration of the cosmos is that of the 'contents of space' -- the distribution of matter, or of creation, as we are wont to designate the assemblage of all that is and ever will be developed. we see matter either agglomerated into rotating, revolving spheres of different density and size, or scattered through space in the form of self-luminous vapor. if we consider first the cosmical vapor dispersed in definite nebulous spots, its state of aggregation will p appear constantly to vary, sometimes appearing separated into round or elliptical disks, single or in pairs, occasionally connected by a thread of light; while, at another time, these nebulae occur in forms of larger dimensions, and are either elongated, or variously branched or fan-shaped or appear like well-defined rings, including a dark interior. it is conjectured that these bodies are undergoing variously developed formative processes, as the cosmical vapor becomes condensed in conformity with the laws of attraction, either round one or more of the nuclei. between two and three thousand of such unresolvable nebulae, in which the most powerful telescopes have hitherto been unable to distinguish the presence of stars, have been counted, and their positions determined. the genetic evolution -- that perpetual state of development which seems to affect this portion of the regions of space -- has led philosophical observers to the discovery of the analogy existing among organic phenomena. as in our forests we see the same kind of tree in all the various stages of its growth, and are thus enabled to form an idea of progressive, vital development, so do we also in the great garden of the universe, recognise the most different phases of sidereal formation. the process of condensation, which formed a part of the doctrines of anaximenes and of the ionian school, appears to be going on before our eyes. this subject of investigation and conjecture is especially attractive to the imagination, for in the study of the animated circles of nature, and of the action of all the moving forces of the universe, the charm that exercises the most powerful influence on the mind is derived less from a knowledge of that which 'is' than from a perception of that which 'will be', even though the latter be nothing more than a new condition of a known material existence; for of actual creation, of origin, the beginning of existence from non-existence, we have no experience, and can therefore form no conception. a comparison of the various causes influencing the development manifested by the greater or less degree of condensation in the interior of nebulae, no less than a successive course of direct observations, have led to the belief that changes of form have been recognized first in andromeda, next in the constallation argo, and in the isolated filamentous portion of the nebula in orion. but want of uniformity in the power of the instruments employed, different conditions of our atmosphere, and other optical relations, render a part of the results invalid as historical evidence. p 'nebulous stars' must not be confounded either with irregularly-shaped nebulous spots, properly so called, whose separate parts have an unequal degree of brightness (and which may, perhaps, become concentrated into stars as their circumference contracts), nor with the so-called planetary nebulae, whose circular or slightly oval disks manifest in all their parts a perfectly uniform degree of faint light. 'nebulous stars' are not merely accidental bodies projected upon a nebulous ground, but are a part of the nebulous matter constituting one mass with the body which it surrounds. the not unfrequently considerable magnitude of their apparent diameter, and the remote distance from which they are revealed to us, show that both the planetary nebulae and the nebulous stars must be of enormous dimensions. new and ingenious considerations of the different influence exercised by distance* on the intensity of light of a disk of appreciable diameter, and of a single self-luminous point, render it not improbable that the planetary nebulae are very remote nebulous stars, in which the difference between the central body and the surrounding nebulous covering can no longer be detected by our telescopic instruments. [footnote] * the optical considerations relative to the difference presented by a single luminous point, and by a disk subtending an appreciable angle, in which the intensity of light is constant at every distance, are explained in arago's 'analyse des travaux de sir william herschel' ('annuaire du bureau des long.', , p. - , and ). the magnificent zones of the southern heavens, between degrees and degrees, are especially rich in nebulous stars, and in compressed unresolvable nebua e. the larger of the two magellanic clouds, which circle round the starless, desert pole of the south, appears, according to the most recent researches,* as "a collection of clusters of stars, composed of globular clusters and nebulae of different magnitude, and of large nebulous spots p not resolvable, which, producing a general brightness in the field of view, form, as it were, the back-ground of the picture." [footnote] *the two magellanic clouds, nubecula major and nubecula minor, are very remarkable objects. the larger of the two is an accumulated mass of stars, and consists of clusters of stars of irregular form, either conical masses or nebulae of different magnitudes and degrees of condensation. this is interspersed with nebulous spots, not resolvable into stars, but which are probably 'star dust', appearing only as a general radiance upon the telescopic field of a twenty-feet reflector, and forming a luminous ground on which other objects of striking and indescribable form are scattered. in no other portion of the heavens are so many nebulous and stellar masses thronged together in an equally small space. nubecula minor is much less beautiful, has more unresolvable nebulous light, while the stellar masses are fewer and fainter in intensity. -- (from a letter of sir john herschel, feldhuysen, cape of good hope, th june, .) the appearance of these clouds, of the brightly-beaming constellation argo, of the milky way between scorpio, the centaur, and the southern cross, the picturesque beauty, if one may so speak, of the whole expanse of the southern celestial hemisphere, has left upon my mind an ineffaceable impression. the zodiacal light, which rises in a pyramidal form, and constantly contributes, by its mild radiance, to the external beauty of the tropical nights, is either a vast nebulous ring, rotating between the earth and mars, or, less probably, the exterior stratum of the solar atmosphere. besides these luminous clouds and nebulae of definite form, exact and corresponding observations indicate the existence and the general distribution of an apparently non-luminous, infinitely-divided matter, which posssesses a force of resistance and manifests its presence in encke's, and perhaps also in biela's comet, by diminishing their eccentricity and shortening their period of revolution. of this impending, ethereal, and cosmical matter, it may be supposed that it is in motion; that it gravitates, notwithstanding its original tenuity; that it is condensed in the vicinity of the great mass of the sun; and, finally, that it may, for myriads of ages, have been augmented by the vapor emanating from the tails of comets. if we now pass from the consideration of the vaporous matter of the immeasurable regions of space [(greek)*] -- whether scattered without definite form and limits, it exists as a cosmical other, or is condensed into nebulous spots, and becomes comprised among the solid agglomerated bodies of the universe -- we approach a class of phenomena exclusively designated by the form of stars, or as the sidereal world. [footnote] *i should have made use, in the place of garden of the universe, of the beautiful expression [greek], borrowed by hesychius from an unknown poet, if [greek] had not rather signified in general an inclosed space. the connection with the german 'garten' and the english 'garden', 'gards' in gothic (derived according to jacob grimm, from 'gairdan', 'to gird'), is, however, evident, as is likewise the affinity with the slavonic 'grad', 'gorod', and as pott remarks, in his 'etymol. forschungen', th. i., s. (etymol. researches), with the latin 'chors', whence we have the spanish 'corte', the french 'cour', and the english word 'court', together with the ossetic 'khart'. to these may be further added the scandinavian 'gard',** 'gard', a place inclosed, as a court, or a country seat, and the persian 'gerd', 'gird', a district, a circle, a princely country seat, a castle or city, as we find the term applied to the names of places in firdusi's schahnameh, as 'siyawakschgird', 'darabgird', etc. ** (this word is written 'gaard' in the danish) -- tr. p here, too, we find differences existing in the solidity or density of the spheroidally agglomerated matter. our own solar system presents all stages of 'mean' density (or of the relation of 'volume' to 'mass'.) on comparing the planets from mercury to mars with the sun and with jupiter, and these two last named with the yet inferior density of saturn, we arrive, by a descending scale -- to draw our illustration from the terrestrial substances -- at the respective densities of antimony, honey, water, and pine wood. in comets, which actually constitute the most considerable portion of our solar system with respect to the number of individual forms, the concentrated part, usually termed the 'head', or 'nucleus', transmits sidereal light unimpaired. the mass of a comet probably in no case equals the five thousandth part of that of the earth, so dissimilar are the formative processes manifested in the original and perhaps still progressive agglomerations of matter. in proceeding from general to special considerations, it was particularly desirable to draw attention to this diversity, not merely as a possible, but as an actually proved fact. the purely speculative conclusions arrived at by wright, kant, and lambert, concerning the general structural arrangement of the universe, and of the distribution of matter in space, have been confirmed by sir william herschel, on the more certain path of observation and measurement. that great and enthusiastic, although cautious observer, was the first to sound the depths of heaven in order to determine the limits and form of the starry stratum which we inhabit, and he, too, was the first who ventured to throw the light of investigation upon the relations existing between the position and distance of remote nebulae and our own portion of the sidereal universe. william herschel, as is well expressed in the elegant inscription on his monument at upton, broke through the inclosures of heaven ('caelorum perrupit claustra'), and, like another columbus, penetrated into an unknown ocean, from which he beheld coasts and groups of islands, whose true position it remains for future ages to determine. considerations regarding the different intensity of light in stars, and their relative number, that is to say, their numerical frequency on telescopic fields of equal magnitude, have led to the assumption of unequal distances and distribution in space in the strata which they compose. such assumptions, in as far as they may lead us to draw the limits of the individual portions of the universe, can not offer the same degree of mathematical certainty as that which may be attained in all that p relates to our solar system, whether we consider the rotation of double stars with unequal velocity round one common center of gravity, or the apparent or true movements of all the heavenly bodies. if we take up the physical description of the universe from the remotest nebulae, we may be inclined to compare it with the mythical portions of history. the one begins in the obscurity of antiquity, the other in that of inaccessible space; and at the point where reality seems to flee before us, imagination becomes doubly incited to draw from its own fullness, and give definite outline and permanence to the changing forms of objects. if we compare the regions of the universe with one of the island-studded seas of our own planet, we may imagine matter to be distributed in groups, either as unresolvable nebulae of different ages, condensed around one or more nuclei, or as already agglomerated into clusters of stars, or isolated spheroidal bodies. the cluster of stars, to which our cosmical island belongs, forms a lens-shaped, flattened stratum, detached on every side, whose major axis is estimated at seven or eight hundred, and its minor one at a hundred and fifty times the distance of sirius. it would appear, on the supposition that the parallax of sirius is not greater than that accurately determined for the brightest star in the centaur ( ". ), that light traverses one distance of sirius in three years, while it also follows, from bessel's earlier excellent memoir* on the parallax of the remarkable star cygni ( ". ), (whose considerable motion might lead to the inference of great proximity), that a period of nine years and a quarter is required for the transmission of light from this star to our planet. [footnote] *see maclear's "results from to ," in the 'trans. of the astronomical soc.', vol. xii., p. , on 'a' centauri, the probable mean error being ". . for cygni, see bessel, in schumacher's 'jahrbuch', , s. , and schumacher's 'astron. nachr.', bd. xviii., s. , , probable mean error, ". . with reference to the relative distances of stars of different magnitudes, how those of the third magnitude may probably be three times more remote, and the manner in which we represent to ourselves the material arrangement of the starry strata, i have found the following remarkable passage in kepler's 'epitome astronomiae copernicanae', , t. i., lib. , p. - : "sol hic noster nil aliud est quam una ex fixis, nobis major et clarior visa, quia propior quam fixa. pone terram stare ad latus, una semi-diametro via e lactea e, tunc ha ec via lactea apparebit circulus parvus, vel ellipsis parva, tota declinans ad latus alterum; eritque simul uno intuitu conspicua, quae nunc no potest nisi dimidia conspici quovis momento. itaque fix arum spha era non tantum orbe stellarum, sed etiam circulo lactis versus not deorsum est terminata." our starry stratum is a disk of inconsiderable thickness, divided a p third of its length into two branches; it is supposed that we are near this division, and nearer to the region of sirius than to the constellation aquila, almost in the middle of the stratum in the line of its thickness or minor axis. this position of our solar system, and the form of the whole discoidal stratum, have been inferred from sidereal scales, that is to say, from that method of counting the stars to which i have already alluded, and which is based upon the equidistant subdivision of the telescopic field of view. the relative depth of the stratum in all directions is measured by the greater or smaller number of stars appearing in each division. these divisions give the length of the ray of vision in the same manner as we measure the depth to which the plummet has been thrown, before it reaches the bottom, although in the case of a starry stratum there can not, correctly speaking, be any idea of depth, but merely of outer limits. in the direction of the longer axis, where the stars lie behind one another, the more remote ones appear closely crowded together, united, as it were, by a milky-white radiance or luminous vapor, and are perspectively grouped, encircling as in a zone, the visible vault of heaven. this narrow and branched girdle, studded with a radiant light, and here and there interrupted by dark spots, deviates only by a few degrees from forming a perfect large circle round the concave sphere of heaven, owing to our being near the center of the large starry cluster, and almost on the plane of the milky way. if our planetary system were far 'outside' this cluster, the milky way would appear to telescopic vision as a ring, and at a still greater distance as a resolvable discoidal nebula. among the many self-luminous moving suns, erroneously called 'fixed stars', which constitute our cosmical island, our own sun is the only one known by direct observation to be a 'central body' in its relations to spherical agglomerations of matter directly depending upon and revolving round it, either in the form of planets, comets, or aerolite asteroids. as far as we have hitherto been able to investigate 'multiple' stars (double stars or suns), these bodies are not subject, with respect to relative motion and illumination, to the same planetary dependence that characterizes our own solar system. two or more self-luminous bodies, whose planets and moon, if such exist, have hitherto escaped our telescopic powers of vision, certainly revolve around one common center of gravity; but this is in a portion of space which is probably occupied merely by unagglomerated matter or cosmical vapor, while in our system p the center of gravity is often comprised within the innermost limits of a 'visible' central body. if, therefore, we regard the sun and the earth, or the earth and the moon, as double-stars, and the whole of our planetary solar system as a multiple cluster of stars, the analogy thus suggested must be limited to the universality of the laws of attraction in different systems, being alike applicable to the independent processes of light and to the method of illumination. for the generalization of cosmical views, corresponding with the plan we have proposed to follow in giving a delineation of nature or of the universe, the solar system to which the earth belongs may be considered in a two-fold relation: first, with respect to the different classes of individually agglomerated matter, and the relative size, conformation, density, and distance of the heavenly bodies of this system; and secondly, with reference to other portions of our starry cluster, and of the changes of position of its central body, the sun. the solar system, that is to say, the variously-formed matter circling round the sun, consists, according to the present state of our knowledge of 'eleven primary planets',* eighteen satellites p or secondary planets, and myriads of comets, three of which, known as the "planetary comets," do not pass beyond the narrow limits of the orbits described by the principal planets. [footnote] * (since the publication of baron humboldt's work in , several other planets have been discovered, making the number of those belonging to our planetary system 'sixteen' instead of 'eleven'. of these, astrea, hebe, flora, and iris are members of the remarkable group of asteroids between mars and jupiter. astrea and hebe were discovered by hencke at driesen, the one in and the other in ; flora and iris were both discovered in by mr. hind, at the south villa observatory, regent's park. it would appear from the latest determinations of their elements, that the small planets have the following order with respect to mean distance from the sun: flora, iris, vesta, hebe, astrea, juno, ceres, pallas. of these, flora has the shortest period (about / years). the planet neptune, which, after having been predicted by several astronomers, was actually observed on the th of september, , is situated on the confines of our planetary system beyond uranus. the discovery of this planet is not only highly interesting from the importance attached to it as a question of science, but also from the evidence it affords of the care and unremitting labor evinced by modern astronomers in the investigation and comparison of the older calculations, and the ingenious application of the results thus obtained to the observation of new facts. the merit of having paved the way for the discovery of the planet neptune is due to m. bouvard, who, in his persevering and assiduous efforts to deduce the entire orbit of uranus from observations made during the forty years that succeeded the discovery of that planet in , found the results yielded by theory to be at variance with fact, in a degree that had no parallel in the history of astronomy. this startling discrepancy, which seemed only to gain additional weight from every attempt made by m. bouvard to correct his calculations, led leverrier, after a careful modification of the tables of bouvard, to establish the proposition that there was "a formal incompatibility between the observed motions of uranus and the hypothesis that he was acted on 'only' by the sun and known planets, according to the law of universal gravitation." pursuing this idea, leverrier arrived at the conclusion that the disturbing cause must be a 'planet', and finally, after an amount of labor that seems perfectly overwhelming, he, on the st of august, , laid before the french institute a paper, in which he indicated the exact spot in the heavens where this new planetary body would be found, giving the following data for its various elements: mean distance from the sun, . times that of the earth; period of revolution, . years; mean long., jan. st, , degrees '; mass, / th; heliocentric long., jan st , degrees '. essential difficulties still intervened, however, and as the remoteness of the planet rendered it improbable that its disk would be discernible by any telescopic instrument, no other means remained for detecting the suspected body but its planetary motion, which could only be ascertained by mapping, after every observation, the quarter of the heavens scanned, and by a comparison of the various maps. fortunately for the verification of leverrier's predictions, dr. bremiker had just completed a map of the precise region in which it was expected the new planet would apper, this being one of a series of maps made for the academy of berlin, of the small stars along the entire zodiac. by means of this valuable assistance, dr. galle, of the berlin observatory, was led, on the th of september, , by the discovery of a star of the eighth magnitude, not recorded in dr. bremiker's map, to make the first observation of the planet predicted by leverrier. by a singular coincidence, mr. adams, of cambridge, had predicted the appearance of the planet simultaneously with m. leverrier; but by the concurrence of several circumstances much to be regretted, the world at large were not made acquainted with mr. adams's valuable discovery until subsequently to the period at which leverrier published his observations. as the data of leverrier and adams stand at present, there is a discrepancy between the predicted and the true distance, and in some other elements of the planet; it remains therefore, for these or future astronomers to reconcile theory with fact, or perhaps, as in the case of uranus, to make the new planet the means of leading to yet greater discoveries. it would appear from the most recent observations, that the mass of neptune, instead of being, as at first stated, / th, is only about / th that of the sun, while its periodic time is now given with a greater probability at years, and its mean distance from the sun nearly . the planet appears to have a ring, but as yet no accurate observations have been made regarding its system of satellites. see 'trans. astron. soc.', and 'the planet neptune', , by j. p. nicholl.) -- tr. we may, with no incondsiderable degree of probability, include within the domain of our sun, in the immediate sphere of its central force, a rotating ring of vaporous matter, lying probably between the orbits of venus and mars, but certainly beyond that of the earth,* which appears to us in p a pyramidal form, and is known as the 'zodiacal light'; and a host of very small asteroids, whose orbits either intersect, or very nearly approach, that of our earth, and which present us with the phenomena of aerolites and falling or shooting stars. [footnote] * "if there should be molecules in the zones diffused by the atmosphere of the sun of too volatile a nature either to combine with one another or with the planets, we must suppose that they would, in circling round that luminary, present all the appearances of zodiacal light, without opposing any appreciable resistance to the different bodies composing the planetary system, either owing to their extreme rarity, or to the similarity existing between their motion and that of the planets with which they come in contact." -- laplace, 'expos. du syst. du monde' (ed. ), p. . when we consider the complication of variously-formed bodies which revolve round the sun in orbits of such dissimilar eccentricity--although we may not be disposed, with the immortal author of the 'mecanique celeste', to regard the largr number of comets as nebulous stars, passing from one central system to another,* we yet can not fail to acknowledge that the planetary system, especially so called (that is, the group of heavenly bodies which, together with their satellites, revolve with but slightly eccentric orbits round the sun), constitutes but a small portion of the whole system with respect to individual numbers, if not to mass. [footnote] *laplace, 'exp. du syst. du monde', p. , . it has been proposed to consider the telescopic planets, vesta, juno, ceres, and pallas, with their more closely intersecting, inclined, and eccentric orbits, as a zone of separation, or as a middle group in space; and if this view be adopted, we shall discover that the interior planetary group (consisting of mercury, venus, the earth, and mars) presents several very striking contrasts* when compared with the exterior group, comprising jupiter, saturn, and uranus. [footnote] *littrow, 'astronomie', , bd.xi., . mÃ�Â�dler, 'astron.', , Ã�¤ . laplace, 'exp. du syst. du monde', p. . the planets nearest the sun, and consequently included in the inner group, are of more moderate size, denser, rotate more slowly and with nearly equal velocity (their periods of revolution being almost all about hours), are less compressed at the poles, and with the exception of one, are without satellites. the exterior planets, which are further removed from the sun, are very considerably larger, have a density five times less, more than twice as great a velocity in the period of their rotation round their axes, are more compressed at the poles, and if six satellites may be ascribed to uranus, have a quantitative preponderance in the number of their attendant moons, which is as seventeen to one. p such general considerations regarding certain characteristic properties appertaining to whole groups, can not, however, be applied with equal justice to the individual planets of every group, nor to the relations between the distances of the revolving planets from the central body, and their absolute size, density, period or rotation, eccentricity, and the inclination of their orbits and the axes. we know as yet of no inherent necessity, no mechanical natural law, similar to the one which teaches us that the squares of the periodic times are proportional to the cubes of the major axes, by which the above-named six elements of the planetary bodies and the form of their orbit are made dependent either on one another, or on their mean distance from the sun. mars is smaller than the earth and venus, although further removed from the sun than these last-named planets, approaching most nearly in size to mercury, the nearest planet to the sun. saturn is smaller than jupiter, and yet much larger than uranus. the zone of the telescopic planets, which have so inconsiderable a volume, immediately procede jupiter (the greatest in size of any of the planetary bodies), if we consider them with regard to distance from the sun; and yet the disks of these small asteroids, which scarcely admit of measurement, have an areal surface not much more than half that of france, madagascar, or borneo. however striking may be the extremely small density of all the colossal planets, which are furthest removed from the sun, we are yet unable in this respect to recognize any regular succession.* [footnote] *see kepler, on the increasing density and volume of the planets in proportion with their increase of distance from the sun, which is described as the densest of all the heavenly bodies; in the 'epitome astran. copern. in' vii. 'libros digesta', - , p. . leibnitz also inclined to the opinions of kepler and otto von guericke, that the planets increase in volume in proportion to their increase of distance from the sun. see his letter to the magdeburg burgomaster (mayence, ), in leibnitz, 'deutschen schriften, herausg. von guhrauer', th. i., . uranus appears to be denser than saturn, even if we adopt the smaller mass, / , assumed by lamont; and, notwithstanding the inconsiderable difference of density observed in the innermost planetary group,* we find both venus and mars less dense than the earth, which lies between them. [footnote] *on the arrangement of masses, see encke, in schum., 'astr. nachr', nr. , . the time of rotation certainly diminishes with increasing solar distance, but yet it is greater in mars than in the earth, and in saturn than in jupiter. the elliptic p orbits of juno, pallas, and mercury have the greatest degree of eccentricity, and mars and venus, which immediately follow each other, have the least. mercury and venus exhibit the same contrasts that may be observed in the four smaller planets, or asteroids, whose paths are so closely interwoven. the eccentriciities of juno and pallas are very nearly identical, and reach three times as great as those of ceres and vesta. the same may be said of the inclination of the orbits of the planets toward the plane of projection of the ecliptic, or in the position of their axes of rotation with relation to their orbits, a position on which the relations of climate, seasons of the year, and length of the days depend more than on eccentricity. those planets that have the most elongated elliptic orbits, as juno, pallas, and mercury, have also, although not to the same degree their orbits most strongly inclined toward the ecliptic. pallas has a comet-like inclination nearly twenty-six times greater than that of jupiter, while in the little planet vesta, which is so near pallas, the angle of inclination scarcely by six times exceeds that of jupiter. an equally irregular succession is observed in the position of the axes of the few planets (four or five) whose planes of rotation we know with any degree of certainty. it would appear from the position of the satellites of uranus, two of which, the second and fourth, have been recently observed with certainty, that the axis of this, the outermost of all the planets is scarcely inclined as much as degrees toward the plane of its orbit, while saturn is placed between this planet, whose axis almost coincides with the plane of its orbit, and jupiter, whose axis of rotation is nearly perpendicular to it. in this enumeration of the forms which compose the world in space, we have delineated them as possessing an actual existence, and not as objects of intellectual contemplation, or as mere links of a mental and causal chain of connection. the planetary system, in its relations of absolute size and relative position of the axes, density, time of rotation, and different degrees of eccentricity of the orbits, does not appear to offer to our apprehension any stronger evidence of a natural necessity than the proportion observed in the distribution of land and water on the earth, the configuration of continents, or the height of mountain chains. in these respects we can discover no common law in the regions of space or in the inequalities of the earth's crust. they are 'facts' in nature that have arisen from the conflict of manifold forces acting under unknown p conditions, although man considers as 'accidental' whatever he is unable to explain in the planetary formation on purely genetic principles. if the planets have been formed out of separate rings of vaporous matter revolving round the sun, we may conjecture that the different thickness, unequal density, temperature, and electro-magnetic tension of these rings may have given occasion to the most various agglomerations of matter, in the same manner as the amount of tangential velocity and small variations in its direction have produced so great a differencein the forms and inclinations of the elliptic orbits. attractions of mass and laws of gravitation have no doubt exercised an influence here, no less than in the geognostic relations of the elevations of continents; but we are unable from the present forms to draw any conclusions regarding the series of conditions through which they have passed. even the so-called law of the distances of the planets from the sun, the law of progression (which led kepler to conjecture the existence of a planet supplying the link that was wanting in the chain of connection between mars and jupiter), has been found numerically inexact for the distances between mercury, venus, and the earth, and a variance with the conception of a series, owing to the necessity for a supposition in the case of the first member. the hitherto disscovered principal planets that revolve round our sun are attended certainly by fourteen, and probably by eighteen secondary planets (moons or satellites). the principal planets are, therefore, themselves the central bodies of subordinate systems. we seem to recognize in the fabric of the universe the same process of arrangement so frequently exhibited in the development of organic life, where we find in the manifold combinations of groups of plants or animals the same typical form repeated in the 'subordinate classes'. the secondary planets or satellites are more frequent in the external region of the planetary system, lying beyond the intersecting orbits of the smaller planets or asteroids; in the inner region none of the planets are attended by satellites, with the exception of the earth, whose moon is relatively of great magnitude, since its diameter is equal to a fourth of that of the earth, while the diameter of the largest of all known secondary planets -- the sixth satellite of saturn -- is probably about one seventeenth, and the largest of jupiter's moons, the third, only about one twenty-sixth part that of the primary planet or central body. the planets which are attended by the largest number of satellites are most remote from the sun, p and are at the same time the largest, most compressed at the poles, and the least dense. according to the most recent measurements of mÃ�Â�dler, uranus has a greater planetary compression than any other of the planets, viz., / . d. in our earth and her moon, whose mean distance from one another amounts to , miles, we find that the differences of mass* and diameter between the two are much less considerable than are usually observed to exist between the principal planets and their attendant satellites, or between bodies of different orders in the solar system. [footnote] *if, according to burckhardt's determination, the moon's radius be . and its volume / . th, its density will be . , or nearly five ninths. compare, also, wilh. beer and h. madler, 'der mond', , , and madler, 'ast.', . the material contents of the moon are, according to hansen, nearly / th (and Ã�Â�dler / . th) that of the earth, and its mass equal to / . d that of the earth. in the largest of jupiter's moons, the third, the relations of volume to the central body are / th, and of mass / th. on the polar flattening of uranus, see schum, 'astron. nachr.', , no. . while the density of the moon is five ninths less than that of the earth, it would appear, if we may sufficiently depend upon the determinations of their magnitudes and masses, that the second of jupiter's moons is actually denser than that great planet itself. among the fourteen satellites that have been investigated with any degree of certainty, the system of the seven satellites of saturn presents an instance of the greatest possible contrast, both in absolute magnitude and in distance from the central body. the sixth of these satellites is probably not much smaller than mars, while our moon has a diameter which does not amount to more than half that of the latter planet. with respect to volume, the two outer, the sixth and seventh of saturn's satellites, approach the nearest to the third and brightest of jupiter's moons. the two innermost of these satellites belong perhaps, together with the remote moons of uranus to the smallest cosmical bodies of our solar system, being only made visible under favorable circumstances by the most powerful instruments. they were first discovered by the forty-foot telescope of william herschel in , and were seen again by john herschel at the cape of good hope, by vico at rome, and by lamont at munich. determinations of the 'true' diameter of satellites, made by the measurement of the apparent size of their small disks, are subjected to many optical difficulties; but numerical astronomy, whose task it is to predetermine by calculation the motions of the heavenly bodies as they will appear when viewed from the earth, is directed almost p exclusively to motion and mass, and but little to volume. the absolute distance of a satellite from its central body is greatest in the case of the outermost or seventh satellite of saturn, its distance from the body round which it revolves amounting to more than two millions of miles, or ten times as great a distance as that of our moon from the earth. in the case of jupiter we find that the outermost or fourth attendant moon is only , , miles from that planet, while the distance between uranus and its sixth satellite (if the latter really exist) amounts to as much as , , miles. if we compare, in each of these subordinate systems, the volume of the satellite, we discover the existence of entirely new numerical relations. the distances of the outermost satellites of uranus, saturn, and jupiter are when expressed in semi-diameters of the main planets, as , , and . the outermost satellite of saturn appears, therefore, to be removed only about one fifteenth further from the center of that planet than our moon is from the earth. the first or innermost of saturn's satellites is nearer to its central body than any other of the secondary planets, and presents, moreover, the only instance of a period of revolution of less than twenty-four hours. its distance from the center of saturn may, according to mÃ�Â�dler and wilhelm beer, be expressed as . semi-diameters of that planet, or as , miles. its distance from the surface of the main planet is therefore , miles, and from the outer-most edge of the ring only miles. the traveler may form to himself an estimate of the smallness of this amount by remembering the statement of an enterprising navigator, captain beechey, that he had in three years passed over , miles. if, instead of absolute distances, we take the semi-diameters of the principal planets, we shall find that even the first or nearest of the moons of jupiter (which is , miles further removed from the center of that planet than our moon is from that of the earth) is only six semi-diameters of jupiter from its center, while our moon is removed from us fully / d semi-diameters of the earth. in the subordinate systems of satellites, we find that the same laws of gravitation which regulate the revolutions of the principal planets round the sun likewise govern the mutual relations existing between these planets among one another and with reference to their attendant satellites. the twelve moons of saturn, jupiter, and the earth all most like the primary planets from west to east, and in elliptic orbits, deviating p but little from circles. it is only in the case of one moon, and perhaps in that of the first and innermost of the satellites of saturn ( . ), that we discover an eccentricity greater than that of jupiter; according to the very exact observations of bessel, the eccentricity of the sixth of saturn's satellites ( . ) exceeds that of the earth. on the extremest limits of the planetary system, where, at a distance nineteen times greater than that of our earth, the centripetal force of the sun is greatly diminished, the satellites of uranus (which most striking contrasts from the facts observed with regard to other secondary planets. instead, as in all other satellites, of having their orbits but slightly inclined toward the ecliptic and (not excepting even saturn's ring, which may be regarded as a fusion of agglomerated satellites) moving from west to east, the satellites of uranus are almost perpendicular to the ecliptic, and move retrogressively from east to west, as sir john herschel has proved by observations continued during many years. if the primary and secondary planets have been formed by the condensation of rotating rings of solar and planetary atmospheric vapor, there must have existed singular causes of retardation or impediment in the vaporous rings revolving round uranus, by which, under the relations with which we are unacquainted, the revolution of the second and fourth of its satellites was made to assume a direction opposite to that of the rotation of the central planet. it seems highly probable that the period of rotation of 'all' secondary planets is equal to that of their revolution round the main planet, and therefore that they always present to the latter the same side. inequalities, occasioned by sight variations in the revolution, give rise to fluctuations of from degrees to degrees, or to an apparent libration in longitude as well as in latitude. thus, in the case of our moon, we sometimes observe more than the half of its surface, the eastern and northern edges being more visible at one time, and the western or southern at another. by means of this libration* we are enabled to see the annular mountain malapert (which occasionally conceals the moon's south pole), the arctic landscape round the crater of gioja, and the large gray plane near endymion which exceeds in superficial extent the 'mare vaporum'. [footnote] *beer and madler, op. cit., , s. , and Ã�¤ , s. ; and ix their 'phys. kenntniss der himml. korper', s. und , tab. (physical history of the heavenly bodies). three sevenths of the moon's surface are entirely p concealed from our observation, and must always remain so, unless new and unexpected disturbing causes come into play. these cosmical relations involuntarily remind us of nearly similar conditions in the intellectual world, where, in the domain of deep research into the mysteries and the primeval creative forces of nature, there are regions similarly turned away from us, and apparently unattainable, of which only a narrow margin has revealed itself, for thousands of years, to the human mind, appearing, from time to time, either glimmering in true or delusive light. we have hitherto considered the primary planets, their satellites, and the concentric rings which belong to one, at least, of the outermost planets, as products of tangential force, and as closely connected together by mutual attraction; it therefore now only remains for us to speak of the unnumbered host of 'comets' which constitute a portion of the cosmical bodies revolving in independent orbits round the sun. if we assume an equable distribution of their orbits, and the limits of their perihelia, or greatest proximities to the sun, and the possibility of their remaining invisible to the inhabitants of the earth, and base our estimates on the rules of the calculus of probabilities, we shall obtain as the result an amount of myriads perfectly astonishing. kepler, with his usual animation of expression, said that there were more comets in the regions of space than fishes in the depths of the ocean. as yet, however, there are scarcely one hundred and fifty whose paths have been calculated, if we may assume at six or seven hundred the number of comets whose appearance and passage through known constellations have been ascertained by more or less precise observations. while the so-called classical nations of the west, the greeks and romans, although they may occasionally have indicated the position in which a comet first appeared, never afford any information regarding its apparent path, the copious literature of the chinese (who observed nature carefully, and recorded with accuracy what they saw) contains circumstantial notices of the constellations through which each comet was observed to pass. these notices go back to more than five hundred years before the christian era, and many of them are still found to be of value in astronomical observations.* [footnote] *the first comets of whose orbits we have any knowledge, and which were calculated from chinese observations, are those of (under gordian ii.), (under justinian), , , , , , and . see john russell hind, in schum., 'astron. nachr.', , no. . while the comet of (which, according to du sejour, continued during twenty-four hours within a distance of , , miles from the earth) terrified louis i. of france to that degree that he busied himself in building churches and founding monastic establishments, in the hope of appeasing the evils threatened by its appearance, the chinese astronomers made observations on the path of this cosmical body, whose tail extended over a space of degrees, appearing sometimes single and sometimes multiple. the first comet that has been calculated solely from european observations was that of , known as halley's comet, from the belief long, but erroneously, entertained that the period when it was first observed by that astronomer was its first and only well-attested appearance. see arago, in the 'annuaire', , p. , and langier, 'comptes rendus des seances de l'acad.', , t. xvi., . p although comets have a smaller mass than any other cosmical bodies -- being, according to our present knowledge, probably not equal to / th part of the earth's mass -- yet they occupy the largest space, as their tails in several instances extend over many millions of miles. the cone of luminous vapor which radiates from them has been found, in some cases (as in and ), to equal the length of the earth's distance from the sun, forming a line that intersects both the orbits of venus and mercury. it is even probable that the vapor of the tails of comets mingled with our atmosphere in the years and . comets exhibit such diversities of form, which appear rather to appertain to the individual than the class, that a description of one of these "wandering light-clouds," as they were already called by xenophanes and theon of alexandria, contemporaries of pappus, can only be applied with caution to another. the faintest telescopic comets are generally devoid of visible tails, and resemble herschel's nebulous stars. they appear like circular nebulae of faintly-glimmering vapor, with the light concentrted toward the middle. this is the most simple type; but it can not, however, be regarded as rudimentary, since it might equally be the type of an older cosmical body, exhausted by exhalation. in the larger comets we may distinguish both the so-called "head" or "nucleus," and the single or multiple tail, which is characteristically denominated by the chinese astronomers "the brush" ('sui'). the nucleus generally presents no definite outline, although, in a few rare cases, it appears like a star of the first or second magnitude, and has even been seen in bright sunshine;* as, p for instance, in the large comets of , , , , and . [footnote] *arago, 'annuaire', , p. , . the phenomenon of the tail of a comet being visible in bright sunshine, which is recorded of the comet of , occurred again in the case of the large comet of , whose nucleus and tail were seen in north america on the th of february (according to the testimony of j. g. clarke, of portland, state of maine), between and o'clock in the afternoon.(a) the distance of the very dense nucleus from the sun's light admitted of being measured with much exactness. the nucleus and tail appeared like a very pure white cloud, a darker space intervening between the tail and the nucleus. ('amer. journ. of science', vol. xiv., no. , p. .) [footnote] (a) [the translator was at new bedford, massachusetts, u.s., on the th february, , and distinctly saw the comet, between and in the afternoon. the sky at the time was intensely blue, and the sun shining with a dazzling brightness unknown in european climates.] -- tr this latter circumstance indicates, in particular individuals, a denser mass, capable of reflecting light with greater intensity. even in herschel's large telescope, only two comets, that discovered in sicily in , and the splendid one of , exhibited well-defined disks;* the one at an angle of second, and the other at . seconds, whence the true diameters are assumed to be and miles. [footnote] *'phil. trans.' for , part ii., p. , and for , part i., p. . the diameters found by herschel for the nuclei were and english miles. for the magnitudes of the comets of and , see arago, 'annuaire', , p. . the diameters of the less well-defined nuclei of the comets of and did not appear to exceed or miles. in several comets that have been investigated with great care, especially in the above-named one of , which continued visible for so long a period, the nucleus and its nebulous envelope were entirely separated from the tail by a darker space. the intensity of light in the nucleus of comets does not augment toward the center in any uniform degree, brightly shining zones being in many cases separated by concentric nebulous envelopes. the tails sometimes appear single, sometimes, although more rarely, double; and in the comets of and the branches were of different lengths; in one instance ( ) the tail had six branches, the whole forming an angle of degrees. the tails have been sometimes straight, sometimes curved, either toward both sides, or toward the side appearing to us as the exterior (as in ), or convex toward the direction in which the comet is moving (as in that of ); and sometimes the tail has even appeared like a flame in motion. the tails are always turned away from the sun, so that their line of prolongation passes through its center; a fact which, according to edward biot, was noticed by the chinese astronomers as early as , but was first generally made known in europe by fracastoro and peter apian in the sixteenth century. these emanations may be regarded as conoidal envelopes of greater of less thickness, p and, considered in this manner, they furnish a simple explanation of many of the remarkable optical phenomena already spoken of. comets are not only characteristically different in form, some being entirely without a visible tail, while others have a tail of immense length (as in the instance of the comet of , whose tail measured degrees), but we also see the same comets undergoing successive and rapidly-changing processes of configuration. these variations of form have been most accurately and admirably described in the comet of , by hensius, at st. petersburg, and in halley's comet, on its last reappearance in , by bessel, at konigsberg. a more or less well-defined tuft of rays emanated from that part of the nucleus which was turned toward the sun; and the rays being bent backward, formed a part of the tail. the nucleus of halley's comet; with its emanations, presented the appearance of a burning rocket, the end of which was turned sideways by the force of the wind. the rays issuing from the head were seen by arago and myself, at the observatory at paris, to assume very different forms on successive nights.* [footnote] *arago, 'des changements physiques de la comete de halley du - oct., . 'annuaire', , p. , . the ordinary direction of the emanations was noticed even in nero's time. "comae radios solis effugiunt." -- seneca, 'nat. quaest.', vii., . the great konigsberg astronomer concluded from many measurements, and from theoretical considerations, "that the cone of light issuing from the comet deviated considerably both to the right and the left of the true direction of the sun, but that it always returned to that direction, and passed over to the opposite side, so that both the cone of light and the body of the comet from whence it emanated experienced a rotatory, or, rather, a vibratory motion in the plane of the orbit." he finds that "the attractive force exercised by the sun on heavy bodies is inadequate to explain such vibrations, and is of opinion that they indicate a polar force, which turns one semi-diameter of the comet toward the sun, and strives to turn the opposite side away from that luminary. the magnetic polarity possessed by the earth may present some analogy to this, and, should the sun have an opposite polarity, an influence might be manifested, resulting in the precession of the equinoxes." this is not the place to enter more fully upon the grounds on which explanations of this subject have been based; but observations so remarkable,* and views of so exalted p a character, regarding the most wonderful class of the cosmical bodies belonging to our solar system, ought not to be entirely passed over in this sketch of a general picture of nature. [footnote] *bessel, in schumacher, 'astr. nachr.', , no. - , s. , , , , , und . also in schumacher, 'jahrb.', , s. , . william herschel, in his observations on the beautiful comet of , believed that he had discovered evidences of the rotation of the nucleus and tail ('phil. trans.' for , part i., p. ). dunlop, at paramatta thought the same with reference to the third comet of . although, as a rule, the tails of comets increase in magnitude and brilliancy in the vicinity of the sun, and are directed away from that central body, yet the comet of offered the remarkable example of two tails, one of which was turned toward the sun, and the other away from it, forming with each other an angle of degrees. modifications of polarity and the unequal manner of its distribution, and of the direction in which it is conducted, may in this rare instance have occasioned a double, unchecked, continuous emanation of nebulous matter.* [footnote] *bessel, in 'astr. nachr.', , no. , s. . schum, 'jahrb.', s. . see, also lehmann, 'ueber cometenschweife' (on the tails of comets), in bode, 'astron. jahrb. fur' , s. . aristotle, in his 'natural philosophy', makes these emanations the means of bringing the phenomena of comets into a singular connection with the existence of the milky way. according to his views, the innumerable quantity of stars which compose this starry zone give out a self-luminous, incandescent matter. the nebulous belt which separates the different portions of the vault of heaven was therefore regarded by the stagirite as a large comet, the substance of which was incessantly being renewed.* [footnote] *aristot., 'meteor.', i., , - , und - (ed. ideler, t. i., p. - ). biese, 'phil. des aristoteles', bd. ii., s. . since aristotle exercised so great an influence throughout the whole of the middle ages, it is very much to be regretted that he was so averse to those grander views of the elder pythagoreans, which inculcated ideas so nearly approximating to truth respecting the structure of the universe. he asserts that comets are transitory meteors belonging to our atmosphere in the very book in which he cites the opinion of the pythagorean school, according to which these cosmical bodies are supposed to be planets having long periods of revolution. (aristot., i., , .) this pythagorean doctrine, which, according to the testimony of apollonius myndius, was still more ancient, having originated with the chaldeans, passed over to the romans, who in this instance, as was their usual practice, were merely the copiers of others. the myndian philosopher describes the path of comets as directed toward the upper and remote regions of heaven. hence seneca says, in his 'nat. quaest.', vii., : "cometes non est species falsa, sed proprium sidus sicut solis et lunae: altiora mundi secat et tunc demum apparet quum in imum cursum sui venit;" and again (at vii., ), "cometes aternos esse et sortis ejusdem, cujus caetera (sidera), etiamsi faciem illis non habent similem." pliny (ii., ) also refers to apollonius myndius, when he says, "sunt qui et haec sidera perpetua esse credant suoque ambitu ire, sed non nisi relicta a sole cerni." p the occulation of the fixed stars by the nucleus of a comet, or by its innermost vaporous envelopes, might throw some light on the physical character of these wonderful bodies; but we are unfortunately deficient in observations by which we may be assured* that the occulation was perfectly central; for, as it has already been observed, the parts of the envelope contiguous to the nucleus are alternately composed of layers of dense or very attenuated vapor. [footnote] *olbers, in 'astr. nachr.', , s. , . arago, 'de la constitution physique des cometes; annuaire de' , p. , . the ancients were struck by the phenomenon that it was possible to see through comets as through a flame. the earliest evidence to be met with of stars having been seen through comets is that of democritus (aristot., 'meteor.', i., , ), and the statement leads aristotle to make the not unimportant remark, that he himself had observed the occulation of one of the stars of gemini by jupiter. seneca only speaks decidedly of the transparence of the tail of comets. "we may see," says he, "stars through a comet as through a cloud ('nat. quaest.', vii., ); but we can ony see through the rays of the tail, and not through the body of the comet itself: 'non in ea parte qua sidus ipsum est spissi et solidi ignis, sed qua rarus splendor occurrit et in crines dispergitur. per intervalla ignium, non er ipsos, vides" (vii., ). the last remark is unnecessary, since, as galileo observed in the 'saggiatore (lettera a monsignor cesarini', ), we can certainly see through a flame when it is not of too great a thickness'. on the other hand the carefully conducted measurements of bessel prove, beyond all doubt, that on the th of september, , the light of a star of the tenth magnitude, which was then at a distance of ". from the central point of the head of halley's comet, passed through very dense nebulous matter, without experiencing any deflection during its passage.* [footnote] *bessel, in the 'astron. nachr.', , no. , s. , . struve, in 'recueil des mem. de l'acad. de st. peterab.', , p. , , and 'astr. nachr.', , no. , s. , writes as follows: "at dorpat the star was in conjunction only ". from the brightest point of the comet. the star remained continually visible, and its light was not perceptibly diminished, while the nucleus of the comet seemed to be almost extinguished before the radiance of the small star of the ninth or tenth magnitude." if such an absence of refracting power must be ascribed to the nucleus of a comet, we can scarcely regard the matter composing comets as a gaseous fluid. the question here arises whether this absence of refracting power may not be owing to the extreme tenuity of the fluid; or does the comet consist of separated particles, constituting a cosmical stratum of clouds, which, like the clouds of our atmosphere, that exercise no influence on the p zenith distance of the stars, does not affect the ray of light passing through it? in the passage of a comet over a star, a more or less considerable diminution of light has often been observed; but this has been justly ascribed to the brightness of the ground from which the star seems to stand forth during the passage of the comet. the most important and decisive observations that we possess on the nature and the light of comets are due to arago's polarization experiments. his polariscope instructs us regarding the physical constitution of the sun and comets, indicating whether a ray that reaches us from a distance of many millions of miles transmits light directly or by reflection; and if the former, whther the source of light is a solid, a liquid, or a gaseous body. his apparatus was used at the paris observatory in examining the light of capella and that of the great comet of . the latter showed polarized, and therefore reflected light, while the fixed star, as was to be expected, appeared to be a self-luminous sun.* [footnote] *on the d of july, , arago made the first attempt to analyze the light of comets by polarization, on the evening of the sudden appearance of the great comet. i was present at the paris observatory, and was fully convinced, as were also matthieu and the late bouvard of the dissimilarity in the intensity of the light seen in the polariscope, when the instrument received cometary light. when it received light from capella, which was near the comet, and at an equal altitude, the images were of equal intensity. on the reappearance of halley's comet in , the instrument was altered so as to give, according to arago's chromatic polarization, two images of complementary colors (green and red). ('annales de chimie', t. xiii., p. ; 'annuaire', , p. .) "we must conclude from these observations," says arago, "that the cometary light was not entirely composed of rays having the properties of direct light, there being light which was reflected specularly or polarized, that is, coming from the sun. it can not be stated with absolute certainty that comets shine only with borrowed light, for bodies, in becoming self-luminous, do not, on that account, lose the power of reflecting foreign light." the existance of polarized cometary light announced itself not only by the inequality of the images, but was proved with greater certainty on the reappearance of halley's comet, in the year , by the more striking contrast of the complementary colors, deduced from the laws of chromatic polarization discovered by arago in . these beautiful experiments still leave it undecided whether, in addition to this reflected solar light, comets may not have light of their own. even in the case of the planets, as, for instance, in venus, an evolution of independent light seems very probable. the variable intensity of light in comets can not always be p explained by the position of their orbits and their distance from the sun. it would seem to indicate, in some individuals, the existence of an inherent process of condensation, and an increased or diminished capacity of reflecting borrowed light. in the comet of , and in that which has a period of three years, it was observed first by hevelius that the nucleus of the comet diminished at its perihelion and enlarged at its aphelion, a fact which, after remaining long unheeded, was again noticed by the talented astronomer valz at nismes. the regularity of the change of volume, according to the different degrees of distance from the sun, appears very striking. the physical explanation of the phenomenon can not, however, be sought in the condensed layers of cosmical vapor occurring in the vicinity of the sun, since it is difficult to imagine the nebulous envelope of the nucleus of the comet to be vesicular and impervious to the other.* [footnote] *arago, in the 'annuaire', , p. - . sir john herschel, 'astron.', . the dissimilar eccentricity of the orbits of comets has, in recent times ( ), in the most brilliant manner enriched our knowledge of the solar system. encke has discovered the existence of a comet of so short a period of revolution that it remains entirely within the limits of our planetary system, attaining its aphelion between the orbits of the smaller planets and that of jupiter. its eccentricity must be assumed at . , that of juno (which has the greatest eccentricity of any of the planets) being . . encke's comet has several times, although with difficulty, been observed by the naked eye, as in europe in , and according to rumker, in new holland in . its period of revolution is about / d years; but, from a careful comparison of the epochs of its return to its perihelion, the remarkable fact has been discovered that these periods have diminished in the most regular manner between the years and , the diminution amounting, in the course of years, to about / th days. the attempt to bring into unison the results of observation and calculation in the investigation of all the planetary disturbances, with the view of explaining this phenomenon, has led to the adoption of the very probable hypothesis that there exists dispersed in space a vaporous substance capable of acting as a resisting medium. this matter diminished the tangential force, and with it the major axis of the comet's orbit. the value of the constant of the resistance appears to be somewhat different before and after the perihelion; and this may, perhaps, be ascribed p to the altered form of the small nebulous star in the vicinity of the sun, and to the action of the unequal density of the strata of cosmical ether.* [footnote] *encke, in the 'astronomiche nachrichten', , no. , s. - . these facts, and the investigations to which they have led, belong to the most interesting results of modern astronomy. encke's comet has been the means of leading astronomers to a more exact investigation of jupiter's mass (a most important point with reference to the calculation of perturbations); and, more recently, the course of this comet has obtained for us the first determination, although only an approximative one, of a smaller mass for mercury. the discovery of encke's comet, which had a period of only / d years, was speedily followed, in , by that of another, biela's comet, whose period of revolution is / th years, and which is likewise planetary, having its aphelion beyond the orbit of jupiter, but within that of saturn. it has a fainter light than encke's comet, and, like the latter, its motion is direct, while halley's comet moves in a course opposite to that pursued by the planets. biela's comet presents the first certain example of the orbit of a comet intersecting that of the earth. this position, with reference to our planet, may therefore be productive of danger, if we can associate an idea of danger with so extraordinary a natural phenomenon, whose history presents no parallel, and the results of which we are consequently unable correctly to estimate. small masses endowed with enormous velocity may certainly exercise a considerable power; but laplace has shown that the mass of the comet of is probably not equal to / th that of the earth, or about / th that of the moon.* [footnote] *laplace, 'expos. du syst. du monde', p. , . we must not confound the passage of biela's comet through the earth's orbit with its proximity to, or collision with our globe. when this passage took place, on the th of october, , it required a full month before the earth would reach the point of intersection of the two orbits. these two comets of short periods of revolution also intersect each other, and it has been justly observed,* that amid the many perturbations experienced by such small bodies from the largr planets, there is a 'possibility' -- supposing a meeting of these comets to occur in october -- that the inhabitants of the earth may witness the extraordinary spectacle of an encounter between two cosmical bodies, and possibly of their reciprocal penetration and amalgamation, or of their destruction by means of exhausting emanations. [footnote] *littrow, 'beschreibende astron.', , s. . on the inner comet recently discovered by m. faye, at the observatory of paris, and whose eccentricity is . , its distance at its perihelion . , and its distance at its aphelion . , see schumacher, 'astron. nachr.', , no. . regarding the supposed identity of the comet of with the third comet of , see 'astr. nachr.', , no. ; and on the identity of the comet of and the fourth comet of , see no. or the last mentioned work. events of this nature, resulting either from deflection occasioned by disturbing masses or primevally intersecting orbits, must have been of frequent occurrence in the course of millions of years in the immeasurable regions of ethereal space; but they must be regarded as isolated occurrences, exercising no more general or alternative effects on cosmical relations than the breaking forth or extinction of a volcano within the limited sphere of our earth. a third interior comet, having likewise a short period of revolution was discovered by faye on the d of november, , at the observatory at paris. its elliptic path, which approaches much more nearly to a circle than that of any other known comet, is included within the orbits of mars and saturn. this comet, therefore, which, according to goldschmidt, passes beyond the orbit of jupiter, is one of the few whose perihelia are beyond mars. its period of revolution is / years, and it is not improbable that the form of its present orbit may be owing to its great approximation to jupiter at the close of the year . if we consider the comets in their inclosed elliptic orbits as members of our solar system, and with respect to the length of their major axes, the amount of their eccentricity, and their periods of revolution, we shall probably find that the three planetary comets of encke, biela, and faye are most nearly approached in these respects, first, by the comet discovered in by messier, and which is regarded by clausen as identical with the third comet of ; and next, by the fourth comet of the last-mentioned year, discovered by blaupain, but considered by clausen as identical with that of the year , and whose orbit appears, like that of lexell's comet, to have suffered great variations from the proximity and attraction of jupiter. the two last-named comets would likewise seem to have a period of revolution not exceeding five or six years, and their aphelia are in the vicinity of jupiter's orbit. among the comets that have a period of revolution of from seventy to p seventy-six years, the first in point of importance with respect to theoretical and physical astronomy is halley's comet, whose last appearance, in , was much less brilliant than was to be expected from preceding ones; next we would notice olbers's comet, discovered on the th of march, ; and, lastly, the comet discovered by pons in the year , and whose elliptic orbit has been determined by encke. the two latter comets were invisible to the naked eye. we now know with certainty of nine returns of halley's large comet, it having recently been proved by laugier's calculations*, that in the chinese table of comets, first made known to us by edward biot, the comet of is identical with halley's; its periods of revolution have varied in the interval between and from . to . years, the mean being . . [footnote] *laugier, in the 'comptes rendus des seances de l'academie', , t. xvi., p. . a host of other comets may be contrasted with the cosmical bodies of which we have spoken, requiring several thousand years to perform their orbits, which it is difficult to determine with any degree of certainty. the beautiful comet of requires, according to argelander, a period of years for its revolution, and the colossal one of as much as years, according to encke's calculation. these bodies respectively recede, therefore, and times further than uranus from the sun, that is to say, , and , millions of miles. at this enormous distance the attractive force of the sun is still manifested; but while the velocity of the comet of at its perihelion is miles in a second, that is, thirteen times greater than that of the earth, it scarcely moves ten feet in the second when at its aphelion. this velocity is only three times greater than that of water in our most sluggish european rivers, and equal only to half that which i have observed in the cassiquiare, a branch of the orinoco. it is highly probable that, among the innumerable host of uncalculated or undiscovered comets, there are many whose major axes greatly exceed that of the comet of . in order to form some idea by numbers, i do not say of the sphere of attraction, but of the distance in space of a fixed star, or other sun, from the aphelion of the comet of (the furthest receding cosmical body with which we are acquainted in our solar system), it must be remembered that, according to the most recent determinations of parallaxes, the nearest fixed star is full times further removed from our sun than the comet in its aphelion. the comet's distance is only p times that of uranus, while 'a' centauri is , and cygni , times that of uranus, according to bessel's determinations. having considered the greatest distances of comets from the central body, it now remains for us to notice instances of the greatest proximity hitherto measured. lexell and burckhardt's comet of , so celebrated on account of the disturbances it experienced from jupiter, has approached the earth within a smaller distance than any other comet. on the th of june, , its distance from the earth was ony six times than of the moon. the same comet passed twice, viz., in and , through the system of jupiter's four satellites without producing the slightest notable change in the well-known orbits of these bodies. the great comet of approached at its perihelion eight or nine times nearer to the surface of the sun than lexell's comet did to that of our earth, being on the th of december a sixth part of the sun's diameter, or seven tenths of the distance of the moon from that luminary. perihelia occurring beyond the orbit of mars can seldom be observed by the inhabitants of the earth, owing to the faintness of the light of distant comets; and among those already calculated the comet of is the only one which has its perihelion between the orbits of pallas and jupiter; it was even observed beyond the latter. since scientific knowledge, although frequently blended with vague and superficial views, has been more extensively diffused through wider circles of social life, apprehensions of the possible evils threatened by comets have acquired more weight as their direction has become more definite. the certainty that there are within the known planetary orbits comets which revisit our regions of space at short intervals -- that great disturbances have been produced by jupiter and saturn in their orbits, by which such as were apparently harmless have been converted into dangerous bodies -- the intersection of the earth's orbit by biela's comet -- the cosmical vapor, which, acting as a resisting and impeding medium, tends to contract all orbits -- the individual difference of comets, which would seem to indicate considerable decreasing gradations in the quantity of the mass of the nucleus, are all considerations more than equivalent, both as to number and variety, to the vague fears entertained in early ages of the general conflagration of the world by 'flaming swords', and stars with 'fiery streaming hair'. as the consolatory considerations which may be derived from the calculus of probabilities address themselves to reason and to p meditative understanding only, and not to the imagination or to a desponding condition of mind, modern science has been accused, and not entirely without reason, of not attempting to allay apprehensions which it has been the very means of exciting. it is an inherent attribute of the human mind to experience fear, and not hope or joy, at the aspect of that which is unexpected and extraordinary.* [footnote] *fries, 'vorlesungen uber die sternkunde', , s. - (lectures on the science of astronomy). an infelicitously chosen instance of the good omen of a comet may be found in seneca, 'nat. quest.', vii., and . the philosopher thus writes of the comet: "quem nos neronis principatu latissimo vidimus et qui cometis detraxit infamiam." the strange form of a large comet, its faint nebulous light, and its sudden appearance in the vault of heaven, have in all regions been almost invariably regarded by the people at large as some new and formidable agent inimical to the existing state of things. the sudden occurrence and short duration of the phenomenon lead to the belief of some equally rapid reflection of its agency in terrestrial matters, whose varied nature renders it easy to find events that may be regarded as the fulfillment of the evil foretold by the appearance of these mysterious cosmical bodies. in our own day, however, the public mind has taken another and more cheerful, although singular, turn with regard to comets; and in the german vineyards in the beautiful valleys of the rhine and moselle, a belief has arisen, ascribing to these once ill-omened bodies a beneficial influence on the ripening of the vine. the evidence yielded by experience, of which there is no lack in these days, when comets may so frequently be observed, has not been able to shake the common belief in the meteorological myth of the existence of wandering stars capable of radiating heat. this material taken from pages - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- from comets i would pass to the consideration of a far more enigmatical class of agglomerated matter -- the smallest of all asteroids, to which we apply the name 'aÃ�Â�rolites', or 'meteoric stones',* when they reach our atmosphere in a fragmentary condition. [footnote] * (much valuable information may be obtained regarding the origin and composition of aÃ�Â�rolites or meteoric stones in memoirs on the subject, by baumbeer and other writers, in the numbers of poggendorf's 'annalen', from to the present time.) -- tr. if i should seem to dwell on the specific enumeration of these bodies, and of comets, longer than the general nature of this work might warrant, i have not done so undesignedly. the diversity existing in the individual characteristics of comets has already been noticed. the imperfect knowledge we possess of their physical character renders it p diifficult in a work like the present, to give the proper degree of circumstantiality to the phenomena, which, although of frequent recurrence, have been observed with such various degrees of accuracy, or to separate the necessary from the accidental. it is only with respect to measurements and computations that the astronomy of comets has made any marked advancement, and, consequently, a scientific consideration of these bodies must be limited to a specification of the differences of physiognomy and conformation in the nucleus and tail, the instances of great approximation to other cosmical bodies, and of the extremes in the length of their orbits and in their periods of revolution. a faithful delineation of these phenomena, as well as of those which we proceed to consider, can only be given by sketching individual features with the animated circumstantiality of reality. shooting stars, fire-balls, and meteoric stones are, with great probability, regarded as small bodies moving with planetary velocity, and revolving in obedience to the laws of general gravity in conic sections round the sun. when these masses meet the earth in their course, and are attracted by it, they enter within the limits of our atmosphere in a luminous condition, and frequently let fall more or less strongly heated stony fragments, covered with a shining black crust. when we enter into a careful investigation of the facts observed at those epochs when showers of shooting stars fell periodically in cumana in , and in north america during the years and , we shall find that 'fire-balls' can not be considered separately from shooting stars. both these phenomena are frequently not only simultaneous and blended together, but they likewise are often found to merge into one another, the one phenomenon gradually assuming the character of the other alike with respect to the size of their disks, the emanation of sparks, and the velocities of their motion. although exploding smoking luminous fire-balls are sometimes seen, even in the brightness of tropical daylight,* equaling in size the apparent p diameter of the moon, innumerable quantities of shooting stars have, on the other hand, been observed to fall in forms of such extremely small dimensions that they appear only as moving points or 'phosphorescent lines.'** [footnote] *a friend of mine, much accustomed to exact trigonometrical measurements, was in the year at popayan, a city which is degrees ' north latitude, lying at an elevation of feet above the level of the sea, and at noon, when the sun was shining brightly in a cloudless sky, saw his room lighted up by a fire-ball. he had his back to the window at the time, and on turning round, perceived that great part of the path traversed by the fire-ball was still illuminated by the brightest radiance. different nations have had the most various terms to express these phenomena: the germans use the word 'sternschnuppe', literally 'star snuff' -- an expression well suited to the physical views of the vulgar in former times, according to which, the lights in the firmament were said to undergo a process of 'snuffing' or cleaning; and other nations generally adopt a term expressive of a 'shot' or 'fall' of stars, as the swedish 'stjernifall', the italian 'stella cadente', and the english 'star shoot.' in the woody district of the orinoco, on the dreary banks of the cassiquiare, i heard the natives in the mission of vasiva use terms still more inelegant than the german 'star snuff.' ('relation historique du voy. aux rÃ�Â�gions equinox.', t. ii., p. .) these same tribes term the pearly drops of dew which cover the beautiful leaves of the heliconia 'star spit.' in the lithuanian mythology, the imagination of the people has embodied its ideas of the nature and signification of falling stars under nobler and more graceful symbols. the parcÃ�¾, 'werpeja', weave in heaven for the new-born child its thread of fate, attaching each separate thread to a star. when death approaches the person, the thread is rent, and the star wanes and sinks to the earth. jacob grimm, 'deutsche mythologie', , s. . [footnote] ** according to the testimony of professor denison olmsted, of yale college, new haven, connecticut. (see poggend., 'annalen der physik', bd. xxx., s. .) kepler, who excluded fire-balls and shooting stars from the domain of astronomy, because they were, according to his views, "meteors arising from the exhalations of the earth, and blending with the higher ether," expresses himself, however, generally with much caution. he says: "stellÃ�¾ cadentes sunt materia viscida inflammata. earum aliquÃ�¾ inter cadendum absumuntur, aliquÃ�¾ verÃ�Â� in terram cadunt, pondere suo tractÃ�¾. nec est dissimile vero, quasdam conglobatas esse ex materia fÃ�¾culentÃ�Â�, in ipsam auram Ã�¾theream immixta: exque aÃ�Â�theris regione, tractu rectilineo, per aÃ�Â�rem trajicere, ceu minutos competas, occultÃ�Â� causa motus utrorumque." -- kepler, 'epit. astron. copernicanÃ�¾', t. i., p. . it still remains undertermined whether the many luminous bodies that shoot across the sky may not vary in their nature. on my return from the equinoctial zones, i was impressed with an idea that in the torrid regions of the tropics i had more frequently than in our colder latitudes seen shooting stars fall as if from a height of twelve or fifteen thousand feet; that they were of brighter colors, and left a more brilliant line of light in their track; but this impression was no doubt owing to the greater transparency of the tropical atmosphere*, which enables the eye to penetrate further into distance. [footnote] *'relation historique', t. i., p. , , . if in falling stars, as in comets, we distinguish between the head or nucleus and the tail, we shall find that the greater transparency of the atmosphere in tropical climates is evinced in the greater length and brilliancy of the tail which may be observed in those latitudes. the phenomenon is therefore not necessarily more frequent there, because it is oftener seen and continues longer visible. the influence exercised on shooting stars by the character of the atmosphere is shown occasionally even in our temperate zone, and at very small distances apart. wartmann relates that on the occasion of a november phenomenon at two places lying very near each other, geneva and aux planchettes, the number of the meteors counted were as to . (wartmann, 'mÃ�Â�m. sur les etoiles filantes', p. .) the tail of a shooting star (or its 'train'), on the subject of which brandes has made so many exact and delicate observations, is in no way to be ascribed to the continuance of the impression produced by light on the retina. it sometimes continues visible a whole minute, and in some rare instances longer than the light of the nucleus of the shooting star; in which case the luminous track remains motionless. (gilb., 'ann.', bd. xiv., s. .) this circumstance further indicates the analogy between large shooting stars and fire-balls. admiral krusenstern saw, in his voyage round the world, the train of a fire-ball shine for an hour after the lluminous body itself had disappeared, and scarcely move throughout the whole time. ('reise', th. i., s. .) sir alexander burnes gives a charming description of the transparency of the clear atmosphere of bokhara, which was once so favorable to the pursuit of astronomical observations. bokhara is situated in degrees ' north latitude, and at an elevation of feet above the level of the sea. "there is a constant serenity in its atmosphere, and an admirable clearness in the sky. at night, the stars have uncommon luster, and the milky way shines gloriously in the firmament. there is also a never-ceasing display of the most brilliant meteors, which dart like rockets in the sky; ten or twelve of them are sometimes seen in an hour, assuming every color -- fiery red, blue, pale, and faint. it is a noble country for astronomical science, and great must have been the advantage enjoyed by the famed observatory of samarkand." (burnes, 'travels into bokhara', vol. ii. ( ), p. .) a mere traveler must not be reproached for calling ten or twelve shooting stars in an hour "many," since it is only recently that we have learned, from careful observations on this subject in europe, that eight is the mean number which may be seen in an hour in the field of vision of one individual (quetelet, 'corresp. mathÃ�Â�m.', novem., , p. ); this number is, however, limited to five or six by that diligent observer, olbers. (schum., 'jahrb.', , s. .) p sir alexander burnes likewise extols as a consequence of the purity of the atmosphere in bokhara the enchanting and constantly-recurring spectacle of variously-colored shooting stars. the connection of meteoric stones with the grander phenomenon of fire-balls -- the former being known to be projected from the latter with such force as to penetrate from ten to fifteen feet into the earth -- has been proved, among many other instances, in the falls of azzzuerolites at barbotan, in the department des landes ( th july, ), at siena ( th june, ), at weston, in connecticut, u. s. ( th december, ), and at juvenas in the department of ardÃ�Â�che ( th june, ). meteoric stones are in some instances thrown from dark clouds suddenly formed in a clear sky, and fall with a noise resembling thunder. whole districts have thus occasionally been covered with thousands of fragmentary masses, of uniform character but unequal magnitudes, that p have been hurled from one of these moving clouds. in less frequent cases, as in that which occurred on the th of september, , at kleinwenden, near mÃ�Â�hilhausen, a large aÃ�Â�rolite fell with a thundering crash while the sky was clear and cloudless. the intimate affinity between fire-balls and shooting stars is further proved by the fact that fire-balls, from which meteoric stones have been thrown have occasionally been found, as at angers, on the th of june, , having a diameter scarcely equal to that of the small fire-works called roman candles. the formative power, and the nature of the physical and chemical processes involved in these phenomena are questions all equally shrouded in mystery, and we are as yet ignorant whether the particles composing the dense mass of meteoric stones are originally, as in comets, separated from one another when they become luminous to our sight, or whether in the case of smaller shooting stars, any compace substance actually falls, or, finally, whether a meteor is composed only of a smoke-like dust, containing iron and nickel; while we are wholly ignorant of what takes place within the dark cloud from which a noise like thunder is often heard for many minutes before the stones fall.* [footnote] *on 'mÃ�Â�teoric dust', see arago, in the 'annuaire' for , p. . i haave very recently endeavored to show, in another work ('asie centrale', t. i., p. ). how the scythian saga of the sacred gold, which fell burning from heaven, and remained in the possession of the golden horde of the paralatÃ�¾ (herod., iv., - ), probably originated in the vague recollection of the fall of an aÃ�Â�rolite. the ancients had also some strange fictions (dio cassius, lxxv., ) or silver which had fallen from heaven, and with which it had been attempted, under the emperor severus, to cover bronze coins; metallic iron was however, known to exist in meteoric stones. (plin., ii., .) the frequently-recurring expression 'lapidibus pluit' must not always be understood to refer to falls of aÃ�Â�rolites. in liv., xxv., , it probably refers to pumice ('rapilli') ejected from the volcano, mount albanus (monte cavo), which was not wholly extinguished at the time. (see heyne, 'opuscula acad.', t. iii., p. ; and my 'relation hist.', t. i., p. .) the contest of hercules with the ligyans, on the road from the caucasus to the hesperides, belongs to a different sphere of ideas, being an attempt to explain mythically the origin of the round quartz blocks in the ligyan field of stones at the mouth of the rhone, which aristotle supposes to have been ejected from a fissure during an earthquake, and posidonius to have been caused by the force of the waves of an inland piece of water. in the fragments that we still possess of the play of Ã�®schylus, the 'prometheus delivered', every thing proceeds, however, in part of the narration, as in a fall of aÃ�Â�rolites, for jupiter draws together a cloud, and causes the "district around to be covered by a shower of round stones". posidonius even ventured to deride the geognostic myth of the blocks and stones. the lygian field of stones was, however, very naturally and well described by the ancients. the district is now known as 'la crau.' (see guerin, 'mesures baromÃ�Â�triques dans les alpes, et mÃ�Â�tÃ�Â�orologie d'avignon', , chap. xii., p. .) p we can ascertain by measurement the enormous, wonderful, and wholly planetary velocity of shooting stars, fire-valls and meteoric stones, and we can gain a knowledge of what is the general and uniform character of the phenomenon, but not of the genetically cosmical process and the results of the metamorphoses. if meteoric stones while revolving in space are already consolidated into dense masses,* less dense, however, p than the mean density of the earth, they must be very small nuclei, which surrounded by inflammable vapor or gas, form the innermost part of fire-balls, from the height and apparent diameter of which we may, in the case of the largest, estimate that the actual diameter varies from to about feet. [footnote] *the specific weight of aÃ�Â�rolites varies from . (alais) to . (tabor). their general density may be set down as , water being . as to what has been said in the text of the actual diameters of fire-balls, we must remark, that the numbers have been taken from the few measurements that can be relied upon as correct. these give for the fire-ball of weston, connecticut ( th december, ), only ; for that observed by le roi ( th july, ) about and for that estimated by sir charles blagden ( th january, ) feet in diameter. brandes ('unterhaltungen' bd.i., s. ) ascribes a diameter varying from to feet to shooting stars, and a luminous train extending from to miles. there are, however, ample optical causes for supposing that the apparent diameter of fire-balls and shooting stars has been very much overrated. the volume of the largest fire-ball yet observed can not be compared with that of ceres, estimating generally so exact and admirable treatise, 'on the connection of the physical sciences', , p. .) with the view of elucidating what has been stated in the text regarding the large zÃ�Â�rolite that fell into the bed of the river narni, but has not again been found, i will give the passage made known by pertz, from the 'chronicon benedicti, monachi sancti andreÃ�¾ in mont soracte', a ms. belonging to the tenth century, and preserved in the chigi library at rome. the barbarous latin of that age has been left unchanged. "anno , temporibus domini johannis decimi pape, in anno pontificatus illius visa sunt signa. nam juxta urben romam lapides plurimi de cÃ�¾lo cadere visi sunt. in civilate quÃ�¾ vocatur narnia tam diri ac tetri, ut nihil aliud credatur, quam de infernalibus locis deducti essent. nam ita ex illis lapidibus unus omnium maximum est, ut decidens in flumen narnus, ad mensuram unius cubiti super aquas fluminus usque hodie videretur. nam et ignitÃ�¾ita ut pene terra contingeret. alianno , temporibus domini johannis decimi pape, in anno pontificatus illius visa sunt signa. nam juxta urben romam lapides plurimi de cÃ�¾lo cadere visi sunt. in civilate quÃ�¾ vocatur narnia tam diri ac tetri, ut nihil aliud credatur, quam de infernalibus locis deducti essent. nam ita ex illis lapidibus unus omnium maximum est, ut decidens in flumen narnus, ad mensuram unius cubiti super aquas fluminus usque hodie videretur. nam et ignitÃ�¾ ita ut pene terra contingeret. ali cadentes," etc. (pertz, 'monum. germ. hist. scriptores', t. iii., p. .) on the aÃ�Â�rolites of gos potamus, which fell, according to the parian chroniccle, in the olympiad, see bÃ�Â�ckh, 'corp. inscr. graec', t. ii., p. , , ; also aristot., 'meteor.', i., (ideler's 'comm.', t. i., p. - ); stob., 'eel. phys.', i., , p. (heeren); plut., 'lys.', c. ; diog. laert., ii., ; and see, also, subsequent notes in this work. according to a mongolisn tradition, a black fragment of a rock, forty feet in height, fell from heaven on a plain near the source of the great yellow river in western china. (abel rÃ�Â�musat, in lamÃ�Â�therie, 'jour. de phys.', , mai p. .) the largest meteoric masses as yet known are those of otumpa, in chaco, and of bahia, in brazil, described by rubi de celis as being from to / feet in length. the meteoric stone of gos potamos, celebrated in antiquity, and even mentioned in the chronicle of the parian marbles, which fell about the year in which socrates was born, has been described as of the size of two mill-stones, and equal in weight to a full wagon load. notwithstanding the failure that has attended the efforts of the african traveler, brown, i do not wholly relinquish the hope that, even after the lapse of years, this thracian meteoric mass, which it would be so difficult to destroy, may be found, since the region in which it fell is now bcome so easy of access to european travelers. the huge aÃ�Â�rolite which in the beginning of the tenth century fell into the river at narni, projected between three and four feet above the surface of the water, as we learn from a document lately discovered by pertz. it must be remarked that these meteoric bodies, whether in ancient or modern times can only be regarded as the principal fragments of masses that have been broken up by the explosion either of a fire-ball of a dark cloud. on considering the enormous velocity with which, as has been mathematically proved, meteoric stones reach the earth from the extremest confines of the atmosphere, and the lengthened course traversed by fire-balls through the denser strata of the air, it seems more than improbable that these metalliferous stony masses, containing perfectly-formed crystals of olivine, labradorite, and pyroxene, should in so short a period of time has been converted from a vaporous condition to a solid nucleus. moreover, that which falls from meteoric masses, even where the internal composition is chemically different, exhibits almost always the peculiar character of a fragment, being of a prismatic or truncated pyramidal form, with broad, somewhat curved faces, and rounded angles. but whence comes this form, which was first recognized by schreiber as characteristic of the 'severed' part of a rotating planetary body? here, as in the sphere of organic life, all that appertains to the history of development remains hidden in obscurity. meteoric masses become luminous and kindle at heights which p must be regarded as almost devoid of air, of occupied by an atmosphere that does not even contain / th part of oxygen. the recent investigations of biot on the important phenomenon of twilight* have considerably lowered the lines which had, perhaps with some degree of temerity, been usually termed the boundaries of the atmosphere; but processes of light may be evolved independently of the presence of oxygen, and poisson conjectured that aÃ�Â�roliteswere ignited far beyond the range of our atmosphere. numerical calculation and geometrical measurement are the only means by which as in the case of the larger bodies of our solar system, we are enabled to impart a firm and safe basis to our investigations of meteoric stones. [footnote] *biot, 'traitÃ�Â� d'astronomie physique' ( Ã�Â�me Ã�Â�d.), , t. i., p. , , , . my lamented friend poisson endeavored, in a singular manner, to solve the difficulty attending an assumption of the spontaneous ignition of meteoric stones at an elevation where the density of the atmosphere is almost null. these are his words: "it is difficult to attribute, as is uaually done, the incandescence of aÃ�Â�rolites to friction against the molecules of the atmosphere at an elevation above the earth where the density of the air is almost null. may we not suppose that the electric fluid, in a neutral condition, forms a kind of atmosphere, extending far beyond the mass of our atmosphere, yet subject to terrestrial attraction, although physically imponderable, and consequently following our globe in its motion? according to this hypothesis, the bodies of which we have been speaking would, on entering this imponderable atmosphere, decompose the neutral fluid by their unequal action on the two electricities, and they would thus be heated, and in a state of incandescence, by becoming electrified." (poisson, 'rech. sur la probabilitÃ�Â� des jugements', , p. .) although halley pronounced the great fire-ball of , whose motion was opposite to that of the earth in its orbit,* to be a cosmical body, chadni, in , first recognized, with ready acuteness of mind, the connection between fire-balls and the stones projected from the atmosphere, and the motions of the former bodies in space.** [footnote] *'philos. transact.', vol. xxix., p. - . [footnote] **the first edition of chlandni's important treatise, 'ueber den ursprung der von pallas gefundenen und anderen eisenmassen' (on the origin of the masses of iron found by pallas, and other similar masses), appeared two months prior to the shower of stones at siena, and two years before lichtenberg stated, in the 'gÃ�Â�ttingen taschenbuch', that "stones reach our atmosphere from the remoter regions of space.' comp., also, olbers's letter to benzenberg, th nov., , in benzenberg's 'treatise on shooting stars', p. . a brilliant confirmation of the cosmical origin of these phenomena has been afforded by denison olmsted, at new haven, connecticut, who has shown on the concurrent authority of all eye-witnesses, that during the celebrated fall of shooting stars on the night between the th p and th of november, , the fire-balls and shooting stars all emerged from one and the same quarter of the heavens, namely, in the vicinity of the star 'gamma' in the constellation leo, and did not deviate from this point, although the star changed its apparent height and azimuth during the time of the observation. such an independence of the earth's rotation shows that the luminous body must have reached our atmosphere from 'without.' according to encke's computation* of the whole p number of observations made in the united states of north america, between the thirty-fifth and the forty-second degrees of latitude, it would appear that all these meteors came from the same point of space in the direction in which the earth was moving at the time. [footnote] *encke, in poggend., 'annalen', bd. xxxiii. ( ), s. . arago, in the 'annuaire' for , p. . two letters which i wrote to benzenberg, may and october , , on the conjectural precession of the nodes in the orbit of periodical falls of shooting stars. (benzenberg's 'sternsch.', s. and .) olbers subsequently adopted this opinion of the gradual retardation of the november phenomenon. ('astron. nachr.', , no. , s. .) if i may venture to combine two of the falls of shooting stars mentioned by the arabian writers with the epochs found by boguslawski for the fourteenth century, i obtain the following more or less accordant elements of the movements of the nodes: in oct., , on the night in which king ibrahim ben ahmed died, there fell a heavy shower of shooting stars, "like a fiery rain;" and this year was, therefore, called the year of stars. (conde, 'hist. de la domin.' de los arabes', p. .) on the th of oct., , the stars were in motion all night. "they fell like locusts." ('comptes rendus', , t. i., p. ; and frÃ�¾hn, in the 'bull. de l'acadÃ�Â�mie de st. pÃ�Â�tersbourg', t. iii., p. .) on the st oct., o.s., , "'die sequente post festum xi. millia virginum ab hora matutina usque ad horam primam visÃ�¾ sunt quasi stellÃ�¾ de cÃ�¾lo cadere continuo, et in tanta multitudine, quod nemo narrare suf ficit.'" this remarkable notice, of which we shall speak more fully in the subsequent part of this work, was found by the younger von boguslawski, in benesse (de horowic) de weitmil or weithmÃ�Â�l, 'chronicon ecclesiÃ�¾ pragensis', p. . this chronicle may also be found in the second part of 'scriptores rerum bohemicarum', by pelzel and dobrowsky, . (schum., 'astr. nachr.', dec., .) on the night between the th and th of november, , many falling stars were observed at manheim, southern germany, by hemmer (kÃ�Â�mtz, 'meteor.', th. iii., s. .) after midnight, on the th of november, , occurred the extraordinary fall of stars at cumana, which bonpland and myself have described, and which was observed over a great part of the earth. ('relat. hist.', t. i., p. - .) between the th and th of november, , shooting stars, intermingled with fire-balls, were seen in large numbers by kloden, at potsdam. (gilbert's 'ann.', bd. lxxii., s. .) on the th of november, , at o'clock in the morning, a great shower of falling stars was seen by captain bÃ�Â�rard, on the spanish coast, near carthagena del levante. ('annuaire', , p. .) in the night between the th and th of november, , occurred the phenomenon so admirably described by professor olmsted, in north america. in the night of the - th of november, , a similar fall of shooting stars was seen in north america, although the numbers were not quite so considerable. (poggend., 'annalen', bd. xxxiv., s. .) on the th of november, , a barn was set on fire by the fall of a sporadic fire-ball, at belley, in the department de l'ain. ('annuaire', , p. .) in the year , the stream showed itself most decidedly on the night of the - th of november. ('astron. nachr.', , no. .) on the recurrence of falls of shooting stars in north america, in the month of november of the years and , and in the analogous falls observed at bremen in , a like general parallelism of the orbits, and the same direction of the meteors from the constellation leo, were again noticed. it has been supposed that a greater parallelism was observable in the direction of periodic falls of shooting stars than in those of sporadic occurrence; and it has further been remarked, that in the periodically-recurring falls in the month of august, as, for instance, in the year , the meteors came principally from one point between perseus and taurus, toward the latter of which constellations in the earth was then moving. this peculiarity of the phenomenon, manifested in the retrograde direction of the orbits in november and august, should be thoroughly investigated by accurate observations, in order that it may either be fully confirmed or refuted. the heights of shooting stars, that is to say, the heights of the points at which they begin and cease to be visible, vary exceedingly, fluctuating between and miles. this important result, and the enormous velocity of these problematical asteroids, were first ascertained by benzenberg and brandes, by simultaneous observations and determinations of parallax at the extremities of a base line of , feet in length.* [footnote] *i am well aware that, among the shooting stars simultaneously observed in silesia, in , at the suggestion of professor brandes some appeared to have an elevation of to , or even miles. (brandes, 'unterhaltungen fÃ�Â�r freunde der astronomie und physik', heft i., s. . instructive narratives for the lovers of astronomy and physics.) but olbers considered that all determinations for elevations beyond miles must be doubtful, owing to the smallness of the parallax. the relative velocity of motion is from to miles in a second, and consequently equal to planetary velocity. this planetary velocity,* as well as the direction of the orbits p of fire-balls and shooting stars, which has frequently been observed to be opposite to that of the earth, may be considered as conclusive arguments against the hypothesis that aÃ�Â�rolites derive their origin from the so-called active 'lunar volcanoes.' [footnote] *the planetary velocity of translation, the movement in the orbit, is in mercury . , in venus . , and in the earth . miles in a second. numerical views regarding a greater or lesser volcanic force on a small cosmical body, not surrounded by any atmosphere, must, from their nature, be wholly arbitrary. we may imagine the reaction of the interior of a planet on its crust ten or even a hundred times greater than that of our present terrestrial volcanoes; the direction of masses projected from a satellite revolving from west to east might appear retrogressive, owing to the earth in its orbit subsequently reaching that point of space at which these bodies fall. if we examine the whole sphere of relations which i have touched upon in this work, in order to escape the charge of having made unproved assertions, we shall find that the hypothesis of the selenic origin of meteoric stones* depends upon a number of conditions p whose accidental coincidence could alone convert a possible into an actual fact. [footnote] *chladni states that an italian physicist, paolo maria terzago, on the occasion of the fall of an aÃ�Â�rolite at milan in , by which a franciscan monk was killed, was the first who surmised that aÃ�Â�rolites were of selenic origin. he says, in a memoir entitled 'musÃ�¾um septalianum, manfredi septalÃ�¾, patricii mediolanensis, industrioso labore constructum' (tortona, , p. ), "labant philosophorum mentes sub horum lapidum ponderibus; ni dicire velimus, lunan terram alteram, sine mundum esse, ex cujus montibus divisa frustra in inferiorem nostrum hunc orben dela bantur." without any previous knowledge of this conjecture, olbers was led, in the year (after the celebrated fall at siena on the th of june, ), into an investigation of the amount of the initial tangential force that would be requisite to bring to the earth masses projected from the moon. this ballistic problem occupied, during ten or twelve years, the attention of the geometricians laplace, biot, brandes, and poisson. the opinion which was then so prevalent, but which has since been abandoned, of the existence of active volcanoes in the moon, where air and water are absent, led to a confusion in the minds of the generality of persons between mathematical possibilities and physical probabilities. olbers, brandes, and chladni thought "that the velocity of to miles, with which fire-balls and shooting stars entered our atmosphere," furnished a refutation to the view of their selenic origin. according to olbers, it would require to reach the earth, setting aside the resistance of the air, an initial velocity of feet in the second; according to laplace, ; to biot, ; and to poisson, . laplace states that this velocity is only five or six times as great as that of a cannon ball; but olbers has shown "that, with such an initial velocity as or feet in a second, meteoric stones would arrive at the surface of our earth with a velocity of only , feet (or . german geographical mile). but the measured velocity of meteoric stones averages five such miles, or upward of , feet to a second; and, consequently, the original velocity of projection from the moon must be almost , feet, and therefore fourteen times greater than laplace asserted." (olbers, in schum, 'jahrb.', , p. - ; and in gehler, 'neues physik.' 'wÃ�Â�rterbuche', bd. vi., abth. , s. - .) if we could assume volcanic forces to be still active on the moon's surface, the absence of atmospheric resistance would certainly give to their projectile force an advantage over that of our terrestrial volcanoes; but even in respect to the measure of the latter force (the projectile force of our own volcanoes), we have no observations on which any reliance can be placed, and it has probably been exceedingly overrated. dr. peters, who accurately observed and measured the phenomena presented by Ã�®tna, found that the greatest velocity of any of the stones projected from the crater was only feet to a second. observations on the peak of teneriffe, in , gave feet. although laplace, at the end of his work ('expos. du syst. du monde', ed. de , p. ), cautiously observes, regarding aÃ�Â�rolites, "that in all probability they come from the depths of space," yet we see from another passage (chap. vi., p. ) that, being probably unacquainted with the extraordinary planetary velocity of meteoric stones, he inclines to the hypothesis of their lunar origin, always, however, assuming that the stones projjected from the moon "become satellites of our earth, describing around it more or less eccentric orbits, and thus not reaching its atmosphere until several or even many revolutions have been accomplished." as an italian at tortona had the fancy that aÃ�Â�rolites came from the moon, so some of the greek philosophers thought they came from the sun. this was the opinion of diogenes laertius (ii., ) regarding the origin of the mass that fell at "gos potamos (see note, p. ). pliny, whose labors in recording the opinions and statements of preceding writers are astonishing, repeats the theory, and derides it the more freely, because he, with earlier writers (diog. laert., and , p. , hÃ�Â�bner), accuses anaxagoras of having predicted the fall of aÃ�Â�rolites from the sun: "celebrant grÃ�¾ci anaxagoram clazomenium olympiadis septuagesimÃ�¾ octavÃ�¾ secundo anno prÃ�¾dixisse cÃ�¾lestium litterarum scientia quibus diebus saxum casurum esse e sole, idque factum interdia in thraciÃ�¾ parte ad gos flumen. quod si quis prÃ�¾dictum credat, simul fateatur necesse est, majoris miraculi divinitatem anaxagorÃ�¾ fuisse, solvique rerum naturÃ�¾ intellectum, et confundi omnia, si aut ipse sol lapis esse aut unquam lapidem in eo fuisse credatur; decidere tamen crebro non erit dubium." the fall of a moderate-sized stone, which is preserved in the gymnasium at abydos, is also reported to have been foretold by anaxagoras. the fall of aÃ�Â�rolites in bright sunshine, and when the moon's disk was invisible, probably led to the idea of sun-stones. moreover, according to one of the physical dogmas of anaxagoras, which brought on him the persecution of the theologians (even as they have attacked the geologists of our own times), the sun was regarded as "a molten fiery mass" ([greed words]). in accordance with these views of anaxagoras, we find euripides, in 'phaÃ�Â�ton', terming the sun "a golden mass;" that is to say, a fire-colored, brightly-shining matter, but not leading to the inference that aÃ�Â�rolites are golden sun-stones. (see note to page .) compare valckenaer, 'diatribe in eurip. perd. dram. reliquias', , p. . diog. laert., ii., . hence, among the greek philosophers, we find four hypotheses regarding the origin of falling stars: a telluric origin from ascending exhalations; masses of stone raised by hurricane (see aristot., 'meteor., lib. i., cap. iv., - , and cap. vii., ); a solar origin; and, lastly, an origin in the regions of space, as heavenly bodies which had long remained invisible. respecting this last opinion, which is that of diogenes of apollonia, and entirely accords with that of the present day, see pages and . it is worthy of remark, that in syria, as i have been assured by a learned orientalist, now resident at smyrna, andrea de nericat, who instructed me in persian, there is a popular belief that aÃ�Â�rolites chiefly fall on clear moonlight nights. the ancients, on the contrary, especially looked for their fall during lunar eclipses. (see pliny, xxxvii., , p. . solinus, c. . salm., 'exere.', p. ; and the passages collected by ukert, in his 'geogr. der griechen und rÃ�Â�mer', th. ii., , s. , note .) on the improbability that meteoric masses are formed from metal-dissolving gases, which, according to fusinieri, may exist in the highest strata of our atmosphere, and previously diffused through an almost boundless space, may suddenly assume a solid condition, and on the penetration and misceability of gases, see my ' relat. hist.', t. i., p. . p the view of the original existence of p small planetary masses in space is simpler, and at the same time, more analogous with those entertained concerning the formation of other portions of the solar system. it is very probable that a large number of these cosmical bodies traverse space undestroyed by the vicinity of our atmosphere, and revolve round the sun without experiencing any alteration but a slight increase in the eccentricity of their orbits, occasioned by the attraction of the earth's mass. we may, consequently, suppose the possibility of these bodied remaining invisible to us during many years and frequent revolutions. the supposed phenomenon of ascending shooting stars and fire-balls, which chladni has unsuccessfully endeavored to explain on the hypothesis of the 'reflection' of strongly compressed air, appears at first sight as the consequence of some unknown tngential force propelling bodies from the earth; but bessel has shown by theoretical deductions, confirmed by feldt's carefully-conducted calculations, that, owing to the absence of any proofs of the simultaneous occurrence of the observed disappearances, the assumptiopn of an ascent of shooting stars was rendered wholly improbable, and inadmissible as a result of observation.* [footnote] *bessel, in schum., 'astr. nachr.', , no und , s. und . at the conclusion of the memoir there is a comparison of the sun's longitudes with the epochs of the november phenomenon, from the period of the first observations in cumana in , the opinion advanced by olbers that the explosion of shooting stars and ignited fire-balls not moving in straight lines may impel meteors upward in the manner of rockets, and influence the direction of their orbits, must be made the subject of future researches. shooting stars fall either seprately and in inconsiderable numbers, that is, sporadically, or in swarms of many thousands. p the latter, which are compared by arabian authors to swarms of locusts, are periodic in their occurrence, and move in streams, generally in a parallel direction. among periodic falls, the most celebrated are that known as the november phenomenon, occurring from about the th to the th of november, and that of the festival of st. lawrence (the th of august), whose "fiery tears" were noticed in former times in a church calendar of england, no less than in old traditionary legends, as a meteorological event of constant recurrence.* [footnote] *dr. thomas forster ('the pocket encyclopedia of natural phenomena' , p. ) states that a manuscript is preserved in the library of christ's college, cambridge,** written in the tenth century by a monk, and entitled 'ephemerides rerum naturalium', in which the natural phenomena for each day of the year are inscribed as, for instance, the first flowering of plants, the arrival of birds, etc.; the th of august is distinguished by the word "meteorodes." it was this indication, and the tradition of the fiery tears of st. lawrence, that chiefly induced dr. forster to undertake his extremely zealous investigation of the august phenomena. (quetelet, 'correspond. mathÃ�Â�m.', sÃ�Â�rie iii., t. i., , p. .) [further footnote] **[no such manuscript is at present known to exist in the library of that college. for this information i am indebted to the inquiries of mr. cory, of pembroke college, the learned editor of 'hieroglyphics of horapollo nilous', greek and english, .] -- tr. notwithstanding the great quantity of shooting stars and fire-balls of the most various dimensions, which, according to klÃ�Â�den, were seen to fall at potsdam on the night between the th and th of november, , and on the same night of the year in throughout the whole of europe, from portsmouth to orenburg on the ural river, and even in the southern hemisphere, as in the isle of france, no attention was directed to the 'periodicity' of the phenomenon, and no idea seems to have been entertained of the connection existing between the fall of shooting stars and the recurrence of certain days, until the prodigious swarm of shooting stars which occurred in north america between the th and th of november, , and was observed by olmsted and palmer. the stars fell on this occasion, like flakes of snow, and it was calculated that at least , had fallen during a period of nine hours. palmer, of new haven, connecticut, was led, in consequence of this splendid phenomenon, to the recollection of the fall of meteoric stones in , first described by ellicot and myself,* and which, by p a comparison of the facts i had adduced, showed that the phenomenon had been simultaneously seen in the new continent, from the equator to new herrnhut in greenland ( degrees ' north latitude), and between degrees and degrees longitude. [footnote] *humb., 'rel. hist.', t. i., p. - . ellicot in the 'transactions of the american society', , vol. vi., . . arago makes the following observations in reference to the november phenomena: "we thus become more and more confirmed in the belief that there exists a zone composed of millions of small bodies, whose orbits cut the plane of the ecliptic at about the point which out earth annually occupies between the th and th of november. it is a new planetary world beginning to be revealed to us." ('annuaire', , p. .) the identity of the epochs was recognized with astonishment. the stream which had been seen from jamaica to boston ( degrees ' north latitude) to traverse the whole vault of heaven on the th and th of november, , was again observed in the united states in , on the night between the th and th of november, although on this latter occasion it showed itself with somewhat less intensity. in europe the periodicity of the phenomenon has since been manifested with great regularity. another and a like regularly recurring phenomenon is that noticed in the month of august, the meteoric stream of st. lawrence, appearing between the th and th of august. muschenbrock,* as early as in the middle of the last century, drew attention to the frequency of meteors in the month of august' but their certain periodic return about the time of st. lawrence's day was first shown by quetelet, olbers, and benzenberg. [footnote] *compare muschenbroek, 'introd. ad phil. nat.', , t. ii., p. ; howard, 'on the climate of london', vol. ii., p. , observations of the year ; seven years, therefore aftr the earliest observations of brandes (benzenberg, 'Ã�Â�ber sternschnuppen', s. - ); the august observations of thomas forster, in quetelet, op. cit., p. - ; those of adolph erman, boguslawski, and kreil, in schum., 'jahrb.', , s. - . regarding the point of origin in perseus, on the th of august, , see the accurate measurements of bessel and erman (schum., 'astr. nachr.', no. und ); but on the th of august, , the path does not apper to have been retrograde; see arago in 'comptes rendus', , t. ii., p. . we shall, no doubt, in time, discover other periodically appearing streams,* probably about the d to the p. th of april, between the th and th of december, and, to judge by the number of true falls of aÃ�Â�rolites enumerated by capocci, also between the th and th of november, of about the th of july. [footnote] *on the th of april, , "innumerable eyes in france saw stars falling from heaven as thickly as hail" ('ut grando, nisi lucerent, pro densitate putaretur'; baldr., p. ), and this occurrence was regarded by the council of clermont as indicative of the great movement in christendom. (wilken, 'gesch. der kreuzzÃ�Â�ge', bd. i., s. .) on the th of april, , a great fall of stars was observed in virginia and massachusetts; it was "a fire of rockets that lasted two hours." arago was the first to call attention to the "trainÃ�Â�e d'asteroÃ�Â�des," as a recurring phenomenon. ('annuaire', , p. .) the falls of aÃ�Â�rolites in the beginning of the month of december are also deserving of notice. in reference to their periodic recurrence as a meteoric stream, we may mention the early observation of brandes on the night of the th and th of december, (when he counted falling stars), and very probably the enormous fall of aÃ�Â�rolites that occurred at the rio assu, near the village of macao, in the brazils, on the th of december, . (brandes, 'unterhalt. fÃ�Â�r freunde der physik', , heft i., s. , and 'comptes rendus', t. v., p. .) capocci, in the interval between and , a space of thirty years, has discovered twelve authenticated cases of aÃ�Â�rolites occurring between the th and th of november, besides others on the th of november, the th of august, and the th of july. ('comptes rendus', t. xi., p. .) it is singular that in the portion of the earth's path corresponding with the months of january and february, and probably also with march, no 'periodic' streams of falling stars of aÃ�Â�rolites have as yet been noticed; although when in the south sea in the year , i observed on the th of march a remarkably large number of falling stars, and they were seen to fall as in a swarm in the city of quito, shortly before the terrible earthquake of riobamba on the th of february, . from the phenomena hitherto observed, the following epochs seem especially worthy of remark: d to the th of april. th of july ( th to the th of july?). (quet., 'corr.', , p. .) th of august. th to the th of november. th to the th of november. th to the th of december. when we consider that the regions of space must be occupied by myriads of comets, we are led by analogy, notwithstanding the differences existing between isolated comets and rings filled with asteroids, to regard the frequency of these meteoric streams with less astonishment than the first consideration of the phenomenon would be likely to excite. although the phenomena hitherto observed appear to have been independent of the distance from the pole, the temperature of the air, and other climatic relations, there is, however, one perhaps accidentally coincident phenomenon which must not be wholly disregarded. the northern light, the aurora borealis, was unusually brilliant on the occurrence of the borealis, was unusually brilliant on the occurrence of the splendid fall of meteors of the th and th november, , described by olmsted. it was also observed at bremen in , where the periodic meteoric fall was, however, less remarkable than at richmond, near london. i have mentioned in another work the singular fact observed by admiral wrangel, and frequently confirmed to me by himself,* that when he p was on the siberian coast of the polar sea, he observed, during an aurora borealis, certain portions of the vault of heaven which were not illuminated, light up and continue luminous whenever a shooting star passed over them. [footnote] *ferd. v. wrangle, 'reise lÃ�Â�ngs der nordkÃ�Â�ste von sibirien in den jahren', - , th. ii., s. . regarding the recurrence of the denser swarm of the november stream after an interval of thirty-three years, see olbers, in 'jahrb.', , s. . i was informed in cumana that shortly before the fearful earthquake of , and consequently thirty-three years (the same interval) before the great fall of stars on the th and th of november, , a similar fiery manifestation had been observed in the heavens. but it was on the st of october, , and not in the beginning of november, that the earthquake occurred. possibly some traveler in quito may yet be able to ascertain the day on which the volcano of cayambe, which is situated there, was for the space of an hour enveloped in falling stars, so that the inhabitants endeavored to appease heaven by religious processions. ('relat. hist.', t. i., chap. iv., p ; chap. x., p. and .) the different meteoric streams, each of which is composed of myriads of small cosmical bodies, probably intersect our earth's orbit in the same manner as biela's comet. according to this hypothesis, we may represent to ourselves these asteroid-meteors as composing a closed ring or zone, within which they all pursue one common orbit. the s aller planets between mars and jupiter present us if we except pallas with an analogous relation in their constantly intersecting orbits. as yet, however, we have no certain knowledge as to whether changes in the periods at which the stream becomes visible, or the 'retardations' of the phenomena of which i have already spoken, indicate a regular precession of oscillation of the nodes -- that is to say, of the points of intersection of the earth's orbit and of that of the ring; or whether this ring or zone attains so considerable a degree of breadth from the irregular grouping and distances apart of the small bodies, that it requires several days for the earth to traverse it. the system of saturn's satellites shows us likewise a group of immense width, composed of most intimately-connected cosmical bodies. in this system, the orbit of the outermost (the seventh) satellite has such a vast diameter, that the earth, in her revolution round the sun, requires three days to traverse an extent of space equal to this diameter. if, therefore, in one of these rings, which we regard as the orbit of a periodical stream, the asteroids should be so irregularly distributed as to consist of but few groups sufficiently dense to give rise to these phenomena, we may easily understand why we so seldom witness such glorious spectacles as those exhibited in the november months of and . the acute mind of olbers led him almost to predict that the next appearance of the phenomenon of shooting stars and fire-balls intermixed, falling like flakes of snow, would not recur until between the th and th of november, . p the stream of the november asteroids has occasionally only been visible in a small section of the earth. thus, for instance, a very splendid 'meteoric shower' was seen in england in the year , while a most attentive and skillful observer at braunsberg, in prussia only saw on the same night, which was there uninterruptedly clear, a few sporadic shooting stars fall between seven o'clock in the evening and sunrise the next morning. bessel* concluded from this "that a dense group of the bodies composing the great ring may have reached that part of the earth in which england is situated, while the more eastern districts of the earth might be passing at the time through a part of the meteoric ring proportionally less densely studded with bodies." [footnote] *from a letter to myself, dated jan. th, . the enormous swarm of falling stars in november, , was almost exclusively seen in america, where it was witnessed from new herrnhut in greenland to the equator. the swarms of and were visible only in europe, and those of and only in the united states of north america. if the hypothesis of a regular progression or oscillation of the nodes should acquire greater weight, special interest will be attached to the investigation of older observations. the chinese annals, in which great falls of shooting stars, as well as the phenomena of comets, are recorded, go back beyond the age of tyrtÃ�¾s, or the second messenian war. they give a description of two streams in the month of march, one of which is years anterior to the christian era. edward biot has observed that among the fifty-two phenomena which he has collected from the chinese annals, those that were of most frequent recurrence are recorded at periods nearly corresponding with the th and d of july, o.s., and might consequently be identical with the stream of st. lawrence's day, taking into account that it has advanced since the epochs* indicated. [footnote] *lettre de m. edouard biot Ã�Â� m. quetelet, sur les anciennes apparitions d'etoiles filantes en chine, in the 'bull. de l'acadÃ�Â�mie de bruxelles', , t. x., no. , p. . on the notice from the 'chronicon ecclesiÃ�¾ pragensis', see the younger boguslawski, in poggend., 'annalen', bd. xlviii., s. . if the fall of shooting stars of the st of october, , o.s. (a notice of which was found by the younger von boguslawski, in benessius de horowic's 'chronicon ecclesiÃ�¾ pragensis'), be identical with our november phenomenon, although the occurrence in the fourteenth century was seen in broad daylight, we find by the precession in years that this system of meteors, or, rather, its common center of gravity, must describe p a retrograde orbit round the sun. it also follows, from the views thus developed, that the non-appearance, during certain years, in any portion of the earth, of the two streams hitherto observed in november and about the time of st. lawrence's day, must be ascribed either to an interruption in the meteoric ring, that is to say, to intervals occurring between the asteroid groups, or, according to poisson to the action of the larger planets* on the form and position of this annulus. [footnote] *"it appears that an apparently inexhaustible number of bodies, too small to be observed, are moving in the regions of space, either around the sun or the planets, or perhaps even around their satellites. it is supposed that when these bodies come in contact with our atmosphere, the difference between their velocity and that of our planet is so great, that the friction which they experience from their contact with the air heats them to incandescence, and sometimes causes their explosion. if the group of falling stars form an annulus around the sun, its velocity of circulation may be very different from that of our earth; and the displacements it may experience in space, in consequence of the actions of the various planets, may render the phenomenon of its intersecting the planes of the ecliptic possible at some epochs, and altogether impossible at others." -- poisson, 'recherches sur la probabilitÃ�Â� des jugements', p. , . the solid masses which are observed by night to fall to the earth from fire-balls, and by day generally when the sky is clear, from a cark small cloud, are accompanied by much candescence. they undeniably exhibit a great degree of general identity with respect to their external form, the character of their crust, and the chemical composition of their principal constituents. these characteristics of identity have been observed at all the different epochs and in the most various parts of the earth in which these meteoric stones have been found. this striking and early-observed analogy of physiognomy in the denser meteoric masses is, however, met by many exceptions regarding individual points. what differences, for instance, do we not find between the malleable masses of for instance, do we not find between the malleable masses of iron of hradeschina in the district of agram, those from the shores of the sisim in the government of jeniseisk, rendered so celebrated by pallas, or those which i brought from mexico,* all of which contain per cent. of iron, from the aÃ�Â�rolites of siena, in which the iron scarcely amounts to per cent., or the earthy aÃ�Â�rolite of alais (in the department du gard), which broke up in water, or, lastly, from those of jonzac and javenas, which contained no metallic iron, but presented a p mixture of oryctognostically distinct crystalline compoonents! [footnote] *humboldt, 'essai politique sur la nouv. espagne' ( de Ã�Â�dit.), t. iii. p. . these differences have led mineralogists to separate these cosmical masses into two classes, namely, those containing nickelliferous meteoric iron, and those consisting of fine or coarsely-granular meteoric dust. the crust or rind of aÃ�Â�rolites is peculiarly characteristic of these bodies, being only a few tenths of a line in thickness, often glossy and pitch-like, and occasionally veined.* [footnote] *the peculiar color of their crust was observed even as early as in the time of pliny (ii., and ): "colore adusto." the phrase "lateribus pluisse" seems also to refer to the burned outer surface of aÃ�Â�rolites. there is only one instance on record, as far as i am aware (the aÃ�Â�rolite of chantonnay, in la vendÃ�Â�e), in which the rind was absent, and this meteor, like that of juvenas, presented likewise the peculiarity of having pores and vesicular cavities. in all other cases the black crust is divided from the inner light-gray mass by as sharply-defined a line of separation as is the black leaden-colored investment of the white granit blocks* which i brought from the cataracts of the orinoco, and which are also associated with many other cataracts, as, for instance, those of the nile and of the congo river. [footnote] * humb., 'rel. hist.', t. ii., chap xx., p. - . the greatest heat employed in our porcelain ovens would be insufficient to produce any thing similar to the crust of meteoric stones, whose interior remains wholly unchanged. here and there, facts have been observed which would seem to indicate a fusion together of the meteoric fragments; but, in general, the character of the aggregate mass, the absence of compression by the fall, and the inconsiderable degree of heat possessed by these bodies when they reach the earth, are all opposed to the hypothesis of the interior being in a state of fusion during their short passage from the boundary of the atmosphere to our earth. the chemical elements of which these meteoric masses consist, and on which berzelius has thrown so much light, are the same as those distributed throughout the earth's crust, and are fifteen in number, namely, iron, nickel, cobalt, manganese, chromium, copper, arsenic, zinc, potash, soda, sulphur, phosphorus, and carbon, constituting altogether nearly one third of all the known simple bodies. notwithstanding this similarity with the primary elements into which inorganic bodies are chemically reducible, the aspect of aÃ�Â�rolites, owing to the mode in which their constituent parts are compounded, presents, generally, some features foreign to our telluric rocks and minerals. the pure native iron, which is almost always p found incorporated with aÃ�Â�rolites, imparts to them a peculiar, but not consequently, a 'selenic' character; for in other regions of space, and in other cosmical bodies besides our moon, water may be wholly absent, and processes of oxydation of rare occurence. cosmical gelatinous vesicles, similar to the organic 'nostoc' (masses which have been supposed since the middle ages to be connected with shooting stars), and those pyrites of sterlitamak, west of the uralian mountains, which are said to have constituted the interior of hailstones,* must both be classed among the mythical fables of meteorology. [footnote] *gustav rose, 'reise nach dem ural', bd. ii., s. . some few aÃ�Â�rolites, as those composed of a finely granular tissue of olivine, augite, and labradorite blended together* (as the meteoric stone found at juvenas, in the department de l'ardÃ�Â�che, which resembled dolorite), are the only ones, as gustav rose has remarked, which have a more familiar aspect. [footnote] *gustav rose, in poggend., 'ann.', , bd. iv., x. - . rammelsberg, 'erstes suppl. zum chem. handwÃ�Â�rterbuche der mineralogie', , s. . "it is," says the clear-minded observer olbers, "a remarkable but hitherto unregarded fact, that while shells are found in secondary and tertiary formations, no 'fossil meteoric stones' have as yet been discovered. may we conclude from this circumstance that previous to the present and last modification of the earth's surface no meteoric stones fell on it, although at the present time it appears probable, from the researches of schreibers, that fall annually?" (olbers, in schum., 'jahrb.', , s. .) problematical nickelliferous masses of native iron have been found in northern asia (at the gold-washing establishment at petropawlowsk, eighty miles southeast of kusnezk), imbedded thirty-one feet in the ground, and more recently in the western carpathians (the mountain chain of magura, at szlanicz), both of which are remarkably like meteoric stones. compart erman, 'archiv fÃ�Â�r wissenschaftliche kunde von russland', bd. i., s. , and haidinger, 'bericht Ã�Â�ber szlaniczer schÃ�Â�rfe in ungarn.' these bodiescontain, for instance, crystalline substances, perfectly similar to those of our earth's crust; and in the siberian mass of meteoric iron investigated by pallas, the olivine only differs from common olivine by the absence of nickel, which is replaced by the oxyd of tin.* [footnote] *berzelius, 'jahresber.', bd. xv., s. und . rammelsberg, 'handwÃ�Â�rterb., abth. ii., s. - . as meteoric olivine, like our basalt, contains from to per cent. of magnesia, constituting, according to berzelius, almost the half of the earthy components of meteoric stones, we can not be surprised at the great quantity of silicate of magnesia found in these cosmical bodies. if the zÃ�Â�rolite of juvenas contain separable crystals of augite and labradorite, the numerical relation of the constituents p render it at least probable that the meteoric masses of chateau-renard may be a compound of diorite, consisting of hornblende and albite, and those of blansko and chantonnay compounds of hornblende and labradorite. the proofs of the telluric and atmospheric origin of auerolites, which it is attempted to base upon the oryctognostic analogies presented by these bodies, do not appear to me to possess any great weight. recalling to mind the remarkable interview between newton and conduit at kensington,* i would ask why the elementary substances that compose one group of cosmical bodies, or one planetary system, may not, in a great measure, be identical? [footnote] * "sir isaac newton said he took all the planets to be composed of the same matter with the earth, viz., earth, water, and stone, but variously connected." -- turner, 'collections for the history of grantham, containing authentic memoirs of sir isaac newton', p. . why should we not adopt this view, since we may conjecture that these planetary bodies, like all the larger or smaller agglomerated masses revolving round the sun, have been thrown off from the once far more expanded solar atmosphere, and been formed from vaporous rintgs describing their orbits round the central body? we are not, it appears to me, more justified in applying the term telluric to the nickel and iron, the olivine and pyroxene (augite), found in meteoric stones, than in indicating the german plants which i found beyond the obi as european species of the flora of northern asia. if the elementary substances composing a group of cosmical bodies of different magnitudes be identical, why should they not likewise, in obeying the laws of mutual attraction, blend together under definite relations of mixture, composing the white glittring snow and ice in the polar zones of the planet mars, or constituting in the smaller cosmical masses mineral bodies inclosing crystals of olivine, augite, and labradorite? even in the domain of pure conjecture we should not suffer ourselves to be led away by unphilosophical and arbitrary views devoid of the support of inductive reasoning. remarkable obscurations of the sun's disk, during which the stars have been seen at mid-day (as, for instance, in the obscuration of , which continued for three days, and occurred about the time of the eventful battle of mÃ�Â�hlberg), can not be explained as arising from volcanic ashes or mists, and were regarded by kepler as owing either to a 'materia cometica', or to a black cloud formed by the sooty exhalations of the solar body. the shorter obscurations of and , which continued, the one only three, and the other six p hours, were supposed by chladni and schnurrer to be occasioned by the passage of meteoric masses before the sun's disk. since the period that streams of meteoric shooting stars were first considered with reference to the direction of their orbit as a closed ring, the epochs of these mysterious celestial phenomena have been observed to present a remarkable connection with the regular recurrence of swarms of shooting stars adolph erman has evinced great acuteness of mind in his accurate investigation of the facts hitherto observed on this subject, and his researches have enabled him to discover the connection of the sun's conjunction with the august asteroids on the th of february, and with the november asteroids on the th of may, the latter period corresponding with the days of st. mamert (may th), st. pancras (may th), and st. servatius (may th), which according to popular belief, were accounted "cold days."* [footnote] adolph erman, in poggend., 'annalen', , bd. xlviii., s. - . biot had previously thrown doubt regarding the probability of the november stream reappearing in the beginning of may ('comptes rendus', , t. ii., p. ). mÃ�Â�dler has examined the mean depression of temperature on the three ill-named days of may by berlin observations for eighty-six years ('verhandl. des vereins zur bedfÃ�Â�rd, des gartenbaues', , s. ), and found a retrogression of temperature amounting to . degrees fahr. from the th to the th of may, a period at which nearly the most rapid advance of heat takes place. it is much to be desired that this phenomenon of depressed temperature, which some have felt inclined to attribute to the melting of the ice in the northeast of europe, should be also investigated in very remote spots, as in america, or in the southern hemisphere. (comp. 'bull. de l'acad. imp. de st. pÃ�Â�tersbourg', , t. i., no. .) the greek natural philosophers, who were but little disposed to pursue observations, but evinced inexhaustible fergility of imagination in giving the most various interpretation of half-perceived facts, have, however, left some hypotheses regarding shooting stars and meteoric stones which strikingly accord with the views now almost universally admitted of the cosmical process of these phenomena. "falling stars," says plutarch, in his life of lysander,* are, according to the opinion of some physicists, not eruptions of the ethereal fire extinguished in the air immediately after its ignition, nor yet an inflammatory combustion of the air, which is dissolved in large quantities in the upper regions of space, but these meteors are rather a fall of celestial bodies, which, in consequence of a certain intermission in the rotatory force, and by the impulse of some irregular movements, have been hurled down not only to the inhabited portions of the earth, but also beyond it into the great ocean, where we can not find them." [footnote] *plut., 'vitÃ�¾ par, in lysandro', cap. . the statement of damachos (daÃ�Â�machos), that for seventy days continuously there was a fiery cloud seen in the sky, emitting sparks like falling stars, and which then, sinking nearer to the earth, let fall the stone of Ã�®gos potamos, "which, however, was only a small part of it," is extremely improbable, since the direction and velocity of the fire-cloud would in that case of necessity have to remain for so many days the same as those of the earth; and this, in the fire-ball of the th of july, , described by halley ('trans.', vol. xxix., p. ), lasted only a few minutes. it is not altogether certain whether daÃ�Â�machos, the writer, [greek words], was the same person as daÃ�Â�machos of platÃ�¾a, who was sent by selencus to india to the son of androcottos, and who ws charged by strabo with being "a speaker of lies" (p. , casaub.). from another passage of plutarch ('compar. solonis c. cop.', cap. ) we should almost believe that he was. at all events, we have here only the evidence of a very late author, who wrote a century and a half after the fall of aÃ�Â�rolites occurred in thrace, and whose authenticity is also doubted by plutarch. diogenes of apollonia* expresses himself still more explicitly. [footnote] *stob., ed. heeren, i., , p. ; plut., 'de plac. philos.', ii., . according to his views, "stars that are 'invisible', and, consequently, have no name, move in space together with those that are visible. these invisible stars frequently fall burning at Ã�®gos potamos." the apollonian, who held all other stellar bodies, when luminous, to be of a pumice-like nature, probably grounded his opinions regarding shooting stars and meteoric masses on the doctrine of anaxagoras the clazomenian, who regarded all the bodies in the universe "as fragments of rocks, which the fiery ether, in the force of its gyratory motion, had torn from the earth and converted into stars." in the ionian school, therefore, according to the testimony transmitted to us in the views of diogenes of apollonia, aÃ�Â�rolites and stars were ranged in one and the same class; both, when considered with reference to their primary origin, being equally telluric, this being understood only so far as the earth was then regarded as a central body,* p forming all things around it in the same manner was we, according to our present views, suppose the planets of our system to have originated in the expanded atmosphere of another central body, the sun. [footnote] *the remarkable passage in plut., 'de plac. philos.', ii., , runs thus: "anaxagoras teaches that the surrounding ether is a fiety substance, which, by the power of its rotation, tears rocks from the earth, inflames them, and converts them into stars." applying an ancient fable to illustrate a physical dogma, the clazomenian appears to have ascribed the fall of the nemÃ�¾an lion to the peloponnesus from the moon to such a rotatory or centrifugal force. (Ã�®lian., xii., ; plut., 'de facie in orge lunÃ�¾' c. ; schol. ex cod. paris., in 'apoll. argon.', lib. i., p. , ed. schaef., t. ii., p. ; meineke, 'annal. alex.', , p. .) here, instead of stones from the moon, we have an animal from the moon! according to an acute remark of bÃ�Â�ckh, the ancient mythology of the nemÃ�¾an lunar lion has an astronomical origin, and is symbolically connected in chronology with the cycle of intercalation of the lunar year, with the moon-worship at nemÃ�¾a, and the games by which it was accompanied. these views must not, therefore, be confounded with what is commonly termed the telluric or atmospheric origin of meteoric stones, nor yet with the singular opinion of aristotle, which supposed the enormous mass of Ã�®gos potamos to have been raised by a hurricane. that rrogant spirit of incredulity, which rejects facts without attempting to investigate them, is in some cases almost more injurious than an unquestioning credulity. both are alike detrimental to the force of investigation. notwithstanding that for more than two thousand years the annals of different nations had recorded falls of meteoric stones, many of which had been attested beyond all doubt by the evidence of irreproachable eye-witnesses -- notwithstanding the important part enacted by the bÃ�¾tylia in the meteor-worship of the ancients -- notwithstanding the fact of the companions of cortez having see an aÃ�Â�rolite at cholula which had fallen on the neighboring pyramid -- notwithstanding that califs and mongolian chiefs had caused swords to be forged from recently-fallen meteoric stones -- nay, notwithstanding that several persons had been struck dead by stones falling from heaven, as for instance, a monk at crema on the th of september, , another monk at milan in , and two swedish sailors on board ship in , yet this great cosmical phenomenon remained almost wholly unheeded, and its intimate connection drawn to the subject by chladni, who had already gained immortal renown by his discovery of the sound-figures. he who is penetrated with a sense of this mysterious connection, and whose mind is open to deep impressions of nature, will feel himself moved by the deepest and most solemn emotion at the sight of every star that shoots across the vault of heaven, no less than at the glorious spectacle of meteoric swarms in the november phenomenon or on st. lawrence's day. here motion is suddenly revealed in the midst of nocturnal rest. the still radiance of the vault of heaven is for a moment animated with life and movement. in the mild radiance left on the track of the shooting star, imagination pictures the lengthened path of the meteor through the vault of heaven, p while, every where around, the luminous asteroids proclaim the existence of one common material universe. if we compare the volume of the innermost of saturn's satellites, or that of ceres, with the immense volume of the sun, all relations of magnitude vanish from our minds. the extinction of suddenly resplendent stars in cassiopeia, cygnus, and serpentarius have already led to the assumption of other and non-luminous cosmical bodies. we now know that the meteoric asteroids, spherically agglomerated into small masses, revolve round the sun, intersect, like comets, the orbits of the luminous larger planets, and become ignited either in the vicinity of our atmosphere or in its upper strata. the only media by which we are brought in connection with other planetary bodies, and with all portions of the universe beyond our atmosphere, are light and heat (the latter of which can scarcely be separated from the former),* and those mysterious powers of attraction exercised by remote masses, according to the quantity of their constituents, upon our globe, the ocean, and the strata of our atmosphere. [footnote' *the following remarkable passage on the radiation of heat from the fixed stars, and on their low combustion and vitality -- one of kepler's many aspirations -- occurs in the 'paralipom. in vitell. astron. parsopticqa', , propos. xxxii., p. : "luciis proprium est calor, sydera omnia calefaciunt. de syderum luce claritatis ratio testatur, calorem universorum in minori esse proportione ad calorem unius solis, quam ut ab homine, cujus est certa caloris mensura, utrque simul percipi et judicari possit. de cincindularum lucula tenuissima negare non potes, quin cum calore sit. vivunt enim et moventur, hoc auten non sine calefactione perficitur. sic neque putrescentium lignorum lux sui calore destituitur; nam ipsa puetredo quidam lentus ignis est. inest et stirpibus suus calor." (compare kepler, 'epit. astron. copernicanÃ�¾', , t. i., lib. i., p. .) another and different kind of cosmical, or, rather, material mode of contact is, however, opened to us, if we admit falling stars and meteoric stones to be planetary asteroids. they not only act upon us merely from a distance by the excitement of luminous or calorific vibrations, or in obedience to the laws of mutual attraction, but they acquire an actual material existence for us, reaching our atmosphere from the remoter regions of universal space, and remaining on the earth itself. meteoric stones are the only means by which we can be brought in possible contact with that which is foreign to our own planet. accustomed to gain our knowledge of what is not telluric solely through measurement, calculations, and the deductions of reason, we experience a sentiment of astonishment at finding that we may examine, weigh, and analyze bodies that appertain p to the outer world. this awakens, by the power of the imagination, a meditative, spiritual train of thought, where the untutored mind perceives only scintillations of light in the firmament, and sees in the blackened stone that falls from the exploded cloud nothing beyond the rough product of a powerful natural force. although the asteroid-swarms, on which we have been led, from special predilection, to dwell somewhat at length, approximate to a certain degree, in their inconsiderable mass and the diversity of their orbits, to comets, they present this essential difference from the latter bodies, that our knowledge of their existence is almost entirely limited to the moment of their destruction, that is, to the period when, drawn within the sphere of the earth's attraction they become luminous and ignite. in order to complete our view of all that we have learned to consider as appertaining to our solar system, which now, since the discovery of the small planets, of the interior comets of short revolutions, and of the meteoric asteroids, is so rich and complicated in its form, it remains for us to speak of the ring of zodiacal light, to which we have already alluded. those who have lived for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidally upward, illumines a part of the uniform length of tropical nights. i have seen it shine with an intensity of light equal to the milky way in sagittarius, and that not only in the rare and dry atmosphere of the summits of the andes, at an elevation of from thirteen to fifteen thousand feet, but even on the boundless grassy plains, the illanos of venezuela, and on the sea-shore, beneath the ever-clear sky of cumana. this phenomenon was often rendered especially beautiful by the passage of light, fleecy clouds, which stood out in picturesque and bold relief from the luminous back-ground. a notice of this aÃ�Â�rial spectacle is contained in a passage in my journal, while i was on the voyage from lima to the western coasts of mexico: "for three or four nights (between Ã�¼degrees and Ã�¼degrees north latitude) the zodiacal light has appeared in greater splendor than i have ever observed it. the transparency of the atmosphere must be remarkably great in this part of the southern ocean, to judge by the radiance of the stars and nebulous spots. from the th to the th of march a regular interval of three quarters of an hour occurred between the disappearance of the sun's disk in the ocean and the first manifestation of the zodiacal p light, although the night was already perfectly dark. an hour after sunset it was seen in great briliancy between aldebaran and the pleiades; and on the th of march it attained an altitude of Ã�¼degrees 'minutes. narrow elongated clouds are scattered over the beautiful deep azure of the distant horizon, flitting past the zodiacal light as before a golden curtain. above these, other clouds are from time to time reflecting the most brightly variegated colors. it seems a second sunset. on this side of the vault of heaven the lightness of the night appears to increase almost as much as at the first quarter of the moon. toward o'clock the zodiacal light generally becomes very faint in this part of the southern ocean, and at midnight i have scarcely been able to trace a vestige of it. on the th of march, when most strongly luminous a faint reflection was visible in the east." in our gloomy so-called "temperate" northern zone, the zodiacal light is only distinctly visible in the beginning of spring, after the evening twilight, in the western part of the sky, and at the close of autumn, before the dawn of day, above the eastern horizon. it is difficult to understand how so striking a natural phenomenon should have failed to attract the attention of physicists and astronomers until the middle of the seventeenth century, or how it could have escaped the observation of the atabian natural philosophers in ancient bactria, on the euphrates, and in the south of spain. almost equal surprise is excited by the tardiness of observation of the nebulous spots in andromeda and orion, first described by simon marius and huygens. the earliest explicit descriptions of the zodiacal light occurs in childrey's 'britannia baconica',* in the year . p [footnote] *"there is another thing which i recommend to the observation of mathematical men, which is that in february, and for a little before and a little after that month (as i have observed several years together), about six in the evening, when the twilight hath almost deserted the horizon, you shall see a plainly discernible way of the twilight striking up toward the pleiades, and seeming almost to touch them. it is so observed any clear night, but it is best illac nocte. there is no such way to be observed at any other time of the year (that i can perceive), nor any other way at that time to be perceived darting up elsewhere; and i believe it hath been, and will be constantly visible at that time of the year; but what the cause of it in nature should be, i can not yet imagine, but leave it to future inquiry." (childrey, 'britannia baconica', , p. .) this is the first view and a simple description of the phenomenon. (cassini, 'dÃ�Â�couverte de la lumi dfd Ã�Â�leste qui paroÃ�Â�t dans le zodiaque', in the 'mÃ�Â�m. de l'acad.', t. viii., , p . mairan, 'traitÃ�Â�phys de l'aurore borÃ�Â�ale', , . .) in this remarkable work by childrey there are to be found (p. ) very clear accounts of the epochs of maxima and minima diurnal and annual temperatures, and of the retardation of the extremes of the effects in meteorological processes. it is, however, to be regretted that our baconian-philosophy-loving author, who was lord henry somerset's chaplain, fell into the same error as bernardin de st. pierre, and regarded the earth as elongated at the poles (see p. ). at the first he believes that the earth was spherical, but supposes that the uninterrupted and increasing addition of layers of ice at both poles has changed its figure; and that as the ice is formed from water, the quantity of that liquid is every where diminishing. the first observation of the phenomenon may have been made two or three years prior to this period; but, notwithstanding, the merit of having (in the spring of ) been the first to investigate the phenomenon in all its relations in space is incontestably due to dominicus cassini. the light which he saw at bologna in , and which was observed at the same time in persia by the celebrated traveler chardin (the court astrologers of ispahan called this light, which had never before been observed, 'nyzek', a small lance), was not the zodiacal light, as has often been asserted,* but the p enormous tail of a comet, whose head was concealed in the vapory mist of the horizon, and which, from its length and appearance, presented much similarity to the great comet of . [footnote] *dominicus cassini ('mÃ�Â�m. de l'acad.', t. viii., , p. ), and mairan ('aurore bor.', p. ), have even maintained that the phenomenon observed in persia in was the zodiacal light. delambre ('hist. de l'astron. moderne', t. ii., p. ), in very decided trms ascribes the discovery of this light to the celebrated traveler chardin; but in the 'couronnement de soliman', and in several passages of the narrative of his travels (Ã�Â�d. de langlÃ�Â�s. t. iv., p. ; t. x., p. ), he only applies the term niazouk (nyzek), or "petite lance," to "the great and famous comet which appeared over nearly the whole world in , and whose head was so hidden in the wewst that it could not be perceived in the horizon of ispahan" ('atlas du voyage de chardin', tab. iv.; from the observations at schiraz). the head or nucleus of the comet was, however, visible in the brazils and in india (pingrÃ�Â�, 'comÃ�Â�togr.', t. ii., p. ). regarding the conjectured identity of the last great comet of march, , with this, which cassini mistook for the zodiacal light, see schum., 'astr. nachr.', , no. and . in persian, the term "nizehi Ã�Â�teschÃ�Â�n"(fiery spears or lances) is also applied to the rays of the rising or setting sun, in the same way as "nayÃ�Â�zik," according to freytag's arabic lexicon, signifies "stellÃ�¾ cadentes." the comparison of comets to lances and swords was, however, in the middle ages, very common in all languages. the great comet of , which was visible from april to june, was always termed by the italian writers of that time 'il signor astone' (see my 'examen critique de l'hist. de la gÃ�Â�ographie', t. v., p. ). all the hypotheses that have been advanced to show that descartes (cassini, p. ; mairan, p. ), and even kepler (delambre, t. i., p. ), were acquainted with the zodiacal light, appear to me altogether untenable. descartes ('principes', iii., art. , ) is very obscure in his remarks on comets, observing that their tails are formed "by oblique rays, which, falling on different parts of the planetary orbs, strike the eye laterally by extraordinary refraction," and that they might be seen morning and evening, "like a long beam," when the sun is between the comet and the earth. this passage no more refers to the zodiacal light than those in which kepler ('epit. astron. copernicanÃ�¾', t. i., p. , and t. ii., p. ) speaks of the existence of a solar atmosphere (limbus circa solem, coma lucida), which, in eclipses of the sun, prevents it "from being quite night:" and even more uncertain, or indeed erroneous, is the assumption that the "trabes quas [greek word] vocant" (plin., ii., and ) had reference to the tongue-shaped rising zodiacal light, as cassini (p. , art. xxxi.) and mairan (p. ) have maintained. every where among the ancients the trabes are associated with the bolides (ardores et faces) and other fiery meteors, and even with long-barbed comets. (regarding [greek words] . see schÃ�Â�fer, 'schol. par. ad apoll. rhod.', , t. ii., p. ; pseudo-aristot., 'de mundo, , ; 'comment. alex. joh. philop. et olymp. in aristot. meteor.', lib. i., cap. vii., , p. , ideler; seneca, 'nat. quÃ�¾st.', i., .) we may conjecture, with much probability, that the remarkable light on the elevated plains of mexico, seen for forty nights consecutively i n , and observed in the eastern horizon rising pyramidally from the earth, was the zodiacal light. i found a notice of this phenomenon in an ancient aztec ms., the 'codextelleriano-remensis',* preserved in the royal library at paris. [footnote] *humboldt, 'monumens des peuples indigÃ�Â�nes de l'amÃ�Â�rique', t. ii., p. . the rare manuscript which belonged to the archbishop of rheims, le tellier, contains various kinds of extracts from an aztec ritual, an astrological calendar, and historical annals, extending from to , and embracing a notice of different natural phenomena, epochs of earthquakes and comets (as, for instance, those of and ), and of (which are important in relation to mexican chronology) solar eclipses. in camargo's manuscript 'historia de tlascala', the light rising in the east almost to the zenith is, singularly enough, described as "sparkling, and as if sown with stars." the description of this phenomenon, which lasted forty days, can not in any way apply to volcanic eruptions of popcatepetl, which lies very near, in the southeastery direction. (prescott, 'history of the conquest of mesico', vol. i., p. .) later commentators have confounded this phenomenon, which montezuma regarded as a warning of his misfortunes, with the "estrella que humeava" (literally, 'which spring forth'; mexican 'choloa, to leap or spring forth'). with respect to the connection of this vapor with the star citlal choloha (venus) and with "the mountain of the star" (citialtepetl, the volcano of orizaba), see my 'monumens', t. ii., p. . this phenomenon, whose primordial antiquity can scarcely be doubted, and which was first noticed in europe by childrey and dominicus cassini, is not the luminous solar atmosphere itself, since this can not, in accordance with mechanical laws, be more compressed than in the relation of to , and consequently can not be diffused beyond / ths of mercury's heliocentric distance. these same laws teach us that the altitude of the extreme boundaries of the atmosphere of a cosmical p body above its equator, that is to say, the point at which gravity and centrifugal force are in equilibrium, must be the same as the altitude at which a satellite would rotate round the central body simultaneously with the diurnal revolution of the latter.* [footnote] *laplace, 'expos. du syst. du monde', p. ; 'mÃ�Â�canique cÃ�Â�leste', t. ii., p. and ; schubert, 'astr.', bd. iii., Ã�¤ . this limitation of the solar atmosphere in its present concentrated condition is especially remarkable when we compare the central body of our system with the nucleus of other nebulous stars. herschel has discovered several, in which the radius of the nebulous matter surrounding the star appeared at an angle of ". on the assumption that the parallax is not fully equal to ", we find that the outermost nebulous layer of such a star must be times further from the central body than our earth is from the sun. if, therefore, the nebulous star were to occupy the place of our sun, its atmosphere would not only include the orbit of uranus, but even extend eight times beyond it.Ã�Â¥ [footnote] *arago, in the 'annuaire', , p. . compare sir john herschel's considerations on the volume and faintness of light of planetary nebulÃ�¾, in mary somerville's 'connection of the physical sciences', , p. . the opinion that the sun is a nebulous star, whose atmosphere presents the phenomenon of zodiacal light, did not originate with dominicus cassini, but was first promulgated by mairan in ('traitÃ�Â� de l'aurore bor.', p. and ; arago, in the 'annuaire', , p. ). it is a renewal of kepler's views. considering the narrow limitation of the sun's atmosphere, which we have just described, we may with much probability regard the existence of a very compressed annulus of nebulous matter,* revolving freely in space between the orbits of venus and mars, as the material cause of the zodiacal light. [footnote] *cominicus cassini was the first to assume, as did subsequently laplace, schubert, and poisson, the hypothesis of a separate ring to explain the form of the zodiacal light. he says distinctly, "if the orbits of mercury and venus were visible (throughout their whole extent), we should invariably observe them with the same figure and in the same position with regard to the sun, and at the same time of the year with the zodiacal light." ('mÃ�Â�m. de l'acad.', t. viii., , p. , and biot, in the 'comptes rendus', , t. iii., p. .) cassini believed that the nebulous ring of zodiacal light consisted of innumerable small planetary bodies revolving round the sun. he even went so far as to believe that the fall of fire-balls might be connected with the passage of the earth through the zodiacal nebulous ring. olmsted, and especially biot (op. cit., p. ), have attempted to establish its connection with the november phenomenon -- a connection which olbers doubts. (schum., 'jahrb.', , s. .) regarding the question whether the place of the zodiacal light perfectly coincides with that of the sun's equator, see houzeau, in schum., 'astr. nachr.', , no. , s. . as p yet we certainly know nothing definite regarding its actual material dimensions; its augmentation* by emanations from the tails of myriads of comets that come within the sun's vicinity; the singular changes affecting its expansion, since it sometimes does not apper to extend beyond our earth's orbit; or, lastly, regarding its conjectural intimate connection with the more condensed cosmical vapor in the vicinity of the sun. [footnote] *sir john herschel, 'astron.', Ã�¤ . the nebulous particles composing this ring, and revolving round the sun in accordance with planetary laws, may either be self-luminous or receive light from that luminary. even in the case of a terrestrial mist (and this fact is very remarkable), which occurred at the time of the new moon at midnight in , the phosphorescence was so intense that objects could be distinctly recognized at a distance of more than feet. i have occasionally been astonished in the tropical climates of south america, to observe the variable intensity of the zodiacal light. as i passed the nights, during many months, in the open air, on the shores of rivers and on ilanos, i enjoyed ample opportunities of carefully examining this phenomenon. when the zodiacal light had been most intense, i have observed that it would be perceptibly weakened for a few minutes, until it again suddenly shone forth in full brilliancy. in some few instances i have thought that i could perceive -- not exactly a reddish coloration, nor the lower portion darkened in an arc-like form, nor even a scintillation, as mairan affirms he has observed -- but a kind of flickering and wavering of the light.* [footnote] *arago, in the 'annuaire', , p. . several physical facts appear to indicate that, in a mechanical separation of matter into its smallest particles, if the mass be very small in relation to the surface, the electrical tension may increase sufficiently for the production of light and heat. experiments with a large concave mirror have not hitherto given any positive evidence of the presence of radiant heat in the zodiacal light. (lettre de m. matthiessen Ã�Â� m. arago, in the 'comptes rendus', t. xvi., , avril, p. .) must we suppose that changes are actually in progress in the nebulous ring? or is it not more probable that, although i could not, by my meteorological instruments, detect any change of heat or moisture near the ground, and small stars of the fifth and sixth magnitudes appeared to shine with equally undiminished intensity of light, processes of condensation may be going on in the uppermost strata of the air, by means of which the transparency, or rather, the reflection of light, may be modified in some peculiar and unknown manner? p an assumption of the existence of such meteorological causes on the confines of our atmosphere is strengthened by the "sudden flash and pulsation of light," which, according to the acute observations of olbers, vibrated for several seconds through the tail of a comet, which appeared during the continuance of the pulsations of light to be lengthened by several degrees, and then again contracted.* [footnote] *"what you tell me of the changes of light in the zodiacal light, and of the causes to which you ascribe such changes within the tropics, is of the greatr interest to me, since i have been for a long time past particularly attentive, every spring, to this phenomenon in our northern latitudes. i, too, have always believed that the zodiacal light rotated; but i assumed (contrary to poisson's opinion, which you have communicated to me) that it completely extended to the sun, with considerably augmenting brightness. the light circle which, in total solar eclipses, is seen surrounding the darkened sun, i have regarded as the brightest portion of the zodiacal light. i have convinced my self that this light is very different in different years, often for several successive years being very bright and diffused, while in othr years it is scarcely perceptible. i tyhink that i find the first trace of an allusion to the zodiacal light in a letter from rothmann to tycho, in which he mentions that in the spring he has observed the twilight did not close until the sun was Ã�¼degrees below the horizon. rothmann must certainly have confounded the disappearance of the setting zodiacal light in the vapors of the western horizon with the actual cessation of twilight. i have failed to observe the pulsations of the light, probably on account of the faintness with which it appears in these countries. you are, however, certainly right in ascribing those rapid variations in the light of the heavenly bodies, which you have perceived in tropical climates, to our own atmosphere, and especially to its higher regions. this is especially in the clearest weather, that these tails exhibit pulsations, commencing from the head, as being the lowest part, and vibrating in one or two seconds through the entire tail, which thus appears rapidly to become some degrees longer, but again as rapidly contracts. that these undulations, which were formerly noticed with attention by robert hooke, and in more recent times by schrÃ�Â�ter and chladni, 'do not actually occur in the tails of the comets', but are produced by our atmosphere, is obvious when we recollect that the individual parts of those tails (which are many millions of miles in length) lie 'at very different distances' from us, and that the light from their extreme points can only reach us at intervals of time which differ several minutes from one another. whether what you saw on the orinoco, not at intervals of seconds, but of minutes, were actual coruscations of the zodiacal light, or whether they belonged exclusively to the upper strata of our atmosphere, i will not attempt to decide; neither can i explain the remarkable 'lightness of whole nights', nor the anomalous augmentation and prolongation of the twilight in the year , particularly if, as has been remarked, the lightest part of these singular twilights did not coincide with the sun's place below the horizon." (from a lettr written by dr. olbers to myself, and dated bremen, marth th, .) as, however, the separate particles of a comet's tail, measuring millions of miles, p are very unequally distant from earth, it is not possible, according to the laws of the velocity and transmission of light, that we should be able, in so short a period of time, to perceive any actual changes in a cosmical body of such vast extent. there considerations in no way exclude the realith of the changes that have been observed in the emanations from the more condensed envelopes around the nucleus of a comet, nor that of the sudden irradiation of the zodiacal light, from internal molecular motion, nor of the increased or diminished reflection of light in the cosmical vapor of the luminous ring, but should simply be the means of drawing our attention to the differences existing between that which appertains to the air of heaven (the realms of universal space) and that which belongs to the strata of our terrestrial atmosphere. it is not possible, as well-attested facts prove, perfectly to explain the operations at work in the much-contested upper boundaries of our atmosphere. the extraordinary lightness of whole nights in the year , during which small print might be read at midnight in the latitudes of italy and the north of germany is a fact directly at variance with all that we know, according to the most recent and acute researches on the crepuscular theory, and of the height of the atmosphere.* [footnote] *biot, 'traitÃ�Â� d'astron. physique', Ã�Â�me Ã�Â�d., , t. i., p. , and . the phenomena of light depend upon conditions still less understood, and their variability at twilight, as well as in the zodiacal light, excite our astonishment. we have hitherto considered that which belongs to our solare system -- that world of material forms governed by the sun -- which includes the primary and secondary planets, comets of short and long periods of revolution, meteoric asteroids, which move thronged together in streams, either sporadically or in closed rings, and finally a luminous nebulous ring, that revolves round the sun in the vicinity of the earth, and for which, owing to its position, we may retain the name of zodiacal light. every where the law of periodicity governs the motions of these bodies, however different may be the amount of tangential velocity, or the quantity of their agglomerated material parts; the meteoric asteroids which enter our atmosphere from the external regions of universal space are alone arrested in the course of their planetary revolution, and retained within the sphere of a larger planet. in the solar system, whose boundaries determine the attractive force of the central body, comets are made to revolve in their elliptical p orbits at a distance times greater than that of uranus; may, in those comets whose nucleus appears to us, from its inconsiderable mass, like a mere passing cosmical cloud, the sun exercises its attractive force on the outermost parts of the emanations radiating from the tail over a space of many millions of miles. central forces, therefore, at once constitute and maintain the system. our sun may be considered as at rest when compared to all the large and small, dense and almost vaporous cosmical bodies tht appertain to and revolve around it; but it actually rotates around the common center of gravity of the whole system, which occasionally falls within itself, that is to say, remains within the material circumference of the sun, whatever changes may be assumed by the position of the planets. a very different phenomenon is that presented by the translatory motion of the sun, that is, the progressive motion of the center of gravity of the whole solar system in universal space. its velocity is such* that, according to bessel, the relative motion of the sun, and that of cygni, is not less in one day than , , geographical miles. [footnote] *bessel, in schum., 'jahrb. fÃ�Â�r' , s. ; probably four millions of miles daily, in a relative velocity of at the least , , miles, or more than couble the velocity of revolution of the earth in her orbit round the sun. this change of the entire solar system would remain unknown to us, if the admirable exactness of our astronomical instruments of measurement, and the advancement recently made in the art of observing, did not cause our advance toward remote stars to be perceptible, like an approximation to the objects of a distant shore in apparent motion. the proper motion of the star cygni, for instance, is so considerable, that it has amounted to a whole degree in the course of years. the amount or quantity of these alterations in the fixed stars (that is to say, the changes in the relative position of self-luminous stars toward each other), can be determined with a greater degree of certainty than we are able to attach to the genetic explanation of the phenomenon. after taking into consideration what is due to the precession of the equinoxes, and the nutation of the earth's axis produced by the action of the sun and moon on the spheroidal figure of our globe, and what may be ascribed to the transmission of light, that is to say, to its aberration, and to the parallax formed by the diametrically opposite position of the earth in its course round the sun, we still find that there is a residual portion p of the annual motion of the fixed stars due to the translation of the whole solar system in universal space, and to the true proper motion of the stars. the difficult problem of numerically separating these two elements, the true and the apparent motion, has been effected by the careful study of the direction of the motion of certain individual stars, and by the consideration of the fact that, if all the stars were in a state of absolute rest, they would appear perspectively to recede from the point in space toward which the sun was directing its course. but the ultimate result of this investigation, confirmed by the calculus of probabilities, is, that our solar system and the stars both change their places in space. according to the admirable researches of d'argelander at abo, who has extended and more perfectly developed the work begun by william herschel and prevost, the sun moves in the direction of the constellation hercules, and probably, from the combination of the observations made of stars, toward a point lying (at the equinox of . ) at Ã�¼degrees .' r.a., and Ã�¼degrees .' n.d. it is extremely difficult, in investigations of this nature, to separate the absolute from the relative motion, and to determine what is aloone owing to the solar system.* [footnote] *regarding the motion of the solar system, according to bradley, tobias mayer, lambert, lalande, and william herschel, see arago in the 'annuaire', , p. - ' argelander, in schum., 'astron. nachr ., no. , , , and in the treatise 'von der eigenen bewegung des sonnensystems' (on the proper motion of the solar system), , s. , respecting perseus as the central body of the whole stellar stratum, likewise otho struve, in the 'bull. de l'acad. de st. pÃ�Â�tersb.', , t. x., no. , p. - . the last-named astronomer has found, by a mo re recent combination, Ã�¼degrees ' r.a.+ Ã�¼degrees ' decl. for the direction of the sun's motion; and, taking the mean of his own results with that of argelander, we have, by a combination of stars, the formula Ã�¼degrees ' r.a.+ Ã�¼degrees ' decl. if we consider the proper, and not the perspective motions of the stars, we shall find many that appear to be distributed in groups, having an opposite direction; and facts hitherto observed do not, at any rate, render it a necessary assumption that all parts of our starry stratum, or the whole of the stellar islands filling space, should move round one large unknown luminous or non-luminous central body. the tendency of the human mind to investigate ultimate and highest causes certainly inclines the intellectual activity, no less than the imagination of mankind, to adopt such an hypothesis. even the stagirite proclaimed that "every thing which is moved must be referable to a motor, and that there would be no end to p the concatenation of causes if there were not one primordial immovable morot."* [footnote] *aristot., 'de cÃ�¾lo', iii., , p. , bekker: 'phys.', viii., t, p. . this material taken from pages - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- the manifold translatory changes of the stars, not those produced by the parallaxes at which they are seen from the changing position of the spectator, but the true changes constantly going on in the regions of space, afford us incontrovertible evidence of the 'dominion of the laws of attraction' in the remotest regions of space, beyond the limits of our solar system. the existence of these laws is revealed to us by many phenomena, as, for instance, by the motion of double stars, and by the amount of retarded or accelerated motion in different parts of their elliptic orbits. human inquiry need no longer pursue this subject in the domain of vague conjecture, or amid the undefined analogies of the ideal world; for even here the progress made in the method of astronomical observations and calculations has enabled astronomy to take up its position on a firm basis. it is not only the discovery of the astounding numbers of double and multiple stars revolving round a center of gravity lying 'without' their system ( such systems having been discovered up to ), but rather the extension of our knowledge regarding the fundamental forces of the whole material world, and the proofs we have obtained of the universal empire of the laws of attraction, that must be ranked among the most brilliant discoveries of the age. the periods of revolution of colored stars present the greatest differences; thus, in some instances, the period extends to years, as in Ã�¹pi of corona, and in others to several thousands,, as in of cetus, of gemini, and of pisces. since herschel's measurements in , the satellite of the nearest star in the triple system of [greek letter] of cancer has completed more than one entire revolution. by a skillful combination of the altered distances and angles of position,* the elements of these orbits may be found, conclusions drawn regarding the absolute distance of the double stars from the earth, and comparisons made between their mass and that of the sun. [footnote] *savary, in the 'connaissance des tems', , p. and . encke, 'berl. jahrb.', , s. , etc. arago, in the 'annuaire' , p. , . john herschel, in the 'memoirs of the astronom. soc.', vol. v., p. . whether, however, here and in our solar system, quantity of matter is the only standard of the amount of attractive force, or whether 'specific' forces of attraction proportionate to the mass may not at the same time come into operation, as bessel was the first to conjecture, are questions p whose practical solution must be left to future ages.* [footnote] * bessel, 'untersuchung. des theils der planetarischen storungen, welche aus der bewegung der sonne entstchen' (an investigation of the portion of the planetary disturbances depending on the motion of the sun) in 'abh. der berl. akad. der wissensch.', (mathem. classe), s. - . the question has been raised by john tobias mayer, in 'comment. soc. reg. gotting.', - , vol. xvi., p. - . when we compare our sun with the other fixed stars, that is, with other self-luminous suns in the lenticular starry stratum of which our system forms a part, we find, at least in the case of some, that channels are opened to us, which may lead, at all events, to an 'approximate' and limited knowledge of their relative distances, volumes, and masses, and of the velocities of their translatory motion. if we assume the distance of uranus from the sun to be nineteen times that of the earth, that is to say, nineteen times as great as that of the sun from the earth, the central body of our planetary system will be , times the distance of uranus from the star 'a' in the constellation centaur, almost , from cygni, and , from vega in the constellation lyra. the comparison of the volume of the sun with that of the fixed stars of the first magnitude is dependent upon the apparent diameter of the latter bodies -- an extremely undertain optical element. if even we assume, with herschel, that the apparent diameter of arcturus is only a tenth part of a second, it still follows that the true diameter of this star is eleven times greater than that of the sun.* [footnote] *'philos. trans.' for , p. . arago, in the 'annuaire', , p. . in order to obtain a clearer idea of the distances ascribed in a rather earlier part of the text to the fixed stars, let us assume that the earth is a distance of one foot from the sun; uranus is then feet, and vega lyrae is geographical miles from it. the distance of the star cygni, made known by bessel, has led approximately to a knowledge of the quantity of matter contained in this body as a double star. notwithstanding that, since bradley's observations, the portion of the apparent orbit traversed by this star is not sufficiently great to admit of our arriving with perfect exactness at the true orbit nd the major axis of this star, it has been conjectured with much probability by the great konigsberg astronomer,* "that the mass of this double star can not be very considerably larger or smaller than half of the mass of the sun." [footnote] *bessel, in schum., 'jahrb.', , s. . this result is from actual measurement. the analogies deduced from the relatively larger mass of those planets in our solar system that are attended by satellites, and from the fact that struve has discovered six times more double stars among p the brighter than among the telescopic fixed stars, have led other astronomers to conjecture that the average mass of the larger number of the binary stars exceeds the mass of the sun.* [footnote] *mÃ�Â�dler, 'astron.', s. ; also in schum, 'jahrb.', , s. . we are, however, far from having arrived at general results regarding this subject. our sun, according to argenlander, belongs, with reference to proper motion in space, to the class of rapidly-moving fixed stars. the aspect of the starry heavens, the relative position of stars and nebullae, the distribution of their luminous masses, the picturesque beauty, if i may so express myself, of the whole firmament, depend in the course of ages conjointly upon the proper motion of the stars and nebulae, the translation of our solar system in space, the appearance of new stars, and the disappearance or sudden diminution in the intensity of the light of others, and lastly and specially, on the changes which the earth's axis experiences from the attraction of the sun and moon. the beautiful stars in the constellation of the centaur and the southern cross will at some future time be visible in our northern latitudes, while other stars, as sirius and the stars in the belt of orion, will in their turn disappear below the horizon. the places of the north pole will successively be indicated by the stars Ã�§ beta and a alpha cephei, and Ã�¶ delta cygni, until after a period of , years, vega in lyra will shine forth as the brightest of all possible pole stars. these data give us some idea of the extent of the motions which, divided into infinitely small portions of time, proceed without intermission in the great chronometer of the universe. if for a moment we could yield to the power of fancy, and imagine the acuteness of our visual organs to be made equal with the extremest bounds of telescopic vision, and bring together that which is now divided by long periods of time, the apparent rest that reigns in space would suddenly disappear. we should see the countless host of fixed stars moving in thronged groups in different directions; nebulae wandering through space, and becoming condensed and dissolved like cosmical clouds; the vail of the milky way separated and broken up in many parts, and 'motion' ruling supreme in every portion of the vault of heave, even as on the earth's surface, where we see it unfolded in the germ, the leaf, and the blossom, the organisms of the vegetable world. the celebrated spanish botanist cavanilles was the first who entertained the idea of "seeing grass grow," and he directed the horizontal micrometer threads of a powerfully magnifying glass at one time to p the apex of the shoot of a bambusa, and at another on the rapidly-growing stem of an american aloe ('agave americana', precisely as the astronomer places his cross of net-work against a culminating star. in the collective life of physical nature, in the organic as in the sidereal world, all things that have been, that are, and will be, are alike dependent on motion. the breaking up of the milky way, of which i have just spoken, requires special notice. william herschel, our safe and admirable guide to this portion of the regions of space, has discovered by his star-guagings that the telescopic breadth of the milky way extends from six to seven degrees beyond what is indicated by our astronomical maps and by the extent of the sidereal radiance visible to the naked eye.* [footnote] *sir william herschel, in the 'philos. transact.' for , part ii p. . the two brilliant nodes in which the branches of the zone unite, in the region of cepheus and cassiopeia, and in the vicinity of scorpio and sagittarius, appear to exercise a powerful attraction on the contiguous stars; in the most brilliant part, however between beta and [greek symbol] cygni, one half of the , stars that have been discovered in a breadth of degrees are directed toward one side, and the remainder to the other. it is in this part that herschel supposes the layer to be broken up.* [footnote] *arago, in the 'annuaire', , p. the number of telescopic stars in the milky way uninterrupted by any nebulae is estimated at millions. in order, i will not say, to realize the greatness of this number, but, at any rate, to compare it with something analogous, i will call attention to the fact that there are not in the whole heavens more than about stars between the first and the sixth magnitudes, visible to the naked eye. the barren astonishment excited by numbers and dimensions in space, when not considered with reference to applications engaging the mental and perceptive powers of man, is awakened in both extremes of the universe, in the celestial bodies as in the minutest animalcules.* [footnote] *sir john herschel, in a letter from feldhuysen, dated jan. th, . nicholl, 'architecture of the heavens', , p. . (see, also, some separate notices by sir william herschel on the starless space which separates us by a great distance from the milky way, in the 'philos. transact.' for , part ii., p. .) a cubic inch of the polishing slate of bilin contains, according to ehrenberg, , millions of the silicious shells of galionellae. the stellar milky way, in the region of which, according to argelander's admirable observations, the brightest stars of the firmament appear to be congregated, is almost at right angles p with another milky way, composed of nebulae. the former constitutes, according to sir john herschel's views, an annulus, that is to say, an independent zone, somewhat remote from our lenticular-shaped starry stratum, and similar to saturn's ring. our planetary system lies in an eccentric direction, nearer to the region of the cross than to the diametrically opposite point, cassiopeia.* [footnote] *sir john herschel, 'astronom.', ; likewise in his 'observations on nebulae and clusters of stars' ('phil. transact.', , part ii., p. , fig. ): "we have here a brother system, bearing a real physical resemblance and strong analogy of structure to our own." an imperfectly seen nebulous spot, discovered by messier in , appeared to present a remarkable similarity to the form of our starry stratum and the divided ring of our milky way.* [footnote] *sir william herschel, in the 'phil. trans.' for , part i., p. . sir john herschel, 'astron.', . ("the 'nebulous' region of the heavens forms 'a nebulous milky way', composed of distinct nebulae, as the other of stars." the same observation was made in a letter he addressed to me in march, .) the milky way composed of nebulae does not belong to our starry stratum, but surrounds it at a great distance without being physically connected with it, passing almost in the form of a large cross through the dense nebulae of virgo, especially in the northern wing, through comae berenicis, ursa major, andromeda's girdle, and pisces boreales. it probably intersects the stellar milky way in cassiopeia, and connects its dreary poles (rendered starless from the attractive forces by which stellar bodies are made to agglomerate into groups) in the least dense portion of the starry stratum. we see from these considerations that our starry cluster, which bears traces in its projecting branches of having been subject in the course of time to various metamorphoses, and evinces a tendency to dissolve and separate, owing to secondary centers of attraction -- is surrounded by two rings, one of which, the nebulous zone, is very remote, while the other is nearer, and composed of stars alone. the latter, which we generally term the milky way, is composed of nebulous stars, averaging from the tenth to the eleventh degree of magnitude,* but appearing, when considered individually, of very different magnitudes, while isolated starry clusters (starry swarms) almost always exhibit throughout a character of great uniformity in magnitude and brilliancy. [footnote] *sir john herschel, 'astron.', . in whatever part the vault of heaven has been pierced by powerful and far-penetrating telescopic instruments, stars or luminous nebulae are every where discoverable, the former, in p some cases, not exceeding the twentieth or twenty-fourth degree of telescopic magnitude. a portion of the nebulous vapor would probably be found resolvable into stars by more powerful optical instruments. as the retina retains a less vivid impression of separate than of infinitely near luminous points, less strongly marked photometric relations are excited in the latter case, as arago has recently shown.* [footnote] *arago, in the 'annuaire', , p. - , - , and - . the definite or amorphous cosmical vapor so universally diffused, and which generates heat through condensation, probably modifies the transparency of the universal atmosphere, and diminishes that uniform intensity of light which, according to halley and olbers, should arise, if every point throughout the depths of space were filled by an infinite series of stars.* [footnote] *olbers, on the transparency of celestial space, in bode's 'jahrb.', , s. - . the assumption of such a distribution in space is, however, at variance with observation, which shows us large starless regions of space, 'openings' in the heavens, as william herschel terms them -- one, four degrees in width, in scorpio, and another in serpentarius. in the vicinity of both, near their margin, we find unresolvable nebulae, of which that on the western edge of the opening scorpio is one of the most richly thronged of the clusters of small stars by which the firmament is adorned. herschel ascribes these openings or starless regions to the attractive and agglomerative forcesof the marginal groups.* [footnote] *"an opening in the heavens," william herschel, in the 'phil. trans.' for , vol. lxxv., part i., p. . le francais lalande, in the 'connaiss. des tems pour l'an.' viii., p. . arago, in the 'annuaire', , p. . "they are parts of our starry stratum," says he, with his usual graceful animation of style, "that have experienced great devastation from time." if we picture to ourselves the telescopic stars lying behind one another as a starry canopy spread over the vault of heaven, these starless regions in scorpio and serpentarius may, i think, be regarded as tubes through which we may look into the remotest depths of space. other stars may certainly lie in those parts where the strata forming the canopy are interrupted, but these are unattainable by our instruments. the aspect of fiery meteors had led the ancients likewise to the idea of clefts or openings ('chasmata') in the vault of heaven. these openings were, however, only regarded as transient, while the reason of their being luminous and fiery, instead of obscure, was supposed to be owing to the p translucent illuminated ether which lay beyond them.* [footnote] *aristot., 'meteor.', ii.,, , . seneca, 'natur. quaest.', i., , . "coelum discessisse," in cic., 'de divin.', i., . derham, and even huygens, did not appear disinclined to explain in a similar manner the mild radiance of the nebulae.* [footnote] *arago, in the 'annuaire', , p. . when we compare the stars of the first magnitude, which, on an average, are certainly the nearest to us, with the non-nebulous telescopic stars, and further, when we compare the nebulous stars with unresolvable nebulae, for instance, with the nebula in andromeda, or even with the so-called planetary nebulous vapor, a fact is made manifest to us by the consideration of the varying distances and the boundlessness of space, which shows the world of phenomena, and that which constitutes its causal reality, to be dependent upon the 'propagation of light'. the velocity of this propagation is according to struve's most recent investigations, , geographical miles in a second, consequently almost a million of times greater than the velocity of sound. according to the measurements of maclear, bessel, and struve, of the parallaxes and distances of three fixed stars of very unequal magnitudes ('a' centauri, cygni, and 'a' lyrae), a ray of light requires respectively , / , and years to reach us from these three bodies. in the short but memorable period between and , from the time of cornelius gemma and tycho brahe to that of kepler, three new stars suddenly appeared in cassiopeia and cygnus, and in the foot of serpentarius. a similar phenomenon exhibited itself at intervals in , in the constellation vulpis. in recent times, even since , sir john herschel has observed, at the cape of good hope, the brilliant star [greek symbol] in argo increase in splendor from the second to the first magnitude.* [footnote] *in december, , sir john herschel saw the star [greek symbol] argo, which till that time appeared as of the second magnitude, and liable to no change, rapidly increase till it became of the first magnitude. in january, , the intensity of its light was equal to that of 'a' centauri. according to our latest information, maclear in march, , found it as bright as canopus; and even 'a' crucis looked faint by [greek symbol] argo. these events in the universe belong, however, with reference to their historical reality, to other periods of time than those in which the phenomena of light are first revealed to the inhabitants of the earth: they reach us like the voices of the past. it has been truly said, that with our large and powerful telescopic instruments we penetrate alike through the boundaries of time and space: we measure the former through the latter, for in the course of an p hour a ray of light traverses over a space of millions of miles. while according to the theogony of hesiod, the dimensions of the universe were supposed to be expressed by the time occupied by bodies in falling to the ground ("the brazen anvil was not more than nine days and nine nights in falling from heaven to earth"), the elder herschel was of opinion* that light required almost two millions of years to pass to the earth from the remotest luminous vapor reached by his forty-foot reflector. [footnote] *"hence it follows that the rays of light of the remotest nebulae must have been almost two millions of years on their way, and that consequently, so many years ago, this object must already have had an existence in the sidereal heaven, in order to send out those rays by which we now perceive it." william herschel, in the 'phil. trans.' for , p. . john herschel, 'astron.', . arago, in the 'annuaire', , p. , , and - . much, therefore, has vanished long before it is rendered visible to us -- much that we see was once differently arranged from what it now appears. the aspect of the starry heavens presents us with the spectacle of that which is only apparently simultaneous, and however much we may endeavor, by the aid of optical instruments, to bring the mildly-radiant vapor of nebulous masses or the faintly-glimmering starry clusters nearer, and diminish the thousands of years interposed between us and them, that serve as a criterion of their distance, it still remains more than probable, from the knowledge we possess of the velocity of the transmission of luminous rays, that the light of remote heavenly bodies presents us with the most ancient perceptible evidence of the existence of matter. it is thus that the reflective mind of man is led from simple premises to rise to those exalted heights of nature, where in the light-illumined realms of space, "myriads of worlds are bursting into life like the grass of the night."* [fotnote] *from my brother's beautiful sonnet "freiheit und gesetz." (wilhelm von humboldt, 'gesammelte werke', bd. iv., s. , no. .) from the regions of celestial forms, the domain of uranus, we will now descend to the more contracted sphere of terrestrial forces -- to the interior of the earth itself. a mysterious chain links together both classes of phenomena. according to the ancient signification of the titanic myth,* the powers of organic life, that is to say, the great order of nature, depend upon the combined action of heaven and earth. [footnote] *otfried muller, 'prolegomena', s. . if we suppose that the earth, like all the other planets, primordially belonged, according to its origin, to the central body, the sun, and to the solar atmosphere that has been separated into nebulous p rings, the same connection with this continguous sun, as well as with all the remote suns that shine in the firmament, is still revealed through the phenomena of light and radiating heat. the difference in the degree of these actions must not lead the physicist, in his delineation of nature, to forget the connection and the common empire of similar forces in the universe. a small fraction of telluric heat is derived from the regions of universal space in which our planetary system is moving, whose temperature (which according to fourier, is almost equal to our mean icy polar heat) is the result of the combined radiation of all the stars. the causes that more powerfully excite the light of the sun in the atmosphere and in the upper strata of our air, that give rise to heat-engendering electric and magnetic currents, and awaken and genially vivify the vital spark in organic structures on the earth's surface, must be reserved for the subject of our future consideration. as we purpose for the present to confine ourselves exclusively within the telluric sphere of nature, it will be expedient to cast a preliminary glance over the relations in space of solids and fluids, the form of the earth, its mean density, and the partial distribution of this density in the interior of our planet, its temperature and its electro-magnetic tension. from the consideration of these relations in space, and of the forces inherent in matter, we shall pass to the reaction of the interior on the exterior of our globe; and to the special consideration of a universally distributed natural power -- subterranean heat; to the phenomena of earthquakes, exhibited in unequally expanded circles of commotion, which are not referable to the action of dynamic laws alone; to the springing forth of hot wells; and, lastly, to the more powerful actions of volcanic processes. the crust of the earth, which may scarcely have been perceptibly elevated by the sudden and repeated, or almost uninterrupted shocks by which it has been moved from below, undergoes, nevertheless, great changes in the course of centuries in the relations of the elevation of solid portions, when compared with the surface of the liquid parts, and even in the form of the bottom of the sea. in this manner simultaneous temporary or permanent fissures are opened, by which the interior of the earth is brought in contact with the external atmosphere. molten masses, rising from an unknown depth, flow in narrow streams along the declivity of mountains, rushing impetuously onward, or moving slowly and gently, until the fiery source is quenched in the midst of exhalations, and the lava becomes incrusted, as it were, by p the solidification of its outer surface. new masses of rocks are thus formed before our eyes, while the older ones are in their turn converted into other forms by the greater or lesser agency of platonic forces. even where no disruption takes place the crystalline moleculres are displaced, combining to form bodies of denser texture. the water presents structures of a totally different nature, as, for instance, concretions of animal and vegetable remains, of earthy, calcareous, or aluminous precipitates, agglomerations of finely-pulverized mineral bodies, covered with layers of the silicious shields of infusoria, and with transported soils containing the bones of fossil animal forms of a more ancient world. the study of the strata which are so differently formed and arranged before our eyes, and of all that has been so variously dislocated, conforted, and upheaved, by mutual compression and volcanic force, leads the reflective observer, by simple analogies, to draw a comparison between the present and an age that has long passed. it is by a combination of actual phenomena, by an ideal enlargement of relations in space, and of the amount of active forces, that we are able to advance into the long sought and indefinitely anticipated domain of geognosy, which has only within the last half century been based on the solid foundation of scientific deduction. it has been acutely remarked, "that notwithstanding our continual employment of large telescopes, we are less acquainted with the exterior than with the interior of other planets, excepting, perhaps, our own satellite." they have been weighed, and their volume measured; and their mass and density are becoming known with constantly-increasing exactness; thanks to the progress made in astronomical observation and calculation. their physical character is, however, hidden in obscurity, for it is only in our own globe that we can be brought in immediate contact with all the elements of organic and inorganic creation. the diversity of the most heterogenous substances, their admixtures and metamorphoses, and the ever-changing play of the forces called into action, afford to the human mind both nourishment and enjoyment, and open an immeasurable field of observation, from which the intellectual activity of man derives a great portion of its grandeur and power. the world of perceptive phenomena is reflected in the depths of the ideal world, and the richness of nature and the mass of all that admits of classification gradually become the objects of inductive reasoning. i would here allude to the advantage, of which i have already p spoken, possessed by that portion of physical science whose origin is familiar to us, and is connected with our earthly existence. the physical description of celestial bodies from the remotely-glimmering nebulae with their suns, to the central body of our own system, is limited, as we have seen, to general conceptions of the volume and quantity of matter. no manifestation of vital activity is there presented to our senses. it is only from analogies, frequently from purely ideal combinations, that we hazard conjectures on the specific elements of matter, or on their various modifications in the different planetary bodies. but the physical knowledge of the heterogeneous nature of matter, its chemical differences, the regular forms in which its molecules combine together, whether in crystals or granules; its relations to the deflected or decomposed waves of light by which it is penetrated; to radiating, transmitted, or polarized heat; and to the brilliant or invisible, but not, on that account, less active phenomena of electro-magnetism -- all this inexhaustible treasure, by which the enjoyment of the contemplation of nature is so much heightened, is dependent on the surface of the planet which we inhabit, and more on its solid than on its liquid parts. i have already remarked how greatly the study of natural objects and forces, and the infinite diversity of the sources they open for our consideration, strengthen the mental activity, and call into action every manifestation of intellectual progress. these relations require, however, as little comment as that concatenation of causes by which particular nations are permitted to enjoy a superiority over others in the exercise of a material power derived from their command of a portion of these elementary forces of nature. if, on the one hand, it were necessary to indicate the difference existing between the nature of our knowledge of the earth and of that of the celestial regions and their contents, i am no less desirous, on the other hand, to draw attention to the limited boundaries of that portion of spacefrom which we derive all our knowledge of the heterogeneous character of matter. this has been somewhat inappropriately termed the earth's crust; it includes the strata most contiguous to the upper surface of our planet, and which have been laid open before us by deep fissure-like valleys, or by the labors of man, in the bores and shafts formed by miners. these labors* do not extend beyond a vertical depth of somewhat more than feet (about one third of a geographical mile) below the p level of the sea, and consequently only about / th of the earth's radius. [footnote] *in speaking of the greatest depths within the earth reached by human labor, we must recollect that there is a difference between the 'absolute depth' (that is to say, the depth below the earth's surface at that point) and the 'relative depth' (or that beneath the level of the sea). the greatest relative depth that man has hitherto reached is probably the bore at the new salt-works at minden, in prussia: in june, , it was exactly feet, the absolute depth being feet. the temperature of the water at the bottom was degrees f., which assuming the mean temperature of the air at . degrees gives an augmentation of temperature of degree for every feet. the absolute depth of the artesian well of grenelle, near paris, is only feet. according to the account of the missionary imbert, the fire-springs, "ho-tsing." of the chinese, which are sunk to obtain [carbureted] hydrogen gas for salt-boiling, far exceed our artesian springs in depth. in the chinese province of szu-tschuan these fire-springs are very commonly of the depth of more than feet; indeed, at tseu-lieu-tsing (the place of continual flow) there is a ho-tsing which, in the year , was found to be feet deep. (humboldt, 'asie centrale', t. ii., p. and . 'annales de l'association de la propagation de la foi', , no. , p. .) [footnote continues] the relative depth reached at mount massi, in tuscany, south of volterra, amounts, according to matteuci, to only feet. the boring at the new salt-works near minden is probably of about the same relative depth as the coal-mine at apendale, near newcastle-under-lyme, in staffordshire, where men work yards below the surface of the earth. (thomas smith, 'miner's guide', , p. .) unfortunately, i do not know the exact height of its mouth above the level of the sea. the relative depth of the monk-wearmouth mine, near newcastle, is only feet. (phillips, in the 'philos. mag.', vol. v., , p. .) that of the liege coal-mine, 'l'esperance' at seraing, is, according to m. gernaert, ingenieur des mines, feet in depth. the works of greatest absolute depth that have ever been formed are for the most part situated in such elevated plains or valleys that they either do not descend so low as the level of the sea, or at most reach very little below it. thus the eselchacht, at kuttenberg, in bohemia, a mine which can not now be worked, had the enormous absolute depth of feet. (fr. a. schmidt, 'berggestze der oter mon.', abth. i., bd. i., s. xxxii.) also, at st. daniel and at geish, on the rorerbubel, in the 'landgericht' (or provincial district) of kitzbuhl, there were, in the sixteenth century, excavations of feet. the plans of the works of the rorerbubel are still preserved. (see joseph von sperges, 'tyroler bergwerksgeschichte', s. . compare, also, humboldt, 'gutachten uber Ã�ºerantreibung des meissner stollens in die freiberger erzrevier', printed in herder, 'uber herantreibung des meissner stollens in die freiberger erzrevier', printed in herder, 'uber den jetz begonnenen erbstollen', , s. cxxiv.) we may presume that the knowledge of the extraordinary depth of the rorerbuhel reached england at an early period, for i find it remarked in gilbert, 'de magnete', that men have penetrated or even feet into the crust of the earth. ("exigua videtur terrae portio, quae unquam hominibus spectanda emerget aut eruitur; cum profundinus in ejus viscera, ultra efflorescentis extremitatis corruptelam, aut propter aquas in magnis fodin, tanquam per venas scaturientesaut propter seris salubrioris ad vitam operariorum sustinendam necessarii defectum, aut propter ingentex sumptus ad tantos labores exantlandos, multasque difficultates, ad profundiores terrz' partes penetrre non possumus; adeo ut quadrigentas aut [quod rarissime] quingentas orgyas in quibusdam metallis descendisse, stupendus omnibus videatur connatus." -- guilielmi gilberti, colcestrensis, 'de magnete physiologia nova'. lond., , p. .) [footnote continues] the absolute depth of the mines in the saxon erzgebirge, near freiburg, are: in the thurmhofer mines, feet; in the honenbirker mines, feet; the relative depths are only and feet, if, in order to calculate the elevation of the mine's mouth above the level of the sea, we regard the elevation of freiburg as determined by reich's recent observations to be feet. the absolute depth of the celebrated mine of joachimsthal, in bohemia (verkreuzung des jung hauer zechen-und andreasganges), is full feet; so that, as von dechen's measurements show that its surface is about feet above the level of the sea, it follows that the excavations have not as yet reached that point. in the harz, the samson mine at andreasberg has an absolute depth of feet. in what was formerly spanish america, i know of no mine deeper than the valenciana, near guanaxuato (mexico), where i found the absolute depth of the planes de san bernardo to be feet; but these planes are feet above the level of the sea. if we compare the depth of the old kuttenberger mine (a depth greater than the height of our brocken, and only feet less than that of vesuvius) with the loftiest structures that the hands of man have erected (with the pyramid of cheops and with the cathedral of strasburg), we find that they stand in the ratio of eight to one. in this note i have collected all the certain information i could find regarding the greatest absolute and relative depths of mines and borings. in descending eastward from jerusalem toward the dead sea, a view presents itself to the eye, which, according to our present hypsometrical knowledge of the surface of our planet, is unrivaled in any country; as we approach the open ravine through which the jordan takes its course, we tread, with the open sky above us, on rocks which, according to the barometric measurements of berton and russegger are feet below the level of the mediterranean. (humboldt, 'asie centrale', th. ii., p. .) the crystalline masses that have been erupted from active volcanoes, and are generally similar to the rocks on the upper surface, have come from depths which, although not accurately determined, must certainly be sixty times greater than those to which human labor has been enabled to penetrate. we are able to give in numbers the depth of the shaft where the strata of coal, after penetrating a certain way, rise again at a distance that admits of being accurately defined by measurements. these dips show that the carboniferous strata, together with the fossil organic remains which they contain, must lie, as, for instance, in belgium, more than five or six thousand feet* below the present level p of the sea, and that the calcareous and the curved strata of the devonian basin penetrate twice that depth. [footnote] *basin-shaped curved strata, which dip and reappear at measureable distances, although their deepest portions are beyond the reach of the miner, afford sensible evidence of the nature of the earth's crust at great depths below its surface. testimony of this kind possesses, consequently, a great geognostic interest. i am indebted to that excellent geognosist, von dechen, for the following observations. "the depth of the coal basin of liege, at mont st. gilles, which i, in conjunction with our friend von oeynhausen, have ascertained to be feet below the surface, extends feet below the surface of the sea, for the absolute height of mont st. gilles certainly does not much exceed feet; the coal basin of mons is fully feet deeper. but all these depths are trifling compared with those which are presented by the coal strata of saar-revier (saarbrucken). i have found after repeated examinations, that the lowest coal stratum which is known in the neighborhood of duttweiler, near bettingen, northeast of saarlouis, must descend to depths of , and , feet (or . geographical miles) below the level of the sea." this result exceeds, by more than feet, the assumption made in the text regarding the basin of the devonian strata. this coal-field is therefore sunk as far below the surface of the sea as chimborazo is elevated above it -- at a depth at which the earth's temperature must be as high as Ã�¼degrees f. hence, from the highest pinnacles of the himalaya to the lowest basins containing the vegetation of an earlier world, there is a vertical distance of about , feet, or of the th part of the earth's radius. if we compare these subterranean basins with the summits of montains that have hitherto been considered as the most elevated portions of the raised crust of the earth, we obtain a distance of , feet (about seven miles), that is, about the / th of the earth's radius. these, therefore, would be the limits of vertical depth and of the superposition of mineral strata to which geognostical inquiry could penetrate, even if the general elevation of the upper surface of the earth were equal to the height of the dhawalagigi in the himalaya, or of the sorata in bolivia. all that lies at a greater depth below the level of the sea than the shafts or the basins of which i have spoken, the limits to which man's labors have penetrated, or than the depths to which the sea has in some few instances been sounded (sir james ross was unable to find bottom with , feet of line), is as much unknown to us as the interior of the other planets of our solar system. we only know the mass of the whole earth and its mean density by comparing it with the open strata, which alone are accessible to us. in the interior of the earth, where all knowledge of its chemical and mineralogical character fails, we are again limited to as pure conjecture, as in the remotest bodies that revolve round the sun. we can determine nothing with certainty regarding the depth at which the geological strata must be supposed to be in state of softening or of liquid fusion, of the cavities occupied by elastic vapor, of the condition of fluids when heated under an enormous pressure, or of the law of the increase p of density from the upper surface to the center of the earth. the consideration of the increase of heat with the increase of depth toward the interior of our planet, and of the reaction of the interior on the external crust, leads us to the long series of volcanic phenomena. these elastic forces are manifested in earthquakes, eruptions of gas, hot wells, mud volcanoes and lava currents from craters of eruption and even in producing alterations in the level of the sea.* [footnote] * [see daubeney 'on volcanoes', d edit., , p. , etc., on the so called 'mud volcanoes', and the reasons advanced in favor of adopting the term "salses" to designate these phenomena.] -- tr. large plains and variously indented continents are raised or sunk, lands are separated from seas, and the ocean itself, which is permeated by hot and cold currents, coagulates at both poles, converting water into dense masses of rock, which are either stratified and fixed, or broken up into floating banks. the boundaries of sea and land, of fluids and solids, are thus variously and frequently changed. plains have undergone oscillatory movements, being alternately elevated and depressed. after the elevation of continents, mountain chains were raised upon long fissures, mostly parallel, and in that case, probably cotemporaneous; and salt lakes and inland seas, long inhabited by the same creatures, were forcibly separated, the fossil remains of shells and zoophytes still giving evidence of their original connection. thus, in following phenomena in their mutual dependence, we are led from the consideration of the forces acting in the interior of the earth to those which cause eruptions on its surface, and by the pressure of elastic vapors give rise to burning streams of lava that flow from open fissures. the same powers that raised the chains of the andes and the hiimalaya to the regions of perpetual snow, have occasioned new compositions and new textures in the rocky masses, and have altered the strata which had been previously deposited from fluids impregnated with organic substances. we here trace the series of formations, divided and superposed according to their age, and depending upon the changes of configuration of the surface, the dynamic relations of upheaving forces, and the chemical action of vapors issuing from the fissures. the form and distribution of continents, that is to say, of that solid portion of the earth's surface which is suited to the luxurious development of vegetable life, are associated by intimate connection and reciprocal action with the encircling p sea in which organic life is almost entirely limited to the animal world. the liquid element is again covered by the atmosphere, an aÃ�Â�rial ocean in which the mountain chains and high plains of the dry land rise like shoals, occasioning a variety of currents and changes of temperature, collecting vapor from the region of clouds, and distributing life and motion by the action of the streams of water which flow from their declivities. while the geography of plants and animals depends on these intricate relations of the distribution of sea and land, the configuration of the surface, and the direction of isothermal lines (or zones of equal mean annual heat), we find that the case is totally different when we consider the human race -- the last and noblest subject in a physical description of the globe. the characteristic differences in races, and their relative numerical distribution over the earth's surface, are conditions affected not by natural relations alone, but at the same time and specially, by the progress of civilization, and by moral and intellectual cultivation on which depends the political superiority that distinguishes national progress. some few races, clinging, as it were, to the soil, are supplanted and ruined by the dangerous vicinity of others more civilized than themselves, until scarce a trace of their existence remains. other races, again, not the strongest in numbers, traverse the liquid element, and thus become the first to acquire, although late, a geographical knowledge of at least the maritime lands of the whole surface of our globe, from pole to pole. i have thus, before we enter on the individual characters of that portion of the delineation of nature which includes the sphere of telluric phenomena, shown generally in what manner the consideration of the form of the earth and the incessant action of electro-magnetism and subterranean heat may enable us to embrace in one view the relations of horizontal expansion and elevation on the earth's surface, the geognostic type of formations, the domain of the ocean (of the liquid portions of the earth), the atmosphere with its meteorological processes, the geographical distribution of plants and animals, and, finally, the physical gradations of the human race, which is, exclusively and every where, susceptible of intellectual culture. this unity of contemplation presupposes a connection of phenomena according to their internal combination. a mere tabular arrangement of these facts would not fulfill the object i have proposed to myself, and would not satisfy that requirement for cosmical presentation awakened in me by the p aspect of nature in my journeyings by sea and land, by the careful study of forms and forces, and by a vivid impression of the unity of nature in the midst of the most varied portions of the earth. in the rapid advance of all branches of physical science, much that is deficient in this attempt will, perhaps, at no remote period, be corrected and rendered more perfect, for it belongs to the history of the development of knowledge that portions which have long stood isolated become gradually connected, and subject to higher laws. i only indicate the empirical path in which i and many others of similar pursuits with myself are advancing, full of expectation that, as plato tells us socrates once desired, "nature may be interpreted by reason alone."* [footnote] *plato, 'phaedo', p. . (arist., 'metaph.', p. .) compare hegel, 'philosophie der geschichte', , s. . the delineation of the principal characteristics of telluric phenomena must begin with the form of our planet and its relations in space. here too, we may say that it is not only the mineralogical character of rocks, whether they are crystalline, granular, or densely fossiliferous, but the geometrical form of the earth itself, which indicates the mode of its origin, and is, in fact, its history. an elliptical spheroid of revolution gives evidence of having once been a soft or fluid mass. thus the earth's compression constitutes one of the most ancient geognostic events, as every attentive reader of the book of nature can easily discern; and an analogous fact is presented in the case of the moon, the perpetual direction of whose axes toward the earth, that is to say, the increased accumulation of matter on that half of the moon which is turned toward us, determines the relations of the periods of rotation and revolution, and is probably contemporaneous with the earliest epoch in the formative history of this satellite. the mathematical figure of the earth is that which it would have were its surface covered entirely by water in a state of rest; and it is this assumed form to which all geodesical measurements of degrees refer. this mathematical surface is different from that true physical surface which is affected by all the accidents and inequalities of the solid parts.* [footnote] *bessel, 'allgemeine betrachtungen uber gradmessungen nach astronomisch-geodÃ�Â�tischen arbeiten', at the conclusion of bessel and baeyer, 'gradmessung in ostpreussen', s. . regarding the accumulation of matter on the side of the moon turned toward us (a subject noticed in an earlier part of the text), see laplace, 'expos. du syst. du monde', p. . the whole figure of the earth is determined when we know the amount of the p compression at the poles and the equatorial diameter; in order, however, to obtain a perfect representation of its form it is necessary to have measurements in two directions, perpendicular to one another. eleven measurements of degrees (or determinations of the curvature of the earth's surface in different parts), of which nine only belong to the present century, have made us acquainted with the size of our globe, which pliny names "a point in the immeasurable universe."* [footnote] *plin., ii., . seneca, 'nat. quaest., praef., c. ii. "el mundo espoco" (the earth is small and narrow), writes columbus from jamaica to queen isabella on the th of july, : not because he entertained the philosophic views of the aforesaid romans, but because it appeared advantageous to him to maintain that the journey from spain was not long, if, as he observes, "we seek the east from the west." compare my 'examen crit. de l'hist. de la geogr. du me siecle', t.i., p. , and t. ii., p. , where i have shown that the opinion maintained by delisle, freret, and gosselin, that the excessive differences in the statements regarding the earth's circumference, found in the writings of the greeks, are only apparent, and dependent on different values being attached to the stadia, was put forward as early as by jaime ferrer, in a proposition regarding the determination of the line of demarkation of the papal dominions. if these measurements do not always accord in the curvatures of different meridians under the same degree of latitude, this very circumstance speaks in favor of the exactness of the instruments and the methods employed, and of the accuracy and the fidelity to nature of these partial results. the conclusion to be drawn from the increase of forces of attraction (in the direction from the equator to the poles) with respect to the figure of a planet is dependent on the distribution of density in its interior. newton, from theoretical principles, and perhaps likewise prompted by cassini's discovery, previously to , of the compression of jupiter,* determined, in his immortal work, 'philosophiae naturalis principia', that the compression of the earth, as a homogeneous mass, was / th. [footnote] *brewster, 'life of sir isaac newton', , p. . "the discovery of the spheroidal form of jupiter by cassini had probably directed the attention of newton to the determination of its cause, and consequently, to the investigation of the true figure of the earth." although cassini did not announce the amount of the compression of jupiter ( / th) till ('anciens memoires de l'acad. des sciences', t. ii., p. ), yet we know from lalande ('astron.', me ed., t. iii., p. ) that moraldi possessed some printed sheets of a latin work, "on the spots of the planets," commenced by cassini, from which it was obvious that he was aware of the compression of jupiter before the year , and therefore at least twenty-one years before the publication of newton's 'principia'. actual mesurements, p made by the aid of new and more perfect analysis, have, however, shown that the compression of the poles of the terrestrial spheroid, when the density of the strata is regarded as increasing toward the center, is very nearly / th. three methods have been employed to investigate the curvature of the earth's surface, viz., measurements of degrees, oscillations of the pendulum, and observations of the inequalities in the moon's orbit. the first is a direct geometrical and astronomical method, while in the other two we determine from accurately observed movements the amount of the forces which occasion those movements, and from these forces we arrive at the cause from whence they have originated, viz., the compression of our terrestrial spheroid. in this part of my delineation of nature, contrary to my usual practice, i have instanced methods because their accuracy affords a striking illustration of the intimate connection existing among the forms and forces of natural phenomena, and also because their application has given occasion to improvements in the exactness of instruments (as those employed in the measurements of space) in optical and chronological observations; to greater perfection in the fundamental branches of astronomy and mechanics in respect to lunar motion and to the resistance experienced by the oscillations of the pendulum; and to the discovery of new and hitherto untrodden paths of analysis. with the exception of the investigations of the parallax of stars, which led to the discovery of aberration and nutation, the history of science presents no problem in which the object attained -- the knowledge of the compression and of the irregular form of our planet -- is so far exceeded in importance by the incidental gain which has accrued, through a long and weary course of investigation, in the general furtherance and improvement of the mathematical and astronomical sciences. the comparison of eleven measurements of degrees (in which are included three extra-european, namely, the old peruvian and two east indian) gives, according to the most strictly theoretical requirements allowed for by bessel,* a compression p of / th. [footnote] *according to bessel's examination of ten measurements of degrees, in which the error discovered by poissant in the calculation of the french measurements is taken into consideration (schumacher, 'astron. nachr.', , no. , s. ), the semi-axis major of the elliptical spheroid of revolution to which the irregular figure of the earth most closely approximates is , , . toises, or , , feet; the semi-axis minor, , , , toises, or , , feet; and the amount of compression or eccentricity / . d; the length of a mean degree of the meridian, , . toises, or , feet, with an error of + . toises, or . feet, whence the length of a geographical mile is . toises, or . feet. previous combinations of measurements of degrees varied between / d and / th; thus walbeck ('de forma of magnitudine telluris in demensis arcubus meridiani definiendis', ) gives / th: ed. schmidt ('lehrbuch der mathem. und phys. geographie', , s. ) gives / d, as the mean of seven measures. respecting the influence of great differences of longitude on the polar compression, see 'bibliotheque universelle', t. xxxiii., p. , and t. xxxv., p. : likewise 'connaissance des tems', , p. . from the lunar inequalities alone, laplace ('exposition du syst. du monde', p. ) found it, by the older tables of burg, to be / th; and subsequently, from the lunar observations of burckhardt and bouvard, he fixed it at / . th ('mecanique celeste', t. v., p. and ). in accordance with this, the polar radius is , toises ( , feet), or about / miles, shorter than the equatorial radius of our terrestrial spheroid. the excess at the equator in consequence of the curvature of the upper surface of the globe amounts, consequently, in the direction of gravitation, to somewhat more than / th times the height of mont blanc, or only / times the probable height of the summit of the chawalagiri, in the himalaya chain. the lunar inequalities (perturbation in the moon's latitude and longitude) give according to the last investigations of laplace, almost the same result for the ellipticity as the measurements of degrees, viz., / th. the results yielded by the oscillation of the pendulum give, on the whole, a much greater amount of compression, viz., / th.* [footnote] *the oscillations of the pendulum give / . th as the general result of sabine's great expedition ( and , from the equator to degrees north latitude); according to freycinet, / . d, exclusive of the experiments instituted at the isle of france, guam, and mowi (mawi); according to forster, / . th; according to duperrey, / . th; and according to lutke ('partie nautique', , p. ), / th, calculated from eleven stations. on the other hand, mathieu ('connais. des temps', , p. ) fixed the amount at / . d, from observations made between formentera and dunkirk; and biot, at / th, from observations between formentera and the island of ust. compare baily, 'report on pendulum experiments', in the 'memoirs of the royal astronomical society', vol. vii., p. ; also borenius, in the 'bulletin de l'acad. de st. petersbourg', , t. i., p. . the first proposal to apply the length of the pendulum as a standard of measure, and to establish the third part of the seconds pendulum (then supposed to be every where of equal length) as a 'pes horarius', or general measure, that might be recovered at any age and by all nations, is to be found in huygens's 'horologium oscillatorium', , prop. . a similar wish was afterward publicly expressed, in , on a monument erected at the equator by bouguer, la condamine, and godin. on the beautiful marble tablet which exists, as yet uninjured, in the old jesuits' college at quito, i have myself read the inscription, 'penduli simplicis aequinoctialis unius minuti secundi archetypus, mensurae naturalis exemplar, utinam universalis!' from an observation made by la condamine, in his 'journal du voyage a l'equateur', , p. , regarding parts of the inscription that were not filled up, and a slight difference between bonguer and himself respecting the numbers, i was led to expect that i should find considerable discrepancies between the marble tablet and the inscription as it had been described in paris; but, after a careful comparison, i merely found two "ex arca graduum plusquam trium," and the date of instead of . the latter circumstance is singular, because la condamine returned to europe in november, , bouguer in june of the same year, and godin had left south america in july, . the most necessary and useful amendment to the numbers on this inscription would have been the astronomical longitude of quito. (humboldt, 'recueil d'observ. astron.', t. ii., p. - .) nouet's latitudes, engraved on egyptian monuments, offer a more recent example of the danger presented by the grave perpetuation of false or careless results. galileo, who first observed when a boy (having, probably, suffered his thoughts to wander from the service) that the height of the vaulted roof of a church might be measured by the time of the vibration of the chandeliers suspended at different altitudes, could hardly have anticipated that the pendulum would one day be carried from pole to pole, in order to determine the form of the earth, or, rather, that the unequal density of the strata of the earth affects the length of the seconds pendulum by means of intricate forces of local attraction, which are, however, almost regular in large tracts of land. these geognostic relations of an instrument intended for the measurement of time -- this property of the pendulum, by which, like a sounding line, it searches unknown depths, and reveals in volcanic islands,* or in the declivity of elevated continental mountain chains,** dense masses of basalt and melaphyre instead of cavities, render it difficult, notwithstanding the admirable simplicity of the method, to arrive at any great result regarding the figure of the earth from observation of the oscillations of the pendulum. [footnote] *respecting the augmented intensity of the attraction of gravitation in volcanic islands (st. helena, ualan, fernando de noronha, isle of france, guam, mowe, and galapagos), rawak (lutke, p. ) being an exception, probably in consequence of its proximity to the highland of new guinea, see mathieu, in delambre, 'hist. de l'astronomie, au me siecle', p. . [footnote] **numerous observations also show great irregularities in the length of the pendulum in the midst of continents, and which are ascribed to local attractions. (delambre, 'mesure de la meridienne', t. iii., p. ; biot, in the 'mem. de l'academie des sciences', t. viii., , p. and .) in passing over the south of france and lombardy from west to east, we find the minimum intensity of gravitation at bordeaux; from thence it increases rapidly as we advance eastward, through figeac, clermont-ferrand, milan, and padua; and in the last town we find that the intensity has attained its maximum. the influence of the southern declivities of the alps is not merely t on the general size of their mass, but (much more), in the opinion of elie de beaumont ('rech. sur les revol. de la surface du globe', , p. ), on the rocks of melaphyre and serpentine, which have elevated the chain. on the declivity of ararat, which with caucasus may be said to lie in the center of gravity of the old continent formed by europe, asia, and africa, the very exact pendulum experiments of fedorow give indications, not of subterranean cavities, but of dense volcanic masses. (parrot, 'reise zum ararat', bd. ii., s. .) in the geodesic operations of carlini and plana, in lombardy, differences ranging from " to ". have been found between direct observations of latitude and the results of these operations. (see the instances of andrate and mondovi, and those of milan and padua, in the 'operations geodes. et astron. pour la mesure d'un arc du parallele moyen', t. ii., p. ; 'effemeridi astron. di milano', , p. .) the latitude of milan, deduced from that of berne, according to the , is Ã�¼degrees ' ", while, according to direct astronomical observations, it is degrees ' ". as the perturbations extend in the plain of lombardy to parma, which is far south of the po (plana, 'operat. geod.', t. ii., p. ), it is probable that there are deflecting causes 'concealed beneath the soil of the plain itself'. struve has made similar experiments [with corresponding results] in the most level parts of eastern europe. (schumacher, 'astron. nachrichten', , no. , s. .) regarding the influence of dense masses supposed to lie at a small depth, equal to the mean height of the alps, see the analytical expressions given by hossard and rozet, in the 'comptes rendus', t. xviii., , p. , and compare them with poisson, 'traite de mecanique' ( me ed., t. i., p. . the earliest observations on the influence which different kinds of rocks exercise on the vibration of the pendulum are those of thomas young, in the 'philos. transactions' for , p. - . in drawing conclusions regarding the earth's curvature from the length of the pendulum, we ought not to overlook the possibility that its crust may have undergone a process of hardening previously to metallic and dense basaltic masses having penetrated from great depths, through open clefts, and approached near the surface. in the astronomical part of the determination of degrees of latitude, mountain chains, or the denser strata of the earth, likewise exercise, although in a less degree, an unfavorable influence on the measurement. as the form of the earth exerts a powerful influence on the motions of other cosmical bodies, and especially on that of its own neighboring satellite, a more perfect knowledge of the motion of the latter will enable us reciprocally to draw an inference regarding the figure of the earth. thus, as laplace ably remarks,* "an astronomer, without leaving his observatory, may, by a comparison of lunar theory with true observations, not only be enabled to determine the form and size of the earth, but also its distance from the sun and moon -- results that otherwise could only be arrived at by long and arduous expeditions to the most remote parts of both hemispheres." [footnote] *laplace, 'expos. du syst. du monde', p. . p the compression which may be inferred from lunar inequalities affords an advantage not yielded by individual measurements of degrees or experiments with the pendulum, since it gives a mean amount which is referable to the whole planet. the comparison of the earth's compression with the velocity of rotation shows, further, the increase of density from the strata from the surface toward the center -- an increase which a comparison of the ratios of the axes of jupiter and saturn with their times of rotation likewise shows to exist in these two large planets. thus the knowledge of the external form of planetary bodies leads us to draw conclusions regarding their internal character. the northern and southern hemispheres appear to present nearly the same curvature under equal degrees of latitude, but, as has already been observed, pendulum experiments and measurements of degrees yield such different results for individual portions of the earth's surface that no regular figure can be given which would reconcile all the results hitherto obtained by this method. the true figure of the earth is to a regular figure as the uneven surfaces of water in motion are on the even surface of water at rest. when the earth had been measured, it still had to be weighed. the oscillations of the pendulum* and the plummet have here likewise served to determine the mean density of the earth, either in connection with astronomical and geodetic operations, with the view of finding the deflection of the plummet from a vertical line in the vicinity of a mountain, or by a comparison of the length of the pendulum in a plain and on the summit of an elevation, or, finally, by the employment of a torsion balance, which may be considered as a horizontally vibrating pendulum for the measurement of the relative density of neighbouring strata. [footnote] *la caille's pendulum measurements at the cape of good hope, which have been calculated with much care by mathieu (delambre, 'hist. de l'astron. au me siecle', p. ), give a compression of / . th; but, from several comparisons of observations made in equal latitudes in the two hemispheres (new holland and the malouines (falkland islands), compared with barcelona, new york, and dunkirk), there is as yet no reason for supposing that the mean compression of the southern hemisphere is greater than that of the northern. (biot, in the 'mem. de l'acad. des sciences', t. viii., , p. - .) of these three methods* the p last is the most certain, since it is independent of the difficult determination of the density of the mineral masses of which the spherical segment of the mountain consists near which the observations are made. [footnote] *the three methods of observation give the following results: ( .) by the deflection of the plumb-line in the proximity of the shehallien mountain (gaelic, thichallin) in perthshire, r. , as determined by maskelyne, hutton, and playfair ( - and ), according to a method that had been proposed by newton; ( .) by pendulum vibrations on mountains, . (carlini's observations on mount cenis compared with biot's observations at bordeaux, 'effemer. astron. di milano', , p. ); ( .) by the torsion balance used by cavendish, with an apparatus originally devised by mitchell, . (according to hutton's revision of the calculation, . , and according to that of eduard schmidt, . ; 'lehrbuch der math. geographie', bd. i., s. ); by the torsion balance, according to reich, . . in the calculation of these experiments of professor reich, which have been made with masterly accuracy, the original mean result was . (with a probable error of only . ), a result which, being increased by the quantity by which the earth's centrifugal force diminishes the force of gravity for the latitude of freiberg ( degrees '), becomes changed to . . the employment of cast iron instead of lead has not presented any sensible difference, or none exceeding the limits of errors of observation, hence disclosing no traces of magnetic influences. (reich, 'vrsuche uber die mittlere dichtigheit der erde', , s. , , and .) by the assumption of too slight a degree of ellipticity of the earth, and by the uncertainty of the estimations regarding the density of rocks on its surface, the mean density of the earth, as deduced from experiments on and near mountains, was found about one sixth smaller than it really is, namely, . (laplace, 'mecan. celeste', t. v., p. ), or . . (eduard schmidt, 'lehrb. der math. geogr.', bd. i., und .) on halley's hypothesis of the earth being a hollow sphere (noticed in page ), which was the germ of franklin's ideas concerning earthquakes, see 'philos. trans.' for the year , vol. xvii., p. ('on the structure of the internal parts of the earth, and the concave habited 'arch of the shell'). halley regarded it as more worthy of the creator "that the earth, like a house of several stories, should be inhabited both without and within. for light in the hollow sphere (p. ) provision might in some manner be contrived." according to the most recent experiments of reich, the result obtained is . ; that is to say, the mean density of the whole earth is . times greater than tht of pure water. as according to the nature of the mineralogical strata constituting the dry continental part of the earth's surface, the mean density of this portion scarcely amounts to . , and the density of the dry and liquid surface conjointly to scarcely . , it follows that the elliptical unequally compressed layers of the interior must greatly increase in density toward the center, either through pressure or owing to the heterogeneous nature of the substances. here again we see that the vertical, as well as the horizontally vibrating pendulum, may justly be termed a geognostical instrument. the results obtained by the employment of an instrument of this kind have led celebrated physicists, according to the difference of the hypothesis from which they started, to adopt p entirely opposite views regarding the nature of the interior of the globe. it has been computed at what depths liquid or even gaseous substances would, from the pressure of their own superimposed strata, attain a density exceeding that of platinum or even iridium; and in order that the compression which has been detrmined within such narrow limits might be brought into harmony with the assumption of simple and infinitely compressible matter, leslie has ingeniously conceived the nucleus of the world to be a hollow sphere, filled with an assumed "imponderable matter, having an enormous force of expansion." these venturesome and arbitrary conjectures have given rise, in wholly unscientific circles, to still more fantastic notions. the hollow sphere has by degrees been peopled with plants and animals, and two small subterranean revolving planets -- pluto and proserpine -- were imaginatively supposed to shed over it their mild light; as, however, it was further imagined that an ever-uniform temperature reigned in these internal regions, the air, which was made self-luminous by compression, might well render the planets of this lower world unnecessary. near the north pole, at degrees latitude, whence the polar light emanates, was an enormous opening, through which a descent might be made into the hollow sphere, and sir humphrey davy and myself were even publicly and frequently invited by captain symmes to enter upon this subterranean expedition: so powerful is the morbid inclination of men to fill unknown spaces with shapes of wonder, totally unmindful of the counter evidence furnished by well-attested facts and universally acknowledged natural laws. even the celebrated halley, at the end of the seventeenth century, hollowed out the earth in his magnetic speculations. men were invited to believe that a subterranean freely-rotating nucleus occasions by its position the diurnal and annual changes of magnetic declination. it has thus been attempted in our own day, with tedious solemnity, to clothe in a scientific garb the quaintly-devised fiction of the humorous holbert.* [footnote] *[the work referred to, one of the wittiest productions of the learned norwegian satirist and dramatist holberg, was written in latin, and first appeared under the following title: 'nicolai klimii iter subterraneum novam telluris theoriam ac historiam quintae monarchi nicolai klimii iter subterraneum novam telluris theoriam ac historiam quintae monarchi ad huc nobis incognitae exhibens e bibliotheca b. abelini. hafniae et lipsiae sunt. jac. preuss', . an admirable danish translation of this learned but severe satire on the institutions, morals, and manners of the inhabitants of the upper earth, appeared at copenhagen in , and was entitled 'niels klim's underjordiske reise ocd ludwig holberg, oversal after den latinske original of jens baggesen'. holberg, who studied for a time at oxford, was born at bergen in , and died in as rector of the university of copenhagen.] -- tr. p the figure of the earth and the amount of solidification (density) which it has acquired are intimately connected with the forces by which it is animated, in so far, at least, as they have been excited or awakened from without, through its planetry position with reference to a luminous central body. compression, when considered as a consequence of centrifugal force acting on a rotating mass, explains the earlier condition of fluidity of our planet. during the solidification of this fluid, which is commonly conjectured to have been gaseous and primordially heated to a very high temperature, an enormous quantity of latent heat must have been liberated. if the process of solidification began as fourier conjectures, by radiation from the cooling surface exposed to the atmosphere, the particles near the center would have continued fluid and hot. as, after long emanation of heat from the center toward the exterior, a stable condition of the temperature of the earth would at length be established, it has been assumed that with increasing depth the subterranean heat likewise uninterruptedly increases. the heat of the water which flows from deep borings (artesian wells), direct experiments regarding the temperature of rocks in mines, but, above all, the volcanic activity of the earth, shown by the flow of molten masses from open fissures, afford unquestionable evidence of this increase for very considerable depths from the upper strata. according to conclusions based certainly upon mere analogies, this increase is probably much greater toward the center. that which has been learned by an ingenious analytic calculation, expressly perfected for this class of investigations,* p regarding the motion of heat in homogeneous metallic spheroids, must be applied with much caution to the actual character of our planet, considering our present imperfect knowledge of the substances of which the earth is composed, the difference in the capacity of heat and in the conducting power of different superimposed masses, and the chemical changes experienced by solid and liquid masses from any enormous compression. [footnote] *here we must notice the admirable analytical labors of fourier, biot, laplace, poisson, duhamel, and lame. in his 'theorie mathematique de la chaleur', , p. , - , , and - (see, also, de la rive's abstract in the 'bibliotheque universelle de geneve', poisson has developed an hypothesis totally different from fourier's view ('theorie analytique de la chaleur'.) he denies the present fluid state of the earth's center; he believes that "in cooling by radiation to the medium surrounding the earth, the parts which were first solidified sunk, and that by a double descending and ascending current, the great inequality was lessened which would have taken place in a solid body cooling from the surface." it seems more probable to this great geometer that the solidification began in the parts lying nearest to the center: "the phenomenon of the increase of heat with the depth does not extend to the whole mass of the earth, and is merely a consequence of the motion of our planetary system in space, of which some parts are of a very different temperature from others, in consequence of stellar heat (chaleur stellaire)." thus, according to poisson, the warmth of the water of our artesian wells is merely that which has penetrated into the earth from without; and the earth itself "might be regarded as in the same circumstances as a mass of rock conveyed from the equator to the pole in so short a time as not to have entirely cooled. the increase of temperature in such a block would not extend to the central strata." the physical doubts which have reasonably been entertained against this extraordinary cosmical view (which attributes to the regions of space that which probably is more dependent on the first transition of matter condensing from the gaseo-fluid into the solid state) will be found collected in poggendorf's 'annalen', bd. xxxix., s - . it is with the greatest difficulty that our powers of comprehension can conceive the boundary line which divides the fluid mass of the interior from the hardened mineral masses of the external surface, or the gradual increase of the solid strata, and the condition of semi-fluidity of the earthy substances, these being conditions to which known laws of hydraulics can only apply under considerable modifications. the sun and moon, which cause the sea to ebb and flow, most probably also affect these subterranean depths. we may suppose that the periodic elevations and depressions of the molten mass under the already solidified strata must have caused inequalities in the vaulted surface from the force of pressure. the amount and action of such oscillations must, however, be small; and if the relative position of the attracting cosmical bodies may here also excite "spring tides," it is certainly not to these, but to more powerful internal forces, that we must ascribe the movements that shake the earth's surface. there are groups of phenomena to whose existence it is necessary to draw attention, in order to indicate the universality of the influence of the attraction of the sun and moon on the external and internal conditions of the earth, however little we may be able to determine the quantity of this influence. according to tolerably accordant experiments in artesian wells, it has been shown that the heat increases on an average about degree for every . feet. if this increase can be reduced p to arithmetical relations, it will follow, as i have already observed,* that a stratum of granite would be in a state of fusion at a depth of nearly twenty-one geographical miles, or between four and five times the elevation of the highest summit of the hinalaya. [footnote] *see the introduction. this increase of temperature has been found in the puits de grenelle, at paris, at . feet; in the boring at the new salt-works at minden, almost . ; at pregny, near geneva, according to auguste de la rive and marcet, notwithstanding that the mouth of the boring is feet above the level of the sea, it is also . feet. this coincidence between the results of a method first proposed by arago in the year ('annuaire du bureau des longitudes', , p. ), for three different mines, of the absolute depths of , , and feet respectively, is remarkable. the two points on the earth, lying at a small vertical distance from each other, whose annual mean temperatures are most accurately known, are probably at the spot on which the paris observatory stands, and the caves de l'observatoire beneath it; the mean temperature of the former is . Ã�¼degrees, and of the latter . Ã�¼degrees, the difference being . Ã�¼degrees for feet, or degree for . feet. (poisson, 'theorie math. de la chaleur', p. and .) in the course of the last seventeen years, from causes not yet perfectly understood, but probably not connected with the actual temperature of the caves, the thermometer standing there has risen very nearly . degrees. although in artesian wells there are sometimes slight errors from the lateral permeation of water, these errors are less injurious to the accuracy of conclusions than those resulting from currents of cold air, which are almost always present in mines. the general result of reich's great work on the temperature of the mines in the saxony mining districts gives a somewhat slower increase of the terrestrial heat, or degree to . feet. (reich, 'beob. uber die temperatur des gesteins in verschielen en tiefen', , s. .) phillips, however, found (pogg., 'annalen', bd. xxxiv., s. ), in a shaft of the coal-mine of monk-wearmouth, near newcastle, in which, as i have already remarked, excavations are going on at a depth of about feet below the level of the sea, an increase of degree to . feet, a result almost identical with that found by arago in the puits de grenell. we must distinguish in our globe three different modes for the transmission of heat. the first is periodic, and affects the temperature of the terrestrial strata according as the heat penetrates from above downward or from below upward, being influenced by the different positions of the sun and the seasons of the year. the second is likewise an effect of the sun, although extremely slow: a portion of the heat that has penetrated into the equatorial regions moves in the interior of the globe toward the poles, where it escapes into the atmosphere and the remoter regions of space. the third mode of transmission is the slowest of all, and is derived from the secular cooling of the globe, and from the small portion of the primitive heat which is still being disengaged from the surface. p this loss experienced by the central heat must have been very considerable in the earliest epochs of the earth's revolutions, but within historical periods it has hardly been appreciable by our instruments. the surface of the earth is therefore situated between the glowing heat of the inferior strata and the universal regions of space, whose temperature is probably below the freezing-point of mercury. the periodic changes of temperature which have been occasioned on the earth's surface by the sun's position and by meteorological processes, are continued in its interior, although to a very inconsiderable depth. the slow conducting power of the ground diminishes this loss of heat in the winter, and is very favorable to deep-rooted trees. points that lie at very different depths on the same vertical line attain the maximum and minimum of the imparted temperature at very different periods of time. the further they are removed from the surface, the smaller is this difference between the extremes. in the latitudes of our temperate zone (between degrees and degrees), the stratum of invariable temperature is at a depth of from to feet, and at half that depth the oscillations of the thermometer, from the influence of the seasons, scarcely amount to half a degree. in tropical climates this invariable stratum is only one foot below the surface, and this fact has been ingeniously made use of by boussingault to obtain a convenient, and as he believes, certain determination of the mean temperature of the air of different places.* [footnote] *boussingault, 'sur la profondeus a laquelle se trouve la couche de temperature invariable, entre les tropiques', in the 'annales de chimie et de physique', t. liii., , p. - . this mean temperature of the air at a fixed point, or at a group of contiguous points on the surface, is to a certain degree the fundamental element of the climate and agricultural relations of a district; but the mean temperature of the whole surface is very different from that of the globe itself. the questions so often agitated, whether the mean temperature has experienced any considerable differences in the course of centuries, whether the climate of a country has deteriorated, and whether the winters have not become milder and the summers cooler, can only be answered by means of the thermometer; this instrument has, however, scarcely been invented more than two centuries and a half, and its scientific application hardly dates back years. the nature and novelty of the means interpose, therefore, very narrow limits to our investigation regarding the temperature p of the air. it is quite otherwise, however, with the solution of the great problem of the internal heat of the whole earth. as we may judge of uniformity of temperature from the unaltered time of vibration of a pendulum, so we may also learn, from the unaltered rotatory velocity of the earth, the amount of stability in the mean temperature of our globe. this insight into the relations between the 'length of the day' and the 'heat of the earth' is the result of one of the most brilliant applications of the knowledge we had long possessed of the planet. the rotatory velocity of the earth depends on its volume; and since, by the gradual cooling of the mass by radiation, the axis of rotation would become shorter, the rotatory velocity would necessarily increase, and the length of the day diminish, with a decrease of the temperature. from the comparison of the secular inequalities in the motions of the moon with the eclipses observed in ancient times, it follows that, since the time of hipparchus, that is, for full years, the length of the day has certainly not diminished by the hundredth part of a second. the decrease of the mean heat of the globe during a period of years has not, therefore, taking the extremest limits, diminished as much as / th of a degree of fahrenheit.* [footnote] *laplace, 'exp. du syst. du monde', p. and ; 'mecanique celeste', t. v., p. and . it should be remarked that the fraction / th of a degree of fahrenheit of the mercurial thermometer, given in the text as the limit of the stability of the earth's temperature since the days of hipparchus, rests on the assumption that the dilation of the substances of which the earth is composed is equal to that of glass, that is to say, / , th for degree. regarding this hypothesis, see arago in the 'annuaire' for , p. - . this invariability of form presupposes also a great invariability in the distribution of relations of density in the interior of the globe. the translatory movements, which occasion the eruptions of our present volcanoes and of ferruginous lava, and the filling up of previously empty fissures and cavities with dense masses of stone, are consequently only to be regarded as slight superficial phenomena affecting merely one portion of the earth's crust, which, from their smallness when compared to the earth's radius, become wholly insignificant. i have described the internal heat of our planet, both with reference to its cause and distribution, almost solely from the results of fourier's admirable investigations. poisson doubts the fact of the uninterrupted increase of the earth's heat p from the surface to the center, and is of opinion that all heat has penetrated from without inward, and that the temperature of the globe depends upon the very high or very low temperature of the regions of space through which the solar temperature of the regions of space, through which the solar system has moved. this hypothesis, imagined by one of the most acute mathematicians of our time, has not satisfied physicists or geologists, or scarcely indeed any one besides its author. but, whatever may be the cause of the internal heat of our planet, and of its limited or unlimited increase in deep strata, it leads us, in this general sketch of nature, through the intimate connection of all primitive phenomena of matter, and through the common bond by which molecular forces are united, into the mysterious domain of magnetism. changes of temperature call forth magnetic and electric currents. terrestrial magnetism, whose main character, expressed in the three-fold manifestation of its forces, is incessant periodic variability, is ascribed either to the heated mass of the earth itself,* or to those galvanic currents which we consider as electricity in motion, that is, electricity moving in a closed circuit.** [footnote] *william gilbert, of colchester, whom galileo pronounced "great to a degree that might be envied," said "magnus magnes ipse est globus terrestris." he ridicules the magnetic mountains of frascatori, the great contemporary of columbus, as being magnetic poles: "rejicienda est vulgaris opinio de montibus magneticis, aut rupe aliqua magnetica, aut polo phantastico a polo mundi distante." he assumes the declination of the magnetic needle at any give point on the surface of the earth to be invariable (variatio uniuscujusque loci constans est), and refers the curvatures of the isogonic lines to the configuration of continents and the relative positions of sea basins, which possess a weaker magnetic force than the solid masses rising above the ocean. (gilbert, 'de magnete', ed. , p. , , and .) [footnote] ** gauss, 'allgemcine theorie des erdmagnetismus', in the 'resultate aux den beob. des magnet. vereins', , s. , p. . the mysterious course of the magnetic needle is equally affected by time and space, by the sun's course, and by changes of place on the earth's surface. between the tropics, the hour of the day may be known by the direction of the needle as well as by the oscillations of the barometer. it is affected instantly, but only transiently, by the distant northern light as it shoots from the pole, flashing in beams of colored light across the heavens. when the uniform horary motion of the needle is disturbed by a magnetic storm, the perturbation manifests itself 'simultaneously', in the strictest sense of the word, over hundreds and thousands of miles of sea and land, or propagates itself by degrees, in short intervals of time, in p every direction over the earth's surface.* [footnote] *there are also perturbations which are of a local character, and do not extend themselves far, and are probably less deep-seated. some years ago i described a rare instance of this kind, in which an extraordinary disturbance was felt in the mines at freiberg, but was not perceptible at berlin. ('lettre de m. de humboldt a son altesse royale le duc de sussex sur les moyens propres a perfectionner la connaissance du magnetisme terrestre', in becquerel's 'traite experimental de l'electricite' t. vii., p. .) magnetic storms which were simultaneously felt from sicily to upsala, did not extend from upsala to alten. (gauss and weber, 'resultate des magnet. vereins', , ; lloyd, in the 'comptes rendus de l'acad. des sciences', t. xii., , sem. ii., p. and .) among the numerous examples that have been recently observed, of perturbations occurring simultaneously and extending over wide portions of the earth's surface, and which are collected in sabine's important work ('observ. on days of unusual magnetic disturbance', ), one of the most remarkable is that of the th of september, , which was observed at toronto in canada, at the cape of good hope, at prague, and partially in van diemen's land. the english sunday, on which it is deemed sinful, after midnight on saturday, to register an observation, and to follow out the great phenomena of creation in their perfect development, interrupted the observations in van diemen's land, where in consequence of the difference of the longitude, the magnetic storm fell on the sunday. ('observ.', p. xiv., , , and .) in the former case, the simultaneous manifestation of the storm may serve, within certain limitations, like jupiter's satellites, fire-signals, and well-observed falls of shooting stars, for the geographical determination of degrees of longitude. we here recognize with astonishment that the perturbations of two small magnetic needles, even if suspended at great depths below the surface, can measure the distances apart at which they are placed, teaching us, for instance, how far kasan is situated east of gottingen or of the banks of the seine. there are also districts in the earth where the mariner, who has been enveloped for many days in mist, without seeing either the sun or stars, and deprived of all means of determining the time, may know with certainty, from the variations in the inclination of the magnetic needle, whether he is at the north or the south of the port he is desirous of entering.* [footnote] *i have described, in lametherie's 'journal de physique', , t. lix., p. , the application (alluded to in the text) of the magnetic inclination to the determination of latitude along a coast running north and south, and which, like that of chili and peru, is for a part of the year enveloped in mist ('garua'). in the locality i have just mentioned, this application is of the greater importance, because, in consequence of the strong current running northward as far as to cape parena, navigators incur a great loss of time if they approach the coast to the north of the haven they are seeking. in the south sea, from callao de lima harbor to truxillo, which differ from each other in latitude by degrees ' i have observed a variation of the magnetic inclination amounting to degrees (centesimal division); and from callao to guayaquil, which differ in latitude by degrees ', a variation of . degrees. (see my 'relat. hist.', t. iii., p. .) at guarmey ( degrees ' south lat.), huaura ( degrees ' south lat.), and chancay ( degrees ' south lat.), huaura ( degrees ' south lat.), and chancay ( degrees ' south lat.), the inclinations are . degrees, degrees, and . degrees of the centesimal division. the determination of position by means of the magnetic inclination has this remarkable feature connected with it, that where the ship's course cuts the isoclinalline almost perpendicularly, it is the only one that is independent of all determination of time, and consequently, of observations of the sun or stars. it is only lately that i discovered, for the first time, that as early as at the close of the sixteenth century, and consequently hardly twenty years after robert norman had invented the inclinatorium, william gilbert, in his great work, 'de magnete', proposed to determine the latitude by the inclination of the magnetic needle. gilbert ('physiologia nova de magnete', lib. v., cap. , p. ) commends the method as applicable "aÃ�Â�re caliginoso." edward wright, in the introduction which he added to his master's great work, describes this proposal as "worth much gold." as he fell into the same error with gilbert, of presuming that the isoclinal lines coincided with the geographical parallel circles, and that the magnetic and geographical equators were identical, he did not perceive that the proposed method had only a local and very limited application. p when the needle, by its sudden disturbance in its horary course, indicates the presence of a magnetic storm, we are still unfortunately ignorant whether the seat of the disturbing cause is to be sought in the earth itself or in the upper regions of the atmosphere. if we regard the earth as a true magnet, we are obliged, according to the views entertained by friedrich gauss (the acute propounder of a generaltheory of terrestrial magnetism), to ascribe to every portion of the globe measuring one eighth of a cubic meter (or / ths of a french cubic foot) in volume, an average amount of magnetism equal to that contained in a magnetic rod of lb. weight.* [footnote[ *gauss and weber, 'resultate des magnet. vereins', , , s. . if iron and nickel, and probably, also, cobalt (but not chrome, as has long been believed),* are the only substances which become permanently magnetic, and retain polarity from a certain coerceive force, the phenomena of arago's magnetism of rotation and of faraday's induced currents show, on the other hand, that all telluric substances may possibly be made transitorily magnetic. according to faraday ('london and edinburgh philosophical magazine', , vol. viii., p. ), pure cobalt is totally devoid of magnetic power. i know, however, that other celebrated chemists (heinrich rose and wohler) do not admit this as absolutely certain. if out of two carefully-purified masses of cobalt totally free from nickel, one appears altogether non-magnetic (in a state of equilibrium), i think it probable that the other owes its magnetic property to a want of purity; and this opinion coincides with faraday's view. according to the experiments of the p first-mentioned of these great physicists, water, ice, glass, and carbon affect the vibrations of the needle entirely in the same manner as mercury in the rotation experiments.* [footnote] *arago, in the 'annales de chimie', t. xxxii., p. ; brewster, 'treaties on magnetism', , p. ; baumgartner, in the 'zeitschrift fur phys. und mathem.', bd. ii., s. . almost all substances show themselves to be, in a certain degree, magnetic when they are conductors, that is to say, when a current of electricity is passing through them. although the knowledge of the attracting power of native iron magnets or loadstones appears to be of very ancient date among the nations of the west, there is strong historical evidence in proof of the striking fact that the knowledge of the directive power of a magnetic needle and of its relation to terrestrial magnetism was peculiar to the chinese, a people living in the extremest eastern portions of asia. more than a thousand years before our era, in the obscure age of codrus, and about the time of the return of the heraclidae to the peloponnesus, the chinese had already magnetic carriages, on which the movable arm of the figure of a man continually pointed to the south, as a guide by which to find the way across the boundless grass plains of tartary; nay, even in the third century of our era, therefore at least years before the use of the mariner's compass in european seas, chinese vessels navigated the indian ocean* under the direction of magnetic needles pointing to the south. [footnote] *humboldt, 'examen critique de l'hist. de la geographie', t. iii., p. . i have shown, in another work, what advantages this means of topographical direction, and the early knowledge and application of the magnetic needle gave the chinese geographers over the greeks and romans, to whom, for instance, even the true direction of the apennines and pyrenees always remained unknown.* [footnote] *'asie centrale', t. i., introduction, p. xxxviii-xlii. the western nations, the greeks and the romans, knew that magnetism could be communicated to iron, 'and that that metal would retain it for a length of time'. ("sola haec materia ferri vires, a maguete lapide accipit, 'retinetque longo tempore." plin., xxxiv., .) the great discovery of the terrestrial directive force depended, therefore, alone on this, that no one in the west had happened to observe an elongated fragment of magnetic iron stone, or a magnetic iron rod, floating, by the aid of a piece of wood, in water, or suspended in the air by a thread, in such a position as to admit of free motion. the magnetic power of our globe is manifested on the terrestrial surface in three classes of phenomena, one of which exhibits itself in the varying intensity of the force, and the two others in the varying direction of the inclination, and in p the horizontal deviation from the terrestrial meridian of the spot. their combined action may therefore be graphically represented by three systems of lines, the 'isodynamic, isoclinic', and 'isogonic' (or those of equal force, equal inclination, and equal declination). the distances apart, and the relative positions of these moving, oscillating, and advancing curves, do not always remain the same. the total deviation (variation or declination of the magnetic needle) has not at all changed, or, at any rate, not in any appreciable degree, during a whole century, at any particular point on the earth's surface,* as, for instance, the western part of the antilles, or spitzbergen. [footnote] *a very slow secular progression, or a local invariability of the magnetic declination, prevents the confusion which might arise from terrestrial influences in the boundaries of land, when, with an utter disregard for the correction of declination, estates are, after long intervals, measured by the mere application of the compass. "the whole mass of the bottomless pit of endless litigation by the invariability of the magnetic declination in jamica and the surrounding archipelago during the whole of the last century, all surveys of property there having been conducted solely by the compass." see robertson in the 'philosophical transactions' for , part ii., p. , 'on the permanency of the compass in jamaica since '. in the mother country (england) the magnetic declination has varied by fully degrees during the period. in like manner, we observe that the isogonic curves, when they pass in their secular motion from the surface of the sea to a continent or an island of considerable extent, continue for a long time in the same position, and become inflected as they advance. these gradual changes in the forms assumed by the lines in their translatory motions, and which so unequally modify the amount of eastern and western declination, in the course of time render it difficult to trace the transitions and analogies of forms in the graphic representations belonging to different centuries. each branch of a curve has its history, but this history does not reach further back among the nations of the west than the memorable epoch of the th of september, , when the re-discoverer of the new world found a line of no variation degrees west of the meridian of the island of flores, one of the azores.* [footnote] *i have elsewhere shown that, from the documents which have come down to us regarding the voyages of columbus, we can, with much certainty, fix upon three places 'in the atlantic line of no declination' for the th of september, , the st of may, , and the th of august, . the atlantic line of no declination at that period ran from northeast to southwest. it then touched the south american continent a little east of cape codera, while it is not observed to reach that continent on the northern coast of the brazils. (humboldt, 'examen critique de l'hist. de la geogr.', t. iii., p. - .) from gilbert's 'physiologia nova de magnete', we see plainly (and the fact is very remarkable) that in the declination was still null in the region of the azores, just as it had been in the time of columbus (lib. , cap. ). i believe that in my 'examen critique' (t. iii., p. ) i have proved from documents that the celebrated line of demarkation by which pope alexander vi. divided the western hemisphere between portugal and spain was not drawn through the most western point of the azores, because columbus wished to convert a physical into a political division. he attached great importance to the zone (raya) "in which the compass shows no variation, where air and ocean, the later covered with pastures of sea-weed, exhibit a peculiar constitution, where cooling winds begin to blow, and where [as erroneous observations of the polar star led him to imagine] the form (sphericity) of the earth is no longer the same." the whole of europe, excepting a small p part of russia, has now a western declination, while at the close of the seventeenth century the needle first pointed due north, in london in , and in paris in , there being thus a difference of twelve years, notwithstanding the small distance between these two places. in eastern russia, to the east of the mouth of the volga, of saratow, nischni-nowgorod, and archangel, the easterly declination of asia is advancing toward us. two admirable observers, hansteen and adolphus erman, have made us acquainted with the remarkable double curvature of the lines of declination in the vast region of northern asia; these being concave toward the pole between obdorsk, on the oby, and turuchansk, and convex between the lake of baikal and the gulf of ochotsk. in this portion of the earth, in northern asia, between the mountains of werchojansk, jakutsk, and the northern korea, the isogonic lines form a remarkable closed system. this oval configuration* recurs regularly and over a great extent of the south sea, almost as far as the meridian of pitcairn and the group of the marquesas islands, between degrees north and degrees p south lat. [footnote] *to determine whether the two oval systems of isogonic lines, so singularly included each within itself, will continue to advance for centuries in the same inclosed form, or will unfold and expand themselves, is a question of the highest interest in the problem of the physical causes of terrestrial magnetism. in the eastern asiatic nodes the declination increases from without inward, while in the node or oval system of the south sea the opposite holds good; in fact, at the present time, in the whole south sea to the east of the meridian of kamt-schatka, there is no line where the declination is null, or, indeed, in which it is less than degrees (erman, in pogg., 'annal.', bd. xxxi, ). yet cornelius schouten, on easter sunday, , appears to have found the declination null somewhere to the southeast of nukahiva, in degrees south lat. and degrees west long., and consequently in the middle of the present closed isogonal system. (hansteen, 'magnet. der erde', Ã�¤ .) it must not be forgotten, in the midst of all these considerations, that we can only follow the direction of the magnetic lines in their progress as they are projected upon the surface of the earth. one would almost be inclined to regard this singular configuration of closed, almost concentric, lines of declination as the effect of a local character of that portion of the globe; but if, in the course of centuries, these apparently isolated systems should also advance, we must suppose, as in the case of all great natural forces, that the phenomenon arises from some general cause. the horary variations of the declination, which, although dependent upon true time, are apparently governed by the sun, as long as it remains above the horizon, diminish in angular value with the magnetic latitude of place. near the equator, for instance, in the island of rawak, they scarcely amount to three or four minutes, while they are from thirteen to fourteen minutes in the middle of europe. as in the whole northern hemisphere the north point of the needle moves from east to west on an average from / in the morning until / at mid-day, while in the southern hemisphere the same north point moves from west to east,* attention has recently been drawn, with much justice, to the fact that there must be a region of the earth between the terrestrial and the magnetic equator where no horary deviations in the declination are to be observed. [footnote] *arago, in the 'annuaire', , p. , and , p. - . this fourth curve, which might be called the 'curve of no motion', or, rather, 'the line of no variation of horary declination', has not yet been discovered. the term 'magnetic poles' has been applied to those points of the earth's surface where the horizontal power disappears, and more importance has been attached to these points than properly appertains to them;* and in like manner, the curve, where the inclination of the needle is null, has been termed the 'magnetic equator'. [footnote] *gauss, 'allg. theorie des erdmagnet.', . the position of this line and its secular change of configuration have been made an object of careful investigation in modern times. according to the admirable work of duperrey,* who crossed the magnetic equator six times between and , the nodes of the two equators, that is to say, the two points at which the line without inclination intersects the terrestrial equator, and consequently passes from one henisphere into the other, are so unequally placed, that in the node near the island of st. thomas, on the western p coast of africa, was / degrees distant from the node in the south sea, close to the little islands of gilbert, nearly in the meridian of the viti group. [footnote] *duperrey, 'de la configuration de l'equateur magnetique', in the 'annales de chimie', t. xlv., p. and . (see also, morlet, in the 'memoires presentes par divers savans a l'acad. roy. des sciences', t. iii., p. . in the beginning of the present century, at an elevation of , feet above the level of the sea, i made an astronomical determination of the point ( degrees ' south lat., degrees ' west long. from paris), where, in the interior of the new continent, the chain of the andes is intersected by the magnetic equator between quito and lima. to the west of this point, the magnetic equator continues to traverse the south sea in the southern hemisphere, at the same time slowly drawing near the terrestrial equator. it first passes into the northern hemisphere a little before it approaches the indian archipelago, just touches the southern points of asia, and enters the african continent to the west of socotora, almost in the straits of bab-el-mandeb, where it is most distant from the terrestrial equator. after intersecting the unknown regions of the interior of africa in a southwest direction, the magnetic equator re-enters the south tropical zone in the gulf of guinea, and retreats so far from the terrestrial equator that it touches the brazilian coast near os ilheos, north of porto seguro, in degrees south lat. from thence to the elevated plateaux of the cordilleras, between the silver mines of micuipampa and caxamarca, the ancient seat of the incas, where i observed the inclination, the line traverses the whole of south america, which in these latitudes is as much a magnetic 'terra incognita' as the interior of africa. the recent observations of sabine* have shown that the node near the island of st. thomas has moved degrees from east to west between and . [footnote] *see the remarkable chart of isoclinic lines in the atlantic ocean for the years and , in sabine's 'contributions to terrestrial magnetism', , p. . it would be extremely important to know whether the opposite pole, near the gilbert islands, in the south sea, has aproached the meridian of the carolinas in a westerly direction. these general remarks will be sufficient to connect the different systems of isoclinic non-parallel lines with the great phenomenon of equilibrium which is manifested in the magnetic equator. it is no small advantage, in the exposition of the laws of terrestrial magnetism, that the magnetic equator (whose oscillatory change of form and whose nodal motion exercise an influence on the inclination of the needle in the remotest districts of the world, in consequence of the altered magnetic latitudes)* should traverse the p ocean throughout its whole course, excepting about one fifth, and consequently be made so much more accessible, owing to the remarkable relations in space between the sea and land, and to the means of which we are now possessed for determining with much exactness both the declination and the inclination at sea. [footnote] *humboldt, 'ueber die seculÃ�Â�re verÃ�Â�nderung der magnetischen inclination' (on the secular change in the magnetic inclination), in pogg. 'annal.', bd. sv., s. . we have described the distribution of magnetism on the surface of our planet according to the two forms of 'declination' and 'inclination'; it now, therefore, remains for us to speak of the 'intensity of the force' which is graphically expressed by isodynamic curves (or lines of equal intensity). the investigation and measurement of this force by the oscillations of a vertical or horizontal needle have only excited a general and lively interest in its telluric relations since the beginning of the nineteenth century. the application of delicate optical and chronometrical instruments has rendered the measurement of this horizontal power susceptible of a degree of accuracy far surpassing that attained in any other magnetic determinations. the isogonic lines are the more important in their immediate application to navigation, while we find from the most recent views that isodynamic lines, especially those which indicate the horizontal force, are the most valuable elements in the theory of terrestrial magnetism.* [footnote] *gauss, 'resultate der beob. des magn. vereins', , ; sabine, 'report on the variations of the magnetic intensity', p. . one of the earliest facts yielded by observation is, that the intensity of the total force increases from the equator toward the pole.* [footnote] *the following is the history of the discovery of the law that the intensity of the force increases (in general) with the magnetic latitude. when i was anxious to attach myself, in , to the expedition of captain bandin, who intended to circumnavigate the globe, i was requested by borda, who took a warm interest in the success of my project, to examine the oscillations of a vertical needle in the magnetic meridian in different latitudes in each hemisphere, in order to determine whether the intensity of the force was the same, or whether it varied in different places. during my travels in the tropical regions of america, i paid much attention to this subject. i observed that the same needle, which in the space of ten minutes made oscillations in paris, in the havana, and in mexico, performed only oscillations during the same period at st. carlos del rio negro ( degree ' north lat. and degrees ' west long. from paris), on the magnetic equator, i.e., the line in which the inclination = ; in peru ( degrees ' south lat. and degrees ' west long. from paris) only ;while at lima ( degrees ' south lat.) the number rose to . i found, in the years intervening between and , that the whole force, if we assume it at . on the magnetic equator in the peruvian andes, between micuipampa and caxamarca, may be expressed at paris by . , in mexico by . , in san carlos del rio negro by . , and in lima by . . when i developed this law of the variable intensity of terrestrial magnetic force, and supported it by the numerical value of observations instituted in different places, in a memoir read before the paris institute on the th frimaire, an. xiii. (of which the mathematical portion was contributed by m. biot), the facts were regarded as altogether new. it was only after the reading of the paper, as biot expressly states (lametherie, 'journal de physique', t. lix., p. , note ) and as i have repeated in 'the relation historique', t. i., p. , note , that m. de rossel communicated to biot his oscillation experiments made six years earlier (between and ) in van diemen's land, in java, and in amboyna. these experiments gave evidence of the same law of decreasing force in the indian archipelago. it must, i think be supposed, that this excellent man, when he wrote his work, was not aware of the regularity of the augmentation and diminution of the intensity as before the reading of my paper he never mentioned this (certainly not unimportant) physical law to any of our mutual friends, la place, delambre, prony, or biot. it was not till , four years after my return from america that the observations made by m. de rossel were published in the 'voyage de l'entrecasteaux', t. ii., p. , , , , and . up to the present day it is still usual, in all the tables of magnetic intensity which have been published in germany (hausteen, 'magnet. der erde', , s. ; gauss, 'beob. des magnet. vereins', , s. - ; erman, 'physikal. beob.', , s. - ), in england (sabine, 'report on magnet. intensity', , p. - ; 'contributions to terrestrial magnetism', ), and in france (becquerel, 'traite de electr. et de magnet.', t. vii., p. - ), to reduce the oscillations observed in any part of the earth to the standard of force which i found on the magnetic equator in northern peru, so that, according to the unit thus arbitrarily assumed, the intensity of the magnetic force at paris is put down as . . the observations made by lamanon in the unfortunate expedition of la perouse, during the stay at teneriffe ( ), and on the voyage to macao ( ), are still older than those of admiral rossel. they were sent to the academy of sciences, and it is known that they were in the possession of condorcet in the july of (becquerel, t. vii., p. ); but, notwithstanding the most careful search, they are not now to be found. from a copy of a very important letter of lamanon, now in the possession of captain duperrey, which was addressed to the then perpetual secretary of the academy of sciences, but was omitted in the narrative of the 'voyage de la perouse', it is stated "that the attractive force of the magnet is less in the tropics than when we approach the poles, and that the magnetic intensity deduced from the number of oscillations of the needle of the inclination-compass varies and increases with the latitude." if the academicians, while they continued to expect the return of the unfortunate la perouse, had felt themselves justified, in the course of , in publishing a truth which had been independently discovered by no less than three different travelers, the theory of terrestrial magnetism would have been extended by the knowledge of a new class of observations, dating eighteen years earlier than they now do. this simple statement of facts may probably justify the observations contained in the third volume of my 'relation historique' p. ): "the observations on the variation of terrestrial magnetism, to which i have devoted myself for thirty-two years, by means of instruments which admit of comparison with one another, in america, europe, and asia, embrace an area extending over degrees of longitude, from the frontier of chinese dzoungarie to the west of the south sea bathing the coasts of mexico and peru, and reaching from degrees north lat. to degrees south lat. i regard the discovery of the law of the decrement of magnetic force from the pole to the equator as the most important result of my american voyage." although not absolutely certain, it is very probable that condorcet read lamanon's letter of july, , at a meeting of the paris academy of sciences; and such a simple reading i regard as a sufficient act of publication. ('annuaire du bureau des longitudes', , p. .) the first recognition of the law belongs, therefore, beyond all question, to the comparison of la perouse; but, long disregarded or forgotten, the knowledge of the law that the intensity of the magnetic force of the earth varied with the latitude, did not, i conceive, acquire an existence in science until the publication of my observations from to . the object and the length of this note will not be indifferent to those who are familiar with the connection with it, and who, from their own experience, are aware that we are apt to attach some value to that which has cost us the uninterrupted labor of five years, under the pressure of a tropical climate, and of perilous mountain expeditions. p the knowledge which we possess of the quantity of this increase, and of all the numerical relations of the law of intensity p affecting the whole earth, is especially due, since , to the unwearied activity of edward sabine, who, after having observed the oscillations of the same needles at the american north pole, in greenland, at spitzbergen, and on the coasts of guinea and brazil, has continued to collect and arrange all the facts capable of explaining the direction of the isodynamic system in zones for a small part of south america. these lines are not parallel to lines of equal inclination (isoclinic line), and the intensity of the force is not at its minimum at the magnetic equator, as has been supposed, nor is it even equal at all parts of it. if we compare erman's observations in the southern part of the atlantic ocean, where a faint zone ( . ) extends from angola over the island of st. helena to the brazilian coast, with the most recent investigations of the celebrated navigator james clark ross, we shall find that on the surface of our planet the force increases almost in the relation of : toward the magnetic south pole, where victoria land extends from cape crozier toward the volcano erebus, which has been raised to an elevation of , feet above the ice.* [footnote] *from the observations hitherto collected, it appears that the maximum of intensity for the whole surface of the earth is . , and the minimum . . both phenomena occur in the southern hemisphere; the former in degrees ' s. lat., and degrees 'e. long. from paris, near mount crozier, west-northwest of the south magnetic pole, at a place where captain james ross found the inclination of the needle to be degrees ' (sabine, 'contributions to terrestrial magnetism', , no. , p. ); the latter, observed by erman at degrees ' s. lat., and degrees ' w. long. from paris, miles eastward from the brazilian coast of espiritu santo (erman, 'phys. beob.', , s. ), at a point where the inclination is only degrees '. the actual ratio of the two intensities is therefore as to . . it was long believed that the greatest intensity of the magnetic force was only two and a half times as great as the weakest exhibited on the earth's surface. (sabine, 'report on magnetic intensity', p. .) if the intensity near the magnetic south pole p be expressed by . (the unit still employed being the intensity which i discovered on the magnetic equator in northern peru), sabine found it was only . at the magnetic north pole near melville island ( degrees ' north lat.), while it is . at new york, in the united states, which has almost the same latitude as naples. the brilliant discoveries of oersted, arago, and faraday have established a more intimate connection between the electric tension of the atmosphere and the magnetic tension of our terrestrial globe. while oestred has discovered that electricity excites magnetism in the neighborhood of the conducting body, faraday's experiments have elicited electric currents from the liberated magnetism. magnetism is one of the manifold forms under which electricity reveals itself. the ancient vague presentiment of the identity of electric and magnetic attraction has been verified in our own times. "when electrum (amber)," says pliny, in the spirit of the ionic natural philosophy of thales,* is 'animated' by friction and heat, it will attract bark and dry leaves precisely as the loadstone attracts iron." [footnote] *of amber (succinum, glessum) pliny observes (xxxvii., ), "genera ejus plura. attritu digitorum accepta caloris anima trahunt in se paleas ac folia arida quae levia sunt, ac ut magnes lapis ferri ramenta quoque." (plato, 'in timaeo', p. . martin, 'etude sur le timee', t. ii., p. - . strabo, xv., p. , casaub,; clemens alex., 'strom.', ii., p. , where, singularly enough, a difference is made between [greek words]) when thales, in aristot., 'de anima', , , and hippias, in diog. laert., i., , describe the magnet and amber as possessing a soul, they refer only to a moving principle. the same words may be found in the literature of an asiatic nation, and occur in a eulogium on the loadstone by the chinese physicist kuopho.* [footnote] *"the magnet attracts iron as amber does the smallest grain of mustard seed. it is like a breath of wind which mysteriously penetrates through both, and communicates itself with the rapidity of an arrow." these are the words of kuopho, a chinese panegyrist on the magnet, who wrote in the beginning of the fourth century. (klaproth, 'lettre a m. a. de humboldt, sur l'invention de la boussole', , p. .) i observed with astonishment, p on the woody banks of the orinoco, in the sports of the natives, that the excitement of electricity by friction was known to these savage races, who occupy the very lowest place in the scale of humanity. children may be seen to rub the dry, flat, and shining seeds or husks of a trailing plant (probably a 'negretia') until they are able to attract threads of cotton and pieces of bamboo cane. that which thus delights the naked copper-colored indian is calculated to awaken in our minds a deep and earnest impression. what a chasm divides the electric pastime of these savages from the discovery of a metallic conductor discharging its electric shocks, or a pile composed of many chemically-decomposing substances, or a light-engendering magnetic apparatus! in such a chasm lie buried thousands of years that compost the history of the intellectual development of mankind! the incessant change or oscillatory motion which we discover in all magnetic phenomena, whether in those of the inclincation, declination, and intensity of these forces, according to the hours of the day and the night, and the seasons and the course of the whole year, leads us to conjecture the existence of very various and partial systems of electric currents on the surface of the earth. are these currents, as in seebeck's experiments, thermo-magnetic, and excited directly from unequal distribution of heat? or should we not rather regard them as induced by the position of the sun and by solar heat?* [footnote] *"the phenomena of periodical variations depend manifestly on the action of solar heat, operating probably through the medium of thermo-electric currents induced on the earth's surface. beyond this rude guess, however, nothing is as yet known of their physical cause. it is even still a matter of speculation whether the solar influence be a principal or only a subordinate cause in the phenomena of terrestrial magnetism." ('observations to be made in the antarctic expedition', , p. .) have the rotation of the planets, and the different degrees of velocity which the individual zones acquire, according to their respective distances from the equator, any influence on the distribution of magnetism? must we seek the seat of these currents, that is to say, of the disturbed electricity, in the atmosphere, in the regions of planetary space, or in the polarity of the sun and moon? galileo, in his celebrated 'dialogo', was inclined to ascribe the parallel direction of the axis of the earth to a magnetic point of attraction seated in universal space. if we represent to ourselves the interior of the earth as fused and undergoing an enormous pressure, and at a degree of temperature the amount of which we are unable to assign, p we must renounce all idea of a magnetic nucleus of the earth. all magnetism is certainly not lost until we arrive at a white heat,* and it is manifested when iron is at a dark red heat, however different, therefore, the modifications may be which are excited in substances in their molecular state, and in the coercive force depending upon that condition in experiments of this nature, there will still remain a considerable thickness of the terrestrial stratum, which might be assumed to be the seat of magnetic currents. [footnote] *barlow, in the 'philos. trans.' for , pt. i., p. ; sir david brewster, 'treatise on magnetism', p. . long before the times of gilbert and hooke, it was taught in the chinese work 'ow-thea-tsou' that heat diminished the directive force of the magnetic needle. (klaproth, 'lettre a m. a. de humboldt, sur l'invention de la boussole', p. .) the old explanation of the horary variations of declination by the progressive warming of the earth in the apparent revolution of the sun from east to west must be limited to the uppermost surface, since thermometers sunk into the earth, which are now being accurately observed at so many different places, show how slowly the solar heat penetrates even to the inconsiderable depth of a few feet. moreover, the thermic condition of the surface of water, by which two thirds of our planet is covered, is not favorable to such modes of explanation, when we have reference to an immediate action and not to an effect of induction in the aÃ�Â�rial and aqueous investment of our terrestrial globe. in the present condition of our knowledge, it is impossible to afford a satisfactory reply to all questions regarding the ultimate physical causes of these phenomena. it is only with reference to that which presents itself in the triple manifestations of the terrestrial force, as a measurable relation of space and time, and as a stable element in the midst of change, that science has recently made such brilliant advances by the aid of the determination of mean numerical values. from toronto in upper canada to the cape of good hope and van diemen's land, from paris to pekin, the earth has been covered, since , with magnetic observatories,* in which every regular p or irregular manifestation of the terrestrial force is detected by uninterrupted and simultaneous observations. a variation p of / th of the magnetic intensity is measured, and at certain epochs, observations are made at intervals of / minutes, and continued for twenty-four hours consecutively. [footnote] *as the first demand for the establishment of these observatories (a net-work of stations, provided with similar instruments) proceeded from me, i did not dare to cherish the hope that i should live long enough to see the time when both hemispheres should be uniformly covered with magnetic houses under the associated activity of able physicists and astronomers. this has, however, been accomplished, and chiefly through the liberal and continued support of the russian and british governments. [footnote continues] in the years and , i and my friend and fellow-laborer, herr oltmanns, while at berlin, observed the movements of the needle, especially at the times of the solstices and equinoxes, from hour to hour, and often from half hour to half hour, for five or six days and nights uninterruptedly. i had persuaded myself that continuous and uninterrupted observations of several days and nights (observatio perpetua) were preferable to the single observations of many months. the apparatus, a prony's magnetic telescope, suspended in a glass case by a thread devoid of torsion, allowed angles of seven or eight seconds to be read off on a finely-divided scale, placed at a proper distance, and lighted at night by lamps. magnetic perturbations (storms), which occasionally recurred at the same hour on several successive nights, led me even then to desire extremely that similar apparatus should be used to the east and west of berlin, in order to distinguish general terrestrial phenomena from those which are mere local disturbances, depending on the inequality of heat in different parts of the earth, or on the cloudiness of the atmosphere. my departure to paris, and the long period of political disturbance that involved the whole of the west of europe, prevented my wish from being then accomplished. (oersted's great discovery ( ) of the intimate connection between electricity and magnetism again excited a general interest (which had long flagged) in the periodical variations of the electro-magnetic tension of the earth. arago, who many years previously had commenced in the observatory at paris, with a new and excellent declination instrument by gambey, the longest uninterrupted series of horary observations which we possess in europe, showed by a comparison with simultaneous observations of perturbation made at kasan, what advantages might be obtained from corresponding measurements of declination. when i returned to berlin, after an eighteen years' residence in france, i had a small magnetic house erected in the autumn of , not only with the view of carrying on the work commenced in , but more with the object that simultaneous observations at hours previously determined might be made at berlin, paris, and freiburg, at a depth of fathoms below the surface. the simultaneous occurrence of the perturbations, and the parallelism of the movements for october and december, , were then graphically represented. (pogg., 'annalen', bd. xix., s. , taf. i.-iii.) an expedition into northern asia, undertaken in , by command of the emperor of russia, soon gave me an opportunity of working out my plan on a larger scale. the plan was laid before a select committee of one of the imperial academies of science, and, under the protection of the director of the mining department, count von cancrin, and the excellent superintendence of professor kupffer, magnetic stations were appointed over the whole of northern asia, from nicolajeff, in the line through catharinenburg, barnaul, and nertschinsk, to pekin. [footnote continues] the year ('gottinger gelehrte anzeigen', st. ) is distinguished as the great epoch in which the profound author of a general theory of terrestrial magnetism, friedrich gauss, erected apparatus, constructed on a new principle, in the gottingen observatory. the magnetic observatory was finished in , and in the same year gauss distributed new instruments, with instructions for their use, in which the celebrated physicist, wilhelm weber, took extreme interest, over a large portion of germany and sweden, and the whole of italy. ('resultate der beob. des magnetischen verceins in jahr' , s. , and poggend., 'annalen.' bd. xxxiii., s. .) in the magnetic association that was now formed with gottingen for its center, simultaneous observations have been undertaken four times a year since , and continued uninterruptedly for twenty-four hours. the periods, however, do not coincide with those of the equinoxes and solstices, which i had proposed and followed out in . up to this period, great britain, in possession of the most extensive commerce and the largest navy in the world, had taken no part in the movement which since had begun to yield important results for the more fixed ground-work of terrestrial magnetism. i had the good fortune, by a public appeal from berlin which i sent in april , to the duke of sussex, at that time president of the royal society (lettre de m. de humboldt a s. a. r. le duc de sussex, sur les moyens propres a perfectionner la connaissance du magnetisme terrestre par l'establissement des stations magnetiques et d'observations correspondantes), to excite a friendly interest in the undertaking which it had so long been the chief object of my wish to carry out. in my letter to the duke of sussex i urged the establishment of permanent stations in canada, st. helena, the cape of good hope, the isle of france, ceylon, and new holland, which five years previously i had advanced as good positions. the royal society appointed a joint physical and meteorological committee, which not only proposed to the government the establishment of fixed magnetic observatories in both hemispheres, but also the equipment of a naval expedition for magnetic observations in the antarctic seas. it is needless to proclaim the obligations of science to the great activity of sir john herschel, sabine, airy, and lloyd, as well as the powerful support that was afforded by the british association for the advancement of science at their meeting held at newcastle in . in june, , the antarctic magnetic expedition, under the command of captain james clark ross, was fully arranged; and now, since its successful return, we reap the double fruits of the highly important geographical discoveries around the south pole, and a series of simultaneous observations at eight or ten magnetic stations. a great english astronomer and physicist has calculated* that the mass of observations which are in progress will accumulate in the course of three years to , , . [footnote] *see the article on 'terrestrial magnetism', in the 'quarterly review' , vol. lxvi., p. - . never before has so noble and cheerful a spirit presided over the inquiry into the 'quantitative' relations of the laws of the phenomena of nature. we are, therefore, justified in hoping that these laws, when compared with those which govern the atmosphere and the remoter regions of space, may, by degrees, lead us to a more intimate acquaintance with the genetic conditions of magnetic phenomena. as yet we can only boast of having opened a greater number of paths which may possibly lead to an explanation of this subject. in the physical science of terrestrial p magnetism, which must not be confounded with the purely mathematical branch of the study, those persons only will obtain perfect satisfaction who, as in the science of the meteorological processes of the atmosphere conveniently turn aside the practical bearing of all phenomena that can not be explained according to their own views. terrestrial magnetism, and the electro-dynamic forces computed by the intellectual ampere,* stand in simultaneous and intimate connection with the terrestrial or polar light, as well as with the internal and external heat of our planet, whose magnetic poles may be considered as the poles of cold.** [footnote] *instead of ascribing the internal heat of the earth to the transition of matter from a vapor-like fluid to a solid condition, which accompanies the formation of the planets, ampere has propounded the idea, which i regard as highly improbable, that the earth's temperature may be the consequence of the continuous chemical action of a nucleus of the metals of the earths and alkalies on the oxydizing external crust. "it can not be doubted," he observes in his masterly 'theorie des phenomenes electro-dynamiques', , p. , "that electro-magnetic currents exist in the interior of the globe, and that these currents are the cause of its temperature. they arise from the action of a central metallic nucleus, composed of the metals discovered by sir humphrey davy, acting on the surrounding oxydized layer." [footnote] **the remarkable connection between the curvature of the magnetic lines and that of my isothermal lines was first detected by sir david brewster. see the 'transactions of the royal society of edinburgh', vol. ix., , p. , and 'treatise on magnetism', , p. , , , and . this distinguished physicist admist two cold poles (poles of maximum cold) in the northern hemisphere, an american one near cape walker ( degrees lat., degrees w. long.), and an asiatic one ( degrees lat., degrees e. long.); whence arise, according to him, two hot and two cold meridians, i.e., meridians of greatest heat and cold. even in the sixteenth century, acosts ('historia natural de las indias', , lib. i., cap. ), grounding his opinion on the observations of a very experienced portuguese pilot, taught that there were four lines without declination. it would seem from the controversy of henry bond (the author of 'the longitude found', ) with beckborrow, that this view in some measure influenced halley in his theory of four magnetic poles. see my 'examen critique de l'hist. de la geographie', t. iii., p. . the bold conjecture hazarded one hundred and twenty-eight years since by halley,* that the aurora borealis was a magnetic phenomenon, has acquired empirical certainty from faraday's brilliant discovery of the evolution of light by magnetic forces. [footnote] *halley, in the 'philosophical transactions', vol. xxix. (for - ), no. . the northern light is preceded by premonitory signs. thus, in the morning before the occurrence of the phenomenon, the irregular horary course of the magnetic needle generally indicates a disturbance of the equilibrium in the distribution of p terrestrial magnetism.* [footnote] *[the aurora borealis of october th, , which was one of the most brilliant ever known in this country, was preceded by great magnetic disturbance. on the d of october the maximum of the west declination was degrees '; on the d the position of the magnet was continually changing, and the extreme west declinations were between degrees ' and degrees ';on the night between the d and th of october, the changes of position were very large and very frequent, the magnet at times moving across the field so rapidly that a difficulty was experienced in following it. during the day of the th of october there was a constant change of position, but after midnight, when the aurora began perceptibly to decline in brightness, the disturbance entirely ceased. the changes of position of the horizontal-force magnet were as large and as frequent as those of the declination magnet, but the vertical-force magnet was at no time so much affected as the other two instruments. see 'on the aurora borealis, as it was seen on sunday evening, october th, , at blackheath,' by james glaisher, esq., of the royal observatory, greenwich, in the 'london, edinburgh, and dublin philos. mag and journal of science for nov.', , by john h. morgan, esq. we must not omit to mention that magnetic disturbance is now registered by a 'photographic' process: the self-registering photographic apparatus used for this purpose in the observatory at greenwich was designed by mr. brooke, and another ingenious instrument of this kind has been invented by mr. f. ronalds, of the richmond observatory.] -- tr. when this disturbance attains a great degree of intensity, the equilibrium of the distribution is restored by a discharge attended by a development of light "the aurora* itself is, therefore, not to be regarded as an externally manifested cause of this disturbance, but rather as a result of telluric activity, manifested on the one side by the appearance of the light, and on the other by the vibrations of the magnetic needle." [footnote] *dove, in poggend., 'annalen', bd. xx., s. ; bd. xix., s. . "the declination needle acts in very nearly the same way as an atmospheric electrometer, whose divergence in like manner shows the increased tension of the electricity before this has become so great as to yield a spark." see also, the excellent observations of professor kÃ�Â�wmtz, in his 'lehrbuch der meteorologie', bd. iii., s. - , and sir david brewster, in his 'treatise on magnetism', p. . regarding the magnetic properties of the galvanic flame, or luminous arch from a bunsen's carbon and zinc battery, see casselmann's 'beobachtungen' (marburg, ), s. - . the splendid appearance of colored polar light is the act of discharge, the termination of a magnetic storm, as in an electrical storm a development of light -- the flash of lightning -- indicates the restoration of the disturbed equilibrium in the distribution of the electricity. an electric storm is generally confined to a small space beyond the limits of which the condition of the atmospheric electricity remains unchanged. a magnetic storm, on the other hand, p shows its influence on the course of the needle over large portions of continents, and, as arago first discovered far from the spot where the evolution of light was visible. it is not improbable that, as heavily-charged threatening clouds, owing to frequent transitions of the atmospheric electricity to an opposite condition, are not always discharged, accompanied by lightning, so likewise magnetic storms may occasion far-extending disturbances in the horary course of the needle, without there being any positive necessity that the equilibrium of the distribution should be restored by explosion, or by the passage of luminous effusions from one of the poles to the equator, or from pole to pole. in collecting all the individual features of the phenomenon in one general picture, we must not omit to describe the origin and course of a perfectly developed aurora borealis. low down in the distant horizon, about the part of the heavens which is intersected by the magnetic meridian, the sky which was previously clear is at once overcast. a dense wall of bank of cloud seems to rise gradually higher and higher, until it attains an elevation of or degrees. the color of the dark segment passes into brown or violet; and stars are visible through the cloudy stratum, as when a dense smoke darkens the sky. a broad, brightly-luminous arch, first white, then yellow, encircles the dark segment; but as the brilliant arch appears subsequently to the smoky gray segment, we can not agree with argelander in ascribing the latter to the effect of mere contrast with the bright luminous margin.* [footnote] *argelander, in the important observations on the northern light embodied in the 'vortrÃ�Â�gen gehalten in der physikalish-okonomischen gessellschaft zu konigsberg', bd. i., , s. - . the highest point of the arch of light is, according to accurate observations made on the subject,* not generally in the magnetic meridian itself, but from degrees to degrees toward the direction of the magnetic declination of the place.** [footnote] *for an account of the results of the observations of lottin, bravais, and siljerstrom, who spent a winter at bosekop, on the coast of lapland ( degrees n. lat.), and in nights saw the northern lights times, see the 'comptes rendus de l'acad. des sciences', t. x., p. , and martins's 'meteorologie', , p. . see also, argelander in the 'vortragen geh. in der konigsberg gessellschaft', bd. i., s. . [footnote] **[professor challis of cambridge, states that in the aurora of october th, , the streamers all converged toward a single point of the heavens, situated in or very near a vertical circle passing through the magnetic pole. around this point a corona was formed, the rays of which diverged in all directions from the center, leaving a space free from light: its azimuth was degrees ' from south to east, and its altitude degrees '. see professor challis, in the 'athenaeum', oct. , .] -- tr. in the northern latitudes, p in the immediate vicinity of the magnetic pole, the smoke-like conical segment appears less dark, and sometimes is not even seen. where the horizontal force is the weakest, the middle of the luminous arch deviates the most from the magnetic meridian. the luminous arch remains sometimes for hours together flashing and kindling in ever-varying undulations, before rays and streamers emanate from it, and shoot up to the zenith. the more intense the discharges of the northern light, the more bright is the play of colors, through all the varying gradations from violet and bluish white to green and crimson. even in ordinary electricity excited by friction, the sparks are only colored in cases where the explosion is very violent after great tension. the magnetic columns of flame rise eithr singly from the luminous arch, blended with black rays similar to thick smoke, or simultaneously in many opposite points of the horizon, uniting together to torm a flickering sea of flame, whose brilliant beauty admits of no adequate description, as the luminous waves are every moment assuming new and varying forms. the intensity of this light is at times so great, that lowenorn (on the th of june, ) recognized the coruscation of the polar light n bright sunshine. motion renders the phenomenon more visible. round the point in the vault of heaven which corresponds to the direction of the inclination of the needle, the beams unite together to form the so-called corona, the crown of the northern light, which encircles the summit of the heavenly canopy with a milder radiance and unflickering emanations of light. it is only in rare instances that a perfect crown or circle is formed, but on its completion the phenomenon has invariably reached its maximum, and the radiations become less frequent, shorter, and more colorless. the crown and the luminous arches break up, and the whole vault of heaven becomes covered with irregularly-scattered, broad, faint, almost ashy-gray luminous immovable patches, which in their turn disappear, leaving nothing but a trace of the dark, smoke-like segment on the horizon. there often remains nothing of the whole spectacle but a white, delicate cloud with feathery edges, or divided at equal distances into small roundish groups like cirio-cumuli. this connection of the polar light with the most delicate cirrous clouds deserves special attention, because it shows that the electro-magnetic evolution of light is a part of a meteorological process. terrestrial magnetism here manifests its influence p on the atmosphere and on the condensation of aqueous vapor. the fleecy clouds seen in iceland by thienemann, and which he considered to be the northern light, have been seen in recent times by franklin and richardson near the american north pole, and by admiral wrangel on the siberian coast of the polar sea. all remarked "that the aurora flashed forth in the most vivid beams when masses of cirrous strata were hovering in the upper regions of the air, and when these were so thin that their presence could only be recognized by the formation of a halo round the moon." these clouds sometimes range themselves, even by day in a similar manner to the beams of the aurora, and then disturb the course of the magnetic needle in the same manner as the latter. on the morning after every distinct nocturnal aurora, the same superimposed strata of clouds have still been observed that had previously been luminous.* [footnote] *john franklin, 'narrative of a journey to the shores of the polar sea, in the years - ', p. and ; thienemann in the 'edinburgh philosophical journal', vol. xx., p. ; farquharson, in vol. vi., p. , of the same journal; wrangel, 'phys. beob.', s. . parry even saw the great arch of the northern light continue throughout the day. ('journal of the royal institution of great britain', , jan., p. .) the apparently converging polar zones (streaks of clouds in the direction of the magnetic meridian), which constantly occupied my attention during my journeys on the elevated plateaux of mexico and in northern asia, belong probably to the same group of ciurnal phenomena.* [footnote] *on my return from my american travels, i described the delicate cirro-cumulus cloud, which appears uniformly divided, as if by the action of repulsive forces, under the name of polar bands ('bandes polaires'), because their perspective point of convergence is mostly at first in the magnetic pole, so that the parallel rows of fleecy clouds follow the magnetic meridian. one peculiarity of this mysterious phenomenon is the oscillation, or occasionally the gradually progressive motion, of the point of convergence. it is usually observed that the bands are only fully developed in one region of the heavens, and they are seen to move first from south to north, and then gradually from east to west. i could not trace any connection between the advancing motion of the bands and alterations of the currents of air in the higher regions of the atmosphere. they occur when the air is extremely calm and the heavens are quite serene, and are much more common under the tropics than in the temperate and frigid zones. i have seen this phenomenon on the andes, almost under the equator, at an elevation of , feet, and in northern asia, in the plains of krasnojarski, south of buchtarminsk, so similarly developed, that we must regard the influences producing it as very widely distributed, and as depending on general natural forces. see the important observations of kamtz ('vorlesungen uber meteorologie', , s. ), and the more recent ones of martins and bravais ('meteorologie', , p. ). in south polar bands, composed of very delicate clouds, observed by arqago at paris on the d of june, , dark rays shot upward from an arch running east and west. we have already made mention of black rays, resembling dark smoke, as occurring in brilliant nocturnal northern lights. p southern lights have often been seen in england by the intelligent and indefatigable observer dalton and northern lights have been observed in the southern hemisphere as far as degrees latitude (as on the th of january, ). on occasions that are by no means of rare occurrence, the equilibrium at both poles has been simultaneously disturbed. i have discovered with certainty that northern polar lights have been seen within the tropics in mexico and peru. we must distinguish between the sphere of simultaneous visibility of the phenomenon and the zones of the earth where it is seen almost nightly. every observer no doubt sees a separate aurora of his own, as he sees a separate rainbow. a great portion of the earth simultaneously engenders these phenomena of emanations of light. many nights may be instanced in which the phenomenon has been simultaneously observed in england and in pennsylvania, in rome and in pekin. when it is stated that auroras diminish with the decrease of latitude, the latitude must be understood to be magnetic, and as measured by its distance from the magnetic pole. in iceland, in greenland, newfoundland, on the shores of the slave lake, and at fort enterprise in northern canada, these lights appear almost every night at certain seasons of the year, celebrating with their flashing beams, according to the mode of expression common to the inhabitants of the shetland isles, "a merry dance in heaven."* [footnote] *the northrn lights are called by the shetland islanders "the merry dancers." (kendal, in the 'quarterly journal of science', new series, vol. iv., p. .) while the aurora is a phenomenon of rare occurrence in italy, it is frequently seen in the latitude of philadelphia ( degrees '), owing to the southern position of the american nagnetic pole. in the districts which are remarkable, in the new continent and the siberian coasts, for the frequent occurrence of this phenomenon, there are special regions or zones of longitude in which the polar light is particularly bright and brilliant.* [footnote] *see muncke's excellent work in the new edition of gehler's 'physik worterbuch', bd. vii., i., s - , and especially s. . the existence p of local influences can not, therefore, be denied in these cases. wrangel saw the brilliancy diminish as he left the shores of the polar sea, about mischne-kolymsk. the observations made in the north polar expedition appear to prove that in the immediate vicinity of the magnetic pole the development of light is not in the least degree more intense or frequent than at some distance from it. the knowledge which we at present possess of the altitude of the polar light is based on measurements which from their nature, the constant oscillation of the phenomenon of light, and the consequent uncertainty of the angle of parallax, are not deserving of much confidence. the results obtained, setting aside the older data, fluctuate between several miles and an elevation of or feet; and, in all probability, the northern lights at different times occur at very different elevations.* [footnote] *farquharson in the 'edinburgh philos. journal', vol. xvi., p. ; 'philos. transact.' for , p. . [the height of the bow of light of the aurora seen at the cambridge observatory, march , , was determined by professors challis, of cambridge, and chevallier, of durham, to be miles above the surface of the earth. see the notice of this meteor in 'an account of the aurora borealis of oct. , ', by john h. morgan, esq., .] -- tr.] the most recent observers are disposed to place the phenomenon in the region of clouds, and not on the confines of the atmosphere; and they even believe that the rays of the aurora may be affected by winds and currents of air, if the phenomenon of light, by which alone the existence of an electro-magnetic current is appreciable, be actually connected with matrial groups of vesicles of vapor in motion, or, more correctly speaking, if light penetrate them, passing from one vesicle to another. franklin saw near great bear lake a beaming northern light, the lower side of which he thought illuminated a stratum of clouds, while, at a distance of only eighteen geographical miles, kendal, who was on watch throughout the whole night, and never lost sight of the sky, perceived no phenomenon of light. the assertion, so frequently maintained of late, that the rays of the aurora have been seen to shoot down to the ground between the spectator and some neighboring hill, is open to the charge of optical delusion, as in the cases of strokes of lightning or of the fall of fire-balls. whether the magnetic storms, whose local character we have illustrated by such remarkable examples, share noise as well as light in common with electric storms, is a question p that has become difficult to answer, since implicit confidence is no longr yielded to the relations of greenland whale-fishers and siberian fox-hunters. northern lights appear to have become less noisy since their occurrences have been more accurately recorded. parry, franklin, and richardson, near the north pole; thienemann in iceland; gieseke in greenland; lotur, and bravais, near the north cape; wrangel and anjou, on the coast of the polar sea, have together seen the aurora thousands of times, but never heard any sound attending the phenomenon. if this negative testimony should not be deemed equivalent to the positive counter-evidence of hearne on the mouth of the copper river and of henderson in iceland, it must be remembered that, although hood heard a noise as of quickly-moved musket-balls and a slight cracking sound during an aurora, he also noticed the same noise on the following day, when there was no northern light to be seen; and it must not be forgotten that wrangel and gieseke were fully convinced that the sound they had heard was to be ascribed to the contraction of the ice and the crust of the snow on the sudden cooling of the atmosphere. the belief in a crackling sound has arisen, not among the people generally, but rather among learned travelers, because in earlier times the northern light was declared to be an effect of atmospheric electricity, on account of the luminous manifestation of the electricity in rarefied space, and the observers found it easy to hear what they wished to hear. recent experiments with very sensitive electrometers have hitherto, contrary to the expectation generally entertained, yielded only negative results. the condition of the electricity in the atmosphere* p is not found to be changed during the most intense aurora; but, on the other hand, the three expressions of the power of terrestrial magnetism, declination, inclination and intensity, are all affected by polar light, so that in the same night, and at different periods of the magnetic development, the same end of the needle is both attracted and repelled. [footnote] *[mr. james glaisher, of the royal observatory, greenwich, in his interesting 'remarks on the weather during the quarter ending december st, ', says, "it is a fact well worthy of notice, that from the beginning of this quarter till the th of december, the electricity of the atmosphere was almost always in a neutral state, so that no signs of electricity were shown for several days together by any of the electrical instruments." during this period there were 'eight' exhibitions of the aurora borealis, of which one was the peculiarly bright display of the aurora borealis, of which one was the peculiarly bright display of the meteor on the th of october. these frequent exhibitions of brilliant aurorae seem to depend upon many remarkable meteorological relations, for we find, according to mr. glaisher's statement in the paper to which we have already alluded, that the previous fifty years afford no parallel season to the closing one of . the mean temperature of evaporation and of the dew point, the mean elastic force of vapor, the mean reading of the barometer, and the mean daily range of the readings of the thermometers in air, were all greater at greenwich during that season of than the average range of many preceding years.] -- tr. the assertion made by parry, on the strength of the data yielded by his observations in the neighborhood of the magnetic pole at melville island, that the aurora did not disturb, but rather exercised a calming influence on the magnetic needle, has been satisfactorily refuted by parry's own more exact researches,* detailed in his journal, and by the admirable observations of richardson, hood, and franklin in northern canada, and lastly by bravais and lottin in lapland. [footnote] *kamtz, 'lehrbuch der meteorologie', bd. iii., s. and . the process of the aurora is, as has already been observed, the restoration of a disturbed condition of equilibrium. the effect on the needle is different according to the degree of intensity of the explosion. it was only unappreciable at the gloomy winter station of bosekop when the phenomenon of light was very faint and aptly compared to the flame which rises in the closed circuit of a voltaic pile between two points of carbon at a considerable distance apart, or, according to fizeau, to the flame rising between a silver and a carbon point, and attracted or repelled by the magnet. this analogy certainly sets aside the necessity of assuming the existence of metallic vapors in the atmosphere, which some celebrated physicists have regarded as the substratum of the northern light. when we apply the indefinite term 'polar light' to the luminous phenomenon which we ascribe to a galvanic current, that is to say, to the motion of electricity in a closed circuit, we merely indicate the local direction in which the evolution of light is most frequently, although by no means invariably, seen. this phenomenon derives the greater part of its importance from the fact that the earth becomes 'self-luminous', and that as a planet, besides the light which it receives from the central body, the sun, it shows itself capable in itself of developing light. the intensity of the terrestrial light, or, rather the luminosity which is diffused, exceeds, in cases of the brightest colored radiation toward the zenith, the light of the moon in its first quarter. occasionally, as on the th of january, , printed characters could be read without difficulty. this almost uninterrupted development of light p in the earth leads us by analogy to the remarkable process exhibited in venus. the portion of this planet which is not illumined by the sun often shines with a phosphorescent light of its own. it is not improbable that the moon, jupiter, and the comets shine with an independent light, besides the reflected solar light visible through the polariscope. without speaking of the problematical but yet ordinary mode in which the sky is illuminated, when a low cloud may be seen to shine with an uninterrupted flickering light for many minutes together, we still meet with other instances of terrestrial development of light in our atmosphere. in this category we may reckon the celebrated luminous mists seen in and ; the steady luminous appearance exhibited without any flickeriing in great clouds observed by rozier and beccaria; and lastly, as arago* well remarks, the faint diffused light which guides the steps of the traveler in cloudy, starless, and moonless nights in autumn and winter, even when there is no snow on the ground. [footnote] *arago, on the dry fogs of and , which illuminated the night, in the 'annuaire du bureau des longitudes', , p. and ; and, regarding extraordinary luminous appearances in clouds without storms, see 'notices sur la tonnerre', in the 'annuaire pour l'an. ', p. - . as in polar light or the electro-magnetic storm, a current of brilliant and often colored light streams through the atmosphere in high latitudes, so also in the torrid zones between the tropics, the ocean simultaneously develops light over a space of many thousand square miles. here the magical effect of light is owing to the forces of organic nature. foaming with light, the eddying waves flash in phosphorescent sparks over the wide expanse of waters, where every scintillation is the vital manifestation of an invisible animal world. so varied are the sources of terrestrial light! must we still suppose this light to be latent, and combined in vapors, in order to explain 'moser's images produced at a distance' -- a discovery in which reality has hitherto manifested itself like a mere phantom of the imagination. as the internal heat of our planet is connected on the one hand with the generation of electro-magnetic currents and the process of terrestrial light (a consequence of the magnetic storm), it, on the other hand, discloses to us the chief source of geognostic phenomena. we shall consider these in their connection with and their transition from merely dynamic disturbances, from the elevation of whole continents and mountain chains to the development and effusion of gaseous and p liquid fluids, of hot mud, and of those heated and molten earths which become solidified into crystalline mineral masses. modern geognosy, the mineral portion of terrestrial physics, has made no slight advance in having investigated this connection of phenomena. this investigation has led us away from the delusive hypothesis, by which it was customary formerly to endeavor to explain, individually every expression of force in the terrestrial globe: it shows us the connection of the occurrence of heterogeneous substances with that which only appertains to changes in space (disturbances or elevations), and groups together phenomena which at first sight appeared most heterogeneous, as thermal springs, effusion of carbonic acid and sulphurous vapor, innocuous salses (mud eruptions), and the dreadful devastation of volcanic mountains.* [footnote] *[see mantell's 'wonders of geology', , vol. i., p. , , ; also lyell's 'principles of geology', vol. ii., and daubeney 'on volcanoes', d ed., , part ii., ch. xxxii., xxxiii.] -- tr. in a general view of nature, all these phenomena are fused together in one sole idea of the reaction of the interior of a planet on its external surface. we thus recognize in the depths of the earth, and in the increase of temperature with the increase of depth from the surface, not only the germ of disturbing movements, but also of the gradual elevation of whole continents (as mountain chains on long fissures), of volcanic eruptions, and of the manifold production of mountains and mineral masses. the influence of this reaction of the interior on the exterior is not, however, limited to inorganic nature alone. it is highly probable that, in an earlier world, more powerful emanations of carbonic acid gas, blended with the atmosphere, must have increased the assimilation of carbon in vegetables, and that an inexhaustible supply of combustible matter (lignites and carboniferous formations) must have been thus buried in the upper strata of the earth by the revolutions attending the destruction of vast tracts of forest. we likewise perceive that the destiny of mankind is in part dependent on the formation of the external surface of the earth, the direction of mountain tracts and high lands, and on the distribution of elevated continents. it is thus granted to the inquiring mind to pass from link to link along the chain of phenomena until it reaches the period when, in the solidifying process of our planet, and in its first transition from the gaseous form to the agglomeration of matter, that portion of the inner heat of the earth was developed, which does not belong to the action of the sun. this material taken from pages - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p in order to give a general delineation of the causal connection of geognostical phenomena, we will begin with those whose chief characteristic is dynamic, consisting in motion and in change in space. earthquakes manifest themselves by quick and successive vertical, or horizontal, or rotatory vibrations.* [footnote] *[see daubeney 'on volcanoes', d ed., , p. .] -- tr. in the very considerable number of earthquakes which i have experienced in both hemispheres, alike on land and at sea, the two first-named kinds of motion have often appeared to me to occur simultaneously. the mine-like explosiion -- the vertical action from below upward -- was most strikingly manifested in the overthrow of the town of riobamba in , when the bodies of many of the inhabitants were found to have been hurled to cullea, a hill several hundred feet in neight, and on the opposite side of the river lican. the propagation is most generally effected by undulations in a linear direction,* with a velocity of from twenty to twenty-eight miles in a minute, but partly in circles of commotion or large ellipses, in which the vibrations are propagated with decreasing intensity from a center toward the circumference. [footnote] *[on the linear direction of earthquakes, see daubeney 'on volcanoes', p. .] -- tr. there are districts exposed to the action of two intersecting circles of commotion. in northern asia, where the father of history,* and subsequently theophylactus simocatta,** described the districts of scythia as free from earthquakes, i have observed the metalliferous portion of the altai mountains under the influence of a two-fold focus of commotion, the lake of baikal, and the volcano of the celestial mountain (thianschan).*** [footnote] *herod, iv., . the prostration of the colossal statue of memnon, which has been again restored (letronne, 'la statue vocale de memnon', , p. , ), presents a fact in opposition to the ancient prejudice that egypt is free from earthquakes (pliny, ii., ); but the valley of the nile does lie external to the circle of commotion of byzantium, the archipelago, and syria (ideler ad aristot., 'meteor.', p. ). [footnote] **saint-martin, in the learned notes to lebeau, 'hist. du bas empire', t. ix., p. . [footnote] ***humboldt, 'asie centrale', t. ii., p. - . in regard to the difference between agitation of the surface and of the strata lying beneath it, see gay-lussac, in the 'annales de chimie et de physique', t. xxii., p. . when the circles of commotion intersect one another -- when, for instance, an elevated plain lies between two volcanoes simultaneously in a state of eruption, several wave-systems may exist together, as in fluids, and not mutually disturb one another. we may even suppose 'interference' p to exist here, as in the intersecting waves of sound. the extent of the propagated waves of commotion will be increased on the upper surface of the earth, according to the general law of mechanics, by which, on the transmission of motion in elastic bodies, the stratum lying free on the one side endeavors to separate itself from the other strata. waves of commotion have been investigated by means of the pendulum and the seismometer* with tolerable accuracy in respect to their direction and total intensity, but by no means with reference to the internal nature of their alternations and their periodic intumescence. [footnote] *[this instrument, in its simplest form, consists merely of a basin filled with some viscid liquid, which, on the occurrence of a shock of an earthquake of sufficient force to disturb the equilibrium of the building in which it is placed, is tilted on one side, and the liquid made to rise in the same direction, thus showing by its height the degree of the disturbance. professor j. forbes has invented an instrument of this nature, although on a greatly improved plan. it consists of a vertical metal rod, having a ball of lead movable upon it. it is supported upon a cylindrical steel wire, which may be compressed at pleasure by means of a screw. a lateral movement, such as that of an earthquake, which carries forward the base of the instrument, can only act upon the ball through the medium of the elasticity of the wire, and the direction of the displacement will be indicated by the plane of vibration of the pendulum. a self-registering apparatus is attached to the machine. see professor j. forbes's account of his invention in 'edinb. phil. trans.', vol. xv., part i.] -- tr. in the city of quito, which lies at the foot of a still active volcano (the rucu pichincha), and at an elevation of feet above the level of the sea, which has beautiful cupolas, high vaulted churches, and massive edifices of several stories, i have often been astonished that the violence of the nocturnal earthquakes so seldom causes fissures in the walls, while in the peruvian plains oscillations apparently much less intense injure low reed cottages. the natives, who have experienced many hundred earthquakes, believe that the difference depends less upon the length or shortness of the waves, and the slowness or rapidity of the horizontal vibrations.* than on the uniformity of the motion in opposite directions. [footnote] * "tutissimum est cum vibrat crispante aedificiorum crepitu; et cum intumescit assurgens alternoque motu residet, innoxium et cum concurrentia tecta contrario ictu arietant; quoniam alter motus alteri renititur. undantis inclinatio et fluctus more quaedam volutatio investa est, aut cum in unam partem totus se motus impellitae -- plin., ii., . the circling rotatory commotions are the most uncommon, but, at the same time, the most dangerous. walls were observed to be twisted, but not thrown down; rows of trees turned from their previous parallel direction; p and fields covered with different kinds of plants found to be displaced in the great earthquake of riobamba, in the province of quito, on the th of february, , and in that of calabria, between the th of february and the th of march, . the phenomenon of the inversion or displacement of fields and pieces of land, by which one is made to occupy the place of another, is connected with a translatory motion or penetration of separate terrestrial strata. when i made the plan of the ruined town of riobamba, one particular spot was pointed out to me, where all the furniture of one house had been found under the ruins of another. the loose earth had evidently moved like a fluid in currents, which must be assumed to have been directed first downward, then horizontally, and lastly upward. it was found necessary to appeal to the 'audiencia', or council of justice, to decide upon the contentions that arose regarding the proprietorship of objects that had been removed to a distance of many hundred roises. in countries where earthquakes are comparatively of much less frequent occurrence (as for instance, in southern europe), a very general belief prevails, although unsupported by the authority of inductive reasoning,* that a calm, an oppressive p heat and a misty horizon, are always the forerunners of this phenomenon. [footnote] *even in italy they have begun to observe that earthquakes are unconnected with the state of the weather, that is to say, with the appearance of the heavens immediately before the shock. the numerical results of friedrich hoffmann ('hinterlassene werke', bd. ii., - ) exactly correspond with the experience of the abbate scina of palermo. i have myself several times observed reddish clouds on the day of an earthquake, and shortly before it on the th of november, , i experienced two sharp shocks at the moment of a loud clap of thunder. ('relat. hist.', liv. iv., chap. .) the turin physicist, vassalli eaudi, observed volta's electrometer to be strongly agitated during the protracted earthquake of pignerol, which lasted from the d of april to the th of may, ; 'journal de physique', t. lxvii., p. . but these indications presented by clouds, by modifications of atmospheric electricity, or by calms, can not be regarded as 'generally' or 'necessarily' connected with earthquakes, since in quito, peru, and chili, as well as in canada and italy, many earthquakes are observed along with the purest and clearest skies, and with the freshest land and sea breezes. but if no meteorological phenomenon indicates the coming earthquake either on the morning of the shock or a few days previously, the influence of certain periods of the year (the vernal and autumnal equinoxes), the commencement of the rainy season in the tropics after long drought, and the change of the monsoons (according to general belief), can not be overlooked, even though the genetic connection of meteorological processes with those going on in the interior of our globe is still enveloped in obscurity. numerical inquiries on the distribution of earthquakes throughout the course of the year, such as those of von hoff, peter merian, and friedrich hoffmann, bear testimony to their frequency at the periods of equinoxes. it is singular that pliny, at the end of his fanciful theory of earthquakes, names the entire frightful phenomenon a subterranean storm; not so much in consequence of the rolling sound which frequently accompanies the shock, as because the elastic forces, concussive by their tension, accumulate in the interior of the earth when they are absent in the atmosphere! "ventos in causa esse non dubium reor. neque enim unquam intemiscunt terre, nisi sopito mari, coeloque adeo tranquillo, ut volatus avium non pendeant, subtracto omni spiritu qui vehit; nec unquam nisi post ventos conditos, scilicet in venas et cavernas ejus occulto afflatu. neque aliad est in terra tremor, quam in nube toonitruum; nec hiatus aliud quam cum fulmen erumpit, incluso spiritu luctante et ad libertatem exire nitente." (plin., ii., .) the germs of almost every thing that has been observed of imagined on the causes of earthquakes, up to the present day, may be found in seneca, 'nat. quaest.', vi., - . the fallacy of this popular opinion is not only refuted by my own experience, but likewise by the observations of all those who have lived many years in districts where, as in cumana, quito, peru, and chili, the earth is frequently and violently agitated. i have felt earthquakes in clear air and a fresh east wind, as well as in rain and thunder storms. the regularity of the horary changes in the declination of the magnetic needle and in the atmospheric pressure remained undisturbed between the tropics on the days when earthquakes occurred.* [footnote] *i have given proof that the course of the horary variations of the barometer is not affected before or after earthquakes, in my 'relat. hist.', t. i., p. and . these facts agree with the observations made by adolph erman (in the temperate zone, on the th of march, ) on the occasion of an earthquake at irkutsk, near the lake of baikal. during the violent earthquake of cumana, on the th of november, , i found the declination and the intensity of the magnetic force alike unchanged, but, to my surprise, the inclination of the needle was diminished about degrees.* [footnonte] *humboldt, 'relat. hist.', t. i., p. - . there was no ground to suspect an error in the calculation, and yet, in the many other earthquakes which i have experienced on the elevated plateaux of quito and lima, the inclination as well as the other elements of terrestrial magnetism remained always unchanged. although, in general, the processes at work within the interior of the earth may not be announced by any meteorological phenomena or any special appearance of the sky, it is, on the contrary, not improbable, as we shall soon see, that in cases of violent earthquakes some effect may be imparted to the atmosphere, in consequence of which they can not always act in a purely dynamic manner. p during the long-continued trembling of the ground in the piedmontese valleys of pelis and clusson, the greatest changes in the electric tension of the atmosphere were observed while the sky was cloudless. the intensity of the hollow noise which generally accompanies an earthquake does not increase in the same degree as the force of the oscillations. i have ascertained with certainty that the great shock of the earthquake of riobamba ( th feb., ) -- one of the most fearful phenomena recorded in the physical history of our planet -- was not accompanied by any noise whatever. the tremendous noise ('el gram ruido') which was heard below the soil of the cities of quito and ibarra, but not at tacunga and hambato, nearer the center of the motion, occurred between eighteen and twenty minutes 'after' the actual catastrophe. in the celebrated earthquake of lima and callao ( th of october, ), a noise resembling a subterranean thunder-clap was heard at truxillo a quarter of an hour after the shock, and unaccompanied by any trembling of the ground. in like manner, long after the great earthquake in new granada, on the th of november, , described by boussingault, subterranean detonations were heard in the whole valley of cauca during twenty or thirty seconds, unattended by motion. the nature of the noise varies also very much, being either rolling, or rustling, or clanking like chains when moved, or like near thunder, as, for instance, in the city of quito; or, lastly, clear and ringing, as if obsidian or some other vitrified masses were struck in subterranean cavities. as solid bodies are excellent conductors of sound, which is propagated in burned clay, for instance, ten or twelve times quicker than in the air, the subterranean noise may be heard at a great distance from the place where it has originated. in caracas, in the grassy plains of calabozo, and on the banks of the rio apure, which falls into the orinoco, a tremendously loud noise, resembling thunder, was heard, unaccompanied by an earthquake, over a district of land square miles in extent, on the th of april, , while at a distance of miles to the north-east, the volcano of st. vincent, in the small antilles, poured forth a copious stream of lava. with respect to distance, this was as if an eruption of vesuvius had been heard in the north of france. in the year , on the great eruption of the volcano of cotopaxi, subterranean noises, resembling the discharge of cannon, were heard in honda, on the magdalena river. the crater of cotopaxi lies not only , feet higher than honda, but these two points are separated by the colossal p mountain chain of quito, pasto, and popayan, no less than by numerous valleys and clefts, and they are miles apart. the sound was certainly not propagated through the air, but through the earth, and at a great depth. during the violent earthquake of new granada, in february, , subterranean thunder was heard simultaneously at popayan, bogota, santa marta, and caracas (where it continued for seven hours without any movement of the ground), in haiti, jamaica, and on the lake of nicaragua. these phenomena of sound, when unattended by any perceptible shocks, produce a peculiarly deep impression even on persons who have lived in countries where the earth has been frequently exposed to shocks. a striking and unparalleled instance of uninterrupted subterranean noise, unaccompanied by any trace of an earthquake, is the phenomenon known in the mexican elevated plateaux by the name of the "roaring and the subterranean thunder) ('bramidos y truenos subterraneos') of guanaxuato.* [footnote] *on the 'bramidos' of guanaxuato, see my 'essai polit. sur la nouv. espagne', t. i., p. . the subterranean noise, unaccompanied with any appreciable shock, in the deep mines and on the surface (the town of guanaxuata lies feet above the level of the sea), was not heard in the neighboring elevated plains, but only in the mountainous parts of the sierra, from the cuesta de los aguilares, near marfil, to the north of santa rosa. there were individual parts of the sierra - miles northwest of guanaxuata, to the other side of chichimequillo, near the boiling spring of san jose de comgngillas, to which the waves of sound did not extend. extremely stringent measures were adopted by the magistrates of the large mountain towns on the th of january , when the terror produced by these subterranean thunders was at its height. "the flight of a wealthy family shall be punished with a fine of piasters, and that of a poor family with two months' imprisonment. the militia shall bring back the fugitives." one of the most remarkable points about the whole affair is the opinion which the magistrates (el cabildo) cherished of their own superior knowledge. in one of their 'proclamas', i find the expression, "the magistrates, in their wisdom (en su sabiduria), will at once know when there is actual danger, and will give orders for flight; for the present, let processions be instituted." the terror excited by the tremor gave rise to a famine, since it prevented the importation of corn from the table-lands, where it abounded. the ancients were also aware that noises sometimes existed without earthquakes. -- aristot., 'meteor.', ii., p. ; plin., ii., . the singular noise that was heard from march, , to september, , in the dalmatian island meleda (sixteen miles from ragusa) and on which partsch has thrown much light, was occasionally accompanied by shocks. this celebrated and rich mountain city lies far removed from any active volcano. the noise began about midnight on the th of january, , and continued for a month. i have been enabled to give a circumstantial p description of it from the report of many witnesses, and from the documents of the municipality, of which i was allowed to make use. from the th to the th of january, it seemed to the inhabitants as if heavy clouds lay beneath their feet, from which issued alternate slow rolliing sounds and short, quick claps of thunder. the noise abated as gradually as it had begun. it was limited to a small space, and was not heard in a basaltic district at the distance of a few miles. almost all the inhabitants, in terror, left the city, in which large masses of silver ingots were stored; but the most courageous, and those more accustomed to subterranean thunder, soon returned, in order to drive off the bands of robbers who had attempted to possess themselves of the treasures of the city. neither on the surface of the earth, nor in mines feet in depth, was the slightest shock to be perceived. no similar noise had ever before been heard on the elevated tableland of mexico, nor has this terrific phenomenon since occurred there. thus clefts are opened or closed in the interior of the earth, by which waves of sound penetrate to us or are impeded in their propagation. the activity of an igneous mountain, however terrific and picturesque the spectacle may be which it presents to our contemplation, is always limited to a very small space. it is far otherwise with earthquakes, which although scarcely perceptible to the eye, nevertheless simultaneously propagate their waves to a distance of many thousand miles. the great earthquake which destroyed the city of lisbon on the st of november, , and whose effects were so admirably investigated by the distinguished philosopher emmanuel kant, was felt in the alps, on the coast of sweden, in the antilles, antigua, barbadoes, and martinique; in the great canadian lakes, in thuringia, in the flat country of northern germany, and in the small inland lakes on the shores of the baltic.* [footnote] *[it has been computed that the shock of this earthquake pervaded an area of , miles, or the twelfth part of the circumference of the globe. this dreadful shock lasted only five minutes: it happened about nine o'clock in the morning of the feast of all saints, whien almost the whole population was within the churches, owing to which circumstance no less than , persons perished by the fall of these edifices. see daubeney 'on volcanoes', p. - .] -- tr. remote springs were interrupted in their flow, a phenomenon attending earthquakes which had been noticed among the ancients by demetrius the callatian. the hot springs of toplitz dried up, and returned, inundating every thing around, and having their waters colored with iron ocher. in cadiz p the sea rose to an elevation of sixty-four feet, while in the antilles, where the tide usually rises only from twenty-six to twenty-eight inches, it suddenly rose above twenty feet, the water being of an inky blackness. it has been computed that on the st of november, , a portion of the earth's surface four times greater than that of europe, was simultaneously shaken. as yet there is no manifestation of force known to us, including even the murderous inventions of our own race, by which a greater number of people have been killed in the short space of a few minutes: sixty thousand were destroyed in sicily in , from thirty to forty thousand in the earthquake of riobamba in , and probably five times as many in asia minor and syria, under tiberius and justinian the elder, about the years and . there are instances in which the earth has been shaken for many successive days in the chain of the andes in south america, but i am only acquainted with the following cases in which shocks that have been felt almost every hour for months together have occurred far from any volcano, as, for instance, on the eastern declivity of the alpine chain of mount cenis, at fenestrelles and pignerol, from april, ; between new madrid and little prairie,* north of cincinnati in the united states of america, in december, , as well as through the whole winter of ; and in the pachalik of aleppo, in the months of august and september, . [footnote] *drake, 'nat. and statist. view of cincinnati', p. - ; mitchell, in the 'transactions of the lit. and philos. soc. of new york', vol. i., p. - . in the piedmonese county of pignerol, glasses of water, filled to the very brim, exhibited for hours a continuous motion. as the mass of the people are seldom able to rise to general views, and are consequently always disposed to ascribe great phenomena to local telluric and atmospheric processes, wherever the shaking of the earth is continued for a long time, fears of the eruption of a new volcano are awakened. in some few cases, this apprehension has certainly proved to be well grounded, as, for instance, in the sudden elevation of volcanic islands, and as we see in the elevation of the volcano of jorullo, a mountain elevated feet above the ancient level of the neighboring plain, on the th of september , after ninety days of earthquake and subterranean thunder. if we could obtain information regarding the daily condition of all the earth's surface, we should probably discover that the earth is almost always undergoing shocks at some point of its superficies, and is continually influenced by the reaction p of the interior on the exterior. the frequency and general prevalence of a phenomenon which is probably dependent on the raised temperature of the deepest molten strata explain its independence of the nature of the mineral masses in which it manifests itself. earthquakes have even been felt in the loose alluvial strata of holland, as in the neighborhood of middleburg and vliessingen on the d of february, . granite and mica slate are shaken as well as limestone and sandstone, or as trachyte and amygdaloid. it is not, therefore, the chemical nature of the constituents, but rather the mechanical structure of the rocks, which modifies the propagation of the motion, the wave of commotion. where this wave proceeds along a coast, or at the foot and in the direction of a mountain chain, interruptions at certain points have sometimes been remarked, which manifested themselves during the course of many centuries. the undulation advances in the depths below, but is never felt at the same points on the surface. the peruvians* say of these unmoved upper strata that "they form a bridge." [footnote] *in spanish they say, 'rocas que hacen puente'. with this phenomenon of non-propagation through superior strata is connected the remarkable fact that in the beginning of this century shocks were felt in the deep silver mines at marienberg, in the saxony mining district, while not the slightest trace was perceptible at the surface. the miners ascended in a state of alarm. conversely, the workmen in the mines of falun and persberg felt nothing of the shocks which in november, , spread dismay among the inhabitants above ground. as the mountain chains appear to be raised on fissures, the walls of the cavities may perhaps favor the direction of undulations parallel to them; occasionally, however, the waves of commotion intersect several chains almost perpenducularly. thus we see them simultaneously breaking through the littoral chain of venezuela and the sierra parime. in asia, shocks of earthquakes have been propagated from lahore and from the foot of the himalaya ( d of january, ) transversely across the chain of the hindoo chou to badakschan, the upper oxus, and even to bokhara.* [footnote] *sir alex. burnes, 'travels in bokhara', vol. i., p. ; and wathen, 'mem. on the usbek state', in the 'journal of the asiatic society of bengal', vol. iii., p. . the circles of commotion unfortunately expand occasionally in consequence of a single and usually violent earthquake. it is only since the destruction of cumana, on the th of december, , that shocks on the southern coast have been felt in the mica slate rocks of the peninsula of maniquarez, situated opposite to the chalk hills of the main land. the advance p from south to north was very striking in the almost uninterrupted undulations of the soil in the alluvial valleys of the mississippi, the arkansas, and the ohio, from to . it seemed here as if subterranean obstacles were gradually overcome, and that the way being once opened, the undulatory movement could be freely propagated. although earthquakes appear at first sight to be simply dynamic phenomena of motion, we yet discover, from well-attested facts, that they are not only able to elevate a whole district above its ancient level (as for instance, the ulla bund, delta of the indus, or the coast of chili, in november, ), but we also find that various substances have been ejected during the earthquake, as hot water at catania in ; hot steam at new madrid, in the valley of the mississippi, in ; irrespirable gases, 'mofettes', which injured the flocks grazing in the chain of the andes; mud, black smoke, and even flames, at messina in , and at cumana on the th of november, . during the great earthquake of lisbon, on the st of november, , flames and columns of smoke were seen to rise from a newly-formed fissure in the rock of alvidras, near the city. the smoke in this case became more dense as the subterranean noise increased in intensity.* [footnote] * 'philos. transaci.', vol. xlix. p. . at the destruction of riobamba, in the year , when the shocks were not attended by any outbreak of the neighboring volcano, a singular mass called the 'moya' was uplifted from the earth in numerous continuous conical elevations, the whole being composed of carbon, crystals of augite, and the silicious shields of infusoria. the eruption of carbonic acid gas from fissures in the valley of the magdalene, during the earthquake of new granada, on the th of november, , suffocated many snakes, rats, and other animals. sudden changes of weather, as the occurrence of the rainy season in the tropics, at an unusual period of the year, have sometimes succeeded violent earthquakes in quito and peru. do gaseous fluids rise from the interior of the earth, and mix with the atmosphere? or are these meteorological processes the action of atmospheric electricity disturbed by the earthquake? in the tropical regions of america, where sometimes not a drop of rain falls for ten months together, the natives consider the repeated shocks of earthquakes, which do not endanger the low reed huts, as auspicious harbingers of fruitfulness and abundant rain. p the intimate connection of the phenomena which we have considered is still hidden in obscurity. elastic fluids are doublessly the cause of the slight and perfectly harmless trembling of the earth's surface, which has often continued several days (as in , at scaccia, in sicily, before the volcanic elevation of the island of julia), as well as of the terrific explosions accompanied by loud noise. the focus of this destructive agent, the seat of the moving force, lies far below the earth's surface; but we know as little of the extent of this depth as we know of the chemical nature of these vapors that are so highly compressed. at the edges of two craters, vesuvius, and the towering rock which projects beyond the great abyss of pichincha, near quito, i have felt periodic and very regular shocks of earthquakes, on each occasion from to seconds before the burning scoriae or gases were erupted. the intensity of the shocks was increased in proportion to the time intervening between them, and, consequently, to the length of time in which the vapors were accumulating. this simple fact, which has been attested by the evidence of so many travelers, furnishes us with a general solution of the phenomenon, in showing that active volcanoes are to be considered as safety-valves for the immediate neighborhood. the danger of earthquakes increases when the openings of the volcano are closed, and deprived of free communication with the atmosphere; but the destruction of lisbon, of caraccas, of lima, of cashmir in ,* and of so many cities of calabria, syria, and asia minor, shows us, on the whole, that the force of the shock is not the greatest in the neighborhood of active volcanoes. [footnote] *on the frequency of earthquakes in cashmir, see troyer's german translation of the ancient 'radjataringini', vol. ii., p. , and carl hugel, 'reisen', bd. ii., s. . as the impeded activity of the volcano acts upon the shocks of the earth's surface, so do the latter react on the volcanic phenomena. openings of fissures favor the rising of cones of eruption, and the processes which take place in these cones, by forming a free communication with the atmosphere. a column of smoke, which had been observed to rise for months together from the volcano of pasto, in south america, suddenly disappeared, when on the th of february, , the province of quito, situated at a distance of miles to the south, suffered from the great earthquake of riobamba. after the earth had continued to tremble for some time through out the whole of syria, in the cyclades, and in euboea, the shocks suddenly ceased on the eruption of a stream of hot mud p on the lelantine plains near chalcia.* [footnote] * strabo, lib. i., p. , casaub. that the expression [greek words] does not mean erupted mud, but lava, is obvious from a passage in strabo, lib. vi., p. . compare walter, in his 'abnahme der vulkanischen thatigkeit in historischen zeiten' (on the decrease of volcanic activity during historical times), , s. . the intelligent geographer of amasea, to whom we are indebted for the notice of this circumstance, further remarks: "since the craters of aetna have been opened, which yield a passage to the escape of fire, and since burning masses and water have been ejected, the country near the sea-shore has not been so much shaken as at the time previous to the separation of sicily from lower italy, when all communications with the external surface were closed." we thus recognize in earthquakes the existence of a volcanic force, which, although every where manifested, and as generally diffused as the internal heat of our planet, attains but rarely, and then only at separate points, sufficient intensity to exhibit the phenomenon of eruptions. the formation of veins, that is to say, the filling up of fissures with crystalline masses bursting forth from the interior (as basalt, melaphyre, and greenstone), gradually disturbs the free intercommunication of elastic vapors. this tension acts in three different ways, either in causing disruptions, or sudden and retroversed elevations, or, finally, as was first observed in a great part of sweden, in producing changes in the relative level of the sea and land, which, although continuous, are only appreciable at intervals of long period. before we leave the important phenomena which we have considered not so much in their individual characteristics as in their general physical and geognostical relations, i would advert to the deep and peculiar impression left on the mind by the first earthquake which we experience, eeven where it is not attended by any subterranean noise.* [footnote] *[dr. tschudi, in his interesting work, 'travels in peru', translated from the german by thomasina ross, p. , , describes strikingly the effect of an earthquake upon the native and upon the stranger. "no familiarity with the phenomenon can blunt this feeling. the inhabitant of lima, who from childhood has frequently witnessed these convulsions of nature, is roused from his sleep by the shock, and rushes from his apartment with the cry of 'misericordia!' the foreigner from the north of europe, who knows nothing of earthquakes but by description, waits with impatience to feel the movement of the earth, and longs to hear with his own ear the subterranean sounds which he has hitherto considered fabulous. with levity he treats the apprehension of a coming convulsion, and laughs at the fears of the natives: but, as soon as his wish is gratified, he is terror-stricken, and is involuntarily prompted to seek safety in flight."] -- tr. this impression is not, p in my opinion, the result of a recollection of those fearful pictures of devastation presented to our imaginations by the historical narratives of the past, but is rather due to the sudden revelation of the delusive nature of the inherent faith by which we had clung to a belief in the immobility of the solid parts of the earth. we are accustomed from early childhood to draw a contrast between the mobility of water and the immobility of the soil on which we tread; and this feeling is confirmed by the evidence of our senses. when, therefore, we suddenly feel the ground move beneath us, a mysterious and natural force, with which we are previously unacquainted, is revealed to us as an active disturbance of stability. a moment destroys the illusion of a whole life; our deceptive faith in the repose of nature vanishes, and we feel transported, as it were, into a realm of unknown destructive forces. every sound -- the faintest motion in the air -- arrests our attention, and we no longer trust the ground on which we stand. animals, especially dogs and swine, participate in the same anxious disquietude; and even the crocodiles of the orinoco, which are at other times as dumb as our little lizards, leave the trembling bed of the river, and run with loud cries into the adjacent forests. to man the earthquake conveys an idea of some universal and unlimited danger. we may flee from the crater of a volcano in active eruption, or from the dwelling whose destruction is threatened by the approach of the lava stream; but in an earthquake, direct our flight whithersoever we will, we still feel as if we trod upon the very focus of destruction. this condition of the mind is not of long duration, although it takes its origin in the deepest recesses of our nature; and when a series of faint shocks succeed one another, the inhabitants of the country soon lose every trace of fear. on the coasts of peru, where rain and hail are unknown, no less than the rolling thunder and the flashing lightning, these luminous explosions of the atmosphere are replaced by the subterranean noises which accompany earthquakes.* [footnote] *["along the whole coast of peru the atmosphere is almost uniformly in a state of repose. it is not illuminated by the lightning's flash, or disturbed by the roar of the thunder; no deluges of rain, no fierce hurricanes, destroy the fruits of the fields, and with them the hopes of the husbandman. but the mildness of the elements above ground is frightfully counterbalanced by their subterranean fury. lima is frequently visited by earthquakes, and several times the city has been reduced to a mass of ruins. at an average, forty-five shocks may be counted on in the year. most of them occur in the later part of october, in november, december, january, may, and june. experience gives reason to expect the visitation of two desolating earthquakes in a century. the period between the two is from forty to sixty years. the most considerable catastrophes experienced in lima since europeans have visited the west coast of south america happened in the years , , , , , . there is reason to fear that in the course of a few years this city may be the prey of another such visitation."] --tr. long habit, and the very p prevalent opinion that dangerous shocks are only to be apprehended two or three times in the course of a century, cause faint oscillations of the soil to be regarded in lima with scarcely more attention than a hail storm in the temperate zone. having thus taken a general view of the activity -- the inner life, as it were -- of the earth, in respect to its internal heat, its electro-magnetic tension, its emanation of light at the poles, and its irregularly-recurring phenomena of motion, we will now proceed to the consideration of the material products, the chemical changes in the earth's surface, and the composition of the atmosphere, which are all dependent on planetary vital activity. we see issue from the ground steam and gaseous carbonic acid, almost always free from the admixture of nitrogen;* carbureted hydrogen gas, which has been used in the chinese province sse-tschuan** for several thousand years, and recently in the village of fredonia, in the state of new york, united states, in cooking and for illumination; sulphureted hydrogen gas and sulphurous vapors; and, more rarely,*** sulphurous and hydrochloric acids.**** [footnote] * bischof's comprehensive work, 'warmelchere des inneren erdkorpers'. [footnote] **on the artesian fire-springs (ho-tsing) in china, and the ancient use of portable gas (in bamboo canes) in the city of khiung-tsheu, see klaproth, in my 'asie centrale', t. iii., p. - . [footnote] *** boussingault ('annales de chimie', t. lii., p. ) observed no evolution of hydrochloric acid from the volcanoes of new granada, while monticelli found it in enormous quantity in the eruption of vesuvius in . [footnote] ****[of the gaseous compounds of sulphur, one, sulphurous acid, appears to predominate chiefly in volcanoes possessing a certain degree of activity, while the other, sulphureted hydrogen, has been most frequently perceived among those in a dormant condition. the occurrence of abundant exhalations of sulphuric acid, which have been hitherto noticed chiefly in extinct volcanoes, as for instance, in a stream issuing from that of purace, between bogota and quito, from extinct volcanoes in java, is satisfactorily explained in a recent paper by m. dumas, 'annales de chimie', dec., . he shows that when sulphureted hydrogen, at a temperature above degrees fahr., and still better when near degrees, comes in contact with certain porous bodies, a catalytic action is set up, by which water, sulphuric acid, and sulphur are produced. hence probably the vast deposits of sulphur, associated with sulphates of lime and strontian, which are met with in the western parts of sicily.] -- tr. such effusions p from the fissures of the earth not only occur in the districts of still burning or long-extinguished volcanoes, but they may likewise be observed occasionally in districts where neither trachyte nor any other volcanic rocks are exposed on the earth's surface. in the chain of quindiu i have seen sulphur deposited in mica slate from warm sulphurous vapor at an elevation of feet* above the level of the sea, while the same species of rock, which was formerly regarded as primitive, contains, in the cerro cuello, near tiscan, south of quito, an immense deposit of sulphur imbedded in pure quartz. [footnote] * humboldt, 'recucil d'observ. astronomiques', t. i., p. ('nivellement barometrique de la cordillere des andes', no. ). exhalations of carbonic acid ('mofettes') are even in our days to be considered as the most important of all gaseous emanations, with respect to their number and the amount of their effusion. we see in germany, in the deep valleys of the eifel, in the neighborhood of the lake of laach,* in the crater-like valley of the wehr and in western bohemia, exhalations of carbonic acid gas manifest themselves as the last efforts of volcanic activity in or near the foci of an earlier world. [footnote] *[the lake of laach, in the district of the eifel, is an expanse of water two miles in circumference. the thickness of the vegetation on the sides of its crater-like basin renders it difficult to discover the nature of the subjacent rock, but it is probably composed of black cellular augitic lava. the sides of the crater present numerous loose masses, which appear to have been ejected, and consist of glassy feldspar, ice-spar, sodalite, hauyne, spinellane, and leucite. the resemblance between these products and the masses formerly ejected from vesuvius is most remarkable. (daubeney 'on volcanoes', p. .) dr. hibbert regards the lake of laach as formed in the first instance by a crack caused by the cooling of the crust of the earth, which was widened afterward into a circular cavity by the expansive force of elastic vapors. see 'history of the extinct volcanoes of the basin of neuwied', .] -- tr. in those earlier periods, when a higher terrestrial temperature existed, and when a great number of fissures still remained unfilled, the processes we have described acted more powerfully, and carbonic acid and hot steam were mixed in larger quantities in the atmosphere, from whence it follows, as adolph bronguiart has ingeniously shown,* that the primitive vegetable world must have exhibited almost every where, and independently of geographical position, the most luxurious abundance and the fullest development of organism. [footnote] *adolph bronguiart, in the 'annales des sciences naturelles', t. xv., p. . in these constantly warm and damp atmospheric strata, saturated with p carbonic acid, vegetation must have attained a degree of vital activity, and derived the superabundance of nutrition necessary to furnish materials for the formation of the beds of lignite (coal) constituting the inexhaustible means on which are based the physical power and prosperity of nations. such masses are distributed in basins over certain parts of europe, occurring in large quantities in the british islands, in belgium, in france, in the provinces of the lower rhine, and in upper silesia. at the same primitive period of universal volcanic activity, those enormous quantities of carbon must also have escaped from the earth which are contained in limestone rocks, and which, if seprated from oxygen and reduced to a solid form, would constitute about the eighth part of the absolute bulk of these mountain masses.* [footnote] * bischof, op. cit., s. , anm. . that portion of the carbon which was not taken up by alkaline earths, but remained mixed with the atmosphere, as carbonic acid, was gradually consumed by the vegetation of the earlier stages of processes of vegetable life, only retained the small quantity which it now possesses, and which is not injurious to the sulphurous vapor have occasioned the destruction of the species of mollusca and fish which inhabited the inland waters of the earlier world, and have given rise to the formation of the contorted beds of gypsum, which have doubtless been frequently affected by shocks of earthquakes. gaseous and liquid fluids, mud, and molten earths, ejected from the craters of volcanoes, which are themselves only a kind of "intermittent springs," rise from the earth under precisely analogous physical relations.* [footnote] *humboldt, 'asie centrale', t. i., p. . all these substances owe their temperature and their chemical character to the place of their origin. the 'mean' temperature of aqueous springs is less than that of the air at the point whence they emerge, if the water flow from a height; but their heat increases with the depth of the strata with which they are in contact at their origin. we have already spoken of the numerical law regulating this increase. the blending of waters that have come from the height of a mountain with those that have sprung from the depths of the earth, render it difficult to determine the position of the 'isogeothermal lines'* (lines of equal internal p terrestrial temperature, when this determination is to be made from the temperature of flowing springs. [footnote] *on the theory of isogeothermal (chthonisothermal) lines, consult the ingenious labors of kupffer, in pogg, 'annalen', bd xv., s. , and bd xxxii., s. , in the 'voyage dans l'oural', p. - , and in the 'edinburgh journal of science', new series, vol. iv., p. . see, also, kamtz, 'lehrb. der meteor.', bd. ii., s. ; and, on the ascent of the chthonisothermal lines in mountainous districts, bischof, s. - . such at any rate, is the result i have arrived at from my own observations and those of my fellow-travelers in northern asia. the temperature of springs, which has become the subject of such continuous physical investigation during the last half century, depends, like the elevation of the line of perpetual snow, on very many simultaneous and deeply-involved causes. it is a function of the temperature of the stratum in which they take their rise, of the specific heat of the soil, and of the quantity and temperature of the meteoric water,* which is itself different from the temperature of the lower strata of the atmosphere, according to the different modes of its origin in rain, snow, or hail.** [footnote] *leop. v. buch, in pogg., 'annalen', bd. xii., s. . [footnote] ** on the temperature of the drops, of rain in cumana, which fell to degrees, when the temperature of the air shortly before had been degrees and degrees, and during the rain sank to degrees, see my 'relat. hist.', t. ii., p. . the rain-drops, while falling, change the normal temperature they originally possessed, which depends on the height of the clouds from which they fell, and their heating on their upper surface by the solar rays. the rain-drops, on their first production, have a higher temperature than the surrounding medium in the superior strata of our atmosphere, in consequence of the liberation of their latent heat; and they continue to rise in temperature, since, in falling through lower and warmer strata, vapor is precipitated on them, and they thus increase in size (bischof, 'warmelehre des inneren erdkorpers' s. ); but this additional heating is compensated for by evaporation. the cooling of the air by rain (putting out of the question what probably belongs to the electric process in storms) is effected by the drops, which are themselves of lower temperature, in consequence of the cold situation in which they were formed, and bring down with them a portion of the higher colder air, and which finally, by moistening the ground, give rise to evaporation. the cooling of the air by rain (putting out of the question what probably belongs to the electric process in storms) is effected by the drops, which are themselves of lower temperature, in consequence of the cold situation in which they were formed, and bringi down with them a portion of the higher colder air, and which finally, by moistening the ground, give rise to evaporation. these are the ordinary relations of the phenomenon. when, as occasionally happens, the rain-drops are warmer than the lower strata of the atmosphere (humboldt, 'rel. hist.', t. iii., p. ), the cause must probably be sought in higher warmer currents, or in a higher temperature of widely-extended and not very thick clouds, from the action of the sun's rays. how, moreover, the phenomenon of supplementary rainbows, which are explained by the interference of light, is connected with the original and increasing size of the falling drops, and how an optical phenomenon, if we know how to observe it accurately, may enlighten us regarding a meteorological process, according to diversity of zone, has been shown, with much talent and ingenuity, by arago, in the 'annuaire' for , p. . cold springs can only indicate the mean atmospheric temperature p when they are unmixed with the waters rising from great depths, or descending from considerable mountain elevations, and when they have passed through a long course at a depth from the surface of the earth which is equal in our latitudes to or feet, and according to boussingault, to about one foot in the equinoctial regions,* these being the depths at which the invariability of the temperature begins in the temperate and torrid zones, that is to say, the depths at which horary, diurnal, and monthly changes of heat in the atmosphere cease to be perceived. [footnote] * the profound investigations of boussingault fully convince me, that in the tropics, the temperature of the ground, at a very slight depth, exactly corresponds with the mean temperature of the air. the following instances are sufficient to illustrate this fact: ________________________________________________________ stations temperature at mean height, in within french foot temperature english tropic [ . of the of the feet, above zones. english foot] air. the level below the of the sea. earth's surface. ________________________________________________________ guayaquil . . anserma nuevo . . zupia . . popayan . . quito . . ________________________________________________________ the doubts about the temperature of the earth within the tropics, of which i am probably, in some degree, the cause, by my observations on the cave of caripe (cueva del guacharo), 'rel. hist.', t. iii., p. - ), are resolved by the consideration that i compared the presumed mean temperature of the air of the convent of caripe, . degrees, not with the temperature of the air of the cave, . degrees, but with the temperature of the subterranean stream, . degrees, although i observed ('rel. hist.', t. iii., p. and ) that mountain water from a great height might probably be mixed with the water of the cave. hot springs issue from the most various kinds of rocks. the hottest permanent springs that have hitherto been observed are, as my own researches confirm, at a distance from all volcanoes. i will here advert to a notice in my journal of the aguas calientes de las trincheras', in south america, between porto cabello and nueva valencia, and the 'aguas de comangillas', in the mexican territory, near guanaxuato; the former of these, which issued from granite, had a temperature of . degrees; the latter, issuing from basalt, . degrees. the depth of the source from whence the water flowed with this temperature, judging from what we know of the law of the increase of heat in the interior of the earth, was probably feet, or above two miles. if the universally-diffused terrestrial heat be the cause of thermal springs, as of active volcanoes, the rocks can only exert an influence by the different capacities p for heat and by their conducting powers. the hottest of all permanent springs (between degrees and degrees) are likewise, in a most remarkable degree, the purest, and such as hold in solution the smallest quantity of mineral substances. their temperature appears, on the whole, to be less constant than that of springs between degrees and degrees, which in europe, at least, have maintained, in a most remarkable manner, their 'invariability of heat and mineral contents' during the last fifty or sixty years, a period in which thermometrical measurements and chemical analyses have been applied with increasing exactness. boussingault found in that the thermal springs of las tricheras had risen degrees during the twenty-three years that had intervened since my travels in .* [footnote] *boussingault, in the 'annales de chimie', t. lii., p. . the spring of chaudes aigues, in auvergne, is only degrees. it is also to be observed, that while the aguas calientes de las trincheras, south of porto cabello (venezuela), springing from granite cleft in regular beds, and far from all volcanoes, have a temperature of fully . degrees, all the springs which rise in the vicinity of still active volcanoes (pasto, cotopaxi, and tunguragua) have a temperature of only - degrees. this calmly-flowing spring is therefore now nearly degrees hotter than the intermittent fountains of the geyser and the strokr, whose temperature has recently been most carefully determined by krug of nidda. a very striking proof of the origin of hot springs by the sinking of cold meteoric water into the earth, and by its contact with a volcanic focus, is afforded by the volcano of jorulla in mexico, which was unknown before my american journey. when, in september, , jorullo was suddenly elevated into a mountain feet above the level of the surrounding plain, two small rivers, the 'rio de cuitimba' and 'rio de san pedro', disappeared, and some time afterward burst forth again, during violent shocks of an earthquake, as hot springs, whose temperature i found in to be . degrees. the springs in greece still evidently flow at the same places as in the times of hellenic antiquity. the spring of erasinos, two hours' journey to the south of argos, on the declivity of chaon, is mentioned by herodotus. at delphi we still see cassotis (now the springs of st. nicholas) rising south of the lesche, and flowing beneath the temple of apollo; castalia, at the foot of phaedriadae; pirene, near acro-corinth; and the hot baths of aedipsus, in euboea, in which sulla bathed during the mithridatic war.* [footnote] *cassotis (the spring of st. nicholas) and castalia, at the phaedriadae, mentioned in pausanias, x., , , and x., , ; pirene (acro-corinth), in strabo, p. ; the spring of erasinos, at mount chaon, south of argos, in herod., vi., , and pausanias, ii., , ; the springs of aedipsus in euboea, some of which have a temperature of degrees, while in others it ranges between ) qne degrees, in strabo, p. and , and athenaeus, ii., , ; the hot springs of thermopylae, at the foot of oeta, with a temperature of degrees. all from manuscript notes by professor curtius, the learned companion of otfried muller. i advert with pleasure to these p facts, as they show us that, even in a country subject to frequent and violent shocks of earthquakes, the interior of our planet has retained for upward of years its ancient configuration in reference to the course of the open fissures that yield a passage to these waters. the 'fontaine jaillissante' of lillers, in the department des pas de calais, which was bored as early as the year , still rises to the same height and yields the same quantity of water; and, as another instance, i may mention that the admirable geographer of the caramanian coast, captain beaufort, saw in the district of phaselis the same flame fed by emissions of inflammable gas which was described by pliny as the flame of the lycian chimera.* [footnnote] (pliny, ii., ; seneca, 'epist.' , , ed. ruhkopf (beaufort, 'survey of the coast of karamania', , art. yanar, near delktasch, the ancient phaselis, p. ). see also ctesias, 'fragm.', cap. p. , ed. bahr; strabo, lib. xiv., p. , casaub. ["not far from the deliktash, on the side of a mountain, is the perpetual fire described by captain beaufort. the travelers found it as brilliant as ever, and even somewhat increased; for, besides the large flame in the corner of the ruins described by beaufort, there were small jets issuing from crevices in the side of the crater-like cavity five or six feet deep. at the bottom was a shallow pool of sulphureous and turbid water, regarded by the turks as a sovereign remedy for all skin complaints. the soot deposited from the flames was regarded as efficacious for sore eyelids, and valued as a dye for the eyebrows." see the highly interesting and accurate work, 'travels in lycia', by lieut. spratt and professor e. forbes.] -- tr. the observation made by arago in , that the deepest artesian wells are the warmest,* threw great light on the origin of thermal springs, and on the establishment of the law that terrestrial heat increases with increasing depth. [footnote] *arago, in the 'annuaire pour' , p. . it is a remarkable fact, which has but recently been noticed, that at the close of the third century, st. patricus,* probably bishop of pertusa, was led to adopt very correct views regarding the phenomenon of the hot springs at carthage. [footnote] *'acta s. patricii', p. , ed. ruinart, t. ii., p. , mazochi. dureau de la malle was the first to draw attention to this remarkable passage in the 'recherches sur la topographie de carthage', , p. . (see, also, seneca, 'nat. quaest.', iii., .) on being asked what was the cause of boiling water bursting from the earth, he replied, "fire is nourished in the clouds and in the interior p of the earth, as aetna and other mountains near naples may teach you. the subterranean waters rise as if through siphons. the cause of hot springs is this: waters which are more remote from the subterranean fire are colder, while those which rise nearer the fire are heated by it, and bring with them to the surface which we inhabit an insupportable degree of heat." as earthquakes are often accompanied by eruptions of water and vapors, we recognize in the 'salses',* of small mud volcanoes, a transition from the changing phenomena presented by these eruptions of vapor and thermal springs to the more powerful and awful activity of the streams of lava that flow from volcanic mountains. [footnote] *[true volcanoes, as we have seen, generate sulphureted hydrogen and muriatic acid, upheave tracts of land, and omit streams of melted feldspathic materials; salses, on the contrary, disengage little else but carbureted hydrogen, together with bitumen and other products of the distillation of coal, and pour forth no other torrents except of mud, or argillaceous materials mixed up with water. daubeney, op cit., p. .] -- tr. if we consider these mountains as springs of molten earths producing volcanic rocks, we must remember that thermal water, when impregnated with carbonic acid and sulphurous gases, are continually forming horizontally ranged strata of limestone (travertine) or conical elevations, as in northern africa (in alberia), and in the banos of caxamarca, on the western declivity of the peruvian cordilleras. the travertine of van diemen's land (near hobart town) contains, according to charles darwin, remains of a vegetation that no longer exists. lava and travertine, which are constantly forming before our eyes, present us with the two extremes of geognostic relations. 'salses' deserve more attention than they have hitherto received from geognosists. their grandeur has been overlooked because of the two conditions to which they are subject; it is only the more peaceful state, in which they may continue for centuries, which has generally been described: their origin is, however, accompanied by earthquakes, subterranean thunder, the elevation of a whole district, and lofty emissions of flame of short duration. when the mud volcano of jokmali began to form on the th of november, , in the peninsula of abscheron, on the caspian sea, east of baku, the flames flashed up to an extraordinary height for three hours, while during the next twenty hours they scarcely rose three feet above the crater, from which mud was ejected. near the village of baklichli, west of baku, the flames rose so high that p they could be seen at a distance of twenty-four miles. enormous masses of rock were torn up and scattered around. similar masses may be seen round the now inactive mud volcano of monte ziblo, near sassuolo, in northern italy. the secondary condition of repose has been maintained for upward of fifteen centuries in the mud volcanoes of girgenti, the 'macalubi', in sicily, which have been described by the ancients. these salses consist of many contitiguous conical hills, from eight to ten, or even thirty feet in height, subject to variations of elevation as well as of form. streams of argillaceous mud, attended by a periodic development of gas, flow from the small basins at the summits, which are filled with water; the mud, although usualy cold is sometimes at a high temperature, as at damak, in the province of samarang, in the island of java. the gases that are developed with loud noise differ in their nature consisting for instance, of hydrogen mixed with naphtha, or of carbonic acid, or, as parrot and myself have shown (in the peninsula of taman, and in the 'volcancitos de turbaco', in south america), of almost pure nitrogen.* [footnote] *humboldt, 'rel. hist.', t. iii., p. - ; 'asie centrale', t. i., p. ; t. ii., p. - ; 'vues des cordilleres', pl. xli. regarding the 'macalubi', the 'overthrown' or 'inverted', from the word 'khalaba'), and on "the earth ejecting fluid earth," see solinus, cap. : "idem ager agrigentinus eructat limosas scaturigenes, et ut venae fontium sufficiunt rivis subjinistrandis, ita in hac sicilae parte solo munquam deficiente, aeterna rejectatione terram terra evomit." mud volcanoes, after the first violent explosion of fire, which is not, perhaps, in an equal degree common to all, present to the spectator an image of the uninterrupted but weak activity of the interior of our planet. the communication with the deep strata in which a high temperature prevails is soon closed, and the coldness of the mud emissions of the salses seems to indicate that the seat of the phenomenon can not be far removed from the surface during their ordinary condition. the reaction of the interior of the earth on its external surface is exhibited with totally different force in true volcanoes or igneous mountains, at points of the earth in which a permanent, or, at least, continually-renewed connection with the volcanic force is manifested. we must here carefully distinguish between the more or less intensely developed volcanic phenomena, as for instance, between earthquakes, thermal, aqueous, and gaseous springs, mud volcanoes, and the appearance of bell-formed or dome-shaped trachytic rocks without openings; the opening of these rocks, or of the elevated beds of basalt, as p craters of elevation; and, lastly, the elevation of a permanent volcano in the crater of elevation, or among the 'debris' of its earlier formation. at different periods, and in different degrees of activity and force, the permanent volcanoes emit steam acids, luminous scoriae, or, when the resistance can be overcome, narrow, band-like streams of molten earths. elastic vapors sometimes elevate either separate portions of the earth's crust into dome-shaped unopened masses of feldspathic trachyte and dolerite (as in puy de dome and chimborazo), in consequence of some great or local manifestation of force in the interior of our planet, or the upheaved strata are broken through and curved in such a manner as to form a steep rocky ledge on the opposite inner side, which then constitutes the inclosure of a crater of elevation. if this rocky ledge has been uplifted from the bottom of the sea, which is by no means always the case, it determines the whole physiognomy and form of the island. in this manner has arisen the circular form of palma, which has been described with such admirable accuracy by leopold von buch, and that of nisyros,* in the aegean sea. [footnote] *see the interesting little map of the island of nisyros, in roise's 'reisen auf den griechischen inseln', bd. ii., , s. . sometimes half of the annular ledge has been destroyed, and in the bay formed by the encroachment of the sea corallines have built their cellular habitations. even on continents craters of elevation are often filled with water, and embellish in a peculiar manner the character of the landscape. their origin is not connected with any determined species of rock: they break out in basalt, trachyte, leucitic porphyry (somma), or in doleritic mixtures of augite and labradorite; and hence arise the different nature and external conformation of these inclosures of craters. no phenomena of eruption are manifested in such craters, as they open no permanent channel of communication with the interior, and it is but seldom that we meet with traces of volcanic activity either in the neighborhood or in the interior of these craters. the force which was able to produce so important an action must have been long accumulating in the interior before it could overpower the resistance of the mass pressing upon it; it sometimes, for instance, on the origin of new islands, will raise granular rocks and conglomerated masses (strata of tufa filled with marine plants) above the surface of the sea. the compressed vapors escape through the crater of elevation, but a large mass soon falls back and closes the opening, which had been only formed by these manifestations of force. no volcano can, therefore, p be produced.* [footnote] *leopold von buch, 'phys. beschreibung der canarischen inseln', s. ; and his memoir 'uber erhebungscratere und vulcane', in poggend., 'annal.', bd. xxxvii., s. . in his remarks on the separation of sicily from calabria, strbo gives an excellend description of the two modes in which islands are formed: "some islands," he observes (lib. vi., p. , ed. casaub.), "are fragments of the continent, others have arisen from the sea, as even at the present time is known to happen; for the islands of the great ocean, lying far from the main land, have probably been raised from its depths, while, on the other hand, those near promontories appear (according to reason) to have been separated from the continent." a volcano, properly so called, exists only where a permanent connection is established between the interior of the earth and the atmosphere, and the reaction of the interior on the surface then continues during long periods of time. it may be interrupted for centuries, as in the case of vesuvius fisove,* and then manifest itself with renewed activity. [footnote] *ocre fisove (mons vesuvius) in the umbrian language. (lassen 'deutung der eugubinischen tafeln in rhein. museum', , s. .) the word 'ochre' is very probaby genuine umbrian, and means, according to festus, 'mountain'. aetna would be a burning and shining mountain, if voss is correct in stating that [greek work] is an hellenic sound, and is connected with [greed word] and [greek word]; but the intelligent writer parthey doubts this hellenic origin on etymological grounds, and also because etna was by no means regarded as a luminous beacon for ships or wanderers, in the same manner as the ever-travailing stromboli (strongyle), to which homer seems to refer in the odyssey (xii., , , and ), and its geographical position was not so well determined. i suspect that tna would be found to be a sicilian word, if we had any fragmentary materials to refer to. according to diodorus (v., ), the sicani, or aborigines preceding the sicilians, were compelled to fly to the western part of the island, in the consequence of successive eruptions extending over many years. the most ancient eruption of mount aetna on record is that mentioned by pindar and schylus, as occurring under hiero, in the second year of the th olympiad. it is probable that hesiod was aware of the devastating eruptions of aetna before the period of greek immigration. there is, however, some doubt regarding the work [greek word] in the text of hesiod, a subject into whci i have entered at some length in another place. (humboldt, 'examen crit. de le geogr.', t. i., p. .) in the time of nero, men were disposed to rank aetna among the volcanic mountains which were graduallybecoming extinct,* and subsequently aelian** even maintained that mariners could no longer see the sinking summit of the mountain from so great a distance at sea. [footnote] *seaeca. 'epist.', . [footnote] ** aelian, 'var. hist.', viii., . where these evidences -- these old scaffoldings of eruption, i might almost say -- still exist, the volcano rises from a crater of elevation, while a high rocky wall surrounds, like an amphitheater, the isolated conical mount, and forms around it a kind of easing of highly elevated p strata. occasionally not a trace of this inclosure is visible, and the volcano, which is not always conical rises immediately from the neighboring plateau in an elongated form, as in the case of pichincha,* at the foot of which lies the city of quito. [footnote] *[this mountain contains two funnel-shaped craters, apparently resulting from two set of eruptions: the western nearly circular, and having in its center a cone of eruption, from the summit and sides of which are no less than seventy vents, some in activity and others extinct. it is probable that the larger number of the vents were produced at periods anterior to history. caubney, op. cit., p. .] -- tr. as the nature of rocks, or the mixture (grouping) of simple minerals into granite, gneiss, and mica slate, or into trachyte, basalt, and dolorite, is independent of existing climates, and is the same under the most varied latitudes of the earth, so also we find every where in inorganic nature that the same laws of configuration regulate the reciprocal superposition of the strata of the earth's crust, cause them to penetrate one another in the form of veins, and elevate them by the agency of elastic forces. this constant recurrence of the same phenomena is most strikingly manifested in volcanoes. when the mariner, amid the islands of some distant archipelago, is no longer guided by the light of the same stars with which he had been familiar in his native latitude, and sees himself surrounded by palms and other forms of an exotic vegetation, he still can trace, reflected in the individual characteristics of the landscape, the forms of vesuvius, of the come-shaped summits of auvergne, the craters of elevation in the canaries and azores, or the fissures of eruption in iceland. a glance at the satellite of our planet will impart a wider generalization to this analogy of configuration. by means of the charts that have been drawn in accordance with the observations made with large telescopes, we may recognize in the moon, where water and air are both absent, vast craters of elevation surrounding or supporting conical mountains, thus affording incontrovertible evidence of the effects produced by the reaction of the interior on the surface, favored by the influence of a feebler force of gravitation. although vocanoes are justy termed in many languages "fire-emitting mountains," mountains of this kind are not formed by the gradual accumulation of ejected currents of lava, but their origin seems rather to be a general consequence of the sudden elevation of soft masses of trachyte or labradoritic augite. the amount of the elevating force is manifested p by the elevation of the volcano, which varies from the inconsiderable height of a hill (as the volcano of cosima, one of the japanese kurile islands) to that of a cone above , feet in height. it has appeared to me that relations of height have a great influence on the occurrence of eruptions, which are more frequent in low than in elevated volcanoes. i might instance the series presented by the following mountains: stromboli, feet; guacamayo, in the province of quixos, from which detonations are heard almost daily (i myself often heard them at chillo, near quito, a distance of eighty-eight miles); vesuvius, feet; aetna, feet; the peak of teneriffe, , feet; and cotopaxi, , feet. if the focus of these volcanoes be at an equal depth below the surface, a greater force must be required where the fused masses have to be raised to an elevation six or eight times greater than that of the lower eminences. while the volcano stromboli (strongyle) has been incessantly active since the homeric ages, and has served as a beacon-light to guide the mariner in the tyrrhenian sea, loftier volcanoes have been characterized by loong intervals of quiet. thus we see that a whole century often intervenes between the eruptions of most of the colossi which crown the summits of the cordilleras of the andes. where we meet with exceptions to this law, to which i long since drew attention, they must depend upon the circumstance that the connections between the volcanic foci and the crater of eruption can not be considered as equaly permanent in the case of all volcanoes. the channel of communication may be closed for a time in the case of the lower ones, so that they less frequently come to a state of eruption, although they do not, on that account, approach more nearly to their final extinction. these relations between the absolute height and the frequency of volcanic eruptions, as far as they are externally perceptible, are intimately connected with the consideration of the local conditions under which lava currents are erupted. eruptions from the crater are very unusual in many mountains, generally occurring from lateral fissures (as was observed in the case of aetna, in the sixteenth century, by the celebrated historian bembo, when a youth*), whenever the sides p of the upheaved mountain were least able, from their configuration and position, to offer any resistance. [footnote] *petri bembi opuscula ('aetna dialogus'), basil, , p. : "quicquid in aetnae matris utero coulescit, nunquam exit ex cratere superiore, quod vel eo inscondere gravis materia non queat, vel, quia inferius alia spiramenta sunt, non fit opus. despumant flammis urgentibus ignei rivi pigro fluxu totas delambentes plagas, et in lapidem indurescunt." cones of eruption are sometimes uplifted on these fissures; the larger ones, which are erroneously termed 'new volcanoes', are ranged together in line marking the direction of a fissure, which is soon reclosed, while the smaller ones are grouped together covering a whole district with their dome-like or hive-shaped forms. to the latter belong the 'hornitos de jorullo',i the cone of vesuvius erupted in october, , that of awatscha, according to postels, and those of the lava-field mentioned by erman, near the baidar mountains, in the peninsula of kamtschatka. [footnote] see my drawing of the volcano of jorullo, of its 'hornitos', and of the uplifted 'malpays', in my 'vues de cordilleres', pl. xliii., p. . [burckhardt states that during the twenty-four years that have intervened since baron humboldt's visit to jorullo, the 'hornitos' have either wholly disappeared or completely changed their forms. see 'aufenthalt und reisen in mexico in und '.] -- tr. when volcanoes are not isolated in a plain, but surrounded, as in the double chain of the andes of quito, by a table-land having an elevation from nine to thirteen thousand feet, this circumstance may probably explain the cause why no lava streams are formed* during the most dreadful eruption of ignited scoriae accompanied by detonations heard at a distance of more than a hundred miles. [footnote] * humboldt, 'essaii sur la geogr. des plantes et tableau phys. des regions equinoxiales', , p. , and 'essai geogn. sur le gisement des roches', p. . most of the volcanoes in java demonstrate that the cause of the perfect absence of lava streams in volcanoes of incessant activity is not alone to be sought for in their form, position, and height. leop. von buch, 'descr. phys. des iles canaries', p. ; reinwardt and hoffmann, in poggened., 'annalen.', bd. xii., s. . such are the volcanoes of popayan, those of the elevated plateau of los pastos and of the andes of quito, with the exception, perhaps, in the case of the latter, of the volcano of antisana. the height of the cone of cinders, and the size and form of the crater, are elements of configuration which yield an especial and individual character to volcanoes, although the cone of cinders and the crater are both wholly independent of the dimensions of the mountain. vesuvius is more than three times lower than the peak of teneriffe; its cone of cinders rises to one third of the height of the whole mountain, while the cone of cinders of the peak is only / d of its altitude. [footnote] * [it may be remarked in general, although the rule is liable to exceptions, that the dimensions of a crater are in an inverse ratio to the elevation of the mountain. daubeney, op. cit., p. .] -- tr. in a much higher volcano than that of teneriffe, the rueu pichincha, other relations occur p which approach more nearly to that of vesuvius. among all the volcanoes that i have seen in the two hemispheres, the conical form of cotopaxi is the most beautifully regular. a sudden fusion of the snow at its cone of cinders announces the proximity of the eruption. before the smoke is visible in the rarefied strata of air surrounding the summit and the opening of the crater, the walls of the cone of cinders are sometimes in a state of glowing heat, when the whole mountain presents an appearance of the most fearful and portentous blackness. the crater, which, with very few exceptions, occupies the summit of the volcano, forms a deep, caldron-like valley, which is often accessible, and whose bottom is subject to constant alterations. the great or lesser depth of the crater is in many volcanoes likewise a sign of the near or distant occurrence of an eruption. long, narrow fissures, from which vapors issue forth, or small rounding hollows filled with molten masses, alternately open and close in the caldron-like valley; the bottom rises and sinks, eminences of scoriae and cones of eruption are formed, rising sometimes far over the walls of the crater, and continuing for years together to impart to the volcano a peculiar character, and then suddenly fall together and disappear during a new eruption. the openings of these cones of eruption, which rise from the bottom of the crater, must not, as is too often done, be confounded with the crater which incloses them. if this be inaccessible from extreme depth and from the perpendicular descent, as in the case of the volcano of rucu pichincha, which is , feet in height, the traveler may look from the edge on the summit of the mountains which rise in the sulphurous atmosphere of the valley at his feet; and i have never beheld a grander or more remarkable picture than that presented by this volcano. in the interval between two eruptions, a crater may either present no luminous appearance, showing merely open fissures and ascending vapors, or the scarcely heated soil may be covered by eminences of scoriae, that admit of being approached without danger, and thus present to the geologist the spectacle of the eruption of burning and fused masses, which fall back on the ledge of the cone of scoriae, and whose appearance is regularly announced by small wholly local earthquakes. lava sometimes streams forth from the open fissures and small hollows, without breaking through or escaping beyond the sides of the crater. if, however, it does break through, the newly-opened terrestrial stream generally flows in such a quiet and well-defined course, that the deep valley, which we term the crater, remains accessible p even during periods of eruption. it is impossible, without an exact representation of the configuration -- the normal type, as it were, of fire-emitting mountains, to form a just idea of those phenomena which, owing to fantastic descriptions and an undefined phraseology, have long been comprised under the head of 'craters, cones of eruption', and 'volcanoes'. the marginal ledges of craters vary much less than one would be led to suppose. a comparison of saussure's measurements with my own yields the remarkable result, for instance, that in the course of forty-nine years (from to ), the elevation of the northwestern margin of mount vesuvius ('rocca del palo') may be considered to have remained unchanged.* [footnote] *see the ground-work of my measurements compared with those of saussure and lord minto, in the 'abhandlungen der akademie der wiss. zu berlin' for the years and . volcanoes which, like the chain of the andes, lift their summits high above the boundaries of the region of perpetual snow, present peculiar phenomena. the masses of snow, by their sudden fusion during eruptions, occasion not only the most fearful inundations and torrents of water, in which smoking scoriae are borne along on thick masses of ice, but they likewise exercise a constant action, while the volcano is in a state of perfect repose, by infiltration into the fissures of the trachytic rock. cavities which are either on the declivity or at the foot of the mountain are gradually converted into subterranean resevoirs of water, which communicate by numerous narrow openings with mountain streams, as we see exemplified in the highlands of quito. the fishes of these rivulets multiply, especially in the obscurity of the hollows; and when the shocks of earthquakes, which precede all eruptions in the andes, have violently shaken the whole mass of the volcano, these subterranean caverns are suddenly opened, and water, fishes, and tufaceous mud are all ejected together. it is through this singular phenomenon* that the inhabitants of the highlands of quito became acquainted with the existence of the little cyclopic fishes, termed by them the prenadilla. [footnote] *pimelodes cyclopum. see humboldt, 'recueil d'observations de zoologie et d'anatomie comparee', t. i., p. - . on the night between the th and th of june, , when the summit of carguairazo, a mountain , feet in height, fell in, leaving only two huge masses of rock remaining of the ledge of the crater, a space of nearly thirty-two square miles was overflowed and devastated by streams of liquid tufa and argillaceous mud ('lodazales'), containing large quantities of dead fish. p in like manner, the putrid fever, which raged seven years previously in the mountain town of ibarra, north of quito, was ascribed to the ejection of fish from the volcano of imbaburu.* [footnote] *[it would appear, as there is no doubt that these fishes proceed from the mountain itself, that there must be large lakes in the interior, which in ordinary season are out of the immediate influence of the volcanic action. see daubeney, op. cit., p. , .] -- tr. water and mud, which flow not from the crater itself, but from the hollows in the trachytic mass of the mountain, can not, strictly speaking, be classed among volcanic phenomena. they are only indirectly connected with the volcanic activity of the mountain, resembling, in that respect, the singular meteorological process which i have designated in my earlier writings by the term of 'volcanic storm'. the hot stream which rises from the crater during the eruption and spreads itself in the atmosphere, condenses into a cloud, and surrounds the column of fire and cinders which rises to an altitude of many thousand feet. the sudden condensation of the vapors, and, as gay-lussac has shown, the formation of a cloud of enormous extent, increase the electric tension. forked lightning flashes from the column of cinders, and it is then easy to distinguish (as at the close of the eruption of mount vesuvius, in the latter end of october, ) the rolling thunder of the volcanic storm from the detonations in the interior of the mountain. the flashes of lightning that darted from the volcanic cloud of steam, as we learn from olafsen's report, killed eleven horses and two men, on the eruption of the volcano of katlagia, in iceland, on the th of october, . having thus delineated the structure and dynamic activity of volcanoes, it now remains for us to throw a glance at the differences existing in their material products. the subterranean forces sever old combinations of matter in order to produce new ones, and they also continue to act upon matter as long as it is in a state of liquefaction from heat, and capable of being displaced. the greater or less pressure under which merely softened or wholly liquid fluids are solidified, appears to constitute the main difference in the formation of plutonic and volcanic rocks. the mineral mass which flows in narrow, elongated streams from a volcanic opening (an earth-spring), is called lava. where many such currents meet and are arrested in their course, they expand in width, filling large basins, in which they become solidified in superimposed strata. these few sentences describe the general character of the products of volcanic activity. p rocks which are merely broken through by the volcanic action are often inclosed in the igneous products. thus i have found angular fragments of feldspathic syenite imbedded in the black augitic lava of the volcano of jorullo, in mexico; but the masses of dolomite and granular limestone, which contain magnificent clusters of crystalling fossils (vesuvian and garnets, covered with mejonite, nepheline, and sodalite), are not the ejected products of vesuvius, these belonging rather to very generally distributed formations, viz., strata of tufa, which are more ancient than the elevation of the somma and of vesuvius, and are probably the products of a deep-seated and concealed submarine volcanic action.* [footnote] *leop. von buch, in poggend., 'annalen', bd. xxxvii., s. . we find five metals among the products of existing volcanoes, iron, copper, lead, arsenic, and selenium, discovered by stromeyer in the crater of volcano.* [footnote] *[the little island of volcano is separated from lipari by a narrow channel. it appears to have exhibited strong signs of volcanic activity long before the christian era, and still emits gaseous exhalations. stromeyer detected the presence of selenium in a mixture of sal ammoniac and sulphur. another product, supposed to be peculiar to this volcano, is boracic acid, which lines the sides of the cavities in beautiful white silky crystals. daubeney, op. cit., p. .] -- tr. the vapors that rise from the 'fumarolles' cause the sublimation of the chlorids of iron, copper, lead, and ammonium; iron glancei and chlorid of sodium (the latter often in large quantities) fill the cavities of recent lava streams and the fissures of the margin of the crater. [footnote] *regarding the chemical origin of iron glance in volcanic masses, see mitscherlich, in poggend., 'annalen', bd. xv., s. ; and on the liberation of hydrochloric acid in the crater, see gay-lussac, in the 'annals de chimique et de physique', t. xxii., p. . the mineral composition of lava differs according to the nature of the crystalline rock of which the volcano is formed, the height of the point where the eruption occurs, whether at the foot of the mountain or in the neighborhood of the crater, and the condition of temperature of the interior. vitreous volcanic formations, obsidian, pearl-stone, and pumice, are entirely wanting in some volcanoes, while in the case of others they only proceed from the crater, or, at any rate, from very considerable heights. these important and involved relations can only be explained by very accurate crystallographic and chemical investigations. my fellow-traveler in siberia, gustav rose, and subsequently hermann abich, have already been able, by their fortunate and ingenious researches, to throw much light on the structural relations of the various kinds of volcanic rocks. p the greater part of the ascending vapor is mere steam. when condensed, this forms springs, as in pantellaria,iwhere they are used by the goatherds of the island. [footnote] *[steam issues from many parts of this insular mountain, and several hot springs gush forth from it, which form together a lake feet in circumference. daubeney, op. cit.] -- tr. on the morning of the th of october, , a current was seen to flow from a lateral fissure of the crater of vesuvius, and was loong supposed to have been boiling water; it was, however, shown, by monticelli's accurate investigations, to consist of dry ashes, which fell like sand, and of lava pulverized by friction. the ashes, which sometimes darken the air for hours and days together, and produce great injury to the vineyards and olive groves by adhering to the leaves, indicate by their columnar ascent, impelled by vapors, the termination of every great eqrthquake. this is the magnificent phenomenon which pliny the younger, in his celebrated letter to cornelius tacitus, compares, in the case of vesuvius, to the form of a lofty and thickly-branched and foliaceous pine. that which is described as flames in the eruption of scoriae, and the radiance of the glowing red clouds that hover over the crater, can not be ascribed to the effect of hydrogen gas in a state of combustion. they are rather reflections of light which issue from molten masses, projected high in the air, and also reflections from the burning depths, whence the glowing vapors ascend. we will not, however, attempt to decide the nature of the flames, which are occasionally seen now, as in the time of strabo, to rise from the deep sea during the activity of littoral volcanoes, or shortly before the elevation of a volcanic island. when the questions are asked, what is it that burns in the volcano? what excites the heat, fuses together earths and metals, and imparts to lava currents of thick layers a degree of heat that lasts for many years? it is necessarily implied that volcanoes must be connected with the existence of substances capable of maintaining combustion, like the beds of coal in subterranean fires. [footnote] *see the beautiful experiments on the cooling of masses of rock, in bischof's 'warmelehre', s. , , - . according to the different phases of chemical science, bitumen, pyrites, the moist admixture of finely-pulverized sulphur and iron, pyrophoric substances, and the metals of the alkalies and earths, have in turn been designated as the cause of intensely active volcanic phenomena. the great chemist, sir humphrey davy, to whom we are indebted for the knowledge of the most combustible metallic p substances, has himself renounced his bold chemical hypothesis in his last work ('consolation in travel, and last days of a philosopher') -- a work which can not fail to excite in the reader a feeling of the deepest melancholy. the great mean density of the earth ( . ), when compared with the specific weight of potassium ( . ), of sodium (-. ), or of the metals of the earths ( . ), and the absence of hydrogen gas in the gaseous emanations from the fissures of craters, and from still warm streams of lava, besides many chemical considerations, stand in opposition with the earlier conjectures of davy and ampere.* [footnote] *see berzelius and wohler, in poggend., 'annalen', bd. i., s. , and bd. xi., s. ; gay-lussac, in the 'annals de chimie', t. x., xii., p. ; and bischof's 'reasons against the chemical theory of volcanoes', in the english edition of his 'warmelehre', p. - . if hydrogen were evolved from erupted lava, how great must be the quantity of the gas disengaged, when, the seat of the volcanic activity being very low, as in the case of the remarkable eruption at the foot of the skaptar jokul in iceland (from the th of june to the d of august, , described by mackenzie and soemund magnussen), a space of many square miles was covered by streams of lava, accumulated to the thickness of several hundred feet! similar difficulties are opposed to the assumption of the penetration of the atmospheric air into the crater, or, as it is figuratively expressed, the 'inhalation of the earth', when we have regard to the small quantity of nitrogen emitted. so general, deep-seated, and far-propagated an activity as that of volcanoes, can not assuredly have its source in chemical affinity, or in the mere contact of individual or merely locally distributed substances. modern geognosy* rather seeks the cause of this activity in the increased temperature with the increase of depth at all degrees of latitude, in that powerful internal heat which our planet owes to its first solidification, its formation in the regions of space, and to the spherical contraction of p matter revolving elliptically in a gaseous condition. [footnote] *[on the various theories that have been advanced in explanation of volcanic action, see daubeney 'on volcanoes', a work to which we have made continual reference during the preceding pages, as it constitutes the most recent and perfect compendium of all the important facts relating to this subject, and is peculiarly adapted to serve as a source of reference to the 'cosmos', since the learned author in many instances enters into a full exposition of the views advanced by baron humboldt. the appendix contains several valuable notes with reference to the most recent works that have appeared on the continent, on subjects relating to volcanoes; among others, an interesting notice of professor bischof's views "on the origin of the carbonic acid discharged from volcanoes," as enounced in his recently published work, 'lehrbuch der chemischen und physikalischen geologie'.] -- tr. we have thus mere conjecture and supposition side by side with certain knowledge. a philosophical study of nature strives ever to elevate itself above the narrow requirements of mere natural description, and does not consist, as we have already remarked, in the mere accumulation of isolated facts. the inquiring and active spirit of man must be suffered to pass from the present to the past, to conjecture all that can not yet be known with certainty, and still to dwell with pleasure on the ancient myths of geognosy which are presented to us under so many various forms. if we consider volcanoes as irregular intermittent springs, emitting a fluid mixture of oxydized metals, alkalies, and earths, flowing gently and calmy wherever then find a passage, or being upheaved by the powerful expansive force of vapors, we are involuntarily led to remember the geognostic visions of plato, according to which hot springs, as well as all volcanic igneous streams, were eruptions that might be traced back to one generally distributed subterranean cause, 'pyriphlegethon'.* [footnote] *according to plato's geognostic views, as developed in the 'phaedo', pyriphlegethon plays much the same part in relation to the activity of volcanoes that we now ascribe to the augmentation of heat as we descend from the earth's surface, and to the fused condition of its internal strata. ('phaedo', ed. ast, p. and ; annot., p. and .) "within the earth, and all around it, are larger and smaller caverns. water flows there in abundance; also much fire and large streams of fire, and streams of moist mud (some purer and others more filthy), like those in sicily, consisting of mud and fire, preceding the great eruption. these streams fill all places that fall in the way of their course. pyriphlegethon flows forth into an extensive district burning with a fierce fire, where it forms a lake larger than our sea, boiling with water and mud. from thence it moves in circles round the earth, turbid and muddy." this stream of molten earth and mud is so much the general cause of volcanic phenomena, that plato expressly adds, "thus is pyriphlegethon constituted, from which also the streams of fire ([greek words]), wherever they reach the earth ([greek words]), inflate such parts (detached fragments)." volcanic scoriae and lava streams are therefore portions of pyriphlegethon itself, portions of the subterranean molten and ever-undulating mass. that {greek words] are lava streams, and not, as schneider, passow, and schleiermacher will have it, "fire-vomiting mountains," is clear enough from many passages, some of which have been collected by ukert ('geogr. der griechen und romer', th. ii., s. ): [greek word] is the volcanic phenomenon in reference to its most striking characteristic, the lava stream. hence the expression, the [greek word] of aetna. aristot. 'mirab. ausc.', t. ii., p. ; sect. , bekker; thucyd., iii., ; theophrast., 'de lap'., , p. , schneider; diod., v., , and xiv., , where are the remarkable words, "many places near the sea, in the neighborhood of aetna, were leveled to the ground, [greek words];" strabo, vi., p. ; xiii., p. , and where there is a notice of the celebrated burning mud of the lelantine plains, in euboea, i., p. , casaub.; and appian, 'de bello civili', v., . the blame which aristotle throws on the geognostical fantasies of the phaedo ('meteor.', ii., , ) is especially applied to the sources of the rivers flowing over the earth's surface. the distinct statement of plato, that "in sicily eruptions of wet mud precede the glowing (lava) stream," is very remarkable. observations on aetna could not have led to such a statement, unless pumice and ashes, formed into a mud-like mass by admixture with melted snow and water, during the volcano-electric storm in the crater of eruption, were mistaken for ejected mud. it is more probable that plato's streams of moist mud ([greek words]) originated in a faint recollection of the salses (mud volcanoes) of agrigentum, which, as i have already mentioned, eject argillaceous mud with a loud noise. it is much to be regretted, in reference to this subject, that the work of theophrastus [greek words] 'on the volcanic stream in sicily', to which diog. laert., v., , refers, has not come down to us. p the different volcanoes over the earth's surface, when they are considered independently of all climatic differences, are acutely and characteristically classified as central and linear volcanoes. under the first name are comprised those which constitute the central point of many active mouths of eruption, distributed almost regularly in all directions; under the second, those lying at some little distance from one another, forming, as it were, chimneys or vents along an extended fissure. linear volcanoes again admit of further subdivision, namely, those which rise like separate conical islands from the bottom of the sea, being generally parallel with a chain of primitive mountains, whose foot they appear to indicate, and those volcanic chains which are elevated on the highest ridges of these mountain chains, of which they form the summits.* [footnote] *leopold von buch, 'physikal. beschreib. der canarischen inseln', s. - . i doubt if we can agree with the ingenious charles darwin ('geological observations on volcanic islands', , p. ) in regarding central volcanoes in general as volcanic chains of small extent on parallel fissures. friedrich hoffman believes that in the group of the lipari islands, which he has so admirably described, and in which two eruption fissures intersect near panaria, he has found an intermediate link between the two principal modes in which volcanoes appear, namely, the central volcanoes and volcanic chains of von buch (poggendorf, 'annalen der physik', bd. xxvi., s. - ). the peak of teneriffe, for instance, is a central volcano, being the central point of the volcanic group to which the eruption of palma and landerote may be referred. the long, rampart-like chain of the andes, which is sometimes single, and sometimes divided into two or three parallel branches, connected by various transverse ridges, presents, from the south of chili to the northwest coast of america, one of the grandest instances of a continental volcanic chain. the proxiimity of p active volcanoes is always manifested in the chain of the andes by the appearance of certain rocks (as dolerite, melaphyre, trachyte, andesite, and dioritic porphyry), which divide the so-called primitive rocks, the transition slates and sandstones, and the stratified formations. the constant recurrence of this phenomenon convinced me long since that these sporadic rocks were the seat of volcanic phenomena, and were connected with volcanic eruptions. at the foot of the grand tunguragua, near penipe, on the banks of the rio puela, i first distinctly observed mica slate resting on granite, broken through by a volcanic rock. in the volcanic chain of the new continent, the separate volcanoes are occasionally, when near together in mutual dependence upon one another; and it is even seen that the volcanic activity for centuries together has moved on in one and the same direction, as for instance, from north to south in the province of quito.* [footnote] (humboldt, 'geognost. beobach, uber die vulkane des hochlandes von quito', in poggend., 'annal. der physik', bd. xliv., s. . the focus of the volcanic action lies below the whole of the highlands of this province; the only channels of communication with the atmosphere are, however, those mountains which we designate by special names, as the mountains of pichincha, cotopaxi, and tunguragua, and which, from their grouping, elevation, and form, constitute the grandest and most picturesque spectacle to be found in any volcanic district of an equally limited extent. experience shows us, in many instances, that the extremities of such groups of volcanic chains are connected together by subterranean communications; and this fact reminds us of the ancient and true expression made use of by seneca,* that the igneous mountain is only the issue of the more deeply-seated volcanic forces. [footnote] *seneca, while he speaks very clearly regarding the problematical sinking of aetna, says in his th letter, "though this might happen, not because the mountain's height is lowered, but because the fires are weakened, and do not blaze out with their former vehemence; and for which reason it is that such vast clouds of smoke are not seen in the day-time. yet neither of these seem incredible, for the mountain may possibly be consumed by being daily devoured, and the fire not be so large as formerly, since it is not self-generated here, but is kindled in the distant bowels of the earth, and there rages, being fed with continual fuel, not with that of the mountain, through which it only makes its passage." the subterranean communication, "by galleries," between the volcanoes of sicily, lipari, pithecusa (ischia), and vesuvius, "of the last of which we may conjecture that it formerly burned and presented a fiery circle," seems fully understood by strabl (lib. i., p. and ). he terms the whole district "sub-igneous." in the mexican highlands a mutual dependence is p also observed to exist among the volcanic mountains orizaba, popocatepel, jorullo, and colima; and i have shown* that they all lie in one direction between degrees ' and degrees ' north latitude, and are situated in a transverse fissure running from sea to sea. [footnote] *humboldt, 'essai politique sur la nouv. espagne', t. ii., p. - . the volcano of jorullo broke forth on the th of september, , exactly in this direction, and over the same transverse fissure, being elevated to a height of feet above the level of the surrounding plain. the mountain only once emitted an eruption of lava, in the same manner as is recorded of mount epomeo in ischia, in the year . but although jorullo, which is eighty miles from any active volcano, is in the strict sense of the word a new mountain, it must not be compared with monte nuovo, near puzzuolo, which first appeared on the th of september, , and is rather to be classed among craters of elevation. i believe that i have furnished a more natural explanation of the eruption of the mexican volcano, in comparing its appearance to the elevation of the hill of methone, now methana, in the peninsula of troezene. the description given by strabo and pausanias of this elevation, led one of the roman poets, most celebrated for his richness of fancy, to develop views which agree in a remarkable manner with the theory of modern geognosy. "near troezene is a tumulus, steep and devoid of trees, once a plain, now a mountain. the vapors inclosed in dark caverns in vain seek a passage by which they may escape. the heavier earth, inflated by the force of the compressed vapors, expands like a bladder filled with air, or like a goat-skin. the ground has remained thus inflated, and the high projecting eminence has been solidified by time into a naked rock." thus picturesquely, and, as analogous phenomena justify us in believing, thus truly has ovid described that great natural phenomenon which occurred years before our era, and consequently, years bfore the volcanic separation of thera (santorino) and therasia, between troezene and epidaurus, on the same spot where russegger has found veins of trachyte.* [footnote] *ovid's description of the eruption of methone ('metam.', xv., p. - ): "near troezene stands a hill, exposed in air to winter winds, of leafy shadows bare: this once was level ground; but (strange to tell) th' included vapors, that in caverns dwell, laboring with colic pangs, and close confined, in vain sought issue for the rumbling wind: yet still they heaved for vent, and heaving still, enlarged the concave and shot up the hill, as breath extends a bladder, or the skins of goats are blown t'inclose the hoarded wines; the mountain yet retains a mountain's face, and gathered rubbish heads the hollow space." 'dryden's translation'. [footnote continues] this description of a dome-shaped elevation on the continent is of great importance in a geognostical point of view, and coincides to a remarkable degree with aristotle's account ('meteor.', ii., , - ) of the upheaval of islands of eruption: "the heaving of the earth does not cease till the wind [(greek word)] which occasions the shocks has made its escape into the crust of the earth. it is not long ago since this actually happened at heraclea in pontus, and a similar event formerly occurred at hiera, one of the aeolian islands. a portion of the earth swelled up, and with loud noise rose into the form of a hill, till the mighty urging blast [(greek word)] found an outlet, and ejected sparks and ashes which covered the neighborhood of lipari, and even extended to several italian cities." in this description, the vesicular distension of the earth's crust (a stage at which many trachytic mountains have remained) is very well distinguished from the eruption itself. strabo, lib. i., p. (casaubon), likewise describes the phenomenon as it occurred at methone: near the town, in the bay of hermione, there arose a flaming eruption; a fiery mountain, seven (?) stadia in height, was then thrown up, which during the day was inaccessible from its heat and sulphureous stench, but at night evolved an agreeable odor (?) , and was so hot that the sea boiled for a distance of five stadia, and was turbid for full twenty stadia, and also was filled with detached masses of rock. regarding the present mineralogical character of the peninsula of methana, see fiedler, 'reise durch griechenland', th. i., s. - . p santorino is the most important of all the 'islands of eruption' belonging to volcanic chains.* [footnote] *[i am indebted to the kindness of professor e. forbes for the following interesting account of the island of santorino, and the adjacent islands of neokaimeni and microkaimeni. "the aspect of the bay is that of a great crater filled with water, thera and therasia forming its walls, and the other islands being after-productions in its center. we sounded with fathoms of line in the middle of the bay, between therasia and the main islands, but got no bottom. both these islands appear to be similarly formed of successive strata of volcanic ashes, which, being of the most vivid and variegated colors, present a striking contrast to the black and cindery aspect of the central isles. neokaimeni, the last-formed island, is a great heap of obsidian and scoriae. so, also, is the greater mass, microkaimeni, which rises up in a conical form, and has a cavity or crater. on one side of this island, however, a section is exposed, and cliffs of fine pumiceous ash appear stratified in the greater islands. in the main island, the volcanic strata abut against the limestone mass of mount st. elias in such a way as to lead to the inference that they were deposited in a sea bottom in which the present mountain rose as a submarine mass of rock. the people at santorino assured us that subterranean noises are not unfrequently heard, especially during calms and south winds, when they say the water of parts of the bay becomes the color of sulphur. my own impression is, that this group of islands, constitutes a crater of elevation, of which the outer ones are the remains of the walls, while the central group are of later origin, and consist partly of upheaved sea bottoms and partly of erupted matter -- erupted, however, beneath the surface of the water."] -- tr. it combines within itself p the history of all islands of elevation. for upward of years, as far as history and tradition certify, it would appear as if nature were striving to form a volcano in the midst of the crater of elevation."* [footnote] *leop. von buch, 'physik. beschr. der canar. inseln', s. - , and particularly the french translation of this excellent work, p. ; and his memoir in poggendorf's 'annalen', bd. xxxviii., s. . a submarine island has quite recently made its appearance within the crater of santorino. in it was still fifteen fathoms below the surface of the sea, but in it had risen to within three or four. it rises steeply like a great cone, from the bottom of the sea, and the continuous activity of the submarine crater is obvious from the circumstance that sulphurous acid vapors are mixed with the sea water, in the eastern bay of neokaimeni, in the same manner as at vromolimni, near methana. coppered ships lie at anchor in the bay in order to get their bottoms cleaned and polished by this natural (volcanic) process. (virlet, in the 'bulletin de la societe geologique de france', t. iii., p. , and fiedler 'reise durch griechenland', th. ii., s. and .) similar insular elevations, and almost always at regular intervals of or years,* have been manifested in the island of st. michael, in the azores; but in this case the bottom of the sea has not been elevated at exactly the same parts.** [footnote] *appearance of a new island near st. miguel, one of the azores, th of june, , st of december, , th of june, . [footnote] **[my esteemed friend, dr. webster, professor of chemistry and mineralogy at harvard college, cambridge, massachusetts, u. s., in his 'description of the island of st. michael, etc.', boston, , gives an interesting account of the sudden appearance of the island named sabrina which was about a mile in circumference, and two or three hundred feet above the level of the ocean. after continuing for some weeks, it sank into the sea. dr. webster describes the whole of the island of st. michael as volcanic, and containing a number of conical hills of trachyte, several of which have craters, and appear at some former time to have been the openings of volcanoes. the hot springs which abound in the island are impregnated with sulphureted hydrogen and carbonic acid gases, appearing to attest the existence of volcanic action.] -- tr. the island which captain tillard named 'sabrina', appeared unfortunately at a time (the th of january, ) when the political relations of the maritime nations of western europe prevented that attention being bestowed upon the subject by scientific institutions which was afterward directed to the sudden appearance (the d of july, ), and the speedy destruction of the igneous island of ferdinandea in the sicilian sea, between the limestone shores of sciacca and the purely volcanic island of pantellaria.* [footnote] *prevost, in the bulletin de la societe geologique, t. iii., p. ; friedrich hoffman, 'hinterlassene werke.' bd. ii., s. - . p the geographical distribution of the volcanoes which have been in a state of activity during historical times, the great number of insular and littoral volcanic mountains, and the occasional, although ephemeral, eruptions in the bottom of the sea, early led to the belief that volcanic activity was connected with the neighborhood of the sea, and was dependent upon it for its continuance. "for many hundred years," says justinian, or rather trogus pompeius, whom he follows,* "aetna and the aeolian islands have been burning, and how could this have continued so long if the fire had not been fed by the p neighboring sea?"** [footnote] *"accedunt vicini et perpetui aetnae montis ignes et insularum aeolidum, veluti ipsis undis alatur incendium; neque enim aliter durare tot seculis tantus ignis potuisset, nisi humoris nutrimentis aleretur." (justin, 'hist. philipp.', iv., i.) the volcanic theory with which the physical description of sicily here begins is extremely intricate. deep fissured; violent motion of the waves of the sea, which, as they strike together, draw down the air (the wind) for the maintenance of the fire: such are the elements of the theory of trogus. since he seems from pliny (xi., ) to have been a physiognomist, we may presume that his numerous lost works were not confined to history alone. the opinion that air is forced into the interior of the earth, there to act on the vocanic furnaces, was connected by the ancients with the supposed influence of winds from different quarters on the intensity of the fires burning in tna, hiera, and stromboli. (see the remarkable passage in strabo, liv. vi., aetna.) the mountain island of stromboli (strongyle) was regarded therefore, as the dwelling-place of aeolus, "the regulator of the winds," in consequence of the sailors foretelling the weather from the activity of the volcanic eruptions of this island. the connection between the eruption of a small volcano with the state of the barometer and the direction of the wind is still generally recognized (leop. von buch, 'descr. phys. des iles canaries', p. ; hoffmann, in poggend., 'annalen', bd. xxvi., s. viii), although our present knowledge of volcanic phenomena, and the slight changes of atmospheric pressure accompanying our winds, do not enable us to offer any satisfactory explanation of the fact. bembo, who during his youth was brought up in sicily by greek refugees, gave an agreeable narrative of his wanderings, and in his 'aetna dialogus' (written in the middle of the sixteenth century) advances the theory of the penetration of sea water to the very center of the volcanic action, and of the necessity of the proximity of the sea to active volcanoes. in ascending aetna the following question was proposed: "explaina potius nobis quae petimus, ea incendia unde oriantur et orta quomodo perdurent. in omni tellure nuspiam majores fistulae aut meatus ampliores sunt quam in locis, quae vel mari vicina sunt, vel a mari protinus alluntur: mare erodit illa facillime pergitque in viscera terrae. itaque cum in aliena regna sibi viam faciat, ventis etiam facit; ex quo fit, ut loca quaeque maritima maxime terrae motibus subjecta sint, parum mediterranea. habes quum in sulfuris venas venti furentes inciderint, unde incendia oriantur tn tuae. vides, quae mare in radicibus habeat, quae sulfurea sit, quae cavernosa, quae a mari aliquando perforata ventos admiscrit aestuantes, per quos idonea flammae materies incenderetur." [footnote] **[although extinct volcanoes seem by no means confined to the neighborhood of the present seas, being often scattered over the most inland portions of our existing continents, yet it will appear that, at the time at which they were in an active state, the greater part were in the neighborhood either of the sea, or of the extensive salt or fresh water lakes, which existed at that period over much of what is now dry land. this may be seen either by referring to dr. boue's map of europe, or to that published by mr. lyell in the recent edition of his 'principles of geology' ( ), from both of which it will become apparent that, at a comparatively recent epoch, those parts of france, of germany, of hungary, and of italy, which afford evidences of volcanic action now extinct, were covered by the ocean. daubeney 'on volcanoes', p. .] -- tr. in order to explain the necessity of the vicinity of the sea, recourse has been had, even in modern times, to the hypothesis of the penetration of sea water into the foci of volcanic agency, that is to say, into deep-seated terrestrial strata. when i collect together all the facts that may be derived from my own observation and the laborious researches of others, it appears to me that every thing in this great quantity of aqueous vapors, which are unquestionably exhaled from volcanoes even when in a state of rest, be derived from sea water impregnated with salt, or rather, perhaps with fresh meteoric water; or whether the expansive force of the vapors (which, at a depth of nearly , feet, is equal to atmospheres) would be able at different depths to counterbalance the hydrostatic pressure of the sea, and thus afford them, under certain conditions, a free access to the focus;* or whether the formation of metallic chlorids, the presence of chlorid of sodium in the fissures of the crater, and the frequent mixture of hydrochloric acid with the aqueous vapors, necessarily imply access of sea water; or, finally, whether the repose of volcanoes (either when temporary, or permanent and complete) depends upon the closure of the channels by which the sea or meteoric water was conveyed, or whether the absence of flames and of exhalations of hydrogen (and sulphureted hydrogen gas seems more characteristic of solfataras than of active volcanoes) is not directly at variance p with the hypothesis of the decomposition of great masses of water?** [footnote] * compare gay-lussac, 'sur les volcans', in the 'annales de chimie', t. xxii., p. , and bischof, 'warmelehre', s. . the eruptions of smoke and steam which have at different periods been seen in lancerote, iceland, and the kurile islands, during the eruption of the neighboring volcanoes, afford indications of the reaction of volcanic foci through tense columns of water; that is to say, these phenomena occur when the expansive force of the vapor exceeds the hydrostatic pressure. [footnote] ** [see daubeney 'on volcanoes', part iii., ch. xxxvi., xxxviii., xxxix.] -- tr. the discussion of these important physical questions does not come within the scope of a work of this nature; but, while we are considering these phenomena, we would enter somewhat more into the question of the geographical distribution of still active volcanoes. we find, for instance, that in the new world, three, viz., jorullo, popocatepetl, and the volcano of de la fragua, are situated at the respective distances of , , and miles from the sea-coast, while in central asia, as abel remusat* first made known to geognosists, the thianschan (celestial mountains), in which are situated the lava-emitting mountain of pe-schan, the solfatara of urumtsi, and the still active igneous mountain (ho-tscheu) of turfan, lie at an almost equal distance ( to miles) from the shores of the polar sea and those of the indian ocean. [footnote] *abel remusat, 'lettre a m. cordier', in the 'annales de chimie', t. v., p. . pe-schan is also fully miles distant from the caspian sea,* and and miles from the seas of issikul and balkasch. [footnote] *humboldt, 'asie centrale', t. ii., p. - , - , - , and - . the existence of active volcanoes in kordofan, miles from the red sea, has been recently contradicted by ruppell, 'reisen in nubien', , s. . it is a fact worthy of notice, that among the four great parallel mountain chains which traverse the asiatic continent from east to west, the altai, the thianschan, the kuen-lun, and the himalaya, it is not the latter chain, which is nearest to kuen-lun, at the distance of and miles from the sea, which have fire-emitting mountains like aetna and vesuvius, and generate ammonia like the volcano of guatimala. chinese writers undoubtedly speak of lava streams when they describe the emissions of smoke and flame, which, issuing from pe-schan, devastated a space measuring ten li* in the first and seventh centuries of our era. [footnote] *[a 'li' is a chinese measurement, equal to about one thirtieth of a mile.] -- tr. burning masses of stone flowed, according to their description "like thin melted fat." the facts that have been enumerated, and to which sufficient attention has not been bestowed, render it probable that the vicinity of the sea, and the penetration of sea water to the foci of volcanoes, are not absolutely necessary to the eruption of p subterranean fire, and that littoral situations only favor the eruption by forming the margin of a deep sea basin, which, covered by strata of water, and lying many thousand feet lower than the interior continent, can offer but an inconsiderable degree of resistance. the present active volcanoes, which communicate by permanent craters simultaneously with the interior of the earth and with the atmosphere, must have been formed at a subsequent period, when the upper chalk strta and all the tertiary formations were already present: this is shown to be the fact by the trachytic and basaltic eruptions which frequently form the walls of the crater of elevation. melaphyres extend to the middle tertiary formations, but are found already in the jura limestone, where they break through the variegated sandstone.* [footnote] *dufrenoy et elie de beaumont, 'explication de la carte geologique de la france', t. i., p. . we must not confound the earlier outpourings of granite, quartzose porphyry, and euphotide from temporary fissures in the old transition rocks with the present active volcanic craters. the extinction of volcanic activity is either only partial -- in which case the subterranean fire seeks another passage of escape in the same mountain chain -- or it is total, as in auvergne. more recent examples are recorded in historical times, of the total extinction of the volcano of mosychlos,* on the island sacred to hephaestos (vulcan), whose "high whirling flames" were known to sophocles; and of the volcano of medina, which according to burckhardt, still continued to pour out a stream of lava on the d of november, . [footnote] *sophocl., 'philoct.', v. and . on the supposed epoch of the extinction of the lemnian fire in the time of alexander, compare buttmann, in the 'museum der alterhumswissenschaft', bd. i., , s. ; dureau de la malle, in malte-brun, 'annales des voyages', t. ix., , p. ; ukert in bertuch, 'geogr. ephemeriden', bd. xxxix., , s. ; rhode, 'res lemnicae', , p. ; and walter, 'ueber abnahame der vulken. thatigkeit in historischen zeiten', , s. . the chart of lemmos, constructed by choiseul, makes it extremely probable that the extinct crater of mosychlos, and the island of chryse, the desert habitation of philoctetes (otfried muller, 'minyer', s. ), have been long swallowed up by the sea. reefs and shoals, to the northeast of lemnos, still indicate the spot where the aegean sea once possessed an active volcano like aetna, vesuvius, stromboli, and volcano (in the lipari isles). every stage of volcanic activity, from its first origin to its extinction, is characterized by peculiar products; first by ignited scoriae, streams of lava consisting of trachyte, pyroxene, and obsidian, and by rapilli and tufaceous ashes, accompanied by the development p of large quantities of pure aqueous vapor; subsequently, when the volcano becomes a solfatara, by aqueous vapors mixed with sulphureted hydrogen and carbonic acid gases; and, finally, when it is completely cooled, by exhalations of carbonic acid alone. there is a remarkable class of igneous mountains which do not eject lava, but merely devastating streams of hot water,* impregnated with burning sulphur and rocks reduced to a state of dust (as, for instance, the galungung in java); but whether these mountains present a normal condition, or only a certain transitory modification of the volcanic process, must remain undecided until they are visited by geologists possessed of a knowledge of chemistry in its present condition. [footnote] *compare reinwardt and hoffmann, in poggendorf's 'annalen', bd. xii., s. ; leop. von buch, 'descr. des iles canaries', p. - . the eruptions of argillaceous mud at carguairazo, when that volcano was destroyed in , the lodazales of igualata, and the moya of pelileo -- all on the table-land of quito -- are volcanic phenomena of a similar nature. i have endeavored in the above remarks to furnish a general description of volcanoes -- comprising one of the most important sections of the history of terrestrial activity -- and i have based my statements partly on my own observations, but more in their general bearing on the results yielded by the labors of my old friend, leopold von buch, the greatest geognosist of our own age, and the first who recognized the intimate connection of volcanic phenomena, and their mutual dependence upon one another, considered with reference to their relations in space. volcanic action, or the reaction of the interior of a planet on its external crust and surface, was long regarded only as an isolated phenomenon, and was considered solely with respect to the disturbing action of the subterranean force; and it is only in recent times that -- greatly to the advantage of geognostical views based on physical analogies -- volcanic forces have been regarded as 'forming new rocks, and transforming those that already existed'. we here arrive at the point to which i have already alluded, at which a well-grounded study of the activity of volcanoes, whether igneous or merely such as emit gaseous exhalations, leads us, on the one hand, to the mineralogical branch of geognosy (the science of the texture and the succession of terrestrial strata), and, on the other, to the science of geographical forms and outlines -- the configuration of continents and insular groups elevated above the level p of the sea. this extended insight into the connection of natural phenomena is the result of the philosophical direction which has been so generally assumed by the more earnest study of geognosy. increased cultivation of science and enlargement of political views alike tend to unite elements that had long been divided. this material taken from pages - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p if, instead of classifying rocks according to their varieties of form and superposition into stratified and unstratified, schistose and compact, normal and abnormal, we investigate those phenomena of formation and transformation which are still going on before our eyes, we shall find that rocks admit of being arranged according to four modes of origin. 'rocks of eruption', which have issued from the interior of the earth either in a state of fusion from volcanic action, or in a more or less soft, viscous condition, from plutonic action. 'sedimentary rocks', which have been precipitated and deposited on the earth's surface from a fluid, in which the most minute particles were either dissolved or held in suspension constituting the greater part of the secondary (or flotz) and tertiary groups. 'transformed or metamorphic rocks',* in which the internal texture and the mode of stratification have been changed, either p by contact or proximity with a plutonic or volcanic endogenous rock of eruption,** or, what is more frequently the case, by a gaseous sublimation of substances*** which accompany certain masses erupted in a hot, fluid condition. [footnote] *[as the doctrine of mineral metamorphism is now exciting very general attention, we subjoin a few explanatory observations by the 'new philos. journ.', jan., : "in its widest sense, mineral metamorphism means every change of aggregation, structure, or chemical condition which rocks have undergone subsequently to their deposition and stratification, or the effects which have been produced by other forces than gravity and cohesion. there fall under this definition, the discoloration of the surface of black limestone by the loss of carbon; the formation of brownish-red crusts on rocks of limestone, sandstone, many slate structures, serpentine, granite, etc., by the decomposition of iton pyrites, or magnetic iron, finely disseminated in the mass of the rock; the conversion of anhydrite into gypsum, in consequence of the absorption of water; the crumbling of many granites and porphyries into gravel, occasioned by the decomposition of the mica and feldspar. in its more limited sense, the term metamorphic is confined to those changes of the rock which are produced, not by the effect of the atmosphere or of water on the exposed surfaces, but which are produced, directly or indirectly, by agencies seated in the interior of the earth. in many cases the mode of change may be explained by our physical or chemical theories, and may be viewed as the effect of temperature or of electro-chemical actions. adjoining rocks, or connecting communications with the interior of the earth, also distinctly point out the seat from which the change proceeds. in many other cases the metamorphic process itself remains a mystery, and from the nature of the products alone do we conclude that such a metamorphic action has taken place.] -- tr. [footnote] ** in a plan of the neighborhood of tezcuco, totonilco, and moran ('atlas geographique et physique', pl. vii.), which i originally ( ) intended for a work which i never published, entitled 'pasigrafia geognostica destinada al uso de los jovenes del colegio de mineria de mexico', i names (in ) the plutonic and volcanic eruptive rocks 'endogenous' (generated in the interior), and the sedimentary and flotz rocks 'exogenous' (or generated externally on the surface of the earth). pasiward, [upward arrow] and the latter by the same symbol directed downward [downward arrow]. these signs have at least some advantage over the ascending lines, which in the older systems represent arbitrarily and ungracefully the horizontally ranged sedimentary strata, and their penetration through masses of basalt, porphyry, and syenite. the names proposed in the pasigraphico-geognostic plan were borrowed from de candolle's nomenclature, in which 'endogenous' is synonymous with monocotyledonous, and 'exogenous' with dicotyledonous plants. mohl's more accurate examination of vegetable tissues has, however, shown that the growth of monocotyledons from within, and dicotyledons from without, is not strictly and generally true for vegetable organisms (link, 'elementa philosophiae botanicae', t. i., , p. ; endlicher and unger, 'grundzugeder botanik', , s. ; and jussieu, 'traite de botanique', t. i., p. ). the rocks which i have termed endogenous are characteristically distinguished by lyell, in his 'principles of geology', , vol. iii., p. , as "nether-formed" or "hypogene rocks." [footnote] *** compare leop. von buch, 'ueber dolomit als gebirgsart', , s. ; and his remarks on the degree of fluidity to be ascribed to plutonic rocks at the period of their eruption, as well as on the formation of gneiss from schist, through the action of granite and of the substances upheaved with it, to be found in the 'abhandl. der akad. der wissensch. zu berlin' for the year , s. und , and in the 'jahrbuch fur wissenschaftliche kritik', , s. . 'conglomerates'; coarse or finely granular sandstones, or breccias composed of mechanically-divided masses of the three previous species. these four modes of formation -- by the emission of volcanic masses, as narrow lava streams; by the action of these masses on rocks previously hardened; by mechanical separation or chemical precipitation from liquids impregnated with carbonic acid; and, finally, by the cementation of disintegrated rocks of heterogeneous nature -- are phenomena and formative processes which must merely be regarded as a faint reflection of that more energetic activity which must have characterized the chaotic condition of the earlier world under wholly different conditions of pressure and at a higher temperature, not only in the whole crust of the earth, but likewise in the more p extended atmosphere, overloaded with vapors. the vast fissures which were formerly open in the solid crust of the earth have since been filled up or closed by the protrusion of elevated mountain chains, or by the penetration of veins of rocks of eruption (granite, porphyry, basalt, and melaphyre); and while, scarcely more than four volcanoes remaining through which fire and stones are erupted, the thinner, more fissured, and unstable crust of the earth was anciently almost every where covered by channels of communication between the fused interior and the external atmosphere. gaseous emanations rising from very unequal depths, and therefore conveying substances differing in their chemical nature, imparted greater activity to the plutonic processes of formation and transformation. the sedimentary formations, the deposits of liquid fluids from cold and hot springs, which we daily see producing the travertine strata near rome, and near hobart town in van diemen's land, afford but a faint idea of the flotz formation. in our seas, small banks of limestone, almost equal in hardness at some parts to carrara marble,* are in the course of formation, by gradual precipitation, accumulation, and cementation -- processes whose mode of action has not been sufficiently well investigated. [footnote] darwin, 'volcanic islands', , p. and . the sicilian coast, the island of ascension, and king george's sound in australia, are instances of this mode of formation. on the coasts of the antilles, these formations of the present ocean contain articles of pottery, and other objects of human industry, and in guadaloupe even human skeletons of the carib tribes.* [footnote] *[in most instances the bones are dispersed; but a large slab of rock, in which considerable portion of the skeleton of a female is embedded, is preserved in the british museum. the presence of these bones has been explained by the circumstance of a battle, and the massacre of a tribe of gallibis by the caribs, which took place near the spot in which they are found, about years ago; for, as the bodies of the slain were interred on the sea-shore, their skeletons may have been subsequently covered by sand-drift, which has since consolidated into limestone. dr. moultrie, of the medical college, charleston, south carolina, u.s., is, however, of opinion that these bones did not belong to individuals of the carib tribe, but of the peruvian race, or of a tribe possessing a similar craniological development.] --tr. the negroes of the french colonies designate these formations by the name of 'maconne-bon-dieu'.* moreau de jonnes, 'hist. phys. des antilles', t. i., p. , , and ; humboldt, 'relation historique', t. iii., p. . a small colitic bed, formed in lancerote, one of the canary islands, and which, notwithstanding p its recent formation, bears a resemblance to jura limestone, has been recognized as a product of the sea and of tempests.* [footnote] *near teguiza. leop. von buch, 'canarische inseln', s. . composite rocks are definite associations of certain crytonostic, simple minerals, as feldspar, mica, solid silex, augite, and nepheline. rocks very similar to these consisting of the same elements, but grouped differently, are still formed by volcanic processes, as in the earlier periods of the world. the character of rocks, as we have already remarked is so independent of geographical relations of space,* that the geologist recognizes with surprise, alike to the north or the south of the equator, in the remotest and most dissimilar zones, the familiar aspect, and the repetition of even the most minute characteristics in the periodic stratification of the silurian strata, and in the effects of contact with augitic masses of eruption. [footnote] *leop. von buch, op. cit., p. . we will now enter more fully into the consideration of the four modes in which rocks are formed -- the four phases of their formative processes manifested in the stratified and unstratified portions of the earth's surface; thus, in the 'endogenous' or 'erupted rocks', designated by modern geognosists as compact and abnormal rocks, we may enumerate the following principal groups as immediate products of terrestrial activity: . 'granite and syenite' of very different respective ages; the granite is frequently the more recent,* traversing the syenite in veins, and being, in that case, the active upheaving agent. "where the granite occurs in large, insulated masses of a faintly-arched, ellipsoidal form, it is covered by a crust of shell cleft into blocks, instances of which are met with alike in the hartz district, in mysore, and in lower peru. [footnote] *bernhard cotta, 'geognosie', , s. . this surface of the granite, owing to the great expansion that accompanied its first upheaval."* [footnote] *leop. von buch, 'ueber granit and gneiss', in the 'abhandl. der berl. akad.' for the year , s. . both in northern asia,* on the charming and romantic shores of the lake of kolivan, on the northwest declivity of p. the altai mountains, and at las trincheras, on the slop of the littoral chain of caraccas,** i have seen granite divided into ledges, owing probably to a similar contraction, although the divisions appeared to penetrate far into the interior. [footnote] * in the projecting mural masses of granite of lake kolivan, divided into narrow parallel beds, there are numerous crystals of feldspar and albite, and a few of titanium (humboldt, 'asie centrale', t. i., p. , gustav rose, 'reise mach dem ural', bd. i., s. ). [footnote] *humboldt, 'relation historique', t. ii., p. further to the south of lake kolivan, toward the boundaries of the chinese province ili (between buchtarminsk and the river narym), the formation of the erupted rock, in which there is no gneiss, is more remarkable than i ever observed in any other part of the earth. the granite, which is always covered with scales and characterized by tabular divisions, rises in the steppes, either in small hemispherical eminences, scarcely six or eight feet in height, or like basalt, in mounds, terminating on either side of their bases in narrow streams.* [footnote] ** see the sketch of biri-tau, which i took from the south side, where the kirghis tents stood, and which is given in rose's 'reise', bd. i., s. . on spheres of granite scaling off concentrically, see my 'relat. hist.', t. ii., p. , and 'essai geogn. sur les gisement des roches', p. . at the cataracts of the orinoco, as well as in the district of the fichtelgebirge (seissen), in galicia, and between the pacific and the highlands of mexico (on the papagallo), i have seen granite in large, flattened spherical masses, which could be divided, like basalt, into concentric layers. in the valley of irtysch, between buchtarminsk and ustkamenogorsk, granite covers transition slate for a space of four miles,* penetrating into it from above in narrow, variously ramified, wedge-like veins. [footnote] *humboldt, 'asie centrale', t. i., p. - , and the drawings in rose's 'reise', bd. i., s. , in which we see the curvature in the layers of granite which leop. von buch has pointed out as chracteristic. i have only instanced these peculiarities in order to designate the individual character of one of the most generally diffused erupted-rocks. as granite is superposed on slate in siberia and in the departement de finisterre (isle de mihau), so it covers the jura limestone in the mountains of oisons (fermonts), and syenite, and indirectly also chalk, in saxony, near weinbohla.* [footnote] *this remarkable superposition was first described by weiss in krsten's 'archiv fur bergbau und hÃ�¬ttenwesen', bd. xvi., , s. . near mursinsk, in the uralian district, granite is of a drusous character, and here the pores, like the fissures and cavities of recent volcanic products, inclose many kinds of magnificent crystals, especially beryls and topazes. . 'quartzose porphyry' is often found in the relation of veins to other rocks. the base is generally a finely granular mixture of the same elements which occur in the larger imbedded p crystals. in granitic porphyry that is very poor in quartz, the feldspathic base is almost granular and laminated.* [footnote] *dufrenoy et elie de beaumont, 'geologie de la france', t. i., p. . . 'greenstones, diorite', are granular mixtures of white albite and blackish-green hornblende, forming dioritic porphyry when the crystals are deposited in a base of denser tissue. the greenstones, either pure, or inclosing laminae of diallage (as in the fichtelgebirge), and passing into serpentine, have sometimes penetrated, in the form of strata, into the old stratified fissures of green argillaceous slate, but they more frequently traverse the rocks in veins, or appear as globular masses of greenstone, similar to domes of basalt and porphyry.* [footnote] *these intercalated beds of diorite play an important part in the mountain district of nailau, near steben, where i was engaged in mining operations in the last century, and with which the happiest associations of my early life are connected. compare hoffmann, in poggendorf's 'annalen', bd. xvi., s. . 'hypersthene rock' is a granular mixture of labradorite and hypersthene. 'euphotide' and serpentine, containing sometimes crystald of augite and uralite instead of diallage, are thus nearly allied to another more frequent, and i might almost say, more 'energetic' eruptive rock -- augitic porphyry.* [footnote] *in the southern and bashkirian portion of the ural. rose, 'reise', bd. ii., s. . 'melaphyre', augitic, uralitic, and oligoklastic porphyries. to the last-named species belongs the genuine 'verd-antique', so celebrated in the arts. 'basalt', containing olivine and constituents which gelatinize in acids; phonolithe (porphyritic slate), trachyte, and colerite; the first of these rocks is only paartially, and the second always, divided into thin laminae, which give them an appearance of stratification when extended over a large space. mesotype and nepheline constitute, according to girard, an important part in the composition and internal texture of basalt. the nepheline contained in basalt reminds the geognosist both of the miascite of the ilmen mountains in the ural,* which has been confounded with granite, and sometimes contains zirconium, and of the pyroxenic nepheline discovered by gumprecht near lobau and chemnitz. [footnote] *g. rose, 'reise nach dem ural', bd. ii., s. - . respecting the identity of eleolite and uepheline (the latter containing rather the more lime), see scheerer, in poggend., 'annalen', bd. xlix., s. - . to the second or sedimentary rocks belong the greater part of the formations which have been comprised under the old p systematic, but not very correct designation of 'transition, flot' or 'secondary', and 'tertiary formations'. if the erupted rocks had not exercised an elevating, and, owing to the simultaneous shock of the earth, a disturbing influence on these sedimentary formations, the surface of our planet would have consisted of strata arranged in a uniformly horizontal direction above one another. deprived of mountain chains, on whose declivities the gradations of vegetable forms and the scale of the diminishing heat of the atmosphere appear to be picturesquely reflected -- furrowed ony here and there by valleys of erosion, formed by the force of fresh water moving on in gentle undulations, or by the accumulation of detritus, resulting from the action of currents of water -- continents would have presented no other appearance from pole to pole than the dreary uniformity of the llanos of south america or the steppes of northern asia. the vault of heaven would everywhere have appeared to rest on vast plains, and the stars to rise as if they emerged from the depths of ocean. such a condition of things could not, however, have generally prevailed for any length of time in the earlier periods of the world, since subterranean forces must have striven in all epochs to exert a counteracting influence. sedimentary strta have been either precipitated or deposited from liquids, according as the materials entering into their composition are supposed, whether as limestone or argillaceous slate, to be either chemically dissolved or suspended and commingled. but earth, when dissolved in fluids impregnated with carbonic acid, must be regarded as undergoing a mechanical process while they are being precipitated, deposited, and accumulated into strata. this view is of some importance with respect to the envelopment of organic bodies in petrifying calcareous beds. the most ancient sediments of the transition and secondary formations have probably been formed from water at a more or less high temperature, and at a time when the heat of the upper surface of the earth was still very considerable. considered in this point of view, a plutonic action seems to a certain extent also to have taken place in the sedimentary strata, especially the more ancient; but these strata appear to have been hardened into a schistose structure, and under great pressure, and not to have been solidified by cooling, like the rocks that have issued from the interior, as, for instance, granite, porphyry, and basalt. by degrees, as the waters lost their temperature, and were able to absorb a copious supply of the carbonic acid gas with which p the atmosphere was overcharged, they became fitted to hold in solution a larger quantity of lime. 'the sedimentary strata', setting aside all other exogenous, purely mechanical deposits of sand or detritus, are as follows: 'schist', of the lower and upper transition rock, compositing the silurian and devonian formations; from the lower silurian strata, which were once termed cambrian, to the upper strata of the old red sandstone or devonian formation, immediately in contact with the mountain limestone. 'carboniferous deposits': 'limestones' imbedded in the transition and carboniferous formations; zechstein, muschelkalk, jura formation and chalk, also that portion of the tertiary formation which is not included in sandstone and conflomerate. 'travertine', fresh-water limestone, and silicious concretions of hot springs, formations which have not been produced under the pressure of a large body of sea water, but almost in immediate contact with the atmosphere, as in shallow marshes and streams. 'infusorial deposits': geognostical phenomena, whose great importance in proving the influence of organic activity in the formation of the solid part of the earth's crust was first discovered at a recent period by my highly-gifted friend and fellow-traveler, ehrenberg. if, in this short and superficial view of the mineral constituents of the earth's crust, i do not place immediately after the simple sedimentary rocks the conglomerates and sandstone formations which have also been deposited as sedimentary strata from liquids, and which have been imbedded alternately with schist and limestone, it is only because they contain, together with the detritus of eruptive and sedimentary rocks, also the detritus of gneiss, mica slate, and other metamorphic masses. the obscure process of this metamorphism, and the action if produces, must therefore compose the third class of the fundamental forms of rock. endogenous or erupted rocks (granite, porphyry, and melaphyre) produce, as i have already frequently remarked, not only cynamical, shaking, upheaving actions, either vertically or laterally displacing the strata, but they also occasion changes in their chemical composition as well as in the nature of their internal structure; new rocks being thus formed, as gneiss, mica slate, and granular limestone (carrara and parian marble). the old silurian or devonian transition schists, the belemnitic limestone of tarantaise, and the dull gray calcareous p sandstone ('macigno'), which contains alggae found in the northern apennines, often assume a new and more brilliant appearance after their metamorphosis, which renders it difficult to recognize them. the theory of metamorphism was not established until the individual phases of the change were followed step by step, and direct chemical experiments on the difference in the fusion point, in the pressure and time of cooling, were brought in aid of mere inductive conclusions. where the study of chemical combinations is regulated by leading ideas,* it may be the means of throwing a clear light on the wide field of geognosy, and over the vast laboratory of nature in which rocks are continually being formed and modified by the agency of subterranean forces. [footnote] *see the admirable researches of mitscherlich, in the 'abhandl. der berl. akad.' for the years and , s. - ; and in poggend., 'annalen', bd. x., s. - ; bd. xi., s. - ; bd. sli., s. - (gustav rose, 'ueber gildung des kalkspaths und aragonits', in poggend., 'annalen', bd. xli., s, - ; haidinger, in the 'transactions of the royal society of edinburgh', , p. .) the philosopohical inquirer will escape the deception of apparent analogies, and the danger of being led astray by a narrow view of natural phenomena, if he constantly bear in view the complicated conditions which may, by the intensity of their force, have modified the counteracting effect of those individual substances whose nature is better known to us. simple bodies have, no doubt, at all periods, obeyed the same laws of attraction, and, wherever apparent contradictions present themselves, i am confident that chemistry will in most cases be able to trace the cause to some corresponding error in the experiment. observations made with extreme accuracy over large tracts of land, show that erupted rocks have not been produced in an irregular and unsystematic manner. in parts of the globe most remote from one another, we often find that granite, basalt, and diorite have exercised a regular and uniform metamorphic action, even in the minutest details, on the strata of argillaceous slate, dense limestone, and the grains of quartz in sandstones. as the same endogenous rock manifests almost every where the same degree of activity, so on the contrary, different rocks belonging to the same class, whether to the endogenous or the erupted, exhibit great differences in their character. intense heat has undoubtedly influenced all these phenomena, but the degree of fluidity (the more or less perfect mobility of the particles -- their more viscous composition) has varied very considerably from the granite to the basalt, while at different geological p periods (or metamorphic phases of the earth's crust) other substances dissolved in vapors have issued from the interior of the earth simultaneously with the eruption of granite, basalt, greenstone porphyry, and serpentine. this seems a fitting place again to draw attention to the fact that, according to the admirable views of modern geognosy, the metamorphism of rocks is not a mere phenomenon of contact, limited to the effect produced by the apposition of two rocks, since it comprehends all the generic phenomena that have accompanied the appearance of a particular erupted mass. even where there is no immediate contact, the proximity of such a mass gives rise to modifications of solidification, cohesion, granulation, and crystallization. all eruptive rocks penetrate, as ramifying veins either into the sedimentary strata, or into other equally endogenous masses; but there is a special importance to be attached to the difference manifested between 'plutonic' rocks* (granite, porphyry, and serpentine) and those termed 'volcanic' in the strict sense of the word (as trachyte, basalt, and lava). [footnote] ([lyell, 'principales of geology', vol. i.i., p. and .] -- tr. the rocks produced by the activity of our present volcanoes appear as band-like streams, but by the confluence of several of them they may form an extended basin. wherever it has been possible to trace basaltic eruptions, they have generally been found to terminate in slender threads. examples of these narrow openings may be found in three places in germany: in the 'pflaster-kaute', at marksuhl, eight miles from eisenach; in the blue 'kuppe', near eschwege, on the banks of the werra; and in the druidical stone on the hollert road (siegen), where the basalt has broken through the variegated sandstone and graywacke slate, and has spread itself into cup-like fungoid enlargements, which are either grouped together like rows of columns, or are sometimes stratified in thin laminae. the case is otherwise with granite, syenite, quartzose porphyry, serpentine, and the whole series of unstratified compact rocks, to which, from a predilection for a mythological nomenclature, the term plutonic has been applied. these, with the exception of occasional veins, were probably not erupted in a state of fusion, but merely in a softened condition; not from narrow fissures, but from long and widely-extending gorges. they have been protruded, but have not flowed forth, and are found not in streams like lava, but in extended masses.* [footnote] *the description here given of the relation of position under which granite occurs, expresses the general or leading character of the whole formation. but its aspect at some places leads to the belief that it was occasionally more fluid at the period of its eruption. the description given by rose, in his 'reise nach dem ural', bd. i., s. , of part of the narym chain, near the frontiers of the chinese territories, as well as the evidence afforded by trachyte, as described by dufrenoy and elie de beaumont, in their 'description geologique de la france', t. i., p. . having already spoken in the text of the narrow apertures through which the basalts have sometimes been effused, i will here notice the large fissures, which have acted as conducting passages for melaphyres, which must not be confounded with basalts. see murchison's interesting account ('the silurian system', p. ) of a fissure feet wide, through which melaphyre has been ejected, at the coal-mine at cornbrook, hoar edge. some groups of dolerite and trachyte indicate p a certain degree of basaltic fluidity; others, which have been expanded into vast craterless domes, appear to have been only in a softened condition at the time of their elevation. other trachytes, like those of the andes, in which i have frequently perceived a striking analogy with the greenstones and syenitic porphyries (which are argentiferous, and without quartz), are deposited in the same manner as granite and quartzose porphyry. experiments on the changes which the texture and chemical constitution of rocks experience from the action of heat, have shown that volcanic masses* (diorite, augitic porphyry, basalt, and the lava of aetna) yield different products, according to the difference of the pressure under which they have been fused, and the length of time occupied during their cooling; thus, where the cooling was rapid, they form a black glass, having a homogeneous fracture, and where the cooling was slow, a stony mass of granular crystalline structure. [footnote] *sir james hall, in the 'edin. trans.', vol. v., p. , and vol. vi., p. ; gregory watt, in the 'phil. trans. of the roy. soc. of london for' , part ii., p. ; dartigues and fleurieu de bellevue, in the 'journal de physique', t. lx., p. ; bischof, 'warmelchre', s. und . in the latter case, the crystals are formed partly in cavities and partly inclosed in the matrix. the same materials yield the most dissimilar products, a fact that is of the greatest importance in reference to the study of the nature of erupted rocks, and of the metamorphic action which they occasion. carbonate of lime, when fused under great pressure, does not lose its carbonic acid, but becomes, when cooled, granular limestone; when the crystallization has been effected by the dry method, saccharoidal marble; while by the humid method, calcareous spar and aragonite and produced, the former under a lesser degree of temperature than the latter.* [footnote] *gustav rose, in poggend., 'annalen.' bd. xliii., s . differences of temperature p likewise modify the direction in which the different particles arrange themselves in the act of crystallization, and also affect the form of the crystal.* [footnote] *on the dimorphism of sulphur, see mitscherlich, 'lehrbuch der chemie', - . even when a body is not in a fluid condition, the smallest particles may undergo certain relations in their various modes of arrangement, which are manifested by the different action on light.* [footnote] *on gypsum as a uniaxal crystal, and on the sulphate of magnesia, and the oxyds of zinc and nickel, see mitscherlich, in poggend., 'annalen.' bd. xi., s. . the phenomena presented by devitrification, and by the formation of steel by cementation and casting -- the transition of the fibrous in the granular tissue of the iron, from the action of heat* and probably, also, by regular and long-continued concussions -- likewise throw a considerable degree of light on the geological process of metamorphism. [footnote] *coste, 'versuche am creusot uber das bruchig werden des stabeisens.' elie de beaumont, 'mem. geol.', t. ii., p. . heat may even simultaneously induce opposite actions in crystalline bodies; for the admirable experiments of mitscherlich have established the fact* that calcareous spar, without altering its condition of aggregation, expands in the direction of one of its axes and contracts in the other. [footnote] * mitscherlich, 'ueber die ausdehnung der krystallisirten korper durch die warmelehre', in poggend., 'annalen', bd. x., s. . if we pass from these general considerations to individual examples, we find that schist is converted, by the vicinity of plutonic erupted rocks, into a bluish-black, glistening roofing slate. here the planes of stratification are intersected by another system of divisional stratification, almost at right angles with the former,* and thus indicating an action subsequent to the alteration. [footnote] * on the double system of divisional planes, see elie de beaumont, 'geologie de la france', p. ; credner, 'geognosie thuringens und des harzes', s. ; and romer, 'das rheinische uebergangsgebirge', . s. und . the penetration of silica causes the argillaceous schist to be traversed by quartz, transforming it, in part, into whetstone and silicious schist; the latter sometimes containing carbon, and being then capable of producing galvanic effects on the nerves. the highest degree of silicifaction of schist is that observed in ribbon jasper, a material highly valuable in the arts,* and which is produced in the oural mountains p by the contact and eruption of augitic porphyry (at orsk), of dioritic porphyry (at aufschkul), or of a mass of hypersthenic rock conglomerated into spherical masses (at bogoslowsk). at monte serrato, in the island of elba, according to frederic hoffman, and in tuscany, according to alexander brongniart, it is formed by contact with euphotide and serpentine. [footnote] *the silica is not merely colored by peroxyd of iron, but is accompanied by clay, lime, and potash. rose, 'reise', bd. ii., s. . on the formation of jasper by the action of dioritic porphyry, augite, and by persthene rock, see rose, bd. ii., s. , , und . see, also, bd. i., s. , where there is a drawing of the porphyry spheres between which jasper occurs, in the calcareous graywacke of bogoslowsk, being produced by the plutonic influence of the augitic rock; bd. ii., s. ; and likewise humboldt, 'asie centrale', t. i., p. . the contact and plutonic action of granite have sometimes made argillaceous schist granular, as was observed by gustav rose and myself in the altai mountains (within the fortress of buchtarminsk),* and have transformed it into a mass resembling granite, consisting of a mixture of feldspar and mica, in which larger laminae of the latter were again imbedded.** [footnote] *rose, 'reise nach dem ural', bd. i., s. - . [footnote] **in respect to the volcanic origin of mica, it is important to notice that crystals of mica are found in the basalt of the bohemian mittelgebirge, in the lava that in was ejected from vesuvius (monticelli, 'storia del vesuvio negli anni e ', ), and in fragments of agrillaceous alte imbedded in scoriaceous basalt at hohenfels, not far from gerolstein, in the eifel (see mitscherlich, in leonhard, 'basalt-gebilde', s. ). on the formation of feldspar in argillaceous schist, through contact with porphyry, occurring between urval and poÃ�Â�et (forez), see dufrenoy, in 'geol. de la france', t. i., p. . it is probably to a similar contact that certain schists near paimpol, in brittany, with whose appearance i was much struck, while making a geological pedestrian tour through that interesting country with professor kunth, owe their amygdaloid and cellular character, t. i., p. . most geognosists adhere, with leopold von buch, to the well-known hypothesis "that all the gneiss in the silurian strata of the transition formation, between the icy sea and the gulf of finland, has been produced by the metamorphic action of granite.* [footnote] * leopold von buch, in the 'abhandlungen der akad. der wissenschaft zu berlin, aus dem jahr' , s. , and in the 'jahrbuchern fur wissenschaftliche kritik jahrg.' , s. . in the alps, at st. gothard, calcareous marl is likewise changed from granite into mica slate, and then transformed into gneiss." similar phenomena of the formation of gneiss and mica slate through granite present themselves in the oolitic group of the tarantaise,* in which belemnites are p found in rocks, which have some claim to be considered as mica slate, and in the schistose group in the western part of the island of elba, near the promontory of calamita, and the fichtelgebirge in baireuth, between loomitz and markleiten.** [footnote] * elie de beaumont, in the 'annales des sciences naturelles', t. xv., p. - . "in approaching the primitive masses of mont rosa, and the mountains situated to the west of coni, we perceive that the secondary strata gradually lose the characters inherent in their mode of deposition. frequently assuming a character apparently arising from a perfectly distinct cause, but not losing their stratification, they somewhat resemble in their physical structure a brand of half-consumed wood, in which we can follow the traces of the ligneous fibers beyond the spots which continue to present the natural characters of wood." (see, also, the 'annales des sciences naturelles', t. xiv., p. - , and von dechen, 'geognosie', s. .) among the most striking proofs of the transformation of rocks by plutonic action, we must place the belemites in the schists of nuffenen (in the alpine valley of eginen and in the gries-glaciers), and the belemnites found by m. charpentier in the so-called primitive limestone on the western descent of the col de la seigne, between the enclove de monjovet and the 'chalet' of la lanchette, and which he showed to me at bex in the autumn of ('annales de chimie', t. xxiii., p. ). [footnote] ** hoffmann, in poggend., 'annalen', bd. xvi., s. , "strate of transition argillaceous schist in the fichtelgebirge, which can be traced for a length of miles, are transformed into gneiss only at the two extremities, where they come in contact with granite. we can there follow the gradual formation of the gneiss, and the development of the mica and of the feldspathic amygdaloids, in the interior of the argillaceous schist, which indeed contains in itself almost all the elements of these substances." jasper, which,* as i have already remarked, is a production formed by the volcanic action of augitic porphyry, could only be obtained in small quantities by the ancients, while another material, very generally and efficiently used by them in the arts, was granular or saccharoidal marble, which is likewise to be regarded solely as a sedimentary stratum altered by terrestrial heat and by proximity with erupted rocks. [footnote] * among the works of art which have come down to us from the ancient greeks and romans, we observe that none of any size -- as columns or large vases -- are formed from jasper; and even at the present day, this substance, in large masses, is only obtained from the ural mountains. the material worked as jasper from the rhubarb mountain (raveniaga sopka), in altai, is a beautiful ribboned porphyry. the word 'jasper' is derived from the semitic languages; and from the confused description of theophrastus ('de lapidibus', and ) and pliny (xxxvii., and ), who rank jasper among the "opaque gems," the name appears to have been given to fragments of 'jaspachat', and to a substance which the ancients termed 'jasponyx', which we now know as 'opal-jasper'. pliny considers a piece of jasper eleven inches in length so rare as to require his mentioning that he had actually seen such a specimen: "magnitudinem jaspidis undecim unciarum vidimus, formatamque inde effigem neronis thoracatam." according to theophrastus, the stone which he calls emerald, and from which large obelists were cut, must have been an imperfect jasper. this opinion is corroborated by the accurate observations on the phenomena of contact, by the remarkable experiments on fusion p made by sir james hall more than half a century ago, and by the attentive study of granitic veins, which has contributed so largely to the establishment of modern geognosy. sometimes the erupted rock has not transformed the compact into granular limestone to any great depth from the point of contact. thus, for instance, we meet with a slight transformation -- a penumbra -- as at belfast, in ireland, where the basaltic veins traverse the chalk, and, as in the compact calcareous beds, which have been partially inflected by the contact of syenitic granite, at the bridge of boscampo and the cascade of conzocoli, in the tyrol (rendered celebrated by the mention made of it by count mazari peucati).* [footnote[ *humboldt, 'lettre a m. brochant de villiers', in the 'annales de chimie et de physique', t. xxiii., p. ; leop. von buch, 'geog. briefe uber das sudliche tyrol', s. , , und . another mode of transformation occurs where all the strata of the compact limestone have been changed into granular limestone by the action of granite, and syenitic or dioritic porphyry.* [footnote] *on the transformation of compact into granular limestone by the action of granite, in the pyrenees at the 'montagnes de rancie', see dufrenoy, in the 'memoires geologiques', t. ii., p. ; and on similar changes in the 'montagnes de l'oisans', see elie de beaumont, in the 'mem. geolog.', t. ii., p. - ; on a similar effect produced by the action of dioritic and pyroxenic porphyry (the 'ophite' described by elie de beaumont, in the 'geologie de la france', t. i., p. ), between tolosa and st. sebastian, see dufrenoy, in the 'mem. geolog.', t. ii., p. ; and by syenite in the isle of skye, where the fossils in the altered limestone may still be distinguished, see von dechen, in his 'geognosie', p. . in the transformation of chalk by contact with basalt, the transposition of the most minute particles in the processes of crystallization and granulation is the more remarkable, because the excellent microscopic investigations of ehrenberg have shown that the particles of chalk previously existed in the form of closed rings. see poggend., 'annalen der physic', bd. xxxix., s. ; and on the rings of aragonite deposited from solution, see gustav rose in vol. xlii., p. , of the same journal. i would here wish to make special mention of parian and carrara marbles, which have acquired such celebrity from the noble works of art into which they have been converted, and which have too long been considered in our geognostic collections as the main types of primitive limestone. the action of granite has been manifested sometimes by immediate contact, as in the pyrenees,* and sometimes, as in the main land of greece, and in the insular groups in the gean sea, through the intermediate layers of gneiss or mica slate. [footnote] *beds of granular limestone in the granite at port d'oo and in the mont de labourd. see charpentier, 'constitution geologique des pyrenes', p. , . both cases presuppose a simultaneous but heterogeneous process of transformation. p in attica, in the island of euboea, and in the peloponnesus, it has been remarked, "that the limestone, when superposed on mica slate, is beautiful and crystalline in proportion to the purity of the latter substance and to the smallness of its argillaceous contents; and, as is well known, this rock, together with beds of gneiss, appears at many points, at a considerable depth below the surface, in the islands of paros and antiparos."* [footnote] *leop. von buch, 'descr. des canaries', p. ; fiedler, 'reise durch das konigreich griechenland', th. ii., s., , , und . we may here infer the existence of an imperfectly metamorphosed flotz formation, if faith can be yielded to the testimony of origen, according to whom, the ancient eleatic, xenophanes of colophon* (who supposed the whole earth's crust to have been once covered by the sea), declared that marine fossils had been found in the quarries of syracuse, and the impression of a fish (a sardine) in the deepest rocks of paros. [footnote] *i have previously alluded to the remarkable passage in origen's 'philosophumena', cap. ('opera', ed. delarue, t. i., p. ). from the whole context, it seems very improbable that xenophanes meant an impression of a laurel ([greek words]) instead of an impression of a fish ([greek words]). delarue is wrong in blaming the correction of jacob gronovius in changing the laurel into a sardel. the petrifaction of a fish is also much more probable than the natural picture of silenus, which, according to pliny (lib. xxxvi., ), the quarry-men are stated to have met with in parian marble from mount marpessos. 'servius ad virg., aen.', vi., . the carrara or luna marble quarries, which constituted the principal source from which statuary marble was derived even prior to the time of augustus, and which will probably continue to do so until the quarries of paros shall be reopened, are beds of calcareous sandstone -- macigno -- altered by plutonic action, and occurring in the insulated mountain of apuana, between gneiss-like mica and talcose schist.* [footnote] *on the geognostic relations of carrara ('the city of the moon', strabo, lib. v., p. ), see savi 'osservazioni sui terreni antichi toscani', in the 'nuova giornale de' letterati di pisa', and hoffmann, in karsten's 'archiv fur mineralogie', bd. vi., s. - , as well as in his 'geogn. reise durch italien', s. - . whether at some points granular limestone may not have been formed in the interior of the earth, and been raised by gneiss and syenite to the surface, where it forms vein-like fissures,* is a question on which i can not hazard an opinion, owing to my own want of personal knowledge of the subject. [footnote] *according to the assumption of an excellent and very experienced observer, karl von leonhard. see his 'jahrbuch fur mineralogie', s. , and bernhard cotta, 'geognosie', s. . p according to the admirable observations of leopold von buch, the masses of dolomite found in southern tyrol, and on the italian side of the alps, present the most remarkable instance of metamorphism produced by massive eruptive rocks on compact calcareous beds. the formation of the limestone seems to have proceeded from the fissures which traverse it in all directions. the cavities are every where covered with rhomboidal crystals of magnesian bitter spar, and the whole formation, without any trace of strtification, or of the fossil remains which it once contained, consists only of a granular aggregation of crystals of dolomite. talc laminae lie scattered here and there in the newly-formed rock, traversed by masses of serpentine. in the valley of the fassa, dolomite rises perpendicularly in smooth walls of dazzling whiteness to a height of many thousand feet. it forms sharply-pointed conical mountains, clustered together in large numbers, but yet not in contact with each other. the contour of their forms recalls to mind the beautiful landscape with which the rich imagination of leonardi da vinci has embellished the back-ground of the portrait of mona lisa. the geognostic phenomena which we are now describing, and which excite the imagination as well as the powers of the intellect, are the result of the action of augite porphyry manifested in its elevating, destroying, and transforming force.* [footnote] *leop. von buch, 'geognostische briefe an alex. von humboldt', , s. and ; also in the 'annalen de chemie', t. xxiii., p. , and in the 'abhandl. der berliner akad. aus der jahren 'und' , s. - ; von dechen, 'geognosie.' s. - . the process by which limestone is converted into dolomite is not regarded by the illustrious investigator who first drew attention to the phenomenon as the consequence of the tale being derived from the black porphyry, but rather as a transformatiion simultaneous with the appearance of this erupted stone through wide fissures filled with vapors. it remains for future inquirers to determine how transformation can have been effected without contact with the endogenous stone, where strata of dolomite are found to be interspersed in imestone. where, in this case, are we to seek the concealed channels by which the plutonic action is conveyed? even here it may not, however, be necessary, in conformity with the old roman adage, to believe "that much that is alike in nature may have been formed in wholly different ways." when we find, over widely extended parts of the earth, that two phenomena are always associated together, as, for instance, the occurrence of melaphyre p and the transformation of compact limestone into a crystaline mass differing in its chemical character, we are, to a certain degree, justified in believing, where the second phenomenon is manifested unattended by the appearance of the first, that this apparent contradiction is owing to the absence, in certain cases, of some of the conditions attendant upon the exciting causes. who would call in question the volcanic nature and igneous fluidity of basalt merely because there are some rare instances in which basaltic veins, traversing beds of coal or strata of sandstone and chalk, have not materially deprived the coal of its carbon, nor broken and slacked the sandstone, not converted the chalk into granular marble? wherever we have obtained even a faint light to guide us in the obscure domain of mineral formation, we ought not ungratefully to disregard it, because there may be much that is still unexplained in the history of the relations of the transitions, or in the isolated interposition of beds of unaltered strata. after having spoken of the alteration of compact carbonate of lime into granular limestone and dolomite, it still remains for us to mention a third mode of transformation of the same mineral, which is ascribed to the emission, in the ancient periods of the world, of the vapors of sulphuric acid. this transformation of limestone into gypsum is analogous to the penetration of rock salt and sulphur, the latter being deposited from sulphureted aqueous vapor. in the lofty cordilleras of quindin, far from all volcanoes, i have observed deposits of sulphur in fissures in gneiss, while in sicily (at cattolica, near girgenti), sulphur, gypsum, and rock salt belong to the most recent secondary strata, the chalk formations.* [footnote] *horrman, 'geogn. reise', edited by von dechen, s. - , and - ; poggend., 'annalen der physik', bd. xxvi., s. . i have also seen on the edge of the crater of vesuvius, fissures filled with rock salt, which occurred in such considerable masses as occasionally to lead to its being disposed of by contraband trade. on both declivities of the pyrenees, the connection of diorite and pyroxene, and colomite, gypsum, and rock salt, can not be questioned;* and here, as in the other phenomena which we have been considering, every thing bears evidence of the action of subterranean forces on the sedimentary strata of the ancient sea. [footnote] *dufrenoy, in the 'memoires geologiques', t. ii., p. and . there is much difficulty in explaining the origin of the beds of pure quartz, which occur in such large quantities in south america, and impart so peculiar a character to the chain of p the andes.* [footnote] *humboldt, 'essai geogn. sur le gisement des roches', p. ; 'asie centrale', t. iii., p. . in descending toward the south sea, from caxamarca toward guangamarca, i have observed vast masses of quartz, from to feet in height, superposed sometimes on porphyry devoid of quartz, and sometimes on diorite. can these beds have been transformed from sandstone, as elie de beaumont conjectures in the case of the quartz strata on the col de la poissonniere, east of brianÃ�Â�on?* [footnote] *elie de beaumont, in the 'annales des sciences naturelles', t. xv., p. ; murchison, 'silurian system', p. . in the brazils, in the diamond district of minas geraes and st. paul, which has recently been so accurately investigated by clausen, plutonic action has developed in dioritic veins sometimes ordinary mica, and sometimes specular iron in quartzose itacolumite. the diamonds of grammagoa are imbedded in strata of solid silica, and are occasionally enveloped in laminae of mica, like the garnets found in mica slate. the diamonds that occur furthest to the north, as those discovered in at degrees lat., on the european slope of the uralian mountains, bear a geognostic relation to the black carboniferous dolomite of adolffskoi* and to augitic porphyry, although more accurate observations are required in order fully to elucidate this subject. [footnote] *rose, 'reise nach dem ural', bd. i., s. und . among the most remarkable phenomena of contact, we must, finally, enumerate the formation of garnets in argillaceous schist in contact with basalt and dolerite (as in northumberland and the island of anglesea), and the occurrence of a vast number of beautiful and most various crystals, as garnets, vesuvian, augite, and ceylanite, on the surfaces of contact between the erupted and sedimentary rock, as, for instance, on the junction of the syenite of monzon with dolomite and compact limestone. [footnote] *leop. von buch, 'briefe', s. - . see also, elie de beaumont 'on the contact of granite with the beds of the jura', in the 'mem. geol.' t. ii., p. . in the island of elba, masses of serpentine, which perhaps nowhere more clearly indicate the character of erupted rocks, have occasioned the sublimation of iron glance and red oxyd of iron in fissures of calcareous sandstone. [footnote] *hoffman, 'reise', s. und . we still daily find the same iron glance formed by sublimation from the vapors and the walls of the fissures of open veins on the margin of the crater, and in the fresh lava currents of the volcanoes of stromboli, vesuvius, and aetna.* [footnote] *on the chemical process in the formation of specular iron, see gay lussac, in the 'annales de chimie', t. xxii., p. , and mitscherlich, in poggend., 'annalen', bd. xv., s. . moreover, crystals of olivine have been formed (probaby by sublimation) in the cavities of the obsidian of cerro del jacal, which i brought from mexico (gustav rose, in poggend., 'annalen', bd. x., s. ). hence olivine occurs in basalt, lava, obsidian, artificial scoriae in meteoric stones, in the syenite of elfdale, and (as hyalosiderite) in the wacke of the kaiserstuhl. the veins that p are thus formed beneath our eyes by volcanic forces, where the contiguous rock has already attained a certain degree of solidification, show us how, in a similar manner, mineral and metallic veins may have been every where formed in the more ancient periods of the world, where the solid but thinner crust of our planet, shaken by earthquakes, and rent and fissured by the change of volume to which it was subjected in cooling, may have presented many communications with the interior, and many passages for the escape of vapors impregnated with earthy and metallic substances. the arrangement of the particles in layers parallel with the margins of the beins, the regular recurrence of analogous layers on the opposite sides of the veins (on their different walls), and, finally, the elongated cellular cavities in the middle, frequently afford direct evidence of the plutonic process of sublimation in metalliferous veins. as the traversing rocks must be of more recent origin than the traversed, we learn from the relations of stratification existing between the porphyry and the argentiferous ores in the saxon mines (the richest and most important in germany), that these formations are at any rate more recent than the vegetable remains found in carboniferous strata and in the red sandstone.* [footnote] *constantin von veust, 'ueber die porphyrgebilde', , s. - ; also his 'belenchtung der werner'schen gangtheorie', , s. ; and c. von wissenbach, 'abbildungen merkwurdiger gangverhaltnisse', , fig. . the ribbon-like structure of the veins is, however, no more to be regarded of general occurrence than the periodic order of the different members of these masses. all the facts connected with our geological hypotheses on the formation of the earth's crust and the metamorphism of rocks have been unexpectedly elucidated by the ingenious idea which led to a comparison of the slags or scoriae of our smelting furnaces with natural minerals, and to the attempt of reproducing the latter from their elements.* [footnote] *mitscherlich, 'ueber die kunstliche darstellung der mineralien', in the 'abhandl. der akademie der wiss. zu berlin', - , s. - . in all these operations, the same affinities manifest themselves which determine chemical combinations both in our laboratories and in the interior of the earth. the most considerable part of p the simple minerals which characterize the more generally diffused plutonic and erupted rocks, as well as those on which they have exercised a metamorphic action, have been produced in a crystalline state, and with perfect identify, in artificial mineral products. we must, however, distinguish here between the scoriae accidentally formed, and those which have been designedly produced by chemists. to the former belong feldspar, mica, augite, olivine, hornblende, crystallized oxyd of iron, magnetic iron in octahedral crystals, and metallis titanium;* to the latter, garnets, idocrase, rubies (equal in hardness to those found in the east), olivine, and augite.** [footnote] *in scoriae crystals of feldspar have been discovered by heine in the refuse of a furnace for copper fusing, near sangerhausen, and analyzed by kersten (poggend., 'annalen', bd. xxxiii., s. ); crystals of augite in scoriae at sahle (mitscherlich, in the 'abhandl. der akad. zu berlin', - , s. ); of oliving by seifstrom (leonhard, 'basalt-gebilde', bd. ii., s. ); of mica in old scoriae of schloss garpenberg (mitscherlich, in leonhard, op. cit., s. ); of magnetic iron in the scoriae of chatillon sur seine (leonhard, s. ); and of micaceous iron in potter's clay (mitscherlich, in leohnard, op. cit., s. ). [see ebelmer's papers in 'ann. de chimie et de physique', ; also 'report on the crystalline slags', by john percy, m.d., f.r.s., and william hallows miller, m.a., . dr. percy, in a communication with which he has kindly favored me, says that the minerals which he has found artificially produced and proved by analysis are humboldtilite, gehlenite, olivine, and magnetic oxyd of iron, in octahedral crystals. he suggests that the circumstance of the production of gehlenite at a high temperature in an iron furnace may possibly be made available by geologists in explaining the formation of the rocks in which the natural mineral occurs, as in fassathal in the tyrol.] -- tr. [footnote] **of minerals purposely produced, we may mention idocrase and garnet (mitscherlich, in poggend., 'annalen der physik', bd. xxxii., s. ); ruby (gaudin, in the 'comptes rendus de l'academie de science', t. iv., part i., p. ); olivine and augite (mitscherlich and berthier, in the 'annales de chimie et de physique', t. xxiv., p. ). notwithstanding the greatest possible similarity in crystalline form, and perfect identity in chemical composition, existing, according to gustav rose, between augite and hornblende, hornblende has never been found accompanying augite in scoriae, nor have chemists ever succeeded in artificially producing either hornblende or feldspar (mitscherlich in poggend., 'annalen', bd. xxxiii., s. , and rose, 'reise nach dem ural', bd. ii., s. und ). see also, beaudant, in the 'mem. de l'acad. des sciences', t. viii., p. , and becquerel's ingenious experiments in his 'trait de l'electricite,' t. i., p. ; t. iii., p. ; and t. v., p. and . these minerals constitute the main constituents of granite, gneiss, and mica schist, of basalt, dolerite, and many porphyries. the artificial production of feldspar and mica is of most especial geognostic importance with reference to the theory of the formation of gneiss by the metamorphic agency of argillaceous schist, which contains all the constituents of granite, p potash not excepted.* [footnote] *d'aubuisson, in the 'journal de physique', t. lxviii., p. . it would not be very surprising, therefore, as is well observed by the distinguished geognosist, von dechen, if we were to meet with a fragment of gneiss formed on the walls of a smelting furnace which was built of argillaceous slate and graywacke. after having taken this general view of the three classes of erupted, sedimentary, and metamorphic rocks of the earth's crust, it still remains for us to consider the fourth class, comprising 'conglomerates', or 'rocks of detrius'. the very term recalls the destruction which the earth's crust has suffered, and likewise, perhaps reminds us of the process of cementation, which has connected together, by means of oxyd of iron, or of some argillaceous and calcareous substances, the sometimes rounded and sometimes angular portions of fragments. conglomerates and rocks of detritus, when considered in the widest sense of the term, manifest characters of a double origin. the substances which enter into their mechanical composition have not been alone accumulated by the action of the waves of the sea or currents of fresh water, for there are some of these rocks the formation of which can not be attributed to the action of water. "when basaltic islands and trachytic rocks rise on fissures, friction of the elevated rock against the walls of the fissures causes the elevated rock to be inclosed by conglomerates composed of its own matter. the granules composing the sandstones of many formations have been separated rather by friction against the erupted volcanic or plutonic rock than destroyed by the erosive force of a neighboring sea. the existence of these friction 'conglomerates', which are met with in enormous masses in both hemispheres, testifies the intensity of the force with which the erupted rocks have been propelled from the interior through the earth's crust. this detritus has subsequently been taken up by the waters, which have then deposited it in the strata which it still covers."* [footnote] *leop. von buck, 'geognost. briefe', s. - , where it is also shown why the new red sandstone (the 'todtliegende' of the thuringian flotz formation) and the coal measures must be regarded as produced by erupted porphyry. sandstone formations are found imbedded in all strata, from the lower silurian transition stone to the beds of the tertiary formations, superposed on the chalk. they are found on the margin of the boundless plains of the new continent, both within and without the tropics, extending like breast-works along the ancient shore, against which the sea once broke its foaming waves. p if we cast a glance on the geographical distribution of rocks, and their relations in space, in that portion of the earth's crust which is accessible to us, we shall find that the most universally distributed chemical substance is 'silicic acid', generally in a variously-colored and opaque form. next to solid silicic acid we must reckon carbonate of lime, and then the combinations of silicic acid with alumina, potash, and soda, with lime, magnesia, and oxyd of iron. the substances which we designate as 'rocks' are determinate associations of a small number of minerals, in which some combine parasitically, as it were, with others, but only under definite relations; thus, for instance, although quartz (silica), feldspar, and mica are the principal constituents of granite, these minerals also occur, either individually or collectively, in many other formations. by way of illustrating how the quantitative relations of one feldspathic rock differ from another, richer in mica than the former, i would mention that, according to mitscherlich, three times more alumina and one third more silica than that ossessed by feldspar, give the constituents that enter into the composition of mica. potash is contained in both -- a substance whose existence in many kinds of rocks is probably antecedent to the dawn of vegetation on the earth's surface. the order of succession, and the relative age of the different formations, may be recognized by the superposition of the sedimentary, metamorphic, and conglomerate strata; by the nature of the formations traversed by the erupted masses, and -- with the greatest certainty -- by the presence of organic remains and the differences of their structure. the application of botanical and zoological evidence to determine the relative age of rocks -- this chronometry of the earth's surface, which was already present to the lofty mind of hooke -- indicates one of the most glorious epochs of modern geognosy, which has finally, on the continent at least, been emancipated from the sway of semitic doctrines. palaeontological investigations have imparted a vivifying breath of grace and diversity to the science of the solid structure of the earth. the fossiliferous strata contain, entombed within them, the floras and faunas of by-gone ages. we ascend the stream of time, as in our study of the relations of superposition we descend deeper and deeper through the different strata, in which lies revealed before us a past world of animal and vegetable life. far-extending disturbances, the elevation of great mountain chains, whose relative ages we are able to define, attest the p destruction of ancient and the manifestation of recent organisms. a few of these older structures have remained in the midst of more recent species. owing to the limited nature of our knowledge of existence, and from the figurative terms by which we seek to hide our ignorance, we apply the appellation 'recent structure' to the historical henomena of transition manifested in the organisms as well as in the forms of primitive seas and of elevated lands. in some cases these organized structures have been preserved perfect in the minutest details of tissues, integument, and articulated parts, while in others, the animal, passing over soft argillaceous mud, has left nothing but the traces of its course,* or the remains of its undigested food, as in the coprolites.** [footnote] *[in certain localities of the new red sandstone, in the valley of the connecticut, numerous tridactyl markings have been occasionally observed on the surface of the slabs of stone when split asunder, in like manner as the ripple-marks appear on the successive layers of sandstone in tilgate forest. some remarkably distinct impressions of this kind, at turner's falls (massachusetts), happening to attract the attention of dr. james deane, of greenfield, that sagacious observer was struck with their resemblance to the foot-marks left on the mud-banks of the adjacent river by the aquatic birds which had recenty frequented the spot. the specimens collected were submitted to professor g. hitchcock, who followed up the inquiry with a zeal and success that have led to the most interesting results. no reasonable doubt now exists that the imprints in question have been produced by the tracks of bipeds impressed on the stone when in a soft state. the announcement of this extraordinary phenomenon was first made by professor hitchcock, in the 'american journal of science' (january, ), and that eminent geologist has since published full descriptions of the different species of imprints which he has detected, in his splendid work on the geology of massachusetts. -- mantell's 'medals of creation', vol. ii., p. . in the work of dr. mantell above referred to, there is, in vol. ii., p. , an admirable diagram of a slab from turner's falls, covered with numerous foot-marks of birds, indicating the track of ten or twelve individuals of different sizes.] -- tr. [footnote] **[from the examination of the fossils spoken of by geologists under the name of 'coprolites', it is easy to determine the nature of the food of the animals, and some other points; and when, as happened occasionally, the animal was killed while the process of digestion was going on, the stomach and intestines being partly filled with half-digested food, and exhibiting the coprolites actually 'in situ', we can make out with certainty not only the true nature of the food, but the proportionate size of the stomach, and the length and nature of the intestinal canal. within the cavity of the rib of an extinct animal, the palaeontologist thus finds recorded, in indelible characters, some of those hieroglyphics upon which he founds his history. -- 'the ancient world', by d. t. ansted, , p. .] -- tr. in the lower jura formations (the lias of lyme regis), the ink bag of the sepia has been so wonderfully preserved, that the material, which myriads p of years ago might have served the animal to conceal itself from its enemies, still yields the color with which its image may be drawn.* [footnote] *a discovery made by miss mary anning, who was likewise the discoverer of the coprolites of fish. these coprolites, and the excrements of the ichthyosauri, have been found in such abundance in england (as, for instance, near lyme regis), that, according to buckland's expression, they lie like potatoes scattered in the ground. see buckland, 'geology considered with reference to natural theology', vol. i., p. - and . with respect to the hope expressed by hooke "to raise a chronology" from the mere study of broken and fossilized shells "and to state the interval of time wherein such or such castrophes and mutations have happened," see his 'posthumous works, lecture', feb. , . [still more wonderful is the preservation of the substance of the animal of certain cephalopodes in the oxford clay. in some specimens recently obtained, and described by professor owen, not only the ink bag, but the muscular mantle, the head, and its crown of arms, are all preserved in connection with the belemnite shell, while one specimen exhibits the large eyes and the funnel of the animal, and the remains of two fins, in addition to the shell and the ink bag. see ansted's 'ancient world', p. .] -- tr. in other strata, again, nothing remains but the faint impression of a muscle shell; but even this, if it belong to a main dividion of mollusca,* may serve to show the traveler, in some distant land, the nature of the rock in which it is found, and the organic remains with which it is associated. [footnote] *leop. von buch, in the 'abhandlungen der akad. der wiss. zu berlin in dem jahr' , s. . its discovery gives the history of the country in which it occurs. the analytic study of primitive animal and vegetable life has taken a double direction: the one is purely morphological, and embraces, especially, the natural history and physiology of organisms, filling up the chasms in the series of still living species by the fossil structures of the primitive world. the second is more specially geognostic, considering fossil remains in their relations to the superposition and relative age of the sedimentary formations. the former has long predominated over the latter, and an imperfect and superficial comparison of fossil remains with existing species has led to errors, which may still be traced in the extraordinary names applied to certain natural bodies. it was sought to identify all fossil species with those still extant in the same manner as, in the sixteenth century, men were led by false analogies to compare the animals of the new continent with those of the old. peter camper, sommering, and blumenbach had the merit of being the first, by the scientific application of a more accurate p comparative anatomy, to throw light on the osteological branch of palaeontology -- the archaeology of organic life; but the actual geognostic views of the doctrine of fossil remains, the felicitous combination of the zoological character with the order of succession, and the relative ages of strata, are due to the labors of george cuvier and alexander brongniart. the ancient sedimentary formations and those of transition rocks exhibit, in the organic remains contained within them, a mixture of structures very variously situated on the scale of progressively-developed organisms. these strata contain but few plants, as, for instance, some species of fuci, lycopodiaceae which were probably arborescent, equisetaceae, and tropical ferns; they present, however, a singular association of animal forms, consisting of crustacea (trilobites with reticulated eyes, and calymene), brachiopoda ('spirifer, orthis'), elegant sphaeronites, nearly allied to the crinoidea,* orthoceraitites, of the family of the cephalopoda, corals, and, blended with these low organisms, fishes of the most singular forms, imbedded in the upper silurian formations. [footnote] *leop. von buch, 'gebirgsformationen von russland', , s. - . the family of the cephalaspides, whose fragments of the species 'pterichtys' were long held to be trilobites, belongs exclusively to the devonian period (the old red), manifesting, according to agassiz, as peculiar a type among fishes as do the ichthyosauri and plesiosauri among reptiles.* [footnote] *agassiz, 'monographie des poissons fossiles du vieux gres rouge', p. vi. and . the goniatites, of the tribe of ammonites,* a are manifested in the transition chalk, in the graywacke of the devonian periods, and even in the latest silurian formations. [footnote] *leop. von buch, in the 'abhandl. der berl. akad.', , s. - ; beyrich, 'beitr. zur kenntniss des rheinischen uebergangagebirges', , s. . the dependence of physiological gradation upon the age of the formations, which has not hitherto been shown with perfect certainty in the case of invertebrata,* is most regularly manifested in vertebrated animals. [footnote] *agassiz, 'recherches sur les poissons fossiles', t. i., 'introd.', p. xviii.; davy, 'consolation in travel', dial. iii. the most ancient of these, as we have already seen, are fishes; next in the order of succession of formation, passing from the lower to the upper, come reptiles and mammalia. the first reptile (a saurian, the monitor of cuvier), which excited the attention of leibnitz,* is found in cuperiferous schist of the zechstein of thuringa; the palaeosaurus and thecodontosaurus of bristol are, according to murchison, of the same age. [footnote] *a protosaurus, according to hermann von meyer. the rib of a saurian asserted to have been found in the mountain limestone (carbonate of lime) of northumberland (herm. von meyer, 'palaeologica', s. ), is regarded by lyell ('geology', , vol. i., p. ) as very doubtful. the discoverer himself referred it to the alluvial strata which cover the mountain limestone. the saurians are found in large numbers in the muschelkalk,* in the keuper, and in the oolitic formations, where they are the most numerous. [footnote] *f. von alberti, 'monographie des bunten sandsteins, muschelkalks und keupers', , s. und . at the period of these formations there existed pleiosauri, having long, swan-like necks consisting of thirty vertebrae; megalosauri, monsters resembling the crocodile, forty-five feet in length, and having feet whose bones were like those of terrestrial mammalia, eight species of large-eyed ichthyosauri, the geosaurus or 'lacerta gigantea', of sommering, and finally, seven remarkable species of pterodactyles,* of saurians furnished with membranous wings. [footnote] *see hermann von meyer's ingenious considertions regarding the organization of the flying saurians, in his 'palaeologica', s. - . in the fossil specimen of the pterodactylus crassirostris, which, as well as the loonger known p. longirostris (ornithocephalus of sommering), was found at solenhofen, in the lithographic slate of the upper jura formation, professor goldfuss has even discovered traces of the membranous wing, "with the impressions of curling tufts of hair, in some places a full inch in length." in the chalk the number of the crocodilial saurians diminishes, although this epoch is characterized by the so-called crocodile of maestricht (the mososaurus of conybeare), and the colossal, probably graminivorous iguandon. cuvier has found animals belonging to the existing families of the crocodile in the tertiary formation, and scheuchzer's 'antediluvian man' ('homo diluvii testis'), a large salamander allied to the axolotl, which i brought with me from the large mexican lakes, belongs to the most recent fresh-water formations of oeningen.* [footnote] *[ansted's 'ancient world', p. .] -- tr. the determination of the relative ages of organisms by the superposition of the strata has led to important results regarding the relations which have been discovered between extinct families and species (the latter being but few in number) and those which still exist. ancient and modern observations concur in showing that the fossil floras and faunas differ more from the present vegetable and animal forms in proportion as they belong to lower, that is, more ancient sedimentary formations. the numerical relations first deduced by cuvier p from the great phenomena of the metamorphism of organic life,* have led, through the admirable labors of deshayes and lyell, to the most marked results, especially with reference to the different groups of the tertiary formations, which contain a considerable number of accurately investigated structures. [footnote] *cuvier, 'recherches sur les ossemens fossiles', t. i., p. - . see, also, the geological scale of epochs in phillips's 'geology', , p. - . agassiz, who has examined species of fossil fishes, and who estimates the number of living species which have either been described or are preserved in museums at , expressly says, in his masterly work, that, "with the exception of a few small fossil fishes peculiar to the argillaceous geodes of greenland, he has not found any animal of this class in all the transition, secondary or tertiary formations, which is specifically identical with any still extant fish." he subjoins the important observation "that in the lower tertiary formations, for instance, in the coarse granular calcareous beds, and in the london clay,* one third of the fossil fishes belong to wholly extinct families. [footnote] *[see 'wonders of geology', vol. i., p. .] -- tr. not a single species of a still extant family is to be found under the chalk, while the remarkable family of the 'sauroidi' (fishes with enameled scales), almost allied to reptiles, and which are found from the coal beds -- in which the larger species lie -- to the chalk, where they occur individually, bear the same relation to the two families (the lepidosteus and polypterus) which inhabit the american rivers and the nile, as our present elephants and tapirs do to the mastodon and anaplotheriun of the primitive world."* [footnote] *agassiz, 'poissons fossiles', t. i., p. , and t. iii., p. - ; buckland, 'geology', vol. i., p. - . the beds of chalk which contain two of these sauroid fishes and gigantic reptiles, and a whole extinct world of corals and muscles, have been proved by ehrenberg's beautiful discoveries to consist of microscopic polythalamia, many of which still exist in our seas, and in the middle latitudes of the north sea and baltic. the first group of tertiary formations above the chalk, which has been designated as belonging to the 'eocene period', does not, therefore, merit that designation, since "the 'dawn of the world' in which we live extends much further back in the history of the past than we have hitherto supposed."* [footnote] *ehrenberg, 'ueber noch jetzt lebende thierarten der kreidelnldung', in the 'abhandl. der berliner akad.', , s. . as we have already seen, fishes, which are the most ancient of all vertebrata, are found in the silurian transition strata, p and then uninterruptedly on through all formations to the strata of the tertiary period, while saurians begin with the zechstone. in like manner, we find the first mammalia ('thylacotherium prevostii', and 't. bucklandii', which are nearly allied according to valenciennes,* with marsupial animals) in the oolitic formations (stonesfield schist), and the first birds in the most ancient cretaceous strata.** [footnote] *valenciennes, in the 'comptes rendus de l'academie des sciences', t. vii., , part ii., p. . [footnote] **in the weald clay; bendant, 'geologie', p. . the ornitholites increase in number in the gypsum of the tertiary formations. cuvier, 'ossemens fossiles', t. ii., p. - . such are, according to the present state of our knowledge, the lowest* limits of fishes, saurians, mammalia, and birds. [footnote] *[recent collections from the southern hemisphere show that this distribution was not so universal during the earlier epochs as has generally been supposed. see papers by darwin, sharpe, morris, and mccoy, in the 'geological journal'.] -- tr'. although corals and serpulidae occur in the most ancient formations simultaneously with highly-developed cephalopodes and crustaceans, thus exhibiting the most various orders grouped together, we yet discover very determinate laws in the case of many individual groups of one and the same orders. a single species of fossil, as goniatites, trilobites, or nummulites, sometimes constitutes whole mountains. where different families are blended together, a determinate succession of organisms has not only been observed with reference to the superposition of the formations, but the association of certain families and species has also been noticed in the lower strata of the same formation. by his acute discovery of the arrangement of the lobes of their chamber-sutures, leopold von buch has been enabled to divide the innumerable quantity of ammonites into well-characterized families, and to show that ceratites appertain to the muschelkalk, arietes to the lias, and goniatites to transition limestone and graywacke.* [footnote] *leop. von buch, in the 'abhandl. der berl. akad.', , s. - . the lower limits of belemnites are, in the keuper, covered by jura limestone, and their upper limits in the chalk formations.* [footnote] *quenstedt, 'flotzgebirge wurtembergs', , s. . it appears, from what we now know of this subject, that the waters must have been inhabited at the same epoch, and in the most widely-remote districts of the world, by shell-fish, which were at any rate, in part, identical with the fossil remains found in england. leopold von buch has discovered exogyra and trigonia in the southern hemisphere (volcano of p maypo in chili), and d'orbigny has described ammonites and gryphites from the himalaya and the indian plains of cutch, these remains being identical with those found in the old jurassic sea of germany and france. the strata which are distinguished by definite kinds of petrifacations, or by the fragments contained within them, form a geognostic horizon, by which the inquirer may guide his steps, and arrive at certain conclusions regarding the identity or relative age of the formations, the periodic recurrence of certain strata, their parallelism, or their total suppression. if certain strata, their parallelism, or their total suppression. if we classify the type of the sedimentary structures in the simplest mode of generalization, we arrive at the following series in proceeding from below upward: . the so-called 'transition rocks', in the two divisions of upper and lower graywacke (silurian and devonian systems), the latter being formerly designated as old red sandstone. . the 'lower trias',* comprising mountain limestone, coal-measures, together with the lower new red sandstone (todtliegende and zechstein).** . the 'upper trias', including variegated sandstone,** muschelkalk, and keuper. . 'jura limestone' (lias and oolite). . 'green sandstone', the quader sanstein, upper and lower chalk, terminating the secondary formations, which begin with limestone. . 'tertiary formations' in three divisions, distinguished as granular limestone, the lignites, and the sub-apennine gravel of italy. [footnote] *quenstedt, 'flotzgebirge wurtembergs', , s. . [footnote] ** murchison makes two divisions of the 'bunter sandstone', the upper being the same as the 'trias' of alberti, while the lower division, to which the 'vosges sandstone' of elie de beaumont belongs -- the 'zeckstein' and the 'todtliegende' -- he forms his 'permian' system. he makes the secondary formations commence with the 'upper trias', that is to say, with the upper division of our (german) bunter sandstone, while the permian system, the carboniferous or mountain limestone, and the devonian and silurian strata, constitute his 'palaeozoic formatiions'. according to these views, the chalk and jura constitute the upper, and the keuper, the muschelkalk, and the bunter sandstone the lower secondary formations, while the permian system and the carboniferous limestone are the upper, and the devonian and silurian strata are the lower palaeooic formation. the fundamental principles of this general classification are developed in the great work in which this indefatigable british geologist purposes to describe the geology of a large part of eastern europe. then follow, in the alluvial beds, the colossal bones of the mammalia of the primitive world, as the mastodon, dinothrium p missurium, and the megatherides, among which is owen's sloth-like mylodon, eleven feet in the length.* [footnote] *[see mantell's 'wonders of geology', vol. i., p. .] -- tr. besides these extinct families, we find the fossil remains of still extant animals, as the elephant, rhinoceros, ox, horse, and stag. the field near bogota, called the 'campo de gigantes', which is filled with the bones of mastodons, and in which i caused excavations to be made, lies feet above the level of the sea, while the osseous remains, found in the elevated plateaux of mexico, belong to true elephants of extinct species.* [footnote] *cuvier, 'ossemens fossiles', , t. i., p. , , and . see, also, humboldt, 'ueber die hochebene von bogota', in the 'deutschen vierteljahrs-schrift', , bd. i., s. . the projecting spurs of the himalaya, the sewalik hills, which have been so zealously investigated by captain cantley* and dr. falconer, and the cordilleras, whose elevations are probably, of very different epochs, contain, besides numerous mastodons, the sivatherium, and the gigantic land tortoise of the primitive world ('colossochelys'), which is twelve feet in length and six in height, and several extant families, as elephants, rhinoceroses, and giraffes; and it is a remarkable fact, that these remains are found in a zone which still enjoys the same tropical climate which must be supposed to have prevailed at the period of the mastodons.** [footnote] *[the fossil fauna of the sewalik range of hills, skirting the southern base of the himalaya, has proved more abundant in genera and species of mammalia than that of any other region yet explored. as a general expression of the leading features, it may be stated, that it appears to have been composed of representative forms of all ages, from the 'oldest of the tertiary period down to the modern', and of 'all the geographical' divisions of the old continent grouped together into one comprehensive fauna. 'fauna antiqua sivaliensis', by hugh falconer, m.d., and major p. t. cautley.] -- tr. having thus passed in review both the inorganic formations of the earth's crust and the animal remains which are contained within it, another branch of the history of the organic life still remains for our consideration, viz., the epoch of vegetation, and the successive floras that have occurred simultaneously with the increasing extent of the dry land and the modifications of the atmosphere. the oldest transition strata, as we have already observed, contain merely cellular marine plants, and it is only in the devonian system that a few cryptogamic forms of vascular plants (calamites and lycopodiaceae) have been observed.* [footnote] *beyrich, in karsteu's 'archiv fur mineralogie', , bd. xviii., s. . nothing appears to corroborate p the theoretical views that have been started regarding the simplicity of primitive forms of organic life, ow that vegetable preceded animal life, and that the former was necessarily dependent upon the latter. the existence of races of men inhabiting the icy regions of the north polar lands, and whose nutriment is solely derived from fish and cetaceans, shows the possibility of maintaining life independently of vegetable substances. after the devonian system and the mountain limestone, we come to a formation, the botanical analysis of which has made such brilliant advances in modern times.* [footnote] *by the important labors of count sternberg, adolphe brongniart, goppert, and lindley. the coal measures contain not only fern-like cryptogamic plants and phanerogamic monocotyledons (grasses, yucc-like liliaceae and palms), but also gymnospermic dicotyledons (coniferae and cycadeae), amounting in all to nearly species, as characteristic of the coal formations. of these we will only enumerate arborescent calamites and lycopodiaceae, scaly lepidodendra, sigillariae, which attain a height of sixty feet, and are sometimes found standing upright, being distinguished by a double system of vascular bundles, cactus-like stigmariae, a great number of ferns, in some cases the stems, and in others the fronds alone being found, indicating by their abundance the insular form of the dry land,* cycadeae** especially palms, although fewer in number.*** [footnote] *see robert brown's 'botany of congo', p. , and the memoir of the unfortunate e'urville, 'de la distribution des fougeres sur la surface du globe terrestre'. [footnote] **such are the cycadeae discovered by count sternberg in the old carboniferous formation at radnitz, in bohemia, and described by corda (two species of cycatides and zamites cordai. see goppert, 'fossile cycadeen in den arbeiten der schles. gesellschaft, fur waterl. cultur im jahr' , s. , , and ). a cycadea (pterophyllum gonorchachis, gopp.) has also been found in the carboniferous formations in upper silesia, at konigshutte. [footnote] ***lindley, 'fossil flora', no. xv., p. . asterophyllites, having whorl-like leaves, and allied to the naiades, with araucaria-like coniferae',* which exhibit faint traces of annual rings. [footnote] *'fossil coniferae', in buckland's 'geology', p. - . witham has the great merit of having first recognized the existence of coniferae in the early vegetation of the old carboniferous formation. almost all the trunks of trees found in this formation were previously regarded as palms. the species of the genus 'araucaria' are, however, not peculiar to the coal formations of the british islands; they likewise occur in upper silesia. this difference of character from our present vegtation, minifested in the vegetative forms which were so luxuriously developed on the drier p and more elevated portions of the old red sandstone, was maintained through all the subsequent epochs to the most recent chalk formations; amid the peculiar characteristics exhibited in the vegetable forms contained in the coal measures, there is, however, a strikingly-marked prevalence of the same families, if not of the same species,* in all parts of the earth as it then existed, as in new holland, canada, greenland, and melville island. [footnote[ *adolphe brongniart, 'prodrome d'une hist. des vegetaux fossiles', p. ; buckland, 'geology', p. ; endlicher and unger, 'grundzuge der botanik', , s. . the vegetation of the primitive period exhibits forms which, from their simultaneous affinity with several families of the present world, testify that many intermediate links must have become extinct in the scale of organic development. thus, for example, to mention only two instances, we would notice the lepidodendra, which, according to lindley, occupy a place between the coniferae and the lycopodiaceae*, and the araucariae and pines, which exhibit some peculiarities in the union of their vascular bundles. [footnote] *"by means of lepidodendron, a better passage is established from flowering to flowerless plants than by either equisetum or cycas, or any other known genus." -- lindley and hutton, 'fossil flora', vol. ii., p. . even if we limit our consideration to the present world alone, we must regard as highly important the discovery of cycadeae and coniferae side by side with sagenariae and lepidodendra in the ancient coal measures. the coniferae are not ony allied to cupuliferae and betulinae, with which we find them associated in lignite formations, but also with lycopodiaceae. the family of the sago-like cycadeae approaches most nearly to palms in its external appearance, while these plants are specially allied to coniferae in respect to the structure of their blossoms and seed.* [footnote] *kunth, 'anordnung der pflanzenfamilien', in his 'handb. der botanik', s. und . where many beds of coal are superposed over one another, the families and species are not always blended, being most frequently grouped together in separate genera; lycopodiaceae and certain ferns being alone found in one bed, and stigmariae and sigillariae in another. in order to give some idea of the luxuriance of the vegetation of the primitive world, and of the immense masses of vegetable matter which was doubtlessly accumulated in currents and converted in a moist condition into coal,* i would instance the saarbrucker coal measures, p where beds are superposed on one another, exclusive of a great many which are less than a foot in thickness; the coal beds at johnstone, in scotland, and those in the creuzot, in burgundy, are some of them, respectively, thirty and fifty feet in thickness,** while in the forests of our temperate zones, the carbon contained in the trees growing over a certain area would hardly suffice, in the space of a hundred years, to cover it with more than a stratum of seven french lines in thickness.*** [footnote] that coal has not been formed from vegetable fibers charred by fire, but that it has more probably been produced in the moist way by the action of sulphuric acid, is strikingly demonstrated by the excellent observation made by goppert (karsten, 'archiv fu mineralogie', bd. xviii., s. ), on the conversion of a fragment of amber-tree into black coal. the coal and the unaltered amber lay side by side. regarding the part which the lower forms of vegetation may have had in the formation of coal beds, see link, in the 'abhandl. der berliner akademie der wissenschaften', , s. . [footnote] **[the actual total thickness of the different beds in england varies considerably in different districts, but appears to amount in the lancashire coal field to as much as feet. -- ansted's 'ancient world', p. . for an enumeration of the thickness of coal measures in america and the old continent, see mantell's 'wonders of geology', vol. ii., p. .] -- tr. [footnote] ***see the accurate labors of chevandier, in the 'comptes rendus de l'academie des sciences', , t. xviii., part i., p. . in comparing this bed of carbon, seven lines in thickness, with beds of coal, we must not omit to consider the enormous pressure to which the latter have been subjected from superimposed rock, and which manifests itself in the flattened form of the stems of the trees found in these subterranean regions. "the so-called 'wood-hills' discovered in by sirowatskoi, on the south coast of the island of new siberia, consist, according to hedenstrom, of horizontal strata of sandstone, aolternating with bituminous trunks of trees, forming a mound thirty fathoms in neight; at the summit the stems were in a vertical position. the bed of driftwood is visible at five wersts' distance." -- see wrangel, 'reise iangs der nordkuste von siberien, in den jahren' - , th. i., s. . near the mouth of the mississippi, and in the "wood hills" of the siberian polar sea, described by admiral wrangel, the vast number of trunks of trees accumulated by river and sea water currents affords a striking instance of theenormous quantities of drift-wood which must have favored the formation of carboniferous deposition in the island waters and insular bays. there can be no doubt that these beds owe a considerable portion of the substances of which they consist to grasses, small branching shrubs, and cryptogamic plants. the association of palms and coniferae, which we have indicated as being characteristic of the coal formations, is discoverable throughout almost all formations to the tertiary period. in the present condition of the world, these genera p appear to exhibit no tendency whatever to occur associated together. we have so accustomed ourselves, although erroneously, to regard coniferae as a northern form, that i experienced a feeling of surprise when, in ascending from the shores of the south pacific toward chilpansingo and the elevated valleys of mexico, between the 'venta de la moxonera' and the 'alto de los caxones', feet above the level of the sea, i rode a whole day through a dense wood of pinus occidentalis, where i observed that these trees, which are so similar to the weymouth pine, were associated with fan palms* ('corypha dulcis'), swarming with brightly-colored parrots. [[footnote] *this corypha is the 'soyate' (in aztec, zoyatl), or the 'palma dulce' of the natives. see humboldt and bonplaud, 'synopsis plant. aequinoct. orbis novi', t. i., p. . professor buschmann, who is profoundly acquainted with the american languages, remarks, that the 'palma soyate' is so named in yepe's 'vocabulario de la lengua othomi', and that the aztec word zoyatl (molina, 'vocabulario en lengua mexicana y castellana', p. ) recurs in names of places, such as zoyatitlan and zoyapanco, near chiapa. south america has oaks, but not a single species of pine; and the first time that i again saw the familiar form of a fir-tree, it was thus associated with the strange appearance of the fan palm.* [footnote] *near baracoa and cayos de moya. see the admiral's journal of the th and th of november, , and humboldt, 'examen critique de l'hist. de la geographie du nouveau continent', t. ii., p. , and . iii., p. . columbus, who invariably paid the most remarkable attention to all natural objects, was the first to observe the difference between 'podocarpus' and 'pinus'. "i find," said he, "en la tierra aspera del cibao pinos que no ilevan pinas (fir cones), pero portal orden compuestos por naturaleza, que (los frutos) parecen azeytunas del axarafe de sevilla." the great botanist, richard, when he published his excellent memoir on cycadeae and coniferae, little imagined that before the time of l'heritier, and even before the end of the fifteenth century, a navigator had separated 'podocarpus' from the abietineae. christopher columbus, in his first voyage of discovery, saw coniferae and palms growing together on the northeastern extremity of the island of cuba, likewise within the tropics, and scarcely above the level of the sea. this acute observer, whom nothing escaped, mentions the fact in his journal as a remarkable circumstance, and his friend anghiera, the secretary of frdinand the catholic, remarks with astonishment "that 'palmeta' and 'pineta' are found associated together in the newly-discovered land." it is a matter of much importance to geology to compare the present distribution of plants over the earth's surface with that exhibited in the fossil floras of the primitive world. the temperate zone of the southern hemisphere, which is so rich in seas and islands, and where p tropical forms blend so remarkably with those of colder parts of the earth, presents according to darwin's beautiful and animated descriptions,* the most instructive materials for the study of the present and the past geography of plants. [footnote] *charles darwin, 'journal of the voyages of the adventure and beagle', , p. . the history of the primordial ages is, in the strict sense of the word, a part of the history of plants. cycadeae, which, from the number of their fossil species, must have occupied a far more important part in the extinct than in the present vegetable world, are associated with the nearly allied coniferae from the coal formations upward. they are almost wholly absent in the epoch of the variegated sandstone which contains coniferae of rare and luxuriant structure ('voltizia, haidingera, albertia'); the cycadeae, however, occur most frequently in the keuper and lias strata, in which more than twenty different forms appear. in the chalk, marine plants and naiades predominate. the forests of cycadeae of the jura formations had, therefore, long disappeared, and even in the more ancient tertiary formations they are quite subordinate to the coniferae and palms.* [footnote] *goppert describes three other cycadeae (species of cycadites and pterophyllum), found in the brown carboniferous schistose clay of alt-sattel and commotau, in bohemia. they very probably belong to the eocene period. goppert, 'fossile cycadeen', s. . the lignites, or beds of brown coal* which are present in all divisions of the tertiary period, present, among the most ancient cryptogamic land plants, some few palms, many coniferae having distinct annual rings, and foliaceous shrubs of a more or less tropical character. [footnote] *['medals of creation', vol. i., ch. v., etc. 'wonders of geology', vol. i., p. , .] -- tr. in the middle tertiary period we again find palms and cycadeae fully established, and finally a great similarity with our existing flora, manifested in the sudden and abundant occurrence of our pines and firs, cupuliferae, maples, and poplars. the dicotyledonous stems found in lignite are occasionally distinguished by colossal size and great age. in the trunk of a tree found at bonn, noggerath counted annual rings.* [footnote] *buckland, 'geology', p. . in the north of france, at yseux, near abbeville, oaks have been discovered in the turf moors of the somme which measured fourteen feet in diameter, a thickness which is very remarkable in the old continent and without the tropics. according to goppert's excellent investigations, which, it is hoped, may soon be illustrated by plates, it would appear that "all the amber of the baltic comes from p a coniferous tree, which, to judge by the still extant remains of wood and the bark at different ages, approaches very nearly to our white and red pines, although forming a distinct species. the amber-tree of the ancient world ('pinites succifer') abounded in resin to a degree far surpassing that manifested by any extant coniferous tree; for not only were large masses of amber deposited in and upon the bark, but also in the wood itself, following the course of the medullary rays, which, together with ligneous cells, are still discernible under the microscope, and peripherally between the rings, being some times both yellow and white." "among the vegetable forms inclosed in amber are male and femald blossoms of our native needle-wood trees and cupuliferae, while fragments which are recognized as belonging to thuia, cupressus, ephedera, and castania vesca, blended with those of junipers and firs, indicate a vegetation different from that of the coasts and plains of the baltic."* [footnote] *{the forests of amber-pines, 'pinites succifer', were in the southeastern part of what is now the bed of the baltic, in about degrees n. lat., and degrees e. long. the different colors of amber are derived from local chemical admixture. the amber contains fragments of vegetable matter, and from these it has been ascertained tht the amber-pine forests contained four other species of pine (besides the 'pinites succier'), several cypresses, yews, and junipers, with oaks, poplars, beeches, etc. -- altogether forty-eight species of trees and shrubs, constituting a flora of north american chracter. there are also some ferns, mosses, fungi, and liverworts. see professor goppert, 'geol. trans.', . insects, spiders, small crustaceans, leaves, and fragments of vegetable tissue, are imbedded in some of the masses. upward of species of insects have been observed; most of them belong to species, and even genera, that appear to be distinct from any now known, but others are nearly related to indigenous species, and some are identical with existing forms, that inhabit more southern climes. -- 'wonders of geology', vol. i., p. , etc.] -- tr. we have now passed through the whole series of formations comprised in the geological portion of the present work, proceeding from the oldest erupted rock and the most ancient sedimentary formations to the alluvial land on which are scattered those large masses of rock, the causes of whose general distribution have been so long and variously discussed, and which are, in my opinion, to be ascribed rather to the penetration and violent outpouring of pent-up waters by the elevation of mountain chains than to the motion of floating blocks of ice.* [footnote] *leopold von buch, in the 'abhandl. der akad. der wissensch. zu berlin', - , s. ; and in poggend., 'annalen', bd. ix., s. ; elie de beaumont, in the 'annales des sciences naturelles', t. xix., p. . the most ancient structures of the transition formation p with which we are acquainted are slate and graywacke, which contain some remains of sea weeds from the silurian or cambrian sea. on what did these so-called 'most ancient' formations rest, if gneiss and mica schist must be regarded as changed sedimentary strata? dare we hazard a conjecture on that which can not be an object of actual geognostic observation? according to an ancient indian myth, the earth is borne up by an elephant, who in his turn is supported by a gigantic tortoise, in order that he may not fall; but it is not permitted to the credulous brahmins to inquire on what the tortoise rests. we venture here upon a somewhat similar problem, and are prepared to meet with opposition in our endeavors to arrive at its soluion. in the first formation of the planets, as we stated in the astronomical portion of this work, it is probable that nebulous rings revolving round the sun were agglomerated into spheroids, and consolidated by a gradual condensation proceeding from the exterior toward the center. what we term the ancient silurian strata are thus only the upper portions of the solid crust of the earth. the erupted rocks which have broken through and upheaved these strata have been elevated from depths that are wholly inaccessible to our research; they must, therefore, have existed under the silurian strata, and been composed of the same association of minerals which we term granite, augite, and quartzose porphyry, when they are made known to us by eruption through the surface. basing our inquiries on analogy, we may assume that the substances which fill up deep fissures and traverse the sedimentary strata are merely the ramifications of a lower deposit. the foci of active volcanoes are situated at enormous depths, and judging from the remarkable fragments which i have found in various parts of the earth incrusted in lava currents, i should deem it more than probable tht a primordial granite rock forms the substratum of the whole stratified edifice of fossil remains.* [footnote] *see elie de beaumont, 'descr. geol. de la france', t. i., p. ; beaudant, 'geologie', , p. . basalt containing olivine first shows itself in the period of the chalk trachyte still later, while eruptions of granite belong, as we learn from the products of their metamorphic action to the epoch of the oldest sedimentary strata of the transition formation. where knowledge can not be attained from immediate perceptive evidence, we may be allowed from induction, no less than from a careful comparison of facts, to hazard a conjecture by which granite would be restored p to a portion of its contested right and title to be considered as a 'primordial' rock. the recent progress of geognosy, that is to say, the more extended knowledge of the geognostic epochs characterized by differences of mineral formations, by the peculiarities and succession of the organisms contained within them, and by the position of the strata, whether uplifted or inclined horizontally, leads us, by means of the causal connection existing among all natural phenomena, to the distribution of solids and fluids into the continents and seas which constitute the upper crust of our planet. we here touch upon a point of contact between geological and geographical geognosy which would constitute the complete history of the form and extent of continents. the limitation of the solid by the fluid parts of the earth's surface and their mutual relations of area, have varied very considerably in the long series of geognostic epochs. they were very different, for instance, when carboniferous strata were horizontally deposited on the inclined beds of the mountain limestone and old red sandstone; when lias and oolite lay on a substratum of keuper and muschelkalk, and the chalk rested on the slopes of green sandstone and jura limestone. if, with elie de beaumont, we term the waters in which the jura limestone and chalk formed a soft deposit the 'jurassic or oolitic', and the 'cretaceous seas', the outlines of these formations will indicate, for the two corresponding epochs, the boundaries between the already dried land and the ocean in which these rocks were forming. an ingenious attempt has been made to craw maps of this physical portion of primitive geography and we may consider such diagrams as more correct than those of the wanderings of io or the homeric geography, since the latter are merely graphic representations of mythical images, while the former are based upon positive facts deduced from the science of geology. the results of the investigations made regarding the areal relations of the solid portions of our planet are as follows: in the most ancient times, during the silurian and devonian transition epochs, and in the secondary formations, including the trias, the continental portions of the earth were limited to insular groups covered with vegetation; these islands at a subsequent period became united, giving rise to numerous lakes and deeply-indented bays; and finally, when the chains of the pyrenees, apennines, and carpathian mountains were elevated about the period of the more ancient tertiary formations, large continents appeared, having almost their present p size.* [footnote] *[these movements, described in so few words, were doubtless going on for many thousands and tens of thousands of revolutions of our planet. they were accompanied, also, by vast but slow changes of other kinds. the expansive force employed in lifting up, by mighty movements, the northern portion of the continent of asia, found partial vent; and from partial subsqueous fissures there were poured out the tabular masses of basalt occurring in central india, while an extensive area of depression in the indian ocean, marked by the coral islands of the laccadives, the maldives, the great chagos bank, and some others, were in the course of depression by a counteracting movement. -- ansted's 'ancient world', p. , etc.] -- tr. in the silurian epoch, as well as in that in which the cycadeae flourished in such abundance, and gigantic saurians were living, the dry land, from pole to pole, was probably less than it now is in the south pacific and the indian ocean. we shall see, in a subsequent part of this work, how this preponderating quantity of water, combined with other causes, must have contributed to raise the temperature and induce a greater uniformity of climate. here we would only remark in considering the gradual extension of the dry land, that, shortly before the 'disturbances' which at longer or shorter intervals caused the sudden destruction of so great a number of colossal vertebrata in the 'diluvial period', some parts of the present continental masses must have been completely separated from one another. there is a great similarity in south america and australia between still living and extinct species of animals. in new holland, fossil remains of the kangaroo have been found, and in new zealand the semi-foxxilized bones of an enormous bird, resembling the ostrich, the dinornis of owen,* which is nearly allied to the present spteryx, and but little so to the recently extinct dronte (dodo) of the island of rodriguez. [[footnote] *[see 'american journal of science', vol. xiv., p. ; and 'medals of creation', vol. ii., p. ; 'trans. zoolog. society of london', vol. ii; 'wonders of geology', vol. i., p. .] -- tr. the form of the continental portions of the earth may, perhaps, in a great measure, owe their elevation above the surrounding level of the water to the eruption of quartzose porphyry, which overthrew with violence the first great vegetation from which the matrial of our present coal measures was formed. the portions of the earth's surface which we term plains are nothing more than the broad summits of hills and mountains whose bases rest on the bottom of the ocean. every plain is, therefore, when considered according to its submarine relations, an 'elevated plateau', whose inequalities have been covered over by horizontal deposition of new sedimentary formations and by the accumulation of alluvium. p among the general subjects of contemplation appertaining to a work of this nature, a prominent place must be given, first, in the consideration of the 'quantity' of the land raised above the level of the sea, and next, to the individual configuration of each part, either in relation to horizontal extension (relations of form) or to vertical elevation (hypsometrical relations of mountain-chains). our planet has two envelopes, of which one, which is general -- the atmosphere -- is composed of an elastic fluid, and the other -- the sea -- is only locally distributed, surrounding, and therefore modifying, the form of the land. these two envelopes of air and sea constitute a natural whole, on which depend the difference of climate on the earth's surface, according to the relative extension of the aqueous and solid parts, the form and aspect of the land, and the direction and elevation of mountain chains. a knowledge of the reciprocal action of air, sea, and land teaches us that great meteorological phenomena can not be comprehended when considered independently of geognostic relations. meteorology, as well as the geography of plants and animals, has only begun to make actual progress since the mutual dependence of the phenomena to be investigated has been fully recognized. the word climate has certainly special reference to the character of the atmosphere, but this character is itself dependent on the perpetually concurrent influences of the ocean, which is universally and deeply agitated by currents having a totally opposite temperature, and of radiation from the dry land, which varies greatly in form, elevation, color, and fertility, whether we consider its bare, rocky portions, or those that are covered with arborescent or herbaceous vegetation. in the present condition of the surface of our planet, the area of the solid is to that of the fluid parts as : / ths (according to rigaud, as : ).* [footnote] *see 'transactions of the cambridge philosophical society', vcl. vi., part ii., , p. . other writers have given the ratio as : . the islands form scarcely / d of the continental masses, which are so unequally divided that they consist of three times more land in the northern than in the southern hemisphere; the latter being, therefore, pre-eminently oceanic. from degrees south latitude to the antarctic pole the earth is almost entirely covered with water. the fluid element predominates in like manner between the eastern shores of the old and the western shores of the new continent, being only interspersed with some few insular groups. the learned hydrographer fleurieu has very justly named this p vast oceanic basis, which, under the tropics, extends over Ã�¼degrees of longitude, the 'great ocean', in contradistinction to all other seas. the southern and western hemispheres (reckoning the latter from the meridian of teneriffe) are therefore more rich in water than in any other region of the whole earth. these are the main points involved in the consideration of the relative quantity of land and sea, a relation which exercises so important an influence on the distribution of temperature, the variations in atmospheric pressure, the direction of the winds, and the quantity of moisture contained in the air, with which the development of vegetation is so essentially connected. when we consider that nearly three fourths of the upper surface of our planet are covered with water,* we shall be less surprised at the imperfect condition of meteorology before the beginning of the present century, since it is only during the subsequent period that numerous accurate observations on the temperature of the sea at different latitudes and at different seasons have been made and numerically compared together. [footnote] *in the middle ages, the opinion prevailed that the sea covered one seventh of the surface of the globe, an opinion which cardinal d'ailly ('imago mundi', cap. ) founded on the fourth apocryphal book of esdras. columbus, who derived a great portion of his cosmographical knowledge from the cardinal's work, was much interested in upholding this idea of the smallness of the sea, to which the misunderstood expression of "the ocean stream" contributed not a little. see humboldt, 'examen critique de l'hist. de la geographie', t. i., p. . the horizontal configuration of continents in their general relations of extension was already made a subject of intellectual contemplation by the ancient greeks. conjectures were advanced regarding the maximum of the extension from west to east, and dicaearchus placed it, according to the testimony of agathemerus, in the latitude of rhodes, in the direction of a line passing from the pillars of hercules to thine. this line, which has been termed 'the parallel of the diaphragm of dicaearchus', is laid down with an astronomical accuracy of position, which, as i have stated in another work, is well worthy of exciting surprise and admiration.* [footnote] *agathemerus, in hudson, 'geographi minores', t. ii., p. . see humboldt, 'asie centr.', t. i., p. - . strabo, who was probably influenced by eratosthenes, appears to have been so firmly convinced that this parallel of degrees was the maximum of the extension of the then existing world, that he supposed it had some intimate connection with the form of the earth, and therefore places under this line the continent whose existence p he divined in the northern hemisphere, between theria and the coasts of thine.* [footnote] *strabo, lib. i., p. , casaub. see humboldt, 'examen crit.', t. i., p. . as we have already remarked, one hemisphere of the earth (whether we divide the sphere through the equator or through the meridian of teneriffe) has a much greater expansion of elevated land than the opposite one: these two vast ocean-girt tracts of land, which we term the eastern and western, or the old and new continents, present, however, conjointly with the most striking contrasts of configuration and position of their axes, some similarities of form, especially with reference to the mutual relations of their opposite coasts. in the eastern continent, the predominating direction -- the position of the major axis -- inclines from east to west (or, more correctly speaking, from southwest to northeast), while in the western continent it inclines from south to north (or, rather, from south-southeast to north-northwest). both terminate to the north at a parallel coinciding nearly with that of Ã�¼degrees, while they extend to the south in pyramidal points, having submarine prolongations of islands and shoals. such, for instance, are the archipelago of tierra del fuego, the lagullas bank south of the cape of good hope, and van diemen's land, separated from new holland by bass's straits. northern asia extends to the above parallel at cape taimura, which, according to krusenstern, is degrees ', while it falls below it from the mouth of the great tschukotsehja river eastward to behring's straits, in the eastern extremity of asia -- cook's east cape -- which, according to beechey, is only degrees e.* [footnote] *on the mean latitude of the northern asiatic shores, and the true name of cape taimura (cape siewere-wostotschnoi), and cape northeast (schalagskoi mys), see humboldt, 'asie centrale', t. iii., p. , . the northern shore of the new continent follows with tolerable exactness the parallel of degrees, since the lands to the north and south of barrow's strait, from boothia felix and victoria land, are merely detached islands. the pyramidal configuration of all the southern extremities of continents belongs to the 'similtudines physicae in configuratione mundi', to which bacon already called attention in his 'novum organon', and with which reinhold foster, one of cook's companions in his second voyage of circumnavigation, connected some ingenious considerations. on looking eastward from the meridian of teneriffe, we perceive that the southern extremities of the three continents, viz., africa as the extreme p of the old world, australia, and south america, successively approach nearer toward the south pole. new zealand, whose length extends fully degrees of latitude, forms an intermediate link between australia and south america, likewise terminating in an island, new leinster. it is also a remarkable circumstance that the greatest extension toward the south falls in the old continent, under the same meridian in which the extremest projection toward the north pole is manifested. this will be perceived on comparing the cape of good hope and the lagullas bank with the north cape of europe, and the peninsula of malacca with cape taimura in siberia.* [footnote] *humboldt, 'asie centrale', t. i., p. - . the southern point of america, and the archipelago which we call terra del fuego, lie in the meridian of the northwestern part of baffin's bay, and of the great polar land, whose limits have not as yet been ascertained, and which, perhaps, belongs to west greenland. we know not whether the poles of the earth are surrounded by land or by a sea of ice. toward the north pole the parallel of degrees ' has been reached, but toward the south pole only that of degrees '. the pyramidal terminations of the great continents are variously repeated on a smaller scale, not only in the indian ocean and in the peninsulas of arabia, hindostan, and malacca, but also, as was remarked by eratosthenes and polybius, in the mediterranean, where these writers had ingeniously compared together the forms of the iberian, italian, and hellenic peninsulas.* [footnote] *strabo, lib. ii., p. , , cassaub. europe, whose area is five times smaller than that of asia, may almost be regarded as a multifariously articulated western peninsula of the more compact mass of the ontinent of asia, the climatic relations of the former being to those of the latter as the peninsula of brittany is to the rest of france. [footnote] *humboldt, 'asie centrale', t. iii., p. . as early as the year , in my work 'de distributione geographica plantarum, secundum caels temperiem et altitudinem montium', i directed attention to the important influence of compact and of deeply-articulated continents on climate and human civilization, "regiones vel per sinus lunatos in longa cornua porrectae, angulois littorum recessibus quasi membratim discerptae, vel spatia patentia in immensum, quorum littora nullis incisa angulis ambit sine aufractu oceanus" (p. , ). on the relations of the extent of coast to the area of a continent (considered in some degree as a measure of the accessibility of the interior), see the inquiries in berghaus, 'annalen der erdkunde', bd. xii., , s. , and 'physikal. atlas', , no. iii., s. . the influence exercised by the articulation and higher development of the form of a continent on the moral and intellectual condition of nations was remarked by strabo,* who extols p the varied form of our small continent as a special advantage. [footnote] *strabo, lib. ii., p. , . casaub. africa* and south america, which manifest so great a resemblence in their configuration, are also the two continents that exhibit the simplest littoral outlines. [footnote] *of africa, pliny says (v. ), "nec alia pars terrarum paudiores recipit sinus." the small indian peninsula on this side the ganges present, in its triangular outline, a third analogous form. in ancient greece there prevailed an opinion of the regular configuration of the dry land. there were four gulfs or bays, among which the persian gulf was placed in opposition to the hyrcanian or caspian sea (arrian, vii., ; plut., 'in vita alexandri', cap. ; dionys. perieg., v. and , p. , , bernh.). these four bays and the isthmuses were, according to the optical fancies of agesianax, supposed to be reflected in the moon (plut., 'de facie in orbem lunae', p. , ). respecting the 'terra quadrifida', or four divisions of the dry land, of which two lay north and two south of the equator, see macrobius, 'comm. in somnium scipionis', ii., . i have submitted this portion of the geography of the ancients, regarding which great confusion prevails, to a new and careful examination, in my 'examen crit. de l'hist. de la geogr.', t. i., p. , , - , as also in 'asie centr.', t. ii., p. - . it is only the eastern shores of asia, which, broken as it were by the force of the currents of the ocean* ('fractas ex aequore terra'), exhibit a richly-variegated configuration, peninsulas and contiguous islands alternating from the equator to degrees north latitude. [footnote] *fleurieu, in 'voyage de marchand autour du monde', t. iv., p. - . our atlantic ocean presents all the indications of a valley. it is as if a flow of eddying waters had been directed first toward the northeast, then toward the northwest, and back again to the northeast. the parallelism of the coasts north of degrees south latitude, the projecting and receding angles, the convexity of brazil opposite to the gulf of guinea, that of africa under the same parallel, with the gulf of the antilles, all favor this apparently speculative view.* [footnote] *humboldt, in the 'journal de physique', liii., , p. ; and 'rel. hist.', t. ii., p. ; t. iii., p. , . in this atlantic valley, as is almost every where the case in the configuration of large continental masses, coasts deeply indented, and rich in islands, are situated opposite to those possessing a different character. i long since drew attention to the geognostic importance of entering into a comparison of the western coast of africa and of south america within the tropics. the deeply curved indentation of the african continent at fernando po, degrees ' north latitude, is repeated on the coast of the pacific at degrees ' south latitude, between the valley of arica and the morro de juan diaz, where the peruvian coast suddenly changes the direction from wouth to north which it had previously followed, and inclines to the northwest. this change p of direction extends in like manner to the chain of the andes, which is divided into two parallel branches affecting not only the littoral portions,* but even the eastern cordilleras. [footnote] *humboldt, in poggendorf's 'annalen der physik', bd. xl., s. . on the remarkable fiord formation at the southeast end of america, see darwin's journal ('narrative of the voyages of the adventure and beagle', vol. iii.), , p. . the parallelism of the two mountain chains is maintained from degrees north latitude. the change in the direction of the coast at arica appears to be in consequence of the altered course of the fissure, above which the cordillera of the andes has been upheaved. in the latter, civilization had its earliest seat in the south american plateaux where the small alpine lake of titicaca bathes the feet of the colossal mountains of sorata and illimani. further to the south, from valdiva and chiloÃ�Â� ( degrees to degrees south latitude), through the archipelago 'de los chonos' to 'terra del fuego', we find repeated that singular configuration of 'fiords' (a blending of narrow and deeply-indented bays), which in the northern hemisphere characterizes the western shores of norway and scotland. these are the most general considerations suggested by the study of the upper surface of our planet with reference to the form of continents, and their expansion in a horizontal direction. we have collected facts and brought forward some analogies of configuration in distant parts of the earth, but we do not venture to regard them as fixed laws of form. when the traveler on the declivity of an active volcano, as, for instance, of vesuvius, examines the frequent partial elevations by which portions of the soil are often permanently upheaved several feet above their former level, either immediately precediing or during the continuance of an eruption, thus forming roof-like or flattened summits, he is taught how accidental conditions in the expression of the force of subterranean vapors, and in the resistance to be overcome, may modify the feeble perturbations in the equilibrium of the internal elastic forces of our planet may have inclined them more to its norther than to its southern direction, and caused the continent in the eastern part of the globe to present a broad mass, whose major axis is almost parallel with the equator, while in the western and more oceanic part the southern extremity is extremely narrow. very little can be empirically determined regarding the causal connection of the phenomena of the formation of continents, or of the analogies and contrasts presented by their p configuration. all that we know regarding this subject resolves itself into this one point, that the active cause is subterranean; that continents did not arise at once in the form they now present, but were, as we have already observed, increased by degrees by means of numerous oscillatory elevations and depressions of the soil, or were formed by the fusion of separate smaller continental masses. their present form is, therefore, the result of two causes, which have exercised a consecutive action the one on the other; the first is the expression of subterranean force, whose direction we term accidental, owing to our inability to defint it, from its removal from within the sphere of our comprehension, while the second is derived from forces acting on the surface, among which volcanic eruptions, the elevation of mountains, and currents of sea water play the principal parts. how totally different would be the condition of the temperature of the earth, and consequently, of the state of vegetation, husbandry, and human society, if the major axis of the new continent had the same direction as that of the old continent; if, for instance, the cordilleras, instead of having a southern direction, inclined from east to west; if there had been no radiating tropical continent, like africa, to the south of europe; and if the mediterranean, which was once connected with the caspian and red seas, and which has become so powerful a means of furthering the intercommunication of nations, had never existed, or if it had been elevated like the plains of lombardy and cyrene? the changes of the reciprocal relations of height between the fluid and solid portions of the earth's surface (changes which, at the same time, determine the outlines of continents, and the greater or lesser submersion of low lands) are to be ascribed to numerous unequally working causes. the most powerful have incontestably been the force of elastic vapors inclosed in the interior of the earth, the sudden change of temperature of certain dense strata,* the unequal secular loss of p heat experienced by the crust and nucleus of the earth, occasioning ridges in the solid surface, local modifications of gravitation,** and, as a consequence of these alterations, in the curvature of a portion of the liquid element. [footnote] *de la beche, 'sections and views illustrative of geological phenomena', , tab. ; charles babbage, 'observations on the temple of serapis at pozzuoli, near naples, and on certain causes which may produce geological cycles of great extent', . "if a stratum of sandstone five miles in thickness should have its temperature raised about degrees, its surface would rise twenty-five feet. heated beds of clay would, on the contrary, occasion a sinking of the ground by their contraction." see bischof, 'wurmelehre des innern unseres erdkorpers', s. , concerning the calculations for the secular elevation of sweden, on the supposition of a rise by so small a quantity as degrees in a stratum of about , feet in thickness, and heated to a state of fusion. [footnote] **the opinion so implicitly entertained regarding the invariability of the force of gravity at any given point of the earth's surface, has in some degree been controverted by the gradual rise of large portions of the earth's surface. see bessel, 'ueber maas und gewicht', in schumacher's 'jahrbuch fur' , s. . according to the views generally adopted by geognosists in the present day and which are supported by the observation of a series of well-attested facts, no less than by analogy with the most important volcanic phenomena, it would appear that the elevation of continents is actual, and not merely apparent or owing to the configuration of the upper surface of the sea. the merit of having advanced this view beloongs to leopold von buch, the narrative of his memorable 'travels through norway and sweden' in and .* [footnnote] *th. ii. ( ), s. . see hallstrom, in 'kongl. vetenskaps-academiens handlingar' (stockh.), , p. ; lyell in the 'philos. trans.' for ; blom (amtmann in budskerud), 'stat. beschr. von norwegen', , s. - . if not before von buch's travels through scandinavia, at any rate before their publication, playfair, in , in his illustrations of the huttonian theory, Ã�¤ , and according to keilhau ('om landjardens stigning in norge', in the 'nyt magazine fur naturvidenskaberne'), and the dane jessen, even before the time of playfair, had expressed the opinion that it was not the sea which was sinking, but the solid land of sweden which was rising. their ideas, however, were wholly unknown to our great geologist, and exerted no influence on 'norge fremstillet efter dets naturlige og borgerlige tilstand', kjobenh., , sought to explain the causes of the changes in the relative levels of the land and sea, basing his views on the early calculations of celsius, kalm, and dalin. he broaches some confused ideas regarding the possibility of an internal growth of rocks, but finally declares himself in favor of an upheaval of the land by earthquakes, "although," he observes, "no such rising was apparent immediately after the earthquake of egersund, yet the earthquake may have opened the way for other causes producing such an effect." while the whole coast of sweden and finland, from solvitzborg, on the limits of northern scania, past gefle to tornea, and from tornea to abo, experiences a gradual rise of four feet in a century, the southern part of sweden is, according to neilson, undergoing a simultaneous depression.* [footnote] *see berzelius, 'jahrsbericht uber die fortschritte der physichen wiss.', no. , s. . the islands of saltholm, opposite to copenhagen, and bjornholm, however, rise but very little -- bjornholm scarcely one foot in a century. see forchhammer, in 'philos. magazine', d series, vol. ii., p. . the maximum of this elevating p force appears to be in the north of lapland, and to diminish gradually to the south toward calmar and solvitzborg. lines marking the ancient level of the sea in pre-historic times are indicated throughout the whole of norway,* from cape lindesnaes to the extremity of the north cape, by banks of shells identical with those of the present seas, and which have lately been most accurately examined by bravais during his long winter sojourn at bosekop. [footnote] *keilhan, in 'nyt mag. fur naturvid.', , bd. i., p. - ; bd. ii., p. ; bravais, 'surles lignes d'ancien niveau de la mer', , p. - . see, also, darwin, "on the parallel roads of glen-roy and lochaber," in 'philos. trans. for' , p. . these banks lie nearly feet above the present mean level of the sea, and reappear, according to keilhau and eugene robert, in a north-northwest direction on the coasts of spitzbergen, opposite the north cape. leopold von buch, who was the first to draw attention to the high banks of shells at tromsoe (latitude degrees '), has, however, shown that the more ancient elevations on the north sea appertain to a different class of phenomena, from the regular and gradual retrogressive elevations of the swedish shores in the gulf of bothnia. this latter phenomenon, which is well attested by historical evidence, must not be confounded with the changes in the level of the soil occasioned by earthquakes, as on the shores of chili and of cutch, and which have recently given occasion to similar observations in other countries. it has been found that a perceptible sinking resulting from a disturbance of the strata of the upper surface sometimes occurs, corresponding with an elevation elsewhere, as, for instance, in west greenland, according to pingel and graah, in dalmatia and in scania. since it is highly probable that the oscillatory movements of the soil, and the rising and sinking of the upper surface, were more strongly marked in the early periods of our planet than at present, we shall be less surprised to find in the interior of continents some few portions of the earth's surface lying below the general level of existing seas. instances of this kind occur in the soda lakes described by general andreossy, the small bitter lakes in the narrow isthmus of suez, the caspian sea, the sea of tiberias, and especially the dead sea.* [footnote] *humboldt, 'asie centrale', t. ii., p. - ; t. iii., p. - . the depression of the dead sea has been successively determined by the barometrical measurements of count berton, by the more careful ones of russegger, and by the trigonometrical survey of lieutenant symond, of the royal navy, who states that the difference of level between the surface of the dead sea and the highest houses of jaffa is about feet. mr. alderson, who communicated this result to the geographical society of london in a letter, of the contents of which i was informed by my friend, captain washington, was of opinion (nov. , ) that the dead sea lay about feet under the level of the mediterranean. a more recent communication of lieutenant symond (jameson's 'edinburgh new philosophical journal', vol. xxxiv., , p. ) gives feet as the final result of two very accordant trigonometrical operations. the level of the water in the two last-named seas is p and feet below the level of the mediterranean. if we could suddenly remove the alluvial soil which covers the rocky strata in many parts of the earth's surface, we should discover how great a portion of the rocky crust of the earth was then below the present level of the sea. the periodic, although irregularly alternating rise and fall of the water of the caspian sea, of which i have myself observed evident traces in the northern portions of its basin, appears to prove,* as do also the observations of darwin on the coral seas,** that without earthquakes, properly so- called, the surface of the earth is capable of the same gentle and progressive oscillations as those which must have prevailed so generally in the earliest ages, when the surface of the hardening crust of the earth was less compact than at present. [footnote] *'sur la mobilite du fond de la mer caspienne', in my 'asie centr.', t. ii., p. - . the imperial academy of sciences of st. petersburgh in , at my request, charged the learned physicist lenz to place marks indicating the mean level of the sea, for definite epochs, in different places near baku, in the peninsula of abscheron. in the same manner, in an appendix to the instructions given to captain (now sir james c.) ross for his antarctic expedition, i urged the necessity of causing marks to be cut in the rocks of the southern hemisphere, as had already been done in sweden and on the shores of the caspian sea. had this measure been adopted in the early voyages of bougainville and cook, we should now know whether the secular relative changes in the level of the seas and land are to be considered as a general, or merely a local natural phenomenon, and whether a law of direction can be recognized in the points which have simultaneous elevation or depression. [footnote] **on the elevation and depression of the bottom of the south sea, and the diffrent areas of alternate movements, see darwin's 'journal', p. , - . the phenomena to which we would here direct attention remind us of the instability of the present order of things, and of the changes to which the outlines and configuration of continents are probably still subject at long intervals of time. that which may scarcely be perceptible in one generation, accumulates during periods of time, whose duration is revealed to us by the movement of remote heavenly bodies. the eastern coast of the scandinavian peninsula has probably risen p about feet in the space of years; and in , years, if the movement be regular, parts of the bottom of the sea which lie nearest the shores, and are in the present day covered by nearly fifty fathoms of water, will come to the surface and constitute dry land. but what are such intervals of time compared to the length of the geognostic periods revealed to us in the stratified series of formations, and in the world of extinct and varying organisms! we have hitherto only considered the phenomena of elevation; but the analogies of observed facts lead us with equal justice to assume the possibility of the depression of whole tracts of land. the mean elevation of the non-mountainous parts of france amounts to less than feet. it would not, therefore, require any long period of time, compared with the old geognostic periods, in which such great changes were brought about in the interior of the earth, to effect the permanent submersion of the northwestern part of europe, and induce essential alterations in its littoral relations. the depression and elevation of the solid or fluid parts of the earth -- phenomena which are so opposite in their action that the effect of elevation in one part is to produce an apparent depression in another -- are the causes of all the changes which occur in the configuration of continents. in a work of this general character, and in an impartial exposition of the phenomena of nature, we must not overlook the 'possibility' of a diminution of the quantity of water, and a constant depression of the level of seas. thgere can scarcely be a doubt that, at the period when the temperature of the surface of the earth was higher, when the waters were inclosed in larger and deeper fissures, and when the atmosphere possessed a totally different character from what it does at present, great changes must have occurred in the level of seas, depending upon the increase and decrease of the liquid parts of the earth's surface. but in the actual condition of our planet, there is no direct evidence of a real continuous increase or decrease of the sea, and we have no proof of any gradual change in its level at certain definite points of observation, as indicated by the mean range of the barometer. according to experiments made by daussy and antonio nobile, an increase in the height of the barometer would in itself be attended by a depression in the level of the sea. but as the mean pressure of the atmosphere at the level of the sea is not the same at all latitudes, owing to meteorological causes depending upon the direction of the wind and varying degrees of moisture, the p barometer alone can not afford a certain evidence of the general change of level in the ocean. the remarkable fact that some of the ports in the mediterranean were repeatedly left dry during several hours at the beginning of this century, appears to show that currents may by changes occurring in their direction and force, occasion a 'local'' retreat of the sea, and a permanent drying of a small portion of the shore, without being followed by any actual diminution of water, or any permanent depression of the ocean. we must, however, be very cautious in applying the knowledge which we have lately arrived at, regarding these involved phenomena, since we might otherwise be led to ascribe to water as the elder element, what ought to be referred to the two other elements, earth and air. as the 'external' configuration of continents, which we have already described in their horizontal expansion, exercises, by their variously indented littoral outlines, a favorable influence on climate, trade, and the progress of civilization, so likewise does their internal articulation, or the vertical elevation of the soil (chains of mountains and elevated plateaux), give rise to equally important results. whatever produces a polymorphic diversity of forms on the surface of our planetary habitation -- such as mountains, lakes, grassy savannas, or even deserts encircled by a band of forests -- impresses some peculiar character on the social condition of the inhabitants. ridges of high land covered by snow impede intercourse; but a blending of low, discontinued mountain chains* and tracts of valleys, as we see so happily presented in the west and south of europe, tends to the multiplication of meteorological processes and the products of vegetation, and, from the variety manifested in different kinds of cultivation in each district, even under the same degree of latitude, gives rise to wants that stimulate the activity of the inhabitants. [footnote] *humboldt, 'rel. hist.', t. iii., p. - . see also, the able remarks on the configuration of the earth, and the position of its lines of elevation in albrechts von roon, 'grundzugen der erd volker und staatenkunde', abth. i., , s. , , . thus the awful revolutions, during which, by the action of the interior on the crust of the earth, great mountain chains have been elevated by the sudden upheaval of a portion of the oxydized exterior of our planet, have served, after the establishment of repose, and on the revival of organic life, to furnish a richer and more beautiful variety of individual forms, and in a great measure to remove from the earth that aspect of dreary p uniformity which exercises so impoverishing an influence on the physical and intellectual powers of mankind. according to the grand views of elie de beaumont, we must ascribe a relative age to each system of mountain chains* on the supposition that their elevation must necessarily have occurred between the period of the deposition of the vertically elevated strata and that of the horizontally inclined strata running at the base of the mountains. [footnnote] *leop. von buch, 'ueber die geognostischen systeme von deutschland', in his 'geogn. briefen an alexander von humboldt', , s. - ; elie de beaumont, 'recherches sur les revolutions de la surface du globe', , p. - . the ridges of the earth's crust -- elevations of strata which are of the same geognostic age -- appear, moreover, to follow one common direction. the line of strike of the horizontal strata is not always parallel with the axis of the chain, but intersects it, so that, according to my views,* the phenomenon of elevation of the strata, which is even found to be repeated in the neighboring plains, must be more ancient than the elevation of the chain. [footnote] *humboldt, 'asie centrale', t. i., p. - . see, also my 'essai sur le gisement des roches', , p. , and 'relat. hist.', t. iii., p. - . the main direction of the whole continent of europe (from southwest to northeast) is opposite to that of the great fissures which pass from northwest to southeast, from the mouths of the rhine and elbe, through the adriatic and red seas, and through the mountain system of putschi-koh in luristan, toward the persian gulf and the indian ocean. this almost rectangular intersection of geodesic lines exercises an important influence on the commercial relations of europe, asia, and the northwest of africa, and on the progress of civilization on the formerly more flourishing shores of the mediterranean.* [footnote] *'asie centrale', t. i., p. , . the adriatic sea likewise follows a direction from s.e. to n.w. since grand and lofty mountain chains so strongly excite our imagination by the evidence they afford of great terrestrial revolutions, and when considered as the boundaries of climates, as lines of separation for waters, or as the site of a different form of vegetation, it is the more necessary to demonstrate, by a correct numerical estimation of their volume, how small is the quantity of their elevated mass when compared with the area of the adjacent continnents. the mass of the pyrenees, for instance, the mean elevation of whose summits, and the real quantity of whose base have been ascertained by accurate measurements, would if scattered over p the surface of france, only raise its mean level about feet. the mass of the eastern and western alps would in like manner only increase the height of europe about / feet above its present level. i have found by a laborious investigation,* which from its nature, can only give a maximum limit, that the center of gravity of the volume of the land raised above the present level of the sea in europe and north america is respectively situated at an elevation of and feet, while it is at and feet in asia and south america. [footnote] *'de la hauteur moyenne des continents', in my 'asie centrale', t. i., p. - , - . the results which i have obtained are to be regarded as the extreme value ('nombres-limites'). laplace's estimate of the mean height of continents at feet is at least three times too high. the immortal author of the 'mecanique celeste' (t. v., p. ) was led to this conclusion by hypothetical views as to the mean depth of the sea. i have shown ('asie centr.', t. i., p. ) that the old alexandrian mathematicians, on the testimony of plutarch ('in aemilio paulo', cap. ), believed this depth to depend on the height of the mountains. the height of the center of gravity of the volume of the continental masses is probably subject to slight variations in the course of many centuries. these numbers show the low level of norther regions. in asia the vast steppes of siberia are compensated for by the great elevations of the land (between the himalaya, the north thibetian chain of kuen-lun, and the celestial mountains), from degrees ' to degrees north latitude. we may, to a certain extent, trace in these numbers the portions of the earth in which the plutonic forces were most intensely manifested in the interior by the upheaval of continental masses. there are no reasons why these plutonic forces may not, in future ages, add new mountain systems to those which elie de beaumont has shown to be of such different ages, and inclined in such different directions. why should the crust of the earth have lost its property of being elevated in the ridges? the recently-elevated mountain systems of the alps and the cordilleras exhibit in mont blanc and monte rosa, in sorata, illimani, and chimborazo, colossal elevations which do not favor the assumption of a decrease in the intensity of the subterranean forces. all geognostic phenomena indicate the periodic alternation of activity and repose;* but the quiet we now enjoy is only apparent. [footnote] *'zweiter geologischer brief von elie de beaumont an alexander von humboldt', in poggendorf's 'annalen', bd. xxv., s. - . the tremblings which still agitate the surface under all latitudes, and in every species of rock, the elevation of sweden, the appearance of new islands of eruption, are all conclusive as to the unquiet condition of our planet. p the two envelopes of the solid surface of our planet -- the liquid and the aeriform -- exhibit, owing to the mobility of their particles, their currents, and their atmospheric relations, many analogies combined with the contrasts which arise from the great difference in the condition of their aggregation and elasticity. the depths of ocean and of air are alike unknown to us. at some few places under the tropics no bottom has been found with soundings of , (or more than four miles), while in the air, if, according to wollaston, we may assume that it has a limit from which waves of sound may be reverberated, the phenomenon of twilight would incline us to assume a height at least nine times as great.* [footnote] *[see wilson's paper, 'on wollaston's argument from the limitation of the atmosphere as to the finite divisibility of matter.' -- 'trans. of the royal society of edinb.', vol. xvi., p. , .] -- tr. the aÃ�Â�rial ocean rests partly on the solid earth, whose mountain chains and elevated plateaux rise, as we have already seen, like green wooded shoals, and partly on the sea, whose surface forms a moving base, on which rest the lower, denser, and more saturated strata of air. proceeding upward and downward from the common limit of the aÃ�Â�rial and liquid oceans, we find that the strata of air and water are subject to determinate laws of decrease of temperature. this decrease is much less rapid in the air than in the sea, which has a tendency under all latitudes to maintain its temperature in the strata of water most contiguous to the atmosphere, owing to the sinking of the heavier and more cooled particles. a large series of the most carefully conducted observations on temperature shows us that in the ordinary and mean condition of its surface, the ocean from the equator to the forty-eighth degree of north and south latitude is somewhat warmer than the adjacent strata of air.* [footnnote[ *hamboldt, 'relation hist.', t. iii., chap. xxix., p. - . owing to this decrease of temperature at increasing depths, fishes and other inhabitants of the sea, the nature of whose digestive and respiratory organs fits them for living in deep water, may even, under the tropics, find the low degree of temperature and the coolness of climate characteristic of more temperate and more northern latitudes. this circumstance, which is analogous to the prevalence of a mild and even cold air on the elevated plains of the torrid zone, exercises a special influence on the migration and geographical distribution of many marine animals. moreover, the depths at which fishes live, modify, by the increase of pressure, their cutaneous respiration, and the p oxygenous and nitrogenous contents of the swimming bladders. as fresh and salt water do not attain the maximum of their density at the same degree of temperature, and as the saltness of the sea lowers the thermometrical degree corresponding to this point, we can understand how the water drawn from breat depths of the sea during the voyages of the kotzebue and dupetit-thouars could have been found to have only the temperature of degrees and . degrees. this icy temperatureof sea water, which is likewise manifested at the depths of tropical seas, first led to a study of the lower polar currents, which move from both poles toward the equator. without these submarine currents, the tropical seas at those depths could only have a temperature equal to the local maximum of cold possessed by the falling particles of water at the radiating and cooled surface of the tropical sea. in the mediterranean, the cause of the absence of such a refrigeration of the lower strata is ingeniously explained by arago, on the assumption that the entrance of the deeper polar currents into the straits of gibraltar, where the water at the surface flows in from the atlantic ocean from west to east, is hindered by the submariine counter-currents which move from east to west, from the mediterranean into the atlantic. the ocean, which acts as a general equalizer and moderator of climates, exhibits a most remarkable uniformity and constancy of temperature, especially between degrees north and degrees south latitude,* over spaces of many thousands of square miles, at a distance from land where it is not penetrated by currents of cold and heated water. [footnote] *see the series of observations made by me in the south sea, from degrees ' to degrees ' n. lat., in my 'asie centrale', t. iii., p. . it has therefore, been justly observed, that an exact and long-continued investigation of these thermic relations of the tropical seas might most easily afford a solution to the great and much-contested problem of the permanence of climates and terrestrial temperatures.* [footnote] *we might (by means of the temperature of the ocean under the tropics) enter into the consideration of a question which has hitherto remained unanswered, namely, that of the constancy of terrestrial temperatures, without taking into account the very circumscribed local influences arising from the diminution of wood in the plains and on mountains, and the drying up of lakes and marshes. each age might easily transmit to the succeeding one some few data, which would perhaps furnish the most simple, exact, and direct means of deciding whether the sun, which is almost the sole and exclusive source of the heat of our planet, changes its physical constitution and splendor, like the greater number of the stars, or whether, on the contrary, that luminary has attained to a permanent condition." -- arago, in the 'comptes rendus des seances de l'acad. des sciences', t. ii., p. , . great changes in the luminous disk of the sun would, p if they were of long duration, be reflected with more certainty in the mean temperature of the sea than in that of the solid land. the zones at which occur the maxima of the oceanic temperature and of the density (the saline contents) of its waters, do not correspond with the equator. the two maxima are separated from one another, and the waters of the highest temperature appear to form two nearly parallel lines north and south of the geographical equator. lenz, in his voyage of circumnavigation, found in the pacific the maxima of density in degrees north and degrees south latitude, while its minimum was situated a few degrees to the south of the equator. in the region of calms the solar heat can exercise but little influence on evaporation, because the stratum of air impregnated with saline aqueous vapor, which rests on the surface of the sea, remains still and unchanged. the surface of all connected seas must be considered as having a general perfectly equal level with respect to their mean elevation. local causes (probably prevailing winds and currents) may, however, produce permanent, although trifling changes in the level of some deeply indented bays, as for instance, the red sea. the highest level of the water at the isthmus of suez is at different hours of the day from to feet above that of the mediterranean. the form of the straits of bab-el-mandeb, through which the waters appear to find an easier ingress than egress, seems to contribute to this remarkable phenomenon, which was known to the ancients.* [[footnote] *humboldt, 'asie centrale', t. ii., p. , . the admirable geodetic operations of coraboeuf and delcrois show that no perceptible difference of level exists between the upper surfaces of the atlantic and the mediterranean, along the chain of the pyrenees, or between the coasts of northern holland and marseilles.* [footnote] *see the numerical results in p. - of the volume just named. from the geodesical levelings which, at my request, my friend general bolivar caused to be taken by lloyd and falmare, in the years and , it was ascertained that the level of the pacific is at the utmost / feet higher than that of the caribbean sea; and even that at different hours of the day each of the seas is in turn the higher, according to their respective hours of flood and ebb. if we reflect that in a distance of miles, comprising stations of observation, an error of three feet would be very apt to occur, we may say that in these new operations we have further confirmation of the equilibrium of the waters which communicate round cape horn. (arago, in the 'annuaire du bureau des longitudes pour' , p. .) i had inferred from barometrical observations instituted in and , that if there were any difference between the level of the pacific and the atlantic (carribean sea), it could not exceed three meters (nine feet three inches). see my 'relat. hist.', t. iii., p. - , and 'annales de chimie', t. i., p. - . the measurements, which appear to establish an excess of height for the waters of the gulf of mexico, and for those of the northern part of the adriatic sea, obtained by combining the trigonometrical operations of delcrois and choppin with those of the swiss and austrian engineers, are open to many doubts. notwithstanding the form of the adriatic, it is improbable that the level of its waters in its northern portion should be feet higher than that of the mediterranean at marseilles, and feet higher than the level of the atlantic ocean. see my 'asie centrale', t. ii., p. . p disturbances of equilibrium and consequent movements of the waters are partly irregular and transitory, dependent upon winds, and producing waves which sometimes, at a distance from the shore and during a storm, rise to a height of more than feet; partly regular and periodic, occasioned by the position and attraction of the sun and moon, as the ebb and flow of the tides; and partly permanent, although less intense, occurring as oceanic currents. the phenomena of tides, which prevail in all seas (with the exception of the smaller ones that are completely closed in, and where the ebbing and flowing waves are scarcely or not at all perceptible), have been perfectly explained by the newtonian doctrine, and thus brought "within the domain of necessary facts." each of these periodically-recurring oscillations of the waters of the sea has a duration of somewhat more than half a day. although in the open sea they scarcely attain an elevation of a few feet, they often rise considerably higher where the waves are opposed by the configuration of the shores, as for instance, at st. malo and in nova scotia, where they reach the respective elevation of feet, and of to feet. "it has been shown by the analysis of the great geometrician laplace, that, supposing the depth to be wholly inconsiderable when compared with the radius of the earth, the stability of the equilibrium of the sea requires that the density of its fluid should be less than that of the earth; and, as we have already seen, the earth's density is in fact five times greater than that of water. the elevated parts of the land can not therefore be overflowed, nor can the remains of marine animals found on the summits of mountains have been conveyed to those localities by any previous high tides.* [footnote] *bessel, 'ueber fluth und ebbe', in schumacher's 'ahrbuch', , s. . it is no slight this material taken from pages - cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p [balance of p is in file " humboldt"] it is no slight p evidence of the importance of analysis, which is too often regarded with contempt among the unscientific, that laplace's perfect theory of tides has enabled us, in our astronomical ephemerides, to predict the height of spring-tides at the periods of new and full moon, and thus put the inhabitants of the sea-shore on their guard against the increased danger attending these lunar revolutions. oceanic currents, which exercise so important an influence on the intercourse of nations and on the climatic relations of adjacent coasts, depend conjointly upon various causes, differing alike in nature and importance. among these we may reckon the periods at which tides occur in their progress round the earth; the duration and intensity of prevailing winds; the modifications of density and specific gravity which the particles of water undergo in consequence of differences in the temperature and in the relative quantity of saline contents at different latitudes and depths;* and, lastly, the horary variations of the atmospheric pressure, successively propagated from east to west, and occurring with such regularity in the tropics. [footnote] *the relative density of the particles of water depends simultaneously on the temperature and on the amount of the saline contents -- a circumstance that is not sufficiently borne in mind in considering the cause of currents. the submarine current, which brings the cold polar water to the equatorial regions, would follow an exactly opposite course, that is to say, from the equator toward the poles, if the difference in saline contents were alone concerned. in this view, the geographical distribution of temperature and of density in the water of the ocean, under the different zones of latitude and longitude, is of great importance. the numerous observations of lenz (poggendorf's 'annalen', bd. xx., , s. ), and those of captain beechey, collected in his 'voyage to the pacific', vol. ii., p. , deserve particular attention. see humboldt, 'relat. hist.', t. i., p. , and 'asie centrale', t. iii., p. . these currents present a remarkable spectacle; like rivers of uniform breadth, they cross the sea in different directions, while the adjacent strata of water, which remain undisturbed, form, as it were, the banks of these moving streams. this diffrence between the moving waters and those at rest is most strikingly manifested where long lines of sea-weed, borne onward by the current, enable us to estimate its velocity. in the lower strata of the atmosphere, we may sometimes, during a storm, observe similar phenomena in the limited aerial current, which is indicated by a narrow line of trees, which are often found to be overthrown in the midst of a dense wood. the general movement of the sea from east to west between p the tropics (termed the equatorial or rotation currnt) is considered to be owing to the propagation of tides and to the trade winds. its direction is changed by the resistance it experiences from the prominent eastern shores of continents. the results recently obtained by daussy regarding the velocity of this current, estimated from observations made on the distances traversed by bottles that had purposely been thrown into the sea, agree within one eighteenth with the velocity of motion ( french nautical miles, toises each, in hours) which i had found from a comparison with earlier experiments.* [footnote] *humboldt, 'relat. hist.', t. i., p. ; 'nouvelles annales des voyages', , p. . christopher columbus, during his third voyage, when he was seeking to enter the tropics in the meridian of teneriffe, wrote in his journal as follows:* "i regard it as proved that the waters of the sea move from east to west, as do the heavens ('las aguas van con los cielos'), that is to say, like the apparent motion of the sun, moon, and stars." [footnote] *humboldt, 'examen crit. de l'hist. de la geogr.', t. iii., p. . columbus adds shortly after (navarrete, 'coleccion de los viages y descubrimientos de los espanoles', t. i., p. ), that the movement is strongest in the caribbean sea. in fact, rennell terms this region, "not a current, but a sea in motion". ('investigation of currents', p. ). - . the narrow currents, or true oceanic rivers which traverse the sea, bring warm water into higher and cold water into lower latitudes. to the first class belongs the celebrated gulf stream,* which was known to anghiera, and more especially to sir humphrey gilbert in the sixteenth century. [footnote] *humboldt, 'examen critique', t. ii., p. ; 'relat. hist.', t. i., p. - . [footnote] *petrus martyr de anghiera, 'de rebus oceanicis et orbe novo', bas., , dec. iii., lib. vi., p. . see humboldt, 'examen critique', t. ii., p. - , and t. iii., p. . its first impulse and origin is to be sought to the south of the cape of good hope; after a long circuit it pours itself from the caribbean sea and the mexican gulf through the straits of the bahamas, and, following a course from south-southwest to north-northeast, continues to recede from the shores of the united states, until, further deflected to the eastward by the banks of newfoundland, it approaches the european coasts, frequently throwing a quantity of tropical seeds ('mimosa scandens, guilandina bonduc, dolichos urens') on the shores of ireland, the hebrides, and norway. the northeastern prolongation tends to mitigate the cold of the ocean, and to ameliorate the climate on the most northern extremity of scandinavia. at the point where the gulf stream p is deflected from the banks of newfoundland toward the east, it sends off branches to the south near the azores.* [footnote] *humboldt, 'examen crit.', t. iii., p. - this is the situation of the sargasso sea, or that great bank of weeds which so vividly occupied the imagination of christopher columbus, and which oviedo calls the sea-weed meadows ('praderias de yerva'). a host of small marine animals inhabits these tently-moved and evergreen masses of 'fucus natans', one of the most generally distributed of the social plants of the sea. the counterpart of this current (which in the atlantic ocean, between africa, america, and europe, belongs almost exclusively to the northern hemisphere) is to be found in the south pacific, where a current prevails, the effect of whose low temperature on the climate of the adjacent shores i had an opportunity of observing in the autumn of . it brings the cold waters of the high southern latitudes to the coast of chili, follows the shores of this continent and of peru, first from south to north, and is then deflected from the bay of arica onward from south-southeast to north-northwest. at certain seasons of the year the temperature of this cold oceanic current is, in the tropics, only degrees, while the undisturbed adjacent water exhibits a temperature of . degrees and . degrees. on that part of the shore of south america south of payta, which inclines furthest westward, the current is suddenly deflected in the same direction from the shore, turning so sharply to the west that a ship sailing northward passes suddenly from cold into warm water. it is not known to what depth cold and warm oceanic currents propagate their motion; but the deflection experienced by the south african current, from the lagullas bank, which is fully from to fathoms deep, would seem to imply the existence of a far-extending propagation. sand banks and shoals lying beyond the line of these currents may, as was first discovered by the admirable benjamin franklin, be recognized by the coldness of the water over them. this depression of the temperature appears to me to depend upon the fact that, by the propagation of the motion of the sea, deep waters rise to the margin of the banks and mix with the upper strata. my lamented friend, sir humphrey davy, ascribed this phenomenon (the knowledge of which is often of great practical utility in securing the safety of the navigator) to the descent of the particles of water that had been cooled by nocturnal radiation p and which remain nearer to the surface, owing to the hinderance placed in the way of their greater descent by the intervention of sand-banks. by his observations franklin may be said to have converted the thermometer into a sounding line. mists are frequently found to rest over these depths, owing to the condensation of the vapor of the atmosphere by the cooled waters. i have seen such mists in the south of jamaica, and also in the pacific, defining with sharpness and clearness the form of the shoals below them, appearing to the eye as the aerial reflection of the bottom of the sea. a still more striking effect of the cooling produced by shoals is manifested in the higher strata of air, in a somewhat analogous manner to that observed in the case of flat coral reefs, or sand islands. in the open sea, far from the land, and when the air is calm, clouds are often observed to rest over the spots where shoals are situated, and their bearing may then be taken by the compass in the same manner as that of a high mountain or isolated peak. although the surface of the ocean is less rich in living forms than that of continents, it is not improbable that, on a further investigation of its depths, its interior may be found to possess a greater richness of organic life than any other portion of our planet. charles darwin, in the agreeable narrative of his extensive voyages, justly remarks that our forests do not conceal so many animals as the low woody regions of the ocean, where the sea-weed rooted to the bottom of the shoals, and the severed branches of fuci, loosened by the force of the waves and currents, and swimming free, unfold their delicate foliage, upborne by air-cells.* [footnote] *[see 'structure and distribution of coral reefs', by charles darwin, london, . also, 'narrative of the surveying voyage of h.m.s. "fly" in the eastern archipelago, during the years ' - , by j. b. jukes, naturalist to the expedition, .] -- tr. the application of the microscope increases, in the most striking manner, our impression of the rich luxuriance of animal life in the ocean, and reveals to the astonished senses a consciousness of the universality of life. in the oceanic depths, far exceeding the height of our loftiest mountain chains, every stratum of water is animated with polygastric sea-worms, cyclidiae and ophrydinae. the waters swarm with countless hosts of small luminiferous animalcules, mammaria (of the order of acalephae), crustacea, peridinea, and circling nereides, which when attracted to the surface by peculiar meteorological conditions, convert every wave into a foaming band of flashing light. p the abundance of those marine animalcules, and the animal matter yielded by their rapid decomposition are so vast that the sea water itself becomes a nutrient fluid to many of the larger animals. however much this richness in animated forms, and this multitude of the most various and highly-developed microscopic organisms may agreeably excite the fancy, the imagination is even more seriously, and, i might say, more solemnly moved by the impression of boundlessness and immeasureability, which are presented to the mind by every sea voyage. all who possess an ordinary degree of mental activity, and delight to create to themselves an inner world of thought, must be penetrated with the sublime image of the infinite, when gazing around them on the vast and boundless sea, when involuntarily the glance is attracted to the distant horizon, where air and water blend together, and the stars continually rise and set before the eyes of the mariner. this contemplation of the eternal play of the elements is clouded, like every human joy, by a touch of sadness and of longing. a peculiar predilection for the sea, and a grateful remenbrance of the impression which it has excited in my mind, when i have seen it in the tropics in the calm of nocturnal rest, or in the fury of the tempest, have alone induced me to speak of the individual enjoyment afforded by its aspect before i entered upon the consideration of the favorable influence which the proximity of the ocean has incontrovertibly exercised on the cultivation of the intellect and character of many nations, by the multiplication of those bands which ought to encircle the whole of humanity, by affording additional means of arriving at a knowledge of the configuration of the earth, and furthering the advancement of astronomy, and of all other mathematical and physical sciences. a portion of this influence was at first limited to the mediterranean and the shores of southwestern africa, but from the sixteenth century it has widely spread, extending to nations who live at a distance from the sea, in the interior of continents. since columbus was sent to "unchain the ocean"* (as the unknown voice whispered to him in a dream when he lay on a sick-bed near p the river belem), man has ever boldly ventured onward toward the discovery of unknown regions. [footnote] *the voice addressed him in these words, "maravillosamente dios hizo sonar tu nombre en la tierra; de los atamientos de la mar oceana, que estaban cerrados con cadenas tan fuertes, te diÃ�Â� las llaves" -- "god will cause thy name to be wonderfully resounded through the earth, and give thee the keys of the gates of the ocean, which are closed with strong chains." the dream of columbus is related in the letter to the catholic monarchs of july the th, . (humboldt, 'examen critique', t. iii., p. .) the second external and general covering of our planet, the aerial ocean, in the lower strata, and on the shoals of which we live, presents six classes of natural phenomena, which manifest the most intimate connection with one another. they are dependent on the chemical composition of the atmosphere, the variations in its transparency, polarization, and color, its density or pressure, its temperature and humidity, and its electricity. the air contains in oxygen the first element of physical animal life, and besides this benefit, it possesses another, which may be said to be of a nearly equally high character, namely, that of conveying sound; a faculty by which it likewise becomes the conveying sound; a faculty by which it likewise becomes the conveyer of speech and the means of communicating thought, and consequently of maintaining social intercourse. if the earth were deprived of an atmosphere, as we suppose our moon to be, it would present itself to our imagination as a soundless desert. the relative quantities of the substances composing the strata of air accessible to us have, since the beginning of the nineteenth century, become the object of investigations, in which gay-lussac and myself have taken an active part; it is however, only very recently that the admirable labors of dumas and boussingault have, by new and more accurate methods, brought the chemical analysis of the atmosphere to a high degree of perfection. according to this analysis, a volume of dry air contains . of oxygen, and . of nitrogen, besides from two to five thousandth parts of carbonic acid gas, a still smaller quantity of carbureted hydrogen gas,* and, according to the important experiments of saussure and liebig, traces of ammoniacal vapors,** from which plants derive their nitrogenous contents. [footnote] *boussingault, 'recherches sur la composition de l'atmosphere', in the 'annales de chimie et de physique', t. lvii., , p. - ; and lxxi. , p. . according to boussingault and lewy, the proportion of carbonic acid in the atmosphere at audilly, at a distance, therefore, from the exhalations of a city, varied only between . and . in volume. [footnote] **liebig, in his important work, entitles 'die organische chemie in ihrer anwendung auf agricultur und physiologie', , s. - . on the influence of atmospheric electricity in the production of nitrate of ammonia, which, coming into contact with carbonate of lime, is changed into carbonate of ammonia, see boussingault's 'economie rurale consideree dans ses rapports avec la chimie et la meteorologie', , t. ii., p. , , and t. i., p. . some observations of lewy render it probable that the quantity of oxygen varies perceptibly p but slightly, over the sea and in the interior of continents, according to local conditions or to the seasons of the year. we may easily conceive that changes in the oxygen held in solution in the sea, produced by microscopic animal organisms, may be attended by alterations in the strata of air in immediate contact with it.* [footnote] *lewy, in the 'comptes rendus de l'acad. des sciences', t. xvii., part ii., p. - . the air which martins collected at faulhorn at an elevation of feet, contained as much oxygen as the air at paris.* [footnote] *dumas, in the 'annales de chimie, e serie', t. iii., , p. . the admixture of carbonate of ammonia in the atmosphere may probably be considered as older than the existence of organic beings on the surface of the earth. the sources from which carbonic acid* may be yielded to the atmosphere are most numerous. [footnote] *in this enumeration, the exhalation of carbonic acid by plants during the night, while they inhale oxygen, is not taken into account, because the increase of carbonic acid from this source is amply counter-balanced by the respiratory process of plants during the day. see boussingault's 'econ. rurale', t. i., p. - , and liebig's 'organische chemie', s. , . in the first place we would mention the respiration of animals, who receive the carbon which they inhale from vegetable food, while vegetables receive it from the atmosphere; in the next place, carbon is supplied from the interior of the earth in the vicinity of exhausted volcanoes and thermal springs, from the decomposition of a small quantity of carbureted hydrogen gas in the atmosphere, and from the electric discharges of clouds, which are of such frequent occurrence within the tropics. besides these substances, which we have considered as appertaining to the atmosphere at all heights that are accessible to us, there are others accidentally mixed with them, especially near the ground, which sometimes, in the form of miasmatic and gaseous contagia, exercise a noxious influence on animal organization. their chemical nature has not yet been ascertained by direct analysis; but, from the consideration of the processes of decay which are perpetually going on in the animal and vegetable substances with which the surface of our planet is covered, and judging from analogies deduced from the comain of pathology, we are led to infer the existence of such noxious local admixtures. ammoniacal and other nitrogenous vapors, sulphureted hydrogen gas, and compounds analogous to the polybasic ternary and quaternary compounds analogous to the polybasic ternary and quaternary combinations of the vegetable kingdom, may produce miasmata,* p which, under various forms, may generate ague and typhus fever (not by any means exclusively on wet, marshy ground, or on coasts covered by putrescent mollusca, and low bushes of 'rhizophora mangle' and avicennia). [footnote] *gay-lussac, in 'annales de chimie', t. liii., p. ; payen, mem. sur la composition chimique des vegetaux, p. , ; liebig, 'org. chemie', s. - ; boussingault, 'econ. rurale', t. i., p. - . fogs which have a peculiar smell at some seasons of the year, remind us of these accidental admixtures in the lower strata of the atmosphere. winds and currents of air caused by the heating of the ground even carry up to a considerable elevation solid substances reduced to a fine powder. the dust which darkens the air for an extended area, and falls on the cape verd islands, to which darwin has drawn attention, contains, according to ehrenberg's discovery, a host of silicious-shelled infusoria. as principal features of a general descriptive picture of the atmosphere, we may enumerate: . 'variations of atmospheric pressure': to which belong the horary oscillations, occurring with such regularity in the tropics, where they produce a kind of ebb and flow in the atmosphere, which can not be ascribed to the attraction of the moon,* and which differs so considerably according to geographical latitude, the seasons of the year, and the elevation above the level of the sea. [footnote] *bouvard, by the application of the formulae, in , which laplace had deposited with the board of longitude shortly before his death, found that the portion of the horary oscillations of the pressure of the atmosphere, which depends on the attraction of the moon, can not raise the mercury in the barometer at paris more than the . of a millimeter, while eleven years' observations at the same place show the mean barometric oscillation, from a.m. to p.m., to be . millim., and from p.m. to p.m., . millim. see 'memoires de l'acad. des sciences', t. vii., , p. . . 'climatic distribution of heat', which depends on the relative position of the transparent and opaque masses (the fluid and solid parts of the surface of the earth), and on the hypsometrical configuration of continents; relations which determine the geographical position and curvature of the isothermal lines (or curves of equal mean annual temperature) both in a horizontal and vertical direction, or on a uniform plane, or in different superposed strata of air. . 'the distribution of the humidity of the atmosphere'. the quantitative relations of the humitidy depend on the differences in the solid and oceanic surfaces; on the distance from the equator and the level of the sea; on the form in which the p aqueous vapor is precipitated, and on the connection existing between these deposits and the changes of temperature, and the direction and succession of winds. . 'the electric condition of the atmosphere'. the primary cause of this condition, when the heavens are serene, is still much contested. under this head we must consider the relation of ascending vapors to the electric charge and the form of the clouds, according to the different periods of the day and year; the difference between the cold and warm zones of the earth, or low and high lands; the frequency or rarity of thunder storms, their periodicity and formation in summer and winter; the causal connection of electricity, with the infrequent occurrence of hail in the night, and with the phenomena of water and sand spouts, so ably investigated by peltier. the horary oscillations of the barometer, which in the tropics present two maxima (viz., at or / p.m., and a.m., occurring, therefore, in almost the hottest and coldest hours), have long been the object of my most careful diurnal and nocturnal observations.* [footnote] *'observations faites pour constater la marche des variations horaires du barometre sous les tropiques', in my 'relation historique du voyage aux regions equinoxiales', t. iii., p. - . their regularity is so great, that, in the daytime especially, the hour may be ascertained from the height of the mercurial column without an error, on the average, of more than fifteen or seventeen minutes. in the torrid zones of the new continent, on the coasts as well as at elevations of nearly , feet above the level of the sea, where the mean temperature falls to . degrees, i have found the regularity of the ebb and flow of the aerial ocean undisturbed by storms, hurricanes, rain, and earthquakes. the amount of the daily oscillations diminishes from . to . french lines from the equator to degrees north latitude, where bravais made very accurate observations at bosekop.* [footnote] *bravais, in daemtz and martins, 'meteorologie', p. . at halle ( degrees ' n. lat.), the oscillation still amounts to . lines. it would seem that a great many observations will be required in order to obtain results that can be trusted in regard to the hours of the maximum and minimum on mountains in the temperate zone. see the observations of horary variations, collected on the faulhorn in , , and (martins, 'meteorologie', p. .) the supposition that, much nearer the pole, the height of the barometer is really less at a.m. than at p.m., and consequently, that the maximum and minimum influences of these hours p are inverted, is not confirmed by parry's observations at port bowen ( degrees '). the mean height of the barometer is somewhat less under the equator and in the tropics, owing to the effect of the rising current,* than in the temperate zones, and it appears to attain its maximum in western europe between the parallels of degrees and degrees. [footnote] *humboldt, 'essai sur la geographie des plantes', , p. ; and in 'rel. hist.', t. iii., p. ; and on the diminuation of atmospheric pressure in the tropical portions of the atlantic, in poggend., 'annalen der physik', bd. xxxvii., s. - , and s. - . if with kÃ�Â�mtz we connect together by 'isobarometric' lines those places which present the same mean difference between the monthly extremes of the barometer, we shall have curves whose geographical position and inflections yield important conclusions regarding the influence exercised by the form of the land and the distribution of seas on the oscillations of the atmosphere. hindostan with its high mountain chains and triangular peninsulas, and the eastern coasts of the new continent, where the warm gulf stream turns to the east at the newfoundland banks, exhibit greater isobarometric oscillations than do the group of the antilles and western europe. the prevailing winds exercise a principal influence on the diminution of the pressure of the atmosphere, and this, as we have already mentioned, is accompanied, according to daussey, by an elevation of the mean level of the sea.Ã�Â¥ [footnote] *dausay, in the 'comptes rendus', t. iii., p. . as the most important fluctuations of the pressure of the atmosphere, whether occurring with horary or annual regularity, or accidentally, and then often attended by violence and danger,* are like all the other phenomena of the weather, mainly owing to the heating force of the sun's rays, it has long been suggested (partly according to the idea of lambert) that the direction of the wind should be compared with the height of the barometer, alternations of temperature, and the increase and decrease of humidity. [footnote] *dove, 'ueber die sturme', in poggend., 'annalen', bd. lii., s. . tables of atmospheric pressure during different winds, termed 'barometric windroses', afford a deeper insight into the connection of meteorological phenomena.* [footnote] *leopold von buch, 'barometrische windrose', in 'abhandl. der akad. der wiss. zu berlin aus den jahren', - , s. . dove has, with admirable sagacity, recognized, in the "law of rotation" in both hemispheres, which he himself established, the cause of many important processes in the aerial ocean.* [footnote] *see dove, 'meteorologishe untersuchungen', , s. - ; and the excellent observations of kÃ�Â�mtz on the descent of the west wind of the upper current in high latitudes, and the general phenomena of the direction of the wind, in his 'vorlesungen uber Ã�µeterologie', , s. - , - , - , - ; and in schumacher's 'jahrbuch fur' , s. - . a very satisfactory and vivid representation of meteorological phenomena is given by dove, in his small work entitled 'witterungsverhÃ�Â�ltnisse von berlin', . on the knowledge of the earlier navigators of the rotation of the wind, see churruca, 'viage at magellanes', , p. ; and on a remarkable expression of columbus, which his son don fernando colon has presented to us in his 'vida del almirante', cap. , see humboldt, 'examen critique de l'hist. de geographie', t. iv., p. . the difference of temperature between the p equatorial and polar regions engenders two opposite currents in the upper strata of the atmosphere and on the earth's surface. owing to the difference between the rotatory velocity at the poles and at the equator, the polar current is deflected eastward, and the equatorial current westward. the great phenomena of atmospheric pressure, the warming and cooling of the strata of air, the aqueous deposits, and even, as dove has correctly represented, the formation and appearance of clouds, alike depend on the opposition of these two currents, on the place where the upper one descends, and on the displacement of the one by the other. thus the figures of the clouds, which form an animated part of the charms of a landscape, announce the processes at work in the upper regions of the atmosphere, and, when the air is calm, the clouds will often present, on a bright summer sky, the "projected image" of the radiating soil below. where this influence of radiation is modified by the relative position of large continental and oceanic surfaces, as between the eastern shore of africa and the western part of the indian peninsula, its effects are manifested in the indian monsoons, which change with the periodic variations in the sun's declination,* and which were known to the greek navigators under the name of 'hippalos'. [footnote] *'monsun' (malayan 'musim', the 'hippalos' of the greeks) is derived from the arabic word 'mausim', a set time or season of the year, the time of the assemblage of pilgrims at mecca. the word has been applied to the seasons at which certain winds prevail, which are, besides, named from places lying in the direction from whence they come; thus, for instance, there is the 'mausim' of aden, of guzerat, malabar, etc. (lassen, 'indische alterthumskunde', bd. i., , s. ). on the contrasts between the solid or fluid substrata of the atmosphere, see dove, in 'der abhandl. der akad. der wiss. zu berlin aus dem jahr' , s. . in the knowledge of the monsoons, which undoubtedly dates back thousands of years among the inhabitants of hindostan and china, of the eastern parts of the arabian gulf and of the western shores of the malayan p sea, and in the still more ancient and more general acquaintance with land and sea winds, lies concealed, as it were, the germ of that meteorological sciences which is now making such rapid progress. the long chain of 'magnetic stations' extending from moscow to pekin, across the whole of northern asia, will prove of immense importance in determining the 'law of the winds', since these stations have also for their object the investigation of general meteorological relations. the comparison of observations made at places lying so many hundred miles apart, will decide, for instance, whether the same east wind blows from the elevated desert of gobi to the interior of russia, or whether the direction of the aerial current first began in the middle of the series of the stations, by the descent of the air from the higher regions. by means of such observations, we may learn, in the strictest sense, 'whence' the wind cometh. if we only take the results on which we may depend from those places in which the observations on the direction of the winds have been continued more than twenty years, we shall find (from the most recent and careful calculations of wilhelm mahlmann) that in the middle latitudes of the temperate zone, in both continents, the prevailing aerial current has a west-southwest direction. our insight into the 'distribution of heat' in the atmosphere has been rendered more clear since the attempt has been made to connect together by lines those places where the mean annual summer and winter temperatures have been ascertain by correct observations. the system of 'isothermal, osotheral' and 'isochimenal' lines, which i first brought into use in , may, perhaps, if it be gradually perfected by the united efforts of investigators, serve as one of the main foundations of 'comparative climatology'. terrestrial magnetism did not acquire a right to be regarded as a science until partial results were graphically connected in a system of lines of 'equal declination, equal inclinatiion', and 'equal intensity'. the term 'climate', taken in its most general sense, indicated all the changes in the atmosphere which sensibly affect our organs, as temperature, humidity, variations in the barometrical pressure, the calm state of the air or the action of opposite winds, the amount of electric tension, the purity of the atmosphere or its admixture with more or less noxious gaseous exhalations, and, finally, the degree of ordinary transparency and clearness of the sky, which is not only important with respect to the increased radiation from the earth, the organic development of plants, and the ripening of fruits, but p also with reference to its influence on the feelings and mental condition of men. if the surface of the earth consisted of one and the same homogeneous fluid mass, or of strata of rock having the same color, density, smoothness, and power of absorbing heat from the solar rays, and of radiating it in a similar manner through the atmosphere, the isothermal, isotheral, and isochimenal lines would all be parallel to the equator. in this hypothetical condition of the earth's surface, the power of absorbing and emitting light and heat would every where be the same under the same latitudes. the mathematical consideration of climate, which does not exclude the supposition of the existence of currents of heat in the interior, or in the external crust of the earth, nor of the propagation of heat by atmospheric currents, proceeds from this mean, and, as it were, primitive condition. whatever alters the capacity for absorption and radiation, at places lying under the same parallel of latitude, gives rise to inflections in the isothermal lines. the nature of these inflections, the angles at which the isothermal, isotheral, or isochimenal lines intersect the parallels of latitude, their convexity or concavity with respect to the pole of the same hemisphere, are dependent on causes which more or less modify the temperature under different degrees of longitude. the progress of 'climatology' has been remarkably favored by the extension of european civilization to two opposite coasts, by its transmission from our western shores to a continent which is bounded on the east by the atlantic ocean. when, after the ephemeral colonization from iceland and greenland, the british laid the foundation of the first permanent settlements on the shores of the united states of america, the emigrants (whose numbers were rapidly increased in consequence either of religious persecution, fanaticism, or love of freedom, and who soon spread over the vast extent of territory lying between the carolinas, virginia, and the st. lawrence) were astonished to find themselves exposed to an intensity of winter cold far exceeding that which prevailed in italy, france, and scotland, situated in corresponding parallels of latitude. but, however much a consideration of these climatic relations may have awakened attention, it was not attended by any practical results until it could be based on the numerical data of 'mean annual temperature'. if, between degrees and degrees north latitude, we compair nain, on the coast of labrador, with gottenburg; halifax with bordeaus; new p york with naples; st. augustine, in florida, with cairo, we find that, under the same degrees of latitude, the differences of the mean annual temperature between eastern america and western europe, proceeding from north to south, are successively . degrees, . degrees, . degrees, and almost degrees. the gradual decrease of the differences in this series extending over degrees of latitude is very striking. further to the south, under the tropics, the isothermal lines are every where parallel to the equator in both hemispheres. we see, from the above examples, that the questions often asked in society, how many degrees america (without distinguishing between the eastern and western shores) is colder than europe? and how much the mean annual temperature of canada and the united states is lower than that of corresponding latitudes in europe? are, when thus 'generally expressed', devoid of meaning. there is a separate difference for each parallel of latitude, and without a special comparison of the winter and summer temperatures of the opposite coasts, it will be impossible to arrive at a correct idea of climatic relations, in their influence on agriculture and other industrial pursuits, or on the individual comfort or discomfort of manking in general. in enumerating the causes which produce disturbances in the form of the isothermal lines, i would distinguish between those which 'raise' and those which 'lower' the temperature. to the first class belong the proximity of a western coast in the temperate zone; the divided configuration of a continent into peninsulas, with deeply-indented bays and inland seas; the aspect of the position of a portion of the land with reference either to a sea of ice spreading far into the polar circle, or to a mass of continental land of considerable extent, lying in the same meridian, either under the equator, or, at least, within a portion of the tropical zone; the prevalence of southerly or westerly winds on the western shore of a continent in the temperate northern zone; chains of mountains acting as protecting salls against the winds coming from colder regions; the infrequency of swamps, which, in the spring and beginning of summer, long remain covered with ice, and the absence of woods in a dry, sandy soil; finally the constant serenity of the sky in the summer months, and the vicinity of an oceanic current, bringing water which is of a higher temperature than that of the surrounding sea. among the causes which tend to 'lower' the mean annual temperature i include the following: elevation above the level of the sea, when not forming part of an extended plain; the p vicinity of an eastern coast in high and middle latitudes; the compact configuration of a continent having no littoral curvatures or bays; the extension of land toward the poles into the region of perpetual ice, without the intervention of a sea remaining open in the winter; a geographical position, in which the equatorial and tropical regions are occupied by the sea, and consequently, the absence, under the same meridian, of a continental tropical land having a strong capacity for the absorption and radiation of heat; mountain chains, whose mural form and direction impede the access of warm winds, the vicinity of isolated peaks, occasioning the descent of cold currents of air down their declivities; extensive woods, which hinder the isolation of the soil by the vital activity of their foliage, which produces great evaporation, owing to the extension of these organs, and increases the surface that is cooled by radiation, acting consequently in a three-fold manner, by shade, evaporation, and radiation; the frequency of swamps or marshes, which in the north form a kind of subterranean glacier in the plains, lasting till the middle of the summer; a cloudy summer sky, which weakens the action of the solar rays; and, finally, a very clear winter sky, favoring the radiation of heat.* [footnote] *humboldt, 'recherches sur les causes des inflexions des lignes isothermes', in 'asie centr.', t. iii., p. - , , , . the simultaneous action of these disturbing causes, whether productive of an increase or decrease of heat, determines, as the total effect, the inflection of the isothermal lines, especially with relation to the expansion and configuration of solid continental masses, as compared with the liquid oceanic. these perturbations give rise to convex and concave summits of the isothermal curves. there are, however, different orders of disturbing causes, and each one must, therefore, be considered separately, in order that their total effect may afterward be investigated with reference to the motion (direction, local curvature) of the isothermal lines, and the actions by which they are connected together, modified, destroyed, or increased in intensity, as manifested in the contact and intersection of small oscillatory movements. such is the method by which, i hope, it may some day be possible to connect together, by empirical and numerically expressed laws, vast series of apparently isolated facts, and to exhibit the mutual dependence which must necessarily exist among them. the trade winds -- easterly winds blowing within the tropics -- give rise, in both temperate zones, to the west, or west-southwest p sinds which prevail in those regions, and which are land winds to eastern coasts, and sea winds to western coasts, estending over a space which, from the great mass and the sinking of its cooled particles, is not capable of any considerable degree of cooling, and hence it follows that the east winds of the continent must be cooler than the west winds, where their temperature is not affected by the occurrence of oceanic currents near the shore. cook's young companion on his second voyage of circumnavigation, the intelligent george forster, to whom i am indebted for the lively interest which prompted me to undertake distant travels, was the first who drew attention, in a definite manner, to the climatic differences of temperature existing in the eastern and western coasts of both continents, and to the similarity of temperature of the western coast of north america in the middle latitudes, with that of western europe.* [footnote] *george forster, 'klein schriften', th. iii., , s. ; dove, in schumacher's 'jahrbuch fur', s. ; kÃ�Â�mtz, 'meteorologie', bd. ii., s. , , , and ; arago, in the 'comptes rendus', t. i., p. . even in northern latitudes exact observations show a striking difference between the 'mean annual temperature' of the east and west coasts of america. the mean annual temperature of nain, in (lat. degrees '), is fully . degrees 'below' the freezing point, while on the northwest coast, at new archangel, in russian america (lat. degrees '), it is . degrees 'above' this point. at the first-named place, the mean summer temperature hardly amounts to degrees, while at the latter place it is degrees. pekin ( degrees '), on the eastern coast of asia, has a mean annual tempeerature of . degrees, which is degrees below that of naples, situated somewhat further to the north. the mean winter temperature of pekin is at least . degrees below the freezing point, while in western europe, even at paris ( degrees '), it is nearly degrees above the freezing point. pekin has also a mean winter cold which is . degrees lower than that of copenhagen, lying degrees further to the north. we have already seen the slowness with which the great mass of the ocean follows the variations of temperature in the atmosphere, and how the sea acts in equalizing temperatures, moderating simultaneously the severity of winter and the heat of summer. hence arises a second more important contrast -- that, namely, between insular and littoral climates enjoyed by all articulated continents having deeply indented bays and peninsulas, and between the climate of the interior of great masses of solid land. this remarkable contrast has been fully p developed by leopold von buch in all its various phenomena, both with respect to its influence on vegetation and agriculrure, on the transparency of the atmosphere, the radiation of the soil, and the elevation of the line of perpetual snow. in the interior of the asiatic continent, tobolsk, barnaul on the oby, and irkutsk, have the same mean summer heat as berlin, munster, and cherbourg in normandy, the thermometer sometimes remaining for weeks together at degrees or degrees, while the mean winter temperature is, during the coldest month, as low as - . degrees to - degrees. these continental climates have therefore justly been termed 'excessive' by the great mathematician and physicist buffon; and the inhabitants who live in countries having such 'excessive' climates seem almost condemned, as dante expresses himself, "a sofferir tormenti caldi e geli."* [fiitbite] *dante, 'divina commedia, purgatorio', canto iii. in no portion of the earth, neither in the canary islands, in spain, nor in the south of france, have i ever seen more luxuriant fruit, especially grapes, than in astrachan, near the shores of the caspian sea ( degrees '). although the mean annual temperature is about Ã�¼degrees, the mean summer heat rises to Ã�¼degrees, as at bordeaux, while not only there, but also further to the south, as at kislar on the mouth of the terek (in the latitude of avignon and rimini), the thermometer sinks in the winter to - degrees or - degrees. ireland, guernsey, and jersey, the peninsula of brittany, the coasts of normandy, and of the south of england, present, by the mildness of their winters, and by the low temperature and clouded sky of their summers, the most striking contrast to the continental climate of the interior of eastern europe. in the northeast of ireland ( degrees '), lying under the same parallel of latitude as konigsberg in prussia, the myrtle blooms as luxuriantly as in portugal. the mean temperature of the month of august, which in hungary rises to degrees, scarcely reaches degrees at dublin, which is situated on the same isothermal line of degrees; the mean winter temperature, which falls to about degrees at pesth, is degrees at dublin (whose mean annual temperature is not more than degrees); . degrees higher than that of milan, pavia, padua, and the whole of lombardy, where the mean annual temperature is upward of Ã�¼degrees. at stromness, in the orkneys, scarcely half a degree further south than stockholm, the winter temperature is degrees, and consequently higher than that of paris, and neary as high as that of london. p even in the faroe islands, at degrees latitude, the inland waters never freeze, owing to the favoring influence of the west winds and of the sea. on the charming coasts of devonshire, near salcombe bay, which has been termed, on account of the mildness of its climate, the 'montpellier of the north', the agave mexicana has been seen to blossoom in the open air, while orange-trees trained against espaliers, and only slightly protected by matting, are found to bear fruit. there, as well as at penzance and gosport, and at cherbourg on the coast of normandy, the mean winter temperature exceeds degrees, falling short by only . degrees of the mean winter temperature of montpellier and florence.* [footnote] *humboldt, 'sur les lignes isothermes', in the 'memoires de physique et de chimie de la societe d'arcueil', t. iii., paris, , p. - ; knight, in the 'transactions of the horticultural society of london', vol. i, p. ; watson, 'remarks on the geographical distribution of british plants', , p. ; trevelyan, in jemieson's 'edinburgh new phil. journal', no. , p. ; mahlmann in his admirable german translation of my 'asie centrale', th. ii., s. . these observations will suffice to show the important influence exercised on vegetation and agriculture, on the cultivation of fruit, and on the comfort of mankind, by differences in the distribution of the same mean annual temperature, through the different seasons of the year. the lines which i have termed 'isochimenal' and 'isotheral' (lines of equal winter and equal summer temperature) are by no means parallel with the 'isothermal' lines (lines of equal annual temperature). if, for instance, in countries where myrtles grow wild, and the earth does not remain covered with snow in the winter, the temperature of the summer and autumn is barely sufficient to bring apples to perfect ripeness, and if, again, we observe that the grape rarely attains the ripeness necessary to convert it into wine, either in islands or in the vicinity of the sea, even when cultivated on a western coast, the reason must not be sought only in the low degree of summer heat, indicated, in littoral situations, by the thermometer when suspended in the shade, but likewise in another cause that has not hitherto been sufficiently considered, although it exercises an active influence on many other phenomena (as, for instance, in the inflammation of a mixture of chlorine and hydrogen), namely the difference between direct and diffused light, or that which prevails when the sky is clear and when it is overcast by mist. i long since endeavored to attract the attention of physicists and physiologists* to this p difference, and to the 'unmeasured' heat which is locally developed in the living vegetable cell by the action of direct light. [footnote] *"haec de temperie aeris, qui terram late circumfundit, ac in quo, longe a solo, instrumenta nostra meteorologica suspensa habemus. sed alia est caloris vis, quem radii solis nullis nubibus velati, in foliis ipsia et fructibus maturescentibus, magis minusve coloratis, gignunt, quemque, ut egregia demonstrant experimenta amicissimorum gay-lussacii et thenardi de combustione chlori et hydrogenis, ope thermometri metiri nequis. etenim locis planis et montanis, vento libe spirante, circumfusi aeris temperies cadem esse potest coelo sudo vel nebuloso; ideoque ex observationibus solis thermometricis, nullo adhibito photometro, haud cognosces, quam ob causam galliae septentrionalis tractur armoricanus et nervicus, versus littora, coe temperato sed sole raro utentia, vitem fere non tolerant. egent enim stirpes non solum caloris stimulo, sed et lucis, quae magis intensa locis excelsis quam planis, duplici modo plantas movet, vi sua tum propria, tum calorem in superficie earum excitante." -- humboldt, 'de distributione geographica plantarum', , p. - . if, in forming a thermic scale of different kinds of cultivation,* we begin with those plants which require the hottest climate, as the vanilla, the cacao, banana, and cocoa-nut, and proceed to the pine-apples, the sugar-cane, coffee, fruit-bearing date-trees, the cotton-tree, citrons, olives, edible chestnuts, and fines producing potable wine, an exact geographical consideration of the limits of cultivation, both on plains and on the declivities of mountains, will teach us that other climatic relations besides those of mean annual temperature are involved in these phenomena. [footnote] *humboldt, op. cit., p. - ; meyen, in his 'grundriss der pflanzengeographie', s. - ; boussingault, 'economie rurale', t. ii., p. . taking an example, for instance, from the cultivation of the vine, we find that, in order to procure 'potable' wine,* it is requisite that the mean annual heat should exceed degrees, that the winter temperature upward of degrees. [footnote] *the following table illustrates the cultivation of the vine in europe, and also the depreciation of its produce according to climatic relations. see my 'asie centrale', t. iii., p. . the examples quoted in the text for bordeaux and potsdam are, in respect of numerical relation, alike applicable to the countries of the rhine and maine ( degrees ' to degrees ' n. lat.). cherbourg in normandy, and ireland, show in th most remarkable manner how, with thermal relations very nearly similar to those prevailing in the interior of the continent (as estimated by the thermometer in the shade), the results are nevertheless extremely different as regards the ripeness or the unripeness of the fruit of the vine, this difference undoubtedly depending on the circumstance whether the vegetation of the plant proceeds under a bright sunny sky, or under a sky that is habitually obscured by clouds: [nb table will line up in courier point] _____________________________________________________________________ places. lat- ele- mean win- spring. sum- aut- number of the it- va- of the ter. mer. umn. years of the tude tion. year. observation _____________________________________________________________________ deg ' eng.ft. fahr. bordeaux . . . . . . stras- . . . . . . bourg heid- . . . . . . elberg manheim . . . . . . wurzburg . . . . . . frank- fort on maine . . . . . . berlin . . . . . . cher- bourg (no wine) .... . . . . . dublin (ditto) .... . . . . . ___________________________________________________________________ the great accordance in the distribution of the annual temperature through the different seasons, as presented by the results obtained for the valleys of the rhine and maine, tends to confirm the accuracy of these meteorological observations. the months of december, january, and february are reckoned as winter months. when the different qualities of the wines produced in franconia, and in the countries around the baltic, are compared with the mean summer and autumn temperature of wurzburg and berlin, we are almost surprised to find a difference of only about two degrees. the difference in the spring is about four degrees. the influence of late may frosts on the flowering season, and after a correspondingly cold winter, is almost as important an element as the time of the subsequent ripening of the grape. the difference alluded to in the text between the true temperature of the surface of the ground and the indications of a thermometer suspended in the shade and protected from extraneous influences, is inferred by dove from a consideration of the results of fifteen years' observations made at the chiswick gardens. see dove, in 'bericht uber die verhandl. der berl. akad. der wiss.', august, , s. . at bordeaux, in the valley of the garonne ( degrees ' lat.), the mean annual winter, summer, and autumn temperatures are respectively degrees, degrees, degrees, and degrees. in the plains near the p baltic ( degrees ' lat.), where a wine is produced that can scarcely be considered potable, these numbers are as follows: . degrees, degrees, . degrees, and . degrees. if it should appear strange that the great differences indicated by the influence of climate on the production of wine should not be more clearly manifested by our thermometers, the circumstance will appear less singular when we remember that a thermometer standing in the shade, and protected from the effect of direct insolation and nocturnal radiation can not, at all seasong of the year, and during all periodic changes of heat, indicate the true superficial temperature of the ground exposed to the whole effect of the sun's rays. the same relations which exist between the equable littoral climate of the peninsula of brittany, and the lower winter and p higher summer temperature of the remainder of the continent of france, are likewise manifested in some degree, between europe and the great continent of asia, of which the former may be considered to constitute the western peninsula. europe owes its milder climate, in the first place, to its position with respect to africa, whose wide extent of tropical land is favorable to the ascending current, while the equatorial region to the south of asia is almost wholly oceanic; and next to its deeply-articulated configuration, to the vicinity of the ocean on its western shores; and, lastly, to the existence of an open sea, which bounds its northern confines. europe would therefore become colder* if africa were to be overflowed by the ocean; of if the mythical atlantis were to arise and connect europe with north america; or if the gulf stream were no longer to diffuse the warming influence of its waters into the north sea; or if, finally, another mass of solid land should be upheaved by volcanic action, and interposed between the scandinavian peninsula and spitzbergen. [footnote] *see my memoir, 'ueber die haupt-ursachen der temperaturverschiedenheit auf der erdoberflÃ�Â�che', in the 'abhandl. der akad. der wissensch. zu berlin von dem jahr' , s. . if we observe that in europe the mean annual temperature falls as we proceed, from west to east, under the same parallel of latitude, from the atlantic shores of france through germany, poland, and russia, toward the uralian mountains, the main cause of this phenomenon of increasing cold must be sought in the form of the continent (which becomes less indented, and wider, and more compact as we advance), in the increasing distance from seas, and in the diminished influence of westerly winds. beyond the uralian mountains these winds are converted into cool land-winds, blowing over extended tracts covered with ice and show. the cold of western siberia is to be ascribed to these relations of configuration and atmospheric currents, and not -- as hippocrates and trogus pompeius, and even celebrated travelers of the eighteenth century conjectures -- to the great elevation of the soil above the level of the sea.* [footnote] *the general level of siberia, from tobolsk, tomsk, and barnaul, from the altai mountains to the polar sea, is not so high as that of mauheim and dresden; indeed, irkutsk, far to the east of the jenisei, is only feet above the level of the sea, or about one third lower than munich. if we pass from the differences of temperature manifested in the plains to the inequalities of the polyhedric form of the surface of our planet, we shall have to consider mountains either in relation to their influence on the climate of neighboring p valleys, or according to the effects of the hyposometrical relations on their own summits, which often spread into elevated plateaux. the division of mountains into chains separates the earth's surface into different basins, which are often narrow and walled in, forming caldron-like valleys, and (as in greece and in part of asia minor) constitute an individual local climate with respect to heat, moisture, transparancy of atmosphere, and frequency of winds and storms. these circumstances have at all times exercised a powerful influence on the character and cultivation of natural products, and on the manners and institutions of neighboring nations, and even on the feelings with which they regard one another. this character of 'geographical individuality' attains its maximum, if we may be allowed so to speak, in countries where the differences in the configuration of the soil are the greatest possible, either in a vertical or horizontal direction, both in relief and in the articulation of the continent. the greatest contrast to these varieties in the relations of the surface of the earth are manifested in the steppes of northern asia, the grassy plains (savannahs, llanos, and pampas) of the new continent, the heath ('ericeta') of europe, and the sandy and stony deserts of africa. the law of the decrease of heat with the increase of elevation at different latitudes is one of the most important subjects involved in the study of meteorological processes, of the geography of plants, of the theory of terrestrial refraction, and of the various hypotheses that relate to the determination of the height of the atmosphere. in the many mountain journeys which i have undertaken, both within and without the tropics, the investigation of this law has always formed a special object of my researches.* [footnote] *humboldt, 'recueil d'observations astronomiques', t. i., p. - ; 'relation historique', t. i., p. , , ; biot, in 'connaissance des temps pour l'an' , p. - . since we have acquired a more accurate knowledge of the true relations of the distribution of heat on the surface of the earth, that is to say, of the inflections of isothermal and isotheral lines, and their unequal distance apart in the different eastern and western systems of temperature in asia, central europe, and north america, we can no longer ask the general question, what fraction of the mean annual or summer temperature corresponds to the difference of one degree of geographical latitude, taken in the same meridian? in each system of 'isothermal' lines of equal curvature there reigns a p close and necessary connection between three elements, namely, the decrease of heat in a vertical direction from below upward, the difference of temperature for every one degree of geographical latitude, and the uniformity in the mean temperature of a mountain station, and the latitude of a point situated at the level of the sea. in the system of eastern america, the mean annual temperature from the coast of labrador to boston changes . Ã�¼degrees for every degree of latitude; from boston to charleston about . degrees; from charleston to the tropic of cancer, in cuba, the variation is less rapid, being only . degrees. in the tropics this diminution is so much greater, that from the havana to cumana the variation is less than . degrees for every degree of latitude. the case is quite different in the isothermal system of central europe. between the parallels of degrees and degrees i found that the decrease of temperature was very regularly . degrees for every degree of latitude. but as, on the other hand, in central europe the decrease of heat is . degrees for about every feet of vertical elevation, it follows that a difference of elevation of about feet corresponds to the difference of one degree of latitude. the same mean annual temperature as that occurring at the convent of st. bernard, at an elevation of feet, in lat. degrees ' should therefore be met with at the level of the sea in lat. degrees '. in that part of the cordilleras which falls within the tropics, the observations i made at various heights, at an elevation of upward of , feet, gave a decrease of degree for every feet; and my friend boussingault found, thirty years afterward, as a mean result, feet. by a comparison of places in the cordilleras, lying at an equal elevation above the level of the sea, either on the declivities of the mountains or even on extensive elevated plateaux, i observed that in the latter there was an increase in the annual temperature varying from . degrees to . degrees. this difference would be still greater if it were not for the cooling effect of nocturnal radiation. as the different climates are arranged in successive strata, the one above the other, from the cacao woods of the valleys to the region of perpetual snow, and as the temperature in the tropics varies but little throughout the year, we may form to ourselves a tolerably correct representation of the climatic relations to which the inhabitants of the large cities in the andes are subjected, by comparing these climates with the temperatures of particular months in the plains of france and italy. while p the heat which prevails daily on the woody shores of the orinoco exceeds by . degrees that of the month of august at palermo, we find, on ascending the chain of the andes, at popayan, at an elevation of feet, the temperature of the three summer months of marseilles; at quito, at an elevation of feet, that of the close of may at paris; and on the paramos, at a height of , feet, where only stunted alpine shrubs grow, though flowers still bloom in abundance, that of the beginning of april at paris. the intelligent observer, peter martyr de aughiera, one of the friends of christopher columbus, seems to have been the first who recognized (in the expedition undertaken by rodrigo enrique colmenares, in october, ) that the limit of perpetual snow continues to ascend as we approach the equator. we read, in the fine work 'de rebus oceanicis',* "the river gaira comes from a mountain in the sierra nevada de santa maria, which, according to the testimony of the companions of colmenares, is higher than any other mountain hitherto discovered. [footnote] *anglerius, 'de rebus oceanicis', dec. xi., lib. ii., p. (ed. col., ). in the sierra de santa marta, the highest point of which appears to exceed , feet (see my 'relat. hist.', t. ii., p. ), there is a peak that is still called pico de gaira. it must undoubtedly be so if 'it retain snow perpetually' in a zone which is not more than degrees from the equinoctial line." the lower limit of perpetual snow, in a given latitude, is the lowest line at which snow continues during summer, or, in other words, it is the maximum of height to which the snow-line recedes in the course of the year. but this elevation must be distinguished from three other phenomena, namely, the annual fluctuation of the snow-line, the occurrence of sporadic falls of snow, and the existence of glaciers, which appear to be peculiar to the temperate and cold zones. this last phenomenon, since saussure's immortal work on the alps, has received much light, in recent times, from the labors of venetz, charpentier, and the intrepid and persevering observer agassiz. we know only the 'lower', and not the 'upper' limit of perpetual snow; for the mountains of the earth do not attain to those ethereal regions of the rarefied and dry strata of air, in which we may suppose, with bouguer, that the vesicles of aqueous vapor are converted into crystals of ice, and thus rendered perceptible to our organs of sight. the lower limit of snow is not, however, a mere function of geographical latitude or of mean annual temperature; nor is it at the equator, or p even, in the region of the tropics, that this limit attains its greatest elevation above the level of the sea. the phenomenon of which we are treating is extremely complicated, depending on the general relations of temperature and humidity, and on the form of the mountains. on submitting these relations to the test of special analysis, as we may be permitted to do from the number of determinations that have recently been made,* we shall find that the controlling causes are the differences in the temperature of different seasons of the year; the direction of the prevailing winds and their relations to this land and sea; the degree of dryness or humitidy in the upper strata of the air; the absolute thickness of the accumulated masses of fallen snow; the relation of the s-line to the total height of the mountain; the relative position of the latter in the chain to which it belongs, and the steepness of its declivity; the vicinity of either summits likewise perpetually covered with show; the expansion, position, and elevation of the plains from which the snow mountain rises as an isolated peak or as a portion of a chain; whether this plain be part of the sea-coast, or of the interior of a continent; whether it be covered with wood or waving grass; and whether, finally, it consist of a dry and rocky soil, or of a wet and marshy bottom. [footnote] *see my table of the height of the line of perpetual snow, in both hemispheres, from degrees ' north lat. to degrees ' south lat., in my 'asie centrale', t. iii., p. . the snow-line which, under the equator in south america, attains an elevation equal to that of the summit of mont blanc in the alps, and descends, according to recent measurements, about feet lower toward the northern tropic in the elevated plateaux of mexico (in degrees north latitude), rises, according to pentland, in the southern tropical zone ( degrees ' to degrees south latitude), being more than feet higher in the maritime and western branch of the cordilleras of chili than under the equator near quito on chimborazo, cotopaxi, and antisana. dr. gilles even asserts that much further to the south, on the declivity of the volcano of peuquenes (latitude degrees), he found the snow-line at an elevation of between , and , feet. the evaporation of the snow in the extremely dry air of the summer, and under a cloudless sky, is so powerful, that the volcano of aconcagua, northeast of valparaiso (latitude degrees '), which was found in the expedition of the beagle to be more than feet higher than chimborazo, was on one occasion seen free from snow.Ã�Â¥ [footnote] *darwin, 'journal of the voyages of the adventure and beagle', p. . as the volcano of aconcagua was not at that time in a state of eruption, we must not ascribe the remarkable phenomenon of this absence of snow to the internal heat of the mountain (to the escape of heated air through fissures), as is sometimes the case with cotopaxi. gilles, in the 'journal of natural science', , p. . in p an almost equal northern latitude (from degrees ' to degrees), the snow'line on the southern declivity of the himalaya lies at an elevation of , feet, which is about the same as the height which we might have assigned to it from a comparison with other mountain chains; on the northern declivity, however, under the influence of the high lands of thibet (whose mean elevation appears to be about , feet), the snow-line is situated at a height of , feet. this phenomenon, which has long been contested both in europe and in india, and whose causes i have attempted to develop in various works, published since ,* possesses other grounds of interest than p those of a purely physical nature, since it exercises no inconsiderable degree of influence on the mode of life of numerous tribes -- the meteorological processes of the atmosphere being the controlling causes on which depend the agricultural or pastoral pursuits of the inhabitants of extensive tracts of continents. [footnote] *see my 'second memoire sur les montagnes de inde', in the 'annales de chemie et de physique', t. xiv., p. - ; and 'asie centrale', t. iii., p. - . while the most learned and experienced travelers in india, colebrooke, webb, and hodgson, victor jacquemont, fobes royle, carl von hugel, and vigne, who have all personally examined the himalaya range, are agreed, regarding the greater elevation of the snow-line on the thibeta=ian side, the accuracy of this statement is called in question by john gerard, by the geognoist macclelland, the editor of the 'calcutta journal', and by captain thomas hutton, assistant surveyor of the agra division. the appearance of my work on central asia gave rise to a rediscussion of this question. a recent number (vol. iv., january, ) of macclelland and griffith's 'calcutta journal of natural history' contains, however, a very remarkable and decisive notice of the determination of the snow-line in the himalaya. mr. batten, of the bengal service, writes as follows from camp semulka, on the cosillah river, kumaon: "in the july, , no. of your valuable journal of natural history, which i have only lately had the opportunity of seeing, i read captain hutton's paper on the snow of the himalayas, and as i differed almost entirely from the conclusions so confidently drawn by that gentleman, i thought it right, for the interest of scientific truth, to prepare some kind of answer; as however, on a more attentive perusal, i find that you yourself appear implicitly to adopt captain hutton's views, and actually use these words, 'we have long been conscious of the error here so well ppointed out by captain hutton, 'in common with every one who has visited the himalayas,' i feel more inclined to address you, in the first instance, and to ask whether you will publish a short reply which i meditate; and whether your not to captain hutton's paper was written after your own full and careful examination of the subject, or merely on a general kind of acquiscence with the fact and opinions of your able contributor, who is so well known and esteemed as a collector of scientific data? now i am one who have visited the himalaya on the western side; i have crossed the borendo or booria pass into the buspa valley, in lower kanawar, returning into the rewaien mountains of ghurwal by the koopin pass; i have visited the source of the jumna at jumnootree; and, moving eastward, the sources of the kalee or mundaknee branch of the ganges at kadarnath; of the bishnoo gunga, or aluknunda, at buddrinath and mana; of the pindur at the foot of the great peak nundidavi; of the dhoulee branch of the ganges, beyond neetee, crossing and recrossing the pass of that name into thibet; of the goree or great branch of the sardah, or kalee, near oonta dhoora, beyond melum. i have also, in my official capacity made the settlement of the bhote mehals of this province. my residence of more than six years in the hills has thrown me constantly in the way of european and native travelers, nor have i neglected to acquire information from the recorded labors of others. yet, with all this experience, i am prepared to affirm that 'the perpetual snow-line is at a higher elevation' on the northern slope of 'the himalaya' than on the southern slope. "the facts mentioned by captain hutton appear to me only to refer to the northern sides of all mountains in these regions, and not to affect, in any way the reports of captain webb and others, on which humboldt formed his theory. indeed how can any facts of one observer in one place falsify the facts of another observer in another place? i willingly allow that the north side of a hill retains the snow longer and deeper than the south side, and this observation applies equally to heights in bhote; but humboldt's theory is on the question of the perpetual snow-line, and captain hutton's reference to simla and mussooree, and other mountain sites, are out of place in this question, or else he fights against a shadow, or an objectioon of his own creation. in no part of his paper does he quote accurately the dictum which he wishes to oppose." if the mean altitude of the thibetian highlands be , feet, they admit of comparison with the lovely and fruitful plateau of caxamarca in peru. but at this estimate they would still be feet lower than the plateau of bolivia at the lake of titicaca, and the causeway of the town of potosi. ladak, as appears from vigne's measurement, by determining the boiling-point, is feet high. this is probably also the altitude of h'lassa (yul-sung), a monastic city, which chinese writers describe as the 'realm of pleasure', and which is surrounded by vineyards. must not these lie in deep valleys? as the quantity of moisture in the atmosphere increases with the temperature, this element, which is so important for the whole organic creation, must vary with the hours of the day, the seasons of the year, and the differences in latitude and elevation. our knowledge of the hygrometric relations of the earth's surface has been very materially augmented of late years by the general application of august's psychrometer, framed in accordance with the views of dalton and daniell, for determining the relative quantity of vapor, or the p condition of moisture of the atmosphere, by means of the difference of the 'dew point' and of the temperature of the air. temperature, atmospheric pressure, and the direction of the wind, are all intimately connected with the vivifying action of atmospheric moisture. this influence is not, however, so much a consequence of the quantity of moisture held in solution in different zones, as of the nature and frequency of the precipitation which moistens the ground, whether in the form of dew, mist, rain, or snow. according to the exposition made by dove of the law of rotation, and to the general views of this distinguished physicist,* it would appear that, in our northern zone, "the elastic force of the vapor is greatest with a southwest, and least with a northeast wind. on the western side of the windrose this elasticity diminishes, while it increases on the eastern side; on the former side, for instance, the cold, dense, and dry current of air repels the warmer, lighter current containing an abundance of aqueous vapor, while on the eastern side it is the former current which is repulsed by the latter. [footnote] *see dove, 'meteorologische vergleichung von nordamerika und europa', in schumacher's 'jahrbuch fur' , s. ; and his 'meteorologische untersuchungen', s. . the agreeable and fresh verdure which is observed in many trees in districts within the tropics, where, for five or seven months of the yeqar, not a cloud is seen on the vault of heaven, and where no perceptible dew or rain falls, proves that the leaves are capable of extyracting water from the atmosphere by a peculiar vital process of their own, which perhaps is not alone that of producing cold by radiation. the absence of rain in the arid plains of cumana, coro, and ceara in north brazil, forms a striking contrast to the quanitity of rain which falls in some tropical regions, as, for instance, in the havana, where it would appear, from the average of six years' observation by ramong de la sagra, the mean annual quantity of rain is inches, equal to four or five times that which falls at paris or at geneva.* [footnote] *the mean annual quantity of rain that fell in paris between and was found by arago to be inches; in london, between and , it was determined by howard at inches; while at geneva the mean of thirty-two years' observation was . inches. in hindostan, near the coast, the quantity of rain is from to inches; and in the island of cuba, fully inches fell in the year . with regard to the distribution of the quantity of rain in central europe, at different periods of the year, see the admirable researches of gasparin, schuow, and bravais, in the 'bibliotheque universelle', t. xxxvviii., p. and ; 'tableau du climat de l'italie', p. ; and martins's notes to his excellent french translation of kÃ�Â�mtz's 'vorlesungen uber meteorologie', p. . on the declivity of the cordilleras, p the quantity of rain, as well as the temperature, diminishes with the increase in the elevation.* [footnote] *according to boussingault ('economie rurale', t. ii., p. ), the mean quantity of rain that fell at marmato (latitude degrees ', altitude feet, and mean temperature degrees) in the years and was inches, while at santa fe de bogota (latitude degrees ', altitude feet, and mean temperature degrees) it only amounted to / inches. my south american fellow-traveler, caldas, found that, at santa fe de bogota, at an elevation of almost feet, it did not exceed inches, being consequently little more than on some parts of the western shore of europe. boussingault occasionally observed at quito that saussure's hygrometer receded to degrees with a temperature of from . degrees to . degrees. gay-lussac saw the same hygrometer standing at . degrees in his great aerostatic ascent in a stratum of air feet high, and with a temperature of . degrees. the greatest dryness that has yet been observed on the surface of the globe in the low lands is probably that which gustav rose, ehrenberg, and myself found in northern asia, between the valleys of the irtisch and the oby. in the steppe of platowskaja, after southwest winds had blown for a long time from the interior of the continent, with a temperature of . degrees, we found the dew point at degrees. the air contained only / ths of aqueous vapor.* [footnote] *for the particulars of this observation, see my 'asie centrale', t. iii., p. - and ; and regarding the amount of vapor in the atmosphere in the lowlands of tropical south america, consult my 'relat. hist.', t. i., p. - ; t. ii., p. , . the accurate observers kÃ�Â�mtz, bravais, and martins have raised doubts during the last few years regarding the greater dryness of the mountain air, which appeared to be proved by the hygrometric measurements made by saussure and myself in the higher regions of the alps and the cordilleras. the strata of air at zurich and on the faulhorn, which can not be considered as an elevated mountain when compared with non-european elevations, furnished the data employed in the comparisons made by these observers.* [footnote] *kÃ�Â�mtz, 'vorlesungen uber meteorologie', s. . in the tropical region of the paramos (near the region where snow begins to fall, at an elevation of between , and , feet), some species of large flowering myrtle-leaved alpine shrubs are almost constantly bathed in moisture; but this fqact does not actually prove the existence of any great and absolute quantity of aqueous vapor at such an elevation, merely affording p an evidence of the frequency of aqueous precipitation, in like manner as do the frequent mists with which the lovely plateau of bogota is covered. mists arise and disappear several times in the course of an hour in such elevations as these, and with a calm state of the atmosphere. these rapid alternations characterize the paramos and the elevated plains of the chain of the andes. 'the electricity of the atmosphere', whether considered in the lower or in the upper strata of the clouds, in its silent problematical diurnal course, or in the explosion of the lightning and thunder of the tempest, appears to stand in a manifold relation to all phenomena of the distribution of heat, of the pressure of the atmosphere and its disturbances, of hydrometeoric exhibitions, and probably, also, of the magnetism of the external crust of the earth. it exercises a powerful influence on the whole animal and vegetable world; not merely by meteorological processes, as precipitations of aqueous vapor, and of the acids and ammoniacal compounds to which it gives rise, but also directly as an electric force acting on the nerves, and promoting the circulation of the organic juices. this is not a place in which to renew the discussion that has been started regarding the actual source of atmospheric eletricity when the sky is clear, a phenomenon that has alternately been ascribed to the evaporation of impure fluids impregnated with earths and salts,* to the growth of plants,** or to some other chemical decompositions on the surface of the earth, to the unequal distribution of heat in the strata of the air,*** and, finally, according to peltier's intelligent researches,**** to the agency of a constant charge of negative electricity in the terrestrial globe. [footnote] *regarding the conditions of electricity from evaporation at high temperatures, see peltier, in the 'annales de chimie', t. lxxv., p. . [footnote] **pouillet, in the 'annales de chimie', t. xxxv., p. . [footnote] ***de la rive, in his admirable 'essai historique sur l'electricite', p. . [footnote] ****peltier, in the 'comptes rendus de l'acad. des sciences', t. xii., p. ; becquerel, 'traite de l'electricite et du magnetisme', t. iv., p. . limiting itself to results yielded by electrometric observations, such, for instance, as are furnished by the ingenious electro-magnetic apparatus first proposed by colladon, the physical description of the universe should merely notice the incontestable increase of intensity in the general positive electricity of the atmosphere,* accompanying an increase of altitude and and the absence of trees, its daily variations (which, according to clark's experiments at dublin, p take place at more complicated periods than those found by saussure and myself), and its variations in the different seasons of the year, at different distances from the equator, and in the different relations of continental or oceanic surface. [footnote] *duprez, 'sur l'electricite de l'air' (bruxelles, ), p. - . the electric equilibrium is less frequently disturbed where the aerial ocean rests on a liquid base than where it impends over the land; and it is very striking to observe how, in extensive seas, small insular groups affect the condition of the atmosphere, and occasion the formation of storms. in fogs, and in the commencement of falls of snow, i have seen, in a long series of observations, the previously permanent positive electricity rapidly pass into the negative condition, both on the plains of the colder zones, and in the paramos of the cordilleras, at elevations varying from , to , feet. the alternate transition was precisly similar to that indicated by the electrometer shortly before and during a storm.* [footnote] *humboldt, 'relation historique', t. iii., p. . i here only refer to those of my experiiments in which the three-foot metallic conductor of saussure's electrometer was neither moved upward nor downward, nor, according to volta's proposal, armed with burning sponge. those of my readers who are well acquainted with the 'quaestiones vexatae' of atmospheric electricity will understand the grounds for this limitation. respecting the formation of storms in the tropics, see my 'rel. hist.', t. ii., p. and - . when the vesicles of vapor have become condensed into clouds, having definite outlines, the electric tension of the external surface will be increased in proportion to the amount of electricity which passes over to it from the separate vesicles of vapor.* [footnote] *gay-lussac, in the 'annales de chimie et de physique', t. viii., p. . in consequence of the discordant views of lame, becquerel, and peltier, it is difficult to come to a conclusion regarding the cause of the specific distribution of electricity in clouds, some of which have a positive, and others a negative tension. the negative electricity of the air, which near high water-falls is caused by a disintegration of the drops of water -- a fact originally noticed by tralles, and confirmed by myself in various latitudes -- is very remarkable, and is sufficiently intense to produce an appreciable effect on a delicate electrometer at a distance of or feet. slate-gray clouds are charged, according to peltier's experiments at paris, with negative, and white, red, and orange-colored clouds with positive electricity. thunder clouds not only envelop the highest summits of the chain of the andes (i have myself seen the electric effect of lightning on one of the rocky pinnacles which project upward of , feet above the crater of the volcano of toluca), but they have also been observed at a vertical height of , feet over the low p lands in the temperate zone.* [footnote] *arago, in the 'annuaire du bureau des longitudes pour' , p. . sometimes, however, the stratum of cloud from which the thunder proceeds sinks to a distance of , or, indeed, only feet above the plain. according to arago's investigations -- the most comprehensive that we possess on this difficult branch of meteorology -- the evolution of light (lightning) is of three kinds -- zigzag, and sharply defined at the edges; in sheets of light, illuminating a whole cloud, which seems to open and refeal the light within it; and in the form of fire-balls.* [footnote] *arago, op. cit., p. - . (see also, p. - .) the duration of the two first kinds scarcely continues the thousandth part of a second; but the globular lightning moves much more slowly remaining visible for several seconds. occasionally (as is proved by the recent observations, which have confirmed the description given by nicholson and beccaria of this phenomenon), isolated clouds, standing high above the horizon, continue uninterruptedly for some time to emit a luminous radiance from their interior and from their margins, although there is no thunder to be heard, and no indication of a storm; in some cases even hail-stones, drops of rain, and flakes of snow have been seen to fall in a luminous condition, when the phenomenon was not preceded by thunder. in the geographical distribution of storms, the peruvian coast, which is not visited by thunder or lightning, presents the most striking contrast to the rest of the tropical zone, in which, at certain seasons of the year, thunder-storms occur almost daily, about four or five hours after the sun has reached the meridian. according to the abundant evidence collected by arago* from the testiimony of navigators (scoresby, parry, ross, and franklin), there can be no doubt that, in general, electric explosions are extremely rare in high northern regions (between degrees and degrees latitude). [footnote] *arago, op. cit., p. - . the learned academician von baer, who has done so much for the meteorology of northern asia, has not taken into consideration the extreme rarity of storms in iceland and greenland; he has only remarked ('bulletin de l'academie de st. petersbourg', , mai) that in nova zembla and spitzbergen it is sometimes heard to thunder. 'the meteorological portion' of the descriptive history of nature which we are now concluding shows that the processes of the absorption of light, the liberation of heat, and the variations in the elastic and electric tension, and in the hygrometric condition of the vast aerial ocean, are all so intimately connected together, that each individual meteorological process is modified by the action of all the others. the complicated p nature of these disturbing causes (which involuntarily remind us of those which the near and especially the smallest cosmical bodies, the satellites, comets, and shooting stars, are subjected to in their course) increases the difficulty of giving a full explanation of these involved meteorological phenomena, and likewise limits, or wholly precludes, the possibility of that predetermination of atmospheric changes which would be so important for horticulture, agriculture, and navigation, no less than for the comfort and enjoyment of life. those who place the value of meteorology in this problematic species of prediction rather than in the knowledge of the phenomena themselves, are firmly convinced that this branch of science, on account of which so many expeditions to distant mountainous regions have been undertaken, has not made any very considerable progress for centuries past. the confidence which they refuse to the physicist they yield to changes of the moon, and to certain days marked in the calendar by the superstition of a by-gone age. "great local deviations from the distribution of the mean temperature are of rare occurrence, the variations being in general uniformly distributed over extensive tracts of land. the deviation, after attaining its maximum at a certain point, gradually decreases to its limits; when these are passed, however, decided deviations are observed in the 'opposite direction'. similar relations of weather extend more frequently from south to north than from west to east. at the close of the year (when i had just completed my siberian journey), the maximum of cold was at berlin, while north america enjoyed an unusually high temperature. it is an entirely arbitrary assumption to believe that a hot summer succeeds a severe winter, and that a cool summer is preceded by a mild winter." opposite relations of weather in contiguous countries, or in two corn-growing continents, give rise to a beneficient equalization in the prices of the products of the vine, and of agricultural and horticultural cultivation. it has been justy remarked, that it is the barometer alone which indicates to us the changes that occur in the pressure of the air throughout all the aerial strata from the place of observation to the extremest confines of the atmosphere, while* the thermometer and psychrometer only acquaint us with all the variations occurring in the local heat and moisture of the lower strata of p air in contact with the ground. [footnote] *kÃ�Â�mtz, in schumacher's 'jahrbuch fur' , s. . regarding the opposite distribution of heat in the east and the west of europe and north america, see dove, 'repertorium der physik', bd. iii., s. - . the simultaneous thermic and hygrometric modifications of the upper regions of the air can only be learned (when direct observations on mountain stations or aerostatic ascents are impracticable) from hypothetical combinations, by making the barometer serve both as a thermometer and an hygrometer. important changes of weather are not owing to merely local causes, situated at the place of observation, but are the consequence of a disturbance in the equilibrium of the aerial currents at a great distance from the surface of the earth, in the higher strata of the atmosphere, bringing cold or warm, dry or moist air, rendering the sky cloudy or serene, and converting the accumulated masses of clouds into light feathery 'cirri'. as, therefore, the inaccessibility of the phenomenon is added to the manifold nature and complication of the disturbances, it has always appeared to me that meteorology must first seek its foundation and progress in the torrid zone, where the variations of the atmospheric pressure, the course of hydro-meteors, and the phenomena of electric explosion, are all of periodic occurrence. as we have now passed in review the whole sphere of inorganic terrestrial life, and have briefly considered our planet with reference to its form, its internal heat, its electro-magnetic tension, its phenomena of polar light, the volcanic reaction of its interior on its variously composed solid crust, and, lastly, the phenomena of its two-fold envelopes -- the aerial and liquid ocean -- we might, in accordance with the older method of treating physical geography, consider that we had completed our descriptive history of the globe. but the nobler aim i have proposed to myself, of raising the contemplation of nature to a more elevated point of view, would be defeated, and this delineation of nature would appear to lose its most attractive charm, if it did not also include the sphere of organic life in the many stages of its typical development. the idea of vitality is so intimatey associated with the idea of the existence of the active, ever-blending natural forces which animate the terrestrial sphere, that the creation of plants and animals is ascribed in the most ancient mythical representations of many nations to these forces, while the condition of the surface of our planet, before it was animated by vital forms, is regarded as coeval with the epoch of a chaotic conflict of the struggling elements. but the empirical domain of objective contemplation, and the delineation of our planet in its present condition, do not include a consideration p of the mysterious and insoluble problems of origin and existence. a cosmical history of the universe, resting upon facts as its basis, has, from the nature and limitations of its sphere, necessarily no connection with the obscure domain embraced by a 'history of organisms',* if we understand the word 'history' in its broadest sense. [footnote] *the 'history of plants', which endlicher and unger have described in a most masterly manner ('grundzuge der botanik', , s. - ), i myself separated from the 'geography of plants' half a century ago. in the aphorisms appended to my 'subterranean flora', the following passage occurs: "geognosia naturam animantem et inanimam vel, ut vocabulo minus apto, ex antiquitate saltem haud petito, utar, corpora vitur capita: geographia oryctologica quam simpliciter geognosiam vel geologiam dicunt, virque acutissimus wernerus egregie digessit; geographia zoologica, cujus doctrinae fundamenta zimmermannus et treviranus jecerunt; et geographic plantarum quam aequales nostri diu intactam reliquerunt. geographia plantarum vincula et cognationem tradit, quibus omnia vegetabilia inter se connexa sint, terraetractur quos teneant, in aerem atmosphaericum quae sit eorum vis ostendit, saxa atque rupes quibus potissimum algarum primordiis radicibusque destruantur docet, et quo pacto in telluris superficie humus nascatur, commemorat. est itaque quod differat inter geognosiam et physiographiam, 'historia naturalis' perperam nuncupatam quum zoognosia, phytognosia, et oryctognosia, quae quidem omnes in naturae investigatione versantur, non nisi singulorum animalium, plantarum, rerum metallicarum vel (venia sit verbo) fossilium formas, anatomen, vires scrutautur. historia telluris, geognosiae magis quam physiographiae affinis, nemini adhuc tenata, plantarum animaliumque genera orbem inhabitantia primaevum, migrationes eorum compluriumque interitum, ortum quem montes, valles, saxorum strata et vemae metalliferae ducunt, aerem, mutatis temporum vicibus, modo purum, modo vitiatum, terrae superficiem humo plantisque paulatim obtectam, fluminum inundantium impetu denuo nudatam, iterumque siccatam et gramine vestitam commemorat. igitur historia zoolopgica, historia plantarum et historia oryctologica, quae non nisi pristinum orbis terrae statum indicant, a geognosia probe distinguendae." -- humboldt, 'flora friburgensis subterranea, cui accedunt aphorismi ex physiologia chemica plantarum', , p. ix.-x. respecting the "spontaneous motion." which is referred to in a subsequent part of the text, see the remarkable passage in aristotle, 'de coelo,' ii., , p. , bekker, where the distinction between animate and inanimate bodies is made to depend on the internal or external position of the seat of the determining motion. "no movement," says the stagirite, "proceeds from the vegetable spirit, because plants are buried in a still sleep, from which nothing can arouse them" (aristotle, 'de generat. animal.', v. i., p. , bekker); and again, "because plants have no desires which incite them to spontaneous motion." (arist., 'de somno et vigil'., cap. i., p. , bekker.) it must, however, be remembered, that the inorganic crust of the earth contains within it the same elements that enter into the structure of animal and vegetable organs. a physical cosmography would therefore be incomplete p if it were to omit a consideration of these forces, and of the substances which enter into solid and fluid combinations in organic tissues, under conditiions which, from our ignorance of their actual nature, we designate by the vague term of 'vital forces', and group into various systems in accordance with more or less perfectly conceived analogies. the natural tendency of the human mind involuntarily prompts us to follow the physical phenomena of the earth, through all their varied series, until we reach the final stage of the morphological evolution of vegetable forms, and the self-determining powers of motion in animal organisms. and it is by these links that 'the geography of organic beings -- of plants and animals' -- is connected with the delineation of the inorganic phenomena of our terrestrial globe. without entering on the difficult question of 'spontaneous motion', or, in other words, on the difference between vegetable and animal life, we would remark, that if nature had endowed us with microscopic powers of vision, and the integuments of plants had been rendered perfectly transparent to our eyes, the vegetable world would present a very different aspect from the apparent immobility and repose in which it is now manifested to our senses. the interior portion of the cellular structure of their organs is incessantly animated by the most varied currents, either rotating, ascending and descending, remifying, and ever changing their direction, as manifested in the motion of the granular mucus of marine plants (naiades, characeae, hydrocharidae), and in the hairs of phanerogamic land plants; in the molecular motion first discovered by the illustrious botanist robert brown, and which may be traced in the ultimate portions of every molecule of matter, even when separated from the organ; in the gyratory currents of the globules of cambium ('cyclosis') circulating in their peculiar vessels; and, finally, in the singularly articulated self-unrolling filamentous vessels in the antheridia of the chara, and in the reproductive organs of liverworts and algae, in the structural conditions of which meyen, unhappily too early lost to science, believed that he recognized an analogy with the spermatozoa of the animal kingdom.* [footnote] *["in certain parts, probably, of all plants, are found peculiar spiral filaments, having a striking resemblance to the spermatozoa of animals. they have been long known in the organs called the antheridia of mosses, hepaticcae, and characeae, and have more recently been discovered in peculiar cells on the germinal frond of ferns, and on the very young leaves of the buds of phanerogamia. they are found in peculiar cells, and when these are placed in water they are torn by the filament, which commences an active spiral motion. the signification of these organs is at present quite unknown; they appear, from the researches of nÃ�Â�geli, to resemble the cell mucilage, or proto-plasma, in composition, and are developed from it. schleiden regards them as mere mucilaginous deposits, similar to those connected with the circulation in cells, and he contends that the movement of these bodies in water is analogous to the molecular motion of small particles of organic and inorganic substances, and depends on mechanical causes." -- 'outlines of structural and physiological botany', by a. henfrey, f.l.s., etc., , p. .] -- tr. if to these p manifold currents and gyratory movements we add the phenomena of endosmosis, nutrition, and growth, we shall have some idea of those forces which are ever active amid the apparent repose of vegetable life. since i attempted in a former work, 'ansichten der natur' (views of nature), to delineate the universal diffusion of life over the whole surface of the earth, in the distribution of organic forms, both with respect to elevation and depth, our knowledge of this branch of science has been most remarkably increased by ehrenberg's brilliant discovery "on microscopic life in the ocean, and in the ice of the polar regions" -- a discovery based, not on deductive conclusions, but on direct observation. the sphere of vitality, we might almost say, the horizon of life, has been expanded before our eyes. "not only in the polar regions is there an uninterrupted development of active microscopic life, where larger animals can no longer exist, but we find that the microscopic animals collected in the antarctic expedition of captain james ross exhibit a remarkable abundance of unknown and often most beautiful forms. even in the residuum obtained from the melted ice, swimming about in round fragments in the latitude of degrees ', there were found upward of fifty species of silicious-shelled polygastria and coscinodiscae with their green ovaries, and therefore living and able to resist the extreme severity of the cold. in the gulf of erebus, sixty-eight silicious-shelled polygastria and phytolitharia, and only one calcareous-shelled polythalamia, were brought up by lead sunk to a depth of from to feet." the greater number of the oceanic microscopic forms hitherto discovered have been silicious-shelled, although the analysis of sea water does not yield silica as the main constituent, and it can only be imagined to exist in it in a state of suspension. it is not only at particular points in inland seas, or in the vicinity of the land, that the ocean is densely inhabited by living atoms, invisible to the naked eye, but samples of p water taken up by schayer on his return from van diemen's land (south of the cape of good hope, in degrees latitude, and under the tropics in the atlantic) show that the ocean in its ordinary condition, without any apparent discoloration, contains numerous microscopic moving organisms, which bear no resemblance to the swimming fragmentary silicious filaments of the genus chaetoceros, similar to the oscillatoriae so common in our fresh waters. some few polygastria, which have been found mixed with sand and excrements of penguins in cockburn island, appear to be spread over the whole earth, while others seem to be peculiar to the polar regions.* [footnote] *see ehrenberg's treatise 'ueber das kleinste leben im ocean', read before the academy of science at berlin on the th of may, . [dr. j. hooker found diatomaceae in countless numbers between the parallels of degrees and degrees south, where they gave a color to the sea, and also the icebergs floating in it. the death of these bodies in the south arctic ocean is producing a submarine deposit, consisting entirely of the silicious particles of which the skeletons of these vegetables are composed. this deposit exists on the shores of victoria land and at the base of the volcanic mountain erebus. dr. hooker accounted for the fact that the skeletons of diatomaceae had been found in the lava of volcanic mountains, by referring to these deposits at mount erebus, which lie in such a position as to render it quite possible that the skeletons of these vegetables should pass into the lower fissures of the mountain, and then passing into the stream of lava, be thrown out, unacted upon by the heat to which they have been exposed. see dr. hooker's paper, read before the british association at oxford, july, .] -- tr. we thus find from the most recent observations that animal life predominates amid the eternal night of the depths of ocean, while vegetable life, which is so dependent on the periodic action of the solar rays, is most prevalent on continents. the mass of vegetation on the earth very far exceeds that of animal organisms; for what is the volume of all the large living cetacea and pachydermata when compared with the thickly-crosded colossal trunks of trees, of from eight to twelve feet in diameter, which fill the vast forests covering the tropical region of south america, between the orinoco, the amazon, and the rio de madeira? and although the character of different portions of the earth depends on the combination of external phenomena, as the outlines of mountains -- the physiognomy of plants and animals -- the azure of the sky -- the forms of the clouds -- and the transparency of the atmosphere -- it must still be admitted that the vegetable mantle with which the earth is decked constitutes the main feature of the picture. animal forms are inferior in mass, and their powers of motion often withdraw them from our sight. the p vegetable kingdom, on the contrary, acts upon our imagination by its continued presence and by the magnitude of its forms; for the size of a tree indicates its age, and here alone age is associated with the expression of a constantly renewed vigor.* [footnote] *humboldt, 'ansichten der natur' ( te ausgabe, ), bd. ii. s. . in the animal kingdom (and this knowledge is also the result of ehrenberg's discoveries), the form which we term microscopic occupy the largest space, in consequence of their rapid propagation.* [footnote] *on multiplication by spontaneous division of the mother-corpuscle and intercalation of new substance, see ehrenberg 'van den jetzt lebenden thierarten der kreidebildung', in the 'abhandl. der berliner akad. der wiss.', , s. . the most powerful productive faculty in nature is that manifested in the vorticellae. estimations of the greatest possible development of masses will be found in chrenberg's great work 'die infusionsthierchen als volkommne organismen', , s. xiii., xix., and . "the milky way of these organisms comprises the genera monas, vibrio, bacterium, and bodo." the universality of life is so profusely distributed throughout the whole of nature, that the smaller infusoria live as parasites on the larger, and are themselves inhabited by others, s. , , and . the minutest of the infusoria, the monadidae, have a diameter which does not exceed / th of a line, and yet these silicious-shelled organisms form in humid districts subterranean strata of many fathoms in depth. the strong and beneficial influence exercised on the feelings of mankind by the consideration of the diffusion of life, throughout the realms of nature is common to every zone, but the impression thus produced is most powerful in the equatorial regions, in the land of palms, bamboos, and arborescent ferns, where the ground rises from the shore of seas rich in mollusca and corals to the limits of perpetual snow. the local distribution of plants embraces almost all heights and all depths. organic forms not only descend into the interior of the earth, where the industry of the miner has laid open extensive excavations and sprung deep shafts, but i have also found snow-white stalactiitic columns encircled by the delicate web of an usnea, in caves where meteoric water could alone penetrate through fissures. podurellae penetrate into the icy crevices of the glaciers on mount rosa, the grindelwald, and the upper aar; the chionaea nivalis (formerly known as protococcus), exist in the polar snow as well as in that of our high mountains. the redness assumed by the snow after lying on the ground for soome time was known to aristotle, and was probably observed by him on the mountains of macedonia.* [footnote] *aristot., 'hist. animal.', v. xix., p. , bekk. p while, on the loftiest summits of the alps, only lecideae, parmeliae, and umbilicariae cast their colored but scanty covering over the rocks, exposed by the melted snow, beautiful phanerogamic plants, as the culcitium rufescens, sida pinchinchensis, and saxifraga boussingaulti, are still found to flourish in the tropical region of the chain of the andes, at an elevation of more than , feet. thermal springs contain small insects (hydroporus thermalis), gallionellae, oscillatoria and confervae, while their waters bathe the root-fibers of phanerogamic plants. as air and water are aniimated at different temperatures by the presence of vital organisms, so likewise is the interior of the different portions of animal bodies. animalcules have been found in the blood of the frog and the salmon; according to nordmann, the fluids in the eyes of fishes are often filled with a worm that lives by suction (diplostomum), while in the gills of the bleak the same observer has discovered a remarkable double aniimalcule (diplozoon paradoxum), having a cross-shaped form with two heads and two caudal extremities. although the existence of meteoric infusoria is more than doubtful, it can not be denied that, in the same manner as the pollen of the flowers of the pine is observed every year to fall from the atmosphere, minute infusorial animalcules may likewise be retained for a time in the strata of the air, after having been passively borne up by currents of aqueous vapor.* [footnote] *ehrenberg, op. cit., s. xiv., p. and . the rapid multiplication of microscopic organisms is, in the case of some (as, for instance, in wheat-eels, wheel-animals, and water-bears or tardigrade animalcules), accompanied by a remarkable tenacity of life. they have been seen to come to life from a state of apparent death after being dried for twenty-eight days in a vacuum with chloride of line and sulphuric acid, and after being exposed to a heat of degrees. see the beautiful experiments of doyere, in 'mem. sur les tardigrades et sur leur propriete de revenir a la vie', , p. , , , . compare, also, ehrenberg, s. - , on the revival of animalcules that had been dried during a space of many years. this circumstance merits serious attention in reconsidering the old discussion respecting 'spontaneous generation',* and the p more so, as ehrenberg, as i have already remarked, has discovered that the nebulous dust or sand which mariners often encounter in the vicinity of the cape verd islands, and even at a distance of geographical miles from the african shore, contains the remains of eighteen species of silicious-shelled polygastric animalcules. [footnote] *on the supposed "primitive transformation" of organized or unorganized matter into plants and animals, see ehrenberg, in poggendorf's 'annalen der physik', bd. xxiv., s. - , and also his 'infusionsthierchen', s. , , and joh. muller, 'physiologie des menschen' ( te aufl., ), bd. i., s. - . it appears to me worthy of notice that one of the early fathers of the church, st. augustine, in treating of the question how islands may have been covered with new animals and plants after the flood, shows himself in no way disinclined to adope the view of the so-called "spontaneous generation" ('generatio aequivoca, spontanea aut primaria'). "if," says he, "animals have not been brought to remote islands by angels, or perhaps by inhabitants of continents addicted to the chase, they must have been spontaneously produced upon the earth; although here the question certainly arises, to what purpose, then, were animals of all kinds assembled in the ark?" "si e terra exort" sunt (bestiae) secundum originem primam, quando dixit deus" 'producat terra animam vivam!' multo clarius apparet, non tam reparandorum animalium causa, quam figurandarum variarum gentium (?) propter ecclesiae sacramentumin arca fuisse omnia genera, si in insulis quo transire non possent, multa animalia terra produxit." augustinus, 'de civitate dei', lib. xvi., cap. : 'opera, ed. monach. ordinis s. benedicti', t. vii., venet., , p. . two centuries before the tiime of the bishop of hippo, we find, by extracts from trogus pompeius, that the 'generatio primaria' was brought forward in connection with the earliest drying up of the ancient world, and of the high table-land of asia, precisely in the same manner as the terraces of paradise, in the theory of the great linnaeus, and in the visionary hypotheses entertained in the eighteenth century regarding the fabled atlantis: "quod si omnes quondam terrae submersae profundo fuerunt, profecto editissilimam quamque partem decurrentibus aquis primum detectam; humillimo autem solo eandem aquam diutissime immoratam, et quanto prior quaeque pars terrarum siccata sit, tanto prius animalia generare coepisse. porro scythiam adeo editiorem omnibus terris esse ut cuncta flumina ibi nata in maeotium, tum deinde in ponticum et aegyptium mare decurrant." -- justinus, lib. ii., cap. . the erroneous supposition that the land of scythia is an elevated table-land, is so ancient that we meet with it most clearly expressed in hippocrates, 'de aere et aquis', cap. , , coray. "scythia," says he, "coonsists of high and naked plains, which, without being crowned with mountains, ascend higher and higher toward the north." vital organisms, whose relations in space are comprised under the head of the geography of plants and animals, may be considered either according to the difference and relative numbers of the types (their arrangement into genera and species), or according to the number of individuals of each species on a given area. in the mode of life of plants as in that of animals, an important difference is noticed; they either exist in an isolated state, or live in a social condition. those species of plants which i have termed 'social'* uniformly cover vast extents of land. [footnote] *humboldt, 'aphorismi ex physiologia chemica plantarum', in the 'flora fribergensis subterranea', , p. . among these we may reckon many of the marine algae -- cladoniae and mosses, which extend over the desert steppes of northern asia -- grasses, and cacti growing p together like the pipes of an organ -- avicennim and mangroves in the tropics -- and forests of coniferae and of birches in the plains of the baltic and in siberia. this mode of geographical distribution determines, together with the individual form of the vegetable world, the size and type of leaves and flowers, in fact, the principal physiognomy of the district,* its characteracter being but little, if at all, influenced by the ever-moving forms of animal life, which, by their beauty and diversity, so powerfully affect the feelings of man, whether by exciting the sensations of admiration or horror. [footnote] *on the physiognomy of plants, see humboldt, 'anischten der natur', bd. ii., s. - . agricultural nations increase artificially the predominance of social plants, and thus augment, in many parts of the temperate and northern zones, the natural aspect of uniformity; and while their labors tend to the extirpation of some wild plants, they likewise lead to the cultivation of others, which follow the colonist in his most distant migration. the luxuriant zone of the tropics offers the strongest resistance to these changes in the natural distribution of vegetable forms. observers who in short periods of time have passed over vast tracts of land, and ascended lofty mountains, in which climates were ranged, as it were in strata one above another, must have been early impressed by the regularity with which vegetable forms are distributed. the results yielded by their observations furnished the rough materials for a science, to which no name had as yet been given. the same zones of regions of vegetation which, in the sixteenth century, cardinal bembo, when a youth,*described on the declivity of aetna, were observed on mount ararat by tournefort. [footnote] *aetna dialogus.' 'opuscula', basil., , p. , . a very beautiful geography of the plants of mount aetna has recently been published by philippi. see 'linnaea', , s. . he ingeniously compared the alpine flora with the flora of plains situated in different latitudes, and was the first to observe the influence exercised in mountainous regions, on the distribution of plants by the elevation of the ground above the level of the sea, and by the distance from the poles in flat countries. menzel, in an inedited work on the flora of japan, accidentally made use of the term 'geography of plants'; and the same expression occurs in the fanciful but graceful work of bernardin de st. pierre, 'etudes de la nature'. a scientific treatment of the subject began, however, only when the geography of plants was intimately associated with the study of the distribution p of heat over the surface of the earth, and when the arrangement of vegetable forms in natural families admitted of a numerical estimate being made of the different forms which increase of decrease as we recede from the equator toward the poles, and of the relations in which, in diffrent parts of the earth, each family stood with reference to the whole mass of phanerogamic indigenous plants of the same region. i consider it a happy circumstance that, at the time during which i devoted my attention almost exclusively to botanical pursuits, i was led by the aspect of the grand and strongly characterized features of tropical scenery to direct my investigations toward these subjects. the study of the geographical distribution of animals, regarding which buffon first advanced general, and, in most instances, very correct views, has been considerably aided in its advance by the progress made in modern times in the geography of plants. the curves of the isothermal lines, and more especially those of the isochimenal lines, correspond with the limits which are seldom passed by certain species of plants, and of animals which do not wander far from their fixed habitation either with respect to elevation or latitude.* [footnote] *[the following valuable remarks by professor forbes, on the correspondence existing between the distribution of existing faunas and floras of the british islands, and the geological changes that have affected their area, will be read with much interest; they have been copied, by the author's permission, from the 'survey report', p. : "if the view i have put forward respecting the origin of the flora of the british mountains be true -- and every geological and botanical probability, so far as the are is concerned, favors it -- then must we endeavour to find some more plausible cause than any yet shown for the presence of numerous species of plants, and of some animals, on the higher parts of alpine ranges in europe and asia, specifically identical with animals and plants indigenous in the regions very far north, and not found in the intermediate lowlands. tournefort first remarked and humboldt, the great organizer of the science of natural history geography, demonstrated, that zones of elevation on mountains correspond to parallels of latitude, the higher with the more northern or southern, as the case might be. it is well known that this correspondence is recognized in the general 'facies' of the flora and fauna, dependent on generic identities. but when announcing and illustrating the law that climatal zones of animal and vegetable life are mutually repeated or represented by elevation and latitude, naturalists have not hitherto sufficiently (if at all) distinguished between the evidence of that law, as exhibited by 'representative species' and by 'identical'. in reality, the former essentially depend on the law, the latter being an 'accident' not necessarily dependent upon it, and which has hitherto not been accounted for. in the case of the alpine flora of britain, the evidence of the activity of the law, and the influence of the accident, are inseparable, the law being maintained by a transported flora, for the transmission of which i have shown we can not account by an appeal to unquestionable geological events. in the case of the alps and carpathians, and some other mountain ranges, we find the law maintained partly by a representative flora, special in its region, i.e., by specific centers of their own, and partly by an assemblage more or less limited in the several ranges of identical species, these latter in several cases so numerous that ordinary modes of transportation now in action can no more account for their presence than they can for the presence of a norwegian flora on the british mountains. now i am prepared to maintain that the same means which introduced a sub-arctic (now mmountain) flora into britain, acting at the same epoch, originated the identity, as far as it goes, of the alpine floras of middle europe and central asia; for, now that we know the vast area swept by the glacial sea, including almost the whole of central and northern europe, and belted by land, since greatly uplifted, which then presented to the water's edge those climatal lconditions for which a sub-arctic flora -- destined to become alpine -- was specially organized, the difficulty of deriving such a flora from its paarent north, and of diffusing it over the snowy hills bounding this glacial ocean, vanishes, and the presence of identical species at such distant pooints remain no longer a mystery. moreover, when we consider that conditions during the epoch referred to, the undoubted evidences of continental observers, on the boounds of asia by sir roderick murchison, in america by mr. lyell, mr. logan, captain bayfield, and others, and that the botanical (and zoological as well) region, essentially northern and alpine, designated by professor schouw that 'of saxifrages and mosses,' and first in his classification, exists now only on the flanks of the great area which suffered such conditions; and that, though similar conditions reappear, the relationship of alpine and arctic vegetation in the southern hemisphere, with that in the northern, is entirely maintained by 'representative', and not by identical species (the general truth of my explanation of alpine floras, including identical species, becomes so strong, that the view proposed acquires fair claims to be ranked as a theory, and not considered merely a convenient or bold hypothesis."] -- tr. the p elk, for instance, lives in the scandinavian peninsula, almost ten degrees further north than in the interior of siberia, where the line of equal winter temperature is so remarkably concave. plants migrate in the germ; and, in the case of many species, the seeds are furnished with organs adapting them to be conveyed to a distace through the air. when once they have taken root, they become dependent on the soil and on the strata of air surrounding them. animals, on the contrary, can at pleasure migrate from the equator toward the poles; and this they can more especially doo where the isothermal lines are much inflected, and where hot summers succeed a great degree of winter cold. the royal tiger, which in no respect differs from the bengal species, penetrates every summer into p the north of asia as far as the latitudes of berlin and hamburg, a fact of which ehrenberg and myself have spoken in other works.* [footnote] *ehrenberg, in the 'annales des sciences naturelles', t. xxi., p. , ; humboldt, 'asie centrale', t. i., p. - , and t. iii., p. - . the grouping or association of diffrent vegetable species, to which we are accustomed to apply the term 'floras', do not appear to me, from what i have observed in different portions of the earth's surface, to manifest such a predominance of individual families as to justify us in marking the geographical distinctions between the regions of the umbellatae, of the solidaginae, of the labiatae, or the scitamineae. with reference to this subject, my views differ from those of several of my friends, who rank among the most distinguished of the botanists of germany. the character of the floras of the elevated plateaux of mexico, new granada, and quito, of european russia, and of northern asia, consists, in my opinion, not so much in the relatively larger number of the species presented by one or two natural families, as in the more complicated relations of the coexistence of many families, and in the relative numerical value of their species. the gramineae and the cyperaceae undoubtedly predominate in meadow lands and stppes, as do coniferae, cupuliferae, and betulineae in our northern woods; but this predominance of certain forms is only apparent, and owing to the aspect imparted by the social plants. the north of europe, and that portion of siberia which is situated to the north of the altai mountains, have no greater right to the appellation of a region of gramineae and coniferae than have the boundless llanos between the orinoco and the mountain chain of caraccas, or the pine forests of mexico. it is the coexistence of forms which may partially replace each other, and their relative numbers and association, which give rise either to the general impression of luxuriance and diversity, or of poverty and uniformity in the contemplation of the vegetable world. in this fragmentary sketch of the phenomena of organization, i have ascended from the simplest celli -- the first manifestation of life -- progressively to higher structures. "the p association of mucous granules constitutes a definitely-formed cytoblase, around which a vesicular membrane forms ia closed well," this cell being either produced from another pre-existing cell,** or being due to a cellular formation, which, as in the case of the fermentation-fungus, is concealed in the obscurity of some unknown chemical process.*** [footnote] *schleiden, 'ueber die entwicklungsweise der pflanzenzellen', in muller's 'archiv fur anatomie und physiologie', , s. - ; also his 'grundzuge der wissenschaftlichen botanik', th. i., s. , and th. ii., s . schwann, 'mikroscopische untersucungen uber die uebereinstimmung in der struktur und dem wachsthum der thiere und pflanzen', , s. , . compare also, on similar propagation, joh. muller 'physiologie des menschen', , th. ii., s. . [footnote] **schleiden, 'grundzuge der wissenschaftlichen botanik', , th. i., s. - . [footnote] ***[on cellular formation, see henfrey's 'outlines of structural and physiological botany', op. cit., p. - .] -- tr. but in a work like the present we can venture on no more than an allusion to the mysteries that involve the question of modes of origin; the geography of animal and vegetable organisms must limit itself to the consideration of germs already developed, of their haabitation and transplantation, either by voluntary or involuntary migrations, their numerical relation, and their distribution over the surface of the earth. the general picture of nature which i have endeavored to delineate would be incomplete if i did not venture to trace a few of the most marked features of the human race, considered with reference to physical gradations -- to the geographical distribution of contemporaneous types -- to the influence exercised upon man by the forces of nature, and the reciprocal, although weaker action which he in his turn exercises on these natural forces. dependent, although in a lesser degree than plants and animals, on the soil, and on the meteorological processes of the atmosphere with which he is surroounded -- escaping more readily from the control of natural forces, by activity of mind and the advance of intellectual cultivation, no less than by his wonderful capacity of adapting himself to all climates -- man every where becomes most essentially associated with terrestrial life. it is by these relations that the obscure and much-contested problem of the possibility of one common descent enters into the sphere embraced by a general physical cosmography. the investigation of this problem will impart a nobler, and, if i may so express myself, more purely human interest to the closing pages of this section of my work. the vast domain of language, in whose varied structure we see mysteriously reflected the destinies of nations, is most intimately associated with the affinity of races; and what even slight differences of races may effect is strikingly manifested in the history of the hellenic nations in the zenith of their intellectual cultivation. the most important questions of the civilization of mankind are connected with the ideas of races, p community of language, and adherence to one original direction of the intellectual and moral faculties. as long as attention was directed solely to the extremes in varieties of color and of form, and to the vividness of the first impression of the senses, the observer was naturally disposed to regard races rather as originally different species than as mere varieties. the permanence of certain types* in the midst of the most hostile influences, especially of climate, appeared to favor such a view, notwithstanding the shortness of the interval of time from which the historical evidence was derived. [footnote] *tacitus, in his speculations on the inhabitants of britain ('agricola', cap. ii.), distinguishes with much judgment between that which may be owing to the local climatic relations, and that which, in the immigrating races, may be owing to the unchangeable influence of a hereditary and transmitted type. "britanniam qui mortales initio coluerunt, indigenae an advecti, ut inter barbaros, parum coompertum. habitus corporis varii, alque ex eo argumenta; namque rutilae caledoniam habitantium comae, magni artus germanicam originem adseverant. silu ram colorati vultus et torti plerumque crines, et posita contra hispania, iberos veteres trajecisse, easque cedes occupasse fidem faciunt: proximi gallis, et similes sunt: seu durante originis vi; seu procurrentibus in diversa terris, positio coeli corporibus habitum dedit." regarding the persistency of types of conformation in the hot and cold regions of the earth, and in the mountainous districts of the new continent, see my 'relation historique', t. i., p. , , and t. ii., p. , . in my opinion, however, more powerful reasons can be advanced in support of the theory of the unity of the human race, as, for instance, in the many intermediate gradations* in the color of the skin and in the form of the skull, which have been made known to us in recent times by the rapid progress of geographical knowledge -- the analogies presented by the varieties in the species of many wild and domesticated animals -- and the more correct observations collected regarding the limits of fecundity in hybrids.** [footnote] on the american races generally, see the magnificent work of samuel george morton, entitled 'crania americana', , p. , ; and on the skulls brought by pentland from the highlands ot titicaca, see the 'dublin journal of medical and chemical science', vol. v., , p. ; also alcide d'orbigny, 'l'homme americain considere sous ses rapports physiol. et mor.', , p. ; and the work by prince maximilian of wied, which is well worthy of notice for the admirable ethnographical remarks in which it abounds, entitled 'reise in das innere von nordamerika' ( ). [footnote] ** rudolph wagner, 'ueber blendlinge und bastarderzeugung', in his notes to the german translation of prichard's 'physical history of mankind', vol. i., p. - . the greater number of the contrasts which were formerly supposed to exist, have disappeared before the laborious researches of tiedemann on the brain of negroes and of europeans, and the anatomical investigations p of vrolik and weber on the form of the pelvis. on comparing the dark-colored african nations, on whose physical history the admirable work of prichard has thrown so much light, with the races inhabiting the islands of the south-indian and west-australian archipelago, and with the papuas and alfourous (haroforas, endamenes), we see that a black skin, woolly hair, and a negro-like cast of countenance are not necessarily connected together.* [footnote] *prichard, op. cit., vol. ii., p. . so long as only a small portion of the earth was known to the western nations, partial views necessarily predominated, and tropical heat and a black skin consequently appeared inseparable. "the ethiopians," said the ancient tragic poet theodectes of phaselis,* "are colored by the near sun-god in his course with a sooty luster, and their hair is dried and crisped with the heat of his rays." [footnote] *onesicritus, in strabo, xv., p. , , casaub. welcker, 'griechische tragodien', abth. iii., s. , conjectures that the verses of theodectes, cited by strabo, are taken from a list tragedy, which probably bore the title of "memnon." the campaigns of alexander, which gave rise to so many new ideas regarding physical geography, likewise first excited a discussion on the problematical influence of climate on races. "families of animals and plants," writes one of the greatest anatomists of the day, johannes muller, in his noble and comprehensive work, 'physiologie des menschen', "undergo, within certain limitations peculiar to the different races and species, various modifications in their distribution over the surface of the earth, propagating these variations as organic types of species.* [footnote] *[in illustration of this, the conclusions of professor edward forbes respecting the origin and diffusion of the british flora may be cited. see the 'survey memoir' already quoted, 'on the connection between the distribution of the existing fauna and flora of the british islands, etc.', p. . " . the flora and fauna, terrestrial and marine, of the british islands and seas, have originated, so far as that area is concerned, since the melocene epoch. . the assemblages of animals and plants compositing that fauna and flora did not appear in the area they now inhabit simultaneously, but at several distinct points in time. . both the fauna and flora of the british islands and seas are composed partly of species which, either permanently or for a time, appeared in that area before the glacial epoch; partly of such as inhabited it during that epoch; and in great part of those which did not appear there until afterward, and whose appearance on the earth was coeval with the elevation of the bed of the glacial sea and the consequent climatal changes. . the greater part of the terrestrial animals and flowering plants now inhabiting the british islands are members of specific centers beyond their area, and have migrated to it over continuous land before, during, or after the glacial epoch. . the climatal conditions of the area under discussion, and north, east, and west of it, were severer during the glacial epoch, when a great part of the space now occupied by the british isles was under water, than they are now or were before; but there is good reason to believe that, so far from those conditions having continued severe, or having gradually diminished in severity southward of britain, the cold region of the glacial epoch came directly into contact with a region of more southern and thermal character than that in which the most southern beds of glacial drift are now to be met with. . this state of things did not materially differ from that now existing, under corresponding latitudes, in the north american, atlantic, and arctic seas, and on their bounding shores. . the alpine floras of europe and asia, so far as they are identical with the flora of the arctic and sub-arctic zones of the old world, are fragments of a flora which was diffused from the north, either by means of transport not now in action on the temperate coasts of europe, or over continuous land which no longer exists. the deep sea fauna is in like manner a fragment of the general glacial fauna. . the floras of the islands of the atlantic region, between the gulf-weed bank and the old world, are fragments of the great mediterranean flora, anciently diffused over a land consistuted out of the upheaval and never again subjerged bed of the (shallow) meiocene sea. this great flora, in the epoch anterior to, and probably, in part, during the glacial period, had a greater extension northward than it now presents. . the termination of the glacial epoch in europe was marked by a recession of an arctic fauna and flora northward, and of a fauna and flora of the mediterranean type southward; and in the interspace thus produced there appeared on land the germanic fauna and flora, and in the sea that fauna termed celtic. . the causes which thus preceded the appearance of a new assemblage of organized beings were the destruction of many species of animals, and probably also of plants, either forms of extremely local distribution, or such as were not capable of enduring many changes of conditions -- species, in short, with very limited capacity for horizontal or vertical diffusion. . all the changes before, during, and after the glacial epoch appear to have been gradual, and not sudden, so that no marked line of demarkation can be drawn between the creatures inhabiting the same element and the same locality during two proximate periods."] -- tr. the different races of mankind are forms of one sole species, by the union of two of whose members descendants are propagated. they are not different species of a genus, since in that case their hybrid descendants would remain unfruitful. but whether the human races have descended from several primitive races of men, or from one alone, is a question that can not be determined from experience."* [footnote] *joh. muller, 'physiologie des menschen', bd. ii., s. . geographical investigations regarding the ancient 'seat', the so-called 'cradle of the human race', are not devoid of a mythical p character. "we do not know," says wilhelm von humboldt, in an unpublished work 'on the varieties of languages and nations', "either from history or from authentic tradition, any period of time in which the human race has not been divided into social groups. whether the gregarious condition was original, or of subsequent occurrence, we have no historic evidence to show. the separate mythical relations found to exist independently of one another in different parts of the earth, appear to refute the first hypothesis, and concur in ascribing the generation of the whole human race to the union of one pair. the general prevalence of this myth has cause it to be regarded as a traditionary record transmitted from the primitive man to his descendants. but this very circumstance seems rather to prove that it has no historical foundation, but has simply arisen from an identity in the mode of intellectual conception, which has every where led man to adopt the same conclusion regarding identical phenomena; in the same manner as many myths have doubtlessly arisen, not from any historical connection existing between them, but rather from an identity in human thought and imagination. another evidence in favor of the purely mythical nature of this belief is afforded by the fact that the first origin of mankind -- a phenomenon which is wholly beyond the sphere of experience -- is explained in perfect conformity with existing views, being considered on the principle of the colonization of some desert island or remote mountainous valley at a period when mankind had already existed for thousands of years. it is in vain that we direct our thoughts to the solution of the great problem of the first origin, since man is too intimately associated with his own race and with the relations of time to conceive of the existence of an individual independently of a preceding generation and age. a solution of those difficult questions, which can not be determined by inductive reasoning or by experience -- whether the belief in this presumed traditional condition be actually based on historical evidence, or whether mankind inhabited the earth in gregarious associations from the origin of the race -- can not, therefore, be determined from philological data, and yet its elucidation ought not to be sought from other sources." the distribution of mankind is therefore only a distribution into 'varieties', which are commonly designated by the somewhat indefinite term 'races'. as in the vegetable kingdom, and in the natural history of birds and fishes, a classification into many small families is based on a surer foundation than p where large sections are separated into a few but large divisions; so it also appears to me, that in the determination of races a preference should be given to the establishment of small families of nations. whether we adopt the old classification of my master, blumenbach, and admit 'five' races (the caucasian, mongolian, american, ethiopian, and malayan), or that of prichard, into 'seven races'* (the iranian, turanian, american, hottentots and bushmen, negroes, papuas, and alfourons), we fail to recognize any typical sharpness of definition, or any general or well-established principle in the division of these groups. [footnote] *prichard, op. cit., vol. i., p. . the extremes of form and color are certainly separated, but without regard to the races, which can not be included in any of these classes, and which have been alternately termed scythian and allophyllic. iranian is certainly a less objectionable term for the european nations than caucasian; but it may be maintained generally that geographical denominations are very vague when used to express the points of departure of races, more especially where the country which has given its name to the race, as, for instance, turan (mawerannahr), has been inhabited at different periods* by indo-germanic and finnish, and not by mongolian tribes. [footnote] *the late arrival of the turkish and mongolian tribes on the oxus and on the kirghis steppes is opposed to the hypothesis of niebuhr, according to which the scythians of herodotus and hippocrates were mongolians. it seems far more probable that the scythians (scoloti) should be referred to the indo-germanic massagetae (alani). the mongolian, true tartars (the latter term was afterward falsely given to purely turkish tribes in russia and siberia), were settled, at that period, far in the eastern part of asia. see my 'asie centrale', t. i., p. , ; 'examen critique de l'histoire de la geogr.', th. ii., p. . a distinguished philologist, professor buschmann, calls attention to the circumstance that the poet firdousi, in his half-mythical prefatory remarks in the 'schahnameh', mentions "a fortress of the alani" on the sea-shore, in which selm took refuge, this prince being the eldest son of the king feridun, who in all probability lived two hundred years before cyrus. the kirghis of the scythian steppe were originally a finnish tribe; their three hordes probably constitute in the present day the most numerous nomadic nation, and their tribe dwelt, in the sixteenth century, in the same steppe in which i have myself seen them. the byzantine menander (p. - , ed. nieb.) expressly states that the chacan of the turks (thu-khiu), in , made a present of a kirghis slave to zemarchus, the embassador of ustinish ii.; he terms her a [greek word]; and we find in abulgasi ('historia mongolorum et tatarorum') that the kirghis are called kirkiz. similarity of manners, where the nature of the country determines the principal characteristics, is a very uncertain evidence of identity of race. the life of the steppes produces among the turks (ti tukiu), the baschkirs (fins), the kirghis, the torgodi and dsungari (mongolians), the same habits of nomadic life, and the same use of felt tents, carried on wagons and pitched among herds of cattle. p languages, as intellectual creations of man, and as closely interwoven with the development of mind, are, independently of the 'national' form which they exhibit, of the greatest importance in the recognition of similarities or differences in races. this importance is especially owing to the clew which a community of descent affords in treading that mysterious labyrinth in which the connection of physical powers and intellectual forces manifests itself in a thousand different forms. the brilliant progress made within the last half century, in germany, in philosophical philology, has greatly facilitated our investigations into the 'national' character* of languages and the influence exercised by descent. [footnote] *wilhelm von humboldt, 'ueber die verschiedenheit der menschlichen sprachbaues', in his great work 'ueber die kawi-sprache auf der insel java', bd. i., s. xxi., xlviii., and ccxiv. but here, as in all domains of ideal speculation, the dangers of deception are closely linked to the rich and certain profit to be derived. positive ethnographical studies, based on a thorough knowledge of history, teach us that much caution should be applied in entering into these comparisons of nations, and of the languages employed by them at certain epochs. subjection, long association, the influence of a foreign religion, the blending of races, even when only including a small number of the more influential and cultivated of the immigrating tribes, have produced, in both continents, similarly recurring phenomena; as, for instance, in introducing totally different families of languages among one and the same race, and idioms, having one common root, among nations of the most different origin. great asiatic conquerors have exercised the most powerful influence on phenomena of this kind. but language is a part and parcel of the history of the development of mind; and however happily the human intellect, under the most dissimilar physical conditions, may unfettered pursue a self-chosen track, and strive to free itself from the dominion of terrestrial influences, this emancipation is never perfect. there ever remains, in the natural capacities of the mind, a trace of something that has been derived from the influences of race or of climate, whether they be associated with a land gladdened by cloudless azure skies, or with the vapory atmosphere of an insular region. as, therefore, richness and grace of language are unfolded from the most luxuriant p depths of thought, we have been unwilling wholly to disregard the bond which so closely links together the physical world with the sphere of intellect and of the feelings by depriving this general picture of nature of those brighter lights and tints which may be borrowed from considerations, however slightly indicated, of the relations existing between races and languages. while we maintain the unity of the human species, we at the same time repel the depressing assumption of superior and inferior races of men.* [footnote] *the very cheerless, and, in recent times, too often discussed doctrine of the unequal rights of men to freedom, and of slavery as an institution in conformity with nature, is unhappily found most systematically developed in aristotle's 'politica', i., , , . there are nations more susceptible of cultivation, more highly civilized, more enobled by mental cultivation than others, but none in themselves nobler than others. all are in like degree designed for freedom; a freedom which, in the ruder conditions of society, belongs only to the individual, but which, in social states enjoying political institutions, appertains as a right to the whole body of the community. "if we would indicate an idea which, throughout the whole course of history, has ever more and more widely extended its empire, or which, more than any other, testifies to the much-contested and still more decidedly misunderstood perfectibility of the whole human race, it is that of establishing our common humanity -- of striving to remove the barriers which prejudice and limited views of every kind have erected among men, and to treat all mankind, without reference to religion, nation, or color, as one fraternity, one great community, fitted for the attainment of one object, the unrestrained development of the physical powers. this is the ultimate and highest aim of society, identical with the direction implanted by nature in the mind of man toward the indefinite extension of his existence. he regards the earth in all its limits, and the heavens as far as his eye can scan their bright and starry depths, as inwardly his own, given to him as the objects of his contemplation, and as a field for the development of his energies. even the child longs to pass the hills or the seas which inclose his narrow home; yet, when his eager steps have borne him beyond those limits, he pines, like the plant, for his native soil; and it is by this touching and beautiful attribute of man -- this longing for that which is unknown, and this fond remembrance of that which is lost -- that he is spared from an exclusive attachment to the present. p thus deeply rooted in the innermost nature of man, and even enjoined upon him by his highest tendencies, the recognition of the bond of humanity becomes one of the noblest leading principles in the history of mankind."* [footnote] *wilhelm von humboldt, 'ueber die kawi-sprache', bd. iii., s. . i subjoin the following extract from this work: "the impetuous conquests of alexander, the more politic and premeditated extension of territory made by the romans, the wild and cruel incursions of the mexicans, and the despotic acquisitions of the incas, have in both hemispheres contributed to put an end to the separate existence of many tribes as independent nations, and tended at the same time to establish more extended international amalgamation. men of great and strong minds, as well as whole nations, acted under the influence of one idea, the purity of which was, however, utterly unknown to them. it was christianity which first promulgated the truth of its exalted charity, although the seed sown yielded but a slow and scanty harvest. before the religion of christ manifested its form, its existence was only revealed by a faint foreshadowing presentiment. in recent times, the idea of civilization has acquired additional intensity, and has given rise to a desire of extending more widely the relations of national intercourse and of intellectual cultivation; even selfishness begins to learn that by such a course its interests will be better served than by violent and forced isolation. language more than any other attribute of mankind, binds together the whole human race. by its idiomatic properties it certainly seems to separate nations, but the reciprocal understanding of foreign languages connects men together on the other hand without injuring individual national characteristics." with these words, which draw their charm from the depths of feeling, let a brother be permitted to close this general description of the natural phenomena of the universe. from the remotest nebulae and from the revolving double stars, we have descended to the minutest organisms of animal creation, whether manifested in the depths of ocean or on the surface of our globe, and to the delicate vegetable germs which clothe the naked declivity of the ice-crowned mountain summit; and here we have been able to arrange these phenomena according to partially known laws; but other laws of a more mysterious nature rule the higher spheres of the organic world, in which is comprised the human species in all its varied conformation, its creative intellectual power, and the languages to which it has given existence. a physical delineation of nature terminates at the point where the sphere of intellect begins, and a new world of mind is opened to our view. it marks the limit, but does not pass it. p is blank p additional notes to the present edition. march, . __________ gigantic birds of new zealand. -- vol. i., p. . an extensive and highly interesting collection of bones, referrible to several species of the 'moa' (dinornis of owen), and to three or four other genera of birds, formed by mr. walter mantell, of wellington, new zealand, has recently arrived in england, and is now deposited in the british museum. this series consists of between and speciments, belonging to different parts of the skeletons of many individuals of various sizes and ages. some of the largest vertebrae, tibiae, and femora equal in magnitude the most gigantic previously known, while others are not larger than the corresponding bones of the living apteryx. among these relics are the 'skulls' and 'mandibles' of two genera, the 'dinornis' and 'palapteryx'; and of an extinct genus, 'notornis', allied to the 'rallidae'; and the mandibles of a species of 'nestor', a genus of nocturnal owl-like parrots, of which only two living species are known.* [footnote] *see professor owen's memoir on these fossil remains, in 'zoological transactions', . these osseous remains are in a very different state of preservation from any previously received from new zealand; they are light and porous, and of a light fawn-color; the most delicate processes are entire, and the articulating surfaces smooth and uninjured; 'fragments of egg-shells', and even the bony rings of the trachea and air tubes, are preserved'. the bones were dug up by mr. walter mantell from a bed of marly sand, containing magnetic iron, crystals of hornblende and augite, and the detritus of augitic rocks and earthy volcanic tuff. the sand had filled up all the cavities and cancelli, but was in no instance consolidated or aggregated together; it was, therefore, easily removed by a soft brush, and the bones perfectly cleared without injury. the spot whence these precious relics of the colossal birds that once inhabited the islands of new zealand were obtained, is a flat tract of land, near the embouchure of a river, named waingongoro, not far from wanganui, which has its rise in the volcanic regions of mount egmont. the natives affirm that this level tract was one of the places first dwelt upon by their remote ancestors; and this tradition is corroborated by the existence of numerous heaps and pits of ashes and charred bones indicating ancient fires, long burning on the same spot. in these fire-heaps mr. mantell found burned bones of 'men, moas', and 'dogs'. the fragments of egg-shells, imbedded in the ossiferous deposits, had escaped the notice of all previous naturalists. they are, unfortunately, very small portions, the largest being only four inches long, but they afford a chord by which to estimate the size of the original. mr. mantell observes that the egg of the moa must have been so large that a hat would form a good egg-cup for it. these relics evidently belong to two or more species, perhaps genera. in some examples the external p surface is smooth; in others it is marked with short intercepted linear grooves, resembling the eggs of some of the struthiouidae, but distinct from all known recent types. in this valuable collection only one bone of a mammal has been detected, namely, 'the femur of a dog'. an interesting memoir on the probable geological position and age of the ornithic bone deposits of new zealand, by dr. mantell, based on the observations of his enterprising son, it published in the quarterly journal of the geological society of london ( ). it appears that in many instances the bones are imbedded in sand and clay, which lie beneath a thick deposit of volcanic detritus, and rest on an argillaceous stratum abounding in marine shells. the specimens found in the rivers and streams have been washed out of their banks by the currents which now flow through channels from ten to thirty feet deep, formed in the more ancient alluvial soil. dr. mantell concludes that the islands of new zealand were densely peopled at a period geologically recent, though historically remote, by tribes of gigantic brevi-pennate birds allied to the ostrich tribe, all, or almost all, of species and genera now extinct; and that, subsequently to the formation of the most ancient ornithic deposit, the sea-coast has been elevated from fifty to one hundred feet above its original level; hence the terraces of shingle and loam which now skirt the maritime districts. the existing rivers and mountain torrents flow in deep gulleys which they have eroded in the course of centuries in these pleistocene strata, in like manner as the river courses of auvergne, in central france, are excavated in the mammiferous tertiary deposits of that country. the last of the gigantic birds were probably exterminated, like the dodo, by human agency: some small species allied to the apteryx may possibly be met with in the unexplored parts of the middle island. the dodo. -- a most valuable and highly interesting history of the dodo and its kindred* has recently appeared in which the history, affinities, and osteology of the 'dodo, solitaire', and other extinct birds of the islands mauritius, rodriguez, and bourbon are admirably elucidated by h. g. strickland (of oxford), and dr. g. a. melville. [footnote] *'the dodo and its kindred'. by messrs. strickland and melville. vol. to. with numerous plates. reeves, london, . the historical part is by the former, the osteological and physiological portion by the latter eminent anatomist. we would earnestly recommend the reader interested in the most perfect history that has ever appeared, of the extinction of a race of large animals, of which thousands existed but three centuries ago, to refer to the original work. we have only space enough to state that the authors have proved, upon the most incontrovertible evidence, that the dodo was neither a vulture, ostrich, nor galline, as previously anatomists supposed, but a 'frugiverous pigeon'. this section from pp - of: cosmos: a sketch of the physical description of the universe, vol. by alexander von humboldt translated by e c otte from the harper & brothers edition of cosmos, volume -------------------------------------------------- p index to vol. i. ------------------- abich, hermana, structural relations of volcanic rocks, . acosta, joseph de, historia natural de las indias, , . adams, mr., planet neptune. see note by translator, , . aegos potamos, on the aerolite of, , . aelian on mount aetna, . aerolites (shooting stars, meteors, meteoric stones, fire-balls, etc), general description of, - ; physical character, - ; dates of remarkable falls, , ; their planetary velocity, - ; ideas of the ancients on, , ; november and august periodic falls of shooting stars, - , - ; their direction from one point in the heavens, ; altitude, ; orbit, ; chinese notices of, ; media of communication with other planetary bodies, ; their essential difference from comets, ; specific weights, , ; large meteoric stones on record, ; chemical elements, , - ; crust, , ; deaths occasioned by, . aeschylus, "prometheus delivered," . aetna, mount, its elevation, , ; supposed extinction by the ancients, ; its eruptions from lateral fissures, ; similarity of its zones of vegetation to those of ararat, . agassiz, researches on fossil fishes, , - . alexander, influence of his campaigns on physical science, . alps, the, elevation of, , . amber, researches on its vegetable origin, ; goppert on the amber-tree of the ancient world (pinites succifer), . ampere, andre marie, , , . anaxagoras on aerolites, ; on the surrounding ether, . andes, the, their altitude, etc. see cordilleras. anghiera, peter martyr de, remarked that the palmeta and pineta were found associated together, , ; first recognized ( ) that the limit of perpetual snow continues to ascend as we approach the equator, . animal life, its universality, - ; as viewed with microscopic powers of vision, - ; rapid propagation and tenacity of life in animalcules, - ; geography of, - . anning, miss mary, discovery of the ink bag of the sepia, and of coprolites of fish, in the lias of lyme regis, , . austed's, d. r., "ancient world." see notes by translator, , , , , . aplan, peter, on comets, . apollonius myndius, described the paths of comets, . arago, his ocular micrometer, ; chromatic polarization, ; optical considerations, ; on comets, - ; polarization experiments on the light of comets, ; aerolites, ; on the november fall of meteors, ; zodiacal light, ; motion of the solar system, , ; on the increase of heat at increasing depths, , ; magnetism of rotation, , ; horary observations of declination at paris compared with simultaneous perturbations at kasan, ; discovery of the influence of magnetic storms on the course of the needle, , ; on south polar bands, ; on terrestrial light, ; phenomenon of supplementary rainbows, ; observed the deepest artesian wells to be the warmest, ; explanation of the absence of a refrigeration of temperature in the lower strata of the mediterranean, ; observations on the mean annual quantity of rain in paris, ; his investigations on the evolution of lightning, . argelander on the comet of , ; on the motion of the solar system, , ; on the light of the aurora, , . aristarchus of samos, the pioneer of the copernican system, . aristotle, ; his definition of cosmos, ; use of the term history, ; on comets, , ; on the ligyan field of stones, ; aerolites, ; on the stone of aegos potamos, ; aware that noises sometimes existed without earthquakes, ; his account of the upheavals of islands of eruption, ; "spontaneous motion," ; noticed the redness assumed by long fallen snow, . artesian wells, temperature of, , . astronomy, results of, - ; phenomena of physical astronomy, , . atmosphere, the general description of, , ; its composition and admixture, ; variation of pressure, - ; climatic distribution of heat, , - ; distribution of humidity, , , ; electric condition, , - . p august, his psychometer, . augustine, st., his views on spontaneous generation, , . aurora borealis, general description of - ; origin and course, , ; altitude, ; brilliancy coincident with the fall of shooting stars, , ; whether attended with crackling sound, , ; intensity of the light, . bacon, lord, , ; novum organon, . baer, von, . barometer, the increase of its height attended by a depression of the level of the sea, ; horary oscillations of, , batten, mr., letter on the snow-line of the two sides of the himalayas, , . beaufort, capt., observed the emissions of inflammable gas on the caramanian coast, as described by pliny, . see also, note by translator, . beaumont, elie de, on the uplifting of mountain chains, , ; influence of the rocks of melaphyre and serpentine, on pendulum experiments, ; conjectures on the quartz strata of the col de la poissoniere, . baccaria, observation of steady luminous appearance in the clouds, ; of lightning clouds, unaccompanied by thunder or indication of storm, . beechey, capt., ; observations on the temperature and density of the water of the ocean under different zones of longitude and latitude, . bembo, cardinal, his observations on the eruptions of mount aetna, ; theory of the necessity of the proximity of volcanoes to the sea, ; vegetation on the declivity of aetna, . berard, capt., shooting stars, . berton, count, his barometrical measurements of the dead sea, . berzelins on the chemical elements of aerolites, , . benzenberg on meteors and shooting stars, , ; their periodic return in autgust, . bessel's theory on the oscillations of the pendulum, ; pendulum experiments, ; on the parallax of cygni, ; on halley's comet, , , ; on the ascent of shooting stars, ; on their partial visibility, ; velocity of the sun's translatory motion, ; mass of the star cygni, ; parallaxes and distances of fixed stars, ; comparison of measurements of degrees, , . biot on the phenomenon of twilight, ; on the zodical light, ; pendulum experiments at bordeaux, . biot, edward, chinese observations of comets, , ; of aerolites, . bischof on the interior heat of the globe, , , , , . blumenbach, his classification of the races of men, . bockh, origin of the ancient myth of the nemean lunar lion, , . boguslawski, falls of shooting stars, , . bonpland, m., and humboldt, on the pelagic shells found on the ridge of the andes, . boussingault, on the depth at which is found the mean annual temperature within the tropics, ; on the volcanoes of new granada, ; on the temperature of the earth in the tropics, , ; temperature of the thermal springs of las trincheras, ; his investigations on the chemical analysis of the atmosphere, , ; on the mean annual quantity of rain in different parts of south america, , . bouvard, m., ; his observations on that portion of the horary oscillations of the pressure of the atmosphere, which depends on the attraction of the moon . bramidos y truenos of guanaxuato, , . brandes, falls of shooting stars, , ; height and velocity of shooting stars, ; their periodic falls, , . bravais, on the aurora, ; on the daily oscillations of the barometer in degrees north latitude, ; distribution of the quantity of rain in central europe, ; doubts on the greater dryness of mountain air, . brewster, sir david, first detected the connection between the curvature of magnetic lines and my isothermal lines, . brongniart, adolphe, luxuriance of the primitive vegetable world, ; fossil flora contained in coal measures, . brongniart, alexander, formation of ribbon jasper, ; one of the founders of the archaeology of organic life, . brown, robert, first discoverer of molecular motion, . buch's, leopold von, theory on the elevation of continents and mountain chains, ; on the craters and circular form of the island of palma, ; on volcanoes, , , , , ; on metamorphic rocks, - , , , ; on the origin of various conglomerates and rocks of detritus, ; classification of ammonites, , ; physical causes of the elevation of continents, ; on the changes in height of the swedish coasts, . buckland, ; on the fossil flora of the coal measures, . buffon, his views on the geographical distribution of animals, . burckhardt, on the volcano of medina, ; on the hornitos de jerullo, see note by translator, . burnes, sir alexander, on the purity of the atmosphere in bokhara, ; propagation of shocks of earthquakes, . p caile, la, pendulum measurements at the cape of good hope, . caldas, quantity of rain at santa fe de bogota, . camargo's ms. 'historia de tiascala', . capocci, his observations on periodic falls of aerolites, . carlini, geodesic experiments in lombardy, ; mount cenis, . carrara marble, , . carus, his definition of "nature," . caspian sea, its periodic rise and fall, . cassini, dominicus, on the zodiacal light, , ; hypothesis on ; his discovery of the spheroidal form of jupiter, . cautley, capt, and dr. falconer, discovery of gigantic fossils in the himalayas. cavanilles, first entertained the idea of seeing grass grow, . cavendish, use of the torsion balance to determine the mean density of the earth, . challis, professor, on the aurora, march and oct. th, , see note by translator, , . chardin, noticed in persia the famous comet of , called "nyzek" or "petite lance," . charpentier, m., belemnites found in the primitive limestone of the col de la seigne, ; glaciers, . chemistry as distinguished from physics, ; chemical affinity, . chevandier, calculations on the carbon contained in the trees of the forests of our temperate zones, . childrey first described the zodical light in his britannia baconica, . chinese accounts of comets, , , ; shooting stars, : "fire springs," ; knowledge of the magnetic needle, ; electro-magnetism, , . chladni on meteoric stones, etc., , ; on the selenic origin of aerolites, ; on the supposed phenomenon of ascending shooting stars, ; on the obscuration of the sun's disk, ; sound-figures, ; pulsations in the tails of comets, . choiseul, his chart of lemnos, . chromatic polarization. see polarization. cirro-cumulus cloud. see clouds. cirrous strata. see clouds. clark, his experiments on the variations of atmospheric electricity, , . clarke, j. g., of maine, u.s., on the comet of , . climatic distribution of heat, , - ; of humidity, , , . climatology, - ; climate, general sense of, , . clouds, their electric tension, color, and height, , ; connection of cirrous strata with the aurora borealis, ; cirro-cumulus cloud, phenomena of, ; luminous, ; dove on their formation and appearance, , ; often present on a bright summer sky the "projected image" of the soil below, ; volcanic, . coal formations, ancient vegetable remains in, , . coal mines, depth of, - . colebrooke on the snow-line of the two sides of the himalayas, . colladon, electro-magnetic apparatus, . columbus, his remark that "the earth is small and narrow," ; found the compass showed no variation in the azores, , ; of lava streams, ; noticed conifers and palms growing together in cuba, ; remarks in his journal on the equatorial currents, ; of the sargasso sea, ; his dream, , . comets, general description of, - ; biela's , , , ; blaupain's ; clausen's ; encke's, , , , - ; faye's , ; halley's, , , - ; lexell's and burchardt's , ; messier's ; olbera's, ; pons's ; famous one of , seen in persia, called "nyzek," or "petit lance," ; comet of , ; their nucleus and tail, , ; small mass, ; diversity of form, - ; light, - ; velocity, ; comets of short period, - ; long period, - ; number, ; chinese observations on, - ; value of a knowledge of their orbits, ; possibility of collision of blela's and encke's comets, , ; hypothesis of a resisting medium conjectured from the diminishing period of the revolution of encke's comet, ; apprehensions of their collision with the earth, , , ; their popular supposed influence on the vintage, . compass, early use of by the chinese, ; permanency in the west indies, . condamine, la, inscription on a marble tablet at the jesuit's college, quito on the use of the pendulum as a measure of seconds, , . conde, notice of a heavy shower of shooting stars, oct., , . coraboeuf and delcrois, geodetic operations, . cordilleras, scenery of, , , ; vegetation, , ; intensity of the zodiacal light, . cosmography, physical, its object and ultimate aims, - ; materials, . cosmos, the author's object, , ; primitive signification and precise definition of the word, ; how employed by greek and roman writers, , ; derivation, . craters. see volcanoes. curtius, professor, his notes on the temperature of various springs in greece, , . cuvier, one of the founders of the archaeology of organic life, ; discovery of fossil crocodiles in the tertiary formations, . dainachos on the phenomena attending the fall of the stone of aegos potamos, , . dalman on the existence of chionaea araneoides in polar snow, . dalton, observed the southern lights in england, . dante, quotation from, . darwin, charles, fossil vegetation in the travertine of van diemen's land, ; central volcanoes regarded as volcanic chains of small extent on parallel fissures, ; instructive materials in the temperate zones of the southern hemisphere for the study of the present and past geography of plants, , ; on the fiord formation at the southeast end of america, ; on the elevation and depression of the bottom of the south sea, ; rich luxuriance of animal life in the ocean, , ; on the volcano of aconcagua, . daubeney on volcanos. see translator's notes, , , , , , , , , , , , , , . daussy, his barometric expriments, ; observations on the velocity of the equatorial current, . davy, sir humphrey, hypothesis on active volcanic phenomena, ; on the low temperature of water on shoals, . dead sea, its depression below the level of the mediterranean, , . dechen, von, on the depth of the coal-basin of liege, . delcrois. see coraboeuf. descartes, his fragments of a contemplated work, entitled "monde," ; on comets, . deshayes and lyell, their investigations on the numerical relations of extinct and existing organic life, . dicaearchus, his "parallel of the diaphragm," . diogenes laertius, on the aerolite of aegos potamos, , , . d'orbigny, fossil remains from the himalaya and the indian plains of cutch, . dove on the similar action of the declination needle to the atmospheric electrometer, ; "law of rotation," ; on the formation and appearance of clouds, ; on the difference between the true temperature of the surface of the ground and the indications of a thermometer suspended in the shade, ; hygrometric windrose, . doyere, his beautiful experiments on the tenacity of life in animalcules, . drake, shaking of the earth for successive days in the united states ( - ), . dufrenoy et elie de beaumont, geologie de la france, , , , , , . dumas, results of his chemical analysis of the atmosphere, . dunlop on the comet of , . duperrey on the configuration of the magnetic equator, ; pendulum oscillations, . duprez, influence of trees on the intensity of electricity in the atmosphere, . eandi, vassalli, electric perturbation during the protracted earthquake of pignorol, . earth, survey of its crust, ; relative magnitude, etc., in the solar system, - ; general description of terrestrial phenomena, - ; geographical distribution, , ; its mean density, - ; internal heat and temperature, - ; electro-magnetic activity, - ; conjectures on its early high temperature, ; interior increase of heat with increasing depth, ; greatest depths reached by human labor, - ; methods employed to investigate the curvature of its surface, - ; reaction of the interior on the external crust, , - ; general delineation of its reaction, - ; fantastic views on its interior, . earthquakes, general account of, - ; their manifestations, - ; of riobamba, , , , , ; lisbon, , , , ; calabria, ; their propagation, , , ; waves of commotion, , , ; action on gaseous and aqueous springs, , , ; salses and mud volcanoes, - ; erroneous popular belief on, - ; noise accompanying earthquakes, - ; their vast destruction of life, , ; volcanic force, , ; deep and peculiar impression produced on men and animals, , . ehrenberg, his discovery of infusoria in the polishing slate of bilin, ; infusorial deposits, , ; brilliant discovery of microscopic life in the ocean and in the ice of the polar regions, ; rapid propogation of animalcules and their tenacity of life, - ; transformation of chalk, . electricity, magnetic, - ; conjectured electric currents, , ; electric storms, ; atmospheric , . elevations, comparative, of mountains in the two hemispheres, , . encke, ; his computation that the showers of meteors, in , proceeded from the same point of space in the direction in which the earth was moving at the time, , . ennius, . epicharmus, writings of, . equator, advantages of the countries bordering on, , ; their organic richness and fertility, , ; magnetic equator, - . erman, adolph, on the three cold days of may ( th- th), ; lines of declination in northern asia, ; in the southern parts of the atlantic, ; observations during the earthquake of irkutsk, on the non-disturbance of the horary changes of the magnetic needle, . eruptions and exhalations (volcanic), lava, gaseous and liquid fluids, hot mud, mud mofettes, etc., , [other page numbers obscured in paper copy] p ethnographical studies, their importance and teaching, , . euripides, his phaeton, . falconer, dr., fossil researches in the himalayas, . faraday, radiating heat, electro-magnetism etc., , , ; brilliant discovery of the evolution of light by magnetic forces, . farquharson on the connection of cirrous clouds with the aurora, ; its altitude, . federow, his pendulum experiments, . feldt on the ascent of shooting stars, . ferdinandes, igneous island of, . floras, geographical distribution of, . forbes, professor e., reference to his travels in lycia, ; account of the island of santorino, , . forbes, professor j., his improved selsmometer, ; on the correspondence existing between the distribution of existing floras in the british islands, , ; on the origin and diffusion of the british flora, , . forster, george, remarked the climatic difference of temperature of the eastern and western coasts of both continents, . forster, dr. thomas, monkish notice of "meteorodes," . fossil remains of tropical plants and animals found in northern regions, , - ; of extinct vegetation in the travertine of van diemen's land, ; fossil human remains, . foster, reinhold, pyramidal configuration of the southern extremities of continents, , . fourier, temperature of our planetary system, , , . fracastoro on the direction of the tails of comets from the sun, . fraehn, fall of stars, . franklin, benjamin, existence of sandbanks indicated by the coldness of the water over them, . franklin, capt., on the aurora, , , , ; rarity of electric explosions in high northern regions, . freycinet, pendulum oscillations, . fusinieri on meteoric masses, . galileo, , . galle, dr., . galvant, aloysio, accidental discovery of galvanism, . gaseous emanations, fluids, mud, and molten earth, , . gasparin, distribution of the quantity of rain in central europe, . gauss, friedrich, on terrestrial magnetism, ; his erection. in , of a magnetic observatory on a new principle, , . gay-lussac, , , , , , , , , . geognostic or geological description of the earth's surface, - . geognosy (the study of the textures and position of the earth's surface), its progress, . geography, physical, - ; of animal life, - ; of plants, - . geographics, ritter's (carl), "geography in relation to nature and the history of man," , ; varenius (bernhard), general and comparative geography, , . gerard, capts. a. g. and j. g., on the snow-line and vegetation of the himalayas, , , , . german scientific works, their defects, . geyser, intermittent fountains of, . gieseke on the aurora, . gilbert, sir humphrey, gulf stream, . gilbert, william, of colchester, terrestrial magnetism, , , , , . gillies, dr., on the snow-line of south america, , . gioja, crater of, . girard, composition and texture of basalt, . glaisher, james, on the aurora borealis of oct. , . see translator's notes, , . goldfuss, professor, examination of fossil specimens of the flying saurians, . goppert on the conversion of a fragment of amber-tree into black coal, ; eyeadeae, ; on the amber-tree of the baltic, , . gothe, , , . greek philosophers, their use of the term cosmos, , ; hypotheses on aerolites, , , . grimm, jacob, graceful symbolism attached to falling stars in the lithuanian mythology, , . gulf stream, its origin and course, . gumprecht, pyroxenic nepheline, . guanaxuato, striking subterranean noise at, . hall, sir james, his experiments on mineral fusion, . halley, comet, , , - ; on the meteor of , , ; on the light of stars, ; hypothesis of the earth being a hollow sphere, ; his bold conjecture that the aurora borealis was a magnetic phenomenon, . hansteen on magnetic lines of declination in northern asia, . hausen on the material contents of the moon, . hedenstrom on the so-called "wood hills" of new siberia, . hegel, quotation from his "philosophy of history," . heine, discovery of crystals of feldspar in scoriae, . hemmer, falling stars, . hencke, planets discovered by. see note by translator, , . henfrey, a., extract from his outlines of structural and physiological botany. see notes by translator, , , . p hensius on the variations of form in the comet of , . herodotus, described scythia as free from earthquakes, ; scythian saga of the sacred gold, which fell burning from heaven, . herschel, sir william, map of the world, ; inscription on his monument at upton, ; satellites of saturn, ; diameters of comets, ; on the comet of , ; star guagings, ; starless space, , ; time required for light to pass to the earth from the remotest luminous vapor, . herschel, sir john, letter on magellanic clouds, ; satellites of saturn, ; diameter of nebulous stars, ; stellar milky way, , ; light of isolated starry clusters, ; observed at the cape, the star pi in argo increase in splendor, ; invariability of the magnetic declination in the west indes, . hesiod, dimensions of the universe, . hevellus on the comet of , . hibbert, dr., on the lake of laach. see note by translator, . himalayas, the, their altitude, ; scenery and vegetation, , ; temperature, , ; variations of the snow-line on their northern and southern declivities, - , . hind, mr., planets discovered by. see translator's note, , . hindoo civilization, its primitive seat, , . hippalos, or monsoons, . hippocrates, his erroneous supposition that the land of scythia is an elevated table-land, . hoff, numerical inquiries on the distribution of earthquakes throughout the year, . hoffman, friedrich, observations on earthquakes, - ; on eruption fissures in the lipari islands, . holberg, his satire, "travels of nic. klimius, in the world under ground." see translator's note, , . hood on the aurora, , . hooke, robert, pulsations in the tails of comets, ; his anticipation of the application of botannical and zoological evidence to determine the relative age of rocks, - . ho-tsings, chinese fire-springs, their depth, ; chemical composition, . howard on the climate of london, ; mean annual quantity of rain in london, . hugel, carl von, on the elevation of the valley of kashmir, , ; on the snow-line of the himalayas, . humboldt, alexander von, works by referred to in various notes: annales de chimie et de physique, , . annales des science naturelles, . ansichten der natur, , , . asie centrale, , , , , , , , , , , , , , , , , , , , , , , - , , , , , , , , . atlas geographique et physique du nouveau continent, , . de distributione geographica plantrum, secundum coeli temperiem, et altitudinem montium, , , . examen critique de l'histoire de la geographie, , , , , , , , , , , . essai geognostique sur le gisement des roches, , , , . essai politique sur la nouvelle espagne, , . essai sur la geographie des plantes, , , . flora friburgensis subterranea, , . journal de physique, , . lettre au duc de sussex, sur les moyens propres a perfectionner la connaissance du magnetisme terrestre, , . monumens des peuples indigenes de l'amerique, . nouvelles annales des voyages, . recueil d'observations astronomiques, , , , . recueil d'observations de zoologi et d'anatomie comparee, . relation historique du voyage aux regions equinoxiales, , , , , , , , , , , , , , , , , - , , , , , , . tableau physique des regions equinoxiales, , . vues des cordilleres, , . humboldt, wilhelm von, on the primitive seat of hindoo civilization, ; sonnet, extract from, ; on the gradual recognition by the human race of the bond of humanity, , . humidity, , - . hutton, capt. thomas, his paper on the snow-line of the himalayas, , . huygens, polarization of light, ; nebulous spots, . hygrometry, , ; hygrometric wind-rose, . imagination, abuse of, by half-civilized nations, . imbert, his account of chinese "fire-springs," . ionian school of natural philosophy, , , , . isogenic, isoclinical, isodynamic, etc. see lines. jacquemont, victor, his barometrical observations on the snow-line of the himalayas, , . jasper, its formation, - . jessen on the gradual rise of the coast of sweden, . jorullo, hornitos de, . p justinian, conjectures on the physical causes of volcanic eruptions, . kamtz, isobarometric lines, ; doubts on the greater dryness of mountain air, . kant, emmanuel, "on the theory and structure of the heavens," , ; earthquake at lisbon, . kelihau on the ancient sea-line of the coast of spitzbergen, . kepler on the distances of stars, ; on the density of the planets, ; law of progression, ; on the number of comets, ; shooting stars, ; on the obscuration of the sun's disk, ; on the radiations of heat from the fixed stars, ; on a solar atmosphere, . kloden, shooting stars, , . knowledge, superficial, evils of, . krug of nidda, temperature of the geyser and the strokr intermittent fountains, . krusenstern, admiral, on the train of a fire-ball, . kuopho, a chinese physicist on the attraction of the magnet, and of amber, . kupffer, magnetic stations in northern asia, . lamanon, . lambert, suggestion that the direction of the wind be compared with the height of the barometer, alterations of temperature, humidity, etc., . lamont, mass of uranus, ; satellites of saturn, . language and thought, their mutual alliance, ; author's praise of his native language, . languages, importance of their study, , . laplace, his "systeme du monde," , , , ; mass of the comet of , ; on the required velocity of masses projected from the moon, , ; on the altitude of the boundaries of the atmosphere of cosmical bodies, ; zodiacal light, ; lunar inequalities, ; the earth's form and size inferred from lunar inequalities, , ; his estimate of the mean height of mountains, ; density of the ocean required to be less than the earth's for the stability of its equilibrium, ; results of his perfect theory of tides, . latin writers, their use of the term "mundus," , . latitudes, northern, obstacles they present to a discovery of the laws of nature, ; earliest acquaintance with the governing forces of the physical world, there displayed, ; spread from thence of the germs of civilization, . latitudes, tropical, their advantages for the contemplation of nature, ; powerful impressions, from their organic richness and fertility, ; facilities they present for a knowledge of the laws of nature, ; brilliant display of shooting stars, . laugier, his calculations to prove halley's comet identical with the comet of , described in chinese tables, . lava, its mineral composition, . lavoisier, . lawrence (st.), fiery tears, ; meteoric stream, . leibnitz, his conjecture that the planets increase in volume in proportion to their increase of distance from the sun, . lenz, observations on the mean level of the caspian sea, ; maxims of density of the oceanic temperature, ; temperature and density of the ocean under different zones of latitude and longitude, . leonhard, karl von, assumption on formations of granular limestone, . leverrier, planet neptune. see translator's note, , . lewy, observations on the varying quantity of oxygen in the atmosphere, according to local conditions, or the seasons, , . lichtenberg, on meteoric stones, . liebig on traces of ammonical vapors in the atmosphere, . light, chromatic polarization of, ; transmission, ; of comets, - ; of fixed stars, ; extraordinary lightness, instances of, - ; propagation of ; speed of transit, , . see aurora, zodiacal light, etc. lignites or beds of brown coal, , . lines, isogonic (magnetic equal deviation), , - ; isoclinal (magnetis equal inclination), , , - ; isodynamic (or magnetic equal force), , - ; isogeothermal (chthonisothermal), ; isobarometric, ; isothermal, isotheral, and isochimenal, , , , . line of no variation of horary declination, ; lower limit of perpetual snow, - ; phosphorescent, . lisbon, earthquake of, , , , . lord on the limits of the snow-line on the himalayas, . lottin, his observations of the aurora, with bravais and siljerstrom, on the coast of lapland, , , . lowenorn, recognized the coruscation of the polar light in bright sunshine, . lyell, charles, investigations on the numerical relations of extinct and organic life, , ; nether-formed or hypogene rocks, ; uniformity of the production of erupted rocks, . see notes by translator, , , . mackenzie, description of a remarkable eruption in iceland, . maclear on a centauri, ; parallaxes and distances of fixed stars, ; increase in brightness of 'pi' argo, . madler, planetary compression of uranus, ; distance of the innermost satellite of saturn from the centre of that planet, ; material contents of the moon, ; its libration, ; mean depression of temperature on the three cold days of may ( th- th), ; conjecture that the average mass of the larger number of binary stars exceeds the mass of the sun, . magellanic clouds, . magnetic attraction, ; declination, - ; horary motion, - ; horary variations , ; magnetic storms, , , , ; their intimate connection with the aurora, - ; represented by three systems of lines, see lines; movement of oval systems, ; magnetic equator, - ; magnetic poles, , ; observatories, - ; magnetic stations, , , . magnetism, terrestrial, - , ; electro, - . magnussen, soemund, description of remarkable eruption in iceland, . mahlmann, wilhelm, south west direction of the aÃ�Â�rial current in the middle latitudes of the temperate zone, . mairan on the zodiacal light, , , ; his opinion that the sun is a nebulous star, . malapert, annular mountain, . malle, dureau de la, . man, general view of, - ; proofs of the flexibility of his nature, ; results of his intellectual progress, , ; geographical distribution of races, - ; on the assumption of superior and inferior races, - ; his gradual recognition of the bond of humanity, , . mantell, dr., his "wonders of geology," see notes by translator, , , , , , , , , ; "medals of creation," , , , . margarita philosophica by gregory reisch, . marius, simon, first described the nebulous spots in andromeda and orion, . martins, observations on polar bands, ; found that air collected at faulhorn contained as much oxygen as the air of paris, ; on the distribution of the quantity of rain in central europe, ; doubts on the greater dryness of mountain air, . matthessen, letter to arago on the zodiacal light, . mathieu on the augmented intensity of the attraction of gravitation in volcanic islands, . mayer, tobias, on the motion of the solar system, , . mean numerical values, their necessity in modern physical science, . melloni, his discoveries on radiating heat and electro-magnetism, . menzel, unedited work by, on the flora of japan, . messier, comet, ; nebulous spot resembling our starry stratum, . metamorphic rocks. see rocks. meteorology, - . meteors, see aÃ�Â�rolites; meteoric infusoria, , . methone, hill of, . meyen on forming a thermal scale of cultivation, ; on the reproductive organs of liverworts and algae, . meyer, hermann von, on the organization of flying saurians, . milky way, its figure, ; views of aristotle on, ; vast telescopic breadth, ; milky way of nebulous spots at right angles with that of the stars, . minerals, artificially formed, , . mines, greatest depth of, , ; temperature, . mist, phosphorescent, . mitchell, protracted earthquake shocks in north america, . mitscherlich on the chemical origin of iron glance in volcanic masses, ; chemical combinations, a means of throwing a clear light on geognosy, ; on gypsum, as a uniaxal crystal, ; experiments on the simultaneously opposite actions of heat on crystalline bodies, ; formation of crystals of mica, ; on artificial mineral products, , . mofettes (exhalations of carbonic acid gas), - . monsoons (indian), , . monticelli on the current of hydrochloric acid from the crater of vesuvius, ; crystals of mica found in the lava of vesuvius, . moon, the, its relative magnitude, ; density, ; distance from the earth, ; its libration, , ; its light compared with that of the aurora, , ; volcanic action in, . moons or satellites, their diameter, distances, rotation, etc., - . morgan, john h. "on the aurora borealis of oct. , ." see translator's notes, , . morton, samuel george, his magnificent work on the american races, . moser's images, . mountains, in asia, america, and europe, their altitude, scenery, and vegetation, - , , ; their influence on climate, natural productions, and on the human race, its trade, civilization, and social condition, , , , , ; zones of vegetation on the declivities of , , - ; snow-line of, - , , . mud volcanoes. see salses and volcanoes. muller, johannes, on the modifications of plants and aniimals within certain limitations, . muncke on the appearance of auroras in certain districts, . murchison, sir r., account of a large fissure through which melaphyre had been ejected, ; classification of fossiliferous strata, ; on the age of the palaeosaurus and thecodontosaurus of bristol, . muschenbroek on the frequency of meteors in august, . myndius, apollonius, on the pythagorean doctrine of comets, , . nature, result of a rational inquiry into, ; emotions excited by her contemplation, ; striking scenes, ; their sources of enjoyment, , ; magnificence of the tropical scenery, , , , ; religious impulses from a communion with nature, ; obstacles to an active spirit of inquiry, ; mischief of inaccurate observations, ; higher enjoyments of her study, ; narrow-minded views of nature, ; lofty impressions produced on the minds of laborious observers, ; nature defined, ; her studies inexhaustible, ; general observations, their great advantages, ; how to be correctly comprehended, ; her most vivid impressions earthly, . nature, philosophy of, , ; physical description of, , , . nebulae, - ; nebulous milky way at right angles with that of the stars, - ; nebulous spots, conjectures on, - ; nebulous stars and planetary nebulae, , , ; nebulous vapor, - , , ; their supposed condensation in conformity with the laws of attraction, . neilson, gradual depression of the southern part of sweden, . nericat, andrea de, popular belief in syria on the fall of aerolites, . newton, discussed the question on the difference between the attraction of masses and molecular attraction, ; newtonian axiom confirmed by bessel, ; his edition of the geography of varenius, ; principia mathematica, ; considered the planets to be composed of the same matter with the earth, ; compression of the earth, . nicholl, j. p., note from his account of the planet neptune, , . nicholson, observations of lighting clouds, unaccompanied by thunder or indications of storm, . nobile, antonio, experiments of the height of the barometer, and its influence on the level of the sea, . noggerath counted annual rings in the trunk of a tree at bonn, . nordmann on the existence of animalcules in the fluids of the eyes of fishes, . norman, robert, invented the inclinatorium, . observations, scientific, mischief of inaccurate, ; tendency of unconnected, . ocean, general view of, - ; its extent as compared with the dry land, , ; its depth, , ; tides, , ; decreasing temperature at increased depths, ; uniformity and constancy of temperature in the same spaces, ; its currents and their various causes, - ; its phosphorescence in the torrid zone, ; its action on climate, , - ; influence on the mental and social condition of the human race, , , , , ; richness of its organic life, , ; oceanic microscopic forms, , ; sentiments excited by its contemplation, . oersted, electro-magnetic discoveries, , . olbers, comets, , ; aerolites, , ; on their planetary velocity, ; on the supposed phenomena of ascending shooting stars, ; their periodic return in august, ; november stream, ; prediction of a brilliant fall of shooting stars in nov., , ; absence of fossil meteoric stones in secondary and tertiary formations, ; zodiacal light, its vibration through the tails of comets, ; on the transparency of celestial space, . olmsted, denison of new haven, connecticut, observations of aerolites, , , , . oltmanns, herr, observed continuously with humboldt, at berlin, the movements of the declination needle, , . ovid, his description of the volcanic hill of methone, . oviedo describes the weed of the gulf stream as praderias de yerva (sea weed meadows), . palaeontology, - . pallas, meteoric iron, . palmer, new haven, connecticut, on the prodigious swarm of shooting stars, nov. and , , ; on the non-appearance in certain years of the august and november fall of aerolites, . parallaxes of fixed stars, , ; of the solar system, , . perry, capt., on auroras, their connection with magnetic perturbations, , ; whether attended with any sound, ; seen to continue throughout the day, ; barometric observation at port bowen, , ; rarity of electric explosions in northern regions, . patricius, st., his accurate conjectures on the hot springs of carthage, , . peltier on the actual source of atmospheric electricity, , . pendulum, its scientific uses, ; experiments with, , , , ; employed to investigate the curvature of the earth's surface, ; local attraction, its influence on the pendulum, and geognostic knowledge deduced from, , , , ; experiments of bessel, . pentland, his measurements of the andes, . percy, dr., on minerals artifically produced. see note by translator, . permian system of murchison, . perouse, la, expedition of, . persia, great comet seen in ( ), , . pertz on the large aerolite that fell in the bed of the river narni, . peters, dr., velocity of stones projected from aetna, . peucati, count mazari, partial infection of calcareous beds by the contact of syenitic granite in the tyrol, . phillips on the temperature of a coalmine at increasing depths, . philolaus, his astronomical studies, ; his fragmentary writings, - . philosophy of nature, first germ, . phosphorescence of the sea in the torrid zones, . physics, their limits, ; influence of physical science on the wealth and prosperity of nations, ; province of physical science, ; distinction betweeen the physical 'history' and physical 'description' of the world, , ; physical science, characteristics of its modern progress, . pindar, . plans, geodesic experiments in lombardy, . planets, - ; present number discovered, . (see note by translator on the most recent discoveries, , ); sir isaac newton on their composition, ; limited physical knowledge of, , ; ceres, - ; earth, - ; juno, , - , ; jupiter, , , - , ; mars, , - , ; mercury, , - ; pallas, , ; saturn, , - ; venus, - , ; uranus, - ; planets which have the largest number of moons, , . plants, geographical distribution of, - . plato on the heavenly bodies, etc., ; interpretation of nature, ; his geognostic views on hot springs, and volcanic igneous streams, , . pliny the elder, his natural history, ; on comets, ; aerolites, , , ; magnetism, ; attraction of amber, ; on earthquakes, , ; on the flame of inflammable gas, in the district of phasells, ; rarity of jasper, ; on the configuration of africa, . pliny the younger, his description of the great eruption of mount vesuvius, and the phenomenon of volcanic ashes, . plutarch, truth of his conjecture that falling stars are celestial bodies, , . poisson on the planet jupiter, ; conjecture on the spontaneous ignition of meteoric stones, ; zodiacal light, ; theory on the earth's temperature, , , , , . polarization, chromatic, results of its discovery, ; experiments on the light of comets, , . polybius, . posidonius on the ligyran field of stones, , . pouilet on the actual source of atmospheric electricity, . prejudices against science, how originated, ; against the study of the exact sciences, why fallacious, - . prichard, his physical history of mankind, . pseudo-plato, . psychrometer, , . pythagoras, first employed the word cosmos in its modern sense, . pythagoreans, their study of the heavenly bodies, ; doctrine on comets, . quarterly review, article on terrestrial magnetism, . quetelet on aerolites, ; their periodic return in august, . races, human, their geographical distribution, and unity, , . rain drops, temperature of, ; mean annual quantity in the two hemispheres, , . reich, mean density of the earth, as ascertained by the torsion balance, ; temperature of the mines in saxony, . reisch, gregory, his "margarita philosophica," . remusat, abel, mongolian tradition on the fall of an aerolite, ; active volcanoes in central asia, at great distances from the sea, . richardson, magnetic phenomena attending the aurora, ; whether accompanied by sound ; influence on the magnetic needle of the aurora, . riohamba, earthquake at, , , , , . ritter, carl, on his "geography in relation to nature and the history of man," , . robert, eugene, on the ancient sea-line on the coast of spitzbergen, . robertson on the permanency of the compass in jamaica, . rocks, their nature and configuration, ; geognostical classification into four groups, - ; i. rocks of eruption, , - ; ii. sedimentary rocks, , , ; iii. transformed, or metamorphic rocks, , , , - ; iv. conglomerates, or rocks of detritus, , ; their changes from the action of heat, , ; phenomena of contact, - ; effects of pressure and the rapidity of cooling, , . rose, gustav, on the chemical elements, etc., of various aerolites, ; on the structural relations of volcanic rocks, ; on crystals of feldspar and albite found in granite, ; relations of position in which granite occurs, - ; chemical process in the formation of various minerals, - . ross, sir james, his soundings with , feet of line, ; magnetic observations at the south pole, ; important results of the antarctic magnetic expedition in , ; rarity of electric explosions in high northern regions, . rossell, m. de, his magnetic oscillation experiments, and their date of publication, , . rothmann, confounded the setting zodiscal light with the cessation of twilight, . rozier, observation of a steady luminous appearance in the clouds, . rumker, encke's comet, . ruppell denies the existence of active volcanoes in kordofan, . sabine, edward, observations on days of unusual magnetic disturbances, ; recent magnetic observations, , , , . sagra, ramon de la, observations on the mean annual quantity of rain in the havana, . saint pierre, bernardin de, paul and virginia, ; studies of nature, . salses or mud volcanoes, - ; striking phenomena attending their origin, , . salt works, depth of , ; temperature, . santorino, the most important of the islands of eruption, , ; description of. see note by translator, . sargasso sea, its situation, . satellites revolving round the primary planets, their diameter, distance, rotation, etc., , ; saturn's - , ' earth's see moon, jupiter's, , ; uranus, - . saurians, flying, fossil remains of, , . saussure, measurements of the marginal ledge of the crater of mount vesuvius, ; traces of ammoniacal vapors in the atmosphere, ; hygrometric measurements with humboldt, - . schayer, microscopic organisms in the ocean, , . scheerer on the identity of eleolite and nepheline, . schelling on nature, ; quotation from his giordino bruino, . scheuchzner's fossil salamander, conjectured to be an antediluvian man, . schiller, quotation from, . schnurrer on the obscuration of the sun's disk, . schouten, cornelius, in found the declination null in the pacific, . schouw, distribution of the quantity of rain in central europe, . schrieber on the fragmentary character of meteoric stones, . scientific researches, their frequent result, ; scientific knowledge a requirement of the present age, , ; scientific terms, their vagueness and misapplication, , . scina, abbate, earthquakes unconnected with the state of the weather, , . scoresby, rarity of electric explosions in high northern regions, . sea. see ocean. seismometer, the, . seleucus of erythrea, his astronomical studies, . seneca, noticed the direction of the tails of comets, ; his views on the nature and paths of comets, , ; omens drawn from their sudden appearance, ; the germs of later observations on earthquakes found in his writings, ; problematical extinction and sinking of mount aetna, , . shoals, atmospheric indications of their vicinity, . sidereal systems, , . siljerstrom, his observations on the aurora, with lottin and bravais, on the coast of lapland, . sirowatskoi, "wood hills" in new siberia, . snow-line of the himalayas, - , , ; of the andes, ; redness of long-fallen snow, . solar system, general description, - ; its position in space, ; its transistory motion, - . solinus on mud volcanoes, . sommering on the fossil remains of the large vertebrata, . somerville, mrs., on the volume of fire-balls and shooting stars, ; faintness of light of planetary nebulae, . southern celestial hemisphere, its picturesque beauty, , . spontaneous generation, , . springs, hot and cold, - ; intermittent, ; causes of their temperature, - ; thermal, , ; deepest artesian wells the warmest, observed by arago, ; salses, - ; influence of earthquake shocks on hot springs, , - . stars, general account of, - ; fixed , , ; double and multiple, , ; nebulous, , , , ; their translatory motion, - ; parallaxes and distances, - ; computations of bessel and herschel on their diameter and volume, ; immense number in the milky way, , ; star dust, ; star gaugings, ; starless spaces, , ; telescopic stars, ; velocity of the propagation of light of, , ; apparition of new stars, . storms, magnetic and volcanic. see magnetism, volcanoes. strabo, observed the cessation of shocks of erthquake on the eruption of lava, ; on the mode in which islands are formed, ; description of the hill of methone, ; volcanic theory, ; divined the existence of a continent in the northern hemisphere between theria and thine, ; extolled the varied form of our small continent as favorable to the moral and intellectual development of its people, , . struve, otho, on the proper motion of the solar system, ; investigations on the propagation of light, ; parallaxes and distances of fixed stars, ; observations on halley's comet, . studer, professor, on mineral metamorphism. see note by translator, . sun, magnitude of its volume compared with that of the fixed stars, ; obscuration of its disk, ; rotation round the center of gravity of the whole solar system, ; velocity of its translatory motion, ; narrow limitations of its atmosphere as compared with the nucleus of other nebulous stars, ; "sun stones" of the ancients, ; views of the greek philosophers on the sun, . symond, lieut., his trigonometrical survey of the dead sea, , . tacitus, distinguished local climatic relations from those of race, . temperature of the globe, see earth and ocean; remarkable uniformity over the same spaces of the surface of the ocean, ; zones at which occur the maxima of the oceanic temperature, ; causes which lower the temperature, , ; temperature of various places, annual, and in the different seasons, , - ; thermic scale of temperature, , ; of continental climates as compared with insular and littoral climates, , ; law of decrease with increase of elevation, ; depression of, by shoals, ; refrigeration of the lower strata of the ocean, . teneriffe, peak of its striking scenery, . theodectes of phaselis on the color of the ethiopians, . theon of alexandria described comets as "wandering light clouds," . theophylactus described scythia as free from earthquakes, . thermal scales of cultivated plants, , . thermal springs, their temperature, constancy, and change, - ; animal and vegetable life in, . thermometer, . thibet, habitability of its elevated plateaux, , . thienemann on the aurora, , . thought, results of its free action, , ; union with language, . tiberias, sea of, its depression below the level of the mediterranean, . tides of the ocean, their phenomena, , . tillard, capt., on the sudden appearance of the island of sabrina, . tournefort, zones of vegetation on mount ararat, . tralles, his notice of the negative electricity of the air near high waterfalls, . translator, notes by, ; on the increase of the earth's internal heat with increase of depth, ; silicious infusoria and animalculites, ; chemical analysis of an aerolite, ; on the recent discoveries of planets, , ; observed the comet of , at new bedford, massachusetts, in bright sunshine, ; on meteoric stones, ; on a ms., said to be in the library of christ's college, cambridge, ; on the term "salses," ; on holberg's satire, "travels in the world under ground," ; on the aurora borealis of oct. , , , , ; on the electricity of the atmosphere during the aurora, ; on volcanic phenomena, , ; description of the seismometer, ; on the great earthquake of lisbon, ; impression made on the natives and foreigners by earthquakes in peru, ; earthquakes at lima, , ; on the gaseous compounds of sulphur, , ; on the lake of lasch, its craters, ; on the emissions of inflammable gas in the district of phasells, ; on true volcanoes as distinguished from salses, ; on the volcano of pichincha, ; on the hornitos de jorullo, as seen by humboldt, ; general rule on the dimensions of craters, ; on the ejection of fish from the volcano of imbaburn, ; on the little isle of volcano, ; volcanic steam of pantellaria, ; on daubeney's work "on volcanoes," ; account of the island of santorino, ; on the vicinity of extinct volcanoes to the sea, ; meaning of the chinese term "li," ; on mineral metamorphism, ; on fossil human remains found in guadaloupe, ; on minerals artifically produced , ; fossil organic structures, , ; on coprolites, ; geognostic distribution of fossils, ; fossil fauna of the sewalik hills, ; thickness of coal measures, ; on the amber pine forests of the baltic, , ; elevation of mountain chains, , ; the dinornis of owen, ; depth of the atmosphere, ; richness of organic life in the ocean, ; on filaments of plants resembling the spermatozoa of animals, ; on the diatomaceae in the south arctic ocean, ; on the distribution of the floras and faunas of the british isles, , ; on the origin and diffusion of the british flora, , . translatory motion of the solar system, - . trogus, pompeius, on the supposed necessity that volcanoes were dependent on their vicinity to the sea for their continuance, , ; views of the ancients on spontaneous generation, . tropical latitudes, their advantages for the contemplation of nature, ; powerful impressions from their organic richness and fertility, ; facilities they present for a knowledge of the laws of nature ; transparency of the atmosphere, ; phosphorescence of the sea, . tschudi, dr., extract from his "travels in peru." see translator's note, , , . turner, note on sir isaac newton, . universality of animated life, , . valz on the comet of , . varenius, bernhard, his excellent general and comparative geography, , ; edited by newton, . vegetable world, as viewed with microscopic powers of vision, ; its predominance over animal life, . vegetation, its varied distribution on the earth's surface, - , ; richness and fertility in the tropics, - ; zones of vegetation on the declivities of mountains, - , - . see aetna, cordilleras, himalayas, mountains. vico, satellites of saturn, . vigne, measurement of ladak, . vine, thermal scale of its cultivation, . volcanoes, , , , , , , , - ; author's application of the term volcanic, ; active volcanoes, safety-valves for their immediate neighborhood, ; volcanic eruptions, , - ; mud volcanoes or salses, - ; traces of volcanic action on the surface of the earth and moon, ; influence of relations of height on the occurrence of eruptions, - ; volcanic storm, ; volcanic ashes, ; classification of volcanoes into central and linear, ; theory of the necessity of their proximity to the sea, - ; geographical distribution of still active volcanoes, - ; metamorphic action on rocks, - . vrolik, his anatomical investigations on the form of the pelvis, , . wagner, rudolph, notes on the races of africa, . walter on the decrease of volcanic activity, . wartmann, meteors, , . weber, his anatomical investigations on the form of the pelvis, . webster, dr. (of harvard college, u.s.), account of the island named sabrina. see note by translator, . winds, - ; monsoons, , ; trade winds, -, ; law of rotation, importance of its knowledge, - . wine on the temperature required for its cultivation, ; thermic table of mean annual heat, . wolleston on the limitation of the atmosphere, . wrangel, admiral, on the brilliancy of the aurora borealis, coincident with the fall of shooting stars, , ; observations of the aurora, , ; wood hills of the siberian polar sea, . xenophanes of colophon, described comets as wandering light clouds, ; marine fossils found in marble quarries, . young, thomas, earliest observer of the influence different kinds of rocks exercise on the vibrations of the pendulum, . yul-sung, described by chinese writers as "the realm of pleasure," . zimmerman, carl, hypsometrical remarks on the elevation of the himalayas, . zodiacal light, conjectures on, - ; general account of, - ; beautiful appearance, , ; first described in childrey's britannia baconica, ; probable causes, ; intensity in tropical climates, . zones, of vegetation, on the declivities of mountains, - ; of latitude, their diversified vegetation, ; of the southern heavens, their magnificence, , ; polar, , . end of vol. i. [illustration: pike's peak, colorado] earth and sky every child should know easy studies of the earth and the stars for any time and place by julia ellen rogers author of "the tree book," "the shell book," "key to the nature library," "trees every child should know." illustrated by thirty-one pages of photographs and drawings [illustration] new york grosset & dunlap publishers copyright, , by doubleday, page & company published, october, all rights reserved, including that of translation into foreign languages, including the scandinavian acknowledgments a number of the photographs in this volume are used by permission of the american museum of natural history. the star maps and drawings of the constellations are by mrs. jerome b. thomas. the poem by longfellow, quoted in part, is with the permission of the publishers, houghton, mifflin & co. * * * * * contents _part i. the earth_ page the great stone book the fossil fish the crust of the earth what is the earth made of? the first dry land a study of granite metamorphic rocks the air in motion the work of the wind rain in summer, _by henry w. longfellow_ what becomes of the rain? the soil in fields and gardens the work of earthworms quiet forces that destroy rocks how rocks are made getting acquainted with a river the ways of rivers the story of a pond the riddle of the lost rocks the question answered glaciers among the alps the great ice sheet following some lost rivers the mammoth cave of kentucky land building by rivers the making of mountains the lava flood of the northwest the first living things an ancient beach at ebb tide the lime rocks the age of fishes king coal how coal was made the most useful metal the age of reptiles the age of mammals the horse and his ancestors the age of man _part ii. the sky_ every family a "star club" the dippers and the pole star constellations you can always see winter constellations orion, his dogs, and the bull seven famous constellations the twenty brightest stars how to learn more illustrations pike's peak _frontispiece_ facing page sand dunes in arizona grand cañon of the colorado castles carved by rain and wind where all the water comes from the richest gold and silver mines rocks being ground to flour a pond made by a glacier the struggle between a stream and its banks ripple marks and glacial striæ glacial grooves and markings crinoid and ammonite fossil corals, coquina, hippurite limestone fossil fish meteorite eocene fish and trilobite how coal was made banded sandstone. opalized wood allosaurus a three-horned dinosaur remains of brontosaurus restoration of brontosaurus ornitholestes, a small dinosaur a mammoth an ancestor of the horse orion, his dogs, and the bull other fanciful sketches of constellations the sky in winter the sky in spring the sky in summer the sky in autumn part i the earth * * * * * the great stone book "the crust of our earth is a great cemetery where the rocks are tombstones on which the buried dead have written their own epitaphs. they tell us who they were, and when and where they lived."--_louis agassiz._ deep in the ground, and high and dry on the sides of mountains, belts of limestone and sandstone and slate lie on the ancient granite ribs of the earth. they are the deposits of sand and mud that formed the shores of ancient seas. the limestone is formed of the decayed shells of animal forms that flourished in shallow bays along those shores. and all we know about the life of these early days is read in the epitaphs written on these stone tables. under the stratified rocks, the granite foundations tell nothing of life on the earth. but the sea rolled over them, and in it lived a great variety of shellfish. evidently the earliest fossil-bearing rocks were worn away, for the rocks that now lie on the granite show not the beginnings, but the high tide of life. the "lost interval" of which geologists speak was a time when living forms were few in the sea. in the muddy bottoms of shallow, quiet bays lie the shells and skeletons of the creatures that live their lives in those waters and die when they grow old and feeble. we have seen the fiddler crabs by thousands on such shores, young and old, lusty and feeble. we have seen the rocks along another coast almost covered by the coiled shells of little gray periwinkles, and big clumps of black mussels hanging on the piers and wharfs. all these creatures die, at length, and their shells accumulate on the shallow sea bottom. who has not spent hours gathering dead shells which the tide has thrown up on the beach? who has not cut his foot on the broken shells that lie in the sandy bottom we walk on whenever we go into the surf to swim or bathe? read downward from the surface toward the earth's centre-- table of contents ------+----------------------+-----------+----------------------- part | _rock systems_ | _dominant | _dominant plants_ | | animals_ | ------+----------------------+-----------+----------------------- vii. | recent | man | flowering kinds |{ quaternary | | vi. | { pliocene | mammals | early flowering |{ tertiary { miocene | | | { eocene | | v. | mesozoic | reptiles | cycads iv. | carboniferous | amphibians| ferns and conifers iii. | devonian | fishes | ferns ii. | silurian | molluscs | seaweeds i. | fire-formed | no life | no life ------+----------------------+-----------+----------------------- it is by dying that the creatures of the sea write their epitaphs. the mud or sand swallows them up. in time these submerged banks may be left dry, and become beds of stone. then some of the skeletons and shells may be revealed in blocks of quarried stone, still perfect in form after lying buried for thousands of years. the leaves of this great stone book are the layers of rock, laid down under water. between the leaves are pressed specimens--fossils of animals and plants that have lived on the earth. the fossil fish i remember seeing a flat piece of stone on a library table, with the skeleton of a fish distinctly raised on one surface. the friend who owned this strange-looking specimen told me that she found it in a stone quarry. she brought home a large piece of the slate, and a stone-mason cut out the block with the fish in it, and her souvenir made a useful and interesting paper-weight. the story of that fish i heard with wonder, and have never forgotten. i had never heard of fossil animals or plants until my good neighbour talked about them. she showed me bits of stone with fern leaves pressed into them. one piece of hard limestone was as full of little sea-shells as it could possibly be. one ball of marble was a honeycombed pattern, and called "fossil coral." the fossil fish was once alive, swimming in the sea, and feeding on the things it liked to eat, as all happy fishes do. near shore a river poured its muddy water into the sea, and the sandy bottom was covered with the mud that settled on it. at last the fish grew old, and perhaps a trifle stupid about catching minnows. it died, and sank to the muddy floor of the sea. its horny bones were not dissolved by the water. they remained, and the mud filtered in and filled all the spaces. soon the fish was buried completely by the sediment the river brought. years, thousands of them, went by, and the layer of mud was so thick and heavy above the skeleton of the fish that it bore a weight of tons there, under the water. the close-packed mud became a stiff clay. after more thousands of years, the sea no longer came so far ashore, for the river had built up a great delta of land out of mud. the clay in which the fish was hidden hardened into slate. water crept down in the loose upper layers, dissolving out salt and other minerals, and having harder work to soak through, the lower it went. the water left some of the minerals it had accumulated, calcium and silica and iron, in the lower rock beds, making them harder than they were before, and heavier and less porous. when the river gorge was cut through these layers of rock, the colour and thickness of each kind were laid bare. centuries after, perhaps thousands of years, indeed, the quarrymen cut out the layers fit for building stones, flags for walks and slates for roofing. in the splitting of a flagstone, the long-buried skeleton of the fish came to light. under our feet the earth lies in layers. under the soil lie loose beds of clay and sand and gravel, and under these loose kinds of earth are close-packed clays, sandstones, limestones, shales, often strangely tilted away from the horizontal line, but variously fitted, one layer to another. under these rocks lie the foundations of the earth--the fire-formed rocks, like granite. the depth of this original rock is unknown. it is the substance out of which the earth is made, we think. all the layered rocks are made of particles of the older ones, stolen by wind and water, and finally deposited on the borders of lakes and seas. so our rivers are doing to-day what they have always done--they are tearing down rocks, grinding and sifting the fragments, and letting them fall where the current of fresh water meets a great body of water that is still, or has currents contrary to that of the river. do you see a little dead fish in the water? it is on the way to become a fossil, and the mud that sifts over it, to become a layer of slate. every seashore buries its dead in layers of sand and mud. the crust of the earth it is hard to believe that our solid earth was once a ball of seething liquid, like the red-hot iron that is poured out of the big clay cups into the sand moulds at an iron foundry. but when a mountain like vesuvius sets up a mighty rumbling, and finally a mass of white-hot lava bursts from the centre and streams down the sides, covering the vineyards and olive orchards, and driving the people out of their homes in terror, it seems as if the earth's crust must be but a thin and frail affair, covering a fiery interior, which might at any time break out. the people who live near volcanoes might easily get this idea. but they do not. they go back as soon as the lava streams are cooled, and rebuild their homes, and plant more orchards and vineyards. "it is _so_ many years," say they to one another, "since the last bad eruption. vesuvius will probably sleep now till we are dead and gone." this is good reasoning. there are few active volcanoes left on the earth, compared with the number that were once active, and long ago became extinct. and the time between eruptions of the active ones grows longer; the eruptions less violent. terrible as were the recent earthquakes of san francisco and messina, this form of disturbance of the earth's crust is growing constantly less frequent. the earth is growing cooler as it grows older; the crust thickens and grows stronger as centuries pass. we have been studying the earth only a few hundred years. the crust has been cooling for millions of years, and mountain-making was the result of the shrinking of the crust. that formed folds and clefts, and let masses of the heated substance pour out on the surface. my first geography lesson i shall never forget. the new teacher had very bright eyes and _such_ pretty hands! she held up a red apple, and told us that the earth's substance was melted and burning, inside its crust, which was about as thick, in proportion to the size of the globe, as the skin of the apple. i was filled with wonder and fear. what if we children jumped the rope so hard as to break through the fragile shell, and drop out of sight in a sea of fiery metal, like melted iron? some of the boys didn't believe it, but they were impressed, nevertheless. the theory of the heated interior of the earth is still believed, but the idea that flames and bubbling metals are enclosed in the outer layer of solid matter has generally been abandoned. the power that draws all of its particles toward the earth's centre is stated by the laws of gravitation. the amount of "pull" is the measure of the weight of any substance. lift a stone, and then a feather pillow, much larger than the stone. one is strongly drawn to the earth; the other not. one is _heavy_, we say, the other _light_. if a stone you can pick up is heavy, how much heavier is a great boulder that it takes a four-horse team to haul. what tremendous weight there is in all the boulders scattered on a hillside! the hill itself could not be made level without digging away thousands of tons of earth. the earth's outer crust, with its miles in depth of mountains and level ground, is a crushing weight lying on the heated under-substance. every foot of depth adds greatly to the pressure exerted upon the mass, for the attraction of gravitation increases amazingly as the centre of the earth is approached. it is now believed that the earth is solid to its centre, though heated to a high degree. terrific pressure, which causes this heat, is exerted by the weight of the crust. a crack in the crust may relieve this pressure at some point, and a mass of substance may be forced out and burst into a flaming stream of lava. such an eruption is familiar in volcanic regions. the fact that red-hot lava streams from the crater of vesuvius is no proof that it was seething and bubbling while far below the surface. volcanoes, geysers, and hot springs prove that the earth's interior is hot. the crust is frozen the year around in the polar regions, and never between the tropics of cancer and capricorn. the sun's rays produce our different climates, but they affect only the surface. underground, there is a rise of a degree of temperature for every fifty feet one goes down. the lowest mine shaft is about a mile deep. that is only one four-thousandth of the distance to the earth's centre. by an easy computation we could locate the known melting-point for metals and other rock materials. but one degree for each fifty feet of depth below the surface may not be correct for the second mile, as it is for the first. again, the melting-point is probably a great deal higher for substances under great pressure. the weight of the crust is a burden the under-rocks bear. probably the pressure on every square inch reaches thousands of tons. could any substance become liquid with such a weight upon it, whatever heat it attained? nobody can answer this question. the theory that volcanoes are chimneys connecting lakes of burning lava with the surface of the earth is discredited by geologists. the weight of the overlying crust would, they think, close such chambers, and reduce liquids to a solid condition. since the first land rose above the sea, the crust of the earth has gradually become more stable, but even now there is scarcely a day when the instruments called seismographs do not record earthquake shocks in some part of the earth; and the outbreaks of vesuvius and Ætna, the constant boiling of lava in the craters of the hawaiian islands and other volcanic centres, prove that even now the earth's crust is very thin and unstable. the further back in time we go, the thinner was the crust, the more frequent the outbursts of volcanic activity, the more readily did wrinkles form. the shores of new jersey and of greenland are gradually sinking, and the sea coming up over the land. certain parts of the world are gradually rising out of the sea. in earlier times the rising or the sinking of land over large areas happened much more frequently than now. what is the earth made of? "baking day" is a great institution in the comfortable farm life of the american people. the big range oven is not allowed to grow cold until rows of pies adorn the pantry shelves, and cakes, tarts, and generous loaves of bread are added to the store. cookies, perhaps, and a big pan full of crisp, brown doughnuts often crown the day's work. no gallery of art treasures will ever charm the grown-up boys and girls as those pantry shelves charmed the bright-eyed, hungry children, who were allowed to survey the treasure-house, and sample its good things while they were still warm. you could count a dozen different kinds of cakes and pies, rolls and cookies on those pantry shelves, yet several of them were made out of the same dough. instead of a loaf of bread, mother could make two or three kinds of coffee cake, or cinnamon rolls, or currant buns, or parker-house rolls. even the pastry, which made the pies and tarts, was not so different from the bread dough, for each was made of flour, and contained, besides the salt, "shortening," which was butter or lard. sugar was used in everything, from the bread, which had a table-spoonful, to the cookies, which were finished with a sifting of sugar on top. how much of the food we eat is made of a very few staple foodstuffs,--starch, sugar, fats! so in the wonderful earth and all that grows out of it and lives upon it. only seventy different elements have been discovered, counting, besides the earth, the water and the air, and even the strange wandering bodies, called meteorites, that fall upon the earth out of the sky. like the flour in the different cakes and pies, the element carbon is found in abundance and in strangely different combinations. as a gas, in combination with oxygen, it is breathed out of our lungs, and out of chimneys where coal and wood are burned. it forms a large part of the framework of trees and other plants, and remains as charcoal when the wood is slowly burned under a close covering. there is a good proportion of carbon in animal bodies, in the bones as well as the soft parts, and carbon is plentiful in the mineral substances of the earth. the chemist is the man who has determined for us the existence and the distribution of the seventy elements. he finds them in the solid substances of the globe and in the water that covers four-fifths of its surface; in the atmosphere that covers sea and land, and in all the living forms of plants and animals that live in the seas and on the land. by means of an instrument called the spectroscope, the heavenly bodies are proved to be made of the same substances that are found in the rocks. the sun tells what it is made of, and one proof that the earth is a child of the sun is in the fact that the same elements are found in the substance of both. of the seventy elements, the most important are these: oxygen, silicon, aluminum, iron, manganese, calcium, magnesium, potassium, sodium, carbon, hydrogen, phosphorus, sulphur, chlorine, nitrogen. _oxygen_ is the most plentiful and the most important element. one-fifth of the air we breathe is oxygen; one-third of the water we drink. the rock foundations of the earth are nearly one-half oxygen. no fire can burn, no plant or animal can grow, or even decay after it dies, unless oxygen is present and takes an active part in each process. strangely enough, this wonderful element is invisible. we open a window, and pure air, rich in oxygen, comes in and takes the place of the bad air but we cannot see the change. water we see, but if the oxygen and the hydrogen which compose the colourless liquid were separated, each would become at once an invisible gas. the oxygen of solid rocks exists only in combination with other elements. _silicon_ is the element which, united with oxygen, makes the rock called quartz. on the seashore the children are busy with their pails and shovels digging in the white, clean sand. these grains are of quartz,--fine crystals of a rock which forms nearly three-quarters of the solid earth's substance. not only in rocks, but out here in the garden, the soil is full of particles of sand. you cannot get away from it. _aluminum_ is a light, bluish-white metal which we know best in expensive cooking utensils. it is more abundant even than iron, but processes of extracting it from the clay are still expensive. it is oftenest found in combination with oxygen and silicon. while nearly one-tenth of the earth's crust is composed of the metal aluminum, four-fifths and more is composed of the minerals called silicates of aluminum--oxygen, silicon, and aluminum in various combinations. it is more plentiful than any other substance in rocks and in the clays and ordinary soils, which are the finely ground particles of rock material. _iron_ is one of the commonest of elements. we know it by its red colour. a rusty nail is covered with oxide of iron, a combination which is readily formed wherever iron is exposed to the action of water or air. you have seen yellowish or red streaks in clefts of the rocks. this shows where water has dissolved out the iron and formed the oxide. the red colour of new jersey soil is due to the iron it contains. indeed, the whole earth's crust is rich in iron which the water easily dissolves. the roots of plants take up quantities of iron in solution and this mounts to the blossoms, leaves, and fruit. the red or yellow colour of autumn leaves, of apples, of strawberries, of tulips, and of roses, is produced by iron. the rosy cheeks of children are due to iron in the food they eat and in the water they drink. the doctor but follows the suggestion of nature when he gives a pale and listless person a tonic of iron to make his blood red. iron is rarely found free, but it forms about five per cent. of the crust of the earth, and it is believed to form at least one-fifth of the unknown centre of the earth, the bulk of the globe, the weight of which we know, but concerning the substance of which we can say little that is positive. _manganese_ is not a conspicuous element, but is found united with oxygen in purplish or black streaks on the sides of rocks. it is somewhat like iron, but much less common. _calcium_ is the element that is the foundation of limestones. the skeletons and shells of animals are made of calcite, a common mineral formed by the uniting of carbon, oxygen, and calcium. marbles are, perhaps, the most permanent form of the limestone rocks. "hard" water has filtered through rocks containing calcite, and absorbed particles of this mineral. from water thus impregnated, all animal life on the earth obtains its bone-building and shell-building materials. _carbon_ forms a large part of the tissues of plants and animals, and in the remains of these it is chiefly found in the earth's crust. when these burn or decay, the carbon remains as charcoal or escapes to the air in union with oxygen as the well known carbonic acid gas. this is one of the most important foods of plants. joined with calcium it forms the mineral calcite, or carbonate of lime. _hydrogen_ is one of the two gases that unite to form water. oxygen is the other. many kinds of rock contain a considerable amount of water. surface water sinks into porous soils and rocks, and accumulates in pockets and veins which feed springs, and are the reserve water supply that keeps our rivers flowing, even through dry weather. more water is held by absorption in the earth's solid crust than in all the oceans and seas and great lakes. hydrogen, combined with carbon, occurs in solid rocks where the remains of plants and animals have slowly decayed. from such processes the so-called hydrocarbons, rock oil and natural gas, have accumulated. when such decay goes on above ground, these valuable products escape into the air. marsh gas, whose feeble flame above decaying vegetation is the will-o'-the-wisp of swamps, is an example. _magnesium_, _potassium_, and _sodium_ are found in equal quantities in the earth's crust, but never free. in union with chlorine, each forms a soluble salt, and is thus found in water. common salt, chloride of sodium, is the most abundant of these. water dissolves salt out of the rocks, and carries it into the sea. clouds that rise by the evaporation of ocean water leave the salt behind, hence the seas are becoming more and more salty, for the rivers carry salt to the oceans, which hold fast all they get. _phosphorus_ is an element found united with oxygen in the tissues of both plants and animals. it is most abundant in bones. rocks containing fossil bones are rich in lime phosphates, which are important commercial fertilizers for enriching the soil. beds of these rocks are found and mined in south carolina and elsewhere. _sulphur_ is well known as a yellow powder found most plentifully in rocks that are near volcanoes. it is a needed element in plant and animal bodies. it occurs in rocks, united with many different elements. in union with oxygen and a metal it forms the group of minerals called sulphates. in union with iron it forms sulphide of iron. the "fool's gold" which captain john smith's colonists found in the sand at jamestown, was this worthless iron pyrites. _chlorine_ is a greenish, yellow gas, very heavy, and dangerous to inhale. if it gets into the lungs, it settles into the lowest levels, and one must stand on one's head to get it out. as an element of the earth's crust it is not very plentiful, but it is a part of all the chlorides of sodium, magnesium, and potassium. in salt, it forms two per cent. of the sea water. it is much less abundant in the rocks. to these elements we might add _nitrogen_, that invisible gas which forms nearly four-fifths of our atmosphere, and is a most important element of plant food in the soil. most of the seventy elements are very rare. many are metals, like gold and iron and silver. some are not metals. some are solid. a few are liquid, like the metal mercury, and several are gaseous. some are free and pure, and show no disposition to unite with others. nuggets of gold are examples of this. some exist only in union with other elements. this is the common rule among the elements. changes are constantly going on. the elements are constantly abandoning old partnerships and forming new ones. growth and decay of plant and animal life are but parts of the great programme of constant change which is going on and has been in progress since the world began. the first dry land when the earth's crust first formed it was still hot, though not so hot as when it was a mass of melted, glowing substance. as it moved through the cold spaces of the sky, it lost more heat and its crust became thicker. at length the cloud masses became condensed enough to fall in torrents of water, and a great sea covered all the land. this was before any living thing, plant or animal, existed on our planet. can you imagine the continents and islands that form the land part of a map or globe suddenly overwhelmed by the oceans, the names and boundaries of which you have taken such pains to learn in the study of geography? the globe would be one blank of blue water, and geography would be abolished--and there would be nobody to study it. possibly the fishes in the sea might not notice any change in the course of their lives, except when they swam among the ruins of buried cities and peered into the windows of high buildings, or wondered what new kind of seaweed it was when they came upon a submerged forest. in that old time of the great sea that covered the globe, we are told that there was a dense atmosphere over the face of the deep. so things were shaping themselves for the far-off time when life should exist, not only in the sea, where the first life did appear, but on land. but it took millions of years to fit the earth for living things. the cooling of the earth made it shrink, and the crust began to be folded into gentle curves, as the skin of a shrunken apple becomes wrinkled on the flesh. some of these creases merely changed the depth of water on the sea bottom; but one ridge was lifted above the water. the water parted and streamed down its sloping sides, and a granite reef, which shone in the sunshine, became the first dry land. it lay east and west, and stretched for many miles. it is still dry land and is a part of our own continent. now it is but a small part of the country, but it is known by geologists, who can tell its boundaries, though newer land joins it on every side. it is named the laurentian hills, on geological maps. its southern border reaches along the northern boundary of the great lakes to the head-waters of the mississippi river. from this base, two ridges are lifted, forming a colossal v. one extends northeast to nova scotia; the other northwest to the arctic seas. the v encloses hudson bay. besides this first elongated island of bare rocks, land appeared in a strip where now the blue ridge mountains stretch from new england to georgia. the other side of the continent lifted up two folds of the crust above sea level. they are the main ridges of the colorado and the wasatch mountains. possibly the main ridge of the sierra nevada rose also at this time. the ozark group of mountains, too, showed as a few island peaks above the sea. these first rocks were rapidly eaten away, for the atmosphere was not like ours, but heavily charged with destructive gases, which did more, we believe, to disintegrate the exposed rock surfaces than did the two other forces, wind and water, combined. the sediment washed down to the sea by rains, accumulated along the shores, filling the shallows and thus adding to the width of the land areas. the ancient granite ridge of the laurentian hills is now low, and slopes gently. this is true of all very old mountains. the newer ones are high and steep. it takes time to grind down the peaks and carry off the waste material loosened by erosion. far more material than could have been washed down the slopes of the first land ridges came directly from the interior of the earth, and spread out in vast, submarine layers upon the early crust. volcanic craters opened under water, and poured out liquid mineral matter, that flowed over the sea bottom before it cooled. imagine the commotion that agitated the water as these submerged chimneys blew off their lids, and discharged their fiery contents! it was long before the sea was cool enough to be the home of living things. the layers of rock that formed under the sea during this period of the earth's history are of enormous thickness. they were four or five miles deep along the laurentian hills. they broadened the original granite ridge by filling the sea bottom along the shores. the backbones of the appalachian system and the cordilleras were built up in the same way--the oldest rocks were worn away, and their débris built up newer ones in strata. when these layers of rock became dry land, the earth's crust was much more stable and cool than it had ever been before. the vast rock-building of that era equals all that has been done since. the layers of rocks formed since then do not equal the total thickness of these first strata. so we believe that the time required to build those archæan rock foundations equals or surpasses the vast period that has elapsed since the archæan strata were formed. the northern part of north america has grown around those old granite ridges by the gradual rising of the shores. the geologist may walk along the laurentian hills, that parted the waters into a northern and a southern ocean. he crosses the rocky beds deposited upon the granite; then the successive beds formed as the land rose and the ocean receded. age after age is recorded in the rocks. gradually the sea is crowded back, and the land masses, east, west, and north, meet to form the continent. nowhere on the earth are the steps of continental growth shown in unbroken sequence as they are in north america. how long ago did those first islands appear above the sea? nobody ventures a definite answer to this question. no one has the means of knowing. but those who know most about it estimate that at the least one hundred million years have passed since then--one hundred thousand thousand years! a study of granite in every village cemetery it is easy to find shafts of gray or speckled granite, the polished surfaces of which show that the granite is made of small bits of different coloured minerals, cemented together into solid rock. outside the gate you will usually find a place where monuments and gravestones may be bought. here there is usually a stonecutter chipping away on a block with his graving tools. he is a man worth knowing, and because his work is rather monotonous he will probably be glad to talk to a chance visitor and answer questions about the different kinds of stone on which he works. there are bits of granite lying about on the ground. if you have a hand-glass of low power, such as the botany class uses to examine the parts of flowers, it will be interesting to look through it and see the magnified surface of a flake of broken granite. here are bits of glassy quartz, clear and sparkling in the sun. black and white may be all the colours you make out in this specimen, or it may be that you see specks of pink, dark green, gray, and smoky brown, all cemented together with no spaces that are not filled. the particles of quartz are of various colours, and are very hard. they scratch glass, and you cannot scratch them with the steel point of your knife, as you can scratch the other minerals associated with the grains of quartz. granite is made of quartz, feldspar, and mica, sometimes with added particles of hornblende. feldspar particles have as wide a range of colour as quartz, but it is easy to tell the two apart. a knife will scratch feldspar, as it is not so hard as quartz. the crystals of feldspar have smooth faces, while quartz breaks with a rough surface as glass does. feldspar loses its glassy lustre when exposed to the weather, and becomes dull, with the soft lustre of pearl. mica may be clear and glassy, and it ranges in colour from transparency through various shades of brown to black. it has the peculiarity of splitting into thin, leaf-like, flexible sheets, so it is easy to find out which particles in a piece of granite are mica. one has only to use one's pocket knife with a little care. hornblende is a dark mineral which contains considerable iron. it is found in lavas and granites, where it easily decays by the rusting of the iron. it is not unusual to see a rough granite boulder streaked with dark red rust from this cause. the crumbling of granite is constantly going on as a result of the exposure of its four mineral elements to the air. quartz is the most stable and resistant to weathering. soil water trickling over a granite cliff has little effect on the quartz particles; but it dissolves out some of the silicon. the bits of feldspar are even more resistant to water than quartz is, but the air causes them to decay rapidly, and finally to fall away in a sort of mealy clay. mica, like feldspar, decays easily. its substance is dissolved by water and carried away to become a kind of clay. the hornblende rusts away chiefly under the influence of moist air and trickling water. we think of granite as a firm, imperishable kind of rock, and use it in great buildings like churches and cathedrals that are to stand for centuries. but the faces that are exposed to the air suffer, especially in regions having a moist climate. the signs of decay are plainly visible on the outer surfaces of these stones. fortunate it is that the weathering process cannot go very deep. the glassy polish on a smooth granite shaft is the silicon which acts as a cement to bind all the particles together. it is resistant to the weather. a polished shaft will last longer than an unpolished one. granites differ in colouring because the minerals that compose them, the feldspars, quartzes, micas, and hornblendes, have each so wide a range of colour. again, the proportions of the different mineral elements vary greatly in different granites. a banded granite the colours of which give it a stratified appearance is called a gneiss. we have spoken before of the seventy elements found in the earth's crust. a mineral is a union of two or more of these different elements; and we have found four minerals composing our granite rock. it may be interesting to go back and inquire what elements compose these four minerals. quartz is made of silicon and oxygen. feldspar is made of silicon, oxygen, and aluminum. mica is made of silicon, oxygen, and carbon, with some mingling of potassium and iron and other elements in differing proportions. hornblende is made of silicon, oxygen, carbon, and iron. the crumbling of a granite rock separates the minerals that compose it, reducing some to the condition of clay, others to grains of sand. some of the elements let go their union and become free to form new unions. water and wind gather up the fragments of crumbling granite and carry them away. the feldspar and mica fragments form clay; the quartz fragments, sand. all of the sandstones and slates, the sand-banks and sand beaches, are made out of crumbled granite, the rocky foundations of the earth. metamorphic rocks in the dawn of life on the earth, soft-bodied creatures, lowest in the scale of being, inhabited the sea. the ancient volcanoes the subterranean eruptions of which had spread layers of mineral substance on the ocean floor, and heated the water to a high degree, had subsided. the ocean was sufficiently cool to maintain life. the land was being worn down, and its débris washed into the ocean. the first sand-banks were accumulating along sandy shores. the finer sediment was carried farther out and deposited as mud-banks. these were buried under later deposits, and finally, by the rising of the earth's crust, they became dry land. time and pressure converted the sand-banks into sandstones; the mud-banks into clay. the remains of living creatures utterly disappeared, for they had no hard parts to be preserved as fossils. the shrinking of the earth's crust had crumpled into folds of the utmost complexity those horizontal layers of lava rock poured out on the ocean floor. next, the same forces attacked the thick rock layers formed out of sediment--the aqueous or water-formed sandstones and clays. the core of the globe contracts, and the force that crumples the crust to fit the core generates heat. the alkaline water in the rocks joins with the heat produced by the crumpling and crushing forces, acting downward, and from the sides, to transform pure sandstone into glassy quartzite, and clay into slate. in other words, water-formed rocks are baked until they become fire-formed rocks. they are what the geologist calls _metamorphic_, which means _changed_. in many mountainous regions there are breaks through the strata of sandstone and slates and limestones, through which streams of lava have poured forth from the heated interior. along the sides of these fissures the hot lava has changed all the rocks it touched. the heat of the volcanic rock matter has melted the silica in the sand, which has hardened again into a crystalline substance like glass. have you ever visited a brick-yard? here men are sifting clay dug out of a pit or the side of a hill, adding sand from a sand-bank, and in a big mixing box, stirring these two "dry ingredients" with water into a thick paste. this dough is moulded into bricks, sun-dried, and then baked in kilns themselves built of bricks. at the end of the baking, the soft, doughy clay block is transformed into a hard, glassy, or dull brick. from aqueous rock materials, fire has produced a metamorphic rock. volcanic action is imitated in this common, simple process of brickmaking. milwaukee brick is made of clay that has no iron in it. for this reason the bricks are yellow after baking. most bricks are red, on account of the iron in the clay, which is converted into a red oxide, or rust, by water and heat. common flower pots and the tiles used in draining wet land are not glazed, as hard-burned bricks are. the baking of these clay things is done with much less heat. they are left somewhat porous. but the tiles of roofs are baked harder, and get a surface glaze by the melting of the glassy particles of the sand. as bricks vary in colour and quality according to the materials that compose them, so the metamorphic rocks differ. the white sand one sees on many beaches is largely quartz. this is the substance of pure, white sandstone. metamorphism melts the silica into a glassy liquid cement; the particles are bound close together on cooling. the rock becomes a white, granular quartzite, that looks like loaf sugar. if banded, it is called gneiss. such rocks take a fine polish. pure limestone is also white and granular. when metamorphosed by heat, it becomes white marble. the glassy cement that holds the particles of lime carbonate shows as the glaze of the polished surface. it is silica. one sees the same mineral on the face of polished granite. clays are rarely pure. kaolin is a white clay which, when baked, becomes porcelain. china-ware is artificially metamorphosed kaolin. in the early rocks the clay beds were transformed by heat into jasper and slates. in beds where clay mingled with sand, in layers, gneiss was formed. if mica is a prominent element, the metamorphic rock is easily parted into overlapping, scaly layers. it is a mica schist. if hornblende is the most abundant mineral, the same scaly structure shows in a dark rock called hornblende schist, rich in iron. a schist containing much magnesia is called serpentine. the bricks of the wall, the tiles on the roof, the flower pots on the window sill, and the dishes on the breakfast table, are examples of metamorphic rocks made by man's skill, by the use of fire and water acting on sand and clay. pottery has preserved the record of civilization, from the making of the first crude utensils by cave men to the finest expression of decorative art in glass and porcelain. the choicest material of the builder and the sculptor is limestone baked by the fires under the earth's crust into marble. the most enduring of all the rocks are the foundation granite, and the metamorphic rocks that lie next to them. over these lie thick layers of sedimentary rocks laid down by water. in them the record of life on the earth is written in fossils. the air in motion most of the beautiful things that surround us and make our lives full of happiness appeal to one or more of our five senses. the green trees we can see, the bird songs we hear, the perfume of honey-laden flowers we smell, the velvety smoothness of a peach we feel, and its rich pulp we taste. but over all and through all the things we see and feel and hear and taste and smell, is the life-giving air, that lies like a blanket, miles in depth, upon the earth. the substance which makes the life of plants and animals possible is, when motionless, an invisible, tasteless, odourless substance, which makes no sound and is not perceptible to the touch. air fills the porous substance of the earth's crust for a considerable distance, and even the water has so much air in it that fishes are able to breathe without coming to the surface. it is not a simple element, like gold, or carbon, or calcium, but is made up of several elements, chief among which are nitrogen and oxygen. four-fifths of its bulk is nitrogen and one-fifth oxygen. there is present in air more or less of watery vapour and of carbon dioxide, the gas which results from the burning or decay of any substance. although no more than one per cent. of the air that surrounds us is water, yet this is a most important element. it forms the clouds that bear water back from the ocean and scatter it in rain upon the thirsty land. solid matter in the form of dust, and soot from chimneys, accumulates in the clouds and does a good work in condensing the moisture and causing it to fall. it is believed that the air reaches to a height of one hundred to two hundred miles above the earth's surface. if a globe six feet in diameter were furnished with an atmosphere proportionately as deep as ours, it would be about an inch in depth. at the level of the sea the air reaches its greatest density. two miles above sea-level it is only two-thirds as dense. on the tops of high mountains, four or five miles above sea level, the air is so rarefied as to cause the blood to start from the nostrils and eyelids of explorers. the walls of the little blood-vessels are broken by the expansion of the air that is inside. at the sea-level air presses at the rate of fifteen pounds per square foot in all directions. as one ascends to higher levels, the air pressure becomes less and less. the barometer is the instrument by which the pressure of air is measured. a glass tube, closed at one end, and filled with mercury, the liquid metal often called quicksilver, is inverted in a cup of the same metal, and so supported that the metal is free to flow between the two vessels. the pressure of air on the surface of the mercury in the cup is sufficient at the sea-level to sustain a column of mercury thirty inches high in the tube. as the instrument is carried up the side of a mountain the mercury falls in the tube. this is because the air pressure decreases the higher up we go. if we should descend into the shaft of the deepest mine that reaches below the sea level, the column of air supported by the mercury in the cup would be a mile higher, and for this reason its weight would be correspondingly greater. the mercury would thus be forced higher in the tube than the thirty-inch mark, which indicates sea-level. another form of barometer often seen is a tube, the lower and open end of which forms a u-shaped curve. in this open end the downward pressure of the air rests upon the mercury and holds it up in the closed end, forcing it higher as the instrument is carried to loftier altitudes. at sea level a change of feet in altitude makes a change of an inch in the height of the mercury in the column. the glass tube is marked with the fractions of inches, or of the metre if the metric system of measurements is used. it is a peculiarity of air to become heated when it is compressed, and cooled when it is allowed to expand again. it is also true that when the sun rises, the atmosphere is warmed by its rays. this is why the hottest part of the day is near noon when the sun's rays fall vertically. the earth absorbs a great deal of the sun's heat in the daytime and through the summer season. when it cools this heat is given off, thus warming the surrounding atmosphere. in the polar regions, north and south, the air is far below freezing point the year round. in the region of the equator it rarely falls below degrees, a temperature which we find very uncomfortable, especially when there is a good deal of moisture in the air. if we climb a mountain in mexico, we leave the sultry valley, where the heat is almost unbearable, and very soon notice a change. for every three hundred feet of altitude we gain there is a fall of one degree in the temperature. before we are half way up the slope we have left behind the tropical vegetation, and come into a temperate zone, where the plants are entirely different from those in the lower valley. as we climb, the vegetation becomes stunted, and the thermometer drops still lower. at last we come to the region of perpetual snow, where the climate is like that of the frozen north. so we see that the air becomes gradually colder as we go north or south from the equator, and the same change is met as we rise higher and higher from the level of the sea. it is only when air is in motion that we can feel and hear it, and there are very few moments of the day, and days of the year, when there is not a breeze. on a still day fanning sets the air in motion, and creates a miniature breeze, the sound of which we hear in the swishing of the fan. the great blanket of air that covers the earth is in a state of almost constant disturbance, because of the lightness of warm air and the heaviness of cold air. these two different bodies are constantly changing places. for instance, the heated air at the equator is constantly being crowded upward by cold air which settles to the level of the earth. cold streams of air flow to the tropics from north and south of the equator, and push upward the air heated by the sun. this constant inrush of air from north and south forms a double belt of constant winds. if the earth stood still, no doubt the direction would be due north and due south for these winds; but the earth rotates rapidly from west to east upon its axis, carrying with it everything that is securely fastened to the surface: the trees, the houses, etc. but the air is not a part of the earth, not even so much as the seas, the waters of which must stay in their proper basins, and be whirled around with other fixed objects. the earth whirls so rapidly that the winds from north and south of the equator lag behind, and thus take a constantly diagonal direction. instead of due south the northern belt of cold air drifts south-west and the southern belt drifts northwest. these are called the trade winds. near the equator they are practically east winds. the belt of trade winds is about fifty degrees wide. it swings northward in our summer and southward in our winter, its centre following the vertical position of the sun. near the centre of the course which marks the meeting of the northern with the southern winds is a "belt of calms" where the air draws upward in a strong draught. the colder air of the trade winds is pushing up the columns of light, heated air. this strip is known by sailors as "the doldrums," or "the region of equatorial calms." though never wider than two or three hundred miles, this is a region dreaded by captains of sailing-vessels, for they often lie becalmed for weeks in an effort to reach the friendly trade winds that help them to their desired ports. vessels becalmed are at the mercy of sudden tempests which come suddenly like thunder-storms, and sometimes do great damage to vessels because they take the sailors unawares and allow no time to shorten sail. until late years the routes of vessels were charted so that sailors could take advantage of the trade winds in their long voyages. it was necessary in the days of sailing-vessels for the captain to understand the movements of winds which furnished the motive power that carried his vessel. fortunate it was for him that there were steady winds in the temperate zones that he could take advantage of in latitudes north of the tropic of cancer and south of the tropic of capricorn. what becomes of the hot air that rises in a constant stream above the "doldrums," pushed up by the cooler trade winds that blow in from north and south? naturally this air cannot ascend very high, for it soon reaches an altitude in which its heat is rapidly lost, and it would sink if it were not constantly being pushed by the rising column of warm air under it. so it turns and flows north and south at a level above the trade winds. not far north of the tropic of cancer it sinks to the level of sea and land, and forms a belt of winds that blows ships in a northeasterly direction. between trades and anti-trades is another zone of calms,--near the tropics of cancer and of capricorn. the land masses of the continents with their high mountain ranges interfere with these winds, especially in the northern hemisphere, but in the southern pacific and on the opposite side of the globe the "roaring forties," as these prevailing westerly winds are known by the sailors, have an almost unbroken waste of seas over which they blow. in the long voyages between england and australia, and in the indian trade, the ships of england set their sails to catch the roaring forties both going and coming. they accomplish this by sailing past the cape of good hope on the outward voyage and coming home by way of cape horn, thus circling the globe with every trip. in the north atlantic, traffic is now mostly carried on in vessels driven by engines, not by sails. yet the westerly winds that blow from the west indies diagonally across the atlantic are still useful to all sailing craft that are making for british ports. from the north and from the south cold air flows down into the regions of warmer climate. these polar winds are not so important to sea commerce, but they do a great work in tempering the heat in the equatorial regions. we cannot know how much our summers are tempered by the cool breath of winds that blow over polar ice-fields. and the cold regions of the earth, in their brief summer, enjoy the benefits of the warm breezes that flow north and south from the heated equatorial regions. the land, north and south, is made habitable by the clouds. they gather their burdens of vapour from the warm seas, the wind drifts them north and south, where they let it fall in rains that make and keep the earth green and beautiful. from the clouds the earth gathers, like a great sponge, the water that stores the springs and feeds rivers and lakes. how necessary are the winds that transport the cloud masses! the air is the breath of life to all living things on our planet. mars is one of the sun's family so provided. plants or animals could probably live on the planet mars. do we think often enough of this invisible, life-giving element upon which we depend so constantly? the open air which the wind purifies by keeping it in motion is the best place in which to work, to play, and to sleep, when work and play are done and we rest until another day comes. indoors we need all the air we can coax to come in through windows and doors. fresh air purifies air that is stale and unwholesome from being shut up. nobody is afraid, nowadays, to breathe night air! what a foolish notion it was that led people to close their bedroom windows at night. clean air, in plenty, day and night, we need. air and sunshine are the two best gifts of god. the work of the wind when the march wind comes blustering down the street, rudely dashing a cloud of dust in our faces, we are uncomfortable and out of patience. we duck our heads and cover our faces, but even then we are likely to get a cinder in one eye, to swallow germs by the dozens, and to get a gray coating of plain, harmless dust. we welcome the rain that lays the dust, or its feeble imitation, the water sprinkler, that brings us temporary relief. on the quietest day, even after a thorough sweeping and dusting of the library, you are able to write your name plainly on the film of dust that lies on the polished table. take a book from the open shelves, and blow into the trough of its top. this is always dusty. where does the dust come from? this is the house-keeper's riddle. the answer is not a hard one. i look out of my window on a street which is famous as the road washington took on his retreat from white plains to trenton. it has always been the main thoroughfare between new york and philadelphia, and now is the route that automobiles follow. a constant procession of vehicles passes my house, and to-day each one approaches in a cloud of dust. the air is gray with suspended particles of dirt. the wind carries the successive clouds, and they roll up against the houses like breakers on the beach. windows and doors are loose enough to let dust sift in. when a door opens, the cloud enters and lights on rugs and carpets and curtains. any ledge collects its share of dust. the beating of carpets and rugs disturbs the accumulated dust of many months. [illustration: in this lonely arizona desert the wind drifts the sand into dunes, just as it does on the toe of cape cod] [illustration: the grand canyon of the colorado shows on a magnificent scale the work of water in cutting away rock walls] the wind sweeps the ploughed field, and takes all the dust it can carry. it blows the finest top soil from our gardens into the street. it blows soil from other fields and gardens into ours, so the level of our land is not noticeably lowered. the wind strips the high land and drops its burden on lower levels. this is one of the big jobs the water has to do, and the wind is a valuable helper. to tear down the mountains and fill in the valleys is the great work of the two partners, wind and water. dead, still air holds the finest dust, without letting it fall. the buoyancy of the particles overcomes their weight. we see them in a sunbeam, like shining points of precious metal, and watch them. a light breeze picks up bits of soil and litter, from the smallest up to a certain size and weight. if the velocity of the wind increases, its carrying power increases. it is able to carry bits that are larger and heavier. the following table is exact and interesting: _velocity_ _pressure_ _in miles_ _in pounds_ _per hour_ _per sq. ft._ light breeze strong breeze strong gale hurricane the terrible paths of hurricanes are seen in forest countries. the trees are uprooted, as if a great roller had crushed them, throwing the tops all in one direction, and leaving the roots uncovered, and a sunken pocket where each tree stood. on a steep, rocky slope, the uprooting of scattered trees often loosens tons of rock, and sends the mass thundering down the mountain-side. much more destruction may be accomplished by one brief tornado than by years of wear by ordinary breezes. the wind does much to help the waves in their patient beating on rocky shores. if the wind blows from the ocean and the tide is landward, the two forces combine, and the loose rocks are thrown against the solid beach with astonishing force. even the gravel and the sharp sand are tools of great usefulness to the waves in grinding down the resisting shore. up and back they are swept by the water, and going and coming they have their chance to scratch or strike a blow. boulders on the beach become pockmarked by the constant sand-blast that plays upon them. the lower windows of exposed seaside houses are dimmed by the sand that picks away the smooth surface outside, making it ground glass by the same process used in the factory. lighthouses have this difficulty in keeping their windows clear. the "lantern" itself is sometimes reached by the sand grains. that is the cupola in which burns the great light that warns vessels away from the rocks and tells the captain where he is. in the far western states the telegraph poles and fence posts are soon cut off at the ground by the flinty knives the wind carries. these are the grains of sand that are blown along just above the ground. the trees are killed by having their bark girdled in this way. the sand-storms which in the orange and lemon region of california are called "santa anas" sometimes last two or three days, and damage the trees by piercing the tender bark with the needle-pointed sand. wind-driven soil, gathered from the sides of bare hills and mountains, fills many valleys of china with a fine, hard-packed material called "loess." in some places it is hundreds of feet deep. the people dig into the side of a hill of this loess and carry out the diggings, making themselves homes, of many rooms, with windows, doors, and solid walls and floors, all in one solid piece, like the chambered house a mole makes underground in the middle of a field. so compact is the loess that there is no danger of a cave-in. the hills of sand piled up on the southern shore of lake michigan, and at provincetown, at the toe of cape cod, are the work of the wind. on almost any sandy shore these "dunes" are common. the long slope is toward the beach that furnishes the sand. the wind does the building. up the slope it climbs, then drops its burden, which slides to the bottom of an abrupt landward steep. there is a gradual movement inland if the strongest winds come from the water. the shifting of the dunes threatens to cover fertile land near them. in the desert regions, the border-land is always in danger of being taken back again, even though it has been reclaimed from the desert and cultivated for long years. besides tearing down, carrying away, and building up again the fragments of the earth's crust, the wind does much that makes the earth a pleasant planet to live on. it drives the clouds over the land, bringing rains and snows and scattering them where they will bless the thirsty ground and feed the springs and brooks and rivers. it scatters the seeds of plants, and thus plants forests and prairies and lovely mountain slopes, making the wonderful wild gardens that men find when they first enter and explore a new region. the trade winds blow the warm air of the tropics north and south, making the climate of the northern countries milder than it would otherwise be. sea winds blow coolness over the land in summer, and cool lake breezes temper the inland regions. from the snow-capped mountains come the winds that refresh the hot, tired worker in the valleys. everywhere the wind blows, the life-giving oxygen is carried. this is what we mean when we speak of fresh air. stagnant air is as unwholesome as stagnant water. constant moving purifies both. so we must give the wind credit for some of the greatest blessings that come into our lives. light and warmth come from the sun. pure water and pure air are gifts the bountiful earth provides. without them there would be no life on the earth. rain in summer how beautiful is the rain! after the dust and heat, in the broad and fiery street, in the narrow lane, how beautiful is the rain! how it clatters along roofs, like the tramp of hoofs! how it gushes and struggles out from the throat of the overflowing spout! across the window-pane it pours and pours; and swift and wide, with a muddy tide, like a river down the gutter roars the rain, the welcome rain! the sick man from his chamber looks at the twisted brooks; he can feel the cool breath of each little pool; his fevered brain grows calm again, and he breathes a blessing on the rain. --henry w. longfellow. what becomes of the rain? the clouds that sail overhead are made of watery vapour. sometimes they look like great masses of cotton-wool against the intense blue of the sky. sometimes they are set like fleecy plumes high above the earth. sometimes they hang like a sullen blanket of gray smoke, so low they almost touch the roofs of the houses. indeed, they often rest on the ground and then we walk through a dense fog. in their various forms, clouds are like wet sponges, and when they are wrung dry they disappear--all their moisture falls upon the earth. when the air is warm, the water comes in the form of rain. if it is cold, the drops are frozen into hail, sleet, or snow. all of the water in the oceans, in the lakes and rivers, great and small, all over the earth, comes from one source, the clouds. in the course of a year enough rain and snow fall to cover the entire surface of the globe to a depth of forty inches. this quantity of water amounts to , barrels on every acre. what becomes of it all? we can easily understand that all the seas and the other bodies of water would simply add forty inches to their depth, and many would become larger, because the water would creep up on their gradually sloping shores. we have to account for the rain and the snow that fall upon the dry land and disappear. go out after a drenching rainstorm and look for the answer to this question. the gullies along the street are full of muddy, running water. there are pools of standing water on level places, but on every slope the water is hurrying away. the ground is so sticky that wagons on country roads may mire to the hubs in the pasty earth. there is no use in trying to work in the garden or to mow the lawn. the sod is soft as a cushion, and the garden soil is water-soaked below the depth of a spading-fork. the sun comes out, warm and bright, and the flagstones of the sidewalk soon begin to steam like the wooden planks of the board walk. the sun is changing the surface water into steam which rises into the sky to form a part of another bank of clouds. the earth has soaked up quantities of the water that fell. if we followed the racing currents in the gullies we should find them pouring into sewer mains at various points, and from these underground pipes the water is conducted to some outlet like a river. all of the streams are swollen by the hundreds of brooks and rivulets that are carrying the surface water to the lowest level. [illustration: rain and wind are the sculptors that have carved these strange castles out of a rocky table] [illustration: all the water in the seas, lakes, rivers, and springs came out of the clouds] so we can see some of the rainfall going back to the sky, some running off through rivulets to the sea, and some soaking into the ground. it will be interesting to follow this last portion as it gradually settles into the earth. the soil will hold a certain quantity, for it is made up of fine particles, all separated by air spaces, and it acts like a sponge. in seasons of drought and great heat the sun will draw this soil water back to the surface, by forming cracks in the earth, and fine, hair-like tubes, through which the vapour may easily rise. the gardener has to rake the surface of the beds frequently to stop up these channels by which the sun is stealing the precious moisture. the water that the surface soil cannot absorb sinks lower and lower into the ground. it finds no trouble to settle through layers of sand, for the particles do not fit closely together. it may come to a bed of clay which is far closer. here progress is retarded. the water may accumulate, but finally it will get through, if the clay is not too closely packed. again it may sink rapidly through thick beds of gravel or sand. reaching another bed of clay which is stiffer by reason of the weight of the earth above it, the water may find that it cannot soak through. the only way to pass this clay barrier is to fill the basin, and to trickle over the edge, unless a place is found in the bottom where some looser substance offers a passage. let us suppose that a concave clay basin of considerable depth is filled with water-soaked sand. at the very lowest point on the edge of this basin a stream will slowly trickle out, and will continue to flow, as long as water from above keeps the bowl full. it is not uncommon to find on hillsides, in many regions, little brooks whose beginnings are traceable to springs that gush out of the ground. the spring fills a little basin, the overflow of which is the brook. if the source of this spring could be traced underground, we might easily follow it along some loose rock formation until we come to a clay basin like the one described above. we might have to go down quite a distance and then up again to reach the level of this supply, but the level of the water at the mouth of the spring can never be higher than the level of the water in the underground supply basin. often in hot summers springs "go dry." the level of water in the supply basin has fallen below the level of the spring. we must wait until rainfall has added to the depth of water in the basin before we can expect any flow into the pool which marks the place where the brook begins. suppose we had no beds of clay, but only sand and gravel under the surface soil. we should then expect the water to sink through this loose material without hindrance, and, finding its way out of the ground, to flow directly into the various branches of the main river system of our region. after a long rain we should have the streams flooded for a few days, then dry weather and the streams all low, many of them entirely dry until the next rainstorm. instead of this, the soil to a great depth is stored with water which cannot get away, except by the slow process by which the springs draw it off. this explains the steady flow of rivers. what should we do for wells if it were not for the water basins that lie below the surface? a shallow well may go dry. its owner digs deeper, and strikes a lower "vein" of water that gives a more generous supply. in the regions of the country where the drift soil, left by the great ice-sheet, lies deepest, the glacial boulder clay is very far down. the surface water, settling from one level to another, finally reaches the bottom of the drift. wells have to be deep that reach this water bed. the water follows the slope of this bed and is drained into the ocean, sometimes by subterranean channels, because the bed of the nearest river is on a much higher level. so we must not think that the springs contain only the water that feeds the rivers. they contain more. the layers of clay at different levels, from the surface down to the bottom of the drift, form water basins and make it possible for people to obtain a water supply without the expense of digging deep wells. the clayey subsoil, only a few feet below the surface, checks the downward course of the water, so that the sun can gradually draw it back, and keep a supply where plant roots can get it. the vapour rising keeps the air humid, and furnishes the dew that keeps all plant life comfortable and happy even through the hot summer months. under the drift lie layers of stratified rock, and under these are the granites and other fire-formed rocks, the beginning of those rock masses which form the solid bulk of the globe. we know little about the core of the earth, but the granites that are exposed in mountain ridges are found to have a great capacity for absorbing water, so it is not unlikely that much surface water soaks into the rock foundations and is never drained away into the sea. the water in our wells is often hard. it becomes so by passing through strata of soil and rock made, in part, at least, of limestone, which is readily dissolved by water which contains some acid. soil water absorbs acids from the decaying vegetation,--the dead leaves and roots of plants. rain water is soft, and so is the water in ponds that have muddy basins, destitute of lime. water in the springs and wells of the mid-western states is "hard" because it percolates through limestone material. in many parts of this country the well water is "soft," because of the scarcity of limestone in the soil. i have seen springs around which the plants and the pebbles were coated with an incrustation of lime. "petrified moss" is the name given to the plants thus turned to stone. the reason for this deposit is clear. underground water is often subjected to great pressure, and at this time it is able to dissolve much more of any mineral substance than under ordinary conditions. when the pressure is released, the water is unable to hold in solution the quantity of mineral it contains; therefore, as it flows out through the mouth of the spring, the burden of mineral is laid down. the plants coated with the lime gradually decay, but their forms are preserved. there are springs the water of which comes out burdened with iron, which is deposited as a yellowish or red mineral on objects over which it flows. ponds fed by these springs accumulate deposits of the mineral in the muddy bottoms. some of the most valuable deposits of iron ore have accumulated in bogs fed by iron-impregnated spring water. in a similar way lime deposits called marl or chalk are made. the soil in fields and gardens city and country teachers are expected to teach classes about the formation and cultivation of soil. it is surprising how much of the needed materials can be brought in by the children, even in the cities. the beginning is a flowering plant growing in a pot. a window box is a small garden. a garden plot is a miniature farm. _materials to collect for study indoors._ a few pieces of different kinds of rock: granite, sandstone, slate; gravelly fragments of each, and finer sand. pebbles from brooks and seashore. samples of clays of different colors, and sands. samples of sandy and clay soils, black pond muck, peat and coal. rock fossils. a box of moist earth with earthworms in it. _keep it moist._ a piece of sod, and a red clover plant with the soil clinging to its roots. _what is soil?_ it is the surface layer of the earth's crust, sometimes too shallow on the rocks to plough, sometimes much deeper. under deep soil lies the "subsoil," usually hard and rarely ploughed. _what is soil made of?_ ground rock materials and decayed remains of animal and plant life. by slow decay the soil becomes rich food for the growing of new plants. wild land grows up to weeds and finally to forests. the soil in fields and gardens is cultivated to make it fertile. plants take fertility from the soil. to maintain the same richness, plant food must be put back into the soil. this is done by deep tillage, and by mixing in with the soil manures, green crops, like clover, and commercial fertilizers. _plants must be made comfortable, and must be fed._ few plants are comfortable in sand. it gets hot, it lets water through, and it shifts in wind and is a poor anchor for roots. clay is so stiff that water cannot easily permeate it; roots have the same trouble to penetrate it and get at the food it is rich in. air cannot get in. sand mixed with clay makes a mellow soil, which lets water and air pass freely through. the roots are more comfortable, and the tiny root hairs can reach the particles of both kinds of mineral food. but the needful third element is decaying plant and animal substances, called "humus." these enrich the soil, but they do a more important thing: their decay hastens the release of plant food from the earthy part of the soil, and they add to it a sticky element which has a wonderful power to attract and hold the water that soaks into the earth. _what is the best garden soil?_ a mixture of sand, clay, and humus is called "loam." if sand predominates, it is a sandy loam--warm, mellow soil. if clay predominates, we have a clay loam--a heavy, rich, but cool soil. all gradations between the two extremes are suited to the needs of crops, from the melons on sandy soil, to celery that prefers deep, cool soil, and cranberries that demand muck--just old humus. _how do plant roots feed in soil?_ by means of delicate root hairs which come into contact with particles of soil around which a film of soil water clings. this fluid dissolves the food, and the root absorbs the fluid. plants can take no food in solid form. hence it is of the greatest importance to have the soil pulverized and spongy, able to absorb and hold the greatest amount of water. the moisture-coated soil particles must have air-spaces between them. air is as necessary to the roots as to the tops of growing plants. _why does the farmer plough and harrow and roll the land?_ to pulverize the soil; to mellow and lighten it; to mix in thoroughly the manure he has spread on it, and to reach, if he can, the deeper layers that have plant food which the roots of his crops have not yet touched. killing weeds is but a minor business, compared with tillage. later, ploughing or cultivating the surface lightly not only destroys the weeds, but it checks the loss of water by evaporation from the cracks that form in dry weather. raking the garden once a day in dry weather does more good than watering it. the "dust mulch" acts as a cool sunguard over the roots. _the process of soil-making._ if the man chopping wood in the yosemite valley looks about him he can see the soil-making forces at work on a grand scale. the bald, steep front of el capitan is of the hardest granite, but it is slowly crumbling, and its fragments are accumulating at the bottom of the long slope. rain and snow fill all crevices in the rocks. frost is a wonderful force in widening these cracks, for water expands when it freezes. the loosened rock masses plough their way down the steep, gathering, as they go, increasing power to tear away any rocks in their path. wind blows finer rock fragments along, and they lodge in cracks. fine dust and the seeds of plants are lodged there. the rocky slopes of the yosemite valley are all more or less covered with trees and shrubs that have come from wind-sown seeds. these plants thrust their roots deeper each year into the rock crevices. the feeding tips of roots secrete acids that eat away lime and other substances that occur in rocks. dead leaves and other discarded portions of the trees rot about their roots, and form soil of increasing depth. the largest trees grow on the rocky soil deposited at the base of the slope. the tree's roots prevent the river from carrying it off. when granite crumbles, its different mineral elements are separated. clear, glassy particles of quartz we call sand. dark particles of feldspar become clay, and may harden into slate. sand may become sandstone. exposed slate and sandstone are crumbled by exposure to wind and frost and moving water, and are deposited again as sand-bars and beds of clay. the most interesting phase of soil study is the discovery of what a work the humble earthworm does in mellowing and enriching the soil. the work of earthworms the farmer and the gardener should expect very poor crops if they planted seed without first ploughing or spading the soil. next, its fine particles must be separated by the breaking of the hard clods. a wise man ploughs heavy soil in the fall. it is caked into great clods which crumble before planting time. the water in the clods freezes in winter. the expansion due to freezing makes this soil water a force that separates the fine particles. so the frost works for the farmer. just under the surface of the soil lives a host of workers which are our patient friends. they work for their living, and are perhaps unconscious of the fact that they are constantly increasing the fertility of the soil. they are the earthworms, also called fishworms, which are distributed all over the world. they are not generally known to farmers and gardeners as friendly, useful creatures, and their services are rarely noticed. we see robins pulling them out of the ground, and we are likely to think the birds are ridding us of a garden pest. what we need is to use our eyes, and to read the wonderful discoveries recorded in a book called "vegetable mould and earthworms," written by charles darwin. the benefits of ploughing and spading are the loosening and pulverizing of the packed earth; the mixing of dead leaves and other vegetation on and near the surface with the more solid earth farther down; the letting in of water and air; and the checking of loss of water through cracks the sun forms by baking the soil dry. the earthworm is a creature of the dark. it cannot see, but it is sufficiently sensitive to light to avoid the sun, the rays of which would shrivel up its moist skin. having no lungs or gills, the worm uses the skin as the breathing organ; and it must be kept moist in order to serve its important use. this is why earthworms are never seen above ground except on rainy days, and never in the top soil if it has become dry. in seasons of little rain, they go down where the earth is moist, and venture to the surface only at night, when dew makes their coming up possible. earthworms have no teeth, but they have a long snout that protrudes beyond the mouth. their food is found on and in the surface soil. they will eat scraps of meat by sucking the juices, and scrape off the pulp of leaves and root vegetables in much the same way. much of their subsistence is upon organic matter that can be extracted from the soil. quantities of earth are swallowed. it is rare that an earthworm is dug up that does not show earth pellets somewhere on their way through the long digestive canal. the rich juices of plant substance are absorbed from these pellets as they pass through the body. earthworms explore the surface of the soil by night, and pick up what they can find of fresh food. nowhere have i heard of them as a nuisance in gardens, but they eagerly feed on bits of meat, especially fat, and on fresh leaves. they drag all such victuals into their burrows, and begin the digestion of the food by pouring on it from their mouths a secretion somewhat like pancreatic juice. the worms honeycomb the earth with their burrows, which are long, winding tubes. in dry or cold weather these burrows may reach eight feet under ground. they run obliquely, as a rule, from the surface, and are lined with a layer of the smooth soil, like soft paste, cast from the body. the lining being spread, the burrow fits the worm's body closely. this enables it to pass quickly from one end to the other, though it must wriggle backward or forward without turning around. at the lower end of the burrow, an enlarged chamber is found, where hibernating worms coil and sleep together in winter. at the top, a lining of dead leaves extends downward for a few inches, and in day time a plug of the same material is the outside door. at night the worm comes to the surface, and casts out the pellets of earth swallowed. the burrow grows in length by the amount of earth scraped off by the long snout and swallowed. the daily amount of excavation done is fairly estimated by the castings observed each morning on the surface. one earthworm's work for the farmer is not very much, but consider how many are at work, and what each one is doing. it is boring holes through the solid earth, and letting in the surface water and the air. it is carrying the lower soil up to the surface, often the stubborn subsoil, that no plough could reach. it is burying and thus hastening the decay of plant fibre, which lightens heavy soil and makes it rich because it is porous. moreover, the earthworms are doing over and over again this work of fining and turning over the soil, which the plough does but seldom. by the continuous carrying up of their castings, the earthworms gradually bury manures spread on the surface. the collapse of their burrows and the making of new ones keep the soil constantly in motion. the particles are being loosened and brought into contact with the soil water, that dissolves, and thus frees for the use of feeding roots, the plant food stored in the rock particles that compose the mineral part of the soil. the weight of earth brought to the surface by worms in the course of a year has been carefully estimated. darwin gives seven to eighteen tons per acre as the lowest and highest reports, based on careful collecting of castings by four observers, working on small areas of totally different soils. in england, earthworms have done a great deal more toward burying boulders and ancient ruins than any other agency. they eagerly burrow under heavy objects, the weight of which causes them to crush the honeycombed earth. undiscouraged, the earthworms repeat their work. "long before man existed, the land was regularly ploughed, and continues still to be ploughed by earthworms. it may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures." after years of study, charles darwin came to this conclusion. the more we study the lives of these earth-consuming creatures, the more fully do we believe what the great nature student said. the fertile soil is made of rock meal and decayed leaves and roots. only recently have ploughs been invented. but the great forest crops have grown in soil made mellow by the earthworm's ploughing. quiet forces that destroy rocks wind and water are the blustering active agents we see at work tearing down rocks and carrying away their particles. they do the most of this work of levelling the land; but there are quiet forces at work which might not attract our attention at all, and yet, without their help, wind and running water would not accomplish half the work for which they take the credit. the air contains certain destructive gases which by their chemical action separate the particles of the hardest rocks, causing them to crumble. now the wind blows away these crumbling particles, and the solid unchanged rock beneath is again exposed to the crumbling agencies. the changes in temperature between day and night cause rocks to contract and expand, and these changes put a strain upon the mineral particles that compose them. much scaling of rock surfaces is due to these causes. building a fire on top of a rock, and then dashing water upon the heated mass, shatters it in many directions. this process merely intensifies the effect produced by the mild changes of winter and summer. water is present in most rocks, in surprising quantities, often filling the spaces in porous rocks like sandstones. when winter brings the temperature down to the freezing point, the water near the surface of the rock first feels it. ice forms, and every particle of water is swollen by the change. a strain is put upon the mineral particles against which the particles of ice crowd for more room. frost is a very powerful agent in the crumbling of rocks, as well as of stubborn clods of earth. in warm climates, and in desert regions where there is little moisture in the rocks, this destructive action of freezing water is not known. in cold countries, and in high altitudes, where the air is heavy with moisture, its greatest work is done. some kinds of rock decay when they become dry, and resist crumbling better when they absorb a certain amount of moisture. alternate wetting and drying is destructive to certain rocks. one of the unnoticed agents of rock decay is the action of lowly plants. mosses grow upon the faces of rocks, thrusting their tiny root processes into pits they dig deeper by means of acids secreted by the delicate tips. you have seen shaded green patches of lichens, like little rugs, of different shapes, spread on the surface of rocks. but you cannot see so well the work these growths are doing in etching away the surface, and feeding upon the decaying mineral substance. mosses and lichens do a mighty work, with the help of water, in reducing rocks to their original elements, and thus forming soil. no plants but lichens and mosses can grow on the bare faces of rocks. as their root-like processes lengthen and go deeper into the rock face, particles are pried off, and the under-substance is attacked. higher plants then find a footing. have you not seen little trees growing on a patch of moss which gets its food from the air and the rock to which it clings? the spongy moss cushion soaks up the rain and holds it against the rock face. a streak of iron in the rock may cause the water to follow and rust it out, leaving a distinct crevice. now the roots of any plant that happens to be growing on the moss may find a foot-hold in the crack. streaks of lime in a rock readily absorb water, which gradually dissolves and absorbs its particles, inviting the roots to enter these new passages and feed upon the disintegrating minerals. dead leaves decay, and the acids the trickling water absorbs from them are especially active in disintegrating lime rocks. from such small beginnings has resulted the shattering of great rock masses by the growth of plants upon them. tree roots that grow in rock crevices exert a power that is irresistible. the roots of smaller plants do the same great work in a quieter way. when a hurricane or a flood tears down the mountain-side, sweeping everything before it, trees, torn out by the roots, drag great masses of rock and soil into the air, and fling them down the slope. wind and water thus finish the destruction which the humble mosses and lichens began. what seemed an impregnable fortress of granite has crumbled into fragments. its particles are reduced to dust, or are on the way to this condition. the plant food locked up in granite boulders becomes available to hungry roots. forests, grain-fields, and meadows cover the work of destructive agencies with a mantle of green. how rocks are made the granite shaft is made out of the original substance of the earth's crust. its minerals are the elements out of which all of the rock masses of the earth are formed, no matter how different they look from granite. sandstone is made of particles of quartz. clay and slate are made out of feldspar and mica. iron ore comes from the hornblende in granite. the mineral particles, reassembled in different proportions, form all of the different rocks that are known. here in my hand is a piece of pudding-stone. it is made of pebbles of different sizes, each made of different coloured minerals. the pebbles are cemented together with a paste that has hardened into stone. this kind of rock the geologists call _conglomerate_. pudding-stone is the common name, for the pebbles in the pasty matrix certainly do suggest the currants and the raisins that are sprinkled through a christmas pudding. under the seashores there are forming to-day thick beds of sand. the rivers bring the rock material down from the hills, and it is sorted and laid down. the moving water drops the heaviest particles near shore, and carries the finer ones farther out before letting them fall. [illustration: the town of cripple creek, colorado, which has grown up like magic since , covers the richest gold and silver mines in the world] [illustration: the level valley is filled up with fine rock flour washed from the sides of the neighboring mountains] the hard water, that comes through limestone rocks, adds lime in solution to the ocean water. all the shellfish of the sea, and the creatures with bony skeletons, take in the bone-building, shell-making lime with their food. generations of these inhabitants of the sea have died, and their shells and bones have accumulated and been transformed into thick beds of limestone on the ocean floor. this is going on to-day; but the limestone does not accumulate as rapidly as when the ocean teemed with shell-bearing creatures of gigantic size. of these we shall speak in another chapter. the fine dust that is blown into the ocean from the land, and that makes river water muddy, accumulates on the sea bottom as banks of mud, which by the burden of later deposits is converted into clay. sandstone is but the compressed sand-bank. in the study of mountains, geologists have discovered that old seashores were thrown up into the first great ridges that form the backbone of a mountain system. the rocky mountains, and the appalachian system on the east, were made out of thick strata of rocks that had been formed by accumulations of mud and sand--the washings of the land--on the opposite shores of a great mid-continental sea, that stretched from the crest of one great mountain system across to the other, and north and south from the laurentian hills to the gulf of mexico. the great weight of the accumulating layers of rock materials on one side, and the wasted land surfaces on the other, made the sea border a line of greatest weakness in the crust of the earth. the shrinking of the globe underneath caused the break; mashing and folding followed, throwing the ridge above sea-level, and making dry land out of rock waste which had been accumulating, perhaps for millions of years, under the sea. the wrinkling of the earth's crust was the result of crushing forces which produced tremendous heat. streams of lava sprang out through the fissures and poured streams of melted rock down the sides of the fold, quite burying, in many places, the layers of limestone, sandstone, and clay. between the strata of water-formed rocks there were often created chimney-like openings, into which molten rock from below was forced, forming, when cool, veins and dikes of rock material, specimens of the substance of the earth's interior. tremendous pressure and heat, acting upon stratified rocks saturated with water transform them into very different kinds of rock. limestone, subjected to these forces, is changed into marble. clays are transformed into slates. sandstone is changed into quartzite, the sand grains being melted so as to become no longer visible to the naked eye. the anthracite coal of the pennsylvania mountains is the result of heat and pressure acting upon soft coal. associated with these beds of hard coal are beds of black lead, or graphite, the substance used in making "lead" pencils. we believe that the same forces that operated to transform clay rocks into slate, and limestone into marble, transformed soft coal into hard, and hard coal into graphite, in the days when the earth was young. the word _sedimentary_ is applied to rocks which were originally laid down under water, as sediment, brought by running water, or by wind, or by the decay of organic substances. _stratified_ rocks are those which are arranged in layers. sedimentary rocks will fall into this class. _aqueous_ rocks are those which are formed under water. most of the stratified and sedimentary rocks, but not all, may be included under this term. rocks that are made out of fragments of other rocks torn down by the agencies of erosion are called _fragmental_. wind, water, and ice are the three great agencies that wear away the land, bring rock fragments long distances, and deposit them where aqueous rocks are being formed. volcanic eruptions bring material from the earth's interior. this material ranges all the way from huge boulders to the finest impalpable dust, called volcanic ashes. rivers of ice called glaciers crowd against their banks, loosening rock masses and carrying away fragments of all sizes, in their progress down the valley. brooks and rivers carry the pebbles and the larger rock masses they are able to loosen from their walls and beds, and grind them smooth as they move along toward lower levels. the air itself causes rocks to crumble; percolating water robs them of their soluble salts, reducing even solid granite to a loose mass of quartz grains and clay. plants and animals absorb as food the mineral substances of rocks, when they are dissolved in water. they transform these food elements into their own body substance, and finally give back their dead bodies, the mineral substances of which are freed by decay to return to the earth, and become elements of rock again. the decay of rock is well shown by the materials that accumulate at the base of a cliff. angular fragments of all sizes, but all more or less flattened, come from strata of shaly rock, that can be seen jutting out far above. a great deal of this sort of material is found mingled with the soil of the northeastern states. round pebbles in pudding-stone have been formed in brook beds and deposited on beaches where they have become caked in mud and finally consolidated into rock. if the beach chanced to be sandy instead of muddy, a matrix of sandy paste holds the larger pebbles in place. limestone paste cements together the pebbles of limestone conglomerates. in st. augustine many of the houses are built of coquina rock, a mass of broken shells which have become cemented together by lime mud, derived from their own decay. on the slopes of volcanoes, rock fragments of all kinds are cemented together by the flowing lava. so we see that there are pudding-stones of many kinds to be found. if some solvent acid is present in the water that percolates through these rocks it may soften the cement and thus free the pebbles, reducing the conglomerate again to a mere heap of shell fragments, or gravel, or rounded pebbles. the story of rock formation tells how fire and water, and the two combined, have made, and made over, again and again, the substance of the earth's crust. chemical and physical changes constantly tear down some portions of the earth to build up others. the constant, combined effort of wind and water is to level the earth and fill up the ocean bed. rocks are constantly being formed; the changes that have been going on since the world began are still in progress. we can see them all about us on any and every day of our lives. getting acquainted with a river i have two friends whose childhood was spent in a home on the banks of a noble eastern river. their father taught the boy and the girl to row a boat, and later each learned the more difficult art of managing a canoe. on holidays they enjoyed no pleasure so much as a picnic on the river-bank at some point that could be reached by rowing. as they grew older, longer trips were planned, and the river was explored as far as it was navigable by boat or canoe. last summer when vacation came, these two carried out a long-cherished plan to find the beginning of the river--to follow it to its source. so they left home, and canoed up-stream, until the stream became a brook, so shallow they could go no farther. then they followed it on foot--wading, climbing, making little détours, but never losing the little river. at last they came to the beginning of it--a tiny rivulet trickled out of the side of a hill, filling a wooden keg that formed a basin, where thirsty passers-by could stoop and drink. they decided to mark the spring, so that people who found it later, and were refreshed by its clear water, might know that here was born the greatest river of a great state. but they were not the original discoverers. above the spring, a board was nailed to a tree, saying that this is the headwater of the river with the beautiful indian name, susquehanna. it was a dry summer, and the overflow of the basin was almost all drunk up by the thirsty ground. they could scarcely follow it, except by the groove cut by the rivulet in seasons when the flow was greater. they followed the runaway brook, through the grass roots, that almost hid it. as the ground grew steeper, it hurried faster. soon it gathered the water of other springs, which hurried toward it in small rivulets, because its level was lower. water always seeks the lowest level it can find. sometimes marshy spots were reached where water stood in the holes made by the feet of cattle that came there to drink. the water was muddy, and seemed to stand still. but it was settling steadily, and at one side the little river was found, flowing away with the water it drew from the swampy, springy ground. all the mud was gone, now; the water was clear. it flowed in a bed with a stony floor, and there were rough steps where the water fell down in little sheets, forming a waterfall, the first of many that make this river beautiful in the upper half of its course. to get from the high level of that hillside spring to the low level of the sea, the water has to make a fall of twenty-three hundred feet, but it makes the descent gradually. it could not climb over anything, but always found a way to get around the rocks and hills that stood in its way. when the flat marsh land interfered, the water poured in and overflowed the basin at the lowest margin. in the rocky ground the two explorers found that the stream had widened its channel by entering a narrow crevice and wearing away its walls. the continual washing of the water wears away stone. rocks are softened by being wet. streaks of iron in the hardest granite will rust out and let the water in. then the lime in rocks is easily dissolved. every dead leaf the river carried along added an acid to the water, and this made easier the process of dissolving the limestone. every crumbling rock gives the river tools that it uses like hammer and chisel and sandpaper to smooth all the uneven surfaces in its bed, to move stumbling blocks, and to dig the bed deeper and wider. the steeper the slope is, the faster the stream flows, and the larger the rocks it can carry. rocks loosened from the stream bed are rolled along by the current. then bang! against the rocks that are not loose, and often they are able to break them loose. the fine sand is swept along, and its sharp points strike like steel needles, and do a great work in polishing roughness and loosening small particles from the stream bed. the bigger pebbles of the stream have banged against the rock walls, with the same effect, smoothing away unevenness and pounding fragments loose, rolling against one another, and getting their own rough corners worn away. the makers of stone marbles learned their business from a brook. they cut the stone into cubical blocks, and throw them into troughs, into which is poured a stream of running water. the blocks are kept in motion, and the grinding makes each block help the rest to grind off the eight corners and the twelve ridges of each one. the water becomes muddy with the fine particles, just as the drip from a grindstone becomes unclean when an axe is ground. pretty soon all the blocks in the trough are changed into globes--the marbles that children buy at the shops when marble season comes around. i suppose if the troughs are not watched and emptied in time, the marbles would gradually be ground down to the size of peas, then to the size of small bird shot, and finally they would escape as muddy water and fine sand grains. sure it is that the sandy shores that line most rivers are the remnants of hard rocks that have been torn out and ground up by the action of the current. not very many miles from its first waterfall the stream had grown so large that my two friends knew that they would soon find their canoes. the plan now was to float down the curious, winding river and to learn, if the river and the banks could tell them, just why the course was so crooked on the map. they came into a broad, level valley where streams met them, coming out of deep clefts between the hills they were leaving behind them. the banks were pebbly, but blackened with slimy mud that made the water murky. the current swerved from one side to the other, sometimes quite close to the bank, where the river turned and formed a deep bend. on this side the bank was steep, the roots of plants and trees exposed. on the opposite side a muddy bank sloped gently out into the stream. here building up was going on, to offset the tearing down. the sharp bends are made sharper, once the current is deflected from the middle of the stream to one side. at length the loops bend on each other and come so near together that the current breaks through, leaving a semicircular bayou of still water, and the river's course straightened at that place. it must have been in a spring flood that this cut-off was made, and, the break once made was easily widened, for the soil is fine mud which, when soaked, crumbles and dissolves into muddy water. stately and slow that river moves down to the bay, into which it empties its load. the rain that falls on hundreds of square miles of territory flows into the streams that feed this trunk. the little spring that is the headwater of the system is but one of many pockets in the hillsides that hold the water that soaks into the ground and give it out by slow degrees. surface water after a rain flows quickly into the streams. it is the springs that hold back their supply and keep the rivers from running dry in hot weather. do they feel now that they know their river? are they ready to leave it, and explore some other? indeed, no. they are barely introduced to it. all kinds of rivers are shown by the different parts of this one. it is a river of the mountains and of the lowland. it flows through woods and prairies, through rocky passes and reedy flats. it races impetuously in its youth, and plods sedately in later life. the trees and the other plants that shadow this stream, and live by its bounty, are very different in the upland and in the lowland. the scenery along this stream shows endless variety. up yonder all is wild. down here great bridges span the flood, boats of all kinds carry on the commerce between two neighbour cities. a great park comes down to the river-bank on one side. canoes are thick as they can paddle on late summer afternoons. no one can ever really know a river well enough to feel that it is an old story. there is always something new it has to tell its friends. so my two explorers say, and they know far more about their friendly river than i do. the ways of rivers a canal is an artificial river, built to carry boats from one place to another. its course is, as nearly as possible, a straight line between two points. a river, we all agree, is more beautiful than a canal, for it winds in graceful curves, in and out among the hills, its waters seeking the lowest level, always. no artist could lay out curves more beautiful than the river forms. these curves change from year to year, some slowly, some more rapidly. it is not hard to understand just why these changes take place. some rivers are dangerous for boating at certain points. the current is strong, and there are eddies and whirlpools that have to be avoided, or the boat becomes unmanageable. people are drowned each season by trusting themselves to rivers the dangerous tricks of which they do not know. deep holes are washed out of the bed of the stream by whirling eddies. the pot-holes of which people talk are deep, rounded cavities, ground out of the rocky stream-bed by the scouring of sand and loose stones driven by whirling eddies in shallow basins. every year deepens each pot-hole until some change in the stream-bed shifts the eddy to another place. no stream finds its channel ready-made; it makes its own, and constantly changes it. the current swings to one side of the channel, lifting the loose sediment and grinding deeper the bed of the stream. the water lags on the opposite side, and sediment falls to the bottom. so the building-up of one side is going on at the same time that the tearing-down process is being carried on on the other. with the lowering of the bed the river swerves toward one bank, and a hollow is worn by slow degrees. the current swings into this hollow, and in passing out is thrown across the stream to the opposite bank. here its force wears away another hollow; and so it zigzags down-stream. the deeper the hollows, the more curved becomes the course, if the general fall is but moderate. it is toward the lower courses of the stream that the winding becomes more noticeable. the sediment that is carried is deposited at the point where the current is least strong, so that while the outcurves become sharper by the tearing away of the stream's bank, the incurves become sharper by the building up of this bank. the mississippi below memphis is thrown into a wonderful series of curves by the erosion and the deposit caused by the current zigzagging back and forth from one bank to the other. gradually the curves become loops. the river's current finally jumps across the meeting of the curves, and abandons the circular bend. it becomes a bayou or lagoon of still water, while the current flows on in the straightened channel. all rivers that flow through flat, swampy land show these intricate winding channels and many lagoons that have once been curves of the river. no one would ever mistake a river for a lake or any other body of water, yet rivers differ greatly in character. one tears its way along down its steep, rock-encumbered channel between walls that rise as vertical precipices on both sides. the roaming, angry waters are drawn into whirlpools in one place. they lie stagnant as if sulking in another, then leap boisterously over ledges of rock and are churned into creamy foam at the bottom. outside the mountainous part of its course this same river flows broad and calm through a mud-banked channel, cut by tributary streams that draw in the water of low, sloping hills. the missouri is such a wild mountain stream at its headwaters. we who have seen its muddy waters from sioux city to st. louis would hardly believe that its impetuous and picturesque youth could merge into an old age so comfortable and placid and commonplace. this thing is true of all rivers. they flow, gradually or suddenly, from higher to lower levels. to reach the lowest level as soon as possible is the end each river is striving toward. if it could, each river would cut its bed to this depth at the first stage of its course. its tools are the rocks it carries, great and small. the force that uses these tools is the power of falling water, represented by the current of the stream. the upper part of a river such as the missouri or mississippi engages in a campaign of widening and deepening its channel, and carrying away quantities of sediment. the lower reaches of the stream flow through more level country; the current is checked, and a vast burden of sediment is laid down. instead of tearing away its banks and bottom, the river fills up gradually with mud. the current meanders between banks of sediment over a bottom which becomes shallower year by year. the rocky mountains are being carried to the gulf of mexico. the commerce of the river is impeded by mountain débris deposited as mud-banks along the river's lower course. many rivers are quiet and commonplace throughout their length. they flow between low, rounded hills, and are joined by quiet streams, that occupy the separating grooves between the hills. this is the oldest type of river. it has done its work. rainfall and stream-flow have brought the level of the land nearly to the level of the stream. very little more is left to be ground down and carried away. the landscape is beautiful, but it is no longer picturesque. wind and water have smoothed away unevennesses. trees and grass and other vegetation check erosion, and the river has little to do but to carry away the surface water that falls as rain. but suppose our river, flowing gently between its grassy banks, should feel some mighty power lifting it up, with all its neighbour hills and valleys, to form a wrinkle in the still unstable crust of the earth. away off at the river's mouth the level may not have changed, or that region may have been depressed instead of elevated by the shrinking process. suppose the great upheaval has not severed the upper from the lower courses of the stream. with tremendous force and speed, the current flows from the higher levels to the lower. the river in the highlands strikes hard to reach the level of its mouth. it grinds with all its might, and all its rocky tools, upon its bed. all the mud is scoured out, and then the underlying rocks are attacked. if these rocks are soft and easily worn away, the channel deepens rapidly. one after another the alternating layers are excavated, and the river flows in a canyon which deepens more and more. as the level is lowered, the current of the stream becomes slower and the cutting away of its bed less rapid. the stream is content to flow gently, for it has almost reached the old level, on which it flowed before the valley became a ridge or table-land. the rivers that flow in canyons have been thousands of years in carving out their channels, yet they are newer, geologically speaking, than the streams that drain the level prairie country. the earth has risen, and the canyons have been carved since the prairies became rolling, level ground. [illustration: this little pond is a basin hollowed by the same glacier that scattered the stones and rounded the hills] [illustration: every stream is wearing away its banks, while trees and grass blades are holding on to the soil with all their roots] the colorado river flows through a canyon with walls that in places present sheer vertical faces a mile in depth, and so smooth that no trail can be found by which to reach from top to bottom. the region has but slight erosion by wind, and practically none by rain. the local rainfall is very slight. so the river is the one force that has acted to cut down the rocks, and its force is all expended in the narrow area of its own bed. had frequent rains been the rule on the colorado plateau, the angles of the mesas would have been rounded into hills of the familiar kind so constantly a part of the landscape in the eastern half of the continent. the colorado is an ancient river which has to carry away the store of moisture that comes from the pacific ocean and falls as snow on the high peaks of the rocky mountains. similar river gorges with similar stories to tell are the arkansas, the platte, and the yellowstone. all cut their channels unaided through regions of little rain. when the earth's crust is thrown up in mountain folds, and between them valleys are formed, the level of rivers is sometimes lowered and the rapidity of their flow is checked. a stream which has torn down its walls at a rapid rate becomes a sluggish water-course, its current clogged with sediment, which it has no power to carry farther. when such a river begins to build and obstruct its own waters it bars its progress and may form a lake as the outlet of its tributary streams. many ancient rivers have been utterly changed and some obliterated by general movements of the earth's crust. the story of a pond look out of the car window as you cross a flat stretch of new prairie country, and you see a great many little ponds of water dotting the green landscape. forty years ago iowa was a good place to see ponds of all shapes and sizes. the copious rainfall of the early spring gathered in the hollows of the land, and the stiff clay subsoil prevented the water from soaking quickly into the ground. the ponds might dry away during the hot, dry summer, leaving a baked clay basin, checked with an intricate system of cracks. or if rains were frequent and heavy, they might keep full to the brim throughout the season. tall bulrushes stood around the margins of the largest ponds, and water-lilies blossomed on the surface during the summer. the bass and the treble of the spring chorus were made by frogs and toads and little hylas, all of which resorted to the ponds to lay their eggs, in coiled ropes or spongy masses, according to their various family traditions. on many a spring night my zoölogy class and i have visited the squashy margins of these ponds, and, by the light of a lantern, seen singing toads and frogs sitting on bare hummocks of grass roots that stood above the water-line. the throat of each musician was puffed out into a bag about the size and shape of a small hen's egg; and all were singing for dear life, and making a din that was almost ear-splitting at close range. so great was the self-absorption of these singers that we could approach them, daze them with the light of the lantern, and capture any number of them with our long-handled nets before they noticed us. but it was not easy to persuade them to sing in captivity, no matter how many of the comforts of home we provided in the school aquariums. so, after some very interesting nature studies, we always carried them back and liberated them, where they could rejoin their kinsfolk and neighbours. it was when we were scraping the mud from our rubber boots that we realized the character of the bottoms of our prairie ponds. the slimy black deposit was made partly of the clay bottom, but largely of decaying roots and tops of water plants of various kinds. whenever it rained or the wind blew hard, the bottom was stirred enough to make the water muddy; and on the quietest days a pail of pond water had a tinge of brown because there were always decaying leaves and other rubbish to stain its purity. the farmers drained the ponds as fast as they were able, carrying the water, by open ditches first, and later by underground tile drains, to lower levels. finally these trunk drain pipes discharged the water into streams or lakes. to-day a large proportion of the pond areas of iowa has disappeared; the hollow tile of terra-cotta has been the most efficient means of converting the waste land, covered by ponds, into fertile fields. but the ponds that have not been drained are smaller than they used to be, and are on the straight road to extinction. this process one can see at any time by visiting a pond. every year a crop of reeds and a dozen other species of vigorous water plants dies at the top and adds the substance of their summer growth to the dust and other refuse that gathers in the bottom of the pond. each spring roots and seeds send up another crop, if possible more vigorous than the last, and this top growth in turn dies and lies upon the bottom. the pond level varies with the rainfall of the years, but it averages a certain depth, from which something is each year subtracted by the accumulations of rotting vegetable matter in the bottom. evaporation lowers the water-level, especially in hot, dry summers. from year to year the water plants draw in to form a smaller circle, the grassy meadow land encroaches on all sides. the end of the story is the filling up of the pond basin with the rotting substance of its own vegetation. this is what is happening to ponds and inland marshes by slow degrees. the tile drain pipes obliterate the pond in a single season. nature is more deliberate. she may require a hundred years to fill up a single pond which the farmer can rid himself of by a few days of work and a few rods of tiling. the riddle of the lost rocks outside of my window two robins are building a nest in the crotch of a blossoming red maple tree. and just across the hedge, men are digging a big square hole in the ground--the cellar of our neighbour's new house. it looks now as if the robins would get their house built first, for they need but one room, and they do not trouble about a cellar. i shall watch both houses as they grow through the breezy march days. the brown sod was first torn up by a plough, which uncovered the red new jersey soil. two men, with a team hitched to a scraper, have carried load after load of the loose earth to a heap on the back of the lot, while two other men with pickaxes dug into the hard subsoil, loosening it, so that the scraper could scoop it up. this subsoil is heavy, like clay, and it breaks apart into hard clods. at the surface the men found a network of tree roots, about which the soil easily crumbled. often i hear a sharp, metallic stroke, unlike the dull sound of the picks striking into the earth. the digger has struck a stone, and he must work around it, pry it up and lift it out of the way. a row of these stones is seen at one side of the cellar hole, ranged along the bank. they are all different in size and shape, and red with clay, so i can't tell what they are made of. but from this distance i see plainly that they are irregular in form and have no sharp corners. the soil strewn along the lot by the scraper is full of stones, mostly irregular, but some rounded; some are as big as your head, others grade down to the sizes of marbles. when i went down and examined this red earth, i found pebbles of all shapes and sizes, gravel in with the clay, and grains of sand. this rock-sprinkled soil in new jersey is very much like soil which i know very well in iowa; it looks different in colour, but those pebbles and rock fragments must be explained in the same way here as there. these are not native stones, the outcrop of near-by hillsides, but strangers in this region. the stones in iowa soil are also imported. the prairie land of iowa has not many big rocks on the surface, yet enough of them to make trouble. the man who was ploughing kept a sharp lookout, and swung his plough point away from a buried rock that showed above ground, lest it should break the steel blade. one of the farmer's jobs for the less busy season was to go out with sledge and dynamite sticks, and blast into fragments the buried boulders too large to move. sometimes building a hot fire on the top of it, and throwing on water, would crack the stubborn "dornick" into pieces small enough to be loaded on stone-boats. i remember when the last giant boulder whose buried bulk scarcely showed at the surface, was fractured by dynamite. its total weight proved to be many tons. we hauled the pieces to the great stone pile which furnished materials for walling the sides of a deep well and for laying the foundation of the new house. yet for years stones have been accumulating, all of them turned out of the same farm, when pastures and swampy land came under the plough. draw a line on the map from new york to st. louis, and then turn northward a little and extend it to the yellowstone park. the boulder-strewn states lie north of this line, and are not found south of it, anywhere. canada has boulders just like those of our northern states. the same power scattered them over all of the vast northern half of north america and a large part of europe. what explanation is there for this extensive distribution of unsorted débris? the question answered the rocks tell their own story, partly, but not wholly. they told just enough to keep the early geologists guessing; and only very recently has the guessing come upon the truth. these things the rocks told: . we have come from a distance. . we have had our sharp corners worn off. . many of us have deep scratches on our sides. . at various places we have been dumped in long ridges, mixed with much earth. . a big boulder is often balanced on another one. the first thing the geologist noted was the fact that these boulders are strangers--that is, they are not the native rocks that outcrop on hillsides and on mountain slopes near where they are found. far to the north are beds of rock from which this débris undoubtedly came. could a flood have scattered them as they are found? no, for water sorts the rock débris it deposits, and it rounds and polishes rock fragments, instead of scratching and grooving them and leaving them angular, as these are. professor agassiz went to switzerland and studied the glaciers. he found unsorted rock fragments where the glacier's nose melted, and let them fall. they were worn and scratched and grooved, by being frozen into the ice, and dragged over the rocky bed of the stream. the rocky walls of the valley were scored by the glacier's tools. rounded domes of rock jutted out of the ground, in the paths of the ice streams, just like the granite outcrop in central park in new york, and many others in the region of scattered boulders. after long studies in europe and in north america, professor agassiz declared his belief that a great ice-sheet once covered the northern half of both countries, rounding the hills, scooping out the valleys and lake basins, and scattering the boulders, gravel, and clay, as it gradually melted away. the belief of professor agassiz was not accepted at once, but further studies prove that he guessed the riddle of the boulders. the rich soil of the northern states is the glacial drift--the mixture of rock fragments of all sizes with fine boulder clay, left by the gradual melting of the great ice-sheet as it retreated northward at the end of the "glacial epoch." glaciers among the alps switzerland is a little country without any seacoast, mountainous, with steep, lofty peaks, and narrow valleys. the climate is cool and moist, and snow falls the year round on the mountain slopes. a snow-cap covers the lower peaks and ridges. above the level of nine thousand feet the bare peaks rise into a dry atmosphere; but below this altitude, and above the six thousand-foot mark, lies the belt of greatest snowfall. peaks between six and nine thousand feet high are buried under the alpine snow-field, which adds thickness with each storm, and is drained away to feed the rushing mountain streams in the lower valleys. the snow that falls on the steep, smooth slope clings at first; but as the thickness and the weight of these snow banks increase, their hold on the slope weakens. they may slip off, at any moment. the village at the foot of the slope is in danger of being buried under a snow-slide, which people call an avalanche. "challanche" is another name for it. the hunter on the snow-clad mountains dares not shout for fear that his voice, reëchoing among the silent mountains, may start an avalanche on its deadly plunge into the valley. on the surface of the snow-field, light snow-flakes rest. under them the snow is packed closer. deeper down, the snow is granular, like pellets of ice; and still under this is ice, made of snow under pressure. the weight of the accumulated snow presses the underlying ice out into the valleys. these streams are the glaciers--rivers of ice. the glaciers of the alps vary in length from five to fifteen miles, from one to three miles in width, and from two hundred to six hundred feet in thickness. they flow at the rate of from one to three feet a day, going faster on the steeper slopes. it is hard to believe that any substance as solid and brittle as ice can flow. its movement is like that of stiff molasses, or wax, or pitch. the tremendous pressure of the snow-field pushes the mass of ice out into the valleys, and its own weight, combined with the constant pressure from behind, keeps it moving. the glacier's progress is hindered by the uneven walls and bed of the valley, and by any decrease in the slope of the bed. when a flat, broad area is reached, a lake of ice may be formed. these are not frequent in the alps. the water near the banks and at the bottom of a river does not flow as swiftly as in the middle and at the surface of the stream. the flow of ice in a glacier is just so. friction with the banks and bottom retards the ice while the middle parts go forward, melting under the strain, and freezing again. there is a constant readjusting of particles, which does not affect the solidity of the mass. the ice moulds itself over any unevenness in its bed if it cannot remove the obstruction. the drop which would cause a small waterfall in a river, makes a bend in the thick body of the ice river. great cracks, called _crevasses_, are made at the surface, along the line of the bend. the width of the v-shaped openings depends upon the depth of the glacier and the sharpness of the bend that causes the breaks. rocky ridges in the bed of the ice-stream may cause crevasses that run lengthwise of the glacier. snow may fill these chasms or bridge them over. the hunter or the tourist who ventures on the glacier is in constant danger, unless he sees solid ice under him. men rope themselves together in climbing over perilous places, so that if one slips into a crevasse his mates can save him. a glacier tears away and carries away quantities of rock and earth that form the walls of its bed. as the valley narrows, tremendous pressure crowds the ice against the sides, tearing trees out by the roots and causing rock masses to fall on the top of the glacier, or to be dragged along frozen solidly into its sides. the weight of the ice bears on the bed of the glacier, and its progress crowds irresistibly against all loose rock material. the glacier's tools are the rocks it carries frozen into its icy walls and bottom. these rocks rub against the walls, grinding off débris which is pushed or carried along. no matter how heavy the boulders are that fall in the way of the ice river, the ice carries them along. it cannot drop them as a river of water would do. slowly they travel, and finally stop where the nose of the glacier melts and leaves all débris that the mountain stream, fed by the melting of the ice, cannot carry away. the bedrock under a glacier is scraped and ground and scored by the glacier's tools--the rock fragments frozen into the bottom of the ice. these rocks are worn away by constant grinding, just as a steel knife becomes thin and narrow by use. scratches and scorings and polished surfaces are found in all rocks that pass one another in close contact. its worn-out tools the glacier drops at the point where its ice melts. this great, unsorted mass of rock meal and coarser débris the stream is gradually scattering down the valley. the name "moraine" has been given to the earth rubbish a glacier collects and finally dumps. the _top moraine_ is at the surface of the ice. the _lateral moraines_, one at each side, are the débris gathered from the sides of the valley. the _ground moraine_ is what débris the ice pushes and drags along on the bottom. the _terminal moraine_ is the dumping-ground of this mass of material, where the ice river melts. glaciers, like other rivers, often have tributary streams. a _median moraine_, seen as a dark streak running lengthwise on the surface of a glacier, means that two branch glaciers have united to form this one. go back far enough and you will reach the place where the two streams come together. the two lateral moraines that join form the middle line of débris, the median moraine. three ice-streams joined produce two top moraines. they locate the lateral moraines of the middle glacier. the surface of a glacier is often a mass of broken and rough ice, forming a series of pits and pinnacles that make crossing impossible. the sun melts the surface, forming pools and percolating streams of water, that honeycomb the mass. underneath, the ice is tunnelled, and a rushing stream flows out under the end of the glacier. it is not clear, but black with mud, called _boulder clay_, or _till_, made of ground rock, and mixed with fragments of all shapes and sizes. this is the meal from the glacier's mill, dumped where the water can sift it. "balanced rocks" are boulders, one upon another, that once lay on a glacier, and were left in this strange, unstable position when the supporting ice walls melted away from them. in bronx park in new york the "rocking stone" always attracts attention. the glacier that lodged it there, also rounded the granite dome in central park and scattered the rock-strewn boulder clay on long island. doubtless in an earlier day the edges of this glacier were thrust out into the atlantic, not far from the great south bay, and icebergs broke off and floated away. [illustration: potsdam sandstone showing ripple marks] [illustration: _by permission of the american museum of natural history_ glacial striæ on lower helderberg limestone] [illustration: glacial grooves in the south meadow, central park, new york] [illustration: _by permission of the american museum of natural history_ mt. tom, west d st., new york] glaciers are small to-day compared with what they were long ago, in europe and in america. the climate became warmer, and the ice-cap retreated. old moraines show that the ice rivers of the alps once came much farther down the valleys than they do now. smooth, deeply scored domes of rock, the one in central park and the bald head of mount tom, are just like those that lie in alpine valleys from which the glaciers have long ago retreated. there are old moraines far up the sides of valleys, showing that once the glaciers were far deeper than now. no other power could have brought rocks from strata higher up the mountains, and lodged them thus. nearer home, mt. shasta and mt. rainier still have glaciers that have dwindled in size, until they bear little comparison to the gigantic ice-streams that once filled the smooth beds their puny successors flow into. remnants of glaciers lie in the hollows of the sierras. we must go north to find the snow-fields of alaska and glaciers worthy to be compared with those ancient ice rivers whose work is plainly to be seen, though they are gone. the great ice-sheet greenland is green only along its southern edge, and only in summer, so its name is misleading. it is a frozen continent lying under a great ice-cap, which covers , square miles and is several thousand feet in thickness. the top of this icy table-land rises from five thousand to ten thousand feet above the sea-level. the long, cold winters are marked by great snowfall, and the drifts do not have time to melt during the short summer; and so they keep getting deeper and deeper. streams of ice flow down the steeps into the sea, and break off by their weight when they are pushed out into the water. these are the icebergs which float off into the north atlantic, and are often seen by passengers on transatlantic steamers. long ago greenland better deserved its name. explorers who have climbed the mountain steeps that guard the unknown ice-fields of the interior have discovered, a thousand feet above the sea-level, an ancient beach, strewn with shells of molluscs like those which now inhabit salt water, and skeletons of fishes lie buried in the sand. it is impossible to think that the ocean has subsided. the only explanation that accounts for the ancient beach, high and dry on the side of greenland's icy mountain is that the continent has been lifted a thousand feet above its former level. this is an accepted fact. we know that climate changes with changed altitude as well as latitude. going up the side of a mountain, even in tropical regions, we may reach the snow-line in the middle of summer. magnolia trees and tree ferns once grew luxuriantly in greenland forests. their fossil remains have been found in the rocks. this was long before the continent was lifted into the altitude of ice and snow. and it is believed that the climate of northern latitudes has become more severe than formerly from other causes. it is possible that the earth's orbit has gradually changed in form and position. if greenland should ever subside until the ancient beach rests again at sea-level, the secrets of that unknown land would be revealed by the melting of the glacial sheet that overspreads it. possibly it would turn out to be a mere flock of islands. we can only guess. north america had, not so long ago, two-thirds of its area covered with an ice-sheet like that of greenland, and a climate as cold as greenland's. at this time the land was lifted two to three thousand feet higher than its present level. all of the rain fell as snow, and the ice accumulated and became thicker year by year. instead of glaciers filling the gorges, a great ice flood covered all the land, and pushed southward as far as the ohio river on the east and yellowstone park in the west. the rocky mountains and some parts of the appalachian system accumulated snow and formed local glaciers, separated from the vast ice-sheet. the unstable crust of the earth began to sink at length, and gradually the ice-sheet's progress southward was checked, and it began to recede by melting. all along the borders of this great fan-shaped ice-field water accumulated from the melting, and flooded the streams which drained it to the atlantic and the gulf. icebergs broken off of the edge of the retiring ice-sheet floated in a great inland sea. the land sank lower and lower until the general level was five hundred to one thousand feet lower than it now is. the climate became correspondingly warm, and the icebergs melted away. then the land rose again, and in time the inland sea was drained away into the ocean, except for the waters that remained in thousands of lakes great and small that now occupy the region covered by the ice. ancient sea beaches mark the level of high water at the time that the flood followed the melting ice. on the shores of lake champlain, but nearly five hundred feet higher than the present level of the lake, curious geologists have found many kinds of marine shells on a well-marked old sea beach. the members of one exploring party in the same region were surprised and delighted to come by digging upon the skeleton of a whale that had drifted ashore in the ancient days when the inland sea joined the atlantic. lake ontario's ancient beach is five hundred feet above the present water-level; lake erie's is two hundred fifty feet above it; lake superior's three hundred thirty feet higher than the present beach. no doubt when the water stood at the highest level, the great lakes formed one single sheet of water which settled to a lower level as the rivers flowing south cut their channels deep enough to draw off the water toward the gulf. lake winnipeg is now the small remnant of a vast lake the shores of which have been traced. the minnesota river finally made its way into the mississippi and drained this great area the stranded beaches of which still remain. the name of agassiz has been given to the ancient lake formed by the glacial flood and drained away thousands of years ago but not until it had built the terraced beach which locates it on the geological map of the region. when the ice-sheet came down from the north it dragged along all of the soil and loose rock material that lay in its path. with the boulders frozen into its lower surface it scratched and grooved the firm bedrock over which it slid, and rounded it to a smooth and billowy surface. the progress of the ice-sheet was southward, but it spread like a fan so that its widening border turned to east and west. when it reached its southernmost limit and began to melt, it laid down a great ridge of unsorted rock material, remnants of which remain to this day,--the terminal moraine of the ancient ice-sheet. the line of this ancient deposit starts on long island, crosses new jersey and pennsylvania, then dips southward, following the general course of the ohio river to its mouth, forming bluffs in southern ohio, indiana, and illinois. the line bends upward as it crosses central missouri, a corner of kansas, and eastern nebraska, parallel with the course of the missouri. as the ice-sheet melted, boulders were dropped all over the northern states and canada. these were both angular and rounded. in some places they are scattered thickly over the surface and are so numerous as to be a great hindrance to agriculture. in many places great boulders of thousands of tons weight are perched on very slight foundations, just where they lodged when the ice went off and left them, after carrying them hundreds of miles. around them are scattered quantities of loose rock material, not scored or ground as are those which were carried on the under-surface of the glacial ice. these unscarred fragments rode on the top of the ice. they were a part of the top moraine of the glacial sheet. the finest material deposited is rock meal, ground by the great glacial mill, and called "boulder clay." it is a stiff, dense, stony paste in which boulders of all sizes, gravel, pebbles, and cobblestones are cemented. the "drift" of the ice-sheet is the rubbish, coarse and fine, it left behind as it retreated. below the ohio river there is a deep soil produced by the decay of rocks that lie under it. north of ohio is spread that peculiar mixture of earth and rock fragments which was transported from the north and spread over the land which the ice-sheet swept bare and ground smooth and polished. the drift has been washed away in places by the floods that followed the ice. granite domes are thus exposed, the grooves and scratches of which tell in what direction the ice flood was travelling. miles away from that scored granite, but in the same direction as the scratches, scattered fragments of the same foundation rock cover fields and meadows. thus, much of the drift material can be traced to its original home, and the course of the ice-sheet can be determined. many immense boulders the home of which was in the northern highlands of canada rode southward, frozen into icebergs that floated in the great inland sea. great quantities of débris were added to the original glacial drift through the agency of these floating ice masses, which melted by slow degrees. following some lost rivers what would you think if the boat in which you were floating down a pleasant river should suddenly grate upon sand, and you should look over the gunwale and find that here the waters sank out of sight, the river ended? i believe you would rub your eyes, and feel sure that you were dreaming. do not all rivers flow along their beds, growing larger with every mile, and finally empty their waters into a sea, or bay, or lake, or flow into some larger stream? this is the way of most rivers, but there are exceptions. in the far west there are some great rivers that absolutely disappear before they reach a larger body of water. they simply sink away into the sand, and sometimes reappear to finish their courses after flowing underground for miles. do you know the name of one great western river of which i am thinking? is there any stream in your neighbourhood which has such peculiar ways? down in kentucky there is a region where, it is said, one may walk fifty miles without crossing running water. in the middle of our country, in the region of plentiful rainfall, and in a state covered with beautiful woodlands and famous for blue grass and other grain crops, it is amazing that, over a large area, brooks and larger streams are lacking. in most of the state there is plenty of water flowing in streams like those in other parts of the eastern half of the united states. in the near neighbourhood of this peculiar section of the state the streams come to an end suddenly, pouring their water into funnel-shaped depressions of the ground called sink-holes. after a heavy rain the surface water, accumulating in rivulets, may also be traced to small depressions which seem like leaks in the earth's crust, into which the water trickles and disappears. it must have been noticed by the early settlers who came over the mountains from the eastern colonies, and settled in the new, wild, hilly country, which they called kentucky. the first settlers built their log cabins along the streams they found, and shot deer and wild turkey and other game that was plentiful in the woods. the deer showed them where salt was to be found in earthy deposits near the streams; for salt is necessary to every creature. deer trails led from many directions to the "salt licks" which the wild animals visited frequently. perhaps the same pioneers who dug the salt out of the earth found likewise deposits of _nitre_, called also _saltpetre_, a very precious mineral, for it is one of the elements necessary in the manufacture of gunpowder. with the indians all about him, and often showing themselves unfriendly, the pioneer counted gunpowder a necessity of life. he relied on his gun to defend and to feed his family. there were men among those first settlers who knew how to make gunpowder, and saltpetre was one of the things that had to be carried across the mountains into kentucky, until they found it in the hills. no wonder that prospectors went about looking for nitre beds in the overhanging ledges of rocks along stream-beds. in such situations the deposits of nitre were found. the earth was washed in troughs of running water to remove the clayey impurity. after a filtering through wood-ashes, the water which held the nitre in solution was boiled down, and left to evaporate, after which the crystals of saltpetre remained. solid masses of saltpetre weighing hundreds of pounds were sometimes found in protected corners under shelving rocks. it was no doubt in the fascinating hunt for lumps of this pure nitre that the early prospectors discovered that the streams which disappeared into the sink-holes made their way into caverns underground. digging in the sides of ravines often made the earthy wall cave in, and the surprised prospector stood at the door of a cavern. the discoverer of a cave had hopes that by entering he might find nitre beds richer than those he could uncover on the surface, and this often turned out to be true. the hope of finding precious metals and beds of iron ore also encouraged the exploration of these caves. by the time the war of was declared, the mining of saltpetre was a good-sized industry in kentucky. most of the mineral was taken out of small caves, and shipped, when purified, over the mountains, on mule-back by trails, and in carts over good roads that were built on purpose to bring this mineral product to market. as long as war threatened the country, the government was ready to buy all the saltpetre the kentucky frontiersmen could produce. and the miners were constantly in search of richer beds that promised better returns for their labour. it was this search that led to the exploration of the caves discovered, although the explorer took his life in his hands when he left the daylight behind him and plunged into the under-world. not all lost rivers tell as interesting stories or reveal as valuable secrets as did those the neighbours of daniel boone traced along their dark passages underground, and finally saw emerge as hillside springs, in many cases, to feed kentucky rivers. but it is plain that no river sinks from sight unless it finds porous or honeycombed rocks that let it through. the water seeks the nearest and easiest route to the sea. its weight presses toward the lowest level, always. the more water absorbs of acid, the more powerfully does it attack and carry away the substance of lime rocks through which it passes. the mammoth cave of kentucky there is no more fertile soil in the country than that of the famous blue grass region of kentucky. the surface soil rests upon a deep foundation of limestone rocks, and very gradually the plant food locked up in these underlying strata is pulled up to the surface by the soil water, and greedily appropriated by the roots of the plants. part of the water of the abundant rainfall of this region soaks into the layers of the lime rock, carrying various acids in solution which give it power to dissolve the limestone particles, and thus to make its way easily through comparatively porous rock to the very depths of the earth. so it has come about that the surface of the earth is undermined. vast empty chambers have been carved by the patient work of trickling water, which has carried away the lime that once formed solid and continuous layers of the earth's crust. we must believe that the work has taken thousands of years, at least, for no perceptible change has come to these wonderful caves since the discovery and exploration of them a century and more ago. the streams that flow into the region of these caves disappear suddenly into sink-holes and flow through caverns. after wearing away their subterranean channels, leaping down from one level to another, forming waterfalls and lakes, some emerge finally through hillsides in the form of springs. the cavern region of kentucky covers eight thousand square miles. the underground chambers found there are in the limestone rock which varies from ten to four hundred feet in thickness, and averages a little less than two hundred feet. over this territory the number of sink-holes average one hundred to the square mile; and the streams that have poured their water into these basins have made a network of open caverns one hundred thousand miles in length. a great many small caverns have been thoroughly explored and are famous for their beauty. the diamond cave is one of the most splendid, for it is lined with walls and pillars of alabaster that sparkle in the torchlight with crystals that look like veritable diamonds. beautiful springs and waterfalls are found in many caves, but the grandest of all is the mammoth cave, beside which no other is counted worthy to be compared. great tales the miners told of the wonder and the beauty of these caverns, the walls of which were supported by arching alabaster columns and wonderful domes, of indescribable beauty of form and colouring. in , the year that washington died, a pioneer discovered the entrance to a cave, the size and beauty of which surpassed anything he had seen before. after exploring it for a short distance he returned home and took his whole family with him to enjoy the first view of the wonderful cavern he had discovered. they carried pine knots and a lighted torch, by which they made their way for some distance, but the torch was accidentally extinguished and they groped their way in darkness and missed the entrance. without anything to guide them, they wandered in darkness for three days, and were almost dead when at last they stumbled upon the exit. this is the doorway of the mammoth cave of kentucky, one of the wonders of the world. this was a terrible experience. the next persons who attempted to explore the new cave were better provisioned against the chance of spending some time underground. the pioneers found rich deposits of nitre in the "great cave," as they called it. scientists visited it and explored many of its chambers. the reputation of this cavern has been spread by thousands of visitors who have come from all over the world to see it. the cave has not yet been completely explored. the regular tours, on which the guides conduct visitors, cover but a small part of the one hundred and fifty miles measured by the two hundred or more avenues. the passages wind in and out, crossing each other, sometimes at different levels, and forming a network of avenues in which the unaccustomed traveller would surely be lost. the old guides know every inch of their regular course, and their quaint and edifying talk adds greatly to the pleasure of the visitors. from the hotel, parties are organized for ten o'clock in the morning and seven o'clock in the evening. each visitor is provided with a lard-oil lamp. the guide carries a flask of oil and plenty of matches. no special garb is necessary, though people usually dress for comfort, and wear easy shoes. the temperature of the cave is uniform winter and summer, varying between fifty-three and fifty-four degrees fahrenheit. the cave entrance is an arch of seventy-foot span in the hillside. a winding flight of seventy stone steps leads the party around a waterfall, into a great chamber under the rocks. then the way goes through a narrow passage, where the guide unlocks an iron gate to let them in. the visitors now leave all thoughts of daylight behind, for the breeze that put out their lights as they entered the cave is past, and they stand in the rotunda, a vast high-ceilinged chamber, silent and impressive, with walls of creamy limestone, encrusted with gypsum, which has been stained black by manganese. from the vestibule on, each passage and each room has a name, based upon some historic event or some fancied resemblance. the giant's coffin is a great kite-shaped rock lying in one of the rooms of the cave. the star chamber has a wonderful crystal-studded dome in which the guide produces the effect of a sunrise by burning coloured lights. bonfires built at suitable points produce wonderful shadow effects, which are like nothing else in the world. the old saltpetre vats which the visitors pass in taking the "long route" through the cave, point them back to the days during the war of , when this valuable mineral was extracted from the earth in the floor of the cave. the industry greatly enriched the thrifty owners of the cave, but the works were abandoned after peace was declared. it must be a wonderful experience to walk steadily for nine hours over the long route, for so pure is the air and so wonderful is the scenery that people rarely complain of fatigue when the experience is over. there is no dust on the floors of these subterranean chambers, and they are not damp except near places where water trickles, here and there, in rivulets and cascades. pools of water at the bottoms of pits so deep that a lighted torch requires several seconds to reach the bottom, and rivers and lakes of considerable size, show where some of the surface water goes to. a strange underground suction creates whirlpools in some of these streams. people go in boats holding twenty passengers for a row on echo river, and the guide dips up with a net the blind fish and crayfish and cave lizards which inhabit these subterranean waters. the echoes in various chambers of the mammoth cave are remarkable. in some of them a song by a single voice comes back with full chords, as if several voices carried the different parts. the single notes of flute and cornet are returned with the same beautiful harmonies. a pistol shot is given back a dozen times, the sound rebounding like a ball from rock to rock of the arching walls. the vibrations of the water made by the rower's paddles reëcho in sounds like bell notes, and they are multiplied into harmonies that suggest the chimes in the belfry of a cathedral. the walls of various chambers differ from each other according to the minerals that compose them. some are creamy white limestone arches, some are walled with black gypsum, some are hung with great curtains of stalagmites, solid but suggesting the lightness and grace of folds of crêpe. under such hangings the floor is built up in stalactites. the mineral-laden water, the constant drip of which has produced a hanging, icicle-like stalagmite, has built up the stalactite to meet it. probably nothing is more beautiful than the flower-like crystals that bloom all over the walls of a chamber called "mary's bower." the floor, even, sparkles with jewels that have fallen from the wonderful and delicate flower clusters built from deposits of the lime-laden water which goes on building and replacing the bits that fall. "martha's vineyard" is decorated with nodules, like bunches of grapes, that glisten as if the dew were on them. the white gypsum in some caves makes the walls look as if they were carved out of snow. still others have clear, transparent crystals that make them gleam in the torches' light as if the walls were encrusted with diamonds. the cave region of indiana is also famous. the great wyandotte cave in crawford county is the most noted of many similar caverns. in some of the chambers, bats are found clinging to the ceiling, heads downward, like swarms of bees. the caverns of luray, in virginia, are complex and wonderful in their structure, and famous for the beautiful stalactites and stalagmites they contain. but there is no cave in this country so wonderful and so grand in its dimensions as the mammoth cave in kentucky. land-building by rivers once a year, when the rainy season comes in the mountainous country south of egypt, the old nile floods its banks and spreads its slimy waters over the land, covering the low plains to the very edge of the sahara desert. the people know it is coming, and are prepared for this flood. we should think such an overflow of our nearest river a monstrous calamity, but the egyptians bless the river which blesses them. they know that without the nile's overflow their country would be added to the desert of sahara. in a short time after the overflow, the river reaches its highest point and begins to ebb. canals lying parallel to its course are filled with water which is saved for use in the hot, dry summer. as the flood goes down, a deposit of slimy mud lies as a rich fertilizer on the land. it is this and the water which the earth has absorbed that make egypt one of the most fertile agricultural countries in the world. the region covered by the nile's overflow is the flood plain of this river. on this plain the pyramids, the sphinx, and other famous monuments of egypt stand. the statue of rameses ii. built , years ago, has its base buried nine feet deep in the rich soil made of nile sediment. a well dug in this region goes through forty feet of this soil before striking the underlying sand. how many years ago did the first nile overflow take place? we may begin our calculation by finding out the average yearly deposit. it is a slow process that accumulates but nine feet in , years. if you were in egypt when the nile went back into its banks, you would see that the scum it leaves in a single overflow adds not a great deal to the thickness of the soil. possibly floods have varied in their deposits from year to year, so that any calculation of the time it took to build that forty feet of surface soil must be but a rough estimate. this much we know: it has been an uninterrupted process which has taken place within the present geological epoch, "the age of man." not all the rich sediment the nile brings down is left on the level flood plain along its course. a vast quantity is dumped at the river's mouth, where the tides of the mediterranean check the river's current. thus the great delta is formed. the broad river splits into many mouths that spread out like a fan and build higher and broader each year the mud-banks between the streams. upper egypt consists of river swamps. lower egypt, from cairo to the sea, is the delta built by the river itself on sea bottom. from the head of the delta, where the river commences to divide, to the sea, is an area of , square miles made out of material contributed by upper egypt, and built by the river. layer upon layer, it is constantly forming, but most rapidly during the season of floods. coming closer home, let us look at the map of the mississippi valley. begin as far north as st. louis. for the rest of its course the mississippi river flows through a widening plain of swamp land, flooded in rainy seasons. through this swampy flood plain the river meanders; its current, heavily loaded with sediment, swings from one side to the other of the channel, building up here, wearing away there, and straightening its course when the curves become so sharp that their sides meet. then the current breaks through the thin wall, and a bayou of still water is left behind. below baton rouge the mississippi breaks into many mouths, that spread and carry the water of the great river into the gulf of mexico. the nile delta is triangular, like delta, [greek: d], the fourth letter of the greek alphabet; but the mississippi's delta is very irregular. the main mouth of the river flows fifty miles out into the gulf between mud-banks, narrow and low. at the tip it branches into several streams. from the mouth of the ohio to the gulf, the mississippi flood plain covers , square miles. over this area, sediment to an average depth of fifty feet has been laid down. in earlier times the river flooded this whole area, when freshets swelled its tributaries in the spring. the flood plain then became a sea, in the middle of which the river's current flowed swiftly. the slow-flowing water on each side of the main current let go of its burden of sediment and formed a double ridge. between these two natural walls the main river flowed. when its level fell, two side streams, running parallel with the main river drained the flood plains on each side into the main tributaries to right and left. these natural walls deposited when the river was in flood are called _levees_. each heavy flood builds them higher, and the bed of the stream rises by deposits of sediment. so it happens that the level of the river bed is higher than the level of its flood plain. this is an interesting fact in geology. but the people who have taken possession of the rich flood plain of the mississippi river, who have built their homes there, drained and cultivated the land, and built cities and towns on the areas reclaimed from swamps, recognize the elevation of the river bed as the greatest danger that threatens them. suppose a flood should come. even if it does not overflow the levees, it may break through the natural banks and thus overflow the cities and the farm lands to left and right. instead of living in constant fear of such a calamity, the people of the mississippi flood plain have sought safety by making artificial levees, to make floods impossible. these are built upon the natural levees. as the river bed rises by the deposit of mud, the levees are built higher to contain the rising waters. no longer does the rich soil of the mississippi flood plain receive layers of sediment from the river's overflow. the river very rarely breaks through a levee. the united states government has spent great sums in walling in the river, and each state along its banks does its share toward paying for this self-protection. by means of _jetties_ the river's current is directed into a straightened course, and its power is expended upon the work of deepening its own channel and carrying its sediment to the gulf. much as the river has been forced to do in cleaning its own main channel, dredging is needed at various harbours to keep the river deep enough for navigation. the forests of the mountain slopes in colorado are being slaughtered, and the headwaters of the missouri are carrying more and more rocky débris to choke the current of the mississippi. colorado soil is stolen to build land in the vast delta, which is pushing out into the gulf at the rate of six miles in a century--a mile in every sixteen years. the mississippi delta measures , square miles. with the continued denuding of mountain slopes, we shall expect the rate of delta growth to be greatly increased, until reforesting checks the destructive work of wind and water. the making of mountains the gradual thickening and shrinking of the earth's crust as it cools have made the wrinkles we call mountain systems. through millions of years the globe has been giving off heat to the cold sky spaces through which it swings in its orbit around the sun. the cooling caused the contraction of the outer layer to fit the shrinking of the mass. when a plump peach dries on its pit, the skin wrinkles down to fit the dried flesh. the fruit shrinks by loss of water, just as the face of an old person shrinks by loss of fat. the skin becomes wrinkled in both cases. the weakest places in the earth's crust were the places to crumple, because they could not resist the lateral pressure that was exerted by the shrinking process. along the shores of the ancient seas the rivers piled great burdens of sediment. this caused the thin crust to sink and to become a basin alongside of a ridge. the wearing away of the land in certain places lightened and weakened the crust at these places, so that it bent upward in a ridge. perhaps the first wrinkles were not very high and deep. the gradual cooling must have exerted continued pressure, and the wrinkles have become larger. it is not likely that new wrinkles would be formed as long as the old ones would crumple and draw up into narrower, steeper slopes, in response to the lateral crushing. we can imagine those first mountains rising as folds under the sea. gradually their bases were narrowed, and their crests lifted out of the water. they rose as long, narrow islands, and grew in size as time went on. why is the trend of the great mountain systems almost always north and south? study the map of the continents and see how few cross ranges are shown, and how short they are, compared with the others. the molten globe bulged at its equator, as it rotated on its axis. the moon added its strong pulling force to make it bulge still more. as the crust thickened, it became less responsive to the two forces that caused it to bulge. the shrinkage was greatest where the globe had been most pulled out of shape. the rate of the earth's rotation is believed to have diminished. every change tended to let the earth draw in its (imaginary) belt, a notch at a time. the forces of contraction acted along the line of the equator, and formed folds running toward the poles. in this early time the great mountain systems were born, and they grew in size gradually, from small beginnings. these mountains of upheaval, made by the bending of the earth's crust, and the formation of alternating ridges and depressed valleys, are many. the earth is old and much wrinkled. other mountains have been formed by forces quite different. volcanic mountains have been far more numerous in ages gone than they are now. mt. hood and mt. rainier are peaks built up by the materials thrown out of the craters of volcanoes dead these thousands of years. vesuvius is at present showing us how volcanic mountains are made. each eruption builds larger the cone--that is, the chimney through which the molten rocks, the ashes, and the steam are ejected. side craters may open, the main cone be broken and its form changed, but the mass of lava and stones and ashes grows with each eruption. the mountain grows by the additions it receives. Ætna is a mountain built of lava. a third mountain system grew, not by addition, but by subtraction. the catskills illustrate this type. this group of mountains is the remnant of a table-land made of level layers of red sandstone. the rest of the high plain has been cut down and carried away, leaving these picturesque hills, the survival of which is as much a mystery as the disappearance of the balance of the plateau of which they were once a part. the fold that forms a typical mountain ridge has a cone of granite, the original rock foundation of the earth, and on this are layers of stratified rock, ancient deposits of sediment carried to the sea by streams. when exposed to wind and rain, the ridge is gradually worn down. in some places the water cuts away the soft rock and forms a stream-bed, that cuts deeper and deeper, using the rock fragments as its tools. often the layers of aqueous rocks are cut through, and the granite exposed. sometimes the hardest stratified rock-beds resist the water and the wind and are left as a series of ridges along the sides of the main range. the crumpling forces may crack the ridge open for its whole length, and one side of the chasm may slip down and the other go up. the result is a sheer wall of exposed rock strata, layers of which correspond with those that lie far below the top of the portion that slid down in the great upheaval and subsidence that parted them. these slips are known as _faults_. the lava flood of the northwest we know little about the substance that occupies the four thousand miles of distance between the surface and the centre of our earth. we know that the terrible weight borne by the central mass compresses it, so that the interior must grow denser as the core is approached. scientists have weighed the earth, and tell us that the crust is lighter than the rest. the supposition is that there is a great deal of iron in the interior, and possibly precious metals, too. our deepest wells and mines go down about a mile, then digging stops, on account of the excessive heat. but the crumpling of the crust, and the wearing away of the folded strata by wind and running water, have laid bare rocks several miles in thickness on the slopes of mountains, and exposed the underlying granite, on which the first sedimentary rocks were deposited. on this granite lie stratified rocks, which are crystalline in texture. these are the beds, sometimes miles in depth, called _metamorphic_ rocks, formed by water, then transformed by heat. the wearing away of rocks by wind and water has furnished the materials out of which the aqueous rocks have been made. layers upon layers of sandstone, shales, limestone, and the like, are exposed when a river cuts a canyon through a plateau. the layered deposits of débris at the mouth of the river make new aqueous rocks out of old. every sandy beach is sandstone in the making. this work is never ended. in the early days the earth's crust often gaped open in a mighty crater and let a flood of lava overspread the surface. the ocean floor often received this flood of melted rock. in many places the same chimney opened again and again, each time spreading a new layer of lava on top of the old, so that the surface has several lava sheets overlying the aqueous strata. if the hardened lava sheet proved a barrier to the rising tide of molten lava in the chimney it was often forced out in sheets between the layers of aqueous rocks. wherever the heated material came into contact with aqueous rocks it transformed them, for a foot or more, into crystalline, metamorphic rocks. a chimney of lava is called a _dike_. in mountainous countries dikes are common. sometimes small, they may also be hundreds of feet across, often standing high above the softer strata, which rains have worn away. dikes often look like ruined walls, and may be traced for miles where they have been overturned in the mountain-making process. the great lava flood of the northwest happened when the coast range was born. along the border of the pacific ocean vast sedimentary deposits had accumulated during the cretaceous and tertiary periods. then the mighty upheaval came, the mountain ridge rose at the end of the miocene epoch and stretched itself for hundreds of miles through the region which is now the coast of california and oregon. great fissures opened in the folded crust, and floods of lava overspread an area of , square miles. a dozen dead craters show to-day where those immense volcanic chimneys were. the depth of the lava-beds is well shown where the columbia river has worn its channel through. walls of lava three thousand feet in thickness rise on each side of the river. they are made of columns of basalt, fitted together, like cells of a honeycomb, and jointed, forming stone blocks laid one upon another. the lava shrinks on cooling and forms prisms. in ireland, the giants' causeway is a famous example of basaltic formation. in oregon, the walls of the des chutes river show thirty lava layers, each made of vertical basalt columns. the palisades of the hudson, mt. tom, and mt. holyoke are examples on the eastern side of the continent of basaltic rocks made by lava floods. northern california, northwestern nevada, and large part of idaho, montana, oregon, and washington are included in the basin filled with lava at the time of the great overflow, which extended far into british columbia. it is probable that certain chimneys continued to discharge until comparatively recent times. mt. rainier, mt. shasta, and mt. hood are among dead volcanoes. quite a different history has the great deccan lava-field of india, which covers a larger area than the basin of our northwest, and is in places more than a mile in depth. it has no volcanoes, nor signs of any ever having existed. the floods alone overspread the region, which shows no puny "follow-up system" of scattered craters, intermittently in eruption. the first living things strange days and nights those must have been on the earth when the great sea was still too hot for living things to exist in it. the land above the water-line was bare rocks. these were rapidly being crumbled by the action of the air, which was not the mild, pleasant air we know, but was full of destructive gases, breathed out through cracks in the thin crust of the earth from the heated mass below. if you stand on the edge of a lava lake, like one of those on the islands of the hawaiian group, the stifling fumes that rise might make you feel as if you were back at the beginning of the earth's history, when the solid crust was just a thin film on an unstable sea of molten rock, and this volcano but one of the vast number of openings by which the boiling lava and the condensed gases found their way to the surface. then the rivers ran black with the waste of the rocky earth they furrowed, and there was no vegetation to soften the bleakness of the landscape. the beginnings of life on the earth are a mystery. nobody can guess the riddle. the earliest rocks were subjected to great heat. it is not possible that life could have existed in the heated ocean or on the land. gradually the shores of the seas became filled up with sediment washed down by the rivers. layer on layer of this sediment accumulated, and it was crumpled by pressure, and changed by heat, so that if any plants or animals had lived along those old shores their remains would have been utterly destroyed. rocks that lie in layers on top of these oldest, fire-scarred foundations of the earth show the first faint traces of living things. limestone and beds of iron ore are signs of the presence of life. the first animals and plants lived in the ancient seas. from the traces that are left, we judge that the earliest life forms were of the simplest kind, like some plants and animals that swim in a drop of water. have you ever seen a drop of pond water under a compound microscope? it is a wonder world you look into, and you forget all the world besides. you are one of the wonderful higher animals, the highest on the earth. you focus on a shapeless creature that moves about and feels and breathes, but hasn't any eyes or mouth or stomach--in fact, it is the lowest form of animal life, and one of the smallest. it is but one of many animal forms, all simple in structure, but able to feed and grow and reproduce their kind. gaze out of the window on the garden, now. the flowering plants, the green grass, and the trees are among the highest forms of plants. in the drop of water under the microscope tiny specks of green are floating. they belong to the lowest order of plants. among the plant and the animal forms that have been studied and named, are a few living things the places of which in the scale are not agreed upon. some say they are animals; some believe they are plants. they are like both in some respects. it is probable that the first living things were like these confusing, minute things--not distinctly plants or animals, but the parent forms from which, later on, both plants and animals sprang. the lowest forms of life, plant and animal, live in water to-day. they are tiny and their bodies are made of a soft substance like the white of an egg. if these are at all like the living creatures that swarmed in the early seas, no wonder they left no traces in the rocks of the early part of the age when life is first recorded by fossils. soft-bodied creatures never do. some of the animals and the plants in the drop of water under the microscope have body walls of definite shapes, made of lime, or of a glassy substance called silica. when they die, these "skeletons" lie at the bottom of the water, and do not decay, as the living part of the body does, because they are mineral. gradually a number of these shells, or hard skeletons, accumulate. in a glass of pond water they are found at the bottom, amongst the sediment. in a pond how many thousands of these creatures must live and their shells fall to the bottom at last, buried in the mud! so it is easy to understand why the first creatures on earth left no trace. the first real fossils found in the rocks are the hard shells or skeletons of the first plants and animals that had hard parts. an ancient beach at ebb tide when the tide is out, the rocks on the maine coast have plenty of living creatures to prove this northern shore inhabited. starfishes lurk in the hollows, and the tent-shaped shells of the little periwinkle encrust the wet rocks. mussels cling to the rocks in clumps, fastened to each other by their ropes of coarse black hair. the furry coating of sea mosses that encrust the rocks is a hiding-place for many kinds of living things, some soft-bodied, some protected by shells. the shallow water is the home of plants and animals of many different kinds. as proof of this one finds dead shells and fragments of seaweeds strewn on the shore after a storm. along the outer shores of the cape cod peninsula and down the jersey coast, the sober colouring of the shells of the north gives way to a brighter colour scheme. in the warmer waters, life becomes gayer, if we may judge by the rich tints that ornament the shells. the kinds of living creatures change. they are larger and more abundant. the seaweeds are more varied and more luxuriant in growth. when we reach the shores of the west indian islands and the keys of florida the greatest abundance and variety of living forms are found. the submerged rocks blossom with flower-like sea anemones of every colour. corals, branching like trees and bushes on the sea floor, form groves under water. among them brilliant-hued fishes swim, and highly ornamented shells glide, as people know who have gazed through the glass bottoms of the boats built especially to show visitors the wonderful sea gardens at nassau, bahama islands, and at santa catalina island, southern california. on every beach the skeletons of animals which die help to build up the land; though the process is not so rapid in the north as on the shores that approach the tropics. the coast of florida has a rim of island reefs around it built out of coral limestone. indeed, the peninsula was built by coral polyps. houses in st. augustine are built of coquina rock, which is simply a mass of broken shells held together by a lime cement. every sea beach is packed with shells and other remnants of animals and plants that live in the shallow waters. deeper and deeper year by year the sand buries these skeletons, and many of them are preserved for all time. thus what is sandy beach to-day may, a few million years from now, be uncovered as a ledge of sandstone with the prints of waves distinctly shown, and fossil shells of molluscs, skeletons of fishes, and branches of seaweed--all of them different from those then growing upon the earth. in the neighbourhood of cincinnati there have been uncovered banks of stone accumulated along the border of an ancient sea. from the sides of granite highlands streams brought down the sand built into these oldest sandstone rocks. the fine mud which now appears as beds of slate was the decay of feldspar and hornblende in the same granite. limestone beds are full of the fossil shells of creatures that lived in the shallow seas. their skeletons, accumulating on the bottom, formed deep layers of limestone mud. these rocks preserve, by the fossils they contain, a great variety of shellfish which had limy skeletons. the sea fairly swarmed along its shallow margin with these creatures. we might not recognize many of the shells and other curious fossils we find in the rock uncovered by the workmen who are cutting a railroad embankment. they are not exactly like the living forms that grow along our beaches to-day, but they are enough like them for us to know that they lived along the seashore, and if we had time to examine all the rocks of this kind preserved in a museum we should decide that seashore life was quite as abundant then as it is now. the pressed specimens of plants of those earliest seashores are mere imprints showing that they were pulpy and therefore gradually decayed. only their shape is recorded by dark stains made by each branching part. the decay of the vegetable tissue painted the outline on the rock which when split apart shows us what those ancient seaweeds looked like. they belonged to the group of plants we call kelp, or tangle, which are still common enough in the sea, though the forms we now have are not exactly like the old ones. seaweeds belong to the very lowest forms of plants. [illustration: crinoid from indiana] [illustration: _by permission of the american museum of natural history_ ammonite from jurassic of england] [illustration: _by permission of the american museum of natural history_ fossil corals coquina, hippurite limestone] the limestones are full of fossils of corals. indeed, there must have been reefs like those that skirt florida to-day built by these lime-building polyps. their forms are so well preserved in the rocks that it is possible to know just how they looked when they grew in the shallows. one very common kind is called a cup coral, because the polyp formed a skeleton shaped like a cup. the body wall surrounded the skeleton, and the arms or tentacles rose from the centre of the funnel-like depression in the top. little cups budded off from their parents, but remained attached, and at length the skeletons of all formed great masses of limy rock. some cup corals grew in a solid mass, the new generation forming an outer layer, thus burying the parent cups. a second type of corals in these oldest limestones is the honeycomb group. the colonies of polyps lived in tubes which lengthened gradually, forming compact, limy cylinders like organ pipes, fitted close together. the living generation always inhabited the upper chambers of the tubes. a third type is the chain coral, made of tubes joined in rows, single file like pickets of a fence. but these walls bend into curious patterns, so that the cross-section of a mass of them looks like a complex pattern of crochet-work, the irregular spaces fenced with chain stitches. each open link is a pit in which a polyp lived. among the corals are sprays of a fine feathery growth embedded in the limestone. fine, straight, splinter-like branches are saw-toothed on one or both edges. these limy fossils might not be seen at all, were they not bedded in shales, which are very fine-grained. here again are the skeletons of animals. each notch on each thread-like branch was the home of a tiny animal, not unlike a sea anemone and a coral polyp. to believe this story it is necessary only to pick up a bit of dead shell or floating driftwood on which a feathery growth is found. these plumes, like "sea mosses," as they are called, are not plants at all, but colonies of polyps. each one lived in a tiny pit, and these pits range one above the other, so as to look like notches on the thread-like divisions of the stem. put a piece of this so-called "sea moss" in a glass of sea water, and in a few moments of quiet you will see, by the use of a magnifying glass, the spreading arms of the polyp thrust out of each pit. the ancient seas swarmed with these living hydrozoans, and their remains are found preserved as fossils in the shales which once were beds of soft mud. the hard shells of sea urchins and starfishes are made of lime. in the ancient seas, starfishes were rare and sea urchins did not exist, but all over the sea bottom grew creatures called crinoids, the soft parts of which were enclosed in limy protective cases and attached to rocks on the sea bottom by means of jointed stems. no fossils are more plentiful in the early limestones than these wonderful "stone lilies." indeed, the crinoidal limestone seemed to be built of the skeletons of these animals. the lily-like body was flung open, as a lily opens its calyx, when the creature was feeding. but any alarm caused the tentacles to be drawn in, and the petal-like divisions of the body wall to close tightly together, till that wall looked like an unopened bud. on the bottom of the atlantic, near the bahama islands, these stone lilies are still found growing. their jointed stems and body parts are as graceful in form and motion as any lily. the creature's mouth is in the centre of the flower-like top, and it feeds like the sea urchin, on particles obtained in the sea water. the old limestones contain great quantities of "lamp shells," which are old-fashioned bivalves. their shells remind us of our bivalve clams and scallops, but the internal parts were very different. the gills of clams and oysters are soft parts. inside of the lamp shells are coiled, bony arms, supporting the fringed gills. it is fortunate for us that a few lamp shells still live in the seas. by studying the soft parts of these living remnants of a very old race we can know the secrets of the lives of those ancient lamp shells, the soft parts of which were all washed away, and the fossil shells of which are preserved. gradually the lamp shells died out, and the modern bivalves have come to take their places. just so, the ancient crinoids are now almost extinct; the sea urchins and the starfishes have succeeded them. the chambered nautilus has its shell divided by partitions and it lives in the outer chamber, a many-tentacled creature, that is a close relative of the soft-bodied squid. in the ancient seas the same family was represented by huge creatures the shells of which were chambered, but not coiled. their abundance and great size are proved by the rocks in which their fossils are preserved. some of them must have been the rulers of the sea, as sharks and whales are to-day. fossil specimens have been found more than fifteen feet long and ten inches in diameter in the ancient rocks of some of the western states. it is possible to read from the lowest rock formations upward, the rise of these sea giants and their gradual decline. certain strata of limestone contain the last relics of this race, after which they became extinct. as the straight-chambered forms diminished, great coiled forms became more abundant, but all died out. one of the most abundant fossil animals in ancient rocks is called a trilobite. its body is divided by two grooves into three parts, a central ridge extending the whole length of the body and two side ridges. the front portion of the shell formed the head shield, and behind the main body part was a little tail shield. the skeleton was formed of many movable jointed plates, and the creature had eyes set in the head shield just as the king crab's are set. jointed legs in pairs fringed each side of the body. each leg had two branches, one for walking, the other for swimming. a pair of feelers rose from the head. the body could be rolled into a ball when danger threatened, by bringing head and tail together. these remarkable, extinct trilobites were the first crustaceans. their nearest living relative to-day is the horseshoe crab. the fresh-water crayfish and the lobster are more distant relatives: so are the shrimps and the prawns. no such abundance of these creatures exists to-day as existed when the trilobites thronged the shallows. so well preserved are these skeletons that, although there are no living trilobites for comparison, it is possible to find out from the fossil enough about their structure to know how they fed and lived their lives along with the straight-horns which were the scavengers of those early seas and the terror of smaller creatures. the trilobites throve, and, dying, left their record in the rocks; then disappeared entirely. we find their fossils in a great variety of forms, shapes, and sizes. the smallest is but a fraction of an inch long, the largest twenty inches long. the ancient rocks, in which these lower forms of life have left their fossils, are known as the silurian system. the time in which these rocks were accumulating under the seas covers a vast period. we call it the age of invertebrates, because these soft-bodied, hard-shelled animals, the crinoids, the molluscs, and the trilobites, with bony external skeletons and no backbones, were the most abundant. they overshadowed all other forms of life. the rocks of this wonderful series were formed on the shores of a great inland sea that covered the central portion of north america. in the ages that followed, these rocks were covered deeply with later sediments. but the upheavals of the crust have broken open and erosion has uncovered these strata in different regions. geologists have found written there, page upon page, the record of life as it existed in the early seas. the lime rocks "hard" water and "soft" water are very different. the rain that falls and fills our cisterns is not softer or more delightful to use than the well water in some favoured regions. in it, soap makes beautiful, creamy suds, and it is a real pleasure to put one's hands into it. but in hard water soap seems to curdle, and some softening agent like borax has to be added or the water will chap the hands. there is little satisfaction in using water of this kind for any purpose. hard water was as soft as any when it fell from the sky; but the rain water trickled into the ground and passed through rocks containing lime. some of this mineral was absorbed, for lime is readily soluble in water. clear though it may be, water that has lime in it has quite a different feeling from rain water. blow the breath into a basin of hard water, and a milky appearance will be noted. the carbonic acid gas exhaled from the lungs unites with the invisible lime, causing it to become visible particles of carbonate of lime, which fall to the bottom of the basin. nearly all well water is hard. so is the water of lakes and rivers and the ocean, for limestone is one of the most widely distributed rocks in the surface of the earth. rain water makes its way into the earth's crust, absorbs mineral substances, and collects in springs which feed brooks and rivers and lakes. wells are holes in the ground which bore into water-soaked strata of sand. we gain something from the lime dissolved in hard water, for it is an essential part of our food. we must drink a certain amount of water each day to keep the body in perfect health. the lime in this water goes chiefly to the building of our bones. plant roots take up lime in the water that mounts as sap through the plant bodies. we get some of the lime we need in vegetable foods we eat. all of the kingdom of vertebrate animals, from the lowest forms to the highest, all of the shell-bearing animals of sea and land, require lime. many of the lower creatures especially these in the sea, such as corals and their near relatives, encase themselves in body walls of lime. they absorb the lime from the sea water, and deposit it as unconsciously as we build the bony framework of our bodies. all the bone and shell-bearing creatures that die on the earth and in the sea restore to the land and to the water the lime taken by the creatures while they lived. carbonic acid gas in the water greatly hastens the dissolving of dead shells. carbonic acid gas, whether free in the air, or absorbed by percolating water, hastens the dissolving of skeletons of creatures that die upon land. then the raw materials are built again into lime rocks underground. the lime rocks are the most important group in the list of rocks that form the crust of the earth. they are made of the elements calcium, carbon, and oxygen, yet the different members of this calcite group differ widely in composition and appearance. so do oyster shells and beef bones, though both contain quantities of carbonate of lime. calcite is a soft mineral, light in weight, sometimes white, but oftener of some other colour. it may be found crystallized or not. whenever a drop of acid touches it, a frothy effervescence occurs. the drop of acid boils up and gives off the pungent odour of carbonic acid gas. the reason that calcite is hard to find in rocks is that percolating water, charged with acids, is constantly stealing it, and carrying it away into the ocean. the rocks that contain it crumble because the limy portions have been dissolved out. some limestones resist the destructive action of water. when they are impregnated with silica they become transformed into marble, which takes a high polish like granite. acids must be strong to make any impression on marble. the thick beds of pure limestone that underlie the surface soil in kentucky and in parts of virginia sometimes measure several hundred feet in thickness, a single stratum often being twenty feet thick. they are all horizontal, for they were formed on sea bottom, and have not been crumpled in later time. the dead bodies of sea creatures contributed their shells and skeletons to the lime deposit on the sea bottom. who can estimate the time it took to form those thick, solid layers of lime rock? the animals were corals, crinoids, and molluscs. little sand and clay show in the lime rock of this period, before the marshes of the carboniferous age took the place of the ancient inland sea of the subcarboniferous period, the sedimentary accumulations of which we are now talking about. the living corals one sees in the shallow water of the florida coast to-day are building land by building up their limy skeletons. the reefs are the dead skeletons of past generations of these tiny living things. they take in lime from the water, and use it as we use lime in building our bones. in each case it is an unconscious process of animal growth--not a "building process" like a mason's building of a wall. many people think that the coral polyp builds in this way. they give it credit for patience in a great undertaking. all the polyp does is to feed on whatever the water supplies that its digestive organs can use. it is like a sea anemone in appearance and in habits of life. it is not at all like an insect. yet it is common to hear people speak of the "coral insect"! do not let any one ever hear you repeat such a mistake. southern florida is made out of coral rock, but thinly covered with soil. it was made by the growth of reef after reef, and it is still growing. the cretaceous period of the earth's eventful history is named for the lime rock which we know as chalk. beds of this recent kind of limestone are found in england and in france, pure white, made of the skeletons of the smallest of lime-consuming creatures, foraminifera. they swarmed in deep water, and so did minute sponge animalcules and plant forms called diatoms that took silica from the water, and formed their hard parts of this glassy substance. the result is seen in the nodules of flint found in the soft, snow-white chalk. did you ever use a piece of chalk that scratched the black-board? the flint did it. have you ever seen the chalk cliffs of dover? when you do see them, notice how they gleam white in the sun. see how the rains have sculptured those cliffs. the prominences left standing out are strengthened by the flint they contain. chalk beds occur in texas and under our great plains; but the principal rocks of the age in america were sandstones and clays. the age of fishes the first animal with a backbone recorded its existence among the fossils found in rocks of the upper silurian strata. it is a fish; but the earliest fossils are very incomplete specimens. we know that these old-fashioned fishes were somewhat like the sturgeons of our rivers. their bodies were encased in bony armour of hard scales, coated with enamel. the bones of the spine were connected by ball and socket joints, and the heads were movable. in these two particulars the fishes resembled reptiles. the modern gar-pike has a number of the same characteristics. another backboned creature of the ancient seas was the ancestral type of the shark family. in some points this old-fashioned shark reminds us of birds and turtles. these early fishes foreshadowed all later vertebrates, not yet on the earth. after them came the amphibians, then the reptiles, then the birds, and latest the mammals. the race of fishes began, no doubt, with forms so soft-boned that no fossil traces are preserved in the rocks. when those with harder bones appeared, the fossil record began, and it tells the story of the passing of the early, unfish-like forms, and the coming of new kinds, great in size and in numbers, that swarmed in the seas, and were tyrants over all other living things. they conquered the giant straight-horns and trilobites, former rulers of the seas. [illustration: _by permission of the american museum of natural history_ a sixteen-foot fossil fish from cretaceous of kansas, with a modern tarpon] [illustration: _by permission of the american museum of natural history_ cañon diablo meteorite from arizona] one of these giant fishes fifteen to twenty feet long, three feet wide, had jaws two feet long, set with blade-like teeth. devonian rocks in ohio have yielded fine fossils of gigantic fishes and sharks. devonian fishes were unlike modern kinds in these particulars, the spinal column extended to the end of the tail, whether the fins were arranged equally or unequally on the sides; the paired side fins look like limbs fringed with fins. every devonian fish of the gar type seems to have had a lung to help out its gill-breathing. in these traits the first fishes were much like the amphibians. they were the parent stock from which branched later the true fishes and the amphibians, as a single trunk parts into two main boughs. the trunk is the connecting link. the sea bottom was still thronged with crinoids, and lamp shells, and cup corals. shells of both clam and snail shapes are plentiful. the chambered straight-horns are fewer and smaller, and coiled forms of this type of shell are found. trilobite forms are smaller, and their numbers decrease. the first land plants appeared during this age. ferns and giant club mosses and cycads grew in swampy ground. this was the beginning of the wonderful fern forests that marked the next age, when coal was formed. the rocks that bear the record of these living things in their fossils, form strata of great thickness that overlie the silurian deposits. there is no break between them. so we understand that the sea changed its shore-line only when the silurian deposits rose to the water-level. the devonian sea was smaller than the silurian. a great tract of devonian deposits occupies the lower half of the state of new york, canada between lakes erie and huron, and the northern portions of indiana and illinois. these older layers of the stratified rock are covered with the deposits of later periods. rivers that cut deep channels reveal the earlier rocks as outcrops along their canyon walk. the record of the age of fishes is, for the most part, still an unopened book. the pages are sealed, waiting for the geologist with his hammer to disclose the mysteries. king coal in this country, and in this age, who can doubt that coal is king? it is one of the few necessities of life. in various sections of the country, layers of coal have been discovered--some near the surface, others deep underground. these are the storehouses of fuel which the coal miners dig out and bring to the surface, and the railroads distribute. from pennsylvania and ohio to alabama stretches the richest coal-basin. illinois and indiana contain another. from iowa southward to texas another broad basin lies. central michigan and nova scotia each has isolated coal-basins. all these have been discovered and mined, for they lie in the oldest part of the country. in the west, coal-beds have been discovered in several states, but many regions have not yet been explored. vast coal-fields, still untouched, have been located in alaska. the government is trying to save this fuel supply for coming generations. many of the richest coal-beds from nova scotia southward dip under the ocean. they have been robbed by the erosive action of waves and running water. glaciers have ground away their substance, and given it to the sea. much that remains intact must be left by miners on account of the difficulties of getting out coal from tilted and contorted strata. as a rule, the first-formed coal is the best. the western coal-fields belong to the period following the carboniferous age. although conditions were favourable to abundant coal formation, western coal is not equal to the older, eastern coal. it is often called _lignite_, a word that designates its immaturity compared with anthracite. coal formed in the triassic period is found in a basin near richmond, virginia. there is an abundance of this coal, but it has been subjected to mountain-making pressure and heat, and is extremely inflammable. the miners are in constant danger on account of coal gas, which becomes explosive when the air of the shaft reaches and mingles with it. this the miner calls "fire damp." north carolina has coal of the same formation, that is also dangerous to mine, and very awkward to reach, on account of the crumpling of the strata. there are beds of coal so pure that very little ash remains after the burning. five per cent, of ash may be reasonably expected in pure coal, unmixed with sedimentary deposits. such coal was formed in that part of the swamp which was not stirred by the inflow of a river. wherever muddy water flowed in among the fallen stems of plants, or sand drifted over the accumulated peat, these deposits remained, to appear later and bother those who attempt to burn the coal. [illustration: eocene fish] [illustration: _by permission of the american museum of natural history_ trilobite from the niagara limestone, upper silurian, of western new york] [illustration: sigillaria, stigmaria and lepidodendron] [illustration: _by permission of the american museum of natural history_ coal fern] you know pure coal, that burns with great heat and leaves but little ashes. you know also the other kind, that ignites with difficulty, burns with little flame, gives out little heat, and dying leaves the furnace full of ashes. you are trying to burn ancient mud that has but a small proportion of coal mixed with it. the miners know good coal from poor, and so do the coal dealers. it is not profitable to mine the impure part of the vein. it costs as much to mine and ship as the best quality, and it brings a much lower price. the deeper beds of coal are better than those formed in comparatively recent time and found lying nearer the surface. in many bogs a layer of embedded root fibres, called peat, is cut into bricks and dried for burning. deeper than peat-beds lie the _lignites_, which are old beds of peat, on the way to become coal. _soft coal_ is older than lignite. it contains thirty to fifty per cent. of volatile matter, and burns readily, with a bright blaze. the richest of this bituminous coal is called _fat_, or _fusing coal_. the bitumen oozes out, and the coal cakes in burning. ordinary soft coal contains less, but still we can see the resinous bitumen frying out of it as it burns. there is more heat and less volatile matter in _steam coal_, so-called because it is the fuel that most quickly forms steam in an engine. _hard coal_ contains but five to ten per cent. of volatile matter. it is slow to ignite and burns with a small blue blaze. from peat to anthracite coal i have named the series which increases gradually in the amount of heat it gives out, and increases and then decreases in its readiness to burn and in the brightness of its flame. anthracite coal has the highest amount of fixed carbon. this is the reason why it makes the best fuel, for fixed carbon is the substance which holds the store of imprisoned sunlight, liberated as heat when the coal burns. tremendous pressure and heat due to shrinking of the earth's crust have crumpled and twisted the strata containing coal in eastern pennsylvania, and thus changed bituminous coal into anthracite. ohio beds, formed at the same time, but undisturbed by heat and pressure, are bituminous yet. the coal-beds of rhode island are anthracite, but the coal is so hard that it will not burn in an open fire. the terrible, mountain-making forces that crumpled these strata and robbed the coal of its volatile matter, left so little of the gas-forming element, that a very special treatment is necessary to make the carbon burn. it is used successfully in furnaces built for the smelting of ores. the last stage in the coal series is a black substance which we know as black lead, or graphite. we write with it when we use a "lead" pencil. this is anthracite coal after all of the volatile matter has been driven out of it. it cannot burn, although it is solid carbon. the beds of graphite have been formed out of coal by the same changes in the earth's crust which have converted soft coal into anthracite. the tremendous pressure that bears on the coal measures has changed a part of the carbon into liquid and gaseous form. lakes of oil have been tapped from which jets of great force have spouted out. such accumulations of oil usually fill porous layers of sandstone and are confined by overlying and underlying beds of impervious clay. gas may be similarly confined until a well is drilled, relieving the pressure, and furnishing abundant or scanty supply of this valuable fuel. western pennsylvania coal-fields have beds of gas and oil. if mountain-making forces had broken the strata, as in eastern pennsylvania, the gas and the oil would have been lost by evaporation. this is what happened in the anthracite coal-belt. how coal was made the broad, rounded dome of a maple tree shades my windows from the intense heat of this august day. the air is hot, and every leaf of the tree's thatched roof is spread to catch the sunlight. the carbon in the air is breathed in through openings on the under side of each leaf. the sap in the leaf pulp uses the carbon in making starch. the sun's heat is absorbed. it is the energy that enables the leaf-green to produce a wonderful chemical change. out of soil water, brought up from the roots, and the carbonic acid gas, taken in from the air, rich, sugary starch is manufactured in the leaf laboratory. this plant food returns in a slow current, feeding the growing cells under the bark, from leaf tip to root tip, throughout the growing tree. the sap builds solid wood. the maple tree has been built out of muddy water and carbon gas. it stands a miracle before our eyes. in its tough wood fibres is locked up all the heat its leaves absorbed from the sun, since the day the maple seed sprouted and the first pair of leaves lifted their palms above the ground. if my maple tree should die, and fall, and lie undisturbed on the ground, it would slowly decay. the carbon of its solid frame would pass back into the air, as gas, and the heat would escape so gradually that i could not notice it at all, unless i thrust my hand into the warm, crumbling mass. if my tree should be cut down to-day and chopped into stove wood, it would keep a fire in my grate for many months. burning destroys wood substance a great deal faster than decay in the open air does, but the processes of rotting and burning are alike in this: each process releases the carbon, and gives it back to the air. it gives back also the sun's heat, stored while the tree was growing. there is left on the ground, and in the ashes on the hearth, only the mineral substance taken up in the water the roots gathered underground. if my tree stood in swampy ground and fell over under a high wind, the water that covered it and saturated its substance would prevent decay. the carbon would not be allowed to escape as a gas to the air; the woody substance would become gradually changed into _peat_. in this form it might remain for thousands of years, and finally be changed into coal. whether it was burned while yet in the condition of peat, or millions of years later, when it was transformed into coal, the heat stored in its substance was liberated by the burning. the carbon and the heat went back to the air. every green plant we see spreads its leaves to the sun. every stick of wood we burn, and every lump of coal, is giving back, in the form of light and heat, the energy that came from sunshine and was captured by the green leaves. how long the wood has held this store of heat we may easily compute, for we can read the age of a tree. but the age of coal we cannot accurately state. the years probably should be counted by millions, instead of thousands. the great inland sea that covered the middle portion of the continent during the silurian and the devonian periods, became shallow by the deposit of vast quantities of sediment. along the lines of the deposits of greatest thickness, a crumpling of the earth's crust lifted the first fold of the alleghany mountains as a great sea wall on the east, and on the western shore another formed the beginning of the ozark mountain system in missouri. an island was lifted out of the sea, forming the elevated ground on which the city of cincinnati now stands. various other ridges and islands divided the ancient sea into much smaller bodies of water. hemmed in by land these shallow sea-basins gradually changed into fresh-water lakes, for they no longer had connection with the ocean, and all the water they received came from rain. after centuries of freshets, and of filling in with the rock débris brought by the streams, they became great marshes, in which grew water-loving plants. generation after generation of these plants died, and their remains, submerged by the water, were converted into peat. in the course of ages this peat became coal. this is the history of the coal measures. there is no guesswork here. the stems of plants do not lose their microscopic structure in all the ages it has taken to transform them to coal. a thin section of coal shows under a magnifier the structure of the stems of the coal-forming plants. moreover, the veins of coal preserve above or below them, in shales that were once deposits of mud, the branching trunks of trees, perfectly fossilized. there are no better proofs of the vegetable origin of coal than the lumps themselves. but they are plain to the naked eye, while the coal tells its story to the man with the microscope. the fossil remains of the plants that flourished when coal was forming are gigantic, compared with plants of the same families now living. we must conclude that the climate was tropical, the air very heavy with moisture, and charged much more heavily than it is now with carbonic acid gas. these conditions produced, in rapid succession, forests of tree ferns and horsetails and giant club mosses. these are the three types of plants out of which the coal was made. they were all rich in resin, which makes the coal burn readily. the ferns had stems as large as tree trunks. some have been found that are eighteen inches in diameter. we know they are ferns, because the leaves are found with their fruits attached to them in the manner of present-day ferns. the stems show the well known scar by which fern leaves are joined. and the wood of these fossil fern stems is tubular in structure, just as the wood of living ferns is to-day. among the ferns which dominated these old marsh forests grew one kind, the scaly leaves of which covered the stems and bore their fruits on the branching tips. these giants, some of them with trunks four feet in diameter, belong to the same group of plants as our creeping club mosses, but in the ancient days they stood up among the other ferns as trees forty or fifty feet high. the giant scouring rushes, or horsetails, had the same general characteristics as the little reed-like plants we know by those names to-day. the highest plants of the coal period were leafy trees with nut-like fruits, that resemble the yew trees of the present. these gigantic trees were the first conifers upon the earth. they foreshadowed the pines and the other cone-bearing evergreens. their leaves were broad and their fruits nut-like. the japanese ginkgo, or maidenhair fern tree, is an old-fashioned conifer somewhat like those first examples of this family. trunks sixty to seventy feet long, crowned with broad leaves and a spike of fruit, have been found lying in the upper layers of the coal-seams, and in sandstone strata that lie between the strata of coal. peculiar circular discs, which the microscope reveals along the sides of the wood fibres of these fossil trees, prove the wood structure to be like that of modern conifers. generation after generation of forests lived and died in the vast spreading swamps of this era. the land sank, and freshets came here and there, drowning out all plant life, and covering the layers of peat with beds of sand or mud. when the water went down, other forests took possession, and a new coal-bed was started. it is plainly seen that flooding often put an end to coal formation. fifteen seams of coal, one above another, is the greatest number that have been found. the veins vary from one inch to forty feet in thickness. these are separated by layers of sandstone or shale, which accumulated as sediment, covering the stumps of dead tree ferns and other growths, and preserving them as fossils to tell the story of those bygone ages as plainly as any other record could have done. fresh-water animals succeeded those of salt water in the swamps that formed the coal measures. overhead, the first insects flitted among the branches of the tree ferns. dragon-flies darted above the surface and dipped in water as they do to-day. spiders, scorpions, and cockroaches, all air-breathing insects, were represented, but none of the higher, nectar-loving insects, like flies and bees and butterflies, were there. flowering plants had not yet appeared on the earth. snakelike amphibians, some fishlike, some lizard-like, and huge crocodilian forms appeared for the first time. these air-breathing swamp-dwellers could not have lived in salt water. fresh-water molluscs and land shells appear for the first time as fossils in the rocks of the coal measures. on the shores of the ocean, the rocks of this period show that trilobites, horseshoe crabs, and fishes still lived in vast numbers, and corals continued to form limestone. the old types of marine animals changed gradually, but the coal measures show strikingly different fossils. these rocks bear the first record of fresh-water and land animals. the most useful metal it is fortunate for us all that, out of the half-dozen so-called useful metals, iron, which is the most useful of them all to the human race, should be also the most plentiful and the cheapest. aluminum is abundant in the common clay and soil under our feet. but separating it is still an expensive process; so that this metal is not commercially so plentiful as iron is, nor is it cheap. all we know of the earth's substance is based on studies of the superficial part of its crust, a mere film compared with the eight thousand miles of its diameter. nobody knows what the core of the earth--the great globe under this surface film--is made of; but we know that it is of heavier material than the surface layer; and geologists believe that iron is an important element in the central mass of the globe. one thing that makes this guess seem reasonable is the great abundance of iron in the earth's crust. another thing is that meteors which fall on the earth out of the sky prove to be chiefly composed of iron. all of their other elements are ones which are found in our own rocks. if we believe that the earth itself is a fragment of the sun, thrown off in a heated condition and cooling as it flew through space, we may consider it a giant meteor, made of the substances we find in the chance meteor that strikes the earth. iron is found, not only in the soil, but in all plant and animal bodies that take their food from the soil. the red colour in fruits and flowers, and in the blood of the higher animals, is a form in which iron is familiar to us. it does more, perhaps, to make the world beautiful than any other mineral element known. but long before these benefits were understood, iron was the backbone of civilization. it is so to-day. iron, transformed by a simple process into steel, sustains the commercial supremacy of the great civilized nations of the world. the railroad train, the steel-armoured battleship, the great bridge, the towering sky-scraper, the keen-edged tool, the delicate mechanism of watches and a thousand other scientific instruments--all these things are possible to-day because iron was discovered and has been put to use. it was probably one of the cave men, poking about in his fire among the rocks, who discovered a lump of molten metal which the heat had separated from the rest of the rocks. he examined this "clinker" after it cooled, and it interested him. it was a new discovery. it may have been he, or possibly his descendants, who learned that this metal could be pounded into other shapes, and freed by pounding from the pebbles and other impurities that clung to it when it cooled. the relics of iron-tipped spears and arrows show the skill and ingenuity of our early ancestors in making use of iron as a means of killing their prey. the earliest remains of this kind have probably been lost because the iron rusted away. man was pretty well along on the road to civilization before he learned where iron could be found in beds, and how it could be purified for his use. we now know that certain very minute plants, which live in quiet water, cause iron brought into that water to be precipitated, and to accumulate in the bottom of these boggy pools. in ancient days these bog deposits of iron often alternated with coal layers. millions of years have passed since these two useful substances were laid down. to-day the coal is dug, along with the bog iron. the coal is burned to melt the iron ore and prepare it for use. it is a fortunate region that produces both coal and iron. bituminous coal is plentiful, and scattered all over the country, while anthracite is scarce. the cheapest iron is made in alabama, which has its ore in rich deposits in hillsides, and coal measures close by, furnishing the raw material for coke. the result is that the region of birmingham has become the centre of great wealth through the development of iron and coal mines. where water flows over limestone rock, and percolates through layers of this very common mineral, it causes the iron, gathered in these rock masses, to be deposited in pockets. all along the appalachian mountains the iron has been gathered in beds which are being mined. these beds of ore are usually mixed with clay and other earthy substances from which the metal can be separated only by melting. the ore is thrown into a furnace where the metal melts and trickles down, leaving behind the non-metallic impurities. it is drawn off and run into moulds, where it cools in the form of "pig" iron. the first fuel used in the making of pig iron from the ore was charcoal. in america the early settlers had no difficulty in finding plenty of wood. indeed, the forests were weeds that had to be cut down and burned to make room for fields of grain. the finding of iron ore always started a small industry in a colony. if there was a blacksmith, or any one else among the small company who understood working in iron, he was put in charge. to make the charcoal, wood was cut and piled closely in a dome-shaped heap, which was tightly covered with sods, except for a small opening near the ground. in this a fire was built, and smothered, but kept going until all the wood within the oven was charred. this fuel burned readily, with an intense heat, and without ashes. sticks of charcoal have the form of the wood, and they are stiff enough to hold up the ore of iron so that it cannot crush out the fire. for a long time american iron was supplied by little smelters, scattered here and there. the workmen beat the melted metal on the forge, freeing it from impurities, and shaping the pure metal into useful articles. sometimes they made it into steel, by a process learned in the old world. the american iron industry, which now is one of the greatest in the world, centres in pittsburg, near which great deposits of iron and coal lie close together. the making of coke from coal has replaced the burning of charcoal for fuel. when the forests were cut away by lumbermen, the supply of charcoal threatened to give out, and experiments were made in charring coal, which resulted in the successful making of coke, a fuel made from coal by a process similar to the making of charcoal from wood. the story of the making of coke out of hard and soft coal is a long one, for it began as far back as the beginning of the nineteenth century. in the first boat-load of anthracite coal was sent to philadelphia from a little settlement along the lehigh river. a mine had been opened, the owner of which believed that the black, shiny "rocks" would burn. his neighbours laughed at him, for they had tried building fires with them, and concluded that it could not be done. in philadelphia, the owners of some coke furnaces gave the new fuel a trial, in spite of the disgust of the stokers, who thought they were putting out their fires with a pile of stones. after a little, however, the new fuel began to burn with the peculiar pale flame and intense heat that we know so well, and the stokers were convinced that here was a new fuel, with possibilities in it. but it was hard for people to be patient with the slow starting of this hard coal. not until did it come into general favour, following the discovery that if hot air was supplied at the draught, instead of cold, anthracite coal became a perfect fuel. at connellsville, pennsylvania, a vein of coal was discovered which made coke of the very finest quality. around this remarkable centre, coke ovens were built, and iron ore was shipped in, even from the rich beds of the lake superior country. but it was plain to see that connellsville coal would become exhausted; and so experiments in coke-making from other coals were still made. when soft coal burns, a waxy tar oozes out of it, which tends to smother the fire. early experiments with coal in melting iron ore indicated that soft coal was useless as a substitute for charcoal and coke; but later experiments proved that coke of fine quality can be made out of this bituminous soft coal, by drawing off the tar which makes the trouble. new processes were invented by which valuable gas and coal tar are taken out of bituminous coal, leaving, as a residue, coke that is equal in quality to that made from the connellsville coal. fortunes have been made out of the separation of the elements of the once despised soft coal. for the coke and each of its by-products, coal tar and coal gas, are commercial necessities of life. the impurities absorbed by the melting iron ore include carbon, phosphorus, and silicon. carbon is the chief cause of the brittleness of cast iron. the puddling furnace was invented to remove this trouble. the melted ore was stirred on a broad, basin-like hearth, with a long-handled puddling rake. the flames swept over the surface, burning the carbon liberated by the stirring. it was a hard, hot job for the man at the rake, but it produced forge iron, that could be shaped, hot or cold, on the anvil. the next improvement was the process of pressing the hot iron between grooved rollers to rid it of slag and other foreign matters collected in the furnace. the old way was to hammer the metal free from such impurities. this was slow and hard work. iron was an expensive and scarce metal until the hot blast-furnace cheapened the process of smelting the ore. the puddling furnace and the grooved rollers did still more to bring it into general use. the railroads developed with the iron industry. soon they required a metal stronger than iron. steel was far too expensive, though it was just what was needed. efforts were made to find a cheap way to change iron into steel. sir henry bessemer solved the problem by inventing the bessemer converter. it is a great closed retort, which is filled with melted pig iron. a draught admits air, and the carbon is all burned out. then a definite amount of carbon, just the amount required to change iron into steel, is added, by throwing in bars of an alloy of carbon and manganese. the latter gives steel its toughness, and enables it to resist greater heat without crystallizing and thus losing its temper. when the carbon has been put in, the retort is closed. the molten metal absorbs the alloy, and the product is bessemer steel. in fifteen minutes pig iron can be transformed into ingot steel. the invention made possible the use of steel in the construction of bridges, high buildings, and ships. it made this age of the world the age of steel. the age of reptiles two big and interesting reptiles we see in the zoo, the crocodile and its cousin, the alligator. in the everglades of florida both are found. the crocodile of the nile is protected by popular superstition, so it is in better luck than ours. the alligators have been killed off for their skins, and it is only a matter of time till these lumbering creatures will be found only in places where they are protected as the remnants of a vanished race. giant reptiles of other kinds are few upon the earth now. the _boa constrictor_ is the giant among snakes. the great tropical turtles represent an allied group. most of the turtles, lizards, and snakes are small, and in no sense dominant over other creatures. the rocks that lie among the coal measures contain fossils of huge animals that lived in fresh water and on land, the ancestors of our frogs, toads, and salamanders, a group we call amphibians. some of these animals had the form of snakes; some were fishlike, with scaly bodies; others were lizard-like or like huge crocodiles. these were the ancestors of the reptiles, which became the rulers of land and sea during the mesozoic era. the rocks that overlie the coal measures contain fossils of these gigantic animals. strange crocodile-like reptiles, with turtle-like beaks and tusks, but no teeth, left their skeletons in the mud of the shores they frequented. and others had teeth in groups--grinders, tearers, and cutters--like mammals. these had other traits like the old-fashioned, egg-laying mammals, the duck-billed platypus, for example, that is still found in australia. along with the remains of these creatures are found small pouched mammals, of the kangaroo kind, in the rocks of europe and america. these land animals saw squatty cycads, and cone-bearing trees, the ancestors of our evergreens, growing in forests, and marshes covered with luxuriant growths of tree ferns and horsetails, the fallen bodies of which formed the recent coal that is now dug in virginia and north carolina. ammonites, giant sea snails, with chambered shells that reached a yard and more in diameter, and gigantic squids, swam the seas. sea urchins, starfish, and oysters were abundant. insects flitted through the air, but no birds appeared among the trees or beasts in the jungles. over all forms of living creatures reptiles ruled. they were remarkable in size and numbers. there were swimming, running, and flying forms. [illustration: banded sandstone from calico cañon, south dakota] [illustration: _by permission of the american museum of natural history_ opalized wood from utah] [illustration: _by permission of the american museum of natural history_ restoration of a carnivorous dinosaur, allosaurus, from the upper jurassic and lower cretaceous of wyoming. when erect the animal was about feet high] the fish-reptile, _ichthyosaurus_, was a hump-backed creature, thirty to forty feet long, with short neck, very large head, and long jaw, set with hundreds of pointed teeth. its eye sockets were a foot across. the four short limbs were strong paddles, used for swimming. the long, slender tail ended in a flat fin. perfect skeletons of this creature have been found. its rival in the sea was the lizard-like _plesiosaurus_, the small head of which was mounted on a long neck. the tail was short, but the paddles were long and powerful. no doubt this agile creature held its own, though somewhat smaller than the more massively built ichthyosaurus. the land reptiles called _dinosaurs_ were the largest creatures that have ever walked the earth. in the american museum of natural history, in new york, the mounted skeleton of the giant dinosaur fairly takes one's breath away. it is sixty-six feet long, and correspondingly large in every part, except its head. this massive creature was remarkably short of brains. the strangest thing about the land reptiles is the fact that certain of them walked on their hind legs, like birds, and made three-toed tracks in the mud. indeed, these fossil tracks, found in slate, were called bird tracks, until the bones of the reptile skeleton with the bird-like foot were discovered. certain long grooves in the slate, hitherto unexplained, were made by the long tail that dragged in the mud. when the mud dried, and was later covered with sediment of another kind, these prints were preserved, and when the bed of rock was discovered by quarrymen, the two kinds split apart, showing the record of the stroll of a giant along the river bank in bygone days. the flying reptiles were still more bird-like in structure, though gigantic in size. imagine the appearance of a great lizard with bat-like, webbed wings and bat-like, toothed jaws! the first feathered fossil bird was discovered in the limestone rock of bavaria. it was a wonderfully preserved fossil, showing the feathers perfectly. three fingers of each "hand" were free and clawed, so that the creature could seize its prey, and yet use its feathered wings in flight. the small head had jaws set with socketed teeth, like a reptile's, and the long, lizard tail of twenty-one bones had a pair of side feathers at each joint. this _archeopteryx_ is the reptilian ancestor of birds. during this age of the world, one branch of the reptile group established the family line of birds. the bird-like reptiles are the connecting link between the two races. how much both birds and reptiles have changed from that ancient type, their common ancestor! i have mentioned but a few of the types of animals that make the reptilian age the wonder of all time. one after another skeletons are unearthed and new species are found. the connecticut river valley, with its red sandstones and shales of the mesozoic era, is famous among geologists, because it preserves the tracks of reptiles, insects, and crustaceans. these signs tell much of the life that existed when these flakes of stone were sandy and muddy stretches not many bones have been found, however. the thickness of these rocks is between one and two miles. the time required to accumulate so much sediment must have been very great. [illustration: _by permission of the american museum of natural history_ model of a three-horned dinosaur, _triceratops_, from cretaceous of montana. animal in life about feet long] [illustration: _by permission of the american museum of natural history_ mounting the forelegs of _brontosaurus_, the aquatic dinosaur] it is not clear just what caused the race of giant reptiles to decline and pass away. the climate did not materially change. perhaps races grow old, and ripe for death, after living long on the earth. it seems as if their time was up; and the clumsy giants abdicated their reign, leaving dominion over the sea, the air, and the land to those animals adapted to take the places they were obliged to vacate. the age of mammals the warm-blooded birds and mammals followed the reptiles. this does not mean that all reptiles died, after having ruled the earth for thousands of years. it means that changes in climate and other life conditions were unfavourable to the giants of the cold-blooded races, and gradually they passed away. they are represented now on the earth by lesser reptiles, which live comfortably with the wild creatures of other tribes, but which in no sense rule in the brute creation. they live rather a lurking, cautious life, and have to hide from enemies, except a few more able kinds, provided with means of defense. there were mammals on the earth in the days of reptilian supremacy, but they were small in size and numbers, and had to avoid any open conflict with the giant reptiles, or be worsted in a fight. now the time came when the ruling power changed hands. the mammals had their turn at ruling the lower animals. it was the beginning of things as they are to-day, for mammals still rule. but many millions of years have probably stood between the age when this group of animals first began to swarm over the earth, and the time when man came to be ruler over all created things. among the reptiles of the period when the sea, the land, and the air were swarming with these great creatures were certain kinds that had traits of mammals. others were bird-like. from these reptilian ancestors birds and mammals have sprung. no one doubts this. the fossils prove it, step by step. yet the rocks surprise the geologist with the suddenness with which many new kinds of mammals appeared on the earth. possibly the rocks containing the bones of so many kinds were fortunately located. the spots may have been morasses where migrating mammals were overwhelmed while passing. possibly conditions favored the rapid development of new kinds, and the multiplication of their numbers. warm, moist climate furnished abundant succulent plant food for the herbivors, and these in turn furnished prey for the carnivors. the coal formed during the tertiary period gives added proof that the plant life was luxuriant. the kinds of trees that grew far north of our present warm zones have left in the rocks evidence in the form of perfect leaves and cones and other fruits. for instance, magnolias grew in greenland, and palm trees in dakota. the temperature of greenland was thirty degrees warmer than it is now. our northern states lie in a belt that must have had a climate much like that of florida now. europe was correspondingly mild. a special chapter tells of the gradual development of the horse. one hundred different kinds of mammals have been found in the eocene rocks, many of which have representative species at the same time in europe and america. the rocks of asia probably have similar records. the eocene rocks, lowest of the tertiary strata, contain remains of animals the families of which are now extinct. next overlying the eocene, the miocene rocks have fossils of animals belonging to modern families--rhinoceroses, camels, deer, dogs, cats, horses--but the genera of which are now extinct. the pliocene strata (above the miocene) contains fossils of animals so closely related to the wild animals now on the earth as to belong to the same genera. they differ from modern kinds only in the species, as the red squirrel is a different species from the gray. so the record in the rocks shows a gradual approach of the mammals to the kinds we know, a gradual passing of the mighty forms that ruled by size and strength, and the coming of forms with greater intelligence, adapted to the change to a colder climate. it sometimes happens that a farmer, digging a well on the prairie, strikes the skeleton of a monster mammal, called the _mastodon_. this very thing happened on a neighbour's farm when i was a girl, in iowa. everybody was excited. the owner of the land dug out every bone, careful that the whole skeleton be found. as he expected, the director of a museum was glad to pay a high price for the bones. [illustration: _by permission of the american museum of natural history_ restoration of an aquatic dinosaur, _brontosaurus excelsus_, from the upper jurassic and lower cretaceous of wyoming. the animal in life was over feet long] [illustration: _by permission of the american museum of natural history_ restoration of the small carnivorous dinosaur, _ornitholestes hermanui_, catching a primitive bird _archæopteryx_. upper jurassic and lower cretaceous] the mastodon was about the size of an elephant, with massive limbs, and large, heavy head that bore two stout, up-curved tusks of ivory. the creature moved in herds like the buffalo from swamp to swamp; and old age coming on, the individual, unable to keep up with the herd, sank to his death in the boggy ground. the peat accumulated over his bones, undisturbed until thousands of years elapse, and the chance digging of a well discovers his skeleton. frozen in the ice of northern siberia, near the mouths of rivers, a number of mammoths have been found. these are creatures of the elephant family, and belonging to the extinct race that lived in the quaternary period, just succeeding the tertiary. the ice overtook the specimens, and they have been in cold storage ever since. for this reason, both flesh and bones are preserved, a rare thing to happen, and rarer still to be seen by a scientist. the ignorant natives made a business of watching the ice masses at the river mouth for dark spots that showed where a mammoth was encased in the ice. if an iceberg broke off near such a place, the sun might thaw the ice front of the glacier, until the hairy monster could at length be reached. his long hair served for many uses, and the wool that grew under the hair was used as a protection from the arctic winter. the frozen flesh was eaten; the bones carved into useful tools; but the chief value of the find was in the great tusks of ivory, that curved forward and pointed over the huge shoulders. it was worth a fortune to get a pair and sell them to a buyer from st. petersburg. one of the finest museum specimens of the mammoth was secured by buying the tusks of the dealer, and by his aid tracing the location of the carcass, which was found still intact, except that dogs had eaten away part of one foreleg, bone and all. from this carefully preserved specimen, models have been made, exactly copying the shape and the size of the animal, its skin, hair, and other details. the sabre-toothed tiger, the sharp tusks of which, six to eight inches long, made it a far more ferocious beast than any modern tiger of tropical jungles, was a quaternary inhabitant of europe and america. so was a smaller tiger, and a lion. the irish elk, which stood eleven feet high, with antlers that spread ten feet apart at the tips, was monarch in the deer family, which had several different species on both continents. wild horses and wild cattle, one or two of great size, roamed the woods, while rhinos and the hippopotamus kept near the water-courses. hyenas skulked in the shadows, and acted as scavengers where the great beasts of prey had feasted. sloths and cuirassed animals, like giant armadillos, lived in america. among bears was one, the cave bear, larger than the grizzly. true monkeys climbed the trees. flamingo, parrots, and tall secretary birds followed the giant _gastornis_, the ancestor of wading birds and ostriches, which stood ten feet high, but had wings as small and useless as the auk of later times. with the entrance of the modern types of trees, came other flowering plants, and with them the insects that live on the nectar of flowers. through a long line of primitive forms, now extinct, flowering plants and their insect friends conform to modern types. the record is written in the great stone book. the age of mammals in america and europe ended with the gradual rise of the continental areas, and a fall of temperature that ushered in the ice age. with the death of tropical vegetation, the giant mammals passed away. the horse and his ancestors every city has a horse market, where you may look over hundreds of animals and select one of any colour, size, or kind. the least in size and weight is the shetland pony, which one man buys for his children to drive or ride. another man wants a long-legged, deep-chested hunter. another wants heavy draught-horses, with legs like great pillars under them, and thick, muscular necks--horses weighing nearly a ton apiece and able to draw the heaviest trucks. what a contrast between these slow but powerful animals and the graceful, prancing racer with legs like pipe-stems--fleet and agile, but not strong enough to draw a heavy load! all these different breeds of horses have been developed since man succeeded in capturing the wild horse and making it help him. man himself was still a savage, and he had to fight with wild beasts, as if he were one of them, until he discovered that he could conquer them by some power higher than physical strength. from this point on, human intelligence has been the power that rules the lower animals. its gradual development is the story of the advance of civilization on the earth. through unknown thousands of years it has gone on, and it is not yet finished. [illustration: _by permission of the american museum of natural history_ restoration of a siberian mammoth, _elephas primogenius_, pursued by men of the old stone age of europe. late pleistocene epoch] [illustration: _by permission of the american museum of natural history_ restoration of a small four-toed ancestor of the horse family, _eohippus venticolus_. lower eocene of wyoming] just when and where and how our savage ancestors succeeded in taming the wild horse of the plains and the forests of europe or asia is unknown. man first made friends with the wild sheep, which were probably more docile than wild oxen and horses. we can imagine cold and hungry men seeking shelter from storms in rocky hollows, where sheep were huddled. how warm the woolly coats of these animals felt to their human fellow-creatures crowded in with them in the dark! it is believed that the primitive men who used stone axes as implements and weapons, learned to use horses to aid them in their hunting, and in their warfare with beasts and other men. gradually these useful animals were adapted to different uses; and at length different breeds were evolved. climate and food supply had much to do with the size and the character of the breeds. in the shetland islands the animals are naturally dwarfed by the cold, bleak winters, and the scant vegetation on which they subsist. in middle europe, where the summers are long and the winters mild, vegetation is luxurious, and the early horses developed large frames and heavy muscles. the shetland pony and the percheron draught-horse are the two extremes of size. what man has done in changing the types of horses is to emphasize natural differences. the offspring of the early heavy horses became heavier than their parents. the present draught-horse was produced, after many generations, all of which gradually approached the type desired. the slender racehorses, bred for speed and endurance rather than strength, are the offspring of generations of parents that had these qualities strongly marked. hence came the english thoroughbred and the american trotter. we can read in books the history of breeds of horses. our knowledge of what horses were like in prehistoric times is scant. it is written in layers of rock that are not very deep, but are uncovered only here and there, and only now and then seen by eyes that can read the story told by fossil skeletons of horses of the ages long past. geologists have unearthed from time to time skeletons of horses. it was professor marsh who spent so much time in studying the wonderful beds of fossil mammals in the western part of this country, and found among them the skeletons of many species of horses that lived here with camels and elephants and rhinoceroses and tigers, long before the time of man's coming. how can any one know that these bones belonged to a horse's skeleton? because some of them are like the bones of a modern horse. it is an easy matter for a student of animal anatomy to distinguish a horse from a cow by its bones. the teeth and the foot are enough. these are important and distinguishing characters. it is by peculiarities in the formation of the bones of the foot that the different species of extinct horses are recognized by geologists. wild horses still exist in the wilds of russia. remains of the same species have been dug out of the soil and found in caves in rocky regions. deeper in the earth are found the bones of horses differing from those now living. the bones of the foot indicate a different kind of horse--an unknown species. but in the main features, the skeleton is distinctly horse-like. in rocks of deeper strata the fossil bones of other horses are found. they differ somewhat from those found in rocks nearer the surface of the earth, and still more from those of the modern horse. the older the rocks, the more the fossil horse differs from the modern. could you think of a more interesting adventure than to find the oldest rocks that show the skeletons of horses? the foot of a horse is a long one, though we think of it as merely the part he walks on. a horse walks on the end of his one toe. the nail of the toe we call the "hoof." the true heel is the hock, a sharp joint like an elbow nearly half way up the leg. along each side of the cannon, the long bone of this foot, lies a splint of bone, which is the remnant of a toe, that is gradually being obliterated from the skeleton. these two splints in the modern horse's foot tell the last chapter of an interesting story. the earliest american horse, the existence of which is proved by fossil bones, tells the first chapter. the story has been read backward by geologists. it is told by a series of skeletons, found in successive strata of rock. the "bad lands" of the arid western states are rich in fossil remains of horses. below the surface soil lie the rocks of the quaternary period, which included the drift laid down by the receding glaciers and the floods that followed the melting of the ice-sheet. under the quaternary lie the tertiary rocks. these comprise three series, called the eocene, miocene, and pliocene, the eocene being the oldest. in the middle region of north america, ponds and marshy tracts were filled in during the tertiary period, by sediment from rivers; and in these beds of clay and other rock débris the remains of fresh-water and land animals are preserved. raised out of water, and exposed to erosive action of wind and water, these deposits are easily worn away, for they have not the solidity of older rocks. they are the crumbly bad lands of the west, cut through by rivers, and strangely sculptured by wind and rain. here the fossil horses have been found. _eohippus_, the dawn horse, is the name given a skeleton found in in the lower eocene strata in wyoming. this specimen lay buried in a rock formation ages older than that in which the oldest known skeleton of this family had been found. its discovery made a great sensation among scientists. this little animal, the skeleton of which is no larger than that of a fox, had four perfect toes, and a fifth splint on the forefoot, and three toes on the hind foot. the teeth are herbivorous. _orohippus_, with a larger skeleton, was found in the middle eocene strata of wyoming. its feet are like those of its predecessor, except that the splint is gone. the teeth as well as the feet are more like those of the modern horse. _mesohippus_, the three-toed horse, found in the miocene, shows the fourth toe reduced to splints, and the skeleton as big as that of a sheep. in this the horse family becomes fairly established. _hypohippus_, the three-toed forest horse, found in the middle miocene strata of colorado, is a related species, but not a direct ancestor of the modern horse. _neohipparion_, the three-toed desert horse, from the upper miocene strata, shows the three toes still present. but the pliocene rocks contain fossils showing gradual reduction of the two side toes, modification of the teeth, and increase in size of the skeleton. _protohippus_ and _pliohippus_, the one-toed species from the pliocene strata, illustrate these changes. they were about the size of small ponies. _equus_, the modern horse, was represented in the pliocene strata by a species, now extinct, called _equus scotti_. this we may regard as the true wild horse of america, for it was as large as the domesticated horse, and much like it, though more like a zebra in some respects. no one can tell why these animals, once abundant in this country, became extinct at the end of the tertiary period. but this is undoubtedly true. the types described form a series showing how the ancestors of the modern horse, grazing on the marshy borders of ancient ponds, lived and died, generation after generation, through a period covering thousands, possibly millions, of years. along the sides of the crumbling buttes these ancient burying-grounds are being uncovered. within a dozen years several expeditions, fitted out by the american museum of natural history, have searched the out-cropping strata in dakota and wyoming for bones of mammals known to have lived at the time the strata were forming in the muddy shallows along the margins of lake and marsh. duplicate skeletons of the primitive horse types above have been found, and vast numbers of their scattered bones. each summer geological excursions will add to the wealth of fossils of this family collected in museums. the tertiary rocks in europe yield the same kind of secrets. the region of paris overlies the estuary of an ancient river. when the strata are laid bare by the digging of foundations for buildings, bones are found in abundance. cuvier was a famous french geologist who made extensive studies of the remains of the prehistoric animals found in this old burial-place called by scientists the paris basin. he believed that the dead bodies floated down-stream and accumulated in the mud of the delta, where the tide checked the river's current. skeletons of the hipparion, a graceful, three-toed horse, were found in numbers in the strata of the miocene time. this animal lived in europe while the pliohippus and the protohippus were flourishing in america. a great number of species of tapir-like animals left their bones in the paris basin, among them a three-hoofed animal which may have been the connecting link between the horse and the tapir families. cuvier found the connecting link between tapirs and cud-chewing mammals. the age of man the hairy, woolly mammoth was one of the giant mammals that withstood the cold of the great ice flood, when the less hardy kinds were cut off by the changing climate of the northern half of europe and america. in caves where the wild animals took refuge from their enemies, skeletons of men have been found with those of the beasts. with these chance skeletons have been found rude, chipped stone spear-heads, hammers, and other tools. with these the savage ancestors of our race defended themselves, and preyed on such animals as they could use for food. they hunted the clumsy mammoth successfully, and shared the caverns in the rocks with animals like the hyena, the sabre-toothed tiger, and the cave bear, which made these places their homes. in california a human skull was found in the bed of an ancient river, which was buried by a lava flow from craters long ago extinct. with this buried skull a few well-shaped but rough stone tools were found. this man must have lived when the great ice flood was at its height. in southern france, caves have been opened that contained bones and implements of men who evidently lived by fishing and hunting. bone fish-hooks showed skill in carving with the sharp edges of flint flakes. a spirited drawing of a mammoth, made on a flat, stone surface, is a proof that savage instincts were less prominent in these cave men than in those who fought the great reindeer and the mammoth farther north. in later times men of higher intelligence formed tribes, tamed the wild horse, the ox, and the sheep, and made friends with the dog. great heaps of shells along the shores show where the tribes assembled at certain times to feast on oysters and clams. bones of animals used as food, and tools, are found in these heaps, called "kitchen-middens." these are especially numerous in northern europe. the stone implements used by these tribes were smoothly polished. a higher intelligence expressed itself also by the making of utensils out of clay. this pottery has been found in shell heaps. so the rude cave man, who was scarcely less a wild beast than the animals which competed with him for a living and a shelter from storms and cold, was succeeded by a higher man who brought the brutes into subjection by force of will and not by physical strength. the lake-dwellers, men of the bronze age, built houses on piles in the lakes of central europe. about sixty years ago the water was low, and these relics of a vanished race were first discovered. the lake bottoms were scraped for further evidences of their life. tools of polished stone and of bronze were taken up in considerable numbers. stored grains and dried fruits of several kinds were found. ornamental trinkets, weapons of hunters and warriors, and agricultural tools tell how the people lived. their houses were probably built over the water as a means of safety from attack of beasts or hostile men. in our country the mound-builders have left the story of their manners of life in the spacious, many-roomed tribal houses, built underground, and left with a great variety of relics to the explorers of modern times. these people worked the copper mines, and hammered and polished lumps of pure metal into implements for many uses. with these are tools of polished stone. stores of corn were found in many mounds scattered in the mississippi valley. the cliff-dwellers of the mesas of arizona and new mexico had habits like those of the mound-builders, and the aztecs, a vanished race in the southwest, at whose wealth and high civilization the invading spaniards under cortez marvelled. the plastered stone houses of the cliff-dwelling indians had many stories and rooms, each built to house a tribe, not merely a family. the pueblo, the moqui, and the zuni indians build similar dwellings to-day, isolated on the tops of almost inaccessible mesas. millions of years have passed since life appeared on the earth. gradually higher forms have followed lower ones in the sea and on the land. but not all of the lower forms have gone. all grades of plants and animals still flourish, but the dominant class in each age is more highly organized than the class that ruled the preceding age. to discover the earth's treasure, and to turn it to use; to tame wild animals and wild plants, and make them serve him; to create ever more beautiful and more useful forms in domestication; to find out the earth's life story, by reading the pages of the great stone book--these are undertakings that waited for man's coming. * * * * * part ii the sky * * * * * every family a "star club" the best family hobby we have ever had is the stars. we have a star club with no dues to pay, no officers to boss us, and only three rules: . we shall have nothing but "fun" in this club--no hard work. therefore no mathematics for us! . we can't afford a telescope. therefore we must be satisfied with what bright eyes can see. . no second-hand wonders for us! we want to see the things ourselves, instead of depending on books. you can't imagine what pleasure we have had in one short year! the baby, of course, was too young to learn anything, and besides he was in bed long before the stars came out. but ruth, our seven-year-old, knows ten of the fifteen brightest stars; and she can pick out twelve of the most beautiful groups or constellations. we grown-ups know all of the brightest stars, and all forty-eight of the most famous constellations. and the whole time we have given to it would not exceed ten minutes a day! and the best part is the _way_ we know the stars. the sky is no longer bewildering to us. the stars are not cold, strange, mysterious. they are friends. we know their faces just as easily as you know your playmates. for instance, we know sirius, because he is the brightest. we know castor and pollux, because they are twins. we know regulus, because he is in the handle of the sickle. and some we know by their colours. they are just as different as president taft, "ty" cobb, horace fletcher and maude adams. and quite as interesting! what's more, none of us can ever get lost again. no matter what strange woods or city we go to, we never get "turned around." or if we do, we quickly find the right way by means of the sun or the stars. then, too, our star club gives us all a little exercise when we need it most. winter is the time when we all work hardest and have the fewest outdoor games. winter is also the best time for young children to enjoy the stars, because it gets dark earlier in winter--by five o'clock, or long before children go to bed. it is pleasant to go out doors for half an hour before supper and learn one new star or constellation. again, it is always entertaining because every night you find the old friends in new places. no two nights are just the same. the changes of the moon make a great difference. some nights you enjoy the moonlight; other nights you wish there were no moon, because it keeps you from spying out some new star. we have a little magazine that tells us all the news of the stars and the planets and the comets _before_ the things happen! we pay a dollar a year for it. it is called the _monthly evening sky map_. when we first became enthusiastic about stars, the father of our family said: "well, i think our star club will last about two years. i judge it will cost us about two dollars and we shall get about twenty dollars worth of fun out of it." but in all three respects father was mistaken. part of the two dollars father spoke of went for a book called "the friendly stars," and seventy-five cents we spent for the most entertaining thing our family ever bought--a planisphere. this is a device which enables us to tell just where any star is, at any time, day or night, the whole year. it has a disc which revolves. all we have to do is to move it until the month and the day come right opposite the very hour we are looking at it, and then we can tell in a moment which stars can be seen at that time. then we go down the street where there is a good electric light at the corner and we hold our planisphere up, almost straight overhead. the light shines through, so that we can read it, and it is just as if we had a map of the heavens. we can pick out all the interesting constellations and name them just as easily as we could find the great lakes or rocky mountains in our geography. we became so eager not to miss any good thing that father got another book. every birthday in our family brought a new star book, until now we have about a dozen--all of them interesting and not one of them having mathematics that children cannot understand. so i think we have spent on stars fifteen dollars more than we needed to spend (but i'm glad we did it), and i think we have had about two hundred dollars worth of fun! yes, when i think what young people spend on ball games, fishing, tennis, skating, and all the other things that children love, i am sure our family has had about two hundred dollars worth of fun out of stars. and there is more to come! you would laugh to know why i enjoy stars so much. i have always studied birds and flowers and trees and rocks and shells so much that i was afraid to get interested in stars. i thought it wouldn't rest me. but it's a totally different kind of science from any i ever studied! there are no families, genera, and species among the stars, thank heaven! that's one reason they refresh me. another is that no one can press them and put them in a herbarium, or shoot them and put them in a museum. and another thing about them that brings balm to my spirit is that no human being can destroy their beauty. no one can "sub-divide" capella and fill it with tenements. no one can use vega for a bill-board. ah, well! we must not be disturbed if every member of our family has a different point of view toward the stars; we can all enjoy and love them in our own ways. how would you like to start a star club like ours? you ought to be able to persuade your family to form one, because it need not cost a cent. perhaps this book will interest them all, but the better way is for you to read about one constellation and then go out with some of the family and find it. this book does not tell about wonderful things you can never see; it tells about the wonderful things all of us can see. i wish you success with your star club. perhaps your uncles and aunts will start clubs, too. we have three star clubs in our family--one in new york, one in michigan, and one in colorado. last winter the "colorado star gazers" sent this challenge to the "new jersey night-owls:" "_we bet you can't see venus by daylight!_" that seemed possible, because during that week the "evening star" was by far the brightest object in the sky. but father and daughter searched the sky before sunset in vain, and finally we had to ask the "moonstruck michiganders" how to see venus while the sun was shining. back came these directions on a postal-card: "wait until it is dark and any one can see venus. then find some tree, or other object, which is in line with venus and over which you can just see her. put a stake where you stand. next day go there half an hour before sunset, and stand a little to the west. you will see venus as big as life. the next afternoon you can find her by four o'clock. and if you keep on you will see her day before yesterday!" that was a great "stunt." we did it; and there are dozens like it you can do. and that reminds me that father was mistaken about our interest lasting only two years. we know that it will not die till we do. for, even if we never get a telescope, there will always be new things to see. our club has still to catch algol, the "demon's eye," which goes out and gleams forth every three days, because it is obscured by some dark planet we can never see. and we have never yet seen mira the wonderful, which for some mysterious reason dies down to ninth magnitude and then blazes up to second magnitude every eleventh month. ah, yes, the wonders and the beauties of astronomy ever deepen and widen. better make friends with the stars now. for when you are old there are no friends like old friends. the dippers and the pole star i never heard of any boy or girl who didn't know the big dipper. but there is one very pleasant thing about the dipper which children never seem to know. with the aid of these seven magnificent stars you can find all the other interesting stars and constellations. so true is this that a book has been written called "the stars through a dipper." to illustrate, do you know the _pointers_? i mean the two stars on the front side of the dipper. they point almost directly toward the pole star, or north star, the correct name of which is polaris. most children can see the pole star at once because it is the only bright star in that part of the heavens. but if you can't be sure you see the right one, a funny thing happens. your friend will try to show you by pointing, but even if you look straight along his arm you can't always be sure. and then, if he tries to tell you how far one star is from another, he will try to show you by holding his arms apart. but that fails also. and so, we all soon learn the easiest and surest way to point out stars and measure distances. the easiest way to tell any one how to find a star is to get three stars in a straight line, or else at right angles. the surest way to tell any one how far one star is from another is by "degrees." you know what degrees are, because every circle is divided into of them. and if you will think a moment, you will understand why we can see only half the sky at any one time, or degrees, because the other half of the sky is on the other side of the earth. therefore, if you draw a straight line from one horizon, clear up to the top of the sky and down to the opposite horizon, it is degrees long. and, of course, it is only half that distance, or degrees, from horizon to zenith. (horizon is the point where earth and sky seem to meet, and zenith is the point straight over your head.) now ninety degrees is a mighty big distance in the sky. the pole star is nothing like ninety degrees from the dipper. it is only twenty-five degrees, or about five times the distance between the pointers. and now comes the only thing i will ask you to remember. look well at the two pointers, because the distance between them, five degrees, is the most convenient "foot rule" for the sky that you will ever find. most of the stars you will want to talk about are from two to five times that distance from some other star that you and your friends are sure of. perhaps this is a little hard to understand. if so, read it over several times, or get some one to explain it to you, for when you grasp it, it will unlock almost as many pleasures as a key to the store you like the best. now, let's try our new-found ruler. let us see if it will help us find the eighth star in the dipper. that's a famous test of sharp eyes. i don't want to spoil your pleasure by telling you too soon where it is. perhaps you would rather see how sharp your eyes are before reading any further. but if you can't find the eighth star, i will tell you where to look. look at the second star in the dipper, counting from the end of the handle. that is a famous star called mizar. now look all around mizar, and then, if you can't see a little one near it, try to measure off one degree. to do this, look at the pointers and try to measure off about a fifth of the distance between them. then look about one degree (or less) from mizar, and i am sure you will see the little beauty--its name is alcor, which means "the cavalier" or companion. the two are sometimes called "_the horse and rider_"; another name for alcor is saidak, which means "the test." i shall be very much disappointed if you cannot see saidak, because it is not considered a hard test nowadays for sharp eyes. aren't these interesting names? mizar, alcor, saidak. they sound so arabian, and remind one of the "arabian nights." at first, some of them will seem hard, but you will come to love these old names. i dare say many of these star names are , years old. shepherds and sailors were the first astronomers. the sailors had to steer by the stars, and the shepherds could lie on the ground and enjoy them without having to twist their necks. they saw and named alcor, thousands of years before telescopes were invented, and long before there were any books to help them. they saw the demon star, too, which i have never seen. it needs patience to see those things; sharp eyes are nothing to be proud of, because they are given to us. but patience is something to be eager about, because it costs us a lot of trouble to get it. let's try for it. we've had a test of sight. now let's have a test of patience. it takes more patience than sharpness of sight to trace the outline of the little dipper. it has seven stars, too, and the pole star is in the end of the handle. do you see two rather bright stars about twenty-five degrees from the pole? i hope so, for they are the only brightish stars anywhere near polaris. well, those two stars are in the outer rim of the little dipper. now, i think you can trace it all; but to make sure you see the real thing, i will tell you the last secret. the handle of the big dipper is bent _back_; the handle of the little dipper is bent _in_. now, if you have done all this faithfully, you have worked hard enough, and i will reward you with a story. once upon a time there was a princess named callisto, and the great god jupiter fell in love with her. naturally, jupiter's wife, juno, wasn't pleased, so she changed the princess into a bear. but before this happened, callisto became the mother of a little boy named arcas, who grew up to be a mighty hunter. one day he saw a bear and he was going to kill it, not knowing that the bear was really his own mother. luckily jupiter interfered and saved their lives. he changed arcas into a bear and put both bears into the sky. callisto is the big bear, and arcas is the little bear. but juno was angry at that, and so she went to the wife of the ocean and said, "please, never let these bears come to your home." so the wife of the ocean said, "i will never let them sink beneath the waves." and that is why the big and the little dipper never set. they always whirl around the pole star. and that is why you can always see them, though some nights you would have to sit up very late. is that a true story? no. but, i can tell you a true one that is even more wonderful. once upon a time, before the bear story was invented and before people had tin dippers, they used to think of the little dipper as a little dog. and so they gave a funny name to the pole star. they called it cynosura, which means "the dog's tail." we sometimes say of a great man, "he was the cynosure of all eyes," meaning that everybody looked at him. but the original cynosure was and is the pole star, because all the stars in the sky seem to revolve around it. the two dippers chase round it once every twenty-four hours, as you can convince yourself some night when you stay up late. so that's all for to-night. what! you want another true story? well, just one more. once upon a time the big dipper was a perfect cross. that was about , years ago. fifty thousand years from now the big dipper will look like a steamer chair. how do i know that? because, the two stars at opposite ends of the dipper are going in a direction different from the other five stars. how do i know that? why, i don't know it. i just believe it. there are lots of things i don't know, and i'm not afraid to say so. i hope you will learn how to say "i don't know." it's infinitely better than guessing; it saves trouble, and people like you better, because they see you are honest. i don't know how the stars in the big dipper are moving, but the men who look through telescopes and study mathematics say the end stars do move in a direction opposite to the others, and they say the dipper _must_ have looked like a cross, and will look like a dipper long, long after we are dead. and i believe them. constellations you can always see there are forty-eight well known constellations, but of these only about a dozen are easy to know. i think a dozen is quite enough for children to learn. and therefore, i shall tell you how to find only the showiest and most interesting. the best way to begin is to describe the ones that you can see almost every night in the year, because you may want to begin any month in the year, and you might be discouraged if i talked about things nobody could see in that month. there are five constellations you can nearly always see, and these are all near the pole star. doubtless you think you know two of them already--the big and the little dipper. ah, i forgot to tell you that these dippers are not the real thing. they are merely parts of bigger constellations and their real names are great bear and little bear. the oldest names are the right ones. thousands of years ago, when the greeks named these groups of stars, they thought they looked like two bears. i can't see the resemblance. but for that matter all the figures in the sky are disappointing. the people who named the constellations called them lions, and fishes, and horses, and hunters, and they thought they could see a dolphin, a snake, a dragon, a crow, a crab, a bull, a ram, a swan, and other things. but nowadays we cannot see those creatures. we can see the stars plainly enough, and they do make groups, but they do not look like animals. i was greatly disappointed when i was told this; but i soon got over it, because new wonders are always coming on. i think the only honest thing to do is to tell you right at the start that you cannot see these creatures very well. you will spoil your pleasure unless you take these resemblances good-naturedly and with a light heart. and you will also spoil your pleasure if you scold the ancients for naming the constellations badly. nobody in the world would change those old names now. there is too much pleasure in them. besides, i doubt if we could do much better. i believe those old folks were better observers than we. and i believe they had a lighter fancy. let us, too, be fanciful for once. i have asked my friend, mrs. thomas, to draw her notion of some of these famous creatures of the sky. you can draw your idea of them too, and it is pleasant to compare drawings with friends. there is only one way to see anything like a great bear. you have to imagine the dipper upside down and make the handle of the dipper serve for the bear's tail. what a funny bear to drag a long tail on the ground! miss martin says he looks more like a chubby hobby-horse. you will have to make the bowl of the dipper into hind legs and use all the other stars, somehow, to make a big, clumsy, four-legged animal. and what a monster he is! he measures twenty-five degrees from the tip of his nose to the root of his tail. yes, all those miscellaneous faint stars you see near the big dipper belong to the great bear. [illustration: orion fighting the bull. above are orion's two dogs] [illustration: the little bear, the queen in her chair, the twins and the archer] how the great bear looked to the people who named it thousands of years ago, we probably shall never know. they left no books or drawings, so far as i know. but in every dictionary and book on astronomy you can find these bears and other animals drawn so carefully and beautifully that it seems as if they _must_ be in the sky, and we must be too dull to see them. it is not so. look at the pictures in this book and, you will see that the stars do not outline the animals. many of them come at the wrong places. and so it is with all the costly books and charts and planispheres. it is all very interesting, but it isn't true. it's just fancy. and when you once understand that it isn't true, you will begin to enjoy the fancy. many a smile you will have, and sometimes a good laugh. for instance, the english children call the dipper "charles's wain" or "the wagon." and the romans called it "the plough." they thought of those seven stars as oxen drawing a plough. well, that's enough about the two bears. i want to tell you about the other three constellations you can nearly always see. these are the chair, the charioteer, and perseus (pronounced _per'soos_). the chair is the easiest to find, because it is like a very bad w, and it is always directly opposite the big dipper, with the pole star half way between the two constellations. there are five stars in the w, and to make the w into a chair you must add a fainter star which helps to make the square bottom of the chair. but what a crazy piece of furniture! i have seen several ways of drawing it, but none of them makes a comfortable chair. i should either fall over backward, or else the back of the chair would prod me in the small of my back. the correct name of this constellation is cassiopeia's chair. i think this is enough to see and enjoy in one night. to-morrow night let us look for the charioteer. i love the charioteer for several reasons. one is that it makes a beautiful pentagon, or five-sided figure, with its five brightest stars. another is that it contains the second-brightest star in the northern part of the heavens, capella. the only star in the north that is brighter is vega, but vega is bluish white or creamy. if you haven't already found the five-sided figure, i will tell you how to find capella. suppose you had a gun that would shoot anything as far as you wish. shoot a white string right through the pointers and hit the pole star. then place your gun at the pole star and turn it till it is at right angles to that string you shot. aim away from the big dipper, shoot a bullet forty-five degrees and it will hit capella. if that plan doesn't work, try this. start with the star that is at the bottom of the dipper and nearest the handle. draw a line half-way between the two pointers and keep on till you come to the first bright star. this is capella, and the distance is about fifty degrees. capella means "a kid," or "little goat," and that reminds me of the third reason why i enjoy so much the constellation of which capella is the brightest star. in the old times they sometimes called this five-sided figure "the goat-carrier." and the shepherds thought they could see a man carrying a little goat in his left hand. i am sure you can see the kid they meant. it is a triangle of faint stars which you see near capella. that's enough for to-night. to-morrow night let us look for perseus. i dare say you know that old tale about perseus rescuing the princess who was chained upon a rock. (he cut off the snaky head of medusa and showed it to the dragon that was going to devour the princess, and it turned the monster to stone. remember?) well, there are constellations named after all the people in that story, but although they contain many showy stars, i could never make them look like a hero, a princess, a king, and a queen. i do not even try to trace out all of perseus. for i am satisfied to enjoy a very beautiful part of it which is called the "arc of perseus." an arc, you know, is a portion of a circle. and the way to find this arc is to draw a curve from capella to cassiopeia. on nights that are not very clear i can see about seven stars in this arc of perseus. and the reason i love it so much is that it is the most beautiful thing, when seen through an opera-glass, that i know. you could never imagine that a mere opera-glass would make such a difference. the moment i put it to my eyes about a dozen more stars suddenly leap into my sight in and near the arc of perseus. that's enough. no more stories to-night. winter constellations by winter constellations i mean those you can see in winter at the pleasantest time--the early evening. and i want you to begin with the northern cross. i hope you can see this before christmas, for, after that, it may be hidden by trees or buildings in the west and you may not see it again for a long while. this is because the stars seem to rise in the east and set in the west. to prove this, choose some brilliant star you can see at five or six o'clock; get it in line with some bush or other object over which you can just see it. put a stake where you stand, and then go to the same spot about eight o'clock or just before you go to bed. you can tell at once how much the star seems to have moved westward. another thing, every star rises four minutes later every night, and therefore the sky looks a little different at the same hour every evening. the stars in the north set for a short time only, but when those toward the south set they are gone a long time. for instance, the brightest star of all is sirius, the dog star, which really belongs to the southern hemisphere. there are only about three months in the year when children who go to bed by seven o'clock can see it--january, february, and march. so now you understand why i am so eager that you should not miss the pleasure of seeing the famous northern cross. but although it is a big cross, and easy to find, after you know it, i have never yet known a boy who could show it to another boy simply by pointing at it. the surest and best way to find it is learn three bright stars first--altair, vega, and deneb. altair is the brightest star in the milky way. it is just at the edge of the milky way, and you are to look for three stars in a straight line, with the middle one brightest. those three stars make the constellation called "the eagle." the body of the eagle is altair, and the other two stars are the wings. i should say that altair is about five degrees from each of his companions. it is worth half an hour's patient search to find the eagle. now these three stars in the eagle point straight toward the brightest star in the northern part of the sky--vega. to make sure of it, notice four fainter stars near it which make a parallelogram--a sort of diamond. these stars are all part of a constellation called "the lyre." if you try to trace out the old musical instrument, you will be disappointed; but here is a game worth while. can you see a small triangle made by three stars, of which vega is one? well, one of those stars is double, and with an opera-glass you can see which it is. on very clear nights some people with very sharp eyes can see them lying close together, but i never could. at last we are ready to find the celebrated northern cross. first draw a line from altair to vega. then draw a line at right angles to this, until you come to another bright star--deneb--which is about as far from vega as vega is from altair. now this beautiful star, deneb, is the top of the northern cross. i can't tell you whether the cross will be right or wrong side up when you see it, or on its side. for every constellation is likely to change its position during the night, as you know from watching the dipper. but you can tell the cross by these things. there are six stars in it. it is like a kite made of two sticks. there are three stars in the crosspiece and four in the long piece. deneb, the brightest star in the cross, is at the top of the long stick. but you mustn't expect to see a perfect cross. there is one star that is a little out of place, and sometimes my fingers fairly "itch to put it where it belongs." it is the one that ought to be where the long stick of your kite is tacked to the crosspiece. and one of the stars is provokingly faint, but you can see it. counting straight down the long piece, it is the third one from deneb that is faint. it is where it ought to be, but i should like to make it brighter. have you the cross now? if not, have patience. you can't be a "true sport" unless you are patient. you can't be a great ball-player, or hunter, or any thing else, without resisting, every day, that sudden impulse to "quit the game" when you lose. be a "good loser," smile and try again. that is better than to give up, or to win by cheating or sharp practice. this is the last thing i want you to see in the northern part of the sky; and if you have done a good job, let us celebrate by having a story. once upon a time a cross didn't mean so much to the world as it does now. that was before christ was born. in those old times people did not think of the northern cross as a cross. they thought of it as a swan, and you can see the swan if you turn the cross upside down. deneb will then be in the tail of the swan, and the two stars which used to be at the tips of the crosspiece now become the wings. is that a true story? yes. if we lived in arabia the children there could tell us what deneb means. it means "the tail." another story? well, do you see the star in the beak of the swan, or foot of the cross? what color is it? white? well, they say this white star is really made up of two stars--one yellow and the other blue. that is one reason i want to buy a telescope when i can afford it, for even the smallest telescope will show that. and mr. serviss says that even a strong field-glass will help any one see this wonder. i can't tell you about all the winter constellations in one chapter. we have made friends of the northern ones. now let's see the famous southern ones. and let's start a new chapter. orion, his dogs, and the bull the most gorgeous constellation in the whole sky is orion. i really pity any one who does not know it, because it has more bright stars in it than any other group. besides, it doesn't take much imagination to see this mighty hunter fighting the great bull. i dare say half the people in the united states know orion and can tell him as quick as they see him by the famous "belt of orion." this belt is made of three stars, each of which is just one degree from the next. that is why the english people call these three stars "the ell and yard." another name for them is "the three kings." you can see the "sword of orion" hanging down from his belt. as soon as you see these things you will see the four bright stars that outline the figure of the great hunter, but only two of them are of the first magnitude. the red one has a hard name--betelgeuse (pronounced _bet-el-guz´_). that is a frenchified word from the arabic, meaning "armpit," because this star marks the right shoulder of orion. the other first-magnitude star is the big white one in the left foot. its name is rigel (pronounced _re´-jel_) from an arabian word meaning "the foot." you can see the giant now, i am sure. over his left arm hangs a lion's skin which he holds out to shield him from the bull's horns. see the shield--about four rather faint stars in a pretty good curve? now look for his club which he holds up with his right hand so as to smite the bull. see the arm and the club--about seven stars in a rather poor curve--beyond the red star betelgeuse? now you have him, and isn't he a wonder! it is even easier to see the bull which is trying to gore orion. look where orion is threatening to strike, and you will see a v. how many stars in that v? five. and which is the brightest? that red one at the top of the left branch of the v? yes. that v is the face of the bull and that red star is the baleful eye of the angry bull which is lowering his head and trying to toss orion. the name of that red eye is aldebaran (pronounced _al-deb´-ar-an_). i wish aldebaran meant "red eye," but it doesn't. it is an old arabian word meaning the "hindmost," or the "follower," because every evening it comes into view about an hour after you can see the famous group of stars called the pleiades, which are in the shoulder of the bull. i do not care to trace the outline of this enormous bull, but his horns are a great deal longer than you think at first. if you will extend the two arms of that v a long way you will see two stars which may be called the tips of his horns. one of these stars really belongs in another constellation--our old friend the charioteer, the one including capella. wow! what a pair of horns! but now we come to the daintiest of all constellations--the seven sisters, or pleiades (pronounced _plee´-a-deez_). i can see only six of them, and there is a famous old tale about the "lost pleiad." but i needn't describe them. every child finds them by instinct. some compare them to a swarm of bees; some to a rosette of diamonds; some to dewdrops. but i would not compare them to a dipper as some do, because the real little dipper is very different. the light that seems to drip from the pleiades is quivering, misty, romantic, magical. no wonder many children love the pleiades best of all the constellations. no wonder the poets have praised them for thousands of years. the oldest piece of poetry about them that i know of was written about , years before christ. you can find it in the book of job. but the most poetic description of the pleiades that i have ever read is in tennyson's poem "locksley hall," in which he says they "glitter like a swarm of fireflies tangled in a silver braid." there are a great many old tales about the lost pleiad. one is that she veiled her face because the ancient city of troy was burned. another story says she ceased to be a goddess when she married a man and became mortal. some people think she was struck by lightning. others believe the big star, canopus, came by and ran away with her. still others declare she was a new star that appeared suddenly once upon a time, and after a while faded away. for myself, i do not believe any of these stories. one reason why i don't is that a seventh star is really there, and many people can really see it. indeed, there are some people so sharp-eyed that on clear nights they can see anywhere from eight to eleven. and, what is more, they can draw a map or chart showing just where each star seems to them to be. but the most wonderful stories about the pleiades are the true stories. one is that there are really more than , stars among the pleiades. some of them can be seen only with the biggest telescopes. others are revealed only by the spectroscope. and some can be found only by means of photography. but the most amazing thing about the pleiades is the distances between them. they look so close together that you would probably say "the moon seems bigger than all of them put together." sometimes the moon comes near the pleiades, and you expect that the moon will blot them all out. but the astronomers say the full moon sails through the pleiades and covers only one of them at a time, as a rule. they even say it is possible for the moon to pass through the pleiades without touching one of them! i should like to see that. if anything like it is going to occur, the magazine i spoke of in the first chapter will tell me about it. and you'd better believe i will stay up to see that, if it takes all night! there are two more constellations in the southern part of the sky that ought to be interesting, because they are the two hunting dogs that help orion fight the bull. but i can't trace these animals, and i don't believe it is worth while. the brightest stars in them everybody can see and admire--sirius, the bigger dog, and procyon, the smaller dog. every one ought to know sirius, because he is the brightest star of all. (of course, he is not so bright as venus and jupiter, but they are planets.) to find him, draw a line from the eye of the bull through the belt of orion and extend it toward the southeast about twenty degrees. they call him the dog star because he follows the heels of orion. and people still call the hottest days of summer "dog days" because years before christ the romans noticed that the dog star rose just before the sun at that time. the romans thought he chased the sun across the sky all day and therefore was responsible for the great heat. but that was a foolish explanation. and so is the old notion that dogs are likely to go mad during the dog days "because the dog star is in the ascendant." so is the idea that sirius is an unlucky star. there are no lucky or unlucky stars. these are all superstitions, and we ought to be ashamed to believe any superstition. yet for thousands of years before we had public schools and learned to know better, people believed that every one was born under a lucky star or an unlucky one, and they believe that farmers ought to plant or not plant, according to the size of the moon. now we know that is all bosh. those old superstitions have done more harm than good. one of the most harmful was the belief in witches. let us resolve never to be afraid of these old tales, but laugh at them. why should anybody be afraid of anything so lovely as sirius? i used to think sirius twinkled more than any other star. but that was bad reasoning on my part. i might have noticed that every star twinkles more near the horizon than toward the zenith. i might have noticed that stars twinkle more on clear, frosty nights than when there is a little uniform haze. and putting those two facts together i might have reasoned that the stars never really twinkle at all; they only seem to. i might have concluded that the twinkling is all due to the atmosphere--that blanket of air which wraps the earth around. the nearer the earth, the thicker the air, and the more it interferes with the light that comes to us from the stars. they say that sirius never looks exactly alike on two successive nights. "it has a hundred moods," says mr. serviss, "according to the state of the atmosphere. by turns it flames, it sparkles, it glows, it blazes, it flares, it flashes, it contracts to a point, and sometimes when the air is still, it burns with a steady white light." (quotation somewhat altered and condensed.) it is a pity that so fine a star as procyon should be called the "smaller dog," because it suffers unjustly by comparison with sirius. if it were in some other part of the sky we might appreciate it more, because it is brighter than most of the fifteen first-magnitude stars we can see. my brother william has grown to love it, but perhaps that is because he always "sympathizes with the under dog." he was the youngest brother and knows. and curiously enough he was nicknamed "the dog"--just why, i don't know. to find procyon, drawn a line from sirius northeast about twenty degrees. and to make sure, draw one east from betelgeuse about the same distance. these three stars make a triangle of which the sides are almost equal. the name procyon means "before the dog" referring to the fact that you can see him fifteen or twenty minutes earlier every night than you can see sirius. the only kind word about procyon i have heard in recent years was in connection with that miserable business of dr. cook and the north pole. a captain somebody-or-other was making observations for dr. cook, and he wanted to know what time it was. he had no watch and didn't want to disturb any one. so he looked out of the window and saw by the star procyon that it was eleven o'clock. that sounds mysterious, but it is easy if you have a planisphere like ours. last winter when we were all enjoying orion, the bull, and the two dogs, i used to whirl the planisphere around to see where they would be at six o'clock at night, at eight, at ten, at midnight, and even at six o'clock in the morning. and so, if i waked up in the night i could tell what time it was without even turning my head. sometimes i looked out of my window, saw orion nearly overhead and knew it must be midnight. and sometimes i woke up just before daybreak and saw the great bull backing down out of sight in the west, the mighty hunter still brandishing his club, and his faithful dogs following at his heels. seven famous constellations there are only seven more constellations that seem to me interesting enough for every one to know and love all his life. these are: the lion (spring) the twins (spring) the virgin (summer) the herdsman (summer) the northern crown (summer) the scorpion (summer) southern fish (autumn) i have named the seasons when, according to some people, these constellations are most enjoyable. but these are not the only times when you can see them. (if you had that seventy-five-cent planisphere, now, you could always tell which constellations are visible and just where to find them.) no matter what time of year you read this chapter, it is worth while to go out and look for these marvels. you can't possibly miss them all. have you ever seen a sickle in the sky? it's a beauty, and whenever i have seen it it has been turned very conveniently for me, because i am left-handed. it is so easy to find that i am almost ashamed to tell. but if you need help, draw a line through the pointers backward, away from the pole star, about forty degrees, and it will come a little west of the sickle. the sickle is only part of the lion--the head and the forequarters. only fanciful map-makers can trace the rest of the lion. the bright star at the end of the handle is regulus, which means "king," from the stupid old notion that this star ruled the lives of men. to this day people speak of the "royal star," meaning regulus. and at the end of this chapter i will tell you about three other stars which the persians called "royal stars." another constellation which children particularly love is the twins--castor and pollux. but the sailors got there first! for thousands of years the twins have been supposed to bring good luck to sailors. i don't believe a word of it. but i do know that sailors gloat over castor and pollux, and like them better than any other stars. the whole constellation includes all the stars east of the bull and between the charioteer and procyon. but another way to outline the twins is to look northeast of orion where you will see two rows of stars that run nearly parallel. to me the brothers seem to be standing, but all the old picture-makers show them sitting with their arms around each other, the two brightest stars being their eyes. the eyes are about five degrees apart--the same as the pointers. pollux is now brighter than castor, but for thousands of years it was just the other way. it is only within three hundred years that this change has taken place. whether castor has faded or pollux brightened, or both, i do not know. anyhow, castor is not quite bright enough to be a first magnitude star. three hundred years is a short time in the history of man, and only a speck in the history of the stars. three hundred years ago they killed people in europe just because of the church they went to. that was why the pilgrim fathers sailed from england in , and made the first permanent settlement in america, except, of course, jamestown, va., in . there are plenty of stories about old castor and pollux, and, like all the other myths, they conflict, more or less. but all agree that these two brothers went with jason in the ship argo, shared his adventures and helped him get the golden fleece. and all agree that castor and pollux were "born fighters." and that is why the roman soldiers looked up to these stars and prayed to them to help them win their battles. now for the four summer constellations every one ought to know. the first thing to look for is two famous red or reddish stars--arcturus and antares. the way you find arcturus is amusing. look for the big dipper and find the star at the bottom of the dipper nearest the handle. got it? now draw a curve that will connect it with all the stars in the handle, and when you come to the end of the handle keep on till you come to the first very bright star--about twenty-five degrees. that is the monstrous star arcturus, probably the biggest and swiftest star we can ever see with the naked eye in the northern hemisphere. he is several times as big as our sun, and his diameter is supposed to be several million miles. he is called a "runaway sun," because he is rushing through space at the rate of between two hundred and three hundred miles a second. that means between seventeen and thirty-four million miles a day! he is coming toward us, too! at such a rate you might think that arcturus would have smashed the earth to pieces long ago. but he is still very far away, and there is no danger. some people say that if job were to come to life, the sky would seem just the same to him as it did , years ago. the only difference he might notice would be in arcturus. that would seem to him out of place by a distance about three times the apparent diameter of the moon. some people believe this because job said, "canst thou guide arcturus with his sons?" and therefore they imagine that he meant this red star. but i believe he meant the big dipper. for in king james's time, when the bible was translated into english, the word "arcturus" meant the big dipper or rather the great bear. and for centuries before it meant the great bear. one proof of it is that "arcturus" comes from an old greek word meaning "bear"--the same word from which we get arctic. it is only within a few hundred years that astronomers have agreed to call the great bear "ursa major," and this red star arcturus. so i think all the books which say job mentioned this red star are mistaken. i believe webster's dictionary is correct in this matter, and i believe the revised version translates job's hebrew phrase more correctly when it says, "canst thou guide the bear with her train?" anyhow, arcturus is a splendid star--the brightest in the constellation called the "herdsman" or boötes. it is not worth while to trace the herdsman, but here is an interesting question. is arcturus really red? the books mostly say he is yellow. they say he looks red when he is low in the sky, and yellow when he is high. how does he look to you? more yellow than red? well, there's no doubt about antares being red. to find him, draw a long line from regulus through arcturus to antares, arcturus being more than half way between the other two. but if regulus and the sickle are not visible, draw a line from altair, at right angles to the eagle, until you come to a bright star about sixty degrees away. you can't miss antares, for he is the only red star in that part of the sky. antares belongs to a showy constellation called the scorpion. i cannot trace all the outline of a spider-like creature, but his poisonous tail or "stinger" is made by a curved line of stars south and east of antares. and you can make a pretty fan by joining antares to several stars in a curve which are west of antares and a little north. there is an old tale that this scorpion is the one that stung orion to death when he began to "show off" and boast that there was no animal in the world that could kill him. another very bright star in the southern part of the sky is spica. to find it, start with the handle of the dipper, and making the same backward curve which helped you to find arcturus, keep on till you come to the white star spica--say thirty degrees beyond arcturus. this is the brightest star in the constellation called "the virgin." it is not worth while trying to trace her among nearly forty faint stars in this neighbourhood. but she is supposed to be a winged goddess who holds up in her right hand an _ear of wheat_, and that is what spica means. now for an autumn constellation--the southern fish. i don't care if you fail to outline a fish, but i do want you to see the bright star that is supposed to be in the fish's mouth. and i don't want you to balk at its hard name--fomalhaut (pronounced _fo´-mal-o_). it is worth a lot of trouble to know it as a friend. to find it, you have to draw an exceedingly long line two-thirds of the way across the whole sky. start with the pointers. draw a line through them and the pole star and keep clear on until you come to a solitary bright star rather low down in the south. that is fomalhaut. it looks lonely and is lonely, even when you look at it through a telescope. and now for the last story. once upon a time the persians thought there must be four stars that rule the lives of men. so they picked out one in the north and one in the south and one in the east and one in the west, just as if they were looking for four bright stars to mark the points of the compass. if you want to find them yourself without my help don't read the next sentence, but shut this book and go out and see. then write down on a piece of paper the stars you have selected and compare them with the list i am about to give. here are the four royal stars of the persians: fomalhaut for the north, regulus for the south, aldebaran for the east, and antares for the west. why doesn't this list agree with yours? because persia is so far south of where we live. ah, there are very few things that are absolutely true. let's remember that and not be too sure: for everything depends upon the point of view! i hope you will see fomalhaut before christmas, before he disappears in the west. he is with us only five months and is always low--near the horizon. but the other seven months in the year he gladdens the children of south america and the rest of the southern hemisphere, for they see him sweeping high and lonely far up into their sky and down again. but the loveliest of all the constellations described in this chapter is the northern crown. it is not a perfect crown--only about half a circle--but enough to suggest a complete ring. look for it east of arcturus. i can see seven or eight stars in the half-circle, one of which is brighter than all the others. that one is called "the pearl." the whole constellation is only fifteen degrees long, but "fine things come in small packages"; and children grow to love the northern crown almost as much as they love the pleiades. the twenty brightest stars if you have seen everything i have described so far, you have reason to be happy. for now you know sixteen of the most famous constellations and fifteen of the twenty brightest stars. there are only twenty stars of the first magnitude. "magnitude" ought to mean size, but it doesn't. it means brightness--or rather the apparent brightness--of the stars when seen by us. the word magnitude was used in the old days before telescopes, when people thought the brighter a star is the bigger it must be. now we know that the nearer a star is to us the brighter it is, and the farther away the fainter. some of the bright stars are comparatively near us, some are very far. deneb and canopus are so far away that it takes over three hundred years for their light to reach us. what whoppers they must be--many times as big as our sun. here is a full list of the twenty stars of the first magnitude arranged in the order of their brightness. you will find this table very useful. ----------------+---------------+-------------+-------------------------- stars | pronounced |constellation| interesting facts ----------------+---------------+-------------+-------------------------- sirius | _sir´i-us_ | big dog | brightest star. nearest | | | star visible in northern | | | hemisphere canopus* | _ca-no´pus_ | ship argo | perhaps the largest body | | | in universe alpha centauri* | _al´fa | | | sen-taw´re_ | centaur | nearest star. light four | | | years away vega | _ve´ga_ | lyre | brightest star in the | | | northern sky. bluish capella | _ca-pell´a_ | charioteer | rivals vega, but opposite | | | the pole. yellowish arcturus | _ark-tu´rus_ | herdsman | swiftest of the bright | | | stars. miles a second rigel | _re´jel_ | orion | brightest star in orion. | | | white star in left foot procyon | _pro´si-on_ | little dog | before the dog. rises a | | | little before sirius achernar* | _a-ker´nar_ | river po | means the end of the river beta centauri* | _ba´ta | | | sen-taw´re_ | centaur | this and its mate point to | | | the southern cross altair | _al-tare´_ | eagle | helps you find vega and | | | northern cross betelgeuse | _bet-el-guz´_ | orion | means "armpit." the red | | | star in the right shoulder alpha crucis* | _al´fa | southern | | cru´sis_ | cross | at the base of the most | | | famous southern | | | constellation aldebaran | _al-deb´a-ran_| bull | the red eye in the v pollux | _pol´lux_ | twins | brighter than castor spica | _spi´ca_ | virgin | means ear of wheat antares | _an-ta´rez_ | scorpion | red star. name means | | | "looks like mars" fomalhaut | _fo´mal-o_ | southern | | | fish | the lonely star in the | | | southern sky deneb | _den´eb_ | swan | top of northern cross, | | | or tail of swan regulus | _reg´u-lus_ | lion | the end of the handle | | | of the sickle ----------------+---------------+-------------+-------------------------- the five stars marked * belong to the southern hemisphere, and we can never see them unless we travel far south. last winter i went to florida and saw canopus, but to see the southern cross you should cross the tropic of cancer. how to learn more all i can hope to do in this book is to get you enthusiastic about astronomy. i don't mean "gushy." look in the dictionary and you will find that the enthusiast is not the faddist. he is the one who sticks to a subject for a lifetime. nor do i care a rap whether you become an astronomer--or even buy a telescope. there will be always astronomers coming on, but there are too few people who know and love even a few of the stars. i want you to make popular astronomy a life-long hobby. perhaps you may have to drop it for ten or fifteen years. never mind, you will take up the study again. i can't expect you to read a book on stars if you are fighting to make a living or support a family, unless it really rests you to read about the stars. it does rest me. when things go wrong at the office or at home, i can generally find rest and comfort from music. and if the sky is clear, i can look at the stars, and my cares suddenly seem small and drop away. let me tell you why and how you can get the very best the stars have to teach you, without mathematics or telescope. follow this programme and you need never be afraid of hard work, or of exhausting the pleasures of the subject. go to your public library and get one of the books i recommend in this chapter, and read whatever interests you. i don't care whether you take up planets before comets or comets before planets, but whatever you do do it well. soak the interesting facts right in. nail them down. see everything the book talks about. make notes of things to watch for. get a little blank book and write down the date you first saw each thing of interest. write down the names of the constellations you love most. before you lay down any star book you are reading, jot down the most wonderful and inspiring thing you have read--even if you have only time to write a single word that may recall it all to you. treasure that little note book as long as you live. every year it will get more precious to you. now for the books: . _martin._ _the friendly stars._ harper & brothers, new york, . this book teaches you first the twenty brightest stars and then the constellations. i cannot say that this, or any other, is the "best book," but it has helped me most, and i suppose it is only natural that we should love best the first book that introduces us to a delightful subject. . _serviss._ _astronomy with the naked eye._ harper & brothers, new york, . this teaches you the constellations first and the brightest stars incidentally. also it gives the old myths. . _serviss._ _astronomy with an opera-glass._ d. appleton & co., new york, . . _serviss._ _pleasures of the telescope._ d. appleton & co., new york, . . _milham._ _how to identify the stars._ the macmillan co., new york, . this gives a list of eighty-eight constellations, including thirty-six southern ones, and has tracings of twenty-eight. . _elson._ _star gazer's handbook._ sturgis & walton co., new york, . about the briefest and cheapest. has good charts and makes a specialty of the myths. . _serviss._ _curiosities of the sky._ harper & brothers, new york. tells about comets, asteroids, shooting stars, life on mars, nebulæ, temporary stars, coal-sacks, milky way, and other wonders. . _ball._ _starland._ ginn & co., boston, new york, etc., . this tells about a great many interesting experiments in astronomy that children can make. * * * * * if i had only a dollar or less to spend on astronomy i should buy a planisphere. i got mine from thomas whittaker, no. bible house, new york. it cost seventy-five cents, and will tell you where to find any star at any time in the year. it does not show the planets, however. a planisphere that will show the planets costs about five dollars. however, there are only two very showy planets, viz., venus and jupiter. any almanac will tell you (for nothing) when each of these is morning star, and when each of them is evening star. the best newspaper about stars, as far as i know, is a magazine called _the monthly evening sky map_, published by leon barritt, nassau st., new york. it costs a dollar a year. it gives a chart every month, showing all the planets, and all the constellations. also it tells you about the interesting things, like comets, before they come. good-bye. i hope you will never cease to learn about and love the earth and the sky. perhaps you think you have learned a great deal already. but your pleasures have only begun. wait till you learn about how the world began, the sun and all his planets, the distances between the stars, and the millions of blazing suns amid the milky way! the end [illustration: the sky in winter] note.--these simplified star maps are not as accurate as a planisphere, but they may be easier for children. all star maps are like ordinary maps, except that east and west are transposed. the reason for this is that you can hold a star map over your head, with the pole star toward the north, and the map will then match the sky. these maps contain some constellations that are only for grown-ups to study. the winter constellations every child should know are: auriga, the charioteer canis major, the big dog canis minor, the little dog cassiopeia, the queen in her chair cygnus, the swan leo, the lion orion, the hunter perseus, which has the arc taurus, the bull ursa major, the great bear ursa minor, the little bear [illustration: the sky in spring] note.--once upon a time all the educated people spoke latin. it was the nearest approach to a universal language. so most of the constellations have latin names. the english, french and german names are all different, but if all children would learn the latin names they could understand one another. the spring constellations every child should know are: leo, the lion lyra, the lyre cassiopeia, the queen in her chair scorpio, the scorpion ursa major, the great bear ursa minor, the little bear virgo, the virgin [illustration: the sky in summer] note.--every sky map is good for three months, in this way: if this is correct on june st at p.m., it will be correct july st at p.m., and august st at p.m. this is because the stars rise four minutes earlier every night. thus, after thirty days, any star will rise thirty times four minutes earlier, or minutes, or two hours. children need not learn all the summer constellations. the most interesting are: auriga, the charioteer canis major, the big dog cygnus, the swan lyra, the lyre scorpio, the scorpion [illustration: the sky in autumn] note.--this book tells how to find all the most interesting stars and constellations without maps, but many people prefer them. how to use star maps is explained under "the sky in winter." the autumn constellations most interesting to children are: aquila, the eagle auriga, the charioteer cassiopeia, the queen in her chair cygnus, the swan lyra, the lyre perseus, which has the arc taurus, the bull ursa major, the great bear ursa minor, the little bear transcriber's notes page "streams, runing" corrected to "streams, running" page "where he globe" corrected to "where the globe" page "ceatures to prove" corrected to "creatures to prove" page "this consellation is" corrected to "this constellation is" page "everybirth day" corrected to "every birthday" http://www.archive.org/details/gradualacceptan stim) [transcriber's note: obvious printer errors have been corrected without note. other questionable items are marked with a [transcriber's note].] the gradual acceptance of the copernican theory of the universe dorothy stimson, ph.d. new york copyright by dorothy stimson trade selling agents the baker & taylor co., fourth ave., new york to my father and mother [illustration: the systems of the world in according to father riccioli (reduced facsimile of the frontispiece in riccioli: _almagestum novum_. bologna, .)] explanation "astrea, goddess of the heaven, wearing angel's wings and gleaming everywhere with stars, stands at the right; on the left is argus of the hundred eyes, not tense, but indicating by the position of the telescope at his knee rather than at the eyes in his head, that while observing the work of god's hand, he appears at the same time to be worshipping as in genuflexion." (riccioli: _alm. nov._, _præfatio_, xvii). he points to the cherubs in the heavens who hold the planets, each with its zodiacal sign: above him at the top is mars, then mercury in its crescent form, the sun, and venus also in the crescent phase; on the opposite side are saturn in its "tripartite" form (the ring explanation was yet to be given), the sphere of jupiter encircled by its four satellites, the crescent moon, its imperfections clearly shown, and a comet. thus father riccioli summarized the astronomical knowledge of his day. the scrolls quote psalms : , "day unto day uttereth speech and night unto night showeth knowledge." astrea holds in her right hand a balance in which riccioli's theory of the universe (an adaptation of the tychonic, see p. ) far outweighs the copernican or heliocentric one. at her feet is the ptolemaic sphere, while ptolemy himself half lies, half sits, between her and argus, with the comment issuing from his mouth: "i will arise if only i am corrected." his left hand rests upon the coat of arms of the prince of monaco to whom the _almagestum novum_ is dedicated. at the top is the hebrew _yah-veh_, and the hand of god is stretched forth in reference to the verse in the book of wisdom ( : ): "but thou hast ordered all things in measure, and number and weight." contents illustrations preface part i. an historical sketch of the heliocentric theory of the universe. chapter i. the development of astronomical thought to : preliminary review chapter ii. copernicus and his times chapter iii. later development and scientific defense of the copernican theory part ii. the reception of the copernican theory. chapter i. opinions and arguments in the sixteenth century chapter ii. bruno and galileo chapter iii. the opposition and their arguments chapter iv. the gradual acceptance of the copernican theory chapter v. the church and the new astronomy: conclusion appendices: translations by the writer. a. ptolemy: _almagest_. bk. i, chap. : that the earth has no movement of rotation b. copernicus: _de revolutionibus_, dedication to the pope c. bodin: _universæ naturæ theatrum_, bk. v, sections and in part, and section entire d. fienus: _epistolica quæstio_: is it true that the heavens are moved and the earth is at rest? bibliography index illustrations facsimile of the frontispiece "the systems of the world" in riccioli: _almagestum novum_, _frontispiece_ photographic facsimile (reduced) of a page from a copy of copernicus: _de revolutionibus_, as "corrected" in the th century according to the directions of the congregations of the index in p. photographic facsimile (reduced) of another "corrected" page from the same copy p. preface this study does not belong in the field of astronomy, but in that of the history of thought; for it is an endeavor to trace the changes in people's beliefs and conceptions in regard to the universe as these were wrought by the dissolution of superstition resulting from the scientific and rationalist movements. the opening chapter is intended to do no more than to review briefly the astronomical theories up to the age of copernicus, in order to provide a background for the better comprehension of the work of copernicus and its effects. such a study has been rendered possible only by the generous loan of rare books by professor herbert d. foster of dartmouth college, professor edwin e. slosson of columbia university, doctor george a. plimpton and major george haven putnam, both of new york, and especially by the kindly generosity of professor david eugene smith of teachers college who placed his unique collection of rare mathematical books at the writer's disposal and gave her many valuable suggestions as to available material. professors james t. shotwell and harold jacoby of columbia university have read parts of this study in manuscript. the writer gratefully acknowledges her indebtedness not only to these gentlemen, but to the many others, librarians and their assistants, fellow-students and friends, too numerous to mention individually, whose ready interest and whose suggestions have been of real service, and above all to professor james harvey robinson at whose suggestion and under whose guidance the work was undertaken, and to the reverend doctor henry a. stimson whose advice and criticism have been an unfailing source of help and encouragement. part one an historical sketch of the heliocentric theory of the universe. chapter i. the development of astronomical thought to a.d. _a preliminary sketch of early theories as a background._ the appearances in the heavens have from earliest historic ages filled men with wonder and awe; then they gradually became a source of questioning, and thinkers sought for explanations of the daily and nightly phenomena of sun, moon and stars. scientific astronomy, however, was an impossibility until an exact system of chronology was devised.[ ] meanwhile men puzzled over the shape of the earth, its position in the universe, what the stars were and why the positions of some shifted, and what those fiery comets were that now and again appeared and struck terror to their hearts. [footnote : the earliest observation ptolemy uses is an egyptian one of an eclipse occurring march , b.c. (cumont: ). [in these references, the roman numerals refer to the volume, the arabic to the page, except as stated otherwise. the full title is given in the bibliography at the back under the author's name.]] in answer to such questions, the chaldean thinkers, slightly before the rise of the greek schools of philosophy, developed the idea of the seven heavens in their crystalline spheres encircling the earth as their center.[ ] this conception seems to lie back of both the later egyptian and hebraic cosmologies, as well as of the ptolemaic. through the visits of greek philosophers to egyptian shores this conception helped to shape greek thought and so indirectly affected western civilization. thus our heritage in astronomical thought, as in many other lines, comes from the greeks and the romans reaching europe (in part through arabia and spain), where it was shaped by the influence of the schools down to the close of the middle ages when men began anew to withstand authority in behalf of observation and were not afraid to follow whither their reason led them. [footnote : warren: . see "calendar" in hastings: _ency. of religion and ethics_.] but not all greek philosophers, it seems,[ ] either knew or accepted the babylonian cosmology.[ ] according to plutarch, though thales ( ?- ? b.c.) and later the stoics believed the earth to be spherical in form, anaximander ( - ? b.c.) thought it to be like a "smooth stony pillar," anaximenes ( th cent.) like a "table." beginning with the followers of thales or perhaps parmenides (?- b.c.), as diogenes laërtius claims,[ ] a long line of greek thinkers including plato ( ?- ? b.c.) and aristotle ( - b.c.) placed the earth in the center of the universe. whether plato held that the earth "encircled" or "clung" around the axis is a disputed point;[ ] but aristotle claimed it was the fixed and immovable center around which swung the spherical universe with its heaven of fixed stars and its seven concentric circles of the planets kept in their places by their transparent crystalline spheres.[ ] [footnote : for a summary of recent researches, see the preface of heath: _aristarchus of samos_. for further details, see heath: _op. cit._, and the writings of kugler and schiaparelli.] [footnote : see plutarch: _moralia: de placitas philosophorum_, lib. i et ii, (v. - , - ).] [footnote : diogenes laërtius: _de vitis_, lib. ix, c. ( ).] [footnote : plato: _timæus_, sec. (iii, in jowett's translation).] [footnote : aristotle: _de mundo_, c. et (iii, and ).] the stars were an even greater problem. anaximenes thought they were "fastened like nails" in a crystalline firmament, and others thought them to be "fiery plates of gold resembling pictures."[ ] but if the heavens were solid, how could the brief presence of a comet be explained? [footnote : plutarch: _op. cit._, lib. iii, c. (v, - ).] among the philosophers were some noted as mathematicians whose leader was pythagoras (c. b.c.). he and at least one of the members of his school, eudoxus ( ?- ? b.c.), had visited egypt, according to diogenes laërtius,[ ] and had in all probability been much interested in and influenced by the astronomical observations made by the egyptian priests. on the same authority, pythagoras was the first to declare the earth was round and to discuss the antipodes. he too emphasized the beauty and perfection of the circle and of the sphere in geometry, forms which became fixed for years as the fittest representations of the perfection of the heavenly bodies. [footnote : diogenes laërtius: _de vitis_, lib. viii, c. , et ( , ).] there was some discussion in diogenes' time as to the author of the theory of the earth's motion of axial rotation. diogenes[ ] gives the honor to philolaus ( th cent. b.c.) one of the pythagoreans, though he adds that others attribute it to icetas of syracuse ( th or th cent. b.c.). cicero, however, states[ ] the position of hicetas of syracuse as a belief in the absolute fixedness of all the heavenly bodies except the earth, which alone moves in the whole universe, and that its rapid revolutions upon its own axis cause the heavens apparently to move and the earth to stand still. [footnote : diogenes: _op. cit._, lib. viii, c. ( ).] [footnote : cicero: _academica_, lib. ii, c. ( ).] other thinkers of syracuse may also have felt the egyptian influence; for one of the greatest of them, archimedes (c. - b.c.), stated the theory of the earth's revolution around the sun as enunciated by aristarchus of samos. (perhaps this is the "hearth-fire of the universe" around which philolaus imagined the earth to whirl.[ ]) in _arenarius_, a curious study on the possibility of expressing infinite sums by numerical denominations as in counting the sands of the universe, archimedes writes:[ ] "for you have known that the universe is called a sphere by several astrologers, its center the center of the earth, and its radius equal to a line drawn from the center of the sun to the center of the earth. this was written for the unlearned, as you have known from the astrologers.... [aristarchus of samos][ ] concludes that the world is many times greater than the estimate we have just given. he supposes that the fixed stars and the sun remain motionless, but that the earth following a circular course, revolves around the sun as a center, and that the sphere of the fixed stars having the same sun as a center, is so vast that the circle which he supposes the earth to follow in revolving holds the same ratio to the distance of the fixed stars as the center of a sphere holds to its circumference." [footnote : plutarch: _op. cit._, lib. ii (v. - ).] [footnote : archimedes: _arenarius_, c. . delambre: _astr. anc._, i, .] [footnote : this is the only account of his system. even the age in which he flourished is so little known that there have been many disputes whether he was the original inventor of this system or followed some other. he was probably a contemporary of cleanthes the stoic in the rd century b.c. he is mentioned also by ptolemy, diogenes laërtius and vitruvius. (schiaparelli: _die vorlaufer des copernicus im alterthum_, . see also heath: _op. cit._)] these ancient philosophers realized in some degree the immensity of the universe in which the earth was but a point. they held that the earth was an unsupported sphere the size of which eratosthenes (c. - b.c.) had calculated approximately. they knew the sun was far larger than the earth, and cicero with other thinkers recognized the insignificance of earthly affairs in the face of such cosmic immensity. they knew too about the seven planets, had studied their orbits, and worked out astronomical ways of measuring the passage of time with a fair amount of accuracy. hipparchus and other thinkers had discovered the fact of the precession of the equinoxes, though there was no adequate theory to account for it until copernicus formulated his "motion of declination." the pythagoreans accepted the idea of the earth's turning upon its axis, and some even held the idea of its revolution around the motionless sun. others suggested that comets had orbits which they uniformly followed and therefore their reappearance could be anticipated.[ ] [footnote : plutarch: _op. cit._: bk. iii, c. (v, - ).] why then was the heliocentric theory not definitely accepted? in the first place, such a theory was contrary to the supposed facts of daily existence. a man did not have to be trained in the schools to observe that the earth seemed stable under his feet and that each morning the sun swept from the east to set at night in the west. sometimes it rose more to the north or to the south than at other times. how could that be explained if the sun were stationary? study of the stars was valuable for navigators and for surveyors, perhaps, but such disturbing theories should not be propounded by philosophers. cleanthes,[ ] according to plutarch,[ ] "advised that the greeks ought to have prosecuted aristarchus the samian for blasphemy against religion, as shaking the very foundations of the world, because this man endeavoring to save appearances, supposed that the heavens remained immovable and that the earth moved through an oblique circle, at the same time turning about its own axis." few would care to face their fellows as blasphemers and impious thinkers on behalf of an unsupported theory. eighteen hundred years later galileo would not do so, even though in his day the theory was by no means unsupported by observation. [footnote : the stoic contemporary of aristarchus, author of the famous stoic hymn. see diogenes laërtius: _de vitis_.] [footnote : plutarch: _de facie in orbe lunæ_, (v, ).] furthermore, one of the weaknesses of the greek civilization militated strongly against the acceptance of this hypothesis so contrary to the evidence of the senses. experimentation and the development of applied science was practically an impossibility where the existence of slaves made manual labor degrading and shameful. men might reason indefinitely; but few, if any, were willing to try to improve the instruments of observation or to test their observations by experiments. at the same time another astronomical theory was developing which was an adequate explanation for the phenomena observed up to that time.[ ] this theory of epicycles and eccentrics worked out by apollonius of perga (c. b.c.) and by hipparchus (c. b.c.) and crystallized for posterity in ptolemy's great treatise on astronomy, the _almagest_, (c. a.d.) became the fundamental principle of the science until within the last three hundred years. the theory of the eccentric was based on the idea that heavenly bodies following circular orbits revolved around a center that did not coincide with that of the observer on the earth. that would explain why the sun appeared sometimes nearer the earth and sometimes farther away. the epicycle represented the heavenly body as moving along the circumference of one circle (called the epicycle) the center of which moves on another circle (the deferent). with better observations additional epicycles and eccentric were used to represent the newly observed phenomena till in the later middle ages the universe became a "----sphere with centric and eccentric scribbled o'er, cycle and epicycle, orb in orb"--[ ] [footnote : young: .] [footnote : milton: _paradise lost_, bk. viii, ll. - .] yet the heliocentric theory was not forgotten. vitruvius, a famous roman architect of the augustan age, discussing the system of the universe, declared that mercury and venus, the planets nearest the sun, moved around it as their center, though the earth was the center of the universe.[ ] this same notion recurs in martianus capella's book[ ] in the fifth century a.d. and again, somewhat modified, in the th century in tycho brahe's conception of the universe. [footnote : vitruvius: _de architectura_, lib. ix, c. ( ).] [footnote : martianus capella: _de nuptiis_, lib. viii, ( ).] ptolemy devotes a column or two of his _almagest_[ ] (to use the familiar arabic name for his _syntaxis mathematica_) to the refutation of the heliocentric theory, thereby preserving it for later ages to ponder on and for a copernicus to develop. he admits at the outset that such a theory is only tenable for the stars and their phenomena, and he gives at least three reasons why it is ridiculous. if the earth were not at the center, the observed facts of the seasons and of day and night would be disturbed and even upset. if the earth moves, its vastly greater mass would gain in speed upon other bodies, and soon animals and other lighter bodies would be left behind unsupported in the air--a notion "ridiculous to the last degree," as he comments, "even to imagine it." lastly, if it moves, it would have such tremendous velocity that stones or arrows shot straight up in the air must fall to the ground east of their starting point,--a "laughable supposition" indeed to ptolemy. [footnote : ptolemy: _almagest_, lib. i, c. , ( , - ). translated in appendix b.] this book became the great text of the middle ages; its author's name was given to the geocentric theory it maintained. astronomy for a thousand years was valuable only to determine the time of easter and other festivals of the church, and to serve as a basis for astrology for the mystery-loving people of europe. to the arabians in syria and in spain belongs the credit of preserving for europe during this long period the astronomical works of the greeks, to which they added their own valuable observations of the heavens--valuable because made with greater skill and better instruments,[ ] and because with these observations later scientists could illustrate the permanence or the variability of important elements. they also discovered the so-called "trepidation" or apparent shifting of the fixed stars to explain which they added another sphere to ptolemy's eight. early in the sixth century uranus translated aristotle's works into syrian, and this later was translated into arabic.[ ] albategnius[ ] (c. - [transcriber's note: ] a.d.), the arabian prince who was the greatest of all their astronomers, made his observations from aracte and damascus, checking up and in some cases amending ptolemy's results.[ ] [footnote : whewell: i, .] [footnote : whewell: i, .] [footnote : berry: .] [footnote : his book _de motu stellarum_, translated into latin by plato tiburtinus (fl. ) was published at nuremberg ( ) by melancthon with annotations by regiomontanus. _ency. brit._ th. edit.] then the center of astronomical development shifted from syria to spain and mainly through this channel passed on into western europe. the scientific fame of alphonse x of castile ( - a.d.) called the wise, rests chiefly upon his encouragement of astronomy. with his support the alfonsine tables were calculated. he is said[ ] to have summoned fifty learned men from toledo, cordova and paris to translate into spanish the works of ptolemy and other philosophers. under his patronage the university of salamanca developed rapidly to become within two hundred years one of the four great universities of europe[ ]--a center for students from all over europe and the headquarters for new thought, where columbus was sheltered,[ ] and later the copernican system was accepted and publicly taught at a time when galileo's views were suppressed.[ ] [footnote : vaughan: i, .] [footnote : graux: .] [footnote : graux: .] [footnote : rashdall: ii, pt. i, .] popular interest in astronomy was evidently aroused, for sacrobosco (to give john holywood[ ] his better known latin name) a scotch professor at the sorbonne in paris in the th century, published a small treatise _de sphæri mundo_ that was immensely popular for centuries,[ ] though it was practically only an abstract of the _almagest_. whewell[ ] tells of a french poem of the time of edward i entitled _ymage du monde_, which gave the ptolemaic view and was illustrated in the manuscript in the university of cambridge with a picture of the spherical earth with men upright on it at every point, dropping balls down perforations in the earth to illustrate the tendency of all things toward the center. of the same period ( th century) is an arabian compilation in which there is a reference to another work, the book of hammarmunah the old, stating that "the earth turns upon itself in the form of a circle, and that some are on top, the others below ... and there are countries in which it is constantly day or in which at least the night continues only some instants."[ ] apparently, however, such advanced views were of no influence, and the ptolemaic theory remained unshaken down to the close of the th century. [footnote : _dict. of nat. biog._] [footnote : mss. of it are extremely numerous. it was the second astronomical book to be printed, the first edition appearing at ferrara in . editions appeared before . it was translated into italian, french, german, and spanish, and had many commentators. _dict. of nat. biog._] [footnote : whewell: i, .] [footnote : blavatski: ii, , note.] aside from the adequacy of this explanation of the universe for the times, the attitude of the church fathers on the matter was to a large degree responsible for this acquiescence. early in the first century a.d., philo judæus[ ] emphasized the minor importance of visible objects compared with intellectual matters,--a foundation stone in the church's theory of an homocentric universe. clement of alexandria (c. a.d.) calls the heavens solid since what is solid is capable of being perceived by the senses.[ ] origen (c. -c. .) has recourse to the holy scriptures to support his notion that the sun, moon, and stars are living beings obeying god's commands.[ ] then lactantius thunders against those who discuss the universe as comparable to people discussing "the character of a city they have never seen, and whose name only they know." "such matters cannot be found out by inquiry."[ ] the existence of the antipodes and the rotundity of the earth are "marvelous fictions," and philosophers are "defending one absurd opinion by another"[ ] when in explanation why bodies would not fall off a spherical earth, they claim these are borne to the center. [footnote : philo judæus: _quis rerum divinarum hæres._ (iv, ).] [footnote : clement of alexandria: _stromatum_, lib. v, c. , (iii, ).] [footnote : origen: _de principiis_, lib. i, c. , (xi, ).] [footnote : lactantius: _divinarum institutionum_, lib. iii, c. (vi, ).] [footnote : ibid: lib. iii, c. , (vi, - ).] how clearly even this brief review illustrates what henry osborn taylor calls[ ] the fundamental principles of patristic faith: that the will of god is the one cause of all things (voluntate dei immobilis manet et stat in sæculum terra.[ ] ambrose: _hexæmeron_.) and that this will is unsearchable. he further points out that augustine's and ambrose's sole interest in natural fact is as "confirmatory evidence of scriptural truth." the great augustine therefore denies the existence of antipodes since they could not be peopled by adam's children.[ ] he indifferently remarks elsewhere:[ ] "what concern is it to me whether the heavens as a sphere enclose the earth in the middle of the world or overhang it on either side?" augustine does, however, dispute the claims of astrologers accurately to foretell the future by the stars, since the fates of twins or those born at the same moment are so diverse.[ ] [footnote : taylor: _mediæval mind_, i, .] [footnote : by the will of god the earth remains motionless and stands throughout the age.] [footnote : augustine: _de civitate dei_, lib. xvi, c. , ( , p. ).] [footnote : augustine: _de genesi_, ii, c. , (v. , p. ). (white's translation).] [footnote : augustine: _civitate dei_, lib. v, c. , (v. , p. ).] philastrius (d. before a.d.) dealing with various heresies, denounces those who do not believe the stars are fixed in the heavens as "participants in the vanity of pagans and the foolish opinions of philosophers," and refers to the widespread idea of the part the angels play in guiding and impelling the heavenly bodies in their courses.[ ] [footnote : philastrius: _de hæresibus_, c. , (v. , p. ).] it would take a brave man to face such attitudes of scornful indifference on the one hand and denunciation on the other, in support of a theory the church considered heretical. meanwhile the church was developing the homocentric notion which would, of course, presuppose the central position in the universe for man's abiding place. in the pseudo-dionysius[ ] is an elaborately worked out hierarchy of the beings in the universe that became the accepted plan of later centuries, best known to modern times through dante's blending of it with the ptolemaic theory in the _divine comedy_.[ ] isidore of seville taught that the universe was created to serve man's purposes,[ ] and peter lombard ( th cent.) sums up the situation in the definite statement that man was placed at the center of the universe to be served by that universe and in turn himself to serve god.[ ] supported by the mighty thomas aquinas[ ] this became a fundamental church doctrine. [footnote : pseudo-dionysius: _de coelesti ierarchia_, (v. , p. ).] [footnote : milman: viii, p. - . see the _paradiso_.] [footnote : isidore of seville: _de ordine creaturarum_, c. , sec. , (v. , p. ).] [footnote : lombard: _sententia_, bk. ii, dist. i, sec. , (v. , p. ).] [footnote : aquinas: _summa theologica_, pt. i, qu. , art. . (_op. om. caietani_, v, ).] an adequate explanation of the universe existed. aristotle, augustine, and the other great authorities of the middle ages, all upheld the conception of a central earth encircled by the seven planetary spheres and by the all embracing starry firmament. in view of the phrases used in the bible about the heavens, and in view of the formation of fundamental theological doctrines based on this supposition by the church fathers, is it surprising that any other than a geocentric theory seemed untenable, to be dismissed with a smile when not denounced as heretical? small wonder is it, in the absence of the present day mechanical devices for the exact measurement of time and space as aids to observation, that the ptolemaic, or geocentric, theory of the universe endured through centuries as it did, upheld by the authority both of the church and, in essence at least, by the great philosophers whose works constituted the teachings of the schools. chapter ii. copernicus and his times. during these centuries, one notable scholar at least stood forth in open hostility to the slavish devotion to aristotle's writings and with hearty appreciation for the greater scientific accuracy of "infidel philosophers among the arabians, hebrews and greeks."[ ] in his _opus tertium_ ( ), roger bacon also pointed out how inaccurate were the astronomical tables used by the church, for in , according to these tables "christians will fast the whole week following the true easter, and will eat flesh instead of fasting at quadragesima for a week--which is absurd," and thus christians are made foolish in the eyes of the heathen.[ ] even the rustic, he added, can observe the phases of the moon occurring a week ahead of the date set by the calendar.[ ] bacon's protests were unheeded, however, and the church continued using the old tables which grew increasingly inaccurate with each year. pope sixtus iv sought to reform the calendar two centuries later with the aid of regiomontanus, then the greatest astronomer in europe ( );[ ] the lateran council appealed to copernicus for help ( ), but little could be done, as copernicus replied, till the sun's and the moon's positions had been observed far more precisely;[ ] and the modern scientific calendar was not adopted until under pope gregory xiii. [footnote : roger bacon: _opus tertium_, , - .] [footnote : ibid: .] [footnote : ibid: .] [footnote : delambre: _moyen age_, .] [footnote : prowe: ii, - .] what was the state of astronomy in the century of copernicus's birth? regiomontanus--to use johann müller's latin name--his teacher pürbach, and the great cardinal nicolas of cues were the leading astronomers of this fifteenth century. pürbach[ ] ( - ) died before he had fulfilled the promise of his youth, leaving his _epitome of ptolemy's almagest_ to be completed by his greater pupil. in his _theorica planetarum_ ( ) pürbach sought to explain the motions of the planets by placing each planet between the walls of two curved surfaces with just sufficient space in which the planet could move. as m. delambre remarked:[ ] "these walls might aid the understanding, but one must suppose them transparent; and even if they guided the planet as was their purpose, they hindered the movement of the comets. therefore they had to be abandoned, and in our own modern physics they are absolutely superfluous; they have even been rather harmful, since they interfered with the slight irregularities caused by the force of attraction in planetary movements which observations have disclosed." this scheme gives some indication of the elaborate devices scholars evolved in order to cope with the increasing number of seeming irregularities observed in "the heavens," and perhaps it makes clearer why copernicus was so dissatisfied with the astronomical hypothesis of his day, and longed for some simpler, more harmonious explanation. [footnote : delambre: _moyen age_, - .] [footnote : delambre: _moyen age_, .] regiomontanus[ ] ( - ) after pürbach's death, continued his work, and his astronomical tables (pub. ) were in general use throughout europe till superseded by the vastly more accurate copernican tables a century later. it has been said[ ] that his fame inspired copernicus (born three years before the other's death in ) to become as great an astronomer. m. delambre hails him as the wisest astronomer europe had yet produced[ ] and certainly his renown was approached only by that of the great cardinal. [footnote : it has been claimed that regiomontanus knew of the earth's motion around the sun a hundred years before copernicus; but a german writer has definitely disproved this claim by tracing it to its source in schöner's _opusculum geographicum_ ( ) which states only that he believed in the earth's axial rotation. ziegler: .] [footnote : ibid: .] [footnote : delambre: _op. cit._: .] both janssen,[ ] the catholic historian, and father hagen[ ] of the vatican observatory, together with many other catholic writers, claim that a hundred years before copernicus, cardinal nicolas cusanus[ ] (c. - ) had the courage and independence to uphold the theory of the earth's motion and its rotation on its axis. as father hagen remarked: "had copernicus been aware of these assertions he would probably have been encouraged by them to publish his own monumental work." but the cardinal stated these views of the earth's motions in a mystical, hypothetical way which seems to justify the marginal heading "paradox" (in the edition of ).[ ] and unfortunately for these writers, the jesuit father, riccioli, the official spokesman of that order in the th century after galileo's condemnation, speaking of this paradox, called attention, also, to a passage in one of the cardinal's sermons as indicating that the latter had perhaps "forgotten himself" in the _de docta ignorantia_, or that this paradox "was repugnant to him, or that he had thought better of it."[ ] the passage he referred to is as follows: "prayer is more powerful than all created things. although angels, or some kind of beings, move the spheres, the sun and the stars; prayer is more powerful than they are, since it impedes motion, as when the prayer of joshua made the sun stand still."[ ] this may explain why copernicus apparently disregarded the cardinal's paradox, for he made no reference to it in his book; and the statement itself, to judge by the absence of contemporary comment, aroused no interest at the time. but of late years, the cardinal's position as stated in the _de docta ignorantia_ has been repeatedly cited as an instance of the church's friendly attitude toward scientific thought,[ ] to show that galileo's condemnation was due chiefly to his "contumacy and disobedience." [footnote : janssen: _hist. of ger._, i, .] [footnote : _cath. ency._: "cusanus."] [footnote : from cues near treves.] [footnote : cusanus: _de docta ignorantia_, bk. ii, c. - : "centrum igitur mundi, coincideret cum circumferentiam, nam si centrum haberet et circumferentiam, et sic intra se haberet suum initium et finem et esset ad aliquid aliud ipse mundus terminatus, et extra mundum esset aluid et locus, quæ omnia veritate carent. cum igitur non sit possibile, mundum claudi intra centrum corporale et circumferentiam, non intelligitur mundus, cuius centrum et circumferentia sunt deus: et cum hic non sit mundus infinitus, tamen non potest concipi finitus, cum terminis careat, intra quos claudatur. terra igitur, quæ centrum esse nequit, motu omni carere non potest, nam eam moveri taliter etiam necesse est, quod per infinitum minus moveri posset. sicut igitur terra non est centram mundi.... unde licet terra quasi stella sit, propinquior polo centrali, tamen movetur, et non describit minimum circulum in motu, ut est ostensum.... terræ igitur figura est mobilis et sphærica et eius motus circularis, sed perfectior esse posset. et quia maximum in perfectionibus motibus, et figuris in mundo non est, ut ex iam dictis patent: tunc non est verum quod terra ista sit vilissima et infima, nam quamvis videatur centralior, quo'ad mundum, est tamen etiam, eadem ratione polo propinquior, ut est dictum." (pp. - ).] [footnote : riccioli: _alm. nov._, ii, .] [footnote : cusanus: _opera_, : excitationum, lib. vii, ex sermone: _debitores sumus_: "est enim oratio, omnibus creaturis potentior. nam angeli seu intelligentiæ, movent orbes, solem et stellas: sed oratio potentior, quia impedit motum, sicut oratio josuæ, fecit sistere solem."] [footnote : di bruno: , a; walsh: _an early allusion_, - .] copernicus[ ] himself was born in thorn on february , ,[ ] seven years after that hansa town founded by the teutonic order in had come under the sway of the king of poland by the second peace of thorn.[ ] his father,[ ] niklas koppernigk, was a wholesale merchant of cracow who had removed to thorn before , married barbara watzelrode of an old patrician thorn family, and there had served as town councillor for nineteen years until his death in .[ ] thereupon his mother's brother, lucas watzelrode, later bishop of ermeland, became his guardian, benefactor and close friend.[ ] [footnote : _nicolaus coppernicus_ (berlin, - ; vol.; pt. i, biography, pt. ii, sources), by dr. leopold prowe gives an exhaustive account of all the known details in regard to copernicus collected from earlier biographers and tested most painstakingly by the documentary evidence dr. prowe and his fellow-workers unearthed during a lifetime devoted to this subject. (_allgemeine deutsche biographie._) the manuscript authority dr. prowe cites (prowe: i, - and footnotes), requires the double p in copernicus's name, as copernicus himself invariably used the two p's in the latinized form _coppernic_ without the termination _us_, and usually when this termination was added. also official records and the letters from his friends usually give the double p; though the name is found in many variants--koppernig, copperinck, etc. his signatures in his books, his name in the letter he published in , and the latin form of it used by his friends all bear testimony to his use of the double p. but custom has for so many centuries sanctioned the simpler spelling, that it seems unwise not to conform in this instance to the time-honored usage.] [footnote : prowe: i, .] [footnote : _ency. brit._: "thorn."] [footnote : prowe: i, - .] [footnote : these facts would seem to justify the poles today in claiming copernicus as their fellow-countryman by right of his father's nationality and that of his native city. dr. prowe, however, claims him as a "prussian" both because of his long residence in the prussian-polish bishopric of ermeland, and because of copernicus's own reference to prussia as "unser lieber vaterland." (prowe: ii, .)] [footnote : prowe: i, - .] after the elementary training in the thorn school,[ ] the lad entered the university at cracow, his father's former home, where he studied under the faculty of arts from - .[ ] nowhere else north of the alps at this time were mathematics and astronomy in better standing than at this university.[ ] sixteen teachers taught these subjects there during the years of copernicus's stay, but no record exists of his work under any of them.[ ] that he must have studied these two sciences there, however, is proved by rheticus's remark in the _narratio prima_[ ] that copernicus, after leaving cracow, went to bologna to work with dominicus maria di novara "non tarn discipulus quam adjutor." he left cracow without receiving a degree,[ ] returned to thorn in when he and his family decided he should enter the church after first studying in italy.[ ] consequently he crossed the alps in and was that winter matriculated at bologna in the "german nation."[ ] the following summer he received word of his appointment to fill a vacancy among the canons of the cathedral chapter at ermeland where his uncle had been bishop since .[ ] he remained in italy, however, about ten years altogether, studying civil law at bologna, and canon law and medicine at padua,[ ] yet receiving his degree as doctor of canon law from the university of ferrara in .[ ] he was also in rome for several months during the jubilee year, . [footnote : ibid: i, .] [footnote : ibid: i, - .] [footnote : ibid: i, .] [footnote : ibid: i, - .] [footnote : rheticus: _narratio prima_, (thorn edit.).] [footnote : prowe: i, .] [footnote : ibid: i, .] [footnote : ibid: i, .] [footnote : ibid: i, . this insured him an annual income which amounted to a sum equalling about $ today. later he received a sinecure appointment besides at breslau. (holden in _pop. sci._, .)] [footnote : prowe: i, .] [footnote : ibid: i, .] at this period the professor of astronomy at bologna was the famous teacher dominicus maria di novara ( - ), a man "ingenio et animo liber" who dared to attack the immutability of the ptolemaic system, since his own observations, especially of the pole star, differed by a degree and more from the traditional ones.[ ] he dared to criticise the long accepted system and to emphasize the pythagorean notion of the underlying harmony and simplicity in nature[ ]; and from him copernicus may have acquired these ideas, for whether they lived together or not in bologna, they were closely associated. it was here, too, that copernicus began his study of greek which later was to be the means[ ] of encouraging him in his own theorizing by acquainting him with the ancients who had thought along similar lines. [footnote : ibid: i, and note. little is known about him today, except that he was primarily an observer, and was highly esteemed by his immediate successors; see gilbert: _de magnete_.] [footnote : clerke in _ency. brit._, "novara."] [footnote : prowe: i, .] in the spring of the year ( ) following his visit to rome,[ ] copernicus returned to the chapter at frauenburg to get further leave of absence to study medicine at the university of padua.[ ] whether he received a degree at padua or not and how long he stayed there are uncertain points.[ ] he was back in ermeland early in . [footnote : prowe: i, .] [footnote : ibid, .] [footnote : ibid: i, .] his student days were ended. and now for many years he led a very active life, first as companion and assistant to his uncle the bishop, with whom he stayed at schloss heilsberg till after the bishop's death in ; then as one of the leading canons of the chapter at frauenburg, where he lived most of the rest of his life.[ ] as the chapter representative for five years (at intervals) he had oversight of the spiritual and temporal affairs of two large districts in the care of the chapter.[ ] he went on various diplomatic and other missions to the king of poland,[ ] to duke albrecht of the teutonic order,[ ] and to the councils of the german states.[ ] he wrote a paper of considerable weight upon the much needed reform of the prussian currency.[ ] his skill as a physician was in demand not only in his immediate circle[ ] but in adjoining countries, duke albrecht once summoning him to königsberg to attend one of his courtiers.[ ] he was a humanist as well as a catholic churchman, and though he did not approve of the protestant revolt, he favored reform and toleration.[ ] gassendi claims that he was also a painter, at least in his student days, and that he painted portraits well received by his contemporaries.[ ] but his interest and skill in astronomy must have been recognized early in his life for in the committee of the lateran council in charge of the reform of the calendar summoned him to their aid.[ ] [footnote : prowe: i, - .] [footnote : ibid: ii, - , , .] [footnote : ibid: ii, - .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : ibid: ii, - .] [footnote : ibid: ii, - .] [footnote : holden in _pop. sci._, .] [footnote : prowe: ii, - .] he was no cloistered monk devoting all his time to the study of the heavens, but a cultivated man of affairs, of recognized ability in business and statesmanship, and a leader among his fellow canons. his mathematical and astronomical pursuits were the occupations of his somewhat rare leisure moments, except perhaps during the six years with his uncle in the comparative freedom of the bishop's castle, and during the last ten or twelve years of his life, after his request for a coadjutor had resulted in lightening his duties. in his masterwork _de revolutionibus_[ ] there are recorded only of his own astronomical observations, and these extend over the years from to . the first was made at bologna, the second at rome in , and seven of the others at frauenburg, where the rest were also probably made. it is believed the greater part of the _de revolutionibus_ was written at heilsburg[ ] where copernicus was free from his chapter duties, for as he himself says[ ] in the dedication to the pope (dated ) his work had been formulated not merely nine years but for "more than three nines of years." it had not been neglected all this time, however, as the original ms. (now in the prague library) with its innumerable changes and corrections shows how continually he worked over it, altering and correcting the tables and verifying his statements.[ ] [footnote : copernicus: _de revolutionibus_, thorn edit., . the last two words of the full title: _de revolutionibus orbium coelestium_ are not on the original ms. and are believed to have been added by osiander. prowe: ii, , note.] [footnote : ibid: ii, - .] [footnote : copernicus: dedication, . (thorn edit.)] [footnote : prowe: ii, - .] copernicus was a philosopher.[ ] he thought out a new explanation of the world machine with relatively little practical work of his own,[ ] though we know he controlled his results by the accumulated observations of the ages.[ ] his instruments were inadequate, inaccurate and out of date even in his time, for much better ones were then being made at nürnberg[ ]; and the cloudy climate of ermeland as well as his own active career prevented him from the long-continued, painstaking observing, which men like tycho brahe were to carry on later. despite such handicaps, because of his dissatisfaction with the complexities and intricacies of the ptolemaic system and because of his conviction that the laws of nature were simple and harmonious, copernicus searched the writings of the classic philosophers, as he himself tells us,[ ] to see what other explanation of the universe had been suggested. "and i found first in cicero that a certain nicetas had thought the earth moved. later in plutarch i found certain others had been of the same opinion." he quoted the greek referring to philolaus the pythagorean, heraclides of pontus, and ecphantes the pythagorean.[ ] as a result he began to consider the mobility of the earth and found that such an explanation seemingly solved many astronomical problems with a simplicity and a harmony utterly lacking in the old traditional scheme. unaided by a telescope, he worked out in part the right theory of the universe and for the first time in history placed all the then known planets in their true positions with the sun at the center. he claimed that the earth turns on its axis as it travels around the sun, and careens slowly as it goes, thus by these three motions explaining many of the apparent movements of the sun and the planets. he retained,[ ] however, the immobile heaven of the fixed stars (though vastly farther off in order to account for the non-observance of any stellar parallax[ ]), the "perfect" and therefore circular orbits of the planets, certain of the old eccentrics, and new epicycles in place of all the old ones which he had cast aside.[ ] he accepted the false notion of trepidation enunciated by the arabs in the th century and later overthrown by tycho brahe.[ ] his calculations were weak.[ ] but his great book is a sane and modern work in an age of astrology and superstition.[ ] his theory is a triumph of reason and imagination and with its almost complete independence of authority is perhaps as original a work as an human being may be expected to produce. [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : rheticus: _narratio prima_.] [footnote : prowe: ii, .] [footnote : copernicus: dedication, - . see appendix b.] [footnote : for a translation of this dedication in full, see appendix b. in the original ms. occurs a reference (struck out) to aristarchus of samos as holding the theory of the earth's motion. (prowe: ii, , note.) the finding of this passage proves that copernicus had at least heard of aristarchus, but his apparent indifference is the more strange since an account of his teaching occurs in the same book of plutarch from which copernicus learned about philolaus. but the chief source of our knowledge about aristarchus is through archimedes, and the editio princeps of his works did not appear till , a year after the death of copernicus. c.r. eastman in _pop. sci._ : .] [footnote : delambre: _astr. mod._ pp. xi-xii.] [footnote : as the earth moves, the position in the heavens of a fixed star seen from the earth should differ slightly from its position observed six months later when the earth is on the opposite side of its orbit. the distance to the fixed stars is so vast, however, that this final proof of the earth's motion was not attained till when bessel ( - ) observed stellar parallax from königsberg. berry: - .] [footnote : _commentariolus_ in prowe: iii, .] [footnote : holden in _pop. sci._, .] [footnote : delambre: _astr. mod._, p. xi.] [footnote : snyder: .] copernicus was extremely reluctant to publish his book because of the misunderstandings and malicious attacks it would unquestionably arouse.[ ] possibly, too, he was thinking of the hostility already existing between himself and his bishop, dantiscus,[ ] whom he did not wish to antagonize further. but his devoted pupil and friend, rheticus, aided by tiedeman giese, bishop of culm and a lifelong friend, at length ( ) persuaded him.[ ] so he entrusted the matter to giese who passed it on to rheticus, then connected with the university at wittenberg as professor of mathematics.[ ] rheticus, securing leave of absence from melancthon his superior, went to nürnberg to supervise the printing.[ ] this was done by petrejus. upon his return to wittenberg, rheticus left in charge johann schöner, a famous mathematician and astronomer, and andreas osiander, a lutheran preacher interested in astronomy. the printed book[ ] was placed in copernicus's hands at frauenburg on may th, , as he lay dying of paralysis.[ ] [footnote : copernicus: dedication, .] [footnote : prowe: ii, - .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : four other editions have since appeared; at basel, , amsterdam , warsaw , and thorn . for further details, see prowe: ii, - , and thorn edition pp. xii-xx. the edition cited in this study is the thorn one of .] [footnote : prowe: ii, - .] copernicus passed away that day in ignorance that his life's work appeared before the world not as a truth but as an hypothesis; for there had been inserted an anonymous preface "ad lectorem de hypothesibus huius opera" stating this was but another hypothesis for the greater convenience of astronomers.[ ] "neque enim necesse est eas hypotheses esse veras, imo ne verisimiles quidem, sed sufficit hoc unum, si calculum observationibus congruentem exhibeant."[ ] for years copernicus was thought to have written this preface to disarm criticism. kepler sixty years later ( ) called attention to this error,[ ] and quoted osiander's letters to copernicus and to rheticus of may, , suggesting that the system be called an hypothesis to avert attacks by theologians and aristotelians. he claimed that osiander had written the preface; but kepler's article never was finished and remained unpublished till .[ ] giese and rheticus of course knew that the preface falsified copernicus's work, and giese, highly indignant at the "impiety" of the printer (who he thought had written it to save himself from blame) wrote rheticus urging him to write another "præfatiunculus" purging the book of this falsehood.[ ] this letter is dated july , , and the book had appeared in april. apparently nothing was done and the preface was accepted without further challenge. [footnote : copernicus: _de revolutionibus_, i. "to the reader on the hypotheses of this book."] [footnote : "for it is not necessary that these hypotheses be true, nor even probable, but this alone is sufficient, if they show reasoning fitting the observations."] [footnote : kepler: _apologia tychonis contra ursum_ in _op. om._: i, - .] [footnote : prowe: ii, , note.] [footnote : ibid: ii, - .] it remains to ask whether people other than copernicus's intimates had known of his theory before . peucer, melancthon's nephew, declared copernicus was famous by ,[ ] and the invitation from the lateran council committee indicates his renown as early as . in vienna in [ ] there was found a _commentariolus_, or summary of his great work,[ ] written by copernicus for the scholars friendly to him. it was probably written soon after , and gives a full statement of his views following a series of seven axioms or theses summing up the new theory. this little book probably occasioned the order from pope clement vii in to widmanstadt to report to him on the new scheme.[ ] this widmanstadt did in the papal gardens before the pope with several of the cardinals and bishops, and was presented with a book as his reward. [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : a second copy was found at upsala shortly afterwards, though for centuries its existence was unknown save for two slight references to such a book, one by gemma frisius, the other by tycho brahe. prowe: ii, .] [footnote : ibid: ii, - .] in , the cardinal bishop of capua, nicolas von schönberg, apparently with the intent to pave the way for the theory at rome, wrote for a report of it.[ ] it is not known whether the report was sent, and the cardinal died the following year. but that copernicus was pleased by this recognition is evident from the prominence he gave to the cardinal's letter, as he printed it in his book at the beginning, even before the dedication to the pope. [footnote : prowe: ii, , note.] the most widely circulated account at this time, however, was the _narratio prima_, a letter from georg joachim of rhaetia (better known as rheticus), written in october, , from frauenburg to johann schöner at nürnberg.[ ] rheticus,[ ] at twenty-five years of age professor of mathematics at wittenberg, had gone uninvited to frauenburg early that summer to visit copernicus and learn for himself more in detail about this new system. this was rather a daring undertaking, for not only were luther and melancthon outspoken in their condemnation of copernicus, but rheticus was going from wittenberg, the headquarters of the lutheran heresy, into the bishopric of ermeland where to the bishop and the king his overlord, the very name of luther was anathema. nothing daunted, rheticus departed for frauenberg and could not speak too highly of the cordial welcome he received from the old astronomer. he came for a few weeks, and remained two years to return to wittenberg as an avowed believer in the system and its first teacher and promulgator. not only did he write the _narratio prima_ and an _encomium borussæ_, both extolling copernicus, but what is more important, he succeeded in persuading him to allow the publication of the _de revolutionibus_. rheticus returned to his post in , to resign it the next year and become dean of the faculty of arts. in all probability the conflict was too intense between his new scientific beliefs and the statements required of him as professor of the old mathematics and astronomy. [footnote : prowe: ii, - .] [footnote : ibid: ii, - .] his colleague, erasmus reinhold, continued to teach astronomy there, though he, too, accepted the copernican system.[ ] he published a series of tables (_tabulæ prutenicæ_, ) based on the copernican calculations to supersede the inaccurate ones by regiomontanus; and these were in general use throughout europe for the next seventy-odd years. as he himself declared, the series was based in its principles and fundamentals upon the observations of the famous nicolaus copernicus. the almanacs deduced from these calculations probably did more to bring the new system into general recognition and gradual acceptance than did the theoretical works.[ ] [footnote : ibid: ii, .] [footnote : holden in _pop. sci._, .] opposition to the theory had not yet gathered serious headway. there is record[ ] of a play poking fun at the system and its originator, written by the elbing schoolmaster (a dutch refugee from the inquisition) and given in by the villagers at elbing ( miles from frauenburg). elbing and ermeland were hostile to each other, copernicus was well known in elbing though probably from afar, for there seems to have been almost no personal intercourse between canons and people, and the spread of luther's teachings had intensified the hostility of the villagers towards the church and its representatives. but not until giordano bruno made the copernican system the starting-point of his philosophy was the roman catholic church seriously aroused to combat it. possibly osiander's preface turned opposition aside, and certainly the non-acceptance of the system as a whole by tycho brahe, the leading astronomer of europe at that time, made people slow to consider it. [footnote : prowe: ii, - .] chapter iii. the later development and scientific defense of the copernican system. copernicus accomplished much, but even his genius could not far outrun the times in which he lived. when one realizes that not only all the astronomers before him, but he and his immediate successor, tycho brahe, made all their observations and calculations unaided by even the simplest telescope, by logarithms or by pendulum clocks for accurate measurement of time,[ ] one marvels not at their errors, but at the greatness of their genius in rising above such difficulties. this lack of material aids makes the work of tycho brahe,[ ] accounted one of the greatest observers that has ever lived,[ ] as notable in its way perhaps as that of copernicus. [footnote : burckhardt: .] [footnote : the two standard lives of tycho brahe are the _vita tychonis brahei_ by gassendi ( ) till recently the sole source of information, and dreyer's _tycho brahe_ ( ) based not only on gassendi but on the documentary evidence disclosed by the researches of the th century. for tycho's works i have used the _opera omnia_ published at frankfort in . the danish royal scientific society has issued a reprint ( ) of the rare edition of the _de nova stella_.] [footnote : bridges: .] his life[ ] was a somewhat romantic one. born of noble family on december th, , at knudstrup in denmark, tyge brahe, the second of ten children,[ ] was early practically adopted by his father's brother. his family wished him to become a statesman and sent him in to the university at copenhagen to prepare for that career. a partial eclipse of the sun on august st, as foretold by the astronomers thrilled the lad and determined him to study a science that could foretell the future and so affect men's lives.[ ] when he was sent to leipsic with a tutor in to study law, he devoted his time and money to the study of mathematics and astronomy. two years later when eighteen years of age, he resolved to perform anew the task of hipparchos and ptolemy and make a catalogue of the stars more accurate than theirs. his family hotly opposed these plans; and for six years he wandered through the german states, now at wittenberg, now at rostock (where he fought the duel in which he lost part of his nose and had to have it replaced by one of gold and silver)[ ] or at augsburg--everywhere working on his chosen subjects. but upon his return to denmark ( ) he spent two years on chemistry and medicine, till the startling appearance of the new star in the constellation of cassiopæa (november, ) recalled him to what became his life work.[ ] [footnote : dreyer: - .] [footnote : gassendi: .] [footnote : dreyer: .] [footnote : gassendi: - .] [footnote : dreyer: - .] through the interest and favor of king frederick ii, he was given the island of hveen near elsinore, with money to build an observatory and the pledge of an annual income from the state treasury for his support.[ ] there at uraniborg from to he and his pupils made the great catalogue of the stars, and studied comets and the moon. when he was forced to leave hveen by the hostility and the economical tendencies of the young king,[ ] after two years of wandering he accepted the invitation of the emperor rudolphus and established himself at prague in bohemia. among his assistants at prague was young johann kepler who till tycho's death (on october , ) was his chief helper for twenty months, and who afterwards completed his observations, publishing the results in the rudolphine tables of . [footnote : ibid: .] [footnote : ibid: - .] this "phoenix among astronomers"--as kepler calls him,[ ]--was the father of modern practical astronomy.[ ] he also propounded a third system of the universe, a compromise between the ptolemaic and the copernican. in this the tychonic system,[ ] the earth is motionless and is the center of the orbits of the sun, the moon, and the sphere of the fixed stars, while the sun is the center of the orbits of the five planets.[ ] mercury and venus move in orbits with radii shorter than the sun's radius, and the other three planets include the earth within their circuits. this system was in harmony with the bible and accounted as satisfactorily by geometry as either of the other two systems for the observed phenomena.[ ] to tycho brahe, the ptolemaic system was too complex,[ ] and the copernican absurd, the latter because to account for the absence of stellar parallax it left vacant and purposeless a vast space between saturn and the sphere of the fixed stars,[ ] and because tycho's observations did not show any trace of the stellar parallax that must exist if the earth moves.[ ] [footnote : kepler: _tabulæ rudolphinæ_. title page.] [footnote : dreyer: - .] [footnote : as stated in his book on the comet of (pub. ).] [footnote : dreyer: - .] [footnote : schiaparelli in snyder: .] [footnote : brahe: _op. om._, pt. i, p. .] [footnote : ibid: - .] [footnote : the tychonic system has supporters to this day. see chap. viii.] though tycho thus rejected the copernican theory, his own proved to be the stepping stone toward the one he rejected,[ ] for by it and by his study of comets he completely destroyed the ideas of solid crystalline spheres to the discredit of the scholastics; and his promulgation of a third theory of the universe helped to diminish men's confidence in authority and to stimulate independent thinking. [footnote : dreyer: .] copernicus worked out his system by mathematics with but slight aid from his own observations. it was a theory not yet proven true. tycho brahe, though denying its validity, contributed in his mass of painstaking, accurate observations the raw material of facts to be worked up by kepler into the great laws of the planets attesting the fundamental truth of the copernican hypothesis. johann kepler[ ] earned for himself the proud title of "lawmaker for the universe" in defiance of his handicaps of ill-health, family troubles, and straitened finances.[ ] born in weil, wurtemberg, (december , ) of noble but indigent parents, he was a sickly child unable for years to attend school regularly. he finally left the monastery school in mulifontane in and entered the university at tübingen to stay for four and a half years. there he studied philosophy, mathematics, and theology (he was a lutheran) receiving the degree of master of arts in . while at the university he studied under mæstlin, professor of mathematics and astronomy, and a believer in the copernican theory. because of mæstlin's teaching kepler developed into a confirmed and enthusiastic adherent to the new doctrine. [footnote : the authoritative biography is the _vita_ by frisch in vol. viii, pp. - of _op. om. kep._] [footnote : frisch: viii, . [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] in he reluctantly abandoned his favorite study, philosophy, and accepted a professorship in mathematics at grætz in styria. two years later he published his first work: _prodromus dissertationum continens mysterium cosmographicum_ etc. ( ) in which he sought to prove that the creator in arranging the universe had thought of the five regular bodies which can be inscribed in a sphere according to which he had regulated the order, the number and the proportions of the heavens and their movements.[ ] the book is important not only because of its novelty, but because it gave the copernican doctrine public explanation and defense.[ ] kepler himself valued it enough to reprint it with his _harmonia mundi_ twenty-five years later. and it won for him appreciative letters from various scientists, notably from tycho brahe and galileo.[ ] [footnote : delambre: _astr. mod._ - .] [footnote : frisch: viii, .] [footnote : ibid: viii, .] as kepler, a lutheran, was having difficulties in grætz, a catholic city, he finally accepted tycho's urgent invitation to come to prague.[ ] he came early in , and after some adjustments had been made between the two,[ ] he and his family settled with tycho that autumn to remain till the latter's death the following november. kepler himself then held the office of imperial mathematician by appointment for many years thereafter.[ ] [footnote : ibid: viii, - .] [footnote : dreyer: - .] [footnote : frisch: viii, .] with the researches of tycho's lifetime placed at his disposal, kepler worked out two of his three great planetary laws from tycho's observations of the planet mars. yet, as m. bertrand remarks,[ ] it was well for kepler that his material was not too accurate or its variations (due to the then unmeasured force of attraction) might have hindered him from proving his laws; and luckily for him the earth's orbit is so nearly circular that in calculating the orbit of mars to prove its elliptical form, he could base his work on the earth's orbit as a circle without vitiating his results for mars.[ ] that a planet's orbit is an ellipse and not the perfect circle was of course a triumph for the new science over the scholastics and aristotelians. but they had yet to learn what held the planets in their courses. [footnote : bertrand: p. - .] [footnote : the two laws first appeared in in his _physica coelestis tradita commentarius de motu stellæ martis_. (frisch: viii, .) the third he enunciated in his _harmonia mundi_, . (ibid: viii, - .)] from kepler's student days under mæstlin when as the subject of his disputation he upheld the copernican theory, to his death in , he was a staunch supporter of the new teaching.[ ] in his _epitome astronomiæ copernicanæ_ ( ) he answered objections to it at length.[ ] he took infinite pains to convert his friends to the new system. it was in vain that tycho on his deathbed had urged kepler to carry on their work not on the copernican but on the tychonic scheme.[ ] [footnote : "cor et animam meam": kepler's expression in regard to the copernician theory. ibid: viii, .] [footnote : ibid: viii, .] [footnote : ibid: viii, .] kepler had reasoned out according to physics the laws by which the planets moved.[ ] in italy at this same time galileo with his optic tube (invented ) was demonstrating that venus had phases even as copernicus had declared, that jupiter had satellites, and that the moon was scarred and roughened--ocular proof that the old system with its heavenly perfection in number ( planets) and in appearance must be cast aside. within a year after galileo's death newton was born[ ] (january , ). his demonstration of the universal application of the law of gravitation ( ) was perhaps the climax in the development of the copernican system. complete and final proof was adding in the succeeding years by roemer's ( - ) discovery of the velocity of light, by bradley's ( - ) study of its aberration,[ ] by bessel's discovery of stellar parallax in ,[ ] and by foucault's experimental demonstration of the earth's axial motion with a pendulum in .[ ] [footnote : kepler: _op. om._, i, : _præfatio ad lectorem_.] [footnote : berry: .] [footnote : berry: .] [footnote : ibid: .] [footnote : jacoby: .] part two the reception of the copernican theory. chapter i. opinions and arguments in the sixteenth century. during the lifetime of copernicus, roman catholic churchmen had been interested in his work: cardinal schönberg wrote for full information, widmanstadt reported on it to pope clement vii and copernicus had dedicated his book to pope paul iii.[ ] but after his death, the church authorities apparently paid little heed to his theory until some fifty years later when giordano bruno forced it upon their attention in his philosophical teachings. osiander's preface had probably blinded their eyes to its implications. [footnote : see before, p. .] the protestant leaders were not quite so urbane in their attitude. while copernicus was still alive, luther is reported[ ] to have referred to this "new astrologer" who sought to prove that the earth and not the firmament swung around, saying: "the fool will overturn the whole science of astronomy. but as the holy scriptures state, joshua bade the sun stand still and not the earth." melancthon was more interested in this new idea, perhaps because of the influence of rheticus, his colleague in the university of wittenberg and copernicus's great friend and supporter; but he too preferred not to dissent from the accepted opinion of the ages.[ ] informally in a letter to a friend he implies the absurdity of the new teaching,[ ] and in his _initia doctrinæ physicæ_ he goes to some pains to disprove the new assumption not merely by mathematics but by the bible, though with a kind of apology to other physicists for quoting the divine witness.[ ] he refers to the phrase in psalm xix likening the sun in its course "to a strong man about to run a race," proving that the sun moves. another psalm states that the earth was founded not to be moved for eternity, and a similar phrase occurs in the first chapter of ecclesiastes. then there was the miracle when joshua bade the sun stand still. while this is a sufficient witness to the truths there are other proofs: first, in the turning of a circumference, the center remains motionless. next, changes in the length of the day and of the seasons would ensue, were the position of the earth in the universe not central, and it would not be equidistant from the two poles. (he has previously disposed of infinity by stating that the heavens revolve around the pole, which could not happen if a line drawn from the center of the universe were infinitely projected).[ ] furthermore, the earth must be at the center for its shadow to fall upon the moon in an eclipse. he refers next to the aristotelian statement that to a simple body belongs one motion: the earth is a simple body; therefore it can have but one motion. what is true of the parts applies to the whole; all the parts of the earth are borne toward the earth and there rest; therefore the whole earth is at rest. quiet is essential to growth. lastly, if the earth moved as fast as it must if it moves at all, everything would fly to pieces.[ ] [footnote : luther: _tischreden_, iv, ; "der narr will die ganze kunst astronomiæ umkehren. aber wie die heilige schrift anzeigt, so heiss josua die sonne still stehen, und nicht das erdreich."] [footnote : "non est autem hominis bene instituti dissentire a consensu tot sæculorum." præfatio philippi melanthonis, , in sacro-busto: _libellus de sphæra_ (no date).] [footnote : "vidi dialogum et fui dissuassor editionis. fabula per sese paulatim consilescet; sed quidam putant esse egregiam _katorthoma_ rem tam absurdam ornare, sicut ille sarmaticus astronomis qui movet terram et figet solem. profecto sapientes gubernatores deberent ingeniorum petulantia cohercere." _epistola b. mithobio_, oct. . p. melancthon: _opera_: iv, .] [footnote : "quamquam autem rident aliqui physicum testimonia divina citantem, tamen nos honestum esse censemus, philosophiam conferre ad coelestia dicta, et in tanta caligine humanæ mentis autoritatem divinam consulere ubicunque possumus." melancthon: _initia doctrinæ physicæ_: bk. i, .] [footnote : ibid: .] [footnote : ibid: - .] melancthon thus sums up the usual arguments from the scriptures, from aristotle, ptolemy and the then current physics, in opposition to this theory. not only did he publish his own textbook on physics, but he republished sacrobosco's famous introduction to astronomy, writing for it a preface urging diligent study of this little text endorsed by so many generations of scholars. calvin, the great teacher of the protestant revolt, apparently was little touched by this new intellectual current.[ ] he did write a semi-popular tract[ ] against the so called "judicial" astrology, then widely accepted, which he, like luther, condemns as a foolish superstition, though he values "la vraie science d'astrologie" from which men understand not merely the order and place of the stars and planets, but the causes of things. in his _commentaries_, he accepts the miracle of the sun's standing still at joshua's command as proof of the faith christ commended, so strong that it will remove mountains; and he makes reference only to the time-honored ptolemaic theory in his discussion of psalm xix.[ ] [footnote : farrar: _hist. of interpretation_: preface, xviii: "who," asks calvin, "will venture to place the authority of copernicus above that of the holy spirit?"] [footnote : calvin: _oeuvres françois_: _traité ... contre l'astrologie_, - .] [footnote : calvin: _op. om._ in _corpus reformatorum_: vol. , - ; vol. , - .] for the absolute authority of the pope the protestant leaders substituted the absolute authority of the bible. it is not strange, then, that they ignored or derided a theory as yet unsupported by proof and so difficult to harmonize with a literally accepted bible. how widespread among the people generally did this theory become in the years immediately following the publication of the _de revolutionibus_? m. flammarion, in his _vie de copernic_ ( ), refers[ ] to the famous clock in the strasburg cathedral as having been constructed by the university of strasburg in protest against the action taken by the holy office against galileo, (though the clock was constructed in and galileo was not condemned until ). this astronomical clock constructed only thirty years after the death of copernicus, he claims represented the copernican system of the universe with the planets revolving around the sun, and explained clearly in the sight of the people what was the thought of the makers. lest no one should miscomprehend, he adds, the portrait of copernicus was placed there with this inscription: nicolai copernici vera effigies, ex ipsius autographo depicta. [footnote : p. - : "ce planétaire ... represente le système du monde tel qu'il a été expliqué par copernic."] this would be important evidence of the spread of the theory were it true. but m. flammarion must have failed to see a brief description of the strasburg clock written in by charles schwilgué, son of the man who renovated its mechanism in - . he describes the clock as it was before his father made it over and as it is today. originally constructed in , it was replaced in by an astrolabe based on the ptolemaic system; six hands with the zodiacal signs of the planets gave their daily movements and, together with a seventh representing the sun, revolved around a map of the world.[ ] when m. schwilgué repaired the clock in , he changed it to harmonize with the copernican system.[ ] [footnote : schwilgué: p. .] [footnote : ibid: p. .] but within eighteen years after the publication of the _de revolutionibus_, proof of its influence is to be found in such widely separated places as london and the great spanish university of salamanca. in , robert recorde, court physician to edward and to mary and teacher of mathematics, published in london his _castle of knowledge_, an introduction to astronomy and the first book printed in england describing the copernican system.[ ] he evidently did not consider the times quite ripe for a full avowal of his own allegiance to the new doctrine, but the remarks of the _maister_ and the _scholler_ are worth repeating:[ ] "maister: ... howbeit copernicus a man of great learning, of much experience, and of wonderfull diligence in observation, hath renewed the opinion of aristarchus samius, affirming that the earth, not onely moveth circularly about his owne centre, but also may be, yea and is, continually out of the precise centre of the world eight and thirty hundred thousand miles: but because the understanding of that controversie depends of profounder knowledge than in this introduction may be uttered conveniently, i wil let it passe til some other time. "scholler: nay sit, in good faith, i desire not to heare such vaine fantasies, so farre against the common reason, and repugnant to the content of all the learned multitude of writers, and therefore let it passe for ever and a day longer. "maister: you are too yong to be a good judge in so great a matter: it passeth farre your learning, and their's also, that are much better learned than you, to improuve his supposition by good arguments, and therefore you were best condemne nothing that you do not well understand: but an other time, as i saide, i will so declare his supposition, that you shall not onely wonder to heare it, but also peradventure be as earnest then to credite it, as you are now to condemne it: in the meane season let us proceed forward in our former order...." [footnote : _dict. of nat. biog._: "recorde."] [footnote : quoted (p. ), from the edition of in the library of mr. george a. plimpton. see also recorde's _whetstone of witte_ ( ) as cited by berry, .] this little book, reprinted in and in , and one of the most popular of the mathematical writings in england during that century, must have interested the english in the new doctrine even before bruno's emphatic presentation of it to them in the eighties. yet the english did not welcome it cordially. one of the most popular books of this period was sylvester's translation ( ) of dubartas's _the divine weeks_ which appeared in france in , a book loved especially by milton.[ ] dubartas writes:[ ] "those clerks that think--think how absurd a jest! that neither heavens nor stars do turn at all, nor dance around this great, round earthly ball, but the earth itself, this massy globe of our's, turns round about once every twice twelve hours! and we resemble land-bred novices new brought aboard to venture on the seas; who at first launching from the shore suppose the ship stands still and that the firm earth goes." [footnote : dubartas: _the divine weeks_ (sylvester's trans. edited by haight): preface, pp. xx-xxiii and note.] [footnote : _op. cit._: .] quite otherwise was the situation in the sixteenth century at the university of salamanca. a new set of regulations for the university, drawn up at the king's order by bishop covarrubias, was published in . it contained the provision in the curriculum that "mathematics and astrology are to be given in three years, the first, astrology, the second, euclid, ptolemy or copernicus _ad vota audientium_," which also indicates, as vicente de la fuente points out, that at this university "the choice of the subject-matter to be taught lay not with the teachers but with the students, a rare situation."[ ] one wonders what happened there when the professors and students received word[ ] from the cardinal nuncio at madrid in that the congregations of the index had decreed the copernican doctrine was thereafter in no way to be held, taught or defended. [footnote : la fuente: _historia de la universidades ... de españa_: ii, .] [footnote : _doc. _ in favaro: .] one of the graduates of this university, father zuñiga,[ ] (better known as didacus à stunica), wrote a commentary on job that was licensed to be printed in , but was not published until at toledo. another edition appeared at rome seven years later. it evidently was widely read for it was condemned _donec corrigatur_ by the index in and the mathematical literature of the next half century contains many allusions to his remarks on job: ix: ; "who shaketh the earth out of her place, and the pillars thereof tremble." after commenting here upon the greater clarity and simplicity of the copernican theory, didacus à stunica then states that the theory is not contradicted by solomon in ecclesiastes, as that "text signifieth no more but this, that although the succession of ages, and generations of men on earth be various, yet the earth itself is still one and the same, and continueth without any sensible variation" ... and "it hath no coherence with its context (as philosophers show) if it be expounded to speak of the earth's immobility. the motion that belongs to the earth by way of speech is assigned to the sun even by copernicus himself, and those who are his followers.... to conclude, no place can be produced out of holy scriptures which so clearly speaks the earth's immobility as this doth its mobility. therefore this text of which we have spoken is easily reconciled to this opinion. and to set forth the wonderful power and wisdom of god who can indue the frame of the whole earth (it being of monstrous weight by nature) with motion, this our divine pen-man added; 'and the pillars thereof tremble:' as if he would teach us, from the doctrine laid down, that it is moved from its foundations."[ ] [footnote : _diccionario enciclopédico hispano-americano de literatura, ciencias y artes_ (barcelona, ).] [footnote : quoted in salusbury: _math. coll._: i, - ( ), as a work inaccessible to most readers at that time because of its extreme rarity. it remained on the index until the edition of .] french thinkers, like the english, did not encourage the new doctrine at this time. montaigne[ ] was characteristically indifferent: "what shall we reape by it, but only that we neede not care which of the two it be? and who knoweth whether a hundred yeares hence a third opinion will arise which happily shall overthrow these two præcedent?" the famous political theorist, jean bodin, ( - ), was as thoroughly opposed to it as dubartas had been. in the last year of his life, bodin wrote his _universæ naturæ theatrum_[ ] in which he discussed the origin and composition of the universe and of the animal, vegetable, mineral and spiritual kingdoms. these five books (or divisions) reveal his amazing ideas of geology, physics and astronomy while at the same time they show a mind thoroughly at home in hebrew and arabian literature as well as in the classics. his answer to the copernican doctrine is worth quoting to illustrate the attitude of one of the keenest thinkers in a brilliant era: "theorist: since the sun's heat is so intense that we read it has sometimes burned crops, houses and cities in scythia,[ ] would it not be more reasonable that the sun is still and the earth indeed revolves? "mystic: such was the old idea of philolaus, timæus, ecphantes, seleucus, aristarchus of samos, archimedes and eudoxus, which copernicus has renewed in our time. but it can easily be refuted by its shallowness although no one has done it thoroughly. "the.: what arguments do they rely on who hold that the earth is revolved and that the sun forsooth is still? "mys.: because the comprehension of the human mind cannot grasp the incredible speed of the heavenly spheres and especially of the tenth sphere which must be ten times greater than that of the eighth, for in twenty-four hours it must traverse , , miles, so that the earth seems like a dot in the universe. this is the chief argument. besides this, we get rid entirely of epicycles in representing the motions of the planets and what is taught concerning the motion of trepidation in the eighth sphere vanishes. also, there is no need for the ninth and tenth spheres. there is one argument which they have omitted but which seems to me more efficacious than any, viz.: rest is nobler than movement, and that celestial and divine things have a stable nature while elemental things have motion, disturbance and unrest; therefore it seems more probable that the latter move rather than the former. but while serious absurdities result from the idea of eudoxus, far more serious ones result from that of copernicus. "the.: what are these absurdities? "mys.: eudoxus knew nothing of trepidation, so his idea seems to be less in error. but copernicus, in order to uphold his own hypothesis, claims the earth has three motions, its diurnal and annual ones, and trepidation; if we add to these the pull of weight towards the center, we are attributing four natural motions to one and the same body. if this is granted, then the very foundations of physics must fall into ruins; for all are agreed upon this that each natural body has but one motion of its own, and that all others are said to be either violent or voluntary. therefore, since he claims the earth is agitated by four motions, one only can be its own, the others must be confessedly violent; yet nothing violent in nature can endure continuously. furthermore the earth is not moved by water, much less by the motion of air or fire in the way we have stated the heavens are moved by the revolutions of the enveloping heavens. copernicus further does not claim that all the heavens are immobile but that some are moved, that is, the moon, mercury, venus, mars, jupiter and saturn. but why such diversity? no one in his senses, or imbued with the slightest knowledge of physics, will ever think that the earth, heavy and unwieldy from its own weight and mass, staggers up and down around its own center and that of the sun; for at the slightest jar of the earth, we would see cities and fortresses, towns and mountains thrown down. a certain courtier aulicus, when some astrologer in court was upholding copernicus's idea before duke albert of prussia, turning to the servant who was pouring the falernian, said: "take care that the flagon is not spilled."[ ] for if the earth were to be moved, neither an arrow shot straight up, nor a stone dropped from the top of a tower would fall perpendicularly, but either ahead or behind. with this argument ptolemy refuted eudoxus. but if we search into the secrets of the hebrews and penetrate their sacred sanctuaries, all these arguments can easily be confirmed; for when the lord of wisdom said the sun swept in its swift course from the eastern shore to the west, he added this: terra vero stat æternam. lastly, all things on finding places suitable to their natures, remain there, as aristotle writes. since therefore the earth has been alloted a place fitting its nature, it cannot be whirled around by other motion than its own. "the.: i certainly agree to all the rest with you, but aristotle's law i think involves a paralogism, for by this argument the heavens should be immobile since they are in a place fitting their nature. "mys.: you argue subtly indeed, but in truth this argument does not seem necessary to me; for what aristotle admitted, that, while forsooth all the parts of the firmament changed their places, the firmament as a whole did not, is exceedingly absurd. for either the whole heaven is at rest or the whole heaven is moved. the senses themselves disprove that it is at rest; therefore it is moved. for it does not follow that if a body is not moved away from its place, it is not moved in that place. furthermore, since we have the most certain proof of the movement of trepidation, not only all the parts of the firmament, but also the eight spheres, must necessarily leave their places and move up and down, forward and back."[ ] [footnote : montaigne: _essays_: bk. ii, c. : _an apologie of raymonde sebonde_ (ii, ).] [footnote : this book, published at frankfort in , was translated into french by m. fougerolles and printed in lyons that same year. it has become extremely rare since its "atheistic atmosphere" (peignot: _dictionnaire_) caused the roman church to place it upon the index by decree of , where it has remained to this day.] [footnote : cromer in history of poland.] [footnote : cromer in history of poland.[a]] [footnote a: i could not find this reference in either of martin kromer's books; _de origine et rebus gestis polonorum, ad _, or in his _res publicæ sive status regni poloniæ_.] [footnote : bodin: _univ. nat. theatrum_: bk. v, sec. (end).] this was the opinion of a profound thinker and experienced man of affairs living when tycho brahe and bruno were still alive and kepler and galileo were beginning their astronomical investigations. but he was not alone in his views, as we shall see; for at the close of the sixteenth century, the copernican doctrine had few avowed supporters. the roman church was still indifferent; the protestants clinging to the literal interpretation of the bible were openly antagonistic; the professors as a whole were too aristotelian to accept or pay much attention to this novelty, except kepler and his teacher mæstlin (though the latter refused to uphold it in his textbook);[ ] while astronomers and mathematicians who realized the insuperable objections to the ptolemaic conception, welcomed the tychonic system as a _via media_; and the common folk, if they heard of it at all, must have ridiculed it because it was so plainly opposed to what they saw in the heavens every day. in the same way their intellectual superiors exclaimed at the "delirium" of those supporting such a notion.[ ] one thinker, however was to see far more in the doctrine than copernicus himself had conceived, and by giordano bruno the roman church was to be aroused. [footnote : delambre: _astr. mod._: i, .] [footnote : justus-lipsius: _physiologiæ stoicorum_: bk. ii, dissert. (dedication , louvain), (iv, ); "vides deliria, quomodo aliter appellent?"] chapter ii. bruno and galileo. when the roman catholic authorities awoke to the dangers of the new teaching, they struck with force. the first to suffer was the famous monk-philosopher, giordano bruno, whose trial by the holy office was premonitory of trouble to come for galileo.[ ] [footnote : berti: .] after an elementary education at naples near his birth-place, nola,[ ] filippo bruno[ ] entered the dominican monastery in or when about fourteen years old, assuming the name giordano at that time. before , when he entered the priesthood, he had fully accepted the copernican theory which later became the basis of all his philosophical thought. bruno soon showed he was not made for the monastic life. various processes were started against him, and fleeing to rome he abandoned his monk's garments and entered upon the sixteen years of wandering over europe, a peripatetic teacher of the philosophy of an infinite universe as deduced from the copernican doctrine and thus in a way its herald.[ ] he reached geneva in (where he did not accept calvinism as was formerly thought),[ ] but decided before many months had passed that it was wise to depart elsewhere because of the unpleasant position in which he found himself there. he had been brought before the council for printing invectives against one of the professors, pointing out some twenty of his errors. the council sent him to the consistory, the governing body of the church, where a formal sentence of excommunication was passed against him. when he apologized it was withdrawn. probably a certain stigma remained, and he left geneva soon thereafter with a warm dislike for calvinism. after lecturing at the university of toulouse he appeared in paris in , where he held an extraordinary readership. two years later he was in england, for he lectured at oxford during the spring months and defended the copernican theory before the polish prince alasco during the latter's visit there in june.[ ] [footnote : mcintyre: - .] [footnote : four lives of bruno have been written within the last seventy-five years. the first is _jordano bruno_ by christian bartholmèss ( vol., paris ). the next, _vita di giordano bruno da nola_ by domenico berti ( , turin), quotes in full the official documents of his trial. frith's _life of giordano bruno_ (london, ), has been rendered out of date by j.l. mcintyre's _giordano bruno_ (london, ), which includes a critical bibliography. in addition, w.r. thayer's _throne makers_ (new york, ), gives translations of bruno's confessions to the venetian inquisition. bruno's latin works (_opera latina conscripta_), have been republished by fiorentino ( vol., naples, ), and the _opere italiane_ by gentile ( vol., naples, ).] [footnote : bartholmèss: i, .] [footnote : libri: iv, .] [footnote : mcintyre: - .] to bruno belongs the glory of the first public proclamation in england of the new doctrine,[ ] though only gilbert[ ] and possibly wright seem to have accepted it at the time. upon bruno's return to london, he entered the home of the french ambassador as a kind of secretary, and there spent the happiest years of his life till the ambassador's recall in october, . it was during this period that he wrote some of his most famous books. in _la cena de la ceneri_ he defended the copernican theory, incidentally criticising the oxford dons most severely,[ ] for which he apologized in _de la causa, principio et uno_. he developed his philosophy of an infinite universe in _de l'infinito e mondi_, and in the _spaccio de la bestia trionphante_ "attacked all religions of mere credulity as opposed to religions of truth and deeds."[ ] this last book was at once thought to be a biting attack upon the roman church and later became one of the grounds of the inquisition's charges against him. during this time in london also, he came to know sir philip sydney intimately, and fulk greville as well as others of that brilliant period. he may have known bacon;[ ] but it is highly improbable that he and shakespeare met,[ ] or that shakespeare ever was influenced by the other's philosophy.[ ] [footnote : bartholmèss: i, .] [footnote : gilbert: _de magnete_ (london, ).] [footnote : berti: , doc. xiii.] [footnote : mcintyre: - .] [footnote : bartholmèss: i, .] [footnote : beyersdorf: _giordano bruno und shakespear_, - .] [footnote : such passages as _troilus and cressida_: act i, sc. ; _king john_, act iii, sc. ; and _merry wives_, act iii, sc. , indicate that shakespeare accepted fully the ptolemaic conception of a central, immovable earth. see also beyersdorf: _op. cit._] leaving paris soon after his return thither, bruno wandered into southern germany. at marburg he was not permitted to teach, but at wittenberg the lutherans cordially welcomed him into the university. after a stay of a year and a half, he moved on to prague for a few months, then to helmstadt, frankfort and zurich, and back to frankfort again where, in , he received an invitation from a young venetian patrician, moecenigo, to come to venice as his tutor. he re-entered italy, therefore, in august, much to the amazement of his contemporaries. it is probable that moecenigo was acting for the inquisition.[ ] at any rate, he soon denounced bruno to that body and in may, , surrendered him to it.[ ] [footnote : mcintyre: .] [footnote : ibid: - .] in his trial before the venetian inquisition,[ ] bruno told the story of his life and stated his beliefs in answer to the charges against him, based mainly on travesties of his opinions. in this statement as well as in _la cena de le ceneri_, and in _de immenso et innumerabilis_,[ ] bruno shows how completely he had not merely accepted the copernican doctrine, but had expanded it far beyond its author's conception. the universe according to copernicus, though vastly greater than that conceived by aristotle and ptolemy, was still finite because enclosed within the sphere of the fixed stars. bruno declared that not only was the earth only a lesser planet, but "this world itself was merely one of an infinite number of particular worlds similar to this, and that all the planets and other stars are infinite worlds without number composing an infinite universe, so that there is a double infinitude, that of the greatness of the universe, and that of the multitude of worlds."[ ] how important this would be to the church authorities may be realized by recalling the patristic doctrine that the universe was created for man and that his home is its center. of course their cherished belief must be defended from such an attack, and naturally enough, the copernican doctrine as the starting point of bruno's theory of an infinite universe was thus brought into question;[ ] for, as m. berti has said,[ ] bruno's doctrine was equally an astro-theology or a theological astronomy. [footnote : see official documents in berti: - .] [footnote : bruno: _de immenso et innumerabilis_: lib. iii, cap. (vol. , pt. , - ).] [footnote : thayer: .] [footnote : berti: .] [footnote : ibid: .] the roman inquisition was not content to let the venetian court deal with this arch heretic, but wrote in september, , demanding his extradition. the venetian body referred its consent to the state for ratification which the doge and council refused to grant. finally, when the papal nuncio had represented that bruno was not a venetian but a neapolitan, and that cases against him were still outstanding both in naples and in rome, the state consented, and in february of the next year, bruno entered rome, a prisoner of the inquisition. nothing further is known about him until the congregations took up his case on february th, . perhaps pope clement had hoped to win back to the true faith this prince of heretics.[ ] however bruno stood firm, and early in the following year he was degraded, sentenced and handed over to the secular authorities, who burned him at the stake in the campo di fiori, february , .[ ] all his books were put on the index by decree of february , , (where they remain to this day), and as a consequence they became extremely rare. it is well to remember bruno's fate, when considering galileo's case, for galileo[ ] was at that time professor of mathematics in the university of padua and fully cognizant of the event. [footnote : fahie: - .] [footnote : thayer: .] [footnote : the publication of a. favaro's _galileo e l'inquisizione: documenti del processo galileiano ... per la prima volta integralmente pubblicati_, (firenze, ), together with that of the national edition (in vols.) of galileo's works, edited by favaro (firenze, completed ), renders somewhat obsolete all earlier lives of galileo. the more valuable, however, of these books are: martin's _galilée_ (paris, ), a scholarly catholic study containing valuable bibliographical notes; anon. (mrs. olney): _private life of galileo_, based largely on his correspondence with his daughter from which many extracts are given; and von gebler's _galileo galilei and the roman curia_ (trans. by mrs. sturge, london, ), which includes in the appendix the various decrees in the original. fahie's _life of galileo_ (london, ), is based on favaro's researches and is reliable. the documents of the trial have been published in part by de l'epinois, von gebler and berti, but favaro's is the complete and authoritative edition.] galileo's father, though himself a skilled mathematician, had intended that his son (born at pisa, february , ), should be a cloth-dealer, but at length permitted him to study medicine instead at the university of pisa, after an elementary education at the monastery of vallombrosa near florence. at the tuscan court in pisa, galileo received his first lesson in mathematics, which thereupon became his absorbing interest. after nearly four years he withdrew from the university to florence and devoted himself to that science and to physics. his services as a professor at this time were refused by five of the italian universities; finally, in , he obtained the appointment to the chair of physics at pisa. he became so unpopular there, however, through his attacks on the aristotelian physics of the day, that after three years he resigned and accepted a similar position at padua.[ ] he remained here nearly eighteen years till his longing for leisure in which to pursue his researches, and the patronage of his good friend, the grand duke of tuscany, brought him a professorship at the university of pisa again, this time without obligation of residence nor of lecturing. he took up his residence in florence in ; and later ( ), purchased a villa at arcetri outside the city, in order to be near the convent where his favorite daughter "suor maria celeste" was a religious.[ ] [footnote : fahie: - .] [footnote : ibid: .] during the greater part of his lectureship at padua, galileo taught according to the ptolemaic cosmogony out of compliance with popular feeling, though himself a copernican. in a letter to kepler (august , )[ ] he speaks of his entire acceptance of the new system for some years; but not until after the appearance of the new star in the heavens in and , and the controversy that its appearance aroused over the aristotelian notion of the perfect and unchangeable heavens, did he publicly repudiate the old scheme and teach the new. the only information we have as to how he came to adopt the copernican scheme for himself is the account given by "_sagredo_," galileo's spokesman in the famous _dialogue on the two principal systems_ ( ): "being very young and having scarcely finished my course of philosophy which i left off, as being set upon other employments, there chanced to come into these parts a certain foreigner of rostock, whose name as i remember, was christianus vurstitius, a follower of copernicus, who in an academy made two or three lectures upon this point, to whom many flock't as auditors; but i thinking they went more for the novelty of the subject than otherwise, did not go to hear him; for i had concluded with myself that that opinion could be no other than a solemn madnesse. and questioning some of those who had been there, i perceived they all made a jest thereof, except one, who told me that the business was not altogether to be laugh't at, and because this man was reputed by me to be very intelligent and wary, i repented that i was not there, and began from that time forward as oft as i met with anyone of the copernican persuasion, to demand of them, if they had always been of the same judgment; and of as many as i examined, i found not so much as one, who told me not that he had been a long time of the contrary opinion, but to have changed it for this, as convinced by the reasons proving the same: and afterwards questioning them, one by one, to see whether they were well possest of the reasons of the other side, i found them all to be very ready and perfect in them; so that i could not truly say that they had took up this opinion out of ignorance, vanity, or to show the acuteness of their wits. on the contrary, of as many of the peripateticks and ptolemeans as i have asked (and out of curiosity i have talked with many) what pains they had taken in the book of copernicus, i found very few that had so much as superficially perused it: but of those whom, i thought, had understood the same, not one; and moreover, i have enquired amongst the followers of the peripatetick doctrine, if ever any of them had held the contrary opinion, and likewise found that none had. whereupon considering that there was no man who followed the opinion of copernicus that had not been first on the contrary side, and that was not very well acquainted with the reasons of aristotle and ptolemy; and on the contrary, that there is not one of the followers of ptolemy that had ever been of the judgment of copernicus, and that had left that to embrace this of aristotle, considering, i say, these things, i began to think that one, who leaveth an opinion imbued with his milk, and followed by very many, to take up another owned by very few, and denied by all the schools, and that really seems a very great parodox, must needs have been moved, not to say forced, by more powerful reasons. for this cause i am become very curious to dive, as they say, into the bottom of this business ... and bring myself to a certainty in this subject."[ ] [footnote : galileo: _opere_, x, .] [footnote : 'the second day' in salusbury: _math. coll._ i, - .] galileo's brilliant work in mechanics and his great popularity--for his lectures were thronged--combined with his skilled and witty attacks upon the accepted scientific ideas of the age, embittered and antagonized many who were both conservative and jealous.[ ] the jesuits particularly resented his influence and power, for they claimed the leadership in the educational world and were jealous of intruders. furthermore, they were bound by the decree of the fiftieth general congregation of their society in to defend aristotle, a decree strictly enforced.[ ] while a few of the jesuits were friendly disposed to galileo at first, the controversies in which he and they became involved and their bitter attacks upon him made him feel by that they were among his chief enemies.[ ] [footnote : fahie: .] [footnote : conway: - .] [footnote : conway: - .] early in , galileo heard a rumor of a spy-glass having been made in flanders, and proceeded to work one out for himself according to the laws of perspective. the fifth telescope that he made magnified thirty diameters, and it was with such instruments of his own manufacture that he made in the next three years his famous discoveries: jupiter's four satellites (which he named the medicean planets), saturn's "tripartite" character (the rings were not recognized as such for several decades thereafter), the stars of the milky way, the crescent form of venus, the mountains of the moon, many more fixed stars, and the spots on the sun. popular interest waxed with each new discovery and from all sides came requests for telescopes; yet there were those who absolutely refused even to look through a telescope lest they be compelled to admit aristotle was mistaken, and others claimed that jupiter's moons were merely defects in the instrument. the formal announcement of the first of these discoveries was made in the _sidereus nuncius_ ( ), a book that aroused no little opposition. kepler, however, had it reprinted at once in prague with a long appreciative preface of his own.[ ] [footnote : fahie: - .] the following march galileo went to rome to show his discoveries and was received with the utmost distinction by princes and church dignitaries alike. a commission of four scientific members of the roman college had previously examined his claims at cardinal bellarmin's suggestion, and had admitted their truth. now pope paul v gave him long audiences; the academia dei lincei elected him a member, and everywhere he was acclaimed. nevertheless his name appears on the secret books of the holy office as early as may of that year ( ).[ ] already he was a suspect. [footnote : doc. in favaro: .] his _delle macchie solari_ ( ) brought on a sharp contest over the question of priority of discovery between him and the jesuit father, christopher scheiner of ingolstadt, from which galileo emerged victorious and more disliked than before by that order. opposition was becoming active; father castelli, for instance, the professor of mathematics at pisa and galileo's intimate friend, was forbidden to discuss in his lectures the double motion of the earth or even to hint at its probability. this same father wrote to his friend early in december, , to tell him of a dinner-table conversation on this matter at the tuscan court, then wintering at pisa. castelli told how the dowager grand duchess cristina had had her religious scruples aroused by a remark that the earth's motion must be wrong because it contradicted the scriptures, a statement that he had tried to refute.[ ] galileo wrote in reply (december , ), the letter[ ] that was to cause him endless trouble, in which he marked out the boundaries between science and religion and declared it a mistake to take the literal interpretation of passages in scripture that were obviously written according to the understanding of the common people. he pointed out in addition how futile the miracle of the sun's standing still was as an argument against the copernican doctrine for, even according to the ptolemaic system, not the sun but the _primum mobile_ must be stayed for the day to be lengthened. [footnote : fahie: .] [footnote : galileo: _opere_, v, - .] father castelli allowed others to read and to copy this supposedly private letter; copies went from hand to hand in florence and discussion ran high. on the fourth sunday in december, , father caccini of the dominicans preached a sermon in the church of s.m. novella on joshua's miracle, in which he sharply denounced the copernican doctrine taught by galileo as heretical, so he believed.[ ] the copernicans found a neapolitan jesuit who replied to caccini the following sunday from the pulpit of the duomo.[ ] [footnote : doc. in favaro: - .] [footnote : doc. in favaro: .] in february ( ), came the formal denunciation of galileo to the holy office at rome by father lorini, a dominican associate of caccini's at the convent san marco. the father sent with his "friendly warning," a copy of the letter to castelli charging that it contained "many propositions which were either suspect or temerarious," and, he added, "though the _galileisti_ were good christians they were rather stubborn and obstinate in their opinions."[ ] the machinery of the inquisition began secretly to turn. the authorities failed to get the original of the letter, for castelli had returned that to galileo at the latter's request.[ ] pope paul sent word to father caccini to appear before the holy office in rome to depose on this matter of galileo's errors "pro exoneratione suæ conscientiæ."[ ] this he did "freely" in march and was of course sworn to secrecy. he named a certain nobleman, a copernican, as the source of his information about galileo, for he did not know the latter even by sight. this nobleman was by order of the pope examined in november after some delay by the inquisitor at florence. his testimony was to the effect that he considered galileo the best of catholics.[ ] [footnote : ibid: : "amorevole avviso."] [footnote : ibid: , , .] [footnote : ibid: .] [footnote : ibid: .] meanwhile the consultors of the holy office had examined lorini's copy of the letter and reported the finding of only three objectionable places all of which, they stated, could be amended by changing certain doubtful phrases; otherwise it did not deviate from the true faith. it is interesting to note that the copy they had differed in many minor respects from the original letter, and in one place heightened a passage with which the examiners found fault as imputing falsehood to the scriptures although they are infallible.[ ] galileo's own statement ran that there were many passages in the scriptures which according to the literal meaning of the words, "hanno aspetto diverso dal vero...." the copy read, "molte propositioni falso quanto al nudo senso delle parole." [footnote : ibid: - , see original in galileo: _opere_, v, - .] rumors of trouble reached galileo and, urged on by his friends, in he wrote a long formal elaboration of the earlier letter, addressing this one to the dowager grand duchess, but he had only added fuel to the fire. at the end of the year he voluntarily went to rome, regardless of any possible danger to himself, to see if he could not prevent a condemnation of the doctrine.[ ] it came as a decided surprise to him to receive an order to appear before cardinal bellarmin on february , ,[ ] and there to learn that the holy office had already condemned it two days before. he was told that the holy office had declared: first, "that the proposition that the sun is the center of the universe and is immobile is foolish and absurd in philosophy and formally heretical since it contradicts the express words of the scriptures in many places, according to the meaning of the words and the common interpretation and sense of the fathers and the doctors of theology; and, secondly, that the proposition that the earth is not the center of the universe nor immobile receives the same censure in philosophy and in regard to its theological truth, it at least is erroneous in faith."[ ] [footnote : doc. in favaro: .] [footnote : ibid: .] exactly what was said at that meeting between the two men became the crucial point in galileo's trial sixteen years later, hence a somewhat detailed study is important. at the meeting of the congregation on february th, the pope ordered cardinal bellarmin to summon galileo and, in the presence of a notary and witnesses lest he should prove recusant, warn him to abandon the condemned opinion and in every way to abstain from teaching, defending or discussing it; if he did not acquiesce, he was to be imprisoned.[ ] the secret archives of the vatican contain a minute reporting this interview (dated february , ), in which the cardinal is said to have ordered galileo to relinquish this condemned proposition, "nec eam de cætero, quovis modo, teneat, doceat aut defendat, verbo aut scriptis," and that galileo promised to obey.[ ] rumors evidently were rife in rome at the time as to what had happened at this secret interview, for galileo wrote to the cardinal in may asking for a statement of what actually had occurred so that he might silence his enemies. the cardinal replied: "we, robert cardinal bellarmin, having heard that signor galileo was calumniated and charged with having abjured in our hand, and also of being punished by salutary penance, and being requested to give the truth, state that the aforesaid signor galileo has not abjured in our hand nor in the hand of any other person in rome, still less in any other place, so far as we know, any of his opinions and teachings, nor has he received salutary penance nor any other kind; but only was he informed of the declaration made by his holiness and published by the sacred congregation of the index, in which it is stated that the doctrine attributed to copernicus,--that the earth moves around the sun and that the sun stands in the center of the world without moving from the east to the west, is contrary to the holy scriptures and therefore cannot be defended nor held (non si possa difendere né tenere). and in witness of this we have written and signed these presents with our own hand, this th day of may, . robert cardinal bellarmin."[ ] [footnote : ibid: .] [footnote : doc. in favaro: - .] [footnote : ibid: .] galileo's defense sixteen years later[ ] was that he had obeyed the order as given him by the cardinal and that he had not "defended nor held" the doctrine in his _dialoghi_ but had refuted it. the congregation answered that he had been ordered not only not to hold nor defend, but also not to treat in any way (quovis modo) this condemned subject. when galileo disclaimed all recollection of that phrase and produced the cardinal's statement in support of his position, he was told that this document, far from lightening his guilt, greatly aggravated it since he had dared to deal with a subject that he had been informed was contrary to the holy scriptures.[ ] [footnote : ibid: - .] [footnote : ibid: .] to return to . on the third of march the cardinal reported to the congregation in the presence of the pope that he had warned galileo and that galileo had acquiesced.[ ] the congregation then reported its decree suspending "until corrected" "nicolai copernici de revolutionibus orbium coelestium, et didaci astunica in job," and prohibiting "epistola fratris pauli antonii foscarini carmelitæ," together with all other books dealing with this condemned and prohibited doctrine. the pope ordered this decree to be published by the master of the sacred palace, which was done two days later.[ ] but this prohibition could not have been widely known for two or three years; the next year mulier published his edition of the _de revolutionibus_ at amsterdam without a word of reference to it; in thomas feyens, professor at louvain, heard vague rumors of the condemnation and wondered if it could be true;[ ] and the following spring fromundus, also at louvain and later a noted antagonist of the new doctrine, wrote to feyens asking: "what did i hear lately from you about the copernicans? that they have been condemned a year or two ago by our holy father, pope paul v? until now i have known nothing about it; no more have this crowd of german and italian scholars, very learned and, as i think, very catholic, who admit with copernicus that the earth is turned. is it possible that after a lapse of time as considerable as this, we have nothing more than a rumor of such an event? i find it hard to believe, since nothing more definite has come from italy. definitions of this sort ought above all to be published in the universities where the learned men are to whom the danger of such an opinion is very great."[ ] [footnote : ibid: .] [footnote : doc. in favaro: .] [footnote : monchamp: .] [footnote : fromundus: _de cometa anni_ : chap. vii, p. . (from the private library of dr. e.e. slosson. a rare book which lecky could not find. _history of rationalism in europe_, i, , note.)] galileo meanwhile had retired to florence and devoted himself to mechanical science, (of which his work is the foundation) though constantly harassed by much ill health and many family perplexities. at the advice of his friends, he allowed the attacks on the copernican doctrine to go unanswered,[ ] till with the accession to the papacy in of cardinal barberini, as urban viii, a warm admirer and supporter of his, he thought relief was in sight. he was further cheered by a conversation cardinal di zollern reported having had with pope urban, in which his holiness had reminded the cardinal how he (the pope) had defended copernicus in the time of paul v, and asserted that out of just respect owed to the memory of copernicus, if he had been pope then, he would not have permitted his opinion to be declared heretical.[ ] feeling that he now had friends in power, galileo began his great work, _dialogo sopra i due sistemi massimi del mondo_, a dialogue in four "days" in which three interlocutors discuss the arguments for and against the copernican theory, though coming to no definite conclusion. sagredo was an avowed copernican and galileo's spokesman, salviati was openminded, and the peripatetic was simplicio, appropriately named for the famous sicilian sixth century commentator on aristotle.[ ] [footnote : in the congregation issued the changes it required to have made in the _de revolutionibus_. they are nine in all, and consist mainly in changing assertion of the earth's movement to hypothetical statement and in striking out a reference to the earth as a planet. doc. in favaro: - . see illustration, p. .] [footnote : doc. in favaro: .] [footnote : galileo: _dialogo_: to the reader.] [illustration: a "corrected" page from the _de revolutionibus_. a photographic facsimile (reduced) of a page from mulier's edition ( ) of the _de revolutionibus_ as "corrected" according to the _monitum_ of the congregations in . the first writer underlined the passages to be deleted or altered with marginal notes indicating the changes ordered; the second writer scratched out these passages, and wrote out in full the changes the other had given in abbreviated form. the _notæ_ are mulier's own, and so were not affected by the order. the effect of the page is therefore somewhat contradictory!] in he brought the completed manuscript to riccardi, master of the sacred palace, for permission to print it in rome. after much reading and re-reading of it both by riccardi and his associate, father visconti, permission was at length granted on condition that he insert a preface and a conclusion practically dictated by riccardi, emphasizing its hypothetical character.[ ] the pope's own argument was to be used: "god is all-powerful; all things are therefore possible to him; ergo, the tides cannot be adduced as a necessary proof of the double motion of the earth without limiting god's omnipotence--which is absurd."[ ] galileo returned to florence in june with the permission to print his book in rome. meanwhile the plague broke out. he decided to print it in florence instead, and on writing to riccardi for that permission, the latter asked for the book to review it again. the times were too troublesome to risk sending it, so a compromise was finally effected: galileo was to send the preface and conclusion to rome and riccardi agreed to instruct the inquisitor at florence as to his requirements and to authorize him to license the book.[ ] the parts were not returned from rome till july, , and the book did not appear till february of the following year, when it was published at florence with all these licenses, both the roman and the florentine ones. [footnote : doc. in favaro: .] [footnote : fahie: .] [footnote : ibid: .] the _dialogo_ was in italian so that all could read it. it begins with an outline of the aristotelian system, then points out the resemblances between the earth and the planets. the second "day" demonstrates the daily rotation of the earth on its axis. the next claims that the necessary stellar parallax is too minute to be observed and discusses the earth's annual rotation. the last seeks to prove this rotation by the ebb and flow of the tides. it is a brilliant book and received a great reception. the authorities of the inquisition at once examined it and denounced galileo (april , ) because in it he not merely taught and defended the "condemned doctrine but was gravely suspected of firm adherence to this opinion."[ ] other charges made against him were that he had printed the roman licenses without the permission of the congregation, that he had printed the preface in different type so alienating it from the body of the book, and had put the required conclusion into the mouth of a fool (simplicio), that in many places he had abandoned the hypothetical treatment and asserted the forbidden doctrine, and that he had dealt indecisively with the matter though the congregation had specifically condemned the copernican doctrine as contrary to the express words of the scripture.[ ] [footnote : doc. in favaro: - . [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] [footnote : ibid: .] the pope became convinced that galileo had ridiculed him in the character of simplicio to whom galileo had naturally enough assigned the pope's syllogistic argument. on the rd of september, he ordered the inquisitor of florence to notify galileo (in the presence of concealed notary and witnesses in case he were "recusant") to come to rome and appear before the sacred congregation before the end of the next month;[ ] the publication and sale of the _dialogo_ meanwhile being stopped at great financial loss to the printer.[ ] galileo promised to obey; but he was nearly seventy years old and so much broken in health that a long difficult journey in the approaching winter seemed a great and unnecessary hardship, especially as he was loath to believe that the church authorities were really hostile to him. delays were granted him till the pope in december finally ordered him to be in rome within a month.[ ] the florentine inquisitor replied that galileo was in bed so sick that three doctors had certified that he could not travel except at serious risk to his life. this certificate declared that he suffered from an intermittent pulse, from enfeebled vital faculties, from frequent dizziness, from melancholia, weakness of the stomach, insomnia, shooting pains and serious hernia.[ ] the answer the pope made to this was to order the inquisitor to send at galileo's expense a commissary and a doctor out to his villa to see if he were feigning illness; if he were, he was to be sent bound and in chains to rome at once; if [transcriber's note: 'he' missing] were really too ill to travel, then he was to be sent in chains as soon as he was convalescent and could travel safely.[ ] galileo did not delay after that any longer than he could help, and set out for rome in january in a litter supplied by the tuscan grand duke.[ ] the journey was prolonged by quarantine, but upon his arrival (february , ), he was welcomed into the palace of niccolini, the warm-hearted ambassador of the grand duke. [footnote : ibid: - .] [footnote : galileo: _opere_, xv, .] [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : ibid: .] [footnote : ibid: - .] four times was the old man summoned into the presence of the holy office, though never when the pope was presiding. in his first examination held on the th of april, he told how he thought he had obeyed the decree of as his _dialogo_ did not defend the copernican doctrine but rather confuted it, and that in his desire to do the right, he had personally submitted the book while in manuscript to the censorship of the master of the sacred palace, and had accepted all the changes he and the florentine inquisitor had required. he had not mentioned the affair of because he thought that order did not apply to this book in which he proved the lack of validity and of conclusiveness of the copernican arguments.[ ] with remarkable, in fact unique, consideration, the holy office then assigned galileo to a suite of rooms within the prisons of the holy office, allowed him to have his servant with him and to have his meals sent in by the ambassador. on the th after his examination, they even assigned as his prison, the ambassador's palace, out of consideration for his age and ill-health. [footnote : ibid: - .] in his second appearance (april ), galileo declared he had been thinking matters over after re-reading his book (which he had not read for three years), and freely confessed that there were several passages which would mislead a reader unaware of his real intentions, into believing the worse arguments were the better, and he blamed these slips upon his vain ambition and delight in his own skill in debate.[ ] he thereupon offered to write another "day" or two more for the _dialogo_ in which he would completely refute the two "strong" copernican arguments based on the sun's spots and on the tides.[ ] ten days later, at his third appearance, he presented a written statement of his defence in which he claimed that the phrase _vel quovis modo docere_ was wholly new to him, and that he had obeyed the order given him by cardinal bellarmin over the latter's own signature. however he would make what amends he could and begged the cardinals to "consider his miserable bodily health and his incessant mental trouble for the past ten months, the discomforts of a long hard journey at the worst season, when years old, together with the loss of the greater part of the year, and that therefore such suffering might be adequate punishment for his faults which they might condone to failing old age. also he commended to them his honor and reputation against the calumnies of his ill-wishers who seek to detract from his good name."[ ] to such a plight was the great man brought! but the end was not yet. [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : ibid: - .] nearly a month later (june ), by order of the pope, galileo was once again interrogated, this time under threat of torture.[ ] once again he declared the opinion of ptolemy true and indubitable and said he did not hold and had not held this doctrine of copernicus after he had been informed of the order to abandon it. "as for the rest," he added, "i am in your hands, do with me as you please." "i am here to obey."[ ] then by the order of the pope, ensued galileo's complete abjuration on his knees in the presence of the full congregation, coupled with his promise to denounce other heretics (i.e., copernicans).[ ] in addition, because he was guilty of the heresy of having held and believed a doctrine declared and defined as contrary to the scriptures, he was sentenced to "formal imprisonment" at the will of the congregation, and to repeat the seven penitential psalms every week for three years.[ ] [footnote : ibid: .] [footnote : doc. in favaro: .] [footnote : doc. in favaro: .] [footnote : ibid: .] at galileo's earnest request, his sentence was commuted almost at once, to imprisonment first in the archiepiscopal palace in siena (from june -december ), then in his own villa at arcetri, outside florence, though under strict orders not to receive visitors but to live in solitude.[ ] in the spring his increasing illness occasioned another request for greater liberty in order to have the necessary visits from the doctor; but on march , , this was denied him with a stern command from the pope to refrain from further petitions lest the sacred congregation be compelled to recall him to their prisons in rome.[ ] [footnote : ibid: , .] [footnote : ibid: .] the rule forbidding visitors seems not to have been rigidly enforced all the time, for milton visited him, "a prisoner of the inquisition" in ;[ ] yet father castelli had to write to rome for permission to visit him to learn his newly invented method of finding longitude at sea.[ ] when in florence on a very brief stay to see his doctor, galileo had to have the especial consent of the inquisitor in order to attend mass at easter. he won approval from the holy congregation, however, by refusing to receive some gifts and letters brought him by some german merchants from the low countries.[ ] he was then totally blind, but he dragged out his existence until january , (the year of newton's birth), when he died. as the pope objected to a public funeral for a man sentenced by the holy office, he was buried without even an epitaph.[ ] the first inscription was made years later, and in , his remains were removed to santa croce after the congregation had first been asked if such action would be unobjectionable.[ ] [footnote : milton: _areopagitica_: .] [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : fahie: .] [footnote : doc. in favaro: ; and fahie: .] pope urban had no intention of concealing galileo's abjuration and sentence. instead, he ordered copies of both to be sent to all inquisitors and papal nuncios that they might notify all their clergy and especially all the professors of mathematics and philosophy within their districts, particularly those at florence, padua and pisa.[ ] this was done during the summer and fall of . from wilna in poland, cologne, paris, brussels, and madrid, as well as from all italy, came the replies of the papal officials stating that the order had been obeyed.[ ] he evidently intended to leave no ground for a remark like that of fromundus about the earlier condemnation. [footnote : doc. in favaro: , .] [footnote : ibid: - .] galileo was thus brought so low that the famous remark, "eppur si muove," legend reports him to have made as he rose to his feet after his abjuration, is incredible in itself, even if it had appeared in history earlier than its first publication in .[ ] but his discoveries and his fight in defence of the system did much both to strengthen the doctrine itself and to win adherents to it. the appearance of the moon as seen through a telescope destroyed the aristotelian notion of the perfection of heavenly bodies. jupiter's satellites gave proof by analogy of the solar system, though on a smaller scale. the discovery of the phases of venus refuted a hitherto strong objection to the copernican system; and the discovery of the spots on the sun led to his later discovery of the sun's axial rotation, another proof by analogy of the axial rotation of the earth. yet he swore the ptolemaic conception was the true one. [footnote : fahie: , note.] the abjuration of galileo makes a pitiful page in the history of thought and has been a fruitful source of controversy[ ] for nearly three centuries. he was unquestionably a sincere and loyal catholic, and accordingly submitted to the punishment decreed by the authorities. but in his abjuration he plainly perjured himself, however fully he may be pardoned for it because of the extenuating circumstances. had he not submitted and been straitly imprisoned, if not burned, the world would indeed have been the poorer by the loss of his greatest work, the _dialoghi delle nuove scienze_, which he did not publish until .[ ] [footnote : for full statement, see martin: - .] [footnote : gebler: .] even more hotly debated has been the action of the congregations in condemning the copernican doctrine, and sentencing galileo as a heretic for upholding it.[ ] though both paul v and urban viii spurred on these actions, neither signed either the decree or the sentence, nor was the latter present at galileo's examinations. pope urban would prefer not so openly to have changed his position from that of tolerance to his present one of active opposition caused partly by his piqued self-respect[ ] and partly by his belief that this heresy was more dangerous even than that of luther and calvin.[ ] it is a much mooted question whether the infallibility of the church was involved or not. though the issue at stake was not one of faith, nor were the decrees issued by the pope _ex cathedra_, but by a group of cardinals, a fallible body, yet they had the full approbation of the popes, and later were published in the index preceded by a papal bull excommunicating those who did not obey the decrees contained therein.[ ] it seems to be a matter of the letter as opposed to the spirit of the law. de morgan points out that contemporary opinion as represented by fromundus, an ardent opponent of galileo, did not consider the decree of the index or of the inquisition as a declaration of the church,[ ]--a position which galileo himself may have held, thus explaining his practical disregard of the decree of after he was persuaded the authorities were more favorably disposed to him. but m. martin, himself a catholic, thinks[ ] that theoretically the congregations could punish galileo only for disobedience of the secret order,--but even so his book had been examined and passed by the official censors. [footnote : see gebler: - ; white: i, - ; also martin.] [footnote : martin: ; and salusbury: _math. coll._ "to the reader."] [footnote : galileo: _opere_, xv, .] [footnote : putnam: i, .] [footnote : de morgan: i, .] [footnote : martin: .] when the index was revised under pope benedict xiv in , largely through the influence of the jesuit astronomer boscovich, so it is said,[ ] the phrase prohibiting all books teaching the immobility of the sun, and the mobility of the earth was omitted from the decrees.[ ] but in , the master of the sacred palace refused to permit the publication in rome of a textbook on astronomy by canon settele, who thereupon appealed to the congregations. they granted his request in august, and two years later, issued a decree approved by pope pius vii ordering the master of the sacred palace in future "not to refuse license for publication of books dealing with the mobility of the earth and the immobility of the sun according to the common opinion of modern astronomers" on that ground alone.[ ] the next edition of the _index librorum prohibitorum_ ( ) did not contain the works of copernicus, galileo, foscarini, à stunica and kepler which had appeared in every edition up to that time since their condemnation in , (kepler's in ). [footnote : _cath. ency._: "boscovich."] [footnote : doc. in favaro: .] [footnote : ibid: , .] chapter iii. the opposition and their arguments. the protestant leaders had rejected the copernican doctrine as contrary to the scriptures. the roman congregations had now condemned galileo for upholding this doctrine after they had prohibited it for the same reasons. these objections are perhaps best summarized in that open letter foscarini wrote to the general of his order, the carmelites, at naples in january, ,[ ]--the letter that was absolutely prohibited by the index in march, . he gave these arguments and answered them lest, as he said, "whilst otherwise the opinion is favored with much probability, it be found in reality to be extremely repugnant (as at first sight it seems) not only to physical reasons and common principles received on all hands (which cannot do so much harm), but also (which would be of far worse consequence) to many authorities of sacred scripture. upon which account many at first looking into it explode it as the most fond paradox and monstrous _capriccio_ that ever was heard of." "yet many modern authors," he says further on, "are induced to follow it, but with much hesitancy and fear, in regard that it seemeth in their opinion so to contradict the holy scriptures that it cannot possibly be reconciled to them." consequently foscarini argued that the theory is either true or false; if false, it ought not to be divulged; if true, the authority of the sacred scriptures will not oppose it; neither does one truth contradict another. so he turned to the bible. [footnote : in salusbury: _math. coll._: i, - .] he found that six groups of authorities seemed to oppose this doctrine. ( ) those stating that the earth stands fast, as eccles. : . ( ) those stating that the sun moves and revolves; as psalm xix, isaiah xxxviii, and the miracle in josh. x: - . ( ) those speaking of the heaven above and the earth beneath, as in joel ii. also christ came _down_ from heaven. ( ) those authorities who place hell at the center of the world, a "common opinion of divines," because it ought to be in the lowest part of the world, that is, at the center of the sphere. then by the copernican hypothesis, hell must either be in the sun; or, if in the earth, if the earth should move about the sun, then hell within the earth would be in heaven, and nothing could be more absurd. ( ) those authorities opposing heaven to earth and earth to heaven, as in gen. i, mat. vi, etc. since the two are always mutually opposed to each other, and heaven undoubtedly refers to the circumference, earth must necessarily be at the center. ( ) those authorities ("rather of fathers and divines than of the sacred scriptures") who declare that after the day of judgment, the sun shall stand immovable in the east and the moon in west. foscarini then lays down in answer six maxims, the first of which is that things attributed to god must be expounded metaphorically according to our manner of understanding and of common speech. the other maxims are more metaphysical, as that everything in the universe, whether corruptible or incorruptible, obeys a fixed law of its nature; so, for example, fortune is _always_ fickle. in concluding his defense, he claims among other things, that the copernician is a more admirable hypothesis than the ptolemaic, and that it is an easy way into astronomy and philosophy. then he adds that there may be an analogy between the seven-branched candlestick of the old testament and the seven planets around the sun, and possibly the arrangement of the seeds in the "indian figg," in the pomegranate and in grapes is all divine evidence of the solar system. with such an amusing reversion to mediæval analogy his spirited letter ends. some or all of these scriptural arguments appear in most of the attacks on the doctrine even before its condemnation by the index in was widely known. besides these objections, aristotle's and ptolemy's statements were endlessly repeated with implicit faith in their accuracy. even sir francis bacon ( - ) with all his modernity of thought, failed in this instance to recognize the value of the new idea and, despite his interest in galileo's discoveries, harked back to the time-honored objections. at first mild in his opposition, he later became emphatically opposed to it. in the _advancement of learning_[ ] ( ), he speaks of it as a possible explanation of the celestial phenomena according to astronomy but as contrary to natural philosophy. some fifteen years later in the _novum organon_,[ ] he asserts that the assumption of the earth's movement cannot be allowed; for, as he says in his _thema coeli_,[ ] at that time he considered the opinion that the earth is stationary the truer one. finally, in his _de augmentis scientiarum_[ ] ( - ) he speaks of the old notions of the solidity of the heavens, etc., and adds, "it is the absurdity of these opinions that has driven men to the diurnal motion; which i am convinced is most false." he gives his reasons in the _descriptio globi intellectualis_[ ] (ch. - ): "in favor of the earth [as the center of the world] we have the evidence of our sight, and an inveterate opinion; and most of all this, that as dense bodies are contracted into a narrow compass, and rare bodies are widely diffused (and the area of every circle is contracted to the center) it seems to follow almost of necessity that the narrow space about the middle of the world be set down as the proper and peculiar place for dense bodies." the sun's claims to such a situation are satisfied through having two satellites of its own, venus and mercury. copernicus's scheme is inconvenient; it overloads the earth with a triple motion; it creates a difficulty by separating the sun from the number of the planets with which it has much in common; and the "introduction of so much immobility into nature ... and making the moon revolve around the earth in an epicycle, and some other assumptions of his are the speculations of one who cares not what fictions he introduces into nature, provided his calculations answer." the total absence of all reference to the scriptures is the unique and refreshing part of bacon's thought. [footnote : bk. ii: sec. , § .] [footnote : bk. ii, ch. .] [footnote : _phil. works_: .] [footnote : bk. iii.] [footnote : _phil. works_: - .] all the more common arguments against the diurnal rotation of the earth are well stated in an interesting little letter ( ) by thomas feyens, or fienus, a professor at the school of medicine in the university of louvain.[ ] thus catholic, protestant, and unbeliever, feyens, melancthon, bacon and bodin, all had recourse to the same arguments to oppose this seemingly absurd doctrine. [footnote : translated in appendix c. for criticism, see monchamp: - .] froidmont, or fromundus, the good friend and colleague of feyens at louvain, was also much interested in these matters, so much so that some thought he had formerly accepted the copernican doctrine and "only fled back into the camp of aristotle and ptolemy through terror at the decree of the s. congregation of cardinals."[ ] his indignant denial of this imputation of turn-coat in is somewhat weakened by reference to his _saturnalitæ coenæ_[ ] ( ) in which he suggests that, if the copernican doctrine is admitted, then hell may be in the sun at the center of the universe rather than in the earth, in order to be as far as possible from paradise. he also refers in his _de cometa_ ( ) to the remark of justus-lipsius[ ] that this paradox was buried with copernicus, saying "you are mistaken, o noble scholar: it lives, and it is full of vigor even now among many,"[ ] thus apparently not seeing serious objection to it. m. monchamp summarizes froidmont's point of view as against aristotle and ptolemy, half for copernicus and wholly for tycho brahe. [footnote : fromundus: _vesta_: ad lectorem.] [footnote : monchamp: .] [footnote : justus-lipsius: iv, .] [footnote : monchamp: .] froidmont's best known books are the two he wrote in answer to a defense of the copernican position first by philip lansberg, then by his son. the _ant-aristarchus sive orbis terræ immobilis, liber unicus in quo decretum s. congregationis s.r.e. cardinal. an. , adversus pythagorico-copernicanus editum, defenditur_, appeared in before galileo's condemnation. the jesuit cavalieri wrote to galileo in may about it thus:[ ] "i have run it through, and verily it states the copernican theory and the arguments in its favor with so much skill and efficacy that he seems to have understood it very well indeed. but he refutes them with so little force that he seems rather to be of an opinion contrary to that expressed in the title of his book. i have given it to m. césar. if you wish it, i will have it sent to you. the arguments he brings against copernicus are those you have so masterfully stated and answered in your _dialogo_." nearly a year later, galileo wrote to gassendi and diodati that he had received this book a month before and, although he had been unable to read much of it on account of his eye trouble, it seemed to him that of all the opponents of copernicus whom he had seen, fromundus was the most sensible and efficient.[ ] again he wrote in january, , regretting that he had not seen it till six months after he had published his dialogues, for he would have both praised it and commented upon certain points. "as for fromundus (who however shows himself to be a man of great talent) i wish he had not fallen into what seems to me a truly serious error, although a rather common one, in order to refute the copernican opinion, of beginning by poking scorn and ridicule at those who consider it true, and then (what seems to me still less becoming) of basing his attack chiefly on the authority of the scriptures, and finally of deducing from this that in this respect it is an opinion little short of heretical. to argue in this way is clearly not praiseworthy;" for as galileo goes on to show, if the scriptures are the word of god, the heavens themselves are his handiwork. why is the one less noble than the other?[ ] [footnote : ibid: .] [footnote : galileo: _opere_: xv, .] [footnote : ibid: xiv, - .] froidmont replied in to lansberg's reply with his second attack, _vesta, sive ant-aristarchi vindex_, in which he laid even more emphasis upon the theological and scriptural objections. yet, in ignorance of galileo's condemnation, he considers the charge of heresy too strong. "the partisans of this system do not after all disdain the authority of the scriptures, although they appear to interpret it in a way rather in their favor." he also, and rightly, denies the existence at that time of any conclusive proof.[ ] [footnote : monchamp: - .] in spite of froidmont's position, the university of louvain was not cordial in its response to the papal nuncio's announcement in september, , of galileo's abjuration and sentence, in marked contrast to the reply sent by the neighboring university of douay. the latter body, in a letter signed by matthæus kellison (sept. , ), declared the condemned theory "should be discarded and hissed from the schools; and that in the english college there in douay, this paradox never had been approved and never would be, but had always been opposed and always would be."[ ] [footnote : doc. in favaro: - , , .] this opposition in the universities in belgium continued throughout the century to be based not so much on scientific grounds as upon the bible. this may be seen in the manuscript reports of lectures in physics and astronomy given at liège in , and at louvain between - , though one of these does not mention the decree of .[ ] the general congregation of the society of jesus in drew up a list of the propositions proscribed in their teaching, though, according to m. monchamp (himself a catholic) not thereby implying a denial of any probability they might have. the th proposition ran: "terra movetur motu diurno; planetæ, tanquam viventia, moventur ab intrinseco. firmamentum stat."[ ] the jesuit astronomer tacquet in his textbook (antwerp, ) respected this decision, acknowledging that no scientific reason kept him from defending the theory, but solely his respect for the christian faith.[ ] [footnote : monchamp: , .] [footnote : ibid: - .] [footnote : ibid: - .] one of the pupils of the jesuits revolted however. martin van welden, appointed professor of mathematics at louvain in , debated a series of theses in january, . the second read: "indubitum est systhema copernici de planetarum motu circa sole; inter quos merito terra censetur." his refusal to alter the wording except to change _indubitum_ to _certum_ brought on a stormy controversy within the faculty which eventually reached the council of brabant and the papal nuncio at brussels.[ ] the professor finally submitted, though he was not forbidden to teach the copernician system, nor did the faculty affirm its falsity, merely that it was contrary to the roman decree. the professor re-opened the matter with a similar thesis in july, thereby arousing a second controversy that this time reached even the privy council. once more he submitted, but solely with an apology for having caused a disagreement. his new theses in contained no explicit mention of the copernician system; at least he had learned tact.[ ] [footnote : ibid: - .] [footnote : monchamp: .] the absorption of the german states in the thirty years war may account for the apparent absence there of copernican discussion until after the peace of westphalia. a certain georgius ludovicus agricola gave a syllogistic refutation of the doctrine as his disputation at the university of wittenberg in . while he acknowledged its ingenuity, he preferred to it "the noblest, truest, and divinely inspired system" of tycho brahe. the four requirements of an acceptable astronomical hypothesis according to this student are: ( ) that it suit all the observations of all the ages; ( ) that as far as possible, it be simple and clear; ( ) that it be not contrary to the principles of physics and optics; ( ) that it be not contrary to the holy scriptures. as the copernican theory does not meet all these tests, it is unsatisfactory. incidentally, he considers it "ridiculous to include the earth among the planets, because then we would be living in heaven, forsooth, since we would be in a star." he decides finally "that the decree of march, , condemning the copernican opinion was not unjust, nor was galileo unfairly treated."[ ] [footnote : agricola: _disputatio_.] two years later appeared a textbook at nürnberg, by a jesuit father, based on the twelfth century sacrobosco treatise and without a single reference so far as i could find, to copernicus![ ] another publication of the same year was a good deal more up to date. this was a kind of catechism in german by johann-henrich voight[ ] explaining for the common people various scientific and mathematical problems in a hundred questions and answers. he himself, a royal swedish astronomer, obviously preferred the tychonic system, but he left his reader free to choose between that and the copernican one, both of which he carefully explained.[ ] he made an interesting summary in parallel columns of the arguments for and against the earth's motion which it seems worth while to repeat as an instance of what the common people were taught: reasons for asserting the earth is motionless: . david in psalm : god has founded the earth and it shall not be moved. . joshua bade the sun stand still--which would not be notable were it not already at rest. . the earth is the heaviest element, therefore it more probably is at rest. . everything loose on the earth seeks its rest on the earth, why should not the whole earth itself be at rest? . we always see half of the heavens and the fixed stars also in a great half circle, which we could not see if the earth moved, and especially if it declined to the north and south.... . a stone or an arrow shot straight up falls straight down. but if the earth turned under it, from west to east, it must fall west of its starting point. . in such revolutions houses and towers would fall in heaps. . high and low tide could not exist; the flying of birds and the swimming of fish would be hindered and all would be in a state of dizziness. reasons for the belief that the earth is moved: . the sun, the most excellent, the greatest and the midmost star, rightly stands still like a king while all the other stars with the earth swing round it. . that you believe that the heavens revolve is due to ocular deception similar to that of a man on a ship leaving shore. . that joshua bade the sun stand still moses wrote for the people in accordance with the popular misconception. . as the planets are each a special created thing in the heavens, so the earth is a similar creation and similarly revolves. . the sun fitly rests at the center as the heart does in the middle of the human body. . since the earth has in itself its especial _centrum_, a stone or an arrow falls freely out of the air again to its own _centrum_ as do all earthly things. . the earth can move five miles in a second more readily than the sun can go forty miles in the same time. and similarly on both sides.[ ] [footnote : schotto: _organum mathematicum_ ( ).] [footnote : voight: _der kunstgünstigen einfalt mathematischer raritäten erstes hundert_. (hamburg, ).] [footnote : voight: _op. cit._: .] [footnote : ibid: - .] another writer preferring the tychonic scheme was longomontanus, whose _astronomica danica_ (amsterdam, ) upheld this theory because it "obviates the absurdities of the copernican hypothesis and most aptly corresponds to celestial appearances," and also because it is "midway between that and the ptolemaic one."[ ] even though he speaks of the "apparent motion of the sun," he attributed diurnal motion to the heavens, and believed the earth was at the center of the universe because ( ), from the account of the creation, the heaven and the earth were first created, and what could be more likely than that the heavens should fill the space between the center (the earth) and the circumference? ( ) and because of the incredibly enormous interval between the sphere of the fixed stars and the earth necessitated by copernican doctrine.[ ] [footnote : longomontanus: _op. cit._: .] [footnote : longomontanus: _op. cit._: .] the high-water mark of opposition after galileo's condemnation was reached in the _almagestum novum_ (bologna, ) by father riccioli of the society of jesus. it was the authoritative answer of that order, the leaders of the church in matters of education, to the challenges of the literary world for a justification of the condemnation of the copernican doctrine and of galileo for upholding it. father riccioli had been professor of philosophy and of mathematics for six years and of theology for ten when by order of his superiors, he was released from his lectureship to prepare a book containing all the material he could gather together on this great controversy of the age.[ ] he wrote it as he himself said, as "an _apologia_ for the sacred congregation of the cardinals who officially pronounced these condemnations, not so much because i thought such great height and eminence needed this at my hands but especially in behalf of catholics; also out of the love of truth to which every non-catholic, even, should be persuaded and from a certain notable zeal and eagerness for the preservation of the sacred scriptures intact and unimpaired; and lastly because of that reverence and devotion which i owe from my particular position toward the holy, catholic and apostolic church."[ ] [footnote : riccioli: _alm. nov._: præfatio, i, xviii.] [footnote : riccioli: _alm. nov._: ii, .] this monumental work, the most important literary production of the society in the th century,[ ] is abundant witness to riccioli's remarkable erudition and industry. nearly one-fifth of the total bulk of the two huge volumes is devoted to a statement of the copernican controversy. this is prefaced by a brief account of his own theory of the universe--the invention of which is another proof of the ability of the man--for his scientific training prevented his acceptance of the aristotelian-ptolemaic theory in the light of galileo's discoveries; his position as a jesuit and a faithful son of the church precluded him from adopting the system condemned by its representatives; and tycho brahe's scheme was not wholly to his liking. therefor he proposed an adaptation of the last-named, more in accordance, as he thought, with the facts.[ ] where tycho had all the planets except the earth and the moon encircle the sun, and that in turn, together with the moon and the sphere of the fixed stars, sweep around the earth as the center of the universe, riccioli made only mars, mercury and venus encircle the sun,--mars with an orbit the radius of which included the earth within its sweep, the other two planets with orbital radii shorter than that of the sun, and so excluding the earth. this he did, ( ) because both jupiter and saturn have their own kingdoms in the heavens, and mars, mercury and venus are but satellites of the sun; ( ) because there are greater varieties of eccentricity among these three than the other two; ( ) because saturn and jupiter are the greatest planets and with the sphere of the fixed stars move more slowly; ( ) mars belongs with the sun because of their related movements; and ( ) because it is likely that one of the planets would have much in common both with saturn and jupiter and with mercury and venus also.[ ] [footnote : _cath. ency._: "riccioli," and walsh: catholic churchmen in science: . ( nd series, .)] [footnote : riccioli: _alm. nov._: ii, - ; see frontispiece.] [footnote : riccioli: _alm. nov._: ii, - ; see frontispiece.] then he takes up the attack upon the copernican doctrine. m. delambre summarizes and comments upon of his arguments against it,[ ] and riccioli himself claims[ ] to have stated " new arguments in behalf of copernicus and against him." but these sound somewhat familiar to the reader of anti-copernican literature: as, for instance, "which is more natural, straight or circular movement?" or, the copernican argument that movement is easier if the object moved is smaller involves a matter of faith since it implies a question of god's power; for to god all is alike, there is no hard nor easy.[ ] although diurnal movement is useful to the earth alone and so, according to the copernicans, the earth should have the labor of it, riccioli argues that everything was created for man; let the stars revolve around him. the sun may be nobler than the earth, but man is nobler than the sun.[ ] if the earth's movement were admitted, ptolemy's defense would be broken down through the elimination of the epicycles of the superior planets: here, if ever, the copernicans appear to score, as riccioli himself admits,[ ] but he calls to his aid tycho brahe and the bible. "to invoke such aids is to avow his defeat" is m. delambre's comment at this point.[ ] there are many more arguments, of which the foregoing are but instances chosen more or less at random; but no one of them is of especial weight or novelty. [footnote : delambre: _astr. mod._: i, - .] [footnote : riccioli: _apologia_: .] [footnote : riccioli: _alm. nov._: ii, , .] [footnote : riccioli: _alm. nov._: ii, - .] [footnote : ibid: ii, - .] [footnote : delambre: _op. cit._: i, .] to strengthen his case, riccioli listed the supporters of the heliocentric doctrine throughout the ages, with those of the opposite view. if a man's fame adds to the weight of his opinion, the modern reader will be inclined to think the copernicans have the best of it, for omitting the ancients, most of those opposing it are obscure men.[ ] [footnote : ibid: i, .] in favor of the copernican doctrine [references omitted].[ ] copernicus rheticus mæstlin kepler rothman galileo gilbert (diurnal motion) foscarini didacus stunica (_sic_) ismael bullialdus jacob lansberg peter herigonus gassendi,--"but submits his intellect captive to the church decrees." descartes "inclines to this belief." a.l. politianus bruno against the hypothesis of the earth's movement. aristotle ptolemy theon the alexandrine regiomontanus alfraganus macrobius cleomedes petrus aliacensis george buchanan maurolycus clavius barocius michael neander telesius martinengus justus-lipsius scheiner tycho tasso scipio claramontius michael incofer fromundus jacob ascarisius julius cæsar la galla tanner bartholomæus amicus antonio rocce marinus mersennius polacco kircher spinella pineda lorinis mastrius bellutris poncius delphinus elephantutius [footnote : riccioli: _alm. nov._: ii, .] riccioli nevertheless viewed the copernican system with much sympathy. after a full statement of it, he comments: "we have not yet exhausted the full profundities of the copernican hypothesis, for the deeper one digs into it, the more ingenious and valuable subtilties may one unearth." then he adds that "the greatness of copernicus has never been sufficiently appreciated nor will it be,--that man who accomplished what no astronomer before him had scarcely been able even to suggest without an insane machinery of spheres, for by a triple motion of the earth he abolished epicycles and eccentrics. what before so many atlases could not support, this one hercules has dared to carry. would that he had kept himself within the limits of his hypothesis!"[ ] [footnote : riccioli: _op. cit._: ii, , .] his conclusions seem to show that only his position as a jesuit restrained him from being a copernican himself.[ ] "i. if the celestial phenomena alone are considered, they are equally well explained by the two hypotheses [ptolemaic and copernican]. ii. the physical evidence as explained in the two systems with exception of percussion and the speed of bodies driven north or south, and east or west, is all for immobility. iii. one might waver indifferently between the two hypotheses aside from the witness of the scriptures, which settles the question. iv. there are in addition plenty of other motives besides scriptural ones for rejecting this system." (!) but with the scriptural evidence he adduces the decree of the index under paul v against the doctrine, and the sentence of galileo, so that "the sole possible conclusion is that the earth stands by nature immobile in the center of the universe, and the sun moves around it with both a diurnal and an annual motion."[ ] [footnote : delambre: _astr. mod._: i, .] [footnote : riccioli: _op. cit._: ii, (condensed), .] even this great book was as insufficient to stop the criticism of the action of the congregations, as it was to stop the spread of the doctrine. so once again the father took up the cudgels in defense of the church. the full title of his _apologia_ runs: "an apologia in behalf of an argument from physical mathematics against the copernican system, directed against that system by a new argument from the reflex motion of falling weights." (venice, ). he states in this that his _almagestum novum_ had received the approbation of professors of mathematics at bologna, of one at pisa, and of another at padua, and he quotes the conclusion from _nicetas orthodoxus_ ("a diatribe by julius turrinus, doctor of mathematics, philosophy, medicine, law, and greek letters"): "that the sun is revolved by diurnal and by annual motion, and that the earth is at rest i firmly hold, infallibly believe, and openly confess, not because of mathematical reasons, but solely at the command of faith, by the authority of the scriptures, and the nod of approval (_nutu_) of the roman see, whose rules laid down at the dictation of the spirit of truth, may i, as befits everyone, uphold as law."[ ] [footnote : riccioli: _apologia_: .] riccioli further on proceeds to answer his objecters, declaring that "the church did not decide _ex cathedra_ that the scripture concerning movement should be interpreted literally; that the censure was laid by qualified theologians and approved by eminent cardinals, and was not merely provisional, nor for the time being absolute, since the contrary could never be demonstrated; and that while it was the primary intent of the inquisitors to condemn the opinion as heretical and directly contrary to the scriptures ... they added that it was absurd and false also in philosophy, in order, not to avert any objections which could be on the side of philosophy or astronomy, but only lest any one should say that scripture is opposed to philosophy."[ ] these answers are indicative of the type of criticism with which the church had to cope even at that time.[ ] [footnote : ibid: .] [footnote : one bit of contemporary opinion on riccioli and his work has come down to us. a canon at liège, réné-françois sluse, wrote asking a friend (about ) to sound wallis, the english mathematician, as to his opinion of the _almagestum novum_, and of this argument based on the acceleration of movement in falling bodies. wallis himself replied that he thought the argument devoid of all value. the canon at once wrote, "i do not understand how a man as intelligent as riccioli should think he could bring to a close a matter so difficult [the refutation] by a proof as futile as this." monchamp: - . for a full, annotated list of books published against the copernican system between - , see martin: _galilée_: - .] chapter iv. the gradual acceptance of the copernican system. just as tycho brahe's system proved to be for some a good half-way station between the improbable ptolemaic and the heretical copernican system;[ ] so the cartesian philosophy helped others to reconcile their scientific knowledge with their reverence for the scriptures, until newton's work had more fully demonstrated the scientific truth. [footnote : see moxon: _advice, a tutor to astronomy and geography_ ( ): .] its originator, réné descartes[ ] ( - ) was in holland when word of galileo's condemnation reached him in , as he was seeking in the bookshops of amsterdam and leyden for a copy of the _dialogo_.[ ] he at once became alarmed lest he too be accused of trying to establish the movement of the earth, a doctrine which he had understood was then publicly taught even in rome, and which he had made the basis of his own philosophy. if this doctrine were condemned as false, then his philosophy must be also; and, true to his training by the jesuits, rather than go against the church he would not publish his books. he set aside his _cosmos_, and delayed the publication of the _méthode_ for some years in consequence, even starting to translate it into latin as a safeguard.[ ] his conception of the universe, the copernican one modified to meet the requirements of a literally interpreted bible, was not printed until , when it appeared in his _principes_.[ ] [footnote : haldane's _descartes_ ( ) is the most recent and authoritative account based upon descartes's works as published in the adams-tannery edition (paris, . foll.). this edition supersedes that of cousin. [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] [footnote : haldane: .] [footnote : ibid: .] [footnote : descartes: _principes_, pt. iii, chap. .] according to this statement which he made only as a possible explanation of the phenomena and not as an absolute truth, while there was little to choose between the tychonic and the copernican conceptions, he inclined slightly toward the former. he conceived of the earth and the other planets as each borne along in its enveloping heaven like a ship by the tide, or like a man asleep on a ship that was sailing from calais to dover. the earth itself does not move, but it is transported so that its position is changed in relation to the other planets but not visibly so in relation to the fixed stars because of the vast intervening spaces. the laws of the universe affect even the most minute particle, and all alike are swept along in a series of vortices, or whirlpools, of greater or less size. thus the whole planetary system sweeps around the sun in one great vortex, as the satellites sweep around their respective planets in lesser ones. in this way descartes worked out a mechanical explanation of the universe of considerable importance because it was a rational one which anyone could understand. its defects were many, to be sure, as for example, that it did not allow for the elliptical orbits of the planets;[ ] and one critic has claimed that this theory of a motionless earth borne along by an enveloping heaven was comparable to a worm in a dutch cheese sent from amsterdam to batavia,--the worm has travelled about leagues but without changing its place![ ] but this theory fulfilled descartes's aim: to show that the universe was governed by mechanical laws of which we can be absolutely certain and that galileo's discoveries simply indicated this.[ ] [footnote : haldane: .] [footnote : monchamp: , note.] [footnote : haldane: .] this exposition of the copernican doctrine strongly appealed to the literary world of the th and th centuries in western europe, especially in the netherlands, in the paris salons and in the universities.[ ] m. monchamp cites a number of contemporary comments upon its spread, in one of which it is claimed that in , the university of louvain had for the preceding forty years been practically composed of cartesians.[ ] for the time being, this theory was a more or less satisfactory explanation of the universe according to known laws; it answered to galileo's observations; it was in harmony with the scriptures, and its vortices paved the way for the popular acceptance of newton's law of universal gravitation. [footnote : ibid: , .] [footnote : monchamp: - .] protestant england was of course little disturbed by the decree against the copernican doctrine, a fact that makes it possible, perhaps, to see there more clearly the change in people's attitude from antagonism to acceptance, than in catholic europe where fear of the church's power, and respect for its decisions inhibited honest public expression of thought and conviction. while in england also the literal interpretation of the scriptures continued to be with the common people a strong objection against the doctrine, the rationalist movement of the late seventeenth and eighteenth centuries along with newton's great work, helped win acceptance for it among the better educated classes. bruno had failed to win over his english hearers, and in when the _de magnete_ was published, william gilbert, ( - ) was apparently the only supporter of the earth's movement then in england,[ ] and he advocated the diurnal motion only.[ ] not many, however, were as outspoken as bacon in denunciation of the system; they were simply somewhat ironically indifferent. an exception to this was dean wren of windsor (father of the famous architect). he could not speak strongly enough against it in his marginal notes on browne's _pseudodoxia epidemica_. as dr. johnson wrote,[ ] sir thomas browne ( - ) himself in his zeal for the old errors, did not easily admit new positions, for he never mentioned the motion of the earth but with contempt and ridicule. this was not enough for the dean, who wrote in the margin of browne's book, at such a passage,[ ] that there were "eighty-odd expresse places in the bible affirming in plaine and overt terms the naturall and perpetuall motion of sun and moon" and that "a man should be affrighted to follow that audacious and pernicious suggestion which satan used, and thereby undid us all in our first parents, that god hath a double meaning in his commands, in effect condemning god of amphibologye. and all this boldness and overweaning having no other ground but a seeming argument of some phenomena forsooth, which notwithstanding we know the learned tycho, prince of astronomers, who lived fifty-two years since copernicus, hath by admirable and matchlesse instruments and many yeares exact observations proved to bee noe better than a dreame." [footnote : berry quotes (p. ) a passage from thomas digges (d. ) with the date : "but in this our age, one rare witte (seeing the continuall errors that from time to time more and more continually have been discovered, besides the infinite absurdities in their theoricks, which they have been forced to admit that would not confess any mobility in the ball of the earth) hath by long studye, paynfull practise, and rare invention delivered a new theorick or model of the world, shewing that the earth resteth not in the center of the whole world or globe of elements, which encircled or enclosed in the moone's orbit, and together with the whole globe of mortality is carried round about the sunne, which like a king in the middst of all, rayneth and giveth laws of motion to all the rest, sphærically dispersing his glorious beames of light through all this sacred celestiall temple." browne also refers to digges (i, ).] [footnote : gilbert: _de magnete_, bk. vi, c. - ( - ).] [footnote : johnson: _life_, in browne: i, xvii.] [footnote : browne: i, .] richard [transcriber's note: robert] burton ( - ) in _the anatomy of melancholy_ speaks of the doctrine as a "prodigious tenent, or paradox," lately revived by "copernicus, brunus and some others," and calls copernicus in consequence the successor of atlas.[ ] the vast extent of the heavens that this supposition requires, he considers "quite opposite to reason, to natural philosophy, and all out as absurd as disproportional, (so some will) as prodigious, as that of the sun's swift motion of the heavens." if the earth is a planet, then other planets may be inhabited (as christian huygens argued later on); and this involves a possible plurality of worlds. burton laughs at those who, to avoid the church attitude and yet explain the celestial phenomena, invent new hypotheses and new systems of the world, "correcting others, doing worse themselves, reforming some and marring all," as he says of roeslin's endeavors. "in the meantime the world is tossed in a blanket amongst them; they hoyse the earth up and down like a ball, make it stand and goe at their pleasure."[ ] he himself was indifferent. [footnote : burton: _anatomy of melancholy_, i, ; i, . first edition, ; reprinted , , , , - , , .] [footnote : ibid: i, , .] others more sensitive to the implications of this system, might exclaim with george herbert ( - ):[ ] "although there were some fourtie heav'ns, or more, sometimes i peere above them all; sometimes i hardly reach a score, sometimes to hell i fall. "o rack me not to such a vast extent, those distances belong to thee. the world's too little for thy tent, a grave too big for me." [footnote : herbert: ii, .] or they might waver, undecided, like milton who had the archangel answer adam's questions thus:[ ] "but whether thus these things, or whether not, whether the sun predominant in heaven rise on the earth, or earth rise on the sun, hee from the east his flaming robe begin, or shee from west her silent course advance with inoffensive pace that spinning sleeps on her soft axle, while she paces ev'n and bears thee soft with the smooth air along, solicit not thy thoughts with matters hid, leave them to god above, him serve and feare; of other creatures, as him pleases best, wherever plac't, let him dispose; joy thou in what he gives to thee, this paradise and the fair eve: heaven is for thee too high to know what passes there: be lowlie wise." ( ) [footnote : milton: _paradise lost_, bk. viii, lines _et seq._ the great puritan divine, john owen ( - ), accepts the miracle of the sun's standing still without a word of reference to the new astronomy. (_works_: ii, .) farrar states that owen declared newton's discoveries were against the evident testimonies of scripture (farrar: _history of interpretation_: xviii.), but i have been unable to verify this statement. owen died before the _principia_ was published in .] whewell thinks[ ] that at this time the diffusion of the copernican system was due more to the writings of bishop wilkins than to those of any one else, for their very extravagances drew stronger attention to it. the first, "the discovery of a new world: or a discourse tending to prove that there may be another habitable world in the moon," appeared in ; and a third edition was issued only two years later together with the second book; "discourse concerning a new planet--that 'tis probable our earth is one of the planets." in this latter, the bishop stated certain propositions as indubitable; among these were, that the scriptural passages intimating diurnal motion of the sun or of the heavens are fairly capable of another interpretation; that there is no sufficient reason to prove the earth incapable of those motions which copernicus ascribes to it; that it is more probable the earth does move than the heavens, and that this hypothesis is exactly agreeable to common appearances.[ ] and these books appeared when political and constitutional matters, and not astronomical ones, were the burning questions of the day in england. [footnote : whewell: i, .] [footnote : wilkins: _discourse concerning a new planet_.] the spread of the doctrine was also helped by thomas salusbury's translations of the books and passages condemned by the index in and . this collection, "intended for gentlemen," he published by popular subscription immediately after the restoration,[ ] a fact that indicates that not merely mathematicians (whom whewell claims[ ] were by that time all decided copernicans) but the general public were interested and awake.[ ] [footnote : salusbury: _math. coll._: to the reader.] [footnote : whewell: i, .] [footnote : one london bookseller in advertised for sale "spheres according to the ptolmean, tychonean and copernican systems with books for their use." (moxon: .) in in london appeared the third edition of gassendi's _institutio_, the textbook of astronomy in the universities during this period of uncertainty. it too wavers between the tychonic and the copernican systems.] the appearance of newton's _principia_ in with his statement of the universal application of the law of gravitation, soon ended hesitancy for most people. twelve years later, john keill, ( - ), the scotch mathematician and astronomer at oxford, refuted descartes's theory of vortices and opened the first course of lectures delivered at oxford on the new newtonian philosophy.[ ] not only were his lectures thronged, but his books advocating the copernican system in full[ ] went through several editions in relatively few years. [footnote : _dict. of nat. biog._: "keill."] [footnote : keill: _introductio ad veram astronomiam_.] in the colonies, yale university which had hitherto been using gassendi's textbook, adopted the newtonian ideas a few years later, partly through the gift to the university of some books by sir isaac himself, and partly through the enthusiasm of two young instructors there, johnson and brown, who in - widened the mathematical course by including the new theories.[ ] the text they used was by rohault, a cartesian, edited by samuel clarke with critical notes exposing the fallacies of cartesianism. this "disguised newtonian treatise" was used at yale till . the university of pennsylvania used this same text book even later.[ ] [footnote : cajori: - .] [footnote : cajori: .] in pope ( - ) refers to "our copernican system,"[ ] and addison ( - ) in the _spectator_ (july , ) writes this very modern passage: "but among this set of writers, there are none who more gratify and enlarge the imagination, than the authors of the new philosophy, whether we consider their theories of the earth or heavens, the discoveries they have made by glasses, or any other of their contemplations on nature.... but when we survey the whole earth at once, and the several planets that lie within its neighborhood, we are filled with a pleasing astonishment, to see so many worlds hanging one above another, and sliding around their axles in such an amazing pomp and solemnity. if, after this, we contemplate those wide fields of æther, that reach in height as far as from saturn to the fixed stars, and run abroad almost to an infinitude, our imagination finds its capacity filled with so immense a prospect, as puts it upon the stretch to comprehend it. but if we yet rise higher, and consider the fixed stars as so many vast oceans of flame, that are each of them attended with a different set of planets, and still discover new firmaments and new lights, that are sunk farther in those unfathomable depths of æther, so as not to be seen by the strongest of our telescopes, we are lost in such a labyrinth of suns and worlds, and confounded with the immensity and magnificence of nature. "nothing is more pleasant to the fancy, than to enlarge itself by degrees, in its contemplation of the various proportions which its several objects bear to each other, when it compares the body of man to the bulk of the whole earth, the earth to the circle it describes round the sun, that circle to the sphere of the fixed stars, the sphere of the fixed stars to the circuit of the whole creation, the whole creation itself to the infinite space that is everywhere diffused around it; ... but if, after all this, we take the least particle of these animal spirits, and consider its capacity wrought into a world, that shall contain within those narrow dimensions a heaven and earth, stars and planets, and every different species of living creatures, in the same analogy and proportion they bear to each other in our own universe; such a speculation, by reason of its nicety, appears ridiculous to those who have not turned their thoughts that way, though, at the same time, it is founded on no less than the evidence of a demonstration."[ ] [footnote : pope: _works_, vi, .] [footnote : addison: _spectator_, no. , (iv, - ). an interesting contrast to this passage and a good illustration of how the traditional phraseology continued in poetry is found in addison's famous hymn, written a year later: "whilst all the stars that round her [earth] burn and all the planets in their turn, confirm the tidings as they roll, and spread the truth from pole to pole. "what though in solemn silence all move round this dark terrestrial ball; what though no real voice nor sound amidst their radiant orbs be found; "in reason's ear they all rejoice, and utter forth a glorious voice; forever singing, as they shine, 'the hand that made us is divine'."] a little later, cotton mather declared ( ) that the "copernican hypothesis is now generally preferred," and "that there is no objection against the motion of the earth but what has had a full solution."[ ] soon the semi-popular scientific books took up the newtonian astronomy. one such was described as "useful for all sea-faring men, as well as gentlemen, and others."[ ] "newtonianisme pour les dames" was advertised in france in the forties.[ ] by when pope wrote the _universal prayer_: "yet not to earth's contracted span thy goodness let me bound or think thee lord alone of man, when thousand worlds are round," the copernican-newtonian astronomy had become a commonplace to most well-educated people in england. to be sure, the great john wesley ( ) considered the systems of the universe merely "ingenious conjectures," but then, he doubted whether "more than probabilities we shall ever attain in regard to things at so great a distance from us."[ ] [footnote : mather: _christian philosopher_, , .] [footnote : leadbetter: _astronomy_ ( ).] [footnote : in de maupertius: _ouvrages divers_, (at the back).] [footnote : wesley: _compendium of natural philosophy_, i, , .] the old phraseology, however, did recur occasionally, especially in poetry and in hymns. for instance, a hymnal (preface dated ) contains such choice selections as: "before the pondr'ous earthly globe in fluid air was stay'd, before the ocean's mighty springs their liquid stores display'd"-- and: "who led his blest unerring hand or lent his needful aid when on its strong unshaken base the pondr'ous earth was laid?"[ ] [footnote : dobell: _hymns_, no. , no. .] but too much importance should not be attributed to such passages; though poetry and astronomy need not conflict, as keble illustrated:[ ] "ye stars that round the sun of righteousness in glorious order roll...." [footnote : keble: _christian year_, .] by the middle of the th century in england, one could say with horne "that the newtonian system had been in possession of the chair for some years;"[ ] but it had not yet convinced the common people, for as pike wrote in , "many common christians to this day firmly believe that the earth really stands still and that the sun moves all round the earth once a day: neither can they be easily persuaded out of this opinion, because they look upon themselves bound to believe what the scripture asserts."[ ] [footnote : horne: _fair, candid, impartial statement ..._, .] [footnote : pike: _philosophia sacra_, .] there was, however, just at this time a little group of thinkers who objected to newton's scheme, "because of the endless uninterrupted flux of matter from the sun in light, an expense which should destroy that orb."[ ] these hutchinsonians conceived of light as pure ether in motion springing forth from the sun, growing more dense the further it goes till it becomes air, and, striking the circumference of the universe (which is perhaps an immovable solid), is thrown back toward the sun and melted into light again. its force as its tides of motion strike the earth and the other planets produces their constant gyrations.[ ] men like duncan forbes, lord president of the court of sessions, and george horne, president of magdalen college, oxford, as a weapon against rationalism, favored this notion that had been expounded by john hutchinson ( - ) in his _moses's principia_ ( ).[ ] they were also strongly attracted by the scriptural symbolism with which the book abounds. leslie stephen summarizes their doctrines as ( ) extreme dislike for rationalism, ( ) a fanatical respect for the letter of the bible, and ( ) an attempt to enlist the rising powers of scientific enquiry upon the side of orthodoxy.[ ] this "little eddy of thought"[ ] was not of much influence even at that time, but it has a certain interest as indicating the positions men have taken when on the defensive against new ideas. [footnote : forbes: _letter_, ( ).] [footnote : see wesley: i, - .] [footnote : _dict. of nat. biog._: "hutchinson."] [footnote : stephen: _hist. of eng. thought_: i, .] [footnote : ibid: .] chapter v. the church and the new astronomy: conclusion. astronomical thought on the continent was more hampered, in the catholic countries especially, by the restrictive opinions of the church. yet in , when the decree prohibiting all books dealing with the copernican doctrine was removed from the index, that system had already long been adopted by the more celebrated academies of europe, for so mme. de premontval claimed in ; and it was then reaching out to non-scientific readers, through simple accounts for "ladies and others not well versed in these somewhat technical matters."[ ] the great landmark in the development of the doctrine was the publication of newton's _principia_ in , though its effect in europe was of course slower in being felt than it was in england. newton's work and that of the astronomers immediately following him was influential except where the church's prohibitions still held sway. [footnote : de premontval: _le méchaniste philosophe_, , . (the hague, ).] during this period, the books published in free holland were more outspoken in their radical acceptance or in their uncertainty of the truth than were those published in the catholic countries. christian huygens's treatises on the plurality of worlds not only fully accepted the copernican doctrine, but like those of bishop wilkins in england, deduced therefrom the probability that the other planets are inhabited even as the earth is. a writer[ ] on the sphere in stated the different theories of the universe so that his readers might choose the one that to them appeared the most probable. he himself preferred the cartesian explanation as the simplest and most convenient of all, "though it should be held merely as an hypothesis and not as in absolute agreement with the truth." pierre bayle[ ] also explained the different systems, but appears himself to waver between the copernican and the tychonic conceptions. he used, however, the old word "perigee" (nearness to the earth) rather than the newtonian "perihelion" (nearness to the sun). his objections to the copernican doctrine have a familiar ring: it is contrary to the evidence of the senses; a stone would not fall back to its starting-place, nor could a bird return to her nest; the earth would not be equidistant from the horizon and the two poles; and lastly it is contrary to the scriptures. only a few years later, however, de maupertius wrote that no one at that day ( ) doubted any longer the motion of the earth around its axis, and he believed with newton that the laws of gravity applied to the universe as well as to the earth. then he proceeded to explain the copernican system which he favored on the ground of its greater probability.[ ] [footnote : de brisbar: _calendrier historique_, (leyden), - .] [footnote : bayle: _système abregé de philosophie_ (the hague, ), iv, - .] [footnote : de maupertius: _eléments de géographie_, xv, - .] even in , mme. de premontval thought it wiser to publish in holland her little life of her father, _le méchaniste philosophe_. this jean piegeon, she claimed, was the first man in france to make spheres according to the copernican system. an orphan, he was educated by a priest; then took up carpentry and mechanics. when he tried to make a celestial sphere according to the ptolemaic system, he became convinced of its falsity because of its complexities. therefore he plunged into a study of the new system which he adopted. his first copernican sphere was exhibited before louis xiv at versailles in and was bought by the king and presented to the académie des sciences.[ ] the second was taken to canada by one of the royal officials. public interest in his work was keen; even peter the great, who was then in paris, visited his workroom.[ ] m. piegeon also wrote a book on the copernican system.[ ] [footnote : de premontval: .] [footnote : ibid: .] [footnote : ibid: .] it seems, however, as though m. piegeon were slightly in advance of his age, or more daring, perhaps, than his contemporaries, for there was almost no outspoken support of the copernican system at this time in france. even cassini of the french académie des sciences did not explicitly support it, though he spoke favorably of it and remarked that recent observations had demonstrated the revolutions of each planet around the sun in accordance with that supposition.[ ] but the great orator, bossuet, ( - ), clung to the ptolemaic conception as alone orthodox, and scriptural.[ ] abbé fénelon ( - ) writing on the existence of god, asked: "who is it who has hung up this motionless ball of the earth; who has placed the foundations for it," and "who has taught the sun to turn ceasely [transcriber's note: ceaselessly] and regularly in spaces where nothing troubles it?"[ ] and a writer on the history of the heavens as treated by poets, philosophers and moses ( ), tells gassendi, descartes and many other great thinkers that their ideas of the heavens are proved vain and false by daily experience as well as by the account of creation; for the most enlightened experience is wholly and completely in accord with the account of moses. this book was written, the author said, for young people students of philosophy and the humanities, also for teachers.[ ] [footnote : cassini: _de l'origine et du progrès ..._, .] [footnote : shields: . i have failed to find this reference in bossuet's works.] [footnote : fénelon: _oeuvres_, i, and .] [footnote : pluche: _histoire du ciel_: viii, ix, xiii.] the jesuit order, still a power in europe in the early th century, was bound to the support of the traditional view, which led them into some curious positions in connection with the discoveries made in astronomy during this period. thus the famous jesuit astronomer boscovich ( - ) published in rome in a study of the ellipticity of the orbits of planets which necessitated the use of the copernican position; he stated he had assumed it as true merely to facilitate his labors. in the second edition ( ) published some years after the removal from the index of the decree against books teaching the copernican doctrine (at his instigation, it is claimed),[ ] he added a note to this passage asking the reader to remember the time and the place of its former publication.[ ] just at the end of the preceding century, one of the seminary fathers at liège maintained that were the earth to move, being made up of so many and divers combustible materials, it would soon burst into flames and be reduced to ashes![ ] [footnote : _cath. ency._: "boscovich."] [footnote : _opera_: iii ( ).] [footnote : cited in monchamp: note.] during the th century at louvain the copernican doctrine was warmly supported, but as a theory. a ms. of a course given there in has come down to us, in which the professor, while affirming its hypothetical character, described it as a simple, clear and satisfactory explanation of the phenomena, then answered all the objections made against it by theologians, physicists, and astronomers.[ ] a few years earlier, ( ) a jesuit at liège, though well acquainted with newton's work, declared: "for my part i do not doubt the least in the world that the earth is eternally fixed, for god has founded the terrestrial globe, and it will not be shaken."[ ] another priest stated in the first chapter of his astronomy that the sun and the planets daily revolve around the earth; then later on, he explained the copernican and the tychonic schemes and the cartesian theory of motion with evident sympathy.[ ] two others, one a jesuit in at naples,[ ] the other in at verona, frankly preferred the tychonic system, and the latter called the system found by "tommaso copernico" a mere fancy.[ ] still another priest, evidently well acquainted with bradley's work, as late as in declared that there was nothing decisive on either side of the great controversy between the systems.[ ] at this time, however, a father was teaching the copernican system at liège without differentiating between thesis and hypothesis.[ ] and a jesuit, while he denied ( ) universal gravitation, the earth's movement, and the plurality of inhabited worlds, declared that the roman congregation had done wrong in charging these as heretical suggestions. in fact, m. monchamp, himself a catholic priest at louvain, declared that the newtonian proofs were considered by many in the th century virtually to abrogate the condemnation of and ; hence the professors of the seminary at liège had adopted the copernican system.[ ] [footnote : ibid: .] [footnote : ibid: .] [footnote : fontana: _institutio_, ii, - .] [footnote : ferramosca: _positiones ..._: .] [footnote : piccoli: _la scienza_, , .] [footnote : spagnio, _de motu_, .] [footnote : monchamp: .] [footnote : monchamp: .] the famous french astronomer lalande, in rome in when the inquisition first modified its position, tried to persuade the authorities to remove galileo's book also from the index; but his efforts were unavailing, because of the sentence declared against its author.[ ] in canon settele was not allowed by the master of the sacred palace to publish his textbook because it dealt with the forbidden subject. his appeal to the congregation itself resulted, as we have seen, in the decree of removing this as a cause for prohibition. yet as late as in , when a statue to copernicus was being unveiled at warsaw, and a great convocation had met in the church for the celebration of the mass as part of the ceremony, at the last moment the clergy refused in a body to attend a service in honor of a man whose book was on the index.[ ] [footnote : bailly: ii, , note.] [footnote : flammarion: - .] thus the roman catholic church by reason of its organization and of its doctrine requiring obedience to its authority was more conspicuous for its opposition as a body to the copernican doctrine, even though as individuals many of its members favored the new system. but the protestant leaders were quite as emphatic in their denunciations, though less influential because of the protestant idea of the right to individual belief and interpretation. luther, melancthon, calvin, turrettin,[ ] owen, and wesley are some of the notable opponents to it. and when the scientific objections had practically disappeared, those who interpreted the scriptures literally were still troubled and hesitant down to the present day. not many years ago, people flocked to hear a negro preacher of the south, brother jasper, uphold with all his ability that the sun stood still at joshua's command, and that today "the sun do move!" far more surprising is this statement in the new _catholic encyclopedia_ under "faith," written by an english dominican: "if, now, the will moves the intellect to consider some debatable point--_e.g._, the copernican and ptolemaic theories of the relationship between the sun and the earth--it is clear that the intellect can only assent to one of these views in proportion that it is convinced that the particular view is true. but neither view has, as far as we can know, more than probable truth, hence of itself the intellect can only give in its partial adherence to one of these views, it must always be precluded from absolute assent by the possibility that the other may be right. the fact that men hold more tenaciously to one of these than the arguments warrant can only be due to some extrinsic consideration, _e.g._, that it is absurd not to hold to what a vast majority of men hold." [footnote : shields: .] in astronomical thought as in many another field, science and reason have had a hard struggle in men's minds to defeat tradition and the weight of verbal inspiration. within the roman catholic church opposition to this doctrine was officially weakened in , but not completely ended till the publication of the index in --the first edition since the decrees of and which did not contain the works of copernicus, galileo, foscarini, à stunica and kepler. since then, roman catholic writers have been particularly active in defending and explaining the positions of the church in these matters. they have not agreed among themselves as to whether the infallibility of the church had been involved in these condemnations, nor as to the reasons for them. as one writer has summarized these diverse positions,[ ] they first claimed that galileo was condemned not for upholding a heresy, but for attempting to reconcile these ideas with the scriptures,--though in fact he was sentenced specifically for heresy. in their next defense they declared galileo was not condemned for heresy, but for contumacy and want of respect to the pope.[ ] this statement proving untenable, others held that it was the result of a persecution developing out of a quarrel between aristotelian professors and those professors who favored experiment,--a still worse argument for the church itself. then some claimed that the condemnation was merely provisional,--a position hardly warranted by the wording of the decrees themselves and flatly contradicted by father riccioli, the spokesman of the jesuit authorities.[ ] more recently, roman catholics have held that galileo was no more a victim of the roman church than of the protestant--which fails to remove the blame of either. the most recent position is that the condemnation of the doctrine by the popes was not as popes but as men simply, and the church was not committed to their decision since the popes had not signed the decrees. but two noted english catholics, roberts and mivart, publicly stated in that the infallibility of the papacy was fully committed in these condemnations by what they termed incontrovertible evidence.[ ] [footnote : white: i, - .] [footnote : see di bruno: _catholic belief_, a.] [footnote : riccioli: _apologia_, .] [footnote : white: i, . see the answer by wegg-prosser: _galileo and his judges_.] one present-day catholic calls the action of the congregations "a theoretical mistake;"[ ] another admits it was a deplorable mistake, but practically their only serious one;[ ] and a third considers it "providential" since it proved conclusively "that whenever there is apparent contradiction between the truths of science and the truths of faith, either the scientist is declaring as proved what in reality is a mere hypothesis, or the theologian is putting forth his own personal views instead of the teaching of the gospel."[ ] few would accept today, however, the opinion of the anonymous writer in the _dublin review_ in the forties that "to the pontiffs and dignitaries of rome we are mainly indebted for the copernican system" and that the phrases "heretical" and "heresy" in the sentence of were but the _stylus curiæ_, for it was termed heresy only in the technical sense.[ ] [footnote : donat: .] [footnote : walsh: _popes and science_, .] [footnote : conway: .] [footnote : anon.: _galileo--the roman congregation_, , .] the majority of protestants, with the possible exception of the lutherans, were satisfied with the probable truth of the copernican doctrine before the end of the th century. down to the present day, however, there have been isolated protests raised against it, usually on technical grounds supported by reference to the scriptures. de morgan refers to one such, "an inquiry into the copernican system ... wherein it is proved in the clearest manner, that the earth has only her diurnal motion ... with an attempt to point out the only true way whereby mankind can receive any real benefit from the study of the heavenly bodies, by john cunningham, london, ." de morgan adds that "the true way appears to be the treatment of heaven and earth as emblematical of the trinity."[ ] another, by "anglo-american," is entitled "copernicus refuted; or the true solar system" (baltimore, ). it begins thus: "one of these must go, the other stand still, it matters not which, so choose at your will; but when you find one already stuck fast, you've only got hobson's choice left at last." [footnote : de morgan: i, .] this writer admits the earth's axial rotation, but declares the earth is fixed as a pivot in the center of the universe, because the poles of the earth are fixed and immovable, and that the sun as in the tychonic scheme encircles the earth and is itself encircled by five planets.[ ] his account of the origin of the copernican system is noteworthy: it was originated by pythagoras and his deciples but lay neglected because it was held to be untenable in their time; it was "revived when learning was at its lowest ebb by a monk in his cloister, copernicus, who in ransacking the contents of the monastery happened to lay his hands on the ms. and then published it to the world with all its blunders and imperfections!"[ ] one might remark that the anglo-american's own learning was at very low ebb. [footnote : "anglo-american": - .] [footnote : ibid: .] the tychonic scheme was revived also some years later by a dane, zytphen ( ).[ ] three years after, an assembly of lutheran clergy met together at berlin to protest against "science falsely so-called,"[ ] but were brought into ridicule by pastor knap's denunciations of the copernican theory as absolutely incompatible with belief in the bible. a carl schoepffer had taken up the defense of the tychonic scheme in berlin before this ( ) and by his lecture was in its seventh edition. in it he sought to prove that the earth revolves neither upon its own axis nor yet about the sun. he had seen foucault's pendulum demonstration of the earth's movement, but he held that something else, as yet unexplained, caused the deviation of the pendulum, and that the velocity of the heavens would be no more amazing than the almost incredible velocity of light or of electricity.[ ] his lecture, curiously enough, fell into the hands of the late general john watts de peyster of new york, who had it translated and published in together with a supplement by frank allaben.[ ] both these gentlemen accepted its scientific views and deductions, but the general refused to go as far as his colleague in the latter's enthusiastic acceptance of the verbal inspiration of the scriptures as a result of these statements.[ ] a few months later, they published a supplementary pamphlet claiming to prove the possibility of the sun's velocity by the analogy of the velocity of certain comets.[ ] a professor j.r. lange of california (a german), attracted by these documents, sent them his own lucubrations on this subject. he considered newton's doctrine of universal attraction "nonsense," and had "absolute proof" in the fixity of the pole star that the earth does not move.[ ] in a letter to general de peyster, he wrote: "let us hope and pray that the days of the pernicious copernican system may be numbered,"[ ]--but he did not specify why he considered it pernicious. the general was nearly eighty years old when he became interested in these matters, and he did not live long thereafter to defend his position. his biographers make no mention of it. the other men seem almost obsessed, especially lange;--like the italian painter, sindico, who bombarded the director of the paris observatory in with many letters protesting against the copernican system.[ ] [footnote : de morgan: ii, .] [footnote : white: i, .] [footnote : schoepffer: _the earth stands fast_, title-page, - .] [footnote : ibid: supplement by allaben, , .] [footnote : ibid: note by j.w. de p., .] [footnote : de peyster and allaben: _algol_, preface.] [footnote : lange: _the copernican system: the greatest absurdity in the history of human thought_.] [footnote : de peyster and allaben: _algol_, .] [footnote : sindico: _refutation du système de copernic...._] german writers, whether lutherans or not, appear to have opposed the system more often in the last century than have the writers of other nationalities. besides those already mentioned, one proposed an ingenious scheme in which the sun moves through space followed by the planets as a comet is by its tail, the planets revolving in a plane perpendicular to that of the sun's path. a diagram of it would be cone-shaped. he included in this pamphlet, besides a list of his own books, (all published in leipsic), a list of twenty-six titles from to , books and pamphlets evidently opposed in whole or in part to the modern astronomy, and seventeen of these were in german or printed in germany.[ ] in this country at st. louis was issued an _astronomische unterredung_ ( ) by j.c.w.l.; according to the late president white, a bitter attack on modern astronomy and a decision by the scriptures that the earth is the principal body of the universe, that it stands fixed, and that the sun and the moon only serve to light it.[ ] [footnote : tischner: _le système solaire se mouvant_. ( ).] [footnote : white: i, .] such statements are futile in themselves nowadays, and are valuable only to illustrate the advance of modern thought of which these are the little eddies. while modern astronomers know far more than copernicus even dreamed of, much of his work still holds true today. the world was slow to accept his system because of tradition, authority, so-called common sense, and its supposed incompatibility with scriptural passages. catholic and protestant alike opposed it on these grounds; but because of its organization and authority, the roman catholic church had far greater power and could more successfully hinder and delay its acceptance than could the protestants. consequently the system won favor slowly at first through the indifference of the authorities, then later in spite of their active antagonism. scholars believed it long before the universities were permitted to teach it; and the rationalist movement of the th century, the revolt against a superstitious religion, helped to overturn the age-old conception of the heavens and to bring newtonian-copernicanism into general acceptance. the elements of this traditional conception are summarized in the fifth book of bodin's _universæ naturæ theatrum_, a scholar's account of astronomy at the close of the sixteenth century.[ ] man in his terrestrial habitation occupies the center of a universe created solely to serve him, god presides over all from the empyrean above, sending forth his messengers the angels to guide and control the heavenly bodies. such had been the thought of christians for more than a thousand years. then came the influence of a new science. tycho brahe "broke the crystal spheres of aristotle"[ ] by his study of the comet of ; galileo's telescopes revealed many stars hitherto unknown, and partly solved the mysteries of the milky way; kepler's laws explained the courses of the planets, and newton's discovery of the universal application of the forces of attraction relieved the angels of their duties among the heavens. thinkers like bruno proposed the possibility of other systems and universes besides the solar one in which the earth belongs. and thus not only did man shrink in importance in his own eyes; but his conception of the heavens changed from that of a finite place inexplicably controlled by the mystical beings of a supernatural world, to one of vast and infinite spaces traversed by bodies whose density and mass a man could calculate, whose movements he could foretell, and whose very substance he could analyze by the science of today. this dissolution of superstition, especially in regard to comets was notably rapid and complete after the comet of .[ ] thus the rationalist movement with the new science opened men's minds to a universe composed of familiar substances and controlled by known or knowable laws with no tinge remaining of the supernatural. today a man's theological beliefs are not shaken by the discovery of a new satellite or even a new planet, and the appearance of a new comet merely provides the newspaper editor with the subject of a passing jest. [footnote : see translated sections in appendix c.] [footnote : robinson: .] [footnote : ibid: .] yet it was fully one hundred and fifty years after the publication of the _de revolutionibus_ before its system met with the general approval of scholars as well as of mathematicians; then nearly a generation more had to elapse before it was openly taught even at oxford where the roman catholic and lutheran churches had no control. during the latter part of this period, readers were often left free to decide for themselves as to the relative merits of the tychonic and copernican or copernican-cartesian schemes. but it took fully fifty years and more, besides, before these ideas had won general acceptance by the common people, so wedded were they to the traditional view through custom and a superstitious reverence for the bible. briefly then, the _de revolutionibus_ appeared in ; and quietly won some supporters, notably bruno, kepler and galileo; the congregations of the index specifically opposed it in and ; however it continued to spread among scholars and others with the aid of cartesianism for another fifty years till the appearance of newton's _principia_ in . then its acceptance rapidly became general even in catholic europe, till it was almost a commonplace in england by , two hundred years after its first formal promulgation, and had become strong enough in europe to cause the congregations in to modify their stand. thereafter opposition became a curiosity rather than a significant fact. only the roman church officially delayed its recognition of the new astronomy till the absurdity of its obsolete position was brought home to it by canon settele's appeal in . fifteen years later the last trace of official condemnation was removed, a little over two hundred years after the decrees had first been issued, and just before bessel's discovery of stellar parallax at length answered one of the strongest and oldest arguments against the system. since then have come many _apologias_ in explanation and extenuation of the church's decided stand in this matter for so many generations. though galileo himself was forced to his knees, unable to withstand his antagonists, his work lived on after him; he and copernicus, together with kepler and newton stand out both as scientists and as leaders in the advance of intellectual enlightenment. the account of their work and that of their less well-known supporters, compared with that of their antagonists, proves the truth of the ancient greek saying which rheticus used as the motto for the _narratio prima_, the first widely known account of the copernican system: "one who intends to philosophize must be free in mind." appendix a. ptolemy: _syntaxis mathematica (almagest)_ "that the earth has no movement of rotation," in _opera quæ exstant omnia_, edidit heiberg, leipsic, , bk. i, sec. : (i, - ); compared with the translation into french by halma, paris, . by proofs similar to the preceding, it is shown that the earth cannot be transported obliquely nor can it be moved away from the center. for, if that were so, all those things would take place which would happen if it occupied any other point than that of the center. it seems unnecessary to me, therefore, to seek out the cause of attraction towards the center when it is once evident from the phenomena themselves, that the earth occupies the center of the universe and that all heavy bodies are borne towards it; and this will be readily understood if it is remembered that the earth has been demonstrated to have a spherical shape, and according to what we have said, is placed at the center of the universe, for the direction of the fall of heavy bodies (i speak of their own motions) is always and everywhere perpendicular to an uncurved plane drawn tangent to the point of intersection. obviously these bodies would all meet at the center if they were not stopped by the surface, since a straight line drawn to the center is perpendicular to a plane tangent to the sphere at that point. those who consider it a paradox that a mass like the earth is supported on nothing, yet not moved at all, appear to me to argue according to the preconceptions they get from what they see happening to small bodies about them, and not according to what is characteristic of the universe as a whole, and this is the cause of their mistake. for i think that such a thing would not have seemed wonderful to them any longer if they had perceived that the earth, great as it is, is merely a point in comparison to the surrounding body of the heaven. they would find that it is possible for the earth, being infinitely small relative to the universe, to be held in check and fixed by the forces exercised over it equally and following similar directions by the universe, which is infinitely great and composed of similar parts. there is neither up nor down in the universe, for that cannot be imagined in a sphere. as to the bodies which it encloses, by a consequence of their nature it happens that those that are light and subtle are as though blown by the wind to the outside and to the circumference, and seem to appear to us to go _up_, because that is how we speak of the space above our heads that envelops us. it happens on the other hand that heavy bodies and those composed of dense parts are drawn towards the middle as towards a center, and appear to us to fall _down_, because that it is the word we apply to what is beneath our feet in the direction of the center of the earth. but one should believe that they are checked around this center by the retarding effect of shock and of friction. it would be admitted then that the entire mass of the earth, which is considerable in comparison to the bodies falling on it, could receive these in their fall without acquiring the slightest motion from the shock of their weight or of their velocity. but if the earth had a movement which was common to it and to all other heavy bodies, it would soon seemingly outstrip them as a result of its weight, thus leaving the animals and the other heavy bodies without other support than the air, and would soon touch the limits of the heaven itself. all these consequences would seem most ridiculous if one were only even imagining them. there are those who, while they admit these arguments because there is nothing to oppose them, pretend that nothing prevents the supposition, for instance, that if the sky is motionless, the earth might turn on its axis from west to east, making this revolution once a day or in a very little less time, or that, if they both turn, it is around the same axis, as we have said, and in a manner conformable to the relations between them which we have observed. it has escaped these people that in regard to the appearances of the planets themselves, nothing perhaps prevents the earth from having the simpler motion; but they do not realize how very ridiculous their opinion is in view of what takes place around us and in the air. for if we grant them that the lightest things and those composed of the subtlest parts do not move, which would be contrary to nature, while those that are in the air move visibly more swiftly than those that are terrestrial; if we grant them that the most solid and heavy bodies have a swift, steady movement of their own, though it is true however that they obey impelling forces only with difficulty; they would be obliged to admit that the earth by its revolution has a movement more rapid than the movements taking place around it, since it would make so great a circuit in so short a time. thus the bodies which do not rest on it would appear always to have a motion contrary to its own, and neither the clouds, nor any missile or flying bird would appear to go towards the east, for the earth would always outstrip them in this direction, and would anticipate them by its own movement towards the east, with the result that all the rest would appear to move backwards towards the west. if they should say that the atmosphere is carried along by the earth with the same speed as the earth's own revolution, it would be no less true that the bodies contained therein would not have the same velocity. or if they were swept along with the air, no longer would anything seem to precede or to follow, but all would always appear stationary, and neither in flight nor in throwing would any ever advance or retreat. that is, however, what we see happening, since neither the retardation nor the acceleration of anything is traceable to the movement of the earth. appendix b. "to his holiness, paul iii, supreme pontiff, preface by nicholas copernicus to his books on revolutions." (a translation of the _præfatio_ in copernicus: _de revolutionibus_; pp. - .) "i can certainly well believe, most holy father, that, while mayhap a few will accept this my book which i have written concerning the revolutions of the spheres of the world, ascribing certain motions to the sphere of the earth, people will clamor that i ought to be cast out at once for such an opinion. nor are my ideas so pleasing to me that i will not carefully weigh what others decide concerning them. and although i know that the meditations of philosophers are remote from the opinions of the unlearned, because it is their aim to seek truth in all things so far as it is permitted by god to the human reason, nevertheless i think that opinions wholly alien to the right ought to be driven out. thus when i considered with myself what an absurd fairy-tale people brought up in the opinion, sanctioned by many ages, that the earth is motionless in the midst of the heaven, as if it were the center of it, would think it if i were to assert on the contrary that the earth is moved; i hesitated long whether i would give to the light my commentaries composed in proof of this motion, or whether it would indeed be more satisfactory to follow the example of the pythagoreans and various others who were wont to pass down the mysteries of philosophy not by books, but from hand to hand only to their friends and relatives, as the letter of lysis to hipparchus proves.[ ] but verily they seemed to me not to have done this, as some think, from any dislike to spreading their teachings, but lest the most beautiful things and those investigated with much earnestness by great men, should be despised by those to whom spending good work on any book is a trouble unless they make profit by it; or if they are incited to the liberal study of philosophy by the exhortations and the example of others, yet because of the stupidity of their wits they are no more busily engaged among philosophers than drones among bees. when therefore i had pondered these matters, the scorn which was to be feared on account of the novelty and the absurdity of the opinion impelled me for that reason to set aside entirely the book already drawn up. [footnote : see prowe: _nic. cop._: iii, - .] "but friends, in truth, have brought me forth into the light again, though i long hesitated and am still reluctant; among these the foremost was nicholas schönberg, cardinal of capua, celebrated in all fields of scholarship. next to him is that scholar, my very good friend, tiedeman giese, bishop of culm, most learned in all sacred matters, (as he is), and in all good sciences. he has repeatedly urged me and, sometimes even with censure, implored me to publish this book and to suffer it to see the light at last, as it has lain hidden by me not for nine years alone, but also into the fourth 'novenium'. not a few other scholars of eminence also pleaded with me, exhorting me that i should no longer refuse to contribute my book to the common service of mathematicians on account of an imagined dread. they said that however absurd in many ways this my doctrine of the earth's motion might now appear, so much the greater would be the admiration and goodwill after people had seen by the publications of my commentaries the mists of absurdities rolled away by the most lucid demonstrations. brought to this hope, therefore, by these pleaders, i at last permitted my friends, as they had long besought me, to publish this work. "but perhaps your holiness will not be so shocked that i have dared to bring forth into the light these my lucubrations, having spent so much work in elaborating them, that i did not hesitate even to commit to a book my conclusions about the earth's motion, but that you will particularly wish to hear from me how it came into my mind to dare to imagine any motion of the earth, contrary to the accepted opinion of mathematicians and in like manner contrary to common sense. so i do not wish to conceal from your holiness that nothing else moved me to consider some other explanation for the motions of the spheres of the universe than what i knew, namely that mathematicians did not agree among themselves in their examinations of these things. for in the first place, they are so completely undecided concerning the motion of the sun and of the moon that they could not observe and prove the constant length of the great year.[ ] next, in determining the motions of both these and the five other planets, they did not use the same principles and assumptions or even the same demonstrations of the appearances of revolutions and motions. for some used only homocentric circles; others, eccentrics and epicycles, which on being questioned about, they themselves did not fully comprehend. for those who put their trust in homocentrics, although they proved that other diverse motions could be derived from these, nevertheless they could by no means decide on any thing certain which in the least corresponded to the phenomena. but these who devised eccentrics, even though they seem for the most part to have represented apparent motions by a number [of eccentrics] suitable to them, yet in the meantime they have admitted quite a few which appear to contravene the first principles of equality of motion. another notable thing, that there is a definite symmetry between the form of the universe and its parts, they could not devise or construct from these; but it is with them as if a man should take from different places, hands, feet, a head and other members, in the best way possible indeed, but in no way comparable to a single body, and in no respect corresponding to each other, so that a monster rather than a man would be constructed from them. thus in the process of proof, which they call a system, they are found to have passed over some essential, or to have admitted some thing both strange and scarcely relevant. this would have been least likely to have happened to them if they had followed definite principles. for if the hypotheses they assumed were not fallacious, everything which followed out of them would have been verified beyond a doubt. however obscure may be what i now say, nevertheless in its own place it will be made more clear. [footnote : _i.e._, the , solar years in which all the heavenly bodies complete their circuits and return to their original positions.] "when therefore i had long considered this uncertainty of traditional mathematics, it began to weary me that no more definite explanation of the movement of the world machine established in our behalf by the best and most systematic builder of all, existed among the philosophers who had studied so exactly in other respects the minutest details in regard to the sphere. wherefore i took upon myself the task of re-reading the books of all the philosophers which i could obtain, to seek out whether any one had ever conjectured that the motions of the spheres of the universe were other than they supposed who taught mathematics in the schools. and i found first that, according to cicero, nicetas had thought the earth was moved. then later i discovered according to plutarch that certain others had held the same opinion; and in order that this passage may be available to all, i wish to write it down here: "but while some say the earth stands still, philolaus the pythagorean held that it is moved about the element of fire in an oblique circle, after the same manner of motion that the sun and moon have. heraclides of pontus and ecphantus the pythagorean assign a motion to the earth, not progressive, but after the manner of a wheel being carried on its own axis. thus the earth, they say, turns itself upon its own center from west to east."[ ] [footnote : plutarch: _moralia: de placitis philosophorum_, lib. iii, c. (v. ).] when from this, therefore, i had conceived its possibility i myself also began to meditate upon the mobility of the earth. and although the opinion seemed absurd, yet because i knew the liberty had been accorded to others before me of imagining whatsoever circles they pleased to explain the phenomena of the stars, i thought i also might readily be allowed to experiment whether, by supposing the earth to have some motion, stronger demonstrations than those of the others could be found as to the revolution of the celestial sphere. thus, supposing these motions which i attribute to the earth later on in this book, i found at length by much and long observation, that if the motions of the other planets were added to the rotation of the earth and calculated as for the revolution of that planet, not only the phenomena of the others followed from this, but also it so bound together both the order and magnitude of all the planets and the spheres and the heaven itself, that in no single part could one thing be altered without confusion among the other parts and in all the universe. hence, for this reason, in the course of this work i have followed this system, so that in the first book i describe all the positions of the spheres together with the motions i attribute to the earth; thus this book contains a kind of general disposition of the universe. then in the remaining books, i bring together the motions of the other planets and all the spheres with the mobility of the earth, so that it can thence be inferred to what extent the motions and appearances of the other planets and spheres can be solved by attributing motion to the earth. nor do i doubt that skilled and scholarly mathematicians will agree with me if, what philosophy requires from the beginning, they will examine and judge, not casually but deeply, what i have gathered together in this book to prove these things. in order that learned and unlearned may alike see that in no way whatsoever i evade judgment, i prefer to dedicate these my lucubrations to your holiness rather than to any one else; especially because even in this very remote corner of the earth in which i live, you are held so very eminent by reason of the dignity of your position and also for your love of all letters and of mathematics that, by your authority and your decision, you can easily suppress the malicious attacks of calumniators, even though proverbially there is no remedy against the attacks of sycophants. [illustration: a photographic facsimile (reduced) of a page from mulier's edition ( ) as "corrected" according to the _monitum_ of the congregations in . the first writer merely underlined the passage with marginal comment that this was to be deleted by ecclesiastical order. the second writer scratched out the passage and referred to the second volume of riccioli's _almagestum novum_ for the text of the order. the earlier writer was probably the librarian of the florentine convent from which this book came, and wrote this soon after . the later writer did his work after , when riccioli's book was published. this copy of the _de revolutionibus_ is now in the dartmouth college library.] if perchance there should be foolish speakers who, together with those ignorant of all mathematics, will take it upon themselves to decide concerning these things, and because of some place in the scriptures wickedly distorted to their purpose, should dare to assail this my work, they are of no importance to me, to such an extent do i despise their judgment as rash. for it is not unknown that lactantius, the writer celebrated in other ways but very little in mathematics, spoke somewhat childishly of the shape of the earth when he derided those who declared the earth had the shape of a ball.[ ] so it ought not to surprise students if such should laugh at us also. mathematics is written for mathematicians to whom these our labors, if i am not mistaken, will appear to contribute something even to the ecclesiastical state the headship of which your holiness now occupies. for it is not so long ago under leo x when the question arose in the lateran council about correcting the ecclesiastical calendar. it was left unsettled then for this reason alone, that the length of the year and of the months and the movements of the sun and moon had not been satisfactorily determined. from that time on, i have turned my attention to the more accurate observation of these, at the suggestion of that most celebrated scholar, father paul, a bishop from rome, who was the leader then in that matter. what, however, i may have achieved in this, i leave to the decision of your holiness especially, and to all other learned mathematicians. and lest i seem to your holiness to promise more about the value of this work than i can perform, i now pass on to the undertaking. [footnote : these two sentences the congregations in ordered struck out, as part of their "corrections."] appendix c. the drama of universal nature: in which are considered the efficient causes and the ends of all things, discussed in a connected series of five books, by jean bodin, (frankfort, ). _book v_: on the celestial bodies: their number, movement, size, harmony and distances compared with themselves and with the earth. sections and (in part) and (entire). bodin, jean: _universæ naturæ theatrum in quo rerum omnium effectrices causa et fines contemplantur, et continuæ series quinque libris discutiuntur_. frankfort, . book v translated into english by the writer and compared with the french translation by françois de fougerolles, (lyons, ). _section _: on the definition and the number of the spheres. mystagogue: ... now to prove that the heavens have a nature endowed with intelligence i need no other argument than that by which theophrastus and alexander prove they are living, for, they say, if the heavens did not have intelligence, they would be greatly inferior in dignity and excellence to men. that is why aben-ezra,[ ] having interpreted the hebrew of these two words of the psalm: "the heavens declare," has written that the phrase _sapperim_ (declare) in the judgment of all hebrews is appropriate to such great intelligence. also he who said "when the morning stars sang together and shouted for joy,"[ ] indicated a power endowed with intelligence, as did the master of wisdom[ ] also when he said that god created the heavens with intelligence. [footnote : as rabbi david testified on the th psalm [these footnotes are by bodin].] [footnote : job: .] [footnote : proverbs.] theodore. i have learned in the schools that the spheres are not moved of themselves but that they have separate intelligences who incite them to movement. myst. that is the doctrine of aristotle. but theophrastus and alexander,[ ] (when they teach that the spheres are animated bodies) explain adequately that the spheres are agitated by their own coëssential soul. for if the sky were turned by an intelligence external to it, its movement would be accidental with the result that it, and the stars with it, would not be moved otherwise, than as a body without soul. but accidental motion is violent. and nothing violent in nature can be of long duration. on the contrary there is nothing of longer duration, nor more constant, than the movement of the heavens. [footnote : metaphysics: ii. c. , de coelo. i. c. .] theo. what do you call fixed stars? myst. celestial beings who are gifted with intelligence and with light, and who are in continual motion. this is sufficiently indicated by the words of daniel[ ] when he wrote, that the souls of those who have walked justly in this life, and who have brought men back to the path of virtue, all have their seat and dwelling (like the gleaming stars) among the heavens. by these words one can plainly understand the essence and figure of the angels as well as of the celestial beings; for while other beings have their places in this universe assigned to them for their habitation, as the fish the sea, the cattle the fields, and the wild beasts the mountains and forests, even as origen,[ ] eusebius, and diodorus say, so the stars are assigned positions in the heavens. this can also be understood by the curtains of the tabernacle which moses, the great lawgiver, had ornamented with the images of cherubim showing that the heavens were indicated by the angelic faces of the stars. while st. augustine,[ ] jerome,[ ] thomas aquinas[ ] and scotus most fitly called this universe a being, nevertheless albertus, damascenus, and thomas aquinas deny that the heavenly bodies are animated. but thomas aquinas shows himself in this inconsistent and contradictory, for he confesses that spiritual substances are united with the heavenly bodies, which could not be unless they were united in the same hypostasis of an animated body. if this body is animated, it must necessarily be living and either rational or irrational. if, on the other hand, this spiritual substance does not make the same hypostasis with the celestial body, it will necessarily be that the movement of the sky is accidental, as coming from the mover outside to the thing moved, no more nor less than the movement of a wheel comes from the one who turns it: as this is absurd, what follows from it is necessarily absurd also. [footnote : in his last chapter.] [footnote : which is confirmed by pico of mirandola: heptaplus: bk. v.] [footnote : enchiridion: cap. ; gen.: and .] [footnote : on psalm: audite coeli.] [footnote : summa: pt. , art. , ques. .] theo. how many spheres are there? myst. it is difficult to determine their number because of the variety of opinions among the authorities, each differing from the other, and because of the inadequacy of the proofs of such things. for eudoxus has stated that the spheres with their deferents are not more than three and twenty in number. calippus has put it at thirty, and aristotle[ ] at forty-seven, which alexander aphrodisiensis[ ] has amended by adding to it two more on the advice of sosigenes. ptolemy holds that there are celestial spheres not including the bodies of the planets. johan regiomontanus says , an opinion which is followed by nearly all, because in the time of ptolemy they did not yet know that the eighth sphere and all the succeeding ones are carried around by the movement of the trepidation. thus he held that the moon has five orbits, mercury six, venus, mars, jupiter, and saturn each four, aside from the bodies of the planets themselves, for beyond these are still the spheres and deferents of the eighth and ninth spheres. but copernicus, reviving eudoxus' idea, held that the earth moved around the motionless sun; and he has also removed the epicycles with the result that he has greatly reduced their number, so that one can scarcely find eight spheres remaining. [footnote : metaphy. xii.] [footnote : in his commentaries on book xii of metaph. where he gives the opinion of calippus and eudoxus.] theo. what should one do with such a variety of opinions? myst. have recourse to the sacred fountain of the hebrews to search out the mysteries of a thing so deeply hidden from man; for from them we may obtain an absolutely certain decision. the tabernacle which the great lawgiver moses ordered to be made[ ] was like the archetype of the universe, with its ten curtains placed around it each decorated with the figures of cherubim thus representing the ten heavens with the beauty of their resplendent stars. and even though aben-ezra did not know of the movement of trepidation, nevertheless he interpreted this passage, "the heavens are the work of thy fingers" as indicating the number of the ten celestial spheres. the pythagoreans seem also to have agreed upon the same number since, besides the earth and the eight heavens, they imagine a sphere anticthon because they did not then clearly understand the celestial movements. they thought however, all should be embraced in the tenth. [footnote : ex. xviii and following. philo judæus in the allegories.] theo. the authority of such writers has indeed so great weight with me that i place their statements far in advance of the arguments of all others. nevertheless if it can be done, i should wish to have this illustrated and confirmed by argument in order to satisfy those who believe nothing except on absolute proof. myst. it can indeed be proved that there are ten mobile spheres in which the fiery bodies accomplish their regular courses. yet by these arguments that ultimate, motionless sphere which embraces and encircles all from our terrestial abode to its circumference within its crystalline self, encompassing plainly the utmost shores and limits of the universe, cannot be proved. for as it has been shown before [in book i] the elemental world was inundated by celestial waters from above. nor can it apparently be included in the number of the spheres since (as we will point out later) as great a distance exists between it and the nearest sphere as between the ocean and the starry heaven. furthermore it has been said before that the essence of the spheres consists of fire and water which is not fitting for the celestial waters above. theo. by what arguments then can it be proved there are ten spheres? myst. the ancients knew well that there were the seven spheres of the planets, and an eighth sphere of the fixed stars which, down to the time of eudoxus and meto, they thought had but one simple movement. these men were the first who perceived by observation that the fixed stars were carried backward quite contrary to the movement of the primum mobile. after them came timochares, hipparchus, and menelaus, and later ptolemy, who confirmed these observations perceiving that the fixed stars (which people had hitherto thought were fixed in their places) had been separated from their station. for this reason they thought best to add a ninth sphere to the eight inferior ones. much later an arabian and a spanish king, mensor and alphonse, great students of the celestial sciences, in their observations noticed that the eighth sphere with the seven following moved in turning from the north to the east, then towards the south, and so to the west, finally returning to the north, and that such a movement was completed in years. this johannus regiomontanus, a franconian, has proved, with a skill hitherto equalled only by that of those who proved the ninth sphere, which travels from west to east. from this it is necessarily concluded that there are ten spheres. theo. why so? myst. because every natural body[ ] has but one movement which is its own by nature; all others are either voluntary or through violence, contrary to the nature of a mobile object; for just as a stone cannot of its own impulse ascend and descend, so one and the same sphere cannot of itself turn from the east to the west and from the west to the east and still less from the north to the south and south to north. [footnote : aristotle: metaph. ii and xii and de coelo i.] theo. what then? myst. it follows from this that the extremely rapid movement by which all the spheres are revolved in twenty-four hours, belongs to the primum mobile, which we call the tenth sphere, and which carries with it all the nine lesser spheres; that the second or planetary movement, that is, from west to east, is communicated to the lesser spheres and belongs to the ninth sphere; that the third movement, resembling a person staggering, belongs to the eighth sphere with which it affects the other lesser spheres and makes them stagger in a measure outside of the poles, axes and centres of the greater spheres. _section _: on the position of the universe according to its divisions. * * * * theo. does it not also concern physics to discuss those things that lie outside the universe? myst. if there were any natural body beyond the heavens, most assuredly it would concern physics, that is, the observer and student of nature. but in the book of origins,[ ] the master workman is said to have separated the waters and placed the firmament in between them. the hebrew philosophers declare that the crystalline sphere which ezekiel[ ] called the great crystal and upon which he saw god seated, as he wrote, is as far beyond the farthermost heaven as our ocean is far from that heaven, and that this orb is motionless and therefore is called god's throne. for "seat" implies quiet and tranquility which could be proper for none other than the one immobile and immutable god. this is far more probable and likely than aristotle's absurd idea, unworthy the name of a philosopher, by which he placed the eternal god in a moving heaven as if he were its source of motion and in such fashion that he was constrained of necessity to move it. we have already refuted this idea. it has also been shown that these celestial waters full of fertility and productiveness sometimes are spread abroad more widely and sometimes less so, as though obviously restrained, whence the heavens are said to be closed[ ] and roofed[ ] with clouds or that floods burst forth out of the heaven to inundate the earth. finally we read in the holy scriptures that the eternal god is seated upon the flood. [footnote : gen.: .] [footnote : chap. and . exod.: .] [footnote : i kings: . deut.: .] [footnote : psalm .] theo. why then are not eleven spheres counted? myst. because the crystalline sphere is said to have been separated from the inferior waters by the firmament, and it therefore cannot be called a heaven. furthermore motion is proper to all the heavens, but the crystalline one is stationary. that is why rabi akiba called[ ] it a marble counterpart of the universe. this also is signified in the construction of the altar which was covered with a pavilion in addition to its ten curtains for, as it is stated elsewhere,[ ] god covers the heavens with clouds, and the scriptures often make mention of the waters beyond the heavens.[ ] there are those, however, who teach that the hebrew word _scamajim_ may be applied only to a dual number, so that they take it to mean the crystalline sphere and the starry one. but i think those words in solomon's speech[ ] "the heaven of heaven, and the heavens of the heavens" refer in the singular to the crystalline sphere, in the plural to the ten lesser spheres. [footnote : according to maymon: perplexorum, iii.] [footnote : psalm .] [footnote : psalm . gen. and .] [footnote : also in psalm and .] theo. it does not seem so marvelous to me that an aqueous or crystalline sphere exists beyond the ten spheres, as that it is as far beyond the furthermost sphere as the ocean is far this side of it, that is, as astrologists teach, terrestrial diameters. myst. it is written most plainly that the firmament holds the middle place between the two waters. therefore god is called[ ] in hebrew _helion_, the sun, that is, the most high, and under his feet the heaven is spread like a crystal,[ ] although he is neither excluded nor included in any part of the universe, it is however consistent with his majesty to be above all the spheres and to fill heaven and earth with his infinite power as isaiah[ ] indicated when he writes: "his train filled the temple;" it is the purest and simplest act, the others are brought about by forces and powers. he alone is incorporeal, others are corporeal or joined to bodies. he alone is eternal, others according to their nature are transitory and fleeting unless they are strengthened by the creator's might; wherefore the chaldean interpreter is seen everywhere to have used the words, majesty, glory or power in place of the presence of god. [footnote : psalm .] [footnote : exod. . ezek. , .] [footnote : isa. .] theo. nevertheless so vast and limitless a space must be filled with air or fire, since there are no spheres there, nor will nature suffer any vacuum. myst. if then the firmament occupies the middle position between the two waters, then by this hypothesis you must admit that the space beyond the spheres is empty of elemental and celestial bodies; otherwise you would have to admit that the last sphere extends on even to the crystalline orb, which can in no way be reconciled with the holy scriptures and still less with reason because of the incredible velocity of this sphere. therefore it is far more probable that this space is filled with angels. theo. is there some medium between god and the angels which shares in the nature of both? myst. what is incorporeal and indivisible cannot communicate any part of its essence to another; for if a creature had any part of the divine essence, it would be all god, since god neither has parts nor can be divided, therefore he must be separated from all corporeal contact or intermixture. _section _: on guardian angels. theo. what then in corporeal nature is closest to god? myst. the two seraphim, who stand near the eternal creator,[ ] and who are said to have six wings, two wherewith to fly, the others to cover head and feet. by this is signified the admirable swiftness with which they fulfill his commands, yet head and feet are veiled for so the purpose of their origin and its earliest beginning are not known to us. also they have eyes scattered in all parts of their bodies to indicate that nothing is hidden from them. and they also pour oil for lighting through a funnel into the seven-branched candlestick; that is, strength and power are poured forth by the creator to the seven planets, so that we should turn from created things to the worship and love of the creator. [footnote : isa. . ezek. and . zach. . exod. , .] theo. since nothing is more fitting for the divine goodness than to create, to generate, and to pile up good things for all, whence comes the destruction of the world and the ruin of all created things? myst. it is true plato and aristotle attributed the cause of all ills to the imperfection of matter in which they thought was some _kakopoion_,[ ] but that is absurd since it is distinctly written: all that god had made was good, or as the hebrews express it, beautiful,--so evil is nothing-else than the absence[ ] or privation of good. [footnote : maleficium quidam, _i.e._, some evil-power. job .] [footnote : augustine against faustus wrote that vanity is not produced from the dust, nor evil from the earth.] theo. can not wicked angels be defined without privation since they are corporeal essences? myst. anything that exists is said to be good and to be a participant by its existence in the divine goodness; and even as in a well regulated republic, executioners, lictors, and corpse-bearers are no less necessary than magistrates, judges and overseers; so in the republic of this world, for the generation, management and guardianship of things god has gathered together angels as leaders and directors for all the celestial places, for the elements, for living beings, for plants, for minerals, for states, provinces, families and individuals. and not only has he done this, but he has also assigned his servants, lictors, avengers and others to places where they may do nothing without his order, nor inflict any punishment upon wicked men unless the affair has been known fully and so decided. thus god is said[ ] to have made leviathan, which is the outflow of himself, that is, the natural rise and fall of all things. "i have created a killer,"[ ] he said, "to destroy," and so also behemoth, and the demons cleaving to him, which are often called ravens, eagles and lions, and which are said to beg their food of god, that is, the taking of vengeance upon the wicked whose punishment and death they feed upon as upon ordinary fare. from these, therefore, or rather from ourselves, come death, pestilence, famine, war and those things we call ills, and not from the author of all good things except by accident. for so god says of himself:[ ] "i am the god making good and creating evil, making light and creating darkness." for when he withdraws his spirit, evil follows the good; when he takes the light away, darkness is created; as when one removes the pillars of a building, the ruin of a house follows. if he takes the vital spark away, death follows; nor can he be said to do evil[ ] to anyone in taking back what is his own. [footnote : job and . isa. . ezek. .] [footnote : isa. .] [footnote : isa. .] [footnote : job .] theo. when the legislator asked him to disclose his face to his gaze, why did the architect of the universe and the author of all things reply: "my face is to be seen by no mortal man, but only my back?" myst. this fine allegory signifies that god cannot be known from superior or antecedent causes but from behind his back, that is, from results, for a little later he adds, "i will cover thine eyes with my hand." thus the hand signifies those works which he has placed before anyone's eyes, and it indicates that he places man not in an obscure corner but in the center of the universe so that he might better and more easily than in heaven contemplate the universe and all his works through the sight of which, as through spectacles, the sun, that is, god himself, may be disclosed. and therefore we undertook this disputation concerning nature and natural things, so that even if they are but slightly explained, nevertheless we may attain from this disquisition an imperfect knowledge of the creator and may break forth in his praises with all our might, that at length by degrees we may be borne on high and be blessed by the divine reward; for this is indeed the supreme and final good for a man. here endeth the drama of nature which jean bodin wrote while all france was aflame with civil war. finis appendix d. a translation of a letter by thomas feyens on the question: is it true that the heavens are moved and the earth is at rest? (february, ) (_thomæ fieni epistolica quæstio_: an verum sit, coelum moveri et terram quiescere? londini, .) to the eminent and noble scholars, tobias matthias and george gays: it is proved that the heavens are moved and the earth is stationary: first; by authority; for besides the fact that this is asserted by aristotle and ptolemy whom wellnigh all philosophers and mathematicians have followed by unanimous consent, except for copernicus, bernardus patricius[ ] and a very few others, the holy scriptures plainly attest it in at least two places which i have seen. in joshua,[ ] are the words: steteruntque sol et luna donec ulcisceretur gens de inimicis suis. and a little further on: stetit itaque sol in medio coeli, et non festinavit occumbere spatio unius diei, et non fuit antea et postea tam longa dies. the scriptures obviously refer by these words to the motion of the _primum mobile_ by which the sun and the moon are borne along in their diurnal course and the day is defined; and it indicates that the heavens are moved as well as the _primum mobile_. then ecclesiastes, chapter ,[ ] reads: generatio præterit, et generatio advenit, terra autem semper stat, oritur sol et occidit, et ad locum suum revertitur. [footnote : feyens probably refers here to francesco patrizzi, who was an enemy of the peripatetics and a great supporter of platonism. he died in at rome, where clement viii had conferred on him the chair of philosophy.] [footnote : joshua x: - .] [footnote : ecclesiastes i: .] secondly, it is proved by reason. all the heavens and stars were made in man's behalf and, with other terrestrial bodies, are the servants of man to warm, light, and vivify him. this they could not do unless in moving they applied themselves by turns to different parts of the world. and it is more likely that they would apply themselves by their own movement to man and the place in which man lives, than that man should come to them by the movement of his own seat or habitation. for they are the servants of man; man is not their servant; therefore it is more probable that the heavens are moved and the earth is at rest than that the reverse is true. thirdly; no probable argument can be thought out from philosophy to prove that the earth is moved and the heavens are at rest. nor can it be done by mathematics. by saying that the heavens are moved and the earth is at rest, all phenomena of the heavenly bodies can be solved. just as in the same way in optics all can be solved by saying either that sight comes from the thing to the eye, or that rays go from the eye to the thing seen; so is it in astronomy. therefore one ought rather to abide in the ancient and general opinion than in one received recently without justification. fourthly; the earth is the center of the universe; all the heavenly bodies are observed to be moved around it; therefore it itself ought to be motionless, for anything that moves, it seems, should move around or above something that is motionless. fifthly; if the earth is moved in a circle, either it moves that way naturally or by force, either by its own nature or by the nature of another. it is not by its own nature, for straight motion from above downward is natural to it; therefore circular motion could not be natural to it. further, the earth is a simple body; and a simple body can not have two natural motions of distinct kinds or classes. nor is it moved by another body; for by what is it moved? one has to say it is moved either by the sun or by some other celestial body; and this cannot be said, since either the sun or that body is said to be at rest or in motion. if it is said to be at rest, then it cannot impart movement to another. if it is said to be in motion, then it can not move the earth, because it ought to move either by a motion similar to its own or the opposite of it. it is not similar, since thus it would be observed to move neutrally as when two boats moving in the same direction, appear not to move but to be at rest. it is not the opposite motion, since nothing could give motion contrary to its own. and because galileo seems to say, in so far as i have learned from your lordships, that the earth was moved by the sun; i prove anyway that this is not true since the movement of the sun and of the earth ought to be from contrary and distinct poles. the sun, however, can not be the cause of the other's movement because it is moved above different poles. lastly, the earth follows the motion of no other celestial body; since if it is moved, it moves in hours, and all the other celestial bodies require the space of many days, months and years. ergo. finally, if the earth is moved by another, its motion would be violent; but this is absurd, for no violence can be regular and perpetual. sixthly; even so it is declared that the earth is moved. nevertheless, it must be admitted to this that either the planets themselves or their spheres are moved, for in no other way can the diversities of aspects among themselves be solved; nor can a reason be given why the sun does not leave the ecliptic and the moon does; and how a planet can be stationary or retrograde, high or low,--and many other phenomena. for this reason those who said the earth moved, as bernardus patricius and the others said, claimed that the _primum mobile_, forsooth, was stationary and that the earth was moved in its place; yet they could not in the least deny that the planets themselves were moved, but admitted it. that is the reason why both ancient and modern mathematicians, aside from the motion of the _primum mobile_, were forced to admit and consider the peculiar movements of the planets themselves. if therefore it must be acknowledged, and it is certain, that the stars and the celestial bodies are moved; then it is more probable that all movement perceived in the universe belongs rather to the heavenly bodies than to the earth. for if movement were ascribed to all the rest, why for that same reason is not diurnal rotation ascribed rather to the _primum mobile_ than to the earth, particularly when our senses seem to decide thus? although one may well be mistaken, sometimes, concerning other similar movements; yet it is not probable that all ages could be at fault, or should be, about the movements of its most important objects, of course the celestial luminaries. seventhly; it is proved by experience. for if the earth is moved, then an arrow shot straight up on high could never fall back to the place whence it was shot, but should fall somewhere many miles away. but this is not so. ergo. this can be answered and is so customarily in this way: this does not follow because the air is swept along with the earth, and so, since the air which carries the arrow is turning in the same way with the earth, the arrow also is borne along equally with it, and thus returns to the same spot. this in truth is a pure evasion and a worthless answer for many reasons. it is falsely observed that the air is moved and by the same motion as the earth. for what should move the earth? truly, if the air is moved by the same motion as the earth, either it ought to be moved by the earth itself, or by that other which moves the earth, or by itself. it is not moved by itself; since it has another motion, the straight one of course natural to itself, and also since it has a nature, an essence and qualities all different from the nature and the essence of the earth; therefore it could not by its own nature have the same motion as that other, but of necessity ought to have a different one. nor is it moved by any other that may move the earth; as that which moves the earth could not at the same time and with like motion move the air. for since the air is different from the earth in essence, in both active and passive qualities, and in kind of substance, it can not receive the impelling force of the acting body, or that force applied in the same way as the earth, and so could not be moved in the same way. the virtues [of bodies] acting and of moving diversely are received by the recipients according to the diversity of their dispositions. also it can not be moved by the earth; since if it were moved by the earth, it must be said to be moved by force, but such motion appears to be impossible. ergo. the minor premise is proved: for if air is thus moved by the earth by force the air ought to be moved more rapidly than the earth, because air is larger [than the earth]. for what is outside is larger than what is inside. when, however, what is larger and what is outside is driven around equally rapidly with what is less, and what is inside, then the former is moved much more rapidly. thus it is true that the sphere of saturn in its daily course is moved far faster than the sphere of the moon. but it is impossible that the one driven should move more rapidly than the one driving; therefore the air is not moved by the earth's violence. thus would it be if the air were moved with the earth, or by itself, or by force. thus far, then, the force of the original argument remains; since of its own motion, indeed, it could not be in every way conformable to the motion of the earth as i have shown; and this because the air differs from the earth in consistency of substance, in qualities and in essence. but the air ought at all events to move more sluggishly than the earth. it follows from this that an arrow shot straight up could not return to its starting point; for the earth, moving like the air, on account of the other's slower rate leaves it behind, and the arrow also which is carried away from it. besides, if the air does not move so rapidly as the earth, a man living in a very high tower, however quiet the air, ought then always to feel the strongest wind and the greatest disturbance of the air. since mountains and towers are moved with the earth, and the air would not be accompanying them at an equal speed, it would necessarily follow that they would precede the air by cleaving and cutting and ploughing through it which ought to make a great wind perceptible. eighthly; if a person stood in some very high tower or other high place and aimed from that tower at some spot of earth perpendicularly below his eye, and allowed a very heavy stone to fall following that perpendicular line, it is absolutely certain that that stone would land upon the spot aimed at perpendicularly underneath. but if the earth is moved, it would be impossible for the stone to strike that spot. this i prove first: because either the air moves at an unequal rate with the earth; or it moves equally rapidly. if not equally, then it is certain the stone could not land at that spot, since the earth's movement would outstrip the stone borne by the air. if equally rapidly, then again the stone could not land at that spot, since although the air was moving in itself at an equal speed, yet on that account it could not carry the stone thus rapidly with itself and carrying it downward falling by its own weight, for the stone tending by gravity towards the center resists the carrying of the air. you will say: if the earth is moved in a circle, so are all its parts; wherefore that stone in falling not only moves in a circle by the carrying of the air, but also in a circle because of its own nature as being part of the earth and having the same motion with it. verily this answer is worthless. for although the stone is turned in a circle by its own nature like the earth, yet its own natural gravity impeded it so that it is borne along that much the less swiftly, unlike the air or the earth, both of which are in their natural places and which in consequence have no gravity as a stone falling from on high has. lastly; because although the stone is moved in the world by its own nature like the whole earth, yet it is not borne along as swiftly as the whole earth. for as one stone by its own weight falls from the heaven following its own direct motion straight to the center just as a part of the earth, so also the whole earth itself would fall; and yet it would not fall so swiftly as the whole earth, for although the stone would be borne along in its sphere like the whole earth just as a part of it, yet it would not be borne along as swiftly as the whole earth; and so, in whatever way it is said, the motion of the earth ought always to outstrip the stone and leave it a long distance behind. thus a stone could never fall at the point selected or a point perpendicularly beneath it. this is false. ergo. ninthly: if the earth is moved in a circular orbit, it ought to pass from the west through the meridian to the east; consequently the air ought to move by the same path. but if this were so, then if an archer shot toward the east, his arrow ought to fly much farther than if he shot toward the west. for when he shot toward the east, the arrow would fly with the natural movement of the air and would have that supporting it. but when he shot toward the west, he would have the motion of the air against him and then the arrow would struggle against it. but it is certain the arrow ought to go much farther and faster when the movement of the air is favorable to it then when against it, as is obvious in darts sent out with a favoring wind. ergo. similarly not a few other arguments can be worked out, but there are none as valuable for proof as the foregoing ones. though these were written by me with a flying pen far from books and sick in bed with a broken leg, yet they seem to me to have so much value that i do not see any way by which they could rightly be refuted. these i have written for your gracious lordships in gratitude for your goodwill on the occasion of our conversation at your dinner four days ago; and i ask for them that you meditate on them justly and well. bibliography (of references cited.) i general works. addis and arnold: _catholic dictionary_, nd edit. london, . bailly: _histoire de l'astronomie moderne depuis la fondation de l'ecole d'alexandrie, jusqu' a l'epoque de _. vol. paris, . berry, arthur: _short history of astronomy_. new york, . cajori, florian: _the teaching and history of mathematics in the united states_. washington, . (bureau of education, no. .) delambre, j.b.j.: _histoire de l'astronomie ancienne_. paris, . ----: _histoire de l'astronomie du moyen age_. paris, . ----: _histoire de l'astronomie moderne_. paris, . de morgan, augustus: _a book of paradoxes_. vol. nd edit. ed. by david eugene smith. chicago, . di bruno, joseph faà: _catholic belief, or a short and simple exposition of catholic doctrine_. author's american edit. th thousand. new york, [ .] jacoby, harold: _astronomy, a popular handbook_. new york, . janssen, j.: _history of the german people at the close of the middle ages_. trans. by mitchell and christie. vol. st. louis, no date. lecky, wm. e. hartpole: _history of england in the th century_. vol. new edit. new york, . libri, c.: _histoire des sciences mathématiques en italie depuis la renaissance des lettres_. me édit. vol. halle, . milman, henry h.: _history of latin christianity_. vol. in . new york, . owen, john: _the skeptics of the italian renaissance_. nd edit. new york, . peignot, g.: _dictionnaire critique littéraire et bibliographique des principaux livres condamnés au feu, supprimés on censurés_. vol. paris, . putnam, george haven: _the censorship of the church of rome_. vol. new york, . rashdall, hastings: _universities of europe in the middle ages_. vol. oxford, . smith, david eugene: _rara arithmetica_. boston, . snyder, carl: _the world machine: the cosmic mechanism_. london, . stephen, leslie: _history of english thought in the th century_. vol. rd edit. new york, . taylor, henry osborne: _the mediæval mind_. nd edit. london, . walsh, j.j.: _catholic churchmen in science_. nd series. philadelphia, . ----: _the popes and science_. knights of columbus edit. new york, . wegg-prosser, f.r.: _galileo and his judges_. london. . whewell, william: _history of the inductive sciences from the earliest to the present time_. new edit. revised. vol. london, . white, andrew d.: _history of the welfare of science with theology in christendom_. vol. new york, . windle, b.c.a.: _a century of scientific thought and other essays_. london, . young, charles: _manual of astronomy_. boston, . ii special works. allaben, frank: _john watts de peyster_. vol. new york, . ----: see de peyster. anon: _galileo--the roman inquisition: a defence of the catholic church from the charge of having persecuted galileo for his philosophical opinions_. reprinted from the _dublin review_ with an introduction by an "american catholic." cincinnati, . baudrillart, henri: _jean bodin et son temps: tableau des théories politiques et des idées economiques au me siècle_. paris, . bartholmèss, christian: _jordano bruno_. vol. paris, . berti, domenico: _vita di giordano bruno da nola_. turin, . bertrand, m.j.: _copernic et ses travaux_ (fév. ) in _mémoires sur les mathématiques_. ----: _le procès de galilée_ (oct. ), in _eloges académiques, nouvelle série_. paris, . ----: _notice sur la vie et les travaux de kepler_. (dec. ) in _mémoires de l'académie des sciences_, xxxv. paris, . betten, francis s. (s.j.): _the roman index of forbidden books briefly explained for catholic booklovers and students_. th edit. enlarged. st. louis, . beyersdorf, robert: _giordano bruno und shakespeare_. leipsic, . blavatsky, h.p.: _the secret doctrine_. vol. point loma, cal., . brewster, david: _martyrs of science: lives of galileo, tycho brahe and kepler_. london, . bridges, j.h.: _tycho brahe_, in _contemp. rev._: : - (feb. ). brinton, daniel g. and davidson, thomas: _giordano bruno, philosopher and martyr_. philadelphia, . burckhardt, f.: _zur erinnerungaan tycho brahe. vortrag , oct. , in den naturforschen der gesellschaft in basel_. vol. , basel, . chasles, philarète: _galilei, sa vie, son procès et ses contemporains_. paris, . conway, bertrand l. (c.s.p.): _the condemnation of galileo_. pamphlet. new york, . cumont, franz: _astrology and religion among the greeks and romans_. new york, . davidson: see brinton. de l'epinois, henri: _galilée, son procès, sa condemnation, d'après des documents inédits_, in _revue des quest. hist._, iii, - . paris, . desdouits, théophile: _la légende tragique de jordano bruno_. paris, . dreyer, j.l.e.: _tycho brahe: a picture of scientific life and work in the th century_. edinburgh, . eastman, charles r.: _earliest predecessors of copernicus_, in _pop. sci._ lviii: - (april, ). fahie, j.j.: _galileo, his life and work_. london, . flammarion, camille: _vie de copernic et histoire de la découverte du système du monde_. paris, . frisch: _vita joannis kepler_ in _opera omnia kepleri_. viii, - . frith, i.: _life of giordano bruno the nolan_. london, . graux, charles: _l'université de salamanque_ in _notices bibliographiques_. paris, . haldane, elizabeth s.: _descartes, his life and times_. london, . heath, thomas l.: _aristarchus of samos, the ancient copernicus_. oxford, . holden, e.s.: _copernicus_ in _pop. sci._ lxv: - (june, .) la fuente, (vicente de): _historia de las universidades ... de españa_. vol. . martin, henri t.: _galilée, les droits de la science et la méthode des sciences physiques_. paris, . mcintyre, j. lewis: _giordano bruno_. london, . monchamp, georges: _galilée et la belgique, essai historique sur les vicissitudes du système du copernic en belgique_. saint-trond, . parchappe, max: _galilée, sa vie, ses découvertes et ses travaux_. paris, . prowe, leopold: _nicolaus coppernicus_, vol.: i and ii, biography, ; iii, sources, . berlin. r----: _beitrage zur beantwortung der frage nach der nationalität des nicolaus copernicus_. pamphlet. breslau, . reusch, f.h.: _der process galilei's und die jesuiten_. bonn, . robinson, james howard: _the great comet of : a study in the history of rationalism_. northfield, minn., . schiaparelli, g.v.: _die vorlaufer des copernicus im alterthum_, trans. by m. curtze. leipsic, . ----: _studj cosmologici: opinioni e ricerche degli antichi sulle distance e sulle grandezze dei corpi celesti_. pamphlet. . schwilgué, charles: _description abregée de l'horloge astronomique de la cathédrale de strasbourg_, me édit. strasbourg, . shields, charles w.: _the final philosophy_. new york, . small, robert: _account of the astronomical discoveries of kepler,--including an historical review of the systems which had successively prevailed before his time_. london, . thayer, william roscoe: _throne-makers_. new york, . pp. - : giordano bruno: his trial, opinions and death. walsh, j.j.: _an early allusion to accurate methods in diagnosis_. pamphlet. . warren, william f.: _the earliest cosmologies_. new york, . vaughan, roger bede: _life and labours of s. thomas of aquin_. vol. london, . von gebler, karl: _galileo galilei and the roman curia_, trans. by mrs. sturge. london, . ziegler, alexander: _regiomontanus, ein geistiger vorlaufer des columbus_. dresden. . iii sources. _a: pre-copernican (chapters i and ii)._ archimedes: _arenarius_, vol. ii in _opera omnia_ ed. heiberg, leipsic, . aquinas, thomas: _summa theologica_, vol. v in _opera omnia ... cum commentariis ... caietani...._ rome, . aristotle: _de mundo_, vol. iii in _opera omnia_. paris, . augustine: _de civitate dei_, vol. xli in migne: _patr. lat._ (cf. trans. in vol. ii in nicene and post-nicene christian library. new york, .) ----: _de genesi_, vol. xxxiv in migne: _pair. lat._ bacon, roger: _opus tertium_, vol. i in _opera quædam hactenus inedita_, ed. by brewer. london, . capella, martianus: _de nuptiis philologiæ et mercurii et de septem artibus liberalibus, libri novem_. ed. by kopp. frankfort, . cicero: _academica_, ed. by j.s. reid, london, . (cf. trans. by yonge in bohn classical library, london, .) clement of alexander: _stromatum_, vol. iii in _opera omnia_, leipsic, (cf. trans. by williams, vol. ii in _writings_, edinburgh, .) cusanus, nicolaus: _de docta ignorantia_, and _sermones_, in _opera_. basle, [ .] diogenes laërtius: _de clarorum philosophorum vitis_, ed. cobet. paris, . (cf. trans. by yonge in bohn classical library, london, .) st. dionysius the areopagite: _de coelesti ierarchia_, vol. cxxii in migne: _patr. lat._ st. isidore: _de ordine creaturarum_, vol. lxxxiii in migne: _patr. lat._ lactantius: _divinarum institutionum_, vol. vi in migne: _patr. lat._ (cf. trans. by fletcher, vol. xxi in ante-nicene christian library, edinburgh, .) lombard, peter: _sententiæ_, vol. cxcii in migne: _patr. lat._ origen: _de principiis_, vol. xi in migne: _patr. græc._ (cf. trans. vol. x in ante-nicene christian library, edinburgh, .) st. philastrius: _de hæeresibus_, vol. xii in migne: _patr. lat._ philo judæus: _de mundi creatione_ (vol. i), and _quis rerum divinarum hæres_ (vol. iv) in _opera omnia_, erlangæ, . (cf. trans. by yonge, london, .) plato: _timæus_, vol. iv in _opera_ ed. burnet, oxford, [ .] (cf. trans. by jowett, vol. iii of _dialogues_, rd edit. revised. new york, ). plutarch: _moralia_, ed. bernardakis, teubner, leipsic, . (cf. trans. ed. by goodwin, boston, .) ptolemy, claudius: _syntaxis mathematica_, vol. i in _opera quæ supersunt omnia_, vol., teubner, leipsic, . (cf. trans. into french by halma, vol., paris, .) sacro bosco: _libellus de sphæra_, venice, ; wittenberg, [ ]; wittenberg, ; paris, ; venice, ; wittenberg, . scotus, joannus: _depositiones super ierarchias sancti dionysii_, vol. cxxii in migne: _patr. lat._ seneca: _naturalium quæstionum libros viii_, ed. gercke, vol. ii in _opera quæ supersunt_, teubner, leipsic, . (cf. trans. by clarke, london, .) vitruvius: _de architectura libri decem_, teubner, leipsic, . (cf. trans. by gwilt, london, .) _b: copernican and post-copernican._ addison, joseph: _the spectator, no. _, vol. iv in _works_. new edit. with notes. vol. london, . agricola, georgius ludovicus: _de systemate mundi copernico, disputatio astronomica_. pamphlet. wittenberg, . allaben, see schoepffer. "anglo-american": _copernicus refuted: or the true solar system_. pamphlet. baltimore, . bacon, francis: _philosophical works_. reprinted from texts and translations of ellis and spedding, ed. by robertson. london, . barocio, francisco: _cosmographia in quatuor libros_. venice, . bayle, pierre: _système abregé de philosophie_, vol. iii in _oeuvres diverses_. vol. the hague, . bodin, jean: _universæ naturæ theatrum in quo rerum omnium effectrices causa et fines contemplantur et continuæ series quinque libris discutiuntur_. frankfort, . ----: _universæ naturæ theatrum_, trans. into french by françois de fougerolles. lyons, . boscovich, rogerio josepho (s.j.): _de determinanda orbita planetæ ope catoptricæ_. rome, . ----: _opera pertinentia ad opticam et astronomiam_. vol. bassan, . bottrigaro, hercole: _trattato della descrittone della sfera celeste in piano di cl. tolomeo tradotto in parlare italiano_. bologna, . brahe, tychonis: _opera omnia, sive astronomiæ instauratæ progymnasmata_. frankfort, . browne, thomas: _pseudodoxia epidemica_ in _works_, ed. by s. wilkins. vol. london, . bruno, giordano: _de immenso et innumerabilis_, in _opera latina conscripta_, ed. by fiorentino. naples, . ----: _la cena de le ceneri_, in _opere italiane_, ed. by gentile. bari, . burnet, thomas: _the sacred theory of the earth_. th edit., vol. london, . burton, richard [transcriber's note: robert]: _anatomy of melancholy_. th edit. corrected, vol. london, . calvin, jean: _commentaria_ in _opera omnia_ in _corpus reformatorum_, vol. lix. brunswick, . ----: _traité ou avertissement contre l'astrologie qu'on appelle judiciaire et autre curiosités qui regnent aujourd'hui au monde_, in _oeuvres françois_, ed. by p.l. jacob. paris, . canevari, petro, giovannelli, andrea, giovannelli, benedicto: _de observationibus astronomicis. dissertatio habita in seminario romano_. rome, . cassini, g.d.: _de l'origine et du progrès de l'astronomie et de son usage dans la géographie et dans la navigation_, in _recueil d'observations faites en plusieurs voyages par ordre de sa majesté pour perfectionner l'astronomie et la géographie, par mm. de l'académie royale des sciences_. paris, . cavalieri, bonaventura: _sfera astronomica, lettore primario delle matematiche nello studio di bologna ... cavate da ms. dell'autore da antonio manari_. rome, . copernicus, nicolas: _de revolutionibus orbium coelestium, libri sex_. nürnberg, . ----: _astronomia instaurata, libris sex comprehensa, qui de revolutionibus orbium coelestium, inscribuntur. nunc demum post ab obitu authoris annum integritati suæ restituta, notisque illustrata, opera et studio nicolai mulerii_. amsterdam, . ----: _de revolutionibus orbium coelestium. libri sex. accedit g.j. rhetici narratio prima, cum copernici nonnullis scriptis minoribus nunc primum collectis, ejusque vita_. (in latin and polish). warsaw, . ----: _de revolutionibus orbium coelestium, libri sex_, with rheticus, george joachim: _narratio prima_. thorn, . ----: see also vol. iii, sources, of prowe: _nicolaus coppernicus_. cromer, martin: _de origine et rebus gestibus polonorum libri xxx. tertium ab authore diligenter recogniti_. basel, . ----: _poloniæ_, in _res publicæ sive status regni poloniæ, lituanæ, prussiæ, livoniæ, etc. diversorum autorum_. lugd: batavorum, . dubartas, w. desaluste: _the divine weeks_, trans. by josuah sylvester, ( ) ed. by t.w. haight. waukesha, wis. . de brisbar, j.: _calendrier historique ... avec un traité historique de la sphère_. me édit. leyden, . de maupertius: _eléments de géographie_, in _ouvrages divers_. amsterdam, . de premontval, mme.: _le méchaniste philosophe, mémoir ... de la vie et des ouvrages du sr. jean piegeon, mathématicien, membre de la société des arts, auteur des premières sphères mouvantes qui ayent été faites en france, selon l'hypothèse de copernic_. the hague, . depeyster, j.w., allaben, f.: _algol: the "ghoul" or "demon" star, a supplement to "the earth stands fast."_ pamphlet. new york, . descartes, réné: _les principes de la philosophie_, vol. iii in _oeuvres_ ed. by cousin. ii vol. paris, . di gallo, marco antonio giovanni gianesimi: _opinione sopra il movimento della terra e degli astri_. pamphlet. bassano, . dobell, john (ed.): _hymns_. no title-page. preface dated england, . favaro, antonio: _galileo e l'inquisizione, documenti de processo galileiano ... per la prima volta integralmente pubblicati_. florence, . fénelon, f. de s. de la mothe: _traité de l'existence et des attributs de dieu_, in vol. i, _oeuvres_. vol. paris, . ferramosca, aegidius leognanus: _positiones suas physioastronomicas de sphæra coelesti publice demonstrandas et propugnandis in collegio neapolitano soc. jesu_. naples, . fienus, thomas, fromundus, liberti: _de cometa anni , dissertationes. ejusdem thomæ fieni epistolica quæstio, an verum sit coelum moveri, et terram quiescere?_ london, . bound with fromundus: _meteorologicorum_. fienus, thomas: _epistolica quæstio_. see above. fontana, cajetano: _institutio physico-astronomica_. mutinæ, . forbes, duncan: _a letter to a bishop concerning some important discoveries in philosophy and theology_, in _works_. dublin, . foscarini, paolo antonio: _an epistle concerning the pythagorian and copernican opinion of the mobility of the earth and stability of the sun ... in which the authorities of sacred scriptures ... are reconciled. written to the most reverend father sebastiano fontoni, general of the order of carmelites, jan., , naples_, in salusbury: _math. coll._, q.v. fromondus, liberti: _ant-aristarchus sive orbis-terræ immobilis: liber unicus in quo decretum s. congregationis s.r.e. cardinal, an adversus pythagorico-copernicanos editum defenditur_. antwerp, . ----: _meteorologicorum libri sex. cui accessit in hac ultima editione thomæ fieni et lib. fromondi dissertationes de cometa anni , et clarorum virorum judicia de pluvia purpurea bruxelliensis_. london, . ----: _vesta, sive ant-aristarchi vindex adversus iac. lansbergium ... in quo decretum ... et alterum anno adversus copernicanos terræ motores editum, iterum defenditur_. antwerp, . see also fienus. gadbury, john and timothy: _george hartgill's astronomical tables_. london, . galilei, galileo: _opere_, edizione nazionale, ed. by favaro. vol. florence, - . ----: _dialogo sopra i due massimi sistemi del monde, tolemaico, e copernicano_. florence, . trans. in salusbury: _math. coll._, q.v. ----: _lettera a madama cristina di lorena, granduchessa di toscana_, in vol. v, _opere_; trans. in salusbury: _math. coll._, q.v. ----: _sidereus nuncius ... atque medicea sidera_, in vol. iii, _opere_. _accusation, condemnation and abjuration of galileo galilei before the holy inquisition at rome_, . pamphlet. london, . see also favaro. gassendi, petro: _institutio astronomica juxta hypothesis quam veterum quam copernici ac tychonis_. rd edit. hagæ-comitum, . ----: _institutio astronomica...._ th edit. london, . ----: _institutio astronomica juxta hypothesis tam veterum quam recentiorum cui accesserunt galileo galilei; nuncius sidereus, et johannis kepleri: dioptrice_. rd edit. corrected. london, . ----: _vita tychonis brahei, equitis dani, astrononum coryphæi._, nd edit. corrected. hagæ-comitum, . george, earl of macclesfield: _speech in the house of peers, mar. , _. pamphlet. london, . gilbert, william: _de magnete, magnetis qui corporibus, et de magno magnete tellure physilogia nova_. london, , reprinted berlin . trans. by p.f. mottelay, new york, . herbert, george: _man_, in _english works_ ed. by g.h. palmer, boston, . horne, george: _commentary on book of psalms_. vol. oxford, . ----: _a fair, candid and impartial state of the case between sir isaac newton and mr. hutchinson_. pamphlet. oxford, . hutchinson, john: _moses's principia_. london, . huygens, christian: _the celestial worlds discover'd. trans. from the latin_. london, . ----: _nouveau traité de la pluralité des mondes ... traduit du latin en françois par m.d._ amsterdam, . _index librorum prohibitorum ... usque _, (appendix to june, ). rome, . ---- _usque _. rome, . ---- _usque _. rome, . ---- _usque _. rome, . ---- _benedicti xiv_. rome, . ---- _pii sexti_. rome, . ---- _pii septimi_. rome, . ---- _editum _. mechlin, . ---- _gregorii xvi_. rome, . ---- _leonis xiii recognitus pii x_. rd edit. rome, . justus-lipsius: _physiologiæ stoicorum_, vol. iv, in _opera omnia_, vols. vesaliæ, . keble, john: _christian year_. ed. by lock. london, . keill, john: _introductio ad veram astronomiam, seu lectiones astronomicæ habitæ in schola astronomica academiæ oxoniensis_. oxford, . kepler, joannis: _opera omnia_, edidit frisch. vol. frankfort a.m. - . ----: _abstract of the "introduction upon mars"_, trans. in salusbury: _math. coll._, q.v. ----: _tabulæ rudolphinæ ... a phoenice illo astronomorum tychone ... primum concepta ... ... observatioribus siderum ... post annum præcipue ... traducta in germaniam ... . tabulas ipsas ... jussu et stipendiis ... imp. rudolphi_. ulm, . kircher, athanasius (s.j.): _iter exstaticum coeleste_, enlarged by gaspare schotto, s.j. herbipoli, . kromer, see cromer. la galla, julius cæsar: _de phoenomenis in orbe lunæ novi telescopii usu a gallileo gallileo. physica disputatio_. venice, . lambert: _système du monde_. me édit. berlin, . lange, j.r.l.: _the copernican system: the greatest absurdity in the history of human thought_. no place, . leadbetter, charles: _astronomy of the satellites of the earth, jupiter and saturn, grounded upon sir isaac newton's theory of the earth's satellites_. london, . longomontanus, christianus: _astronomica danica_. amsterdam, . luther, martin: _tischreden oder colloquia_, ed. by forstemann. vol. leipsic, . mather, cotton: _the christian philosopher, a collection of the best discoveries in nature with religious improvements_. london, . melancthon, philip: _initia doctrinæ physicæ_, nd edit. wittenberg, . milton, john: _areopagitica_, ed. by hales. oxford, . ----: _paradise lost_, in _complete poetical works_, ed. by beeching. london, . montaigne, michel e. de: _apologie of raymond sebonde_, vol. ii in _essayes_, trans. by florio. vol. london, . moxon, joseph: _a tutor to astronomie and geographie, or an easie and speedy way to know the use of both the globes, celestial and terrestrial_. nd edit. london, . mulerius, nicolaus: _tabulæ friscæ lunæ-solares quadruplices è fontibus cl. ptolemæi, regis alfonsi, nic. copernici et tychonis brahe_. amsterdam, . piccioli, gregorio: _la scienza dei cieli e dei corpi celesti, e loro meravigliosa posizione, moto, e grandezza: epilogata colle sue figure quattro più famosi sistemi dell'universo tolemaico, copernicano, ticonico, e novissimo. colla patente dimostrazione della quieta di nostra terra, e che poco più, o meno ci apparisce ella oggidi nella sua superfizie tal quale era avanti l'universal diluvio_. verona, . pike, samuel: _philosophica sacra: or the principles of natural philosophy extracted from divine revelation_. london, . pluche: _histoire du ciel considéré selon les idées des poêtes, des philosophes et de moïse_. vol. paris, . pope, alexander: _letter_ in vol. vi, _works_, new edit. by croker and elwin. london, . record, robert: _the castle of knowledge_. rd edit. london, . reisch, gregorius: _margarita filosofica..._ trans. into italian by gallucci. venice, . rheticus, georgius joachim: _de libris revolutionum ad joannem schönerum narratio prima_, , in copernicus: _de revolutionibus_, thorn, . riccioli, giovanni baptista (s.j.): _almagestum novum, astronomiam veterem novamque completens observationibus aliorum et propriis, novisque theorematibus, problematibus ac tabulis promotam_. vol. bologna, . ----: _apologia pro argumento physicomathematico contra systema copernicanum adiecto contra illud novo argumento ex reflexo motu gravium decidentium_. venice, . spooner, w.w.: _great copernican myth_; a review of algol by de peyster and allaben. pamphlet. tivoli, n.y., . salusbury, thomas: _mathematical collections and translations, first tome_. london, . schoepffer, c.: _the earth stands fast_, trans. for and ed. by j.w. de peyster with notes and supplement by frank allaben. pamphlet. new york, . schotto, gaspar (s.j.): _organum mathematicum. opus posthumum_, herbipoli, . simpson, thomas: _essays on several curious and useful subjects in speculative and mix'd mathematicks_. london, . sindico, pierre: _refutation du système de copernic exposé en dix-sept lettres qui été adressées à feu m. le verrier_. paris, . spagnio, andrea: _de motu_. rome, . tischner, august: _le système solaire se mouvant_. pamphlet. leipsic, . toland, john: _miscellaneous works_. vol. london, . vitali, hieronymo: _lexicon mathematicum_. rome, . voight, johann-henrich: _der kunstgünstigen einfalt mathematischer raritäten erstes hundert: allen kunstgünstigen zum lustigen und nutzbaren gebrauch mit fleiss und mühe zusammen geordnet und furgetragen_. hamburg, . wesley, john: _sermon_, vol. vii in _works_. th edit. vol. london, . ----: _survey of the wisdom of god in the creation, or a compendium of natural philosophy_. vol. in . nd edit. bristol, . whiston, william: _a new theory of the earth_. th edit. london, . wilkins, g.: _the first book: the discovery of a new world_. rd edit. london, . ----: _the second book: discourse concerning a new planet, that 'tis probable our earth is one of the planets_. london, . (bound with _first book_.) "w.r.": _the new astronomer, or astronomy made easy by such instruments that readily shew by observation the stars...._ london, . index addison, j., - . agricola, g.l., . albategnius, . allaben, f., . alphonse x of castile, , . ambrose, . arabian astronomers, , , , . archimedes, . aristarchus of samos, - n., , n., , . aristotle, , , , , , , , , . augustine, , , . bacon, francis, , - . bacon, roger, . bayle, pierre, - . bellarmin, cardinal, , - , . benedict xiv, . bessel, , . bodin, jean, - , - , - . boscovich, , . bossuet, . bradley, , . browne, thomas, - . bruno, , , - , , , , , . burton, richard [transcriber's note: robert], . calvin, , , . cartesian-copernicans, - , , , , . cassini, g.d., - . castelli, , . church fathers, - , . cicero, , , , . cleanthes, . clement of alexandria, . clement viii, n. congregations of the index, , - , - , , , , , , . copernicus, , , , , , , , - , , , , , , , , , . name, n. life, - . theory, , - , , , , - , , - . opponents, , , - , , - , - , , - , , , - . supporters, , , - , , - , - , , - , - , , , - , - , - , - , - . dante, . delambre, , . de maupertius, . de peyster, j.w., . de premontval, mme., . _de revolutionibus_, , , , , , - , - . descartes, , , . didacus à stunica, , , , , . digges, thomas, n. diogenes laërtius, . dominicus maria di novara, , . dubartas, . fénelon, . feyens, thomas, , , - . flammarion, . forbes, duncan, . foscarini, , , - , , . foucault, , . froidmont, see fromundus. fromundus, , , - , . galileo, , , - , , , - , , , , , , , , , , , . gassendi, , , . gilbert, wm., , , . greek philosophers, - , , , . herbert, george, - . hipparchos, , . hicetas, , . horne, george, . hutchinson, john, . huygens, christian, , . index, , , - , , , , . inquisition, , , , - , - , , , . isidore of seville, . jasper, bro., . jesuits, , , , , , , - , . johnson, s., . justus-lipsius, , . keble, j., . keill, j., - . kepler, , , - , , , , , , , , , . knap, . kromer, m., n. lactantius, , . lalande, . lange, j.r., . lansberg, - , . leo x, . liège, univ. of, , - . longomontanus, . louvain, univ. of, , , - , , . luther, , , , . lutherans, , , . mæstlin, , , , . martianus capella, . mather, cotton, . melancthon, , - , . milton, , , . mivart, . montaigne, . _narratio prima_, , . newton, , , , , . nicolas cusanus, , . origen, . osiander, , . owen, j., n., . paul iii, . paul v, - , , , . peter lombard, . peter the great, . philastrius, . philo judæus, . philolaus, , . piegeon, j., . pike, s., . pius vii, . plato, , . plutarch, , , , . pope, alexander, , . pseudo-dionysius, . ptolemy, n., , , , - , , , . theory, , , , , , , , , , , , - . pürbach, . pythagoras, , , . pythagoreans, , . recorde, r., - . regiomontanus, , , , , . reinhold, erasmus, . rheticus, - , , , . riccioli, , , - , , . roberts, . roemer, . sacrobosco, , , . salamanca, univ. of, , . schoepffer, c., . schwilgué, . settele, , . shakespeare, . sindico, . stephen, leslie, . thomas aquinas, . turrettin, . turrinus, j., . tycho brahe, , - , , , . theory, , , , , , , , , , , . urban viii, - . van welden, m., - . vitruvius, . voight, j.h., - . von schönberg, n., , , . wallis, n. wesley, j., , . whewell, , . widmanstadt, , . wilkins, bp., - , . wren, dean, - . yale, univ. of, . zytphen, . transcriber's note greek letters used to identify stars (bayer designation), are replaced with the full name of the greek letter, e.g. alpha centauri. the single greek word in the text is transliterated within braces, {komê}. minor punctuation and hyphenation inconsistencies have been corrected. the spelling "bernices" for "berenices" has been retained throughout. the following minor typographical errors have been corrected: p : "establish" changed to "established" p : "firmanent" changed to "firmament" p : "they thoughts" changed to "thy thoughts" p : "suen" changed to "seuen" p : "consequenc" changed to "consequence" p : "geographieal" changed to "geographical" p : "lyrae" changed to "lyræ" for consistency p : removed redundant word "degrees" following the degree symbol the astronomy of milton's 'paradise lost' [illustration: a typical sun-spot] the astronomy of milton's 'paradise lost' by thomas n. orchard, m.d. member of the british astronomical association _these are thy glorious works, parent of good, almighty! thine this universal frame, thus wondrous fair: thyself how wondrous then! unspeakable._ longmans, green, and co. london, new york, and bombay all rights reserved contents chapter page i. a short historical sketch of astronomy ii. astronomy in the seventeenth century iii. milton's astronomical knowledge iv. milton and galileo v. the seasons vi. the starry heavens vii. the starry heavens viii. description of celestial objects mentioned in 'paradise lost' ix. milton's imaginative and descriptive astronomy illustrations _plates_ a typical sun-spot _frontispiece_ venus on the sun's disc _to face page_ cluster in hercules " great nebula in orion " a portion of the moon's surface " _in text_ fig. page . the ptolemaic system of the universe . milton's division of universal space . a binary star system-- ophiuchi . the orbits of the components of gamma virginis . apparent orbit of the companion of sirius . a sun-spot magnified . the corona during the eclipse of may . a portion of the milky way preface many able and cultured writers have delighted to expatiate on the beauties of milton's 'paradise lost,' and to linger with admiration over the lofty utterances expressed in his poem. though conscious of his inability to do justice to the sublimest of poets and the noblest of sciences, the author has ventured to contribute to miltonic literature a work which he hopes will prove to be of an interesting and instructive character. perhaps the choicest passages in the poem are associated with astronomical allusion, and it is chiefly to the exposition and illustration of these that this volume is devoted. the writer is indebted to many authors for information and reference, and especially to miss agnes m. clerke, professors masson and young, mr. james nasmyth, mr. g. f. chambers, and sir robert ball. also to the works of the late mr. r. a. proctor, sirs w. and j. herschel, admiral smyth, professor grant, mr. j. r. hind, sir david brewster, rev. a. b. whatton, and prebendary webb. most of the illustrations have been supplied by the publishers: messrs. macmillan and w. hunt & co. have kindly permitted the reproduction of some of their drawings. manchester, _march _. chapter i a short historical sketch of astronomy astronomy is the oldest and most sublime of all the sciences. to a contemplative observer of the heavens, the number and brilliancy of the stars, the lustre of the planets, the silvery aspect of the moon, with her ever-changing phases, together with the order, the harmony, and unison pervading them all, create in his mind thoughts of wonder and admiration. occupying the abyss of space indistinguishable from infinity, the starry heavens in grandeur and magnificence surpass the loftiest conceptions of the human mind; for, at a distance beyond the range of ordinary vision, the telescope reveals clusters, systems, galaxies, universes of stars--suns--the innumerable host of heaven, each shining with a splendour comparable with that of our sun, and, in all likelihood, fulfilling in a similar manner the same beneficent purposes. the time when man began to study the stars is lost in the antiquity of prehistoric ages. the ancient inhabitants of the earth regarded the heavenly bodies with veneration and awe, erected temples in their honour, and worshipped them as deities. historical records of astronomy carry us back several thousand years. during the greater part of this time, and until a comparatively recent period, astronomy was associated with astrology--a science which originated from a desire on the part of mankind to penetrate the future, and which was based upon the supposed influence of the heavenly bodies upon human and terrestrial affairs. it was natural to imagine that the overruling power which governed and directed the course of sublunary events resided in the heavens, and that its decrees might be understood by watching the movements of the heavenly bodies under its control. it was, therefore, believed that by observing the configuration of the planets and the positions of the constellations at the instant of the birth of an individual, his horoscope, or destiny, could be foretold; and that by making observations of a somewhat similar nature the occurrence of events of public importance could be predicted. when, however, the laws which govern the motions of the heavenly bodies became better known, and especially after the discovery of the great law of gravitation, astrology ceased to be a belief, though for long after it retained its power over the imagination, and was often alluded to in the writings of poets and other authors. in the early dawn of astronomical science, the theories upheld with regard to the structure of the heavens were of a simple and primitive nature, and might even be described as grotesque. this need occasion no surprise when we consider the difficulties with which ancient astronomers had to contend in their endeavours to reduce to order and harmony the complicated motions of the orbs which they beheld circling around them. the grouping of the stars into constellations having fanciful names, derived from fable or ancient mythology, occurred at a very early period, and though devoid of any methodical arrangement, is yet sufficiently well-defined to serve the purposes of modern astronomers. several of the ancient nations of the earth, including the chaldeans, egyptians, hindus, and chinese, claim to have been the earliest astronomers. chinese records of astronomy reveal an antiquity of near , years b.c., but they contain no evidence that their authors possessed any scientific knowledge, and they merely record the occurrence of solar eclipses and the appearances of comets. it is not known when astronomy was first studied by the egyptians; but what astronomical information they have handed down is not of a very intelligible kind, nor have they left behind any data that can be relied upon. the great pyramid, judging from the exactness with which it faces the cardinal points, must have been designed by persons who possessed a good knowledge of astronomy, and it was probably made use of for observational purposes. it is now generally admitted that correct astronomical observations were first made on the plains of chaldea, records of eclipses having been discovered in chaldean cities which date back , years b.c. the chaldeans were true astronomers: they made correct observations of the risings and settings of the heavenly bodies; and the exact orientation of their temples and public buildings indicates the precision with which they observed the positions of celestial objects. they invented the zodiac and gnomon, made use of several kinds of dials, notified eclipses, and divided the day into twenty-four hours. to the greeks belongs the credit of having first studied astronomy in a regular and systematic manner. thales ( b.c.) was one of the earliest of greek astronomers, and may be regarded as the founder of the science among that people. he was born at miletus, and afterwards repaired to egypt for the purpose of study. on his return to greece he founded the ionian school, and taught the sphericity of the earth, the obliquity of the ecliptic, and the true causes of eclipses of the sun and moon. he also directed the attention of mariners to the superiority of the lesser bear, as a guide for the navigation of vessels, as compared with the great bear, by which constellation they usually steered. thales believed the earth to be the centre of the universe, and that the stars were composed of fire; he also predicted the occurrence of a great solar eclipse. thales had for his successors anaximander, anaximenes, and anaxagoras, who taught the doctrines of the ionian school. the next great astronomer that we read of is pythagoras, who was born at samos b.c. he studied under thales, and afterwards visited egypt and india, in order that he might make himself familiar with the scientific theories adopted by those nations. on his return to europe he founded his school in italy, and taught in a more extended form the doctrines of the ionian school. in his speculations with regard to the structure of the universe he propounded the theory (though the reasons by which he sustained it were fanciful) that the sun is the centre of the planetary system, and that the earth revolves round him. this theory--the accuracy of which has since been confirmed--received but little attention from his successors, and it sank into oblivion until the time of copernicus, by whom it was revived. pythagoras discovered that the morning and evening stars are one and the same planet. among the famous astronomers who lived about this period we find recorded the names of meton, who introduced the metonic cycle into greece and erected the first sundial at athens; eudoxus, who persuaded the greeks to adopt the year of - / days; and nicetas, who taught that the earth completed a daily revolution on her axis. the alexandrian school, which flourished for three centuries prior to the christian era, produced men of eminence whose discoveries and investigations, when arranged and classified, enabled astronomy to be regarded as a true theoretical science. the positions of the fixed stars and the paths of the planets were determined with greater accuracy, and irregularities of the motions of the sun and moon were investigated with greater precision. attempts were made to ascertain the distance of the sun from the earth, and also the dimensions of the terrestrial sphere. the obliquity of the ecliptic was accurately determined, and an arc of the meridian was measured between syene and alexandria. the names of aristarchus, eratosthenes, aristyllus, timocharis, and autolycus, are familiarly known in association with the advancement of the astronomy of this period. we now reach the name of hipparchus of bithynia ( b.c.), the most illustrious astronomer of antiquity, who did much to raise astronomy to the position of a true science, and who has also left behind him ample evidence of his genius 'as a mathematician, an observer, and a theorist.' we are indebted to him for the earliest star catalogue, in which he included , stars. he discovered the precession of the equinoxes, and determined the motions of the sun and moon, and also the length of the year, with greater precision than any of his predecessors. he invented the sciences of plane and spherical trigonometry, and was the first to use right ascensions and declinations. the next astronomer of eminence after hipparchus was ptolemy ( a.d.), who resided at alexandria. he was skilled as a mathematician and geographer, and also excelled as a musician. his chief discovery was an irregularity of the lunar motion, called the '_evection_.' he was also the first to observe the effect of the refraction of light in causing the apparent displacement of a heavenly body from its true position. ptolemy devoted much of his time to extending and improving the theories of hipparchus, and compiled a great treatise, called the 'almagest,' which contains nearly all the knowledge we possess of ancient astronomy. ptolemy's name is, however, most widely known in association with what is called the ptolemaic theory. this system, which originated long before his time, but of which he was one of the ablest expounders, was an attempt to establish on a scientific basis the conclusions and results arrived at by early astronomers who studied and observed the motions of the heavenly bodies. ptolemy regarded the earth as the immovable centre of the universe, round which the sun, moon, planets, and the entire heavens completed a daily revolution in twenty-four hours. after the death of ptolemy no worthy successor was found to occupy his place, the study of astronomy began to decline among the greeks, and after a time it ceased to be cultivated by that people. the arabs next took up the study of astronomy, which they prosecuted most assiduously for a period of four centuries. their labours were, however, confined chiefly to observational work, in which they excelled; unlike their predecessors, they paid but little attention to speculative theories--indeed, they regarded with such veneration the opinions held by the greeks, that they did not feel disposed to question the accuracy of their doctrines. the most eminent astronomer among the arabs was albategnius ( a.d.). he corrected the greek observations, and made several discoveries which testified to his abilities as an observer. ibn yunis and abul wefu were arab astronomers who earned a high reputation on account of the number and accuracy of their observations. in persia, a descendant of the famous genghis khan erected an observatory, where astronomical observations were systematically made. omar, a persian astronomer, suggested a reformation of the calendar which, if it had been adopted, would have insured greater accuracy than can be attained by the gregorian style now in use. in , ulugh beg, who resided at samarcand, made many observations, and constructed a star catalogue of greater exactness than was known to exist prior to his time. the arabs may be regarded as having been the custodians of astronomy until the time of its revival in another quarter of the globe. after the lapse of many centuries, astronomy was introduced into western europe in , and from that date to the present time its career has been one of triumphant progress. in , a translation of ptolemy's 'almagest' from arabic into latin was accomplished by order of the german emperor, frederick ii.; and in alphonso x., king of castile, himself a zealous patron of astronomy, caused a new set of astronomical tables to be constructed at his own expense, which, in honour of his majesty, were called the 'alphonsine tables.' purbach and regiomontanus, two german astronomers of distinguished reputation, and waltherus, a man of considerable renown, made many important observations in the fifteenth century. the most eminent astronomer who lived during the latter part of this century was copernicus. nicolas copernicus was born february , , at thorn, a small town situated on the vistula, which formed the boundary between the kingdoms of prussia and poland. his father was a polish subject, and his mother of german extraction. having lost his parents early in life, he was educated under the supervision of his uncle lucas, bishop of ermland. copernicus attended a school at thorn, and afterwards entered the university of cracow, in , where he devoted four years to the study of mathematics and science. on leaving cracow he attached himself to the university of bologna as a student of canon law, and attended a course of lectures on astronomy given by novarra. in the ensuing year he was appointed canon of frauenburg, the cathedral city of the diocese of ermland, situated on the shores of the frisches haff. in the year he was at rome, where he lectured on mathematics and astronomy. he next spent a few years at the university of padua, where, besides applying himself to mathematics and astronomy, he studied medicine and obtained a degree. in copernicus returned to his native country, and was appointed medical attendant to his uncle, the bishop of ermland, with whom he resided in the stately castle of heilsberg, situated at a distance of forty-six miles from frauenburg. copernicus lived with his uncle from till , and during that time prosecuted his astronomical studies, and undertook, besides, many arduous duties associated with the administration of the diocese; these he faithfully discharged until the death of the bishop, which occurred in . after the death of his uncle he took up his residence at frauenburg, where he occupied his time in meditating on his new astronomy and undertaking various duties of a public character, which he fulfilled with credit and distinction. in he was appointed administrator-general of the diocese. though a canon of frauenburg, copernicus never became a priest. after many years of profound meditation and thought, copernicus, in a treatise entitled 'de revolutionibus orbium celestium,' propounded a new theory, or, more correctly speaking, revived the ancient pythagorean system of the universe. this great work, which he dedicated to pope paul iii., was completed in ; but he could not be prevailed upon to have it published until , the year in which he died. in copernicus had an apoplectic seizure, followed by paralysis and a gradual decay of his mental and vital powers. his book was printed at nuremberg, and the first copy arrived at frauenburg on may , , in time to be touched by the hands of the dying man, who in a few hours after expired. the house in which copernicus lived at allenstein is still in existence, and in the walls of his chamber are visible the perforations which he made for the purpose of observing the stars cross the meridian. copernicus was the means of creating an entire revolution in the science of astronomy, by transferring the centre of our system from the earth to the sun. he accounted for the alternation of day and night by the rotation of the earth on her axis, and for the vicissitudes of the seasons by her revolution round the sun. he devoted the greater part of his life to meditating on this theory, and adduced several weighty reasons in its support. copernicus could not help perceiving the complications and entanglements by which the ptolemaic system of the universe was surrounded, and which compared unfavourably with the simple and orderly manner in which other natural phenomena presented themselves to his observation. by perceiving that mars when in opposition was not much inferior in lustre to jupiter, and when in conjunction resembled a star of the second magnitude, he arrived at the conclusion that the earth could not be the centre of the planet's motion. having discovered in some ancient manuscripts a theory, ascribed to the egyptians, that mercury and venus revolved round the sun, whilst they accompanied the orb in his revolution round the earth, copernicus was able to perceive that this afforded him a means of explaining the alternate appearance of those planets on each side of the sun. the varied aspects of the superior planets, when observed in different parts of their orbits, also led him to conclude that the earth was not the central body round which they accomplished their revolutions. as a combined result of his observation and reasoning copernicus propounded the theory that the sun is the centre of our system, and that all the planets, including the earth, revolve in orbits around him. this, which is called the copernican system, is now regarded as, and has been proved to be, the true theory of the solar system. tycho brahÉ was a celebrated danish astronomer, who earned a deservedly high reputation on account of the number and accuracy of his astronomical observations and calculations. the various astronomical tables that were in use in his time contained many inaccuracies, and it became necessary that they should be reconstructed upon a more correct basis. tycho possessed the practical skill required for this kind of work. he was born december , , at knudstorp, near helsingborg. his father, otto brahé, traced his descent from a swedish family of noble birth. at the age of thirteen tycho was sent to the university of copenhagen, where it was intended he should prepare himself for the study of the law. the prediction of a great solar eclipse, which was to happen on august , , caused much public excitement in denmark, for in those days such phenomena were regarded as portending the occurrence of events of national importance. tycho looked forward with great eagerness to the time of the eclipse. he watched its progress with intense interest, and when he perceived all the details of the phenomenon occur exactly as they were predicted, he resolved to pursue the study of a science by which, as was then believed, the occurrence of future events could be foretold. from copenhagen tycho brahé was sent to leipsic to study jurisprudence, but astronomy absorbed all his thoughts. he spent his pocket-money in purchasing astronomical books, and, when his tutor had retired to sleep, he occupied his time night after night in watching the stars and making himself familiar with their courses. he followed the planets in their direct and retrograde movements, and with the aid of a small globe and pair of compasses was able by means of his own calculations to detect serious discrepancies in the alphonsine and prutenic tables. in order to make himself more proficient in calculating astronomical tables he studied arithmetic and geometry, and learned mathematics without the aid of a master. having remained at leipsic for three years, during which time he paid far more attention to the study of astronomy than to that of law, he returned to his native country in consequence of the death of an uncle, who bequeathed him a considerable estate. in denmark he continued to prosecute his astronomical studies, and incurred the displeasure of his friends, who blamed him for neglecting his intended profession and wasting his time on astronomy, which they regarded as useless and unprofitable. not caring to remain among his relatives, tycho brahé returned to germany, and arrived at wittenberg in . whilst residing here he had an altercation with a danish gentleman over some question in mathematics. the quarrel led to a duel with swords, which terminated rather unfortunately for tycho, who had a portion of his nose cut off. this loss he repaired by ingeniously contriving one of gold, silver, and wax, which was said to bear a good resemblance to the original. from wittenberg tycho proceeded to augsburg, where he resided for two years. here he made the acquaintance of several men distinguished for their learning and their love of astronomy. during his stay at augsburg he constructed a quadrant of fourteen cubits radius, on which were indicated the single minutes of a degree; he made many valuable observations with this instrument, which he used in combination with a large sextant. in tycho returned to denmark, where his fame as an astronomer had preceded him, and was the means of procuring for him a hearty welcome from his relatives and friends. in , when returning one night from his laboratory--for tycho studied alchemy as well as astronomy--he beheld what appeared to be a new and brilliant star in the constellation cassiopeia, which was situated overhead. he directed the attention of his companions to this wonderful object, and all declared that they had never observed such a star before. on the following night he measured its distance from the nearest stars in the constellation, and arrived at the conclusion that it was a fixed star, and beyond our system. this remarkable object remained visible for sixteen months, and when at its brightest rivalled sirius. at first it was of a brilliant white colour, but as it diminished in size it became yellow; it next changed to a red colour, resembling aldebaran; afterwards it appeared like saturn, and as it grew smaller it decreased in brightness, until it finally became invisible. in tycho brahé married a peasant-girl from the village of knudstorp. this imprudent act roused the resentment of his relatives, who, being of noble birth, were indignant that he should have contracted such an alliance. the bitterness and mutual ill-feeling created by this affair became so intense that the king of denmark deemed it advisable to endeavour to bring about a reconciliation. after this tycho returned to germany, and visited several cities before deciding where he should take up his permanent residence. his fame as an astronomer was now so great that he was received with distinction wherever he went, and on the occasion of a visit to hesse-cassel he spent a few pleasant days with william, landgrave of hesse, who was himself skilled in astronomy. frederick ii., king of denmark, having recognised tycho brahé's great merits as an astronomer, and not wishing that his fame should add lustre to a foreign court, expressed a desire that he should return to his native country, and as an inducement offered him a life interest in the island of huen, in the sound, where he undertook to erect and equip an observatory at his own expense; the king also promised to bestow upon him a pension, and grant him other emoluments besides. tycho gladly accepted this generous offer, and during the construction of the observatory occupied his time in making a magnificent collection of instruments and appliances adapted for observational purposes. this handsome edifice, upon which the king of denmark expended a sum of , _l._, was called 'uranienburg' ('the citadel of the heavens'). here tycho resided for a period of twenty years, during which time he pursued his astronomical labours with untiring energy and zeal, and made a large number of observations and calculations of much superior accuracy to any that existed previously, which were afterwards of great service to his successors. during his long residence at huen, tycho was visited by many distinguished persons, who were attracted to his island home by his fame and the magnificence of his observatory. among them was james vi. of scotland, who, whilst journeying to the court of denmark on the occasion of his marriage to a danish princess, paid tycho a visit, and enjoyed his hospitality for a week. the king was delighted with all that he saw, and on his departure presented tycho with a handsome donation, and at his request composed some latin verses, in which he eulogised his host and praised his observatory. the island of huen is situated about six miles from the coast of zealand, and fourteen from copenhagen. it has a circumference of six miles, and consists chiefly of an elevated plateau, in the centre of which tycho erected his observatory, the site of which is now marked by two pits and a few mounds of earth--all that remains of uranienburg. all went well with tycho brahé during the lifetime of his noble patron; but in frederick ii. died, and was succeeded by his son, a youth eleven years of age. the danish nobles had long been jealous of tycho's fame and reputation, and on the death of the king an opportunity was afforded them of intriguing with the object of accomplishing his downfall. several false accusations were brought against him, and the court party made the impoverished state of the treasury an excuse for depriving him of his pension and emoluments granted by the late king. tycho was no longer able to bear the expense of maintaining his establishment at huen, and fearing that he might be deprived of the island itself, he took a house in copenhagen, to which he removed all his smaller instruments. during his residence in the capital he was subjected to annoyance and persecution. an order was issued in the king's name preventing him from carrying on his chemical experiments, and he besides suffered the indignity of a personal assault. tycho brahé resolved to quit his ungrateful country and seek a home in some foreign land, where he should be permitted to pursue his studies unmolested and live in quietness and peace. he accordingly removed from the island of huen all his instruments and appliances that were of a portable nature, and packed them on board a vessel which he hired for the purpose of transport, and, having embarked with his family, his servants, and some of his pupils and assistants, 'this interesting barque, freighted with the glory of denmark,' set sail from copenhagen about the end of , and having crossed the baltic in safety, arrived at rostock, where tycho found some old friends waiting to receive him. he was now in doubt as to where he should find a home, when the austrian emperor rudolph, himself a liberal patron of science and the fine arts, having heard of tycho brahé's misfortunes, sent him an invitation to take up his abode in his dominions, and promised that he should be treated in a manner worthy of his reputation and fame. tycho resolved to accept the emperor's kind invitation, and in the spring of arrived at prague, where he found a handsome residence prepared for his reception. he was received by the emperor in a most cordial manner and treated with the greatest kindness. an annual pension of three thousand crowns was settled upon him for life, and he was to have his choice of several residences belonging to his majesty, where he might reside and erect a new observatory. from among these he selected the castle of benach, in bohemia, which was situated on an elevated plateau and commanded a wide view of the horizon. during his residence at benach tycho received a visit from kepler, who stayed with him for several months in order that he might carry out some astronomical observations. in the following year kepler returned, and took up his permanent residence with tycho, having been appointed assistant in his observatory, a post which, at tycho's request, was conferred upon him by the emperor. tycho brahé soon discovered that his ignorance of the language and unfamiliarity with the customs of the people caused him much inconvenience. he therefore asked permission from the emperor to be allowed to remove to prague. this request was readily granted, and a suitable residence was provided for him in the city. in the meantime his family, his large instruments, and other property, having arrived at prague, tycho was soon comfortably settled in his new home. though tycho brahé continued his astronomical observations, yet he could not help feeling that he lived among a strange people; nor did the remembrance of his sufferings and the cruel treatment he received at the hands of his fellow-countrymen subdue the affection which he cherished towards his native land. pondering over the past, he became despondent and low-spirited; a morbid imagination caused him to brood over small troubles, and gloomy, melancholy thoughts possessed his mind--symptoms which seemed to presage the approach of some serious malady. one evening, when visiting at the house of a friend, he was seized with a painful illness, to which he succumbed in less than a fortnight. he died at prague on october , , when in his fifty-fifth year. the emperor rudolph, when informed of tycho brahé's death, expressed his deep regret, and commanded that he should be interred in the principal church in the city, and that his obsequies should be celebrated with every mark of honour and respect. tycho brahé stands out as the most romantic and prominent figure in the history of astronomy. his independence of character, his ardent attachments, his strong hatreds, and his love of splendour, are characteristics which distinguish him from all other men of his age. this remarkable man was an astronomer, astrologer, and alchemist; but in his latter years he renounced astrology, and believed that the stars exercised no influence over the destinies of mankind. as a practical astronomer, tycho brahé has not been excelled by any other observer of the heavens. the magnificence of his observatory at huen, upon the equipment and embellishment of which it is stated he expended a ton of gold; the splendour and variety of his instruments, and his ingenuity in inventing new ones, would alone have made him famous. but it was by the skill and assiduity with which he carried out his numerous and important observations that he has earned for himself a position of the most honourable distinction among astronomers. in his investigation of the lunar theory tycho brahé discovered the moon's _annual equation_, a yearly effect produced by the sun's disturbing force as the earth approaches or recedes from him in her orbit. he also discovered another inequality in the moon's motion, called the _variation_. he determined with greater exactness astronomical refractions from an altitude of ° downwards to the horizon, and constructed a catalogue of stars. he also made a vast number of observations on planets, which formed the basis of the 'rudolphine tables,' and were of invaluable assistance to kepler in his investigation of the laws relating to planetary motion. tycho brahé declined to accept the copernican theory, and devised a system of his own, which he called the 'tychonic.' by this arrangement the earth remained stationary, whilst all the planets revolved round the sun, who in his turn completed a daily revolution round the earth. all the phenomena associated with the motions of those bodies could be explained by means of this system; but it did not receive much support, and after the copernican theory became better understood it was given up, and heard of no more. we now arrive at the name of kepler, one of the very greatest of astronomers, and a man of remarkable genius, who was the first to discover the real nature of the paths pursued by the earth and planets in their revolution round the sun. after seventeen years of close observation, he announced that those bodies travelled round the sun in elliptical or oval orbits, and not in circular paths, as was believed by copernicus. in his investigation of the laws which govern the motions of the planets he formulated those famous theorems known as 'kepler's laws,' which will endure for all time as a proof of his sagacity and surpassing genius. prior to the discovery of those laws the sun, though acknowledged to be the centre of the system, did not appear to occupy a central position as regards the motions of the planets; but kepler, by demonstrating that the planes of the orbits of all the planets, and the lines connecting their apsides, passed through the sun, was enabled to assign the orb his true position with regard to those bodies. john kepler was born at weil, in the duchy of wurtemberg, december , . his parents, though of noble family, lived in reduced circumstances, owing to causes for which they were themselves chiefly responsible. in his youth kepler suffered so much from ill-health that his education had to be neglected. in he was sent to a monastic school at maulbronn, which had been established at the reformation, and was under the patronage of the duke of wurtemberg. afterwards he studied at the university of tubingen, where he distinguished himself and took a degree. kepler devoted his attention chiefly to science and mathematics, but paid no particular attention to the study of astronomy. maestlin, the professor of mathematics, whose lectures he attended, upheld the copernican theory, and kepler, who adopted the views of his teacher, wrote an essay in favour of the diurnal rotation of the earth, in which he supported the more recent astronomical doctrines. in , a vacancy having occurred in the professorship of astronomy at gratz consequent upon the death of george stadt, kepler was appointed his successor. he did not seek this office, as he felt no particular desire to take up the study of astronomy, but was recommended by his tutors as a man well fitted for the post. he was thus in a manner compelled to devote his time and talents to the science of astronomy. kepler directed his attention to three subjects--viz. 'the number, the size, and the motion of the orbits of the planets.' he endeavoured to ascertain if any regular proportion existed between the sizes of the planetary orbits, or in the difference of their sizes, but in this he was unsuccessful. he then thought that, by imagining the existence of a planet between mars and jupiter, and another between venus and mercury, he might be able to attain his object; but he found that this assumption afforded him no assistance. kepler then imagined that as there were five regular geometrical solids, and five planets, the distances of the latter were regulated by the size of the solids described round one another. the discovery afterwards of two additional planets testified to the absurdity of this speculation. a description of these extraordinary researches was published, in , in a work entitled 'prodromus of cosmographical dissertations; containing the cosmographical mystery respecting the admirable proportion of the celestial orbits, and the genuine and real causes of the number, magnitude, and periods of the planets, demonstrated by the five regular geometrical solids.' this volume, notwithstanding the fanciful speculations which it contained, was received with much favour by astronomers, and both tycho brahé and galileo encouraged kepler to continue his researches. galileo admired his ingenuity, and tycho advised him 'to lay a solid foundation for his views by actual observation, and then, by ascending from these, to strive to reach the causes of things.' kepler spent many years in these fruitless endeavours before he made those grand discoveries in search of which he laboured so long. the religious dissensions which at this time agitated germany were accompanied in many places by much tumult and excitement. at gratz the catholics threatened to expel the protestants from the city. kepler, who was of the reformed faith, having recognised the danger with which he was threatened, retired to hungary with his wife, whom he had recently married, and remained there for near twelve months, during which time he occupied himself with writing several short treatises on subjects connected with astronomy. in he returned to gratz and resumed his professorship. in the year kepler set out to pay tycho brahé a visit at prague, in order that he might be able to avail himself of information contained in observations made by tycho with regard to the eccentricities of the orbits of the planets. he was received by tycho with much cordiality, and stayed with him for four months at his residence at benach, tycho in the meantime having promised that he would use his influence with the emperor rudolph to have him appointed as assistant in his observatory. on the termination of his visit kepler returned to gratz, and as there was a renewal of the religious trouble in the city, he resigned his professorship, from which he only derived a small income, and, relying on tycho's promise, he again journeyed to prague, and arrived there in . kepler was presented to the emperor by tycho, and the post of imperial mathematician was conferred upon him, with a salary of florins a year, upon condition that he should assist tycho in his observatory. this appointment was of much value to kepler, because it afforded him an opportunity of obtaining access to the numerous astronomical observations made by tycho, which were of great assistance to him in the investigation of the subject which he had chosen--viz. the laws which govern the motions of the planets, and the form and size of the planetary orbits. as an acknowledgment of the emperor's great kindness, the two astronomers resolved to compute a new set of astronomical tables, and in honour of his majesty they were to be called the 'rudolphine tables.' this project pleased the emperor, who promised to defray the expense of their publication. logomontanus, tycho's chief assistant, had entrusted to him that portion of the work relating to observations on the stars, and kepler had charge of the part which embraced the calculations belonging to the planets and their orbits. this important work had scarcely been begun when the departure of logomontanus, who obtained an appointment in denmark, and the death of tycho brahé in october , necessitated its suspension for a time. kepler was appointed chief mathematician to the emperor in succession to tycho--a position of honour and distinction, and to which was attached a handsome salary, that was paid out of the imperial treasury. but owing to the continuance of expensive wars, which entailed a severe drain upon the resources of the country, the public funds became very low, and kepler's salary was always in arrear. this condition of things involved him in serious pecuniary difficulties, and the responsibility of having to maintain an increasing family added to his anxieties. it was with the greatest difficulty that he succeeded in obtaining payment of even a portion of his salary, and he was reduced to such straits as to be under the necessity of casting nativities in order to obtain money to meet his most pressing requirements. in kepler published his great work, entitled 'the new astronomy; or, commentaries on the motions of mars.' it was by his observation of mars, which has an orbit of greater eccentricity than that of any of the other planets, with the exception of mercury, that he was enabled, after years of patient study, to announce in this volume the discovery of two of the three famous theorems known as kepler's laws. the first is, that all the planets move round the sun in elliptic orbits, and that the orb occupies one of the foci. the second is, that the radius-vector, or imaginary line joining the centre of the planet and the centre of the sun, describes equal areas in equal times. the third law, which relates to the connection between the periodic times and the distances of the planets, was not discovered until ten years later, when kepler, in , issued another work, called the 'harmonies of the world,' dedicated to james i. of england, in which was contained this remarkable law. these laws have elevated astronomy to the position of a true physical science, and also formed the starting-point of newton's investigations which led to the discovery of the law of gravitation. kepler's delight on the discovery of his third law was unbounded. he writes: 'nothing holds me. i will indulge in my sacred fury. i will triumph over mankind by the honest confession that i have stolen the golden vases of the egyptians to build up a tabernacle for my god far away from the confines of egypt. if you forgive me, i rejoice; if you are angry, i can bear it. the die is cast; the book is written, to be read either now or by posterity i care not which. it may well wait a century for a reader, as god has waited six thousand years for an observer.' when kepler presented his celebrated book to the emperor, he remarked that it was his intention to make a similar attack upon the other planets, and promised that he would be successful if his majesty would undertake to find the means necessary for carrying on operations. but the emperor had more formidable enemies to contend with nearer home than jupiter and saturn, and no funds were forthcoming to assist kepler in his undertaking. the chair of mathematics in the university of linz having become vacant, kepler offered himself as a candidate for the appointment, which he was anxious to obtain; but the emperor rudolph was averse to his leaving prague, and encouraged him to hope that the arrears of his salary would be paid. but past experience led kepler to have no very sanguine expectations on this point; nor was it until after the death of rudolph, in , that he was relieved from his pecuniary embarrassments. on the accession of rudolph's brother, matthias, to the austrian throne, kepler was reappointed imperial mathematician; he was also permitted to hold the professorship at linz, to which he had been elected. kepler was not loth to remove from prague, where he had spent eleven years harassed by poverty and other domestic afflictions. having settled with his family at linz, kepler issued another work, in , entitled 'epitome of the copernican astronomy,' in which he gave a general account of his astronomical observations and discoveries, and a summary of his opinions with regard to the theories which in those days were the subject of controversial discussion. almost immediately after its publication it was included by the congregation of the index, at rome, in the list of prohibited books. this occasioned kepler considerable alarm, as he imagined it might interfere with the sale of his works, or give rise to difficulties in the issue of others. he, however, was assured by his friend remus that the action of the papal authorities need cause him no anxiety. the emperor matthias died in , and was succeeded by ferdinand iii., who not only retained kepler in his office, but gave orders that all the arrears of his salary should be paid, including those which accumulated during the reign of rudolph; he also expressed a desire that the 'rudolphine tables' should be published without delay and at his cost. but other obstacles intervened, for at this time germany was involved in a civil and religious war, which interfered with all peaceful vocations. kepler's library at linz was sealed up by order of the jesuits, and the city was for a time besieged by troops. this state of public affairs necessitated a considerable delay in the publication of the 'tables.' the 'rudolphine tables' were published at ulm in . they were commenced by tycho brahé, and completed by kepler, who made his calculations from tycho's observations, and based them upon his own great discovery of the ellipticity of the orbits of the planets. they are divided into four parts. the first and third parts contain logarithmic and other tables for the purpose of facilitating astronomical calculations; in the second are tables of the sun, moon, and planets; and in the fourth are indicated the positions of one thousand stars as determined by tycho. kepler made a special journey to prague in order to present the 'tables' to the emperor, and afterwards the grand duke of tuscany sent him a gold chain as an acknowledgment of his appreciation of the completion of this great work. albert wallenstein, duke of friedland, an accomplished scholar and a man fond of scientific pursuits, made kepler a most liberal offer if he would take up his residence in his dominions. after duly considering this proposal, kepler decided to accept the duke's offer, provided it received the sanction of the emperor. this was readily given, and kepler, in , removed with his family from linz to sagan, in silesia. the duke of friedland treated him with great kindness and liberality, and through his influence he was appointed to a professorship in the university of rostock. though kepler was permitted to retain the pension bestowed upon him by the late emperor rudolph, he was unable after his removal to silesia to obtain payment of it, and there was a large accumulation of arrears. in a final endeavour to recover the amount owing to him he travelled to ratisbon, and appealed to the imperial assembly, but without success. the fatigue which kepler endured on his journey, combined with vexation and disappointment, brought on a fever, which terminated fatally. he died on november , , when in the sixtieth year of his age, and was interred in st. peter's churchyard, ratisbon. kepler was a man of indomitable energy and perseverance, and spared neither time nor trouble in the accomplishment of any object which he took in hand. in thinking over the form of the orbits of the planets, he writes: 'i brooded with the whole energy of my mind on this subject--asking why they are not other than they are--the number, the size, and the motions of the orbits.' but many fanciful ideas passed through kepler's imaginative brain before he hit upon the true form of the planetary orbits. in his 'mysterium cosmographicum' he asserts that the five kinds of regular polyhedral solids, when described round one another, regulated the distances of the planets and size of the planetary orbits. in support of this theory he writes as follows: 'the orbit of the earth is the measure of the rest. about it circumscribe a dodecahedron. the sphere including this will be that of mars. about mars' orbit describe a tetrahedron; the sphere containing this will be jupiter's orbit. round jupiter's describe a cube; the sphere including this will be saturn's. within the earth's orbit inscribe an icosahedron; the sphere inscribed in it will be venus's orbit. in venus inscribe an octahedron; the sphere inscribed in it will be mercury's.' the above quotation is an instance of kepler's wild and imaginative genius, which ultimately led him to make those sublime discoveries associated with planetary motion which are known as 'kepler's laws.' he describes himself as 'troublesome and choleric in politics and domestic matters;' but in his relations with scientific men he was affable and pleasant. he showed no jealousy of a rival, and was always ready to recognise merit in others; nor did he hesitate to acknowledge any error of his own when more recent discoveries proved that he was wrong. some of his works contain passages, written in a jocular strain, indicative of a bright and cheerful temperament. the following characteristic paragraph refers to the opinions of the epicureans with regard to the appearance of a new star, which they ascribed to a fortuitous concourse of atoms: 'when i was a youth, with plenty of idle time on my hands, i was much taken with the vanity, of which some grown men are not ashamed, of making anagrams by transposing the letters of my name written in latin so as to make another sentence. out of ioannes keplerus came _serpens in akuleo_ (a serpent in his sting); but not being satisfied with the meaning of these words, and being unable to make another, i trusted the thing to chance, and, taking out of a pack of playing-cards as many as there were letters in the name, i wrote one upon each, and then began to shuffle them, and at each shuffle to read them in the order they came, to see if any meaning came of it. now, may all the epicurean gods and goddesses confound this same chance, which, although i have spent a good deal of time over it, never showed me anything like sense, even from a distance. so i gave up my cards to the epicurean eternity, to be carried away into infinity; and it is said they are still flying about there, in the utmost confusion, among the atoms, and have never yet come to any meaning. i will tell those disputants, my opponents, not my own opinion, but my wife's. yesterday, when weary with writing, and my mind quite dusty with considering these atoms, i was called to supper, and a salad i had asked for was set before me. "it seems, then," said i aloud, "that if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar and oil, and slices of egg, had been flying about in the air from all eternity, it might at last happen by chance that there would come a salad." "yes," says my wife, "but not so nice and well dressed as this of mine is."' notwithstanding the frequent interruptions which, owing to various reasons, retarded his labours, kepler was able to bring to a successful completion the numerous and important works upon which he was engaged during his lifetime, the voluminous nature of which may be imagined when it is stated that he published thirty-three separate works, besides leaving behind twenty-two volumes of manuscript. during his researches on the motions of mars, kepler discovered that the planet sometimes travelled at an accelerated rate of speed, and at another time its pace was diminished. at one time he observed it to be in advance of the place where he calculated it should be found, and at another time it was behind it. this caused him considerable perplexity, and, feeling convinced in his mind that the form of the planet's orbit could not be circular, he was compelled to turn his attention to some other closed curve, by which those inequalities of motion could be explained. after years of careful observation and study, kepler arrived at the conclusion that the form of the planet's orbit is an ellipse, and that the sun occupies one of the foci. he afterwards determined that the orbits of all the planets are of an elliptical form. having discovered the true form of the planetary orbits, kepler next endeavoured to ascertain the cause which regulates the unequal motion that a planet pursues in its path. he observed that when a planet approached the sun its motion was accelerated, and as it receded from him its pace became slower. this he explained in his next great discovery by proving that an imaginary line, or radius-vector, extending from the centre of the sun to the centre of the planet 'describes equal areas in equal times.' when near the sun, or at perihelion, a planet traverses a larger portion of its arc in the same period of time than it does when at the opposite part of its orbit, or when at aphelion; but, as the areas of both are equal, it follows that the planet does not always maintain the same rate of speed, and that its velocity is greatest when nearest the sun, and least when most distant from him. by the application of his first and second laws kepler was able to formulate a third law. he found that there existed a remarkable relationship between the mean distances of the planets and the times in which they complete their revolutions round the sun, and discovered 'that the squares of the periodic times are to each in the same proportion as the cubes of the mean distances.' the periodic time of a planet having been ascertained, the square of the mean distance and the mean distance itself can be obtained. it is by the application of this law that the distances of the planets are usually calculated. these discoveries are known as kepler's laws, and are usually classified as follows:-- . 'the orbit described by every planet is an ellipse, of which the centre of the sun occupies one of the foci. . 'every planet moves round the sun in a plane orbit, and the radius-vector, or imaginary line joining the centre of the planet and the centre of the sun, describes equal areas in equal times. . 'the squares of the periodic times of any two planets are proportional to the cubes of their mean distances from the sun.'[ ] these remarkable discoveries do not embrace all the achievements by which kepler has immortalised his name, and earned for himself the proud title of 'legislator of the heavens;' he predicted transits of mercury and venus, made important discoveries in optics, and was the inventor of the astronomical telescope. galileo galilei, the famous italian astronomer and philosopher, and the contemporary of kepler and of milton, was born at pisa on february , . his father, who traced his descent from an ancient florentine family, was desirous that his son should adopt the profession of medicine, and with this intention he entered him as a student at the university of pisa. galileo, however, soon discovered that the study of mathematics and mechanical science possessed a greater attraction for his mind, and, following his inclinations, he resolved to devote his energies to acquiring proficiency in those subjects. in his attention was attracted by the oscillation of a brass lamp suspended from the ceiling of the cathedral at pisa. galileo was impressed with the regularity of its motion as it swung backwards and forwards, and was led to imagine that the pendulum movement might prove a valuable method for the correct measurement of time. the practical application of this idea he afterwards adopted in the construction of an astronomical clock. having become proficient in mathematics, galileo, whilst engaged in studying the writings of archimedes, wrote an essay on 'the hydrostatic balance,' and composed a treatise on 'the centre of gravity in solid bodies.' the reputation which he earned by these contributions to science procured for him the appointment of lecturer on mathematics at the university of pisa. galileo next directed his attention to the works of aristotle, and made no attempt to conceal the disfavour with which he regarded many of the doctrines taught by the greek philosopher; nor had he any difficulty in exposing their inaccuracies. one of these, which maintained that the heavier of two bodies descended to the earth with the greater rapidity, he proved to be incorrect, and demonstrated by experiment from the top of the tower at pisa that, except for the unequal resistance of the air, all bodies fell to the ground with the same velocity. as the chief expounder of the new philosophy, galileo had to encounter the prejudices of the followers of aristotle, and of all those who disliked any innovation or change in the established order of things. the antagonism which existed between galileo and his opponents, who were both numerous and influential, was intensified by the bitterness and sarcasm which he imparted into his controversies, and the attitude assumed by his enemies at last became so threatening that he deemed it prudent to resign the chair of mathematics in the university of pisa. in the following year he was appointed to a similar post at padua, where his fame attracted crowds of pupils from all parts of europe. in galileo visited rome. he was received with much distinction by the different learned societies, and was enrolled a member of the lyncæan academy. in two years after his visit to the capital he published a work in which he declared his adhesion to the copernican theory, and openly avowed his disbelief in the astronomical facts recorded in the scriptures. galileo maintained that the sacred writings were not intended for the purpose of imparting scientific information, and that it was impossible for men to ignore phenomena witnessed with their eyes, or disregard conclusions arrived at by the exercise of their reasoning powers. the champions of orthodoxy having become alarmed, an appeal was made to the ecclesiastical authorities to assist in suppressing this recent astronomical heresy, and other obnoxious doctrines, the authorship of which was ascribed to galileo. in , galileo was summoned before the inquisition to reply to the accusation of heresy. 'he was charged with maintaining the motion of the earth and the stability of the sun; with teaching this doctrine to his pupils; with corresponding on the subject with several german mathematicians; and with having published it, and attempted to reconcile it to scripture in his letters to mark velser in .' these charges having been formally investigated by the inquisition, cardinal bellarmine was authorised to communicate with galileo, and inform him that unless he renounced the obnoxious doctrines, and promised 'neither to teach, defend, or publish them in future,' it was decreed that he should be committed to prison. galileo appeared next day before the cardinal, and, without any hesitation, pledged himself that for the future he would adhere to the pronouncement of the inquisition. having, as they imagined, silenced galileo, the inquisition resolved to condemn the entire copernican system as heretical; and in order to effectually accomplish this, besides condemning the writings of galileo, they inhibited kepler's 'epitome of the copernican system,' and copernicus's own work, 'de revolutionibus orbium celestium.' whether it was that galileo regarded the inquisition as a body whose decrees were too absurd and unreasonable to be heeded, or that he dreaded the consequences which might have followed had he remained obstinate, we know that, notwithstanding the pledges which he gave, he was soon afterwards engaged in controversial discussion on those subjects which he promised not to mention again. on the accession of his friend cardinal barberini to the pontifical throne in , under the title of urban viii., galileo undertook a journey to rome to offer him his congratulations upon his elevation to the papal chair. he was received by his holiness with marked attention and kindness, was granted several prolonged audiences, and had conferred upon him several valuable gifts. notwithstanding the kindness of pope urban and the leniency with which he was treated by the inquisition, galileo, having ignored his pledge, published in a book, in dialogue form, in which three persons were supposed to express their scientific opinions. the first upheld the copernican theory and the more recent philosophical views; the second person adopted a neutral position, suggested doubts, and made remarks of an amusing nature; the third individual, called simplicio, was a believer in ptolemy and aristotle, and based his arguments upon the philosophy of the ancients. as soon as this work became publicly known, the enemies of galileo persuaded the pope that the third person held up to ridicule was intended as a representation of himself--an individual regardless of scientific truth, and firmly attached to the ideas and opinions associated with the writings of antiquity. almost immediately after the publication of the 'dialogues' galileo was summoned before the inquisition, and, notwithstanding his feeble health and the infirmities of advanced age, he was, after a long and tedious trial, condemned to abjure by oath on his knees his scientific beliefs. 'the ceremony of galileo's abjuration was one of exciting interest and of awful formality. clothed in the sackcloth of a repentant criminal, the venerable sage fell upon his knees before the assembled cardinals, and, laying his hand upon the holy evangelists, he invoked the divine aid in abjuring, and detesting, and vowing never again to teach the doctrines of the earth's motion and of the sun's stability. he pledged himself that he would nevermore, either in words or in writing, propagate such heresies; and he swore that he would fulfil and observe the penances which had been inflicted upon him.' 'at the conclusion of this ceremony, in which he recited his abjuration word for word and then signed it, he was conveyed, in conformity with his sentence, to the prison of the inquisition.'[ ] galileo's sarcasm, and the bitterness which he imparted into his controversies, were more the cause of his misfortunes than his scientific beliefs. when he became involved in difficulties he did not possess the moral courage to enable him to abide by the consequences of his acts; nor did he care to become a martyr for the sake of science, his submission to the inquisition having probably saved him from a fate similar to what befell bruno. though it would be impossible to justify galileo's want of faith in his dealings with the inquisition, yet one cannot help sympathising deeply with the aged philosopher, who, in this painful episode of his life, was compelled to go through the form of making a retractation of his beliefs under circumstances of a most humiliating nature. but the persecution of galileo did not delay the progress of scientific inquiry nor retard the advancement of the copernican theory, which, after the discovery by newton of the law of gravitation, was universally adopted as the true theory of the solar system. ferdinand, duke of tuscany, having exerted his influence with pope urban on behalf of galileo, he was, after a few days' incarceration, released from prison, and permission was given him to reside at siena, where he remained for six months. he was afterwards allowed to return to his villa at arcetri, and, though regarded as a prisoner of the inquisition, was permitted to pursue his studies unmolested for the remainder of his days. galileo died at arcetri on january , , when in the seventy-eighth year of his age. though not the inventor, he was the first to construct a refracting telescope and apply it to astronomical research. with this instrument he made a number of important discoveries which tended to confirm his belief in the truthfulness of the copernican theory. on directing his telescope to the sun, he discovered movable spots on his disc, and concluded from his observation of them that the orb rotated on his axis in about twenty-eight days. he also ascertained that the moon's illumination is due to reflected sunlight, and that her surface is diversified by mountains, valleys, and plains. on the night of january , , galileo discovered the four moons of jupiter. this discovery may be regarded as one of his most brilliant achievements with the telescope; and, notwithstanding the improvement in construction and size of modern instruments, no other satellite was discovered until near midnight on september , , when mr. e. e. barnard, with the splendid telescope of the lick observatory, added 'another gem to the diadem of jupiter.' the phases of venus and mars, the triple form of saturn, and the constitution of the milky way, which he found to consist of a countless multitude of stars, were additional discoveries for our knowledge of which we are indebted to galileo and his telescope. galileo made many other important discoveries in mechanical and physical science. he detected the law of falling bodies in their accelerated motion towards the earth, determined the parabolic law of projectiles, and demonstrated that matter, even if invisible, possessed the property of weight. in these pages a short historical description is given of the progress made in astronomical science from an early period to the time in which milton lived. the discoveries of copernicus, kepler, and galileo had raised it to a position of lofty eminence, though the law of gravitation, which accounts for the form and permanency of the planetary orbits, still remained undiscovered. theories formerly obscure or conjectural were either rejected or elucidated with accuracy and precision, and the solar system, having the sun as its centre, with his attendant family of planets and their satellites revolving in majestic orbits around him, presented an impressive spectacle of order, harmony, and design. chapter ii astronomy in the seventeenth century the seventeenth century embraces the most remarkable epoch in the whole history of astronomy. it was during this period that those wonderful discoveries were made which have been the means of raising astronomy to the lofty position which it now occupies among the sciences. the unrivalled genius and patient labours of the illustrious men whose names stand out in such prominence on the written pages of the history of this era have rendered it one of the most interesting and elevating of studies. though copernicus lived in the preceding century, yet the names of tycho brahé, kepler, galileo, and newton, testify to the greatness of the discoveries that were made during this period, which have surrounded the memories of those men with a lustre of undying fame. foremost among astronomers of less conspicuous eminence who made important discoveries in this century we find the name of huygens. christian huygens was born at the hague in . he was the second son of constantine huygens, an eminent diplomatist, and secretary to the prince of orange. huygens studied at leyden and breda, and became highly distinguished as a geometrician and scientist. he made important investigations relative to the figure of the earth, and wrote a learned treatise on the cause of gravity; he also determined with greater accuracy investigations made by galileo regarding the accelerated motion of bodies when subjected to the influence of that force. huygens admitted that the planets and their satellites attracted each other with a force varying according to the inverse ratio of the squares of their distances, but rejected the mutual attraction of the molecules of matter, believing that they possessed gravity towards a central point only, to which they were attracted. this supposition was at variance with the newtonian theory, which, however, was universally regarded as the correct one. huygens originated the theory by which it is believed that light is produced by the undulatory vibration of the ether; he also discovered polarization. up to this time the method adopted in the construction of clocks was not capable of producing a mechanism which measured time with sufficient accuracy to satisfy the requirements of astronomers. huygens endeavoured to supply this want, and applied his mechanical ingenuity in constructing a clock that could be relied upon to keep accurate time. though the pendulum motion was first adopted by galileo, he was unable to arrange its mechanism so that it should keep up a continuous movement. the oscillation of the pendulum ceased after a time, and a fresh impulse had to be applied to set it in motion. consequently, galileo's clock was of no service as a timekeeper. huygens overcame this difficulty by so arranging the mechanism of his clock that the balance, instead of being horizontal, was directed perpendicularly, and prolonged downwards to form a pendulum, the oscillations of which regulated the downward motion of the weight. this invention, which was highly applauded, proved to be of great service everywhere, and was especially valuable for astronomical purposes. huygens next directed his attention to the construction of telescopes, and displayed much skill in the grinding and polishing of lenses. he made several instruments superior in power and accuracy to any that existed previously, and with one of these made some remarkable discoveries when observing the planet saturn. the telescopic appearance of saturn is one of the most beautiful in the heavens. the planet, surrounded by two brilliant rings, and accompanied by eight attendant moons, surpasses all the other orbs of the firmament as an object of interest and admiration. to the naked eye, saturn is visible as a star of the first magnitude, and was known to the ancients as the most remote of the planets. travelling in space at a distance of nearly one thousand millions of miles from the sun, the planet accomplishes a revolution of its mighty orbit in twenty-nine and a half years. galileo was the first astronomer who directed a telescope to saturn. he observed that the planet presented a triform appearance, and that on each side of the central globe there were two objects, in close contact with it, which caused it to assume an ovoid shape. after further observation, galileo perceived that the lateral bodies gradually decreased in size, until they became invisible. at the expiration of a certain period of time they reappeared, and were observed to go through a certain cycle of changes. by the application of increased telescopic power it was discovered that the appendages were not of a rounded form, but appeared as two small crescents, having their concave surfaces directed towards the planet and their extremities in contact with it, resembling the manner in which the handles are attached to a cup. these objects were observed to go through a series of periodic changes. after having become invisible, they reappeared as two luminous straight bands, projecting from each side of the planet; during the next seven or eight years they gradually opened out, and assumed a crescentic form; they afterwards began to contract, and on the expiration of a similar period, during which time they gradually decreased in size, they again became invisible. it was perceived that the appendages completed a cycle of their changes in about fifteen years. in , huygens, with a telescope constructed by himself, was enabled to solve the enigma which for so many years baffled the efforts of the ablest astronomers. he announced his discovery in the form of a latin cryptograph which, when deciphered, read as follows:-- 'annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam inclinatio.' 'the planet is surrounded by a slender flat ring everywhere distinct from its surface, and inclined to the ecliptic.' huygens perceived the shadow of the ring thrown on the planet, and was able to account in a satisfactory manner for all the phenomena observed in connection with its variable appearance. the true form of the ring is circular, but by us it is seen foreshortened; consequently, when the earth is above or below its plane, it appears of an elliptical shape. when the position of the planet is such that the plane of the ring passes through the sun, the edge of the ring only is illumined, and then it becomes invisible for a short period. in the same manner, when the plane of the ring passes through the earth, the illumined edge of the ring is not of sufficient magnitude to appear visible, but as the enlightened side of the plane becomes more inclined towards the earth, the ring comes again into view. when the plane of the ring passes between the earth and the sun, the unillumined side of the ring is turned towards the earth, and during the time it remains in this position it is invisible. huygens discovered the sixth satellite of saturn (titan), and also the great nebula in orion. johann hevelius, a celebrated prussian astronomer, was born at dantzig in , and died in that city in . he was a man of wealth, and erected an observatory at his residence, where, for a period of forty years, he carried out a series of astronomical observations. he constructed a chart of the stars, and in order to complete his work, formed nine new constellations in those spaces in the celestial vault which were previously un-named. they are known by the names camelopardus, canes venatici, coma bernices, lacerta, leo minor, lynx, monoceros, sextans, and vulpecula. he also executed a chart of the moon's surface, wrote a description of the lunar spots, and discovered the libration of the moon in longitude. on may , , hevelius observed a transit of mercury, a description of which he published, and included with it horrox's treatise on the first-recorded transit of venus. this work, after having passed through several hands, became the property of hevelius, who was capable of appreciating its merits. the manuscript was sent to him by huygens, and in acknowledging it he writes: 'how greatly does my mercury exult in the joyous prospect that he may shortly fold within his arms horrox's long looked-for and beloved venus! he renders you unfeigned thanks that by your permission this much-desired union is about to be celebrated, and that the writer is able, with your concurrence, to introduce them both together to the public.' hevelius made numerous researches on comets, and suggested that the form of their paths might be a parabola. giovanni domenico cassini was born at perinaldo, near nice, in . he studied at genoa and bologna, and was afterwards appointed to the chair of astronomy at the latter university. he was a man of high scientific attainments, and made many important astronomical discoveries. in he became director of the royal observatory at paris, and devoted a long life to trying and difficult observations, which in his later years deprived him of his eyesight. in cassini proved beyond doubt that jupiter rotated on his axis, and also assigned his period of rotation with considerable accuracy. he published tables of the planet's satellites, and determined their motions from observations of their eclipses. he ascertained the periods of rotation of venus and mars; executed a chart of the lunar surface, and observed an occultation of jupiter by the moon. cassini discovered the dual nature of saturn's ring, having perceived that instead of one there are two concentric rings separated by a dark space. he also discovered four of the planet's satellites--viz. japetus, rhea, dione, and tethys. he made a near approximation to the solar parallax by means of researches on the parallax of mars, and investigated some irregularities of the moon's motion. cassini discovered the belts of jupiter, and also the zodiacal light, and established the coincidence of the nodes of the lunar equator and orbit. jaques cassini, son of giovanni, was born at paris in . he followed in his father's footsteps, and wrote several treatises on astronomical subjects. he investigated the period of the rotation of venus on her axis, and upheld the results arrived at by his father, which were afterwards confirmed by observations made by schroeter. cassini made some valuable researches with regard to the proper motion of the stars, and demonstrated that their change of position on the celestial vault was real, and not caused by a displacement of the ecliptic. he attempted to ascertain the apparent diameter of sirius, and made observations with regard to the visibility of the stars. the cassini family produced several generations of eminent astronomers, whose discoveries and investigations were of much value in advancing the science of astronomy. olaus roemer, an eminent danish astronomer, was born at copenhagen september , . when picard, a french astronomer, visited denmark in , for the purpose of ascertaining the exact position of 'uranienburg,' the site of tycho brahé's observatory, he made the acquaintance of roemer, who was engaged in studying mathematics and astronomy under erasmus bartolinus. having perceived that the young man was gifted with no ordinary degree of talent, he secured his services to assist him in his observations, and, on the conclusion of his labours, picard was so much impressed with the ability displayed by roemer, that he invited him to accompany him to france. this invitation he accepted, and took up his residence in the french capital, where he continued to prosecute his astronomical studies. in roemer communicated to the academy of sciences a paper, in which he announced his discovery of the progressive transmission of light. it was believed that light travelled instantaneously, but roemer was able to demonstrate the inaccuracy of this conclusion, and determined that light travels through space with a measurable velocity. by diligently observing the eclipses of jupiter's satellites, roemer perceived that sometimes they occurred before, and sometimes after their predicted times. this irregularity, he discovered, depended upon the position of the earth with regard to jupiter. when the earth, in traversing her orbit, moved round to the opposite side of the sun, thereby bringing jupiter into conjunction, an eclipse occurred sixteen minutes twenty-six seconds later than it did when jupiter was in opposition or nearest to the earth. as there existed an impression that light travelled instantaneously, it was believed that an eclipse occurred at the moment it was perceived in the telescope. this, however, was not so. roemer, after a long series of observations, concluded that the discrepancies were due to the fact that light travels with a measurable velocity, and that it requires a greater length of time, upwards of sixteen minutes, to traverse the additional distance--the diameter of the earth's orbit--which intervenes between the earth and jupiter, when the planet is in conjunction, as compared with the distance between the earth and jupiter, when the latter is in opposition. this discovery of roemer's was the means of enabling the velocity of light to be ascertained, which, according to recent calculations, is about , miles a second. as an acknowledgment of the importance of his communication, roemer was awarded a seat in the academy, and apartments were assigned to him at the royal observatory, where he carried on his astronomical studies. in roemer returned to denmark, and was appointed professor of mathematics in the university of copenhagen; he was also entrusted with the care of the city observatory--a duty which his reputation as an astronomer eminently qualified him to undertake. the transit instrument--a mechanism of much importance to astronomers--was invented by roemer in ; it consists of a telescope fixed to a horizontal axis, and adjusted so as to revolve in the plane of the meridian. it is employed in observing the passage of the heavenly bodies across the observer's meridian. to note accurately by means of the astronomical clock the exact instant of time at which a celestial body crosses the centre of the field of view is the essential part of a transit observation. small transit instruments are employed for taking the time and for regulating the observatory clock, but large instruments are used for delicate and exact observations of right ascensions and declinations of stars of different magnitudes. meridian, and altitude and azimuth circles, are important astronomical appliances, which owe their existence to the inventive skill of this distinguished astronomer. roemer resided for many years at the observatory in the city of copenhagen, where he pursued his astronomical studies until the time of his death, which occurred in . he meritoriously attempted to determine the parallax of the fixed stars; and it is said that the astronomical calculations and observations which he left behind him were so voluminous as to equal in number those made by tycho brahé, nearly all of which perished in a great conflagration that destroyed the observatory and a large portion of the city of copenhagen in . among other astronomers of this century whose names deserve recording were descartes and gassendi, whose mathematical researches in their application to astronomy were of much value; fabricius, torricelli, and maraldi, who by their observations and investigations added many facts to the general knowledge of the science; and bayer, to whom belongs the distinction of having constructed the first star-atlas. in our own country during this period astronomy was cultivated by a few enthusiastic men, who devoted their time and talents to promoting the advancement of the science. it, however, received no recognition as a subject of study at any of the universities, and no public observatory existed in great britain. though it was not until towards the close of the century that the attention of all europe was directed to england in admiration of the discoveries of the illustrious newton, yet astronomy had its humble votaries, and chief among those was a young clergyman of the name of horrox. jeremiah horrox was born at toxteth, near liverpool, in --close on three centuries ago. little is known of his family. his parents have been described as persons who occupied a humble position in life, but, as they were able to give their son a classical education which fitted him for one of the learned professions, it is probable they were not so obscure as they have been represented to be. having received his early education at toxteth, horrox afterwards proceeded to cambridge, and was entered as a student at emmanuel college on may , , when in his fourteenth year. at the university he devoted himself to the study of classics, especially latin, which in those days was the language adopted by men of learning, when engaged in writing works of a philosophical and scientific character. after having remained at cambridge for three years, horrox returned to his native county, and was appointed curate of hoole, a place about eight miles distant from preston. hoole is described as a narrow low-lying strip of land consisting largely of moss, and almost converted into an island by the waters of martin mere on the south, and the ribble on the north; and, though doubtless an open and favourable situation for astronomical observation, it could not have been attractive as a place of residence. yet it was here on november , , that horrox made his famous observation of the first recorded transit of venus, an occurrence with which his name will be for ever associated. it was while at cambridge that horrox first turned his attention to the study of astronomy. his love of the sublime, and the captivating influence exerted on his mind by the contemplation of the heavenly bodies, induced him to adopt astronomy as a pursuit congenial to his tastes, and capable of exercising his highest mental powers. having this object in view, he applied himself with much earnestness to the study of mathematics; he had, however, to rely mainly upon his own exertions, for at that time no branch of physical or mathematical science was taught at cambridge, and consequently he obtained no professional instruction. it was so also with astronomy, which, as a science, was scarcely known in this country; no regular record of astronomical observations was kept by any individual observer, and no public observatory existed in england or in france. the disadvantages and obstacles which horrox had to encounter may be best described by quoting his own words. he writes: 'there were many hindrances. the abstruse nature of the study, my inexperience and want of means dispirited me. i was much pained not to have any one to whom i could look for guidance, or indeed for the sympathy of companionship in my endeavours, and i was assailed by the languor and weariness which are inseparable from every great undertaking. what then was to be done? i could not make the pursuit an easy one, much less increase my fortune, and least of all imbue others with a love for astronomy; and yet to complain of philosophy on account of its difficulties would be foolish and unworthy. i determined, therefore, that the tediousness of study should be overcome by industry; my poverty--failing a better method--by patience; and that instead of a master i would use astronomical books. armed with these weapons i would contend successfully; and, having heard of others acquiring knowledge without greater help, i would blush that any one should be able to do more than i, always remembering that word of virgil's-- totidem nobis animaeque manusque. having heard much praise bestowed upon the works of lansberg, a flemish astronomer, horrox thought it would be to his advantage to procure a copy of his writings. this he succeeded in obtaining after some difficulty, and devoted a considerable time to calculating ephemerides, based upon the lansberg tables, but after making a number of computations he discovered that they were unreliable and inaccurate. in the year horrox made the acquaintance of william crabtree, a devoted astronomer, who lived at broughton, a suburb of manchester. a close friendship soon existed between the two men, and they carried on an active correspondence about matters relating to the science which they both loved so well. crabtree, who was an unbeliever in lansberg, urged horrox to discard the flemish astronomer's works, and devote his talents to the study of tycho brahé and kepler. this advice led horrox to make a more rigorous examination of the lansberg tables, and after comparing them with the observations made by crabtree, which coincided with his own, he resolved to renounce them. acting on the advice of his friend, horrox directed his attention to the writings of kepler. the youthful astronomer soon realised their value, and was charmed with the accuracy of observation and inductive reasoning displayed in the elucidation of those general laws which constituted a new era in the history of astronomy. the rudolphine tables, which were the astronomical calculations commenced by tycho brahé, and completed by kepler, were regarded by horrox as much superior to those of lansberg; but it occurred to him that they might be improved by changing some of the numbers, and yet retaining the hypotheses. to this task he applied himself with much earnestness and assiduity, and after close application and laborious study he accomplished the arduous undertaking of bringing those tables to a high state of perfection. in his investigation of the lunar theory, horrox outstripped all his predecessors, and sir isaac newton distinctly affirms he was the first to discover that the moon's motion round the earth is in the form of an ellipse with the centre in the lower focus. besides having made this discovery, horrox was able to explain the causes of the inequalities of the moon's motion, which render the exact computation of her elements so difficult. the annual equation, an irregularity discovered by tycho brahé, which is produced by the increase and decrease of the sun's disturbing force as the earth approaches or recedes from him in her orbit, had its value first assigned by horrox. this he calculated to be eleven minutes sixteen seconds, which is within four seconds of what it has since been proved to be by the most recent observations. the evection, an irregular motion of the moon discovered by ptolemy, whereby her mean longitude is increased or diminished, was explained by horrox as depending upon the libratory motion of the apsides, and the change which takes place in the eccentricity of the lunar orbit. these discoveries were made by horrox before he attained the age of twenty years, and if his reputation had alone rested upon them his name would have been honourably associated with those who have attained to the highest eminence in astronomy. another achievement which adds lustre to horrox's name consists in his detection of the inequality in the mean motions of jupiter and saturn. he also directed his attention to the study of cometary bodies, and arrived at certain conclusions with regard to the nature of their movements. at first, he believed like kepler that comets were projected in straight lines from the sun; this supposition having been upheld on account of the great elongation of their orbits. he next perceived that their velocity increased as they approached the sun, and decreased as they receded from him. afterwards he says, 'they move in an elliptic figure or near it,' and finally he arrived at the conclusion that 'comets move in elliptical orbits, being carried round the sun with a velocity which is probably variable.' this theory has been verified by numerous observations, and is now generally accepted by astronomers. horrox also made a series of observations on the tides. he notified the extent of their rise and fall at different periods, and investigated other phenomena associated with their ebb and flow. after having continued his observations for some time, he wrote to his friend crabtree, and informed him that he had perceived many interesting details which had not been previously described, and he hoped to be able to arrive at some important conclusions with regard to their nature and cause. unfortunately, horrox's writings on this subject, along with many other important papers, have been lost or destroyed. we are therefore ignorant of the result of his researches, which were the first undertaken by any person for the purpose of scientific inquiry. from his study of the lansberg and rudolphine tables, horrox arrived at the conclusion that a transit of venus would occur on november , . this transit was for some unaccountable reason overlooked by kepler, who predicted one in , and the next not until . the transit of was not visible in europe. we are indebted to horrox for a description of the transit of --the first that was ever observed of which there is any record; and were it not for the accuracy of his calculations, the occurrence of the phenomenon would have been unperceived, and no history of the conjunction would have been handed down to posterity. as soon as horrox had assured himself of the time when the transit would take place, he wrote to crabtree to inform him of the date, and asked him to make observations with his telescope, and especially to examine the diameter of the planet, which he thought had been over-estimated. he also requested him to write to dr. foster of cambridge, and inform him of the expected event, as it was desirable that the transit should be observed from several places in consequence of the possibility of failure, owing to an overcast sky. his letter is dated october , . he says: 'my reason for now writing is to advise you of a remarkable conjunction of the sun and venus on the th of november, when there will be a transit. as such a thing has not happened for many years past, and will not occur again in this century, i earnestly entreat you to watch attentively with your telescope in order to observe it as well as you can. 'notice particularly the diameter of venus, which is stated by kepler to be seven minutes, and by lansberg to be eleven, but which i believe to be scarcely greater than one minute.' in describing the method which he adopted for observing the transit, horrox writes as follows: 'having attentively examined venus with my instrument, i described on a sheet of paper a circle, whose diameter was nearly equal to six inches--the narrowness of the apartment not permitting me conveniently to use a larger size. i divided the circumference of this circle into degrees in the usual manner, and its diameter into thirty equal parts, which gives about as many minutes as are equivalent to the sun's apparent diameter. each of these thirty parts was again divided into four equal portions, making in all one hundred and twenty; and these, if necessary, may be more minutely subdivided. the rest i left to ocular computation, which, in such small sections, is quite as certain as any mechanical division. suppose, then, each of these thirty parts to be divided into sixty seconds, according to the practice of astronomers. when the time of the observation approached, i retired to my apartment, and, having closed the windows against the light, i directed my telescope--previously adjusted to a focus--through the aperture towards the sun, and received his rays at right angles upon the paper already mentioned. the sun's image exactly filled the circle, and i watched carefully and unceasingly for any dark body that might enter upon the disc of light. 'although the corrected computation of venus' motions which i had before prepared, and on the accuracy of which i implicitly relied, forbade me to expect anything before three o'clock in the afternoon of the th, yet since, according to the calculations of most astronomers, the conjunction should take place sooner--by some even on the rd--i was unwilling to depend entirely on my own opinion, which was not sufficiently confirmed, lest by too much self-confidence i might endanger the observation. anxiously intent, therefore, on the undertaking through the greater part of the rd, and on the whole of the th, i omitted no available opportunity of observing her ingress. i watched carefully on the th from sunrise to nine o'clock, and from a little before ten until noon, and at one in the afternoon, being called away in the intervals by business of the highest importance, which for these ornamental pursuits i could not with propriety neglect.[ ] but during all this time i saw nothing in the sun except a small and common spot, consisting as it were of three points at a distance from the centre towards the left, which i noticed on the preceding and following days. this evidently had nothing to do with venus. about fifteen minutes past three in the afternoon, when i was again at liberty to continue my labours, the clouds, as if by divine interposition, were entirely dispersed, and i was once more invited to the grateful task of repeating my observations. i then beheld a most agreeable spectacle--the object of my sanguine wishes; a spot of unusual magnitude and of a perfectly circular shape, which had already fully entered upon the sun's disc on the left, so that the limbs of the sun and venus precisely coincided, forming an angle of contact. not doubting that this was really the shadow of the planet, i immediately applied myself sedulously to observe it. 'in the first place, with respect to the inclination, the line of the diameter of the circle being perpendicular to the horizon, although its plane was somewhat inclined on account of the sun's altitude, i found that the shadow of venus at the aforesaid hour--namely, fifteen minutes past three--had entered the sun's disc about ° ´, certainly between ° and °, from the top towards the right. this was the appearance in the dark apartment; therefore, out of doors, beneath the open sky, according to the laws of optics, the contrary would be the case, and venus would be below the centre of the sun, distant ° ´ from the lower limbs or the nadir, as the arabians term it. the inclination remained to all appearances the same until sunset, when the observation was concluded. 'in the second place, the distance between the centres of venus and the sun i found by three observations to be as follows:-- the hour. | distance of the centres. | at · by the clock | ´ ´´ " · " | ´ ´´ " · " | ´ ´´ " · the apparent sunset. | the true setting being · , and the apparent about minutes later, the difference being caused by refraction. the clock therefore was sufficiently correct. 'in the third place i found after careful and repeated observation that the diameter of venus, as her shadow was depicted on the paper, was larger indeed than the thirtieth part of the solar diameter, though not more so than the sixth, or at the utmost the fifth of such a part. therefore let the diameter of the sun be to the diameter of venus as ´ to ´ ´´. certainly her diameter never equalled ´ ´´, scarcely perhaps ´ ´´, and this was evident as well when the planet was near the sun's limb as when far distant from it. [illustration: venus on the sun's disc.] 'this observation was made in an obscure village where i have long been in the habit of observing, about fifteen miles to the north of liverpool, the latitude of which i believe to be ° ´, although by common maps it is stated at ° ´, therefore the latitude of the village will be ° ´, and longitude of both ° ´ from the fortunate islands, now called the canaries. this is ° ´ to the west of uraniburg in denmark, the longitude of which is stated by brahé, a native of the place, to be ° ´ from these islands. 'this is all i could observe respecting this celebrated conjunction during the short time the sun remained in the horizon: for although venus continued on his disc for several hours, she was not visible to me longer than half an hour on account of his so quickly setting. nevertheless, all the observations which could possibly be made in so short a time i was enabled by divine providence to complete so effectually that i could scarcely have wished for a more extended period. the inclination was the only point upon which i failed to attain the utmost precision; for, owing to the rapid motion of the sun it was difficult to observe with certainty to a single degree, and i frankly confess that i neither did nor could ascertain it. but all the rest is sufficiently accurate, and as exact as i could desire.' besides having ascertained that the diameter of venus subtends an angle not much greater than one minute of arc, horrox reduced the horizontal solar parallax from fifty-seven seconds as stated by kepler to fourteen seconds, a calculation within one and a half second of the value assigned to it by halley sixty years after. he also reduced the sun's semi-diameter. crabtree, to whom horrox refers as 'his most esteemed friend and a person who has few superiors in mathematical learning,' made preparations to observe the transit similar to those already described. but the day was unfavourable, dark clouds obscured the sky and rendered the sun invisible. crabtree was in despair, and relinquished all hope of being able to witness the conjunction. however, just before sunset there was a break in the clouds, and the sun shone brilliantly for a short interval. crabtree at once seized his opportunity, and to his intense delight observed the planet fully entered upon the sun's disc. instead of proceeding to take observations, he was so overcome with emotion at the sight of the phenomenon, that he continued to gaze upon it with rapt attention, nor did he recover his self-possession until the clouds again hid from his view the setting sun.[ ] crabtree's observation of the transit was, however, not a fruitless one. he drew from memory a diagram showing the exact position of venus on the sun's disc, which corresponded in every respect with horrox's observation; he also estimated the diameter of the planet to be / that of the sun, which when calculated gives one minute three seconds; horrox having found it to be one minute twelve seconds. this transit of venus is remarkable as having been the first ever observed of which there is any record, and for this we are indebted to the genius of horrox, who by a series of calculations, displaying a wonderfully accurate knowledge of mathematics, was enabled to predict the occurrence of the phenomenon on the very day, and almost at the hour it appeared, and of which he and his friend crabtree were the only observers. having thought it desirable to write an account of the transit, horrox prepared an elegant latin treatise, entitled 'venus in sole visa'--'venus seen in the sun;' but not knowing what steps to take with regard to its publication, he requested crabtree to communicate with his bookseller and obtain his advice on the matter. in the meantime horrox returned to toxteth, and arranged to fulfil a long-promised visit to crabtree, which he looked forward to with much pleasure, as it would afford him an opportunity of discussing with his friend many matters of interest to both. this visit was frustrated in a manner altogether unexpected. for we read that horrox was seized with a sudden and severe illness, the nature of which is not known, and that his death occurred on the day previous to that of his intended visit to his friend at broughton. he expired on january , , when in the rd year of his age. his death was a great grief to crabtree, who, in one of his letters, describes it as 'an irreparable loss:' and it is believed that he only survived him a few years.[ ] of the papers left by horrox, only a few have been preserved, and these were discovered in crabtree's house after his death. among them was his treatise on the transit of venus which, with other papers, was purchased by dr. worthington, fellow of emmanuel college, cambridge, a man of learning, who was capable of appreciating their value. ultimately, the treatise fell into the possession of hevelius, a celebrated german astronomer, who published it along with a dissertation of his own, describing a transit of mercury. horrox did not live to see any of his writings published, nor was any monument erected to his memory until nearly two hundred years after his death. but his name, though long forgotten except by astronomers, is now engraved on marble in westminster abbey. had his life been spared, it would have been difficult to foretell to what eminence and fame he might have risen, or what further discoveries his genius might have enabled him to make. few among english astronomers will hesitate to rank him next with the illustrious newton, and all will agree with herschel, who called him 'the pride and the boast of british astronomy.' william gascoigne was born in , in the parish of rothwell, in the county of york, and afterwards resided at middleton, near leeds. he was a man of an inventive turn of mind, and possessed good abilities, which he devoted to improving the methods of telescopic observation. at an early age he was occupied in observing celestial objects, making researches in optics, and acquiring a proficient knowledge of astronomy. among his acquaintances were crabtree and horrox, with whom he carried on a correspondence on matters appertaining to their favourite study. the measurement of small angles was found at all times to be one of the greatest difficulties which astronomers had to contend with. tycho brahé was so misled by his measurements of the apparent diameters of the sun and moon, that he concluded a total eclipse of the sun was impossible. gascoigne overcame this difficulty by his invention of the micrometer. this instrument, when applied to a telescope, was found to be of great service in the correct measurement of minute angles and distances, and was the means of greatly advancing the progress of practical astronomy in the seventeenth century. a micrometer consists of a short tube, across the opening of which are stretched two parallel wires; these being intersected at right angles by a third. the wires are moved to or from each other by delicately constructed screws, to which they are attached. each revolution, or part of a revolution, of a screw indicates the distance by which the wires are moved. this apparatus, when placed in the focus of a lens, gives very accurate measurements of the diameters of celestial objects. it was successfully used by gascoigne in determining the apparent diameters of the sun, moon, and several of the planets, and the mutual distances of the stars which form the pleiades. crabtree, after having paid gascoigne a visit in , describes in a letter to horrox the impression created on his mind by the micrometer. he writes: 'the first thing mr. gascoigne showed me was a large telescope, amplified and adorned with new inventions of his own, whereby he can take the diameters of the sun or moon, or any small angle in the heavens or upon the earth, most exactly through the glass to a second.' the micrometer is now regarded as an indispensable appliance in the observatory; the use of a spider web reticule instead of wire having improved its efficiency. gascoigne was one of the earliest astronomers who recognised the value of the keplerian telescope for observational purposes, and sherburn affirms that he was the first to construct an instrument of this description having two convex lenses. whether this be true or not, it is certain that he applied the micrometer to the telescope, and was the first to use telescopic sights, by means of which he was able to fix the optical axis of his telescope, and ascertain by observation the apparent positions of the heavenly bodies. crabtree, in a letter to gascoigne, says: 'could i purchase it with travel, or procure it with gold, i would not be without a telescope for observing small angles in the heavens; or want the use of your device of a glass in a cane upon the movable ruler of your sextant, as i remember for helping to the exact point of the sun's rays.' it was not known until the beginning of the eighteenth century that gascoigne had invented and used telescopic sights for the purpose of making accurate astronomical observations. the accidental discovery of some documents which contained a description of his appliances was the means by which this became known. townley states that gascoigne had completed a treatise on optics, which was ready for publication, but that no trace of the manuscript could be discovered after his death. having embraced the royalist cause, william gascoigne joined the forces of charles i., and fell in the battle of marston moor on july , . the early death of this young and remarkably clever man was a severe blow to the science of astronomy in england. the invention of logarithms, by baron napier, of merchistoun, was found to be of inestimable value to astronomers in facilitating and abbreviating the methods of astronomical calculation. by the use of logarithms, arithmetical computations which necessitated laborious application for several months could with ease be completed in as many days. it was remarked by laplace that this invention was the means of doubling the life of an astronomer, besides enabling him to avoid errors and the tediousness associated with long and abstruse calculations. thomas harriot, an eminent mathematician, and an assiduous astronomer, made some valuable observations of the comet of . he was one of the earliest observers who made use of the telescope, and it was claimed on his behalf that he discovered jupiter's satellites, and the spots on the sun, independently of galileo. other astronomers have been desirous of sharing this honour, but it has been conclusively proved that galileo was the first who made those discoveries. the investigations of norwood and gilbert, the mechanical genius of hooke, and the patient researches of flamsteed--the first astronomer royal--were of much value in perfecting many details associated with the study of astronomy. the royal observatory at greenwich was founded in . the building was erected under a warrant from charles ii. it announces the desire of the sovereign to build a small observatory in the park at greenwich, 'in order to the finding out of the longitude for perfecting the art of navigation and astronomy.' this action on the part of the king may be regarded as the first public acknowledgment of the usefulness of astronomy for national purposes. since its erection, the observatory has been presided over by a succession of talented men, who have raised it to a position of eminence and usefulness unsurpassed by any similar institution in this or any other country. the well-known names of flamsteed, halley, bradley, and airy, testify to the valuable services rendered by those past directors of the greenwich observatory in the cause of astronomical science. if we take a general survey of the science of astronomy as it existed from to --a period that embraced the time in which milton lived--we shall find that it was still compassed by ignorance, superstition, and mystery. astrology was zealously cultivated; most persons of rank and position had their nativity or horoscope cast, and the belief in the ruling of the planets, and their influence on human and terrestrial affairs, was through long usage firmly established in the public mind. indeed, at this time, astronomy was regarded as a handmaid to astrology; for, with the aid of astronomical calculation, the professors of this occult science were enabled to predict the positions of the planets, and by this means practised their art with an apparent degree of truthfulness. although over one hundred years had elapsed since the death of copernicus, his theory of the solar system did not find many supporters, and the old forms of astronomical belief still retained their hold on the minds of the majority of philosophic thinkers. this can be partly accounted for, as many of the ptolemaic doctrines were at first associated with the copernican theory, nor was it until a later period that they were eliminated from the system. though copernicus deserved the credit of having transferred the centre of our system from the earth to the sun, yet his theory was imperfect in its details, and contained many inaccuracies. he believed that the planets could only move round the sun in circular paths, nor was he capable of conceiving of any other form of orbit in which they could perform their revolutions. he was therefore compelled to retain the use of cycles and epicycles, in order to account for irregularities in the uniformly circular motions of those bodies. we are indebted to the genius of kepler for having placed the copernican system upon a sure and irremovable basis, and for having raised astronomy to the position of a true physical science. by his discovery that the planets travel round the sun in elliptical orbits, he was enabled to abolish cycles and epicycles, which created such confusion and entanglement in the system, and to explain many apparent irregularities of motion by ascribing to the sun his true position with regard to the motions of the planets. after the death of kepler, which occurred in , the most eminent supporter of the copernican theory was the illustrious galileo, whose belief in its accuracy and truthfulness was confirmed by his own discoveries. five of the planets were known at this time--viz. mercury, venus, mars, jupiter, and saturn; the latter, which revolves in its orbit at a profound distance from the sun, formed what at that time was believed to be the boundary of the planetary system. the distance of the earth from the sun was approximately known, and the orb was observed to rotate on his axis. it was also ascertained that the moon shone by reflected light, and that her surface was varied by inequalities resembling those of our earth. the elliptical form of her orbit had been discovered by horrox, and her elements were computed with a certain degree of accuracy. the cloudy luminosity of the milky way had been resolved into a multitude of separate stars, disclosing the immensity of the stellar universe. the crescent form of the planet venus, the satellites of jupiter and of saturn, and the progressive motion and measurement of light, had also been discovered. observations were made of transits of mercury and venus, and refracting and reflecting telescopes were invented. the law of universal gravitation, a power which retains the earth and planets in their orbits, causing them year after year to describe with unerring regularity their oval paths round the sun, was not known at this time. though newton was born in , he did not disclose the results of his philosophic investigations until --thirteen years after the death of milton--when, in the 'principia,' he announced his discovery of the great law of universal gravitation. kepler, though he discovered the laws of planetary motion, was unable to determine the motive force which guided and retained those bodies in their orbits. it was reserved for the genius of newton to solve this wonderful problem. this great philosopher was able to prove 'that every particle of matter in the universe attracts every other particle with a force proportioned to the mass of the attracting body, and inversely as the square of the distance between them.' newton was capable of demonstrating that the force which guides and retains the earth and planets in their orbits resides in the sun, and by the application of this law of gravitation he was able to explain the motions of all celestial bodies entering into the structure of the solar system. this discovery may be regarded as the crowning point of the science of astronomy, for, upon the unfailing energy of this mysterious power depend the order and stability of the universe, extending as it does to all material bodies existing in space, guiding, controlling, and retaining them in their several paths and orbits, whether it be a tiny meteor, a circling planet, or a mighty sun. the nature of cometary bodies and the laws which govern their motions were at this time still enshrouded in mystery, and when one of those erratic wanderers made its appearance in the sky it was beheld by the majority of mankind with feelings of awe and superstitious dread, and regarded as a harbinger of evil and disaster, the precursor of war, of famine, or the overthrow of an empire. newton, however, was able to divest those bodies of the mystery with which they were surrounded by proving that any conic section may be described about the sun, consistent with the law of gravitation, and that comets, notwithstanding the eccentricity of their orbits, obey the laws of planetary motion. beyond the confines of our solar system, little was known of the magnitude and extent of the sidereal universe which occupies the infinitude of space by which we are surrounded. the stars were recognised as self-luminous bodies, inconceivably remote, and although they excited the curiosity of observers, and conjectures were made as to their origin, yet no conclusive opinions were arrived at with regard to their nature and constitution, and except that they were regarded as glittering points of light which illumine the firmament, all else appertaining to them remained an unravelled mystery. even copernicus had no notion of a universe of stars. galileo, by his discovery that the galaxy consists of a multitude of separate stars too remote to be defined by ordinary vision, demonstrated how vast are the dimensions of the starry heavens, and on what a stupendous scale the universe is constructed. but at this time it had not occurred to astronomers, nor was it known until many years after, that the stars are suns which shine with a splendour resembling that of our sun, and in many instances surpassing it. it was not until this truth became known that the glories of the sidereal heavens were fully comprehended, and their magnificence revealed. it was then ascertained that the minute points of light which crowd the fields of our largest telescopes, in their aggregations forming systems, clusters, galaxies, and universes of stars, are shining orbs of light, among the countless multitudes of which our sun may be numbered as one. chapter iii milton's astronomical knowledge it would be reasonable to imagine that milton's knowledge of astronomy was comprehensive and accurate, and superior to that possessed by most scientific men of his age. his scholarly attainments, his familiarity with ancient history and philosophy, his profound learning, and the universality of his general knowledge, would lead one to conclude that the science which treats of the mechanism of the heavens, and especially the observational part of it--which at all times has been a source of inspiration to poets of every degree of excellence--was to him a study of absorbing interest, and one calculated to make a deep impression upon his devoutly poetical mind. the serious character of milton's verse, and the reverent manner in which celestial incidents and objects are described in it, impress one with the belief that his contemplation of the heavens, and of the orbs that roll and shine in the firmament overhead, afforded him much enjoyment and meditative delight. for no poet, in ancient or in modern times, has introduced into his writings with such frequency, or with such pleasing effect, so many passages descriptive of the beauty and grandeur of the heavens. no other poet, by the creative effort of his imagination, has soared to such a height; nor has he ever been excelled in his descriptions of the celestial orbs, and of the beautiful phenomena associated with their different motions. in his minor poems, which were composed during his residence at horton, a charming rural retreat in buckinghamshire, where the freshness and varied beauty of the landscape and the attractive aspects of the midnight sky were ever before him, we find enchanting descriptions of celestial objects, and especially of those orbs which, by their brilliancy and lustre, have always commanded the admiration of mankind. for example, in 'l'allegro' there are the following lines:-- right against the eastern gate where the great sun begins his state, robed in flames and amber light, the clouds in thousand liveries dight; and in 'il penseroso'-- to behold the wandering moon, riding near her highest noon, like one that had been led astray through the heaven's wide pathless way, and oft as if her head she bowed, stooping through a fleecy cloud. in the happy choice of his theme, and by the comprehensive manner in which he has treated it, milton has been enabled by his poetic genius to give to the world in his 'paradise lost' a poem which, for sublimity of thought, loftiness of imagination, and beauty of expression in metrical verse, is unsurpassed in any language. it is, however, our intention to deal only with those passages in the poem in which allusion is made to the heavenly bodies, and to incidents and occurrences associated with astronomical phenomena. in the exposition and illustration of these it has been considered desirable to adopt the following general classification:-- . to ascertain the extent of milton's astronomical knowledge. . to describe the starry heavens and the celestial objects mentioned in 'paradise lost.' . to exemplify the use which milton has made of astronomy in the exercise of his imaginative and descriptive powers. in the earlier half of the seventeenth century the ptolemaic theory--by which it was believed that the earth was the immovable centre of the universe, and that round it all the heavenly bodies completed a diurnal revolution--still retained its ascendency over the minds of men of learning and science, and all the doctrines associated with this ancient astronomical creed were still religiously upheld by the educated classes among the peoples inhabiting the different civilised regions of the globe. the copernican theory--by which the sun is assigned the central position in our system, with the earth and planets revolving in orbits round him--obtained the support of a few persons of advanced views and high scientific attainments, but its doctrines had not yet seriously threatened the supremacy of the older system. though upwards of one hundred years had elapsed since the death of copernicus, yet the doctrines associated with the system of which he was the founder were but very tardily adopted up to this time. there were several reasons which accounted for this. the copernican system was at first imperfect in its details, and included several of the ptolemaic, doctrines which rendered it less intelligible, and retarded its acceptance by persons who would otherwise have been inclined to adopt it. copernicus believed that the planets travelled round the sun in circular paths. this necessitated the retention of cycles and epicycles, which gave rise to much confusion; nor was it until kepler made his great discovery of the ellipticity of the planetary orbits that they were eliminated from the system. as the ptolemaic system of the universe held complete sway over the minds of men for upwards of twenty centuries, it was difficult to persuade many persons to renounce the astronomical beliefs to which they were so firmly attached, in favour of those of any other system; so that the overthrow of this venerable theory required a lengthened period of time for its accomplishment. it was thus in his earlier years, when milton devoted his time to the study of literature and philosophy, which he read extensively when pursuing his academic career at christ's college, cambridge, and afterwards at horton, where he spent several years in acquiring a more proficient knowledge of the literary, scientific, and philosophical writings of the age, that he found the beliefs associated with the ptolemaic theory adopted without doubt or hesitation by the numerous authors whose works he perused. his knowledge of italian enabled him to become familiar with dante--one of his favourite authors, whose poetical writings were deeply read by him, and who, in the elaboration of his poem, the 'divina commedia,' included the entire ptolemaic cosmology. in england the copernican theory had few supporters, and the majority of those who represented the intellect and learning of the country still retained their adherence to the old form of astronomical belief. we therefore find that milton followed the traditional way of thinking by adopting the views associated with the ptolemaic theory. according to the ptolemaic system, the earth was regarded as the immovable centre of the universe, and surrounding it were ten crystalline spheres, or heavens, arranged in concentric circles, the larger spheres enclosing the smaller ones; and within those was situated the cosmos, or mundane universe, usually described as 'the heavens and the earth.' to each of the first seven spheres there was attached a heavenly body, which was carried round the earth by the revolution of the crystalline. st sphere: that of the moon. nd sphere: that of the planet mercury. rd sphere: that of the planet venus. th sphere: that of the sun; regarded as a planet. th sphere: that of the planet mars. th sphere: that of the planet jupiter. th sphere: that of the planet saturn. th sphere: that of the fixed stars. [illustration: fig. ] the eighth sphere included all the fixed stars, and was called the firmament, because it was believed to impart steadiness to the inner spheres, and, by its diurnal revolution, to carry them round the earth, causing the change of day and night. the separate motions of the spheres, revolving with different velocities, and at different angles to each other, accounted for the astronomical phenomena associated with the orbs attached to each. according to ptolemy's scheme, the eighth sphere formed the outermost boundary of the universe; but later astronomers added to this system two other spheres--a _ninth_, called the _crystalline_, which caused precession of the equinoxes; and a _tenth_, called the _primum mobile_, or first moved, which brought about the alternation of day and night, by carrying all the other spheres round the earth once in every twenty-four hours. the primum mobile enclosed, as if in a shell, all the other spheres, in which was included the created universe, and, although of vast dimensions, its conception did not overwhelm the mind in the same manner that the effort to comprehend infinitude does. beyond this last sphere there was believed to exist a boundless, uncircumscribed region, of immeasurable extent, called the empyrean, or heaven of heavens, the incorruptible abode of the deity, the place of eternal mysteries, which the comprehension of man was unable to fathom, and of which it was impossible for his mind to form any conception. such were the imaginative beliefs upon which this ancient astronomical theory was founded, that for a period of upwards of two thousand years held undisputed sway over the minds of men, and exercised during that time a predominating influence upon the imagination, thoughts, and conceptions of all those who devoted themselves to literature, science, and art. of the truthfulness of this assertion there is ample evidence in the poetical, philosophical, and historical writings of ancient authors, whose ideas and conceptions regarding the created universe were limited and circumscribed by this form of astronomical belief. in the works of more recent writers we find that it continued to assert its influence; and among our english poets, from chaucer down to shakespeare, there are numerous references to the natural phenomena associated with this system, and most frequently expressed by poetical allusions to 'the music of the spheres.' the ideas associated with the ptolemaic theory were gratifying to the pride and vanity of man, who could regard with complacency the paramount importance of the globe which he inhabited, and of which he was the absolute ruler, fixed in the centre of the universe, and surrounded by ten revolving spheres, that carried along with them in their circuit all other celestial bodies--sun, moon, and stars, which would appear to have been created for his delectation, and for the purpose of ministering to his requirements. but when the copernican theory became better understood, and especially after the discovery of the law of universal gravitation, this venerable system of the universe, based upon a pile of unreasonable and false hypotheses, after an existence of over twenty centuries, sank into oblivion, and was no more heard of. milton's ptolemaism is apparent in some of his shorter pieces, and also in his minor poems, 'arcades' and 'comus.' his 'ode on the nativity' is written in conformity with this belief, and the expression, ring out ye crystal spheres, indicates a poetical allusion to this theory. but as milton grew older his ptolemaism became greatly modified, and there are good reasons for believing that in his latter years he renounced it entirely in favour of copernicanism. when on his continental tour in , he made the acquaintance of eminent men who held views different from those with which he was familiar; and in his interview with galileo at arcetri, the aged astronomer may have impressed upon his mind the superiority of the copernican theory, in accounting for the occurrence of celestial phenomena, as compared with the ptolemaic. on his return to england from the continent, milton took up his residence in london, and lived in apartments in a house in st. bride's churchyard. having no regular vocation, and not wishing to be dependent upon his father, he undertook the education of his two nephews, john and edward phillips, aged nine and ten years respectively. from st. bride's churchyard he removed to a larger house in aldersgate, where he received as pupils the sons of some of his most intimate acquaintances. in the list of subjects which milton selected for the purpose of imparting instruction to those youths he included astronomy and mathematics, which formed part of the curriculum of this educational establishment. the text-book from which he taught his nephews and other pupils astronomy was called 'de sphæra mundi,' a work written by joannes sacrobasco (john holywood) in the thirteenth century. this book was an epitome of ptolemy's 'almagest,' and therefore entirely ptolemaic in its teaching. it enjoyed great popularity during the middle ages, and is reported to have gone through as many as forty editions. the selection of astronomy as one of the subjects in which milton instructed his pupils affords us evidence that he must have devoted considerable time and attention to acquiring a knowledge of the facts and details associated with the study of the science. in the attainment of this he had to depend upon his own exertions and the assistance derived from astronomical books; for at this time astronomy received no recognition as a branch of study at any of the universities; and in britain the science attracted less attention than on the continent, where the genius of kepler and galileo elevated it to a position of national importance. we shall find as we proceed that milton's knowledge of astronomy was comprehensive and accurate; that he was familiar with the astronomical reasons by which many natural phenomena which occur around us can be explained; and that he understood many of the details of the science which are unknown to ordinary observers of the heavens. it is remarkable how largely astronomy enters into the composition of 'paradise lost,' and we doubt if any author could have written such a poem without possessing a knowledge of the heavens and of the celestial orbs such as can only be attained by a proficient and intimate acquaintance with this science. the arguments in favour of or against the ptolemaic and copernican theories were well known to milton, even as regards their minute details; and in book viii. he introduces a scientific discussion based upon the respective merits of those theories. the configuration of the celestial and terrestrial spheres, and the great circles by which they are circumscribed, he also knew. the causes which bring about the change of the seasons; the obliquity of the ecliptic; the zodiacal constellations through which the sun travels, and the periods of the year in which he occupies them, are embraced in milton's knowledge of the science of astronomy. the motions of the earth, including the precession of the equinoxes; the number and distinctive appearances of the planets, their direct and retrograde courses, and their satellites, are also described by him. the constellations, and their relative positions on the celestial sphere; the principal stars, star-groups, and clusters, and the galaxy, testify to milton's knowledge of astronomy, and to the use which he has made of the science in the elaboration of his poem. the names of fourteen of the constellations are mentioned in 'paradise lost.' these, when arranged alphabetically, read as follows:-- andromeda, aries, astrea, centaurus, cancer, capricornus, gemini, leo, libra, ophiuchus, orion, scorpio, taurus, and virgo. milton's allusions to the zodiacal constellations are chiefly associated with his description of the sun's path in the heavens; but with the celestial sign libra (the _scales_) he has introduced a lofty and poetical conception of the means by which the creator made known his will when there arose a contention between gabriel and satan on his discovery in paradise. the eternal, to prevent such horrid fray, hung forth in heaven his golden scales, yet seen betwixt astrea[ ] and the scorpion sign, wherein all things created first he weighed, the pendulous round earth with balanced air in counterpoise, now ponders all events, battles and realms. in these he put two weights, the sequel each of parting and of fight: the latter quick up flew, and kicked the beam.--iv. - . orion, the finest constellation in the heavens, did not escape milton's observation, and there is one allusion to it in his poem. it arrives on the meridian in winter, where it is conspicuous as a brilliant assemblage of stars, and represents an armed giant, or hunter, holding a massive club in his right hand, and having a shield of lion's hide on his left arm. a triple-gemmed belt encircles his waist, from which is suspended a glittering sword, tipped with a bright star. the two brilliants betelgeux and bellatrix form the giant's shoulders, and the bright star rigel marks the position of his advanced foot. the rising of orion was believed to be accompanied by stormy and tempestuous weather. milton alludes to this in the following lines:-- when with fierce winds orion armed hath vexed the red sea coast, whose waves o'erthrew busiris and his memphian chivalry.--i. - . andromeda is described as being borne by aries, and in 'ophiuchus huge' milton locates a comet which extends the whole length of the constellation. it is evident that milton possessed a precise knowledge of the configuration and size of the constellations, and of the positions which they occupy relatively to each other on the celestial sphere. though milton was conversant with the copernican theory, and entertained a conviction of its accuracy and truthfulness, and doubtless recognised the superiority of this system, which, besides conveying to the mind a nobler conception of the universe and of the solar system--though it diminished the importance of the earth as a member of it--was capable of explaining the occurrence of celestial phenomena in a manner more satisfactory than could be arrived at by the ptolemaic theory. notwithstanding this, he selected the ptolemaic cosmology as the scientific basis upon which he constructed his 'paradise lost,' and in its elaboration adhered with marked fidelity to this system. there were many reasons why milton, in the composition of an imaginative poem, should have chosen the ptolemaic system of the universe rather than the copernican. this form of astronomical belief was adopted by all the authors whose works he perused and studied in his younger days, including his favourite poet, dante; and his own poetic imaginings, as indicated by his early poems, were in harmony with the doctrines of this astronomical creed, a long acquaintance with which had, without doubt, influenced his mind in its favour. this system of revolving spheres, with the steadfast earth at its centre, and the whole enclosed by the primum mobile, constituted a more attractive and picturesque object for poetic description than the simple and uncircumscribed arrangement of the universe expressed by the copernican theory. it also afforded him an opportunity of localising those regions of space in which the chief incidents in his poem are described--viz. heaven, or the empyrean, chaos, hell, and the mundane universe. milton's ptolemaism, with its adjuncts, may be understood by the following: all that portion of space above the newly created universe, and beyond the primum mobile, was known as heaven, or the empyrean--a region of light, of glory, and of happiness; the dwelling-place of the deity, who, though omnipresent, here visibly revealed himself to all the multitude of angels whom he created, and who surrounded his throne in adoration and worship. underneath the universe there existed a vast region of similar dimensions to the empyrean, called chaos, which was occupied by the embryo elements of matter, that with incessant turmoil and confusion warred with each other for supremacy--a wild abyss-- the womb of nature and perhaps her grave.--ii. . the lower portion of this region was divided off from the remainder, and embraced the locality known as hell--the place of torment, where the rebellious angels were driven and shut in after their expulsion from heaven. as far removed from god and light of heaven as from the centre thrice to the utmost pole.--i. - . the new universe, which included the earth and all the orbs of the firmament known as the starry heavens, was created out of chaos, and hung, as if suspended by a golden chain, from the empyrean above; and although its magnitude and dimensions were inconceivable, yet, according to the ptolemaic theory, it was enclosed by the tenth sphere or primum mobile. by this partitioning of space milton was able to contrive a system which fulfilled the requirements of his great poem. the annexed diagram explains the relative positions of the different regions into which space was divided. though there are traces of copernicanism found in 'paradise lost,' yet milton has very faithfully adhered to the ptolemaic mechanism and nomenclature throughout his poem. in his description of the creation, the earth is formed first, then the sun, followed by the moon, and afterwards the stars, all of which are described as being in motion round the earth. allusion is also made to this ancient system in several prominent passages, and in the following lines there is a distinct reference to the various revolving spheres. [illustration: fig. ] they pass the planets seven, and pass the fixed, and that crystalline sphere whose balance weighs the trepidation talked, and that first moved.--iii. - . the seven planetary spheres are first mentioned; then the eighth sphere, or that of the fixed stars; then the ninth, or crystalline, which was believed to cause a shaking, or trepidation, to account for certain irregularities in the motions of the stars; and, lastly, the tenth sphere, or primum mobile, called the 'first moved' because it set the other spheres in motion. to an uninstructed observer, the apparent motion of the heavenly bodies round the earth would naturally lead him to conclude that, of the two theories, the ptolemaic was the correct one. we therefore find that milton adopted the system most in accord with the knowledge and intelligence possessed by the persons portrayed by him in his poem; and in describing the natural phenomena witnessed in the heavens by our first parents, he adheres to the doctrines of the ptolemaic system, as being most in harmony with the simple and primitive conceptions of those created beings. to their upward gaze, the orbs of heaven appeared to be in ceaseless motion; the solid earth, upon which they stood, was alone immovable and at rest. day after day they observed the sun pursue his steadfast course with unerring regularity: his rising in the east, accompanied by the rosy hues of morn; his meridian splendour, and his sinking in the west, tinting in colours of purple and gold inimitable the fleecy clouds floating in the azure sky, as he bids farewell for a time to scenes of life and happiness, rejoicing in the light and warmth of his all-cheering beams. with the advent of night they beheld the moon, now increasing, now waning, pursue her irregular path, also to disappear in the west; whilst, like the bands of an army marshalled in loose array, the constellations of glittering stars, with stately motion, traversed their nocturnal arcs, circling the pole of the heavens. by referring to book viii., - , we find an account of an interesting scientific discussion, or conversation, between adam and raphael regarding the merits of the ptolemaic and copernican systems, and of the relative importance and size of the heavenly bodies. by it we are afforded an opportunity of learning how accurate and precise a knowledge milton possessed of both theories, and in what clear and perspicuous language he expresses his arguments in favour of or against the doctrines associated with each. we may, with good reason, regard the views expressed by adam as representing milton's own opinions, which were in conformity with the copernican theory; and in the angel's reply, though of an undecided character, we are able to perceive how aptly milton describes the erroneous conclusions upon which the ptolemaic theory was based. in this scientific discussion, it would seem rather strange that adam, the first of men, should have been capable of such philosophic reasoning, propounding, as if by intuition, a theory upon which was founded a system that had not been discovered until many centuries after the time that astronomy became a science. by attributing to adam such a degree of intelligence and wisdom, the poet has taken a liberty which enabled him to carry on this discussion in a manner befitting the importance of the subject. in the following lines adam expresses to his angel-guest, in forcible and convincing language, his reasons in support of the copernican theory:-- when i behold this goodly frame, this world, of heaven and earth consisting, and compute their magnitudes--this earth, a spot, a grain, an atom, with the firmament compared and all her numbered stars, that seem to roll spaces incomprehensible (for such their distance argues, and their swift return diurnal) merely to officiate light round this opacous earth, this punctual spot, one day and night, in all her vast survey useless besides--reasoning, i oft admire, how nature, wise and frugal could commit such disproportions, with superfluous hand so many nobler bodies to create, greater so manifold, to this one use, for aught appears, and on their orbs impose such restless revolution day by day repeated, while the sedentary earth, that better might with far less compass move, served by more noble than herself, attains her end without least motion, and receives, as tribute, such a sumless journey brought of incorporeal speed, her warmth and light; speed, to describe whose swiftness number fails.--viii. - . we are enabled to perceive that milton had formed a correct conception of the magnitude and proportions of the universe, and also of the relative size and importance of the earth, which he describes as 'a spot, a grain, an atom,' when compared with the surrounding heavens. he expresses his surprise that all the stars of the firmament, whose distances are so remote, and whose dimensions so greatly exceed those of this globe, should in their diurnal revolution have 'such a sumless journey of incorporeal speed imposed upon them' merely to officiate light to the earth, 'this punctual spot;' and reasoning, wonders how nature, wise and frugal in her ways, should commit such disproportions, by adopting means so great to accomplish a result so small, when motion imparted to the sedentary earth would with greater ease produce the same effect. the inconceivable velocity with which it would be necessary for those orbs to travel in order to accomplish a daily revolution round the earth might be described as almost spiritual, and beyond the power of calculation by numbers. the angel, after listening to adam's argument, expresses approval of his desire to obtain knowledge, but answers him dubiously, and at the same time criticises in a severe and adverse manner the ptolemaic theory. to ask or search i blame thee not; for heaven is as the book of god before thee set, wherein to read his wondrous works, and learn his seasons, hours, or days, or months, or years. this to attain, whether heaven move or earth, imports not, if thou reckon right; the rest from man or angel the great architect did wisely to conceal, and not divulge his secrets, to be scanned by them who ought rather admire. or, if they list to try conjecture, he his fabric of the heavens hath left to their disputes, perhaps to move his laughter at their quaint opinions wide hereafter, when they come to model heaven, and calculate the stars; how they will wield the mighty frame; how build, unbuild, contrive to save appearances; how gird the sphere with centric and eccentric scribbled o'er cycle and epicycle, orb in orb.--viii. - . when, with the advancement of science, astronomical observations were made with greater accuracy, it was discovered that uniformity of motion was not always maintained by those bodies which were believed to move in circles round the earth. it was observed that the sun, when on one side of his orbit, had an accelerated motion, as compared with the speed at which he travelled when on the other side. the planets, also, appeared to move with irregularity: sometimes a planet was observed to advance, then become stationary, and afterwards affect a retrograde movement. those inequalities of motion could not be explained by means of the revolution of crystalline spheres alone, but were accounted for by imagining the existence of a small circle, or epicycle, whose centre corresponded with a fixed point in the larger circle, or eccentric, as it was called. this small circle revolved on its axis when carried round with the larger one, and round it the planet also revolved, which when situated in its outer portion would have a forward, and when in its inner portion a retrograde, motion. the theory of eccentrics and epicycles was sufficient for a time to account for the inequalities of motion already described, and by this means the ptolemaic system was enabled to retain its ascendency for a longer period than it otherwise would have done. but more recent discoveries brought to light discrepancies and difficulties which were explained away by adding epicycle to epicycle. this created a most complicated entanglement, and hastened the downfall of a system which, after an existence of many centuries, sank into oblivion, and is now remembered as a belief of bygone ages. the devices which the upholders of this system were compelled to adopt, in order 'to save appearances,' with 'centric and eccentric,' cycle and epicycle, 'orb in orb,' are in this manner appropriately described by milton, as indicating the confusion arising from a theory based upon false hypotheses. continuing his reply, the angel says:-- already by thy reasoning this i guess, who art to lead thy offspring, and supposest that bodies bright and greater should not serve the less not bright, nor heaven such journies run, earth sitting still, when she alone receives the benefit. consider, first, that great or bright infers not excellence. the earth, though, in comparison of heaven, so small, nor glistering, may of solid good contain more plenty than the sun that barren shines, whose virtue on itself works no effect, but in the fruitful earth; there first received, his beams, inactive else, their vigour find, yet not to earth are those bright luminaries officious, but to thee, earth's habitant. and, for the heaven's wide circuit, let it speak the maker's high magnificence, who built so spacious, and his line stretched out so far, that man may know he dwells not in his own-- an edifice too large for him to fill, lodged in a small partition; and the rest ordained for uses to his lord best known, the swiftness of those circles attribute, though numberless, to his omnipotence, that to corporeal substances could add speed almost spiritual. me thou think'st not slow, who since the morning-hour set out from heaven where god resides, and ere midday arrived in eden--distance inexpressible by numbers that have name. but this i urge, admitting motion in the heavens, to show invalid that which thee to doubt it moved; not that i so affirm, though so it seem to thee who hast thy dwelling here on earth. god, to remove his ways from human sense, placed heaven from earth so far, that earthly sight, if it presume, might err in things too high, and no advantage gain.--viii. - . notwithstanding the angel's severe criticism of the ptolemaic system, he does not unreservedly support the conclusions arrived at by adam, but endeavours to show that his reasoning may not be altogether correct. he questions the validity of his argument that bodies of greater size and brightness should not serve the smaller, though not bright, and that heaven should move, while the earth remained at rest. he argues that great or bright infers not excellence, and that the earth, though small, may contain more virtue than the sun, that 'barren shines,' whose beams create no beneficial effect, except when directed on the fruitful earth. he reminds adam that those bright luminaries minister not to the earth, but to himself, 'earth's habitant,' and directs his attention to the magnificence and extent of the surrounding universe, of which he occupies but a small portion. the diurnal swiftness of the orbs that move round the earth he attributes to god's omnipotence, that to material bodies 'could add speed almost spiritual.' the angel, after alluding to his rapid flight through space, suggests that god placed heaven so far from earth that man might not presume to inquire into things which it would be of no advantage for him to know. he then suddenly changes to the copernican system, which he lucidly describes in the following lines:-- what if the sun be centre to the world, and other stars by his attractive virtue and their own incited, dance about him various rounds? their wandering course, now high, now low, then hid, progressive, retrograde, or standing still, in six thou seest; and what if, seventh to these the planet earth, so steadfast though she seem, insensibly three different motions move? which else to several spheres thou must ascribe, moved contrary with thwart obliquities, or save the sun his labour, and that swift nocturnal and diurnal rhomb supposed invisible else above all stars, the wheel of day and night; which needs not thy belief, if earth, industrious of herself, fetch day travelling east, and with her part averse from the sun's beam meet night, her other part still luminous by his ray. what if that light, sent from her through the wide transpicuous air, to the terrestrial moon be as a star, enlightening her by day, as she by night this earth--reciprocal, if land be there, fields and inhabitants? her spots thou seest as clouds, and clouds may rain, and rain produce fruits in her softened soil, for some to eat allotted there; and other suns, perhaps, with their attendant moons, thou wilt descry, communicating male and female light-- which two great sexes animate the world, stored in each orb perhaps with some that live. for such vast room in nature unpossessed by living soul, desert and desolate, only to shine, yet scarce to contribute each orb a glimpse of light, conveyed so far down to this habitable, which returns light back to them, is obvious to dispute.--viii. - . the copernican theory, which is less complicated and more easily understood than the ptolemaic, is described by milton with accuracy and methodical skill. the sun having been assigned that central position in the system which his magnitude and importance claim as his due, the planets circling in orbits around him have their motions described in a manner indicative of the precise knowledge which milton acquired of this theory. at this time the law of gravitation was unknown, and, although the ellipticity of the orbits of the planets had been discovered by kepler, the nature of the motive force which guided and retained them in their paths still remained a mystery. it was believed that the planets were whirled round the sun, as if by the action of magnetic fibres; a mutual attractive influence having been supposed to exist between them and the orb, similar to that of the opposite poles of magnets. milton alludes to this theory in the following lines:-- they, as they move their starry dance in numbers that compute days, months, and years, towards his all-cheering lamp turn swift their various motions, or are turned by his magnetic beam.--iii. - . an important advance upon this theory was made by horrox, who, in his study of celestial dynamics, attributed the curvilineal motion of the planets to the influence of two forces, one projective, the other attractive. he illustrated this by observing the path described by a stone when thrown obliquely into the air. he perceived that its motion was governed by the impulse imparted to it by the hand, and also by the attractive force of the earth. under these two influences, the stone describes a graceful curve, and in its descent falls at the same angle at which it rose. hence arises the general law: 'when two spheres are mutually attracted, and if not prevented by foreign influences, their straight paths are deflected into curves concave to each other, and corresponding with one of the sections of a cone, according to the velocity of the revolving body. if the velocity with which the revolving body is impelled be equal to what it would acquire by falling through half the radius of a circle described from the centre of deflection, its orbit will be circular; but if it be less than that quantity, its path becomes elliptical.' newton afterwards embraced this law in his great principle of gravitation, and demonstrated that the force which guides and retains the earth and planets in their orbits resides in the sun. by the orb's attractive influence a planet, after having received its first impulse, is deflected from its original straight path, and bent towards that luminary, and by the combined action of the projective and attractive forces is made to describe an orbit which, if elliptical, has one of its foci occupied by the sun. so evenly balanced are those two forces, that one is unable to gain any permanent ascendency over the other, and consequently the planet traverses its orbit with unerring regularity, and, if undisturbed by external influences, will continue in its path for all time. milton describes the position of the planets in the sky as-- now high, now low, then hid; and their motions-- progressive, retrograde, or standing still. it is evident that milton was familiar with the apparently irregular paths pursued by the planets when observed from the earth. he knew of their stationary points, and also the backward loopings traced out by them on the surface of the sphere. if observed from the sun, all the planets would be seen to follow their true paths round that body; their motion would invariably lie in the same direction, and any variation in their speed as they approached perihelion or aphelion would be real. but the planets, when observed from the earth, which is itself in motion, appear to move irregularly. sometimes they remain stationary for a brief period, and, instead of progressing onward, affect a retrograde movement. this irregularity of motion is only apparent, and can be explained as a result of the combined motions of the earth and planets, which are travelling together round the sun with different velocities, and in orbits of unequal magnitude. in his allusion to the copernican system the 'planet' 'earth' is described by milton as seventh. this is not strictly accurate, as only five planets were known--viz. mercury, venus, mars, jupiter, and saturn; but to make up the number milton has included the moon, which may be regarded as the earth's planet. the three motions ascribed to the earth are--( ) the diurnal rotation on her axis; ( ) her annual revolution round the sun; ( ) precession of the equinoxes. the rotation of the earth on her axis may be likened to the spinning motion of a top, and is the cause of the alternation of day and night. this rotatory motion is sustained with such exact precision that, during the past , years, it has been impossible to detect the minutest difference in the time in which the earth accomplishes a revolution on her axis, and therefore the length of the sidereal day, which is minutes seconds shorter than the mean solar day, is invariable. in this motion of the earth we have a time-measuring unit which may be regarded as absolutely correct. the earth completes a revolution of her orbit in - / days. in this period of time she accomplishes a journey of millions of miles, travelling at the average rate of , miles an hour. the change of the seasons, and the lengthening and shortening of the day, are natural phenomena, which occur as a consequence of the earth's annual revolution round the sun. precession is a retrograde or westerly motion of the equinoctial points, caused by the attraction of the sun, moon, and planets on the spheroidal figure of the earth. by this movement the poles of the earth are made to describe a circular path in that part of the heavens to which they point; so that, after the lapse of many years, the star which is known as the pole star will not occupy the position indicated by its name, but will be situated at a considerable distance from the pole. these motions, milton says, unless attributed to the earth, must be ascribed to several spheres crossing and thwarting each other obliquely; but the earth, by rotating from west to east, will of herself fetch day, her other half, averted from the sun's rays, being enveloped in night. thus saving the sun his labour, and the 'primum mobile,' 'that swift nocturnal and diurnal rhomb,' which carried all the lower spheres along with it, and brought about the change of day and night. milton's allusion to the occurrence of natural phenomena in the moon similar to those which happen on the earth is in keeping with the opinions entertained regarding our satellite, galileo having imagined that he discovered with his telescope continents and seas on the lunar surface, which led to the belief that the moon was the abode of intelligent life. ... and other suns, perhaps, with their attendant moons, thou wilt descry communicating male and female light.--viii. - . milton in these lines refers to jupiter and saturn, and their satellites, which had been recently discovered; those of the former by galileo, and four of those of the latter by cassini. the existence of male and female light was an idea entertained by the ancients, and which is mentioned by pliny. the sun was regarded as a masculine star, and the moon as feminine; the light emanating from each being similarly distinguished, and possessing different properties. milton supposes that, as the earth receives light from the stars, she returns light back to them. but in his time little was known about the stars, nor was it ascertained how distant they are. the angel, in bringing to a conclusion his conversation with adam, deems it unadvisable to vouchsafe him a decisive reply to his inquiry regarding the motions of celestial bodies, and in the following lines gives a beautifully poetical summary of this elevated and philosophic discussion:-- but whether thus these things, or whether not, whether the sun, predominant in heaven, rise on the earth, or earth rise on the sun; he from the east his flaming round begin, or she from west her silent course advance with inoffensive pace that spinning sleeps on her soft axle, whilst she paces even, and bears thee soft with the smooth air along-- solicit not thy thoughts with matters hid.--viii. - . in this scientific discourse between adam and raphael, in which they discuss the structural arrangement of the heavens and the motions of celestial bodies, we are afforded an opportunity of learning what exact and comprehensive knowledge milton possessed of both the ptolemaic and copernican theories. the concise and accurate manner in which he describes the doctrines belonging to each system indicates that he must have devoted considerable time and attention to making himself master of the details associated with both theories, which in his time were the cause of much controversy and discussion among philosophers and men of science. the ptolemaic system, with its crystalline spheres revolving round the earth, the addition to those of cycles and epicycles, and the heaping of them upon each other, in order to account for phenomena associated with the motions of celestial bodies, are concisely and accurately described. the unreasonableness of this theory, when compared with the copernican, is clearly delineated by milton where adam is made to express his views with regard to motion in the heavens. his argument, declared in logical and persuasive language, demonstrates how contrary to reason it would be to imagine that the entire heavens should revolve round the earth to bring about a result which could be more easily attained by imparting motion to the earth herself. the inconceivable velocity with which it would be necessary for the celestial orbs to travel in order to accomplish their daily revolution is described by him as opposed to all reason, and entailing upon them a journey which it would be impossible for material bodies to perform. none the less accurate is milton's description of the copernican system. he describes the sun as occupying that position in the system which his magnitude and supreme importance claim as his sole right, having the planets with their satellites, that from his lordly eye keep distance due.--iii. , circling in majestic orbits around him, acknowledging his controlling power, and bending to his firm but gentle sway. their positions, their paths, and their motions, real and apparent, are described in flowing and harmonious verse. chapter iv milton and galileo after the death of his mother, which occurred in , milton expressed a desire to visit the continent, where there were many places of interest which he often longed to see. having obtained the consent of his kind and indulgent father, he set out on his travels in april , accompanied by a single man-servant, and arrived in paris, where he only stayed a few days. during his residence in the french capital he was introduced by lord scudamore, the english ambassador at the court of versailles, to hugo grotius, one of the most distinguished scholars and philosophic thinkers of his age. from paris milton journeyed to nice, where he first beheld the beauty of italian scenery and the classic shores of the mediterranean sea. from nice he sailed to genoa and leghorn, and after a short stay at those places continued his journey to florence, one of the most interesting and picturesque of italian cities. situated in the valley of the arno, and encircled by sloping hills covered with luxuriant vegetation, the sides of which were studded with residences half-hidden among the foliage of gardens and vineyards, florence, besides being famed for its natural beauty, was at that time the centre of italian culture and learning, and the abode of men eminent in literature and science. here milton remained for a period of two months, and enjoyed the friendship and hospitality of its most noted citizens, many of whom delighted to honour their english visitor. he was warmly welcomed by the members of the various literary academies, who admired his compositions and conversation; the flattering encomiums bestowed upon him by those learned societies having been amply repaid by milton in choice and elegant latin verse. among those who resided in the vicinity of florence was the illustrious galileo, who in his sorrow-stricken old age was held a prisoner of the inquisition for having upheld and taught scientific doctrines which were declared to be heretical. after his abjuration he was committed to prison, but on the intervention of influential friends was released after a few days' incarceration, and permitted to return to his home at arcetri. he was, however, kept under strict surveillance, and forbidden to leave his house or receive any of his intimate friends without having first obtained the sanction of the ecclesiastical authorities. after several years of close confinement at arcetri, during which time he suffered much from rheumatism and continued ill-health, aggravated by grief and mental depression consequent upon the death of his favourite daughter, galileo applied for permission to go to florence in order to place himself under medical treatment. this request was granted by the pope subject to certain conditions, which would be communicated to him when he presented himself at the office of the inquisition at florence. these were more severe than he anticipated. he was forbidden to leave his house or receive any of his friends there, and those injunctions were so strictly adhered to that during passion week he had to obtain a special order so that he might be able to attend mass. at the expiration of a few months galileo was ordered to return to arcetri, which he never left again. an affliction, perhaps the most deplorable that can happen to any human being, was added to the burden of galileo's misfortunes and woes. a disorder which had some years previously injured the sight of his right eye returned in . in the following year the left eye became similarly affected, with the result that in a few months galileo became totally blind. his friends at first hoped that the disease was cataract, and that some relief might be afforded by means of an operation; but it was discovered to be an opacity of the cornea, which at his age was considered unamenable to treatment. this sudden and unexpected calamity was to galileo a most deplorable occurrence, for it necessitated the relinquishment of his favourite pursuit, which he followed with such intense interest and delight. his friend castelli writes: 'the noblest eye is darkened which nature ever made; an eye so privileged, and gifted with such rare qualities that it may with truth be said to have seen more than all of those eyes who are gone, and to have opened the eyes of all who are to come.' galileo endured his affliction with patient resignation and fortitude, and in the following extract from a letter by him he acknowledges the chastening hand of a divine providence: 'alas! your dear friend and servant galileo has become totally blind, so that this heaven, this earth, this universe, which with wonderful observations i had enlarged a hundred and a thousand times beyond the belief of bygone ages, henceforward for me is shrunk into the narrow space which i myself fill in it. so it pleases god; it shall then please me also.' the rigorous curtailment of his liberty which prompted galileo to head his letters, 'from my prison at arcetri,' was relaxed when total blindness had supervened upon the infirmities of age. permission was given him to receive his friends, and he was allowed to have free intercourse with his neighbours. milton, during his stay at florence, visited galileo at arcetri. we are ignorant of the details of this eventful and interesting interview between the aged and blind astronomer and the young english poet, who afterwards immortalised his name in heroic verse, and who in his declining years suffered from an affliction similar to that which befel galileo, and to which he alludes so pathetically in the following lines:-- thee i revisit safe, and feel thy sovran vital lamp; but thou revisitest not these eyes, that roll in vain to find thy piercing ray, and find no dawn; so thick a drop serene hath quenched their orbs, or dim suffusion veiled.--iii. - . we can imagine that galileo's astronomical views, which at that time were the subject of much discussion among scientific men and professors of religion, and on account of which he suffered persecution, were eagerly discussed. it is also probable that the information communicated by galileo, or by some of his followers, may have persuaded milton to entertain a more favourable opinion of the copernican theory. the interesting discoveries made by galileo with his telescope without doubt formed a pleasant subject of conversation, and milton enjoyed the privilege of listening to a detailed description of these from the lips of the aged astronomer. the telescope, its principle, its mechanism, and the method of observing, were most probably explained to him; and we can believe that an opportunity was afforded him of examining those in galileo's observatory, and of perhaps testing their magnifying power upon some celestial object favourably situated for observation. though milton has not favoured us with any details of his visit to galileo, yet it was one which made a lasting impression upon his mind, and was never afterwards forgotten by him. 'there it was,' he writes, 'i found and visited the famous galileo, grown old, a prisoner of the inquisition for thinking in astronomy otherwise than the franciscan and dominican licensers thought.' in years long after, when milton, himself feeble and blind, sat down to compose his 'paradise lost,' the remembrance of the tuscan artist and his telescope was still fresh in his memory. by the invention of the telescope and its application to astronomical research, a vast amount of information and additional detail have been learned regarding the bodies which enter into the formation of the solar system; and by its aid many new ones were also discovered. on sweeping the heavens with the instrument, the illimitable extent of the sidereal universe became apparent, and numberless objects of interest were brought within the range of vision the existence of which had not been previously imagined. the galilean telescope was invented in . but the magnifying power of certain lenses, and their combination in producing singular visual effects, are alluded to in the writings of several early authors. the value of single lenses as an aid to sight had been long known, and spectacles were in common use in the fourteenth century. several mathematicians have described the wonderful optical results obtained from glasses concave and convex, of parabolic and circular forms, and from 'perspective glasses,' in which were embodied the principle of the telescope. it is asserted that our countryman, roger bacon ( ), had some notion of the properties of the telescope; but among those familiar with the combination of lenses the two men who made the nearest approach to the invention of the instrument were baptista porta and gerolamo fracastro. the latter, who died in , writes as follows: 'for which reason those things which are seen at the bottom of water appear greater than those which are at the top; and if anyone look through two eye-glasses, one placed upon the other, he will see everything much larger and nearer.' it is doubtful if fracastro had any notion of constructing a mechanism which might answer the purpose of a telescopic tube. baptista porta ( ) is more explicit in what he describes. he writes: 'concave lenses show distant objects most clearly, convex those which are nearer; whence they may be used to assist the sight. with a concave glass distant objects will be seen, small, but distinct; with a convex one, those near at hand, larger, but confused; if you know _rightly_ how to combine one of each sort, you will see both far and near objects larger and clearer.' he then goes on to say: 'i shall now endeavour to show in what manner we may continue to recognise our friends at the distance of several miles, and how those of weak sight may read the most minute letters from a distance. it is an invention of great utility, and grounded on optical principles; nor is at all difficult of execution; but it must be so divulged as not to be understood by the vulgar, and yet be clear to the sharp-sighted.' after this, he proceeds to describe a mechanism the details of which are confusing and unintelligible, nor did it appear to bear any resemblance to a telescopic tube. in a work published by thomas digges in , he makes the following allusion to his father's experiments with the lenses: 'my father, by his continuall painfull practices, assisted with demonstrations mathematicall, was able, and sundry times hath by proportionall glasses, duely situate in convenient angles, not only discouered things farre off, read letters, numbered peeces of money with the verye coyne and superscription thereof cast by some of his freends of purpose, upon downes in open fields; but also seuen miles off, declared what hath beene doone at that instant in priuate places.' it must be admitted that if leonard digges had not constructed a telescope, he knew how to combine lenses by the aid of which a visual effect was created similar to that produced by the use of the instrument. the inventor of the telescope was a dutchman named hans lippershey, who carried on the business of a spectacle-maker in the town of middelburg. his discovery was purely accidental. it is said that the instrument--which was directed towards a weather-cock on a church spire, of which it gave a large and inverted image--was for some time exhibited in his shop as a curiosity before its importance was recognised. the marquis spinola, happening to see this philosophical toy, purchased it, and presented it to prince maurice of nassau, who imagined it might be of service for the purpose of military reconnoitring. the value of the invention was, however, soon realised, and in the following year telescopes were sold in paris. in , galileo, when on a visit to a friend at venice, received intelligence of the invention of an instrument by a dutch optician which possessed the power of causing distant objects to appear much nearer than when observed by ordinary vision. the accuracy of this information was confirmed by letters which he received from paris; and this general report, galileo asserted, was all he knew of the subject. fuccarius, in a disparaging letter, says that one of the dutch telescopes had been brought to venice, and that he himself had seen it. this statement is not incompatible with galileo's affirmation that he had not seen the original instrument, and knew no more about it than what had been communicated to him in the letters from the french capital. it was insinuated by fuccarius that galileo had seen the telescope at venice, but, as he denied this, we should not hesitate to believe in his veracity. immediately after his return to padua, galileo began to think how he might be able to contrive an instrument with properties similar to the one of which he had been informed; and in the following words describes the process of reasoning by which he arrived at a successful result: 'i argued in the following manner. the contrivance consists either of one glass or of more--one is not sufficient, since it must be either convex, concave, or plane. the last does not produce any sensible alteration in objects; the concave diminishes them. it is true that the convex magnifies, but it renders them confused and indistinct; consequently, one glass is insufficient to produce the desired effect. proceeding to consider two glasses, and bearing in mind that the plane causes no change, i determined that the instrument could not consist of the combination of a plane glass with either of the other two. i therefore applied myself to make experiments on combinations of the two other kinds, and thus obtained that of which i was in search.' galileo's telescope consisted of two lenses--one plano-convex, the other plano-concave, the latter being held next the eye. these he fixed in a piece of organ pipe, which served the purpose of a tube, the glasses being distant from each other by the difference of their focal lengths. an exactly similar principle is adopted in the construction of an opera-glass, which can be accurately described as a double galilean telescope. galileo must be regarded as the inventor of this kind of telescope, which in one respect differed very materially from the one constructed by the dutch optician. if what has been said with regard to the _inverted_ weather-cock be true, then lippershey's telescope was made with two convex lenses, distant from each other by the sum of their focal lengths, and all objects observed with it were seen inverted. refracting astronomical telescopes are now constructed on this principle, it having been discovered that for observational purposes they possess several advantages over the galilean instrument. when galileo had completed his first telescope he returned with it to venice, where he exhibited it to his friends. the sensation created by this small instrument, which magnified only three times, was most extraordinary, and almost amounted to a frenzy. crowds of the principal citizens of venice flocked to galileo's house in order that they might see the magical tube about which such wonderful reports were circulated; and for upwards of a month he was daily occupied in describing his invention to attentive audiences. at the expiration of this time the doge of venice, leonardo deodati, hinted that the senate would not be averse to receive the telescope as a gift. galileo readily acquiesced with this desire, and, as an acknowledgment of his merits, a decree was issued confirming his appointment as professor at padua for life, and increasing his salary from to , florins. the public excitement created by the telescope showed no signs of abatement. sirturi mentions that, having succeeded in constructing an instrument, he ascended the tower of st. mark's at venice, hoping to be able to use it there without interruption. he was, however, detected by a few individuals, and soon surrounded by a crowd, which took possession of his telescope, and detained him for several hours until their curiosity was satisfied. eager inquiries having been made as to where he lodged, sirturi, fearing a repetition of his experience in the church tower, decided to quit venice early next morning, and betake himself to a quieter and less frequented neighbourhood. the instrument was at first called galileo's tube; the double eye-glass; the perspective; the trunk; the cylinder. the appellation _telescope_ was given it by demisiano. galileo next directed his attention to the construction of telescopes, and applied his mechanical skill in making instruments of a larger size, one of which magnified _eight_ times. 'and at length,' he writes, 'sparing neither labour nor expense, he completed an instrument that was capable of magnifying more than _thirty_ times.' galileo now commenced an exploration of the celestial regions with his telescope, and on carefully examining some of the heavenly bodies, made many wonderful discoveries which added greatly to the fame and lustre of his name. the first celestial object to which galileo directed his telescope was the moon. he was deeply interested to find how much her surface resembled that of the earth, and was able to perceive lofty mountain ranges, the illumined peaks of which reflected the sunlight, whilst their bases and sides were still enveloped in dark shadow; great plains which he imagined were seas, valleys, elevated ridges, depressions, and inequalities similar to what are found on our globe. galileo believed the moon to be a habitable world, and concluded that the dark and luminous portions of her surface were land and water, which reflected with unequal intensity the light of the sun. the followers of aristotle received the announcement of these discoveries with much displeasure. they maintained that the moon was perfectly spherical and smooth--a vast mirror, the dark portions of which were the reflection of our terrestrial mountains and forests--and accused galileo 'of taking a delight in distorting and ruining the fairest works of nature.' he appealed to the unequal condition of the surface of our globe, but this was of no avail in altering their preconceived notions of the lunar surface. perhaps the most important discovery made by galileo with the telescope was that of the four moons of jupiter. on the night of january , , when engaged in observing the planet, his attention was attracted by three small stars which appeared brighter than those in their immediate neighbourhood. they were all in a straight line and parallel with the ecliptic; two of them were situated to the east, and one to the west of jupiter. on the following night he was surprised to find all three to the west of the planet, and nearer to each other. this caused him considerable perplexity, and he was at a loss to understand how jupiter could be east of the three stars, when on the preceding night he was observed to the west of two of them. galileo was unable to reconcile the altered positions of those bodies with the apparent motion of jupiter among the fixed stars as indicated by the astronomical tables. the next opportunity he had of observing them was on the th, when two stars only were visible, and they were to the east of the planet. as it was impossible for jupiter to move from west to east on january and from east to west on the th, he concluded that it was the motion of the stars and not that of jupiter which accounted for the observed phenomena. galileo watched the stars attentively on successive evenings and discovered a fourth, and on observing how they changed their positions relatively to each other he soon arrived at the conclusion that the stars were four moons which revolved round jupiter after the manner in which the moon revolves round the earth. having assured himself that the four new stars were four moons that with periodical regularity circled round the great planet, galileo named them the medicean stars in honour of his patron, cosmo de' medici, grand duke of tuscany. he also published an essay entitled 'nuncius sidereus,' or the 'sidereal messenger,' which contained an account of this important discovery. the announcement of galileo's discovery of the four satellites of jupiter created a profound sensation, and its significance became at once apparent. aristotelians and ptolemaists received the information with much disfavour and incredulity, and many persons positively refused to believe galileo, whom they accused of inventing fables. on the other hand, the upholders of the copernican theory hailed it with satisfaction, as it declared that jupiter with his four moons constituted a system of greater magnitude and importance than that of our globe with her single satellite, and that consequently the earth could not be regarded as the centre of the universe. when kepler heard of this remarkable discovery, he wrote to galileo and expressed himself in the following characteristic manner: 'i was sitting idle at home thinking of you, most excellent galileo, and your letters, when the news was brought me of the discovery of four planets by the help of the double eye-glass. wachenfels stopped his carriage at my door to tell me, when such a fit of wonder seized me at a report which seemed so very absurd, and i was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or i of listening.... i am so far from disbelieving in the existence of the four circumjovial planets, that i long for a telescope to anticipate you, if possible, in discovering two round mars (as the proportion seems to me to require), six or eight round saturn, and perhaps one each round mercury and venus.' the intelligence of galileo's discoveries was received by his opponents in a spirit entirely different from that manifested by kepler. the principal professor of philosophy at padua, when requested to look at the moon and planets through galileo's glass, persistently declined, and did his utmost to persuade the grand duke that the four satellites of jupiter could not possibly exist. francesco sizzi, a florentine astronomer, argued that, as there are seven apertures in the head, seven known metals, and seven days in the week, so there could only be seven planets. to these absurd remarks galileo replied by saying that, 'whatever their force might be as a reason for believing beforehand that no more than seven planets would be discovered, they hardly seemed of sufficient weight to destroy the new ones when actually seen.' another individual, named christmann, writes: 'we are not to think that jupiter has four satellites given him by nature in order, by revolving round him, to immortalize the name of the medici, who first had notice of the observation. these are the dreams of idle men, who love ludicrous ideas better than our laborious and industrious correction of the heavens. nature abhors so horrible a chaos, and to the truly wise such vanity is detestable.' martin horky, a _protégé_ of kepler's, issued a pamphlet in which he made a violent attack on galileo. he says: 'i will never concede his four new planets to that italian from padua though i die for it.' he then asks the following questions, and replies to them himself: ( ) whether they exist? ( ) what they are? ( ) what they are like? ( ) why they are? 'the first question is soon disposed of by horky's declaring positively that he has examined the heavens with galileo's own glass, and that no such thing as a satellite about jupiter exists. to the second, he declared solemnly that he does not more surely know that he has a soul in his body than that reflected rays are the sole cause of galileo's erroneous observations. in regard to the third question, he says that these planets are like the smallest fly compared to an elephant; and, finally, concludes on the fourth, that the only use of them is to gratify galileo's "thirst of gold," and to afford himself a subject of discussion.'[ ] galileo did not condescend to take any notice of this scurrilous production; but horky, who imagined that he had done something clever, sent a copy of his pamphlet to kepler. in a few days after he called to see him, and was received with such a storm of indignation that he begged for mercy and implored his forgiveness. kepler forgave him, but insisted on his making amends. he writes: 'i have taken him again into favour upon this preliminary condition, to which he has agreed--that i am to show him jupiter's satellites, _and he is to see them_, and own that they are there.' the evidence in support of the existence of jupiter's satellites became so conclusive that the opponents of galileo were compelled to renounce their disbelief in those bodies, whether real or pretended. the grand duke, preferring to trust to his eyes rather than believe in the arguments of the professor at padua, observed the satellites on several occasions, along with galileo, at pisa, and on his departure bestowed upon him a gift of one thousand florins. several of galileo's enemies, as a result of their observations, now arrived at the conclusion that his discovery was incomplete, and that jupiter had more than four satellites in attendance upon him. scheiner counted five, rheita nine, and other observers increased the number to twelve. but it was found to be quite as hazardous to exceed the number stated by galileo as it was to deny the existence of any; for, when jupiter had traversed a short distance of his path among the fixed stars, the only bodies that accompanied him were his four original attendants, which continued to revolve round him with unerring regularity in every part of his orbit. galileo did not afford his opponents much time to oppose or controvert with argument the discoveries made by him with the telescope before his announcement of a new one attracted public attention from those already known. he, however, exercised greater caution in disclosing the results of his observations, as other persons laid claim to having made similar discoveries prior to the time at which his were announced. he therefore adopted a method in common use among astronomers in those days, by which the letters in a sentence announcing a discovery were transposed so as to form an anagram. galileo announced his next discovery in this manner, and which read as follows:-- smaismrmilme poeta leumi bvne nugttaviras. this, when deciphered, formed the sentence:-- altissimum planetam tergeminum observavi. i have observed that the remotest planet is triple. galileo perceived that saturn presented a triform appearance, and that, instead of one body, there were three, all in a straight line, and apparently in contact with each other, the middle one being larger than the two lateral ones. in a letter to kepler he remarked: 'now i have discovered a court for jupiter, and two servants for this old man, who aid his steps and never quit his side.' kepler, who excelled as an imaginative writer, replied: 'i will not make an old man of saturn, nor slaves of his attendant globes; but rather let this tricorporate form be geryon--so shall galileo be hercules, and the telescope his club, armed with which he has conquered that distant planet, and dragged him from the remotest depths of nature, and exposed him to the view of all.' continuing his observations, galileo perceived that the two lateral objects gradually decreased in size, and at the expiration of two years entirely disappeared, leaving the central globe visible only. he was unable to assign any reason for this peculiar occurrence, which caused him much perplexity, and he expresses himself thus: 'what is to be said concerning so strange a metamorphosis? are the two lesser stars consumed after the manner of the solar spots? have they vanished and suddenly fled? has saturn, perhaps, devoured his own children? or were the appearances, indeed, illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom i have shown them? now, perhaps, is the time to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of their existence. i do not know what to say in a case so surprising, so unlooked-for, and so novel. the shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me.' after a certain interval those bodies reappeared; but galileo's glass was not sufficiently powerful to enable him to ascertain their nature nor solve the mystery, which for upwards of half a century perplexed the ablest astronomers. the elucidation of this inexplicable phenomenon was reserved for christian huygens, who, with an improved telescope of his own construction, was able to declare that saturn's appendages were portions of a ring which surrounds the planet, and is everywhere distinct from its surface. galileo next directed his attention to the planet venus, and as a result of his observations was led to communicate to the public another anagram:-- haec immatura a me jam frustra leguntur oy. this, when rendered correctly, reads:-- cynthiae figuras aemulatur mater amorum. venus rivals the appearances of the moon. the phases of venus were one of the most interesting of galileo's discoveries with the telescope. when observed near inferior conjunction the planet presents the appearance of a slender crescent, resembling the moon when a few days old. travelling from this point to superior conjunction, the illumined portion of her disc gradually increases, until it becomes circular, like the full moon. this changing appearance of venus afforded galileo irresistible proof that the planet is an opaque body, which derives its light from the sun, and that it circles round the orb--convincing evidence of the accuracy and truthfulness of the copernican theory. it was in this manner that galileo announced his discovery of the phases of venus, the peerless planet of our morning and evening skies, whose slender crescent forms such a beautiful object in the telescope, and who, as she traverses her orbit, exhibits all the varied changes of form presented by the moon in her monthly journey round the earth. these varying aspects of venus were not unknown to milton; and, indeed, he may have been informed of them by galileo in his conversation with him at arcetri; nor has he failed to introduce an allusion to this beautiful phenomenon in his poem. in his description of the creation, after the sun was formed, he adds:-- hither, as to their fountain, other stars repairing, in their golden urns draw light, and hence the morning planet gilds her horns.--vii. - . galileo also discovered that the planet mars does not always present the appearance of a circular disc. when near opposition the full disc of the planet is visible, but at all other times it is gibbous, and approaches nearest to that of a half-moon when at the quadratures. in the year , on directing his telescope to the sun, galileo detected dark spots on the solar disc. similar spots, sufficiently large to be distinguished by the naked eye, had been observed from time to time for centuries prior to the invention of the telescope, but nothing was known of their nature. in kepler observed a spot on the sun, which he thought was the planet mercury in conjunction with the orb; the short time during which it was visible, in consequence of clouds having obscured the face of the luminary, prevented him from being able to determine the accuracy of his surmise, but since then it has been ascertained that no transit of mercury took place at that time, and kepler afterwards acknowledged that he had arrived at an erroneous conclusion. galileo was much puzzled in trying to find out the true nature of the spots. at first he was led to imagine that planets like mercury and venus revolved round the sun at a short distance from the orb, and that their dark bodies, travelling across the solar disc, gave rise to the phenomenon of the spots. after further observation, he ascertained that the spots were in actual contact with the sun; that they were irregular in shape and size, and continued to appear and disappear. sometimes a large spot would break up into several smaller ones, and at other times three or four small spots would unite to form a large one. they all had a common motion, and appeared to rotate with the sun, from which galileo concluded that the orb rotated on his axis in about twenty-eight days. galileo believed that the spots were clouds floating in the solar atmosphere, and that they intercepted a portion of the light of the sun. the milky way, that wondrous zone of light which encircles the heavens, remained for many ages a source of perplexity to ancient astronomers and philosophers, who, in their endeavours to ascertain its nature, had arrived at various absurd and erroneous conclusions. on directing his telescope to this luminous tract, galileo discovered, to his inexpressible admiration, that it consists of a vast multitude of stars, too minute to be visible to the naked eye. he also discerned that its milky luminosity is created by the blended light of myriads of stars, so remote as to be incapable of definition by his telescope. in his 'nuncius sidereus' he gives an account of his observations of the galaxy and expresses his satisfaction that he has been enabled to terminate an ancient controversy by demonstrating to the senses the stellar structure of the milky way. when engaged in exploring the celestial regions with his telescope, galileo observed a marked difference in the appearance of the fixed stars, as compared with that of the planets. each of the latter showed a rounded disc resembling that of a small moon, but the stars exhibited no disc, and shone as vivid sparkling points of light; all of them, whether of large or small magnitude, presenting the same appearance in the telescope. this led him to conclude that the fixed stars were not illumined by the sun, because their brilliancy in all their changes of position remained unaltered. but, in the case of the planets, he found that their lustre varied according to their distance from the sun; consequently, he believed they were opaque bodies which reflected the solar rays. on directing his telescope to the pleiades, which, to the naked eye, appear as a group of seven stars, he succeeded in counting forty lucid points. the nebula praesepe in cancer, he was also able to resolve into a cluster of stars. galileo made many other observations of the heavenly bodies with his telescope, all of which he describes as having afforded him 'incredible delight.' shortly before the failure of his eyesight, galileo discovered the moon's diurnal libration, a variation in the visible edges of the moon caused by its oscillatory motion, and the diurnal rotation of the earth on her axis. though milton has not favoured us with any interesting details of his interview with galileo, nor expressed his opinions with regard to the controversies which at that time agitated both the religious and scientific worlds of thought, and which eventually culminated in a storm of rancour and hatred that burst over the devoted head of the aged astronomer, and brought him to his knees, yet he informs us that he 'found and visited' galileo, whom he describes as 'grown old,' and cynically remarks that he 'was held a prisoner of the inquisition for thinking in astronomy otherwise than the franciscan and dominican licensers thought.' milton does not allude to his blindness, and yet it would be natural to imagine that, had his host suffered from this affliction at the time of his visit, he would have referred to it. we learn that milton arrived in italy in the spring of . in , the affection which, in the preceding year, deprived galileo of the use of his right eye, attacked the left also, which began to grow dim, and in the course of a few months became sightless; so that, although milton has not alluded to this calamity, galileo had become totally blind at the time of his visit. how much milton was impressed with the fame of galileo and his telescope becomes apparent on referring to his 'paradise lost.' in it he alludes to the instrument upon three different occasions, twice when in the hands of galileo; and the remembrance of the same artist was doubtless in his mind when he mentions the 'glazed optic tube' in another part of his poem. the interval that elapsed from the date of milton's visit to galileo in , to the publication of 'paradise lost' in , included a period of about thirty years, yet this length of time did not erase from milton's memory his recollection of galileo and of his pleasant sojourn at florence. the first allusion in the poem to the italian astronomer is in the lines in which milton describes the shield carried by satan:-- the broad circumference hung on his shoulders like the moon, whose orb through optic glass the tuscan artist views at evening, from the top of fesolé, or in valdarno, to descry new lands, rivers, or mountains, in her spotty globe.--i. - . galileo is described as having observed the moon from the heights of fesolé, which formed part of the suburbs of florence, or from valdarno, the valley of the arno, in which the city is situated. the belief that galileo had discovered continents and seas on the moon justified milton in imagining the existence of rivers and mountains on the lunar surface. the expression 'spotty globe' is more descriptive of the appearance of our satellite when observed with the telescope, than when seen with the naked eye. galileo's attention was attracted by the freckled aspect of the moon--a visual effect created by the number of extinct volcanoes scattered over the surface of the orb. in his next allusion to the telescope milton associates galileo's name with the instrument:-- as when by night the glass of galileo, less assured, observes imagined lands and regions in the moon.--v. - . in these lines milton describes with accuracy the extent of galileo's knowledge of our satellite. the conclusions which the italian astronomer arrived at with regard to its habitability were not supported by telescopic evidence sufficient to justify such a belief. galileo writes: 'had its surface been absolutely smooth it would have been but a vast, unblessed desert, void of animals, of plants, of cities and men; the abode of silence and inaction--senseless, lifeless, soulless, and stripped of all those ornaments which now render it so variable and so beautiful:'-- there lands the fiend, a spot like which perhaps astronomer in the sun's lucent orb through his glazed optic tube yet never saw.--iii. - . milton may have remembered that galileo was the first astronomer who directed a telescope to the sun; and that he discovered the dark spots frequently seen on the solar disc. anyone who has read a history of the life of galileo, and contemplated the career of this remarkable man, his ardent struggles in the cause of freedom and philosophic truth, his victories and reverses, his brilliant astronomical discoveries, and his investigation of the laws of motion, and other natural phenomena, will arrive at the conclusion that he merited the distinction conferred upon him by our great english poet, when he included him among the renowned few whose names are found in the pages of 'paradise lost.' chapter v the seasons the great path of the sun among the constellations as seen from the earth is called the ecliptic. it is divided into °, and again into twelve equal parts of °, called signs. as one half of the ecliptic is north, and the other half south, of the equator, the line of intersection of their planes is at two points which are known as the equinoctial points, because, when the sun on his upward and downward journey arrives at either of them the days and nights are of equal length all over the world. the equinoctial points are not stationary, but have a westerly motion of ´´ annually along the ecliptic; at this rate they will require a period of , years to complete an entire circuit of the heavens. milton alludes to the ecliptic when he mentions the arrival of satan upon the earth:-- down from the ecliptic, sped with hoped success, throws his steep flight in many an airy wheel, nor staid till on niphates top he lights.--iii. - . extending for ° on each side of the ecliptic is a zone or belt called the zodiac, the mesial line of which is occupied by the sun, and within this space the principal planets perform their annual revolutions. it was for long believed that the paths of all the planets lay within the zodiac, but on the discovery of the minor planets, ceres, pallas, and juno, it was ascertained that they travelled beyond this zone. the stars situated within the zodiac are divided into twelve groups or constellations, which correspond with the twelve signs, and each is named after an animal or some figure which it is supposed to resemble. the zodiac is of great antiquity; the ancient egyptians and hindoos made use of it, and there are allusions to it in the earliest astronomical records. the twelve constellations of the zodiac bear the following names:-- aries the ram taurus the bull gemini the twins cancer the crab leo the lion virgo the virgin libra the balance scorpio the scorpion sagittarius the archer capricornus the goat aquarius the water-bearer pisces the fishes in close association with the sun's annual journey are the seasons, upon the regular sequence of which mankind depend for the various products of the soil essential for the maintenance and enjoyment of life. the revolution of the earth in her orbit, and the inclination of her axis to her annual path, causing the plane of the equator to be inclined - / ° to that of the ecliptic, are the reasons which account for the succession of the seasons--spring, summer, autumn, and winter. owing to the position of the earth's axis with regard to her orbit, the sun appears to travel - / ° north and - / ° south of the equator. when, on june , the orb attains his highest northern altitude, we have the summer solstice and the longest days; when, by retracing his steps, he declines - / ° below the equator, at which point he arrives on december , we have the winter solstice and the shortest days. intermediate between those two seasons are spring and autumn. when the sun, on his journey northward, reaches the equator, we have the vernal equinox, and at this period of the year the days and nights are of equal length all over the globe. in a similar manner, when, on his return journey, the sun is again on the equator, the autumnal equinox occurs. in summer the north pole is inclined towards the sun, consequently his rays fall more direct and impart much more heat to the northern hemisphere than in winter, when the pole is turned away from the sun. this difference in the incidence of the solar rays upon the surface of the globe, along with the increased length of the day, mainly accounts for the high temperature of summer as compared with that of winter. astronomically, the seasons commence at the periods of the equinoxes and solstices. spring begins on march , the time of the vernal equinox; summer on june , at the summer solstice; autumn on september , at the autumnal equinox; and winter on december , at the winter solstice. this conventional division of the year is not equally applicable to all parts of the globe. in the arctic and antarctic regions spring and autumn are very brief, the summer is short and the winter of long duration. in the tropics, owing to the comparatively slight difference in the obliquity of the sun's rays, one season is, as regards temperature, not much different from the other; but in the temperate regions of the earth the vicissitudes of the seasons are more perceptible and can be best distinguished by the growth of vegetation, and the changes observable in the foliage of shrubs and trees. in spring there is the budding, in summer the blossom, in autumn the fruit-bearing, and in winter the leafless condition of deciduous trees, and the repose of vegetable life. the legendary belief that before the fall there reigned on the earth a perpetual spring, is introduced by milton in his poem when he describes the pleasant surroundings associated with the happy conditions of life that existed in paradise:-- thus was this place, a happy rural seat of various view: groves whose rich trees wept odorous gums and balm; others whose fruit, burnished with golden rind, hung amiable--hesperian fables true, if true here only--and of delicious taste. betwixt them lawns, or level downs, and flocks grazing the tender herb, were interposed, or palmy hillock; or the flowery lap of some irriguous valley spread her store, flowers of all hue, and without thorn the rose. another side, umbrageous grots and caves of cool recess, o'er which the mantling vine lays forth her purple grape, and gently creeps luxuriant; meanwhile murmuring waters fall down the slope hill dispersed, or in a lake that to the fringèd bank with myrtle crowned her crystal mirror holds, unite their streams. the birds their quire apply; airs, vernal airs, breathing the smell of field and grove, attune the trembling leaves, while universal pan, knit with the graces and the hours in dance, led on the eternal spring.--iv. - . in sad contrast with this charming sylvan scene, we turn to the unhappy consequences which ensued as a result of the first act of transgression. milton describes a change of climate characterised by extremes of heat and cold which succeeded the perpetual spring. the sun was made to shine so that the earth should be exposed to torrid heat and icy cold unpleasant to endure. the pale moon and the planets were given power to combine with noxious effect, and the fixed stars to shed their malignant influences:-- the sun had first his precept so to move, so shine, as might affect the earth with cold and heat scarce tolerable, and from the north to call decrepit winter, from the south to bring solstitial summer's heat. to the blanc moon her office they prescribed; to the other five their planetary motions and aspects, in sextile, square, and trine, and opposite, of noxious efficacy, and when to join in synod unbenign; and taught the fixed their influence malignant when to shower-- which of them rising with the sun or falling, should prove tempestuous. to the winds they set their corners, when with bluster to confound sea, air, and shore; the thunder when to roll with terror through the dark aerial hall.--x. - . we are here afforded an opportunity of learning that milton possessed some knowledge of astrology, to which he makes allusion in other parts of his poem besides. in his time, astrology was believed in by many persons, and there were few learned men but who knew something of that occult science. milton may be included among those who devoted some attention to astrology. of this there is ample evidence, by the manner in which he expresses himself in words and phrases in common use among astrologers. the professors of this art recognised five planetary aspects, viz., opposition, conjunction, sextile, square, and trine, each possessing its peculiar kind of influence on events. the moon, the planets, and the constellations in their conjunctions and configurations, were believed to reveal to those who could understand the significance of their aspects, the destiny of individuals and the occurrence of future events. the inauspicious influences of the heavenly bodies are described by milton as contributing to the general disarrangement of the happy condition of things that existed before the fall. after having described the adverse physical changes which occurred in nature as a consequence of the fall, milton makes use of his astronomical knowledge in explaining how they were brought about, and suggests two hypotheses: ( ) a change of position of the earth's axis; ( ) an alteration of the sun's path from the equinoctial road:-- some say he bid his angels turn askance the poles of earth twice ten degrees and more from the sun's axle; they with labour pushed oblique the centric globe: some say the sun was bid turn reins from the equinoctial road like distant breadth--to taurus with the seven atlantic sisters, and the spartan twins, up to the tropic crab; thence down amain by leo, and the virgin, and the scales, as deep as capricorn; to bring in change of seasons to each clime. else had the spring perpetual smiled on earth with vernant flowers.--x. - . in support of the theory of a perpetual spring, milton assumes that the earth's axis was directed at right angles to her orbit, and that the plane of the equator coincided with that of the ecliptic. consequently, the sun's path remained always on the equator, where his rays were vertical, and north and south of this line each locality on the earth enjoyed one constant season, the character of which depended upon its geographical position. in what are now the temperate regions of the globe there was one continuous season, similar in climate and length of day to what is experienced at the vernal equinox, when the sun is for a few days on the equator. there was then no winter, no summer, nor autumn; and, consequently, the growth of vegetation must have taken place under conditions of climate entirely different to what exist on the earth at the present time. the change of position of the earth's axis, 'twice ten degrees and more from the sun's axle,' is described by milton as having been accomplished by the might of angels, who 'with labour pushed oblique the centric globe.' ( ) according to the ptolemaic belief, the sun revolved round the earth, but his course was altered from the equinoctial road to the path that he now pursues, which is the ecliptic. instead of remaining on the equator, he travels an equal distance from this line upwards and downwards in each hemisphere. the path of the sun in the heavens is described by milton with marked precision, and he mentions in regular order the names of the zodiacal constellations through which the orb travels. passing through taurus with the seven atlantic sisters (the pleiades) and the spartan twins (gemini), he enters the tropic crab (cancer), in which constellation he attains his highest northern altitude; thence downwards he travels through leo, virgo, and the scales (libra), as deep as capricornus, reaching his lowest point of declination at the winter solstice; and were it not for this alteration of the sun's path, the poet informs us that perpetual spring would have reigned upon the earth. milton was evidently well acquainted with the astronomical reasons (the revolution of the earth in her orbit and the obliquity of the ecliptic) by which the occurrence and regular sequence of the seasons can be explained. the path of the sun in the heavens; his upward and downward course from the equator; the names of the constellations through which the orb travels, and the periods of the year at which he enters them, were also familiar to him. the grateful change of the seasons, and the varied aspects of nature peculiar to each, which give a charm and freshness to the rolling year, must have been to milton a source of pleasure and delight, and have stimulated his poetic fancy. his observation of natural phenomena, and his keen perception of the pleasing changes which accompany them, are described in the following lines:-- as, when from mountain-tops the dusky clouds ascending, while the north wind sleeps, o'erspread heaven's cheerful face, the louring element scowls o'er the darkened landskip snow or shower, if chance the radiant sun, with farewell sweet, extend his evening beam, the fields revive, the birds their notes renew, and bleating herds attest their joy, that hill and valley rings.--ii. - . the ancient poets virgil and ovid describe the earth as having been created in the spring; and associated with this season, which to the heart inspires vernal delight and joy--iv. - , were the graces and the hours, which danced hand in hand as they led on the eternal spring. milton alludes to the seasons on several occasions throughout his poem, and to the natural phenomena associated with them:-- as bees in springtime when the sun with taurus rides, pour forth their populous youth about the hive in clusters; they among fresh dews and flowers fly to and fro, or on the smoothèd plank the suburb of their straw-built citadel new rubbed with balm, expatiate and confer their state affairs.--i. - . the sun is in the constellation taurus in april, when the warmth of his rays begins to impart new life and activity to the insect world after their long winter's sleep. in his description of the repast partaken by the angel raphael with adam and eve in paradise, milton writes:-- raised of grassy turf their table was, and mossy seats had round, and on her ample square, from side to side, all autumn piled, though spring and autumn here danced hand in hand.--v. - . in describing beelzebub when about to address the stygian council, he says:-- his look drew audience and attention still as night or summer's noontide air, while thus he spake.--ii. - . the failing vision from which milton suffered in his declining years was succeeded by total blindness. this sad affliction he alludes to in the following lines:-- thus with the year seasons return; but not to me returns day, or the sweet approach of even or morn, or sight of vernal bloom, or summer's rose.--iii. - . we are able to perceive how much milton was impressed with the beautiful seasons, and the varying aspects of the year which accompany them, and how his poetic imagination luxuriated in the changing variety of nature observable in earth and sky that from day to day afforded him exquisite delight; and, although his poem was written when blindness had overtaken him, yet those glad remembrances remained as fresh in his memory as when in his youth he roamed among the flowery meadows, the vocal woodlands, and the winding lanes of buckinghamshire. the idea expressed by milton that the primitive earth enjoyed a perpetual spring, though pleasing to the imagination, and well adapted for poetic description, is not sustained by any astronomical testimony. indeed, the position of the earth, with her axis at right angles to her orbit, is one which may be regarded as being ill adapted for the support and maintenance of life on her surface, just as her present position is the best that can be imagined for fulfilling this purpose. astronomy teaches us to rely with certainty upon the permanence and regular sequence of the seasons. the position of the earth's axis as she speeds along in her orbit through the unresisting ether remains unchanged, and her rapid rotation has the effect of increasing its stability. yet, the earth performs none of her motions with rigid precision, and there is a very slow alteration of the position of her axis occurring, which, if unchecked, would eventually produce a coincidence of the equator and the ecliptic. instead of a succession of the seasons, there would then be perpetual spring upon the earth, and, although it would require a great epoch of time to bring about such a change, there would result a condition of things entirely different to what now exists on the globe. but, before the ecliptic can have approached sufficiently near the equator to produce any appreciable effect upon the climate of the earth, its motion must cease, and after remaining stationary for a time, it will begin to recede to its former position. the seasons must therefore follow each other in regular sequence, and throughout all time, reminding us of the promise of the creator, 'that while the earth remaineth seed-time and harvest, and cold and heat, and summer and winter shall not cease.' chapter vi the starry heavens the celestial vault, that, like a circling canopy of sapphire hue, stretches overhead from horizon to horizon, resplendent by night with myriad stars of different magnitudes and varied brilliancy, forming clusterings and configurations of fantastic shape and beauty, arrests the attention of the most casual observer. but to one who has studied the heavens, and followed the efforts of human genius in unravelling the mysteries associated with those bright orbs, the impression created on his mind as he gazes upon them in the still hours of the night, when the turmoil of life is hushed in repose, is one of wonder and longing to know more of their being and the hidden causes which brought them forth. here, we have poetry written in letters of gold on the sable vestment of night; music in the gliding motion of the spheres; and harmony in the orbital sweep of sun, planet, and satellite. milton was not only familiar with 'the face of the sky,' as it is popularly called, but also knew the structure of the celestial sphere, and the great circles by which it is circumscribed. two of those--the colures--he alludes to in the following lines, when he describes the manner in which satan, to avoid detection, compassed the earth, after his discovery by gabriel in paradise, and his flight thence:-- the space of seven continued nights he rode with darkness--thrice the equinoctial line he circled, four times crossed the car of night from pole to pole, traversing each colure.--ix. - . aristarchus of samos believed the stars were golden studs, that illumined the crystal dome of heaven; but modern research has transformed this conception of the ancient astronomer's into a universe of blazing suns rushing through regions of illimitable space. in milton's time astronomers had arrived at no definite conclusion with regard to the nature of the stars. they were known to be self-luminous bodies, situated at a remote distance in space, but it had not been ascertained with any degree of certainty that they were suns, resembling in magnitude and brilliancy our sun. indeed, little was known of those orbs until within the past hundred years, when the exploration of the heavens by the aid of greatly increased telescopic power, was the means of creating a new branch of astronomical science, called sidereal astronomy. we are indebted to sir william herschel, more than to any other astronomer, for our knowledge of the stellar universe. it was he who ascertained the vastness of its dimensions, and attempted to delineate its structural configuration. he also explored the star depths, which occupy the infinitude of space by which we are surrounded, and made many wonderful discoveries, which testify to his ability as an observer, and to his greatness as an astronomer. william herschel was born at hanover, november , . his father was a musician in the band of the hanoverian guard, and trained his son in his own profession. after four years of military service, young herschel arrived in england when nineteen years of age, and maintained himself by giving lessons in music. we hear of him first at leeds, where he followed his profession, and instructed the band of the durham militia. from leeds he went to halifax, and was appointed organist there; on the expiration of twelve months he removed to bath, and was elected to a similar post at the octagon chapel in that city. here, fortune smiled upon him, and he became a busy and prosperous man. besides attending to his numerous private engagements, he organised concerts, oratorios, and other public musical entertainments, which gained him much popularity among the cultivated classes which frequented this fashionable resort. notwithstanding his numerous professional engagements, herschel was able to devote a portion of his time to acquiring knowledge on other subjects. he became proficient in italian and greek, studied mathematics, and read books on astronomy. in he borrowed a small telescope, which he used for observational purposes, and was so captivated with the appearances presented by the celestial bodies, that he resolved to dedicate his life to acquiring 'a knowledge of the construction of the heavens.' this resolution he nobly adhered to, and became one of the most distinguished of astronomers. like many other astronomers, herschel possessed the requisite skill which enabled him to construct his own telescopes. being desirous of possessing a more powerful instrument, and not having the means to purchase one, he commenced the manufacture of specula, the grinding and polishing of which had to be done by hand, entailing the necessity of tedious labour and the exercise of much patience. after repeated failures he at length completed a - / -foot gregorian reflector, and with this instrument made his first survey of the heavens. having perceived the desirability of possessing a more powerful telescope, he equipped himself with a reflector of twenty feet focal length, and it was with this instrument that he made those wonderful discoveries which established his reputation as a great astronomer. on march , , when examining the stars in the constellation gemini, herschel observed a star which presented an appearance slightly different to that of the other stars by which it was surrounded; it looked larger, had a perceptible disc, and its light became fainter when viewed with a higher magnifying power. after having carefully examined this object, herschel arrived at the conclusion that he had discovered a comet. he communicated intelligence of his discovery to the royal society, and, a notification of it having been sent to the continental observatories, this celestial visitor was subjected to a close scrutiny; its progressive motion among the stars was carefully observed, and an orbit was assigned to it. after it had been under observation for some time, doubts were expressed as to its being a comet, these were increased on further examination, and eventually it was discovered that this interesting object was a new planet. this important discovery at once raised herschel to a position of eminence and distinction, and from a star-gazing musician he became a famous astronomer. a new planet named uranus was added to our system, which completes a revolution round the sun in a little over eighty-four years, and at a distance of near , millions of miles beyond the orbit of saturn. herschel's name became a household word. george iii. invited him to court in order that he might obtain from his own lips an account of his discovery of the new planet; and so favourable was the impression made by herschel upon the king, that he proposed to create him royal astronomer at windsor, and bestow upon him a salary of _l._ a year. herschel decided to accept the proffered appointment, and, with his sister caroline, removed from bath to datchet, near windsor, in , and from there to slough in . in he married the wealthy widow of a london merchant, by whom he had one son, who worthily sustained his father's high reputation as an astronomer. herschel was created a knight in , and in was elected first president of the royal astronomical society. he died at slough on august , , when in the eighty-fourth year of his age, and was buried in upton churchyard. it is inscribed on his tomb, that 'he burst the barriers of heaven;' the lofty praise conveyed by this expression is not greater than what herschel merited when we consider with what unwearied assiduity and patience he laboured to accomplish the results described in the words which have been quoted. by a method called 'star-gauging' he accomplished an entire survey of the heavens and examined minutely all the stars in their groups and aggregations as they passed before his eye in the field of the telescope. he sounded the depths of the milky way, and explored the wondrous regions of that shining zone, peopled with myriads of suns so closely aggregated in some of its tracts as to suggest the appearance of a mosaic of stars. he resolved numerous nebulæ into clusters of stars, and penetrated with his great telescope depth after depth of space crowded with 'island universes of stars,' beyond which he was able to discern luminous haze and filmy streaks of light, the evidence of the existence of other universes plunged in depths still more profound, where space verges on infinity. in his exploration of the starry heavens herschel's labours were truly amazing. on four different occasions he completed a survey of the firmament, and counted the stars in several thousand gauge-fields; he discovered , nebulæ, double stars, and attempted to ascertain the approximate distances of the stars by a comparison of their relative brightness. it had long been surmised, though no actual proof was forthcoming, that the law of gravitation by which the order and stability of our system are maintained exercises its potent influence over other material bodies existing in space, and that other systems, though differing in many respects from that of ours, and presenting a more complex arrangement in their structure, perform their motions subject to the guidance of this universal law. the uncertainty with regard to the controlling influence of gravity was removed by herschel when he made his important discovery of binary star systems. the components of a binary star are usually in such close proximity that, to the naked eye, they appear as one star, and sometimes, even with telescopic aid, it is impossible to distinguish them individually; but when observed with sufficient magnifying power they can be easily perceived as two lucid points. double stars were for a long time believed to be a purely optical phenomenon--an effect created by two stars projected on the sphere so as to appear nearly in the same line of vision, and, although apparently almost in contact, situated at great distances apart. at one time herschel entertained a similar opinion with regard to those stars. in he undertook an extensive exploration of the heavens with the object of discovering double stars. as a result of his labours he presented to the royal society in a list of newly discovered double stars, and in three years after he supplemented this list with another which contained more new stars. he carefully measured the distances by which the component stars were separated, and determined their position angles, in order that he might be able to detect the existence of any sensible parallax. on repeating his observations twenty years after, he discovered that the relative positions of many of the stars had changed, and in he made the important announcement of his discovery that the components of many double stars form independent systems, held together in a mutual bond of union and revolving round one common centre of gravity. the importance of this discovery, which we owe to herschel's sagacity and accuracy of observation, cannot be over-estimated; what was previously conjecture and surmise, now became precise knowledge established upon a sure and accurate basis. it was ascertained that the law of gravity exerts its power in regulating and controlling the motions of all celestial bodies within the range of telescopic vision, and that the order and harmony which pervade our system are equally present among other systems of suns and worlds distributed throughout the regions of space. the spectacle of two or more suns revolving round each other, forming systems of greater magnitude and importance than that of ours, conveyed to the minds of astronomers a knowledge of the mechanism of the heavens which had hitherto been unknown to them. during the many years which herschel devoted to the exploration of the starry heavens, and when engaged night after night in examining and enumerating the various groups and clusters of stars which passed before his eye in the field of his powerful telescope, he did not fail to remember the sublime object of his life, and to which he made all his other investigations subordinate, viz., the delineation of the structural configuration of the heavens, and the inclusion of all aggregations, groups, clusters, and galaxies of stars which are apparently scattered promiscuously throughout the regions of space into one grand harmonious design of celestial architecture. having this object in view, he explored the wondrous zone of the milky way, gauged its depths, measured its dimensions, and, in attempting to unravel the intricacies of its structure, penetrated its recesses far beyond the limit attained by any other observer. acting on the assumption that the stars are uniformly distributed throughout space, herschel, by his method of star-gauging, concluded that the sidereal system consists of an irregular stratum of evenly distributed suns, resembling in form a cloven flat disc, and that the apparent richness of some regions as compared with that of others could be accounted for by the position from which it was viewed by an observer. the stars would appear least numerous where the visual line was shortest, and, as it became lengthened, they would increase in number until, by crowding behind each other as a greater depth of stratum was penetrated, they would, when very remote, present the appearance of a luminous cloud or zone of light. after further observation herschel was compelled to relinquish his theory of equal star distribution, and found, as he approached the galaxy, that the stars became much more numerous, and that in the milky way itself there was evidence of the gravitation of stars towards certain regions forming aggregations and clusters which would ultimately lead to its breaking up into numerous separate sidereal systems. as he extended his survey of the heavens and examined with greater minuteness the stellar regions in the galactic tract, he discovered that by his method of star-gauging he was unable to define the complexity of structure and variety of arrangement which came under his observation; he also perceived that the star-depths are unfathomable, and discerned that beyond the reach of his telescope there existed systems and galaxies of stars situated at an appalling distance in the abysmal depths of space. though the magnitude of that portion of the sidereal heavens which came under his observation was inconceivable as regards its dimensions, herschel was able to perceive that it formed but a part--and most probably a small part--of the stellar universe, and that without a more extended knowledge of this universe, which at present is unattainable, it would be impossible to determine its structural configuration or discover the relationships that exist among the sidereal systems and galactic concourses of stars distributed throughout space. herschel ultimately abandoned his star-gauging method of observation and confined his attention to exploring the star depths and investigating the laws and theories associated with the bodies occupying those distant regions. since all the planets if viewed from the sun would be seen to move harmoniously and in regular order round that body, so there may be somewhere in the universe a central point, or, as some persons imagine, a great central sun, round which all the systems of stars perform their majestic revolutions with the same beautiful regularity; having their motions controlled by the same law of gravitation, and possessing the same dynamical stability which characterises the mechanism of the solar system. the extent of the distance which intervenes between our system and the fixed stars constituted a problem which exercised the minds of astronomers from an early period until the middle of the present century. tycho brahé, who repudiated the copernican theory, asserted as one of his reasons against it that the distances by which the heavenly bodies are separated from each other were greater than even the upholders of this theory believed them to be. although the distance of the sun from the earth was unknown, tycho was aware that the diameter of the earth's orbit must be measured by millions of miles, and yet there was no perceptible motion or change of position of the stars when viewed from any point of the vast circumference which she traverses. consequently, the earth, if viewed from the neighbourhood of a star, would also appear motionless, and the dimensions of her orbit would be reduced to that of a point. this seemed incredible to tycho, and he therefore concluded that the copernican theory was incorrect. the conclusion that the stars are orbs resembling our sun in magnitude and brilliancy was one which, tycho urged, should not be hastily adopted; and yet, if it were conceded that the earth is a body which revolves round the sun, it would be necessary to admit that the stars are suns also. if the earth's orbit, as seen from a star, were reduced to a point, then the sun, which occupies its centre, would be reduced to a point of light also, and, when observed from a star of equal brilliancy and magnitude, would have the same resemblance that the star has when viewed from the earth, which may be regarded as being in proximity to the sun. tycho brahé would not admit the accuracy of these conclusions, which were too bewildering and overwhelming for his mental conception. but the investigations of later astronomers disclosed the fact that the heavenly bodies are situated at distances more remote from each other than had been previously imagined, and that the reasons which led tycho to reject the copernican theory were based upon erroneous conclusions, and could, with greater aptitude, be employed in its support. it was ascertained that the distance of the sun from the earth, which at different periods was surmised to be ten, twenty, and forty millions of miles, was much greater than had been previously estimated. later calculations determined it to be not less than eighty millions of miles, and, according to the most recent observations, the distance of the sun from the earth is believed to be about ninety-three millions of miles. having once ascertained the distance between the earth and the sun, astronomers were enabled to determine with greater facility the distances of other heavenly bodies. it was now known that the diameter of the earth's orbit exceeded millions of miles, and yet, with a base line of such enormous length, and with instruments of the most perfect construction, astronomers were only able to perceive the minutest appreciable alteration in the positions of a few stars when observed from opposite points of the terrestrial orbit. it had long been the ambitious desire of astronomers to accomplish, if possible, a measurement of the abyss which separates our system from the nearest of the fixed stars. no imaginary measuring line had ever been stretched across this region of space, nor had its unfathomed depths ever been sounded by any effort of the human mind. the stars were known to be inconceivably remote, but how far away no person could tell, nor did there exist any guide by which an approximation of their distances could be arrived at. in attempting to calculate the distances of the stars, astronomers have had recourse to a method called 'parallax,' by which is meant the apparent change of position of a heavenly body when viewed from two different points of observation. the annual parallax of a heavenly body is the angle subtended at that body by the radius of the earth's orbit. the stars have no diurnal parallax, because, owing to their great distance, the earth's radius does not subtend any measurable angle, but the radius of the earth's orbit, which is immensely larger, does, in the case of a few stars, subtend a very minute angle. 'this enormous base line of millions of miles is barely sufficient, in conjunction with the use of the most delicate and powerful astronomical instruments, to exhibit the minutest measureable displacement of two or three of the nearest stars.'--proctor. the efforts of early astronomers to detect any perceptible alteration in the positions of the stars when observed from any point of the circumference of the earth's orbit were unsuccessful. copernicus ascribed the absence of any parallax to the immense distances of the stars as compared with the dimensions of the terrestrial orbit. tycho brahé, though possessing better appliances, and instruments of more perfect construction, was unable to perceive any annual displacement of the stars, and brought this forward as evidence against the copernican theory. galileo suggested a method of obtaining the parallax of the fixed stars, by observing two stars of unequal magnitude apparently near to each other, though really far apart. those, when observed from different points of the earth's orbit, would appear to change their positions relatively to each other. the smaller and more distant star would remain unaltered, whilst the larger and nearer star would have changed its position with respect to the other. by continuing to observe the larger star during the time that the earth accomplished a revolution of her orbit, galileo believed that its parallax might be successfully determined. though he did not himself put this method into practice, it has been tried by others with successful results. in , hooke made the first attempt to ascertain the parallax of a fixed star, and selected for this purpose gamma draconis, a bright star in the head of the dragon. this constellation passed near the zenith of london at the time that he made his observations, and was favourably situated, so as to avoid the effects of refraction. hooke made four observations in the months of july, august, and october, and believed that he determined the parallax of the star; but it was afterwards discovered that he was in error, and that the apparent displacement of the star was mainly due to the aberration of light--a phenomenon which was not discovered at that time. a few years later, picard, a french astronomer, attempted to find the parallax of alpha lyræ, but was unsuccessful. in - , roemer, a danish astronomer, observed irregularities in the declinations of the stars which could neither be ascribed to parallax or refraction, and which he imagined resulted from a changing position of the earth's axis. one of the principal causes which baffled astronomers in their endeavours to determine the parallax of the fixed stars was a phenomenon called the 'aberration of light,' which was discovered and explained by bradley in . the peculiar effect of aberration was perceived by him when endeavouring to obtain the parallax of gamma draconis. owing to the progressive transmission of light, conjointly with the motion of the earth in her orbit, there results an apparent slight displacement of a star from its true position. the extent of the displacement depends upon the ratio of the velocity of light as compared with the speed of the earth in her orbit, which is as , to . as a consequence of this, each star describes a small ellipse in the course of a year, the central point of which would indicate the place occupied by the star if the earth were at rest. the shifting position of the star is very slight, and at the end of a year it returns to its former place. prior to the discovery of aberration, astronomers ascribed the apparent displacement of the stars arising from this cause as being due to parallax--a conclusion which led to erroneous results; but after bradley's discovery this source of error was avoided, and it was found that the parallax of the stars had to be considerably reduced. bessel was the first astronomer who merited the high distinction of having determined the first reliable stellar parallax, and by this achievement he was enabled to fathom the profound abyss which separates our solar system from the stars. frederick william bessel was born in at minden, in westphalia. it was his intention to pursue a mercantile career, and he commenced life by becoming apprenticed to a firm of merchants at bremen. soon afterwards he accompanied a trading expedition to china and the east indies, and while on this voyage picked up a good deal of information with regard to many matters which came under his observation. he acquired a knowledge of spanish and english, and made himself acquainted with the art of navigation. on his return home, bessel endeavoured to determine the longitude of bremen. the only appliances which he made use of were a sextant constructed by himself, and a common clock; and yet, with those rude instruments, he successfully accomplished his object. during the next two years he devoted all his spare time to the study of mathematics and astronomy, and, having obtained possession of harriot's observations of the celebrated comet of --known as halley's comet--bessel, after much diligent application and careful calculation, was enabled to deduce from them an orbit, which he assigned to that remarkable body. this meritorious achievement was the means of procuring for him a widely known reputation. a vacancy for an assistant having occurred at schröter's observatory at lilienthal, the post was offered to bessel and accepted by him. here he remained for four years, and was afterwards appointed director of the new prussian observatory at königsberg, where he pursued his astronomical labours for a period of upwards of thirty years. bessel directed his energies chiefly to the study of stellar astronomy, and made many observations in determining the number, the exact positions, and proper motions of the stars. he was remarkable for the precision with which he carried out his observations, and for the accuracy which characterised all his calculations. in bessel, by the exercise of his consummate skill, endeavoured to solve a problem which for many years baffled the efforts of the ablest astronomers, viz., the determination of the parallax of the fixed stars. this had been so frequently attempted, and without success, that the results of any new observations were received with incredulity before their value could be ascertained. bessel was ably assisted by joseph frauenhofer, an eminent optician of munich, who constructed a magnificent heliometer for the observatory at königsberg, and in its design introduced a principle which admirably adapted it for micrometrical measurement. the star selected by bessel is a binary known as cygni, the components being of magnitudes · and respectively. it has a large proper motion, which led him to conclude that its parallax must be considerable. this star will always be an object of interest to astronomers, as it was the first of the stellar multitude that revealed to bessel the secret of its distance. bessel commenced his observations in october , and continued them until march . during this time he made measurements, and, before arriving at a conclusive result, carefully considered every imaginable cause of error, and rigorously calculated any inaccuracies that might arise therefrom. finally, he determined the parallax of the star to be ´´· --a result equivalent to a distance about , times that of the earth from the sun. in - m. peters, of the pulkova observatory, arrived at an almost similar result, having obtained a parallax of ´´· ; but by more recent observations the parallax of the star has been increased to about half a second. about the same time that bessel was occupied with his observation of cygni, professor henderson, of edinburgh, when in charge of the observatory at the cape of good hope, directed his attention to alpha centauri, one of the brightest stars in the southern hemisphere. during - he made a series of observations of the star, with the object of ascertaining its mean declination; and, having been informed afterwards of its large proper motion, he resolved to make an endeavour to determine its parallax. this he accomplished after his return to scotland, having been appointed astronomer royal in that country. by an examination of the observations made by him at the cape, he determined the parallax of alpha centauri to be ´´· , but later astronomers have reduced it to ´´· . professor henderson's detection of the parallax of alpha centauri was communicated to the astronomical society two months after bessel announced his determination of the parallax of cygni. the parallax of cygni assigns to the star a distance of forty billions of miles from the earth, and that of alpha centauri--regarded as the nearest star to our system--a distance of twenty-five billions of miles. it is utterly beyond the capacity of the human mind to form any adequate conception of those vast distances, even when measured by the velocity with which the ether of space is thrilled into light. light, which travels twelve millions of miles in a minute, requires - / years to cross the abyss which intervenes between alpha centauri and the earth, and from cygni the period required for light to reach our globe is rather less than double that time. the parallax of more than a dozen other stars has been determined, and the light passage of a few of the best known is estimated as follows:--sirius, eight years; procyon, twelve; altair, sixteen; aldebaran, twenty-eight; capella, thirty; regulus, thirty-five; polaris, sixty-three; and vega, ninety-six years. it does not always follow that the brightest stars are those situated nearest to our system, though in a general way this may be regarded as correct. the diminishing magnitudes of the stars can be accounted for mainly by their increased distances, rather than by any difference in their intrinsic brilliancy. we should not err by inferring that the most minute stars are also the most remote; the telescope revealing thousands that are invisible to the naked eye. there are, however, exceptions to this general rule, and there are many stars of small magnitude less remote than those whose names have been enumerated, and whose light passage testifies to their profound distances and surpassing magnitude when compared with that of our sun. sirius, 'the leader of the heavenly host,' is distant fifty billions of miles. the orb shines with a brilliancy far surpassing that of the sun, and greatly exceeds him in mass and dimensions. arcturus, the bright star in boötes, whose golden yellow light renders it such a conspicuous object, is so far distant that its measurement gives no reliable parallax; and if we may infer from what little we know of the stars, arcturus is believed to be the most magnificent and massive orb entering into the structure of that portion of the sidereal system which comes within our cognisance. judging by its relative size and brightness, this star is ten thousand times more luminous, and may exceed the sun one million times in volume. deneb, in the constellation of the swan, though a first-magnitude star, possesses no perceptible proper motion or parallax--a circumstance indicative of amazing distance, and magnitude equalling, or surpassing, arcturus and sirius. canopus, in the constellation argo, in the southern hemisphere, the brightest star in the heavens with the exception of sirius, possesses no sensible parallax; consequently, its distance is unknown, though it has been estimated that its light passage cannot be less than sixty-five years. by establishing a mean value for the parallax of stars of different magnitudes, it was believed that an approximation of their distances could be obtained by calculating the time occupied in their light passage. the light period for stars of the first magnitude has been estimated at thirty-six and a half years; this applies to the brightest stars, which are also regarded as the nearest. at the distance indicated by this period, the sun would shrink to the dimensions of a seventh-magnitude star and become invisible to the naked eye; this of itself affords sufficient proof that the great luminary of our system cannot be regarded as one of the leading orbs of the firmament. stars of the second magnitude have a mean distance of fifty-eight light years, those of the third magnitude ninety-two years, and so on. m. peters estimated that light from stars of the sixth magnitude, which are just visible to the naked eye, requires a period of years to accomplish its journey hither; whilst light emitted from the smallest stars visible in large telescopes does not reach the earth until after the lapse of thousands of years from the time of leaving its source. the profound distances of the nearest stars by which we are surrounded lead us to consider the isolated position of the solar system in space. a pinnacle of rock, or forsaken raft floating in mid-ocean, is not more distant from the shore than is the sun from his nearest neighbours. the inconceivable dimensions of the abyss by which the orb and his attendants are surrounded in utter loneliness may be partially comprehended when it is known that light, which travels from the sun to the earth--a distance of ninety-three millions of miles--in eight minutes, requires a period of four and a third years to reach us from the nearest fixed star. a sphere having the sun at its centre and this nearest star at its circumference would have a diameter of upwards of fifty billions of miles; the volume of the orb when compared with the dimensions of this circular vacuity of space is as a small shot to a globe miles in diameter. it has been estimated by father secchi that, if a comet when at aphelion were to arrive at a point midway between the sun and the nearest fixed star, it would require one hundred million years in the accomplishment of its journey thither. and yet the sun is one of a group of stars which occupy a region of the heavens adjacent to the milky way and surrounded by that zone; nor is his isolation greater than that of those stars which are his companions, and who, notwithstanding their profound distance, influence his movements by their gravitational attraction, and in combination with the other stars of the firmament control his destiny. ancient astronomers, for the purpose of description, have mapped out the heavens into numerous irregular divisions called 'constellations.' they are of various forms and sizes, according to the configuration of the stars which occupy them, and have been named after different animals, mythological heroes, and other objects which they appear to resemble. in a few instances there does exist a similitude to the object after which a constellation is called; this is evident in the case of corona borealis (the northern crown), in which there can be seen a conspicuous arrangement of stars resembling a coronet, and in the constellations of the dolphin and scorpion, where the stars are so distributed that the forms of those creatures can be readily recognised. there is some slight resemblance to a bear in ursa major, and to a lion in leo, and no great effort of the mind is required to imagine a chair in cassiopeia, and a giant in orion; but in the majority of instances it is difficult to perceive any likeness of the object after which a constellation is named, and in many cases there is no resemblance whatever. the constellations are sixty-seven in number: excluding those of the zodiac, which have been already mentioned, the constellations of the northern hemisphere number twenty-nine. the most important of these are ursa major and minor, andromeda, cassiopeia, cepheus, cygnus, lyra, aquila, auriga, draco, boötes, hercules, pegasus, and corona borealis. to an observer of the nocturnal sky the stars appear to be very unequally distributed over the celestial sphere. in some regions they are few in number and of small magnitude, whilst in other parts of the heavens, and especially in the vicinity of the milky way, they are present in great numbers and form groups and aggregations of striking appearance and conspicuous brilliancy. on taking a casual glance at the midnight sky on a clear moonless night, one is struck with the apparent countless multitude of the stars; yet this impression of their vast number is deceptive, for not more than two thousand stars are usually visible at one time. much, however, depends upon the keenness of vision of the observer, and the transparency of the atmosphere. argelander counted at bonn more than , stars, and hozeau, near the equator, where all the stars of the sphere successively appear in view, enumerated , stars. this number may be regarded as including all the stars in the heavens that are visible to the naked eye. with the aid of an opera glass thousands of stars can be seen that are imperceptible to ordinary vision. argelander, with a small telescope of - / inches aperture, was able to count , stars in the northern hemisphere. large telescopes reveal multitudes of stars utterly beyond the power of enumeration, nor do they appear to diminish in number as depth after depth of space is penetrated by powerful instruments. the star-population of the heavens has been reckoned at , , , but this estimate is merely an assumption; recent discoveries made by means of stellar photography indicate that the stars exist in myriads. it is reasonable to believe that there is a limit to the sidereal universe, but it is impossible to assign its bounds or comprehend the apparently infinite extent of its dimensions. scintillation or twinkling of the stars is a property which distinguishes them from the planets. it is due to a disturbed condition of the atmosphere and is most apparent when a star is near the horizon; at the zenith it almost entirely vanishes. humboldt states that in the clear air of cumana, in south america, the stars do not twinkle after they reach an elevation of ° above the horizon. the presence of moisture in the atmosphere intensifies scintillation, and this is usually regarded as a prognostication of rain. white stars twinkle more than red ones. the occurrence of scintillation can be accounted for by the fact that the stars are visible as single points of light which twinkle as a whole, but in the case of the sun, moon, and planets, they form discs from which many points of light are emitted; they, therefore, do not scintillate as a whole, for the absence of rays of light from one portion of their surface is compensated by those from other parts of their discs, giving a mean average which creates a steadiness of vision. the stars are divided into separate classes called 'magnitudes,' by which their relative apparent size and degree of brightness are distinguished. the magnitude of a star does not indicate its mass or dimensions, but its light-giving power, which depends partly upon its size and distance, though mainly upon the intensity of its luminosity. the most conspicuous are termed stars of the first magnitude; there are ten of those in the northern hemisphere, and an equal number south of the equator, but they are not all of the same brilliancy. sirius outshines every other star of the firmament, and arcturus has no rival in the northern heavens. the names of the first-magnitude stars north of the equator are: arcturus, capella, vega, betelgeux, procyon, aldebaran, altair, pollux, regulus, and deneb. the next class in order of brightness are called second-magnitude stars; they are fifty or sixty in number, the most important of which is the pole star. the stars diminish in luminosity by successive gradations, and when they sink to the sixth magnitude reach the utmost limit at which they appear visible to the naked eye. in great telescopes this classification is carried so low as to include stars of the eighteenth and twentieth magnitudes. entering into the structure of the stellar universe we have single stars, double stars, triple, quadruple, and multiple stars, temporary, periodical, and variable stars, star-groups, star-clusters, galaxies, and nebulæ. single or insulated stars include all those orbs sufficiently isolated in space so as not to be perceptibly influenced by the attraction of other similar bodies. they are believed to constitute the centres of planetary systems, and fulfil the purpose for which they were created by dispensing light and heat to the worlds which circle around them. the sun is an example of this class of star, and constitutes the centre of the system to which the earth belongs. reasoning from analogy, it would be natural to conclude that there are other suns, numberless beyond conception, the centres of systems of revolving worlds, and although we are utterly unable to catch a glimpse of their planetary attendants, even with the aid of the most powerful telescopes, yet they have in a few instances been _felt_, and have afforded unmistakable indications of their existence. since the sun must be regarded as one of the stellar multitude that people the regions of space, and whose surpassing splendour when contrasted with that of other luminaries can be accounted for by his proximity to us, it would be of interest to ascertain his relative importance when compared with other celestial orbs which may be his peers or his superiors in magnitude and brilliancy. the sun is one of a widely scattered group of stars situated in the plane of the milky way and surrounded by that zone, and, as a star among the stars, would be included in the constellation of the centaur. although regarded as one of the leading orbs of the firmament, and of supreme importance to us, astronomers are undecided whether to classify the sun with stars of greater magnitude and brightness, or assign him a position among minor orbs of smaller size. much uncertainty exists with regard to star magnitudes. this arises from inability on the part of astronomers to ascertain the distances of the vast majority of stars visible to the naked eye, and also on account of inequality in their intrinsic brilliancy. among the stars there exists an indefinite range of stellar magnitudes. there are many stars known whose dimensions have been ascertained to greatly exceed those of the sun, and there are others of much smaller size. no approximation of the magnitude of telescopic stars can be arrived at; many of them may rival sirius, canopus, and arcturus, in size and splendour, their apparent minuteness being a consequence of their extreme remoteness. if the sun were removed a distance in space equal to that of many of the brightest stars, he would in appearance be reduced to a minute point of light or become altogether invisible; and there are other stars, situated at distances still more remote, of which sufficient is known to justify us in arriving at the conclusion that the sun must be ranked among the minor orbs of the firmament, and that many of the stars surpass him in brilliancy and magnitude. double stars.--to the unaided eye, these appear as single points of light; but, when observed with a telescope of sufficient magnifying power, their dual nature can be detected. the first double star discovered was mizar, the middle star of the three in ursa major which form the tail of the bear. the components are of the fourth and fifth magnitudes, of a brilliant white colour, and distant fourteen seconds of arc. in , cassini perceived stars which appeared as single points of light when viewed with the naked eye, but when observed with the telescope presented the appearance of being double. the astronomer bode, in , published a list of eighty double stars, and, in a few years after, sir william herschel discovered several hundreds more of those objects. they are now known to exist in thousands, mr. burnham, of the lick observatory, having, by his keen perception of vision, contributed more than any other observer to swell their number. all double stars are not binaries; many of them are known as 'optical doubles'--an impression created by two stars when almost in the same line of vision, and, though apparently near, are situated at a great distance apart and devoid of any physical relationship. binary stars consist of two suns which revolve round their common centre of gravity, and form real dual systems. the close proximity of the components of double stars impressed the minds of some astronomers with the belief that a physical bond of union existed between them. in the interval between and , bradley detected a change of ° in the position angle of the two stars forming castor, and was very nearly discovering their physical connection. in , the rev. john michell wrote: 'it is highly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together do really consist of stars placed near together and under the influence of some general law.' afterwards he says: 'it is not improbable that a few years may inform us that some of the great number of double and triple stars which have been observed by mr. herschel are systems of bodies revolving about each other.' christian mayer, a german astronomer, formed a list of stellar pairs, and announced, in , the supposed discovery of 'satellites' to many of the principal stars. his observations were, however, not exact enough to lead to any useful results, and the existence of his 'planet stars' was at that time derided, and believed to find a place only in his imagination. the conclusions arrived at by some astronomers with regard to double stars were afterwards confirmed by herschel, when, by his observation of a change in the relative positions of many of their components, he was able to announce that they form independent systems in mutual revolution, and are controlled by the law of gravitation. the number of binary stars in active revolution is known to exceed ; but, besides these, there are doubtless numerous other compound stars which, on account of their extreme remoteness and the close proximity of their components, are irresolvable into pairs by any optical appliances which we possess. the revolution of two suns in one sphere presents to our observation a scheme of creative design entirely different to the single-star system with which we are familiar--one of a higher and more complex order in the ascending scale of celestial architecture. for, if we assume that around each revolving sun there circles a retinue of planetary worlds, it is obvious that a much more complicated arrangement must exist among the orbs which enter into the formation of such a system than is found among those which gravitate round our sun. the common centre of gravity of a binary system is situated on a line between both stars, and distant from each in inverse proportion to their respective masses. when the stars are of equal mass their orbits are of equal dimensions, but when the mass of one star exceeds that of the other, the orbit of the larger star is proportionately diminished as compared with the circumference traversed by the smaller star. when their orbits are circular--a rare occurrence--both stars pursue each other in the same path, and invariably occupy it at diametrically opposite points; nor is it possible for one star to approach the other by the minutest interval of space in any duration of time, so long as the synchronous harmony of their revolution remains undisturbed. [illustration: fig. .--a binary star system-- ophiuchi (_drawn by mr. j. e. gore._)] when a pair of suns move in an ellipse, their orbits intersect and are of equal dimensions when the stars are of equal mass, their common centre of gravity being then at a point equidistant from each. consequently, neither star can approach or recede from this point without the other affecting a similar motion, they must be at periastron and apastron together, and any acceleration or retardation of speed must occur simultaneously with each. stars of unequal magnitude always maintain a proportionate distance from their common focus, and both simultaneously occupy corresponding parts of their orbits. the nature of the motions of those distant suns, and the form of the orbits which they traverse, have been investigated by several eminent astronomers, and although the subject is one of much difficulty, on account of their extreme remoteness and the minute angles which have to be dealt with, necessitating the carrying out of very refined observations, yet a considerable amount of information has been obtained with regard to the paths which they pursue in the accomplishment of their revolutions round each other. the orbits of about sixty stellar pairs have been computed, but only with partial success. some stars have shown themselves to be totally regardless of theory and computation, and have shot ahead far beyond the limits ascribed to them, whilst others, by the slowness of their motions, have upset the calculations of astronomers as much in the opposite direction. so that out of this number the orbits of not more than half a dozen are satisfactorily known. the dimensions of stellar orbits are of very varied extent. some pairs are apparently so close that the best optical means which we possess are incapable of dividing them, whilst others revolve in wide and spacious orbits. the most marked peculiarity of the orbits of binary stars is their high eccentricity; they are usually much more eccentric than are those of the planets, and in some instances approach in form that of a comet. the finest binary star in the northern heavens is castor, the brighter of the two leading stars in the constellation gemini. the components are of the second and third magnitudes, and over five seconds apart. they are of a brilliant white colour, and form a beautiful object in the telescope. in bradley determined the relative positions of those stars, and on comparing the results obtained by him with recent measurements it was found that they had altered to the extent of °. travelling at the same rate of speed, they will require a period of about years to complete an entire circuit of their orbits. this pace, however, has not been maintained, for, their periastron having occurred in , they travelled more rapidly in the last century than they are doing at present, and, as their orbits are so eccentric that when at apastron the stars are twice as remote from each other as at periastron, they will for the next three and a half centuries continue to slacken their pace, until they shall have reached the most remote points of their orbits, when they will again begin to approach with an increasing velocity; so that the time in which an entire revolution can be accomplished will not be much less than , years.[ ] as the distance of castor is unknown, it is impossible to compute the combined mass of its components. they are very remote, their light period being estimated at forty-four years. castor is doubtless a more massive orb than our sun, and possesses a higher degree of luminosity. alpha centauri, in the southern hemisphere, is the brightest binary, and also the nearest known star in the heavens; its estimated distance being twenty-five billions of miles. both components equal stars of the first magnitude, and are of a brilliant white colour. since they were first observed, in , they have completed two revolutions, and are now accomplishing a third. the eccentricity of their orbit approaches in form that of faye's comet, which travels round the sun; consequently the stars, when at apastron, are twice their periastron distance. their period of revolution is about eighty-eight years. the mean radius of their orbit corresponds to a span of , millions of miles, so that those orbs are sometimes as close to each other as jupiter is to the sun, and never so far distant as uranus.[ ] their combined mass is twice that of the sun, and the luminosity of each star is slightly greater. the double star cygni--one of the nearest to our system--is believed to be a binary the components of which move in an orbit of more spacious dimensions than that of any other known revolving pair. though they have been under continuous observation since , it is only within the last few years that any orbital motion has been perceived. some observers are disinclined to admit the accuracy of this statement; whilst others believe that the stars have executed a hyperbolic sweep round their common centre of gravity and are now separating. the radius of the orbit in which those bodies travel is sixty-five times the distance of the earth from the sun; which means that they travel in an orbit twice the width of that of the planet neptune. it has been estimated that they complete a revolution in about eight centuries. the united mass of the system is about one-half that of the sun, and in point of luminosity they are much inferior to that orb. the star ophiuchi (fig. ) may be regarded as typical of a binary system. the components are five seconds apart, and of the fourth and sixth magnitudes. their light period is stated to be twenty years, and the combined mass of the system is nearly three times that of the sun. the pair travel in an orbit from fourteen to forty-two times the radius of the earth's orbit; so that when at apastron they are three times as distant from each other as when at periastron. they complete a revolution in eighty-eight years. the accompanying diagram (fig. ) is a delineation of the beautiful orbits of the components of gamma virginis. these may be described as elongated ellipses. both stars being of equal mass, their orbits are of equal dimensions, and their common centre of gravity at a point equidistant from each. any approach to, or recession from this point, must occur simultaneously with each; they must always occupy corresponding parts of their orbits, and be in apastron and at periastron in the same period of time. the ellipse described by this pair is the most eccentric of known binary orbits, and approaches in form the path pursued by encke's comet round the sun. these orbs complete a revolution in years, and when in apastron are seventeen times more remote from each other than when at periastron. [illustration: fig. .--the orbits of the components of gamma virginis.] from his observation of the motion of sirius in , bessel was led to believe that the brilliant orb was accompanied by another body, whose gravitational attraction was responsible for the irregularities observed in the path of the great dog-star when pursuing his journey through space. the elements of this hypothetical body were afterwards computed by peters and auwers, and its exact position assigned by safford in . on january , , mr. alvan clarke, of cambridgeport, massachusetts, when engaged in testing a recently constructed telescope of great power, directed it on sirius, and was enabled by good fortune to discover the companion star at a distance of ten seconds from its primary. since its discovery, the star has pursued with such precision the theoretical path previously assigned to it that astronomers have had no hesitation in identifying it as the hypothetical body whose existence bessel had correctly surmised. [illustration: fig. .--apparent orbit of the companion of sirius. (_drawn by mr. burnham._)] the sirian satellite is a yellow star of the eighth magnitude, and shines with a feeble light when contrasted with the surpassing brilliancy of its neighbour. astronomers were for some time in doubt as to whether the uneven motion which characterised the path of sirius could be ascribed to the attraction of its obscure attendant, which presented such a marked contrast to its primary, and several observers were inclined to believe that the disturbing body still remained undiscovered. when, however, the density of the lesser star became known, it was discovered that, weight for weight, that of sirius exceeded it only in the proportion of two to one, though as a light-giver the great orb is believed to be , times more luminous. the sirian satellite revolves round its primary in about fifty years, and at a distance twenty-eight times that of the earth from the sun. the surpassing brilliancy of sirius as compared with that of the other stars of the firmament has rendered it at all times an object of interest to observers. the egyptians worshipped the star as sothis, and it was believed to be the abode of the soul of isis. the nations inhabiting the region of the nile commenced their year with the heliacal rising of sirius, and its appearance was regarded as a sure forerunner of the rising of the great river, the fertilising flood of which was attributed to the influence of this beautiful star. it is believed that the mazzaroth in job is an allusion to this brilliant orb. among the romans sirius was regarded as a star of evil omen; its appearance above the horizon after the summer solstice was believed to be associated with pestilence and fevers, consequent upon the oppressive heat of the season of the year. the _dies caniculares_, or dog-days, were reckoned to begin twenty days before, and to continue for twenty days after, the heliacal rising of sirius, the dog-star. during those days a peculiar influence was believed to exist which created diseases in men and madness among dogs. homer alludes to the star 'whose burning breath taints the red air with fevers, plagues and death.' sirius, which is in canis major (one of orion's hunting dogs), is a far more glorious orb than our sun. according to recent photometric measurements it emits seventy times the quantity of light, and is three times more massive than the great luminary of our system. at the distance of sirius (fifty billions of miles) the sun would shrink to the dimensions of a third-magnitude star, and the light of seventy such stars would be required to equal in appearance the brilliant radiance of the great dog-star. the orb, with his retinue of attendant worlds--some of which are reported as having been seen--is travelling through space with a velocity of not less than , miles a minute. an irregularity of motion resembling that of sirius has been detected with regard to procyon, the lesser dog-star. but in this case the companion star has not as yet been seen, though a careful search has been made for it with the most powerful of telescopes. should it be a planetary body, illumined by its primary, its reflected light would not appear visible to us, even if it were much less remote than it is. we are able only to perceive the effulgence of brilliant suns scattered throughout the regions of space; but besides those, there are doubtless many faintly luminous orbs and opaque bodies of vast dimensions occupying regions unknown to us, but by a knowledge of the existence of which an enlarged conception is conveyed to our minds of the greatness of the universe. the most rapid of known revolving pairs is delta equulei. the components are so close that only the finest instruments can separate them, and this they cannot do at all times. they accomplish a revolution in eleven and a half years. the slowest revolving pair is zeta aquarii. the motion of the components is so tardy that to complete a circuit of their orbits they require a period of about sixteen centuries. other binary stars have had different periods assigned to them; eleven pairs have been computed to revolve round each other in less than fifty years, and fifteen in less than but more than fifty. there are other compound stars whose motions appear to be much more leisurely than those just mentioned, and although no orbital movement has, so far, been detected among them, yet, so vast is the scale upon which the sidereal system is constructed, that thousands of years must elapse before they can have accomplished a revolution of their orbits. the pole star is an optical double, but the components are of very unequal magnitude. the pole star itself is of the second magnitude, but its companion is only of the ninth, and on account of its minuteness is regarded as a good test for telescopes of small aperture. mizar, in the constellation ursa major, is a beautiful double star. the components are wide apart, and can be easily observed with a small instrument. there is a remarkable star in the constellation of the lyre (epsilon lyræ), described as a double double. this object can just be distinguished by a person with keen eyesight as consisting of two stars; when observed with a telescope they appear widely separated, and each star is seen to have a companion, the entire system forming two binary pairs in active revolution. the pair which first cross the meridian complete a revolution in about , years; the second pair have a more rapid motion, and accomplish it in half that time. the two pairs are believed to be physically connected, and revolve round their common centre of gravity in a period of time not much under one million years. cor caroli, in canes venatici, is a pleasing double star, the components being of a pale white and lilac colour. albireo, in the constellation of the swan, is one of the loveliest of double stars. the larger component is of the third magnitude, and of a golden yellow colour; the smaller of the sixth magnitude, and of a sapphire blue. epsilon boötis, known also as mirac, and called by admiral smyth 'pulcherrima,' on account of its surpassing beauty, is a delicate object of charming appearance. the components of this lovely star are of the third and seventh magnitudes: the primary orange, the secondary sea-green. the late mr. r. a. proctor, in describing a binary star system, writes as follows: 'if we regard a pair of stars as forming a double sun, round which--or, rather, round the common centre of which--other orbs revolve as planets, we are struck by the difference between such a scheme and our own solar system; but we find the difference yet more surprising when we consider the possibility that in some such schemes each component sun may have its own distinct system of dependent worlds. in the former case the ordinary state of things would probably be such that both suns would be above the horizon at the same time, and then, probably, their distinctive peculiarities would only be recognisable when one chanced to pass over the disc of the other, as our moon passes over the sun's disc in eclipses. for short intervals of time, however, at rising or setting, one or other would be visible alone; and the phenomena of sunset and sunrise must therefore be very varied, and also exquisitely beautiful, in worlds circling round such double suns. but when each sun has a separate system, even more remarkable relations must be presented. for each system of dependent worlds, besides its own proper sun, must have another sun--less splendid, perhaps (because farther off), but still brighter beyond comparison than our moon at the full. and, according to the position of any planet of either system, there will result for the time being either an interchange of suns, instead of the change from night to day, or else double sunlight during the day, and a corresponding intensified contrast between night and day. where the two suns are very unequal or very differently coloured, or where the orbital path of each is very eccentric, so that they are sometimes close together and at others far apart, the varieties in the worlds circling round either, or around the common centre of both, must be yet more remarkable. "it must be confessed," we may well say with sir john herschel, "that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in."' anyone who takes a cursory glance at the heavens on a clear night can readily perceive that there exists considerable diversity of colour among the stars. the contrast between some is pronounced and well marked, whilst others exhibit refined gradations of hue. the most numerous class of stars are those which are described as white or colourless. they comprise about one-half of the stars visible to the naked eye. among the most conspicuous examples of this type are sirius--whose diamond blaze is sometimes mingled with an occasional flash of blue and red--altair, spica, castor, regulus, rigel, all the stars of ursa major with the exception of one, and vega--a glittering gem of pale sapphire, almost colourless. the light emitted by stars of this class gives a continuous spectrum, the predominating element being hydrogen, having a very elevated temperature and under relatively high pressure. the vapours of iron, sodium, magnesium, and other metals, are indicated as existing in small quantities. the second class of stars is that to which our sun belongs. they are of a yellow colour, and embrace two-thirds of the remaining stars. the most prominent examples of this type are arcturus, capella, aldebaran, procyon, and pollux. hydrogen does not predominate so much in these as in the sirian stars, and their spectra resemble closely the solar spectrum, indicating that they are composed of elements similar to those which exist in the sun. the star which bears the nearest resemblance to our sun, both as regards the colour of its light and physical structure, is capella, the most conspicuous star in the constellation auriga, and one of the leading brilliants in the northern hemisphere. its spectrum presents all the characteristics observed in the solar spectrum, and there exists an almost identical similarity in their physical constitution, though capella is a much more magnificent orb than the sun. the third class of stars includes those which are of a ruddy hue, such as betelgeux in the right shoulder of orion, antares in scorpio, and alpha herculis. their spectra present a banded or columnar appearance, and there is greater absorption, especially of the blue rays of light. it is believed that the temperature of stars of this colour is not so elevated as that of those belonging to the other two orders, and that this is a sufficient reason to account for the different appearance of their spectra. the aid of a good telescope is, however, necessary to enable us to perceive the varied colours and tints of the sparkling gems with which nature has adorned her star-built edifice of the universe. most of the precious stones on earth have their counterparts in the heavens, presenting in a jewelled form contrasts of colour, pleasing harmonies, and endless variety of shade. the diamond, sapphire, emerald, amethyst, topaz, and ruby sparkle among crowds of stars of more sombre hue. agate, chalcedony, onyx, opal, beryl, lapis-lazuli, and aquamarine are represented by the radiant sheen emanating from distant suns, displaying an inexhaustible variety of colour, blended in tints of untold harmony. it is among double stars that the richest and most varied colours predominate. there are pairs of white, yellow, orange, and red stars; yellow and blue, yellow and pale emerald, yellow and rose red, yellow and fawn, green and gold, azure and crimson, golden and azure, orange and emerald, orange and lilac, orange and purple, orange and green, white and blue, white and lilac, lilac and dark purple, &c., &c. there are companion stars revolving round their primaries, coloured olive, lilac, russet, fawn, dun, buff, grey, and other shades indistinguishable by any name. our knowledge of binary star systems brings us to what may be regarded as the threshold of the fabric of the heavens. for it is known that other systems exist into the construction of which numerous stars enter. these form intricate and complex stellar arrangements, in which the component stars are physically united and retained in their orbits by their mutual attraction. chapter vii the starry heavens triple, quadruple, and multiple stars.--these, when observed with the naked eye, appear as single stars, but, when examined with a high magnifying power, each lucid point can be resolved into several component stars. they vary in number from three to half a dozen or more, and form systems of a more complex character than what are observed in the case of binary stars. in the usual construction of a triple system, the secondary star of a binary is resolvable into two, each star being in mutual revolution, whilst they both gravitate round their primary. by another arrangement, a close pair control the movements of a distant attendant. one of the most interesting of triple stars is the tricoloured gamma andromedæ. the brilliant components of this system have their counterparts in the topaz, the emerald, and the sapphire--the larger star is of the third magnitude and of a golden yellow colour; the secondary of the fifth magnitude and of an emerald green. these stars are ten seconds apart, and, though they have been under observation since , no orbital movement has as yet been detected, but their common proper motion indicates their close relationship and physical connection. in , otto struve discovered that the companion star is itself double, and round it there gravitates a sapphire sun, which is believed to accomplish a revolution of its orbit in about years. if round those suns there should be circling planetary systems of worlds inhabited by intelligent beings, the varied effects produced by the light emanating from those different coloured orbs would be of a very beautiful and pleasing nature. a system suggestive of the endless variety of stellar arrangement that exists throughout the sidereal regions is apparent in the case of the triple star zeta cancri. two of the stars, of magnitudes six and seven, form a binary in rapid revolution, the components of which complete a circuit of their orbits in fifty-eight years, whilst the more distant third star, of almost similar magnitude, accomplishes a wide orbital ellipse round the other two in or years. these stars have been closely observed by astronomers during the past forty years, with the result that their motions have appeared most perplexing, and complicated beyond precedent. 'if this be really a ternary system,' wrote sir john herschel, 'connected by the mutual attraction of its parts, its perturbations will present one of the most intricate problems in physical astronomy.' the second star revolves round its primary, whilst the third pursues a retrograde course, but its path, instead of being even, presents the appearance of a series of circular loopings, in traversing which the star alternately quickens and slackens its pace, or at times appears to be stationary. astronomers have arrived at the conclusion that these perturbations are produced by the presence of a fourth member, which, though invisible, is probably the most massive of the system--perhaps a magnificent world teeming with animated beings, and attended by three suns which gravitate round it, dispensing light and heat to meet the requirements of the various forms of life which exist on its surface. in this system we have an arrangement the reverse of what exists in the solar system, where all the planets revolve round a predominant sun; but here there is a strange verification of the old ptolemaic belief with regard to the path of a sun, though in this instance there are three suns circling round a dark globe which they illumine and vivify. triple stars occur with comparative frequency throughout the heavens. in monoceros there is a fine triple star, discovered by herschel, which he describes as 'one of the most beautiful sights in the heavens.' the stars xi and beta scorpii form triple systems in which the components are differently arranged. in xi the primary and secondary consist of two revolving stars which control the movements of a distant attendant; in beta the primary and secondary stars are in mutual revolution, whilst round the former there circles a very close minute companion. there are doubtless many binary stars which, if examined with adequate telescopic power, would resolve themselves into triple and multiple systems, but the profound distances of those objects render the detection of their components a most difficult task. quadruple stars are usually arranged in pairs, _i.e._ the primary and secondary of a binary system are each resolvable into two, forming two pairs, each pair being in mutual revolution, while they both gravitate round their common centre of gravity. epsilon lyræ, which has been described as a double double, is an example of a quadruple system, and nu scorpii is of a similar construction, but more beautiful because its components are in closer proximity to each other. close upon twenty of those double double systems have been discovered in different parts of the heavens. one of the most interesting of quadruple systems is theta orionis, which is situated in the great nebula, by which it is surrounded. this star, when observed with a telescope of low power, can be at once resolved into four separate lucent points, so arranged as to form a quadrilateral figure or trapezium. they are of the fifth, sixth, seventh, and eighth magnitudes, and described as pale white, garnet, faint lilac, and red. though they have been under careful observation for upwards of two centuries, no perceptible motion has been perceived as occurring among them, nor has there been any change in their relative positions--they appear to be perfectly motionless; but we must not infer from this that no physical bond of union exists between them, for they are situated at an amazing distance from the earth. ascending higher in the scale of celestial architecture, we have multiple stars forming systems still more elaborate and complex, into the structure of which numerous stars enter, and they, as they increase in number, gradually merge into star-clusters. if we assume that around each of the components of a multiple star there circles a retinue of planetary worlds, we are confronted with a most perplexing problem as to how the dynamical stability of a system so different from, and so vastly more complicated than, that of our solar system is maintained--where, as it were, suns and planets intermingle--how numerous circling orbs can accomplish their revolutions without being swayed and deflected from their paths by the gravitational attraction of adjacent members of the same system. perplexing though the arrangement of such a scheme may be to our conception, yet, each orb has been weighed, poised, and adjusted by infinite wisdom, to perform its intricate motions in synchronous harmony with other members of the system--all moving in unison like the parts of a complicated piece of mechanism, and maintained in stable equilibrium by their mutual attraction-- mystical dance, which yonder starry sphere of planets and of fixed in all her wheels resembles nearest; mazes intricate, eccentric, intervolved, yet regular then most, when most irregular they seem; and in their motions harmony divine so smooths her charming tones that god's own ear listens delighted.--v. - . all the natural phenomena with which we are familiar would, in the case of planets revolving round the component suns of a multiple system, be of a different kind or altogether absent. instead of being illumined by one sun, those worlds would, at certain times, have several suns--some more distant than others--above their horizons, and upon very rare occasions, if ever, would there be an entire absence of all of those orbs from their skies. consequently there would be no year such as we are familiar with; no regular sequence of seasons similar to what is experienced on earth; no alternation of day and night, for there would be '_no night there_,' though, in the absence of the primary orb, the light emitted by distant suns, whilst sufficient to banish night, and beyond comparison brighter than the moon when at full, would, in the diminution of its intensity from that of noonday, be as grateful a change as that of from day to night which occurs on our globe. should those suns be differently coloured, each emitting its own peculiar shade of light as it appears above the horizon, the varied aspects of the perpetual day enjoyed by the inhabitants of those circling worlds present to the imagination harmonies of light and shade over which it is pleasant to linger. temporary, periodical, and variable stars.--it may seem remarkable that among so many thousands of stars which spangle the firmament, there should occur no very perceptible change or variation in their aspect and brilliancy. from age to age they present the same appearance, shine with the same undiminished splendour, and rise and set with the same regularity. so that from time immemorial the stars have been regarded by mankind as the embodiment of all that is eternal and unchangeable. yet, the serenity of the celestial regions does not always remain undisturbed--at occasional times a 'nova,' or new star, blazes forth unexpectedly in the heavens, and perplexes astronomers; and, after shining with a varying degree of brilliancy for a few weeks or months, gradually diminishes in size and brightness and eventually becomes lost to sight. a record has been kept of about twenty temporary stars that have been observed at various periods since the time that reliable data of those objects have been published. pliny mentions the appearance of a new star in the time of hipparchus ( b.c.); it was seen in the constellation of the scorpion, and it is said that it was the apparition of this star which induced the celebrated astronomer to construct what is known as the earliest star catalogue. a new star is said to have become visible when the emperor honorius ruled, and another during the reign of the emperor otho, about a.d. in may a new star appeared in aries, and in july another was observed in scorpio, which resembled saturn. the most remarkable star of this kind was one observed by tycho brahé, which appeared in the constellation cassiopeia. he first perceived it on november , . in lustre it equalled jupiter, and when at its brightest rivalled venus; it was visible at noonday, and at night its light could be perceived through strata of cloud which rendered all other stars invisible. the star maintained its brilliancy for three weeks, when it became of a yellowish colour and perceptibly decreased in size; it afterwards assumed a ruddy hue resembling aldebaran, and, diminishing gradually in magnitude and brightness, ceased to be visible in march . it twinkled more than the other stars, and during the time it could be perceived its position remained unchanged. in a conspicuous new star burst forth in ophiuchus. it surpassed in brilliancy stars of the first magnitude, and outshone the planet jupiter, which was in its proximity. kepler observed this star, and described it as 'sparkling like a diamond with prismatic tints.' it soon began to decline after its appearance; in march it had shrunk to the dimensions of a third-magnitude star, and in a year later it became entirely lost to view. other stars of the same class, though of a less conspicuous character, have been observed at occasional times. anthelme, a carthusian monk, discovered one near beta cygni in ; another appeared in ophiuchus in ; one in scorpio in ; one in corona borealis in ; in cygnus in ; in andromeda in ; and in auriga in . various theories have been advanced in order to account for the sudden outbursts of those stars, the light from which has probably occupied not much less than one hundred years in its passage hither. it has been suggested that the collision of two suns, or of two great masses of matter, would create such phenomena; but, apart from the improbability of such a catastrophe occurring among the celestial orbs, the rapid subsidence in the luminosity of the observed objects would indicate that the outburst was produced by causes of a more rapidly transitory nature than what would result from the collision of two condensed masses of matter. a collision occurring between two swarms of meteors has been suggested as one way of accounting for the sudden appearance of those stars; but another, and more plausible, explanation is that they are produced by a great eruption of glowing gas from the interior of a sun, causing an enormous increase in its luminosity, which subsides after a time, and is succeeded by a normal condition of things. it has been observed that all those temporary stars, with the exception of two, have appeared in the region of the milky way. in this luminous zone the condensation of small gaseous stars and nebulæ is more pronounced than in any other part of the heavens, and this would seem to indicate that there may be cosmical changes taking place among them which need not be associated with the occurrence of catastrophes resulting in the conflagration of worlds, and that nature, in accomplishing her purposes, does not overstep the uniform working of her laws, upon which depend the stability and existence of the universe. periodical and variable stars are distinguished from other similar objects by the fluctuations which occur in the quantity of light emitted by them. the difference in the luminosity of some stars is at times so marked that, in a few weeks or months, they decline from the first or second magnitudes to invisibility, and, after the expiration of a certain period, they again gradually regain their pristine condition. when these changes take place with regular recurrence, they are called 'periodical;' when they occur in a variable and uncertain manner, they are called 'irregular.' about stars are known as variable, but the majority of them are telescopic objects. their periodical changes of brilliancy present every degree of variety; in some stars they are scarcely perceptible and occur at long intervals; in others, changes of brightness occur in a few hours or days, by which the light emitted is intensified many hundreds of times. some stars accomplish their cycle of change in a few days, many in a few weeks or months, and there are others which do not complete their periods until the expiration of a number of years. one of the most remarkable of variable stars is called mira 'the wonderful,' in the constellation cetus. when at its maximum brilliancy it shines for two or three weeks as a star of the second magnitude. it then begins to gradually decline, and at the end of three months becomes invisible. it remains invisible for five months, and then reappears, and during the ensuing three months it regains by degrees its former brilliancy. mira completes a cycle of its changes in days, and, during that time, oscillates between a star of the second and tenth magnitude. the variability of mira ceti was first observed by david fabricius in the sixteenth century. another remarkable star is eta argus, which is surrounded by the great nebula in the constellation argo navis. it is invisible to the naked eye, but in the telescope it has a reddish appearance, and is slightly brighter than the stars in its vicinity. it was first observed by halley in , and it was then of the fourth magnitude. in it had risen to the second magnitude, and maintained its position as a star of this class until , when, on december of that year, its brilliancy suddenly increased, and it equalled in a short time alpha centauri. it reached its maximum in , and then it was surpassed only by sirius. it maintained its brilliancy for about ten years. in , it declined to the second magnitude, in to the third, and, gradually diminishing, it became invisible to the naked eye in . it is now of the seventh magnitude, and is again increasing, and may soon resume its position among the other stars. it is believed to have a period of seventy years, and in that time its light ebbs and flows between the seventh and first magnitudes. the most interesting variable star in the heavens is algol (the demon), in the constellation perseus. its light fluctuations can be observed without the aid of a telescope, and it completes a cycle of its changes in two or three days. for about two days and thirteen hours it is conspicuously visible as a star of the second magnitude; it then begins to decline, and in about four hours sinks to the dimensions of a fourth-magnitude star; it remains in this condition for twenty minutes, and then increases gradually until, at the expiration of four hours, it regains its former brilliancy, which it sustains for two days and thirteen hours, when it again goes through the same cycle of changes in a precisely similar manner to what has been described. astrologers have ascribed many evil influences to the demon star, which adorned the head of medusa; nor did it escape the observation of ancient astronomers that this malevolent orb is--as a modern writer amusingly remarks--slowly winking at us from out the depths of space. variable stars are found in greater numbers in some parts of the heavens than in others. those of a white colour, and with shorter and more regular periods, are most numerous in the region of the milky way; those that are small, with long periods and of a reddish hue, are more widely removed from that zone. stars of this class are all very remote, and no attempt has as yet been made to ascertain the parallax of algol. several theories have been suggested in order to account for the periodical brilliancy of those stars. it has been suggested that the stars have opaque non-luminous patches on their surfaces, and that during axial rotation their light ebbs and flows according as the dark or bright portions are turned towards us. this theory is highly improbable. another and more plausible reason, especially with regard to short period variables, is, that around those stars there revolve opaque bodies or satellites which at times intercept a portion of their light by producing a partial eclipse of their discs, similar to that caused by the dark body of the moon when passing between the sun and the earth. it is now known that in the case of variables of the algol type, the periodical fluctuations of their light arises from this cause, and that round algol there is a dark world or satellite travelling, which completes a revolution of its orbit in about sixty-nine hours, and that, during each circuit, it intercepts one half of the light of its primary by partially eclipsing the orb, and thereby creating a diminution in its apparent magnitude which becomes perceptible at recurring intervals. star groups.--these are plentifully scattered over the heavens and, by their conspicuous brilliancy, add to the grandeur and magnificence of the midnight sky. the hyades in taurus, of which aldebaran is the chief, forming the eye of the bull, attract attention. the stars in coma bernices form a rich group; the sickle in leo, the seven stars in ursa major, and those in cassiopeia and aquila are familiarly known to all observers. besides these, there are many other groups and aggregations of stars which adorn the celestial vault and enhance the beauty of the heavens. star clusters.--on observing the heavens on a clear, dark night, there can be seen in different parts of the sky closely aggregated groups of stars called clusters. in some instances the component stars are so near together that the naked eye is unable to discern the individual members of the cluster. they then assume an indistinct, hazy, cloudlike appearance. upwards of clusters are known to astronomers, the majority of which are very remote. many of them contain thousands of stars compressed into a very small space, and others are so distant that the largest telescopes are incapable of resolving their nebulous appearance into separate stars. star clusters have been arranged into two classes, 'irregular' and 'globular;' but no sharp line of demarcation exists between them, though each have their distinctive peculiarities. irregular clusters consist of aggregations of stars brought promiscuously together, and presenting an appearance devoid of any structural arrangement. they are of different shapes and sizes, possess no distinct outline, and are not condensed towards their centre, like those that are globular. on examination, they present an intricate reticulated appearance; streams and branches of stars extend outwards from the parent cluster, sometimes in rows and sinuous lines, and, in other instances, diverging from a common centre, forming sprays. sometimes the stars are seen to follow each other on the same curve which terminates in loops and arches of symmetrical proportions. there are three conspicuous clusters in the northern sky that are visible to the naked eye--viz. the pleiades in taurus, the great cluster in the sword-handle of perseus, and praesepe in cancer, commonly called the beehive. the cluster which from time immemorial has had bestowed upon it the chief attention of mankind are the beautiful pleiades or seven sisters, and intertwined among its stars are the legendary and mythological beliefs of ancient nations and untutored tribes inhabiting the different regions of the globe. when viewed with a telescope of moderate size the cluster appears as a scattered group, and numerous stars become visible that are imperceptible to ordinary vision. in the sword-handle of perseus there is a cluster which, to the naked eye, appears as a small patch of luminous cloud. this inconspicuous object when observed with an instrument of moderate power is resolved into a magnificent assemblage of stars, and presents a spectacle which creates in the mind of the beholder mingled feelings of admiration and amazement. no telescope has yet penetrated its utmost depths, or revealed all the glories of this shining region, crowded with glittering points of light comparable in number to the pebbles strewn on the shore of a troubled sea. the cluster praesepe in cancer is visible on a clear night to the unaided eye as a small nebula. this object attracted the attention of galileo, to which he applied his newly invented telescope, and was delighted to find that his glass was capable of resolving it into a group of stars thirty-six in number, and all of comparatively large magnitude. the disappearance of praesepe in consequence of the condensation of vapour in the atmosphere was regarded by the ancients as a sure indication of approaching rain. in the same constellation, near the crab's southern claw, there is another rich cluster, which consists of stars of the ninth and tenth magnitudes. in sobieski's shield there is a magnificent fan-shaped cluster of minute stars with a prominent one in its centre; and in the constellation of the southern cross there is a cluster which, on account of the varied colours of its component stars, has been compared by sir john herschel to 'a piece of rich fancy jewellery;' eight of the principal stars being coloured red, green, and blue. globular clusters.--these have been described by herschel as 'the most magnificent objects that can be seen in the heavens.' they are all very remote, of a rounded form, and when viewed with a telescope present the appearance of 'a ball of stars.' in some clusters the constituent stars are distinguishable as minute points of light; in others, more remote, they are of a coarse granular texture, and in those still more distant they resemble a 'heap of golden sand.' some clusters are situated at such a profound distance in space that it is impossible with the most powerful of telescopes to define their stellar structure; all that can be distinguished of these is a cloudy luminosity resembling in appearance an irresolvable nebula. globular clusters usually present a radiated appearance. rays, branches, and spiral-shaped streams of stars appear to flow from the circumference of some; and, in other instances, fantastic appendages of stars project outwards from the parent cluster. there doubtless exists much variety in the structural arrangement of these clusters, and an equal diversity in the magnitude and number of the stars which enter into their formation. the stars in some clusters may equal those of the first magnitude, and in others they may not exceed in dimensions the minor planets. in the telescope they vary in size from the eleventh to the fifteenth magnitude; the smaller stars occupy the centre of a cluster, whilst the larger ones are found near its circumference. globular clusters are more condensed towards their centre than those of irregular shape, and some have a nucleated appearance. this apparent condensation is not altogether owing to the depth of star strata as viewed from the circumference of the cluster, but there appears to exist an attractive force (probably gravitational) which draws the stars towards its centre, and if this 'clustering power' were not opposed by some other counteracting force, those bodies would coalesce into one mass. it may be 'that a centrifugal impulse predominates by which full-grown orbs are driven from the nursery of suns in which they were reared to seek their separate fortunes and enter on an independent career elsewhere.' it is not known how the dynamical equilibrium of a star cluster is maintained; and on account of its extreme distance no motion is perceptible among its component stars. the laws by which those stellar aggregations are produced and governed are wrapped in obscurity, and the nature of the motions of their stars, whether towards concentration or diffusion, cannot at present be ascertained. if those globular clusters could be observed sufficiently near, they would most probably expand into vast systems of suns occupying immense regions of space. the largest and most magnificent globular cluster in the heavens is omega centauri, in the southern hemisphere. to the naked eye it resembles a round, indistinct, cometary object, about equal to a star of the fourth magnitude; but when observed with a powerful telescope it appears as a globe of considerable dimensions composed of innumerable stars of the thirteenth and fifteenth magnitudes, all exceedingly minute and gathered into small knots and groups. a remarkable cluster in toucani is described by sir john herschel as 'most magnificent; very large; very bright, and very much compressed in the middle.' the interior mass consists of closely aggregated pale rose-coloured stars, surrounded by others of a pure white which embrace the remainder of the cluster. there is a fine globular cluster in sagittarius between the archer's head and the bow. it was observed by hevelius in . the central portion is very much compressed, and consists of excessively minute stars enclosed by others of larger size. in aquarius there is a magnificent ball of stars of a beautiful spherical form, which sir j. herschel compared to a heap of fine sand. numerous other clusters are profusely distributed over the heavens, occupying regions in the profound depths of space which can only be reached by the aid of most powerful instruments. the finest and most remarkable object of this class visible in the northern heavens is the great cluster which lies between eta and zeta herculis. it was discovered by halley in , who writes: 'this is but a little patch, but it shows itself to the naked eye when the sky is serene and the moon absent.' when observed with a powerful telescope its magnificence at once becomes apparent to the beholder. 'perhaps,' says dr. nichol, 'no one ever saw it for the first time through a telescope without uttering a shout of wonder.' at its circumference the stars are rather scattered, but towards the centre they appear so closely aggregated that their combined effulgence forms a perfect blaze of light. sir william herschel estimated that there are , stars in the cluster, each a magnificent world but unaccompanied by any planetary attendants. [illustration: cluster in hercules] as a result of more recent investigations this number has been considerably reduced, and it is now generally believed that about , stars enter into the formation of the cluster. as its distance from the earth is unknown, it follows that there must be some uncertainty attached to any conclusions that may be arrived at with regard to this superb object. miss agnes clerke estimates the number of the constituent stars at , , and in support of her conclusion this talented lady writes as follows: 'the apparent diameter of this object, including most of the "scattered stars in streaky masses and lines" which form a sort of "glory" round it, is ´; that of its truly spherical portion may be put at ´. now, a globe subtending an angle of ´ must have (because the sine of that angle is to radius nearly as to : ) a real diameter / of its distance from the eye, which, if we assume to be such as would correspond to a parallax of / of a second, we find that the cluster, outliers apart, measures , millions of miles across. light, in other words, occupies thirty-six days in traversing it, but sixty-five years in journeying thence hither. its components may be regarded, on an average, as of the twelfth magnitude; for, although the divergent stars rank much higher in the scale of brightness, the central ones, there is reason to believe, are notably fainter. the sum total of their light, if concentrated into one stellar point, would at any rate very little (if at all) exceed that of a third-magnitude star. and one star of the third is equivalent to just four thousand stars of the twelfth magnitude. hence we arrive at the conclusion that the stars in the hercules cluster number much more nearly four than fourteen thousand.' for what purpose do those thousands of clustering orbs shine? who can tell? night is unknown in the regions illumined by their brilliant radiance. this stupendous aggregation of suns testifies to the magnificence of the starry heavens, and to the omnipotence of the creator. galaxies.--these consist of vast aggregations of stars which form separate 'island universes' floating in the depths of space; they are believed to equal in magnitude and magnificence the milky way--the galaxy to which our system belongs. nebulÆ.--we now reach the last, and what are believed to be the most distant of the known contents of the heavens. they are all exceedingly remote, devoid of any perceptible motion, faintly luminous, and, with the exception of two of their number, invisible to the naked eye. halley was the first astronomer who paid any attention to those objects. in he enumerated six of them, but of this number only two can, in a strict sense, be regarded as nebulæ, the others since then have been resolved into magnificent star clusters. in , messier catalogued nebulæ, and the herschels--father and son--in their survey of the stellar regions, discovered , of those objects. there are now , known nebulæ in the heavens, but the majority of them are not of much interest to astronomers. prior to the invention of the spectroscope it was believed that all nebulæ were irresolvable star clusters, but the analysis of their light by this instrument indicated that their composition was not stellar but gaseous. their spectra consist of a few bright lines revealing the presence of hydrogen, nitrogen, and other gaseous elements. much that is mysterious and uncertain is associated with those objects which appear to lie far beyond the limits of our sidereal system. it is now generally believed that they exhibit the earliest stage in the formation of stars and planets--inchoate worlds in process of slow evolution, which will eventually condense into systems of suns, and planetary worlds. nebulæ present every variety of form. some are annular, elliptic, circular, and spiral; others are fan-shaped, cylindrical, and irregular, with tufted appendages, rays, and filaments. a fancied resemblance to different animated creatures has been observed in some. in taurus there is a nebula called the 'crab' on account of its likeness to the crustacean; another is called the 'owl nebula' from its resemblance to the face of that bird. the orion nebula suggests the opened jaws of a fish or sea monster, hence called the fish-mouth nebula. there is a horse-shoe nebula, a dumb-bell nebula, and many others of various shapes and forms. they are classified as follows: ( ) annular nebulæ, ( ) elliptic nebulæ, ( ) spiral nebulæ, ( ) planetary nebulæ, ( ) nebulous stars, ( ) large irregular nebulæ. annular nebulÆ.--these resemble in appearance an oval-shaped luminous ring; they are comparatively few in number, and not more than a dozen have been discovered in the whole heavens. the most remarkable object of this class is the ring nebula, which is situated between the stars beta and gamma lyræ. it is visible in a moderate-sized telescope as a well-defined, flat, oval ring; its central part is not quite dark but is occupied by a filmy haze of luminous matter which is prolonged inwards from the margin of the ring. when examined with a high power the edges of the ring have a fringed appearance, and numerous glittering stellar points become visible both within and without its circumference. this nebulous ring, though a small object in the telescope, is of enormous magnitude, and if it were not more distant than cygni, one of the nearest of the fixed stars, its diameter would not be less than , millions of miles, but it has been estimated by herschel that it is times more remote than sirius. how stupendous, then, must be its dimensions, and how bewildering to our conception is the profound immensity of space in which it is located! an annular nebula similar to that of lyra, but on a smaller scale, is found in cygnus, and within it there can be seen a conspicuous star. another exists in scorpio which contains two stars situated within the ring at diametrically opposite points to each other. elliptical nebulÆ.--the most interesting object of this class is the great nebula in andromeda, called 'the transcendentally beautiful queen of the nebulæ'--an appellation which it scarcely merits. this object, which is plainly visible to the naked eye, is of an oval shape, of a milky white colour, and is situated near the most northern star of the three which form the girdle of andromeda. it was known to the ancients, and ali sufi, a persian astronomer who flourished in the tenth century, alludes to it; but it did not attract much attention until the seventeenth century. simon marius was the first to observe this object with a telescope. this he did on december , ; he describes it as shining with a pale white light resembling in appearance the flame of a candle when seen through a semi-transparent piece of horn. when examined with a high magnifying power it is seen to occupy a largely extended area measuring ° in length and - / ° in breadth. its luminosity increases from the circumference to the centre, where there can be seen a small nucleus with an ill-defined boundary, which has the appearance of being granular, but its composition is not stellar. two dark channels running almost parallel to each other and to the axis of the nebula have been observed by bond; these, when prolonged, form into curves which terminate in two great rings. they are wide rifts which separate streams of nebulous matter, and are indicative that some formative processes may be going on within the nebula. astronomers have been baffled in their attempts to discover the nature of the andromeda nebula. though great telescopes have been able to render visible thousands of stars over and around it, yet the nebula itself is irresolvable and bears no trace of stellar formation; neither, according to dr. huggins, is its spectrum gaseous, a circumstance which deepens the mystery associated with this object. its distance is unknown, and its dimensions cannot be ascertained. other elliptical nebulæ are found in different regions of the heavens. in ursa major there is an oval nebula resembling that of andromeda, but on a much smaller scale. it possesses a nucleus, and on the photographic plate there can be detected the presence of spiral structure, indicating the existence of streams of nebulous matter. adjacent to this nebula is another of the same class with a double nucleus, and associated with it is a nebulous star. spiral nebulÆ.--the great reflector of earl rosse at parsonstown was the successful means by which nebulæ of this form were discovered. this powerful telescope was capable of defining with greater accuracy the structural formation of those objects than any other instrument in use. it was ascertained that spiral coils and convoluted whorls enter into the structure of most nebulæ, indicating a similarity in the process of change which may be going on in these vast accumulations of cosmical matter. the most interesting specimen of a spiral nebula is situated in canes venatici. it consists of spiral coils emanating from a centre with a nucleus and surrounded by a narrow luminous ring. in appearance it resembles the coiled mainspring of a watch. planetary nebulÆ.--these have been so named on account of the resemblance which they bear to the discs of planets. they are of uniform brightness, circular in shape, with sharply-defined edges, and are frequently of a bluish colour. they are more numerous than annular nebulæ; three-fourths of their number are in the southern hemisphere, and they are situated in or very near the milky way. those objects were first described by sir william herschel, who was rather perplexed as to what was their real nature and how he should classify them. he remarked that they could not be planets belonging to far-off suns, nor distant comets, nor distended stars. consequently, he concluded rightly that they were nebulæ. when observed with large telescopes, they lose their planetary aspect, and their sharpness of outline is less apparent; their discs become broken up into bright and dark portions, and in some, numerous minute stars have been observed, whilst others have well-defined nuclei. the most prominent nebula of this class is situated in the constellation ursa major, and is called the owl nebula, from its fancied resemblance to the face of that bird. sir john herschel describes it as 'a most extraordinary object, a large, uniform nebulous disc, quite round, very bright, not sharply defined, but yet very suddenly fading away to darkness.' when examined in with earl rosse's reflector, two bright stars were discovered in its interior; each was in the centre of a circular dark space surrounded by whorls of nebulous matter--hence the origin of its name. this nebula gives a bright line spectrum indicative of gaseous composition. it is believed to consist chiefly of hydrogen and other gases which form a globe of such stupendous magnitude that, if we surmise its distance from the earth to be sixty-five light years--an estimate much too low--'its diameter would exceed that of the orbit of neptune upwards of times.'[ ] within its compass the orbs of hundreds of solar systems as large as that of ours would be able to perform their revolutions, having spacious intervals existing between each system. another interesting planetary nebula is in the constellation of the dragon, near to the pole of the ecliptic; it is slightly oval, of a pale blue colour, and contains a star of the eleventh magnitude in its centre. it gives a gaseous spectrum. attempts have been made to determine its parallax, but without success, and during the eighty years it has been under observation it has remained apparently motionless. its light period, if estimated at years, would indicate the existence of a globe with a diameter equal to forty-four diameters of the orbit of the planet neptune.[ ] a nebula of this class was discovered by sir john herschel in the centaur. he described it as resembling uranus, but larger; its colour was of a beautiful rich blue, and its light equalled that of a star of the seventh magnitude. nebulous stars.--these stars are each surrounded by a luminous haze several minutes of arc in diameter and of a circular form. sir william herschel, by his observation of those objects, arrived at the conclusion 'that there exists in space a shining fluid of a nature totally unknown to us, and that the nebulosity about those stars was not of a starry nature.' thirteen stars of this type have been enumerated by him and many others have since been discovered. the 'glow' which surrounds them has been observed in a few instances to have vanished without leaving any trace of nebulosity behind, but the causes which have brought about such a result are entirely unknown. the nature of those stars is involved in considerable obscurity, and one class of nebula would seem to merge into the other; nebulous stars with faint aureolæ do not differ much from small nebulæ interspersed with stellar points. large irregular nebulÆ.--these are found in both hemispheres, and are remarkable on account of the varied appearances which they present, and the large extent of space which many of them occupy. in some, the nebulous matter of which they are composed can be seen like masses of tufted flocculi, sometimes piled up, and at other times promiscuously scattered, resembling in appearance the foam on the crested billows of a surging ocean rendered suddenly motionless, or cirro-cumuli floating in a tranquil sky. islands of light with intervening dark channels, promontories projecting into gulfs of deep shade, sprays of luminous matter, convoluted filaments, whorls, wreaths, and spiral streams all enter into the structural formation of a great nebula. the great nebula in argo, in the southern hemisphere, is one of the most remarkable objects of this class. it consists of bright irregular masses of luminous matter, streaks and branches, and occupies an area about equal to one square degree. at its eastern border is situated the variable star eta argus, which fluctuates between the first and seventh magnitudes in a period of about seventy years. a rich portion of the galaxy lies in front of the nebula, which creates an effect as if it were studded over with stars. sir john herschel, in describing this nebula, writes as follows:--'the whole is situated in a very rich and brilliant part of the milky way, so thickly strewed with stars that, in the area occupied by the nebula, not less than , have been actually counted. yet it is obvious that these have no connection whatever with the nebula, being, in fact, only a simple continuation over it of the general ground of the galaxy. the conclusion can hardly be avoided that, in looking at it, we see through and beyond the milky way, far out into space, through a starless region, disconnecting it altogether from our system. it is not easy for language to convey a full impression of the beauty and sublimity of the spectacle which this nebula offers as it enters the field of view of a telescope, fixed in right ascension, by the diurnal motion, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, and in a part of the heavens otherwise full of interest.' another large bright nebula (called doradus), also in the southern hemisphere, is composed of a series of loops with intricate windings forming a kind of open network against the background of the sky which it adorns. sir john herschel describes it as one of the most extraordinary objects in the heavens. the 'crab' nebula in taurus, the 'horse-shoe' nebula in sobieski's shield, and the 'dumb-bell' nebula in vulpecula are remarkable objects, but the assistance of a powerful telescope is required to bring out their distinctive features. the 'crab' nebula is partially resolvable into stars; the other two are believed to be gaseous. the largest and most remarkable of all the nebulæ is that known as the great nebula in orion, which was discovered and delineated by huygens in the middle of the seventeenth century. it is perceptible to the naked eye, and when viewed with a glass of low power can be seen as a circular luminous haze surrounding the multiple star theta orionis--one of the stars in the giant's sword, and which is of itself a remarkable object. the most conspicuous part of the nebula bears a slight resemblance to the wing of a bird; it consists of flocculent masses of nebulous matter possessing a faint greenish tinge. sir john herschel compared it to a surface studded over with flocks of wool, or to the breaking up of a mackerel sky when the clouds of which it consists begin to assume a cirrous appearance. its brightest portion is occupied by four conspicuous stars, which form a trapezium; around each there is a dark space free from nebulosity, a circumstance which would seem to indicate that the stars possess the power either of absorbing or of repelling the nebulous matter in their immediate vicinity. when observed with a powerful telescope, this nebula appears to be of vast dimensions, and, with its effluents, occupies an area of ° by - / °. irregular branching masses, streams, sprays, filaments, and curved spiral wreaths project outward from the parent mass, and become gradually lost in the surrounding space. this object remained for long a profound mystery; no telescope was capable of resolving it, nor was it known what this 'unformed fiery mist, the chaotic material of future suns,' was, until the spectroscope revealed that it consists of a stupendous mass of incandescent gases--nitrogen, hydrogen, and other elementary substances, occupying a region of space believed by some to equal in extent the whole stellar system to which our sun belongs. in the southern hemisphere, near to the pole of the equator, are two nebulous clouds of unequal size; the larger having an area about four times that of the smaller. they are known as the magellanic clouds, having been called after the navigator magellan. both are visible on a moonless night, but in bright moonlight the smaller disappears. sir john herschel, when at the cape of good hope, examined those objects with his powerful telescope. he described them 'as consisting of swarms of stars, globular clusters, and nebulæ of various kinds, some portions of them being quite irresolvable, and presenting the same milky appearance in the telescope that the nebulæ themselves do to the naked eye.' these are believed to be other universes of stars sunk in the profound depths of space, our knowledge of their existence being dependent upon the faint nebulous light which left them, perhaps, several thousand years ago. [illustration: great nebula in orion] the description of the various kinds of nebulæ leads us to consider what is called the nebular hypothesis. that the stars and solar system had at some time in the past a beginning, is as much a matter of certainty as that they will at some future time cease to be. stars, like organic beings, have their birth, grow and arrive at maturity, then decline into a state of decrepitude, and finally die out. the duration of the life of a star, which may be reckoned by millions of years, depends upon the length of time during which it can maintain a temperature that renders it capable of emitting light. by the constant radiation of its heat into space, a condition of its constituent particles consequent upon the gradual contraction of its mass will ultimately occur, which will result in the exhaustion of its stores of thermal energy, the extinction of its light, and the reduction of what was once a brilliant orb to the condition of a mass of cold, opaque, inert matter. inquiries as to the origin of the stars have led scientific men to conclude that they have been evolved from gaseous nebulæ, and these have therefore been regarded as indicating the earliest stage in the formation of suns and planets. it is believed that the condensation of those attenuated masses of luminous matter into stars is capable of accounting for the generation and formation of all the shining orbs which enter into the structure of the starry heavens. in the evolution of a 'cosmos out of a chaos' we should expect to find stars presenting every stage of development--some in an embryo state and others more advanced; stars in full vigour and activity, stars that have passed the meridian of life, and stars in a condition of decay and on the verge of extinction. the observations of astronomers have led them to conclude that this condition of 'youth and age' exists among the stellar multitude; but the characteristics by which it is distinguished are neither very obvious nor reliable. the nebular theory is incapable of proof or demonstration; but modern discoveries tend to support the accuracy of its conclusions, and its principles have now been adopted by the majority of philosophic thinkers. the physical changes which are going on in the nebulæ towards stellar evolution, or in fully formed stars towards dissolution, are so slow that the life of an individual, or even the historical records of the past, are incapable of furnishing any evidence of alteration in their condition. a period of time infinitely greater than what has elapsed since the birth of science must pass before anything can be known of the life history of the stars; indeed, the allotted span of man's existence on this planet may have terminated ere the evolution of a large nebula into a star cluster can have taken place. the nebular hypothesis was first propounded by kant, who suggested that the sun and planets originated from a vast and diffused mass of cosmical matter. this theory was afterwards supported by herschel and by the great french astronomer laplace. as a result of close and continued observation of the different classes of nebulæ, herschel arrived at the conclusion that there exists in space a widely diffused 'shining fluid,' of a nature totally unknown to us, and that the nebulosity which he perceived to surround some stars was not of a starry nature. he further adds that this self-luminous matter 'seemed more fit to produce a star by its condensation than to depend on the star for its existence.' his sagacious conclusion with regard to the non-stellar nature of this nebulous matter was afterwards confirmed by the spectroscope; for at that time it was believed that even the faintest nebulæ were irresolvable star clusters. in herschel read a paper before the royal society in which he propounded his famous nebular hypothesis, and stated his reasons for believing that nebulæ, by their gradual condensation, were transformed into stars. having assumed that there exists a highly attenuated self-luminous substance diffused over vast regions of space, he endeavoured to show that by the law of attraction its particles would have a tendency to coalesce and form aggregations of nebulous matter, and that each of these, by the continued action of the same force, would gradually condense and ultimately acquire the consistence of a star. in the case of large irregular nebulæ, numerous centres of attraction would originate in the mass, round which the nebulous particles of matter would arrange themselves; each nucleus, when condensation had been completed, would become a star, and the entire nebula would in this manner be transformed into a cluster of stars. herschel believed that he could trace the different stages of nebular condensation which result in the evolution of a star. in large, faintly luminous nebulæ the process of condensation had only commenced; in others that were smaller and brighter it was in a more advanced stage; in those that contained nuclei there was evidence of nascent stars; and, finally, there could be seen in some nebulæ minute stellar points--new-born suns--interspersed among the haze of the transforming mass. by this theory herschel was able to account for the phenomena associated with nebulous stars and the supposed changes which were observed in some nebulæ. the nebular hypothesis as described by herschel was not received with much favour, nor did it unsettle much the belief that all nebulæ were vast stellar aggregations, and that their cloudy luminosity was a consequence of the inadequacy of telescopic power to resolve them into their component stars. laplace, who was highly gifted as a geometrician, demonstrated how the solar system could have been evolved in accordance with dynamical principles from a slowly rotating and slowly contracting spheroidal nebula. the rotatory motion of a nebula, in obedience to a well-known mechanical law, increases as its density becomes greater, and this goes on until the tangential force at the equator overcomes the gravitational attraction at its centre. when this occurs, a revolving ring of nebulous matter is thrown off from the parent mass, and by this means equilibrium is restored between the two forces. as the rotatory velocity of the nebula continues to increase with its contraction, another ring is cast off, and in this manner a succession of revolving rings may be detached from the condensing spheroid; each newly-formed ring being nearer to the centre of the contracting mass and revolving in a shorter period than its predecessor. in the evolution of our system, the central mass of the nebula became the sun and each of the revolving rings, by their condensation into one mass, formed a planet. in a similar manner, though on a diminished scale, the elementary planets, whilst in a nebulous state, parted with annular portions of their substance, out of which were evolved their systems of satellites. this theory furnished a plausible reason, which was capable of explaining how the orbs which constitute the solar system came into existence, and, though hypothetical, yet the manner in which it accounted for the orderly and symmetrical genesis of the system rendered it attractive and fascinating to scientific minds. the evidence in support of the nebulous origin of the solar system, if not conclusive, is of much weight and importance. the remarkable harmony with which the orbs of the system perform their motions is strongly indicative of their common origin and that their evolution occurred in subordination to the law of universal gravitation. the following are the characteristic points in favour of this theory:-- . all the planets revolve round the sun in the same direction, and they all occupy nearly the same plane. . their satellites, with the exception of those of uranus and neptune, perform their revolutions in obedience to the same law. . the rotation on their axes of the sun, planets, and satellites is in the same direction as their orbital motion. between the orbits of mars and jupiter there revolves a remarkable group of small planets or planetoids. on account of the absence of a planet in this region, where, according to the laws of planetary distances, one ought to be found, the existence of those small bodies was suspected for some years prior to their discovery. the first was detected by piazzi at palermo in ; two others were discovered by olbers in and , and one by harding in . for some time it was believed that no more planetoids existed, but in a fifth was detected by hencke, and from that year until now upwards of of those small bodies have been discovered. their magnitudes are of varied extent; the diameter of the largest is believed not to exceed miles, and that of the smaller ones from twenty to thirty miles. it was surmised at one time, when only a few of those bodies were known, that they were the fragments of a planet which met with some terrible catastrophe; but since the discovery of so many other planetoids this theory cannot be maintained. according to the nebular hypothesis, these bodies are the consolidated portions of a nebulous ring which remained separate instead of having coalesced into one mass so as to form a planet. the uniform condensation of the ring would result in the formation of a multitude of small planets similar to what are found between the orbits of mars and jupiter. in saturn's ring we have a remarkable instance of annular consolidation in which the form of the ring has been preserved. the ring is believed to consist of myriads of minute bodies, each of which travels in an orbit of its own as it pursues its path round the planet; the close approximation and exceeding minuteness of those moving objects create the appearance of a solid continuous ring. though, by means of the nebular hypothesis, it is impossible to explain all the phenomena associated with the motions of the orbs which enter into the structure of the solar system, yet this does not detract much from the merits of the theory, the fundamental principles of which are based upon the evolution of the solar system from a rotating nebula. the retrograde motions of the satellites of uranus and neptune, the velocity of the inner martian moon, and other abnormalities in the system, have not as yet been explained, but doubtless there are reasons by which those peculiarities can be accounted for if they were only known, '_felix qui potuit cognoscere causas omnium rerum_.' no attempt has been made to supplant the nebular hypothesis by any other theory of cosmical evolution. modern investigations and discoveries have strengthened its position, and at present it is the only means by which we can account for the existence of the visible material universe by which we are surrounded. in the days when milton lived--three hundred years ago--the nocturnal heavens presented the same appearance to an observer as they do at the present time. the stars pursued their identical paths, and looked down upon the earth with the same aspect of serene tranquillity, regardless of the vicissitudes which affect the inhabitants of this terrestrial sphere. the constellations that adorn the celestial vault duly appeared in their seasons, and in the ascending scale of heaven the stars that usher evening rose.--iv. - . the winter glories of orion, the scintillating brilliancy of sirius, and the spangled firmament, bearing no impress of change or variation which would lead one to conclude that the heavens were other than eternal, attracted then, as now, the admiration of beholders. apart from the orbs which constitute the solar system, little was known of the sidereal heavens beyond the visual effect created by the nocturnal aspect of the star-lit sky. though ancient philosophers hazarded an opinion that the stars were suns, they received but scant attention from early astronomers, by whom they were merely regarded as convenient fixed points which enabled them to determine with greater accuracy the positions of the planets and the paths traced out by them in the heavens. the ptolemaists, who believed in the diurnal revolution of the spheres, assigned to the stars a very subordinate place in their cosmology, which was the one adopted by milton; and although copernicus relegated them to their proper location in space, yet he had no clear conception of a universe of stars. tycho brahé, who declined to accept the copernican theory, disbelieved that the stars were suns, and galileo, who discovered the stellar nature of the milky way, remarked that the stars were not illumined by the sun's rays in the same manner that the planets are, but expressed no opinion with regard to their physical constitution. it is only within the past fifty years that proof has been obtained of the real nature of the stars. by the spectroscopic analysis of their light it has been ascertained that the elements of matter which enter into their composition exist in a condition similar to what is found in the sun. the stars are therefore suns, many of them surpassing in magnitude and brilliancy the great luminary of our system. though milton makes frequent allusion to the magnificence of the starry heavens, we have no evidence that he regarded the stars as suns, nor does he refer to them as such in any part of his poem.[ ] what impressed him most was their number and brilliancy, to which reference is made in the following passages: about him all the sanctities of heaven stood thick as stars.--iii. - . and sowed with stars the heavens thick as a field.--vii. . amongst innumerable stars, that shone stars distant, but nigh hand seemed other worlds.--iii. - . her reign with thousand lesser lights dividual holds, with thousand thousand stars, that then appeared spangling the hemisphere.--vii. - . milton describes the number of the fallen angels as an host innumerable as the stars of night.--v. - , and the attention of satan is directed by the archangel uriel to the multitude of stars formed from the chaotic elements of matter: numberless as thou seest, and how they move; each had his place appointed, each his course; the rest in circuit walls this universe.--iii. - . though milton was doubtless familiar with the leading orbs of the firmament and knew their names, and the constellations in which they are situated, yet he makes no direct allusion to any of them in his poem. neither arcturus, which is mentioned in the book of job, nor sirius, which attracted the attention of homer, who compared the brightness of achilles' armour to the dazzling brilliancy of the dog-star, finds a place in 'paradise lost.' and yet the superior magnitude and brilliancy of some stars when compared with those of others did not escape milton's observation when, in describing the lofty eminence of satan in heaven, prior to his fall, he represents him as brighter once amidst the host of angels than that star the stars among.--vii. - . there is but one star to which milton makes individual allusion, and, though not of any conspicuous brilliancy, yet it is one of much importance to astronomers-- the fleecy star that bears andromeda far off atlantic seas beyond the horizon.--iii. - . this is alpha arietis, the first point in the constellation of that name, which signifies the ram, and from which the right ascensions of the stars are measured on the celestial sphere. in the time of hipparchus the ecliptic intersected the celestial equator in aries, which indicated the commencement of the astronomical year and the occurrence of the vernal equinox; but, owing to precession, this point is now ° westward of aries and in the constellation pisces. the star was called hamal by the arabs, signifying a sheep, and the animal is represented as looking backwards. manilius writes:-- first aries, glorious in his golden wool, looks back and wonders at the mighty bull. aries is associated with the legend of the golden fleece, in quest of which jason and his valiant crew sailed in the ship 'argo.' in the autumn, andromeda is situated above aries, and would seem to be borne by the latter, which accounts for milton's description of the relative positions of those two constellations. milton alludes to the starry sphere in several passages in his poem, and also mentions the starry pole above which he soared in imagination up to the empyrean or heaven of heavens. his contemplation of the galaxy must have impressed his mind with the magnitude and extent of the sidereal universe, for he was aware that this luminous zone which encircles the heavens consists of myriads of stars, so remote as to be incapable of definition by unaided vision. milton's description of this vast assemblage of stars is worthy of its magnificence, and the purpose with which he poetically associates this glorified highway testifies to the sublimity of his thoughts and to the originality of his genius. in those parts of his poem in which he describes the glories of the celestial regions, and instances the beautiful phenomena associated with the individual orbs of the firmament, we are able to perceive with what exquisite delight he beheld them all. the invention of the telescope, and the important discoveries made by kepler, galileo, and newton in the seventeenth century, were the means of effecting a rapid advance in the science of astronomy; but that branch of it known as sidereal astronomy was not then in existence. the star depths, owing to inadequate telescopic power, remained unexplored, and the secrets associated with those distant regions were inviolable, and lay beyond the reach of human knowledge. the physical constitution of the stars was unknown, nor was it ascertained with any degree of certainty that they were suns. the knowledge possessed by astronomers in those days was but meagre compared with what is now known of the sidereal heavens. milton's astronomical knowledge, we find, was commensurate with what was known of the stellar universe, and this he has conspicuously displayed in his poem. chapter viii description of celestial objects mentioned in 'paradise lost' the sun the surpassing splendour of the sun, as compared with that of any of the other orbs of the firmament, is not more impressive than his stupendous magnitude, and the important functions which it is his prerogative to fulfil. situated at the centre of our system--of which he may be regarded as 'both eye and soul'--the orb has a diameter approaching , , miles, and a mass times greater than that of all the planets combined. these, by his attractive power, he retains in their several paths and orbits, and even far distant neptune acknowledges his potent sway. with prodigal liberality he dispenses his vast stores of light and heat, which illumine and vivify the worlds circling around him, and upon the constant supply of which all animated beings depend for their existence. deprived of the light of the sun, this world would be enveloped in perpetual darkness, and we should all miserably perish. the sun is distant from the earth about , , miles. his diameter is , miles, or nearly four times the extent of the radius of the moon's orbit. the mass of the orb exceeds that of the earth , times, and in volume , , times. the sun is a sphere, and rotates on his axis from west to east in days hours. the velocity of a point at the solar equator is , miles an hour. the density of the sun is only one-fourth that of the earth, or, in other words, bulk for bulk, the earth is four times heavier than the sun. the force of gravity at the sun's surface is twenty-seven times greater than it is on the earth; it would therefore be impossible for beings constituted as we are to exist on the solar surface. the dazzling luminous envelope which indicates to the naked eye the boundary of the solar disc is called the photosphere. it is most brilliant at the centre of the sun, and diminishes in brightness towards the circumference, where its luminosity is but one-fourth that of the central portion of the disc. the photosphere consists of gaseous vapours or clouds, of irregular form and size, separated by less brilliant interstices, and glowing white with the heat derived from the interior of the sun. in the telescope the photosphere is not of uniform brilliancy, but presents a mottled or granular appearance, an effect created by the intermixture of spaces of unequal brightness. small nodules of intense brilliance, resembling 'rice-grains,' but which, according to nasmyth, are of a willow-leaf shape with pointed extremities, which form a network over portions of the photosphere, are sprinkled profusely over a more faintly luminous background. these 'grains' consist of irregular rounded masses, having an area of several hundred miles. by the application of a high magnifying power they can be resolved into 'granules'--minute luminous dots which constitute one-fifth of the sun's surface and emit three-fourths of the light. this granulation is not uniform over the surface of the photosphere; in some parts it is indistinct, and appears to be replaced by interlacing filamentous bands, which are most apparent in the penumbræ of the spots and around the spots themselves. the 'granules' are the tops of ascending masses of intensely luminous vapour; the comparatively dark 'pores' consist of similar descending masses, which, having radiated their energy, are returning to be again heated underneath the surface of the photosphere. in certain regions of the photosphere several dark patches are usually visible, which are called 'sun-spots.' at occasional times they are almost entirely absent from the solar disc. it has been observed that they occupy a zone extending from ° to ° north and south of the solar equator, but are not found in the equatorial and polar regions of the sun. a sun-spot is usually described as consisting of an irregular dark central portion, called the _umbra_; surrounding it is an edging or fringe less dark, consisting of filaments radiating inwards called the _penumbra_. within the umbra there is sometimes seen a still darker spot, called the _nucleus_. the umbra is generally uniformly dark, but at times filmy luminous clouds have been observed floating over it. the nucleus is believed to be the orifice of a tubular depression in the floor of the umbra, prolonged downwards to an unknown depth. the penumbra is brightest at its inner edge, where the filaments present a marked contrast when compared with the dark cavity of the umbra which they surround and overhang. sometimes lengthened processes unite with those of the opposite side and form bands and 'bridges' across the umbra. the darkest portion of the penumbra is its external edge, which stands out conspicuously against the adjoining bright surface of the sun. one penumbra will sometimes enclose several umbræ whilst the nuclei may be entirely wanting. [illustration: fig. .--a sun-spot magnified. (_janssen._)] sun-spots usually appear in groups; large isolated spots are of rare occurrence, and are generally accompanied by several smaller ones of less perfect formation. the exact moment of the origin of a sun-spot cannot be ascertained, because it arises from an imperceptible point; it grows very rapidly, and often attains its full size in a day. prior to its appearance there is an unusual disturbance of the solar surface over the site of the spot: luminous ridges, called _faculæ_, and dark 'pores' become conspicuous, between which greyish patches appear, that seem to lie underneath a thin layer of the photosphere; this is rapidly dispelled and a fully formed spot comes into view. when a sun-spot has completed its period of existence, the photospheric matter overwhelms the penumbra, and rushes into the umbra, which it obliterates, causing the spot to disappear. the duration of sun-spots is subject to considerable variation; some last for weeks or months, and others for a few days or hours. a spot when once fully formed maintains its shape, which is usually rounded, until the period of its breaking up. spots of long duration rotate with the sun. those which become visible at the edge of the sun's limb have been observed to travel across his disc in less than a fortnight, disappearing at the margin of the opposite limb; afterwards, if sufficiently long-lived, they have reappeared in twelve or thirteen days on the surface of the orb where first observed. it was by observation of the spots that the period of the axial rotation of the sun became known. sun-spots vary very much in size--some are only a few hundred miles in width, whilst others have a diameter of , or , miles or upwards. in some instances the umbra alone has a breadth of , or , miles--three times the extent of the diameter of the earth. spots of this size are visible to the naked eye when the sun is partially obscured by fog, or when his brilliancy is diminished by vapours near the horizon. a year seldom passes without the occurrence of several of such spots being recorded. the largest sun-spot ever observed had a diameter of about , miles. a group of spots, including their penumbræ, will occupy an area of many millions of square miles. by long observation it has been ascertained that sun-spots increase and diminish in number with periodical regularity, and that a maximum sun-spot period occurs at the end of each eleven years. when spots are numerous on the sun's disc there is great disturbance of the solar surface, accompanied by fierce rushes of intensely heated gases. this solar activity is known to influence terrestrial magnetism by causing a marked oscillation of the magnetic needle, and giving rise to so-called 'magnetic storms,' accompanied by magnificent displays of auroræ, with variations in electrical earth-currents. it would therefore appear that sun-spots have a pronounced effect upon magnetic terrestrial phenomena, but how this is produced remains unknown. besides sun-spots, there are seen on the solar disc bright flocculent streaks or ridges of luminous matter called _faculæ_; they are found over the whole surface of the sun, but are most numerous near the limb and in the immediate vicinity of the spots. they have been compared to immense waves--vast upheavals of photospheric matter, indicative of enormous pressure, and often extending in length for many thousands of miles. nearly all observers have arrived at the conclusion that sun-spots are depressions or cavities in the photosphere, but considerable difference of opinion exists as to how they are formed. the most commonly accepted theory is that they are caused by the pressure of descending masses of vapour having a reduced temperature, which absorb the light and prevent it reaching us. our knowledge of the sun is insufficient to admit of any accurate conclusion on this point; though we are able to perceive that the surface of the orb is in a state of violent agitation and perpetual change, yet his great distance and intense luminosity prevent our capability of perceiving the ultimate minuter details which go to form the _texture_ of the solar surface. 'bearing in mind that a second of arc on the sun represents miles, it follows that an object miles in diameter is about the _minimum visible_ even as a mere mathematical point, and that anything that is sufficiently large to give the slightest impression of shape and extension of surface must have an area of at least a quarter of a million square miles; ordinarily speaking, we shall not gather much information about any object that covers less than a million.'[ ] since the british islands have only an area of , square miles, it is evident that on the surface of the sun there are many phenomena and physical changes occurring which escape our observation. though the changes which occur in the spots and faculæ appear to be slow when observed through the telescope, yet in reality they are not so. tremendous storms and cyclones of intensely heated gases, which may be compared to the flames arising from a great furnace, sweep over different areas of the sun with a velocity of hundreds of miles an hour. vast ridges and crests of incandescent vapour are upheaved by the action of internal heat, which exceeds in intensity the temperature at which the most refractory of terrestrial substances can be volatilised; and downrushes of the same photospheric matter take place after it has parted with some of its stores of thermal energy. sun-spots of considerable magnitude have been observed to grow rapidly and then disappear in a very short period of time; occasionally a spot is seen to divide into two or more portions, the fragments flying asunder with a velocity of not less than , miles an hour. it is by these upheavals and convulsions of the solar atmosphere that the light and heat are maintained which illumine and vivify the worlds that gravitate round the sun. during total eclipses of the sun, several phenomena become visible which have enabled astronomers to gain some further knowledge of the nature of the solar appendages. the most important of these is the chromosphere, which consists of layers of incandescent gases that envelop the photosphere and completely surround the sun. its average depth is from , to , miles, and when seen during an eclipse is of a beautiful rose colour, resembling a sheet of flame. as seen in profile at the edge of the sun's disc, it presents an irregular serrated appearance, an effect created by the protuberance of luminous ridges and processes--masses of flame which arise from over its entire surface. the chromosphere consists chiefly of glowing hydrogen, and an element called _helium_, which has been recently discovered in a terrestrial substance called cleveite; there are also present the vapours of iron, calcium, cerium, titanium, barium, and magnesium. from the surface of this ocean of fire, jets and pointed spires of flaming hydrogen shoot up with amazing velocity, and attain an altitude of ten, twenty, fifty, and even one hundred thousand miles in a very short period of time. they are, however, of an evanescent nature, change rapidly in form and appearance, and often in the course of an hour or two die down so as not to be recognisable. these _prominences_, as they are called, have been divided into two classes. some are in masses that float like clouds in the atmosphere, which they resemble in form and appearance; they are usually attached to the chromosphere by a single stem, or by slender columns; occasionally they are entirely free. these are called _quiescent_ prominences; they consist of clouds of hydrogen, and are of more lasting duration than the other variety, called _eruptive_ or metallic prominences. the latter are usually found in the vicinity of sun-spots, and, besides hydrogen, contain the vapours of various metals. they are of different forms, and present the appearance of filaments, spikes, and jets of liquid fire; others are pyramidal, convoluted, and parabolic. these outbursts, bending over like the jets from a fountain, and descending in graceful curves of flame, ascend from the surface of the chromosphere with a velocity often exceeding miles in a second, and frequently reach an enormous height, but are of transient duration. they are closely connected with sun-spots, and are evidence of the tremendous forces that are in action on the surface of the sun. the corona is an aureole of light which is seen to surround the sun during a total eclipse. it is an impressive and beautiful phenomenon, and is only visible when the sun is concealed behind the dark body of the moon. professor young gives the following graphic description of the corona: 'from behind it [the moon] stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre. the portion nearest the sun is of dazzling brightness, but still less brilliant than the prominences, which blaze through it like carbuncles. generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance and is far more irregular in form. usually there are several "rifts," as they have been called, like narrow beams of darkness, extending from the very edge of the sun to the outer night, and much resembling the cloud shadows which radiate from the sun before a thundershower. but the edges of these rifts are frequently curved, showing them to be something else than real shadows. sometimes there are narrow bright streamers as long as the rifts, or longer. these are often inclined, or occasionally even nearly tangential to the solar surface, and frequently are curved. on the whole, the corona is usually less extensive and brilliant over the solar poles, and there is a recognisable tendency to accumulation above the middle latitudes, or spot zones; so that, speaking roughly, the corona shows a disposition to assume the form of a quadrilateral or four-rayed star, though in almost every individual case this form is greatly modified by abnormal streamers at some point or other.' the corona surrounds the sun and its other envelopes to a depth of many thousands of miles. it consists of various elements which exist in a condition of extreme tenuity; hydrogen, helium, and a substance called coronium appear to predominate, whilst finely divided shining particles of matter and electrical discharges resembling those of an aurora assist in its illumination. [illustration: fig. .--the corona during the eclipse of may .] we possess no knowledge of the physical structure of the interior of the sun, nor have we any terrestrial analogy to guide us as to how matter would behave when subjected to such conditions of extreme temperature and pressure as exist in the interior of the orb. yet we are justified in concluding that the sun is mainly a gaseous sphere which is slowly contracting, and that the energy expended in this process is being transformed into heat so extreme as to render the orb a great fountain of light. milton in his poem makes more frequent allusion to the sun than to any of the other orbs of the firmament, and, in all his references to the great luminary, describes him in a manner worthy of his unrivalled splendour, and of his supreme importance in the system which he upholds and governs. after having alighted on mount niphates, satan is described as looking sometimes towards heaven and the full-blazing sun, which now sat high in his meridian tower.--iv. - . he then addresses him thus:-- o thou that with surpassing glory crowned, look'st from thy sole dominion like the god of this new world--at whose sight all the stars hide their diminished heads--to thee i call, but with no friendly voice, and add thy name, o sun, to tell thee how i hate thy beams, that bring to my remembrance from what state i fell, how glorious once above thy sphere.--iv. - . on another occasion:-- the golden sun in splendour likest heaven allured his eye.--iii. - . in describing the different periods of the day, milton seldom fails to associate the sun with these times, and rightly so, since they are brought about by the apparent diurnal journey of the orb across the heavens. commencing with morning, he says:-- meanwhile, to re-salute the world with sacred light, leucothea waked, and with fresh dews embalmed the earth.--xi. - . soon as they forth were come to open sight of day-spring, and the sun--who, scarce up-risen, with wheels yet hovering o'er the ocean-brim, shot parallel to the earth his dewy ray, discovering in wide landskip all the east of paradise and eden's happy plains.--v. - or some renowned metropolis with glistering spires and pinnacles adorned, which now the rising sun gilds with his beams.--iii. - . while now the mounted sun shot down direct his fervid rays, to warm earth's inmost womb.--v. - . for scarce the sun hath finished half his journey, and scarce begins his other half in the great zone of heaven.--v. - . to sit and taste, till this meridian heat be over, and the sun more cool decline.--v. - . and the great light of day yet wants to run much of his race, though steep. suspense in heaven, held by thy voice, thy potent voice he hears, and longer will delay, to hear thee tell his generation, and the rising birth of nature from the unapparent deep.--vii. - . the declining day and approach of evening are described as follows:-- meanwhile in utmost longitude, where heaven with earth and ocean meets, the setting sun slowly descended, and with right aspect against the eastern gate of paradise levelled his evening rays.--iv. - . the sun now fallen beneath the azores; whether the prime orb, incredible how swift, had thither rolled diurnal, or this less volubil earth, by shorter flight to the east, had left him there arraying with reflected purple and gold the clouds that on his western throne attend.--iv. - . the parting sun beyond the earth's green cape and verdant isles hesperian sets, my signal to depart.--viii. - . now was the sun in western cadence low from noon, and gentle airs due at their hour to fan the earth now waked, and usher in the evening cool.--x. - . for the sun, declined, was hasting now with prone career to the ocean isles, and in the ascending scale of heaven the stars that usher evening rose.--iv. - . in the combat between michael and satan, which ended in the overthrow of the rebel angels, milton, in his description of their armour, says:-- two broad suns their shields blazed opposite.--vi. - , and in describing the faded splendour of the ruined archangel, the poet compares him to the sun when seen under conditions which temporarily deprive him of his dazzling brilliancy and glory:-- as when the sun new-risen looks through the horizontal misty air shorn of his beams, or, from behind the moon in dim eclipse, disastrous twilight sheds on half the nations, and with fear of change perplexes monarchs.--i. - . this passage affords us an example of the sublimity of milton's imagination and of his skill in adapting the grandest phenomena in nature to the illustration of his subject. the moon the moon is the earth's satellite, and next to the sun is the most important of the celestial orbs so far as its relations with our globe are concerned. besides affording us light by night, the moon is the principal cause of the ebb and flow of the tide--a phenomenon of much importance to navigators. the moon is almost a perfect sphere, and is , miles in diameter. the form of its orbit is that of an ellipse with the earth in the lower focus. it revolves round its primary in days hours, at a mean distance of , miles, and with a velocity of , miles an hour. its equatorial velocity of rotation is miles an hour. the density of the moon is · that of water, or · that of the earth; eighty globes, each of the weight of the moon, would be required to counterbalance the weight of the earth, and fifty globes of a similar size to equal it in dimensions. the orb rotates on its axis in the same period of time in which it accomplishes a revolution of its orbit; consequently the same illumined surface of the moon is always directed towards the earth. to the naked eye the moon appears as large as the sun, and it very rapidly changes its form and position in the sky. its motions, which are of a very complex character, have been for many ages the subject of investigation by mathematicians and astronomers, but their difficulties may now be regarded as having been finally overcome. the phases of the moon are always interesting and very beautiful. the orb is first seen in the west, after sunset, as a delicate slender crescent of pale light; each night it increases in size, whilst it travels eastward, until it attains the figure of a half moon; still growing larger as it pursues its course, it finally becomes a full resplendent globe, rising about the time that the sun sets and situated directly opposite to him. then, in a reverse manner, after full moon, it goes through the same phases, until, as a slender crescent, it becomes invisible in the solar rays; afterwards to re-appear in a few days, and, in its monthly round, to undergo the same cycle of changes. the phases of the moon depend upon the changing position of the orb with regard to the sun. the moon shines by reflected light derived from the sun, and as one half of its surface is always illumined and the other half totally dark, the crescent increases or diminishes when, by the moon's change of position, we see more or less of the bright side. visible at first as a slender crescent near the setting sun, the angular distance from the orb and the width of the crescent increase daily, until, at the expiration of seven days, the moon is distant one quarter of the circumference of the heavens from the sun. the moon is then a semi-circle, or in quadrature. at the end of other seven days, the distance of the moon from the sun is at its greatest--half the circumference of its orbit. it is then visible as a circular disc and we behold the orb as full moon. the waning moon, as it gradually decreases, presents the same aspects reversed, and, finally, its slender crescent disappears in the sun's rays. the convex edge of the crescent is always turned towards the sun. the rising of the moon in the east and its setting in the west is an effect due to the diurnal rotation of the earth on her axis, but the orb can be perceived to have two motions besides: one from west to east, which carries it round the heavens in · days, and another from north to south. the west to east motion is steady and continuous, but, owing to the sun's attractive force, the moon is made to swerve from its path, giving rise to irregularities of its motion called perturbations. the most important of these is the _annual equation_, discovered by tycho brahé--a yearly effect produced by the sun's disturbing influence as the earth approaches or recedes from him in her orbit; another irregularity, called the _evection_, is a change in the eccentricity of the lunar orbit, by which the mean longitude of the moon is increased or diminished. _elliptic inequality_, _parallactic inequality_, the _variation_, and _secular acceleration_, are other perturbations of the lunar motion, which depend directly or indirectly on the attractive influence of the sun and the motion of the earth in her orbit. as the plane of the moon's orbit is inclined at an angle of rather more than ° to the ecliptic, it follows that the orb, in its journey round the earth, intersects this great circle at two points called the 'nodes.' when crossing the ecliptic from south to north the moon is in its ascending node, and when crossing from north to south in its descending node. in december the moon reaches the most northern point of its course, and in june the southernmost. consequently we have during the winter nights the greatest amount of moonlight, and in summer the least. in the evenings the moonlight is least in march and greatest in september, when we have what is called the harvest moon. the telescopic appearance of the moon is very interesting and beautiful, especially if the orb is observed when waxing and waning. as no aqueous vapour or cloud obscures the lunar surface, all its details can be perceived with great clearness and distinctness. indeed, the topography of the moon is better known than that of the earth, for the whole of its surface has been mapped and delineated with great accuracy and precision. the moon is in no sense a duplicate of its primary, and no analogy exists between the earth and her satellite. evidence is wanting of the existence of an atmosphere surrounding the moon; no clouds or exhalations can be perceived, and no water is believed to exist on the lunar surface. consequently there are no oceans, seas, rivers, or lakes; no fertile plains or forest-clad mountains, such as are found upon the earth. indeed, all the conditions essential for the support and maintenance of organic life by which we are surrounded appear to be nonexistent on the moon. our satellite has no seasons; its axial rotation is so slow that one lunar day is equal in length to fourteen of our days; this period of sunshine is succeeded by a night of similar duration. the alternation of such lengthened days and nights subjects the lunar surface to great extremes of heat and cold. when viewed with a telescope, the surface of the moon is perceived to consist of lofty mountain chains with rugged peaks, numerous extinct volcanoes called crater mountains, hills, clefts, chasms, valleys, and level plains--a region of desolation, presenting to our gaze the shattered and upturned fragments of the moon's crust, convulsed by forces of a volcanic nature which have long since expended their energies and died out. the mountain ranges on the moon resemble those of the earth, but they have a more rugged outline, and their peaks are more precipitous, some of them rising to a height of , feet. they are called the lunar alps, apennines, and cordilleras, and embrace every variety of hill, cliff, mound, and ridge of comparatively low elevation. the plains are large level areas, which are situated on various parts of the lunar surface; they are of a darker hue than the mountainous regions by which they are surrounded, and were at one time believed to be seas. they are analogous to the prairies, steppes, and deserts of the earth. _valleys._--some of these are of spacious dimensions; others are narrow, and contract into gorges and chasms. clefts or rills are long cracks or fissures of considerable depth, which extend sometimes for hundreds of miles across the various strata of which the moon's crust is composed. the characteristic features of the moon's surface are the crater mountains: they are very numerous on certain portions of the lunar disc, and give the moon the freckled appearance which it presents in the telescope, and which galileo likened to the eyes in the feathers of a peacock's tail. they are believed to be of volcanic origin, and have been classified as follows: 'walled plains, mountain rings, ring plains, crater plains, craters, craterlets, and crater cones.' upwards of , of these mountains have been enumerated, and , are known to have a diameter exceeding nine miles. walled plains consist of circular areas which have a width varying from miles to a few hundred yards. they are enclosed by rocky ramparts, whilst the centre is occupied by an elevated peak. the depth of these formations, which are often far below the level of the moon's surface, ranges from , to , feet. mountain rings, ring plains, and crater plains resemble those already described, but are on a smaller scale; the floors of the larger ones are frequently occupied by craters and craterlets. the latter exist in large numbers, and some portions of the moon's surface appear honeycombed with them, the smaller craters resting on the sides of larger ones and occupying the bottoms of the more extensive areas. there is no kind of formation on the earth's surface that can be compared with these crater mountains, which indicate that the moon was at one time a fiery globe convulsed by internal forces which found an outlet in the numerous volcanoes scattered over her surface. the most remarkable of these volcanic mountains have been named after distinguished men. ( ) copernicus is one of the most imposing; its crater is miles in diameter, and situated at its centre is a mountain with six peaks , feet in height. the ring by which it is surrounded rises , feet above the floor of the crater, and consists of terraces believed to have been created by the partial congelation and periodic subsidence of a lake of molten lava which occupied the enclosed area. ( ) tycho is one of the most magnificent and perfect of lunar volcanoes, and is also remarkable as being a centre from which, when the moon is full, there radiates a number of bright streaks which extend across the lunar surface, over mountain and valley, through ring and crater, for many hundreds of miles. their nature is unknown, and nothing resembling them is found on the earth. tycho has a diameter of miles and a depth of , feet. the peak which rises from the floor of the crater attains a height of , feet, and the rampart consists of a series of terraces which give variety to the appearance of the inner wall. the surface of the moon round tycho is honeycombed with small volcanoes. ( ) clavius is one of the most extensive of the walled plains; it has a diameter of miles and an area of , square miles. the rocky annulus which surrounds it is very lofty and precipitous, and at one point reaches a height of , feet. upwards of craters have been counted within this space, one of the peaks attaining to an elevation of , feet above the level floor of the plain. it is believed that the lowest depths of this wild and precipitous region are never penetrated by sunlight, they are so overshadowed by towering crag and fell which intercept the solar rays; and, as there is no atmosphere to cause reflection, they are consequently enveloped in perpetual darkness. ( ) plato has a diameter of about miles and an area of , square miles; its central peak rises to a height of , feet. it has an irregular rampart which is broken up into terraces averaging about , feet high; three cones, each with an elevation of from , to , feet, rest on its western border. ( ) theophilus is the deepest of the visible craters on the moon. it has a diameter of miles, and the inner edge of the ring rises from the level floor to a height ranging from , to , feet. a group of mountains occupies the centre of the area, the highest peak of which reaches an elevation of , feet. cyrillus and catharina, two adjacent craters, are each about , feet deep and connected by a wide valley. ( ) aristarchus is the brightest spot on the moon, and appears almost dazzling in the telescope. the crater has a diameter of miles, the centre of which is occupied by a steep mountain. the rampart on the western side rises to a height of , feet, on the east it becomes a plateau which connects it with a smaller crater called herodotus. bright streaks radiate from aristarchus when there is full moon, and extend for a considerable distance over the surface of the orb. though the face of the moon has been carefully scanned for two centuries and a half, and selenographers have mapped and delineated her features with the utmost accuracy and precision, yet no perceptible change of a reliable character has been perceived to occur on any part of the orb. the surface of the hemisphere directed towards the earth appears to be an alternation of desert plains, craggy wildernesses, and extinct volcanoes--a region of desolation unoccupied by any living thing, and 'upon which the light of life has never dawned.' owing to the absence of an atmosphere, there is neither diffuse daylight nor twilight on the moon. every portion of the lunar surface not exposed to the sun's rays is shrouded in darkness, and black shadows can be observed fringing prominences of silvery whiteness. if the moon were enveloped in an atmosphere similar to that which surrounds the earth, the reflection and diffusion of light among the minute particles of watery vapour which permeate it would give rise to a gradual transition from light to darkness; the lunar surface would be visible when not illumined by the direct rays of the sun, and before sunrise and after sunset, dawn and twilight would occur as upon the earth. but upon the moon there is no dawn, and the darkness of night envelops the orb until the appearance of the edge of the sun's disc above the horizon, then his dazzling rays illumine the summits and loftiest peaks of the lunar mountains whilst yet their sides and bases are wrapped in deep gloom. since the pace of the sun across the lunar heavens is times slower than it is with us, there is continuous sunshine on the moon for hours, and this long day--equal to about a fortnight of our time--is succeeded by a night of similar duration. as there is no atmosphere overhead to diffuse or reflect the light, the sun shines in a pitch-black sky, and at lunar noonday the planets and constellations can be seen displaying a brilliancy of greater intensity than can be perceived on earth during the darkest night. every portion of the moon's surface is bleak, bare, and untouched by any softening influences. no gentle gale ever sweeps down her valleys or disturbs the dead calm that hangs over this world; no cloud ever tempers the fierce glare of the sun that pours down his unmitigated rays from a sky of inky blackness; no refreshing shower ever falls upon her arid mountains and plains; no sound ever breaks the profound stillness that reigns over this realm of solitude and desolation. [illustration: a portion of the moon's surface] as might be expected, milton makes frequent allusion to the moon in 'paradise lost,' and does not fail to set forth the distinctive charms associated with the unrivalled queen of the firmament. the majority of poets would most likely regard a description of evening as incomplete without an allusion to the moon. milton has adhered to this sentiment, as may be perceived in the following lines:-- till the moon, rising in clouded majesty, at length apparent queen, unveiled her peerless light, and o'er the dark her silver mantle threw.--iv. - . now reigns full-orbed the moon, and with more pleasing light, shadowy sets off the face of things.--v. - . the association of the moon with the nocturnal revels and dances of elves and fairies is felicitously expressed in the following passage:-- or faëry elves, whose midnight revels, by a forest side or fountain, some belated peasant sees, or dreams he sees, while overhead the moon sits arbitress, and nearer to the earth wheels her pale course.--i. - . in contrast with this, we have milton's description of the moon when affected by the demoniacal practices of the 'night-hag' who was believed to destroy infants for the sake of drinking their blood, and applying their mangled limbs to the purposes of incantation. the legend is of scandinavian origin and the locality lapland:-- nor uglier follow the night-hag, when called in secret, riding through the air she comes, lured with the smell of infant blood, to dance with lapland witches, while the labouring moon eclipses at their charms.--ii. - . in his description of the massive shield carried by satan, the poet compares it with the full moon:-- his ponderous shield ethereal temper, massy, large, and round, behind him cast. the broad circumference hung on his shoulders like the moon.--i. - . the phases displayed by the moon in her monthly journey round the earth, and which lend a variety of charm to the appearances presented by the orb, are poetically described by milton in the following lines:-- but there the neighbouring moon (so call that opposite fair star) her aid timely interposes, and her monthly round still ending, still renewing, through mid-heaven with borrowed light her countenance triform hence fills and empties, to enlighten the earth, and in her pale dominion checks the night.--iii. - . it is interesting to observe how aptly milton describes the subdued illumination of the moon's reflected light, as compared with the brilliant radiance of the blazing sun, and how the distinguishing glory peculiar to each orb is appropriately set forth in the various passages in which they are described; their contrasted splendour enhancing rather than detracting from the grandeur and beauty belonging to each. the planet earth[ ] no lovelier planet circles round the sun than the planet earth, with her oceans and continents, her mountains, valleys, rivers, lakes, and plains; surrounded by heaven's azure, radiant with the sunlight of her day and adorned by night with countless sparkling points of gold. this beautiful world, the abode of man, is of paramount importance to us, and is the only part of the universe of which we have any direct knowledge. the earth may be regarded as one of the sun's numerous family, and is situated third in order from the refulgent orb, round which it revolves in an elliptical orbit at a mean distance of , , miles. the earth is nearest to the sun at the end of december, and furthest away at the beginning of july; the difference between those distances is , , miles--the extent of the eccentricity of the planet's orbit. the figure of the earth is that of an oblate spheroid; it is slightly flattened at the poles and bulges at the equator. its polar or shortest diameter is , miles, its equatorial diameter is , miles--greater than the other by miles. the circumference of the earth at the equator is , miles, and the total area of its surface is , , square miles. its mean density is - / times greater than that of water. the two principal motions performed by the earth are: ( ) rotation on its axis; ( ) its annual revolution round the sun. the earth always rotates in the same manner, and in the same direction, from west to east. as the axis of rotation corresponds with the shortest diameter of the planet, it affords strong evidence that the earth assumed its present shape whilst rapidly rotating round its axis when in a fluid or plastic condition. this would accord with the nebular hypothesis. the ends of the earth's axis are called the poles of the earth; one is the north, the other the south pole. the north pole is directed towards a star in the lesser bear called the pole star. the south pole is directed to a corresponding opposite part of the heavens. the earth's axis is inclined ° ´ to the plane of the ecliptic, and is always directed to the same point in the heavens. the earth accomplishes a revolution on its axis in hours minutes seconds mean solar time, which is the length of the sidereal day. this rate of rotation is invariable. at the equator, where the circumference of the globe exceeds , miles, the velocity of a point on its surface is upwards of , miles an hour, but, as the poles are approached, the tangential velocity diminishes, and at those points it is entirely absent. the earth accomplishes a revolution of her orbit in days hours minutes; in her journey round the sun she travels a circuit of , , miles at an average pace of , miles an hour. the earth has other slight motions called _perturbations_, which are produced by the gravitational attraction of other members of the solar system. the most important of these is precession of the equinoxes, which is caused by the attraction of the sun, moon, and planets, on the protuberant equatorial region of the globe. this attraction has a tendency to turn the earth's axis at right angles to her orbit, but it only results in the slow rotation of the pole of the equator round that of the ecliptic, which is occurring at the rate of ° in years, and will require a period of , years to complete an entire revolution of the heavens. the spot on earth round which is centred the chief interest in milton's poem is paradise, which was situated in the east of eden, a district of central asia. it was here where god ordained that man should first dwell--a place created for his enjoyment and delight. satan, after his soliloquy on mount niphates, directs his way to paradise, and arrives first in eden, where he beholds from a distance the happy garden-- so on he fares, and to the border comes of eden, where delicious paradise, now nearer, crowns with her enclosure green, as with a rural mound, the champain head of a steep wilderness, whose hairy sides with thicket overgrown, grotesque and wild, access denied; and overhead upgrew insuperable highth of loftiest shade, cedar, and pine, and fir, and branching palm, a sylvan scene, and, as the ranks ascend, shade above shade, a woody theatre of stateliest view. yet higher than their tops the verdurous wall of paradise up-sprung; which to our general sire gave prospect large into his nether empire neighbouring round. and higher than that wall, a circling row of goodliest trees, loaden with fairest fruit, blossoms and fruits at once of golden hue, appeared, with gay enamelled colours mixed; on which the sun more glad impressed his beams than in fair evening cloud, or humid bow, when god hath showered the earth: so lovely seemed that landskip. and of pure now purer air meets his approach, and to the heart inspires vernal delight and joy, able to drive all sadness but despair. now gentle gales, fanning their odoriferous wings, dispense native perfumes, and whisper whence they stole those balmy spoils.--iv. - . satan, having gained admission to the garden by overleaping the tangled thicket of shrubs and bushes which formed an impenetrable barrier and prevented any access to the enclosure within, he flew up on to the tree of life-- beneath him, with new wonder, now he views, to all delight of human sense exposed, in narrow room nature's whole wealth; yea, more!-- a heaven on earth: for blissful paradise of god the garden was, by him in the east of eden planted, eden stretched her line from auran eastward to the royal towers of great seleucia, built by grecian kings, or where the sons of eden long before dwelt in telassar. in this pleasant soil his far more pleasant garden god ordained. out of the fertile ground he caused to grow all trees of noblest kind for sight, smell, taste; and all amid them stood the tree of life, high eminent, blooming ambrosial fruit of vegetable gold; and next to life, our death, the tree of knowledge, grew fast by-- knowledge of good, bought dear by knowing ill. southward through eden went a river large, nor changed his course, but through the shaggy hill passed underneath ingulfed; for god had thrown that mountain, as his garden mould, high raised upon the rapid current, which, through veins of porous earth with kindly thirst up-drawn, rose a fresh fountain, and with many a rill watered the garden; thence united fell down the steep glade, and met the nether flood, which from his darksome passage now appears, and now, divided into four main streams, runs diverse, wandering many a famous realm and country whereof here needs no account; but rather to tell how, if art could tell how, from that sapphire fount the crisped brooks, boiling on orient-pearl and sands of gold, with mazy error under pendent shades ran nectar, visiting each plant, and fed flowers worthy of paradise, which not nice art in beds and curious knots, but nature boon poured forth profuse on hill, and dale, and plain, both where the morning sun first warmly smote the open field, and where the unpierced shade imbrowned the noontide bowers.--iv. - . milton's description of paradise is not less remarkable in its way than the lurid scenes depicted by him in pandemonium. the versatility of his poetic genius is nowhere more apparent than in the charming pastoral verse contained in this part of his poem. the poet has lavished the whole wealth of his luxuriant imagination in his description of eden and blissful paradise with its 'vernal airs' and 'gentle gales,' its verdant meads, and murmuring streams, 'rolling on orient-pearl and sands of gold;' its stately trees laden with blossom and fruit; its spicy groves and shady bowers, over which there breathed the eternal spring. in book ix. satan expresses himself in an eloquent apostrophe to the primitive earth, over which he previously wandered for seven days-- o earth, how like to heaven, if not preferred more justly, seat worthier of gods, as built with second thoughts, reforming what was old! for what god, after better, worse would build? terrestrial heaven, danced round by other heavens, that shine, yet bear their bright officious lamps, light above light, for thee alone, as seems, in thee concentring all their precious beams of sacred influence! as god in heaven is centre, yet extends to all, so thou centring receiv'st from all those orbs; in thee, not in themselves, all their known virtue appears, productive in herb, plant, and nobler birth of creatures animate with gradual life of growth, sense, reason, all summed up in man, with what delight i could have walked thee round, if i could joy in aught--sweet interchange of hill and valley, rivers, woods, and plains, now land, now sea, and shores with forest crowned, rocks, dens, and caves.--ix. - . though it is impossible to regard the earth as possessing the importance ascribed to it by the ancient ptolemaists; nevertheless, our globe is a great and mighty world, and appears to be one of the most favourably situated of all the planets, being neither near the sun nor yet very far distant from the orb; and although, when compared with the universe, it is no more than a leaf on a tree in the midst of a vast forest; still, it is not the least important among other circling worlds, and unfailingly fulfils the part allotted to it in the great scheme of creation. the planet hesperus this is the beautiful morning and evening star, the peerless planet that ushers in the twilight and the dawn, the harbinger of day and unrivalled queen of the evening. venus, called after the roman goddess of love, and also identified with the greek aphrodite of ideal beauty, is the name by which the planet is popularly known; but milton does not so designate it, and the name 'venus' is not found in 'paradise lost.' the ancients called it lucifer and phosphor when it shone as a morning star before sunrise, and hesperus and vesper when it became visible after sunset. it is the most lustrous of all the planets, and at times its brilliancy is so marked as to throw a distinct shadow at night. venus is the second planet in order from the sun. its orbit lies between that of mercury and the earth, and in form approaches nearer to a circle than that of any of the other planets. it travels round the sun in · days, at a mean distance of , , miles, and with an average velocity of , miles an hour. its period of rotation is unknown. by the observation of dusky spots on its surface, it has been surmised that the planet completes a revolution on its axis in - / hours; but other observers doubt this and are inclined to believe that it always presents the same face to the sun. when at inferior conjunction venus approaches nearer to the earth than any other planet, its distance then being , , miles. its greatest elongation varies from ° to ° ´; it therefore can never be much more than three hours above the horizon before sunrise, or after sunset. venus is a morning star when passing from inferior to superior conjunction, and during the other half of its synodical period it is an evening star. the planet attains its greatest brilliancy at an elongation ° west or east of the sun--five weeks before and after inferior conjunction. it is at these periods, when at its greatest brilliancy, that it casts a shadow at night. though so pleasing an object to the unaided eye, venus, when observed with the telescope, is often a source of disappointment--this is on account of its dazzling brilliancy, which renders any accurate definition of its surface impossible. sir john herschel writes: 'the intense lustre of its illuminated part dazzles the sight, and exaggerates every imperfection of the telescope; yet we see clearly that its surface is not mottled over with permanent spots like the moon; we notice in it neither mountains nor shadows, but a uniform brightness, in which sometimes we may indeed fancy, or perhaps more than fancy, brighter or obscurer portions, but can seldom or never rest fully satisfied of the fact.' it is believed that the surface of the planet is invisible on account of the existence of a cloud-laden atmosphere by which it is enveloped, and which may serve as a protection against the intense glare of the sunshine and heat poured down by the not far-distant sun. schröter, a german astronomer, believed that he saw lofty mountains on the surface of the planet, but their existence has not been confirmed by any other observer. the sun if viewed from venus would have a diameter nearly half as large again as when seen from the earth; it is therefore probable that the planet is subjected to a much higher temperature than what is experienced on our globe. the phases of venus are similar to those exhibited by the moon, and are caused by a change in position of the illumined hemisphere of the planet with regard to the earth. at superior conjunction the whole enlightened disc of the planet is turned towards the earth, but is invisible by being lost in the sun's rays. shortly before or after it arrives at this point, its form is gibbous, the illumined portion being less than a circle but greater than a semi-circle. at its greatest elongation west or east of the sun the planet resembles the moon in quadrature--a half moon--and between those points and inferior conjunction it is visible as a beautiful crescent. it becomes narrower and sharper as it approaches inferior conjunction, until it resembles a curved luminous thread prior to its disappearance at the conjunction. after having passed this point it reappears on the other side of the sun as the morning star. it would be only natural to imagine that this peerless orb, the most beautiful and lustrous of the planets, upon which men have gazed with longing admiration, and designated the emblem of 'all beauty and all love,' should have impressed milton's poetical imagination with its charming appearance, and stimulated the flow of his captivating muse. he addresses the orb as fairest of stars, last in the train of night, if better thou belong not to the dawn, sure pledge of day, that crown'st the smiling morn with thy bright circlet, praise him in thy sphere while day arises, that sweet hour of prime.--v. - . in these lines the poet alludes to venus as the morning star. in the other passages in his poem milton associates the planet sometimes with the morning and at other times with the evening-- his countenance, as the morning star that guides the starry flock.--v. - . or if the star of evening and the moon haste to thy audience, night with her will bring silence, and sleep listening to thee will watch.--vii. - . and hence the morning planet gilds her horns.--vii. . the sun was sunk and after him the star of hesperus, whose office is to bring twilight upon the earth, short arbiter twixt day and night.--ix. - . and bid haste the evening star on his hill top to light the bridal lamp.--viii. - . milton knew of the phases of venus and was aware that at certain times the planet was visible in the telescope as a beautiful crescent. the line in which he mentions her as gilding her horns is an allusion to this appearance of venus. the pleiades the beautiful cluster of the pleiades or seven sisters has been regarded with hallowed veneration from time immemorial. the happy influences believed to be shed down upon the earth by those stars and their close association with human destinies have rendered them objects of almost sacred interest among the different races of mankind. in every region of the globe and in every clime, among civilised nations and savage fetish-worshipping tribes, the same benign influences were ascribed to the stars which form this interesting group. in greek mythology they were known as the seven daughters of atlas and pleione. different versions are given of their fate. by some writers it is said they died from grief in consequence of the death of their sisters, the hyades, or on account of the fate of their father, who, for treason, was condemned by zeus to bear on his head and hands the vault of heaven, on the mountains of north-west africa which bear his name. according to others they were the companions of diana, and, in order to escape from orion, by whom they were pursued, the gods translated them to the sky. all writers agree in saying that after their death or translation they were transformed into stars. their names are alcyone, electra, maia, merope, sterope, taygeta, and celaeno. the seventh atlantid is said to be the 'lost pleiad,' but it can be perceived without difficulty by a person possessing good eyesight. in the book of job there is a beautiful allusion to the pleiades (chap. xxxviii.) when god speaks out of the whirlwind and asks the patriarch to answer him-- canst thou bind the sweet influences of the pleiades, or loose the bands of orion? canst thou bring forth mazzaroth in his season? or canst thou guide arcturus with his sons? knowest thou the ordinances of heaven? canst thou set the dominion thereof in the earth? admiral smyth says that this noble passage is more correctly rendered as follows: canst thou bind the delightful teemings of cheemah? or the contractions of chesil canst thou open? canst thou draw forth mazzaroth in his season or ayeesh and his sons canst thou guide? he writes: 'in this very early description of the cardinal constellations, _cheemah_ denotes taurus with the pleiades; _chesil_ is scorpio; mazzaroth is sirius in "the chambers of the south;" and ayeesh the greater bear, the hebrew word signifying a _bier_, which was shaped by the four well-known bright stars, while the three forming the tail were considered as children attending a funeral.' the greeks at an early period were attracted by this cluster of stars, and hesiod alludes to them in his writings. one passage converted into rhyme reads as follows: there is a time when forty days they lie, and forty nights, conceal'd from human eye; but in the course of the revolving year, when the swain sharps the scythe, again appear. their heliacal rising was considered a favourable time for setting out on a voyage, and their midnight culmination, which occurred shortly after the middle of november, was celebrated by some nations with festivals and public ceremonies. considerable diversity of opinion existed among the ancients with regard to the number of stars which constitute this group. it was affirmed by some that only six were visible, whilst others maintained that seven could be seen. ovid writes: quae septem dici, sex tamen esse solent. homer and attalus mention six; hipparchus and aratus seven. the legend with regard to the lost pleiad would seem to indicate that, during a period in the past, the star possessed a superior brilliancy and was more distinctly visible than it is at the present time. this may have been so, for, should it belong to the class of variable stars, there would be a periodic ebb and flow of its light, by which its fluctuating brilliance could be explained. when looked at directly only six stars can be seen in the group, but should the eye be turned sideways more than this number become visible. several observers have counted as many as ten or twelve, and it is stated by kepler that his tutor, maestlin, was able to enumerate fourteen stars and mapped eleven in their relative positions. with telescopic aid the number is largely increased--galileo observed thirty-six with his instrument and hooke, in , counted seventy-eight. large modern telescopes bring into view several thousand stars in this region. the pleiades are situated at a profound distance in space. their light period is estimated at years, indicating a distance of , billions of miles. our sun if thus far removed would be reduced to a tenth-magnitude star. 'there can be little doubt,' says miss agnes clerke, 'that the solar brilliancy is surpassed by sixty to seventy of the pleiades. and it must be in some cases enormously surpassed; by alcyone , , by electra , by maia nearly times. sirius itself takes a subordinate rank when compared with the five most brilliant members of a group, the real magnificence of which we can thus in some degree apprehend.' this is the only star cluster which can be perceived to be moving in space, or which has an ascertained common proper motion. its constituents form a magnificent system in which the stars bear a mutual relationship to each other, and perform intricate internal revolutions, whilst they in systemic union drift along through the depths of space. there are two allusions to the pleiades in 'paradise lost.' in describing the path of the newly created sun, milton introduces them as indicative of the joyfulness associated with the birth of the universe-- first in his east the glorious lamp was seen, regent of day, and all the horizon round invested with bright rays, jocund to run his longitude through heaven's high road; the grey dawn, and the pleiades before him danced, shedding sweet influence.--vii. - . it was believed that the earth was created in the spring; and towards the end of april this group rises a little before the sun and precedes him in his course, 'shedding sweet influences.' the ancients believed that the good or evil influences of the stars were exercised not in the night but during the day, when their rays mingled with those of the sun. the pernicious influence of the dog-star is mentioned by latin writers as being most pronounced during the dog-days, at the end of summer and commencement of autumn, the time of the heliacal rising of this star. the other allusion to the pleiades is in book x., line , where milton, in describing the altered path of the sun consequent upon the fall, mentions how the orb travels through taurus with the seven atlantic sisters--the seven daughters of atlas, the pleiades, which are situated on the shoulder of the animal representing this zodiacal constellation. the galaxy the galaxy or milky way is the great luminous zone encircling the heavens, which can be seen extending across the sky from horizon to horizon. its diffused nebulous appearance caused the ancients much perplexity, and many quaint opinions were hazarded as to the nature of this celestial highway; but the mystery associated with it was not solved until galileo directed his newly invented telescope to this lucent object, when, to his intense delight, he discovered that it consists of myriads of stars--millions upon millions of suns so distant as to be individually indistinguishable to ordinary vision, and so closely aggregated, that their blended light gives rise to the milky luminosity signified by its name. this stelliferous zone almost completely encircles the sphere, which it divides into two nearly equal parts, and is inclined at an angle of ° to the celestial equator. in centaurus it divides into two portions, one indistinct and of interrupted continuity, the other bright and well defined; these, after remaining apart for °, reunite in cygnus. the milky way is of irregular outline and varies in breadth from ° to °; it intersects the equinoctial in the constellations monoceros and aquila, and approaches in cassiopeia to within ° of the north pole of the heavens; an equal distance intervenes between it and the south pole. its poles are in coma bernices and cetus. the stars in the galactic tract are very unevenly distributed; in some of its richest regions as many stars as are visible to the naked eye on a clear night have been counted within the space of a square degree. in other parts they are much less numerous, and there have been observed besides, adjacent to the most luminous portions of the zone, dark intervals and winding channels almost entirely devoid of stars. an instance of this kind occurs in the constellation of the southern cross, where there exists in a rich stellar region a large oval-shaped dark vacuity, ° by ° in extent, that appears to be almost entirely denuded of stars. in looking at it, an impression is created that one is gazing into an empty void of space far beyond the milky way. this gulf of cimmerian darkness was called by early navigators the coal sack. similar dark spaces, though not of such magnitude, are seen in ophiuchus, scorpio, and cygnus. the galaxy, when viewed with a powerful telescope, is found to consist of congeries of stars, vast stellar aggregations, great luminous tracts resolvable into clouds of stars of overpowering magnificence, superb clusters of various orders, and convoluted nebulous streams wandering 'with mazy error' among 'islands of light and lakes of darkness,' resolved by the telescope into banks of shining worlds. the concourses of stars which enter into the formation of this wonderful zone exhibit in a marvellous degree the amazing profusion in which these orbs exist in certain regions of space; yet those multitudes of stars perform their motions in harmonious unison and in orderly array, and by their mutual attraction sustain the dynamical equilibrium of this stupendous galactic ring, the diameter of which, according to one authority, is not traversed by light in less than , years. [illustration: fig. .--a portion of the milky way.] sir william herschel, to whom we are indebted for most of what we know of the milky way, commenced a series of observations in with the object of acquiring a knowledge of the structure of the sidereal heavens. in the accomplishment of this object, to which he devoted a considerable part of his life, he undertook a systematic survey of that portion of the galaxy which is visible in the northern hemisphere. by a method called star-gauging, which consisted in the enumeration of the stars in each successive telescopic field as the instrument moved slowly over the region under observation, he found that the depth of the star strata could be approximately ascertained by counting the stars along the line of vision; those were most numerous where the visual line appeared of the greatest length and fewest in number where it was shortest. herschel perceived the internal structure of the galaxy to be exceedingly intricate and complex, and that it embraced within its confines an endless variety of systems, clusters, and groups, branches, sprays, arches, loops, and streaming filaments of stars, all of which combined to form this luminous zone. 'it is indeed,' says a well-known astronomer, 'only to the most careless glance, or when viewed through an atmosphere of imperfect transparency, that the milky way seems a continuous zone. let the naked eye rest thoughtfully on any part of it, and, if circumstances be favourable, it will stand out rather as an accumulation of patches and streams of light of every conceivable variety of form and brightness, now side by side, now heaped on each other; again spanning across dark spaces, intertwining and forming a most curious and complex network; and at other times darting off into the neighbouring skies in branches of capricious length and shape which gradually thin away and disappear.' sir john herschel, who was occupied for four years at the cape of good hope in exploring the celestial regions of the southern hemisphere, describes the coming on of the milky way as seen in his -foot reflector. he first remarks 'that all the stars visible to us, whether by unassisted vision or through the best telescopes, belong to and form part of a vast stratum or considerably flattened and unsymmetrical congeries of stars in which our system is deeply and eccentrically plunged; and, moreover, situated near a point where the stratum bifurcates or spreads itself out into two sheets.' 'as the main body of the milky way comes on the frequency and variety of those masses (nebulous) increases; here the milky way is composed of separate or slight or strongly connected clouds of semi-nebulous light, and, as the telescope moves, the appearance is that of clouds passing in a scud, as sailors call it.' the milky way is like sand, not strewed evenly as with a sieve, but as if flung down by handfuls (and both hands at once), leaving dark intervals, and all consisting of stars of the fourteenth, sixteenth, twentieth magnitudes down to nebulosity, in a most astonishing manner. after an interval of comparative poverty, the same phenomenon, and even more remarkable, i cannot say it is nebulous, it is all resolved, but the stars are inconceivably numerous and minute; there must be millions and all almost equally massed together. yet they nowhere run to nuclei or clusters much brighter in the middle. towards the end of the seventeenth hour (right ascension) the globular clusters begin to come in; they consist of stars of excessive minuteness, but yet not more so than the ground of the milky way, on which not only they appear projected, but of which it is very probable they form a part. 'from the foregoing analysis of the telescopic aspect of the milky way in this interesting region, i think it can hardly be doubted that it consists of portions differing exceedingly in distance, but brought by the effect of projection into the same, or nearly the same, visual line; in particular, that at the anterior edge of what we have called the main stream, we see foreshortened a vast and illimitable area scattered over with discontinuous masses and aggregates of stars in the manner of the cumuli of a mackerel sky, rather than of a stratum of regular thickness and homogeneous formation.' the profound distance at which the stars of the galaxy are situated in space precludes the possibility of our obtaining any definite knowledge of their magnitude and of the extent of the intervals by which they are separated from each other, nor can we learn anything of the details associated with the systems and combinations into which they enter. it is believed that the majority of the stars in the milky way equal or surpass the sun in brilliancy and splendour. they are tenth to fifteenth magnitude stars; now, the sun at the distance indicated by these magnitudes would in the telescope appear a much fainter object; he would not reach the fifteenth magnitude. consequently, the galactic stars are regarded as his peers or superiors in magnitude and brilliancy. those myriads of suns are all in motion--in nature a stationary body is unknown--and they are sufficiently far apart so as not to be unduly influenced by their mutual gravitational attraction; a distance perhaps equal to that which separates our sun from the nearest fixed star may intervene between each of those orbs. in the deepest recesses of the milky way, sir william herschel was able to count stars receding in regular order behind each other; between each there existed an interval of space, probably not less extensive than the interstellar spaces among the stars by which we are surrounded. the richest galactic regions in the northern hemisphere are found in perseus, cygnus, and aquila. night after night could be spent in sweeping the telescope over fields where the stars can be seen in amazing profusion. in the interval of a quarter of an hour, sir william herschel observed , stars pass before him in the telescope, and on another occasion he perceived , stars in the space of forty-one minutes. in the constellation of the swan there is a region about ° in breadth which contains , stars. photography reveals in a remarkable manner the amazing richness of this stelliferous zone; the impress of the stars on the sensitive plate of the camera, in some instances, resembles a shower of descending snowflakes. though sir william herschel was able to fathom the galaxy in most of its tracts, yet there were regions which his great telescopes were unable to penetrate entirely through. in cepheus there is a spot where he observed the stars become 'gradually less till they escape the eye so that appearances here favour the idea of a succeeding more distant clustering part.' he perceived another in scorpio 'where, through the hollows and deep recesses of its complicated structure, we behold what has all the appearance of a wide and indefinitely prolonged area strewed over with discontinuous masses and clouds of stars which the telescope at length refuses to analyse.' the great cluster in perseus, which lies in the milky way, also baffled the penetrative capacity of herschel's instruments. we cannot help quoting professor nichol's description of herschel's observation of this remarkable object. he says: 'in the milky way, thronged all over with splendours, there is one portion not unnoticed by the general observer, the spot in the sword-hand of perseus. that spot shows no stars to the naked eye; the milky light which glorifies it comes from regions to which unaided we cannot pierce. but to a telescope of considerable power the space appears lighted up with unnumbered orbs; and these pass on through the depths of the infinite, until, even to that penetrating glass, they escape all scrutiny, withdrawing into regions unvisited by its power. shall we adventure into these deeper retirements? then, assume an instrument of higher efficacy, and lo! the change is only repeated; those scarce observed before appear as large orbs, and, behind, a new series begins, shading gradually away, leading towards farther mysteries! the illustrious herschel penetrated on one occasion into this spot, until he found himself among depths whose light could not have reached him in much less than , years; no marvel that he withdrew from the pursuit, conceiving that such abysses must be endless!' the milky way may be regarded as a universe by itself, and our sun as one of its myriad stars. milton was aware of the stellar constitution of the milky way, which was one of galileo's discoveries. the poet gives a singularly accurate description of this luminous path, which he glorifies as the way by which the deity returned up to the heaven of heavens after he finished his great work of creation-- so sung the glorious train ascending: he through heaven, that opened wide her blazing portals, led to god's eternal house direct the way-- a broad and ample road, whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powdered with stars.--vii. - . comets records of the appearance of these remarkable objects have been handed down from earliest times; and when one of those mysterious visitors, travelling from out the depths of space, became visible in our skies, it was regarded with apprehension and dread as betokening the occurrence of calamities and direful events among the nations of the earth. the word comet is derived from the greek {komê}, signifying 'hair,' to which the hazy, luminous appearance of those objects bears some resemblance. a comet consists of a bright central part called the _nucleus_; this is surrounded by layers of nebulous matter called the _coma_, and both combined form the _head_, from which a long appendage extends called the _tail_. the nucleus and tail are not essential parts of a comet, for many have been observed in which both have been wanting. the tail is frequently very conspicuous, and presents considerable diversity both as regards its appearance and length. in some comets it is entirely absent, and in others it has been observed to stretch over an arc of sixty or seventy degrees, indicating a length of to million miles. sometimes it is straight, and at other times it is curved at the extremity; it has been observed bifurcated into two branches; and, on rare occasions, comets have been seen with two or more tails. the tail of a comet is always directed away from the sun; it increases in size as the comet approaches the orb, and diminishes as it recedes from him. this depends upon the degree of heat to which the comet is exposed, which has the effect of driving off or evaporating some of the matter composing the head. during the time the comet is travelling round the sun there is a continuous emission of this highly attenuated matter, which is visible as the tail, but when the comet begins to recede from the orb and reaches cooler regions of space the tail diminishes in size as the temperature becomes reduced, and ultimately it disappears. the appearance of a comet in the sky is often sudden and unexpected, and one of those erratic wanderers may become visible at any time and in any part of the heavens. it was remarked by kepler that there are as many comets in the sky as there are fishes in the ocean. this may or may not be true, for they only become visible when they approach the sun, and the time during which they remain so does not usually exceed a few weeks or months. ancient astronomers were much perplexed with the motions of comets, which appeared to be much more irregular than those of other celestial bodies and unconformed to any known laws. tycho brahé believed that comets moved in circular orbits, and kepler imagined that they travelled in straight lines outwards from the sun. newton, however, was able to demonstrate that any conic section can be described about the sun consistent with the law of gravitation, and that the orbits of comets correspond with three of the four sections into which a cone can be divided. consequently, they obey the laws of planetary motion. comets which move in ellipses of known eccentricity and return with periodical regularity may be regarded as belonging to the solar system. twenty of these are known, and eleven of them have more than once passed their perihelion. those most familiarly known complete their periods in years as follows:--encke's · ; swift's, · ; winnecke's, · ; tempel's, ; brorsen's, · ; faye's, · ; tuttle's, · , and halley's, . comets with parabolic and hyperbolic orbits may be regarded as stray objects which visit our system once, and depart never to return again. besides those already mentioned there are many comets with orbits of such marked eccentricity that their ellipses when near perihelion cannot be distinguished from parabolæ. the great comets of , , , , , and traverse orbits approaching this form, and some of them require hundreds and thousands of years to accomplish a circuit of their paths. numerous instances of the appearance of remarkable comets have been recorded in the annals of ancient nations. the earliest records of comets are by the chinese, who were careful observers of celestial phenomena. a comet is said to have appeared at the time of the birth of mithridates ( b.c.), which had a disc as large as that of the sun; a great comet also became visible in the heavens about the time of the death of julius cæsar ( b.c.), and another was seen in the reign of justinian ( a.d.). a remarkable comet was observed in , and in , the year in which the turks obtained possession of constantinople and threatened to overrun europe, a great comet appeared, which was regarded by christendom with ominous forebodings. the celebrated astronomer halley was the first to predict the return of a comet. having become acquainted with newton's investigations, which showed that the forms of the orbits of comets were either parabolæ or extremely elongated ellipses, he subjected the next great comet, which appeared in , to a series of observations, calculated its orbit, and predicted that it would return to perihelion in seventy-five or seventy-six years. on referring to past records he discovered that a great comet appeared in , which pursued a path similar to the one traced out for his comet, another was seen in , and one in . halley perceived that the intervals between those dates corresponded to a period of about seventy-six years, the time which he calculated would be required for his comet to complete a revolution of its orbit. he therefore had no hesitation in predicting that the comet would appear again in . halley knew that he would not be alive to witness the event, and alludes to it in the following sentence: 'wherefore if it should return according to our prediction about the year , impartial posterity will not refuse to acknowledge that this was first discovered by an englishman.' as the time approached when the comet should be drawing near to our system, much interest was excited among astronomers, who would have an opportunity afforded them of testing the accuracy of halley's prediction. an eminent french mathematician named clairaut computed anew, by a method rather different to that adopted by halley, the retarding effect of the attraction of the planets upon the speed of the comet, and arrived at the conclusion that it would reach perihelion about the middle of april ; but, owing to unknown influences--uranus and neptune not having been discovered--it might be a month before or behind the calculated time. clairaut made this announcement on november , . astronomers were now intently on the look-out for the comet, and night after night the sky was swept by telescopes in search of the expected visitor, which for upwards of seventy years had been pursuing its solitary path invisible to mortal eyes. but the mental vision of the mathematician did not fail to follow this celestial object, which was now announced as being on the confines of our system. the comet was first observed on december , , it soon became conspicuous in the heavens, and reached perihelion on march , , a month before the time assigned to it by clairaut but within the limit of error allowed for unknown influences. halley's comet returned again in , and may be expected about the year . the periodic appearance of this comet has been traced back to the year . the celebrated comet of was noted as having been the one which afforded newton an opportunity of making observations which led to his discovery that comets describe orbits round the sun in conformity with the different sections of a cone. the comet of was observed for many weeks in the northern heavens as a brilliant object with a beautiful fan-shaped tail; it completes a revolution of its orbit in about , years. the comet of was also a splendid object. it possessed a tail million miles in length, and approached within , miles of the sun. the heat to which it was exposed was sufficient to volatilize the most infusible substances known to exist. donati's comet of will be long remembered as one of the most impressive of celestial spectacles: its tail extended over an area of forty degrees, and enveloped the star arcturus, which could be seen shining through it with undiminished brilliancy. its period is estimated to be , years. a great comet appeared in , through the tail of which the earth passed without any perceptible effect having resulted. no remarkable comets have appeared during recent years. in , , and , several were observed, and that of was the first successfully photographed. comets consist of cosmical matter which exists in a condition of extreme tenuity, and especially so in the coma and tail. sir john herschel described them as almost spiritual in texture, and small stars have been seen shining through their densest parts without any perceptible diminution of their light. the nucleus is believed to be composed of a congeries of meteoric fragments, and these, when exposed to the sun's heat, throw off luminous nebulous particles that are swept by some repulsive force into space and form the appendage known as the tail. comets may be regarded as celestial objects that are perfectly innocuous. neither fear nor dread need be apprehended from their visits; they come to please and instruct, not to injure or destroy. milton does not fail to introduce into his poem several allusions to comets, and in doing so expresses the ideas and sentiments which in his time were associated with those objects. in describing the hostile meeting between satan and death before the gates of hell, he writes: on the other side, incensed with indignation, satan stood unterrified, and like a comet burned, that fires the length of ophiuchus huge in the arctic sky, and from his horrid hair shakes pestilence and war.--ii. - . this passage is eminently descriptive of the appearance of a great comet, and the occasion on which it is introduced adds to the intensity of the lurid imaginings and feelings of terror and dismay with which these objects have always been regarded. the comparison of the enraged prince of hell with one of those mysterious and fiery looking visitors to our skies was a grand conception of the poet's, and one worthy of the mighty combatant. ophiuchus (the serpent-bearer) is a large constellation which occupies a rather barren region of the heavens to the south of hercules. it has a length of about forty degrees, and is represented by the figure of a man bearing a serpent in both hands. it is not easy to imagine why milton should have assigned the comet to this uninteresting constellation; he may possibly have seen one in this part of the sky, or his poetical ear may have perceived that the expression 'ophiuchus huge,' which has about it a ponderous rhythm, was well adapted for the poetic description of a comet. the only other allusion in the poem to a comet is near its conclusion, when the cherubim descend to take possession of the garden, prior to the removal of adam and eve-- high in front advanced, the brandished sword of god before them blazed, fierce as a comet; which with torrid heat, and vapour as the lybian air adust began to parch that temperate clime.--xii. - . falling stars on any clear night an observer can, by attentively watching the heavens, perceive a few of those objects which become visible for a moment as a streak of light and then vanish. they are the result of the combustion of small meteoric masses having a celestial origin, and travelling with cosmical velocity, and which, in their headlong flight, become so heated by contact with the earth's atmosphere that they are converted into glowing vapour. this vapour when it cools condenses into fine powder or dust, and gradually descends upon the earth's surface, where it can be detected. shooting stars become visible at a height varying between twenty and one hundred and thirty miles, and their average velocity has been estimated at about thirty miles a second. though casual falling stars can be seen at all times in every part of the heavens, yet there are certain periods at which they appear in large numbers, and have been observed to radiate from certain well-defined parts of the sky. when the radiant point is overhead, the falling stars spread out and resemble a parachute of fire; but when it is below the horizon, the stars ascend upwards like rockets into the sky. the radiant point is fixed among the stars, so that at the commencement of a shower it may be overhead, and before the termination of the display it may have travelled below the horizon. the radiant is usually named after the constellation in which it is observed. the november meteors are called leonids, because they radiate from a point in the constellation leo; those in taurus are called taurids; in perseus, perseids; in lyra, lyraïds; and in andromeda, andromedes, because their radiant points are situated in those constellations. the falling stars that have attracted most attention are those which appear on or about november . every year at this period they can be seen in greater or less numbers, and on referring to numerous past records it has been ascertained that a magnificent display of those objects occurs every thirty-three years. the earliest historical allusion to this meteoric shower is by theophanes, who wrote that in the year a.d. the sky at constantinople appeared to be on fire with falling stars. in the year a.d. another remarkable display took place, and from that time until twelve conspicuous displays are recorded as having occurred at recurring intervals of thirty-three years. the grandest display of this kind that was ever witnessed occurred in . it was visible over nearly the whole of the american continent, and, having commenced at midnight, lasted for four or five hours. the falling stars were so numerous that they appeared to rain upon the earth, and caused the utmost consternation and terror among those who witnessed the phenomenon, many persons having imagined that the end of the world was at hand. the regular recurrence of these meteoric displays has been satisfactorily explained by the assumption that round the sun there travels in an elliptical orbit with planetary velocity a vast shoal of meteoric bodies some millions of miles in length and several hundred thousand miles in breadth. the nearest point of their orbit to the sun coincides with the earth's orbit, and the most distant part extends beyond the orbit of uranus. these bodies accomplish a circuit of their orbit in - / years. the earth in her annual revolution intersects the path of the meteors, and when this occurs some falling stars can always be seen; but when the intersection happens at the time the shoal is passing, then there results a grand meteoric display. numerous other meteoric swarms travel in orbital paths round the sun. milton, in his poem, alludes to falling stars upon two occasions. in describing the fall of mulciber from heaven he says:-- from morn to noon he fell, from noon to dewy eve, a summer's day; and with the setting sun dropt from the zenith like a falling star, on lemnos the Ægaean isle.--i. - . the rapid flight of the archangel uriel from the sun to the earth is described in the following lines:-- thither came uriel, gliding through the even on a sunbeam, swift as a shooting star in autumn thwarts the night, when vapours fired impress the air, and shows the mariner from what point of his compass to beware impetuous winds.--iv. - . milton mentions the season of the year in which those stars are most frequently seen, and refers to an ancient belief by which they were regarded as the precursors of stormy weather. a translation from virgil contains a similar allusion to them-- oft shalt thou see ere brooding storms arise, star after star glide headlong down the skies. the standard borne by the cherub azazel is described as having-- shone like a meteor streaming to the wind.--i. . chapter ix milton's imaginative and descriptive astronomy the theme chosen by milton for his great epic, viz. the fall of man and his expulsion from paradise--perhaps the most momentous incident in the history of the human race--was one worthy of the genius of a great poet and in the treatment of which milton has been sublimely successful. the newly created earth; the untainted loveliness of the paradise in which our first parents dwelt during their innocence; their temptation; their fall and removal from the happy garden, furnished a theme which afforded him an opportunity for the display of his unrivalled poetic genius. though the chief interest in the poem is centred in the garden of eden and its occupants, yet milton was enabled, by the comprehensive manner in which he treated his subject, to introduce into his work a cosmology which embraced not only the system to which our globe belongs, but the entire starry heavens by which we are surrounded. but the universality of his genius did not rest here. in the utterance of his sacred song he soared beyond the starry sphere, describing himself as wrapt above the pole--the starry pole--up to the empyrean, or heaven of heavens, the ineffable abode of the deity and the blissful habitation of angelic beings who, in adoration and worship, surround the throne of the most high. descending to that nether world at the opposite pole of the universe, in the lowest depth of chaos, the place prepared by eternal justice for the rebellious, he unfolds to our horror-stricken gaze the terrors of this infernal region; its fiery deluge of ever-burning sulphur; its 'regions of sorrow;' its 'doleful shades'--the unhappy abode of fallen angels who 'in floods and whirlwinds of tempestuous fire,' alternated by exposure to unendurable cold and icy torment, experience the direful consequences of their apostacy. milton's 'paradise lost' may be regarded as the loftiest intellectual effort in the whole range of literature. in it we find all that was known of science, philosophy, and theology. the theme, founded upon a bible narrative, itself written under divine inspiration, embraces the entire system of christian doctrine as revealed in the scriptures, and many of the noblest passages in the sacred volume are introduced into the poem expressed in the lofty utterance of flowing and harmonious verse. the choicest classical writings of greek and latin authors; the mythological and traditional beliefs of ancient nations; historical incidents of valour and renown and all that was great and good in the annals of mankind were laid under contribution by milton in the illustration and embellishment of his poem. in order to obtain a basis or foundation upon which to construct his great epic, milton found it necessary to localise the regions of space in which the principal events mentioned in his poem are described as having occurred. the unfathomable abyss of space may be regarded as an uncircumscribed sphere boundless on all sides round, and so far as we can comprehend of infinite extent. this sphere milton divided into two hemispheres--an upper and a lower. the upper was called heaven, or the empyrean--a glorified region of boundless dimensions; the lower hemisphere embraced chaos--a dark, fathomless abyss in which the elements of matter existed in a state of perpetual tumult and wild uproar. the occurrence of a rebellion in heaven necessitated a further division of the sphere. the revolt, headed by lucifer, one of the highest archangels, afterwards known as satan, who drew after him a third of the angelic host, contested the supremacy of heaven with michael and the angels which kept their loyalty. after two days' battle-- him the almighty power hurled headlong flaming from the ethereal sky, with hideous ruin and combustion, down to bottomless perdition; there to dwell in adamantine chains and penal fire.--i. - . having been precipitated over the crystal wall of heaven into the deep abyss, milton says:-- nine days they fell; confounded chaos roared, and felt tenfold confusion in their fall through his wild anarchy; so huge a rout encumbered him with ruin. hell at last, yawning, received them whole, and on them closed.--vi. - . hell, milton locates in the lowest depth of chaos, a region cut off from the body of chaos, through which the expelled angels fell for nine days before reaching their destined habitation. there are now three divisions of space: heaven, chaos, and hell. but a fourth is required to enable milton to complete his scheme for the delineation of his poem. the earth and starry universe were not as yet called into existence, but after the overthrow of the rebellious angels, god, by circumscribing a portion of chaos situated immediately underneath the empyrean, created the mundane universe, or the 'heavens and the earth.'[ ] this new universe he reclaimed from chaos, and with the embryo elements of matter-- his dark materials to create new worlds.--ii. . he formed the earth and all the countless shining orbs visible overhead, and the myriads more which the telescope reveals, scattered in apparently endless profusion over the circular immensity of space. it is this new universe--the earth and starry heavens--that claims our chief attention, and in the delineation of milton's imaginative and descriptive powers it is to this latest manifestation of divine wisdom and might that our remarks shall principally apply. after the expulsion of the rebel angels from heaven, god sent his son, the messiah to create the new universe--a work of omnipotence described by milton in a manner worthy of so magnificent a display of almighty power-- meanwhile the son on his great expedition now appeared, girt with omnipotence, with radiance crowned of majesty divine: sapience and love immense; and all his father in him shone. about his chariot numberless were poured cherub and seraph, potentates and thrones, and virtues, winged spirits, and chariots winged from the armoury of god, where stand of old myriads, between two brazen mountains lodged against a solemn day, harnessed at hand, celestial equipage; and now came forth spontaneous, for within them spirit lived, attendant on their lord. heaven opened wide her ever-during gates, harmonious sound! on golden hinges moving, to let forth the king of glory, in his powerful word and spirit, coming to create new worlds. on heavenly ground they stood, and from the shore they viewed the vast immeasurable abyss outrageous as a sea, dark, wasteful, wild, up from the bottom turned by furious winds and surging waves, as mountains to assault heaven's highth, and with the centre mix the pole. 'silence, ye troubled waves, and thou deep, peace!' said then the omnific word: 'your discord end!' nor stayed; but on the wings of cherubim uplifted, in paternal glory rode far into chaos, and the world unborn; for chaos heard his voice. him all his train followed in bright procession, to behold creation, and the wonders of his might. then stayed the fervid wheels, and in his hand he took the golden compasses, prepared in god's eternal store, to circumscribe this universe, and all created things. one foot he centred, and the other turned round through the vast profundity obscure; and said, 'thus far extend, thus far thy bounds; this be thy just circumference, o world!' thus god the heaven created, thus the earth, matter unformed and void. darkness profound covered the abyss; but on the watery calm his brooding wings the spirit of god outspread, and vital virtue infused, and vital warmth, throughout the fluid mass; but downward purged the black, tartareous, cold, infernal dregs, adverse to life; then founded, then conglobed like things to like; the rest to several place disparted, and between spun out the air; and earth self balanced on her centre hung.--vii. - . milton begins his narrative of the creation by describing the progress of the deity on his great expedition, accompanied by hosts of angels and surrounded with all the solemn pomp and splendour of heaven. the brilliant throng having passed through heaven's gates, which opened wide their portals, they beheld in front of them the dark abyss of chaos--a tempest-tossed sea of warring elements upturned in wild confusion. at god's instant command silence and peace reigned over the deep, and tranquil calm succeeded noisy discord. then on the wings of cherubim he rode far into chaos, and with his golden compasses decreed the dimensions of the universe by circumscribing the vast vacuity of space. into the elements which hasted to their several places, his spirit infused vital warmth and caused the formless mass of matter to assume the figure of a sphere, and thus the earth poised on her axis unsupported, and in darkness shrouded hung suspended in space. the placing of the golden compasses in the hands of the creator, with which he measured out the heavens, is a noble conception on the part of milton, and one most appropriate, since the construction of the universe is based upon the principles of geometrical science. 'let there be light!' said god; and forthwith light ethereal, first of things, quintessence pure, sprung from the deep; and from her native east to journey through the aëry gloom began, sphered in a radiant cloud; for yet the sun was not; she in a cloudy tabernacle sojourned the while. god saw the light was good; and light from darkness by the hemisphere divided; light the day, and darkness night he named. thus was the first day even and morn: nor passed uncelebrated, nor unsung by the celestial quires, when orient light exhaling first from darkness they beheld; birthday of heaven and earth; with joy and shout the hollow universal orb they filled, and touched their golden harps, and hymning praised god and his works: creator him they sung, both when first evening was, and when first morn.--vii. - . the appearance of light, which sprung into existence at the fiat of the creator, was the next great event witnessed by beholding angels--birthday of heaven and earth, first morning and first evening, which the celestial choirs celebrated with praise and shouts of joy. the creation of the firmament was the great work of the second day. again god said, 'let there be firmament amid the waters, and let it divide the waters from the waters!' and god made the firmament, expanse of liquid, pure, transparent, elemental air, diffused in circuit to the uttermost convex of this great round--partition firm and sure, the waters underneath from those above dividing; for as the earth, so he the world built on circumfluous waters calm, in wide crystalline ocean, and the loud misrule of chaos far removed, lest fierce extremes contiguous might distemper the whole frame: and heaven he named the firmament. so even and morning chorus sung the second day.--vii. - . after describing the gathering of the waters off the face of the globe into seas, causing the dry land to appear, which at the word of god became clothed with vegetation, rendering the earth a habitable abode, milton proceeds to describe the creation of the heavenly bodies-- again the almighty spake: 'let there be lights high in the expanse of heaven, to divide the day from night; and let them be for signs, for seasons, and for days, and circling years; and let them be for lights, as i ordain their office in the firmament of heaven, to give light on the earth!' and it was so. and god made two great lights, great for their use to man, the greater to have rule by day, the less by night, altern; and made the stars, and set them in the firmament of heaven to illuminate the earth, and rule the day in their vicissitude, and rule the night, and light from darkness to divide. god saw, surveying his great work, that it was good: for, of celestial bodies, first, the sun, a mighty sphere he framed, unlightsome first, though of ethereal mould; then formed the moon globose, and every magnitude of stars, and sowed with stars the heaven thick as a field. of light by far the greater part he took, transplanted from her cloudy shrine, and placed in the sun's orb, made porous to receive and drink the liquid light; firm to retain her gathered beams, great palace now of light. hither, as to their fountain, other stars repairing, in their golden urns draw light, and hence the morning planet gilds her horns; by tincture or reflection they augment their small peculiar, though, from human sight so far remote, with diminution seen. first in his east the glorious lamp was seen, regent of day, and all the horizon round invested with bright rays, jocund to run his longitude through heaven's high road; the grey dawn, and the pleiades before him danced, shedding sweet influence. less bright the moon, but opposite in levelled west was set his mirror, with full face borrowing her light from him; for other light she needed none in that aspect, and still that distance keeps till night; then in the east her turn she shines, revolved on heaven's great axle, and her reign with thousand lesser lights dividual holds, with thousand thousand stars that then appeared spangling the hemisphere. then first adorned with their bright luminaries, that set and rose, glad evening and glad morn crowned the fourth day.--vii. - . the first creation was light, and milton, according to scriptural testimony, ascribes its origin to the bidding of the creator. 'god said, let there be light; and there was light!' the sun he describes as a mighty sphere, but at first non-luminous. there was light, but no sun. the reason usually given in explanation of this phenomenon is, that the heavenly bodies were created at the same time as the earth, but were rendered invisible by a canopy of vapour and cloud which enveloped the newly-formed globe; and that afterwards, when it dispersed, they appeared in the firmament, shining in all their pristine splendour. milton does not, however, adhere to this view of things, but says that light for the first three days sojourned in a cloudy shrine or tabernacle, and was afterwards transplanted in the sun, which became a great palace of light. he expresses himself in a somewhat similar manner in book iii., which opens with an address to light--one of the most beautiful passages in the poem, in which he alludes to his blindness when expressing his thoughts and sentiments with regard to this ethereal medium, which conveys to us the pleasurable sensation of vision-- hail, holy light! offspring of heaven first-born! or of the eternal co-eternal beam, may i express thee unblamed? since god is light, and never but in unapproached light dwelt from eternity--dwelt then in thee, bright effluence of bright essence increate! or hear'st thou rather, pure ethereal stream, whose fountain who shall tell? before the sun, before the heavens thou wert, and at the voice of god, as with a mantle, didst invest the rising world of waters dark and deep, won from the void and formless infinite.--iii. - . the sun having become a lucent orb, milton poetically describes how the planets repair to him as to a fountain, and in their golden urns draw light; and how the morning planet venus gilds her horns illumined by his rays. the poet associates joyous ideas with the new-born universe. the sun, now the glorious regent of day, begins his journey in the east, lighting up the horizon with his beams; whilst before him danced the grey dawn, and the pleiades shedding sweet influences. there existed an ancient belief that the earth was created in the spring, and in april the sun is in the zodiacal constellation taurus, in which are also situated the pleiades; they rise a little before the orb, and precede him in his path through the heavens. the stars of this group have always been regarded with a peculiar sacredness, and their rays, mingling with those of the sun, were believed to shed sweet influences upon the earth. the moon, less bright, with borrowed light, in her turn shines in the east, and, with the thousand thousand luminaries that spangle the firmament, reigns over the night. we learn in book iii. that the archangel uriel, who was beguiled by satan, witnessed the creation, and described how the heavenly bodies were brought into existence, he having perceived what we should call the gaseous elements of matter rolled into whorls and vortices which became condensed into suns and systems of worlds. this mighty angel says:-- i saw when, at his word the formless mass, this world's material mould, came to a heap: confusion heard his voice, and wild uproar stood ruled, stood vast infinitude confined; till at his second bidding darkness fled, light shone, and order from disorder sprung. swift to their several quarters hasted then the cumbrous elements, earth, flood, air, fire; and this ethereal quintessence of heaven flew upward, spirited with various forms, that rolled orbicular, and turned to stars numberless, as thou seest, and how they move; each had his place appointed, each his course; the rest in circuit walls this universe.--iii. - . in his sublime description of the creation milton has adhered with marked fidelity to the mosaic version, as narrated in the first two chapters of genesis, when god, by specific acts in certain stated periods of time, created the visible universe and all that it contains. the successive acts of creation are described in words almost identical with those of scripture, embellished and adorned with all the wealth of expression which our language is capable of affording. the several scenes presented to the imagination, and witnessed by hosts of admiring angels as each portion of the magnificent work was accomplished, are full of a grandeur and majesty worthy of the loftiest conceivable effort of divine power and might. the return of the creator after the completion of his great work is described by milton in a manner worthy of the progress of deity through the celestial regions. the whole creation rang with jubilant delight, and the bright throng which witnessed the wonders of his might followed him with acclamation, ascending by the glorified path of the milky way up to his high abode--the heaven of heavens-- here finished he, and all that he had made viewed, and behold! all was entirely good. so even and morn accomplished the sixth day: yet not till the creator from his work desisting, though unwearied, up returned, up to the heaven of heavens, his high abode, thence to behold this new created world, the addition of his empire, how it showed in prospect from his throne, how good, how fair, answering his great idea. up he rode, followed with acclamation, and the sound symphonious of ten thousand harps, that tuned angelic harmonies: the earth, the air resounded (thou remember'st, for thou heard'st) the heavens and all the constellations rung, the planets in their stations listening stood, while the bright pomp ascended jubilant. 'open ye everlasting gates!' they sung; 'open ye heavens! your living doors; let in the great creator, from his work returned magnificent, his six days' work, a world; open, and henceforth oft; for god will deign to visit oft the dwellings of just men, delighted; and with frequent intercourse thither will send his winged messengers on errands of supernal grace.' so sung the glorious train ascending: he through heaven, that opened wide her blazing portals, led to god's eternal house direct the way-- a broad and ample road, whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powdered with stars.--vii. - . milton, throughout his description of the creation, sustains with lofty eloquence his sublime conception of this latest display of almighty power; and invests with becoming majesty all the acts of the creator, who, when he finished his great work, saw that all was entirely good. shortly after the creation of the new universe, satan, having escaped from hell, plunged into the abyss of chaos, and, after a long and arduous journey upwards, in which he had to fight his way through the surging elements that raged around him like a tempestuous sea, he reached the upper confines of this region where less confusion prevailed, and where a glimmering dawn of light penetrated its darkness and gloom, indicating that the limit of the empire of chaos and ancient night had been reached by the adventurous fiend. pursuing his way with greater ease, he leisurely beholds the sight which is opening to his eyes--a sight rendered more glorious by his long sojourn in darkness. he sees:-- far off the empyreal heaven, extended wide in circuit, undetermined square or round, with opal towers and battlements adorned of living sapphire, once his native seat, and, fast by, hanging in a golden chain, this pendent world, in bigness as a star of smallest magnitude close by the moon.--ii. - . he gazes upon his native heaven where once he dwelt, and observes the pendent world in quest of which he journeyed hither--hung by a golden chain from the empyrean and no larger than a star of the smallest magnitude when close by the moon. in this passage milton does not allude to the earth, which was invisible, but to the entire starry heavens--the newly created universe reclaimed from chaos, which, when contrasted with the empyrean, appeared in size no larger than the minutest star when compared with the full moon. pursuing his journey, the new universe as it is approached expands into a globe of vast dimensions; its convex surface--round which the chaotic elements in stormy aspect lowered--seemed a boundless continent, dark, desolate, and starless, except on the side next to the wall of heaven, which though far-distant afforded it some illumination by its reflected light. satan, having alighted on this convex shell which enclosed the universe, wandered long over its bleak and dismal surface, until his attention was attracted by a gleam of light which appeared through an opening at its zenith right underneath the empyrean. thither he directed his steps, and perceived a structure resembling a staircase, or ladder, which formed the only means of communication between heaven and the new creation, and upon which angels descended and ascended-- far distant he descries, ascending by degrees magnificent up to the wall of heaven, a structure high; at top whereof, but far more rich, appeared the work as of a kingly palace gate, with frontispiece of diamond and gold embellished; thick with sparkling orient gems the portal shone, inimitable on earth by model, or by shading pencil drawn. the stairs were such as whereon jacob saw angels ascending and descending, bands of guardians bright, when he from esau fled to padan aram, in the field of luz dreaming by night under the open sky, and waking cried, '_this is the gate of heaven._'--iii. - . sometimes this mysterious structure was drawn up to heaven and invisible. at the time that satan reached the opening, the stairs were lowered, and standing at their base he looked down with wonder upon the entire starry universe-- such wonder seized, though after heaven seen, the spirit malign, but much more envy seized, at sight of all this world beheld so fair, round he surveys (and well might, where he stood so high above the circling canopy of night's extended shade) from eastern point of libra to the fleecy star that bears andromeda far off atlantic seas beyond the horizon; then from pole to pole he views in breadth, and without longer pause, down right into the world's first region throws his flight precipitant, and winds with ease through the pure marble air his oblique way amongst innumerable stars, that shone stars distant, but nigh hand seemed other worlds, or other worlds they seemed, or happy isles, like those hesperian gardens famed of old, fortunate fields, and groves, and flowery vales; thrice happy isles! but who dwelt happy there he staid not to inquire: above them all the golden sun, in splendour likest heaven allured his eye: thither his course he bends through the calm firmament, (but up or down by centre or eccentric hard to tell or longitude) where the great luminary, aloof the vulgar constellations thick, that from his lordly eye keep distance due, dispenses light from far. they, as they move their starry dance in numbers that compute days, months, and years, towards his all-cheering lamp turn swift their various motions, or are turned by his magnetic beam, that gently warms the universe, and to each inward part with gentle penetration, though unseen, shoots invisible virtue even to the deep; so wondrously was set his station bright.--iii. - . the ptolemaic cosmology having been adopted by milton in the elaboration of his poem, he describes the universe in conformity with the doctrines associated with this form of astronomical belief. to each of the first seven spheres which revolved round the steadfast earth there was attached a heavenly body; the eighth sphere embraced all the fixed stars, a countless multitude; the ninth the crystalline; and enclosing all the other spheres as if in a shell was the tenth sphere, or primum mobile, which in its diurnal revolution carried round with it all the other spheres. the nine inner spheres were transparent, but the tenth was an opaque solid shell-like structure, which enclosed the new universe and constituted the boundary between it and chaos underneath and the empyrean above. it was on the surface of this sphere that satan wandered until he discovered the opening at its zenith, where, by means of a staircase or ladder, communication was maintained with the empyrean. standing on the lower steps of this structure he paused for a moment to look down into the glorious universe which lay beneath him-- another heaven from heaven-gate not far, founded in view on the clear hyaline the glassy sea.--vii. - . he beholds it in all its dimensions, from pole to pole, and longitudinally from libra to aries, then without hesitation precipitates himself down into the world's first region, and winds his way with ease among the fixed stars. around him he sees innumerable shining worlds, sparkling and glittering in endless profusion over the circumscribed immensity of space--mighty constellations that shone from afar; clustering aggregations of stars; floating islands of light; twinkling systems rising out of depths still more profound, and a zone luminous with the light of myriads of lucid orbs verging on the confines of the universe. all these worlds the fiend passed unheeded, nor stayed he to inquire who dwelt happy there. in splendour above them all the sun attracted his attention and, directing his course towards the great luminary of our system, he alights on the surface of the orb. milton now makes a digression in order to describe what satan observed in the sun after having landed there. the poet embraces an opportunity for exercising his imaginative and descriptive powers by giving an ideal description of what, judging from the appearance of the orb, might be the natural condition of things existing on his surface-- there lands the fiend, a spot like which perhaps astronomer in the sun's lucent orb through his glazed optic tube, yet never saw. the place he found beyond expression bright, compared with aught on earth, metal or stone; not all parts like, but all alike informed with radiant light, as glowing iron with fire; if metal, part seemed gold, part silver clear; if stone, carbuncle most or chrysolite, ruby or topaz, to the twelve that shone in aaron's breastplate, and a stone besides, imagined rather oft than elsewhere seen; that stone, or like to that, which here below philosophers in vain so long have sought, in vain, though by their powerful art they bind volatile hermes, and call up unbound in various shapes old proteus from the sea, drained through a limbec to his native form. what wonder then if fields and regions here breathe forth elixir pure, and rivers run potable gold, when, with one virtuous touch, the arch-chemic sun, so far from us remote, produces, with terrestrial humour mixed, here in the dark so many precious things of colour glorious, and effect so rare? here matter new to gaze the devil met undazzled; far and wide his eye commands; for sight no obstacle found here, nor shade, but all sunshine, as when his beams at noon culminate from the equator, as they now shot upward still direct, whence no way round shadow from body opaque can fall; and the air, nowhere so clear sharpened his visual ray to objects distant far, whereby he soon saw within here a glorious angel stand.--iii. - . the physical structure of the interior of the sun is unknown; all that we see of the orb is the photosphere--the dazzling luminous envelope which indicates to the eye the boundary of the solar disc, and which is the source of light and heat. milton, in his imaginative and beautifully poetical description of the sun, is not more fanciful in his conception of the nature of the refulgent orb than a renowned astronomer (sir william herschel) who writes in the following strain: 'a cool, dark, solid globe, its surface diversified with mountains and valleys, clothed in luxuriant vegetation and richly stored with inhabitants, protected by a heavy cloud-canopy from the intolerable glare of the upper luminous region, where the dazzling coruscations of a solar aurora some thousands of miles in depth evolved the stores of light and heat which vivify our world.' satan, disguised as a cherub, makes himself known to uriel, regent of the sun. the upright seraph in response to his request directs him to the earth, the abode of man-- look downward on that globe, whose hither side with light from hence, though but reflected, shines, that place is earth, the seat of man; that light his day, which else, as the other hemisphere, night would invade; but there neighbouring moon (so call that opposite fair star) her aid timely interposes, and her monthly round still ending, still renewing, through mid-heaven, with borrowed light her countenance triform hence fills and empties, to enlighten the earth, and in her pale dominion checks the night.--iii. - . it would be impossible not to feel impressed with the accuracy and comprehensiveness of milton's astronomical knowledge; and how he has united in charming poetic expression the dry details of science with the divine inspiration of the heavenly muse. the distinctive appearances of the sun, moon, planets, and stars; their functional importance as regards this terrestrial sphere; the splendour and lustre peculiar to each; and the glory displayed in the entire created heavens, are portrayed with a skill indicative of a masterly knowledge of the science of astronomy. descend from heaven, urania, by that name if rightly thou art called, whose voice divine following, above the olympian hill i soar, above the flight of pegasean wing! the meaning, not the name, i call; for thou nor of the muses nine, nor on the top of old olympus dwell'st; but heavenly-born, before the hills appeared or fountain flowed, thou with eternal wisdom didst converse, wisdom thy sister, and with her didst play in presence of the almighty father, pleased with thy celestial song. up led by thee, into the heaven of heavens i have presumed, an earthly guest, and drawn empyreal air, thy tempering. with like safety guided down, return me to my native element; lest, from this flying steed unreined, (as once belerophon, though from a lower clime) dismounted, on the aleian field i fall, erroneous there to wander, and forlorn. half yet remains unsung, but narrower bound within the visible diurnal sphere. standing on earth, not rapt above the pole, more safe i sing with mortal voice, unchanged to hoarse or mute, though fallen on evil days, on evil days though fallen, and evil tongues, in darkness, and with dangers compassed round, and solitude; yet not alone, while thou visit'st my slumbers nightly, or when morn purples the east. still govern thou my song, urania, and fit audience find though few.--vii. - . the muses were greek mythological divinities who possessed the power of inspiring song, and were the patrons of poets and musicians. according to hesiod they were nine in number and presided over the arts. urania was the goddess of astronomy, and calliope the goddess of epic poetry. they are described as the daughters of zeus, and homer alludes to them as the goddesses of song who dwelt on the summit of mount olympus. they were the companions of apollo, and accompanied with song his playing on the lyre at the banquets of the immortals. milton does not invoke the mythological goddess, but urania the heavenly muse, whose aid he also implores at the commencement of his poem prior to his flight above the aonian mount. under her divine guidance he ascended to the heaven of heavens and breathed empyreal air, her tempering; in like manner he requests her to lead him down to his native element lest he should meet with a fate similar to what befell bellerophon. half his task he has completed, the other half, confined to narrower bounds within the visible diurnal sphere, remains unsung, and in its fulfilment he still implores his celestial patroness to govern his song. the natural phenomena which occur as a consequence of the motions of the heavenly bodies and the diurnal rotation of the earth on her axis, are accompanied by agreeable alternations in the aspect of nature with which every one is familiar. the rosy footsteps of morn; the solar splendour of noonday; the fading hues of even; and night with her jewelled courts and streams of molten stars, have been sung with rapturous admiration by poets of every nation and in every age. they, as ardent lovers of nature, have described in choicest language the pleasing vicissitudes brought about by the real and apparent motions of the celestial orbs. in this respect milton is unsurpassed by any poet in ancient or in modern times. the occasions on which he describes the heavenly bodies, or alludes to them in association with other phenomena, testify to the felicity of his thoughts and to the greatness of his poetic genius. surely no poet has ever given us a lovelier description of evening, or has added more to its exquisite beauty by his allusion to the celestial orbs, than milton when he describes the first evening in paradise-- now came still evening on, and twilight gray had in her sober livery all things clad; silence accompanied; for beast and bird, they to their grassy couch, these to their nests were slunk, all but the wakeful nightingale. she all night long her amorous descant sung; silence was pleased. now glowed the firmament with living sapphires: hesperus that led the starry host, rode brightest, till the moon, rising in clouded majesty, at length apparent queen, unveiled her peerless light, and o'er the dark her silver mantle threw.--iv. - . in the avowal of her conjugal love, eve, with charming expression, associates the orbs of the firmament with the delightful appearances of nature which presented themselves to her observation after she awoke to the consciousness of intelligent existence. sweet is the breath of morn, her rising sweet, with charm of earliest birds: pleasant the sun, when first on this delightful land he spreads his orient beams, on herb, tree, fruit, and flower, glistering with dew; fragrant the fertile earth after soft showers; and sweet the coming on of grateful evening mild; then silent night, with this her solemn bird, and this fair moon, and these the gems of heaven, her starry train: but neither breath of morn, when she ascends with charm of earliest birds; nor rising sun on this delightful land; nor herb, fruit, flower, glistering with dew; nor fragrance after showers; nor grateful evening mild; nor silent night, with this her solemn bird; nor walk by moon, or glittering star-light, without thee is sweet. but wherefore all night long shine these? for whom this glorious sight, when sleep hath shut all eyes?--iv. - . one of the charms of milton's verse is the devoutly poetical sentiment which pervades it. his thoughts, though serious, are not austere or gloomy, and it is in his loftiest musings that his reverence becomes most apparent. this feeling is conspicuous in adam's reply to the inquiry addressed to him by eve-- daughter of god and man, accomplished eve, these have their course to finish round the earth by morrow evening, and from land to land in order, though to nations yet unborn, ministering light prepared, they set and rise; lest total darkness should by night regain her old possession, and extinguish life in nature and all things; which these soft fires not only enlighten, but with kindly heat of various influence foment and warm, temper or nourish, or in part shed down their stellar virtue on all kinds that grow on earth, made hereby apter to receive perfection from the sun's more potent ray. these, then, though unbeheld in deep of night, shine not in vain; nor think, though men were none, that heaven would want spectators, god want praise: millions of spiritual creatures walk the earth unseen, both when we wake, and when we sleep: all these with ceaseless praise his works behold both day and night. how often from the steep of echoing hill or thicket, have we heard celestial voices to the midnight air, sole, or responsive each to other's note singing their great creator! oft in bands while they keep watch, or nightly rounding walk, with heavenly touch of instrumental sounds in full harmonic number joined, their songs divide the night, and lift our thoughts to heaven.--iv. - . the morning hymn of praise which adam and eve offer up in concert to their maker contains their loftiest thoughts and most reverent sentiments, expressed in melodiously flowing verse. in their solemn invocations they call upon the orbs of the firmament to join in praising and extolling the creator, and in their devout enthusiasm and adoration address by name those that are most conspicuous. hesperus, 'fairest of stars,' is asked to praise him in her sphere. the sun, great image of his maker, is told to acknowledge him his greater, and to sound his praise in his eternal course. the moon, the fixed stars, and the planets are called upon to resound the praise of the creator, whose glory is declared in the heavens-- fairest of stars, last in the train of night, if better thou belong not to the dawn, sure pledge of day, that crown'st the smiling morn with thy bright circlet, praise him in thy sphere while day arises, that sweet hour of prime. thou sun, of this great world both eye and soul, acknowledge him thy greater; sound his praise in thy eternal course, both when thou climb'st, and when high noon hast gained, and when thou fall'st. moon, that now meet'st the orient sun, now fliest with the fixed stars, fixed in their orb that flies; and ye five other wandering fires, that move in mystic dance, not without song, resound his praise, who out of darkness called up light.--v. - . milton's conception of celestial distances, and of the vast regions of interstellar space, is finely described in the following lines:-- down thither prone in flight he speeds, and through the vast ethereal sky sails between worlds and worlds, with steady wing now on the polar winds; then with quick fan winnows the buxom air, till, within soar of towering eagles.--v. - . as in their morning, so in their evening devotions, our first parents never fail to introduce a reference to the celestial orbs as indicating the power and goodness of the creator, made manifest in the beauty and greatness of his works-- thus, at their shady lodge arrived, both stood, both turned, and under open sky adored the god that made both sky, air, earth and heaven which they beheld; the moon's resplendent globe, and starry pole.--iv. - . the numerous extracts contained in this volume impress upon one's mind how largely astronomy enters into the composition of 'paradise lost,' and of how much assistance the knowledge of this science was to milton in the elaboration of his poem. indeed, it would be hard to imagine how such a work could have been written except by a poet who possessed a proficient and comprehensive knowledge of astronomy. the chief characteristic of milton's poetry is its sublimity, which is the natural outcome of the magnificence of his conceptions and of his own pure imaginative genius. among all the fields of literature, science, and philosophy explored by him, he found none more congenial to his tastes, or that afforded his imagination more freedom for its loftiest flights, than the sublimest of sciences--astronomy. whether we admire most the accuracy of his astronomical knowledge, or the wonderful creations of his poetic fancy, or his beautiful descriptions of the celestial orbs, it is apparent that in this domain of science, as a poet, he stands alone and without a rival. in his choice of the ptolemaic cosmology milton adopted a system with which he had been familiar from his youth--the same which his favourite poet dante introduced into his poem, 'the divina commedia,' and which was well adapted for poetic description. the picturesque conception of ten revolving spheres, carrying along with them the orbs assigned to each, which, by their revolution round the steadfast earth, brought about with unfailing regularity the successive alternation of day and night, and in every twenty-four hours exhibited the pleasing vicissitudes of dawn, of sunshine, of twilight, and of darkness, relieved by the soft effulgence of the nocturnal sky, afforded milton a favourable basis upon which to construct a cosmical epic. the copernican theory--with which he was equally conversant, and in the accuracy and truthfulness of which he believed--though less complicated than the ptolemaic in its details, did not possess the same attractiveness for poetic description that belonged to the older system. according to this theory there is, surrounding us on all sides, a boundless uncircumscribed ocean of space, to which it is impossible to assign any conceivable limit; in every effort to comprehend its dimensions or fathom its depths, the mind recoils upon itself, baffled and discomfited, with a conscious feeling that there can be no nearer approach to the end when end there is none that can be conceived of. interspersed throughout the regions of this azure vast of space is the stellar universe, which to our comprehension is as infinite as the abyss in which it exists. the solar system, though of magnificent dimensions, is but a unit in the astronomical whole, in which are embraced millions of other similar units--other solar systems, perhaps differing in construction from that of ours, with billions of miles of interstellar space intervening between each; yet so vast are the dimensions of the celestial sphere that those distances when measured upon it sink into utter insignificance. as the receding depths of space are penetrated by powerful telescopes, they are found to be pervaded with stars and starry archipelagoes, distributed in profusion over the circular immensity and extending away into abysmal depths, beyond the reach of visibility by any optical means which we possess. to the universe there is no known end--nowhere in imagination can its boundary be reached! this bewildering conception of the cosmos did not trouble the minds of pre-copernican thinkers. they regarded the steadfast earth as the most important body in the universe; nor were the celestial orbs which circled round it believed to be very far distant. tycho brahé imagined that the stars were not much more remote than the planets. epicurus thought the stars were small crystal mirrors in the sky which reflected the solar rays, and the venerable bede remarked that they needed assistance from the sun's light in order to render them more luminous. the adoption of the ptolemaic system by milton afforded greater scope for the exercise of his imaginative powers, and enabled him to bring within the mental grasp of his readers a conception of the universe which was not lost in the immensity associated with the copernican view of things. besides, it also furnished him with a distinctly defined basis upon which to erect the superstructure of his poem. above the circumscribed universe was heaven or the empyrean; underneath it was chaos, from which it had been reclaimed, and in the lowest depth of which milton located the infernal world called hell. these four regions embraced universal space; and in the elaboration of his great epic milton relied upon his imaginative genius, his brilliant scholarship, his vast erudition, and the divine inspiration of the heavenly muse. with these, aided by the power and vigour of his intellect, he was enabled to produce a cosmical epic that surpassed all previous efforts of a similar kind, and which still remains without a parallel. one of the distinguishing features of milton's mind was his wonderful imagination, and in its exercise he beheld those sublime celestial and terrestrial visions on which he reared fabrics of splendour and beauty, described in harmonious numbers with the fervid eloquence and charm of a true poet. an example of the loftiness and originality of his imagination is afforded us in his description of the creation, the main facts of which he derived from the first two chapters of genesis, and upon these he elaborated in full and striking detail his magnificent conception of the efforts of divine might, which in six successive creative acts called into existence the universe and all that it contains. the rising of the earth out of chaos; the creation of light and of the orbs of the firmament; the joyfulness associated with the onward career of the new-born sun; the subdued illumination of the full-orbed moon, and the thousand thousand stars that spangle the nocturnal sky--all these afforded milton a rich field in which his imagination luxuriated, and in the description of which he found subject-matter worthy of his gifted intellect. milton gives an ampler and more detailed description of the new universe in his narration of satan's journey through space in search of this world, and brings more vividly before the imagination of his readers the glories of the celestial regions. the fiend, having emerged from the dark abyss of chaos into a region of light, first beheld the new creation from such a distance that to his view it appeared as a star suspended by a golden chain from the empyrean. this stellar conception of the poet's harmonised with the views of the ptolemaists, who believed that the universe was of limited extent, and though its dimensions were vast beyond comprehension, it was, nevertheless, enclosed by the tenth sphere or primum mobile. it was on the surface of this sphere that satan alighted, and over which he wandered, until attracted by a beam of light that appeared through an opening at its zenith, where, by means of a stair or ladder, communication was maintained between the new universe and heaven above. hither the undaunted fiend hied, and, standing on the lower steps of this structure, momentarily paused to gaze upon the glorious sight which burst upon his view before directing his flight down into the newly created universe. milton then describes his progress through the stellar regions, his landing in the sun and what he saw there, and the termination of his journey when he descends from the ecliptic down to the earth. in doing so the poet gives a wonderfully beautiful description of the starry universe, of the sun, moon, and earth (book iii. - ), enhanced and adorned with his own poetic imaginings derived from fable, philosophy, and science. milton makes more frequent allusion to the sun than to any of the other orbs of the firmament. this we should expect: the poet always gives the orb the precedence which is his due, and never fails, when the occasion requires it, to surround him with the 'surpassing glory' which marks his pre-eminence above all other occupants of the sky. the moon, his consort--peerless in the subdued effulgence of her borrowed light; the beautiful star of evening, hesperus; the sidereal heavens with their untold glories; the galaxy, overpowering in the magnificence of its clouds and streams of stars--all these have their beauties and charms mirrored in the pages of this remarkable poem. that the observation of the celestial orbs, their phases, and the varied phenomena which occur as a consequence of their motions, were to milton an unfailing source of enjoyment and of meditative delight, is evident from the frequency with which he alludes to them. the following lines also testify to this:-- for wonderful indeed are all his works, pleasant to know, and worthiest to be all had in remembrance always with delight! but what created mind can comprehend their number, or the wisdom infinite that brought them forth, but hid their causes deep?--iii. - . it is very pleasant, as milton says, to sit and rightly spell of every star that heaven doth show. it is also pleasant to know the astronomy of his 'paradise lost,' and to linger over the delightful and harmonious utterances associated with the sublimest of sciences, expressed in the melodious language of england's greatest epic poet. printed by spottiswoode and co., new-street square london footnotes: [ ] chambers's _handbook of astronomy_. [ ] brewster's _martyrs of science_. [ ] the transit occurred on a sunday, and the 'business of the highest importance' to which horrox alludes was his clerical duties. [ ] a fresco by the late mr. ford maddox-brown, depicting crabtree observing the transit of venus, adorns the interior of the manchester town hall. [ ] william crabtree died on august , , aged years. [ ] the constellation virgo. [ ] _life of galileo_ (library of useful knowledge). [ ] miss clerke's _system of the stars_. [ ] miss clerke's _system of the stars_. [ ] miss clerke's _system of the stars_. [ ] _ibid._ [ ] an expression in book viii. - would seem to indicate that this was inaccurate, but the lines 'and other suns perhaps with their attendant moons, thou wilt descry,' are an allusion to the planets jupiter and saturn, whose satellites had been recently discovered. [ ] mr. e. w. maunder, in _knowledge_, march . [ ] though not a celestial body, it is considered desirable to describe the earth as a member of the solar system. [ ] see diagram, chap. iii. p. . sir william herschel his life and works [illustration: sir william herschel] sir william herschel his life and works by edward s. holden united states naval observatory, washington [illustration: coelis exploratis] new york charles scribner's sons and broadway copyright, , by charles scribner's sons. press of j. j. little & co., nos. to astor place, new york. please see the end of the text for transcriber's notes preface. in the following account of the life and works of sir william herschel, i have been obliged to depend strictly upon data already in print--the _memoir_ of his sister, his own scientific writings and the memoirs and diaries of his cotemporaries. the review of his published works will, i trust, be of use. it is based upon a careful study of all his papers in the _philosophical transactions_ and elsewhere. a life of herschel which shall be satisfactory in every particular can only be written after a full examination of the materials which are preserved at the family seat in england; but as two generations have passed since his death, and as no biography yet exists which approaches to completeness, no apology seems to me to be needed for a conscientious attempt to make the best use of the scanty material which we do possess. this study will, i trust, serve to exhibit so much of his life as belongs to the whole public. his private life belongs to his family, until the time is come to let the world know more of the greatest of practical astronomers and of the inner life of one of its most profound philosophers,--of a great and ardent mind, whose achievements are and will remain the glory of england. contents. page chapter i. early years; - , chapter ii. life in bath; - , chapter iii. life at datchet, clay hall, and slough; - , chapter iv. review of the scientific labors of herschel, bibliography, index of names, life and works of william herschel. chapter i. early years; - . of the great modern philosophers, that one of whom least is known, is william herschel. we may appropriate the words which escaped him when the barren region of the sky near the body of _scorpio_ was passing slowly through the field of his great reflector, during one of his sweeps, to express our own sense of absence of light and knowledge: _hier ist wahrhaftig ein loch im himmel._ herschel prepared, about the year , a biographical memorandum, which his sister carolina placed among his papers. this has never been made public. the only thoroughly authentic sources of information in possession of the world, are a letter written by herschel himself, in answer to a pressing request for a sketch of his life, and the _memoir and correspondence of caroline herschel_ (london, ), a precious memorial not only of his life, but of one which otherwise would have remained almost unknown, and one, too, which the world could ill afford to lose. the latter, which has been ably edited by mrs. mary cornwallis herschel,[ ] is the only source of knowledge in regard to the early years of the great astronomer, and together with the all too scanty materials to be gained from a diligent search through the biography of the time, affords the data for those personal details of his life, habits, and character, which seem to complete the distinct, though partial conception of him which the student of his philosophical writings acquires. the letter referred to was published in the göttingen magazine of science and literature, iii., , shortly after the name of herschel had become familiar to every ear through his discovery of _uranus_, but while the circumstances of the discovery, and the condition of the amateur who made it, were still entirely unknown. the editor (lichtenberg) says: "herr herschel was good enough to send me, some time since, through herr magellan, copies of his dissertations on double stars, on the parallax of the fixed stars, and on a new micrometer. in the letter which conveyed to him my thanks for his gift, i requested him to note down a few facts in regard to his life, for publication in this magazine, since various accounts, more or less incorrect, had appeared in several journals. in answer, i received a very obliging letter from him and what follows is that portion of it relating to my request, which was sent me with full permission to make it public." "datchet, near windsor, _nov. , ._ "i was born in hanover, november, . my father, who was a musician, destined me to the same profession, hence i was instructed betimes in his art. that i might acquire a perfect knowledge of the theory as well as of the practice of music, i was set at an early age to study mathematics in all its branches--algebra, conic sections, infinitesimal analysis, and the rest. "the insatiable desire for knowledge thus awakened resulted next in a course of languages; i learned french, english, and latin, and steadfastly resolved henceforth to devote myself wholly to those sciences from the pursuit of which i alone looked for all my future happiness and enjoyment. i have never been either necessitated or disposed to alter this resolve. my father, whose means were limited, and who consequently could not be as liberal to his children as he would have desired, was compelled to dispose of them in one way or another at an early age; consequently in my fifteenth year i enlisted in military service, only remaining in the army, however, until i reached my nineteenth year, when i resigned and went over to england. "my familiarity with the organ, which i had carefully mastered previously, soon procured for me the position of organist in yorkshire, which i finally exchanged for a similar situation at bath in , and while here the peculiar circumstances of my post, as agreeable as it was lucrative, made it possible for me to occupy myself once more with my studies, especially with mathematics. when, in the course of time, i took up astronomy, i determined to accept nothing on faith, but to see with my own eyes everything which others had seen before me. having already some knowledge of the science of optics, i resolved to manufacture my own telescopes, and after many continuous, determined trials, i finally succeeded in completing a so-called newtonian instrument, seven feet in length. from this i advanced to one of ten feet, and at last to one of twenty, for i had fully made up my mind to carry on the improvement of my telescopes as far as it could possibly be done. when i had carefully and thoroughly perfected the great instrument in all its parts, i made systematic use of it in my observations of the heavens, first forming a determination never to pass by any, the smallest, portion of them without due investigation. this habit, persisted in, led to the discovery of the new planet (_georgium sidus_). this was by no means the result of chance, but a simple consequence of the position of the planet on that particular evening, since it occupied precisely that spot in the heavens which came in the order of the minute observations that i had previously mapped out for myself. had i not seen it just when i did, i must inevitably have come upon it soon after, since my telescope was so perfect that i was able to distinguish it from a fixed star in the first minute of observation. "now to bring this sketch to a close. as the king had expressed a desire to see my telescope, i took it by his command to greenwich, where it was compared with the instruments of my excellent friend, dr. maskelyne, not only by himself, but by other experts, who pronounced it as their opinion that my instrument was superior to all the rest. thereupon the king ordered that the instrument be brought to windsor, and since it there met with marked approval, his majesty graciously awarded me a yearly pension, that i might be enabled to relinquish my profession of music, and devote my whole time to astronomy and the improvement of the telescope. gratitude, as well as other considerations specified by me in a paper presented to the royal society, of which i am a member, has induced me to call the new planet _georgium sidus_. "'georgium sidus.--jam nunc assuesce vocari.'--(_virgil._) and i hope it will retain the name." we know but little of the family of herschel. the name is undoubtedly jewish, and is found in poland, germany, and england. we learn that the ancestors of the present branch left moravia about the beginning of the xviith century, on account of their change of religion to protestantism. they became possessors of land in saxony. hans herschel, the great-grandfather of william, was a brewer in pirna (a small town near dresden). of the two sons of hans, one, abraham (born in , died ), was employed in the royal gardens at dresden, and seems to have been a man of taste and skill in his calling. of his eldest son, eusebius, there appears to be little trace in the records of the family. the second son, benjamin, died in infancy; the third, isaac, was born in (jan. ), and was thus an orphan at eleven years of age. isaac was the father of the great astronomer. he appears to have early had a passionate fondness for music, and this, added to a distaste for his father's calling, determined his career. he was taught music by an oboe-player in the royal band, and he also learned the violin. at the age of twenty-one he studied music for a year under the cappelmeister pabrich, at potsdam, and in august, , he became oboist in the band of the guards, at hanover. in august, , he married anna ilse moritzen. she appears to have been a careful and busy wife and mother, possessed of no special faculties which would lead us to attribute to her care any great part of the abilities of her son. she could not herself write the letters which she sent to her husband during his absences with his regiment. it was her firm belief that the separations and some of the sorrows of the family came from too much learning; and while she could not hinder the education of the sons of the family, she prevented their sisters from learning french and dancing. it is but just to say that the useful accomplishments of cooking, sewing, and the care of a household, were thoroughly taught by her to her two daughters. the father, isaac, appears to have been of a different mould, and to him, no doubt, the chief intellectual characteristics of the family are due. his position obliged him to be often absent from hanover, with his regiment, but his hand appears to have been always present, smoothing over difficulties, and encouraging his sons to such learning and improvement as was to be had. his health was seriously injured by the exposures of the campaigns, and he was left, after the seven years' war, with a broken constitution. after his final return home, in , his daughter gives this record of him-- "copying music employed every vacant moment, even sometimes throughout half the night. . . . with my brother [dietrich]--now a little engaging creature of between four and five years old--he was very much pleased, and [on the first evening of his arrival at home] before he went to rest, the adempken (a little violin) was taken from the shelf and newly strung, and the daily lessons immediately commenced. . . . i do not recollect that he ever desired any other society than what he had opportunities of enjoying in many of the parties where he was introduced by his profession, though far from being of a morose disposition; he would frequently encourage my mother in keeping up a social intercourse among a few acquaintances, whilst his afternoon hours generally were taken up in giving lessons to some scholars at home, who gladly saved him the troublesome exertion of walking. . . . he also found great pleasure in seeing dietrich's improvement, who, young as he was, and of the most lively temper imaginable, was always ready to receive his lessons, leaving his little companions with the greatest cheerfulness to go to his father, who was so pleased with his performances that he made him play a solo on the adempken in rake's concert, being placed on a table before a crowded company, for which he was very much applauded and caressed, particularly by an english lady, who put a gold coin in his little pocket. "it was not long before my father had as many scholars as he could find time to attend. and when they assembled at my father's to make little concerts, i was frequently called to join the second violin in an overture, for my father found pleasure in giving me sometimes a lesson before the instruments were laid by, after practising with dietrich, for i never was missing at those hours, sitting in a corner with my knitting and listening all the while." here, as in all her writing, carolina is simple, true, direct to awkwardness, and unconsciously pathetic even in joy. the family of isaac and anna herschel consisted of ten children. six of these lived to adult age. they were: . sophia elizabeth; born , married griesbach, a musician in the guard, by whom she had children. five of her sons were afterwards musicians at the court, in england, where they obtained places through the influence of william. . henry anton jacob; born , november . . frederic william (the astronomer) born , november . . john alexander; born , november . . carolina lucretia; born , march . . dietrich; born , september . of this family group, the important figures to us are william, alexander, and carolina. jacob was organist at the garrison church of hanover in , a member of the guards' band in , and first violin in the hanover court orchestra in . afterwards he joined his brother william in bath, but again returned to hanover. in he published in amsterdam his opus i., a set of six quartettes, and later, in london, he published two symphonies and six trios. he appears to have been a clever musician, and his letters to his younger brother william are full of discussion on points of musical composition, etc. he died in . dietrich, the youngest brother, shared in the musical abilities of his family, and when only fifteen years old was so far advanced as to be able to supply his brother jacob's place in the court orchestra, and to give his lessons to private pupils. there is no one of the family, except the eldest daughter, whom we do not know to have possessed marked ability in music, and this taste descended truly for four generations. in the letters of chevalier bunsen,[ ] he describes meeting, in , the eldest granddaughter of william herschel, who, he says, "is a musical genius." three members of the family, william, alexander, and carolina, formed a group which was inseparable for many years, and while the progress of the lives of alexander and carolina was determined by the energy and efforts of william, these two lent him an aid without which his career would have been strangely different. it is necessary to understand a little better the early life of all three. the sons of the herschel family all attended the garrison school in hanover until they were about fourteen years old. they were taught the ordinary rudiments of knowledge--to read, to write, to cipher--and a knowledge of french and english was added. william especially distinguished himself in his studies, learning french very rapidly, and studying latin and arithmetic with his master out of hours. the household life seems to have been active, harmonious, and intelligent, especially during the presence of the father, who took a great delight in the rapid progress of all his sons in music, and who encouraged them with his companionship in their studies and in their reading on all intellectual subjects. from the _memoir_ of carolina, on which we must depend for our knowledge of this early life, we take the following paragraph: "my brothers were often introduced as solo performers and assistants in the orchestra of the court, and i remember that i was frequently prevented from going to sleep by the lively criticism on music on coming from a concert, or by conversations on philosophical subjects, which lasted frequently till morning, in which my father was a lively partaker and assistant of my brother william, by contriving self-made instruments. . . . often i would keep myself awake that i might listen to their animating remarks, for it made _me so happy_ to see _them so happy_. but generally their conversation would branch out on philosophical subjects, when my brother william and my father often argued with such warmth that my mother's interference became necessary, when the names leibnitz, newton, and euler sounded rather too loud for the repose of her little ones, who ought to be in school by seven in the morning. but it seems that on the brothers retiring to their own room, where they shared the same bed, my brother william had still a great deal to say; and frequently it happened that when he stopped for an assent or reply, he found his hearer was gone to sleep, and i suppose it was not till then that he bethought himself to do the same. "the recollection of these happy scenes confirms me in the belief, that had my brother william not then been interrupted in his philosophical pursuits, we should have had much earlier proofs of his inventive genius. my father was a great admirer of astronomy, and had some knowledge of that science; for i remember his taking me, on a clear frosty night, into the street, to make me acquainted with several of the most beautiful constellations, after we had been gazing at a comet which was then visible. and i well remember with what delight he used to assist my brother william in his various contrivances in the pursuit of his philosophical studies, among which was a neatly turned -inch globe, upon which the equator and ecliptic were engraved by my brother." the mechanical genius was not confined to william, for we read that alexander used often to "sit by us and amuse us and himself by making all sorts of things out of pasteboard, or contriving how to make a twelve-hour cuckoo clock go a week." this ability of alexander's was turned later to the best account when he became his brother william's right hand in the manufacture of reflectors, eye-pieces, and stands in england. his abilities were great, and a purpose which might otherwise have been lacking was supplied through the younger brother's ardor in all that he undertook. his musical talent was remarkable; he played "divinely" on the violoncello. he returned to hanover in , where he lived in comfortable independence, through the never-failing generosity of his brother, until his death in . a notice of him in a bristol paper says: "died, march , , at hanover, alexander herschel, esqr., well known to the public of bath and bristol as a performer and elegant musician; and who for forty-seven years was the admiration of the frequenters of concerts and theatres of both those cities as principal violoncello. to the extraordinary merits of mr. herschel was united considerable acquirement in the superior branches of mechanics and philosophy, and his affinity to his brother, sir william herschel, was not less in science than in blood." we shall learn more of the sister, carolina, as time goes on. now in these early years she was a silent and persistent child, growing up with a feeling that she was uncared for and neglected, and lavishing all her childish affection, as she did all that of her womanly life, on her brother william. throughout her long life, "my brother" was william, "my nephew" _his_ son. the brothers jacob and william were, with their father, members of the band of the guards in , when the regiment was ordered to england, and they were absent from hanover a year. william (then seventeen years old) went as oboist, and out of his scanty pay brought back to hanover, in , only one memento of his stay--a copy of locke _on the human understanding_. he appears to have served with the guard during part of the campaign of . his health was then delicate, and his parents "determined to remove him from the service--a step attended by no small difficulties."[ ] this "removal" was hurriedly and safely effected, so hurriedly that the copy of locke was not put in the parcels sent after him to hamburg by his mother; "she, dear woman, knew no other wants than good linen and clothing." thus, at last, the young william herschel, the son of an oboe-player in the king's guard, is launched in life for himself, in the year , at the age of nineteen. all his equipment is the "good linen and clothing," a knowledge of french, latin, and english, some skill in playing the violin, the organ, and the oboe, and an "uncommon precipitancy" in doing what there is to be done. a slender outfit truly; but we are not to overlook what he said of himself on another occasion. "i have, nevertheless, several resources in view, and do not despair of succeeding pretty well in the end." from to --three years--we know nothing of his life. we can imagine what it was. his previous visit to england had given him a good knowledge of the language, and perhaps a few uninfluential acquaintances. on his return he would naturally seek these out, and, by means of his music, he could gain a livelihood. we first hear of him as charged with the organization of the music of a corps of the militia of durham, under the auspices of the earl of darlington. "la manière dont il remplit cette mission, le fit connaître avantageusement."[ ] the nature of the service of these militia corps, which were then forming all over england, is well described in the autobiography of gibbon. every county-gentleman felt constrained to serve his country, and the regimental mess-rooms were filled with men of rank and fashion. in we hear of him again. he has attracted the notice of those about him. "about the year , as miller[ ] was dining at pontefract with the officers of the durham militia, one of them, knowing his love of music, told him they had a young german in their band as a performer on the hautboy, who had only been a few months in england, and yet spoke english almost as well as a native, and who was also an excellent performer on the violin; the officer added that if miller would come into another room, this german should entertain him with a solo. the invitation was gladly accepted, and miller heard a solo of giardini's executed in a manner that surprised him. he afterwards took an opportunity of having some private conversation with the young musician, and asked him whether he had engaged himself for any long period to the durham militia. the answer was, 'only from month to month.' 'leave them, then,' said the organist, 'and come and live with me. i am a single man, and think we shall be happy together; and, doubtless, your merit will soon entitle you to a more eligible situation.' the offer was accepted as frankly as it was made, and the reader may imagine with what satisfaction dr. miller must have remembered this act of generous feeling when he hears that this young german was herschel, the astronomer. 'my humble mansion,' says miller, 'consisted, at that time, but of two rooms. however, poor as i was, my cottage contained a library of well-chosen books; and it must appear singular that a foreigner who had been so short a time in england should understand even the peculiarities of the language so well as to fix upon swift for his favorite author.' "he took an early opportunity of introducing his new friend at mr. cropley's concerts; the first violin was resigned to him; 'and never,' says the organist, 'had i heard the concertos of corelli, geminiani, and avison, or the overtures of handel performed more chastely, or more according to the original intention of the composers, than by mr. herschel. i soon lost my companion; his fame was presently spread abroad; he had the offer of pupils, and was solicited to lead the public concerts both at wakefield and halifax. a new organ for the parish church of halifax was built about this time, and herschel was one of the seven candidates for the organist's place. they drew lots how they were to perform in succession. herschel drew the third, the second fell to dr. wainwright of manchester, whose finger was so rapid that old snetzler, the organ-builder, ran about the church exclaiming: '_te tevel! te tevel! he run over te keys like one cat; he will not give my piphes room for to shpeak._' 'during mr. wainwright's performance,' says miller, 'i was standing in the middle aisle with herschel. 'what chance have you,' said i, 'to follow this man?' he replied, 'i don't know; i am sure fingers will not do.' on which he ascended the organ loft, and produced from the organ so uncommon a fulness, such a volume of slow, solemn harmony, that i could by no means account for the effect. after this short _ex tempore_ effusion, he finished with the old hundredth psalm-tune, which he played better than his opponent. "'_ay, ay_,' cried old snetzler, '_tish is very goot, very goot indeet; i vil luf tish man, for he gives my piphes room for to shpeak._' having afterwards asked mr. herschel by what means, in the beginning of his performance, he produced so uncommon an effect, he replied, 'i told you fingers would not do!' and producing two pieces of lead from his waistcoat pocket, 'one of these,' said he, 'i placed on the lowest key of the organ, and the other upon the octave above; thus by accommodating the harmony, i produced the effect of four hands, instead of two.'"[ ] the dates in this extract are not so well defined as might be wished. herschel had certainly been more than a few months in england at the time of his meeting with dr. miller, which was probably about . the appointment as organist at halifax was in , and the pupils and public concerts must have filled up the intervening five years. during a part of this time he lived in leeds, with the family of mr. bulman, whom he afterwards provided with a place as clerk to the octagon chapel, in his usual generous manner. all during his life he was placing some of the less fortunate and energetic members of his family. we cannot be too grateful to dr. miller, who, seeing his opportunity, used it. their frank friendship does honor to both. herschel's organ-playing, which no doubt had been begun when his brother was the organist of the garrison chapel at hanover, must have been perfected at this time, and it was through his organ-playing that he was able to leave the needy life in yorkshire. he was sure to have emerged sooner or later, but every year spared to him as a struggling musician was a year saved to astronomy. during all this period, a constant correspondence was maintained between the family at hanover and the absent son. many of william's letters were written in english, and addressed to his brother jacob, and treated of such subjects as the theory of music, in which he was already far advanced. his little sister was still faithful to the memory of her _dearest_ brother, and his father, whose health was steadily declining, became painfully eager for his return. in (april ), he returned to hanover on a very brief visit. he was attached to england, he was prospering there, and he had no inclination towards returning to a life in hanover. his sister says: "of the joys and pleasures which all felt at this long-wished-for meeting with my--let me say my _dearest_--brother, but a small portion could fall to my share; for with my constant attendance at church and school, besides the time i was employed in doing the drudgery of the scullery, it was but seldom i could make one in the group when the family were assembled together. "in the first week, some of the orchestra were invited to a concert, at which some of my brother william's compositions, overtures, etc., and some of my eldest brother jacob's were performed, to the great delight of my dear father, who hoped and expected that they would be turned to some profit by publishing them, but there was no printer who bid high enough. "sunday, the th, was the--to me--eventful day of my confirmation, and i left home not a little proud and encouraged by my dear brother william's approbation of my appearance in my new gown." the engagement of herschel at halifax did not long continue. in he obtained an advantageous engagement as oboist at bath, and soon after the position of organist at the octagon chapel was offered to him and accepted. this was a great and important change. bath was then, as now, one of the most beautiful cities in england, and the resort of the fashion and rank of the kingdom, who came to take the waters. it is beautifully situated on both sides of the avon, and has many fine walks and public buildings. the aspect of the city is markedly cheerful and brilliant, owing to the nature of the white stone of which the principal houses are built, and to the exquisite amphitheatre of hills in which they lie. the society was then gay and polite, and herschel was at once thrown into a far more intelligent atmosphere than that he had just left in yorkshire. it was easy to get new books, to see new faces, to hear new things. the assembly rooms (built in ) were noted for their size and elegance; the theatre was the best out of london. his position as organist of the fashionable chapel placed him in the current. his charming and engaging manners made him friends. his talents brought him admirers and pupils, and pupils brought him money.[ ] he began in a life of unceasing activity, which continued. in he published in london a symphony (in c) for two violins, viola, bass, two oboes, and two horns, and in the same year two military concertos for two oboes, two horns, two trumpets, and two bassoons.[ ] he wrote pieces for the harp, glees, "catches," and other songs for the voice. one of these, the _echo catch_, was published and had even considerable vogue. a competent musical critic writes to me of this work: "the counterpoint is clear and flowing, and is managed with considerable taste and effect. it would be difficult to explain the great cleverness shown in the construction of the _catch_ without diagrams to illustrate the movements of the parts. it is certainly an ingenious bit of musical writing." when he left bath (in ), many of these musical writings were lost, in his great haste to take up his new profession. one, specially, his sister remembers to have written out for the printer, "but he could not find a moment to send it off, nor answer the printer's letters." this was a four-part song, "in thee i bear so dear a part." he wrote very many anthems, chants, and psalm-tunes for the excellent cathedral choir of the octagon chapel. unfortunately, most of this music is now not to be found. a notice of herschel's life which appeared in the _european magazine_ for , january, gives a very lively picture of his life at this time, and it is especially valuable as showing how he appeared to his cotemporaries. "although mr. herschel loved music to an excess, and made a considerable progress in it, he yet determined with a sort of enthusiasm to devote every moment he could spare from business to the pursuit of knowledge, which he regarded as the sovereign good, and in which he resolved to place all his views of future happiness in life.". . . "his situation at the octagon chapel proved a very profitable one, as he soon fell into all the public business of the concerts, the rooms, the theatre, and the oratorios, besides many scholars and private concerts. this great run of business, instead of lessening his propensity to study, increased it, so that many times, after a fatiguing day of fourteen or sixteen hours spent in his vocation, he would retire at night with the greatest avidity to _unbend the mind_, if it may be so called, with a few propositions in maclaurin's _fluxions_, or other books of that sort." it was in these years that he mastered italian and made some progress in greek. "we may hazard a natural conjecture respecting the course of herschel's early studies. music conducted him to mathematics, or, in other words, impelled him to study smith's _harmonics_. now this robert smith was the author of _a complete system of optics_, a masterly work, which, notwithstanding the rapid growth of that branch of the science, is not yet wholly superseded. it seems to us not unlikely that herschel, studying the _harmonics_, conceived a reverence for the author, who was at that time still living, so that from the _philosophy of music_ he passed to the _optics_, a work on which smith's great reputation chiefly rested; and thus undesignedly prepared himself for the career on which he was shortly about to enter with so much glory."[ ] there is no doubt that this conjecture is a true one. the _optics_ of dr. smith is one of the very few books quoted by herschel throughout his writings, and there is every evidence of his complete familiarity with its conclusions and methods; and this familiarity is of the kind which a student acquires with his early text-books. one other work he quotes in the same way, lalande's _astronomy_, and this too must have been deeply studied. during the years - , while herschel was following his profession and his studies at bath, the family life at hanover went on in much the same way. in his father isaac had a stroke of paralysis, which ended his violin-playing forever, and forced him to depend entirely upon pupils and copying of music for a livelihood. he died on march , , leaving behind him a good name, and living in the affectionate remembrance of his children and of all who knew him. carolina had now lost her best friend, and transferred to her brother william the affection she had before divided between him and her father. "my father wished to give me something like a polished education, but my mother was particularly determined that it should be a rough, but at the same time a useful one; and nothing farther she thought was necessary but to send me two or three months to a sempstress to be taught to make household linen. . . . my mother would not consent to my being taught french, and my brother dietrich was even denied a dancing-master, because she would not permit my learning along with him, though the entrance had been paid for us both; so all my father could do for me was to indulge me (and please himself) sometimes with a short lesson on the violin, when my mother was either in good humor or out of the way. though i have often felt myself exceedingly at a loss for the want of those few accomplishments of which i was thus, by an erroneous though well-meant opinion of my mother, deprived, i could not help thinking but that she had cause for wishing me not to know more than was necessary for being useful in the family; for it was her certain belief that my brother william would have returned to his country, and my eldest brother not have looked so high, if they had had a little less learning. * * * * * but sometimes i found it scarcely possible to get through with the work required, and felt very unhappy that no time at all was left for improving myself in music or fancy work, in which i had an opportunity of receiving some instruction from an ingenious young woman whose parents lived in the same house with us. but the time wanted for spending a few hours together could only be obtained by our meeting at daybreak, because by the time of the family's rising at seven, i was obliged to be at my daily business. though i had neither time nor means for producing anything immediately either for show or use, i was content with keeping samples of all possible patterns in needlework, beads, bugles, horse-hair, etc., for i could not help feeling troubled sometimes about my future destiny; yet i could not bear the idea of being turned into an abigail or housemaid, and thought that with the above and such like acquirements, with a little notion of music, i might obtain a place as governess in some family where the want of a knowledge of french would be no objection." a change was soon to come in her life too; her brother william wrote to propose that she should join him at bath-- . . . "to make the trial, if, by his instruction, i might not become a useful singer for his winter concerts and oratorios; he advised my brother jacob to give me some lessons by way of beginning; but that if, after a trial of two years, we should not find it answer our expectation, he would bring me back again. this at first seemed to be agreeable to all parties, but by the time i had set my heart upon this change in my situation, jacob began to turn the whole scheme into ridicule, and, of course, he never heard the sound of my voice except in speaking, and yet i was left in the harassing uncertainty whether i was to go or not. i resolved at last to prepare, as far as lay in my power, for both cases, by taking, in the first place, every opportunity, when all were from home, to imitate, with a gag between my teeth, the solo parts of concertos, _shake and all_, such as i had heard them play on the violin; in consequence i had gained a tolerable execution before i knew how to sing. i next began to knit ruffles, which were intended for my brother william, in case i remained at home--else they were to be jacob's. for my mother and brother d. i knitted as many cotton stockings as would last two years at least." in august, , her brother arrived at hanover, to take her back to england with him. the journey to london was made between august th and th, and soon after they went together to herschel's house, no. new king's street, bath. footnotes: [ ] wife of major john herschel, of the royal engineers, grandson of sir william. [ ] page . [ ] _memoir_ of carolina herschel, p. . sir george airy, astronomer royal, relates in the _academy_ that this "removal" was a desertion, as he was told by the duke of sussex that on the first visit of herschel to the king, after the discovery of the _georgium sidus_, the pardon of herschel was handed to him by the king himself, written out in due form. [ ] fÉtis; _biographie universelle des musiciens_, tome v. ( ) p. . [ ] dr. miller, a noted organist, and afterwards historian of doncaster. [ ] _the doctor_; by robert southey, edition of , p. . [ ] he frequently gave thirty-five and thirty-eight lessons a week to pupils at this time. [ ] according to fÉtis. a search for these in london has led me to the belief that fÉtis, who is usually very accurate, is here mistaken, and that these writings are by jacob herschel. [ ] _foreign quarterly review_, volume . chapter ii. life in bath; - . it was to a busy life in bath that herschel took his sister carolina, then twenty-two years old. she was a perfectly untried girl, of very small accomplishments and outwardly with but little to attract. the basis of her character was the possibility of an unchanging devotion to one object; for the best years of her life this object was the happiness and success of her brother william, whom she profoundly loved. her love was headstrong and full of a kind of obstinate pride, which refused to see anything but the view she had adopted. as long as her life continued to be with her dearest brother, all was well with her. she had a noble aim, and her heart was more than full. later on, this very singleness of character brought her other years of wretchedness. it is necessary to understand the almost spaniel-like allegiance she gave, in order to comprehend the value which her services were to herschel. she supplied him with an aid which was utterly loyal, entire, and devoted. her obedience was unquestioning, her reverence amounted almost to adoration. in their relation, he gave everything in the way of incentive and initiative, and she returned her entire effort loyally. at first her business was to gain a knowledge of the language, and to perfect herself in singing, so that she might become a soloist in the concerts and oratorios which he was constantly giving. in the beginning it was not easy. . . . "as the season for the arrival of visitors to the baths does not begin till october, my brother had leisure to try my capacity for becoming a useful singer for his concerts and oratorios, and being very well satisfied with my voice, i had two or three lessons every day, and the hours which were not spent at the harpsichord, were employed in putting me in the way of managing the family. . . . on the second morning, on meeting my brother at breakfast, he began immediately to give me a lesson in english and arithmetic, and showed me the way of booking and keeping accounts of cash received and laid out. . . . by way of relaxation we talked of astronomy and the bright constellations with which i had made acquaintance during the fine nights we spent on the postwagen travelling through holland. "my brother alexander, who had been some time in england, boarded and lodged with his elder brother, and, with myself, occupied the attic. the first floor, which was furnished in the newest and most handsome style, my brother kept for himself. the front room, containing the harpsichord, was always in order to receive his musical friends and scholars at little private concerts or rehearsals. . . . sundays i received a sum for the weekly expenses, of which my housekeeping book (written in english) showed the amount laid out, and my purse the remaining cash. one of the principal things required was to market, and about six weeks after coming to england i was sent alone among fishwomen, butchers, basket-women, etc., and i brought home whatever in my fright i could pick up. . . . my brother alex., who was now returned from his summer engagement, used to watch me at a distance, unknown to me, till he saw me safe on my way home. but all attempts to introduce any order in our little household proved vain, owing to the servant my brother then had. and what still further increased my difficulty was, that my brother's time was entirely taken up with business, so that i only saw him at meals. breakfast was at seven o'clock or before--much too early for me, who would rather have remained up all night than be obliged to rise at so early an hour. . . . "the three winter months passed on very heavily. i had to struggle against _heimwehe_ (home sickness) and low spirits, and to answer my sister's melancholy letters on the death of her husband, by which she became a widow with six children. i knew too little english to derive any consolation from the society of those who were about me, so that, dinner-time excepted, i was entirely left to myself." so the winter passed. "the time when i could hope to receive a little more of my brother's instruction and attention was now drawing near; for after easter, bath becomes very empty, only a few of his scholars, whose families were resident in the neighborhood, remaining. but i was greatly disappointed; for, in consequence of the harassing and fatiguing life he had led during the winter months, he used to retire to bed with a basin of milk or glass of water, and smith's _harmonics_ and _optics_, ferguson's _astronomy_, etc., and so went to sleep buried under his favorite authors; and his first thoughts on rising were how to obtain instruments for viewing those objects himself of which he had been reading. there being in one of the shops a two-and-a-half-foot gregorian telescope to be let, it was for some time taken in requisition, and served not only for viewing the heavens, but for making experiments on its construction. . . . it soon appeared that my brother was not contented with knowing what former observers had seen, for he began to contrive a telescope eighteen or twenty feet long (i believe after huyghens' description). . . . i was much hindered in my musical practice by my help being continually wanted in the execution of the various contrivances, and i had to amuse myself with making the tube of pasteboard for the glasses, which were to arrive from london, for at that time no optician had settled at bath. but when all was finished, no one besides my brother could get a glimpse of jupiter or saturn, for the great length of the tube would not allow it to be kept in a straight line. this difficulty, however, was soon removed by substituting tin tubes. . . . my brother wrote to inquire the price of a reflecting mirror for (i believe) a five or six foot telescope. the answer was, there were none of so large a size, but a person offered to make one at a price much above what my brother thought proper to give. . . . about this time he bought of a quaker, resident at bath, who had formerly made attempts at polishing mirrors, all his rubbish of patterns, tools, hones, polishers, unfinished mirrors, etc., but all for small gregorians, and none above two or three inches diameter. "but nothing serious could be attempted, for want of time, till the beginning of june, when some of my brother's scholars were leaving bath; and then, to my sorrow, i saw almost every room turned into a workshop. a cabinet-maker making a tube and stands of all descriptions in a handsomely furnished drawing-room; alex. putting up a huge turning machine (which he had brought in the autumn from bristol, where he used to spend the summer) in a bedroom, for turning patterns, grinding glasses, and turning eye-pieces, etc. at the same time music durst not lie entirely dormant during the summer, and my brother had frequent rehearsals at home, where miss farinelli, an italian singer, was met by several of the principal performers he had engaged for the winter concerts." finally, in , he had made himself a gregorian telescope,[ ] and had begun to view the heavens. he was then thirty-six years old. the writer in the _european magazine_ describes this period: "all this time he continued his astronomical observations, and nothing now seemed wanting to complete his felicity, but sufficient leisure to enjoy his telescopes, to which he was so much attached, that at the theatre he used frequently to run from the harpsichord to look at the stars, during the time between the acts." in an extract from his _journal no. _, now at the rooms of the royal society, may be seen a copy of his first observation of the nebula of _orion_, on march , . this was made with his five-and-a-half-foot gregorian reflector. it was at this time ( ), between the acts of the theatre, that he made his first review of the heavens, with a newtonian telescope, of an aperture of four and a half inches and a magnifying power of times. this telescope was one of the first made by himself. the review consisted of the examination of every star in the sky of the first, second, third, and fourth magnitudes, and of all planets visible. there are no records of these observations now extant, and they are noteworthy only as a preparation for more serious work. he was carrying out his resolve to see everything for himself. his assiduity may be judged of by the fact that between and herschel had observed a single object--the nebula of _orion_--no less than fourteen times. the success of his first telescopes incited him to new efforts. his house became a complete _atelier_, where everything that could tend to excellence in this manufacture was tried and re-tried a hundred different ways. when a difficulty arose, experiments were begun which continued till it was conquered. when a success was gained, it was prosecuted to the utmost. in the first seven-foot reflector was made, in a ten-foot was finished, in a "very good" ten-foot took its place. it must not be thought that the telescopes mentioned were the only ones completed. on the contrary, they were but the best ones selected out of many. in a new house had been engaged, which had "more room for workshops," and whose roof gave space for observing. the grass-plat near it was soon utilized to hold the stand of a twenty-foot telescope, which he had even then projected. his projects were unending, no success was final; his mind was at the height of activity; his whole effort was thrown into every undertaking. the mirrors for all these telescopes were made by hand. every portion of the grinding down to rough dimensions, the shaping to something near the correct form, the polishing till the accurately exact curves were obtained, all this must be done by hand. the machines for the purpose were not invented until .[ ] alexander and william worked together at this, but most of the work was done by the latter. the sister's part was to attend in the workshop and lend a hand wherever and whenever it was needed. . . . "my time was taken up with copying music and practising, besides attendance on my brother when polishing, since by way of keeping him alive i was constantly obliged to feed him by putting the victuals by bits into his mouth. this was once the case when, in order to finish a seven-foot mirror, he had not taken his hands from it for sixteen hours together. in general he was never unemployed at meals, but was always at those times contriving or making drawings of whatever came in his mind. generally i was obliged to read to him whilst he was at the turning-lathe, or polishing mirrors, _don quixote_, _arabian nights' entertainment_, the novels of sterne, fielding, etc.; serving tea and supper without interrupting the work with which he was engaged, . . . and sometimes lending a hand. i became, in time, as useful a member of the workshop as a boy might be to his master in the first year of his apprenticeship. . . . but as i was to take a part the next year in the oratorios, i had, for a whole twelvemonth, two lessons per week from miss fleming, the celebrated dancing-mistress, to drill me for a gentlewoman (god knows how she succeeded). so we lived on without interruption. my brother alex. was absent from bath for some months every summer, but when at home he took much pleasure in executing some turning or clockmaker's work for his brother." news from hanover put a sudden stop, for a time, to all these labors. the mother wrote, in the utmost distress, to say that dietrich had disappeared from his home, it was supposed with the intention of going to india "with a young idler not older than himself." his brother immediately left the lathe at which he was turning an eye-piece in cocoa-nut, and started for holland, whence he proceeded to hanover, failing to meet his brother, as he expected. meanwhile the sister received a letter to say that dietrich was "laid up very ill" at an inn in wapping. alexander posted to town, removed him to a lodging, and, after a fortnight's nursing, brought him to bath, where, on his brother william's return, he found him being well cared for by his sister. about this time another change was made to the house new king street, which was the last move in bath. it was here that the _georgium sidus_ was discovered. the music still went on. the oratorios of the _messiah_, _judas maccabeus_, and _samson_ were to be performed under herschel's direction, with an orchestra of nearly one hundred pieces. the scores and vocal parts of these carolina copied with her own hands, and the _soprani_ were instructed by her, she being the leading soloist. along with the music went the astronomy. not only were new telescopes made, but they were made for immediate use. the variable star _mira ceti_ was observed, and a long series of lunar observations begun. "in , , and i measured the heights of about one hundred mountains of the moon, by three different methods. "some of these observations are given in _philosophical transactions_, vol. lxx., but most remain uncalculated in my journal _till some proper opportunity."[ ]_ while herschel was measuring these lunar mountains, in december, , he made by chance an acquaintance of much value to him. dr. william watson, a fellow of the royal society, distinguished for his researches in electricity, happened to see him at his telescope, and this led to a visit and an invitation to herschel to join the philosophical society of bath, then forming. this he gladly did, and it was of use to him in many ways. he there formed acquaintance with men of his own way of thinking, and he himself became known. better than all, he learned to measure himself with other men, and by his early papers read to the society, he gained skill in putting his thoughts before his hearers. this skill he never lost, and the merely literary art of his memoirs would make his papers remarkable without their other merits. he is always clear, and in his early papers especially, he appeals to his particular audience--the royal society--in a way which shows that he is conscious of all its weaknesses as well as of its dignity. later, his tone slightly changed. he became less anxious to win his audience, for he had become an authority. this knowledge lent a quiet strength to his style, but never induced the slightest arrogance of spirit or manner. the bath philosophical society has left no printed proceedings. herschel was one of its earliest members, and many papers were communicated to it by his hand. these appear to have been of a very miscellaneous nature. some of them at least would be of the highest interest to us now. in the _philosophical transactions_ for , p. , herschel tells us that he communicated to that society "certain mathematical papers" relating to central forces other than the force of gravity, which are or may be concerned in the construction of the sidereal heavens. this early idea was still entertained by herschel in , and the mathematical papers referred to must be contained in the _minutes_ of the society, which on its dissolution were torn from the minute-book and returned to the writers. the earliest published writing of herschel is the answer to the prize question in the "ladies' diary" for , proposed by the celebrated landen, namely: "the length, tension, and weight of a musical string being given, it is required to find how many vibrations it will make in a given time, when a small given weight is fastened to its middle and vibrates with it." in the _philosophical transactions_ of the royal society for , are two papers of his. the title of the first is, _astronomical observations on the periodical star in collo ceti_, by mr. william herschel, of bath. this was communicated to the society by dr. william watson, jr., and was read may , , at the same time as the other paper on the mountains of the moon. it is to be noted that herschel was at this time plain "mr. william herschel, of bath." it was only in that he became "dr. herschel," through the oxford degree of ll.d. neither of these two papers is specially remarkable on its purely astronomical side. the problems examined were such as lay open before all, and the treatment of them was such as would naturally be suggested. the second of these two contained, however, a short description of his newtonian telescope, and he speaks of it with a just pride: "i believe that for distinctness of vision this instrument is perhaps equal to any that was ever made." he was, at least, certain of having obtained excellence in the making of his instruments. in his next paper, however, read january , , a subject is approached which shows a different kind of thought. it is the first obvious proof of the truth of the statement which he made long afterwards ( ), when he said: "a knowledge of the construction of the heavens has always been the ultimate object of my observations." the title of this paper was _astronomical observations on the rotation of the planets round their axes, made with a view to determine whether the earth's diurnal motion is perfectly equable_. here the question is a difficult and a remote one, and the method adopted for its solution is perfectly suitable in principle. it marks a step onward from mere observations to philosophizing upon their results. in practical astronomy, too, we note an advance. not only are his results given, but also careful estimates of the errors to be feared in them, and a discussion of the sources of such errors. the same volume of the _philosophical transactions_ which contains this paper, also contains another, _account of a comet_, read april , . this comet was the major planet _uranus_, or, as herschel named it, _georgium sidus_. he had found it on the night of tuesday, march , . "in examining the small stars in the neighborhood of h _geminorum_, i perceived one that appeared visibly larger than the rest; being struck with its uncommon appearance, i compared it to h _geminorum_ and the small star in the quartile between _auriga_ and _gemini_, and finding it so much larger than either of them, i suspected it to be a comet." the "comet" was observed over all europe. its orbit was computed by various astronomers, and its distance from the sun was found to be nineteen times that of our earth. this was no comet, but a new major planet. the discovery of the amateur astronomer of bath was the most striking since the invention of the telescope. it had absolutely no parallel, for every other major planet had been known from time immemorial.[ ] the effect of the discoveries of galileo was felt almost more in the moral than in the scientific world. the mystic number of the planets was broken up by the introduction of four satellites to _jupiter_. that _venus_ emulated the phases of our moon, overthrew superstition and seated the copernican theory firmly. the discovery of "an innumerable multitude of fixed stars" in the milky way confounded the received ideas. this was the great mission of the telescope in galileo's hands. the epoch of mere astronomical discovery began with the detection of the large satellite of _saturn_ by huyghens, in . even then superstition was not dead. huyghens did not search for more moons, because by that discovery he had raised the number of known satellites to six,[ ] and because these, with the six planets, made "the perfect number twelve." from to cassini discovered four more moons revolving about _saturn_. since no new body had been added to the solar system. it was thought complete for nearly a century. in england, the remarkable discoveries of bradley ( - ) had been in the field of practical astronomy, and his example had set the key-note for further researches. france was just about beginning the brilliant period of her discoveries in mathematical astronomy, and had no observatory devoted to investigations like herschel's, with the possible exception of darquier's and flaugergues'. the observatories of schroeter and von hahn, in germany, were not yet active. the field which herschel was created to fill was vacant, the whole world over. it was especially so in england. the royal observatory at greenwich, under maskelyne, a skilful observer, whose work was mostly confined to meridian observations, was no rival to a private observatory like herschel's. the private observatories themselves were but small affairs; those of the king, at kew, of dr. wilson, at glasgow, of mr. aubert, at loampit hill, of the count von bruhl, in london, being perhaps the most important. the whole field was open. what was perhaps more remarkable, there was in england, during herschel's lifetime, no astronomer, public or private, whose talents, even as an observer, lay in the same direction. it hardly need be said that as a philosopher in his science, he had then no rival, as he has had none since. his only associates even, were michell and wilson.[ ] without depreciating the abilities of the astronomers of england, his cotemporaries, we may fairly say that herschel stood a great man among a group of small ones. let us endeavor to appreciate the change effected in the state of astronomy not only in england but in the whole world, simply by the discovery of _uranus_. suppose, for example, that the last planet in our system had been _saturn_. no doubt herschel would have gone on. in spite of one and another difficulty, he would have made his ten-foot, his twenty-foot telescopes. his forty-foot would never have been built, and the two satellites which he found with it might not have been discovered. certainly _mimas_ would not have been. his researches on the construction of the heavens would have been made; those were in his brain, and must have been ultimated. the mass of observations of _saturn_, of _jupiter_, of _mars_, of _venus_, would have been made and published. the researches on the sun, on the "invisible rays" of heat, on comets and nebulæ--all these might have been made, printed, and read. but these would have gone into the _philosophical transactions_ as the work of an amateur astronomer, "mr. herschel, of bath." they would have been praised, and they would have been doubted. it would have taken a whole generation to have appreciated them. they would have been severely tried, entirely on their merits, and finally they would have stood where they stand to-day--unrivalled. but through what increased labors these successes would have been gained! it is not merely that the patronage of the king, the subsidies for the forty-foot telescope (£ , ), the comparative ease of herschel's life would have been lacking. it is more than this. it would have been necessary for him to have created the audience to which he appealed, and to have conquered the most persistent of enemies--indifference. certainly, if herschel's mind had been other than it was, the discovery of _uranus_, which brought him honors from every scientific society in the world, and which gave him authority, might have had a hurtful effect. but, as he was, there was nothing which could have aided his career more than this startling discovery. it was needed for him. it completed the solar system far more by affording a free play to a profoundly philosophical mind, than by occupying the vacant spaces beyond _saturn_. his opportunities would have been profoundly modified, though his personal worth would have been the same. "the star that from the zenith darts its beams, visible though it be to half the earth, though half a sphere be conscious of its brightness, is yet of no diviner origin, no purer essence, than the one that burns like an untended watchfire, on the ridge of some dark mountain; or than those that seem humbly to hang, like twinkling winter lamps, among the branches of the leafless trees." to show how completely unknown the private astronomer of bath was at this time, i transcribe a sentence from bode's account of the discovery of _uranus_. "in the _gazette littéraire_ of june, , this worthy man is called mersthel; in julius' _journal encyclopédique_, hertschel; in a letter from mr. maskelyne to m. messier, herthel; in another letter of maskelyne's to herr mayer, at mannheim, herrschell; m. darquier calls him hermstel. what may his name be? he must have been born a _german_."[ ] this obscurity did not long continue. the news spread quickly from fashionable bath to london. on the th of december, , herschel was elected a fellow of the royal society, to which he was formally "admitted" may , . he was forty-three years old. he also received the copley medal in for his "discovery of a new and singular star."[ ] . . . "he was now frequently interrupted by visitors who were introduced by some of his resident scholars, among whom i remember sir harry engelfield, dr. blagden, and dr. maskelyne. with the latter he was engaged in a long conversation, which to me sounded like quarrelling, and the first words my brother said after he was gone were: 'that is a devil of a fellow.'. . . "i suppose their names were often not known, or were forgotten; for it was not till the year or that a memorandum of the names of visitors was thought of.". . . "my brother now applied himself to perfect his mirrors, erecting in his garden a stand for his twenty-foot telescope; many trials were necessary before the required motions for such an unwieldy machine could be contrived. many attempts were made by way of experiment before an intended thirty-foot telescope could be completed, for which, between whiles (not interrupting the observations with seven, ten, and twenty-foot, and writing papers for both the royal and bath philosophical societies), gauges, shapes, weight, etc., of the mirror were calculated, and trials of the composition of the metal were made. in short, i saw nothing else and heard nothing else talked of but these things when my brothers were together. alex. was always very alert, assisting when anything new was going forward, but he wanted perseverance, and never liked to confine himself at home for many hours together. and so it happened that my brother william was obliged to make trial of my abilities in copying for him catalogues, tables, etc., and sometimes whole papers which were lent him for his perusal. among them was one by mr. michell and a catalogue of christian mayer, in latin, which kept me employed when my brother was at the telescope at night. when i found that a hand was sometimes wanted when any particular measures were to be made with the lamp micrometer, etc., or a fire to be kept up, or a dish of coffee necessary during a long night's watching, i undertook with pleasure what others might have thought a hardship. . . . since the discovery of the _georgium sidus_ [march , ], i believe few men of learning or consequence left bath before they had seen and conversed with its discoverer, and thought themselves fortunate in finding him at home on their repeated visits. sir william watson was almost an intimate, for hardly a day passed but he had something to communicate from the letters which he received from sir joseph banks, and other members of the royal society, from which it appeared that my brother was expected in town to receive the gold medal. the end of november was the most precarious season for absenting himself. but sir william watson went with him, and it was arranged so that they set out with the diligence at night, and by that means his absence did not last above three or four days, when my brother returned alone, sir william remaining with his father. "now a very busy winter was commencing; for my brother had engaged himself to conduct the oratorios conjointly with ronzini, and had made himself answerable for the payment of the engaged performers, for his credit ever stood high in the opinion of every one he had to deal with. (he lost considerably by this arrangement.) but, though at times much harassed with business, the mirror for the thirty-foot reflector was never out of his mind, and if a minute could but be spared in going from one scholar to another, or giving one the slip, he called at home to see how the men went on with the furnace, which was built in a room below, even with the garden. "the mirror was to be cast in a mould of loam, of which an immense quantity was to be pounded in a mortar and sifted through a fine sieve. it was an endless piece of work, and served me for many an hour's exercise; and alex. frequently took his turn at it, for we were all eager to do something towards the great undertaking. even sir william watson would sometimes take the pestle from me when he found me in the work-room, where he expected to find his friend, in whose concerns he took so much interest that he felt much disappointed at not being allowed to pay for the metal. but i do not think my brother ever accepted pecuniary assistance from any one of his friends, and on this occasion he declined the offer by saying it was paid for already. "among the bath visitors were many philosophical gentlemen who used to frequent the levées at st. james's, when in town. colonel walsh, in particular, informed my brother that from a conversation he had had with his majesty, it appeared that in the spring he was to come with his seven-foot telescope to the king. similar reports he received from many others, but they made no great impression nor caused any interruption in his occupation or study, and as soon as the season for the concerts was over, and the mould, etc., in readiness, a day was set apart for casting, and the metal was in the furnace. unfortunately it began to leak at the moment when ready for pouring, and both my brothers and the caster, with his men, were obliged to run out at opposite doors, for the stone flooring (which ought to have been taken up) flew about in all directions as high as the ceiling. before the second casting was attempted, everything which could insure success had been attended to, and a very perfect metal was found in the mould. "but a total stop and derangement now took place, and nearly six or seven months elapsed before my brother could return to the undisturbed enjoyment of his instruments and observations. for one morning in passion week, as sir william watson was with my brother, talking about the pending journey to town, my eldest nephew arrived to pay us a visit, and brought the confirmation that his uncle was expected with his instrument in town. . . . we had not one night in the week, except friday, but what was set apart for an oratorio either at bath or bristol. soon after easter, a new organ being erected in st. james's church, it was opened with two performances of the 'messiah;' this again took up some of my brother's time.". . . in may of herschel went to london. "but when almost double the time had elapsed which my brother could safely be absent from his scholars, alex., as well as myself, were much at a loss how to answer their inquiries, for, from the letters we received, we could learn nothing but that he had been introduced to the king and queen, and had permission to come to the concerts at buckingham house, where the king conversed with him about astronomy." it was during his absence at this time that the three following letters were written and received: "dear lina:-- "i have had an audience of his majesty this morning, and met with a very gracious reception. i presented him with the drawing of the solar system, and had the honor of explaining it to him and the queen. my telescope is in three weeks' time to go to richmond, and meanwhile to be put up at greenwich, where i shall accordingly carry it to-day. so you see, lina, that you must not think of seeing me in less than a month. i shall write to miss lee myself; and other scholars who inquire for me, you may tell that i cannot wait on them till his majesty shall be pleased to give me leave to return, or rather to dismiss me, for till then i must attend. i will also write to mr. palmer to acquaint him with it. "i am in a great hurry, therefore can write no more at present. tell alexander that everything looks very likely as if i were to stay here. the king inquired after him, and after my great speculum. he also gave me leave to come to hear the griesbachs play at the private concert which he has every evening. my having seen the king need not be kept a secret, but about my staying here it will be best not to say anything, but only that i must remain here till his majesty has observed the planets with my telescope. "yesterday i dined with colonel walsh, who inquired after you. there were mr. aubert and dr. maskelyne. dr. maskelyne in public declared his obligations to me for having introduced to them the high powers, for mr. aubert has so much succeeded with them that he says he looks down upon , , or with contempt, and immediately begins with . he has used , very completely, and seen my fine double stars with them. all my papers are printing, with the postscript and all, and are allowed to be very valuable. you see, lina, i tell you all these things. you know vanity is not my foible, therefore i need not fear your censure. farewell. "i am, your affectionate brother, "wm. herschel. "saturday morning, "probably _may , _." to miss herschel. "monday evening, _june , ._ "dear lina:-- "i pass my time between greenwich and london agreeably enough, but am rather at a loss for work that i like. company is not always pleasing, and i would much rather be polishing a speculum. last friday i was at the king's concert to hear george play. the king spoke to me as soon as he saw me, and kept me in conversation for half an hour. he asked george to play a solo-concerto on purpose that i might hear him; and george plays extremely well, is very much improved, and the king likes him very much. these two last nights i have been star-gazing at greenwich with dr. maskelyne and mr. aubert. we have compared our telescopes together, and mine was found very superior to any of the royal observatory. double stars which they could not see with their instruments i had the pleasure to show them very plainly, and my mechanism is so much approved of that dr. maskelyne has already ordered a model to be taken from mine, and a stand to be made by it to his reflector. he is, however, now so much out of love with his instrument that he begins to doubt whether it _deserves_ a new stand. "i am introduced to the best company. to-morrow i dine at lord palmerston's, next day with sir joseph banks, etc., etc. among opticians and astronomers nothing now is talked of but _what they call_ my great discoveries. alas! this shows how far they are behind, when such trifles as i have seen and done are called _great_. let me but get at it again! i will make such telescopes, and see such things--that is, i will endeavor to do so." to miss herschel. "_july , ._" "dear carolina:-- "i have been so much employed that you will not wonder at my not writing sooner. the letter you sent me last monday came very safe to me. as dr. watson has been so good as to acquaint you and alexander with my situation, i was still more easy in my silence to you. last night the king, the queen, the prince of wales, the princess royal, princess sophia, princess augusta, etc., duke of montague, dr. heberden, m. de luc, etc., etc., saw my telescope, and it was a very fine evening. my instrument gave general satisfaction. the king has very good eyes, and enjoys observations with telescopes exceedingly. "this evening, as the king and queen are gone to kew, the princesses were desirous of seeing my telescope, but wanted to know if it was possible to see without going out on the grass, and were much pleased when they heard that my telescope could be carried into any place they liked best to have it. about eight o'clock it was moved into the queen's apartments, and we waited some time in hopes of seeing _jupiter_ or _saturn_. meanwhile i showed the princesses, and several other ladies who were present, the speculum, the micrometers, the movements of the telescopes, and other things that seemed to excite their curiosity. when the evening appeared to be totally unpromising, i proposed an artificial _saturn_ as an object, since we could not have the real one. i had beforehand prepared this little piece, as i guessed by the appearance of the weather in the afternoon we should have no stars to look at. this being accepted with great pleasure, i had the lamps lighted up which illuminated the picture of a _saturn_ (cut out in pasteboard) at the bottom of the garden wall. the effect was fine, and so natural that the best astronomer might have been deceived. their royal highnesses and other ladies seemed to be much pleased with the artifice. "i remained in the queen's apartment with the ladies till about half after ten; when in conversation with them i found them extremely well instructed in every subject that was introduced, and they seemed to be most amiable characters. to-morrow evening they hope to have better luck, and nothing will give me greater happiness than to be able to show them some of those beautiful objects with which the heavens are so gloriously ornamented." carolina's diary goes on: "sir william watson returned to bath after a fort-night or three weeks' stay. from him we heard that my brother was invited to greenwich with the telescope, where he was met by a numerous party of astronomical and learned gentlemen, and trials of his instrument were made. in these letters he complained of being obliged to lead an idle life, having nothing to do but to pass between london and greenwich. sir william received many letters, which he was so kind as to communicate to us. by these, and from those to alexander or to me, we learned that the king wished to see the telescope at windsor. at last a letter, dated july , arrived from therese, and from this and several succeeding ones we gathered that the king would not suffer my brother to return to his profession again, and by his writing several times for a supply of money we could only suppose that he himself was in uncertainty about the time of his return. "in the last week of july my brother came home, and immediately prepared for removing to datchet, where he had taken a house with a garden and grass-plat annexed, quite suitable for the purpose of an observing-place. sir william watson spent nearly the whole time at our house, and he was not the only friend who truly grieved at my brother's going from bath; or feared his having perhaps agreed to no very advantageous offers; their fears were, in fact, not without reason. . . . the prospect of entering again on the toils of teaching, etc., which awaited my brother at home (the months of leisure being now almost gone by), appeared to him an intolerable waste of time, and by way of alternative he chose to be royal astronomer, with a salary of £ a year. sir william watson was the only one to whom the sum was mentioned, and he exclaimed, 'never bought monarch honor so cheap!' to every other inquirer, my brother's answer was that the king had provided for him." on the st of august, , the family removed to datchet. the last musical duty was performed on whit-sunday, , in st. margaret's chapel, bath, when the anthem for the day was of herschel's own composition. the end of the introductory epoch of his life is reached. henceforth he lived in his observatory, and from his forty-fourth year onwards he only left it for short periods to go to london to submit his classic memoirs to the royal society. even for these occasions he chose periods of moonlight, when no observations could be made. he was a private man no longer. henceforth he belongs to the whole world. footnotes: [ ] probably on the model of one of short's gregorian telescopes, which were then the best instruments of the kind. [ ] for a description of the main points of herschel's processes of making reflectors, which will illustrate his strong mechanical talents, see _encyclopædia britannica_, eighth edition, article _telescope_. [ ] these have never been published, nor is it likely at this day, when our measuring instruments are so greatly improved, that they would be of any material value to science, although of interest as giving the proofs of herschel's assiduity and skill. he was always more than the maker of telescopes, for he was never content until they were applied to the problems of astronomy. [ ] arago has implied that if herschel had directed his telescope to _uranus_ only eleven days earlier than he did, this discovery would have escaped him, since at that time (march , ) the planet was at its _station_, and had no motion relative to the star. this is an entire misconception, since the new planet was detected by its physical appearance, and not by its motion. does any one suppose that "a new and singular star" like this would have been once viewed and then forgotten? [ ] four of _jupiter_, one of the earth, and one of _saturn_. [ ] john michell had been a member of the royal society since : he died in . he was a philosophical thinker, as is shown by his memoirs on the distances of the stars, and by his invention of the method for determining the earth's density. it is not certain that he was personally known to herschel, although his writings were familiar to the latter. alexander wilson was professor of astronomy at glasgow, and is chiefly known to us by his theory of the nature of the solar spots, which was adopted and enlarged by herschel. he died in ; but the families of wilson and herschel remained close friends. [ ] _berliner jahrbuch_, , p. . in the _connaissance des tems_ for he is called "horochelle." [ ] at the presentation sir joseph banks, the president of the royal society, said: "in the name of the royal society i present to you this gold medal, the reward which they have assigned to your successful labors, and i exhort you to continue diligently to cultivate those fields of science which have produced to you a harvest of so much honor. your attention to the improvement of telescopes has already amply repaid the labor which you have bestowed upon them; but the treasures of the heavens are well known to be inexhaustible. who can say but your new star, which exceeds _saturn_ in its distance from the sun, may exceed him as much in magnificence of attendance? who knows what new rings, new satellites, or what other nameless and numberless phenomena remain behind, waiting to reward future industry and improvement?" chapter iii. life at datchet, clay hall, and slough; - . the new house at datchet, which was occupied from till , was a source of despair to carolina herschel, who looked upon its desolate and isolated condition with a housekeeper's eyes. this was nothing to her brother, who gayly consented to live upon "eggs and bacon," now that he was free at last to mind the heavens. the ruinous state of the place had no terrors in his eyes, for was there not a laundry which would serve as a library, a large stable which was just the place for the grinding of mirrors, and a grass-plat for the small twenty-foot reflector? here they set to work at astronomy; the brother with the twenty-foot, the sister aiding him, and at odd times sweeping for comets. in the course of her life she discovered no less than eight, and five of these were first seen by her. * * * * * in herschel wrote his paper "on three volcanoes in the moon," which he had observed in april of that year. in this he mentions previous observations of the same sort. i do not remember that the following account of these has ever been put on record in english. baron von zach writes from london to bode:[ ] "probably you have heard also of the volcanoes in the moon, which herschel has observed. . . . i will give you an account of it as i heard it from his own lips. dr. lind, a worthy physician in windsor, who has made himself known through his two journeys in china, and who is a friend of our herschel's, was with his wife one evening on a visit to herschel in datchet [ , may ]. on this evening there was to be an occultation of a star at the moon's dark limb. this was observed by herschel and doctor lind. mrs. lind wished also to see what was occurring, and placed herself at a telescope and watched attentively. "scarcely had the star disappeared before mrs. lind thought she saw it again, and exclaimed that the star had gone in front of, and not behind the moon. this provoked a short astronomical lecture on the question, but still she would not credit it, because she _saw_ differently. finally herschel stepped to the telescope, and in fact he saw a bright point on the dark disc of the moon, which he followed attentively. it gradually became fainter and finally vanished.". . . the life at datchet was not free from its annoyances. "much of my brother's time was taken up in going, when the evenings were clear, to the queen's lodge, to show the king, etc., objects through the seven-foot. but when the days began to shorten, this was found impossible, for the telescope was often (at no small expense and risk of damage) obliged to be transported in the dark back to datchet, for the purpose of spending the rest of the night with observations on double stars for a second catalogue. my brother was, besides, obliged to be absent for a week or ten days, for the purpose of bringing home the metal of the cracked thirty-foot mirror, and the remaining materials from his work-room. before the furnace was taken down at bath, a second twenty-foot mirror, twelve inches diameter, was cast, which happened to be very fortunate, for on the st of january, , a very fine one cracked by frost in the tube. . . . "in my brother's absence from home i was, of course, left alone to amuse myself with my own thoughts, which were anything but cheerful. i found i was to be trained for an assistant astronomer, and, by way of encouragement, a telescope adapted for 'sweeping,' consisting of a tube with two glasses, such as are commonly used in a 'finder,' was given me. i was 'to sweep for comets,' and i see, by my journal, that i began august d, , to write down and describe all remarkable appearances i saw in my 'sweeps,' which were horizontal. but it was not till the last two months of the same year that i felt the least encouragement to spend the star-light nights on a grass-plot covered with dew or hoar-frost, without a human being near enough to be within call. i knew too little of the real heavens to be able to point out every object so as to find it again, without losing much time by consulting the atlas. but all these troubles were removed when i knew my brother to be at no great distance making observations, with his various instruments, on double stars, planets, etc., and when i could have his assistance immediately if i found a nebula or cluster of stars, of which i intended to give a catalogue; but, at the end of , i had only marked fourteen, when my sweeping was interrupted by being employed to write down my brother's observations with the large twenty-foot. i had, however, the comfort to see that my brother was satisfied with my endeavors to assist him when he wanted another person either to run to the clocks, write down a memorandum, fetch and carry instruments, or measure the ground with poles, etc., etc., of which something of the kind every moment would occur. for the assiduity with which the measurements on the diameter of the _georgium sidus_, and observations of other planets, double stars, etc., etc., were made, was incredible, as may be seen by the various papers that were given to the royal society in , which papers were written in the daytime, or when cloudy nights interfered. besides this, the twelve-inch speculum was perfected before the spring, and many hours were spent at the turning-bench, as not a night clear enough for observing ever passed but that some improvements were planned for perfecting the mounting and motions of the various instruments then in use, or some trials were made of new constructed eye-pieces, which were mostly executed by my brother's own hands. wishing to save his time, he began to have some work of that kind done by a watchmaker who had retired from business and lived on datchet common; but the work was so bad, and the charges so unreasonable, that he could not be employed. it was not till some time afterwards, in his frequent visits to the meetings of the royal society (made in moonlight nights), that he had an opportunity of looking about for mathematical workmen, opticians, and founders. but the work seldom answered expectation, and it was kept, to be executed with improvements by alexander during the few months he spent with us. "the summer months passed in the most active preparation for getting the large twenty-foot ready against the next winter. the carpenters and smiths of datchet were in daily requisition, and, as soon as patterns for tools and mirrors were ready, my brother went to town to have them cast, and, during the three or four months alexander could be absent from bath, the mirrors and optical parts were nearly completed. "but that the nights after a day of toil were not given to rest, may be seen by the observations on _mars_, of which a paper, dated december , , was given to the royal society. some trouble, also, was often thrown away, during those nights, in the attempt to teach me to remeasure double stars with the same micrometers with which former measures had been taken, and the small twenty-foot was given me for that purpose. . . . i had also to ascertain their places by a transit instrument lent for that purpose by mr. dalrymple; but, after many fruitless attempts, it was seen that the instrument was, perhaps, as much in fault as my observations." in herschel says: "i have now finished my third review of the heavens. the first was made with a newtonian telescope something less than seven feet focal length, a power of , and an aperture of four and a half inches. it extended only to stars of the first, second, third, and fourth magnitudes. my second review was made with an instrument much superior to the other, of . inches focus, . inches aperture, and power . it extended to all the stars of harris's maps and the telescopic ones near them, as far as the eighth magnitude. the catalogue of double stars and the discovery of the _georgium sidus_, were the results of that review. the third was with the same instrument and aperture, but with a power of . this review extended to all the stars of flamsteed's catalogue, together with every small star about them, to the amount of a great many thousands of stars. i have, many a night, in the course of eleven or twelve hours of observation, carefully and singly examined not less than celestial objects, besides taking measures, and sometimes viewing a particular star for half an hour together." the fourth review began with the twenty-foot, in . "my brother began his series of sweeps when the instrument was yet in a very unfinished state, and my feelings were not very comfortable when every moment i was alarmed by a crack or fall, knowing him to be elevated fifteen feet or more on a temporary cross-beam, instead of a safe gallery. the ladders had not even their braces at the bottom; and one night, in a very high wind, he had hardly touched the ground before the whole apparatus came down. some laboring men were called up to help in extricating the mirror, which was, fortunately, uninjured, but much work was cut out for carpenters next day. i could give a pretty long list of accidents which were near proving fatal to my brother as well as myself. to make observations with such large machinery, where all around is in darkness, is not unattended with danger, especially when personal safety is the last thing with which the mind is occupied; even poor piazzi did not go home without getting broken shins by falling over the rack-bar. "in the long days of the summer months many ten and seven foot mirrors were finished; there was nothing but grinding and polishing to be seen. for ten-foot, several had been cast with ribbed backs, by way of experiment, to reduce the weight in large mirrors. in my leisure hours i ground seven-foot and plain mirrors from rough to fining down, and was _indulged_ with polishing and the last finishing of a very beautiful mirror for sir william watson. "an account of the discoveries made with the twenty-foot and the improvements of the mechanical parts of the instrument during the winter of is given with the catalogue of the first , new nebulæ. by which account it must plainly appear that the expenses of these improvements, and those which were yet to be made in the apparatus of the twenty-foot (which, in fact, proved to be a model of a larger instrument), could not be supplied out of a salary of £ a year, especially as my brother's finances had been too much reduced during the six months before he received his _first_ quarterly payment of _fifty pounds_ (which was michaelmas, ). travelling from bath to london, greenwich, windsor, backwards and forwards, transporting the telescope, etc., breaking up his establishment at bath and forming a new one near the court, all this, even leaving such personal conveniences as he had for many years been used to, out of the question, could not be obtained for a trifle; a good large piece of ground was required for the use of the instruments, and a habitation in which he could receive and offer a bed to an astronomical friend, was necessary after a night's observation. "it seemed to be supposed that enough had been done when my brother was enabled to leave his profession that he might have time to make and sell telescopes. the king ordered four ten-foot himself, and many seven-foot besides had been bespoke, and much time had already been expended on polishing the mirrors for the same. but all this was only retarding the work of a thirty or forty foot instrument, which it was my brother's chief object to obtain as soon as possible; for he was then on the wrong side of forty-five, and felt how great an injustice he would be doing to himself and to the cause of astronomy by giving up his time to making telescopes for other observers. "sir william watson, who often in the lifetime of his father came to make some stay with us at datchet, saw my brother's difficulties, and expressed great dissatisfaction. on his return to bath he met, among the visitors there, several belonging to the court, to whom he gave his opinion concerning his friend and his situation very freely. in consequence of this, my brother had soon after, through sir j. banks, the promise that £ , would be granted for enabling him to make himself an instrument. "immediately every preparation for beginning the great work commenced. a very ingenious smith (campion), who was seeking employment, was secured by my brother, and a temporary forge erected in an upstairs room." the sale of these telescopes of herschel's must have produced a large sum, for he had made before more than two hundred seven-feet, one hundred and fifty ten-feet, and eighty twenty-feet mirrors. for many of the telescopes sent abroad no stands were constructed. the mirrors and eye-pieces alone were furnished, and a drawing of the stand sent with them by which the mirrors could be mounted. in the cost of a seven-foot telescope, six and four-tenths inches aperture, stand, eye-pieces, etc., complete, was two hundred guineas, a ten-foot was six hundred guineas, and a twenty-foot about , to , guineas. he had made four ten-foot telescopes like this for the king. in schroeter got the mirrors and eye-pieces only for a four-and-three-quarter-inch reflector for five guineas; those for his seven-foot telescope were twenty-three guineas. later a seven-foot telescope, complete, was sold for one hundred guineas, and the twenty-five-foot reflector, made for the madrid observatory, cost them , francs = $ , .[ ] it was ordered in , but not delivered for several years, the spanish government being short of money. for a ten and a seven foot telescope, the prince of canino paid £ , . von magellan writes to bode concerning a visit to herschel:[ ] "i spent the night of the th of january at herschel's, in datchet, near windsor, and had the good luck to hit on a fine evening. he has his twenty-foot newtonian telescope in the open air and mounted in his garden very simply and conveniently. it is moved by an assistant, who stands below it. . . . near the instrument is a clock regulated to sidereal time. . . . in the room near it sits herschel's sister, and she has flamsteed's atlas open before her. as he gives her the word, she writes down the declination and right ascension and the other circumstances of the observation. in this way herschel examines the whole sky without omitting the least part. he commonly observes with a magnifying power of one hundred and fifty, and is sure that after four or five years he will have passed in review every object above our horizon. he showed me the book in which his observations up to this time are written, and i am astonished at the great number of them. each sweep covers ° ' in declination, and he lets each star pass at least three times through the field of his telescope, so that it is impossible that anything can escape him. he has already found about double stars and almost as many nebulæ. i went to bed about one o'clock, and up to that time, he had found that night four or five new nebulæ. the thermometer in the garden stood at ° fahrenheit; but, in spite of this, herschel observes the whole night through, except that he stops every three or four hours and goes in the room for a few moments. for some years herschel has observed the heavens every hour when the weather is clear, and this always in the open air, because he says that the telescope only performs well when it is at the same temperature as the air. he protects himself against the weather by putting on more clothing. he has an excellent constitution, and thinks about nothing else in the world but the celestial bodies. he has promised me in the most cordial way, entirely in the service of astronomy, and without thinking of his own interest, to see to the telescopes i have ordered for european observatories, and he will himself attend to the preparation of the mirrors." it was at this time, , may , that herschel married. his wife was the daughter of mr. james baldwin, a merchant of the city of london, and the widow of john pitt, esq. she is described as a lady of singular amiability and gentleness of character. she was entirely interested in his scientific pursuits, and the jointure which she brought removed all further anxiety about money affairs. they had but one child, john frederick william, born march , .[ ] * * * * * the house at datchet became more and more unfit for the needs of the family, and in june, , a move was made to clay hall, in old windsor. the residence here was but short, and finally a last change was made to slough on april, d, . the ardor of the work during these years can be judged of by a single sentence from carolina herschel's diary: "the last night at clay hall was spent in sweeping till daylight, and by the next evening the telescope stood ready for observation at slough." from until his death, herschel remained at slough; his life, truly speaking, was in his observatory. it is indeed true, as arago has said in his eloquent tribute to him: "on peut dire hardiment du jardin et de la petite maison de slough, que c'est le lieu du monde où il a été fait le plus de découvertes. le nom de ce village ne périra pas; les sciences le transmettront religieusement à nos derniers neveux." herschel's first contribution to the _philosophical transactions_ was printed in the volume for , his last in that for . of these thirty-nine volumes, there are only two ( and ) which contain no paper from his hand, and many volumes contain more than one, as he published no less than sixty-eight memoirs in this place. and yet it must not be thought that his was an austere and grave existence. music, which he loved to enthusiasm, was still a delight to him. all the more that his devotion was free. the glimpses which we get of his life with his friends show him always cheerful, ardent, and devoted. even in his later years, he had not lost a "boyish earnestness to explain;" his simplicity and the charm of his manner struck every one. "herschel, you know, and everybody knows, is one of the most pleasing and well-bred natural characters of the present age," says dr. burney, who had opportunity to know. the portrait which is given in the frontispiece must have been painted about this time ( ), and the eager, ardent face shows his inner life far better than any words can do. even in his scientific writings, which everything conspired to render grave and sober, the almost poetic nature of his mind shows forth. in one of his (unpublished) note-books, now in the royal society's library, i found this entry: " th sweep--november , .--the nebula of _orion_, which i saw by the front view, was so glaring and beautiful that i could not think of taking any place of its extent." he was quite alone under the perfectly silent sky when this was written, and he was at his post simply to make this and other such observations. but the sky was beautiful to him, and his faithful sister, carolina, sitting below, has preserved for us the words as they dropped from his lips. on the th of january, , herschel discovered two satellites to _uranus_. after he had well assured himself of their existence, but before he communicated his discovery to the world, he made this crucial test. he prepared a sketch of _uranus_ attended by his two satellites, as it would appear on the night of february , , and when the night came, "the heavens displayed the original of my drawings, by showing in the situation i had delineated them _the georgian planet attended by two satellites_. i confess that this scene appeared to me with additional beauty, as the little secondary planets seemed to give a dignity to the primary one which raises it into a more conspicuous situation among the great bodies of the solar system.". . . in a memoir of , he has a few sentences which show the living way in which the heavens appeared to him: "this method of viewing the heavens seems to throw them into a new kind of light. "they are now seen to resemble a luxuriant garden, which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. for is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens selected from every stage through which the plant passes in the course of its existence be brought at once to our view?" the thought here is no less finely expressed than it is profound. the simile is perfect, if we have the power to separate among the vast variety each state of being from every other, and if the very luxuriance of illustration in the heavens does not bewilder and overpower the mind. it was precisely this discriminating power that herschel possessed in perfection. there is a kind of humor in the way he records a change of opinion: "i formerly supposed the surface of _saturn's_ ring to be rough, owing to luminous points like mountains seen on it, till one of these was kind enough to venture off the edge of the ring and appear as a satellite." in he replies with a certain concealed sharpness to the idea that he used magnifying powers which were too high. there is a tone almost of impatience, as if he were conscious he was replying to a criticism based on ignorance: "we are told that we gain nothing by magnifying too much. i grant it; but shall never believe i magnify too much till by experience i find that i can see better with a lower power." ( .) by , when he returns to this subject, in answer to a formal request to explain his use of high magnifiers, he is quite over any irritation, and treats the subject almost with playfulness: "soon after my first essay of using high powers with the newtonian telescope, i began to doubt whether an opinion which has been entertained by several eminent authors, 'that vision will grow indistinct when the optic pencils are less than the fiftieth part of an inch,' would hold good in all cases. i perceived that according to this criterion i was not entitled to see distinctly with a power of much more than about in a seven-foot telescope of an aperture of six and four-tenths inches, whereas in many experiments i found myself very well pleased with magnifiers which far exceeded such narrow limits. this induced me, as it were, by way of apology to myself for seeing well where i ought to have seen less distinctly, to make a few experiments." it is needless to say that these experiments proved that from the point of view taken by herschel, he was quite right, and that his high powers had numerous valuable applications. he goes on to say: "had it not been for a late conversation with some of my highly esteemed and learned friends, i might probably have left the papers on which these experiments were recorded, among the rest of those that are laid aside, when they have afforded me the information i want." the last sentence seems to be a kind of notice to his learned friends that there is yet more unsaid. as a warning to those to whose criticisms he had replied, he gives them this picture of the kind of assiduity which will be required, if some of his observations on double stars are to be repeated: "it is in vain to look for these stars if every circumstance is not favorable. the observer as well as the instrument must have been long enough out in the open air to acquire the same temperature. in very cold weather an hour at least will be required." ( .) we may gain some further insight into his character from the following chance extracts from his writings: "i have all along had truth and reality in view as the sole object of my endeavors." ( .) "not being satisfied when i thought it possible to obtain more accurate measures, i employed [a more delicate apparatus]." ( .) "to this end i have already begun a series of observations upon several zones of double stars, and should the result of them be against these conjectures, i shall be the first to point out their fallacy." ( .) "there is a great probability of succeeding still farther in this laborious but delightful research, so as to be able at last to say not only how much the annual parallax _is not_, but how much it really _is_." ( .) the nature of his philosophizing, and the limits which he set to himself, may be more clearly seen in further extracts: "by taking more time [before printing these observations] i should undoubtedly be enabled to speak more confidently of the _interior_ _construction of the heavens_, and of its various _nebulous_ and sidereal strata. as an apology for this prematurity it may be said that, the end of all discoveries being communication, we can never be too ready in giving facts and observations, whatever we may be in reasoning upon them." ( .) "in an investigation of this delicate nature we ought to avoid two opposite extremes. if we indulge a fanciful imagination, and build worlds of our own, we must not wonder at our going wide from the path of truth and nature. on the other hand, if we add observation to observation without attempting to draw not only certain conclusions but also conjectural views from them, we offend against the very end for which only observations ought to be made. i will endeavor to keep a proper medium, but if i should deviate from that, i could wish not to fall into the latter error." ( .) "as observations carefully made should always take the lead of theories, i shall not be concerned if what i have to say contradicts what has been said in my last paper on this subject." ( .) no course of reasoning could be more simple, more exact, more profound, and more beautiful than this which follows: "as it has been shown that the spherical figure of a cluster is owing to the action of central powers, it follows that those clusters which, _cæteris paribus_, are the most complete in this figure, must have been the longest exposed to the action of these causes. thus the maturity of a sidereal system may be judged from the disposition of the component parts. "hence planetary nebulæ may be looked on as very aged. though we cannot see any individual nebula pass through all its stages of life, we can select particular ones in each peculiar stage." ( .) there is something almost grandiose and majestic in his statement of the ultimate destiny of the galaxy: "to him the fates were known of orbs dim hovering on the skirts of space." "--since the stars of the milky way are permanently exposed to the action of a power whereby they are irresistibly drawn into groups, we may be certain that from mere clustering stars they will be gradually compressed, through successive stages of accumulation, till they come up to what may be called the ripening period of the globular form, and total insulation; from which it is evident that the milky way must be finally broken up and cease to be a stratum of scattered stars. "the state into which the incessant action of the clustering power has brought it at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that since the breaking up of the milky way affords a proof that it cannot last forever, it equally bears witness that its past duration cannot be admitted to be infinite." ( .) herschel's relations with his cotemporaries were usually of the most pleasant character, though seldom intimate. this peace was broken but by one unpleasant occurrence. in the _philosophical transactions_ for , schroeter had communicated a series of observations made with one of herschel's own telescopes on the atmospheres of _venus_, the moon, etc. it was not only an account of phenomena which had been seen; it was accompanied by measures, and the computations based on these led to heights and dimensions for mountains on _venus_ which were, to say the least, extravagant. the adjective will not seem too strong when we say that the very existence of the mountains themselves is to-day more than doubtful. the appearances seen by schroeter were described by him in perfectly good faith, and similar ones have been since recorded. his reasoning upon them was defective, and the measures which he made were practically valueless. this paper, printed in the _transactions_ of the royal society, to which schroeter had not before contributed, appears to have irritated herschel. no doubt there were not wanting members of his own society who hinted that on the continent, too, there were to be found great observers, and that here, at least, herschel had been anticipated even in his own field. i have always thought that the memoir of herschel which appeared in the next volume of the _transactions_ ( ), _observations on the planet venus_, was a rejoinder intended far more for the detractors at home than for the astronomer abroad. the review is conceived in a severe spirit. the first idea seems to be to crush an opposition which he feels. the truth is established, but its establishment is hardly the _first_ object. it seems as if herschel had almost allowed himself to be forced into a position of arrogance, which his whole life shows was entirely foreign to his nature. all through the review he does not once mention schroeter's name. he says: "a series of observations on _venus_, begun by me in april, , has been continued down to the present time. . . . the result of my observations would have been communicated long ago if i had not flattered myself with the hope of some better success concerning the diurnal motion of _venus_, which has still eluded my constant attention as far as concerns its period and direction. . . . even at this present time i should hesitate to give the following extracts if it did not seem incumbent on me to examine by what accident i came to overlook mountains in this planet of such enormous height as to exceed four, five, or even six times the perpendicular height of chimboraço, the highest of our mountains. . . . the same paper contains other particulars concerning _venus_ and _saturn_. all of which being things of which i have never taken any notice, it will not be amiss to show, by what follows, that neither want of attention, nor a deficiency of instruments, would occasion my not perceiving these mountains of more than twenty-three miles in height, this jagged border of _venus_, and these flat, spherical forms on _saturn_." the reply of schroeter ( ) is temperate and just. it does him honor, and he generously gives full justice to his critic. it would hardly be worth while to mention this slight incident if it were not that during these years there certainly existed a feeling that herschel undervalued the labors of his cotemporaries. this impression was fostered no doubt by his general habit of not quoting previous authorities in the fields which he was working. a careful reading of his papers will, i think, show that his definite indebtedness to his _cotemporaries_ was vanishingly small. the work of michell and wilson he alludes to again and again, and always with appreciation. certainly he seems to show a vein of annoyance that the papers of christian mayer, _de novis in coelo sidereo phænomenis_ ( ), and _beobachtungen von fixsterntrabanten_ ( ), should have been quoted to prove that the method proposed by herschel in for ascertaining the parallax of the fixed stars by means of observations of those which were double, was not entirely original with himself. there is direct proof that it was so,[ ] and if this was not forthcoming it would be unnecessary, as he has amply shown in his catalogue of double stars. one is reminded of his remarks on the use of the high magnifying powers by the impatience of his comments. his proposal to call the newly discovered minor planets _asteroids_ ( ) was received as a sign that he wished to discriminate between the discoveries of piazzi and olbers and his own discovery of uranus.[ ] he takes pains to quietly put this on one side in one of his papers, showing that he was cognizant of the existence of such a feeling. i am tempted to resurrect from a deserved obscurity a notice of herschel's _observations on the two lately discovered celestial bodies_ (_philosophical transactions_, ), printed in the first volume of the _edinburgh review_, simply to show the kind of envy to which even he, the glory of england, was subject. the reviewer sets forth the principal results of herschel's observations, and, after quoting his definition of the new term asteroid, goes on to say: "if a new name must be found, why not call them by some appellation which shall, in some degree, be descriptive of, or at least consistent with, their properties? why not, for instance, call them _concentric comets_, or _planetary comets_, or _cometary planets_? or, if a single term must be found, why may we not coin such a phrase as _planetoid_ or _cometoid_?" then follows a general arraignment of herschel's methods of expression and thought, as distinguished from his powers of mere observation. this distinction, it may be said, exists only in the reviewer's mind; there was no such distinction in fact. if ever a series of observations was directed by profound and reasonable thought, it was herschel's own. "dr. herschel's passion for coining words and idioms has often struck us as a weakness wholly unworthy of him. the invention of a name is but a poor achievement for him who has discovered whole worlds. why, for instance, do we hear him talking of the _space-penetrating power_ of his instrument--a compound epithet and metaphor which he ought to have left to the poets, who, in some future age, shall acquire glory by celebrating his name. the other papers of dr. herschel, in the late volumes of the _transactions_, do not deserve such particular attention. his catalogue of new nebulæ, though extremely valuable to the practical astronomer, leads to no general conclusions of importance, and abounds with the defects which are peculiar to the doctor's writings--a great prolixity and tediousness of narration--loose and often unphilosophical reflections, which give no very favorable idea of his scientific powers, however great his merit may be as an observer--above all, that idle fondness for inventing names without any manner of occasion, to which we have already alluded, and a use of novel and affected idioms. * * * * * "to the speculations of the doctor on the nature of the sun, we have many similar objections; but they are all eclipsed by the grand absurdity which he has there committed, in his hasty and erroneous theory concerning the influence of the solar spots on the price of grain. since the publication of gulliver's voyage to laputa, nothing so ridiculous has ever been offered to the world. we heartily wish the doctor had suppressed it; or, if determined to publish it, that he had detailed it in language less confident and flippant." one is almost ashamed to give space and currency to a forgotten attack, but it yields a kind of perspective; and it is instructive and perhaps useful to view herschel's labors from all sides, even from wrong and envious ones. the study of the original papers, together with a knowledge of the circumstances in which they were written, will abundantly show that herschel's ideas sprung from a profound meditation of the nature of things in themselves. what the origin of trains of thought prosecuted for years may have been we cannot say, nor could he himself have expressed it. a new path in science was to be found out, and he found it. it was not in his closet, surrounded by authorities, but under the open sky, that he meditated the construction of the heavens. as he says, "my situation permitted me not to consult large libraries; nor, indeed, was it very material; for as i intended to view the heavens myself, nature, that great volume, appeared to me to contain the best catalogue." his remarkable memoirs on the invisible and other rays of the solar spectrum were received with doubt, and with open denial by many of the scientific bodies of europe. the reviews and notices of his work in this direction were often quite beyond the bounds of a proper scientific criticism; but herschel maintained a dignified silence. the discoveries were true, the proofs were open to all, and no response was needed from him. he may have been sorely tempted to reply, but i am apt to believe that the rumors that reached him from abroad and at home did not then affect him as they might have done earlier. he was at his grand climacteric, he had passed his sixty-third year, his temper was less hasty than it had been in his youth, and his nerves had not yet received the severe strain from whose effects he suffered during the last years of his life. * * * * * we have some glimpses of his personal life in the reminiscences of him in the _diary and letters_ of madame d'arblay, who knew him well: " .--in the evening mr. herschel came to tea. i had once seen that very extraordinary man at mrs. de luc's, but was happy to see him again, for he has not more fame to awaken curiosity than sense and modesty to gratify it. he is perfectly unassuming, yet openly happy, and happy in the success of those studies which would render a mind less excellently formed presumptuous and arrogant. "the king has not a happier subject than this man, who owes it wholly to his majesty that he is not wretched; for such was his eagerness to quit all other pursuits to follow astronomy solely, that he was in danger of ruin, when his talents and great and uncommon genius attracted the king's patronage. he has now not only his pension, which gives him the felicity of devoting all his time to his darling study, but he is indulged in license from the king to make a telescope according to his new ideas and discoveries, that is to have no cost spared in its construction, and is wholly to be paid for by his majesty. "this seems to have made him happier even than the pension, as it enables him to put in execution all his wonderful projects, from which his expectations of future discoveries are so sanguine as to make his present existence a state of almost perfect enjoyment. mr. locke himself would be quite charmed with him. "he seems a man without a wish that has its object in the terrestrial globe. at night mr. herschel, by the king's command, came to exhibit to his majesty and the royal family the new comet lately discovered by his sister, miss herschel; and while i was playing at piquet with mrs. schwellenburg, the princess augusta came into the room and asked her if she chose to go into the garden and look at it. she declined the offer, and the princess then made it to me. i was glad to accept it for all sorts of reasons. we found him at his telescope. the comet was very small, and had nothing grand or striking in its appearance; but it is the first lady's comet, and i was very desirous to see it. mr. herschel then showed me some of his new discovered universes, with all the good humor with which he would have taken the same trouble for a brother or a sister astronomer; there is no possibility of admiring his genius more than his gentleness." "_ , december th_.--this morning my dear father carried me to dr. herschel. that great and very extraordinary man received us almost with open arms. he is very fond of my father, who is one of the council of the royal society this year, as well as himself. . . . at this time of day there was nothing to see but his instruments; those, however, are curiosities sufficient. . . . i wished very much to have seen his sister, . . . but she had been up all night, and was then in bed." "_ , september_.--dr. herschel is a delightful man; so unassuming with his great knowledge, so willing to dispense it to the ignorant, and so cheerful and easy in his general manners, that, were he no genius, it would be impossible not to remark him as a pleasing and sensible man." "_ , october d_.--we returned to windsor at noon, and mrs. de luc sent me a most pressing invitation to tea and to hear a little music. two young ladies were to perform at her house in a little concert. dr. herschel was there, and accompanied them very sweetly on the violin; his new-married wife was with him, and his sister. his wife seems good-natured; she was rich, too! and astronomers are as able as other men to discern that gold can glitter as well as stars." dr. burney to madame d'arblay. "chelsea college, _september , _. "* * * * * "i drove through slough in order to ask at dr. herschel's door when my visit would be least inconvenient to him--that night or next morning. the good soul was at dinner, but came to the door himself, to press me to alight immediately and partake of his family repast; and this he did so heartily that i could not resist. * * * * * * * "i expected (not knowing that herschel was married) only to have found miss herschel; but there was a very old lady, the mother, i believe, of mrs. herschel, who was at the head of the table herself, and a scots lady (a miss wilson, daughter of dr. wilson, of glasgow, an eminent astronomer), miss herschel, and a little boy. they rejoiced at the accident which had brought me there, and hoped i would send my carriage away and take a bed with them. they were sorry they had no stables for my horses. "we soon grew acquainted--i mean the ladies and i--and before dinner was over we seemed old friends just met after a long absence. mrs. herschel is sensible, good-humored, unpretending, and well bred; miss herschel all shyness and virgin modesty; the scots lady sensible and harmless; and the little boy entertaining, promising, and comical. herschel, you know, and everybody knows, is one of the most pleasing and well-bred natural characters of the present age, as well as the greatest astronomer. "your health was drunk after dinner (put that into your pocket), and after much social conversation and a few hearty laughs, the ladies proposed to take a walk, in order, i believe, to leave herschel and me together. we walked and talked round his great telescopes till it grew damp and dusk, then retreated into his study to philosophize. * * * * * "he made a discovery to me, which, had i known it sooner, would have overset me, and prevented my reading any part of my work.[ ] he said that he had almost always had an aversion to poetry, which he regarded as the arrangement of fine words, without any useful meaning or adherence to truth; but that when truth and science were united to these fine words, he liked poetry very well." , december . dr. burney to madame d'arblay. "herschel has been in town for short spurts, and back again two or three times, leaving mrs. herschel behind (in town) to transact law business. i had him here two whole days." the reading of the manuscript of the _poetical history of astronomy_ was continued, "and herschel was so humble as to confess that i knew more of the history of astronomy than he did, and had surprised him with the mass of information i had got together. "he thanked me for the entertainment and instruction i had given him. 'can anything be grander?' and all this before he knows a word of what i have said of himself--all his discoveries, as you may remember, being kept back for the twelfth and last book." dr. burney to madame d'arblay. "slough, _monday morning._ _july , _, in bed at dr. herschel's, half-past five, where i can neither sleep nor lie idle. "my dear fanny:--i believe i told you on friday that i was going to finish the perusal of my astronomical verses to the great astronomer on saturday. * * * * * "after tea dr. herschel proposed that we two should retire into a quiet room in order to resume the perusal of my work, in which no progress has been made since last december. the evening was finished very cheerfully; and we went to our bowers not much out of humor with each other or the world. . . . after dinner we all agreed to go to the terrace [at windsor]--mr., mrs., and miss h., with their nice little boy, and three young ladies. here i met with almost everybody i wished and expected to see previous to the king's arrival. * * * * * "but now here comes will, and i must get up, and make myself up to go down to the perusal of my last book, entitled _herschel_. so good-morrow." "chelsea, _tuesday._ "not a moment could i get to write till now. . . . i must tell you that herschel proposed to me to go with him to the king's concert at night, he having permission to go when he chooses, his five nephews (griesbachs) making a principal part of the band. 'and,' says he, 'i know you will be welcome.'" an intimacy was gradually established between herschel and dr. burney. they saw each other often at the meetings of the royal society, and herschel frequently stayed at the doctor's house. "on the first evening herschel spent at chelsea, when i called for my argand lamp, herschel, who had not seen one of those lamps, was surprised at the great effusion of light, and immediately calculated the difference between that and a single candle, and found it sixteen to one."[ ] in we find herschel as a witness for his friend james watt, in the celebrated case of watt _vs._ bull, which was tried in the court of common pleas. and from muirhead's life of watt, it appears that herschel visited watt at heathfield in . a delightful picture of the old age of herschel is given by the poet campbell,[ ] whose nature was fitted to perceive the beauties of a grand and simple character like herschel's: "[brighton], _september , _. . . . "i wish you had been with me the day before yesterday, when you would have joined me, i am sure, deeply in admiring a great, simple, good old man--dr. herschel. do not think me vain, or at least put up with my vanity, in saying that i almost flatter myself i have made him my friend. i have got an invitation, and a pressing one, to go to his house; and the lady who introduced me to him, says he spoke of me as if he would really be happy to see me. . . . i spent all sunday with him and his family. his son is a prodigy in sciences, and fond of poetry, but very unassuming. . . . now, for the old astronomer himself. his simplicity, his kindness, his anecdotes, his readiness to explain--and make perfectly conspicuous too--his own sublime conceptions of the universe are indescribably charming. he is seventy-six, but fresh and stout; and there he sat, nearest the door, at his friend's house, alternately smiling at a joke, or contentedly sitting without share or notice in the conversation. any train of conversation he follows implicitly; anything you ask he labors with a sort of boyish earnestness to explain. "i was anxious to get from him as many particulars as i could about his interview with buonaparte.[ ] the latter, it was reported, had astonished him by his astronomical knowledge. "'no,' he said, 'the first consul did surprise me by his quickness and versatility on all subjects; but in science he seemed to know little more than any well-educated gentleman, and of astronomy much less for instance than our own king. his general air,' he said, 'was something like affecting to know more than he did know.' he was high, and tried to be great with herschel, i suppose, without success; and 'i remarked,' said the astronomer, 'his hypocrisy in concluding the conversation on astronomy by observing how all these glorious views gave proofs of an almighty wisdom.' i asked him if he thought the system of laplace to be quite certain, with regard to the total security of the planetary system from the effects of gravitation losing its present balance? he said, no; he thought by no means that the universe was secured from the chance of sudden losses of parts. "he was convinced that there had existed a planet between _mars_ and _jupiter_, in our own system, of which the little asteroids, or planetkins, lately discovered, are indubitably fragments; and 'remember,' said he, 'that though they have discovered only four of those parts, there will be thousands--perhaps thirty thousand more--yet discovered.' this planet he believed to have been lost by explosion. "with great kindness and patience he referred me, in the course of my attempts to talk with him, to a theorem in newton's 'principles of natural philosophy' in which the time that the light takes to travel from the sun is proved with a simplicity which requires but a few steps in reasoning. in talking of some inconceivably distant bodies, he introduced the mention of this plain theorem, to remind me that the progress of light could be measured in the one case as well as the other. then, speaking of himself, he said, with a modesty of manner which quite overcame me, when taken together with the greatness of the assertion: 'i have looked _further into space than ever human being did before me_. i have observed stars, of which the light, it can be proved, must take two millions of years to reach this earth.' "i really and unfeignedly felt at this moment as if i had been conversing with a supernatural intelligence. 'nay, more,' said he, 'if those distant bodies had ceased to exist two millions of years ago, we should still see them, as the light would travel after the body was gone. . . .' these were herschel's words; and if you had heard him speak them, you would not think he was apt to tell more than the truth. "after leaving herschel i felt elevated and overcome; and have in writing to you made only this memorandum of some of the most interesting moments of my life." campbell's conscientious biographer appears to have felt that the value of this charming account of his interview with herschel was in its report of astronomical facts and opinions, and he adds a foot-note to explain that "herschel's opinion never amounted to more than _hypothesis_ having some degree of probability. sir john herschel remembers his father saying, 'if that hypothesis were true, and _if_ the planet destroyed were as large as the earth, there must have been at least thirty-thousand such fragments,' but always as an hypothesis--he was never heard to declare any degree of conviction that it was so." for us, the value of this sympathetic account of a day in herschel's life is in its conception of the simplicity, the modesty, the "boyish earnestness," the elevation of thought and speech of the old philosopher; and in the impression made on the feelings, not the mind, of the poet, then thirty-five years old. in a letter to alison, campbell reverts with great pleasure to the day spent with herschel: "sydenham, _december , _. "my dearest alison:-- * * * * * "i spent three weeks with my family at brighton, in charming weather, and was much pleased with, as well as benefited by, the place. there i met a man with whom you will stare at the idea of my being congenial, or having the vanity to think myself so--the great herschel. he is a simple, great being. . . . i once in my life looked at newton's _principia_, and attended an astronomical class at glasgow; wonderful it seemed to myself, that the great man condescended to understand my questions; to become apparently earnest in communicating to me as much information as my limited capacity and preparation for such knowledge would admit. he invited me to see him at his own abode, and so kindly that i could not believe that it was mere good breeding; but a sincere wish to see me again. i had a full day with him; he described to me his whole interview with buonaparte; said it was not true, as reported, that buonaparte understood astronomical subjects deeply, but affected more than he knew. "in speaking of his great and chief telescope, he said with an air, not of the least pride, but with a greatness and simplicity of expression that struck me with wonder, 'i have looked further into space than ever human being did before me. i have observed stars, of which the light takes _two millions_ of years to travel to this globe.' i mean to pay him a reverential visit at slough, as soon as my book is out, this winter." * * * * * in carolina herschel has this entry in her diary: "_october_ .--my brother came from brighton. the same night two parties from the castle came to see the comet, and during the whole month my brother had not an evening to himself. as he was then in the midst of polishing the forty-foot mirror, rest became absolutely necessary after a day spent in that most laborious work; and it has ever been my opinion that on the th of october his nerves received a shock of which he never got the better afterwards." in the spring of he was quite seriously ill; but in may the observing went on again. in and his principal investigations were upon physical subjects (newton's rings), and in the only long series of observations was upon the comet of that year. after the state of herschel's health required that his observations should be much less frequent. much of the time after he was absent, and his work at home consisted largely in arranging the results of his previous labors, and in computations connected with them. all through the years to , herschel's health was very feeble. the severe winter of - had told materially upon him. in , however, he undertook to repolish the forty-foot mirror, but was obliged to give it over. he now found it necessary to make frequent little excursions for change of air and scene. his faithful sister remained at home, bringing order into the masses of manuscript, and copying the papers for the royal society. she was sick at heart, fearing that each time she saw her brother it would be the last. in she says: "feb. , i went to my brother and remained with him till the d. we spent our time, though not in idleness, in sorrow and sadness. he is not only unwell, but low in spirits." in (december ), herschel went to london to have his portrait painted by artaud. while he was in london his will was made.[ ] in there is a glimmer of the old-time light. in a note herschel says: "lina:--there is a great comet. i want you to assist me. come to dine and spend the day here. if you can come soon after one o'clock, we shall have time to prepare maps and telescopes. i saw its situation last night. it has a long tail. "_july , ._" this note has been carefully kept by his sister, and on it she has written: "i keep this as a relic. every line _now_ traced by the hand of my dear brother becomes a treasure to me." so the next three years passed away. sir william[ ] was daily more and more feeble. he spent his time in putting his works in order, but could devote only a few moments each day to this. his sister says: "_aug. th_, _ th_, _ th_, and _ th_ [ ], i went as usual to spend some hours of the forenoon with my brother. "_aug. th._--i hastened to the spot where i was wont to find him, with the newspaper which i was to read to him. but instead i found mrs. monson, miss baldwin, and mr. bulman, from leeds, the grandson of my brother's earliest acquaintance in this country. i was informed my brother had been obliged to return to his room, whither i flew immediately. lady h. and the housekeeper were with him, administering everything which could be thought of for supporting him. i found him much irritated at not being able to grant mr. bulman's request for some token of remembrance for his father. as soon as he saw me, i was sent to the library to fetch one of his last papers and a plate of the forty-foot telescope. but for the universe i could not have looked twice at what i had snatched from the shelf, and when he faintly asked if the breaking up of the milky way was in it, i said 'yes,' and he looked content. i cannot help remembering this circumstance; it was the last time i was sent to the library on such an occasion. that the anxious care for his papers and workrooms never ended but with his life, was proved by his frequent whispered inquiries if they were locked and the key safe, of which i took care to assure him that they were, and the key in lady herschel's hands. "after half an hour's vain attempt to support himself, my brother was obliged to consent to be put to bed, leaving no hope ever to see him rise again." on the th of august, , herschel died peacefully at the age of eighty-four years. his remains lie in the little church at upton, near windsor, where a memorial tablet has been erected by his son. the epitaph is as follows:[ ] h. s. e. gulielmus herschel eques guelphicus hanoviæ natus angliam elegit patriam astronomis ætatis suæ præstantissimis merito annumeratus ut leviora sileantur inventa planetam ille extra saturni orbitam primus detexit novis artis adjumentis innixus quæ ipse excogitavit et perfecit coelorum perrupit claustra et remotiora penetrans et explorans spatia incognitos astrorum ignes astronomorum oculis et intellectui subjecit qua sedulitate qua solertia corporum et phantasmatum extra systematis nostri fines lucentium naturam indagaverit quidquid paulo audacius conjecit ingenita temperans verecundia ultro testantur hodie æquales vera esse quæ docuit pleraque siquidem certiora futuris ingeniis subsidia debitura est astronomia agnoscent forte posteri vitam utilem innocuam amabilem non minus felici laborum exitu quam virtutibus ornatam et vere eximiam morte suis et bonis omnibus deflenda nec tamen immatura clausit die xxv augusti a. d. ci[c]i[c]cccxxii Ætatis vero suæ lxxxiv. footnotes: [ ] bode's _jahrbuch_, , p. . [ ] zach's _monatlich correspondenz_, , p. . [ ] bode's _jahrbuch_, , p. . [ ] through sir john herschel there is preserved to us an incident of his early boyhood, which shows the nature of the training his young mind received in the household at slough. walking with his father, he asked him "what was the oldest of all things?" the father replied, after the socratic manner, "and what do you suppose is the oldest of all things?" the boy was not successful in his answers, whereon the old astronomer took up a small stone from the garden walk: "there, my child, there is the oldest of all the things that i certainly know." on another occasion the father asked his son, "what sort of things do you think are most alike?" the boy replied, "the leaves of the same tree are most like each other." "gather, then, a handful of leaves from that tree," rejoined the philosopher, "and choose two which are alike."--_monthly notices royal astronomical society_, vol. xxxii., page . [ ] _memoir of caroline herschel_, p. . [ ] "of late years these expectations have been more than accomplished by the discovery of no fewer than four planetary bodies, almost all in the same place; but so small that dr. herschel refuses to honor them with the name of planets, and chooses to call them asteroids, though for what reason it is not easy to determine, unless it be to deprive the discoverers of these bodies of any pretence for rating themselves as high in the list of astronomical discoverers as himself."--_history of the royal society_, by thomas thomson, p. . this work was published in , and therefore during the lifetime of herschel. [ ] _poetical history of astronomy_: this work was nearly completed, but was never published. the whole of it was read to herschel, in order that burney might have the benefit of his criticism on its technical terms. [ ] _memoirs of dr. burney_, vol. iii., p. . [ ] life and letters of thomas campbell, edited by william beattie, vol. ii., p. . [ ] this interview must have taken place in , during herschel's journey to paris. we have no other record of it. [ ] the will of herschel was dated december th, . "the personal effects were sworn under £ , . the copyhold and other lands and tenements at upton-cum-chalvey, in the county of bucks, and at slough, he decrees to his son, with £ , in the per cent. reduced annuities. £ , are given to his brother johann dietrich, and annuities of £ each to his brother johann alexander and to his sister carolina; £ each to his nephews and nieces, and the residue (with the exception of astronomical instruments, telescopes, observations, etc., which he declares to have given, on account of his advanced age, to his son for the purpose of continuing his studies) is left solely to lady herschel."--_gentleman's magazine_, vol. xcii., , p. . it is not necessary to say here how nobly sir john herschel redeemed the trust confided in him. all the world knows of his survey of the southern heavens, in which he completed the review of the sky which had been begun and completed for the northern heavens by the same instruments in his father's hands. a glance at the bibliography at the end of this book will show the titles of several papers by sir john, written with the sole object of rendering his father's labors more complete. [ ] he was created a knight of the royal hanoverian guelphic order in , and was the first president of the royal astronomical society in , his son being its first foreign secretary. [ ] bode's _jahrbuch_, , p. . chapter iv. review of the scientific labors of william herschel. in this chapter i shall endeavor to give such explanations as will enable the general reader to follow the course of discovery in each branch of astronomy and physics, regularly through the period of herschel's life, and up to the state in which he left it. a more detailed and precise account, which should appeal directly to the professional astronomer, will not be needed, since arago has already fulfilled this want in his "_analyse de la vie et des travaux de sir william herschel_," published in . the few misconceptions there contained will be easily corrected by those to whom alone they are of consequence. the latter class of readers may also consult the abstracts of herschel's memoirs, which have been given in "_a subject-index and a synopsis of the scientific writings of sir william herschel_," prepared by dr. hastings and myself, and published by the smithsonian institution. an accurate sketch of the state of astronomy in england and on the continent, in the years - , need not be given. it will be enough if we remember that of the chief observatories of europe, public and private, no one was actively devoted to such labors as were undertaken by herschel at the very beginning of his career. his observations on variable stars, indeed, were in the same line as those of pigott; flaugergues and darquier, in france, had perhaps preceded him in minute scrutiny of the sun's surface, etc.; but, even in that department of observation, he at once put an immense distance between himself and others by the rapid and extraordinary advances in the size and in the excellence of his telescopes. before his time the principal aids to observation were the gregorian and newtonian telescopes of short, and the small achromatics of dollond.[ ] we have seen, in what goes before, how his patient zeal had succeeded in improving upon these. there was no delay, and no rest. steadily the art of making reflectors was urged forward, until he had finally in his hands the forty-foot telescope. it must be admitted that this was the limit to which the manufacture of powerful telescopes could be pushed in his generation. the optical and mechanical difficulties which prevented a farther advance required time for their solution; and, indeed, some of these difficulties are scarcely solved at this day. it may fairly be said that no reflector larger than three feet in aperture has yet realized our expectations. _the improvement of telescopes and optical apparatus._ it will be of interest to give in this place some connected account of the large forty-foot reflector, of four feet aperture, made by herschel. its history extends from to . its manufacture was considered by his cotemporaries as his greatest triumph. as a machine, it was extremely ingenious in all its parts, as may be seen from the elaborate description and plates of it published in the _philosophical transactions_ for . one of its mirrors certainly had good definition, for, by means of it, the two small satellites of _saturn_ (_mimas_ and _enceladus_) were discovered, and these discoveries alone would make it famous. perhaps more was expected of it by the public in general than it absolutely performed. its merits were after a while decried, and herschel even felt obliged to state why he did not always employ it in his observations. his reasons were perfectly valid, and such as any one may understand. the time required to get so large a machine into working order was a serious tax; it required more assistants than his twenty-foot telescope, and he says, "i have made it a rule never to employ a larger telescope when a smaller will answer the purpose." it still remains as a remarkable feat of engineering and an example of great optical and mechanical skill. it led the way to the large reflectors of lord rosse, some sixty years later, and several of the forty-foot telescopes of the present day even have done less useful work. its great feat, however, was to have added two satellites to the solar system. from the published accounts of it the following is taken: "when i resided at bath i had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. this i acquired by degrees at that place, where in my leisure hours, by way of amusement, i made several two-foot, five-foot, seven-foot, ten-foot, and twenty-foot newtonian telescopes, beside others, of the gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. in this way i made not less than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the gregorian telescopes.[ ] "the number of stands i invented for these telescopes it would not be easy to assign. . . . in i began to construct a thirty-foot aërial reflector, and having made a stand for it, i cast the mirror thirty-six inches in diameter. this was cracked in cooling. i cast it a second time, and the furnace i had built in my house broke." soon after, the georgian planet was discovered, and this interrupted the work for a time. "in the year i finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. after two years' observation with it, the great advantage of such apertures appeared so clearly to me that i recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which i wished to undertake, the president of the royal society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. his majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal bounty. "in consequence of this arrangement i began to construct the forty-foot telescope about the latter end of .[ ] the woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. in the whole of the apparatus none but common workmen were employed, for i made drawings of every part of it, by which it was easy to execute the work, as i constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. while the stand of the telescope was preparing, i also began the construction of the great mirror, of which i inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where i then lived, to my present situation at slough. "here, soon after my arrival, i began to lay the foundation upon which by degrees the whole structure was raised as it now stands, and the speculum being highly polished and put into the tube, i had the first view through it on february , . i do not, however, date the completing of the instrument till much later. for the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it. "a second mirror was cast january , , but it cracked in cooling. february we recast it, and it proved to be of a proper degree of strength. october it was brought to a pretty good figure and polish, and i observed the planet _saturn_ with it. but not being satisfied, i continued to work upon it till august , , when it was tried upon the fixed stars, and i found it to give a pretty sharp image. large stars were a little affected with scattered light, owing to many remaining scratches on the mirror. august the th, , having brought the telescope to the parallel of _saturn_, i discovered a _sixth_ satellite of that planet, and also saw the spots upon _saturn_ better than i had ever seen them before, so that i may date the finishing of the forty-foot telescope from that time." another satellite of _saturn_ was discovered with the forty-foot on the th of september ( ). it was used for various observations so late as . on january , of that year, herschel observed the nebula of _orion_ with it. this was one of his last observations. the final disposition of the telescope is told in the following extract from a letter of sir john herschel's to mr. weld, secretary of the royal society: "collingwood, _march , _. . . . "in reply to your queries, respecting the forty-foot reflecting telescope constructed by my father, i have to state that king george iii. munificently defrayed the _entire_ cost of that instrument (including, of course, all preparatory cost in the nature of construction of tools, and of the apparatus for casting, grinding, and figuring the reflectors, of which two were constructed), at a total cost of £ , . the woodwork of the telescope being so far decayed as to be dangerous, in the year i pulled it down, and piers were erected on which the tube was placed, _that_ being of iron and so well preserved, that, although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain inclosed within it, to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery. . . . the mirror and the rest of the polishing apparatus are on the premises. the iron grinding tools and polishers are placed underneath the tube, let into the ground, and level with the surface of the gravelled area in which it stands.". . . the closing of the tube was done with appropriate ceremony on new-year's-day, , when, after a procession through it by the family at slough, a poem, written by sir john, was read, the machinery put into its present position, and the tube sealed. the memoir on the forty-foot telescope shows throughout that herschel's prime object was not the making of the telescope itself, but that his mind was constantly directed towards the uses to which it was to be put--towards the questions which he wished it to answer. again and again, in his various papers, he returns to the question of the _limit of vision_. as bessel has said: "the naked eye has its limit of vision in the stars of the sixth magnitude. the light of fainter stars than these does not affect the retina enough for them to be seen. a very small telescope penetrates to smaller, and, in general, without doubt, to more distant stars. a more powerful one penetrates deeper into space, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions. whoever has followed the history of the series of herschel's telescopes will have observed this. but herschel was not content with the bare fact, but strove ever to know _how far_ a telescope of a certain construction and size could penetrate, compared with the naked and unassisted eye. these investigations were never for the discovery of new facts concerning the working of his instruments; it was for the knowledge of the distribution of the fixed stars in space itself that he strove. . . . herschel's instruments were designed to aid vision to the last extent. they were only secondarily for the taking of measures. his efforts were not for a knowledge of the _motions_, but of the _constitution_ and _construction_ of the heavenly bodies." besides the stands for his telescopes, which were both ingenious and convenient, herschel devised many forms of apparatus for facilitating the art of observation. his micrometers for measuring position angles, his lamp micrometer, the method of limiting apertures, and the methods he used for viewing the sun may be mentioned among these. points in practical astronomy are considered all through the years of observation. a reference to his original papers will show how numerous, how varied, and how valuable these are. i cannot forbear quoting here the account of a precaution observed during his examination of the belts on _saturn_ ( ). it is the most striking example of how fully herschel realized that the eye of the observer is a material part of the optical apparatus of astronomy. simple as this principle may appear, it was an absolute novelty in his day. in making these observations, he says: "i took care to bend my head so as to receive the picture of the belt in the same direction as i did formerly. this was a precaution that occurred to me, as there was a possibility that the vertical diameter of the retina might be more or less sensitive than the horizontal one." astronomers will recognize in this the first suggestion of the processes which have led to important results in the hands of dr. otto struve and others in the comparison of the measures of double stars by different observers, each of whom has a personal habit of observation, which, if not corrected, may affect his results in the way which herschel was striving to avoid. _researches on the relative brightness of the stars: variable stars._ no research of herschel's was more laborious than the elaborate classification of the stars according to their comparative brightness, which he executed during the years to . it was directly in the line of his main work--to find out the construction of the heavens. his first paper had been upon the variable star _mira ceti_. here was a sun, shining by its native brightness, which waxed and waned like the moon itself. this star is periodic. it is for a long period invisible to the unassisted eye. then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy. from this it decreases for nearly three months, and after becoming invisible, remains so for five or six months. its whole period is about days. are all other stars constant in brightness? the example of _mira ceti_ and of other known variables makes this at least doubtful. but the sun itself may vary for all that we know. it is a simple star like the rest. this question of variability in general is an important one, then. it can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. no such general catalogue existed before herschel's time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation. the method adopted by herschel was perfectly simple in principle, though most laborious in practice. suppose any number of stars, a, b, c, d, e, . . . etc., near enough to each other to be well compared. the process consists simply in writing down the names of the stars, a, b, c, etc., in the order of their relative brightness. thus if for a group of eight stars we have found at one epoch a, b, c, d, e, f, g, h, and if at another time the order was a, b, c, d, f, e, g, h, symptoms of variability are pointed out. repeated observations, where the same star is found in different sequences, will decide the question. thus, for the stars visible to the naked eye, we know exactly the state of the sky in herschel's day, now nearly a century ago. any material change cannot escape us. these catalogues have been singularly overlooked by the observers of our generation who have followed this branch of observation, and it was not till that they received proper attention and a suitable reduction (at the hands of mr. c. s. pierce). we owe to herschel the first trustworthy account of the stars visible to the naked eye, and since the date of his labors (about ) we have similar views published by argelander ( ), heis ( ), argelander and schÖnfeld ( ), gould ( and ), and houzeau ( ). thus his labors have been well followed up. in the prosecution of this work herschel found stars whose light was progressively diminishing, others which regularly increased, one star whose light periodically varies (_[alpha] herculis_), and at least one star ( _herculis_) which has utterly disappeared. on october , , and april , , he observed this latter star, but in may, , it had totally vanished. there was no trace remaining. the discovery of the variability of _[alpha] herculis_ was a more important one than would at first sight appear. up to that time the only variable stars known were seven in number. their periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days. these periods seemed to fall into two groups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days. _[alpha] herculis_ came to occupy the middle place between these groups, its period being about sixty days. the cause of these strange and regular variations of brightness was supposed by herschel to be the rotation of the star bodily on an axis, by which revolution different parts of its surface, of different brilliancy, were successively and periodically presented to us. this explanation it might have been difficult to receive, when the periods of the known variables were so markedly various in length. his own discovery came to bridge over the interval, and quite confirmed him in his belief. he returned to the subject of the revolution of stars about their axes again and again, and connected it with the revolution of satellites. he found that the satellites of _jupiter_ and one of _saturn's_ periodically changed in brightness, and by quite simple means showed that their periods of rotation were at least approximately the same as their periods of revolution about their primaries. in this case, as in every other, he considered a discovery in each and every one of its possible bearings. there are no instances where he has singularly overlooked the consequences of his observations. _researches on double stars._ the double stars were the subject of herschel's earliest and of his latest papers. in he published his "_catalogue of double stars_," and his last published memoir ( ) was on the same subject. the question of determining the parallax of stars first brought herschel to the discovery of double stars. if two stars, a and b, appear very close together, and if, in reality, the star b is very many times more distant from the earth than a, although seen along the same line of sight, then the revolution of the earth in its orbit will produce changes in the relative situation of a and b, and, in fact, b will describe a small orbit about a, due to this revolution. this idea had been proposed by galileo, and measures on this plan had been made by long, with negative results. but herschel, in reviewing their work, declares that the stars chosen by long were not suitable to the purpose. it is necessary, among other things, to the success of this method, that it should be certain that the star b is really very much more distant than the star a. the only general test of the distance of stars is their brilliancy, and herschel decided to use only stars for this research which had two components very greatly different in brightness. a must be very bright (and presumably near to us), and b must be very close to a, and very faint (and thus, presumably, very distant). it was in the search for such pairs of stars that the _catalogue of double stars_ ( ) was formed. herschel's first idea of a double star made such pairs as he found, to consist of two stars _accidentally_ near to each other. a was near to us, and appeared projected in a certain place on the celestial sphere. b was many times more distant, but, by chance, was seen along the same line, and made with a an _optical_ double. if the two stars were at the same distance from the earth, if they made part of the same physical system, if one revolved around the other, then this method of gaining a knowledge of their distance failed. even in his first memoir on the subject, a surmise that this latter state might occur in some cases, was expressed by herschel. the notes on some of the pairs declare that a motion of one of them was suspected. but this motion might be truly orbital--of one star about the other as a centre--or it might simply be that one star was moving by its own _proper_ motion, and leaving the other behind. it was best to wait and see. the first catalogue of double stars contained two hundred and three instances of such associations. these were observed from time to time, and new pairs discovered. the paper of michell, "an inquiry into the probable parallax and magnitude of the fixed stars, from the quantity of light which they afford, and the particular circumstances of their situation" ( ), was read and pondered. by herschel had become certain that there existed in the heavens real pairs of stars, both at the same distance from the earth, which were physically connected with each other. the arguments of michell have been applied by bessel to the case of one of herschel's double stars, in much the same order in which the argument ran in herschel's own mind, as follows: the star _castor_ (_[alpha] geminorum_) is a double star, where a is of the second, and b of the fourth, magnitude. to the naked eye these two appear as one star. with a telescope this is seen to be two stars, some " apart. in the whole sky there are not above fifty such stars as the brighter of the two, and about four hundred of the brilliancy of b. these fifty and four hundred stars are scattered over the vault of heaven, almost at random. no law has yet been traced by which we can say that here or here there shall be a bright star like a, or a fainter one like b. in general the distribution appears to be fortuitous. how then can we account for one of the four hundred stars like b placed so close to one of the fifty like a? the chances are over four hundred thousand to one that the association in position is not accidental. this argument becomes overwhelming when the same association is found in many other cases. there were two hundred and three doubles in the catalogue of alone, and many thousands are now known. by a process like this, herschel reached his grand discovery of true binary systems, where one sun revolves about another. for he saw that if the two stars are near together in space, they could not stand still in face of each other, but that they must revolve in true orbits. here was the discovery which came to take the place of the detection of the parallaxes of the fixed stars. he had failed in one research, but he was led to grand conclusions. was the force that these distant pairs of suns obeyed, the force of gravitation? this he could not settle, but his successors have done so. it was not till about that savary, of the paris observatory, showed that one of herschel's doubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large. herschel himself lived to see some of his double stars perform half a revolution. of herschel's discoveries, arago thinks this has "le plus d'avenir." it may well be so. the laws which govern our solar system have been extended, through his researches, to regions of unknown distance. the binary stars will afford the largest field for research into the laws which govern them, and together with the clusters and groups, they will give a firm basis by which to study the distribution of stars in general, since here we have the great advantage of knowing, if not the real distance of the two stars from the earth, at least that this distance is alike for both. _researches on planets and satellites._ after herschel's first publication on the mountains of the moon ( ), our satellite appears to have occupied him but little. the observation of volcanoes ( ) and of a lunar eclipse are his only published ones. the planets _mercury_, _venus_, _mars_, and _jupiter_, although they were often studied, were not the subjects of his more important memoirs. the planet _saturn_, on the contrary, seems never to have been lost sight of from the time of his first view of it in . the field of discovery always appears to be completely occupied until the advent of a great man, who, even by his way of putting old and familiar facts, shows the paths along which discoveries must come, if at all. this faculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality. herschel's paper on _saturn_, in , is an admirable example of this. herschel's observations on _saturn_ began in . from to he published six memoirs on the figure, the ring, and the satellites of this planet. the spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the constitution of the rings to his work. the sixth and seventh satellites, _mimas_ and _enceladus_, were discovered by him in . the periods of rotation of the ball and of the ring were also fixed. in his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive. in his paper of , herschel shows that the most distant satellite of _saturn_--_japetus_--turns once on its axis in each revolution about its primary, just as our moon does. he says of this: "i cannot help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondary planets of our solar system; and we may conjecture that probably most of the satellites are governed by the same law; especially if it be founded on such a construction of their figure as makes them more ponderous towards their primary planets." i believe the last suggestion to have been the first statement of the possible arrangement of matter in satellites, which was afterwards so forcibly maintained by hansen in his theory of the moon. hansen's researches show the consequences of such an arrangement, although they do not prove its existence. it should be recorded that the explanation which is to-day received of the belts and bands upon _jupiter_, is, i believe, first found in herschel's memoir on _venus_ ( ). his memoir of , on the changeable brightness of the satellites of _jupiter_, has already been referred to. the times of the rotation of the satellites on their axes was first determined by herschel from these observations, which also contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet. herschel discovered in january, , the two brighter satellites of _uranus_, now called _oberon_ and _titania_. they are among the faintest objects in the solar system. a later discussion of all his observations led him to the belief that there were four more, and he gives his observations and computations in full. he says that of the existence of additional satellites he has no doubt. of these four, three were exterior to the most distant satellite _oberon_, the other was "interior" to _titania_. it was not until that even _oberon_ and _titania_ were again observed (by sir john herschel) with a telescope of twenty feet, similar to that which had discovered them, and not until was the true state of this system known, when mr. lassell discovered _ariel_ and _umbriel_, two satellites interior to _titania_, neither of which was herschel's "interior" satellite. in and later years mr. lassell, by the aid of telescopes constructed by himself, fully settled the fact that only four satellites of this planet existed. in i examined the observations of herschel on his supposed "interior" satellite, thinking that it might be possible that among the very few glimpses of it which he recorded, some might have belonged to _ariel_ and some to _umbriel_, and that by combining rare and almost accidental observations of two satellites which really existed, he had come to announce the existence of an "interior" satellite which had no existence in fact. such i believe to be the case. in , april , herschel describes an interior satellite in the position angle °, distant " from the planet. at that instant _umbriel_, one of mr. lassell's satellites, was in the position °, and distant " from _uranus_, in the most favorable position for seeing it. the observation of , march , _may_ belong to _ariel_. at the best the investigation is of passing interest only, and has nothing to do with the question of the discovery of the satellites. herschel discovered the two brighter ones, and it was only sixty years later that they were properly re-observed by mr. lassell, who has the great honor of having added as many more, and who first settled the vexed question of satellites _exterior_ to _oberon_, and this with a reflecting telescope made by himself, which is unequalled by any other of its dimensions. _researches on the nature of the sun._ in the introduction to his paper on the _nature and construction of the sun and fixed stars_ ( ), herschel recounts what was known of the nature of the sun at that time. newton had shown that it was the centre of the system; galileo and his successors had determined its rotation, the place of its equator, its real diameter, magnitude, density, distance, and the force of gravity on its surface. he says: "i should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "the spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things." "the sun itself has been called a globe of fire, though, perhaps, metaphorically." "it is time now to profit by the observations we are in possession of. i have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena." herschel then refers to the theories advanced by his friend, prof. wilson, of glasgow, in . wilson maintained that the spots were depressions below the sun's atmosphere, vast hollows as it were, at the bases of which the true surface of the sun could be seen. the essence of his theory was the existence of two different kinds of matter in the sun: one solid and non-luminous--the nucleus--the other gaseous and incandescent--the atmosphere. vacant places in the atmosphere, however caused, would show the black surface of the solid mass below. these were the spots. no explanation could be given of the _faculæ_, bright streaks, which appear on the sun's surface from time to time; but his theory accounted for the existence of the black _nuclei_ of the spots, and for the existence of the _penumbræ_ about these. the penumbra of a spot was formed by the thinner parts of the atmosphere about the vacancy which surrounded the nucleus. this theory of wilson's was adopted by herschel as a basis for his own, and he brought numerous observations to confirm it, in the modified shape which he gave to it. according to herschel, the sun consisted of three essentially different parts. first, there was a solid nucleus, non-luminous, cool, and even capable of being inhabited. second, above this was an atmosphere proper; and, lastly, outside of this was a layer in which floated the clouds, or bodies which gave to the solar surface its intense brilliancy: "according to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions" above it. the two atmospheric layers, which will be of varying thickness about a spot, will account for all the shades of darkness seen in the penumbra. ascending currents from the solar surface will elevate certain regions, and may increase the solar activity near by, and will thus give rise to faculæ, which herschel shows to be elevated above the general surface. it will not be necessary to give a further account of this theory. the data in the possession of the modern theorist is a thousand-fold that to be derived from herschel's observations, and, while the subject of the internal construction of the sun is to-day unsettled, we know that many important, even fundamental, portions of his theory are untenable. a remark of his should be recorded, however, as it has played a great part in such theories: "that the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. many of the operations of nature are carried on in her great laboratory which we cannot comprehend. perhaps the many telescopic comets may restore to the sun what is lost by the emission of light." arguments in favor of the habitability of both sun and moon are contained in this paper; but they rest more on a metaphysical than a scientific basis, and are to-day justly forgotten. _researches on the motion of the sun and of the solar system in space._ in herschel writes, in regard to some of his discoveries of double stars: "these may serve another very important end. i will just mention it, though it is foreign to my present purpose. several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. . . . if this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change, due to the motion of the whole solar system." in he published his paper _on the proper motion of the solar system_, which contained the proofs of his surmises of a year before. that certain of the stars had in fact a _proper_ motion had been well established by the astronomers of the eighteenth century. after all allowances had been made for the effects of precession and other displacements of a star's position which were produced by motions of the earth, it was found that there were still small outstanding differences which must be due to the motion of the star itself--its proper motion. the quantity of this motion was not well known for any star when herschel's researches began. before they were concluded, however, maskelyne had deduced the proper motions of thirty-six stars--the fundamental stars, so called--which included in their number _sirius_, _procyon_, _arcturus_, and generally the brightest stars. it is _à priori_ evident that stars, in general, must have proper motions, when once we admit the universality of gravitation. that any fixed star should be entirely at rest would require that the attractions on all sides of it should be exactly balanced. any change in the position of this star would break up this balance, and thus, in general, it follows that stars must be in motion, since all of them cannot occupy such a critical position as has to be assumed. if but one fixed star is in motion, this affects all the rest, and we cannot doubt but that every star, our sun included, is in motion by an amount which varies from small to great. if the sun alone had a motion, and the other stars were at rest, the consequence of this would be that all the fixed stars would appear to be retreating _en masse_ from that point in the sky towards which we were moving. those nearest us would move more rapidly, those more distant less so. and in the same way, the stars from which the solar system was receding would seem to be approaching each other. if the stars, instead of being quite at rest, as just supposed, had motions proper to themselves, then we should have a double complexity. they would still appear to an observer in the solar system to have motions, and part of these motions would be truly proper to the stars, and part would be due to the advance of the sun itself in space. observations can show us only the _resultant_ of these two motions. it is for reasoning to separate this resultant into its two components. at first the question is to determine whether the results of observation indicate any solar motion at all. if there is none, the proper motions of stars will be directed along all possible lines. if the sun does truly move, then there will be a general agreement in the resultant motions of the stars near the ends of the line along which it moves, while those at the sides, so to speak, will show comparatively less systematic effect. it is as if one were riding in the rear of a railway train and watching the rails over which it has just passed. as we recede from any point, the rails at that point seem to come nearer and nearer together. if we were passing through a forest, we should see the trunks of the trees from which we were going apparently come nearer and nearer together, while those on the sides of us would remain at their constant distance, and those in front would grow further and further apart. these phenomena, which occur in a case where we are sensible of our own motion, serve to show how we may deduce a motion, otherwise unknown, from the appearances which are presented by the stars in space. in this way, acting upon suggestions which had been thrown out previously to his own time by lambert, mayer, and bradley, herschel demonstrated that the sun, together with all its system, was moving through space in an unknown and majestic orbit of its own. the centre round which this motion is directed cannot yet be assigned. we can only know the point in the heavens towards which our course is directed--"the apex of solar motion." by a study of the proper motions assigned by maskelyne to the brighter stars, herschel was able to define the position of the solar apex with an astonishing degree of accuracy. his calculations have been several times repeated with the advantage of modern analytical methods, and of the hundred-fold material now at our disposition, but nothing essential has been added to his results of , which were based upon such scanty data; and his paper of contains the announcement of the discovery itself. his second paper on the _direction_ and _velocity_ of the solar system ( ) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. for sustained reflection and high philosophic thought it is to be ranked with the researches of newton in the _principia_. _researches on the construction of the heavens._ herschel's papers on the construction of the heavens, as he named it, extended over his whole scientific life. by this he specially means the method according to which the stars, the clusters, the nebulæ, are spread through the regions of space, the causes that have led to this distribution, and the laws to which it is subjected. no single astronomical fact is unimportant in the light which it may throw on the scheme of the whole, and each fact is to be considered in this light. as an instance: his discovery of the variable star _[alpha] herculis_, which has a period of sixty days, was valuable in itself as adding one more to the number of those strange suns whose light is now brighter, now fainter, in a regular and periodic order. but the chief value of the discovery was that now we had an instance of a periodic star which went through all its phases in sixty days, and connected, as it were, the stars of short periods (three to seven days) with those of very long ones (three hundred to five hundred days), which two groups had, until then, been the only ones known. in the same way all his researches on the parallaxes of stars were not alone for the discovery of the distance of any one or two single stars, but to gain a unit of celestial measure, by means of which the depths of space might be sounded. astronomy in herschel's day considered the bodies of the solar system as separated from each other by distances, and as filling a cubical space. the ideas of near and far, of up and down, were preserved, in regard to them, by common astronomical terms. but the vast number of stars seemed to be thought of, as they appear in fact to exist, lying on the surface of a hollow sphere. the immediate followers of bradley used these fixed stars as points of reference by which the motions within the solar system could be determined, or, like lacaille and lalande, gathered those immense catalogues of their positions which are so indispensable to the science. michell and herschel alone, in england, occupied their thoughts with the nature and construction of the heavens--the one in his study, the other through observation.[ ] they were concerned with all three of the dimensions of space. in his memoir of , herschel says: "hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere, in the centre of which the eye of an observer might be supposed to be placed. "it is true the various magnitudes of the fixed stars even then plainly suggested to us, and would have better suited, the idea of an expanded firmament of three dimensions; but the observations upon which i am now going to enter still farther illustrate and enforce the necessity of considering the heavens in this point of view. in future, therefore, we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. the surface of a globe or map, therefore, will but ill delineate the interior parts of the heavens." herschel's method of study was founded on a mode of observation which he called _star-gauging_. it consisted in pointing a powerful telescope toward various parts of the heavens, and ascertaining by actual count how thick the stars were in each region. his twenty-foot reflector was provided with such an eye-piece that, in looking into it, he saw a portion of the heavens about ' in diameter. a circle of this size on the celestial sphere has about one quarter the apparent surface of the sun, or of the full moon. on pointing the telescope in any direction, a greater or less number of stars were visible. these were counted, and the direction in which the telescope pointed was noted. gauges of this kind were made in all parts of the sky, and the results were tabulated in the order of right ascension. the following is an extract from the gauges, and gives the average number of stars in each field at the points noted in right ascension and north polar distance: ---------------------------------------------------------- | n. p. d. || | n. p. d. r. a. | ° to °. || r. a. | ° to °. | no. of stars. || | no. of stars. ------------|-----------------||-----------|-------------- h. m. | || h. m. | | . || | . | . || | . | . || | . | . || | . | . || | . | . || | . ---------------------------------------------------------- in this small table, it is plain that a different law of clustering or of distribution obtains in the two regions. such differences are still more marked, if we compare the extreme cases found by herschel, as r. a. = h m, n. p. d. = ° ', number of stars per field = ; and r. a. = h m, n. p. d. = ° ', number of stars = . . the number of stars in certain portions is very great. for example, in the milky way, near _orion_, six fields of view promiscuously taken gave , , , , , and stars each, or a mean of stars per field. the most vacant space in this neighborhood gave stars. so that as herschel's sweeps were two degrees wide in declination, in one hour ( °) there would pass through the field of his telescope , or more stars. in some of the sweeps this number was as great as , stars in a quarter of an hour. when herschel first applied his telescope to the milky way, he believed that it completely resolved the whole whitish appearance into small stars. this conclusion he subsequently modified. he says: "it is very probable that the great stratum called the milky way is that in which the sun is placed, though perhaps not in the very centre of its thickness. "we gather this from the appearance of the galaxy, which seems to encompass the whole heavens, as it certainly must do if the sun is within it. for, suppose a number of stars arranged between two parallel planes, indefinitely extended every way, but at a given considerable distance from each other; and calling this a sidereal stratum, an eye placed somewhere within it will see all the stars in the direction of the planes of the stratum projected into a great circle, which will appear lucid on account of the accumulation of the stars, while the rest of the heavens, at the sides, will only seem to be scattered over with constellations, more or less crowded according to the distance of the planes, or number of stars contained in the thickness or sides of the stratum. "if the eye were placed somewhere without the stratum, at no very great distance, the appearance of the stars within it would assume the form of one of the smaller circles of the sphere, which would be more or less contracted according to the distance of the eye; and, if this distance were exceedingly increased, the whole stratum might at last be drawn together into a lucid spot of any shape, according to the length, breadth, and height of the stratum. "suppose that a smaller stratum should branch out from the former in a certain direction, and that it also is contained between two parallel planes, so that the eye is contained within the great stratum somewhere before the separation, and not far from the place where the strata are still united. then this second stratum will not be projected into a bright circle like the former, but it will be seen as a lucid branch proceeding from the first, and returning into it again at a distance less than a semicircle. if the bounding surfaces are not parallel planes, but irregularly curved surfaces, analogous appearances must result." the milky way, as we see it, presents the aspect which has been just accounted for, in its general appearance of a girdle around the heavens and in its bifurcation at a certain point, and herschel's explanation of this appearance, as just given, has never been seriously questioned. one doubtful point remains: are the stars scattered all through space? or are they near its bounding planes, or clustered in any way within this space so as to produce the same result to the eye as if uniformly distributed? herschel assumed that they were nearly equably arranged all through the space in question. he only examined one other arrangement, _viz._, that of a ring of stars surrounding the sun, and he pronounced against such an arrangement, for the reason that there is absolutely nothing in the size or brilliancy of the sun to cause us to suppose it to be the centre of such a gigantic system. no reason, except its importance to us personally, can be alleged for such a supposition. every star will have its own appearance of a galaxy or milky way, which will vary according to the situation of the star. such an explanation will account for the general appearances of the milky way and of the rest of the sky, supposing the stars equally or nearly equally distributed in space. on this supposition, the system must be deeper where the stars appear most numerous. herschel endeavored, in his early memoirs, to explain this inequality of distribution on the fundamental assumption that the stars were nearly equably distributed in space. if they were so distributed, then the number of stars visible in any gauge would show the thickness of the stellar system in the direction in which the telescope was pointed. at each pointing, the field of view of the instrument includes all the visible stars situated within a cone, having its vortex at the observer's eye, and its base at the very limits of the system, the angle of the cone (at the eye) being '. then the cubes of the perpendiculars let fall from the eye, on the plane of the bases of the various visual cones, are proportional to the solid contents of the cones themselves, or, as the stars are supposed equally scattered within all the cones, the cube roots of the numbers of stars in each of the fields express the relative lengths of the perpendiculars. a _section_ of the sidereal system along any great circle can be constructed from the data furnished by the gauges in the following way: the solar system is within the mass of stars. from this point lines are drawn along the different directions in which the gauging telescope was pointed. on these lines are laid off lengths proportional to the cube roots of the number of stars in each gauge. the irregular line joining the terminal points will be approximately the bounding curve of the stellar system in the great circle chosen. within this line the space is nearly uniformly filled with stars. without it is empty space. a similar section can be constructed in any other great circle, and a combination of all such would give a representation of the shape of our stellar system. the more numerous and careful the observations, the more elaborate the representation, and the gauges of herschel are sufficient to mark out with great precision the main features of the milky way, and even to indicate some of its chief irregularities. on the fundamental assumption of herschel (equable distribution), no other conclusion can be drawn from his statistics but the one laid down by him. this assumption he subsequently modified in some degree, and was led to regard his gauges as indicating not so much the _depth of the system_ in any direction, as the _clustering power or tendency_ of the stars in those special regions. it is clear that if in any given part of the sky, where, on the average, there are ten stars (say) to a field, we should find a certain small portion having or more to a field, then, on herschel's first hypothesis, rigorously interpreted, it would be necessary to suppose a spike-shaped protuberance directed from the earth, in order to explain the increased number of stars. if many such places could be found, then the probability is great that this explanation is wrong. we should more rationally suppose some real inequality of star distribution here. it is, in fact, in just such details that the method of herschel breaks down, and a careful examination of his system leads to the belief that it must be greatly modified to cover all the known facts, while it undoubtedly has, in the main, a strong basis. the stars are certainly not uniformly distributed, and any general theory of the sidereal system must take into account the varied tendency to aggregation in various parts of the sky. in , herschel published an important memoir on the same subject, in which his first method was largely modified, though not abandoned. its fundamental principle was stated by him as follows: "it is evident that we cannot mean to affirm that the stars of the fifth, sixth, and seventh magnitudes are really smaller than those of the first, second, or third, and that we must ascribe the cause of the difference in the apparent magnitudes of the stars to a difference in their relative distances from us. on account of the great number of stars in each class, we must also allow that the stars of each succeeding magnitude, beginning with the first, are, one with another, further from us than those of the magnitude immediately preceding. the relative magnitudes give only relative distances, and can afford no information as to the real distances at which the stars are placed. "a standard of reference for the arrangement of the stars may be had by comparing their distribution to a certain properly modified equality of scattering. the equality which i propose does not require that the stars should be at equal distances from each other, nor is it necessary that all those of the same nominal magnitude should be equally distant from us." it consisted in allotting a certain equal portion of space to every star, so that, on the whole, each equal portion of space within the stellar system contains an equal number of stars. the space about each star can be considered spherical. suppose such a sphere to surround our own sun. its radius will not differ greatly from the distance of the nearest fixed star, and this is taken as the unit of distance. suppose a series of larger spheres, all drawn around our sun as a centre, and having the radii , , , , etc. the contents of the spheres being as the cubes of their diameters, the first sphere will have × × = times the volume of the unit sphere, and will therefore be large enough to contain stars; the second will have times the volume, and will therefore contain stars, and so on with the successive spheres. for instance, the sphere of radius has room for stars, but of this space parts belong to the spheres inside of it; there is, therefore, room for stars between the spheres of radii and . herschel designates the several distances of these layers of stars as orders; the stars between spheres and are of the first order of distance, those between and of the second order, and so on. comparing the room for stars between the several spheres with the number of stars of the several magnitudes which actually exists in the sky, he found the result to be as follows: -------------------------------------------------------- order of | number of | | number of distance. | stars there | magnitude. | stars of that | is room for. | | magnitude. -------------------------------------------------------- ........ | | | ........ | | | ........ | | | ........ | | | ........ | | | , ........ | | | , ........ | , | | , ........ | , | | --------------------------------------------------------- the result of this comparison is, that if the order of magnitudes could indicate the distance of the stars, it would denote at first a gradual and afterward a very abrupt condensation of them, at and beyond the region of the sixth-magnitude stars. if we assume the brightness of any star to be inversely proportional to the square of its distance, it leads to a scale of distance different from that adopted by herschel, so that a sixth-magnitude star on the common scale would be about of the eighth order of distance according to this scheme--that is, we must remove a star of the first magnitude to eight times its actual distance to make it shine like a star of the sixth magnitude. on the scheme here laid down, herschel subsequently assigned the _order_ of distance of various objects, mostly star-clusters, and his estimates of these distances are still quoted. they rest on the fundamental hypothesis which has been explained, and the error in the assumption of equal intrinsic brilliancy for all stars affects these estimates. it is perhaps probable that the hypothesis of equal brilliancy for all stars is still more erroneous than the hypothesis of equal distribution, and it may well be that there is a very large range indeed in the actual dimensions and in the intrinsic brilliancy of stars at the same order of distance from us, so that the tenth-magnitude stars, for example, may be scattered throughout the spheres which herschel would assign to the seventh, eighth, ninth, tenth, eleventh, twelfth, and thirteenth magnitudes. however this may be, the fact remains that it is from herschel's groundwork that future investigators must build. he found the whole subject in utter confusion. by his observations, data for the solution of some of the most general questions were accumulated, and in his memoirs, which struve well calls "immortal," he brought the scattered facts into order and gave the first bold outlines of a reasonable theory. he is the founder of a new branch of astronomy. _researches for a scale of celestial measures. distances of the stars._ if the stars are _supposed_ all of the same absolute brightness, their brightness to the eye will depend only upon their distance from us. if we call the brightness of one of the fixed stars at the distance of _sirius_, which may be used as the unity of distance, , then if it is moved to the distance , its apparent brightness will be one-fourth; if to the distance , one-ninth; if to the distance , one-sixteenth, and so on, the apparent brightness diminishing as the square of the distance increases. the distance may be taken as an order of magnitude. stars at the _distances_ two, three, four, etc., herschel called of the second, third, and fourth magnitudes. by a series of experiments, the details of which cannot be given here, herschel determined the space-penetrating power of each of his telescopes. the twenty-foot would penetrate into space seventy-five times farther than the naked eye; the twenty-five foot, ninety-six times; and the forty-foot, one hundred and ninety-two times. if the seventh-magnitude stars are those just visible to the naked eye, and if we still suppose all stars to be of equal intrinsic brightness, such seventh-magnitude stars would remain visible in the forty-foot, even if removed to , times the distance of _sirius_ ( , = × ). if, further, we suppose that the visibility of a star is strictly proportional to the total intensity of the light from it which strikes the eye, then a condensed cluster of , stars of the , th magnitude could still be seen in the forty-foot at a distance where each star would have become , times fainter, that is, at about times the distance of _sirius_ ( × = , ). the light from the nearest star requires some three years to reach the earth. from a star , times farther it would require about , years, and for such a cluster as we have imagined no less than , years are needed. that is, the light by which we see such a group has not just now left it. on the contrary, it has been travelling through space for centuries and centuries since it first darted forth. it is the ancient history of such groups that we are studying now, and it was thus that herschel declared that telescopes penetrated into time as well as into space. other more exact researches on the relative light of stars were made by herschel. these were only one more attempt to obtain a scale of celestial distances, according to which some notion of the limits and of the interior dimensions of the universe could be gained. two telescopes, _exactly equal_ in every respect, were chosen and placed side by side. pairs of stars which were _exactly equal_, were selected by means of them. by diminishing the aperture of one telescope directed to a bright star, and keeping the other telescope unchanged and directed to a fainter star, the two stars could be equalized in light, and, from the relative size of the apertures, the relative light of this pair of stars could be accurately computed, and so on for other pairs. this was the first use of the method of _limiting apertures_. his general results were that the stars of the first magnitude would still remain visible to the naked eye, even if they were at a distance from us _twelve_ times their actual distance. this method received a still further development at his hands. he did not leave it until he had gained all the information it was capable of giving. he prepared a set of telescopes collecting , , , etc. ( × , × , × , etc.), times as much light as the naked eye. these were to extend the determinations of distance to the telescopic stars. for example, a certain portion of the heavens which he examined contained no star visible to the naked eye, but many telescopic stars. we cannot say that no one of these is as bright in itself as some of our first-magnitude stars. the smallest telescope of the set showed a large number of stars; these must, then, be _twice_ as far from us, on the average, as the stars just visible to the naked eye. but first-magnitude stars, like _sirius_, _procyon_, _arcturus_, etc., become just visible to the eye if removed to twelve times their present distance. hence the stars seen in this first telescope of the set were between twelve and twenty-four times as far from us as _arcturus_, for example. "at least," as herschel says, "we are certain that if stars of the size and lustre of _sirius_, _arcturus_, etc., were removed into the profundity of space i have mentioned, they would then appear like the stars which i saw." with the next telescope, which collected nine times more light than the eye, and brought into view objects three times more distant, other and new stars appeared, which were then ( × ) thirty-six times farther from us than _arcturus_. in the same way, the seven-foot reflector showed stars times, the ten-foot times, the twenty-foot times farther from us than the average first-magnitude star. as the light from such a star requires three years to reach us, the light from the faintest stars seen by the twenty-foot would require , years ( × ). but herschel was now ( ) convinced that the twenty-foot telescope could not penetrate to the boundaries of the milky way; the faintest stars of the galaxy must then be farther from us even than nine hundred times the distance of _arcturus_, and their light must be at least , years old when it reaches us. there is no escaping a certain part of the consequences established by herschel. it is indeed true that unless a particular star is of the same intrinsic brightness as our largest stars, this reasoning does not apply to it; in just so far as the average star is less bright than the average brightness of our largest stars, will the numbers which herschel obtained be diminished. but for every star of which his hypothesis is true, we may assert that his conclusions are true, and no one can deny, with any show of reason, that, on the whole, his suppositions must be valid. on the whole, the stars which we call faint are farther from us than the brighter ones; and, on the whole, the brilliancy of our brightest and nearest stars is not very far from the brilliancy of the average star in space. we cannot yet define the word _very_ by a numerical ratio. the _method_ struck out by herschel was correct; it is for his successors to look for the special cases and limitations, to answer the question, at a certain distance from us, what are the variations which actually take place in the brilliancy and the sizes of stars? the answer to this question is to be found in the study of the clusters of regular forms, where we _know_ the stars to be all at the same distance from us. _researches on light and heat, etc._ frequently in the course of his astronomical work, herschel found himself confronted by questions of physics which could not be immediately answered in the state of the science at that time. in his efforts to find a method for determining the dimensions of the stellar universe, he was finally led, as has been shown, to regard the brightness of a star as, in general, the best attainable measure of its distance from us. his work, however, was done with telescopes of various dimensions and powers, and it was therefore necessary to find some law for comparing the different results among themselves as well as with those given by observations with an unassisted eye. this necessity prompted an investigation, published in , in which, after drawing the distinction between absolute and intrinsic brightness, herschel gave an expression for the _space-penetrating power_ of a telescope. the reasoning at the base of this conception was as follows. the ratio of the light entering the eye when directed toward a star, to the whole light given out by the star, would be as the area of the pupil of the eye to the area of the whole sphere having the star as a centre and our distance from the star as a radius. if the eye is assisted by a telescope, the ratio is quite different. in that case the ratio of the light which enters the eye to the whole light, would be as the area of the mirror or object-glass to the area of the whole sphere having the star as a centre and its distance as a radius. thus the light received by the _eye_ in the two cases would be as the area of the pupil is to the area of the object-glass. for instance, if the pupil has a diameter of two-fifths of an inch, and the mirror a diameter of four inches, then a hundred times as much light would enter the eye when assisted by the telescope as when unarmed, since the _area_ of the pupil is one-hundredth the _area_ of the objective. if a particular star is just visible to the naked eye, it will be quite bright if viewed with this special telescope, which makes it one hundred times more brilliant in appearance. if we could move the star bodily away from us to a distance ten times its present distance, we could thus reduce its brightness, as seen with the telescope, to what it was at first, as seen with the eye alone, _i. e._, to bare visibility. moving the star to ten times its present distance would increase the surface of the sphere which it illuminates a hundred-fold. we cannot move any special star, but we can examine stars of all brightnesses, and thus (presumably) of all distances. herschel's argument was, then, as follows: since with such a telescope one can see a star ten times as far off as is possible to the naked eye, this telescope has the power of penetrating into space ten times farther than the eye alone. but this number ten, also, expresses the ratio of the diameter of the objective to that of the pupil of the eye, consequently the general law is that the _space-penetrating power_ of a telescope is found by dividing the diameter of the mirror in inches by two-fifths. the diameter of the pupil of the eye (two-fifths of an inch) herschel determined by many measures. this simple ratio would only hold good, however, provided no more light were lost by the repeated reflections and refractions in the telescope than in the eye. that light must be so lost was evident, but no data existed for determining the loss. herschel was thus led to a long series of photometric experiments on the reflecting powers of the metals used in his mirrors, and on the amount of light transmitted by lenses. applying the corrections thus deduced experimentally, he found that the space-penetrating power of his twenty-foot telescope, with which he made his star-gauges, was sixty-one times that of the unassisted eye, while the space-penetrating power of his great forty-foot telescope was one hundred and ninety-two times that of the eye. in support of his important conclusions herschel had an almost unlimited amount of experimental data in the records of his observations, of which he made effective use. by far the most important of herschel's work in the domain of pure physics was published in the same year ( ), and related to radiant heat. the investigation of the space-penetrating powers of telescopes was undertaken for the sole purpose of aiding him in measuring the dimensions of the stellar universe, and there was no temptation for him to pursue it beyond the limits of its immediate usefulness. but here, though the first hint leading to remarkable discoveries was a direct consequence of his astronomical work, the novelty and interest of the phenomena observed induced him to follow the investigation very far beyond the mere solution of the practical question in which it originated. having tried many varieties of shade-glasses between the eye-piece of his telescope and the eye, in order to reduce the inordinate degree of heat and light transmitted by the instrument when directed towards the sun, he observed that certain combinations of colored glasses permitted very little light to pass, but transmitted so much heat that they could not be used; while, on the other hand, different combinations and differently colored glasses would stop nearly all the heat, but allow an inconveniently great amount of light to pass. at the same time he noticed, in the various experiments, that the images of the sun were of different colors. this suggested the question as to whether there was not a different heating power proper to each color of the spectrum. on comparing the readings of sensitive thermometers exposed in different portions of an intense solar spectrum, he found that, beginning with the violet end, he came to the maximum of light long before that of heat, which lay at the other extremity, that is, near the red. by several experiments it appeared that the maximum of illumination, _i. e._, the yellow, had little more than half the heat of the full red rays; and from other experiments he concluded that even the full red fell short of the maximum of heat, which, perhaps, lay even a little beyond the limits of the visible spectrum. "in this case," he says, "radiant heat will at least partly, if not chiefly, consist, if i may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum[ ] as to be unfit for vision. and admitting, as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumination should be in the middle of the refrangible rays; as those which have greater or less momenta are likely to become equally unfit for the impression of sight." in his second paper on this subject, published in the same year, herschel describes the experiments which led to the conclusion given above. this paper contains a remarkably interesting passage which admirably illustrates herschel's philosophic method. "to conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired whether light be essentially different from radiant heat? in answer to which i would suggest that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one. . . . if this be a true account of the solar heat, for the support of which i appeal to my experiments, it remains only for us to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat." we now know that the reasoning and conclusion here given are entirely correct, but they have for their basis only a philosophical conception, and not a series of experiments designed especially to test their correctness. such an experimental test of this important question was the motive for a third and last paper in this department of physics. this paper was published in volume ninety of the _philosophical transactions_, and gave the results of two hundred and nineteen quantitative experiments. here we are at a loss to know which to admire most--the marvellous skill evinced in acquiring such accurate data with such inadequate means, and in varying and testing such a number of questions as were suggested in the course of the investigation--or the intellectual power shown in marshalling and reducing to a system such intricate and apparently self-contradictory phenomena. it is true that this discussion led him to a different conclusion from that announced in the previous paper, and, consequently, to a false conclusion; but almost the only escape from his course of reasoning lay in a principle which belongs to a later period of intellectual development than that of herschel's own time. herschel made a careful determination of the quantitative distribution of light and of heat in the prismatic spectrum, and discovered the surprising fact that not only where the light was at a maximum the heat was very inconsiderable, but that where there was a maximum exhibition of heat, there was not a trace of light. "this consideration," he writes, "must alter the form of our proposed inquiry; for the question being thus at least partly decided, since it is ascertained that we have rays of heat which give no light, it can only become a subject of inquiry whether some of these heat-making rays may not have a power of rendering objects visible, superadded to their now already established power of heating bodies. this being the case, it is evident that the _onus probandi_ ought to lie with those who are willing to establish such an hypothesis, for it does not appear that nature is in the habit of using one and the same mechanism with any two of our senses. witness the vibration of air that makes sound, the effluvia that occasion smells, the particles that produce taste, the resistance or repulsive powers that affect the touch--all these are evidently suited to their respective organs of sense." it is difficult to see how the fallacy of this argument could have been detected by any one not familiar with the fundamental physiological law that the nature of a sensation is in no wise determined by the character of the agent producing it, but only by the character of the nerves acted upon; but, as already intimated, this law belongs to a later epoch than the one we are considering. herschel thus finally concluded that light and radiant heat were of essentially different natures, and upon this supposition he explained all of the phenomena which his numerous experiments had shown him. so complete and satisfactory did this work appear to the scientific world, that for a long time the question was looked upon as closed, and not until thirty-five years later was there any dissent. then the italian physicist, melloni, with instrumental means a thousand times more delicate than that of herschel, and with a far larger store of cognate phenomena, collected during the generation which had elapsed, to serve as a guide, discovered the true law. this, as we have seen, was at first adopted by herschel on philosophical grounds, and then rejected, since he did not at that time possess the key which alone could have enabled him to properly interpret his experiments. it is well to summarize the capital discoveries in this field made by herschel, more particularly because his claims as a discoverer seem to have been strangely overlooked by historians of the development of physical science. he, before any other investigator, showed that radiant heat is refracted according to the laws governing the refraction of light by transparent media; that a portion of the radiation from the sun is incapable of exciting the sensation of vision, and that this portion is the less refrangible; that the different colors of the spectrum possess very unequal heating powers, which are not proportional to their luminosity; that substances differ very greatly in their power of transmitting radiant heat, and that this power does not depend solely upon their color; and that the property of diffusing heat is possessed to a varying degree by different bodies, independently of their color. nor should we neglect to emphasize, in this connection, the importance of his measurements of the intensity of the heat and light in the different portions of the solar spectrum. it is the more necessary to state herschel's claims clearly, as his work has been neglected by those who should first have done him justice. in his "history of physics," poggendorff has no reference to herschel. in the collected works of verdet, long bibliographical notes are appended to each chapter, with the intention of exhibiting the progress and order of discovery. but all of herschel's work is overlooked, or indexed under the name of his son. one little reference in the text alone shows that his very name was not unknown. even in the great work of helmholtz on physiological optics, herschel's labors are not taken account of. it is easy to account for this seemingly strange neglect. herschel is known to this generation only as an astronomer. a study of his memoirs will show that his physical work alone should give him a very high rank indeed, and i trust that the brief summaries, which alone can be given here, will have made this plain. * * * * * we may conclude from the time expended, the elaborate nature of the experiments involved, and the character of the papers devoted to their consideration, that the portion of herschel's researches in physics which interested him to the greatest degree, was the investigation of the optical phenomena known as newton's rings. in he obtained the two object-glasses of huyghens, which were in the possession of the royal society, for the purpose of repeating newton's experiments, and in he read the last of his three papers on the subject. sir isaac newton had given some of his most vigorous efforts to the study of the phenomena of interference of light, which are exemplified in the colors of thin and of thick plates. the colors of thin plates are most conveniently studied in the regular form which they present when produced by a thin plate of air, limited on one side by a plane polished surface, and on the other by a spherical surface of long radius, such as the exterior surface of a convex lens, for example. the colors are then arranged in concentric circles, and, though others had so produced them before newton, these rings have, ever since the publication of his remarkable work, been known by his name. to explain the phenomena, newton was obliged to supplement his theory of the corpuscular nature of light, by supposing that the inconceivably minute particles constituting light are not always equally susceptible of reflection, but that they have periodically recurring "fits of easy reflection" and of "easy transmission." this conception, though by no means unphilosophical, seemed to herschel too artificial and improbable for ready acceptance, and his effort was to supply a more probable explanation. the developments of optical science have justified herschel in his objections, but we cannot accord to him must any considerable part in making clear the true nature of the phenomenon. indeed, it must be recognized that his position was distinctly less advanced than that of newton. that great philosopher announced the true law governing the relation between the color and the thickness of the film. herschel did not recognize such a relation. newton showed exactly how the phenomenon depended upon the obliquity at which it was viewed. herschel found no place in his theory for this evident variation. in the series of experiments described in the first paper on this subject, herschel mistook the locus of a certain set of rings which he was observing. this mistake, though so slight as hardly to be detected without the guidance of the definite knowledge acquired in later times, not only vitiated the conclusion from the experiments, but gave an erroneous direction to the whole investigation. to him these experiments proved that newton's conception of a periodic phenomenon was untenable. thus cut loose from all hypothesis, his fertility in ideas and ingenuity in experimentation are as striking as ever. he tried the effect of having a polished metal as one of the surfaces limiting the thin plate of air. observing the so-called "blue bow" of newton at the limit of total reflection in a prism, he was led to the discovery of its complement, the "red bow" by refraction. here he thought he had found the solution of his problem, and attributed the rings to the reflection of the light which passed through in the red bow. though mistaken, he had presented to the world of science two experiments which have since played very prominent parts in the undulatory theory of light, namely, the rings formed upon polished metal, and the bands produced by a thin plate near the critical angle. as in his later researches upon the nature of radiant heat, he was wrong in his conclusions, and perhaps with less excuse. his experiments were skilfully devised and most ingenious. his philosophizing was distinctly faulty. we can see not only that he was wrong, but exactly where he began to go wrong. yet these papers are full of interest to the physicist, and by no means deserve the neglect into which they have fallen. _researches on the dimensions of the stars._ herschel examined a number of bright stars, using extremely high magnifying powers, in order to determine whether the stars have sensible dimensions. in a good telescope stars present round and pretty uniformly illuminated disks. if these disks really represent the angular diameter of the stars, they should admit of magnifying, like other objects; but, instead of this, herschel found that they appeared smaller as the telescopic power was increased. he accordingly called the disk of light seen in the telescope a spurious disk. this singular phenomenon gave its discoverer a ready criterion for determining whether a small bright body has an appreciable size, or only impresses the sense of sight by virtue of its intrinsic brightness. if the first were the case, the apparent size would increase with increased magnifying power, while, if the angular dimensions were inappreciable, the apparent size would, on the contrary, diminish with additional magnifying. an occasion for using this criterion came in the first years of this century, with the discovery of three small planets having orbits lying between those of _mars_ and _jupiter_. herschel gave the name _asteroids_ to these bodies. as the appropriateness of this term had been violently assailed, the discovery of _juno_, in , the third one of the group, led to a careful experimental study of the defining power of the telescope used, and of the laws governing the phenomena of spurious disks. with a telescope of about nine inches in aperture, herschel found that if _juno_ subtended an angle greater than a quarter of a second of arc, a certain indication of the fact would have shown itself in the course of the experiments. this conclusion was a justification of the name asteroid, since the appearance of the new planet was strictly stellar. on other grounds, a better name might have been selected. in the paper giving the results of the experiments, the phenomena of the spurious disks are very completely described; but they did not attract the attention which they deserved, and they only became an object of especial interest to students of physics when they were again studied by the famous german optician fraunhofer, a generation later. incidentally the experiments are of interest, as yielding us a measure of the excellence of herschel's telescopes, and a measure which is quite independent of the keenness of his vision. from them we may be sure that the efficiency of the nine-inch mirror used was not sensibly less than that of the highest theoretically attainable excellence. in this connection, too, we may refer to the _philosophical transactions_ for , pp. and , where herschel gives observations of both _enceladus_ and _mimas_ seen in contact with the ball of _saturn_. i have never seen so good definition, telescopic and atmospheric, as he must have had on these occasions. _researches on the spectra of the fixed stars._ the spectroscope was applied by secchi to the study of the spectra of the fixed stars visible to the naked eye in the years to . he examined the nature of the spectrum of each of the larger stars, and found that these stars could be arranged in three general classes or _types_. his results have been verified and extended by other astronomers, and his classification has been generally accepted. according to secchi, the lucid stars may be separated into three groups, distinguished by marked differences in their spectra. secchi's type i. contains stars whose spectra are like those of _sirius_, _procyon_, and _[alpha] lyræ_; his type ii. stars like _arcturus_ and _aldebaran_; his type iii. stars like _[alpha] orionis_. herschel also made some trials in this direction. in the _philosophical transactions_ for (p. ), he says: "by some experiments on the light of a few of the stars of the first magnitude, made in , by a prism applied to the eye-glasses of my reflectors, adjustable to any angle and to any direction, i had the following analyses: "the light of _sirius_ consists of red, orange, yellow, green, blue, purple, and violet. _[alpha] orionis_ contains the same colors, but the red is more intense, and the orange and yellow are less copious in proportion than they are in _sirius_. _procyon_ contains all the colors, but proportionately more blue and purple than _sirius_. _arcturus_ contains more red and orange, and less yellow in proportion than _sirius_. _aldebaran_ contains much orange and very little yellow. _[alpha] lyræ_ contains much yellow, green, blue, and purple." here the essential peculiarities of the spectrum of each of the stars investigated by herschel is pointed out, and if we were to use his observations alone to classify these stars into types, we should put _sirius_ and _procyon_ into one type of stars which have "all the colors" in their spectra; _arcturus_ and _aldebaran_ would represent another group of stars, with a deficiency of yellow and an excess of orange and red in the spectrum; and _[alpha] orionis_ would stand as a type of those stars with an excess of red and a deficiency of orange. _[alpha] lyræ_ would represent a sub-group of the first class. herschel's immediate object was not classification, and his observations are only recorded in a passing way. but the fact remains that he clearly distinguished the essential differences of the spectra of these stars, and that he made these observations in support of his statement that the fixed stars, "like the planets, also shine with differently colored light. that of _arcturus_ and _aldebaran_, for instance, is as different from the light of _sirius_ and _capella_ as that of _mars_ and _saturn_ is from the light of _venus_ and _jupiter_." of course, no special discovery can be claimed for him on these few instances. we can see, however, a good example of the manner in which he examined a subject from every side, and used the most remote evidence exactly in its proper place and time. _researches on the variable emission of light and heat from the sun._ it is certainly a remarkable fact that herschel was the first observer to recognize the real importance of the aperture or diameter of a telescope. before his time it was generally assumed that this element only conditioned the amount of light transmitted to the eye, or, in other words, merely determined the brightness of the image. hence the conclusion that if an object is sufficiently bright, the telescope may be made as small as desired without loss of power. thus, in observing the sun, astronomers before herschel had been accustomed to reduce the aperture of their telescopes, in order to moderate the heat and light transmitted. scheiner, it is true, nearly two centuries before the time we are considering, had invented a method for observing the sun without danger, still employing the full aperture. this was by projecting the image of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened. for special purposes this ingenious method has even been found useful in modern times, though for sharpness of definition it bears much the same relation to the more usual manner of observing, that a photographic picture does to direct vision. although herschel saw the advantages of using the whole aperture of a telescope in such observations, the practical difficulties in the way were very great. we have noted his attempts to find screens which would effectively cut off a large portion of the heat and light without impairing vision, and have considered, somewhat in detail, the remarkable discoveries in radiant heat to which these attempts led him. his efforts were not unsuccessful. a green glass smoked, and a glass cell containing a solution of black writing ink in water--were found to work admirably. thus provided with more powerful instrumental means than had ever been applied to the purpose, herschel turned his attention to the sun. in a very short time he exhausted nearly all there was to be discovered, so that since the publication of his last paper on this subject, in , until the present time, there has been but a single telescopic phenomenon, connected with the physical appearance of the sun, which was unknown to herschel. that phenomenon is the frequent occurrence of a darker central shade or kernel in large spots, discovered by dawes about . herschel, though observing a hundred and ninety years after the earliest discovery of sun spots, seems to have been the first to suspect their periodic character. to establish this as a fact, and to measure the period, was left for our own times and for the indefatigable observer schwabe. the probable importance of such a period in its relation to terrestrial meteorology was not only clearly pointed out by herschel, but he even attempted to demonstrate, from such data as were obtainable, the character of this influence. perhaps no one thing which this great philosopher has done better exhibits the catholic character of his mind than this research. when the possible connection of solar and terrestrial phenomena occurred to him as a question to be tested, there were no available meteorological records, and he could find but four or five short series of observations, widely separated in time. to an ordinary thinker the task would have seemed hopeless until more data had been collected. but herschel's fertile mind, though it could not recall lost opportunities for solar observations, did find a substitute for meteorological records in the statistics of the prices of grain during the various epochs. it is clear that the price of wheat must have depended upon the supply, and the supply, in turn, largely upon the character of the season. the method, as ingenious as it is, failed in herschel's hands on account of the paucity of solar statistics; but it has since proved of value, and has taken its place as a recognized method of research. _researches on nebulæ and clusters._ when herschel first began to observe the nebulæ in , there were very few of these objects known. the nebulæ of _orion_ and _andromeda_ had been known in europe only a little over a hundred years. in messier published a list of sixty-eight such objects which he had found in his searches for comets, and twenty-eight nebulæ had been found by lacaille in his observations at the cape of good hope. in the mere discovery of these objects herschel quickly surpassed all others. in he published a catalogue of one thousand new nebulæ, in a catalogue of a second thousand, and in one of five hundred. in all he discovered and described two thousand five hundred and eight new nebulæ and clusters. this branch of astronomy may almost be said to be proper to the herschels, father and son. sir john herschel re-observed all his father's nebulæ in the northern hemisphere, and added many new ones, and in his astronomical expedition to the cape of good hope he recorded almost an equal number in the southern sky. of the six thousand two hundred nebulæ now known the herschels discovered at least eight-tenths. the mere discovery of twenty-five hundred nebulæ would have been a brilliant addition to our knowledge of celestial statistics. herschel did more than merely point out the existence and position of these new bodies. each observation was accompanied by a careful and minute description of the object viewed, and with sketches and diagrams which gave the position of the small stars in it and near it.[ ] as the nebulæ and clusters were discovered they were placed in classes, each class covering those nebulæ which resembled each other in their general features. even at the telescope herschel's object was not discovery merely, but to know the inner constitution of the heavens. his classes were arranged with this end, and they are to-day adopted. they were: class i. "bright nebulæ ( in all). ii. "faint nebulæ ( in all). iii. "very faint nebulæ ( in all). iv. "planetary nebulæ, stars with burs, with milky chevelure, with short rays, remarkable shapes, etc. ( in all). v. "very large nebulæ ( in all). vi. "very compressed and rich clusters of stars ( in all). vii. "pretty much compressed clusters ( in all). viii. "coarsely scattered clusters of stars" ( in all). the lists of these classes were the storehouses of rich material from which herschel drew the examples by which his later opinions on the physical conditions of nebulous matter were enforced. as the nebulæ were discovered and classified they were placed upon a star-map in their proper positions ( ), and, as the discoveries went on, the real laws of the distribution of the nebulæ and of the clusters over the surface of the sky showed themselves more and more plainly. it was by this means that herschel was led to the announcement of the law that the spaces richest in nebulæ are distant from the milky way, etc. by no other means could he have detected this, and i believe this to have been the first example of the use of the graphical method, now become common in treating large masses of statistics. it is still in his capacity of an observer--an acute and wise one--that herschel is considered. but this was the least of his gifts. this vast mass of material was not left in this state: it served him for a stepping-stone to larger views of the nature and extent of the nebulous matter itself. his views on the nature of nebulæ underwent successive changes. at first he supposed all nebulæ to be but aggregations of stars. the logic was simple. to the naked eye there are many groups of stars which appear nebulous. _praesepe_ is, perhaps, the best example. the slightest telescopic power applied to such groups alters the nebulous appearance, and shows that it comes from the combined and confused light of discrete stars. other groups which remain nebulous in a seven-foot telescope, become stellar in a ten-foot. the nebulosity of the ten-foot can be resolved into stars by the twenty-foot, and so on. the nebulæ which remained still unresolved, it was reasonable to conclude, would yield to higher power, and generally a nebula was but a group of stars removed to a great distance. an increase of telescopic power was alone necessary to demonstrate this.[ ] "nebulæ can be selected so that an insensible gradation shall take place from a coarse cluster like the _pleiades_ down to a milky nebulosity like that in _orion_, every intermediate step being represented. this tends to confirm the hypothesis that all are composed of stars more or less remote." so, at first, herschel believed that his twenty-foot telescope was of power sufficient to fathom the milky way, that is, to see through it and beyond it, and to reduce all its nebulosities to true groups of stars. in he published a memoir on _nebulous stars_, in which his views were completely changed. he had found a nebulous star, the sixty-ninth of his class iv., to which his reasons would not apply. in the centre of it was a bright star; around the star was a halo gradually diminishing in brightness from the star outward, and perfectly circular. it was clear the two parts, star and nebula, were connected, and thus at the same distance from us. there were two possible solutions only. either the whole mass was, _first_, composed of stars, in which case the nucleus would be enormously larger than the other stars of its stellar magnitude elsewhere in the sky, or the stars which made up the halo indefinitely small; or, _second_, the central nucleus was indeed a star, but a star surrounded with "a shining fluid, of a nature totally unknown to us." the long strata of nebulæ, which he had before described under the name of "telescopic milky ways," might well be accounted for by masses of this fluid lying beyond the regions of the seventh-magnitude stars. this fluid might exist independently of stars. if it is self-luminous, it seems more fit to produce a star by its condensation, than to depend upon the star for its own existence. such were a few of the theorems to which his discovery of this nebula led him. the hypothesis of an elastic _shining fluid_ existing in space, sometimes in connection with stars, sometimes distinct from them, was adopted and never abandoned. how well the spectroscope has confirmed this idea it is not necessary to say. we know the shining fluid does exist, and in late years we have seen the reverse of the process imagined by herschel. a star has actually, under our eyes, become a planetary nebula, and the cycle of which he gave the first terms is complete. in five separate memoirs ( , , , , and ) herschel elaborated his views of the sidereal system. the whole extent of his views must be gained from the extended memoirs themselves. here only the merest outline can be given. in there is a marshaling of the various objects beyond our solar system. the stars themselves may be _insulated_, or may belong to _binary_ or _multiple_ systems, to _clusters_ and groups, or to grand groups like the milky way. nebulæ may have any of the forms which have been described; and, in , he gives examples of immense spaces in the sky covered with diffused and very faint nebulosity. "its abundance exceeds all imagination."[ ] these masses of nebular matter are the seats of attracting forces, and these forces must produce condensation. when a nebula has more than one preponderating seat of attracting matter, it may in time be divided, and the double nebulæ have had such an origin. when nebulæ appear to us as round masses, they are in reality globular in form, and this form is at once the effect and the proof of a gravitating cause. the central brightness of nebulæ points out the seat of the attraction; and the completeness of the approximation to a spherical form points out the length of time that the gravitating forces have been at work. those nebulæ (and clusters) which are most perfect in the globular form, have been longest exposed to central forces. the planetary nebulæ are the oldest in our system. they must have a rotatory motion on their axes. by progressive condensation planetary nebulæ may be successively converted into bright stellar nebulæ, or into nebulous stars, and these again, by the effects of the same cause, into insulated or double stars. this chain of theorems, laid down in the memoir of , is enforced in with examples which show how the nebulous appearance may grow into the sidereal. herschel selects from the hundreds of instances in his note-books, nebulæ in every stage of progress, and traces the effect of condensation and of clustering power through all its course, even to the final breaking up of the milky way itself. the memoirs of and add little to the general view of the physical constitution of the heavens. they are attempts to gain a scale of celestial measures by which we may judge of the distances of the stars and clusters in which these changes are going on. there is little to change in herschel's statement of the general construction of the heavens. it is the groundwork upon which we have still to build. every astronomical discovery and every physical fact well observed is material for the elaboration of its details or for the correction of some of its minor points. as a scientific conception it is perhaps the grandest that has ever entered into the human mind. as a study of the height to which the efforts of one man may go, it is almost without a parallel. the philosopher who will add to it to-day, will have his facts and his methods ready to his hands. herschel presents the almost unique example of an eager observer marshaling the multitude of single instances, which he himself has laboriously gathered, into a compact and philosophic whole. in spite of minor errors and defects, his ideas of the nature of the sidereal universe have prevailed, and are to-day the unacknowledged basis of our every thought upon it. some of its most secret processes have been worked out by him, and the paths which he pointed out are those along which our advances must be made. in concluding this condensed account of herschel's scientific labors, it behoves us to remember that there was nothing due to accident in his long life. he was born with the faculties which fitted him for the gigantic labors which he undertook, and he had the firm basis of energy and principle which kept him steadily to his work. as a practical astronomer he remains without an equal. in profound philosophy he has few superiors. by a kindly chance he can be claimed as the citizen of no one country. in very truth his is one of the few names which belong to the whole world. footnotes: [ ] james short, f.r.s. ( - ), and john dollond, f.r.s. ( - ), were the most celebrated makers of telescopes of their day. the six-foot newtonian reflectors of short (aperture . inches), and the forty-six-inch achromatics of dollond (aperture . inches), were highly esteemed. the royal observatory of greenwich possessed, in , one of each class. in a comparative trial of short's telescope, at greenwich, and one of herschel's first telescopes, the latter was adjudged greatly superior. [ ] at least _one_ of these telescopes had the principal mirror made of glass instead of metal.--_philosophical transactions_, . [ ] the following extract from fourier's _Éloge_ of herschel is of interest in this connection. the sum first appropriated by the king was £ , . this was afterwards raised to £ , , and a sum of £ yearly was given for maintenance. "l'histoire doit conserver à jamais la réponse de ce prince à un étranger célèbre [lalande?] qui le remerciait des sommes considérables accordées pour les progrès de l'astronomie. 'je fais les dépenses de la guerre,' dit le roi, 'parcequ'elles sont nécessaires; quant à celles des sciences, il m'est agréable des les ordonner; leur objet ne coûte point des larmes, et honore l'humanité.'" lalande's own account is a little different. he says the king exclaimed: "ne vaut-il pas mieux employer son argent à cela qu'à faire tuer des hommes?" [ ] the memoirs on the parallaxes of stars, written by various astronomers from to , were mainly directed to the improvement of the methods, or to the discovery of the parallax of some particular star. for example, lacaille's observations of _sirius_, at the cape of good hope, had resulted in a parallax of " for that star--a quantity over forty times too large. [ ] herschel accepted, as did all his cotemporaries, the newtonian or corpuscular theory of light. [ ] thus the position of small stars critically situated in the centre, or on the edges of the nebulæ was always noted. many of the descriptions are given in the published papers, but the publication of the diagrams would be an immense help to this branch of astronomy. d'arrest in his reduction of herschel's nebula observations ( ) writes: "gewiss wäre es vom höchsten interesse für die entwickelung, welche hoffentlich auch dieser zweig der beobachtenden astronomie zukünftig erhalten wird, wenn die herschel'schen beobachtungen in der ausführlichkeit in welcher sie, verschiedenen andeutungen zufolge, _handschriftlich_ vorhanden sind, veröffentlicht würden. es schliesst sich dieser wunsch in betreff der nebelflecken lebhaft an den an, welcher, schon vor einem jahrzehnt nach veröffentlichung der noch unedirten _star-gauges_ von gewichtigerer seite her geäussert wurde." in this all must agree who have a knowledge of the direction in which we must look for advances in the difficult and important questions of the distance, the motions, and the changes of the nebulæ. almost the only aid to be looked for from the older observations must come from such diagrams, and we may safely say that the publication of this priceless material, just as it stands, would carry our exact data back from to , or no less than forty-seven years. [ ] long after herschel had abandoned this idea, it continued current among astronomers. the successes of lord rosse's telescope perpetuated to the middle of the nineteenth century an erroneous view which herschel had given up in . [ ] these have never been re-observed. they should be sought for with a powerful refractor, taking special precautions against the illumination of the field of view from neighboring bright stars. herschel's reflectors were specially open to illusions produced in this way. his observations probably will remain untested until some large telescope is used in the way he adopted, _i. e._, in sweeping. bibliography. i.--list of the published writings of william herschel on astronomical subjects. [in chronological order.] _n.b.--in general, translations and abstracts of those which appeared in periodicals are not noticed here. i have made exceptions in the more important cases._ [solution of a prize question. _see_ this book, page .] _ladies' diary_, . astronomical observations on the periodical star in _collo ceti_. _phil. trans._, , p. . astronomical observations relating to the mountains of the moon. _phil. trans._, , p. . astronomical observations on the rotation of the planets round their axes, made with a view to determine whether the earth's diurnal motion is perfectly equable. _phil. trans._, , p. . account of a comet. [dated th march, . this was _uranus_.] _phil. trans._, , p. . on the parallax of the fixed stars. _phil. trans._, , p. . catalogue of double stars. _phil. trans._, , p. : translation in _bode's jahrbuch_, , p. . description of a lamp micrometer and the method of using it. _phil. trans._, , p. . a paper to obviate some doubts concerning the great magnifying powers used. _phil. trans._, , p. . a letter from william herschel, esq., f.r.s., to sir joseph banks, _bart._, p.r.s. _phil. trans._, , p. . aus einem schreiben des hrn. herschel an mich [bode], datirt london, den ten august, . [this is a letter forwarding herschel's memoir on the parallax of the fixed stars, etc.] _bode's jahrbuch_, , p. . on the diameter and magnitude of the _georgium sidus_, with a description of the dark and lucid disk and periphery micrometers. _phil. trans._, , p. . on the proper motion of the sun and solar system, with an account of several changes that have happened among the fixed stars since the time of mr. flamsteed. _phil. trans._, , p. . _bode's jahrbuch_, , p. , p. . astronomische nachrichten und entdeckungen, aus einem französischen schreiben desselben an mich [bode], datirt datchet, nahe bey windsor, den . mai, . [this letter is on the subject of the use of high magnifying powers, and gives a _résumé_ of his recent papers.] _bode's jahrbuch_, , p. . on the remarkable appearances at the polar regions of the planet _mars_, the inclination of its axis, the position of its poles and its spheroidical figure; with a few hints relating to its real diameter and atmosphere. _phil. trans._, , p. . account of some observations tending to investigate the construction of the heavens. _phil. trans._, , p. . [_bode's jahrbuch_, , p. , has a summary of this paper by baron von zach. see, also, _bode's jahrbuch_, , p. .] catalogue of double stars. _phil. trans._, , p. . on the construction of the heavens. _phil. trans._, , p. . _bode's jahrbuch_, , p. . see, also, _same_, , p. , and , p. . aus einem schreiben des hrn. herschel an mich [bode], datirt clay hall, nahe bey windsor, den . jul., . [this is a letter forwarding two memoirs, and giving the prices of telescopes] _bode's jahrbuch_, , p. . catalogue of one thousand new nebulæ and clusters of stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. , and _same_, , p. . investigation of the cause of that indistinctness of vision which has been ascribed to the smallness of the optic pencil. _phil. trans._, , p. . remarks on the new comet [ , ii.]. _phil. trans._, , p. . [letter from herschel to bode on the discovery of two satellites to _uranus_, dated slough, , feb. .] _bode's jahrbuch_, , p. . an account of the discovery of two satellites revolving round the _georgian planet_. _phil. trans._, , p. . _bode's jahrbuch_, , p. . an account of three volcanoes in the moon. _phil. trans._, , p. . _bode's jahrbuch_, , p. . note on m. mÉchain's comet. [ , i.] [added to preceding paper.] _phil. trans._, , p. . on the _georgian planet_ and its satellites. _phil. trans._, , p. . _bode's jahrbuch_, , p. . observations on a comet [ , ii.]. _phil. trans._, , p. . catalogue of a second thousand of new nebulæ and clusters of stars, with a few introductory remarks on the construction of the heavens. _phil. trans._, , p. . _bode's jahrbuch_, , p. . also, _same_, , p. . account of the discovery of a sixth and seventh satellite of the planet _saturn_, with remarks on the construction of its ring, its atmosphere, its rotation on an axis, and its spheroidical figure. _phil. trans._, , p. . _bode's jahrbuch_, , p. ; _same_, , p. ; , p. . on the satellites of the planet _saturn_, and the rotation of its ring on an axis. _phil. trans._, , p. . on nebulous stars properly so called. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the ring of _saturn_ and the rotation of the fifth satellite upon its axis. _phil. trans._, , p. . _bode's jahrbuch_, , p. . miscellaneous observations. [account of a comet], p. [ , i.]. [on the periodical appearance of omicron ceti], p. . [on the disappearance of the th _herculis_], p. . [remarkable phenomenon in an eclipse of the moon], p. . _phil. trans._, , p. . observations on the planet _venus_. _phil. trans._, , p. . observations of a quintuple belt on the planet _saturn_. _phil. trans._, , p. . _bode's jahrbuch_, , p. . account of some particulars observed during the late eclipse of the sun. [ , september th.] _phil. trans._, , p. . on the rotation of the planet _saturn_ upon its axis. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the nature and construction of the sun and fixed stars. _phil. trans._, , p. . _bode's jahrbuch_, ii. suppl. band, p. . description of a forty-foot reflecting telescope. _phil. trans._, , p. . _bode's jahrbuch_, iii. suppl. band, p. . additional observations on the comet. [ , i.] _phil. trans._, , p. . on the method of observing the changes that happen to the fixed stars; with some remarks on the stability of the light of our sun. to which is added a catalogue of comparative brightness for ascertaining the permanency of the lustre of stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the periodical star _[alpha] herculis_; with remarks tending to establish the rotatory motion of the stars on their axes; to which is added a second catalogue of the comparative brightness of the stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . a third catalogue of the comparative brightness of the stars, with an introductory account of an index to mr. flamsteed's observations of the fixed stars, contained in the second volume of the historia coelestis. to which are added several useful results derived from that index. _phil. trans._, , p. . _bode's jahrbuch_, , p. . observations of the changeable brightness of the satellites of _jupiter_, and of the variation in their apparent magnitudes, with a determination of the time of their rotatory motions on their axes. to which is added a measure of the diameter of the second satellite, and an estimate of the comparative size of all the four. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the discovery of four additional satellites of the _georgium sidus_. the retrograde motion of its old satellites announced, and the cause of their disappearance at certain distances from the planet explained. _phil. trans._, , p. . _bode's jahrbuch_, , p. . a fourth catalogue of the comparative brightness of the stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the power of penetrating into space by telescopes, with a comparative determination of the extent of that power in natural vision, and in telescopes of various sizes and constructions, illustrated by select observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . investigation of the powers of the prismatic colors to heat and illuminate objects, with remarks that prove the different refrangibility of radiant heat. to which is added an inquiry into the method of viewing the sun advantageously with telescopes of large apertures and high magnifying powers. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . experiments on the refrangibility of the invisible rays of the sun. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . experiments on the solar and on the terrestrial rays that occasion heat, with a comparative view of the laws by which light and heat, or rather the rays that occasion them, are subject, in order to determine whether they are the same or different. _phil. trans._, , pp. - , - . _gilbert annal._, x. ( ), pp. - ; _same_, xii. ( ), pp. - . observations tending to investigate the nature of the sun, in order to find the causes or symptoms of its variable emission of light and heat, with remarks on the use that may possibly be drawn from solar observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. , and , p. . ueber den nebelfleck der sten classe des herschel'schen verzeichniss, und ueber _ceres_ und _pallas_, vom herrn doctor herschel, aus zwey briefen desselben. _bode's jahrbuch_, , p. . additional observations tending to investigate the symptoms of the variable emission of the light and heat of the sun, with trials to set aside darkening glasses by transmitting the solar rays through liquids, and a few remarks to remove objections that might be made against some of the arguments contained in the former paper. _phil. trans._, , pp. - . observations on the two lately discovered celestial bodies [_ceres and pallas_]. _phil. trans._, , pp. - . _nicholson journal_, iv. ( ), pp. - , - . catalogue of five hundred new nebulæ, nebulous stars, planetary nebulæ, and clusters of stars, with remarks on the construction of the heavens. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . observations of the transit of _mercury_ over the sun's disk, to which is added an investigation of the causes which often prevent the proper action of mirrors. _phil. trans._, , pp. - . account of the changes which have happened during the last twenty-five years in the relative situation of double stars, with an investigation of the cause to which they are owing. _phil. trans._, , pp. - . _bode's jahrbuch_, , pp. - . continuation of the account of the changes that have happened in the relative situation of double stars. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . aus einem schreiben des herrn doctor herschel, datirt slough, bey windsor, den . may, . [relates to his theory of the relation between the solar radiation and the price of wheat.] _bode's jahrbuch_, , p. . experiments for ascertaining how far telescopes will enable us to determine very small angles, and to distinguish the real from the spurious diameters of celestial and terrestrial objects, with an application of the results of those experiments to a series of observations on the nature and magnitude of mr. harding's lately discovered star [_juno_ ( ),]. _phil. trans._, , pp. - . on the direction and velocity of the motion of the sun and solar system. _phil. trans._, , pp. - . _bode's jahrbuch_, iv. suppl. band, p. . observations on the singular figure of the planet _saturn_. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . on the quantity and velocity of solar motion. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . observations and remarks on the figure, climate, and atmosphere of _saturn_ and its ring. _phil. trans._, , pp. - . _gilbert annal._, xxxiv. ( ), pp. - . _bode's jahrbuch_, , p. . experiments for investigating the cause of the colored concentric rings discovered by sir i. newton between two object-glasses laid one upon another. _phil. trans._, , pp. - . _annal. de chimie_, lxx., , pp. - , - ; _same_, lxxi., , pp. - . observations on the nature of the new celestial body [_vesta_] discovered by dr. olbers, and of the comet which was expected to appear last january in its return from the sun. [ , ii.] _phil. trans._, , pp. - . observations of a comet [ , i.] made with a view to investigate its magnitude and the nature of its illumination, to which is added an account of a new irregularity lately perceived in the apparent figure of the planet _saturn_. _phil. trans._, , pp. - . _gilbert annal._, xxxvi. ( ), pp. - . _zach, monat. corresp._, xx. ( ), pp. - . continuation of experiments for investigating the cause of colored concentric rings and other appearances of a similar nature. _phil. trans._, , pp. - . supplement to the first and second part of the paper of experiments for investigating the cause of colored concentric rings between object-glasses, and other appearances of a similar nature. _phil. trans._, , pp. - . _gilbert annal._, xlvi., , pp. - . astronomical observations relating to the construction of the heavens, arranged for the purpose of a critical examination, the result of which appears to throw some new light upon the organization of the celestial bodies. _phil. trans._, , pp. - . _journ. de phys._, lxxv., , pp. - . observations of a comet, with remarks on the construction of its different parts [ , i.]. _phil. trans._, , pp. - . _journ. de phys._, lxxvii., , pp. - . _zach, monat. corresp._, xxviii., , pp. - , - . _bode's jahrbuch_, , p. . observations of a second comet, with remarks on its construction [ , ii.]. _phil. trans._, , pp. - . _nicholson journ._, xxxv., , pp. - . _bode's jahrbuch_, , p. . astronomical observations relating to the sidereal part of the heavens, and its connection with the nebulous part, arranged for the purpose of a critical examination. _phil. trans._, . pp. - . _bode's jahrbuch_, , pp. - . a series of observations of the satellites of the _georgian planet_, including a passage through the node of their orbits, with an introductory account of the telescopic apparatus that has been used on this occasion, and a final exposition of some calculated particulars deduced from the observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. - . astronomical observations and experiments tending to investigate the local arrangement of the celestial bodies in space, and to determine the extent and condition of the milky way. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . astronomical observations and experiments selected for the purpose of ascertaining the relative distances of clusters of stars, and of investigating how far the power of our telescopes may be expected to reach into space, when directed to ambiguous celestial objects. _phil. trans._, , pp. - . on the places of one hundred and forty-five new double stars ( ). _mem. roy. ast. soc._, , , pp. - . ii.--list of works relating to the life and writings of william herschel. [arranged alphabetically by authors.] _n.b.--in general, the notices of his life to be found in encyclopædias of biography, etc., are not included here._ arago (f.) analyse de la vie et des travaux de sir william herschel [from _annuaire du bureau des longitudes_, ]. paris, . mo. [see also the _annuaire_ for , for an account of herschel's work on double stars.] arago (f.) biographies of distinguished scientific men. translated by admiral w. h. smyth, rev. b. powell, and robert grant, esq. herschel. first series, p. . boston, . vo. arago (f.) herschel. [translated from the french.] smithsonian report. . p. . vo. auwers (a.) william herschel's verzeichnisse von nebelflecken und sternhaufen bearbeitet von a. auwers. from the _königsberg observations_. . folio. bessel (f. w.) sir william herschel. [from the _königsberger allgemeine zeitung_, , , no. , _et seq._, reprinted in his] _abhandlungen_, vol iii., p. . leipzig, . to. d'arrest (h. l.) verzeichniss von sir william herschel's nebelflecken _erster_ und _vierter_ classe, aus den beobachtungen berechnet und auf reducirt. _abhandlungen der math. phys. classe der k. sächs gesells. d. wissenschaften_, band iii. [ ], p. . dunkin (e.) obituary notices of astronomers, p. . sir william herschel, k.c.h., f.r.s., - . london, . mo. fÉtis (f. j.) biographie universelle des musiciens [article herschel]. paris, - . vo. forbes (j. d.) sir william herschel [being § of dissertation vi.]. encyclopædia britannica, eighth edition. vol. i.,_dissertations_, p. . fourier (j.) Éloge historique de sir william herschel, prononcé dans la séance publique de l'académie royale des sciences le juin, . _historie de l'académie royale des sciences de l'institut de france_, _tome_ vi., _année_ , p. lxi. harding (c. l.) des herrn dr. herschel's untersuchungen über die natur der sonnenstrahlen, aus dem englischen übersetzt. erstes heft. [translations from _phil. trans._, .] celle, . mo. hastings (c. s.) see holden _and_ hastings. herschel (carolina.) an account of a new comet. [ , ii.] _phil. trans._, , vol. lxxvii., p. . herschel (carolina.) an account of the discovery of a comet. [ , i.] _phil. trans._, , vol. lxxxiv., p. . herschel (carolina.) account of the discovery of a comet. [ , ii.] _phil. trans._, , vol. lxxxvi., p. . herschel (carolina.) catalogue of stars taken from flamsteed's observations contained in the second volume of his _historia coelestis_, and not inserted in the british catalogue; to which is added a collection of errata which should be noticed in the same volume; with remarks by w. herschel. london, . folio. herschel (carolina.) verzeichniss von sternen flamsteeds von denen keine beobachtungen in der _hist. coel. brit._ vorkommen. _bode's jahrbuch_, , p. [herschel (carolina.)] [notice of her life.] _monthly notices roy. ast. soc._, vol. , p. ; _also_, _memoirs roy. ast. soc._, vol. , p. . [herschel (carolina.)] memoir and correspondence of caroline herschel. by mrs. john herschel. with portraits. london, . mo. herschel (j. f. w.) article _telescope_, in encyclopædia britannica, eighth edition. [this article (illustrated) gives most of the important features of sir william herschel's manner of grinding and polishing specula.] herschel (j. f. w.) catalogue of nebulæ and clusters of stars. [general and systematic reduction of all sir w. herschel's observations brought into connection with all other similar ones.] _phil. trans._, . page . to. herschel (j. f. w.) a synopsis of all sir william herschel's micrometrical measurements, etc., of double stars, together with a catalogue of those stars . . . for . _mem. roy. ast. soc._, vol. , p. . lond., . to. herschel (j. f. w.) additional identifications of double stars in the synoptic catalogues of sir william herschel's micrometrical measurements, etc. _monthly notices roy. ast. soc._, vol. , p. . london, . vo. herschel (mrs. john.) memoir and correspondence of caroline herschel. with portraits. london, . mo. herschel (w.) [solution of a prize question. _see_ this book, page .] _ladies' diary_, . herschel (w.) the favorite eccho catch . . . and the preceding glee [by s. leach]. to which is added the . . . catch sung by three old women . . . in the pantomime called "the genius of nonsense" [by h. harington]. london, (?). obl. folio. [a ms. copy of this was kindly furnished me by dr. r. garnett, of the british museum.] herschel (w.) _göttingen magazin der wissenschaften und literatur_ ( ), vol. iii., p. . lichtenberg and forster, editors. [letter from herschel, giving a brief account of his life. _see_ this book, page .] herschel (w.) i. _manuscripts in possession of the royal society._ . a series of register sheets in which are entered up _all_ the observations of _each_ nebula, copied _verbatim_ from the sweeps. . a similar set of register sheets for messier's nebulæ. . a general index of the , nebulæ of w. herschel; given the class and number, to find the general number. . an index list; given the general number, to find the class and number. . a more complete list like . . a manuscript catalogue of all the nebulæ and clusters, reduced to , , and arranged in zones of ° in polar distance; by miss carolina herschel. . the original sweeps with the -foot reflector at slough, in three small to and four folio vols. of ms. ii. _manuscripts in possession of the royal astronomical society._ this library contains "the whole series of autograph observations of each double star [observed by herschel], brought together on separate sheets by sir william herschel and miss carolina herschel." [herschel (w.)] some account of the life and writings of william herschel, esq. [with a portrait.] the _european magazine and london review_ for january, . vo. [herschel (w.)] _edinburgh review_, vol. i., p, . [a review of herschel's memoir, "observations on the two lately discovered bodies," from _phil. trans._, . _see_ this book, page .] [herschel (w.)] "sir william herschel, from a london paper." [this is a short obituary notice "furnished by a gentleman well acquainted with sir william and his family, and its accuracy may be relied on."] _niles' register_, vol. , p. . nov. , . vo. [herschel (w.)] obituary: sir william herschel, knt., ll.d., f.r.s. _the gentleman's magazine and historical chronicle_, vol. xcii., , p. . vo. [herschel (w.)] _annual register_, , p. . vo. [herschel (w.)] w. herschel's sämmtliche schriften, erster band. ueber den bau des himmels. mit kupfertafeln. [edited by j. w. pfaff. a second edition was published in .] dresden and leipzig, . vo. [herschel (w.)] _new york mirror_, vol. vi., - , p. . [herschel (w.)] _living age_, vol. ii., p. ( ). vo. [reprinted from _chambers' journal_.] [herschel (w.)] _foreign quarterly review_, vol. , p. . vo. [review of arago's "analyse de la vie et des travaux de sir william herschel."] [herschel (w.)] arago's life of herschel. _eclectic museum_, vol. ii., p. . [reprinted from the _foreign quarterly review_, vol. .] holden (e. s.) on the inner satellites of _uranus_. [reduction of sir william herschel's observations.] _proceedings amer. assn. adv. science_, _august_, , p. . vo. holden (e. s.) index catalogue of books and memoirs relative to nebulæ, clusters, etc. _smithsonian miscellaneous collections_, no. , pp. - . [abstracts of sir william herschel's memoirs (on nebulæ) in the _philosophical transactions_.] washington, . vo. holden (e. s.) and c. s. hastings. a subject-index and a synopsis of the scientific writings of sir william herschel. [reprinted from the _report_ of the smithsonian institution ( ).] washington, . vo. krafft (j. g. f.) kurze nachricht von dem berühmten astronomen herschel und einigen seiner entdeckungen. bayreuth, . vo. peirce (c. s.) photometric researches. [a reduction of herschel's observations on the comparative brightness of the stars.] _annals harvard college observatory_, vol. ix. leipzig, . to. sommer (g. m.) william herschel . . . ueber den bau des himmels; drei abhandlungen aus dem englischen uebersetzt, nebst einem authentischen auszug aus kants allgemeiner naturgeschichte und theorie des himmels. koenigsberg, . vo. struve (w.) Études d'astronomie stellaire. sur la voie lactée et sur la distance des étoiles fixes. [p. _et seq._ contains an elaborate review of the construction of the heavens according to herschel.] st. petersburg, . vo. wolf (r.) william herschel. zurich, . vo. zach (f. von.) dr. william herschel [translated from _public characters_ and printed in zach's _monatlich correspondenz_, , part i., p. _et seq._] iii.--list of the published portraits of william herschel. _artist_, mme. dupiery. _engraver_, thÖnert. vo. early portrait. some copies in red. profile. _artist_, f. rehburg. _engraver_, f. w. bollinger. vo. late portrait. _artist_, ----? _engraver_, c. westermayr. vo. medallion. _artist_, c. brand. _engraver_, ----? vo. lithograph. _artist_, ----? _engraver_, j. sewell. vo. profile, . _artist_, ----? _engraver_, ----? vo. profile. _artist_, f. bonneville. _engraver_, f. bonneville. vo. profile. _artist_, j. russell, r.a. _engraver_, e. scriven. vo. engraved from a crayon in the possession of his son, and published by the s. d. u. k. in the _gallery of portraits_, vol. . _artist_, ----? _engraver_, ----? vo. _european magazine_, jan., . this is a bust in profile, showing the left side of the face. _artist_, ----? _engraver_, thomson. vo. published by caxton, . this must have been engraved before since the legend is william herschel, ll.d., f.r.s. _artist_, lady gordon. from the painting by abbott in the national portrait gallery. _engraver_, joseph brown. vo. published in memoir of caroline herschel. this is of the date , or thereabouts. _see_ frontispiece. _artist_, ----? _engraver_, c. mÜller. to. medallion, (?). _artist_, ----? _engraver_, h. pinhas. to. legend in russian. _artist_, baisch. _engraver_, ----? to. lithograph. _artist_, h. grÉvedon. _engraver_, ----? fol. lithograph. _artist_, ----? _engraver_, f. mÜller. fol. _artist_, abbott. _engraver_, ryder. fol. . _artist_, j. boilly. _engraver_, ----? fol. . lithograph. _artist_, ----? _engraver_, j. godby. fol. r. w. s. lutwidge, esq., f.r.a.s., has an original seal with a head of sir william herschel, which is shown on the title-page of this work. a cut of it has been courteously furnished me by john browning, esq., f.r.a.s., etc. in a bust of herschel was made by lockie for sir william watson. a picture of herschel was painted by mr. artaud about the beginning of . a portrait of herschel by abbott is in the national portrait gallery, london. there are no doubt many other paintings in england, though i can find notices of these only. the royal society of london has nearly a hundred portraits of its most distinguished members, but owns none of sir william herschel. index of names. _n.b.--this index is intended to refer to the proper names occurring in the body of the work only, and not to the bibliography._ airy (sir george), . alison (sir archibald), . arago (françois), , , . artaud (m.), . aubert (alexander), , , . baldwin (miss), . banks (sir joseph), , , . bessel (f. w.), , . blagden (dr.), . bonaparte (napoleon), , . bradley (james), , . bruhl (count von), . bulman (mr.), , , . bunsen (chevalier), . burney (dr.), , , , , , . campbell (thomas), . cassini (j. d.), . cropley (mr.), . dalrymple (mr.), . d'arblay (madame), _et seq._ darlington (earl of), . darquier, , . d'arrest (h. l.), . dawes (w. r.), . de luc (m.), . de luc (mrs.), , . dollond (j.), . engelfield (sir harry), . farinelli (miss), . flaugergues (h.), , . fleming (miss), . fourier (j.), . frauenhofer (j.), . galileo, , . george iii., , , , . griesbach (george), . griesbachs (the), , . hansen (p. a.), . hastings (c. s.), . heberden (dr.), . helmholtz (h.), . herschel (abraham) [ - ], . herschel (alexander) [ - ], , , , , , , , , , , , , , . herschel (benjamin), . herschel (carolina) [ - ], , , , , , , , , , , ; discovers five comets, , , , , , , , , , . herschel (carolina), her _memoir_ quoted, , , , , , , , , , , , , , , , . herschel (dietrich), , , , , , . herschel (eusebius), . herschel (hans) [_circa_ ], . herschel (isaac) [ - ], , , . herschel (jacob) [ - ], , , , , . herschel (sir john frederick william) [ - ], , , , , , , , . herschel (lady), , , , , . herschel (major john), . herschel (mrs. mary cornwallis), . herschel (sophia elizabeth), b. , married griesbach, . herschel (william), born , november ; . oboist in the band of the guards ( ), and goes to england for the first time, returning in , . deserts from the guards and goes to england ( ), . organizes the band of the durham militia ( ), . leaves the band and lives with dr. miller, . leads the public concerts at wakefield and halifax, . organist at halifax ( ), . organist of the octagon chapel at bath ( ), . his musical writings, . studies smith's harmonies and optics, . visits hanover, august, , . hires a small telescope, . makes his first telescope ( ), . visits hanover ( ?), . st review of the heavens, , . d review of the heavens, . d review of the heavens ( ), . th review of the heavens ( ), . manufacture of telescopes, , , , , , , , , , , , . moves to new king st., bath, . conducts oratorios of handel, . begins astronomical _measures_ ( ), . joins philosophical society of bath, . first published scientific writing ( ), . first communication to the royal society ( ), . discovery of _uranus_ ( , march ), . its effect on his career, . elected a member of the royal society ( ), and receives the copley medal, . attempts a thirty-foot reflector, . goes to london, (may, june, july), . appointed royal astronomer (£ ), , . removes to datchet, , august , . his assiduity, , , , . his mechanical genius, , , . cost of his telescopes, . marries mrs. john pitt, _née_ baldwin ( ), . only child born ( ), . removes to slough ( ), . ll.d. (_oxon._), , . his account of the discovery of _uranus_ ( , march ), . discovers two satellites to _uranus_, , jan. , . discovers two satellites to _saturn_, , august-september, . invents machines for making reflectors ( ), . began forty-foot telescope, , finished it, , . biographical letter ( ), . list of published portraits of him, . value of his sister's assistance to him, . letters to carolina herschel, , , , . his personal character ( - ), _et seq._ his relations to his cotemporaries, , , , , , , , , , . list of writings relating to him and to his works, . his literary skill, . examples of his style, _et seq._ failure of health, _et seq._ created a knight of the royal hanoverian guelphic order ( ), . herschel (sir william), first president of the royal astronomical society ( ), . his will, . his death, august th, , . his epitaph, . list of his scientific writings, . review of his scientific labors, . the improvement of telescopes and apparatus, . the relative brightness of the stars; variable stars, . researches on double stars, . researches on planets and satellites, . researches on the nature of the sun, , , . the motion of the solar system in space, . researches on the construction of the heavens, . scale of celestial measures; distances of the stars, . researches on light, heat, etc., . researches on the dimensions of the stars, . on the spectra of the fixed stars, . on the variable emission of light and heat from the sun, . researches on nebulæ and clusters, . huyghens (c.), , . king george iii., , , , . lacaille (n. l.), , . lalande (jerome), , , . lambert (j. h.), . lassell (w.), . lee (miss), . lichtenberg (herr), . lind (dr. and mrs.), . long (dr.), . magellan (herr), , . maskelyne (nevil), , , , , , , . mayer (christian), , . melloni (m.), . messier (c.), . michell (john), , , , . miller (dr.), , , , . monson (mrs.), . moritzen (anna ilse), m. isaac herschel, , . napoleon i., , . newton (sir isaac), , , , . olbers (william), . pabrich (cappelmeister), . palmerston (lord), . piazzi (joseph), , . pierce (charles s.), . pigott (j.), . poggendorff (j. g.), . ronzoni (signor), . rosse (lord), , . savary (m.), . secchi (angelo), , . scheiner (c.), . schroeter (j. h.), , , , , . schwabe (h.), . short (james), . smith (dr. robert), . snetzler (herr), , . struve (otto von), . thomson (thomas), . verdet (e.), . wainwright (dr.), . walsh (colonel), . watson (sir william), , , , , , , , , . watt (james), . weld (r.), . wilson (alexander), , , . zach (baron von), . the end. start of transcriber's notes: i have used [alpha] to represent the greek letter used in the text. page line in the original text, left as is. übersetzt uebersetzt biographie universelle des musiciens biographie universelle des musiciens koenigsberg, . vo. königsberger allgemeine zeitung fraunhofer index frauenhofer (j.), . vol. (all other occurances are) vol. index ronzoni (signor), . ronzini original page line original text replaced with ornamented. ornamented." [c] for letter c reversed ci[c]i[c]cccxxii den . may den . mai suppl. band., p. . suppl. band, p. . end of transcriber's notes: side-lights on astronomy and kindred fields of popular science essays and addresses by simon newcomb contents preface i. the unsolved problems of astronomy ii. the new problems of the universe iii. the structure of the universe iv. the extent of the universe v. making and using a telescope vi. what the astronomers are doing vii. life in the universe viii. how the planets are weighed ix. the mariner's compass x. the fairyland of geometry xi. the organization of scientific research xii. can we make it rain? xiii. the astronomical ephemeris and nautical almanac xiv. the world's debt to astronomy xv. an astronomical friendship xvi. the evolution of the scientific investigator xvii. the evolution of astronomical knowledge xviii. aspects of american astronomy xix. the universe as an organism xx. the relation of scientific method to social progress xxi. the outlook for the flying-machine illustrations simon newcomb photograph of the corona of the sun, taken in tripoli during total eclipse of august , . a typical star cluster-centauri the glass disk the optician's tool the optician's tool grinding a large lens image of candle-flame in object-glass testing adjustment of object-glass a very primitive mounting for a telescope the huyghenian eye-piece section of the primitive mounting spectral images of stars, the upper line showing how they appear with the eye-piece pushed in, the lower with the eye-piece drawn out the great refractor of the national observatory at washington the "broken-backed comet-seeker" nebula in orion dip of the magnetic needle in various latitudes star spectra professor langley's air-ship preface in preparing and issuing this collection of essays and addresses, the author has yielded to what he could not but regard as the too flattering judgment of the publishers. having done this, it became incumbent to do what he could to justify their good opinion by revising the material and bringing it up to date. interest rather than unity of thought has determined the selection. a prominent theme in the collection is that of the structure, extent, and duration of the universe. here some repetition of ideas was found unavoidable, in a case where what is substantially a single theme has been treated in the various forms which it assumed in the light of constantly growing knowledge. if the critical reader finds this a defect, the author can plead in extenuation only the difficulty of avoiding it under the circumstances. although mainly astronomical, a number of discussions relating to general scientific subjects have been included. acknowledgment is due to the proprietors of the various periodicals from the pages of which most of the essays have been taken. besides harper's magazine and the north american review, these include mcclure's magazine, from which were taken the articles "the unsolved problems of astronomy" and "how the planets are weighed." "the structure of the universe" appeared in the international monthly, now the international quarterly; "the outlook for the flying-machine" is mainly from the new york independent, but in part from mcclure's magazine; "the world's debt to astronomy" is from the chautauquan; and "an astronomical friendship" from the atlantic monthly. simon newcomb. washington, june, . i the unsolved problems of astronomy the reader already knows what the solar system is: an immense central body, the sun, with a number of planets revolving round it at various distances. on one of these planets we dwell. vast, indeed, are the distances of the planets when measured by our terrestrial standards. a cannon-ball fired from the earth to celebrate the signing of the declaration of independence, and continuing its course ever since with a velocity of eighteen hundred feet per second, would not yet be half-way to the orbit of neptune, the outer planet. and yet the thousands of stars which stud the heavens are at distances so much greater than that of neptune that our solar system is like a little colony, separated from the rest of the universe by an ocean of void space almost immeasurable in extent. the orbit of the earth round the sun is of such size that a railway train running sixty miles an hour, with never a stop, would take about three hundred and fifty years to cross it. represent this orbit by a lady's finger-ring. then the nearest fixed star will be about a mile and a half away; the next more than two miles; a few more from three to twenty miles; the great body at scores or hundreds of miles. imagine the stars thus scattered from the atlantic to the mississippi, and keep this little finger-ring in mind as the orbit of the earth, and one may have some idea of the extent of the universe. one of the most beautiful stars in the heavens, and one that can be seen most of the year, is a lyrae, or alpha of the lyre, known also as vega. in a spring evening it may be seen in the northeast, in the later summer near the zenith, in the autumn in the northwest. on the scale we have laid down with the earth's orbit as a finger-ring, its distance would be some eight or ten miles. the small stars around it in the same constellation are probably ten, twenty, or fifty times as far. now, the greatest fact which modern science has brought to light is that our whole solar system, including the sun, with all its planets, is on a journey towards the constellation lyra. during our whole lives, in all probability during the whole of human history, we have been flying unceasingly towards this beautiful constellation with a speed to which no motion on earth can compare. the speed has recently been determined with a fair degree of certainty, though not with entire exactness; it is about ten miles a second, and therefore not far from three hundred millions of miles a year. but whatever it may be, it is unceasing and unchanging; for us mortals eternal. we are nearer the constellation by five or six hundred miles every minute we live; we are nearer to it now than we were ten years ago by thousands of millions of miles, and every future generation of our race will be nearer than its predecessor by thousands of millions of miles. when, where, and how, if ever, did this journey begin--when, where, and how, if ever, will it end? this is the greatest of the unsolved problems of astronomy. an astronomer who should watch the heavens for ten thousand years might gather some faint suggestion of an answer, or he might not. all we can do is to seek for some hints by study and comparison with other stars. the stars are suns. to put it in another way, the sun is one of the stars, and rather a small one at that. if the sun is moving in the way i have described, may not the stars also be in motion, each on a journey of its own through the wilderness of space? to this question astronomy gives an affirmative answer. most of the stars nearest to us are found to be in motion, some faster than the sun, some more slowly, and the same is doubtless true of all; only the century of accurate observations at our disposal does not show the motion of the distant ones. a given motion seems slower the more distant the moving body; we have to watch a steamship on the horizon some little time to see that she moves at all. thus it is that the unsolved problem of the motion of our sun is only one branch of a yet more stupendous one: what mean the motions of the stars--how did they begin, and how, if ever, will they end? so far as we can yet see, each star is going straight ahead on its own journey, without regard to its neighbors, if other stars can be so called. is each describing some vast orbit which, though looking like a straight line during the short period of our observation, will really be seen to curve after ten thousand or a hundred thousand years, or will it go straight on forever? if the laws of motion are true for all space and all time, as we are forced to believe, then each moving star will go on in an unbending line forever unless hindered by the attraction of other stars. if they go on thus, they must, after countless years, scatter in all directions, so that the inhabitants of each shall see only a black, starless sky. mathematical science can throw only a few glimmers of light on the questions thus suggested. from what little we know of the masses, distances, and numbers of the stars we see a possibility that the more slow-moving ones may, in long ages, be stopped in their onward courses or brought into orbits of some sort by the attraction of their millions of fellows. but it is hard to admit even this possibility in the case of the swift-moving ones. attraction, varying as the inverse square of the distance, diminishes so rapidly as the distance increases that, at the distances which separate the stars, it is small indeed. we could not, with the most delicate balance that science has yet invented, even show the attraction of the greatest known star. so far as we know, the two swiftest-moving stars are, first, arcturus, and, second, one known in astronomy as groombridge, the latter so called because it was first observed by the astronomer groombridge, and is numbered in his catalogue of stars. if our determinations of the distances of these bodies are to be relied on, the velocity of their motion cannot be much less than two hundred miles a second. they would make the circuit of the earth every two or three minutes. a body massive enough to control this motion would throw a large part of the universe into disorder. thus the problem where these stars came from and where they are going is for us insoluble, and is all the more so from the fact that the swiftly moving stars are moving in different directions and seem to have no connection with each other or with any known star. it must not be supposed that these enormous velocities seem so to us. not one of them, even the greatest, would be visible to the naked eye until after years of watching. on our finger-ring scale, groombridge would be some ten miles and arcturus thirty or forty miles away. either of them would be moving only two or three feet in a year. to the oldest assyrian priests lyra looked much as it does to us to-day. among the bright and well-known stars arcturus has the most rapid apparent motion, yet job himself would not to-day see that its position had changed, unless he had noted it with more exactness than any astronomer of his time. another unsolved problem among the greatest which present themselves to the astronomer is that of the size of the universe of stars. we know that several thousand of these bodies are visible to the naked eye; moderate telescopes show us millions; our giant telescopes of the present time, when used as cameras to photograph the heavens, show a number past count, perhaps one hundred millions. are all these stars only those few which happen to be near us in a universe extending out without end, or do they form a collection of stars outside of which is empty infinite space? in other words, has the universe a boundary? taken in its widest scope this question must always remain unanswered by us mortals because, even if we should discover a boundary within which all the stars and clusters we ever can know are contained, and outside of which is empty space, still we could never prove that this space is empty out to an infinite distance. far outside of what we call the universe might still exist other universes which we can never see. it is a great encouragement to the astronomer that, although he cannot yet set any exact boundary to this universe of ours, he is gathering faint indications that it has a boundary, which his successors not many generations hence may locate so that the astronomer shall include creation itself within his mental grasp. it can be shown mathematically that an infinitely extended system of stars would fill the heavens with a blaze of light like that of the noonday sun. as no such effect is produced, it may be concluded that the universe has a boundary. but this does not enable us to locate the boundary, nor to say how many stars may lie outside the farthest stretches of telescopic vision. yet by patient research we are slowly throwing light on these points and reaching inferences which, not many years ago, would have seemed forever beyond our powers. every one now knows that the milky way, that girdle of light which spans the evening sky, is formed of clouds of stars too minute to be seen by the unaided vision. it seems to form the base on which the universe is built and to bind all the stars into a system. it comprises by far the larger number of stars that the telescope has shown to exist. those we see with the naked eye are almost equally scattered over the sky. but the number which the telescope shows us become more and more condensed in the milky way as telescope power is increased. the number of new stars brought out with our greatest power is vastly greater in the milky way than in the rest of the sky, so that the former contains a great majority of the stars. what is yet more curious, spectroscopic research has shown that a particular kind of stars, those formed of heated gas, are yet more condensed in the central circle of this band; if they were visible to the naked eye, we should see them encircling the heavens as a narrow girdle forming perhaps the base of our whole system of stars. this arrangement of the gaseous or vaporous stars is one of the most singular facts that modern research has brought to light. it seems to show that these particular stars form a system of their own; but how such a thing can be we are still unable to see. the question of the form and extent of the milky way thus becomes the central one of stellar astronomy. sir william herschel began by trying to sound its depths; at one time he thought he had succeeded; but before he died he saw that they were unfathomable with his most powerful telescopes. even today he would be a bold astronomer who would profess to say with certainty whether the smallest stars we can photograph are at the boundary of the system. before we decide this point we must have some idea of the form and distance of the cloudlike masses of stars which form our great celestial girdle. a most curious fact is that our solar system seems to be in the centre of this galactic universe, because the milky way divides the heavens into two equal parts, and seems equally broad at all points. were we looking at such a girdle as this from one side or the other, this appearance would not be presented. but let us not be too bold. perhaps we are the victims of some fallacy, as ptolemy was when he proved, by what looked like sound reasoning, based on undeniable facts, that this earth of ours stood at rest in the centre of the heavens! a related problem, and one which may be of supreme importance to the future of our race, is, what is the source of the heat radiated by the sun and stars? we know that life on the earth is dependent on the heat which the sun sends it. if we were deprived of this heat we should in a few days be enveloped in a frost which would destroy nearly all vegetation, and in a few months neither man nor animal would be alive, unless crouching over fires soon to expire for want of fuel. we also know that, at a time which is geologically recent, the whole of new england was covered with a sheet of ice, hundreds or even thousands of feet thick, above which no mountain but washington raised its head. it is quite possible that a small diminution in the supply of heat sent us by the sun would gradually reproduce the great glacier, and once more make the eastern states like the pole. but the fact is that observations of temperature in various countries for the last two or three hundred years do not show any change in climate which can be attributed to a variation in the amount of heat received from the sun. the acceptance of this theory of the heat of those heavenly bodies which shine by their own light--sun, stars, and nebulae--still leaves open a problem that looks insoluble with our present knowledge. what becomes of the great flood of heat and light which the sun and stars radiate into empty space with a velocity of one hundred and eighty thousand miles a second? only a very small fraction of it can be received by the planets or by other stars, because these are mere points compared with their distance from us. taking the teaching of our science just as it stands, we should say that all this heat continues to move on through infinite space forever. in a few thousand years it reaches the probable confines of our great universe. but we know of no reason why it should stop here. during the hundreds of millions of years since all our stars began to shine, has the first ray of light and heat kept on through space at the rate of one hundred and eighty thousand miles a second, and will it continue to go on for ages to come? if so, think of its distance now, and think of its still going on, to be forever wasted! rather say that the problem, what becomes of it? is as yet unsolved. thus far i have described the greatest of problems; those which we may suppose to concern the inhabitants of millions of worlds revolving round the stars as much as they concern us. let us now come down from the starry heights to this little colony where we live, the solar system. here we have the great advantage of being better able to see what is going on, owing to the comparative nearness of the planets. when we learn that these bodies are like our earth in form, size, and motions, the first question we ask is, could we fly from planet to planet and light on the surface of each, what sort of scenery would meet our eyes? mountain, forest, and field, a dreary waste, or a seething caldron larger than our earth? if solid land there is, would we find on it the homes of intelligent beings, the lairs of wild beasts, or no living thing at all? could we breathe the air, would we choke for breath or be poisoned by the fumes of some noxious gas? to most of these questions science cannot as yet give a positive answer, except in the case of the moon. our satellite is so near us that we can see it has no atmosphere and no water, and therefore cannot be the abode of life like ours. the contrast of its eternal deadness with the active life around us is great indeed. here we have weather of so many kinds that we never tire of talking about it. but on the moon there is no weather at all. on our globe so many things are constantly happening that our thousands of daily journals cannot begin to record them. but on the dreary, rocky wastes of the moon nothing ever happens. so far as we can determine, every stone that lies loose on its surface has lain there through untold ages, unchanged and unmoved. we cannot speak so confidently of the planets. the most powerful telescopes yet made, the most powerful we can ever hope to make, would scarcely shows us mountains, or lakes, rivers, or fields at a distance of fifty millions of miles. much less would they show us any works of man. pointed at the two nearest planets, venus and mars, they whet our curiosity more than they gratify it. especially is this the case with venus. ever since the telescope was invented observers have tried to find the time of rotation of this planet on its axis. some have reached one conclusion, some another, while the wisest have only doubted. the great herschel claimed that the planet was so enveloped in vapor or clouds that no permanent features could be seen on its surface. the best equipped recent observers think they see faint, shadowy patches, which remain the same from day to day, and which show that the planet always presents the same face to the sun, as the moon does to the earth. others do not accept this conclusion as proved, believing that these patches may be nothing more than variations of light, shade, and color caused by the reflection of the sun's light at various angles from different parts of the planet. there is also some mystery about the atmosphere of this planet. when venus passes nearly between us and the sun, her dark hemisphere is turned towards us, her bright one being always towards the sun. but she is not exactly on a line with the sun except on the very rare occasions of a transit across the sun's disk. hence, on ordinary occasions, when she seems very near on a line with the sun, we see a very small part of the illuminated hemisphere, which now presents the form of a very thin crescent like the new moon. and this crescent is supposed to be a little broader than it would be if only half the planet were illuminated, and to encircle rather more than half the planet. now, this is just the effect that would be produced by an atmosphere refracting the sun's light around the edge of the illuminated hemisphere. the difficulty of observations of this kind is such that the conclusion may be open to doubt. what is seen during transits of venus over the sun's disk leads to more certain, but yet very puzzling, conclusions. the writer will describe what he saw at the cape of good hope during the transit of december , . as the dark planet impinged on the bright sun, it of course cut out a round notch from the edge of the sun. at first, when this notch was small, nothing could be seen of the outline of that part of the planet which was outside the sun. but when half the planet was on the sun, the outline of the part still off the sun was marked by a slender arc of light. a curious fact was that this arc did not at first span the whole outline of the planet, but only showed at one or two points. in a few moments another part of the outline appeared, and then another, until, at last, the arc of light extended around the complete outline. all this seems to show that while the planet has an atmosphere, it is not transparent like ours, but is so filled with mist and clouds that the sun is seen through it only as if shining in a fog. not many years ago the planet mars, which is the next one outside of us, was supposed to have a surface like that of our earth. some parts were of a dark greenish gray hue; these were supposed to be seas and oceans. other parts had a bright, warm tint; these were supposed to be the continents. during the last twenty years much has been learned as to how this planet looks, and the details of its surface have been mapped by several observers, using the best telescopes under the most favorable conditions of air and climate. and yet it must be confessed that the result of this labor is not altogether satisfactory. it seems certain that the so-called seas are really land and not water. when it comes to comparing mars with the earth, we cannot be certain of more than a single point of resemblance. this is that during the martian winter a white cap, as of snow, is formed over the pole, which partially melts away during the summer. the conclusion that there are oceans whose evaporation forms clouds which give rise to this snow seems plausible. but the telescope shows no clouds, and nothing to make it certain that there is an atmosphere to sustain them. there is no certainty that the white deposit is what we call snow; perhaps it is not formed of water at all. the most careful studies of the surface of this planet, under the best conditions, are those made at the lowell observatory at flagstaff, arizona. especially wonderful is the system of so-called canals, first seen by schiaparelli, but mapped in great detail at flagstaff. but the nature and meaning of these mysterious lines are still to be discovered. the result is that the question of the real nature of the surface of mars and of what we should see around us could we land upon it and travel over it are still among the unsolved problems of astronomy. if this is the case with the nearest planets that we can study, how is it with more distant ones? jupiter is the only one of these of the condition of whose surface we can claim to have definite knowledge. but even this knowledge is meagre. the substance of what we know is that its surface is surrounded by layers of what look like dense clouds, through which nothing can certainly be seen. i have already spoken of the heat of the sun and its probable origin. but the question of its heat, though the most important, is not the only one that the sun offers us. what is the sun? when we say that it is a very hot globe, more than a million times as large as the earth, and hotter than any furnace that man can make, so that literally "the elements melt with fervent heat" even at its surface, while inside they are all vaporized, we have told the most that we know as to what the sun really is. of course we know a great deal about the spots, the rotation of the sun on its axis, the materials of which it is composed, and how its surroundings look during a total eclipse. but all this does not answer our question. there are several mysteries which ingenious men have tried to explain, but they cannot prove their explanations to be correct. one is the cause and nature of the spots. another is that the shining surface of the sun, the "photosphere," as it is technically called, seems so calm and quiet while forces are acting within it of a magnitude quite beyond our conception. flames in which our earth and everything on it would be engulfed like a boy's marble in a blacksmith's forge are continually shooting up to a height of tens of thousands of miles. one would suppose that internal forces capable of doing this would break the surface up into billows of fire a thousand miles high; but we see nothing of the kind. the surface of the sun seems almost as placid as a lake. yet another mystery is the corona of the sun. this is something we should never have known to exist if the sun were not sometimes totally eclipsed by the dark body of the moon. on these rare occasions the sun is seen to be surrounded by a halo of soft, white light, sending out rays in various directions to great distances. this halo is called the corona, and has been most industriously studied and photographed during nearly every total eclipse for thirty years. thus we have learned much about how it looks and what its shape is. it has a fibrous, woolly structure, a little like the loose end of a much-worn hempen rope. a certain resemblance has been seen between the form of these seeming fibres and that of the lines in which iron filings arrange themselves when sprinkled on paper over a magnet. it has hence been inferred that the sun has magnetic properties, a conclusion which, in a general way, is supported by many other facts. yet the corona itself remains no less an unexplained phenomenon. [illustration with caption: photograph of the corona of the sun, taken in tripoli during total eclipse of august , ] a phenomenon almost as mysterious as the solar corona is the "zodiacal light," which any one can see rising from the western horizon just after the end of twilight on a clear winter or spring evening. the most plausible explanation is that it is due to a cloud of small meteoric bodies revolving round the sun. we should hardly doubt this explanation were it not that this light has a yet more mysterious appendage, commonly called the gegenschein, or counter-glow. this is a patch of light in the sky in a direction exactly opposite that of the sun. it is so faint that it can be seen only by a practised eye under the most favorable conditions. but it is always there. the latest suggestion is that it is a tail of the earth, of the same kind as the tail of a comet! we know that the motions of the heavenly bodies are predicted with extraordinary exactness by the theory of gravitation. when one finds that the exact path of the moon's shadow on the earth during a total eclipse of the sun can be mapped out many years in advance, and that the planets follow the predictions of the astronomer so closely that, if you could see the predicted planet as a separate object, it would look, even in a good telescope, as if it exactly fitted over the real planet, one thinks that here at least is a branch of astronomy which is simply perfect. and yet the worlds themselves show slight deviations in their movements which the astronomer cannot always explain, and which may be due to some hidden cause that, when brought to light, shall lead to conclusions of the greatest importance to our race. one of these deviations is in the rotation of the earth. sometimes, for several years at a time, it seems to revolve a little faster, and then again a little slower. the changes are very slight; they can be detected only by the most laborious and refined methods; yet they must have a cause, and we should like to know what that cause is. the moon shows a similar irregularity of motion. for half a century, perhaps through a whole century, she will go around the earth a little ahead of her regular rate, and then for another half-century or more she will fall behind. the changes are very small; they would never have been seen with the unaided eye, yet they exist. what is their cause? mathematicians have vainly spent years of study in trying to answer this question. the orbit of mercury is found by observations to have a slight motion which mathematicians have vainly tried to explain. for some time it was supposed to be caused by the attraction of an unknown planet between mercury and the sun, and some were so sure of the existence of this planet that they gave it a name, calling it vulcan. but of late years it has become reasonably certain that no planet large enough to produce the effect observed can be there. so thoroughly has every possible explanation been sifted out and found wanting, that some astronomers are now inquiring whether the law of gravitation itself may not be a little different from what has always been supposed. a very slight deviation, indeed, would account for the facts, but cautious astronomers want other proofs before regarding the deviation of gravitation as an established fact. intelligent men have sometimes inquired how, after devoting so much work to the study of the heavens, anything can remain for astronomers to find out. it is a curious fact that, although they were never learning so fast as at the present day, yet there seems to be more to learn now than there ever was before. great and numerous as are the unsolved problems of our science, knowledge is now advancing into regions which, a few years ago, seemed inaccessible. where it will stop none can say. ii the new problems of the universe the achievements of the nineteenth century are still a theme of congratulation on the part of all who compare the present state of the world with that of one hundred years ago. and yet, if we should fancy the most sagacious prophet, endowed with a brilliant imagination, to have set forth in the year the problems that the century might solve and the things which it might do, we should be surprised to see how few of his predictions had come to pass. he might have fancied aerial navigation and a number of other triumphs of the same class, but he would hardly have had either steam navigation or the telegraph in his picture. in an article appeared in harper's magazine depicting some anticipated features of life in a.d. . we have since made great advances, but they bear little resemblance to what the writer imagined. he did not dream of the telephone, but did describe much that has not yet come to pass and probably never will. the fact is that, much as the nineteenth century has done, its last work was to amuse itself by setting forth more problems for this century to solve than it has ever itself succeeded in mastering. we should not be far wrong in saying that to-day there are more riddles in the universe than there were before men knew that it contained anything more than the objects they could see. so far as mere material progress is concerned, it may be doubtful whether anything so epoch-making as the steam-engine or the telegraph is held in store for us by the future. but in the field of purely scientific discovery we are finding a crowd of things of which our philosophy did not dream even ten years ago. the greatest riddles which the nineteenth century has bequeathed to us relate to subjects so widely separated as the structure of the universe and the structure of atoms of matter. we see more and more of these structures, and we see more and more of unity everywhere, and yet new facts difficult of explanation are being added more rapidly than old facts are being explained. we all know that the nineteenth century was marked by a separation of the sciences into a vast number of specialties, to the subdivisions of which one could see no end. but the great work of the twentieth century will be to combine many of these specialties. the physical philosopher of the present time is directing his thought to the demonstration of the unity of creation. astronomical and physical researches are now being united in a way which is bringing the infinitely great and the infinitely small into one field of knowledge. ten years ago the atoms of matter, of which it takes millions of millions to make a drop of water, were the minutest objects with which science could imagine itself to be concerned, now a body of experimentalists, prominent among whom stand professors j. j. thompson, becquerel, and roentgen, have demonstrated the existence of objects so minute that they find their way among and between the atoms of matter as rain-drops do among the buildings of a city. more wonderful yet, it seems likely, although it has not been demonstrated, that these little things, called "corpuscles," play an important part in what is going on among the stars. whether this be true or not, it is certain that there do exist in the universe emanations of some sort, producing visible effects, the investigation of which the nineteenth century has had to bequeath to the twentieth. for the purpose of the navigator, the direction of the magnetic needle is invariable in any one place, for months and even years; but when exact scientific observations on it are made, it is found subject to numerous slight changes. the most regular of these consists in a daily change of its direction. it moves one way from morning until noon, and then, late in the afternoon and during the night, turns back again to its original pointing. the laws of this change have been carefully studied from observations, which show that it is least at the equator and larger as we go north into middle latitudes; but no explanation of it resting on an indisputable basis has ever been offered. besides these regular changes, there are others of a very irregular character. every now and then the changes in the direction of the magnet are wider and more rapid than those which occur regularly every day. the needle may move back and forth in a way so fitful as to show the action of some unusual exciting cause. such movements of the needle are commonly seen when there is a brilliant aurora. this connection shows that a magnetic storm and an aurora must be due to the same or some connected causes. those of us who are acquainted with astronomical matters know that the number of spots on the sun goes through a regular cycle of change, having a period of eleven years and one or two months. now, the curious fact is, when the number and violence of magnetic storms are recorded and compared, it is found that they correspond to the spots on the sun, and go through the same period of eleven years. the conclusion seems almost inevitable: magnetic storms are due to some emanation sent out by the sun, which arises from the same cause that produces the spots. this emanation does not go on incessantly, but only in an occasional way, as storms follow each other on the earth. what is it? every attempt to detect it has been in vain. professor hale, at the yerkes observatory, has had in operation from time to time, for several years, his ingenious spectroheliograph, which photographs the sun by a single ray of the spectrum. this instrument shows that violent actions are going on in the sun, which ordinary observation would never lead us to suspect. but it has failed to show with certainty any peculiar emanation at the time of a magnetic storm or anything connected with such a storm. a mystery which seems yet more impenetrable is associated with the so-called new stars which blaze forth from time to time. these offer to our sight the most astounding phenomena ever presented to the physical philosopher. one hundred years ago such objects offered no mystery. there was no reason to suppose that the creator of the universe had ceased his functions; and, continuing them, it was perfectly natural that he should be making continual additions to the universe of stars. but the idea that these objects are really new creations, made out of nothing, is contrary to all our modern ideas and not in accord with the observed facts. granting the possibility of a really new star--if such an object were created, it would be destined to take its place among the other stars as a permanent member of the universe. instead of this, such objects invariably fade away after a few months, and are changed into something very like an ordinary nebula. a question of transcendent interest is that of the cause of these outbursts. it cannot be said that science has, up to the present time, been able to offer any suggestion not open to question. the most definite one is the collision theory, according to which the outburst is due to the clashing together of two stars, one or both of which might previously have been dark, like a planet. the stars which may be actually photographed probably exceed one hundred millions in number, and those which give too little light to affect the photographic plate may be vastly more numerous than those which do. dark stars revolve around bright ones in an infinite variety of ways, and complex systems of bodies, the members of which powerfully attract each other, are the rule throughout the universe. moreover, we can set no limit to the possible number of dark or invisible stars that may be flying through the celestial spaces. while, therefore, we cannot regard the theory of collision as established, it seems to be the only one yet put forth which can lay any claim to a scientific basis. what gives most color to it is the extreme suddenness with which the new stars, so far as has yet been observed, invariably blaze forth. in almost every case it has been only two or three days from the time that the existence of such an object became known until it had attained nearly its full brightness. in fact, it would seem that in the case of the star in perseus, as in most other cases, the greater part of the outburst took place within the space of twenty-four hours. this suddenness and rapidity is exactly what would be the result of a collision. the most inexplicable feature of all is the rapid formation of a nebula around this star. in the first photographs of the latter, the appearance presented is simply that of an ordinary star. but, in the course of three or four months, the delicate photographs taken at the lick observatory showed that a nebulous light surrounded the star, and was continually growing larger and larger. at first sight, there would seem to be nothing extraordinary in this fact. great masses of intensely hot vapor, shining by their own light, would naturally be thrown out from the star. or, if the star had originally been surrounded by a very rare nebulous fog or vapor, the latter would be seen by the brilliant light emitted by the star. on this was based an explanation offered by kapteyn, which at first seemed very plausible. it was that the sudden wave of light thrown out by the star when it burst forth caused the illumination of the surrounding vapor, which, though really at rest, would seem to expand with the velocity of light, as the illumination reached more and more distant regions of the nebula. this result may be made the subject of exact calculation. the velocity of light is such as would make a circuit of the earth more than seven times in a second. it would, therefore, go out from the star at the rate of a million of miles in between five and six seconds. in the lapse of one of our days, the light would have filled a sphere around the star having a diameter more than one hundred and fifty times the distance of the sun from the earth, and more than five times the dimensions of the whole solar system. continuing its course and enlarging its sphere day after day, the sight presented to us would have been that of a gradually expanding nebulous mass--a globe of faint light continually increasing in size with the velocity of light. the first sentiment the reader will feel on this subject is doubtless one of surprise that the distance of the star should be so great as this explanation would imply. six months after the explosion, the globe of light, as actually photographed, was of a size which would have been visible to the naked eye only as a very minute object in the sky. is it possible that this minute object could have been thousands of times the dimensions of our solar system? to see how the question stands from this point of view, we must have some idea of the possible distance of the new star. to gain this idea, we must find some way of estimating distances in the universe. for a reason which will soon be apparent, we begin with the greatest structure which nature offers to the view of man. we all know that the milky way is formed of countless stars, too minute to be individually visible to the naked eye. the more powerful the telescope through which we sweep the heavens, the greater the number of the stars that can be seen in it. with the powerful instruments which are now in use for photographing the sky, the number of stars brought to light must rise into the hundreds of millions, and the greater part of these belong to the milky way. the smaller the stars we count, the greater their comparative number in the region of the milky way. of the stars visible through the telescope, more than one-half are found in the milky way, which may be regarded as a girdle spanning the entire visible universe. of the diameter of this girdle we can say, almost with certainty, that it must be more than a thousand times as great as the distance of the nearest fixed star from us, and is probably two or three times greater. according to the best judgment we can form, our solar system is situate near the central region of the girdle, so that the latter must be distant from us by half its diameter. it follows that if we can imagine a gigantic pair of compasses, of which the points extend from us to alpha centauri, the nearest star, we should have to measure out at least five hundred spaces with the compass, and perhaps even one thousand or more, to reach the region of the milky way. with this we have to connect another curious fact. of eighteen new stars which have been observed to blaze forth during the last four hundred years, all are in the region of the milky way. this seems to show that, as a rule, they belong to the milky way. accepting this very plausible conclusion, the new star in perseus must have been more than five hundred times as far as the nearest fixed star. we know that it takes light four years to reach us from alpha centauri. it follows that the new star was at a distance through which light would require more than two thousand years to travel, and quite likely a time two or three times this. it requires only the most elementary ideas of geometry to see that if we suppose a ray of light to shoot from a star at such a distance in a direction perpendicular to the line of sight from us to the star, we can compute how fast the ray would seem to us to travel. granting the distance to be only two thousand light years, the apparent size of the sphere around the star which the light would fill at the end of one year after the explosion would be that of a coin seen at a distance of two thousand times its radius, or one thousand times its diameter--say, a five-cent piece at the distance of sixty feet. but, as a matter of fact, the nebulous illumination expanded with a velocity from ten to twenty times as great as this. the idea that the nebulosity around the new star was formed by the illumination caused by the light of the explosion spreading out on all sides therefore fails to satisfy us, not because the expansion of the nebula seemed to be so slow, but because it was many times as swift as the speed of light. another reason for believing that it was not a mere wave of light is offered by the fact that it did not take place regularly in every direction from the star, but seemed to shoot off at various angles. up to the present time, the speed of light has been to science, as well as to the intelligence of our race, almost a symbol of the greatest of possible speeds. the more carefully we reflect on the case, the more clearly we shall see the difficulty in supposing any agency to travel at the rate of the seeming emanations from the new star in perseus. as the emanation is seen spreading day after day, the reader may inquire whether this is not an appearance due to some other cause than the mere motion of light. may not an explosion taking place in the centre of a star produce an effect which shall travel yet faster than light? we can only reply that no such agency is known to science. but is there really anything intrinsically improbable in an agency travelling with a speed many times that of light? in considering that there is, we may fall into an error very much like that into which our predecessors fell in thinking it entirely out of the range of reasonable probability that the stars should be placed at such distances as we now know them to be. accepting it as a fact that agencies do exist which travel from sun to planet and from star to star with a speed which beggars all our previous ideas, the first question that arises is that of their nature and mode of action. this question is, up to the present time, one which we do not see any way of completely answering. the first difficulty is that we have no evidence of these agents except that afforded by their action. we see that the sun goes through a regular course of pulsations, each requiring eleven years for completion; and we see that, simultaneously with these, the earth's magnetism goes through a similar course of pulsations. the connection of the two, therefore, seems absolutely proven. but when we ask by what agency it is possible for the sun to affect the magnetism of the earth, and when we trace the passage of some agent between the two bodies, we find nothing to explain the action. to all appearance, the space between the earth and the sun is a perfect void. that electricity cannot of itself pass through a vacuum seems to be a well-established law of physics. it is true that electromagnetic waves, which are supposed to be of the same nature with those of light, and which are used in wireless telegraphy, do pass through a vacuum and may pass from the sun to the earth. but there is no way of explaining how such waves would either produce or affect the magnetism of the earth. the mysterious emanations from various substances, under certain conditions, may have an intimate relation with yet another of the mysteries of the universe. it is a fundamental law of the universe that when a body emits light or heat, or anything capable of being transformed into light or heat, it can do so only by the expenditure of force, limited in supply. the sun and stars are continually sending out a flood of heat. they are exhausting the internal supply of something which must be limited in extent. whence comes the supply? how is the heat of the sun kept up? if it were a hot body cooling off, a very few years would suffice for it to cool off so far that its surface would become solid and very soon cold. in recent years, the theory universally accepted has been that the supply of heat is kept up by the continual contraction of the sun, by mutual gravitation of its parts as it cools off. this theory has the advantage of enabling us to calculate, with some approximation to exactness, at what rate the sun must be contracting in order to keep up the supply of heat which it radiates. on this theory, it must, ten millions of years ago, have had twice its present diameter, while less than twenty millions of years ago it could not have existed except as an immense nebula filling the whole solar system. we must bear in mind that this theory is the only one which accounts for the supply of heat, even through human history. if it be true, then the sun, earth, and solar system must be less than twenty million years old. here the geologists step in and tell us that this conclusion is wholly inadmissible. the study of the strata of the earth and of many other geological phenomena, they assure us, makes it certain that the earth must have existed much in its present condition for hundreds of millions of years. during all that time there can have been no great diminution in the supply of heat radiated by the sun. the astronomer, in considering this argument, has to admit that he finds a similar difficulty in connection with the stars and nebulas. it is an impossibility to regard these objects as new; they must be as old as the universe itself. they radiate heat and light year after year. in all probability, they must have been doing so for millions of years. whence comes the supply? the geologist may well claim that until the astronomer explains this mystery in his own domain, he cannot declare the conclusions of geology as to the age of the earth to be wholly inadmissible. now, the scientific experiments of the last two years have brought this mystery of the celestial spaces right down into our earthly laboratories. m. and madame curie have discovered the singular metal radium, which seems to send out light, heat, and other rays incessantly, without, so far as has yet been determined, drawing the required energy from any outward source. as we have already pointed out, such an emanation must come from some storehouse of energy. is the storehouse, then, in the medium itself, or does the latter draw it from surrounding objects? if it does, it must abstract heat from these objects. this question has been settled by professor dewar, at the royal institution, london, by placing the radium in a medium next to the coldest that art has yet produced--liquid air. the latter is surrounded by the only yet colder medium, liquid hydrogen, so that no heat can reach it. under these circumstances, the radium still gives out heat, boiling away the liquid air until the latter has entirely disappeared. instead of the radiation diminishing with time, it rather seems to increase. called on to explain all this, science can only say that a molecular change must be going on in the radium, to correspond to the heat it gives out. what that change may be is still a complete mystery. it is a mystery which we find alike in those minute specimens of the rarest of substances under our microscopes, in the sun, and in the vast nebulous masses in the midst of which our whole solar system would be but a speck. the unravelling of this mystery must be the great work of science of the twentieth century. what results shall follow for mankind one cannot say, any more than he could have said two hundred years ago what modern science would bring forth. perhaps, before future developments, all the boasted achievements of the nineteenth century may take the modest place which we now assign to the science of the eighteenth century--that of the infant which is to grow into a man. iii the structure of the universe the questions of the extent of the universe in space and of its duration in time, especially of its possible infinity in either space or time, are of the highest interest both in philosophy and science. the traditional philosophy had no means of attacking these questions except considerations suggested by pure reason, analogy, and that general fitness of things which was supposed to mark the order of nature. with modern science the questions belong to the realm of fact, and can be decided only by the results of observation and a study of the laws to which these results may lead. from the philosophic stand-point, a discussion of this subject which is of such weight that in the history of thought it must be assigned a place above all others, is that of kant in his "kritik." here we find two opposing propositions--the thesis that the universe occupies only a finite space and is of finite duration; the antithesis that it is infinite both as regards extent in space and duration in time. both of these opposing propositions are shown to admit of demonstration with equal force, not directly, but by the methods of reductio ad absurdum. the difficulty, discussed by kant, was more tersely expressed by hamilton in pointing out that we could neither conceive of infinite space nor of space as bounded. the methods and conclusions of modern astronomy are, however, in no way at variance with kant's reasoning, so far as it extends. the fact is that the problem with which the philosopher of konigsberg vainly grappled is one which our science cannot solve any more than could his logic. we may hope to gain complete information as to everything which lies within the range of the telescope, and to trace to its beginning every process which we can now see going on in space. but before questions of the absolute beginning of things, or of the boundary beyond which nothing exists, our means of inquiry are quite powerless. another example of the ancient method is found in the great work of copernicus. it is remarkable how completely the first expounder of the system of the world was dominated by the philosophy of his time, which he had inherited from his predecessors. this is seen not only in the general course of thought through the opening chapters of his work, but among his introductory propositions. the first of these is that the universe--mundus--as well as the earth, is spherical in form. his arguments for the sphericity of the earth, as derived from observation, are little more than a repetition of those of ptolemy, and therefore not of special interest. his proposition that the universe is spherical is, however, not based on observation, but on considerations of the perfection of the spherical form, the general tendency of bodies--a drop of water, for example--to assume this form, and the sphericity of the sun and moon. the idea retained its place in his mind, although the fundamental conception of his system did away with the idea of the universe having any well-defined form. the question as attacked by modern astronomy is this: we see scattered through space in every direction many millions of stars of various orders of brightness and at distances so great as to defy exact measurement, except in the case of a few of the nearest. has this collection of stars any well-defined boundary, or is what we see merely that part of an infinite mass which chances to lie within the range of our telescopes? if we were transported to the most distant star of which we have knowledge, should we there find ourselves still surrounded by stars on all sides, or would the space beyond be void? granting that, in any or every direction, there is a limit to the universe, and that the space beyond is therefore void, what is the form of the whole system and the distance of its boundaries? preliminary in some sort to these questions are the more approachable ones: of what sort of matter is the universe formed? and into what sort of bodies is this matter collected? to the ancients the celestial sphere was a reality, instead of a mere effect of perspective, as we regard it. the stars were set on its surface, or at least at no great distance within its crystalline mass. outside of it imagination placed the empyrean. when and how these conceptions vanished from the mind of man, it would be as hard to say as when and how santa claus gets transformed in the mind of the child. they are not treated as realities by any astronomical writer from ptolemy down; yet, the impressions and forms of thought to which they gave rise are well marked in copernicus and faintly evident in kepler. the latter was perhaps the first to suggest that the sun might be one of the stars; yet, from defective knowledge of the relative brightness of the latter, he was led to the conclusion that their distances from each other were less than the distance which separated them from the sun. the latter he supposed to stand in the centre of a vast vacant region within the system of stars. for us the great collection of millions of stars which are made known to us by the telescope, together with all the invisible bodies which may be contained within the limits of the system, form the universe. here the term "universe" is perhaps objectionable because there may be other systems than the one with which we are acquainted. the term stellar system is, therefore, a better one by which to designate the collection of stars in question. it is remarkable that the first known propounder of that theory of the form and arrangement of the system which has been most generally accepted seems to have been a writer otherwise unknown in science--thomas wright, of durham, england. he is said to have published a book on the theory of the universe, about . it does not appear that this work was of a very scientific character, and it was, perhaps, too much in the nature of a speculation to excite notice in scientific circles. one of the curious features of the history is that it was kant who first cited wright's theory, pointed out its accordance with the appearance of the milky way, and showed its general reasonableness. but, at the time in question, the work of the philosopher of konigsberg seems to have excited no more notice among his scientific contemporaries than that of wright. kant's fame as a speculative philosopher has so eclipsed his scientific work that the latter has but recently been appraised at its true value. he was the originator of views which, though defective in detail, embodied a remarkable number of the results of recent research on the structure and form of the universe, and the changes taking place in it. the most curious illustration of the way in which he arrived at a correct conclusion by defective reasoning is found in his anticipation of the modern theory of a constant retardation of the velocity with which the earth revolves on its axis. he conceived that this effect must result from the force exerted by the tidal wave, as moving towards the west it strikes the eastern coasts of asia and america. an opposite conclusion was reached by laplace, who showed that the effect of this force was neutralized by forces producing the wave and acting in the opposite direction. and yet, nearly a century later, it was shown that while laplace was quite correct as regards the general principles involved, the friction of the moving water must prevent the complete neutralization of the two opposing forces, and leave a small residual force acting towards the west and retarding the rotation. kant's conclusion was established, but by an action different from that which he supposed. the theory of wright and kant, which was still further developed by herschel, was that our stellar system has somewhat the form of a flattened cylinder, or perhaps that which the earth would assume if, in consequence of more rapid rotation, the bulging out at its equator and the flattening at its poles were carried to an extreme limit. this form has been correctly though satirically compared to that of a grindstone. it rests to a certain extent, but not entirely, on the idea that the stars are scattered through space with equal thickness in every direction, and that the appearance of the milky way is due to the fact that we, situated in the centre of this flattened system, see more stars in the direction of the circumference of the system than in that of its poles. the argument on which the view in question rests may be made clear in the following way. let us chose for our observations that hour of the night at which the milky way skirts our horizon. this is nearly the case in the evenings of may and june, though the coincidence with the horizon can never be exact except to observers stationed near the tropics. using the figure of the grindstone, we at its centre will then have its circumference around our horizon, while the axis will be nearly vertical. the points in which the latter intersects the celestial sphere are called the galactic poles. there will be two of these poles, the one at the hour in question near the zenith, the other in our nadir, and therefore invisible to us, though seen by our antipodes. our horizon corresponds, as it were, to the central circle of the milky way, which now surrounds us on all sides in a horizontal direction, while the galactic poles are degrees distant from every part of it, as every point of the horizon is degrees from the zenith. let us next count the number of stars visible in a powerful telescope in the region of the heavens around the galactic pole, now our zenith, and find the average number per square degree. this will be the richness of the region in stars. then we take regions nearer the horizontal milky way--say that contained between degrees and degrees from the zenith--and, by a similar count, find its richness in stars. we do the same for other regions, nearer and nearer to the horizon, till we reach the galaxy itself. the result of all the counts will be that the richness of the sky in stars is least around the galactic pole, and increases in every direction towards the milky way. without such counts of the stars we might imagine our stellar system to be a globular collection of stars around which the object in question passed as a girdle; and we might take a globe with a chain passing around it as representative of the possible figure of the stellar system. but the actual increase in star-thickness which we have pointed out shows us that this view is incorrect. the nature and validity of the conclusions to be drawn can be best appreciated by a statement of some features of this tendency of the stars to crowd towards the galactic circle. most remarkable is the fact that the tendency is seen even among the brighter stars. without either telescope or technical knowledge, the careful observer of the stars will notice that the most brilliant constellations show this tendency. the glorious orion, canis major containing the brightest star in the heavens, cassiopeia, perseus, cygnus, and lyra with its bright-blue vega, not to mention such constellations as the southern cross, all lie in or near the milky way. schiaparelli has extended the investigation to all the stars visible to the naked eye. he laid down on planispheres the number of such stars in each region of the heavens of degrees square. each region was then shaded with a tint that was darker as the region was richer in stars. the very existence of the milky way was ignored in this work, though his most darkly shaded regions lie along the course of this belt. by drawing a band around the sky so as to follow or cover his darkest regions, we shall rediscover the course of the milky way without any reference to the actual object. it is hardly necessary to add that this result would be reached with yet greater precision if we included the telescopic stars to any degree of magnitude--plotting them on a chart and shading the chart in the same way. what we learn from this is that the stellar system is not an irregular chaos; and that notwithstanding all its minor irregularities, it may be considered as built up with special reference to the milky way as a foundation. another feature of the tendency in question is that it is more and more marked as we include fainter stars in our count. the galactic region is perhaps twice as rich in stars visible to the naked eye as the rest of the heavens. in telescopic stars to the ninth magnitude it is three or four times as rich. in the stars found on the photographs of the sky made at the harvard and other observatories, and in the stargauges of the herschels, it is from five to ten times as rich. another feature showing the unity of the system is the symmetry of the heavens on the two sides of the galactic belt let us return to our supposition of such a position of the celestial sphere, with respect to the horizon, that the latter coincides with the central line of this belt, one galactic pole being near our zenith. the celestial hemisphere which, being above our horizon, is visible to us, is the one to which we have hitherto directed our attention in describing the distribution of the stars. but below our horizon is another hemisphere, that of our antipodes, which is the counterpart of ours. the stars which it contains are in a different part of the universe from those which we see, and, without unity of plan, would not be subject to the same law. but the most accurate counts of stars that have been made fail to show any difference in their general arrangement in the two hemispheres. they are just as thick around the south galactic poles as around the north one. they show the same tendency to crowd towards the milky way in the hemisphere invisible to us as in the hemisphere which we see. slight differences and irregularities, are, indeed, found in the enumeration, but they are no greater than must necessarily arise from the difficulty of stopping our count at a perfectly fixed magnitude. the aim of star-counts is not to estimate the total number of stars, for this is beyond our power, but the number visible with a given telescope. in such work different observers have explored different parts of the sky, and in a count of the same region by two observers we shall find that, although they attempt to stop at the same magnitude, each will include a great number of stars which the other omits. there is, therefore, room for considerable difference in the numbers of stars recorded, without there being any actual inequality between the two hemispheres. a corresponding similarity is found in the physical constitution of the stars as brought out by the spectroscope. the milky way is extremely rich in bluish stars, which make up a considerable majority of the cloudlike masses there seen. but when we recede from the galaxy on one side, we find the blue stars becoming thinner, while those having a yellow tinge become relatively more numerous. this difference of color also is the same on the two sides of the galactic plane. nor can any systematic difference be detected between the proper motions of the stars in these two hemispheres. if the largest known proper motion is found in the one, the second largest is in the other. counting all the known stars that have proper motions exceeding a given limit, we find about as many in one hemisphere as in the other. in this respect, also, the universe appears to be alike through its whole extent. it is the uniformity thus prevailing through the visible universe, as far as we can see, in two opposite directions, which inspires us with confidence in the possibility of ultimately reaching some well-founded conclusion as to the extent and structure of the system. all these facts concur in supporting the view of wright, kant, and herschel as to the form of the universe. the farther out the stars extend in any direction, the more stars we may see in that direction. in the direction of the axis of the cylinder, the distances of the boundary are least, so that we see fewer stars. the farther we direct our attention towards the equatorial regions of the system, the greater the distance from us to the boundary, and hence the more stars we see. the fact that the increase in the number of stars seen towards the equatorial region of the system is greater, the smaller the stars, is the natural consequence of the fact that distant stars come within our view in greater numbers towards the equatorial than towards the polar regions. objections have been raised to the herschelian view on the ground that it assumes an approximately uniform distribution of the stars in space. it has been claimed that the fact of our seeing more stars in one direction than in another may not arise merely from our looking through a deeper stratum, as herschel supposed, but may as well be due to the stars being more thinly scattered in the direction of the axis of the system than in that of its equatorial region. the great inequalities in the richness of neighboring regions in the milky way show that the hypothesis of uniform distribution does not apply to the equatorial region. the claim has therefore been made that there is no proof of the system extending out any farther in the equatorial than in the polar direction. the consideration of this objection requires a closer inquiry as to what we are to understand by the form of our system. we have already pointed out the impossibility of assigning any boundary beyond which we can say that nothing exists. and even as regards a boundary of our stellar system, it is impossible for us to assign any exact limit beyond which no star is visible to us. the analogy of collections of stars seen in various parts of the heavens leads us to suppose that there may be no well-defined form to our system, but that, as we go out farther and farther, we shall see occasional scattered stars to, possibly, an indefinite distance. the truth probably is that, as in ascending a mountain, we find the trees, which may be very dense at its base, thin out gradually as we approach the summit, where there may be few or none, so we might find the stars to thin out could we fly to the distant regions of space. the practical question is whether, in such a flight, we should find this sooner by going in the direction of the axis of our system than by directing our course towards the milky way. if a point is at length reached beyond which there are but few scattered stars, such a point would, for us, mark the boundary of our system. from this point of view the answer does not seem to admit of doubt. if, going in every direction, we mark the point, if any, at which the great mass of the stars are seen behind us, the totality of all these points will lie on a surface of the general form that herschel supposed. there is still another direct indication of the finitude of our stellar system upon which we have not touched. if this system extended out without limit in any direction whatever, it is shown by a geometric process which it is not necessary to explain in the present connection, but which is of the character of mathematical demonstration, that the heavens would, in every direction where this was true, blaze with the light of the noonday sun. this would be very different from the blue-black sky which we actually see on a clear night, and which, with a reservation that we shall consider hereafter, shows that, how far so-ever our stellar system may extend, it is not infinite. beyond this negative conclusion the fact does not teach us much. vast, indeed, is the distance to which the system might extend without the sky appearing much brighter than it is, and we must have recourse to other considerations in seeking for indications of a boundary, or even of a well-marked thinning out, of stars. if, as was formerly supposed, the stars did not greatly differ in the amount of light emitted by each, and if their diversity of apparent magnitude were due principally to the greater distance of the fainter stars, then the brightness of a star would enable us to form a more or less approximate idea of its distance. but the accumulated researches of the past seventy years show that the stars differ so enormously in their actual luminosity that the apparent brightness of a star affords us only a very imperfect indication of its distance. while, in the general average, the brighter stars must be nearer to us than the fainter ones, it by no means follows that a very bright star, even of the first magnitude, is among the nearer to our system. two stars are worthy of especial mention in this connection, canopus and rigel. the first is, with the single exception of sirius, the brightest star in the heavens. the other is a star of the first magnitude in the southwest corner of orion. the most long-continued and complete measures of parallax yet made are those carried on by gill, at the cape of good hope, on these two and some other bright stars. the results, published in , show that neither of these bodies has any parallax that can be measured by the most refined instrumental means known to astronomy. in other words, the distance of these stars is immeasurably great. the actual amount of light emitted by each is certainly thousands and probably tens of thousands of times that of the sun. notwithstanding the difficulties that surround the subject, we can at least say something of the distance of a considerable number of the stars. two methods are available for our estimate--measures of parallax and determination of proper motions. the problem of stellar parallax, simple though it is in its conception, is the most delicate and difficult of all which the practical astronomer has to encounter. an idea of it may be gained by supposing a minute object on a mountain-top, we know not how many miles away, to be visible through a telescope. the observer is allowed to change the position of his instrument by two inches, but no more. he is required to determine the change in the direction of the object produced by this minute displacement with accuracy enough to determine the distance of the mountain. this is quite analogous to the determination of the change in the direction in which we see a star as the earth, moving through its vast circuit, passes from one extremity of its orbit to the other. representing this motion on such a scale that the distance of our planet from the sun shall be one inch, we find that the nearest star, on the same scale, will be more than four miles away, and scarcely one out of a million will be at a less distance than ten miles. it is only by the most wonderful perfection both in the heliometer, the instrument principally used for these measures, and in methods of observation, that any displacement at all can be seen even among the nearest stars. the parallaxes of perhaps a hundred stars have been determined, with greater or less precision, and a few hundred more may be near enough for measurement. all the others are immeasurably distant; and it is only by statistical methods based on their proper motions and their probable near approach to equality in distribution that any idea can be gained of their distances. to form a conception of the stellar system, we must have a unit of measure not only exceeding any terrestrial standard, but even any distance in the solar system. for purely astronomical purposes the most convenient unit is the distance corresponding to a parallax of ", which is a little more than , times the sun's distance. but for the purposes of all but the professional astronomer the most convenient unit will be the light-year--that is, the distance through which light would travel in one year. this is equal to the product of , miles, the distance travelled in one second, by , , , the number of seconds in a year. the reader who chooses to do so may perform the multiplication for himself. the product will amount to about , times the distance of the sun. [illustration with caption: a typical star cluster--centauri] the nearest star whose distance we know, alpha centauri, is distant from us more than four light-years. in all likelihood this is really the nearest star, and it is not at all probable that any other star lies within six light-years. moreover, if we were transported to this star the probability seems to be that the sun would now be the nearest star to us. flying to any other of the stars whose parallax has been measured, we should probably find that the average of the six or eight nearest stars around us ranges somewhere between five and seven light-years. we may, in a certain sense, call eight light-years a star-distance, meaning by this term the average of the nearest distances from one star to the surrounding ones. to put the result of measures of parallax into another form, let us suppose, described around our sun as a centre, a system of concentric spheres each of whose surfaces is at the distance of six light-years outside the sphere next within it. the inner is at the distance of six light-years around the sun. the surface of the second sphere will be twelve light-years away, that of the third eighteen, etc. the volumes of space within each of these spheres will be as the cubes of the diameters. the most likely conclusion we can draw from measures of parallax is that the first sphere will contain, beside the sun at its centre, only alpha centauri. the second, twelve light-years away, will probably contain, besides these two, six other stars, making eight in all. the third may contain twenty-one more, making twenty-seven stars within the third sphere, which is the cube of three. within the fourth would probably be found sixty-four stars, this being the cube of four, and so on. beyond this no measures of parallax yet made will give us much assistance. we can only infer that probably the same law holds for a large number of spheres, though it is quite certain that it does not hold indefinitely. for more light on the subject we must have recourse to the proper motions. the latest words of astronomy on this subject may be briefly summarized. as a rule, no star is at rest. each is moving through space with a speed which differs greatly with different stars, but is nearly always swift, indeed, when measured by any standard to which we are accustomed. slow and halting, indeed, is that star which does not make more than a mile a second. with two or three exceptions, where the attraction of a companion comes in, the motion of every star, so far as yet determined, takes place in a straight line. in its outward motion the flying body deviates neither to the right nor left. it is safe to say that, if any deviation is to take place, thousands of years will be required for our terrestrial observers to recognize it. rapid as the course of these objects is, the distances which we have described are such that, in the great majority of cases, all the observations yet made on the positions of the stars fail to show any well-established motion. it is only in the case of the nearer of these objects that we can expect any motion to be perceptible during the period, in no case exceeding one hundred and fifty years, through which accurate observations extend. the efforts of all the observatories which engage in such work are, up to the present time, unequal to the task of grappling with the motions of all the stars that can be seen with the instruments, and reaching a decision as to the proper motion in each particular case. as the question now stands, the aim of the astronomer is to determine what stars have proper motions large enough to be well established. to make our statement on this subject clear, it must be understood that by this term the astronomer does not mean the speed of a star in space, but its angular motion as he observes it on the celestial sphere. a star moving forward with a given speed will have a greater proper motion according as it is nearer to us. to avoid all ambiguity, we shall use the term "speed" to express the velocity in miles per second with which such a body moves through space, and the term "proper motion" to express the apparent angular motion which the astronomer measures upon the celestial sphere. up to the present time, two stars have been found whose proper motions are so large that, if continued, the bodies would make a complete circuit of the heavens in less than , years. one of these would require about , ; the other about , years for the circuit. of other stars having a rapid motion only about one hundred would complete their course in less than a million of years. quite recently a system of observations upon stars to the ninth magnitude has been nearly carried through by an international combination of observatories. the most important conclusion from these observations relates to the distribution of the stars with reference to the milky way, which we have already described. we have shown that stars of every magnitude, bright and faint, show a tendency to crowd towards this belt. it is, therefore, remarkable that no such tendency is seen in the case of those stars which have proper motions large enough to be accurately determined. so far as yet appears, such stars are equally scattered over the heavens, without reference to the course of the milky way. the conclusion is obvious. these stars are all inside the girdle of the milky way, and within the sphere which contains them the distribution in space is approximately uniform. at least there is no well-marked condensation in the direction of the galaxy nor any marked thinning out towards its poles. what can we say as to the extent of this sphere? to answer this question, we have to consider whether there is any average or ordinary speed that a star has in space. a great number of motions in the line of sight--that is to say, in the direction of the line from us to the star--have been measured with great precision by campbell at the lick observatory, and by other astronomers. the statistical investigations of kaptoyn also throw much light on the subject. the results of these investigators agree well in showing an average speed in space--a straight-ahead motion we may call it--of twenty-one miles per second. some stars may move more slowly than this to any extent; others more rapidly. in two or three cases the speed exceeds one hundred miles per second, but these are quite exceptional. by taking several thousand stars having a given proper motion, we may form a general idea of their average distance, though a great number of them will exceed this average to a considerable extent. the conclusion drawn in this way would be that the stars having an apparent proper motion of " per century or more are mostly contained within, or lie not far outside of a sphere whose surface is at a distance from us of light-years. granting the volume of space which we have shown that nature seems to allow to each star, this sphere should contain , stars in all. there are about , stars known to have so large a proper motion as ". but there is no actual discordance between these results, because not only are there, in all probability, great numbers of stars of which the proper motion is not yet recognized, but there are within the sphere a great number of stars whose motion is less than the average. on the other hand, it is probable that a considerable number of the , stars lie at a distance at least one-half greater than that of the radius of the sphere. on the whole, it seems likely that, out to a distance of or even light-years, there is no marked inequality in star distribution. if we should explore the heavens to this distance, we should neither find the beginning of the milky way in one direction nor a very marked thinning out in the other. this conclusion is quite accordant with the probabilities of the case. if all the stars which form the groundwork of the milky way should be blotted out, we should probably find , , , perhaps even more, remaining. assigning to each star the space already shown to be its quota, we should require a sphere of about light-years radius to contain such a number of stars. at some such distance as this, we might find a thinning out of the stars in the direction of the galactic poles, or the commencement of the milky way in the direction of this stream. even if this were not found at the distance which we have supposed, it is quite certain that, at some greater distance, we should at least find that the region of the milky way is richer in stars than the region near the galactic poles. there is strong reason, based on the appearance of the stars of the milky way, their physical constitution, and their magnitudes as seen in the telescope, to believe that, were we placed on one of these stars, we should find the stars around us to be more thickly strewn than they are around our system. in other words, the quota of space filled by each star is probably less in the region of the milky way than it is near the centre where we seem to be situated. we are, therefore, presented with what seems to be the most extraordinary spectacle that the universe can offer, a ring of stars spanning it, and including within its limits by far the great majority of the stars within our system. we have in this spectacle another example of the unity which seems to pervade the system. we might imagine the latter so arranged as to show diversity to any extent. we might have agglomerations of stars like those of the milky way situated in some corner of the system, or at its centre, or scattered through it here and there in every direction. but such is not the case. there are, indeed, a few star-clusters scattered here and there through the system; but they are essentially different from the clusters of the milky way, and cannot be regarded as forming an important part of the general plan. in the case of the galaxy we have no such scattering, but find the stars built, as it were, into this enormous ring, having similar characteristics throughout nearly its whole extent, and having within it a nearly uniform scattering of stars, with here and there some collected into clusters. such, to our limited vision, now appears the universe as a whole. we have already alluded to the conclusion that an absolutely infinite system of stars would cause the entire heavens to be filled with a blaze of light as bright as the sun. it is also true that the attractive force within such a universe would be infinitely great in some direction or another. but neither of these considerations enables us to set a limit to the extent of our system. in two remarkable papers by lord kelvin which have recently appeared, the one being an address before the british association at its glasgow meeting, in , are given the results of some numerical computations pertaining to this subject. granting that the stars are scattered promiscuously through space with some approach to uniformity in thickness, and are of a known degree of brilliancy, it is easy to compute how far out the system must extend in order that, looking up at the sky, we shall see a certain amount of light coming from the invisible stars. granting that, in the general average, each star is as bright as the sun, and that their thickness is such that within a sphere of light-years there are , , , stars, if we inquire how far out such a system must be continued in order that the sky shall shine with even four per cent of the light of the sun, we shall find the distance of its boundary so great that millions of millions of years would be required for the light of the outer stars to reach the centre of the system. in view of the fact that this duration in time far exceeds what seems to be the possible life duration of a star, so far as our knowledge of it can extend, the mere fact that the sky does not glow with any such brightness proves little or nothing as to the extent of the system. we may, however, replace these purely negative considerations by inquiring how much light we actually get from the invisible stars of our system. here we can make a definite statement. mark out a small circle in the sky degree in diameter. the quantity of light which we receive on a cloudless and moonless night from the sky within this circle admits of actual determination. from the measures so far available it would seem that, in the general average, this quantity of light is not very different from that of a star of the fifth magnitude. this is something very different from a blaze of light. a star of the fifth magnitude is scarcely more than plainly visible to ordinary vision. the area of the whole sky is, in round numbers, about , times that of the circle we have described. it follows that the total quantity of light which we receive from all the stars is about equal to that of , stars of the fifth magnitude--somewhat more than of the first magnitude. this whole amount of light would have to be multiplied by , , to make a light equal to that of the sun. it is, therefore, not at all necessary to consider how far the system must extend in order that the heavens should blaze like the sun. adopting lord kelvin's hypothesis, we shall find that, in order that we may receive from the stars the amount of light we have designated, this system need not extend beyond some light-years. but this hypothesis probably overestimates the thickness of the stars in space. it does not seem probable that there are as many as , , , stars within the sphere of light-years. nor is it at all certain that the light of the average star is equal to that of the sun. it is impossible, in the present state of our knowledge, to assign any definite value to this average. to do so is a problem similar to that of assigning an average weight to each component of the animal creation, from the microscopic insects which destroy our plants up to the elephant. what we can say with a fair approximation to confidence is that, if we could fly out in any direction to a distance of , , perhaps even of , , light-years, we should find that we had left a large fraction of our system behind us. we should see its boundary in the direction in which we had travelled much more certainly than we see it from our stand-point. we should not dismiss this branch of the subject without saying that considerations are frequently adduced by eminent authorities which tend to impair our confidence in almost any conclusion as to the limits of the stellar system. the main argument is based on the possibility that light is extinguished in its passage through space; that beyond a certain distance we cannot see a star, however bright, because its light is entirely lost before reaching us. that there could be any loss of light in passing through an absolute vacuum of any extent cannot be admitted by the physicist of to-day without impairing what he considers the fundamental principles of the vibration of light. but the possibility that the celestial spaces are pervaded by matter which might obstruct the passage of light is to be considered. we know that minute meteoric particles are flying through our system in such numbers that the earth encounters several millions of them every day, which appear to us in the familiar phenomena of shooting-stars. if such particles are scattered through all space, they must ultimately obstruct the passage of light. we know little of the size of these bodies, but, from the amount of energy contained in their light as they are consumed in the passage through our atmosphere, it does not seem at all likely that they are larger than grains of sand or, perhaps, minute pebbles. they are probably vastly more numerous in the vicinity of the sun than in the interstellar spaces, since they would naturally tend to be collected by the sun's attraction. in fact there are some reasons for believing that most of these bodies are the debris of comets; and the latter are now known to belong to the solar system, and not to the universe at large. but whatever view we take of these possibilities, they cannot invalidate our conclusion as to the general structure of the stellar system as we know it. were meteors so numerous as to cut off a large fraction of the light from the more distant stars, we should see no milky way, but the apparent thickness of the stars in every direction would be nearly the same. the fact that so many more of these objects are seen around the galactic belt than in the direction of its poles shows that, whatever extinction light may suffer in going through the greatest distances, we see nearly all that comes from stars not more distant than the milky way itself. intimately connected with the subject we have discussed is the question of the age of our system, if age it can be said to have. in considering this question, the simplest hypothesis to suggest itself is that the universe has existed forever in some such form as we now see it; that it is a self-sustaining system, able to go on forever with only such cycles of transformation as may repeat themselves indefinitely, and may, therefore, have repeated themselves indefinitely in the past. ordinary observation does not make anything known to us which would seem to invalidate this hypothesis. in looking upon the operations of the universe, we may liken ourselves to a visitor to the earth from another sphere who has to draw conclusions about the life of an individual man from observations extending through a few days. during that time, he would see no reason why the life of the man should have either a beginning or an end. he sees a daily round of change, activity and rest, nutrition and waste; but, at the end of the round, the individual is seemingly restored to his state of the day before. why may not this round have been going on forever, and continue in the future without end? it would take a profounder course of observation and a longer time to show that, notwithstanding this seeming restoration, an imperceptible residual of vital energy, necessary to the continuance of life, has not been restored, and that the loss of this residuum day by day must finally result in death. the case is much the same with the great bodies of the universe. although, to superficial observation, it might seem that they could radiate their light forever, the modern generalizations of physics show that such cannot be the case. the radiation of light necessarily involves a corresponding loss of heat and with it the expenditure of some form of energy. the amount of energy within any body is necessarily limited. the supply must be exhausted unless the energy of the light sent out into infinite space is, in some way, restored to the body which expended it. the possibility of such a restoration completely transcends our science. how can the little vibration which strikes our eye from some distant star, and which has been perhaps thousands of years in reaching us, find its way back to its origin? the light emitted by the sun , years ago is to-day pursuing its way in a sphere whose surface is , light-years distant on all sides. science has nothing even to suggest the possibility of its restoration, and the most delicate observations fail to show any return from the unfathomable abyss. up to the time when radium was discovered, the most careful investigations of all conceivable sources of supply had shown only one which could possibly be of long duration. this is the contraction which is produced in the great incandescent bodies of the universe by the loss of the heat which they radiate. as remarked in the preceding essay, the energy generated by the sun's contraction could not have kept up its present supply of heat for much more than twenty or thirty millions of years, while the study of earth and ocean shows evidence of the action of a series of causes which must have been going on for hundreds of millions of years. the antagonism between the two conclusions is even more marked than would appear from this statement. the period of the sun's heat set by the astronomical physicist is that during which our luminary could possibly have existed in its present form. the period set by the geologist is not merely that of the sun's existence, but that during which the causes effecting geological changes have not undergone any complete revolution. if, at any time, the sun radiated much less than its present amount of heat, no water could have existed on the earth's surface except in the form of ice; there would have been scarcely any evaporation, and the geological changes due to erosion could not have taken place. moreover, the commencement of the geological operations of which we speak is by no means the commencement of the earth's existence. the theories of both parties agree that, for untold aeons before the geological changes now visible commenced, our planet was a molten mass, perhaps even an incandescent globe like the sun. during all those aeons the sun must have been in existence as a vast nebulous mass, first reaching as far as the earth's orbit, and slowly contracting its dimensions. and these aeons are to be included in any estimate of the age of the sun. the doctrine of cosmic evolution--the theory which in former times was generally known as the nebular hypothesis--that the heavenly bodies were formed by the slow contraction of heated nebulous masses, is indicated by so many facts that it seems scarcely possible to doubt it except on the theory that the laws of nature were, at some former time, different from those which we now see in operation. granting the evolutionary hypothesis, every star has its lifetime. we can even lay down the law by which it passes from infancy to old age. all stars do not have the same length of life; the rule is that the larger the star, or the greater the mass of matter which composes it, the longer will it endure. up to the present time, science can do nothing more than point out these indications of a beginning, and their inevitable consequence, that there is to be an end to the light and heat of every heavenly body. but no cautious thinker can treat such a subject with the ease of ordinary demonstration. the investigator may even be excused if he stands dumb with awe before the creation of his own intellect. our accurate records of the operations of nature extend through only two or three centuries, and do not reach a satisfactory standard until within a single century. the experience of the individual is limited to a few years, and beyond this period he must depend upon the records of his ancestors. all his knowledge of the laws of nature is derived from this very limited experience. how can he essay to describe what may have been going on hundreds of millions of years in the past? can he dare to say that nature was the same then as now? it is a fundamental principle of the theory of evolution, as developed by its greatest recent expounder, that matter itself is eternal, and that all the changes which have taken place in the universe, so far as made up of matter, are in the nature of transformations of this eternal substance. but we doubt whether any physical philosopher of the present day would be satisfied to accept any demonstration of the eternity of matter. all he would admit is that, so far as his observation goes, no change in the quantity of matter can be produced by the action of any known cause. it seems to be equally uncreatable and indestructible. but he would, at the same time, admit that his experience no more sufficed to settle the question than the observation of an animal for a single day would settle the question of the duration of its life, or prove that it had neither beginning nor end. he would probably admit that even matter itself may be a product of evolution. the astronomer finds it difficult to conceive that the great nebulous masses which he sees in the celestial spaces--millions of times larger than the whole solar system, yet so tenuous that they offer not the slightest obstruction to the passage of a ray of light through their whole length--situated in what seems to be a region of eternal cold, below anything that we can produce on the earth's surface, yet radiating light, and with it heat, like an incandescent body--can be made up of the same kind of substance that we have around us on the earth's surface. who knows but that the radiant property that becquerel has found in certain forms of matter may be a residuum of some original form of energy which is inherent in great cosmical masses, and has fed our sun during all the ages required by the geologist for the structure of the earth's crusts? it may be that in this phenomenon we have the key to the great riddle of the universe, with which profounder secrets of matter than any we have penetrated will be opened to the eyes of our successors. iv the extent of the universe we cannot expect that the wisest men of our remotest posterity, who can base their conclusions upon thousands of years of accurate observation, will reach a decision on this subject without some measure of reserve. such being the case, it might appear the dictate of wisdom to leave its consideration to some future age, when it may be taken up with better means of information than we now possess. but the question is one which will refuse to be postponed so long as the propensity to think of the possibilities of creation is characteristic of our race. the issue is not whether we shall ignore the question altogether, like eve in the presence of raphael; but whether in studying it we shall confine our speculations within the limits set by sound scientific reasoning. essaying to do this, i invite the reader's attention to what science may suggest, admitting in advance that the sphere of exact knowledge is small compared with the possibilities of creation, and that outside this sphere we can state only more or less probable conclusions. the reader who desires to approach this subject in the most receptive spirit should begin his study by betaking himself on a clear, moonless evening, when he has no earthly concern to disturb the serenity of his thoughts, to some point where he can lie on his back on bench or roof, and scan the whole vault of heaven at one view. he can do this with the greatest pleasure and profit in late summer or autumn--winter would do equally well were it possible for the mind to rise so far above bodily conditions that the question of temperature should not enter. the thinking man who does this under circumstances most favorable for calm thought will form a new conception of the wonder of the universe. if summer or autumn be chosen, the stupendous arch of the milky way will pass near the zenith, and the constellation lyra, led by its beautiful blue vega of the first magnitude, may be not very far from that point. south of it will be seen the constellation aquila, marked by the bright altair, between two smaller but conspicuous stars. the bright arcturus will be somewhere in the west, and, if the observation is not made too early in the season, aldebaran will be seen somewhere in the east. when attention is concentrated on the scene the thousands of stars on each side of the milky way will fill the mind with the consciousness of a stupendous and all-embracing frame, beside which all human affairs sink into insignificance. a new idea will be formed of such a well-known fact of astronomy as the motion of the solar system in space, by reflecting that, during all human history, the sun, carrying the earth with it, has been flying towards a region in or just south of the constellation lyra, with a speed beyond all that art can produce on earth, without producing any change apparent to ordinary vision in the aspect of the constellation. not only lyra and aquila, but every one of the thousand stars which form the framework of the sky, were seen by our earliest ancestors just as we see them now. bodily rest may be obtained at any time by ceasing from our labors, and weary systems may find nerve rest at any summer resort; but i know of no way in which complete rest can be obtained for the weary soul--in which the mind can be so entirely relieved of the burden of all human anxiety--as by the contemplation of the spectacle presented by the starry heavens under the conditions just described. as we make a feeble attempt to learn what science can tell us about the structure of this starry frame, i hope the reader will allow me to at least fancy him contemplating it in this way. the first question which may suggest itself to the inquiring reader is: how is it possible by any methods of observation yet known to the astronomer to learn anything about the universe as a whole? we may commence by answering this question in a somewhat comprehensive way. it is possible only because the universe, vast though it is, shows certain characteristics of a unified and bounded whole. it is not a chaos, it is not even a collection of things, each of which came into existence in its own separate way. if it were, there would be nothing in common between two widely separate regions of the universe. but, as a matter of fact, science shows unity in the whole structure, and diversity only in details. the milky way itself will be seen by the most ordinary observer to form a single structure. this structure is, in some sort, the foundation on which the universe is built. it is a girdle which seems to span the whole of creation, so far as our telescopes have yet enabled us to determine what creation is; and yet it has elements of similarity in all its parts. what has yet more significance, it is in some respects unlike those parts of the universe which lie without it, and even unlike those which lie in that central region within it where our system is now situated. the minute stars, individually far beyond the limit of visibility to the naked eye, which form its cloudlike agglomerations, are found to be mostly bluer in color, from one extreme to the other, than the general average of the stars which make up the rest of the universe. in the preceding essay on the structure of the universe, we have pointed out several features of the universe showing the unity of the whole. we shall now bring together these and other features with a view of showing their relation to the question of the extent of the universe. the milky way being in a certain sense the foundation on which the whole system is constructed, we have first to notice the symmetry of the whole. this is seen in the fact that a certain resemblance is found in any two opposite regions of the sky, no matter where we choose them. if we take them in the milky way, the stars are more numerous than elsewhere; if we take opposite regions in or near the milky way, we shall find more stars in both of them than elsewhere; if we take them in the region anywhere around the poles of the milky way, we shall find fewer stars, but they will be equally numerous in each of the two regions. we infer from this that whatever cause determined the number of the stars in space was of the same nature in every two antipodal regions of the heavens. another unity marked with yet more precision is seen in the chemical elements of which stars are composed. we know that the sun is composed of the same elements which we find on the earth and into which we resolve compounds in our laboratories. these same elements are found in the most distant stars. it is true that some of these bodies seem to contain elements which we do not find on earth. but as these unknown elements are scattered from one extreme of the universe to the other, they only serve still further to enforce the unity which runs through the whole. the nebulae are composed, in part at least, of forms of matter dissimilar to any with which we are acquainted. but, different though they may be, they are alike in their general character throughout the whole field we are considering. even in such a feature as the proper motions of the stars, the same unity is seen. the reader doubtless knows that each of these objects is flying through space on its own course with a speed comparable with that of the earth around the sun. these speeds range from the smallest limit up to more than one hundred miles a second. such diversity might seem to detract from the unity of the whole; but when we seek to learn something definite by taking their average, we find this average to be, so far as can yet be determined, much the same in opposite regions of the universe. quite recently it has become probable that a certain class of very bright stars known as orion stars--because there are many of them in the most brilliant of our constellations--which are scattered along the whole course of the milky way, have one and all, in the general average, slower motions than other stars. here again we have a definable characteristic extending through the universe. in drawing attention to these points of similarity throughout the whole universe, it must not be supposed that we base our conclusions directly upon them. the point they bring out is that the universe is in the nature of an organized system; and it is upon the fact of its being such a system that we are able, by other facts, to reach conclusions as to its structure, extent, and other characteristics. one of the great problems connected with the universe is that of its possible extent. how far away are the stars? one of the unities which we have described leads at once to the conclusion that the stars must be at very different distances from us; probably the more distant ones are a thousand times as far as the nearest; possibly even farther than this. this conclusion may, in the first place, be based on the fact that the stars seem to be scattered equally throughout those regions of the universe which are not connected with the milky way. to illustrate the principle, suppose a farmer to sow a wheat-field of entirely unknown extent with ten bushels of wheat. we visit the field and wish to have some idea of its acreage. we may do this if we know how many grains of wheat there are in the ten bushels. then we examine a space two or three feet square in any part of the field and count the number of grains in that space. if the wheat is equally scattered over the whole field, we find its extent by the simple rule that the size of the field bears the same proportion to the size of the space in which the count was made that the whole number of grains in the ten bushels sown bears to the number of grains counted. if we find ten grains in a square foot, we know that the number of square feet in the whole field is one-tenth that of the number of grains sown. so it is with the universe of stars. if the latter are sown equally through space, the extent of the space occupied must be proportional to the number of stars which it contains. but this consideration does not tell us anything about the actual distance of the stars or how thickly they may be scattered. to do this we must be able to determine the distance of a certain number of stars, just as we suppose the farmer to count the grains in a certain small extent of his wheat-field. there is only one way in which we can make a definite measure of the distance of any one star. as the earth swings through its vast annual circuit round the sun, the direction of the stars must appear to be a little different when seen from one extremity of the circuit than when seen from the other. this difference is called the parallax of the stars; and the problem of measuring it is one of the most delicate and difficult in the whole field of practical astronomy. the nineteenth century was well on its way before the instruments of the astronomer were brought to such perfection as to admit of the measurement. from the time of copernicus to that of bessel many attempts had been made to measure the parallax of the stars, and more than once had some eager astronomer thought himself successful. but subsequent investigation always showed that he had been mistaken, and that what he thought was the effect of parallax was due to some other cause, perhaps the imperfections of his instrument, perhaps the effect of heat and cold upon it or upon the atmosphere through which he was obliged to observe the star, or upon the going of his clock. thus things went on until , when bessel announced that measures with a heliometer--the most refined instrument that has ever been used in measurement--showed that a certain star in the constellation cygnus had a parallax of one-third of a second. it may be interesting to give an idea of this quantity. suppose one's self in a house on top of a mountain looking out of a window one foot square, at a house on another mountain one hundred miles away. one is allowed to look at that distant house through one edge of the pane of glass and then through the opposite edge; and he has to determine the change in the direction of the distant house produced by this change of one foot in his own position. from this he is to estimate how far off the other mountain is. to do this, one would have to measure just about the amount of parallax that bessel found in his star. and yet this star is among the few nearest to our system. the nearest star of all, alpha centauri, visible only in latitudes south of our middle ones, is perhaps half as far as bessel's star, while sirius and one or two others are nearly at the same distance. about stars, all told, have had their parallax measured with a greater or less degree of probability. the work is going on from year to year, each successive astronomer who takes it up being able, as a general rule, to avail himself of better instruments or to use a better method. but, after all, the distances of even some of the stars carefully measured must still remain quite doubtful. let us now return to the idea of dividing the space in which the universe is situated into concentric spheres drawn at various distances around our system as a centre. here we shall take as our standard a distance , times that of the sun from the earth. regarding this as a unit, we imagine ourselves to measure out in any direction a distance twice as great as this--then another equal distance, making one three times as great, and so indefinitely. we then have successive spheres of which we take the nearer one as the unit. the total space filled by the second sphere will be times the unit; that of the third space times, and so on, as the cube of each distance. since each sphere includes all those within it, the volume of space between each two spheres will be proportional to the difference of these numbers--that is, to , , , etc. comparing these volumes with the number of stars probably within them, the general result up to the present time is that the number of stars in any of these spheres will be about equal to the units of volume which they comprise, when we take for this unit the smallest and innermost of the spheres, having a radius , times the sun's distance. we are thus enabled to form some general idea of how thickly the stars are sown through space. we cannot claim any numerical exactness for this idea, but in the absence of better methods it does afford us some basis for reasoning. now we can carry on our computation as we supposed the farmer to measure the extent of his wheat-field. let us suppose that there are , , stars in the heavens. this is an exceedingly rough estimate, but let us make the supposition for the time being. accepting the view that they are nearly equally scattered throughout space, it will follow that they must be contained within a volume equal to , , times the sphere we have taken as our unit. we find the distance of the surface of this sphere by extracting the cube root of this number, which gives us . we may, therefore, say, as the result of a very rough estimate, that the number of stars we have supposed would be contained within a distance found by multiplying , times the distance of the sun by ; that is, that they are contained within a region whose boundary is , , times the distance of the sun. this is a distance through which light would travel in about years. it is not impossible that the number of stars is much greater than that we have supposed. let us grant that there are eight times as many, or , , , . then we should have to extend the boundary of our universe twice as far, carrying it to a distance which light would require years to travel. there is another method of estimating the thickness with which stars are sown through space, and hence the extent of the universe, the result of which will be of interest. it is based on the proper motion of the stars. one of the greatest triumphs of astronomy of our time has been the measurement of the actual speed at which many of the stars are moving to or from us in space. these measures are made with the spectroscope. unfortunately, they can be best made only on the brighter stars--becoming very difficult in the case of stars not plainly visible to the naked eye. still the motions of several hundreds have been measured and the number is constantly increasing. a general result of all these measures and of other estimates may be summed up by saying that there is a certain average speed with which the individual stars move in space; and that this average is about twenty miles per second. we are also able to form an estimate as to what proportion of the stars move with each rate of speed from the lowest up to a limit which is probably as high as miles per second. knowing these proportions we have, by observation of the proper motions of the stars, another method of estimating how thickly they are scattered in space; in other words, what is the volume of space which, on the average, contains a single star. this method gives a thickness of the stars greater by about twenty-five per cent, than that derived from the measures of parallax. that is to say, a sphere like the second we have proposed, having a radius , times the distance of the sun, and therefore a diameter , , times this distance, would, judging by the proper motions, have ten or twelve stars contained within it, while the measures of parallax only show eight stars within the sphere of this diameter having the sun as its centre. the probabilities are in favor of the result giving the greater thickness of the stars. but, after all, the discrepancy does not change the general conclusion as to the limits of the visible universe. if we cannot estimate its extent with the same certainty that we can determine the size of the earth, we can still form a general idea of it. the estimates we have made are based on the supposition that the stars are equally scattered in space. we have good reason to believe that this is true of all the stars except those of the milky way. but, after all, the latter probably includes half the whole number of stars visible with a telescope, and the question may arise whether our results are seriously wrong from this cause. this question can best be solved by yet another method of estimating the average distance of certain classes of stars. the parallaxes of which we have heretofore spoken consist in the change in the direction of a star produced by the swing of the earth from one side of its orbit to the other. but we have already remarked that our solar system, with the earth as one of its bodies, has been journeying straightforward through space during all historic times. it follows, therefore, that we are continually changing the position from which we view the stars, and that, if the latter were at rest, we could, by measuring the apparent speed with which they are moving in the opposite direction from that of the earth, determine their distance. but since every star has its own motion, it is impossible, in any one case, to determine how much of the apparent motion is due to the star itself, and how much to the motion of the solar system through space. yet, by taking general averages among groups of stars, most of which are probably near each other, it is possible to estimate the average distance by this method. when an attempt is made to apply it, so as to obtain a definite result, the astronomer finds that the data now available for the purpose are very deficient. the proper motion of a star can be determined only by comparing its observed position in the heavens at two widely separate epochs. observations of sufficient precision for this purpose were commenced about at the greenwich observatory, by bradley, then astronomer royal of england. but out of stars which he determined, only a few are available for the purpose. even since his time, the determinations made by each generation of astronomers have not been sufficiently complete and systematic to furnish the material for anything like a precise determination of the proper motions of stars. to determine a single position of any one star involves a good deal of computation, and if we reflect that, in order to attack the problem in question in a satisfactory way, we should have observations of , , of these bodies made at intervals of at least a considerable fraction of a century, we see what an enormous task the astronomers dealing with this problem have before them, and how imperfect must be any determination of the distance of the stars based on our motion through space. so far as an estimate can be made, it seems to agree fairly well with the results obtained by the other methods. speaking roughly, we have reason, from the data so far available, to believe that the stars of the milky way are situated at a distance between , , and , , times the distance of the sun. at distances less than this it seems likely that the stars are distributed through space with some approach to uniformity. we may state as a general conclusion, indicated by several methods of making the estimate, that nearly all the stars which we can see with our telescopes are contained within a sphere not likely to be much more than , , times the distance of the sun. the inquiring reader may here ask another question. granting that all the stars we can see are contained within this limit, may there not be any number of stars outside the limit which are invisible only because they are too far away to be seen? this question may be answered quite definitely if we grant that light from the most distant stars meets with no obstruction in reaching us. the most conclusive answer is afforded by the measure of starlight. if the stars extended out indefinitely, then the number of those of each order of magnitude would be nearly four times that of the magnitude next brighter. for example, we should have nearly four times as many stars of the sixth magnitude as of the fifth; nearly four times as many of the seventh as of the sixth, and so on indefinitely. now, it is actually found that while this ratio of increase is true for the brighter stars, it is not so for the fainter ones, and that the increase in the number of the latter rapidly falls off when we make counts of the fainter telescopic stars. in fact, it has long been known that, were the universe infinite in extent, and the stars equally scattered through all space, the whole heavens would blaze with the light of countless millions of distant stars separately invisible even with the telescope. the only way in which this conclusion can be invalidated is by the possibility that the light of the stars is in some way extinguished or obstructed in its passage through space. a theory to this effect was propounded by struve nearly a century ago, but it has since been found that the facts as he set them forth do not justify the conclusion, which was, in fact, rather hypothetical. the theories of modern science converge towards the view that, in the pure ether of space, no single ray of light can ever be lost, no matter how far it may travel. but there is another possible cause for the extinction of light. during the last few years discoveries of dark and therefore invisible stars have been made by means of the spectroscope with a success which would have been quite incredible a very few years ago, and which, even to-day, must excite wonder and admiration. the general conclusion is that, besides the shining stars which exist in space, there may be any number of dark ones, forever invisible in our telescopes. may it not be that these bodies are so numerous as to cut off the light which we would otherwise receive from the more distant bodies of the universe? it is, of course, impossible to answer this question in a positive way, but the probable conclusion is a negative one. we may say with certainty that dark stars are not so numerous as to cut off any important part of the light from the stars of the milky way, because, if they did, the latter would not be so clearly seen as it is. since we have reason to believe that the milky way comprises the more distant stars of our system, we may feel fairly confident that not much light can be cut off by dark bodies from the most distant region to which our telescopes can penetrate. up to this distance we see the stars just as they are. even within the limit of the universe as we understand it, it is likely that more than one-half the stars which actually exist are too faint to be seen by human vision, even when armed with the most powerful telescopes. but their invisibility is due only to their distance and the faintness of their intrinsic light, and not to any obstructing agency. the possibility of dark stars, therefore, does not invalidate the general conclusions at which our survey of the subject points. the universe, so far as we can see it, is a bounded whole. it is surrounded by an immense girdle of stars, which, to our vision, appears as the milky way. while we cannot set exact limits to its distance, we may yet confidently say that it is bounded. it has uniformities running through its vast extent. could we fly out to distances equal to that of the milky way, we should find comparatively few stars beyond the limits of that girdle. it is true that we cannot set any definite limit and say that beyond this nothing exists. what we can say is that the region containing the visible stars has some approximation to a boundary. we may fairly anticipate that each successive generation of astronomers, through coming centuries, will obtain a little more light on the subject--will be enabled to make more definite the boundaries of our system of stars, and to draw more and more probable conclusions as to the existence or non-existence of any object outside of it. the wise investigator of to-day will leave to them the task of putting the problem into a more positive shape. v making and using a telescope the impression is quite common that satisfactory views of the heavenly bodies can be obtained only with very large telescopes, and that the owner of a small one must stand at a great disadvantage alongside of the fortunate possessor of a great one. this is not true to the extent commonly supposed. sir william herschel would have been delighted to view the moon through what we should now consider a very modest instrument; and there are some objects, especially the moon, which commonly present a more pleasing aspect through a small telescope than through a large one. the numerous owners of small telescopes throughout the country might find their instruments much more interesting than they do if they only knew what objects were best suited to examination with the means at their command. there are many others, not possessors of telescopes, who would like to know how one can be acquired, and to whom hints in this direction will be valuable. we shall therefore give such information as we are able respecting the construction of a telescope, and the more interesting celestial objects to which it may be applied. whether the reader does or does not feel competent to undertake the making of a telescope, it may be of interest to him to know how it is done. first, as to the general principles involved, it is generally known that the really vital parts of the telescope, which by their combined action perform the office of magnifying the object looked at, are two in number, the objective and the eye-piece. the former brings the rays of light which emanate from the object to the focus where the image of the object is formed. the eye-piece enables the observer to see this image to the best advantage. the functions of the objective as well as those of the eye-piece may, to a certain extent, each be performed by a single lens. galileo and his contemporaries made their telescopes in this way, because they knew of no way in which two lenses could be made to do better than one. but every one who has studied optics knows that white light passing through a single lens is not all brought to the same focus, but that the blue light will come to a focus nearer the objective than the red light. there will, in fact, be a succession of images, blue, green, yellow, and red, corresponding to the colors of the spectrum. it is impossible to see these different images clearly at the same time, because each of them will render all the others indistinct. the achromatic object-glass, invented by dollond, about , obviates this difficulty, and brings all the rays to nearly the same focus. nearly every one interested in the subject is aware that this object-glass is composed of two lenses--a concave one of flint-glass and a convex one of crown-glass, the latter being on the side towards the object. this is the one vital part of the telescope, the construction of which involves the greatest difficulty. once in possession of a perfect object-glass, the rest of the telescope is a matter of little more than constructive skill which there is no difficulty in commanding. the construction of the object-glass requires two completely distinct processes: the making of the rough glass, which is the work of the glass-maker; and the grinding and polishing into shape, which is the work of the optician. the ordinary glass of commerce will not answer the purpose of the telescope at all, because it is not sufficiently clear and homogeneous. optical glass, as it is called, must be made of materials selected and purified with the greatest care, and worked in a more elaborate manner than is necessary in any other kind of glass. in the time of dollond it was found scarcely possible to make good disks of flint-glass more than three or four inches in diameter. early in the present century, guinand, of switzerland, invented a process by which disks of much larger size could be produced. in conjunction with the celebrated fraunhofer he made disks of nine or ten inches in diameter, which were employed by his colaborer in constructing the telescopes which were so famous in their time. he was long supposed to be in possession of some secret method of avoiding the difficulties which his predecessors had met. it is now believed that this secret, if one it was, consisted principally in the constant stirring of the molten glass during the process of manufacture. however this may be, it is a curious historical fact that the most successful makers of these great disks of glass have either been of the family of guinand, or successors, in the management of the family firm. it was feil, a son-in-law or near relative, who made the glass from which clark fabricated the lenses of the great telescope of the lick observatory. his successor, mantois, of paris, carried the art to a point of perfection never before approached. the transparency and uniformity of his disks as well as the great size to which he was able to carry them would suggest that he and his successors have out-distanced all competitors in the process. he it was who made the great -inch lens for the yerkes observatory. as optical glass is now made, the material is constantly stirred with an iron rod during all the time it is melting in the furnace, and after it has begun to cool, until it becomes so stiff that the stirring has to cease. it is then placed, pot and all, in the annealing furnace, where it is kept nearly at a melting heat for three weeks or more, according to the size of the pot. when the furnace has cooled off, the glass is taken out, and the pot is broken from around it, leaving only the central mass of glass. having such a mass, there is no trouble in breaking it up into pieces of all desirable purity, and sufficiently large for moderate-sized telescopes. but when a great telescope of two feet aperture or upward is to be constructed, very delicate and laborious operations have to be undertaken. the outside of the glass has first to be chipped off, because it is filled with impurities from the material of the pot itself. but this is not all. veins of unequal density are always found extending through the interior of the mass, no way of avoiding them having yet been discovered. they are supposed to arise from the materials of the pot and stirring rod, which become mixed in with the glass in consequence of the intense heat to which all are subjected. these veins must, so far as possible, be ground or chipped out with the greatest care. the glass is then melted again, pressed into a flat disk, and once more put into the annealing oven. in fact, the operation of annealing must be repeated every time the glass is melted. when cooled, it is again examined for veins, of which great numbers are sure to be found. the problem now is to remove these by cutting and grinding without either breaking the glass in two or cutting a hole through it. if the parts of the glass are once separated, they can never be joined without producing a bad scar at the point of junction. so long, however, as the surface is unbroken, the interior parts of the glass can be changed in form to any extent. having ground out the veins as far as possible, the glass is to be again melted, and moulded into proper shape. in this mould great care must be taken to have no folding of the surface. imagining the latter to be a sort of skin enclosing the melted glass inside, it must be raised up wherever the glass is thinnest, and the latter allowed to slowly run together beneath it. [illustration with caption: the glass disk.] if the disk is of flint, all the veins must be ground out on the first or second trial, because after two or three mouldings the glass will lose its transparency. a crown disk may, however, be melted a number of times without serious injury. in many cases--perhaps the majority--the artisan finds that after all his months of labor he cannot perfectly clear his glass of the noxious veins, and he has to break it up into smaller pieces. when he finally succeeds, the disk has the form of a thin grindstone two feet or upward in diameter, according to the size of the telescope to be made, and from two to three inches in thickness. the glass is then ready for the optician. [illustration with caption: the optician's tool.] the first process to be performed by the optician is to grind the glass into the shape of a lens with perfectly spherical surfaces. the convex surface must be ground in a saucer-shaped tool of corresponding form. it is impossible to make a tool perfectly spherical in the first place, but success may be secured on the geometrical principle that two surfaces cannot fit each other in all positions unless both are perfectly spherical. the tool of the optician is a very simple affair, being nothing more than a plate of iron somewhat larger, perhaps a fourth, than the lens to be ground to the corresponding curvature. in order to insure its changing to fit the glass, it is covered on the interior with a coating of pitch from an eighth to a quarter of an inch thick. this material is admirably adapted to the purpose because it gives way certainly, though very slowly, to the pressure of the glass. in order that it may have room to change its form, grooves are cut through it in both directions, so as to leave it in the form of squares, like those on a chess-board. [illustration with caption: the optician's tool.] it is then sprinkled over with rouge, moistened with water, and gently warmed. the roughly ground lens is then placed upon it, and moved from side to side. the direction of the motion is slightly changed with every stroke, so that after a dozen or so of strokes the lines of motion will lie in every direction on the tool. this change of direction is most readily and easily effected by the operator slowly walking around as he polishes, at the same time the lens is to be slowly turned around either in the opposite direction or more rapidly yet in the same direction, so that the strokes of the polisher shall cross the lens in all directions. this double motion insures every part of the lens coming into contact with every part of the polisher, and moving over it in every direction. then whatever parts either of the lens or of the polisher may be too high to form a spherical surface will be gradually worn down, thus securing the perfect sphericity of both. [illustration with caption: grinding a large lens.] when the polishing is done by machinery, which is the custom in europe, with large lenses, the polisher is slid back and forth over the lens by means of a crank attached to a revolving wheel. the polisher is at the same time slowly revolving around a pivot at its centre, which pivot the crank works into, and the glass below it is slowly turned in an opposite direction. thus the same effect is produced as in the other system. those who practice this method claim that by thus using machinery the conditions of a uniform polish for every part of the surface can be more perfectly fulfilled than by a hand motion. the results, however, do not support this view. no european optician will claim to do better than the american firm of alvan clark & sons in producing uniformly good object-glasses, and this firm always does the work by hand, moving the glass over the polisher, and not the polisher over the glass. having brought both flint and crown glasses into proper figure by this process, they are joined together, and tested by observations either upon a star in the heavens, or some illuminated point at a little distance on the ground. the reflection of the sun from a drop of quicksilver, a thermometer bulb, or even a piece of broken bottle, makes an excellent artificial star. the very best optician will always find that on a first trial his glass is not perfect. he will find that he has not given exactly the proper curves to secure achromatism. he must then change the figure of one or both the glasses by polishing it upon a tool of slightly different curvature. he may also find that there is some spherical aberration outstanding. he must then alter his curve so as to correct this. the correction of these little imperfections in the figures of the lenses so as to secure perfect vision through them is the most difficult branch of the art of the optician, and upon his skill in practising it will depend more than upon anything else his ultimate success and reputation. the shaping of a pair of lenses in the way we have described is not beyond the power of any person of ordinary mechanical ingenuity, possessing the necessary delicacy of touch and appreciation of the problem he is attacking. but to make a perfect objective of considerable size, which shall satisfy all the wants of the astronomer, is an undertaking requiring such accuracy of eyesight, and judgment in determining where the error lies, and such skill in manipulating so as to remove the defects, that the successful men in any one generation can be counted on one's fingers. in order that the telescope may finally perform satisfactorily it is not sufficient that the lenses should both be of proper figure; they must also both be properly centred in their cells. if either lens is tipped aside, or slid out from its proper central line, the definition will be injured. as this is liable to happen with almost any telescope, we shall explain how the proper adjustment is to be made. the easiest way to test this adjustment is to set the cell with the two glasses of the objective in it against a wall at night, and going to a short distance, observe the reflection in the glass of the flame of a candle held in the hand. three or four reflections will be seen from the different surfaces. the observer, holding the candle before his eye, and having his line of sight as close as possible to the flame, must then move until the different images of the flame coincide with each other. if he cannot bring them into coincidence, owing to different pairs coinciding on different sides of the flame, the glasses are not perfectly centred upon each other. when the centring is perfect, the observer having the light in the line of the axes of the lenses, and (if it were possible to do so) looking through the centre of the flame, would see the three or four images all in coincidence. as he cannot see through the flame itself, he must look first on one side and then on the other, and see if the arrangement of the images seen in the lenses is symmetrical. if, going to different distances, he finds no deviation from symmetry, in this respect the adjustment is near enough for all practical purposes. a more artistic instrument than a simple candle is a small concave reflector pierced through its centre, such as is used by physicians in examining the throat. [illustration with caption: image of candle-flame in object-glass.] [illustration with caption: testing adjustment of object-glass.] place this reflector in the prolongation of the optical axis, set the candle so that the light from the reflector shall be shown through the glass, and look through the opening. images of the reflector itself will then be seen in the object-glass, and if the adjustment is perfect, the reflector can be moved so that they will all come into coincidence together. when the objective is in the tube of the telescope, it is always well to examine this adjustment from time to time, holding the candle so that its light shall shine through the opening perpendicularly upon the object-glass. the observer looks upon one side of the flame, and then upon the other, to see if the images are symmetrical in the different positions. if in order to see them in this way the candle has to be moved to one side of the central line of the tube, the whole objective must be adjusted. if two images coincide in one position of the candle-flame, and two in another position, so that they cannot all be brought together in any position, it shows that the glasses are not properly adjusted in their cell. it may be remarked that this last adjustment is the proper work of the optician, since it is so difficult that the user of the telescope cannot ordinarily effect it. but the perpendicularity of the whole objective to the tube of the telescope is liable to be deranged in use, and every one who uses such an instrument should be able to rectify an error of this kind. the question may be asked, how much of a telescope can an amateur observer, under any circumstances, make for himself? as a general rule, his work in this direction must be confined to the tube and the mounting. we should not, it is true, dare to assert that any ingenious young man, with a clear appreciation of optical principles, could not soon learn to grind and polish an object-glass for himself by the method we have described, and thus obtain a much better instrument than galileo ever had at his command. but it would be a wonderful success if his home-made telescope was equal to the most indifferent one which can be bought at an optician's. the objective, complete in itself, can be purchased at prices depending upon the size. [footnote: the following is a rough rule for getting an idea of the price of an achromatic objective, made to order, of the finest quality. take the cube of the diameter in inches, or, which is the same thing, calculate the contents of a cubical box which would hold a sphere of the same diameter as the clear aperture of the glass. the price of the glass will then range from $ to $ . for each cubic inch in this box. for example, the price of a four-inch objective will probably range from $ to $ . very small object-glasses of one or two inches may be a little higher than would be given by this rule. instruments which are not first-class, but will answer most of the purposes of the amateur, are much cheaper.] [illustration with caption: a very primitive mounting for a telescope.] the tube for the telescope may be made of paper, by pasting a great number of thicknesses around a long wooden cylinder. a yet better tube is made of a simple wooden box. the best material, however, is metal, because wood and pasteboard are liable both to get out of shape, and to swell under the influence of moisture. tin, if it be of sufficient thickness, would be a very good material. the brighter it is kept, the better. the work of fitting the objective into one end of a tin tube of double thickness, and properly adjusting it, will probably be quite within the powers of the ordinary amateur. the fitting of the eye-piece into the other end of the tube will require some skill and care both on his own part and that of his tinsmith. although the construction of the eye-piece is much easier than that of the objective, since the same accuracy in adjusting the curves is not necessary, yet the price is lower in a yet greater degree, so that the amateur will find it better to buy than to make his eye-piece, unless he is anxious to test his mechanical powers. for a telescope which has no micrometer, the huyghenian or negative eye-piece, as it is commonly called, is the best. as made by huyghens, it consists of two plano-convex lenses, with their plane sides next the eye, as shown in the figure. [illustration with caption: the huyghenian eye-piece.] so far as we have yet described our telescope it is optically complete. if it could be used as a spy-glass by simply holding it in the hand, and pointing at the object we wish to observe, there would be little need of any very elaborate support. but if a telescope, even of the smallest size, is to be used with regularity, a proper "mounting" is as essential as a good instrument. persons unpractised in the use of such instruments are very apt to underrate the importance of those accessories which merely enable us to point the telescope. an idea of what is wanted in the mounting may readily be formed if the reader will try to look at a star with an ordinary good-sized spy-glass held in the hand, and then imagine the difficulties he meets with multiplied by fifty. the smaller and cheaper telescopes, as commonly sold, are mounted on a simple little stand, on which the instrument admits of a horizontal and vertical motion. if one only wants to get a few glimpses of a celestial object, this mounting will answer his purpose. but to make anything like a study of a celestial body, the mounting must be an equatorial one; that is, one of the axes around which the telescope moves must be inclined so as to point towards the pole of the heavens, which is near the polar star. this axis will then make an angle with the horizon equal to the latitude of the place. the telescope cannot, however, be mounted directly on this axis, but must be attached to a second one, itself fastened to this one. [illustration with caption: section of the primitive mounting. p p. polar axis, bearing a fork at the upper end a. declination axis passing through the fork e. section of telescope tube c. weight to balance the tube.] when mounted in this way, an object can be followed in its diurnal motion from east to west by turning on the polar axis alone. but if the greatest facility in use is required, this motion must be performed by clock-work. a telescope with this appendage will commonly cost one thousand dollars and upward, so that it is not usually applied to very small ones. we will now suppose that the reader wishes to purchase a telescope or an object-glass for himself, and to be able to judge of its performance. he must have the object-glass properly adjusted in its tube, and must use the highest power; that is, the smallest eye-piece, which he intends to use in the instrument. of course he understands that in looking directly at a star or a celestial object it must appear sharp in outline and well defined. but without long practice with good instruments, this will not give him a very definite idea. if the person who selects the telescope is quite unpractised, it is possible that he can make the best test by ascertaining at what distance he can read ordinary print. to do this he should have an eye-piece magnifying about fifty times for each inch of aperture of the telescope. for instance, if his telescope is three inches clear aperture, then his eye-piece should magnify one hundred and fifty times; if the aperture is four inches, one magnifying two hundred times may be used. this magnifying power is, as a general rule, about the highest that can be advantageously used with any telescope. supposing this magnifying power to be used, this page should be legible at a distance of four feet for every unit of magnifying power of the telescope. for example, with a power of , it should be legible at a distance of feet; with a power of , at feet, and so on. to put the condition into another shape: if the telescope will read the print at a distance of feet for each inch of aperture with the best magnifying power, its performance is at least not very bad. if the magnifying power is less than would be given by this rule, the telescope should perform a little better; for instance, a three-inch telescope with a power of should make this page legible at a distance of feet, or four feet for each unit of power. the test applied by the optician is much more exact, and also more easy. he points the instrument at a star, or at the reflection of the sun's rays from a small round piece of glass or a globule of quicksilver several hundred yards away, and ascertains whether the rays are all brought to a focus. this is not done by simply looking at the star, but by alternately pushing the eye-piece in beyond the point of distinct vision and drawing it out past the point. in this way the image of the star will appear, not as a point, but as a round disk of light. if the telescope is perfect, this disk will appear round and of uniform brightness in either position of the eye-piece. but if there is any spherical aberration or differences of density in different parts of the glass, the image will appear distorted in various ways. if the spherical aberration is not correct, the outer rim of the disk will be brighter than the centre when the eye-piece is pushed in, and the centre will be the brighter when it is drawn out. if the curves of the glass are not even all around, the image will appear oval in one or the other position. if there are large veins of unequal density, wings or notches will be seen on the image. if the atmosphere is steady, the image, when the eye-piece is pushed in, will be formed of a great number of minute rings of light. if the glass is good, these rings will be round, unbroken, and equally bright. we present several figures showing how these spectral images, as they are sometimes called, will appear; first, when the eye-piece is pushed in, and secondly, when it is drawn out, with telescopes of different qualities. we have thus far spoken only of the refracting telescope, because it is the kind with which an observer would naturally seek to supply himself. at the same time there is little doubt that the construction of a reflector of moderate size is easier than that of a corresponding refractor. the essential part of the reflector is a slightly concave mirror of any metal which will bear a high polish. this mirror may be ground and polished in the same way as a lens, only the tool must be convex. [illustration with caption: spectral images of stars; the upper line showing how they appear with the eye-piece pushed in, the lower with the eye-piece drawn out. a the telescope is all right b spherical aberration shown by the light and dark centre c the objective is not spherical but elliptical d the glass not uniform--a very bad and incurable case e one side of the objective nearer than the other. adjust it] of late years it has become very common to make the mirror of glass and to cover the reflecting face with an exceedingly thin film of silver, which can be polished by hand in a few minutes. such a mirror differs from our ordinary looking-glass in that the coating of silver is put on the front surface, so that the light does not pass through the glass. moreover, the coating of silver is so thin as to be almost transparent: in fact, the sun may be seen through it by direct vision as a faint blue object. silvered glass reflectors made in this way are extensively manufactured in london, and are far cheaper than refracting telescopes of corresponding size. their great drawback is the want of permanence in the silver film. in the city the film will ordinarily tarnish in a few months from the sulphurous vapors arising from gaslights and other sources, and even in the country it is very difficult to preserve the mirror from the contact of everything that will injure it. in consequence, the possessor of such a telescope, if he wishes to keep it in order, must always be prepared to resilver and repolish it. to do this requires such careful manipulation and management of the chemicals that it is hardly to be expected that an amateur will take the trouble to keep his telescope in order, unless he has a taste for chemistry as well as for astronomy. the curiosity to see the heavenly bodies through great telescopes is so wide-spread that we are apt to forget how much can be seen and done with small ones. the fact is that a large proportion of the astronomical observations of past times have been made with what we should now regard as very small instruments, and a good deal of the solid astronomical work of the present time is done with meridian circles the apertures of which ordinarily range from four to eight inches. one of the most conspicuous examples in recent times of how a moderate-sized instrument may be utilized is afforded by the discoveries of double stars made by mr. s. w. burnham, of chicago. provided with a little six-inch telescope, procured at his own expense from the messrs. clark, he has discovered many hundred double stars so difficult that they had escaped the scrutiny of maedler and the struves, and gained for himself one of the highest positions among the astronomers of the day engaged in the observation of these objects. it was with this little instrument that on mount hamilton, california--afterward the site of the great lick observatory--he discovered forty-eight new double stars, which had remained unnoticed by all previous observers. first among the objects which show beautifully through moderate instruments stands the moon. people who want to see the moon at an observatory generally make the mistake of looking when the moon is full, and asking to see it through the largest telescope. nothing can then be made out but a brilliant blaze of light, mottled with dark spots, and crossed by irregular bright lines. the best time to view the moon is near or before the first quarter, or when she is from three to eight days old. the last quarter is of course equally favorable, so far as seeing is concerned, only one must be up after midnight to see her in that position. seen through a three or four inch telescope, a day or two before the first quarter, about half an hour after sunset, and with a magnifying power between fifty and one hundred, the moon is one of the most beautiful objects in the heavens. twilight softens her radiance so that the eye is not dazzled as it will be when the sky is entirely dark. the general aspect she then presents is that of a hemisphere of beautiful chased silver carved out in curious round patterns with a more than human skill. if, however, one wishes to see the minute details of the lunar surface, in which many of our astronomers are now so deeply interested, he must use a higher magnifying power. the general beautiful effect is then lessened, but more details are seen. still, it is hardly necessary to seek for a very large telescope for any investigation of the lunar surface. i very much doubt whether any one has ever seen anything on the moon which could not be made out in a clear, steady atmosphere with a six-inch telescope of the first class. next to the moon, saturn is among the most beautiful of celestial objects. its aspect, however, varies with its position in its orbit. twice in the course of a revolution, which occupies nearly thirty years, the rings are seen edgewise, and for a few days are invisible even in a powerful telescope. for an entire year their form may be difficult to make out with a small telescope. these unfavorable conditions occur in and . between these dates, especially for some years after , the position of the planet in the sky will be the most favorable, being in northern declination, near its perihelion, and having its rings widely open. we all know that saturn is plainly visible to the naked eye, shining almost like a star of the first magnitude, so that there is no difficulty in finding it if one knows when and where to look. in - its oppositions occur in the month of september. in subsequent years, it will occur a month later every two and a half years. the ring can be seen with a common, good spy-glass fastened to a post so as to be steady. a four or five-inch telescope will show most of the satellites, the division in the ring, and, when the ring is well opened, the curious dusky ring discovered by bond. this "crape ring," as it is commonly called, is one of the most singular phenomena presented by that planet. it might be interesting to the amateur astronomer with a keen eye and a telescope of four inches aperture or upward to frequently scrutinize saturn, with a view of detecting any extraordinary eruptions upon his surface, like that seen by professor hall in . on december th of that year a bright spot was seen upon saturn's equator. it elongated itself from day to day, and remained visible for several weeks. such a thing had never before been known upon this planet, and had it not been that professor hall was engaged in observations upon the satellites, it would not have been seen then. a similar spot on the planet was recorded in , and much more extensively noticed. on this occasion the spot appeared in a higher latitude from the planet's equator than did professor hall's. at this appearance the time of the planet's revolution on its axis was found to be somewhat greater than in , in accordance with the general law exhibited in the rotations of the sun and of jupiter. notwithstanding their transient character, these two spots have afforded the only determination of the time of revolution of saturn which has been made since herschel the elder. [illustration with caption: the great refractor of the national observatory at washington] of the satellites of saturn the brightest is titan, which can be seen with the smallest telescope, and revolves around the planet in fifteen days. iapetus, the outer satellite, is remarkable for varying greatly in brilliancy during its revolution around the planet. any one having the means and ability to make accurate photometrical estimates of the light of this satellite in all points of its orbit, can thereby render a valuable service to astronomy. the observations of venus, by which the astronomers of the last century supposed themselves to have discovered its time of rotation on its axis, were made with telescopes much inferior to ours. although their observations have not been confirmed, some astronomers are still inclined to think that their results have not been refuted by the failure of recent observers to detect those changes which the older ones describe on the surface of the planet. with a six-inch telescope of the best quality, and with time to choose the most favorable moment, one will be as well equipped to settle the question of the rotation of venus as the best observer. the few days near each inferior conjunction are especially to be taken advantage of. the questions to be settled are two: first, are there any dark spots or other markings on the disk? second, are there any irregularities in the form of the sharp cusps? the central portions of the disk are much darker than the outline, and it is probably this fact which has given rise to the impression of dark spots. unless this apparent darkness changes from time to time, or shows some irregularity in its outline, it cannot indicate any rotation of the planet. the best time to scrutinize the sharp cusps will be when the planet is nearly on the line from the earth to the sun. the best hour of the day is near sunset, the half-hour following sunset being the best of all. but if venus is near the sun, she will after sunset be too low down to be well seen, and must be looked at late in the afternoon. the planet mars must always be an object of great interest, because of all the heavenly bodies it is that which appears to bear the greatest resemblance to the earth. it comes into opposition at intervals of a little more than two years, and can be well seen only for a month or two before and after each opposition. it is hopeless to look for the satellites of mars with any but the greatest telescopes of the world. but the markings on the surface, from which the time of rotation has been determined, and which indicate a resemblance to the surface of our own planet, can be well seen with telescopes of six inches aperture and upward. one or both of the bright polar spots, which are supposed to be due to deposits of snow, can be seen with smaller telescopes when the situation of the planet is favorable. the case is different with the so-called canals discovered by schiaparelli in , which have ever since excited so much interest, and given rise to so much discussion as to their nature. the astronomer who has had the best opportunities for studying them is mr. percival lowell, whose observatory at flaggstaff, arizona, is finely situated for the purpose, while he also has one of the best if not the largest of telescopes. there the canals are seen as fine dark lines; but, even then, they must be fifty miles in breadth, so that the word "canal" may be regarded as a misnomer. although the planet jupiter does not present such striking features as saturn, it is of even more interest to the amateur astronomer, because he can study it with less optical power, and see more of the changes upon its surface. every work on astronomy tells in a general way of the belts of jupiter, and many speculate upon their causes. the reader of recent works knows that jupiter is supposed to be not a solid mass like the earth, but a great globe of molten and vaporous matter, intermediate in constitution between the earth and the sun. the outer surface which we see is probably a hot mass of vapor hundreds of miles deep, thrown up from the heated interior. the belts are probably cloudlike forms in this vaporous mass. certain it is that they are continually changing, so that the planet seldom looks exactly the same on two successive evenings. the rotation of the planet can be very well seen by an hour's watching. in two hours an object at the centre of the disk will move off to near the margin. the satellites of this planet, in their ever-varying phases, are objects of perennial interest. their eclipses may be observed with a very small telescope, if one knows when to look for them. to do this successfully, and without waste of time, it is necessary to have an astronomical ephemeris for the year. all the observable phenomena are there predicted for the convenience of observers. perhaps the most curious observation to be made is that of the shadow of the satellite crossing the disk of jupiter. the writer has seen this perfectly with a six-inch telescope, and a much smaller one would probably show it well. with a telescope of this size, or a little larger, the satellites can be seen between us and jupiter. sometimes they appear a little brighter than the planet, and sometimes a little fainter. of the remaining large planets, mercury, the inner one, and uranus and neptune, the two outer ones, are of less interest than the others to an amateur with a small telescope, because they are more difficult to see. mercury can, indeed, be observed with the smallest instrument, but no physical configurations or changes have ever been made out upon his surface. the question whether any such can be observed is still an open one, which can be settled only by long and careful scrutiny. a small telescope is almost as good for this purpose as a large one, because the atmospheric difficulties in the way of getting a good view of the planet cannot be lessened by an increase of telescopic power. uranus and neptune are so distant that telescopes of considerable size and high magnifying power are necessary to show their disks. in small telescopes they have the appearance of stars, and the observer has no way of distinguishing them from the surrounding stars unless he can command the best astronomical appliances, such as star maps, circles on his instrument, etc. it is, however, to be remarked, as a fact not generally known, that uranus can be well seen with the naked eye if one knows where to look for it. to recognize it, it is necessary to have an astronomical ephemeris showing its right ascension and declination, and star maps showing where the parallels of right ascension and declination lie among the stars. when once found by the naked eye, there will, of course, be no difficulty in pointing the telescope upon it. of celestial objects which it is well to keep a watch upon, and which can be seen to good advantage with inexpensive instruments, the sun may be considered as holding the first place. astronomers who make a specialty of solar physics have, especially in this country, so many other duties, and their view is so often interrupted by clouds, that a continuous record of the spots on the sun and the changes they undergo is hardly possible. perhaps one of the most interesting and useful pieces of astronomical work which an amateur can perform will consist of a record of the origin and changes of form of the solar spots and faculae. what does a spot look like when it first comes into sight? does it immediately burst forth with considerable magnitude, or does it begin as the smallest visible speck, and gradually grow? when several spots coalesce into one, how do they do it? when a spot breaks up into several pieces, what is the seeming nature of the process? how do the groups of brilliant points called faculae come, change, and grow? all these questions must no doubt be answered in various ways, according to the behavior of the particular spot, but the record is rather meagre, and the conscientious and industrious amateur will be able to amuse himself by adding to it, and possibly may make valuable contributions to science in the same way. still another branch of astronomical observation, in which industry and skill count for more than expensive instruments, is the search for new comets. this requires a very practised eye, in order that the comet may be caught among the crowd of stars which flit across the field of view as the telescope is moved. it is also necessary to be well acquainted with a number of nebulae which look very much like comets. the search can be made with almost any small telescope, if one is careful to use a very low power. with a four-inch telescope a power not exceeding twenty should be employed. to search with ease, and in the best manner, the observer should have what among astronomers is familiarly known as a "broken-backed telescope." this instrument has the eye-piece on the end of the axis, where one would never think of looking for it. by turning the instrument on this axis, it sweeps from one horizon through the zenith and over to the other horizon without the observer having to move his head. this is effected by having a reflector in the central part of the instrument, which throws the rays of light at right angles through the axis. [illustration: the "broken-backed comet-seeker"] how well this search can be conducted by observers with limited means at their disposal is shown by the success of several american observers, among whom messrs. w. r. brooks, e. e. barnard, and lewis swift are well known. the cometary discoveries of these men afford an excellent illustration of how much can be done with the smallest means when one sets to work in the right spirit. the larger number of wonderful telescopic objects are to be sought for far beyond the confines of the solar system, in regions from which light requires years to reach us. on account of their great distance, these objects generally require the most powerful telescopes to be seen in the best manner; but there are quite a number within the range of the amateur. looking at the milky way, especially its southern part, on a clear winter or summer evening, tufts of light will be seen here and there. on examining these tufts with a telescope, they will be found to consist of congeries of stars. many of these groups are of the greatest beauty, with only a moderate optical power. of all the groups in the milky way the best known is that in the sword-handle of perseus, which may be seen during the greater part of the year, and is distinctly visible to the naked eye as a patch of diffused light. with the telescope there are seen in this patch two closely connected clusters of stars, or perhaps we ought rather to say two centres of condensation. another object of the same class is proesepe in the constellation cancer. this can be very distinctly seen by the naked eye on a clear moonless night in winter or spring as a faint nebulous object, surrounded by three small stars. the smallest telescope shows it as a group of stars. of all stellar objects, the great nebula of orion is that which has most fascinated the astronomers of two centuries. it is distinctly visible to the naked eye, and may be found without difficulty on any winter night. the three bright stars forming the sword-belt of orion are known to every one who has noticed that constellation. below this belt is seen another triplet of stars, not so bright, and lying in a north and south direction. the middle star of this triplet is the great nebula. at first the naked eye sees nothing to distinguish it from other stars, but if closely scanned it will be seen to have a hazy aspect. a four-inch telescope will show its curious form. not the least interesting of its features are the four stars known as the "trapezium," which are located in a dark region near its centre. in fact, the whole nebula is dotted with stars, which add greatly to the effect produced by its mysterious aspect. the great nebula of andromeda is second only to that of orion in interest. like the former, it is distinctly visible to the naked eye, having the aspect of a faint comet. the most curious feature of this object is that although the most powerful telescopes do not resolve it into stars, it appears in the spectroscope as if it were solid matter shining by its own light. the above are merely selections from the countless number of objects which the heavens offer to telescopic study. many such are described in astronomical works, but the amateur can gratify his curiosity to almost any extent by searching them out for himself. [illustration with caption: nebula in orion] ever since a red spot, unlike any before noticed, has generally been visible on jupiter. at first it was for several years a very conspicuous object, but gradually faded away, so that since it has been made out only with difficulty. but it is now regarded as a permanent feature of the planet. there is some reason to believe it was occasionally seen long before attention was first attracted to it. doubtless, when it can be seen at all, practice in observing such objects is more important than size of telescope. vi what the astronomers are doing in no field of science has human knowledge been more extended in our time than in that of astronomy. forty years ago astronomical research seemed quite barren of results of great interest or value to our race. the observers of the world were working on a traditional system, grinding out results in an endless course, without seeing any prospect of the great generalizations to which they might ultimately lead. now this is all changed. a new instrument, the spectroscope, has been developed, the extent of whose revelations we are just beginning to learn, although it has been more than thirty years in use. the application of photography has been so extended that, in some important branches of astronomical work, the observer simply photographs the phenomenon which he is to study, and then makes his observation on the developed negative. the world of astronomy is one of the busiest that can be found to-day, and the writer proposes, with the reader's courteous consent, to take him on a stroll through it and see what is going on. we may begin our inspection with a body which is, for us, next to the earth, the most important in the universe. i mean the sun. at the greenwich observatory the sun has for more than twenty years been regularly photographed on every clear day, with the view of determining the changes going on in its spots. in recent years these observations have been supplemented by others, made at stations in india and mauritius, so that by the combination of all it is quite exceptional to have an entire day pass without at least one photograph being taken. on these observations must mainly rest our knowledge of the curious cycle of change in the solar spots, which goes through a period of about eleven years, but of which no one has as yet been able to establish the cause. this greenwich system has been extended and improved by an american. professor george e. hale, formerly director of the yerkes observatory, has devised an instrument for taking photographs of the sun by a single ray of the spectrum. the light emitted by calcium, the base of lime, and one of the substances most abundant in the sun, is often selected to impress the plate. the carnegie institution has recently organized an enterprise for carrying on the study of the sun under a combination of better conditions than were ever before enjoyed. the first requirement in such a case is the ablest and most enthusiastic worker in the field, ready to devote all his energies to its cultivation. this requirement is found in the person of professor hale himself. the next requirement is an atmosphere of the greatest transparency, and a situation at a high elevation above sea-level, so that the passage of light from the sun to the observer shall be obstructed as little as possible by the mists and vapors near the earth's surface. this requirement is reached by placing the observatory on mount wilson, near pasadena, california, where the climate is found to be the best of any in the united states, and probably not exceeded by that of any other attainable point in the world. the third requirement is the best of instruments, specially devised to meet the requirements. in this respect we may be sure that nothing attainable by human ingenuity will be found wanting. thus provided, professor hale has entered upon the task of studying the sun, and recording from day to day all the changes going on in it, using specially devised instruments for each purpose in view. photography is made use of through almost the entire investigation. a full description of the work would require an enumeration of technical details, into which we need not enter at present. let it, therefore, suffice to say in a general way that the study of the sun is being carried on on a scale, and with an energy worthy of the most important subject that presents itself to the astronomer. closely associated with this work is that of professor langley and dr. abbot, at the astro-physical observatory of the smithsonian institution, who have recently completed one of the most important works ever carried out on the light of the sun. they have for years been analyzing those of its rays which, although entirely invisible to our eyes, are of the same nature as those of light, and are felt by us as heat. to do this, langley invented a sort of artificial eye, which he called a bolometer, in which the optic nerve is made of an extremely thin strip of metal, so slight that one can hardly see it, which is traversed by an electric current. this eye would be so dazzled by the heat radiated from one's body that, when in use, it must be protected from all such heat by being enclosed in a case kept at a constant temperature by being immersed in water. with this eye the two observers have mapped the heat rays of the sun down to an extent and with a precision which were before entirely unknown. the question of possible changes in the sun's radiation, and of the relation of those changes to human welfare, still eludes our scrutiny. with all the efforts that have been made, the physicist of to-day has not yet been able to make anything like an exact determination of the total amount of heat received from the sun. the largest measurements are almost double the smallest. this is partly due to the atmosphere absorbing an unknown and variable fraction of the sun's rays which pass through it, and partly to the difficulty of distinguishing the heat radiated by the sun from that radiated by terrestrial objects. in one recent instance, a change in the sun's radiation has been noticed in various parts of the world, and is of especial interest because there seems to be little doubt as to its origin. in the latter part of an extraordinary diminution was found in the intensity of the sun's heat, as measured by the bolometer and other instruments. this continued through the first part of , with wide variations at different places, and it was more than a year after the first diminution before the sun's rays again assumed their ordinary intensity. this result is now attributed to the eruption of mount pelee, during which an enormous mass of volcanic dust and vapor was projected into the higher regions of the air, and gradually carried over the entire earth by winds and currents. many of our readers may remember that something yet more striking occurred after the great cataclasm at krakatoa in , when, for more than a year, red sunsets and red twilights of a depth of shade never before observed were seen in every part of the world. what we call universology--the knowledge of the structure and extent of the universe--must begin with a study of the starry heavens as we see them. there are perhaps one hundred million stars in the sky within the reach of telescopic vision. this number is too great to allow of all the stars being studied individually; yet, to form the basis for any conclusion, we must know the positions and arrangement of as many of them as we can determine. to do this the first want is a catalogue giving very precise positions of as many of the brighter stars as possible. the principal national observatories, as well as some others, are engaged in supplying this want. up to the present time about , stars visible in our latitudes have been catalogued on this precise plan, and the work is still going on. in that part of the sky which we never see, because it is only visible from the southern hemisphere, the corresponding work is far from being as extensive. sir david gill, astronomer at the cape of good hope, and also the directors of other southern observatories, are engaged in pushing it forward as rapidly as the limited facilities at their disposal will allow. next in order comes the work of simply listing as many stars as possible. here the most exact positions are not required. it is only necessary to lay down the position of each star with sufficient exactness to distinguish it from all its neighbors. about , stars were during the last half-century listed in this way at the observatory of bonn by argelander, schonfeld, and their assistants. this work is now being carried through the southern hemisphere on a large scale by thome, director of the cordoba observatory, in the argentine republic. this was founded thirty years ago by our dr. b. a. gould, who turned it over to dr. thome in . the latter has, up to the present time, fixed and published the positions of nearly half a million stars. this work of thome extends to fainter stars than any other yet attempted, so that, as it goes on, we have more stars listed in a region invisible in middle northern latitudes than we have for that part of the sky we can see. up to the present time three quarto volumes giving the positions and magnitudes of the stars have appeared. two or three volumes more, and, perhaps, ten or fifteen years, will be required to complete the work. about twenty years ago it was discovered that, by means of a telescope especially adapted to this purpose, it was possible to photograph many more stars than an instrument of the same size would show to the eye. this discovery was soon applied in various quarters. sir david gill, with characteristic energy, photographed the stars of the southern sky to the number of nearly half a million. as it was beyond his power to measure off and compute the positions of the stars from his plates, the latter were sent to professor j. c. kapteyn, of holland, who undertook the enormous labor of collecting them into a catalogue, the last volume of which was published in . one curious result of this enterprise is that the work of listing the stars is more complete for the southern hemisphere than for the northern. another great photographic work now in progress has to do with the millions of stars which it is impossible to handle individually. fifteen years ago an association of observatories in both hemispheres undertook to make a photographic chart of the sky on the largest scale. some portions of this work are now approaching completion, but in others it is still in a backward state, owing to the failure of several south american observatories to carry out their part of the programme. when it is all done we shall have a picture of the sky, the study of which may require the labor of a whole generation of astronomers. quite independently of this work, the harvard university, under the direction of professor pickering, keeps up the work of photographing the sky on a surprising scale. on this plan we do not have to leave it to posterity to learn whether there is any change in the heavens, for one result of the enterprise has been the discovery of thirteen of the new stars which now and then blaze out in the heavens at points where none were before known. professor pickering's work has been continually enlarged and improved until about , photographic plates, showing from time to time the places of countless millions of stars among their fellows are now stored at the harvard observatory. not less remarkable than this wealth of material has been the development of skill in working it up. some idea of the work will be obtained by reflecting that, thirty years ago, careful study of the heavens by astronomers devoting their lives to the task had resulted in the discovery of some two or three hundred stars, varying in their light. now, at harvard, through keen eyes studying and comparing successive photographs not only of isolated stars, but of clusters and agglomerations of stars in the milky way and elsewhere, discoveries of such objects numbering hundreds have been made, and the work is going on with ever-increasing speed. indeed, the number of variable stars now known is such that their study as individual objects no longer suffices, and they must hereafter be treated statistically with reference to their distribution in space, and their relations to one another, as a census classifies the entire population without taking any account of individuals. the works just mentioned are concerned with the stars. but the heavenly spaces contain nebulae as well as stars; and photography can now be even more successful in picturing them than the stars. a few years ago the late lamented keeler, at the lick observatory, undertook to see what could be done by pointing the crossley reflecting telescope at the sky and putting a sensitive photographic plate in the focus. he was surprised to find that a great number of nebulae, the existence of which had never before been suspected, were impressed on the plate. up to the present time the positions of about of these objects have been listed. keeler found that there were probably , nebulae in the heavens capable of being photographed with the crossley reflector. but the work of taking these photographs is so great, and the number of reflecting telescopes which can be applied to it so small, that no one has ventured to seriously commence it. it is worthy of remark that only a very small fraction of these objects which can be photographed are visible to the eye, even with the most powerful telescope. this demonstration of what the reflecting telescope can do may be regarded as one of the most important discoveries of our time as to the capabilities of astronomical instruments. it has long been known that the image formed in the focus of the best refracting telescope is affected by an imperfection arising from the different action of the glasses on rays of light of different colors. hence, the image of a star can never be seen or photographed with such an instrument, as an actual point, but only as a small, diffused mass. this difficulty is avoided in the reflecting telescope; but a new difficulty is found in the bending of the mirror under the influence of its own weight. devices for overcoming this had been so far from successful that, when mr. crossley presented his instrument to the lick observatory, it was feared that little of importance could be done with it. but as often happens in human affairs outside the field of astronomy, when ingenious and able men devote their attention to the careful study of a problem, it was found that new results could be reached. thus it was that, before a great while, what was supposed to be an inferior instrument proved not only to have qualities not before suspected, but to be the means of making an important addition to the methods of astronomical investigation. in order that our knowledge of the position of a star may be complete, we must know its distance. this can be measured only through the star's parallax--that is to say, the slight change in its direction produced by the swing of our earth around its orbit. but so vast is the distance in question that this change is immeasurably small, except for, perhaps, a few hundred stars, and even for these few its measurement almost baffles the skill of the most expert astronomer. progress in this direction is therefore very slow, and there are probably not yet a hundred stars of which the parallax has been ascertained with any approach to certainty. dr. chase is now completing an important work of this kind at the yale observatory. to the most refined telescopic observations, as well as to the naked eye, the stars seem all alike, except that they differ greatly in brightness, and somewhat in color. but when their light is analyzed by the spectroscope, it is found that scarcely any two are exactly alike. an important part of the work of the astro-physical observatories, especially that of harvard, consists in photographing the spectra of thousands of stars, and studying the peculiarities thus brought out. at harvard a large portion of this work is done as part of the work of the henry draper memorial, established by his widow in memory of the eminent investigator of new york, who died twenty years ago. by a comparison of the spectra of stars sir william huggins has developed the idea that these bodies, like human beings, have a life history. they are nebulae in infancy, while the progress to old age is marked by a constant increase in the density of their substance. their temperature also changes in a way analogous to the vigor of the human being. during a certain time the star continually grows hotter and hotter. but an end to this must come, and it cools off in old age. what the age of a star may be is hard even to guess. it is many millions of years, perhaps hundreds, possibly even thousands, of millions. some attempt at giving the magnitude is included in every considerable list of stars. the work of determining the magnitudes with the greatest precision is so laborious that it must go on rather slowly. it is being pursued on a large scale at the harvard observatory, as well as in that of potsdam, germany. we come now to the question of changes in the appearance of bright stars. it seems pretty certain that more than one per cent of these bodies fluctuate to a greater or less extent in their light. observations of these fluctuations, in the case of at least the brighter stars, may be carried on without any instrument more expensive than a good opera-glass--in fact, in the case of stars visible to the naked eye, with no instrument at all. as a general rule, the light of these stars goes through its changes in a regular period, which is sometimes as short as a few hours, but generally several days, frequently a large fraction of a year or even eighteen months. observations of these stars are made to determine the length of the period and the law of variation of the brightness. any person with a good eye and skill in making estimates can make the observations if he will devote sufficient pains to training himself; but they require a degree of care and assiduity which is not to be expected of any one but an enthusiast on the subject. one of the most successful observers of the present time is mr. w. a. roberts, a resident of south africa, whom the boer war did not prevent from keeping up a watch of the southern sky, which has resulted in greatly increasing our knowledge of variable stars. there are also quite a number of astronomers in europe and america who make this particular study their specialty. during the past fifteen years the art of measuring the speed with which a star is approaching us or receding from us has been brought to a wonderful degree of perfection. the instrument with which this was first done was the spectroscope; it is now replaced with another of the same general kind, called the spectrograph. the latter differs from the other only in that the spectrum of the star is photographed, and the observer makes his measures on the negative. this method was first extensively applied at the potsdam observatory in germany, and has lately become one of the specialties of the lick observatory, where professor campbell has brought it to its present degree of perfection. the yerkes observatory is also beginning work in the same line, where professor frost is already rivalling the lick observatory in the precision of his measures. let us now go back to our own little colony and see what is being done to advance our knowledge of the solar system. this consists of planets, on one of which we dwell, moons revolving around them, comets, and meteoric bodies. the principal national observatories keep up a more or less orderly system of observations of the positions of the planets and their satellites in order to determine the laws of their motion. as in the case of the stars, it is necessary to continue these observations through long periods of time in order that everything possible to learn may be discovered. our own moon is one of the enigmas of the mathematical astronomer. observations show that she is deviating from her predicted place, and that this deviation continues to increase. true, it is not very great when measured by an ordinary standard. the time at which the moon's shadow passed a given point near norfolk during the total eclipse of may , , was only about seven seconds different from the time given in the astronomical ephemeris. the path of the shadow along the earth was not out of place by more than one or two miles but, small though these deviations are, they show that something is wrong, and no one has as yet found out what it is. worse yet, the deviation is increasing rapidly. the observers of the total eclipse in august, , were surprised to find that it began twenty seconds before the predicted time. the mathematical problems involved in correcting this error are of such complexity that it is only now and then that a mathematician turns up anywhere in the world who is both able and bold enough to attack them. there now seems little doubt that jupiter is a miniature sun, only not hot enough at its surface to shine by its own light the point in which it most resembles the sun is that its equatorial regions rotate in less time than do the regions near the poles. this shows that what we see is not a solid body. but none of the careful observers have yet succeeded in determining the law of this difference of rotation. twelve years ago a suspicion which had long been entertained that the earth's axis of rotation varied a little from time to time was verified by chandler. the result of this is a slight change in the latitude of all places on the earth's surface, which admits of being determined by precise observations. the national geodetic association has established four observatories on the same parallel of latitude--one at gaithersburg, maryland, another on the pacific coast, a third in japan, and a fourth in italy--to study these variations by continuous observations from night to night. this work is now going forward on a well-devised plan. a fact which will appeal to our readers on this side of the atlantic is the success of american astronomers. sixty years ago it could not be said that there was a well-known observatory on the american continent. the cultivation of astronomy was confined to a professor here and there, who seldom had anything better than a little telescope with which he showed the heavenly bodies to his students. but during the past thirty years all this has been changed. the total quantity of published research is still less among us than on the continent of europe, but the number of men who have reached the highest success among us may be judged by one fact. the royal astronomical society of england awards an annual medal to the english or foreign astronomer deemed most worthy of it. the number of these medals awarded to americans within twenty-five years is about equal to the number awarded to the astronomers of all other nations foreign to the english. that this preponderance is not growing less is shown by the award of medals to americans in three consecutive years-- , , and . the recipients were hale, boss, and campbell. of the fifty foreign associates chosen by this society for their eminence in astronomical research, no less than eighteen--more than one-third--are americans. vii life in the universe so far as we can judge from what we see on our globe, the production of life is one of the greatest and most incessant purposes of nature. life is absent only in regions of perpetual frost, where it never has an opportunity to begin; in places where the temperature is near the boiling-point, which is found to be destructive to it; and beneath the earth's surface, where none of the changes essential to it can come about. within the limits imposed by these prohibitory conditions--that is to say, within the range of temperature at which water retains its liquid state, and in regions where the sun's rays can penetrate and where wind can blow and water exist in a liquid form--life is the universal rule. how prodigal nature seems to be in its production is too trite a fact to be dwelt upon. we have all read of the millions of germs which are destroyed for every one that comes to maturity. even the higher forms of life are found almost everywhere. only small islands have ever been discovered which were uninhabited, and animals of a higher grade are as widely diffused as man. if it would be going too far to claim that all conditions may have forms of life appropriate to them, it would be going as much too far in the other direction to claim that life can exist only with the precise surroundings which nurture it on this planet. it is very remarkable in this connection that while in one direction we see life coming to an end, in the other direction we see it flourishing more and more up to the limit. these two directions are those of heat and cold. we cannot suppose that life would develop in any important degree in a region of perpetual frost, such as the polar regions of our globe. but we do not find any end to it as the climate becomes warmer. on the contrary, every one knows that the tropics are the most fertile regions of the globe in its production. the luxuriance of the vegetation and the number of the animals continually increase the more tropical the climate becomes. where the limit may be set no one can say. but it would doubtless be far above the present temperature of the equatorial regions. it has often been said that this does not apply to the human race, that men lack vigor in the tropics. but human vigor depends on so many conditions, hereditary and otherwise, that we cannot regard the inferior development of humanity in the tropics as due solely to temperature. physically considered, no men attain a better development than many tribes who inhabit the warmer regions of the globe. the inferiority of the inhabitants of these regions in intellectual power is more likely the result of race heredity than of temperature. we all know that this earth on which we dwell is only one of countless millions of globes scattered through the wilds of infinite space. so far as we know, most of these globes are wholly unlike the earth, being at a temperature so high that, like our sun, they shine by their own light. in such worlds we may regard it as quite certain that no organized life could exist. but evidence is continually increasing that dark and opaque worlds like ours exist and revolve around their suns, as the earth on which we dwell revolves around its central luminary. although the number of such globes yet discovered is not great, the circumstances under which they are found lead us to believe that the actual number may be as great as that of the visible stars which stud the sky. if so, the probabilities are that millions of them are essentially similar to our own globe. have we any reason to believe that life exists on these other worlds? the reader will not expect me to answer this question positively. it must be admitted that, scientifically, we have no light upon the question, and therefore no positive grounds for reaching a conclusion. we can only reason by analogy and by what we know of the origin and conditions of life around us, and assume that the same agencies which are at play here would be found at play under similar conditions in other parts of the universe. if we ask what the opinion of men has been, we know historically that our race has, in all periods of its history, peopled other regions with beings even higher in the scale of development than we are ourselves. the gods and demons of an earlier age all wielded powers greater than those granted to man--powers which they could use to determine human destiny. but, up to the time that copernicus showed that the planets were other worlds, the location of these imaginary beings was rather indefinite. it was therefore quite natural that when the moon and planets were found to be dark globes of a size comparable with that of the earth itself, they were made the habitations of beings like unto ourselves. the trend of modern discovery has been against carrying this view to its extreme, as will be presently shown. before considering the difficulties in the way of accepting it to the widest extent, let us enter upon some preliminary considerations as to the origin and prevalence of life, so far as we have any sound basis to go upon. a generation ago the origin of life upon our planet was one of the great mysteries of science. all the facts brought out by investigation into the past history of our earth seemed to show, with hardly the possibility of a doubt, that there was a time when it was a fiery mass, no more capable of serving as the abode of a living being than the interior of a bessemer steel furnace. there must therefore have been, within a certain period, a beginning of life upon its surface. but, so far as investigation had gone--indeed, so far as it has gone to the present time--no life has been found to originate of itself. the living germ seems to be necessary to the beginning of any living form. whence, then, came the first germ? many of our readers may remember a suggestion by sir william thomson, now lord kelvin, made twenty or thirty years ago, that life may have been brought to our planet by the falling of a meteor from space. this does not, however, solve the difficulty--indeed, it would only make it greater. it still leaves open the question how life began on the meteor; and granting this, why it was not destroyed by the heat generated as the meteor passed through the air. the popular view that life began through a special act of creative power seemed to be almost forced upon man by the failure of science to discover any other beginning for it. it cannot be said that even to-day anything definite has been actually discovered to refute this view. all we can say about it is that it does not run in with the general views of modern science as to the beginning of things, and that those who refuse to accept it must hold that, under certain conditions which prevail, life begins by a very gradual process, similar to that by which forms suggesting growth seem to originate even under conditions so unfavorable as those existing in a bottle of acid. but it is not at all necessary for our purpose to decide this question. if life existed through a creative act, it is absurd to suppose that that act was confined to one of the countless millions of worlds scattered through space. if it began at a certain stage of evolution by a natural process, the question will arise, what conditions are favorable to the commencement of this process? here we are quite justified in reasoning from what, granting this process, has taken place upon our globe during its past history. one of the most elementary principles accepted by the human mind is that like causes produce like effects. the special conditions under which we find life to develop around us may be comprehensively summed up as the existence of water in the liquid form, and the presence of nitrogen, free perhaps in the first place, but accompanied by substances with which it may form combinations. oxygen, hydrogen, and nitrogen are, then, the fundamental requirements. the addition of calcium or other forms of matter necessary to the existence of a solid world goes without saying. the question now is whether these necessary conditions exist in other parts of the universe. the spectroscope shows that, so far as the chemical elements go, other worlds are composed of the same elements as ours. hydrogen especially exists everywhere, and we have reason to believe that the same is true of oxygen and nitrogen. calcium, the base of lime, is almost universal. so far as chemical elements go, we may therefore take it for granted that the conditions under which life begins are very widely diffused in the universe. it is, therefore, contrary to all the analogies of nature to suppose that life began only on a single world. it is a scientific inference, based on facts so numerous as not to admit of serious question, that during the history of our globe there has been a continually improving development of life. as ages upon ages pass, new forms are generated, higher in the scale than those which preceded them, until at length reason appears and asserts its sway. in a recent well-known work alfred russel wallace has argued that this development of life required the presence of such a rare combination of conditions that there is no reason to suppose that it prevailed anywhere except on our earth. it is quite impossible in the present discussion to follow his reasoning in detail; but it seems to me altogether inconclusive. not only does life, but intelligence, flourish on this globe under a great variety of conditions as regards temperature and surroundings, and no sound reason can be shown why under certain conditions, which are frequent in the universe, intelligent beings should not acquire the highest development. now let us look at the subject from the view of the mathematical theory of probabilities. a fundamental tenet of this theory is that no matter how improbable a result may be on a single trial, supposing it at all possible, it is sure to occur after a sufficient number of trials--and over and over again if the trials are repeated often enough. for example, if a million grains of corn, of which a single one was red, were all placed in a pile, and a blindfolded person were required to grope in the pile, select a grain, and then put it back again, the chances would be a million to one against his drawing out the red grain. if drawing it meant he should die, a sensible person would give himself no concern at having to draw the grain. the probability of his death would not be so great as the actual probability that he will really die within the next twenty-four hours. and yet if the whole human race were required to run this chance, it is certain that about fifteen hundred, or one out of a million, of the whole human family would draw the red grain and meet his death. now apply this principle to the universe. let us suppose, to fix the ideas, that there are a hundred million worlds, but that the chances are one thousand to one against any one of these taken at random being fitted for the highest development of life or for the evolution of reason. the chances would still be that one hundred thousand of them would be inhabited by rational beings whom we call human. but where are we to look for these worlds? this no man can tell. we only infer from the statistics of the stars--and this inference is fairly well grounded--that the number of worlds which, so far as we know, may be inhabited, are to be counted by thousands, and perhaps by millions. in a number of bodies so vast we should expect every variety of conditions as regards temperature and surroundings. if we suppose that the special conditions which prevail on our planet are necessary to the highest forms of life, we still have reason to believe that these same conditions prevail on thousands of other worlds. the fact that we might find the conditions in millions of other worlds unfavorable to life would not disprove the existence of the latter on countless worlds differently situated. coming down now from the general question to the specific one, we all know that the only worlds the conditions of which can be made the subject of observation are the planets which revolve around the sun, and their satellites. the question whether these bodies are inhabited is one which, of course, completely transcends not only our powers of observation at present, but every appliance of research that we can conceive of men devising. if mars is inhabited, and if the people of that planet have equal powers with ourselves, the problem of merely producing an illumination which could be seen in our most powerful telescope would be beyond all the ordinary efforts of an entire nation. an unbroken square mile of flame would be invisible in our telescopes, but a hundred square miles might be seen. we cannot, therefore, expect to see any signs of the works of inhabitants even on mars. all that we can do is to ascertain with greater or less probability whether the conditions necessary to life exist on the other planets of the system. the moon being much the nearest to us of all the heavenly bodies, we can pronounce more definitely in its case than in any other. we know that neither air nor water exists on the moon in quantities sufficient to be perceived by the most delicate tests at our command. it is certain that the moon's atmosphere, if any exists, is less than the thousandth part of the density of that around us. the vacuum is greater than any ordinary air-pump is capable of producing. we can hardly suppose that so small a quantity of air could be of any benefit whatever in sustaining life; an animal that could get along on so little could get along on none at all. but the proof of the absence of life is yet stronger when we consider the results of actual telescopic observation. an object such as an ordinary city block could be detected on the moon. if anything like vegetation were present on its surface, we should see the changes which it would undergo in the course of a month, during one portion of which it would be exposed to the rays of the unclouded sun, and during another to the intense cold of space. if men built cities, or even separate buildings the size of the larger ones on our earth, we might see some signs of them. in recent times we not only observe the moon with the telescope, but get still more definite information by photography. the whole visible surface has been repeatedly photographed under the best conditions. but no change has been established beyond question, nor does the photograph show the slightest difference of structure or shade which could be attributed to cities or other works of man. to all appearances the whole surface of our satellite is as completely devoid of life as the lava newly thrown from vesuvius. we next pass to the planets. mercury, the nearest to the sun, is in a position very unfavorable for observation from the earth, because when nearest to us it is between us and the sun, so that its dark hemisphere is presented to us. nothing satisfactory has yet been made out as to its condition. we cannot say with certainty whether it has an atmosphere or not. what seems very probable is that the temperature on its surface is higher than any of our earthly animals could sustain. but this proves nothing. we know that venus has an atmosphere. this was very conclusively shown during the transits of venus in and . but this atmosphere is so filled with clouds or vapor that it does not seem likely that we ever get a view of the solid body of the planet through it. some observers have thought they could see spots on venus day after day, while others have disputed this view. on the whole, if intelligent inhabitants live there, it is not likely that they ever see sun or stars. instead of the sun they see only an effulgence in the vapory sky which disappears and reappears at regular intervals. when we come to mars, we have more definite knowledge, and there seems to be greater possibilities for life there than in the case of any other planet besides the earth. the main reason for denying that life such as ours could exist there is that the atmosphere of mars is so rare that, in the light of the most recent researches, we cannot be fully assured that it exists at all. the very careful comparisons of the spectra of mars and of the moon made by campbell at the lick observatory failed to show the slightest difference in the two. if mars had an atmosphere as dense as ours, the result could be seen in the darkening of the lines of the spectrum produced by the double passage of the light through it. there were no lines in the spectrum of mars that were not seen with equal distinctness in that of the moon. but this does not prove the entire absence of an atmosphere. it only shows a limit to its density. it may be one-fifth or one-fourth the density of that on the earth, but probably no more. that there must be something in the nature of vapor at least seems to be shown by the formation and disappearance of the white polar caps of this planet. every reader of astronomy at the present time knows that, during the martian winter, white caps form around the pole of the planet which is turned away from the sun, and grow larger and larger until the sun begins to shine upon them, when they gradually grow smaller, and perhaps nearly disappear. it seems, therefore, fairly well proved that, under the influence of cold, some white substance forms around the polar regions of mars which evaporates under the influence of the sun's rays. it has been supposed that this substance is snow, produced in the same way that snow is produced on the earth, by the evaporation of water. but there are difficulties in the way of this explanation. the sun sends less than half as much heat to mars as to the earth, and it does not seem likely that the polar regions can ever receive enough of heat to melt any considerable quantity of snow. nor does it seem likely that any clouds from which snow could fall ever obscure the surface of mars. but a very slight change in the explanation will make it tenable. quite possibly the white deposits may be due to something like hoar-frost condensed from slightly moist air, without the actual production of snow. this would produce the effect that we see. even this explanation implies that mars has air and water, rare though the former may be. it is quite possible that air as thin as that of mars would sustain life in some form. life not totally unlike that on the earth may therefore exist upon this planet for anything that we know to the contrary. more than this we cannot say. in the case of the outer planets the answer to our question must be in the negative. it now seems likely that jupiter is a body very much like our sun, only that the dark portion is too cool to emit much, if any, light. it is doubtful whether jupiter has anything in the nature of a solid surface. its interior is in all likelihood a mass of molten matter far above a red heat, which is surrounded by a comparatively cool, yet, to our measure, extremely hot, vapor. the belt-like clouds which surround the planet are due to this vapor combined with the rapid rotation. if there is any solid surface below the atmosphere that we can see, it is swept by winds such that nothing we have on earth could withstand them. but, as we have said, the probabilities are very much against there being anything like such a surface. at some great depth in the fiery vapor there is a solid nucleus; that is all we can say. the planet saturn seems to be very much like that of jupiter in its composition. it receives so little heat from the sun that, unless it is a mass of fiery vapor like jupiter, the surface must be far below the freezing-point. we cannot speak with such certainty of uranus and neptune; yet the probability seems to be that they are in much the same condition as saturn. they are known to have very dense atmospheres, which are made known to us only by their absorbing some of the light of the sun. but nothing is known of the composition of these atmospheres. to sum up our argument: the fact that, so far as we have yet been able to learn, only a very small proportion of the visible worlds scattered through space are fitted to be the abode of life does not preclude the probability that among hundreds of millions of such worlds a vast number are so fitted. such being the case, all the analogies of nature lead us to believe that, whatever the process which led to life upon this earth--whether a special act of creative power or a gradual course of development--through that same process does life begin in every part of the universe fitted to sustain it. the course of development involves a gradual improvement in living forms, which by irregular steps rise higher and higher in the scale of being. we have every reason to believe that this is the case wherever life exists. it is, therefore, perfectly reasonable to suppose that beings, not only animated, but endowed with reason, inhabit countless worlds in space. it would, indeed, be very inspiring could we learn by actual observation what forms of society exist throughout space, and see the members of such societies enjoying themselves by their warm firesides. but this, so far as we can now see, is entirely beyond the possible reach of our race, so long as it is confined to a single world. viii how the planets are weighed you ask me how the planets are weighed? i reply, on the same principle by which a butcher weighs a ham in a spring-balance. when he picks the ham up, he feels a pull of the ham towards the earth. when he hangs it on the hook, this pull is transferred from his hand to the spring of the balance. the stronger the pull, the farther the spring is pulled down. what he reads on the scale is the strength of the pull. you know that this pull is simply the attraction of the earth on the ham. but, by a universal law of force, the ham attracts the earth exactly as much as the earth does the ham. so what the butcher really does is to find how much or how strongly the ham attracts the earth, and he calls that pull the weight of the ham. on the same principle, the astronomer finds the weight of a body by finding how strong is its attractive pull on some other body. if the butcher, with his spring-balance and a ham, could fly to all the planets, one after the other, weigh the ham on each, and come back to report the results to an astronomer, the latter could immediately compute the weight of each planet of known diameter, as compared with that of the earth. in applying this principle to the heavenly bodies, we at once meet a difficulty that looks insurmountable. you cannot get up to the heavenly bodies to do your weighing; how then will you measure their pull? i must begin the answer to this question by explaining a nice point in exact science. astronomers distinguish between the weight of a body and its mass. the weight of objects is not the same all over the world; a thing which weighs thirty pounds in new york would weigh an ounce more than thirty pounds in a spring-balance in greenland, and nearly an ounce less at the equator. this is because the earth is not a perfect sphere, but a little flattened. thus weight varies with the place. if a ham weighing thirty pounds were taken up to the moon and weighed there, the pull would only be five pounds, because the moon is so much smaller and lighter than the earth. there would be another weight of the ham for the planet mars, and yet another on the sun, where it would weigh some eight hundred pounds. hence the astronomer does not speak of the weight of a planet, because that would depend on the place where it was weighed; but he speaks of the mass of the planet, which means how much planet there is, no matter where you might weigh it. at the same time, we might, without any inexactness, agree that the mass of a heavenly body should be fixed by the weight it would have in new york. as we could not even imagine a planet at new york, because it may be larger than the earth itself, what we are to imagine is this: suppose the planet could be divided into a million million million equal parts, and one of these parts brought to new york and weighed. we could easily find its weight in pounds or tons. then multiply this weight by a million million million, and we shall have a weight of the planet. this would be what the astronomers might take as the mass of the planet. with these explanations, let us see how the weight of the earth is found. the principle we apply is that round bodies of the same specific gravity attract small objects on their surface with a force proportional to the diameter of the attracting body. for example, a body two feet in diameter attracts twice as strongly as one of a foot, one of three feet three times as strongly, and so on. now, our earth is about , , feet in diameter; that is , , times four feet. it follows that if we made a little model of the earth four feet in diameter, having the average specific gravity of the earth, it would attract a particle with one ten-millionth part of the attraction of the earth. the attraction of such a model has actually been measured. since we do not know the average specific gravity of the earth--that being in fact what we want to find out--we take a globe of lead, four feet in diameter, let us suppose. by means of a balance of the most exquisite construction it is found that such a globe does exert a minute attraction on small bodies around it, and that this attraction is a little more than the ten-millionth part of that of the earth. this shows that the specific gravity of the lead is a little greater than that of the average of the whole earth. all the minute calculations made, it is found that the earth, in order to attract with the force it does, must be about five and one-half times as heavy as its bulk of water, or perhaps a little more. different experimenters find different results; the best between . and . , so that . is, perhaps, as near the number as we can now get. this is much more than the average specific gravity of the materials which compose that part of the earth which we can reach by digging mines. the difference arises from the fact that, at the depth of many miles, the matter composing the earth is compressed into a smaller space by the enormous weight of the portions lying above it. thus, at the depth of miles, the pressure on every cubic inch is more than tons, a weight which would greatly condense the hardest metal. we come now to the planets. i have said that the mass or weight of a heavenly body is determined by its attraction on some other body. there are two ways in which the attraction of a planet may be measured. one is by its attraction on the planets next to it. if these bodies did not attract one another at all, but only moved under the influence of the sun, they would move in orbits having the form of ellipses. they are found to move very nearly in such orbits, only the actual path deviates from an ellipse, now in one direction and then in another, and it slowly changes its position from year to year. these deviations are due to the pull of the other planets, and by measuring the deviations we can determine the amount of the pull, and hence the mass of the planet. the reader will readily understand that the mathematical processes necessary to get a result in this way must be very delicate and complicated. a much simpler method can be used in the case of those planets which have satellites revolving round them, because the attraction of the planet can be determined by the motions of the satellite. the first law of motion teaches us that a body in motion, if acted on by no force, will move in a straight line. hence, if we see a body moving in a curve, we know that it is acted on by a force in the direction towards which the motion curves. a familiar example is that of a stone thrown from the hand. if the stone were not attracted by the earth, it would go on forever in the line of throw, and leave the earth entirely. but under the attraction of the earth, it is drawn down and down, as it travels onward, until finally it reaches the ground. the faster the stone is thrown, of course, the farther it will go, and the greater will be the sweep of the curve of its path. if it were a cannon-ball, the first part of the curve would be nearly a right line. if we could fire a cannon-ball horizontally from the top of a high mountain with a velocity of five miles a second, and if it were not resisted by the air, the curvature of the path would be equal to that of the surface of our earth, and so the ball would never reach the earth, but would revolve round it like a little satellite in an orbit of its own. could this be done, the astronomer would be able, knowing the velocity of the ball, to calculate the attraction of the earth as well as we determine it by actually observing the motion of falling bodies around us. thus it is that when a planet, like mars or jupiter, has satellites revolving round it, astronomers on the earth can observe the attraction of the planet on its satellites and thus determine its mass. the rule for doing this is very simple. the cube of the distance between the planet and satellite is divided by the square of the time of revolution of the satellite. the quotient is a number which is proportional to the mass of the planet. the rule applies to the motion of the moon round the earth and of the planets round the sun. if we divide the cube of the earth's distance from the sun, say , , miles, by the square of / , the days in a year, we shall get a certain quotient. let us call this number the sun-quotient. then, if we divide the cube of the moon's distance from the earth by the square of its time of revolution, we shall get another quotient, which we may call the earth-quotient. the sun-quotient will come out about , times as large as the earth-quotient. hence it is concluded that the mass of the sun is , times that of the earth; that it would take this number of earths to make a body as heavy as the sun. i give this calculation to illustrate the principle; it must not be supposed that the astronomer proceeds exactly in this way and has only this simple calculation to make. in the case of the moon and earth, the motion and distance of the former vary in consequence of the attraction of the sun, so that their actual distance apart is a changing quantity. so what the astronomer actually does is to find the attraction of the earth by observing the length of a pendulum which beats seconds in various latitudes. then, by very delicate mathematical processes, he can find with great exactness what would be the time of revolution of a small satellite at any given distance from the earth, and thus can get the earth-quotient. but, as i have already pointed out, we must, in the case of the planets, find the quotient in question by means of the satellites; and it happens, fortunately, that the motions of these bodies are much less changed by the attraction of the sun than is the motion of the moon. thus, when we make the computation for the outer satellite of mars, we find the quotient to be / that of the sun-quotient. hence we conclude that the mass of mars is / that of the sun. by the corresponding quotient, the mass of jupiter is found to be about / that of the sun, saturn / , uranus / , neptune / . we have set forth only the great principle on which the astronomer has proceeded for the purpose in question. the law of gravitation is at the bottom of all his work. the effects of this law require mathematical processes which it has taken two hundred years to bring to their present state, and which are still far from perfect. the measurement of the distance of a satellite is not a job to be done in an evening; it requires patient labor extending through months and years, and then is not as exact as the astronomer would wish. he does the best he can, and must be satisfied with that. ix the mariner's compass among those provisions of nature which seem to us as especially designed for the use of man, none is more striking than the seeming magnetism of the earth. what would our civilization have been if the mariner's compass had never been known? that columbus could never have crossed the atlantic is certain; in what generation since his time our continent would have been discovered is doubtful. did the reader ever reflect what a problem the captain of the finest ocean liner of our day would face if he had to cross the ocean without this little instrument? with the aid of a pilot he gets his ship outside of sandy hook without much difficulty. even later, so long as the sun is visible and the air is clear, he will have some apparatus for sailing by the direction of the sun. but after a few hours clouds cover the sky. from that moment he has not the slightest idea of east, west, north, or south, except so far as he may infer it from the direction in which he notices the wind to blow. for a few hours he may be guided by the wind, provided he is sure he is not going ashore on long island. thus, in time, he feels his way out into the open sea. by day he has some idea of direction with the aid of the sun; by night, when the sky is clear he can steer by the great bear, or "cynosure," the compass of his ancient predecessors on the mediterranean. but when it is cloudy, if he persists in steaming ahead, he may be running towards the azores or towards greenland, or he may be making his way back to new york without knowing it. so, keeping up steam only when sun or star is visible, he at length finds that he is approaching the coast of ireland. then he has to grope along much like a blind man with his staff, feeling his way along the edge of a precipice. he can determine the latitude at noon if the sky is clear, and his longitude in the morning or evening in the same conditions. in this way he will get a general idea of his whereabouts. but if he ventures to make headway in a fog, he may find himself on the rocks at any moment. he reaches his haven only after many spells of patient waiting for favoring skies. the fact that the earth acts like a magnet, that the needle points to the north, has been generally known to navigators for nearly a thousand years, and is said to have been known to the chinese at a yet earlier period. and yet, to-day, if any professor of physical science is asked to explain the magnetic property of the earth, he will acknowledge his inability to do so to his own satisfaction. happily this does not hinder us from finding out by what law these forces act, and how they enable us to navigate the ocean. i therefore hope the reader will be interested in a short exposition of the very curious and interesting laws on which the science of magnetism is based, and which are applied in the use of the compass. the force known as magnetic, on which the compass depends, is different from all other natural forces with which we are familiar. it is very remarkable that iron is the only substance which can become magnetic in any considerable degree. nickel and one or two other metals have the same property, but in a very slight degree. it is also remarkable that, however powerfully a bar of steel may be magnetized, not the slightest effect of the magnetism can be seen by its action on other than magnetic substances. it is no heavier than before. its magnetism does not produce the slightest influence upon the human body. no one would know that it was magnetic until something containing iron was brought into its immediate neighborhood; then the attraction is set up. the most important principle of magnetic science is that there are two opposite kinds of magnetism, which are, in a certain sense, contrary in their manifestations. the difference is seen in the behavior of the magnet itself. one particular end points north, and the other end south. what is it that distinguishes these two ends? the answer is that one end has what we call north magnetism, while the other has south magnetism. every magnetic bar has two poles, one near one end, one near the other. the north pole is drawn towards the north pole of the earth, the south pole towards the south pole, and thus it is that the direction of the magnet is determined. now, when we bring two magnets near each other we find another curious phenomenon. if the two like poles are brought together, they do not attract but repel each other. but the two opposite poles attract each other. the attraction and repulsion are exactly equal under the same conditions. there is no more attraction than repulsion. if we seal one magnet up in a paper or a box, and then suspend another over the box, the north pole of the one outside will tend to the south pole of the one in the box, and vice versa. our next discovery is, that whenever a magnet attracts a piece of iron it makes that iron into a magnet, at least for the time being. in the case of ordinary soft or untempered iron the magnetism disappears instantly when the magnet is removed. but if the magnet be made to attract a piece of hardened steel, the latter will retain the magnetism produced in it and become itself a permanent magnet. this fact must have been known from the time that the compass came into use. to make this instrument it was necessary to magnetize a small bar or needle by passing a natural magnet over it. in our times the magnetization is effected by an electric current. the latter has curious magnetic properties; a magnetic needle brought alongside of it will be found placing itself at right angles to the wire bearing the current. on this principle is made the galvanometer for measuring the intensity of a current. moreover, if a piece of wire is coiled round a bar of steel, and a powerful electric current pass through the coil, the bar will become a magnet. another curious property of magnetism is that we cannot develop north magnetism in a bar without developing south magnetism at the same time. if it were otherwise, important consequences would result. a separate north pole of a magnet would, if attached to a floating object and thrown into the ocean, start on a journey towards the north all by itself. a possible method of bringing this result about may suggest itself. let us take an ordinary bar magnet, with a pole at each end, and break it in the middle; then would not the north end be all ready to start on its voyage north, and the south end to make its way south? but, alas! when this experiment is tried it is found that a south pole instantly develops itself on one side of the break, and a north pole on the other side, so that the two pieces will simply form two magnets, each with its north and south pole. there is no possibility of making a magnet with only one pole. it was formerly supposed that the central portions of the earth consisted of an immense magnet directed north and south. although this view is found, for reasons which need not be set forth in detail, to be untenable, it gives us a good general idea of the nature of terrestrial magnetism. one result that follows from the law of poles already mentioned is that the magnetism which seems to belong to the north pole of the earth is what we call south on the magnet, and vice versa. careful experiment shows us that the region around every magnet is filled with magnetic force, strongest near the poles of the magnet, but diminishing as the inverse square of the distance from the pole. this force, at each point, acts along a certain line, called a line of force. these lines are very prettily shown by the familiar experiment of placing a sheet of paper over a magnet, and then scattering iron filings on the surface of the paper. it will be noticed that the filings arrange themselves along a series of curved lines, diverging in every direction from each pole, but always passing from one pole to the other. it is a universal law that whenever a magnet is brought into a region where this force acts, it is attracted into such a position that it shall have the same direction as the lines of force. its north pole will take the direction of the curve leading to the south pole of the other magnet, and its south pole the opposite one. the fact of terrestrial magnetism may be expressed by saying that the space within and around the whole earth is filled by lines of magnetic force, which we know nothing about until we suspend a magnet so perfectly balanced that it may point in any direction whatever. then it turns and points in the direction of the lines of force, which may thus be mapped out for all points of the earth. we commonly say that the pole of the needle points towards the north. the poets tell us how the needle is true to the pole. every reader, however, is now familiar with the general fact of a variation of the compass. on our eastern seaboard, and all the way across the atlantic, the north pointing of the compass varies so far to the west that a ship going to europe and making no allowance for this deviation would find herself making more nearly for the north cape than for her destination. the "declination," as it is termed in scientific language, varies from one region of the earth to another. in some places it is towards the west, in others towards the east. the pointing of the needle in various regions of the world is shown by means of magnetic maps. such maps are published by the united states coast survey, whose experts make a careful study of the magnetic force all over the country. it is found that there is a line running nearly north and south through the middle states along which there is no variation of the compass. to the east of it the variation of the north pole of the magnet is west; to the west of it, east. the most rapid changes in the pointing of the needle are towards the northeast and northwest regions. when we travel to the northeastern boundary of maine the westerly variation has risen to degrees. towards the northwest the easterly variation continually increases, until, in the northern part of the state of washington, it amounts to degrees. when we cross the atlantic into europe we find the west variation diminishing until we reach a certain line passing through central russia and western asia. this is again a line of no variation. crossing it, the variation is once more towards the east. this direction continues over most of the continent of asia, but varies in a somewhat irregular manner from one part of the continent to another. as a general rule, the lines of the earth's magnetic force are not horizontal, and therefore one end or the other of a perfectly suspended magnet will dip below the horizontal position. this is called the "dip of the needle." it is observed by means of a brass circle, of which the circumference is marked off in degrees. a magnet is attached to this circle so as to form a diameter, and suspended on a horizontal axis passing through the centre of gravity, so that the magnet shall be free to point in the direction indicated by the earth's lines of magnetic force. armed with this apparatus, scientific travellers and navigators have visited various points of the earth in order to determine the dip. it is thus found that there is a belt passing around the earth near the equator, but sometimes deviating several degrees from it, in which there is no dip; that is to say, the lines of magnetic force are horizontal. taking any point on this belt and going north, it will be found that the north pole of the magnet gradually tends downward, the dip constantly increasing as we go farther north. in the southern part of the united states the dip is about degrees, and the direction of the needle is nearly perpendicular to the earth's axis. in the northern part of the country, including the region of the great lakes, the dip increases to degrees. noticing that a dip of degrees would mean that the north end of the magnet points straight downward, it follows that it would be more nearly correct to say that, throughout the united states, the magnetic needle points up and down than that it points north and south. going yet farther north, we find the dip still increasing, until at a certain point in the arctic regions the north pole of the needle points downward. in this region the compass is of no use to the traveller or the navigator. the point is called the magnetic pole. its position has been located several times by scientific observers. the best determinations made during the last eighty years agree fairly well in placing it near degrees north latitude and degrees longitude west from greenwich. this point is situated on the west shore of the boothian peninsula, which is bounded on the south end by mcclintock channel. it is about five hundred miles north of the northwest part of hudson bay. there is a corresponding magnetic pole in the antarctic ocean, or rather on victoria land, nearly south of australia. its position has not been so exactly located as in the north, but it is supposed to be at about degrees of south latitude and degrees of east longitude from greenwich. the magnetic poles used to be looked upon as the points towards which the respective ends of the needle were attracted. and, as a matter of fact, the magnetic force is stronger near the poles than elsewhere. when located in this way by strength of force, it is found that there is a second north pole in northern siberia. its location has not, however, been so well determined as in the case of the american pole, and it is not yet satisfactorily shown that there is any one point in siberia where the direction of the force is exactly downward. [illustration with caption: dip of the magnetic needle in various latitudes. the arrow points show the direction of the north end of the magnetic needle, which dips downward in north latitudes, while the south end dips in south latitudes.] the declination and dip, taken together, show the exact direction of the magnetic force at any place. but in order to complete the statement of the force, one more element must be given--its amount. the intensity of the magnetic force is determined by suspending a magnet in a horizontal position, and then allowing it to oscillate back and forth around the suspension. the stronger the force, the less the time it will take to oscillate. thus, by carrying a magnet to various parts of the world, the magnetic force can be determined at every point where a proper support for the magnet is obtainable. the intensity thus found is called the horizontal force. this is not really the total force, because the latter depends upon the dip; the greater the dip, the less will be the horizontal force which corresponds to a certain total force. but a very simple computation enables the one to be determined when the value of the other is known. in this way it is found that, as a general rule, the magnetic force is least in the earth's equatorial regions and increases as we approach either of the magnetic poles. when the most exact observations on the direction of the needle are made, it is found that it never remains at rest. beginning with the changes of shortest duration, we have a change which takes place every day, and is therefore called diurnal. in our northern latitudes it is found that during the six hours from nine o'clock at night until three in the morning the direction of the magnet remains nearly the same. but between three and four a.m. it begins to deviate towards the east, going farther and farther east until about a.m. then, rather suddenly, it begins to swing towards the west with a much more rapid movement, which comes to an end between one and two o'clock in the afternoon. then, more slowly, it returns in an easterly direction until about nine at night, when it becomes once more nearly quiescent. happily, the amount of this change is so small that the navigator need not trouble himself with it. the entire range of movement rarely amounts to one-quarter of a degree. it is a curious fact that the amount of the change is twice as great in june as it is in december. this indicates that it is caused by the sun's radiation. but how or why this cause should produce such an effect no one has yet discovered. another curious feature is that in the southern hemisphere the direction of the motion is reversed, although its general character remains the same. the pointing deviates towards the west in the morning, then rapidly moves towards the east until about two o'clock, after which it slowly returns to its original direction. the dip of the needle goes through a similar cycle of daily changes. in northern latitudes it is found that at about six in the morning the dip begins to increase, and continues to do so until noon, after which it diminishes until seven or eight o'clock in the evening, when it becomes nearly constant for the rest of the night. in the southern hemisphere the direction of the movement is reversed. when the pointing of the needle is compared with the direction of the moon, it is found that there is a similar change. but, instead of following the moon in its course, it goes through two periods in a day, like the tides. when the moon is on the meridian, whether above or below us, the effect is in one direction, while when it is rising or setting it is in the opposite direction. in other words, there is a complete swinging backward and forward twice in a lunar day. it might be supposed that such an effect would be due to the moon, like the earth, being a magnet. but were this the case there would be only one swing back and forth during the passage of the moon from the meridian until it came back to the meridian again. the effect would be opposite at the rising and setting of the moon, which we have seen is not the case. to make the explanation yet more difficult, it is found that, as in the case of the sun, the change is opposite in the northern and southern hemispheres and very small at the equator, where, by virtue of any action that we can conceive of, it ought to be greatest. the pointing is also found to change with the age of the moon and with the season of the year. but these motions are too small to be set forth in the present article. there is yet another class of changes much wider than these. the observations recorded since the time of columbus show that, in the course of centuries, the variation of the compass, at any one point, changes very widely. it is well known that in the needle pointed east of north in the mediterranean, as well as in those portions of the atlantic which were then navigated. columbus was therefore much astonished when, on his first voyage, in mid-ocean, he found that the deviation was reversed, and was now towards the west. it follows that a line of no variation then passed through the atlantic ocean. but this line has since been moving towards the east. about it passed the meridian of paris. during the two hundred and forty years which have since elapsed, it has passed over central europe, and now, as we have already said, passes through european russia. the existence of natural magnets composed of iron ore, and their property of attracting iron and making it magnetic, have been known from the remotest antiquity. but the question as to who first discovered the fact that a magnetized needle points north and south, and applied this discovery to navigation, has given rise to much discussion. that the property was known to the chinese about the beginning of our era seems to be fairly well established, the statements to that effect being of a kind that could not well have been invented. historical evidence of the use of the magnetic needle in navigation dates from the twelfth century. the earliest compass consisted simply of a splinter of wood or a piece of straw to which the magnetized needle was attached, and which was floated in water. a curious obstacle is said to have interfered with the first uses of this instrument. jack is a superstitious fellow, and we may be sure that he was not less so in former times than he is today. from his point of view there was something uncanny in so very simple a contrivance as a floating straw persistently showing him the direction in which he must sail. it made him very uncomfortable to go to sea under the guidance of an invisible power. but with him, as with the rest of us, familiarity breeds contempt, and it did not take more than a generation to show that much good and no harm came to those who used the magic pointer. the modern compass, as made in the most approved form for naval and other large ships, is the liquid one. this does not mean that the card bearing the needle floats on the liquid, but only that a part of the force is taken off from the pivot on which it turns, so as to make the friction as small as possible, and to prevent the oscillation back and forth which would continually go on if the card were perfectly free to turn. the compass-card is marked not only with the thirty-two familiar points of the compass, but is also divided into degrees. in the most accurate navigation it is probable that very little use of the points is made, the ship being directed according to the degrees. a single needle is not relied upon to secure the direction of the card, the latter being attached to a system of four or even more magnets, all pointing in the same direction. the compass must have no iron in its construction or support, because the attraction of that substance on the needle would be fatal to its performance. from this cause the use of iron as ship-building material introduced a difficulty which it was feared would prove very serious. the thousands of tons of iron in a ship must exert a strong attraction on the magnetic needle. another complication is introduced by the fact that the iron of the ship will always become more or less magnetic, and when the ship is built of steel, as modern ones are, this magnetism will be more or less permanent. we have already said that a magnet has the property of making steel or iron in its neighborhood into another magnet, with its poles pointing in the opposite direction. the consequence is that the magnetism of the earth itself will make iron or steel more or less magnetic. as a ship is built she thus becomes a great repository of magnetism, the direction of the force of which will depend upon the position in which she lay while building. if erected on the bank of an east and west stream, the north end of the ship will become the north pole of a magnet and the south end the south pole. accordingly, when she is launched and proceeds to sea, the compass points not exactly according to the magnetism of the earth, but partly according to that of the ship also. the methods of obviating this difficulty have exercised the ingenuity of the ablest physicists from the beginning of iron ship building. one method is to place in the neighborhood of the compass, but not too near it, a steel bar magnetized in the opposite direction from that of the ship, so that the action of the latter shall be neutralized. but a perfect neutralization cannot be thus effected. it is all the more difficult to effect it because the magnetism of a ship is liable to change. the practical method therefore adopted is called "swinging the ship," an operation which passengers on ocean liners may have frequently noticed when approaching land. the ship is swung around so that her bow shall point in various directions. at each pointing the direction of the ship is noticed by sighting on the sun, and also the direction of the compass itself. in this way the error of the pointing of the compass as the ship swings around is found for every direction in which she may be sailing. a table can then be made showing what the pointing, according to the compass, should be in order that the ship may sail in any given direction. this, however, does not wholly avoid the danger. the tables thus made are good when the ship is on a level keel. if, from any cause whatever, she heels over to one side, the action will be different. thus there is a "heeling error" which must be allowed for. it is supposed to have been from this source of error not having been sufficiently determined or appreciated that the lamentable wreck of the united states ship huron off the coast of hatteras occurred some twenty years ago. x the fairyland of geometry if the reader were asked in what branch of science the imagination is confined within the strictest limits, he would, i fancy, reply that it must be that of mathematics. the pursuer of this science deals only with problems requiring the most exact statements and the most rigorous reasoning. in all other fields of thought more or less room for play may be allowed to the imagination, but here it is fettered by iron rules, expressed in the most rigid logical form, from which no deviation can be allowed. we are told by philosophers that absolute certainty is unattainable in all ordinary human affairs, the only field in which it is reached being that of geometric demonstration. and yet geometry itself has its fairyland--a land in which the imagination, while adhering to the forms of the strictest demonstration, roams farther than it ever did in the dreams of grimm or andersen. one thing which gives this field its strictly mathematical character is that it was discovered and explored in the search after something to supply an actual want of mathematical science, and was incited by this want rather than by any desire to give play to fancy. geometricians have always sought to found their science on the most logical basis possible, and thus have carefully and critically inquired into its foundations. the new geometry which has thus arisen is of two closely related yet distinct forms. one of these is called non-euclidian, because euclid's axiom of parallels, which we shall presently explain, is ignored. in the other form space is assumed to have one or more dimensions in addition to the three to which the space we actually inhabit is confined. as we go beyond the limits set by euclid in adding a fourth dimension to space, this last branch as well as the other is often designated non-euclidian. but the more common term is hypergeometry, which, though belonging more especially to space of more than three dimensions, is also sometimes applied to any geometric system which transcends our ordinary ideas. in all geometric reasoning some propositions are necessarily taken for granted. these are called axioms, and are commonly regarded as self-evident. yet their vital principle is not so much that of being self-evident as being, from the nature of the case, incapable of demonstration. our edifice must have some support to rest upon, and we take these axioms as its foundation. one example of such a geometric axiom is that only one straight line can be drawn between two fixed points; in other words, two straight lines can never intersect in more than a single point. the axiom with which we are at present concerned is commonly known as the th of euclid, and may be set forth in the following way: we have given a straight line, a b, and a point, p, with another line, c d, passing through it and capable of being turned around on p. euclid assumes that this line c d will have one position in which it will be parallel to a b, that is, a position such that if the two lines are produced without end, they will never meet. his axiom is that only one such line can be drawn through p. that is to say, if we make the slightest possible change in the direction of the line c d, it will intersect the other line, either in one direction or the other. the new geometry grew out of the feeling that this proposition ought to be proved rather than taken as an axiom; in fact, that it could in some way be derived from the other axioms. many demonstrations of it were attempted, but it was always found, on critical examination, that the proposition itself, or its equivalent, had slyly worked itself in as part of the base of the reasoning, so that the very thing to be proved was really taken for granted. [illustration with caption: fig. ] this suggested another course of inquiry. if this axiom of parallels does not follow from the other axioms, then from these latter we may construct a system of geometry in which the axiom of parallels shall not be true. this was done by lobatchewsky and bolyai, the one a russian the other a hungarian geometer, about . to show how a result which looks absurd, and is really inconceivable by us, can be treated as possible in geometry, we must have recourse to analogy. suppose a world consisting of a boundless flat plane to be inhabited by reasoning beings who can move about at pleasure on the plane, but are not able to turn their heads up or down, or even to see or think of such terms as above them and below them, and things around them can be pushed or pulled about in any direction, but cannot be lifted up. people and things can pass around each other, but cannot step over anything. these dwellers in "flatland" could construct a plane geometry which would be exactly like ours in being based on the axioms of euclid. two parallel straight lines would never meet, though continued indefinitely. but suppose that the surface on which these beings live, instead of being an infinitely extended plane, is really the surface of an immense globe, like the earth on which we live. it needs no knowledge of geometry, but only an examination of any globular object--an apple, for example--to show that if we draw a line as straight as possible on a sphere, and parallel to it draw a small piece of a second line, and continue this in as straight a line as we can, the two lines will meet when we proceed in either direction one-quarter of the way around the sphere. for our "flat-land" people these lines would both be perfectly straight, because the only curvature would be in the direction downward, which they could never either perceive or discover. the lines would also correspond to the definition of straight lines, because any portion of either contained between two of its points would be the shortest distance between those points. and yet, if these people should extend their measures far enough, they would find any two parallel lines to meet in two points in opposite directions. for all small spaces the axioms of their geometry would apparently hold good, but when they came to spaces as immense as the semi-diameter of the earth, they would find the seemingly absurd result that two parallel lines would, in the course of thousands of miles, come together. another result yet more astonishing would be that, going ahead far enough in a straight line, they would find that although they had been going forward all the time in what seemed to them the same direction, they would at the end of , miles find themselves once more at their starting-point. one form of the modern non-euclidian geometry assumes that a similar theorem is true for the space in which our universe is contained. although two straight lines, when continued indefinitely, do not appear to converge even at the immense distances which separate us from the fixed stars, it is possible that there may be a point at which they would eventually meet without either line having deviated from its primitive direction as we understand the case. it would follow that, if we could start out from the earth and fly through space in a perfectly straight line with a velocity perhaps millions of times that of light, we might at length find ourselves approaching the earth from a direction the opposite of that in which we started. our straight-line circle would be complete. another result of the theory is that, if it be true, space, though still unbounded, is not infinite, just as the surface of a sphere, though without any edge or boundary, has only a limited extent of surface. space would then have only a certain volume--a volume which, though perhaps greater than that of all the atoms in the material universe, would still be capable of being expressed in cubic miles. if we imagine our earth to grow larger and larger in every direction without limit, and with a speed similar to that we have described, so that to-morrow it was large enough to extend to the nearest fixed stars, the day after to yet farther stars, and so on, and we, living upon it, looked out for the result, we should, in time, see the other side of the earth above us, coming down upon us? as it were. the space intervening would grow smaller, at last being filled up. the earth would then be so expanded as to fill all existing space. this, although to us the most interesting form of the non-euclidian geometry, is not the only one. the idea which lobatchewsky worked out was that through a point more than one parallel to a given line could be drawn; that is to say, if through the point p we have already supposed another line were drawn making ever so small an angle with cd, this line also would never meet the line ab. it might approach the latter at first, but would eventually diverge. the two lines ab and cd, starting parallel, would eventually, perhaps at distances greater than that of the fixed stars, gradually diverge from each other. this system does not admit of being shown by analogy so easily as the other, but an idea of it may be had by supposing that the surface of "flat-land," instead of being spherical, is saddle-shaped. apparently straight parallel lines drawn upon it would then diverge, as supposed by bolyai. we cannot, however, imagine such a surface extended indefinitely without losing its properties. the analogy is not so clearly marked as in the other case. to explain hypergeometry proper we must first set forth what a fourth dimension of space means, and show how natural the way is by which it may be approached. we continue our analogy from "flat-land" in this supposed land let us make a cross--two straight lines intersecting at right angles. the inhabitants of this land understand the cross perfectly, and conceive of it just as we do. but let us ask them to draw a third line, intersecting in the same point, and perpendicular to both the other lines. they would at once pronounce this absurd and impossible. it is equally absurd and impossible to us if we require the third line to be drawn on the paper. but we should reply, "if you allow us to leave the paper or flat surface, then we can solve the problem by simply drawing the third line through the paper perpendicular to its surface." [illustration with caption: fig. ] now, to pursue the analogy, suppose that, after we have drawn three mutually perpendicular lines, some being from another sphere proposes to us the drawing of a fourth line through the same point, perpendicular to all three of the lines already there. we should answer him in the same way that the inhabitants of "flat-land" answered us: "the problem is impossible. you cannot draw any such line in space as we understand it." if our visitor conceived of the fourth dimension, he would reply to us as we replied to the "flat-land" people: "the problem is absurd and impossible if you confine your line to space as you understand it. but for me there is a fourth dimension in space. draw your line through that dimension, and the problem will be solved. this is perfectly simple to me; it is impossible to you solely because your conceptions do not admit of more than three dimensions." supposing the inhabitants of "flat-land" to be intellectual beings as we are, it would be interesting to them to be told what dwellers of space in three dimensions could do. let us pursue the analogy by showing what dwellers in four dimensions might do. place a dweller of "flat-land" inside a circle drawn on his plane, and ask him to step outside of it without breaking through it. he would go all around, and, finding every inch of it closed, he would say it was impossible from the very nature of the conditions. "but," we would reply, "that is because of your limited conceptions. we can step over it." "step over it!" he would exclaim. "i do not know what that means. i can pass around anything if there is a way open, but i cannot imagine what you mean by stepping over it." but we should simply step over the line and reappear on the other side. so, if we confine a being able to move in a fourth dimension in the walls of a dungeon of which the sides, the floor, and the ceiling were all impenetrable, he would step outside of it without touching any part of the building, just as easily as we could step over a circle drawn on the plane without touching it. he would simply disappear from our view like a spirit, and perhaps reappear the next moment outside the prison. to do this he would only have to make a little excursion in the fourth dimension. [illustration with caption: fig. ] another curious application of the principle is more purely geometrical. we have here two triangles, of which the sides and angles of the one are all equal to corresponding sides and angles of the other. euclid takes it for granted that the one triangle can be laid upon the other so that the two shall fit together. but this cannot be done unless we lift one up and turn it over. in the geometry of "flat-land" such a thing as lifting up is inconceivable; the two triangles could never be fitted together. [illustration with caption: fig ] now let us suppose two pyramids similarly related. all the faces and angles of the one correspond to the faces and angles of the other. yet, lift them about as we please, we could never fit them together. if we fit the bases together the two will lie on opposite sides, one being below the other. but the dweller in four dimensions of space will fit them together without any trouble. by the mere turning over of one he will convert it into the other without any change whatever in the relative position of its parts. what he could do with the pyramids he could also do with one of us if we allowed him to take hold of us and turn a somersault with us in the fourth dimension. we should then come back into our natural space, but changed as if we were seen in a mirror. everything on us would be changed from right to left, even the seams in our clothes, and every hair on our head. all this would be done without, during any of the motion, any change having occurred in the positions of the parts of the body. it is very curious that, in these transcendental speculations, the most rigorous mathematical methods correspond to the most mystical ideas of the swedenborgian and other forms of religion. right around us, but in a direction which we cannot conceive any more than the inhabitants of "flat-land" can conceive up and down, there may exist not merely another universe, but any number of universes. all that physical science can say against the supposition is that, even if a fourth dimension exists, there is some law of all the matter with which we are acquainted which prevents any of it from entering that dimension, so that, in our natural condition, it must forever remain unknown to us. another possibility in space of four dimensions would be that of turning a hollow sphere, an india-rubber ball, for example, inside out by simple bending without tearing it. to show the motion in our space to which this is analogous, let us take a thin, round sheet of india-rubber, and cut out all the central part, leaving only a narrow ring round the border. suppose the outer edge of this ring fastened down on a table, while we take hold of the inner edge and stretch it upward and outward over the outer edge until we flatten the whole ring on the table, upside down, with the inner edge now the outer one. this motion would be as inconceivable in "flat-land" as turning the ball inside out is to us. xi the organization of scientific research the claims of scientific research on the public were never more forcibly urged than in professor ray lankester's recent romanes lecture before the university of oxford. man is here eloquently pictured as nature's rebel, who, under conditions where his great superior commands "thou shalt die," replies "i will live." in pursuance of this determination, civilized man has proceeded so far in his interference with the regular course of nature that he must either go on and acquire firmer control of the conditions, or perish miserably by the vengeance certain to be inflicted on the half-hearted meddler in great affairs. this rebel by every step forward renders himself liable to greater and greater penalties, and so cannot afford to pause or fail in one single step. one of nature's most powerful agencies in thwarting his determination to live is found in disease-producing parasites. "where there is one man of first-rate intelligence now employed in gaining knowledge of this agency, there should be a thousand. it should be as much the purpose of civilized nations to protect their citizens in this respect as it is to provide defence against human aggression." it was no part of the function of the lecturer to devise a plan for carrying on the great war he proposes to wage. the object of the present article is to contribute some suggestions in this direction; with especial reference to conditions in our own country; and no better text can be found for a discourse on the subject than the preceding quotation. in saying that there should be a thousand investigators of disease where there is now one, i believe that professor lankester would be the first to admit that this statement was that of an ideal to be aimed at, rather than of an end to be practically reached. every careful thinker will agree that to gather a body of men, young or old, supply them with laboratories and microscopes, and tell them to investigate disease, would be much like sending out an army without trained leaders to invade an enemy's country. there is at least one condition of success in this line which is better fulfilled in our own country than in any other; and that is liberality of support on the part of munificent citizens desirous of so employing their wealth as to promote the public good. combining this instrumentality with the general public spirit of our people, it must be admitted that, with all the disadvantages under which scientific research among us has hitherto labored, there is still no country to which we can look more hopefully than to our own as the field in which the ideal set forth by professor lankester is to be pursued. some thoughts on the question how scientific research may be most effectively promoted in our own country through organized effort may therefore be of interest. our first step will be to inquire what general lessons are to be learned from the experience of the past. the first and most important of these lessons is that research has never reached its highest development except at centres where bodies of men engaged in it have been brought together, and stimulated to action by mutual sympathy and support. we must call to mind that, although the beginnings of modern science were laid by such men as copernicus, galileo, leonardo da vinci, and torricelli, before the middle of the seventeenth century, unbroken activity and progress date from the foundations of the academy of sciences of paris and the royal society of london at that time. the historic fact that the bringing of men together, and their support by an intelligent and interested community, is the first requirement to be kept in view can easily be explained. effective research involves so intricate a network of problems and considerations that no one engaged in it can fail to profit by the suggestions of kindred spirits, even if less acquainted with the subject than he is himself. intelligent discussion suggests new ideas and continually carries the mind to a higher level of thought. we must not regard the typical scientific worker, even of the highest class, as one who, having chosen his special field and met with success in cultivating it, has only to be supplied with the facilities he may be supposed to need in order to continue his work in the most efficient way. what we have to deal with is not a fixed and permanent body of learned men, each knowing all about the field of work in which he is engaged, but a changing and growing class, constantly recruited by beginners at the bottom of the scale, and constantly depleted by the old dropping away at the top. no view of the subject is complete which does not embrace the entire activity of the investigator, from the tyro to the leader. the leader himself, unless engaged in the prosecution of some narrow specialty, can rarely be so completely acquainted with his field as not to need information from others. without this, he is constantly liable to be repeating what has already been better done than he can do it himself, of following lines which are known to lead to no result, and of adopting methods shown by the experience of others not to be the best. even the books and published researches to which he must have access may be so voluminous that he cannot find time to completely examine them for himself; or they may be inaccessible. all this will make it clear that, with an occasional exception, the best results of research are not to be expected except at centres where large bodies of men are brought into close personal contact. in addition to the power and facility acquired by frequent discussion with his fellows, the appreciation and support of an intelligent community, to whom the investigator may, from time to time, make known his thoughts and the results of his work, add a most effective stimulus. the greater the number of men of like minds that can be brought together and the larger the community which interests itself in what they are doing, the more rapid will be the advance and the more effective the work carried on. it is thus that london, with its munificently supported institutions, and paris and berlin, with their bodies of investigators supported either by the government or by various foundations, have been for more than three centuries the great centres where we find scientific activity most active and most effective. looking at this undoubted fact, which has asserted itself through so long a period, and which asserts itself today more strongly than ever, the writer conceives that there can be no question as to one proposition. if we aim at the single object of promoting the advance of knowledge in the most effective way, and making our own country the leading one in research, our efforts should be directed towards bringing together as many scientific workers as possible at a single centre, where they can profit in the highest degree by mutual help, support, and sympathy. in thus strongly setting forth what must seem an indisputable conclusion, the writer does not deny that there are drawbacks to such a policy, as there are to every policy that can be devised aiming at a good result. nature offers to society no good that she does not accompany by a greater or less measure of evil the only question is whether the good outweighs the evil. in the present case, the seeming evil, whether real or not, is that of centralization. a policy tending in this direction is held to be contrary to the best interests of science in quarters entitled to so much respect that we must inquire into the soundness of the objection. it would be idle to discuss so extreme a question as whether we shall take all the best scientific investigators of our country from their several seats of learning and attract them to some one point. we know that this cannot be done, even were it granted that success would be productive of great results. the most that can be done is to choose some existing centre of learning, population, wealth, and influence, and do what we can to foster the growth of science at that centre by attracting thither the greatest possible number of scientific investigators, especially of the younger class, and making it possible for them to pursue their researches in the most effective way. this policy would not result in the slightest harm to any institution or community situated elsewhere. it would not be even like building up a university to outrank all the others of our country; because the functions of the new institution, if such should be founded, would in its relations to the country be radically different from those of a university. its primary object would not be the education of youth, but the increase of knowledge. so far as the interests of any community or of the world at large are concerned, it is quite indifferent where knowledge may be acquired, because, when once acquired and made public, it is free to the world. the drawbacks suffered by other centres would be no greater than those suffered by our western cities, because all the great departments of the government are situated at a single distant point. strong arguments could doubtless be made for locating some of these departments in the far west, in the mississippi valley, or in various cities of the atlantic coast; but every one knows that any local advantages thus gained would be of no importance compared with the loss of that administrative efficiency which is essential to the whole country. there is, therefore, no real danger from centralization. the actual danger is rather in the opposite direction; that the sentiment against concentrating research will prove to operate too strongly. there is a feeling that it is rather better to leave every investigator where he chances to be at the moment, a feeling which sometimes finds expression in the apothegm that we cannot transplant a genius. that such a proposition should find acceptance affords a striking example of the readiness of men to accept a euphonious phrase without inquiring whether the facts support the doctrine which it enunciates. the fact is that many, perhaps the majority, of the great scientific investigators of this and of former times have done their best work through being transplanted. as soon as the enlightened monarchs of europe felt the importance of making their capitals great centres of learning, they began to invite eminent men of other countries to their own. lagrange was an italian transplanted to paris, as a member of the academy of sciences, after he had shown his powers in his native country. his great contemporary, euler, was a swiss, transplanted first to st. petersburg, then invited by frederick the great to become a member of the berlin academy, then again attracted to st. petersburg. huyghens was transplanted from his native country to paris. agassiz was an exotic, brought among us from switzerland, whose activity during the generation he passed among us was as great and effective as at any time of his life. on the continent, outside of france, the most eminent professors in the universities have been and still are brought from distant points. so numerous are the cases of which these are examples that it would be more in accord with the facts to claim that it is only by transplanting a genius that we stimulate him to his best work. having shown that the best results can be expected only by bringing into contact as many scientific investigators as possible, the next question which arises is that of their relations to one another. it may be asked whether we shall aim at individualism or collectivism. shall our ideal be an organized system of directors, professors, associates, assistants, fellows; or shall it be a collection of individual workers, each pursuing his own task in the way he deems best, untrammelled by authority? the reply to this question is that there is in this special case no antagonism between the two ideas. the most effective organization will aim both at the promotion of individual effort, and at subordination and co-operation. it would be a serious error to formulate any general rule by which all cases should be governed. the experience of the past should be our guide, so far as it applies to present and future conditions; but in availing ourselves of it we must remember that conditions are constantly changing, and must adapt our policy to the problems of the future. in doing this, we shall find that different fields of research require very different policies as regards co-operation and subordination. it will be profitable to point out those special differences, because we shall thereby gain a more luminous insight into the problems which now confront the scientific investigator, and better appreciate their variety, and the necessity of different methods of dealing with them. at one extreme, we have the field of normative science, work in which is of necessity that of the individual mind alone. this embraces pure mathematics and the methods of science in their widest range. the common interests of science require that these methods shall be worked out and formulated for the guidance of investigators generally, and this work is necessarily that of the individual brain. at the other extreme, we have the great and growing body of sciences of observation. through the whole nineteenth century, to say nothing of previous centuries, organizations, and even individuals, have been engaged in recording the innumerable phases of the course of nature, hoping to accumulate material that posterity shall be able to utilize for its benefit. we have observations astronomical, meteorological, magnetic, and social, accumulating in constantly increasing volume, the mass of which is so unmanageable with our present organizations that the question might well arise whether almost the whole of it will not have to be consigned to oblivion. such a conclusion should not be entertained until we have made a vigorous effort to find what pure metal of value can be extracted from the mass of ore. to do this requires the co-operation of minds of various orders, quite akin in their relations to those necessary in a mine or great manufacturing establishment. laborers whose duties are in a large measure matters of routine must be guided by the skill of a class higher in quality and smaller in number than their own, and these again by the technical knowledge of leaders in research. between these extremes we have a great variety of systems of co-operation. there is another feature of modern research the apprehension of which is necessary to the completeness of our view. a cursory survey of the field of science conveys the impression that it embraces only a constantly increasing number of disconnected specialties, in which each cultivator knows little or nothing of what is being done by others. measured by its bulk, the published mass of scientific research is increasing in a more than geometrical ratio. not only do the publications of nearly every scientific society increase in number and volume, but new and vigorous societies are constantly organized to add to the sum total. the stately quartos issued from the presses of the leading academies of europe are, in most cases, to be counted by hundreds. the philosophical transactions of the royal society already number about two hundred volumes, and the time when the memoirs of the french academy of sciences shall reach the thousand mark does not belong to the very remote future. besides such large volumes, these and other societies publish smaller ones in a constantly growing number. in addition to the publications of learned societies, there are journals devoted to each scientific specialty, which seem to propagate their species by subdivision in much the same way as some of the lower orders of animal life. every new publication of the kind is suggested by the wants of a body of specialists, who require a new medium for their researches and communications. the time has already come when we cannot assume that any specialist is acquainted with all that is being done even in his own line. to keep the run of this may well be beyond his own powers; more he can rarely attempt. what is the science of the future to do when this huge mass outgrows the space that can be found for it in the libraries, and what are we to say of the value of it all? are all these scientific researches to be classed as really valuable contributions to knowledge, or have we only a pile in which nuggets of gold are here and there to be sought for? one encouraging answer to such a question is that, taking the interests of the world as a whole, scientific investigation has paid for itself in benefits to humanity a thousand times over, and that all that is known to-day is but an insignificant fraction of what nature has to show us. apart from this, another feature of the science of our time demands attention. while we cannot hope that the multiplication of specialties will cease, we find that upon the process of differentiation and subdivision is now being superposed a form of evolution, tending towards the general unity of all the sciences, of which some examples may be pointed out. biological science, which a generation ago was supposed to be at the antipodes of exact science, is becoming more and more exact, and is cultivated by methods which are developed and taught by mathematicians. psychophysics--the study of the operations of the mind by physical apparatus of the same general nature as that used by the chemist and physicist--is now an established branch of research. a natural science which, if any comparisons are possible, may outweigh all others in importance to the race, is the rising one of "eugenics,"--the improvement of the human race by controlling the production of its offspring. no better example of the drawbacks which our country suffers as a seat of science can be given than the fact that the beginning of such a science has been possible only at the seat of a larger body of cultivated men than our land has yet been able to bring together. generations may elapse before the seed sown by mr. francis galton, from which grew the eugenic society, shall bear full fruit in the adoption of those individual efforts and social regulations necessary to the propagation of sound and healthy offspring on the part of the human family. but when this comes about, then indeed will professor lankester's "rebel against nature" find his independence acknowledged by the hitherto merciless despot that has decreed punishment for his treason. this new branch of science from which so much may be expected is the offshoot of another, the rapid growth of which illustrates the rapid invasion of the most important fields of thought by the methods of exact science. it is only a few years since it was remarked of professor karl pearson's mathematical investigations into the laws of heredity, and the biological questions associated with these laws, that he was working almost alone, because the biologists did not understand his mathematics, while the mathematicians were not interested in his biology. had he not lived at a great centre of active thought, within the sphere of influence of the two great universities of england, it is quite likely that this condition of isolation would have been his to the end. but, one by one, men were found possessing the skill and interest in the subject necessary to unite in his work, which now has not only a journal of its own, but is growing in a way which, though slow, has all the marks of healthy progress towards an end the importance of which has scarcely dawned upon the public mind. admitting that an organized association of investigators is of the first necessity to secure the best results in the scientific work of the future, we meet the question of the conditions and auspices under which they are to be brought together. the first thought to strike us at this point may well be that we have, in our great universities, organizations which include most of the leading men now engaged in scientific research, whose personnel and facilities we should utilize. admitting, as we all do, that there are already too many universities, and that better work would be done by a consolidation of the smaller ones, a natural conclusion is that the end in view will be best reached through existing organizations. but it would be a great mistake to jump at this conclusion without a careful study of the conditions. the brief argument--there are already too many institutions--instead of having more we should strengthen those we have--should not be accepted without examination. had it been accepted thirty years ago, there are at least two great american universities of to-day which would not have come into being, the means devoted to their support having been divided among others. these are the johns hopkins and the university of chicago. what would have been gained by applying the argument in these cases? the advantage would have been that, instead of so-called universities which appear to-day in the annual report of the bureau of education, we should have had only . the work of these would have been strengthened by an addition, to their resources, represented by the endowments of baltimore and chicago, and sufficient to add perhaps one professor to the staff of each. would the result have been better than it actually has been? have we not gained anything by allowing the argument to be forgotten in the cases of these two institutions? i do not believe that any who carefully look at the subject will hesitate in answering this question in the affirmative. the essential point is that the johns hopkins university did not merely add one to an already overcrowded list, but that it undertook a mission which none of the others was then adequately carrying out. if it did not plant the university idea in american soil, it at least gave it an impetus which has now made it the dominant one in the higher education of almost every state. the question whether the country at large would have reaped a greater benefit, had the professors of the university of chicago, with the appliances they now command, been distributed among fifty or a hundred institutions in every quarter of the land, than it has actually reaped from that university, is one which answers itself. our two youngest universities have attained success, not because two have thus been added to the number of american institutions of learning, but because they had a special mission, required by the advance of the age, for which existing institutions were inadequate. the conclusion to which these considerations lead is simple. no new institution is needed to pursue work on traditional lines, guided by traditional ideas. but, if a new idea is to be vigorously prosecuted, then a young and vigorous institution, specially organized to put the idea into effect, is necessary. the project of building up in our midst, at the most appropriate point, an organization of leading scientific investigators, for the single purpose of giving a new impetus to american science and, if possible, elevating the thought of the country and of the world to a higher plane, involves a new idea, which can best be realized by an institution organized for the special purpose. while this purpose is quite in line with that of the leading universities, it goes too far beyond them to admit of its complete attainment through their instrumentality. the first object of a university is the training of the growing individual for the highest duties of life. additions to the mass of knowledge have not been its principal function, nor even an important function in our own country, until a recent time. the primary object of the proposed institution is the advance of knowledge and the opening up of new lines of thought, which, it may be hoped, are to prove of great import to humanity. it does not follow that the function of teaching shall be wholly foreign to its activities. it must take up the best young men at the point where universities leave them, and train them in the arts of thinking and investigating. but this training will be beyond that which any regular university is carrying out. in pursuing our theme the question next arises as to the special features of the proposed association. the leading requirement is one that cannot be too highly emphasized. how clearly soever the organizers may have in their minds' eye the end in view, they must recognize the fact that it cannot be attained in a day. in every branch of work which is undertaken, there must be a single leader, and he must be the best that the country, perhaps even the world, can produce. the required man is not to be found without careful inquiry; in many branches he may be unattainable for years. when such is the case, wait patiently till he appears. prudence requires that the fewest possible risks would be taken, and that no leader should be chosen except one of tried experience and world-wide reputation. yet we should not leave wholly out of sight the success of the johns hopkins university in selecting, at its very foundation, young men who were to prove themselves the leaders of the future. this experience may admit of being repeated, if it be carefully borne in mind that young men of promise are to be avoided and young men of performance only to be considered. the performance need not be striking: ex pede herculem may be possible; but we must be sure of the soundness of our judgment before accepting our hercules. this requires a master. clerk-maxwell, who never left his native island to visit our shores, is entitled to honor as a promoter of american science for seeing the lion's paw in the early efforts of rowland, for which the latter was unable to find a medium of publication in his own country. it must also be admitted that the task is more serious now than it was then, because, from the constantly increasing specialization of science, it has become difficult for a specialist in one line to ascertain the soundness of work in another. with all the risks that may be involved in the proceeding, it will be quite possible to select an effective body of leaders, young and old, with whom an institution can begin. the wants of these men will be of the most varied kind. one needs scarcely more than a study and library; another must have small pieces of apparatus which he can perhaps design and make for himself. another may need apparatus and appliances so expensive that only an institution at least as wealthy as an ordinary university would be able to supply them. the apparatus required by others will be very largely human--assistants of every grade, from university graduates of the highest standing down to routine drudges and day-laborers. workrooms there must be; but it is hardly probable that buildings and laboratories of a highly specialized character will be required at the outset. the best counsel will be necessary at every step, and in this respect the institution must start from simple beginnings and grow slowly. leaders must be added one by one, each being judged by those who have preceded him before becoming in his turn a member of the body. as the body grows its members must be kept in personal touch, talk together, pull together, and act together. the writer submits these views to the great body of his fellow-citizens interested in the promotion of american science with the feeling that, though his conclusions may need amendment in details, they rest upon facts of the past and present which have not received the consideration which they merit. what he most strongly urges is that the whole subject of the most efficient method of promoting research upon a higher plane shall be considered with special reference to conditions in our own country; and that the lessons taught by the history and progress of scientific research in all countries shall be fully weighed and discussed by those most interested in making this form of effort a more important feature of our national life. when this is done, he will feel that his purpose in inviting special consideration to his individual views has been in great measure reached. xii can we make it rain? to the uncritical observer the possible achievements of invention and discovery seem boundless. half a century ago no idea could have appeared more visionary than that of holding communication in a few seconds of time with our fellows in australia, or having a talk going on viva voce between a man in washington and another in boston. the actual attainment of these results has naturally given rise to the belief that the word "impossible" has disappeared from our vocabulary. to every demonstration that a result cannot be reached the answer is, did not one lardner, some sixty years ago, demonstrate that a steamship could not cross the atlantic? if we say that for every actual discovery there are a thousand visionary projects, we are told that, after all, any given project may be the one out of the thousand. in a certain way these hopeful anticipations are justified. we cannot set any limit either to the discovery of new laws of nature or to the ingenious combination of devices to attain results which now look impossible. the science of to-day suggests a boundless field of possibilities. it demonstrates that the heat which the sun radiates upon the earth in a single day would suffice to drive all the steamships now on the ocean and run all the machinery on the land for a thousand years. the only difficulty is how to concentrate and utilize this wasted energy. from the stand-point of exact science aerial navigation is a very simple matter. we have only to find the proper combination of such elements as weight, power, and mechanical force. whenever mr. maxim can make an engine strong and light enough, and sails large, strong, and light enough, and devise the machinery required to connect the sails and engine, he will fly. science has nothing but encouraging words for his project, so far as general principles are concerned. such being the case, i am not going to maintain that we can never make it rain. but i do maintain two propositions. if we are ever going to make it rain, or produce any other result hitherto unattainable, we must employ adequate means. and if any proposed means or agency is already familiar to science, we may be able to decide beforehand whether it is adequate. let us grant that out of a thousand seemingly visionary projects one is really sound. must we try the entire thousand to find the one? by no means. the chances are that nine hundred of them will involve no agency that is not already fully understood, and may, therefore, be set aside without even being tried. to this class belongs the project of producing rain by sound. as i write, the daily journals are announcing the brilliant success of experiments in this direction; yet i unhesitatingly maintain that sound cannot make rain, and propose to adduce all necessary proof of my thesis. the nature of sound is fully understood, and so are the conditions under which the aqueous vapor in the atmosphere may be condensed. let us see how the case stands. a room of average size, at ordinary temperature and under usual conditions, contains about a quart of water in the form of invisible vapor. the whole atmosphere is impregnated with vapor in about the same proportion. we must, however, distinguish between this invisible vapor and the clouds or other visible masses to which the same term is often applied. the distinction may be very clearly seen by watching the steam coming from the spout of a boiling kettle. immediately at the spout the escaping steam is transparent and invisible; an inch or two away a white cloud is formed, which we commonly call steam, and which is seen belching out to a distance of one or more feet, and perhaps filling a considerable space around the kettle; at a still greater distance this cloud gradually disappears. properly speaking, the visible cloud is not vapor or steam at all, but minute particles or drops of water in a liquid state. the transparent vapor at the mouth of the kettle is the true vapor of water, which is condensed into liquid drops by cooling; but after being diffused through the air these drops evaporate and again become true vapor. clouds, then, are not formed of true vapor, but consist of impalpable particles of liquid water floating or suspended in the air. but we all know that clouds do not always fall as rain. in order that rain may fall the impalpable particles of water which form the cloud must collect into sensible drops large enough to fall to the earth. two steps are therefore necessary to the formation of rain: the transparent aqueous vapor in the air must be condensed into clouds, and the material of the clouds must agglomerate into raindrops. no physical fact is better established than that, under the conditions which prevail in the atmosphere, the aqueous vapor of the air cannot be condensed into clouds except by cooling. it is true that in our laboratories it can be condensed by compression. but, for reasons which i need not explain, condensation by compression cannot take place in the air. the cooling which results in the formation of clouds and rain may come in two ways. rains which last for several hours or days are generally produced by the intermixture of currents of air of different temperatures. a current of cold air meeting a current of warm, moist air in its course may condense a considerable portion of the moisture into clouds and rain, and this condensation will go on as long as the currents continue to meet. in a hot spring day a mass of air which has been warmed by the sun, and moistened by evaporation near the surface of the earth, may rise up and cool by expansion to near the freezing-point. the resulting condensation of the moisture may then produce a shower or thunder-squall. but the formation of clouds in a clear sky without motion of the air or change in the temperature of the vapor is simply impossible. we know by abundant experiments that a mass of true aqueous vapor will never condense into clouds or drops so long as its temperature and the pressure of the air upon it remain unchanged. now let us consider sound as an agent for changing the state of things in the air. it is one of the commonest and simplest agencies in the world, which we can experiment upon without difficulty. it is purely mechanical in its action. when a bomb explodes, a certain quantity of gas, say five or six cubic yards, is suddenly produced. it pushes aside and compresses the surrounding air in all directions, and this motion and compression are transmitted from one portion of the air to another. the amount of motion diminishes as the square of the distance; a simple calculation shows that at a quarter of a mile from the point of explosion it would not be one ten-thousandth of an inch. the condensation is only momentary; it may last the hundredth or the thousandth of a second, according to the suddenness and violence of the explosion; then elasticity restores the air to its original condition and everything is just as it was before the explosion. a thousand detonations can produce no more effect upon the air, or upon the watery vapor in it, than a thousand rebounds of a small boy's rubber ball would produce upon a stonewall. so far as the compression of the air could produce even a momentary effect, it would be to prevent rather than to cause condensation of its vapor, because it is productive of heat, which produces evaporation, not condensation. the popular notion that sound may produce rain is founded principally upon the supposed fact that great battles have been followed by heavy rains. this notion, i believe, is not confirmed by statistics; but, whether it is or not, we can say with confidence that it was not the sound of the cannon that produced the rain. that sound as a physical factor is quite insignificant would be evident were it not for our fallacious way of measuring it. the human ear is an instrument of wonderful delicacy, and when its tympanum is agitated by a sound we call it a "concussion" when, in fact, all that takes place is a sudden motion back and forth of a tenth, a hundredth, or a thousandth of an inch, accompanied by a slight momentary condensation. after these motions are completed the air is exactly in the same condition as it was before; it is neither hotter nor colder; no current has been produced, no moisture added. if the reader is not satisfied with this explanation, he can try a very simple experiment which ought to be conclusive. if he will explode a grain of dynamite, the concussion within a foot of the point of explosion will be greater than that which can be produced by the most powerful bomb at a distance of a quarter of a mile. in fact, if the latter can condense vapor a quarter of a mile away, then anybody can condense vapor in a room by slapping his hands. let us, therefore, go to work slapping our hands, and see how long we must continue before a cloud begins to form. what we have just said applies principally to the condensation of invisible vapor. it may be asked whether, if clouds are already formed, something may not be done to accelerate their condensation into raindrops large enough to fall to the ground. this also may be the subject of experiment. let us stand in the steam escaping from a kettle and slap our hands. we shall see whether the steam condenses into drops. i am sure the experiment will be a failure; and no other conclusion is possible than that the production of rain by sound or explosions is out of the question. it must, however, be added that the laws under which the impalpable particles of water in clouds agglomerate into drops of rain are not yet understood, and that opinions differ on this subject. experiments to decide the question are needed, and it is to be hoped that the weather bureau will undertake them. for anything we know to the contrary, the agglomeration may be facilitated by smoke in the air. if it be really true that rains have been produced by great battles, we may say with confidence that they were produced by the smoke from the burning powder rising into the clouds and forming nuclei for the agglomeration into drops, and not by the mere explosion. if this be the case, if it was the smoke and not the sound that brought the rain, then by burning gunpowder and dynamite we are acting much like charles lamb's chinamen who practised the burning of their houses for several centuries before finding out that there was any cheaper way of securing the coveted delicacy of roast pig. but how, it may be asked, shall we deal with the fact that mr. dyrenforth's recent explosions of bombs under a clear sky in texas were followed in a few hours, or a day or two, by rains in a region where rain was almost unknown? i know too little about the fact, if such it be, to do more than ask questions about it suggested by well-known scientific truths. if there is any scientific result which we can accept with confidence, it is that ten seconds after the sound of the last bomb died away, silence resumed her sway. from that moment everything in the air--humidity, temperature, pressure, and motion--was exactly the same as if no bomb had been fired. now, what went on during the hours that elapsed between the sound of the last bomb and the falling of the first drop of rain? did the aqueous vapor already in the surrounding air slowly condense into clouds and raindrops in defiance of physical laws? if not, the hours must have been occupied by the passage of a mass of thousands of cubic miles of warm, moist air coming from some other region to which the sound could not have extended. or was jupiter pluvius awakened by the sound after two thousand years of slumber, and did the laws of nature become silent at his command? when we transcend what is scientifically possible, all suppositions are admissible; and we leave the reader to take his choice between these and any others he may choose to invent. one word in justification of the confidence with which i have cited established physical laws. it is very generally supposed that most great advances in applied science are made by rejecting or disproving the results reached by one's predecessors. nothing could be farther from the truth. as huxley has truly said, the army of science has never retreated from a position once gained. men like ohm and maxwell have reduced electricity to a mathematical science, and it is by accepting, mastering, and applying the laws of electric currents which they discovered and expounded that the electric light, electric railway, and all other applications of electricity have been developed. it is by applying and utilizing the laws of heat, force, and vapor laid down by such men as carnot and regnault that we now cross the atlantic in six days. these same laws govern the condensation of vapor in the atmosphere; and i say with confidence that if we ever do learn to make it rain, it will be by accepting and applying them, and not by ignoring or trying to repeal them. how much the indisposition of our government to secure expert scientific evidence may cost it is strikingly shown by a recent example. it expended several million dollars on a tunnel and water-works for the city of washington, and then abandoned the whole work. had the project been submitted to a commission of geologists, the fact that the rock-bed under the district of columbia would not stand the continued action of water would have been immediately reported, and all the money expended would have been saved. the fact is that there is very little to excite popular interest in the advance of exact science. investigators are generally quiet, unimpressive men, rather diffident, and wholly wanting in the art of interesting the public in their work. it is safe to say that neither lavoisier, galvani, ohm, regnault, nor maxwell could have gotten the smallest appropriation through congress to help make discoveries which are now the pride of our century. they all dealt in facts and conclusions quite devoid of that grandeur which renders so captivating the project of attacking the rains in their aerial stronghold with dynamite bombs. xiii the astronomical ephemeris and the nautical almanac [footnote: read before the u s naval institute, january , .] although the nautical almanacs of the world, at the present time, are of comparatively recent origin, they have grown from small beginnings, the tracing of which is not unlike that of the origin of species by the naturalist of the present day. notwithstanding its familiar name, it has always been designed rather for astronomical than for nautical purposes. such a publication would have been of no use to the navigator before he had instruments with which to measure the altitudes of the heavenly bodies. the earlier navigators seldom ventured out of sight of land, and during the night they are said to have steered by the "cynosure" or constellation of the great bear, a practice which has brought the name of the constellation into our language of the present day to designate an object on which all eyes are intently fixed. this constellation was a little nearer the pole in former ages than at the present time; still its distance was always so great that its use as a mark of the northern point of the horizon does not inspire us with great respect for the accuracy with which the ancient navigators sought to shape their course. the nautical almanac of the present day had its origin in the astronomical ephemerides called forth by the needs of predictions of celestial motions both on the part of the astronomer and the citizen. so long as astrology had a firm hold on the minds of men, the positions of the planets were looked to with great interest. the theories of ptolemy, although founded on a radically false system, nevertheless sufficed to predict the position of the sun, moon, and planets, with all the accuracy necessary for the purposes of the daily life of the ancients or the sentences of their astrologers. indeed, if his tables were carried down to the present time, the positions of the heavenly bodies would be so few degrees in error that their recognition would be very easy. the times of most of the eclipses would be predicted within a few hours, and the conjunctions of the planets within a few days. thus it was possible for the astronomers of the middle ages to prepare for their own use, and that of the people, certain rude predictions respecting the courses of the sun and moon and the aspect of the heavens, which served the purpose of daily life and perhaps lessened the confusion arising from their complicated calendars. in the signs of the zodiac and the different effects which follow from the sun and moon passing from sign to sign, still found in our farmers' almanacs, we have the dying traces of these ancient ephemerides. the great kepler was obliged to print an astrological almanac in virtue of his position as astronomer of the court of the king of austria. but, notwithstanding the popular belief that astronomy had its origin in astrology, the astronomical writings of all ages seem to show that the astronomers proper never had any belief in astrology. to kepler himself the necessity for preparing this almanac was a humiliation to which he submitted only through the pressure of poverty. subsequent ephemerides were prepared with more practical objects. they gave the longitudes of the planets, the position of the sun, the time of rising and setting, the prediction of eclipses, etc. they have, of course, gradually increased in accuracy as the tables of the celestial motions were improved from time to time. at first they were not regular, annual publications, issued by governments, as at the present time, but the works of individual astronomers who issued their ephemerides for several years in advance, at irregular intervals. one man might issue one, two, or half a dozen such volumes, as a private work, for the benefit of his fellows, and each might cover as many years as he thought proper. the first publication of this sort, which i have in my possession, is the ephemerides of manfredi, of bonn, computed for the years to , in two volumes. of the regular annual ephemerides the earliest, so far as i am aware, is the connaissance des temps or french nautical almanac. the first issue was in the year , by picard, and it has been continued without interruption to the present time. its early numbers were, of course, very small, and meagre in their details. they were issued by the astronomers of the french academy of sciences, under the combined auspices of the academy and the government. they included not merely predictions from the tables, but also astronomical observations made at the paris observatory or elsewhere. when the bureau of longitudes was created in , the preparation of the work was intrusted to it, and has remained in its charge until the present time. as it is the oldest, so, in respect at least to number of pages, it is the largest ephemeris of the present time. the astronomical portion of the volume for fills more than seven hundred pages, while the table of geographical positions, which has always been a feature of the work, contains nearly one hundred pages more. the first issue of the british nautical almanac was that for the year and appeared in . it differs from the french almanac in owing its origin entirely to the needs of navigation. the british nation, as the leading maritime power of the world, was naturally interested in the discovery of a method by which the longitude could be found at sea. as most of my hearers are probably aware, there was, for many years, a standing offer by the british government, of ten thousand pounds for the discovery of a practical and sufficiently accurate method of attaining this object. if i am rightly informed, the requirement was that a ship should be able to determine the greenwich time within two minutes, after being six months at sea. when the office of astronomer royal was established in , the duty of the incumbent was declared to be "to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens, and the places of the fixed stars in order to find out the so much desired longitude at sea for the perfecting the art of navigation." about the middle of the last century the lunar tables were so far improved that dr. maskelyne considered them available for attaining this long-wished-for object. the method which i think was then, for the first time, proposed was the now familiar one of lunar distances. several trials of the method were made by accomplished gentlemen who considered that nothing was wanting to make it practical at sea but a nautical ephemeris. the tables of the moon, necessary for the purpose, were prepared by tobias mayer, of gottingen, and the regular annual issue of the work was commenced in , as already stated. of the reward which had been offered, three thousand pounds were paid to the widow of mayer, and three thousand pounds to the celebrated mathematician euler for having invented the methods used by mayer in the construction of his tables. the issue of the nautical ephemeris was intrusted to dr. maskelyne. like other publications of this sort this ephemeris has gradually increased in volume. during the first sixty or seventy years the data were extremely meagre, including only such as were considered necessary for the determination of positions. in the subject of improving the nautical almanac was referred by the lord commissioners of the admiralty to a committee of the astronomical society of london. a subcommittee, including eleven of the most distinguished astronomers and one scientific navigator, made an exhaustive report, recommending a radical rearrangement and improvement of the work. the recommendations of this committee were first carried into effect in the nautical almanac for the year . the arrangement of the navigator's ephemeris then devised has been continued in the british almanac to the present time. a good deal of matter has been added to the british almanac during the forty years and upwards which have elapsed, but it has been worked in rather by using smaller type and closer printing than by increasing the number of pages. the almanac for contains five hundred and seventeen pages and that for five hundred and nineteen pages. the general aspect of the page is now somewhat crowded, yet, considering the quantity of figures on each page the arrangement is marvellously clear and legible. the spanish "almanaque nautico" has been issued since the beginning of the century. like its fellows it has been gradually enlarged and improved, in recent times, and is now of about the same number of pages with the british and american almanacs. as a rule there is less matter on a page, so that the data actually given are not so complete as in some other publications. in germany two distinct publications of this class are issued, the one purely astronomical, the other purely nautical. the astronomical publication has been issued for more than a century under the title of "berliner astronomisches jahrbuch." it is intended principally for the theoretical astronomer, and in respect to matter necessary to the determinations of positions on the earth it is rather meagre. it is issued by the berlin observatory, at the expense of the government. the companion of this work, intended for the use of the german marine, is the "nautisches jahrbuch," prepared and issued under the direction of the minister of commerce and public works. it is copied largely from the british nautical almanac, and in respect to arrangement and data is similar to our american nautical almanac, prepared for the use of navigators, giving, however, more matter, but in a less convenient form. the right ascension and declination of the moon are given for every three hours instead of for every hour; one page of each month is devoted to eclipses of jupiter's satellites, phenomena which we never consider necessary in the nautical portion of our own almanac. at the end of the work the apparent positions of seventy or eighty of the brightest stars are given for every ten days, while it is considered that our own navigators will be satisfied with the mean places for the beginning of the year. at the end is a collection of tables which i doubt whether any other than a german navigator would ever use. whether they use them or not i am not prepared to say. the preceding are the principal astronomical and nautical ephemerides of the world, but there are a number of minor publications, of the same class, of which i cannot pretend to give a complete list. among them is the portuguese astronomical ephemeris for the meridian of the university of coimbra, prepared for portuguese navigators. i do not know whether the portuguese navigators really reckon their longitudes from this point: if they do the practice must be attended with more or less confusion. all the matter is given by months, as in the solar and lunar ephemeris of our own and the british almanac. for the sun we have its longitude, right ascension, and declination, all expressed in arc and not in time. the equation of time and the sidereal time of mean noon complete the ephemeris proper. the positions of the principal planets are given in no case oftener than for every third day. the longitude and latitude of the moon are given for noon and midnight. one feature not found in any other almanac is the time at which the moon enters each of the signs of the zodiac. it may be supposed that this information is designed rather for the benefit of the portuguese landsman than of the navigator. the right ascensions and declinations of the moon and the lunar distances are also given for intervals of twelve hours. only the last page gives the eclipses of the satellites of jupiter. the fixed stars are wholly omitted. an old ephemeris, and one well known in astronomy is that published by the observatory of milan, italy, which has lately entered upon the second century of its existence. its data are extremely meagre and of no interest whatever to the navigator. the greater part of the volume is taken up with observations at the milan observatory. since taking charge of the american ephemeris i have endeavored to ascertain what nautical almanacs are actually used by the principal maritime nations of europe. i have been able to obtain none except those above mentioned. as a general rule i think the british nautical almanac is used by all the northern nations, as already indicated. the german nautical jahrbuch is principally a reprint from the british. the swedish navigators, being all well acquainted with the english language, use the british almanac without change. the russian government, however, prints an explanation of the various terms in the language of their own people and binds it in at the end of the british almanac. this explanation includes translations of the principal terms used in the heading of pages, such as the names of the months and days, the different planets, constellations, and fixed stars, and the phenomena of angle and time. they have even an index of their own in which the titles of the different articles are given in russian. this explanation occupies, in all, seventy-five pages--more than double that taken up by the original explanation. one of the first considerations which strikes us in comparing these multitudinous publications is the confusion which must arise from the use of so many meridians. if each of these southern nations, the spanish and portuguese for instance, actually use a meridian of their own, the practice must lead to great confusion. if their navigators do not do so but refer their longitudes to the meridian of greenwich, then their almanacs must be as good as useless. they would find it far better to buy an ephemeris referred to the meridian of greenwich than to attempt to use their own the northern nations, i think, have all begun to refer to the meridian of greenwich, and the same thing is happily true of our own marine. we may, therefore, hope that all commercial nations will, before long, refer their longitudes to one and the same meridian, and the resulting confusion be thus avoided. the preparation of the american ephemeris and nautical almanac was commenced in , under the superintendence of the late rear-admiral, then lieutenant, charles henry davis. the first volume to be issued was that for the year . both in the preparation of that work and in the connected work of mapping the country, the question of the meridian to be adopted was one of the first importance, and received great attention from admiral davis, who made an able report on the subject. our situation was in some respects peculiar, owing to the great distance which separated us from europe and the uncertainty of the exact difference of longitude between the two continents. it was hardly practicable to refer longitudes in our own country to any european meridian. the attempt to do so would involve continual changes as the transatlantic longitude was from time to time corrected. on the other hand, in order to avoid confusion in navigation, it was essential that our navigators should continue to reckon from the meridian of greenwich. the trouble arising from uncertainty of the exact longitude does not affect the navigator, because, for his purpose, astronomical precision is not necessary. the wisest solution was probably that embodied in the act of congress, approved september , , on the recommendation of lieutenant davis, if i mistake not. "the meridian of the observatory at washington shall be adopted and used as the american meridian for all astronomical purposes, and the meridian of greenwich shall be adopted for all nautical purposes." the execution of this law necessarily involves the question, "what shall be considered astronomical and what nautical purposes?" whether it was from the difficulty of deciding this question, or from nobody's remembering the law, the latter has been practically a dead letter. surely, if there is any region of the globe which the law intended should be referred to the meridian of washington, it is the interior of our own country. yet, notwithstanding the law, all acts of congress relating to the territories have, so far as i know, referred everything to the meridian of greenwich and not to that of washington. even the maps issued by our various surveys are referred to the same transatlantic meridian. the absurdity culminated in a local map of the city of washington and the district of columbia, issued by private parties, in , in which we find even the meridians passing through the city of washington referred to a supposed greenwich. this practice has led to a confusion which may not be evident at first sight, but which is so great and permanent that it may be worth explaining. if, indeed, we could actually refer all our longitudes to an accurate meridian of greenwich in the first place; if, for instance, any western region could be at once connected by telegraph with the greenwich observatory, and thus exchange longitude signals night after night, no trouble or confusion would arise from referring to the meridian of greenwich. but this, practically, cannot be done. all our interior longitudes have been and are determined differentially by comparison with some point in this country. one of the most frequent points of reference used this way has been the cambridge observatory. suppose, then, a surveyor at omaha makes a telegraphic longitude determination between that point and the cambridge observatory. since he wants his longitude reduced to greenwich, he finds some supposed longitude of the cambridge observatory from greenwich and adds that to his own longitude. thus, what he gives is a longitude actually determined, plus an assumed longitude of cambridge, and, unless the assumed longitude of cambridge is distinctly marked on his maps, we may not know what it is. after a while a second party determines the longitude of ogden from cambridge. in the mean time, the longitude of cambridge from greenwich has been corrected, and we have a longitude of ogden which will be discordant with that of omaha, owing to the change in the longitude of cambridge. a third party determines the longitudes of, let us suppose, st. louis from washington, he adds the assumed longitudes of washington from greenwich which may not agree with either of the longitudes of cambridge and gets his longitude. thus we have a series of results for our western longitude all nominally referred to the meridian of greenwich, but actually referred to a confused collection of meridians, nobody knows what. if the law had only provided that the longitude of washington from greenwich should be invariably fixed at a certain quantity, say degrees ', this confusion would not have arisen. it is true that the longitude thus established by law might not have been perfectly correct, but this would not cause any trouble nor confusion. our longitude would have been simply referred to a certain assumed greenwich, the small error of which would have been of no importance to the navigator or astronomer. it would have differed from the present system only in that the assumed greenwich would have been invariable instead of dancing about from time to time as it has done under the present system. you understand that when the astronomer, in computing an interior longitude, supposes that of cambridge from greenwich to be a certain definite amount, say h m s, what he actually does is to count from a meridian just that far east of cambridge. when he changes the assumed longitude of cambridge he counts from a meridian farther east or farther west of his former one: in other words, he always counts from an assumed greenwich, which changes its position from time to time, relative to our own country. having two meridians to look after, the form of the american ephemeris, to be best adapted to the wants both of navigators and astronomers was necessarily peculiar. had our navigators referred their longitudes to any meridian of our own country the arrangement of the work need not have differed materially from that of foreign ones. but being referred to a meridian far outside our limits and at the same time designed for use within those limits, it was necessary to make a division of the matter. accordingly, the american ephemeris has always been divided into two parts: the first for the use of navigators, referred to the meridian of greenwich, the second for that of astronomers, referred to the meridian of washington. the division of the matter without serious duplication is more easy than might at first be imagined. in explaining it, i will take the ephemeris as it now is, with the small changes which have been made from time to time. one of the purposes of any ephemeris, and especially of that of the navigators, is to give the position of the heavenly bodies at equidistant intervals of time, usually one day. since it is noon at some point of the earth all the time, it follows that such an ephemeris will always be referred to noon at some meridian. what meridian this shall be is purely a practical question, to be determined by convenience and custom. greenwich noon, being that necessarily used by the navigator, is adopted as the standard, but we must not conclude that the ephemeris for greenwich noon is referred to the meridian of greenwich in the sense that we refer a longitude to that meridian. greenwich noon is h m s, washington mean time; so the ephemeris which gives data for every greenwich noon may be considered as referred to the meridian of washington giving the data for h m s, washington time, every day. the rule adopted, therefore, is to have all the ephemerides which refer to absolute time, without any reference to a meridian, given for greenwich noon, unless there may be some special reason to the contrary. for the needs of the navigator and the theoretical astronomer these are the most convenient epochs. another part of the ephemeris gives the position of the heavenly bodies, not at equidistant intervals, but at transit over some meridian. for this purpose the meridian of washington is chosen for obvious reasons. the astronomical part of our ephemeris, therefore, gives the positions of the principal fixed stars, the sun, moon, and all the larger planets at the moment of transit over our own meridian. the third class of data in the ephemeris comprises phenomena to be predicted and observed. such are eclipses of the sun and moon, occultations of fixed stars by the moon, and eclipses of jupiter's satellites. these phenomena are all given in washington mean time as being most convenient for observers in our own country. there is a partial exception, however, in the case of eclipses of the sun and moon. the former are rather for the world in general than for our own country, and it was found difficult to arrange them to be referred to the meridian of washington without having the maps referred to the same meridian. since, however, the meridian of greenwich is most convenient outside of our own territory, and since but a small portion of the eclipses are visible within it, it is much the best to have the eclipses referred entirely to the meridian of greenwich. i am the more ready to adopt this change because when the eclipses are to be computed for our own country the change of meridians will be very readily understood by those who make the computation. it may be interesting to say something of the tables and theories from which the astronomical ephemerides are computed. to understand them completely it is necessary to trace them to their origin. the problem of calculating the motions of the heavenly bodies and the changes in the aspect of the celestial sphere was one of the first with which the students of astronomy were occupied. indeed, in ancient times, the only astronomical problems which could be attacked were of this class, for the simple reason that without the telescope and other instruments of research it was impossible to form any idea of the physical constitution of the heavenly bodies. to the ancients the stars and planets were simply points or surfaces in motion. they might have guessed that they were globes like that on which we live, but they were unable to form any theory of the nature of these globes. thus, in the almagest of ptolemy, the most complete treatise on the ancient astronomy which we possess, we find the motions of all the heavenly bodies carefully investigated and tables given for the convenient computation of their positions. crude and imperfect though these tables may be, they were the beginnings from which those now in use have arisen. no radical change was made in the general principles on which these theories and tables were constructed until the true system of the world was propounded by copernicus. on this system the apparent motion of each planet in the epicycle was represented by a motion of the earth around the sun, and the problem of correcting the position of the planet on account of the epicycle was reduced to finding its geocentric from its heliocentric position. this was the greatest step ever taken in theoretical astronomy, yet it was but a single step. so far as the materials were concerned and the mode of representing the planetary motions, no other radical advance was made by copernicus. indeed, it is remarkable that he introduced an epicycle which was not considered necessary by ptolemy in order to represent the inequalities in the motions of the planets around the sun. the next great advance made in the theory of the planetary motion was the discovery by kepler of the celebrated laws which bear his name. when it was established that each planet moved in an ellipse having the sun in one focus it became possible to form tables of the motions of the heavenly bodies much more accurate than had before been known. such tables were published by kepler in , under the name of rudolphine tables, in memory of his patron, the emperor rudolph. but the laws of kepler took no account of the action of the planets on one another. it is well known that if each planet moved only under the influence of the gravitating force of the sun its motion would accord rigorously with the laws of kepler, and the problems of theoretical astronomy would be greatly simplified. when, therefore, the results of kepler's laws were compared with ancient and modern observations it was found that they were not exactly represented by the theory. it was evident that the elliptic orbits of the planets were subject to change, but it was entirely beyond the power of investigation, at that time, to assign any cause for such changes. notwithstanding the simplicity of the causes which we now know to produce them, they are in form extremely complex. without the knowledge of the theory of gravitation it would be entirely out of the question to form any tables of the planetary motions which would at all satisfy our modern astronomers. when the theory of universal gravitation was propounded by newton he showed that a planet subjected only to the gravitation of a central body, like the sun, would move in exact accordance with kepler's laws. but by his theory the planets must attract one another and these attractions must cause the motions of each to deviate slightly from the laws in question. since such deviations were actually observed it was very natural to conclude that they were due to this cause, but how shall we prove it? to do this with all the rigor required in a mathematical investigation it is necessary to calculate the effect of the mutual action of the planets in changing their orbits. this calculation must be made with such precision that there shall be no doubt respecting the results of the theory. then its results must be compared with the best observations. if the slightest outstanding difference is established there is something wrong and the requirements of astronomical science are not satisfied. the complete solution of this problem was entirely beyond the power of newton. when his methods of research were used he was indeed able to show that the mutual action of the planets would produce deviations in their motions of the same general nature with those observed, but he was not able to calculate these deviations with numerical exactness. his most successful attempt in this direction was perhaps made in the case of the moon. he showed that the sun's disturbing force on this body would produce several inequalities the existence of which had been established by observation, and he was also able to give a rough estimate of their amount, but this was as far as his method could go. a great improvement had to be made, and this was effected not by english, but by continental mathematicians. the latter saw, clearly, that it was impossible to effect the required solution by the geometrical mode of reasoning employed by newton. the problem, as it presented itself to their minds, was to find algebraic expressions for the positions of the planets at any time. the latitude, longitude, and radius-vector of each planet are constantly varying, but they each have a determined value at each moment of time. they may therefore be regarded as functions of the time, and the problem was to express these functions by algebraic formulae. these algebraic expressions would contain, besides the time, the elements of the planetary orbits to be derived from observation. the time which we may suppose to be represented algebraically by the symbol t, would remain as an unknown quantity to the end. what the mathematician sought to do was to present the astronomer with a series of algebraic expressions containing t as an indeterminate quantity, and so, by simply substituting for t any year and fraction of a year whatever-- , , , for example, the result would give the latitude, longitude, or radius-vector of a planet. the problem as thus presented was one of the most difficult we can perceive of, but the difficulty was only an incentive to attacking it with all the greater energy. so long as the motion was supposed purely elliptical, so long as the action of the planets was neglected, the problem was a simple one, requiring for its solution only the analytic geometry of the ellipse. the real difficulties commenced when the mutual action of the planets was taken into account. it is, of course, out of the question to give any technical description or analysis of the processes which have been invented for solving the problem; but a brief historical sketch may not be out of place. a complete and rigorous solution of the problem is out of the question--that is, it is impossible by any known method to form an algebraic expression for the co-ordinates of a planet which shall be absolutely exact in a mathematical sense. in whatever way we go to work the expression comes out in the form of an infinite series of terms, each term being, on the whole, a little smaller as we increase the number. so, by increasing the number of these various terms, we can approach nearer and nearer to a mathematical exactness, but can never reach it. the mathematician and astronomer have to be satisfied when they have carried the solution so far that the neglected quantities are entirely beyond the powers of observation. mathematicians have worked upon the problem in its various phases for nearly two centuries, and many improvements in detail have, from time to time, been made, but no general method, applicable to all cases, has been devised. one plan is to be used in treating the motion of the moon, another for the interior planets, another for jupiter and saturn, another for the minor planets, and so on. under these circumstances it will not surprise you to learn that our tables of the celestial motions do not, in general, correspond in accuracy to the present state of practical astronomy. there is no authority and no office in the world whose duty it is to look after the preparations of the formulae i have described. the work of computing them has been almost entirely left to individual mathematicians whose taste lay in that direction, and who have sometimes devoted the greater part of their lives to calculations on a single part of the work. as a striking instance of this, the last great work on the motion of the moon, that of delaunay, of paris, involved some fifteen years of continuous hard labor. hansen, of germany, who died five years ago, devoted almost his whole life to investigations of this class and to the development of new methods of computation. his tables of the moon are those now used for predicting the places of the moon in all the ephemerides of the world. the only successful attempt to prepare systematic tables for all the large planets is that completed by le verrier just before his death; but he used only a small fraction of the material at his disposal, and did not employ the modern methods, confining himself wholly to those invented by his countrymen about the beginning of the present century. for him jacobi and hansen had lived in vain. the great difficulty which besets the subject arises from the fact that mathematical processes alone will not give us the position of a planet, there being seven unknown quantities for each planet which must be determined by observations. a planet, for instance, may move in any ellipse whatever, having the sun in one focus, and it is impossible to tell what ellipse it is, except from observation. the mean motion of a planet, or its period of revolution, can only be determined by a long series of observations, greater accuracy being obtained the longer the observations are continued. before the time of bradley, who commenced work at the greenwich observatory about , the observations were so far from accurate that they are now of no use whatever, unless in exceptional cases. even bradley's observations are in many cases far less accurate than those made now. in consequence, we have not heretofore had a sufficiently extended series of observations to form an entirely satisfactory theory of the celestial motions. as a consequence of the several difficulties and drawbacks, when the computation of our ephemeris was started, in the year , there were no tables which could be regarded as really satisfactory in use. in the british nautical almanac the places of the moon were derived from the tables of burckhardt published in the year . you will understand, in a case like this, no observations subsequent to the issue of the tables are made use of; the place of the moon of any day, hour, and minute of greenwich time, mean time, was precisely what burckhardt would have computed nearly a half a century before. of the tables of the larger planets the latest were those of bouvard, published in , while the places of venus were from tables published by lindenau in . of course such tables did not possess astronomical accuracy. at that time, in the case of the moon, completely new tables were constructed from the results reached by professor airy in his reduction of the greenwich observations of the moon from to . these were constructed under the direction of professor pierce and represented the places of the moon with far greater accuracy than the older tables of burckhardt. for the larger planets corrections were applied to the older tables to make them more nearly represent observations before new ones were constructed. these corrections, however, have not proved satisfactory, not being founded on sufficiently thorough investigations. indeed, the operation of correcting tables by observation, as we would correct the dead-reckoning of a ship, is a makeshift, the result of which must always be somewhat uncertain, and it tends to destroy that unity which is an essential element of the astronomical ephemeris designed for permanent future use. the result of introducing them, while no doubt an improvement on the old tables, has not been all that should be desired. the general lack of unity in the tables hitherto employed is such that i can only state what has been done by mentioning each planet in detail. for mercury, new tables were constructed by professor winlock, from formulae published by le verrier in . these tables have, however, been deviating from the true motion of the planet, owing to the motion of the perihelion of mercury, subsequently discovered by le verrier himself. they are now much less accurate than the newer tables published by le verrier ten years later. of venus new tables were constructed by mr. hill in . they are more accurate than any others, being founded on later data than those of le verrier, and are therefore satisfactory so far as accuracy of prediction is concerned. the place of mars, jupiter, and saturn are still computed from the old tables, with certain necessary corrections to make them better represent observations. the places of uranus and neptune are derived from new tables which will probably be sufficiently accurate for some time to come. for the moon, pierce's tables have been employed up to the year inclusive. commencing with the ephemeris for the year , hansen's tables are introduced with corrections to the mean longitude founded on two centuries of observation. with so great a lack of uniformity, and in the absence of any existing tables which have any other element of unity than that of being the work of the same authors, it is extremely desirable that we should be able to compute astronomical ephemerides from a single uniform and consistent set of astronomical data. i hope, in the course of years, to render this possible. when our ephemeris was first commenced, the corrections applied to existing tables rendered it more accurate than any other. since that time, the introduction into foreign ephemerides of the improved tables of le verrier have rendered them, on the whole, rather more accurate than our own. in one direction, however, our ephemeris will hereafter be far ahead of all others. i mean in its positions of the fixed stars. this portion of it is of particular importance to us, owing to the extent to which our government is engaged in the determination of positions on this continent, and especially in our western territories. although the places of the stars are determined far more easily than those of the planets, the discussion of star positions has been in almost as backward a state as planetary positions. the errors of old observers have crept in and been continued through two generations of astronomers. a systematic attempt has been made to correct the places of the stars for all systematic errors of this kind, and the work of preparing a catalogue of stars which shall be completely adapted to the determination of time and longitude, both in the fixed observatory and in the field, is now approaching completion. the catalogue cannot be sufficiently complete to give places of the stars for determining the latitude by the zenith telescope, because for such a purpose a much greater number of stars is necessary than can be incorporated in the ephemeris. from what i have said, it will be seen that the astronomical tables, in general, do not satisfy the scientific condition of completely representing observations to the last degree of accuracy. few, i think, have an idea how unsystematically work of this kind has hitherto been performed. until very lately the tables we have possessed have been the work of one man here, another there, and another one somewhere else, each using different methods and different data. the result of this is that there is nothing uniform and systematic among them, and that they have every range of precision. this is no doubt due in part to the fact that the construction of such tables, founded on the mass of observation hitherto made, is entirely beyond the power of any one man. what is wanted is a number of men of different degrees of capacity, all co-operating on a uniform system, so as to obtain a uniform result, like the astronomers in a large observatory. the greenwich observatory presents an example of co-operative work of this class extending over more than a century. but it has never extended its operations far outside the field of observation, reduction, and comparison with existing tables. it shows clearly, from time to time, the errors of the tables used in the british nautical almanac, but does nothing further, occasional investigations excepted, in the way of supplying new tables. an exception to this is a great work on the theory of the moon's motion, in which professor airy is now engaged. it will be understood that several distinct conditions not yet fulfilled are desirable in astronomical tables; one is that each set of tables shall be founded on absolutely consistent data, for instance, that the masses of the planets shall be the same throughout. another requirement is that this data shall be as near the truth as astronomical data will suffice to determine them. the third is that the results shall be correct in theory. that is, whether they agree or disagree with observations, they shall be such as result mathematically from the adopted data. tables completely fulfilling these conditions are still a work of the future. it is yet to be seen whether such co-operation as is necessary to their production can be secured under any arrangement whatever. xiv the world's debt to astronomy astronomy is more intimately connected than any other science with the history of mankind. while chemistry, physics, and we might say all sciences which pertain to things on the earth, are comparatively modern, we find that contemplative men engaged in the study of the celestial motions even before the commencement of authentic history. the earliest navigators of whom we know must have been aware that the earth was round. this fact was certainly understood by the ancient greeks and egyptians, as well as it is at the present day. true, they did not know that the earth revolved on its axis, but thought that the heavens and all that in them is performed a daily revolution around our globe, which was, therefore, the centre of the universe. it was the cynosure, or constellation of the little bear, by which the sailors used to guide their ships before the discovery of the mariner's compass. thus we see both a practical and contemplative side to astronomy through all history. the world owes two debts to that science: one for its practical uses, and the other for the ideas it has afforded us of the immensity of creation. the practical uses of astronomy are of two kinds: one relates to geography; the other to times, seasons, and chronology. every navigator who sails long out of sight of land must be something of an astronomer. his compass tells him where are east, west, north, and south, but it gives him no information as to where on the wide ocean he may be, or whither the currents may be carrying him. even with the swiftest modern steamers it is not safe to trust to the compass in crossing the atlantic. a number of years ago the steamer city of washington set out on her usual voyage from liverpool to new york. by rare bad luck the weather was stormy or cloudy during her whole passage, so that the captain could not get a sight on the sun, and therefore had to trust to his compass and his log-line, the former telling him in what direction he had steamed, and the latter how fast he was going each hour. the result was that the ship ran ashore on the coast of nova scotia, when the captain thought he was approaching nantucket. not only the navigator but the surveyor in the western wilds must depend on astronomical observations to learn his exact position on the earth's surface, or the latitude and longitude of the camp which he occupies. he is able to do this because the earth is round, and the direction of the plumb-line not exactly the same at any two places. let us suppose that the earth stood still, so as not to revolve on its axis at all. then we should always see the stars at rest and the star which was in the zenith of any place, say a farm-house in new york, at any time, would be there every night and every hour of the year. now the zenith is simply the point from which the plumb-line seems to drop. lie on the ground; hang a plummet above your head, sight on the line with one eye, and the direction of the sight will be the zenith of your place. suppose the earth was still, and a certain star was at your zenith. then if you went to another place a mile away, the direction of the plumb-line would be slightly different. the change would, indeed, be very small, so small that you could not detect it by sighting with the plumb-line. but astronomers and surveyors have vastly more accurate instruments than the plumb-line and the eye, instruments by which a deviation that the unaided eye could not detect can be seen and measured. instead of the plumb-line they use a spirit-level or a basin of quicksilver. the surface of quicksilver is exactly level and so at right angles to the true direction of the plumb-line or the force of gravity. its direction is therefore a little different at two different places on the surface, and the change can be measured by its effect on the apparent direction of a star seen by reflection from the surface. it is true that a considerable distance on the earth's surface will seem very small in its effect on the position of a star. suppose there were two stars in the heavens, the one in the zenith of the place where you now stand, and the other in the zenith of a place a mile away. to the best eye unaided by a telescope those two stars would look like a single one. but let the two places be five miles apart, and the eye could see that there were two of them. a good telescope could distinguish between two stars corresponding to places not more than a hundred feet apart. the most exact measurements can determine distances ranging from thirty to sixty feet. if a skilful astronomical observer should mount a telescope on your premises, and determine his latitude by observations on two or three evenings, and then you should try to trick him by taking up the instrument and putting it at another point one hundred feet north or south, he would find out that something was wrong by a single night's work. within the past three years a wobbling of the earth's axis has been discovered, which takes place within a circle thirty feet in radius and sixty feet in diameter. its effect was noticed in astronomical observations many years ago, but the change it produced was so small that men could not find out what the matter was. the exact nature and amount of the wobbling is a work of the exact astronomy of the present time. we cannot measure across oceans from island to island. until a recent time we have not even measured across the continent, from new york to san francisco, in the most precise way. without astronomy we should know nothing of the distance between new york and liverpool, except by the time which it took steamers to run it, a measure which would be very uncertain indeed. but by the aid of astronomical observations and the atlantic cables the distance is found within a few hundred yards. without astronomy we could scarcely make an accurate map of the united states, except at enormous labor and expense, and even then we could not be sure of its correctness. but the practical astronomer being able to determine his latitude and longitude within fifty yards, the positions of the principal points in all great cities of the country are known, and can be laid down on maps. the world has always had to depend on astronomy for all its knowledge concerning times and seasons. the changes of the moon gave us the first month, and the year completes its round as the earth travels in its orbit. the results of astronomical observation are for us condensed into almanacs, which are now in such universal use that we never think of their astronomical origin. but in ancient times people had no almanacs, and they learned the time of year, or the number of days in the year, by observing the time when sirius or some other bright star rose or set with the sun, or disappeared from view in the sun's rays. at alexandria, in egypt, the length of the year was determined yet more exactly by observing when the sun rose exactly in the east and set exactly in the west, a date which fixed the equinox for them as for us. more than seventeen hundred years ago, ptolemy, the great author of the almagest, had fixed the length of the year to within a very few minutes. he knew it was a little less than / days. the dates of events in ancient history depend very largely on the chronological cycles of astronomy. eclipses of the sun and moon sometimes fixed the date of great events, and we learn the relation of ancient calendars to our own through the motions of the earth and moon, and can thus measure out the years for the events in ancient history on the same scale that we measure out our own. at the present day, the work of the practical astronomer is made use of in our daily life throughout the whole country in yet another way. our fore-fathers had to regulate their clocks by a sundial, or perhaps by a mark at the corner of the house, which showed where the shadow of the house fell at noon. very rude indeed was this method; and it was uncertain for another reason. it is not always exactly twenty-four hours between two noons by the sun, sometimes for two or three months the sun will make it noon earlier and earlier every day; and during several other months later and later every day. the result is that, if a clock is perfectly regulated, the sun will be sometimes a quarter of an hour behind it, and sometimes nearly the same amount before it. any effort to keep the clock in accord with this changing sun was in vain, and so the time of day was always uncertain. now, however, at some of the principal observatories of the country astronomical observations are made on every clear night for the express purpose of regulating an astronomical clock with the greatest exactness. every day at noon a signal is sent to various parts of the country by telegraph, so that all operators and railway men who hear that signal can set their clock at noon within two or three seconds. people who live near railway stations can thus get their time from it, and so exact time is diffused into every household of the land which is at all near a railway station, without the trouble of watching the sun. thus increased exactness is given to the time on all our railroads, increased safety is obtained, and great loss of time saved to every one. if we estimated the money value of this saving alone we should no doubt find it to be greater than all that our study of astronomy costs. it must therefore be conceded that, on the whole, astronomy is a science of more practical use than one would at first suppose. to the thoughtless man, the stars seem to have very little relation to his daily life; they might be forever hid from view without his being the worse for it. he wonders what object men can have in devoting themselves to the study of the motions or phenomena of the heavens. but the more he looks into the subject, and the wider the range which his studies include, the more he will be impressed with the great practical usefulness of the science of the heavens. and yet i think it would be a serious error to say that the world's greatest debt to astronomy was owing to its usefulness in surveying, navigation, and chronology. the more enlightened a man is, the more he will feel that what makes his mind what it is, and gives him the ideas of himself and creation which he possesses, is more important than that which gains him wealth. i therefore hold that the world's greatest debt to astronomy is that it has taught us what a great thing creation is, and what an insignificant part of the creator's work is this earth on which we dwell, and everything that is upon it. that space is infinite, that wherever we go there is a farther still beyond it, must have been accepted as a fact by all men who have thought of the subject since men began to think at all. but it is very curious how hard even the astronomers found it to believe that creation is as large as we now know it to be. the greeks had their gods on or not very far above olympus, which was a sort of footstool to the heavens. sometimes they tried to guess how far it probably was from the vault of heaven to the earth, and they had a myth as to the time it took vulcan to fall. ptolemy knew that the moon was about thirty diameters of the earth distant from us, and he knew that the sun was many times farther than the moon; he thought it about twenty times as far, but could not be sure. we know that it is nearly four hundred times as far. when copernicus propounded the theory that the earth moved around the sun, and not the sun around the earth, he was able to fix the relative distances of the several planets, and thus make a map of the solar system. but he knew nothing about the scale of this map. he knew, for example, that venus was a little more than two-thirds the distance of the earth from the sun, and that mars was about half as far again as the earth, jupiter about five times, and saturn about ten times; but he knew nothing about the distance of any one of them from the sun. he had his map all right, but he could not give any scale of miles or any other measurements upon it. the astronomers who first succeeded him found that the distance was very much greater than had formerly been supposed; that it was, in fact, for them immeasurably great, and that was all they could say about it. the proofs which copernicus gave that the earth revolved around the sun were so strong that none could well doubt them. and yet there was a difficulty in accepting the theory which seemed insuperable. if the earth really moved in so immense an orbit as it must, then the stars would seem to move in the opposite direction, just as, if you were in a train that is shunting off cars one after another, as the train moves back and forth you see its motion in the opposite motion of every object around you. if then the earth at one side of its orbit was exactly between two stars, when it moved to the other side of its orbit it would not be in a line between them, but each star would have seemed to move in the opposite direction. for centuries astronomers made the most exact observations that they were able without having succeeded in detecting any such apparent motion among the stars. here was a mystery which they could not solve. either the copernican system was not true, after all, and the earth did not move in an orbit, or the stars were at such immense distances that the whole immeasurable orbit of the earth is a mere point in comparison. philosophers could not believe that the creator would waste room by allowing the inconceivable spaces which appeared to lie between our system and the fixed stars to remain unused, and so thought there must be something wrong in the theory of the earth's motion. not until the nineteenth century was well in progress did the most skilful observers of their time, bessel and struve, having at command the most refined instruments which science was then able to devise, discover the reality of the parallax of the stars, and show that the nearest of these bodies which they could find was more than , times as far as the , , of miles which separate the earth from the sun. during the half-century and more which has elapsed since this discovery, astronomers have been busily engaged in fathoming the heavenly depths. the nearest star they have been able to find is about , times the sun's distance. a dozen or a score more are within , , times that distance. beyond this all is unfathomable by any sounding-line yet known to man. the results of these astronomical measures are stupendous beyond conception. no mere statement in numbers conveys any idea of it. nearly all the brighter stars are known to be flying through space at speeds which generally range between ten and forty or fifty miles per second, some slower and some swifter, even up to one or two hundred miles a second. such a speed would carry us across the atlantic while we were reading two or three of these sentences. these motions take place some in one direction and some in another. some of the stars are coming almost straight towards us. should they reach us, and pass through our solar system, the result would be destructive to our earth, and perhaps to our sun. are we in any danger? no, because, however madly they may come, whether ten, twenty, or one hundred miles per second, so many millions of years must elapse before they reach us that we need give ourselves no concern in the matter. probably none of them are coming straight to us; their course deviates just a hair's-breadth from our system, but that hair's-breadth is so large a quantity that when the millions of years elapse their course will lie on one side or the other of our system and they will do no harm to our planet; just as a bullet fired at an insect a mile away would be nearly sure to miss it in one direction or the other. our instrument makers have constructed telescopes more and more powerful, and with these the whole number of stars visible is carried up into the millions, say perhaps to fifty or one hundred millions. for aught we know every one of those stars may have planets like our own circling round it, and these planets may be inhabited by beings equal to ourselves. to suppose that our globe is the only one thus inhabited is something so unlikely that no one could expect it. it would be very nice to know something about the people who may inhabit these bodies, but we must await our translation to another sphere before we can know anything on the subject. meanwhile, we have gained what is of more value than gold or silver; we have learned that creation transcends all our conceptions, and our ideas of its author are enlarged accordingly. xv an astronomical friendship there are few men with whom i would like so well to have a quiet talk as with father hell. i have known more important and more interesting men, but none whose acquaintance has afforded me a serener satisfaction, or imbued me with an ampler measure of a feeling that i am candid enough to call self-complacency. the ties that bind us are peculiar. when i call him my friend, i do not mean that we ever hobnobbed together. but if we are in sympathy, what matters it that he was dead long before i was born, that he lived in one century and i in another? such differences of generation count for little in the brotherhood of astronomy, the work of whose members so extends through all time that one might well forget that he belongs to one century or to another. father hell was an astronomer. ask not whether he was a very great one, for in our science we have no infallible gauge by which we try men and measure their stature. he was a lover of science and an indefatigable worker, and he did what in him lay to advance our knowledge of the stars. let that suffice. i love to fancy that in some other sphere, either within this universe of ours or outside of it, all who have successfully done this may some time gather and exchange greetings. should this come about there will be a few--hipparchus and ptolemy, copernicus and newton, galileo and herschel--to be surrounded by admiring crowds. but these men will have as warm a grasp and as kind a word for the humblest of their followers, who has merely discovered a comet or catalogued a nebula, as for the more brilliant of their brethren. my friend wrote the letters s. j. after his name. this would indicate that he had views and tastes which, in some points, were very different from my own. but such differences mark no dividing line in the brotherhood of astronomy. my testimony would count for nothing were i called as witness for the prosecution in a case against the order to which my friend belonged. the record would be very short: deponent saith that he has at various times known sundry members of the said order; and that they were lovers of sound learning, devoted to the discovery and propagation of knowledge; and further deponent saith not. if it be true that an undevout astronomer is mad, then was father hell the sanest of men. in his diary we find entries like these: "benedicente deo, i observed the sun on the meridian to-day.... deo quoque benedicente, i to-day got corresponding altitudes of the sun's upper limb." how he maintained the simplicity of his faith in the true spirit of the modern investigator is shown by his proceedings during a momentous voyage along the coast of norway, of which i shall presently speak. he and his party were passengers on a norwegian vessel. for twelve consecutive days they had been driven about by adverse storms, threatened with shipwreck on stony cliffs, and finally compelled to take refuge in a little bay, with another ship bound in the same direction, there to wait for better weather. father hell was philosopher enough to know that unusual events do not happen without cause. perhaps he would have undergone a week of storm without its occurring to him to investigate the cause of such a bad spell of weather. but when he found the second week approaching its end and yet no sign of the sun appearing or the wind abating, he was satisfied that something must be wrong. so he went to work in the spirit of the modern physician who, when there is a sudden outbreak of typhoid fever, looks at the wells and examines their water with the microscope to find the microbes that must be lurking somewhere. he looked about, and made careful inquiries to find what wickedness captain and crew had been guilty of to bring such a punishment. success soon rewarded his efforts. the king of denmark had issued a regulation that no fish or oil should be sold along the coast except by the regular dealers in those articles. and the vessel had on board contraband fish and blubber, to be disposed of in violation of this law. the astronomer took immediate and energetic measures to insure the public safety. he called the crew together, admonished them of their sin, the suffering they were bringing on themselves, and the necessity of getting back to their families. he exhorted them to throw the fish overboard, as the only measure to secure their safety. in the goodness of his heart, he even offered to pay the value of the jettison as soon as the vessel reached drontheim. but the descendants of the vikings were stupid and unenlightened men--"educatione sua et professione homines crassissimi"--and would not swallow the medicine so generously offered. they claimed that, as they had bought the fish from the russians, their proceedings were quite lawful. as for being paid to throw the fish overboard, they must have spot cash in advance or they would not do it. after further fruitless conferences, father hell determined to escape the danger by transferring his party to the other vessel. they had not more than got away from the wicked crew than heaven began to smile on their act--"factum comprobare deus ipse videtur"--the clouds cleared away, the storm ceased to rage, and they made their voyage to copenhagen under sunny skies. i regret to say that the narrative is silent as to the measure of storm subsequently awarded to the homines crassissimi of the forsaken vessel. for more than a century father hell had been a well-known figure in astronomical history. his celebrity was not, however, of such a kind as the royal astronomer of austria that he was ought to enjoy. a not unimportant element in his fame was a suspicion of his being a black sheep in the astronomical flock. he got under this cloud through engaging in a trying and worthy enterprise. on june , , an event occurred which had for generations been anticipated with the greatest interest by the whole astronomical world. this was a transit of venus over the disk of the sun. our readers doubtless know that at that time such a transit afforded the most accurate method known of determining the distance of the earth from the sun. to attain this object, parties were sent to the most widely separated parts of the globe, not only over wide stretches of longitude, but as near as possible to the two poles of the earth. one of the most favorable and important regions of observation was lapland, and the king of denmark, to whom that country then belonged, interested himself in getting a party sent thither. after a careful survey of the field he selected father hell, chief of the observatory at vienna, and well known as editor and publisher of an annual ephemeris, in which the movements and aspects of the heavenly bodies were predicted. the astronomer accepted the mission and undertook what was at that time a rather hazardous voyage. his station was at vardo in the region of the north cape. what made it most advantageous for the purpose was its being situated several degrees within the arctic circle, so that on the date of the transit the sun did not set. the transit began when the sun was still two or three hours from his midnight goal, and it ended nearly an equal time afterwards. the party consisted of hell himself, his friend and associate, father sajnovics, one dominus borgrewing, of whom history, so far as i know, says nothing more, and an humble individual who in the record receives no other designation than "familias." this implies, we may suppose, that he pitched the tent and made the coffee. if he did nothing but this we might pass him over in silence. but we learn that on the day of the transit he stood at the clock and counted the all-important seconds while the observations were going on. the party was favored by cloudless weather, and made the required observations with entire success. they returned to copenhagen, and there father hell remained to edit and publish his work. astronomers were naturally anxious to get the results, and showed some impatience when it became known that hell refused to announce them until they were all reduced and printed in proper form under the auspices of his royal patron. while waiting, the story got abroad that he was delaying the work until he got the results of observations made elsewhere, in order to "doctor" his own and make them fit in with the others. one went so far as to express a suspicion that hell had not seen the transit at all, owing to clouds, and that what he pretended to publish were pure fabrications. but his book came out in a few months in such good form that this suspicion was evidently groundless. still, the fears that the observations were not genuine were not wholly allayed, and the results derived from them were, in consequence, subject to some doubt. hell himself considered the reflections upon his integrity too contemptible to merit a serious reply. it is said that he wrote to some one offering to exhibit his journal free from interlineations or erasures, but it does not appear that there is any sound authority for this statement. what is of some interest is that he published a determination of the parallax of the sun based on the comparison of his own observations with those made at other stations. the result was ". . it was then, and long after, supposed that the actual value of the parallax was about ". , and the deviation of hell's result from this was considered to strengthen the doubt as to the correctness of his work. it is of interest to learn that, by the most recent researches, the number in question must be between ". and ". , so that in reality hell's computations came nearer the truth than those generally current during the century following his work. thus the matter stood for sixty years after the transit, and for a generation after father hell had gone to his rest. about it was found that the original journal of his voyage, containing the record of his work as first written down at the station, was still preserved at the vienna observatory. littrow, then an astronomer at vienna, made a critical examination of this record in order to determine whether it had been tampered with. his conclusions were published in a little book giving a transcript of the journal, a facsimile of the most important entries, and a very critical description of the supposed alterations made in them. he reported in substance that the original record had been so tampered with that it was impossible to decide whether the observations as published were genuine or not. the vital figures, those which told the times when venus entered upon the sun, had been erased, and rewritten with blacker ink. this might well have been done after the party returned to copenhagen. the case against the observer seemed so well made out that professors of astronomy gave their hearers a lesson in the value of truthfulness, by telling them how father hell had destroyed what might have been very good observations by trying to make them appear better than they really were. in i paid a visit to vienna for the purpose of examining the great telescope which had just been mounted in the observatory there by grubb, of dublin. the weather was so unfavorable that it was necessary to remain two weeks, waiting for an opportunity to see the stars. one evening i visited the theatre to see edwin booth, in his celebrated tour over the continent, play king lear to the applauding viennese. but evening amusements cannot be utilized to kill time during the day. among the works i had projected was that of rediscussing all the observations made on the transits of venus which had occurred in and , by the light of modern discovery. as i have already remarked, hell's observations were among the most important made, if they were only genuine. so, during my almost daily visits to the observatory, i asked permission of the director to study hell's manuscript, which was deposited in the library of the institution. permission was freely given, and for some days i pored over the manuscript. it is a very common experience in scientific research that a subject which seems very unpromising when first examined may be found more and more interesting as one looks further into it. such was the case here. for some time there did not seem any possibility of deciding the question whether the record was genuine. but every time i looked at it some new point came to light. i compared the pages with littrow's published description and was struck by a seeming want of precision, especially when he spoke of the ink with which the record had been made. erasers were doubtless unknown in those days--at least our astronomer had none on his expedition--so when he found he had written the wrong word he simply wiped the place off with, perhaps, his finger and wrote what he wanted to say. in such a case littrow described the matter as erased and new matter written. when the ink flowed freely from the quill pen it was a little dark. then littrow said a different kind of ink had been used, probably after he had got back from his journey. on the other hand, there was a very singular case in which there had been a subsequent interlineation in ink of quite a different tint, which littrow said nothing about. this seemed so curious that i wrote in my notes as follows: "that littrow, in arraying his proofs of hell's forgery, should have failed to dwell upon the obvious difference between this ink and that with which the alterations were made leads me to suspect a defect in his sense of color." the more i studied the description and the manuscript the stronger this impression became. then it occurred to me to inquire whether perhaps such could have been the case. so i asked director weiss whether anything was known as to the normal character of littrow's power of distinguishing colors. his answer was prompt and decisive. "oh yes, littrow was color-blind to red. he could not distinguish between the color of aldebaran and the whitest star." no further research was necessary. for half a century the astronomical world had based an impression on the innocent but mistaken evidence of a color-blind man--respecting the tints of ink in a manuscript. it has doubtless happened more than once that when an intimate friend has suddenly and unexpectedly passed away, the reader has ardently wished that it were possible to whisper just one word of appreciation across the dark abyss. and so it is that i have ever since felt that i would like greatly to tell father hell the story of my work at vienna in . xvi the evolution of the scientific investigator [footnote: presidential address at the opening of the international congress of arts and science, st. louis exposition, september : .] as we look at the assemblage gathered in this hall, comprising so many names of widest renown in every branch of learning--we might almost say in every field of human endeavor--the first inquiry suggested must be after the object of our meeting. the answer is that our purpose corresponds to the eminence of the assemblage. we aim at nothing less than a survey of the realm of knowledge, as comprehensive as is permitted by the limitations of time and space. the organizers of our congress have honored me with the charge of presenting such preliminary view of its field as may make clear the spirit of our undertaking. certain tendencies characteristic of the science of our day clearly suggest the direction of our thoughts most appropriate to the occasion. among the strongest of these is one towards laying greater stress on questions of the beginnings of things, and regarding a knowledge of the laws of development of any object of study as necessary to the understanding of its present form. it may be conceded that the principle here involved is as applicable in the broad field before us as in a special research into the properties of the minutest organism. it therefore seems meet that we should begin by inquiring what agency has brought about the remarkable development of science to which the world of to-day bears witness. this view is recognized in the plan of our proceedings by providing for each great department of knowledge a review of its progress during the century that has elapsed since the great event commemorated by the scenes outside this hall. but such reviews do not make up that general survey of science at large which is necessary to the development of our theme, and which must include the action of causes that had their origin long before our time. the movement which culminated in making the nineteenth century ever memorable in history is the outcome of a long series of causes, acting through many centuries, which are worthy of especial attention on such an occasion as this. in setting them forth we should avoid laying stress on those visible manifestations which, striking the eye of every beholder, are in no danger of being overlooked, and search rather for those agencies whose activities underlie the whole visible scene, but which are liable to be blotted out of sight by the very brilliancy of the results to which they have given rise. it is easy to draw attention to the wonderful qualities of the oak; but, from that very fact, it may be needful to point out that the real wonder lies concealed in the acorn from which it grew. our inquiry into the logical order of the causes which have made our civilization what it is to-day will be facilitated by bringing to mind certain elementary considerations--ideas so familiar that setting them forth may seem like citing a body of truisms--and yet so frequently overlooked, not only individually, but in their relation to each other, that the conclusion to which they lead may be lost to sight. one of these propositions is that psychical rather than material causes are those which we should regard as fundamental in directing the development of the social organism. the human intellect is the really active agent in every branch of endeavor--the primum mobile of civilization--and all those material manifestations to which our attention is so often directed are to be regarded as secondary to this first agency. if it be true that "in the world is nothing great but man; in man is nothing great but mind," then should the key-note of our discourse be the recognition of this first and greatest of powers. another well-known fact is that those applications of the forces of nature to the promotion of human welfare which have made our age what it is are of such comparatively recent origin that we need go back only a single century to antedate their most important features, and scarcely more than four centuries to find their beginning. it follows that the subject of our inquiry should be the commencement, not many centuries ago, of a certain new form of intellectual activity. having gained this point of view, our next inquiry will be into the nature of that activity and its relation to the stages of progress which preceded and followed its beginning. the superficial observer, who sees the oak but forgets the acorn, might tell us that the special qualities which have brought out such great results are expert scientific knowledge and rare ingenuity, directed to the application of the powers of steam and electricity. from this point of view the great inventors and the great captains of industry were the first agents in bringing about the modern era. but the more careful inquirer will see that the work of these men was possible only through a knowledge of the laws of nature, which had been gained by men whose work took precedence of theirs in logical order, and that success in invention has been measured by completeness in such knowledge. while giving all due honor to the great inventors, let us remember that the first place is that of the great investigators, whose forceful intellects opened the way to secrets previously hidden from men. let it be an honor and not a reproach to these men that they were not actuated by the love of gain, and did not keep utilitarian ends in view in the pursuit of their researches. if it seems that in neglecting such ends they were leaving undone the most important part of their work, let us remember that nature turns a forbidding face to those who pay her court with the hope of gain, and is responsive only to those suitors whose love for her is pure and undefiled. not only is the special genius required in the investigator not that generally best adapted to applying the discoveries which he makes, but the result of his having sordid ends in view would be to narrow the field of his efforts, and exercise a depressing effect upon his activities. the true man of science has no such expression in his vocabulary as "useful knowledge." his domain is as wide as nature itself, and he best fulfils his mission when he leaves to others the task of applying the knowledge he gives to the world. we have here the explanation of the well-known fact that the functions of the investigator of the laws of nature, and of the inventor who applies these laws to utilitarian purposes, are rarely united in the same person. if the one conspicuous exception which the past century presents to this rule is not unique, we should probably have to go back to watt to find another. from this view-point it is clear that the primary agent in the movement which has elevated man to the masterful position he now occupies is the scientific investigator. he it is whose work has deprived plague and pestilence of their terrors, alleviated human suffering, girdled the earth with the electric wire, bound the continent with the iron way, and made neighbors of the most distant nations. as the first agent which has made possible this meeting of his representatives, let his evolution be this day our worthy theme. as we follow the evolution of an organism by studying the stages of its growth, so we have to show how the work of the scientific investigator is related to the ineffectual efforts of his predecessors. in our time we think of the process of development in nature as one going continuously forward through the combination of the opposite processes of evolution and dissolution. the tendency of our thought has been in the direction of banishing cataclysms to the theological limbo, and viewing nature as a sleepless plodder, endowed with infinite patience, waiting through long ages for results. i do not contest the truth of the principle of continuity on which this view is based. but it fails to make known to us the whole truth. the building of a ship from the time that her keel is laid until she is making her way across the ocean is a slow and gradual process; yet there is a cataclysmic epoch opening up a new era in her history. it is the moment when, after lying for months or years a dead, inert, immovable mass, she is suddenly endowed with the power of motion, and, as if imbued with life, glides into the stream, eager to begin the career for which she was designed. i think it is thus in the development of humanity. long ages may pass during which a race, to all external observation, appears to be making no real progress. additions may be made to learning, and the records of history may constantly grow, but there is nothing in its sphere of thought, or in the features of its life, that can be called essentially new. yet, nature may have been all along slowly working in a way which evades our scrutiny, until the result of her operations suddenly appears in a new and revolutionary movement, carrying the race to a higher plane of civilization. it is not difficult to point out such epochs in human progress. the greatest of all, because it was the first, is one of which we find no record either in written or geological history. it was the epoch when our progenitors first took conscious thought of the morrow, first used the crude weapons which nature had placed within their reach to kill their prey, first built a fire to warm their bodies and cook their food. i love to fancy that there was some one first man, the adam of evolution, who did all this, and who used the power thus acquired to show his fellows how they might profit by his example. when the members of the tribe or community which he gathered around him began to conceive of life as a whole--to include yesterday, to-day, and to-morrow in the same mental grasp--to think how they might apply the gifts of nature to their own uses--a movement was begun which should ultimately lead to civilization. long indeed must have been the ages required for the development of this rudest primitive community into the civilization revealed to us by the most ancient tablets of egypt and assyria. after spoken language was developed, and after the rude representation of ideas by visible marks drawn to resemble them had long been practised, some cadmus must have invented an alphabet. when the use of written language was thus introduced, the word of command ceased to be confined to the range of the human voice, and it became possible for master minds to extend their influence as far as a written message could be carried. then were communities gathered into provinces; provinces into kingdoms, kingdoms into great empires of antiquity. then arose a stage of civilization which we find pictured in the most ancient records--a stage in which men were governed by laws that were perhaps as wisely adapted to their conditions as our laws are to ours--in which the phenomena of nature were rudely observed, and striking occurrences in the earth or in the heavens recorded in the annals of the nation. vast was the progress of knowledge during the interval between these empires and the century in which modern science began. yet, if i am right in making a distinction between the slow and regular steps of progress, each growing naturally out of that which preceded it, and the entrance of the mind at some fairly definite epoch into an entirely new sphere of activity, it would appear that there was only one such epoch during the entire interval. this was when abstract geometrical reasoning commenced, and astronomical observations aiming at precision were recorded, compared, and discussed. closely associated with it must have been the construction of the forms of logic. the radical difference between the demonstration of a theorem of geometry and the reasoning of every-day life which the masses of men must have practised from the beginning, and which few even to-day ever get beyond, is so evident at a glance that i need not dwell upon it. the principal feature of this advance is that, by one of those antinomies of human intellect of which examples are not wanting even in our own time, the development of abstract ideas preceded the concrete knowledge of natural phenomena. when we reflect that in the geometry of euclid the science of space was brought to such logical perfection that even to-day its teachers are not agreed as to the practicability of any great improvement upon it, we cannot avoid the feeling that a very slight change in the direction of the intellectual activity of the greeks would have led to the beginning of natural science. but it would seem that the very purity and perfection which was aimed at in their system of geometry stood in the way of any extension or application of its methods and spirit to the field of nature. one example of this is worthy of attention. in modern teaching the idea of magnitude as generated by motion is freely introduced. a line is described by a moving point; a plane by a moving line; a solid by a moving plane. it may, at first sight, seem singular that this conception finds no place in the euclidian system. but we may regard the omission as a mark of logical purity and rigor. had the real or supposed advantages of introducing motion into geometrical conceptions been suggested to euclid, we may suppose him to have replied that the theorems of space are independent of time; that the idea of motion necessarily implies time, and that, in consequence, to avail ourselves of it would be to introduce an extraneous element into geometry. it is quite possible that the contempt of the ancient philosophers for the practical application of their science, which has continued in some form to our own time, and which is not altogether unwholesome, was a powerful factor in the same direction. the result was that, in keeping geometry pure from ideas which did not belong to it, it failed to form what might otherwise have been the basis of physical science. its founders missed the discovery that methods similar to those of geometric demonstration could be extended into other and wider fields than that of space. thus not only the development of applied geometry but the reduction of other conceptions to a rigorous mathematical form was indefinitely postponed. there is, however, one science which admitted of the immediate application of the theorems of geometry, and which did not require the application of the experimental method. astronomy is necessarily a science of observation pure and simple, in which experiment can have no place except as an auxiliary. the vague accounts of striking celestial phenomena handed down by the priests and astrologers of antiquity were followed in the time of the greeks by observations having, in form at least, a rude approach to precision, though nothing like the degree of precision that the astronomer of to-day would reach with the naked eye, aided by such instruments as he could fashion from the tools at the command of the ancients. the rude observations commenced by the babylonians were continued with gradually improving instruments--first by the greeks and afterwards by the arabs--but the results failed to afford any insight into the true relation of the earth to the heavens. what was most remarkable in this failure is that, to take a first step forward which would have led on to success, no more was necessary than a course of abstract thinking vastly easier than that required for working out the problems of geometry. that space is infinite is an unexpressed axiom, tacitly assumed by euclid and his successors. combining this with the most elementary consideration of the properties of the triangle, it would be seen that a body of any given size could be placed at such a distance in space as to appear to us like a point. hence a body as large as our earth, which was known to be a globe from the time that the ancient phoenicians navigated the mediterranean, if placed in the heavens at a sufficient distance, would look like a star. the obvious conclusion that the stars might be bodies like our globe, shining either by their own light or by that of the sun, would have been a first step to the understanding of the true system of the world. there is historic evidence that this deduction did not wholly escape the greek thinkers. it is true that the critical student will assign little weight to the current belief that the vague theory of pythagoras--that fire was at the centre of all things--implies a conception of the heliocentric theory of the solar system. but the testimony of archimedes, confused though it is in form, leaves no serious doubt that aristarchus of samos not only propounded the view that the earth revolves both on its own axis and around the sun, but that he correctly removed the great stumbling-block in the way of this theory by adding that the distance of the fixed stars was infinitely greater than the dimensions of the earth's orbit. even the world of philosophy was not yet ready for this conception, and, so far from seeing the reasonableness of the explanation, we find ptolemy arguing against the rotation of the earth on grounds which careful observations of the phenomena around him would have shown to be ill-founded. physical science, if we can apply that term to an uncoordinated body of facts, was successfully cultivated from the earliest times. something must have been known of the properties of metals, and the art of extracting them from their ores must have been practised, from the time that coins and medals were first stamped. the properties of the most common compounds were discovered by alchemists in their vain search for the philosopher's stone, but no actual progress worthy of the name rewarded the practitioners of the black art. perhaps the first approach to a correct method was that of archimedes, who by much thinking worked out the law of the lever, reached the conception of the centre of gravity, and demonstrated the first principles of hydrostatics. it is remarkable that he did not extend his researches into the phenomena of motion, whether spontaneous or produced by force. the stationary condition of the human intellect is most strikingly illustrated by the fact that not until the time of leonardo was any substantial advance made on his discovery. to sum up in one sentence the most characteristic feature of ancient and medieval science, we see a notable contrast between the precision of thought implied in the construction and demonstration of geometrical theorems and the vague indefinite character of the ideas of natural phenomena generally, a contrast which did not disappear until the foundations of modern science began to be laid. we should miss the most essential point of the difference between medieval and modern learning if we looked upon it as mainly a difference either in the precision or the amount of knowledge. the development of both of these qualities would, under any circumstances, have been slow and gradual, but sure. we can hardly suppose that any one generation, or even any one century, would have seen the complete substitution of exact for inexact ideas. slowness of growth is as inevitable in the case of knowledge as in that of a growing organism. the most essential point of difference is one of those seemingly slight ones, the importance of which we are too apt to overlook. it was like the drop of blood in the wrong place, which some one has told us makes all the difference between a philosopher and a maniac. it was all the difference between a living tree and a dead one, between an inert mass and a growing organism. the transition of knowledge from the dead to the living form must, in any complete review of the subject, be looked upon as the really great event of modern times. before this event the intellect was bound down by a scholasticism which regarded knowledge as a rounded whole, the parts of which were written in books and carried in the minds of learned men. the student was taught from the beginning of his work to look upon authority as the foundation of his beliefs. the older the authority the greater the weight it carried. so effective was this teaching that it seems never to have occurred to individual men that they had all the opportunities ever enjoyed by aristotle of discovering truth, with the added advantage of all his knowledge to begin with. advanced as was the development of formal logic, that practical logic was wanting which could see that the last of a series of authorities, every one of which rested on those which preceded it, could never form a surer foundation for any doctrine than that supplied by its original propounder. the result of this view of knowledge was that, although during the fifteen centuries following the death of the geometer of syracuse great universities were founded at which generations of professors expounded all the learning of their time, neither professor nor student ever suspected what latent possibilities of good were concealed in the most familiar operations of nature. every one felt the wind blow, saw water boil, and heard the thunder crash, but never thought of investigating the forces here at play. up to the middle of the fifteenth century the most acute observer could scarcely have seen the dawn of a new era. in view of this state of things it must be regarded as one of the most remarkable facts in evolutionary history that four or five men, whose mental constitution was either typical of the new order of things, or who were powerful agents in bringing it about, were all born during the fifteenth century, four of them at least, at so nearly the same time as to be contemporaries. leonardo da vinci, whose artistic genius has charmed succeeding generations, was also the first practical engineer of his time, and the first man after archimedes to make a substantial advance in developing the laws of motion. that the world was not prepared to make use of his scientific discoveries does not detract from the significance which must attach to the period of his birth. shortly after him was born the great navigator whose bold spirit was to make known a new world, thus giving to commercial enterprise that impetus which was so powerful an agent in bringing about a revolution in the thoughts of men. the birth of columbus was soon followed by that of copernicus, the first after aristarchus to demonstrate the true system of the world. in him more than in any of his contemporaries do we see the struggle between the old forms of thought and the new. it seems almost pathetic and is certainly most suggestive of the general view of knowledge taken at that time that, instead of claiming credit for bringing to light great truths before unknown, he made a labored attempt to show that, after all, there was nothing really new in his system, which he claimed to date from pythagoras and philolaus. in this connection it is curious that he makes no mention of aristarchus, who i think will be regarded by conservative historians as his only demonstrated predecessor. to the hold of the older ideas upon his mind we must attribute the fact that in constructing his system he took great pains to make as little change as possible in ancient conceptions. luther, the greatest thought-stirrer of them all, practically of the same generation with copernicus, leonardo and columbus, does not come in as a scientific investigator, but as the great loosener of chains which had so fettered the intellect of men that they dared not think otherwise than as the authorities thought. almost coeval with the advent of these intellects was the invention of printing with movable type. gutenberg was born during the first decade of the century, and his associates and others credited with the invention not many years afterwards. if we accept the principle on which i am basing my argument, that in bringing out the springs of our progress we should assign the first place to the birth of those psychic agencies which started men on new lines of thought, then surely was the fifteenth the wonderful century. let us not forget that, in assigning the actors then born to their places, we are not narrating history, but studying a special phase of evolution. it matters not for us that no university invited leonardo to its halls, and that his science was valued by his contemporaries only as an adjunct to the art of engineering. the great fact still is that he was the first of mankind to propound laws of motion. it is not for anything in luther's doctrines that he finds a place in our scheme. no matter for us whether they were sound or not. what he did towards the evolution of the scientific investigator was to show by his example that a man might question the best-established and most venerable authority and still live--still preserve his intellectual integrity--still command a hearing from nations and their rulers. it matters not for us whether columbus ever knew that he had discovered a new continent. his work was to teach that neither hydra, chimera nor abyss--neither divine injunction nor infernal machination--was in the way of men visiting every part of the globe, and that the problem of conquering the world reduced itself to one of sails and rigging, hull and compass. the better part of copernicus was to direct man to a view-point whence he should see that the heavens were of like matter with the earth. all this done, the acorn was planted from which the oak of our civilization should spring. the mad quest for gold which followed the discovery of columbus, the questionings which absorbed the attention of the learned, the indignation excited by the seeming vagaries of a paracelsus, the fear and trembling lest the strange doctrine of copernicus should undermine the faith of centuries, were all helps to the germination of the seed--stimuli to thought which urged it on to explore the new fields opened up to its occupation. this given, all that has since followed came out in regular order of development, and need be here considered only in those phases having a special relation to the purpose of our present meeting. so slow was the growth at first that the sixteenth century may scarcely have recognized the inauguration of a new era. torricelli and benedetti were of the third generation after leonardo, and galileo, the first to make a substantial advance upon his theory, was born more than a century after him. only two or three men appeared in a generation who, working alone, could make real progress in discovery, and even these could do little in leavening the minds of their fellowmen with the new ideas. up to the middle of the seventeenth century an agent which all experience since that time shows to be necessary to the most productive intellectual activity was wanting. this was the attrition of like minds, making suggestions to one another, criticising, comparing, and reasoning. this element was introduced by the organization of the royal society of london and the academy of sciences of paris. the members of these two bodies seem like ingenious youth suddenly thrown into a new world of interesting objects, the purposes and relations of which they had to discover. the novelty of the situation is strikingly shown in the questions which occupied the minds of the incipient investigators. one natural result of british maritime enterprise was that the aspirations of the fellows of the royal society were not confined to any continent or hemisphere. inquiries were sent all the way to batavia to know "whether there be a hill in sumatra which burneth continually, and a fountain which runneth pure balsam." the astronomical precision with which it seemed possible that physiological operations might go on was evinced by the inquiry whether the indians can so prepare that stupefying herb datura that "they make it lie several days, months, years, according as they will, in a man's body without doing him any harm, and at the end kill him without missing an hour's time." of this continent one of the inquiries was whether there be a tree in mexico that yields water, wine, vinegar, milk, honey, wax, thread and needles. among the problems before the paris academy of sciences those of physiology and biology took a prominent place. the distillation of compounds had long been practised, and the fact that the more spirituous elements of certain substances were thus separated naturally led to the question whether the essential essences of life might not be discoverable in the same way. in order that all might participate in the experiments, they were conducted in open session of the academy, thus guarding against the danger of any one member obtaining for his exclusive personal use a possible elixir of life. a wide range of the animal and vegetable kingdom, including cats, dogs and birds of various species, were thus analyzed. the practice of dissection was introduced on a large scale. that of the cadaver of an elephant occupied several sessions, and was of such interest that the monarch himself was a spectator. to the same epoch with the formation and first work of these two bodies belongs the invention of a mathematical method which in its importance to the advance of exact science may be classed with the invention of the alphabet in its relation to the progress of society at large. the use of algebraic symbols to represent quantities had its origin before the commencement of the new era, and gradually grew into a highly developed form during the first two centuries of that era. but this method could represent quantities only as fixed. it is true that the elasticity inherent in the use of such symbols permitted of their being applied to any and every quantity; yet, in any one application, the quantity was considered as fixed and definite. but most of the magnitudes of nature are in a state of continual variation; indeed, since all motion is variation, the latter is a universal characteristic of all phenomena. no serious advance could be made in the application of algebraic language to the expression of physical phenomena until it could be so extended as to express variation in quantities, as well as the quantities themselves. this extension, worked out independently by newton and leibnitz, may be classed as the most fruitful of conceptions in exact science. with it the way was opened for the unimpeded and continually accelerated progress of the last two centuries. the feature of this period which has the closest relation to the purpose of our coming together is the seemingly unending subdivision of knowledge into specialties, many of which are becoming so minute and so isolated that they seem to have no interest for any but their few pursuers. happily science itself has afforded a corrective for its own tendency in this direction. the careful thinker will see that in these seemingly diverging branches common elements and common principles are coming more and more to light. there is an increasing recognition of methods of research, and of deduction, which are common to large branches, or to the whole of science. we are more and more recognizing the principle that progress in knowledge implies its reduction to more exact forms, and the expression of its ideas in language more or less mathematical. the problem before the organizers of this congress was, therefore, to bring the sciences together, and seek for the unity which we believe underlies their infinite diversity. the assembling of such a body as now fills this hall was scarcely possible in any preceding generation, and is made possible now only through the agency of science itself. it differs from all preceding international meetings by the universality of its scope, which aims to include the whole of knowledge. it is also unique in that none but leaders have been sought out as members. it is unique in that so many lands have delegated their choicest intellects to carry on its work. they come from the country to which our republic is indebted for a third of its territory, including the ground on which we stand; from the land which has taught us that the most scholarly devotion to the languages and learning of the cloistered past is compatible with leadership in the practical application of modern science to the arts of life; from the island whose language and literature have found a new field and a vigorous growth in this region; from the last seat of the holy roman empire; from the country which, remembering a monarch who made an astronomical observation at the greenwich observatory, has enthroned science in one of the highest places in its government; from the peninsula so learned that we have invited one of its scholars to come and tells us of our own language; from the land which gave birth to leonardo, galileo, torricelli, columbus, volta--what an array of immortal names!--from the little republic of glorious history which, breeding men rugged as its eternal snow-peaks, has yet been the seat of scientific investigation since the day of the bernoullis; from the land whose heroic dwellers did not hesitate to use the ocean itself to protect it against invaders, and which now makes us marvel at the amount of erudition compressed within its little area; from the nation across the pacific, which, by half a century of unequalled progress in the arts of life, has made an important contribution to evolutionary science through demonstrating the falsity of the theory that the most ancient races are doomed to be left in the rear of the advancing age--in a word, from every great centre of intellectual activity on the globe i see before me eminent representatives of that world--advance in knowledge which we have met to celebrate. may we not confidently hope that the discussions of such an assemblage will prove pregnant of a future for science which shall outshine even its brilliant past. gentlemen and scholars all! you do not visit our shores to find great collections in which centuries of humanity have given expression on canvas and in marble to their hopes, fears, and aspirations. nor do you expect institutions and buildings hoary with age. but as you feel the vigor latent in the fresh air of these expansive prairies, which has collected the products of human genius by which we are here surrounded, and, i may add, brought us together; as you study the institutions which we have founded for the benefit, not only of our own people, but of humanity at large; as you meet the men who, in the short space of one century, have transformed this valley from a savage wilderness into what it is today--then may you find compensation for the want of a past like yours by seeing with prophetic eye a future world-power of which this region shall be the seat. if such is to be the outcome of the institutions which we are now building up, then may your present visit be a blessing both to your posterity and ours by making that power one for good to all man-kind. your deliberations will help to demonstrate to us and to the world at large that the reign of law must supplant that of brute force in the relations of the nations, just as it has supplanted it in the relations of individuals. you will help to show that the war which science is now waging against the sources of diseases, pain, and misery offers an even nobler field for the exercise of heroic qualities than can that of battle. we hope that when, after your all too-fleeting sojourn in our midst, you return to your own shores, you will long feel the influence of the new air you have breathed in an infusion of increased vigor in pursuing your varied labors. and if a new impetus is thus given to the great intellectual movement of the past century, resulting not only in promoting the unification of knowledge, but in widening its field through new combinations of effort on the part of its votaries, the projectors, organizers and supporters of this congress of arts and science will be justified of their labors. xvii the evolution of astronomical knowledge [footnote: address at the dedication of the flower observatory, university of pennsylvania, may , --science, may , ] assembled, as we are, to dedicate a new institution to the promotion of our knowledge of the heavens, it appeared to me that an appropriate and interesting subject might be the present and future problems of astronomy. yet it seemed, on further reflection, that, apart from the difficulty of making an adequate statement of these problems on such an occasion as the present, such a wording of the theme would not fully express the idea which i wish to convey. the so-called problems of astronomy are not separate and independent, but are rather the parts of one great problem, that of increasing our knowledge of the universe in its widest extent. nor is it easy to contemplate the edifice of astronomical science as it now stands, without thinking of the past as well as of the present and future. the fact is that our knowledge of the universe has been in the nature of a slow and gradual evolution, commencing at a very early period in human history, and destined to go forward without stop, as we hope, so long as civilization shall endure. the astronomer of every age has built on the foundations laid by his predecessors, and his work has always formed, and must ever form, the base on which his successors shall build. the astronomer of to-day may look back upon hipparchus and ptolemy as the earliest ancestors of whom he has positive knowledge. he can trace his scientific descent from generation to generation, through the periods of arabian and medieval science, through copernicus, kepler, newton, laplace, and herschel, down to the present time. the evolution of astronomical knowledge, generally slow and gradual, offering little to excite the attention of the public, has yet been marked by two cataclysms. one of these is seen in the grand conception of copernicus that this earth on which we dwell is not a globe fixed in the centre of the universe, but is simply one of a number of bodies, turning on their own axes and at the same time moving around the sun as a centre. it has always seemed to me that the real significance of the heliocentric system lies in the greatness of this conception rather than in the fact of the discovery itself. there is no figure in astronomical history which may more appropriately claim the admiration of mankind through all time than that of copernicus. scarcely any great work was ever so exclusively the work of one man as was the heliocentric system the work of the retiring sage of frauenburg. no more striking contrast between the views of scientific research entertained in his time and in ours can be found than that afforded by the fact that, instead of claiming credit for his great work, he deemed it rather necessary to apologize for it and, so far as possible, to attribute his ideas to the ancients. a century and a half after copernicus followed the second great step, that taken by newton. this was nothing less than showing that the seemingly complicated and inexplicable motions of the heavenly bodies were only special cases of the same kind of motion, governed by the same forces, that we see around us whenever a stone is thrown by the hand or an apple falls to the ground. the actual motions of the heavens and the laws which govern them being known, man had the key with which he might commence to unlock the mysteries of the universe. when huyghens, in , published his systema saturnium, where he first set forth the mystery of the rings of saturn, which, for nearly half a century, had perplexed telescopic observers, he prefaced it with a remark that many, even among the learned, might condemn his course in devoting so much time and attention to matters far outside the earth, when he might better be studying subjects of more concern to humanity. notwithstanding that the inventor of the pendulum clock was, perhaps, the last astronomer against whom a neglect of things terrestrial could be charged, he thought it necessary to enter into an elaborate defence of his course in studying the heavens. now, however, the more distant objects are in space--i might almost add the more distant events are in time--the more they excite the attention of the astronomer, if only he can hope to acquire positive knowledge about them. not, however, because he is more interested in things distant than in things near, but because thus he may more completely embrace in the scope of his work the beginning and the end, the boundaries of all things, and thus, indirectly, more fully comprehend all that they include. from his stand-point, "all are but parts of one stupendous whole, whose body nature is and god the soul." others study nature and her plans as we see them developed on the surface of this little planet which we inhabit, the astronomer would fain learn the plan on which the whole universe is constructed. the magnificent conception of copernicus is, for him, only an introduction to the yet more magnificent conception of infinite space containing a collection of bodies which we call the visible universe. how far does this universe extend? what are the distances and arrangements of the stars? does the universe constitute a system? if so, can we comprehend the plan on which this system is formed, of its beginning and of its end? has it bounds outside of which nothing exists but the black and starless depths of infinity itself? or are the stars we see simply such members of an infinite collection as happen to be the nearest our system? a few such questions as these we are perhaps beginning to answer; but hundreds, thousands, perhaps even millions, of years may elapse without our reaching a complete solution. yet the astronomer does not view them as kantian antinomies, in the nature of things insoluble, but as questions to which he may hopefully look for at least a partial answer. the problem of the distances of the stars is of peculiar interest in connection with the copernican system. the greatest objection to this system, which must have been more clearly seen by astronomers themselves than by any others, was found in the absence of any apparent parallax of the stars. if the earth performed such an immeasurable circle around the sun as copernicus maintained, then, as it passed from side to side of its orbit, the stars outside the solar system must appear to have a corresponding motion in the other direction, and thus to swing back and forth as the earth moved in one and the other direction. the fact that not the slightest swing of that sort could be seen was, from the time of ptolemy, the basis on which the doctrine of the earth's immobility rested. the difficulty was not grappled with by copernicus or his immediate successors. the idea that nature would not squander space by allowing immeasurable stretches of it to go unused seems to have been one from which medieval thinkers could not entirely break away. the consideration that there could be no need of any such economy, because the supply was infinite, might have been theoretically acknowledged, but was not practically felt. the fact is that magnificent as was the conception of copernicus, it was dwarfed by the conception of stretches from star to star so vast that the whole orbit of the earth was only a point in comparison. an indication of the extent to which the difficulty thus arising was felt is seen in the title of a book published by horrebow, the danish astronomer, some two centuries ago. this industrious observer, one of the first who used an instrument resembling our meridian transit of the present day, determined to see if he could find the parallax of the stars by observing the intervals at which a pair of stars in opposite quarters of the heavens crossed his meridian at opposite seasons of the year. when, as he thought, he had won success, he published his observations and conclusions under the title of copernicus triumphans. but alas! the keen criticism of his successors showed that what he supposed to be a swing of the stars from season to season arose from a minute variation in the rate of his clock, due to the different temperatures to which it was exposed during the day and the night. the measurement of the distance even of the nearest stars evaded astronomical research until bessel and struve arose in the early part of the present century. on some aspects of the problem of the extent of the universe light is being thrown even now. evidence is gradually accumulating which points to the probability that the successive orders of smaller and smaller stars, which our continually increasing telescopic power brings into view, are not situated at greater and greater distances, but that we actually see the boundary of our universe. this indication lends a peculiar interest to various questions growing out of the motions of the stars. quite possibly the problem of these motions will be the great one of the future astronomer. even now it suggests thoughts and questions of the most far-reaching character. i have seldom felt a more delicious sense of repose than when crossing the ocean during the summer months i sought a place where i could lie alone on the deck, look up at the constellations, with lyra near the zenith, and, while listening to the clank of the engine, try to calculate the hundreds of millions of years which would be required by our ship to reach the star a lyrae, if she could continue her course in that direction without ever stopping. it is a striking example of how easily we may fail to realize our knowledge when i say that i have thought many a time how deliciously one might pass those hundred millions of years in a journey to the star a lyrae, without its occurring to me that we are actually making that very journey at a speed compared with which the motion of a steamship is slow indeed. through every year, every hour, every minute, of human history from the first appearance of man on the earth, from the era of the builders of the pyramids, through the times of caesar and hannibal, through the period of every event that history records, not merely our earth, but the sun and the whole solar system with it, have been speeding their way towards the star of which i speak on a journey of which we know neither the beginning nor the end. we are at this moment thousands of miles nearer to a lyrae than we were a few minutes ago when i began this discourse, and through every future moment, for untold thousands of years to come, the earth and all there is on it will be nearer to a lyrae, or nearer to the place where that star now is, by hundreds of miles for every minute of time come and gone. when shall we get there? probably in less than a million years, perhaps in half a million. we cannot tell exactly, but get there we must if the laws of nature and the laws of motion continue as they are. to attain to the stars was the seemingly vain wish of an ancient philosopher, but the whole human race is, in a certain sense, realizing this wish as rapidly as a speed of ten miles a second can bring it about. i have called attention to this motion because it may, in the not distant future, afford the means of approximating to a solution of the problem already mentioned--that of the extent of the universe. notwithstanding the success of astronomers during the present century in measuring the parallax of a number of stars, the most recent investigations show that there are very few, perhaps hardly more than a score, of stars of which the parallax, and therefore the distance, has been determined with any approach to certainty. many parallaxes determined about the middle of the nineteenth century have had to disappear before the powerful tests applied by measures with the heliometer; others have been greatly reduced and the distances of the stars increased in proportion. so far as measurement goes, we can only say of the distances of all the stars, except the few whose parallaxes have been determined, that they are immeasurable. the radius of the earth's orbit, a line more than ninety millions of miles in length, not only vanishes from sight before we reach the distance of the great mass of stars, but becomes such a mere point that when magnified by the powerful instruments of modern times the most delicate appliances fail to make it measurable. here the solar motion comes to our help. this motion, by which, as i have said, we are carried unceasingly through space, is made evident by a motion of most of the stars in the opposite direction, just as passing through a country on a railway we see the houses on the right and on the left being left behind us. it is clear enough that the apparent motion will be more rapid the nearer the object. we may therefore form some idea of the distance of the stars when we know the amount of the motion. it is found that in the great mass of stars of the sixth magnitude, the smallest visible to the naked eye, the motion is about three seconds per century. as a measure thus stated does not convey an accurate conception of magnitude to one not practised in the subject, i would say that in the heavens, to the ordinary eye, a pair of stars will appear single unless they are separated by a distance of or seconds. let us, then, imagine ourselves looking at a star of the sixth magnitude, which is at rest while we are carried past it with the motion of six to eight miles per second which i have described. mark its position in the heavens as we see it to-day; then let its position again be marked five thousand years hence. a good eye will just be able to perceive that there are two stars marked instead of one. the two would be so close together that no distinct space between them could be perceived by unaided vision. it is due to the magnifying power of the telescope, enlarging such small apparent distances, that the motion has been determined in so small a period as the one hundred and fifty years during which accurate observations of the stars have been made. the motion just described has been fairly well determined for what, astronomically speaking, are the brighter stars; that is to say, those visible to the naked eye. but how is it with the millions of faint telescopic stars, especially those which form the cloud masses of the milky way? the distance of these stars is undoubtedly greater, and the apparent motion is therefore smaller. accurate observations upon such stars have been commenced only recently, so that we have not yet had time to determine the amount of the motion. but the indication seems to be that it will prove quite a measurable quantity and that before the twentieth century has elapsed, it will be determined for very much smaller stars than those which have heretofore been studied. a photographic chart of the whole heavens is now being constructed by an association of observatories in some of the leading countries of the world. i cannot say all the leading countries, because then we should have to exclude our own, which, unhappily, has taken no part in this work. at the end of the twentieth century we may expect that the work will be repeated. then, by comparing the charts, we shall see the effect of the solar motion and perhaps get new light upon the problem in question. closely connected with the problem of the extent of the universe is another which appears, for us, to be insoluble because it brings us face to face with infinity itself. we are familiar enough with eternity, or, let us say, the millions or hundreds of millions of years which geologists tell us must have passed while the crust of the earth was assuming its present form, our mountains being built, our rocks consolidated, and successive orders of animals coming and going. hundreds of millions of years is indeed a long time, and yet, when we contemplate the changes supposed to have taken place during that time, we do not look out on eternity itself, which is veiled from our sight, as it were, by the unending succession of changes that mark the progress of time. but in the motions of the stars we are brought face to face with eternity and infinity, covered by no veil whatever. it would be bold to speak dogmatically on a subject where the springs of being are so far hidden from mortal eyes as in the depths of the universe. but, without declaring its positive certainty, it must be said that the conclusion seems unavoidable that a number of stars are moving with a speed such that the attraction of all the bodies of the universe could never stop them. one such case is that of arcturus, the bright reddish star familiar to mankind since the days of job, and visible near the zenith on the clear evenings of may and june. yet another case is that of a star known in astronomical nomenclature as groombridge, which exceeds all others in its angular proper motion as seen from the earth. we should naturally suppose that it seems to move so fast because it is near us. but the best measurements of its parallax seem to show that it can scarcely be less than two million times the distance of the earth from the sun, while it may be much greater. accepting this result, its velocity cannot be much less than two hundred miles per second, and may be much more. with this speed it would make the circuit of our globe in two minutes, and had it gone round and round in our latitudes we should have seen it fly past us a number of times since i commenced this discourse. it would make the journey from the earth to the sun in five days. if it is now near the centre of our universe it would probably reach its confines in a million of years. so far as our knowledge goes, there is no force in nature which would ever have set it in motion and no force which can ever stop it. what, then, was the history of this star, and, if there are planets circulating around, what the experience of beings who may have lived on those planets during the ages which geologists and naturalists assure us our earth has existed? was there a period when they saw at night only a black and starless heaven? was there a time when in that heaven a small faint patch of light began gradually to appear? did that patch of light grow larger and larger as million after million of years elapsed? did it at last fill the heavens and break up into constellations as we now see them? as millions more of years elapse will the constellations gather together in the opposite quarter and gradually diminish to a patch of light as the star pursues its irresistible course of two hundred miles per second through the wilderness of space, leaving our universe farther and farther behind it, until it is lost in the distance? if the conceptions of modern science are to be considered as good for all time--a point on which i confess to a large measure of scepticism--then these questions must be answered in the affirmative. the problems of which i have so far spoken are those of what may be called the older astronomy. if i apply this title it is because that branch of the science to which the spectroscope has given birth is often called the new astronomy. it is commonly to be expected that a new and vigorous form of scientific research will supersede that which is hoary with antiquity. but i am not willing to admit that such is the case with the old astronomy, if old we may call it. it is more pregnant with future discoveries today than it ever has been, and it is more disposed to welcome the spectroscope as a useful handmaid, which may help it on to new fields, than it is to give way to it. how useful it may thus become has been recently shown by a dutch astronomer, who finds that the stars having one type of spectrum belong mostly to the milky way, and are farther from us than the others. in the field of the newer astronomy perhaps the most interesting work is that associated with comets. it must be confessed, however, that the spectroscope has rather increased than diminished the mystery which, in some respects, surrounds the constitution of these bodies. the older astronomy has satisfactorily accounted for their appearance, and we might also say for their origin and their end, so far as questions of origin can come into the domain of science. it is now known that comets are not wanderers through the celestial spaces from star to star, but must always have belonged to our system. but their orbits are so very elongated that thousands, or even hundreds of thousands, of years are required for a revolution. sometimes, however, a comet passing near to jupiter is so fascinated by that planet that, in its vain attempts to follow it, it loses so much of its primitive velocity as to circulate around the sun in a period of a few years, and thus to become, apparently, a new member of our system. if the orbit of such a comet, or in fact of any comet, chances to intersect that of the earth, the latter in passing the point of intersection encounters minute particles which cause a meteoric shower. but all this does not tell us much about the nature and make-up of a comet. does it consist of nothing but isolated particles, or is there a solid nucleus, the attraction of which tends to keep the mass together? no one yet knows. the spectroscope, if we interpret its indications in the usual way, tells us that a comet is simply a mass of hydrocarbon vapor, shining by its own light. but there must be something wrong in this interpretation. that the light is reflected sunlight seems to follow necessarily from the increased brilliancy of the comet as it approaches the sun and its disappearance as it passes away. great attention has recently been bestowed upon the physical constitution of the planets and the changes which the surfaces of those bodies may undergo. in this department of research we must feel gratified by the energy of our countrymen who have entered upon it. should i seek to even mention all the results thus made known i might be stepping on dangerous ground, as many questions are still unsettled. while every astronomer has entertained the highest admiration for the energy and enthusiasm shown by mr. percival lowell in founding an observatory in regions where the planets can be studied under the most favorable conditions, they cannot lose sight of the fact that the ablest and most experienced observers are liable to error when they attempt to delineate the features of a body , , or , , miles away through such a disturbing medium as our atmosphere. even on such a subject as the canals of mars doubts may still be felt. that certain markings to which schiaparelli gave the name of canals exist, few will question. but it may be questioned whether these markings are the fine, sharp, uniform lines found on schiaparelli's map and delineated in lowell's beautiful book. it is certainly curious that barnard at mount hamilton, with the most powerful instrument and under the most favorable circumstances, does not see these markings as canals. i can only mention among the problems of the spectroscope the elegant and remarkable solution of the mystery surrounding the rings of saturn, which has been effected by keeler at allegheny. that these rings could not be solid has long been a conclusion of the laws of mechanics, but keeler was the first to show that they really consist of separate particles, because the inner portions revolve more rapidly than the outer. the question of the atmosphere of mars has also received an important advance by the work of campbell at mount hamilton. although it is not proved that mars has no atmosphere, for the existence of some atmosphere can scarcely be doubted, yet the mount hamilton astronomer seems to have shown, with great conclusiveness, that it is so rare as not to produce any sensible absorption of the solar rays. i have left an important subject for the close. it belongs entirely to the older astronomy, and it is one with which i am glad to say this observatory is expected to especially concern itself. i refer to the question of the variation of latitudes, that singular phenomenon scarcely suspected ten years ago, but brought out by observations in germany during the past eight years, and reduced to law with such brilliant success by our own chandler. the north pole is not a fixed point on the earth's surface, but moves around in rather an irregular way. true, the motion is small; a circle of sixty feet in diameter will include the pole in its widest range. this is a very small matter so far as the interests of daily life are concerned; but it is very important to the astronomer. it is not simply a motion of the pole of the earth, but a wobbling of the solid earth itself. no one knows what conclusions of importance to our race may yet follow from a study of the stupendous forces necessary to produce even this slight motion. the director of this new observatory has already distinguished himself in the delicate and difficult work of investigating this motion, and i am glad to know that he is continuing the work here with one of the finest instruments ever used for the purpose, a splendid product of american mechanical genius. i can assure you that astronomers the world over will look with the greatest interest for professor doolittle's success in the arduous task he has undertaken. there is one question connected with these studies of the universe on which i have not touched, and which is, nevertheless, of transcendent interest. what sort of life, spiritual and intellectual, exists in distant worlds? we cannot for a moment suppose that our little planet is the only one throughout the whole universe on which may be found the fruits of civilization, family affection, friendship, the desire to penetrate the mysteries of creation. and yet this question is not to-day a problem of astronomy, nor can we see any prospect that it ever will be, for the simple reason that science affords us no hope of an answer to any question that we may send through the fathomless abyss. when the spectroscope was in its infancy it was suggested that possibly some difference might be found in the rays reflected from living matter, especially from vegetation, that might enable us to distinguish them from rays reflected by matter not endowed with life. but this hope has not been realized, nor does it seem possible to realize it. the astronomer cannot afford to waste his energies on hopeless speculation about matters of which he cannot learn anything, and he therefore leaves this question of the plurality of worlds to others who are as competent to discuss it as he is. all he can tell the world is: he who through vast immensity can pierce, see worlds on worlds compose one universe; observe how system into system runs, what other planets circle other suns, what varied being peoples every star, may tell why heaven has made us as we are. xviii aspects of american astronomy [footnote: address delivered at the university of chicago, october , , in connection with the dedication of the yerkes observatory. printed in the astro physical journal. november, .] the university of chicago yesterday accepted one of the most munificent gifts ever made for the promotion of any single science, and with appropriate ceremonies dedicated it to the increase of our knowledge of the heavenly bodies. the president of your university has done me the honor of inviting me to supplement what was said on that occasion by some remarks of a more general nature suggested by the celebration. one is naturally disposed to say first what is uppermost in his mind. at the present moment this will naturally be the general impression made by what has been seen and heard. the ceremonies were attended, not only by a remarkable delegation of citizens, but by a number of visiting astronomers which seems large when we consider that the profession itself is not at all numerous in any country. as one of these, your guests, i am sure that i give expression only to their unanimous sentiment in saying that we have been extremely gratified in many ways by all that we have seen and heard. the mere fact of so munificent a gift to science cannot but excite universal admiration. we knew well enough that it was nothing more than might have been expected from the public spirit of this great west; but the first view of a towering snowpeak is none the less impressive because you have learned in your geography how many feet high it is, and great acts are none the less admirable because they correspond to what you have heard and read, and might therefore be led to expect. the next gratifying feature is the great public interest excited by the occasion. that the opening of a purely scientific institution should have led so large an assemblage of citizens to devote an entire day, including a long journey by rail, to the celebration of yesterday is something most suggestive from its unfamiliarity. a great many scientific establishments have been inaugurated during the last half-century, but if on any such occasion so large a body of citizens has gone so great a distance to take part in the inauguration, the fact has at the moment escaped my mind. that the interest thus shown is not confined to the hundreds of attendants, but must be shared by your great public, is shown by the unfailing barometer of journalism. here we have a field in which the non-survival of the unfit is the rule in its most ruthless form. the journals that we see and read are merely the fortunate few of a countless number, dead and forgotten, that did not know what the public wanted to read about. the eagerness shown by the representatives of your press in recording everything your guests would say was accomplished by an enterprise in making known everything that occurred, and, in case of an emergency requiring a heroic measure, what did not occur, showing that smart journalists of the east must have learned their trade, or at least breathed their inspiration, in these regions. i think it was some twenty years since i told a european friend that the eighth wonder of the world was a chicago daily newspaper. since that time the course of journalistic enterprise has been in the reverse direction to that of the course of empire, eastward instead of westward. it has been sometimes said--wrongfully, i think--that scientific men form a mutual admiration society. one feature of the occasion made me feel that we, your guests, ought then and there to have organized such a society and forthwith proceeded to business. this feature consisted in the conferences on almost every branch of astronomy by which the celebration of yesterday was preceded. the fact that beyond the acceptance of a graceful compliment i contributed nothing to these conferences relieves me from the charge of bias or self-assertion in saying that they gave me a new and most inspiring view of the energy now being expended in research by the younger generation of astronomers. all the experience of the past leads us to believe that this energy will reap the reward which nature always bestows upon those who seek her acquaintance from unselfish motives. in one way it might appear that little was to be learned from a meeting like that of the present week. each astronomer may know by publications pertaining to the science what all the others are doing. but knowledge obtained in this way has a sort of abstractness about it a little like our knowledge of the progress of civilization in japan, or of the great extent of the australian continent. it was, therefore, a most happy thought on the part of your authorities to bring together the largest possible number of visiting astronomers from europe, as well as america, in order that each might see, through the attrition of personal contact, what progress the others were making in their researches. to the visitors at least i am sure that the result of this meeting has been extremely gratifying. they earnestly hope, one and all, that the callers of the conference will not themselves be more disappointed in its results; that, however little they may have actually to learn of methods and results, they will feel stimulated to well-directed efforts and find themselves inspired by thoughts which, however familiar, will now be more easily worked out. we may pass from the aspects of the case as seen by the strictly professional class to those general aspects fitted to excite the attention of the great public. from the point of view of the latter it may well appear that the most striking feature of the celebration is the great amount of effort which is shown to be devoted to the cultivation of a field quite outside the ordinary range of human interests. the workers whom we see around us are only a detachment from an army of investigators who, in many parts of the world, are seeking to explore the mysteries of creation. why so great an expenditure of energy? certainly not to gain wealth, for astronomy is perhaps the one field of scientific work which, in our expressive modern phrase, "has no money in it." it is true that the great practical use of astronomical science to the country and the world in affording us the means of determining positions on land and at sea is frequently pointed out. it is said that an astronomer royal of england once calculated that every meridian observation of the moon made at greenwich was worth a pound sterling, on account of the help it would afford to the navigation of the ocean. an accurate map of the united states cannot be constructed without astronomical observations at numerous points scattered over the whole country, aided by data which great observatories have been accumulating for more than a century, and must continue to accumulate in the future. but neither the measurement of the earth, the making of maps, nor the aid of the navigator is the main object which the astronomers of to-day have in view. if they do not quite share the sentiment of that eminent mathematician, who is said to have thanked god that his science was one which could not be prostituted to any useful purpose, they still know well that to keep utilitarian objects in view would only prove & handicap on their efforts. consequently they never ask in what way their science is going to benefit mankind. as the great captain of industry is moved by the love of wealth, and the political leader by the love of power over men, so the astronomer is moved by the love of knowledge for its own sake, and not for the sake of its useful applications. yet he is proud to know that his science has been worth more to mankind than it has cost. he does not value its results merely as a means of crossing the ocean or mapping the country, for he feels that man does not live by bread alone. if it is not more than bread to know the place we occupy in the universe, it is certainly something which we should place not far behind the means of subsistence. that we now look upon a comet as something very interesting, of which the sight affords us a pleasure unmixed with fear of war, pestilence, or other calamity, and of which we therefore wish the return, is a gain we cannot measure by money. in all ages astronomy has been an index to the civilization of the people who cultivated it. it has been crude or exact, enlightened or mingled with superstition, according to the current mode of thought. when once men understand the relation of the planet on which they dwell to the universe at large, superstition is doomed to speedy extinction. this alone is an object worth more than money. astronomy may fairly claim to be that science which transcends all others in its demands upon the practical application of our reasoning powers. look at the stars that stud the heavens on a clear evening. what more hopeless problem to one confined to earth than that of determining their varying distances, their motions, and their physical constitution? everything on earth we can handle and investigate. but how investigate that which is ever beyond our reach, on which we can never make an experiment? on certain occasions we see the moon pass in front of the sun and hide it from our eyes. to an observer a few miles away the sun was not entirely hidden, for the shadow of the moon in a total eclipse is rarely one hundred miles wide. on another continent no eclipse at all may have been visible. who shall take a map of the world and mark upon it the line on which the moon's shadow will travel during some eclipse a hundred years hence? who shall map out the orbits of the heavenly bodies as they are going to appear in a hundred thousand years? how shall we ever know of what chemical elements the sun and the stars are made? all this has been done, but not by the intellect of any one man. the road to the stars has been opened only by the efforts of many generations of mathematicians and observers, each of whom began where his predecessor had left off. we have reached a stage where we know much of the heavenly bodies. we have mapped out our solar system with great precision. but how with that great universe of millions of stars in which our solar system is only a speck of star-dust, a speck which a traveller through the wilds of space might pass a hundred times without notice? we have learned much about this universe, though our knowledge of it is still dim. we see it as a traveller on a mountain-top sees a distant city in a cloud of mist, by a few specks of glimmering light from steeples or roofs. we want to know more about it, its origin and its destiny; its limits in time and space, if it has any; what function it serves in the universal economy. the journey is long, yet we want, in knowledge at least, to make it. hence we build observatories and train observers and investigators. slow, indeed, is progress in the solution of the greatest of problems, when measured by what we want to know. some questions may require centuries, others thousands of years for their answer. and yet never was progress more rapid than during our time. in some directions our astronomers of to-day are out of sight of those of fifty years ago; we are even gaining heights which twenty years ago looked hopeless. never before had the astronomer so much work--good, hard, yet hopeful work--before him as to-day. he who is leaving the stage feels that he has only begun and must leave his successors with more to do than his predecessors left him. to us an interesting feature of this progress is the part taken in it by our own country. the science of our day, it is true, is of no country. yet we very appropriately speak of american science from the fact that our traditional reputation has not been that of a people deeply interested in the higher branches of intellectual work. men yet living can remember when in the eyes of the universal church of learning, all cisatlantic countries, our own included, were partes infidelium. yet american astronomy is not entirely of our generation. in the middle of the last century professor winthrop, of harvard, was an industrious observer of eclipses and kindred phenomena, whose work was recorded in the transactions of learned societies. but the greatest astronomical activity during our colonial period was that called out by the transit of venus in , which was visible in this country. a committee of the american philosophical society, at philadelphia, organized an excellent system of observations, which we now know to have been fully as successful, perhaps more so, than the majority of those made on other continents, owing mainly to the advantages of air and climate. among the observers was the celebrated rittenhouse, to whom is due the distinction of having been the first american astronomer whose work has an important place in the history of the science. in addition to the observations which he has left us, he was the first inventor or proposer of the collimating telescope, an instrument which has become almost a necessity wherever accurate observations are made. the fact that the subsequent invention by bessel may have been independent does not detract from the merits of either. shortly after the transit of venus, which i have mentioned, the war of the revolution commenced. the generation which carried on that war and the following one, which framed our constitution and laid the bases of our political institutions, were naturally too much occupied with these great problems to pay much attention to pure science. while the great mathematical astronomers of europe were laying the foundation of celestial mechanics their writings were a sealed book to every one on this side of the atlantic, and so remained until bowditch appeared, early in the present century. his translation of the mecanique celeste made an epoch in american science by bringing the great work of laplace down to the reach of the best american students of his time. american astronomers must always honor the names of rittenhouse and bowditch. and yet in one respect their work was disappointing of results. neither of them was the founder of a school. rittenhouse left no successor to carry on his work. the help which bowditch afforded his generation was invaluable to isolated students who, here and there, dived alone and unaided into the mysteries of the celestial motions. his work was not mainly in the field of observational astronomy, and therefore did not materially influence that branch of science. in professor airy, afterwards astronomer royal of england, made a report to the british association on the condition of practical astronomy in various countries. in this report he remarked that he was unable to say anything about american astronomy because, so far as he knew, no public observatory existed in the united states. william c. bond, afterwards famous as the first director of the harvard observatory, was at that time making observations with a small telescope, first near boston and afterwards at cambridge. but with so meagre an outfit his establishment could scarcely lay claim to being an astronomical observatory, and it was not surprising if airy did not know anything of his modest efforts. if at this time professor airy had extended his investigations into yet another field, with a view of determining the prospects for a great city at the site of fort dearborn, on the southern shore of lake michigan, he would have seen as little prospect of civic growth in that region as of a great development of astronomy in the united states at large. a plat of the proposed town of chicago had been prepared two years before, when the place contained perhaps half a dozen families. in the same month in which professor airy made his report, august, , the people of the place, then numbering twenty-eight voters, decided to become incorporated, and selected five trustees to carry on their government. in a city charter was obtained from the legislature of illinois. the growth of this infant city, then small even for an infant, into the great commercial metropolis of the west has been the just pride of its people and the wonder of the world. i mention it now because of a remarkable coincidence. with this civic growth has quietly gone on another, little noted by the great world, and yet in its way equally wonderful and equally gratifying to the pride of those who measure greatness by intellectual progress. taking knowledge of the universe as a measure of progress, i wish to invite attention to the fact that american astronomy began with your city, and has slowly but surely kept pace with it, until to-day our country stands second only to germany in the number of researches being prosecuted, and second to none in the number of men who have gained the highest recognition by their labors. in professor albert hopkins, of williams college, and professor elias loomis, of western reserve college, ohio, both commenced little observatories. professor loomis went to europe for all his instruments, but hopkins was able even then to get some of his in this country. shortly afterwards a little wooden structure was erected by captain gilliss on capitol hill, at washington, and supplied with a transit instrument for observing moon culminations, in conjunction with captain wilkes, who was then setting out on his exploring expedition to the southern hemisphere. the date of these observatories was practically the same as that on which a charter for the city of chicago was obtained from the legislature. with their establishment the population of your city had increased to . the next decade, to , was that in which our practical astronomy seriously commenced. the little observatory of captain gilliss was replaced by the naval, then called the national observatory, erected at washington during the years - , and fitted out with what were then the most approved instruments. about the same time the appearance of the great comet of led the citizens of boston to erect the observatory of harvard college. thus it is little more than a half-century since the two principal observatories in the united states were established. but we must not for a moment suppose that the mere erection of an observatory can mark an epoch in scientific history. what must make the decade of which i speak ever memorable in american astronomy was not merely the erection of buildings, but the character of the work done by astronomers away from them as well as in them. the national observatory soon became famous by two remarkable steps which raised our country to an important position among those applying modern science to practical uses. one of these consisted of the researches of sears cook walker on the motion of the newly discovered planet neptune. he was the first astronomer to determine fairly good elements of the orbit of that planet, and, what is yet more remarkable, he was able to trace back the movement of the planet in the heavens for half a century and to show that it had been observed as a fixed star by lalande in , without the observer having any suspicion of the true character of the object. the other work to which i refer was the application to astronomy and to the determination of longitudes of the chronographic method of registering transits of stars or other phenomena requiring an exact record of the instant of their occurrence. it is to be regretted that the history of this application has not been fully written. in some points there seems to be as much obscurity as with the discovery of ether as an anaesthetic, which took place about the same time. happily, no such contest has been fought over the astronomical as over the surgical discovery, the fact being that all who were engaged in the application of the new method were more anxious to perfect it than they were to get credit for themselves. we know that saxton, of the coast survey; mitchell and locke, of cincinnati; bond, at cambridge, as well as walker, and other astronomers at the naval observatory, all worked at the apparatus; that maury seconded their efforts with untiring zeal; that it was used to determine the longitude of baltimore as early as by captain wilkes, and that it was put into practical use in recording observations at the naval observatory as early as . at the cambridge observatory the two bonds, father and son, speedily began to show the stuff of which the astronomer is made. a well-devised system of observations was put in operation. the discovery of the dark ring of saturn and of a new satellite to that planet gave additional fame to the establishment. nor was activity confined to the observational side of the science. the same decade of which i speak was marked by the beginning of professor pierce's mathematical work, especially his determination of the perturbations of uranus and neptune. at this time commenced the work of dr. b. a. gould, who soon became the leading figure in american astronomy. immediately on graduating at harvard in , he determined to devote all the energies of his life to the prosecution of his favorite science. he studied in europe for three years, took the doctor's degree at gottingen, came home, founded the astronomical journal, and took an active part in that branch of the work of the coast survey which included the determination of longitudes by astronomical methods. an episode which may not belong to the history of astronomy must be acknowledged to have had a powerful influence in exciting public interest in that science. professor o. m. mitchell, the founder and first director of the cincinnati observatory, made the masses of our intelligent people acquainted with the leading facts of astronomy by courses of lectures which, in lucidity and eloquence, have never been excelled. the immediate object of the lectures was to raise funds for establishing his observatory and fitting it out with a fine telescope. the popular interest thus excited in the science had an important effect in leading the public to support astronomical research. if public support, based on public interest, is what has made the present fabric of american astronomy possible, then should we honor the name of a man whose enthusiasm leavened the masses of his countrymen with interest in our science. the civil war naturally exerted a depressing influence upon our scientific activity. the cultivator of knowledge is no less patriotic than his fellow-citizens, and vies with them in devotion to the public welfare. the active interest which such cultivators took, first in the prosecution of the war and then in the restoration of the union, naturally distracted their attention from their favorite pursuits. but no sooner was political stability reached than a wave of intellectual activity set in, which has gone on increasing up to the present time. if it be true that never before in our history has so much attention been given to education as now; that never before did so many men devote themselves to the diffusion of knowledge, it is no less true that never was astronomical work so energetically pursued among us as at the present time. one deplorable result of the civil war was that gould's astronomical journal had to be suspended. shortly after the restoration of peace, instead of re-establishing the journal, its founder conceived the project of exploring the southern heavens. the northern hemisphere being the seat of civilization, that portion of the sky which could not be seen from our latitudes was comparatively neglected. what had been done in the southern hemisphere was mostly the occasional work of individuals and of one or two permanent observatories. the latter were so few in number and so meagre in their outfit that a splendid field was open to the inquirer. gould found the patron which he desired in the government of the argentine republic, on whose territory he erected what must rank in the future as one of the memorable astronomical establishments of the world. his work affords a most striking example of the principle that the astronomer is more important than his instruments. not only were the means at the command of the argentine observatory slender in the extreme when compared with those of the favored institutions of the north, but, from the very nature of the case, the argentine republic could not supply trained astronomers. the difficulties thus growing out of the administration cannot be overestimated. and yet the sixteen great volumes in which the work of the institution has been published will rank in the future among the classics of astronomy. another wonderful focus of activity, in which one hardly knows whether he ought most to admire the exhaustless energy or the admirable ingenuity which he finds displayed, is the harvard observatory. its work has been aided by gifts which have no parallel in the liberality that prompted them. yet without energy and skill such gifts would have been useless. the activity of the establishment includes both hemispheres. time would fail to tell how it has not only mapped out important regions of the heavens from the north to the south pole, but analyzed the rays of light which come from hundreds of thousands of stars by recording their spectra in permanence on photographic plates. the work of the establishment is so organized that a new star cannot appear in any part of the heavens nor a known star undergo any noteworthy change without immediate detection by the photographic eye of one or more little telescopes, all-seeing and never-sleeping policemen that scan the heavens unceasingly while the astronomer may sleep, and report in the morning every case of irregularity in the proceedings of the heavenly bodies. yet another example, showing what great results may be obtained with limited means, is afforded by the lick observatory, on mount hamilton, california. during the ten years of its activity its astronomers have made it known the world over by works and discoveries too varied and numerous to be even mentioned at the present time. the astronomical work of which i have thus far spoken has been almost entirely that done at observatories. i fear that i may in this way have strengthened an erroneous impression that the seat of important astronomical work is necessarily connected with an observatory. it must be admitted that an institution which has a local habitation and a magnificent building commands public attention so strongly that valuable work done elsewhere may be overlooked. a very important part of astronomical work is done away from telescopes and meridian circles and requires nothing but a good library for its prosecution. one who is devoted to this side of the subject may often feel that the public does not appreciate his work at its true relative value from the very fact that he has no great buildings or fine instruments to show. i may therefore be allowed to claim as an important factor in the american astronomy of the last half-century an institution of which few have heard and which has been overlooked because there was nothing about it to excite attention. in the american nautical almanac office was established by a congressional appropriation. the title of this publication is somewhat misleading in suggesting a simple enlargement of the family almanac which the sailor is to hang up in his cabin for daily use. the fact is that what started more than a century ago as a nautical almanac has since grown into an astronomical ephemeris for the publication of everything pertaining to times, seasons, eclipses, and the motions of the heavenly bodies. it is the work in which astronomical observations made in all the great observatories of the world are ultimately utilized for scientific and public purposes. each of the leading nations of western europe issues such a publication. when the preparation and publication of the american ephemeris was decided upon the office was first established in cambridge, the seat of harvard university, because there could most readily be secured the technical knowledge of mathematics and theoretical astronomy necessary for the work. a field of activity was thus opened, of which a number of able young men who have since earned distinction in various walks of life availed themselves. the head of the office, commander davis, adopted a policy well fitted to promote their development. he translated the classic work of gauss, theoria motus corporum celestium, and made the office a sort of informal school, not, indeed, of the modern type, but rather more like the classic grove of hellas, where philosophers conducted their discussions and profited by mutual attrition. when, after a few years of experience, methods were well established and a routine adopted, the office was removed to washington, where it has since remained. the work of preparing the ephemeris has, with experience, been reduced to a matter of routine which may be continued indefinitely, with occasional changes in methods and data, and improvements to meet the increasing wants of investigators. the mere preparation of the ephemeris includes but a small part of the work of mathematical calculation and investigation required in astronomy. one of the great wants of the science to-day is the reduction of the observations made during the first half of the present century, and even during the last half of the preceding one. the labor which could profitably be devoted to this work would be more than that required in any one astronomical observatory. it is unfortunate for this work that a great building is not required for its prosecution because its needfulness is thus very generally overlooked by that portion of the public interested in the progress of science. an organization especially devoted to it is one of the scientific needs of our time. in such an epoch-making age as the present it is dangerous to cite any one step as making a new epoch. yet it may be that when the historian of the future reviews the science of our day he will find the most remarkable feature of the astronomy of the last twenty years of our century to be the discovery that this steadfast earth of which the poets have told us is not, after all, quite steadfast; that the north and south poles move about a very little, describing curves so complicated that they have not yet been fully marked out. the periodic variations of latitude thus brought about were first suspected about , and announced with some modest assurance by kustner, of berlin, a few years later. the progress of the views of astronomical opinion from incredulity to confidence was extremely slow until, about , chandler, of the united states, by an exhaustive discussion of innumerable results of observations, showed that the latitude of every point on the earth was subject to a double oscillation, one having a period of a year, the other of four hundred and twenty-seven days. notwithstanding the remarkable parallel between the growth of american astronomy and that of your city, one cannot but fear that if a foreign observer had been asked only half a dozen years ago at what point in the united states a great school of theoretical and practical astronomy, aided by an establishment for the exploration of the heavens, was likely to be established by the munificence of private citizens, he would have been wiser than most foreigners had he guessed chicago. had this place been suggested to him, i fear he would have replied that were it possible to utilize celestial knowledge in acquiring earthly wealth, here would be the most promising seat for such a school. but he would need to have been a little wiser than his generation to reflect that wealth is at the base of all progress in knowledge and the liberal arts; that it is only when men are relieved from the necessity of devoting all their energies to the immediate wants of life that they can lead the intellectual life, and that we should therefore look to the most enterprising commercial centre as the likeliest seat for a great scientific institution. now we have the school, and we have the observatory, which we hope will in the near future do work that will cast lustre on the name of its founder as well as on the astronomers who may be associated with it. you will, i am sure, pardon me if i make some suggestions on the subject of the future needs of the establishment. we want this newly founded institution to be a great success, to do work which shall show that the intellectual productiveness of your community will not be allowed to lag behind its material growth the public is very apt to feel that when some munificent patron of science has mounted a great telescope under a suitable dome, and supplied all the apparatus which the astronomer wants to use, success is assured. but such is not the case. the most important requisite, one more difficult to command than telescopes or observatories, may still be wanting. a great telescope is of no use without a man at the end of it, and what the telescope may do depends more upon this appendage than upon the instrument itself. the place which telescopes and observatories have taken in astronomical history are by no means proportional to their dimensions. many a great instrument has been a mere toy in the hands of its owner. many a small one has become famous. twenty years ago there was here in your own city a modest little instrument which, judged by its size, could not hold up its head with the great ones even of that day. it was the private property of a young man holding no scientific position and scarcely known to the public. and yet that little telescope is to-day among the famous ones of the world, having made memorable advances in the astronomy of double stars, and shown its owner to be a worthy successor of the herschels and struves in that line of work. a hundred observers might have used the appliances of the lick observatory for a whole generation without finding the fifth satellite of jupiter; without successfully photographing the cloud forms of the milky way; without discovering the extraordinary patches of nebulous light, nearly or quite invisible to the human eye, which fill some regions of the heavens. when i was in zurich last year i paid a visit to the little, but not unknown, observatory of its famous polytechnic school. the professor of astronomy was especially interested in the observations of the sun with the aid of the spectroscope, and among the ingenious devices which he described, not the least interesting was the method of photographing the sun by special rays of the spectrum, which had been worked out at the kenwood observatory in chicago. the kenwood observatory is not, i believe, in the eye of the public, one of the noteworthy institutions of your city which every visitor is taken to see, and yet this invention has given it an important place in the science of our day. should you ask me what are the most hopeful features in the great establishment which you are now dedicating, i would say that they are not alone to be found in the size of your unequalled telescope, nor in the cost of the outfit, but in the fact that your authorities have shown their appreciation of the requirements of success by adding to the material outfit of the establishment the three men whose works i have described. gentlemen of the trustees, allow me to commend to your fostering care the men at the end of the telescope. the constitution of the astronomer shows curious and interesting features. if he is destined to advance the science by works of real genius, he must, like the poet, be born, not made. the born astronomer, when placed in command of a telescope, goes about using it as naturally and effectively as the babe avails itself of its mother's breast. he sees intuitively what less gifted men have to learn by long study and tedious experiment. he is moved to celestial knowledge by a passion which dominates his nature. he can no more avoid doing astronomical work, whether in the line of observations or research, than a poet can chain his pegasus to earth. i do not mean by this that education and training will be of no use to him. they will certainly accelerate his early progress. if he is to become great on the mathematical side, not only must his genius have a bend in that direction, but he must have the means of pursuing his studies. and yet i have seen so many failures of men who had the best instruction, and so many successes of men who scarcely learned anything of their teachers, that i sometimes ask whether the great american celestial mechanician of the twentieth century will be a graduate of a university or of the backwoods. is the man thus moved to the exploration of nature by an unconquerable passion more to be envied or pitied? in no other pursuit does success come with such certainty to him who deserves it. no life is so enjoyable as that whose energies are devoted to following out the inborn impulses of one's nature. the investigator of truth is little subject to the disappointments which await the ambitious man in other fields of activity. it is pleasant to be one of a brotherhood extending over the world, in which no rivalry exists except that which comes out of trying to do better work than any one else, while mutual admiration stifles jealousy. and yet, with all these advantages, the experience of the astronomer may have its dark side. as he sees his field widening faster than he can advance he is impressed with the littleness of all that can be done in one short life. he feels the same want of successors to pursue his work that the founder of a dynasty may feel for heirs to occupy his throne. he has no desire to figure in history as a napoleon of science whose conquests must terminate with his life. even during his active career his work may be such a kind as to require the co-operation of others and the active support of the public. if he is disappointed in commanding these requirements, if he finds neither co-operation nor support, if some great scheme to which he may have devoted much of his life thus proves to be only a castle in the air, he may feel that nature has dealt hardly with him in not endowing him with passions like to those of other men. in treating a theme of perennial interest one naturally tries to fancy what the future may have in store if the traveller, contemplating the ruins of some ancient city which in the long ago teemed with the life and activities of generations of men, sees every stone instinct with emotion and the dust alive with memories of the past, may he not be similarly impressed when he feels that he is looking around upon a seat of future empire--a region where generations yet unborn may take a leading part in moulding the history of the world? what may we not expect of that energy which in sixty years has transformed a straggling village into one of the world's great centres of commerce? may it not exercise a powerful influence on the destiny not only of the country but of the world? if so, shall the power thus to be exercised prove an agent of beneficence, diffusing light and life among nations, or shall it be the opposite? the time must come ere long when wealth shall outgrow the field in which it can be profitably employed. in what direction shall its possessors then look? shall they train a posterity which will so use its power as to make the world better that it has lived in it? will the future heir to great wealth prefer the intellectual life to the life of pleasure? we can have no more hopeful answer to these questions than the establishment of this great university in the very focus of the commercial activity of the west. its connection with the institution we have been dedicating suggests some thoughts on science as a factor in that scheme of education best adapted to make the power of a wealthy community a benefit to the race at large. when we see what a factor science has been in our present civilization, how it has transformed the world and increased the means of human enjoyment by enabling men to apply the powers of nature to their own uses, it is not wonderful that it should claim the place in education hitherto held by classical studies. in the contest which has thus arisen i take no part but that of a peace-maker, holding that it is as important to us to keep in touch with the traditions of our race, and to cherish the thoughts which have come down to us through the centuries, as it is to enjoy and utilize what the present has to offer us. speaking from this point of view, i would point out the error of making the utilitarian applications of knowledge the main object in its pursuit. it is an historic fact that abstract science--science pursued without any utilitarian end--has been at the base of our progress in the utilization of knowledge. if in the last century such men as galvani and volta had been moved by any other motive than love of penetrating the secrets of nature they would never have pursued the seemingly useless experiments they did, and the foundation of electrical science would not have been laid. our present applications of electricity did not become possible until ohm's mathematical laws of the electric current, which when first made known seemed little more than mathematical curiosities, had become the common property of inventors. professional pride on the part of our own henry led him, after making the discoveries which rendered the telegraph possible, to go no further in their application, and to live and die without receiving a dollar of the millions which the country has won through his agency. in the spirit of scientific progress thus shown we have patriotism in its highest form--a sentiment which does not seek to benefit the country at the expense of the world, but to benefit the world by means of one's country. science has its competition, as keen as that which is the life of commerce. but its rivalries are over the question who shall contribute the most and the best to the sum total of knowledge; who shall give the most, not who shall take the most. its animating spirit is love of truth. its pride is to do the greatest good to the greatest number. it embraces not only the whole human race but all nature in its scope. the public spirit of which this city is the focus has made the desert blossom as the rose, and benefited humanity by the diffusion of the material products of the earth. should you ask me how it is in the future to use its influence for the benefit of humanity at large, i would say, look at the work now going on in these precincts, and study its spirit. here are the agencies which will make "the voice of law the harmony of the world." here is the love of country blended with love of the race. here the love of knowledge is as unconfined as your commercial enterprise. let not your youth come hither merely to learn the forms of vertebrates and the properties of oxides, but rather to imbibe that catholic spirit which, animating their growing energies, shall make the power they are to wield an agent of beneficence to all mankind. xix the universe as an organism [footnote: address before the astronomical and astrophysical society of america, december , ] if i were called upon to convey, within the compass of a single sentence, an idea of the trend of recent astronomical and physical science, i should say that it was in the direction of showing the universe to be a connected whole. the farther we advance in knowledge, the clearer it becomes that the bodies which are scattered through the celestial spaces are not completely independent existences, but have, with all their infinite diversity, many attributes in common. in this we are going in the direction of certain ideas of the ancients which modern discovery long seemed to have contradicted. in the infancy of the race, the idea that the heavens were simply an enlarged and diversified earth, peopled by beings who could roam at pleasure from one extreme to the other, was a quite natural one. the crystalline sphere or spheres which contained all formed a combination of machinery revolving on a single plan. but all bonds of unity between the stars began to be weakened when copernicus showed that there were no spheres, that the planets were isolated bodies, and that the stars were vastly more distant than the planets. as discovery went on and our conceptions of the universe were enlarged, it was found that the system of the fixed stars was made up of bodies so vastly distant and so completely isolated that it was difficult to conceive of them as standing in any definable relation to one another. it is true that they all emitted light, else we could not see them, and the theory of gravitation, if extended to such distances, a fact not then proved, showed that they acted on one another by their mutual gravitation. but this was all. leaving out light and gravitation, the universe was still, in the time of herschel, composed of bodies which, for the most part, could not stand in any known relation one to the other. when, forty years ago, the spectroscope was applied to analyze the light coming from the stars, a field was opened not less fruitful than that which the telescope made known to galileo. the first conclusion reached was that the sun was composed almost entirely of the same elements that existed upon the earth. yet, as the bodies of our solar system were evidently closely related, this was not remarkable. but very soon the same conclusion was, to a limited extent, extended to the fixed stars in general. such elements as iron, hydrogen, and calcium were found not to belong merely to our earth, but to form important constituents of the whole universe. we can conceive of no reason why, out of the infinite number of combinations which might make up a spectrum, there should not be a separate kind of matter for each combination. so far as we know, the elements might merge into one another by insensible gradations. it is, therefore, a remarkable and suggestive fact when we find that the elements which make up bodies so widely separate that we can hardly imagine them having anything in common, should be so much the same. in recent times what we may regard as a new branch of astronomical science is being developed, showing a tendency towards unity of structure throughout the whole domain of the stars. this is what we now call the science of stellar statistics. the very conception of such a science might almost appall us by its immensity. the widest statistical field in other branches of research is that occupied by sociology. every country has its census, in which the individual inhabitants are classified on the largest scale and the combination of these statistics for different countries may be said to include all the interest of the human race within its scope. yet this field is necessarily confined to the surface of our planet. in the field of stellar statistics millions of stars are classified as if each taken individually were of no more weight in the scale than a single inhabitant of china in the scale of the sociologist. and yet the most insignificant of these suns may, for aught we know, have planets revolving around it, the interests of whose inhabitants cover as wide a range as ours do upon our own globe. the statistics of the stars may be said to have commenced with herschel's gauges of the heavens, which were continued from time to time by various observers, never, however, on the largest scale. the subject was first opened out into an illimitable field of research through a paper presented by kapteyn to the amsterdam academy of sciences in . the capital results of this paper were that different regions of space contain different kinds of stars and, more especially, that the stars of the milky way belong, in part at least, to a different class from those existing elsewhere. stars not belonging to the milky way are, in large part, of a distinctly different class. the outcome of kapteyn's conclusions is that we are able to describe the universe as a single object, with some characters of an organized whole. a large part of the stars which compose it may be considered as divisible into two groups. one of these comprises the stars composing the great girdle of the milky way. these are distinguished from the others by being bluer in color, generally greater in absolute brilliancy, and affected, there is some reason to believe, with rather slower proper motions the other classes are stars with a greater or less shade of yellow in their color, scattered through a spherical space of unknown dimensions, but concentric with the milky way. thus a sphere with a girdle passing around it forms the nearest approach to a conception of the universe which we can reach to-day. the number of stars in the girdle is much greater than that in the sphere. the feature of the universe which should therefore command our attention is the arrangement of a large part of the stars which compose it in a ring, seemingly alike in all its parts, so far as general features are concerned. so far as research has yet gone, we are not able to say decisively that one region of this ring differs essentially from another. it may, therefore, be regarded as forming a structure built on a uniform plan throughout. all scientific conclusions drawn from statistical data require a critical investigation of the basis on which they rest. if we are going, from merely counting the stars, observing their magnitudes and determining their proper motions, to draw conclusions as to the structure of the universe in space, the question may arise how we can form any estimate whatever of the possible distance of the stars, a conclusion as to which must be the very first step we take. we can hardly say that the parallaxes of more than one hundred stars have been measured with any approach to certainty. the individuals of this one hundred are situated at very different distances from us. we hope, by long and repeated observations, to make a fairly approximate determination of the parallaxes of all the stars whose distance is less than twenty times that of a centauri. but how can we know anything about the distance of stars outside this sphere? what can we say against the view of kepler that the space around our sun is very much thinner in stars than it is at a greater distance; in fact, that, the great mass of the stars may be situated between the surfaces of two concentrated spheres not very different in radius. may not this universe of stars be somewhat in the nature of a hollow sphere? this objection requires very careful consideration on the part of all who draw conclusions as to the distribution of stars in space and as to the extent of the visible universe. the steps to a conclusion on the subject are briefly these: first, we have a general conclusion, the basis of which i have already set forth, that, to use a loose expression, there are likenesses throughout the whole diameter of the universe. there is, therefore, no reason to suppose that the region in which our system is situated differs in any essential degree from any other region near the central portion. again, spectroscopic examinations seem to show that all the stars are in motion, and that we cannot say that those in one part of the universe move more rapidly than those in another. this result is of the greatest value for our purpose, because, when we consider only the apparent motions, as ordinarily observed, these are necessarily dependent upon the distance of the star. we cannot, therefore, infer the actual speed of a star from ordinary observations until we know its distance. but the results of spectroscopic measurements of radial velocity are independent of the distance of the star. but let us not claim too much. we cannot yet say with certainty that the stars which form the agglomerations of the milky way have, beyond doubt, the same average motion as the stars in other regions of the universe. the difficulty is that these stars appear to us so faint individually, that the investigation of their spectra is still beyond the powers of our instruments. but the extraordinary feat performed at the lick observatory of measuring the radial motion of groombridge, a star quite invisible to the naked eye, and showing that it is approaching our system with a speed of between fifty and sixty miles a second, may lead us to hope for a speedy solution of this question. but we need not await this result in order to reach very probable conclusions. the general outcome of researches on proper motions tends to strengthen the conclusions that the keplerian sphere, if i may use this expression, has no very well marked existence. the laws of stellar velocity and the statistics of proper motions, while giving some color to the view that the space in which we are situated is thinner in stars than elsewhere, yet show that, as a general rule, there are no great agglomerations of stars elsewhere than in the region of the milky way. with unity there is always diversity; in fact, the unity of the universe on which i have been insisting consists in part of diversity. it is very curious that, among the many thousands of stars which have been spectroscopically examined, no two are known to have absolutely the same physical constitution. it is true that there are a great many resemblances. alpha centauri, our nearest neighbor, if we can use such a word as "near" in speaking of its distance, has a spectrum very like that of our sun, and so has capella. but even in these cases careful examination shows differences. these differences arise from variety in the combinations and temperature of the substances of which the star is made up. quite likely also, elements not known on the earth may exist on the stars, but this is a point on which we cannot yet speak with certainty. perhaps the attribute in which the stars show the greatest variety is that of absolute luminosity. one hundred years ago it was naturally supposed that the brighter stars were the nearest to us, and this is doubtless true when we take the general average. but it was soon found that we cannot conclude that because a star is bright, therefore it is near. the most striking example of this is afforded by the absence of measurable parallaxes in the two bright stars, canopus and rigel, showing that these stars, though of the first magnitude, are immeasurably distant. a remarkable fact is that these conclusions coincide with that which we draw from the minuteness of the proper motions. rigel has no motion that has certainly been shown by more than a century of observation, and it is not certain that canopus has either. from this alone we may conclude, with a high degree of probability, that the distance of each is immeasurably great. we may say with certainty that the brightness of each is thousands of times that of the sun, and with a high degree of probability that it is hundreds of thousands of times. on the other hand, there are stars comparatively near us of which the light is not the hundredth part of the sun. [illustration with caption: star spectra] the universe may be a unit in two ways. one is that unity of structure to which our attention has just been directed. this might subsist forever without one body influencing another. the other form of unity leads us to view the universe as an organism. it is such by mutual action going on between its bodies. a few years ago we could hardly suppose or imagine that any other agents than gravitation and light could possibly pass through spaces so immense as those which separate the stars. the most remarkable and hopeful characteristic of the unity of the universe is the evidence which is being gathered that there are other agencies whose exact nature is yet unknown to us, but which do pass from one heavenly body to another. the best established example of this yet obtained is afforded in the case of the sun and the earth. the fact that the frequency of magnetic storms goes through a period of about eleven years, and is proportional to the frequency of sun-spots, has been well established. the recent work of professor bigelow shows the coincidence to be of remarkable exactness, the curves of the two phenomena being practically coincident so far as their general features are concerned. the conclusion is that spots on the sun and magnetic storms are due to the same cause. this cause cannot be any change in the ordinary radiation of the sun, because the best records of temperature show that, to whatever variations the sun's radiation may be subjected, they do not change in the period of the sun-spots. to appreciate the relation, we must recall that the researches of hale with the spectro-heliograph show that spots are not the primary phenomenon of solar activity, but are simply the outcome of processes going on constantly in the sun which result in spots only in special regions and on special occasions. it does not, therefore, necessarily follow that a spot does cause a magnetic storm. what we should conclude is that the solar activity which produces a spot also produces the magnetic storm. when we inquire into the possible nature of these relations between solar activity and terrestrial magnetism, we find ourselves so completely in the dark that the question of what is really proved by the coincidence may arise. perhaps the most obvious explanation of fluctuations in the earth's magnetic field to be inquired into would be based on the hypothesis that the space through which the earth is moving is in itself a varying magnetic field of vast extent. this explanation is tested by inquiring whether the fluctuations in question can be explained by supposing a disturbing force which acts substantially in the same direction all over the globe. but a very obvious test shows that this explanation is untenable. were it the correct one, the intensity of the force in some regions of the earth would be diminished and in regions where the needle pointed in the opposite direction would be increased in exactly the same degree. but there is no relation traceable either in any of the regular fluctuations of the magnetic force, or in those irregular ones which occur during a magnetic storm. if the horizontal force is increased in one part of the earth, it is very apt to show a simultaneous increase the world over, regardless of the direction in which the needle may point in various localities. it is hardly necessary to add that none of the fluctuations in terrestrial magnetism can be explained on the hypothesis that either the moon or the sun acts as a magnet. in such a case the action would be substantially in the same direction at the same moment the world over. such being the case, the question may arise whether the action producing a magnetic storm comes from the sun at all, and whether the fluctuations in the sun's activity, and in the earth's magnetic field may not be due to some cause external to both. all we can say in reply to this is that every effort to find such a cause has failed and that it is hardly possible to imagine any cause producing such an effect. it is true that the solar spots were, not many years ago, supposed to be due in some way to the action of the planets. but, for reasons which it would be tedious to go into at present, we may fairly regard this hypothesis as being completely disproved. there can, i conclude, be little doubt that the eleven-year cycle of change in the solar spots is due to a cycle going on in the sun itself. such being the case, the corresponding change in the earth's magnetism must be due to the same cause. we may, therefore, regard it as a fact sufficiently established to merit further investigation that there does emanate from the sun, in an irregular way, some agency adequate to produce a measurable effect on the magnetic needle. we must regard it as a singular fact that no observations yet made give us the slightest indication as to what this emanation is. the possibility of defining it is suggested by the discovery within the past few years, that under certain conditions, heated matter sends forth entities known as rontgen rays, becquerel corpuscles and electrons. i cannot speak authoritatively on this subject, but, so far as i am aware, no direct evidence has yet been gathered showing that any of these entities reach us from the sun. we must regard the search for the unknown agency so fully proved as among the most important tasks of the astronomical physicist of the present time. from what we know of the history of scientific discovery, it seems highly probable that, in the course of his search, he will, before he finds the object he is aiming at, discover many other things of equal or greater importance of which he had, at the outset, no conception. the main point i desire to bring out in this review is the tendency which it shows towards unification in physical research. heretofore differentiation--the subdivision of workers into a continually increasing number of groups of specialists--has been the rule. now we see a coming together of what, at first sight, seem the most widely separated spheres of activity. what two branches could be more widely separated than that of stellar statistics, embracing the whole universe within its scope, and the study of these newly discovered emanations, the product of our laboratories, which seem to show the existence of corpuscles smaller than the atoms of matter? and yet, the phenomena which we have reviewed, especially the relation of terrestrial magnetism to the solar activity, and the formation of nebulous masses around the new stars, can be accounted for only by emanations or forms of force, having probably some similarity with the corpuscles, electrons, and rays which we are now producing in our laboratories. the nineteenth century, in passing away, points with pride to what it has done. it has become a word to symbolize what is most important in human progress yet, perhaps its greatest glory may prove to be that the last thing it did was to lay a foundation for the physical science of the twentieth century. what shall be discovered in the new fields is, at present, as far without our ken as were the modern developments of electricity without the ken of the investigators of one hundred years ago. we cannot guarantee any special discovery. what lies before us is an illimitable field, the existence of which was scarcely suspected ten years ago, the exploration of which may well absorb the activities of our physical laboratories, and of the great mass of our astronomical observers and investigators for as many generations as were required to bring electrical science to its present state. we of the older generation cannot hope to see more than the beginning of this development, and can only tender our best wishes and most hearty congratulations to the younger school whose function it will be to explore the limitless field now before it. xx the relation of scientific method to social progress [footnote: an address before the washington philosophical society] among those subjects which are not always correctly apprehended, even by educated men, we may place that of the true significance of scientific method and the relations of such method to practical affairs. this is especially apt to be the case in a country like our own, where the points of contact between the scientific world on the one hand, and the industrial and political world on the other, are fewer than in other civilized countries. the form which this misapprehension usually takes is that of a failure to appreciate the character of scientific method, and especially its analogy to the methods of practical life. in the judgment of the ordinary intelligent man there is a wide distinction between theoretical and practical science. the latter he considers as that science directly applicable to the building of railroads, the construction of engines, the invention of new machinery, the construction of maps, and other useful objects. the former he considers analogous to those philosophic speculations in which men have indulged in all ages without leading to any result which he considers practical. that our knowledge of nature is increased by its prosecution is a fact of which he is quite conscious, but he considers it as terminating with a mere increase of knowledge, and not as having in its method anything which a person devoted to material interests can be expected to appreciate. this view is strengthened by the spirit with which he sees scientific investigation prosecuted. it is well understood on all sides that when such investigations are pursued in a spirit really recognized as scientific, no merely utilitarian object is had in view. indeed, it is easy to see how the very fact of pursuing such an object would detract from that thoroughness of examination which is the first condition of a real advance. true science demands in its every research a completeness far beyond what is apparently necessary for its practical applications. the precision with which the astronomer seeks to measure the heavens and the chemist to determine the relations of the ultimate molecules of matter has no limit, except that set by the imperfections of the instruments of research. there is no such division recognized as that of useful and useless knowledge. the ultimate aim is nothing less than that of bringing all the phenomena of nature under laws as exact as those which govern the planetary motions. now the pursuit of any high object in this spirit commands from men of wide views that respect which is felt towards all exertion having in view more elevated objects than the pursuit of gain. accordingly, it is very natural to classify scientists and philosophers with the men who in all ages have sought after learning instead of utility. but there is another aspect of the question which will show the relations of scientific advance to the practical affairs of life in a different light. i make bold to say that the greatest want of the day, from a purely practical point of view, is the more general introduction of the scientific method and the scientific spirit into the discussion of those political and social problems which we encounter on our road to a higher plane of public well being. far from using methods too refined for practical purposes, what most distinguishes scientific from other thought is the introduction of the methods of practical life into the discussion of abstract general problems. a single instance will illustrate the lesson i wish to enforce. the question of the tariff is, from a practical point of view, one of the most important with which our legislators will have to deal during the next few years. the widest diversity of opinion exists as to the best policy to be pursued in collecting a revenue from imports. opposing interests contend against one another without any common basis of fact or principle on which a conclusion can be reached. the opinions of intelligent men differ almost as widely as those of the men who are immediately interested. but all will admit that public action in this direction should be dictated by one guiding principle--that the greatest good of the community is to be sought after. that policy is the best which will most promote this good. nor is there any serious difference of opinion as to the nature of the good to be had in view; it is in a word the increase of the national wealth and prosperity. the question on which opinions fundamentally differ is that of the effects of a higher or lower rate of duty upon the interests of the public. if it were possible to foresee, with an approach to certainty, what effect a given tariff would have upon the producers and consumers of an article taxed, and, indirectly, upon each member of the community in any way interested in the article, we should then have an exact datum which we do not now possess for reaching a conclusion. if some superhuman authority, speaking with the voice of infallibility, could give us this information, it is evident that a great national want would be supplied. no question in practical life is more important than this: how can this desirable knowledge of the economic effects of a tariff be obtained? the answer to this question is clear and simple. the subject must be studied in the same spirit, and, to a certain extent, by the same methods which have been so successful in advancing our knowledge of nature. every one knows that, within the last two centuries, a method of studying the course of nature has been introduced which has been so successful in enabling us to trace the sequence of cause and effect as almost to revolutionize society. the very fact that scientific method has been so successful here leads to the belief that it might be equally successful in other departments of inquiry. the same remarks will apply to the questions connected with banking and currency; the standard of value; and, indeed, all subjects which have a financial bearing. on every such question we see wide differences of opinion without any common basis to rest upon. it may be said, in reply, that in these cases there are really no grounds for forming an opinion, and that the contests which arise over them are merely those between conflicting interests. but this claim is not at all consonant with the form which we see the discussion assume. nearly every one has a decided opinion on these several subjects; whereas, if there were no data for forming an opinion, it would be unreasonable to maintain any whatever. indeed, it is evident that there must be truth somewhere, and the only question that can be open is that of the mode of discovering it. no man imbued with a scientific spirit can claim that such truth is beyond the power of the human intellect. he may doubt his own ability to grasp it, but cannot doubt that by pursuing the proper method and adopting the best means the problem can be solved. it is, in fact, difficult to show why some exact results could not be as certainly reached in economic questions as in those of physical science. it is true that if we pursue the inquiry far enough we shall find more complex conditions to encounter, because the future course of demand and supply enters as an uncertain element. but a remarkable fact to be considered is that the difference of opinion to which we allude does not depend upon different estimates of the future, but upon different views of the most elementary and general principles of the subject. it is as if men were not agreed whether air were elastic or whether the earth turns on its axis. why is it that while in all subjects of physical science we find a general agreement through a wide range of subjects, and doubt commences only where certainty is not attained, yet when we turn to economic subjects we do not find the beginning of an agreement? no two answers can be given. it is because the two classes of subjects are investigated by different instruments and in a different spirit. the physicist has an exact nomenclature; uses methods of research well adapted to the objects he has in view; pursues his investigations without being attacked by those who wish for different results; and, above all, pursues them only for the purpose of discovering the truth. in economic questions the case is entirely different. only in rare cases are they studied without at least the suspicion that the student has a preconceived theory to support. if results are attained which oppose any powerful interest, this interest can hire a competing investigator to bring out a different result. so far as the public can see, one man's result is as good as another's, and thus the object is as far off as ever. we may be sure that until there is an intelligent and rational public, able to distinguish between the speculations of the charlatan and the researches of the investigator, the present state of things will continue. what we want is so wide a diffusion of scientific ideas that there shall be a class of men engaged in studying economic problems for their own sake, and an intelligent public able to judge what they are doing. there must be an improvement in the objects at which they aim in education, and it is now worth while to inquire what that improvement is. it is not mere instruction in any branch of technical science that is wanted. no knowledge of chemistry, physics, or biology, however extensive, can give the learner much aid in forming a correct opinion of such a question as that of the currency. if we should claim that political economy ought to be more extensively studied, we would be met by the question, which of several conflicting systems shall we teach? what is wanted is not to teach this system or that, but to give such a training that the student shall be able to decide for himself which system is right. it seems to me that the true educational want is ignored both by those who advocate a classical and those who advocate a scientific education. what is really wanted is to train the intellectual powers, and the question ought to be, what is the best method of doing this? perhaps it might be found that both of the conflicting methods could be improved upon. the really distinctive features, which we should desire to see introduced, are two in number: the one the scientific spirit; the other the scientific discipline. although many details may be classified under each of these heads, yet there is one of pre-eminent importance on which we should insist. the one feature of the scientific spirit which outweighs all others in importance is the love of knowledge for its own sake. if by our system of education we can inculcate this sentiment we shall do what is, from a public point of view, worth more than any amount of technical knowledge, because we shall lay the foundation of all knowledge. so long as men study only what they think is going to be useful their knowledge will be partial and insufficient. i think it is to the constant inculcation of this fact by experience, rather than to any reasoning, that is due the continued appreciation of a liberal education. every business-man knows that a business-college training is of very little account in enabling one to fight the battle of life, and that college-bred men have a great advantage even in fields where mere education is a secondary matter. we are accustomed to seeing ridicule thrown upon the questions sometimes asked of candidates for the civil service because the questions refer to subjects of which a knowledge is not essential. the reply to all criticisms of this kind is that there is no one quality which more certainly assures a man's usefulness to society than the propensity to acquire useless knowledge. most of our citizens take a wide interest in public affairs, else our form of government would be a failure. but it is desirable that their study of public measures should be more critical and take a wider range. it is especially desirable that the conclusions to which they are led should be unaffected by partisan sympathies. the more strongly the love of mere truth is inculcated in their nature the better this end will be attained. the scientific discipline to which i ask mainly to call your attention consists in training the scholar to the scientific use of language. although whole volumes may be written on the logic of science there is one general feature of its method which is of fundamental significance. it is that every term which it uses and every proposition which it enunciates has a precise meaning which can be made evident by proper definitions. this general principle of scientific language is much more easily inculcated by example than subject to exact description; but i shall ask leave to add one to several attempts i have made to define it. if i should say that when a statement is made in the language of science the speaker knows what he means, and the hearer either knows it or can be made to know it by proper definitions, and that this community of understanding is frequently not reached in other departments of thought, i might be understood as casting a slur on whole departments of inquiry. without intending any such slur, i may still say that language and statements are worthy of the name scientific as they approach this standard; and, moreover, that a great deal is said and written which does not fulfil the requirement. the fact that words lose their meaning when removed from the connections in which that meaning has been acquired and put to higher uses, is one which, i think, is rarely recognized. there is nothing in the history of philosophical inquiry more curious than the frequency of interminable disputes on subjects where no agreement can be reached because the opposing parties do not use words in the same sense. that the history of science is not free from this reproach is shown by the fact of the long dispute whether the force of a moving body was proportional to the simple velocity or to its square. neither of the parties to the dispute thought it worth while to define what they meant by the word "force," and it was at length found that if a definition was agreed upon the seeming difference of opinion would vanish. perhaps the most striking feature of the case, and one peculiar to a scientific dispute, was that the opposing parties did not differ in their solution of a single mechanical problem. i say this is curious, because the very fact of their agreeing upon every concrete question which could have been presented ought to have made it clear that some fallacy was lacking in the discussion as to the measure of force. the good effect of a scientific spirit is shown by the fact that this discussion is almost unique in the history of science during the past two centuries, and that scientific men themselves were able to see the fallacy involved, and thus to bring the matter to a conclusion. if we now turn to the discussion of philosophers, we shall find at least one yet more striking example of the same kind. the question of the freedom of the human will has, i believe, raged for centuries. it cannot yet be said that any conclusion has been reached. indeed, i have heard it admitted by men of high intellectual attainments that the question was insoluble. now a curious feature of this dispute is that none of the combatants, at least on the affirmative side, have made any serious attempt to define what should be meant by the phrase freedom of the will, except by using such terms as require definition equally with the word freedom itself. it can, i conceive, be made quite clear that the assertion, "the will is free," is one without meaning, until we analyze more fully the different meanings to be attached to the word free. now this word has a perfectly well-defined signification in every-day life. we say that anything is free when it is not subject to external constraint. we also know exactly what we mean when we say that a man is free to do a certain act. we mean that if he chooses to do it there is no external constraint acting to prevent him. in all cases a relation of two things is implied in the word, some active agent or power, and the presence or absence of another constraining agent. now, when we inquire whether the will itself is free, irrespective of external constraints, the word free no longer has a meaning, because one of the elements implied in it is ignored. to inquire whether the will itself is free is like inquiring whether fire itself is consumed by the burning, or whether clothing is itself clad. it is not, therefore, at all surprising that both parties have been able to dispute without end, but it is a most astonishing phenomenon of the human intellect that the dispute should go on generation after generation without the parties finding out whether there was really any difference of opinion between them on the subject. i venture to say that if there is any such difference, neither party has ever analyzed the meaning of the words used sufficiently far to show it. the daily experience of every man, from his cradle to his grave, shows that human acts are as much the subject of external causal influences as are the phenomena of nature. to dispute this would be little short of the ludicrous. all that the opponents of freedom, as a class, have ever claimed is the assertion of a causal connection between the acts of the will and influences independent of the will. true, propositions of this sort can be expressed in a variety of ways connoting an endless number of more or less objectionable ideas, but this is the substance of the matter. to suppose that the advocates on the other side meant to take issue on this proposition would be to assume that they did not know what they were saying. the conclusion forced upon us is that though men spend their whole lives in the study of the most elevated department of human thought it does not guard them against the danger of using words without meaning. it would be a mark of ignorance, rather than of penetration, to hastily denounce propositions on subjects we are not well acquainted with because we do not understand their meaning. i do not mean to intimate that philosophy itself is subject to this reproach. when we see a philosophical proposition couched in terms we do not understand, the most modest and charitable view is to assume that this arises from our lack of knowledge. nothing is easier than for the ignorant to ridicule the propositions of the learned. and yet, with every reserve, i cannot but feel that the disputes to which i have alluded prove the necessity of bringing scientific precision of language into the whole domain of thought. if the discussion had been confined to a few, and other philosophers had analyzed the subject, and showed the fictitious character of the discussion, or had pointed out where opinions really might differ, there would be nothing derogatory to philosophers. but the most suggestive circumstance is that although a large proportion of the philosophic writers in recent times have devoted more or less attention to the subject, few, or none, have made even this modest contribution. i speak with some little confidence on this subject, because several years ago i wrote to one of the most acute thinkers of the country, asking if he could find in philosophic literature any terms or definitions expressive of the three different senses in which not only the word freedom, but nearly all words implying freedom were used. his search was in vain. nothing of this sort occurs in the practical affairs of life. all terms used in business, however general or abstract, have that well-defined meaning which is the first requisite of the scientific language. now one important lesson which i wish to inculcate is that the language of science in this respect corresponds to that of business; in that each and every term that is employed has a meaning as well defined as the subject of discussion can admit of. it will be an instructive exercise to inquire what this peculiarity of scientific and business language is. it can be shown that a certain requirement should be fulfilled by all language intended for the discovery of truth, which is fulfilled only by the two classes of language which i have described. it is one of the most common errors of discourse to assume that any common expression which we may use always conveys an idea, no matter what the subject of discourse. the true state of the case can, perhaps, best be seen by beginning at the foundation of things and examining under what conditions language can really convey ideas. suppose thrown among us a person of well-developed intellect, but unacquainted with a single language or word that we use. it is absolutely useless to talk to him, because nothing that we say conveys any meaning to his mind. we can supply him no dictionary, because by hypothesis he knows no language to which we have access. how shall we proceed to communicate our ideas to him? clearly there is but one possible way--namely, through his senses. outside of this means of bringing him in contact with us we can have no communication with him. we, therefore, begin by showing him sensible objects, and letting him understand that certain words which we use correspond to those objects. after he has thus acquired a small vocabulary, we make him understand that other terms refer to relations between objects which he can perceive by his senses. next he learns, by induction, that there are terms which apply not to special objects, but to whole classes of objects. continuing the same process, he learns that there are certain attributes of objects made known by the manner in which they affect his senses, to which abstract terms are applied. having learned all this, we can teach him new words by combining words without exhibiting objects already known. using these words we can proceed yet further, building up, as it were, a complete language. but there is one limit at every step. every term which we make known to him must depend ultimately upon terms the meaning of which he has learned from their connection with special objects of sense. to communicate to him a knowledge of words expressive of mental states it is necessary to assume that his own mind is subject to these states as well as our own, and that we can in some way indicate them by our acts. that the former hypothesis is sufficiently well established can be made evident so long as a consistency of different words and ideas is maintained. if no such consistency of meaning on his part were evident, it might indicate that the operations of his mind were so different from ours that no such communication of ideas was possible. uncertainty in this respect must arise as soon as we go beyond those mental states which communicate themselves to the senses of others. we now see that in order to communicate to our foreigner a knowledge of language, we must follow rules similar to those necessary for the stability of a building. the foundation of the building must be well laid upon objects knowable by his five senses. of course the mind, as well as the external object, may be a factor in determining the ideas which the words are intended to express; but this does not in any manner invalidate the conditions which we impose. whatever theory we may adopt of the relative part played by the knowing subject, and the external object in the acquirement of knowledge, it remains none the less true that no knowledge of the meaning of a word can be acquired except through the senses, and that the meaning is, therefore, limited by the senses. if we transgress the rule of founding each meaning upon meanings below it, and having the whole ultimately resting upon a sensuous foundation, we at once branch off into sound without sense. we may teach him the use of an extended vocabulary, to the terms of which he may apply ideas of his own, more or less vague, but there will be no way of deciding that he attaches the same meaning to these terms that we do. what we have shown true of an intelligent foreigner is necessarily true of the growing child. we come into the world without a knowledge of the meaning of words, and can acquire such knowledge only by a process which we have found applicable to the intelligent foreigner. but to confine ourselves within these limits in the use of language requires a course of severe mental discipline. the transgression of the rule will naturally seem to the undisciplined mind a mark of intellectual vigor rather than the reverse. in our system of education every temptation is held out to the learner to transgress the rule by the fluent use of language to which it is doubtful if he himself attaches clear notions, and which he can never be certain suggests to his hearer the ideas which he desires to convey. indeed, we not infrequently see, even among practical educators, expressions of positive antipathy to scientific precision of language so obviously opposed to good sense that they can be attributed only to a failure to comprehend the meaning of the language which they criticise. perhaps the most injurious effect in this direction arises from the natural tendency of the mind, when not subject to a scientific discipline, to think of words expressing sensible objects and their relations as connoting certain supersensuous attributes. this is frequently seen in the repugnance of the metaphysical mind to receive a scientific statement about a matter of fact simply as a matter of fact. this repugnance does not generally arise in respect to the every-day matters of life. when we say that the earth is round we state a truth which every one is willing to receive as final. if without denying that the earth was round, one should criticise the statement on the ground that it was not necessarily round but might be of some other form, we should simply smile at this use of language. but when we take a more general statement and assert that the laws of nature are inexorable, and that all phenomena, so far as we can show, occur in obedience to their requirements, we are met with a sort of criticism with which all of us are familiar, but which i am unable adequately to describe. no one denies that as a matter of fact, and as far as his experience extends, these laws do appear to be inexorable. i have never heard of any one professing, during the present generation, to describe a natural phenomenon, with the avowed belief that it was not a product of natural law; yet we constantly hear the scientific view criticised on the ground that events may occur without being subject to natural law. the word "may," in this connection, is one to which we can attach no meaning expressive of a sensuous relation. the analogous conflict between the scientific use of language and the use made by some philosophers is found in connection with the idea of causation. fundamentally the word cause is used in scientific language in the same sense as in the language of common life. when we discuss with our neighbors the cause of a fit of illness, of a fire, or of cold weather, not the slightest ambiguity attaches to the use of the word, because whatever meaning may be given to it is founded only on an accurate analysis of the ideas involved in it from daily use. no philosopher objects to the common meaning of the word, yet we frequently find men of eminence in the intellectual world who will not tolerate the scientific man in using the word in this way. in every explanation which he can give to its use they detect ambiguity. they insist that in any proper use of the term the idea of power must be connoted. but what meaning is here attached to the word power, and how shall we first reduce it to a sensible form, and then apply its meaning to the operations of nature? whether this can be done, i do not inquire. all i maintain is that if we wish to do it, we must pass without the domain of scientific statement. perhaps the greatest advantage in the use of symbolic and other mathematical language in scientific investigation is that it cannot possibly be made to connote anything except what the speaker means. it adheres to the subject matter of discourse with a tenacity which no criticism can overcome. in consequence, whenever a science is reduced to a mathematical form its conclusions are no longer the subject of philosophical attack. to secure the same desirable quality in all other scientific language it is necessary to give it, so far as possible, the same simplicity of signification which attaches to mathematical symbols. this is not easy, because we are obliged to use words of ordinary language, and it is impossible to divest them of whatever they may connote to ordinary hearers. i have thus sought to make it clear that the language of science corresponds to that of ordinary life, and especially of business life, in confining its meaning to phenomena. an analogous statement may be made of the method and objects of scientific investigation. i think professor clifford was very happy in defining science as organized common-sense. the foundation of its widest general creations is laid, not in any artificial theories, but in the natural beliefs and tendencies of the human mind. its position against those who deny these generalizations is quite analogous to that taken by the scottish school of philosophy against the scepticism of hume. it may be asked, if the methods and language of science correspond to those of practical life, why is not the every-day discipline of that life as good as the discipline of science? the answer is, that the power of transferring the modes of thought of common life to subjects of a higher order of generality is a rare faculty which can be acquired only by scientific discipline. what we want is that in public affairs men shall reason about questions of finance, trade, national wealth, legislation, and administration, with the same consciousness of the practical side that they reason about their own interests. when this habit is once acquired and appreciated, the scientific method will naturally be applied to the study of questions of social policy. when a scientific interest is taken in such questions, their boundaries will be extended beyond the utilities immediately involved, and one important condition of unceasing progress will be complied with. xxi the outlook for the flying-machine mr. secretary langley's trial of his flying-machine, which seems to have come to an abortive issue for the time, strikes a sympathetic chord in the constitution of our race. are we not the lords of creation? have we not girdled the earth with wires through which we speak to our antipodes? do we not journey from continent to continent over oceans that no animal can cross, and with a speed of which our ancestors would never have dreamed? is not all the rest of the animal creation so far inferior to us in every point that the best thing it can do is to become completely subservient to our needs, dying, if need be, that its flesh may become a toothsome dish on our tables? and yet here is an insignificant little bird, from whose mind, if mind it has, all conceptions of natural law are excluded, applying the rules of aerodynamics in an application of mechanical force to an end we have never been able to reach, and this with entire ease and absence of consciousness that it is doing an extraordinary thing. surely our knowledge of natural laws, and that inventive genius which has enabled us to subordinate all nature to our needs, ought also to enable us to do anything that the bird can do. therefore we must fly. if we cannot yet do it, it is only because we have not got to the bottom of the subject. our successors of the not distant future will surely succeed. this is at first sight a very natural and plausible view of the case. and yet there are a number of circumstances of which we should take account before attempting a confident forecast. our hope for the future is based on what we have done in the past. but when we draw conclusions from past successes we should not lose sight of the conditions on which success has depended. there is no advantage which has not its attendant drawbacks; no strength which has not its concomitant weakness. wealth has its trials and health its dangers. we must expect our great superiority to the bird to be associated with conditions which would give it an advantage at some point. a little study will make these conditions clear. we may look on the bird as a sort of flying-machine complete in itself, of which a brain and nervous system are fundamentally necessary parts. no such machine can navigate the air unless guided by something having life. apart from this, it could be of little use to us unless it carried human beings on its wings. we thus meet with a difficulty at the first step--we cannot give a brain and nervous system to our machine. these necessary adjuncts must be supplied by a man, who is no part of the machine, but something carried by it. the bird is a complete machine in itself. our aerial ship must be machine plus man. now, a man is, i believe, heavier than any bird that flies. the limit which the rarity of the air places upon its power of supporting wings, taken in connection with the combined weight of a man and a machine, make a drawback which we should not too hastily assume our ability to overcome. the example of the bird does not prove that man can fly. the hundred and fifty pounds of dead weight which the manager of the machine must add to it over and above that necessary in the bird may well prove an insurmountable obstacle to success. i need hardly remark that the advantage possessed by the bird has its attendant drawbacks when we consider other movements than flying. its wings are simply one pair of its legs, and the human race could not afford to abandon its arms for the most effective wings that nature or art could supply. another point to be considered is that the bird operates by the application of a kind of force which is peculiar to the animal creation, and no approach to which has ever been made in any mechanism. this force is that which gives rise to muscular action, of which the necessary condition is the direct action of a nervous system. we cannot have muscles or nerves for our flying-machine. we have to replace them by such crude and clumsy adjuncts as steam-engines and electric batteries. it may certainly seem singular if man is never to discover any combination of substances which, under the influence of some such agency as an electric current, shall expand and contract like a muscle. but, if he is ever to do so, the time is still in the future. we do not see the dawn of the age in which such a result will be brought forth. another consideration of a general character may be introduced. as a rule it is the unexpected that happens in invention as well as discovery. there are many problems which have fascinated mankind ever since civilization began which we have made little or no advance in solving. the only satisfaction we can feel in our treatment of the great geometrical problems of antiquity is that we have shown their solution to be impossible. the mathematician of to-day admits that he can neither square the circle, duplicate the cube or trisect the angle. may not our mechanicians, in like manner, be ultimately forced to admit that aerial flight is one of that great class of problems with which man can never cope, and give up all attempts to grapple with it? [illustration with caption: professor langley's air-ship] the fact is that invention and discovery have, notwithstanding their seemingly wide extent, gone on in rather narrower lines than is commonly supposed. if, a hundred years ago, the most sagacious of mortals had been told that before the nineteenth century closed the face of the earth would be changed, time and space almost annihilated, and communication between continents made more rapid and easy than it was between cities in his time; and if he had been asked to exercise his wildest imagination in depicting what might come--the airship and the flying-machine would probably have had a prominent place in his scheme, but neither the steamship, the railway, the telegraph, nor the telephone would have been there. probably not a single new agency which he could have imagined would have been one that has come to pass. it is quite clear to me that success must await progress of a different kind from that which the inventors of flying-machines are aiming at. we want a great discovery, not a great invention. it is an unfortunate fact that we do not always appreciate the distinction between progress in scientific discovery and ingenious application of discovery to the wants of civilization. the name of marconi is familiar to every ear; the names of maxwell and herz, who made the discoveries which rendered wireless telegraphy possible, are rarely recalled. modern progress is the result of two factors: discoveries of the laws of nature and of actions or possibilities in nature, and the application of such discoveries to practical purposes. the first is the work of the scientific investigator, the second that of the inventor. in view of the scientific discoveries of the past ten years, which, after bringing about results that would have seemed chimerical if predicted, leading on to the extraction of a substance which seems to set the laws and limits of nature at defiance by radiating a flood of heat, even when cooled to the lowest point that science can reach--a substance, a few specks of which contain power enough to start a railway train, and embody perpetual motion itself, almost--he would be a bold prophet who would set any limit to possible discoveries in the realm of nature. we are binding the universe together by agencies which pass from sun to planet and from star to star. we are determined to find out all we can about the mysterious ethereal medium supposed to fill all space, and which conveys light and heat from one heavenly body to another, but which yet evades all direct investigation. we are peering into the law of gravitation itself with the full hope of discovering something in its origin which may enable us to evade its action. from time to time philosophers fancy the road open to success, yet nothing that can be practically called success has yet been reached or even approached. when it is reached, when we are able to state exactly why matter gravitates, then will arise the question how this hitherto unchangeable force may be controlled and regulated. with this question answered the problem of the interaction between ether and matter may be solved. that interaction goes on between ethers and molecules is shown by the radiation of heat by all bodies. when the molecules are combined into a mass, this interaction ceases, so that the lightest objects fly through the ether without resistance. why is this? why does ether act on the molecule and not the mass? when we can produce the latter, and when the mutual action can be controlled, then may gravitation be overcome and then may men build, not merely airships, but ships which shall fly above the air, and transport their passengers from continent to continent with the speed of the celestial motions. the first question suggested to the reader by these considerations is whether any such result is possible; whether it is within the power of man to discover the nature of luminiferous ether and the cause of gravitation. to this the profoundest philosopher can only answer, "i do not know." quite possibly the gates at which he is beating are, in the very nature of things, incapable of being opened. it may be that the mind of man is incapable of grasping the secrets within them. the question has even occurred to me whether, if a being of such supernatural power as to understand the operations going on in a molecule of matter or in a current of electricity as we understand the operations of a steam-engine should essay to explain them to us, he would meet with any more success than we should in explaining to a fish the engines of a ship which so rudely invades its domain. as was remarked by william k. clifford, perhaps the clearest spirit that has ever studied such problems, it is possible that the laws of geometry for spaces infinitely small may be so different from those of larger spaces that we must necessarily be unable to conceive them. still, considering mere possibilities, it is not impossible that the twentieth century may be destined to make known natural forces which will enable us to fly from continent to continent with a speed far exceeding that of the bird. but when we inquire whether aerial flight is possible in the present state of our knowledge, whether, with such materials as we possess, a combination of steel, cloth, and wire can be made which, moved by the power of electricity or steam, shall form a successful flying-machine, the outlook may be altogether different. to judge it sanely, let us bear in mind the difficulties which are encountered in any flying-machine. the basic principle on which any such machine must be constructed is that of the aeroplane. this, by itself, would be the simplest of all flyers, and therefore the best if it could be put into operation. the principle involved may be readily comprehended by the accompanying figure. a m is the section of a flat plane surface, say a thin sheet of metal or a cloth supported by wires. it moves through the air, the latter being represented by the horizontal rows of dots. the direction of the motion is that of the horizontal line a p. the aeroplane has a slight inclination measured by the proportion between the perpendicular m p and the length a p. we may raise the edge m up or lower it at pleasure. now the interesting point, and that on which the hopes of inventors are based, is that if we give the plane any given inclination, even one so small that the perpendicular m p is only two or three per cent of the length a m, we can also calculate a certain speed of motion through the air which, if given to the plane, will enable it to bear any required weight. a plane ten feet square, for example, would not need any great inclination, nor would it require a speed higher than a few hundred feet a second to bear a man. what is of yet more importance, the higher the speed the less the inclination required, and, if we leave out of consideration the friction of the air and the resistance arising from any object which the machine may carry, the less the horse-power expended in driving the plane. [illustration] maxim exemplified this by experiment several years ago. he found that, with a small inclination, he could readily give his aeroplane, when it slid forward upon ways, such a speed that it would rise from the ways of itself. the whole problem of the successful flying-machine is, therefore, that of arranging an aeroplane that shall move through the air with the requisite speed. the practical difficulties in the way of realizing the movement of such an object are obvious. the aeroplane must have its propellers. these must be driven by an engine with a source of power. weight is an essential quality of every engine. the propellers must be made of metal, which has its weakness, and which is liable to give way when its speed attains a certain limit. and, granting complete success, imagine the proud possessor of the aeroplane darting through the air at a speed of several hundred feet per second! it is the speed alone that sustains him. how is he ever going to stop? once he slackens his speed, down he begins to fall. he may, indeed, increase the inclination of his aeroplane. then he increases the resistance to the sustaining force. once he stops he falls a dead mass. how shall he reach the ground without destroying his delicate machinery? i do not think the most imaginative inventor has yet even put upon paper a demonstratively successful way of meeting this difficulty. the only ray of hope is afforded by the bird. the latter does succeed in stopping and reaching the ground safely after its flight. but we have already mentioned the great advantages which the bird possesses in the power of applying force to its wings, which, in its case, form the aeroplanes. but we have already seen that there is no mechanical combination, and no way of applying force, which will give to the aeroplanes the flexibility and rapidity of movement belonging to the wings of a bird. with all the improvements that the genius of man has made in the steamship, the greatest and best ever constructed is liable now and then to meet with accident. when this happens she simply floats on the water until the damage is repaired, or help reaches her. unless we are to suppose for the flying-machine, in addition to everything else, an immunity from accident which no human experience leads us to believe possible, it would be liable to derangements of machinery, any one of which would be necessarily fatal. if an engine were necessary not only to propel a ship, but also to make her float--if, on the occasion of any accident she immediately went to the bottom with all on board--there would not, at the present day, be any such thing as steam navigation. that this difficulty is insurmountable would seem to be a very fair deduction, not only from the failure of all attempts to surmount it, but from the fact that maxim has never, so far as we are aware, followed up his seemingly successful experiment. there is, indeed, a way of attacking it which may, at first sight, seem plausible. in order that the aeroplane may have its full sustaining power, there is no need that its motion be continuously forward. a nearly horizontal surface, swinging around in a circle, on a vertical axis, like the wings of a windmill moving horizontally, will fulfil all the conditions. in fact, we have a machine on this simple principle in the familiar toy which, set rapidly whirling, rises in the air. why more attempts have not been made to apply this system, with two sets of sails whirling in opposite directions, i do not know. were there any possibility of making a flying-machine, it would seem that we should look in this direction. the difficulties which i have pointed out are only preliminary ones, patent on the surface. a more fundamental one still, which the writer feels may prove insurmountable, is based on a law of nature which we are bound to accept. it is that when we increase the size of any flying-machine without changing its model we increase the weight in proportion to the cube of the linear dimensions, while the effective supporting power of the air increases only as the square of those dimensions. to illustrate the principle let us make two flying-machines exactly alike, only make one on double the scale of the other in all its dimensions. we all know that the volume and therefore the weight of two similar bodies are proportional to the cubes of their dimensions. the cube of two is eight. hence the large machine will have eight times the weight of the other. but surfaces are as the squares of the dimensions. the square of two is four. the heavier machine will therefore expose only four times the wing surface to the air, and so will have a distinct disadvantage in the ratio of efficiency to weight. mechanical principles show that the steam pressures which the engines would bear would be the same, and that the larger engine, though it would have more than four times the horse-power of the other, would have less than eight times. the larger of the two machines would therefore be at a disadvantage, which could be overcome only by reducing the thickness of its parts, especially of its wings, to that of the other machine. then we should lose in strength. it follows that the smaller the machine the greater its advantage, and the smallest possible flying-machine will be the first one to be successful. we see the principle of the cube exemplified in the animal kingdom. the agile flea, the nimble ant, the swift-footed greyhound, and the unwieldy elephant form a series of which the next term would be an animal tottering under its own weight, if able to stand or move at all. the kingdom of flying animals shows a similar gradation. the most numerous fliers are little insects, and the rising series stops with the condor, which, though having much less weight than a man, is said to fly with difficulty when gorged with food. now, suppose that an inventor succeeds, as well he may, in making a machine which would go into a watch-case, yet complete in all its parts, able to fly around the room. it may carry a button, but nothing heavier. elated by his success, he makes one on the same model twice as large in every dimension. the parts of the first, which are one inch in length, he increases to two inches. every part is twice as long, twice as broad, and twice as thick. the result is that his machine is eight times as heavy as before. but the sustaining surface is only four times as great. as compared with the smaller machine, its ratio of effectiveness is reduced to one-half. it may carry two or three buttons, but will not carry over four, because the total weight, machine plus buttons, can only be quadrupled, and if he more than quadruples the weight of the machine, he must less than quadruple that of the load. how many such enlargements must he make before his machine will cease to sustain itself, before it will fall as an inert mass when we seek to make it fly through the air? is there any size at which it will be able to support a human being? we may well hesitate before we answer this question in the affirmative. dr. graham bell, with a cheery optimism very pleasant to contemplate, has pointed out that the law i have just cited may be evaded by not making a larger machine on the same model, but changing the latter in a way tantamount to increasing the number of small machines. this is quite true, and i wish it understood that, in laying down the law i have cited, i limit it to two machines of different sizes on the same model throughout. quite likely the most effective flying-machine would be one carried by a vast number of little birds. the veracious chronicler who escaped from a cloud of mosquitoes by crawling into an immense metal pot and then amused himself by clinching the antennae of the insects which bored through the pot until, to his horror, they became so numerous as to fly off with the covering, was more scientific than he supposed. yes, a sufficient number of humming-birds, if we could combine their forces, would carry an aerial excursion party of human beings through the air. if the watch-maker can make a machine which will fly through the room with a button, then, by combining ten thousand such machines he may be able to carry a man. but how shall the combined forces be applied? the difficulties i have pointed out apply only to the flying-machine properly so-called, and not to the dirigible balloon or airship. it is of interest to notice that the law is reversed in the case of a body which is not supported by the resistance of a fluid in which it is immersed, but floats in it, the ship or balloon, for example. when we double the linear dimensions of a steamship in all its parts, we increase not only her weight but her floating power, her carrying capacity, and her engine capacity eightfold. but the resistance which she meets with when passing through the water at a given speed is only multiplied four times. hence, the larger we build the steamship the more economical the application of the power necessary to drive it at a given speed. it is this law which has brought the great increase in the size of ocean steamers in recent times. the proportionately diminishing resistance which, in the flying-machine, represents the floating power is, in the ship, something to be overcome. thus there is a complete reversal of the law in its practical application to the two cases. the balloon is in the same class with the ship. practical difficulties aside, the larger it is built the more effective it will be, and the more advantageous will be the ratio of the power which is necessary to drive it to the resistance to be overcome. if, therefore, we are ever to have aerial navigation with our present knowledge of natural capabilities, it is to the airship floating in the air, rather than the flying-machine resting on the air, to which we are to look. in the light of the law which i have laid down, the subject, while not at all promising, seems worthy of more attention than it has received. it is not at all unlikely that if a skilful and experienced naval constructor, aided by an able corps of assistants, should design an airship of a diameter of not less than two hundred feet, and a length at least four or five times as great, constructed, possibly, of a textile substance impervious to gas and borne by a light framework, but, more likely, of exceedingly thin plates of steel carried by a frame fitted to secure the greatest combination of strength and lightness, he might find the result to be, ideally at least, a ship which would be driven through the air by a steam-engine with a velocity far exceeding that of the fleetest atlantic liner. then would come the practical problem of realizing the ship by overcoming the mechanical difficulties involved in the construction of such a huge and light framework. i would not be at all surprised if the result of the exact calculation necessary to determine the question should lead to an affirmative conclusion, but i am quite unable to judge whether steel could be rolled into parts of the size and form required in the mechanism. in judging of the possibility of commercial success the cheapness of modern transportation is an element in the case that should not be overlooked. i believe the principal part of the resistance which a limited express train meets is the resistance of the air. this would be as great for an airship as for a train. an important fraction of the cost of transporting goods from chicago to london is that of getting them into vehicles, whether cars or ships, and getting them out again. the cost of sending a pair of shoes from a shop in new york to the residence of the wearer is, if i mistake not, much greater than the mere cost of transporting them across the atlantic. even if a dirigible balloon should cross the atlantic, it does not follow that it could compete with the steamship in carrying passengers and freight. i may, in conclusion, caution the reader on one point. i should be very sorry if my suggestion of the advantage of the huge airship leads to the subject being taken up by any other than skilful engineers or constructors, able to grapple with all problems relating to the strength and resistance of materials. as a single example of what is to be avoided i may mention the project, which sometimes has been mooted, of making a balloon by pumping the air from a very thin, hollow receptacle. such a project is as futile as can well be imagined; no known substance would begin to resist the necessary pressure. our aerial ship must be filled with some substance lighter than air. whether heated air would answer the purpose, or whether we should have to use a gas, is a question for the designer. to return to our main theme, all should admit that if any hope for the flying-machine can be entertained, it must be based more on general faith in what mankind is going to do than upon either reasoning or experience. we have solved the problem of talking between two widely separated cities, and of telegraphing from continent to continent and island to island under all the oceans--therefore we shall solve the problem of flying. but, as i have already intimated, there is another great fact of progress which should limit this hope. as an almost universal rule we have never solved a problem at which our predecessors have worked in vain, unless through the discovery of some agency of which they have had no conception. the demonstration that no possible combination of known substances, known forms of machinery, and known forms of force can be united in a practicable machine by which men shall fly long distances through the air, seems to the writer as complete as it is possible for the demonstration of any physical fact to be. but let us discover a substance a hundred times as strong as steel, and with that some form of force hitherto unsuspected which will enable us to utilize this strength, or let us discover some way of reversing the law of gravitation so that matter may be repelled by the earth instead of attracted--then we may have a flying-machine. but we have every reason to believe that mere ingenious contrivances with our present means and forms of force will be as vain in the future as they have been in the past. plato's doctrine respecting the rotation of the earth, and aristotle's comment upon that doctrine. by george grote, esq. london: john murray, albemarle street. . _the right of translation is reserved._ london: printed by william clowes and sons, stamford street, and charing cross. examination of the three following questions:-- . whether the doctrine of the earth's rotation is affirmed or implied in the platonic timÆus? . if affirmed or implied, in what sense? . what is the cosmical function which plato assigns to the earth in the timÆus? preface. the following paper was originally intended as an explanatory note on the platonic timæus, in the work which i am now preparing on plato and aristotle. interpreting, differently from others, the much debated passage in which plato describes the cosmical function of the earth, i found it indispensable to give my reasons for this new view. but i soon discovered that those reasons could not be comprised within the limits of a note. accordingly i here publish them in a separate dissertation. the manner in which the earth's rotation was conceived, illustrates the scientific character of the platonic and aristotelian age, as contrasted with the subsequent development and improvement of astronomy. plato--on the earth's rotation. in plato, timæus, p. b, we read the following words--[greek: gê=n de\ tropho\n me\n ê(mete/ran, ei(llome/nên de\ peri\ to\n dia\ panto\s po/lon tetame/non phu/laka kai\ dêmiourgo\n nukto/s te kai\ ê(me/ras e)mêchanê/sato, prô/tên kai\ presbuta/tên theô=n, o(/soi e)nto\s ou)ra/nou gego/nasi.] i give the text as it stands in stallbaum's edition. the obscurity of this passage is amply attested by the numerous differences of opinion to which it has given rise, both in ancient and in modern times. various contemporaries of plato ([greek: e)/nioi]--aristot. de coelo, ii. , p. b. ) understood it as asserting or implying the rotatory movement of the earth in the centre of the kosmos, and adhered to this doctrine as their own. aristotle himself alludes to these contemporaries without naming them, and adopts their interpretation of the passage; but dissents from the doctrine, and proceeds to impugn it by arguments. cicero mentions (academic ii. ) that there were persons who believed plato to have indicated the same doctrine obscurely, in his timaeus: this passage must undoubtedly be meant. plutarch devotes a critical chapter to the enquiry, what was plato's real doctrine as to the cosmical function of the earth--its movement or rest (quaestion. platonic. vii. , p. .) there exists a treatise, in doric dialect, entitled [greek: ti/maio** tô= lo/krô peri\ psucha=s ko/smô kai\ phu/sios], which is usually published along with the works of plato. this treatise was supposed in ancient times to be a genuine production of the lokrian timaeus, whom plato introduces as his spokesman in the dialogue so called. as such, it was considered to be of much authority in settling questions of interpretation as to the platonic timaeus. but modern critics hold, i believe unanimously, that it is the work of some later pythagorean or platonist, excerpted or copied from the platonic timaeus. this treatise represents the earth as being in the centre and at rest. but its language, besides being dark and metaphorical, departs widely from the phraseology of the platonic timaeus: especially in this--that it makes no mention of the cosmical axis, nor of the word [greek: i)llome/nên] or [greek: ei(loume/nên]. alexander of aphrodisias (as we learn from simplikius ad aristot. de coelo, fol. ) followed the construction of plato given by aristotle. "it was improbable (he said) that aristotle could be ignorant either what the word signified, or what was plato's purpose" ([greek: a)lla\ tô=| a)ristote/lei, phêsi\n, ou(/tô le/gonti _i)/llesthai_, ou)k eu)/logon a)ntile/gein; ô(s a)lêthô=s ga\r ou)/te tê=s le/xeôs to\ sêmaino/menon ei)ko\s ê)=n a)gnoei=n au)to\n, ou)/te to\n pla/tônos skopo/n.] this passage is not given in the scholia of brandis). alexander therefore construed [greek: i)llome/nên] as meaning or implying rotatory movement, though in so doing he perverted (so simplikius says) the true meaning to make it consonant with his own suppositions. proklus maintains that aristotle has interpreted the passage erroneously,--that [greek: i)llome/nên] is equivalent to [greek: sphiggome/nên] or [greek: xunechome/nên]--and that plato intends by it to affirm the earth as at rest in the centre of the kosmos (ad timaeum, book iv., p. ed. schneider). simplikius himself is greatly perplexed, and scarcely ventures to give a positive opinion of his own. on the whole, he inclines to believe that [greek: i)llome/nên] might possibly be understood, by superficial readers, so as to signify rotation, though such is not its proper and natural sense: that some platonists did so misunderstand it: and that aristotle accepted their sense for the sake of the argument, without intending himself to countenance it (ad aristot. de coelo, p. ). both proklus and simplikius, we must recollect, believed in the genuineness of the doric treatise ascribed to timaeus locrus. reasoning upon this basis, they of course saw, that if aristotle had correctly interpreted plato, plato himself must have interpreted _incorrectly_ the doctrine of timaeus. they had to ascribe wrong construction either to plato or to aristotle: and they could not bear to ascribe it to plato. alkinous, in his eisagôge (c. ) gives the same interpretation as proklus. but it is remarkable that in his paraphrase of the platonic words, he calls the earth [greek: ê(me/ras phu/lax kai\ nukto/s]: omitting the significant epithet [greek: dêmiourgo/s]. in regard to modern comments upon the same disputed point, i need only mention (besides those of m. cousin, in the notes upon his translation of the 'timæus', and of martin in his 'Études sur le timée') the elaborate discussion which it has received in the two recent dissertations 'ueber die kosmischen systeme der griechen,' by gruppe and boeckh. gruppe has endeavoured, upon the evidence of this passage, supported by other collateral proofs, to show that plato, towards the close of his life, arrived at a belief, first, in the rotation of the earth round its own axis, next, at the double movement of the earth, both rotation and translation, round the sun as a centre (that is, the heliocentric or copernican system): that plato was the first to make this discovery, but that he was compelled to announce it in terms intentionally equivocal and obscure, for fear of offending the religious sentiments of his contemporaries ('die kosmischen systeme der griechen, von o. f. gruppe,' berlin, ). to this dissertation m. boeckh--the oldest as well as the ablest of all living philologists--has composed an elaborate reply, with his usual fulness of illustrative matter and sobriety of inference. opinions previously delivered by him (in his early treatises on the platonic and pythagoreian philosophy) had been called in question by gruppe: he has now re-asserted them and defended them at length, maintaining that plato always held the earth to be stationary and the sidereal sphere rotatory--and answering or extenuating the arguments which point to an opposite conclusion ('untersuchungen über das kosmische system des platon, von august boeckh,' berlin, ). gruppe has failed in his purpose of proving that plato adopted either of the two above-mentioned doctrines--either the rotation of the earth round its own axis, or the translation of the earth round the sun as a centre. on both these points i concur with boeckh in the negative view. but though i go along with his reply as to its negative results, i cannot think it satisfactory in its positive aspect as an exposition of the doctrine proclaimed in the platonic timæus: nor can i admit that the main argument of m. boeckh's treatise is sufficient to support the inference which he rests upon it. moreover, he appears to me to set aside or explain away too lightly the authority of aristotle. i agree with alexander of aphrodisias and with gruppe who follows him, in pronouncing aristotle to be a good witness, when he declares what were the doctrines proclaimed in the platonic timæus; though i think that gruppe has not accurately interpreted either timæus or aristotle. the capital argument of boeckh is as follows: "the platonic timæus affirms, in express and unequivocal terms, the rotation of the outer celestial sphere (the sidereal sphere or aplanes) in twenty-four hours, as bringing about and determining the succession of day and night. whoever believes this cannot at the same time believe that the earth revolves round its own axis in twenty-four hours, and that the succession of day and night is determined thereby. the one of these two affirmations excludes the other; and, as the first of the two is proclaimed, beyond all possibility of doubt, in the platonic timæus, so we may be sure that the second of the two cannot be proclaimed in that same discourse. if any passage therein seems to countenance it, we must look for some other mode of interpreting the passage." this is the main argument of m. boeckh, and also of messrs. cousin and martin. the latter protests against the idea of imputing to plato "un mélange monstrueux de deux systêmes incompatibles" (Études sur le timée, vol. ii. p. - ). as applied to any person educated in the modern astronomy, the argument is irresistible. but is it equally irresistible when applied to plato and to plato's time? i think not. the incompatibility which appears so glaring at present, did not suggest itself to him or to his contemporaries. to prove this we have only to look at the reasoning of aristotle, who (in the treatise de coelo, ii. - , p. . b. , . a. ) notices and controverts the doctrine of the rotation of the earth, with express reference to the followers of the platonic timæus--and who (if we follow the view of martin) imputes this doctrine with wilful falsehood to plato, for the purpose of contemptuously refuting it "pour se donner le plaisir de la réfuter avec dédain." granting the view of m. boeckh (still more that of martin) to be correct, we should find aristotle arguing thus:--"plato affirms the diurnal rotation of the earth round the centre of the cosmical axis. this is both incredible, and incompatible with his own distinct affirmation that the sidereal sphere revolves in twenty-four hours. it is a glaring inconsistency that the same author should affirm both the one and the other." such would have been aristotle's reasoning, on the hypothesis which i am considering; but when we turn to his treatise we find that he does not employ this argument at all. he contests the alleged rotation of the earth upon totally different arguments--chiefly on the ground that rotatory motion is not natural to the earth, that the kind of motion natural to the earth is rectilineal, towards the centre; and he adds various corollaries flowing from this doctrine which i shall not now consider. at the close of his refutation, he states in general terms that the celestial appearances, as observed by scientific men, coincided with his doctrine. hence we may plainly see that aristotle probably did not see the incompatibility, supposed to be so glaring, upon which m. boeckh's argument is founded. to say the least, even if he saw it, he did not consider it as glaring and decisive. he would have put it in the foreground of his refutation, if he had detected the gross contradiction upon which m. boeckh insists. but aristotle does not stand alone in this dulness of vision. among the various commentators, ancient and modern, who follow him, discussing the question now before us, not one takes notice of m. boeckh's argument. he himself certifies to us this fact, claiming the argument as his own, and expressing his astonishment that all the previous critics had passed it over, though employing other reasons much weaker to prove the same point. we read in m. boeckh's second 'commentatio de platonico systemate coelestium globorum et de verâ indole astronomiæ philolaicæ,' heidelberg, , p. , the following words:-- "non moveri tellurem, proclus et simplicius ostendunt ex phædone. parum firmum tamen argumentum est ex phædone ductum ad interpretandum timæi locum: nec melius alterum, quod locrus timæus, quem plato sequi putabatur, terram stare affirmat: quia, ut nuper explicuimus, non plato ex locro, sed personatus locrus ex platone, sua compilavit. at omnium firmissionum et certissimum argumentum ex ipso nostro dialogo sumptum, _adhuc, quod jure mirere, nemo reperit_. etenim, quum, paulo supra, orbem stellarum fixarum, quem græci [greek: a)planê=] appellant, dextrorsum ferri quotidiano motu plato statuebat, non poterat ullum terræ motum admittere; quia, _qui hunc admittit, illum non tollere non potest_." (this passage appears again cited by m. boeckh himself in his more recent dissertation 'untersuchungen über das kosmische system des platon,' p. ). the writers named (p. ) as having discussed the question, omitting or disregarding this most cogent argument, are names extending from aristotle down to ruhnken and ideler. it is honourable to the penetration of m. boeckh that he should have pointed out, what so many previous critics had overlooked, that these two opinions are scientifically incompatible. he wonders, and there may be good ground for wondering, how it happened that none of these previous writers were aware of the incompatibility. but the fact that it did not occur to them, is not the less certain, and is of the greatest moment in reference to the question now under debate; for we are not now inquiring what is or is not scientifically true or consistent, but what were the opinions of plato. m. boeckh has called our attention to the fact, that these two opinions are incompatible; but can we safely assume that plato must have perceived such incompatibility between them? surely not. the pythagoreans of his day did not perceive it; their cosmical system included both the revolution of the earth and the revolution of the sidereal sphere round the central fire, ten revolving bodies in all (aristotel. metaphysic. i. , p. a. . de coelo, ii. , p. b. ). they were not aware that the revolutions of the one annulled those of the other as to effect, and that their system thus involved the two contradictory articles, or "mélange monstrueux," of which martin speaks so disdainfully. nay, more, their opponent, aristotle, while producing other arguments against them, never points out the contradiction. since it did not occur to them, we can have no greater difficulty in believing that neither did it occur to plato. indeed, the wonder would rather be if plato _had_ seen an astronomical incompatibility which escaped the notice both of aristotle and of many subsequent writers who wrote at a time when astronomical theories had been developed and compared with greater fulness. even ideler, a good astronomer as well as a good scholar, though he must surely have known that plato asserted the rotation of the sidereal sphere (for no man can read the 'timæus' without knowing it), ascribed to him also the other doctrine inconsistent with it, not noticing such inconsistency until m. boeckh pointed it out. it appears to me, therefore, that m. boeckh has not satisfactorily made good his point--"plato cannot have believed in the diurnal rotation of the earth, because he unquestionably believed in the rotation of the sidereal sphere as causing the succession of night and day." for, though the two doctrines really are incompatible, yet the critics antecedent to m. boeckh took no notice of such incompatibility. we cannot presume that plato saw what aristotle and other authors, even many writing under a more highly developed astronomy, did not see. we ought rather, i think, to presume the contrary, unless plato's words distinctly attest that he did see farther than his successors. now let us examine what plato's words do attest:--[greek: gê=n de\ tropho\n me\n ê(mete/ran, ei(llome/nên] (al. [greek: ei(lome/nên, i)llome/nên]) [greek: de\ peri\ to\n dia\ panto\s po/lon tetame/non phu/laka kai\ dêmiourgo\n nukto/s te kai\ ê(me/ras e)mêchanê/sato, prô/tên kai\ presbuta/tên theô=n, o(/soi e)nto\s ou)ra/nou gego/nasi.] i explain these words as follows:-- in the passage immediately preceding, plato had described the uniform and unchanging rotation of the outer sidereal sphere, or circle of the same, and the erratic movements of the sun, moon, and planets, in the interior circles of the diverse. he now explains the situation and functions of the earth. being the first and most venerable of the intra-kosmic deities, the earth has the most important place in the interior of the kosmos--the centre. it is packed, fastened, or rolled, close round the axis which traverses the entire kosmos; and its function is to watch over and bring about the succession of night and day. _plato conceives the kosmic axis itself as a solid cylinder revolving or turning round, and causing thereby the revolution of the circumference or the sidereal sphere._ the outer circumference of the kosmos not only revolves round its axis, but obeys a rotatory impulse emanating from its axis, like the spinning of a teetotum or the turning of a spindle. plato in the republic illustrates the cosmical axis by comparison with a spindle turned by necessity, and describes it as causing by its own rotation the rotation of all the heavenly bodies (republ. x. p. , c. a). [greek: e)k de\ tô=n a)/krôn tetame/non a)na/gkês a)/trakton, di' ou(= pa/sas e)pistre/phesthai ta\s peri/phoras . . . , kuklei=sthai de\ dê\ strepho/menon to\n a)/trakton o(/lon me\n tê\n au)tê\n phoran . . . . stre/phesthai de\ au)to\n e)n toi=s a)na/gkês go/nasin.][ ] [footnote : proklus in his commentary on the platonic timæus (p. , schn.) notes this passage of the republic as the proper comparison from which to interpret how plato conceived the cosmical axis. in many points he explains this correctly; but he omits to remark that the axis is expressly described as revolving, and as causing the revolution of the peripheral substance:-- ----[greek: to\n de\ a)/xona mi/an theo/têta sunagôgo\n me\n tô=n ke/ntrôn tou= panto\s sunektikê\n de\ tou= o(/lou ko/smou, _kinêtikê\n de\ tô=n thei/ôn periphorô=n_, peri\ ê(\n ê( chorei/a** tô=n o(/lôn, peri\ ê(\n ai( a)nakuklê/seis, a)ne/chousan to\n o(/lon ou)rano\n,** ê(\n kai\ a)/tlanta dia\ tou=to proseirê/kasin, ô(s a)/trepton kai\ a)/truton e)ne/rgeian e)/chousan. kai\ me/ntoi kai\ to\ tetame/non** e)ndei/knutai** titê/nion ei)=nai tê\n mi/an _tau/tên du/namin, tê\n phrourêtikê\n tê=s a)nakuklê/seôs tô=n o(/lôn_.] here proklus recognises the efficacy of the axis in producing and maintaining the revolution of the kosmos, but he does not remark that it initiates this movement by revolving itself. the [greek: theotê\s], which proklus ascribes to the axis, is invested in the earth packed round it, by the platonic timæus.] now the function which plato ascribes to the earth in the passage of the timæus before us is very analogous to that which in the republic he ascribes to necessity--the active guardianship of the axis of the kosmos and the maintenance of its regular rotation. with a view to the exercise of this function, the earth is planted in the centre of the axis, the very root of the kosmic soul (plato, timæus, p. b). it is even "packed close round the axis," in order to make sure that the axis shall not be displaced from its proper situation and direction. the earth is thus not merely active and influential, but is really the chief regulator of the march of the kosmos, being the immediate neighbour and auxiliary of the kosmic soul. such a function is worthy of "the first and eldest of intra-kosmic deities," as plato calls the earth. with perfect propriety he may say that the earth, in the exercise of such a function, "is guardian and artificer of day and night." this is noway inconsistent with that which he says in another passage, that the revolutions of the outer sidereal sphere determine day and night. for these revolutions of the outer sidereal sphere depend upon the revolutions of the axis, which latter is kept in uniform position and movement by the earth grasping it round its centre and revolving with it. the earth does not determine days and nights by means of its own rotations, but by its continued influence upon the rotations of the kosmic axis, and (through this latter) upon those of the outer sidereal sphere. it is important to attend to the circumstance last mentioned, and to understand in what sense plato admitted a rotatory movement of the earth. in my judgment, the conception respecting the earth and its functions, as developed in the platonic timæus, has not been considered with all its points taken together. one point among several, and that too the least important point, has been discussed as if it were the whole, because it falls in with the discussions of subsequent astronomy. thus plato admits the rotation of the earth, but he does not admit it as producing any effects, or as the primary function of the earth: it is only an indirect consequence of the position which the earth occupies in the discharge of its primary function--of keeping the cosmical axis steady, and maintaining the uniformity of its rotations. if the cosmical axis is to revolve, the earth, being closely packed and fastened round it, must revolve along with it. if the earth stood still, and resisted all rotation of its own, it would at the same time arrest the rotations of the cosmical axis, and of course those of the entire kosmos besides. the above is the interpretation which i propose of the passage in the platonic timæus, and which i shall show to coincide with aristotle's comment upon it. messrs. boeckh and martin interpret differently. they do not advert to the sense in which plato conceives the axis of the kosmos--not as an imaginary line, but as a solid revolving cylinder; and moreover they understand the function assigned by the platonic timæus to the earth in a way which i cannot admit. they suppose that the function assigned to the earth is not to keep up and regularize, but to withstand and countervail, the rotation of the kosmos. m. boeckh comments upon gruppe, who had said (after ideler) that when the earth is called [greek: phu/laka _kai\ dêmiourgo\n_ nukto\s kai\ ê(me/ras], plato must have meant to designate some active function ascribed to it, and not any function merely passive or negative. i agree with gruppe in this remark, and i have endeavoured to point out what this active function of the earth is, in the platonic theory. but m. boeckh (untersuchungen, &c., p. - ) controverts gruppe's remark, observing, first, that it is enough if the earth is in any way necessary to the production of the given effect; secondly, that if active force be required, the earth (in the platonic theory) does exercise such, by its purely passive resistance, which is in itself an energetic putting forth of power. m. boeckh's words are:--"es kommt nur darauf an, dass er ein werk, eine wirkung, hervorbringt oder zu einer wirkung beiträgt, die ohne ihn nicht wäre: dann ist er durch seine wirksamkeit ein werkmeister der sache, sey es auch ohne active thätigkeit, durch bloss passiven widerstand, der auch eine mächtige kraft-äusserung ist. die erde ist werkmeisterin der nacht und des tages, wie martin (b. ii. p. ) sehr treffend sagt 'par son énergique existence, c'est à dire, par son immobilité même:' denn sie setzt der täglichen bewegung des himmels beständig eine gleiche kraft in entgegengesetzter richtung entgegen. so _muss_ nach dem zusammenhange ausgelegt werden: so meint es platon klar und ohne verhüllungen: denn wenige zeilen vorher hat er gesagt, nacht und tag, das heisst ein sterntag oder zeittag, sei ein umlauf des kreises des selbigen--_das ist, eine tägliche umkreisung des himmels von osten nach westen, wodurch also die erde in stillstand versetzt ist:_ und diese tägliche bewegung des himmels hat er im vorhergehenden immer und immer gelehrt." . . . . "indem platon die erde nennt [greek: ei(lome/nên], nicht [greek: peri\ to\n e(autê=s po/lon], sondern [greek: peri\ to\n dia\ panto\s po/lon tetame/non], setzt er also die tägliche bewegung des himmels voraus" (p. - ).[ ] [footnote : "we are only required to show, that the earth produces a work or an effect,--or contributes to an effect which would not exist without such help: the earth is then, through such operation, an _artificer_ of what is produced, even without any positive activity, by its simply passive resistance, which indeed is in itself a powerful exercise of force.** the earth is artificer of night and day, according to the striking expression of martin, 'par son énergique existence, c'est-à-dire, par son immobilité même:' for the earth opposes, to the diurnal movement of the heavens, a constant and equal force in the opposite direction. this explanation _must_ be the true one required by the context: this is plato's meaning, plainly and without disguise: for he has said, a few lines before, that night and day (that is, a sidereal day, or day of time) is a diurnal revolution of the heaven from east to west, whereby accordingly the earth is assumed as at rest: and this diurnal movement of the heaven he has taught over and over again in the preceding part of his discourse."--"since therefore plato calls the earth [greek: ei(lome/nên], not [greek: peri\ to\n e(autê=s po/lon], but [greek: peri\ to\n dia\ panto\s po/lon tetame/non], he implies thereby the diurnal movement of the heaven."] i not only admit but put it in the front of my own case, that plato in the timæus assumes the diurnal movement of the celestial sphere; but i contend that he also assumes the diurnal rotation of the earth. m. boeckh founds his contrary interpretation upon the unquestionable truth that these two assumptions are inconsistent; and upon the inference that because the two cannot stand together in fact, therefore they cannot have stood together in the mind of plato. in that inference i have already stated that i cannot acquiesce. but while m. boeckh takes so much pains to vindicate plato from one contradiction, he unconsciously involves plato in another contradiction, for which, in my judgment, there is no foundation whatever. m. boeckh affirms that the function of the earth (in the platonic timæus) is to put forth a great force of passive resistance--"to oppose constantly, against the diurnal movement of the heavens, an equal force in an opposite direction." is it not plain, upon this supposition, that the kosmos would come to a standstill, and that its rotation would cease altogether? as the earth is packed close or fastened round the cosmical axis, so, if the axis endeavours to revolve with a given force, and the earth resists with equal force, the effect will be that the two forces will destroy one another, and that neither the earth nor the axis will move at all. there would be the same nullifying antagonism as if,--reverting to the analogous case of the spindle and the verticilli (already alluded to) in the tenth book of the republic,--as if, while ananké turned the spindle with a given force in one direction, klotho (instead of lending assistance) were to apply her hand to the outermost verticillus with equal force of resistance in the opposite direction (see reipubl. x. p. d). it is plain that the spindle would never turn at all. here, then, is a grave contradiction attaching to the view of boeckh and martin as to the function of the earth. they have not, in my judgment, sufficiently investigated the manner in which plato represents to himself the cosmical axis: nor have they fully appreciated what is affirmed or implied in the debated word [greek: ei(lo/menon--ei(lou/menon--i)llo/menon]. that word has been explained partly by ruhnken in his notes on timæi lexicon, but still more by buttmann in his lexilogus, so accurately and copiously as to leave nothing further wanting. i accept fully the explanation given by buttmann, and have followed it throughout this article. after going over many other examples, buttmann comes to consider this passage of the platonic timæus; and he explains the word [greek: ei(lome/nên] or [greek: i)llo/menên] as meaning--"_sich drängen oder gedrängt werden_ um die axe: d. h. von allen seiten her an die axe. auch lasse man sich das praesens nicht irren: die kräfte, welche den weltbau machen und zusammen halten, sind als fortdauernd thätig gedacht. die erde drängt sich (ununterbrochen) an den pol, _macht, bildet eine kugel um ihn_. welcher gebrauch völlig entspricht dem wonach dasselbe verbum ein _einwickeln_, _einhüllen_, bedeutet. auch hier mengt sich in der vorstellung einiges hinzu, was auf ein _biegen_ _winden_, und mitunter auf ein _drehen_ führt: was aber _überall nur ein durch die sache selbst hinzutretender begriff ist_," p. . and again, p. , he gives the result--that the word has only "die bedeutung _drängen_, _befestigen_, nebst den davon ausgehenden--die von _drehen_, _winden_, aber ihm _gänzlich fremd_ sind, _und nur aus der natur der gegenstände in einigen fällen als nebengedanken hinzutreten_."[ ] [footnote : "to _pack itself_, or to _be packed_, round the axis: that is, upon the axis from all sides. we must not be misled by the present tense: for the forces, which compose and hold together the structure of the universe, are conceived as continuously in active operation. the earth _packs itself_, or _is packed_, on to the axis--_makes or forms a ball round the axis:_ which corresponds fully to that other usage of the word, in the sense of _wrapping up_ or _swathing round_. here too there is a superadded something blended with the idea, which conducts us to _turning_, _winding_, and thus to _revolving_: but this is every where nothing more than an accessory notion, suggested by the circumstances of the case. the word has only the meaning, to _pack_, to _fasten_--the senses, to _wind_, to _revolve_, are altogether foreign to it, and can only be superadded as accessory ideas, in certain particular instances, by the special nature of the case."] in these last words buttmann has exactly distinguished the true, constant, and essential meaning of the word, from the casual accessories which become conjoined with it by the special circumstances of some peculiar cases. the constant and true meaning of the word is, _being packed or fastened close round_, _squeezing or grasping around_. the idea of _rotating_ or _revolving_ is quite foreign to this meaning, but may nevertheless become conjoined with it, in certain particular cases, by accidental circumstances. let us illustrate this. when i say that a body _a_ is [greek: ei(lo/menon] or [greek: i)llo/menon] (packed or fastened close round, squeezing or grasping around), another body _b_, i affirm nothing about revolution or rotation. this is an idea foreign to the proposition _per se_, yet capable of being annexed or implicated with it under some accidental circumstances. whether in any particular case it be so implicated or not depends on the question "what is the nature of the body _b_, round which i affirm _a_ to be fastened?" . it may be an oak tree or a pillar, firmly planted and stationary. . it may be some other body, moving, but moving in a rectilinear direction. . lastly, it may be a body rotating or intended to rotate, like a spindle, a spit, or the rolling cylinder of a machine. in the first supposition, all motion is excluded: in the second, rectilinear motion is implied, but rotatory motion is excluded: in the third, rotatory motion is implied as a certain adjunct. the body which is fastened round another, must share the motion or the rest of that other. if the body _b_ is a revolving cylinder, and if i affirm that _a_ is packed or fastened close round it, i introduce the idea of rotation; though only as an accessory and implied fact, in addition to that which the proposition affirms. the body _a_, being fastened round the cylinder _b_, must either revolve along with it and round it, or it must arrest the rotation of _b_. if the one revolves, so must the other; both must either revolve together, or stand still together. this is a new fact, distinct from what is affirmed in the proposition, yet implied in it or capable of being inferred from it through induction and experience. here we see exactly the position of plato in regard to the rotation of the earth. he does not affirm it in express terms, but he affirms what implies it. for when he says that the earth is packed, or fastened close round the cosmical axis, he conveys to us by implication the knowledge of another and distinct fact--that the earth and the cosmical axis must either revolve together or remain stationary together--that the earth must either revolve along with the axis or arrest the revolutions of the axis. it is manifest that plato does not mean the revolutions of the axis of the kosmos to be arrested: they are absolutely essential to the scheme of the timæus--they are the grand motive-agency of the kosmos. he must, therefore, mean to imply that the earth revolves along with and around the cosmical axis. and thus the word [greek: ei(lo/menon] or [greek: i)llo/menon], according to buttmann's doctrine, becomes accidentally conjoined, through the specialities of this case, with an accessory idea of rotation or revolution; though that idea is foreign to its constant and natural meaning. now if we turn to aristotle, we shall find that he understood the word [greek: ei(lo/menon] or [greek: i)llo/menon], and the proposition of plato, exactly in this sense. here i am compelled to depart from buttmann, who affirms (p. ), with an expression of astonishment, that aristotle misunderstood the proposition of plato, and interpreted [greek: ei(lo/menon] or [greek: i)llo/menon] as if it meant directly as well as incontestably, _rotating_ or _revolving_. proklus, in his commentary on the timæus, had before raised the same controversy with aristotle--[greek: i)llome/nên de\, tê\n sphiggome/nên dêloi= kai\ sunechome/nên ou) ga\r ô(s a)ristote/lês oi)/etai, tê\n kinoume/nên] (procl. p. ). let us, therefore, examine the passages of aristotle out of which this difficulty arises. the passages are two, both of them in the second book de coelo; one in cap. , the other in cap. (p. b. , a. ). . the first stands--[greek: e)/nioi de\ kai\ keime/nên (tê\n gê=n) e)pi\ tou= ke/ntrou phasi\n au)tê\n i)/llesthai peri\ to\n dia\ panto\s tetame/non po/lon, ô(/sper e)n tô=| timai/ô|** ge/graptai.** such is the reading of bekker in the berlin edition: but he gives various readings of two different mss.--the one having [greek: i)/llesthai kai\ kinei=sthai]--the other ei(lei=sthai** kai\ kinei=sthai]. . the second stands, beginning chap. --[greek: ê(mei=s de\ le/gômen prô=ton po/teron] (the earth) [greek: e)/chei ki/nêsin ê)\ me/nei; katha/per ga\r ei)/pomen, oi( me\n au)tê\n e(\n tô=n a)/strôn poiou=sin, oi( d' e)pi\** tou= me/sou the/ntes i)/llesthai kai\ kinei=shai/ phasi peri\ to\n po/lon me/son.] now, in the first of these two passages, where aristotle simply brings the doctrine to view without any comment, he expressly refers to the timæus, and therefore quotes the expression of that dialogue without any enlargement. he undoubtedly understands the affirmation of plato--that the earth was fastened round the cosmical axis--as implying that it rotated along with the rotations of that axis. aristotle thus construes [greek: i)/llesthai], _in that particular proposition_ of the timæus, as implying rotation. but he plainly did not construe [greek: i)/llesthai] as naturally and constantly either denoting or implying rotation. this is proved by his language in the second passage, where he reproduces the very same doctrine with a view to discuss and confute it, and without special reference to the platonic timæus. here we find that he is not satisfied to express the doctrine by the single word [greek: i)/llesthai]. he subjoins another verb--[greek: i)/llesthai kai\ kinei=sthai]: thus bringing into explicit enunciation the fact of rotatory movement, which, while [greek: i)/llesthai] stood alone, was only known by implication and inference from the circumstances of the particular case. if he had supposed [greek: i)/llesthai] by itself to signify _revolving_ the addition of [greek: kinei=sthai] would have been useless, unmeaning, and even impertinent. aristotle, as boeckh remarks, is not given to multiply words unnecessarily. it thus appears, when we examine the passages of aristotle, that he understood [greek: i)/llesthai] quite in conformity with buttmann's explanation. rotatory movement forms no part of the meaning of the word; yet it may accidentally, in a particular case, be implied as an adjunct of the meaning, by virtue of the special circumstances of that case. aristotle describes the doctrine as held by _some persons_. he doubtless has in view various platonists of his time, who adopted and defended what had been originally advanced by plato in the timæus. m. boeckh, in a discussion of some length (untersuch. p. - ), maintains the opinion that the reading in the first passage of aristotle is incorrect; that the two words [greek: i)/llesthai kai\ kinei=sthai] ought to stand in the first as they do in the second,--as he thinks that they stood in the copy of simplikius: that aristotle only made reference to plato with a view to the peculiar word [greek: i)/llesthai], and not to the general doctrine of the rotation of the earth: that he comments upon this doctrine as held by others, but not by plato--who (according to boeckh) was known by everyone not to hold it. m. boeckh gives this only as a conjecture, and i cannot regard his arguments in support of it as convincing. but even if he had convinced me that [greek: i)/llesthai kai\ kinei=sthai] were the true reading in the first passage, as well as in the second, i should merely say that aristotle had not thought himself precluded by the reference to the timæus from bringing out into explicit enunciation what the platonists whom he had in view knew to be implied and intended by the passage. this indeed is a loose mode of citation, which i shall not ascribe to aristotle without good evidence. in the present case such evidence appears to me wanting.[ ] [footnote : exactness of citation is not always to be relied on among ancient commentators. simplikius cites this very passage of the timæus with more than one inaccuracy.--(ad aristot. de coelo, fol. .)] m. martin attributes to aristotle something more than improper citation. he says (Êtudes sur le timée, vol. ii. p. ), "si aristote citait l'opinion de la rotation de la terre comme un titre de gloire pour platon, je dirais--il est probable que la vérité l'y a forcé. mais aristote, qui admettait l'immobilité complète de la terre, attribue à platon l'opinion contraire, _pour se donner le plaisir de la réfuter avec dédain_." a few lines before, m. martin had said that the arguments whereby aristotle combated this opinion ascribed to plato were "very feeble." i am at a loss to imagine in which of aristotle's phrases m. martin finds any trace of disdain or contempt, either for the doctrine or for those who held it. for my part, i find none. the arguments of aristotle against the doctrine, whatever be their probative force, are delivered in that brief, calm, dry manner which is usual with him, without a word of sentiment or rhetoric, or anything [greek: e)/xô tou= pra/gmatos]. indeed, among all philosophers who have written much, i know none who is less open to the reproach of mingling personal sentiment with argumentative debate than aristotle. plato indulges frequently in irony, or sneering, or rhetorical invective; aristotle very rarely. moreover, even apart from the question of contempt, the part which m. martin here assumes aristotle to be playing, is among the strangest anomalies in the history of philosophy. aristotle holds, and is anxious to demonstrate, the doctrine of the earth's immobility; he knows (so we are required to believe) that plato not only holds the same doctrine, but has expressly affirmed it in the timæus: he might have produced plato as an authority in his favour, and the passage of the timæus as an express declaration; yet he prefers to pervert, knowingly and deliberately, the meaning of this passage, and to cite plato as a hostile instead of a friendly authority--simply "to give himself the pleasure of contemptuously refuting plato's opinion!" but this is not all. m. martin tells us that the arguments which aristotle produces against the doctrine are, after all, very feeble. but he farther tells us that there was one argument which might have been produced, and which, if aristotle had produced it, would have convicted plato of "an enormous contradiction" (p. ) in affirming that the earth revolved round the cosmical axis. aristotle might have said to plato--"you have affirmed, and you assume perpetually throughout the timæus, the diurnal revolution of the outer sidereal sphere; you now assert the diurnal revolution of the earth at the centre. here is an enormous contradiction; the two cannot stand together."--yet aristotle, having this triumphant argument in his hands, says not a word about it, but contents himself with various other arguments which m. martin pronounces to be very feeble. perhaps m. martin might say--"the contradiction exists; but aristotle was not sharpsighted enough to perceive it; otherwise he would have advanced it." i am quite of this opinion. if aristotle had perceived the contradiction, he would have brought it forward as the strongest point in his controversy. his silence is to me a proof that he did not perceive it. but this is a part of my case against m. martin. i believe that plato admitted both the two contradictory doctrines without perceiving the contradiction; and it is a strong presumption in favour of this view that aristotle equally failed to perceive it--though in a case where, according to m. martin, he did not scruple to resort to dishonest artifice. it appears to me that the difficulties and anomalies, in which we are involved from supposing that aristotle either misunderstood or perverted the meaning of plato--are far graver than those which would arise from admitting that plato advanced a complicated theory involving two contradictory propositions, in the same dialogue, without perceiving the contradiction; more especially when the like failure of perception is indisputably ascribable to aristotle--upon every view of the case. m. cousin maintains the same interpretation of the platonic passage as boeckh and martin, and defends it by a note on his translation of the timæus (p. ). the five arguments which he produces are considered both by himself and by martin to be unanswerable. as he puts them with great neatness and terseness, i here bestow upon them a separate examination. . "platon a toujours été considéré dans l'antiquité comme partisan de l'immobilité absolue de la terre." m. cousin had before said, "aristote se fonde sur ce passage pour établir que platon a fait tourner la terre sur elle-même: mais aristote est, dans l'antiquité, le seul qui soutienne cette opinion." my reply is, that aristotle is himself a portion and member of antiquity, and that the various platonists, whom he undertakes to refute, are portions of it also. if m. cousin appeals to the authority of antiquity, it must be to antiquity, not merely _minus_ aristotle and these contemporary platonists, but _against_ them. now these are just the witnesses who had the best means of knowledge. besides which, aristotle himself, adopting and anxious to demonstrate the immobility of the earth, had every motive to cite plato as a supporter, if plato was such--and every motive to avoid citing plato as an opponent, unless the truth of the case compelled him to do so. i must here add, that m. cousin represents aristotle as ascribing to plato the doctrine that "la terre tourne sur elle-même." this is not strictly exact. aristotle understands the platonic timæus as saying, "that the earth is packed and moved _round the axis of the kosmos_"--a different proposition. . "dans plusieurs endroits de ses ouvrages où platon parle de l'équilibre de la terre, il ne dit pas un mot de sa rotation." i know of only _one_ such passage--phædon, p. --where undoubtedly plato does not speak of the rotation of the earth; but neither does he speak of the rotation of the sidereal sphere and of the kosmos--nor of the axis of the kosmos. it is the figure and properties of the earth, considered in reference to mankind who inhabit it, that plato sketches in the phædon; he takes little notice of its cosmical relations, and gives no general theory about the kosmos. m. cousin has not adverted to the tenth book of the republic, where plato does propound a cosmical theory, expressly symbolising the axis of the kosmos with its rotatory functions. . "si la _terre suit le mouvement de l'axe du monde_, le mouvement de la huitième sphère, qui est le même, devient nul par rapport à elle, et les étoiles fixes, qui appartiennent à elle, demeurent en apparence dans une immobilité absolue: ce qui est contraire à _l'expérience et au sens commun_, et à l'opinion de platon, exprimée dans ce même passage." this third argument of m. cousin is the same as that which i have already examined in remarking upon m. boeckh. the diurnal rotation of the earth cannot stand in the same astronomical system with the diurnal rotation of the sidereal sphere. incontestably true (i have already said) as a point of science. but the question here is, not what opinions are scientifically consistent, but what opinions were held by plato, and whether he detected the inconsistency between the two. i have shown grounds for believing that he did not--and not he alone, but many others along with him, aristotle among the number. how, indeed, can this be denied, when we find m. boeckh announcing that he is the _first_ among all the critics on the timæus, who has brought forward the inconsistency as a special ground for determining what plato's opinion was--that no other critic before him had noticed it? the first words of this argument deserve particular attention, "si la terre suit le mouvement de l'axe du monde." here we have an exact recital of the doctrine proclaimed by the platonic timæus, and ascribed to him by aristotle (quite different from the doctrine "que la terre tourne sur elle-même"). m. cousin here speaks very distinctly about the cosmical axis, and about its movement; thus implying that plato conceived it as a solid revolving cylinder. this, in my judgment, is the most essential point for clearing up the question in debate. the cosmical axis being of this character, when plato affirms that the earth is _packed or fastened round it_ (_se roule_--cousin: _se serre et s'enroule_--martin: _drängt sich, macht eine kugel um ihn_--buttmann), i maintain that, in the plainest construction of the word, the earth does and must follow the movement of the axis--or arrest the movement of the axis. the word [greek: ei(lome/nên] or [greek: i)llome/nên] has no distinct meaning at all, if it does not mean this. the very synonyms ([greek: sphiggome/nên, peridedeme/nên], &c.), which the commentators produce to prove that plato describes the earth as at rest, do really prove that he describes it as rotating round and with the cosmical axis. we ought not to be driven from this plain meaning of the word, by the assurance of m. cousin and others that plato cannot have meant so, because it would involve him in an astronomical inconsistency. . "les divers mouvemens des huit sphères expliquent toutes les apparences célestes; il n'y a donc aucune raison pour donner un mouvement à la terre." the terms of this fourth argument, if literally construed, would imply that plato had devised a complete and satisfactory astronomical theory. i pass over this point, and construe them as m. cousin probably intended: his argument will then stand thus--"the movement of the earth does not add anything to plato's power of explaining astronomical appearances; therefore plato had no motive to suggest a movement of the earth." i have already specified the sense in which i understand the platonic timæus to affirm, or rather to imply, the rotation of the earth; and that sense is not open to the objections raised in m. cousin's fourth and fifth arguments. the rotation of the earth, as it appears in the platonic timæus, explains nothing, and is not intended to explain anything. it is a consequence, not a cause: it is a consequence arising from the position of the earth, as packed or fastened round the centre of the cosmical axis, whereby the earth participates, of necessity and as a matter of course, in the movements of that axis. the _function_ of the earth, thus planted in the centre of the kosmos, is to uphold and regulate the revolutions of the cosmical axis; and this function explains, in the scheme of the platonic timæus, why the axis revolves uniformly and constantly without change or displacement. now upon these revolutions of the cosmical axis all the revolutions of the exterior sphere depend. this is admitted by m. cousin himself in argument . there is therefore every reason why plato should assign such regulating function to the earth, the "first and oldest of intra-kosmic deities." the movement of the earth (as i before observed) is only an incidental consequence of the position necessary for the earth to occupy in performing such function. . "enfin platon assigne un mouvement aux étoiles fixes, et deux mouvemens aux planètes; puisqu'il ne range la terre ni avec les unes ni avec les autres, il y a lieu de croire qu'elle ne participe à aucun de leurs mouvemens." in so far as this argument is well-founded, it strengthens my case more than that of m. cousin. the earth does not participate in the movements either of the fixed stars or of the planets; but it does participate in the revolutions of the cosmical axis, upon which these movements depend--the movements of the outer sphere, wholly and exclusively--the movements of the planets, to a very great degree, but not exclusively. the earth is not ranked either among the fixed stars or among the planets; it is a body or deity _sui generis_, having a special central function of its own, to regulate that cosmical axis which impels the whole system. the earth has a motion of its own, round and along with the cosmical axis to which it is attached; but this motion of the earth (i will again repeat, to prevent misapprehension) is a fact not important by itself, nor explaining anything. the grand and capital fact is the central position and regulating function of the earth, whereby all the cosmical motions, first those of the axis, next those of the exterior kosmos, are upheld and kept uniform. m. cousin adds, as a sixth argument:-- "on peut ajouter à ces raisons que platon aurait nécessairement insisté sur le mouvement de la terre, s'il l'avait admis; et que ce point étoit trop controversé de son temps et trop important en lui-même, pour qu'il ne fît que l'indiquer en se servant d'une expression équivoque." in the first place, granting plato to have believed in the motion of the earth, can we also assume that he would necessarily have asserted it with distinctness and emphasis, as m. cousin contends? i think not. gruppe maintains exactly the contrary; telling us that plato's language was intentionally obscure and equivocal--from fear of putting himself in open conflict with the pious and orthodox sentiment prevalent around him. i do not carry this part of the case so far as gruppe, but i admit that it rests upon a foundation of reality. when we read (plutarch, de facie in orbe lunæ, p. ) how the motion of the earth, as affirmed by aristarchus of samos (doubtless in a far larger sense than plato ever imagined, including both rotation and translation), was afterwards denounced as glaring impiety, we understand the atmosphere of religious opinion with which plato was surrounded. and we also perceive that he might have reasons for preferring to indicate an astronomical heresy in terms suitable for philosophical hearers, rather than to proclaim it in such emphatic unequivocal words, as might be quoted by some future melêtus in case of an indictment before the dikasts. we must remember that plato had been actually present at the trial of sokrates. he had heard the stress laid by the accusers on astronomical heresies, analogous to those of anaxagoras, which they imputed to sokrates--and the pains taken by the latter to deny that he held such opinions (see the platonic apology). the impression left by such a scene on plato's mind was not likely to pass away: nor can we be surprised that he preferred to use propositions which involved and implied, rather than those which directly and undisguisedly asserted, the heretical doctrine of the earth's rotation. that his phraseology, however indirect, was perfectly understood by contemporary philosophers, both assentient and dissentient, as embodying his belief in the doctrine--is attested by the two passages of aristotle. upon these reasons alone i should dissent from m. cousin's sixth argument. but i have other reasons besides. he rests it upon the two allegations that the doctrine of the earth's motion was the subject of much controversial debate in plato's time, and of great importance in itself. now the first of these two allegations can hardly be proved, as to the time of plato; for aristotle, when he is maintaining the earth's immobility, does not specify any other opponents than the pythagoreians and the followers of the platonic timæus. and the second allegation i believe to be unfounded, speaking with reference to the platonic timæus. in the cosmical system therein embodied, the rotation of the earth round the cosmical axis, though a real part of the system, was in itself a fact of no importance, and determining no results. the capital fact of the system was the position and function of the earth, packed close round the centre of the cosmical axis, and regulating the revolutions of that axis. plato had no motive to bring prominently forward the circumstance that the earth revolved itself along with the cosmical axis, which circumstance was only an incidental accompaniment. i have thus examined all the arguments adduced by m. cousin, and have endeavoured to show that they fail in establishing his conclusion. there is, however, one point of the controversy in which i concur with him more than with boeckh and martin. this point is the proper conception of what plato means by the _cosmical axis_. boeckh and martin seem to assume this upon the analogy of what is now spoken of as the axis of the earth: m. boeckh (p. ) declares the axis of the kosmos to be a prolongation of that axis. but it appears to me (and m. cousin's language indicates the same) that plato's conception was something very different. the axis of the earth (what astronomers speak of as such) is an imaginary line traversing the centre of the earth; a line round which the earth revolves. now the cosmical axis, as plato conceives it, is a solid material cylinder, which not only itself revolves, but causes by this revolution the revolution of the exterior circumference of the kosmos. this is a conception entirely different from that which we mean when we speak of the axis of the earth. it is, however, a conception symbolically enunciated in the tenth book of the republic, where the spindle of necessity is said to be composed of adamant, hard and solid material, and to cause by its own rotation the rotation of all the _verticilli_ packed and fastened around it. what is thus enunciated in the republic is implied in the timæus. for when we read therein that the earth is packed or fastened round the cosmical axis, how can we understand it to be packed or fastened round an imaginary line? i will add that the very same meaning is brought out in the translation of cicero--"_trajecto axe sustinetur_" (terra). the axis, round which the earth is fastened, and which sustains the earth, must be conceived, not as an imaginary line, but as a solid cylinder, itself revolving; while the earth, being fastened round it, revolves round and along with it. the axis, in the sense of an imaginary line, cannot be found in the conception of plato. those contemporaries of plato and aristotle, who all agreed in asserting the revolution of the celestial sphere, did not all agree in their idea of the force whereby such revolution was brought about. some thought that the poles of the celestial sphere exercised a determining force: others symbolised the mythical atlas, as an axis traversing the sphere from pole to pole and turning it round. (aristotel. de motu animal. . p. a. - .) aristotle himself advocated the theory of a _primum movens immobile_ acting upon the sphere from without the sphere. even in the succeeding centuries, when astronomy was more developed, aratus, eratosthenes, and their commentators, differed in their way of conceiving the cosmical axis. most of them considered it as solid: but of these, some thought it was stationary, with the sphere revolving round it--others that it revolved itself: again, among these latter, some believed that the revolutions of the axis determined those of the surrounding sphere--others, that the revolutions of the sphere caused those of the axis within it. again, there were some physical philosophers who looked at the axis as airy or spiritual--[greek: to\ dia\ me/sou tê=s sphai/ras diê=kon pneu=ma]. then there were geometers who conceived it only as an imaginary line. (see the phaenomena of aratus - --with the scholia thereon; achilles tatius ad arati phaenom. apud petavium--uranolog. p. ; also hipparchus ad arat. ib. p. .) i do not go into these dissentient opinions farther than to show, how indispensable it is, when we construe the passage in the platonic timaeus, [greek: peri\ to\n dia\ panto\s po/lon tetame/non], to enquire in what sense plato understood the cosmical axis: and how unsafe it is to assume at once that he must have conceived it as an imaginary line. proklus argues that because the earth is mentioned by plato in the phædon as stationary in the centre of the heaven, we cannot imagine plato to affirm its rotation in the timæus. i agree with m. boeckh in thinking this argument inconclusive; all the more, because, in the phædon, not a word is said either about the axis of the kosmos, or about the rotation of the kosmos; all that sokrates professes to give is [greek: tê\n i)de/an tê=s gê=s kai\ tou\s to/pous au)tê=s**] (p. e). no cosmical system or theory is propounded in that dialogue. when we turn to the phædrus, we find that, in its highly poetical description, the rotation of the heaven occupies a prominent place. the internal circumference of the heavenly sphere, as well as its external circumference or back ([greek: nô=ton]), are mentioned; also its periodical rotations, during which the gods are carried round on the back of the heaven, and contemplate the eternal ideas occupying the super-celestial space (p. , ), or the plain of truth.[ ] but the purpose of this poetical representation appears to be metaphysical and intellectual, to illustrate the antithesis presented by the world of ideas and truth on one side--against that of sense and appearances on the other. astronomically and cosmically considered, no intelligible meaning is conveyed. nor can we even determine whether the rotations of the heaven, alluded to in the phædrus, are intended to be diurnal or not; i incline to believe not ([greek: me/chri tê=s _e(te/ras_ perio/dou]--p. --which can hardly be understood of so short a time as one day). lastly, nothing is said in the phædrus about the cosmical axis; and it is upon this that the rotations of the earth intimated in the timæus depend. [footnote : whether [greek: e)sti/a] in the phædrus, which is said "to remain alone stationary in the house of the gods," can be held to mean the earth, is considered by proklus to be uncertain (p. ).] among the different illustrations, given by plato in his different dialogues respecting the terrestrial and celestial bodies, i select the tenth book of the republic as that which is most suitable for comparison with the timæus, because it is only therein that we learn how plato conceived the axis of the kosmos. m. boeckh (untersuchungen, p. ) wishes us to regard the difference between the view taken in the phædon, and that in the republic, as no way important; he affirms that the adamantine spindle in the republic is altogether mythical or poetical, and that plato conceives the axis as not being material. on this point i dissent from m. boeckh. the mythical illustrations in the tenth book of the republic appear to me quite unsuitable to the theory of an imaginary, stationary, and immaterial axis. here i much more agree with gruppe (p. , - ), who recognises the solid material axis as an essential feature of the cosmical theory in the republic; and recognises also the marked difference between that theory and what we read in the phædon. yet, though gruppe is aware of this important difference between the republic and the phædon, he still wishes to illustrate the timæus by the latter and not by the former. he affirms that the earth in the timæus is conceived as unattached, and freely suspended, the same as in the phædon; but that in the timæus it is conceived, besides, as revolving on its own axis, which we do not find in the phædon (p. , ). here i think gruppe is mistaken. in construing the words of timæus, [greek: ei(lome/nên (i)llome/nên) peri\ to\n dia\ panto\s po/lon tetame/non], as designating "the unattached earth revolving round its own axis," he does violence not less to the text of plato than to the expository comment of aristotle. neither in the one nor the other is anything said about _an axis of the earth_; in both, the cosmical axis is expressly designated; and, if gruppe is right in his interpretation of [greek: ei(lome/nên], we must take plato as affirming, not that the earth is fastened round the cosmical axis, but that it revolves, though unattached, around that axis, which is a proposition both difficult to understand, and leading to none of those astronomical consequences with which gruppe would connect it. again, when gruppe says that [greek: ei(lome/nên peri\] does _not_ mean _packed or fastened round_, but that it _does_ mean _revolving round_, he has both the analogies of the word and the other commentators against him. the main proof, if not the only proof, which he brings, is that aristotle so construed it. upon this point i join issue with him. i maintain that aristotle does _not_ understand [greek: ei(lome/nên] or [greek: i)llome/nên peri\] as naturally meaning _revolving round_, and that he does understand the phrase as meaning _fastened round_. when we find him, in the second passage of the treatise de coelo, not satisfied with the verb [greek: i)/llesthai] alone, but adding to it the second verb [greek: _kai\ kinei=sthai_], we may be sure that he did not consider [greek: i)/llesthai] as naturally and properly denoting _to revolve_ or _move round_. agreeing as i do with gruppe in his view, that the interpretation put by aristotle is the best evidence which we can follow in determining the meaning of this passage in the timæus, i contend that the authority of aristotle contradicts instead of justifying the conclusion at which he arrives. aristotle understands [greek: i)llome/nên] as meaning _packed or fastened round_; he does not understand it as meaning, when taken by itself, _revolving round_. the two meanings here indicated are undoubtedly distinct and independent. but they are not for that reason contradictory and incompatible. it has been the mistake of critics to conceive them as thus incompatible; so that if one of the two were admitted, the other must be rejected. i have endeavoured to show that this is not universally true, and that there are certain circumstances in which the two meanings not only may come together, but must come together. such is the case when we revert to plato's conception of the cosmical axis as a solid revolving cylinder. that which is packed or fastened around the cylinder must revolve around it, and along with it. both m. boeckh and gruppe assume the incompatibility of the two meanings; and we find the same assumption in plutarch's criticisms on the timæus (plutarch. quæst. platon. p. c), where he discusses what plato means by [greek: o)/rgana chro/nou]; and in what sense the earth as well as the moon can be reckoned as [greek: o)/rganon chro/nou] (timæus, p. e, d). plutarch inquires how it is possible that the earth, if stationary and at rest, can be characterised as "among the instruments of time;" and he explains it by saying that this is true in the same sense as we call a gnomon or sun-dial an instrument of time, because, though itself never moves, it marks the successive movements of the shadow. this explanation might be admissible for the phrase [greek: o)/rganon chro/nou]; but i cannot think that the immobility of the earth can be made compatible with the attribute which plato bestows upon it of being [greek: phu/lax kai\ _dêmiourgo\s_ nukto\s te kai\ ê(me/ras]. the difficulty, however, vanishes when we understand the function ascribed by plato to the earth as i have endeavoured to elucidate it. the earth not only is not at rest, but cannot be at rest, precisely because it is packed round the solid revolving cosmical axis, and must revolve along with it. the function of the earth, as the first and oldest of intra-kosmic deities, is to uphold and regulate the revolutions of this axis, upon which depend the revolutions of the sidereal sphere or outer shell of the kosmos. it is by virtue of this regulating function (and not by virtue of its rotation) that the earth is the guardian and artificer of night and day. it is not only "an instrument of time," but the most potent and commanding among all instruments of time. what has just been stated is, in my belief, the theory of the platonic timæus, signified in the words of that dialogue, and embodied in the comment of aristotle. the commentators, subsequent to aristotle, so far as we know them, understood the theory in a sense different from what plato intended. i think we may see how this misconception arose. it arose from the great development and elaboration of astronomical theory during the two or three generations immediately succeeding plato. much was added by eudoxus and others, in their theory of concentric spheres: more still by others of whom we read in cicero (academ. ii. .) "hicetas syracusius, ut ait theophrastus, coelum, solem, lunam, stellas, supera denique omnia, stare censet, neque praeter terram rem ullam in mundo moveri: quae cum circum axem se summâ celeritate convertat et torqueat, eadem effici omnia, quae si stante terrâ coelum moveretur. atque hoc etiam platonem in timaeo dicere quidam arbitrantur, sed paullo obscurius." the same doctrine is said to have been held by herakleides of pontus, the contemporary of aristotle, and by others along with him. (simplikius ad aristot. physic. p. --de coelo, p. --plutarch. plac. phil. iii. .) the doctrine of the rotation of the earth here appears along with another doctrine--the immobility of the sidereal sphere and of the celestial bodies. the two are presented together, as correlative portions of one and the same astronomical theory. there are no celestial revolutions, and therefore there is no solid celestial axis. moreover, even aristarchus of samos (who attained to a theory substantially the same as the copernican, with the double movement of the earth, rotation round its own axis, and translation round the sun as a centre) comes within less than a century after plato's death. though the _quidam_ alluded to by cicero looked upon the obscure sentence in plato's timaeus as a dim indication of the theory of hicetas, yet the two agree only in the supposition of a rotation of the earth, and differ essentially in the pervading cosmical conceptions. hicetas states distinctly that which his theory denies, as well as that which it affirms. the negation of the celestial rotations, is in his theory a point of capital and coordinate importance, on which he contradicts both plato and aristotle as well as the apparent evidence of sense. i cannot suppose that this theory can have been proclaimed or known to aristotle when his works were composed: for the celestial revolutions are the keystone of his system, and he could hardly have abstained from combating a doctrine which denied them altogether. in the hands of hicetas (perhaps in those of herakleides, if we may believe what is said about him) astronomy appears treated as a science by itself, with a view "to provide such hypotheses as may save the phenomena" ([greek: sô/zein ta\ phaino/mena], simpl. ad aristot. de coelo, p. , schol. brandis). it becomes detached from those religious, ethical, poetical, teleological, arithmetical decrees or fancies, in which we see it immersed in the platonic timaeus, and even (though somewhat less) in the aristotelian treatise de coelo. hence the meaning of plato, obscurely announced from the beginning, ceased to be understood: the solid revolving axis of the kosmos, assumed without being expressly affirmed in his timaeus, dropped out of sight: the doctrine of the rotation of the earth was presented in a new point of view, as a substitute for the celestial revolutions. but no proper note was taken of this transition. the doctrine of plato was assumed to be the same as that of hicetas. when we read plutarch's criticism (quæst. plat. p. c) upon the word [greek: i)llome/nên], we see that he puts to himself the question thus--"does plato in the timæus conceive the earth as kept together and stationary--or as turning round and revolving, agreeably to the subsequent theory of aristarchus and seleukus?" here we find that plutarch conceives the alternative thus--either the earth does not revolve at all, or it revolves as aristarchus understood it. one or other of these two positions must have been laid down by plato in the timæus.--so we read in plutarch. but the fact is, that plato meant neither the one nor the other. the rotation of the earth round the solid cosmical axis, which he affirms in the timæus--is a phenomenon utterly different from the rotation of the earth as a free body round the imaginary line called its own axis, which was the doctrine of aristarchus. when expositors in plutarch's day, and since his day, enquired whether or not the platonic timæus affirmed the rotation of the earth, they meant to designate the rotation of the earth in the sense of aristarchus, and in the sense in which modern astronomy understands that capital fact. now speaking the language of modern astronomy, i think it certain that the rotation of the earth is _not_ to be found affirmed in the platonic timæus; and i agree with m. boeckh when he says (untersuch. p. ), "granting that aristotle ascribed to plato the doctrine of the rotation of the earth, he at least did not ascribe to him the doctrine as gruppe assumes, and as now understood." as between gruppe--who holds that the platonic timæus affirms the rotation of the earth, and that aristotle ascribes it to him, in our sense of the words--and m. boeckh, who denies this--i stand with the latter for the negative. but when m. boeckh assumes that the only alternative doctrine is the immobility of the earth, and tries to show that this doctrine is proclaimed in the platonic timæus--nay, that no opposite doctrine _can_ be proclaimed, because the discourse expressly announces the rotation of the sidereal heaven in twenty-four hours--i am compelled to dissent from him as to the conclusion, and to deny the cogency of his proof. m. boeckh has hardly asked himself the question, whether there was not some other sense in which plato might have affirmed it in the timæus. i have endeavoured to show that there was another sense; that there are good analogies in plato to justify the belief that he intended to affirm the doctrine in that other sense; and that the comments of aristotle--while thoroughly pertinent, if we thus understand the passage in the timæus--become either irrelevant, dishonest, or absurd, if we construe the passage as signifying either what is maintained by m. boeckh or what is maintained by gruppe. the eminent critics, whose opinions i here controvert, have been apparently misled by the superior astronomical acquirements of the present age, and have too hastily made the intellectual exigencies of their own minds a standard for all other minds, in different ages as well as in different states of cultivation. the question before us is, not what doctrines are scientifically true or scientifically compatible with each other, but what doctrines were affirmed or implied by plato. in interpreting him, we are required to keep our minds independent of subsequent astronomical theories. we must look, first and chiefly, to what is said by plato himself; next, if that be obscure, to the construction and comments of his contemporaries so far as they are before us. in no case is this more essential than in the doctrine of the rotation of the earth, which in the modern mind has risen to its proper rank in scientific importance, and has become connected with collateral consequences and associations foreign to the ideas of the ancient pythagoreans, or plato, or aristotle. unless we disengage ourselves from these more recent associations, we cannot properly understand the doctrine as it stands in the platonic timæus. this doctrine, as i have endeavoured to explain it, leads to an instructive contrast between the cosmical theories of plato (in the timæus) and aristotle. plato conceives the kosmos as one animated and intelligent being or god, composed of body and soul. its body is moved and governed by its soul, which is fixed or rooted in the centre, but stretches to the circumference on all sides, as well as all round the exterior. it has a perpetual movement of circular rotation in the same unchanged place, which is the sort of movement most worthy of a rational and intelligent being. the revolutions of the exterior or sidereal sphere (circle of the same) depend on and are determined by the revolutions of the solid cylinder or axis, which traverses the kosmos in its whole diameter. besides these, there are various interior spheres or circles (circles of the different), which rotate by distinct and variable impulses in a direction opposite to the sidereal sphere. this latter is so much more powerful than they, that it carries them all round with it; yet they make good, to a certain extent, their own special opposite movement, which causes their positions to be ever changing, and the whole system to be complicated. but the grand capital, uniform, overpowering, movement of the kosmos, consists in the revolution of the solid axis, which determines that of the exterior sidereal sphere. the impulse or stimulus to this movement comes from the cosmical soul, which has its root in the centre. just at this point is situated the earth, "the oldest and most venerable of intra-kosmic deities," packed round the centre of the axis, and having for its function to guard and regulate those revolutions of the axis, and through them those of the outer sphere, on which the succession of day and night depends--as well as to nurse mankind. in all this we see that the ruling principle and force of the kosmos ([greek: to\ ê(gemoniko\n tou= ko/smou]) is made to dwell in and emanate from _its centre_. when we come to aristotle, we find that the ruling principle or force of the kosmos is placed, not in its centre, but in its circumference. he recognises no solid revolving axis traversing the whole diameter of the kosmos the interior of the kosmos is occupied by the four elements--earth, water, air, fire--neither of which can revolve except by violence or under the pressure of extraneous force. to each of them rectilinear motion is _natural_; earth moves naturally towards the centre--fire moves naturally towards the circumference, away from the centre. but the peripheral substance of the kosmos is radically distinct from the four elements: rotatory motion in a circle is _natural_ to it, and is the only variety of motion natural to it. that it is moved at all, it owes to a _primum movens immobile_ impelling it: but the two are coeternal, and the motion has neither beginning nor end. that when moved, its motion is rotatory and not rectilinear, it owes to its own nature. it rotates perpetually, through its own nature and inherent virtue, not by constraining pressure communicated from a centre or from a soul. if constraint were required--if there were any contrary tendency to be overcome--the revolving periphery would become fatigued, and would require periods of repose; but, since in revolving it only obeys its own peculiar nature, it persists for ever without knowing fatigue. this peripheral or fifth essence, perpetually revolving, is the divine, venerable, and commanding portion of the kosmos, more grand and honourable than the interior parts or the centre. aristotle lays this down (de coelo, ii. , p. , b. ) in express antithesis to the pythagoreans, who (like plato) considered the centre as the point of grandeur and command, placing fire in the centre for that reason. the earth has no positive cosmical function in aristotle; it occupies the centre because all its parts have a natural movement towards the centre: and it is unmoved because there _must be_ something in the centre which is always stationary, as a contrary or antithesis to the fifth essence or peripheral substance of the kosmos, which is in perpetual rotation by its own immutable nature. i do not here go farther into the exposition of these ancient cosmical theories. i have adverted to aristotle's doctrine only so far as was necessary to elucidate, by contrast, that which i believe to be the meaning of the platonic timæus about the rotation of the earth. london: printed by w. clowes and sons, stamford street, and charing cross. **************************************************************** transcriber's note the text is based on versions made available by the internet archive. for the greek transcriptions the following conventions have been used: ) is for smooth breathing; ( for hard; + for diaeresis; / for acute accent; \ for grave; = for circumflex; | for iota subscript. ch is used for chi, ph for phi, ps for psi, th for theta; ê for eta and ô for omega; u is used for upsilon in all cases. corrections to the text, indicated with ** text correction ti/ma/iô ti/maio chore/ia chorei/a ou)/ranon ou)rano\n tetamenon tetame/non e)ndeiknutai e)ndei/knutai] forge force tima/iô| timai/ô| gegraptai ge/graptai] e(ilei=sthai ei(lei=sthai e)/pi\ e)pi\ a)utê=s au)tê=s